1 Introduction
Environmental pollution and energy shortage are becoming more and more serious, posing great challenges to global sustainable development
[1-3]. The continuous accumulation of organic pollutants in water and the widespread presence of volatile organic compounds (VOCs), nitrogen oxides (NO
x) and carbon dioxide (CO
2) in the atmosphere seriously threaten ecological security and public health
[4-7]. Traditional physical, chemical, and biological treatment technologies are often energy-intensive, inefficient, and prone to secondary pollution, struggling to degrade stable pollutants
[8-12]. Thus, efficient, clean, and sustainable conversion technologies are urgently needed. In this context, photocatalysis, as a green and energy-saving advanced oxidation process, has shown great potential and broad application prospects
[13-16]. It directly harnesses solar energy to drive pollutant degradation and resource-oriented conversion under mild conditions, avoiding secondary pollution
[17-23], and has been widely studied in wastewater treatment, air purification, hydrogen production, CO
2 reduction, and high-value organic synthesis
[24-29].
Photocatalyst is the core of photocatalytic process, and its performance directly determines the photocatalytic efficiency
[30]. At present, the commonly used photocatalysts mainly include TiO
2, bismuth compounds, metal sulfide and other semiconductor materials
[31-36]. TiO
2 is widely studied and applied because of its high chemical stability, low cost, non-toxic and other advantages
[37]. However, traditional photocatalysts generally have a narrow light response range, mostly limited to the ultraviolet region, and low utilization of solar energy
[38]. The recombination rate of photogenerated electron hole pairs is high, which leads to the problem of low quantum efficiency, and it is difficult to achieve efficient conversion in CO
2 reduction and other reactions
[39,40]. Traditional reactors such as tank and tube reactors further hinder industrialization due to uneven catalyst dispersion, poor mass transfer, and difficulty in continuous production
[41,42]. Furthermore, among the forms of photocatalysts, powder photocatalysts account for the majority, but they have certain shortcomings: (1) Difficult to recycle. The particles of powder catalysts are tiny and easily dispersed, making it difficult to completely separate them through conventional methods during reuse, resulting in significant loss and increasing operating costs, as well as potentially affecting the purity of the products. (2) Prone to agglomeration. Due to their high surface energy, they are prone to agglomeration, leading to a decrease in specific surface area and coverage of active sites, significantly reducing the photocatalytic activity. (3) Poor applicability in continuous flow. In continuous reaction systems for pollutant degradation and hydrogen production, suspended solids are easily carried away by the fluid and may cause pipeline blockage. This problem makes it difficult to maintain long-term stable operation of the system.
Additive manufacturing, also known as 3D printing, has developed rapidly in recent years
[43]. The basic principle of this technology is to achieve rapid fabrication from scratch by computer generated geometric information or by obtaining reverse geometric information through 3D scanning of the object
[44]. It was first proposed in the 1980s, inspired by the slicing forming method. After decades of development, it has formed mainstream technologies such as direct inkjet writing (DIW) fused deposition modeling (FDM), stereolithography (SLA), and selective laser sintering (SLS), which can process various materials including plastics, metals, ceramics, and biological materials
[45]. As an emerging technology based on the principle of layer-by-layer stacking, 3D printing has significant advantages over traditional manufacturing
[46-49]. First, it can achieve a high degree of customization and precise molding of complex structures. At the same time, its material utilization rate is very high, far beyond the traditional subtractive manufacturing
[50]. Secondly, it does not rely on molds. This allows it to quickly switch between small-batch production and multi-variety production, thereby reducing initial costs
[51]. Finally, its digital drive model supports the integrated manufacturing of complex structures such as hollow meshes and internal flow channels. At the same time, the multi-head synergy technology can realize the accurate composite of a variety of materials
[52]. These features make it possible to integrate functional devices.
3D printing technology based on photocatalysis provides a new solution for the design of traditional photocatalysts and the fabrication of photocatalytic reactors. With the precise prototyping capability of 3D printing, photocatalytic materials and devices with specific microstructures and macroscopic shapes can be customized. This not only optimizes the capture, transmission and utilization efficiency of light, but also regulates the transfer process of reactants and products. At the same time, by introducing functional materials, the performance of photocatalysts can be further improved and multifunctional integration can be achieved
[53]. Specifically, 3D printing can be used to fabricate photocatalysts with graded porous structure to increase their specific surface area and promote adequate contact between reactants and catalysts
[54]. The constructed microchannel structure can also optimize the flow state of gas or liquid, thereby realizing a continuous and controllable photocatalytic reaction process
[55]. In addition, with the multi-material molding ability of 3D printing, the photocatalyst can be combined with auxiliary materials such as conductive materials and high thermal conductivity materials to construct new photocatalytic systems suitable for different scenarios, which will further promote the development and practical application of this technology
[56-58]. With the rapid development and wide application of 3D printing technology, the attention in the field of photocatalyst research has increased significantly. The number of relevant publications has shown a steady increase over the years, reflecting that this cross-cutting research direction is gaining increasing academic attention (
Fig. 1).
3D printing technology based on photocatalysis has achieved significant research progress, yet it remains at an early developmental stage
[59]. The integration of 3D printing and photocatalysis is not merely a simple technological overlay, but a targeted breakthrough addressing the core pain points of traditional photocatalytic technology. It can not only solve the problems of difficult recovery and easy agglomeration faced by traditional powder photocatalysts through macro-micro structural formation, but also address the deficiencies of fixed reactor structures and insufficient coordination among various parts through customized design. Therefore, systematically reviewing the technological progress, core challenges, and application potential in this interdisciplinary field holds crucial theoretical and practical significance for promoting the transition of photocatalytic technology from the laboratory to industrial applications
[60]. As shown in
Fig. 2, this review will comprehensively summarize and deeply explore various applications of 3D printing technology in the field of photocatalysis. Firstly, this review introduces the basic principles of photocatalytic technology and 3D printing technology, and briefly introduces their historical development and influencing factors. Subsequently, several representative 3D printing technologies are introduced in detail, including the research and application of fused deposition modeling, direct ink writing, laser etching and stereolithography in the direct shaping of photocatalysts and the construction of photocatalyst support. Then, the latest progress and application of 3D printing technology in the fabrication of photocatalytic reactors and the construction of photocatalytic systems are analyzed.
2 Fundamentals of photocatalytic technology and 3D printing technology
2.1 Overview of photocatalytic technology
Photocatalytic technology, as a highly promising cutting-edge science, was first discovered by Akira Fujishima when he found that TiO
2 electrodes could decompose water to produce hydrogen and oxygen under ultraviolet light
[69-71]. This phenomenon is known as the Hondo-Fujishima effect. This discovery marked the beginning of the development of photocatalytic technology in the scientific community. The publication of relevant research results in a magazine in 1972 officially announced the birth of photocatalytic technology. After that, researchers conducted a series of fundamental studies on photocatalytic technology
[72-75]. Researchers have focused on the photocatalytic mechanism of TiO
2 and discovered that it cannot only decompose water but also degrade organic pollutants and has a bactericidal function. On this basis, researchers began to explore other semiconductor materials, such as ZnO and WO
3 etc. However, these materials have wide band gaps and can only absorb ultraviolet light, with a very low utilization rate of solar energy
[76]. This problem greatly limits the practical application of photocatalytic technology. To break through this core obstacle at the principle level, since the 21st century, researchers have focused on the modification and optimization of photocatalytic materials, optimizing the principle process through targeted measures: The photogenerated carrier separation efficiency is enhanced by element doping
[77], the photocatalytic performance is strengthened by heterostructure construction
[78], and combined with cocatalyst loading, morphology regulation and other methods
[79,80]. It not only broadens the light response range of the material, but also enhances the utilization rate of light energy and the efficiency of carrier separation, fundamentally promoting technological breakthroughs and achieving leapfrog development.
2.1.1 Photocatalytic mechanism
The core principle that supports this series of developments can be broken down into three interlinked steps, as shown in
Fig. 3. The first step is light absorption. When the energy of the irradiated light is equal to or higher than the band gap (Eg) of the photocatalyst, the electrons in the valence band are excited and jump to the conduction band with higher energy, at the same time, holes are generated in the valence band and photo-induced electron-hole pairs are formed. The second step is charge transfer
[81-83]. Direct recombination of electrons and holes results in energy loss. Therefore, it is essential to promote the directional migration of electrons and holes while inhibiting their recombination through approaches like material modification. The third step is surface reaction. Electrons and holes that migrate to the surface of the catalyst, respectively, undergo reduction and oxidation reactions with the species adsorbed on the surface, ultimately enabling key conversion processes—including pollutant degradation, CO
2 reduction, and hydrogen production through photolysis of H
2O
[84,85]. Throughout the entire process, the separation efficiency of electron-hole pairs, the adsorption capacity of surface-active species, and the lifetime of photogenerated carriers are the three core factors determining the efficiency of photocatalytic reactions
[86-90]. At present, although photocatalytic technology has shown great application potential in the fields of environmental governance and energy conversion, its practical implementation still faces urgent problems to be solved, including low quantum efficiency and insufficient catalyst stability
[91-93]. Based on the above analysis of the development history and principle of photocatalytic reaction, researchers have done a lot of work to optimize the efficiency of photocatalysis, mainly including the design of materials and the search for new catalyst materials. Among the diverse material modification strategies for optimizing photocatalytic performance, heterojunction construction has become a research focus due to its remarkable ability to regulate the core factors mentioned above. By rationally assembling two or more semiconductors with complementary band structures, heterojunctions can construct interfacial charge transfer paths or internal electric fields, which effectively promote the directional migration of photoinduced electrons and holes, drastically inhibiting their recombination and thus extending the lifetime of photogenerated carriers. Moreover, the synergistic effect between heterogeneous components can also enhance the surface adsorption capacity for target reactants and adjust the optical absorption properties of the catalyst, further improving the utilization efficiency of incident light. In recent years, various types of heterojunctions (such as Type-II heterojunctions, Z-scheme heterojunctions, and S-scheme heterojunctions) have been widely explored and applied, achieving significant improvements in photocatalytic activity for pollutant degradation, CO
2 reduction, and water splitting. However, challenges such as interfacial compatibility issues, high charge transfer resistance at the heterointerfaces, and poor long-term stability still restrict the practical application of heterojunction-based photocatalysts. Therefore, developing efficient and innovative new catalysts to achieve low-cost and high-efficiency conversion in photocatalysis is of vital importance. A variety of new types of catalysts, including metal oxides, metal sulfides, and bismuth halogen oxide materials, have been developed and applied in the field of photocatalysis
[94-98].
2.1.2 Photocatalysts
As the basis of artificial photosynthesis, the development of photocatalytic materials with appropriate and excellent catalytic performance is particularly crucial. A series of photocatalytic materials with photoresponsivity properties, including semiconductor materials, noble metal coordination compounds and organic molecular materials, have been proven to be ideal for driving photocatalytic reactions. Among these categories, semiconductor materials find extensive application in the photocatalysis field, attributed to their advantages of excellent light absorption capacity, favorable stability, abundant resource reserves, and low material costs
[99-102]. Based on their composition, semiconductor materials can currently be classified into several types of photocatalysts, including metal oxides, metal sulfides, metal oxynitrides/halide oxides and non-metals (
Fig. 4). As the earliest discovered and studied photocatalytic material within metal oxides, TiO
2 demonstrates traits of good chemical stability, non-toxicity, and low cost, thereby securing wide application in the photocatalysis field. However, traditional TiO
2 materials still have limitations in terms of their wide band gap and high recombination rate of photogenerated carriers. Therefore, a series of modification methods have been adopted for the design of materials based on the traditional TiO
2 phase. Liang et al. reviewed the research on black TiO
2, which has been widely used in environmental and energy fields due to its excellent light absorption capacity
[103]. Yang et al. systematically investigated the progress and paradigm shifts in the development of mesoporous TiO
2 photocatalysts
[104]. Mesoporous TiO
2-based structures significantly enhance the activity of photocatalysts due to their tunable pore topology, increased surface-to-volume ratio, and improved mass transfer performance. In addition to the well-known TiO
2 material, several other metal oxide, including ZnO
[105], WO
3 [106] and In
2O
3 [107], have also been widely reported to be applied in the field of photocatalysis. Nevertheless, limited by the inherent band structure of metal oxide semiconductor materials, these materials have poor absorption in the visible light range. Different from metal oxides, metal sulfide semiconductors usually have a smaller band gap, making them widely used in visible light-driven photocatalytic reactions. According to existing reports, metal sulfides, such as CdS
[108] and ZnS
[109], exhibit favorable photocatalytic effects. The application of these materials, however, has been constrained to a certain extent, hindered by issues including the inherent toxicity and low stability of metal sulfides themselves. In view of the problem that metal oxides have poor light absorption capacity due to their large band gap, researchers have proposed the design and research of metal oxynitrides
[110] and metal halide oxides
[111]. Among them, metal oxynitrides formed by doping nitrogen into metal oxides significantly expand the light absorption range to the visible light region
[112]. Taking bismuth oxyhalide, a type of metal halide oxide, as an example, its light absorption shows a red-shift from the ultraviolet to the visible light region as the contained halogen element changes from chlorine to iodine
[113]. In comparison with metal compound materials, non-metallic materials have found extensive application in the photocatalysis field, leveraging advantages of abundant resources, favorable stability, and low cost
[114-117]. Some common non-metallic materials, including graphene
[118], carbon nitride
[119] and black phosphorus
[120], have been confirmed to possess good photocatalytic activity. Wang et al. proposed loading nitrogen-doped graphene quantum dots (NGQDs) onto the surface of Mn
xCd
1-xS solid solution nanowires
[121]. The optimized Mn
0.2Cd
0.8S/NGQDs (M
0.2NG
5) composite achieved a high production rate of 6885 μmol g
-1 h
-1. Jang et al. developed a method to produce one-dimensional (1D) P-doped carbon nitride nanotubes via supramolecular self-assembly, which exhibited good photocatalytic HER activity
[122]. Shen et al. synthesized a Ni
2P-BP photocatalyst by selectively growing Ni
2P on the edges of BP nanosheets, and this catalyst achieved 100% selectivity for the photocatalytic reduction of N
2 to NH
3 [123]. In contrast to traditional photocatalysts, reticular framework materials, particularly metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have emerged as highly promising photocatalytic materials. This status is attributed to their characteristics of ultra-high surface area, customizable pore environment, and modular functionality
[124]. A large number of reports have shown that MOFs, COFs and their composite materials are excellent photocatalytic materials. Chen et al. proposed and constructed a series of covalently linked multi-component Ti-MOF/COF hybrid materials, with the optimal composite achieving a photocatalytic H
2 evolution rate of 12.8 mmol g
-1 h
-1 under simulated sunlight irradiation
[125]. Wang et al. successfully synthesized three MOFs based on tetrakis(4-carboxyphenyl)porphyrin (TCPP) via a solvothermal method, and the Zr-Ni PMOF showed a coupling reaction conversion rate of 89% for benzylamine under visible light, with a selectivity of 95% for the target product
[126]. Deng et al. designed and constructed PDI/COFs hybrid materials with different ratios via a solvothermal method, denoted as PDI/TAPB-PDA (where TAPB=1,3,5-tris(4-aminophenyl)benzene and PDA = terephthaldehyde)
[127]. Among these materials, the optimal composite PDI/TAPB-PDA-5 shows the highest photocatalytic performance for Cr(VI) reduction. To tackle issues including low solar energy utilization efficiency, high recombination rate, and limited activity, considerable research efforts have been devoted to the preparation and selection of catalysts. However, the structural impact of the catalyst substrate itself has been neglected, and traditional methods for the preparation of catalyst matrices remain restricted by factors such as the complexity of catalyst loading, as well as the stability and corrosion rate of the support.
2.1.3 Photocatalytic reactors
A photocatalytic reactor is a device that utilizes semiconductor-based photocatalysts to drive a series of redox reactions under visible light or simulated solar illumination, with the aim of achieving goals like environmental pollutant degradation and energy conversion
[136]. Its core function lies in its comprehensive enhancement of photocatalytic reaction performance by improving light energy utilization efficiency, optimizing the mass transfer process, and refining the contact between the catalyst and reactants
[137]. In recent years, a variety of novel photocatalytic reactor designs have emerged, including photocatalytic membrane reactors that integrate catalytic functionality with separation membranes, microchannel reactors that leverage microscale effects to enhance mass transfer and light absorption, and bionic structured reactors constructed based on 3D printing technology
[138-141]. Among these, the latter simulates the efficient light-harvesting and mass transfer pathways in natural photosynthesis, demonstrating significant advantages. Particularly in the field of photocatalyst design, 3D printing technology has achieved breakthrough progress. It enables the controllable construction of hierarchical porous structures, achieves precise regulation of heterojunction interfaces, and realizes optimization of active site distribution, all of which effectively enhance the performance and stability of catalytically active components
[142]. The efficient implementation of photocatalytic reactions is not only regulated by core factors including catalysts, reactants and reactors, but also highly dependent on the adaptability between catalysts and reaction environments, the optimization of energy input conditions, and the efficiency of mass transfer processes
[143]. These multiple factors collectively constitute the key variables affecting reaction performance. Therefore, the configuration design and system integration of the reactor itself play a crucial role in improving overall catalytic efficiency, and also represent a key direction for the future development of this field.
From the fixed loading of catalysts to the precise control of reaction flow fields, from the improvement of light utilization efficiency to the integrated operation of multiple reaction unit, the limitations of traditional manufacturing methods in achieving structural and functional matching have become increasingly prominent
[144]. For example, mechanical processing is difficult to produce micro-sized or nano-sized irregular flow channels, and injection molding cannot achieve material gradient distribution. These defects make it difficult to effectively transform the optimization of a single catalyst’s performance into an overall efficiency increase of the reaction system, becoming a key bottleneck restricting the industrialization of photocatalytic technology.
Therefore, further expanding the design freedom and manufacturing accuracy of 3D printing technology to the level of photocatalytic reactors and the overall system, and solving the coupling problems of multiple elements through structural innovation, has become a key link in breaking through the bottlenecks of traditional photocatalytic technology applications and promoting its transition from laboratory research to industrial application.
2.1.4 In-situ characterization for revealing mechanisms
As a complement to the prospective methodology introduced in the previous section, this section focuses on in-situ characterization techniques that are already mature in traditional photocatalysis but have not yet been widely adopted in the cross-disciplinary field of 3D printed photocatalysis. Given that the current research emphasis is on verifying the feasibility of 3D printing and demonstrating performance at the macroscale, mechanistic studies are still preliminary. However, as the field matures, in-situ characterization will become an essential tool for decoding microscale mechanisms. The following is a brief overview of validated in-situ techniques, laying the foundation for future research in this interdisciplinary field.
In-situ characterization technology can monitor various changes in the process of photocatalytic reaction in real time, which provides key information for understanding the performance and reaction mechanism of photocatalysts
[145]. By combining different in-situ technologies, we can more comprehensively reveal the various steps in the photocatalytic process, and provide a theoretical basis for the design of more efficient and stable photocatalysts
[146]. In-situ characterization technology has not been widely used in the characterization of photocatalysts combined with 3D printing technology. At this stage, the primary goal of many studies is to verify the feasibility of 3D printing as a manufacturing method in the field of photocatalysis. Therefore, the research focus tends to show the superiority of macro performance, temporarily reducing the urgency of deep mining of micro mechanism. However, it can be predicted that with the further research, more in-situ characterization techniques will be applied to the study of the reaction mechanism of 3D printing photocatalysts. At the same time, a series of in-situ characterization techniques have been widely used in the field of photocatalysis. Therefore, it is necessary to briefly introduce these validated in-situ characterization techniques.
A single photocatalytic reaction usually occurs between microseconds and milliseconds. In situ characterization technology allows real-time monitoring of the structure, electronic properties of the catalyst and the dynamic changes of reaction intermediates under real reaction conditions, so as to directly reveal the reaction mechanism. At present, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (Raman), electron paramagnetic resonance (EPR) and atomic absorption spectroscopy (XAS) are mainly used to identify and quantify the activation/conversion and mass transfer of reactants
[147]. In-situ FTIR is an important tool to study the adsorption, activation and transformation of reactants in photocatalytic reactions. Three operating modes, namely, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), transmission mode and attenuated total reflection (ATR) mode, are usually used to process materials with different properties. Fu et al. studied the photocatalytic oxidation of methanol by TiO
2 using in-situ DRIFTS, and determined that the photocatalytic activity of TiO
2-(101) surface was higher than that of TiO
2-(001) and TiO
2-(100) surfaces through spectral changes
[148]. TiO
2-(101) surface could activate CH
3OH and CH
3O-, while TiO
2-(001) and TiO
2-(100) surfaces mainly activated CH
3O-. Ana et al. monitored the photocatalytic oxidation of cyclohexane in real time by in-situ ATR-FTIR
[149]. The absorption band appears only after oxygen isotope exchange treatment, which indicates that dissolved oxygen is not directly involved in the photocatalytic oxidation of cyclohexane. With the passage of time, the absorption band of cyclohexanone carbonyl (C =
18O) on the surface of TiO
2 catalyst gradually increased, indicating that the formation of cyclohexanone is a continuous process. In-situ Raman technology can identify chemical substances by inelastic light scattering, which can provide information about the structure and surface properties of photocatalysts. Cai and his colleagues prepared 4-NTP monolayer photocatalysts on Au(111) surface by drop coating method and impregnation method to study the photocatalytic coupling reaction
[150]. Through the changes of the spectrum, it was found that most of the surface areas of the drop coating samples had efficient photocatalytic coupling reaction, while most of the surface areas of the impregnation samples had no reaction, and only a few isolated areas had reaction. This shows that the arrangement of molecules has a significant effect on the efficiency of photocatalytic coupling reaction. In-situ EPR technology can detect the free radicals and unpaired electrons produced in the photocatalytic process, so as to provide dynamic information about the generation, migration and reaction of free radicals, and help reveal the mechanism of electron transfer in the photocatalytic reaction. Zhang et al. Synthesized a Cu monatomic catalyst (CuSA-TiO
2) for photocatalytic hydrogen production
[151]. The CuSA-TiO
2 catalyst achieves a photocatalytic hydrogen evolution rate of 356 μmol g
-1 h
-1 under UV-visible light, with a quantum yield of 8.2% at 420 nm. In situ EPR results showed that Cu
2+ was reduced to Cu
+ during the photocatalytic reaction, and the subsequent oxidation process exposed to air showed that Cu species had a reversible redox cycle in the reaction, which might be one of the key factors for the high photocatalytic activity of CuSA-TiO
2. In-situ XAS technology can monitor the changes of the electronic structure and chemical state of the catalyst in the photocatalytic reaction in real time. It can provide information about the oxidation state, coordination environment and electron density of metal ions in the catalyst, which is helpful to understand the activity and stability of the catalyst. Liu et al. observed the changes of Cu during the experiment by in-situ XANES and found that the high activity of Cu/Ti(H
2) catalyst was attributed to the abundant Cu
+ species on its surface, and these Cu
+ species were prone to oxidation during the reaction, thus leading to the deactivation of the catalyst
[152]. By combining different in-situ characterization techniques, we can more comprehensively reveal the various steps in the photocatalytic process, and provide a theoretical basis for the design of more efficient and stable photocatalysts.
2.1.5 DFT calculation for revealing mechanisms
This section introduces density functional theory (DFT) calculations, a core theoretical tool in the field of photocatalysis, and focuses on exploring its potential value and existing challenges in the cross-field of 3D printing and photocatalysis. Although DFT excels in exploring mechanisms at the atomic scale, combining it with the macroscale structural advantages of 3D printing requires multiscale modeling to bridge the gap between atomic simulations and macroscale effects. This topic will be discussed alongside the basic principles and applications of DFT in the following sections.
Thanks to the rapid development of computer technology, theoretical calculation can help researchers in many aspects such as experimental design, result analysis and mechanism exploration at each stage of the experiment
[153,154]. First principles calculations are widely used in the field of photocatalysis, mainly due to the development and improvement of density functional theory (DFT)
[155-157]. The core idea of DFT is to describe the physical properties of a system using the electron density of the system's ground state instead of complex many-body wave functions, which significantly simplifies the calculation
[158]. To solve it practically, DFT employs some key approximations, among which the exchange-correlation functional is the core
[159]. Common approximation methods include the Local Density Approximation (LDA) and the Generalized Gradient Approximation (GGA), with the latter being more commonly used. To calculate the semiconductor band gap more accurately, a hybrid functional is also used
[160]. A typical DFT calculation is a self-consistent iterative process: first, an initial electron density is guessed, then the Kohn-Sham equation is solved to obtain a new electron density, the new and old densities are compared, and the process is repeated until convergence, thereby obtaining the ground state energy and electronic structure of the system. In the field of photocatalysis, DFT calculations can be applied to band structure and state density analysis, reaction path and free energy calculation, heterojunction and interface design, mechanism analysis of cocatalysts, and high-throughput screening of new materials. Wu et al. successfully synthesized a cobalt-based triphenylene framework catalyst with a staggered stacking structure in pure aqueous phase, achieving a synthesis yield as high as 90%
[161]. Density functional theory calculations indicated that the CO yield was mainly influenced by ΔG*H, electronegativity (χ), optical band gap, and the increment of reaction rate, rather than the traditionally believed ΔG (*COOH). This new understanding reveals that the photocatalytic CO
2 reduction is a complex process involving the synergy of multiple factors in both kinetics and thermodynamics, providing a new perspective for the design of novel catalysts. The unique advantage of 3D-printed photocatalysts lies in their designable macroscopic three-dimensional structure. DFT is proficient in simulating catalytic reactions at the atomic scale within a few nanometers. However, to effectively correlate the effects of atomic-scale simulations on the light absorption and mass transfer efficiency of complex three-dimensional structures at the macroscopic millimeter or even centimeter scale is a significant challenge currently faced by computational science.
The disconnection between DFT simulations and macroscale 3D structural effects underscores the pivotal role of multiscale modeling in this interdisciplinary field, yet it faces two major challenges. Firstly, there is the computational complexity of integrating DFT calculations with macroscale fluid dynamics and optical simulations. Secondly, there is a lack of standardized models to correlate 3D printing-induced microstructures with atomic-level catalytic activity. Despite these obstacles, there are significant opportunities. Multiscale modeling can guide the rational design of 3D printed photocatalysts, optimize printing parameters to match active sites identified by DFT, or predict how macroscale porous structures enhance light capture and reactant diffusion. As this interdisciplinary field matures, multiscale modeling will become a crucial bridge to maximize the synergistic effect between 3D printing structural control and atomic-scale activity in photocatalysis.
2.2 Overview of 3D printing technology
3D printing technology, also known as additive manufacturing technology, is a manufacturing technique that builds three-dimensional objects by layering materials based on digital model files
[162-164]. Its core feature is layer-by-layer fabrication and successive addition, which contrasts sharply with the material removal approach of traditional subtractive manufacturing methods
[165-167]. In 1984, Charles Hull invented SLA technology and obtained a patent, which marked the official birth of 3D printing technology. In 1988, Scott Crump proposed FDM technology, promoting the development of low-cost equipment. From the 1990s to the early 21st century, technologies such as SLS and Selective Laser Melting (SLM) emerged one after another, further expanding the range of applicable materials. After 2010, with the popularization of open-source technology and the expiration of patents, the cost of 3D printing has significantly decreased, and its application fields have expanded from industrial prototypes to aerospace, medical care, architecture and other fields
[59,168 -171]. According to the ISO/ASTM 52900:2021 standard, 3D printing technologies can be classified into seven categories, as shown in
Fig. 5 [172]. Vat Polymerization initiates curing reactions by irradiating liquid photopolymers with light sources, with core representative technologies including SLA, Digital Light Processing (DLP), and Continuous Digital Light Processing (CDLP). Material Extrusion relies on heated nozzles to extrude and deposit molten thermoplastic plastics, typically represented by FDM or Fused Filament Fabrication (FFF). Powder Bed Fusion achieves melting and solidification of powder particles using high-energy sources, covering SLS, SLM, Multi Jet Fusion (MJF), and Electron Beam Melting (EBM). Material jetting deposits shapeable materials in the form of droplets and finishes curing. The representative technologies include material jetting (MJ), nanoparticle jetting (NPJ), and on-demand drop (DOD). Binder Jetting realizes the bonding of particulate materials by depositing liquid binders, represented by Binder Jetting (BJ) technology. Direct Energy Deposition synchronously completes the processes of metal melting, deposition, and fusion, with representative technologies including Laser Engineered Net Shaping (LENS) and Electron Beam Additive Manufacturing (EBAM). Sheet Lamination achieves manufacturing through cutting and forming of material sheets and layer-by-layer lamination, with Laminated Object Manufacturing (LOM) technology as the core. The specific details are shown in
Table. 1.Compared with traditional manufacturing methods, 3D printing technology shows significant technical advantages
[173-175]. Firstly, it has extremely high design freedom, which can break through the limitations of traditional processing on molds and tool paths, and directly manufacture parts with complex inner cavities, hollow structures, and topology-optimized shapes, providing unlimited possibilities for structural innovation. Secondly, the production cycle is significantly shortened. The conversion cycle from digital models to physical parts can be reduced to several hours to days, which is especially suitable for rapid prototype verification in the product research and development stage. For small-batch production, there is no need to open molds, which significantly reduces time and economic costs. Thirdly, the material utilization rate is significantly improved. The material utilization rate of traditional subtraction manufacturing is low, while the material utilization rate of 3D printing technology can be significantly improved. For high-priced materials such as titanium alloys and precious metals, the economic benefits are particularly prominent. Fourthly, it has outstanding personalized customization capabilities. Personalized design and production of products can be realized only by modifying digital models without adjusting the production line, which has irreplaceable advantages in the fields of medical implants and personalized consumer goods. Fifthly, it has excellent integrated manufacturing capabilities, which can integrally form structures that need to be assembled with multiple components in traditional manufacturing, reducing assembly links, lowering error accumulation, and improving the reliability of overall performance
[176-178].
Different 3D printing technologies show unique advantages in application scenarios due to differences in principles and material properties. DIW technology in material extrusion, which forms shapes by extruding high-viscosity slurries, can flexibly adjust material composition and structure, and is especially suitable for preparing photocatalytic carriers with multi-level pore structures
[179]. FDM uses low-cost polymer filaments as raw materials, and the equipment is highly popular, suitable for preparing photocatalytic reactor shells or basic structural parts
[180]. The SLA technology in vat photopolymerization, relying on high-precision forming capabilities, can prepare photocatalytic devices with micro-nano scale channels, improving photocatalytic reaction efficiency
[181]. The SLS technology in powder bed fusion can directly sinter ceramic or composite powders to prepare high-strength and high-stability photocatalytic materials, meeting the needs of complex working conditions
[182]. These technologies, with their respective characteristics, have become mainstream means for constructing new catalytic materials and devices in the field of photocatalysis, providing a new technical path for the structural optimization and performance improvement of photocatalytic reaction systems
[183].
3 3D printing technology applied to photocatalysis
The design of photocatalysts and their carriers is the core factor determining the efficiency and stability of photocatalytic reactions
[184]. The density of active sites, electron transfer pathways, and light absorption range of photocatalysts, as well as the specific surface area, pore structure, and surface affinity of the carriers, are coupled with each other and jointly constitute the key factors affecting the performance of photocatalysis
[185]. Traditional preparation methods have significant limitations in constructing complex micro-nano structures, achieving precise multi-component composites, and regulating interface characteristics. The sol-gel method is difficult to control the uniform dispersion of catalyst particles, the impregnation method often leads to insufficient loading of active components and uneven distribution, and the pore volume and pore size distribution of the carrier are difficult to be finely adjusted in the conventional sintering process. These deficiencies directly restrict the separation efficiency of photogenerated carriers and the mass transfer rate of reactants.
Therefore, 3D printing technology, with its flexibility in layer-by-layer manufacturing, compatibility in multi-material co-printing, and designability of cross-scale structures, provides a new approach for the innovative design of photocatalysts and carriers. 3D printing technology can break through the geometric constraints and material limitations of traditional manufacturing processes. By directing the regulation of the structure, it can achieve atomic-level doping of active components, multi-level layered pore structure design of photocatalyst, and multi-level complex carrier framework construction, enabling controllable design for the optimization of material performance.
3.1 Design of photocatalysts
3.1.1 FDM technology for photocatalyst fabrication
Fused Deposition Modeling 3D printing technology has become the most popular 3D printing method due to its simple forming method and low cost. It consists of five parts: nozzle, wire feeding system, drive system, heater and working platform
[186]. The equipment melts filamentous consumables into liquid from the nozzle through an extrusion head equipped with a heater. The extrusion head moves along the contour of each cross-section of the printed object, and the extruded semi-fluid material cools and solidifies into a fine thin layer to cover the printed object
[187]. After the molding is completed, the worktable descends by one height, and the extrusion head repeats the upper steps, depositing layer by layer and stacking layer by layer to form a solid model
[188]. For the commonly used desktop FDM 3D printers, the layer height of the model in the Z-axis direction should reach at least 100 µm, and the accuracy in the X-axis and Y-axis directions can be controlled within 500 µm
[189]. However, the models printed by FDM 3D have clear layering patterns. For objects with high requirements for surface flatness, post-processing methods such as polishing are needed
[190]. The materials of FDM 3D printing technology are polymer filaments with thermoplastic properties, including polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), etc
[191-193]. However, the glass transition temperature and exposed surface area of these thermoplastic polymers are relatively low, making them unsuitable for direct use in catalytic reactions. Meanwhile, they exhibit poor stability and are easily affected by heat and light, thereby altering their physical and chemical properties
[194].
There are two methods for preparing FDM 3D printing catalysts. One is to incorporate the catalytic active components into the organic polymer framework and fully mix them to form a composite material for the manufacturing of integral catalyst 3D printing. This blending process is generally completed during the preparation of printing consumables. Firstly, the polymer materials are melted or dissolved in a solvent and uniformly blended with catalytic materials to form filaments, which are then made into integral catalytic materials through FDM 3D printing. This method is prone to cause the active site to be wrapped by the polymer material, making it difficult to come into contact with the reaction substrate and reducing the activity
[199]. Majooni et al. used FDM 3D printing technology to construct PLA porous scaffolds with various structures and optimized the filling structure
[65]. Among them, the surface area of the helical structure was 3259.2 mm
2, and the loading of nanocomposites was the highest. Acetone etching was used to enhance the surface roughness of the scaffolds, and combined with polydopamine as a biological adhesive, the loading capacity of the Cu-TiO
2/rGO nanocomposite coating was increased by 378% and the leaching amount was reduced by 200%. The composite scaffold could remove 89% methylene blue (MB) under visible light for 60 min, 93% for 120 min, and 91.4% for 30 cycles (
Fig. 6a). Ortega-Columbrans et al. fabricated PLA/TiO
2 composite filament by FDM 3D printing technology, and printed 3D scaffolds with linear and helical filling structures, while fabricating 2D composite membrane by the flow-through method
[195]. Under UV light, the degradation rate of methyl orange was 98% for 2D films, 80% for spiral filled scaffolds, and 60% for linear scaffolds. The 50% degradation time of methyl orange of the spiral scaffold was 6.1 h, which was better than that of the linear scaffold (8.9 h), and the composite wires had good thermal stability (
Fig. 6b, c).
Another rule is to combine the catalytic material on the surface of the catalyst by means of grafting and bonding, etc. to carry out catalytic action. However, this method is prone to causing the shedding of active sites. The preparation process of FDM 3D printing catalysts has strong universality and is compatible with various common desktop 3D printers on the market, making it easy to apply and promote
[200]. Khezri
et al. used FDM 3D printing to fabricate a hollow tubular intelligent dust robot using graphene/PLA as raw material
[196]. The Fe
3O
4 nanoparticles were immobilized on the surface for magnetically controlled recovery, and then coated with a chitosan/carbon nitride (g-C
3N
4) hydrogel layer to desorb picric acid under visible light due to the synergistic adsorption property of chitosan and the photocatalytic activity of g-C
3N
4. The degradation efficiency was up to 38% within 10 min, and the activity was still maintained after 3 cycles of recycling, which provided a new scheme for mobile photocatalytic degradation of pollutants (
Fig. 6d). Munoz et al. used FDM 3D printing to construct G/PLA nanocomposite scaffolds, which were activated by DMF to synthesize amorphous Ag
3PO
4 (a-Ag
3PO
4) and faceted Ag
3PO
4 (f-Ag
3PO
4) nanostructures in situ
[197]. Using this scaffold for visible light photocatalysis, the degradation rate of rhodamine B (RhB) by f-Ag
3PO
4@G/PLA was significantly higher than that by a-Ag
3PO
4@G/PLA, and the photocurrent of water and oxygen development was 18 times higher than that of the latter, and the charge transfer resistance was lower (
Fig. 6e). Mimerand et al. designed single-layer and multi-layer superimposed fractal scaffolds by FDM 3D printing of PLA fractal structures (fracmids and fracones)
[198]. They used plasma grafting technology to fix the ZnO@PAA core-shell nanoparticles on the surface of the scaffold. The prepared composite materials degrade 63% RhB under simulated sunlight for 5.5 h, and can be reused after ultrasonic cleaning. It is proved that the fractal structure printed by FDM can effectively improve the contact efficiency between the catalyst and the pollutants (
Fig. 6f). In another study, they used the same method to print PLA fractal scaffolds, and fixed ZnO, TiO
2 and Fe-MOF nano catalyst by plasma grafting, respectively
[201]. The degradation rate of RhB in the ZnO@PAA2.5 loaded four-layer fractal scaffold was 94.3%, and the removal rate of ciprofloxacin in the Fe-MOF loaded scaffold was 42%. The photoactivity of the ZnO@PAA core-shell catalyst was increased by 20%, and the efficiency of the scaffold was only decreased by 10.6% after three cycles of cycling, reflecting the application value of FDM printing in the fixation and efficient degradation of multitype photocatalysts.
In summary, FDM technology, with its low cost and widespread equipment availability, has become the most easily scalable method for preparing 3D-printed photocatalysts. However, the poor heat resistance of polymer substrates and the tendency for active components to be encapsulated limit its application in high-temperature reactions or scenarios demanding high activity. Additionally, insufficient printing accuracy can easily affect the uniformity of the microstructure, necessitating post-processing optimization such as acetone etching.
3.1.2 DIW technology for photocatalyst fabrication
Ink direct writing is another 3D printing technology based on extrusion
[202]. DIW 3D printing technology uses ink pastes with high solid content to print parts with high aspect ratios or without bridging structures and supports. During the process, the ink is placed in a rubber cylinder. As the printing nozzle moves, the air pressure or mechanical device uses the shear thinning ability of the slurry to extrude it out, completing the layer-on-layer stacking and maintaining the shape. After the solvent evaporates, it densifies or self-cross-links and solidifies to form a three-dimensional object. It can directly extrude masterbatch materials without the need for melting and solidification processes
[203]. In the early stage of the development of ink direct writing 3D printing forming, the related technical concept was first proposed by the team of J. Cesarano at Sandia National Laboratories in the United States
[204]. It was widely used in the forming process of ceramic materials, using ceramic suspension slurry as raw material to prepare uniform, fine and controllable three-dimensional ceramic structures
[205]. The DIW 3D printing technology is typically used in the preparation process of catalysts by compounding catalytic components with materials such as polymers, metal oxides or gels, adding appropriate solvents to prepare 3D printing inks for three-dimensional manufacturing, and then the object undergoes post-processing to obtain the catalyst
[206]. 3D printing ink needs to have its components and quantities adjusted to ensure good fluidity for smooth extrusion. At the same time, it also needs sufficient plasticity to maintain its shape after extrusion molding
[207]. The rheological properties of 3D printing ink can be expressed by the Herschel-Bulkley formula, as shown in formula (1).
where, n represents the shear thinning index, γ represents the shear rate, K represents the viscosity index, τγ represents the yield stress, and τ represents the shear stress. The 3D printing ink slurry needs to flow and deform. Due to the existence of τγ, the shear stress τ must be greater than the yield stress τγ of the material itself. Meanwhile, when 0<n<1, the apparent viscosity τγ of the slurry decreases with the increase of the shear rate, and the shear thinning behavior occurs. In addition to shear thinning and deformation, the printing conditions of 3D printing ink slurry also need to meet the requirement of having sufficient viscoelasticity to maintain its own structure. The viscoelasticity of the slurry is expressed by the following formula (2).
In the formula, φgel represents the volume fraction of solid content at the gel point, φ represents the volume fraction of the solid phase, and y represents the elastic modulus. Therefore, the solid content of the slurry can be increased and the solid content at the gel point can be reduced to improve the elastic modulus of the 3D printing slurry.
DIW technology is one of the most reported methods for manufacturing 3D printed monolithic catalysts at present
[214]. Its greatest advantage is that there are fewer restrictions on the types of materials. The water-based colloid paste system, degradable organic paste system and polyelectrolyte paste system in ink dispensing technology can be matched with the DIW 3D printing process, enriching the selection range of raw materials
[215-217]. Yu et al. prepared a grid-like photocatalytic material A-bio-Cu-BTC. Pollen was used as a biological template to assemble Cu-BTC nanoparticles
[208]. SA, CMC and catalyst were mixed to make printing ink, which was cross-linked and freeze-dried by CaCl
2 after printing. The material is used for organic dye degradation. During recycling, the surface porosity exposes more active sites, the Cu(I)/Cu(II) ratio increases from 0.80 to 1.67 to enhance the catalytic activity, and the tensile strength reaches 0.45 MPa. The specific surface area of Cu-BTC only decreased by 8.9% after soaking in 60℃ deionized water for 24 h, and the hydrolytic stability and mechanical properties were significantly better than those of pure Cu-BTC (
Fig. 7a). Lu et al. prepared 3D-FeO/C electrodes by DIW 3D printing, and mixed Fe
3O
4 nanoparticles, carbon black, PAN and HPMC to make printing ink
[209]. After printing, the macroscopic ordered pore structure was formed by pyrolysis at 600℃. This electrode can efficiently degrade ornidazole and actual pharmaceutical wastewater (TOC removal of 51.13% in 420 min), reduce metal leaching (Fe leaching < 0.2 mg L
-1) by the active site of FeO encapsulated in carbon layer, and enhance mass transfer by macroscopic pore structure. The degradation efficiency was above 92% after 5 cycles (
Fig. 7b). Alves et al. used DIW 3D printing to prepare MOF/MoS
2 composite aerogel
[210]. Alginate, CMC and gelatin were used as substrates, and MOF/MoS
2 was added to make printing ink. After printing, the ink was cross-linked by BaCl
2 and lyophilized. The aerogel was used for MB adsorption and photocatalytic degradation. The adsorption efficiency of 7.5 wt.% MOF/MoS
2 loaded aerogel was 95.86%, and the photocatalytic degradation efficiency was 89%. The efficiency remained high after 5 cycles of recycling (
Fig. 7c). Shao et al. used DIW 3D printing to prepare clay/biochar based monolithic material (CBM), adding ZnCl
2 to the printing ink to activate the sludge, and preparing Fe/CBM-S
x-Zn catalyst by impregnating Fe(NO
3)
3 after printing
[211]. The catalyst was used to activate peroxymonosulfate (PMS) to degrade levofloxacin. The degradation efficiency of Fe/CBM-S
10%-Zn loaded by 10% sludge reached 80% within 20 min, the Fe leaching amount was less than 0.2 mL
-1, and the structure was stable during recycling. Efficient degradation is achieved through free radical (SO
4-·,·OH) and non-free radical (
1O
2) pathways (
Fig. 8a). Martins et al. prepared the In
2O
3 monoliths material by DIW 3D printing, mixing the In
2O
3 powder with Pluronic F-127 to make the printing ink, which was sintered at 500℃ after printing to preserve the active structure
[212]. The material was used for adsorption and degradation of perfluorooctanoic acid (PFOA) in a circulating flow system. The removal rate reached 53% within 3 h, and the removal rate increased to 75% after pyrolysis and regeneration at 500℃. The adsorption capacity reached 0.16 mg g
-1 (
Fig. 8b). Barisic et al. used DIW 3D printing to prepare a TiO
2 mono-material by mixing the TiO
2 powder with a binder to make ink and using a rapid sintering process to retain photoactivity
[213]. The monolithic material was used for the photocatalytic degradation of the organic micropollutant primidone. The quantum yield reached 1.1×10
-5 and the power consumption was only 13 kWh/m
3 in a circulating flow reactor, which was significantly better than that reported in the literature (
Fig. 8c).
Overall, DIW technology's primary strength lies in its extensive material versatility, enabling it to process diverse precursors while precisely engineering hierarchical porous architectures. However, controlling the rheological behavior of inks remains challenging, and following photocatalyst formation, intricate post-treatment steps including thermal annealing or lyophilization are necessary before practical application, which constrains its efficiency for rapid prototyping workflows.
3.1.3 SLS/SLM technology for photocatalyst fabrication
SLM and SLS 3D printing technologies are both forming technologies based on powder bed processing
[218]. SLM technology utilizes a high-energy-density laser beam to generate high temperatures to melt the substrate and form a molten layer. As the object descends one layer, the powder bed is leveled by rollers to continue the cladding of the next layer, without the need for adhesives or low-melting-point materials for bonding. SLM printing technology has been extended to the forming processes of many advanced alloy materials and is widely used in aerospace, shipping, high-precision metal processing and other fields. The printing effect of SLM is affected by the metal powder, particle size, scanning speed and time. Compared with SLM, the difference of the SLS process lies in that SLS uses low-melting-point polymer materials as the binder to bond the main material, and the rest of the process is the same as that of SLM. The materials used in SLM printing have a wide range of sources, and the products have excellent mechanical properties and high precision. The consumables of this process are materials that absorb laser energy and have reduced viscosity, including various polymers, ceramics and metal materials. The printed objects of this technology are immersed in powder, which is conducive to reducing the consumption of support materials and simplifying the operation of subsequent processing
[219].
SLM 3D printing technology is mostly used in the manufacturing of monolithic metal catalysts. Metals and their alloy materials are commonly used filling materials in industrial catalytic reactions. Meanwhile, metal 3D printed components have good electrical conductivity and are often used to prepare catalysts for electrochemical reactions. Before printing, the printing chamber of this technology needs to be filled with argon gas or the printing should be carried out in a vacuum environment to prevent the catalytic material from being oxidized and provide good prerequisite conditions for the generation of catalytic components. At present, SLS/SLM technology applied in the field of photocatalysis is relatively limited, and is mostly used as catalyst carrier. Direct printing of metal catalysts has some applications in the field of water treatment, which may provide a reference for the application of such technology in the preparation of photocatalysts. Zhang et al. used SLM 3D printing to fabricate a 316 L stainless steel microlattice structure to mimic the Douglas fir dual-mode pore design overlapping metamaterials, and made a catalyst by electrochemical deposition of Co loaded on the surface
[220]. The catalyst can be activated by PMS for degradation of sulfamethoxazole. The tensile strength of the structure with 70% overlap rate is 3 times that of the traditional microlattice, the specific surface area per unit volume is increased by 3 times, and the normalized reaction kinetic constant is 4 times that of the traditional structure. It can also run continuously for more than 96 h. It also has high efficiency to remove many kinds of pollutants in actual water (
Fig. 9a-c). Liu et al. used SLM 3D printing technology to prepare MoS
2-stainless steel composite catalyst (3DP MoS
2-SS), and mixed MoS
2 and stainless steel powder and molded by 3D printing
[222]. The k
FLO/S
BET value was 1.60 g/(m
2/min), which was 4.3 times higher than that of MoS
2-SS powder catalyst. Mo
4+ can promote the Fe
2+/Fe
3+ cycle and accelerate the formation of active species. It can also degrade a variety of organic compounds such as 2-chlorophenol and acetaminophen. The degradation efficiency remains above 80% after 4 cycles of recycling, and the leaching amount of metal ions is lower than the drinking water standard. Nurshaun Sreedhar et al. used SLS 3D printing technology to produce polyamide spacers
[221]. They designed a structure based on triple cycle minimum surface (TPMS), coated with polydopamine-polyethyleneimine (PDA/PEI). Then β-FeOOH nanorods were mineralized to make photocatalytic spacers. The spacer could degrade MB and 4-nitrophenol under sunlight and H
2O
2, and the degradation rate reached 98% in 4 h at 25 ppm. The alginate flux recovery rate reached 92%. The β-FeOOH nanorods had stable attachment, and the photocatalytic performance did not decrease significantly after repeated use (
Fig. 9d).
In summary, SLS and SLM technologies enable the direct additive manufacturing of high-strength metal or ceramic-matrix photocatalysts with exceptional thermal stability and chemical corrosion resistance. However, these techniques involve elevated initial equipment investment, prolonged process cycles, and inherent material oxidation propensity of metallic powders, necessitating stringent inert atmosphere control such as argon or vacuum environments. These constraints substantially hinder scalability for industrial production. Currently, their application scope is primarily confined to carrier fabrication; research on the direct printing of active photocatalytic components remains underexplored, exhibiting low technological maturity due to challenges in microstructure precision and functional component integration.
3.1.4 Stereo-lithography technology for photocatalyst fabrication
Stereo-lithography 3D printing is an additive manufacturing process that uses ultraviolet (UV) laser beams to selectively cure liquid photosensitive polymer resins layer by layer to manufacture objects
[223]. The light source generally selects ultraviolet light in the 355 nm and 405 nm bands for curing. Due to the differences in light sources and forming methods, the commonly used methods are divided into SLA, DLP and liquid crystal display forming (LCD)
[224]. SLA uses laser dots as the light source to complete layer-by-layer curing through scanning
[225]. DLP and LCD, on the other hand, use digital micro-mirror devices (DMD) and LED light sources as the light source to form an ultraviolet light surface and project it onto the photosensitive resin tank
[226]. As the curing platform moves, the object is manufactured. The forming speed and efficiency of stereolithography 3D printing are both higher than those of other 3D printing forming technologies. Its precision depends on the resolution of the light spot, not on the diameter of the nozzle or the size of the raw material
[227]. Compared with other 3D printing technologies, through the optimization of processes and parameters, the thickness of each layer (resolution in the Z-axis direction) of stereolithography 3D printing can be reduced to less than 10 μm, effectively eliminating the layering phenomenon caused by layer-by-layer stacking, greatly improving the surface effect of printed products, and simultaneously manufacturing highly precise three-dimensional objects with extremely high resolution
[228]. With the continuous update of technology, micro-nano 3D printing technology capable of material forming and control at the micro and nano scales has emerged
[229-231]. It has been developed based on the evolution of stereolithography 3D printing technology. The material used in stereolithography is photosensitive resin, which is required to have good stability, fast curing rate, low shrinkage rate and appropriate curing depth. It is composed of photoinitiators, prepolymer monomers, oligomers and fillers. The curing process of photosensitive resin is as follows: Ultraviolet light is injected into the interior of the photosensitive resin. Under the action of the photoinitiator, the prepolymer and the active monomer undergo cross-linking polymerization to form a cured layer. The role of the oligomer is to adjust the physical properties such as viscosity and mechanical properties of the photosensitive resin. The filling process mainly plays a role in increasing the curing rate, adjusting the curing depth and regulating the color, etc. At present, the commonly used photosensitive resins are mainly free radical photosensitive resins composed of acrylic monomers and cationic photosensitive resins composed of vinyl ether monomers. The development and research of photosensitive resins are promoting the continuous development of stereolithography and 3D printing technology. Based on stereolithography 3D printing technology, high-precision, small-sized and high-strength additive manufacturing of ceramics and glass has also emerged, and the range of materials is constantly expanding.
During the preparation of monolithic catalysts, the dense and smooth photosensitive resin is prone to clogging and masking the active substances. Therefore, calcination is usually adopted to remove or transform the photosensitive resin material, thereby exposing the active centers
[235]. Finch et al. employed SLA and DLP light-induced 3D printing technology, using PETA resin containing ruthenium(II) complexes as the raw material, to print micro/macro-sized photocatalytic structures
[231]. The uniform distribution of ruthenium was verified by time-of-flight secondary ion mass spectrometry. This structure exhibited photocatalytic activity in the activation of aryl bromides for C-H arylation reactions, where the microscopic structure achieved 75% photocatalytic performance using only 1% of the volume of the macroscopic structure (
Fig. 10a). Warren et al. used DLP 3D printing technology to print catalyst structures using titanium acrylate resin as the raw material, and sintered them at 750℃ to obtain TiO
2 foam containing rutile and anatase phases
[232]. This foam has a porous gyroid structure with a porosity of > 90%. When used in a cyclic flow reactor to degrade carbamazepine, the quantum yield reached 7.6×10
-3, with an energy consumption of 67.6 kWh/m
3. The photocatalytic performance was superior to TiO
2 nanoparticle suspensions, and the mechanical stability was good (
Fig. 10b). Luo et al. used DLP photopolymerization 3D printing to print a helical structure free-standing photocatalytic reactor by mixing the Ni
1@mpgCN
x single-atom catalyst with resin
[233]. When the reactor was used for the continuous stream photocatalytic oxidation of benzyl alcohol to produce benzaldehyde, the activity was higher than that of traditional packed bed reactor, and the quantity was selectively maintained after recycling. No metal ions were dissolved, and the structure was stable in a variety of organic solvents (
Fig. 10c). Huong et al. fabricated gyroid structured photocatalysts using DLP 3D printing WO
3-UiO-66@rGO nanocomposites
[234]. The degradation rate constant of sulfamethoxazole was 0.02955 min
-1 in 60 min, which was 6.2 times that of pure WO
3. After repeated use for 10 times, the photocatalytic activity could be completely recovered, and the mechanical strength of the material met the requirements of the flow reaction (
Fig. 10d). Gracia-Pinilla et al. 3D printed TiO
2-Al
2O
3 resin by DLP and sintered to obtain hollow microstructures (3DHMs), which were loaded with α-Fe
2O
3 and used for solar photo-Fenton-like reaction
[236]. The degradation efficiency of MB in the formulation of TiOFe
0.5 was 95% within 180 min, and the degradation of acetaminophen was still effective at neutral pH. After three successive cycles, the activity remained stable and no significant iron dissolution was observed.
In summary, SLA technology stands out for its exceptional micro-nanoscale resolution capabilities, enabling breakthroughs such as the fabrication of topologically complex microchannel reactors and hierarchical photocatalyst architectures with ultrafine features. These innovations have demonstrated transformative potential in optimizing light-matter interactions and broadening design possibilities for high-efficiency photocatalytic systems. While challenges persist including thermal degradation effects during resin removal that may induce controlled shrinkage and the need for post-processing compensation, alongside issues of limited interfacial adhesion between resin matrices and active components often necessitating surface functionalization strategies, the field is actively innovating. Researchers are developing novel material formulations and hybrid manufacturing approaches to systematically address these limitations. Such progress positions SLA as a frontier technology poised to redefine benchmarks in advanced photocatalytic device engineering, balancing precision manufacturing with functional performance optimization.
3.1.5 Other 3D printing technologies for photocatalyst fabrication
In addition to the above commonly used mainstream technologies, a variety of emerging 3D printing technologies have shown unique potential in photocatalyst design, and their core advantages are closely related to the needs of photocatalysis
[237-239]. In recent years, thanks to the rapid development and wide application of 3D printing technology, some of the latest 3D printing technologies have also been demonstrated in the field of photocatalysis. This subsection will introduce a few works. Laser-induced in situ electrospray printing (E-Jet) is a 3D printing technology that combines the principle of high-energy laser induction and electrospray printing. By controlling the thermal field and flow field distribution of the jet by laser, E-jet can achieve the accurate molding of micro and nano-level porous structures, which can effectively improve the specific surface area of the structure and the exposure rate of photocatalytic sites. Li et al. used this technique to prepare hierarchical porous ZnO structures with a minimum pore size of 130 nm and a maximum pore size of 4 μm by using ZnO ink as raw material and adjusting jet shrinkage and curing with the aid of 2 W laser
[240]. When the structure is used for UV photoelectric sensor, the photoresponse performance is significantly better than that of the sample without laser assistance. Moreover, the porous characteristics improve the separation efficiency of photogenerated carriers, which provides a new idea for the preparation of micro/nano photocatalytic devices (
Fig. 11a, b). 3D concrete printing (3DCP) surface coating technology takes advantage of the non-mold forming characteristics of 3D printed concrete, and directly spray photocatalytic coating on the surface of fresh cement-based matrix to avoid the surface smoothness limitation of traditional mold forming and enhance the bonding force between catalyst and matrix. Zahabizadeh et al. sprayed nano-TiO
2 aqueous solution on the surface of 3D printing cement-based materials
[241]. At the coating rate of 80 mg/cm
2, the degradation rate of RhB reached 61% after 20h of illumination. When the optical power intensity was increased from 1 mW/cm
2 to 8 mW/cm
2, the degradation efficiency was increased by 86%. SEM and EDS analysis confirmed that TiO
2 was uniformly distributed on the surface of the 3D printed matrix without obvious agglomeration (
Fig. 11c). Photopolymerization-assisted in situ growth 3D printing technology can prepare organic-inorganic composite printing ink by optimizing the binder and prepolymerization conditions. Photopolymerization is used to realize the formation of macroscopic ordered structure, followed by solvent heat treatment to promote the in-situ crystallization of active components, which can solve the phase separation problem when COF is combined with zeolite. Feng et al. used this technology to prepare TP-DBS-COF/ZSM-5 composite photocatalyst, in which zeolite enhances the hydrophilic and O
2 affinity of the material, and H
2O
2 production is increased by 52% compared with pure TP-DBS-COF
[242]. After 4 cycles, the crystallinity and catalytic activity of the composite did not decrease significantly, which was suitable for sustainable photocatalytic hydrogen production (
Fig. 11d, e). Kuo et al. used in situ precipitation (ISP) 3D printing to produce a four-layer regular porous Ag@AgBr structure
[243]. In this technology, silver nitrate solution is directly printed on the surface of cured sodium bromide, and the regular porous AgBr structure is formed by in situ precipitation at the liquid-solid interface. After sintering and UV photoreduction, Ag@AgBr photocatalyst is prepared without subsequent complex assembly. The degradation rate of Orange II azo dye remained 84.8% after 5 cycles of degradation. Under UV-VIS light irradiation, Escherichia coli can be completely killed within 120 min, and the bactericidal reaction conforms to the hyperbolic kinetics, which is suitable for the synergistic treatment of water pollution (
Fig. 11f, g).
3.2 Design of photocatalyst carriers
In addition to direct printing of photocatalytically active materials, the preparation of structured substrates or supports by 3D printing, followed by in-situ growth, coating loading and other subsequent processes combined with photocatalytically active components, is another technical direction of great practical value. Based on the structural design freedom of 3D printing, the substrate with specific morphology, mechanical stability, and mass transfer characteristics is constructed, and the photocatalytically active material is directionally combined on the surface or inside the pores of the substrate by interface regulation to form a composite system of substrate and photocatalytic material
[244-246]. On the one hand, 3D printed substrates can optimize the scattering and absorption efficiency of light and the mass transfer path of reactants by adjusting the parameters like porosity, specific surface area, and flow channel geometry. At the same time, the problems of rigid traditional support structure and easy agglomeration and shedding of catalysts can be solved. On the other hand, the choice of substrate materials is diverse, and model materials such as polymers, ceramics, metals, and composites have been widely studied in the field of 3D printing technology
[247]. For the needs of liquid-phase degradation, gas-phase reduction, and photoelectric co-catalysis in the field of photocatalysis, the physicochemical properties of different substrates can be used to match the different photocatalytic reaction requirements, and determine the suitability of the subsequent in-situ growth/loading process
[248]. The common combination methods include in situ growth of semiconductor nanoarrays by hydrothermal method, coating loading by sol-gel method, and electrochemical deposition of metal-based catalysts
[249]. The specific process needs to match the surface properties of the substrate materials and the synthesis requirements of active components
[250].
3.2.1 Polymer carrier
With the advantages of strong compatibility, low cost and light weight of 3D printing process, polymer substrate is suitable for liquid photocatalytic reaction at room temperature. Its porous structure can be accurately designed by adjusting the printing parameters like layer thickness and filling rate, providing sufficient loading sites for photocatalytic materials and ensuring the fluidity of reaction media. FDM and SLA are commonly used. Penas-Garzon’s team prepared PLA substrate with vertical wing by FDM 3D printing
[251]. Graphite phase carbon nitride (CN) powder of urea or dicyandiamide pyrolysis was fixed by polymer adhesion. The CNB-U/PLA material was rich in nitrogen vacancies. The degradation rate of 5mg L
-1 venlafaxine (VFX) was 90% within 30 min, and the modular design kept its high degradation activity in the mixed pollutant system (
Fig. 12a). Melendz-Gonzalez et al. synthesized immobilized Cu
xO semiconductors (3D-Cu
xO, 3D2-Cu
xO) on 3D printed substrates using PLA as substrate by graphite coating, copper electrodeposition and anodization processes
[252]. The materials contain CuO/Cu
2O mixed crystal phases. The band gap width of 1.6-1.8 eV can respond to visible light, among which 3D2-Cu
xO has higher carrier concentration and better separation efficiency, and the degradation rate of 10 mg L
-1 sulfamethoxazole (SMX) can reach 60% after 360 min of visible light irradiation, and the synthesis does not require subsequent heat treatment, taking into account the efficiency and sustainability (
Fig. 12b). Son et al. used FDM technology to prepare ABS-ZnO composite substrate
[253]. After plasma treatment and hydrothermal growth, ZnO nanoflowers (NFs) structure was formed on the surface of the substrate, and the degradation rate of 5 mg L
-1 MB reached 100% within 120 min under UV light. The 3D structure effectively enhances the catalyst loading and stability. Kang’s team constructed the Ni-MOF/BiOI/AgVO
3 heterojunction by cold plasma fixation and c-ISCAP method using LCD 3D printing fractal pyramid substrate
[254]. The double Z-type charge transfer path greatly improved the carrier separation efficiency. The degradation rate of 5 ppm RhB under visible light for 6 h was 100%.
In the field of photocatalytic hydrogen production, Li et al. used DLP 3D printing to prepare substrates with different structures and coated ZnIn
2S
4-Pt-Co catalyst to form thermosetting coatings
[255]. The inverted pyramid structure had a long light reflection path, and the hydrogen production rate per unit mass of catalyst reached 1555 μmol h
-1 g
-1. It is 4.2 times that of pure powder, and does not need to sacrifice agents, providing a new strategy for large-scale hydrogen production. The team of Perera used DLP and PIPS technology to fabricate UIO-66-MOF polymer composite
[256]. The surface of UiO-66 was enriched by HEA monomer, which completely degraded ethylparoxon in 60 min at pH 9-10. The elasticity and stability of the composite material make it suitable for wearable chemical protective equipment.
3.2.2 Carbon carrier
Carbon-based materials themselves are excellent photocatalysts and photocatalyst-supported substrate materials, such as graphene
[257], carbon nanotubes
[258,259], activated carbon
[260] and carbon quantum dots
[261,262], which have the advantages of high specific surface area, excellent conductivity, and high chemical stability
[263]. It can not only provide a loading substrate for photocatalytic materials, but also serve as an electron transport channel to promote the separation of photogenerated charge carriers, which is suitable for energy conversion scenarios like photocatalytic hydrogen production and CO
2 reduction
[264,265]. Guo et al. used lignin as carbon source and used DIW technology, supplemented by 23 wt.% Pluronic F-127 to adjust the rheological properties of ink
[266]. By using the reverse temperature sensitivity of F-127, the printing blockage caused by lignin’s strong rigidity and easy aggregation was solved. Self-supporting lignin scaffolds were successfully prepared. Subsequently, PCS were stabilized at 120℃ to eliminate residual water and volatile impurities, and carbonized under nitrogen atmosphere at 800℃ to obtain carbon scaffolds (PCS). The carbon support retained the interconnected porous structure constructed by DIW printing with a porosity of 55%, and its high specific surface area provided sufficient anchoring sites for TiO
2 nanorods and Pd nanoparticles. At the same time, the graphite-like structure formed by lignin carbonization gives the carrier excellent electrical conductivity, which can be used as an electron transport channel to accelerate the photogenerated electron transfer. Together with the Schottky junction formed by Pd and TiO
2 to inhibit the recombination of charge carriers, the Pd/TiO
2/PCS catalyst is finally constructed in the photocatalytic reduction of 4-nitrophenol (4-NP). The turnover frequency (TOF) reached 8.73 min
-1 and the Pd dissolution amount was less than 0.01 mg L
-1 after 9 cycles, reflecting the dual enhancement of the stability and catalytic efficiency of the active components by carbon support (
Fig. 13a). Kandasamy et al. prepared graphene/MnO
2/Fe
3O
4 hybrid particles by wet impregnation method, mixed with light-curing resin, printed by SLA 3D to obtain graphene-like structure, and carbonized by argon gas at 800℃ to obtain 3D floating carbon support
[267]. After carbonization, the conductivity of the carrier increased from 10 S m
-1 to 15 m
-1, graphene ensured high conductivity, Fe
3O
4 gave magnetism and floatability, and provided a loading substrate for MnO
2 while accelerating charge transfer. The degradation rate of 10 ppm MB under sunlight for 120 min reached 95.93%. The activity decreased by only 2% in 10 cycles. Verma et al. used DLP technology and transparent resin to construct micro-lattice structures
[67]. After clear curing, the PyC scaffold was obtained by carbonization under argon atmosphere at 900℃. Although the scaffolds shrank by 51% during carbonization, they still maintained intact structure and high mechanical stability. The porous structure of the surface can be used for hydrothermal growth of ZnO nanowires, and the multiple light scattering characteristics improve the light absorption efficiency. The high conductivity of PyC accelerates the electron transfer, and the degradation rate of 10 mg L
-1 RhB can reach 97.73% after 180 min of UV irradiation. The degradation rate reached 84.04% in 280 min under natural light, and the activity remained 86.22±2.15% after 7 days of cycling (
Fig. 13b).
3.2.3 Ceramics carrier
Ceramic support has the characteristics of high temperature resistance, chemical stability, and high mechanical strength, which are suitable for high temperature sintering and harsh reaction environment
[268]. 3D printing technology can break through the limitations of traditional ceramic molding, construct complex channels and special topological structures, and optimize photocatalytic performance by adjusting the composition and microstructure. A variety of ceramic carriers such as mullite, Al
2O
3, SiOC, ZnO and various modified ceramics. Mei’s team used photosensitive resin containing Al
2O
3 and SiO
2 powder as raw materials, and used SLA technology to prepare square, round, and diamond array hole sheet scaffolds
[269]. After 1400℃ step-by-step sintering, pure ceramic support was obtained under air atmosphere, and carbon-ceramic support was obtained under nitrogen atmosphere. The resin pyrolysis under nitrogen atmosphere produced 12.86±2.32 wt.% pyrolysis carbon, which inhibited the mass transfer of ceramic powder, increased the specific surface area of the support from 0.067 m
2 g
-1 of pure ceramic to 0.509 m
2 g
-1, and refined the pore size to about 2 μm. The photodegradation efficiency of RhB loaded with MoS
2 was 45.95%, which was 1.97 times that of pure MoS
2, and the efficiency was 82.35% after 5 cycles. UV-Vis DRS confirmed that pyrolysis carbon can enhance light absorption in the full visible light band, which provides ideas for the design of high specific surface area carriers (
Fig. 14a). Huang’s team used DLP 3D printing technology to print four minimal surface structure scaffolds like gyroid and diamond with commercial Al
2O
3/SiO
2 ceramic slurry
[270]. Adding 5 wt.% ammonium bicarbonate as pore-making agent, the mullite carrier was prepared by sintering at 1500℃. Cu(OH)
2 nanoneedles were grown by electrochemical oxidation method, and MoS
2 was supported by sol-gel method to construct a three-pore composite catalyst. Among them, the Diamond structure had the best mechanical properties, with a specific surface area of 89.34 m
2 g
-1 and a RhB degradation rate of 84.68%, which was 2.26 times that of pure MoS
2. The efficiency remained above 85% after 10 cycles. ESR confirmed that •O
2- was the main active species, and the Z-type heterojunction promoted carrier separation to realize the synergistic optimization of mechanical properties and photocatalytic activity (
Fig. 14b). Based on the self-developed DLP 3D printing technology, Hu’s team printed CF and HF scaffolds with Al
2O
3 powder and photosensitive resin, and obtained Al
2O
3 carrier by sintering at 1500℃
[271]. Carbon-bridge g-C
3N
4 was prepared by high temperature roasting, coated with polyvinyl alcohol, and then roasting and fixed on the support at 300℃. The band gap width and carrier recombination rate of g-C
3N
4 were reduced by carbon bridge modification. After negative CNMD, the degradation rate of tetracycline reached 91%, the efficiency of 10 cycles remained above 85%, and the TOC removal rate exceeded 82%. LC-MS confirmed that tetracycline was oxidized, deaminated and mineralized to CO
2 and H
2O, which provided a reference for the development of recyclable non-metallic photocatalyst support (
Fig. 14c). Liu’s team used vat photopolymerization 3D printing technology to print porous preforms with methyl-silsesquioxane resin, and prepared cuboidal and spherical porous SiOC carriers by argon sintering at 900℃
[272]. The Bi
12TiO
20/Bi
4Ti
3O
12 S-type heterojunction was formed by coprecipitation (Bi(NO
3)
3•5H
2O and tetrabutyl titanate) and annealing at 600℃ for 1 h. The heterojunction promotes carrier separation and retains strong REDOX capacity with the help of a built-in electric field. The spherical pore can prolong the light reflection path and increase the retention time of the reactant, and the NO removal efficiency is 16.4%, which is 1.39 times that of the cubic pore. After four cycles, the activity is stable, which provides a scheme for the monolithic catalyst for air pollutant treatment (
Fig. 14d). In order to solve the problems of high carrier recombination rate and weak light absorption in photocatalytic CO
2 conversion, Pan’s team prepared gyroid structure preform by DLP 3D printing with ZnO precursor slurry as raw material
[273]. After heat treatment at 500℃ to remove the organic phase and sintering at 1300℃, P-doped ZnO (P-ZO) support was prepared. Bi
2S
3 nanoarrays were loaded onto the surface of the nanocomposites by hydrothermal method to construct S-shaped heterojunction metamaterials. The P-ZO has a significant photothermal effect, which can be heated to 89.2℃ when coupled with Bi
2S
3. The S-type heterojunction inhibits carrier recombination, and the transient photocurrent density is 1.5 times higher than that of pure P-ZO. Under simulated sunlight, the production of CO and CH
4 reached 8.87 and 1.49 μmol h
-1, respectively, which were 3.45 and 4.65 times of that of pure P-ZO, and the activity of the material was stable for 5 cycles, which provided a new structure design idea for CO
2 conversion by photothermal catalysis.
3.2.4 Others
Substrates with specific functions such as metal, glass, and composite substrates are limited by difficulties in manufacturing and application fields. Although the application scenarios are less extensive than the first three scenarios, they are irreplaceable in the subdivision field with their unique performance. Metal substrates focus on high strength and conductivity, glass substrates focus on high light transmittance, and composite substrates achieve complementary properties through the cooperation of multiple materials, providing more diverse solutions for photocatalytic loading. In order to improve the photocatalytic performance of TiO
2 and optimize the efficiency of the reactor for environmental remediation, Grandcolas et al. used direct metal laser sintering (DMLS) 3D printing of Ti6Al4V alloy to prepare planar and pyramidal (PYR), honeycombed (HON), and octahedral (OCT) 3D structures
[274]. TiO
2 nanotubes were grown by anodization at 60V using a solution of ethylene glycol containing 0.35 wt.% NH
4F and 2 vol% H
2O as the electrolyte. The performance test showed that the photocatalytic efficiency of the 3D structure was significantly improved under the rotation condition. The PYR structure achieved complete degradation of MB within 200 min under UV-LED illumination and rotation mode, while the HON structure only increased the efficiency by 20% due to poor light penetration. This study provides technical ideas for the design of high-efficiency photocatalytic reactors for environmental remediation (
Fig. 15a). Addressing the need for recyclable surface-enhanced Raman scattering (SERS) substrates for point-of-care testing (POC) analytical platforms, Malik et al. fabricated metal brushes using laser powder bed melting (L-PBF) 3D printing of Inconel 625 alloy
[275]. The IN_EuSTAg substrate was obtained by three-step functionalization. The SERS enhancement factor of RhB on the substrate reached 2.02×10
4, and the RhB degradation rate was 92.3% within 180 min under 370 nm UV light. The RHB could be recycled for 3 times, providing a recyclable SERS solution for the POC analysis platform (
Fig. 15b).
Aiming at the problem of low solar energy capture efficiency caused by the change of illumination Angle in PEC water decomposition, Hegde et al. used DLP 3D printing of light-curing silica sol-gels, and prepared transparent 3D silica lattices after aging, degreasing and sintering
[68]. Then, ITO and Mo-BiVO
4 were deposited sequentially by dip coating method to construct 3DP/ITO/Mo-BiVO
4 electrode. The volume current density of the electrode was 1.39 mA/cm
3 under 1.23 V and AM 1.5 G illumination, which was 2.4 times that of the flat glass substrate. The performance fluctuation was only 6% when the illumination Angle changed. The stability test showed no current attenuation and the Faraday efficiency of oxygen precipitation was 91.24%. An angle-independent and efficient solar energy capture scheme is provided for PEC electrodes (
Fig. 15c). Wang et al. focused on the problem of light attenuation and reactor amplification during photocatalysis
[277]. A transparent silica cylinder was printed by stereo-lithography technology and used as a catalyst carrier for MB degradation in a packed bed micro-photocatalytic reactor (MPR). The transparent SiO
2 cylinders (S-1, S-2, S-3) were obtained by printing, degreasing and vacuum sintering at 1280℃, and loaded with P25-TiO
2 for MB degradation in a packed bed micro-photocatalytic reactor (MPR). The results showed that the degradation rate of MB of S-1 was 91.4%, while that of Al
2O
3 was less than 60%, and that of SiO
2 was more than 90% (200-800 nm). The reduction of specific surface area and mass transfer limiting efficiency of S-2 and S-3 were significantly reduced. This study provides key data support for the design optimization and scale up of photocatalytic reactors. In addition, in order to overcome the limitation of traditional glass forming technology for the preparation of complex structures, Kotz et al. used SLA to 3D print SiO
2 nanoparticles and photosensitive monomer paste, light curing green parts
[276]. A transparent fused silica glass with a surface roughness of 2 nm and no pores was prepared by heat degreasing and vacuum sintering at 1300℃. The light transmittance of the glass is consistent with that of commercial fused quartz, and it can withstand 800℃ flame. Colored glass can be made by immersing metal salt with brown parts. Micro-stereo lithography can also realize 80 μm resolution structure, effectively expanding the application scene of glass in the field of optics and microelectromechanical systems (MEMS) (
Fig. 15d).
4 3D printing technology applied to photocatalytic reactors and reaction systems
Photocatalytic reaction is a green and efficient technology for pollution control and energy conversion. The improvement of its reaction efficiency largely depends on the performance of photocatalytic reactor and related reaction system. An ideal photocatalytic reactor should have high light utilization efficiency, good mass transfer effect, uniform catalyst loading and contact mode, and structural stability matching with the reaction system. At the same time, it should also meet the practical requirements such as simple operation and easy expansion. Conventional photocatalytic reactors are usually simple in structure and rely on mechanical processing or traditional molding processes
[278]. Limited by manufacturing methods, it is often difficult to realize the synergistic optimization of optical path distribution, fluid path, and catalyst spatial arrangement. Such reactors often suffer from serious light scattering, insufficient contact between reactants and catalysts, and high local mass transfer resistance, which results in low photocatalytic efficiency. In addition, it is often necessary to assemble multiple components to integrate complex functions, which not only introduces systematic errors, but also limits the miniaturization and customization of reactors
[279].
The emergence of 3D printing technology provides a new approach for the design and manufacture of photocatalytic reactors. Based on its unique principle of layer-by-layer manufacturing and successive assembly, 3D printing can precisely construct complex three-dimensional structures that are difficult to achieve through traditional processes, such as irregular flow channels, micro-nano-level light guiding structures, and gradient porous catalyst carriers
[280]. This enables targeted solutions to the inherent defects of traditional reactors in light utilization, mass transfer efficiency, and structural integration. Through digital design and integrated manufacturing, 3D printing can not only achieve precise matching of reactor structure and photocatalytic reaction mechanism but also simplify the setup process of the reaction system, promoting the development of photocatalytic technology towards efficiency, miniaturization, and customization
[281]. The following text will elaborate on the specific contributions of 3D printing technology in the innovation of photocatalytic reactor design and the optimization of reaction system setup, revealing how it provides key support for the practical process of photocatalytic technology.
4.1 Design of photocatalytic reactors
4.1.1 Fluidized bed reactor
Based on the form of the catalysts, photocatalytic reactors can be classified into fluidized bed reactors and fixed bed reactors. The fluidized bed photocatalytic reactor is a type of photocatalytic reaction device characterized by a “slurry-like” dispersion system. Its core operation mode is that the catalyst particles and the reactant mixture containing the target pollutants are suspended together in a liquid phase or an air-liquid mixed reaction medium, and a pseudo-fluidized state is formed through the disturbance of the fluid
[282]. In terms of the supply method of light radiation, either an external transparent window on the reactor wall can introduce an external light source, or an immersed lamp tube can be directly placed inside the reaction system to provide energy. The regulation of the reaction performance is influenced by multiple factors in a coordinated manner. The concentration of catalyst particles needs to be maintained within a reasonable range. Too low a concentration will lead to insufficient active sites, while too high a concentration will cause enhanced light absorption and scattering due to light obstruction between particles. The matching degree of the wavelength of light radiation, the intensity distribution, and the irradiation path directly determine the efficiency of photon utilization. The flow rate and turbulence degree of the reaction medium, as well as other flow state parameters, significantly affect the mass transfer efficiency and interface contact frequency between the catalyst and the reactants.
The ordinary fluidized bed photocatalytic reactor has significant technical advantages, which can maximize the inherent activity of the catalytic material, expose the active sites on the catalyst surface to the light radiation and the reactant atmosphere to the greatest extent. In addition, it has a simple overall structure and convenient operation, making it suitable for various scenarios of pollutant degradation and energy conversion. However, its inherent drawbacks are also quite prominent. The light scattering effect of suspended particles will lead to local light intensity attenuation, reducing the light utilization efficiency. After the reaction, the separation and recovery of the catalyst require an additional solid-liquid separation process, which not only increases the operation cost but also may cause catalyst loss and secondary pollution due to incomplete separation. Wei et al. used SLS metal 3D printing technology to fabricate three autocatalytic flow-bed reactors of Fe-SCR, Co-SCR, and Ni-SCR, and designed channels with hemispherical convex inside the reactor to expand the specific surface area
[283]. Fe-SCR and Co-SCR can efficiently produce liquid fuels in high pressure Fischer-Trop synthesis and CO
2 hydrogenation. Co-SCR has a C
5+ selectivity of 65%, and Ni-SCR can achieve 65% CH
4 and 71% CO
2 conversion in high temperature CH
4/CO
2 reforming reaction. The ratio of H
2/CO is close to 1, which can withstand the harsh conditions of high temperature and high pressure, and realize the integration of catalyst and reactor functions in ordinary fluidized bed reactor (
Fig. 16a). Zhou et al. prepared two lab-scale 3D-printed plastic sinusoidal flow-through photocatalytic reactors, with Reactor A having no baffles and Reactor B being equipped with baffles
[284]. Both reactors were lined with a P25 TiO
2/PLA coating, and after 168 h of UVA pre-conditioning, they could repeatedly photocatalyzed the degradation of circulating aqueous solutions of MB and phenol (PhOH) without obvious activity loss. Owing to the enhanced lateral mixing of laminar flow by baffles, the degradation rate of MB in Reactor B exhibited much lower dependence on flow rate compared to Reactor A. For Reactor A, the photonic efficiencies for MB and PhOH degradation were 0.025% and 0.052%, respectively, and its photocatalytic space-time yields (PSTY) were 0.98×10
-4 and 1.49×10
-4 m
3 of reaction solution per (m
3 reactor volume·day·kW), respectively. This work confirms that 3D printing enables the rapid and low-cost fabrication of flow-through photocatalytic reactors with stable performance (
Fig. 16b). Ramos et al. fabricated an ABS material plate-packed bed flow-through photocatalytic reactor by fused deposition molding (FFF) 3D printing, filled with 1.0 mm ZnO-coated glass or steel beads for the degradation of para-acetamide
[285]. The apparent reaction rate of the steel bead carrier was about 75% higher than that of the glass bead carrier at the first use, but it was easily inactivated. The apparent first-order rate constant of glass beads can reach 1.9-9.5×10
-4 s
-1, which is 10 times faster than that of conventional suspension bed, which is typical of ordinary packed flow bed reactor (
Fig. 16c, d). Phang et al. used DLP 3D printing technology to fabricate a rese-based flow-bed photocatalytic reactor
[286]. The main body was made of highly heat-resistant resin, the cover was made of transparent resin, and the bottom of the reactor was coated with g-C
3N
4 homo-junction thermosetting coating. Under 50 W LED irradiation, the degradation efficiency of RhB reached 95.62% within 24 h, the kinetic rate constant was 2.1×10
-3 min
-1, the activity remained 98.5% after 5 cycles, and the TOC removal rate was 78.56%. This study provided an example for the sustainable application of ordinary circulating fluidizing-bed reactor.
To overcome these limitations, researchers have proposed microchannel-type fluidized bed photocatalytic reactors. These reactors fix the catalysts on the inner walls of microchannels or on specific substrate materials, allowing the reaction to take place in a transparent and particle-free clean medium. This design not only significantly simplifies the catalyst recovery process, reduces subsequent processing costs, but also significantly improves the penetration depth and utilization efficiency of light radiation by eliminating particle scattering. Moreover, the miniaturization and micro-miniaturization of the reactor provide great portability and safety, and through structural design, it can achieve more precise control of the reaction process, which is precisely the direction that 3D printing technology excels in. However, compared with the “slurry-like” system, the immobilization of the catalyst will result in a relatively reduced total amount of exposed active sites, sacrificing some reaction activity to a certain extent. Roibu et al. printed a PLA material microchannel flow-bed photocatalytic reactor by FDM technology, with a channel cross section of 1×1 mm
[63]. The reduced TiO
2 was made into a film, which was fixed on a glass substrate and integrated into the reactor. With imidacloprid as the target pollutant, the degradation efficiency of P25 coating was 28.4% in the single water flow mode under 395 nm wavelength illumination, and increased to 47.8% after air entry, with a residence time of only 1.1 min. The film maintained stable activity during 150 min operation and belonged to a microchannel flow-bed reactor. Pellejero et al. 3D-printed ABS curved mold, treated by plasma, then loaded Au@POM/TiO
2 by dip coating, and then cast PDMS and dissolved the mold to make microchannel flow-bed reactors with 0.5×0.5 mm and 1×1 mm cross sections
[287]. Under 365 nm LED irradiation, the conversion rate of 4-nitrophenol was 93% when the 1×1 mm design was treated at a flow rate of 2 mL h
-1. The 0.5×0.5 mm design had a larger specific surface area and better performance under a short residence time (6.3 min), achieving uniform fixation of the catalyst in the microchannel and efficient photocatalytic reaction (
Fig. 16e).
4.1.2 Fixed bed reactor
Compared with the fluidized bed photoreactor, the catalyst in the fixed bed photoreactor is loaded on the surface of the carrier or structure inside the reactor in a fixed form, and will not be taken away by the reaction medium. The mass transfer and reaction of the reactants only occur on the surface of the catalyst. The light supply mode is similar to that of the fluidized bed. The light source can be introduced from the outside through the transparent reactor wall, and the immersed light source can be placed inside the reactor to ensure the effective delivery of light to the active site of the catalyst. When using 3D printed integrated catalysts, the key factor affecting the reactor performance is more inclined to the regulation ability of the printing technology on the reactor structure. The geometric parameters of the 3D printed flow channel, such as curvature, section size, and branch Angle, directly determine the flow behavior of the reaction medium. Too narrow flow channel can easily lead to excessive pressure drop, while too wide flow channel may make the residence time of reactants insufficient. The optical properties of the printing material significantly affect the transmission efficiency of optical radiation
[62]. The high transparency resin helps to reduce light attenuation, and the specular reflection of the metal surface helps to optimize the light field distribution. The integration accuracy of the reactor with the light source is also controlled by the print size tolerance. For example, if the alignment deviation between the LED array and the catalytic region is too large, the photon utilization efficiency will be reduced. In addition, the interlayer bonding strength and pore sealing performance of the printed structure directly affect the thermal and corrosion resistance of the reactor. Inter layer defects may trigger leakage or structural damage, especially when dealing with acidic or alkaline reaction media.
The advantages of fixed-bed photoreactors are evident. Catalyst immobilization avoids the difficulty of catalyst separation and recovery in fluidized beds, reducing operational costs and the risk of secondary pollution. The design flexibility of the support structure allows optimization of light absorption and mass transfer processes. Fabian and colleagues prepared catalytic materials using a two-step mixing process, with TiO
2-polypropylene (PP) composite filaments as the core
[62]. First, recycled PP and TiO
2 were melt-compounded into blocks, which were then crushed, pelletized, and extruded into filaments. Subsequently, 3D printing was used to fabricate a cyclone fixed-bed reactor with a cylinder-cone structure, along with six different TiO
2-PP catalytic inserts, including grid-type and double-grid-type. Among them, the grid-type insert, due to optimized flow field, achieved a 19% degradation rate of MB after 76 h. The double-grid insert showed the highest initial rate for nitrobenzene reduction, at 0.53 nmol s
-1. The study achieved layered integration of photocatalysis and acid catalysis by mechanically depositing and immobilizing solid acids on 3D-printed PP cylinders. The quinoline selectivity reached 8%, higher than the 3% in the suspension system. Moreover, no downstream separation was required, effectively addressing issues such as difficult catalyst separation and mass transfer-lighting imbalance in traditional reactors (
Fig. 17a). Pan et al. prepared a biomimetic palisade cell structure BiOBr/Sr
2Nb
2O
7/Al
6Si
2O
13 photocatalytic reactor by combining 3D printing with solvothermal method
[288]. The periodic porous structure regulated photon transfer and reactant adsorption, enabling sufficient light scattering inside and smooth reactant flow. The p-n heterojunction interface electric field formed by BiOBr and SNO (Sr
2Nb
2O
7) effectively separated photogenerated electron-hole pairs. Under simulated sunlight, the reactor achieved a CO production rate of 13.68 μmol g
-1 h
-1 and a CH
4 production rate of 6.37 μmol g
-1 h
-1, which were 3.5 and 8 times higher than those with SNO alone, respectively, enhancing CO
2 reduction performance. After 4 cycles, its photocatalytic performance remained s
table. Additionally, the structural ceramic matrix exhibited good mechanical properties, enabling it to withstand harsh reaction environments (
Fig. 17b).
However, its drawbacks cannot be ignored: the exposure of active sites may be limited after catalyst loading, and the fixed structure may increase local mass transfer resistance. If the carrier pores become clogged, reaction efficiency will be further reduced. For example, although monolithic nickel-based nano-photocatalytic reactors simplify the recovery process, the loading of a single active source may limit the maximization of catalytic activity to some extent.
4.2 Design of photocatalytic reaction systems
Thanks to the excellent structural design flexibility, 3D printing technology can also realize the integrated design and functional integration from the catalyst to the system, providing customized solutions for core photocatalytic reaction process
[292]. This method significantly surpasses the limitations of traditional manufacturing methods in complex structure forming and multi-function integration, and can realize the integrated and collaborative optimization of mass transfer, light transfer and catalysis in the photocatalytic reaction system
[293]. Gok et al. prepared Pc-1@MCM-41 heterogeneous photocatalysts and used 3D printing technology to build a low-cost customized photoreactor
[289]. The PETG reactor consisted of a base, a LED mounting side panel, and a top cover. The total cost of the reactor was about 115 euros. The catalyst achieved 89% conversion and greater than 99% selectivity in 15 min under atmospheric air conditions while maintaining high stability. The 3D printed reactor effectively improved the practicability and scalability of the photocatalytic reaction (
Fig. 18a). In their other work, ZnPc was covalently bonded to functionalized MCM-41-Cl to prepare MCM-41-ZnPc catalyst, which was used to build a photoreactor by 3D printing
[290]. The total cost of this study was 21 euros using PETG as material and the reactor was equipped with a 50 W red LED light source. The system was used for the degradation of MB, and the removal rate reached 86% within 90 min. The pollutants were removed by the synergistic effect of adsorption and photocatalysis, and the activity of the catalyst remained more than 67% after 4 cycles (
Fig. 18b).
In terms of light field control, the use of transparent materials to print built-in optical components such as microlens arrays or optical waveguide structures can realize the redistribution and accurate guidance of light sources, effectively eliminate the light dead zone in the reactor, and ensure uniform irradiation on the catalyst surface. Zheng et al. used 3D printing to fabricate a composite parabolic collector (CPC) photoreactor made of plastic with integrated reflective aluminum strips and quartz tubes, allowing precise control of the structure size
[64]. The reactor was filled with g-C
3N
4/chitosan hydrogel beads (GCHBs) to construct a fluidized bed system for the treatment of phenol, sulfamethoxazole and other pollutants. Under simulated sunlight, the performance of the system was comparable to that of the suspension bed, and the activity was not significantly inhibited when processing water samples from real water plants. The outdoor 12-inch 3D printed CPC reactor could also operate stably, and the global single-day water yield was estimated, which proved the value of light regulation in the large-scale construction of photocatalytic system (
Fig. 18c-e).
Different from a single process optimization, 3D printing can realize the multi-functional integration of photocatalytic system. By incorporating cavities and microfluidic channels into the printed structure, and integrating micro-sensors and micro-actuators, it is possible to construct an intelligent photocatalytic system. This system is capable of real-time monitoring of reaction conditions and dynamic adjustment of operational parameters, thereby achieving integrated perception, control, and catalysis at the system level. Although no such construction method has been applied to 3D printed photocatalytic integrated systems, it can be inspired by applications in other fields. Li et al. fabricated a circular concave support by 3D printing to build a layered 3D solar energy reaction system integrated with phase change material (PCM), and the inner side of the support was covered with PMCB coating
[291]. The system achieved a maximum output voltage of 3.51 V and an evaporation rate of 4.0 kg m
-2 h
-1 under natural sunlight. PCM shortened the capacitor charging time by 57.9% and increased fresh water production by 29.9%. 3D printing support optimizes the sunlight capture efficiency and establishes a photocatalytic reaction system that considers both power output and freshwater production. The ingenious structure of the reaction system realizes the integration of a variety of sensors and actuators (
Fig. 18f, g).
5 Conclusions and perspectives
This review systematically describes the rapidly developing applications of 3D printing technology in photocatalysis, including the design of catalysts and catalytic carriers, as well as the manufacturing of advanced photocatalytic reactors and integrated systems. The core advantage of 3D printing lies in its high-precision geometric control and customized manufacturing ability, which can effectively deal with the problems of low light utilization efficiency, mass transfer limitation, difficulty in catalyst recovery and insufficient stability in traditional photocatalytic technology. In terms of photocatalyst design, a variety of 3D printing techniques such as FDM, DIW, SLS/SLM, and SLA are used to fabricate catalyst structures with high porosity, complex channel networks, and customized specific surface areas. These structures significantly increased the active site exposure and improved reactant diffusion, thereby enhancing the photocatalytic efficiency. In addition, in the development of catalyst support, 3D printing supports the flexible use of a variety of materials (polymers, carbon materials, metals, ceramics, etc.), and can prepare support structures with specific properties according to specific photocatalytic requirements, which provides the possibility to construct high-performance integrated catalytic systems. 3D printing has also promoted innovation in the design of photocatalytic reactors and systems. This technology can be used to construct novel reactor configurations to optimize fluid distribution and light field configuration, thereby improving the overall efficiency of energy and mass transfer. More importantly, it makes it possible to integrate functional components such as sensors, mixers, and light-guiding structures into the reaction system, creating conditions for the development of scalable, efficient, and intelligent photocatalytic processes.
3D printing has brought revolutionary design freedom to the field of photocatalysis, and its layered manufacturing characteristics have solved the complex structural customization that traditional processes cannot achieve. The ability to co-print multiple materials has realized the integrated integration of catalysts, carriers, and functional components, reflecting the unique value of this interdisciplinary field. However, there are still limitations that cannot be ignored. Firstly, the adaptability of the ink system is poor. Most existing printing inks rely on polymer or ceramic slurries, which make it difficult to balance printing smoothness and catalytic activity, and it is difficult to print photocatalysts with a high proportion of active components. Secondly, the contradiction between technical cost and scale is prominent. High-precision technology equipment such as SLA/SLM has a unit price exceeding one million, while the precision defects of low-cost FDM cannot meet high-end demands. Thirdly, the long-term stability lacks verification. Most research only focuses on short-term catalytic activity, and issues such as mechanical strength degradation and active component detachment of 3D printed structures in acid-base environments or continuous flow reactions have not been systematically solved.
Based on the analysis of technological progress and limitations mentioned above, we can further identify future challenges and development opportunities (
Fig. 19). Perhaps we can carry out in-depth work from the following aspects:
1. Currently, the types of 3D printing inks suitable for photocatalysis are relatively limited, and it is often difficult to meet the requirements of printing process and catalytic function at the same time. The development of multifunctional ink with excellent printability, high catalytic efficiency and long-term stability is an important prerequisite for realizing high-performance customized catalytic structure.
2. The photocatalytic system needs to integrate catalytic materials, optical elements and support systems. However, the problems of interface bonding, thermal expansion matching and chemical compatibility between different parts have not been systematically solved. The key of building a complex functional system is to realize multi-material integrated printing and stable integration.
3. In the process of industrial application, the current 3D printing technology is still difficult to maintain high resolution, large size and low cost at the same time. It is an important direction for practical application to develop new printing technology suitable for large-scale production and optimize the balance between printing efficiency and structural performance.
4. Most studies have focused on the initial activity of the catalysts, but there is still a lack of sufficient verification of the mechanical stability and catalytic durability of the printed structures under continuous operation, high temperatures, strong light exposure, and chemical corrosion environments. It is crucial to establish a systematic life evaluation method and develop effective strengthening strategies.
5. By leveraging artificial intelligence, machine learning and computational fluid dynamics, it is possible to achieve collaborative design and optimization of the catalyst’s microstructure, the macroscopic reactor flow channels and the optical path. This significantly shortens the research and development cycle and provides the possibility of constructing intelligent catalytic systems that are customizable on demand and have predictable performance. This significantly shortens the research and development cycle and provides the possibility of constructing intelligent catalytic systems that are customizable on demand and have predictable performance.
6. The development of environment-friendly printing materials and energy-saving printing processes, combined with CO2 conversion, pollutant degradation and other environmental application scenarios, will significantly improve the sustainability of the technology in the whole life cycle.
In conclusion, 3D printing has brought unprecedented design freedom and functional integration capabilities to the field of photocatalysis. The future development urgently requires in-depth collaboration among multiple disciplines such as materials, engineering, chemistry and information, in order to achieve breakthrough progress environmental remediation and energy conversion areas.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article. Jizhou Jiang is the Editor-in-Chief, and Hui Xu is an Associate Editor of this journal and they were not involved in the editorial review or the decision to publish this article.
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