Hydrogel photocatalysts for efficient energy conversion and environmental treatment

Abstract

Photocatalysts have attracted great research interest owing to their excellent properties and potential for simultaneously addressing challenges related to energy needs and environmental pollution. Photocatalytic particles need to be in contact with their respective media to exhibit efficient photocatalytic performances. However, it is difficult to separate nanometer-sized photocatalytic materials from reaction media later, which may lead to secondary pollution and a poor recycling performance. Hydrogel photocatalysts with a three-dimensional (3D) network structures are promising support materials for photocatalysts based on features such as high specific surface areas and adsorption capacities and good environmental compatibility. In this review, hydrogel photocatalysts are classified into two different categories depending on their elemental composition and recent progresses in the methods for preparing hydrogel photocatalysts are summarized. Moreover, current applications of hydrogel photocatalysts in energy conversion and environmental remediation are reviewed. Furthermore, a comprehensive outlook and highlight future challenges in the development of hydrogel photocatalysts are presented.

Graphical abstract

Keywords

hydrogel / photocatalysts / energy conversion / environmental treatment

1 Introduction

Environmental remediation and sustainable energy are pressing issues that require effective, sustainable, and green solutions [1]. Solar energy offers a plentiful, clean, and sustainable source of energy, which might be harnessed to address these issues [2–8]. Photocatalytic technologies effectively use solar energy for sustainable energy conversion (such as H₂ evolution, CO₂ reduction, and nitrogen fixation) and environmental treatments (such as conversion of NO_x and volatile organic compounds, removal of heavy metal ions removal, and degradation of organic pollutants) [9–16]. Therefore, there is an urgent need for efficient photocatalysts and reliable methods to meet growing development requirements.

The Fujishima-Honda effect, whereby H₂ and O₂ are produced under UV light irradiation of a TiO₂ photoelectrode, was first reported in 1972 [17]. Numerous novel materials, such as black TiO₂ [18–21], ZnO₂ [22–25], g-C₃N₄ [26–30], SrTiO₃ [31–35], BiVO₄ [36–39], ZnInS₄ [39,40] and their hybrid materials have been developed for applications in photocatalytic water splitting and environmental remediation. Many effective strategies have also been examined to further increase the efficiency of photocatalysis, including the use of doping, co-catalysts, defect manipulation, junctions, quantum confinement effects, and Z-scheme configurations [41–45]. However, owing to requirements for the material bandgap, specific surface area, and charge separation, most photocatalytic materials are applied in the form of nanometer-sized particles, which are difficult to separate from the reaction media. Additionally, for materials in this form there is the possibility of secondary pollution to the environment and poor recycling performance, which greatly hinders practical applications. Hence, there is an urgent need to find suitable substrates to avoid these limitations.

To date, there have been many reports on nanometer-sized photocatalytic materials loaded into foamed metals, plastics, hydrogels, and blended with polymers or adhesives to form thin films [46–50]. Hydrogels have also been recognized as potential 3D network supports for photocatalysis. Hydrogels feature good flexibility, stretchability, ionic conductivity, and environmental compatibility have high surface areas and adsorption capacities [51–56]. The strategy of combining photocatalysts and hydrogels may contribute to more efficient and environmentally friendly energy conversion and environmental management. Here hydrogel photocatalysts of these new materials are denoted (Fig. 1). The physical and chemical properties of hydrogels can also be tuned to enhance the photocatalytic effect by tailoring of cross-linking points, changing basic building blocks, surface structure, and other modifications. Hydrogels provide a suitable platform for photocatalysts to achieve efficient energy conversion and environmental regulation (Fig. 1).

In this review, various kinds of hydrogel photocatalysts and their applications in energy conversion and environmental treatment are focused on. Design and fabrication methods are described for photocatalyst systems. In addition, progresses toward hydrogels with different functionalities constructed based on conventional and emerging strategies are summarized. Moreover, applications of these hydrogel photocatalysts related to water splitting, CO₂ conversion, wastewater treatment, air purification, and their roles in fundamental studies are highlighted. Furthermore, future challenges and outlook of hydrogel photocatalysts are discussed. It is believed that this review will encourage innovation in the field of hydrogel photocatalysts for energy conversion and environmental remediation.

2 Fabrication of hydrogel photocatalysts

In most cases, hydrogel photocatalysts comprise a nanometer-sized photocatalyst supported by a cross-linked hydrogel network. The three-dimensional (3D) hydrogel network provides a porous skeleton structure that limits catalyst leakage into the reaction media (air or water) and facilitates loading of a large amount of catalyst. The photocatalyst provides active sites for the catalytic reaction. Existing methods for preparing hydrogel photocatalytic materials can be divided into three categories: embedding of photocatalysts in hydrogel networks, in situ synthesis of photocatalysts in hydrogel networks, and self-assembly of hydrogel photocatalysts. The fabrication processes of hydrogels with photocatalysts are illustrated in Fig. 2. In the process of fabricating hydrogel photocatalysts, photocatalysts materials, such as semiconductors, g-C₃N₄, conjugated organic molecules, and their hybrids have been widely studied. In this section, fabrication methods frequently used to fabricate hydrogel photocatalysts are outlined based on different photocatalyst materials.

2.1 Inorganic semiconductor-based hydrogel photocatalysts

Inorganic semiconductor photocatalysts (ISPCs) are considered excellent photocatalysts for H₂ evolution and decomposition of organic compounds in effluents [57–59]. The photocatalytic efficiencies of these materials may be enhanced by appropriate textural design, doping, and formation of a semiconductor heterojunction by combination with metals and/or other semiconductors [60–62]. Generally, there are two main fabrication methods that have been applied in the development of inorganic semiconductor-based hydrogels photocatalysts.

One approach is to gel a mixture of ISPCs and hydrophilic polymers/monomers by self-assembly or introducing cross-linkable elements, which allow the ISPCs to be embedded in the hydrogel network [63–66]. For example, a nanocomposite hydrogel of alginate/carboxymethyl cellulose with encapsulated TiO₂ was successfully synthesized by barium-ion cross linking. This system exhibited excellent photocatalytic activity toward degradation of Congo red dye under direct solar light irradiation (Fig. 3(a)) [67]. In the same way, TiO₂ has been added to a polyaniline (PANI)-phytate hydrogel system (Fig. 3(b)). Efficient removal of organic pollutants is achieved through the synergistic effects of absorption of organic pollutants to the hydrogel and in situ photocatalytic degradation by TiO₂ [68]. The networks of 3D hydrogel photocatalysts have also been formed through self-assembly of TiO₂ and reduced graphene oxide (rGO) [69,70]. As shown in Fig. 3(c), a PANI/TiO₂ composite graphene hydrogel (GH) was successfully prepared by chemical reduction of graphene oxide (GO) followed by H-bond and π-π self-assembly. The rGO and PANI acted as a transmitter for e⁻ and H⁺ to further enhance the photocatalytic performance [71]. Graphene is the most widely used additive for self-assembly of ISPCs to form hydrogel photocatalysts, whereas other conductive materials, such as carbon nanotubes, polypyrrole, and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS), have also been used as additives to prepare hydrogel photocatalysts. Figure 3(d) demonstrates hydrogel photocatalytic composites based on TiO₂(P25) and graphene, multi-walled carbon nanotubes formed by a simple room temperature self-assembly [72].

Another method is in situ synthesis of photocatalysts in a hydrogel network by oxidation, reduction, and sulfuration. By using this approach, hydrogel photocatalysts with tunable catalytic activities based on different active species can be prepared for a variety of applications [73–77]. Recently, a poly 2-hydroxyethyl acrylate (HEA) -co- N-hydroxymethyl acrylamide (HAM) hydrogel photocatalyst, denoted P(HEA-co-HAM)-CdS hydrogel, was fabricated to adsorb and photocatalytically degrade organic pollutants [76]. A Cd precursor was introduced by swelling a preformed hydrogel network, synthesized by 60Co-γ irradiation-induced radical polymerization. The sulfur precursor was added to enable in situ synthesis of CdS photocatalysts (Fig. 4(a)). A Bi₂WO₆ (BWO)/GH photocatalyst is an example of a high performance visible-light-driven photocatalyst with a narrow band gap, which was fabricated by utilizing a simple one-step hydrothermal method. The 3D flower-like structures of BWO were formed in situ by addition of Na₂WO₄ to a mixture of Bi(NO₃)₃ and GO solution (Fig. 4(b)). The solution was then hydrothermally reduced to self-assemble the hydrogel through hydrogen bonding and π-π stacking interactions. The novel structure of the BWO/GH hydrogel photocatalyst improved the light utilization efficiency and absorption of the organic compounds and provided many effective multidimensional electron transfer channels [77]. This hydrogel photocatalyst effectively decomposed methylene blue (MB) and 2, 4-dichlorophenol (2, 4-CDP) under visible light irradiation (λ≥420 nm) in both static and dynamic systems.

TiO₂ is a well-known photocatalytic material in the field of photocatalysis because of its low cost, low-toxicity, chemical stability, and high resistance to photocorrosion [78–87]. Therefore, many hydrogel photocatalytic materials based on TiO₂ have been reported [65,88–91]. In addition to the above-mentioned general preparation strategies, several novel methods have been developed to prepare hydrogel photocatalysts. For example, TiO₂ can be used to induce radical formation under light irradiation to trigger the polymerization reaction. The TiO₂ also acts as a crosslinking site to induce gelling of the solution [92]. In this system, the gelling water provides a source of radicals and the TiO₂ nanosheets act as a stable photocatalysis to support the formation of the hydrogels. Microfluidics have also been used to prepare hydrogel microspheres for various applications. Very recently, a glass capillary-based microfluidic technique was used for highly efficient encapsulation of photocatalytic nanoparticles in a thin shell of hydrogel microcapsules (Fig. 5). An aqueous dispersion of photocatalytic nanoparticles and methacrylic anhydride formed the core and shell of the double emulsion drops, respectively [93]. The hydrogel microcapsules containing photocatalytic nanoparticles were gelatinized by external photopolymerization. These thin shell hydrogel microcapsules with photocatalytic nanoparticle cores more effectively promoted the photocatalytic reaction, absorption, diffusion, and separation of molecular species when compared with the performance of bulk hydrogels.

However, the inherent wide bandgap (3.0–3.2 eV) and low quantum efficiency of TiO₂ materials owing to rapid recombination of photogenerated electrons and holes, limits their practical applications [94–97]. Considerable efforts have been made to develop alternative photocatalysts to overcome these problems and improve the photocatalytic activities. These efforts have focused on new semiconductor photocatalysts that have a strong absorption of visible light. Hydrogel photocatalysts have also been investigated for applications ins degradation of pollutants driven by solar energy [98–103]. For example, β-FeOOH/cellulose composite hydrogels (TCH-Fe) were fabricated from a cellulose solution by regeneration in ethanol and in situ synthesis of β-FeOOH nanoparticles (Fig. 6(a)). The photocatalytic degradation of MB over TCH-Fe was as high as 99.89% within 30 min under visible-light irradiation [99]. The performance remained at about 98% after treatment for 8 h, indicating a highly efficient and stable photodegradation of MB. The layered structure of MoS₂ has a strong absorption in the solar spectrum region owing to its narrow band gap of approximately 1.8 eV. Hence, this material has also been embedded into hydrogels to degrade pollutants. As shown in Fig. 6(b), a MoS₂-rGO composite hydrogel was fabricated through a one-step hydrothermal method of mixing a solution of GO and exfoliated MoS₂ nanosheets [100]. Furthermore, Mu and coworkers reported a silver phosphate/Gh (Ag₃PO₄/GH) that efficiently degraded bisphenol A (BPA) through a synergy of adsorption and photocatalysis [101]. Owing to the high quantum yield under visible light of Ag₃PO₄, this composite hydrogel photocatalyst achieved 100% removal of BPA in a continuous flow reaction system. In addition, hydrogels can also be used to prepare composite structures for photocatalytic materials. Core-shell Fe₃O₄-CuO at carbon hollow spheres have been assembled from metal organic framework (MOF) composite hydrogels through a combination of chemical vapor deposition (CVD) and pyrolysis (Fig. 6(d)) [102]. This work also provides a new route to develop highly active and stable bimetallic hydrogel photocatalysts.

2.2 Organic semiconductor-based hydrogel photocatalysts

Compared with the extensive development of ISPCs, the progress related to organic semiconductor photocatalysts (OSPCs) has been more gradual. However, OSPCs have been intensively examined for heterogeneous photocatalysis under visible light illumination and offer advantages in terms of structural designability and stability over conventional molecular photocatalysts [104–109]. Currently, there are two main classes of polymer-semiconductor photocatalysts, graphitic carbon nitride (g-C₃N₄) and chromophore amphiphiles (based on conjugated molecules), that have been studied for water splitting and aqueous pollutant remediation.

As a metal-free polymer n-type semiconductor, C₃N₄ has shown great promise in the field of photocatalysis since Wang and coworkers first reported photocatalytic H₂ and O₂ evolution over C₃N₄ in 2009 [110]. The methods used to prepare C₃N₄-based hydrogel photocatalysts are similar to the aforementioned techniques. The preparation of graphitic carbon nitride-based hydrogels as photocatalysts has also been reviewed [111,112]. Therefore, detailed synthesis will be omitted. Instead, some of the latest outstanding developments of C₃N₄-based hydrogel photocatalysts are presented. Li and coworkers reported a 3D-2D-3D BiOI/porous g-C₃N₄/GH (BPG) composite photocatalyst based on a two-step hydrothermal method (Fig. 7(a)). The 3D GH had a high absorption capacity. The excellent photocatalytic properties of the heterojunction between the 3D BiOI, which had a flower-like structure, contributed to effective adsorption-photocatalysis and effective degradation of MB and levofloxacin hydrochloride compared with that of BiOI [113]. Co-doping of g-C₃N₄ with non-metal ions can also improve the effectiveness of pure g-C₃N₄. Chu et al. reported the use of P, S, and O co-doped g-C₃N₄ to produce photocatalyst hydrogels. A combination of theoretical calculations and experimental analyses of the P, S, and O doping indicated that rapid charge separation of photoexcited electrons took place across the heptazine rings, which enhanced the photocatalytic activity of doped g-C₃N₄. This doped g-C₃N₄ hydrogel photocatalyst exhibited a high photocatalytic activity for MB removal under simulated solar irradiation and could be easily separated and cleaned for reuse (Fig. 7(b)) [114]. In addition, C₃N₄-based hydrogel photocatalysts, such as g-C₃N₄ at ppy-rGO [115], Fe-g-C₃N4 graphene [116], and N-isopropyl acrylamide/high-substituted hydroxypropyl cellulose/g-C₃N₄ hydrogels [117], were reported for efficient catalytic removal of heavy metal ions and degradation of organic pollutants.

Supramolecular assemblies of chromophore molecules benefit from their good environmental stability, strong visible-light absorption, and tunable redox potentials (molecular orbital energy levels) arising from variable structural functionalization. Hence, these systems have been recognized as promising materials for photocatalytic reactions [118–121]. The n-type organic perylene imides, including perylenediimides (PDIs), perylenemonoimides (PMIs), and their oligomers and analogs, have been proven to have excellent photocatalytic properties [122–124]. Recently, Zhang and coworkers reported a urea-linked PDI polymer photocatalyst (Urea-PDI). Based on the energy band structure, excellent crystallization, and the large molecular dipole of Urea-PDI, the photocatalyst has a highest oxygen evolution rate of 3223.9 μmol/(g‧h) with 100 h of stable performance under visible light irradiation without a cocatalyst [125]. Several reports of PMI-based hydrogel photocatalysts from Stupp and coworkers have inspired many related studies [126–128].

A photocatalyst-embedded hydrogel was reported in 2020 (Fig. 8(a)), which featured a PMI photocatalyst embedded in a polyelectrolyte hydrogel network. The catalyst-loaded hybrid PMI/polyelectrolyte hydrogel was used in photocatalytic hydrogen production and could be reused over multiple cycles as a photosensitizer [128]. To further enhance the performance of hydrogel photocatalysts, a conjugated polymer hydrogel photocatalyst which expanded in water to expose more active sites was reported by Byun and coworkers [129]. This system could also be recovered by solvent exchange. The stable polymer ionic complexation between a benzothiadiazole-based polymer containing cationic side chains as the polycation and poly(acrylic acid) as the polyanion, exhibited a good water compatibility, absorbing up to 470 times of its weight in deionized water (Fig. 8(b)). The excellent swelling performance greatly expanded the availability of active sites and enhanced the photocatalytic activity of the hydrogel photocatalyst. The effectiveness of this material was demonstrated by its application to photodegradation of organic dyes and the formation of the enzyme cofactor nicotinamide adenine dinucleotide by photo-oxidation in water. Furthermore, this hydrogel photocatalyst could be regenerated by a simple solvent exchange with methanol after the reaction.

3 Applications of hydrogel photocatalysts

In this section, current progresses in the application of hydrogel photocatalysts in energy conversion and environmental treatments are highlighted, focusing primarily on hydrogen evolution and CO₂ reduction in Section 3.1, and organic pollutant degradation and removal of metal ions in Section 3.2. In Section 3.3, emerging applications of hydrogel photocatalysts for synergistic water evaporation and energy conversion are introduced.

3.1 Energy conversion

3.1.1 Photocatalytic hydrogen evolution

Hydrogen (H₂) is considered to be an ideal energy storage media due to its high energy capacity and environmental compatibility. The generation of H₂ induced by water splitting over photocatalysts is regarded as a promising strategy for achieving a H₂-based economy. Owing to the inherent water absorption capacity of hydrogels, hydrogel photocatalysts act as reaction centers for hydrogen production by photocatalysis. Li and coworkers reported in situ growth of CdS hydrogels (CdS/HGel) for photocatalytic hydrogen generation [130]. Owing to the high dispersibility of CdS nanoparticles in the hydrogels, high hydrophilic and swelling ability of the hydrogel, and high diffusion rate of reactants, CdS/HGel_PDMA2 had the best photocatalytic hydrogen production rate of 51.75 μmol/h (based on 5 mg catalyst powder) and allowed for easy recovery (Fig. 9(a)). The suppression of charge recombination at the catalyst interface is important for improving photocatalytic efficiency. Cocatalyst deposition is an effective strategy that can improve the activity, stability, and selectivity of primary catalysts in a catalytic reaction. A co-assembled aerogel of spherical Au, Pd, and PdAu with TiO₂ nanoparticles was prepared by light-induced gelation of a hydrogel-precursor (Fig. 9(b1)) [131]. PdAu-TiO₂ aerogels were the most efficient photocatalysts, followed by Pd-TiO₂ and Au-TiO₂, demonstrating that enhanced hot-electron transfer and near-field electromagnetic effects contribute to H₂ formation (Fig. 9(b3)). The efficient reagent mass transport and light-harvesting of the monolithic porous networks also promoted photocatalysis.

As an inspiration for rational design of multicomponent hydrogel photocatalysts for photocatalytic hydrogen evolution, a CdS and ZnS containing hydrogel was obtained by a modified gel crystal growth method [132]. The hydrogel (HR) framework inhibited the agglomeration of the CdS and ZnS nanoparticles. Owing to the synergistic effects of the quantum dots and hydrogel, the composite hydrogel photocatalyst exhibited high rates of H₂ evolution compared with those of non-supported nanoparticles (Fig. 10(a)).

Chromophore-amphiphile conjugated PMI hydrogel photocatalysts also play an important role in photocatalytic hydrogen generation. Weingarten et al. fabricated a hydrogel skeleton based on PMI and a cationic analog with an outer ligand sphere functionalized with primary amines as a supramolecular self-assembly for photocatalytic hydrogen production [127]. The highest catalytic turnover number (TON) of this PMI-based hydrogel photocatalyst was approximately 340 under different charge-screening conditions with poly(diallyldimethylammonium) chloride (PDDA) (Fig. 10). The gel catalyst could also be cast on glass slides for H₂ generation.

3.1.2 Photocatalytic CO₂ conversion

Photocatalytic conversion of CO₂ into renewable fuels driven by sunlight is considered as an ideal scenario for reducing the concentration of carbon dioxide in the atmosphere while also generating energy [133–137]. However, because of the inherent high-water content of hydrogel photocatalysts, there is a possibility that an excess of carbon dioxide may dissolve, reducing the conversion. Therefore, much research on CO₂ conversion is based on aerogels, which are mostly derived from freeze-drying and supercritical drying of hydrogels. Layers of MoS₂ on a hierarchical porous structure of mesoporous TiO₂ and macroporous 3D graphene aerogel (TGM) photocatalysts have been fabricated by freeze-drying gel composites (Fig. 11(a)) [138]. The morphologies of the mesopores and macropores that contribute to the high photocatalytic catalyst performance, can be regulated by adjusting the relative amounts of each component and the configuration of the composite. The TGM photocatalyst has a higher CO photoconversion rate (92.33 μmol CO/(g·h)) and is more stable (i.e., maintains its original conversion rate of over 15 cycles) than other composite combinations. Niederberger’s group reported several studies on the preparation of aerogel photocatalysts based on supercritical drying of hydrogel precursors. Figure 11(b1, b2) shows TiO₂-Au composite aerogel samples before and after photocatalytic reduction of CO₂ with water to methanol with high selectivity and reproducibility [139]. Niederberger’s group also found that translucent nanoparticle-based aerogel monoliths were promising photocatalysts for gas phase reactions such as CO₂ reduction.

3.2 Environmental treatment

3.2.1 Organic pollutant degradation

Owing to their high degree of swelling and adsorption capacity, hydrogels have drawn research interest for applications to adsorption of dyes, metal cations, and other pollutants. Although great progress has been made, many different materials accumulate in hydrogels, making it difficult to selectively target specific contaminants [140–142]. Hydrogel photocatalysts promote synergistic absorption and in situ photocatalytic degradation of pollutants, which is important for environmental treatments, especially wastewater treatments. Zhang et al. prepared dynamic systems for total organic carbon (TOC) removal based on graphitic carbon nitride/SiO₂ (C₃N₄/SiO₂) hybrid hydrogels with 3D network structures (Fig. 12(a)) [143]. The hybrid C₃N₄ /SiO₂ hydrogel photocatalyst had excellent cyclic stability and removal abilities for phenol and MB, with performances 3.1 and 6 times as great as those of pure g-C₃N₄, respectively. The C₃N₄/SiO₂ hybrid hydrogel photocatalyst could be used continuously without adsorption saturation or separation from water, avoiding aggregation and secondary pollution of the photocatalysts. Similarly, agar-C₃N₄ hybrid hydrogel photocatalysts have been prepared by a simple heating-cooling polymerization process. These catalysts exhibit excellent performances in photocatalytic degradation of MB and cyclic stability under visible light [144].

In addition to common colored pollutants, some colorless pollutants can also be degraded using hydrogel photocatalysts, e.g., BPA [63,76,101], phenol [116,144], and sulfonamide antibiotics (SAs). Yang et al. used irradiation polymerization and in situ precipitation methods to form a novel hydrogel photocatalyst [p(HEA/NMMA)-CuS] for efficient photocatalytic sulfamethoxazole (SMX) degradation [145]. The mechanism is detailed in Fig. 12(b). First, the [p(HEA/NMMA)-CuS] hydrogel photocatalyst adsorbed SMX through a process similar to Langmuir monolayer adsorption that followed a pseudo second-order rate equation. Thereafter, a photocatalytic decomposition process of SMX, which followed pseudo-first-order kinetics, was promoted by CuS under visible light irradiation. Theoretical calculations of the frontier electron densities and their degradation pathways supported this mechanism.

3.2.2 Removal of metal ions

Toxic metal ions such as copper (Cu²⁺), arsenic (As), zinc (Zn²⁺), cobalt (Co²⁺), nickel (Ni²⁺), lead (Pb²⁺), cadmium (Cd²⁺), and chromium (Cr⁶⁺) cause serious damage to human health and the environment [146–148]. Among these, Cr(VI), a common heavy metal contaminate, is a threat to human health because of its carcinogenic and bio-accumulative properties. The photocatalytic removal of cadmium ions has received extensive attention owing to its potential for treatment with high efficiency, low energy consumption, and mild reaction conditions. Li et al. fabricated a novel TiO₂ and rGH by π-π conjugation induced overlapping and coalescence among the graphene sheets (Fig. 12(c)) [149]. The TiO₂-rGH 3D structure had an excellent adsorption-photocatalysis performance for removal of Cr(VI) from aqueous solutions. The synergistically enhanced photo-induced charge separation, non-porous surface, and π-π interactions contributed to a removal rate of Cr (VI) from a solution of 100%. Furthermore, using a continuous flow system, the removal rate of cadmium was maintained at 100% over a long time.

3.3 Synergistic water evaporation and energy conversion

In recent years, there has been a great interest in obtaining clean water through solar evaporation. In particular, some hydrogel-based photothermal evaporation materials have shown great potential for solar evaporation [150–154]. On this basis, many photothermal materials are embedded in hydrogels to efficiently absorb sunlight for photothermal evaporation. Photothermal materials, such as plasmonic absorbers, semiconductors, carbon-based materials, and conductive polymers can be embedded in hydrogels to efficiently absorb sunlight for photothermal evaporation.

Gao et al. structured a photothermal catalytic (PTC) gel with a hydrophobic membrane to realize a H₂O-H₂ cogeneration system (HCS) for concurrent photothermal-enhanced solar desalination and hydrogen generation [155]. The PTC gel comprised photothermal and photocatalytic TiO₂/Ag nanofibers and a strong water-absorbing chitosan polymer (Fig. 13(a)). The photocatalytic hydrogen production was enhanced by efficient light absorption of the gel interface with a 3D structure. Furthermore, the porous structure of the array provided effective confinement, interfacial heating, and thermal conductivity. A custom-made device was used in the HCS for parallel freshwater production and hydrogen energy generation. The amount of condensate collected and hydrogen gas generated from the water and seawater sources were both increased (Fig. 13(b3)). Later, the same group developed a defective semiconductor nanosheet aerogel that contained oxygen vacancy defect-rich HNb₃O₈ nanosheets (D-HNb₃O₈) and a polymeric polyacrylamide (PAM) network [156]. The hybrid defective HNb₃O₈ aerogel had a high-performance for photothermal water evaporation and photochemical degradation (photocatalytic degradation of Rhodamine B) driven by light over the entire solar spectrum. These developments have broadened applications of hydrogel photocatalysts and inspired future research into hydrogel photocatalysts.

4 Summary and outlook

Through ongoing efforts, many photocatalytic materials have been developed, including single ISPCs, composite nanostructured hybrid photocatalysts, Z-scheme photocatalytic materials, and organic semiconductor photocatalytic materials. The photocatalytic properties of these systems have contributed to major breakthroughs in energy conversion and environmental treatment. However, the problems of secondary pollution and poor recycling performance are related to the difficulty in separating nanometer-sized photocatalytic materials from their reaction media in practical applications. The emergence of hydrogel photocatalysts offers a good solution to these problems. Many hydrogel photocatalysts have shown an excellent photocatalytic performance and cycling stability.

In this review, different approaches to synthesizing different kinds of hydrogel photocatalyst materials were summarized. Those hydrogel photocatalysts have been developed by incorporating various photocatalysts, such as TiO₂, C₃N₄, CdS, CuS, PMI, and graphene hybrid complexes. In addition, two main applications of hydrogel photocatalysts in energy conversion and environmental treatments were discussed in detail. Recent progresses in hydrogel photocatalysts for various applications were summarized in Table 1. From Table 1, it can be observed that based on the synergistic absorption and catalysis of hydrogel photocatalytic materials, they are found to show great advantages in environmental treatment, especially for water environment. At the same time, the hydrogel contains a large amount of water, which makes it unfavorable for pollutant gas treatment. It is anticipated that this review could provide new insights into the design and fabrication of advanced hydrogel photocatalyst materials for highly efficient photocatalysis.

Despite some achievements in terms of component and structural design of hydrogel photocatalysts in recent years, this field is still in its early stage of development and many challenges remain to improve photocatalytic efficiency and stability to satisfy the demands of practical applications. To address these challenges, efforts are needed in the following aspects:

Current research on hydrogel photocatalysts focuses on random co-mingling of photocatalysts and gel networks, which can limit exposure of photocatalytically active sites. Granular dry gels can result, which can complicate the operation and lead to difficult separation and secondary contamination because of photocatalyst leakage, and poor recyclability.

Research has focused mainly on the design and modification of photocatalysts. However, there have been fewer studies of the intrinsic properties of the hydrogel components. Regulation of the gel network structure and swelling properties and adsorption properties of the gel also synergistically contributes to photocatalyst performance and require further investigation.

To further expand the catalytic performance of hydrogel photocatalytic materials, interfacial modification of hydrogel monomers and photocatalysts together with modulation of the gel network will be likely to offer an effective strategy for achieving efficient and sustainable recycling of photocatalytic gels to overcome the drawbacks of existing hydrogel photocatalysts.

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