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.

Key words： 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).

Fig.1 Hydrogel photocatalysts as an effective strategy for energy conversion and environmental treatment.

Full size|PPT slide

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.

Fig.2 Schematic illustrations of the fabrication process of hydrogel photocatalysts.

Full size|PPT slide

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].

Fig.3 Inorganic semiconductor photocatalysts embedded in hydrogel network.

Full size|PPT slide

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.

Fig.4 In situ synthesized hydrogel photocatalysts.

Full size|PPT slide

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.

Fig.5 Preparation of hydrogel photocatalyst microspheres by microfluidic process (adapted with permission from Ref. [93]).

Full size|PPT slide

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.

Fig.6 Narrow bandgap semiconductor-based hydrogel photocatalysts.

Full size|PPT slide

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.

Fig.7 g-C₃N₄ based hydrogel photocatalysts.

Full size|PPT slide

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.

Fig.8 Chromophore amphiphile conjugated polymer hydrogel photocatalyst.

Full size|PPT slide

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.

Fig.9 Hydrogel photocatalysts for hydrogen generation.

Full size|PPT slide

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.

Fig.10 Multicomponent (individual CdS and ZnS nanocrystals) hydrogel photocatalysts for photocatalytic hydrogen generation (adapted with permission from Ref. [132]).

Full size|PPT slide

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.

Fig.11 Gel-based photocatalyst for CO₂ conversion.

Full size|PPT slide

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.

Fig.12 Removal of metal ions using hydrogel photocatalyst.

Full size|PPT slide

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.

Fig.13 Synergistic water evaporation and H₂ generation.

Full size|PPT slide

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.

Tab.1 Summary of various types of hydrogel photocatalysts employed for different applications

Applications	Materials	References
H2 evolution	CdS/HGel	[130]
	PdAu-TiO2 aerogels	[131]
	CdS and ZnS containing hydrogel	[132]
	PMI-based hydrogel	[119,126–128]
CO2 conversion	Macroporous 3D TGM	[138]
	TiO2-Au composite aerogel	[139]
Organic pollutant degradation	GH-AgBr at rGO	[63]
	ZnO/rGO-rGH hydrogel	[64]
	TiO2 based hydrogel	[65,67–72,88–91,93]
	Fe0 at Guar gum-crosslinked-soya lecithin nanocomposite hydrogel	[66]
	CdS based hydrogel	[73,75,76]
	Chitosan-Gelatin based hydrogels	[74]
	Bi2WO6/GH	[77]
	β-FeOOH at tunicate cellulose nanocomposite hydrogels	[99]
	MoS2-rGO composite hydrogel	[100]
	Ag3PO4/rGH hydrogel	[101]
	AgCl/ZnO nanocomposites hydrogel	[103]
	C3N4 based hydrogel	[113–117,143,144]
	polymer ionic complexation hydrogel photocatalyst	[129]
	p(HEA/NMMA)-CuS hydrogel	[145]
Removal of metal ions	TiO2-rGH 3D structure hydrogel	[149]
Photothermal evaporation	TiO2/Ag nanofibers gel D-HNb3O8 and a PAM network	[155] [156]

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.

Acknowledgments

This work was supported by Japan Science and Technology-Strategic International Collaborative Research Program (JSTSICORP) Grant JPMJSC18H1 and Japan Science and Technology-Program on Open Innovation Platform with Enterprises, Research Institute and Academia (JST-OPERA) Grant JPMJOP1843. This work was also supported by Natural Science Foundation of Hebei province (No.B2021203028).

References

1 Weaver P, Jansen L, van Grootveld G, Sustainable Technology Development. London: Routledge, 2017

2 Lewis N S. Research opportunities to advance solar energy utilization. Science, 2016, 351(6271): aad1920

3 Tao P, Ni G, Song C, Solar-driven interfacial evaporation. Nature Energy, 2018, 3(12): 1031–1041

4 Chen C, Kuang Y, Hu L. Challenges and opportunities for solar evaporation. Joule, 2019, 3(3): 683–718

5 Hosseini S E, Wahid M A. Hydrogen from solar energy, a clean energy carrier from a sustainable source of energy. International Journal of Energy Research, 2020, 44(6): 4110–4131

6 Zhang Y, Ren J, Pu Y, Solar energy potential assessment: a framework to integrate geographic, technological, and economic indices for a potential analysis. Renewable Energy, 2020, 149: 577–586

7 Gong J, Li C, Wasielewski M R. Advances in solar energy conversion. Chemical Society Reviews, 2019, 48(7): 1862–1864

8 Kannan N, Vakeesan D. Solar energy for future world: a review. Renewable & Sustainable Energy Reviews, 2016, 62: 1092–1105

9 Wang Q, Hisatomi T, Jia Q, Scalable water splitting on particulate photocatalyst sheets with a solar-to-hydrogen energy conversion efficiency exceeding 1%. Nature Materials, 2016, 15(6): 611–615

10 Ong W J, Tan L L, Ng Y H, Graphitic carbon nitride (g-C₃N₄)-based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability? Chemical Reviews, 2016, 116(12): 7159–7329

11 Luo J, Zhang S, Sun M, A critical review on energy conversion and environmental remediation of photocatalysts with remodeling crystal lattice, surface, and interface. ACS Nano, 2019, 13(9): 9811–9840

12 Tian B, Tian B, Smith B, Supported black phosphorus nanosheets as hydrogen-evolving photocatalyst achieving 5.4% energy conversion efficiency at 353 K. Nature Communications, 2018, 9(1): 1397

13 Zhang P, Lou X W D. Design of heterostructured hollow photocatalysts for solar-to-chemical energy conversion. Advanced Materials, 2019, 31(29): 1900281

14 Dhiman P, Naushad M, Batoo K M, Nano Fe_xZn_1−xO as a tuneable and efficient photocatalyst for solar powered degradation of bisphenol A from aqueous environment. Journal of Cleaner Production, 2017, 165: 1542–1556

15 Tang X, Huang Z, Cao Y, Mo promotes interfacial interaction and induces oxygen vacancies in 2D/2D of Mo-g-C₃N₄ and Bi₂O₂CO₃ photocatalyst for enhanced NO oxidation. Industrial & Engineering Chemistry Research, 2020, 59(20): 9509–9518

16 Yi J, Liao J, Xia K, Integrating the merits of two-dimensional structure and heteroatom modification into semiconductor photocatalyst to boost NO removal. Chemical Engineering Journal, 2019, 370: 944–951

17 Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature, 1972, 238(5358): 37–38

18 Zhou W, Li W, Wang J, Ordered mesoporous black TiO₂ as highly efficient hydrogen evolution photocatalyst. Journal of the American Chemical Society, 2014, 136(26): 9280–9283

19 Hu Y. A highly efficient photocatalyst—hydrogenated black TiO₂ for the photocatalytic splitting of water. Angewandte Chemie International Edition, 2012, 51(50): 12410–12412

20 Naldoni A, Altomare M, Zoppellaro G, Photocatalysis with reduced TiO₂: from black TiO₂ to cocatalyst-free hydrogen production. ACS Catalysis, 2019, 9(1): 345–364

21 Liu N, Häublein V, Zhou X, “Black” TiO₂ nanotubes formed by high-energy proton implantation show noble-metal-co-catalyst free photocatalytic H₂-evolution. Nano Letters, 2015, 15(10): 6815–6820

22 Hsu C C, Wu N L. Synthesis and photocatalytic activity of ZnO/ZnO₂ composite. Journal of Photochemistry and Photobiology A Chemistry, 2005, 172(3): 269–274

23 Elmolla E S, Chaudhuri M. Degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution by the UV/ZnO photocatalytic process. Journal of Hazardous Materials, 2010, 173(1–3): 445–449

24 Yu W, Zhang J, Peng T. New insight into the enhanced photocatalytic activity of N-, C- and S-doped ZnO photocatalysts. Applied Catalysis B: Environmental, 2016, 181: 220–227

25 Tian C, Zhang Q, Wu A, Cost-effective large-scale synthesis of ZnO photocatalyst with excellent performance for dye photodegradation. Chemical Communications, 2012, 48(23): 2858–2860

26 Fu J, Yu J, Jiang C, g-C₃N₄-based heterostructured photocatalysts. Advanced Energy Materials, 2018, 8(3): 1701503

27 Wen J, Xie J, Chen X, A review on g-C₃N₄-based photocatalysts. Applied Surface Science, 2017, 391: 72–123

28 Ye L, Liu J, Jiang Z, Facets coupling of BiOBr-g-C₃N₄ composite photocatalyst for enhanced visible-light-driven photocatalytic activity. Applied Catalysis B: Environmental, 2013, 142–143: 1–7

29 Li Y, He R, Han P, A new concept: volume photocatalysis for efficient H₂ generation—using low polymeric carbon nitride as an example. Applied Catalysis B: Environmental, 2020, 279: 119379

30 Xu M, Han L, Dong S. Facile fabrication of highly efficient g-C₃N₄/Ag₂O heterostructured photocatalysts with enhanced visible-light photocatalytic activity. ACS Applied Materials & Interfaces, 2013, 5(23): 12533–12540

31 Kato H, Kudo A. Visible-light-response and photocatalytic activities of TiO₂ and SrTiO₃ photocatalysts codoped with antimony and chromium. Journal of Physical Chemistry B, 2002, 106(19): 5029–5034

32 Domen K, Kudo A, Onishi T. Mechanism of photocatalytic decomposition of water into H₂ and O₂ over NiOSrTiO₃. Journal of Catalysis, 1986, 102(1): 92–98

33 Iwashina K, Kudo A. Rh-doped SrTiO₃ photocatalyst electrode showing cathodic photocurrent for water splitting under visible-light irradiation. Journal of the American Chemical Society, 2011, 133(34): 13272–13275

34 Takata T, Jiang J, Sakata Y, Photocatalytic water splitting with a quantum efficiency of almost unity. Nature, 2020, 581(7809): 411–414

35 Xu C, Ravi Anusuyadevi P, Aymonier C, Nanostructured materials for photocatalysis. Chemical Society Reviews, 2019, 48(14): 3868–3902

36 Wei Z, Zhu Y, Guo W, Enhanced twisting degree assisted overall water splitting on a novel nano-dodecahedron BiVO₄-based heterojunction. Applied Catalysis B: Environmental, 2020, 266: 118664

37 Jiang Z, Huang Z, Guo W, Photocatalytic overall water splitting on isolated semiconductor photocatalyst sites in an ordered mesoporous silica matrix: a multiscale strategy. Journal of Catalysis, 2019, 370: 210–223

38 Fang W, Jiang Z, Yu L, Novel dodecahedron BiVO₄:YVO₄ solid solution with enhanced charge separation on adjacent exposed facets for highly efficient overall water splitting. Journal of Catalysis, 2017, 352: 155–159

39 Li Y, Han P, Hou Y, Oriented Zn_mIn₂S_m+3@In₂S₃ heterojunction with hierarchical structure for efficient photocatalytic hydrogen evolution. Applied Catalysis B: Environmental, 2019, 244: 604–611

40 Li Y, Hou Y, Fu Q, Oriented growth of ZnIn₂S₄/In(OH)₃ heterojunction by a facile hydrothermal transformation for efficient photocatalytic H₂ production. Applied Catalysis B: Environmental, 2017, 206: 726–733

41 Meng A, Zhang L, Cheng B, Dual cocatalysts in TiO₂ photocatalysis. Advanced Materials, 2019, 31(30): 1807660

42 Ran J, Zhang J, Yu J, Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting. Chemical Society Reviews, 2014, 43(22): 7787–7812

43 Wang Q, Domen K. Particulate photocatalysts for light-driven water splitting: mechanisms, challenges, and design strategies. Chemical Reviews, 2020, 120(2): 919–985

44 Nakata K, Fujishima A. TiO₂ photocatalysis: design and applications. Journal of Photochemistry and Photobiology C, Photochemistry Reviews, 2012, 13(3): 169–189

45 Chen H, Lee S W, Kim T H, Photocatalytic decomposition of benzene with plasma sprayed TiO₂-based coatings on foamed aluminum. Journal of the European Ceramic Society, 2006, 26(12): 2231–2239

46 Shang J, Li W, Zhu Y. Structure and photocatalytic characteristics of TiO₂ film photocatalyst coated on stainless steel webnet. Journal of Molecular Catalysis A Chemical, 2003, 202(1–2): 187–195

47 Carneiro J O, Teixeira V, Portinha A, Iron-doped photocatalytic TiO₂ sputtered coatings on plastics for self-cleaning applications. Materials Science and Engineering B, 2007, 138(2): 144–150

48 Liu X, Chen Q, Lv L, Preparation of transparent PVA/TiO₂ nanocomposite films with enhanced visible-light photocatalytic activity. Catalysis Communications, 2015, 58: 30–33

49 Zhang R, Ma M, Zhang Q, Multifunctional g-C₃N₄/graphene oxide wrapped sponge monoliths as highly efficient adsorbent and photocatalyst. Applied Catalysis B: Environmental, 2018, 235: 17–25

50 Jiang W, Luo W, Zong R, Polyaniline/carbon nitride nanosheets composite hydrogel: a separation-free and high-efficient photocatalyst with 3D hierarchical structure. Small, 2016, 12(32): 4370–4378

51 Zhang Z, Xiao F, Guo Y, One-pot self-assembled three-dimensional TiO₂-graphene hydrogel with improved adsorption capacities and photocatalytic and electrochemical activities. ACS Applied Materials & Interfaces, 2013, 5(6): 2227–2233

52 Mai N X D, Bae J, Kim I T, A recyclable, recoverable, and reformable hydrogel-based smart photocatalyst. Environmental Science: Nano, 2017, 4(4): 955–966

53 Im J S, Bai B C, In S J, Improved photodegradation properties and kinetic models of a solar-light-responsive photocatalyst when incorporated into electrospun hydrogel fibers. Journal of Colloid and Interface Science, 2010, 346(1): 216–221

54 Lei L, Wang W, Wang C, Hydrogel-supported graphitic carbon nitride nanosheets loaded with Pt atoms as a novel self-water-storage photocatalyst for H₂ evolution. Journal of Materials Chemistry A, Materials for Energy and Sustainability, 2020, 8(45): 23812–23819

55 Lei W, Qi S, Rong Q, Diffusion-freezing-induced microphase separation for constructing large-area multiscale structures on hydrogel surfaces. Advanced Materials, 2019, 31(32): 1808217

56 Xiang Q, Yu J, Jaroniec M. Graphene-based semiconductor photocatalysts. Chemical Society Reviews, 2012, 41(2): 782–796

57 Mills A, Le Hunte S. An overview of semiconductor photocatalysis. Journal of Photochemistry and Photobiology A Chemistry, 1997, 108(1): 1–35

58 Maeda K. Z-scheme water splitting using two different semiconductor photocatalysts. ACS Catalysis, 2013, 3(7): 1486–1503

59 Wang H, Zhang L, Chen Z, Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances. Chemical Society Reviews, 2014, 43(15): 5234–5244

60 Liu G, Yu J C, Lu G Q, Crystal facet engineering of semiconductor photocatalysts: motivations, advances and unique properties. Chemical Communications, 2011, 47(24): 6763–6783

61 Jing L, Qu Y, Wang B, Review of photoluminescence performance of nano-sized semiconductor materials and its relationships with photocatalytic activity. Solar Energy Materials and Solar Cells, 2006, 90(12): 1773–1787

62 Abe R, Sayama K, Sugihara H. Development of new photocatalytic water splitting into H₂ and O₂ using two different semiconductor photocatalysts and a shuttle redox mediator IO₃⁻/I⁻. Journal of Physical Chemistry B, 2005, 109(33): 16052–16061

63 Chen F, An W, Liu L, Highly efficient removal of bisphenol A by a three-dimensional graphene hydrogel—AgBr@rGO exhibiting adsorption/photocatalysis synergy. Applied Catalysis B: Environmental, 2017, 217: 65–80

64 Liu C, Yue M, Liu L, A separation-free 3D network ZnO/rGO-rGH hydrogel: adsorption enriched photocatalysis for environmental applications. RSC Advances, 2018, 8(40): 22402–22410

65 Yun J, Jin D, Lee Y S, Photocatalytic treatment of acidic waste water by electrospun composite nanofibers of pH-sensitive hydrogel and TiO₂. Materials Letters, 2010, 64(22): 2431–2434

66 Sharma G, Kumar A, Sharma S, Fabrication and characterization of novel Fe⁰@Guar gum-crosslinked-soya lecithin nanocomposite hydrogel for photocatalytic degradation of methyl violet dye. Separation and Purification Technology, 2019, 211: 895–908

67 Thomas M, Naikoo G A, Sheikh M U D, Effective photocatalytic degradation of Congo red dye using alginate/carboxymethyl cellulose/TiO₂ nanocomposite hydrogel under direct sunlight irradiation. Journal of Photochemistry and Photobiology A Chemistry, 2016, 327: 33–43

68 Jiang W, Liu Y, Wang J, Separation-free polyaniline/TiO₂ 3D hydrogel with high photocatalytic activity. Advanced Materials Interfaces, 2016, 3(3): 1500502

69 Chen Y, Xiang Z, Wang D, Effective photocatalytic degradation and physical adsorption of methylene blue using cellulose/GO/TiO₂ hydrogels. RSC Advances, 2020, 10(40): 23936–23943

70 Chen X, Chen Q, Jiang W, Separation-free TiO₂-graphene hydrogel with 3D network structure for efficient photoelectrocatalytic mineralization. Applied Catalysis B: Environmental, 2017, 211: 106–113

71 Chen F, An W, Li Y, Fabricating 3D porous PANI/TiO₂-graphene hydrogel for the enhanced UV-light photocatalytic degradation of BPA. Applied Surface Science, 2018, 427: 123–132

72 Hou C, Zhang Q, Li Y, P25-graphene hydrogels: room-temperature synthesis and application for removal of methylene blue from aqueous solution. Journal of Hazardous Materials, 2012, 205–206: 229–235

73 Jiang R, Zhu H, Yao J, Chitosan hydrogel films as a template for mild biosynthesis of CdS quantum dots with highly efficient photocatalytic activity. Applied Surface Science, 2012, 258(8): 3513–3518

74 Kaur K, Jindal R. Comparative study on the behaviour of Chitosan-Gelatin based Hydrogel and nanocomposite ion exchanger synthesized under microwave conditions towards photocatalytic removal of cationic dyes. Carbohydrate Polymers, 2019, 207: 398–410

75 Yang J, Gao J, Wang X, Polyacrylamide hydrogel as a template in situ synthesis of CdS nanoparticles with high photocatalytic activity and photostability. Journal of Nanoparticle Research, 2017, 19(10): 350

76 Zhu H, Li Z, Yang J. A novel composite hydrogel for adsorption and photocatalytic degradation of bisphenol A by visible light irradiation. Chemical Engineering Journal, 2018, 334: 1679–1690

77 Yang J, Chen D, Zhu Y, 3D–3D porous Bi₂WO₆/graphene hydrogel composite with excellent synergistic effect of adsorption-enrichment and photocatalytic degradation. Applied Catalysis B: Environmental, 2017, 205: 228–237

78 Hashimoto K, Irie H, Fujishima A. TiO₂ photocatalysis: a historical overview and future prospects. Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers, 2005, 44(12): 8269–8285

79 Fujishima A, Zhang X, Tryk D A. TiO₂ photocatalysis and related surface phenomena. Surface Science Reports, 2008, 63(12): 515–582

80 Schneider J, Matsuoka M, Takeuchi M, Understanding TiO₂ photocatalysis: mechanisms and materials. Chemical Reviews, 2014, 114(19): 9919–9986

81 Guo Q, Zhou C, Ma Z, Fundamentals of TiO₂ photocatalysis: concepts, mechanisms, and challenges. Advanced Materials: Deerfield Beach, Fla, 2019, 31(50): 1901997

82 Leary R, Westwood A. Carbonaceous nanomaterials for the enhancement of TiO₂ photocatalysis. Carbon, 2011, 49(3): 741–772

83 Jiang Y, Ning H, Tian C, Single-crystal TiO₂ nanorods assembly for efficient and stable cocatalyst-free photocatalytic hydrogen evolution. Applied Catalysis B: Environmental, 2018, 229: 1–7

84 Zhang W, He H, Tian Y, Synthesis of uniform ordered mesoporous TiO₂ microspheres with controllable phase junctions for efficient solar water splitting. Chemical Science (Cambridge), 2019, 10(6): 1664–1670

85 Hu J, Xie J, Jia W, Interesting molecule adsorption strategy induced energy band tuning: boosts 43 times photocatalytic water splitting ability for commercial TiO₂. Applied Catalysis B: Environmental, 2020, 268: 118753

86 Li X, Shi J, Hao H, Visible light-induced selective oxidation of alcohols with air by dye-sensitized TiO₂ photocatalysis. Applied Catalysis B: Environmental, 2018, 232: 260–267

87 Qian R, Zong H, Schneider J, Charge carrier trapping, recombination and transfer during TiO₂ photocatalysis: an overview. Catalysis Today, 2019, 335: 78–90

88 Yue Y, Wang X, Wu Q, Highly recyclable and super-tough hydrogel mediated by dual-functional TiO₂ nanoparticles toward efficient photodegradation of organic water pollutants. Journal of Colloid and Interface Science, 2020, 564: 99–112

89 Arikal D, Kallingal A. Photocatalytic degradation of azo and anthraquinone dye using TiO₂/MgO nanocomposite immobilized chitosan hydrogels. Environmental Technology, 2019, online, doi:10.1080/09593330.2019.1701094

90 Lučić M, Milosavljević N, Radetić M, The potential application of TiO₂/hydrogel nanocomposite for removal of various textile azo dyes. Separation and Purification Technology, 2014, 122: 206–216

91 Zhao K, Feng L, Lin H, Adsorption and photocatalytic degradation of methyl orange imprinted composite membranes using TiO₂/calcium alginate hydrogel as matrix. Catalysis Today, 2014, 236: 127–134

92 Liu M, Ishida Y, Ebina Y, Photolatently modulable hydrogels using unilamellar titania nanosheets as photocatalytic crosslinkers. Nature Communications, 2013, 4(1): 2029

93 Liu J, Chen H, Shi X, Hydrogel microcapsules with photocatalytic nanoparticles for removal of organic pollutants. Environmental Science: Nano, 2020, 7(2): 656–664

94 Khan S, Kubota Y, Lei W, One-pot synthesis of (anatase/bronze-type)-TiO₂/carbon dot polymorphic structures and their photocatalytic activity for H₂ generation. Applied Surface Science, 2020, 526: 146650

95 Wu Q, Huang F, Zhao M, Ultra-small yellow defective TiO₂ nanoparticles for co-catalyst free photocatalytic hydrogen production. Nano Energy, 2016, 24: 63–71

96 Nowotny M K, Sheppard L R, Bak T, Defect chemistry of titanium dioxide. Application of defect engineering in processing of TiO₂-based photocatalysts. Journal of Physical Chemistry C, 2008, 112(14): 5275–5300

97 Elbanna O, Zhu M, Fujitsuka M, Black phosphorus sensitized TiO₂ mesocrystal photocatalyst for hydrogen evolution with visible and near-infrared light irradiation. ACS Catalysis, 2019, 9(4): 3618–3626

98 Su R, Ge S, Li H, Synchronous synthesis of Cu₂O/Cu/rGO@carbon nanomaterials photocatalysts via the sodium alginate hydrogel template method for visible light photocatalytic degradation. Science of the Total Environment, 2019, 693: 133657

99 Wang J, Li X, Cheng Q, Construction of β-FeOOH@tunicate cellulose nanocomposite hydrogels and their highly efficient photocatalytic properties. Carbohydrate Polymers, 2020, 229: 115470

100 Ding Y, Zhou Y, Nie W, MoS₂-GO nanocomposites synthesized via a hydrothermal hydrogel method for solar light photocatalytic degradation of methylene blue. Applied Surface Science, 2015, 357: 1606–1612

101 Mu C, Zhang Y, Cui W, Removal of bisphenol A over a separation free 3D Ag₃PO₄-graphene hydrogel via an adsorption-photocatalysis synergy. Applied Catalysis B: Environmental, 2017, 212: 41–49

102 Qin L, Ru R, Mao J, Assembly of MOFs/polymer hydrogel derived Fe₃O₄-CuO@hollow carbon spheres for photochemical oxidation: freezing replacement for structural adjustment. Applied Catalysis B: Environmental, 2020, 269: 118754

103 Taghizadeh M T, de Siyahi V, Ashassi-Sorkhabi H, ZnO, AgCl and AgCl/ZnO nanocomposites incorporated chitosan in the form of hydrogel beads for photocatalytic degradation of MB, E. coli and S. aureus. International Journal of Biological Macromolecules, 2020, 147: 1018–1028

104 Chen S, Jacobs D L, Xu J, 1D nanofiber composites of perylene diimides for visible-light-driven hydrogen evolution from water. RSC Advances, 2014, 4(89): 48486–48491

105 Chen S, Li Y, Wang C. Visible-light-driven photocatalytic H₂ evolution from aqueous suspensions of perylene diimide dye-sensitized Pt/TiO₂ catalysts. RSC Advances, 2015, 5(21): 15880–15885

106 Chen S, Wang C, Bunes B R, Enhancement of visible-light-driven photocatalytic H₂ evolution from water over g-C₃N₄ through combination with perylene diimide aggregates. Applied Catalysis A, General, 2015, 498: 63–68

107 Yang S, Gong Y, Zhang J, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light. Advanced Materials, 2013, 25(17): 2452–2456

108 Wang Y, Wang X, Antonietti M. Polymeric graphitic carbon nitride as a heterogeneous organocatalyst: from photochemistry to multipurpose catalysis to sustainable chemistry. Angewandte Chemie International Edition, 2012, 51(1): 68–89

109 Wang X, Maeda K, Chen X, Polymer semiconductors for artificial photosynthesis: hydrogen evolution by mesoporous graphitic carbon nitride with visible light. Journal of the American Chemical Society, 2009, 131(5): 1680–1681

110 Wang X, Maeda K, Thomas A, A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nature Materials, 2009, 8(1): 76–80

111 Hu C, Lin Y, Yang H C. Recent developments in graphitic carbon nitride based hydrogels as photocatalysts. ChemSusChem, 2019, 12(9): 1769–1806

112 Jiang W, Zhu Y, Zhu G, Three-dimensional photocatalysts with a network structure. Journal of Materials Chemistry A, Materials for Energy and Sustainability, 2017, 5(12): 5661–5679

113 Li J, Yu X, Zhu Y, 3D–2D-3D BiOI/porous g-C₃N₄/graphene hydrogel composite photocatalyst with synergy of adsorption-photocatalysis in static and flow systems. Journal of Alloys and Compounds, 2021, 850: 156778

114 Chu Y C, Lin T J, Lin Y, Influence of P, S, O-doping on g-C₃N₄ for hydrogel formation and photocatalysis: an experimental and theoretical study. Carbon, 2020, 169: 338–348

115 Liang Y, Wang X, An W, Ag-C₃N₄@ppy-rGO 3D structure hydrogel for efficient photocatalysis. Applied Surface Science, 2019, 466: 666–672

116 Hu J, Zhang P, Cui J, High-efficiency removal of phenol and coking wastewater via photocatalysis-Fenton synergy over a Fe-g-C₃N₄ graphene hydrogel 3D structure. Journal of Industrial and Engineering Chemistry, 2020, 84: 305–314

117 Liu G, Li T, Song X, Thermally driven characteristic and highly photocatalytic activity based on N-isopropyl acrylamide/high-substituted hydroxypropyl cellulose/g-C₃N₄ hydrogel by electron beam pre-radiation method. Journal of Thermoplastic Composite Materials, 2020, online, doi:10.1177/0892705-720944214

118 Zhang G, Lan Z, Wang X. Conjugated polymers: catalysts for photocatalytic hydrogen evolution. Angewandte Chemie International Edition, 2016, 55(51): 15712–15727

119 Wang X, Chen L, Chong S Y, Sulfone-containing covalent organic frameworks for photocatalytic hydrogen evolution from water. Nature Chemistry, 2018, 10(12): 1180–1189

120 Liu D, Wang J, Bai X, Self-assembled PDINH supramolecular system for photocatalysis under visible light. Advanced Materials: Deerfield Beach, Fla, 2016, 28(33): 7284–7290

121 Cohen E, Weissman H, Pinkas I, Controlled self-assembly of photofunctional supramolecular nanotubes. ACS Nano, 2018, 12(1): 317–326

122 Chen S, Slattum P, Wang C, Self-assembly of perylene imide molecules into 1D nanostructures: methods, morphologies, and applications. Chemical Reviews, 2015, 115(21): 11967–11998

123 Krieg E, Bastings M M C, Besenius P, Supramolecular polymers in aqueous media. Chemical Reviews, 2016, 116(4): 2414–2477

124 Singh P, Mittal L S, Vanita V, Self-assembled vesicle and rod-like aggregates of functionalized perylene diimide: reaction-based near-IR intracellular fluorescent probe for selective detection of palladium. Journal of Materials Chemistry B, Materials for Biology and Medicine, 2016, 4(21): 3750–3759

125 Zhang Z, Chen X, Zhang H, A highly crystalline perylene imide polymer with the robust built-in electric field for efficient photocatalytic water oxidation. Advanced Materials, 2020, 32(32): 1907746

126 Weingarten A S, Kazantsev R V, Palmer L C, Supramolecular packing controls H₂ photocatalysis in chromophore amphiphile hydrogels. Journal of the American Chemical Society, 2015, 137(48): 15241–15246

127 Weingarten A S, Kazantsev R V, Palmer L C, Self-assembling hydrogel scaffolds for photocatalytic hydrogen production. Nature Chemistry, 2014, 6(11): 964–970

128 Sai H, Erbas A, Dannenhoffer A, Chromophore amphiphile-polyelectrolyte hybrid hydrogels for photocatalytic hydrogen production. Journal of Materials Chemistry A, Materials for Energy and Sustainability, 2020, 8(1): 158–168

129 Byun J, Landfester K, Zhang K. Conjugated polymer hydrogel photocatalysts with expandable photoactive sites in water. Chemistry of Materials, 2019, 31(9): 3381–3387

130 Li F, Yang J, Gao J, Enhanced photocatalytic hydrogen production of CdS embedded in cationic hydrogel. International Journal of Hydrogen Energy, 2020, 45(3): 1969–1980

131 Luna A L, Matter F, Schreck M, Monolithic metal-containing TiO₂ aerogels assembled from crystalline pre-formed nanoparticles as efficient photocatalysts for H₂ generation. Applied Catalysis B: Environmental, 2020, 267: 118660

132 Jiang Z, Zhang X, Yang G, Hydrogel as a miniature hydrogen production reactor to enhance photocatalytic hydrogen evolution activities of CdS and ZnS quantum dots derived from modified gel crystal growth method. Chemical Engineering Journal, 2019, 373: 814–820

133 Yu J, Low J, Xiao W, Enhanced photocatalytic CO₂-reduction activity of anatase TiO₂ by coexposed {001} and {101} facets. Journal of the American Chemical Society, 2014, 136(25): 8839–8842

134 Habisreutinger S N, Schmidt-Mende L, Stolarczyk J K. Photocatalytic reduction of CO₂ on TiO₂ and other semiconductors. Angewandte Chemie International Edition, 2013, 52(29): 7372–7408

135 Ran J, Jaroniec M, Qiao S. Cocatalysts in semiconductor-based photocatalytic CO₂ reduction: achievements, challenges, and opportunities. Advanced Materials, 2018, 30(7): 1704649

136 Bie C, Zhu B, Xu F, In situ grown monolayer N-doped graphene on CdS hollow spheres with seamless contact for photocatalytic CO₂ reduction. Advanced Materials, 2019, 31(42): 1902868

137 Fu J, Jiang K, Qiu X, Product selectivity of photocatalytic CO₂ reduction reactions. Materials Today, 2020, 32: 222–243

138 Jung H, Cho K M, Kim K H, Highly efficient and stable CO₂ reduction photocatalyst with a hierarchical structure of mesoporous TiO₂ on 3D graphene with few-layered MoS₂. ACS Sustainable Chemistry & Engineering, 2018, 6(5): 5718–5724

139 Rechberger F, Niederberger M. Translucent nanoparticle-based aerogel monoliths as 3-dimensional photocatalysts for the selective photoreduction of CO₂ to methanol in a continuous flow reactor. Materials Horizons, 2017, 4(6): 1115–1121

140 Godiya C B, Martins Ruotolo L A, Cai W. Functional biobased hydrogels for the removal of aqueous hazardous pollutants: current status, challenges, and future perspectives. Journal of Materials Chemistry A, Materials for Energy and Sustainability, 2020, 8(41): 21585–21612

141 Jing G, Wang L, Yu H, Recent progress on study of hybrid hydrogels for water treatment. Colloids and Surfaces A, Physicochemical and Engineering Aspects, 2013, 416: 86–94

142 Mohammadzadeh Pakdel P, Peighambardoust S J. A review on acrylic based hydrogels and their applications in wastewater treatment. Journal of Environmental Management, 2018, 217: 123–143

143 Zhang M, Luo W, Wei Z, Separation free C₃N₄/SiO₂ hybrid hydrogels as high active photocatalysts for TOC removal. Applied Catalysis B: Environmental, 2016, 194: 105–110

144 Zhang M, Jiang W, Liu D, Photodegradation of phenol via C₃N₄-agar hybrid hydrogel 3D photocatalysts with free separation. Applied Catalysis B: Environmental, 2016, 183: 263–268

145 Yang J, Li Z, Zhu H. Adsorption and photocatalytic degradation of sulfamethoxazole by a novel composite hydrogel with visible light irradiation. Applied Catalysis B: Environmental, 2017, 217: 603–614

146 Hua M, Zhang S, Pan B, Heavy metal removal from water/wastewater by nanosized metal oxides: a review. Journal of Hazardous Materials, 2012, 211–212: 317–331

147 Fu F, Wang Q. Removal of heavy metal ions from wastewaters: a review. Journal of Environmental Management, 2011, 92(3): 407–418

148 Tahir M B, Kiran H, Iqbal T. The detoxification of heavy metals from aqueous environment using nano-photocatalysis approach: a review. Environmental Science and Pollution Research International, 2019, 26(11): 10515–10528

149 Li Y, Cui W, Liu L, Removal of Cr(VI) by 3D TiO₂-graphene hydrogel via adsorption enriched with photocatalytic reduction. Applied Catalysis B: Environmental, 2016, 199: 412–423

150 Guo Y, Bae J, Fang Z, Hydrogels and hydrogel-derived materials for energy and water sustainability. Chemical Reviews, 2020, 120(15): 7642–7707

151 Zhou X, Guo Y, Zhao F, Hydrogels as an emerging material platform for solar water purification. Accounts of Chemical Research, 2019, 52(11): 3244–3253

152 Zhou X, Zhao F, Guo Y, Architecting highly hydratable polymer networks to tune the water state for solar water purification. Science Advances, 2019, 5(6): eaaw5484

153 Zhao F, Zhou X, Shi Y, Highly efficient solar vapour generation via hierarchically nanostructured gels. Nature Nanotechnology, 2018, 13(6): 489–495

154 Lei W, Khan S, Chen L, Hierarchical structures hydrogel evaporator and superhydrophilic water collect device for efficient solar steam evaporation. Nano Research, 2021, 14(4): 1135–1140

155 Gao M, Peh C K, Zhu L, Photothermal catalytic gel featuring spectral and thermal management for parallel freshwater and hydrogen production. Advanced Energy Materials, 2020, 10(23): 2000925

156 Yang M, Tan C, Lu W, Spectrum tailored defective 2D semiconductor nanosheets aerogel for full-spectrum-driven photothermal water evaporation and photochemical degradation. Advanced Functional Materials, 2020, 30(43): 2004460