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Energy Mater 2022;2:200001. 10.20517/energymater.2021.24 © The Author(s) 2022.
Open Access Review

Recent advances in photocatalytic renewable energy production

The Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Frontiers Science Center of Biomimetic Catalysis, College of Chemistry and Materials Science, Shanghai Normal University, Shanghai, 200234, China.

#Authors contributed equally.

Correspondence to: Prof. Dieqing Zhang, the Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Frontiers Science Center of Biomimetic Catalysis, College of Chemistry and Materials Science, Shanghai Normal University, No. 100, Guilin Road, Xuhui District, Shanghai, 200234, China.E-mail: happy2002zdq@126.com ; Prof. Hexing Li, the Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Frontiers Science Center of Biomimetic Catalysis, College of Chemistry and Materials Science, Shanghai Normal University, No. 100, Guilin Road, Xuhui District, Shanghai, 200234, China.E-mail: hexing-li@shnu.edu.cn

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    Academic Editors: Yuping Wu, Yuhui Chen | Copy Editor: Xi-Jun Chen | Production Editor: Xi-Jun Chen

    © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, sharing, adaptation, distribution and reproduction in any medium or format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

    Abstract

    The development of green and renewable energy is becoming increasingly more important in reducing environmental pollution and controlling CO2 discharge. Photocatalysis can be utilized to directly convert solar energy into chemical energy to achieve both the conversion and storage of solar energy. On this basis, photocatalysis is considered to be a prospective technology to resolve the current issues of energy supply and environmental pollution. Recently, several significant achievements in semiconductor-based photocatalytic renewable energy production have been reported. This review presents the recent advances in photocatalytic renewable energy production over the last three years by summarizing the typical and significant semiconductor-based and semiconductor-like photocatalysts for H2 production, CO2 conversion and H2O2 production. These reactions demonstrate how the basic principles of photocatalysis can be exploited for renewable energy production. Finally, we conclude our review of photocatalytic renewable energy production and provide an outlook for future related research.

    INTRODUCTION

    The overconsumption of fossil fuels has depleted traditional energy resources and contributed to environmental pollution in water, soil and air environments[1]. It is highly expected that renewable and green energy can be exploited to resolve the energy crisis and environmental pollution. Solar energy is an extremely attractive natural energy source. The amount of solar energy that hits the Earth’s surface each year (1.3 × 105 TW) is much greater than that consumed by humans (1.6 × 101 TW in 2010)[2,3]. The conversion of solar energy to chemical energy via chemical reactions is a prospective method of producing renewable energy. Inspired by the natural photosynthesis of green plants and some other microorganisms, which convert solar energy to chemical energy in the form of carbohydrates or hydrogen, artificial photosynthesis (photocatalysis) is considered a prospective technology for the conversion of solar energy to chemical energy[4-6]. Since Fujishima and Honda discovered water photolysis on TiO2 electrodes in 1972, photocatalysis has received tremendous attention and developed rapidly in recent decades due to its promising applications in renewable energy production[7,8].

    The basic principles of semiconductor-based photocatalysis are presented in Figure 1[9]. In general, a complete photocatalytic process on a semiconductor involves three steps. The first step is light absorption, where photons are absorbed by the semiconductor photocatalyst. If the energy () of the photons is larger than the bandgap energy of the semiconductor, the electrons (e-) are excited and transmitted to the conduction band (CB) from the valence band (VB), leaving holes (h+) in the VB. Pairs of negatively charged electrons and positively charged holes (e--h+ pairs) are generated in this step. The second step is charge separation and transfer. The photogenerated e--h+ pairs are separated and transferred to the surface of the semiconductor, while some photogenerated e- and h+ recombine in the bulk of the photocatalyst (volume recombination). The final step is surface reduction and oxidation reactions. The photogenerated charges on the surface of the semiconductor react with chemical species[9,10]. Meanwhile, some photogenerated e- and h+ recombine without taking part in any chemical reaction (surface recombination).

    Figure 1. Basic principles of semiconductor-driven photocatalysis. Reproduced from Ref.[9] with permission from Wiley.

    Realizing the efficient conversion of solar energy to chemical energy for the production of renewable energy relies on semiconductor photocatalysts[11]. One of the major issues associated with semiconductor photocatalysts is the insufficient rate of light utilization. The positions of the band edge (CB and VB) in a semiconductor should meet the required potentials of redox reactions. However, this means that light with a lower energy than the absorption onset [visible or even near-infrared (NIR) light] cannot be used. In addition, semiconductor photocatalysts suffer from the inefficient separation of photogenerated charge carriers, resulting in poor photoactivity and quantum efficiency[11]. There are two effective strategies for overcoming these issues. The first is the development of semiconductor photocatalysts with excellent light absorption ability to improve the light utilization rate. The second is the construction of a scheme or system (including cocatalysts) to effectively facilitate the separation of photogenerated e--h+ pairs. Current state-of-the-art semiconductor-based nanocomposite systems (e.g., materials with element doping/single atom modification, the construction of heterostructures and the development of cocatalysts) offer intense and wide absorption of light from across the solar spectrum, as well as highly efficient charge separation. These achievements have contributed greatly to the field of renewable energy production.

    Herein, we introduce the advances in photocatalytic renewable energy production made in the last three years. We demonstrate how the basic principles of photocatalysis are used in certain reactions for renewable energy production, such as photocatalytic H2 production, CO2 conversion and H2O2 production, and introduce some of the typical heterogeneous photocatalysts used in these reactions, such as inorganic semiconductors and organic semiconductor-like materials (e.g., metal- and covalent-organic frameworks). Finally, we conclude with the progression made in photocatalytic renewable energy production and provide an outlook for future related research.

    PHOTOCATALYTIC RENEWABLE ENERGY PRODUCTION

    Photocatalytic H2 production

    Basic principles of photocatalytic H2 production

    H2 is a promising clean energy source that does not produce secondary pollution[12]. After light harvesting and the separation and transfer of the photogenerated e--h+ pairs in the aforementioned principle of photocatalysis, the photogenerated e- in the CB reduce adsorbed H+ to H2 [Figure 2] and the photogenerated h+ in the VB oxidize a sacrificial agent (S), such as alcohols (e.g., CH3OH, CH3CH2OH and CH3CH2CH2OH), triethanolamine (TEOA) or triethylamine (TEA) into an oxidative product (S+) or water into oxygen (O2)[13-15]. Nonetheless, the energy level of CB for the H2 evolution reaction should be lower than 0 V vs. the normal hydrogen electrode (NHE) at pH 7. Additionally, the energy level of the VB for the H2O oxidation reaction should be higher than 1.23 V vs. NHE, as the following redox reactions show:

    Figure 2. Schematic energy diagram of photocatalytic H2 production from water-splitting on a semiconductor photocatalyst. CBM: Conduction band minimum; VBM: valence band maximum; Eg: bandgap energy.

    Reduction reaction:
    2H+ + 2e- → H2 E = 0.00 eV(1)

    Oxidation reaction:
    Sacrificial agent (S) + h+ → S+E > 1.23 eV(2)
    or
    H2O + 2h+ → ½O2 + 2H+E = 1.23 eV(3)

    Overall water splitting:
    2H2O → O2 + 2H2E = -1.23 eV(4)

    Recent advances in photocatalytic H2 production

    H2 production from water with sacrificial agents
    I. Metal oxide-based photocatalysts
    Metal oxides have the advantages of low cost, good structural stability and low toxicity. In addition, they are easy to prepare and can be modified via various strategies, which can impart them with improved performances. Moreover, the proper band positions confer metal oxides with good redox capability in the photocatalytic H2 production reaction[16].

    Titanium dioxide (TiO2) is the most frequently investigated semiconductor material for photocatalytic H2 production. Hejazi et al.[17] described an atomic-scale defect engineering method to form and control traps for Pt single atom (SA) sites upon thin sputtered TiO2 layers for photocatalytic H2 production. The density of defect centers can be precisely regulated, resulting in the controlled density of Pt SA sites on TiO2. These decorated Pt SA sites improved the photoactivity of TiO2 by 150 times compared to that on conventional Pt nanoparticle-modified TiO2. Cho et al.[18] reported that a pre-reduced TiO2 support, which can reverse the interaction with Pt nanoparticles and reinforce the metallic state of Pt, resulting in a three-fold increase in H2 production rate compared to that on traditional Pt/TiO2. Pt/TiO2/H2O triple junctions are the active catalytic sites for H2 production in the presence of CH3OH. Méndez-Medrano et al.[19] constructed a heterojunction between small CuO nanoclusters and TiO2 (P25), which induces a photocatalytic activity of H2 production by using CH3OH as a sacrificial agent under visible-light irradiation because of the narrow bandgap (1.7 eV) of CuO. The photogenerated e- was injected from CuO nanoclusters to the CB of TiO2 (P25), resulting in high photoactivity. Osuagwu et al.[20] presented anatase TiO2 nanosheet (NS) layers grown on a Ta2O5 substrate and this TiO2 NSs/Ta2O5 displayed a 170-fold increase in photocatalytic H2 production rate compared to that on TiO2 NSs on a fluorine-doped tin oxide (FTO) substrate with CH3OH as a sacrificial agent. Such drastically enhanced photoactivity can be ascribed to the blocking effect of Ta2O5 for the photogenerated e- in TiO2 NSs.

    Sun et al.[21] synthesized a dodecahedral N-doped C-coated CuO-In2O3 p-n heterojunction photocatalyst, which showed a photocatalytic H2 production rate of 600 µmol·g-1·h-1 and good long-term stability of 50 h in a TEOA aqueous solution. Such a photocatalytic H2 production rate can be attributed to the efficient separation of photogenerated e--h+ pairs and the mediated adsorption behavior (|∆GH*|→0) by coupling the N-doped C layer with the CuO-In2O3 p-n heterojunction. The improved stability may originate from the mitigation of electron deficiency in CuO by the formation of the p-n heterojunction and protection with the N-doped C layer. In 2021, Han et al.[22] proposed a rhombohedral corundum/cubic In2O3 (rh/c-In2O3) phase-junction photocatalyst, which could effectively promote the separation and transfer of photogenerated charges between rh-In2O3 and c-In2O3 with a Z-scheme mechanism. This In2O3 phase-junction photocatalyst showed a H2 production rate of 2244 μmol·g-1·h-1 in a TEOA aqueous solution. A relevant apparent quantum efficiency (AQE) of 35% was achieved at 400 nm, which is ~12 times that on bare c-In2O3.

    II. Sulfide-based photocatalysts
    Sulfides, such as molybdenum disulfide (MoS2), zinc sulfide (ZnS), cadmium sulfide (CdS) and tungsten sulfide (WS2), have been developed for photocatalytic H2 production.

    In 2018, Guo et al.[23] reported a MoS2@TiO2 catalyst for photocatalytic H2 production. The catalyst realized a H2 production rate of 580 mmol·gMoS₂-1·h-1 under simulated solar light irradiation, while methanol was used as a sacrificial hole scavenger. The heterostructure between plasmonic MoS2 and TiO2 regulated the charge transfer pathways, which were responsible for light harvesting and photocatalytic H2 production. Wang et al.[24] embedded edge-enriched ultrathin MoS2 flake cocatalysts into a yolk-shell TiO2 bulk to boost photogenerated e- transfer from the bulk to the TiO2 surface. The as-prepared MoS2/TiO2 hybrid showed a H2 production rate of 2443 µmol·g-1·h-1, which is ~10- and ~4.7-fold greater than that of pure TiO2(247 µmol·g-1·h-1) and bulk MoS2-modified TiO2 (513 µmol·g-1·h-1), respectively. This improved activity was ascribed to the exposed catalytic edges of the ultra-thin MoS2 cocatalysts with strong Ti-S bonds, offering a fast charge-transfer pathway between TiO2 and MoS2. Furthermore, WS2-MoS2 in-plane few-layer heterostructures that function as efficient photocatalysts have been developed recently[25]. The built-in potential at the epitaxially-grown WS2-MoS2 interface facilitated fast separation of the photogenerated e--h+ pairs, resulting in a H2 production rate of 9.83 mmol·g-1·h-1.

    Xiao et al.[26] fabricated a copper nanowire (CuNW)/ZnS hybrid with a core-shell structure by a microwave-assisted hydrothermal route [Figure 3A]. The obtained ZnS with a narrowed bandgap could form a strong coupled interface with the CuNWs. As a result, this catalyst exhibited improved activity and stability for photocatalytic H2 production under the illumination of visible light [Figure 3B and C]. The corresponding H2 production rate reached 10,722 µmol·g-1·h-1 with an AQE of 69% under LED illumination with a wavelength (λ) of 420 nm. Cu+ ions from the CuNWs doped the ZnS shell, lowering the Schottky barrier and enabling photogenerated e- to be injected from Cu+/ZnS to the CuNWs, resulting in efficient charge separation. The core-shell NW structure benefited reactant absorption, charge separation and active site protection. Specifically, the CuNW cores acted as active sites to accept e- for the efficient reduction of H+ to H2 [Figure 3D and E].

    Figure 3. (A) Schematic illustration of CuNW/ZnS core-shell hybrids formed by a microwave-assisted hydrothermal route. (B) Photocatalytic H2 production rate of different samples under LED light irradiation (λ = 420 nm). (C) Cycling activity of CZ and 30 wt.% Cu-Z. (D) Schottky contact, band structure and schematic illustration of the e--h+ separation process under visible-light irradiation. (E) Schematic diagram of the core-shell structure and the proposed photocatalytic mechanism. Reproduced from Ref.[26] with permission from Wiley.

    Wang et al.[27] prepared a zinc oxide/CdS hierarchical composite. The CdS moiety plays a key role in light harvesting. A photocatalytic H2 production rate of 4134 μmol·g-1·h-1 was obtained with Na2S and Na2SO3 as electron donors. The Z-scheme charge migration scheme bestowed the photocatalyst with a strong ability for H2 production and improved the photoactivity. Dai et al.[28] constructed a system composed of a unique pyroelectric substrate, poly(vinylidene fluoride-co-hexafluoropropylene), carbon nanotubes and a CdS photocatalyst for infrared (IR)-light-driven H2 production in the presence of TEA. The photocatalytic H2 production efficiency was improved by more than five times with an AQE of 16.9%. Zhang et al.[29] first converted Cd-based Prussian blue analog cubes into a CdS cage, which were then further transformed into a CdS frame-in-cage. Owing to the novel frame-in-cage structure, a visible-light-driven H2 production rate of 13.6 mmol·g-1·h-1 was achieved while Na2S and Na2SO3 were used as sacrificial hole scavengers, which was much higher compared to that of the CdS cubes and cages.

    III. Carbon nitride-based photocatalysts
    Carbon nitride (C3N4) comprises only earth-abundant C and N elements and has high thermal and chemical stability because of the robust covalent bonds between the C and N atoms in the layered structure [Figure 4][30,31]. Its bandgap energy is ~2.7 eV, meaning that it can absorb visible light. In addition, the CB and VB edge positions of C3N4 are suitable for water reduction and oxidation, respectively[32-34]. Therefore, C3N4 is a promising photocatalyst.

    Figure 4. (A) Triazine and (B) tri-s-triazine (heptazine) structures of graphitic carbon nitride (g-C3N4). Reproduced from Ref.[30] with permission from the American Chemical Society.

    In 2019, Mo et al.[35] proposed a Z-scheme system containing two-dimensional (2D) MnO2/monolayer graphitic carbon nitride with defective Mn3+ active sites for H2 production, while TEOA was used as a sacrificial agent. After Mn3+/Mn4+ redox couples were introduced, these defective Mn3+ active sites could promote H2O adsorption and boost the charge separation and transfer at the interface, resulting in a H2 production rate of 28.0 mmol·g-1·h-1 and an AQE of 23.33% at λ = 420 nm. Wang et al.[36] prepared Se-doped carbon nitride (Se-C3N4) by fluorination, followed by thermal defluorination in Se vapor to realize 2D C3N4 with a strong visible-light absorption band. The formation of cyano groups accompanied by Se doping expanded the absorption edge of the C3N4 from 416 to 584 nm. In addition, a downward electron spin polarization in the C3N4 structure facilitated charge separation and surface catalysis reactions. The visible-light-driven H2 production rate reached 5411.2 μmol·g-1·h-1 using Se-C3N4 with 3 wt.% Pt as a cocatalyst, which was 176.5 times that of the pure C3N4 in the presence of TEOA.

    In 2021, Xu et al.[37] prepared a NIR-active C/K-doped red polymeric carbon nitride (RPCN). The homogeneous and high incorporation of C and K narrowed the bandgap of C3N4 (1.7 eV), thus extending the light absorption edge to the NIR region [Figure 5A]. RPCN displayed a NIR-driven H2 production rate of 140 µmol·g-1·h-1 from water [Figure 5B and C]. The AQE was 0.84% at 700 ± 10 nm and 13% at 500 ± 10 nm [Figure 5D].

    Figure 5. (A) Ultraviolet-Visible-Near Infrared (UV-Vis-NIR) diffuse reflection spectra. Activity of H2 production from water with RPCN under irradiation within different light ranges of (B) 500 ≤ λ ≤ 780 nm and (C) 700 ≤ λ ≤ 780 nm, using ~3 wt.% Pt as the cocatalyst and 10 vol.% TEOA as the sacrificial agent. (D) AQE at various monochromatic wavelengths. Reproduced from Ref.[37] with permission from Wiley. RPCN: Polymeric carbon nitride; TEOA: triethanolamine; AQE: apparent quantum efficiency.

    IV. Metal-organic framework-based photocatalysts
    Metal-organic frameworks (MOFs) are porous materials self-assembled from inorganic metal oxide clusters and organic linkers[38-40]. Owing to their unique properties that include high surface areas, well-defined metal nodes, adjustable chemical composition and enriched functionality, they have been extensively applied in various fields, such as gas separation and storage, sensors, environmental purification and catalysis[3,38,41,42].

    In 2018, Xiao et al.[43] combined surface plasmonic Au with a Pt-MOF Schottky junction to synthesize a Pt@MIL-125/Au catalyst [MIL-125 is a Ti-based MOF consisting of Ti8O8(OH)4 clusters and terephthalate acid linkers]. The spatial separation of the Pt and Au particles steered the formation of electron flow and expedited electron transport. Therefore, Pt@MIL-125/Au showed a much higher photocatalytic H2 production rate compared to catalysts with Pt or Au from water under the illumination of visible light with TEOA as electron donors.

    Chen et al.[39] bestowed the organic linkers (2-aminoterephthalate acid) of NH2-MIL-125(Ti) with mixed-valent copper (Cu2+/Cu+) centers (Cu-NM), which enabled effective electron transfer to the mixed-valent Cu2+/Cu+ centers from the 2-aminoterephthalate acid linkers [Figure 6A and B]. This resulted in increases in the carrier density and lifetime of the photogenerated charges by 7000 and 27 times, respectively. As a result, a much higher rate of H2 production compared to that of MOFs with Pt as a cocatalyst (Pt-NM) was obtained under the illumination of visible light from water when TEA was used as a sacrificial hole scavenger [Figure 6C].

    Figure 6. (A) and (B) Schematic of electron-transfer pathways from an organic linker to a titanium-oxo cluster in NH2-MIL-125(Ti) and to a mixed-valent Cu center, respectively. S represents a sacrificial agent. (C) Activity of NM, Pt-NM and Cu-NM in photocatalytic H2 production under visible-light irradiation. Reproduced from Ref.[39] with permission from Wiley.

    Dong et al.[44] fabricated a CdS-based MOF, DLNU-M-CdS(H2TD) (H2TD = 1,3,4-thiadiazole-2,5-dithiol), for photocatalytic H2 production in the presence of TEOA without the assistance of cocatalysts. During the photocatalytic reaction, the electron-donating process from TEOA to H2TD evoked fast electron transfer in the path of TEOA→H2TD→Cd→H+ to separate photogenerated e--h+ pairs in DLNU-M-Cd(H2TD), resulting in a H2 production rate of 26.1 mmol·g-1·h-1 under the irradiation of UV-visible-light.

    Sun et al.[45] incorporated noble-metal-free Ni2P and Ni12P5 into UiO-66-NH2 [a MOF consisting of Zr6O6(OH)4 clusters and 2-aminoterephthalate acid linkers] to form Ni2P@MOF and Ni12P5@MOF photocatalysts, respectively, for photocatalytic H2 production with TEA as a hole sacrificial agent. Both Ni2P@MOF and Ni12P5@MOF showed enhanced H2 production rates in comparison with the pristine MOF and their physical mixtures. Ni2P and Ni12P5 exhibit a similar capability to Pt in promoting the charge transfer from linkers to clusters and lowering the activation energy of the H2 production reaction. Pt is thermodynamically favorable, while Ni2P is kinetically more suitable for H2 production, resulting in higher activity over Ni2P@MOF than over Pt@MOF.

    Meng et al.[46] immobilized CdS quantum dots and carbon nanodots (CDs) in the cages of MIL-101 (a Cr-based MOF consisting of Cr clusters and terephthalic acid linkers) to form CD/CdS@MIL-101 composites. The H2 production rate on CD/CdS@MIL-101 with a CD content of 5.2 wt.% was 14.66 μmol·h-1 when using lactic acid as a sacrificial agent under the illumination of visible light, which was 8.5 and 18.6 times that on CdS@MIL-101 and pure CdS, respectively. The enhanced photoactivity can be ascribed to the role of electron collection of the CDs, which prolongs the lifetime of the photogenerated charge carriers in the CdS@MIL-101 heterostructure photocatalyst.

    Lin et al.[47] demonstrated porphyrin-based zirconium MOFs (PCN-H2/Ptx:y, x:y = 4:1, 3:2, 2:3 and 0:1) with different ratios of H2TCPP and PtIITCPP [TCPP = tetrakis(4-carboxyphenyl)porphyrinate] linkers. PCN-H2/Pt0:1 displayed the highest H2 production reaction rate (351.08 μmol·h-1·g-1) with TEOA as a sacrificial agent under visible-light irradiation. Such photoactivity can be ascribed to the uniformly dispersed Pt2+ ions in PCN-H2/Pt0:1, which boost the transfer of charges from porphyrins (photosensitizers) to PtII ions (catalytic centers).

    V. Covalent-organic framework-based photocatalysts
    Covalent-organic frameworks (COFs) are another type of porous crystalline polymers that can precisely integrate molecular building blocks into extended 2D or three-dimensional (3D) structures via covalent bonds[48-50]. Because of their characteristics, which include low density, high porosity, structural periodicity and modular functionality, COFs have been extensively studied for gas adsorption and separation[51,52], catalysis[53,54], sensors[55-57], optoelectronics[58,59] and energy storage systems[60-62].

    In 2018, Xie et al.[63] reported a highly ordered COF, CTF-1 (C8N2H4), which can efficiently produce H2 from water. A high AQE for H2 production of 6% at λ = 420 nm was obtained, which was ascribed to the well-defined and ordered structure of CTF-1, as well as low carbonization and superior band positions. In 2021, Chen et al.[64] reported the synthesis of four isostructural porphyrinic 2D COFs (MPor-DETH-COF, M = H2, Co, Ni, Zn; DETH = 2,5-diethoxyterephthalohydrazide) and their application in photocatalytic H2 production [Figure 7]. All four COFs have AA stacking structures with high crystallinity and large surface areas. The introduction of different transition metals into the porphyrin rings regulated the photocatalytic H2 production rate of these COFs in the following order: CoPor-DETH-COF < H2Por-DETH-COF <  NiPor-DETH-COF  < ZnPor-DETH-COF. In the same year, COFs with an ordered arrangement of layered structures were reported for visible-light-driven H2 production with ascorbic acid as a sacrificial agent[65]. The mesoporous channels of a β-ketoenamine-linked COF, including a benzothiadiazole moiety, were filled with polyethylene glycol, which inhibited the dislocation of neighboring layers while retaining the columnar π-orbital arrays to boost free charge transfer. Thus, the H2 production rate was enhanced compared to that of the pure COFs. Yang et al.[66] demonstrated that donor-acceptor-type imine-linked COFs could catalyze H2 production with a rate of 20.7 mmol·g-1·h-1 under the illumination of visible light with ascorbic acids as hole scavengers. The improved photoactivity was ascribed to the extended light harvesting range, enhanced charge separation efficiency and increased hydrophilicity.

    Figure 7. (A) Schematic illustration of the synthesis of MPor-DETH-COFs. (B) Activity of H2 production under visible-light irradiation using H2Por-DETH-COF, CoPor-DETH-COF, NiPor-DETH-COF and ZnPor-DETH-COF. (C) Cycling results of H2 production under visible-light irradiation using ZnPor-DETH-COF. Reproduced from Ref.[64] with permission from Springer Nature.

    VI. Nitride-based photocatalysts
    Nitrides, particularly transition metal nitrides with narrow energy bandgaps and excellent physicochemical properties, can effectively regulate the structures of semiconductor photocatalysts and their photocatalytic performance[67].

    In 2021, Xiao et al.[68] fabricated Mg-Zr-codoped single-crystalline Ta3N5 (Ta3N5:Mg+Zr) nanoparticles for H2 production from H2O. The photoactivity was 45 times that of pristine Ta3N5. Simultaneous defect tuning and surface property optimization generated high concentrations of long-lived electrons and facilitated electron migration to the Pt sites on the surface of the photocatalyst, thus enhancing the photoactivity under the illumination of visible light. In the same year, Wang et al.[69] reported the efficient utilization of photogenerated electrons in a single-crystal BaTaO2N photocatalyst for H2 production via the sequential decoration of a Pt cocatalyst. The H2 production rate in a methanol aqueous solution was improved 100-fold compared to that of BaTaO2N. Its AQE was 6.8% at λ = 420 nm.

    H2 production from overall water splitting
    Photocatalytic H2 production from overall water splitting can effectively avoid the secondary pollution caused by the use of sacrificial hole scavengers, while O2 is produced from water oxidation.

    I. Selenide- and sulfide-based photocatalysts
    Wang et al.[70] developed uniform hollow MoSe2/CdSe nanospheres without any template/surfactant assistance. Owing to the Z-scheme mechanism for charge transfer in the heterostructure, it displayed a H2 production rate of 7120.0 μmol·g-1·h-1 from water splitting under the irradiation of simulated sunlight and the AQE reached 27.2% at λ = 670 nm.

    S 3p and O 2p orbitals were hybridized by Zhang et al.[71] to generate oxysulfides with stabilization of S2- in a sulfide-based photocatalyst. Additional surface modification of the oxysulfides with dual cocatalysts promoted the separation and transfer of photogenerated charges, thereby reducing charge accumulation and inhibiting photocorrosion. The results demonstrated that the pH value played an important role in realizing the efficient production of stoichiometric H2 and O2. Pan et al.[72] reported that Ag dopant and nanohole dual defects in ZnIn2S4 monolayers promoted stoichiometric H2 and O2 production from water under the illumination of visible light. The Ag dopant and nanohole dual defects optimized the light harvesting and carrier dynamics and served as active sites for the oxidation and reduction of water, respectively, thereby resulting in stable activity for photocatalytic water splitting.

    II. C3N4-based photocatalysts
    Chen et al.[73] constructed 3D porous g-C3N4 assembled using highly crystalline and ultrathin NSs. The 3D g-C3N4 NSs could produce H2 and O2 from water splitting with production rates of 101.4 and 49.1 μmol·g-1·h-1, respectively, under the illumination of visible light. These rates are ~11.8 and ~5.1 times that of bulk g-C3N4 and g-C3N4 NSs, respectively. Furthermore, the 3D g-C3N4 NSs showed an AQE of 1.4% under light radiation at λ = 420 nm and maintained good stability for 100 h. Lin et al.[74] demonstrated a one-photon excitation route by coupling a polymeric carbon nitride (PCN) with LaOCl to realize overall water splitting. The modification of LaOCl formed an interfacial electric field that boosted the rapid separation and transfer of photogenerated e--h+ pairs in the catalyst. The water reduction half-reaction occurred on LaOCl and its oxidation half-reaction appeared on the PCN. As a result, this “artificial photosynthesis” catalyst showed H2 and O2 production rates of 22.3 and 10.7 μmol·h-1, respectively, from overall water splitting.

    Chen et al.[75] synthesized a g-C3N4/reduced graphene oxide (rGO)/perylene diimide polymer (PDIP) photocatalyst with a Z-scheme heterostructure to achieve excellent photocatalytic overall water splitting [Figure 8A]. A significant internal electric field was built in the Z-scheme heterostructure [Figure 8B], thereby promoting the high-flux charge transfer and improving the charge separation efficiency by a factor of 8.5. The g-C3N4/rGO/PDIP photocatalyst presented H2 and O2 production rates of 15.80 and 7.80 µmol·h-1, respectively, which are ~12.1 times that of pure g-C3N4. An AQE of 4.94% at λ = 420 nm and a conversion efficiency of solar energy to hydrogen energy of 0.3% were realized [Figure 8C and D]. Besides, it showed a very good stability within 120 h of photocatalytic reaction [Figure 8E].

    Figure 8. (A) Schematic illustration of g-C3N4/rGO/PDIP fabrication. (B) Surface photovoltage (SPV) spectra. (C) Wavelength dependency of AQE for photocatalytic overall water splitting. (D) Photocatalytic activity of overall water splitting under AM 1.5G simulated sunlight irradiation using g-C3N4/rGO/PDIP. (E) Cycling results of photocatalytic overall water splitting. Reproduced from Ref.[75] with permission from Wiley. rGO: Reduced graphene oxide; PDIP: perylene diimide polymer.

    Wu et al.[76] prepared g-C3N4 NSs exfoliated by a femtosecond pulsed laser, which achieved H2 and O2 production rates of 42.6 and 18.7 μmol·g-1·h-1, respectively, toward overall water splitting when Pt was used as a cocatalyst. The laser pulses created cyano (-C≡N) defects that favored the anchoring of divalent atomic Pt cocatalysts, which are different from Pt metal nanoparticles. This provided more active sites for the surface reaction and suppressed the reverse reaction of water splitting. In addition, the -C≡N defects reduced the position of the band edge to improve the oxidation ability of h+.

    Using an electrostatic self-assembly method, Zhao et al.[77] coupled B-doped N-deficient 2D C3N4 NSs to obtain a 2D/2D polymeric Z-scheme heterostructure. The combination of ultrathin nanostructures, the robust interfacial interaction and the staggered band arrangement in the Z-scheme system boosted the separation and transfer of photogenerated e--h+ pairs, thus realizing stoichiometric H2 and O2 production from photocatalytic overall water splitting with Pt and Co(OH)2 cocatalysts.

    III. MOF-based photocatalysts
    In 2020, a Pt@UiO-66-NH2@MnOx photocatalyst with Pt and MnOx cocatalysts was designed by Zhang et al.[78] for the complete spatial separation of photogenerated e--h+ pairs in UiO-66-NH2, thereby realizing H2 and O2 production from water. Compared with UiO-66-NH2, Pt@UiO-66-NH2 and UiO-66-NH2@MnOx photocatalysts, Pt@UiO-66-NH2@MnOx exhibited the highest photoactivity for overall water splitting. As cocatalysts, Pt favored the trapping of photogenerated e-, while MnOx particles tended to collect photogenerated h+. The photogenerated e- and h+ flowed to the inside and outside of the MOF, respectively, accumulating on the respective cocatalysts and further inducing redox reactions.

    Significantly, Hu et al.[79] integrated a H2 evolution reaction (HER)-MOF and a water oxidation reaction (WOR)-MOF into liposomal structures for the spatial separation of photogenerated e--h+ pairs. The HER-MOF consisted of a light-harvesting Zn-porphyrin and a catalytic Pt-porphyrin and was modified with pentafluoropropionic acid, making the HER-MOF hydrophobic and thus promoting its binding to the hydrophobic lipid bilayer of the liposome. The WOR-MOF was composed of a [Ru(2,2′-bipyridine)3]2+-based photosensitizer and an Ir-bipyridine catalytic center and was localized in the hydrophilic interior of the liposome. Owing to the rapid electron transfer from the Zn-porphyrin and [Ru(2,2′-bipyridine)3]2+ antennae to the Pt-porphyrin and Ir-bipyridine reaction centers and the efficient charge separation in the lipid bilayers, this liposome-MOF achieved H2 and O2 production in a ~2:1 ratio with an AQE of ~1.5%.

    IV. Other photocatalysts
    Wang et al.[80] reported quadruple-band InGaN NW arrays, which are composed of In0.35Ga0.65N, In0.27Ga0.73N, In0.20Ga0.80N and GaN sections, with energy bandgaps of 2.1, 2.4, 2.6, and 3.4 eV, respectively. These multiband InGaN NW arrays were modified upon a nonplanar wafer to improve the light absorption. The doping gradient was introduced along the NWs to form a built-in electric field, which separated and extracted photogenerated charge carriers for water redox reactions. This InGaN photocatalyst exhibited a solar-to-hydrogen efficiency of ~5.2%.

    Pan et al.[81] used rGO NSs to promote the transfer of photogenerated charge carriers between H2 producing photocatalysts (e.g., carbon nitride and BiVO4) and O2 evolution photocatalysts (e.g., Fe2O3 and WO3), thereby realizing efficient overall water splitting. Furthermore, Oshima et al.[82] presented HCa2Nb3O10 NSs sensitized by a Ru(II) tris-diimine photosensitizer for overall water splitting under visible-light irradiation, combined with a WO3-based water oxidation photocatalyst and a I3-/I- redox couple. The Pt-intercalated HCa2Nb3O10 NSs, which were further modified with amorphous Al2O3 clusters as H2 production components, realized a dye-based turnover number and frequency of 4580 and 1960 h-1, respectively. Its AQE at 420 nm was 2.4%.

    Zhao et al.[83] presented a practically feasible strategy that mimicked natural photosynthesis known as the hydrogen farm project (HFP). The proposed system comprised solar energy harvesting and H2 production subsystems integrated by a Fe3+/Fe2+ redox ion pair [Figure 9]. The BiVO4 crystals with accurately tuned {110}/{010} facets were used as photocatalysts for this project. The AQE of photocatalytic water oxidation and the complete forward reaction is 71% and there is almost no reverse reaction. A solar-to-chemical conversion efficiency of > 1.9% was also achieved.

    Figure 9. (A) Schematic of hydrogen farm project (HFP) for scalable solar H2 production using particulate photocatalysts for water oxidation and a shuttle ion loop for energy storage. (B) Practical realization for HFP using BiVO4 as a water oxidation photocatalyst, Fe3+/Fe2+ as shuttle ions for energy storage and an electrolysis cell for H2 production. Reproduced from Ref.[83] with permission from Wiley.

    In 2020, Takata et al.[84] used an aluminum-doped strontium titanate (SrTiO3:Al) photocatalyst to realize an AQE of 96% for overall water splitting at wavelengths of 350-360 nm, which was equivalent to an almost uniform internal quantum efficient. The selective photodeposition of Rh/Cr2O3 and CoOOH cocatalysts upon different crystal facets of the photocatalyst for the H2 and O2 production reactions, respectively, boosted the H2 and O2 production reactions separately. Thus, multiple consecutive forward charge transfers without reverse charge transfer were realized, thereby attaining the maximum AQE for overall water splitting.

    Photocatalytic H2 production from hydrogen storage materials
    Achieving the efficient storage and transportation of gaseous H2 is a significant challenge. Storing H2 in a liquid or solid material that holds H2 under ambient conditions and releases it as the conditions change is a safe and efficient method[3]. In particular, many efforts have been made to achieve the release of H2 from H2 storage materials, such as ammonia borane (AB) and formic acid (FA).

    In 2018, Zhang et al.[85] demonstrated the NIR-plasmonic energy up-conversion process in Yb3+/Er3+-doped NaYF4 nanoparticle (NaYF4:Yb-Er NP)@W18O49 NW heterostructures. The improvement of the up-conversion luminescence of the NaYF4:Yb-Er NPs was attributed to the NIR-excited localized surface plasmon resonance (LSPR) of the W18O49 NWs. Simultaneously, this plasmon-enhanced up-conversion luminescence was partly absorbed by the W18O49 NWs, re-exciting its higher energy LSPR [Figure 10A]. Based on this plasmonic energy transfer process, the NaYF4:Yb-Er NP@W18O49 NW heterostructures exhibited an ~35-fold increase in catalytic H2 production from AB [Figure 10B and C].

    Figure 10. (A) Schematic diagram of plasmonic energy upconversion in the NaYF4:Yb-Er NP/W18O49 NW heterostructure system upon irradiation at λ = 980 nm. (B) Amount of H2 production from AB aqueous solution under irradiation at λ = 980 nm: (1) no catalyst; (2) W18O49 NWs; and (3) NaYF4:Yb-Er NW@W18O49 NP heterostructures. (C) Amount of H2 production of W18O49 NWs in 1 h under irradiation at different incident light wavelengths. Reproduced from Ref.[85] with permission from Wiley.

    Very recently, Zhang et al.[86] reported a Pd-decorated Ti-MOF@TpTt composite (Pd@Ti-MOF@TpTt) coated with an ultrathin COF nanobelt [Figure 11A]. This catalyst presented much higher photoactivity for H2 production from AB hydrolysis than that of the other counterparts with fibrillar-like COF shells [Figure 11B]. This improved activity could be ascribed to its high surface area, core-shell structure and type II heterojunction [Figure 11C], which provided more active sites and promoted the separation of photogenerated e--h+ pairs. Finally, Pd@Ti-MOF@TpTt displayed excellent stability for H2 production [Figure 11D].

    Figure 11. (A) Schematic illustration of the synthesis of Pd-decorated Ti-MOF@TpTt hybrids. (B) Activity of H2 production from AB aqueous solution over Pd@Ti-MOF@TpTt and Ti-MOF@TpTt under different conditions. (C) Proposed mechanism for AB hydrolysis catalyzed by Pd@Ti-MOF@TpTt. (D) Cycling results of AB hydrolysis under light irradiation using Pd@Ti-MOF@TpTt. Reproduced from Ref.[86] with permission from Wiley.

    Cao et al.[87] synthesized a CdS/CoP@rGO hybrid for photocatalytic H2 production from FA using ultrasmall CoP nanoparticles as a cocatalyst for the first time. The visible-light-driven H2 production rate using the CdS/CoP@rGO hybrid reached 182 ± 12.5 μmol·mg-1·h-1 without any additives, which was 30 times that of the bare CdS. The system could be sustained for more than seven days.

    Zhang et al.[88] loaded AuPd nanoparticles upon super-small carbon nitride nanospheres (AuPd/CNS) to construct a Mott-Schottky photocatalyst and used it to catalyze H2 production from FA. It showed a turnover frequency of 1017.8 h-1 under visible-light illumination (λ > 420 nm). The alloying, plasmonic and Mott-Schottky effects optimized the electronic structure of Pd in the AuPd/CNS composite, which accelerated the electrons transferred from the carbon nitride and Au to the active Pd sites, thus resulting in improved photoactivity.

    Photocatalytic CO2 conversion

    Basic principles of photocatalytic CO2 conversion

    The overuse of fossil resources has led to excessive CO2 emissions, which have contributed to the greenhouse effect. There are already many technologies for collecting and sequestering CO2, such as scrubbing, mineral carbonation, geological injection and oceanic injection[89]. Nevertheless, these technologies are expensive and may cause the leakage of CO2[89,90]. Alternatively, CO2 can be considered as a low-cost, safe and abundant carbon source that can be converted into valuable energy fuels. This strategy could not only reduce CO2 emissions and alleviate the greenhouse effect, but also mitigate the energy crisis.

    CO2 is highly thermodynamically stable and its C=O bond energy (750 kJ·mol-1) is much larger than that of C-H (411 kJ·mol−1), C-O (327 kJ·mol-1), and C-C (336 kJ·mol-1) single bonds[91,92]. Thus, the photocatalytic conversion of CO2 to hydrocarbons requires high energy input to activate the C=O double bonds and convert them into C-H single bonds. Photogenerated e- with an appropriate reduction potential can supply a driving force for the reduction of CO2. Photocatalytic CO2 conversion takes place through a multi-step reaction pathway with the participation of 2, 6, 8, 12, 14 or 18 e- and H+ [Figure 12][93,94]. Various products, including C1 compounds (e.g., CO, CH4, HCOOH, CH3OH and HCHO) and C2 molecules (e.g., CH2CH2, C2H5OH and CH3COOH)[95], can be generated. Some reactions related to photocatalytic CO2 conversion and the relevant reduction potentials (E0) are listed in Table 1[96,97].

    Figure 12. Schematic energy diagram for CO2 reduction and H2O oxidation on a semiconductor. Reproduced from Refs.[93,94] with permission from Wiley.

    Table 1

    Electrochemical reactions involved in aqueous CO2 and proton reduction with their corresponding reduction potentials E0(V vs. NHE at pH 7)

    EntryEquationProductE0 (V)
    1CO2 + e- → CO2-Carbonate anion radical-1.85
    2CO2 + 2H+ + 2e- → HCOOHFormic acid-0.61
    3CO2 + 2H+ + 2e- → CO + H2OCarbon monoxide-0.53
    4CO2 + 4H+ + 4e- → HCHO + H2OFormaldehyde-0.48
    5CO2 + 4H+ + 4e- → C + 2H2OCarbon-0.20
    6CO2 + 6H+ + 6e- → CH3OH + H2OMethanol-0.38
    7CO2 + 8H+ + 8e- → CH4 + 2H2OMethane-0.24
    82CO2 + 12H+ + 12e- → C2H4 + 4H2OEthylene-0.34
    92CO2 + 12H+ + 12e- → C2H5OH + 3H2OEthanol-0.33
    102CO2 + 14H+ + 14e- → C2H6 + 4H2OEthane-0.27
    113CO2 + 18H+ + 18e- → C3H7OH + 5H2OPropanol-0.32
    122H+ + 2e- → H2Hydrogen-0.42

    As shown in Table 1, a reduction potential of -1.85 V is required for direct single-electron CO2 reduction[98]. So far, almost no photocatalysts have displayed sufficient ability to drive this single-electron transfer process. On the contrary, H+-assisted multi-electron/H+ reduction represents an alternative and more advantageous method.

    Recent advances in photocatalytic CO2 conversion

    Photocatalytic CO2 conversion with H2O
    The utilization of H2O as a reducing agent to photocatalytically convert CO2 is an intriguing process[99] that involves multi-electron reactions [Table 1][100,101]. As a result, various products, such as CO[102,103], CH4 and CH3OH, can be obtained. Taking CO production as an example, the photogenerated h+ oxidize H2O into O2 with the generation of H+ (Equation 5), while the photogenerated e- reduce CO2 to CH4 via a two-electron reaction with two protons (Equation 6):

    H2O + 2h+ → 1/2O2 + 2H+(5)

    CO2 + 2H+ + 2e → CO + H2O(6)

    At present, photocatalytic CO2 conversion with H2O mainly involves two reaction systems[89]. One is a liquid phase system, in which the CO2 conversion efficiency is largely limited due to the low solubility of CO2 (~0.03 M under ambient conditions) with the occurrence of competitive H2 production from H2O. Thus, many efforts have been made to enhance the pressure of CO2 and increase the solubility of CO2 in alkaline systems[104]. The other is a gas-phase system that uses CO2 and H2O vapor and realizes a higher selectivity for CO2 reduction[105]. Multifarious photocatalysts have been explored for CO2 conversion using H2O.

    I. Metal oxide-based photocatalysts
    Sorcar et al.[106] reported bimetallic Cu-Pt nanoparticle-sensitized blue titania (TiO2) that generated large amounts of CH4 and CH3CH3 during artificial-sunlight-driven CO2 reduction with H2O. Within 6 h of the reaction, 3.0 mmol·g-1 of CH4 and 0.15 mmol·g-1 of CH3CH3 were produced.

    Li et al.[107] reported a cake-like porous TiO2 photocatalyst with the surface-localized doping of copper and cobalt. The doped TiO2 photocatalyst boosted the photoreduction of CO2 with water vapor. 1% Cu-doped TiO2 promoted the breaking of C=O bonds. The production rates of CO and CH4 reached 45.31 and 42.35 μmol·h-1, respectively, under simulated sunlight irradiation. This activity was further enhanced by incorporating trace cobalt. In addition to the enhanced performance in terms of the production of CO and CH4, the selectivity was also increased for the production of hydrocarbons (C2+). The production rates of C2H6 and C3H8 reached 89.20 and 3.36 μmol·h-1, respectively, over 0.02% Co-1% Cu/TiO2. The incorporation of copper and cobalt ions realized efficient charge separation in the catalyst, resulting in the generation and enrichment of methyl radicals upon cobalt ions, which induced the production of C2+.

    Atomically dispersed Cu-supported mesoporous TiO2 (mTiO2) was used for the light-driven reduction of CO2 with H2O by Yuan et al.[108]. The authors revealed that the atom-dispersed Cu(II) underwent reduction to Cu(I) and finally to Cu(0), with the mixture of Cu(I) and Cu(0) proving effective for the production of CH4.

    La- and Rh-doped SrTiO3 (SrTiO3:La,Rh) and Mo-doped BiVO4 (BiVO4:Mo) light absorbers were integrated into a photocatalyst by Wang et al.[109]. This photocatalyst converted CO2 and H2O into formate (HCOO-) and O2 and achieved a solar-to-HCOO- conversion efficiency of 0.08% and a selectivity of 97%.

    Z-scheme type photocatalysts composed of g-C3N4 and Au-loaded anatase TiO2 (A-TiO2) were fabricated by Wang et al.[110]. The surface heterojunction between the coexposed {001} and {101} facets in A-TiO2 improved the separation efficiency of the photogenerated e--h+ pairs. The loaded Au gathered and transferred the stimulated electrons originating from A-TiO2 to g-C3N4. The g-C3N4 component trapped the photogenerated e and improved the adsorption ability of CO2. This catalyst exhibited high photoactivity of CO2 conversion under the illumination of visible light, with CH4 and CO formation rates of 37.4 and 21.7 μmol·g-1·h-1, respectively.

    Wang et al.[111] loaded Ag and Co dual cocatalysts on Al-doped SrTiO3 [Figure 13], which brought about an enhanced CO production rate and a corresponding selectivity of 99.8% at λ = 300 nm. The CO production rate of 52.7 μmol·h-1 was 12 times that of the catalyst without Co cocatalysts (4.7 μmol·h-1). The AQE for CO production was ~0.03% at λ = 365 nm, with a selectivity of 98.6% for CO production. The Ag and Co cocatalysts acted as reduction and oxidation sites to promote the production of CO and O2, respectively.

    Figure 13. (A) Cocatalyst loading by the photodeposition method. (B) Photocatalytic CO2 conversion into CO over Al-SrTiO3 modified with Ag and Co using water as the reductant. Reproduced from Ref.[111] with permission from the Royal Society of Chemistry.

    A CuOx@p-ZnO photocatalyst with CuOx uniformly dispersed on polycrystalline ZnO was fabricated by Wang et al.[112]. This catalyst reduced CO2 to C2H4 with a selectivity of 32.9% and the production rate was 2.7 μmol·g-1·h-1 with H2O as a hole scavenger under illumination. During the reduction reaction, a unique Cu+ site was formed on the surface of the CuO matrix and this surface Cu+ site can anchor the generated CO and further stabilize the intermediate *OC-COH of C-C coupling, which thus promoted the production of C2H4.

    II. MOF- and COF-based photocatalysts
    Lu et al.[113] synthesized crystalline porphyrin tetrathiafulvalene COFs for CO2 conversion with H2O without the addition of photosensitizers, sacrificial agents or noble metal cocatalysts. The photogenerated e- were effectively transferred from tetrathiafulvalene to porphyrin, which led to the separation of e--h+ pairs for CO2 reduction and H2O oxidation. As a result, a photocatalytic CO production rate of 123.3 μmol·g-1 with a selectivity of 100% was achieved, along with H2O oxidation to O2 under the illumination of visible light after 60 h.

    Fang et al.[114] reported a pyrazolyl porphyrinic Ni-MOF (PCN-601) that integrated a light photosensitizer, active sites and large surface areas as an excellent and durable photocatalyst for visible-light-driven CO2 conversion with H2O vapor. The production rate of CH4 was much higher than that of similar MOFs based on carboxylate porphyrin.

    Heterometallic Fe2M cluster-based MOF (NNU-31-M, M = Co, Ni and Zn) photocatalysts were fabricated by Dong et al.[115]. The overall conversion of CO2 and H2O into HCOOH and O2 was achieved in the absence of any sacrificial agent and photosensitizer when using these MOFs. Under visible-light irradiation, the heterometallic clusters and photosensitive ligands can generate separated e--h+ pairs. The metal M accepts e- to reduce CO2 and the metal Fe uses h+ to oxidize H2O. NNU-31-Zn showed the highest HCOOH production of 26.3 μmol·g-1·h-1 and a selectivity of 100%.

    Feng et al.[116] synthesized a Zr-based mixed-linker MOF (mPT-MOF), consisting of Zr clusters and 4,4'-(1,10-phenanthroline-3,8-diyl)dibenzoic acid and 2''-nitro-[1,1':4',1'':4'',1'''-quaterphenyl]-4,4'''-dicarboxylic acid linkers. CuI photosensitizers (Cu-PSs) and molecular Re catalysts were incorporated in the MOF to form mPT-Cu/Re for photocatalytic CO2 reduction with water. Installation of the Cu-PSs and molecular Re catalysts in the MOF promoted the transfer of multi-electrons to drive CO2 reduction under visible-light irradiation. A turnover number of 1328 was obtained, which was a 95-fold improvement over the homogeneous counterparts.

    Jiang et al.[117] created “molecular compartments” inside MOF crystals by growing TiO2 inside different pores of a chromium (Cr) terephthalate-based MOF (MIL-101) and its derivatives. This allows for the synergy between the light-absorbing/electron-generating TiO2 units and the catalytic metal Cr clusters in the MOFs, thus facilitating the photoreduction of CO2 to CO and CH4 with the production of O2 from H2O. An AQE of 11.3% for CO2 reduction at λ = 350 nm was obtained over 42%-TiO2-in-MIL-101-Cr-NO2 (42% of TiO2 in a MIL-101 derivative).

    Very recently, Yu et al.[118] integrated MoS2 NSs into defective MOFs (d-UiO-66) to form Mo-O-Zr bimetallic sites between UiO-66 and MoS2. The active interfaces were beneficial for the transfer of photogenerated charge carriers, resulting in enhanced activity. The d-UiO-66/MoS2 composite facilitated visible-light-driven CO2 conversion with H2O to CH3COOH. The production rate of CH3COOH was 39.0 µmol·g-1·h-1 and the selectivity was 94%.

    Wu et al.[119] reported Cu-based MOF, Cu3(BTC)2 (BTC = 1,3,5-benzene tricarboxylate), encapsulated Cu2O nanowires for the photoreduction of CO2 to CH4 with water vapor. Such a MOF not only inhibits the water vapor-induced corrosion of Cu2O but also facilitates CO2 uptake and charge separation [direct transfer of photogenerated electrons from the CB of Cu2O to the LUMO level of non-excited Cu3(BTC)2], thus leading to a 1.9-fold higher photoactivity of CO2 reduction into CH4, compared to that of pure Cu2O.

    III. Nitride-based photocatalysts
    Li et al.[120] incorporated the heteroatom B-Co dimer into a porous C2N to form B-Co@C2N, which was used for the photoreduction of CO2 to C2H6. The combination of B and Co regulated the 3d orbital of the Co atoms to a lower energy level, which impairs the CO adsorption strength in comparison with Co-Co@C2N and results in a low energy barrier of ~0.61 eV for C-C coupling. The H2 production reaction was inhibited owing to the strong adsorption of the *CO2/*COOH intermediates. Furthermore, the light absorbance of B-Co@C2N in the visible and infrared light regions was improved in comparison with that on pure C2N.

    A Au/p-GaN photocatalyst with a plasmonic heterostructure realized the photoreduction of gas-phase CO2 to CO with water oxidation under solar illumination[121]. This heterostructure photocatalyst was composed of metal/insulator/semiconductor components with an Al2O3 layer between the metal nanoparticles and p-GaN, which contributed to the promotion of CO production.

    IV. Other photocatalysts
    Wang et al.[122] fabricated a marigold-like SiC@MoS2 photocatalyst with a unique Z-scheme structure to realize visible-light-driven CO2 conversion with water. The production rates of CH4 and O2 were 323 and 621 μL·g-1·h-1, respectively, with stability over five cycles of 40 h.

    Layered bismuth oxyhalides (BiOX, where X = F, CI, Br and I) were used for the conversion of CO2 with H2O without adding photosensitizers or sacrificial agents[123]. The optimal BiOBr photocatalyst displayed CO and CH4 production rates of 21.6 and 1.2 μmol·g-1·h-1, respectively, under simulated sunlight irradiation.

    SnS-SnS2 heterostructured NS frameworks were grown on FTO substrates for the photoconversion of CO2 with H2O to C2 (acetaldehyde) and C3 (acetone) hydrocarbons[124]. The photoactivity of SnS-SnS2 was improved by increasing the fraction of SnS in the composite through the partial transformation of SnS2 to SnS. SnS provides CO2 adsorption sites with lower activation energy, which is the rate-determining step for CO2 reduction. The Z-scheme charge transfer dynamic in the SnS-SnS2 catalyst drives the water oxidation on SnS2 and CO2 reduction on SnS.

    Photocatalytic CO2 conversion with H2
    Photocatalytic CO2 conversion with H2 is a prospective method for CO2 reduction[125]. Light illumination can achieve a favorable rate and yield in eight-electron CO2 reduction with H2[125,126]. The photogenerated h+ in the active VB of the photocatalysts can react with H2 to produce H+. The produced H+ and photogenerated e- can then convert CO2 into CO, CH3OH and hydrocarbons (e.g., CH4, C2H4, C2H6, C3H6 and C3H8). Generally, CH4 or CO products are produced during photocatalytic CO2 conversion with H2 (Equations 7 and 8)[89].

    CO2 + 4H2 → CH4 + 2H2O(7)

    CO2 + H2 → CO + H2O(8)

    In 2018, Jelle et al.[127] fabricated highly dispersed nanostructured RuO2 catalysts loaded on 3D silicon photonic crystal supports for photocatalytic conversion of CO2 with H2 to CH4. A conversion rate of 4.4 mmol·gcat-1·h-1 was obtained under simulated solar irradiation. Silicon photonic crystals have unique light-harvesting characteristics in the entire spectrum of sunlight, coupled with its large surface area, resulting in the high CH4 production rate. In the same year, Wang et al.[128] reported a defect-rich indium oxide [In2O3-x(OH)y] catalyst for the light-driven reduction of CO2 to CH3OH. The CH3OH production rate and selectivity were 0.06 mmol·gcat-1·h-1 and 50%, respectively, under simulated solar irradiation. Furthermore, in 2019, Yan et al.[129] used the rhombohedral polymorph of an indium sesquioxide photocatalyst for the photocatalytic reduction of CO2 to CH3OH and CO. Notably, the rhombohedral polymorph exhibited higher photoactivity, superior stability and improved selectivity toward CH3OH over CO. In 2020, Yan et al.[130] reported the isomorphic replacement of Lewis acid sites (In3+ ions) in In2O3 with single-site Bi3+ ions to activate CO2 molecules. The as-formed BixIn2-xO3 photocatalyst showed a three orders of magnitude higher photoactivity than In2O3 itself and also exhibited significant photoactivity for CH3OH production. The enhanced photoactivity was attributed to the increased solar energy utilization rate and rapid separation and transfer of photogenerated charges.

    Photocatalytic CO2 conversion with methane reforming
    Methane (CH4) is the second most common greenhouse gas[89]. The photocatalytic conversion of CO2 into syngas (H2 and CO) with CH4 reforming (Equation 9) is considered an effective method to decrease the environmental concentration of these two greenhouse gases. Generally, metal catalysts are highly effective for this reaction.

    CO2 + CH4 → 2H2 + 2CO(9)

    Huang et al.[131] reported the conversion of CO2 by CH4 upon a Ni nanocrystal modified with silica clusters, which exhibited excellent durability for methane reforming (> 700 h). It achieved high H2 and CO production rates of 17.1 and 19.9 mmol·g-1·min-1, respectively, and excellent solar fuel efficiency of 12.5% under solar light irradiation. Even under IR irradiation (λ > 830 nm), the solar-to-fuel efficiency remained at 3.1%.

    Zhou et al.[132] loaded single-atom Ru sites on a Cu nanoparticle surface for photocatalytic CO2 conversion with CH4 reforming. A stability of 50 h and a selectivity of > 99% were achieved. Photoexcited hot carriers and single-atom Ru active sites led to the observed photoactivity.

    Shoji et al.[133] reported a SrTiO3-supported Rh catalyst for UV-light-driven CO2 reduction with CH4 reforming, which cannot be realized by traditional thermal catalysis. The photogenerated h+ and e- were employed for the oxidation of CH4 over SrTiO3 and the reduction of CO2 over Rh, respectively.

    The photoassisted steam reforming and dry CO2 reforming of CH4 at room temperature with high selectivity of syngas were realized in gas-phase catalysis by Zhao et al.[134] for the first time. Bimetallic Rh-V oxide cluster anions (Rh2VO1-3-) were used as catalysts and both the oxidation of CH4 and the reduction of H2O/CO2 were achieved effectively without light irradiation. The key step in controlling the syngas (CO and H2) selectivity in this system was to photoinduce the reaction intermediates (Rh2VO2,3CH2-) into electronically excited states [Figure 14][134].

    Figure 14. Two consecutive catalytic cycles of photoassisted steam reforming or dry CO2 reforming of methane to syngas mediated by Rh2VO1-3- clusters. “UV” represents ultraviolet light (λ = 355 nm photon). Reproduced from Ref.[134] with permission from Wiley.

    Photocatalytic CO2 conversion with other electron donors
    TEOA, TEA, triisopropanolamine (TIPA) and sodium sulfite (Na2SO3) have been reported as efficient electron donors for quenching the photogenerated h+ during photocatalytic CO2 conversion.

    I. TEOA as an electron donor
    The growth and assembly of highly dispersed UiO-66-NH2 nanocrystals upon graphene to form an active photocatalyst for CO2 conversion was reported by Wang et al.[135]. The as-synthesized UiO-66-NH2/graphene photocatalyst displayed both high activity and selectivity for the visible-light-driven conversion of CO2 to HCOOH in the presence of TEOA. The photoreduction efficiency of UiO-66-NH2/graphene for CO2 was ~11 times greater than that of UiO-66-NH2. The strong interaction between UiO-66-NH2 and graphene effectively boosted the transfer of photogenerated e- and inhibited the separation of UiO-66-NH2 from graphene, resulting in its high photoactivity and good cyclability.

    Wang et al.[136] confined highly dispersed nickel cobalt oxyphosphide nanoparticles (NiCoOP NPs) in multichannel hollow carbon fibers (MHCFs) to form a NiCoOP-NP@MHCF catalyst for photocatalytic CO2 reduction. The photoactivity was investigated in a tandem system, with [Ru(bpy)3]Cl2·6H2O (bpy = 2′2-bipyridine) used as a photosensitizer in the presence of TEOA. The as-formed catalyst exhibited considerable activity, offering a CO production rate of 166 μmol·mgcat-1·h-1.

    Wang et al.[137] decorated single-atom Cu sites upon UiO-66-NH2 (Cu SAs/UiO-66-NH2) to promote the photoreduction of CO2 to liquid fuels, with TEOA used as an electron donor. The decoration of single-atom Cu sites on UiO-66-NH2 facilitated the conversion of CO2 to CHO* and CO* intermediates, resulting in good selectivity for CH3OH and CH3CHOH. This photocatalyst realized CH3OH and CH3CH2OH production rates of 5.33 and 4.22 μmol·g-1·h-1, respectively, which were much higher compared to those of pure UiO-66-NH2 and Cu NP-loaded UiO-66-NH2.

    Yang et al.[138] demonstrated that a Ni-based metal-organic layer (MOL) exposing rich (100) crystal facets (Ni-MOL-100) showed much higher CO2-to-CO photoactivity than with rich (010) crystal facets exposed (Ni-MOL-010) and bulky Ni-MOF. Under Xe lamp irradiation (300 mW, λ > 420 nm), the catalytic activity in a [Ru(phen)3](PF6)2, TEOA, CH3CN and H2O system reached 2.5- and 4.6-fold higher than those of Ni-MOL-010 and bulky Ni-MOF, respectively.

    Li et al.[139] presented a bioinspired MOF with flexible Cu and Ni dual-metal-site pairs (DMSPs) that exhibited self-adaptive behavior to fit mutative C1 intermediates, realizing visible-light-driven CO2 reduction to CH4. The Cu and Ni DMSPs were incorporated into MOF-808 to form MOF-808-CuNi, leading to a production rate of 158.7 μmol·g-1·h-1 and a promoted CH4 selectivity of 97.5%. Various C1 intermediates were stabilized by the flexible self-adaptive DMSPs in multistep reactions, resulting in the high selectivity of CH4.

    II. TEA as an electron donor
    Qi et al.[140] proposed a single molecular cage of Ir(III) complex-decorated Zr-based metal-organic cages (IrIII-MOC-NH2) for visible-light-driven CO2 reduction. The IrIII-MOC-NH2 catalyst had high photoactivity and selectivity for CO2-to-CO conversion in the presence of TEA. The selectivity was 99.5% and the turnover frequency reached ~120 h-1, which was 3.4 times that of bulk IrIII-MOC-NH2. The AQE was 6.71%.

    III. TIPA as an electron donor
    Liu et al.[141] reported three functionalized polyoxo-titanium cluster-based photocatalysts for CO2 reduction, namely, Ti6 functionalized with phenylphosphonic acid and Ti8 and Ti6 functionalized with 1,1-ferrocene dicarboxylic acid (Fcdc). The light absorption range of Ti8-Fcdc and Ti6-Fcdc was expanded to the visible light region. The introduction of Fcdc ligands in the photocatalysts boosted the transfer of electrons from the Fcdc ligands to the Ti-oxo nucleus. In particular, both Ti8-Fcdc and Ti6-Fcdc achieved the photocatalytic reduction of CO2 to HCOO- with high selectivity (96.2% and 97.5%, respectively) and photoactivity (170.30 and 350.00 μmol·g-1·h-1, respectively) under visible-light irradiation in the presence of TIPA.

    IV. Sodium sulfite (Na2SO3) as an electron donor
    Zhu et al.[142] prepared a Cu/Cu+-modified Ti3+/TiO2 (Cu/Cu+@TiO2) photocatalyst for photocatalytic CO2 conversion under Xe lamp irradiation with Na2SO3 as an electron donor. The Cu+-O valences inside the TiO2 lattice promoted the transfer of carriers and the Cu on the surface of the catalyst as active sites promoted the reduction of CO2. The synergistic effect between Cu and Cu+ ions increased the charge carrier density. All the photogenerated e in the photocatalyst (100%) were used for CO2 reduction.

    Photocatalytic H2O2 production

    Basic principles of photocatalytic H2O2 production

    Since H2O2 was first synthesized in 1818[143], it has captured more and more attention and has even been listed as one of the 100 most important chemicals[144]. As a green oxidant, H2O2 contains a 47.1% (w/w) active oxygen content. Apart from H2O and O2, no other byproducts are generated during its reactions. As a result, H2O2 has been widely used in organic synthesis[145], wastewater treatment and disinfection[146] and the pulp and paper industry[147]. Moreover, H2O2 has also been employed in energy research as a one-compartment fuel cell[148]. The theoretical maximum output potential of 1.09 V in H2O2 fuel cells is comparable to that in conventional H2 fuel cells (1.23 V). However, unlike H2, H2O2 is completely soluble in water and easy to transport, which paves the way for it to become a desirable alternative energy carrier. These wide applications result in a huge demand for H2O2, with the global annual demand at ~2.2 Mt[149].

    At present, anthraquinone oxidation is the primary technology for industrial H2O2 production[143]. However, its industrial synthetic route is complicated, with high energy consumption and toxic byproducts. Alternative approaches are in development, such as the direct production of H2O2 from O2 and H2, but this method has a high cost, high energy intensity and a risk of explosions. Compared with these two methods, photocatalytic H2O2 production requires only earth-abundant H2O or other sacrificial agents and O2 as raw materials instead of dangerous H2/O2 mixtures and can be operated using semiconductors as catalysts under sunlight irradiation[150-153]. Moreover, no pollutants are produced during this process.

    A detailed illustration of photocatalytic H2O2 production is presented in Figure 15[9]. The photogenerated h+ in the VB oxidizes H2O (or other sacrificial agents) to generate O2 (or other oxidation products) and protons (H+, Equation 10), while the photogenerated e- in the CB reacts with the adsorbed O2 to generate H2O2. Specifically, H2O2 can be produced through either an indirect sequential two-step one-electron reduction route (Equations 11 and 12) or a direct one-step two-electron reduction route (Equation 13). In the indirect two-step one-electron reduction process, the one-electron reduction of O2 produces a superoxide radical (O2•-, Equation 11), which then reacts with two H+ ions and another electron to produce H2O2 (Equation 12). In the direct one-step two-electron reduction of O2 to H2O2, O2 reacts directly with two H+ ions to form H2O2 via a two-electron reduction process (Equation 13). These two processes can both be described by the overall reaction in Equation 14.

    Figure 15. Illustration of photocatalytic H2O2 production. S represents the sacrificial agents. CBM: Conduction band minimum; VBM: valence band maximum.

    2H2O + 4h+ → O2 + 4H+(10)

    O2 + e → O2•-(11)

    O2•- + 2H+ → H2O2(12)

    O2 + 2H+ + 2e- → H2O2(13)

    2H2O + O2 → 2H2O2(14)

    Recent advances in photocatalytic H2O2 production

    H2O2 production from O2 reduction
    I. C3N4-based photocatalysts
    An interfacial Schottky junction consisting of Ti3C2 and porous g-C3N4 NSs was designed by Yang et al.[154] for visible-light-driven H2O2 production. It displayed an H2O2 production rate of 2.20 μmol·L-1·min-1, which was ~2.1 times that of pure g-C3N4. The improved photoactivity was ascribed to the formed interfacial Schottky junction and built-in electric field, which boosted the spatial separation of photogenerated charges.

    Wu et al.[155] introduced alkali metal dopants and N vacancies in C3N4. This extended the light absorption region, shortened the band gap from 2.85 to 2.63 eV and suppressed the recombination of photogenerated charges. The synergistic effect of the dopants and defects brought about a photocatalytic H2O2 production rate of 10.2 mmol·g-1·h-1 using isopropanol as an electron donor, which was 89.5 times that of pure C3N4. Similarly, Xie et al.[156] introduced two types of cooperative N vacancies, that is, NHx and N2C vacancies, into polymeric carbon nitride. It delivered a 15-fold improvement in H2O2 production with excellent stability using ethanol as a sacrificial hole scavenger. The AQE reached 26.78% and 11.86% at 340 and 420 nm, respectively. The NHx and N2C vacancies accelerated the photoexcited charge separation and assisted in activating O2 in the two-electron pathway, respectively.

    Zhang et al.[157] prepared an alkali and sulfur codoped polymeric carbon nitride and used it as a photocatalyst for H2O2 production from the O2 reduction reaction. The photocatalyst realized a H2O2 production rate on the millimolar level under the irradiation of visible light with an AQE of 100% and a selectivity of 96%. Alkali and sulfur dopants in the photocatalyst boosted the separation of interlayer charges and the polarization of trapped electrons for the capture and reduction of O2, respectively.

    Zhou et al.[158] synthesized surface •OH group-functionalized g-C3N4 nanotubes. The nanotube structures provided a high surface area and promoted mass transfer. The •OH groups captured the photogenerated h+ to facilitate the separation of photogenerated charges and were also beneficial in suppressing the self-decomposition of H2O2. Consequently, a H2O2 production rate of 240.36 μmol·g-1·h-1 was obtained.

    Chen et al.[159] prepared a Na+-doped cyano-rich g-C3N4 photocatalyst. The porous g-C3N4 with Na+ dopants and cyano groups simultaneously optimized the photoactivity and selectivity, showing H2O2 production rates of 7.01 mmol·h-1 under visible light irradiation (λ ≥ 420 nm) and 16.05 mmol·h-1 under simulated sun conditions, respectively, and a selectivity of 93% from two-electron O2 reduction.

    II. MOF- and COF-based photocatalysts
    In 2018, Isaka et al.[160] reported the visible-light-driven H2O2 production via two-electron O2 reduction using a MIL-125-NH2 photocatalyst with TEOA or benzyl alcohols as electron donors. Depositing NiO nanoparticles upon MIL-125-NH2 drastically improved its photoactivity. Furthermore, in 2021, Chen et al.[161] introduced a photosensitizer, 4,4,4′,4′′-(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid (L2), into MIL-125 to form L2-functionalized MOFs (MIL-125-L2), which showed a H2O2 production rate of 1654 μmol·L-1·h-1 under the illumination of visible light (λ > 400 nm) with TEOA as a sacrificial hole scavenger. This high photoactivity was ascribed to the visible light absorption of L2, which originated from the π-electron system in L2, making MIL-125-L2 a catalyst for visible light response.

    In 2020, Krishnaraj et al.[162] reported two new 2D COFs, in which the (diarylamino)benzene linkers formed a Kagome lattice and showed strong visible light absorption. The high crystallinity and large surface area of these COFs allowed for effective charge transfer and diffusion. The diarylamine (donor) unit in these COFs efficiently promoted the reduction of O2 to H2O2 using alcohols as sacrificial hole scavengers.

    H2O2 production from O2 reduction coupled with value-added organic chemical synthesis from organic matter oxidation
    The production of H2O2 from O2 reduction coupled with the synthesis of value-added organic chemicals is attractive. On this basis, photogenerated carriers can be fully utilized during the process of photocatalytic renewable energy production, enhancing the processes of solar-to-chemical conversion efficiency.

    I. Reduction of O2 coupled with oxidation of benzyl alcohol
    Benzaldehyde is the simplest and most important aromatic aldehyde in industry. It has a wide range of applications in medicine, dyes, perfumes, resins and other industries[163]. The photocatalytic production of benzaldehyde from the oxidation of benzyl alcohol is a green process (Equation 15).

    O2 + Ph-CH2OH → H2O2 + Ph-CHO(15)

    In 2019, Isaka et al.[164] reported hydrophobic linker-alkylated MOFs, MIL-125-Rn (n = 4 and 7), for the photocatalytic production of H2O2 in a two-phase system (water/benzyl alcohol) [Figure 16A and B]. The hydrophobization of MIL-125-NH2 separated it from the aqueous phase into the benzyl alcohol phase. In this two-phase system, H2O2 was produced in the water phase, while the MOF structure was more stable in the benzyl alcohol phase. This resulted in improved photoactivity [Figure 16C]. The as-prepared hydrophobic MOF also showed its feasible application in H2O2 production in various aqueous solutions, including extremely low pH and NaCl solutions [Figure 16D]. Furthermore, Kawase et al.[165] reported another hydrophobic cluster-alkylated MIL-125-NH2 (OPA/MIL-125-NH2). Its Ti clusters were alkylated by octadecylphosphonic acid (OPA). The activity for H2O2 production was higher than that of the reported MIL-125-Rn in the abovementioned two-phase system[164]. OPA modified the outermost surface of the MOFs while preserving the inner pores, which resulted in enhanced activity. In 2020, Chen et al.[166] further synthesized a hydrophobic OPA/Zr100-xTix-MOF (x = 0, 5, 7.5 and 10), in which Zr clusters were alkylated with OPA, and applied it for H2O2 production in the above-mentioned two-phase system. The optimal OPA/Zr92.5Ti7.5-MOF photocatalyst displayed a H2O2 production rate of 9.7 mmol·L-1·h-1 under the illumination of visible light (λ > 420 nm), which was ~4.5 times that of the Zr100-MOF. H2O2 production cycling tests indicated that it showed good stability.

    Figure 16. (A) Two-phase system (water/benzyl alcohol) for photocatalytic H2O2 production. (B) Alkylation reaction of linkers of MIL-125-NH2 to form MIL-125-Rn (n = 1, 4 or 7). (C) Photocatalytic activity and (D) related mechanism of H2O2 production in the two-phase system under photoirradiation (λ > 420 nm). Reproduced from Ref.[164] with permission from Wiley.

    II. Reduction of O2 coupled with oxidation of benzylamine
    The synthesis of nitriles from the selective oxidation of amines plays a key role in both laboratory and industrial synthetic processes because nitrile is an important intermediate product for the synthesis of several fine chemicals, pharmaceuticals and agrochemicals[167-173]. The photocatalytic oxidation of amines to nitriles is an effective approach for the synthesis of nitriles under mild conditions (Equation 16).

    2O2 + Ph-CH2NH2 → 2H2O2 + Ph-CN(16)

    In 2021, Tian et al.[174] reported that benzylamine oxidation could be used as a half-reaction to couple with H2O2 production from O2 reduction (Equation 13) using defective ZrS3 nanobelts with disulfide (S22-) and sulfide anion (S2-) vacancies [Figure 17A-F]. The defective ZrS3 nanobelts exhibited good photoactivity for H2O2 production and high selectivity (> 99%) for benzonitrile production from benzylamine oxidation [Figure 17G and H]. The S22- vacancies facilitated the separation of e--h+ pairs and the S2- vacancies improved the e- conduction, h+ extraction and benzylamine oxidation kinetics. As a result, the photocatalyst displayed a H2O2 production rate of 78.1 ± 1.5 and a benzonitrile production rate 32.0 ± 1.2 μmol·h-1 under simulated sunlight irradiation.

    Figure 17. Transformation process of monoclinic ZrS3 into hexagonal ZrS2 from the [010] (A-C) and [001] (D-F) views. (A, D) Crystal structure of monolayer ZrS3 with a boundary of 1 × 3 × 1. (B, E) Crystal structure of monolayer ZrS3 after desulfuration of S22- ions. (C, F) Crystal structure of monolayer ZrS2 with a boundary of 1 × 3 × 1. (G) Activity of H2O2 and benzonitrile production for a repeated photoreaction sequence with ZrS1-yS2-x (15/100) under simulated sunlight irradiation [ZrSS2-x annealed for X time was denoted as ZrSS2-x(X), and ZrS1-yS2-x annealed for X min and treated with Y mg Li was denoted as ZrS1-yS2-x(X/Y)]. (H) H2O2 and benzonitrile production rates. Reproduced from Ref.[174] with permission from Springer Nature.

    H2O2 production from O2 and H2O
    To improve the eco-friendliness and sustainability of H2O2 production, earth-abundant water should be used instead of alcohols or other sacrificial agents (e.g., TEOA and TEA). The oxidation of H2O by h+ produces O2 and H+, while the reduction of O2 by e produces H2O2[175]. On this basis, H2O2 can be synthesized from H2O and O2 with 100% efficiency.

    Ma et al.[176] fabricated a protonated TiO2 nanotube with carbon dots for H2O2 production under visible-light irradiation (λ > 420 nm). It showed a H2O2 production rate of 3.42 mmol·gcat-1·h-1 in water, exceeding the values obtained on TiO2 catalysts with noble metals. The protons on the catalyst play a key role in H2O2 production by promoting the reduction of O2 to H2O2 and inhibiting H2O2 decomposition. This catalyst displayed a solar-to-H2O2 apparent energy conversion efficiency of 5.2%.

    Zeng et al.[177] reported a C3N4-grafted cationic polyethylenimine (PEI) molecule for the photocatalytic H2O2 production from H2O and O2. The PEI/C3N4 photocatalyst exhibited a H2O2 production rate of 208.1 μmol·g-1·h-1, which was 25 times that of the pure C3N4. This was ascribed to the simultaneous improvement in charge separation and two-electron O2 reduction selectivity.

    In 2020, Zhao et al.[178] demonstrated a ZIF-8/C3N4 composite for visible-light-driven H2O2 production from H2O and O2. It displayed a H2O2 production rate of 2641 μmol·g-1·h-1 and an AQE of 19.57%. In 2021, Zhao et al.[179] constructed a C3N4-assisted Ni3(HHTP)2 (Ni-CAT) photocatalyst that could catalyze the production of H2O2 from H2O and O2 under visible-light illumination. In this case, the Ni-CAT photocatalyst was the main active component for the reduction of O2, while C3N4-assisted Ni-CAT suppressed the recombination of photogeneration charge carriers by providing electrons. Furthermore, Wu et al.[180] fabricated a metal-free photocatalyst composed of carbon dots, organic dye molecules, procyanidins and 4-methoxybenzaldehyde for direct H2O2 production from seawater. This catalyst exhibited a visible-light-driven H2O2 production rate of 1776 μmol·g-1·h-1, which is ~4.8 times that of the pristine polymer.

    In 2021, Teng et al.[181] fabricated a Sb-single-atom-loaded C3N4 photocatalyst for the production of H2O2 from water and O2 at λ = 420 nm. An AQE of 17.6% and a conversion efficiency of solar energy to chemical energy of 0.61% for H2O2 production were achieved. The formed μ-peroxide at the Sb sites promoted the two-electron O2 reduction reaction and highly concentrated photogenerated h+ at its neighboring N atoms promoted water oxidation reaction, resulting in the excellent photocatalytic activity. Importantly, in the same year, Ye et al.[182] reported a zinc polyphthalocyanine (ZnPPc)-decorated and B-doped carbon nitride (NBCN) hybrid (ZnPPc-NBCN) photocatalyst with a Z-scheme heterostructure. The high redox potential of the photocatalyst was stable during the reaction and the photocatalytic H2O2 production rate from pure water and open air reached 114 μmol·g-1·h-1.

    CONCLUSIONS AND OUTLOOK

    This review summarizes recent significant achievements in photocatalytic renewable energy production based on semiconductor and semiconductor-like driven photocatalysis, particularly for H2 production, CO2 reduction and H2O2 generation. To improve the photocatalytic performance, these research works have mainly focused on solving two scientific issues with photocatalysis, namely, light absorption and photogenerated charge separation. To improve the light utilization rate and/or boost the photogenerated charge separation, various strategies based on the design of photocatalysts have been employed: (1) doping of heteroelements/modification with single atoms; (2) creation of defects; (3) loading of dual cocatalysts; (4) construction of heterostructures (e.g., type II and Z-scheme); (5) fabrication of isotropic facets; and (6) generation of synergistic effects using multicomponents. Meanwhile, the improvement on the following reactions has also been considered: (1) for H2 production from overall water splitting: effective water oxidation half-reaction and separation of the H2 and O2 products; (2) for CO2 conversion: solubility and competitive H2 production in the liquid phase, activation of CO2 molecules and product selectivity; and (3) for H2O2 production from O2 (and water): the selectivity of two-electron O2 reduction, effective water oxidation half-reaction and self-decomposition of the formed H2O2.

    The current semiconductor-based nanocomposite systems mainly consist of single inorganic semiconductor components or organic semiconductor-like polymers, such as MOFs and COFs. Although significant advances in photocatalytic renewable energy production have been achieved, these systems still exhibit some shortcomings, including the photocorrosion of sulfides and the instability of MOF and COF structures. For practical applications, it is necessary to bestow these traditional systems with new features or functionality in future research.

    (1) Semiconductor-based bioinspired photocatalysis is a promising avenue for new photocatalysts with enhanced efficiency and stability, inspired by the abovementioned bioinspired HER-WOR-MOF photocatalyst[79]. Such a system can be constructed by the effective integration of the superiorities from traditional photocatalysis and biological components, that is, the combination of traditional inorganic semiconductors and highly selective bioenzymes.

    (2) Photoelectrocatalysis that can efficiently combine photo- and electric energies represents an important direction, inspired by the aforementioned hydrogen farm project[83]. Such strategies can efficiently enhance the catalytic efficiency and easily separate the catalysts and products.

    (3) Photothermal catalysis is another exciting direction. For example, the conversion of CO2 with H2 or CH4 via thermal catalysis requires high temperatures. Introducing photocatalysis can lower the temperature. Thus, the combination of photocatalysis and thermal catalysis is of great significance for future research.

    DECLARATIONS

    Authors’ contributions

    Wrote the manuscript: Chen X, Zhao J

    Reviewed the manuscript: Zhang D, Li G, Li H

    Availability of data and materials

    Not applicable.

    Financial support and sponsorship

    This work was supported by the National Key Research and Development Program of China (2020YFA0211004), and National Natural Science Foundation of China (22022608, 21876112, 21876113, 92034301), “111” Innovation and Talent Recruitment Base on Photochemical and Energy Materials (No. D18020), Ministry of Education, and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Engineering Research Center of Green Energy Chemical Engineering (No. 18DZ2254200), Shanghai Frontiers Science Research Base of Biomimetic Catalysis and Shanghai government (18SG41).

    Conflicts of interest

    All authors declared that there are no conflicts of interest.

    Ethical approval and consent to participate

    Not applicable.

    Consent for publication

    Not applicable.

    Copyright

    © The Author(s) 2022.

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    Cite This Article

    Chen X, Zhao J, Li G, Zhang D, Li H. Recent advances in photocatalytic renewable energy production. Energy Mater 2022;2:200001. http://dx.doi.org/10.20517/energymater.2021.24

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