Photocatalytic Production of Hydrogen Peroxide over Modified Semiconductor Materials: A Minireview

Abstract

Hydrogen peroxide (H₂O₂) has exhibited huge application value in many fields including chemical synthesis, medicine, environmental remediation, and fuel cells. Traditional anthraquinone method for H₂O₂ commercial production has emerged the drawbacks of toxicity, H₂ consumption and high energy input. Photocatalytic production of H₂O₂, which only requires water, oxygen, solar light and catalyst, is a novel and green technique, and potentially becomes one of the substitutes for anthraquinone method. Herein, we comprehensively review the research progress in the reported semiconductor catalysts, their modification strategies, as well as the related photocatalysis systems and mechanisms for the light driven H₂O₂ production. In detail, the photocatalysts are introduced from different families including ZnO, g-C₃N₄, TiO₂, metal complexes, metal sulfides, Bi containing semiconductors, and carbon materials. In the meantime, their modification strategies are systematically evaluated aiming at the improvement in the structures and the photoelectrical properties of semiconductors, as well as their effective activation of molecular O₂, and inhibition of H₂O₂ decomposition. Finally, this review is concluded with a brief summary and outlook, and the major challenges for the development of photocatalytic H₂O₂ production over the emerging semiconductor photocatalysts.

1 Introduction

Hydrogen peroxide (H₂O₂) is a green and efficient oxidant, which can oxidize various inorganic and organic substrates in liquid-phase reactions under very mild conditions, and generates only one clean byproduct of water (H₂O). H₂O₂ has been widely used in many industrial fields including chemical industry, medicine and biological process, and environmental remediation [1, 2]. Very recently, H₂O₂ is also exploited to be a potential energy carrier for fuel cells [3,4,5]. H₂O₂ exhibits several advantages to become an alternative to H₂ fuel cells: (1) H₂O is the only and clean byproduct in fuel cells; (2) the liquid state of H₂O₂ makes it more convenient and safer in storage and transportation; (3) it can be made into a fuel cell of single-compartment with more simplification and better scales than that of H₂ two-compartment.

H₂O₂ production of industrial scale has been achieved via the anthraquinone oxidation (AO) process [6, 7]. However, anthraquinone oxidation is a multistep method that contains hydrogenation, oxidation and extraction procedures in organic solvents, requiring high energy input and emitting a lot of wastes [7, 8]. Therefore, the AO method aggravates the difficulty and hazards in transport and storage, and hardly satisfies the demands for green production and the cost efficiency. Other alternative methods, such as alcohol oxidation and electrochemical synthesis have also been practiced in industrial production of H₂O₂ [6, 8, 9]. However, the purity and quality of the produced H₂O₂ via those methods is not as good as that via the AO process. Therefore, it is essential to develop cost-effective and eco-friendly methods for H₂O₂ production. During the last two decades, direct synthesis of H₂O₂ from hydrogen (H₂) and oxygen (O₂) in the presence of a catalyst has been regarded as another alternative approach for AO process [10,11,12,13]. The direct method is an innovation that H₂O₂ is simply synthesized from its elements by one step, and the H₂O₂ can be used in an oxidation reaction in situ. There are three major problems for the direct method: (1) inert gas (e.g. N₂, CO₂ and Ar) must be charged in order to keep away from the explosive limit of H₂/O₂ mixture; (2) the noble metals (e.g. Pd, Au and Pt), and their alloys (e.g. Pd–Au and Pd–Pt) act as the active centers on various supports, which increase the cost of catalyst; (3) some noble metals like Pd and Au are also active for the H₂O₂ decomposition, which decreases the synthesis efficiency of H₂O₂. Although the catalytic oxidation of H₂ to generate H₂O₂ has been known since 1914, this technique has not yet been put into industrial practice [14].

In recent years, H₂O₂ production from photocatalysis of semiconductors arouses much attention and is frequently reported due to the sufficient and renewable sunlight as driving force [15]. Photocatalytic production of H₂O₂ over semiconductor is at least known since the report of Baur and Neuweiler in 1927 [16]. This developing technique involves only light, water, molecular O₂ and the catalyst, which is eco-friendly, and is recommended to be applied in the field of oxidations and solar fuels demanding for a mild scale of H₂O₂. In general, the photocatalysis process involves two major half reactions: (1) two-electron reduction of O₂ from the conduction band (CB) (Eq. 1); (2) oxidation of H₂O by holes (h⁺) in valence band (VB) to generate O₂ (Eq. 2). Meanwhile, there are several side reactions, which lower the H₂O₂ selectivity: (1) one-electron reduction of O₂ to generate peroxy radicals (·OOH) (Eq. 3); (2) four-electron reduction of O₂ to generate H₂O (Eq. 4). Therefore, H₂O₂ concentrations in many reported works hardly achieve the mmol/L scale due to the existence of those side reactions. In the field of photocatalytic production of H₂O₂, effective inhibition of the one-electron and the four-electron reductions of O₂ becomes the major challenge for most of the semiconductor photocatalysts. Introducing the new structures or the guest molecules to the host semiconductors is an effective strategy to promote the charge separation and increase the selectivity of the two-electron reduction of O₂ to H₂O₂. In addition, multi-channels for H₂O₂ production are possibly opened by the modified semiconductors. In another aspect, a number of works adopt molecular O₂ (pure oxygen gas) or organic electron donors (alcohols) to enhance the H₂O₂ yield, which are far away from the concept of cost-efficiency and green synthesis. Therefore, saving the pure O₂ or electron donor by using appropriate photocatalyst becomes another challenge for photocatalytic production of H₂O₂.

$$ {\text{O}}_{{2}} + {\text{2H}}^{ + } + {\text{ 2e}}^{ - } \to {\text{ H}}_{{2}} {\text{O}}_{{2}} \left( {0.{\text{68 V vs}}.{\text{ NHE}}} \right) $$

(1)

$$ {\text{H}}_{{2}} {\text{O }} + {\text{ 2h}}^{ + } \to { 1}/{\text{2O}}_{{2}} + {\text{ 2H}}^{ + } \left( {{1}.{\text{23 V vs}}.{\text{ NHE}}} \right) $$

(2)

$$ {\text{O}}_{{2}} + {\text{ H}}^{ + } + {\text{ e}}^{ - } \to {}^{ \cdot }{\text{OOH }}\left( { - \,0.{\text{13 V vs}}.{\text{ NHE}}} \right) $$

(3)

$$ {\text{O}}_{{2}} + {\text{ 4H}}^{ + } + {\text{ 4e}}^{ - } \to {\text{ H}}_{{2}} {\text{O }}\left( {{1}{\text{.23 V vs}}.{\text{ NHE}}} \right) $$

(4)

So far, the reported catalysts for photocatalytic production of H₂O₂ can be classified as graphitic carbon nitride (g-C₃N₄) [15, 17,18,19,20,21,22,23,24,25,26,27], TiO₂ [28,29,30,31,32,33], transition metal sulfide [34,35,36,37,38], BiVO₄ [39], transition metal complexes [40,41,42,43,44] and organic ions [45, 46] based materials. In this minireview, we mainly aim at the recent advances and challenges associated with the photocatalytic H₂O₂ production, as well as the related semiconductor photocatalysts. It contains the preparation of semiconductors, the modification strategies, and the related mechanism of H₂O₂ production over the semiconductors.

2 Photocatalytic Production of H₂O₂ over g-C₃N₄ Based Materials

So far, the reports on g-C₃N₄ as fundamental catalysts for light-driven H₂O₂ production are rapidly increasing in number. G-C₃N₄, consisting of earth-abundant elements only, and possessing 2.7 eV of band gap and graphene-like 2D morphology, has been regarded as an appealing and potential photocatalyst. The conduction band of g-C₃N₄ (− 1.3 V vs. NHE) is suitably located to facilitate O₂ reduction (− 0.28 V vs. NHE), and its lower valence band potential (1.4 V vs. NHE) can prevent the oxidative decomposition of H₂O₂. Many efforts have been made for g-C₃N₄ to solve the problems of the fast charge recombination caused intrinsically by the π–π conjugated electronic system of g-C₃N₄ framework, and the limited inhibition of the one-electron reduction of O₂.

2.1 Single C₃N₄ Photocatalysts

Fabrication of a single C₃N₄ photocatalyst is a simple and low-cost way to enhance the photocatalytic performance. The single C₃N₄ photocatalyst can be fabricated based on the preparation of pristine g-C₃N₄ without adding or doping other species. After the treatments of instruments or chemical reagents, the photocatalysts can achieve the improvements in their frameworks, pores and surface, crystal structure, or photoelectric properties to increase the H₂O₂ productivity.

One of the strategies is to fabricate the g-C₃N₄ of appropriate morphology without changing its intrinsic molecule or crystal structure. Shiraishi et al. earlier used the metal-free polymeric photocatalyst g-C₃N₄ for photocatalytic production of H₂O₂ (Fig. 1a) [20, 47]. They found that g-C₃N₄ with alcohol and O₂ can selectively promote the two-electron reduction of O₂ due to the efficient formation of 1,4-endoperoxide species on its surface, while suppressed the subsequent decomposition of the formed H₂O₂. In addition, the g-C₃N₄ catalyst activated by visible light can oxidize water owing to the positively shifted VB levels, while maintaining high selectivity for two-electron reduction of O₂. This thus facilitated highly efficient production of H₂O₂ with more than 90% selectivity. In order to improve the catalytic activity and the H₂O₂ selectivity of g-C₃N₄, the same group subsequently reported a mesoporous g-C₃N₄ prepared by silica-templated thermal polymerization of cyanamide for photocatalytic production of H₂O₂ [25]. Mesoporous g-C₃N₄ with larger surface area contained primary amine groups on the surface, which decreased the H₂O₂ selectivity and increased the photocatalytic decomposition of the formed H₂O₂. Selectivity for H₂O₂ formation via two-electron reduction of O₂ by the conduction band electrons localized on the 1,4-positions of the melem unit decreased with an increase in the surface area. Therefore, H₂O₂ productivity was adjusted by the surface area and the surface defects of mesoporous g-C₃N₄ (Fig. 1b–d). Ou et al. developed a self-assembly method to prepare a self-supported C₃N₄ aerogel with large surface area, incorporated functional groups and 3D network structure [48]. The C₃N₄ aerogel obtained high photocatalytic activity for hydrogen evolution and H₂O₂ photoproduction.

Fig. 1

Amount and selectivity of H₂O₂ over g-C₃N₄ in a 2-propanol/water/O₂ system under sunlight exposure (orange) without filter and (blue) with filter (λ > 420 nm) (a) [20], copyright 2014 American Chemical Society; amount (b) and selectivity (c) of H₂O₂ over mesoporous g-C₃N₄ with different surfaces areas (x, m² g⁻¹) [25], copyright 2015 American Chemical Society; pathway of selective H₂O₂ production on g-C₃N₄ under visible light irradiation (d) [25], copyright 2015 American Chemical Society

Another strategy is to improve the photoelectric properties of g-C₃N₄ by introduction of vacancies or displacement of atoms. Carbon vacancies method was adopted to modulate g-C₃N₄ with an improvement in electrons transfer and band gap [49]. H₂O₂ generation pathway could be changed from a two-step single-electron indirect reduction to the one-step two-electron direct reduction by the carbon vacancies in g-C₃N₄. Therefore, photoproduction of H₂O₂ was improved by 14 times in the absence of organic scavenger through the carbon vacancy-based strategy. A reduced g-C₃N₄ material was prepared by a thermal treatment with NaBH₄ in N₂ atmosphere [18]. The reduction treatment created nitrogen vacancies followed by a formation of functional group C≡N (Fig. 2), which endowed g-C₃N₄ with a feature of visible light-driven water oxidation capacity. In addition, the reduction treatment facilitated the spatial separation of photo-excited electron and hole, and enhanced the charge transfer. Therefore, an optimal reduced g-C₃N₄ obtained enhanced performance in photocatalytic production of H₂O₂ (170 μmol/L h⁻¹) from pure H₂O and O₂ at ambient atmosphere in the absence of organic electron donors. A parent g-C₃N₄ was treated with a dielectric barrier discharge (DBD) plasma, and a PT-g-C₃N₄ material for photocatalytic production of H₂O₂ was finally obtained [50] (Fig. 3a). Compared with bare g-C₃N₄, PT-g-C₃N₄ improved the grain size, the surface and pore properties, as well as the hydrophilic property. Furthermore, PT-g-C₃N₄ significantly improved the H₂O₂ yield by 13 times based on pristine g-C₃N₄ (Fig. 3b).

Fig. 2

Probable reaction of g-C₃N₄ treated with NaBH₄ [18], copyright 2018 Elsevier

Fig. 3

Mechanism illustration of DBD plasma introducing oxygen-containing functional groups on the surface of g-C₃N₄ (a) [50], copyright 2020 Elsevier; photocatalytic activity of pristine g-C₃N₄ and PT-g-C₃N₄ for photocatalytic production of H₂O₂ (b) [50], copyright 2020 Elsevier

2.2 Modified g-C₃N₄ Photocatalysts

Introducing the guest molecules or semiconductors to g-C₃N₄ host is effective and frequently used to promote the charge separation and the selectivity of the two-electron reduction of O₂ to H₂O₂. Moreover, the bandgap of g-C₃N₄ is easily modulated by doping, hybridization or surface decoration, which can promote the effective utilization of visible light [21]. The introduced species mainly include hetero-elements, nanoparticles, semiconductors, organic compounds, polymers. The modification strategies, photocatalytic performances and properties of the reported g-C₃N₄ based materials for photocatalytic H₂O₂ production are collected in Table 1.

Table 1 Summary of the modification strategies, photocatalytic performances and properties of the reported g-C₃N₄ based materials for photocatalytic H₂O₂ production

2.2.1 Elements Doping

Incorporation of earth-abundant heteroelements (K, P or O) can efficiently improve the crystal structure and the band gap of g-C₃N₄ to enhanced its photocatalytic performance. An in situ incorporation of both potassium and phosphate species into the polymeric C₃N₄ framework was reported [51]. The incorporated K, P and O species introduced the negative surface charge, facilitated the interfacial electron transfer to dioxygen, and inhibited the decomposition of in situ generated H₂O₂. Therefore, the modified C₃N₄ enhanced the apparent quantum yields of H₂O₂ by about 25 and 17 times under monochromatic irradiation of 420 and 320 nm, respectively (Fig. 4a). The high selectivity toward H₂O₂ over H₂ are attributed to the enhanced light absorption, the increased lifetime of the transient species, the effective interfacial charge transfer to dioxygen, and the inhibited decomposition of in situ generated H₂O₂ (Fig. 4b). Later, the same group incorporated the potassium hexafluorophosphate into the C₃N₄ structure to obtain a composite photocatalyst (KPF_CN) [27]. Compared with C₃N₄, the introduction of KPF₆ could increase the absorption of visible light, the charge carrier density and the selective two-electron transfer to O₂, and inhibit the photodecomposition of H₂O₂. The catalyst greatly enhanced the apparent quantum yield of H₂O₂ (26.1 times higher than that of bare C₃N₄) in visible light region (Fig. 4c). The high selectivity for O₂ reduction in KPF_CN attributes to the optimized interactions of O₂ molecules and protons with K⁺ and PF₆ sites, respectively. A series of potassium and phosphorus doped g-C₃N₄ catalysts for H₂O₂ photoproduction was synthesized and reported (Fig. 4d) [52]. The optimal catalyst achieved 5 mM of H₂O₂ for 10 h, which were 5 folds of that over pure g-C₃N₄. Xue et al. prepared a Co_xNi_yP cluster incorporated P-doped g-C₃N₄ (Co_xNi_yP-PCN) photocatalyst by a two-step phosphating method [53] (Fig. 5a–c). It was found that P as a substitution of C in g-C₃N₄ introduced a positive charge center (P⁺) forming a unique bridging effect. The bridging effect with the extended light absorption by P doping and optimized surface redox potential by cocatalyst integration stimulated efficient vectorial charge transfer between PCN and CoNiP and subsequent surface mass exchange (Fig. 5d). As a result, the two-electron reaction pathway for H₂O₂ photogeneration was facilitated.

Fig. 4

Apparent quantum yield of H₂O₂ photoproduction over KPD-CN (a) and the O₂ activation pathway (b) [51], copyright 2017 American Chemical Society; apparent quantum yield of H₂O₂ photoproduction over KPF_CN (c) and the O₂ activation pathway (d) [27], copyright 2018 Elsevier

Fig. 5

Schematic illustration for preparation of CoNiP–PCN (a) [53], copyright 2019 Elsevier; TEM (b) and HRTEM (c) imagines of CoNiP–PCN [53], copyright 2019 Elsevier; proposed mechanism of H₂O₂ photoproduction over CoNiP–PCN (d) [53], copyright 2019 Elsevier

In addition, Hu’s group prepared a hollow Cu doped g-C₃N₄ microspheres, in which Cu species was inserted at the interstitial position through the coordinative Cu(I)-N bonds [23]. With a self-established system (Fig. 6a) for H₂O₂ photoproduction, the Cu doped g-C₃N₄ displayed higher H₂O₂ productivity (4.8 mM) and better structural stability than neat g-C₃N₄. With the aid of DFT simulation (Fig. 6b, c), they concluded that the Cu(I)–N active sites could activate molecular O₂, and built an “electron transfer bridge” to the adsorbed O₂ molecules.

Fig. 6

Schematic diagram of photocatalytic H₂O₂ production (a) [23], copyright 2018 Elsevier; Optimal O₂ adsorption models on g-C₃N₄ (left) and Cu doped g-C₃N₄ (right) (b) [23], copyright 2018 Elsevier; charge density difference of O₂ molecule adsorbed on a Cu⁺ doping site (The yellow and blue isosurfaces represent charge accumulation and depletion in the space, respectively) (c) [23], copyright 2018 Elsevier

2.2.2 Surface Decoration

Surface modification with functional species or supporting nanoparticles on the parent photocatalyst is an efficient approach to improve the photocatalytic activity by promoting the charge separation and selectively catalyzing relevant reactions. The enhancement effect of Au nanoparticles for H₂O₂ generation has been demonstrated over the TiO₂ photocatalysts [54]. Thereby, Zuo et al. carried out photocatalytic production of H₂O₂ over g-C₃N₄ supporting Au nanoparticles [55]. Au nanoparticles showed inert nature for the decomposition of H₂O₂, and thus increased the H₂O₂ yield. A boron nitride quantum dots modified ultrathin porous g-C₃N₄ (BNQDs/UPCN, BU) composite was constructed via two steps [56] (Fig. 7a, b). The superoxide radical (·O²⁻) generation rate over the composite was estimated to be 2.3 times higher than that over bulky g-C₃N₄ (Fig. 7c), owing to that the composite simultaneously promoted the dissociation of excitons and accelerate the transfer of charges (Fig. 7d).

Fig. 7

TEM and HRTEM images of BNQDs/UPCN (a, b) [56], copyright 2019 Elsevier; H₂O₂ photoproduction over different BNQDs/UPCN samples (c) [56], copyright 2019 Elsevier; proposed photocatalytic mechanism in BNQDs/UPCN heterostructure (d) [56], copyright 2019 Elsevier

Polyoxometalates (POMs) are classified as metal–oxygen cluster compounds, which can act as electron reservoirs and exhibit extensive ranges of structures and stable redox states [57]. In particular, POMs contain several empty d orbitals that allow them to accept electrons without causing a structural change. These compounds also have nucleophilic oxygen-enriched surfaces and multi-hydrogen protons [58]. Zhao’s group prepared a photocatalyst (3DOM g-C₃N₄) by the covalent combination of a polyoxometalate cluster of [PW₁₁O₃₉]⁷⁻ with a macroporous g-C₃N₄ through the organic linker strategy [22] (Fig. 7a). The catalyst obtained 2.4 μmol h⁻¹ of light driven H₂O₂ production in the absence of organic electron donors (Fig. 7b). The positive shift of the CB in 3DOM g-C₃N₄-PW₁₁ is likely to improve the selectivity of O₂ reduction to H₂O₂. Later, they prepared a hybrid catalyst of g-C₃N₄-CoWO via the calcination of the 3-amino 1,2,4-triazole and the (NH₄)₈Co₂W₁₂O₄₂ precursors [59] (Fig. 7c). The hybrid catalyst with well-defined and stable structure obtained 9.7 μmol h⁻¹ of H₂O₂ productivity in the absence of organic electron donor under visible light (Fig. 7d).

Organic compounds and organisms are also practiced in photocatalytic H₂O₂ production to become good cocatalysts of g-C₃N₄. An all-solid-state Z-scheme heterojunction (PI-NCN) was constructed by assembling perylene imides (PI) on g-C₃N₄ nanosheets (NCN) via gas soft-template and condensation reaction method [17]. Electrons in conduction band of PI were transferred into the valence band of g-C₃N₄ by photoexcitation, which provided more electrons for the reduction of O₂ to generate more H₂O₂. Therefore, PI could change H₂O₂ generation from single-channel to two-channel route. Fu et al. were inspired by the behavior of chlorella as a biological H₂O₂ generator, and prepared a living chlorella vulgaris and carbon micro particle (needle coke) co-modified g-C₃N₄ (C-N-g-C₃N₄) photocatalyst [60]. The novel material achieved the simultaneous photocatalytic water splitting and biological H₂O₂ generation with H₂O₂ productivity of 0.98 μmol h⁻¹ (Fig. 8).

Fig. 8

Photocatalytic H₂O₂ formation pathway over g-C₃N₄-PW₁₁ catalyst (a) and its H₂O₂ productivity (b) [22], copyright 2017 Elsevier; photocatalytic H₂O₂ formation pathway over g-C₃N₄-CoWO (c) and its H₂O₂ productivity (d) [59], copyright 2019 Elsevier

2.2.3 Hybridization

Hybridization is a widely accepted approach to obtain an efficient and stable photocatalyst of heterojunction. A hybrid catalyst of g-C₃N₄ and carbon nanotubes (g-C₃N₄-CNTs) with well-defined and stable structure was prepared through an amidation reaction [26]. The CNTs covalent combined with g-C₃N₄ promoted the electrons generation (Fig. 9a). Therefore, the single-electron reduction of O₂ to ·O₂⁻ and the sequential two-step single-electron O₂ reduction reaction was promoted successively (Fig. 9a). The hybrid catalyst obtained 32.6 μmol·h⁻¹ of H₂O₂ productivity in the presence of formic acid under visible light. A Cu₂(OH)PO₄/g-C₃N₄ composite was prepared via the hydrothermal and co-calcination procedures for photocatalytic H₂O₂ production [61]. Cu₂(OH)PO₄ could adsorb O₂ molecules, and formed photogenerated electrons to recombine the holes in g-C₃N₄ through a Z-scheme mechanism. The heterojunction catalyst with 20 wt% of Cu₂(OH)PO₄ obtained 7.2 mM of H₂O₂, which was over 13 times higher than that of pure g-C₃N₄. An interfacial Schottky junction composed of Ti₃C₂ nanosheets and porous g-C₃N₄ nanosheets (TC/pCN) was fabricated via an electrostatic self-assembly route [62]. The formation of Schottky junction and subsequent built-in electric field at their interface accelerated the spatial charge separation and restrain the charge recombination (Fig. 9b). TC/pCN exhibited a high H₂O₂ yield (2.20 μmol L⁻¹ min⁻¹) under visible light irradiation (λ > 420 nm), which is about 2.1 times higher than that of bare g-C₃N₄.

Fig. 9

Schematic diagram of photocatalytic H₂O₂ production over g-C₃N₄-CNTs (a) [26], copyright 2018 Elsevier, and TC/pCN (b) [62], copyright 2019 Elsevier

The g-C₃N₄ based materials have unique structure of tri-s-triazine moieties which can promote selective two-electron transfer to O₂ via sequential formation of a superoxo radical and 1,4-endoperoxide species to facilitates H₂O₂ generation under visible light irradiation [63]. The yield and selectivity of H₂O₂ can be enhanced via the improvements in the surface, oxygen affinity, charge separation, proton-coupled electron transfer, and H₂O₂ decomposition retardation of g-C₃N₄ catalysts. Therefore, the strategies of morphology control, hetero-elements doping, surface modification and hybridization for g-C₃N₄ are expected to be further updated and reported. Scaling up of H₂O₂ production can be achieved by the development of g-C₃N₄ materials and their system engineering.

3 Photocatalytic Production of H₂O₂ over TiO₂ Based Materials

TiO₂ based photocatalysts are one of the preferred families for H₂O₂ production due to their merits of chemical stability, low cost and practical application. Before H₂O₂ production over TiO₂ becomes commercial, it should overcome two difficulties including the side reaction of one electron oxygen reduction, and the serious decomposition of H₂O₂ catalyzed by the intermediate of ≡Ti–OOH. The improved strategies and the categorization for TiO₂ are similar to that for the abovementioned g-C₃N₄ catalysts. The modification strategies, photocatalytic performances and properties of the reported TiO₂ based materials for photocatalytic H₂O₂ production are collected in Table 2.

Table 2 Summary of the modification strategies, photocatalytic performances and properties of the reported TiO₂ based materials for photocatalytic H₂O₂ production

Single TiO₂ catalyst has obtained the satisfied yield of H₂O₂. Cai et al. earlier researched photocatalytic production of H₂O₂ over TiO₂ [64]. In their work, the effect of copper ions on the formation of H₂O₂ was investigated. In an O₂ purged solution, H₂O₂ productivity was increased to 20 times in the presence of moderate amount of copper ions. Shiraishi’s group tried to use benzylic alcohols as hydrogen sources for light driven H₂O₂ production with TiO₂ photocatalyst [30]. They revealed that the enhanced H₂O₂ formation was due to the efficient formation of side-on coordinated per oxo species on the photoactivated TiO₂ surface, via the reaction of benzylic alcohol and O₂. The peroxo species was readily transformed to H₂O₂, thus facilitating highly efficient H₂O₂ production. The band gap photoexcitation of TiO₂ also promoted the selectivity of H₂O₂. In another earlier report, the effect of Zn(II) on the formation of H₂O₂ over TiO₂ was investigated [65]. In this work, the researchers provided the mechanism and kinetic of interfacial electron transfer by blocking surface trapping sites for photogenerated carriers (≡Ti–OH).

There are a number of works on the surface modification strategies for TiO₂ to enhance its H₂O₂ photoproduction. For examples, a Pt/TiO₂ photocatalytic system simultaneously achieved the H₂ and the H₂O₂ production [66]. H₂O₂ productivity reached 5096 μmol g⁻¹ h⁻¹, which was attributed to the more favorable two-electron oxidation of water to H₂O₂ than the four-electron oxidation of water to O₂. A negative charged Pd nanoparticles loaded on TiO₂ photocatalyst was prepared by coordinating Pd with surface-anchored organic ligands [67]. The negative charge on the Pd were induced by the electron donation from amine groups of the ligands. For photocatalytic production of H₂O₂, a mechanism was proposed that the electronic tuning of Pd nanoparticles enhanced the charge separation on TiO₂, which improved the selectivity of O₂ reduction to produce H₂O₂. The improved selectivity for H₂O₂ production was over the side reactions such as O₂ reduction to water (Pathway 1, Fig. 10). O₂ reduction occurred on the Pd surface given the high affinity of O₂ to Pd, generating surface-bound superoxide by the first electron transfer. The selectivity for H₂O₂ production is determined by the subsequent competing coordination reactions: μ-peroxo coordination followed by homolytic O–O bond cleavage (Pathway 1, with water as final product, Fig. 10) vs protonation (Pathway 2, with H₂O₂ as final product, Fig. 10). A porous TiO₂ films supporting Au nano island was exploited as the photocatalyst of light driven H₂O₂ production [68] (Fig. 11a). H₂O₂ concentration over the catalyst achieved the mM scale within 5 min, which was 80-folds based on pure TiO₂ (Fig. 11b). The combination of small Au, TiO₂, and large Au species reduced the potential barriers, and thus reduced the recombination of electron–hole pairs. A CuO incorporated TiO₂ catalyst was earlier prepared for light driven H₂O₂ production [32]. Modification of CuO promoted the charge separation and provided active sites for water reduction. In detail, photoexcited electrons in CB of both TiO₂ and CuO, and the accumulation of excess electrons in CuO caused a negative shift in the Fermi level, which gained the required overvoltage necessary for efficient water reduction reaction. Zheng et al. modified TiO₂ with S and N co-doped graphene quantum dots (SNGQD/TiO₂) for photocatalytic production of H₂O₂ [69]. SNGQD induced the extended visible light absorption and enhanced electron migration. The catalyst exhibited 3.2 times of H₂O₂ yield (451 μmol L⁻¹) as that of bare TiO₂ under simulated sunlight irradiation. The increased H₂O₂ was attributed to the boosted two-electron reduction of oxygen, as well as the suppressed decomposition of H₂O₂. Koutecky–Levich plots and DFT calculations demonstrated that the kinetic rate of ORR was accelerated by GQDs with the facilitated charge transfer, and the two-electron ORR pathway rationalized the high selectivity for H₂O₂ formation. In another work [70], a novel nafion coatings on S,N-codoped graphene-quantum-dots-modified TiO₂ (Nf-SNG/TiO₂) catalyst also presented the enhanced photocatalytic performance of H₂O₂ photoproduction.

Fig. 10

Mechanisms of oxygen reduction on the surface of Pd nanoparticles [67], copyright 2019 American Chemical Society

Fig. 11

Schematic diagram of photocatalytic H₂O₂ production over Au/TiO₂ films (a) [68], copyright 2019 American Chemical Society; time-dependent H₂O₂ production over bare TiO₂ and Au/TiO₂ films (b) [68], copyright 2019 American Chemical Society

Hybridization of carbon materials with TiO₂ based photocatalysts has also been attempted in photocatalytic H₂O₂ production. For examples, a hybrid material of proton-form titania nanotube with carbon dot (HTNT-CD) was exploited for H₂O₂ photoproduction [71]. It was demonstrated that the protons of HTNT-CD were crucial for acceleration of the half reaction of molecular O₂ reduction to form H₂O₂, and hindering the H₂O₂ decomposition. The HTNT-CD hybrid obtained 5.2% of the solar-to-H₂O₂ apparent energy conversion efficiency, which was about 5 times of that over P25 catalyst. In addition, a reduced graphene oxide and TiO₂ (rGO/TiO₂) hybrid system further increased H₂O₂ yield to a mmol scale via the adsorption of phosphate on TiO₂ [31].

TiO₂ based materials have the potential to become mature photocatalysts for visible light-driven H₂O₂ production due to the merits of chemical stability and low cost. However, there are two challenges restricting this application of TiO₂: (1) the dominated inefficient single-electron O₂ reduction; (2) the simultaneous decomposition of H₂O₂ by forming peroxide complexes (≡Ti–OOH) [70]. Therefore, more studies are being concentrated on doping, hybridization or surface decoration for TiO₂ to achieve highly selective two-electron reduction of O₂ and inhibition of photodecomposition of H₂O₂.

4 Photocatalytic Production of H₂O₂ Over Transition Metal Complexes

Metal complexes including the metal–organic frameworks (MOFs) are another family for photocatalytic production of H₂O₂. The modification strategies, photocatalytic performances and properties of the reported transition metal complexes for photocatalytic H₂O₂ production are collected in Table 3.

Table 3 Summary of the modification strategies, photocatalytic performances and properties of the reported transition metal complexes for photocatalytic H₂O₂ production

4.1 Metal–Organic Frameworks

Recently, a MIL-125-NH₂ MOFs material for photocatalytic H₂O₂ production in a benzylalcohol/water two-phase system was exploited and reported [72] (Fig. 12a). Hydrophobization of MOF enabled the spontaneous separation of the benzaldehyde formed to the benzylalcohol phase and of the H₂O₂ formed to the aqueous phase (Fig. 12b). The novel system enhanced the photocatalytic efficiency and adapted various mediums including a solution of low pH for H₂O₂ production. Meanwhile, the same group modified MIL-125-NH₂ MOFs via alkylation of octadecylphosphonic acid (OPA/MIL-125-NH₂) as a photocatalyst for H₂O₂ production [73] (Fig. 12c). The enhanced photocatalytic performance originated from Ti cluster-alkylated hydrophobic property, and the faster diffusion of reactants and products in the maintained pores of the MOFs.

Fig. 12

Photographs of two-phase systems composed of an aqueous phase and a benzylalcohol phase containing MIL-125-NH₂ (left) and MIL-125-Rn (right) (a); photocatalytic H₂O₂ production utilizing the two-phase system (b) [72], copyright 2019 Wiley; structures of linker-alkylated MIL-125-NH₂, MIL-125-R7 (top left), cluster-alkylated MIL-125-NH₂, OPA/MIL-125-NH₂ (top right), and photocatalytic H₂O₂ production over the MOFs system [73], copyright 2019 Royal Society of Chemistry

4.2 Novel Transition Metal Complexes

In 2005, Hayes et al. earlier reported Zn(II)-centered complexes acting as photocatalysts for H₂O₂ production in an ultraviolet irradiated environment [74]. In this work, various ligands of amino-substituted isomers including indazole, pyridine, and phenylenediamine et al. were tested to catalyze the reaction. Among them, Zn-5-aminoindazole obtained the greatest first-day production of 63 mM/day with a 37% quantum yield and p-phenylenediamine (PPAM) showed the greatest long-term stability.

After that, most of the reports on applying the transition metal complexes in photocatalytic H₂O₂ production were from Fukuzumi’s group. For examples, they reported photocatalytic H₂O₂ production over a complex catalyst of 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh⁺–NA) with oxalate or oxalic acid as electron donor [45, 46]. QuPh⁺–NA formed the long-lived electron-transfer state upon the photoexcitation with strong oxidation ability. In an oxygen saturated mixed solution of a buffer and acetonitrile, 14% of quantum yield and 93% of H₂O₂ yield were obtained.[Ru(Me₂phen)₃]²⁺ (Me₂phen = 4,7-dimethyl-1,10-phenanthroline) and Ir(OH)₃ was used as photocatalysts for water oxidation in an O₂-saturated H₂SO₄ aqueous solution [42]. H₂O₂ was produced from the formation of [Ru^III(Me₂phen)₃]³⁺ and ·O²⁻, which resulted from the electron transfer from the excited state of [Ru^II(Me₂phen)₃]²⁺ to O₂. Photocatalytic activity was further improved by replacing Ir(OH)₃ nanoparticles by [Co^III(Cp*)(bpy)(H₂O)]²⁺ in the presence of Sc(NO₃)₃ in water (Fig. 13a). After that, they employed nanoparticles composed of earth abundant nickel and iron (NiFe₂O₄) instead of the Ir complex as a water oxidation catalyst for the photocatalytic production of H₂O₂ [43]. During the reaction, NiFe₂O₄ nanoparticles were formed from the corresponding as-prepared NiFe₂O₄. The H₂O₂ productivity also achieved improvement. They also used cyano-bridged a polynuclear complexes (Fe_xCo_1−x)₃ [Co(CN)₆]₂ as effective catalysts for photocatalytic H₂O₂ production in an O₂-saturated aqueous solution in the presence of [Ru(Me₂phen)₃]²⁺ and Sc(NO₃)₃ under visible light irradiation [40]. Cobalt chlorin derivatives (Co^II(Ch_n) (n = 1–3)) was used as catalyst for investigation on the mechanism of photocatalytic H₂O₂ production [44]. Nonsubstituted cobalt chlorin complex (Co^II(Ch₁)) efficiently and selectively catalyzed two-electron reduction of O₂ by a one-electron reductant (1,1′-dimethylferrocene) to produce H₂O₂ in the presence of perchloric acid (HClO₄) in benzonitrile (Fig. 13b). The change in redox property resulted in the enhancement of the catalytic reactivity, where the observed rate constant (k_obs) value of Co^II(Ch₃) was 36-fold larger than that of Co^II(Ch₁) (Fig. 13b).

Fig. 13

Schematic diagram of photocatalytic H₂O₂ production over [Ru^II(Me₂phen)₃]²⁺-Ir(OH)₃-Sc³⁺ system (a) [42], copyright 2013 Royal Society of Chemistry, and (Co^II(Ch_n) system (b) [44], copyright 2015 American Chemical Society

In addition to the above work, an octahedral Cd₃(C₃N₃S₃)₂ coordination polymer was exploited and enhanced photocatalytic H₂O₂ production from methanol/water solution [36] (Fig. 14a, b). Later, the octahedron Cd₃(C₃N₃S₃)₂ was adhered to the reduced graphene (rGO) (xrGO/Cd₃(TMT)₂) to become a improved photocatalyst for visible light-driven H₂O₂ production [37]. The formation of H₂O₂ was 2.5-folds enhanced and its deformation was concurrently suppressed. The enhanced performance mainly resulted from the accelerated charge transfer process, which was originated from the supreme electrically conductive properties of graphene.

Fig. 14

Structure and SEM image of Cd₃(C₃N₃S₃)₂ coordination polymer (a), and its photocatalytic H₂O₂ production pathway (b) [36], copyright 2015 Nature

Transition metal complexes including MOFs materials possess the attractive features that their structures can be modified and regulated from 2 to 3D to achieve desired properties. The transition metal complexes as novel photocatalysts for visible light-driven H₂O₂ production can provide appropriate bandgaps and one-electron components to efficiently and selectively promote the two-electron reduction of O₂ after modification. The unique systems of transition metal complexes are worthy of in-depth studying.

5 Other Semiconductor Materials

The modification strategies, photocatalytic performances and properties of other semiconductor materials for photocatalytic H₂O₂ production are collected in Table 4.

Table 4 Summary of the modification strategies, photocatalytic performances and properties of other materials for photocatalytic H₂O₂ production

5.1 ZnO Based Materials

ZnO is a type of semiconductor to be earlier reported in the field of photocatalytic H₂O₂ production. In 1988, Hoffmann’s group used ZnO contained illuminated aqueous suspension for photocatalytic H₂O₂ production in the presence of O₂ and organic electron donors [28]. They proposed that H₂O₂ could be produced through reduction of O₂ by CB electrons, and the yield of photogenerated CB electrons could be increased by adding electron donors. Later, they used aqueous suspensions of transparent quantum-sized ZnO semiconductor colloids to produce steady-state concentrations of H₂O₂ as high as 2 mM [75]. The initial rate of H₂O₂ production was 100–1000 times faster with quantum-sized ZnO than that with bulky ZnO.

5.2 Transition Metal Sulfide-Based Materials

CdS is employed to be an efficient catalyst since its relatively high CB edge position is advantageous for O₂ reduction and the subsequent H₂O₂ production. Kim et al. started with silica nanocapsules (SNCs) that host CdS photocatalysts on their shell surfaces to achieve photocatalytic production of H₂O₂ through sensitized triplet–triplet annihilation (TTA) upconversion (UC) of low-energy, sub-bandgap photons. They further loaded a graphene oxide nanodisk (GOND) as a co-catalyst (GOND/CdS–SNC) [35]. The photogenerated electrons were efficiently transferred into GOND to retard rapid charge recombination in CdS, which subsequently reduced dioxygen to produce H₂O₂ up to a 100 mmol level per hour. Later, a CdS-reduced graphene oxide (RGO) hybrid achieved photocatalytic production of H₂O₂ under sunlight from water and O₂ without using organic electron donors [34]. The optimal catalyst showed five times of H₂O₂ production higher than CdS nanoparticles. Photocatalytic reaction was mainly proceeded by two-electron reduction of O₂ rather than water oxidation on the catalyst surface. The CB level of CdS was demonstrated to be more negative than the reduction potential of O₂, which was sufficient for the high selectivity for the two-electron reduction of O₂ (Fig. 15). In addition, the photocatalytic system was suitable to be operated at lower temperature and pH.

Fig. 15

Plausible mechanism for production of H₂O₂ by CdS-G hybrid under sunlight [34], copyright 2017 Elsevier

In a recent work of our group, atomic-scale Au modified MoS₂ nanosheets as a photocatalyst for light driven H₂O₂ production was prepared via a simple pathway including the deposition-reduction and immobilization process [38]. Au modification brought out the low recombination rate of e⁻–h⁺ pairs, long lifetime of electrons and more negative flat band potential for MoS₂. The catalyst achieved efficient photocatalytic production of H₂O₂ from H₂O and air in the absence of pure O₂ and organic electron donors. An optimal catalyst enhanced the H₂O₂ productivity by about 2.5 times based on bare MoS₂. The H₂O₂ productivity at pH 9 was further enhanced by 7.4 times based on that at pH 2 (Fig. 16).

Fig. 16

Photocatalytic H₂O₂ production over atomic-scale Au modified MoS₂ nanosheets under different conditions [38] copyright 2019 Elsevier

In recent years, the application potential of transition metal sulfides gradually emerges in the field of photocatalytic H₂O₂ production. The VB tops of some transition metal sulfides locate at an appropriate range, which provide the strong thermodynamic driving force for water oxidation. In addition, the CB levels of some transition metal sulfides are more negative than the reduction potential of O₂, which provide enough potential for O₂ reduction. Therefore, it is advised that more sorts of transition metal sulfides can be explored in photocatalytic H₂O₂ production systems.

5.3 Bi Containing Semiconductors

BiVO₄ loaded with Au nanoparticles was first reported for photocatalytic production of H₂O₂ in pure water with O₂ [39]. The bottom of the BiVO₄ conduction band was more positive than the one-electron reduction potential of O₂ while more negative than the two-electron reduction potential of O₂ (Fig. 17). Therefore, one-electron reduction of O₂ was suppressed and the selectivity for two-electron reduction of O₂ was promoted, resulting in efficient H₂O₂ formation. In a Later work, plasmonic Bi/Bi₂O_2−xCO₃ with surface oxygen vacancies was synthesized for photocatalytic production of H₂O₂, and the role of in situ generated H₂O₂ for photocatalytic removal of gaseous NO_x was investigated [76]. In-situ introduction of plasmonic Bi on the surface of Bi₂O_2−xCO₃ promoted the generation of H₂O₂ at mM scale by capturing electrons from the defect states of Bi₂O_2−xCO₃ via the two-electron reduction of O₂. The dissociation of H₂O₂ was concluded to be interdicted by the in situ formation of Bi, which suppressed the single electron reduction of H₂O₂ to ·OH and enhanced the selectivity of O₂ reduction to H₂O₂. The presence of oxygen vacancies in Bi/Bi₂O_2−xCO₃ was critical to H₂O₂ production selectivity. The above two works presented the feasibility of photocatalytic production of H₂O₂ over the Bi containing semiconductors.

Fig. 17

Energy diagrams for Au/TiO₂ and Au/BiVO₄ and reduction potential of O₂ [39] copyright 2016 American Chemical Society

5.4 Carbon Materials

Carbon family including graphene nanomaterials are emerging photocatalysts consisting of earth-abundant elements. A carbon dot-impregnated waterborne hyperbranched polyurethane was developed as a heterogeneous photocatalyst for solar driven production of H₂O₂ in the presence of C₂H₅OH and O₂ [77]. The carbon dots possessed a suitable bandgap of 2.98 eV, which facilitated effective splitting of both water and ethanol under solar irradiation. In the system, photoreaction of C₂H₅OH with H₂O around room temperature promoted selective H₂O₂ production. In another report, graphene oxide could efficiently catalyzed photogeneration of H₂O₂ to mmol levels in the absence of electron donors [19]. It was found that the dissolved O₂ contributed to the H₂O₂ generation, and H₂O₂ photoproduction was readily enhanced by raising pH.

6 Summary and Outlook

Nowadays, photocatalytic production of H₂O₂ is becoming a research hotspot, because it exhibits cost-efficient and eco-friendly advantages, and can adapt to the applications of environmental remediation, organic synthesis and fuel cells. This minireview covers most of the advanced catalysts and techniques of H₂O₂ photoproduction, and highlights the advanced modification strategies for semiconductor catalysts to enhance their H₂O₂ productivity. So far, various photocatalyst families have been explored, such as ZnO, g-C₃N₄, TiO₂, metal complexes, metal sulfides, Bi containing semiconductors, and carbon materials et al. Modification strategies are mainly classified as structure/morphology modulation, surface decoration, elements doping, and semiconductors hybridization. The modifications mainly aim at increase in the inner space of semiconductors, activation of the molecular O₂, inhibition of the photocarriers recombination, promotion of the electron transfer, and weakening the photocatalytic H₂O₂ decomposition. Photocatalytic H₂O₂ production technique is developing in a challenging stage, and thus it has huge space to become mature. The technique still faces the issues that the H₂O₂ productivity is expected to be increased into a higher scale, the use of electron donors and pure O₂ should be lowered, the selectivity of two-electron reduction of O₂ requires to be further increased over one- or four-electron reduction of O₂. The fundamental solution of all the issues is to explore more efficient semiconductor catalysts, as well as their synthesis and modification strategies. The key avenues should focus on modulating the electron structure of semiconductors, suppressing the decomposition of as-prepared H₂O₂, and enhancing the activation and utilization of O₂. Moreover, the mechanisms of various photocatalytic H₂O₂ production systems are also necessary to be investigated in depth to design and optimize the photocatalysts with high efficiency. In this regard, researchers may employ high-efficiency characterization methods to study the morphology and photoelectric properties of photocatalysts, and use theory calculation approaches to design and study the photocatalytic H₂O₂ production system. We expect that the photocatalytic H₂O₂ production is now in progress along with more and more novel synthetic strategies of photocatalyst and improved procedures.