Graphene-based photocatalysts for oxygen evolution from water

Abstract

Graphene (GR) has triggered new research fields in material science, due to its unique monolayer structure, remarkably high conductivity, superior electron mobility, extremely high specific surface area and chemical stability. It is considered to be an ideal matrix and electron mediator of semiconductor nanoparticles for environmental and energy applications. In particular, GR-based nanocomposites have attracted significant attention when used as photocatalysts. This review will focus on oxygen evolution from water using GR-semiconductor photocatalytic systems. Recent achievements of effective strategies in the fabrication of GR-based semiconductor photocatalysts for oxygen evolution from water are summarized. Furthermore, the morphology control and composition design of semiconductors on GR sheets are also reviewed in relation to their photocatalytic oxygen generation properties. This review ends with a summary and some perspectives on the major challenges and opportunities in the future research.

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H. Pan

H. Pan received his Master's degree from Donghua University at the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering in 2014. Presently he is a PhD student under the supervision of Professor S. Zhu. His current work focuses on graphene-based functional materials.

S. Zhu

S. Zhu received her PhD degree from Shanghai Jiaotong University in 2001. She is presently a professor at the School of Materials Science and Engineering, State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University. Her current fields of interest are graphene-based functional materials, porous carbon, and biomimetic materials based on biological templates.

1. Introduction

Photocatalytic water splitting is of great importance, due to its efficient conversion of solar energy to chemical energy, and it represents a promising technology to solve the global energy and environmental challenges.^1–5 Since the pioneering work reported by Honda,⁶ the concept of sunlight-induced H₂ or O₂ production has stimulated considerable research efforts, leading to the development of numerous photocatalytic catalysts.⁷ The semiconductor's band gap and electronic band edge positions with respect to water oxidation/reduction potential levels are very crucial in determining the feasibility of solar hydrogen/oxygen production. The ideal band gap of the semiconductors should be around 2.0 eV for the effective utilization of solar energy. Moreover, the bottom of the conduction band (CB) must be located at a more negative potential than the reduction potential (H⁺/H₂), whereas the top of the valence band (VB) must be located at a more positive potential than the oxidation potential (H₂O/O₂).^8–10 Unfortunately, rare semiconductor photocatalysts possess suitable redox potentials and band gaps for both simultaneous water reduction and oxidation. As a result, many studies have focused on half of the reaction by using sacrificial reagents as electron donors or acceptors.¹¹ Generally, hydrogen production from water reduction requires two electrons, whereas oxygen evolution from water oxidation is more challenging, because it requires four holes to generate two oxygen–oxygen bonds for the production of one oxygen molecule.^12–18 For this reason, reported active photocatalysts for oxygen evolution from water, such as WO₃ and BiVO₄,^19–21 are rare. Recently, some new semiconductors, such as BiCu₂VO₆, BiZn₂VO₆, TiN_xO_yF_z, nitrogen-doped CsCa₂Ta₃O₁₀, layered double hydroxides (LDH) and Ag₃PO₄ crystals, have also been investigated.^22–27 Evidently, the oxygen evolution reaction is crucial for renewable energy technologies, including water splitting and fuel cells.²⁸ Unfortunately, oxygen-evolving photocatalysts represent a bottleneck for the development of energy-conversion schemes based on sunlight.²⁹

The principle and main process of photocatalytic oxygen generation from water is schematically shown in Fig. 1. Two key requirements should be considered in the fabrication of photocatalysts for efficient solar oxygen generation from water: one is excellent photo-adsorption ability within the solar spectrum, and the other is a low recombination rate of photogenerated electron–hole pairs.^1,30–33 In terms of the first issue, some semiconductors have been found to possess efficient absorption in the visible-light solar spectrum, which occupies ∼43% of solar radiation energy; these include Bi₂WO₆ and Bi₂MoO₆ with an aurivillius structure, and BiVO₄ with a monoclinic scheelite structure.^34–36 However, most of these semiconductors are not ideal photocatalytic materials, because of their low quantum efficiencies. A great deal of effort has been made toward improving their photocatalytic performance. Coupling with other substances to fabricate semiconductor-based nanocomposites, such as the utilization of co-catalysts, Z-scheme photocatalysis, carbon quantum dots (CQD), and graphene (GR), is often viewed as a feasible route for efficient photocatalysis.^37–40 Among these, semiconductors incorporated with GR have attracted considerable interest due to their unique properties.⁴¹ With a flat monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice, GR possesses remarkably high conductivity, superior electron mobility and an extremely high specific surface area (2630 m² g⁻¹), and can be produced on a large scale at low cost.^42–44 Given the excellent electronic conductivity endowed by its two-dimensional planar p-conjugation structure, GR in composites acts not only as a superior supporting matrix for bonding with functional components, but also as an excellent electron mediator to adjust electron transfer. Thus, it restrains the recombination of photoexcited charges and enhances the efficiency of electron–hole separation.⁴⁵

Fig. 1 The principle and main process of photocatalytic oxygen generation from water.

As is known, semiconductors possessing valence band positions that are more positive than the oxidation potential (H₂O/O₂) are potential photocatalysts for producing oxygen from water (Fig. 2); examples include WO₃, Fe₂O₃, TiO₂, and ZnO.^46,47 When integrated with graphene, the unique characteristics of GR make it more attractive for use in oxygen evolution. Photocatalytic systems, such as BiWO₆/GR, hematite/reduced graphene oxide (RGO), NiTi-LDH/RGO, WO₃/GR, BiVO₄/graphene oxide (GO) and Ag₃PO₄/Ag/AgBr/RGO, have been reported to have greatly improved catalytic properties for oxygen evolution.^48–53 The combination of GR in photocatalysts has two evident advantages: one is to widen light absorption range; the other one is to promote a stable catalytic system. For example, it is known that TiO₂ and ZnO can only be excited by ultraviolet (UV) or near UV radiation, owing to their wide band gaps. Inspiringly, the band gap energy values for these GR-based composites (GR nanosheets–TiO₂ and GR nanosheet–ZnO), were measured to be 1.39 and 1.26 eV, respectively. The narrowed band gap originating from GR makes this type of composite a promising candidate for water splitting under sunlight.⁵⁴ Bai et al. found that the incorporation of GR greatly enhanced the stability of Ag₃PO₄, which is otherwise photocorrosive when used for photocatalytic O₂ production.⁵⁵ In addition, the introduction of GR to WO₃ results in an enhanced surface area and an efficient separation of charges. It is worth mentioning that the excess addition of GR leads to a decrease in catalytic efficiency, because of the shielding effect of black GR on the active sites.⁵⁶

Fig. 2 The conduction band and valence band positions of selected semiconductors (V vs. NHE, pH = 7). Reproduced from ref. 8.

Thus, GR-based semiconductor photocatalysts have attracted extensive attention because of their usefulness in environmental and energy applications. This critical review summarizes the recent progress in the fabrication of GR-based semiconductor photocatalysts for oxygen evolution from water under visible-light irradiation. The morphology control and composition design of semiconductors on GR sheets are reviewed in relation to their photocatalytic oxygen generation properties. The importance of the interface between the semiconductors and GR is highlighted. The simultaneous evolution of H₂ and O₂ from water using GR-based materials is also discussed. This review ends with a summary and some perspectives on the challenges and new directions in this emerging area of research.

2. Synthesis of graphene-based photocatalysts

2.1. Solution mixing and in situ growth

Solution mixing is one of the most widely used methods to fabricate GR-based photocatalysts, with easy operation and satisfactory results.^41,57 To improve the photocatalytic properties of WO₃, Ng et al. fabricated WO₃/GR composites by mixing WO₃ powder with GR oxide (GO).⁵⁸ WO₃ was added to the GO suspension and ultrasonicated for 30 min to produce a WO₃/GO dispersion. The dispersion was then exposed to either UV or visible light irradiation for 3 h to obtain WO₃/RGO composites. Although the morphological features of the WO₃/RGO composites were essentially identical, the WO₃ particle sizes were as large as 100 nm and tended to aggregate. Using the same strategy, WO₃/RGO composites were prepared using polyvinylpyrrolidone (PVP) as an intermediate to combine tungsten with RGO. The mixture of GO and ammonium metatungstate hydrate was ultrasonicated and stirred to obtain the WO₃/GO precursor solution. The resultant WO₃/RGO composite was obtained by calcining the precursors at 450 °C for 5 h in air. As a result, WO₃ nanocrystallites ranging from 20–40 nm were uniformly distributed on the RGO.⁵⁹

In 2014, an in situ growth method was developed for the fabrication of visible-light-driven NiTi-LDH/RGO catalysts by anchoring NiTi-LDH nanosheets onto the surface of RGO, which displayed excellent photocatalytic behaviour for the splitting of water into oxygen.⁵⁰ The monolayer RGO suspension was obtained by sonication in deionized water. Titanium, nickel sources (Ni(NO₃)₂·6H₂O, TiCl₄) and urea were dissolved in the RGO suspension. After stirring at 90 °C, the final precipitate was dried in an oven at 60 °C for 24 h. The NiTi-LDH nanosheets on the surface of RGO were highly dispersed and possessed a plate-like morphology with a lateral diameter of 100–200 nm. Elemental mapping images displayed the uniform and homogeneous distribution of both Ni and Ti.

2.2. Hydrothermal and solvothermal approaches

Apart from the approaches mentioned above, the hydrothermal method is another efficient method for the synthesis of GR-based photocatalysts.^60–62 As compared with the solution mixing method, hydrothermal and solvothermal approaches are more attractive, owing to the controllable morphology of the semiconductor particles. More importantly, the nanoparticles bond well to GR even without the use of intermediates. Meng et al. used a hydrothermal process to prepare α-Fe₂O₃/RGO composites, in which hematite nanoparticles are supported on RGO nanosheets. A proper amount of FeCl₃·6H₂O was mixed with GO, and then the mixture was dissolved in deionized water and sonicated. Then, the prepared solution was mixed with ethanol and placed in a boiling aqueous bath for thermal hydrolysis. The sample collected by centrifuging was heated in air at 350 °C for 2 h and then in pure nitrogen at 800 °C for 15 min. The crystalline size of monolithic α-Fe₂O₃ (63.8 nm) was larger than that of α-Fe₂O₃ (41.4 nm) grown on the RGO sheets. The TEM image showed single-crystalline structure features of the α-Fe₂O₃ particles on the RGO sheets. The Fe₂O₃/RGO composite showed an enhanced photocatalytic activity toward water oxidation compared with pristine α-Fe₂O₃ nanoparticles.⁴⁹ Similarly, Niu et al. prepared RGO-cuprous oxide (Cu₂O/RGO) photocatalysts using the same method. Well monodispersed cube-like Cu₂O particles (300–500 nm) were observed to precipitate on the RGO layers.⁶³ Moreover, RGO/TiO₂ microspheres were produced by the non-hydrolytic sol–gel reaction of tetrabutyl orthotitanate (TBOT) and acetone followed by hydrothermal treatment.⁶⁴ The pretreated TiO₂ was prepared by adding TBOT to excess acetone. TiO₂/RGO microspheres were prepared by hydrothermal treatment with temperatures varying from 120 to 180 °C. During this process, ammonia was used as a solution medium. It was found that the pre-treated TiO₂ was smooth and spherical in shape with particle diameters in the range between 1 and 2 μm. After treatment at 120 °C, microspheres with aggregates of nanoflakes on their surface were produced. When the hydrothermal treatment temperature was increased from 150 to 160 °C, rough microspheres with aggregates of nanorods were obtained. More recently, a solvothermal method was developed for the fabrication of Bi₂WO₆/RGO (BWO/RGO) composites. A mixture of GO, Bi(NO₃)₃·5H₂O and Na₂WO₄·2H₂O was sealed into a Teflon-lined autoclave and then maintained at 180 °C. Finally, Bi₂WO₆ nanoparticles deposited on RGO sheets were produced, which showed some wrinkles. Evidently, the solvothermal approach is widely used in the fabrication of these GR-based photocatalysts. Unfortunately, most BWO particles were as large as a few hundred nanometers, and their dispersion on GO sheets were not uniform, owing to the difficulty in controlling the composition and phase of the complex ternary compounds.⁶⁵

2.3. Sonochemical methods

As mentioned above, particles more than several hundred nanometers in size are obtained on GR sheets by using the in situ growth method. Two key issues must be considered for the design and fabrication of GR-based photocatalysts: (i) particle size control and (ii) the interface between the catalyst and GR. Controlling the particle size and improving the interaction between the semiconductor and GR is a challenge. In our previous work, ultrasonic waves proved to be effective in solving this problem.^66–68 Using ultrasonication, we succeeded in a controlled incorporation of TiO₂ nanoparticles onto GR layers homogeneously in a few hours. The average size of the nanoparticles was controlled at around 4–5 nm on the sheets without using any surfactant, which is attributed to pyrolysis and the condensation of the dissolved TiCl₄ into TiO₂ by the ultrasonic waves. The uniform dispersion of TiO₂ nanoparticles on both the GR surface and the interlayers was confirmed by SEM and TEM images. The results suggest that ultrasound is very effective in dispersing TiO₂ nanoparticles on GR layers.⁶⁸

Inspired by the effective ultrasonic waves, we reported the synthesis of a composite (WO₃/GR) consisting of WO₃ nanoparticles and GR sheets using a sonochemical method. The average particle size of the WO₃ was controlled at around 12 nm on the GR sheets without using any surfactant. The composite consisted of nano-WO₃ particles, and the two-dimensional GR sheets are a promising photocatalyst for oxygen production. When used as photocatalyst for water splitting, the amount of evolved O₂ from WO₃/GR with 40 wt% GR inside was considerably higher than that from pure WO₃ and mixed-WO₃/GR, 1.8 times and 2 times as much as that from mixed-WO₃/GR (ca. 214 mmol L⁻¹) and pure WO₃ (ca.186 mmol L⁻¹), respectively. The improved performance is due to the synergistic effects of chemically bonded WO₃ and GR. The sensitization of WO₃ by GR enhanced the visible light absorption properties of WO₃/GR. Moreover, the chemical bonding between WO₃ and GR reduced the recombination of the photo-generated electron–hole pairs, leading to improved photoconversion efficiency. This simple strategy opens up a new method to design more optimized systems for photodissociating water under visible light. The same process has been extended to the fabrication of BWO on the surface of GR sheets. As is expected, the combination of the functionality of BWO with the unique properties of GR results in improved performance in the production of O₂ from water.⁵¹

In addition, some other approaches have been developed to construct GR-based photocatalysts, such as layer-by-layer assembly, template-assisted approaches, and photoassisted methods.^69–73 Usually, the GR-based photocatalysts are prepared by a combination of the abovementioned methods, rather than by an individual method.

3. Morphology control

As is well known, the morphology of semiconductors on GO plays a key role in their photocatalytic performance. In our previous work, BWO nanoneedles were fabricated on the surface of GR sheets by a facile sonochemical method, followed by calcination.⁷⁴ The reduction of GO plays an important role in the fabrication of nanoneedles (Fig. 3a and b) instead of nanoparticles on GR sheets. The oxygen-containing groups show a strong influence on the morphology control of BWO on the surface of GR sheets. As compared with GO, RGO sheets have lower quantities of oxygen-containing groups, providing fewer sites for both the physisorption of Bi ions and the deposition of BWO on the sheets, and this leads to the formation of BWO nanoneedles on the sheets after calcination (Fig. 3c). The nanoneedles have a cross-section area of ∼450 nm at the bottom and a length of 2.5 μm, and they form on the surface of the GR sheets and disperse homogeneously. When used as a photocatalyst for oxygen production, the oriented BWO nanoneedles grown on GR sheets produce 188.9 μmol L⁻¹ oxygen, which is higher than that of particulate BWO on GR (164.8 μmol L⁻¹). The high photocatalytic property is mainly attributed to the improved contact area of oriented BWO on GR sheets, resulting in increased active sites.⁷⁵ Similar results were obtained in photocatalytic systems of TiO₂ nanowires/GR and TiO₂ particles/GR.

Fig. 3 (a and b) The morphology of GO-BWO. (c) Proposed mechanism of the formation of BWO-T nanoneedles on GR sheets. (d and e) The photocatalytic O₂ (d) and H₂ (e) creation activities of Bi₂WO₆ powder (BWO-T), the physically mixed Bi₂WO₆ and the reduced GO (BWO-T/GR). Reproduced from ref. 66.

It is widely recognized that a photocatalyst with high photocatalytic activity requires both high crystallinity and a large surface area to reduce recombination of the photogenerated electron–hole pairs and to increase the density of active surface sites, as well as to enhance the light harvesting. On the basis of the abovementioned consideration, exploring mesoporous materials as photocatalysts might be a significant subject because they not only possess the merits of both high crystallinity and large surface area, but also contain continuous pore channels to provide a short distance for photogenerated charge carrier transfer within the mesoporous framework. In 2013, Huang et al. reported a novel photocatalytic system by hybridizing GR with a mesoporous semiconductor for solar energy conversion.⁴⁷ The composite is composed of highly-ordered mesoporous WO₃ (m-WO₃) and RGO. It was found that the obtained m-WO₃/RGO composite had a greater surface area and pore volume than m-WO₃. Furthermore, the specific surface area and pore volume increased with increasing amounts of RGO. Under visible light irradiation, the amount of oxygen evolving from the optimized photocatalyst (ca. 6 wt% RGO) reached 437.3 μmol g⁻¹ in 5 h when KIO₃ was used as an electron acceptor. In addition to the contribution of RGO, this superior photocatalytic activity could be ascribed to the mesoporous structure of m-WO₃, which provides a large surface area and ordered meso-channels, thus more active sites could be achieved and charges could be efficiently transferred. A similar phenomenon was observed in C₃N₄/GR composites.⁷⁶

4. Composition design

4.1. Nitrogen doping

GO is a p-doped material, because oxygen atoms are more electronegative than carbon atoms.⁷⁷ However, GO itself shows very small catalytic activity toward oxygen evolution from water. Doping with heteroatoms, such as nitrogen (N), has been reported to alter the electrical properties, which can induce a charge rearrangement on the GR sheets and enhance their catalytic activity.⁷⁸ Li et al. reported the synthesis of N-doped GR (GN) by nitrogen plasma treatment of GR, and found that GN exhibited high electrocatalytic activity for the reduction of hydrogen peroxide and fast direct electron transfer kinetics for glucose oxidase.⁷⁹

More recently, Jing et al. prepared GR doped with different amounts of N through a one-pot ammonia-modified hydrothermal process, and then successfully coupled the doped GR with nanocrystalline α-Fe₂O₃ by a common wet-chemical method.⁸⁰ The increased amount of doped quaternary-type N was quite favourable for photogenerated charge transfer and transportation. As a result, the photogenerated charge separation of the resulting GN–Fe₂O₃ nanocomposite was greatly promoted. This was responsible for the visibly improved activities of α-Fe₂O₃ in photoelectrochemical water oxidation to produce O₂. Interestingly, it was suggested for the first time that an increased amount of doped quaternary-type N would be very favourable for photogenerated charge transfer and transportation and for O₂ adsorption, further leading to greatly increased charge separation in the resulting GN–Fe₂O₃ nanocomposite. On the basis of the abovementioned results, it is reasonable to conclude that the photogenerated charge separation of α-Fe₂O₃ could be enhanced after coupling with a certain ratio of GR, particularly with that doped with an appropriate amount of N species.⁸¹

A similar result was found in the synthesis of GR modified with Fe and N (Fe–N–GR) by a rapid heat treatment process.⁸² N atoms were doped into the graphene planes and GO was completely reduced during the heat treatment. Fe atoms in the composite are supported by the doped N atoms via coordination bonds. The oxygen reduction reaction for the Fe–N–GR catalyst had an onset-potential of 850 mV vs. RHE (pH = 0.33).

4.2. Multicomponent synergism

A new visible light photocatalyst for O₂ evolution from water, Ag₃PO₄, is unstable upon photo-illumination, which is easily corroded by the photogenerated electrons.²⁶ Compared with bare Ag₃PO₄, incorporating Ag₃PO₄ with GO not only improves its photocatalytic activity, but also enhances its stability during the photocatalytic process.⁸³ As is known, plasmon-mediated photocatalysis has recently become a rising research star in the harvesting and conversion of solar energy, because photocatalysts containing semiconductors and plasmonic nanostructures of noble metals can enhance photocatalytic activity primarily by extending the optical absorption region and enhancing the concentration of charge carriers through an excitation of surface plasmon resonance (SPR).⁸⁴ In 2012, an Ag₃PO₄/Ag/AgBr/RGO hybrid composite with high visible light photocatalytic O₂-production activity was successfully prepared by a photoassisted deposition–precipitation strategy, followed by facile hydrothermal treatment.⁵³ Under irradiation with >420 nm light, the effectiveness of these processes manifests itself in a 1.3 times higher O₂ evolution yield of Ag₃PO₄/Ag/AgBr/RGO as compared with that of Ag₃PO₄/Ag/AgBr and a 2 times higher yield compared with that of bare Ag₃PO₄ (Fig. 4a). The photocurrent traces show a rapid response both at the start and at the end of illumination, and an improved photocurrent density of the Ag₃PO₄/Ag/AgBr/RGO hybrid over all other composites (Fig. 4b). It is considered that the addition of Ag/AgBr caused conduction band depletion and lowered the valence band of Ag₃PO₄; RGO supports this effect in a synergistic manner through delocalization of the transferred charge. Moreover, RGO provides significant parasitic absorption that partially counters the observed increase in efficiency. The proposed mechanism is shown in Fig. 4c. The as-synthesized N-doped Ag₃PO₄ nanoparticles are denuded of the majority of charge carriers by transfer to the Ag nanoparticles, eliminating the availability of the extra conduction-band electrons for recombination with the photogenerated holes (resulting in increased hole availability for water oxidation). This leads to the pinning of the Ag₃PO₄ conduction band at the silver Fermi level, shifting the Ag₃PO₄ valence band edge downward and rendering the photogenerated holes more active in water oxidation. Charge transfer to the silver creates a substantial negative charge on the very small metal nanoparticles, limiting their beneficial effect on the photocatalyst. Charging of the nanoparticles can be reduced by distribution of the charge onto RGO sheets, further lowering the Ag₃PO₄ valence band position.

Fig. 4 The photocatalytic performance of oxygen evolution and the photocatalytic mechanism model. (a) Photocatalytic O₂ production under visible light irradiation (λ > 420 nm) from a 0.05 M aqueous AgNO₃ solution over bare Ag/AgBr, Ag₃PO₄, Ag₃PO₄/RGO, Ag₃PO₄/Ag/AgBr, and Ag₃PO₄/Ag/AgBr/RGO (values 0, 38, 43, 48, 76 from bottom to top, unit μmol h⁻¹). (b) Transient photocurrent responses of electrodes functionalized with the Ag₃PO₄-based materials in the same order (bottom to top) as in panel (a). Measurements proceeded in a 0.01 M Na₂SO₄ aqueous solution under visible light irradiation (λ > 420 nm, I₀ = 64 mW cm⁻²) at 0.5 V bias vs. SCE. (c) Model of the synergistic increase of photocatalytic activity of Ag₃PO₄ upon functionalization with Ag/AgBr and RGO. Reprinted with permission from ref. 77. Copyright 2012 American Chemical Society.

Further study was reported by Tian for the fabrication of CoPi nanoplates integrated with GO by photochemical deposition from an aqueous solution under visible-light illumination. Compared with GO, the CoPi/GO composites exhibited a 3.6-fold enhancement in the photocurrent. The photocurrent of the CoPi/GO-modified electrode after 2 h of illumination remained similar without any significant sign of catalyst dissolution or degradation. The GO not only serves as a substrate for CoPi growth, but also increases charge transfer in the composites.⁷³

In addition, some metal oxides have been employed as co-catalysts to build novel semiconductor composites for photocatalysis, such as hausmannite (Mn₃O₄), a mixed valence manganese oxide. In 2014, Yin et al. reported the fabrication of a nanocomposite consisting of α-Fe₂O₃, Mn₃O₄ and RGO for photocatalytic water oxidation to produce oxygen.⁸⁵ The α-Fe₂O₃/Mn₃O₄ hybrid prepared by modifying α-Fe₂O₃ with Mn₃O₄ nanoparticles was proved to anchor well on the RGO sheets of the nanocomposite, resulting in superior interfacial contacts between the hybrid and RGO. The typical p-type semiconductor Mn₃O₄ in the nanocomposite forms a heterojunction with α-Fe₂O₃, enhancing the charge transfer. In addition, Mn₃O₄ also acts as a noble-metal free co-catalyst, reducing the oxygen evolution overpotential of hematite. RGO in the nanocomposite serves not only as a superior supporting matrix for anchoring the semiconductor nanoparticles, but also as an excellent electron mediator to adjust the electron transfer. As a result, the nanocomposite exhibited remarkably enhanced photocatalytic activity toward water oxidation compared with bare hematite or α-Fe₂O₃/Mn₃O₄ under UV-vis light irradiation. The quantum efficiency of the optimized photocatalyst reached up to 4.35% at 365 nm.

4.3. Core/shell heterojunction structure

In addition to N-doping and multicomponent synergism systems, core/shell geometry is another intriguing architecture that can improve the efficiency of energy conversion. With α-Fe₂O₃ nanorod cores, GR interlayers, and BiV_1−xMo_xO₄ shells, a core/shell heterojunction array was fabricated by Hou et al. for photoelectrochemical water splitting,⁸⁶ as shown in Fig. 5a. The heterojunction yielded a pronounced photocurrent density of ∼1.97 mA cm⁻² at 1.0 V vs. Ag/AgCl, and a high photoconversion efficiency of ∼0.53% at −0.04 V vs. Ag/AgCl under the irradiation of a Xe lamp (Fig. 5b). The unique core/shell architecture enhances the light absorption due to the “window effect”⁸⁷ behaviour between the α-Fe₂O₃ core and BiV_1−xMo_xO₄ shell, and improves the separation of photogenerated carriers at the α-Fe₂O₃/GR/BiV_1−xMo_xO₄ interface.

Fig. 5 (a) Proposed mechanism of photoelectrochemical water splitting; (b) variation of photocurrent density vs. bias potential (left), and photoconversion efficiency as a function of applied potential (right). Reprinted with permission from ref. 68. Copyright 2012 American Chemical Society.

5. Interface between semiconductors and graphene

It has been reported that the transfer of photoexcited electrons from TiO₂ to nanocarbons, such as carbon nanotubes or GR, hinders the recombination process, thereby enhancing the oxidative reactivity.^88,89 For this reason, GO–TiO₂ composites exhibit excellent photochemical responses under visible light (>400 nm) irradiation. It was found that the unpaired p electrons on GO can bond with the surface Ti atoms of TiO₂ to form Ti–O–C bonds and extend the light absorption range of TiO₂.^90–92 Further studies indicate that the interaction between GR and TiO₂ can significantly affect the interfacial electron transfer properties, which is a key issue for photocatalytic activity.

A similar phenomenon has been observed in a Co₃O₄/N-doped RGO (Co₃O₄/N-RGO) composite synthesized with chemical bonding via a facile hydrothermal method.⁹³ X-ray absorption near edge structure (XANES) measurements were conducted to explore the interaction between GO and Co₃O₄. It was found that the Co₃O₄/N-RGO hybrid exhibited an increase of carbon K-edge peak intensity at ∼288 eV compared to pure N-RGO, corresponding to carbon atoms in GR attached to oxygen or other species. This result indicated the possible existence of Co–O–C or Co–N–C in the hybrid. Oxygen K-edge XANES and Co L-edge XANES measurements also suggest the same chemical bonding between the two components. Liang's report demonstrated that chemical bonding between GR and a semiconductor could be achieved through a hydrothermal synthesis, which is relatively simple. Chemical bonding in GR-based photocatalysts could also be achieved through a simple sonochemical method, demonstrated by our previous fabrication of WO₃/GR used for photocatalytic oxygen evolution from water.⁵¹ The bonding between WO₃ and GR minimizes the interface defects, reducing the recombination of photo-generated charges, responsible for the enhanced O₂ evolution.

Because the bridge facilitates charge transfer and can be engineered through relatively simple methods, including hydrothermal or sonochemical synthesis, this kind of chemical bonding or other intimate interface is generally being considered for GR-based photocatalyst architectures.

6. Simultaneous H₂ and O₂ evolution from water using graphene-based materials

As known, semiconductors with large band gaps or improper band location cannot produce O₂ or H₂ effectively. In fact, semiconductor photocatalysts possessing suitable redox potentials and band gaps for both simultaneous water reduction and oxidation are rare. There are many reports about O₂ production from half reaction systems containing sacrificial reagents, which are generally considered to not accurately reflect their photocatalytic ability. In this part, GR-based photocatalytic systems that simultaneously produce O₂ and H₂ are introduced in detail.

6.1. Graphene oxide photocatalytic systems

The electronic properties of GO are related to the composition of oxygen bonding on the GR sheets.⁹⁴ The high electronegativity of the oxygen atoms on carbon sheets causes the charge flow that exerts p-type semiconductivity to GO.^95,96 As oxygen bonds to GR, the valence band changes from the π-orbital of GR to the O 2p orbital, leading to a larger band gap for a higher oxidation level of GO. Introducing more oxygen enlarges the band gap, and the valence band maximum (VBM) gradually changes from the p orbital of GR to the 2p orbital of oxygen; the p* orbital remains as the conduction band minimum (CBM). Yeh et al. researched the electronic band energy levels of GO specimens with various oxidation levels using electrochemical methods.⁹⁷ The results reflect that with sufficient oxidation, the electronic structure of GO is suitable for both the reduction and oxidation of water under illumination, and the production of H₂ and O₂ gases in the presence of sacrificial reagents. They found that the downward shift in the valence band edge was predominantly responsible for the enlargement of the band gap in the GO sheets. During the photocatalytic reaction, the mutual reduction between GO sheets narrows the band gap, leading to an activity decay of GO in catalysing O₂ evolution from an AgNO₃ solution because of the upward shift of the valence band edge, whereas the activity for H₂ evolution from a methanol solution remains unchanged. Strong evolution of O₂ from a NaIO₃ solution under illumination was observed, probably due to a more effective GO dispersion to suppress mutual reduction.

6.2. Z-Scheme system

Further studies have found that the combination of GO with a Z-scheme system makes it an effective mediator to produce H₂ and O₂ via overall water splitting.⁵² GO was used as a solid-state redox mediator, and BiVO₄ was used as an O₂-generating photocatalyst. The GO–BiVO₄ composite was mixed with a H₂-generating Ru–SrTiO₃–Rh photocatalyst in an aqueous solution with a pH value of 3.5 to enable inter-particulate contact. Under visible light irradiation, the photogenerated electrons transferred from the conduction band of BiVO₄ to GO for accumulation. When Ru–SrTiO₃–Rh came in contact with GO–BiVO₄, the accumulated electrons on GO transferred to Ru–SrTiO₃–Rh and combined with the photogenerated holes. The remaining electrons on Ru–SrTiO₃–Rh and the holes on BiVO₄ subsequently reacted with water, forming H₂ and O₂, respectively. The evolution of H₂ and O₂ in a stoichiometric ratio without decay indicated the occurrence of Z-scheme overall water splitting. The minimum turnover number (TON), which was calculated as the number of moles of reactive electrons per mole of GO, was 3.2 over 24 h (Fig. 6a). The result suggested that photoreduced GO (PRGO) is stable as a solid electron mediator, and the Z-scheme system splits water photocatalytically. The mechanism of water splitting in a Z-scheme photocatalysis system consisting of Ru–SrTiO₃–Rh and PRGO/BiVO₄ is shown in Fig. 6b.

Fig. 6 (a) Overall water splitting under visible-light irradiation by the (Ru–SrTiO₃–Rh)–(PRGO/BiVO₄) system. (b) Schematic image of a suspension of Ru–SrTiO₃ and PRGO/BiVO₄ in water (top); mechanism of water splitting in a Z-scheme photocatalysis system consisting of Ru–SrTiO₃–Rh and PRGO/BiVO₄ under visible-light irradiation (down). Reprinted with permission from ref. 89. Copyright 2011 American Chemical Society.

6.3. Graphene oxide quantum dots

Compared with 2D GR, graphene quantum dots (GQDs), a type of C-dot, exhibit new phenomena due to quantum confinement and edge effects.⁹⁸ GQDs are advantageous because their band gaps can be tuned from 0 eV to that of benzene by varying their sizes.⁹⁹ The incorporation of zero dimension GQDs can extend the photo-response of a photocatalyst to the visible-light range.¹⁰⁰ The combination of GQDs with photocatalysts, such as CdS-modified TiO₂ nanotube arrays (TNAs) and ZnO nanowires, has been reported for photoelectrochemical water splitting.^101,102 Very recently, Yeh et al. reported N-doped GO-quantum dots (NGO-QDs) as photocatalysts for overall water-splitting under visible light illumination.¹⁰³ The NGO-QDs exhibited both p- and n-type conductivities. Visible light (>420 nm) illumination of the NGO-QDs resulted in simultaneous H₂ and O₂ production from pure water at an H₂ : O₂ molar ratio of 2 : 1 (Fig. 7a). Fig. 7b shows the time courses of H₂ production over the GO-QD photocatalysts, and no O₂ evolution was observed. Moreover, the evolution of O₂ over NH₃–NGO-QDs also cannot observe H₂ evolution (Fig. 7c). The results shown in Fig. 7 (b and c) proved that the p- and n-domains were responsible for the evolution of H₂ and O₂, respectively. The authors suggested a p–n type photochemical diode configuration (Fig. 7d), which resulted in an energetic band bending existing at the interface between the semiconductor and the solution. The p–n type photochemical diode configuration provided a favourable situation to achieve vectorial charge displacement for overall water-splitting.

Fig. 7 (a) Time courses of H₂ and O₂ evolution over 1.2 g NGO-QDs suspended in 200 mL of pure water under visible-light (420 nm < λ < 800 nm) irradiation. (b)Time courses of H₂ evolution over 1.2 g GO-QDs under the same conditions as (a). (c) Time courses of O₂ evolution over 1.2 g NH₃–NGO-QDs under the same conditions as (a). (d) The configuration and energy diagram for the NGO-QD photochemical diode. Reprinted with permission from ref. 95. Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

7. Summary and perspective

In summary, GR-based semiconductor photocatalysts for oxygen evolution from water are attracting considerable attention, as they are crucial for renewable energy technologies. Considerable approaches have been developed for the fabrication of these photocatalysts. The morphologies of the semiconductors on GO greatly influence their photocatalytic performance. Composition design is an effective method to enhance the photocatalytic properties, including nitrogen doping, multicomponent synergism systems and core/shell heterojunction structure. With appropriate tuning of the CBM and VBM positions, these composites are suitable for H₂ and O₂ generation.

There are several challenges facing research on GO-based photocatalytic O₂ evolution from water. First, theoretical electronic-structure calculations and experimental identification efforts are required. The dependence of the photocatalytic properties on the particle size and layer number of GO should be explored, as they determine the charge transport mechanism in the GR-based composites.

Second, exploration and identification of the interfacial contact and bonding between RGO and the semiconductors in the composites is needed.¹⁰⁴ The interaction of GR and the semiconductor affects the morphology of the loaded particles, which exerts a strong influence on the photocatalytic performance of the resultant materials. On the other hand, the interface determines the efficiency of the electron–hole separation. To date, only a few techniques have succeeded in directly characterizing the interaction of GR and nanoparticles. Atomic resolution scanning transmission electron microscopy (STEM) and surface-enhanced Raman scattering (SERS) spectra may be the most potential techniques for determining the interaction of GR with nanoparticles, although these techniques only have been applied in metal–GR systems.^105,106

Third, studies on the preparation of a ternary hybrid as a photocatalyst for visible-light-driven water oxidation have been seldom reported,^107,108 particularly, for the design of ternary hybrids with non-metal oxide, such as polymers. It has been reported that PANI–GR–TiO₂, a ternary hybrid, shows remarkable photocatalytic activity and photostability for visible-light photoelectrocatalytic water oxidation.¹⁰⁸ This rationally designed ternary hybrid possesses unique advantages over traditional photocatalysts towards water oxidation.

The final challenge is to design a structure for overall water splitting with simultaneous evolution of H₂ and O₂. Achieving this goal may require further exploiting GO by chemically modifying it; for example, the development of new nanostructured GO composites that efficiently transport charge inside the composites and inject charges for reactions at the water–composite interface. Ultimately, a cheap, reliable and highly efficient photocatalyst for oxygen evolution still needs to be developed, considering its critical role in the development of high performance energy conversion and storage devices.

Acknowledgements

The authors gratefully acknowledge the financial support of the National Science Foundation of China (no. 51072117) and Shanghai Science and Technology Committee (no. 13JC1403300). We also thank the Shanghai Jiao Tong University (SJTU) Instrument Analysis Center for the measurements.

Notes and references

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