A review of TiO2 nanostructured catalysts for sustainable H2 generation

Hydrogen is an attractive alternative to fossil fuels that addresses several environmental and energy shortage issues. Nano-sized TiO₂-based photocatalysts with unique structural and functional properties are the most extensively studied photocatalytic nanomaterials for hydrogen production and pollutant degradation. However, titania is hampered by a wide band gap, low utilization of solar light and a rapid recombination of electron/hole pairs. These issues limit its photocatalytic performance. In this review, we present the latest developments in the fabrication of different higher dimensional TiO₂ nanostructured materials that aim to address these inherent limitations to an otherwise very promising material. Specifically, we will look into critical engineering strategies to enlarge the active surface area, enhance visible light absorption and suppress the recombination of electrons/holes that benefit their photo/photoelectric-catalytic water splitting activity. Finally, the current challenges and perspectives for TiO₂ nanostructures are also discussed. Continuous efforts are necessary to endow TiO₂-based materials with novel advanced functionality and commercialization potential in the coming years.     

SiI2 monolayer as a promising photocatalyst for water splitting hydrogen production under the irradiation of solar light

The feasibility of SiI₂ monolayer as the candidate for photocatalytic water splitting for hydrogen generation under the irradiation of the solar light is explored. The geometrical structure, the electronic and optical properties, the mobility of carrier and strain engineering of the monolayer are investigated based on the first-principles calculations. The results demonstrate SiI₂ monolayer possesses an indirect gap of 2.33 eV (HSE06), and both the band edge and the bandgap match the redox potential conditions of the water splitting for hydrogen generation. There is an obvious optical absorption in the visible light and near-ultraviolet region and can be enhanced by the compressive strain. Moreover, the mobility of the electron is significantly different from that of the hole, implicating that the effective spatial charge separation is expectable and the ratio of the recombination of the photogenerated charge pairs is low. The primary adsorption site of the water molecule is identified. The Gibbs free energy and the adsorption energies are calculated to demonstrate the H₂ generation from the water molecule splitting on the monolayer. All the considered properties support that SiI₂ monolayer can be achieved as a promising candidate for the photocatalytic water splitting for hydrogen production under the irradiation of the solar light.     

A first-principles study of electronic structure and photocatalytic performance of two-dimensional van der Waals MTe2–As (M = Mo,W) heterostructures

To efficiently produce green energy and to overcome energy crises and environmental issues, photocatalytic water splitting has become the core heart of recent research. Fabricating heterostructures with type-II band alignment can enhance the photocatalytic activity. By first-principles computations, we study Mo(W)Te₂–As van der Waals (vdW) heterostructures as promising photocatalysts for overall water splitting. The bandgap, band edge position and optical properties can be modified by biaxial strain. With appropriate compressive strain of 2% and 3%, the WTe₂–As heterostructures show transition from type-I to type-II band alignment, which could slow down electron-hole pair recombination. Compared with Mo(W)Te₂ and As monolayers, the band edge of Mo(W)Te₂–As heterostructures is on favorable positions for straddling the water redox potentials. Moreover, Mo(W)Te₂–As heterostructures fascinatingly show strong absorption peaks in both visible and near ultra-violet region, making them promising candidates for overall water splitting photocatalysts.     

Two dimensional MoSSe/BSe vdW heterostructures as potential photocatalysts for water splitting with high carrier mobilities

Production of hydrogen fuel from water splitting driven by solar energy is an effective technique to overcome the energy crisis and environmental problems in coming decades. To explore low cost and efficient photocatalysts is highly desired. In this work, we study the electronic, optical and photocatalytic properties of MoSSe/BSe (Model-1 and Model-2) vdW heterostructures by PBE and HSE06 functionals using first-principle calculations. The stabilities of these heterostructures are confirmed through phonon spectra and ab initio molecular dynamic simulations. The Model-1 and Model-2 heterostructures have indirect band gaps of 1.95 and 1.54 eV respectively by HSE06 hybrid functional. Interestingly, the transition from indirect to direct band gap occurs in Model-1 after including spin-orbit coupling effect. Remarkably, the high carrier mobilities are quantitatively explored by means of deformation potential theory. Furthermore, the transition from type-I to type-II band alignment happens at compressive strain in both Model-1 and Model-2, which effectively slows down the recombination of electron-hole pairs. Compared to the isolated monolayers, the MoSSe/BSe (Model-1 and Model-2) heterostructures harvest maximum portion of visible spectrum, revealing the outstanding paybacks of high efficiency utilization of solar spectrum. Most intriguingly, the band edges of MoSSe/BSe vdW heterostructures meet the redox potential requirements for water splitting. Our results will be valuable for easing the investigation and applications of MoSSe/BSe heterostructures for optoelectronics and photocatalytic water splitting.     

Free-standing CuS–ZnS decorated carbon nanotube films as immobilized photocatalysts for hydrogen production

Free-standing carbon nanotube films (CNTF) with entangled carbon nanotubes (CNT) were used as conductive supports for the preparation of CuS–ZnS/CNTF composite as immobilized photocatalysts for H₂ production. The surface morphology, crystalline property, surface chemistry, and optical properties of the CuS–ZnS/CNTF photocatalysts were investigated. The effects of forming CuS–ZnS heterojunction and conductive CNTF on the separation of photogenerated charges and photocatalytic hydrogen production activity of CuS–ZnS/CNTF photocatalysts were evaluated by the photocatalytic hydrogen production tests. Conductive CNT films can prevent the recombination of photogenerated electron–hole pairs. The deposition of CuS nanoparticles on the ZnS/CNTF leads to higher photocatalytic activity which can be attributed to the effective electron–hole separation. Introducing ZnS and CuS makes the photocatalyst surface more hydrophilic. The porous structure contributed to the effective contact between the sacrificing agents and the photocatalysts, leading to enhanced H₂ production activity.     

Photophysical and photocatalytic water splitting performance of stibiotantalite type-structure compounds,SbMO4 (M = Nb,Ta)

Metal oxides with ferroelectric properties are considered to be a new family of efficient photocatalysts. Here, we investigate stibiotantalite type-structure compounds, SbMO₄ (M = Nb, Ta), with layered crystal structures, and ferroelectric properties as photocatalysts for hydrogen generation from the splitting of pure water. Both compounds were prepared by a conventional solid-state reaction method, and their optical properties, electronic band structure, and photocatalytic water splitting performance were characterized and evaluated. Diffuse reflectance analysis showed that both compounds have moderate band gaps of 3.7 eV for SbTaO₄ and 3.1 eV for SbNbO₄ (cf. 3.0 eV for TiO₂). Mott–Schottky analysis reveals that their conduction-band edge potentials are higher than the water reduction (hydrogen evolution) potential (0 V vs. RHE), indicating both compounds can generate hydrogen from water splitting. The photocatalytic water splitting performance was conducted by using pure water and UV-light irradiation, and photocatalytic H₂ production was confirmed for both compounds. After loading RuO₂ cocatalyst, the rates of hydrogen evolution of SbNbO₄ and SbTaO₄ were 24 μmol/g h and 58 μmol/g h, respectively. It was concluded that both compounds can be used as photocatalysts for water splitting under UV irradiation. The photocatalytic activity difference in both compounds was discussed with regard to electronic band structure and dipole moment difference, resulting from their crystal structures.     

Novel one- and two-dimensional InSbS3 semiconductors for photocatalytic water splitting: The role of edge electron states

Photocatalytic water splitting has become a promising technology to solve environmental pollution and energy shortage. Exploring stable and efficient photocatalysts are highly desired. Herein, we propose novel low-dimensional InSbS₃ semiconductors with good stability based on density functional theory. Such InSbS₃ structures could be obtained from their bulk crystal by suitable exfoliation methods. Our calculations indicate that two-dimensional (2D) and one-dimensional (1D) InSbS₃ nanostructures have moderate band gaps (2.54 and 1.97 eV, respectively) and suitable band edge alignments, which represents sufficient redox capacity for photocatalytic water splitting. 2D InSbS₃ monolayer possesses oxygen evolution reaction (OER) activity and 1D InSbS₃ single-nanochain possesses hydrogen evolution reaction (HER) activity under acidic conditions. Interestingly, two edge electron states can be introduced when the dimension of InSbS₃ is reduced from 2D to 1D and the new electron states can exist in arbitrary-width nanoribbons, which can effectively promote the process of HER. Moreover, InSbS₃ monolayer and single-nanochain also exhibit large solar-to-hydrogen efficiency, high carrier mobility, and excellent optical absorption properties, which can facilitate the process of photocatalytic reactions. Our findings can stimulate the synthesis and applications of low-dimensional InSbS₃ semiconductors for overall water splitting.     

GaN/Surface-modified graphitic carbon nitride heterojunction: Promising photocatalytic hydrogen evolution materials

The coupling of two-dimensional (2D) layered materials is an effective way to realize photocatalytic hydrogen production. Herein, using first-principles calculations, the photocatalytic properties of GaN/CNs heterojunctions formed by two different graphite-like carbon nitride materials and GaN monolayer are discussed in detail. The results show that the GaN/C₂N heterojunction can promote the effective separation of photogenerated electron and hole pairs, which is attributed to its type-II band orientation and high carrier mobility. However, the low overpotential of GaN/C₂N for photocatalytic hydrogen production limits the photocatalytic performance. On this basis, we adjust the CBM position of the GaN/C₂N heterojunction by applying an electric field to enhance its hydrogen evolution capability. In addition, the GaN/g-C₃N₄ is a type-I heterojunction, which is suitable for the field of optoelectronic devices. This work broadens the field of vision for the preparation of highly efficient photocatalysts.     

Indirect Z-scheme hydrogen production photocatalyst based on two-dimensional GeC/MoSi2N4 van der Waals heterostructures

The stacked two-dimensional materials with suitable band gap are crucial for photocatalytic hydrogen production. Here, using first-principles calculations, the GeC/MoSi₂N₄ heterojunction with a band gap of 1.80 eV is calculated thoroughly. The indirect band alignment of Z-scheme and high carrier mobility boost the separation of electron-hole pairs, allowing more electrons and holes participating in the reactions. Additionally, the band-edge potential perfectly satisfies the requirements for redox potential of water splitting. Furthermore, the Gibbs free energy (−0.552 eV) close to zero indicates the heterojunction can conduct HER exceedingly well, providing a guarantee for photocatalytic hydrogen production. Remarkably, the light absorption coefficient peak is about 1.39 × 10⁵ cm⁻¹ within the visible light range enables the heterojunction to absorb more visible light from the spectrum. In short, results demonstrate the GeC/MoSi₂N₄ heterojunction is a promising photocatalyst for visible light water splitting, which will pave the way for the development of water splitting hydrogen production.     

A ZIF-67-derived lamellar CoP@C cocatalyst for promoting photocatalytic hydrogen evolution from water

Photocatalytic hydrogen evolution from water is one of the top issues to achieve green hydrogen energy and utilize solar energy. Construction of cocatalyst is a major part for efficient photocatalysts. Lamellar flower-like CoP@C cocatalyst is synthesized via the phosphating of cobalt precursor derived from metal-organic framework ZIF-67. Different from usual phosphating of ZIF-67 directly, a typical solvothermal treatment of ZIF-67 contributes to tuning the formation of C nanodots on the lamellar CoP. CoP@C as cocatalyst exhibits a remarkable role of improving photocatalytic activity for hydrogen evolution. CoP@C/CdS composite shows a photocatalytic hydrogen evolution rate of 164.4 mmol g^?1 h^?1, which is much higher than those of pure CdS and other CoP/CdS photocatalysts. The heterojunction and interaction are verified between CoP@C and CdS. Light absorption and photoelectric properties of CoP@C/CdS are enhanced accompanying with strong reduction ability. A type-Ⅱ transfer path of photoelectrons is underway in CoP@C/CdS photocatalyst, accelerating the separation of electron-hole pairs and the transfer of carriers, and further resulting in the promoted photocatalytic performance. This work provides a suitable way to achieve carbon nanodots involved metal compound cocatalysts for efficient hydrogen production.     

等离激元效应促进的光催化分解水制氢

  目的  利用光催化分解水的方式直接将太阳能转化并存储为氢气的化学能,是发展清洁能源促进低碳经济的有效途径。文章围绕等离激元效应对光催化分解水制氢的促进机理进行综述,以推进其产业化应用。  方法  阐释了等离激元效应在光催化分解水反应过程中的微观机制,分析了等离激元粒子在增强太阳光的吸收能力、拓展吸收光谱的响应范围、促进光生电子空穴的分离、提升光生载流子的热力学能、以及提供光催化反应活性中心等方面发挥的重要作用。  结果  通过总结当前等离激元效应促进光催化分解水制氢的研究进展,浅析了目前存在的问题,并对该领域的未来发展趋势进行了展望。  结论  基于等离激元效应在太阳能生产燃料中的巨大应用潜力,不同学科背景的相关学者协同合作,对影响光催化剂效率和寿命的各项决定性因素积极研发攻关,将促进该技术领域获得重要突破。     

Cation exchange synthesis of hollow-structured cadmium sulfide for efficient visible-light-driven photocatalytic hydrogen evolution

Efficient solar absorption and photoinduced charge separation are extremely important for solar-energy conversion on semiconductor photocatalysts. To advance the photocatalytic performance, we developed a general templated-assisted reverse cation exchange strategy to successfully synthesize hollow-structured CdS semiconductors with the textile structural surface. The crystal phase, particle morphology, optical/electrical properties, and photocatalytic performance of the as-syntheszied sample are investiagted by XRD, SEM, TEM, XPS, DSR, PL, ESR photoelectrochemical measurements, and Photocatalytic H₂ evolution test. The final CdS sample exhibits an enhanced photocatalytic hydrogen evolution rate of up to 965 μmol·g⁻¹ h⁻¹, 2.8 times higher than the reported CdS nanorods. Based on the experimental and characterization results, the improved photocatalytic activity of the cadmium sulfide semiconductor can be ascribed to the special hollow cubic structure with a thin shell, which can enhance the light-harvesting ability and provide abundant photocatalytic active sites for facilitating the separation of photogenerated electron/hole pairs. This synthetic strategy may pave a new path for the rational design of efficient sulfur-based semiconductor photocatalysts for solar driven H₂ production.     

High-efficiency hydrogen evolution reaction photocatalyst for water splitting of Type-II β-AsP/g-C3N4 van der Waals heterostructure

Photocatalytic for water splitting to produce hydrogen is recognized as a low-cost, promising and attractive method to solve environmental problems and energy crises, but finding a high-performance photocatalyst is a big challenge. In this work, we designed a type-II β-AsP/g-C₃N₄ van der Waals heterostructure as an efficient photocatalyst and had the first principles calculations to analyze its stability, electronic properties, and photocatalytic performance. The results showed that the photocatalyst of β-AsP/g-C₃N₄ heterostructure met the proper band gap and band edge of the redox potential of water splitting, had effective charge separation of photogenerated electronic holes, and efficient visible light response. Importantly, our research showed that the β-AsP/g-C₃N₄ heterostructure could proceed spontaneously in thermodynamics and had an excellent photocatalytic performance in further study. It had quite good hydrogen evolution performance with the Gibbs free energy of ?0.02 eV, which is closer to zero than ?0.09 eV of Pt (111). The overpotential of its oxygen evolution reaction is as low as 0.57 V. This work showed excellent development prospects for β-AsP/g-C₃N₄ heterostructure in the field of photocatalysts, which will promote the development of g–C₃N₄–based photocatalytic for water splitting.     

Hollow In2O3 nanotubes decorated with Cd0.67Mo0.33Se QDs for enhanced photocatalytic hydrogen production performance

Novel Cd_0.67Mo_0.33Se/In₂O₃ hollow nanotubes were prepared for photocatalytic hydrogen production application. Under visible light irradiation, Cd_0.67Mo_0.33Se/In₂O₃ hollow nanotubes showed enhanced photocatalytic performance. And the apparent quantum efficiency of 34.86% was obtained when irradiated with 420 nm monochromatic light. The modification of Cd_0.67Mo_0.33Se QDs on the surface of In₂O₃ hollow nanotubes effectively improved the utilization rate of light absorption, increased the separation and migration rate of electrons, inhibited the recombination of photo-generated electron and hole pairs, thus enhancing the photocatalytic activity of water splitting to produce hydrogen. It would be an efficient photocatalyst for hydrogen production application in future.     

New gallium chalcogenides/arsenene van der Waals heterostructures promising for photocatalytic water splitting

Single two-dimensional (2D) GaS and GaSe were studied as photocatalysts, yet the overall performance is limited by the low optical absorption and inefficient separation of photogenerated electron-hole pairs. Constructing van der Waals (vdW) heterostructures is an ideal way to overcome the deficiency of single 2D gallium chalcogenides. This work unravels that gallium chalcogenides/arsenene (GaX/As, X = S, Se) are the promising vdW heterostructures that show significantly improved photocatalytic performance by means of first-principles calculations. The GaX/As heterostructures possess suitable band alignment and bandgap satisfying the requirements for photocatalysts. Contrary to the pristine monolayers, the Se_0.5GaS_0.5/As and S_0.5GaSe_0.5/As heterostructures undergo indirect-direct bandgap transition by varying the interlayer distances; moreover, they exhibit high carrier mobility (～2000 cm² V^?1 s^?1 for electrons) and transport anisotropy, efficiently facilitating the migration and separation of photogenerated electron-hole pairs. Finally, all GaX/As heterostructures show significantly enhanced optical absorption beyond the isolated GaX monolayers under visible-light irradiation. These extraordinary properties render GaX/As heterostructures as competitive photocatalysts for water splitting to produce hydrogen.     

Enhanced photocatalytic activity and stability of the reduced graphene oxide loaded potassium niobate microspheres for hydrogen production from water reduction

A facile approach to synthesize reduced graphene oxide (RGO) loaded potassium niobate microspheres was reported. The composition, microstructure and electron-transfer properties of the obtained product were characterized. Compared to pure potassium niobate microspheres and commercial P25 TiO₂, the as-prepared potassium niobate microspheres/RGO composite showed much higher photocatalytic activity for generating hydrogen under UV irradiation. It was ascribed to the enhanced separation efficiency of electron/hole pairs as testified by electrochemical impedance spectrum and fluorescence spectrum. Importantly, the composite photocatalyst was stable and easy to recycle, and the amount of hydrogen evolution did not decrease after six recycles. The results are potentially applicable to a range of semiconductors useful in water reduction as well as other areas of heterogeneous photocatalysis.     

Recent development in band engineering of binary semiconductor materials for solar driven photocatalytic hydrogen production

Photocatalytic hydrogen production from water splitting is a promising approach to develop sustainable renewable energy resources and limits the global warming simultaneously. Despite the significant efforts have been dedicated for the synthesis of semiconductor materials, key challenge persists is lower quantum efficiency of a photocatalyst due to charge carrier recombination and inability of utilizing full spectrum of solar light irradiation. In this review, recent developments in binary semiconductor materials and their application for photocatalytic water splitting toward hydrogen production are systematically discoursed. In the main stream, fundamentals and thermodynamic for photocatalytic water splitting and selection of photo-catalysts has been presented. Developments in the binary photocatalysts and their efficiency enhancements though surface sensitization, surface plasmon resonance (SPR) effect, Schoktty barrier and electrons mediation toward enhanced hydrogen production has been deliberated. Different modification approaches including band engineering, coupling of semiconductor catalysts, construction of heterojunction, Z-scheme formation and step-type photocatalytic systems are also discussed. The binary semiconductor materials such as TiO₂, g-C₃N₄, ZnO, ZnS, Fe₂O₃, CdS, WO₃, rGO, V₂O₅ and AgX (Cl, Br and I) are systematically disclosed. In addition, role of sacrificial reagents for efficient photocatalysis through reforming and hole-scavenger are elaborated. Finally, future perspectives for photocatalytic water splitting towards renewable hydrogen production have been suggested.     

Chemical strategies in molybdenum based chalcogenides nanostructures for photocatalysis

Since the last two decades, plenty of environmental issues have risen up due to the damage which humans have caused to the planet for the sake of development. The continual ignorance of global climate change and the stalemate approach of major oil producing industries led to the catastrophic melting of glaciers in Arctic and Antarctica and very recently the highest mountain peak of Sweden have become 24 m shorter which is the evident outcome of climatic disturbance. The chaotic unbalancing in the reservoirs of natural resources is leading us to the several crisis which has a potential to affect the livelihood. Among the various techniques used for the development of sustainable energy, photocatalysis is regarded as one of the simplest technique which can yield enormous amount of energy by the utilization of solar energy for meeting the world's demand of an energy requirement and which can be exploited in the degradation of toxic pollutants i.e. organic as well as inorganic pollutants for environment remediation. Transition metal chalcogenides (TMCs) have a potential to get adsorb easily and be utilized for solving the energy-related problems. Large number of photocatalysts has been fabricated, among them Molybdenum (Mo) chalcogenides nanostructures, which also belong to the class of TMCs exhibit exceptional properties such as non-toxicity, low cost and structural flexibility which give them edge over the other materials. Furthermore, the tunable band gap of Mo chalcogenides nanostructures makes them the promising candidates for efficient hydrogen evolution via photocatalytic water splitting in the visible light illumination. This review deals with the photocatalytic applications of Mo based chalcogenides nanostructures in efficient hydrogen production via water splitting and degradation of dyes. It also discusses the recent developments in fabricating Molybdenum chalcogenides nanostructures, their role in the photocatalytic water splitting and discusses the efforts which have been made to improve their photocatalytic activity for extending their applications to the scalable point.     

Type-II CeO2(111)/hBN vdW heterojunction for enhanced photocatalytic hydrogen evolution: A first principles study

Researches on environmentally friendly semiconductor photocatalysts for efficient photocatalytic hydrogen evolution have important practical significance. Here, using first-principles calculations, the CeO₂(111)/hBN heterojunction was conceived. The influence of the interface effect on the structural, electronic and optical properties of the heterojunction was investigated in detail. The band gap of the heterojunction is smaller than the two individual components and forms a type-II heterojunction, improving the photocatalytic activity. Furthermore, by doping two C atoms, the band gap of heterojunction was further narrowed. Both the oxidation and reduction potential of CeO₂(111)/hBN heterojunction meet the requirements of water splitting and has certain advantages over other photocatalysts in the ability for photocatalytic hydrogen evolution. The study revealed the possible mechanism of CeO₂(111) and hBN monolayers compositing to facilitate photocatalysis and hydrogen evolution ability, which may provide a possible reference direction for the practical design of more high-quality semiconductor photocatalysts.     

Novel B-site substituted KCuTa3-xNbxO9 solid solution photocatalysts with modulated band structure for visible-light-driven hydrogen evolution

Perovskite-like metal oxides (PLMOs), featuring unique structural and optical properties, exhibit great potential in photocatalytic water splitting field. However, the wide bandgap and strong carrier recombination severely suppress their photocatalytic hydrogen production activity. Thus, design and development of novel PLMO photocatalyst with extended photo-response range and enhanced photo-generated charge separation/transport efficiency remains an ongoing challenge. Herein, a series of novel B-site substituted KCuTa_3-xNb_xO₉ solid solution photocatalysts were synthesized via a simple solid-state reaction method. With an increased content of Nb, a distinct red-shifted of the optical absorption edge of KCuTa_3-xNb_xO₉ solid solution was observed, leading to a decreased bandgap (from 2.69 to 1.91 eV), and a positive shift of the conduction band bottom (from −0.54 to −0.49 eV vs RHE). All of the Nb-substituted KCuTa₃O₉ solid solutions exhibit enhanced separation efficiency of photoinduced charge carriers, which leads to increased hydrogen evolution activity, among which KCuTa_0.75Nb_2.25O₉ exhibits the highest hydrogen evolution rate of 2.16 μmol h⁻¹ under the visible light irradiation (λ > 420 nm), which is approximately 7-fold higher than that of the pure KCuTa₃O₉. This study demonstrates the potential of modulating band structure through constructing solid solutions for efficient perovskite-like metal oxides photocatalysis.