Tantalum (Oxy)Nitride: Narrow Bandgap Photocatalysts for Solar Hydrogen Generation

Photocatalytic water splitting, which directly converts solar energy into hydrogen, is one of the most desirable solar-energy-conversion approaches. The ultimate target of photocatalysis is to explore efficient and stable photocatalysts for solar water splitting. Tantalum (oxy)nitride-based materials are a class of the most promising photocatalysts for solar water splitting because of their narrow bandgaps and sufficient band energy potentials for water splitting. Tantalum (oxy)nitride-based photocatalysts have experienced intensive exploration, and encouraging progress has been achieved over the past years. However, the solar-to-hydrogen (STH) conversion efficiency is still very far from its theoretical value. The question of how to better design these materials in order to further improve their water-splitting capability is of interest and importance. This review summarizes the development of tantalum (oxy)nitride-based photocatalysts for solar water spitting. Special interest is paid to important strategies for improving photocatalytic water-splitting efficiency. This paper also proposes future trends to explore in the research area of tantalum-based narrow bandgap photocatalysts for solar water splitting.     

Recent progress in oxynitride photocatalysts for visible-light-driven water splitting

Photocatalytic water splitting into hydrogen and oxygen is a method to directly convert light energy into storable chemical energy, and has received considerable attention for use in large-scale solar energy utilization. Particulate semiconductors are generally used as photocatalysts, and semiconductor properties such as bandgap, band positions, and photocarrier mobility can heavily impact photocatalytic performance. The design of active photocatalysts has been performed with the consideration of such semiconductor properties. Photocatalysts have a catalytic aspect in addition to a semiconductor one. The ability to control surface redox reactions in order to efficiently produce targeted reactants is also important for photocatalysts. Over the past few decades, various photocatalysts for water splitting have been developed, and a recent main concern has been the development of visible-light sensitive photocatalysts for water splitting. This review introduces the study of water-splitting photocatalysts, with a focus on recent progress in visible-light induced overall water splitting on oxynitride photocatalysts. Various strategies for designing efficient photocatalysts for water splitting are also discussed herein.     

Recent progress in oxynitride photocatalysts for visible-light-driven water splitting

Abstract

Photocatalytic water splitting into hydrogen and oxygen is a method to directly convert light energy into storable chemical energy, and has received considerable attention for use in large-scale solar energy utilization. Particulate semiconductors are generally used as photocatalysts, and semiconductor properties such as bandgap, band positions, and photocarrier mobility can heavily impact photocatalytic performance. The design of active photocatalysts has been performed with the consideration of such semiconductor properties. Photocatalysts have a catalytic aspect in addition to a semiconductor one. The ability to control surface redox reactions in order to efficiently produce targeted reactants is also important for photocatalysts. Over the past few decades, various photocatalysts for water splitting have been developed, and a recent main concern has been the development of visible-light sensitive photocatalysts for water splitting. This review introduces the study of water-splitting photocatalysts, with a focus on recent progress in visible-light induced overall water splitting on oxynitride photocatalysts. Various strategies for designing efficient photocatalysts for water splitting are also discussed herein.     

Dye-sensitized photocatalyst for effective water splitting catalyst

Abstract

Renewable hydrogen production is a sustainable method for the development of next-generation energy technologies. Utilising solar energy and photocatalysts to split water is an ideal method to produce hydrogen. In this review, the fundamental principles and recent progress of hydrogen production by artificial photosynthesis are reviewed, focusing on hydrogen production from photocatalytic water splitting using organic–inorganic composite-based photocatalysts.     

Nanocrystalline zinc indium vanadate: a novel photocatalyst for hydrogen generation

Hydrogen is a future fuel and hence production of cheap hydrogen is an important area of research. Recently, the photocatalysts were used to generate hydrogen from water and hydrogen sulfide splitting under solar light. Hence, we designed Zinc Indium Vanadate, a novel visible light active photocatalyst and used for the generation of hydrogen by using solar light. We have demonstrated the synthesis of ZnIn2V2O9 (ZIV) catalyst by sonochemical route using NH4VO3, In (NO3)3 and Zn(CH3COO)2 as a precursors and PVP as a capping agent. The obtained product was further characterized by XRD, UV-DRS and FESEM. The XRD pattern reveals the existence of monoclinic crystal structure and broader peaks indicating the nanocrystalline nature of the material. The particle size was observed in the range of 50-70 nm. The optical study showed the absorption edge cut off at 520 nm with estimated band gap about 2.3 eV. Considering the band gap in visible range, ZnIn2V2O9 was used as a photocatalyst for photodecomposition of H2S under visible light irradiation to produce hydrogen. We observed excellent photocatalytic activity for the hydrogen generation by using this photocatalyst.     

Design of Heterostructured Hollow Photocatalysts for Solar‐to‐Chemical Energy Conversion

Direct conversion of solar energy into chemical energy in a sustainable manner is one of the most promising solutions to the energy crisis and environmental issues. Fabrication of highly active photocatalysts is of great significance for the practical applications of efficient solar‐to‐chemical energy conversion systems. Among various photocatalytic materials, semiconductor‐based heterostructured photocatalysts with hollow features show distinct advantages. Recent research efforts on rational design of heterostructured hollow photocatalysts toward photocatalytic water splitting and CO₂ reduction are presented. First, both single‐shelled and multishelled heterostructured photocatalysts are surveyed. Then, heterostructured hollow photocatalysts with tube‐like and frame‐like morphologies are discussed. It is intended that further innovative works on the material design of high‐performance photocatalysts for solar energy utilization can be inspired.     

Recent advances in visible light-driven water oxidation and reduction in suspension systems

In order to solve the shortage of sustainable energy and the related concern about combustion of fossil fuels, converting the most abundant solar energy into chemical fuels becomes one of the most promising choices to provide the everlasting and environmentally friendly energy vector along with the minimum impact on environment. Among the methods of converting solar energy into chemical fuels, there is a significant interest in the renewable hydrogen production by photocatalysts from abundant water under visible light irradiation. Therefore, the development of efficient photocatalysts for water reduction and oxidation in a suspension system is the footstone for the development of solar energy conversion. In this review, the fundamental theory of photocatalysis and key factors affecting photocatalysis will be introduced first. Then the new materials development covering inorganic materials (oxides, nitrides and sulfides), carbon-based photocatalysts, and semiconductor-coordination compound photocatalysts developed over the past 10?years will be addressed with discussion about dominating factors in the photochemical process. This review would provide a comprehensive reference to exploring the efficient and novel materials working for the solar energy conversion to affordable and sustainable fuels. Finally, the perspective of the technology is also discussed.     

Smart Utilization of Carbon Dots in Semiconductor Photocatalysis

Efficient capture of solar energy will be critical to meeting the energy needs of the future. Semiconductor photocatalysis is expected to make an important contribution in this regard, delivering both energy carriers (especially H₂) and valuable chemical feedstocks under direct sunlight. Over the past few years, carbon dots (CDs) have emerged as a promising new class of metal‐free photocatalyst, displaying semiconductor‐like photoelectric properties and showing excellent performance in a wide variety of photoelectrochemical and photocatalytic applications owing to their ease of synthesis, unique structure, adjustable composition, ease of surface functionalization, outstanding electron‐transfer efficiency and tunable light‐harvesting range (from deep UV to the near‐infrared). Here, recent advances in the rational design of CDs‐based photocatalysts are highlighted and their applications in photocatalytic environmental remediation, water splitting into hydrogen, CO₂ reduction, and organic synthesis are discussed.     

All‐Solid‐State Z‐Scheme Photocatalytic Systems

The current rapid industrial development causes the serious energy and environmental crises. Photocatalyts provide a potential strategy to solve these problems because these materials not only can directly convert solar energy into usable or storable energy resources but also can decompose organic pollutants under solar‐light irradiation. However, the aforementioned applications require photocatalysts with a wide absorption range, long‐term stability, high charge‐separation efficiency and strong redox ability. Unfortunately, it is often difficult for a single‐component photocatalyst to simultaneously fulfill all these requirements. The artificial heterogeneous Z‐scheme photocatalytic systems, mimicking the natural photosynthesis process, overcome the drawbacks of single‐component photocatalysts and satisfy those aforementioned requirements. Such multi‐task systems have been extensively investigated in the past decade. Especially, the all‐solid‐state Z‐scheme photocatalytic systems without redox pair have been widely used in the water splitting, solar cells, degradation of pollutants and CO₂ conversion, which have a huge potential to solve the current energy and environmental crises facing the modern industrial development. Thus, this review gives a concise overview of the all‐solid‐state Z‐scheme photocatalytic systems, including their composition, construction, optimization and applications.     

High activity of hot electrons from bulk 3D graphene materials for efficient photocatalytic hydrogen production

Design and synthesis of efficient photocatalysts for hydrogen production via water splitting are of great importance from both theoretical and practical viewpoints.Many metal-based semiconductors have been explored for this purpose in recent decades.Here,for the first time,an entirely carbon-based material,bulk three-dimensionally cross-linked graphene (3DG),has been developed as a photocatalyst for hydrogen production.It exhibits a remarkable hydrogen production rate of 270 μmol·h-1·g-1cat under full-spectrum light via a hot/free electron emission mechanism.Furthermore,when combined with the widely used semiconductor TiO2 to form a TiO2/3DG composite,it appears to become a more efficient hydrogen production photocatalyst.The composite achieves a production rate of 1,205 μmol·h-1.g-1cat under ultraviolet-visible (UV-vis) light and a 7.2％ apparent quantum efficiency at 350 nm due to the strong synergetic effects between TiO2 and 3DG.     

2D photocatalysts with tuneable supports for enhanced photocatalytic water splitting

Sustainable hydrogen production is attracting increasing attention and visible-light-driven water splitting is considered as one of the most promising approaches for hydrogen evolution and solar energy storage. Different materials have been screened at mild conditions in recent decades and 2-dimensional (2D) layered materials are considered good candidates for the photocatalytic water splitting reaction. 2D single layer MoS₂ has shown its potential in various catalytic systems, and has also been used in photocatalytic water splitting reaction recently. However, current studies of MoS₂ monolayers give low intrinsic activity, preventing it from practical applications. This is attributed to the rapid recombination of the photo-excited charge carriers at room temperature, resulting in poor quantum efficiency (QE). Herein, a state-of-the-art strategy to prolong the exciton lifetimes is reported, which is achieved by combining the 2D MoS₂ nanosheets with solid state polar-faceted supports. The charge separation process can be facilitated by the strong local polarisation introduced by the polar-faceted supports, and tuned by changing the supports with different surface polarities. Polar oxide surface is the exposure of oxygen-terminated high energetic facet, which is known to give a net dipole moment perpendicular to its surface. Such variation in the surface properties of the support to the above metal would lead to a difference in metal-support interaction(s). The resulting composite structures show great enhancement toward the visible-light-driven photocatalytic water splitting reaction, giving hydrogen and oxygen evolution in a stoichiometric 2:1 ratio at elevated temperatures from pure water. Photocatalytic performances are improved by the prolonged exciton lifetimes and exceptional hydrogen evolution activity of 2977 μmol g⁻¹ h⁻¹ with impressive QEs are obtained over Ru-doped MoS₂ nanosheets on polar ceria support, which is among the best of the reported results of similar catalytic systems to date. More excitingly, the linear relationship between the exciton lifetimes and strength of the local polarisation is also observed, indicating that the rational design of photocatalysts can be simply achieved via engineering their local polarisation by incorporation of polar-faceted materials.     

TiO2光催化活性向可见光区拓展的研究进展

TiO2光催化剂可用于光分解有机污染物,组装太阳能电池,光分解水制氢气或氧气等领域。本文对可见光响应的TiO2光催化剂国内外研究进行了综述,概述了采用敏化,掺杂等方法可使二氧化钛光催化活性从紫外光区拓展到可见光区,并对未来的研究方向进行了展望。     

纳米异质结光催化材料制取太阳能燃料研究进展

光催化制取太阳能燃料主要包括光催化分解H₂O制取H₂及光催化还原CO₂制取碳氢化合物, 是应对能源危机最具前景的方法之一。目前, 太阳能燃料的最高转化效率为5%, 无法满足商业化要求(≥10%)。纳米异质结由于能展现出单组分纳米材料或体相异质结所不具备的独特性质, 更能促进光生电子和空穴快速转移, 提供更多的光生电子或使光生电子具有更强的还原性, 因而能显著提高光催化活性。本文主要综述了几种纳米异质结(I-型、II-型、p-n型及Z-型)的光催化原理及其在制取太阳能燃料方面的研究进展, 并展望了研究发展方向。     

Transition‐Metal‐Based Electrocatalysts as Cocatalysts for Photoelectrochemical Water Splitting: A Mini Review

Converting solar energy into hydrogen via photoelectrochemical (PEC) water splitting is one of the most promising approaches for a sustainable energy supply. Highly active, cost‐effective, and robust photoelectrodes are undoubtedly crucial for the PEC technology. To achieve this goal, transition‐metal‐based electrocatalysts have been widely used as cocatalysts to improve the performance of PEC cells for water splitting. Herein, this Review summarizes the recent progresses of the design, synthesis, and application of transition‐metal‐based electrocatalysts as cocatalysts for PEC water splitting. Mo, Ni, Co‐based electrocatalysts for the hydrogen evolution reaction (HER) and Co, Ni, Fe‐based electrocatalysts for the oxygen evolution reaction (OER) are emphasized as cocatalysts for efficient PEC HER and OER, respectively. Particularly, some most efficient and robust photoelectrode systems with record photocurrent density or durability for the half reactions of HER and OER are highlighted and discussed. In addition, the self‐biased PEC devices with high solar‐to‐hydrogen efficiency based on earth‐abundant materials are also addressed. Finally, this Review is concluded with a summary and remarks on some challenges and opportunities for the further development of transition‐metal‐based electrocatalysts as cocatalysts for PEC water splitting.     

Regulating Water‐Reduction Kinetics in Cobalt Phosphide for Enhancing HER Catalytic Activity in Alkaline Solution

Electrochemical water splitting to produce hydrogen renders a promising pathway for renewable energy storage. Considering limited electrocatalysts have good oxygen‐evolution reaction (OER) catalytic activity in acid solution while numerous economical materials show excellent OER catalytic performance in alkaline solution, developing new strategies that enhance the alkaline hydrogen‐evolution reaction (HER) catalytic activity of cost‐effective catalysts is highly desirable for achieving highly efficient overall water splitting. Herein, it is demonstrated that synergistic regulation of water dissociation and optimization of hydrogen adsorption free energy on electrocatalysts can significantly promote alkaline HER catalysis. Using oxygen‐incorporated Co₂P as an example, the synergistic effect brings about 15‐fold enhancement of alkaline HER activity. Theory calculations confirm that the water dissociation free energy of Co₂P decreases significantly after oxygen incorporation, and the hydrogen adsorption free energy can also be optimized simultaneously. The finding suggests the powerful effectiveness of synergetic regulation of water dissociation and optimization of hydrogen adsorption free energy on electrocatalysts for alkaline HER catalysis.     

Enhanced Solar‐to‐Hydrogen Generation with Broadband Epsilon‐Near‐Zero Nanostructured Photocatalysts

The direct conversion of solar energy into fuels or feedstock is an attractive approach to address increasing demand of renewable energy sources. Photocatalytic systems relying on the direct photoexcitation of metals have been explored to this end, a strategy that exploits the decay of plasmonic resonances into hot carriers. An efficient hot carrier generation and collection requires, ideally, their generation to be enclosed within few tens of nanometers at the metal interface, but it is challenging to achieve this across the broadband solar spectrum. Here the authors demonstrate a new photocatalyst for hydrogen evolution based on metal epsilon‐near‐zero metamaterials. The authors have designed these to achieve broadband strong light confinement at the metal interface across the entire solar spectrum. Using electron energy loss spectroscopy, the authors prove that hot carriers are generated in a broadband fashion within 10 nm in this system. The resulting photocatalyst achieves a hydrogen production rate of 9.5 µmol h^?1 cm^?2 that exceeds, by a factor of 3.2, that of the best previously reported plasmonic‐based photocatalysts for the dissociation of H₂ with 50 h stable operation.     

Nanoarchitectonics for Transition‐Metal‐Sulfide‐Based Electrocatalysts for Water Splitting

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, it is described how researchers are working to improve the performance of TMS‐based materials by manipulating their internal and external nanoarchitectures. A general introduction to the water‐splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS‐based materials are explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in the HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water‐splitting electrocatalysts for both the HER and the OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The aim herein is to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.     

Elaborately Modified BiVO4 Photoanodes for Solar Water Splitting

Photoelectrochemical (PEC) cells for solar‐energy conversion have received immense interest as a promising technology for renewable hydrogen production. Their similarity to natural photosynthesis, utilizing sunlight and water, has provoked intense research for over half a century. Among many potential photocatalysts, BiVO₄, with a bandgap of 2.4–2.5 eV, has emerged as a highly promising photoanode material with a good chemical stability, environmental inertness, and low cost. Unfortunately, its charge transport properties are modest, at most a hole diffusion length (L_p) of ≈70 nm. However, recent rapid developments in multiple modification strategies have elevated it to a position as the most promising metal oxide photoanode material. This review summarizes developments in BiVO₄ photoanodes in the past 10 years, in which time it has continuously broken its own performance records for PEC water oxidation. Effective modification techniques are discussed, including synthesis of nanostructures/nanopores, external/internal doping, heterojunction fabrication, surface passivation, and cocatalysts. Tandem systems for unassisted solar water splitting and PEC production of value‐added chemicals are also discussed.     

Significantly enhanced photocatalytic hydrogen production performance of g-C3N4/CNTs/CdZnS with carbon nanotubes as the electron mediators

The electron mediator can effectively improve the performance of the direct Z-scheme heterojunction photocatalysts.However,it is still a great challenge to select cheap and efficient electron mediators and to design them into the Z-scheme photocatalytic system.In the present paper,the g-C3N4/CNTs/CdZnS Z-scheme photocatalyst was prepared using carbon nanotubes (CNTs) as the electron mediators,and its photocatalytic hydrogen production performance was studied.Compared with single-phase g-C3N4,CdZnS and biphasic g-C3 N4/CdZnS photocatalysts,the photocatalytic hydrogen production performance of the prepared g-C3N4/CNTs/CdZnS has been significantly enhanced.Meanwhile,g-C3N4/CNTs/CdZnS possesses very good photocatalytic hydrogen production stability.The enhanced photocatalytic hydrogen production performance of g-C3 N4/CNTs/CdZnS is attributed to the fact that CNTs,as an electron mediator,can accelerate the recombination of the photogenerated holes in the valence band of g-C3N4 and the photogenerated electrons in the conduction band of CdZnS,which makes the g-C3N4/CNTs/CdZnS Z-scheme photocatalyst be easier to escape the photogenerated electrons,increases the concentration of the photogenerated carriers and prolongs the lifetime of the photogenerated carriers.This work provides a theoretical basis for the further development and design of CNTs as the intermediate electron mediator of the Z-scheme heterojunction photocatalyst.     

Bimetallic Fe-Mo sulfide/carbon nanocomposites derived from phosphomolybdic acid encapsulated MOF for efficient hydrogen generation

To tackle the energy crisis and achieve more sustainable development,hydrogen as a clean and renew-able energy resource has attracted great interest.Searching for cheap but efficient catalysts for hydrogen production from water splitting is urgently needed.In this report,bimetallic Fe-Mo sulfide/carbon nanocomposites that derived from a polyoxometalate phosphomolybdic acid encapsulated metal-organic framework MIL-100(PMA@MIL-100)have been generated and their applications in electrocatalytic hydrogen generation were explored.The PMA@MIL-100 precursor is formed via a simple one-pot hydrothermal synthesis method and the bimetallic Fe-Mo sulfide/carbon nanocomposites were obtained by chemical vapor sulfurization of PMA@MIL-100 at high temperatures.The nanocomposite samples were fully characterized by a series of techniques including X-ray diffraction,Fourier-transform infrared analysis,thermogravimetric analysis,N2 gas sorption,scanning electron microscopy,transmission elec-tron microscopy,X-ray photoelectron spectroscopy,and were further investigated as electrocatalysts for hydrogen production from water splitting.The hydrogen production activity of the best performed bimetallic Fe-Mo sulfide/carbon nanocomposite exhibits an overpotential of-0.321 V at 10 mA cm-2 and a Tafel slope of 62 mV dec-1 with a 53％reduction in overpotential compared to Mo-free counterpart composite.This dramatic improvement in catalytic performance of the Fe-Mo sulfide/carbon composite is attributed to the homogeneous distribution of the nanosized iron sulfide,MoS2 particles,and the for-mation of Fe-Mo-S phases in the S-doped porous carbon matrix.This work has demonstrated a potential approach to fabricate complex heterogeneous catalytic materials for different applications.