L-cysteine-protected ruthenium nanoclusters on CdS as efficient and reusable photocatalysts for hydrogen production

Nanocluster-modified semiconductor-based photocatalysts have been identified as a vital area of research in the area of photocatalytic hydrogen evolution from water. However, the existing ligand protection strategy for synthesizing ultrasmall metal nanoclusters remains confined to a few metals, including Au, Ag, Cu, and their alloys. In this investigation, we describe a facile solution-phase reduction synthesis method for the production of L-cysteine-protected Ru nanoclusters. Our findings demonstrate that these novel Ru nanoclusters function as cocatalysts, which notably increase the photocatalytic activity and photostability of CdS photocatalysts. Moreover, the hybrid CdS photocatalyst modified with Ru nanoclusters exhibits superior activity and stability relative to photoinduced Ru nanoparticles/CdS composite photocatalysts. The simplicity of the synthesized metal nanocluster cocatalyst and its effectiveness in enhancing photocatalyst activity, while reducing the use of precious metals, present new avenues for the development of advanced photocatalysts.     

Shining light on panchromatic ruthenium sensitizers towards dye-sensitized photocatalytic hydrogen evolution

Five new photocatalysts have been synthesized in order to extend the photo response upto visible range, by adsorbing MC113-MC117 ruthenium complexes on TiO₂-Pt composites. Highlight harvesting properties of these ruthenium complexes instigated us to evaluate for photocatalytic activity. The absorption curves of the synthesized photocatalysts extended up to 750 nm. Morphological studies of photocatalysts have been carried out using SEM and powder X-ray crystallography. Among all photocatalysts, MC113PC showed high photocatalytic activity i.e. 9474 TONs. IPCE and fluorescence quenching studies of the catalysts revealed the light harvesting nature and electron injection efficiency. The photocatalytic activity of MC photocatalysts were systematically screened at different pH and employing different sacrificial electron donors (SED) in order to obtain optimal photocatalytic performance.     

Recent trends in photocatalytic water splitting using titania based ternary photocatalysts-A review

Hydrogen is considered as an ideal fuel, and its use has several advantages. While several methods are available for producing hydrogen, photocatalytic water splitting using semiconductor-based photocatalysts is one of the better methods. Among the various semiconductors, titania, having many desirable properties, is a widely explored photocatalyst material to fabricate ternary heterojunctions. Preventing the recombination of photoexcited charge carriers, reducing the band gap, and enhancing the migration of charges are steps needed to improve the efficiency of the photocatalysts. Various modifications have been made to the structural and chemical properties of the photocatalysts. While innovative synthetic protocols can bring about the desired changes, incorporating metal oxides and noble metals with varied morphologies into titania leads to multijunction photocatalysts. Structural modifications to titania include incorporation of various nanostructured materials, noble metal nanoparticles, transition metal chalcogenides, polymer materials, semiconductors like g-C₃N₄, quantum dots, etc.     

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.     

Nanostructured materials for the visible-light driven hydrogen evolution by water splitting: A review

The conversion of abundantly available photonic energy into useful chemical energy is considered to be a greener protocol for addressing the energy shortage. Recently, since most of the emphasis has been centralized on the semiconductor-based photocatalysis; the designing and fabrication of the novel semiconductor photocatalytic material is happening at a blistering rate. Recently, the nanostructured materials have attracted ever-growing research attention as photocatalytic material for hydrogen generation reaction by dissociation of water. Such photocatalytic nanomaterials are known to exhibit superior activity than their corresponding bulk counter-parts because of the improved interfacial charge separation and the broad surface area providing sufficient active sites. However, the improvement in the efficiency and selectivity towards hydrogen production reaction under solar or visible light radiation always remains a challenging assignment. In the present review, the segregation of the so far reported nanostructured photocatalysts into different categories, based on their dimensionality such as 0-D, 1-D and 2-D materials, is implemented. Furthermore, their synthetic route and the photocatalytic hydrogen evolving efficiencies are explored and briefly summarized. Moreover, the methodology of development of nanocomposite materials leading to the construction of heterojunctions including Type-I, Type-II, Type-III, Z-Scheme and S-Scheme system is also discussed. In addition, an in-depth investigation on the charge carrier's generation, separation and their transportation is also reviewed. Finally, the future perspectives regarding the designing of an efficient, stable and economic photoactive nano-architecture material for the efficient hydrogen production via photocatalytic dissociation of water are also pointed.     

New evidence for the regulation of photogenerated electron transfer on surface potential energy controlled co-catalyst on TiO2 – The investigation of hydrogen production over selectively exposed Au facet on Au/TiO2

In this study, Au/TiO₂ samples with different exposed facets ({100}, {100/111}, and {111}) of Au were employed as catalysts for the examination of facet-dependent catalytic activity toward photocatalytic hydrogen evolution from water. By photosensitized using Eosin Y as antenna molecule, Au/TiO₂ series photocatalysts exhibited different photocatalytic hydrogen evolution performances under visible light irradiation. Au{111}/TiO₂ photocatalyst presented the highest photocatalytic hydrogen generation activity among Au/TiO₂ series samples. As evidenced by photoluminescence spectra, photocurrent, electrochemical impedance spectra, and Mott–Schottky characterizations, the difference in photocatalytic activities resulted from the different electron transfer rates from the conduction band of TiO₂ to Au nanoparticles. Au nanoparticles with exposed {111} facets were more effective in trapping electrons due to their higher Fermi level. In addition, the apparent activation energy of Au{111}/TiO₂ sample was the lowest, resulted from the biggest uncoordinated numbers of Au atoms on Au{111} nanoparticles, which was favor in forming the hydrogen–metal bond. This study discloses the facet-dependent effect of noble-metal cocatalyst on semiconductor photocatalysts in photocatalytic water reduction, and will give an insight into design and synthesis of high-efficient noble metal/semiconductor hybrid photocatalysts.     

Recent advances in the design of semiconductor hollow microspheres for enhanced photocatalyticv water splitting

Photocatalytic water splitting is a promising method to produce clean and renewable energy, which provides an alternative solution to solve environmental and resource problems. New catalysts based on semiconductor nanoparticles have received increasing attention since they facilitate all the reactions needed for “artificial photosynthesis”. In recent decades, hollow microspheres have provided an ideal platform for efficient utilization. Scientists are working to understand the basic principles, band structures, and modification strategies of hollow microspheres to enhance photocatalytic performance. In this paper, the research progress of hollow microsphere photocatalysts in the field of water splitting is reviewed. In particular, the photocatalytic principles of hollow microspheres and the methods to improve the performance of semiconductor photocatalysts are discussed in depth. The structural advantages and defects of hollow microspheres and modification methods of semiconductor band structure are introduced. Finally, the remaining challenges are summarized, and some insights into new trends and improvement directions for hollow materials are provided. This review will provide new insights for understanding hollow microspheres and help researchers in related fields to have a deeper and more comprehensive understanding of hollow microspheres in photocatalytic water splitting.     

A BiOCl/β‐FeOOH heterojunction for HER photocatalytic performance under visible‐light illumination

β‐iron oxide hydroxide (β‐FeOOH) had been proven to be an effective co‐catalyst during H₂ evolution reaction (HER) process. In this research, a BiOCl/β‐FeOOH heterojunction was successfully synthesized by a solid‐state doping method. Then, the structure, composition, and photo‐electrochemical properties of the prepared photocatalysts were studied. For the superior HER photocatalytic activity of ultrasmall β‐FeOOH nanoparticles (NPs) and the formation of the BiOCl/β‐FeOOH heterojunction, this heterojunction photocatalyst exhibited very superior photocatalytic performance in the HER process. Especially, when the amount of incorporated β‐FeOOH NPs was appropriate, the BFOH‐2 possessed the highest photocatalytic activity in HER process, and the HER rate was about 16.64 mmol·g^?1·h^?1 during illuminated time of 6 hours under visible light. When appropriate, ultrasmall β‐FeOOH NPs were implanted into the structure of BiOCl, the BiOCl/β‐FeOOH heterojunction interfaces would form for the existence of interfacial interactions. Therefore, this BiOCl/β‐FeOOH heterojunction exhibited superior visible‐light response, fast photo‐carrier migration, and high‐efficient separation of photo‐carriers, so that the BFOH‐2 heterojunction possessed high‐efficient hydrogen evolution reaction (HER) photocatalytic activity.     

Metal-organic frameworks (MOFs)-based efficient heterogeneous photocatalysts: Synthesis,properties and its applications in photocatalytic hydrogen generation,CO2 reduction and photodegradation of organic dyes

Metal-organic frameworks (MOFs) are a new class of functional materials having porous structures that show extraordinary specific surface areas, and tunable surface chemistry; hence, they hold great potential as photocatalysts. This review describes the fundamentals of MOFs and possible new research directions in the area of heterogeneous MOFs that can provide enhanced photocatalytic performance, especially for hydrogen production, degradation of emerging organic pollutants, and CO₂ reduction. The role of MOFs as multifunctional photocatalysts for light-stimulated organic reactions through an effective combination of metal/ligand/guest-based photocatalysts is discussed. Recent literature is discussed critically on the design and selection of materials, with possible directions to improve their catalytic properties. Furthermore, this comprehensive review systematically discusses the current developments of various MOFs-based hybrid nanostructures as multifunctional photocatalysts from different points, including several synthetic methodologies, key features, photocatalytic mechanism, and various influencing parameters to enhance catalytic efficiency. The recent achievements are critically discussed in the designing and selection of MOFs-based functional materials, with directions to effectively improve their catalytic properties for various photocatalytic applications. The article also summarizes with challenges and future prospects for the cost-effective and large scale photocatalytic applications of MOFs-based heterostructured catalysts.     

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.     

Borate particulate photocatalysts for photocatalytic applications: A review

Borate is a kind of wide-bandgap semiconducting material with rich structure and variety, which is usually used in optical properties research. Its unique crystal structure has essential research value for expanding the applications of photocatalysis. This review summarizes the recent research progress of borate photocatalysis, including novel borate photocatalyst, borate-based composite, and borate glass photocatalyst. Furthermore, the energy and environmental photocatalysis applications of borate photocatalysts are discussed, such as overall water splitting reaction, water decomposition to generate hydrogen or oxygen, degradation of pollutants, and others. The strategy of the combination of theoretical calculation and experiment to explore new borate photocatalysts was also introduced. Finally, some developing directions and challenges in borate photocatalytic materials are summarized.     

Spinel nano-ferrites as low-cost (photo)electrocatalysts with unique electronic properties in solar energy conversion systems

Spinel nano-ferrites (MFe₂O₄, M: Co²⁺, Ni²⁺, …) are excellent low-cost candidates as (photo) electrocatalysts in solar-to-hydrogen energy conversion systems owing to their high dual electrochemical and photochemical activities. Their crystal structure consists of alternating tetrahedral and octahedral units that accommodate Fe³⁺/Fe²⁺ ions and another divalent ion in their interstices that give them unique electronic properties for (photo) electrocatalysis. Herein, the role of nano-ferrites as oxygen evolution reaction (OER) catalyst and counter-electrode catalyst in photovoltaic-electrolysis (PV-E) and photoelectrochemical (PEC) water splitting systems are highlighted. In particular, the recent advances on OER (photo) electrocatalysts based on spinel nano-ferrites and their design strategies such as doping, oxygen/cation defect engineering, as well as supporting and compositing with other materials are discussed and summarized. Finally, their advantages and performance limitations were presented, and future perspectives towards their improvement are suggested. Overall, this report is a critical review of the performance of spinel nano-ferrites as truly low-cost candidate catalysts with a wide natural abundance and easy preparation toward clean hydrogen generation.     

Functionalized conjugated polymer with plasmonic Au nanoalloy for photocatalytic hydrogen generation under visible-NIR

Gold based multimetallic nanoalloys decorated on conjugated polymer nanofibers have been prepared using a greener approach without using any reducing agent. The as-prepared nanohybrids exhibited superior catalytic activity for solar hydrogen production under visible light (λ > 420 nm) irradiation and near infrared light irradiation. The UV–Visible diffuse reflectance spectra displayed strong absorption in the visible region which significantly favours the photocatalytic performance. Furthermore, the efficient charge separation suggested by electrochemical impedance measurement and photocurrent response curves for Au₅₀Pt₂₄Pd₂₆/PPy nanohybrids which exhibited significant enhancement in H₂ generation rate compared to Au/PPy nanohybrids. The strong interface contact between Au nanoalloys and PPy nanofibers play an important role for the migration of electron during catalysis. The Mott–Schottky plot revealed that photo generated charge carrier concentration has been increased for Au₅₀Pt₂₄Pd₂₆/PPy nanohybrids (7.93 × 10¹¹cm⁻³) compare to pure PPy (1.43 × 10¹¹ cm⁻³). The present study provides a new prospect for using conducting polymer based hybrid as photocatalysts for efficient solar hydrogen production.     

Enhancing the photocatalytic hydrogen production of the ZnO–TiO2 heterojunction by supporting nanoscale Au islands

In this work, Au was loaded on the ZnO–TiO₂ heterojunction by the deposition-precipitation with urea method to boost its photocatalytic hydrogen production. The synthesized materials were characterized by TEM, ICP-OES, XRD, N₂ adsorption-desorption, UV–vis spectrophotometry, XPS, and (photo)electrochemical measurements. The TEM images confirmed the close contact between ZnO and TiO₂ nanoparticles and showed that although Au nanoparticles agglomerated in the form of islands; they were widely dispersed on the surface of the photocatalysts. Besides, the XPS characterization revealed the enhanced contribution by the metallic Au species as their amount was increased in the composite. The heterojunctions with different Au contents produced higher yield in the photocatalytic production of hydrogen, observing a maximum with the 2-wt.%- Au content (9.13 mmol g⁻¹), being this value 6 times higher than the results obtained with the ZnO–TiO₂ heterojunction. This improvement is associated with the synergistic interaction between the ZnO–TiO₂ heterojunction and Au islands that promoted the separation and transfer of charge carriers. Besides, the (photo)electrochemical characterization showed that the islands acted as “electronic reservoirs”, prolonging the lifetime of the photogenerated electron-hole pairs and creating surface or energy states at the Au/ZnO–TiO₂ heterojunction interface. These states helped improve the charge transfer processes by diminishing the recombination and increasing the photocatalytic hydrogen production.     

Graphene reduction of P25 titania: Ti3+- doped titania/graphene nanohybrids for enhanced photocatalytic hydrogen production

Ti³⁺-doped titania has attracted great attention in recent years by its enhanced photocatalytic performance as compared to the conventional titania systems. In this work, solvothermally reduced hydroxy graphene (RHG), derived from fluorographite (FGT), is used to reduce commercially available P25 titania to produce Ti³⁺-doped, TiO_2-x/RHG nanocomposites, via a facile solid-state route. These nanohybrids exhibit high performance towards photocatalytic hydrogen generation under broad-band irradiation, with a 3497 μmol/g/h hydrogen production rate, without the assistance of any noble metal co-catalyst. This enhanced rate can be attributed to the surface defects generated by Ti³⁺-doping as well as the ability of RHG to perform as a highly efficient scavenger for the photogenerated electrons. It is hypothesized that RHG could serve as one of the most suitable co-catalysts for the semiconductor-based photocatalysts.     

Precursor self-derived Cu@TiO2 hybrid Schottky junction for enhanced solar-to-hydrogen evolution

Solar-to-hydrogen production has attracted increasing attention since it possesses great potential in alleviating energy and environmental crises to some extent. The key issue is to develop efficient photocatalysts exhibiting superior hydrogen production capability. In this work, Cu@TiO₂ hybrid (Cu nanoparticles encapsulated in TiO₂) has been successfully prepared by Cu₂O self-template reduction through solvothermal treatment in ethylene glycol-water mixed solvent. When octahedral Cu₂O is involved in the reaction system, the Cu₂O@Ti-precursor octahedral structure is first formed and subsequently the Cu@TiO₂ hybrid is prepared with the reduction of ethylene glycol (EG). The Cu@TiO₂ hybrid derived with different mass of Cu exhibits improved photocatalytic hydrogen production performance compare to pure TiO₂ and P25. Among those photocatalysts, the Cu@TiO₂-10% (the copper content is 10 wt%) shows the highest hydrogen evolution rate of 4336.7 μmol g^?1 h^?1, and it is twice as much as the pure TiO₂ and 1.9 times as much as P25, respectively. Based on the photo/electrochemical results, an efficient photo-generated electron-hole separation contributes to the enhancement of photocatalytic H₂ evolution upon the Cu@TiO₂ hybrid. When replacing octahedral Cu₂O with cubic and truncated octahedrons ones, the Cu@TiO₂ hybrid photocatalysts are also obtained and they also display superior solar-to-hydrogen evolution than pure TiO₂ and P25. It is expected this work could develop an approach to design Cu-encapsulated hybrid photocatalysts for hydrogen generation.     

Review of photocatalytic water‐splitting methods for sustainable hydrogen production

This paper examines photocatalytic hydrogen production as a clean energy solution to address challenges of climate change and environmental sustainability. Advantages and disadvantages of various hydrogen production methods, with a particular emphasis on photocatalytic hydrogen production, are discussed in this paper. Social, environmental and economic aspects are taken into account while assessing selected production methods and types of photocatalysts. In the first part of this paper, various hydrogen production options are introduced and comparatively assessed. Then, solar‐based hydrogen production options are examined in a more detailed manner along with a comparative performance assessment. Next, photocatalytic hydrogen production options are introduced, photocatalysis mechanisms and principles are discussed and the main groups of photocatalysts, namely titanium oxide, cadmium sulfide, zinc oxide/sulfide and other metal oxide‐based photocatalyst groups, are introduced. After discussing recycling issues of photocatalysts, a comparative performance assessment is conducted based on hydrogen production processes (both per mass and surface area of photocatalysts), band gaps and quantum yields. The results show that among individual photocatalysts, on average, Au–CdS has the best performance when band gap, quantum yield and hydrogen production rates are considered. From this perspective, TiO₂–ZnO has the poorest performance. Among the photocatalyst groups, cadmium sulfides have the best average performance, while other metal oxides show the poorest rankings, on average. Copyright © 2016 John Wiley & Sons, Ltd.     

A review on graphitic carbon nitride (g-C3N4) – metal organic framework (MOF) heterostructured photocatalyst materials for photo(electro)chemical hydrogen evolution

Graphitic carbon nitride (g-C₃N₄)-based heterostructured photocatalysts have recently attracted significant attention for solar water splitting and photocatalytic hydrogen (H₂) evolution, because of their alterable physicochemical, optical and electrical properties, such as tunable band structure, ultrahigh specific surface area and controllable pore size, defect formation and active sites. On the other hand, metal-organic frameworks (MOFs) possess a favorable surface area, permanent porosity and adjustable structures that allow them to be suitable candidates for diverse applications. In this review, we therefore comprehensively discuss the structural properties of heterogeneous g-C₃N₄/MOF-based photocatalysts with a special emphasis on their photocatalytic performance regarding the mechanism of heterogeneous photocatalysis, including advantages, challenges and design considerations.     

Recent advances in covalent organic framework (COF) nanotextures with band engineering for stimulating solar hydrogen production: A comprehensive review

Due to the continuous consumption of fossil fuels, natural reserves are depleting and it has been earnest need for developing new sources of energy. Among the several solar energy conversion techniques, photocatalytic hydrogen (H₂) generation is regarded as one of the most promising routes. Till date, several metal-based semiconductor materials have been investigated, however, H₂ generation is not substantial with the notion of sustainable development. Current research trends show the growing interest in advanced and metal free photocatalyst materials such as covalent organic frameworks (COFs) due to their several benefits such as crystalline porous polymers with pre-designed architectures, large surface area, exceptional stability, and ease of molecular functionalization. By combining COFs with other functional materials, composites may be created that display unique characteristics that exceed those of the separate components. This work provides a comprehensive development on COFs as a photocatalysts and their composites/hybrids for photocatalytic hydrogen generation with a focus on visible-light irradiation. To reduce the dependency on novel metals and overcome the drawbacks of individual material, the creation of composite materials based on covalent-organic frameworks (COFs) are systematically discussed. In addition, advantages in terms of performance, stability, durability of composites/hybrids COFs for photocatalytic hydrogen production in reference to traditional catalysts are investigated. Different composites such as metals loading, morphological development, band engineering, and heterojunctions are systematically discussed. Finally, challenges and opportunities associated with constructing COF-based catalysts as future research prospective for chemistry and materials science are highlighted.     

Hydrogel photocatalysts for efficient energy conversion and environmental treatment

Photocatalysts have attracted great research interest owing to their excellent properties and potential for simultaneously addressing challenges related to energy needs and environmental pollution. Photocatalytic particles need to be in contact with their respective media to exhibit efficient photocatalytic performances. However, it is difficult to separate nanometer-sized photocatalytic materials from reaction media later, which may lead to secondary pollution and a poor recycling performance. Hydrogel photocatalysts with a three-dimensional (3D) network structures are promising support materials for photocatalysts based on features such as high specific surface areas and adsorption capacities and good environmental compatibility. In this review, hydrogel photocatalysts are classified into two different categories depending on their elemental composition and recent progresses in the methods for preparing hydrogel photocatalysts are summarized. Moreover, current applications of hydrogel photocatalysts in energy conversion and environmental remediation are reviewed. Furthermore, a comprehensive outlook and highlight future challenges in the development of hydrogel photocatalysts are presented.