Fabrication of noble-metal-free Cd0.5Zn0.5S/NiS hybrid photocatalyst for efficient solar hydrogen evolution

Up to now, most of the semiconductor photocatalysts can only achieve their high photocatalytic activity for hydrogen production with the loading of noble metals, such as Pt or Ru, as cocatalysts, which drastically increases the total cost of the designed photocatalyst. Herein, we report the design and fabrication of a highly efficient Cd_0.5Zn_0.5S photocatalyst decorated with nanosized NiS surface heterojunctions. The hydrogen evolution rate over this photocatalyst reached 1.4 mmol/h, with a remarkable quantum yield of 33.9%. This efficiency is even much higher than many noble metal loaded photocatalysts. In this hybrid photocatalyst, the nanosized NiS on the surface can serve as electron trapping sites, by which, photogenerated electrons were extracted from Cd_0.5Zn_0.5S substrate, leading to spatially separated photoreduction and oxidation reactions. More interestingly, it was found that NiS played a similar role as noble metal, providing active sites for proton reduction, and hence efficiently enhancing the overall hydrogen production rate. Our work demonstrates the possibility of substitution of noble metal cocatalyst by a properly engineered surface hetero-junction to achieve efficient and low cost photocatalytic hydrogen production.     

Visible light-driven photocatalytic hydrogen production using Cu-doped SrTiO3

A new Cu-doped SrTiO₃ photocatalyst was synthesized and characterized for uses in the visible-light photocatalytic production of hydrogen from water. The photocatalytic activity was assessed based on the characterization of the photocatalysts (band gap energy, surface area, crystallinity, and morphology) and the effects of varying together the calcination temperature, the Cu:Sr mole ratio, and the photocatalyst loading amount. It was determined that the amount of hydrogen evolved was largely dictated by the amount of Cu dopant present in the photocatalysts. The created Box Behnken Design optimization scenarios suggested the conditions: 850°C, 0.01 Cu:Sr, 0.33 g loading as the optimal conditions for maximum hydrogen production holding all studied factors in range, and the conditions: 850°C, 0.01 Cu:Sr, 0.21 g loading as the optimal conditions for the maximum hydrogen production while minimizing Cu dopant and photocatalyst loading.     

Rare earth metals co-doped ZnO/CNTs composite as high performance photocatalyst for hydrogen production from water_triethanolmine mixture

This study investigated the zinc oxide (ZnO) based heterojunction photocatalysts for improved hydrogen production from water splitting. A sol-gel route was adopted to produce terbium (Tb) and samarium (Sm) co-doped ZnO/CNTs composites where CNTs worked as a support material. The built-in redox couples of lanthanides in co-doped TS-ZnO/CNTs composite showed higher hydrogen evolution activity than Sm doped (Sm-ZnO/CNTs) and Tb doped (Tb–ZnO/CNTs) photocatalysts. When triethanolamine was utilized as a sacrificial agent, the TS-ZnO/CNTs photocatalyst result in a remarkable hydrogen evolution rate of 2683 molh^?1g^?1 under visible light illumination. The optimum photocatalyst also showed high stability over five successive hydrogen evolution cycles. The better hydrogen evolution rate with TS-ZnO/CNTs was referred to its fine particle size, high reactive surface area, small optical band gap, suppressed reunification of charge carriers and built-in redox couples. The photocatalytic mechanism, involved in water splitting with TS-ZnO/CNTs photocatalyst, is also deduced in this study. This study can stimulate the attempts towards construction of lanthanides based co-doped semiconductor photocatalysts for efficient hydrogen evolution.     

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.     

Efficient hydrogen production by composite photocatalyst CdS–ZnS/Zirconium–titanium phosphate (ZTP) under visible light illumination

A novel composite CdS–ZnS/Zirconium–titanium phosphate (ZTP) photocatalyst working under visible light was successfully prepared by a two-step sulfidation procedure. The photocatalytic activity of the cadmium sulfide–zinc sulfide supported composite catalyst was evaluated toward hydrogen energy production in the presence of hole scavenger, sulfide (S²⁻) and compared with the activity of neat CdS, ZnS, ZTP, CdS–ZnS, CdS/ZTP and ZnS/ZTP without using any co-catalyst. The photocatalysts were characterized by X-ray diffraction (Small Angle X-ray diffraction Studies and Broad-Angle X-ray Diffraction studies), N₂ adsorption–desorption, diffuse reflectance UV–vis spectroscopy (DRUV-vis), photoluminescence (PL) studies, SEM/EDX, X-ray photoelectron spectroscopic (XPS) studies, transmission electron microscopy (TEM) etc. Amongst all the catalysts, 5CdS–ZnS/ZTP showed highest results toward hydrogen production (2142.7 μmol) with an apparent quantum efficiency of 9.6% under visible light illumination.     

Enhancement of photocatalytic hydrogen production by liquid phase plasma irradiation on metal-loaded TiO2/carbon nanofiber photocatalysts

Enhanced hydrogen production by photocatalytic decomposition was assessed using liquid phase plasma over metal-loaded photocatalysts. Effects of irradiation of the liquid phase plasma were evaluated in the photocatalytic hydrogen production of hydrogen. Carbon nanofiber was introduced as photocatalytic support for the Ni-loaded TiO₂ photocatalyst. The influence of addition of organic reagents into water on hydrogen evolution was also evaluated. The photocatalytic decomposition by irradiation of the liquid phase plasma without photocatalyst produced some hydrogen evolution. The rate of hydrogen evolution was improved by the metal loading on the TiO₂ surface. The carbon nanofiber acted as a useful photocatalytic support for the fixation of TiO₂. Hydrogen evolution was enhanced by the Ni loading on the TiO₂ nanocrystallites supported on the carbon nanofiber support. Hydrogen evolution was increased significantly by the addition of organic reagents, which acted as a type of sacrificial reagent promoting photocatalysis.     

Hydrogen production from water splitting under UV light irradiation over Ag-loaded mesoporous-assembled TiO2-ZrO2 mixed oxide nanocrystal photocatalysts

Hydrogen production from the photocatalytic water splitting reaction is very attractive because it is an environmentally friendly process, where hydrogen is produced from two abundantly renewable sources, i.e. water and solar energy, with the aid of photocatalysts. TiO₂ is the most widely investigated photocatalyst; however, it alone still exhibits low performance to photocatalytically produce hydrogen. Hence, the aim of this work focused on the enhanced photocatalytic hydrogen production over Ag-loaded mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalysts under UV light irradiation. The TiO₂-ZrO₂ mixed oxides with various TiO₂-to-ZrO₂ molar ratios were synthesized by a sol-gel process with the aid of a structure-directing surfactant, followed by Ag loading via a photochemical deposition method. The influences of photocatalyst preparation parameters, i.e. calcination temperature, phase composition, and Ag loading, were studied. The results revealed that the mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalyst with a TiO₂-to-ZrO₂ molar ratio of 93:7 calcined at 500 °C exhibited the highest photocatalytic hydrogen production activity, and the Ag loading of 0.5 wt.% further greatly enhanced the photocatalytic activity of such TiO₂-ZrO₂ mixed oxide photocatalyst.     

Effects of anions on the photocatalytic H2 production performance of hydrothermally synthesized Ni-doped Cd0.1Zn0.9S photocatalysts

A series of Ni²⁺ doped Cd_0.1Zn_0.9S photocatalysts were prepared with different salt by hydrothermal method. The prepared photocatalysts were characterized by XRD, UV–Vis, BET and SEM. The effects of SO₄²⁻, CH₃COO⁻, Cl⁻ and NO₃⁻ anions on the photocatalytic hydrogen production performance of these photocatalysts were examined. Experimental results showed that the photocatalysts prepared with acetates and chlorides have the highest hydrogen production activity, and their initial hydrogen production rate reaches to 76.52 μmol/h and 80.75 μmol/h under visible-light irradiation with the apparent quantum yield of 12.30% and 14.36% at 420 nm without any noble metal loading, respectively. The absence of noble metal is propitious to reduce the cost of photocatalyst preparation.     

Efficient hydrogen evolution by liquid phase plasma irradiation over Sn doped ZnO/CNTs photocatalyst

In this study, the liquid phase plasma (LPP) was irradiated over pure zinc oxide (ZnO), strontium (Sn) doped ZnO, and Sn doped ZnO/CNTs photocatalysts for hydrogen evolution from pure water and from aqueous solution of water-methanol. The possible relationship between hydrogen evolution and optical emissions from LPP for activation of ZnO based photocatalysts was revealed. The role of carbon nanotubes (CNTs) as a support material for improved photocatalytic hydrogen evolution was also investigated in this study. The photocatalytic hydrogen evolution from water mixed methanol under LPP irradiation was compared with pure water splitting. The photolysis produced negligible amount of hydrogen due to minimal photodecomposition of water molecules under LPP irradiation. The plasma born reactive species also played crucial role in photolysis. However, the hydrogen evolution rate increased significantly in the presence of ZnO photocatalyst. Further improvement in hydrogen evolution rate was noticed on Sn doping of ZnO and compositing with CNTs. The highest hydrogen evolution rate of 11.46 mmh⁻¹g⁻¹ from water mixed methanol was achieved with Sn doped ZnO/CNTs photocatalyst. This hydrogen evolution rate from water-methanol solution was 9 times higher than from the splitting of pure water. This hydrogen evolution rate is attributed to excessive production of hydroxyl radicals, red shift in optical band gap of Sn doped ZnO/CNTs photocatalyst, slow electron-hole recombination and fast decomposition of methanol as sacrificial reagent.     

A critical review of metal-doped TiO2 and its structure–physical properties–photocatalytic activity relationship in hydrogen production

Hydrogen production is an effective way to replace the primary energy to provide renewable sources as well as preserve the environment. Significant efforts have been developed to increase the effectiveness of hydrogen production through many methods. However, the challenge is still on-going, which exhibits insufficient efficiency and weak selectivity toward hydrogen production. Photocatalysis is one of the best methods to produce hydrogen as well as sustained the environment. Here, modification of TiO₂ by metal doping photocatalyst is reviewed. The right conclusions only can be obtained if consistent data is used. So, in this review, the data used are only data generated from a research group, Bunsho Ohtani's research group of Hokkaido University, that used titania photocatalyst in the production of hydrogen. The photocatalytic activity of photocatalysts and their relationship with hydrogen production and the factors that affect hydrogen production are discussed critically using fuzzy graph and fuzzy logic modelling. Modification of TiO₂ photocatalyst and its application for the production of hydrogen are studied. The modification is designated as mono-, bi-, and trimetallic metal doping. Moreover, there is no clarification has been done on the factors that affect the photocatalytic activity in hydrogen production. Thus, the mathematical tool, which is the fuzzy logic controller (FLC) is introduced in photocatalysis to provide the future direction of the structure-physical properties-photocatalytic activity relationship of metal-doped TiO₂ photocatalyst. Au/TiO₂ is used as the photocatalyst model towards the production of hydrogen under UV light irradiation in the form of a fuzzy graph. It was found that the low amount of Au metal doping and high surface area are the dominant factors to obtain high-efficiency hydrogen production for Au/TiO₂ photocatalyst.     

光催化法对分解硫化氢制氢的影响

文中采用波长为253.7 nm的紫外光为光源,以Na2S/Na2SO3混合水溶液作为反应介质,采用不同种的光催化剂进行紫外光液相分解硫化氢(H2S)制氢反应。考察了不同种紫外响应的光催化剂对产氢的影响、TiO2-P25光催化剂、TiO2-P25光催化剂加入量、焙烧温度对ZnO光催化剂活性对产氢的影响。研究结果表明,加入光催化剂有助于反应的进行,使反应的产氢量有所提高,不同的光催化剂对分解硫化氢制氢的影响不同;TiO2-P25光催化剂其分解Na2S溶液与紫外光子激发HS-有协同作用;250 mL 0.1 mol/L Na2S水溶液中最佳催化剂用量为0.05 g;不同焙烧温度下制得的ZnO光催化剂对反应体系的产氢速率影响较大,随着焙烧温度的提高,反应的产氢速率也相应提高。     

Biomolecule-assisted hydrothermal synthesis of ZnxCd1−xS nanocrystals and their outstanding photocatalytic performance for hydrogen production

To achieve scalable applications in solar hydrogen production, it is necessary to develop visible-light-responsive photocatalysts that are highly efficient, cost-effective, stable and environmentally-benign. Here narrow bandgap Zn–Cd–S solid solution photocatalysts (E_g = 2.11–2.53 eV) were prepared via a facile and green hydrothermal strategy under mild conditions. Amazingly, over the naked Zn_0.5Cd_0.5S photocatalyst, an extraordinarily high H₂ production activity in Na₂S–Na₂SO₃ aqueous solution is achieved up to 18.3 mmol h^?1 g^?1 with an apparent quantum efficiency of 73.8% per 50 mg under 420 nm light irradiation, which, to our knowledge, outperforms cocatalyst-free metal sulfide photocatalysts previously reported to date. Such super high performance arises from the enhanced visible-light-absorption capacity, suitable conduction and valence band potential together with the facilitated charge transport in Zn–Cd–S solid solutions. This work may open an avenue for the green preparation of inexpensive photocatalysts for solar H₂ production.     

S-doped ZnO nanorods on stainless-steel wire mesh as immobilized hierarchical photocatalysts for photocatalytic H2 production

S-doped ZnO nanorods were grown on stainless steel mesh as immobilized hierarchical photocatalysts for hydrogen production. Properties of the photocatalysts were investigated by field-emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), photoinduced current, and photocatalytic hydrogen evolution test. Effects of polymer additive and doping on the surface texture, surface property, and H₂ production performance of the photocatalysts were studied. Polyethyleneimine helps the growth of nanorods on the entire surface of wire mesh. Photocatalytic H₂ production activity of the photocatalysts changes with dopant content and surface texture modification. Due to increased surface area of the hierarchical photocatalyst, enhanced light trapping and liquid flow among wire-mesh, the highest hydrogen evolution rate of 3640 μmol g⁻¹ h⁻¹ is obtained. The photocatalytic activity of photocatalyst remained at 87% of its original performance after five cycles.     

A perspective on the fabrication of heterogeneous photocatalysts for enhanced hydrogen production

This perspective provides an insight to the possibility of adopting hydrogen as a key energy-carrier and fuel source, through Photocatalytic water splitting in the near future. The need of green and clean energy is increasing to overcome the growing demand of sustainable energy throughout globe, owing to CO₂ emission using fossil fuels. To generate highly efficient and cost-competitive hydrogen, the semiconductor based heterojunction nanomaterials have gained tremendous consideration as a promising way. Currently, the efficiency for hydrogen generation through UV–Vis active photocatalysts is relatively low. The key issues are found to be poor separation of photogenerated electron/hole, less surface area, and low absorption region of electromagnetic spectrum. Such issues arise due to inappropriate band edge potentials and large bandgap of present catalyst. A lot of schemes has been devoted to design and fabricate efficient photocatalysts for improved photocatalytic performance in recent years. However, it seems still a challenge and imperative to greatly comprehend the fundamental aspects, photocatalysis and transfer mechanisms for complete deployment of electron/hole pairs. Further, to produce hydrogen to a larger extent through photocatalytic water splitting, the photocatalyst has been modified through co-catalysts/dopants using numerous techniques including the Z-scheme, hybridization, crystallinity, morphology, tuning of band edge positions, reduction of the band gap, surface structure etc., such that these heterogeneous photocatalysts may have ability to absorb enough light in the UV-VIS-IR region. This type of heterogeneous photocatalysts has the ability to improve the rate of efficiency for hydrogen evolution through absorption of sufficient light of solar spectrum and enhance the separation of charge-carriers by inhibiting recombination of electron/hole pairs. We surmise that taking into account the aforesaid factors should support in scheming an efficient photocatalysts for hydrogen production through water splitting, eventually prompting technological developments in this field.     

High-performance hydrogen evolution of NiB/ZnCdS under visible light irradiation

With the continuous growth of global energy demand, the artificial photosynthetic system of photocatalytic decomposition of hydrogen production has been widely concerned, CdZnS, a photocatalyst based on solid solution, has shown good performance. In order to avoid the rapid recombination of photogenerated electron and hole, metal boride is added as a catalyst to enhance its photocatalytic hydrogen production performance. We prepared CdZnS photocatalyst by hydrothermal method, and synthesized different proportions of NiB/CdZnS catalyst by programmed heating method and calcined it at 773K. Characterized by XRD, XPS, uv–vis diffuse reflection and other methods, CZS solid solution and NiB/CdZnS were synthesized, and the absorption edge of NiB/CZS was redshifted relative to the pure CdZnS. The addition of NiB can effectively inhibit the recombination of electrons and holes to improve the separation efficiency of photogenerated charge, and its content has an impact on photocatalytic activity. We found that under visible light irradiation, when the content is 15 wt%, the hydrogen production was the highest. The optimal quantum efficiency was 13.3%, and the hydrogen production reacheed 8137 μmol/g/h, which is 17 times that of pure CdZnS. The addition of NiB greatly improved the photocatalytic performance of CdZnS.     

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.     

Self-defective ZnS mediated charge transfer for bran-new inter-step mode with boosted photoactivity and enhanced photostability

The appropriate interfacial contact and charges transfer mode of heterojunction photocatalysts were critical for high-efficiency hydrogen production. Inter-step mode heterojunction composite had advantages of enhanced visible-light response, improved charge space separation rate, increased electron utilization, which could also protect catalyst anode from photocorrosion. Zinc-vacancy-rich ZnS decorated CdS heterojunction photocatalyst with inter-step mode was constructed in order to fundamentally enhance photocatalytic performance and overcome photocorrosion of CdS. The charge transfer mode was modulated from pervasive type-II to bran-new inter-step mode by defect engineering. Zinc vacancies functioned as acceptor level for charge separation and up-shifted conduction and valance band energy of ZnS. The defective engineered CdS/ZnS heterojunction displayed a reduced over-potential and enhanced photocatalytic activity. The optimal photocatalytic hydrogen production rate for CdS/ZnS reached 42.1 mmol?g^?1 under visible light without any co-catalyst. An apparent quantum yield (AQY) of 38.75% at 450 nm was achieved, which was 269.3 and 71.9 times higher than pristine zinc-vacancy-rich ZnS and CdS, respectively. Meanwhile, holes aggregated on the surface of CdS were blocked and the oxidation corrosion process was suppressed. The charge transfer mechanism and kinetics of charge transfer and separation in inter-step mode heterojunction photocatalysts were investigated and discussed. This work will accelerate practical applications of photocatalysis with inter-step mode and give deep insights into understanding how inherent acceptor levels play a role in designing defect-engineered semiconductor with enhanced photocatalytic performance.     

CoNi bimetallic alloy cocatalyst-modified TiO2 nanoflowers with enhanced photocatalytic hydrogen evolution

In terms of improving photocatalytic hydrogen production performance, inexpensive and earth-rich cocatalysts have become promising alternatives to precious metals. Herein, a novel CoNi–TiO₂ photocatalyst composed of TiO₂ nanoflowers and CoNi alloy was prepared by hydrothermal and chemical reduction methods. Various characterizations and test results have confirmed that the further improvement of the photocatalytic performance of the CoNi–TiO₂ photocatalyst is mainly due to the fact that the bimetallic CoNi alloy can accelerate charge transfer and inhibit the recombination of photo-induced carriers. The hydrogen production rate of the prepared CoNi–TiO₂ is about 24 times higher than that of the pristine TiO₂, and its hydrogen production rate value can reach 6580.9 μmol g^?1 h^?1, and showing comparable photocatalytic performance to 0.5 wt% Pt–TiO₂. In addition, combined with the characterization results, a probable mechanism for enhanced photocatalytic performance was proposed. This study provides favorable enlightenment for the design of a series of highly efficient non-precious metal TiO₂-based photocatalysts.     

Recent advancements of layered double hydroxide heterojunction composites with engineering approach towards photocatalytic hydrogen production: A review

Photocatalytic hydrogen production has been considered as one of the most promising alternatives for providing clean, sustainable, and renewable energy sources. Tremendous investigation and efforts have been devoted to increase the efficiency of the solar to energy conversion of a photocatalyst. Layered double hydroxide (LDH) received scientific attention for its excellent compositional flexibility and controllable morphology, leading to the facile incorporation of the metal species into their layered structure. The unique multi-structure and the tunability of its band gap make LDH more prominent in the field of photocatalysis. This article highlights the recent developments in the fabrication of LDH-based photocatalyst nanocomposites and the engineering approaches for augmenting their photocatalytic hydrogen production efficiency. The thermodynamics and challenges in photocatalytic water splitting are deliberated to understand the pathways to construct efficient semiconductor photocatalysis system. The efficiency enhancement of LDH-based photocatalysts are comprehensively discussed by giving special attention to the heterojunction engineering of type I, type II, p-n junction, Z-scheme, S-scheme, and R-scheme. Fabrication of the hybrid LDH nanocomposites through band gap engineering and metal loading are summarised. The architectural and morphological tuning of LDH-based composite through the construction of the novel core-shell structure and layer-by-layer nanosheets are also demonstrated. Finally, the future recommendations are outlined to provide insights for their development in the photocatalysis field.     

New insights in the performance and reuse of rGO/TiO2 composites for the photocatalytic hydrogen production

The viability of the photocatalytic hydrogen production is closely related to the performance and long term stability of the photocatalyst. In this work rGO/TiO₂ composites have been synthetized with graphene oxide (GO) ratios from 1% to 10% and experimentally assessed towards hydrogen generation from methanol solutions. The performance of the composite with 2% of rGO (2 GT) has been compared to bare TiO₂ working with 20% volume methanol solution. The hydrogen production initial rate showed similar values with both photocatalysts decreasing after about 24 h. Further analysis of the photocatalytic process at longer times showed the negative influence of hydrogen accumulation in the reaction system. Thus, an experimental procedure with argon purge was developed and the behavior of TiO₂ and 2 GT photocatalysts was compared. It is concluded that TiO₂ keeps its activity after 8 operation cycles while 2 GT performance reduces progressively. This can be attributed to the further reduction of GO and the increase of defects in its structure.