Photodeposition of CoOx and MoS2 on CdS as dual cocatalysts for photocatalytic H2 production

Photocatalytic Hproduction from water splitting has a promising prospect for alleviating energy and environmental issues.However,the fast recombination of photogenerated charge carriers limits the photocatalytic efficiency and its practical application.Cocatalyst engineering is an effective strategy to spatially separate photogenerated charge carriers.In this work,noble-metal-free MoSand CoOcocatalysts are loaded on CdS nanorods by a two-step photodeposition method.The MoSfunctions as the reduction cocatalyst to trap electrons and Co Oas the oxidation cocatalyst to trap holes.Transmission electron microscopy(TEM),inductively coupled plasma(ICP),X-ray photoelectron spectroscopy(XPS)and MottSchottky results demonstrate the effectiveness of photodeposition for loading MoSand CoOdual cocatalysts on CdS and their impact on the photochemical properties.The optimized CdS-MoS-CoOcomposite exhibits a high photocatalytic H-production rate of 7.4 mmol g^(-1)h^(-1)and an apparent quantum efficiency(QE)of 7.6%at 420 nm.Further analysis on time-resolved photoluminescence(TRPL)indicates that the introduction of dual cocatalysts greatly prolongs the lifetime of photogenerated charge carriers and deceases the charge recombination rates,consequently leading to superior photocatalytic H-production performance.This work provides a facile and effective strategy for the construction of highly efficient dual-cocatalyst-modified CdS photocatalyst for high-performance photocatalytic Hproduction.

ZnxCd1–xS quantum dot with enhanced photocatalytic H2-production performance

H2 is an important energy carrier for replacing fossil fuel in the future due to its high energy density and environmental friendliness.As a sustainable H2-generation method,photocatalytic H2 production by water splitting has attracted much interest.Here,oil-soluble ZnxCd1-xS quantum dot(ZCS QD)with a uniform particle size distribution were prepared by a hot-injection method.However,no photocatalytic H2-production activity was observed for the oil-soluble ZCS QD due to its hydrophobicity.Thus,the oil-soluble ZCS QD was converted into a water-soluble ZCS QD by a ligand-exchange method.The water-soluble ZCS QD exhibited excellent photocatalytic H2-production performance in the presence of glycerin and Ni^2+,with an apparent quantum efficiency of 15.9%under irradiation of 420 nm light.Further,the photocatalytic H2-generation activity of the ZCS QD was~10.7 times higher than that of the ZnxCd1-xS relative samples prepared by the conventional co-precipitation method.This work will inspire the design and fabrication of other semiconductor QD photocatalysts because QD exhibits excellent separation efficiency for photogenerated electron-hole pairs due to its small crystallite size.

Bound-state electrons synergy over photochromic high-crystalline C_(3)N_(5) nanosheets in enhancing charge separation for photocatalytic H_(2) production

Solar-driven water splitting for photocatalytic hydrogen evolution is considered a highly promising and costeffective solution to achieve a stable renewable energy supply.However,the sluggish kinetics of electron-hole pairs’separation poses challenges in attaining satisfactory hydrogen production efficiency.Herein,we synthesized the exceptional performance of highly crystalline C_(3)N_(5)(HC–C_(3)N_(5))nanosheet as a photocatalyst,demonstrating a remarkable hydrogen evolution rate of 3.01 mmol h^(-1)g^(-1),which surpasses that of bulk C_(3)N_(5)(B–C_(3)N_(5))by a factor of 3.27.Experimental and theoretical analyses reveal that HC-C_(3)N_(5)nanosheets exhibit intriguing macroscopic photoinduced color changes,effectively broadening the absorption spectrum and significantly enhancing the generation of excitons.Besides,the cyano groups in HC-C_(3)N_(5)efficiently captures and converts photoexcited electrons into bound states,thereby prolonging their lifetimes and effectively separating electrons and holes into catalytically active regions.This research provides valuable insights into the establishment of bound electronic states for developing efficient photocatalysts.

TiO2-based materials for photocatalytic hydrogen production

Hydrogen, the cleanest and most promising energy vector, can be produced by solar into chemical energy conversion, either by the photocatalytic direct splitting of water into Hand O, or, more efficiently,in the presence of sacrificial reagents, e.g., in the so-called photoreforming of organics. Efficient photocatalytic materials should not only be able to exploit solar radiation to produce electron–hole pairs, but also ensure enough charge separation to allow electron transfer reactions, leading to solar energy driven thermodynamically up-hill processes. Recent achievements of our research group in the development and testing of innovative TiO-based photocatalytic materials are presented here, together with an overview on the mechanistic aspects of water photosplitting and photoreforming of organics. Photocatalytic materials were either（i） obtained by surface modification of commercial photocatalysts, or produced（ii） in powder form by different techniques, including traditional sol gel synthesis, aiming at engineering their electronic structure, and flame spray pyrolysis starting from organic solutions of the precursors, or（iii） in integrated form, to produce photoelectrodes within devices, by radio frequency magnetron sputtering or by electrochemical growth of nanotube architectures, or photocatalytic membranes, by supersonic cluster beam deposition.

Building surface defects by doping with transition metal on ultrafine TiO_2 to enhance the photocatalytic H_2 production activity

Inefficient charge separation and limited light absorption are two critical issues associated with high‐efficiency photocatalytic H2production using TiO2.Surface defects within a certain concentration range in photocatalyst materials are beneficial for photocatalytic activity.In this study,surface defects(oxygen vacancies and metal cation replacement defects)were induced with a facile and effective approach by surface doping with low‐cost transition metals(Co,Ni,Cu,and Mn)on ultrafine TiO2.The obtained surface‐defective TiO2exhibited a3–4‐fold improved activity compared to that of the original ultrafine TiO2.In addition,a H2production rate of3.4μmol/h was obtained using visible light(λ>420nm)irradiation.The apparent quantum yield(AQY)at365nm reached36.9%over TiO2‐Cu,significantly more than the commercial P25TiO2.The enhancement of photocatalytic H2production activity can be attributed to improved rapid charge separation efficiency andexpanded light absorption window.This hydrothermal treatment with transition metal was proven to be a very facile and effective method for obtaining surface defects.

Photocatalytic H2 Evolution on TiO2 Assembled with Ti3C2 MXene and Metallic 1T-WS2 as Co-catalysts

The biggest challenging issue in photocatalysis is efficient separation of the photoinduced carriers and the aggregation of photoexcited electrons on photocatalyst’s surface.In this paper,we report that double metallic co-catalysts Ti3C2 MXene and metallic octahedral(1T)phase tungsten disulfide(WS2)act pathways transferring photoexcited electrons in assisting the photocatalytic H2 evolution.TiO2 nanosheets were in situ grown on highly conductive Ti3C2 MXenes and 1T-WS2 nanoparticles were then uniformly distributed on TiO2@Ti3C2 composite.Thus,a distinctive 1T-WS2@TiO2@Ti3C2 composite with double metallic co-catalysts was achieved,and the content of 1T phase reaches 73%.The photocatalytic H2 evolution performance of 1T-WS2@TiO2@Ti3C2 composite with an optimized 15 wt%WS2 ratio is nearly 50 times higher than that of TiO2 nanosheets because of conductive Ti3C2 MXene and 1T-WS2 resulting in the increase of electron transfer efficiency.Besides,the 1T-WS2 on the surface of TiO2@Ti3C2 composite enhances the Brunauer–Emmett–Teller surface area and boosts the density of active site.

Precisely decorating CdS on Zr-MOFs through pore functionalization strategy: A highly efficient photocatalyst for H2 production

Different materials,such as metal sulphides,are often combined with metal‐organic frameworks(MOFs)to develop multi‐functional composites and improve their photocatalytic properties.However,the high interfacial energy barrier limits the formation and nano‐assembly of the heterogeneous junctions between MOFs and metal sulphides.Herein,the heterostructured Zr‐MOF‐S@CdS are successfully constructed through a sequential synthesis method,in which the mesoporous Zr‐MOF are firstly decorated with thioglycolic acid through pore functionalization,and followed by the S^(2-)anion exchange process resulting in the surface close attached growth of CdS onto Zr‐MOF‐S materials.Due to the presence of molecules linkers,the CdS can be precisely decorated onto Zr‐MOF‐S without aggregation,which can provide more active sites.Moreover,the intimate connections and the suitable band structures between two materials can also facilitate the photogenerated electron‐hole pairs separation.Therefore,the resulting Zr‐MOF‐S@CdS with appropriate ratio exhibits high photocatalytic activity for water reduction,in which the H_(2) evolution rate can reach up to 1861.7μmol·g^(‒1)·h^(‒1),4.5 times higher than pure CdS and 2.3 times higher than of Zr‐MOF/CdS,respectively.Considering the promising future of MOF‐based photocatalysts,this work may provide an avenue for the further design and synthesis MOF‐based composite photocatalysts for efficient H_(2) evolution.

Synergistic effect of Co（Ⅱ）-hole and Pt-electron cocatalysts for enhanced photocatalytic hydrogen evolution performance of P-doped g-C3N4

g-C3N4 is a metal-free semiconductor and a potential candidate for photocatalytic H2 production,however,the drawbacks,rapid recombination rate and limited migration efficiency of photogenerated carriers,restrict its photocatalytic activity.Herein,Co(II)as a hole cocatalyst modified P-doped g-C3N4 were successfully prepared to ameliorate the separation efficiency of photoinduced carriers and enhance the photocatalytic hydrogen production.The photocatalytic results demonstrated that the P-doped g-C3N4(PCN)exhibited higher photocatalytic activity compared with pure g-C3N4,while Co(II)/PCN photocatalyst exhibited further enhancement of photocatalytic performance.The proposed possible mechanism based on various characterizations is that P-doping can modulate the electronic structure of g-C3N4 to boost the separation of photogenerated-e-and h+;while the synergistic effect of both Co(II)(as hole cocatalyst)and Pt(as electron cocatalyst)can not only lead to the directional shunting of photogenerated e+-h?pairs,but further accelerate the photogenerated electrons transfer to Pt in order to join the photocatalytic reduction process for hydrogen evolution.As a result,the transportation and separation of photoinduced carriers were accelerated to greatest extent in the Pt/Co(II)/PCN photocatalyst.

A review on 2D MoS_(2) cocatalysts in photocatalytic H_(2) production

Owing to their unique physicochemical,optical and electrical properties,two-dimensional(2D)MoS_(2) cocatalysts have been widely applied in designing and developing highly efficient composite photocatalysts for hydrogen generation under suitable light irradiation.In this review,we first elaborated on the fundamental aspects of 2D MoS_(2) cocatalysts to include the structural design principles,synthesis strategies,strengths and challenges.Subsequently,we thoroughly highlighted and discussed the modification strategies of 2D MoS_(2) H2-evolution cocatalysts,including doping heteroatoms(e.g.metals,non-metals,and co-doping),designing interfacial coupling morphologies,controlling the physical properties(e.g.thickness,size,structural defects or pores),exposing the reactive facets or edge sites,constructing cocatalyst heterojunctions,engineering the interfacial bonds and confinement effects.In the future,the forefront challenges in understanding and in precise controlling of the active sites at molecular level or atomic level should be carefully studied,while various potential mechanisms of photogenerated-electrons interactions should be proposed.The applications of MoS_(2) cocatalyst in the overall water splitting are also expected.This review may offer new inspiration for designing and constructing novel and efficient MoS_(2)-based composite photocatalysts for highly efficient photocatalytic hydrogen evolution.

Fabricating covalent organic framework/CdS S-scheme heterojunctions for improved solar hydrogen generation

The fabrication of S-scheme heterojunctions has received considerable attention as an effective approach to promote the separation and migration of photoexcited electron/hole pairs and retain strong redox abilities.Herein,an imine-based porous covalent organic framework(COF-LZU1)is integrated with controllably fabricated Cd S hollow cubes,resulting in the formation of an S-scheme heterojunction.When the COF content reaches 1.5 wt%,the COF/Cd S heterostructure(1.5%COF/Cd S)achieves the highest hydrogen generation rate of 8670μmol·h^(-1)·g^(-1),which is approximately 2.1 times higher than that of pure Cd S.The apparent quantum efficiency(AQE)of 1.5%COF/Cd S is approximately 8.9%at 420 nm.Further systematic analysis shows that the intimate contact interface and suitable energy band structures between Cd S and COF can induce the formation of an internal electric field at the heterojunction interface,which can effectively drive the spatial separation of photoexcited charge carriers and simultaneously maintain a strong redox ability,thus enhancing the photocatalytic H_(2) evolution performance.

Slow photons for solar fuels

Converting solar energy into hydrogen and hydrocarbon fuels through photocatalytic H2production and CO2photoreduction is a highly promising approach to address growing demand for clean andrenewable energy resources.However,solar‐to‐fuel conversion efficiencies of current photocatalysts are not sufficient to meet commercial requirements.The narrow window of solar energy that can be used has been identified as a key reason behind such low photocatalytic reaction efficiencies.The use of photonic crystals,formed from multiple material components,has been demonstrated to be an effective way of improving light harvesting.Within these nanostructures,the slow‐photon effect,a manifestation of light‐propagation control,considerably enhances the interaction between light and the semiconductor components.This article reviews recent developments in the applications of photonic crystals to photocatalytic H2production and CO2reduction based on slow photons.These advances show great promise for improving light harvesting in solar‐energy conversion technologies.

Highly Selective Photocatalytic Conversion of Glucose on Holo-Symmetrically Spherical Three-Dimensionally Ordered Macroporous Heterojunction Photonic Crystal

Photocatalytic conversion of biomass is considered an effective,clean,and environmentally friendly route to obtain high-valued chemicals and hydrogen.However,the limited conversion efficiency and poor selectivity are still the main bottlenecks for photocatalytic biomass conversion.Herein,we report the highly selective photocatalytic conversion of glucose solution on holosymmetrically spherical three-dimensionally ordered macroporous TiO_(2)-CdSe heterojunction photonic crystal structure(s-TCS).The obtained s-TCS photocatalysts show excellent stability and strong light harvesting,uniform mass diffusion and exchange,and efficient photogenerated electrons/holes separation and utilization.The optimized s-TCS-4 photocatalyst displays the highest photocatalytic performance for glucose oxidation and hydrogen production.The glucose conversion,lactic acid selectivity,and yield on s-TCS-4 are about 95.9%,94.3%,and 96.4%,respectively.The photocatalytic production of lactic acid for s-TCS-4(18.5 g/L)is 2.3 times higher than the pure spherical TiO_(2) photonic crystal without CdSe(s-TiO_(2),8.1 g/L),and the hydrogen production rate of s-TCS-4 is 9.4 times that of s-TiO_(2).For the first time,we reveal that the photocatalytic conversion of glucose to lactic acid is a third-order and four-electron-involved reaction.This work could shed some new light on the efficient photocatalysis conversion of biomass to highly value-added products with high selectivity and yield,and simultaneously sustainable hydrogen evolution.

Highly efficient single-crystalline NaNb_(1-X)Ta_(X)O_(3)(X=0.125)wires:The synergistic effect of tantalum-doping and morphology on photocatalytic hydrogen evolution

For the first time in this work,we manage to synthesize single-crystalline NaNb_(0.875)Ta_(0.125)O_(3) wires by combining the advantages of one-dimensional(1 D)nanostructure and heteroatom doping strategy.Careful Ta doping was performed to figure out the correlation between morphological and structural evolution as well as the photocatalytic performance towards H2 generation.It was found that,the as-prepared NaNb_(0.875)Ta_(0.125)O_(3) wires presented a highest and stable photocatalytic performance,which was appropriately 41 and 2 folds higher than that of bare Na TaO_(3) and Na NbO3.The optical activity was mainly ascribed to the synergistic effect between appropriate Ta doping and perfect 1 D wire-like morphology,which resulted in fewer defects,improved charge transfer efficiency and higher reduction capability of electrons.On the other hand,a possible photocatalytic mechanism of photocatalytic H_(2) production was proposed in detail.This work creates a new perspective into designing multi-component materials and understanding the mechanism of H_(2) evolution,which offers new opportunities for solar-energy conversion.

Slow Photon-Enhanced Heterojunction Accelerates Photocatalytic Hydrogen Evolution Reaction to Unprecedented Rates

In photocatalysis,both the photogenerated charge separation and transport and the induced light utilization greatly influence performance.In this work,highly ordered CdS@ZnO core-shell inverse opal(CdS@ZnO-csIO)nanocomposites have been successfully designed as a model to couple the heterojunction system with the slow photon effect for photocatalytic H2 production.Theoretical calculations and experimentation provide direct evidence for the slow photon effect in the CdS@ZnO-csIO nanocomposites.The type II heterojunction is responsible for promoting the migration and separation of photogenerated charges,and the slow photon effect is in charge of enhancing light harvesting in the CdS@ZnO-csIO nanocomposites.This synergy of two functions gives rise to a significantly enhanced photocatalytic H2 production rate under simulated solar light for the CdS@ZnO-csIO nanocomposites.The highest H2 production rate reaches 48.7 mmol g^(−1)h^(−1)under simulated solar light with the benchmark performance for all reported CdS@ZnO composites.Our work provides proof-of-principle that coupling the heterojunction system with the slow photon effect can greatly enhance the photocatalytic activity of composite photocatalysts.