Design And Synthesis Of Visible-light Responsive Photoanodes For Photoelectrochemical Water Splitting

The global energy shortage and environmental pollution issues have become increasingly serious over the past decades.The development of clean and renewable energy is of urgent importance.Solar energy,which is abundant,green,and sustainable,the development and utilization of solar energy has attracted more and more attention.However,the fluctuation and intermittence features of solar energy make it difficult to replace fossil fuels for industrial and daily-life applications directly.Photoelectrochemical(PEC)water splitting,which converts solar energy into hydrogen(H2)energy,is a highly promising approach for solar energy utilization.In general,a PEC system is composed of a photoanode,photocathode,and electrolyte.As the key component of the PEC system,the development of efficient,stable,and cost-effective photoelectrodes has been pursued intensively for achieving over 10%solar-to-hydrogen(STH)conversion efficiency.Light absorption efficiency,charge transfer efficiency and interface charge transfer efficiency jointly limit the overall performance of photoelectrochemical water splitting.The currently photoelectrochemical semiconductors still face some problems,such as low light absorption efficiency,serious charge recombination,and slow reaction kinetics.In order to achieve wide light absorption,visible light response semiconductor materials are selected as the research object,and the materials are modified through various optimization strategies such as crystal plane adjustment,doping,and construction of heterojunctions to achieve efficient and stable photoelectrochemicalwater splitting is a research hotspot in this field.1.Highly active deficient ternary sulfide photoanode for photoelectrochemical water splittingBecause of its suitable band gap,adjustable chemical composition and morphology,and abundant active sites,metal sulfides are widely used in the fields of photocatalysis,electrocatalysis,photosensitive elements and photodetectors.First,a ternary metal sulfide CdIn2S4 with stable crystal structure and easy control of the central metal d electrons was synthesized.Then,through thermal treatment under reducing atmosphere,sulfur vacancies were successfully introduced on the surface of CdIn2S4.Electron paramagnetic spectroscopy and spherical aberration electron microscopy characterization confirmed the existence of surface sulfur vacancies.The performance of photoelectrocatalytic water splitting shows that the CdIn2S4 photoanode with sulfur vacancies obtains a photocurrent density of 5.73 mA cm-2 under 1.23 V vs.RHE external bias and simulated sunlight,which is currently reported maximum value of metal sulfide as base single-photon absorber.At the same time,the effect of sulfur vacancies on PEC performance and charge transfer kinetics was systematically studied.Our research proves that surface sulfur vacancies can not only increase the interface carrier concentration and increase the photoanode/electrolyte band bending but also effectively control the surface state distribution on the CdIn2S4’ photoanode,which is the reason for the high-efficiency photoelectrochemical water splitting performance.2.Surface states regulation of CdIn2S4/InOx/NiFe-LDH photoanode for photoelectrochemical water splittingThe intrinsic photo-generated charge separation efficiency and surface water oxidation reaction rate of the CdIn2S4 photoanode are low,which makes the photoelectrochemical conversion efficiency of the material low.In addition,metal sulfides are prone to self-oxidation and photo-corrosion due to the accumulation of photo-generated holes on the surface,making the interface structure unstable.A single material has inherent defects,and the design of a high-quality composite interface structure is essential for high-performance photoelectric catalytic water splitting.Here,we use an atomic layer deposition(ALD)ultra-thin InOx passivation layer as a hole tunneling carrier to construct a good contact interface structure between CdIn2S4 and NiFe layered double hydroxide(LDH)nanosheet arrays.To achieve excellent PEC water oxidation.In the composite structure,the ALD ultra-thin InOx protective layer physically isolates CdIn2S4 from the electrolyte to reduce photo-corrosion,and InOx has high hole extraction ability,firmly combined with electrodeposited NiFe-LDH,thereby promoting the transfer of holes from CdIn2S4To the NiFe-LDH active catalytic interface.In addition,NiFe-LDH can not only efficiently transport holes and reduce the overpotential of OER kinetics,but also can regulate the distribution of surface states and increase the concentration of surface states to accelerate the surface OER reaction rate.The CdIn2S4/InOx/NiFe-LDH composite photoanode has a photocurrent of 5.47 mA cm-2 at 1.23 V vs.RHE,and can sustain a certain degree of photocatalytic stability without sacrificial agents.By modifying the passivation layer and OER cocatalyst layer,the CdIn2S4 photoanode not only establishes a good interface structure,reduces charge recombination,but also regulates the distribution of interface carriers,promoting high-concentration carriers to effective and rapid participattion in the surface water oxidation reaction.3.Interfacial Coupling Effect on Electron Transport in Hierarchical TaON/Au/ZnCo-LDH Photoanode with Enhanced Photoelectrochemical Water OxidationThe rational design and construction of interface heterostructures,which can simultaneously accelerate the separation of photogenerated carriers and enhance the kinetics of surface water oxidation,which is necessity for photoelectrochemical(PEC)water oxidation.Herein,we report a new strategy for PEC performance enhancement by combining Schottky heterojunction and semiconductor/water oxidation cocatalysts(SC/WOCs)junction into the TaON photoelectrocatalyst.Compared with the original TaON photoanode,the onset potential of the hierarchical TaON/Au/ZnCo-LDH(LDH=layered double hydroxide)photoanode reveals a negatively shift of 156 mV,and photocurrent density increased 17.3 times under 1.23 V vs.RHE,as well as improved PEC water oxidation stability.By studying the mechanism of the TaON/Au/ZnCo-LDH photoanode PEC performance,we found that the improvement of PEC performance is not limited by the kinetics of surface electrochemical water oxidation but mainly attributed to the improvement of charge separation and transfer.It indicates that Au and ZnCo-LDH worked synergistically.According to the energy band matching principle,the role Au is a charge transport carrier to transfer electrons to TaON,then flow to the counter electrode,while holes accumulate on ZnCo-LDH to achieve effective space charge separation,which is beneficial to improve PEC water oxidation.The present work illustrates the importance and mechanism of interfacial heterojunction in PEC water oxidation,which can provide an effective method for design and preparation of photoanodes with new structures.