Preparation And Property Of Photoanode Materials For Photoelectrochemical Water Splitting Applications

With the continuous development of the world economy and the depletion of traditional fossil fuels,the international community has to face the the serious energy crisis and environmetal pollution.Hence,developing renewable clean energy has become one of the main means to solve this crisis.Particularly,photoelectrochemical water splitting technology is an important research direction in the field of renewable energy.Using this technology,solar energy can be converted into green and pollution-free chemical energy for flexible storage.In a complete process of photoelectrochemical water splitting process,there are two half reactions,i e.oxidation of water on the photoanode and reduction of water on the photocathode.Specificially,the water oxidation reaction requires continuous four-charge injection,which therefore needs higher activation energy in the process of water oxidation.Therefore,it can be seen that the water oxidation process on the photoanode is the control step of the whole water splitting reaction.Therefore,it is of great importance to develop high performance photoelectrochemical water oxidation photoanode materials.Up to now,the wide-band gap semiconductor represented by titanium dioxide is still regarded as one of the most promising photoanode material for photoelectrochemical water oxidation photoanode material.However,as wide-band gap semiconductor,the optical response range of TiO2 is quite narrow,which therefore leads to limited photogenerated carriers.At the same time,as intrinsic semiconductor,serious carrier recombination often happens within the photogenerated carrier migration process.In addition,due to the slow surface water oxidation kinetics of intr:insic semiconductor(like TiO2),serious photo-induced carrier recombination and photo-corrosion always exist at the electrode/electrolyte interfaces.The synergistic hindrance effects of these three factors consequently make the photoelectrochemical catalysis performance of intrinsic semiconductors unsatisfied.Therefore,by applying and discussing the heterojunction engineering and interface engineering methods,great efforts have been made to improve the optical absorption and light capture capbility of the photoanode materials,as well as the photo-generated carriers transport efficiency and the surface water oxidation dynamics.At the same time,it is also of significance to establish the photophysical model of photogenerated carriers' separation-migration processes and the water oxidation reaction taking place at the electrode/electrolyte interfaces.The main research contents of this paper are as follows:(1)Through a facile hot injection and hydrothermal method,we successfully synthesize Cu2ZnSnS4(CZTS)nanoparticles sensitized metal-organic frameworks(MOFs)-derived mesoporous Ti02.The MOFs-derived TiO2 inherits the large specific surface area and abundant porous structures of the MOFs structure.Also,the inc ident light traveling length can be prolonged by enhancing the multiple interparticle light-scattering processes,resulting in the improved light absorption capacity.The sensitization of CZTS nanoparticles effectively enlarges the photoresponse range of TiO2 to the visible light region and facilitates photo induced carrier transport.The heterostructure between CZTS nanoparticles and MOFs-derived TiO2 with matched band gap structure effectively supp resses the reco mbination rate s of photo generated e lectro n/hole pairs and pro longs the lifespan of the carriers.Serving as the photoanode material,the modest sample achieves the largest photocurrent density of 2.15 mA cm-2(at 1.23 V vs.RHE)and power conversion efficiency of0.77%at a bias potential of 0.44 V(vs.RHE).(2)A facile two-step hydrothermal method is applied to synthesize layered MoS2 coupled metal organic framework(MOF)derived dual-phase TiO2(MDT),as a photoanode for photoelectrochemical(PEC)water oxidation.The MOF-derived TiO2 with a controllable pore size effectively prolongs the light traveling le ngth and lacilitates the interfac ial carrier transport.Coupling with MoS2,the visible light response property and the surface water oxidation kinetics of TiO2 can be obviously enhanced.Importantly,the multi-junction between layered MoS2 and dual-phase TiO2 suppresses the recombination rate of photoinduced electron-hole pairs and effectively enhances charge transfer kinetics.Applied as a photoanode for PEC water splitting,it shows a photocurrent density of 1.99 mA cm-2 at 1.23 V vs.RHE.(3)A novel hierarchical structured photoanode based on metal-organic frameworks(MOFs)-derived porous Co3O4-modified TiO2 nanorod array grown on Si(MOFs-derived Co3O4/TiO2/Si)is developed as photoanode for efficiently photoelectrochemical(PEC)water oxidation.The ternary Co3O4/TiO2/Si heterojunction displays enhanced carrier separation performance and electron injection efficiency.In the ternary system,an abnormal type-? heterojunction between TiO2 and Si is introduced,because the conduction band and valence band position of Si are higher than those of TiO2,the photogenerated electrons from TiO2 will rapidly recombine with the photogenerated holes from Si.In that case,an efficient separation of photogenerated electrons of Si/holes from TiO2 at the TiO2/Si interface can be obtained,greatly improving the separation efficiency of photogenerated holes within TiO2 and enhancing the photogenerated electron injection efficiency in Si.Meanwhile,the MOFs-derived Co3O4 obviously improves the optical-re sponse performance and surface water oxidation kinetics due to the large specific surface area and porous channel structure.Compared with MOFs-derived Co3O4/TiO2/FTO photoanode,the synergistic function in the MOFs-derived Co3O4/TiO2/Si NR photoanode brings greatly enhanced photoconversion efficiency of 0.54%(1.04 V vs.RHE)and photocurrent density of 2.71 mA cm-2 in alkaline electrolyte.(4)By virtue of a facile electrostatic deposition-high temperature annealing method,a brand new metal-organic framework(MOF)-derived Co3C nanosheet with narrow bandgap energy is demonstrated to effectively sensitize TiO2 hollow cages as a heterostructured photoanode for PEC water oxidation.It is found that MOF-derived Co3C nanosheet with narrow bandgap characteristic can simultaneously accelerate the surface water oxidation kinetics and extend the light harvesting range of pristine TiO2.Meanwhile.a uniquely matched type-? heterojunction constructed between MOF-derived Co3C and TiO2 results in an evidently spontaneous e-/h+ separation.MOF-derived Co3C/TiO2 heterostructure photoanodes bring about drastically improved PEC water oxidation performance.Specifically,MOF-derived Co3C/TiO2 photoanode with an optimized content of Co3C achieves the highest photocurrent density and charge separation efficiency of2.6 mA cm-2 and 92.6%at 1.23 V vs.RHE,corresponding to 201%and 152%improvement compared with pristine TiO2 nanocages.(5)Thro ugh a facile ion insertio n-thermal c utting method,the Janus-structured cobalt nanoparticle coupled Ti3C2 MXene quantum dot(Co-MQD)Schottky catalyst with a tunable Co loading content can be obtained,serving as photoelectrochemical(PEC)water oxidation photoanode.The introduction of Co terminal not only triggers concomitant surface plasmon effect,but also acts as water oxidation center,which greatly enhances visible light harvesting capability and surface water oxidation kinetics of pristine MQD.Most importantly,due to the rectifying effect of the Schottky-junction introduced by Janus Co-MQD,the photogenerated carrier separation/injection efficiency of Co-MQD can be fundamentally facilitated.Specifically,the modest Co-MQD sample exhibits both superior PEC water oxidation(2.99 mA cm-2 at 1.23 V vs.RHE)and charge migration performance(?separation × ?injection of 87.56%),corresponding to 194%and 236%improvement in comparison with pristine MQD.(6)By means of a facile coprecipitation-calcination method,MOFs-derived Co3O4/N-doped C framework stabilized CsPbBr3 compound can be synthesized and serving as the photoanode material of photoelectrochemcial water oxidation.The Pyrrodic-type N arisen from the MOFs-derived N-C can be spontaneously bonded with the bromine from CsPbBr3 nanocrystal,leading to the formation of unique N-Br Halogen bond,which can efficiently stabilize the CsPbBr3 nanocrystal.Meanwhile,the porous MOFs-derived Co3O4/N-doped C framework can also contribute a lot to the stabilization of CsPbBr3 nanocrystal due to the channel confinement effects.In addition,the system carrie r transpo rtation perfo rmance is greatly enhanced by ratio nally contro lling the energy band structures of CsPbBr3 and Co3O4/N-C and the carrier density can be greatly enhanced by carefully tailoring component of MOFs-derived Co3O4/N-C/CsPbBr3 compounds.The modest Co3O4/N-C/CsPbBr3 sample can achieve the best photocurrent density and charge migration performance of 1.38 mA cm-2 and 41.33%(at 1.23 V vs.RHE),respectively,which are 536%and 122%times higher comparing with pristine CsPbBr3.