Investigation On The Photocatalytic Decomposition Of Water Based On One-dimensional Vertically Oriented TiO₂ Arrays

The photoelectrochemical decomposition of water,which uses solar energy to split water and produce cheap hydrogen as a clean energy carrier,is believed to be able to help solve the crisis of fossil fuel depletion and environment problem.Recently,extensive studies have been carried out on TiO2 based nanomaterials due to their high photochemical stability,nontoxicity and low cost.However,TiO2 is a large band-gap semiconductor(3.2 eV)and its activation is limited only in the UV region.To extend the photoresponse into the visible light region,sensitizing TiO2 NTAs with a narrow band-gap semiconductor is deemed to be a promising strategy.Generally,the photocatalytic ability is not only determined by the absorption of visible light but also the separation and transportion of photoexcited electron-hole pairs.An accurate control and manipulation of the microstructure for structural modification and composite structure,which is helpful to accelerate the separation of charge and improve the efficiency of splitting water.Besides,the structure of photoanode play an important role in the efficiency of watesplitting.The conventional mesoporous TiO2 which possesses larger specific surface area can offer substantial attachment sites for sensitizers.However,random hop of electrons will be likely to occur recombination process due to the abundant grains boundaries.In comparison with the traditional TiO2 nanocrystal-based photoelectrodes,the highly ordered one-dimensional nanostructure TiO2 photoanode provides a unidirectional electrical channel for charge transfer,so that photoinduced electron-hole pairs can be effectively separated.Hence,based on the domestic and foreign research status,the research focus of this work is on the modification and composition of the one-dimensional nanostructure TiO2 photoanode,including the choice and fabrication of semiconductor and modification of microstructure and surface.The main abstract of research work and conclusions are as followed:1.To extend the light absorption width,sensitizing TiO2 nanotube arrays(NTAs)with a narrow band gap semiconductor Cu2O,which result in the formation of heterogeneous structure.Such structure is formed by coating nano-particles of Cu2O onto titanium dioxide nanotube-array walls via multiple-cycle chemical adsorption plus reduction method.The size of Cu2O nano-particles is adjusted by changing the glucose concentration in the reaction solutions.The shape of Cu2O particles changed from cubic to sphere and their size obviously decreased after adding the glucose.The size of Cu2O particles even reduced from sphere to nanoparticle in the wake of the increasing glucose content from 0.5 M to 1.0 M.As a result,coaxial heterogeneous structures photoanode within a homogeneous coating are obtained.To prevent Cu2O nanoparticles growth from aggregating and enable them form very even and uniform particle film with very small particle size,a soaking step was introduced to separate the adsorption and reduction processes.Meanwhile,the soaking step limit the diffusion of N2H4 to the nuclei and thus inhibit the further reduction of Cu+ into Cu.Particularly,by restricting the number of cycles,the thickness of Cu2O coating layer can be precisely controlled.The results show that 10nm film with 8cycles is optimum,with a phontocurrent density of 7.2 mA/cm2 and a stable hydrogen production rate of 3.1 mLcm-2h-1.2.To further extend the the light absorption width of TiO2 nanotubes,sensitizing TiO2 NTAs with two different narrow band gap semiconductor,which result in the formation of three-layers structure.Underpotential electrochemical atomic layer co-deposition is a new method to fabricate nano-film materials.It is based on the combination the surface limited reactions and electrochemical atomic layer co-deposition.Cu2Se/CdSe/TiO2 NTAs photoanode was fabricated through this method.Firstly,a suitable potential of-0.7 V was applied to co-deposit the CdSe nanofilm onto the TiO2 NTAs substrate under the underpotential deposition conditions.The fabricated photoanode CdSe/TiO2 NTAs show a photocurrent density of 7,5mA/cm2 and exted the absorption tail to 650 nm.The microstructure and composition of the Cu2Se top layer are regulated and controlled by doping Cu with various amounts in different zones of the CdSe/TiO2 coaxial heterojunction and then using a simple integral annealing process.Surprisingly,a little effort made to achieve the Cu2Se top layer utilizing such doped CdSe/TiO2 exhibits a significant enhancement in photocatalytic activity.The maximum stable photocurrent density of the sample with doping concentration of 0.6C has reached up to 28 mA/cm2,which shows a 14-fold enhancement in photocurrent density in comparison with the pure TiO2 NTAs.The absorption of Cu2Se/CdSe/TiO2 NTAs photoanode is further red-shifed and has an absorption tail to 800 nm,gaining a band-gap value of 1.5 eV.The Cu2Se/CdSe/TiO2 NTAs show a pronounced response in the visible light region,especially around 500nm with a maximum incident-photon-to-current-conversion efficiency(IPCE)value of 90%obtained,which outclass that of CdSe/TiO2 NTAs with 31.9%.3.We report an epitaxial hetero-structure of CdSe/TiO2 nanotube arrays as efficient photo-anodes via simple room-temperature,low-cost electrochemical deposition.Furthermore,an ultrathin PEDOT surface layer is developed on the epitaxial hetero-structure of CdSe/TiO2 nano-tube arrays with galvanostatic polymerization method.With the help of the CdSe sensitization layer is epitaxially grown on the tube wall of the TiO2 nanotubes,resulting in an ideal coherent grain boundary.The resultant photo-anode produces a photocurrent density of 7.5 mA/cm2.After electroploymerization of PEDOT on the CdSe/TiO2 NTAs using the galvanostatic polymerization with current 500uA,the improvement of photocurrent densities of CdSe/TiO2 NTAs was obvious.In particular,the photocurrent density of 14mA/cm2 at 0V was found for the PEDOT/CdSe/TiO2 NTAs sample in which the PEDOT was deposited for 10min.The PEDOT/CdSe/TiO2 shows the smaller charge transfer resistance and stronger light absorption especially in visible light zone from 700nm to 1000nm.Additionally,the PEDOT coating increases the stability of the photoanode.After 3.5h of continuous illumination,the photocurrent density of PEDOT/CdSe/TiO2 NTAs retains 80%of initial value.Ultrathin PEDOT surface layer function as both a physical passivation barrier and a hole transfer layer.It synergistically prevents photo-degradation of CdSe and promotes the hole-scavenging reactions by efficiently transferring photoinduced holes.4.To invesgate the effect of structure of sensitizer to photocatalytic performance,the CdSe/TiO2 NRs with variable deposition amount of CdSe is fabricated.The results show that one-dimensional rod-sphere structural CdSe/TiO2 NRs perform enhance the photocatalytic ability because of the CdSe ball heighten the absorption of light.Besides,with the increasing deposition amount of CdSe,the CdSe/TiO2 NRs exhibit three-dimensional structure,CdSe particle growing on plannar structure.This structural CdSe/TiO2 NRs show best photocatalytic performance owning to their good carrier transport pathway and high specific surface area.The stable photocurrent density of CdSe(2.5C)/TiO2 NRs reach up to 5mA/cm2,which is ten times than that of pure TiO2 NRs photoanode.