Photoelectrochemical Property Of TiO₂ Nanowires/Nanotubes Hybrid Structured Film

The limit of resources reserves and pollution of fossil energy became the bottleneck for human future.Solar energy as a green and renewable energy has attracted increasing attention.Substantial research development of new solar energy-driven semiconducting materials became the bull's-eye.In recent decades,titanium dioxide?TiO₂?has been widely investigated in water splitting,photocatalysis and photovoltaics as photoelectrode,corresponding to its outstanding PEC performances.In particular,TiO₂ nanotubes/nanowires array structure?denoted as TWTs?on Tifoil produced by electrochemical anodization method has became a hot topic,owing to its large specific surface area,direct paths of electrons and effectively enhance charge collection efficiency.Thus,it shows higher change collection efficiency than other TiO₂ nanometer materials.In this study,we focus on the synthesis and modification of TiO₂ nanowires/nanotubes hybrid structured film and study on its photoelectrochemical property.We deposited Cd S QDs on the photoelectrodes using the successiveionic layer adsorption and reaction?SILAR?method to optimize the photoelectrochemical system.The main contents of this paper are as follows:?1?TiO₂ nanowires/nanotubes hybrid structured film?denoted as TWTs?was prepared by electrochemical anodization of a titanium foil.Firstly,the as-prepared TWTs were annealed in hydrogen at various temperatures.Subsequently,theformation of Cd S,quantum dot?QD?sensitized hydrogen treated TWTs photoelectrode was carried out by SILAR.SEM,TEM and XRD of the samples were characterized.The best performance of the photoelectrode was TWTs annealed in hydrogen at 350 o C with 6 cycles of Cd S.A maximum short-circuit photocurrent density of 3.64 m A cm-2 and photoconversion efficiency of 2.67%?-0.63 V vs Ag/Ag Cl?were obtained under an illumination of AM 1.5 G,which can boost the photocurrent density of the pristine TWTs by up to 600%.The enhancement was attributed to the extension of the light absorption range by hydrogen treatment and Cd S QD sensitization.?2?With?NH4?2TiF6 as both titanium source and crystal controlling agent,{001}facets exposed polyhedrons modified TWTs?denoted as P&TWTs?were prepared by chemical bath deposition?CBD?method.The PEC performances of P&TWTs and TWTs were studied.Jsc and photoconversion efficiency of P&TWTs are 1.36 m A cm-2 and 0.81%,which are about 2.4 times greater than those of TWTs.Then we load Cd S QDs in situ on the surface of the P & TWTs and TWTs.Jsc and photoconversion efficiency of Cd S/P&TWTs are 2.31 m Acm-2 and 1.94%?-0.65 V vs Ag/Ag Cl?,which are about 1.5 times bigger than those of Cd S/TWTs.This enhancement was attributed to the exposure of the highly active {001} facets and the increased specific surface area.Furthermore,possible growth mechanism was proposed by studying on the morphology evolution under different reaction conditions.?3?The uniform exposed facets of TiO₂ nanoparticles attached to TWTs were successfully controlled by introducing F-/NH4+ ions during the TiCl₄ treatment process.We synthesized TWTs photoelectrodes with randomly deposited TiO₂nanoparticles?non-facet?,TiO₂ nanofilms,TiO₂ nanocuboids with exposed {001}facets,and TiO₂ nanocuboids with exposed?111?facets,denoted as PTWTs,FTWTs,C001 TWTs and C111 TWTs,respectively.Then we demonstrated that the C111 TWTs photoelectrode displayed significantly greater PEC performances than did the otherphotoelectrodes,owing to its increased surface-to-volume ratio and highly reactive?111?exposed facets,which effectively increased the utilization rate of the photoelectrode surface.The C111 TWTs photoelectrode achieved a Jsc and photoconversion efficiency of 0.97 m A cm-2 and 0.49%,respectively,which were much higher than those of the other electrodes,i.e.,the C001TWTs?0.76 m A cm-2 and0.46%?,FTWTs?0.66 m A cm-2 and 0.41%?,PTWTs?0.56 m A cm-2 and 0.36%?,and TWTs?0.52 m A cm-2 and 0.31%?photoelectrodes.After deposition Cd S QDs on the photoelectrodes,the Cd S/C111 TWTs photoelectrode presents superior PEC performance,which Jsc and photoconversion efficiency of 3.95 m A cm-2 and 2.05%,respectively.This is because of the exposure of high-energy?111?facets,which are the predominant sources of active sites and which favor Cd S adsorption.?4?[001] preferred oriented TWTs was successfully prepared via introducing TiF62-into system during anodization process.We demonstrated that TiF62-could effectively change the growth patterns of TWTs,i.e.preferred oriented altered from[101] to [001]?denoted as T101 WTs and T001WTs?.T001 WTs achieves a maximum Jsc of 1.81 m A cm-2,? of 0.85% under an illumination of 100 m W cm-2,which are nearly 3.35,2.74 times bigger than those of T101 WTs,respectively.Moreover,a possible mechanism for the observed c-axis preferred oriented anatase TiO₂ nanotubes and nanowires mixed nanostructures morphology is discussed.Furthermore,we successfully sensitized Cd S QDs onto TWTs,and T001WTs/Cd S shows superior PEC performance than that of T101WTs/Cd S.This is attributed to the improved electrical conductivity of c-axis orientation and increased amount of Cd S adsorption,which leads to a decreased electron recombination and improved light absorption capability.T001-2.5WTs/Cd S achieves a maximum Jsc of 4.23 m A cm-2,? of3.08 %,which are nearly 1.75,2.59 times greater than those of T101WTs/Cd S,respectively.