Synthesis And Photoelectrochemical Properties Of CdSe/TiO₂Nanotubes Array Coaxial Heterojunction

The photoinduced water splitting to produce hydrogen by utilizing solar energyeffectively, is considered to be an ideal way to solve energy crisis thoroughly in the future.The key segment is to develop a stable, efficient and inexpensive photocatalyst.In1972, Fujishima and Honda firstly reported the use of n-type TiO₂semiconductorphotoanode for photocatalytic water splitting. Since then, the TiO₂nanomaterials havebecome more and more attractive for their potential versatile application. Among all of theTiO₂nanostructured materials, vertically oriented TiO₂nanotubes （NTs） prepared on a Tifoil tend to be exceedingly popular in the application of photoinduced water splitting onaccount of their high specific surface area and excellent surface adsorption capability.However, due to their limited optical absorption and low quantum efficiency, theirwidespread use was severe limited.In this paper， CdSe, a kind of narrowband gap semiconductor compound, issuccessfully deposited onto the walls of the highly defined and ordered top-opening TiO₂nanotube array. Photoelectrochemistry properties of resulted film electrodes werediscussed and some results were achieved.A two-step anodization method was developed to obtain highly defined and orderedtop-opening TiO₂nanotube array in the electrolyte systems which contain H2O, NH4F andethylene glycol. Analysis results of FE-SEM and XRD indicate that the mixedcrystal-type oxide layers act as protective layers in the subsequent tube growth process,which can efficiently eliminate the undesired disintegration of the top end of nanotubesThe under-potential deposition （UPD） of Cd and Se elements on TiO₂nanotubesarray substrate was studied. CdSe, a kind of narrowband gap semiconductor compound, issuccessfully deposited onto the walls of the highly structured and orderly TiO₂nanotubesarray using a combination of UPD and co-deposition at a suitable potential. FESEM，XRD，EDAX and TEM analyses show that CdSe/TiO₂nanotube arrays with coaxialheterojunction structure have been formed.Contrastive analysis of the UV-vis diffuse spectrum showed that the resulted film hadgood visible response performance after the introduction of epitaxial layer CdSe onto the walls of TiO₂nanotube arrays.The thickness of the epitaxial layer was adjusted through changing the deposit time ata suitable potential. The photoelectrochemical properties of the resulted films with theCdSe/TiO₂nanotube-array coaxial heterojunction were discussed. The results show thatthe maximum photoconversion efficiency （6.45%）, almost a5-fold enhancement inphotoelectric activity comparedto that of a pure TiO₂nanotube array film,can be obtainedfor the sample with the sensitization layer using the deposition time of7h. However, itwas found in this study that as the deposition time increased, the photoconversioneffciency of corresponding sample abnormally minished. The reducedphotoelectrochemical performance of sample deposited CdSe for9h can be attributed to adecrease of surface area of the NTs for harvesting light.In addition, the effect of annealing atmosphere on the photoactivity of as-preparedsamples was also investigated. It suggested that the annealing in nitrogen atmospherecould introduce the N atoms to TiO₂lattice to a certain extent, which was conducive to theimprovement of photoactivity. However, the efficacy tended to decline as increasing thedeposition time.The modification through the Cu doping to coaxial heterojunction structure wastentatively performed at the end of experiment.It was found that sandwich structure ofTiO₂/CdSe/Cu/CdSe could achieve a better photoelectric activity compared to otherstructures. The maximum photoconversion efficiency reached14.20%when the Cudepsition charge was0.6C.