Preparation Of Modified TiO₂ Nanorod Arrays And Their Photoelectrochemical Properties

Titanium dioxide(Ti02)has been widely used in the field of biology,environment and energy,because of its numerous merits such as low cost,ease to produce,nontoxicity,photostability,high catalytic activity and appropriate energy level structure.Oriented single-crystalline TiO2 nanorod(TNR)arrays have attracted more and more attention due to its highly aligned order,controlled length,ease to synthesize and small boundary resistance.One-dimensional structure of nanorods exhibit superior photoelectrochemical(PEC)performance,because they not only provide direct electrical path-ways for efficient transport of photogenerated carriers but also offer large semiconductor-electrolyte interfaces,which can effectively facilitate the transfer of carriers.In this paper,oriented single-crystalline TNR arrays were prepared by a traditional hydrothermal treatment under acidic conditions.Metal/semiconductor-modified TNR compound arrays had been constructed through chemical method.The structural information and PEC performance of the photoanode were systematically investigated.The main contents of the work are as follows:Highly ordered TNR arrays decorated with Ag nanoparticles(NPs)were synthesized on FTO transparent conductive glasses by a hydrothermal treatment and a reduction method.The morphology,structure,composition,optical and PEC properties of the Ag/TNR arrays were investigated in detail.The size and amount of Ag NPs on the TNR can be controlled by changing the number of cycles in AgNO3 solution.Compared with TNR arrays,the Ag/TNR arrays exhibit larger red-shift of the absorption edges and higher absorption intensities in the visible light range.Furthermore,the Ag/TNR arrays sensitized with 5 SILAR cycles show the best photocurrent density of 32 ?A/cm2 at 0 V,which is 5.7 times higher than that of pure TNR.The excellent properties can be ascribed to localized surface plasmon resonance(LSPR)effect of Ag NPs.TNR arrays have been sensitized by CdS,PbS,CdS/PbS,and PbS/CdS quantum-dots(QDs).The TNR arrays were synthesized by a hydrothermal treatment,and QDs were deposited on the arrays by a successive ionic layer adsorption and reaction(SILAR)technique.The structural information and PEC property were systematically investigated.The results demonstrate that TNR/QDs can effectively facilitate photoinduced carrier transport,suppress the recombination rate of electron-hole pairs,and improve the visible light absorption and PEC properties.The TNR/CdS(7)/PbS(3)sample shows the maximal photocurrent density of 0.9l mA/cm2 at 0 V,which is nearly 37.9,1.5,2.6 and 6.5 times higher than those of bare TNR,TNR/CdS(7),TNR/PbS(3)and TNR/PbS(3)/CdS(7),respectively.The enhanced properties are attributed to the narrow band gaps and high absorption coefficients of QDs,and the type-II nanoheterostructures with a staggered alignment of band edges at the hetero-interface that can facilitate the spatial separation of electron-hole pairs.In-situ deposition and growth of Cu2ZnSnS4(CZTS)nanocrystals on TNR arrays were successfully prepared via a facile and effective method.The TNR arrays were synthesized using a hydrothermal treatment,and the CZTS nanocrystals were deposited by a three-step SILAR technique followed by annealing in a sulfur atmosphere.The structure,crystallinity and quality of CZTS nanocrystallites were verified using Raman spectroscopy.And a series of characterization and measurement were explored.The results demonstrate that the CZTS/TNR arrays can effectively facilitate photoinduced carrier transport,reduce the recombination rate of electron-hole pairs,and as a result demonstrate improved light absorption properties and PEC performance.The CZTS/TNR arrays sensitized with 7 SILAR cycles showed a maximal photocurrent density of 2.17 mA/cm2 at 0 V and the longest electron lifetime of 47.39 ms,which is nearly 80 and 2.03 times higher than those of pure TNR,respectively.The improvement in performance can be attributed to the narrow bandgap and high absorption coefficient of CZTS and the type-? nanoheterostructures with a staggered alignment of band edges present at the heterointerface.Surface passivation with ZnS resulted in a further improvement in PEC performance in comparison to the untreated CZTS/TNR arrays with values of 2.53 mA/cm2 and 59.77 ms for the maximal photocurrent density and longest electron lifetime,respectively.This is attributed to that ZnS exhibits a conduction band minimum higher than that of CZTS,which results in the formation of an energy barrier which prevents the injection of electrons from CZTS to the electrolyte.