Preparation Of Modified TiO₂Nanotube Arrays And Their Photoelectrical Properties

TiO₂nanotubes arrays fabricated by anodization method have highly orderedone-dimensional structure, higher electron transport and electron lifetime, which havea better application prospect in dye-sensitized solar cells（DSSCs）. However, becauseof the problems of collapose structure in synthetic process and low dye adsorption,the one-dimensional structure didn’t show advantages. This study carried out researchfor the problem of synthesized TiO₂nanotube arrays and how to improve thephotoelectric conversion efficiency of DSSCs. The structures and influence factorswere investigated by using different electrolytes. The photoelectric conversionefficiencies were improved through using surface modification, ZnO and Sm₂O₃modified TiO₂nanotubes.The TiO₂nanotube arrays were fabricated in H₃PO₄/NH₄F electrolyte by anodicoxidation method, in which the effects of stirring rates, calcination temperatures,H₃PO₄ammounts, F-concentrations, oxidic voltages and reaction times on surfacemorphology, tube length and tube diameter were investigated using FESEM analysis.The results show that stirring rates, F-concentrations and voltages have importanteffects on the morphology and pore diameter, whereas anodization times affect thelength of the TiO₂nanotube arrays. Based on the influencing conditions of singlefactor, we selected three important factors. The optimum experimental conditionswere obtained by using Box-Behnken Design. The optimal factor conditions wereobtained as follows: anodization time of240min, anodization voltage of15.39V,and NH₄F concentration of0.50mol/L. The results of software simulation using theoptimized conditions were as follows: TiO₂nanotube arrays were obtained with anaverage tube length of1.43μm and average tube diameter of33nm. Moreover, theeffects of calcining temperatures and calcining times on the phase transformation andphoto-electrical properties of TiO₂nanotubes were analyzed by XRD and UV-vis.The optimum calcination conditions were obtained at500℃for2h. The mechanismof fabrication of TiO₂nanotube arrays was investigated using real time-currentanalysis.The TiO₂nanotube arrays were constructed in ethylene glycol electrolyte byanodic oxidation method. The effects of F-concentrations, oxidic voltages, amount ofH2O and reaction temperatures were investigated. The results show that the H2Oamount is a key factor to controlling the formation TiO₂nanotube arrays. TiO₂nanotube arrays have smooth tube wall and the most ordered structure obtained in anelectrolyte mixture of ethylene glycol,2vol%H2O and0.3wt%NH₄F at40V with 25℃reaction temperature. The surface of the TiO₂nanotube arrays were treatedwith titanium （IV） n-butoxide using a hydrothermal method in combination with anoxygen-plasma treatment. Comparative studies show that the roughness,hydrophilicity and the dye adsorption are increased. The TiO₂nanotube arrays treatedby the combined methods under optimum conditions exhibited a conversionefficiency is increased from1.89%to3.68%.ZnO/TiO₂nanotube arrays were prepared by a convenient electro-depositiontechnique. The morphology, structure and electrochemical properties wereinvestigated. It is found that ZnO/TiO₂nanotube arrays possess higher roughness,higher dye adsorption, lower band gap energy and longer electron lifetime. Comparedwith that of bare TiO₂nanotube arrays, the photoelectrical performance of thedye-sensitized solar cells is increased from1.71%to3.76%with deposition for60min. This improvement comes from the synergetic effect between ZnO and TiO₂.The highly ordered TiO₂nanotubes grown on Ti substrate were modifiedconveniently with samarium （Sm） by hydrothermal method for use in backilluminated dye-sensitized solar cells. The morphology, crystal phase, fluorescenceand electrochemical properties were investigated. It is found that Sm₂O₃/TiO₂nanotubes can decrease grain size and band gap energy, and transfer ultraviolet lightto visible light. As a down-conversion luminescence material, samarium oxideimproves the UV radiation harvesting. The result shows that Sm₂O₃modified TiO₂nanotubes can increase short-circuit current and photoelectrical properties ofdye-sensitized solar cells. When TiO₂nanotubes are modified using0.02mol/LSm₂O₃, the efficiency is improved from1.86%to3.22%compared to the bare TiO₂nanotubes. These modifications provide the theoretical and experimental referencefor improving photoelectrical conversion efficiency of dye-sensitized solar cells infuture explorations.