Study On Chemical Modification Of TiO₂Solar-Hydrogen Electrode And Its Charge Transfer Mechanism

Hydrogen is the the effective and clean energy in the future. The simplest and themost potential method is the photoelectrochemical generation of hydrogen thoughwater splitting using solar energy. TiO₂is considered to be the first choice forcommercial photoelectrode because of its richness, innoxiousness and stableness.Hower, the poor absorption of sunlight and the high rate of photogeneratedelectron/hole pair recombonation are two major factors limiting further improvementof its photoelectrocatalytic efficiency. In this research, on the base of TiO₂nanotubearrays （TNT） fabricated by electrochemical anodic oxidation, we studied the chemicalmodified TiO₂nanotube arrays （Fe-TNT, CdS-TNT）. The morphologies, spectralproperties, crystal structures, elemental composition and state were characterized bymeans of SEM, UV-Vis DRS, XRD and XPS techniques respectively. Thephotoelectrical properties of electrodes were analyzed by I-t curves, CV curves,Mott-schottky polts and EIS techniques. And we discussed the charge transfercharacteristics of CdS-TNT photoelectrodes combined with the hydrogen productionabilities.TiO₂nanotube arrays were fabricated by electrochemical anodic oxidationmethod, in different electrolyte, different stir methods and on different Ti substrates.The suface morphology of TiO₂nanotube arrays were similar, which were fabricatedin two different fluoride solution（0.2mol/L Na2SO₄+0.5wt.%NaF and0.5mol/LH₃PO₄+0.14mol/L NaF）.The photocurrent density of these two electrodes were bothabout0.54mA/cm². Compared with magnetic stirring method, sonoelectrochemicalmethod can quickly synthesize well-ordered and robust titanium dioxide nanotubulararrays. And the TNT fabricated by sonoelectrochemical anodic oxidation had betterphotoabsorption characteristic and photoelectrical properties. Base on this, TiO₂nanotube arrays were anodized by sonoelectrochemical anodic oxidation. The surfacemorphology of the nanotube arrays fabricated on different titanium substrate weresimilar. But the photoelectric characteristics were quite different.The sample TNT-2was the best one and the photocurrent density is1.58mA/cm². This may due to themicro-elements in Ti foils substrate.The Fe doped TiO₂nanotube arrays were fabricated by direct electrochemicalanodic oxidation method. The CdS sensitized TiO₂nanotube arrays were prepared viasonoelectrochemical anodic oxidation-sequential chemical bath deposition method. Ared shift of the absorption edge toward the visible light region was observed for thechemical modified TiO₂nanotube arrays.But the photocurrent density of Fe-TNTreduced compared with the pure TiO₂nanotube arrays. Fe³⁺will gather in the surface of the nanotube arrays when the doping of iron was higher, and become therecombination centre and reduce the activity.There was no obvious change in thesuface structure of CdS-TNT, but the wall thickness were larger. The CdS depositionrelated to deposit times and condition. The photoelectrical properties of CdS-TNTimproved obviously. Compared with the semples made without ultrasonic condition,the CdS nanoparticles evenly distributed throughout the suface of CdS-TNT depositedin ultrasonic field. CdS quantum dots increased, thus facilitated the separation andtransfer of photogenerated charges.The photocurrent density of the nanotube arraysincreased from0.35mA/cm²（TNT-N） to3.6mA/cm²（15-S-CdS-TNT） and4.8mA/cm²（20-S-CdS-TNT）.The hydrogen production rate of CdS modified TiO₂nanotube arrays increasedsignificantly. The average hydrogen production rate of15-S-CdS-TNT and20-S-CdS-TNT were960ul/cm²·h and1258ul/cm²·h respectively. The photovoltaic conversionefficiency of15-S-CdS-TNT and20-S-CdS-TNT reached to4.4%and5.9%.Combined with the flat band potential and the charge transfer of CdS-TNT, wesummarized the influence on the photoelectric characteristics and the hydrogenproduction performance of the CdS chemical modified TiO₂nanotube arrays.