Study On Behaviors Of Photo-induced Charge Carriers And Photocatatytic Property Of TiO₂-based Composite Nanotube Arrays

It is an important research work to solve the energy and environmental problemsby using solar energy. The essence of the related research is the conversion of solarenergy to chemical energy. Among multiple solutions, using the semiconductorcatalysts is one of the best ways to achieve this goal. There are three stages for asemiconductor to take part in the photocatalytic or photochemical reaction: firstlysemiconductor material absorbs photon energy and electrons in the valance bands areexcited to the conduction bands; secondly photogenerated electrons and holes areseparated on the surface or in the bulk of the semiconductor and the recombination ofthe electrons and holes may also take place in those places; thirdly electrons or holesthat transferred to the surface of the semiconductor take part in the reduction oroxidation reaction. The study of each stage can help us to understand more about thesemiconductor catalysts. Among them the second stage about the separation, transferand recombination of the photogenerated charge carriers are much more complicatedthan the other two. For this reason the second stage becomes an important part inaffecting the efficiency of the photocatalytic or photochemical reaction. It is knownthat the behaviors of the photogenerated charge carriers mostly take place in the spacecharge area of the surface or interface of the semiconductor(s), so the research of thephotoelectric properties of the surface or interface of the semiconductor(s) can bringus a clear way for improving the efficiency of the photocatalytic or photochemical reaction. As a fast and convenient measurement, surface photovoltage (SPV)technique is one of the best way to study the behaviors of the photogenerated chargecarriers as it possesses a lot of merit such as non-contact, high sensitivity,non-destruction. In this technique, the phase spectrum is as important as the SPVspectrum but ignored by researchers for a long time, while it does bring us a newdirection for studying the complicated SPV processes when combined with the SPVvector model.Titania nanotube arrays (TNAs) is a one-dimensional nano-structure material.The precisely oriented nature of the nanotube arrays makes them excellent electronpercolation pathways for vectorial charge transfer between interfaces. Also the doublewall surfaces of the TNAs offer bigger area for the surface related physical orchemical reaction than nanorods or nanowires.Based on the above reasons, TNAs and modified TNAs were adopted for ourstudy and the SPV and transient photovoltage (TPV) measurements were used tostudy the behaviors of the photogenerated charge carriers of the TNAs and modifiedTNAs. Also the photocatalytic degradation of organic pollutant and reduction ofheavy metal pollutant experiments were conducted. Seeking the inherent relationshipbetween the behavior of the photogenerated charge carriers and the photocatalyticreactions is the main purpose in our studies. Based on this, we conducted thefollowing experiments and acquired several new results:1. TNAs were synthesized in a0.3wt.%NH4F+2vol.%H2O+98vol.%ethylene glycol mixed solution via potentiostatic anodization of titanium foil. SPVspectrum, SPV phase spectrum and TPV techniques were used for investigating thebehaviors of photogenerated charge carriers. The results showed that the external biascan barely affect the separation and transfer behavior of the surface states relatedphotogenerated charge carriers; the transfer direction of the photogenerated chargesunder the effect of negative bias was the same as that under the effect of build-inelectric field; while the photogenerated electrons transferred toward the surface and holes toward the bottom of the tubes caused SPV process under the effect of positivebias occupied a high proportion in the overall SPV process.2. α-Fe2O3modified TNAs were obtained by sinking the TNAs film in the FeCl3and NaOH solution alternately. SPV spectrum, SPV phase spectrum and TPVmeasurements were also conducted to investigate the behavior of the photogeneratedcharge carriers in the α-Fe2O3-TNAs. The results showed that when irradiated withconsecutive visible light (400nm <λ <580nm, duration time>0.1ms),photogenerated electrons will transfer toward the surface under the effect ofconcentration gradient while with the irradiation of transient laser pulse (λ=532nm, t=5ns), photogenerated electrons will transfer toward the interface between theα-Fe2O3and TiO2under the effect of interface electric field. Under the irradiation ofUV light, two SPV processes caused by photogenerated electrons in TiO2that transferto α-Fe2O3and toward the bottom of the tubes existed simultaneously.3. TNAs with large open entrances were synthesized in a0.5wt.%NH4F+20vol.%H2O+80vol.%glycerol mixed solution via potentiostatic anodization oftitanium foil, followed by sinking in the Zn2+and Fe3+mixed solution for preparingthe ZnFe2O4modified TNAs. SPV and TPV measurements were used for analyzingthe behaviors of photogenerated electrons and holes. Also the photocatalytic reductionof Cr(VI) and degradation of MO experiments were conducted. The results showedthat the SPV process caused by photogenerated electrons transferring to the surface ofZnFe2O4exits when irradiated with either visible light or UV light, which resulted inthe high efficiency of photocatalytic reduction of Cr(VI). And the lowered efficiencyof photodegradation of MO was also caused by the same reason that photogeneratedelectrons recombined with holes in the ZnFe2O4.4. PANi modified TNAs were synthesized by an electrochemical depositionmethod. It was confirmed by the deposition mechanism and SPV and TPV test resultsthat the conduction bands of TiO2and the HOMO level of PANi were aligned andwhen irradiated with visible or UV light, photogenerated electrons will accumulate on PANi and holes on TiO2, which benefited the photodegradation of alizarin red.