Study Of Photoelectric Properties And Photocatalytic Activity Of Ions Doped TiO₂ Nanomaterials

TiO2-based nanomaterial which can convert solar energy into chemical energy is considered to be an effective, environmentally friendly means of solving the increasingly serious energy shortages and environmental crises because of their excellent oxidative capacity and chemical stability. However, the high recombination probability of photocarriers and poor solar efficiency have hindered their practical application. To solve these shortcomings, much effort has been directed towards the modification of TiO2 by incorporating ions, especially the transition metal ions with 3d orbital. As the ionic radius of transition metal ions is similar to that of titanium ion (61 pm), they can access the TiO2 lattice and substitute some of the lattice titanium atoms. On the one hand, the introduction of transition metal ions may induce several localized occupied states in the gap, result in modest variations of the band gap, which may account for the experimentally observed red shift of the absorption edge of TiO2 toward the visible region. On the other hand, it may favor the formation of lattice defects which enable to trap photogenerated electron or hole, enhance the separation extent and restrain the recombination of the photogenerated electron and hole carriers in TiO2, this has been reported to improve its catalytic activity to a certain extent. It is now commonly recognized that visible-light absorption does not always result in visible-light photocatalytic activity, and the key factor to enhancing the photocatalytic activity lies in effectively combining the photon absorption, bulk diffusion, and surface transfer of photogenerated charge-carriers in the photocatalyst. A detailed explanation of the photovoltaic properties of the charge carriers (including generation, separation and recombination) in photocatalyst is meaningful, and may provide useful information for understanding the catalytic mechanism, the information is important for designing new photocatalyst active under visible light.In this thesis, we study focused on the control of semiconductor TiO2 microstructure, developed the best synthetic method and conditions. The modification of TiO2 by incorporating ions to extend photoresponding range to the visible region, promote the separation of charge and inhibite their recombination. Using transient photovoltaic technology (TPV), surface photovoltage (SPS), field induced surface photovoltage (FISPS) and its phase spectrum, the surface photocurrent (SPC) technology to study the influence of different ions, doping content and valence on morphology, crystal structure, thermal stability, photoelectric properties and photocatalytic activity of TiO2 nanomaterial. The relationship between behavior of photogenerated charges (including separation, transport and recombination) and photocatalytic reactions is also discussed. Specific work includes the following aspects:1. Near-monodisperse Bi-doped anatase TiO2 nanospheres with almost uniform diameters in the range of 117 to 87 nm were prepared simply by introducing different amounts of bismuth nitrate pentahydrate into the reaction system and subsequent calcinations. X-ray diffraction, UV-visible diffuse reflectance spectra, and X-ray photoelectron spectroscopy confirm that the doped ions substitute some of the lattice titanium atoms, and furthermore, Bi3+ and Bi4+ ions coexist. All the Bi-doped TiO2 samples show much better photocatalytic activity than pure TiO2 in the degradation of rhodamine B (RhB) under the irradiation of visible light (λ>420 nm), after detailed studies of the results of surface photovoltage spectroscopy (SPS), and UV-visible diffuse reflectance spectroscopy, we propose that two aspects respond to the visible photocatalytic activity of Bi-doped anatase TiO2 nanospheres. One is the photosensitization of the dye with a compatible energy band, the other is the increase of the separation probability of the electrons transferred from dye in the samples by introducing bismuth atoms.2. Multivalent Mn-TiO2 nanospheres with controllable sizes were prepared with manganous chloride as manganese sources, the diameter of nanospehers can be controlled in the range of 200-300 nm by simply varying the amout of manganous chloride. The samples are further characterized by FESEM, TEM, EDX, XRD, XPS, UV-vis DRS, SPS and SPC techniques to study the influence of Mn doping content and annealing temperature on crystal structure, morphology, thermal stability of the photoinduced charge behavior and photocatalytic activity of TiO2. The results indicate that all the samples exhibit similar XRD patterns that can be indexed to anatase, each Mn-TiO2 nanosphere is assembled by hundreds of nanoparticles, and the average crystallites size is about 15-20 nm, This is why there is no significant change in the BET surface area (SBET) with an increase of Mn/Ti molar ratio. The manganese exists in multivalency (Mn4+/Mn3+) and substitutes for some Ti4+ in the anatase TiO2 lattice, enhance the thermal stability of TiO2 against the structural transformation. Although the absorption edge is extended to the visible-light region up to 700 nm for all the Mn-TiO2 materials, SPS and SPC measurements reveal that the sub-band-gap transition of Mn-TiO2 is not observed, moreover, the intensity of SPV gradually decreased with a increase of Mn contet, which indicate that the presence of Mn formed lattice defects in TiO2, Mn served as electron acceptors and effectively inhibited the charge recombination in TiO2, resulting in a high efficient photocatalytic activity in the degradation of rhodamine B (RhB) under visible-light irradiation (λ＞420 nm). There is an optimal doping amout and calcination temperature(Mn/Ti=0.25%,500℃), while an excessive amount of Mn will act as recombination center of electron-hole pair, reducing the photocatalytic efficiency.3. Carbon-doped anatase TiO2 nanoparticle was prepared by a facile hydrothermal process without adding additional carbon source, the average grain size is 5-8 nm, thus give a wider band gap (3.3 eV) than TiO2 bulk due to quantum size effects. The as-prepared sample shows highly efficient photocatalytic activity, which only requires 4 minutes and is about 11 times higher than that of Degussa P25 TiO2 in degradation of methyl orange (MO) dye under UV light irradiation. Moreover, a highly visible-light activity is also observed. UV-visible diffuse reflectance spectra and X-ray photoelectron spectroscopy confirm that the carbon atoms are incorporated into the interstitial positions of TiO2 lattice and form a strong interaction with titanium atoms and extend photoresponding range to 700 nm. According to surface photovoltage spectra (SPS) and transient photovoltage (TPV) results, the photovoltaic responses of C-TiO2 in the range of 400-500 nm which correspond to the sub-band-gap transition are observed, and TPV response peaks move towards the longer timescale for C-TiO2 (32 ms), while the response time of P25 TiO2 is only 5.1 ms. The above results suggest that the presence of interstitial carbons induce several localized occupied states in the gap, which can accept electron from the TiO2 valence and occur the sub-band-gap transition, thus extend the photoresponding range. On the other hand, the localized occupied states can trap electrons in TiO2 conduction, enhance the separation extent and restrain the recombination of the photo-induced electron and hole carriers in TiO2, thus more electrons and holes can spread to the TiO2 surface to participate in the photocatalytic reaction.