Fabrication And Photoelectric Properties Of Highly Efficient Visible-light Photocatalyst Titanium Dioxide Nanomaterials

TiO₂is one of extensively studied semiconductor photocatalyst with wide band gap, and it has beenreceived extensive attention, because of its important application on environmental purification, thegeneration of clean hydrogen energy, degradation of organic pollutants, solar cells, etc,. However, thewidespread use of TiO₂material is limited by its wide band-gap energy, which only responds to ultravioletlight. Many efforts have been done to shift the photoresponsive range of TiO₂to visible spectral region.Since Asahi reported in Science that nitrogen doped TiO₂narrowed its band gap and expanded its responseto the visible region. Many strategies have been used to improve visible photoresponse of TiO₂, such asnon-metal element doping, metal element doping, composite of narrow band gap semiconductor, noblemetal deposition, surface photosensitization.However, there still exists a lot of controversy about themechanism of visible light photoactivity on these materials. So, further in-depth research is indispensable.In view of the above problem, we perform the following work.In this paper, Specific contents are as follows:TiO₂nanomaterials with surface modification were synthesized by a simple calcination method using thehydrothermally prepared nanotubed titanic acid （denoted as NTA） powder as a precursor. The four sampleswere obtained by the thermal treatment of NTA under the four kinds of atmospheres, respectively. Theresulting samples were investigated by means of transmission electronic microscopy （TEM）, X-raydiffraction （XRD）, diffuse reflectance spectra （DRS）, Fourier Transform Infrared Spectroscopy （FTIR）,X-ray photoelectron spectroscopy （XPS）, and electron paramagnetic resonance （ESR）. The results showthat the four samples after calcination transferred from nanotubes to nanoparticles with size about20-30nmand have uniform crystal structures, namely, anatase TiO₂. The N-NTA-TiO₂sample via nitridation in NH3flow exhibited stronger absorption in the visible light range and the low energy threshold extended to520nm, corresponding to2.4eV. XPS results showed that a substantial amount of nitrogen （up to0.68atomic%） were incorporated into the TiO₂lattice. With the hydrogenation and nitridation co-treatment, theabsorption spectrum of the H, N-NTA-TiO₂sample extends to650nm, corresponding to1.9eV. In addition,the absorption intensity of the H, N-NTA-TiO₂sample evidently enhanced compared with the other threesamples. At the same time, XPS revealed that the substititutional nitrogen content and the interstitial N content in the sample both increased. The total N concentrations are calculated as1.32at%for H,N-NTA-TiO₂. ESR measurement revealed the visible-light absorption of the H-NTA-TiO₂sampleoriginates from single-electron-trapped oxygen vacancies （denoted as V·O）. After nitrogen doping, thetriplet ESR signals obtained from the N-NTA-TiO₂and H, N-NTA-TiO₂samples are attributed to theinteraction between doped-nitrogen and V·O. FTIR results confirmed that a large number of Ti-OH bondslocated on the surface of the four samples. It is believed that the enhanced mechanism of optical absorptionand photoactivity arose from the synergistic effect of the N2p levels near the valence band, surfaced Ti-OHbonds and single-electron-trapped oxygen vacancies. The present study motivates us to further exploreTiO₂-based nanomaterials with high visible light activity, which has promising applications asphotocatalysts, in photovoltaics and in solar hydrogen production from water splitting.In the third chapter, we used sodium titanate nanotubes as precursor, and they were ion-exchanged withammonium fluoride solution, then calcined at450℃in the air preparing nitrogen-doped TiO₂nanomaterials. Subsequently Fe, N codoped TiO₂nanomaterials were prepared by regulating theconcentration of iron ions. It is investigated that the compensation of metal ions doped with nitrogen affectvisible light photoactivity of TiO₂nanomaterials. The specific experimental method was ion-exchangedbetween four kinds of ferric nitrate solution with different concentrations （0.01M/0.05M/0.1M/0.2M） andnitrogen-doped TiO₂samples, and then repeatedly washed three times with distilled water until pH=7.Theresults of XRD analysis showed that its crystal struture would only contain anatase structure without ironoxides after Fe being doped. UV-Vis absorption spectrum showed that the absorption band-edge red-shiftedand the visible light absorption enhanced in Fe, N codoped TiO₂nanomaterials. XPS results showed thatthe doping amount of iron is5.99atomic%, and the visible light absorption of the sample is strongest.In the fourth chapter, we used tetrabutyl titanate as main material, hydrofluoric acid as inducer. TiO₂nanosheets with highly reactive （001） crystal face were prepared by hydrothermal method. Subsequently,N-doped TiO₂nanosheets with highly reactive （001） facets were prepared by calcining them at differenttemperatures （400,500,600） in flowing NH3. The analysis of XRD showed that the crystal structure of thesamples before or after doped was anatase, there was no other phases generated. UV-Vis absorption showedthat the visible light absorption of N-doped TiO₂nanosheets significantly enhanced. The results of electron paramagnetic resonance spectroscopy showed that a large number of single-electron-trapped oxygenvacancies were formed in the process of nitridation, corresponding to g=2.004paramagnetic signal peaks.The sample heat-treated at500℃has the strongest visible lightabsorption, corresponding nitrogen-dopedconcentration and concentration of single-electron-trapped oxygen vacancies reached highest. XPS N1sspectra of the sample showed that doped N species belonged to interstitial nitrogen and the binding energyis located at399-400eV, directly connecting with the lattice oxygen or oxygen vacancies. According to theresults of the above analysis, we think that visible light response of the N-doped TiO₂nanosheets isdetermined by two factors, single-electron-trapped oxygen vacancies and doped N-element, the synergisticaction of two factors improved the photocatalytic activity of the material.