Hydrothermal Synthesis Of Narrow Bandgap Photocatalysts And Their Visible-light Photocatalytic Activity

The global energy crisis and environment pollution have attracted tremendously attentions from many countries. The synthesis and applications of visible-light photocatalyst with high utlization efficiency of solar energy to split water into hydrogen or degrade organic pollutant, has become a hot research covering materials science, chemistry and envoronmental science.In this thesis, the related research and progress on narrow band-gap photocatalyst were reviewed, the main concern is focused on the synthesis through hydrothermal or solvothermal methods, phase structure and photocatalytic properties of visible-light-driven modified TiO₂ photocatalyst, narrow band-gap semiconductor photocatalysts of BiVO₄ and YFeO₃; respectively.Two kinds of visible-light-driven modified TiO₂ photocatalysts, which showed the visible-light response and photocatalytic degradation ability, was synthesized by solvothermal method using Ti(OBu)₄ as starting materials. One is the surface-organics modified TiO₂ prepared in mixed organic solvents, and the other is carbon modified TiO₂ composite (C/TiO₂) prepared by employing CHCl₃ as carbon source.Both the surface-organics modified TiO₂ and carbon modified C/TiO₂ catalysts showed the characteristics of narrow band gap photocatalyst, the high response to visible light and the high photocatalytic activities on photodecomposing methyl orange (MO) under visible-light irradiation. It was suggested that, the modified TiO₂ forms doping energy levels within the band gap which corresponds to the visible light response through the chelation between organics and titania and the synergic effects between carbide materials and titania for carbon modified C/TiO₂ catalysts as well. The results of photocatalytic experiments show that the kinds and amounts of surface adsorbed organics play important role in the degradation of MO under visible light irradiation, and the carboneous species on the titania surface also play an important role in enhancing the photoactivity of TiO₂. It was concluded that visible-light-responsive surface-organics modified and carbon modified TiO₂ photocatalysts with high visible-light photoactivities could be synthesized by simply controlling experimental conditions, such as changing the kinds and amounts of solvents, adding CHCl₃ as carbon source, etc.BiVO₄ particles with different crystalline forms, morphologies and photoactivities were synthesized in the temperature range from 150 to 280℃by hydrothermal methods using Bi(NO₃)₃, NH₄VO₃ as starting materials and NH₃·H₂O as mineralizer.The results showed that the pH values of the precursor solutions have effects on the formation of BiVO₄, and the reaction temperature has significant influence on the crystalline forms and morphologies of as-prepared BiVO₄. Tetregonal, monoclinic and mixed-phase BiVO₄ can be synthesized at different reaction temperatures. Photocatalytic degradation results showed that the pure monoclinic BiVO₄, which prepared at the reaction condition of pH of 5, reaction time of 6 hours and reaction temperature above 200℃, exhibits the best photocatalytic degradation efficiency on MO under visible light irradiation.The preparation conditions of synthesizing high purity YFeO₃ was obtained through modified experimental procedures using Bi(NO₃)₃, Fe(NO₃)₃ as starting materials and NH₃·H₂O as mineralizer. Results showed that YFeO3 products with different crystal forms and morphologies could be prepared by washing the mineralized products and then thermal treating in the ethanol solvent for 6 hours.It was found that the phase and morphologies of as-prepared YFeO₃ products can be adjusted by reaction temperature. the phase purity and crystallinity of as-prepared YFeO₃ products were improved with increased reaction temperature. Visible-light photocatalytic experiments showed that the YFeO₃ sample prepared at 280℃has higher photoactivity than that prepared by solid-state reactions. It was obvious that the phase content of YFeO₃ and its crystallinity were important factors on influencing the photocatalytic properties.