Research On The Optimization Of Titania Based Photocatalysts And Photocatalytic Hydrogen Evolution From Water Splitting

The efficiency of photocatalytic hydrogen evolution from water splitting by utilizing titania（TiO₂）as the photocatalyst can be limited by many factors,such as the photon capture,the fluctuation of surface energy on TiO₂ exposed crystal faces,the band gap（anatase:3.2 eV,rutile:3.0 eV）,and the quick recombination of charge.Therefore,approaches based on corresponding theories have been designed to optimize the above limitations.This study not only helps to achieve effective photocatalytic hydrogen evolution by utilizing TiO₂ as the photocatalyst,but also it enriches the methods of optimization of TiO₂ photocatalyst.The major conclusions are as follows.1,Noble metal deposition and biotemplate approach were employed to optimize the internal and external structures of TiO₂ photocatalyst.The wings of Troides Helena with inverse V-type scales and silver bromide（AgBr）were utilized as the templates or the source of Ag⁰,respectively,to synthesize inverse V-type AgBr/TiO₂ photocatalyst with well distributed AgBr and TiO₂.As a result,in-situ deposition of Ag⁰nanoparticles from AgBr photolysis in TiO₂ can transform inverse V-type AgBr/TiO₂ into inverse V-type Ag⁰/TiO₂.Inverse V-type artificial structure can decrease the reflection of incident light on photocatalyst,and Ag⁰ nanoparticles can provide the photon capture sites to increase the light absorption or form the schottky barrier with TiO₂ to facilitate the charge separation.To accurately duplicate the template and uniformly distribute AgBr,a crosslinking extraction method was designed to synthesize a crosslinked titanium precursor material with uniform hydrolytic rate,good liquidity,and low viscosity from TiCl₃ solution.The density of crosslinked titanium precursor material is 0.826 g/ml,which is lower than the densities of tetrabutyl titanate（1.041 g/ml）and isopropyl titanate（0.935 g/ml）.Soaking Troides Helena templates in 3 wt%NaOH solution for 2 h is optimal to harvest the best products.Adding 0.340mmol AgBr is appropriate to duplicate the template and generate the best concentration of Ag⁰ cocatalyst.The quantum yield of inverse V-type AgBr/TiO₂ at 340 nm wavelength is 7.3%,which is higher than inverse V-type TiO₂（1.3%）,non-inverse V-type AgBr/TiO₂（3.8%）,and AgBr/TiO₂（2.8%）by deposition of AgBr on inverse V-type TiO₂,indicating the cooperation of inverse V-type structure and well distributed Ag⁰ nanoparticles is able to promote the activity of TiO₂ photocatalyst.2,Bromine（Br₂）was utilized to regulate the fluctuation of TiO₂ surface energy during photocatalysis by adsorption on TiO₂ exposed crystal faces.To attain a sustained effect of optimization of the surface energy,silver bromide（AgBr）and copper oxide（CuO）were merged into TiO₂ matrix to synthesize TiO₂/AgBr/Cu_xO（x=1,2）,where AgBr can recover from Ag⁰and Br₂ after photocatalysis by the catalysis of CuO to achieve the goal of immobilization of Br₂.The immobilized Br₂ can be repeatedly utilized to regulate the surface energy of TiO₂ exposed crystal faces.The tests of electron paramagnetic resonance（EPR）proved that TiO₂ is the final electron acceptor in TiO₂/AgBr/Cu_xO,which is the basis of maintaining TiO₂ photocatalytic activity by Br₂ adsorption.Simulation based on DFT theory revealed that Br₂ adsorption on anatase{101}crystal face can decrease its surface energy,which gives another proof of promoting TiO₂photocatalytic activity by regulating its surface energy.The average value of quantum yields at 340 nm wavelength in five measurements for TiO₂/AgBr/Cu_xO is 13.1%,which is higher than the average value of11.2%for TiO₂/Ag⁰/Cu_xO.3,Irregular arrangement of atoms in amorphous semiconductor and Ti³⁺doping were employed to optimize the band gap and inhibit the charge recombination in amorphous TiO₂.The method of anaerobic hydrolysis was designed to dissolve and oxidize metal titanium powder by water to synthesize amorphous TiO₂(a-TiO_2-x)containing Ti³⁺.Light absorption and photocatalytic hydrogen evolution experiments proved that a-TiO_2-x is able to absorb and transfer visible and near infrared light to generate H₂ from water splitting.The calculation of bandgap for a-TiO_2-x or crystalline TiO_2-x(c-TiO_2-x)based on the UV-vis absorption spectrum proved that the bandgap of a-TiO_2-x is 1.5 eV,which is smaller than the bandgap of 2.92 eV for c-TiO_2-x.The tests of photoluminescence spectrum（PL）further revealed that the rate of charge recombination in a-TiO_2-x is slower than that in a-TiO₂ from hydrothermal decomposition of tetrabutyl titanate,indicating Ti³⁺doping is capable of inhibiting the charge recombination.The quantum yield of a-TiO_2-x at 488 nm wavelength is approximately 5.70%,which is 10.9 times to the a-TiO₂（0.52%）.