Surface Modification Of TiO₂ And Photocatalytic Reaction Mechanism

Semiconductor photocatalysis and photoelectrocatalysis have received increasing attention in the field of solar utilization,and they are expected to be a new technology for environmental pollutant treatment and clean energy regeneration.Since the last century,many semiconductor photocatalysts have been studied.Among the many semiconductor photocatalysts,TiO₂ is the most studied because of its high efficiency,stability,low cost and so on.However,due to the fast recombination of the photogenerated charges,and the slow interface charge transfer,the quantum yield of a chemical reaction is usually low.To improve the photocatalytic performance of semiconductors,the author in this thesis mainly focus on the modification of TiO₂ interface with Pt,CuWO4,and several inorganic anions and cations(Cu²⁺,B4O72-,CO32-),and examine their effect and possible mechanisms on organic degradation in water.The dissertation is divided into four sections,as briefly shown below:?1?The effects of Cu²⁺ concentration and light intensity on the photocatalytic degradation of phenol on TiO₂ in different crystal forms have been systematically investigated.The results show that Cu²⁺ ions in low concentration are beneficial to phenol degradation over anatase,rutile,and brookite.However,after reaching a maximum,the rates of phenol degradation decreased with CuCl2 concentration.Furthermore,the rate of phenol degradation increased with the increase of light intensity.Upon the addition of 1 mM CuCl2,the relative quantum yield of phenol degradation over rutile was increased by a factor of approximately 4.A colorimetrical and photoelectrochemical measurement,as well as solid analysis,revealed that the interfacial electron transfer to Cu2_from the irradiated TiO₂ was faster than that to O₂.The resulting Cu+species were then oxidized back to Cu²⁺ by O₂.But at a higher concentration of Cu²⁺,Cu+species disproportionated to Cu²⁺ and metallic Cu,the latter of which shielded the light absorbed by TiO₂.Curiously,Cu²⁺ ions were always negative to the photocatalytic degradation of phenol over P25.This is ascribed to the salt induced aggregation of P25 particles and the formation of metal Cu,both of which reduced the number of photons absorbed by P25,and hence decreased the photocatalytic activity of P25.?2?Since Cu²⁺ ions are harmful to aquatic environment,we decided to modify TiO₂ by using solid CuWO4,rather than Cu²⁺ ions.It was found that the addition of CuWO4 into the aqueous suspension of TiO₂ was also beneficial for phenol degradation.A comparative experiment showed that CuO also had similar effect,but CuO/TiO₂ was much less stable than CuWO4/TiO₂.Furthermore,CuWO4/TiO₂ was more active than TiO₂ for the generation of either OH radicals or H2O₂ in aqueous solution.Through an open circuit measurement,a possible mechanism is proposed,involving electron transfer from the irradiated TiO₂ to CuWO4.This would facilitate the charge separation of TiO₂ and consequently accelerate O₂ reduction and phenol degradation at interfaces.?3?The effect of borate on the photocatalytic activity of Pt/TiO₂ was studied.It was found that there was a synergistic effect between Pt and borate,significantly accelerating the photocatalytic degradation of phenol.The rate of phenol degradation changed with Pt loading,borate concentration,and solution pH.A maximum rate of phenol degradation was obtained at 0.52 wt%Pt,3.0 mM Na2B4O7,and initial pH 9.0,respectively.Under such conditions,the addition of 0.52 wt%Pt,3.0 mM borate,and Pt plus 3.0 mM borate results into a 1.34,0.22,and 4.87 fold increase,respectively,in the rates of phenol degradation on TiO₂.This mutual interaction between Pt and borate was also beneficial for the TiO₂-photocatalyzed degradation of 4-chlorphenol and 2,4-dichlrophenol.A separate photoelectrochemical measurement with TiO₂ and Pt/TiO₂ film electrodes revealed that borate had a positive effect on the hole oxidation of phenol,but not on the hole oxidation of water.Furthermore,upon the addition of borate,the electrochemical reduction of O₂ over TiO₂ and Pt/TiO₂ were slightly inhibited and greatly promoted,respectively.It is proposed that there is a special interaction between Pt and borate that facilitates the reduction of O₂.The borate-mediated hole transfer from TiO₂ to phenol,and the Pt-mediated electron transfer from TiO₂ to O₂ would promote each other.This would result into a significant enhancement in the efficiency of the charge separation,and consequently in the rates of O₂ reduction and phenol oxidation.?4?Carbonate anions are often present in aqueous solution,but their effect on the semiconductor-photocatalyzed reaction has been rarely reported.In this part,we examine the effect of carbonate on the TiO₂ and Pt/TiO₂ photocatalysts,by using organic substrates as the model reaction.The results showed that Na2CO3 had a positive effect on the TiO₂-photocatalyzed degradation of phenol,2,4-dichlorophenol,and H2O₂ in aqueous suspension.Upon the addition of 2 mM Na2CO3,0.52 wt%Pt,and 2 mM Na2CO3 plus 0.52 wt%Pt at initial pH 8.0,the rate of phenol degradation over TiO₂ was increased by 0.78,2.38,5.63 times,respectively.With a TiO₂ and Pt/TiO₂ film electrode,the positive effect of carbonate was also observed from the photoelectrochemical oxidation of phenol,but not for water.However,the rates of phenol degradation over TiO₂ and Pt/TiO₂ became decreased as the carbonate concentration exceeded 5.0 and 2.0 mM,respectively.It is proposed that CO3-· radicals are formed mainly from the hole oxidation of dicarbonate adsorbed on TiO₂,following by reoxidation to dicarbonate by phenol.At a high concentration,the CO3-· radicals would recombine to a peroxocarbonate with a weak reactivity.The carbonate-mediated hole transfer from TiO₂ to phenol,and the Pt-mediated electron transfer from TiO₂ to O₂ would promote each other.This would improve the efficiency of the charge separation,and consequently increase the rates of phenol degradation at interfaces.