Modulation Of The Surface/Interface Structure Of TiO₂ And Their Photocatalytic Reactivity

With the explosion of world population,the energy crisis and environment issues are the swords that hang over the top of human beings.To prevent further deterioration of these problems,researchers have tried their best to find alternative clean energy and depress environmental pollution.Photocatalysis is one of the environmentally friendly technologies to convert solar energy into chemical energy for the promising applications of the photodegradation of target pollutants and photocatalytic splitting water into hydrogen.Since photocatalytic redox reactions take place on the surface of photocatalyst,it is of great importance to study the surface/interface of the semiconductor photocatalyst.This study focuses on the surface/interface modulation of the classical photocatalyst TiO₂,and mainly explores the effects of surface electronic structure and reaction environment on photocatalytic performance.The research contents of this study mainly include:?1?The facet engineering of brookite TiO₂ was carried out to investigate the effect of surface electronic structure on photocatalytic reactivity.The brookite TiO₂nanocrystals mainly exposed with{121}and{211}facet were obtained by adjusting the alkalinity of the precursor solution,which were recorded as T₁₂₁ and T₂₁₁,respectively.Theoretical calculations reveal that the{121}surface contains more undercoordinated Ti atoms and a higher surface energy than that of the{211}surface,and the experimental results show that brookite TiO₂ nanorods exposed with majority{121}facet(T₁₂₁)have a more positive valence band potential;those above mentioned superior properties enable T₁₂₁ to show excellent performance in RhB photodegradation.Nevertheless,the electronic structure analyzed from the Density of State plots revealed that the electron prefers to be dispersed in the bulk for TiO₂covered with a{121}surface,indicating that the electrons might be more reluctant to migrate from bulk to surface,which might be the reason for the poor H₂ productivity of T₁₂₁.In contrast,brookite TiO₂ nanosheets exposed with dominant{211}facets(T₂₁₁)exhibited a more negative conduction band potential resulting in a much higher H₂ evolution rate(801?mol h^-1)in photocatalytic water splitting.Accordingly,combining the analyses of the surface atomic structure and electronic band structure,it is indicated that,for brookite TiO₂,the{121}surface is beneficial for photocatalytic oxidation reactions while the{211}surface can facilitate the photocatalytic reduction process.?2?Based on the brookite TiO₂(T₁₂₁ and T₂₁₁)with adjustable band structure,the Au@TiO₂ system was constructed to study the effect of the surface electronic structure of TiO₂ on the hot electrons transfer process.On the basis of the conclusion in part?1?,T₂₁₁ exhibits a much higher conduction band potential.Thus,the Au@T₂₁₁shows a larger Schottky barrier than Au@T₁₂₁.The effect of the surface electronic structure of TiO₂ on the hot electrons transfer is different under various light irradiation.Under high-intensity photoexcitation(visible light,??29?400 nm,338 mW cm^-2),the accumulation of hot-electrons will negatively shift the E_F of Au and ensure the consecutive injection to the conduction band of TiO₂,thus hot-electrons over T₂₁₁with more reductive potential and better charge transfer will be more promising.Hence,the photocatalytic H₂ production of Au@T₂₁₁ is about 2.5 times higher than that of Au@T₁₂₁ under visible light irradiation.However,under low-intensity light excitation(monochromic light,about 2²0 mW cm^-2),a higher Schottky barrier will impede the hot-electron transfer to some extent due to the low accumulation of hot electrons.Consequently,the discrepancy of AQE measured upon the monochromatic light irradiation between Au@T₂₁₁ and Au@T₁₂₁ is smaller than 2.5 times.Therefore,the surface electronic structure of TiO₂ affects the transfer of hot electrons at the interface of Au/TiO₂ and on the surface of TiO₂ which further impacts on the photocatalytic H₂ evolution reactivity.?3?The reaction environment was modulated to explore the mechanism of the alkali-induced enhancement of H₂ evolution.Experimentally,The H₂ evolution efficiency over Au@TiO₂ measured at pH=13.7 is 280 times higher than that at pH=6.0.In order to study the relationship between the TiO₂ surface and alkaline environment,we constructed the Au@TiO₂ system and investigate the mechanism of alkali-enhanced photocatalytic H₂ evolution?surface potential shift?SPS?or methanol oxidation?MO??.Under visible light irradiation,the architecture of Au@TiO₂ can easily separate the reduction process?occurred on TiO₂ surface,only related to SPS?and the oxidation process?occurred on Au surface,only related to MO?in physical space.To explore the dominant factor for the H₂ evolution enhancement,two investigation schemes are designed.For Au@ST01 system?ST01,a commercial anatase TiO₂?,the dominant role of the SPS on the H₂ evolution enhancement is semiquantitatively calculated via the open-circuit potential test?more than 80%at pH?27?13.5 and exceeds 50%at pH?13.5?.This conclusion is further proved by EIS and photoresponse current tests.For Au@T₁₀₀ and Au@T₁₀₁ system(T₁₀₀ and T₁₀₁,facet-optimized anatase TiO₂),the ratio of H₂ evolution rates(rH₂(Au@T₁₀₀)/rH₂(Au@T₁₀₁))exhibits a irregular fluctuation,which manifests the decisive role of SPS.Evidenced by these two investigation schemes,this work clarifies that the enhancement of H₂ evolution is dominated by the alkali-induced negative shift of the surface potential and meanwhile is complemented by the accelerated methanol oxidation.