Study On The Fabrication Of TiO₂ Nanotube Arrays/Cu-based Semiconductor Composite Electrodes And Their Performance Of Photoelectrocatalytic Reduction Of CO₂

Greenhouse effect and energy shortage have become the two major problems hindering the development of the world.In recent years,the use of photoelectrocatalytic technology to reduce CO₂ to low-carbon organics is expected to alleviate the above problems and has received widespread attention.In this paper,we use n-type TiO₂nanotube arrays?TNTs?and Nb-containing TiO₂ nanotube arrays?TNNTs?as substrates,and prepare various types of composite electrodes with p-n junction by loading p-type copper oxides or delafossite on their surfaces.The performance of photoelectrocatalytic CO₂reduction on different composite electrodes was well studied.The main research contents are as follows:?1?TNTs were prepared by anodizing on Ti sheets,and their surface were electrochemical reductive doped to obtain self-doped TNTs as the base material.Cu was deposited into pores of self-doped TNTs by pulse current method,followed by heat treatment to obtain CuO/TNTs composite electrode.The conductivity of TNTs is greatly improved after reduction,which is beneficial to the deposition of copper nanoparticles,thereby forming a stable embedded structure.PL results show that the synthesized CuO/self-doped TNTs composite electrode has a p-n junction,which possesses a higher separation efficiency of electron-hole pairs and a stronger photo response than CuO/Ti.The performance of this electrode is affected by various factors such as applied potentials,Cu deposition amount and p H.Under the optimal conditions,its methanol yield reaches 30?mol/cm²,much high than CuO/unreduced TNTs,which is only 26?mol/cm².?2?Based on the CuO/TNTs composite electrode as photocathode,a special pulse potential method was used during photoelectrocatalysis CO₂reduction to increase methanol yield and improve the stability of Cu_xO/TNTs electrode.When using traditional constant potential method in photoelectrocatalytic reduction of CO₂,CuO on the electrode surface will be gradually reduced to elemental Cu after 5 h,thereby destroying its catalytic activity.However,abundant Cu⁺will be retained when using pulse potential method,so the activity of the electrode is guaranteed.The effects of positive pulse potential and duration on methanol yield from photoelectrocatalysis CO₂reduction by pulse potential method were well studied.Under the optimal pulse potential condition?-0.5 V,100 s,2.0 V 20 s?,the methanol yield reaches 60?mol/cm²,which is twice that of constant potential method.?3?TNTs foam were prepared by anodizing on Ti foam,and then CuO was loaded on their surface to prepare CuO/TNTs foam electrode.Using the above-mentioned pulse potential method in photoelectrocatalytic CO₂ reduction and studying the effects of CuO loading and Ti foam size?pore size?on its performance.CuO was electrodeposited on TNTs foam and when the deposition amount reaches 20 C,the T2TNTs foam?pore diameter 50?80?m?can be completely covered.At this time,the visible light absorption and photocurrent of the electrode can reach the maximum.Further increasing the CuO loading will cause the decline of the electrode performance due to the thick covering layer and the residual elemental Cu.When different types of Ti foams?different pore sizes?were used as substrates,the highest methanol yield is generated in T2 Ti foam,which is 40?mol/cm²after 5 h.?4?TNNTs were prepared by anodizing on Ti Nb alloy sheets and Cu Fe O₂/TNNTs were then prepared by coating precursor liquid onto TNNTs followed by heat treatment in argon atmosphere.TNNTs exhibit a regular nanotube arrays structure and this structure is well remained after the calcination at 650^oC.In addition,TNNTs show similar semiconductor properties to n-type TiO₂ with a band gap of 3.2 e V.Under the optimal loading amount of Cu Fe O₂,Cu Fe O₂/TNNTs composite electrode shows strong visible light absorption performance and photo response,and the products from photoelectrocatalytic CO₂reduction is ethanol and a small amount of formaldehyde.The highest ethanol yield is generated at-0.4 V,with a yield of 3.3?mol/cm² after 5 h.