Anodic TiO₂ nanotube arrays for photocatalytic CO₂ conversion: comparative photocatalysis and EPR study

Abstract

Titania (TiO2) is a widely used semiconductor for the photocatalytic decomposition of organic impurities in air, water and the conversion of CO2 into hydrocarbon fuel precursors. TiO2 in the form of nanotubes arrays is the most attractive for practical use because of the morphological advantages providing more favorable diffusion of photocatalytic reaction products and a low recombination rate of photogenerated electrons and holes. We have carried out a comparative study of the photocatalytic activity of gas-phase conversion of CO2 to hydrocarbon products and the defect properties of multi-walled and single-walled arrays of TiO2 nanotubes. Methanol and methane have been detected in the CO2 photoreduction process. The photocatalytic evolution rate of multi-walled TiO2 nanotubes is twice as fast for methane as for single-walled TiO2 nanotubes after four hours of irradiation and four times faster for methanol. The type and features of the structural defects have been investigated by EPR spectroscopy. For the first time, it has been shown that Ti3+/oxygen vacancy centers are mainly located inside the outer layer of nanotubes, while carbon dangling bonds have been observed directly on the surface of the inner layer. Carbon defects have been found to be the centers of adsorption and accumulation of photoinduced charge carriers. The results are entirely new; they clarify the role of different types of defects in the photocatalytic conversion of CO2 to hydrocarbon compounds and show good prospects for applying TiO2 nanotube arrays.

Effect of Thermal Treatment of Symmetric TiO2 Nanotube Arrays in Argon on Photocatalytic CO2 Conversion

“…The anodized area of the samples was 7 cm 2 . The geometric-specific surface area of the samples is 0.0054 m 2 /cm 2 corresponding with previously published data [31].…”

“…Registered carbon also can be associated with carbon dangling bonds. Such carbon bonds can accumulate an excess negative charge, thereby contributing to greater photocatalytic activity during CO 2 conversion [31,38].…”

“…During thermal treatment in oxygen-free atmospheres, the IL of a nanotube can be a source of carbon for material modification [26]. We have previously shown that carbon centers on the surface of the IL saved after thermal treatment in the air can enhance the conversion efficiency of CO 2 [31]. As far as we know, the works devoted to the study of symmetric TiO 2 NTs arrays modified by carbon through thermal treatment in inert atmospheres pay attention to the electrochemical properties of the material for supercapacitors or the study of photoactivity by photoelectrochemistry and decomposition of organic dyes.…”

A Carbon‐Free and Free‐Standing Cathode From Mixed‐Phase TiO2 for Photo‐Assisted Li–CO2 Battery

“…The mixed‐phase TiO 2 with an optimum rutile/anatase ratio exhibits much better photocatalytic performance than single‐phase TiO 2 due to the extended light absorption and more efficient separation and transfer of charge carriers in hetero‐phase structure. [ 31 ] Anodic TiO 2 nanotube arrays with 3D network and porous structure show superior performance in such fields as photoelectrocatalytic H 2 production, [ 32 ] CO 2 reduction, [ 33 ] and photoelectrochemical sensing. [ 34 ] For 1D TiO 2 nanotubes, the separation between photo‐generated holes and electrons is orthogonal: holes are driven to the wall/electrolyte interface by the space charge layer, while electrons are transported longitudinally to the back contact; thus, achieving effective separation.…”

Влияние условий синтеза на структурные, оптические и электрофизические свойства нанокомпозитов TiO-=SUB=-2-=/SUB=-/Cu-=SUB=-x-=/SUB=-O

“…В настоящее время широко исследуются одностенные (ОС) и многостенные (МС) нанотрубки TiO 2 . В частности, показано, что МС нанотрубки обладают лучшей фотокаталитической активностью в реакциях преобразования CO 2 по сравнению с ОС [7], однако эффективность подобных материалов в качестве фотокатализаторов попрежнему остается недостаточно высокой. Для улучшения фотокаталитических свойств наноструктур на основе TiO 2 используют различные добавки, в том числе наночастицы других полупроводниковых материалов.…”