Modification of TiO2 with nitrogen and carbon via sol-gel method. Not all generating visible light absorption in TiO2 is doping

JULIÁN A. RENGIFO-HERRERA; MANRIQUE-HOLGUÍN, MANUELA; JOHN J. ALVEAR DAZA; LUIS RENÉ PIZZIO

Buenos Aires

Congreso; WCCE11-11th World congress of Chemical Engineering. XXX Interamerican Congress of Chemical Engineering ? IACCHE; 2023

TiO2 is a metal oxide semiconductor with interesting properties such as high photochemical stability and abundancebeing successfully explored in different photocatalytic reactions to destroy organic pollutants and inactivate bacteria inwater and air interfaces, synthesis of organic molecules, CO2 reduction to solar fuels production, H2 production, andsolar photovoltaic cells [1]. However, TiO2 possess a wide band gap (3.2 eV for anatase and 3.0 eV for rutile) limitingits photocatalytic solar and visible light induced applications. Wet synthesis via sol-gel method to prepare visible-lightabsorbing TiO2 doped either with nitrogen (N), or carbon (C) is one of the most popular procedures reported in theliterature. Often, urea, ammonia (NH3), and aliphatic acids are used as doping precursors. However, some doubts aboutthe formation of N- or C-doped TiO2 via this synthesis procedure has recently arisen suggesting the existence ofheterojunctions with metal-free semiconductors, graphitic species, and surface complexes rather doping [2]. In thisstudy, a multi-technique characterization of nitrogen and carbon modified TiO2 materials synthesized by sol-gel methodusing urea, NH3 and oleic acid (OA) as precursors at annealed at 450 and 550 ○C was realized. It was found that thesynthesis procedure led to the visible light absorption in TiO2 but did not generate nitrogen or carbon doping but ratherheterojunctions of graphitic carbon nitride (g-C3N4)/TiO2, presence of oxygen vacancies and NO-TiO2 surface complexesand graphite-graphite oxide/TiO2 heterojunctions when urea, ammonia and oleic acid were used as precursorrespectively. By electronic spin resonance spectroscopy (ESR) using PNBO as spin trapper, it was detected thepresence of hydroxyl radicals (•OH) only when UVA irradiation was present. Photocatalytic activity at short irradiationtimes under UVA and visible light irradiation (λ>450 nm) was evaluated in the degradation of salicylic acid (SA) and itstransformation into 2,3- and 2,5-dihydrobenzoic acid (2,3-DHBA, 2,5-DHBA) and catechol (CT) allowing to determinethe participation of •OH radicals. Mechanisms about how modified TiO2 absorbs visible light and produces reactiveoxygen species (ROS) are proposed.