Coupled optical absorption, charge carrier separation, and surface electrochemistry in surface disordered/hydrogenated TiO₂ for enhanced PEC water splitting reaction

Abstract

The central governing factors that influence the efficiency of photoelectrochemical (PEC) water splitting reaction are photon absorption, effective charge-carrier separation, and surface electrochemistry. Attempts to improve one of the three factors may debilitate other factors and we explore such issues in hydrogenated TiO2, wherein a significant increase in optical absorption has not resulted in a significant increase in PEC performance, which we attribute to the enhanced recombination rate due to the formation of amorphization/disorderness in the bulk during the hydrogenation process. To this end, we report a methodology to increase the charge-carrier separation with enhanced optical absorption of hydrogenated TiO2. Current methodology involves hydrogenation of non-metal (N and S) doped TiO2 which comprises (1) lowering of the band gap through shifting of the valence band via less electronegative non-metal N, S-doping, (2) lowering of the conduction band level and the band gap via formation of the Ti(3+) state and oxygen vacancies by hydrogenation, and (3) material processing to obtain a disordered surface structure which favors higher electrocatalytic (EC) activity. This design strategy yields enhanced PEC activity (%ABPE = 0.38) for the N-S co-doped TiO2 sample hydrogenated at 800 °C for 24 h over possible combinations of N-S co-doped TiO2 samples hydrogenated at 500 °C/24 h, 650 °C/24 h and 800 °C/72 h. This suggests that hydrogenation at lower temperatures does not result in much increase in optical absorption and prolonged hydrogenation results in an increase in optical absorption but a decrease in charge carrier separation by forming disorderness/oxygen vacancies in the bulk. Furthermore, the difference in double layer capacitance (C(dl)) calculated from electrochemical impedance spectroscopy (EIS) measurements of these samples reflects the change in the electrochemical surface area (ECSA) and facilitates assessing the key role of surface electrochemistry in PEC water splitting reaction. Additionally, we observed a blue-shift of the absorption spectrum and a decrease in both electrochemical (EC) and photoelectrochemical (PEC) activities after the removal of surface layers through focused ion beam (FIB) sputtering suggesting the importance of surface defects and photon absorption.

Rational Regulation of Surface Free Radicals on TiO2 Nanotube Arrays via Ag2O–AgBiO3 towards Enhanced Selective Photoelectrochemical Detection

“…Changes in surface electrochemical reactions deeply affect the active surface free radicals of photocatalysts in the PEC reactions, especially the two reactions in the PEC detection process: (1) the direct organics oxidation by holes and (2) the indirect organics oxidation by hydroxyl radicals formed by holes [ 22 , 25 , 45 , 46 ]. Therefore, the corresponding changes in photocurrents of the as-prepared electrodes were studied by adding the corresponding radical trapping agents in PEC measurements ( Figure 4 a–c).…”

Should All Electrochemical Energy Materials Be Isomaterially Heterostructured to Optimize Contra and Co-varying Physicochemical Properties?

“…Much effort in PEC-OER has gone into improving optical absorption of wide bandgap semiconductors (like TiO 2 ) and the third contra varying property has implications on this front. Many approaches toward improving optical absorption cross-section like incorporating substitutional dopants, surface grafted-dyes, plasmonic nanoparticles (Vayssieres, 2010 Behara et al, 2016a;Upadhyay et al, 2016a,b;Pala, 2017;Aslam et al, 2018), hydrogen treatment to make "black-TiO 2 " (Chen et al, 2011;Behara et al, 2014Behara et al, , 2016a increase electronic conductivity and hence, in effect decrease the driving force for electron-hole separation (Behara et al, 2016a). Considering that half-a-century of research into solar water splitting has not yielded a commercial technology, it is worthwhile to consider if the rich science unraveled can be gainfully utilized toward valorising alternate oxidation reactions (Lhermitte and Sivula, 2019) and technologies like semiconductor photoelectrochemical etching (Horikiri et al, 2018).…”

Dual role of activated carbon as fuel and template for solution combustion synthesis of porous zinc oxide powders

“…Synthesis of nanosized metal oxide powders with enhanced surface area is an ever searching field among the research community because of their superior properties in several applications, such as catalysis, adsorption, optoelectronics, and sensors 1‐5 . Porous crystalline materials offer excellent features such as high specific surface area, easy access to the active sites, and improved transport properties 6 .…”