Ultrafast Charge Carrier Dynamics and Photoelectrochemical Properties of Hydrogen-treated TiO 2 Nanowire Arrays
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Ultrafast Charge Carrier Dynamics and Photoelectrochemical Properties of Hydrogentreated TiO2 Nanowire Arrays Damon A. Wheeler, Gongming Wang, Bob C. Fitzmorris, Staci A. Adams, Yat Li*, Jin Z. Zhang* Department of Chemistry and Biochemistry, University of California, Santa Cruz, Santa Cruz, CA 95064 *Corresponding Author: [email protected], Tel: (831) 459-1952; [email protected], Tel: (831) 459-3776 ABSTRACT Here we report studies of photoelectrochemical (PEC) properties and ultrafast charge carrier relaxation dynamics of hydrogen-treated TiO2 (H:TiO2) nanowire arrays. PEC measurements showed the photocurrent density of the H:TiO2 was approximately double that of TiO2, attributed to increased donor density due to the formation of oxygen vacancies in H:TiO2 due to hydrogen treatment Charge carrier dynamics of H:TiO2, measured using fs transient absorption spectroscopy, showed a fast decay of ~20 ps followed by slower decay persisting to tens of picoseconds. The fast decay is attributed to bandedge electron-hole recombination and the slower decay is attributed to recombination from trap states. Visible absorption is attributed to either electronic transitions from the valence band to oxygen vacancy states or from oxygen vacancy states to the conduction band of the TiO2, which is supported by incident photon to current conversion efficiency (IPCE) data. H:TiO2 represents a unique material with improved photoelectrochemical properties for applications including PEC water splitting, solar cells, and photocatalysis. INTRODUCTION Photoelectrochemical (PEC) water splitting is an attractive addition to our current energy sources because it uses abundant source materials, sunlight and water, to produce hydrogen which burns without producing any CO2. Since the inception of PEC water splitting1 TiO2 has been the cornerstone of PEC research because it is stable in aqueous electrolyte, has high carrier mobility and it has valence and conduction bands which straddle the reduction potentials for hydrogen and oxygen2. The drawback of TiO2 as a photoanode for PEC water splitting is its wide band gap that prevents it from absorbing the majority of the solar spectrum. Many studies have attempted to remedy this issue by sensitizing TiO2 to visible light by combining it with narrow band gap semiconductors3 or by doping TiO2 with other elements such as nitrogen3 or chromium4 to add additional states within the band gap. One of the latest strategies for improving the photoactivity of TiO2 involves treating the TiO2 films with hydrogen gas at elevated temperature rendering the TiO2 black5,6 . The H-treated TiO2 (H:TiO2)prepared in this way exhibited solar to hydrogen (STH) efficiency of 1.1%, the highest reported for a TiO2 photoanode. Interestingly, although the performance was greatly enhanced due to the hydrogen treatment, incident photon to current efficiency (IPCE) measurements taken as a function of incident wavelength showed that the majority of the increase in photocurrent occurs in the UV with performance decreasing for waveleng
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