Photochemistry of hematite photoanodes under zero applied bias

Abstract

Surface photovoltage spectroscopy (SPS) was used to observe photochemical charge separation and oxidation reactions on Fe 2 O 3 nanorod arrays under zero applied bias. Nanorod films were grown from FeCl 3 under hydrothermal conditions followed by calcination at 550 ºC. A negative photovoltage of up -130 mV is observed under 2.0 -4.5 eV (0.1 mW cm -2 ) illumination, confirming 2.0 eV as the effective bandgap of the material, and electrons as majority carriers. SPS in the presence of air, nitrogen, water, oxygen, and under vacuum suggest that the photovoltage is associated with the oxidation of surface water and with reversible surface hole trapping on the 1 min time scale and de-trapping on the 1 h time scale. O 2 promotes water oxidation by increasing the concentration of surface holes. Sacrificial donors KI, H 2 O 2 or potassium hydroxide increases the voltage to -240 and -400 mV, due to improved hole transfer. Cobalt oxide and Co-Pi cocatalysts quench the voltage, which is tentatively attributed to the removal of surface states and enhanced e/h recombination. An energy diagram is used to relate the experimental photovoltage to the built-in potentials at the respective interfaces.

Observation of 4th-order water oxidation kinetics by time-resolved photovoltage spectroscopy

“…To derive the rate-law relationship to chemical reaction mechanisms, the simplified surface charge reactions are displayed in Figure 5D, where the separated charges Q sep are consumed by parallel reactions. The signal of iScience Article TRSPV on semiconductor in the presence of water could be strongly related to the oxidation of surface water, which was similar to the SPV analysis on water oxidation under steady illumination (Shelton et al, 2016).…”

“…The surface photovoltage (SPV) techniques have been previously developed for the investigation of the charge separation and recombination on hematite (Herrmann-Geppert et al, 2013). Moreover, the SPV measured in air, nitrogen, and water by Osterloh et al showed the signal was strongly affected by the oxidation of surface water (Shelton et al, 2016). Inspired by the laser photolysis study in nature Kok cycle (Kern et al, 2018) and time-resolved Fourier transform infrared spectroscopy (Zhang et al, 2014), we employed a time-resolved surface photovoltage (TRSPV) spectroscopy technique to detect electromagnetic signals directly from the surface charges (Dittrich et al, 2008;Kronik and Shapira, 1999).…”

Surface Photovoltage Spectroscopy Observes Sub-Band-Gap Defects in Hydrothermally Synthesized SrTiO3 Nanocrystals

“…A similar surface charge trapping effect had been previously identified for films of Fe 2 O 3 particles and CuWO 4 particles. 46,48 For the SrTiO 3 films studied here, surface trap states may originate from adsorbed H 2 O, oxygen, or carbon species or from Ti 3+ ions. According to the Kubelka−Munk absorption spectrum in Figure 2A, the band gap (E g ) of the SrTiO 3 particle film is 3.2 eV, which agrees well with the literature value for this material.…”

Ferroelectric surface photovoltage enhancement in chromium-doped SrTiO3 nanocrystal photocatalysts for hydrogen evolution

“…This slower timescale is typically associated with the diffusion of charge carriers, as previously seen for other nanostructured light absorbers, incl. HCa 2 Nb 3 O 10 nanosheets, 41 Fe 2 O 3 nanowires 47 and BiVO 4 particles. 40 The data in Fig.…”