Photochemical charge transfer observed in nanoscale hydrogen evolving photocatalysts using surface photovoltage spectroscopy

Abstract

The application of inorganic nanostructures for solar water splitting is currently limited by our understanding of photochemical charge transfer on the nanoscale, where space charge layers are less effective for carrier separation. Here we employ surface photovoltage spectroscopy to measure the internal photovoltages in single crystalline platinum/ruthenium-modified Rh-doped SrTiO 3 nanocrystals for the first time. Voltages of À0.88 V and À1.13 V are found between the absorber and the Ru and Pt cocatalysts, respectively, and a voltage of À1.48 V for a Rh:SrTiO 3 film on an Au substrate. This shows that the Pt and Ru cocatalysts not only improve the redox kinetics but also aid charge separation in the absorber. Voltages of +0.4 V, +0.6 V, and +1.2 V are found for hole injection into KI, K 4 [Fe(CN) 6 ], and methanol, respectively, and a voltage of À0.7 V for electron injection into K 3 [Fe(CN) 6 ]. These voltages correlate well with the photocatalytic performance of the catalyst; they are influenced by the built-in potentials of the donor-acceptor configurations, the physical separation of donors and acceptors, and the reversibility of the redox reaction.The photovoltage data also allowed the identification of a photosynthetic system for hydrogen evolution (80 mmol g À1 h À1) under visible light illumination (4400 nm) from 0.05 M aqueous K 4 [Fe(CN) 6 ]. Broader contextNanostructured light absorbers have advantages for solar water splitting, including shortened carrier collection pathways and improved light distribution. However, the application of nanoscale absorbers for artificial photosynthesis is currently limited by our understanding of photochemical charge transfer on the nanoscale, where space charge layers are not effective for charge separation. Here we employ surface photovoltage spectroscopy (SPS) to observe electron and hole transfer from single crystalline platinum/ruthenium-modified Rh-doped SrTiO 3 nanocrystals for the first time. We find that the absorber-Pt/Ru junctions promote electron-hole separation strongly, allowing open circuit voltages of over 1.0 V. This is comparable to the effect of molecular redox reagents at the absorber surfaces. Overall, we find that nanoscale charge transfer is controlled by the built-in potentials of the absorber/acceptor configuration, by the spatial separation between donors and acceptors and by the reversibility of the redox reactions. These observations aid the understanding of photochemical charge transfer on the nanoscale and contribute to the design of more efficient systems for artificial photosynthesis.

Photoelectrochemical Water Splitting System—A Study of Interfacial Charge Transfer with Scanning Electrochemical Microscopy

“…6 ] 3-can be used as a acceptor of photo-generated holes and electrons for charactering the redox kinetics of photocatalyst in PEC water splitting. [52][53][54] For investigating the catalytic reaction kinetics (reaction 1 in Figure 1b also as a sacrificial agent, which can effectively improve the separation efficiency of interface photo-generated charge. This method may underestimate the actual collected electrons by the fraction that is lost due to recombination (bulk and surface) during transport.…”

Photochemistry of hematite photoanodes under zero applied bias

“…This makes SPS particularly useful to probe intrinsic charge transfer processes in nanostructured light absorbing materials. [31][32][33][34][35][36] Here we apply the technique to thin films of Fe 2 O 3 nanorods grown onto FTO. We find that the photovoltage of the material is very sensitive to the ambient environment in the chamber and to any surface treatment of the films.…”

Preparation and electrochemical performance of uniform RuO2/Ti and RuO2‐IrO2/Ti electrode for electrolysis of NaCl solution

“…The CV curves of the RuO 2 /Ti and RuO 2 ‐IrO 2 /Ti electrodes in 0.5 mol/L Na 2 SO 4 + 0.1 mol/L NaCl solution at different scanning speeds are shown in Figures a and b. As the literature reports, the O evolution potential of the Ti‐based Ru layer is 0.88 V . As can be seen from Figure a, the oxidation peak appears when the electrode potential is E(HgO/Hg) = 0.72 V, and a reduction peak appears at the electrode potential E(HgO/Hg) = 0.76 V. The literature reports that E θ = 1.229 V of electrolyzed water, which indicates that the O evolution is produced by the NaCl solution.…”