Sergey Y. Smolin

Engineering titania nanostructure to tune and improve its photocatalytic activity

Significance This work shows that hole−electron recombination can be controlled by engineering the length of brookite nanorods, and that a variety of organic substrates can be efficiently oxidized as the counterreaction to hydrogen evolution. Both are important steps to developing photocatalysis as a sustainable technology. Electron−hole recombination is a major fundamental limitation in any photocatalytic process. By controlling and reducing it with rod length, we can increase the efficiency of photocatalyzed processes. Also, by utilizing demanding substrates in aqueous media, ethanol, glucose, and glycerol, we make a step toward the photoreforming of more plentiful feedstocks such as, or derived from, biomass. Photocatalytic pathways could prove crucial to the sustainable production of fuels and chemicals required for a carbon-neutral society. Electron−hole recombination is a critical problem that has, so far, limited the efficiency of the most promising photocatalytic materials. Here, we show the efficacy of anisotropy in improving charge separation and thereby boosting the activity of a titania (TiO2) photocatalytic system. Specifically, we show that H2 production in uniform, one-dimensional brookite titania nanorods is highly enhanced by engineering their length. By using complimentary characterization techniques to separately probe excited electrons and holes, we link the high observed reaction rates to the anisotropic structure, which favors efficient carrier utilization. Quantum yield values for hydrogen production from ethanol, glycerol, and glucose as high as 65%, 35%, and 6%, respectively, demonstrate the promise and generality of this approach for improving the photoactivity of semiconducting nanostructures for a wide range of reacting systems.

Enhanced photoelectrochemical water splitting via SILAR-deposited Ti-doped hematite thin films with an FeOOH overlayer

Hematite (α-Fe2O3) has been intensely investigated for photoelectrochemical (PEC) water splitting, but diffusion of metal atoms from underlying layers often obscures the nature of improvements induced by intentional dopants and overlayers. Here we provide insight into the effects of Ti doping and FeOOH overlayers using hematite films with well-controlled, uniform doping profiles fabricated by successive ionic layer adsorption and reaction (SILAR) followed by annealing at 600 °C. An optimum Ti concentration of 4.2% in the film doubled overall charge separation efficiency to 20% compared to undoped hematite by enhancing band bending. With Ti, carriers generated within 3 nm of the depletion region were separated with near-unity efficiency. Moreover, Ti improved interfacial hole transfer efficiency from 1.0 mA cm−2 at 1.50 VRHE, which are competitive with champion planar hematite films. The approach for fabrication and analysis reported here can be applied to a wide range of materials to improve fundamental understanding and performance of PEC systems.

Visible light carrier generation in co-doped epitaxial titanate films

Perovskite titanates such as SrTiO3 (STO) exhibit a wide range of important functional properties, including ferroelectricity and excellent photocatalytic performance. The wide optical band gap of titanates limits their use in these applications; however, making them ill-suited for integration into solar energy harvesting technologies. Our recent work has shown that by doping STO with equal concentrations of La and Cr, we can enhance visible light absorption in epitaxial thin films while avoiding any compensating defects. In this work, we explore the optical properties of photoexcited carriers in these films. Using spectroscopic ellipsometry, we show that the Cr3+ dopants, which produce electronic states immediately above the top of the O 2p valence band in STO reduce the direct band gap of the material from 3.75 eV to 2.4–2.7 eV depending on doping levels. Transient reflectance spectroscopy measurements are in agreement with the observations from ellipsometry and confirm that optically generated carriers...

Ultrafast transient reflectance of epitaxial semiconducting perovskite thin films

Ultrafast pump-probe transient reflectance (TR) spectroscopy was used to study carrier dynamics in an epitaxial perovskite oxide thin film of LaFeO3 (LFO) with a thickness of 40 unit cells (16 nm) grown by molecular beam epitaxy on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT). TR spectroscopy shows two negative transients in reflectance with local maxima at ∼2.5 eV and ∼3.5 eV which correspond to two optical transitions in LFO as determined by ellipsometry. The kinetics at these transients were best fit with an exponential decay model with fast (5–40 ps), medium (∼200 ps), and slow (∼ 3 ns) components that we attribute mainly to recombination of photoexcited carriers. Moreover, these reflectance transients did not completely decay within the observable time window, indicating that ∼10% of photoexcited carriers exist for at least 3 ns. This work illustrates that TR spectroscopy can be performed on thin (<20 nm) epitaxial oxide films to provide a quantitative understanding of recombination lifetimes, which are importan...