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 (TiO 2 ) photocatalytic system. Specifically, we show that H 2 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.titania | brookite | photocatalysis | photoreforming | hydrogen

Hybrid organic/inorganic lead iodide perovskites of the formula APbI3, where A is a molecular cation such as methylammonium, exhibit remarkably slow photoinduced charge carrier recombination rates, for reasons that remain uncertain. Prevalent hypotheses credit this behavior to the unique dipolar nature of the molecular cation. Herein, transient terahertz spectroscopy is applied to solution-processed, all-inorganic, perovskite-phase cesium lead iodide (CsPbI3) thin films, which lack such a dipole. The recombination kinetics are studied as a function of the initial photoinduced carrier concentration and the wavelength of excitation. A kinetic model combining diffusion and recombination is fit to the data, from which the rate constants are determined, revealing a bimolecular recombination rate of 10–10 cm3 s–1, comparable to high-quality, single-crystal, direct-gap semiconductors. This rate, as well as a charge carrier mobility > 30 cm2 V–1 s–1 measured herein for CsPbI3, are similar to values reported for the hybrid perovskites, strongly suggesting that the organic cation does not confer a fundamental advantage.

Perovskite oxides are a promising material class for photovoltaic and photocatalytic applications due to their visible band gaps, nanosecond recombination lifetimes, and great chemical diversity. However, there is limited understanding of the link between composition and static and dynamic optical properties despite the critical role these properties play in the design of light-harvesting devices. To clarify these relationships we systemically studied the optoelectronic properties in La 1-x Sr x FeO 3-δ epitaxial films, uncovering the effects of A-site cation substitution and oxygen stoichiometry. Variable angle spectroscopic ellipsometry was used to measure static optical properties, revealing a linear increase in absorption coefficient at 1.25 eV and a red-shifting of the optical absorption edge with increasing Sr fraction. The absorption spectra can be similarly tuned through the introduction of oxygen vacancies, indicating the critical role that nominal Fe valence plays in optical absorption. Dynamic optoelectronic properties were studied with ultrafast transient reflectance spectroscopy, revealing similar nanosecond photoexcited carrier lifetimes for oxygen deficient and stoichiometric films with the same nominal Fe valence. These results demonstrate that while the static optical absorption is strongly dependent on nominal Fe valence tuned through cation or anion stoichiometry, oxygen vacancies do not appear to play a significantly detrimental role in the recombination kinetics.

A mercury drop covered by an aqueous solution which is brought into contact with a metal tip at a fixed potential can be excited to sustained electromechanical oscillations ("beating mercury heart"). The driving force for these pulsations is the electrocapillary effect. We investigated these excitations for the traditional watch glass geometry and for linear and ring-shaped geometries with different lengths and diameters, respectively, varying the potential of the metal tip. We find standing waves in the linear geometry with the number of nodes depending on the potential. In the ring geometry we observe a fast pulsation mode and a slow mode with 2-fold symmetry axis. In addition, we find solitary waves circulating on the ring.

Ultrafast pump-probe transient reflectance (TR) spectroscopy was used to study carrier dynamics in an epitaxial perovskite oxide thin film of LaFeO 3 (LFO) with a thickness of 40 unit cells (16 nm) grown by molecular beam epitaxy on (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 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 important parameters for the potential utilization of perovskite films in photovoltaic and photocatalytic applications. V C 2014 AIP Publishing LLC.Perovskite oxides are a class of transition metal oxides with the chemical structure ABO 3 . They have garnered much interest because of the diverse range of their physical and magnetic properties, including ferroelectricity, insulator-tometal transitions, ferromagnetism, and superconductivity. 1 Many perovskite oxides exhibit band gaps in the visible range, leading to growing research interest in utilizing perovskite oxides for photovoltaic (PV) and photocatalytic (PC) applications. 2-9 However, understanding of the underlying ultrafast carrier dynamics in these materials is limited to a few studies, 10-12 despite the critical role that carrier lifetimes play in the design of materials for PV and PC applications. 13 Here, we present a study of the carrier dynamics and recombination lifetimes in an epitaxial LaFeO 3 (LFO) thin film using ultrafast pump-probe spectroscopy. LFO is an interesting material system for PV and PC applications because its band gap is in the visible range (2.1-2.6 eV). 14,15 The relative chemical simplicity of LFO, compared to quaternary oxides, makes it an appealing system with which to begin investigations focused on identifying strategies to enhance carrier lifetimes in perovskites. Ultrafast spectroscopy is an ideal technique to study recombination lifetimes and has already been used for this purpose in other perovskite oxides. [10][11][12]16,17 Much of the previous work, however, was carried out on bulk crystals or polycrystalline films, as opposed to strained epitaxial thin films as reported here. The ability to measure carrier dynamics in ultrathin perovskites is a critical step in understanding how heterostructure-based approaches can be used to control lifetimes. To mitigate contributions from the substrate, the ultrafast pump-probe measurements were performed in a reflective geometry, using transient reflectance (TR) spectroscopy. ...