Nanoparticulate dye-semiconductor hybrid materials formed by electrochemical self-assembly as electrodes in photoelectrochemical cells

Dye-sensitized zinc oxide thin films were prepared, characterized and optimized for applications as photoelectrochemically active electrodes. Conditions were established under which crystalline thin films of ZnO with a porous texture were formed by electrochemically induced crystallization controlled by structure-directing agents (SDA). Dye molecules were adsorbed either directly as SDA during this preparation step or, preferably, following desorption of a SDA. The external quantum efficiency (IPCE) could thereby be increased significantly. Particular emphasis was laid on dye molecules that absorb in the red part of the visible spectrum. Model experiments under ultrahigh vacuum (UHV) conditions with dye molecules adsorbed on defined crystal planes of single crystals aimed at a deeper understanding of the coupling of the chromophore electronic π-system within molecular aggregates and to the semiconductor surface. Detailed photoelectrochemical kinetic measurements were used to characterize and optimize the electrochemically prepared dye-sensitized ZnO films. Parallel electrical characterization in vacuum served to distinguish between contributions of charge transport within the ZnO semiconductor matrix and the ions of the electrolyte in the pore system of the electrode

Surface photochemistry on confined systems: UV-laser-induced photodesorption

UV-laser induced desorption of NO from nanostructured palladium aggregates on an epitaxial alumina support has been studied by means of resonance enhanced multiphoton ionisation (REMPI) to detect desorbing molecules quantum state resolved by Fourier-transform infrared reflection absorption spectroscopy (FT-IRAS), X-ray photoemission spectroscopy (XPS) and thermal desorption spectroscopy (TPD). The size of the Pd-aggregates was systematically varied between 5 Å and 80 Å . Different morphologies were chosen depending on the growth conditions of the aggregates by Pd-atom deposition on the support. The aggregates were either amorphous (deposition at 100 K) or ordered (deposition at 300 K) with the aggregates having a cubooctahedral shape with dominating (111) terraces and a minority of (100) terraces. Adsorption is only similar to single crystal data for aggregate sizes beyond 75 Å . For smaller aggregates NO is bound on on-top sites of palladium. On small amorphous aggregates a substantial amount of NO is weakly bound, which has only been observed for stepped single Pd-crystals. Dissociation of NO occurs at elevated temperatures above 350 K. The system was excited with nanosecond laser pulses at 6.4 eV. In contrast to single crystals, desorption of intact NO molecules has been observed for small aggregates with increasing efficiencies with decreasing aggregate size for aggregate sizes of 80 Å and below. Desorption cross sections vary by at least one order of magnitude. Dominantly the weakly bound species desorbs. REMPI data do not show a strong size dependence. Different models are discussed to explain the data, including the role of local effects of the adsorption site, spill-over to the alumina support or formation of oscillations in electron densities

Vanadium Oxide Nanosheets for Flexible Dendrite‐Free Hybrid Aluminium‐Lithium‐Ion Batteries with Excellent Cycling Performance

Aluminum (Al)-ion batteries can be an attractive alternative to lithium-ion batteries because of low costs, high volumetric capacities and dendrite-free formation when Al is used as anode. However, there are limited cathode materials for Al-ion batteries that can deliver satisfactory electrochemical performance, especially cycling stability. The major reason for that is the sluggish kinetics of ion intercalation/deintercalation, resulting from large coulombic attraction between Al3+ and cathodes. Herein, a concept of hybrid Al-Li-ion batteries is proposed to circumvent the poor Al3+ ions insertion/extraction kinetics in Al-ion batteries, and maintain the dendrite-free characteristics of Al-ion batteries. The high volumetric capacity (32.5 mAh/cm3 at 100 mA/cm3, based on the total volume of cathode), enhanced rate capability (21.5 mAh/cm3 at 1000 mA/cm3) and excellent cycling performance (70.1% retention after 3000 cycles) have been achieved in the hybrid Al-Li-ion battery composed of vanadium oxide nanosheets on carbon fibers as cathode and Al as anode in a mixed [EMIM][Cl]/AlCl3/LiCl electrolyte. Combining with the good flexibility of cathode and anode, the hybrid Al-Li-ion battery maintains structural and capacity stability under different bending angles. This study unveils a safe, cost-effective and flexible hybrid Al-Li-ion battery that presents highly competitive advantages among various energy storage devices.NRF (Natl Research Foundation, S’pore)Accepted versio

Lateral velocity distributions in laser-induced desorption of CO from Cr2O3(0001): experiment and theory

Quantum state resolved determination of lateral velocity distributions in laser-induced desorption of CO from a Cr2O3(0001) surface is investigated. Experimentally, Doppler profiles are obtained via a (1+1)-REMPI detection scheme for various rovibrational states and desorption velocities. The experimental findings are directly compared to theoretical results from high-dimensional stochastic quantum dynamical wave packet calculations based on complete first-principles potential energy surfaces. Reasonable quantitative agreement between experiment and theory concerning the total width as well as the full width at half maximum of the lateral velocity distributions is found. Depending on the rotational state a single-peaked or a double-peaked shape of the Doppler profiles is predicted theoretically. In order to support our theoretical predictions we suggest refined experimental investigations of the state-resolved Doppler profiles with an enhanced signal to noise ratio and further theoretical studies concerning the nature of the electronically excited intermediate