Size-dependent recombination dynamics in ZnO nanowires

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 96, 053105 (2010) and may be found at https://doi.org/10.1063/1.3294327.A deep understanding of the recombination dynamics of ZnO nanowires (NWs) is a natural step for a precise design of on-demand nanostructures based on this material system. In this work we investigate the influence of finite-size on the recombination dynamics of the neutral bound exciton around 3.365 eV for ZnO NWs with different diameters. We demonstrate that the lifetime of this excitonic transition decreases with increasing the surface-to-volume ratio due to a surface induced recombination process. Furthermore, we have observed two broad transitions around 3.341 and 3.314 eV, which were identified as surface states by studying the dependence of their life time and intensitiy with the NWs dimensions.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Degradation and regeneration mechanisms of NiO protective layers deposited by ALD on photoanodes

The use of high pH electrolytes requires protective layers to avoid corrosion in photoanodes based on semiconductors like silicon. NiO is one of the materials that comply with the requirements for transparency, conductivity, chemical stability and catalysis on the surface in contact with the electrolyte. Here, NiO layers have been deposited by atomic layer deposition (ALD) at low temperatures, and their stability is analyzed over 1000 hours. Due to the layer structure characteristics, the best overall performance was achieved at 100 °C deposition temperature. By electrochemical measurements progressive time dependent degradation under anodic working conditions is observed, attributed to the formation of higher nickel oxidation states at the electrode/electrolyte interface as a main degradation mechanism, resulting in an OER overpotential increase. Another minor degradation mechanism affects the optical surface quality and gives rise to a loss of photon absorption efficiency on the scale of hundreds of hours. A regeneration process based on in situ periodic cyclic voltammetry, bringing the electrodes to cathodic conditions every 3, 12 or 48 hours, has been shown to partially reverse the main degradation mechanism achieving 85% stability over 1000 hours in a study with over 10 mA cm photocurrent densities

Influence of mismatch on the defects in relaxed epitaxial InGaAs/GaAs(100) films grown by molecular beam epitaxy

Thick (∼3 μm) films of InxGa1−xAs grown on GaAs(100) substrates, across the whole composition range, have been examined by transmission electron microscopy and double‐crystal x‐ray diffraction. The results were compared with the observed growth mode of the material determined by in situ reflection high‐energy electron diffraction in the molecular beam epitaxy growth system. The quality of the material degraded noticeably for compositions up to x∼0.5 associated with an increased density of dislocations and stacking faults. In contrast, improvements in quality as x approached 1.0 were correlated with the introduction of an increasingly more regular array of edge dislocations

Structural Influence of the Anode Materials towards Efficient Zn Deposition/Dissolution in Aqueous Zn-Iodide Flow Batteries

Zinc-iodide flow battery (ZIFB) is one of the best potential candidates for future grid-scale energy storage, due to its eye-catching features of benign, high energy density and non-corrosive nature. However major investigations have not done yet on the negative electrode of this battery where the Zn deposition/dissolution mechanism takes place, which may have an impact on the battery performance. Herein, we have reported a comparative study of different carbon-based anodes which are conventional graphite felt, carbon paper and graphite foil. Single-cell charge/discharge performances among these three different anodes depicts that the cell with planar, hydrophilic graphite foil anode is showing the best energy efficiency and the lowest cell resistance among the carbonaceous electrodes. Zinc dissolution process during discharge process seems to be the bottleneck for having a stable cell, which was corroborated by the use of a Zn foil anode that shows excellent efficiencies along the successive cycles

Influence of In and Ga additives onto SnO2 inkjet-printed semiconductor

Tin oxide is a multifunctional semiconductor that offers excellent capabilities in a variety of applications such as solar cells, catalysis and chemical sensors. In this work, tin-based semiconductors have been obtained by means of solution synthesis and inkjet, and compared to similar materials with In and Ga as additives. The effect of different thermal treatments after deposition is also studied. n-Type behavior with saturation mobility N2 cm2 /Vs has been observed, and suitability as a semiconductor for thin-film transistors (TFTs) demonstrated with on/off ratios of more than 8 decades. Both In and InGa additives are shown to provide superior environmental stability, as well as significant change from depletion to enhancement operation modes in TFTs

High-power positive electrode based on synergistic effect of N- and WO3 -decorated carbon felt for vanadium redox flow batteries

Although Vanadium Redox Flow Batteries (VRFB) are suitable for grid-scale applications, their power-related cost must be reduced in order to boost the use of this technology in the market, allowing their widespread commercialization. One effective way to make the VRFB a competitive and viable solution could be through new strategies for improving the electrocatalytic activity of the electrodes with enhanced electrolyte/electrode interface characteristics. Herein, we report the synergistic effect demonstrated by N- and WO3- decorated carbon-based positive electrode, named HTNW electrode, which demonstrates the feasibility of achieving: i) enhanced electrocatalytic activity, achieving large current density and high reversibility towards VO2+/VO2+ couple (promotion of oxygen and electron transfer processes), ii) decrement of the electron-transfer resistance from 76.18¿O to 13.00¿O for the pristine electrode and HTNW electrodes, respectively; iii) 51% of the electrolyte utilization ratio at high rates (i.e. 200¿mA¿cm-2) with 70% of energy efficiency; iv) increment of more than 50% of the power–peak in comparison with pristine electrode.Peer ReviewedPostprint (author's final draft

Suppressing water migration in aqueous Zn-iodide flow batteries by asymmetric electrolyte formulation

Zinc-iodide flow battery (ZIFB) is under research for the last years due to its suitability as a potential candidate for future electrochemical energy storage. During cycling, one of the biggest challenges that affect the reliable performance of ZIFB is the substantial water migration through the membrane because of differential molar concentrations between anolyte and catholyte that imbalance the osmotic pressures in each compartment. Considering the mass balances, herein we propose to equalize the total ionic concentration of electrolytes by the addition of extra solute into the compartment of lower ion concentration as a way to restrict the water crossover. Experimental validation of this electrolyte concentrations balancing strategy has been carried out by assessing the post-cycled electrolytes, and half-cell charged electrolytes, which confirms the efficient suppression of water migration from catholyte to anolyte. Besides, in-depth analysis of ions and water transport mechanism through Nafion 117 membrane confirms that solvated K+ ions of lower ionic radius compared to solvated Zn2+ ions, are the dominant migrating carrier. Therefore, the addition of extra KI solute is beneficial to suppress the major transport of large hydrated Zn2+ ions along with the higher amount of water. Finally, the improved ZIFB cell behaviour with enhanced electrical conductivity, discharge capacity, and voltage efficiency in the cell assembled with the electrolytes of balanced molar concentrations concludes our present study, proving that tuning the electrolytes concentrations is an effective way to suppress water migration as an appealing method in the prospect of upscaling ZIFB application

Nucleation and growth of GaN nanorods on Si (111) surfaces by plasma-assisted molecular beam epitaxy - The influence of Si- and Mg-doping

The self-assembled growth of GaN nanorods on Si (111) substrates by plasma-assisted molecular beam epitaxy under nitrogen-rich conditions is investigated. An amorphous silicon nitride layer is formed in the initial stage of growth that prevents the formation of a GaN wetting layer. The nucleation time was found to be strongly influenced by the substrate temperature and was more than 30 min for the applied growth conditions. The observed tapering and reduced length of silicon-doped nanorods is explained by enhanced nucleation on nonpolar facets and proves Ga-adatom diffusion on nanorod sidewalls as one contribution to the axial growth. The presence of Mg leads to an increased radial growth rate with a simultaneous decrease of the nanorod length and reduces the nucleation time for high Mg concentrations

Adaptation of Cu(In, Ga)Se2 photovoltaics for full unbiased photocharge of integrated solar vanadium redox flow batteries

The integration of photovoltaics and vanadium redox flow batteries (VRFB) is a promising alternative for the direct conversion and storage of solar energy in a single device, considering their inherent higher energy density versus other redox pairs. However, this integration is not seamless unless the photovoltaic system is customized to the voltage needs of the battery, which unlike artificial photosynthesis, continuously increase with the state-of-charge. We have developed integrated solar VRFB with adapted low-cost Cu(In, Ga)Se2 modules of 3 and 4 series-connected cells (solar efficiency of mini-solar module 8.1%), and considering the voltage requirements (1.3-1.6V), we have evaluated the influence of the photovoltaic operation region on the final efficiency of the solar VRFB. Full unbiased photocharge under 1 Sun illumination has been achieved reaching high energy (77%), solar-to-charge (7.5%) and overall round trip energy conversion efficiencies (5.0%) excelling the values reported in literature for other solar VRFB, thus demonstrating the feasibility and intrinsic potential of adapting low-cost commercial photovoltaics to such energy storage systems.Peer ReviewedPostprint (author's final draft

Hydrothermal Fabrication of Carbon-Supported Oxide-Derived Copper Heterostructures : A Robust Catalyst System for Enhanced Electro-Reduction of CO2 to C2H4

Anthropogenic CO can be converted to alternative fuels and value-added products by electrocatalytic routes. Copper-based catalysts are found to be the star materials for obtaining longer-chain carbon compounds beyond 2e products. Herein, we report a facile hydrothermal fabrication of a highly robust electrocatalyst: in-situ grown heterostructures of plate-like CuO−CuO on carbon black. Simultaneous synthesis of copper-carbon catalysts with varied amounts of copper was conducted to determine the optimum blend. It is observed that the optimum ratio and structure have aided in achieving the state of art faradaic efficiency for ethylene >45 % at −1.6 V vs. RHE at industrially relevant high current densities over 160 to 200 mA ⋅ cm. It is understood that the in-situ modification of CuO to CuO during the electrolysis is the driving force for the highly selective conversion of CO to ethylene through the *CO intermediates at the onset potentials followed by C−C coupling. The excellent distribution of Cu-based platelets on the carbon structure enables rapid electron transfer and enhanced catalytic efficiency. It is inferred that choosing the right composition of the catalyst by tuning the catalyst layer over the gas diffusion electrode can substantially affect the product selectivity and promote reaching the potential industrial scale