Efficient Anodically Grown WO3 for Photoelectrochemical Water Splitting

Abstract The potentiostatic anodization of metallic tungsten has been investigated in different solvent/electrolyte compositions with the aim of improving the photoelectrochemical performances of the tungsten oxide layer. Among the explored electrolytes, the anodization in the NMF/H2O/NH4F solvent mixture was found to produce the most efficient WO3 photoanodes, which, combining spectral sensitivity, high electrochemically active surface and improved charge transfer kinetics, outperform, under simulated solar illumination, most of the reported nanocrystalline substrates produced by anodization in aqueous electrolytes and by sol gel methods. While the preparation of the photoelectrodes is a slow process at room temperature (20 °C), it could be greatly accelerated (x 10) by carrying out the anodization at 40-50 °C, thus proving to be a fast and convenient approach to the production of high performing WO3 photoactive substrates directly connected to a metal electron collector

Mineralogical characterization of fluorescent grossular garnet var. tsavorite from Merelani Hills, Tanzania

Tsavorite is the trade name for the green vanadium–chromium variety of grossular occurring in the Precambrian terrains in the areas of Merelani Hills (Tanzania) and Tsavo Park (Kenya) which are by far the most important source of gem grade specimens of tsavorite used for high jewellery. The tsavorite crystals from Merelani Hills exhibit a pink-red and yellow fluorescence when irradiated by common portable UV lamp, an unusual phenomenon among members of the garnet group. The electron density map calculated from the diffraction data and plotted against a grossular standard shows that an excess of negative charge is clearly pinpointed in the crystallographic site occupied by Al3+. The bulk elemental analysis shows that the most represented end-member, besides grossular, is the vanadium-bearing goldmanite garnet (3.82–4.08 mol %). The fluorometry with an excitation beam at 408 nm indicates a complex emission pattern with the most intense emissions at 701 and 716 nm and subordinately at 592 nm. The colour perception is dominated by the emission yellow band at 592 nm while the contribution of the red band modulates the colour ranging from bright orange to pink-red. The attribution of the emission at 592 nm is related to Mn2+ while the emissions at 701 and 716 nm could be related to the chromium content and/or to a possible fraction of vanadium as V2+. Because of the characteristic colour perceived under UV light, the use of a common led lamp can be useful as a diagnostic tool to easily identify tsavorite

Arc sensitivity to cluster ellipticity, asymmetries and substructures

We investigate how ellipticity, asymmetries and substructures separately affect the ability of galaxy clusters to produce strong lensing events, i.e. gravitational arcs, and how they influence the arc morphologies and fluxes. This is important for those studies aiming, for example, at constraining cosmological parameters from statistical lensing, or at determining the inner structure of galaxy clusters through gravitational arcs. We do so by creating two-dimensional gradually smoothed, differently elliptical and asymmetric versions of some numerical models. On average, we find that the contributions of ellipticity, asymmetries and substructures amount to ~40%, ~10% and ~30% of the total strong lensing cross section, respectively. However, our analysis shows that substructures play a more important role in less elliptical and asymmetric clusters, even if located at large distances from the cluster centers (~1Mpc/h). Conversely, their effect is less important in highly asymmetric lenses. The morphology, position and flux of individual arcs are strongly affected by the presence of substructures in the clusters. Removing substructures on spatial scales <~50kpc/h, roughly corresponding to mass scales <~5 10^{10}M_\odot/h, alters the image multiplicity of ~35% of the sources used in the simulations and causes position shifts larger than 5'' for ~40% of the arcs longer than 5''. We conclude that any model for cluster lens cannot neglect the effects of ellipticity, asymmetries and substructures. On the other hand, the high sensitivity of gravitational arcs to deviations from regular, smooth and symmetric mass distributions suggests that strong gravitational lensing is potentially a powerfull tool to measure the level of substructures and asymmetries in clusters.Comment: 16 pages, 18 figures. Accepted version. Version with full resolution images can be found at http://www.ita.uni-heidelberg.de/~massimo/sub/publications.htm

Remote interfacial electron transfer processes on nanocrystalline TiO

The kinetic study of interfacial electron transfer in sensitized nanocrystalline semiconductor is essential to the design of molecular devices performing specific light induced functions in a microheterogeneous environment. A series of molecular assemblies performing direct and remote charge injection to the semiconductor have been discussed in the context of artificial photosynthesis. A particular attention in this article has been paid to the factors that control the interfacial electron transfer processes in nanocrystalline TiO2 films sensitized with mononuclear and polynuclear transition metal complexes

WO3 colloids for photoelectrochemical uses studied through SdFFF and FlFFF.

Tungsten trioxide (WO3) is an indirect band gap semiconductor with interesting photoelectrochemical properties. Nanocrystalline WO3 thin films are used as high performance photoanodes, whose current–voltage behavior is strongly affected by the crystal size. For this reason the nanoparticle formation, evolution and aggregation are aspects which call for a carefully investigation. In this work, colloidal suspensions of about 20-30 nm WO3 particles were obtained through a simple sol-gel. The WO3 colloidal suspensions were subjected to a controlled temperature aging process whose time evolution in terms of particle size distribution was followed by Sedimentation Field-Flow Fractionation (SdFFF) and Flow Field-Flow Fractionation (FlFFF). The experiments performed at 60 °C showed that in a few hours the initially transparent sol of WO3 particles undergo a progressive increase in mass, giving after 5 hours particles and/or aggregates of about 120 nm, expressed in terms of equivalent spherical diameter. The observed shift in particle size distribution SdFFF maxima, the broadening of the FlFFF fractograms and the SEM observations seem to suggest that primary WO3 nanocrystals formed during the initial stage tend to change their shape spontaneously becoming well-defined square particles which aggregate to form large clusters at a rate determined by the temperature set during the aging process. The particle size distribution of the suspensions were related to the photoelectrochemical properties of the WO3 particles, preparing thin films on a transparent conductive glass from the aged suspensions sampled at regular intervals. The current-voltage polarization curves recorded in a biased photoelectrolytic cell in the potential range 0-1.8 V (vs SCE) showed a diminution of the maximum photocurrent from 3.7 mA cm-2 to 2.8 mA cm-2 with aging times of 1h and 5h respectively. This loss of performance was mainly attributed to the reduction of the electroactive surface area

Size sorting of citrate reduced gold nanoparticles by sedimentation field-flow fractionation

Gold nanoparticles (GNPs) have been synthesized through the citrate reduction method; the citrate/gold(III) ratio was changed from 1:1 up to 10:1 and the size of the resulting nanoparticles was measured by sedimentation field-flow fractionation (SdFFF). Experimental data showed that the GNPs size decreases in the ratio range 1:1–3:1 and then increases from 5:1 to 10:1 passing through a plateau region in between, and is almost independent of the precursor solution concentrations. In the zone of minimum diameters the synthetic process does not produce monodispersed GNPs but often multiple distributions, very close in size, are observed as evidenced by the particle size distributions (PSDs) derived from the SdFFF fractograms. UV–vis spectrophotometry, being the most common technique employed in the optical characterization of nanoparticles suspensions, was used throughout this work. A confirmation of the nucleation–aggregation–fragmentation mechanism was inferred from the cross-correlation between UV–vis and SdFFF results

Sedimentation field flow fractionation and flow field flow fractionation as tools for studying the aging effects of WO3 colloids for photoelectrochemical uses

WO3 colloidal suspensions obtained through a simple sol-gel procedure were subjected to a controlled temperature aging process whose time evolution in terms of particle mass and size distribution was followed by Sedimentation Field Flow Fractionation (SdFFF) and Flow Field Flow Fractionation (FlFFF). The experiments performed at a temperature of 60 °C showed that in a few hours the initially transparent sol of WO3 particles, whose size was less than 25 nm, undergoes a progressive size increase allowing nanoparticles to reach a maximum equivalent spherical size of about 130 nm after 5 hours. The observed shift in particle size distribution maxima (SdFFF), the broadening of the curves (FlFFF) and the SEM-TEM observations suggest a mixed mechanism of growth-aggregation of initial nanocrystals to form larger particles. The photoelectrochemical properties of thin WO3 films obtained from the aged suspensions at regular intervals, were tested in a biased photoelectrocatalytic cell with 1M H2SO4 under solar simulated irradiation. The current-voltage polarization curves recorded in the potential range 0-1.8 V (vs SCE) showed a diminution of the maximum photocurrent from 3.7 mA cm-2 to 2.8 mA cm-2 with aging times of 1 h and 5 h respectively. This loss of performance was mainly attributed to the reduction of the electroactive surface area of the sintered particles as suggested by the satisfactory linear correlation between the integrated photocurrent and the cyclic voltammetry cathodic wave area of the W(VI) ->W(V) process measured in the dark

Electrochromic devices based on wide band-gap nanocrystalline semiconductors functionalized with mononuclear charge transfer compounds.

A series of ruthenium and iron mononuclear complexes were prepared and their spectroeletrochemical behavior characterized oil Optically Transparent Thin Layer Electrodes (OTTLE) and on Fluorine Doped SnO2 (FTO) conductive glasses coated with Sb-doped nanocrystalline SnO2. These systems display a reversible electrochemical response and offer potential application in electrochromic devices. On SnO2 films distinct spectral changes are observed in a narrow potential range (-0.5/0.9 V vs SCE) with switching times of the order of 0.8 s