A fast and low-cost sol-gel synthesis used to deposit a shell of TiO2 anatase onto an array of vertically aligned ZnO nanowires (NWs) is reported in this paper. The influence of the annealing atmosphere (air or N2) and of the NWs preannealing process, before TiO2 deposition, on both the physicochemical characteristics and photoelectrochemical (PEC) performance of the resulting heterostructure, was studied. The efficient application of the ZnO@TiO2 core-shells for the PEC water-splitting reaction, under simulated solar light illumination (AM 1.5G) solar light illumination in basic media, is here reported for the first time. This application has had a dual function: to enhance the photoactivity of pristine ZnO NWs and to increase the photodegradation stability, because of the protective role of the TiO2 shell. It was found that an air treatment induces a better charge separation and a lower carrier recombination, which in turn are responsible for an improvement in the PEC performance with respect to N2-treated core-shell materials. Finally, a photocurrent of 0.40 mA/cm(2) at 1.23 V versus RHE (2.2 times with respect to the pristine ZnO NWs) was obtained. This achievement can be regarded as a valuable result, considering similar nanostructured electrodes reported in the literature for this application.

The interest in thin and ultrathin oxide films is increasing rapidly due to the new properties and the many possible technological applications of such materials. In this frame the interaction with water, a major constituent of our atmosphere, is an essential issue for a better characterization of oxide film-based devices. We report here a detailed high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy study of the reactivity of ultrathin MgO films grown on Ag(100) toward H 2 O. We find that only OH groups are detected at 310 K, while at low temperature molecular adsorption prevails. In the former case we observe a strongly enhanced dissociation probability for monolayer and submonolayer MgO films, indicative of an active role of the Ag substrate in the dissociation process. The active sites are suggested to be low-coordinated ions at the border of monolayer MgO islands. Aging phenomena, previously observed on MgO ultrathin films, 1 have been confirmed. Although their origin could not be definitively determined, our data strongly suggest water adsorption not to be the major cause of this process.

The ability to engineer nearly perfect ultrathin oxide layers, up to the limit of monolayer thickness, is a key issue for nanotechnological applications. Here we face the difficult and important case of ultrathin MgO films on Ag(100), for which no extended and well-ordered layers could thus far be produced in the monolayer limit. We demonstrate that their final morphology depends not only on the usual growth parameters (crystal temperature, metal flux, and oxygen partial pressure), but also on aftergrowth treatments controlling so far neglected thermodynamics constraints. We thus succeed in tuning the shape of the oxide films from irregular, nanometer-sized, monolayer-thick islands to slightly larger, perfectly squared, bilayer islands, to extended monolayers limited apparently only by substrate steps.

Aqueous chemical growth of zinc oxide nanowires is a flexible and effective approach to obtain dense arrays of vertically oriented nanostructures with high aspect ratio. Herein we present a systematic study of the different synthesis parameters that influence the ZnO seed layer and thus the resulting morphological features of the free-standing vertically oriented ZnO nanowires. We obtained a homogeneous coverage of transparent conductive substrates with high-aspect-ratio nanowire arrays (length/diameter ratio of up to 52). Such nanostructured vertical arrays were examined to assess their electric and piezoelectric properties, and showed an electric charge generation upon mechanical compressive stress. The principle of energy harvesting with these nanostructured ZnO arrays was demonstrated by connecting them to an electronic charge amplifier and storing the generated charge in a series of capacitors. We found that the generated charge and the electrical behavior of the ZnO nanowires are strictly dependent on the nanowire length. We have shown the importance of controlling the morphological properties of such ZnO nanostructures for optimizing a nanogenerator device.

Using a methylated Si precursor instead of tetraethoxysilane (TEOS), methyl-imogolite (Me−IMO), a nanotube material with formula (OH) 3 Al 2 O 3 SiCH 3 is obtained in place of the standard imogolite (OH) 3 Al 2 O 3 SiOH (IMO). 21 Postsynthesis grafting of the outer surface of Me−IMO with 3aminopropyltriethoxysilane (3-APS) yields a new hybrid material (Me−IMO−NH 2 ), with an entirely hydrophobic inner surface and a largely aminated outer surface. In this paper, the structure and stability of Me−IMO−NH 2 are studied in detail and compared with those of Me−IMO by means of (i) X-ray photoelectron spectroscopy (XPS), confirming the surface chemical composition of Me−IMO− NH 2 ; (ii) 1 H, 13 C, 27 Al, 29 Si, and heteronuclear correlation (HETCOR) 1 H− 13 C magic angle spinning nuclear magnetic resonance (MAS NMR) experiments, providing evidence for the occurrence of grafting and yielding an estimate of its extent; (iii) infrared spectroscopy, showing that most terminal −NH 2 groups are protonated; (iv) X-ray diffraction (XRD) measurements yielding information on the long-range order; and (v) N 2 adsorption at −196 °C, yielding specific surface area and pore size distribution. Reaction with 3-APS brings about a limited loss in microporosity, probably caused by functionalization at the mouth of pores, and an increased disorder in the alignment of nanotubes, with neither a big loss of specific surface area nor a sizable change in the distance between nanotubes. As a whole, imogolite-type nanotubes appear to be rather prone to functionalization, which seems to allow the possible tailoring of the properties of both inner and outer surfaces.

In spite of the relevance of ultrathin MgO films for the study of model systems as well as for technological applications, great difficulties have been found so far in the growth of extended, well-ordered, ultrathin films. Combining scanning tunneling microscopy, X-ray photoemission spectroscopy, and high-resolution electron energy loss spectroscopy experiments with ab initio calculations, we demonstrate here that the structure of sub-monolayer MgO films grown on Ag(100) by reactive deposition is strongly affected not only by the growth conditions but also by after-growth treatments. The latter ones allow one to quench the thermodynamically most stable configuration at the deposition temperature or let the system evolve toward the low-temperature equilibrium state. Moreover, we give experimental and theoretical evidence of the accumulation of oxygen atoms at the MgO/Ag interface at the highest deposition temperature, which reduces the stress of the oxide film favoring the formation of extended terraces. The result is the possibility to tune the morphology of the films from small islands with corrugated borders, to perfectly square islands of larger size, to MgO terraces several tens of nanometers wide

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