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Cr1.8Ti0.2O3+z (CTO) particles were grown by a sol–gel route followed by two thermal treatments at 1273 K. Magnetic susceptibility studies showed a para- to antiferromagnetic transition at around 3...

Small amounts of water (between and 0.05 and 0.35% V/V) critically determine the morphology and plasmon band of Au/ZnO nanostructures obtained by Au3+ photoreduction on ZnO nanoparticles dispersed in 2-propanol. All the synthesized materials exhibit plasmon induced activity to drive the solvent oxidation; however, the temporal evolution of acetone shows a clear induction time followed by the sudden boost in the rate of the oxidation product, which depends on the photodeposition conditions. Xray photoelectron spectroscopy (XPS) indicates that visible irradiation produces the transformation of surface Au(0) in Au(+). Besides, an increment in the ZnO surface area ascribed to the photoinduced fragmentation of aggregated networks of Au/ZnO nanocomposites is evidenced by XPS and simple adsorption experiments. The changes in the surface properties correlate with the onset in the catalytic activity. Possible mechanisms are discussed to account for the experimental findings. Fil: Aguirre, Matías Ezequiel. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Departamento de Química; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Custo, G. Comisión Nacional de Energía Atómica; Argentina Fil: Goes, Marcio S.. Universidade Estadual Paulista Julio de Mesquita Filho; Brasil Fil: Bueno, Paulo R.. Universidade Estadual Paulista Julio de Mesquita Filho; Brasil Fil: Zampieri, Guillermo Enrique. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Grela, Maria Alejandra. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Departamento de Química; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

The preparation and thermal stability of benzenethiol and benzeneselenol self-assembled monolayers (SAMs) grown on Au(111) have been investigated by electrochemical experiments and high-resolution photoemission spectroscopy. Both techniques confirm the formation of monolayers with high packing densities (θ = 0.27-0.29 ML) and good degrees of order in both cases. Despite many similarities between the two SAMs, the thermal desorption is distinctly different: whereas the benzenethiol SAM desorbs in a single steplike process, the desorption of the benzeneselenol SAM occurs with a much lower activation energy and involves the cleavage of some Se-C bonds and a change in molecular configuration from standing up to lying down. This behavior is explained by considering the different nature of the bonding of the headgroup with the metal surface and with the phenyl ring. Density functional theory calculations show that the breakage of the Se-C bond has a lower activation energy barrier than the breakage of the S-C bond.

Heterogeneous photocatalytic reduction of As(V) and As(III) at different concentrations over TiO(2) under UV light in deoxygenated aqueous suspensions is described. For the first time, As(0) was unambiguously identified together with arsine (AsH(3)) as reaction products. As(V) reduction requires the presence of an electron donor (methanol in the present case) and takes place through the hydroxymethyl radical formed from methanol oxidation by holes or hydroxyl radicals. On the contrary, As(III) reduction takes place through direct reduction by the TiO(2)-conduction band electrons. Detailed mechanisms for the photocatalytic processes are proposed. Although reduction to solid As(0) is convenient for purposes of As removal from water as a deposit on TiO(2), attention must be paid to formation of AsH(3), one of the most toxic forms of As, and strategies for AsH(3) treatment should be considered.

Density functional theory has been used to investigate the adsorption of sulfur atoms in the (√7 × √7)R19.1° unit cell of Ag(111). For the coverages θ = 1/7 and 2/7 the S atoms adsorb at fcc and hcp hollow sites with negligible reconstruction of the surface. For θ = 3/7 a large surface reconstruction occurs. The three Ag atoms around the hcp site that hosts a S atom move vertically and in-plane away from the hollow center provoking two effects: (i) the S atom adsorbed at this site penetrates under the surface and (ii) a top site hosting the third S atom is transformed into a new 3-fold hollow site. A detailed analysis of the different contributions to the adsorption energy shows that the energy cost of deforming the lattice is paid off by the stronger and more favorable bonding of these two S atoms. This picture is confirmed by the new charge distribution after the reconstruction.

We investigated the formation and stability of layers of methylthiolate prepared on the Au(111) surface by the method of immersion in an ethanolic solution of dimethyl disulfide (DMDS). The surface species were characterized by electrochemical reductive desorption and high-resolution photoelectron spectroscopy. Both techniques confirmed the formation of a methylthiolate monolayer at short immersion times (around 1 min). As the immersion time increased, the electrochemical experiments showed the disappearance of the methylthiolate reductive desorption current peak and the appearance of a current peak at ca. −0.9 V which was attributed to sulfur species. At long immersion times, the XPS measurements showed two main components for the S 2p signal: a component at ca. 161 eV which corresponds to atomic sulfur and a component at ca. 162 eV which we attributed to polysulfide species. We propose that the breakage of the S−C bond of methylthiolate is responsible for the appearance of sulfur species on the surface....

The structural phases formed by atomic sulfur on Au(111) due to reaction with molecular S2 have been investigated by qualitative low-energy electron diffraction (LEED), scanning tunneling microscopy, and normal incidence X-ray standing wavefield absorption (NIXSW) combined with X-ray photoelectron spectroscopy (XPS). Three phases are identified with increasing coverage, namely, a newly identified (5 × 5) phase, a ( × )R30° phase, and a “complex” phase. The (5 × 5) phase, with a LEED pattern having the appearance of a “split-spot” ( × )R30° pattern, is interpreted in terms of local ( × )R30° ordering within a (5 × 5) ordered domain structure. The S atoms in the (5 × 5) phase occupy fcc hollow sites 1.56 A above the outermost extended Au(111) bulk atomic scatterer plane. A specific model of the ordering in this phase is proposed that, together with the observed marginal stability of the true, long-range-ordered, ( × )R30° phase, indicates significant short-range S−S repulsion and probably compressive surfac...

In this work, the electrochemical formation of alkanethiolate self-assembled monolayers (SAMs) on Ni(111) and polycrystalline Ni surfaces from alkanethiol-containing aqueous 1 M NaOH solutions was studied by combining Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), electrochemical techniques, and density functional theory (DFT) calculations. Results show that alkanethiolates adsorb on Ni concurrent with NiO electroreduction. The resulting surface coverage depends on the applied potential and hydrocarbon chain length. Electrochemical and XPS data reveal that alkanethiolate electroadsorption at room temperature takes place without S-C bond scission, in contrast to previous results from gas-phase adsorption. A complete and dense monolayer, which is stable even at very high cathodic potentials (-1.5 V vs SCE), is formed for dodecanethiol. DFT calculations show that the greater stability against electrodesorption found for alkanethiolate SAMs on Ni, with respect to SAMs on Au, is somewhat related to the larger alkanethiolate adsorption energy but is mainly due to the larger barrier to interfacial electron transfer present in alkanethiolate-covered Ni. A direct consequence of this work is the possibility of using electrochemical self-assembly as a straightforward route to build stable SAMs of long-chained alkanethiolates on Ni surfaces at room temperature.

Thiourea (TU) adsorption on Ag( 111) from aqueous solutions was investigated by in situ scanning tunneling microscopy operating under potential control. Hexagonal arrangements with d = 0.44, 0.38, and 0.33 nm were imaged at potentials close to -1.2 V (vs SCE), where conjugated voltammetric current peaks are observed. The analysis of in situ Fourier transform infrared reflection-absorption spectra (FT-IRRAS) shows that these current peaks mainly involve electroadsorption/electrodesorption of TU on the Ag(111) surface. Data from ex situ X-ray photolectron spectroscopy of TU-covered Ag( 111) closely resemble those obtained for adsorbed alkanethiols on the same substrate, suggesting that the canonical form of TU is the species adsorbed from aqueous solutions. Experimental evidence of TU degradation into other sulfur species is also observed.