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We discuss here for the first time how to combine iron and titanium metal ions to achieve a high photoelectrochemical activity for TiO2-based photoanodes in water splitting devices. To do so, a wide range of photoelectrode materials with tailored Ti/Fe ratio and element vicinity were synthesized by using the versatility of aqueous sol–gel chemistry in combination with a microwave-assisted crystallization process. At low ferric concentrations, single phase TiO2anatase doped with various Fe amounts were prepared. Strikingly, at higher ferric concentrations, we observed the concomitant crystallization of two polymorphs of Fe2TiO5. The as-synthesized compounds were tested as photoelectrodes and compared with pure nanoparticles of TiO2, Fe2TiO5, and α- or γ-Fe2O3and with corresponding nanocomposites. When TiO2is slightly doped by Fe, the performance of this photoelectrode improves particularly in the low-bias region (<1.0 V vs reversible hydrogen electrode.) The photoanode exhibits a higher photocurrent than nanocomposite with TiO2/Fe2O3and FeTi2O5and more cathodic onset potential. The former can be partly explained by a lower bandgap and a hole with a longer lifetime. For the latter, we propose that the nature of the heterojunction impacts charge carrier recombination. The results presented herein not only answer whether iron and titanium should be combined in the same structure or into heterostructured systems but also on the importance of the arrangement of ions in the structure to improve the performances of the photoanode.

Bismuth vanadate has attractive photocatalytic properties under visible light. The influence of structure and morphology of BiVO4nanomaterials on its photocatalytic properties in the UV and the visible domain was investigated. The selection of different sets of synthetic parameters in aqueous solutionpH or the use of organic additivesallowed the formation of tetragonal zircon, tetragonal scheelite, and monoclinic scheelite structure and different morphologies of that last phase. First, the tetragonal zircon was found to be the only inactive structure. Then, the best material for photocatalytic degradation of rhodamine B in solution and stearic acid deposited directly on the photocatalyst is the core–shell tetragonal zircon–monoclinic scheelite system prepared in the presence of sodium dodecyl sulfate. The enhanced properties are explained by the presence of strong surface acidic sites corresponding to the presence of surface sulfate residues rather than to the specific morphology of the material. Additionally, an EPR study on the ability of BiVO4to generate active surface radical showed that hydroxyl radicals are not generated and that superoxide ion concentration under irradiation is close to the detection threshold. Depending on the selected irradiation wavelength, bismuth vanadate may present a better photocatalytic activity than titanium oxide. It is shown to be equivalent to bismuth tungstate under blue light.

We introduce a magic-angle spinning NMR experiment to estimate specific distances in a solid material between a given site occupied by a quadrupolar nucleus and the nearby spin-1/2 nuclei. The new sequence, called DANTE-S-REDOR, consists of a frequency-selective dephasing experiment where heteronuclear dipolar couplings are reintroduced by applying a symmetry-based sequence (S-REDOR). The selectivity is achieved by applying a pulse train, such as Delays Alternating with Nutations for Tailored Excitation (DANTE), to the quadrupolar nucleus. This new method allows quantitative analysis of proximities in the 3–4 Å range of protons in OH ligands and one of the 51V sites in a complex decavanadate cluster, namely Cs4[H2V10O28]·4H2O. The high selectivity of the DANTE-S-REDOR sequence offers the possibility to investigate a wide range of materials with different quadrupolar nuclei, including polyoxometalates, oxides, zeolites, and aluminophosphates.

Bismuth tungstate has attractive photocatalytic properties under visible light. A better understanding of the origin of that good activity should allow its control and its optimization. An improved photocatalytic activity to different pollutants was obtained using bismuth-based oxide obtained by microwave-assisted synthesis combined with the addition of sodium dodecylsulfate as a size tailoring agent. It is shown using electron paramagnetic resonance that bismuth tungstate is able to generate hydroxyl radicals in aqueous aerated solution, but no superoxide radical anions are formed. The catalytic efficiency relative to TiO2could be associated to differences in the number of excitons generated, to their lifetimes as holes and electrons in the semiconductor, and to valence and conduction band positions.

The aim of this study was to provide new insight into the evaluation of the effect of the crystallinity, size, and morphology of TiO2-anatase nanoparticles on their acid–base properties at the solid–liquid interface. This was achieved through monitoring the evolution in the surface charge density with the solution acidity and point of zero net proton charge (PZNPC) for a set of anatase nanoparticles with a mean size in the 5–20 nm range and various shapes. Different anatase nanoparticles were obtained by a sol–gel synthesis approach using different precursors, pH conditions, and various inorganic or organic additives. The measured PZNPC values were found to vary more than one pH unit depending on the degree of crystallinity and presence of differently exposed surfaces. To discriminate slight differences in the surface reactivity at the solid–water interface, high-resolution titration curves of surface charge for each anatase sample were recorded, and a fine analysis by the titration derivative isotherm summation (TDIS) method and proton affinity distributions (PADs) was performed. They provided accurate data on the strength (pKposition) of various local domains of proton adsorption with their relative surface contributions in relation with the particle morphology. The increase in the proportion of sites that are more present on the {101} faces or that of medium acid sites largely present on the {100} faces shifted the point of zero net proton charge to more acidic pH values for particles possessing a majority of such surfaces. In addition to these experiments, the relative acidity of the surface sites on the different surfaces of anatase was evaluated with the multisite complexation model (i.e., MUSIC model) applied to the theoretically optimized surfaces, and a comparison was drawn.

Various titanium dioxide nanoparticles were prepared by sol–gel method in order to obtain samples showing different sizes and morphologies. An original approach based on the adsorption of gaseous molecules from the gas phase was proposed to gain information about surface energy of nanosized TiO2anatase in terms of interfacial reactivity and heterogeneity. Argon, nitrogen, and ammonia were selected as such surface molecular probes. The mainly observed crystallographic faces of anatase particles were the {101} and {001} surfaces together with the {100} one. Their abundance was correlated with the energy distribution inferred from the local isotherms of argon adsorption in the low-pressure range. The acid character of the anatase surface was probed by nitrogen molecules, and, consequently, the location of polar sites on the particle surface could be determined in correlation with the argon adsorption domains. Moreover, the number and the strength of surface acid sites were evaluated with the aid of two-cycle adsorption of gaseous ammonia supplemented by appropriate flow microcalorimetry measurements. This molecular probe revealed significant differences among the samples depending on their crystal shape or face distribution.

Bismuth-based oxides have attractive photocatalytic properties under visible light. A better understanding of the origin of that good photocatalytic activity should allow its control and its optimization. In this Article, we have studied the impact of surface properties on photocatalytic activity for three bismuth-based oxides Bi2O3, BiVO4, and Bi2WO6. The surface acidity of particles was investigated by pyridine adsorption using infrared spectroscopy and the photocatalytic activity was investigated by the degradation of an aqueous solution of rhodamine B (RhB) and of stearic acid deposited on films under visible light. From the results obtained for the three bismuth-based oxides, we established a relation between surface acidity and photocatalytic mechanism of RhB degradation. The best photocatalytic efficiency for RhB and stearic acid degradation has been obtained with Bi2WO6that also exhibits the highest surface acidity. The most acid sites promote a strong interaction with the pollutant, implying a short distance between the pollutant and the photocatalyst. Consequently, the photogenerated electrons, holes, and radicals can reach more easily the pollutant, leading to an efficient degradation under visible light. The development of strong acid solids such as Bi2WO6emerged as promising materials for the degradation of pollutants. Because the good photocatalytic properties are related to the high acidity of Bi2WO6, we are interested in its origin using the multi site complexation model (MUSIC model). We found high acidity sites that are located on the lateral faces of the Bi2WO6platelets (crystallographic planes (101), (101̅), (100), and (001)).

The development of original nanoparticle size and shape requires a detailed knowledge and rationalization of the formation mechanism. A consolidated mechanism of γ-AlOOH boehmite nanorod formation in aqueous hydrothermal conditions is proposed based on an advanced characterization of the final product, kinetic studies, and surface energy calculations. Boehmite nanorods with a width of about 10 nm and more than 100 nm in length are obtained in slightly acidic conditions from Al(III) molecular precursors at temperature higher than 150 °C. The identification of protoboehmite as an intermediate in the kinetic study combined with the low solubility of aluminum in reacting conditions and the presence of stacking defects in the rods is indicative of an aggregation mechanism. Electron diffraction studies show that the crystallites are oriented in the nanorods with a [001] preferential growth direction. Using DFT calculations, we demonstrated that the nanorod morphology is not the expected thermodynamically stable one in these experimental conditions. An explanation of the oriented aggregation of primary particles is proposed based on an eased depletion of the solvation water molecules of the (001) surface compared to the other surfaces.