Nanocrystalline Brookite with Enhanced Stability and Photocatalytic Activity: Influence of Lanthanum(III) Doping

Metastable TiO₂ polymorphs are more promising materials than rutile for specific applications such as photocatalysis or catalysis support. This was clearly demonstrated for the anatase phase but still under consideration for brookite, which is difficult to obtain as pure phase. Moreover, the surface doping of anatase with lanthanum ions is known to both increase the thermal stability of the metastable phase and improve its photocatalytic activity. In this study, TiO₂ nanoparticles of almost only the brookite structure were prepared by a simple sol–gel procedure in aqueous solution. The nanoparticles were then doped with lanthanum­(III) ions. The thermal stability of the nanoparticles was analyzed by X-ray diffraction and kinetic models were successfully applied to quantify phases evolutions. The presence of surface-sorbed lanthanum­(III) ions increased the phase stability of at least 200 °C and this temperature shift was attributed to the selective phase stabilization of metastable TiO₂ polymorphs. Moreover, the combination of the surface doping ions and the thermal treatment induces the vanishing of the secondary anatase phase, and the photocatalytic tests on the doped brookite nanoparticles demonstrated that the doping increased photocatalytic activity and that the extent depended on the duration of the sintering treatment

Quantitative Analysis of the Proximities of OH Ligands and Vanadium Sites in a Polyoxovanadate Cluster Using Frequency-Selective ¹H–⁵¹V Solid-State NMR Spectroscopy

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

Nanocrystalline Brookite with Enhanced Stability and Photocatalytic Activity: Influence of Lanthanum(III) Doping

Metastable TiO₂ polymorphs are more promising materials than rutile for specific applications such as photocatalysis or catalysis support. This was clearly demonstrated for the anatase phase but still under consideration for brookite, which is difficult to obtain as pure phase. Moreover, the surface doping of anatase with lanthanum ions is known to both increase the thermal stability of the metastable phase and improve its photocatalytic activity. In this study, TiO₂ nanoparticles of almost only the brookite structure were prepared by a simple sol–gel procedure in aqueous solution. The nanoparticles were then doped with lanthanum­(III) ions. The thermal stability of the nanoparticles was analyzed by X-ray diffraction and kinetic models were successfully applied to quantify phases evolutions. The presence of surface-sorbed lanthanum­(III) ions increased the phase stability of at least 200 °C and this temperature shift was attributed to the selective phase stabilization of metastable TiO₂ polymorphs. Moreover, the combination of the surface doping ions and the thermal treatment induces the vanishing of the secondary anatase phase, and the photocatalytic tests on the doped brookite nanoparticles demonstrated that the doping increased photocatalytic activity and that the extent depended on the duration of the sintering treatment

Heteroaggregation and Selective Deposition for the Fine Design of Nanoarchitectured Bifunctional Catalysts: Application to Hydroisomerization

We successfully prepared bifunctional catalysts with the distance between metallic and acid sites tuned at the nanometer scale. Sols of β-zeolite nanoparticles were synthesized and mixed in optimized conditions with a γ-AlOOH boehmite suspension to yield alumina/zeolite aggregates with a nanometer scale intimacy. The composition of the aggregate could be tuned from pure alumina to pure zeolite. Then, by carefully choosing the Pt precursor and the pH conditions, we were able to selectively deposit platinum, either on alumina or in zeolite domains. A subsequent, soft thermo-reduction step was applied that produced well-dispersed Pt nanoparticles either on alumina or in the zeolite nanodomains as confirmed by 3D tomography microscopy experiments. The catalytic properties of the obtained nanostructured catalysts were studied through <i>n</i>-heptane conversion. Comparison of these original bifunctional catalysts with monofunctional or conventional bifunctional catalysts showed the impact of the location of the metallic particles on the selectivity

The Challenge of Studying TiO₂ Nanoparticle Bioaccumulation at Environmental Concentrations: Crucial Use of a Stable Isotope Tracer

The ecotoxicity of nanoparticles (NPs) is a growing area of research with many challenges ahead. To be relevant, laboratory experiments must be performed with well-controlled and environmentally realistic (i.e., low) exposure doses. Moreover, when focusing on the intensively manufactured titanium dioxide (TiO₂) NPs, sample preparations and chemical analysis are critical steps to meaningfully assay NP’s bioaccumulation. To deal with these imperatives, we synthesized for the first time TiO₂ NPs labeled with the stable isotope ⁴⁷Ti. Thanks to the ⁴⁷Ti labeling, we could detect the bioaccumulation of NPs in zebra mussels (D<i>reissena polymorpha</i>) exposed for 1 h at environmental concentrations via water (7–120 μg/L of ⁴⁷TiO₂ NPs) and via their food (4–830 μg/L of ⁴⁷TiO₂ NPs mixed with 1 × 10⁶ cells/mL of cyanobacteria) despite the high natural Ti background, which varied in individual mussels. The assimilation efficiency (AE) of TiO₂ NPs by mussels from their diet was very low (AE = 3.0 ± 2.7%) suggesting that NPs are mainly captured in mussel gut, with little penetration in their internal organs. Thus, our methodology is particularly relevant in predicting NP’s bioaccumulation and investigating the factors influencing their toxicokinetics in conditions mimicking real environments

New Insights Into BiVO₄ Properties as Visible Light Photocatalyst

Bismuth vanadate has attractive photocatalytic properties under visible light. The influence of structure and morphology of BiVO₄ nanomaterials 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 BiVO₄ to 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

Influence of Morphology and Crystallinity on Surface Reactivity of Nanosized Anatase TiO₂ Studied by Adsorption Techniques. 1. The Use of Gaseous Molecular Probes

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 TiO₂ anatase 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