New insights for titanium(iv) speciation in acidic media based on UV-visible and 31P NMR spectroscopies and molecular modeling

Titanium chemistry in aqueous acidic media has been extensively investigated over the last decades. Hydrolyzed species such as Ti(OH)³⁺, TiO²⁺, Ti(OH)₂²⁺ or Ti(OH)₃⁺ have been identified and their equilibria have been studied in nitric and perchloric acid. A predominance of the divalent cations was found for low pH (i.e., pH <2). Nonetheless, recent literature reports the existence of small titanium oxo-clusters in aqueous acidic media for large titanium(iv) concentration (typically., >0.1 mol L⁻¹), as stable precursors for the formation of condensed titanium dioxide. The present paper reconsiders firstly previous knowledge about the speciation of titanium(iv) in non-complexing acidic media by giving evidence for the presence of polynuclear hydrolyzed species, even at very low Ti(iv) concentration (i.e., typically <0.1 mmol L⁻¹). UV-visible absorbance spectra recorded for diluted nitric acid solutions (a model of non-complexing acidic medium) containing titanium(iv) were compared to time-dependent density functional theory (TD-DFT) predicted excitation energies. Experimental and predicted maximal absorbance wavelengths showed significantly improved matches when polynuclear species were considered in TD-DFT calculation. Then, 0.1–12.7 mol L⁻¹ phosphoric acid solutions containing titanium(iv) were studied by means of spectroscopic techniques (UV-visible, NMR) in order to identify qualitatively the presence of titanium(iv) complexes and to link this speciation to the acid concentration. Two different titanium(iv) orthophosphate complexes, potentially polynuclear, were detected, and the presence of free titanium(iv) is also expected for low phosphoric acid concentration (i.e., <0.1 mol L⁻¹). A general complexation scheme for a large range of H₃PO₄ concentration was thus formulated.

A spectroscopic study of titanium(iv) speciation in diluted nitric acid (model of non-complexing medium) and 0.1–12.7 mol L⁻¹ phosphoric acid aqueous solutions. Evidence for the presence of polynuclear species is supported by molecular modeling.

Tuning the shape and crystal phase of TiO2 nanoparticles for catalysis

Synthesis of TiO₂ nanoparticles with tunable shape and crystal phase has attracted considerable attention for the design of highly efficient heterogeneous catalysts. Tailoring the shape of TiO₂, in the crystal phases of anatase, rutile, brookite and TiO₂(B), allows tuning of the atomic configurations on the dominantly exposed facets for maximizing the active sites and regulating the reaction route towards a specific channel for achieving high selectivity. Moreover, the shape and crystal phase of TiO₂ nanoparticles alter their interactions with metal species, which are commonly termed as strong metal-support interactions involving interfacial strain and charge transfer. On the other hand, metal particles, clusters and single atoms interact differently with TiO₂, because of the variation of the electronic structure, while the surface of TiO₂ determines the interfacial bonding via a geometric effect. The dynamic behavior of the metal-titania interfaces, driven by the chemisorption of the reactive molecules at elevated temperatures, also plays a decisive role in elaborating the structure-reactivity relationship.

Elucidation of the Crystal Growth Characteristics of SnO2 Nanoaggregates Formed by Sequential Low-Temperature Sol-Gel Reaction and Freeze Drying

SnO2 nanoparticles are regarded as attractive, functional materials because of their versatile applications. SnO2 nanoaggregates with single-nanometer-scale lumpy surfaces provide opportunities to enhance hetero-material interfacial areas, leading to the performance improvement of materials and devices. For the first time, we demonstrate that SnO2 nanoaggregates with oxygen vacancies can be produced by a simple, low-temperature sol-gel approach combined with freeze-drying. We characterize the initiation of the low-temperature crystal growth of the obtained SnO2 nanoaggregates using high-resolution transmission electron microscopy (HRTEM). The results indicate that Sn (II) hydroxide precursors are converted into submicrometer-scale nanoaggregates consisting of uniform SnO2 spherical nanocrystals (2~5 nm in size). As the sol-gel reaction time increases, further crystallization is observed through the neighboring particles in a confined part of the aggregates, while the specific surface areas of the SnO2 samples increase concomitantly. In addition, X-ray photoelectron spectroscopy (XPS) measurements suggest that Sn (II) ions exist in the SnO2 samples when the reactions are stopped after a short time or when a relatively high concentration of Sn (II) is involved in the corresponding sol-gel reactions. Understanding this low-temperature growth of 3D SnO2 will provide new avenues for developing and producing high-performance, photofunctional nanomaterials via a cost-effective and scalable method.

Forced Convection Nanofluid Heat Transfer as a Function of Distance in Microchannels

As electronic devices become smaller and more powerful, the demand for micro-scale thermal management becomes necessary in achieving a more compact design. One way to do that is enhancing the forced convection heat transfer by adding nanoparticles into the base liquid. In this study, the nanofluid forced convection heat transfer coefficient was measured inside stainless-steel microchannels (ID = 210 μm) and heat transfer coefficient as a function of distance was measured to explore the effects of base liquid, crystal phase, nanoparticle material, and size on heat transfer coefficient. It was found that crystal phase, characteristics of nanoparticles, the thermal conductivity and viscosity of nanofluid can play a significant role on heat transfer coefficient. In addition, the effects of man-made and commercial TiO₂ on heat transfer coefficient were investigated and it was found that man-made anatase TiO₂ nanoparticles were more effective to enhance the heat transfer coefficient, for given conditions. This study also conducted a brief literature review on nanofluid forced convection heat transfer to investigate how nanofluid heat transfer coefficient as a function of distance would be affected by effective parameters such as base liquid, flow regime, concentration, and the characteristics of nanoparticles (material and size).