Dr. Valdes-Morales, Hector

Anatase nanoparticles containing surface oxygen vacancies (VO) and Ti3+ are of great importance for applications in photocatalysis, batteries, catalysis, sensors among other uses. The properties of VO and their dependence on the size of nanoparticles are of great research interest and could allow obtaining advanced functional materials. In this work, a complete set of oxygen vacancies in an anatase nanoparticle of size 1.1 nm was investigated and compared to those of a twice larger nanoparticle, having the same shape and surface hydroxylation pattern. It turned out that the decrease in the size of the anatase nanoparticle strongly facilitated creation of surface oxygen vacancies and Ti3+. After their creation, oxygen vacancies undergo three transformation paths — (1) small repulsion of surrounding Ti cations with retention of the vacancy, (2) transfer of oxygen anion, leading to the movement of oxygen vacancy to a more stable position, and (3) collapse of oxygen vacancy accompanied by structure deformation towards Magneli-like phase.

This study describes theoretical and experimental considerations to optimize the photocatalytic degradation of caffeic acid in water using 3D-BiOBr based materials under visible light irradiation. Three BiOBr materials were synthesized through the solvothermal method using different bromide sources, namely potassium bromide (KBr) and the ionic liquid (IL) 1-butyl-3-methylimidazolium bromide. Morphological and chemical changes were observed in IL based 3D-BiOBr materials. The theoretical optimization of the experimental conditions in heterogeneous photocatalysis tests (pH and dose of catalyst) were simulated using the MODDE 12.0.1 software. A central composite design (CCD) was applied to obtain a response surface to elucidate the optimal conditions. This model predicted that the maximum photocatalytic degradation can be achieved at pH of 6.7 and a photocatalyst dose of 344 mg L−1. The optimal experimental conditions were tested using the three synthesized 3D-BiOBr materials. The results showed that the highest degradation efficiency and mineralization yield were obtained using the BiOBr microspheres synthesized with the IL at 145 °C.

Anatase (001)nanosheets have recently attracted great attention as very active catalysts and photocatalysts. These graphene analogs have very high surface area and unique surface properties. In the present paper, very thin two-layer anatase nanosheets are investigated computationally in the form of quantum dots of various size. Quantum size effect (QSE)was clearly observed for nanosheets with fully hydroxylated edges and size up to 14 nm and the ultimate band gap is around 3.4 eV. Dehydroxylation of nanosheets obscured QSE, decreased band gap and induced visible light absorption. Therefore, contradictory trends reported in experimental studies for anatase QSE can be ascribed to different degree of hydroxylation of the TiO 2 samples surface. All anatase nanosheet quantum dots retained their flat graphene-like shape. These findings demonstrate that dehydroxylated anatase nanosheet quantum dots are prospective visible-light active photocatalysts even if their inherent band gap is considerably larger than for bulk anatase.

In the last decades, air pollution control has received much attention due to the increase of environmental and health problems. The design of new materials with potential applications in air pollution control systems is a challenge nowadays. In this work, BiOI nanostructured materials were synthesized and used for photocatalytic oxidation of nitric oxide (NO). A microwave-assisted solvothermal method was successfully applied for BiOI synthesis at 126 °C, using ethylene glycol (EG) as a solvent. Several samples were prepared by varying the microwave irradiation time between 5 and 120 min. Resulting materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), N2 adsorption-desorption isotherms, diffuse reflectance spectroscopy (DRS) and photoluminescence measurements (PL). The photocatalytic activity of BiOI samples was evaluated in the photo-oxidation reaction of nitric oxide (NO) in gas phase under visible light irradiation. BiOI sample synthesized after 15 min of microwave exposition shows the highest photocatalytic activity, even greater than that obtained when TiO2 Evonik P-25 is used. This nanostructured material was applied into the formulation of two types of materials (ceramic paint and stucco) for its potential use in the construction industry. Preliminary results show that the application of nanostructured BiOI into stucco formulation has a great potential to develop commercial products to remove NO from air.

A facile combined method, namely sonothermal-hydrothermal, was adopted to assemble titanium dioxide (TiO2) nanoparticles on the surface of reduced graphene oxide (RGO) to form nanocomposites. Characterization techniques confirm that RGO-TiO2 composite is well constituted. Enhanced photocatalytic CO2 reduction to methanol by the composites under UVA and visible irradiation suggests the modification in the band gap of the composite and promotion of the separation of photogenerated carriers, yielding methanol production rate of 2.33 mmol g−1 h−1. Theoretical investigation demonstrated that combining RGO with TiO2 resulted in an upward shift of TiO2 bands by 0.2 V due to the contribution of RGO electrons. Relatively strong adsorption of RGO over the (101) anatase surface with the binding energy of approximately 0.4 kcal mol−1 per carbon atom was observed. Consideration of orbitals of TiO2, RGO and RGO-TiO2 composite led to a conclusion that UVA photoreaction proceeds via the traditional mechanism of photogenerated electron transfer to RGO while visible light CO2 reduction proceeds as a result of charge transfer photoexcitation that directly produces electrons in RGO and holes in TiO2. Superior photocatalytic activity of RGO-TiO2 composite in the present study is attributed to the formation of tight contact between its constituents, which is required for efficient electron and charge transfer.