Dr. Valdes-Morales, Hector

2019, Drozd, Valeriya S., Zybina, Nadezhda A., Abramova, Kristina E., Parfenov, Mikhail Yu, Kumar, Umesh, Valdes-Morales, Hector, Smirniotis, Panagiotis G., Vorontsov, Alexander V.

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.

2020, Dr. Valdes-Morales, Hector, Olowoyo, Joshua, Saini, Upasana, Kumar, Manoj, Singh, Hitendra, Omorogie, Martins, Babalola, Jonathan, Vorontsov, Alexander, Kumar, Umesh, Smirniotis, Panagiotis

The development of visible-light active photocatalysts is highly desirable for CO2 reduction to hydrocarbons and alcohols using sunlight. Here, we report the metal-organic frameworks (MOF) of amino-benzene dicarboxylate with titanium oxocluster center (NH2-MIL-125(Ti)) and modified with reduced graphene oxide (RGO), RGO-NH2-MIL-125(Ti), ideal for the visible-light-driven photocatalytic reduction of CO2 to hydrocarbons and methanol. The catalyst provides high quantum efficiency and selectivity for methanol. The cluster model and self-consistent charge density functional tight binding methods were used to investigate the photogenerated charge separation for NH2-MIL-125(Ti). The quantum modelling suggests that holes were accumulated in the central ring Ti8O8(OH)4, where strongly adsorbed electron donor, triethanolamine, undergoes photooxidation while electrons were located in the organic ligand of MOF including the NH2 group. The binding affinity of NH2 reaction sites to CO2 possibly work to improve the photocatalytic reduction of CO2 to methanol. The RGO also play an important role for charge separation and better photocatalytic reduction with RGO-NH2-MIL-125(Ti).

2019, Olowoyo, Joshua O., Kumar, Manoj, Singh, Bhupender, Oninla, Vincent O., Babalola, Jonathan O., Valdes-Morales, Hector, Vorontsov, Alexander V., Kumar, Umesh

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.