Effects of Vacuum Annealing on the Photocatalytic Activity of TiO₂ Nanoparticles Synthesized by the Microwave-Assisted Hydrothermal Method
Mejia Caceres, Lourdes Cristel1 ; Vilca Huayhua, Carlos Adolfo 1; Marcos Da silva, Wellington 2 3; Santina Mohallem, Nelcy Della 4; Almeida Silva, Alysson Martins 5; Da Silva, Sebastiao William6; Huamani Coaquira, Jose Antonio1,7
1 Laboratório de Síntese de Nanomateriais e Caracterização Magnética-LSNCM, Institute of Physics, University of Brasilia, Brasília DF 70910900, Brazil; 2 Graduate Program in Materials science (PPGCIMA), Faculty UnB Planaltina (FUP), University of Brasilia, Brasilia, DF 70910 900, Brazil. 3 International Center of Physics, University of Brasilia, Brasilia DF 70910-900, Brazil. 4 Department of Chemistry, Institute of Exact Sciences, Federal University of Minas Gerais, MG - 31270-901 Belo Horizonte-MG, Brazil. 5 Mechanical Engineering Department, Faculty of Technology, Campus Universitario Darcy Ribeiro, University of Brasilia, Brasilia, DF 70.910-900, Brazil. 6 Laboratory of Optical Spectroscopy, Institute of Physics, University of Brasilia, Brasilia DF 70910-900, Brazil. 7 Facultad de Ciencias, Universidad Nacional de Ingeniería, Av. Túpac Amaru 210, Lima 15333, Peru
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Abstract
The TiO2 system is currently of great technological interest due to its optical and catalytic properties mainly in nanometer scale. In this work we present the study of TiO₂ nanoparticles synthesized by the microwave-assisted hydrothermal method. In order to improve their properties, the as-synthesized NPs were subjected to thermal annealing at 400 °C in air and subsequently under vacuum condition (10⁻² mbar). The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and UV-Vis spectroscopy. XRD analyses confirmed the formation of a phase composition consisting of 87% anatase and 13% brookite, and this result was corroborated by Raman spectroscopy measurements. The air-annealed sample exhibited a crystallite size of approximately 4 nm and it increases to 7 nm after vacuum annealing, in agreement with that observed from TEM image data analysis. XPS results demonstrated that vacuum treatment promoted the formation of oxygen vacancies and the partial reduction of Ti⁴⁺ to Ti³⁺. In addition, the UV-Vis spectra indicated the emergence of intermediate electronic states, leading to an apparent band gap reduction. Tests of photocatalytic activity were carried out subsequently using 3ppm of Rhodamine B dissolved in water. The results indicate that the vacuum-annealed sample achieved a degradation efficiency of 99.6% within 100 min, maintaining a high performance after successive reuse cycles. This improvement is attributed to the higher concentration of structural defects (such as oxygen vacancies, Ti3+ ions). Those defects promote the generation of reactive species, such as •OH and O₂•⁻, which responsible for the photodegradation of the dye. Furthermore, kinetic analysis demonstrated that the vacuum-annealed sample exhibits superior performance in comparison to TiO2 materials reported in the literature.
Keywords: TiO₂ nanoparticles; vacuum annealing; photocatalysis.