Abstract:
The continuous burning of fossil fuels has increased the amount of carbon dioxide (CO2) in the atmosphere, leading to global warming. Photocatalytic reduction of CO2 to fuels using titanium (IV) oxide nanoparticles (TiO2-NPs) has been employed to reduce atmospheric CO2. However, the existing synthetic routes such as sol-gel, microwave, ultrasonic irradiation and spray pyrolysis for TiO2-NPs were cumbersome. Also nanoparticles produced have some limitations such as low surface area, large band gap, high electron-hole recombination and low photocatalytic efficiency. Hence, the aim of this work was to design simple and energy-saving approaches for the synthesis of improved TiO2-NPs for photocatalytic reduction of CO2.
Two kinds of TiO2-NPs were synthesised separately by Sonothermal (S) and Sonothermal-Hydrothermal (SH) methods to produce TiO2-S and TiO2-SH, respectively. The TiO2-S nanoparticles were doped with varying amount of magnesium to obtain Mg-TiO2-S. The TiO2-SH nanoparticles were modified with Reduced Graphene Oxide (RGO) and Carbon Nanotubes (CNTs) to obtain RGO-TiO2-SH and CNT-TiO2-SH nanocomposites, respectively. These nanocatalysts were characterised using X-ray diffraction, transmission electron microscopy, X-ray Photoelectron Spectroscopy (XPS), UV-Visible spectroscopy and surface area analysis. Density Functional Theory (DFT) calculations were carried out using Vienna Ab-initio Simulation Package to establish the electronic and the structural properties of the reactant molecules deposited on modelled TiO2-NPs surfaces. Photocatalytic reduction of CO2 to methanol was performed in acetonitrile-H2O (9:1, v/v) mixture under ultraviolet and visible light. Data obtained were analysed using descriptive statistics.
The TiO2-S and TiO2-SH showed 13.7% and 8.4% rutile phase at phase transition temperature of 450°C, an indication that TiO2-SH had more anatase phase and higher crystallinity. The crystal size in nm of the predominant anatase phase for TiO2-S, TiO2-SH, Mg-TiO2-S, RGO-TiO2-SH and CNT-TiO2-SH were 15.4, 18.1, 15.5, 14.9 and 13.4, respectively. All the nanoparticles were homogenous in nature with the TiO2-NPs attached to either RGO or CNTs. The XPS revealed Ti2p, O1s, Mg2p, and C1s as the chemical states of the elements present in all the prepared nanoparticles. The calculated band gaps in eV were 3.1, 3.0, 3.1, 2.9 and 2.9 for TiO2-S, TiO2-SH, Mg-TiO2-S, RGO-TiO2-SH and CNT-TiO2-SH, respectively. Their corresponding surface areas were 64.5, 73.1, 121.6, 128.6 and 117.1 m2g-1, respectively. These indicated the reduction in the band gap and increase in the surface area of nanoparticles when compared with commercial TiO2-NPs of 3.2 eV and 57.4 m2g-1. The DFT calculations revealed that the anatase phase of TiO2-NPs had higher adsorption energy of -0.49 eV for the reactant molecules than the rutile phase of -0.30 eV. Under ultraviolet light, the methanol production rates from the photocatalytic reduction of CO2 were 1.9, 2.0 and 5.9 mmolg-1h-1, using TiO2-S, TiO2-SH and Mg-TiO2-S, respectively; while 2.3 and 1.5 mmolg-1h-1 were obtained using RGO-TiO2-SH and CNT-TiO2-SH, respectively, under visible light.
The new titanium (IV) oxide nanoparticles and their modified forms were of increased surface area, reduced band gap and lowered electron-hole recombination; thus making the synthetic routes viable and effective.
