Investigating the in-Situ Doping Effect of Niobium Pentoxide Nanostructures on Their Electronic Surface Properties

Abstract

The release of large quantities of effluents to the environment as wastewater affects the quality of drinking water and it is a serious concern for the environment and for the public health. Often wastewater contains substances that are toxic, carcinogenic and resistant to biodegradation [1]. Thereby, the stringent regulations imposed by environmental and governmental agencies to improve the quality of water, require more efficient wastewater treatment techniques. Advanced oxidative processes (AOPs) consist of non-selective and efficient oxidative degradation of organic pollutants using chemical species with high oxidative power such as hydroxyl radicals [1]. In one of the AOP methods, light interacts with semiconductors generating excitons by the transfer of electrons from the valence band to the conduction one. The excitons react with oxygen and water forming hydroxyl radicals, which degrade organic molecules. Actually, photodegradation is an effective-cost method to treat industrial effluents [1]. However, this technology suffers from some challenges such as insufficient utilization of visible-light. Therefore, advances in photodegradation processes require the development of new semiconductors, the evaluation of their electronic and surface properties, to enhance the efficiency of hydroxyl radicals’ generation in presence of UV light and broad spectrum light source. [2][3]. Different metal oxides, were exploited for such a purpose like TiO2, SnO2, ZnO2, Fe3O2, Al2O3, showing different performances in terms of degradation rates [1]. Niobium pentoxide has been investigated for different applications such as humidity, gas and chemical sensing, electrochromic devices and catalysis due to its thermodynamic stability, high corrosion resistance, biocompatibility and Bronsted acidic properties [2]. At the same time, is one of the least studied metal oxides, which makes it attractive for fundamental studies. Nb2O5 is a wide band-gap semiconductor, with a band gap of 3.4 - 4 eV depending on its morphological features, meaning that it can be used as a photocatalyst only when activated with ultraviolet irradiation [4][5][6]. In this work, we investigate the impact of the nature and content of the doping element on the electronic surface properties of Nb2O5 nanostructures aiming at widen its photon absorption in the electromagnetic spectrum.The Nb2O5 nanoparticles were synthesized by hydrothermal synthesis at 150°C, for 2h, by mixing ammonium niobium oxalate with the different percentages of dopant precursors: N, Cu, and Ta. The structural and morphological characterization of the samples was carried out by X-ray diffraction (XRD) and Scanning electronic microscopy (SEM). The optical properties were investigated by UV-Visible-NIR diffuse-reflectance spectroscopy (DRS) to define the effective optical band gap, Ultraviolet photoelectron spectroscopy (UPS) to calculate the work function of the materials and by X-ray photoelectron spectroscopy (XPS) to determine the valence band edge, leading to the complete construction of the energy diagram of the different synthesized nanoparticles. The successful doping of Nb2O5 nanostructures with 1 wt% of Ta, Cu or N lead to a narrowing of the optical band gap as illustrated in Figure 1. The reduction of the optical band gap of Nb2O5 is at least of 0.5 EV through the doping with tantalum but around 1 eV with nitrogen. The correlation between the electronic surface properties and the structure of the materials will be discussed.AcknowledgementsThe authors would like to thank the funding from NSERC (Strategic partnership program, Canada) and FAPESP (BEPE 2018/17279-1, Brazil).