Facile Synthesis of Nb2O5 as a Visible Light-Sensitive Photocatalyst

Abstract

Niobium pentoxide (Nb2O5) is a promising semiconductor in catalysis due to its chemical and physical properties, availability, and non-toxicity. However, because of its large bandgap energy (Eg= 3.1 – 4.0 eV), Nb2O5 does not work as a photocatalyst under visible light, which makes its use of solar light limited. Even though Nb2O5 presents this drawback, its photocatalytic activity under visible light radiation can be enhanced due to the presence of surface groups, known as niobium peroxo complexes, formed by the treatment with hydrogen peroxide. In general, the modified niobia is synthetized in multi-step processes which allow the generation of strongly reactive oxygen species (ROS) (i.e. peroxo and superoxo) on its surface. In contrast, in this study, the peroxo-niobium oxide was prepared by a facile combination of the oxidant peroxide method (OPM) and hydrothermal treatment. The ability of Nb2O5 to form the ROS using this method was investigated in respect to the different Nb:H2O2 molar ratio (1:2 or 1:10) added in the metal oxide synthesis. For this purpose, ammonium niobium oxalate was dissolved in distilled water and hydrogen peroxide was then added to the reaction medium. The niobium precursor in contact with H2O2 forms instantaneously the niobium peroxo complex that was crystallized at 120 ⁰C with a heating ramp of 1 oC/min, for 12h in a home-made teflon lined autoclave. When the desired temperature was reached, the internal pressure of the reactor varied from ca. 10 to 27 atm, depending upon the amount of hydrogen peroxide used to prepare each material. The oxidative properties of the reactive oxygen species formed on the surface of metal oxides were tested in photodegradation of caffeic acid (CA) under visible light. In addition, reuse tests were performed in order to investigate the loss of photoactivity of the best photocatalyst in the CA degradation. XRD patterns revealed that the crystallographic structures of the samples Nb2H and Nb10H contain the Nb2O5 pseudohexagonal-type phase (JCPDS 28-317) and hydrated niobium oxide. Although no changes in the crystalline structure of both materials were observed, the presence of peroxo and superoxo species in the photocatalysts was confirmed by XPS and FTIR. The composites Nb2H and Nb10H presented band gap energies of 3.20 and 3.16 eV, respectively. However, Nb10H sample absorbs visible light at more than 400 nm, suggesting that the synthesis of the Nb2O5 with excess of H2O2 shifts the absorption edge of this material to the visible range, as well as reduces the energy required to the photoactivation of the catalyst. It was found that the photocatalytic activity increased with increasing oxidant concentration in the synthesis medium due to formation of a higher amount of oxygen reactive species on the surface of niobia. The peroxo-niobium groups on the Nb2O5 surface allowed the photocatalysts to be activated by visible light, and the superoxo groups formed can be also be responsible to improve its photocatalytic efficiency. The reuse tests of the Nb10H oxide indicated that the CA removal capacity remained practically constant even after four consecutive cycles. Therefore, the possibility of using solar irradiation to supply energy to active the photocatalyst make this material promising for use in low-cost photocatalytic systems for treatment of wastewater.