Visible active silver sensitized vanadium titanium mixed metal oxide photocatalyst nanoparticles through sol–gel technique

Abstract

Silver sensitized titanium vanadium mixed metal (Ag/TiV) oxides were prepared by nanoscale synthesis route employing the sol–gel technique. It led to the development of 5–20 nm particles with predominantly anatase phase. The physicochemical characterization of the particles was done by X-ray diffractrometry (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), UV–visible diffuse reflectance spectroscopy (UV–Vis DRS) and photoluminescence spectroscopy (PL). The reflectance spectrum shows a red shift in the optical response of the catalyst with its band gap absorption upper limit covering a large portion of the visible spectrum, i.e. λ_abs≥700 nm. The performance of the materials was examined under laboratory visible light and solar radiation exposure. The rate of degradation of methylene blue (MB) and phenol exhibited an increase of about six and four times, respectively, in visible light compared to Degussa P-25. This may be attributed to the increased absorption due to Ti–V mixed metal oxides, favorable electron transfer in the anatase–rutile mixed phase coupled with silver’s scavenging action and reduced electron–hole recombination thereon.

Introduction

TiO₂ photocatalysts show a great promise in environmental applications such as photocatalytic degradation of organic compounds and catalytic reduction of NO_X. In energy applications they are used for solar energy conversion in photoelectrochemical cells [1], [2]. Titania has two important crystalline phases namely anatase and rutile. The pure anatase and the mixed phases (anatase+rutile) have been reported to show photoactivity [3], [4]. However pure anatase shows activity under UV radiation only while mixed phase shows some activity in visible region as well [3]. Anatase TiO₂ has a band gap of 3.2 eV, which sets up an upper limit of its absorption wavelengths at 376 nm. This limit impairs its large scale utilization for solar energy applications because over 90% of terrestrial solar flux has λ>376 nm. Consequently the quantum efficiency of TiO₂ catalysts in solar energy utilization application is low. It is desirable to have a red shift in its response spectrum, which may be achieved by doping (or co-doping) of TiO₂ with oxides of metal elements such as V, Fe, W, Cr, Mo, Nb, Zn, Sn, Zr or non metallic elements such as B, N, S and C [5], [6]. Reducing the optical band gap may enhance the visible light absorption but at the same time it may enhance the electron–hole recombination [5]. This calls for development of photocatalyst, which shows visible light response as well as the decreased charge–carriers’ recombination. The charge–carrier recombination may be reduced by separation of electrons and holes using noble metal electron scavengers like silver [7], [8], [9], [10] and platinum [11].

Recently V-doped TiO₂ has been reported by Kubacka et al. [6] to show higher absorption in the red-shifted spectrum compared to the oxides of N, W and Mo. Pentavalent V-oxides are expected to make a net contribution in photo-generation of holes because V has high charge-to-volume ratio compared to Ti. Also high relative polarizability of V makes transfer of photogenerated electrons easier as well as the scavenging of electrons more probable, thereby increasing the average lifetime of holes. However the low solubility of V in anatase structure (about 5 at%) limits its performance potential. Also a red shift in the absorption spectrum does not ensure a proportional increase in the photocatalytic activity [5], due to carrier recombination.

In the present work silver has been used as an electron scavenger to decrease the charge carrier recombination. It is expected to show higher photoactivity compared to V doped TiO₂. Sol–gel process is considered to be one of the simplest and versatile methods for nanoscale synthesis. It offers a great advantage of incorporation of dopant material at low temperature, while the supporting material is in sol phase, thus giving sufficient temporal and spatial flexibility for interatomic interactions. A modified sol–gel technique has been employed in the present work to prepare Ag deposited TiV mixed metal oxide nanoparticles. The performance of the prepared catalyst was studied by investigating its activity in the degradation of methylene blue (MB) and phenol.