Preparation of Carbon Coated FeWO4 and its Photocatalytic Activity Under Visible Light

Preparation of carbon coated FeWO4 was carried out by heat treatment of WO2.9 with iron acetate at 400 o C, followed by carbonization with poly(vinyl alcohol) (PVA) as carbon precursor at 800 o C in N2 atmosphere. The powders mainly consist of FeWO4. The photocatalytic activities of their samples were confirmed by phenol photodecomposition in its aqueous solution. Carbon coated FeWO4 powders revealed photocatalytic activity under visible light. However, the photocatalytic activity of FeWO4 without carbon coating could not be detected. Carbon coating to the FeWO4 derived the photocatalytic activity under visible light irradiation. An addition of 10% PVA to the carbon coating derived the highest photocatalytic activity to decompose the phenol. TiO2 semiconductor photocatalyst has been applied in the mineralization of undesirable chemical pollutants in water and air [1, 2]. However, a large band gap (3.2 eV) having TiO2 corresponds to the absorbance in the ultra-violet (UV) irradiation. This means that it is photocatalytic activating only under the UV. Improvement of the efficiency of photocatalytic activity becomes an important matter. Improvement of it has been examined by anion doping, cation doping and coupling with other semiconductors [3-7]. For utilizing solar light efficiently, the development of the photocatalyst which works under the visible light is a key subject. It was also reported that several compounds which showed semiconductor characteristics such as TaO4, TaON, and BiNO4 revealed the photocatalytic activity under visible light [8-10]. On the other hand, formation of reduced phases of TiO2 which appear in TinO2n-1 were observed through carbon coating to the TiO2 at high temperature, and they revealed photocatalytic activity under visible light irradiation [11]. The reduced phase of tungsten oxide (W18O49) formed by the process of carbon coating also indicated the photocatalytic activity under the visible light irradiation [12, 13]. The thin carbon layer coated on the surface of W18O49 particles derived the adsorptivity and photocatalytic activity under visible light irradiation. Its photocatalytic activity of carbon coated W18O49 has been improved through iron doping [14]. Carbon coated W18O49 doped iron exhibited higher photocatalytic activity than carbon coated W18O49 without iron. In the formation of carbon coated W18O49 with iron, peaks of iron tungstate (FeWO4) were observed on the XRD patterns of samples prepared. An increase of the addition of iron acetate derived the increasing of deposition of FeWO4. The deposition of FeWO4 is strongly related to the formation of solid solution between iron and W18O49. Some of metal tungstates (MWO4) such as ZnWO4 and PbWO4 have been reported as UV active photocatalyst [15]. Moreover Bi2WO6 is photoactive under visible light [16]. *Address correspondence to this author at the Faculty of Engineering, Oita University, Dannoharu 700, Oita 870-1192, Japan; Tel/Fax: 0081975547904; E-mail: [email protected] FeWO4 might be worked as a photocatalyst similar to aforementioned metal tungstates. In this study, the preparation of carbon coated FeWO4 and its photocatalytic activity under visible light will be discussed. The preparation of FeWO4 was carried out by using heat treatment of WO2.9 and iron (II) acetate. They were mixed in 1:1 mass ratio and then heat treated at 400 o C in N2 atmosphere. Carbon coating to the obtained powders was carried out according to the coating of carbon to the ceramic powders [17]. These powders were mixed with poly (vinyl alcohol) (PVA) in different mass ratios 90/10, 80/20, 70/30, 60/40 and 50/50 and then heat treated at 800 o C in N2 to obtain the carbon coating to the FeWO4. Heat treatment conditions were applied 5 o C /min and 60 ml/min in N2 flow. The sample code was applied according to the following: 11FWPx, where F corresponds to the iron, W corresponds to the tungsten, P corresponds to PVA and x corresponds to amount of addition of PVA, respectively. Preparation of FeWO4 without carbon coating was also carried out, where WO2.9 and iron (II) acetate were mixed in 1:1 mass ratio and heat treated at 900 o C in N2. The sample code 1-1FW900 was applied. The photocatalytic activities of the prepared samples were evaluated through the photodecomposition of phenol in its aqueous solution of concentration 5.3 10 -4 mol/dm 3 . Carbon coated powders dispersed in phenol solutions were kept under dark condition to saturate the phenol onto the thin carbon layer deposited through carbon coating process on the surface of FeWO4 particles. After the saturation of this layer, the fluorescent lamp intensity of 7 W/m 2 (intensity from 400 -700 nm) was used as a visible light irradiation source. The residue concentration of phenol was determined by using high performance liquid chromatography (HPLC) with UV detector on an ODS-100S column using 50% MeOH/ 50% water as a mobile phase at 0.001 dm 3 / min of flow rate. The concentration of phenol was measured at the wavelength = 269 nm. Fig. (1) shows the changes in XRD pattern with addition of PVA. FeWO4 was confirmed as main peaks in 1-1FW900 (Fig. 1a) and carbon coated samples (Fig. 1b-d). The peaks of other chemical compounds were recognized. Traces of Preparation of Carbon Coated FeWO4 and its Photocatalytic Activity The Open Materials Science Journal, 2008, Volume 2 57 Fe2O3 and Fe3O4 obtained from the oxidation of iron acetate were also observed. WO2 was also observed on the carbon coated samples and its deposition decreased with the increase of each addition of PVA. Increasing of amount of PVA derived the deposition of W metal in addition of 40 and 50% PVA. It has reported that the thin carbon layer was formed on the photocatalyst particle by carbon coating [18]. It seemed to carry out the formation of thinner carbon layer in this case. This carbon thin layer will be worked as an adsorption layer of pollutants. The color of 1-1FW900 and carbon coated FeWO4 powders were dark brown and black, respectively. The absorbances of their synthesized samples were measured in the wavelength range 250 -700 nm by using diffuse reflectance spectrophotometer. 1-1FW900 which mainly consists of FeWO4 shows the absorbance in the whole wavelength as shown on Fig. (2). Carbon coated FeWO4 powders also revealed absorbance in the whole range of wavelength, including visible light. Their absorbance spectra are compared to TiO2 with anatase structure (ST-01; Ishiharasangyo Co. Fig. (1). XRD patterns of samples prepared, (a) 1-1FW900 (b) 1-1FWP10 (c) 1-1FWP30 (d) 1-1FWP50. Fig. (2). UV – Vis absorption spectra of samples prepared. 10 20 30 40 50 60 70 80 In te n si ty / a .u . Cu Kα / 2θ  FeWO 4 WO 2 W Fe 3 O 4 Fe 2 O 3