On the photocatalytic activity of the sulfur doped titania nano-porous films derived via micro-arc oxidation

Abstract

Sulfur doped TiO₂ layers containing nano/micro-sized pores were synthesized by micro-arc oxidation process. Effect of the applied voltage and the electrolyte composition on physical and chemical properties of the layers was investigated using SEM, AFM, XRD, XPS, and EDS techniques. A UV–vis spectrophotometer was also used to study optical properties of the layers. It was found that the doped layers were porous with a pore size of 40–170 nm. They consisted of anatase and rutile phases with varying fraction depending on the applied voltage and electrolyte concentration. Our XPS investigations revealed the existence of sulfur in the forms of S⁴⁺ and S⁶⁺ states which substituted Ti⁴⁺ in the titania lattice. The sulfur concentration in the layers also increased with the voltage and the electrolyte concentration. Furthermore, the absorption edge of the doped layers shifted significantly toward longer wavelengths as compared to the pure TiO₂ layers. The band gap energy was calculated as 2.29 eV for sulfur doped TiO₂ layers, respectively. Finally, photocatalytic activity of the layers was studied by measuring the degradation rate of methylene blue on their surface under UV and visible illuminations. The doped layers showed a slightly enhanced photoactivity than the pure layers under UV-irradiation, while their photocatalytic performance was much higher than that of pure layers under visible-irradiation. It was measured that about 92% and 66% of methylene blue was decomposed over doped layers under UV and visible irradiations, respectively.

Introduction

It is well known that metal oxide photocatalysis is a promising approach to remedy the problem of chemical wastes. Among various metal oxides, titanium dioxide (TiO₂) has been found to be capable to decompose different kinds of organic and inorganic wastes in both liquid and gas phases, because it is chemically and biologically inert, photo-catalytically stable, commercially available and inexpensive, and from environmental view point is friendly [1], [2]. For the photocatalytic applications, the band gap energy (E_g) of the semiconductor is critical. Since the E_g of titania is wide, considerable efforts have been directed to extend the absorption edge of TiO₂ toward the visible part of the spectrum in the last three decades. One approach for acquiring a visible response is to introduce defects into the titania lattice through doping titania with metallic [3], [4], [5] and non-metallic species [6], [7], [8] and loading other semiconductors [9], [10], [11], [12] on the surface or into the crystal lattice of titania. Among these methods, modification of TiO₂ with various nonmetal ions is a powerful way to extend the adsorption light from UV to visible region and to reduce the recombination rate of photo-generated electrons and holes of TiO₂. Several works concerning nonmetal cations doping was reported. Ohno reported that S cation-doped TiO₂ powder absorbed visible light more strongly than N, C and the S anion doped TiO₂ powders and showed photocatalytic activity under visible light. They demonstrated that the substitution of Ti⁴⁺ by S⁴⁺ was responsible to the visible light absorbance [13], [14], [15]. Umebayashi et al. who synthesized S:TiO₂ photocatalyst using the ion implantation method, indicated that S is doped in TiO₂ as an anion and replace the lattice oxygen in TiO₂ [16], [17], [18]. Whether it is cationic or anionic, the doping of a foreign element into TiO₂ makes a donor or an acceptor level in the forbidden band and this level induces absorption of visible light. At the same time, however, this level may work as a carrier-recombination center, which decreases photocatalytic activity. Therefore, doping of a foreign element does not necessarily enhance response to visible light. To develop and improve visible-light-responsive photocatalysts, we should clarify what kind of doping or modification of TiO₂ is really effective in increasing photoactivity under visible light [19].

Sulfur doping has been achieved under high temperature process or using expensive precursors or preparation instruments [14], [15], [20], [21] which limits investigations on sulfur doped titania catalysts. Micro-arc oxidation (MAO) is a simple and promising approach for fabrication of different categories of oxide layers. It is an electrochemical technique for formation of anodic films by spark/arc micro-discharges which move rapidly on the vicinity of the anode surface [22], [23], [24], [25]. It is characterized by high productivity, economic efficiency, ecological friendliness, high hardness, good wear resistance, and excellent bonding strength with the substrate [26], [27], [28]. This process is carried out at voltages higher than the breakdown voltage of the gas layer enshrouding the anode. Since the substrate is connected to positive pole of the rectifier (anode), the gas layer consists of oxygen. When the dielectric gas layer completely covers the anode surface, electrical resistance of the electrochemical circuit surges and the process continues providing that the applied voltage overcomes the breakdown voltage of the gas layer. Applying such voltages leads to formation of electrical discharges via which electrical current could pass the gas layer. MAO process is characterized by these electrical sparks [23], [29].

To best of our knowledge, there is just one published report on synthesis of the S:TiO₂ layers by MAO process where the photocatalytic performance and how it is affected by growth parameters has not been investigated; meanwhile, position of the sulfur ions and their valance in the titania lattice were not explained [30]. Hence, this research is the first comprehensive report on synthesis of sulfur loaded TiO₂ photocatalysts by MAO process. The present study sheds light on the effect of the applied voltage and the electrolyte composition on surface morphology, topography, phase structure, stoichiometry, and specially photocatalytic activity of the MAO-grown sulfur doped TiO₂ layers. The sulfur valance is also studied by XPS technique. Finally, the obtained results are compared with the results of pure TiO₂ layers which were synthesized via MAO process in our previous works [31], [32].