Full Length ArticleFabrication and characterization of photoelectrochemically-active Sb-doped Snx-W(100-x)%-oxide anodes: Towards the removal of organic pollutants from wastewater
Graphical abstract
Introduction
One of the most serious environmental concerns of the 21 st century is the increasing presence of synthetic and naturally occurring chemicals in the aquatic environment [1], [2]. These compounds include pharmaceuticals, hormones and phenolic organic contaminants which are prevalent in the wastewater of a number of industries such as oil refining, textiles, pharmaceuticals, pulp and paper, and plastics [3], [4], and have the potential to cause adverse ecological and/or human health effects. Conventional treatment methods such as physical, chemical or biological processes are in certain cases ineffective or insufficient in the removal or destruction of organics [5]. In recent decades, more and more efforts have been put in developing new technologies for the treatment of persistent organic contaminants [6], [7]. Ozonation and Advanced Oxidation Processes (AOPs) such as UV and visible light irradiation-based oxidation (e.g. photocatalysis) and electrochemical methods are among the most investigated techniques for the elimination of organic contaminants [8], [9], [10]. Electrochemical methods have proved to be effective for the treatment of organic-containing wastewaters, especially when it is combined with the photochemical oxidation process [11], [12], [13], [14]. These processes involve the generation of strong oxidants such as hydroxyl radicals (OH), because of the formation of electron-hole pairs in semiconductors upon photo-radiation, and consequent oxidative decomposition of organic molecules. In addition, direct electrochemical oxidation at the electrode surface (anode) is possible. The photo- and electrochemical technology offers many advantages such as low cost, simplicity, versatility, modularity, ease of control, environmental compatibility, and minimum waste production [15], [16].
Metal oxide materials are of great importance for environmental applications because of their photoactivity (capability to generate charge carriers when exposed to photo-radiation), electrochemical activity, and good stability [17]. Since the discovery of photoelectrochemical water splitting using TiO2 electrodes [18], this semiconductor photocatalyst has been of particular interest for wastewater purification as well as production of hydrogen as a renewable energy source [19], [20]. Until now, TiO2 has been the most promising photocatalyst, because of its low cost, non-toxicity and good stability under irradiation [21], [22]. However, the lack of visible light activity due to its relatively wide band gap (3.2 eV) and low quantum efficiency, in addition to its low electrocatalytic activity for the anodic oxidation of organics, limits its practical applications. Therefore, more photo/electroactive metal oxide anode materials are needed. More closely, it is desirable to decrease the bandgap to facilitate the production of photogenerated charge carriers, while increasing (or at least maintaining) the electrochemical activity. In order to address the above-mentioned drawbacks, doping of TiO2 and other metal-oxide photoactive electrode materials with a range of metals has been applied: anion-doping with N [23], C [24], S [25] and I [26], and cation-doping with some transition metals such as Pt [27], Au [28], Ag [29], Mg [30], Mn, Ru, Rh and Ir [31] to name but a few.
Sb-doped SnO2 is considered as one of the most promising electrode materials for electrochemical degradation of organic pollutants [32], [33], [34], while undpoed SnO2 (n-type semiconductor; Eg ≈ 3.5 eV) cannot be used directly as an anode material due to its low conductivity at room temperature [35]. Another approach in the development of more (photo)electroactive anode materials is the use of coupled semiconductors such as SnO2-TiO2 [36], [37], WO3-TiO2 [38], [39], CeO2-TiO2 and ZrO2-TiO2 [37], which allows the desirable matching of their electronic band structure. It is generally accepted that the coupled systems offer higher degradation rate and efficiency in organic wastewater treatment [36]. In recent years, multi-component metal oxide semiconductors have also been extensively investigated for the effect of composition on their band gap and thus, the photocatalytic activity [40], [41], [42], [43], [44], [45], [46]. The ternary and quaternary systems have shown enhanced solar-light response in addition to providing the possibility of band gap tuning, due to their various morphologies [47]. Zhang et al. [20] thoroughly reviewed the research and development of photoelectrocatalytic materials for environmental applications in their feature article.
Because of the interesting properties of doped SnO2 as an electroactive material [48], [49] and WO3 as a photoactive material [50], [51], these two materials were chosen as base electrode materials for their possible use as anodes for the wastewater treatment and disinfection. In this manuscript, we report results on the synthesis of antimony-doped tin-tungsten-oxide electrodes via a thermal deposition method and their characterization by electrochemical, optical, surface/structure techniques. Also, the effect of electrode composition on their photo- and electrochemical activity for degradation of phenol red dye was investigated. Their performance as anode materials for the oxidation of a pharmaceutical compound and for the water disinfection will be presented in separate manuscripts.
Section snippets
Electrode preparation
Sb-doped Snx-W(100-x)%-oxide coatings (x = 0, 20, 40, 60, 80 and 100) were prepared on flat titanium substrates employing a thermal method. First, stock solutions of tin, tungsten and antimony salts were prepared by dissolving SnCl2·2H2O (ACS reagent, ≥98.0%, Sigma Aldrich), Na2WO4·2H2O (Certified ACS, 100.0%, Fisher), and SbCl3 (ACS reagent, ≥99.0%, Sigma Aldrich) salts in HCl (37 wt.%, Fisher) and water. All solutions were prepared using ultra-pure deionized water (resistivity: 18.2 MΩ cm). For
Scanning electron microscopy (SEM)
Fig. 1 shows the SEM images displaying the microstructure of surfaces of as-prepared Sb-doped Sn-W-oxide coatings of different compositions. The morphology of the surfaces is typical of that for metal-oxide coatings [20], and it varies with changes in the coating composition. For the two pure compositions, Sn-oxide (Fig. 1a) and W-oxide (Fig. 1f), the surface displays a cauliflower-like morphology, while a cracked-mud morphology characterizes the mixed compositions in Fig. 1c, d and e. The
Conclusions
Sb-doped Snx-W(100-x)%-oxide coatings (x = 0, 20, 40, 60, 80 and 100) were fabricated on flat titanium substrates by thermal decomposition of metal salts. The electrochemically active surface area and the surface roughness of the coatings were found to be the coating-composition dependant. The XPS results confirmed the existence of Sn2+ (e.g. SnO2) and W6+ (e.g. WO3) as the main oxidation states of the elements in the coatings, while antimony was found to be present as Sb4+ and Sb3+. The optical
Acknowledgements
The present work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and McGill Engineering Doctoral Award (MEDA).
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