Innovative visible light-activated sulfur doped TiO2 films for water treatment
Graphical abstract
Highlights
► The synthesis of S–TiO2 films by a novel sol–gel method. ► The significant shift of the optical absorption edge toward the visible region. ► The markedly enhanced EPR intensity under visible light illumination. ► The effective degradation of MC-LR under visible light irradiation using S–TiO2 film.
Introduction
Nanostructured titanium dioxide (TiO2) is widely used as a photocatalyst for the degradation of environmental contaminants in water and air. The physicochemical properties of TiO2, such as thermal and chemical stability, relatively high photocatalytic activity, low-toxicity, and low cost make TiO2 the most attractive photocatalyst for environmental remediation [1], [2], [3], [4], [5], [6]. However, only UV light (4–5% of solar light) can be used for TiO2 photocatalytic reactions because of its wide bandgap (∼3.2 eV for anatase and 3.0 eV for rutile). Therefore, it is of great importance to extend the photoresponse of TiO2 into the visible spectral range, which comprises the larger portion of the solar spectrum in terms of energy availability (∼45% of solar light) [1], [2]. Recently, intensive efforts have been directed to improve the photocatalytic behavior of TiO2 under visible light using various transition metal ions (Fe, Co, Ag, Ni) [7], [8] and non-metal (C, N, F, S) [1], [9], [10], [11], [12], [13], [14], [15], [16], [17] dopants utilizing different synthetic routes such as seeded growth, chemical vapor deposition (CVD), hydrothermal, and sol–gel methods. Despite its effectiveness, transition metal-doped TiO2 has certain disadvantages such as low thermal stability and enhanced recombination of charge carriers. Synthesis of non-metal doped TiO2 has been suggested in order to circumvent those drawbacks [10], [11], [12]. Since Umebayashi et al. [13], [14], [15] synthesized sulfur doped TiO2 for photocatalytic degradation of methylene blue under visible light, several groups have reported that S-doped TiO2 exhibits enhanced photocatalytic activity under visible light [10], [16], [17], [18], [19], [20]. The presence of both anionic S2− and cationic S4+/S6+ species were experimentally identified on sulfur doped TiO2, depending essentially on the preparation conditions and the sulfur precursor. This behavior can be qualitatively rationalized by the low solubility of S2− ions in the titania lattice due to the relatively larger ionic radius of S2− compared to that of O2− and the concomitant increase of the Ti–S bond formation energy [11], [17]. Both anionic and cationic doping was accordingly proposed through the substitution of S2− and S4+/S6+ for O and Ti ions in the TiO2 lattice, respectively, leading to intra-gap impurity states between the valence and conduction band and/or the effective narrowing of the TiO2 bandgap [13], [14], [15], [16], [17], [18], [19], [20]. In both cases, the optical absorption was found to shift to lower energies promoting the response of the doped material into the visible range.
Very recently, sulfuric acid and/or sulfate precursors, mainly ammonium sulfate (NH4)2SO4, have been applied as sulfur sources for the preparation of S and N–S doped TiO2 that simultaneously inhibit the anatase-rutile phase transformation and enhance the stability of the most photocatalytically active TiO2 phase (anatase) up to high temperatures [21], [22], [23], [24], [25]. Sulfur cations (S4+/S6+) have been invariably identified in these materials, which, however, may not actually substitute for Ti4+ in the titania lattice but rather reflect the formation of sulfate groups on the TiO2 surface [26]. The latter species may substantially enhance surface acidicity and/or charge separation in TiO2, while the observed visible light activity may primarily stem from doping with nitrogen provided by ammonium cations in these modified TiO2 nanomaterials [25]. Furthermore, fine tuning of titania's structural properties is a key aspect for the enhancement of the TiO2 photocatalytic activity. Recent works reported on the use of surfactants as pore directing agents to tailor-design the synthesis of porous nanostructured films and particles using the self-assembly surfactant-based sol–gel method. By this method, high surface area and porosity together with small particle size were thus obtained [1], [5], [27], [28], [29], [30].
In this study, firstly, in order to synthesize the visible light-activated sulfur doped TiO2 films, a sol–gel route based on the self-assembly of an nonionic hydrocarbon surfactant as pore directing agent was developed and sulfuric acid was used as both sulfur and water formation precursor. The heat treatment conditions (mainly the calcination temperature) were carefully controlled in order to maximize sulfur incorporation into the TiO2 crystal lattice and remove residual carbon. The synthesized sulfur doped TiO2 materials were thoroughly characterized by UV–vis diffuse reflectance, XRD, TEM, Raman, AFM, ESEM, XPS, FT-IR, EDX, EPR and porosimetry in order to determine their morphology, structural and electronic properties. Secondly, microcystin-LR (MC-LR) was selected as a target contaminant in order to evaluate the photocatalytic activity of sulfur doped TiO2 films under visible light irradiation. Cyanobacterial harmful algal blooms (CHABs) frequently occur globally in water sources and the presence of their toxic metabolites, cyanotoxins, may severely affect the health of humans and animals when consume toxic water or are exposed to them. In particular, MC-LR is one of the most toxic and commonly found cyanotoxins among CHABs in surface waters that serve as sources of drinking water supply [31]. In this study, the application of the sulfur doped TiO2 films for the treatment of water contaminated with MC-LR was evaluated.
Section snippets
Sol–gel synthesis
A nonionic surfactant polyoxyethylene (80) sorbitan monooleate (Tween 80, Sigma–Aldrich) was used as a pore directing agent. The surfactant was dissolved in isopropyl alcohol (i-PrOH, 99.8%, Pharmco) and then titanium (IV) isopropoxide (TTIP, 97%, Sigma–Aldrich) was added as an alkoxide precursor in a mixture of i-PrOH and Tween80. Finally, sulfuric acid (H2SO4, 95–98%, Pharmco) was added as a sulfur precursor and reagent for in situ formation of water. This solution was stirred for 24 h at room
Physical properties of sulfur doped TiO2
The UV–vis absorption spectra of sulfur doped TiO2 samples calcined at different temperatures in comparison with the reference TiO2 sample are shown in Fig. 1. In order to obtain indirect bandgap values for the TiO2 samples, Tauc plots of the remission function of Kubelka–Munk were used. The plots were obtained from the UV–vis diffuse reflectance and the values were obtained by extrapolating the linear part of each curve from the Kubelka–Munk remission function for S350 and reference TiO2 (see
Conclusions
Visible light-activated sulfur doped TiO2 films were successfully synthesized using a sol–gel method based on the self-assembly technique with a nonionic surfactant to control nanostructure and H2SO4 as a sulfur source. The morphological, structural, optical and porous characteristics of the S-doped TiO2 films were found to depend markedly on the calcination temperature. According to XPS, FT-IR and EDX mapping, sulfur was identified mainly as anionic (S2−) substituents in the TiO2 lattice as
Acknowledgements
This work was funded in part by the U.S. Environmental Protection Agency STAR Grant (RD-83322301), the National Science Foundation through a CAREER award (BES-0448117) to Dionysios D. Dionysiou, the National Science Foundation (US-Ireland Collaborative Research; CBET 1033317) and the European Commission (Clean Water Grant Agreement number 227017). Clean Water is a Collaborative Project co-funded by the Research DG of the European Commission within the joint RTD activities of the Environment.
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