Effect of tungsten-doping on the properties and photocatalytic performance of titania thin films on glass substrates
Graphical abstract
Introduction
Photocatalytic materials [1], [2], [3] are being used currently for self-sterilizing surfaces [4], air purification [5], [6], water purification [7], [8], [9], self-cleaning surfaces [10], [11], [12], and solar cells [13], [14], [15]. Titanium dioxide (TiO2) is one of the most promising photocatalysts owing to its low cost, non-toxicity, and chemical and thermal stability [16], [17], [18]. Work by Fujishima and Honda [19] on water splitting using single-crystal TiO2 showed that high-energy radiation, such as UV, is required to excite electrons from the valence to the conduction band owing to the relatively large band gap (∼3.2 eV [20]). This limits the efficiency of TiO2 as a photocatalyst since UV radiation comprises only ∼5% of the solar spectrum [21].
Several methods have been used to enhance both the spectral response of TiO2 in the visible region and its photocatalytic activity [22], [23], [24]. The most common is the addition of dopants, which are intended to create mid-gap states in the electronic structure, thereby narrowing the band-gap [25], [26]. The dopants also act as trapping centers that separate the hole and electron pairs, thereby extending the lifetime of these charge carriers and enhancing photocatalytic activity [26]. Several dopants, including metals [27], [28], [29], [30] and non-metals [31], [32], [33], have been trialed for this purpose.
Doping with transition metal ions can help to retard the recombination of photogenerated electrons and holes, which thus increases the lifetime of charge carriers [34]. Moreover, tungsten (W) additions are believed to extend the spectral response of TiO2 into the visible region through a positive shift in the conduction band [12], [13]. However, the limited studies on W-doped TiO2 have focused on powders. León-Ramos et al. [35] reported that 0.03 mol% W-doped TiO2, produced hydrothermally, displayed higher photodegradation compared to undoped TiO2. Li et al. [36] demonstrated that 3 mol% WO3-doped TiO2 powders, prepared by sol–gel, were most effective in degrading organic solutions, while Couselo et al. [37] suggested that the optimal performance of 3 mol% W-doped TiO2 powders resulted from two competing factors, namely increasing surface area and decreasing specific activity per adsorption site with increased doping. Recently, Hua et al. [38] reported that sol–gel-derived TiO2 nanoparticles doped with 0.4–1.0 wt% W resulted in enhanced capability of degradation of methyl orange solution owing to the role of W ions in shifting the absorption edge of TiO2 to the visible range. Wu et al. [39] showed that W-doped TiO2 particles produced by the solvothermal method displayed enhanced near-infrared absorption performance and similarly enhanced photocatalytic activity to that of commercial Degussa P25 TiO2 in the degradation of NOx gas. Further, Sathasivam et al. [40] demonstrated that 2.25 at% W-doped TiO2 films prepared by aerosol-assisted chemical vapor deposition were effective in degrading resazurin redox dye since the dopants contributed to an increase in the surface area and limitation in the relative grain boundary regions, which act as charge carrier recombination sites.
Prior studies have shown that doping with W6+ has the potential to modify the photocatalytic properties of TiO2 since the similarity of its crystal radius to that of Ti4+ [41] suggests the potential for substitutional solid solubility. Further, the fact that its crystal radius is smaller than that of the two interstices adjacent to the central Ti ion in the elongated TiO6 octahedron [42] suggests the potential for interstitial solid solubility. Both types of solid solubility would require charge compensation in the form of titanium vacancies or oxygen injection (with ionic compensation) or electrons (with electronic compensation); Frenkel defect formation also is possible. All of these would be expected to have significant effects on the photocatalytic performance.
Most prior work has focused on reducing the optical band gap of TiO2 through W additions, without extended consideration of the effects of doping on the mineralogical, microstructural, and photocatalytic properties. Further, in other studies, the doping levels used have tended to be high (2–20 mol%) and well above the typical semiconducting addition range. This is particularly the case for W-doped thin films, as shown in Table 1 [35], [36], [37], [38], [39], [40], [43], [44], [45]. Previous work by the authors [46] has led to the observation that additions of transition metal dopants typically in excess of ∼1 mol% (metal basis) are capable of exceeding the solubility limits, resulting in precipitation on the grain boundaries. This effect lowers the band gap of thin films but it cannot alter that of the doped TiO2 further than that of the TiO2 at the solubility limit. Hence, the band gap of the thin film and that of the TiO2 comprising the film are different. Further effects may include lattice distortion and partial amorphization. All of these effects can alter the photocatalytic efficiency of the doped films, generally deleteriously.
There are many variables that affect the photocatalytic performance of doped TiO2, including crystal structure, lattice deformation, defect equilibria (structural variables), grain size, surface area, and topography (microstructural variables) [46]. Since previous studies have taken a more focused approach to this range of variables, the present work aims to describe a more comprehensive investigation into the effects of doping on the mineralogical, microstructural, optical, chemical, and photocatalytic performance of TiO2 thin films deposited on the common substrate soda-lime-silica glass. In the present case, doping with W in a compositional range (0.010–0.100 mol%) that is consistent with the formation of a homogeneous solid solution and semiconducting dopant levels is designed to systematize the structural variables while avoiding the effects of mechanistic changes in the microstructural variables, all of which affect the photocatalytic properties. In particular, since the band gap is used widely in the assessment of photocatalytic performance, in the absence of a WO3-TiO2 phase diagram or other solubility data, the low levels of dopant are intended to avoid precipitation, significant modification of the microstructure from precipitation and liquid formation, and artificial alteration of the band gap. In the present work, the recognized potential to influence the photocatalytic performance by W-doping of TiO2, summarized above, has been investigated by the fabrication of thin films by spin coating, followed by annealing at 450 °C for 2 h. The thin films were characterized using different techniques to investigate the effects of W-doping on the mineralogical, morphological, topographical, chemical, optical, and photocatalytic properties.
Section snippets
TiO2 thin film fabrication
To prepare sol-gel solutions for coating, titanium isopropoxide (TIP, reagent-grade, 97 wt%, Sigma-Aldrich) and isopropanol (reagent plus ≥99 wt%, Sigma-Aldrich) were mixed to obtain a titanium concentration of 0.1 M (2.84 g of TIP diluted in 100 mL isopropanol). The solution then was magnetically stirred on a hot plate at 90 °C for 10 min. Different amounts of tungsten hexachloride (WCl6, ≥99.9 wt%, Sigma-Aldrich) were added to obtain W-dopant concentrations of 0.010, 0.025, 0.050, 0.075, and 0.100
Mineralogical characteristics
GAXRD patterns of the annealed thin films (Fig. 1a) showed that the films consisted only of anatase (JCPDS 21-1272). No rutile or W-containing phases were observed, but these phases possibly could be present at levels below the detection limit [47]. However, these data are consistent with the presence of unsaturated solid solutions of W in TiO2 since the peak intensities show a consistent increasing trend. Had saturation solubility been reached, then the intensities would have become consistent
Summary
W-doped TiO2 thin films were fabricated using a sol-gel method followed by deposition by spin coating on soda-lime-silica glass substrates and annealing at 450 °C for 2 h. The major conclusions are as follows:
- (1)
GAXRD Data: W formed a substitutional solid solution in the TiO2 (anatase) lattice. This resulted in lattice contraction, largely owing to Ti vacancy formation.
- (2)
AFM Data: Undoped anhedral TiO2 of grain size ∼25–30 nm converted to euhedral grains of size ∼30-40 nm upon doping. The grains were
Acknowledgments
The authors acknowledge the financial support from the Australian Research Council (ARC) and the characterisation facilities provided by the Australian Microscopy & Microanalysis Research Facilities (AMMRF) node at UNSW Australia.
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