Full Length ArticleEnhanced photoelectrochemical activity of WO3-decorated native titania films by mild laser treatment
Graphical abstract
Introduction
In the forty years following the publication of Fujishima and Honda’s groundbreaking work concerning photoelectrochemical water splitting on TiO2 [1] the possible use of this material for various photoassisted and electrocatalytic applications has been extensively investigated, mainly due to its outstanding assets in terms of stability, low toxicity and cost-effectiveness [2], [3]. Nevertheless, the efficiency of pristine TiO2 catalysts is affected to a large extent not only by its wide bandgap (ca. 3.0 to 3.2 eV) which limits the use of solar energy, but also by the quite high rate of photoinduced charge carriers recombination [4].
To extend the photocatalytic activity of TiO2 towards visible light region, doping with non-metallic elements (such as N, F, C, or S) appears to be a propitious method as it enables the formation of new energy levels either below the conduction band or above the valence one [5], [6], [7], [8], [9]. Similar results were reported for thermally treated TiO2, in vacuum or under hydrogen atmosphere, in which cases oxygen vacancies are induced and Ti4+ species are partially reduced to Ti3+ [10], [11]. For enhancing the efficiency of the photoproduced electron-hole pairs separation, Au or Pt surface modification of TiO2 was also successfully used, the noble metal particles being able to capture conduction band electrons, thus preventing recombination [12], [13].
An up-and-coming approach for improving the overall efficiency of TiO2-based photocatalysts consists in coupling titania with other semiconductors having narrower bandgap and a conduction band lower in energy than that of pristine TiO2 (see [14] and references cited therein). Among such semiconducting materials, WO3 is considered to be one of the most suitable candidates because it not only fulfills the above requirements, but also exhibits high photoelectrochemical robustness, lack of toxicity and good resistance to corrosion [15]. It was well established that the noticeable beneficial effect of WO3, in terms of charge carriers separation efficiency, is due to the fact that both its conduction and valence bands are located at more positive potential values than the corresponding bands of TiO2 [16]. Thus, the conduction band of WO3 may act as a trap for the electrons photogenerated in the titanium oxide, while holes from the valence band of the former can be transferred to the valence band of TiO2 [17].
Heretofore, most of the research work on WO3–TiO2 systems was directed towards obtaining efficient powder photocatalysts [18], [19], [20] as they inherently ensure a higher active surface area. Nevertheless, the continuous concern for environmental sustainability has generated a growing demand for reusable, immobilized catalysts. Consequently, WO3–TiO2 photoactive bilayers have also become more and more attractive and it was observed that WO3-decorated titania films exhibit better photocatalytic activity compared to the case in which WO3 underlies TiO2, this behavior being ascribed to the relatively high compactness of titania layers [21].
In this context, the use of thin native air-formed TiO2 films as substrates for WO3 electrochemical deposition appears to be a very promising approach for obtaining high efficiency WO3/TiO2 photoelectrocatalysts, as such films, due their interconnecting structure, ensure better carrier mobility [22]. More recently, it was also shown that the efficiency of the charge carriers separation can be significantly improved by a mild laser treatment [23].
The purpose of the present work was to assess the possibility of successfully using spontaneously formed TiO2 films as supports for WO3 electrochemical deposition, in view of obtaining efficient WO3/TiO2 photoelectrocatalytic hybrid systems. To this end, the use of electrochemistry for WO3-modification might be an advantage as it enables good electrical contact with the titania substrate. The extent to which the performances of such systems could be improved by a specific laser treatment was also investigated, the anodic methanol photooxidation being used as a test reaction.
Section snippets
Material and methods
Titanium oxide substrates were grown on 1 cm2 Ti (Goodfellow) samples, previously washed in acetone and degreased in tri-chlorethylene for 30 min. To ensure an increased roughness of the surface, titanium plates were subjected to an etching procedure described in the literature [24], [25], consisting in an 1 h treatment at 80 °C with a 10 % oxalic acid solution. The samples were then ultrasonically cleaned for 30 min in doubly distilled water and exposed to air for 24 h in a dry box, in order
Results and discussion
To appraise the beneficial role that WO3-modification might play in improving the photoelectrochemical activity of native formed titanium oxide films, potentiodynamic measurements were performed (sweep rate 10 mV s−1) in a 0.1 M KNO3 + 0.01 M HClO4 solution, under chopped UV-illuminated conditions, and Fig. 1 shows the results obtained by increasing the tungsten oxide deposition charge. Photoanodic effects were recorded, characteristic of a n-type semiconductor and, as expected, it was found
Conclusions
The effectiveness of the electrochemical WO3-modification as a method for improving the photoactivity of native air-formed TiO2 layers was investigated, together with the way in which a mild laser treatment influences the photoelectrocatalytic performances of the WO3/TiO2 systems. It was found that WO3 deposition leads to a significant enhancement of the photocurrent, although for deposition charges above ca. 70 mC cm−2, the higher tungsten oxide loading seems to generate additional
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was performed within the framework of the Electrochemical preparation and characterization of active materials with predetermined features research project of the „Ilie Murgulescu“ Institute of Physical Chemistry of the Romanian Academy.
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