Er-W codoping of TiO2-anatase: Structural and electronic characterization and disinfection capability under UV–vis, and near-IR excitation
Graphical abstract
Introduction
The use of titania based photocatalysts has emerged as a powerful, advanced oxidation process to control environmental and health-related effects derived from chemical contaminants as well as from dangerous microorganisms. A main drawback of all titania based systems is their limited performance upon excitation at wavelengths above the UV limit of ca. 380 nm. This is a well known fact, intimately linked with the optical properties of the material and, particularly, with the relatively high band gap energy presented by all titania polymorphs, always above 3.0 eV [[1], [2], [3]].
Several technologies have been attempted to solve or mitigate this issue, being doping of the titania host a rather successful example. From the initial times of photocatalysis, doping was used to enhance performance under UV excitation. This objective leads to introduce Fe, Cu and other metals in relatively small quantities, below or around 1 at.% (cationic basis) [[1], [2], [3], [4], [5]]. Parallel to such development, effort was carry out with the aim of producing photoactive solids which can profit from a broad range of wavelengths in order to achieve a sunlight-operated catalyst. This was initially achieved mostly by decreasing the band gap of the titania and/or creating gap states with cationic [[6], [7], [8], [9], [10]], anionic [[11], [12], [13], [14]] or cation-anionic co-doping [[15], [16], [17], [18], [19], [20], [21]]. These works concern the use of visible light photons as an additional energy source (to UV photons) of the photocatalytic process. More recently, the use of near IR photons captured the attention of researchers in the photocatalytic field. In this case, the source of the positive effect on photoactivity is under debate but at least originally the IR to UV upconversion process motivated a number of studies. Such works essayed the doping of titania mostly with Er but also co-doping with several rare earth always including Er as central doping cation of the solid catalyst [[22], [23], [24], [25], [26], [27]]. Only few works focussed in the combination of what we can call visible light and near infrared light oriented cations. A representative example is the combination of Fe and Er [28].
Here we study the combination of previously tested and highly active photo-active materials concerning pure anatase powders doped with W (the visible light active cation [7]) or Er (the near IR active cation [22]). Note also that WOx species (called suboxides with W oxidation state below 6) at the surface of titania can also absorb IR photons trough a surface plasmon resonance [29,30]. These materials will be applied in the inactivation of microorganisms, specifically a Gram-negative (Escherichia coli) and a Gram-positive (Staphylococcus aureus) bacterium, that are known to cause hospital-acquired infections because their resistance to most commonly used antibiotics. As well known, Matsunga and others provided the first reports of photocatalytic disinfection [31,32], opening a field subsequently flourishing with a large number of studies summarized in recent reviews [[33], [34], [35], [36]]. Titania based catalysts are a kind of biocidal agent with significant advantages over conventional ones due, in first place, to the (relative) innocuousness of the material for humans, and, on second hand, to the absence of known weaknesses related to the type of organism, as the inactivation of Gram-positive and Gram-negative bacteria, viruses and fungi have been tested with significant success [1,33,[37], [38], [39], [40], [41]].
The work here presented attempts to understand the doping process of the single (Er, W) and codoped (ErW) systems by presenting an exhaustive study of the structural and electronic properties of the materials using x-ray diffraction, surface area measurements, UV–vis, photoluminescence, microscopy as well as x-ray photoelectron and absorption spectroscopies. The combination of techniques provides evidence that the codoping process renders powders with significant differences with respect to the single-doped materials. Such differences are reflected in the important efficiency offered by the codoped system in the photo-elimination of E. coli and S. aureus microorganisms. To interpret the catalytic results, a kinetic modeling was carried out following the work of Marugan et al. [42]. Modeling of the inactivation profiles is grounded in a simplified (Langmuir-Hinshelwood-like multistep-type) reaction mechanism and considers that microorganism death occurs via a sequential attack of photo-radicals by which “undamaged” cells become “damaged” and eventually progress to an “inactivated” state. The utilization of an “adsorption Langmuir-Hinshelwood” type mechanism allows a reasonable and relatively flexible description of the inactivation. Moreover, the advantage of using this approach appears two-fold: first, i) its usefulness for analyzing complete sets of inactivation profiles showing (or lacking) initial smooth/fast decays and final tailing section; and, additionally, ii) the model renders kinetic parameters allowing physical interpretation of the underlying process, in contraposition with many other simple kinetic laws [33,42,43]. Such work is combined with the electron paramagnetic and optical characterization of radical species formed under all illumination conditions relevant for this study. The combination of the structural and electronic characterization of the materials with the charge generation and kinetic analysis under light excitation provides evidence of the physical origin of the functional properties in our titania-based catalysts.
Section snippets
Catalysts preparation
Titania based materials were prepared using a microemulsion synthetic route and calcined at 723 K for 2 h as detailed previously in Ref. [44]. Briefly, titanium tetraisopropoxide was added to an inverse emulsion containing an aqueous solution (0.5 M) of ammonium tungsten oxide (Aldrich) and/or erbium nitrate (Aldrich) dispersed in n-heptane, using Triton X-100 (Aldrich) as surfactant and hexanol as cosurfactant. Water/titanium and water/surfactant molar ratios were, respectively, 18 and 110 for
Physico-chemical properties
Table 1 collects results concerning the chemical composition of the samples. Tungsten was introduced in our powders in significant quantity, around 15 at. % (cationic basis). We doped the system with a relatively large quantity of tungsten as significant activity under both UV and visible illumination was observed for such large W concentration in a significant number of studies of titania based photocatalysts [5,7,8,[52], [53], [54]]. Erbium was close to 2 at.% in our samples, a quantity which
Conclusions
The doping of a nanosized anatase (size near 10 nm) powder by tungsten and/or erbium was analysed in materials prepared by a microemulsion method. Both cations are presented at substitutional positions of the anatase structure rendering substitutionally disordered mixed oxides with anatase structure. More importantly, when both cations are present at the anatase structure, the local order is strongly modified by presence of a direct Er-O-W interaction and the corresponding charge neutrality
Acknowledgements
Financial support by Fundación General CSIC (programa ComFuturo) is acknowledged. Work at the ESRF synchrotron was carried out with the help of the BM26 staff (Dr. D. Barnerjee) and EU support.
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