Visible light activated photocatalytic behaviour of rare earth modified commercial TiO2
Graphical abstract
Introduction
The contamination of natural water reserves, air (indoor and outdoor) and soil is currently one of the major global environmental concerns. Therefore, to further the sustainable development of modern society, there is an urgent need for advances in green technologies to achieve environmental remediation.
Environmental catalysis is one of the most studied processes in order, not only to reduce, but also to prevent the causes of pollution. Amongst these catalytic processes, photocatalysis is expected to be one playing an important role in the 21st century's efforts in reducing environmental pollution [1]. Titanium dioxide (TiO2) is the most investigated photocatalyst; the great interest in TiO2 can be related to the work by Fujishima and Honda, published in 1972 [2]. This described the photo-assisted electrolysis of water upon irradiation of a single-crystal TiO2 electrode (with a Pt counter-electrode), with photons of energy greater than the band gap of TiO2. However, this was not strictly catalysis, the reaction not being thermodynamically feasible without the photons – it was actually an energy storage reaction that can be termed photogalvanic [3]. The first reported works rigorously dealing with “photocatalysis” are those by Doerfler and Hauffe, published in 1964 [4], [5]. In any case, we are still experiencing today an enormous boom in this field of research, with a great number of publications concerning photocatalysis appearing over the past 20 years. Photocatalysis can be described as that phenomenon in which a material (a semiconductor) modifies the rate of a reaction, via the action of light having a suitable wavelength [6], [7]. When a semiconductor is irradiated with photons having energy higher than, or equal to, its energy band gap (Eg), an electron (e−) is able to migrate from the valence band to the conduction band, leaving a hole (h+) behind. Such a photo-generated couple (e−–h+) is able to reduce and/or oxidise a pollutant adsorbed on the photocatalyst surface [8].
As a photocatalytic material, TiO2 is chemically inert and non-toxic; the reactions take place at mild operating conditions (e.g. a low level of solar or artificial illumination, room temperature (RT) and atmospheric pressure); no chemical additive is necessary; volatile organic compounds (VOCs) [9], [10], and even very recalcitrant and persistent pollutants, can be degraded [11]. Moreover, TiO2 heterogeneous photocatalysis is, at the same time, efficient in green chemistry, in fine chemicals, and in advanced oxidation processes (AOP) [12], [13].
The photocatalytic reaction of TiO2 is activated by UVA light, although titania is transparent for most of the visible radiation region, since it is a wide band gap semiconductor material (Eg = 3.2 and 3.0 eV for anatase and rutile, respectively). This means that the photocatalytic reaction is exploited by only 3–5% of the solar spectrum [14]. Several different paths were followed in attempts to create visible-light activated photocatalysts. With the aim of extending the absorption edge of TiO2 into the visible region, this was using doped/modified with non-metal atoms [15], [16], transition metals [17], or, more recently, noble-metal atoms [18], [19]. Anionic doping often had a detrimental effect on the photocatalytic activity (PCA) of TiO2 under UV-light irradiation, because of an enhancement of the charge recombination [20], and also because such TiO2 has a low thermal and chemical stability [21]; moreover, an excessive nitridation treatment might remove the doped N atoms from the TiO2 matrix, leading to a decrease in the visible-light PCA [22].
In this paper we report the synthesis, via solid-state reaction, of visible-light activated photocatalysts, made from a commercial TiO2 powder (Degussa P25) modified with several rare earth elements (RE = cerium, lanthanum, europium and yttrium), and thermally treated at temperatures as high as 900 and 1000 °C, in order to investigate the photocatalytic activity mechanism (PCA) of the RE-modified-TiO2. Degussa P25 has been previously modified with REs, in order to investigate its PCA under UV-light exposure [23], [24], but this is the first time that such a high temperature thermal treatment (essential for co-processing with some materials), together with exclusively visible light irradiation, is assessed in this system. REs were chosen as modifying agents because, due to the transitions of 4f electrons, the optical absorption of titania is increased, thus improving its visible-light response, and promoting the separation of photo-generated electron–hole pairs [25].
Section snippets
Sample preparation and characterisation
Samples were prepared via solid-state reaction of the two crystalline end-members, according to the stoichiometry: Ti1−xRExO2 where RE = Ce, Eu, La, and Y, and x = 0, 0.01, and 0.025 mol. Degussa P25 TiO2 powder was used as the titania source (hereafter designated as P25), while reagent-grade CeO2, Eu2O3, La2O3, and Y2O3 (all supplied by Aldrich) were used as RE precursors. Powders were admixed, and wet ground in a rotary ball mill (30 min with deionised water and sintered zirconia balls). Mixtures
X-ray diffraction
The addition of RE did not delay the anatase-to-rutile phase transition (ART) in all cases, at the thermal treatment temperatures used. Notwithstanding, a delaying effect by REs on the ART with P25 TiO2 has actually been reported by Lin and Yu (CeO2, La2O3, and Y2O3, at a concentration of 0.5 wt%, with calcination temperatures of 650 and 700 °C) [23], and by Du et al. (CeO2 and La2O3, at a concentration of 2 wt%, thermally treated at 800 °C) [24]. The XRD patterns all showed only the rutile phase
Photocatalytic activity
The PCA results in gas–solid phase, under visible light irradiation, are shown in Fig. 6. The untreated P25 powder had a PCA – in terms of acetone formation – of 23 ppm h−1. Hurum et al. [61], using electron paramagnetic resonance (EPR) spectroscopy, claimed that the visible-light response of P25 is due to the presence of small rutile crystallites amongst the anatase, their smaller band gap extended the useful range of PCA into the visible region. Moreover, the points of contact between these
Conclusions
A commercial TiO2 powder, Degussa P25, was modified with a series of REs in order to investigate the PCA mechanism, under visible-light irradiation, of samples treated at high temperature. The mixtures studied were prepared via solid-state reaction of the precursor oxides, and the products of the synthesis were thermally treated at 900 °C and 1000 °C. From a mineralogical point of view, the addition of RE accelerated the ART, at both thermal treatment temperatures, because of the formation of
Acknowledgements
Authors wish to acknowledge PEst-C/CTM/LA0011/2013 programme. M.P. Seabra and R.C. Pullar wish to thank the FCT Ciência2008 programme for supporting this work. Maria Celeste Coimbra de Azevedo (Chemistry Department, University of Aveiro) is gratefully acknowledged for the FT-IR measurements.
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