Nanosized titania modified with tungsten and silver: Microstructural characterisation of a multifunctional material
Introduction
The development of nanotechnologies is currently one of the most interesting topics in the field of sciences. Due to their unique physicochemical, surface, magnetic and catalytic properties, nanoparticles (NPs) provide solutions to problems that cannot be dealt with using conventional technologies. Applications are manifold, and metal oxides continue to play a dominant role [1]. Amongst metal oxides, titanium dioxide (TiO2) deserves a special mention: it arose as the most popular semiconductor material for photocatalytic applications, after the discovery of the “Honda–Fujishima effect” [2]. Nowadays, nano-dimensioned titania is attracting increasing interest owing to its unique physicochemical properties and widespread applications, that include, asides from photocatalysis, solar energy conversion/water splitting, sensors, photochromic devices, antibacterial surfaces and, more recently, electrode material in lithium ion batteries [3], [4], [5], [6], [7], [8], [9], [10], [11]. Its performances – for a given application – depend on the crystalline structure, morphology, and size of the material [12].
Being a wide band gap (Eg) semiconductor (Eg of anatase is 3.2 eV, that of rutile is 3.0 eV), TiO2 requires UV-light to be activated as a photocatalyst. A recently suggested solution for the energy-harvesting of visible-light is the deposition of noble metal NPs onto the surface of a semiconductor, so as to form a metal-semiconductor composite photocatalyst [13]. This works because noble metals, owing to their surface plasmon resonance (SPR) [14], [15], [16], are able to strongly absorb visible-light. Moreover, the photogenerated electrons and holes can be efficiently separated by the metal-semiconductor interface, thus enhancing the photocatalytic property [17].
In the present work, a series of TiO2, 1 mol.% W-doped, 1 mol.% Ag-doped and 1 mol.% W/Ag-co-doped (the ratio between W and Ag was 1:1) TiO2 nanopowders were synthesised via an aqueous sol–gel route [18]. Such materials showed themselves to be excellent photocatalysts (under both UV and visible-light irradiation), antibacterial agents, and also to possess tuneable photochromic and SPR properties [18], [19], [20]. Structural and quantitative phase analysis of these samples is reported previously [19]. Here, the aim was to obtain a quantitative description of the nanopowders’ microstructure, using diffraction data analysed by whole powder pattern modelling (WPPM), considered to be a state-of-the-art methodology [21], [22], [23], [24], [25], [26].
Section snippets
Sample preparation
As we reported previously [18], [19], aqueous titanium(IV)hydroxide sols were made from the acid-catalysed peptisation of hydrolysed titanium(IV)isopropoxide (Ti-i-pr, Ti(OCH(CH3)2)4). The dried gels were thermally treated at 450 and 600 °C in a static air flow furnace – heating rate from room temperature (RT) to the desired temperature at 5 °C min−1, followed by 2 h soaking time, with natural cooling. Undoped titania samples were referred to as Ti450, and Ti600, where the number represents the
Microstructural analysis
The WPPM results are depicted in Fig. 1, Fig. 2, Fig. 3, and in Table 1, Table 2. Samples fired at 450 °C are composed of anatase, rutile, brookite and amorphous phase, in different amounts; at 600 °C, the TiO2 polymorphs present are anatase and rutile, together with amorphous phase (Table 3 and [19]).
The unit cell parameters of anatase and rutile are virtually identical for the whole set of samples, at both firing temperatures (Table 1). The exception to this was anatase (and to a much lesser
Conclusions
The microstructure of titania nanopowders, modified with W, Ag, and W/Ag was characterised via the WPPM method. After firing at 450 °C, the addition of W, Ag, and W/Ag led to a general decrease of anatase and rutile mean domain sizes; whereas after firing at 600 °C there was a common increase of the domain size. An overall linear dependence of the lattice volume expansion, with the lowering of the crystalline domain size (an inverse relationship of cell volume to crystallite size) was observed.
Acknowledgements
M. Ferro and RNME – University of Aveiro, FCT Project REDE/1509/RME/2005 – are acknowledged for HR-TEM analysis. Authors also 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.
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