1. IntroductionRecent improvements

1. Introduction
Recent improvements in the photocatalytic performance of titanium dioxide have focused on controlling particle size, phase composition, particle shape and effective visible light absorption. Photocatalysts, produced by new synthesis methods, are being designed with high levels of crystallinity and small particle size in order to maximize the photocatalytic decomposition of selected organic model pollutants. Ryu and Choi [1] found that among many good commercially available photocatalysts, the flame-made Degussa P25 titanium dioxide showed the best photocatalytic performance under UV irradiation for the majority of organic substrates in aqueous solutions. P25's activity was only exceeded by some commercially available photocatalysts (Hombikat UV100 and Ishihara ST-01) and this is attributed to their higher specific surface area particularly when carboxylic acids (dichloro acetic acid, formic acid) or anionic substrates (acid orange 7, sodium dodecylsulphate) were studied [1]. Flame-made titania photocatalysts synthesized on laboratory scale were found to be very efficient for poorly adsorbing substrates, such as phenols [2], [3] and [4]. It was recently demonstrated by Ohtani et al. [5] that the high activity of Degussa P25 TiO2 is not related to the rutile content as previously suggested. The effective UV catalytic process for the weakly adsorbing substrates is attributed to the high anatase content with relatively large nanoparticles [1]. Our novel sol–gel synthesis method combines the rapid heating and short exposure in the furnace (RHSE) of amorphous titanium oxide-hydroxide precipitate and produces highly efficient titania particles in 30–40 nm diameter range [6]. These sol–gel derived photocatalysts are very hydrophilic like the best flame-made titania photocatalysts.

The deposition of noble metal (Pt, Au, Ag) on titania was demonstrated to be an efficient method for enhancing the photocatalytic decomposition of certain organic substrates under UV irradiation [7], [8], [9], [10] and [11] and recent studies demonstrated that similar enhancement could occur even under visible light irradiation [12] and [13]. This post-synthetic modification significantly changes the decomposition mechanism as the presence of the gold and platinum metal nanoparticles can result in the production of hydrogen [7].

Nitrogen [14], [15], [16], [17], [18], [19], [20], [21], [22], [23] and [24] and iron [25], [26], [27], [28], [29], [30], [31], [32], [33] and [34] are the most efficient and commonly used dopants in TiO2 photocatalysts and are usually introduced into the system during the hydrolysis steps. Ammonia gas [22], [24], [35], [36] and [37], ammonium salts [6] and [15], urea [38], [39] and [40] are frequently used to incorporate nitrogen during the synthesis as it can be incorporated in the crystal lattice by oxygen substitution [16], [23] and [38].

Iron(III)-salts are used during the hydrolysis with the iron(III) ions possibly replacing titanium(IV) ions [26]. Nagaveni and Hegde [33] prepared W, V, Ce, Zr, Fe, and Cu ion-doped anatase TiO2 nanoparticles by a solution combustion method and found that the solid solution formation was limited to a narrow range of concentrations of the dopant ions. In our previous studies [25] and [34] XANES measurements showed that the local structure of iron systematically changes with the variation of the dopant concentrations: at higher Fe-contents, hematite- or goethite-like environments were observed, consistent with the formation of separate X-ray amorphous Fe(III)-containing phases.

Iodine [41], [42], [43], [44], [45], [46] and [47] can be incorporated in the crystal structure as I5+ cation [41] or could exist as I7+ and I− on the surface [43]. Usseglio et al. found that molecular iodine can modify titania [46]. Iodine doping was found to be efficient for the photocatalytic degradation of different substrates such as phenol [41], [44] and [45] and methylene blue [42] in liquid phase and acetone in gas phase [43] under visible light irradiation.

The main aim of this article is to provide a comparative study of the best sol–gel made and flame synthesized bare titania nanoparticles and to compare the effects of iodine, nitrogen, iron doping, and silver and gold deposition under both UV and visible light irradiation for phenol and oxalic acid decomposition. These results will be compared with commercially available photocatalysts, such as Aldrich anatase (TiO2-AA) and Aeroxide (former Degussa) P25 (TiO2-P25). Phenol was selected for this comparative study as a representative model contaminant which is a widely used aromatic compound [41], [44], [45], [47], [48] and [49].

The other model organic substrate was oxalic acid which is a well characterized [10], [30], [50], [51], [52], [53], [54], [55] and [56] strongly adsorbing molecule (unlike phenol) which models organic contaminants which chemisorbs onto titanium dioxide [50] and [51]. Oxalic acid is a common oxidation by-product from the mineralization of organic molecules therefore its decomposition is required for the complete purification of the polluted water. Oxalic acid is also a very good electron acceptor therefore it can be applied as sacrificial agent very efficiently for hydrogen production on noble metal/titania composite samples [7] and [50].