Colloids and Surfaces A: Physicochemical and Engineering Aspects
Preparation and photoactivity of nanostructured TiO2 particles obtained by hydrolysis of TiCl4
Introduction
Photocatalytic processes are rapidly developing as potential techniques for the purification of wastewaters [1], [2]. Due to its (photo)stability and low cost, the most used photocatalyst is titanium dioxide [3] but, unfortunately, most of the TiO2 powders show low values of quantum efficiency [4]. The photoactivity strongly depends on several variables as preparation method, particle size, reactive surface area, ratio between anatase and rutile phases [5], [6]. A way to improve the activity of a photocatalyst is to increase its specific surface area and to decrease the size of the particles.
TiO2 nanoparticles have attracted much attention because of the novel electronic and optical properties originating from the quantum confinement [7], [8]. By decreasing particle size, the band gap of the semiconductor becomes larger as indicated by an absorption shift to shorter wavelengths [9]. The shift of the conduction band to more negative potentials and of the valence band to more positive potentials may favour redox processes that cannot occur in bulk materials [10], [11].
TiO2 nanoparticles have been employed for the photodegradation of various noxious species in aqueous systems [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23] and in gas phase [24], [25], [26]. In most cases, the samples have revealed a high photoactivity compared with that of many commercial samples [21], [22], [23], [24], [25], [26]. The favourable characteristics of the photocatalysts are related to the small size of the crystallites even if in most cases the samples did not consist of discrete nanoparticles but rather of nanostructured particles.
The sol–gel method [27], [28] is widely employed for the preparation of nanoparticles, due to the inexpensive equipment required and the low temperatures involved. The properties of the sol–gel derived samples are strongly dependent not only on the composition, but also on the preparation conditions including the starting materials and solvents, the solution preparation sequence and various other processing conditions as [precursor]/[solvent] ratio, temperature, stirring and aging time. The TiO2 particles obtained by this method are amorphous in nature and require temperatures higher than 623 K to realize the transition from amorphous to anatase phase. Unfortunately, the high calcination temperatures give rise to an increase of the size of the nanoparticles and to a decrease of their specific surface areas [14], [29].
Recently, various nanostructured TiO2 catalysts have been prepared by hydrolysis of titanium isopropoxide or titanium tetrachloride [20]. The samples derived from TiCl4 were the most photoactive and neither filtration nor calcination was needed to obtain a highly efficient anatase phase. The preparation of nanostructured TiO2 photocatalysts using TiCl4 as the precursor appears worthy of attention since this compound does not give rise to formation of organic impurities in the final products. It should be noticed, however, that carbon modified TiO2 samples prepared by hydrolysis of TiCl4 with tetrabutylammonium hydroxide have demonstrated a superior photocatalytic activity under direct artificial and diffused natural light [30].
This paper describes the preparation of efficient nanostructured TiO2 particles obtained by controlling the hydrolysis of TiCl4 in pure water. The samples were characterised by means of X-ray diffractometry (XRD), determination of BET specific surface area (SSA), scanning electron microscopy (SEM) observations and diffuse reflectance spectroscopy (DRS). 4-Nitrophenol (4-NP) photodegradation was employed as a probe reaction to test the photocatalytic activity of the nanostructured particles.
Section snippets
Catalyst preparation
Titanium tetrachloride (Fluka 98%) was used as starting material without any further purification. The experimental procedure for the preparation of the various nanostructured TiO2 catalysts is schematically shown in Scheme 1. TiCl4 was slowly added to distilled water (volume ratios: 1:1, 1:5 or 1:10) at room temperature. The hydrolysis reaction was highly exothermic and produced high quantities of fumes of HCl. After ca. 10 h of continuous stirring, the resulting clear solution was boiled for 2
X-ray diffraction
Fig. 1 shows the X-ray diffraction patterns of some samples prepared by hydrolysis of TiCl4. The powders were obtained by drying at room temperature the TiO2 suspensions or dispersions formed after boiling.
The diffractograms indicate always the presence of a badly crystallised anatase phase although small amount of rutile can be noticed in samples prepared by using high TiCl4/H2O volume ratios. The comparison of the XRD patterns reveals that a better crystallinity was obtained when the TiCl4/H2
Discussion
TiCl4 has been often employed as a precursor for the preparation of nanosized TiO2 particles [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]. Calcination [33], [34], [35] or hydrothermal treatments [38], [39], [40], [41], [42] were normally used to improve the crystallinity of the powders.
In this work, nanostructured TiO2 catalysts were prepared by controlling the hydrolysis of TiCl4 in aqueous solutions. The properties of the various samples depended on the preparation conditions,
Conclusion
Nanostructured TiO2 catalysts were prepared by hydrolysis of TiCl4. The resulting aqueous orthotitanic acid solutions were boiled to form titanium dioxide suspensions. The very mild experimental conditions allowed to obtain samples containing badly crystallized anatase with the crystallite size depending on the TiCl4/H2O ratio.
The results indicated that the photoactivity of the samples mainly depended on a compromise between crystallinity and particle size. In fact, the beneficial role of an
Acknowledgments
The authors thank MIUR (Rome) for financial support. This work has been carried out in the framework of the Italian Interuniversity Consortium “Chemistry for the Environment” INCA.
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