Colloidal stability of nanosized titania aqueous suspensions
Introduction
The study of nanomaterials has reached great importance in the last years, because of their huge potential applications in many fields of science. A fundamental aspect to be considered is the complexity of working with nanopowders, because of their toxicity and their strong tendency to spontaneously agglomerate due to their large surface area and surface properties.1 For this reason, the manipulation of nanopowders in wet conditions is preferred and has demonstrated to allow obtaining higher uniformity.2 At this level, the surface forces play a fundamental role on the stability of suspensions,3, 4, 5, 6, 7 as flocculation may occur spontaneously through London-van der Waals attractive forces. The tendency to coagulate can be overcome by means of electrostatic repulsions between similarly charged particles, repulsive non-DLVO forces arising from solvation of adsorbed layers, or most frequently, by an electrosteric mechanism with the use of polyelectrolytes.
Titania-based materials have attracted great attention due to their photovoltaic and catalytic activity, among other applications in different fields such as clean energy production (solar cells), self-cleaning surfaces, water treatments, etc.8, 9 For such requirements nanosized powders are used as coatings on adequate substrates. The coating processes and treatments must be integrated, as much as possible, into the actual ceramic process/practice, so contributing to exploit the high expertise level presently achieved by the ceramic industry and to make competitive the production costs. Hence, when colloidal suspensions are used to produce such coatings an in-depth study of the surface and colloidal behaviour of those powders is needed as well.
The dispersing conditions of TiO2 particles in either aqueous or non-aqueous media have been studied elsewhere using any of the above mentioned mechanisms of dispersion.10, 11, 12, 13, 14 Although the dispersion of nanosized titania powders has received some attention in the last years it is not yet enough and more work is necessary.15, 16
The aim of this work was to study the colloidal behaviour of nanosized titania powders in aqueous medium in terms of particle size distribution and ζ-potential, evaluating the effect of dispersants nature and concentration and the effect of mixing time using an ultrasounds probe. Two commercial nano-powders were selected taking into account the different crystalline phases of titanium dioxide, rutile and anatase. The synthesis of a third powder with much larger surface area prepared through a sol–gel route was studied for comparison purposes.
Section snippets
Characterisation of starting nanopowders
Three different TiO2 nanopowders were employed in this study: a commercial anatase (Inframat® Advanced Materials™, USA), which is supplied as a sprayed powder to facilitate handling, a commercial rutile (Tayca Corporation, Japan), and an anatase titania cryogel synthesised in the laboratory. The synthesis was done by a sol–gel route in which Ti(OPri)4 was hydrolysed into a flask containing a stirring mixture of H2O and HNO3 at pH 1. The final water-to-alkoxide molar ratio was 30:1. The mixture
Characterisation of starting nanopowders
X-ray diffraction (XRD) patterns of the three considered materials are shown in Fig. 1. It can be noted that the XRD patterns of the two commercial powders are highly crystalline. The XRD pattern of the synthesised cryogel presents the anatase peaks, but the signal is less intense, and the peaks are less defined, indicating lower crystallinity and/or a lower crystallite size.
Microstructural observations made by FEG-SEM for the commercial anatase-TiO2, (Fig. 2) show that the particles are
Conclusions
The colloidal behaviour of commercial nanosized powders of rutile and anatase in water has been studied, as well as that of a cryogel obtained by a sol–gel route. The isoelectric point of these powders occurs at pH values ranging from 5.0 to 6.3, in good agreement with the values typically reported in the literature. Microstructural observations demonstrate that commercial powders are formed by soft agglomerates of small particles with ∼40 nm in diameter, whereas the cryogel is formed by flakes
Acknowledgements
This work has been done as part of the activities of CECERBENCH – Laboratory of the Emilia-Romagna High Technology Network, in the frame of the plan PRRIITT Measure 3.4 Action A – Research and Technology Transfer Laboratory. This work has been also partially supported by Spanish Ministry of Education and Science (MAT2006-01038).
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