Colloidal stability of nanosized titania aqueous suspensions

Abstract

In this work the colloidal behaviour of three different TiO₂ nanopowders in water is studied. A commercial powder of anatase and another of rutile were used for this study. For comparison purposes, a cryogel of anatase synthesised by a particulate sol–gel route and freeze-dried was also studied. All three powders were characterised by scanning electron microscopy, specific surface area measurements and X-ray diffraction. Diluted aqueous suspensions were prepared and characterised in terms of particle size distribution and zeta potential, using dynamic light scattering and laser Doppler velocimetry principles, respectively. All suspensions were prepared using an ultrasounds probe for mixing times ranging from 0 to 5 min. Colloidal stability was studied as a function of pH, type and concentration of dispersants (polyacrylic-based deflocculant and citric acid) and mixing time. Stable suspensions of commercial nanosized powders were obtained with polyelectrolyte contents of 1.0–1.5 wt.%. No stable suspensions of the cryogel were obtained with polyelectrolyte, requiring in this case the use of citric acid as deflocculant. It was observed that neither the size distribution nor the zeta potential values were affected by the sonication time.

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.