Titanium dioxide/cellulose nanocomposites prepared by a controlled hydrolysis method

Abstract

TiO₂/cellulose nanocomposites were prepared through the titanyl sulphate hydrolysis in acidic medium in the presence of cellulosic fibres. The influence of several reaction parameters on the morphological characteristics of the nanocomposites was investigated. There is evidence from this study that in specific experimental conditions, the cellulose fibres promote the nucleation and growth of TiO₂ particles, yielding hybrid materials containing up to 46% TiO₂. Two series of paper handsheets having distinct TiO₂ content have been prepared, one from a selected hybrid composition and the other from mixtures of commercial TiO₂ and cellulose fibres. Comparative optical studies performed on the paper handsheets revealed a much higher opacity for the synthetic sample.

Introduction

Hybrid inorganic/organic materials constitute an interesting new class of functional nanocomposite materials. These materials can show improved optical, thermal and mechanical properties, due to synergistic effects resulting from the physical or chemical interactions which occur between the inorganic and organic components [1]. Although in the last years, considerable work has been reported on the preparation of mineral nanoparticle-reinforced and nanofiber-reinforced (including cellulose) polymer matrix nanocomposites [2], the literature dealing with the preparation of hybrid nanocomposites based on natural cellulose (the most abundant polymer in earth) and mineral nanoparticles is quite scarce [3], [4], [5], [6]. Very recently, we have shown that cellulosic fibres can act as efficient hydrophilic substrates to the nucleation and growth of inorganic pigment particles in aqueous medium [7]. In part, such an approach has been inspired by the fact that paper can be regarded as a composite material containing cellulose fibres and mechanically deposited or mixed mineral pigment particles, such as CaCO₃, kaolin and TiO₂ [8], [9].

Besides its application in papermaking, TiO₂ is widely used as a white pigment in paints, solar cells, ceramics, photocatalysis and supports [8], [9], [10], [11], [12]. When used in paper materials, TiO₂ improves significantly the brightness and opacity of the final product, because of its high refraction index, when compared to cellulose fibres or other fillers [8], [9]. In all these applications, the performance of TiO₂ pigments is strongly dependent on the polymorph used (rutile or anatase), the morphology and the particle size [8], [9], [13], [14]. In this context, nanocrystalline TiO₂ particles are of particular interest because of their specifically size related properties such as surface area, band gap tuning and light scattering coefficient [8]. However, nanosized TiO₂ pigments, mechanically mixed with fibres, are poorly retained, unless agglomeration is promoted by means of polyelectrolytes or other retention aids [8], [9], thus decreasing the optical efficiency of the pigment. One way to overcome this limitation would be the deposition of the pigment in the fibre cell wall, by generating TiO₂ in situ in the presence of the cellulose fibres (TiO₂/cellulose nanocomposite), a strategy that was investigated here.

The synthesis of nanosized titania has been the subject of many research groups. Much attention has been paid to hydrothermal methods using titanium salts [6], [7], [10] and sol–gel methods using titanium alkoxides [15], [16], [3] to produce fine and spherical powders of uniform size. In the present study, TiO₂/cellulose nanocomposites have been prepared through the controlled hydrolysis of TiOSO₄ in the presence of cellulose fibres. This is a rather inexpensive chemical method which can be potentially implemented in an industrial environment using conventional technology. In fact, a large percentage of commercial TiO₂ pigments are produced using the so-called sulphate process, which involves the hydrolysis of TiOSO₄ as an intermediate step.