Surface Sol Gel Synthesis

Nanosized particles of cellulose-based oxides with large surface areas, enhanced stability have been obtained following a surface sol-gel procedure (Caruso and Antonietti 2001): ZrO2 (Huang and Kunitake 2003; Caruso and Schattka 2000), SnO2 (Huang et al. 2005), SnxInyOz (Aoki et al. 2006), TiO2 (Huang and Kunitake 2003 ; Caruso and Schattka 2000)Zand Au-TiO2 (Huang et al. 2004). The oxide particles are hierarchically organized into nanostructures, the mechanism of the adapted surface sol-gel method being represented in Fig. 5.6.

A modified sol-gel method involving cellulose acetate membranes as template, was developed by Caruso and Schattka 2000. Two different strategies were employed: (i) the soaking method, in which homogenous TiO2 and ZrO2 films with small specific surface area (about 22 m2 g-1), but with a substantial porosity have been obtained (ii) the filtering method (Holland et al. 1998) which is a quicker process than the first case, resulting materials with an increased surface area (about 74 m2 g-1) but with smaller pores.

Rod-like cellulose nanocrystals, known as cellulose whiskers act as versatile templates in oxides synthesis (Revol et al. 1994). Depending on cellulose concentration the suspensions are highly, partially or randomly ordered. The type of suspensions used as templates in oxides synthesis tailors the morphology of the obtained oxide: mesoporous, nanotubes and nanowires, respectively. Silica oxides represent an illustrative example. Thus, using liquid crystalline cellulose nanorod gels, accurate silica replicas of the cellulose mesostructure template were obtained, the chiral mesoporous silica having co-aligned cylindrical pores of ca. 15 nm in diameter and wall thickness of 10 nm (Dujardin et al. 2003). On the other hand, randomly suspensions of cellulose whiskers have led, depending on the heating treatment, either to silica nanotubes (600°C) or nanowires (900°C) (Scheel et al. 2009).

The morphology of the oxides could be controlled also through the experimental conditions of the sol gel synthesis, mainly the pH value. This is the case of tungsten oxide (Na2W2O7) (Chai et al. 2007) obtained in the presence of cellulose material which function as template in an one pot sol-gel method: nanotubes of 50 nm diameter and 400 nm length were produced at a pH=6, while at pH = 3 oxide nanoparti-cles with sizes of 1-10 nm are assembled into a dendritic structure.

Hydroxypropyl cellulose (HPC) is a soluble cellulose derivative. This property together with its ability to convert itself into a liquid crystalline phase makes it a very useful organic additive in oxides synthesis. The data extracted from literature (Shimamura et al. 1981; Wojciechowski 2000) mentions that the HPC solutions are thermotropic (temperature dependent order, phase transition temperature 160-180°C) or lyotropic (concentration depended order) liquid crystalline mesophases. The anisotropic order of the HPC crystalline liquid could be exploited in template synthesis of the oxides, but some cautions should be taken into account (for example the control of the temperature and concentration) in order to avoid the crystalline liquid mesophase transformation.

Concentration restrictions of HPC template have been applied in the synthesis of mesoporous titania films (Zhao et al. 2005; Halamus and Wojciechowski 2007). Titania-HPC composite films containing oxide particles with low sizes, large specific surfaces enhanced photocatlytic activity and photo-induced hydrophlicity have been synthesized by keeping the concentration under a critical value (Zhao et al. 2005). Part of composite photocatalytic activity could be attributed to the hydroxy-propyl cellulose, with an intrinsic photoactivity due to the presence the isopropanol groups. After the composite calcinations at 550°C, mesoporous nanocrystalline anastase TiO2 particles with 30-40 nm in diameter were obtained. The temperature control in an adapted sol-gel method in a polysaccharide template synthesis of ZnO nanocrystalline films is useful to prevent the phase transformation of the HPC (Kamalasanan and Chandra 1996; Bobowska et al. 2008). The synthesis was performed at 160°C in order to accelerate the condesation process that grows the ZnO nanoparticles into the HPC matrix and also to disable the phase transformation. Dense and nanocrystaline ZnO films obtained after a calcination treatment at 500°C of the ZnO/HCP composites have shown a good transparency for the visible light and high absorbance for UV light.

Nano (<100 nm) and submicron (>100 nm) fibers of HPC, previously obtained by electrospinning approaches were used as template during an in situ or post synthesis sol gel procedures. ZnO nanorods were perpendicularly grown on a hybrid ultrafine fiber of polyethylene oxide/cellulose acetate following a direct in situ elec-trospinning method (Changangam et al. 2009) . A post synthesis example is the preparation of nano and submicron SnO2-HPC fibers composite with a reduced typical "bead-on-string" morphology of the HPC fibers (Shukla et al. 2005). Subsequent annealing treatment at 450°C has led to a porous network of nano and submicron fibers of SnO2.

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