Nanoporous metal oxides templated by nanocrystalline cellulose

Porous materials play an important role in numerous environmental applications including energy storage, energy conversion and environmental remediation systems. Reducing structural features down to the nanoscale drastically alters materials properties and leads to the enhancement of materials performance. The successful fabrication of efficient functional materials requires a high degree of control over their morphology addressing the needs of target applications. The goal of this work was to develop a versatile general approach towards the synthesis of nanoporous metal oxides by using biogenic cellulose nanocrystals. Nanocrystalline cellulose (NCC) is an abundant biological nanomaterial that can be extracted from natural bulk celluloses. The present thesis demonstrates that the unique properties of NCC enable the efficient synthesis of porous titania and iron oxide (hematite) thin films by using sacrificial templating with cellulose nanocrystals. In particular, this study reveals the mechanism of metal oxide formation in the presence of cellulose, as well as the effect of NCC-templated porous scaffolds on titania performance in photocatalysis and dye sensitized solar cells. Chapter 1 provides general information about properties, application areas and common synthesis methods of nanoporous metal oxides, with an emphasis put on titanium oxide materials and biotemplating approaches. Chapter 2 discusses the basic principles of analytical methods employed to characterize porous nanomaterials. Chapters 3‒6 reveal the experimental procedures towards NCC-templated porous titania and hematite thin films, their characterization and their applications. First, the extraction of cellulose crystals from bulk celluloses is discussed. Different cellulose sources, as well as variable hydrolysis parameters have been employed to define the optimal procedure for the NCC preparation. Cotton fibers have provided the best results regarding the crystallinity, purity and shape of extracted cellulose crystals. Furthermore, repeated washings have been shown to narrow down the size distribution and to improve the crystallinity of cotton NCC. Chapter 4 focuses on the synthesis of porous titania thin films assisted by nanocrystalline cellulose. The tunable porosity of titania thin films is a key factor for successful applications in photovoltaics, sensing and photocatalysis. To synthesize NCC-templated titania, the cellulose nanocrystals are introduced to a titania precursor solution. The colloidal mixtures can be directly spin- or dip- coated on glass, silicon and transparent conducting oxide (TCO) substrates and then calcined to remove the template and to crystallize the titania porous network. The obtained structures are highly porous anatase morphologies having well-defined, narrow pore size distribution. We show that by varying the titania-to-template ratio it is possible to tune the surface area, pore size, pore anisotropy and dimensions of titania crystallites in the films. Post-treatment at high humidity and subsequent slow template removal promote pore widening; this treatment is also beneficial for the multilayer deposition of thick films. The NCC-templated mesoporous titania films show very high activity in the photocatalytic NO (nitrogen(II) oxide) conversion and in the degradation of 4-chlorophenol. Furthermore, the films are successfully applied as anodes in dye-sensitized solar cells. Chapter 5 presents a strategy toward enhancement of the photocatalytic activity of NCC-templated titania thin films by introducing solvothermally synthesized preformed anatase nanoparticles into a sol-gel based biotemplated titania scaffold. The synthesis is based on the self-assembly of two types of precursors, namely crystalline and sol-gel titania, directed by the biogenic NCC template. Due to the shape persistence of the template, the NCC-templated titania scaffolds can accommodate large amounts of preformed titania without a significant reduction of the film porosity. The resulting dual source titania thin films containing different amounts of preformed crystalline species were investigated with time resolved microwave conductivity (TRMC) measurements and tested in the photocatalytic conversion of 4-chlorophenol. The gradual addition of preformed nanoparticles leads to a consistent increase of the mean size of titania crystalline domains, whereas the porosity of the composite is well-preserved due to the rigid nature of the NCC template. The microwave conductivity studies establish increased photoconductivity of the films containing preformed anatase nanoparticles, in comparison to that of films made without the nanoparticles. The synergistic features of the dual source titania, namely the improved crystalline properties brought by the preformed nanocrystals in combination with the high surface area provided by the NCC-templated sol-gel titania, result in a very high photocatalytic activity of the films in the photocatalytic decomposition of 4-chlorophenol. In quantitative terms, the dual source titania films prepared with 75% nanoparticles exhibit a first order degradation rate constant of 0.53 h-1, strongly outperforming the activity of commercial P90 nanopowder showing a rate constant of 0.17 h-1 under the same conditions. We have also adapted the NCC templating protocol for the fabrication of porous iron oxide (hematite) thin films. Chapter 6 discusses the formation of porous iron oxide nanostructures via sol-gel transformations of molecular precursors in the confined space of self-organized cellulose nanocrystals used as a shape-persistent template. The obtained structures are highly porous hematite morphologies featuring pronounced anisotropic porosity. The character of the porous nanostructure depends on the iron salt used as precursor and on the heat treatment, respectively. Moreover, a post-synthetic hydrothermal treatment of the NCC/iron salt composites strongly affects the crystal growth, as well as the porous nanomorphology of the obtained hematite scaffolds. We demonstrate that the hydrothermal treatment alters the crystallization mechanism of the molecular iron precursors, which proceeds via the formation of anisotropic iron oxyhydroxide species. The present study reveals that the nanocellulose templating technique enables a straightforward fabrication of a variety of porous crystalline scaffolds with well-defined mesoporous structure. For the first time the NCC has been used for the fabrication of homogeneous porous metal oxide films on different substrates, in contrast to the previously reported powders or free-standing membranes. The versatility and flexibility of the NCC templating approach offers broad perspectives towards the generalization of this method for the fabrication of different types of nanoporous metal oxides.