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 sign