This thesis is about nanostructured metal oxides, their properties, and some of their applications. Semiconducting metal oxides like TiO₂, ZnO,and SnO₂ have a wide band gap, which means they absorb UV light andgenerate electron-hole pairs. These charge carriers can be harnessed andused for a variety of purposes, such as electricity generation in solar cells,hydrogen production by means of photolysis and electrolysis, andenvironmental remediation by mineralizing pollutants in photocatalyticreactions. However, they are typically not very efficient when comparedwith e.g. noble metals in catalysis or silicon in solar cells , and so a widevariety of strategies have been employed to remedy their weaknesses.Such strategies include structuring the materials at the nanoscale, and thefabrication of composite materials and heterostructures.In this work, some advanced hybrid materials have been studied,composed of metal oxide and various additives, such as reduced grapheneoxide (rGO), other metal oxides, and flavonoids. The materials have beenextensively characterized in order to determine how these additives affectthe processes going on in some of the mentioned applications. Thestudied systems include rGO-ZnO, SnO₂-ZnO, Rh-TiO₂, MoO₂, SiO₂coupled to 3-hydroxyflavone and 7-hydroxyflavone, and 3-hydroxyflavone-TiO₂.Particles of ZnO-encapsulated rGO exhibited a good photocatalyticactivity towards the degradation of rhodamine B and phenol, but it wasfound that the main determinant of the performance was the quality ofthe semiconductor component as opposed to any favorable interactionsbetween ZnO and rGO. However, the incorporation of rGO could stillalmost double the observed performance, which was attributed to apassivation of the defects in the metal oxide host, as well as a beneficialimpact of electrochemical properties such as charge transfer resistance anddouble-layer capacitance of the resulting material.Core-shell nanoparticles consisting of a SnO₂ core and a ZnO shell weresuccessfully synthesized and employed as photocatalyst and as photoanodein dye-sensitized solar cells (DSSCs). The ZnO shell improved theperformance in both photocatalysis and DSSCs by nearly a factor of two,due to a combination of the favorable properties of the two metal oxides ,and the formation of a heterojunction in the interface between them.Rhodium as an additive to TiO₂ nanocrystals proved to effectivelyimprove the response in gas sensing experiments. The rhodium exhibitedcomplex speciation, however, being distributed as a homogeneouscoating of Rh(III) as well as nanocrystals of elemental rhodium,highlighting the need for deep characterization in this class of materials.Metallic MoO₂ nanocrystals were synthesized and tested in photocatalysis.Due to their electronic nature, they cannot support photocatalysisaccording to the traditional reaction scheme, because metals cannotgenerate electron-hole pairs. However, they still exhibited significantphotocatalytic activity towards methylene blue, rhodamine B, andparacetamol. This was attributed to a direct sensitization mechanismwhere the dye is photoexcited and undergoes electron transfer, madepossible due to the comparatively low work function in MoO₂. This alsoenables it to assist in the degradation of non-absorbing molecules in thesolution. 3-hydroxyflavone (3HF) and 7-hydroxyflavone (7HF) were combinedwith MCM-41 silica nanoparticles via a post-doping procedure, and theirphotophysics characterized by steady-state and time-resolvedspectroscopic techniques. Both flavonoid-coated nanoparticles turned outto be highly fluorescent and stable when exposed to air at roomtemperature, showing that organic fluorophore-based solid-state emitterscan be obtained by simple methods. Furthermore, 3HF was coupled toTiO₂ nanoparticles with a similarly simple adsorption procedure. In thiscase the result was a chemisorption of the flavonoid, which appears to bevery similar to a chelation of the metal ions in the metal oxide substrate.The fluorescence in the resulting materials is nearly completely quenched,but when a nanometer-thin layer of Al₂O₃ is applied on the TiO₂, it isinstead strongly enhanced. This work therefore represents a rather noveland facile way to produce flavonoid-metal complexes.