Transition metal-doped semiconductor materials are extensively employed for light harvesting and photocatalytic applications owing to their increased light absorption and charge mobility. In this work, spatial tailoring of the Ni dopant in TiO 2 nanostructures is performed by varying the secondary processing parameters to engineer the resulting optoelectronic properties for select applications. Specifically, the aging of the dried Ti sol and the resulting Ni segregation are observed to be moisturedriven phenomena based on the infrared and time-resolved UV−vis spectroscopy measurements. While X-ray diffraction and scanning transmission electron microscopy coupled with electron energy-loss spectroscopy characterizations show a clear difference in the crystal structures between pristine TiO 2 powders and phase-segregated NiO− TiO 2 , the thermogravimetric measurements reveal substitution of the ethoxy group by ambient moisture, resulting in the ejection of hydroxylated Ni clusters. Furthermore, the doped system could be locked into a metastable state by rapidly annealing the amorphous powders. Finally, the photocatalytic activity of these different TiO 2 :Ni 2+ (15 mol %) nanoparticles under AM 1.5G solar light highlights the relationship between the photocatalytic activity and the dopant position. This ability to spatially control dopants within highly doped materials allows for direct control of specific optoelectronic properties, paramount for photoelectrochemical devices.

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In this work, we examined the effect of nanoparticle size on the thermal conductivity of mesoporous silica materials made from colloidal precursors. Porous thin films were synthesized using a polymer-templating method, employing commercial colloidal silica solutions containing nanoparticles 6, 9, and 22 nm in diameter as the silica source and poly(methyl methacrylate) colloids as the template. The ratio of polymer to silica was then varied to produce films with a range of porosities. The thermal conductivity of the films was measured using time domain thermal reflectance, and the results indicated that, for the particle sizes studied, there was a weak dependence of thermal conductivity on particle size. This weak dependence was associated with increased interfacial scattering of heat carriers at the boundaries of the smaller nanoparticles. This work adds to our understanding of the effect of nanostructuring on heat transport in amorphous material systems and improves our ability to design low thermal conductivity materials.

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