Layered titanate nanostructures offer promising photoelectronic properties that are subject to surface chemistry-induced morphology changes. For a systematic evaluation of the bulk and surface contributions to the photoactivity of these structures, we investigated their photoelectronic properties and in particular their dependence on the condition of the gas-solid interface. We comprehensively explored the stability of Na(2)Ti(3)O(7) nanowires and scrolled up H(2)Ti(3)O(7) nanotubes by means of transmission electron microscopy, Raman, and FT-IR spectroscopy and subjected both titanate sheet-based structures to controlled thermal activation treatment under high vacuum conditions. We found that throughout thermal annealing up to T = 870 K the structure and morphology of Na(2)Ti(3)O(7) nanowires are retained. Consistent with the significant photoluminescence emission that is attributed to radiative exciton annihilation in the bulk, UV-induced charge separation is strongly suppressed in these structures. H(2)Ti(3)O(7) nanotubes, however, undergo transformation into elongated anatase nanocrystals during annealing at temperatures T >OR= 670 K. Photoexcitation experiments in O(2) atmosphere reveal that these structures efficiently sustain the separation of photogenerated charges. Trends in the abundance of trapped holes and scavenged electrons were characterized quantitatively by tracking the concentration of paramagnetic O(-) and O(2)(-) species with electron paramagnetic resonance spectroscopy EPR, respectively. An incisive analysis of these results in comparison to those obtained on airborne anatase nanocrystals underlines the critical role of surface composition and structure on charge separation and, in consequence, on the chemical utilization of photogenerated charge carriers.

To include particle attachment and porosity of nanostructured materials in the discussion of their electronic properties is critical to our understanding of charge transfer across grain boundaries. We report the condensation of isolated TiO(2) nanocrystals via the application of a simple hydration-dehydration cycle. After contact with water and subsequent removal of adsorbed water, these nanocrystals form a mesoporous structure with altered properties as compared with the original material: first, the energy needed for defect formation is substantially reduced, and second, electron paramagnetic resonance measurements reveal the presence of polarizable conduction band electrons not detectable in samples which have not been in contact with water.

Aiming at a comparison of microstructure and paramagnetic properties of mesoporous TiO(2) nanoparticle networks, we subjected entirely different TiO(2-x) precursor structures to vacuum annealing. The transformation of an amorphous TiO(2-x) gel--obtained by sol-gel processing of an ethylene glycol-modified titanium precursor--into a network of interconnected anatase nanocrystals was explored by means of X-ray diffraction, nitrogen sorption, and electron microscopy. Crystalline junctions between the particles emerge from temperature treatment. This process of particle network formation is different from that related to the vapor phase grown anatase nanocrystals where particle-particle interface formation is induced by contact with water. It was found that, after annealing up to 873 K and controlled sample purification in oxygen atmosphere, both types of samples exhibit high concentrations of particle-particle interfaces and comparable properties in terms of surface area, porosity, and microstructure. With electron paramagnetic resonance (EPR) we observed on nonstoichiometric TiO(2-x) networks an identical type of subsurface defect which is related to the presence of solid-solid interfaces.

On TiO2 nanocrystal powders, pressure-induced consolidation and subsequent annealing generates mesopores inside the pellet and leads to the depletion of specific surface area. This loss corresponds to the introduction of particle–particle interfaces and grain boundaries. In previous work, they were found to be susceptible to facilitated lattice oxygen depletion as compared to free particle surfaces during vacuum annealing. For the first time, we determined the adsorption of molecular oxygen on these consolidated and nonstoichiometric nanocrystal samples. As a function of repeated cycles of vacuum annealing and O2 exposure, the amount of paramagnetic and diamagnetic surface oxygen species was determined using mass spectroscopy in combination with FT-IR, electron paramagnetic resonance (EPR), and UV–vis–NIR diffuse reflectance.

We explored the impact of interfacial property changes on aggregation behavior and photoinduced charge separation in mixed metal oxide nanoparticle ensembles. TiO2 and SnO2 nanoparticles were synthesized by metal organic chemical vapor synthesis and subsequently transformed into aqueous colloidal dispersions using formic acid for adjustment of the particles’ surface charge. Surface charge-induced heteroaggregation was found to yield blended nanoparticle systems of exceptionally high mixing quality and, after vacuum annealing, to extremely high concentrations of heterojunctions between TiO2 and SnO2 nanoparticles with dehydroxylated surfaces. For tracking charge transfer processes across heterojunctions, the photogeneration of trapped charge carriers was measured with electron paramagnetic resonance (EPR) spectroscopy. On blended nanoparticles systems with high concentrations of SnO2–TiO2 heterojunctions, we observed an enhanced cross section for interparticular charge separation. This results from an effective interfacial charge transfer across the interfaces and gives rise to substantially increased concentrations of electrons and hole centers. The here presented insights are key to the rational design of particle-based heterojunctions and mesoporous nanoparticle networks and help to engineer composite nanomaterials for photocatalysis and solar energy conversion.