Particle Networks from Powder Mixtures: Generation of TiO₂–SnO₂ Heterojunctions via Surface Charge-Induced Heteroaggregation

Abstract

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.

Defects in Metal Oxide Nanoparticle Powders

“…On the other hand, we have also shown examples of microstructural transformations which can easily occur in the course of nanomaterials processing: mesoporous nanoparticle networks can emerge from solvent-mediated nanoparticle aggregation or from the calcination of nanoparticles which were previously synthesized in solution. In both cases, newly formed solid-solid interfaces may give rise to very different defect related property changes, such as the enhanced recombination of photogenerated charges in semiconducting oxides (TiO 2 , SnO 2 , ZrO 2 ,…) [172,178], to enhanced annihilation of photoexcited states (MgO) [161] or to changed oxygen depletion behavior during vacuum annealing (TiO 2 , SnO 2 , In 2 O 3 ,…) [60,173,179]. To test hypotheses about defects and interface specific functionalities it is desirable to have a system of particles with narrow distributions of size, structure and morphology.…”

“…From a materials' design perspective, however, the development of crystalline nanoparticle networks where the interpenetration of the different material components and the degree of compositional intermixing in the neck region of particle interfaces can be controlled, is key to the rational design of photoelectronically relevant heterojunctions at the nanoscale as well as for functional particle interfaces in general [176][177][178].…”

Heterojunctions in Composite Photocatalysts

“…A viable strategy is to use ultrasonication to mix particle suspensions [4]. Preformed particles are mixed in aqueous solutions, and treatment with an ultrasonic tip sonicator or in an ultrasonic bath is started.…”

“…1 Interfacial contact approach for (a) pure systems and (b) mixed particle systems to get efficiently mixed semiconductor composites. Reproduced from Siedl et al [4] with permission, ©2012 American Chemical Society Heterojunctions in Composite Photocatalysts which is advantageous for photocatalytic application [5]. However, whether ball milling leads to a more efficient intermixing compared to the ultrasonic treatment depends on the materials used.…”

“…Thus, the resulting mechanism (Fig. 12) included both charge carrier excitation in CdS and transfer of hot electrons from Direct reduction of AuCl 4 -using …”