Volcanic Eruption in the Nanoworld: Efficient Oxygen Exchange at the Si/SnO2 Interface.

Here, a phenomenon of efficient oxygen exchange between a silicon surface and a thin layer of tin dioxide during chemical vapor deposition is presented, which leads to a unique Sn:SiO2 layer. Under thermodynamic conditions in the temperature range of 725-735 °C, the formation of nanostructures with volcano-like shapes in "active" and "dormant" states are observed. Extensive characterization techniques, such as electron microscopy, X-ray diffraction, synchrotron radiation-based X-ray photoelectron, and X-ray absorption near-edge structure spectroscopy, are applied to study the formation. The mechanism is related to the oxygen retraction between tin(IV) oxide and silicon surface, leading to the thermodynamically unstable tin(II)oxide, which is immediately disproportionate to metallic Sn and SnO2 localized in the SiO2 matrix. The diffusion of metallic tin in the amorphous silicon oxide matrix leads to larger agglomerates of nanoparticles, which is similar to the formation of a magma chamber during the natural volcanic processes followed by magma eruption, which here is associated with the formation of depressions on the surface filled with metallic tin particles. This new effect contributes a new approach to the formation of functional composites but also inspires the development of unique Sn:SiO2 nanostructures for diverse application scenarios, such as thermal energy storage.

Spectromicroscopy Studies of Silicon Nanowires Array Covered by Tin Oxide Layers.

The composition and atomic and electronic structure of a silicon nanowire (SiNW) array coated with tin oxide are studied at the spectromicroscopic level. SiNWs are covered from top to down with a wide bandgap tin oxide layer using a metal-organic chemical vapor deposition technique. Results obtained via scanning electron microscopy and X-ray diffraction showed that tin-oxide nanocrystals, 20 nm in size, form a continuous and highly developed surface with a complex phase composition responsible for the observed electronic structure transformation. The "one spot" combination, containing a chemically sensitive morphology and spectroscopic data, is examined via photoemission electron microscopy in the X-ray absorption near-edge structure spectroscopy (XANES) mode. The observed spectromicroscopy results showed that the entire SiNW surface is covered with a tin(IV) oxide layer and traces of tin(II) oxide and metallic tin phases. The deviation from stoichiometric SnO2 leads to the formation of the density of states sub-band in the atop tin oxide layer bandgap close to the bottom of the SnO2 conduction band. These observations open up the possibility of the precise surface electronic structures estimation using photo-electron microscopy in XANES mode.

Silicon Suboxides as Driving Force for Efficient Light-Enhanced Hydrogen Generation on Silicon Nanowires.

Efficient light-stimulated hydrogen generation from top-down produced highly doped n-type silicon nanowires (SiNWs) with silver nanoparticles (AgNPs) in water-containing medium under white light irradiation is reported. It is observed that SiNWs with AgNPs generate at least 2.5 times more hydrogen than SiNWs without AgNPs. The authors' results, based on vibrational, UV-vis, and X-ray spectroscopy studies, strongly suggest that the sidewalls of the SiNWs are covered by silicon suboxides, by up to a thickness of 120 nm, with wide bandgap semiconductor properties that are similar to those of titanium dioxide and remain stable during hydrogen evolution in a water-containing medium for at least 3 h of irradiation. Based on synchrotron studies, it is found that the increase in the silicon bandgap is related to the energetically beneficial position of the valence band in nanostructured silicon, which renders these promising structures for efficient hydrogen generation.

Gas sensing properties of individual SnO2 nanowires and SnO2 sol-gel nanocomposites.

This work is an investigation of the properties of semiconductor materials based on metal oxides, their catalytic properties, and their application as gas sensors, which were shown to exhibit high sensitivity, stability, and selectivity to target gases. The aim of this work is the comparison of gas sensing properties of tin dioxide in the form of individual nanowires and nanopowders obtained by sol-gel synthesis. This comparison is necessary because the traditional synthesis procedures of small particle, metal oxide materials seem to be approaching their limit. Because of this, there is increasing interest in the fabrication of functional materials based on nanowires, i.e., quasi-one-dimensional objects. In this work, nanocrystalline tin dioxide samples with different morphology were synthesized. The gas-transport method was used for the fabrication of well-faceted wire-like crystals with diameters ranging between 15-100 nm. The sol-gel method allowed us to obtain fragile gels from powders with grain sizes of about 5 nm. By means of X-ray photoelectron spectroscopy (XPS) it was proven that the nanowires contain considerably smaller amounts of hydroxy groups compared to the nanopowders. This leads to a decrease in the parasitic sensitivity of the sensing materials to humidity. In addition, we demonstrated that the nanowires are characterized by a nearly single-crystalline structure, ensuring higher stability of the sensor response due to the unlikelihood of sample recrystallization. The results from the ammonia detection experiments showed that the ratio of the sensor response to the surface area exhibits similar values for both the individual nanowire and nanopowders-based sensor materials.