Reactive and Organosoluble SnO2 Nanoparticles by a Surfactant-Free Non-Hydrolytic Sol-Gel Route

Nanoparticle processing: Understanding and controlling aggregation

Nanoparticles (NPs) are commonly defined as particles with size <100 nm and are currently of considerable technological and academic interest, since they are often the starting materials for nanotechnology. Novel properties develop as a bulk material is reduced to nanodimensions and is reflected in new chemistry, physics and biology. With reduction in size, a greater function of the atoms is at the surface, and promote different interaction with its environment, as compared to the bulk material. In addition, the reduction in size alters the electronic structure of the material, resulting in novel quantum effects. Size also influences mobility, primarily controlled by Brownian motion for NPs, and relevant in biological and environmental processes. However, the small size also leads to high surface energy, and NPs tend to aggregate, thereby lowering the surface energy. In all applications, the uncontrolled aggregation of NPs can have negative effects and needs to be avoided. There are however examples of controlled aggregation of NPs which give rise to novel effects. This review article is focused on the NP features that influences aggregation. Common strategies for synthesis of NPs from the gas and liquid phases are discussed with emphasis on aggregation during and after synthesis. The theory involving Van der Waals attractive force and electrical repulsive force as the controlling features of the stability of NPs is discussed, followed by examples of how repulsive and attractive forces can be manipulated experimentally to control NP aggregation. In some applications, NPs prepared by liquid methods need to be isolated for further applications. The process of solvent removal introduces new forces such as capillary forces that promote aggregation, in many cases, irreversibly. Strategies for controlling aggregation upon drying are discussed. There are also many methods for redispersing aggregated NPs, which involve mechanical forces, as well as manipulating capillary forces and surface characteristics. We conclude this review with a discussion of aggregation relevant real-world applications of NPs. This review should be relevant for scientists and technologists interested in NPs, since emphasis has been on the practical aspects of NP-based technology, and especially, strategies relevant to controlling NP aggregation.

Heteroleptic Tin(IV) Aminoalkoxides and Aminofluoroalkoxides as MOCVD Precursors for Undoped and F-Doped SnO2 Thin Films

A series of asymmetric and potentially bidentate amino alcohols and amino fluoro alcohols (R_NOH) having a different number of methyl/trifluoromethyl substituents at the α-carbon atom, [HOC(R¹)(R²)CH₂NMe₂] (R¹ = R² = H (dmaeH); R¹ = H, R² = CH₃ (dmapH); R¹ = R² = CH₃ (dmampH); R¹ = H, R² = CF₃ (F-dmapH); R¹ = R² = CF₃ (F-dmampH)) have been used to develop new monomeric and heteroleptic tin(IV) amino(fluoro)alkoxides [Sn(OR)₂(OR_N)₂] (R = Et, Prⁱ, Bu^t). These new complexes, which were thoroughly characterized by spectroscopy (IR and multinuclei NMR (¹H, ¹³C, ¹⁹F, and ¹¹⁹Sn)) as well as single-crystal X-ray studies on representative samples, were investigated for their thermal behavior to determine their suitability as MOCVD precursors for the deposition of metal oxide thin films. The two most suitable compounds, [Sn(OBu^t)₂(dmamp)₂] and [Sn(OBu^t)₂(F-dmamp)₂], were used in a direct liquid injection chemical vapor deposition (DLI-CVD) process to deposit undoped SnO₂ and F-doped SnO₂ thin films, respectively, on silicon and quartz substrates. Film growth rates at different temperatures (from 400 to 700 °C), film thickness, crystalline quality, and surface morphology were investigated. The films deposited on quartz showed high transparency (above 80%) in the visible region and low carbon contamination on the surface (11-13% from XPS), which could easily be removed completely with 2 min of Ar⁺ sputtering.

Tungsten oxide by non-hydrolytic sol–gel: effect of molecular precursor on morphology, phase and photocatalytic performance

The synthesis, characterisation and photocatalytic water splitting activity of blue tungsten oxide materials prepared from W(vi) and W(iv) precursors is reported.

Mesoporous SnO2–SiO2 and Sn–silica–carbon nanocomposites by novel non-hydrolytic templated sol–gel synthesis

A novel non-hydrolytic sol–gel (NHSG) synthesis of mesoporous tin silicate xerogels is presented.

Hybrid organotin and tin oxide-based thin films processed from alkynylorganotins: synthesis, characterization, and gas sensing properties

Hydrolysis-condensation of bis(triprop-1-ynylstannyl)butylene led to nanostructured bridged polystannoxane films yielding tin dioxide thin layers upon UV-treatment or annealing in air. According to Fourier transform infrared (FTIR) spectroscopy, contact angle measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM) data, the films were composed of a network of aggregated "pseudo-particles", as calcination at 600 °C is required to form cassiterite nanocrystalline SnO2 particles. In the presence of reductive gases such as H2 and CO, these films gave rise to highly sensitive, reversible, and reproducible responses. The best selectivity toward H2 was reached at 150 °C with the hybrid thin films that do not show any response to CO at 20-200 °C. On the other hand, the SnO2 films prepared at 600 °C are more sensitive to H2 than to CO with best operating temperature in the 300-350 °C range. This organometallic approach provides an entirely new class of gas-sensing materials based on a class II organic-inorganic hybrid layer, along with a new way to include organic functionality in gas sensing metal oxides.