Continuous Hydrothermal Synthesis of Inorganic Nanoparticles: Applications and Future Directions.

Nanomaterials are at the leading edge of the emerging field of nanotechnology. Their unique and tunable size-dependent properties (in the range 1-100 nm) make these materials indispensable in many modern technological applications. In this Review, we summarize the state-of-art in the manufacture and applications of inorganic nanoparticles made using continuous hydrothermal flow synthesis (CHFS) processes. First, we introduce ideal requirements of any flow process for nanoceramics production, outline different approaches to CHFS, and introduce the pertinent properties of supercritical water and issues around mixing in flow, to generate nanoparticles. This Review then gives comprehensive coverage of the current application space for CHFS-made nanomaterials including optical, healthcare, electronics (including sensors, information, and communication technologies), catalysis, devices (including energy harvesting/conversion/fuels), and energy storage applications. Thereafter, topics of precursor chemistry and products, as well as materials or structures, are discussed (surface-functionalized hybrids, nanocomposites, nanograined coatings and monoliths, and metal-organic frameworks). Later, this Review focuses on some of the key apparatus innovations in the field, such as in situ flow/rapid heating systems (to investigate kinetics and mechanisms), approaches to high throughput flow syntheses (for nanomaterials discovery), as well as recent developments in scale-up of hydrothermal flow processes. Finally, this Review covers environmental considerations, future directions and capabilities, along with the conclusions and outlook.

Photocatalytic water disinfection by simple and low-cost monolithic and heterojunction ceramic wafers.

In this work, the photocatalytic disinfection of Escherichia coli (E. coli) using dual layer ceramic wafers, prepared by a simple and low-cost technique, was investigated. Heterojunction wafers were prepared by pressing TiO2 and WO3 powders together into 2 layers within a single, self-supported monolith. Data modelling showed that the heterojunction wafers were able to sustain the formation of charged species (after an initial "charging" period). In comparison, a wafer made from pure TiO2 showed a less desirable bacterial inactivation profile in that the rate decreased with time (after being faster initially). The more favourable kinetics of the dual layer system was due to superior electron-hole vectorial charge separation and an accumulation of charges beyond the initial illumination period. The results demonstrate the potential for developing simplified photocatalytic devices for rapid water disinfection.

High-Throughput Synthesis, Screening, and Scale-Up of Optimized Conducting Indium Tin Oxides.

A high-throughput optimization and subsequent scale-up methodology has been used for the synthesis of conductive tin-doped indium oxide (known as ITO) nanoparticles. ITO nanoparticles with up to 12 at % Sn were synthesized using a laboratory scale (15 g/hour by dry mass) continuous hydrothermal synthesis process, and the as-synthesized powders were characterized by powder X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray analysis, and X-ray photoelectron spectroscopy. Under standard synthetic conditions, either the cubic In2O3 phase, or a mixture of InO(OH) and In2O3 phases were observed in the as-synthesized materials. These materials were pressed into compacts and heat-treated in an inert atmosphere, and their electrical resistivities were then measured using the Van der Pauw method. Sn doping yielded resistivities of ∼ 10(-2) Ω cm for most samples with the lowest resistivity of 6.0 × 10(-3) Ω cm (exceptionally conductive for such pressed nanopowders) at a Sn concentration of 10 at %. Thereafter, the optimized lab-scale composition was scaled-up using a pilot-scale continuous hydrothermal synthesis process (at a rate of 100 g/hour by dry mass), and a comparable resistivity of 9.4 × 10(-3) Ω cm was obtained. The use of the synthesized TCO nanomaterials for thin film fabrication was finally demonstrated by deposition of a transparent, conductive film using a simple spin-coating process.

Structure-property-composition relationships in doped zinc oxides: enhanced photocatalytic activity with rare earth dopants.

In this paper, we demonstrate the use of continuous hydrothermal flow synthesis (CHFS) technology to rapidly produce a library of 56 crystalline (doped) zinc oxide nanopowders and two undoped samples, each with different particle properties. Each sample was produced in series from the mixing of an aqueous stream of basic zinc nitrate (and dopant ion or modifier) solution with a flow of superheated water (at 450 °C and 24.1 MPa), whereupon a crystalline nanoparticle slurry was rapidly formed. Each composition was collected in series, cleaned, freeze-dried, and then characterized using analytical methods, including powder X-ray diffraction, transmission electron microscopy, Brunauer-Emmett-Teller surface area measurement, X-ray photoelectron spectroscopy, and UV-vis spectrophotometry. Photocatalytic activity of the samples toward the decolorization of methylene blue dye was assessed, and the results revealed that transition metal dopants tended to reduce the photoactivity while rare earth ions, in general, increased the photocatalytic activity. In general, low dopant concentrations were more beneficial to having greater photodecolorization in all cases.