Christopher J. Tighe

Highly Sensitive ZnO Nanorod- and Nanoprism-Based NO2 Gas Sensors: Size and Shape Control Using a Continuous Hydrothermal Pilot Plant

Continuous hydrothermal flow synthesis of crystalline ZnO nanorods and prisms is reported via a new pilot-scale continuous hydrothermal reactor (at nominal production rates of up to 1.2 g/h). Different size and shape particles of ZnO (wurtsite structure) were obtained via altering reaction conditions such as the concentration of either additive H2O2 or metal salt. Selected ZnO samples (used as prepared) were evaluated as solid oxide gas sensors, showing excellent sensitivity toward NO2 gas. It was found that both the working temperature and gas concentration significantly affected the NO2 gas response at concentrations as low as 1 ppm.

High-throughput continuous hydrothermal flow synthesis of Zn-Ce oxides: unprecedented solubility of Zn in the nanoparticle fluorite lattice

High-throughput continuous hydrothermal flow synthesis has been used as a rapid and efficient synthetic route to produce a range of crystalline nanopowders in the Ce–Zn oxide binary system. High-resolution powder X-ray diffraction data were obtained for both as-prepared and heat-treated (850°C for 10 h in air) samples using the new robotic beamline I11, located at Diamond Light Source. The influence of the sample composition on the crystal structure and on the optical and physical properties was studied. All the nanomaterials were characterized using Raman spectroscopy, UV–visible spectrophotometry, Brunauer–Emmett–Teller surface area and elemental analysis (via energy-dispersive X-ray spectroscopy). Initially, for ‘as-prepared’ Ce1−xZnxOy, a phase-pure cerium oxide (fluorite) structure was obtained for nominal values of x=0.1 and 0.2. Biphasic mixtures were obtained for nominal values of x in the range of 0.3–0.9 (inclusive). High-resolution transmission electron microscopy images revealed that the phase-pure nano-CeO2 (x=0) consisted of ca 3.7 nm well-defined nanoparticles. The nanomaterials produced herein generally had high surface areas (greater than 150 m2 g−1) and possessed combinations of particle properties (e.g. bandgap, crystallinity, size, etc.) that were unobtainable or difficult to achieve by other more conventional synthetic methods.

High-throughput powder diffraction on beamline I11 at Diamond

A new capability designed for high-throughput (HT) structural analysis using the synchrotron powder diffraction beamline (I11) at Diamond Light Source is reported. With a high-brightness X-ray beam, multi-analyser detectors and fast data-acquisition procedures, high-quality diffraction data can be collected at a speed of ∼15–30 min per powder pattern for good crystalline materials. Fast sample changing at a rate of a few seconds per specimen is achieved with a robotic arm and pre-loaded capillary specimens on a multi-tray carousel (200-sample capacity). Additional equipment, such as an automatic powder-loading machine and a pre-alignment jig for the sample capillaries, is available to reduce preparation time. For demonstration purposes, the first results presented here are those from standard reference powders of Si, TiO2 and TiO2/Si mixtures, obtained by analysing the data using Le Bail (instrumental calibration) and Rietveld refinements (quantitative agreement within 1%). The HT hardware was then used to study the structural phase evolution of a library of 31 La4Ni3−xFexO10 heterometallic ceramic powders in less than 1 d. The powders were generated from a single heat treatment (at 1348 K in air for 12 h) of nanoceramic oxide co-precipitate precursors, made using a newly developed HT synthesis robot. Crystallographic details (symmetry and lattice parameters) were obtained as a function of Fe concentration. The results revealed that this approach was able to produce a pure Ruddlesden–Popper-type phase with an iron content of up to x = 0.5, significantly higher than has been achieved previously using more conventional synthesis routes and thus demonstrating the power of using the HT approach.

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.

Suspension plasma sprayed coatings using dilute hydrothermally produced titania feedstocks for photocatalytic applications

Titanium dioxide coatings have potential applications including photocatalysts for solar assisted hydrogen production, solar water disinfection and self-cleaning windows. Herein, we report the use of suspension plasma spraying (SPS) for the deposition of conformal titanium dioxide coatings. The process utilises a nanoparticle slurry of TiO2 (ca. 6 and 12 nm respectively) in water, which is fed into a high temperature plasma jet (ca. 7000–20 000 K). This facilitated the deposition of adherent coatings of nanostructured titanium dioxide with predominantly anatase crystal structure. In this study, suspensions of nano-titanium dioxide, made via continuous hydrothermal flow synthesis (CHFS), were used directly as a feedstock for the SPS process. Coatings were produced by varying the feedstock crystallite size, spray distance and plasma conditions. The coatings produced exhibited ca. 90–100% anatase phase content with the remainder being rutile (demonstrated by XRD). Phase distribution was homogenous throughout the coatings as determined by micro-Raman spectroscopy. The coatings had a granular surface, with a high specific surface area and consisted of densely packed agglomerates interspersed with some melted material. All of the coatings were shown to be photoactive by means of a sacrificial hydrogen evolution test under UV radiation and compared favourably with reported values for CVD coatings and compressed discs of P25.

Nanoparticle scaffolds for syngas-fed solid oxide fuel cells

Incorporation of nanoparticles into devices such as solid oxide fuel cells (SOFCs) may provide benefits such as higher surface areas or finer control over microstructure. However, their use with traditional fabrication techniques such as screen-printing is problematic. Here, we show that mixing larger commercial particles with nanoparticles allows traditional ink formulation and screen-printing to be used while still providing benefits of nanoparticles such as increased porosity and lower sintering temperatures. SOFC anodes were produced by impregnating ceria–gadolinia (CGO) scaffolds with nickel nitrate solution. The scaffolds were produced from inks containing a mixture of hydrothermally-synthesised nanoparticle CGO, commercial CGO and polymeric pore formers. The scaffolds were heat-treated at either 1000 or 1300 °C, and were mechanically stable. In situ ultra-small X-ray scattering (USAXS) shows that the nanoparticles begin sintering around 900–1000 °C. Analysis by USAXS and scanning electron microscopy (SEM) revealed that the low temperature heat-treated scaffolds possessed higher porosity. Impregnated scaffolds were used to produce symmetrical cells, with the lower temperature heat-treated scaffolds showing improved gas diffusion, but poorer charge transfer. Using these scaffolds, lower temperature heat-treated cells of Ni–CGO/200 μm YSZ/CGO-LSCF performed better at 700 °C (and below) in hydrogen, and performed better at all temperatures using syngas, with power densities of up to 0.15 W cm−2 at 800 °C. This approach has the potential to allow the use of a wider range of materials and finer control over microstructure.

Continuous hydrothermal synthesis of surface-functionalised nanophosphors for biological imaging

A crystalline and highly luminescent nanoparticle red phosphor with average particle size of 35 nm (nominal 4 mol% Eu in Y2O3) was prepared using flash heat-treatment of a nanoparticle precursor (crystals of the corresponding doped oxyhydroxide). The nanoparticle precursors (which also show strong red emission over the 600–630 nm region under broad UV excitation from transitions of 5D0 → 7F2 in europium) were prepared in a single step using a continuous hydrothermal process (utilising supercritical water) operated at ca. 380 °C and 24.1 MPa. Photoluminescence (PL) and time-resolved PL measurements were performed on selected heat-treated nanomaterials and revealed a significantly extended lifetime of >2.25 ms (bulk material typically ca. 1.7 ms). Increases in the emission lifetime as a function of increased heat-treatment time were attributed to inter-particle effects. Surface-functionalized nanoparticles were prepared and further evaluated as probes for biological imaging with the initial precursor phosphor and the highly luminescent oxide variant both being clearly resolved in cell imaging studies under an excitation of 470 nm, using a wide pass band filter centered at 640 nm. Thus, the method employed herein holds promise for readily formulated stable colloids for luminescent security inks and as biological imaging probes.

Direct Reductive Amination of Carbonyl Compounds Catalyzed by a Moisture Tolerant Tin(IV) Lewis Acid

Abstract Despite the ever‐broadening applications of main‐group ‘frustrated Lewis pair’ (FLP) chemistry to both new and established reactions, their typical intolerance of water, especially at elevated temperatures (>100 °C), represents a key barrier to their mainstream adoption. Herein we report that FLPs based on the Lewis acid iPr3SnOTf are moisture tolerant in the presence of moderately strong nitrogenous bases, even under high temperature regimes, allowing them to operate as simple and effective catalysts for the reductive amination of organic carbonyls, including for challenging bulky amine and carbonyl substrate partners.

Modelling and Simulation of Counter-Current and Confined Jet Reactors for Continuous Hydrothermal Flow Synthesis of Nano-Materials

Abstract Computational fluid dynamics is applied to a comparative study of a counter-current reactor and a confined jet reactor for continuous hydrothermal flow synthesis of nanomaterials under supercritical water conditions. The fluid flow and heat transfer variables including velocity and temperature profiles in both reactor configurations are simulated using ANSYS Fluent package. The tracer concentration profiles are also modelled via solving species equations from which the mixing behaviour in the reactors is investigated. The predicted temperatures are found to be in good agreement with experimental data. The simulation also provides suggestions to improving the reactor designs and process control.

Simulation of Hydrodynamics and Heat Transfer in Confined Jet Reactors of Different Size Scales for Nanomaterial Production

Abstract Continuous hydrothermal flow synthesis (CHFS) is an attractive process for producing high quality inorganic nanoparticles. The direct collection of detailed data about the CHFS process during experiments is not always feasible due to the supercritical conditions, hence affecting optimisation and control and scale-up. In this paper, computational fluid dynamics models are developed for confined jet reactors of a CHFS system for nanomaterial production at both laboratory and pilot-plant scales. The flow, mixing and temperature profiles in the reactors were simulated using the ANSYS Fluent package. The predicted temperature profiles were compared with the available experimental data and the mixing between supercritical water and precursor streams was examined in detail. The hydrodynamic and thermodynamic features of the reactors at both size scales were also compared.