Robert I. Gruar

Electrophoretically deposited TiO2 compact layers using aqueous suspension for dye-sensitized solar cells

TiO2 compact layers (CLs) prepared by electrophoretic deposition (EPD) from an aqueous nanoparticle suspension were used in dye-sensitized solar cells (DSSCs) to prevent charge recombination at the interface between the transparent fluorine-doped tin oxide (FTO) substrate and the electrolyte. The TiO2 nanopowder (ca. 4.5 nm diameter) suspension used in the EPD process was prepared via a continuous hydrothermal flow synthesis pilot plant (at a production rate of ca. 0.38 kg h(-1)). The optimal thickness of the TiO2 CL for DSSCs is about 115 nm. Compared to the DSSCs without a CL, the optimal cell has shown improved short-circuit current density (JSC) and solar energy conversion efficiency by 13.1% and 15.0%, respectively. The mechanism for improved performance has been studied by the measurements of dark current and electrochemical impedance spectra. The interfacial charge transfer resistance at the FTO/electrolyte interface is increased after fabricating a CL in the cell, indicating inhibited electron recombination at the interface.

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.

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.

Numerical Simulation of Fluid Flow and Heat Transfer in a Counter-Current Reactor System for Nanomaterial Production

Continuous hydrothermal flow synthesis (CHFS) systems can provide high quality fine nanoparticles. However, optimisation of the CHFS system including the reactor and heat exchanger design, and their scaling-up for commercial applications have not been studied and cannot be achieved only through laboratory and pilot plant experiments. CFD modelling techniques are being widely used to simulate fluid field, heat and mass transfer in a lot of industrial process equipment. However, the application of CFD to model CHFS systems is still rare. This paper employs CFD methodology to simulate fluid flow and heat transfer in a counter-current reactor and a tubular heat exchanger of a laboratory-scale CHFS system for the production of TiO2 nanoparticles. The distributions of flow and heat transfer variables such as velocity and temperature in both units are obtained using ANSYS Fluent package. The tracer concentration profile is also simulated via solving the species equations to investigate the mixing behaviour in the counter-current reactor. Temperature distributions at different locations in a counter-current reactor and a tubular heat exchanger of a CHFS system were obtained experimentally. The simulated temperatures in both the reactor and the heat exchanger are compared with the available experimental data, which reveals that a good level of agreement is achieved.