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.

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 was used to generate a library of aluminum and gallium-codoped zinc oxide nanoparticles of specific atomic ratios. Resistivities of the materials were determined by Hall Effect measurements on heat-treated pressed discs and the results collated into a conductivity-composition map. Optimal resistivities of ∼9 × 10–3 Ω cm were reproducibly achieved for several samples, for example, codoped ZnO with 2 at% Ga and 1 at% Al. The optimum sample on balance of performance and cost was deemed to be ZnO codoped with 3 at% Al and 1 at% Ga.Keywords: conducting; continuous; doped zinc oxides; high-throughput; hydrothermal;

•SnO2 nanomaterials were synthesized in a continuous hydrothermal process.•Transition metal ions were doped into SnO2 in the range 6 to 10 at%.•All nanopowders showed similarly high surface areas of ca. 102 m2 g−1.•Redox-inactive dopants did not improve the overall capacity vs. undoped SnO2.•Some possibly redox-active dopants showed additional stored capacity.Nine different transition metal doped (<10 at%) tin dioxides and undoped SnO2 nanopowders with similar specific surface areas were made using a continuous hydrothermal process and then investigated as potential negative electrode materials for lithium ion batteries. The as-prepared nanopowders were characterized via a range of analytical techniques including powder X-ray diffraction, X-ray photoelectron spectroscopy, X-ray fluorescence spectrometry, transmission electron microscopy, thermogravimetric analysis and Brunauer-Emmett-Teller surface area measurements. Doped SnO2 materials were grouped into two classes according to the potential redox activity of the dopant (those presumed to be redox inactive: Nb, Ti, Zr; and those presumed to be redox active: Fe, Co, Cu, Zn, Mn, Ni). The role of the transition metal ion dopant on the cycling performance (overall capacity and voltage hysteresis), was elucidated for the first cycle via cyclic voltammetry measurements in half cells versus lithium metal. In particular, the authors were able to evaluate whether the initial Coulombic efficiency and the delithiation potential (vs. Li/Li+) of the doped samples, would be likely to offer any increased energy density (compared to undoped SnO2) for lithium ion batteries.Download high-res image (201KB)Download full-size image

Silicon-doped zinc oxide, Zn1−xSixOy, transparent conducting oxide nanoparticles were prepared using a laboratory scale (production rate of 60 g h−1) continuous hydrothermal flow synthesis (CHFS) process in the dopant range 0.25 to 3.0 at% Si. The resistivity of the materials was assessed as pressed heat-treated pellets, revealing that the sample with the lowest resistivity (3.5 × 10−2 Ω cm) was the 0.25 at% Si doped ZnO sample. The synthesis of this optimum composition was then scaled up to 350 g h−1 using a larger pilot plant CHFS process. Spin coating of a slurry of the resulting nanopowder made on the pilot plant, followed by an appropriate heat-treatment, produced a thin film with an optical transmission >80% and a low resistivity of 2.4 × 10−3 Ω cm, with a carrier concentration of 1.02 × 1020 cm−3 and a mobility of 11 cm2 V−1 s−1. This is a factor of almost twenty times improvement in the resistivity versus the analogous pressed, heat-treated pellet.

Defects play important roles in many catalytic processes, particularly for photocatalytic processes in semiconductors as they can alter the band structures and affect the excited electron–hole recombination pathways/lifetimes of semiconductors. In this report, we described the development of a facile route to the production of highly defective photocatalysts. Firstly, organic species were bound onto the surface of a metal oxide semiconductor catalyst, followed by a relatively low temperature ageing in N2, to remove the organics and to attract oxygen molecules from the surface, generating oxygen vacancies. In particular, we introduced a co-catalyst during the syntheses, which acted as a thermocatalyst to promote full oxidation of the organics, leaving more oxygen vacancies at the surface and to form intimate heterojunctions with host-catalysts to further drive the photocatalytic hydrogen evolution. The hydrogen evolution rate for our developed NiO–TiO2 defective heterojunctions in a sacrificial system was measured at ca. 1.41 mmol g−1 h−1, which was much higher than those of comparable catalysts reported in the literature (that generally display hydrogen evolution rates <0.4 mmol g−1 h−1). Computational simulation, together with other analytical techniques, suggested that the generated surface oxygen vacancies could induce a series of impurity energy levels within the VBM and CBM of TiO2 that narrowed the electron transmission gap in the TiO2 and acted as active sites for the reaction between adsorbed H2O and photoinduced trapped electrons to produce H2.

Ultrafine ruthenium–titanium oxide catalysts were directly produced using a continuous hydrothermal flow synthesis process and assessed as chloride oxidation catalysts. Selectivity towards chlorine (over oxygen) evolution was shown to generally increase with decreasing ruthenium content. The optimum catalyst was then used to make an anode for a light-driven brine-splitting demonstrator device to produce hydrogen and chlorine gases.

Highly monodispersed CuO nanoparticles (NPs) were synthesized via continuous hydrothermal flow synthesis (CHFS) and then tested as catalysts for CO2 hydrogenation. The catalytic behavior of unsupported 11 nm sized nanoparticles from the same batch was characterized by diffuse reflectance infrared fourier transform spectroscopy (DRIFTS), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), and catalytic testing, under CO2/H2 in the temperature range 25–500 °C in consistent experimental conditions. This was done to highlight the relationship among structural evolution, surface products, and reaction yields; the experimental results were compared with modeling predictions based on density functional theory (DFT) simulations of the CuO system. In situ DRIFTS revealed the formation of surface formate species at temperatures as low as 70 °C. DFT calculations of CO2 hydrogenation on the CuO surface suggested that hydrogenation reduced the CuO surface to Cu2O, which facilitated the formation of formate. In situ EXAFS supported a strong correlation between the Cu2O phase fraction and the formate peak intensity, with the maxima corresponding to where Cu2O was the only detectable phase at 170 °C, before the onset of reduction to Cu at 190 °C. The concurrent phase and crystallite size evolution were monitored by in situ XRD, which suggested that the CuO NPs were stable in size before the onset of reduction, with smaller Cu2O crystallites being observed from 130 °C. Further reduction to Cu from 190 °C was followed by a rapid decrease of surface formate and the detection of adsorbed CO from 250 °C; these results are in agreement with heterogeneous catalytic tests where surface CO was observed over the same temperature range. Furthermore, CH4 was detected in correspondence with the decomposition of formate and formation of the Cu phase, with a maximum conversion rate of 2.8% measured at 470 °C (on completely reduced copper), supporting the indication of independent reaction pathways for the conversion of CO2 to CH4 and CO that was suggested by catalytic tests. The resulting Cu NPs had a final crystallite size of ca. 44 nm at 500 °C and retained a significantly active surface.Keywords: CO2 hydrogenation; continuous hydrothermal flow; CuO; DFT; DRIFTS; EXAFS; XRD

A high-throughput synthesis, screening and subsequent scale-up approach was utilised for the optimisation of conductive aluminium and gallium-doped zinc oxide (AZO and GZO, respectively) nanoparticles. AZO and GZO nanoparticles with up to 6 at% dopant (with respect to Zn) were directly synthesised using a laboratory scale continuous hydrothermal process at a rate of 60 g per hour. The resistivities were determined by Hall effect measurements on pressed, heat-treated discs. Both Al- and Ga-doping yielded resistivities of the order of 1 × 10−2 Ω cm for most samples; the lowest resistivity of AZO was 7.0 × 10−3 Ω cm (at 2.5 at% Al doping), and the lowest resistivity of GZO was 9.1 × 10−3 Ω cm (at 3.5 at% Ga doping), which are considered exceptionally conductive for pressed nanopowders. Synthesis of the optimised lab-scale compositions was scaled-up using a pilot-scale continuous hydrothermal process at a production rate of 8 kg per day (by dry mass); results obtained from these nanopowders generally retained resistivity trends observed for the lab-scale analogues.

•Nb-doped LiFePO4 nanoparticles (<100 nm) are synthesised via CHFS.•The fructose precursor provided a continuous carbon coating.•The doped samples display improved discharge capacities at high current rates.•The optimal sample LiFe0.99Nb0.01PO4 achieved 110 mA h g−1 at 10 C.•Conductivity benefit of Nb confirmed by conductive carbon additive.High power, phase-pure Nb-doped LiFePO4 (LFP) nanoparticles are synthesised using a pilot-scale continuous hydrothermal flow synthesis process (production rate of 6 kg per day) in the range 0.01–2.00 at% Nb with respect to total transition metal content. EDS analysis suggests that Nb is homogeneously distributed throughout the structure. The addition of fructose as a reagent in the hydrothermal flow process, followed by a post synthesis heat-treatment, affords a continuous graphitic carbon coating on the particle surfaces. Electrochemical testing reveals that cycling performance improves with increasing dopant concentration, up to a maximum of 1.0 at% Nb, for which point a specific capacity of 110 mAh g−1 is obtained at 10 C (6 min for the charge or discharge). This is an excellent result for a high power cathode LFP based material, particularly when considering the synthesis was performed on a large pilot-scale apparatus.

•Phase-pure and V-doped LiFePO4 nanoparticles (<100 nm) are synthesized via CHFS.•A continuous carbon coating is achieved, giving core/shell nanoparticles.•The doped samples exhibit enhanced capacities at high discharge rates.•Sample LiFe0.95V0.05PO4 has high power performance at current rates up to 1.5 A g−1.•Theoretical and experimental techniques suggest vanadium occupies both the Fe and P sites.A high performance vanadium-doped LiFePO4 (LFP) electrode is synthesized using a continuous hydrothermal method at a production rate of 6 kg per day. The supercritical water reagent rapidly generates core/shell nanoparticles with a thin, continuous carbon coating on the surface of LFP, which aids electron transport dynamics across the particle surface. Vanadium dopant concentration has a profound effect on the performance of LFP, where the composition LiFe0.95V0.05PO4, achieves a specific discharge capacity which is among the highest in the comparable literature (119 mA h g−1 at a discharge rate of 1500 mA g−1). Additionally, a combination of X-ray absorption spectroscopy analysis and hybrid-exchange density functional theory, suggest that vanadium ions replace both phosphorous and iron in the structure, thereby facilitating Li+ diffusion due to Li+ vacancy generation and changes in the crystal structure.

•Layered titanates with high surface area are synthesized via CHFS.•The as-prepared nano-sheets can be used as the active component in the electrode.•The high power performance remains excellent for current rates up to 10 A g−1.•The charge is stored via pseudocapacitance intercalation.Ultra-thin layered sodium titanate nano-sheets were synthesised using a continuous hydrothermal flow process and the as-prepared materials were investigated as an anode material for lithium-ion batteries. In comparison to previous studies on similar materials, the layered titanates herein showed high electrochemical activity at lower potentials. Cyclic voltammetry measurements in the potential range of 0.05–2.1 V vs. Li/Li+, revealed that charge storage occurred from both lithium-ion intercalation as well as pseudocapacitive surface chemical processes. During electrochemical cycling tests, a high specific current of 0.5 A g−1 was applied and the cells achieved a stable specific capacity of ca. 120 mAh g−1 for over 1200 cycles. Even at an applied current of 10 A g−1, the electrode material delivered a stable specific capacity of 38 mAh g−1, which suggests that this material may be suitable for high power applications.

•High surface area VO2 nano-sheets can be directly synthesized via CHFS.•The as-obtained VO2 revealed excellent capacities at high and low rates.•The infinite pseudocapacitive charge storage was ca. 100 mAh g− 1.•At higher rates, the VO2 behaved like an oxide supercapacitor material.Vanadium dioxide (VO2) nano-sheets were directly synthesized via a continuous hydrothermal process and were investigated as electrodes in a wide potential range of 0.05–3 V vs. Li/Li+. The nano-sheets showed excellent capacity retention, with a specific capacity of 350 mAh g− 1 at an applied current of 0.1 A g− 1 and 95 mAh g− 1 at 10 A g− 1. Further electrochemical testing suggested that a significant proportion of the charge storage in the cells was due to pseudocapacitive processes.

•High surface area semi-crystalline Nb2O5 can be directly synthesized via CHFS.•The as-obtained Nb2O5 displayed excellent electrochemical performance.•At high current rates, the Nb2O5 behaved like an oxide supercapacitor material.•Pseudocapacitive reactions contributed to >63% of the total charge at 5 mV s−1.Nano-sized, semi-crystalline niobium pentoxide (Nb2O5) was synthesized in a single step via a continuous hydrothermal process. The nanomaterial was characterized using a range of analytical techniques including powder X-ray diffraction and transmission electron microscopy. The “as-prepared” Nb2O5 nanomaterial was investigated as negative electrode for a lithium-ion battery and was shown to be stable during electrochemical cycling (98.6 % capacity retention after 800 cycles) and showed promising high rate performance, with a specific capacity of 43 mAh g−1 at an applied current of 10,000 mA g−1 (in the wide potential range of 0.05 to 3 V vs Li/Li+). Scan rate tests were used to investigate the proportion of stored charge from diffusion-limited processes and that from surface effects, which showed that at higher currents, charge storage from the latter was dominant.

A series of LiMn1–x–yFexVyPO4 (LMFVP) nanomaterials have been synthesized using a pilot-scale continuous hydrothermal synthesis process (CHFS) and evaluated as high voltage cathodes in Li-ion batteries at a production rate of 0.25 kg h–1. The rapid synthesis and screening approach has allowed the specific capacity of the high Mn content olivines to be optimized, particularly at high discharge rates. Consistent and gradual changes in the structure and performance are observed across the compositional region under investigation; the doping of Fe at 20 at% (with respect to Mn) into lithium manganese phosphate, rather than V or indeed codoping of Fe and V, gives the best balance of high capacity and high rate performance.Keywords: cathode; continuous hydrothermal synthesis; doped LMP; high energy; lithium-ion battery

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.Keywords: continuous hydrothermal; indium tin oxide; nanoparticles; scale-up; transparent conducting oxides

A pilot plant scale continuous hydrothermal flow synthesis (CHFS) process was used to control the crystallite size (and surface area) for nano-TiO2 in the range ca. 5–18 nm. In CHFS, a room temperature solution of titanium oxysulfate (and boric acid) was mixed in flow with a feed of potassium hydroxide solution (at 24.1 MPa), and then this combined stream was mixed with a flow of superheated water (at 400 °C and 24.1 MPa) in a confined jet mixer. Nano-TiO2 particles were formed instantly and then cooled inline before being collected as a slurry from the exit of the process. It was observed that the boric acid concentration in the precursor feed affected reaction pH, which in turn, determined the average crystallite size of the nano-TiO2; as the pH increased, larger crystallites were obtained. The nanomaterials were evaluated in a sacrificial photocatalytic water splitting system (hydrogen evolution), and it was found that TiO2 prepared under mildly acidic conditions, yielded the highest photoactivity.

A new low-temperature continuous approach for the surface modification of hydroxyapatite (HA) is described. In this method, the HA particle surfaces were modified using methacrylic acid, vinylphosphonic acid, adipic acid, citric acid, or polyvinyalcohol, respectively, using a continuous plastic flow synthesis (CPFS) system at a reaction temperature of 70 °C for 5 min. The materials were investigated using a range of analytical techniques, including TEM (transmission electron microscopy), zeta potential, XRD (X-ray diffraction), BET (Brunauer–Emmett–Teller) surface area analysis, FTIR (Fourier transform infrared) spectroscopy, and XPS (X-ray photoelectron spectroscopy). The presence of organic agents in the reagents, resulted in a significant reduction in particle size of the nano-HA rods; TEM studies confirmed the formation of highly dispersed nanorods of HA with average lengths and diameters in the ranges 20–60 nm and 4–10 nm, respectively. XPS analyses suggested that the Ca:P molar ratio decreased from 1.67 to ca. 1.34 by the addition of organic surface agents. The zeta potential measurements revealed that the colloidal stability of surface-modified HA generally increased (under certain conditions) compared to ungrafted HA. The small size and presence of functional groups make these materials potentially suitable for dental restoration fillers and composite bone regeneration applications.

Nb-doped TiO2 (anatase) nanoparticles were synthesized using a continuous hydrothermal flow synthesis reactor using a supercritical water flow as a reagent and crystallizing medium. The as-prepared nano-powders with ca. 25 at% Nb5+ (<6 nm diameter) were used as possible anodes for lithium-ion batteries without any further heat-treatment. Cyclic voltammetry and galvanostatic charge/discharge cycling tests were performed in the range of 1.2 to 3.0 V vs. Li/Li+. The Nb-doped TiO2 samples showed superior capacity retention at high current rates compared to the corresponding undoped nano-TiO2. The superior performance of the doped samples (at specific currents up to 15 A g−1) was attributed to higher electronic conductivity and a greater charge storage contribution from surface effects like pseudocapacitance (Faradaic processes) as well as Helmholtz double layer charge storage.

•Phase-pure and Sn-doped TiO2 nanoparticles (<7 nm) are synthesized via CHFS.•The nanomaterial can be used directly after synthesis as a Li-ion battery anode.•TiO2 retains excellent high power performance at current rates up to 10 A g−1.•Doped Sn4+ in the TiO2 is electrochemically active.A range of phase-pure anatase TiO2 (∼5 nm) and Sn-doped TiO2 nanoparticles with the formula Ti1-xSnxO2 (where x = 0, 0.06, 0.11 and 0.15) were synthesized using a continuous hydrothermal flow synthesis (CHFS) reactor. Charge/discharge cycling tests were carried out in two different potential ranges of 3 to 1 V and also a wider range of 3 to 0.05 V vs Li/Li+. In the narrower potential range, the undoped TiO2 nanoparticles display superior electrochemical performance to all the Sn-doped titania crystallites. In the wider potential range, the Sn-doped samples perform better than undoped TiO2. The sample with composition Ti0.85Sn0.15O2, shows a capacity of ca. 350 mAh g−1 at an applied constant current of 100 mA g−1 and a capacity of 192.3 mAh g−1 at a current rate of 1500 mA g−1. After 500 charge/discharge cycles (at a high constant current rate of 382 mA g−1), the same nanomaterial anode retains a relatively high specific capacity of 240 mAh g−1. The performance of these nanomaterials is notable, particularly as they are processed into electrodes, directly from the CHFS process (after drying) without any post-synthesis heat-treatment, and they are made without any conductive surface coating.

•Fe3O4 nanoparticles with amorphous carbon can be synthesized via a continuous hydrothermal synthesis process.•The carbon coatings can be used to reduce the iron oxide to create iron metal particles.•Depending on iron metal content, an improvement in electrochemical performance can be achieved for the active iron oxide.•This work suggests a simple approach to generate composite metal oxides/metal for use in Li-ion batteries.High capacity, stable Fe3O4/Fe nanocomposites for Li-ion battery anodes were manufactured via heat-treating Fe3O4–C (amorphous) nanoparticles that were made via a continuous hydrothermal flow synthesis (CHFS) reactor. Compared to analogous Fe3O4 nanoparticles, the Fe3O4/Fe nanocomposite anodes (vs. Li/Li+), displayed a high specific capacity of ca. 390 mAh g−1 after 50 cycles, at a modest current rate of 200 mA g−1 (at the highest Fe metal content). The performance of the Fe3O4/Fe materials at higher current rates was also excellent (ca. 260 mAh g−1 at the highest current rate of 2000 mA g−1), which confirms that the presence of Fe metallic particles can significantly improve cycling stability of Li-ion battery anodes by retaining structural metal oxide integrity.

Cobalt oxide (Co3O4) nanoparticles were synthesized from aqueous solutions of cobalt(II) acetate using a laboratory scale continuous hydrothermal flow synthesis reactor incorporating a confined jet (coaxial) mixer. By changing the concentration of the precursor combined with operating under flow rate conditions expected to result in a laminar or turbulent mixing, the size of the crystallites could be controlled in the range of 6.5–16.5 nm (median). A quench stream was employed to rapidly cool down the nascent stream of nanoparticles and to elucidate the mechanisms of nucleation and growth. The results show a clear correlation between increasing precursor concentration and crystallite size, which at lower concentrations in particular, decreased in laminar flow and increased in turbulent flow. The smallest particles of 6.5 nm (median) were produced at a precursor concentration of 0.1 M (at a rate of 20 g·h–1). The materials were characterized using a range of analytical methods including powder X-ray diffraction and transmission electron microscopy.

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.Keywords: continuous hydrothermal; doping; photocatalytic activity; UV attenuator; zinc oxide

A new processing methodology is presented for the direct synthesis of surface-functionalized nanoparticles through modification of a single-step continuous supercritical water process. The processing methodology utilizes inexpensive metal salt precursors that form nanoparticles upon mixing the metal salt solution with a supercritical water flow (24 MPa and 450 °C). Surface functionalization is achieved through introducing a supplementary flow of capping agent (citric acid in this example) to the stream of nascent (newly formed) nanoparticles using a novel reactor design. It was found that certain process attributes were key to effective functionalization of the nascent nanoparticle stream, and that high grafting densities of the capping agent were obtained in a relatively narrow process window. We have also used the core design of the reactor to devise and test a scale-up methodology to produce large quantities of surface-functionalized nanoparticles. A method for scaling-up the reactor is described, using a newly developed pilot plant designed to process flow rates 20× greater than the equivalent laboratory-scale process, which yields products at rates of ca. 1 kg/h (effectively semi-industrial-scale production). The method enables large-scale production without recourse to expensive or environmentally damaging reagents and uses water as the only process solvent, a significant advantage over many methods commonly used to produce surface-functionalized nanoparticles. We report the synthesis and characterization of citrate-functionalized Fe3O4 nanoparticles as a model system and present detailed characterization of the materials obtained at both processing scales.

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.

Robust, bilayer heterojunction photodiodes of TiO2–WO3 were prepared successfully by a simple, low-cost powder pressing technique followed by heat-treatment. Exclusive photoirradiation of the TiO2 side of the photodiode resulted in a rapid colour change (dark blue) on the WO3 surface as a result of reduction of W6+ to W5+ (confirmed by X-ray photoelectron spectroscopy). This colour was long lived and shown to be stable in a dry environment in air for several hours. A similar photoirradiation experiment in the presence of a mask showed that charge transfer across the heterojunction occurred approximately normal to the TiO2 surface, with little smearing out of the mask image. As a result of the highly efficient vectorial charge separation, the photodiodes showed a tremendous increase in photocatalytic activity for the degradation of stearic acid, compared to wafers of the respective individual materials when tested separately.

Continuous hydrothermal flow synthesis (CHFS) has been used as a rapid and clean, synthetic method to produce a range of crystalline nanoparticles in the Ti–Zn–O binary system. The nanopowders were prepared from aqueous solutions of titanium(IV) bis(ammonium lactato)dihydroxide (TIBALD) and hydrated zinc nitrate, respectively, using a CHFS reactor which uses superheated water (at 400 °C and 24.1 MPa) as a reagent and crystallizing medium. The resulting nanopowders were heat-treated at 850 °C for five hours in air to give photoactive semiconductor mixtures of rutile and zinc oxide and/or zinc titanates. The as-prepared powders and their corresponding heat-treated mixed phase photocatalysts were characterized using powder X-ray synchrotron diffraction, transmission electron microscopy, BET surface area measurement, X-ray photoelectron spectroscopy and UV-Vis spectrophotometry. Because of the interest for these materials in UVA and UVB attenuating materials, the UV-Vis profiles of the nanocomposites and solid solutions were studied. Photocatalytic activity of the samples towards the decolourisation of methylene blue dye was also assessed.

•In situ synchrotron imaging reveals new insights into a continuous hydrothermal flow synthesiser.•First comparative images of nano-CeO2 synthesis at 3 (sub- and) supercritical water temperatures.•The supercritical water crystallisation of CeO2 nanoparticles in the flow path spatially mapped.•Distinct intensity distribution shown of crystalline content inside the counter-current mixer.•Particle size maps and HRTEM confirming marginal increase in CeO2 particle size with temperature.In situ high-energy synchrotron X-ray diffraction, a non-destructive synchrotron-based technique was employed to probe inside the steel tubing of a continuous hydrothermal flow synthesis (CHFS) mixer to spatially map, for the first time, the superheated water crystallisation of nanocrystalline ceria (CeO2) at three different (superheated-water) temperatures representing three unique chemical environments within the reactor. Rapid hydrothermal co-precipitation at the three selected temperatures led to similarly sized ceria nanoparticles ranging from 3 to 7 nm. 2D maps of CeO2 formation were constructed from the intensity and corresponding full width at half maximum (FWHM) values of the two most intense ceria reflections (111) and (002) for all three water inlet temperatures (350, 400 and 450 °C at 24 MPa) and subsequent changes in the particle size distribution were analysed. The accompanying high-resolution transmission electron microscopy (HRTEM) and tomographic particle size maps have confirmed that the mean ceria particle size slightly increases with temperature. This X-ray tomographic imaging study amounted to a formidable technical and engineering challenge, nevertheless one that has been met; this represents a significant achievement in imaging science, given the dynamic nature and hostile environment of a working CHFS reactor.

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.

A new continuous supercritical water pilot plant was used for the large-scale production of nanomaterials in the Zn–Ce oxide system. Similar to an existing laboratory continuous process, the pilot plant mixes aqueous solutions of the metal salts at room temperature with a flow of supercritical water (450 °C and 24.1 MPa) in a confined jet mixer, resulting in the formation of nanoparticles in a continuous manner. The Zn–Ce oxide system, as synthesized here under identical concentration conditions than those used in our laboratory scale process (but 17.5 times total flow rate), has been used as a model system to identify differences in particle properties due to the physical enlargement of the mixer. The data collected for the nanoparticles from the pilot plant was compared to previous work using a laboratory scale continuous reactor. In the Ce–Zn binary oxide series, it was shown that Zn had an apparent solubility of about 20 mol% in the CeO2 (fluorite) lattice, whereafter a composite of the two phases was obtained, consistent with the high solubility observed in previous studies using a continuous hydrothermal process. Because of the inherent scalability of the continuous process and excellent mixing characteristics of the confined jet mixer, it was found that the pilot plant nanoparticles were almost indistinguishable from those made on the laboratory scale.

A confined jet reactor (mixer) is presented as a novel solution for the scalable continuous hydrothermal flow synthesis (CHFS) of nanoceramics. In CHFS, nanoceramics are formed upon mixing of two streams consisting of an aqueous metal salt solution at room temperature with a flow of less dense supercritical water (at 240 bar and 450 °C). Upon mixing, hydrolysis and dehydration occurs, resulting in the particles being formed in a continuous manner. The confined jet mixer used herein overcomes previous designs of mixers that can accumulate material internally and block. A method for scaling up the jet mixer (reactor) is described, to determine the size of jet mixer (internal mixer diameter 13.5 mm) prior to its use in a newly commissioned pilot plant designed to process flow rates 40 times greater than the equivalent laboratory-scale process (internal mixer diameter 4.6 mm). It was confirmed that the pilot plant scale mixer allowed safe and continuous operation with no blockages at much higher concentrations (i.e., higher molarity) of metal salt precursor than laboratory scale because of the higher velocities and larger physical dimensions of the mixer. Consequently, the pilot plant was used to manufacture nanoparticles at a rate >400 times that of the laboratory-scale process. The synthesis of zinc oxide nanoparticles was used as a model to compare the properties of particles produced on different production scales. The same model system was also used to assess the limitations of a scale-up strategy based on mass (i.e., increasing the molarity of the metal salt).

High-throughput continuous hydrothermal flow synthesis was used to manufacture 66 unique nanostructured oxide samples in the Ce–Zr–Y–O system. This synthesis approach resulted in a significant increase in throughput compared to that of conventional batch or continuous hydrothermal synthesis methods. The as-prepared library samples were placed into a wellplate for both automated high-throughput powder X-ray diffraction and Raman spectroscopy data collection, which allowed comprehensive structural characterization and phase mapping. The data suggested that a continuous cubic-like phase field connects all three Ce–Zr–O, Ce–Y–O, and Y–Zr–O binary systems together with a smooth and steady transition between the structures of neighboring compositions. The continuous hydrothermal process led to as-prepared crystallite sizes in the range of 2–7 nm (as determined by using the Scherrer equation).Keywords: high-throughput; hydrothermal synthesis; phase diagram; Raman spectroscopy; X-ray diffraction

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.

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.

We describe a simple nanoprecursor route for direct solid-state combinatorial synthesis and discovery of heterometallic materials compositions which are normally difficult to make in a single step. Using a combinatorial robot (incorporating a continuous hydrothermal reactor), co-precipitated nanoprecursors containing different amounts of La, Ni and Fe oxides were made. These samples were divided into two identical cloned libraries, which were heat-treated to bring about solid-state transformations at either 1348 K or 1573 K for 12 h. In each case, experimental conditions were designed to form the corresponding La4Ni3 − xFexO10 phases (x = 0.0–3.0) directly without comminution. Such materials are difficult to make without multiple heating and grinding steps. The heat-treated samples from each library were embedded into a wellplate and analysed by powder X-ray diffraction methods in order to elucidate trends in phase behaviour. Several hitherto unknown phase-pure Ruddlesden Popper type La4Ni3 − xFexO10 compositions were identified and their DC electrical conductivities measured.Highlights► A new high-throughput combinatorial process for materials discovery is described. ► Layered lanthanum nickelates are formed directly, without the need for comminution. ► Fe is doped into Ruddlesden Popper type La4Ni3 − xFexO10 to a maximum of x = 1. ► DC electrical conductivity of all phase pure materials is reported.

A rapid, clean, and continuous hydrothermal route to the synthesis of ca. 14 nm indium oxide (In2O3) nanoparticles using a superheated water flow at 400 °C and 24.1 MPa as a crystallizing medium and reagent is described. Powder X-ray diffraction (XRD) of the particles revealed that they were highly crystalline despite their very short time under hydrothermal flow conditions. Gas sensing substrates were prepared from an In2O3 suspension via drop-coating, and their gas sensing properties were tested for response to butane, ethanol, CO, ammonia, and NO2 gases. The sensors showed excellent selectivity toward ethanol, giving a response of 18–20 ppm.

In situ variable temperature XRD (VT-XRD) measurements on the transformation of nano-precursors to LaNiO phases are presented. Experimental results showed that LaNiO3 and La2NiO4 phases were formed at ca. 700 °C via the reaction of La2O3 and NiO (from the initial nano-precursors), where a relatively low temperature of 700 °C was found for the synthesis of La2NiO4. The formation of La3Ni2O7 at higher temperature (up to 1150 °C) appeared to proceed through a further reaction of La2NiO4 with unreacted NiO, whilst the formation of La4Ni3O10 (at 1075 °C) proceeded via a further decomposition of LaNiO3. Although phase pure La3Ni2O7 and La4Ni3O10 were not directly obtained under the processing conditions herein, the results of this study allow for a better understanding of formation pathways, particularly for the higher order La–Ni–O phases.In situ variable temperature XRD showing the phase formation pathway of Lan+1NinO3n+1 at evaluated temperatures.Highlights► In situ VT-XRD was utilized to study the pathways for Lan+1NinO3n+1 formations. ► LaNiO3 and La2NiO4 phases were formed via the reaction of La2O3 and NiO, respectively. ► La3Ni2O7 phase was formed via further reaction of La2NiO4 phase with unreacted NiO. ► La4Ni3O10 phase was formed via further decomposition of LaNiO3 phase.

A novel and rapid and continuous hydrothermal route to the synthesis of extensive ultra-thin 2D sodium titanate (Na2Ti3O7) nano-sheets using a superheated water flow at 450 °C and 24.1 MPa as a crystallizing medium is described. High resolution electron microscopy of the sheets revealed that they were a few layers thick and largely uncurled, highly crystalline despite their very short time under hydrothermal flow conditions. The sodium titanate sheets possessed excellent photocatalytic activity for decolourisation of methylene blue dye.

We report on the commissioning experimental run of the rapid automated materials synthesis instrument (RAMSI), a combinatorial robot designed to manufacture, clean, and print libraries of nanocrystal precursor solid compositions. The first stage of RAMSI, parallel synthesis, uses a fully automated high throughput continuous hydrothermal (HiTCH) flow reactor for automatic metal salt precursor mixing, hydrothermal flow reaction, and sample slurry collection. The second stage of RAMSI provides integrated automated cleanup, and the third section is a ceramic printing function. Nanocrystal precursor solid ceramics were synthesized from precursor solutions and collected into 50 mL centrifuge tubes where they were cleaned by multiple centrifugation and redispersion cycles (monitored by intelligent scanning turbidimetry) and printed with an automated pipette. Eight unique compositions of a model phosphor library comprising pure nano-Y(OH)3 and Eu3+ doped-yttrium hydroxide, Y(OH)3:Eu3+ nanocrystal precursor solid were synthesized (with 2 centrifuge tubes’ worth collected per composition), processed, and printed in duplicate as 75, 100, and 125 μL dots in a 21.6 ks (6 h) experiment (note: the actual time for synthesis of each sample tube was only 12 min so up to 60 compositions could easily be synthesized in 12 h if one centrifuge tube per composition was collected instead). The Y(OH)3:Eu3+ samples were manually placed in a furnace and heat-treated in air for 14.4 ks (4 h) in the temperature range 200−1200 at 100 °C intervals (giving a total of 84 samples plus one as-prepared pure Y(OH)3 sample). The as-prepared and heat-treated ceramic samples were affixed to 4 mm wide hemispherical wells in a custom-made aluminum well-plate and analyzed using a fluorescence spectrometer. When the library was illuminated with a 254 nm light source (and digitally imaged and analyzed), the 3 mol % Eu3+ sample heat-treated at 1200 °C gave the most intense fluorescence (major red peak at 612 nm); however, an identical nanocrystal precursor heat-treated at only 500 °C (identified as Y2O3:Eu3+ after heat treatment) was the brightest phosphor under illumination of the samples heat-treated at or below 1000 °C.

Herein, we report the rapid single step hydrothermal synthesis of phase pure Bi2MoO6 (koechlinite) and Bi2Mo3O12, via a continuous hydrothermal flow synthesis (CHFS) reactor, which uses supercritical water of 375–450 °C at a pressure of 24.1 MPa as a crystallising medium. The product being obtained as highly crystalline nano-materials with high surface area. Simple variation in synthesis condition and appropriate solution stoichiometry were shown to be sufficient to select the phase of the product. The materials synthesised showed significant photcatalytic activity towards the decolourisation of methylene blue in comparison to a commercial gold standard photocatalyst.

Several commercial titania photocatalyst powders were formed into thin (ca. 350 μm), 25 mm diameter ceramic wafers, sputter deposited with Pt on one side. The activities of each of the ceramic wafers were tested for hydrogen and oxygen evolution from aqueous sacrificial systems. The commercial sample PC50 (Millennium Chemicals, UK) yielded reproducible ceramic wafers with high activity for water photoreduction. Many of the ceramic wafers displayed low water photo-oxidation activities; however, these were greatly increased with addition of a NiO co-catalyst. In a selected case, hydrogen evolution activity was compared between a PC50 wafer and an identical weight of platinised PC50 powder suspension.

Homogenously doped and mixed yttria stabilized zirconia, YSZ (with 3 and 10 mol% Y2O3 known as 3YSZ and 10YSZ) and NiO/10YSZ co-precipitates (nominally corresponding to 7, 12, 24, 30, 35 and 45 vol.% Ni metal), were synthesized using a continuous hydrothermal flow synthesis (CHFS) system which uses a stream of superheated water at 450 °C and 24.1 MPa as a reaction medium to cause rapid precipitation of metal oxide nanoparticle co-precipitates from a mixed metal salt solution. All products were obtained directly from the outlet of the CHFS reactor as a slurry, which was then cleaned and freeze-dried prior to further processing. The highly crystalline nano-powdered products were characterized using a range of analytical methods, including: powder X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), Raman spectroscopy and BET surface area measurements. Spherical primary particles of 10YSZ and 3YSZ were observed under the TEM and found to be 5.0 ± 0.8 nm (range 3.2–6.3 nm) and 6.2 ± 1.4 nm (range 3.3–8.4 nm) in size, with measured BET surface areas of 160.6 and 241.7 m2 g−1, respectively. Sintering of the nano-powder co-precipitates was performed via spark plasma sintering (SPS) at 1100 °C for 1 min, leading to densities of ca. 98% for 10YSZ and ca. 96% for Ni/10YSZ cermets (all NiO was converted into Ni due to the reducing atmosphere of the SPS). The 24% Ni/10YSZ cermet was subjected to focused ion beam tomography, which allowed the 3D arrangement of the conducting Ni metallic network of the dense cermet to be elucidated, and showed that a complete 3D network of Ni existed throughout the dense cermet disk. Electrical conductivity tests showed that the samples exhibited higher than expected electrical conductivity (for such low metal content), e.g., the 24 vol.% Ni-containing sample achieved an electrical conductivity of ∼ 200 S cm−1 at the fuel cell operating temperature, which corresponds to an effective conductivity of ∼ 117 S cm−1 if a porosity of 30% were to be introduced.

A novel High-Throughput Continuous Hydrothermal (HiTCH) flow synthesis reactor was used to make directly and rapidly a 66-sample nanoparticle library (entire phase diagram) of nanocrystalline CexZryYzO2−δ in less than 12 h. High resolution PXRD data were obtained for the entire heat-treated library (at 1000 °C/1 h) in less than a day using the new robotic beamline I11, located at Diamond Light Source (DLS). This allowed Rietveld-quality powder X-ray diffraction (PXRD) data collection of the entire 66-sample library in <1 day. Consequently, the authors rapidly mapped out phase behavior and sintering behaviors for the entire library. Out of the entire 66-sample heat-treated library, the PXRD data suggests that 43 possess the fluorite structure, of which 30 (out of 36) are ternary compositions. The speed, quantity and quality of data obtained by our new approach, offers an exciting new development which will allow structure−property relationships to be accessed for nanoceramics in much shorter time periods.

Nanosized TiO2 powder of high surface area was prepared from an aqueous solution of titanium(IV) bis(ammonium lactato) dihydroxide using a continuous hydrothermal flow synthesis (CHFS) reactor which uses superheated water at 400 °C and 24.1 MPa as a crystallizing medium. Freeze-dried nano-TiO2 was heat-treated in air over a range of temperatures and then the resulting powders were characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), BET surface area measurement, and Raman spectroscopy. The particle size of the ‘as-prepared’ TiO2 made using CHFS was ca. 4.8 nm (by HR-TEM), and grew with increasing heat-treatment temperature. It was found that the onset of the anatase–rutile transition of heat-treated nano-TiO2 in air occurred at 500 °C and reached 100% rutile at 900 °C. The Raman band (Eg) at ∼150 cm−1 of anatase (nano-TiO2) softens as the particle size increases with heat-treatment temperature (up to 600 °C). The photocatalytic activity of the TiO2 powders for the decolourisation of methylene blue dye was assessed. The effects of nano-TiO2 anatase-rutile phase composition, crystallinity, and crystallite size on the catalytic activity were investigated.

Continuous hydrothermal flow synthesis (CHFS) technology has been used as an efficient and direct route to produce a range of largely crystalline magnesium substituted calcium phosphate bioceramics. Initially, magnesium substituted hydroxyapatite, Mg-HA, according to the formula [Ca10−xMgx(PO4)6(OH)2] was prepared in the CHFS system for x = 0.2 [where x:(10 − x) is the Mg:Ca ratio used in the reagents]. Biphasic mixtures of Mg-HA and Mg-whitlockite were obtained corresponding to x values in the range x = 0.4–1.6. The direct synthesis of phase pure crystalline Mg-whitlockite [based on the formula (Ca3−yMgy(HPO4)z(PO4)2−2z/3] was also achieved using the CHFS system for the range y = 0.7–1.6 (this corresponds to the range x = 1.6–5.3). With increasing substitution of magnesium for calcium, the material became ever more amorphous and the BET surface area generally increased. All the as-precipitated powders (without any additional heat treatments) were analyzed using techniques including X-ray powder diffraction, Raman spectroscopy and Fourier transform infra-red spectroscopy. Transmission electron microscopy (TEM) images revealed that in the case of y = 1.2, the Mg-whitlockite material comprised of ca. 28 nm sized spheres. The use of the CHFS system in this context facilitated rapid production of combinations of particle properties (crystallinity, size, shape) that were hitherto unobtainable in a single step process.

A new direct route for the “bottom up” syntheses of phases in the Lan+1NinO3n+1 series (n=1, 2, 3 and ∞) has been achieved via single-step heat treatments of nanosized co-crystallized precursors. The co-crystallized precursors were prepared using a continuous hydrothermal flow synthesis system that uses a superheated water flow at ca. 400 °C and 24.1 MPa to produce nanoparticulate slurries. Overall, a significant reduction in time and number of steps for the syntheses of La3Ni2O7 and La4Ni3O10 was achieved compared with more conventional synthesis methods, which typically require multiple homogenization and reheating steps over several days.Scanning electron micrograph of La4Ni3O10 (bar=1 μm) made by a single heat treatment at 1075 °C in air for 12 h of a 4:3 La:Ni ratio co-crystallite mixture of the metal hydroxides.

•Direct and continuous hydrothermal synthesis of nano-metal oxide anodes.•The V0.8Sn0.2O2 material has a high capacity of 630 mAh g−1 (at 50 mA g−1).•The specific capacity is always higher when Sn4+ is doped into the host material.•The Sn in the anodes acts as electrochemically active alloying component.A series of nano-sized tin-doped metal oxides of titanium(IV), niobium(V) and vanadium(IV), were directly synthesized using a continuous hydrothermal process and used for further testing without any post-treatments. Each of the as-prepared powders was characterized via a range of analytical techniques including powder X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and Brunauer-Emmett-Teller surface area measurements, as well as being investigated as an electrode material in a lithium-ion coin cell (vs lithium metal). All the tin-doped nanomaterials showed higher specific capacities compared to their undoped metal oxide counterparts. The increased charge storage was discussed to originate from the electrochemical activation of the tin dopant as an alloying material. Overall, this work presents a reliable method of combining stable insertion materials with high capacity tin alloying materials under scaled-up conditions.Figure optionsDownload full-size imageDownload high-quality image (98 K)Download as PowerPoint slide

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.

Ultra-fine copper(II) oxide nanoparticles were used for the electrocatalytic reduction of CO2 to formic acid at high Faradaic efficiencies. The nanoparticles were directly synthesised via continuous hydrothermal flow synthesis (CHFS) process, which used water as a solvent and reagent. The as-prepared nanoparticles were subsequently formulated into Nafion based inks. For the electroreduction of CO2, the influence of Nafion fraction on the Faradaic efficiencies and overpotential (for formic acid production), was explored over a wide potential range. The highest Faradaic efficiency for formic acid production (61%) was observed with a 25 wt% Nafion fraction, at a potential of −1.4 V vs. Ag/AgCl. Some insights into the significant increase in Faradaic efficiency for the production of formic acid with the optimum Nafion content, was elucidated with electrochemical impedance spectroscopy.

A novel and rapid and continuous hydrothermal route to the synthesis of extensive ultra-thin 2D sodium titanate (Na2Ti3O7) nano-sheets using a superheated water flow at 450 °C and 24.1 MPa as a crystallizing medium is described. High resolution electron microscopy of the sheets revealed that they were a few layers thick and largely uncurled, highly crystalline despite their very short time under hydrothermal flow conditions. The sodium titanate sheets possessed excellent photocatalytic activity for decolourisation of methylene blue dye.

Robust, bilayer heterojunction photodiodes of TiO2–WO3 were prepared successfully by a simple, low-cost powder pressing technique followed by heat-treatment. Exclusive photoirradiation of the TiO2 side of the photodiode resulted in a rapid colour change (dark blue) on the WO3 surface as a result of reduction of W6+ to W5+ (confirmed by X-ray photoelectron spectroscopy). This colour was long lived and shown to be stable in a dry environment in air for several hours. A similar photoirradiation experiment in the presence of a mask showed that charge transfer across the heterojunction occurred approximately normal to the TiO2 surface, with little smearing out of the mask image. As a result of the highly efficient vectorial charge separation, the photodiodes showed a tremendous increase in photocatalytic activity for the degradation of stearic acid, compared to wafers of the respective individual materials when tested separately.

Nb-doped TiO2 (anatase) nanoparticles were synthesized using a continuous hydrothermal flow synthesis reactor using a supercritical water flow as a reagent and crystallizing medium. The as-prepared nano-powders with ca. 25 at% Nb5+ (<6 nm diameter) were used as possible anodes for lithium-ion batteries without any further heat-treatment. Cyclic voltammetry and galvanostatic charge/discharge cycling tests were performed in the range of 1.2 to 3.0 V vs. Li/Li+. The Nb-doped TiO2 samples showed superior capacity retention at high current rates compared to the corresponding undoped nano-TiO2. The superior performance of the doped samples (at specific currents up to 15 A g−1) was attributed to higher electronic conductivity and a greater charge storage contribution from surface effects like pseudocapacitance (Faradaic processes) as well as Helmholtz double layer charge storage.

Continuous hydrothermal flow synthesis (CHFS) technology has been used as an efficient and direct route to produce a range of largely crystalline magnesium substituted calcium phosphate bioceramics. Initially, magnesium substituted hydroxyapatite, Mg-HA, according to the formula [Ca10−xMgx(PO4)6(OH)2] was prepared in the CHFS system for x = 0.2 [where x:(10 − x) is the Mg:Ca ratio used in the reagents]. Biphasic mixtures of Mg-HA and Mg-whitlockite were obtained corresponding to x values in the range x = 0.4–1.6. The direct synthesis of phase pure crystalline Mg-whitlockite [based on the formula (Ca3−yMgy(HPO4)z(PO4)2−2z/3] was also achieved using the CHFS system for the range y = 0.7–1.6 (this corresponds to the range x = 1.6–5.3). With increasing substitution of magnesium for calcium, the material became ever more amorphous and the BET surface area generally increased. All the as-precipitated powders (without any additional heat treatments) were analyzed using techniques including X-ray powder diffraction, Raman spectroscopy and Fourier transform infra-red spectroscopy. Transmission electron microscopy (TEM) images revealed that in the case of y = 1.2, the Mg-whitlockite material comprised of ca. 28 nm sized spheres. The use of the CHFS system in this context facilitated rapid production of combinations of particle properties (crystallinity, size, shape) that were hitherto unobtainable in a single step process.

A high-throughput synthesis, screening and subsequent scale-up approach was utilised for the optimisation of conductive aluminium and gallium-doped zinc oxide (AZO and GZO, respectively) nanoparticles. AZO and GZO nanoparticles with up to 6 at% dopant (with respect to Zn) were directly synthesised using a laboratory scale continuous hydrothermal process at a rate of 60 g per hour. The resistivities were determined by Hall effect measurements on pressed, heat-treated discs. Both Al- and Ga-doping yielded resistivities of the order of 1 × 10−2 Ω cm for most samples; the lowest resistivity of AZO was 7.0 × 10−3 Ω cm (at 2.5 at% Al doping), and the lowest resistivity of GZO was 9.1 × 10−3 Ω cm (at 3.5 at% Ga doping), which are considered exceptionally conductive for pressed nanopowders. Synthesis of the optimised lab-scale compositions was scaled-up using a pilot-scale continuous hydrothermal process at a production rate of 8 kg per day (by dry mass); results obtained from these nanopowders generally retained resistivity trends observed for the lab-scale analogues.