Continuous Hydrothermal Flow Synthesis Research Articles

High-Throughput Continuous Hydrothermal Synthesis of Transparent Conducting Aluminum and Gallium Co-doped Zinc Oxides.

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.

Rapid synthesis of graphene quantum dots using a continuous hydrothermal flow synthesis approach

A rapid and environmentally benign synthesis of green fluorescent graphene quantum dots (GQD) with low cytotoxicity via Continuous Hydrothermal Flow Synthesis (CHFS) aided by calix[4]arene tetrasulfonic acid (SCX4) as a particle size limiting agent.

Light-driven generation of chlorine and hydrogen from brine using highly selective Ru/Ti oxide redox catalysts

Ultrafine ruthenium–titanium oxide catalysts were directly produced using a continuous hydrothermal flow synthesis process and assessed as chloride oxidation catalysts.

Sustainable Transparent Conducting Oxide Nanomaterials; Aluminium- and Gallium-Co-Doped Zinc Oxide (AGZO)

Transparent conducting oxide (TCO) nanoparticles based on zinc oxide doped with aluminium and gallium (Al- and Ga-co-doped ZnO, AGZO) were synthesized using a continuous hydrothermal flow synthesis (CHFS) reactor by rapid hydrolysis and dehydration of aqueous metal salt precursors, in the presence of formic acid as an in-process reducing agent. The as-synthesised powders were characterized by powder X-ray diffraction (pXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The as-synthesised powders were pressed into ceramic compacts and heat-treatment to ensure phase-pure wurtzite structure. The electrical properties of the materials were evaluated by Hall Probe measurements on the compacts, showing resistivities as low as 4.55 × 10−2 Ω cm, which represents a very high conductivity for a pressed disc based on ZnO.

Nanocomposite YSZ-NiO Particles with Tailored Structure Synthesized in a Two-Stage Continuous Hydrothermal Flow Reactor

The increasing amount of fluctuating electricity generation from renewable sources requires a flexible energy system and storage technologies to ensure that energy services can be covered in a stable and affordable manner. In order to become truly independent from fossil fuels, increasing the performance of energy storage and conversion devices such as fuel cells, electrolyzers and batteries is important. One promising approach to further improve these devices is the use of carefully structured nanosized materials. Nano-composite particles combining different materials in advanced geometries like core-shell structures or surface decorated particles could exhibit better performance compared with single phase materials. To obtain such advanced structures is the aim of the ProEco project (www.proeco.dk). In this project, a two-stage continuous reactor is built and used to synthesize such nano-composites. Here we report on the design of the two-stage continuous hydrothermal flow synthesis reactor and first results on obtaining structured nano-composite consisting of yttria-stabilized zirconia (YSZ) and NiO materials. These materials are commonly applied in the fuel electrodes of today’s state-of-the-art solid oxide fuel and electrolysis cells. The prepared particles were characterized by X-ray powder diffraction, (high resolution) transmission electron microscopy, scanning tunnel transmission microscopy and Raman spectroscopy in order to determine crystal structure, particle size, surface morphology and element distribution.

Aerosol assisted chemical vapour deposition of hydroxyapatite-embedded titanium dioxide composite thin films

This work describes the first Aerosol Assisted Chemical Vapour Deposition (AACVD) synthesis of photocatalytic titanium dioxide thin films embedded with synthetic hydroxyapatite, [Ca10(PO4)(OH)2], nanoparticles. The hydroxyapatite nanoparticles were prepared using a low temperature continuous hydrothermal flow synthesis method; analysis of the hydroxyapatite powder showed that it was phase pure and that the as-prepared material was made up of nano-needles. The nanoparticles were then embedded into TiO2 coatings using the AACVD technique by suspending them in a solution of the titania precursor (titanium tetra-isopropoxide). Results showed that the hydroxyapatite, although present in very low concentrations in the coatings (not detectable by XRD or Raman spectroscopy), heavily affected their morphology, depending on their concentration in the precursor solution. Tests of the photocatalytic activity of the composite films showed that the inclusion of the hydroxyapatite led to an increase in methylene blue photodegradation (up to 50% higher) and that the materials were photostable. This study shows that TiO2 coatings embedded with hydroxyapatite nanoparticles have potential as highly efficient photocatalysts.

Combined EXAFS, XRD, DRIFTS, and DFT Study of Nano Copper-Based Catalysts for CO2 Hydrogenation

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...

High power Nb-doped LiFePO4 Li-ion battery cathodes; pilot-scale synthesis and electrochemical properties

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.

Calixarene Assisted Rapid Synthesis of Silver-Graphene Nanocomposites with Enhanced Antibacterial Activity.

Demonstrated herein is a single rapid approach employed for synthesis of Ag-graphene nanocomposites, with excellent antibacterial properties and low cytotoxicity, by utilizing a continuous hydrothermal flow synthesis (CHFS) process in combination with p-hexasulfonic acid calix[6]arene (SCX6) as an effective particle stabilizer. The nanocomposites showed high activity against E. coli (Gram-negative) and S. aureus (Gram-positive) bacteria. The materials were characterized using a range of techniques including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV-vis spectrophotometry, FT-IR, and X-ray powder diffraction (XRD). This rapid, single step synthetic approach not only provides a facile means of enabling and controlling graphene reduction (under alkaline conditions) but also offers an optimal route for homogeneously producing and depositing highly crystalline Ag nanostructures into reduced graphene oxide substrate.

Simulation, design and proof-of-concept of a two-stage continuous hydrothermal flow synthesis reactor for synthesis of functionalized nano-sized inorganic composite materials

Computational fluid dynamics simulations were employed to evaluate several mixer geometries for a novel two-stage continuous hydrothermal flow synthesis reactor. The addition of a second stage holds the promise of allowing the synthesis of functionalized nano-materials as for example core-shell or decorated particles. Based on the simulation results, a reactor system employing a confined jet mixer in the first and a counter-flow mixer in the second stage was designed and built. The two-stage functionality and synthesis capacity is shown on the example of single- and two-stage syntheses of pure and mixed-phase NiO and YSZ particles.

Transition metal doped anatase nanocrystals: Continuous-flow hydrothermal synthesis and photocatalytic activity

Transition metal doped anatase nanoparticles have been prepared by a continuous-flow hydrothermal method. An aqueous solution containing a titanium source and appropriate amounts of cobalt or nickel nitrate is mixed in-flow with potassium hydroxide as a mineraliser before meeting a pre-heated stream of supercritical water in a counter-current mixing reactor. Mixing of the metal ion containing solution with supercritical water results in the rapid nucleation of anatase nanocrystals. Powder X-ray diffraction and electron microscopic imaging show that the dopant ions are incorporated into the anatase structure. The incorporation of dopant ions leads to reduced crystallite size and increased surface area. The doped anatase nanocrystals exhibit lower band-gap energies than the undoped material. The photocatalytic activity of the doped and undoped samples has been assessed by monitoring the degradation of methylene blue in the presence of the catalysts under UV illumination. Doping of the anatase nanoparticles significantly increases the photocatalytic activity, with nickel doping showing a pronounced improvement over both the undoped and cobalt doped catalyst nanocrystals.

Continuous-flow hydrothermal synthesis for the production of inorganic nanomaterials

As nanotechnology becomes increasingly important and ubiquitous, new and scalable synthetic approaches are needed to meet the growing demand for industrially viable routes to nanomaterial production. Continuous-flow hydrothermal synthesis or supercritical water hydrothermal synthesis (scWHS) is emerging as a versatile solution to this problem. The process was initially developed to take advantage of the tunable chemical and physical properties of superheated water to produce metal oxide nanoparticles by rapid nucleation and precipitation. The development of new mixing regimes and reactor designs has been facilitated by the modelling of reactor systems. These new reactor designs further exploit the properties of supercritical water to promote faster and more uniform mixing of reagent streams. The synthetic approach has been expanded beyond the metal oxide systems for which it was conceived, and now encompasses metal sulfides, metal phosphates, metal nanoparticles and metal-organic frameworks. In many of these cases, some degree of size and shape control can be achieved through careful consideration of both chemistry and reactor design. This review briefly considers the development of scWHS reactor technology, before highlighting some of our recent work in expanding the scope of this synthetic method to include a wide range of materials.

Nucleation and Growth of Cobalt Oxide Nanoparticles in a Continuous Hydrothermal Reactor under Laminar and Turbulent Flow

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 diffra...

High power TiO2 and high capacity Sn-doped TiO2 nanomaterial anodes for lithium-ion batteries

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.

High capacity nanocomposite Fe3O4/Fe anodes for 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.

Synthesis and surface modification of nanophosphorous-based flame retardant agent by continuous flow hydrothermal synthesis

Nanoparticles can provide flame retardance to hosting polymers and act as nano fire extinguishers. Hydroxyapatite (Ca5(OH)(PO4)3) (HA) is not hygroscopic, and is thermally stable up to 800°C, with 18.5wt% phosphorous content. It is this high phosphorous content that can provide HA with flame retardant properties. In this paper, we report on the continuous synthesis of ultrafine HA using a hydrothermal synthesis technique. The HA surface properties were changed from hydrophilic to hydrophobic by post-synthesis surface modification. The ratio of the HA nanoparticles and an intumescent agent known as Exolit AP750 was investigated to yield a self-extinguishing multi-component epoxy nanocomposite for extended application under extreme fire conditions. The HA/AP750/epoxy nanocomposite was able to resist a flame at 1700°C and self-extinguish after the flame had been removed. The nanocomposite showed an enhanced flammability performance in standard cone calorimetry testing and formed a compact and cohesive protective char layer with a 50% decrease in peak heat released compared with virgin epoxy. Our aim was to establish the use of HA as an effective nanofiller with phosphorous-based flame retardant properties. The surface of this nano fire extinguisher was modified effectively with different surfactants for enhanced compatibility with different polymeric matrices.

Greener synthesis of propylene carbonate using graphene-inorganic nanocomposite catalysts

Continuous hydrothermal flow synthesis (CHFS) has been employed as rapid and cleaner route for the synthesis of a novel, highly efficient graphene-inorganic heterogeneous catalyst, denoted as Ce–La–Zr–GO nanocomposite. The catalyst was used for the direct synthesis of propylene carbonate (PC) from the cycloaddition of carbon dioxide (CO2) and propylene oxide (PO) in the absence of any organic solvents. Ce–Zr–La–GO nanocomposite was synthesized from pre-mixed aqueous solution of cerium, lanthanum, zirconium nitrate and GO (synthesized via conventional Hummers method) under alkaline conditions using the CHFS reactor. The resulting nanocomposite catalyst was heat-treated at various temperatures (773, 973 and 1173K) and their catalytic properties were assessed. The as-prepared and the corresponding heat-treated catalysts were characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and Brunauer–Emmett–Teller (BET) surface area measurements. Effect of various parameters such as heat-treatment temperature, catalyst loading, CO2 pressure, reaction time and temperature was systematically studied to determine the optimum reaction conditions. The optimum reaction condition for the direct synthesis of PC was found at 443K, 70bar and 10% (w/w) catalyst loading. Catalyst reusability study was also conducted to investigate the long life stability for the synthesis of PC and it was found that the catalyst could be reused several times without losing it catalytic activity. These studies revealed that Ce–Zr–La–GO nanocomposite catalyst showed high catalytic activity as compared to other reported heterogeneous catalysts with the conversion of PO, selectivity and yield of PC.

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.

Highly pseudocapacitive Nb-doped TiO2 high power anodes for lithium-ion batteries

Nb-doped and pure TiO2 (anatase) nanoparticles were synthesized via a continuous hydrothermal flow synthesis reactor and investigated as electrode material for lithium-ion batteries.

The sequential continuous-flow hydrothermal synthesis of molybdenum disulphide.

Molybdenum disulphide (MoS2) has been widely used as a catalyst and high temperature lubricant. It has been heavily researched recently as a graphene analogue and member of the so-called inorganic fullerenes. Here we report the first continuous flow hydrothermal synthesis of MoS2. With fast reaction times and flexibility the continuous flow hydrothermal system allowed MoS2 to be produced in a stepwise fashion, offering an insight into the mechanism involved. It has been found that the synthesis of MoS2 proceeded via the sulphidation of molybdate anions to thiomolybdate species, which are transformed to amorphous MoS3 by acidification in flow, before further hydrothermal treatment decomposes this amorphous precursor to tangled MoS2 nanosheets.