New generation of continuous hydrothermal flow synthesis materials for environmental applications

The demands for new materials such as carbonaceous nanomaterials and their nanocomposites that are produced in a more efficient, economical, and environmentally friendly manner, as well as the need to integrate them into applications that address global issues, is a current challenge. The focus of this doctoral research project was to develop faster, cleaner synthetic methods to produce materials with superior physical and chemical properties utilising sustainable and/or renewable precursors, ultimately delivering solutions to challenges in environmental applications and beyond. This was accomplished using a continuous hydrothermal flow synthesis approach. Continuous hydrothermal flow synthesis is an unconventional method that uses supercritical water as a reaction environment, and it was designed for the very fast, continuous flow production of high quality and high quantity nanomaterials, with real-time control over the process parameter (temperature, pressure, precursors concentration, pH, and flow rates). For the first time, by developing and applying CHFS methodologies, carbon quantum dots (CQDs) and nitrogen-doped carbon quantum dots (NCQDs) were synthesized. The CHFS CQDs and NCQDs were produced from biomass-related precursors: glucose and citric acid; the synthetic process can be classified not only as green but sustainable too. The reduction of graphene oxide with a non-corrosive and reusable reducing agent (formic acid) was achieved. The reduced graphene oxide with different oxygen content (17.37 at%, 16.82 at% and 13.3 at%) was synthesised. A new CHFS method to produce nano TiO2 (anatase) has been developed to pioneer the one-pot synthesis of carbonaceous nanocomposites of TiO2 with NCQDs and reduced graphene oxide (rGO). All the materials produced in this study have been characterised and tested in environmental-related applications: a) toxic ions sensing (CQDs, NCQDs) with promising limits of detection for hexavalent chromium (LODCQDs= 3.62 ppm and LODNCQDs=0.365 ppm), b) graphene-related membrane-based water treatment with good performances, and photocatalysis where the TiO2 and its carbonaceous nanocomposites were tested for photodegradation of methylene blue with excellent conversions and rates constants (the best photocatalyst, TiO2-NCQDs-rGO(1), showed a conversion of 93.45% and a rate constant of 25.24 x10-5 s-1). This research project designed and engineered new promising carbonaceous materials, expanding the CHFS portfolio to new frontiers. The as-prepared materials exhibited superior physical, chemical, and morphological characteristics demonstrating their potential for environmental applications and beyond

Maximizing Polypropylene Recovery from Waste Carpet Feedstock: A Solvent-Driven Pathway Towards Circular Economy

Here we propose a novel approach for the efficient recovery of polypropylene from waste carpet feedstock utilising a solvent based method operating at 160 °C. The findings contribute to advancing sustainable recycling practices for waste carpet materials and offer valuable insight into the recovery of PP which can also be utilised for other complex waste streams

Nanostructured Al2O3/Graphene Additive in Bio-Based Lubricant: A Novel Approach to Improve Engine Performance

Personal and industrial use of internal combustion engines (ICEs) is projected to continue until 2050 and beyond. Yet demands to reduce global dependence on petrochemicals and fossil fuel-derived lubricants are increasing and environmentally necessary. New strategies for maintaining and enhancing ICE performance by reducing friction, wear, fuel consumption, and exhaust emissions will reduce the depletion of mineral and fossil fuel reserves and environmental pollution. This paper reports the tribological enhancement of nano-bio lubricants formulated using 2D nanocomposites of Al2O3/graphene as novel additives in coconut oil, whose performance as a lubricant compares favourably with the mineral-based engine oil 15W40. Structural, compositional, and morphological characterization of an Al2O3/graphene nanocomposite synthesized via thermal annealing revealed an ultra-fine particle size (<10 nm) with spherical/laminar morphology and a rich sp2 domain, exhibiting a consistent colloidal stability when formulated as nanofluid. Through the use of various characterisation techniques, including friction and wear analysis we gained valuable insight into the tribological mechanism. Our optimisation of 2D tribological system using coconut oil formulation resulted significant reductions in the coefficient of friction (28%), specific fuel consumption (8%), and exhaust pollutants (CO, SO2, and NOx) emissions. This work demonstrates the benefits of using nano-bio lubricant formulated using coconut oil and 2D based hybrids as base stock and additives, delivering solutions to global challenges such as improving fuel consumption while reducing environmental pollution; solutions that can be transferred to other areas where lubricants are a necessity

Next frontiers in cleaner synthesis: 3D printed graphene-supported CeZrLa mixed-oxide nanocatalyst for CO2 utilisation and direct propylene carbonate production

A rapidly-growing 3D printing technology is innovatively employed for the manufacture of a new class of heterogenous catalysts for the conversion of CO2 into industrially relevant chemicals such as cyclic carbonates. For the first time, directly printed graphene-based 3D structured nanocatalysts have been developed combining the exceptional properties of graphene and active CeZrLa mixed-oxide nanoparticles. It constitutes a significant advance on previous attempts at 3D printing graphene inks in that it does not merely explore the printability itself, but enhances the efficiency of industrially relevant reactions, such as CO2 utilisation for direct propylene carbonate (PC) production in the absence of organic solvents. In comparison to the starting powder, 3D printed GO-supported CeZeLa catalysts showed improved activity with higher conversion and no noticeable change in selectivity. This can be attributed to the spatially uniform distribution of nanoparticles over the 2D and 3D surfaces, and the larger surface area and pore volume of the printed structures. 3D printed GO-supported CeZeLa catalysts compared to unsupported 3D printed samples exhibited higher selectivity and yield owing to the great number of new weak acid sites appearing in the supported sample, as observed by NH3-TPD analysis. In addition, the catalyst's facile separation from the product has the capacity to massively reduce materials and operating costs resulting in increased sustainability. It convincingly shows the potential of these printing technologies in revolutionising the way catalysts and catalytic reactors are designed in the general quest for clean technologies and greener chemistry

Continuous Hydrothermal Flow Synthesis of Blue-Luminescent, Excitation-Independent N-doped Carbon Quantum Dots as Nanosensors

Blue-luminescent N-doped carbon quantum dots (NCQDs) exhibiting rarely observed excitation independent optical properties are synthesised from citric acid in the presence of ammonia via a Continuous Hydrothermal Flow Synthesis (CHFS) approach. CHFS is an eco-friendly, rapid synthetic approach (within fractions of a second) facilitating ease of scale-up industrialization as well as offering materials with superior properties. The synthesised CQDs readily disperse in aqueous solution, have an average particle size of 3.3 ± 0.7 nm, with highest emission intensity at 441 nm (and a narrow full width at half maximum, FWHM ~78 nm) under a 360 nm excitation wavelength. Carbon quantum dots, without any further modification, exhibited a high selectivity and sensitivity as a nano-sensor for the highly toxic and carcinogenic chromium(VI) ions. The nano-chemo-sensor delivers significant advantages including simplicity of manufacturing via a continuous, cleaner technology (using targeted biomass precursor), high selectivity, sensitivity and fast response leading to potential applications in environmental industry as well photovoltaics, bio-tagging, bio-sensing and beyond

Engineering Nitrogen-Doped Carbon Quantum Dots: Tailoring Optical and Chemical Properties through Selection of Nitrogen Precursors

The process of N-doping is frequently employed to enhance the properties of carbon quantum dots. However, the precise requirements for nitrogen precursors in producing high-quality N-doped carbon quantum dots (NCQDs) remain undefined. This research systematically examines the influence of various nitrogen dopants on the morphology, optical features, and band structure of NCQDs. The dots are synthesized using an efficient, eco- friendly, and rapid continuous hydrothermal flow technique. This method offers unparalleled control over synthesis and doping, while also eliminating convention-related issues. Citric acid is used as the carbon source, and urea, trizma base, beta-alanine, L-arginine, and EDTA are used as nitrogen sources. Notably, urea and trizma produced NCQDs with excitation-independent fluorescence, high quantum yields (up to 40%), and uniform dots with narrow particle size distributions. Density functional theory (DFT) and time-dependent DFT modelling established that defects and substituents within the graphitic structure have a more significant impact on the NCQDs’ electronic structure than nitrogen-containing functional groups. Importantly, for the first time, this work demonstrates that the conventional approach of modelling single-layer structures is insufficient, but two layers suffice for replicating experimental data. This study, therefore, provides essential guidance on the selection of nitrogen precursors for NCQD customization for diverse applications

Efficient Continuous Hydrothermal Flow Synthesis of Carbon Quantum Dots from a Targeted Biomass Precursor for On−Off Metal Ions Nanosensing

Glucose, a readily available biomass precursor is used for the production of carbon quantum dots (CQDs) via a fast, efficient, and environmentally benign continuous hydrothermal flow synthesis (CHFS) process using supercritical water, an approach that can readily be scaled up for industrialization, producing materials with enhanced properties. The water-soluble CQDs exhibit an average particle size of 2.3 ± 0.5 nm, with an optimum emission intensity at 446 nm on excitation at 360 nm. The as-synthesized CQDs with no extra modification show promising sensitivity and good selectivity for the highly toxic, carcinogenic, and mutagenic chromium(VI) ion (limit of detection of 1.83 ppm) and for the essential bioactive transition metal, iron(II) ion (limit of detection of 6.09 ppm). The life-cycle assessment confirms that in comparison to the conventional batch synthetic method, the continuous hydrothermal flow synthesis process is significantly a more efficient and greener route for the synthesis of carbon quantum dots from the glucose biomass precursor

In-situ continuous hydrothermal synthesis of TiO₂ nanoparticles on conductive N-doped MXene nanosheets for binder-free li-ion battery anodes

Anode materials are key to determining the energy density, cyclability and of life recyclability for Li-ion energy storage systems. High surface area materials, such as MXenes, can be manufactured with improved electrochemical properties that remove the need for polymeric binders or hazardous chemicals that pose a challenge to recycle Li-ion batteries. However, there remains a challenge to produce Li-ion anode materials that are binder free and poses energy storage characteristics that match the current carbon-based electrodes. Here we show the synthesis of N-doped MXene-TiO2 hybrid anode materials using an aqueous route. N-doped TiO2-MXene was modified using a single step continuous hydrothermal process. Capacity tests indicate an improvement from the initial specific energy capacity of 305 mAhg−1 to 369 mAhg−1 after 100 cycles at a charge rate of 0.1 C and a Coulombic efficiency of 99.7%. This compares to 252 mAhg−1 for the unmodified MXene which exhibited significant capacity fade to 140 mAhg−1. The ability to manufacture a Li-ion anode that does not require toxic chemicals for processing into an electrode and exhibits good energy storage characteristics in a binder free system is a significant step forward for energy storage applications

Investigating the effect of N‐doping on carbon quantum dots structure, optical properties and metal ion screening

Carbon quantum dots (CQDs) derived from biomass, a suggested green approach for nanomaterial synthesis, often possess poor optical properties and have low photoluminescence quantum yield (PLQY). This study employed an environmentally friendly, cost-effective, continuous hydrothermal flow synthesis (CHFS) process to synthesise efficient nitrogen-doped carbon quantum dots (N-CQDs) from biomass precursors (glucose in the presence of ammonia). The concentrations of ammonia, as nitrogen dopant precursor, were varied to optimise the optical properties of CQDs. Optimised N-CQDs showed significant enhancement in fluorescence emission properties with a PLQY of 9.6% compared to pure glucose derived-CQDs (g-CQDs) without nitrogen doping which have PLQY of less than 1%. With stability over a pH range of pH 2 to pH 11, the N-CQDs showed excellent sensitivity as a nano-sensor for the highly toxic highly-pollutant chromium (VI), where efficient photoluminescence (PL) quenching was observed. The optimised nitrogen-doping process demonstrated effective and efficient tuning of the overall electronic structure of the N-CQDs resulting in enhanced optical properties and performance as a nano-sensor