Preparation and performance of nanostructured iron oxide thin films for solar hydrogen generation

Nowadays, energy and its resources are of prime importance at the global level. During the last few decades there have been several driving forces for the investigation of new sources of energy. Hydrogen has long been identified as one of the most promising carriers of energy. Photoelectrochemical (PEC) water splitting is one of the most promising means of producing hydrogen through a renewable source. Hematite (α-Fe2O3) is a strong candidate material as photoelectrode for PEC water splitting as it fulfils most of the selection criteria of a suitable photocatalyst material for hydrogen generation such as bandgap, chemical and photelectrochemical stability, and importantly ease of fabrication. This work has explored different preparation techniques for undoped and Si-doped iron oxide thin films using microwave-assisted and conventional preparation methods. Two distinct strategies towards improving PEC performance of hematite photoelectrodes were examined: retaining a finer nanostructure and enhancing the photocatalytic behaviour through doping. By depositing thin films using atmospheric pressure chemical vapour deposition (APCVD) and aerosol-assisted CVD (AACVD) at high temperature, it was shown that a combination of different factors (such as silicon incorporation into the hematite structure and formation of lattice defects, along with a nanostructure of small agglomerate/cluster enhancing hole transportation to the surface) were the contributing factors in improving the PEC performance in hematite films. The role of the Si-containing precursors and their consecutive effect on nanostructure of the hematite films were investigated. Further work is needed to study the decomposition pattern of precursors and consequent effects of Si additives as well as co-dopants on fundamental physical and electrical properties of hematite electrodes. In addition, the feasibility of using microwave annealing for the fabrication of iron oxide thin films prepared by electrodeposition at low temperature was also investigated. Hematite films showed improved PEC performance when microwave assisted annealing was used. Microwave heating decreased the annealing temperature by ~40% while the PEC performance was increased by two-fold. The improved performance is attributed to the lower processing temperatures and rapidity of the microwave method that help to retain the nanostructure of the thin films whilst restricting the grain coalescence to a minimum. Around 60% of the energy can be saved using this low carbon foot-print approach compared to conventional annealing procedures for the lab-scale preparation of hematite films – a trait that will have significant implications for scale-up production. The lower processing temperature requirements of the microwave process can also open up the possibility of fabricating hematite thin films on conducting, flexible, plastic electronic substrates

Facile preparation of β-/γ-MgH2 nanocomposites under mild conditions and pathways to rapid dehydrogenation

A magnesium hydride composite with enhanced hydrogen desorption kinetics can be synthesized via a simple wet chemical route by ball milling MgH2 with LiCl as an additive at room temperature followed by tetrahydrofuran (THF) treatment under an Ar atmosphere. The as-synthesized composite comprises ca. 18 mass% orthorhombic γ-MgH2 and 80 mass% tetragonal β-MgH2 as submicron-sized particles. The β-/γ-MgH2 nanocomposite exhibits a dehydrogenation capacity of 6.6 wt.% and starts to release hydrogen at ~260 °C; ca. 140 °C lower than that of commercial MgH2. The apparent activation energy for dehydrogenation is 115±3 kJ mol-1, which is ca. 46 % lower than that of commercial MgH2. Analysis suggests that the meta-stable γ-MgH2 component either directly dehydrogenates exothermically or first transforms into stable β-MgH2 very close to the dehydrogenation onset. The improved hydrogen release performance can be attributed both to the existence of the MgH2 nanostructure and to the presence of γ-MgH2

Erosion and mechanical properties of hydrothermally-resistant nanostructured zirconia components

Large scale 50 × 50 mm sintered nanostructured zirconia ceramics were fabricated via industrially viable dry pressing routes. The green bodies were sintered by a two-stage process and the optimised sintering conditions are reported. The suitability of nanostructured zirconia for demanding applications in petrochemical valve components was investigated by slurry impingement erosion experiments. Zirconia showed a 60-fold improvement compared to commonly used stellite-coated commercial stainless steel specimens under test conditions while no tetragonal to monoclinic phase transformation was observed after erosion. The enhanced performance was also valid when compared with reported erosion resistant properties of alumina and zirconia components by a factor of 36 and 3, respectively. This suggests nanostructured zirconia as a potential robust alternative material for construction of internal trim components of petrochemical valves

Electromagnetic simulation studies of microwave assisted heating for the processing of nanostructured iron oxide for solar driven water splitting

Microwave assisted preparation has been shown to improve the performance of hematite photoelectrodes for solar driven water splitting. To understand the microwave heating process further, the distribution of the electromagnetic (EM) fields within the material is analysed using finite-difference time-domain (FDTD) EM software. The rate of the increase in temperature is calculated from the simulated EM field distributions. In order to validate the simulation results, the calculated temperatures were compared with the experimental temperatures obtained using a thermal imaging camera

Multiscale numerical and experimental analysis of tribological performance of GO coating on steel substrates

Herein, nano-tribological behaviour of graphene oxide (GO) coatings is evaluated by a combination of nanoscale frictional performance and adhesion, as well as macroscale numerical modelling. A suite of characterisation techniques including atomic force microscopy (AFM) and optical interferometry are used to characterise the coatings at the asperity level. Numerical modelling is employed to consider the effectiveness of the coatings at the conjunction level. The macroscale numerical model reveals suitable deposition conditions for superior GO coatings, as confirmed by the lowest measured friction values. The proposed macroscale numerical model is developed considering both the surface shear strength of asperities of coatings obtained from AFM and the resultant morphology of the depositions obtained from surface measurements. Such a multi-scale approach, comprising numerical and experimental methods to investigate the tribological behaviour of GO tribological films has not been reported hitherto and can be applied to real-world macroscale applications such as the piston ring/cylinder liner conjunction within the modern internal combustion engine

Rapid microwave-assisted bulk production of high-quality reduced graphene oxide for lithium ion batteries

Graphene-based advanced electrodes with improved electrochemical properties have received increasing attention for use in lithium ion batteries (LIBs). The conventional synthesis of graphene via liquid phase exfoliation or chemical reduction of graphene oxide (GO) approaches, however, either involves prolonged processing or leads to the retainment of high-concentration oxygen functional groups (OFGs). Herein, bulk synthesis of high-quality reduced graphene oxide using microwave irradiation (MWrGO) within few seconds is reported. The electromagnetic interaction of GO with microwaves is elucidated at molecular level using reactive molecular dynamic simulations. The simulation suggests that higher power microwave irradiation results in significantly less retainment of OFGs and the formation of structural voids. The synthesized MWrGO samples are thoroughly characterized in terms of structural evolution and physicochemical properties. Specifically, a modified ID/IG-in ratio metric for Raman spectrum, wherein the intensity contribution of D’ peak is deducted from the apparent G peak, is proposed to investigate the structural evolution of synthesized MWrGO, which yields a more reliable evaluation of structural disorder over traditional ID/IG ratio. Li-ion half-cell studies demonstrate that the MWrGO is an excellent candidate for usage as high capacity anode (750.0 mAh g-1 with near-zero capacity loss) and high-performance cathode (high capacity retention of ~70% for LiCoO2 at 10 C) for LIBs

Ultrasonic Deposition of Carbon Nanotubes on Polycrystalline Cubic Boron Nitride Composites

Polycrystalline cubic boron nitride (PcBN) are super-hard materials with high hardness and excellent abrasive resistance, widely used in cutting tools for precision machining of automotive and aerospace parts; however, their brittle properties make them prone to premature failure. Coatings are often applied to PcBN to extend their range of applicability and durability. Conventional coating methods are limited to the thickness range of a few hundred nanometres, poor adhesion to the substrate, and limited stability under ambient conditions. To further the properties of PcBN composites, in this paper, we explore the use of ultrasonic bonding to apply thick coatings (30–80 μm) on PcBN cutting tools. For the first time, a multi-walled carbon nanotube (MWCNT) powder is preplaced on a PcBN substrate to allow an unconventional coating technique to take place. The effects of ultrasonic bonding parameters on the change of mechanical properties of the coated tools are investigated through scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), micro-hardness analyses, and white light interferometry. The structure of the carbon nanotubes is investigated through transmission electron microscopy (pre coating) and cross-section of the bonded MWCNTs is studied via focused ion beam milling and SEM to evaluate the bonding between the multi-walled nanotubes. Optimum processing windows (i.e., bonding speed, energy, and pressure) are discovered for coating MWCNTs on PcBN. Focus ion beam milling analyses revealed a relationship between consolidation parameters and porosity of MW(pCNT) bonds. The proposed method paves the way for the novel design of functional coatings with attunable properties (i.e., thickness and hardness) and therefore improved productivity in the machining of aerospace and automotive parts

Erosion and mechanical properties of hydrothermally-resistant nanostructured zirconia components

This paper was published in the journal Ceramics International and the definitive published version is available at https://doi.org/10.1016/j.ceramint.2018.03.074.Large scale 50 × 50 mm sintered nanostructured zirconia ceramics were fabricated via industrially viable dry pressing routes. The green bodies were sintered by a two-stage process and the optimised sintering conditions are reported. The suitability of nanostructured zirconia for demanding applications in petrochemical valve components was investigated by slurry impingement erosion experiments. Zirconia showed a 60-fold improvement compared to commonly used stellite-coated commercial stainless steel specimens under test conditions while no tetragonal to monoclinic phase transformation was observed after erosion. The enhanced performance was also valid when compared with reported erosion resistant properties of alumina and zirconia components by a factor of 36 and 3, respectively. This suggests nanostructured zirconia as a potential robust alternative material for construction of internal trim components of petrochemical valves

Enhanced catalytic performance of reduced graphene oxide–TiO2 hybrids for efficient water treatment using microwave irradiation

Towards achieving efficient waste water treatment, the degradation of a common water pollutant, Orange G azo dye, was studied using a new hybrid catalyst and microwave irradiation. The fabrication of a hybrid catalyst based on reduced graphene oxide–titania (rGO–TiO2), was first achieved in a single mode microwave cavity by reducing the precursor consisting of graphene oxide (GO) and titania. Catalytic performance was then assessed in both microwave assisted and conventional heat treatment conditions. The hybrid catalyst showed significant improvement under microwave irradiation, with more than 88% dye degradation after 20 minutes of treatment at 120 °C. The microwave effect was found to be more dominant in the early stages of the catalysis – the hybrid catalyst decomposed ∼65% of the dye in just 5 minutes of microwave treatment compared to only 18% degradation obtained during conventional heating. The improved performance with microwaves is mainly attributed to the formation of the hot spots at the surface of the hybrid catalyst which ultimately results in higher degradation rates. The morphological and catalytic properties of the hybrid catalyst are investigated using High Resolution Transmission Electron Microscopy (HRTEM) and UV-Vis Spectroscopy, respectively. Successful reduction of GO to rGO was confirmed using Raman spectroscopy and X-ray diffraction. The outstanding performance of microwave irradiated hybrids offers a viable low energy, low carbon footprint process with a new catalyst for wastewater treatment and for highly polluted wastewater conditions where photocatalysis is deemed not feasible