Movement of palladium nanoparticles in hollow graphitised nanofibres: the role of migration and coalescence in nanocatalyst sintering during the Suzuki–Miyaura reaction

The evolution of individual palladium nanoparticle (PdNP) catalysts, in graphitised nanofibres (GNF), in the liquid-phase Suzuki-Miyaura (SM) reaction has been appraised. The combination of identical location-transmission electron microscopy (IL-TEM) and a nano test tube approach allowed spatiotemporal continuity of observations at single nanopartcile level, revealing that migration and coalescence is the most significant pathway to coarsening of the nanocatalyst, rather than Ostwald ripening. IL-TEM gave unprecedented levels of detail regarding the movement of PdNP on carbon surfaces at the nanoscale, including size-dependent migration and directional movement, opening horizons for optimisation of future catalysts through surface morphology design

Formation of hollow carbon nanoshells from thiol stabilised silver nanoparticles via heat treatment

Uniform, less 10 nm sized, hollow carbon nano-shells (HCNS) have been prepared via a single-step, thermal treatment of alkanethiol stabilised Ag nanoparticles (TS-AgNP). Direct evidence for the formation of spherical HCNS from TS-AgNP is provided by in situ MEMS heating on Si3N4 supports within a TEM, and ex situ thermal processing of TS-AgNP on carbon nanotube supports. A mechanism is proposed for the thermally driven, templated formation of HCNS from the TS-AgNP stabilising layer, with Ag catalysing the graphitisation of carbon in advance of thermally induced AgNP template removal. This facile processing route provides for excellent size control of the HCNS product via appropriate AgNP template selection. However, a rapid rate of heating was found to be crucial for the formation of well-defined HCNS, whilst a slow heating rate gave a much more disrupted product, comprising predominantly lacy carbon with decreased levels of graphitic ordering, reflecting a competition between the thermal transformation of the TS-layer and the rate of removal of the AgNP template

Molybdenum dioxide in carbon nanoreactors as a catalytic nanosponge for the efficient desulfurization of liquid fuels

The principle of a “catalytic nanosponge” that combines the catalysis of organosulfur oxidation and sequestration of the products from reaction mixtures is demonstrated. Group VI metal oxide nanoparticles (CrOx, MoOx, WOx) are embedded within hollow graphitized carbon nanofibers (GNFs), which act as nanoscale reaction vessels for oxidation reactions used in the decontamination of fuel. When immersed in a model liquid alkane fuel contaminated with organosulfur compounds (benzothiophene, dibenzothiophene, dimethyldibenzothiophene), it is found that MoO2@GNF nanoreactors, comprising 30 nm molybdenum dioxide nanoparticles grown within the channel of GNFs, show superior abilities toward oxidative desulfurization (ODS), affording over 98% sulfur removal at only 5.9 mol% catalyst loading. The role of the carbon nanoreactor in MoO2@GNF is to enhance the activity and stability of catalytic centers over at least 5 cycles. Surprisingly, the nanotube cavity can selectively absorb and remove the ODS products (sulfoxides and sulfones) from several model fuel systems. This effect is related to an adsorptive desulfurization (ADS) mechanism, which in combination with ODS within the same material, yields a “catalytic nanosponge” MoO2@GNF. This innovative ODS and ADS synergistic functionality negates the need for a solvent extraction step in fuel desulfurization and produces ultralow sulfur fuel

Steric and electronic control of 1,3-dipolar cycloaddition reactions in carbon nanotube nanoreactors

The use of single-walled carbon nanotubes as effective nanoreactors for preparative bimolecular reactions has been demonstrated for the first time. We show that the extreme spatial confinement of guest reactant molecules inside host carbon nanotubes increases the regioselectivity for 1,4-triazole in thermally initiated azide–alkyne cycloaddition reactions. Through comparison of the internal dimensions of the nanotube and the steric bulk of the reactants, we demonstrate that the formation of the more linear 1,4-regioisomer can be enhanced by up to 55% depending on the extent of spatial restrictions imposed within the nanoreactors. Furthermore, through systematic variation of the substituents in the para-position of the alkyne reactants, we reveal the unexpected influence of the reactants’ electronic properties on the regioselectivity of reactions within nanoreactors, which act to either oppose or promote the preferential formation of the 1,4-regioisomer induced by steric effects, reflecting the unique ability of carbon nanotubes to stabilize the dipole moment of confined reactants. Thus, we show that the observed regioselectivity of azide–alkyne cycloaddition reactions confined within carbon nanotube nanoreactors reflects a subtle interplay between both steric and electronic factors

Synthesis of hydroxylated group IV metal oxides inside hollow graphitised carbon nanofibers: nano-sponges and nanoreactors for enhanced decontamination of organophosphates

The confinement and enhanced catalytic properties of hydroxylated group IV metal oxide nanostructures inside hollow graphitised carbon nanofibers (GNF) has been demonstrated. GNF – a structural analogue of carbon nanotubes – were effectively filled with suitable precursor molecules of metal chlorides from the gas and liquid phases. Subsequent basecatalysed hydrolysis afforded amorphous, nanostructured hydroxylated metal oxide (MOx(OH)y where M = Zr, Ti, and Hf) thin films, which coat the internal surfaces of GNF. This versatile and general strategy allows the chemical composition and morphology of the encapsulated material to be modified by varying the conditions used for hydrolysis and post-synthesis thermal treatment. The increased Lewis acidic properties and high surface area of the zirconium composite promote the catalysed hydrolysis of dimethyl nitrophenyl phosphate (DMNP) – a toxic organophosphorus chemical. A four-fold enhancement in the rate of DMNP hydrolysis relative to its separate constituent components was observed, highlighting the surprising synergistic abilities of this composite material to perform both as a ‘nano-sponge’, absorbing the harmful compounds inside the GNF, and a nanoreactor, enhancing the local concentration of organophosphate around the hydroxylated metal oxide species, leading to improved catalytic performance

Palladium nanoparticles in catalytic carbon nanoreactors: the effect of confinement on Suzuki-Miyaura reactions

We explore the construction and performance of a range of catalytic nanoreactors based on palladium nanoparticles encapsulated in hollow graphitised nanofibres. The optimum catalytic material, with small palladium nanoparticles located almost exclusively at the graphitic step-edges within nanoreactors, exhibits attractive catalytic properties in Suzuki-Miyaura cross-coupling reactions. Confinement of nanoparticles at the step-edges facilitates retention of catalytic centres and recycling of catalytic nanoreactors without any significant loss of activity or selectivity over multiple catalytic cycles. Furthermore, careful comparison of the catalytic properties of palladium nanoparticles either on or in nanoreactors reveals that nanoscale confinement of catalysts fundamentally affects the pathways of the Suzuki-Miyaura reaction, with the yield and selectivity for the cross-coupled product critically dependent on the steric properties of the aryl iodide reactant, whereas no effects of confinement are observed for aryl boronic acid reactants possessing substituents in different positions. These results indicate that the oxidative addition step of the Suzuki-Miyaura reaction occurs at the step-edge of nanofibres, where the mechanisms and kinetics of chemical reactions are known to be sensitive to nanoscale confinement, and thus the extent of confinement in carbon nanoreactors can be discretely controlled by careful selection of the aryl iodide reactant

Comparative nanostructure analysis of gasoline turbocharged direct injection and diesel soot-in-oil with carbon black

Two gasoline turbocharged direct injection (GTDI) and two diesel soot-in-oil samples were compared with one flame-generated soot sample. High resolution transmission electron microscopy imaging was employed for the initial qualitative assessment of the soot morphology. Carbon black and diesel soot both exhibit core-shell structures, comprising an amorphous core surrounded by graphene layers; only diesel soot has particles with multiple cores. In addition to such particles, GTDI soot also exhibits entirely amorphous structures, of which some contain crystalline particles only a few nanometers in diameter. Subsequent quantification of the nanostructure by fringe analysis indicates differences between the samples in terms of length, tortuosity, and separation of the graphitic fringes. The shortest fringes are exhibited by the GTDI samples, whilst the diesel soot and carbon black fringes are 9.7% and 15.1% longer, respectively. Fringe tortuosity is similar across the internal combustion engine samples, but lower for the carbon black sample. In contrast, fringe separation varies continuously among the samples. Raman spectroscopy further confirms the observed differences. The GTDI soot samples contain the highest fraction of amorphous carbon and defective graphitic structures, followed by diesel soot and carbon black respectively. The AD1:AG ratios correlate linearly with both the fringe length and fringe separation

Alumina-graphene nanocomposite coatings fabricated by suspension high velocity oxy-fuel thermal spraying for ultra-low-wear

An alumina coating containing 1 wt.% graphene nanoplatelets (GNPs) is deposited for the first time by suspension high velocity oxy-fuel (SHVOF) thermal spraying – an emerging coating deposition technique. Raman spectroscopy shows the GNPs survive the process albeit with a two-fold increase in the density of defects in the graphitic lattice. Dry-sliding wear testing at a 10 N load yields a two order of magnitude reduction in the specific wear rate for the alumina/GNP composite in comparison to the equivalent pure alumina coating. Testing with varying loads shows that the transition to the severe wear regime is prevented until between 30 and 35 N for the alumina + GNP coating which otherwise occurs between 5 and 7.5 N in the absence of GNPs. A reinforced microstructure, and specifically fracture toughness, explains the improved wear behaviour as opposed to tribofilm formation

Surface modification of mild steel using a combination of laser and electrochemical processes

Traditional methods for achieving hierarchical surface structures include highly specified, deterministic approaches to create features to meet design intention. In this study microstructural alteration was undertaken using laser apparatus and secondary texturing was achieved via succeeding electrochemical processes. Electrochemical jet machining (EJM) was performed on mild steel subjected to laser pre-treatment using power densities of 4167 and 5556 W/cm2 with pulse durations from 0.3 - 1.5 seconds. Results show that in combination, laser pre-treatment and EJM can alter the exposed surface textures and chemistries. Here, machined surface roughness (Sa) was shown to increase from approximately 0.45 µm for untreated surfaces to approximately 18 µm for surfaces subjected to extreme laser pretreatments. After pre-treatments materials were characterised to appraise microstructural changes, shown to be martensite formation, reinforced by complementary simulation data, and significant increases in observable hardness from approximately 261 HV for the asreceived material to over 700 HV after pre-treatment. The greater hardness was retained after EJM. Exposed martensitic lath structures at machined surfaces are shown to be partially responsible for surface roughness increases. The surfaces were explored with energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy demonstrating changes in apparent surface chemistry. This analysis revealed increasing oxide formation at the surface of the pre-treated EJM surface, a further contributory factor to surface roughness increases. This new process chain will be of interest to manufacturers seeking to control surface morphology for applications including micro-injection mould/die manufacture. While demonstrated here for steel similar mechanisms are exploitable in other material systems. A new technique has been demonstrated, resulting from the models and processes presented to couple laser and electrolyte jet processing for complex surface preparation

A Click Chemistry Strategy for the Synthesis of Efficient Photoinitiators for Two‐Photon Polymerization

It is reported that efficient photoinitiators, suitable for two‐photon polymerization, can be obtained using the copper catalyzed azide/alkyne cycloaddition reaction. This click chemistry strategy provides a modular approach to the assembly of photoinitiators that enables the rapid variation of key fragments to produce photoinitiators with desirable properties. To assess the performance of the first‐in‐class photoinitiators generated by this approach, a screening method is developed to enable the rapid determination of polymerization and damage thresholds in numerous photoresists during two‐photon polymerization. The degree of consumption of vinyl groups (DC) and homogeneity of the polymerization are further assessed by micro‐Raman spectroscopy. Finally, more complex structures are fabricated to demonstrate that the efficient two‐photon polymerization of stable 3D microarchitectures can be achieved using triazole‐based photoinitiators