Binary clusters produced with cluster beam deposition for electrochemistry and heterogeneous catalysis

This thesis describes the production, characterisation and catalytic performance of nanoclusters fabricated by cluster beam deposition using the magnetron sputtering, gas condensation technique. $MoS_{2}$-based clusters and Au-based clusters are demonstrated in electrochemistry (HER) and gas phase heterogeneous catalysis (CO oxidation), respectively. The atomic structure analysis of the clusters was performed with aberration-corrected scanning transmission electron microscope with high angle annular dark field (HAADF-STEM). Size-controlled $(MoS_{2}$$)_{300}$ clusters deposited on amorphous carbon present an incomplete multi-layer structure with the absence of extended crystalline order. Such a layered structure was also found in $Ni-MoS_{2}$ hybrid clusters [with a mass corresponding to $(MoS_{2}$$)_{1000}$] produced by dual target magnetron sputtering. Compared with $MoS_{2}$ clusters, a significant enhancement in HER activity by $Ni-MoS_{2}$ hybrid clusters was found. However, both $MoS_{2}$ clusters (Mo:S = 1:0.9) and $Ni-MoS_{2}$ clusters (Mo:S = 1:1.8) present a sulphur-deficient composition. In order to overcome the sulphur deficiency of the $MoS_{2}$ clusters, a sulphur-enrichment technique based on a combination of sulphur addition (by sublimation) and annealing inside the cluster beam vacuum chamber was performed on size-selected $(MoS_{2}$$)_{1000}$ clusters. This process led to a notable increase in extended crystallinity and a moderate increase in size (from 5.5 nm to 6.0 nm in diameter). Compared with $Ni-MoS_{2}$ clusters, the sulphur-enriched $MoS_{2}$ clusters show even more enhancement on the HER activities with more than 30-fold increases in exchange current densities. We have demonstrated a method of inhibiting the sintering of Au clusters in Au-based catalysis by exploring the stabilisation of supported Au clusters against sintering by alloying with Ti. Size-selected $Au_{2057}$ (405, 229 amu) clusters and similar mass Au/Ti nanoalloy clusters (400, 000 amu) were produced by cluster beam deposition onto thin silica films. A strong anchoring effect was found in the case of Au/Ti clusters by HAADF-STEM experiments, consistent with DFT calculations by collaborators. Different sintering mechanisms were revealed between Au cluster dimers and Au/Ti cluster dimers. Preliminary CO oxidation measurements on Au and Au/Ti clusters indicates that Au/Ti clusters are promising as catalysts. Au/Ti clusters show catalytic activity on CO oxidation while Au clusters are non-active due to the serious sintering and the support effect

Insight into the intrinsic mechanism of improving electrochemical performance via constructing the preferred crystal orientation in lithium cobalt dioxide

Surface properties of cathode materials play important roles in the transport of lithium-ions/electrons and the formation of surface passivation layer. Optimizing the exposed crystal facets of cathode materials can promote the diffusion of lithium-ions and enhance cathode surface stability, which may ultimately dominate cathode’s performance and stability in lithium-ion batteries. Here, polycrystalline LiCoO2 (LCO) thin films with (0003) and {101} preferred orientations were prepared as the well-defined model electrodes. In situ Current-Sensing Atomic Force Microscopy (CSAFM) was employed to investigate the lithium de-intercalation and electronic conductivity evolution of the (0003) and {101} facts in organic electrolyte at the nanoscale. It was found that the lithium deintercalation following a “Li-rich core model” in the LCO grains, and the LCO grains with (0003) crystal face show less conductivity than those with {101} faces. Moreover, X-ray Photoelectron Spectroscopy characterization of the charged electrode surface indicates that a denser surface passivation layer is formed on {101} than that on (0003) crystal faces. This is caused by the lower adsorption energy of decomposition molecule on {101} crystal faces and higher work function (due to the surface atomic structure) for {101} crystal faces, as confirmed by Density Functional Theory (DFT) and Kelvin probe force microscopy (KPFM) results. In addition, electrochemical measurements confirm that the thin film electrodes with {101} preferred orientation not only show smaller electrode polarization, but also more readily form a stable surface passivation layer compared with the (0003) preferred orientation. This work highlights the importance of cathode conductivity, and suggests that the LCO {101} facet atomic structure may thermodynamically promote the physical/chemical adsorption and decomposition of electrolyte

High surface area microporous carbon nanocubes from controlled processing of graphene oxide nanoribbons

A new, facile, and template-free method to prepare high surface area microporous carbon nanocubes (CNCs) from a mixture of graphene oxide nanoribbons (NRs), graphene oxide, and carbon dots is reported. The nanoribbons, approximately 30 nm wide and with lengths ranging from a few tens of nanometres up to several micrometres, were obtained from the oxidation of Black Pearls 2000 carbon black in nitric acid solution. The non-purified nanoribbons further contained additional fragments of graphene oxide, and of graphene oxide quantum dots. Slow pyrolysis of the nanoribbon mixture with slow heating rates, e.g., 3 °C/min, yielded carbon nanocubes approximately 250 nm in size with surface areas greater than 900 m2/g. Heating rates of 50 °C/min led to carbons with ∼800 m2/g surface area but bulk morphology. Precipitating the nanoribbons in potassium hydroxide solution, followed by carbonization, yielded microporous nanoparticle aggregates that were 20 nm in size with surface areas greater than 2000 m2/g. The particles exhibited complex, quasi-spherical morphology. Pyrolysis of other products obtained from oxidation in HNO3 of different grades of carbon black, specifically graphene oxide nanoparticles and quantum dots, yielded high surface area microporous carbons but with bulk morphology regardless of the processing conditions. Despite the lower surface area and pore volume of the CNCs in comparison to the nanospheres, the former contained ultramicropores that were highly accessible to CO2 as a molecular probe and had excellent selectivity of CO2 over N2. Hence, CNC materials have promising properties for applications where particle surface-to-volume ratios, high internal surface areas, and abundant super and ultramicropores are desired

Fabrication of electrodes by deposition of lead clusters from the Matrix Assembly Cluster Source (MACS) into porous carbon paper for electrocatalysis

The scaling up of the intensity of beams of atomic clusters (nanoparticle beams) creates a new route to the fabrication of functional nanostructured materials. A challenge is to present, to the directed beam, high surface areas of the desired support material, for decoration by the clusters at local sub-monolayer densities. Then, the clusters and their properties can be preserved. Here we employ the Matrix Assembly Cluster Source (MACS) to demonstrate and characterise the deposition of lead clusters, with size of order 2 nm, into planar sheets of porous carbon paper, a material employed in electrode fabrication. We find that clusters are deposited to a depth comparable with the pore size of the carbon, ~ 50 μm, giving rise to a metal loading of ~ 0.05 mg cm−2 of carbon paper. The functionality of the nanocomposite film so created is demonstrated by its use as an electrode for the electrochemical generation of oxidising species suitable for water purification

Modification of Deposited, Size-Selected MoS2 Nanoclusters by Sulphur Addition: An Aberration-Corrected STEM Study

Molybdenum disulphide (MoS2) is an earth-abundant material which has several industrial applications and is considered a candidate for platinum replacement in electrochemistry. Size-selected MoS2 nanoclusters were synthesised in the gas phase using a magnetron sputtering, gas condensation cluster beam source with a lateral time-of-flight mass selector. Most of the deposited MoS2 nanoclusters, analysed by an aberration-corrected scanning transmission electron microscope (STEM) in high-angle annular dark field (HAADF) mode, showed poorly ordered layer structures with an average diameter of 5.5 nm. By annealing and the addition of sulphur to the clusters (by sublimation) in the cluster source, the clusters were transformed into larger, crystalline structures. Annealing alone did not lead to crystallization, only to a cluster size increase by decomposition and coalescence of the primary clusters. Sulphur addition alone led to a partially crystalline structure without a significant change in the size. Thus, both annealing and sulphur addition processes were needed to obtain highly crystalline MoS2 nanoclusters

Interplay between oxygen doping and ultra-microporosity improves the CO2/N2 separation performance of carbons derived from aromatic polycarboxylates

Microporous carbons were prepared starting from a series of benzene polycarboxylic acids following two strategies: (i) activation- and template-free pyrolysis and (ii) ion-exchange pyrolysis. The proposed synthetic strategies are facile approaches to produce highly microporous carbons that avoid the use of large amounts of corrosive and expensive chemical activators or templates. By varying the number of carboxylic acid groups, the charge balancing species and the degree of deprotonation of the precursors, microporous carbons with diverse morphologies, textural properties and oxygen contents were obtained and their CO2 and N2 sorption properties were assessed. The abundant micropores made the materials suitable for CO2 adsorption at low pressure and ambient temperature, achieving CO2 uptake as high as 4.4 mmol/g at 25 °C and 1 bar, competitive with those reported for porous carbons produced using large excess of alkali metal based activating agents. It was found that high performance, in terms of CO2 uptake and CO2/N2 selectivity, was linked to the simultaneous presence of large ultra-micropore volume and high oxygen content in the sorbents. This suggests that the interplay of ultra-microporosity and oxygen doping matters more than the two features taken singularly in determining the CO2/N2 separation properties of porous carbons at low pressure

On the Use of Carbon Cables from Plastic Solvent Combinations of Polystyrene and Toluene in Carbon Nanotube Synthesis

For every three people on the planet, there are approximately two Tonnes (Te) of plastic waste. We show that carbon recovery from polystyrene (PS) plastic is enhanced by the coaddition of solvents to grow carbon nanotubes (CNTs) by liquid injection chemical vapour deposition. Polystyrene was loaded up to 4 wt% in toluene and heated to 780 °C in the presence of a ferrocene catalyst and a hydrogen/argon carrier gas at a 1:19 ratio. High resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and Raman spectroscopy were used to identify multiwalled carbon nanotubes (MWCNTs). The PS addition in the range from 0 to 4 wt% showed improved quality and CNT homogeneity; Raman "Graphitic/Defective" (G/D) values increased from 1.9 to 2.3; mean CNT diameters increased from 43.0 to 49.2 nm; and maximum CNT yield increased from 11.37% to 14.31%. Since both the CNT diameters and the percentage yield increased following the addition of polystyrene, we conclude that carbon from PS contributes to the carbon within the MWCNTs. The electrical contact resistance of acid-washed Bucky papers produced from each loading ranged from 2.2 to 4.4 Ohm, with no direct correlation to PS loading. Due to this narrow range, materials with different loadings were mixed to create the six wires of an Ethernet cable and tested using iPerf3; the cable achieved up- and down- link speeds of ~99.5 Mbps, i.e., comparable to Cu wire with the same dimensions (~99.5 Mbps). The lifecycle assessment (LCA) of CNT wire production was compared to copper wire production for a use case in a Boeing 747-400 over the lifespan of the aircraft. Due to their lightweight nature, the CNT wires decreased the CO footprint by 21 kTonnes (kTe) over the aircraft's lifespan

Composition-Tuned Pt-Skinned PtNi Bimetallic Clusters as Highly Efficient Methanol Dehydrogenation Catalysts

Platinum is the most active anode and cathode catalyst in next-generation fuel cells using methanol as liquid source of hydrogen. Its catalytic activity can be significantly improved by alloying with 3d metals, although a precise tuning of its surface architecture is still required. Herein, we report the design of a highly active low-temperature (below 0 °C) methanol dehydrogenation anode catalyst with reduced CO poisoning based on ultralow amount of precisely defined PtxNi1–x (x = 0 to 1) bimetallic clusters (BCs) deposited on inert flat oxides by cluster beam deposition. These BCs feature clear composition-dependent atomic arrangements and electronic structures stemming from their nucleation mechanism, which are responsible for a volcano-type activity trend peaking at the Pt0.7Ni0.3 composition. Our calculations reveal that at this composition, a cluster skin of Pt atoms with d-band centers downshifted by subsurface Ni atoms weakens the CO interaction that in turn triggers a significant increase in the methanol dehydrogenation activity

On the Use of Carbon Cables from Plastic Solvent Combinations of Polystyrene and Toluene in Carbon Nanotube Synthesis

For every three people on the planet, there are approximately two Tonnes (Te) of plastic waste. We show that carbon recovery from polystyrene (PS) plastic is enhanced by the coaddition of solvents to grow carbon nanotubes (CNTs) by liquid injection chemical vapour deposition. Polystyrene was loaded up to 4 wt% in toluene and heated to 780 °C in the presence of a ferrocene catalyst and a hydrogen/argon carrier gas at a 1:19 ratio. High resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and Raman spectroscopy were used to identify multiwalled carbon nanotubes (MWCNTs). The PS addition in the range from 0 to 4 wt% showed improved quality and CNT homogeneity; Raman “Graphitic/Defective” (G/D) values increased from 1.9 to 2.3; mean CNT diameters increased from 43.0 to 49.2 nm; and maximum CNT yield increased from 11.37% to 14.31%. Since both the CNT diameters and the percentage yield increased following the addition of polystyrene, we conclude that carbon from PS contributes to the carbon within the MWCNTs. The electrical contact resistance of acid-washed Bucky papers produced from each loading ranged from 2.2 to 4.4 Ohm, with no direct correlation to PS loading. Due to this narrow range, materials with different loadings were mixed to create the six wires of an Ethernet cable and tested using iPerf3; the cable achieved up- and down- link speeds of ~99.5 Mbps, i.e., comparable to Cu wire with the same dimensions (~99.5 Mbps). The lifecycle assessment (LCA) of CNT wire production was compared to copper wire production for a use case in a Boeing 747-400 over the lifespan of the aircraft. Due to their lightweight nature, the CNT wires decreased the CO2 footprint by 21 kTonnes (kTe) over the aircraft’s lifespan.We would like to thank Keysight Technologies for the use of a test model of the B2900A SMU. We would like to acknowledge the assistance provided by Swansea University College of Engineering AIM Facility. We would like to thank TRIMTABS Ltd. for purchasing equipment required for making ethernet cables. Thanks to Swansea Employability Academy (SEA) for the summer placements scheme. Thanks to the Swansea University Texas Strategic Partnership. R.E.P. acknowledges his work was associated with the IMPACT operation. We acknowledge pixabay for use of imagery in the graphical abstract (https://pixabay.com/vectors/airplane-boeing-747-transport-48 11157/ (accessed on 1 December 2021))

Reduced sintering of mass-selected Au clusters on SiO2 by alloying with Ti: an aberration-corrected STEM and computational study

Au nanoparticles represent the most remarkable example of a size effect in heterogeneous catalysis. However, a major issue hindering the use of Au nanoparticles in technological applications is their rapid sintering. We explore the potential of stabilizing Au nanoclusters on SiO2 by alloying them with a reactive metal, Ti. Mass-selected Au/Ti clusters (400 000 amu) and Au2057 clusters (405 229 amu) were produced with a magnetron sputtering, gas condensation cluster beam source in conjunction with a lateral time-of-flight mass filter, deposited onto a silica support and characterised by XPS and LEIS. The sintering dynamics of mass-selected Au and Au/Ti alloy nanoclusters were investigated in real space and real time with atomic resolution aberration-corrected HAADF-STEM imaging, supported by model DFT calculations. A strong anchoring effect was revealed in the case of the Au/Ti clusters, because of a much increased local interaction with the support (by a factor 5 in the simulations), which strongly inhibits sintering, especially when the clusters are more than ∼0.60 nm apart. Heating the clusters at 100 °C for 1 h in a mixture of O2 and CO, to simulate CO oxidation conditions, led to some segregation in the Au/Ti clusters, but in line with the model computational investigation, Au atoms were still present on the surface. Thus size-selected, deposited nanoalloy Au/Ti clusters appear to be promising candidates for sustainable gold-based nanocatalysis