62 results found
Zatko V, Galbiati M, Dubois SM-M, et al., 2019, Band-structure spin-filtering in vertical spin valves based on chemical vapor deposited WS2., ACS Nano, ISSN: 1936-0851
We report on spin transport in WS2-based 2D-magnetic tunnel junctions (2D-MTJs), unveiling a band structure spin filtering effect specific to the transition metal dichalcogenides (TMDCs) family. WS2 mono-, bi-, and trilayers are derived by a chemical vapor deposition process and further characterized by Raman spectroscopy, atomic force microscopy (AFM), and photoluminescence spectroscopy. The WS2 layers are then integrated in complete Co/Al2O3/WS2/Co MTJ hybrid spin-valve structures. We make use of a tunnel Co/Al2O3 spin analyzer to probe the extracted spin-polarized current from the WS2/Co interface and its evolution as a function of WS2 layer thicknesses. For monolayer WS2, our technological approach enables the extraction of the largest spin signal reported for a TMDC-based spin valve, corresponding to a spin polarization of PCo/WS2 = 12%. Interestingly, for bi- and trilayer WS2, the spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction. With the support of ab initio calculations, we propose a model to address the experimentally measured inversion of the spin polarization based on the change in the WS2 band structure while going from monolayer (direct bandgap) to bilayer (indirect bandgap). These experiments illustrate the rich potential of the families of semiconducting 2D materials for the control of spin currents in 2D-MTJs.
Chong JY, Wang B, Sherrell PC, et al., 2019, Fabrication of graphene‐covered micro‐tubes for process intensification, Advanced Engineering Materials, Vol: 21, Pages: 1-6, ISSN: 1438-1656
Graphene is known for its high surface‐area‐to‐mass ratio. However, for graphene to be used in engineering processes such as catalytic reactors or heat exchangers, high surface‐area‐to‐volume ratio is essential. Currently, graphene is only prepared in sheet form, which limits its surface‐area‐to‐volume ratio to around 200 m2 m−3. In this study, we propose and demonstrate a technique based on chemical vapour deposition (CVD) to realise graphene on a copper‐based micro‐tubular substrate to not only substantially increase its surface‐area‐to‐volume ratio to a value over 2000 m2 m−3, but also to eliminate maldistribution of flows commonly unavoidable in flat‐sheet configurations. Our approach uses a dual‐layer micro‐tubular substrate fabricated by a phase‐inversion facilitated co‐extrusion technique. In the substrate, a thin copper outer layer is employed to enable the CVD growth of graphene, and an inner Cu‐Fe layer is adopted to provide a strong mechanical support. Our study shows that this approach is feasible to produce graphene with a very high surface‐area‐to‐volume ratio for possible practical applications in catalytic reactors or heat exchangers, though problems such as the inter‐diffusion between the two metal layers and defects in graphene need to be further addressed. To the best of our knowledge, this study is the first attempt to prepare graphene with high surface‐area‐to‐volume ratio by a CVD route.
Sherrell PC, Palczynski P, Sokolikova MS, et al., 2019, Large-area CVD MoS2/WS2 heterojunctions as a photoelectrocatalyst for salt water oxidation, ACS Applied Energy Materials, Vol: 2, Pages: 5877-5882, ISSN: 2574-0962
Splitting salt water via sunlight into molecular oxygen and hydrogen for use as fuel or as an energy carrier is a clear pathway toward renewable energy. Monolayer MoS2 and WS2 are promising materials for the energetically demanding water oxidation reaction, absorbing ∼10% of incident light in the visible spectrum and possessing chemical stability and band edges more positive than the oxidation potential of water. A heterostructure of MoS2/WS2 forms a type-II heterojunction, supporting fast separation of the photogenerated charge carriers across the junction. Here, we show the role played by defects in determining the efficiency of the photon-driven oxidation process. By reducing the defects in this material system, it is possible to obtain an incident photon-to-current conversion efficiency (IPCE) of ∼1.6% and a visible-light-driven photocurrent density of 1.7 mA/cm2 for water oxidation. The efficiency is one order of magnitude higher than that of photoelectrocatalytic hydrogen reduction and water oxidation supported by liquid-phase exfoliated transition-metal dichalcogenides (TMDs). This result has been achieved with chemically vapor deposited (CVD) MoS2/WS2 heterojunctions, in the form of 100 μm large flakes assembled to form thin films. The large flakes sizes, compared to liquid-phase exfoliated materials (normally <5 μm), and thus the low edge flake density, and the flakes’ atomically sharp and clean interfaces between the flakes are responsible for reducing charge carrier recombination. These results show a general approach to the scalable synthesis of high-crystal-quality low-dimensional semiconductor photoelectrodes for solar energy conversion systems. It also shows the uniqueness of the CVD synthesis process of these materials, which can lead to high quality materials without the need of any postsynthesis treatments.
Wakamura T, Reale F, Palczynski P, et al., 2019, Spin-orbit interaction induced in graphene by transition metal dichalcogenides, Physical review B: Condensed matter and materials physics, Vol: 99, ISSN: 1098-0121
We report a systematic study on strong enhancement of spin-orbit interaction (SOI) in grapheneinduced by transition-metal dichalcogenides (TMDs). Low temperature magnetotoransport mea-surements of graphene proximitized to different TMDs (monolayer and bulk WSe2, WS2and mono-layer MoS2) all exhibit weak antilocalization peaks, a signature of strong SOI induced in graphene.The amplitudes of the induced SOI are different for different materials and thickness, and we findthat monolayer WSe2and WS2can induce much stronger SOI than bulk WSe2, WS2and mono-layer MoS2. The estimated spin-orbit (SO) scattering strength for graphene/monolayer WSe2andgraphene/monolayer WS2reachesª10 meV whereas for graphene/bulk WSe2, graphene/bulk WS2and graphene/monolayer MoS2it is around 1 meV or less. We also discuss the symmetry and typeof the induced SOI in detail, especially focusing on the identification of intrinsic (Kane-Mele) andvalley-Zeeman (VZ) SOI by determining the dominant spin relaxation mechanism. Our findingspave the way for realizing the quantum spin Hall (QSH) state in graphene.
Sokolikova MS, Sherrell PC, Palczynski P, et al., 2019, Direct Solution-Phase Synthesis of 1T’ WSe2 Nanosheets, Nature Communications, Vol: 10, ISSN: 2041-1723
Crystal phase control in layered transition metal dichalcogenides is central for exploiting their different electronic properties. Access to metastable crystal phases is limited as their direct synthesis is challenging, restricting the spectrum of reachable materials. Here, we demonstrate the solution phase synthesis of the metastable distorted octahedrally coordinated structure (1T’ phase) of WSe2 nanosheets. We design a kinetically-controlled regime of colloidal synthesis to enable the formation of the metastable phase. 1T’ WSe2 branched few-layered nanosheets are produced in high yield and in a reproducible and controlled manner. The 1T’ phase is fully convertible into the semiconducting 2H phase upon thermal annealing at 400 °C. The 1T’ WSe2 nanosheets demonstrate a metallic nature exhibited by an enhanced electrocatalytic activity for hydrogen evolution reaction as compared to the 2H WSe2 nanosheets and comparable to other 1T’ phases. This synthesis design can potentially be extended to different materials providing direct access of metastable phases.
Jia J, White ER, Clancy AJ, et al., 2018, Fast exfoliation and functionalisation of two-dimensional crystalline carbon nitride by framework charging, Angewandte Chemie, Vol: 57, Pages: 12656-12660, ISSN: 1521-3757
Two-dimensional (2D) layered graphitic carbon nitride (gCN) nanosheets offer intriguing electronic and chemical properties. However, the exfoliation and functionalisation of gCN for specific applications remain challenging. We report a scalable one-pot reductive method to produce solutions of single- and few-layer 2D gCN nanosheets with excellent stability in a high mass yield (35 %) from polytriazine imide. High-resolution imaging confirmed the intact crystalline structure and identified an AB stacking for gCN layers. The charge allows deliberate organic functionalisation of dissolved gCN, providing a general route to adjust their properties.
Sherrell PC, Sharda K, Grotta C, et al., 2018, Thickness dependant characterization of chemically exfoliated TiS2 nanosheets, ACS Omega, Vol: 3, Pages: 8655-8662, ISSN: 2470-1343
Monolayer TiS2 is the lightest member of the transition metal dichalcogenides family with promising application in energy storage and conversion systems. Use of TiS2 has been limited by the lack of rapid characterisation of layer number via Raman spectroscopy and its easy oxidation in wet environment. Here, we demonstrate layer number dependent Raman modes for TiS2. 1T-TiS2 presents two characteristics Raman active modes, A1g (out-of-plane) and Eg (in-plane). We identified a characteristic peak frequency shift of the Eg mode with the layer number and an unexplored Raman mode at 372 cm-1 whose intensity changes relative to the A1g mode with the thickness of TiS2 sheets. These two characteristic features of the Raman spectra allow the determination of layer numbers between 1 and 5 in exfoliated TiS2. Further, we develop a method to produce oxidation-resistant inks of micron sized mono- and few-layered TiS2 nanosheets at concentrations up to 1 mg/mL .These TiS2 inks can be deposited to form thin films with controllable thickness and nanosheet density over cm2 areas. This opens up pathways for a wider utilization of exofliated TiS2 towards a range of applications.
Mattevi C, Electronic band structure of Two-DimensionalWS2/Graphene van derWaals Heterostructure, Physical Review B, ISSN: 2469-9950
Henck H, Ben Aziz Z, Pierucci D, et al., Electronic band structure of two-dimensional WS2/Graphene van der Waals heterostructures, Physical Review B, ISSN: 2469-9950
Combining single-layer two-dimensional semiconducting transition metal dichalcogenides (TMDs)with graphene layer in van der Waals heterostructuresoffers an intriguing means of controlling the electronic properties through these heterostructures. Here, we report the electronic and structural properties of transferred single layer WS2on epitaxial graphene using micro-Raman spectroscopy, angle-resolved photoemission spectroscopy measurements(ARPES)and Density Functional Theory(DFT)calculations. The results show good electronic properties as well as well-defined band arising from the strong splitting of the single layer WS2valence band at K points, with a maximum splitting of 0.44 eV.By comparing our DFT results with local and hybrid functionals, we find the top valence band of the experimental heterostructure is closeto the calculations forsuspended single layer WS2. .Our results provide an important reference for future studies of electronic properties of WS2and its applications in valleytronic devices.
Wakamura T, Reale F, Palczynski P, et al., 2018, Strong anisotropic spin-orbit interaction induced in graphene by monolayer WS₂, Physical Review Letters, Vol: 120, ISSN: 0031-9007
We demonstrate strong anisotropic spin-orbit interaction (SOI) in graphene induced by monolayer WS₂. Direct comparison between graphene-monolayer WS2 and graphene-bulk WS₂ systems in magnetotransport measurements reveals that monolayer transition metal dichalcogenide can induce much stronger SOI than bulk. Detailed theoretical analysis of the weak antilocalization curves gives an estimated spin-orbit energy (Eso) higher than 10 meV. The symmetry of the induced SOI is also discussed, and the dominant z → −z symmetric SOI can only explain the experimental results. Spin relaxation by the Elliot-Yafet mechanism and anomalous resistance increase with temperature close to the Dirac point indicates Kane-Mele SOI induced in graphene.
Li K, Wang B, Chong JY, et al., 2017, Dynamic microstructure of graphene oxide membranes and the permeation flux, Journal of Membrane Science, Vol: 549, Pages: 385-392, ISSN: 0376-7388
Graphene oxide (GO) membranes have been reported to be a promising separation barrier that can retain small molecules and multi-valent salts because of the well-defined interlayer space between GO flakes. However, while some studies suggested fast liquid transport through the extremely tortuous transport path, contradictory observations (e.g. low permeation flux) have also been obtained. This paper revealed the dynamic microstructure of GO membranes, which affected the membrane performance significantly. We showed that all GO membranes prepared by varied methods and on different substrates presented a severe reduction in water permeability during filtration, due to the compaction of their original loose microstructure. The water flux could drop continuously from tens of LMH bar−1 to <0.1 LMH bar−1 after more than ten hours. This result demonstrated that the structure of GO membranes prepared by current approaches was far from the ideal laminar structure. The high permeability of GO membranes observed could be contributed by the disordered membrane microstructure. Therefore, the transport mechanisms assuming perfect laminar structure in GO membranes, and the fast transport hypothesis may not fully describe the water transport in GO membranes. Interestingly, the loosely packed microstructure of GO membranes was also found reversible depending on the storage conditions.
Reale F, Palczynski P, Amit I, et al., 2017, High-Mobility and High-Optical Quality Atomically Thin WS2, Scientific Reports, Vol: 7, ISSN: 2045-2322
The rise of atomically thin materials has the potential to enable a paradigm shift in modern technologies by introducing multi-functional materials in the semiconductor industry. To date the growth of high quality atomically thin semiconductors (e.g. WS2) is one of the most pressing challenges to unleash the potential of these materials and the growth of mono- or bi-layers with high crystal quality is yet to see its full realization. Here, we show that the novel use of molecular precursors in the controlled synthesis of mono- and bi-layer WS2 leads to superior material quality compared to the widely used direct sulfidization of WO3-based precursors. Record high room temperature charge carrier mobility up to 52 cm2/Vs and ultra-sharp photoluminescence linewidth of just 36 meV over submillimeter areas demonstrate that the quality of this material supersedes also that of naturally occurring materials. By exploiting surface diffusion kinetics of W and S species adsorbed onto a substrate, a deterministic layer thickness control has also been achieved promoting the design of scalable synthesis routes.
Bekaert J, Bignardi L, Aperis A, et al., 2017, Free surfaces recast superconductivity in few-monolayer MgB2: Combined first-principles and ARPES demonstration, Scientific Reports, Vol: 7, ISSN: 2045-2322
Two-dimensional materials are known to harbour properties very different from those of their bulk counterparts. Recent years have seen the rise of atomically thin superconductors, with a caveat that superconductivity is strongly depleted unless enhanced by specific substrates, intercalants or adatoms. Surprisingly, the role in superconductivity of electronic states originating from simple free surfaces of two-dimensional materials has remained elusive to date. Here, based on first-principles calculations, anisotropic Eliashberg theory, and angle-resolved photoemission spectroscopy (ARPES), we show that surface states in few-monolayer MgB2make a major contribution to the superconducting gap spectrum and density of states, clearly distinct from the widely known, bulk-like σ- and π-gaps. As a proof of principle, we predict and measure the gap opening on the magnesium-based surface band up to a critical temperature as high as ~30 K for merely six monolayers thick MgB2. These findings establish free surfaces as an unavoidable ingredient in understanding and further tailoring of superconductivity in atomically thin materials.
Pesci FM, Sokolikova MS, Grotta C, et al., 2017, MoS2/WS2 Heterojunction for Photoelectrochemical Water Oxidation, ACS Catalysis, Vol: 7, Pages: 4990-4998, ISSN: 2155-5435
Sokolikova MS, Sherrell PC, Bemmer VL, et al., 2017, Room-temperature growth of colloidal Bi2Te3 nanosheets, Chemical Communications, Vol: 53, Pages: 8026-8029, ISSN: 1364-548X
In this work, we report the colloidal synthesis of Bi2Te3 nanosheets with controlled thickness, morphology and crystallinity at temperatures as low as 20 °C. Grown at room temperature, Bi2Te3 exhibits two-dimensional morphology with thickness of 4 nm and lateral size of 200 nm. Upon increasing the temperature to 170 °C, the nanosheets demonstrate increased thickness of 16 nm and lateral dimensions of 600 nm where polycrystalline nanosheets (20 °C) are replaced by single crystal platelets (170 °C). Rapid synthesis of the material at moderately low temperatures with controllable morphology, crystallinity and consequently electrical and thermal properties can pave the way toward its large-scale production for practical applications.
Riley DJ, Song W, Lischner, et al., 2017, Tuning the Double Layer of Graphene Oxide through Phosphorus Doping for Enhanced Supercapacitance, ACS Energy Letters, Vol: 2, Pages: 1144-1149, ISSN: 2380-8195
The electrochemical double layer plays a fundamental role in energy storage applications. Control of the distribution of ions in the double layer at the atomistic scale offers routes to enhanced material functionality and device performance. Here we demonstrate how the addition of an element from the third row of the periodic table, phosphorus, to graphene oxide increases the measured capacitance and present density functional theory calculations that relate the enhanced charge storage to structural changes of the electrochemical double layer. Our results point to how rational design of materials at the atomistic scale can lead to improvements in their performance for energy storage.
Amit I, Octon TJ, Townsend NJ, et al., 2017, Role of charge traps in the performance of atomically-thin transistors, Advanced Materials, Vol: 29, ISSN: 1521-4095
Transient currents in atomically thin MoTe2 field-effect transistors (FETs) are measured during cycles of pulses through the gate electrode. The curves of the transient currents are analyzed in light of a newly proposed model for charge-trapping dynamics that renders a time-dependent change in the threshold voltage as the dominant effect on the channel hysteretic behavior over emission currents from the charge traps. The proposed model is expected to be instrumental in understanding the fundamental physics that governs the performance of atomically thin FETs and is applicable to the entire class of atomically thin-based devices. Hence, the model is vital to the intelligent design of fast and highly efficient optoelectronic devices.
Olowojoba GB, Kopsidas S, Eslava S, et al., 2017, A facile way to produce epoxy nanocomposites having excellent thermal conductivity with low contents of reduced graphene oxide, Journal of Materials Science, Vol: 52, Pages: 7323-7344, ISSN: 1573-4803
A well-dispersed phase of exfoliated graphene oxide (GO) nanosheets was initially prepared in water. This was concentrated by centrifugation and was mixed with a liquid epoxy resin. The remaining water was removed by evaporation, leaving a GO dispersion in epoxy resin. A stoichiometric amount of an anhydride curing agent was added to this epoxy-resin mixture containing the GO nanosheets, which was then cured at 90 °C for 1 hour followed by 160 °C for 2 hours. A second thermal treatment step of 200 °C for 30 minutes was then undertaken to reduce further the GO in-situ in the epoxy nanocomposite. An examination of the morphology of such nanocomposites containing reduced graphene oxide (rGO) revealed that a very good dispersion of rGO was achieved throughout the epoxy polymer. Various thermal and mechanical properties of the epoxy nanocomposites were measured and the most noteworthy finding was a remarkable increase in the thermal conductivity when relatively very low contents of rGO were present. For example, a value of 0.25 W/mK was measured at 30 °C for the nanocomposite with merely 0.06 weight percentage (wt%) of rGO present, which represents an increase of ~40% compared with that of the unmodified epoxy polymer. This value represents one of the largest increases in the thermal conductivity per wt% of added rGO yet reported. These observations have been attributed to the excellent dispersion of rGO achieved in these nanocomposites made via this facile production method. The present results show that it is now possible to tune the properties of an epoxy polymer with a simple and viable method of GO addition.
Mattevi C, 2016, Graphitic carbon nitride as a catalyst support in fuel cells and electrolyzers, Electrochimica Acta, Vol: 222, Pages: 44-57, ISSN: 1873-3859
Electrochemical power sources, such as polymer electrolyte membrane fuel cells (PEMFCs), require the use of precious metal catalysts which are deposited as nanoparticles onto supports in order to minimize their mass loading and therefore cost. State-of-the-art/commercial supports are based on forms of carbon black. However, carbon supports present disadvantages including corrosion in the operating fuel cell environment and loss of catalyst activity. Here we review recent work examining the potential of different varieties of graphitic carbon nitride (gCN) as catalyst supports, highlighting their likely benefits, as well as the challenges associated with their implementation. The performance of gCN and hybrid gCN-carbon materials as PEMFC electrodes is discussed, as well as their potential for use in alkaline systems and water electrolyzers. We illustrate the discussion with examples taken from our own recent studies.
Mansor N, Jia J, Miller TS, et al., 2016, Graphitic carbon nitride-graphene hybrid nanostructure as a catalyst support for polymer electrolyte membrane fuel cells, ECS Transactions, Vol: 75, Pages: 885-897, ISSN: 1938-5862
Graphitic carbon nitrides form a class of semiconducting graphene-like polymeric materials with visible light absorption and photocatalytic properties. In addition to high nitrogen content and tunable structure, it was shown that graphitic carbon nitride based on polytrazine imide (PTI) sheets exhibit excellent anti-corrosion ability in ex-situ fuel cell environments. However, in bulk form, their low surface area and poor conductivity limits their applications in fuel cells. In this work, PTI was exfoliated to form an ink made from single to few-layer nanosheets. The ink was then processed to produce 3D networks of carbon nitride nanosheets/reduced graphene oxide (PTI-rGO) hybrid aerogel with large interconnecting pores for fast mass transport of reactants and high surface area. The material was decorated with platinum nanoparticles, and then investigated for its electrochemical properties and applications as a catalyst support for polymer electrolyte membrane (PEM) fuel cells. Initial results show that the cathode catalytic activity of Pt/rGO-PTI hybrid is significantly improved in comparison to Pt/PTI or Pt/rGO. In addition, the in-situ fuel cell performance of Pt/PTI as anode catalyst is comparable to commercial Pt/C especially at low densities, making it attractive as an alternative, durable anode catalyst support material to conventional carbon black.
Pierin G, Grotta C, Colombo P, et al., 2016, Direct Ink Writing of micrometric SiOC ceramic structures using a preceramic polymer, Journal of the European Ceramic Society, Vol: 36, Pages: 1589-1594, ISSN: 0955-2219
© 2016 Elsevier Ltd. In this work we manufactured micro-sized SiOC ceramic components by 3D printing (Direct Ink Writing) of a preceramic polymer. Model porous ceramic scaffolds with the lateral dimension of a few millimeters and composed of a continuous ceramic filament ~120 μm thick were produced, and a suitable rheological behavior was obtained by mixing cross-linked preceramic particles with a siloxane resin dissolved in a solvent. The addition of a low amount (0.025-0.1 wt%) of graphene oxide to the ink formulation further improves the structural stability during pyrolysis reducing the shrinkage of the preceramic polymer. Upon pyrolysis at low temperature (1000 °C), graphene oxide converted into graphene. The resulting scaffolds possess a good compression strength, of ~2.5 MPa for a total porosity of ~64 vol% (~3.1 MPa after the addition of 0.1 wt% graphene oxide).
Mattevi C, Yuji Takakuwa YT, 2016, Valence-band electronic structure evolution of graphene oxide upon thermal annealing for optoelectronics, Physica Status Solidi (A) Applied Research, Vol: 213, Pages: 2380-2386, ISSN: 1862-6319
We report valence-band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. The degree of oxygen functionalization was controlled by annealing temperature, and an electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy. We observed a drastic increase in the density of states around the Fermi level upon thermal annealing at ∼600 °C. The result indicates that while there is an apparent bandgap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of bandgap closure was correlated with the electrical, chemical, and structural properties to determine a set of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of ∼500 °C leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to an as-synthesized counterpart.
Kien-Cuong P, Chang Y-H, McPhail DS, et al., 2016, Amorphous Molybdenum Sulfide on Graphene-Carbon Nanotube Hybrids as Highly Active Hydrogen Evolution Reaction Catalysts, ACS APPLIED MATERIALS & INTERFACES, Vol: 8, Pages: 5961-5971, ISSN: 1944-8244
Olowojoba GB, Eslava S, Gutierrez ES, et al., 2016, In-situ thermally-reduced graphene oxide/epoxy composites: thermal and mechanical properties, Applied Nanoscience, Vol: 6, Pages: 1015-1022, ISSN: 2190-5509
Graphene has excellent mechanical, thermal, optical and electrical properties and this has made it a prime target for use as a filler material in the development of multifunctional polymeric composites. However, several challenges need to be overcome in order to take full advantage of the aforementioned properties of graphene. These include achieving good dispersion and interfacial properties between the graphene filler and the polymeric matrix. In the present work we report the thermal and mechanical properties of reduced graphene oxide/epoxy composites prepared via a facile, scalable and commercially-viable method. Electron micrographs of the composites demonstrate that the reduced graphene oxide (rGO) is well-dispersed throughout the composite. Although no improvements in glass transition temperature, tensile strength, and thermal stability in air of the composites were observed, good improvements in thermal conductivity (about 36%), tensile and storage moduli (more than 13%) were recorded with the addition of 2 wt% of rGO.
Reale F, Sharda K, Mattevi C, 2016, From bulk crystals to atomically thin layers of group VI-transition metal dichalcogenides vapour phase synthesis, Applied Materials Today, Vol: 3, Pages: 11-22, ISSN: 2352-9407
Traditional synthesis methods of bulk semiconductors developed during the1970s and 1980s have recently undergone a resurgence of research interest. Physical vapour deposition (PVD), chemical vapour deposition (CVD) and metal organic chemical vapour deposition (MOCVD) have been extensively rediscovered to achieve three-atom thick metal dichalcogenides. Often defined as “graphene-analogous materials” atomically thin sulfides and selenides of group VI of transition metals have revealed a plethora of unforeseen optical, electrical and mechanical properties which make them unique candidates for future nanotechnologies, ranging from quantum electronics to large area consumer electronics. In the last few years tremendous progress has been achieved in the synthesis of high quality atomic crystals, often inspired by the consolidated synthesis methodologies of their bulk counterparts. Most interestingly, several of these methods are still used and also implemented to synthesize new compounds, expanding the range of accessible 2D materials. We review this progress and we highlight key difference in the coordination chemistry of different transition metals which are responsible for the different synthesis products.
Sherrell PC, Mattevi C, 2016, Mesoscale design of multifunctional 3D graphene networks, Materials Today, Vol: 19, Pages: 428-436, ISSN: 1873-4103
Three-dimensional graphene networks are emerging as a new class of multifunctional constructs with a wide range of potential applications from energy storage to bioelectronics. Their multifunctional characteristics stem from the unique combination of mechanical properties, electrical conductivity, ultra-low density, and high specific surface areas which distinguish them from any polymer, ceramic or metal constructs. The most pressing challenge now is the achievement of ordered structures relying on processes that are highly controllable. Recent progresses in materials templating techniques, including the advent of three-dimensional printing, have accelerated the development of macroscopic architectures with micro-level-controlled features by rational design, with potential for manufacturing.
Pham KC, McPhail DS, Mattevi C, et al., 2016, Graphene-Carbon Nanotube Hybrids as Robust Catalyst Supports in Proton Exchange Membrane Fuel Cells, Journal of the Electrochemical Society, Vol: 163, Pages: F255-F263, ISSN: 0013-4651
Catalyst degradation is one major challenge preventing the worldwide commercialization of the Proton Exchange Membrane Fuel Cells. In this study, we investigate the development of a novel hierarchical carbonaceous support for the platinum catalysts, called graphene-carbon nanotube hybrids (GCNT), and its degradation behavior during an accelerated degradation test. The carbon support is fabricated by growing graphene directly onto carbon nanotubes to form a unique all-carbon nanostructure possessing both an ultra-high density of exposed graphitic edges of graphene and a porous structure of carbon nanotubes. The GCNT-supported platinum catalyst exhibits a higher intrinsic catalytic activity than a carbon black-supported platinum catalyst, and much higher than a CNT-supported platinum catalyst. The enhanced catalytic activity of the GCNT-supported platinum catalyst is explained by the high graphitic edge density which promotes the catalytic reactions on platinum catalyst. The GCNT-supported platinum catalyst also exhibits a superior electrochemical stability over that of the carbon black-supported platinum catalyst, explained by the high crystallinity of the GCNT support. The superior stability is expressed by a lower loss in polarization performance, a smaller increase in charge transfer resistance, a lower loss in the platinum electrochemical surface area, a lower rate of carbon corrosion, and a more stable catalyst microstructure.
Li K, Chong JY, Aba NFD, et al., 2015, UV-enhanced sacrificial layer stabilised graphene oxide hollow fibre membranes for nanofiltration, Scientific Reports, Vol: 5, ISSN: 2045-2322
Graphene oxide (GO) membranes have demonstrated great potential in gas separation and liquid filtration. For upscale applications, GO membranes in a hollow fibre geometry are of particular interest due to the high-efficiency and easy-assembly features at module level. However, GO membranes were found unstable in dry state on ceramic hollow fibre substrates, mainly due to the drying-related shrinkage, which has limited the applications and post-treatments of GO membranes. We demonstrate here that GO hollow fibre membranes can be stabilised by using a porous poly(methyl methacrylate) (PMMA) sacrificial layer, which creates a space between the hollow fibre substrate and the GO membrane thus allowing stress-free shrinkage. Defect-free GO hollow fibre membrane was successfully determined and the membrane was stable in a long term (1200 hours) gas-tight stability test. Post-treatment of the GO membranes with UV light was also successfully accomplished in air, which induced the creation of controlled microstructural defects in the membrane and increased the roughness factor of the membrane surface. The permeability of the UV-treated GO membranes was greatly enhanced from 0.07 to 2.8 L m-2 h-1 bar-1 for water, and 0.14 to 7.5 L m-2 h-1 bar-1 for acetone, with an unchanged low molecular weight cut off (~250 Da).
Ni N, Barg S, Garcia-Tunon E, et al., 2015, Understanding Mechanical Response of Elastomeric Graphene Networks, Scientific Reports, Vol: 5, ISSN: 2045-2322
Ultra-light porous networks based on nano-carbon materials (such as graphene or carbon nanotubes) have attracted increasing interest owing to their applications in wide fields from bioengineering to electrochemical devices. However, it is often difficult to translate the properties of nanomaterials to bulk three-dimensional networks with a control of their mechanical properties. In this work, we constructed elastomeric graphene porous networks with well-defined structures by freeze casting and thermal reduction, and investigated systematically the effect of key microstructural features. The porous networks made of large reduced graphene oxide flakes (>20 μm) are superelastic and exhibit high energy absorption, showing much enhanced mechanical properties than those with small flakes (<2 μm). A better restoration of the graphitic nature also has a considerable effect. In comparison, microstructural differences, such as the foam architecture or the cell size have smaller or negligible effect on the mechanical response. The recoverability and energy adsorption depend on density with the latter exhibiting a minimum due to the interplay between wall fracture and friction during deformation. These findings suggest that an improvement in the mechanical properties of porous graphene networks significantly depend on the engineering of the graphene flake that controls the property of the cell walls.
Aba NFD, Chong JY, Wang B, et al., 2015, Graphene oxide membranes on ceramic hollow fibers - Microstructural stability and nanofiltration performance, JOURNAL OF MEMBRANE SCIENCE, Vol: 484, Pages: 87-94, ISSN: 0376-7388
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