Dr Sam Chen

We have developed a rapid and facile method for preparing free-standing nanocomposite of gold nanoparticles with graphene oxide (Au@GO) in water under continuous flow in the absence of harsh reducing agents and any other auxiliary substances, as a method with favourable green chemistry metrics. This uses a vortex fluidic device (VFD) where induced mechanical energy and photo-contact electrification associated with the dynamic thin film in the rapidly rotating tube tilted at 45° while simultaneously UV irradiated (¿=254 nm, 20 W) results in decomposition of water to hydrogen and hydrogen peroxide with growth of the gold nanoparticles on the surface of the GO. We have established that the resulting Au@GO composite sheets rapidly catalyse the degradation of commercial dyes like methyl orange (MO) and methylene blue (MB) using the hydrogen peroxide generated in situ in the VFD. This process relies on active radicals generated through liquid-solid photo-contact electrification of water in the VFD which dramatically minimises the generation of waste in industrial applications, with the reaction having implications for wastewater treatment.

Controlling the size and morphology of gold nanoparticles occurs in the absence of added reducing agents or other excipients such as surfactants, on UV irradiation (¿ 254 nm) of aqueous auric acid (H[AuCl4]) in a thin film of liquid in a vortex fluidic device (VFD) within a rapidly rotating tilted quartz tube. This involves contact electrification (CE), which occurs at the solid-liquid interface with the oxidation of water photoinduced, forming the hydroxyl radical, OH¿. In air, the redox couple is reduction of 3O2 to the superoxide radical anion, O2-¿, which then reduces Au3+ to elemental gold, as does other reactive oxygen species present, competing with CE reduction of Au3+. The resulting nanogold structures effectively mold the different high-shear topological fluid flows in the VFD, being isolated as ultrathin 2D sheets, prisms, hierarchical structures comprising nanoparticles embedded within these sheets, and rosette and tubular structures, depending on the VFD processing parameters and the concentration of auric acid. Processing under a nitrogen atmosphere while UV irradiated affords mainly 2D gold through the above reduction of Au3+. The findings establish a paradigm for VFD processing in water involving photo-CE, generating hydrogen peroxide and hydrogen gas with the surfaces of the gold nanoparticles pristine.

Chemical vapor deposition (CVD) has great potential to produce graphene films at large-scale. However, CVD production of graphene films usually requires a catalytic metal substrate, such as copper. Recently we have developed a new method to grow graphene films directly on crystalline silicon wafers with a thermally grown 300¿nm oxide layer, using a seeded-CVD growth approach. The use of methane as the feedstock and optimized graphene seeds has led to enhanced film formation, which SEM, X-ray photo-electron and Raman spectroscopies indicate consist of graphene layers formed by the coalescence of expanding "graphene seeds". The resultant films have regions of single graphene crystallites within them as a result of lateral growth of the seeds. In addition, we have observed that the unilateral conductivity of the graphene films is consistent with the presence of graphene nanoribbons and as such has potential application in device fabrication.

The fluorination of self-standing graphene oxide (GO) films under mild conditions at 70 °C for 1h using a mixture of elemental fluorine and nitrogen gases (0.35 F2/0.65 N2 v/v) results in transparent, highly fluorinated (F/C = 1 as seen by XPS) graphene oxide (FGO) films which maintain initial densely packed structure and mechanical strength. Most of the oxygen atoms in the GO film were replaced by fluorine after fluorination, and two types of carbon-fluorine bonds coexist, i.e. primarily weakened C¿F bonds and also pure covalent C¿F bonds as evidenced by FTIR, XPS, NMR, and cyclic voltammetry when FGO is used as electrode in Li/electrolyte (PC:EC:3DMC/LiTFSI 1 M)/FGO cell similar to a lithium battery. To further understand the fluorination mechanism and to distinguish the reaction steps, xenon difluoride XeF2 was used as fluorinating agent for GO and in situ EPR and FTIR analyses suggest hydroxyl groups as preferential reactive sites. These self-standing transparent FGO films exhibit unusual electrochemical properties in lithium battery and can be used as hydrophobic lubricant membranes because of the low and stable coefficient of friction evidenced in the present work.

A "cooling¿contraction" method to separate large-area (up to 4.2¿cm in lateral size) graphene oxide (GO)-assembled films (of nanoscale thickness) from substrates is reported. Heat treatment at 3000¿°C of such free-standing macroscale films yields highly crystalline "macroassembled graphene nanofilms" (nMAGs) with 16¿48¿nm thickness. These nMAGs present tensile strength of 5.5¿11.3¿GPa (with ¿3¿µm gauge length), electrical conductivity of 1.8¿2.1 MS m-1, thermal conductivity of 2027¿2820 W m-1 K-1, and carrier relaxation time up to ¿23¿ps. As a demonstration application, an nMAG-based sound-generator shows a 30 µs response and sound pressure level of 89¿dB at 1 W cm-2. A THz metasurface fabricated from nMAG has a light response of 8.2% for 0.159 W mm-2 and can detect down to 0.01¿ppm of glucose. The approach provides a straightforward way to form highly crystallized graphene nanofilms from low-cost GO sheets.

We describe a method to obtain networks of wrinkles in multilayer graphene flakes (and other layered materials) by thermal contraction of the underlying PDMS substrate they are deposited on. The exfoliated flakes on PDMS are dipped into liquid nitrogen and after removal networks of wrinkles are found. The density of wrinkles can be controlled to some degree by sequential dipping into liquid nitrogen. Atomic force microscopy shows that wrinkles form preferentially along the armchair direction of the graphene lattice in such multilayer graphene platelets. Raman spectra show that the interlayer coupling at a wrinkle in multilayer graphene differs from, and is weaker than, that in undeformed regions. High resolution transmission electron microscopy measurements show that the interlayer distance increases in strained regions, which results in the interlayer coupling being decreased in particular regions of the wrinkles in these multilayer graphenes.

High-quality AB-stacked bilayer or multilayer graphene larger than a centimetre has not been reported. Here, we report the fabrication and use of single-crystal Cu/Ni(111) alloy foils with controllable concentrations of Ni for the growth of large-area, high-quality AB-stacked bilayer and ABA-stacked trilayer graphene films by chemical vapour deposition. The stacking order, coverage and uniformity of the graphene films were evaluated by Raman spectroscopy and transmission electron microscopy including selected area electron diffraction and atomic resolution imaging. Electrical transport (carrier mobility and band-gap tunability) and thermal conductivity (the bilayer graphene has a thermal conductivity value of about 2,300 W m-1 K-1) measurements indicated the superior quality of the films. The tensile loading response of centimetre-scale bilayer graphene films supported by a 260-nm thick polycarbonate film was measured and the average values of the Young's modulus (478 GPa) and fracture strength (3.31 GPa) were obtained.

Lightweight, flexible graphite foils that are chemically inert, high-temperature resistant, and highly electrically and thermally conductive can be used as component materials in numerous applications. "Graphenic" foils can be prepared by thermally transforming graphene oxide films. For this transformation, it is desirable to maintain a densely packed film structure at high heating rates as well as to lower the graphitizing temperatures. In this work, we discuss the pressure-assisted thermal decomposition of graphene oxide films by hot pressing at different temperatures (i.e., 300 °C, 1000 °C, or 2000 °C). The films pressed at 1000 °C or 2000 °C were subsequently heated at 2750 °C to achieve a higher degree of graphitization. The combination of heating and pressing promotes the simultaneous thermal decomposition and graphitic transformation of G-O films. Films pressed at 2000 °C as well as films further graphitized at 2750 °C show high chemical purity, uniformity, and retain their flexibility. For films pressed at 2000 °C and then further heated at 2750 °C, the mechanical performances outperform the reported values of the "graphite" foils prepared by calendering exfoliated graphite flakes; the electrical conductivity is ~3.1 × 105 S/m and the in-plane thermal conductivity is ~1.2 × 103 W/(m·K).

Raman spectra of large graphene bubbles showed size-dependent oscillations in spectral intensity and frequency, which originate from optical standing waves formed in the vicinity of the graphene surface. At a high laser power, local heating can lead to oscillations in the Raman frequency and also create a temperature gradient in the bubble. Based on Raman data, the temperature distribution within the graphene bubble was calculated, and it is shown that the heating effect of the laser is reduced when moving from the center of a bubble to its edge. By studying graphene bubbles, both the thermal conductivity and chemical reactivity of graphene were assessed. When exposed to hydrogen plasma, areas with bubbles are found to be more reactive than flat graphene.

Hydrogenated graphite was synthesized through a Birch-type reduction by treating fluorinated graphite ((CFx)n, x ~ 1.1) with a solution of Li in liquid NH3 followed by the addition of H2O as the proton donor. The conversion was evaluated by Fourier transform infrared spectroscopy, Raman spectroscopy and powder X-ray diffraction. X-ray photoelectron spectroscopy and combustion elemental analysis were used to determine and quantify the chemical composition, giving an empirical formula of C1H0.60O0.06N0.01 for the product with no more than 2 at.% of fluorine atoms remaining. Thermal dehydrogenation of the hydrogenated material ¿ as investigated by thermogravimetric analysis coupled to mass spectrometry ¿ predominately occurs over the range of 350¿600 °C. The product was also analyzed using scanning electron microscopy, atomic force microscopy and transmission electron microscopy, which collectively supported the formation of hydrogenated graphene sheets through a wet-chemical route. To elucidate the structure of the hydrogenated sample, the material was investigated by solid-state nuclear magnetic resonance spectroscopy. Direct pulse and cross-polarization nuclear magnetic resonance measurements, including spin counting, spectral editing and 2D heteronuclear correlation experiments, revealed the nature of the sp3- and sp2-hybridized carbon nuclei, and indicated that methine, methylene and quaternary sp3-carbon atoms were present in the hydrogenated material.

The process of oxidation of a copper surface coated by a layer of graphene in water-saturated air at 50 °C was studied where it was observed that oxidation started at the graphene edge and was complete after 24 h. Isotope labeling of the oxygen gas and water showed that the oxygen in the formed copper oxides originated from water and not from the oxygen in air for both Cu and graphene-coated Cu, and this has interesting potential implications for graphene as a protective coating for Cu in dry air conditions. We propose a reaction pathway where surface hydroxyl groups formed at graphene edges and defects induce the oxidation of Cu. DFT simulation shows that the binding energy between graphene and the oxidized Cu substrate is smaller than that for the bare Cu substrate, which facilitates delamination of the graphene. Using this process, dry transfer is demonstrated using poly(bisphenol A carbonate) (PC) as the support layer. The high quality of the transferred graphene is demonstrated from Raman maps, XPS, STM, TEM, and sheet resistance measurements. The copper foil substrate was reused without substantial weight loss to grow graphene (up to 3 cycles) of equal quality to the first growth after each cycle. It was found that dry transfer yielded graphene with less Cu impurities as compared to methods using etching of the Cu substrate. Using PC yielded graphene with less polymeric residue after transfer than the use of poly(methyl methacrylate) (PMMA) as the supporting layer. Hence, this dry and clean delamination technique for CVD graphene grown on copper substrates is highly advantageous for the cost-effective large-scale production of graphene, where the Cu substrate can be reused after each growth.

Folded graphene in which two layers are stacked with a twist angle between them has been predicted to exhibit unique electronic, thermal, and magnetic properties. We report the folding of a single crystal monolayer graphene film grown on a Cu(111) substrate by using a tailored substrate having a hydrophobic region and a hydrophilic region. Controlled film delamination from the hydrophilic region was used to prepare macroscopic folded graphene with good uniformity on the millimeter scale. This process was used to create many folded sheets each with a defined twist angle between the two sheets. By identifying the original lattice orientation of the monolayer graphene on Cu foil, or establishing the relation between the fold angle and twist angle, this folding technique allows for the preparation of twisted bilayer graphene films with defined stacking orientations and may also be extended to create folded structures of other two-dimensional nanomaterials.

"Paper-like" film material made from stacked and overlapping graphene oxide sheets can be exfoliated (expanded) through rapid heating, and this has until now been done with no control of the final geometry of the expanded graphene oxide material, i.e., the expansion has been physically unconstrained. (As a consequence of the heating and exfoliation, the graphene oxide is "reduced", i.e., the graphene oxide platelets are deoxygenated to a degree.) We have used a confined space to constrain the expanding films to a controllable and uniform thickness. By changing the gap above the film, the final thickness of expanded films prepared from, e.g., a 10 µm-thick graphene oxide film, could be controlled to values such as 20, 30, 50, or 100 µm. When the expansion of the films was unconstrained, the final film was broken into pieces or had many cracks. In contrast, when the expansion was constrained, it never cracked or broke. Hot pressing the expanded reduced graphene oxide films at 1000 °C yielded a highly compact structure and promoted graphitization. Such thickness-controlled expansion of graphene oxide films up to tens or hundreds of times the original film thickness was used to emboss patterns on the films to produce areas with different thicknesses that remain connected "in plane". In another set of experiments, we treated the original graphene oxide film with NaOH before its controlled expansion resulted in a different structure featuring uniformly distributed pores and interconnected layers as well as simultaneous activation of the carbon.

Rational bottom-up construction of two-dimensional (2D) covalent or noncovalent organic materials with precise structural control at the atomic or molecular level remains a challenge. The design and synthesis of metal-organic frameworks (MOFs) based on new building blocks is of great significance in achieving new types of 2D monolayer MOF films. Here, we demonstrate that a complexation between copper(II) ions and tri(ß-diketone) ligands yields a novel 2D MOF structure, either in the form of a powder or as a monolayer film. It has been characterized by Fourier transform infrared, Raman, ultraviolet-visible, X-ray photoelectron, and electron paramagnetic resonance spectroscopies. Selected area electron diffraction and powder X-ray diffraction results show that the MOF is crystalline and has a hexagonal structure. A MOF-based membrane has been prepared by vacuum filtration of an aqueous dispersion of the MOF powder onto a porous Anodisc filter having pore size 0.02 µm. The porous MOF membrane filters gold nanoparticles with a cutoff of ~2.4 nm.

The ability to scale up the production of chemically modified forms of graphene has led to intense interest in the manufacture and commercialization of graphene-based materials. Free-standing film-like materials comprised of stacked and overlapped platelets of graphene oxide (G-O) or thermally and electrically conductive reduced graphene oxide (rG-O) are potentially useful in various applications including filtration membranes, mechanical seals, protective layers, heating elements and components of batteries or supercapacitors as well as in electronics and optoelectronics. The advances in these applications require efficient and low-cost protocols for fabricating certain types of layered materials and, as such, protocols are urgently needed for the reduced forms of G-O. Here we report an efficient and straightforward method to thermally reduce thin films of stacked G-O platelets while still maintaining their structural integrity. By rapidly heating confined G-O films on a hot plate set to 400 °C under an atmosphere of air, G-O films were readily converted to intact, electrically conductive, reduced thin films. The structure and degree of reduction of the resulting free-standing rG-O films were found to be comparable to those obtained by slow annealing at the same temperature.

A vortex fluid device (VFD) with non-thermal plasma liquid processing within dynamic thin films has been developed. This plasma-liquid microfluidic platform facilitates chemical processing which is demonstrated through the manipulation of the morphology and chemical character of colloidal graphene oxide in water.