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Graphene oxide or its reduced derivative (GO/RGO) replace metal oxides in perovskite photovoltaics to achieve energy band alignment for minimization of the energy barriers at the film interfaces allowing efficient charge transport, and eliminate stability issues. However, the power conversion efficiencies fall in a wide range (∼0.6–18%). Therefore, the perovskite growth and nucleation on GO/RGO require fundamental understanding to improve device function for controlled fabrication, which remain a major challenge. We analyze the surface morphology and crystallization of the lead halide perovskites (MAPbX3) at 20–300 °C on GO using X-ray diffraction and photoelectron spectroscopy. To determine defect mechanisms and their composition, we perform in situ transmission infrared and micro Raman spectroscopy, and the cross-sectional scanning microscopy that captures interfacial imperfections with the oxygen defects. We demonstrate the oxygen-induced defects at the MAPbX3/GO interfaces that initiate at room temperature, and occur through the nucleophilic substitution reactions. Unexpectedly, structural defects nucleate in GO forming chemically reduced GO, and modify the surface morphology that yield a poor perovskite growth. Our theoretical studies also reveal that energetically favorable, exothermic reactions between the halides of the perovskite precursors and the oxygen groups of GO generate acidic reaction by-products (i.e. HX), that confirm the formation of oxygen-induced defects.

Conversion of light energy directly into electricity by solar cell devices represents one of the most promising options for highly scalable renewable power. Tremendous effort has been directed at improving photovoltaic (PV) conversion efficiencies, resulting in dramatic device performance increases over the past two decades for novel, cost-effective PV systems. Nevertheless, performance issues related to device stability, scalability, and flexibility prevent these novel designs from achieving their market potential. For mechanically flexible architectures, integration of new materials such as graphene-derived nanomaterials (i.e. graphene/graphite oxide and their modified analogs with other nanocarbons and carbon nanotubes) may be necessary to enhance alternatives to silicon-based PV systems. Among the diverse solar technologies, perovskite solar cells—most notably organometal halides—have stood out from the crowd with solar efficiencies over 20% and potential for highly scalable manufacturing. Here, we review the use of graphene and graphene-derived nanomaterials in new designs of perovskite solar cells associated with organic–inorganic metal halide perovskites utilized as light-harvesting layers, outlining design perspectives, device characterization, and performance. Recent efforts to clarify stability issues and efficiency control mechanisms are also briefly discussed, and we provide some perspective on the currently available literature and future research directions in the field.

Smart device designs with cheap materials and simple processing methods are necessary for cost-effective and efficient solar cell manufacturing. Methylammonium lead halides (MAPbTx, T = I, Br, Cl) perovskites are promising photovoltaic materials, with high power conversion efficiency (PCE>22%). Perovskite instability, however, has been one of the major obstacles for achieving device performance. Another challenge is the uncontrolled chemistry at the interfaces of the electron/hole transporting layers that limit device function. In order to facilitate an efficient charge transport, graphene-derived nanomaterials made from graphene oxide (GO) and reduced graphene oxide (RGO) have replaced metal oxides and polymers. Overcoming the variation in PCE due to the presence of defects at the interfaces of GO and MAPbTx remains a challenge. Therefore, understanding the fundamental nature of defects is necessary to identify the root cause of device performance failure. In order to investigate interfacial defect modification, we utilize an in situ spectroscopy characterization approach to study chemical interactions at GO/MAPbT x interfaces. We find that halide anions of the perovskite precursors determine defect nucleation in GO, and thereby the growth mechanism. These results demonstrate the need for interface characterization to improve the reliability of perovskite photovoltaics.

The fluorination of graphite/graphite oxide (GO) and their derivatives has been widely investigated for how fluorine interacts with sp2/sp3 carbon; however, the mechanism of this interaction has not yet been elucidated. Fluorination of GO (FGO) at either 10 or 15 psi for 24 h, produced two new absorption bands at ∼743 cm−1 and 482 cm−1, and are attributed to the presence of out-of-plane surface fluorine bonds in FGO (absent in fluorographite – FG). IR studies confirmed the stability of the formed C–F bonds and defect formation due to the introduction of oxyfluorinated species into the graphitic carbon through fluorination of epoxides. Fluorination of GO resulted in ∼4–5 times more fluorine incorporation in bulk as compared to FG. (4.57 vs. 0.8 at.% and 6.64 vs. 1.4 at.% at 10 and 15 psi, respectively). PXRD analyses also showed that the interlayer spacing of FGO expanded in the presence of intercalated C–F species and a defect formation was observed with the evidence of increase of the ID/IG ratio from Raman spectra. To this end, understanding the origin of surface C–F bonds and structural changes in FGO therefore leads to new applications such as implementation of FGO for sensing, nano-electronics and energy storage.

It is important to understand the electrolyte–electrode interactions for fabricating graphene oxide (GO)- and ionic liquid (IL)-based ultracapacitors. Therefore, we explored how the type and size of the cations in various ILs determine the nature of processed materials. In all cases, the ILs intercalate into the graphitic structure but marked differences are observed during exfoliation via thermal reduction. The combination of a long alkyl chain ammonium-based cation and a large-volume anion leads to strong interactions and defect formation, as evidenced by CO2 production during annealing. In contrast, using the same anions but different cations stabilize the GO functional groups below 400 °C.

Graphene oxide nanoribbons (GONRs) with a high aspect ratio and high gravimetric density of their edges relative to those of graphene flakes are promising platforms for graphene-based devices. Since the edge chemistry of GONRs determines their final electronic and transport properties, it is important to understand the interactions of oxygen with the edges of the ribbons. Although oxidative unzipping of carbon nanotubes has been studied by Dai’s(1) and Tour’s(2) groups for GONR production, the role of oxygen concentration, the nature of edge oxygen groups, and their effect on the ribbon-edge geometry is still not well understood. We have therefore studied thermal annealing of GONRs, obtained by unzipping few-walled or multi-walled carbon nanotubes, focusing on the reduction process. For this purpose, in situ infrared absorption spectroscopy is used to monitor the edge reconstruction during the thermal reduction process. The ribbon edges of reduced graphene nanoribbons (rGNRs), initially functionalized wit...

Highly oriented graphene nanoribbons sheets and yarns are produced by chemical unzipping of self-standing multiwalled carbon nanotube (MWNT) sheets. The as-produced yarns - after being chemically and thermally reduced - exhibit a good mechanical, electrical, and electrochemical performance.

International audience; Graphene oxide has multiple potential applications. The chemistry of graphene oxide and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This under- standing is crucial to enable future applications of this material. Here, a com- bined experimental and density functional theory study shows that multilayer graphene oxide produced by oxidizing epitaxial graphene via the Hummers method is a metastable material whose structure and chemistry evolve at room temperature with a characteristic relaxation time of about one month. At the quasi-equilibrium, graphene oxide reaches a nearly-stable reduced O/C ratio, and exhibits a structure intensively deprived of epoxide groups and enriched of hydroxyl groups. Our calculations show that the structural and chemical changes are driven by the availability of hydrogen in the oxidized graphitic sheets, which favors the reduction of epoxide groups and the formation of water molecules.

As an atypical reductant, ethanol has recently been considered for reducing the oxygen concentration and restoring the graphitic structure in multilayered graphene oxide (GO) during annealing [Su et al., ACS Nano2010, 4, 5285]. The reaction mechanism is, however, still not well understood, hindering progress in the use of GO. By combining density functional theory (DFT), ab initio molecular dynamics (MD) calculations, and in situ infrared absorption spectroscopy, the thermal evolution of both carbonyl and ether groups in multilayered GO is shown to vary dramatically upon individual intercalation of methanol, ethanol, and water molecules. In hydrated GO, bare etch holes are generated by thermal annealing, as evidenced by the evolution of carbon dioxide (CO2) molecules. In contrast, the replacement of water by methanol or ethanol prevents defect enlargement during annealing. Furthermore, ethanol is found to repair the etch holes by facilitating the formation of new hexagonal carbon rings. Ab initio MD simul...

The fabrication of high performance, graphene-based electrochemical energy storage devices, such as ultracapacitors, depends on the reduction of graphite oxide (GO) and its interaction with ionic liquids (ILs), which may be used as the conductive electrolyte. To explore the physical and chemical interactions between ILs and thermally reduced GO (TRG) as a function of annealing temperature, three ILs with ammonium based structures were selected to differentiate the role of their anions and cations in the exfoliation process. Intercalation was accompanied by either covalent or noncovalent bonding, as supported by thermogravimetric analysis (TGA) and infrared (IR) absorption spectroscopy performed in situ during thermal annealing and by X-ray diffraction (XRD) analysis. Upon IL intercalation, covalent bonding between the IL and TRG prevented exfoliation, while noncovalently physisorbed ILs were readily removed and therefore facilitated exfoliation of the reduced GO. The anion and cation moieties of the ILs i...

Understanding the thermal reduction of graphene oxide (GO) is important for graphene exfoliation, and chemical and morphological modifications. In this process, the role of trapped water and the evolution of oxygen during annealing are still not well-understood. To unravel the complex mechanisms leading to the removal of oxygen in reduced GO, we have performed in situ transmission infrared absorption spectroscopy measurements of GO films upon thermal annealing at 60–850 °C in vacuum (10–3–10–4 Torr). Using cluster-based first-principles calculations, epoxides, ethers (pyrans and furans), hydroxyls, carboxyls, lactols, and various types of ketones and their possible derivatives have been identified from the spectroscopic data. Furthermore, the interactions between randomly arranged nearby oxygen species are found to affect the spectral response (red and blue shifts) and the overall chemistry during annealing. For instance, the initial composition of oxygen species (relative amounts and types of species, su...

A detailed in situ infrared spectroscopy analysis of single layer and multilayered graphene oxide (GO) thin films reveals that the normalized infrared absorption in the carbonyl region is substantially higher in multilayered GO upon mild annealing. These results highlight the fact that the reduction chemistry of multilayered GO is dramatically different from the single layer GO due to the presence of water molecules confined in the ∼1 nm spacing between sheets. IR spectroscopy, XPS analysis, and DFT calculations all confirm that the water molecules play a significant role interacting with basal plane etch holes through passivation, via evolution of CO(2) leading to the formation of ketone and ester carbonyl groups. Displacement of water from intersheet spacing with alcohol significantly changes the chemistry of carbonyl formation with temperature.

Realization of graphene based devices will require a controlled integration of graphene into a device structure with multiple material components of metals and insulators. To investigate the fundamental materials problems of graphene devices, a complimentary team of researchers at UTD is applying experimental and theoretical methods to the graphene/metal and graphene/dielectric interface problems. To assess the large area graphene synthesis and the corresponding material properties, graphene oxides and grain boundaries in graphene are also examined by experiments in close connection with modeling study. Through a strong collaborative research in synthesis, characterization, and modeling, we have developed a fundamental understanding of graphene material properties which can facilitate diverse graphene based devices applications.

Polypyrrole (PPy) nano-networks and nano-fibers were synthesized using interfacial and template polymerization techniques, respectively. The morphology of the PPy nano-networks showed that a homogeneous, three-dimensionally grown nano-fibers were produced. Dodecyl sulfonate was used as surfactant in the interfacial polymerization. Bulk conductivity of PPy nano-networks were in a range of 10−1–10−4 S/cm with a surface area of ca. 480 m2/g. Template synthesis produced one-directional alignment of conducting nano-arrays for the purpose of possible applications of these materials in charge storage devices (i.e., supercapacitors) as electrode materials. Electrochemical and spectroelectrochemical investigations showed that these materials are promising for device applications.

Methylammonium lead iodide (MAPbI3) perovskites are organic–inorganic semiconductors with long carrier diffusion lengths serving as the light-harvesting component in optoelectronics. Through a substitutional growth of MAPbI3 catalyzed by polar protic alcohols, evidence is shown for their substrate- and annealing-free production and use of toxic solvents and high temperature is prevented. The resulting variable-sized crystals (≈100 nm–10 µm) are found to be tetragonally single-phased in alcohols and precipitated as powders that are metallic-lead-free. A comparatively low MAPbI3 yield in toluene supports the role of alcohol polarity and the type of solvent (protic vs aprotic). The theoretical calculations suggest that overall Gibbs free energy in alcohols is lowered due to their catalytic impact. Based on this alcohol-catalyzed approach, MAPbI3 is obtained, which is chemically stable in air up to ≈1.5 months and thermally stable (≤300 °C). This method is amendable to large-scale manufacturing and ultimately can lead to energy-efficient, low-cost, and stable devices.

International audience; The chemistry of graphene oxide (GO) and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is however crucial to enable future applications of the material that typically are subject to environmental conditions. The nature of the initial GO is also highly dependent on the preparation and the form of the initial carbon material. Here, we consider both standard GO made from oxidizing graphite and layered GO made from oxidizing epitaxial graphene on SiC, and examine their evolution under different stimuli. The effect of the solvent on the thermal evolution of standard GO in vacuum is first investigated. In situ infrared absorption measurements clearly show that the nature of the last solvent in contact with GO prior to deposition on a substrate for vacuum annealing studies substantially affect the chemical evolution of the material as GO is reduced. Second, the stability of GO derived from epitaxial graphene (on SiC) is examined as a function of time. We show that hydrogen, in the form of CH, is present after the Hummers process, and that hydrogen favors the reduction of epoxide groups and the formation of water molecules. Importantly, this transformation can take place at room temperature, albeit slowly (~ one month). Finally, the chemical interaction (e.g. bonding) between GO layers in multilayer samples is examined with diffraction (XRD) methods, spectroscopic (IR, XPS, Raman) techniques, imaging (APF) and first principles modeling.

Desalinated seawater is the primary source of drinking water in Qatar. Among all present desalination technologies, reverse osmosis (RO) has been demonstrated as one of the most feasible processes. However, the main limitation with RO and other membrane-based techniques is costly operation and maintenance associated with membrane scaling, fouling, and degradation. Advanced membranes that enable ultrafast permeation while maintaining good mechanical properties, are very important to facilitate both water purification and desalination technologies. Low-dimensional nanomaterials such as carbon nanotubes, cellulose nanocrystals and graphene oxide (GO) have been tested in membranes due to their good mechanical properties and amenable surface functionalization. Specifically, GO nanosheets have recently emerged as a new material for ultrathin, high-flux and energy-efficient sieving membranes due to GO's unique two-dimensional atomically thin structure, outstanding mechanical strength and good flexibility, as well as good dispersion in aqueous solutions. However, selectivity and stability of fully wetted GO membranes in cross-flow conditions has remained challenging and solubility of GO can also lead to membrane disintegration under operation conditions. Herein we present MXenes [1], a new class of 2D carbides, as new promising membrane materials for water desalination applications. For this purpose, Ti3C2-based MXene membranes have been prepared by a vacuum-assisted filtration technique. In order to detect the permeated ions and molecules, we have performed electrical conductivity measurements and UV-Vis analyses. The results have shown that MXene membranes are selective towards ions of different size/charge, such as Cu2+, Mg2+, Na+, K+, SO42-, and Cl-. The permeation data have also shown a cut-off trend around 4 A, and species of a larger size have been sieved out. The transport mechanism through MXene membrane films has been therefore size and charge selective due to the presence of the interlayer slit pores and the negative charges on the hydrophilic Ti3C2-based MXene film surfaces. In this study, we compare MXene membranes with GO membranes to better understand differences in their water desalination performance. Indeed these novel membrane composites are expected to improve the flux, increase the salt rejection efficiency and decrease adhesion of the adsorbed particulates and organic molecules, thus mitigating fouling. Reference: 1.M. Naguib, V.N. Mochalin, M.W. Barsoum, Y. Gogotsi, MXenes: A New Family of Two-Dimensional Materials, Advanced Materials, 26, 992-1005 (2014)

Poly(pyrrole), poly(N-methyl pyrrole), poly(thiophene) and poly(3,4-ethylenedioxythiophene) nano-arrays, nano-fibers and nano-tubes have been successively synthesized using polycarbonate and alumina membranes as templates. Oxidative chemical polymerization method was used to produce nano-structured conducting polymers with diameters in the range of 80–350 nm and the size of the resulting polymer was controlled with the pore size of membrane. Preliminary results showed that highly oriented polymers are promising materials for many applications including supercapacitors, sensors and photonic devices. Copyright © 2006 John Wiley & Sons, Ltd.

Exfoliation, i.e. individual separation of carbon sheets, is of great interest to produce single-layered graphene nanosheets. Chemical or thermal treatments are popular approaches to exfoliate graphite chunks. In general, these conventional methods are assisted with intercalation via covalent or non-covalent functionalization, expansion, and swelling, adsorption of organic molecules in gas phase, solid nanoparticle insertion or direct molecular exfoliation. However, direct covalent modification of graphene is challenging and the zero-band gap of graphene limits its use in field-effect transistors in nanoelectronics. Therefore, the use of a band-gap tunable p-type semiconducting reduced graphene oxide (rGO) is an alternative route. There are several approaches to tune its band gap, including tailoring the chemistry. Critical parameters include the control of oxygen amount determined by the degree and time of oxidation and reduction conditions (e. g. temperature), often leading to nonstoichiometry. This short review therefore highlights the production of rGO focusing primarily on the effect of thermal treatmenton the nature and the role of oxygen during thermal exfoliation of GO. The impact of oxygen functionalization on the modulation of the band gap is also reviewed for chemically and thermally reduced GO, as well as chemically treated rGO followed by a thermal exfoliation.

High power conversion efficiency of perovskite-based solar cells offers promise for low-cost and scalable production of renewable energy. The need to harness solar energy has recently motivated the search to alternate ETL/HTL hybrid materials, specifically graphene/perovskite films. Hybrid organic-inorganic methylammonium lead halides, MAPbX3 (X=I, Br, Cl)/mixed-halides (I3-xClx, I3-xBrx) have been reported as light harvesting layers with their superior optoelectronic properties: tunable bandgap, long electron-hole diffusion lengths and high electron/hole mobility. Nevertheless, halide-based perovskites require in situ investigation for film growth, degradation and perovskite formation mechanisms to overcome detrimental effects of incomplete lead precursor conversion, inconsistent crystallite formation/film uniformity, and weak cation-anion-solvent coordination. Graphene-derived hybrids have recently emerged as an ETL/HTL replacement in these devices, however understanding the origin of interfacial chemical reactions between deposited perovskite films over graphene-derived materials is still lacking. Graphene/perovskite structure-property relationships are, however, not well understood due to unclear chemistry at the ETL/perovskite/HTL interfaces. Moreover, effect of film thickness, lead content, stoichiometry control, overlayer/underlayer morphology/composition, stability issues and cation-anion electrostatic interactions ought to be examined for better charge transport at the graphene/perovskite interfaces. Stability factors also need to be studied for charge mechanisms to unravel device performance challenges. Indeed, underlayer ETLs (TiO2/Al2O3) and overlayer HTLs (spiro-OMeTAD) were rarely studied with graphene. To address scalability and stability issues, we examined degradation, nucleation and growth mechanisms in reduced graphene/graphite oxide (RGO) upon halide-based (I, Cl, Br) perovskite deposition. Chemical interactions were interpreted at perovskite/RGO interfaces for the grain size, orientation, boundaries, and surface/bulk effects using variable-temperature (≤600°C, Ar(g)) in situ spectroscopy (infrared absorption, micro-Raman, UV-vis-NIR, luminescence). Controlled perovskite formation was achieved at room temperature for bromide/chloride-based perovskites resulting in improved chemical stability with heat (vs. iodide derivative). Overall, perovskite decomposition was observed at ~150°C. Oxygen-induced chemical reactions occurred at ≤150°C, eliminated hydroxyls/carboxyls in RGO, and maintained ethers/epoxides upon perovskite decomposition. Poor perovskite formation was observed on RGO due to varying electron affinity and reactivity of precursor halides, resulting in film degradation in air (O2, H2O). Film morphology was explored by characterization techniques such as SEM, XRD, XPS, TEM, and AFM. Keywords: In situ spectroscopy, graphene/perovskite interface, perovskite-based solar cells References: (1) M Acik, SB Darling. J. Mater. Chem. A (2016) Advance Article. Doi: 10.1039/C5TA09911K (2) J Gong, SB Darling, F You, Energy Environ. Sci. (2015) 8, 1953-1968 (3) M Acik, G Lee, C Mattevi, M Chhowalla, K Cho, YJ Chabal. Nature Mater. (2010) 9 (10), 840-845 (4) M Acik, C Mattevi, C Gong, G Lee, K Cho, M Chhowalla, YJ Chabal. Acs Nano (2010) 4 (10), 5861-5868 . “Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Office of Science User Facility under Contract No. DE-AC02-06CH11357. M.A. also acknowledges support from the Joseph Katz Named Fellowship at Argonne National Laboratory.”

Power conversion efficiency in perovskite-based solar cells has recently improved to ≥20%, however, there is insufficient understanding of the underlying optoelectronic device function. Among all perovskite materials as candidates for the light harvesters in such solar devices, organolead halide perovskites, MAPbX3 (X=I, Br, Cl), have stood out with their outstanding optoelectronic properties such as tunable bandgaps, long electron-hole diffusion length and high electron/hole mobility. Indeed, replacement of ETL/HTL with graphene-derived materials (graphene oxide, reduced graphene oxide, n/p-doped graphene, etc.) has emerged as a pathway to improve device performance, and replaced traditional use of TiO2 and Al2O3 in these devices. Nevertheless, unclear film growth, nucleation and degradation mechanisms at the graphene/perovskite hybrid interfaces require understanding of interfacial working mechanisms and perovskite film formation, particularly over oxide-based electron/hole conductors. Moreover, graphene/perovskite structure-property relationships are not well understood due to unclear chemistry/poor characterization at the interfaces of ETL/perovskite/HTL hybrids. To explore interfacial working mechanisms and perovskite film formation in graphene-derived perovskite solar cells, we performed variable temperature (≤600°C) in situ spectroscopy (infrared absorption, micro-Raman, UV-vis-NIR, x-ray photoelectron and luminescence). Our studies targeted perovskite/graphene interfaces and perovskite growth mechanisms to overcome detrimental effects of stability factors such as incomplete lead precursor conversion, inconsistent crystallite formation/film uniformity, and weak cation-anion-solvent coordination. Effect of film thickness, lead content, stoichiometry control, underlayer/overlayer composition, and perovskite growth temperature were optimized for better film efficiency and charge transport. To address film scalability and stability, we studied opto-thermal changes in reduced graphene/graphite oxide (RGO) upon halide-based (CH3NH3PbI3, CH3NH3PbBr3, CH3NH3PbCl3) perovskite deposition, and performed spectroscopic analysis derived from the intensity and peak areas of perovskite vibrational normal modes of C-H (~2800-3200 cm-1) and N-H (~2000-2800 cm-1) and their interfacial reactions with oxygen functional groups in RGO. Controlled perovskite formation was achieved at room temperature for bromide/chloride-based perovskites resulting improved chemical stability with heat (vs. iodide derivative) that were decomposed at ≥150°C. Poor perovskite formation was monitored on RGO resulting in film degradation in air (O2, H2O) by in situ characterization; additional insights were derived from defect analysis from ID/IG ratio variation at perovskite/RGO interfaces. Film morphology and composition was examined by ex situ XRD, SEM, TEM, and AFM studies. Keywords: In situ spectroscopy, graphene/perovskite solar cells, perovskite formation References: (1) M Acik, SB Darling. J. Mater. Chem. A (2016) Advance Article. Doi: 10.1039/C5TA09911K (2) J Gong, SB Darling, F You, Energy Environ. Sci. (2015) 8, 1953-1968 (3) M Acik, G Lee, C Mattevi, M Chhowalla, K Cho, YJ Chabal. Nature Mater. (2010) 9 (10), 840-845 (4) M Acik, C Mattevi, C Gong, G Lee, K Cho, M Chhowalla, YJ Chabal. Acs Nano (2010) 4 (10), 5861-5868. “Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Office of Science User Facility under Contract No. DE-AC02-06CH11357. M.A. also acknowledges support from the Joseph Katz Named Fellowship at Argonne National Laboratory.”

Graphene oxide, a two-dimensional aromatic scaffold decorated by oxygen-containing functional groups, possesses rich chemical properties and may present a green alternative to precious metal catalysts. Graphene oxide-based carbocatalysis has recently been demonstrated for aerobic oxidative reactions. However, its widespread application is hindered by the need for high catalyst loadings. Here we report a simple chemical treatment that can create and enlarge the defects in graphene oxide and impart on it enhanced catalytic activities for the oxidative coupling of amines to imines (up to 98% yield at 5 wt% catalyst loading, under solvent-free, open-air conditions). This study examines the origin of the enhanced catalytic activity, which can be linked to the synergistic effect of carboxylic acid groups and unpaired electrons at the edge defects. The discovery of a simple chemical processing step to synthesize highly active graphene oxide allows the premise of industrial-scale carbocatalysis to be explored.

Point sources exhibit low threshold electron emission due to local field enhancement at the tip. The development and implementation of tip emitters have been hampered by the need to position them sufficiently apart to achieve field enhancement, limiting the number of emission sites and therefore the overall current. Here we report low threshold field (< 0.1V/um) emission of multiple electron beams from atomically thin edges of reduced graphene oxide (rGO). Field emission microscopy (FEM) measurements show evidence for interference from emission sites that are separated by a few nanometers, suggesting that the emitted electron beams may be coherent. Based on our high-resolution transmission electron microscopy, infrared spectroscopy and simulation results, field emission from the rGO edge is attributed to a stable and unique aggregation of oxygen groups in the form of cyclic edge ethers. Such closely spaced electron beams from rGO offer prospects for novel applications and understanding the physics of linear electron sources., 20 pages, 4 figures, Submitted

Infrared absorption of atomic and molecular vibrations in solids can be affected by electronic contributions through non-adiabatic interactions, such as the Fano effect. Typically, the infrared-absorption lineshapes are modified, or infrared-forbidden modes are detectable as a modulation of the electronic absorption. In contrast to such known phenomena, we report here the observation of a giant-infrared-absorption band in reduced graphene oxide, arising from the coupling of electronic states to the asymmetric stretch mode of a yet-unreported structure, consisting of oxygen atoms aggregated at the edges of defects. Free electrons are induced by the displacement of the oxygen atoms, leading to a strong infrared absorption that is in phase with the phonon mode. This new phenomenon is only possible when all other oxygen-containing chemical species, including hydroxyl, carboxyl, epoxide and ketonic functional groups, are removed from the region adjacent to the edges, that is, clean graphene patches are present.

Infrared spectroscopy is used to monitor the evolution of CO2 (2330-2350 cm-1) and CO (2050-2200 cm-1) generated during thermal reduction of graphite oxide. The appearance of CO2 at low temperatures (≤200°C) is associated with oxygen removal in species such as anhydrides, esters, lactols, carboxylic acids and lactones either at the edges or in the distorted basal plane of the GO sheets. At higher temperatures (250°C-750°C), release of CO is observed and may be due to decomposition of CO2 or oxygen-containing species like ethers, carbonyls, phenols or quinones. Observation of these gases is possible in multilayer GO because they are trapped in between the interlayer spacing of GO stacks for a time sufficient for detection.

Graphene edges determine the optical, magnetic, electrical, and electronic properties of graphene. In particular, termination, chemical functionalization and reconstruction of graphene edges leads to crucial changes in the properties of graphene, so control of the edges is critical to the development of applications in electronics, spintronics and optoelectronics. Up to date, significant advances in studying graphene edges have directed various smart ways of controlling the edge morphology. Though, it still remains as a major challenge since even minor deviations from the ideal shape of the edges significantly deteriorate the material properties. In this review, we discuss the fundamental edge configurations together with the role of various types of edge defects and their effects on graphene properties. Indeed, we highlight major demanding challenges to find the most suitable technique to characterize graphene edges for numerous device applications such as transistors, sensors, actuators, solar cells, light-emitting displays, and batteries in graphene technology.

Solution exfoliation of graphite holds promise for large-scale bulk synthesis of graphene. Non-covalent exfoliation is attractive because the electronic structure of graphene is preserved but the yield is low and the lateral dimensions of the sheets are small. Chemical exfoliation via formation of graphite oxide is a highly versatile and scalable route but the covalent functionalization of graphene with oxygen significantly alters the properties. Here, we report a new method for large-scale facile synthesis of micron-sized partially oxidized graphene (POG) sheets with dramatically improved electronic properties compared to other solution-phase exfoliated graphene. Due to low initial oxygen content (∼12%), POG requires only mild annealing (