Chun Kiang Chua

Covalent chemistry on graphene

The recent growth in graphene has witnessed the utilization of graphene as promising materials in a plethora of applications. The potential of graphene could be improved exponentially provided that the processability and band gap could be finely controlled. To this aim, chemical functionalization of graphene remains an important and fundamental approach. This tutorial review aims to provide a broad-based coverage on the recent solution-based functionalization methods of graphene in a concise and mechanistic manner. We focus on the reactions of the graphene sp(2) backbone, such as nucleophilic addition, cycloaddition, free radical additions, substitutions and rearrangements.

Chemically reduced graphene contains inherent metallic impurities present in parent natural and synthetic graphite

Graphene-related materials are in the forefront of nanomaterial research. One of the most common ways to prepare graphenes is to oxidize graphite (natural or synthetic) to graphite oxide and exfoliate it to graphene oxide with consequent chemical reduction to chemically reduced graphene. Here, we show that both natural and synthetic graphite contain a large amount of metallic impurities that persist in the samples of graphite oxide after the oxidative treatment, and chemically reduced graphene after the chemical reduction. We demonstrate that, despite a substantial elimination during the oxidative treatment of graphite samples, a significant amount of impurities associated to the chemically reduced graphene materials still remain and alter their electrochemical properties dramatically. We propose a method for the purification of graphenes based on thermal treatment at 1,000 °C in chlorine atmosphere to reduce the effect of such impurities on the electrochemical properties. Our findings have important implications on the whole field of graphene research.

Graphite Oxides: Effects of Permanganate and Chlorate Oxidants on the Oxygen Composition

Research on graphene materials has refocused on graphite oxides (GOs) in recent years. The fabrication of GO is commonly accomplished by using concentrated sulfuric acid in conjunction with: a) fuming nitric acid and KClO(3) oxidant (Staudenmaier); b) concentrated nitric acid and KClO(3) oxidant (Hofmann); c) sodium nitrate for in situ production of nitric acid in the presence of KMnO(4) (Hummers); or d) concentrated phosphoric acid with KMnO(4) (Tour). These methods have been used interchangeably in the graphene community, since the properties of GOs produced by these different methods were assumed as almost similar. In light of the wide applicability of GOs in nanotechnology applications, in which presence of certain oxygen functional groups are specifically important, the qualities and functionalities of the GOs produced by using these four different methods, side-by-side, was investigated. The structural characterizations of the GOs would be probed by using high resolution X-ray photoelectron spectroscopy, nuclear magnetic resonance, Fourier transform infrared spectroscopy, and Raman spectroscopy. Further electrochemical applicability would be evaluated by using electrochemical impedance spectroscopy and cyclic voltammetry techniques. Our analyses highlighted that the oxidation methods based on permanganate oxidant (Hummers and Tour methods) gave GOs with lower heterogeneous electron-transfer rates and a higher amount of carbonyl and carboxyl functionalities compared with when using chlorate oxidant (Staudenmaier and Hofmann methods). These observations indicated large disparities between the GOs obtained from different oxidation methods. Such insights would provide fundamental knowledge for fine tuning GO for future applications.

Synthesis of Strongly Fluorescent Graphene Quantum Dots by Cage-Opening Buckminsterfullerene

Graphene quantum dots is a class of graphene nanomaterials with exceptional luminescence properties. Precise dimension control of graphene quantum dots produced by chemical synthesis methods is currently difficult to achieve and usually provides a range of sizes from 3 to 25 nm. In this work, fullerene C60 is used as starting material, due to its well-defined dimension, to produce very small graphene quantum dots (∼2-3 nm). Treatment of fullerene C60 with a mixture of strong acid and chemical oxidant induced the oxidation, cage-opening, and fragmentation processes of fullerene C60. The synthesized quantum dots were characterized and supported by LDI-TOF MS, TEM, XRD, XPS, AFM, STM, FTIR, DLS, Raman spectroscopy, and luminescence analyses. The quantum dots remained fully dispersed in aqueous suspension and exhibited strong luminescence properties, with the highest intensity at 460 nm under a 340 nm excitation wavelength. Further chemical treatments with hydrazine hydrate and hydroxylamine resulted in red- and blue-shift of the luminescence, respectively.

Graphene oxide reduction by standard industrial reducing agent: thiourea dioxide

The current fabrication methods of pristine graphene are not feasible for bulk production. The closest approach, which is through chemical reduction of graphene oxide to chemically reduced graphene oxide that resembles pristine graphene, has been widely adopted instead. Herein, we report a new methodology for the reduction of graphene oxide to chemically reduced graphene oxide using a common industrial reductant, thiourea dioxide. The final product has been fully characterized by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, electrochemical impedance spectroscopy (EIS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and cyclic voltammetry. The microscopy techniques revealed reduced graphene of few-layered sheets. Based on the XPS analyses, a C/O ratio as high as 16.0 was achieved. The reduced graphene oxide product demonstrated a good electrochemical performance with a charge transfer resistance as low as 0.11 kΩ based on EIS measurements and a low overpotential for the oxidation of ascorbic acid. Since thiourea is a common industrial reductant, standard protocols are already in place for the waste produced from this methodology. As such, we foresee that this methodology holds the potential for industrial scale reduction of graphene oxide.

Inherently Electroactive Graphene Oxide Nanoplatelets As Labels for Single Nucleotide Polymorphism Detection

Graphene materials are being widely used in electrochemistry due to their versatility and excellent properties as platforms for biosensing. However, no records show the use of inherent redox properties of graphene oxide as a label for detection. Here for the first time we used graphene oxide nanoplatelets (GONPs) as electroactive labels for DNA analysis. The working signal comes from the reduction of the oxygen-containing groups present on the surface of GONPs. The different ability of the graphene oxide nanoplatelets to conjugate to DNA hybrids obtained with complementary, noncomplementary, and one-mismatch sequences allows the discrimination of single-nucleotide polymorphism correlated with Alzheimers disease. We believe that our findings are very important to open a new route in the use of graphene oxide in electrochemistry.

Reduction of graphene oxide with substituted borohydrides

The emergence of graphene as a next-generation material promises enhanced improvement in various fields of materials science. So far, the oxidation of graphite to graphite oxide and consequent reduction to chemically reduced graphene oxide is the most relevant method towards large scale production of graphene materials. Our long-standing aim in the investigation of reductive chemical reactions on graphene surfaces is to evaluate standard, well documented organic synthetic reactions with well-known mechanisms. Sodium borohydride is one of the most common reductants to reduce graphene oxide to chemically reduced graphene oxide, and is only one of the very few with a well-known mechanism. It is well-known in synthetic chemistry that the reducing strength of borohydrides can be fine-tuned by alternating their substituents. This knowledge has not yet been applied to the reduction of graphene oxides. Herein, we expand on the scope for reduction of graphene oxides using various derivatives of borohydrides, specifically sodium cyanoborohydride and sodium triacetoxyborohydride, and investigate the extents of reduction conferred by these variations with comparison to sodium borohydride. The reduced graphenes were characterized by high resolution X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, electrochemical impedance spectroscopy and cyclic voltammetry analyses. Our findings highlighted that sodium triacetoxyborohydride is inclined to react unfavourably with graphene oxide, thus resulting in a graphene material with an almost similar electrochemical characteristic to its precursor. On the other hand, sodium cyanoborohydride conferred obvious reductive effects but was still less effective when compared to sodium borohydride. Our findings bring a deeper understanding of the effects of organo-boron substituents on the extent of graphene oxide reduction. This would allow for potential tailoring of the graphene properties for various applications.

Unusual inherent electrochemistry of graphene oxides prepared using permanganate oxidants

Graphene and graphene oxides are materials of significant interest in electrochemical devices such as supercapacitors, batteries, fuel cells, and sensors. Graphene oxides and reduced graphenes are typically prepared by oxidizing graphite in strong mineral acid mixtures with chlorate (Staudenmaier, Hofmann) or permanganate (Hummers, Tour) oxidants. Herein, we reveal that graphene oxides pose inherent electrochemistry, that is, they can be oxidized or reduced at relatively mild potentials (within the range ±1 V) that are lower than typical battery potentials. This inherent electrochemistry of graphene differs dramatically from that of the used oxidants. Graphene oxides prepared using chlorate exhibit chemically irreversible reductions, whereas graphene oxides prepared through permanganate-based methods exhibit very unusual inherent chemically reversible electrochemistry of oxygen-containing groups. Insight into the electrochemical behaviour was obtained through cyclic voltammetry, chronoamperometry, and X-ray photoelectron spectroscopy experiments. Our findings are of extreme importance for the electrochemistry community as they reveal that electrode materials undergo cyclic changes in charge/discharge cycles, which has strong implications for energy-storage and sensing devices.

Carbocatalysis: The State of “Metal-Free” Catalysis

The rise in global demand for crucial chemical compounds has driven immense research in the fundamental science of catalysis. Graphene and its derivatives (chemically modified graphene, CMGs) have recently emerged as a new class of heterogeneous catalyst that promises economically viable and greener routes to these compounds. Although CMGs possess unique catalytic properties, the actual active sites are often points of discussion. Current minimal understanding on the possible effects of metallic impurities on the electrocatalytic performances of these CMGs calls forth the need to raise awareness on possible metallic impurities misrepresenting the actual chemical catalytic performances of the CMGs. This Minireview highlights the latest advances in the application of CMGs as catalysts, with an emphasis on the possible effects of metallic impurities on CMG catalysis.

Selective Removal of Hydroxyl Groups from Graphene Oxide

Graphene has a wide range of potential applications, thus tremendous efforts have been put into ensuring that the most direct and effective methods for its large-scale production are developed. The formation of graphene materials from graphene oxide through a chemical reduction method is still one of the most preferred routes. Numerous methods starting from various reducing agents have been developed to obtain near-pristine graphene sheets. However, most of the reducing agents are not mechanistically supported by classical organic chemistry knowledge and of those that are supported, they are only theoretically capable of, at most, reducing oxygen-containing groups on graphene oxide to hydroxyl groups. Herein, we present a mechanistically proven method for the selective defunctionalisation of hydroxyl groups from graphene oxide that is based on ethanethiol-aluminium chloride complexes and provides a graphene material with improved properties. The structural, morphological and electrochemical properties of the graphene materials have been fully characterised based on high-resolution X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, electrochemical impedance spectroscopy and cyclic voltammetry techniques. Our analyses showed that the obtained graphene materials exhibited high heterogeneous electron-transfer rates, low charge-transfer resistance and high conductivity as compared to the parent graphene oxide. Moreover, the selective defunctionalisation of hydroxyl groups could potentially allow for the tailoring of graphene properties for various applications.