Colin Hong An Wong

Synthetic routes contaminate graphene materials with a whole spectrum of unanticipated metallic elements

Significance Graphene is well-poised to revolutionize many industries because of its multitude of exceptional properties. Current bulk synthesis of graphene materials typically starts with the oxidation of graphite to graphite oxide followed by a reduction step. Many different methods exist for both the oxidation and reduction steps, leading to highly variable types and amounts of metallic contaminations that originate from the reagents themselves. These impurities are able to alter the graphene materials’ properties significantly, which impacts the range of potential applications for which these graphene materials are suitable. Thus, proper characterization of metallic contamination is highly important to ensure the suitability of a chosen set of synthetic procedures to the final application of the graphene material. The synthesis of graphene materials is typically carried out by oxidizing graphite to graphite oxide followed by a reduction process. Numerous methods exist for both the oxidation and reduction steps, which causes unpredictable contamination from metallic impurities into the final material. These impurities are known to have considerable impact on the properties of graphene materials. We synthesized several reduced graphene oxides from extremely pure graphite using several popular oxidation and reduction methods and tracked the concentrations of metallic impurities at each stage of synthesis. We show that different combinations of oxidation and reduction introduce varying types as well as amounts of metallic elements into the graphene materials, and their origin can be traced to impurities within the chemical reagents used during synthesis. These metallic impurities are able to alter the graphene materials’ electrochemical properties significantly and have wide-reaching implications on the potential applications of graphene materials.

Synthesis of water-tolerant indium homoenolate in aqueous media and its application in the synthesis of 1,4-dicarbonyl compounds via palladium-catalyzed coupling with acid chloride.

The first water-tolerant, ketone-type indium homoenolate was synthesized via the oxidative addition of In/InCl(3) to enones. The reaction proceeds exclusively in aqueous media. Both indium and indium(III) chloride are necessary for the smooth conversion of the reaction. Similar results were obtained when InCl or InCl(2) was used in place of In/InCl(3). The synthetic utility of the indium homoenolate was demonstrated through the synthesis of 1,4-dicarbonyl compounds via palladium-catalyzed coupling of indium homoenolate with acid chloride.

Palladium-catalyzed cross-coupling of indium homoenolate with aryl halide with wide functional group compatibility.

An efficient palladium-catalyzed cross-coupling of indium homoenolate with aryl halide is described. The reactions proceeded efficiently in DMA at 100 °C to afford the desired products of β-aryl ketones in moderate to good yields. Various important functional groups including COR, COOR, CHO, CN, OH, and NO(2) can be well tolerated in the protocol.

Highly conductive graphene nanoribbons from the reduction of graphene oxide nanoribbons with lithium aluminium hydride

Graphene nanoribbons are highly promising materials for nanoelectronics. They are typically prepared via the oxidative treatment of carbon nanotubes, which first leads to graphene oxide nanoribbons with poor conductivity. In order to be applied in nanoelectronic systems, such graphene oxide nanoribbons must be converted to their conductive counterparts through a reduction process. We show here that using LiAlH4 as a reducing agent yields highly conductive graphene nanoribbons with conductivity approaching that of carbon nanotubes and significantly surpassing the conductivity of nanoribbons reduced by hydrazine. These highly conductive graphene nanoribbons are expected to be important elements of nanoelectronic devices.

Direct synthesis of ester-containing indium homoenolate and its application in palladium-catalyzed cross-coupling with aryl halide

An efficient method for the synthesis of ester-containing indium homoenolate via a direct insertion of indium into β-halo ester in the presence of CuI/LiCl was described. The synthetic utility of the indium homoenolate was demonstrated by palladium-catalyzed cross-coupling with aryl halides in DMA with wide functional group compatibility.

So-Called “Metal-Free” Oxygen Reduction at Graphene Nanoribbons is in fact Metal Driven

The oxygen reduction reaction (ORR) is of key importance in the field of electrochemical energy production. Remarkable catalytic properties of “metal‐free” graphene nanoribbons towards the ORR were suggested in previous reports. Herein, we show that the electrocatalytic properties of an undoped supposedly “metal‐free” graphene nanoribbon towards the ORR is actually caused by metallic impurities within the graphene nanoribbons.

Geographical and Geological Origin of Natural Graphite Heavily Influence the Electrical and Electrochemical Properties of Chemically Modified Graphenes

Natural graphite is an important precursor for the production of chemically modified graphenes in bulk quantities for electrochemical applications. These natural graphites have varying fundamental properties due to the different geological processes and environments at their points of origin, which are expected to affect their chemical reactivity and hence the properties of the derived graphene materials. Four different natural graphites with known geographical and geological origins were exposed to a modified Hummers oxidation method and the resulting graphite oxides were studied. The graphite oxides were shown to have different extents of oxidation and types of oxygen groups, which directly influenced their electrochemical properties. These differences were propagated further in the subsequent chemical reduction of the graphite oxides, and the reduced graphene oxides exhibited significantly different reduction efficiencies and electrical conductivities. These findings show that the choice of natural graphite of known origin is important to synthesize chemically modified graphenes with a desired set of properties.

Unscrolling of multi-walled carbon nanotubes: towards micrometre-scale graphene oxide sheets.

Chemical routes toward obtaining graphene materials are commonly used in the field, and one such preparation method is the unzipping of carbon nanotubes into long, thin graphene nanoribbons. In this work, we show that oxidative permanganate treatment of Scroll type multi-walled carbon nanotubes can lead to large, stacked sheets instead of nanoribbons. This difference is suggested to arise from the type of nanotube used, such as the Russian Doll form or Scroll form causing a change in the initial oxidation site location and ultimately leading to an alternate nanotube opening process.

Stripping voltammetry at chemically modified graphenes

Several chemically modified graphene (CMG) materials were used to modify bare glassy carbon (GC) electrodes for the square wave anodic stripping voltammetry detection of cadmium ion concentration in aqueous solution, without the use of additives to amplify the detection signal. These CMGs included graphene oxide, graphite oxide, thermally reduced graphene oxide, chemically reduced graphene oxide and electrochemically reduced graphene oxide. The electrochemical performances of these modified electrodes were compared and the recently claimed advantages of using graphene materials to modify electrodes for the ASV detection of trace metal ions was thus challenged. Two CMG materials were proposed as suitable candidates for further investigations in their application towards real world sample analysis.