Reduced Graphene Oxide Materials Research Articles

Cobalt phthalocyanine-supported reduced graphene oxide: A highly efficient catalyst for heterogeneous activation of peroxymonosulfate for rhodamine B and pentachlorophenol degradation

In the present work, a cobalt phthalocyanine-supported reduced graphene oxide material (rGO-CoPc) was used as a catalyst for the degradation of rhodamine B (RhB) and pentachorophenol (PCP) in the presence of peroxymonosulfate (PMS). Inductively coupled plasma (ICP-OES), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were employed to characterize the properties of the composite and Co loading in the material. The catalytic activity of the rGO1-CoPc1 catalyst (rGO:CoPc = 1:1) was evaluated by UV–vis absorption spectroscopy at varying PMS concentrations and catalyst dosages. The complete dye removal was achieved in 9 min by rGO1-CoPc1/PMS system with a high initiation rate constant of 1.019 min−1 using RhB (25 µM), PMS (0.1 mM) and rGO1-CoPc1 (0.5 mg/mL). The removal efficiency was found to be as high as 100% after 11 cycles, suggesting a long-lasting catalytic activity of the of rGO1-CoPc1/PMS system. Quenching experiments suggested that sulfate (SO4−) radicals are the dominant species in the degradation process although hydroxyl (OH) radicals are also involved in the reaction mechanism. Furthermore, the catalyst was successfully applied for the efficient degradation of pentachlorophenol (PCP). A full degradation of PCP (25 µM) was achieved within 20 min using rGO1-CoPc1 (0.5 mg/mL) and PMS (0.6 mM). This study demonstrated that the use of rGO-CoPc as a heterogeneous catalyst for PMS activation is useful for the advanced oxidation processes (AOP) for practical wastewater treatment.

Oxygen Clusters Distributed in Graphene with “Paddy Land” Structure: Ultrahigh Capacitance and Rate Performance for Supercapacitors

AbstractThe introduction of surface functional groups onto graphene can provide additional pseudocapacitance for supercapacitors. However, the compensation for the loss of electrical conductivity arising from the disruption of the conjugated system remains a big challenge. Here, a novel strategy is reported for the design of oxygen clusters distributed in graphene with “paddy land” structure via a low‐temperature annealing process. Moreover, the distribution, content, and variety of oxygen groups and the conductivity of reduced graphene oxide (RGO) can be easily adjusted by annealing temperature and time. First‐principles calculations demonstrate that “paddy land” structure exhibits conjugated carbon network, ultralow HOMO–LUMO gap, and long span of atomic charge values, which are beneficial for the enhanced pseudocapacitance and rate performance. As a result, the functionalized graphene (GO‐160‐8D) exhibits high specific capacitance of 436 F g−1 at 0.5 A g−1, exceeding the values of previously reported RGO materials, as well as excellent rate performance (261 F g−1 at 50 A g−1) and cycling stability (94% of capacitance retention after 10 000 cycles). The findings may open a door for finely controlling the location and density of functionalities on graphene for applications in energy storage and conversion fields via a green and energy‐efficient process.

Microbial reduction of Ferrihydrite in the presence of reduced Graphene oxide materials: Alteration of Fe(III) reduction rate, biomineralization product and settling behavior

Although graphene-based materials have been shown to physicochemically interact with biotic or abiotic particles after entering into the environment, little is known about their impacts on biotransformation of inorganic colloids. Accordingly, the effects of different reduced graphene oxide (rGO) materials on ferrihydrite reduction were investigated by using Shewanella oneidensis MR-1 and lactate as Fe(III) reducer and electron donor, respectively. rGO at 0.5mgL−1 stimulated the transformation of ferrihydrite to magnetite, possibly by improving electron transfer. In contrast, higher concentrations of rGO (10–100mgL−1) hindered the exposure, accessibility and therefore reduction of ferrihydrite, resulting in the formation of a mixture of goethite and magnetite. The adsorption of soil humic acid by coexisting ferrihydrite and rGO resulted in further inhibition of Fe(III) reduction. On the other hand, the enhanced Fe(III) bioreduction by the less adsorbable aquatic humic acid or simpler flavin molecules was further improved by 0.5mgL−1 rGO but inhibited by 50mgL−1 rGO. Additionally, the settling stability of the rGO-mineral aggregates was altered during bioreduction, which could influence the transport and reactivity of iron mineral and the fate of associated rGO. This study improves our understanding of the impact and behavior of rGO in the environment.

Carbon-Heteroatom Bond Formation by an Ultrasonic Chemical Reaction for Energy Storage Systems.

The direct formation of CN and CO bonds from inert gases is essential for chemical/biological processes and energy storage systems. However, its application to carbon nanomaterials for improved energy storage remains technologically challenging. A simple and very fast method to form CN and CO bonds in reduced graphene oxide (RGO) and carbon nanotubes (CNTs) by an ultrasonic chemical reaction is described. Electrodes of nitrogen- or oxygen-doped RGO (N-RGO or O-RGO, respectively) are fabricated via the fixation between N2 or O2 carrier gas molecules and ultrasonically activated RGO. The materials exhibit much higher capacitance after doping (133, 284, and 74 F g-1 for O-RGO, N-RGO, and RGO, respectively). Furthermore, the doped 2D RGO and 1D CNT materials are prepared by layer-by-layer deposition using ultrasonic spray to form 3D porous electrodes. These electrodes demonstrate very high specific capacitances (62.8 mF cm-2 and 621 F g-1 at 10 mV s-1 for N-RGO/N-CNT at 1:1, v/v), high cycling stability, and structural flexibility.

Enhanced bioreduction of nitrobenzene by reduced graphene oxide materials: effects of surface modification and coexisting soluble electron shuttles.

Reduced graphene oxide (rGO) can enhance the bioreduction of nitrobenzene by Shewanella oneidensis MR-1. The effects of surface modification and coexisting soluble electron shuttles on rGO-enhanced bioreduction of nitrobenzene were investigated here. The results showed that rGO enhanced the secretion of extracellular polymeric substance and the bioreduction of nitrobenzene of several folds. No inhibition effect on nitrobenzene bioreduction was observed even when the concentration of rGO was as high as 200mg/L. The surface modification of rGO might affect the electrical conductivity which was assumed as one of the main factors that contributed to the enhancement of nitrobenzene bioreduction by rGO materials. Moreover, the coexisting electron shuttles further enhanced the rGO-mediated nitrobenzene bioreduction. After the simultaneous addition of flavin mononucleotide (10μM) and rGO (50mg/L), the reduction rate increased 7.8 times to 424.98±7.84mg (nitrobenzene)/(g (dry cell)∙h), which was higher than those ever reported.

P.3.c.002 - Nicotinic alpha7 receptor agonist and modulators improved social but not sensorimotor deficits on subchronic MK-801 model of schizophrenia in rats

Gradually reduced graphene oxide materials were successfully synthesized by performing successive chemical treatments on graphite oxide synthesized using Hummers' method. These materials were deeply characterized using complementary spectroscopic techniques (X-ray induced photoelectron spectroscopy, UV- spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy). It was shown that reduction processes allow removing oxygenated functional groups. Nevertheless, defects in the carbon structure materials formed subsequently to the oxidation step remain after such reducing treatments. CO stripping coupled with in-situ Fourier transform infrared spectroscopy measurements revealed that the reduction extent of the carbon-based materials affects the electron density on Pt thus engendering modifications in the adsorption strength of CO molecule.

Occurrence and suppression of transition behavior of reduced graphene oxide thin film for gas sensing

In this paper, we prepared reduced graphene oxide (RGO) thin film by simple airbrush technology and then studied its gas-sensing behaviors at room temperature. On exposure to dynamic NO2 gas, RGO thin film showed a time-resolved n–p transition process (n-type switched to p-type), due to its weak n-type properties arising from hydrazine-reduction method and strong electron-accepting NO2 molecules. As RGO amount was added, the time taken to run through this transition became longer. On consecutive exposures to NO2 and NH3 gases, p–n transition (p-type switched to n-type) emerged as well. To suppress these transition behaviors, CuCl was incorporated into RGO material, resulting in opposite p-type characteristics for the composite film as well as no occurrence of p–n transition during the sensing tests. Compared to pure RGO counterpart, RGO/CuCl film had a six-fold response enhancement and a likewise good repeatability toward 10 ppm NH3 within four cyclic periods. We expect that the proposed research on transition behaviors and relevant suppression methods will shed new light on gas-sensing mechanisms and improve operation stability of RGO based sensors.

Room temperature synthesis of reduced graphene oxide nanosheets as anode material for supercapacitors

We report a facile approach to large scale synthesis of reduced graphene oxide (RGO) materials at room temperature with hydrazine hydrate. As-prepared GO and RGO materials were characterized by X-ray Photoelectron Spectroscopy, Raman spectroscopy, X-ray Diffraction, and scanning electron microscopy techniques. Electrochemical tests of RGO materials have been performed using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy measurements under aqueous solution. The maximum specific capacitance of 112.6Fg−1 for RGO material has been obtained to be at a current density of 1Ag−1 in 1M Na2SO4 electrolyte solution. The materials showed good cyclic stability up to 500 cycles and the degradation of the specific capacitance is only less than 3%. The simple approach and scalable synthesis of present study, is an efficient route to produce RGO sheets with low cost and is a promising material for potential supercapacitor applications.

Modifying optical properties of reduced/graphene oxide with controlled ozone and thermal treatment in aqueous suspensions

Graphene possesses a number of advantageous properties, however, does not exhibit optical emission, which limits its use in optoelectronics. Unlike graphene, its functional derivative, graphene oxide (GO) exhibits fluorescence emission throughout the visible. Here, we focus on controlled methods for tuning the optical properties of GO. We introduce ozone treatment of reduced graphene oxide (RGO) in order to controllably transform it from non-emissive graphene-like material into GO with a specific fluorescence emission response. Solution-based treatment of RGO for 5–45 min with ∼1.2 g l−1 ozone/oxygen gas mixture yields a drastic color change, bleaching of the absorption in the visible and the stepwise increase in fluorescence intensity and lifetime. This is attributed to the introduction of oxygen-containing functional groups to RGO graphitic platform as detected by the infrared spectroscopy. A reverse process: controllable quenching of this fluorescence is achieved by the thermal treatment of GO in aqueous suspension up to 90 °C. This methodology allows for the wide range alteration of GO optical properties starting from the dark-colored non-emissive RGO material up to nearly transparent highly ozone-oxidized GO showing substantial fluorescence emission. The size of the GO flakes is concomitantly altered by oxidation-induced scission. Semi-empirical PM3 theoretical calculations on HyperChem models are utilized to explore the origins of optical response from GO. Two models are considered, attributing the induced emission either to the localized states produced by oxygen-containing addends or the islands of graphitic carbon enclosed by such addends. Band gap values calculated from the models are in the agreement with experimentally observed transition peak maxima. The controllable variation of GO optical properties in aqueous suspension by ozone and thermal treatments shown in this work provides a route to tune its optical response for particular optoelectronics or biomedical applications.

Reply to the ‘Comment on “Colloidal stability of reduced graphene oxide materials prepared using different reducing agents”’ by M. Moazzami Gudarzi, Environ. Sci.: Nano, 2017, 4, DOI: 10.1039/C6EN00424E

This article provides our response to the comment by Gudarzi regarding our interpretation of the results of particle aggregation in NaCl presented in our recent publication.

Comment on “Colloidal stability of reduced graphene oxide materials prepared using different reducing agents” by Y. Qi, T. Xia, Y. Li, L. Duan and W. Chen, Environ. Sci.: Nano, 2016, 3, 1062

The colloidal stability of graphene oxide and its reduced forms can be described using classical DLVO theory. This contradicts the claims by Qi et al. (Environ. Sci.: Nano, 2016, 3, 1062–1071).

Effect of gradual reduction of graphene oxide on the CO tolerance of supported platinum nanoparticles

Gradually reduced graphene oxide materials were successfully synthesized by performing successive chemical treatments on graphite oxide synthesized using Hummers' method. These materials were deeply characterized using complementary spectroscopic techniques (X-ray induced photoelectron spectroscopy, UV- spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy). It was shown that reduction processes allow removing oxygenated functional groups. Nevertheless, defects in the carbon structure materials formed subsequently to the oxidation step remain after such reducing treatments. CO stripping coupled with in-situ Fourier transform infrared spectroscopy measurements revealed that the reduction extent of the carbon-based materials affects the electron density on Pt thus engendering modifications in the adsorption strength of CO molecule.

Synthesis and electrochemical studies of Ta – Graphene nanocomposite film modified platinum electrode

Tantalum/Reduced Graphene Oxide composite is presented in this work as an electrode material for capacitor application. Reduced graphene oxide material was synthesized by modified Hummer's method and Tantalum doped graphene oxide electrodes are fabricated by an electrophoretic deposition method. The physico-chemical properties of the as-synthesized materials are characterized by X-ray diffraction, N2 sorption analyses, Fourier Transform Infrared and Raman spectroscopic techniques. The structural details are elucidated from the scanning and transmission electron microscopic analyses. The X-ray diffraction results confirmed the structural integrity of graphene after the Ta doping process. Raman analysis confirmed the graphitic nature of graphene with a surface area of 281m2/g. The surface area of the tantalum doped composite decreased to 214m2/g due to the deposition of Ta ions onto the graphene surface. The Ta doped graphene composite exhibited comparatively higher capacitance value of 1420μF/cm2 than graphene (980μF/cm2), indicating that this composite can store more charge in comparison with graphene. This enhancement of capacitance with Ta doped graphene is thus found to be a good candidate for super/ultra capacitor applications

The Effect of KOH Treatment on the Chemical Structure and Electrocatalytic Activity of Reduced Graphene Oxide Materials.

Reduced graphene oxide (rG-O)-based materials have great potential as metal-free electrocatalysts for the oxygen reduction reaction (ORR) owing to their electrical and electrochemical properties and large surface area. Long-term durability and chemical stability of the catalysts in the presence of electrolytes such as aqueous KOH solution are important for their use in practical applications. In this study, three types of rG-O and rG-O-K (rG-O after reaction with KOH) materials were synthesized. The chemical structures, surface areas, and catalytic ORR performances of the rG-O materials were compared with those of the corresponding rG-O-K materials. The onset potentials of the rG-O materials for electrocatalytic reduction of oxygen are almost the same as those of the corresponding rG-O-K materials; however, the current density and the number of transferred electrons are significantly reduced. These data show that the catalytic ORR performance of rG-O-based materials can be altered by KOH.

Enhanced dehydrochlorination of 1,1,2,2-tetrachloroethane by graphene-based nanomaterials

Graphene oxide (GO) and reduced graphene oxide (RGO) materials contain a variety of surface O-functional groups that are chemically reactive. When released into the environment these materials may significantly affect the abiotic transformation of organic contaminants, and therefore, may alter their fate and risks. We found that two GO and five RGO materials that varied in C/O ratio, hydrophobicity, and type/distribution of surface O-functionality all had catalytic effects on the dehydrochlorination of 1,1,2,2-tetrachloroethane (TeCA). Even though the catalytic effects of the materials originated from their deprotonated surface O-functional groups, which served as conjugated bases to catalyze the reaction, the catalytic efficiencies of the materials did not correlate strongly with their surface O contents. The spectroscopic evidence (X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy), surface charge data, and adsorption experiments demonstrated that the catalytic efficiencies of the GO/RGO materials were controlled by a complex interplay of the type and distribution of surface O-functionality, as well as adsorption affinity of the materials. Both Ca2+ and Mg2+ inhibited the catalytic efficiency of the materials by binding to the surface O-functional groups, and consequently, decreasing the basicity of the functional groups. At an environmentally relevant concentration of 10 mg/L, Suwannee River humic acid (used as a model dissolved organic matter) alone had little effect on the dehydrochlorination of TeCA. However, it could inhibit the catalytic efficiency of the GO/RGO materials by coating on their surface and thus, decreasing the adsorption affinity of these materials for TeCA. The findings further underline the potentially important impacts of nanomaterials on contaminant fate and effects in the environment.

Surface characterization of graphene based materials

In the present study, two kind of samples were used: (i) a monolayer graphene film with a thickness of 0.345nm deposited by the CVD method on Cu foil, (ii) graphene flakes obtained by modified Hummers method and followed by reduction of graphene oxide. The inelastic mean free path (IMFP), characterizing electron transport in graphene/Cu sample and reduced graphene oxide material, which determines the sampling depth of XPS and AES were evaluated from relative Elastic Peak Electron Spectroscopy (EPES) measurements with the Au standard in the energy range 0.5–2keV. The measured IMFPs were compared with IMFPs resulting from experimental optical data published in the literature for the graphite sample. The EPES IMFP values at 0.5 and 1.5keV was practically identical to that calculated from optical data for graphite (less than 4% deviation). For energies 1 and 2keV, the EPES IMFPs for rGO were deviated up to 14% from IMFPs calculated using the optical data by Tanuma et al. [1]. Before EPES measurements all samples were characterized by various techniques like: FE-SEM, AFM, XPS, AES and REELS to visualize the surface morphology/topography and identify the chemical composition.

Colloidal stability of reduced graphene oxide materials prepared using different reducing agents

The aggregation tendency of GO/RGO correlates poorly with the degree of reduction but strongly with the types and concentrations of surface O functionalities.

A process to enhance the specific surface area and capacitance of hydrothermally reduced graphene oxide

The impact of post-synthesis processing in reduced graphene oxide materials for supercapacitor electrodes has been analyzed. A comparative study of vacuum, freeze and critical point drying was carried out for hydrothermally reduced graphene oxide demonstrating that the optimization of the specific surface area and preservation of the porous network are critical to maximize its supercapacitance performance. As described below, using a supercritical fluid as the drying medium, unprecedented values of the specific surface area (364 m2 g-1) and supercapacitance (441 F g-1) for this class of materials have been achieved.

3D Bicontinuous Nanoporous Reduced Graphene Oxide for Highly Sensitive Photodetectors

Reduced graphene oxide (RGO) is an important graphene derivative for applications in photonics and optoelectronics because of the band gap created by chemical oxidation. However, most RGO materials made by chemically exfoliated graphite oxide are 2D flakes. Their optoelectronic performance deteriorates significantly as a result of weak light‐matter interaction and poor electrical contact between stacking flakes. Here we report a bicontinuous 3D nanoporous RGO (3D np‐RGO) with high optoelectronic performance for highly sensitive photodetectors. 3D np‐RGO demonstrates a over 40 times higher light absorption than monolayer graphene materials and at least two orders of magnitude higher electron mobility than conventional RGO from discrete RGO flakes. The np‐RGO with an optimal reduction state shows ultrahigh photoresponse of 3.10 3 104 A W−1 at room temperature, approximately four orders of magnitude higher than graphene and other graphene derivatives at similar levels of light intensity radiations, and the excellent external quantum efficiency of 1.04 3 107% better than commercial silicon photodetector. The ultrahigh capability of conversing photons to photocurrent originates from strongly enhanced light absorption, facilitated photocarrier transport, and tunable oxygenous defects and reduction states in the 3D interconnected bicontinuous RGO network.

The effect of the thermal reduction temperature on the structure and sorption capacity of reduced graphene oxide materials

The influence of reduction temperatures on the structure and the sorption capacity of thermally reduced graphene (TRGO) has been investigated systematically. A set of TRGO materials were prepared by thermal treatment of parent graphene oxide (GO) at five temperatures (T=200, 300, 500, 700, and 900°C). Investigations of these materials by X-ray diffraction, Raman spectroscopy and X-ray photoemission spectroscopy methods have shown that both the structure and the residual oxygen functional groups on the TRGO surface can be controlled by varying the temperature of the thermal treatment. The data on the sorption and desorption of 4He, H2, N2, Ne and Kr gases in the temperature interval T=2–290K clearly demonstrate that the sorption capacity of TRGO is closely related to the structural changes induced by the treatment temperatures. It is important that the sorption capacities of TRGOs treated at 300°C and at 900°C significantly increase for all the gases used. The prominent increase in the sorption capacity at 300°C is attributed to the structural disorder and liberation of the pores caused by the removal of intercalated water and labile oxygen functional groups (oFGs) favored at this temperature. At 900°C the sorption capacity increases due to the generation of new defects on the TRGO surface, which provide additional access to the internal space between the folds and sheets of the TRGO structure. By tailoring the structural properties we emphasize the potential of TRGO as a highly efficient sorbent.