Graphene oxide as a promising photocatalyst for CO2 to methanol conversion (original) (raw)
Graphene oxide (GO) was synthesized by a modified Hummer's method. 1,2 1 g of graphite flakes (Sigma-Aldrich) was grounded with 20 g of NaCl (Himedia) in a mortar pastel for 30 minutes. The excess NaCl was washed by rinsing repeatedly with water through vacuum-filtration. The above washed graphite flakes were dried in oven for 30 minutes at 70 0 C. 23 ml of 36N H 2 SO 4 (Rankem) was added to the dried graphite flakes in a 250 ml round bottom flask and stirred for 24 hours at room temperature. Then the temperature was raised to 40 0 C and 100 mg of NaNO 3 (Himedia) was dissolved in the solution. This step is followed by slow addition of 500 mg of KMnO 4 (Himedia) into the reaction mixture, keeping the reaction temperature below 45 0 C. Subsequently, 3 ml of water was added to the flask, followed by another 3 ml after 5 minutes. After next 5 minutes, 40 ml of water was added slowly to it. After 15 minutes the flask was removed from the oil bath and 140 ml of H 2 O and 10 ml of 30% H 2 O 2 were added. Then the flask with the reaction mixture was stirred at room temperature for 10 minutes. Then reaction mixtures were washed by centrifugation with 5% HCl and then further with excess of water. The final precipitate was dispersed in 100 ml of water and ultrasonicated for 30 minutes. Graphite particles in the dispersion were separated by centrifugation at 5000 rpm for 5 minutes and a brown homogeneous supernatant was collected. Then pure GO was collected from the above supernatant solution by centrifugation at 15000 rpm for 30 minutes and re-dispersed in 100 ml of ethanol (~2 mg/ml).
Engineering a Water-Dispersible, Conducting, Photoreduced Graphene Oxide
A critical limitation that has hampered widespread application of the electrically conducting reduced graphene oxide (r-GO) is its poor aqueous dispersibility. Here we outline a strategy to obtain water-dispersible conducting r-GO sheets, free of any stabilizing agents, by exploiting the fact that the kinetics of the photoreduction of the insulating GO is heterogeneous. We show that by controlling UV exposure times and pH, we can obtain r-GO sheets with the conducting sp 2-graphitic domains restored but with the more acidic carboxylic groups, responsible for aqueous dispersibility, intact. The resultant photoreduced r-GO sheets are both conducting and water-dispersible. ■ INTRODUCTION Graphene sheets, one-atom thick, two-dimensional layers of carbon atoms, have gained enormous importance over the past few years due to their unique attributes: high electronic and thermal conductivities and exceptional mechanical strength. 1,2 These properties have led to the development of graphene-based field-effect transistors, 3 ultrasensitive sensors, 4 and electromechanical resonators. 5 Current procedures, such as mechanical exfoliation 6 or chemical vapor deposition, 7 are not ideal for the large-scale manufacture of processable graphene sheets and are unlikely to meet current requirements. 8 The chemical reduction of suspensions of graphene oxide (GO) has emerged as a viable route for large-scale production of graphene sheets. 8,9 Over the years various procedures have been developed for the reduction of GO that include the widely used chemical reduction reaction using either hydrazine or sodium borohydride, plasma or thermally induced reduction, and photochemical methods. 8,10−12 Irrespective of the method of reduction, the resultant reduced GO (r-GO) contains residual oxygen functionalities, holes, and defects, and consequently conductivities are considerably lower than that of graphene obtained by mechanical exfoliation. 12 The conductivity, unlike in graphene where electrons and holes undergo ballistic transport, is by an activated mechanism. 13 Nevertheless, r-GO is a versatile material with conductivities appreciably higher than that of GO and which can tailored over several orders of magnitude by controlling the degree of oxidation. Developing effective techniques to reduce graphene oxide as well as deciphering the underlying reduction mechanism are important both from a fundamental and an applied perspective considering the number of potential applications. The light-induced reduction of GO, by exposure to UV radiation, is especially attractive, for apart from being rapid and facile the avoidance of hazardous chemicals makes it a " green " procedure. 14−20 Recent studies have shed light on the mechanism of the photochemical transformation of GO to r-GO. In GO the sp 2-bonded carbon network of graphite is strongly disrupted, and a significant fraction of this carbon network is bonded to hydroxyl groups (C−OH) or participates in epoxide (C−O−C) groups with minor components such as carboxylic or carbonyl groups populating the edges of the GO sheets. 21,22 In aqueous dispersions the photoreduction of GO on exposure to UV radiation has been shown, by pump−probe femtosecond spectroscopy, to be an indirect process, wherein the transformation to r-GO is initiated after the capture of solvated electrons, produced by the UV photoionization of water. 23,24 It is the chemical potential of the photogenerated solvated electrons that drives the reduction of GO, and not simple heating effects. Earlier studies, too, had indicated that the UV light induced transformation of GO to r-GO in aqueous media is not a thermal event, but the mechanism suggested involved band gap excitation and subsequent photocatalytic reductionthe semiconductor domains of GO catalyzing its own photoreduction. The photoreduction of GO using a semiconductor photocatalyst such as TiO 2 is well documented. 25 It has also been established from laser-induced photolysis of single GO sheets that the photoreduction is both spatially and temporally heterogeneous. 26 Reduction arises from the photoinduced migration and subsequent dissociation of hydroxyl groups located on the basal plane. The last step may also be accompanied by dissociation of carbonyl and
RSC Adv., 2014
Graphene synthesized by the reduction of graphene oxide (GO) features in a myriad of applications ranging from sensors to batteries and catalysts to dye-sensitized solar cells. The exceptional physical and electrochemical properties of graphene originate from the presence of several residual functional groups and the non-stoichiometry in its structure. But, investigating the evolution of graphene from GO has been a daunting task. In this manuscript, simple electrochemical methods are reported to characterize GO subjected to thermal, electrochemical, and chemical reduction. The electrochemical features of these samples along with their FTIR spectra and XRD patterns help to identify the functional groups and provide compelling evidence for the transformation among them during the reduction of GO. The redox features of the voltammograms suggest the conversion of epoxides to carbonyl, carbonyl to carboxylic acid groups, and their subsequent removal with potential cycling. Thermal treatment of GO in the range of 80-150 C causes the conversion of some of the epoxides to carbonyls and removal of water content. At the same time, epoxides are more prevalent in chemically reduced GO. The double layer capacitanceone of the figure of merits that distinguishes graphene from other carbon allotropesgives an indication of the reduced graphene oxide content in the sample. Thus, electrochemical characterization sheds significant light onto the nature of oxygen moieties in non-heat-treated GO (n-HT-GO), thermally reduced GO (t-GO), chemically reduced GO (c-RGO) and electrochemically reduced GO (e-RGO), besides explaining the range of reported electrochemical capacitance.
Solar Energy, 2019
Abstract In this study, TiO2 nanoparticles (NPs) /reduced graphene oxide (RGO) composites were prepared by hydrothermal process and physiochemical, optical, photocatalytic and photoelectrochemical properties of prepared materials was investigated. Photocatalytic activity measurement showed that methylene blue (MB) photodegraded faster by TiO2-RGO composites compared to bare TiO2. Furthermore, photoelectrochemical (PEC) properties of TiO2 and TiO2-RGO electrodes were investigated under the illumination of a 150 W Xe lamp in 1M aqueous solution of KOH as the electrolyte. Moreover, TiO2-RGO electrodes showed greatly improved photocurrent density which is 3.3-fold higher than pure TiO2. Combined analyses of Mott-Schottky plots and electrochemical impedance spectroscopy (EIS) confirmed that RGO in the TiO2-RGO nanocomposite increased the donor concentration (ND), decreased recombination process of charge carriers (τD), thinner the space charge layer (WSCL) and reduced flat band potential (VFb) of the TiO2, thereby greatly enhancing the PEC performances of the TiO2 photoanodes. The improved PEC performance of the TiO2-RGO nanocomposite compared to TiO2 NPs attributed to great enhancement of electron transport through the RGO in the TiO2- RGO film and consequently charge separation.
The Journal of Physical Chemistry C, 2013
We report a simple, facile, and reproducible method for the fabrication of electrochemically reduced graphene oxide (ERGO) films on glassy carbon electrode (GCE) by the self-assembly method. The graphene precursor, graphene oxide (GO), was self-assembled on GCE through a diamine linker which was preassembled on GCE by electrostatic interaction between the positively charged amine and the negatively charged layers of graphene oxide (GO). The oxygen functional groups present on the surface of GO were electrochemically reduced to retain the aromatic backbone of graphene. The attachment of GO followed by its electrochemical reduction was confirmed by ATR-FT-IR spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Raman spectra show that the intensity ratio of D and G bands was increased after the electrochemical reduction of GO. XPS results reveal that the carbon-to-oxygen ratio was increased after the electrochemical reduction of electrostatically assembled GO. Further, Raman and XPS results confirm the removal of oxygen functional groups present on the surface of GO after electrochemical reduction. Impedance spectral studies show that the electron transfer reaction was facile at ERGO modified GCE. Finally, the electrocatalytic activity of ERGO was examined by studying the oxidations of ascorbic acid (AA), dopamine (DA), and uric acid (UA). It enhanced the oxidation currents of AA, DA, and UA when compared to bare GCE. The electrocatalytic activity of the present modified electrode was highly stable.
Chemical Engineering Journal, 2017
A series of TiO 2 /nitrogen (N) doped reduced graphene oxide (TiO 2 /NrGO) nanocomposites with varying concentration and bonding configurations of nitrogen were synthesized by a one-step urea-assisted hydrothermal method, and applied to photoreduction of CO 2 with H 2 O vapor in the gas-phase under the irradiation of a Xe lamp. The effect of the N dopant (doping quantity and bonding configuration) on the catalytic performance of TiO 2 /NrGO was examined. In particular, TiO 2 /NrGO-300, with a 300:1 mass ratio of urea/GO in precursor solution, had the highest CO production yield (356.5 μmole g-1), manifesting a significant 4.4 and 2.2-fold enhancements of CO yield over pure TiO 2 and TiO 2 /rGO, respectively. More significantly, TiO 2 /NrGO showed excellent catalytic stability during the prolonged reaction, while catalytic deactivation was observed for both pristine TiO 2 and TiO 2 /rGO after a few hours. The promoting effects of N dopants on the structure and activity of TiO 2 /NrGO were investigated. It was demonstrated that NrGO with an appropriate N quantity and N-bonding configuration acted as a dual-functional promoter, simultaneously enhancing CO 2 adsorption on the catalyst surface and facilitating electron-hole separation, while eventually boosted the photocatalytic performance. Experimental results in this work provide a better understanding of the critical roles of N dopants in the synthesized composites and also inspire the ongoing interest in better design of other N-doped graphene based materials for photoreduction of CO 2 .
Journal of Applied Physics, 2017
Graphene-based materials are among the most innovative and promising materials for the development of high-performance sensing devices, mainly due to the large surface area and the possibility to modify their reactivity by suitable functionalization. In the field of sensing applications, the peculiarities of innovative materials can be exploited only if chemical and physical properties are fully understood and correlated with each other. To this aim, in this work, graphene oxide (GO) and ethanol-treated GO (GO Et) were investigated from chemical and structural points of view. Electrical characterization was performed by depositing GO and GO Et between two electrodes by dielectrophoresis. All the investigations were repeated on GO materials after thermal treatment in a low temperature range (60 C-300 C). Furthermore, the electrical conductivity of GO was investigated by changing the temperature and the environment (air or N 2) during the characterization: an increase in the conductivity of the as-deposited GO was observed when the device is cooled down and this effect is reversible with the temperature. GO Et and the thermally treated GO and GO Et show an opposite trend, confirming the key role of the oxygen functionalities in the conduction mechanisms and, therefore, in the conductivity of the GO layers. Published by AIP Publishing.
Reduced graphene oxide (rGO)–copper oxide nanocomposites are prepared by covalent grafting of CuO nanorods on the rGO skeleton. Chemical and structural features of rGO–CuO nanocomposites are probed by FTIR, XPS, XRD and HRTEM analyses. Photocatalytic potential of rGO–CuO nanocomposites is explored for reduction of CO 2 into the methanol under the visible light irradiation. The breadth of CuO nanorods and the oxidation state of Cu in the rGO–CuO/Cu 2 O nanocomposites are systematically varied to investigate their photocatalytic activities. The pristine CuO nanorods exhibited very low photocatalytic activity owing to fast recombination of charge carriers and yielded 175 mol g −1 methanol, whereas rGO–Cu 2 O and rGO–CuO exhibited significantly improved photocatalytic activities and yielded five (862 mol g −1) and seven (1228 mol g −1) folds methanol, respectively. The superior photocatalytic activity of CuO in the rGO–CuO nanocomposites was attributed to slow recombination of charge carriers and efficient transfer of photo-generated electrons through the rGO skeleton. This study further excludes the use of scavenging donor.