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Mater Renew Sustain Energy (2015) 4:2 DOI 10.1007/s40243-015-0042-0 OR IGINAL PAPER Copper nanoparticles stabilized by reduced graphene oxide for CO reduction reaction • • • Diego C. B. Alves Rafael Silva Damien Voiry Tewodros Asefa Manish Chhowalla Received: 10 October 2014 / Accepted: 16 January 2015 / Published online: 6 February 2015 The Author(s) 2015. This article is published with open access at Springerlink.com Abstract Carbon dioxide (CO ) is one of the main gases compared to other copper-based electrodes that we have produced by human activity and is responsible for the tested. The CuNPs on rGO also exhibit better stability, green house effect. Numerous routes for CO capture and preserving their catalytic activity without degradation for reduction are currently under investigation. Another ap- several hours. proach to mitigate the CO content in the atmosphere is to convert it into useful species such as hydrocarbon mole- Keywords CO reduction Synthetic photosynthesis cules that can be used for fuel. In this view, copper is one Reduced graphene oxide Copper nanoparticles of the most interesting catalyst materials for CO reduction Electrocatalysis due to its remarkable ability to generate hydrocarbon fuels. However, its utilization as an effective catalyst for CO reduction is hampered by its oxidation and relatively high Introduction voltages. We have fabricated hybrid materials for CO reduction by combining the activity of copper and the The increasing demand for renewable energy has been conductivity of reduced graphene oxide (rGO). Cu motivating researchers around the world to pursue the nanoparticles (CuNPs) deposited on rGO have demon- development of new technologies. Various methods and strated higher current density and lower overpotential devices to generate sustainable forms of energy that are efficient and affordable have been reported [1–4]. In ad- dition to clean energy, the high amount of CO emitted in the atmosphere must also be mitigated [5, 6]. The use of Electronic supplementary material The online version of this article (doi:10.1007/s40243-015-0042-0) contains supplementary CO as a feedstock to generate fuels is intriguing and in- material, which is available to authorized users. spired by photosynthesis [7]. The consumption of CO from the atmosphere via its conversion to fuel could pro- D. C. B. Alves D. Voiry M. Chhowalla Materials Science and Engineering, Rutgers University, 607 vide new sources of energy. In this view, electrochemical Taylor Road, Piscataway, NJ 08854, USA reduction of CO appears promising since it can lead to formation of methane, ethylene, methanol, formate, carbon D. C. B. Alves (&) monoxide, formic acid and other compounds that can be Departamento de Fı ´sica, Universidade Federal de Minas Gerais, Belo Horizonte, MG 31270-901, Brazil potentially used as fuels [8–10]. The applicability of the e-mail: diego.c.b.physicist@gmail.com; CO reduction process is currently limited by the high dbarbosa@fisica.ufmg.br overpotentials even when using noble metal catalysts as active electrodes [11]. R. Silva T. Asefa Department of Chemistry and Chemical Biology, Rutgers Different materials have been investigated as catalysts University, 610 Taylor Road, Piscataway, NJ 08854, USA for CO reduction. Some candidates such as Hg, In and Cd have shown high current efficiency associated with high T. Asefa hydrogen overvoltage but remain poor choice for CO re- Department of Chemical and Biochemical Engineering, Rutgers duction catalysts because of their low selectivity in University, 98 Brett Road, Piscataway, NJ 08854, USA 123 2 Page 2 of 7 Mater Renew Sustain Energy (2015) 4:2 producing the desired products [12]. On the other hand, performing novel chemistry [29, 30]. The partial removal materials such as Pt, Ni, Fe and Ti favor hydrogen evolu- of oxygen functional groups leads to an enhancement in the tion reaction (HER) in aqueous media and also possess p conjugation so that the semi-metallicity of graphene is high affinity to CO, leading to rapid reduction of catalytic partially restored and its conductivity increases by several activity [13]. Copper is the most promising catalyst for orders of magnitude compared to as-prepared GO [31, 32]. CO reduction reaction because it enables the production of Recently, it has been reported that reduced graphene oxide hydrocarbons at significant current densities, exhibits good (rGO) works synergistically with catalyst particles to en- selectivity, and has low surface affinity to CO [14]. The- hance their stability and performance [33–36]. oretical studies have shown that CO acts as an important precursor for hydrocarbon synthesis, which progresses in successive hydrogenation/reduction reactions [15] leading Experimental to the 16 products identified during CO reduction on copper [9]. Despite its rich electrochemistry, Cu suffers The synthesis was performed in two steps as shown in from high overpotentials compared to potentials of stan- Scheme 1. First, an aqueous solution of GO was prepared dard catalysts for hydrocarbon production (0.17 V vs. at a concentration of 1.5 mg/mL and this solution was RHE) [9, 16]. Challenges associated with high overpoten- mixed with 9 mg of CuSO (copper II sulfate). NaOH was tial and low electrochemical stability must be overcome if then added and the mixture was vigorously stirred at 70 C copper is to be seriously considered as a practical catalyst for 5 min. At this step of the synthesis, the color of the for CO reduction reaction [16]. It has been demonstrated solution changed to dark grey. Then, 625 lL of hydrazine that the oxide passivation layer on a Cu film can reduce the was quickly added to the mixture and stirred for additional overpotential for the CO reduction reaction as well en- 40 min. Upon the addition of hydrazine, the color turned hance the stability [17]. Although passivated Cu films are dark violet. The mixture was further washed several times more robust catalysts, it leads to a reduction of the current with EtOH by centrifugation (10 min at 30 k rpm). density due to the copper oxide layer. Cu nanoparticles (NPs) could be beneficial in facilitating the CO reduction Graphene oxide synthesis reaction but their instability caused by combined effects of aggregation and rapid deactivation of the catalytic surface Graphene oxide (GO) was prepared chemically via modified via oxidation has limited their consideration. One strategy Hummer’s method [37]. The graphite powder (Branwell recently proposed to overcome the stability issues was to Graphite Inc.) with size of [425 lm was chemically oxi- use Au Cu NPs that are less prone to oxidation instead Cu dized, exfoliated, and purified by repeated centrifugation. NPs for CO reduction [18]. However, Au is expensive and also known to be a good catalyst for HER that can disrupt Copper nanoparticles passivated with polyallylamine the production of hydrocarbons [19]. In the present work, we demonstrate that reduced graphene oxide decorated Cu nanoparticles are prepared following the procedure with copper NPs (CuNPs on rGO) is a promising catalyst proposed by Wang and Asefa [38], using polyallylamine as system for CO reduction that exhibits remarkable stability passivating agent. In the present case, 220 lL polyallyl- in a medium with low proton availability. CuNPs on rGO amine (Sigma-Aldrich) was completely dissolved in dis- electrodes exhibit lower overpotential for CO reduction tilled water under vigorous stirring at 70 C. Then, 9 mg of reaction and show a strong increase in the current density. copper sulfate (Sigma-Aldrich) was added followed by To assess the stability of the catalysts, we have tested CuNPs on rGO for several hours and found limited de- crease in the current density and negligible change in the overpotential. Graphene oxide (GO) has been investigated for a broad range of applications in electronics [20], biology [21], energy storage [22, 23], and more recently in catalysis [24– 26]. GO consists of atomically thin sheets with lateral di- mensions of a few micrometers and thickness of less than 2 3 1 nm. GO is composed of sp and sp carbon bonds as well as oxygen functional groups such as hydroxyls and car- Scheme 1 Schematic representation of the synthesis method con- bonyls [27, 28]. Its chemically heterogeneous properties sisting of mixing GO with copper(II) and NaOH first. Hydrazine was can be tuned to some extent by varying the oxygen content then added and the mixture was stirred for 40 min. The final material through reduction and offer a remarkable platform for corresponds to rGO nanosheets decorated with Cu NPs 123 Mater Renew Sustain Energy (2015) 4:2 Page 3 of 7 2 700 lL of NaOH 0.5 M. After 5 min, *60 lL of hy- Results and discussion drazine was poured in the mixture. The solution color changed from an initial blue color to brown and 40 min RGO decorated with CuNPs was synthesized by co-re- later the solution color became red. duction of copper(II) and GO using hydrazine solution as reducing agent. Briefly, copper(II) sulfate (9 mg) and GO Copper thin film (3 mg) were dispersed in 10 mL of 0.5 M NaOH at 70 C under vigorous stirring. In basic solution, copper(II) cations A thin film of copper was formed by electrochemical de- remain in close contact with the negatively charged oxygen functional groups of GO. 625 lL of 80 % hydrazine so- position using a solution with 19 mg of Cu(I) in 10 mL acetonitrile. The copper was deposited on a glassy carbon lution was then poured into the mixture to reduce cop- electrode with 3 mm diameter. The deposition was carried per(II) and the GO nanosheets (see Methods for details). out using a constant current of 10 lA for 10 min giving The addition of hydrazine results in a significant change in approximately 4 lg of deposited copper. Considering the the color of the solution, becoming intensely reddish due to density of bulk copper and the diameter of the glassy surface plasmon resonance (SPR) effect of metallic copper carbon, the thickness of the copper film is estimated to be nanoparticles. This effect causes strong absorption around around 70 nm. 580–600 nm [39]. As a result of the co-reduction process, conductive rGO is obtained along with metallic copper Characterization nanoparticles formed via reduction of Cu(II) as shown in Scheme 1. It is worth noting that no additional passivating X-ray photoelectron spectroscopy (XPS) measurements agents, such organic molecules or polymers, were used to were performed with a Thermo Scientific K-Alpha spec- stabilize the as-prepared copper particles. TEM images of trometer. All spectra were taken using an Al Ka microfo- the composite indicate that the CuNPs are uniformly dis- cused monochromatized source (1,486.6 eV) with a tributed on the basal plane of the reduced graphene oxide resolution of 0.6 eV. Raman spectra were obtained using a and it was also observed that some nanoparticles were Renishaw 1,000 system operating at 514 nm. The UV–vis partially wrapped by the rGO sheets (Fig. 1a). TEM ob- absorption spectra were measured with a Lambda 950 servations suggest that rGO acts as a stabilizing support for spectrophotometer (PerkinElmer). Transmission electron the 20–40 nm Cu nanoparticles by immobilizing and pre- microscope (TEM) images of the samples were obtained venting their aggregation into larger particles (inset with a Topcon 002B TEM that was working at an accel- Fig. 1a). eration voltage of 200 kV. Figure 1b presents FTIR spectra of the CuNPs on rGO and bare rGO films reduced by hydrazine. Clear differences Electrochemical measurements between the two spectra can be observed. First, the rGO spectrum reveals the absence of oxygen functionalized The measurements (VersaStat 3 potentiostat from Prince- groups, suggesting a strong reduction [40]. In contrast, the -1 ton Applied Research) consist of linear sweep voltammetry intense bands between 800 and 1,330 cm in the CuNPs/ (LSV) and were performed in two distinct environments rGO spectrum correspond to stretching vibrations (mainly (Argon and CO ) using acetonitrile (10 mL) and tetra-n- from epoxy and ketone groups), which suggests that butylammonium hexafluorophospate (NBu PF ) (387 mg, decoration with CuNPs inhibits a more severe reduction of 4 6 -1 Alfa Aesar) as electrolyte. The electrolyte solution was first GO. However, contributions in the 1,500–1,600 cm degassed using argon for 30 min. Under this condition, no range are typically not related to oxygen groups, and can be reduction reaction is expected. The same procedure was assigned to the asymmetric stretch of sp -hybridized C=C repeated under CO to saturate the solution prior the within the rGO basal plane [41]. The presence of residual measurements. hydroxyl has been demonstrated to be beneficial in cata- All materials were deposited by drop casting on glassy lysis through enhancement of electrochemical stability and carbon electrode (CH Instruments, Inc.) and the current of catalytic activity [41, 42]. In addition, hydroxyl groups that the voltammograms is normalized by the geometric area remain on rGO nanosheets decrease the interface energy -2 2 ? (*7 9 10 cm ) of the glassy carbon. An Ag/Ag between rGO and copper, leading to stabilization of (0.1 m) electrode in acetonitrile and a platinum wire were CuNPs. used as reference and counter electrode, respectively. The To further investigate the structure of copper-decorated reference electrode was calibrated using the well-known rGO, X-ray photoelectron spectroscopy (XPS) was per- reduction oxidation signals of ferrocene (FCN) (Figure formed. From the XPS survey spectrum of CuNPs/rGO S1). Potentials in the voltammograms are corrected to hybrid material shown in Fig. 2, it is possible to identify NHE. signals from oxygen, carbon and copper. The upper left 123 2 Page 4 of 7 Mater Renew Sustain Energy (2015) 4:2 Fig. 2 XPS survey spectrum of CuNPs on rGO. High-resolution spectra from the C1s and Cu2p regions can be deconvoluted with different components from various chemical bonds, as indicated by the fits in the upper portion of the figure at 934.4 eV that are due to the presence of Cu(II) [17]. The latter indicates partial oxidation of the Cu nanoparticle surface in the form of CuO. We believe that the CuO shell is formed during exposure to air after the synthesis since the presence of hydrazine during nanoparticle synthesis is likely to prevent copper from being oxidized. It is also possible to verify the presence of two Cu satellites bands at 940 and 943.9 eV. Moreover, the presence of copper in zero-valence state is also confirmed by our UV–Vis spec- troscopy results (see Supplementary Information, Figure S2) that show an intense absorption band at around 600 nm attributed to the surface plasmon resonance effect (SPR) in metallic copper NPs as previously reported [45–47]. This peak is not observed in the UV–Vis spectrum of the bare rGO, Figure S2. The SPR band was also observed at 560 nm for CuNPs prepared using polyallylamine as cap- Fig. 1 TEM image and FTIR of CuNPs on rGO prepared by hydrazine reduction: a TEM image showing the dispersion of ping agent. The shift in position of copper SPR band be- nanoparticles on rGO. The size distribution histogram indicates an tween CuNPs prepared with polyallylamine and CuNPs on average particle size of about 30 nm. b FTIR spectra of bare rGO and rGO can be attributed to differences in the size of the CuNPs on rGO. The oxygen peaks are virtually absent in bare rGO nanoparticles. That is, our TEM image analyses show that while they are present in Cu NP decorated rGO and are presumably helping the stabilization of the Cu NPs the Cu NPs prepared using polyallylamine are significantly smaller in size than the CuNPs on rGO (Figure S3). inset shows the high-resolution signal of the C1s region from rGO with C–C/C=C, C–OH/C–O, C=O and C=O–OH CO reduction reaction peaks at 284.4, 286, 287.3, 289 and the satellite peak at 290.6 eV. After hydrazine treatment, the carbon/oxygen To gain insight into the catalytic activity of copper ratio in the CuNPs on rGO sample increased from 2.1 up to nanoparticles on rGO, cyclic voltammetry was performed 7.3. This value is in good agreement with those previously in argon (Ar)- and CO -saturated electrolyte solutions reported for mildly reduced GO [43, 44]. On the other (Fig. 3). It has been recently reported that the selectivity hand, peaks from Cu 2p and 2p can be identified at toward the chemical species is highly dependent on the 3/2 1/2 932.5 and 952.4 eV, respectively. The fitting of the Cu morphology of polycrystalline copper [48]. To limit the peaks reveals a main signal at 932.5 eV that can be at- number of products from the CO reduction, all the mea- tributed to the zero-valence states of Cu and smaller peaks surements reported here have been performed in the 123 Mater Renew Sustain Energy (2015) 4:2 Page 5 of 7 2 Fig. 3 Linear sweep voltammetry (LSV) of CuNPs/rGO, Cu thin film and polymer passivated CuNPs under Argon- and CO saturated 2- solutions. The Cu NPs on rGO electrodes exhibit lower overpotential and higher current density compared to other forms of copper tested in this study absence of water. Specifically, the experiments were per- formed in acetonitrile using Tetra-n-butylammonium hex- afluorophosphate (NBu PF , 0.1 M) electrolyte. 4 6 Acetonitrile was chosen because it can dissolve ap- proximately eightfold more CO than water [49], and also because it is a low hydrogen availability medium so that the contribution from HER on the cathodic current can be largely disregarded. In addition, the mechanism for CO Fig. 4 a Linear sweep voltammetry (LSV) curves for CuNPs on rGO as a function of time (0–3 h). The electrode was held at -1.4 V vs. reduction in low hydrogen availability medium is well NHE during testing. The corresponding chronoamperometric curve known. Only three species can be obtained: carbon for CuNPs/rGO is shown in inset. b Turnover frequency (TOF) as a 2- 2- monoxide (CO), carbonate (CO ) or oxalate (C O ) 3 2 4 function of the overpotential calculated for CuNPs/rGO (black) and involve 2 electrons each (Equation I and II). Oxalate for- Cu thin film (red). Inset is the evolution of the turnover from CuNPs on rGO measured over 24 h with an overpotential of 1.4 V. The black mation requires C–C coupling which is known to occur at arrow indicates the change of the electrolyte solution. Calculations higher overpotentials ([0.9 V vs. RHE) [9, 10]. Ex- are based considering the total amount of copper and a 2-electron 2- 2- periments in DMF with low proton content similar to process (CO, CO and C O products) 3 2 4 acetonitrile have demonstrated that oxalate and CO are the two main products of the reaction and only traces of currents obtained from CuNPs on rGO electrodes HCO have been detected [50]. immersed in CO -saturated solution compared to Ar- 2CO þ 2e ! C O ð1Þ 2 2 saturated solution, whereas rGO alone shows a limited catalytic activity (Figure S4). Furthermore, catalytic 2CO þ 2e ! CO þ CO ð2Þ activity of CuNPs on rGO electrodes is significantly The activity of the electrodes can be easily tested by higher for *70 nm-thick Cu film or CuNPs stabilized by comparing the cathodic current in solution saturated with polyallylamine (see Methods for details). Interestingly, a CO with the cathodic current in solution saturated with Ar. significant cathodic current was not observed using an It is worth noting that the potentials to reduce Cu (II) to Cu electrode with passivated CuNPs with polyallylamine. (I) and Cu(I) to Cu metal in organic medium are Therefore, it can be stated that the presence of the polymer approximately -0.3 and -0.6 V vs. NHE, respectively on the surface of CuNPs inhibits the copper activity. These [51]. Thus, at the range of negative potentials used here for results demonstrated the clear superior activity of the CO reduction, the reduction of the copper oxide shell is CuNPs supported on rGO nanosheets. Comparing the thermodynamically favored and the nanoparticles are most electrocatalytic activity of CuNPs on rGO and Cu thin film, likely being reduced to copper metal [48]. From Fig. 3, the it can be observed that higher current density can be CO reduction is clearly demonstrated by the high cathodic achieved with lower overpotentials with rGO decorated 123 2 Page 6 of 7 Mater Renew Sustain Energy (2015) 4:2 (NSF) (Grant Nos: CAREER CHE-1004218, NSF DMR-0968937, with CuNPs. For instance, at -1.2 V vs. NHE the current NSF NanoEHS-1134289, NSF-ACIF for 2010, and NSF special cre- density observed in the CuNPs/rGO electrode is ativity grant in 2011). DCBA and RS acknowledge the CAPES -2 -0.24 mA cm , while the same current density is (Coordenac ¸a ˜o de Aperfeic ¸oamento de Pessoal de Nı ´vel Superior, reached at -1.54 V for Cu thin film. The current density Brazil) and the Fulbright Agency, USA for their fellowships and financial support for his graduate study. obtained in the CuNPs/rGO at -1.54 V vs. NHE is -2 -0.97 mA cm , fourfold better than the Cu thin film. Open Access This article is distributed under the terms of the Stability is an additional parameter in catalysis. To Creative Commons Attribution License which permits any use, dis- demonstrate the potential use of CuNPs on rGO for CO tribution, and reproduction in any medium, provided the original author(s) and the source are credited. reduction reaction, chronoamperometric measurements were carried out to investigate the stability (Fig. 4a). A constant overpotential of 1.4 V vs. NHE was applied on the References CuNPs/rGO electrode for 3 h. Linear sweep voltammetry (LSV) was also performed with intervals of 1 h during the 1. Turner, J.A.: A realizable renewable energy future. 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Materials for Renewable and Sustainable Energy – Springer Journals
Published: Feb 6, 2015
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