Enhanced Activity of Ag Nanoplatelets on Few Layers of Graphene

Research Article pubs.acs.org/journal/ascecg

Enhanced Activity of Ag Nanoplatelets on Few Layers of Graphene Film with Preferential Orientation for Dehydrogenative Silane− Alcohol Coupling Amarajothi Dhakshinamoorthy,*,†,‡ Iván Esteve Adell,‡ Ana Primo,‡ and Hermenegildo Garcia*,‡,§ †

School of Chemistry, Madurai Kamaraj University, Palkalai Nagar, Tamil Nadu, India 625 021 Instituto Universitario de Tecnología Química CSIV-UPV, Universitat Politècnica de València, Av. De los Naranjos s/n, 46022 Valencia, Spain § Centre of Excellence for Advanced Materials Research, King Abdulaziz University, Prince Majid Rd., Jeddah, Saudi Arabia

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ABSTRACT: Ag nanoplatelets (4−7 nm height and 30−50 nm lateral size) supported on a few layers of graphene having preferential facet orientation have been prepared by pyrolysis of nanometric films of chitosan on quartz containing adsorbed Ag+ ions at 900 °C under argon. The strong interaction of Ag nanoplatelets and graphene has been deduced from the morphology of the Ag nanoplatelets, their dimensions, the preferential facet orientation, and the 0.4 eV shift toward higher values of the binding energy of the Ag 3d peak in XPS. The facet-oriented Ag on a few layers of graphene film exhibits for the dehydrogenative coupling of dimethylphenylsilane and n-butanol turnover numbers of 1.62 × 106 that are higher than that of analogous Aggraphene catalysts lacking strong grafting and preferential orientation. KEYWORDS: Heterogeneous catalysis, Graphene films as catalysts, Siloxane synthesis, Dehydrogenative coupling of silanes, Ag nanoplatelets, Facet-oriented Ag particles

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INTRODUCTION Catalysis by metal nanoparticles (MNPs) has been an area of research for more than 30 years trying to increase their catalytic activity to gain control on the particle morphology and dimensions, to develop strategies to tune their selectivity, and to establish the type of organic reactions that can be catalyzed.1−3 In order to facilitate their recovery after a reaction and to increase their stability avoiding their spontaneous tendency to grow in size, MNPs are frequently supported on high surface area solids, resulting in highly active heterogeneous catalysts.4−7 The general outcome of this intensive research is that there is a general relationship among small particle size and high catalytic activity and that the morphology of the particles with defined shapes and geometries can influence the selectivity.8,9 It has also been a general observation that besides morphology and particle size, the nature of the support plays an important role on the resulting catalytic activity of supported MNPs.10−12 Besides organic polymers and large surface area metal oxides, carbonaceous solids have been among the most widely used supports of MNPs for their use as heterogeneous catalysts.4,13,14 A constant target in this field has been to develop more efficient and selective MNPs.15 Up to now, most of the progress has been achieved using samples with small MNP size that have been obtained following suitable preparation methods © 2017 American Chemical Society

and by using adequate supports that allow high dispersion of the supported MNPs.7,16 In this context, recent theoretical calculations and modeling as well as scattered experimental data indicate that different crystallographic facets of MNPs with defined morphologies can exhibit contrasting catalytic activity.17,18 Therefore, some recent studies have been aimed at developing reliable preparation procedures to obtain MNPs with preferential morphology of their crystals and, therefore, exposing preferentially certain facets for catalysis.8,9,19 In connection with the use of supported MNPs in heterogeneous catalysis, one current area of research is the use of graphene (G) as supports.13,14 G is a one atom thick layer of an indefinite number of carbon atoms with sp2 hybridization in an hexagonal arrangement that offers support for MNPs unique properties, including having a large specific surface area (ideally 2650 m2/g), high dispersibility in solvents, high atomic efficiency derived from the exposure of all the G atoms available for interaction with supported MNPs, high adsorption capacity that brings substrates and reagents close to the active sites, and strong MNP-support interactions.13 The extended π orbital of G and the presence of possible defects, Received: November 14, 2016 Revised: December 23, 2016 Published: January 17, 2017 2400

DOI: 10.1021/acssuschemeng.6b02729 ACS Sustainable Chem. Eng. 2017, 5, 2400−2406

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ACS Sustainable Chemistry & Engineering

Scheme 1. Illustration of Preparation Procedure Leading to Formation of Oriented Ag Nanoplatelets on fl-G. (i) Spin Coating of an Aqueous Solution of Chitosan on Clean Quartz. (ii) Adsorption of Ag+ on Chitosan Film. (iii) Pyrolysis of Ag+-Containing Chitosan Film at 900 °C under Inert Atmosphere.

Figure 1. Comparison of the XRD of Ag/ml-G (a) with that corresponding to unoriented Ag/fl-G (b) showing the different relative intensity of the diffraction peaks.

orientations (Ag/fl-G, with Ag meaning oriented Ag nanoplatelets, fl-G meaning a few layers of G) and defined nanoplatelet morphology (4−7 nm height by 30−50 nm lateral dimension) is presented. It has been observed that Ag/fl-G as a catalyst exhibits a turnover number (TON) for dehydrogenative coupling of silanes and alcohols in the range of 106 and higher than the catalytic activity of related Ag/G catalysts prepared following an analogous procedure, but without Ag NPs exhibiting preferential facet orientation. Ag/fl-G can be reused several times with some decrease in its high catalytic activity. Dehydrogenative coupling of silanes with alcohols to form siloxanes is a green alternative to the reaction of chlorosilanes with alcohols, where corrosive HCl is the byproduct. However, in contrast with the reaction of chlorosilanes with alcohols, the dehydrogenative coupling requires the use of a catalyst to occur. We have reported that Cu and Au NPs supported on G can promote this reaction, as well as other Cu-containing solids, and now it is of interest to establish the comparative activity of Ag to catalyze this reaction.8,23,24

such as carbon vacancies and sheet holes as well as the presence of heteroatoms and oxygen functionalities, can establish, according to theory, a strong interaction with metal atoms and clusters located on G, reaching in some cases a high degree of π-d orbital overlapping.13 This strong MNP-G interaction combined with other unique properties of G have frequently led to the observation that MNPs supported on G exhibit higher catalytic activity than analogous catalysts in which MNPs of similar particle size are supported on other carbon forms or even MNPs supported on metal oxides.13,20,21 Continuing with this line of research, we have recently reported that 111 facet-oriented Cu and Au NPs can be prepared on a few layers of G in a one-step synthesis by pyrolysis of a suitable biopolymer precursor containing metal salt.8,9 During pyrolysis, simultaneous formation of G and MNPs occur, and this may result in the spontaneous segregation of two phases when the metals exhibits low solubility in carbon and do not form metal carbides. In addition, preferential facet orientation has been observed for Cu and Au nanoplatelets and interpreted as derived from the reverse template effect caused by the evolving G sheet controlling the growth of MNPs. In the chemical vapor deposition (CVD) preparation of G, a Cu film with 111 facet orientation templates the hexagonal arrangement of carbon atoms growing on top of the Cu film due to the good match between the 111 facet of Cu and the dimensions of the G hexagons being formed.22 Analogously, it is proposed that in the reverse template effect G sheets being formed in advance to the formation of MNP crystallites will template the preferential facet of MNPs by epitaxial growth from the G sheet. In the present article, the preparation of Ag nanoplatelets supported on a few of layers G with preferential facet

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RESULTS AND DISCUSSION The preparation procedure of oriented Ag/fl-G is illustrated in Scheme 1. The process consists in the pyrolysis at 900 °C under an inert atmosphere on quartz of a chitosan film of nanometric thickness containing AgNO3. It was reported in the literature that pyrolysis of chitosan, a natural biopolymer obtained by deacetylation of chitin,25,26 results in the formation of single or fl-G.27 On the basis of this knowledge, in the present case, a chitosan film containing Ag+ ions adsorbed on the polysaccharide fibrils was submitted to pyrolysis, resulting in the simultaneous formation of separate phases of G and Ag 2401


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statistically relevant number of particles, it was estimated that Ag nanoplatelets have an average height of 4−7 nm and the lateral size was between 30 and 50 nm. The nanoplatelet morphology of Ag particles is compatible with the assumption of a favorable interaction between Ag and fl-G since Ag particles are wetting the support. It has been previously proposed that the shape of spherical or hemispherical particles on solid surfaces reports on the affinity of the MNP for the solid surface.3 If this is the case, the nanoplatelet morphology of Ag NPs indicates a high affinity of Ag for G. Figure 3 presents the AFM frontal view of oriented Ag/fl-G films where the homogeneous distribution of Ag NPs supported on G can be seen. Also, Figure 3 presents some representative measurements of fl-G thickness and Ag nanoplatelet height. To explain the lower thickness of fl-G surrounding Ag NPs, it was proposed that during the pyrolysis step and nucleation of Ag NPs, this transition metal is acting as catalytic sites improving graphitization of the carbon residue in contact with the metallic particles, resulting in a lower density of defects and higher evolution of CO and CO2 gases; all these features result in a lower number of G layers of higher quality. The morphology of Ag nanoplatelets and their average particle size was also confirmed by scanning electron microscopy (SEM). Figure 4 presents the images of the oriented Ag/fl-G films at different magnifications showing the distribution of Ag NPs on G and a lateral size distribution about 16 nm. This lateral size of Ag NPs is smaller than the value estimated by AFM since AFM tends to overestimate the lateral size due to the width of the probing tip. XPS confirms the formation of G by measuring the binding energy and shape of the C 1s peak appearing at 284.5 eV. Deconvolution of the experimental C 1s peak shows only a minor percentage of about 10% of carbons bonded to oxygen that appear at 286 eV. The Ag 3d peak in high resolution XPS has a binding energy corresponding to metallic Ag in the zero oxidation state. A shift toward higher values of about 0.4 eV in the binding energy was measured for oriented Ag/fl-G with respect to the binding energy of the bulk Ag metal, suggesting that there is a charge transfer interaction between fl-G acting as electron acceptor and Ag nanoplatelets as electron donor. This shift in binding energy together with the morphology of Ag nanoplatelets wetting the G surface and their relatively small particle size and height in spite of the high pyrolysis temperature all point toward a strong metal support interaction that, together with the preferential facet orientation, can be reflected in the catalytic activity of oriented Ag/fl-G films as is commented below.

nanoplatelets. Phase segregation derives from the low solubility of metallic Ag on carbon, the lack of formation of silver carbide, and the high thermodynamic stability of G. On the basis of the precedent of the formation of oriented Cu nanoplatelets on G,8 we were anticipating that upon segregation Ag nanoplatelets would be uniformly distributed on G with some defined crystallite morphology and preferential facet orientation. The exact Ag loading on Ag/fl-G was determined by ICPOES elemental analysis of Ag after dissolving this element from the films by treatment with aqua regia. Several samples varying on the Ag content from a few nanograms to micrograms per cm2 could be prepared by controlling the concentration of AgNO3 solution in step (ii) of Scheme 1. For mM concentrations of AgNO3, almost complete Ag+ adsorption was observed, while for AgNO3 solutions above this threshold only a fraction of total Ag+ present in the solution is adsorbed. XRD can be a good experimental technique to prove facet orientation of Ag in films having a sufficiently large Ag content to record the corresponding diffraction peaks. As expected, samples having a low Ag content did not exhibit in XRD any detectable diffraction peak (Figure 1). In contrast, for oriented Ag/ml-G having high Ag loading on multilayer G (Ag/ml-G; ml meaning multilayer), XRD shows that in contrast to the characteristic XRD pattern of Ag NPs, the major diffraction peak of Ag/ml-G corresponds to the 111 facet accompanied by a much less intense 200 peak and negligible 220 and 311 peaks. The broad baseline peaking at about 24° also observed in the diffractogram of Ag/ml-G corresponds to the diffraction of mlG. This indicates that for these Ag/ml-G samples with high Ag loading preferential facet orientation is taking place as previously observed for other metals,8 but in this case, besides the 111 facet, 200 planes are also present in the sample. Raman spectroscopy of oriented Ag/fl-G shows the expected peaks for G at about 2750, 1600, and 1350 cm−1 corresponding to the 2D, G, and D bands characteristic of G (Figure 2). The

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CATALYTIC ACTIVITY The catalytic activity of oriented Ag/fl-G was evaluated in the dehydrogenative coupling between dimethylphenylsilane with 1-butanol under an argon atmosphere leading to the formation of the corresponding siloxy butyl ether (eq 1 in Table 1). The activity of oriented Ag/fl-G was compared under identical reaction conditions with that of unoriented Ag NPs supported on G (Ag/fl-G). The results are presented in Table 1. Evolution of H2 was confirmed by GC analysis of the reactor head space during the course of the reaction, and the corresponding data are indicated in Table 1 (footnote b). The oriented Ag/fl-G exhibited 28% conversion of dimethylphenylsilane after 24 h with a TON value of 803,571, which is comparatively higher than that reported for Cu2O/fl-G.8 Furthermore, a maximum TON value of 1,625,000 was achieved for this reaction using

Figure 2. Raman spectrum of Ag/fl-G films showing the characteristic signature of defective G with the presence of 2D, G, and D bands at 2750, 1600, and 1350 cm−1, respectively.

relative intensity of the G vs D peak is about 1.13, indicating that fl-G still contains defects mainly consisting on residual oxygenated functional groups and some nitrogen doping.27 No peaks corresponding to Ag2O in the low frequency side of the Raman spectra were recorded. The thicknesses of the fl-G and the nanoplatelet morphology of Ag particles were determined by AFM with subnanometric vertical resolution. It was determined that the average thickness of fl-G was about 8 nm, although there are some areas, particularly in the surroundings of Ag nanoplatelets, with significantly less thickness of about 3 nm. By measuring a 2402


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Figure 3. Frontal views (a,b) of the border and central part of Ag/fl-G film where the homogeneous distribution of Ag nanoplatelets of height between 4 and 7 nm and lateral sizes of 30−50 nm can be seen (c). Inset in (a) shows the height of the fl-G layer at the border that is about 8 nm.

Figure 4. SEM images at two different magnifications of Ag/fl-G fresh (a,b) and after its use as catalyst in the dehydrogenative coupling (c,d). The insets show the particle size distribution of Ag particles with an average size of 16 nm for the four images but with various distributions. It seems that upon the use as catalyst, larger Ag particles are formed.

based on the total Ag content of the catalysts and not on the surface atoms. Thus, in both cases of oriented and unoriented Ag catalysts, these TON values are the limit lower values. The fact that it has not been possible to determine the percentage of Ag surface atoms by CO adsorption makes not possible to determine the TON values based on surface atoms.

the oriented Ag/fl-G as catalyst after 48 h. The analogous sample of unoriented Ag NPs prepared by polyol reduction of AgNO3 and adsorption of fl-G obtained by pyrolysis of chitosan powder followed by sonication of the carbon residue (see Figure 1b for XRD), Ag/fl-G, exhibited an activity lower than that of oriented Ag/fl-G with the TON value being 1,482,142. It should be noted that these TON values have been calculated 2403


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ACS Sustainable Chemistry & Engineering Table 1. Dehydrogenative Coupling of Dimethylphenylsilane with n-Butanol Using Various Catalystsa

entry 1 2 3 4 5

catalyst

time (h)

conversion (%)b

TONc

24 48 48 48 24

28 (22) 56 (53) 44 51 (44) −

803,571 1,625,000 1,267,857 1,482,142 136,000

−2

Ag/fl-G (60 ng cm , 1 cm × 1 cm) Ag/fl-G (60 ng cm−2, 1 cm × 1 cm) Ag/fl-G (60 ng cm−2, 1 cm × 1 cm)d Ag/fl-Ge (0.04 wt %) Cu2O/fl-Gf

a Reactions conditions: dimethylphenylsilane (1.6 mmol), n-butanol (2 mL), argon atmosphere, 100 °C. bDetermined by gas chromatography. In all the cases, the only product observed was the corresponding butyl silyloxy ether. The number in brackets correspond to the yield of H2 evolved monitored from the headspace of the reactor. cCalculated as moles of dimethylphenylsilane converted by moles of Ag nanoplatelets catalyst. dFirst reuse. eUnoriented Ag NPs on fl-G prepared by pyrolysis of a chitosan powder containing adsorbed AgNO3, followed by sonication of the resulting carbon residue. fReported in ref 8.

after the reaction. Nevertheless, the average particle size was similar to that of the fresh sample. This indicates that the Ag/flG sample should undergo a gradual increase in particle size upon consecutive reuses that should be accompanied by a decrease in the catalytic activity.

When comparing the activity of the two Ag catalysts, it should be noted that the particle size of Ag NPs in Ag/fl-G was 4.7 nm, significantly smaller than that of Ag/fl-G. In addition, the Ag/fl-G catalyst was perfectly suspended in the liquid phase, while Ag/fl-G was a film on a rigid 1 cm × 1 cm quartz plate and stirring and diffusion of reagents was less efficient than for suspended powders. Nevertheless, Ag/fl-G was more efficient. To put these TON values into context, it is worth commenting that previous studies using Cu/G reported a maximum TON value of 2000,23 while oriented Au NPs on fl-G exhibited28 a TON value of 3.7 × 105 that is somewhat lower than that obtained here for Ag. The time conversion plot for this reaction using different catalysts is given in Figure 5. We attribute this higher efficiency of Ag/fl-G with larger particles (30−50 nm) as derived from the preferential orientation and strong grafting of Ag NPs on G.

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CONCLUSIONS

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EXPERIMENTAL SECTION

The present manuscript has shown that similarly to the reported cases of Cu and Au, also Ag nanoplatelets can be formed strongly grafted on a few layers of graphene by pyrolysis at 900 °C under an inert atmosphere of films of chitosan containing adsorbed Ag+. Pyrolysis produces the transformation of chitosan onto a few layers of G, while Ag segregates in an independent phase as small nanoplatelets of 4−7 nm height and 30−50 nm lateral size, becoming preferentially oriented in the 111 facet and less in the 200 plane. The strong Ag-G interaction is revealed, besides the small particle size and preferential orientation, in a 0.4 eV shift of the binding energy of the Ag 3d peak to higher values with respect to bulk Ag. The resulting oriented Ag nanoplatelets exhibit a TON for the dehydrogenative coupling of dimethylphenylsilane and nbutanol over 1.62 × 106 that is higher than analogous Ag/flG catalysts prepared in the form of powders and lacking preferential orientation. Our report constitutes another example showing a new procedure for the preparation of supported metal NPs on G that leads to samples with remarkable physical and catalytic properties.

Figure 5. Time conversion plot for the dehydrogenative coupling reaction between dimethylphenylsilane with n-butanol using (a) oriented Ag/fl-G (60 ng cm−2, 1 cm × 1 cm) an (b) Ag/fl-G (0.04 wt %).

Catalysts Preparation. Synthesis of Few Layers of Graphene (flG). Alginic acid sodium salt from brown algae (Sigma) as powder was placed as a thin bed on a ceramic crucible and pyrolyzed under an argon atmosphere using the following oven program: annealing at 200 °C for 2 h and then heating at 10 °C/min up to 900 °C for a holding time of 6 h. The resulting graphitic powder was sonicated at 700 W for 1 h in water, and the solid residue was removed by centrifugation to obtain a suspension of fl-G dispersed in water. Synthesis of Oriented Ag NPs over Few Layers of Graphene Films (Ag/fl-G). Here, 0.5 g of chitosan from Aldrich (low molecular weight) was dissolved in 25 mL of water with a small quantity of acetic acid (0.23 g), necessary for complete dissolution of chitosan. The solution was filtered through a syringe of 0.45 μm pore diameter to remove impurities present in commercial chitosan. The films were supported on a previously cleaned quartz plate (2 cm × 2 cm) by casting 500 μL of filtered solution at 4000 rpm in 1 min. In the second step of the synthesis, the obtained chitosan films, once dried, were immersed in a silver nitrate solution (concentration ranging from 0.01 to 1 mM) during 1 min.

The Ag/fl-G plate was separated at the end of the reaction, washed with 1-butanol, and reused in a consecutive run, whereby similar catalytic activity was observed (Table 1, entry 3) in spite of the minute amount of Ag present on the film. The total accumulated TON of the two experiments was 2.89 × 106. Stability of Ag/fl-G under the reaction conditions was further checked by determining the particle size distribution of the Ag nanoplatelets after the reaction from the SEM images of the used catalyst. Figure 4 shows selected images of the Ag/fl-G used sample as well as the particle size distribution histogram obtained by measuring a statistical meaningful number of Ag nanoplatelets. The appearance of a few large Ag particles was observed, and the particle size distribution becomes broader 2404



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The pyrolysis of chitosan films containing adsorbed AgNO3 was performed under argon atmosphere using an electrical furnace and the heating program indicated for the preparation of fl-G. The silver content on the films was determined by ICP-OES by immersing the plates into aqua regia at room temperature for 3 h and analyzing the Ag content of the resulting solution. Synthesis of unoriented Ag NPs over Few Layers of Graphene (Ag/fl-G). Here, fl-G from alginate pyrolysis (100 mg) was added to ethylene glycol (40 mL), and the mixture was sonicated at 700 W for 1 h to obtain dispersed fl-G. AgNO3 (0.07 mg for the preparation of the sample at 0.04 wt % Ag) was added to the reaction mixture, and Ag metal reduction was then performed at 120 °C for 24 h under continuous stirring. The Ag/fl-G was finally separated by filtration and washed exhaustively with water and with acetone. The resulting material was dried in a vacuum desiccator at 110 °C to remove the remaining water. Physicochemical Characterization. XRD patterns were obtained with a Philips X’Pert diffractometer using Cu Kα radiation (λ= 1.5418 Å, 40 kV, 40 mA) at a scanning speed of 0.20° per min in the 10−80° 2Θ range. The Raman measurements (Renishaw in via Raman microscope) were carried out at room temperature with the 514.5 nm line of an Ar ion laser as the excitation source. XP spectra were recorded on a SPECS spectrometer equipped with a Phoibos 150 9MCD detector using a nonmonochromatic X-ray source (Al and Mg) operating at 200 W. The samples were evacuated in the prechamber of the spectrometer at 1 × 10−9 mbar. The measured intensity ratios of the components were obtained from the area of the corresponding peaks after nonlinear Shirley-type background subtraction and corrected by the transmission function of the spectrometer. Atomic force microscopy (AFM) measurements were made in air at ambient temperature with multimode nanoscope 3A equipment working in tapping mode. It should be noted that AFM were not measured in a clean room, and therefore, films on glass substrates may contain dust that will be detectable by these techniques. FESEM images were taken with an ULTRA 55 ZEISS Oxford instrument. Samples were prepared by sticking a small piece of quartz having on top Ag/fl-G on a sample holder.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Amarajothi Dhakshinamoorthy: 0000-0003-0991-6608 Hermenegildo Garcia: 0000-0002-9664-493X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

A.D.M. thanks University Grants Commission, New Delhi for the award of Assistant Professorship under its Faculty Recharge Programme. A.D.M. also thanks Department of Science and Technology, India, for the financial support through Fast Track project (SB/FT/CS-166/2013) and the Generalidad Valenciana for financial aid supporting his stay at Valencia through the Prometeo programme. Financial support by the Spanish Ministry of Economy and Competitiveness (Grapas, CTQ2012-32315 and Severo Ochoa) and Generalidad Valenciana (Prometeo 2012-014) is gratefully acknowledged. The research leading to these results has received partial funding from the European Community’s Seventh Framework Programme (FP7/2007−2013) under grant agreement no. 228862. 2405


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ACS Sustainable Chemistry & Engineering (21) Albero, J.; Garcia, H. Doped graphenes in catalysis. J. Mol. Catal. A: Chem. 2015, 408, 296. (22) Mattevi, C.; Kim, H.; Chhowalla, M. A review of chemical vapour deposition of graphene on copper. J. Mater. Chem. 2011, 21, 3324. (23) Blandez, J. F.; Primo, A.; Asiri, A. M.; Alvaro, M.; Garcia, H. Copper Nanoparticles Supported on Doped Graphenes as Catalyst for the Dehydrogenative Coupling of Silanes and Alcohols. Angew. Chem., Int. Ed. 2014, 53 (46), 12581. (24) Dhakshinamoorthy, A.; Concepcion, P.; Garcia, H. Dehydrogenative coupling of silanes with alcohols catalyzed by Cu-3(BTC)(2). Chem. Commun. 2016, 52 (13), 2725. (25) Rinaudo, M. Chitin and chitosan: Properties and applications. Prog. Polym. Sci. 2006, 31 (7), 603. (26) Ravi Kumar, M. N. V. A review of chitin and chitosan applications. React. Funct. Polym. 2000, 46 (1), 1. (27) Primo, A.; Atienzar, P.; Sanchez, E.; Delgado, J. M.; Garcia, H. From biomass wastes to large-area, high-quality, N-doped graphene: catalyst-free carbonization of chitosan coatings on arbitrary substrates. Chem. Commun. 2012, 48 (74), 9254. (28) Primo, A.; Esteve-Adell, I.; Coman, S. N.; Candu, N.; Parvulescu, V. I.; Garcia, H. One-Step Pyrolysis Preparation of 1.1.1 Oriented Gold Nanoplatelets Supported on Graphene and Six Orders of Magnitude Enhancement of the Resulting Catalytic Activity. Angew. Chem., Int. Ed. 2016, 55 (2), 607.

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Enhanced Activity of Ag Nanoplatelets on Few Layers of Graphene

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