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Chemical Physics Letters 420 (2006) 86–89

Gold/carbon nanocomposite foam

Edgar Munoz a, Marta de Val b, Marıa Luisa Ruiz-Gonzalez c, Clarisa Lopez-Gascon d,Marıa Luisa Sanjuan e, M. Teresa Martınez a, Jose M. Gonzalez-Calbet c,

German F. de la Fuente d,*, Mariano Laguna b,*

a Instituto de Carboquımica (CSIC), Miguel Luesma Castan 4, 50018 Zaragoza, Spainb Departamento de Quımica Inorganica, Facultad de Ciencias, Instituto de Ciencia de Materiales de Aragon (Universidad de Zaragoza-CSIC),

50009 Zaragoza, Spainc Departamento de Quımica Inorganica I, Facultad de Ciencias Quımicas, Universidad Complutense, 28040 Madrid, Spain

d Laboratorio de Procesado de Materiales por Laser, Instituto de Ciencia de Materiales de Aragon (Universidad de Zaragoza-CSIC), Marıa de Luna 3,

50018 Zaragoza, Spaine Instituto de Ciencia de Materiales de Aragon (Universidad de Zaragoza-CSIC), 50009 Zaragoza, Spain

Received 8 November 2005Available online 17 January 2006

Abstract

The fabrication of gold/carbon nanocomposite foam by laser ablation of an organometallic gold salt is herein reported. The producedmaterial consists of gold nanoparticles diluted within nanostructured carbonaceous matrices, comprising both disordered carbon andgraphitic structures. It should be noted that these graphitic structures were fabricated in plasma-induced processes performed in airunder ambient conditions. A new family of metal/carbon nanocomposite foam materials can be envisioned by employing the simple,versatile laser ablation technique described in this work.� 2005 Elsevier B.V. All rights reserved.

1. Introduction

Carbon nanostructured materials have been the focus ofconsiderable research due to their outstanding electrical,mechanical, and even magnetic properties, as well as theirpromise for applications [1–8]. Designing synthetic routestowards tailoring both the structure and properties of thesematerials is a key issue in nanotechnology. Thus, therecently reported ability to preferentially produce semicon-ducting carbon nanotubes by a plasma enhanced chemicalvapor deposition method may have a major impact innanoelectronics [9]. On the other hand, different chemicalstrategies for transition-metal-doping of carbon aerogelshave resulted in materials exhibiting novel transport, cata-lytic, and electrochemical properties [2,3,10–12]. Here, we

0009-2614/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2005.12.048

* Corresponding authors. Fax: +34 976 761187 (M. Laguna); fax: +34976 761957 (G.F. de la Fuente).

E-mail addresses: xerman@unizar.es (G.F. de la Fuente), mlaguna@unizar.es (M. Laguna).

show a simple, versatile, one-step method for producingfoam-like assemblies of nanostructured carbon matricescomprising gold nanoparticles by laser ablation of an orga-nometallic gold compound, which can be extended to thefabrication of other metal/carbon nanocomposites. Veryinterestingly, this technique is able to provide graphiticstructures in plasma-induced processes performed underoxidizing atmospheres.

2. Experimental

Suspensions of an organometallic gold salt (bis(acetyl-acetonate)aurate(I) of bis(triphenylphosphine)iminium,PPN[Au(acac)2]) [13] were deposited onto ceramic sub-strates in the form of thick layers that, when dried, wereirradiated in air under ambient conditions with a continu-ous wave Nd:YAG laser (k = 1.064 lm, 30 W) at a scanrate of 25 mm/s (spot size: �60 lm). The resulting materi-als were then collected on an entangled metallic wire net-work placed on top of the ablated surface as soot having

E. Munoz et al. / Chemical Physics Letters 420 (2006) 86–89 87

a filamentous, web-like appearance resembling that of sin-gle-walled carbon nanotube [14] and carbon nanofoam[15,16] materials produced by laser ablation. Milligramquantities of this soot were readily produced by irradiatinga few square centimeters of the target.

The synthesized materials were characterized by scan-ning electron microscopy (SEM, JEOL JSM-6330F fieldemission SEM microscope), transmission electron micros-copy (JEOL JEM-3000F microscope), X-ray diffraction(XRD, CuKa radiation), and micro-Raman spectroscopy(kexc = 514.5 nm).

3. Results and discussion

SEM studies show that the synthesized materials have aspongy texture (Fig. 1a), as a consequence of the aggrega-tion of particles whose diameter ranges between 20 and40 nm. This results in the formation of interconnected‘rosary’-like ensembles (Fig. 1b), similar to those observedin carbon nanofoam [15,16], carbon aerogels [1,17,18], andother carbon materials consisting of arrangements of largercarbon aggregates [8,19–21].

TEM and electron diffraction results indicate that thesynthesized material consists of fcc crystalline gold nano-

Fig. 1. SEM characterization of the produced foam: (a) low magnificationSEM micrograph, which shows the spongy texture of the producedmaterial (scale bar: 1 lm) and (b) a ‘rosary’-like assembly of particlesobserved at high magnification (scale bar: 100 nm).

particles, 3 to 7 nm in diameter (Figs. 2a and 3), dilutedin matrices of both crystalline (graphitic) and disordered(amorphous) carbon (Figs. 2b and 3). The graphene sheetsare organized here in several-layer stackings (Figs. 2b and3), eventually in the form of concentric structures. On theother hand, XRD measurements (Fig. 4) show the charac-teristic peaks for fcc crystalline gold metal following theFm-3m space group (with 2h around 38�, 45�, 65�, and78�), as well as typical features corresponding to amor-phous materials, assigned to the disordered carbonobserved by TEM.

Fig. 2. High resolution TEM micrographs of different components of thegold/carbon nanocomposite foam: (a) fcc gold nanoparticles embedded incarbonaceous matrices and (b) carbonaceous matrices comprising entan-gled graphitic nanostructures. Scale bars: 5 nm.

Fig. 3. TEM overview of the gold/carbon nanocomposite foam. Goldnanoparticles are embedded within carbonaceous matrices comprisingconcentric graphitic nanostructures (scale bar: 10 nm).

0 10 20 30 40 50 60 70 800

20

40

60

80

100

CP

S (

a.u.

)

2-Theta (o)

Fig. 4. XRD pattern of the produced gold/carbon nanocomposite foam.

1200 1300 1400 1500 1600 1700

Raman Shift (cm-1)

Ram

an In

tens

ity (

arb.

uni

ts)

Fig. 5. Raman spectra of gold/carbon nanocomposite foam collected attwo different points of the sample. In the lower graph, the continuous lineis the fit of the experimental spectrum with two asymmetric Fano profiles(D and G bands) plus a Lorentzian sideband.

88 E. Munoz et al. / Chemical Physics Letters 420 (2006) 86–89

Additionally, the Raman spectra of this material (Fig. 5)show two broad bands centered at �1340 (D-band) and�1590 cm�1 (G-band) having equivalent intensities, whichis characteristic of defective graphites [22,23], includingcarbon nanofoam [16], carbon aerogels [17], transition-doped carbon aerogels [24], and multi-walled carbon nano-tubes produced by chemical vapor deposition [25]. Both theup-shift of the G-band frequency with respect to graphite(�1580 cm�1) and the intensity and width of the D-bandare consistent with its attribution to graphitic nanocrystal-lites, whose size can be deduced from the D/G intensityrelation according to the formula L(A) = 44(ID/IG)

�1

[26]. At this point we must note that the spectra of Fig. 5cannot be fitted with only two Lorentzian lines: evenallowing for asymmetry by using Fano profiles, fitting thelow frequency slope requires a third band, centered at

�1300 cm�1. The appearance of this band can be attrib-uted either to impurity ions [22] or to sp3/sp2 mixing, pos-sibly arising from disordered regions connecting graphiticcrystallites [8]. Depending on whether this sideband isincluded or not in the calculation of the D-band intensity,the average size of the graphitic nanocrystalline regionswould vary from 2.3 to 5 nm [26,27].

Although the formation of gold nanoparticles can beexplained in terms of the coalescence of gold atoms andclusters within the generated plasma plume [28,29], thepresence of carbonaceous structures (particularly the gra-phitic stackings) in the produced materials is striking.Combustion of laser-irradiated organic ligands would beexpected under the experimental conditions described here.However, our results seem to indicate that the collectedcarbonaceous material is the product of recombinationand assembly processes of high-temperature, ionized clus-ters, taking place within the plume prior to interacting withthe surrounding gas. The phenyl groups of the ablatedmaterial may provide the required carbon feedstock. Thefinal appearance of the collected material might be theresult of a diffusion-limited aggregation process [15,16].On the other hand, while schwarzite and glassy-carbon-likephases have been proposed for carbon clusters with highsp2 hybridization in carbon nanofoam [15,16] and carbonaerogels [17], respectively, the crystallinity of the graphiticnanostructures described here resembles that observed intransition-metal-doped carbon aerogels thermally annealedin a nitrogen atmosphere [3,10,24,30], as well as in sootproduced by nitrogen-shielded laser irradiation of acety-lene [31]. The growth of the graphitic nanostructuresobserved in our gold/carbon nanocomposite foam occursduring the above mentioned recombination and assemblyprocesses that follow the laser ablation of the target. Therole of the gold nanoparticles in the formation of these gra-phitic nanostructures is subject of current research,

E. Munoz et al. / Chemical Physics Letters 420 (2006) 86–89 89

although published work related to similar plasma assistedprocesses suggests the involvement of metal nanoparticlesas nucleation sites where the growth of graphitic nano-structures may be promoted [35].

4. Conclusions

Organometallic compounds can be employed in the fab-rication of metal/carbon nanocomposite foam using thesimple laser ablation method described here, therefore sug-gesting that the nature and properties of the producedmaterials can be tailored at the molecular level by conve-niently choosing the metals and ligands of the ablated tar-get. Based on the remarkable properties of goldnanoparticles [28,29,32–34], future work will focus on fur-ther investigating the structural, chemical, electronic, andoptical properties of these carbon/gold nanocomposites.

Acknowledgments

Funded by INNOVARAGON (ITA-DGA, 02/COOP-ERA/ES1368), and MEC (BQU2002-04090-C02-01,MAT2002-04630-C02-01, MAT2004-03070-C05-03,TEC2004-05098-C02-02).

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