‘Laser chemistry’ synthesis, physicochemical properties, and chemical processing of nanostructured carbon foams
© Seral-Ascaso et al.; licensee Springer. 2013
Received: 17 November 2012
Accepted: 28 March 2013
Published: 16 May 2013
Laser ablation of selected coordination complexes can lead to the production of metal-carbon hybrid materials, whose composition and structure can be tailored by suitably choosing the chemical composition of the irradiated targets. This ‘laser chemistry’ approach, initially applied by our group to the synthesis of P-containing nanostructured carbon foams (NCFs) from triphenylphosphine-based Au and Cu compounds, is broadened in this study to the production of other metal-NCFs and P-free NCFs. Thus, our results show that P-free coordination compounds and commercial organic precursors can act as efficient carbon source for the growth of NCFs. Physicochemical characterization reveals that NCFs are low-density mesoporous materials with relatively low specific surface areas and thermally stable in air up to around 600°C. Moreover, NCFs disperse well in a variety of solvents and can be successfully chemically processed to enable their handling and provide NCF-containing biocomposite fibers by a wet-chemical spinning process. These promising results may open new and interesting avenues toward the use of NCFs for technological applications.
Laser technologies can be successfully utilized for the production of carbon-nanostructured materials exhibiting fascinating structural and physical properties such as carbon nanotubes , carbon nanohorns , carbon nanofoams , or shell-shaped carbon nanoparticles . Our group discovered the production of metal-nanostructured foams (NCFs) by laser ablation of triphenylphosphine (PPh3)-containing organometallic targets . We then demonstrated that organic ligands can act as efficient carbon sources for the laser ablation production of carbon nanomaterials. Metal-NCFs are three-component materials which consist of amorphous carbon aggregates, metal nanoparticles embedded in amorphous carbon matrices, and graphitic nanostructures. The metal-NCF composition, metal nanoparticle size, and dilution (i.e., metal and carbon content) within the carbon matrices can be tailored by conveniently choosing the metals (Au, Cu) and ligands of the ablated targets . On the other hand, laser ablation of PPh3 resulted in the production of metal-free NCFs consisting of graphitic nanostructures and P-containing amorphous carbon aggregates . We report how our versatile ‘laser chemistry’ approach can be extended to the synthesis of a variety of other metal-NCFs, as well as to metal-free, P-free NCFs, proving that the synthesis of NCFs is not restricted to PPh3-based targets and therefore enabling envisioning the synthesis of metal-carbon hybrids by chemical design. Additionally, physicochemical studies have been performed on metal-free NCFs to evaluate their potential applications. We also show that NCFs can be easily chemically processed in the form of stable NCF dispersions in different solvents and NCF biocomposite fibers, which offer promise for NCF incorporation into different matrices and technological applications.
The structure of the synthesized NCFs was imaged by scanning electron microscopy (SEM, Hitachi S-3400N (Hitachi, Ltd., Chiyoda-ku, Japan), including a Röntec XFlash detector (Röntec GmbH, Berlin, Germany) for energy dispersive X-ray spectroscopy (EDS) analyses), and transmission electron microscopy (TEM, JEOL JEM-3000F microscope, JEOL Ltd., Akishima-shi, Japan, equipped with an Oxford Instruments ISIS 300 X-ray microanalysis system and a Link Pentafet detector, Oxford Instruments, Abingdon, UK, for EDS analyses). NCF thermal stability in air was studied by thermogravimetric analysis (TGA, SETARAM Setsys Evolution, Hillsborough, NJ, USA; samples were analyzed in Pt pans at a heating rate of 10°C/min up to 850°C in an atmosphere of air flowing at 100 mL/min). Micro-Raman spectroscopy studies were carried out using a Dilor XY Raman spectrometer (λexc = 514.5 nm, HORIBA, Ltd., Kyoto, Japan). Elemental analyses of metal-free NCFs were performed using a Thermo Flash EA 1112 Series NC analyzer (Thermo Fisher Scientific, Waltham, MA, USA). The textural properties of NCFs were studied using nitrogen adsorption-desorption isotherms measured at 77 K (Micromeritics ASAP 2020, Norcross, GA, USA) and using the Brunauer-Emmett-Teller (BET) method between 0.05 and 0.3 P/P0 and t-Plot and Barret-Joyner-Halenda (BJH) method. Density values were measured using an AccuPyc II 1340 Micromeritics helium picnometer (Micromeritics, Norcross, GA, USA).
Fiber spinning of NCF biocomposites was performed by injecting 1:4 Au-NCF:sodium alginate (MW: 400K) aqueous dispersions (1 mg/mL Au-NCF prepared by bath sonication) into a coagulation bath (5% CaCl2 solution in 70% methanol) following the carbon nanotube biofiber spinning procedure reported by Razal et al. . The electrical conductivity of the spun fibers was characterized by four-probe resistance measurements using a Keithley 2000 Multimeter (Keithley Instruments, Inc., Cleveland, OH, USA).
Results and discussion
Measured densities of different carbon materials
The laser chemistry approach described in the present work is a versatile method for the synthesis of metal nanoparticles embedded in carbon matrices from molecular precursors. This laser chemistry is very appealing for applications requiring metal nanoparticles largely isolated from each other embedded in solid matrices. Moreover, it can be used for the synthesis of metal-free, P-free NCFs from commercial organic precursors, which would in turn facilitate upscaling their production. On the other hand, the chemical processing capabilities of NCFs ease their handling and may open attractive opportunities toward their incorporation into matrices and applications. Future challenges should deal with the design of production or processing strategies to increase the surface area and conductivity of these materials to enable their use as, for example, electrode materials, in catalysis, or as functional magnetic materials.
This work has been supported by the regional Government of Aragón (Spain, Project PI119/09, and E101 and T87 Research Groups funding) and the Spanish Government and Feder funds through grant MAT2010-19837-C06-06. This work has been funded in part by the European Commission through projects LIFE11/ENV/ES 560 and grant agreement no. 280658. The authors would like to acknowledge the use of Servicio de Microscopia Electrónica (Servicios de Apoyo a la Investigación), Universidad de Zaragoza. The authors also thank the technical assistance provided by the Servicio de Análisis of the Instituto de Carboquímica ICB-CSIC. The authors thank María Jesús Lázaro for kindly providing carbon xerogel and ordered mesoporous carbon samples. Carbon black and activated carbon samples were kindly supplied by Delta Tecnic S.A. and Morgui Clima S.L, respectively.
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