Based on plasma-treated single wall carbon nanotubes (SWCNTs), SWCNT/SiO2and thiol groups-functionalized SWCNT/SiO2hybrids have been fabricated through a sol–gel process. By means of thiol groups, Ag nanoparticles have been in situ synthesized and bonded onto the SiO2shell of SWCNT/SiO2in the absence of external reducing agent, resulting in the stable carbon nanotube/SiO2/Ag nanoparticles hybrids. This strategy provides a facile, low–cost, and green methodology for the creation of carbon nanotube/inorganic oxides-metal nanoparticles hybrids.
KeywordsCarbon nanotubes Nanocomposites Plasma treatments Silica Silver nanoparticles
Carbon nanotube (CNT)/inorganic composites are a new type of functional materials that gained tremendous interest in recent decades owing to their exceptional optical, mechanical, electrical, and thermal properties, thus enabling the use in photochemical, catalytic, and electrochemical technologies [1–3]. To date, varied CNT-based composite structures resulting from the deposition of metallic, semiconducting, and insulating nanoparticles/nano lusters on the CNT side walls have been created successfully [4–6].
To efficiently synthesize CNT-based nanocomposites, it is necessary to activate the graphitic surface of CNT that tend to be chemical inert and lack of dispersibility in solvents. In this direction, many synthesis strategies have been designed to covalently or non-covalently decorate the side walls of the CNTs with molecular or polymeric entities to create dispersible CNT derivatives. One of the most popular protocols is achieved under strong oxidizing conditions, such as refluxing in concentrated HNO3, followed by the use of carboxylic acid chemistry or direct sidewall reactions . Although these strong oxidizing treatments functionalize the CNTs, the defects are introduced to the pristine CNTs simultaneously and thus undesirably compromise the electronic and mechanical properties . To conquer these drawbacks, non-covalent techniques for modifying CNTs surfaces have been developed in recent decades. By means of π–π stacking and/or wrapping interactions in the presence of surfactants and/or polymers, aqueous-based CNT sols can be achieved [5, 8, 9]. These modified CNTs can be further assembled with a variety of nanoparticles or ceramic materials by means of in situ synthesis techniques [5, 8, 9]. The resulting CNT-based hybrids exhibit tailored properties while still reserving nearly all the intrinsic properties of CNTs.
The above described pioneering works are very interesting but unfortunately, during all those processes for modifying CNT surfaces, more chemicals such as modifier agents, surfactants, organic solvents, amphiphilic polymers, or other additives are indispensable. These would inevitably increase the hazard to environment; enhance the preparation cost and complex the functionalization processes. Therefore, it is still a challenging work to develop a facile, low-cost, and green CNT surface modification method for fabricating CNT-based nanocomposites.
Modification of SWCNT by Plasma Treatment
The SWCNTs commercially obtained from Carbon Nanotechnologies Inc. were firstly dispersed in 2 mL of acetone by sonication. When the acetone was evaporated completely under vacuum, the dry samples were transferred to the chamber of plasma cleaner (Harrick Plasmer Cleaner PDC-32G). After evacuating chamber to low-pressure residual air (0.2 mbar), the SWCNTs were subject to plasma treatment at 10.5 W for 20 s. The treatment processes were repeated 3 times to introduce hydroxyl groups homogeneously on the side walls of SWCNTs.
Preparation of SWCNT@SiO2Composite
In a typical process, 5 mg of surface-modified SWCNTs was transferred to 50-mL flask containing 10 mL of absolute ethanol. After sonicating for 5 min, 0.4 mL of ammonia water (28 wt %), and 0.1 mL of TEOS were injected into the mixture under gentle stirring. And then the mixture was kept stirring at ambient temperature for 3 h. The SWCNT@SiO2core/shell heterostructures were obtained after removal of supernatants by three circles of centrifugation and redispersion in absolute ethanol. Following the similar procedures, thiol groups modified SWCNT@SiO2composites (SWCNT@SiO2–SH) can be prepared. In a typical reaction, 0.4 mL of ammonia water (28 wt %), 6 μL of MPTO, and 0.1 mL of TEOS were orderly added to the reactor containing 10 mL of absolute ethanol and 5 mg of plasma-treated SWCNTs under vigorous stirring. After continuous stirring for 3 h at ambient temperature, the resulting SWCNT@SiO2–SH hybrid was collected after purification by three circles of centrifugation and redispersion in absolute ethanol. To immobilize Ag nanoparticles onto the surface of SWCNT@SiO2–SH, the resulting SWCNT@SiO2–SH composites were redispersed in 2 mL of aqueous silver nitride (0.01 M) under vigorous stirring at room temperature for 3 h. After three circles of centrifugation and redispersion in water, the SWCNT@SiO2/Ag nanoparticles composite was obtained.
Samples for transmission electron microscopy (TEM) were deposited onto carbon-coated copper electron microscope grids and dried in air. TEM analysis was performed using JEOL 1200 EX at 120 keV. Fourier Transform Infrared (FTIR) spectra were obtained at a resolution of 1 cm−1with a Bruker FT-IR spectrophotometer between 4,000 and 400 cm−1. The IR measurements of the powder samples were performed in the form of KBr pellets. Energy dispersive X-Ray spectroscopy (EDS) analysis was performed using an OXFORD ISIS system attached to the SEM.
Results and Discussion
Plasma Treatment of SWCNT and Fabrication of SWCNT@SiO2Therby
Fabrication of SWCNT@SiO2/Ag
The resulting SWCNT@SiO2 hybrid is comprised of SWCNT core with a shell of bonded SiO2, which combines specific properties from each component into a single homogeneous phase. Moreover, the straightforward manufacture, mechanical strength, non-toxicity, and diverse surface chemistry of SiO2 offer an ideal basis for advancing novel SWCNT-based hybrids with desirable functionalities and inherent properties. It is worth noting that the hydroxyl groups usually formed on the SiO2 surface by dissociation adsorption of water molecules provide the capability required for the reduction of metal ions, which offers an effective rout to form SiO2/metal nanoparticles nanocomposites at low temperature without applying any external reducing agent or media . Unfortunately, these hetero-nanostructures are not stable owing to the weak interactions between the metal deposit and SiO2 substrate.
To overcome this disadvantage, a promising alternative is to immobilize metal nanoparticles on the SiO2 surface through covalent bonds. Thiol groups tend to interact with metal ions by the cleavage of an S–H bond and the spontaneous formation of an S-metal bond . These combined sites on the SiO2 surface (S-metal) act as nucleation sites, on which the reduced silver species successively deposit and in situ grow into larger metal nanoparticles. In this process, thiol groups are used as a chemical protocol to attach the metal nanoparticles to the SiO2 surface, resulting in a stable heterogeneous nanocomposite containing metal nanoparticles.
Aimed to immobilize Ag nanoparticles on the functional SiO2shell of SWCNT@SiO2–SH, the SWCNT@SiO2–SH composites were redispersed in 2 mL of aqueous silver nitride (0.01 M) under vigorous stirring at room temperature for 3 h. In this process, Ag nanoparticles are in situ formed and immobilized onto the SiO2surface, resulting in the formation of SWCNT@SiO2/Ag hybrids. The Direct evidence for the formation of Ag nanoparticles on the SiO2surface is obtained from TEM observation. Figure 3c, d shows the surfaces of SWCNT@SiO2are decorated with Ag nanoparticles. Although the in situ formed Ag nanoparticles are randomly distributed on the SiO2shell, their size is quite uniform (with an average diameter of ca. 5 nm). The further evidence of the existence of Ag nanoparticles is provided by EDS (Fig. 3e), which reveals the presence of S and Ag on the surface of SWCNT@SiO2/Ag. Similarly, by using different metal oxides and metal precursors, a variety of CNT/inorganic oxide/metal nanoparticles hybrid materials can be expected. It is worth noting that this protocol provides a facile, low-cost, and green alternative to create the CNT-based inorganic oxide heterostructures/metal nanoparticle hybrids.
An effective rout to introduce hydroxyl groups onto the side walls of pristine SWCNTs by means of plasma treatment technique has been demonstrated. By means of a co-condensation process between these hydroxyl groups bearing on the SWCNTs and TEOS (or together with MPTO), an uniform SiO2and thiol groups-functionalized SiO2coating on the SWCNTs can be fabricated effectively. Utilizing SWCNT@SiO2–SH, a stable SWCNT@SiO2/Ag heterogeneous hybrid has been generated via in situ growth process in the absence of any additional reducing agents. These SWCNT-based composites may provide considerable potential in applications like catalysis, bactericide, and electrode materials. Particularly, it is worthy to note that this facile procedure could offer a promising alternative to create varied SWCNT/inorganic oxide (TiO2, GeO2et al.) composites and the corresponding SWCNT/inorganic oxide/metal nanoparticles hybrids.
This work was supported by grants-in-aid for theWorld Class University Program(No. R32-2008-000-10174-0) and theNational Core Research Center Program from MEST (No. R15-2006-022-01001-0), and theBrain Korea 21 program(BK-21).