نانوهیبریدهای نانولوله های کربن/نانوذرات فلزات نجیب: سنتز و کاربرد Noble metal nanoparticles/carbon nanotubes nanohybrids: Synthesis and applications

Introduction Carbon nanotubes (CNTs), including single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs), are important carbon-based materials [1,2]. Since the CNTs were discovered [3,4], they have been intensively studied and have received a great deal of attention for their applications in a wide variety of areas, due to their excellent properties which involving high specific surface area, high electronic conductivity, outstanding chemical and electrochemical stability, one dimensional tubular-structure and so on [5—۸]. On the other hand, owing to their unique electric, magnetic, optical, and catalytic properties, metal nanoparticles (NPs) have emerged as a new class of compounds that are interesting in several areas from chemistry to physics, to material sciences, to biology and medicine [9,10]. Importantly, these properties differ from those of the bulk materials and mainly depend on the particle size and morphology [11—۱۳]. The hot research points in metal NPs focus on the synthesis of noble metal NPs such as Au, Ag, Pt, Pd, Ru and their alloys [14—۱۸].Noble metal NPs/CNTs nanohybrids are a new kind of composite materials which successfully integrate the unique properties of two class materials (CNTs and noble metal NPs) and exhibit some new functions caused by the cooperative effects between the CNTs and noble metal NPs. Therefore, noble metal NPs/CNTs nanohybrids have shown very attractive applications in many fields, especially in heterogeneous catalysis, fuel cells and chemo/biosensors (Fig. 1). Since the first report about the synthesis and application of noble metal NPs/CNTs nanohybrids in 1994 [19], the number of literatures escalates at an enormously increasing rate each year. This review focuses on the recent progress in the synthesis and applications of noble metal NPs/CNTs nanohybrids. Synthesis of noble metal NPs/CNTs nanohybrids Numerous synthetic methods have been developed for the preparation of noble metal NPs/CNTs nanohybrids in the literatures, each providing varying degrees of control of the size and distribution of metal NPs on the surface of CNTs. These strategies can be divided into four categories: electrochemical deposition, electroless deposition, dispersion of noble metal NPs on the functionalized CNTs, physical methods. The particle sizes of noble metal NPs in the representative noble metal NPs/CNTs nanohybrids are summarized in Table 1. In this section, a discussion about the recent progress in preparation of the noble metal NPs/CNTs nanohybrids are made according to the above four categories. Electrochemical deposition method Electrochemistry is a powerful technique for deposition of various NPs (especially metal NPs), as it enables effective control over nucleation and growth of metal NPs. Most research has been conducted on the deposition of noble metal NPs and their alloys, such as Au, Ag, Pt, Pd, and bimetallic PtRu [20—۳۰], those were usually used in heterogeneous catalysis, fuel cells and chemo/biosensors. In this method, the noble metal NPs/CNTs nanohybrids are obtained via reduction of noble metal complexes, such as H[AuCl4], H2[PtCl4], and (NH4)2[PdCl4], by electrons. CNTs typically do not react with the noble metal salts but act as molecular conducting wire and supports for the deposition of noble metal NPs. Importantly, the size of the noble metal NPs and their distribution on the sidewalls of CNTs can be controlled by the concentration of the noble metal salts and various electrochemical deposition parameters, including nucleation potential and deposition time. Besides, electrochemical deposition method also has the following advantages: noble metal NPs with very high purity, forming rapidly and having good adhesion to the CNTs substrate. He et al. took the advantages of substrate-grown-directly CNTs and the high purity of electrochemically deposited NPs to decorate CNTs with Pt or bimetallic Pt—Ru NPs with diameters of 60—۸۰ nm (Fig. 2) [20,29]. After the MWCNTs were pre-treated by electrochemical oxidation, the deposition took place potentiostatically at −۰٫۲۵ V from a solution containing chloroplatinic acid or ruthenium chloride and chloroplatinic acid in 0.5 M H2SO4. Quinn et al. further demonstrated that noble metal (Au, Pt, Pd) NPs/SWCNTs nanohybrids could be synthesized by electrodeposition under direct potential control and the particle size of noble metals NPs and surface coverage could be tuned with potential, deposition time, and metal salt concentration [31]. Usually, high dispersion and small particle size of noble metal NPs on CNTs are desired not only from the extraordinary catalytic activity but also from the low cost. However, the noble metal NPs/CNTs nanohybrids prepared by electrodeposition method usually receive noble metal NPs with big particles size (ranging from tens to hundreds nanometers), as shown in He’s work. It is an challenge to synthesize noble metal NPs with high dispersion and small particle size on CNTs by electrochemical deposition method due to the contradiction between a larger driving force (more negative deposition potential) for critical nuclei formation and an inhibitive mechanism for deterring crystal growth (less deposition charge or smaller current density). In order to solve this problem, Tsai et al. tried to synthesized Pt (PtRu) NPs/CNTs nanohybrids by electrochemical deposition in ethylene glycol (EG) containing H2SO4 aqueous solution. They successfully obtained Pt (PtRu) NPs with small size (4.5—۹٫۵ nm for Pt and 4.8—۵٫۲ nm for PtRu) and uniform dispersion on the surface of CNTs (Fig. 3). It was found that EG not only enhanced the dechlorination of Pt and Ru precursor salts and led to the formation of NPs but also acted as a stabilizing surfactant to prevent the particles from agglomeration during the electrodeposition processes, resulting in a better dispersion and smaller particles size of Pt or PtRu NPs [32,33].