铂纳米粒子链状组装结构的直接制备与表征

Direct preparation and characterization

of chainlike assemblies formed by small

platinum nanoparticles1

Zhirui Guo, Lan Huang, Yu Zhang, Meng Wang, Lina Xu, Ning Gu*

State Key Laboratory of Bioelectronics, Department of Biological Science & Medical Engineering,

Southeast University, Nanjing, 210096, China

Jiangsu Laboratory for Biomaterials and Devices, Southeast University, Nanjing, 210096, Southeast

University, Nanjing, 210096, China.

* Corresponding author, e-mail: guning@https://www.sodocs.net/doc/8e7988736.html,

Abstract

We herein report a direct approach to prepare chainlike nanostructures by the

assembly of small platinum nanoparticles through a polyol process in the presence of

aniline. During the reduction process of H2PtCl6 to Pt nanoparticles by ethylene glycol,

aniline functioned as a stabilizer, which played a key role in the formation of the

chainlike assemblies of Pt nanoparticles. Moreover, investigation shows the size of

the chainlike assemblies of Pt nanoparticles can be tailored by changing the amount of

aniline in solution. The morphology of the as-prepared Pt nanostructures was

observed by the combination of transmission electron microscopy (TEM) and field

emission scanning electron microscopy (FE-SEM). Other characterization techniques

including energy-dispersive X-ray spectroscopy (EDX), Powder X-ray diffraction

(XRD) and UV-vis- near infrared region (NIR) absorption spectroscopy were also

used to analyze these chainlike nanostructures.Furthermore, the corresponding I-V

characteristics were studied with a semiconductor characterization system by bridging

a single chainlike assembly on a gold microelectrode pair. The possible formation

mechanism of these chainlike assemblies of Pt nanoparticles was also discussed. It is

believed that these chainlike Pt nanostructures are valuable for fundamental research

and the construction of nanodevices.

Keywords: platinum nanoparticles; chainlike; assembly; aniline.

1 Introduction

Metal nanoparticles have exhibited numerous novel properties compared with those of their bulk counterparts since the emergence of nanoscience. During the past few decades, numerous researches

1 Support by the National Natural Science Foundation of China (nos. 20573019, 60371027, 60171005 and

90406023) and National High-Tech 863 Program.

have been carried on the preparation of size-controllable, isolated metal nanoparticles and their corresponding properties as well as practical applications [1]; on the other hand, it is also considered coequally significant to construct shape-ordered assemblies using nanoparticles as building blocks because these nanoparticles assemblies not only display collective behaviors different from those of their individual nanoparticles but also serve as the bridges between the nanoscopic units and the macroscopic objects [2-4]. Compared with the two-dimensional (2D) and 3D assemblies of metal nanoparticles, 1D or quasi 1D assemblies showed unique collective vectorial performances such as interparitcle electronic, photonic, energy transfer [5-7] and especially the potential application in serving as the interconnectors like metal nanowires/rods in nanodevices [8]. Thus the fabrication of 1D or quasi 1D assemblies has become an active research area in recent years. As an effective strategy, various materials have been used as templates to obtain low-dimensional assemblies of metal nanoparitlces. For example, 1D Au nanoparticles and Au nanoparticles-poly(pyrrole) were prepared through the linear channels of nanoporous Al2O3[9]. Carbon nanofibers and carbon nanotubes were introduced to load metal nanoparticles effectively [10,11]. Some special biomaterials such as DNA, linear superstructures of peptide and even virus were also exploited as scaffolds to construct low-dimensional metal nanoparticles assemblies through electrostatic interaction [12-14]. On the other hand, some template-free methods were also developed: Taking advantage of the non-isotropy of interparticle dipole interactions under proper conditions, low-dimensional chains of various metal nanoparticles were generated respectively [15,16]. Additionally, metal nanoparticles were also covalently cross-linked into low-dimensional nanostructures by the chemical reactions of the functional molecules [17]. However, most of these approaches mentioned above have to synthesize the metal nanoparticles in advance or the materials used as templates are usually costly and difficult to be removed. In this paper, we report a simple, direct method to prepare chainlike Pt nanoparticles assemblies with nanoscaled diameters by a polyol-process in the presence of aniline. During the reaction procedure we adopted, H2PtCl6 were reduced to small Pt nanoparticles by ethylene glycol at a relatively low temperature and the aniline in the solution functioned as a stabilizer to play a key role in the formation of chainlike Pt nanoparticles assemblies.Further investigation suggests the size of the chainlike assemblies of Pt nanoparticles can be tailored by the amount of aniline in the solution. The as-prepared chainlike Pt nanostructures were characterized by microscopy and spectroscopy including TEM, FE-SEM, EDX, XRD and UV-vis-NIR. The corresponding I-V characteristics of the chainlike assembly were studied with a semiconductor characterization system. Based on the experimental results, a possible formation mechanism of these chainlike nanostructures was also proposed.

2 Experimental section

2.1 M a t e r ia ls

All chemicals were analytical pure grade and purchased from Shanghai Chemical Reagent Co. Ltd. (China). Aniline was distilled twice at a reduced pressure in the presence of zinc powder and stored in brown bottle. Hexachloroplatinic acid (H2PtCl6), ethylene glycol (EG) and anhydrous ethanol were used as received without further purification. The water used was purified through a Millipore system (18.0 M? cm-1). Prior to use, all glasswares were cleaned by immersion in piranha solution (98% H2SO4/30% H2O2, 3:1 in volume ratio) at room temperature overnight and then thoroughly rinsed with Millipore water for several times.

2.2 R e a c t io n p r o c e d u r e

In a typical synthesis procedure, 50 ml of EG solution containing 0.029 mmol H2PtCl6·6H2O in a three-neck flask equipped with a reflux condenser was heated to 95℃. Then, 0.1 M aniline solution in EG was added dropwise to this solution under stirring to obtain a 2:1 molar ratio of aniline to Pt. The reaction solution was then heated at 95℃ without stirring at normal atmosphere for 12 h and then naturally cooled to the room temperature. During this course, the color of the reaction solution turned slowly from light yellow to brown-yellow. At the end of the reaction, a great deal of gray black

precipitates deposited at the flask bottom, which were purified by centrifugation and washed several times by anhydrous ethanol to remove the remaining impurities. Finally, the as-prepared product was dispersed in ethanol for further characterization.

2.3 C h a r a c t e r iza t io n t e c h n iq u e s

Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) photographs were taken by a JEM-2000EX microscope under 120 KV accelerating voltage and carbon-coated copper grids were used as substrates. 2 μl of the sample was dropped onto the copper grid and dried naturally. Scanning electron microscopy (SEM) images were recorded using a LEO-1530VP field-emission scanning electron microscope under an 5.0 KV accelerating voltage and the samples were deposited on silicon substrates by drop-casting, which were also used for elemental analysis by energy-dispersive X-ray spectrograph (EDX) attached to the SEM under 10.0 KV accelerating voltage. Powder X-ray diffraction pattern (XRD) Cu K α, 40 kV, 30mA) was taken by a RIGAKU D/Max-RA X-ray powder diffractometer (Cu K α, 40 kV, 30mA). The optical spectrum of the as-prepared Pt product was recorded by a SHIMADZU UV-Vis-NIR spectrophotometer (UV-3150) and the I-V behavior was measured by a keithyley 4200 semiconductor characterization system.

3 Results and discussion

The morphology of as-prepared Pt products was characterized using both TEM and SEM. The chainlike arrangement of quasi-spherical assemblies with diameters of 200-250 nm can be seen clearly from the typical TEM photograph shown in Fig. 1(a), which accords with that of the SEM image (Fig. 1(c)). From a higher magnification, it is found that such spherical assemblies consist of small Pt nanoparticles Fig. 1 (a) Typical TEM photograph of the obtained Pt products at a 2:1 mole ratio of aniline to Pt. The inset shows an enlarge view of the Pt nanoparticles assemblies and the scale bar is 50 nm. (b) The corresponding SAED of these Pt nanostructures. (c) The SEM image of the

same batch of the Pt nanostructures and their corresponding EDX spectrum (d).

d a

with only several nanometers in diameter (Fig. 1(a), inset). The corresponding SAED pattern reveals ring structure, confirming the polycrystalline nature of these Pt nanostructures (Fig. 1(b)). The EDX spectrum of the chainlike assemblies reveals strong peaks due to element platinum. Moreover, the two quite weak peaks ascribed to carbon and nitrogen are also observed respectively, which suggests these Pt chainlike assemblies are covered with aniline related substance. Fig. 2 displays the powder XRD pattern of the chainlike Pt nanostructures mentioned above. The four diffraction peaks locating at 2θ = 39.66o, 46.12o, 67.16o and 81.51o match well with the {111}, {200}, {220}, and {311} planes of the metal platinum face-centered structure respectively (JCPDS, file no.04-0802). It can be seen that the XRD peaks of the Pt products are quite broader than that of the bulk Pt because of the small size of the Pt nanoparticles [18]. The mean diameter of the Pt nanoparticles was calculated from the linewidth of the {111} plane diffraction according to Scherre equation [19]. The result was 2.3 nm, which is consistent with the TEM characterization.

As shown in Fig.3, The UV-vis-NIR spectrum of the obtained chainlike Pt nanostructures presents the maximum absorption band centering at 804 nm, which is quite different from the previously reported plasmon band of the discrete Pt nanoparticles locating at less than 300 nm [20]. The chainlike shape of the Pt nanoparticles assemblies and the mutual action of the Pt nanoparticles may possibly contribute to this special absorption spectrum pattern.

Fig. 2 Powder XRD pattern of the chainlike Pt nanostructures

Fig. 3 UV-vis-NIR spectrum of the chainlike Pt nanostructures dispersed in ethanol.

In order to examine the role of aniline on the formation of chainlike Pt nanostructures in present synthesis, comparison experiments were performed under the different amounts of aniline in starting reaction solution with otherwise reaction parameters the same. When the molar ratio of aniline to platinum was increased to 4:1 and 6:1, the mean diameters of the resulting chainlike nanostructures decreased to about 170 nm and 100 nm respectively (Fig. 4a, 4b). However, when the molar ratio of aniline to platinum was decreased to 1:1, the mean diameter of the obtained chainlike Pt nanostructures turned broad to more than 300 nm (Fig. 4c). In particularly, the reaction rate between EG and H 2PtCl 6 increased sharply when the aniline was absent. It is found that the whole reaction solution turned colorless and large scale of dark powder aggraded on the flask bottom within 3 h and the resulting products are seen to be irregular aggregates consisted of small Pt nanoparticles (Fig. 4d). Evidently, the presence of aniline in the reaction solution plays a key role in the formation of the chainlike Pt nanostructures and the size of these chainlike Pt nanostructures can be tailored by changing the amount of the aniline in solution.

Fig. 4. Representative TEM photographs of Pt products obtained at (a) 4:1, (b) 6:1, (c) 1:1 molar ratio of aniline to Pt respectively and (d) without the presence of aniline. d c a b

Based on the experimental results mentioned above, a possible mechanism for the formation of chainlike Pt nanoparticles assemblies can be proposed. During the first stage, Pt nanoparticles are produced by the EG reduction of H 2PtCl 6 and stabilized by aniline. Due to the relatively weak interaction of amino to metal surfaces arising from the attracting electron effect of the phenyl ring, the aniline-stabilized Pt nanoparticles tend to assembly and gradually grow into a spherical shape for the purpose of reducing the surface energy of these assemblies [21]. Thereafter these spherical assemblies further aggregates into the chainlike morphology by the interaction each other. Higher amount of aniline may increase the stability of Pt nanoparticles and then limit the size of the spherical Pt nanoparticles assemblies. On the other hand, lower amount of aniline may reduce the stability of the Pt nanoparticles and the larger size spherical assemblies formed. The generation of the Pt nanoparticles will greatly accelerate without the stabilization of aniline due to the autocatalystic reduction of the PtCl 62-. As a result, the irregular aggregates of Pt nanoparticles are produced quite rapidly.

The electric characteristics of the obtained chainlike nanostructures were also studied. A single chainlike assembly of Pt nanoparticles was bridged on top of a gold microelectrode pair by dropping with a micropipette and drying in air naturally (Fig. 5a). The corresponding I-V characteristics were measured by the semiconductor characterization system at room temperature. During the original electrical measurement, no effective data was detected because these current signals were all under the measuring limit of the picoampere levels. However, when focusing an electron beam from SEM at 15 KV to irradiate the contact parts between the same chainlike assembly and the electrodes after the first electric measurement, a nonlinear I-V behavior curve was obtained (Fig. 5b). Thus it indicates the loose physical contact between the chainlike assembly and the gold electrode pair brings on the potential barriers to carrier transport [22]. Work is still needed to reduce the contact resistance for future applications.

4 Conclusions

In summary, we have demonstrated a simple approach to directly obtain chainlike assemblies of Pt nanoparticles using EG as solvent as well as reducing agent in the presence of aniline and no additional templates or shape-directing agents are needed. During the reaction in the presence of aniline, H 2PtCl 6

b

Fig. 5 (a) The SEM image of a chainlike Pt nanoparticles assembly deposited on the top of a gold electrode pair and (b) the I-V behavior curve measured after the electron beam irradiation of the its contact parts with the electrodes.

was reduced to small Pt nanoparticles by EG and aniline functioned as a stabilizer, which played a key role in the formation of the chainlike assemblies of Pt nanoparticles. Preliminary studies suggest the size of these chainlike assemblies can be tailored by changing the amount of the aniline in solution. These as-prepared chainlike Pt nanostructures may find their usage in both fundamental research and the practical construction of various nanodevices.

A c k n o w l e d g m e n t s

We thank Mr. Aiqun Xu and Xun Xiao from the Analysis and Testing Centre of Southeast University for the help with the measurements.

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