Characterization of zinc and zinc cyanide nanoparticles in carbon matrices prepared by solid-phase pyrolysis of zinc-phthalocyanine

Using solid-phase pyrolysis of Zn-phthalocyanine (ZnC32H16N8), we have prepared zinc and zinc cyanide nanoparticles in carbon matrices with a zinc concentration of 3 at %. The structure and composition of samples were investigated by the methods of scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. It is shown that at low pyrolysis temperature (700°C) only the Zn nanoparticles are formed, whereas at higher temperature (900°C) a certain amount of Zn(CN)2 nanoparticles are also synthesized. The mean diameter of nanoparticles is about 150 nm, and their size distribution has a logarithmically normal shape.


INTRODUCTION
In recent years metal nanoparticles have attracted considerable interest due to their applications in various fields [1][2][3][4][5]. In particular, they can be used in spintronics, biomedicine, magnetic paints, supercapacitors, catalysis, as sensors, absorbers of the electromagnetic field energy, etc.
One of convenient methods for producing the metal nanoparticles in carbon matrices is solid-phase pyrolysis of metal-phthalocyanines (MPc = MC 32 N 8 H 16 ), because they contain simultaneously the metal and carbon atoms (in the ratio 1:32) and can form the different metal-carbon structures with given characteristics (see, e.g., [6][7][8][9][10]).
In the present work we show that solid-phase pyrolysis of zinc-phthalocyanine is a simple and efficient method for synthesis of zinc and zinc cyanide nanoparticles in carbon matrices. Prepared samples were investigated by the methods of scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. Note that, in contrast to other metals, there are no any data on zinc cyanide nanoparticles in the literature.

EXPERIMENTAL
In this work the process of solid-phase pyrolysis of zinc-phthalocyanine was analogous to that described in [6][7][8][9][10] and corresponded to the following reaction: ISSN 1068-3372, Journal of Contemporary Physics (Armenian Academy of Sciences), 2016, Vol. 51, No. 2, pp. 191-195 where T pyr is the pyrolysis temperature, t pyr the pyrolysis time, and p pyr the autogenic pressure in a reaction chamber. Pyrolysis was performed in closed quartz ampoules with an initial pressure of ~10 -6 bar. It is evident that the concentration of Zn in the samples is about 3 at %.
The morphology, elemental composition and sizes of prepared nanoparticles were investigated by using scanning electron microscope (SEM) Vega TS 5130 MM (Tescan) with the system of energydispersive X-ray microanalysis INCA Energy 300. The structure of samples was studied with an X-ray diffractometer DRON-3 (CuK α radiation) and a Raman spectrometer Renishaw operating at the exciting radiation of an Ar laser with the wavelength of 514.5 nm.

RESULTS AND DISCUSSION
The prepared samples represent air-stable black powders consisting of a carbon matrix with embedded zinc nanoparticles. It should be noted that carbon matrix is biocompatible and prevents the processes of oxidation and aggregation of metal nanoparticles.
We chose the following pyrolysis conditions: T pyr = 700°C and 900°C, t pyr = 300 min. SEM images of two synthesized samples are shown in Fig. 1. As seen, the nearly spherical nanoparticles are sufficiently homogeneously dispersed in carbon matrix. Note that there is also a certain amount of carbon microspheres of 2-3 μm diameter in the matrix (more about them see in [11,12]).
The results of energy-dispersive microanalysis show that the composition of samples is sufficiently homogenous and they consist of atoms of carbon and zinc (≈3 at %). There is also a few amount of nitrogen (up to 10 at %) which decreases at high pyrolysis temperatures.
Size distributions of nanocomposites samples, determined with use of SEM images are presented in Fig. 2. For each sample the diameters of 150-200 nanoparticles were considered. The distributions have a logarithmically normal shape which is characteristic for the coalescence process [2]. As seen, the mean Essential information on the structure of samples can be obtained from X-ray diffraction spectra (Fig.  3). As seen, there is a considerable difference between samples synthesized at different temperatures. Comparison with tabulated data [13] shows that a broad peak at ≈25° corresponds to partially graphitized carbon microspheres (d 002 ≈ 0.345 nm) [11,12], while four more narrow peaks in the spectrum (Fig. 3a) relate to Zn nanocrystallites with hexagonal symmetry. In addition to these peaks, three new peaks with lower intensity arise in the spectrum (Fig. 3b), which correspond to zinc cyanide (Zn(CN) 2 ) nanocrystallites with a cubic symmetry [13]. In the case of preparation of zinc cyanide nanoparticles the pyrolysis reaction can be represented in the following form: ( ) pyr pyr. In our opinion, the cause of appearance of Zn(CN) 2 nanoparticles at high pyrolysis temperatures is the strong interaction of zinc clusters with surrounding carbon and nitrogen atoms. Besides, relatively low melting points of Zn (420°C) and Zn(CN) 2 (800°C) also promote the increase in this interaction.  Interesting features are observed in Raman spectra of samples (Fig. 4). There are two narrow peaks at ≈1350 cm -1 and ≈1590 cm -1 , which correspond, respectively, to D-and G-bands of partially graphitized carbon microspheres [12]. A very broad band in the range of 1000-3500 cm -1 in Fig. 4a can be caused by luminescence in view of the recombination processes in sp 2 carbon clusters [14][15][16].

CONCLUSION
We have shown that solid-phase pyrolysis of zinc-phthalocyanine is a simple, one-stage and efficient method for synthesis of zinc and zinc cyanide nanoparticles in carbon matrices. It is evident that the concentrations of zinc and zinc cyanide nanoparticles can be controlled by changing the pyrolysis conditions (in particular, there is a strong influence of the pyrolysis temperature). It is also known that mean sizes of nanoparticles strongly depend on the pyrolysis time [8].