TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

27918. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

Synthesis of Nanoparticles by Using RF-Plasma System Barış Daryal¹, Levent Kartal¹,², Yasin Kılıç¹, Güldem Kartal Şireli¹, Servet Timur¹

¹Istanbul Technical University, ²Hitit University - Türkiye

Abstract

In this study, an equipment setup of radio-frequency thermal plasma (RF-Plasma) system was designed and installed for the production of nano-particles. For testing of the scheme, different kinds of particles were synthesized namely, a nano-metal (Ag), a nano-metal oxide (FexOy) and composite nano-particles (Ag/TiO2, CuO/ZnO). The developed RF plasma system was composed of three major units: (1) a high frequency ultrasonic spray for the generation of aerosol, (2) a RF induction system for the thermal decomposition of aerosols (3) an electrostatic filter and particle collection system. During the experiments, the mixture of aerosol was generated from the initial solutions contained metal ions between 0.1 M and 2 M by the ultrasonic atomizer and flowed into the plasma zone with the help of argon carrier gas. The produced particles were characterized via Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The results of characterizations revealed that the particles have a considerably homogeneous particle distribution with the average size below 40 nm. 1. Introduction

Radio frequency (RF) induction thermal plasma technology has arisen as a novel method for the manufacturing of advanced materials and also used for production of numerous kinds of nanoparticles in metallic, metal oxide, metal/metal oxide and metal oxide/metal oxide composites [1].

Silver, silver doped titanium dioxide, iron oxide and composite semiconductor oxides (CuO/ZnO) are commonly investigated due to their unique properties. Silver nanoparticles are usually used for applications such as catalysis, biological labelling and photonics [2,3]. Iron oxide nanoparticles are presently in the use of vitro diagnostics because of their biocompatible properties. Furthermore, these nanoparticles also offer many potential applications namely, catalytic materials, waste water treatment adsorbents, pigments, gas sensors and etc. [4,5] TiO2 and ZnO nanoparticles are usually used as photocatalytic materials. Lately, it has been understood that doping semiconductor oxides (TiO2, ZnO) with metal (Ag) and coupling of semiconductor oxides (CuO/ZnO) can reduce its band gap, extend its absorption range at visible light region [6, 7, 8].

Nanoparticles synthesis via RF thermal plasma method has many benefits, namely high chemical reactivity owing to their high temperatures, energy densities [8, 9], as well as capability of obtaining a homogeneous size with high production rates and so on [10,11]. Besides, it is ability to produce reducing, oxidizing, nitriding, and carburizing environment [12] according to the desired chemical reactions. In addition, RF thermal plasma synthesis is contamination-free since it avoids interior electrodes during the process [13]. In addition to that, various precursors such as vapor, liquid, and powder which are able to control their feed rates, can be used in the RF thermal plasma method [14]. Submicron or nanosized spherical powders with narrow particle size distribution can be formed by using thermal plasma processing [15]. In this study, nano-metal (Ag), nano-metal oxide (FexOy) and composite nano-particles (Ag/TiO2, CuO/ZnO) powders synthesizing by using self-designed RF thermal plasma system including high frequency ultrasonic spray were investigated at the constant 5 min. of running time, plasma temperature and 15 L/min. of argon flow rate process conditions. The produced particles were characterized via Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques.

2. Experimental Procedure

Fig. 1 is the schematic procedure of the experimental apparatus consisted of three major units: (1) a high frequency ultrasonic spray for the aerosol generation from solutions, (2) a RF generator and induction system for the thermal decomposition of aerosols and (3) homemade electrostatic filter for the accumulation of the produced particles. First, the plasma-forming gas is supplied to the torch with water cooling, then applied RF power to the induction coil. The thermal plasma is generated by producing an electromagnetic field into the torch. Then, precursors mixed with carrier gas are introduced into the thermal plasma to induce chemical reactions.

The powders that nucleated from the gas phase leave from the high temperature zone quickly and deposit on the electrostatic filter. The powders were dispersed into ethanol and dropped on a conventional carbon-coated copper grid for TEM measurements.

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280 IMMC 2016 | 18th International Metallurgy & Materials Congress

The crystal structures of synthesized particles were identified by X-ray diffraction (XRD, Philips 1700 diffractometer) using Cu K radiation ( =1.54187 Å, 2 range 10° 90°). The morphology and size of produced powders were studied using a field emission scanning electron microscopy (FE-SEM, Jeol JSM 700F) and Transmission electron microscopy (TEM).

Fig. 1. Schematic drawing of the experimental set-up. (1-Ultrasonic generator, 2-Inductive reactor, 3- Quartz tube, 4- Powder collectors)

The process conditions of particle production experiments are summarized in Table 1.

Table 1: Composition of precursor solutions [5 min. process duration, 15 L/min. Argon flow rate.]

Particle Precursor Concentration [mole/L]

Ag AgNO3 2 M

Ag/TiO2 (AgNO3)/ (TTIP) 0.1 M / 0.1M

FexOy FeCl3.6 H2O 0.1 M and 1M

CuO/ZnO Zn(NO3)2/CuNO3 1 M/1 M

3. Results and Discussion 3.1. Production of Ag nanoparticles

The production of Ag nanoparticles from AgNO3 precursor by using self-designed RF plasma system was investigated at constant process parameters; 2 M AgNO3 concentrations, 5 min. of process duration, 15 l/min. of argon volumetric flow rate.

The possible thermal decomposition of silver nitrate at 440 °C for the formation of Ag metal could be proposed as in Eq.1.

2AgNO3 (l) + O2 (g) = 2Ag (s) + 2NO2 (g) + 2O2 (g) (1)

SEM micrographs of nanoparticles generated at concentrations of 2 M AgNO3 were given in Fig. 3(a) and (b). The spherical like Ag-particles were formed with least than 30 nm.

Fig. 3. SEM images of Ag nanoparticles [2 M AgNO3, 15 l/min. Ar, 5 min.]

The XRD patterns of the product nanoparticles clearly confirm the formation of pure Ag (Fig. 4).

Fig. 4. XRD patterns of Ag nanoparticles [2 M AgNO3, 15 l/min. Ar, 5 min. process time]

3.2. Production of Ag/TiO2 nanoparticles

The production of Ag/TiO2 particles from mixture of inorganic AgNO3 salt and organic TTIP compound were investigated at constant process parameters; 0.1 M Ag+ + 0.1 M Ti4+ concentrations, 5 min. of process time, 15 l/min. of argon flow rate.

The stoichiometric reaction for the decomposition of TTIP could be suggested as in Eq.3 [16]

(Ti[OCH(CH3)2]4)(l) TiO2(s) + 4C3H6(g) + 2H2O(g) (3)

A typical SEM images of the spheres like particles synthesized by using TTIP and AgNO3 are shown in Fig. 5. Two distinct nanoparticles were detected in SEM micrographs (see fig. 5), small shiny Ag and bigger matt TiO2 particles. Two individual nano-sized particles were formed from mixture of AgNO3 and TTIP precursor within the plasma. The formation of distinct Ag and TiO2 particle were the result of the exposure of precursor to a very high temperature. Precursor disintegrated into its elements could combine together in the condensation stage yielding Ag/TiO2 composite nanoparticles. However, it did not happen in this experiment and for the production of Ag/TiO2 composite nanoparticles needs further investigation.

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

28118. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

The obtained results on the disintegration of Ag/TiO2 particles is also reported by Aytekin et al [17].

Fig. 5. SEM images of the Ag and TiO2 nanoparticles [0.1 M Ag+ & 0.1 M Ti4+, 15 l/min. Ar, 5 min.]

3.3. Production of FexOy nanoparticles

The metal oxide (FexOy) nanoparticles production ability of self-designed RF Thermal plasma system investigated using two different 0.01 and 1 M FeCl3 solutions at the constant process parameters; 5 min. of process duration, plasma reaction temperature and 15 l/min of argon flow rate.

SEM images of the iron oxide particles produced at the concentrations of 0.01 and 1 M FeCl3 were given in Fig.6.

Based on SEM observations given in fig. 6, it could be concluded that the increase in the precursor concentrations from 0.01 to 1 M have notable influence on the morphology and size of the particles. It caused the growth of particles and also changed morphology of particles from spherical like to needles form.

(a) 0.01 M

(b) 1 M

0.01 M

1 M

Fig. 6. SEM images of the FexOy nanoparticles [0.01 M, 0.1 M Fe3+,15 l/min. Ar, 5 min.,]

3.4. Production of CuO/ZnO composite nanoparticles

The production of metal oxide/metal oxide (CuO/ZnO) composite nanoparticles examined in RF Thermal plasma system at the constant process parameters; 1 mol/L. Cu2+ and 1 mol/L Zn2+ solution, 5 min. of process duration, plasma temperature and 15 l/min of argon flow rate.

As seen obviously from the SEM images given in Fig. 7 primary particles generated via thermal decomposition of aerosols are under approximately 10 nm. The size of secondary particles, formed agglomerations and sintering of primary particles, are about 35 nm. EDS analyses results given in Fig. 7c showed that the particles composed of only copper, zinc and oxygen.

(a)

(b)

(c)

Fig. 7. SEM and EDS analysis results of the CuO/ZnO nanoparticles [1 M Cu2+ & 1 M Zn2+, 15 l/min. Ar, 5 min. experiment duration]

Fig. 8 presents a TEM images of the ultrafine CuO/ZnO powders. TEM photos given in Fig. 8. showed that particles were almost spherical shape and the average grain size is about 15 nm.

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

282 IMMC 2016 | 18th International Metallurgy & Materials Congress

Fig. 8. TEM images of CuO/ZnO particles produced by using 1 mol/L. Cu2+ and 1 mol/L Zn2+ solution [15 L/min., 5 min.]

4. Conclusions

In the current study, the synthesis of Ag, Ag/TiO2, FexOy, CuO/ZnO composite nanoparticles were investigated in RF inductively coupled thermal plasma incorporating a high frequency ultrasonic spray for the aerosol generation. The study has shown the followings;

• Ag particles with the size of 30 nm were obtained at the initial AgNO3 concentrations of 2 M.

• Composite Ag/TiO2 particles could not be synthesized in RF plasma system. However, the separate Ag and TiO2 particles were generated probably due to the considerably high temperature exposure of the precursor.

• The increase in the solution concentration from 0.1 M to 1 M leaded to increase in the average particle size and also changed morphology of particles from spherical to needle like structure.

• Average grain size of CuO/ZnO secondary particles, generated via agglomeration of primary particles, is about 30 nm, whereas the size of the primary ones is under 10 nm.

Acknowledgements

This work was supported by TUBITAK with project number of 110M687.

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