J Nanopart Res (2011) 13:3825–3833 DOI 10.1007/s11051-011-0305-3
RESEARCH PAPER
Synthesis and characterization of carbon coated nanoparticles produced by a continuous low-pressure plasma process Vineet Panchal • Manoj Neergat Upendra Bhandarkar
•
Received: 24 May 2010 / Accepted: 21 February 2011 / Published online: 6 March 2011 Ó Springer Science+Business Media B.V. 2011
Abstract Core–shell nanoparticles coated with carbon have been synthesized in a single chamber using a continuous and entirely low-pressure plasma-based process. Nanoparticles are formed in an argon plasma using iron pentacarbonyl Fe(CO)5 as a precursor. These particles are trapped in a pure argon plasma by shutting off the precursor and then coated with carbon by passing acetylene along with argon as the main background gas. Characterization of the particles was carried out using TEM for morphology, XPS for elemental composition and PPMS for magnetic properties. Iron nanoparticles obtained were a mixture of FeO and Fe3O4. TEM analysis shows an average size of 7–14 nm for uncoated particles and 15–24 nm for coated particles. The effect of the carbon coating on magnetic properties of the nanoparticles is studied in detail. Keywords Nanoparticles Core–shell structure Chemical vapor deposition Magnetic properties
V. Panchal M. Neergat Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India U. Bhandarkar (&) Department of Mechanical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India e-mail:
[email protected]
Introduction Nanoparticles show special properties as compared to bulk materials which are related to their small dimensions. Nanostructured magnetic materials have potential applications in diverse disciplines such as data storage, magnetic resonance imaging, biotechnology, biomedicine, and catalysis. Magnetic nanoparticles show ferromagnetic behavior with high coercivity or superparamagnetic behavior depending on size and chemical composition (Gavrin and Chien 1990; Bai et al. 2009). In general the properties of these nanoparticles can be altered by changing size, shape, chemical composition, and internal structure. Nanoparticles made from a particular material can also be coated with a different material so that particle morphology, properties and functionality can be changed. This is normally called as particle engineering (Davies et al. 1998). Normally the synthesis of the base magnetic particles and the coating process are carried out separately. For example, (Cao et al. 2008) procured readily available Fe3O4 nanoparticles and first milled them with sodium chlorite. These particles were then reduced in a hydrogen atmosphere for 2 h and then coated with carbon using ethylene, H2 and N2 using thermal CVD for 30 min at 500 °C. Magnetic particles have been coated using various methods such as carbon arc technique (McHenry et al. 1994), chemical vapor deposition (Sano et al. 2003; Cao et al. 2008), gas condensation (Uhm et al. 2009) and sol–gel
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method (Lu et al. 2002). Some others have synthesized and coated particles in a continuous process. For example, Chau et al. (2006) have synthesized SiCcoated Co using a microwave plasma by generating Co particles and coating them in two different zones. Similarly iron nanoparticles coated with carbon were generated in an organo-silicon polymer matrix using laser-induced chemical vapor deposition by Diaz et al. (2005). Carbon coating to these nanoparticles provides biocompatibility and stability in many organic and inorganic media. These carbon encapsulated magnetic nanoparticles can be useful in bioengineering applications such as drug delivery, biosensors, and magnetic contrast agents for magnetic resonance imaging (Song and Chen 2003). In this article the authors report a continuous lowpressure plasma-based process wherein iron nanoparticles are first created, then trapped and finally coated with carbon in an argon plasma by switching on different precursors. The authors have studied the effect of the carbon coating on the magnetic properties of the core nanoparticles.
Experimental details The experimental set-up is a custom designed parallel plate capacitively coupled plasma system typically used for PECVD (Plasma enhanced chemical vapor deposition) processes. A schematic diagram of the set-up is shown in Fig. 1. It consists of two parallel plate electrodes each of 150 mm diameter separated by 40 mm. The upper electrode is powered by an RF power source (frequency: 13.56 MHz, make: 600 W Comdel, USA) through a matching network. Mass flow controllers (range: 0–100 sccm, make: Advanced Energy, USA) are used to control the flow of the process gases namely argon, acetylene, and hydrogen. Argon is the principal gas used to generate the plasma. Argon enters the vacuum chamber via two routes, either directly or through a bubbler containing iron pentacarbonyl. In the chamber, the gas enters the space between the electrodes through the ring shower as shown in Fig. 1. The process is started by initiating a pure argon plasma with a chamber pressure of 200–250 mTorr and an RF power of 150 W. (These are optimized quantities obtained after some parametric runs). The argon flow rate is set at 30 sccm.
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Gas
RF power supply
Turbo pump Fig. 1 Schematic of the process chamber
After the plasma has been established, argon is diverted via the bubbler which insures that Fe(CO)5 is carried with it. In PECVD, precursors (in this case Fe(CO)5) are broken down by energetic electrons in the low-pressure plasma. Radicals so formed can either deposit on the substrate or react amongst themselves to create bigger clusters that become particles. In this case, iron nanoparticles are generated by plasma-induced breaking of bonds in Fe(CO)5 (Kalyanaraman et al. 1998). FeðCOÞ5 ! FeðSÞ þ 5COðgÞ
ð1Þ
Within a few seconds, the argon flow is switched back to direct flow to the chamber instead of through the bubbler. The generated iron particles either remain trapped in the plasma, or fall on the lower electrode. The trapping of particles is thus being exploited in the current method. The fact that particles are generated and trapped in low-pressure plasmas is well known (Boufendi and Bouchoule 1994; Kortshagen 2009). The trapping occurs since the particles get negatively charged in a plasma. Because of the low flow rates involved and the low operating pressures in these processes, the main loss of species from within the plasma is diffusion controlled. Since the walls and electrodes in a plasma are at a lower potential than the plasma, negatively charged particles are trapped in the plasma. They
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remain trapped till they reach a size where the gravitational force is higher than the repulsion experienced by the particles. Keeping the argon plasma on to keep the particles trapped, hydrogen is introduced at 10 sccm to act as a reducing gas. After a 10 min treatment, hydrogen flow is shut off and acetylene (5 sccm flow) is introduced for 10 min. During the entire procedure the argon plasma is maintained to keep the particles trapped. Acetylene gets dissociated in the plasma resulting in formation of carbon coating on particles which is akin to a CVD process. The plasma is then shut off and the particles are collected on an aluminum foil kept on the lower electrode. Since the particles remain trapped as long as the plasma is on (and as long as the gravitational force pulling the particles down is lesser than the repelling electrical force from the electrodes), the particles can be further coated if arrangements for a different precursor can be made. In order to increase the yield of the nanoparticles, more iron pentacarbonyl can be added to the argon flow. However, it is difficult to meter iron pentacarbonyl from the bubbler since it is highly volatile. Even a slight increase in the time that argon flows through the bubbler leads to the formation of a layer on the bottom electrode which is not desired since it cannot be coated uniformly. In this method, the yield remains low but the uniformity in the product is preserved. Both coated and uncoated particles were characterized. Particle images were taken using a Philips CM 200 TEM operated at 200 kV and JEOL HRTEM JEM 2100F operated at 200 kV. Particles were dispersed in propanol and sonicated for 15 min. One drop of the solution was put on a TEM grid for observation under TEM and HRTEM. XPS was used to find chemical composition of the sample. The XPS instrument uses monochromatic Mg Ka radiation (1253.6 eV) at 200 W. A Quantum Design Physical Property Measurement System was used to study magnetic properties.
Results and discussion TEM images of both uncoated and coated nanoparticles obtained by this process are shown in Figs. 2a and 3a, respectively. Size distribution histograms of uncoated and coated nanoparticles are shown in
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Fig. 2 a TEM image of uncoated nanoparticles and inset shows lattice fringe pattern obtained by HRTEM. b Size distribution of nanoparticles
Figs. 2b and 3b, respectively. All the uncoated nanoparticles were in the size range 7–14 nm and the coated nanoparticles were in the size range 15–24 nm. The uncoated nanoparticles are linked together because of their magnetic property. The HRTEM image in the inset of Fig. 2 shows the lattice fringe pattern of the uncoated single nanoparticle indicating crystalline nature of the nanoparticles. The electron diffraction pattern obtained by HRTEM is shown in Fig. 4. The observed rings can be associated with iron oxide (FeO) (111), (200), and (220) planes with interplanar spacing of 0.25, 0.21, and 0.15 nm, respectively. There is no evidence of the presence of metallic iron in the nanopowder. In the TEM image of coated nanoparticles, core–shell structure can be easily seen. The inset figure (Fig. 3a) showed a magnified view of the coated nanoparticles. This protective shell on
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Fig. 4 Electron diffraction pattern obtained for the uncoated nanoparticles
Fig. 3 a TEM image of coated nanoparticles and inset shows magnified image. b Size distribution of nanoparticles
nanoparticles affects the magnetic properties of the nanoparticles. Figures 5 and 6 show the XPS spectra of uncoated and coated iron nanoparticles, respectively. Peak fitting of XPS spectra were performed using XPSpeak 4.1 software and the results were plotted in origin 8. A scan in the binding energy range of iron exhibited strong peaks at 710.6 and 724.6 eV corresponding to Fe 2p3/2 and Fe 2p1/2 in uncoated nanoparticles. These observed binding energies are almost the same as those of Fe3O4 without any peak in between Fe 2p3/2 and Fe 2p1/2 (Faiyas et al. 2010). This confirms the presence of Fe3O4 in the nanopowder. After the coating process, these peaks were observed at 710.9 and 724.3 eV, respectively. The shift from 710.6 to 710.9 eV suggests that the oxidized surface layer of the iron particles is bonding with the carbon atoms. Hence the
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binding energy shifts slightly toward the higher side. XPS probes the material up to 10 nm from the surface layer. Carbon coating thickness on the nanoparticles is around 2–5 nm hence the authors get iron peaks in coated nanoparticles with reduced counts. Carbon and oxygen scans showed the presence of C 1 s and O 1 s peaks in both coated and uncoated nanoparticles as shown in Figs. 5 and 6. C 1 s peak of uncoated nanoparticles can be deconvoluted into three peaks. The peak at 284.6 eV is the pure carbon peak (Liao et al. 1993) whereas the peaks at 285.4 and 289.2 eV can be attributed to C–C (Moncoffre et al. 1985) and C=O (Bou et al. 1991), respectively. Carbon and oxygen are present in the source compound used for iron, i.e., iron pentacarbonyl. Decomposition of iron pentacarbonyl takes place at a temperature above 200 °C and CO decomposition takes place by disproportionate reactions (Kouprine 2006). CO þ CO ! C þ CO2 and CO2 ! CO þ 1=2 O2
ð2Þ
Highly energetic electrons present in the plasma stimulate these reactions as mentioned earlier. Hence the presence of carbon and oxygen is inevitable in the nanopowder. Residual oxygen may also be present in the chamber because of degassing from chamber walls during the process. Besides this, oxygen content
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Fig. 5 XPS spectra of uncoated nanoparticles a iron scan, b carbon scan, and c oxygen scan
Fig. 6 XPS spectra of coated nanoparticles a iron scan, b carbon scan, and c oxygen scan
in both coated and uncoated particles is unavoidable because of exposure of particles to atmosphere before characterization.
O 1 s peak of uncoated nanoparticles contains peaks at 529.7 and 531.3 eV. The peak at 529.7 eV corresponds to Fe3O4 and the one at 531.3 corresponds to
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C=O as expected (Briggs and Seah 1993; Poleunis et al. 1995). The C 1 s peak of coated nanoparticles can be fitted to two peaks with binding energies 288.2 and 284.7 eV. The smaller peak with 288.2 eV energy corresponds to C=O while 284.7 eV is the main carbon peak (Liao et al. 1993). The O 1 s peak in coated nanoparticles is broader as compared to that in uncoated nanoparticles. This peak can be fitted to four peaks with binding energy values 530.0, 530.3, 531.5, and 532.6 eV. These are attributed to FeO, Fe3O4, C=O and C–O, respectively (Barr 1978; Grimal and Marcus 1992; Gardner et al. 1995; Jouan et al. 1993). Quantification of XPS peaks resulted in 49.5% carbon, 26.3% iron and 23.9% oxygen in uncoated iron nanoparticles. In the coated form, carbon content is 56%, iron content is 17%, and oxygen content is 26%. Carbon to iron ratio in uncoated nanoparticles is 2:1 and more than 3:1 in coated nanoparticles because of carbon coating on the nanoparticles. The carbon in the uncoated particles comes from the original precursor, i.e., iron pentacarbonyl. Hysteresis curves obtained using PPMS (Physical property measurement system) for uncoated and coated iron nanoparticles are shown in Figs. 7 and 8, respectively. Because of low yield of coated particles, a 10 9 10 mm2 size aluminum foil with coated particles deposited on it was used for PPMS analysis. Aluminum foil background signal was subtracted from magnetization data of the sample while plotting magnetization versus applied curve. The nanoparticles of ferromagnetic material exhibit unique magnetic
property called superparamagnetism, when the particle size is less than the single domain size. Both uncoated and the carbon-coated iron particles show a very narrow hysteresis loop at room temperature (Figs. 7 and 8), which demonstrate superparamagnetic behavior at room temperature. The hysteresis loop is not saturating at room temperature for the uncoated particles since the particles are not ferromagnetic, but super-paramagnetic in nature. However, the rate of increase in magnetization is considerably reduced and the magnetization appears to saturate. For coated particles, the magnetization does not saturate up to an applied field of 30 kOe (Fig. 8). For uncoated particles, the hysteresis loop at 10 K shows a ferromagnetic behavior with apparent Ms value close to 24 emu/g. The apparent Ms value varies from 24 emu/ g at 10 K to 21 emu/g at 300 K. Remnant magnetization (Mr) values are 8.4094, 2.920, and 0.5157 emu/g at 10 K, 150 K, and 300 K, respectively. The ratio of remnant to saturation magnetization at 10 K is 0.35 which is greater than 0.25 (or 25%) and hence the uncoated iron nanoparticles can be classified as ferromagnetic in nature at 10 K. However, at 150 K and 300 K the values of this ratio are 0.12 and 0.024, respectively, which implies that the uncoated nanoparticles are superparamagnetic at both 150 K and 300 K. As per Gangopadhayay et al. (1992), the single domain size for iron is around 20 nm. Hence the superparamagnetic behavior of the particles is expected since their size is lesser than 20 nm. In the case of coated particles, the magnetization reduces
Fig. 7 Magnetization versus applied field for uncoated nanoparticles at 300, 150, and 10 K
Fig. 8 Magnetization versus applied field for coated nanoparticles at 300, 150, and 10 K
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drastically and is very low at 1.182 emu/g (as compared to 24 emu/g for uncoated nanoparticles). This is because of low-iron content and diamagnetic contribution of carbon coating. Wilson et al. (2004) have also reported lower saturation magnetization because of lower iron content in polymer nanocomposites with embedded iron nanoparticles. Saturation magnetization was 11.5 emu/g for 10% iron content and 0.91 emu/g for 0.5% iron content in the nanocomposites. Jiao et al. (1996) have compared magnetic properties of carbon coated iron, cobalt, and nickel nanoparticles. Carbon-coated iron nanoparticles synthesized by them are in the size range 36–81 nm with an average diameter of 56 nm. These nanoparticles have saturation magnetization of 56.21 emu/g. Similarly, Wu et al. (2002) reported generation of SiO2coated iron nanoparticles of size 35 nm that showed higher saturation magnetization (69–165 emu/g) depending on calcination temperature during synthesis. This shows that size of the nanoparticles plays an important role in determining magnetic properties. Since size of these reported nanoparticles is greater than the single domain size, saturation magnetization exhibited by them is higher than that obtained by us. The 3d electrons in iron are responsible for magnetic property. These 3d electrons readily interact with carbon shell leading to their inability to reorient spin in response to an applied field (Huber 2005). When the particle size is very small (in the nanometer range) this effect is dominant. The shape of the hysteresis loop and low magnetization values (Fig. 8) suggest that coated nanoparticles are superparamagnetic at room temperature and weakly ferromagnetic at 10 K. In order to determine the blocking temperature (the transition temperature where the behavior changes from ferromagnetic to super-paramagnetic), the zerofield-cooled (ZFC) curve is plotted by cooling particles in the absence of external field and then slowly warming in a weak field. During the warming phase, the particles slowly get magnetized and the magnetization reaches a maximum at the blocking temperature. Beyond the blocking temperature the electronic spins have enough thermal energy to reorient randomly and hence magnetization decreases. The particles are again cooled in the same weak magnetic field up to 10 K which is plotted as the field cooled (FC) curve. Figure 9a and b plot the ZFC and FC curves for uncoated and coated particles, respectively. The blocking temperature in ZFC sensitively depends
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on the size of the nanoparticle, anisotropy, and interparticle interaction (Sahoo et al. 2004). The broad peak observed in ZFC curve (Fig. 9) is because of the size polydispersity and interparticle interaction. The blocking temperature of uncoated nanoparticles is around 190 K. He et al. (2006) reported that iron nanoparticles of average size below 5 nm showed a blocking temperature in the range 125–130 K. The nanoparticles in the present case are bigger in size (7–14 nm) which may be a probable cause for the higher blocking temperature. The blocking temperature for coated nanoparticles is in the range from 140 to 160 K. Thus, it was noticed that the carbon coating has reduced the blocking temperature of the nanoparticles. According to Kataby et al. (2002) lower blocking temperature is because of low spin states of iron atoms (when coated by various surfactants in their study). Relatively small thermal fluctuations are sufficient to overcome the weak exchange interaction between the iron atoms in low-spin states. Carbon along with oxygen is known to result in a low-spin state in iron 3d electrons (Zumdhal 2005). The carbon-shell interacts with the d electrons of iron and causes splitting of these d levels resulting in low-spin states. Similarly the coercive field in the case of coated particles is higher than that for the uncoated particles. Figure 10 shows a plot of coercivity at all the three temperatures, i.e., 10, 150, and 300 K for coated and uncoated nanoparticles. This plot indicates that at any particular temperature the coercivity of coated nanoparticles is higher than that for uncoated nanoparticles.
Fig. 9 ZFC-FC curves for a uncoated and b coated nanoparticles
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As long as the particles are trapped, they can be given multiple coatings by changing the precursors. At present, the authors are investigating the use of this method to generate particles with abrasive as well as magnetic properties.
Summary
Fig. 10 Coercivity versus temperature for uncoated and coated nanoparticles
These results show that coercivity of the particles is affected by the core–shell particle structure and can be attributed to the diamagnetic property of the carbon shell. Because of the diamagnetic property of the carbon coating in coated nanoparticles, the applied magnetic field is shielded by the carbon shell. Hence the apparent magnetic field at the core is less than the applied magnetic field in coated nanoparticles. Owing to this shielding, a slightly higher field has to be applied to reduced the remnant magnetization to zero in coated nanoparticles as compared to uncoated nanoparticles. This results in increased coercivity in coated nanoparticles. The temperature dependence of coercivity of the particles can be explained by the superparamagnetic behavior of the particles. From the ZFC curve, the blocking temperature for coated nanoparticles is in the range 140–160 K. Above 160 K, the coated particles become superparamagnetic and there is little change in coercivity value from 150 to 300 K. But in the 10 to 150 K range there is a sharp decrease in the value of coercivity from 1528 Oe to 283 Oe. The same pattern is observed for uncoated nanoparticles. Such coated magnetic particles have a variety of biological applications as mentioned earlier. The current process can essentially generate particles using an inert gas (e.g., argon) plasma along with the necessary precursors. The particles can be kept trapped by maintaining the plasma and shutting the precursor off. By changing precursors or adding separate gases to argon, the trapped particles can be coated by breaking down the new precursors and deposition on to the particles by a PECVD process.
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This article presents a method for the synthesis of core– shell carbon-coated nanoparticles using an RF capacitively coupled low-pressure argon plasma. The nanopowder obtained using iron pentacarbonyl can be trapped and coated with carbon by passing acetylene gas in the same process chamber in a continuous process. Nanoparticles obtained were of 7–14 nm in size in uncoated form and 15–24 nm in coated form. PPMS measurements have shown that the particles are ferromagnetic at lower temperature and superparamagnetic at room temperature. Carbon coating decreases the blocking temperature and increases the coercivity of the nanoparticles. With additional arrangements this process can be extended to give further coatings on the particles to achieve multiple functionalization. Acknowledgments This study has been supported by Department of Science and Technology (DST), Govt. of India (project no. 06-DS-002). The authors would like to acknowledge Sophisticated Analytical Instrument Facility, IIT Bombay, Central Surface Analytical Facility (ESCA) and PPMS Laboratory, Department of Physics, IIT Bombay, for characterization of samples. The authors would also like to thank Prof. Mathur from the Department of Chemistry, IIT Bombay, for providing us with Iron Pentacarbonyl and Prof. K. G. Suresh for the discussion on magnetic properties. The authors also would like to thank Prof. P.S. Gandhi for allowing access to the clean room facility of the Suman Mashruwala MicroEngineering Laboratory (Department of Mechanical Engineering, IIT Bombay).
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