NiOx Nanoparticle Synthesis by Chemical Vapor Deposition from Nickel Acetylacetonate

Ni/NiO nanoparticles were synthesized by metal organics chemical vapor deposition of nickel acetylacetonate in an externally heated tube flow reactor at moderate temperatures, up to 500 ̊C. Particle production and characteristics were studied by evaluating the effects of reactor temperature, precursor concentration, and flow rate through the reactor. In addition, two precursor decomposition methods were examined: thermal decomposition and reduction by hydrogen. Particle production was monitored with a scanning mobility particle sizer, and particle characteristics were studied using transmission electron microscopy, high resolution transmission electron microscopy, selected area electron diffraction, and energy dispersive spectroscopy. The presence of hydrogen in the reaction mixture influenced significantly both particle production and their characteristics.


Introduction
Nickel and nickel oxide nanoparticles show many unique optical, magnetic, electrical and chemical properties [1,2], and thus, these nanoparticles have great potential in applications such as ceramic materials, electronic components, sensors, magnetic data storage materials and catalysts [3,4].However, most physical and chemical properties depend on the size and shape of the nanoparticles [5].Moreover, nickel nanoparticles are unstable in air, and their surface can oxidize to NiO even at room temperature [6].
In recent years, several methods of Ni/NiO nanoparticle synthesis have been reported and, of course, each different method of synthesis imparts the final product with different properties.Liquid phase synthesis is one subset of methods for synthesizing Ni/NiO [7][8][9], but wet chemistry methods can be quite expensive, particularly in the steps that involve solid-liquid separation, washing and drying [10].
Ni/NiO nanoparticles also can be synthesized in the gas phase, and several variants of this method have been used for the preparation of nickel or nickel oxide nano-particles.In one case, Ni/NiO particles were synthesized by spray pyrolysis of water solutions of nickel precursors (e.g., nitrate, chloride, formate or acetate) but the resulting particles typically were in the submicron size range [11] or were polydisperse mixtures of nanosized particles, submicron-sized particles and, under some conditions, even supermicron-sized particles [12].However, Lenggoro et al. [3] developed conditions for synthesizing NiO nanoparticles with controlled morphology by using low-pressure spray pyrolysis with a filter expansion aerosol generator [3].Nickel nanoparticles also have been synthesized by a DC sputtering process in an argon atmosphere [13] or by a laser-assisted photonucleation process [1].Suh et al. synthesized nickel nanoparticles in a tubular furnace reactor reducing NiCl 2 with hydrogen (present in a carrier gas) [2].Another method for preparing Ni/NiO nanostructures is chemical vapor deposition (CVD) of metal organic precursors (MOCVD).MOCVD has a number of advantages over other methods: the process is relatively simple, it uses inexpensive equipment, and particle formation can be controlled by a variety of process parameters like reactor temperature, pre-x cursor concentration, or residence time in the reactor.An overview of Ni precursors used for CVD was presented by Brissonneau and Vahlas [14].Promising Ni precursors for MOCVD include nickelocene [15] or nickel acetylacetonate [16].
In this work, we studied Ni/NiO nanoparticle synthesis in an externally heated tube flow reactor at moderate temperatures using nickel acetylacetonate (NiAA) as a precursor and two different methods of precursor decompositions.The goals of this study were to identify an experimental set-up suitable for the production of nickel or nickel oxide particles with valuable properties for possible applications, and to investigate the influence of process parameters on the rate of particle production and particle characteristics.

Experimental
Particle production was studied by two different methods of NiAA decomposition processes (Figure 1) [17]: 1) thermal decomposition in an inert atmosphere (pyrolysis) reduction with hydrogen (present in the carrier gas) ( Experiments were performed using the apparatus shown in Figure 2. Particles were synthesized in an externally heated tube flow reactor 55 cm in length and an inner diameter (i.d.) of 2.7 cm.The inlet nozzle that introduced the reaction mixture was 15 cm in length and had an i.d. of 1.5 cm.Deoxygenated (1), dry (2) and particle-free nitrogen (F) was used as the carrier gas and was saturated with NiAA vapor in an externally heated saturator (S).The precursor concentration was controlled both by the flow rate through the saturator and by the saturator temperature (T S ).The partial pressure of the precursor vapor was calculated on the basis of the experimental data of Götze et al. [18] Saturated carrier gas was fed axially through a nozzle into the center of the reactor, which was surrounded by a coaxial stream of nitrogen.In reductive decomposition experiments, the stream was composed of a mixture of nitrogen and hydrogen.The mixture of gas and particles leaving the reactor was cooled in a diluter (D) by mixing it with a stream of nitrogen gas.The saturator-reactordiluter system was arranged vertically in this work.Flow rates of individual gas streams were controlled with electronic mass flow meters (Tesla 306 KA/RA), and the temperatures of the saturator and reactor were controlled with electronic temperature controllers (RLC T48).Particle production was monitored with a scanning mobility particle sizer (SMPS), which consisted of a TSI model 3080 electrostatic classifier (EC) and a TSI model 3025 condensation particle counter (CPC).Samples for particle characterization were deposited onto carbon-coated Cu grids by point-to-plate electrostatic precipitator or onto polytetrafluoroethylene (PTFE) or Ag filters.Samples for transmission electron microscopy (TEM) from particles deposited on filters were prepared by dispersion in an organic solvent.The resulting suspension was dropped onto a carbon-coated Cu grid.The morphology of particles was studied using two TEM instruments with different resolutions.Images from the JEOL 2010 were appropriate for characterizing overall morphology; images from the high resolution TEM (HR-

Sample
No.

T R [˚C] T S [˚C]
Q R [cm 3   Generally, particle production (mean particle size and number concentration) increases with increasing saturator temperature (T S ) (Figure 4).The particle concentration increased from 4.92 × 10 6 cm -3 at T S = 150˚C to 2.85 × 10 7 cm -3 at 180˚C.Values of GMD at these T S values were 32 and 156 nm, respectively, and the GSD values were 1.37 and 1.52, respectively.Figure 5 shows the  1.
x influences of the precursor decomposition method and the reactor temperature; the remaining conditions in these experiments were nearly identical (Table 1).It is evident that the rate of particle formation by reduction of NiAA at 400˚C (NiAA7) is much lower (N t = 7.48 × 10 6 cm −3 , GMD = 26 nm, GSD = 1.38) than that at 500˚C (NiAA8, N t = 2.96 × 10 7 cm −3 , GMD = 59 nm, GSD = 1.58).At 500˚C, the mean particle size for reductive precursor decomposition (NiAA8) was smaller than those for thermal decomposition (NiAA4, N t = 2.58 × 10 7 cm −3 , GMD = 185 nm, GSD = 1.51) (Figure 5).The other parameters in Table 1, i.e. reactor flow rate and/or hydrogen concentration variation from 7 to 10 vol%, had not remarkable effect on particle production.

Pyrolysis
The morphology of particles prepared by pyrolysis of NiAA can be seen in Figure 6.The particles have shell-like structures, the mean size of the primary particles is well below 50 nm and they are arranged into clusters and/or chains.The samples (NiAAF1, NiAAF2) deposited on PTFE filter are dark in color, suggesting that they may be contaminated by carbon from incomplete decomposition of the precursor.Formation of carboncoated Ni nanoparticles by thermal decomposition of solid nickel acetate in a closed Swagelok reactor was observed by Pol et al. [19].Electron diffraction pattern

Reduction of NiAA
The morphology of particles obtained by reduction differs significantly from those produced by thermal decomposition and depends also on the reactor temperature.
At T R = 400˚C, primary particles 10 -15 nm in size were agglomerated into clusters and/or chains, and they had Copyright © 2011 SciRes.MSA x also faintly shell-like structures (Figure 8).Particles produced at 500˚C had slightly broader size distribution (from 10 nm to ca.50 nm) and samples consisted of separated nanoparticles of various sizes from the aforementioned range as well as from clusters of small nanoparticles (~10 nm) (Figure 9).Several samples of particles prepared by reduction were also analyzed using HRTEM, which enabled detection of the lattice fringes of several of Ni/NiO x crystalline structures.Figures 10(a-c Comparing lattice fringe images and SAED patterns, we can conclude that the particles consist of a metallic Ni core and a thin surface layer composed of NiO/NiOOH crystalline structures.These observations are in agreement with those of Uchikoshi et al. [20].Ni core and NiO shell nanoparticles produced by DC sputtering were reported by Rellinghaus et al. [13].

Discussion
From a comparison of TEM images with PSD curves, it is evident that SMPS detected already agglomerated primary particles.The size of primary particles on TEM images is much smaller than indicated by the PSD curves in Figure 5.In spite of this, SMPS still provides useful

5).
And particles prepared by reduction of NiAA at 500˚C (Figure 9) have slightly broader size distribution than those prepared by pyrolysis (Figure 6), but they are less aggregated, which corresponds to the much smaller mean particle size indicated by PSD curve NiAA8 in Figure 5.
The shell-like structure of particles seen in samples NiAA4 (Figure 6), and somewhat in NiAA7 (Figure 8) seems to be formed from incomplete decomposition of the precursor by pyrolysis T R = 500˚C, and also by reduction at T R = 400˚C, respectively.This incomplete decomposition is suggested by the fact that deposits of particles from pyrolysis on the PTFE filter were dark in color, while those from reduction at 500˚C were silver in color.Furthermore, it seems that the shell layer also protects particles against oxidation.The O/Ni ratio was lower in the samples of particles with shell-like structures: 0.38 -1.86 in sample NiAA4 (pyrolysis) and 0.89 in sample NiAA7 (reduction, 400˚C), while in the particles without a shell (NiAA8; reduction, 500˚C), it was 3.0.However, the O/Ni ratio also depends on particle size.The lowest value of the O/Ni ratio (0.23) was detected in spot analysis of the large particle in sample NiAA8, and this low value can be attributed to the small surface-to-volume ratio of the relatively large particle.

Conclusions
Ni/NiO x nanoparticles were synthesized by pyrolysis and reduction of NiAA in an externally heated tube flow reactor.Particle production was observed at T R = 400˚C and above, and depends mainly on T R , P NiAA and the type of precursor decomposition method.The size range of primary particles varied from ~10 nm to ~50 nm, and they typically were somewhat agglomerated.Particles prepared by pyrolysis had shell-like structure, and were aggregated into clusters and/or branched chains.EDP consists predominantly of rings or dots characteristic of FCC Ni, while HRTEM lattice images showed lattice fringes of both FCC Ni and NiO, and also of NiOOH.Results obtained by SAED are in qualitative agreement with those obtained by EDS (O to Ni ratio).
Particles produced by reduction differ significantly in morphology, crystallinity and composition.These particles had slightly broader size distribution, they were less agglomerated and the shell-like structure was almost nonexistent.EDPs vary with particle size.SAED, EDS and HRTEM lattice fringes images suggest that particles consist of a metallic core with a surface layer composed of various oxide forms of Ni (NiO, NiOOH).

Figure 1 .
Figure 1.Scheme of the inlet arrangements for used decomposition techniques of NiAA precursor.
Particle generation was monitored by SMPS in the form of particle size distribution (PSD) curves, including the following statistics: total number concentration (N t ), geometric mean diameter (GMD), geometric standard deviation (GSD), etc.Several examples of PSD curves are shown in Figure4and Figure5.dN/dlogd P represents differential number concentration, normalized to one decade of particle size.The influence of reactor temperature on particle production was studied for reductive decomposition of NiAA.Particle generation was observed at temperatures of 400˚C and higher.No particles were detected at T R = 300˚C.For thermal decomposition, experiments were performed only at T R = 500˚C.

Figure 3 .
Figure 3. Axial temperature profile in the axis of the reactor for T R set to 500˚C.

Figure 6 .
Figure 6.Bright field TEM image and SAED pattern of particles prepared by pyrolysis of NiAA, sample NiAA4: T R = 500˚C, T S = 180˚C, Q R = 900 cm 3 /min.

Figure 7
shows HRTEM images of particles of sample NiAAF1.We can see lattice fringes d 111 of FCC Ni (4-0850), see area A and corresponding fast Fourier transformation (FFT) pattern in Figure 7(a) and d 105 of hexagonal NiOOH (6-0075) (see area B and corresponding FFT in Figure 7(a).The crystalline structure of hexagonal NiOOH (6-0075) and FCC NiO (22-1189) can be seen in Figure 7(b).As inset, there is FFT pattern of cubic NiO crystalline structure along [-300]c zone axis.EDS analysis (INCA Oxford) was applied to the samples NiAAF1 and NiAAF2, deposited on filters.The diameter of the spot area varied from ~5 nm to ~30 nm, and O/Ni (atomic%) ratios of 0.38 to 1.86 were obtained.Because the sensitivity of EDS to individual elements of the periodic table depends on the atomic number, the values of the O/Ni ratio do not reflect stoichiometry.Furthermore, the O/Ni ratio could be affected (increased) during the preparation of TEM samples from the particles deposited on filters.

9 . 9 .
The shell-like structure almost disappeared.Particles produced at 400˚C were amorphous, while those synthesized at 500˚C were crystalline.Two EDPs taken by the JEOL 2010 are shown in Figure 9.The EDP of the cluster of nanoparticles is visible on the left-hand side of Figure The innermost ring corresponds to d 111 of FCC NiO (0.241 nm), the third to d 220 FCC NiO (0.145 nm), while the middle resolvable ring (0.206 nm) can be attributed both to d 111 FCC Ni and to d 200 FCC NiO, so it is probable that both crystalline structures are present in the sample.The EDP of a single crystal of FCC Ni from approximately 50 nm nanoparticle is apparent on the right-hand side of Figure Results obtained by SAED are in qualitative agreement with those obtained by EDS.EDS analyses showed an oxygen to nickel ratio (atom%) of 3.0 in the cluster of nanoparticles from Figure 9(a), while the ratio was only 0.23 in the large particle of Figure 9(b).The value of O/Ni ratio in the sample NiAA7 (400˚C) was 0.89.
) show the lattice fringes of cubic NiO (4-0835) with an interplanar spacing of d 200 = 0.208 nm, as well as lattice fringes of cubic Ni (4-0850) with an interplanar distance of d 111 = 0.203 nm and the hexagonal lattice structure of NiOOH (6-0075), confirmed by FFT pattern along the [006]c zone axis.A detailed analysis of the crystalline particle of the sample NiAA10 (Figure 10(d)) shows FCC Ni (4-0850) in the bulk and a NiO/NiOOH crystalline structures on the surface.As inset, there is a FFT pattern of a typical cubic Ni along the [02-2] axis.

Figure 8 .Figure 9 .Figure 10 .
Figure 8. Brigth field TEM image of particles produced by reduction of NiAA, sample NiAA7: T R = 400˚C, T S = 180˚C, Q R = 800 cm 3 /min, c H = 10 vol%.information about the particle formation process.The curves in Figure 5 are in good qualitative agreement with the particle morphology shown in the TEM images.Particles synthesized by reduction at 400˚C (Figure 8) are the smallest, and their clusters are much smaller than those of the other samples (PSD curve NiAA7 in Figure