Specific Solvent Produces Specific Phase Ni Nanoparticles: A Pulsed

Article pubs.acs.org/JPCC

Specific Solvent Produces Specific Phase Ni Nanoparticles: A Pulsed Laser Ablation in Solvents Hyeon Jin Jung and Myong Yong Choi* Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Republic of Korea S Supporting Information *

ABSTRACT: In this paper, we present a simple and controllable preparation of face centered cubic (fcc) and hexagonal close-packed (hcp) Ni nanoparticles by a pulsed Nd:YAG laser ablation method in the following four solvents: deionized water, methanol, hexane, and acetonitrile. We generated Ni/NiO, fcc, and/or hcp Ni nanoparticles by primary ablation to a Ni plate submerged in various solvents, followed by secondary ablation to the colloidal solutions. Interestingly, the phases of Ni nanoparticles prepared via a pulsed laser ablation in liquid (PLAL) show a strong dependence on the solvents used in the ablation processes. Ni/NiO, pure fcc, and a mixture of fcc and hcp Ni nanoparticles were generated in DI water, methanol, and hexane or acetonitrile, respectively. After secondary ablation, however, pure fcc Ni nanoparticles were generated in methanol and hexane, while pure hcp Ni was formed in acetonitrile. We think that the solvent dependence on the phase of Ni nanocrystals is related to the specific heat of solvents which plays an important role kinetically and thermodynamically in the process of cooling the plasma plume where the nanoparticles nucleate and coalesce to a specific phase. The Ni nanoparticles prepared from PLAL were analyzed by X-ray diffraction (XRD) measurement, X-ray photoelectron spectroscopy (XPS), field emission-scanning electron microscopy (FE-SEM), high resolutiontransmission electron microscopy (HR-TEM), selected area electron diffraction (SAED), and fast Fourier transform (FFT) analysis.

1. INTRODUCTION In recent years, synthesizing nanoscale materials for enhanced physical properties and promising applications to technologies has attracted tremendous research interest.1 However, up until now synthesizing nanoparticles with different crystalline structure in a controlled way has been a daunting task. Furthermore, there are still surprises in characterizing the Ni nanoparticles that have three crystal structures: face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal closepacked (hcp). Among these three, fcc Ni is most stable thermodynamically, while bcc and hcp structures are in their metastable phases. Thus, the Ni nanostructures have been synthesized and characterized to be mostly in the most stable phase, fcc. However, recent studies of metastable nanostructures have drawn considerable attention due to their unique properties, such as magnetism,2−4 catalysis,5,6 and potential applications in magnetic data storage,7 biomedical applications,8 hydrogen storages,6,9 and microwave absorption.2 Thus, with an effort under specific conditions, the hcp Ni nanoparticles have recently been fabricated.10−12 Unfortunately, the assignment of the hcp Ni nanoparticles synthesized from a number of different methods was ambiguous due to the following two complications. First, the similarity of the powder X-ray diffraction (XRD) patterns for hcp Ni and Ni3C nanoparticles © 2014 American Chemical Society

makes the assignment of both nanostructures very difficult. Second, the magnetism of hcp Ni nanoparticles prepared in different synthetic conditions varies from nonmagnetic to ferromagnetic similar to that of Ni3C nanoparticles.3,13 The previously presented experiments11 for the synthesis of hcp Ni nanoparticles were mostly hydrothermal methods with the hydrocarbon solvents, which provide the carbon source in Ni3C nanoparticles. This approach would lead to the use of expensive vacuum-based synthesis employed to simplify the interpretation of the assignments of hcp Ni and Ni3C nanoparticles.11 Thus, the synthesis of a simple method for the preparation of hcp Ni nanoparticles in solution phase is needed. In this paper, we present a selective preparation of specific phase Ni nanoparticles via a simple and clean preparation method. This method is called pulsed laser ablation in liquid (PLAL), and it is used to overcome or bypass the difficulties of current methods for synthesizing the hcp Ni nanoparticles. As a result the fcc and hcp Ni allotropes have been selectively prepared via PLAL depending on the solvents used in the PLAL process. Received: March 26, 2014 Revised: May 30, 2014 Published: June 5, 2014 14647

dx.doi.org/10.1021/jp503009a | J. Phys. Chem. C 2014, 118, 14647−14654

The Journal of Physical Chemistry C

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2. EXPERIMENTAL METHODS 2.1. PLAL Conditions. A Q-switched nanosecond laser (1064 nm, 10 Hz, 7 ns) was focused onto a solid Ni (99.98%, Sigma-Aldrich) plate immersed in the following four solvents: deionized (DI) water, methanol, hexane, and acetonitrile (Daejung, HPLC Solvent). Typical experimental schemes are well illustrated elsewhere.14−16 Briefly, the Ni plate was placed centered in a 20 mL Pyrex vial which was filled with 10 mL of one of these solvents with vigorous stirring. The Ni plate was continuously moved by a stepper motor that was controlled by a LabView program. In this work, we have used two PLAL steps to produce Ni nanoparticles. In the first step (we call it “primary ablation”), a typical PLAL involves using a lens with a focal length of 30 mm to focus a pulsed Nd:YAG laser about a 1 mm spot on the surface of a Ni metal plate. The laser ablation lasted for 20 min with a laser pulse energy of 80 mJ pulse−1. In the second step (we call it “secondary ablation”), the ablated colloidal solution, in the absence of the Ni plate, was ablated again for another 20 min with the same laser pulse energy used in the primary ablation. 2.2. Sample Preparation and Characterization. The resulting primary and secondary ablated colloidal solutions were collected at a centrifugation rate of 15 685 relative centrifugal force (rcf) for 10 min. The resulting sediments were first separated, then sonicated after addition of corresponding solvents, and later centrifuged. This procedure was repeated a number of times. The sediments were finally collected and dried on a silicon substrate or a copper grid at room temperature. The morphological and structural information on the nanoparticles produced by PLAL were obtained by using a field emission-scanning electron microscope (FE-SEM, XL30 S FEG, Philips), a transmission electron microscope (TEM, 200 kV, JEM2010, Jeol), a high-resolution-transmission electron microscope (HR-TEM, 300 kV, TECNAI, TF30ST), and fast Fourier transform (FFT) analysis (GATAN, Inc.). The X-ray diffraction (XRD) patterns of the nanoparticles were obtained by using a Bruker AXS D8 DISCOVER with a GADDS diffractometer using Cu Ka (0.1542 nm) radiation with the Bragg angle ranging from 10° to 90°. X-ray photoelectron spectroscope (XPS) spectra were measured by using a Thermo Fisher Scientific (U.K.) ESCALAB 250 XPS System with a hemispherical electron analyzer. Al Kα radiation (hν = 1486.6 eV) was used as an excitation source. The Ni nanoparticles were distributed onto a silicon substrate and subjected to the XPS measurements. The binding energy was calibrated based on the assumption that the C 1s binding energy for the contaminant carbon is 284.8 eV.

Figure 1. XRD patterns of the fcc Ni, hcp Ni, and NiO nanoparticles produced by PLA in the following four solvents: (a) DI water, (b) methanol, (c) hexane, and (d) acetonitrile. The origin of the bands marked with an asterisk is not clearly known at this time. Key: fcc Ni (circles), hcp Ni (squares), and NiO (triangles).

No.01−087−0712). In methanol (Figure 1b), on the other hand, (110), (200), and (220) planes of pure fcc Ni were observed. Samples c and d in Figure 1 consist of the mixture of fcc and hcp Ni nanoparticles. In hexane (Figure 1c), the intensity of diffraction peaks for fcc structures is relatively stronger than that of hcp (JCPDS card No.01−089−7129), while the opposite is true in acetonitrile (Figure 1d). These results indicate that the synthesized nanoparticles show a strong dependence on the solvents used in the PLAL process, where solvents could have played a certain role in the formation of nanoparticles. Here it is worth noting that among various parameters in PLAL assisted synthesistarget materials, temperature and/or pressure of solvent,17 surfactants, wavelength of incident light,18,19 pulsed width of lasers,20−22 repetition rate of lasers and solventsolvent dependence experiments have rarely been investigated.23 As mentioned before, the XRD patterns for hcp Ni differ only subtly from those of Ni3C (JCPDS card No.00−072− 1467). Thus, the XRD patterns obtained from the PLA of Ni in hexane and acetonitrile are similar to the simulated XRD patterns of hcp Ni and Ni3C, which is shown in Figure S1 (Supporting Information). In Figure S1 in the Supporting Information, the (100) and (101) planes of hcp Ni are nearly identical to the (110) and (113) planes of Ni3C; however, the experimental (002) plane of hcp Ni at near 42° somewhat appears to the lower angle of 2θ from the (006) plane of Ni3C. However, it is still ambiguous to distinguish hcp Ni from Ni3C by purely relying on the XRD data of the nanoparticles. It is known that Ni3C is stable below 430 °C under atmosphere24,25 or 500 °C under N2 environment,3 whereas, hcp Ni can be easily transformed to fcc Ni at lower temperatures (below 300 °C).26 To observe any crystal phase transformations, we annealed the samples. Figure 2a,b presents the XRD patterns of the Ni nanoparticles prepared in hexane at various temperatures under vacuum and atmosphere, respectively. The XRD patterns of the samples prepared in acetonitrile are shown in Figure S2, Supporting Information. The peaks, marked with circles and squares, are attributed to the fcc Ni and hcp Ni nanoparticles, respectively. Figure 2a shows that the XRD patterns of the samples begin to change at ∼200 °C. The XRD peaks of hcp Ni completely disappeared at ≤300 °C, showing a full phase transition to fcc Ni. The

3. RESULTS AND DISCUSSION 3.1. Primary Ablation onto the Ni Plate. Figure 1 presents the XRD patterns of samples a−d fabricated from PLA onto the Ni plate surface in DI water, methanol, hexane, and acetonitrile, respectively. According to the diffraction peaks, the synthesized nanoparticles depend strongly on the solvents used in the PLAL process. Figure 1 indicates that the nanoparticles fabricated by this simple one-pot synthesis consist of mixed NiO/fcc, pure fcc, or mixed fcc/hcp phase depending on the solvents. In DI water (Figure 1a), the diffraction peaks at 2θ of 37.2, 43.2, 62.85, 76.3, and 79.55 can be indexed as the (111), (200), (220), (311), and (222) planes of NiO (JCPDS card No. 01−078−0429) and 44.35, 51.8, and 76.3 can be indexed as the (111), (200), and (220) planes of fcc Ni (JCPDS card 14648



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Figure 3. XPS spectra of the nanoparticles produced in the solvents DI water (D), methanol (M), hexane (H), and acetonitrile (A): (a) full spectra, (b) Ni 2p core level spectra, and (c) C 1s core level spectra.

Figure 2. Temperature-dependent XRD patterns of the Ni nanoparticles prepared in hexane under (a) vacuum (50−500 °C) and (b) atmosphere (100−700 °C). Key: fcc Ni (circles), hcp Ni (squares), and NiO (triangles).

eV was previously assigned to sp3 CN bonds,28 confirming that CN bonds are present in the nanoshells prepared in acetonitrile. The carbide C 1s component at 283 eV assigned to Ni3C is clearly missing in the spectra.29,30 Based on the XPS analysis, the synthesized Ni nanoparticles via PLA in various solvents were clearly lacking the NiC bond. Furthermore, possible growth of Ni3C on the surface of the Ni nanoparticles as a core−shell (Ni/Ni3C)31 is also less likely, since the XPS technique is very accurate for obtaining information about the surface due to the small depth (1−2 nm) of the photoelectrons. The size and microstructure of the samples obtained in DI water and methanol were examined by using SEM, TEM, HRTEM, and FFT analysis as shown in the two sections of Figure 4a with DI water and Figure 4b with methanol. The typical SEM and TEM images shown in insets (i) and (ii) in Figure 4a,b indicate that all the particles are spherical. In the HR-TEM image of Figure 4a, the interplanar distances of the small nanoparticles and outer shell of larger nanoparticles were determined to be 2.41 and 2.09 Å, which agrees with the NiO (111) and (200) planes, respectively. Figure 4a presents a highmagnification TEM image for a larger nanoparticle (≳10 nm) that reveals clearly a core−shell structure. The FFT analysis (see inset (iii)) of the HR-TEM image is identical with the XRD patterns presented in Figure 1a. The diffraction spots circled with solid and dotted lines correspond to the NiO (200) and (111) lattice planes, respectively. From the morphological analysis of SEM images (see insets (i)), the surface of the sample (a) appears rough and uneven, while that of the sample (b) has a rather smooth surface. This morphological analysis may suggest a postgrowth process of the NiO shell over the Ni nanocrystalline core in an oxygen-rich environment, e.g., DI water. The magnified HR-TEM image in Figure 4b shows that the lattice spacing of sample (b) is 2.06 Å, which corresponds to the (111) planes of fcc Ni. The FFT patterns of the HRTEM (see inset (iii)) from the selected region marked by a circle in Figure 1b are also consistent with the phase assignment of XRD measurements for fcc Ni (111). Selected area electron diffraction (SAED) patterns (see inset (iv)) marked by a square in Figure 1b confirm the presence of nanocrystalline cubic polycrystalline structures, such as fcc Ni (111), (200), and (220) planes. Figure 5 shows the SEM, HR-TEM images, and SAED patterns for the two samples prepared in hexane and

transformation behavior of the samples under atmosphere, shown in Figure 2b, is very similar to Figure 2a. However, the XRD peaks of NiO (marked with a triangle) appeared at ∼300 °C due to the fact that the fcc Ni nanoparticles were oxidized by oxygen, and its full phase transition to NiO was completed at ∼500 °C. Thus, from the XRD patterns via the annealing analysis, the phases of samples prepared by PLAL in hexane and acetonitrile are a mixture of hcp and fcc Ni nanocrystals, excluding the possibility of being Ni 3C nanoparticles. Furthermore, recently, Bharathan et al. determined that the transition temperature of hcp Ni to NiO under atmosphere was lower than that of fcc Ni,26 in good agreement with our results. To verify this, the sample prepared in methanol, consisting of pure fcc Ni, was annealed up to 700 °C, shown in Figure S3 (Supporting Information). The XRD patterns of NiO appeared at ∼400 °C and those of fcc Ni disappeared at ∼700 °C; while those for the temperature range for hcp Ni were 300−500 °C, as described before. Thus, the results further support the presence of hcp Ni nanoparticles from the synthesis of PLAL. The surface composition and interfacial structure of the samples showing peaks for Ni, O, and C, prepared via PLA in DI water (D), methanol (M), and hexane (H) were further confirmed by XPS spectra, as shown in Figure 3a. In addition, there is a peak at 398.4 eV, assigned to the binding energy of N 1s, from the sample of acetonitrile (A) in Figure 3a. This peak confirms that the surface composition of the nanoparticles synthesized in acetonitrile has a contribution of the N atoms. The peaks shown in Figure S4 (Supporting Information) were deconvoluted into two Gaussian line-shaped peaks centered at 396.9 and 398.4 eV assigned to sp3 CN and sp2 CN bonds, respectively.27,28 Figure 3b shows an expanded view of the peaks at 853.0 and 870.4 eV, attributed to the binding energy of Ni 2p3/2 and Ni 2p1/2, respectively. Figure 3c shows the C 1s core level XPS spectra of the nanoparticles synthesized in various solvents. The peak at 285.5 eV was assigned to the graphite-like or amorphous carbons, including sp3 CC and sp2 CC.28 From this behavior, it is immediately apparent that the bandwidth of the C 1s peak obtained from the sample of acetonitrile is larger than the other. Thus, the peak was deconvoluted into two peaks at 285.5 and 284.3 eV, of which the latter was assigned to sp2 CN bonds.28 The peak at 288.2 14649



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Figure 4. HR-TEM images of the nanoparticles prepared in (a) DI water and (b) methanol (scale bar =5 nm). Insets show the corresponding (i) SEM, (ii) TEM, and (iii, iv) SAED patterns (scale bar = 20 nm).

Figure 5. HR-TEM images of the nanoparticles prepared in (a) hexane and (b) acetonitrile (scale bar = 5 nm). Insets show the corresponding (i) SEM, (ii) TEM, and (iii−vi) SAED patterns (scale bar = 20 nm). The interlayer distance of the layers in a square is measured to be 3.35 Å, in agreement with a plane distance of the graphite (002) planes.

and causes nucleation at the surface of hot nanoparticles in the plasma plume. This nucleation consequently could make nanoshells of graphite or nitrogen-doped graphite. A more thorough study of the formation mechanism of graphite-like spheres by PLAL is underway; however, it is beyond the scope of the present study. 3.2. Secondary Ablation of Colloidal Solution Prepared from Primary Ablation. Figure 6 presents the representative TEM images of the nanoparticles generated from the primary (a and c) and secondary (b and d) ablation in methanol and acetonitrile, respectively. Primary ablation was achieved by ablating a metal plate that is submerged in the corresponding solvents, followed by secondary ablation to the colloidal solution prepared from primary ablation. The size distributions of the corresponding nanoparticles were then statistically analyzed by measuring ∼400 nanoparticles, as shown in the insets of Figure 6. Figure 6 shows the typical distribution histograms fitted with log-normal (red line) confirm that the mean size of the nanoparticles generated from secondary ablation became smaller: from 8.68 ± 0.44 to 7.16 ± 0.17 nm for the nanoparticles prepared in methanol and from 6.32 ± 0.15 to 5.32 ± 0.49 nm in acetonitrile. We think that this size reduction arises from the further ablation, resulting in fragmentation of the nanoparticles formed in primary ablation and renucleate to smaller sizes. Indeed, this size reduction has recently been reported with the reult that additional ablation of laser light selectively heats and vaporizes the larger nanoparticles and the subsequent rapid cooling of the vaporized nanoparticles causes the formation of smaller nanoparticles.36−39 This size reduction was well presented in

acetonitrile, panels a and b, respectively. As with Figure 4, the nanoparticles prepared in both solutions have a spherical morphology, as shown in the insets (SEM (i) and TEM (ii) in Figure 5a and 5b). In the HR-TEM images of Figure 5a,b, the observed lattice pattern of the particles has a spacing of 2.08 Å, which corresponds to fcc Ni (111) planes. However, the observed lattice distances of 1.99 and 2.15 Å in Figure 5a,b correspond to hcp Ni (101) and (002) planes. These data are consistent with the XRD observations in Figure 1c,d. The FFT analysis shown in the insets (iii) and (iv) of Figure 5a shows the diffraction spots of fcc Ni (111) and hcp Ni (101) planes, respectively. Similarly, the FFT diffraction spots shown in the insets (iii), (iv), and (v) in Figure 5b correspond to the lattice planes of fcc Ni (111), hcp Ni (101), and hcp Ni (002), respectively. The FFT pattern shown in inset (vi) of Figure 5b suggests that the particles are hexagonal polycrystalline structures indexed as (101) and (002) planes of hcp Ni. According to the HR-TEM images in Figure 5, the Ni nanoparticles are embedded in some layers, marked by squares. As shown in the squares in Figure 5, the interlayer distance of the nanoshells, 3.35 Å, is in agreement with a plane distance of the graphite (002) planes. The production of carbon/graphite encapsulated metal nanoparticles via PLAL has recently been investigated.32−35 Based on the investigation, we think a probable cause producing the carbon/graphite encapsulated metal nanoparticles is as follows. Briefly, highly focused laser beams on the Ni plate in hexane and acetonitrile result in hightemperature and high-pressure conditions, which produces decomposition of solvent molecules into carbon or nitrogen atoms. Further irradiation evaporates carbon/nitrogen atoms 14650



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Figure 6. TEM images of the primary (a and c) and secondary ablated (b and d) Ni nanoparticles in methanol (a and b) and acetonitrile (c and d) (scale bar = 40 nm). The corresponding size distribution histograms and mean sizes obtained from the log−normal fit curves (red lines) are shown in the insets.

the previous study of the synthesis of TiO2 nanoparticles.40 Furthermore, we observed that the mean sizes of the nanoparticles prepared in different solvents showed solvent dependences and decreased in the following order: methanol, hexane, and acetonitrile, as shown in Table 1. It is probable that this can be related to the cooling efficiency of solvents, as discussed later in more detail. A very interesting phenomenon was observed after secondary ablation to the colloidal solutions obtained from primary ablation. Figure 7a−d presents the XRD patterns of the nanoparticles synthesized via secondary ablation to the colloidal solutions prepared from primary ablation of DI water, Table 1. Mean Size and Crystal Phase of the Ni Nanocrystals Prepared in Different Solvents: Methanol, Hexane, and Acetonitrile after Primary and Secondary Ablation primary ablation solvent

mean size, nm

crystal phase

methanol hexane acetonitrile

8.68 ± 0.44 7.08 ± 0.17 6.32 ± 0.15

fcc fcc > hcp fcc < hcp

Figure 7. XRD patterns of the nanoparticles synthesized after secondary ablation of the colloidal solutions prepared from primary ablation in (a) DI water, (b) methanol, (c) hexane, and (d) acetonitrile. Key: fcc Ni (circles), hcp Ni (squares), and NiO (triangles).

secondary ablation mean size, nm crystal phase 7.16 ± 0.17 6.62 ± 0.32 5.32 ± 0.49

fcc fcc hcp 14651



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of fcc and hcp Ni colloids in acetonitrile. Thus, the fast cooling time can be related to the specific heat of solvents used in the PLA solvents, whose values are decreasing in the following order: DI water (4.2 J/(g·K)), methanol (2.5 J/(g·K)), hexane (2.3 J/(g·K)), and acetonitrile (2.2 J/(g·K)). Accordingly, the laser-induced plasma plume generated by PLAL will rapidly be quenched in the confining liquid in the following order: acetonitrile, hexane, methanol, and DI water, in favor of forming metastable hcp Ni nanocrystals in solvents with a lower specific heat. The thermodynamic and kinetic factors of PLAL can both influence the phase transformation based on the understandings of the evolution of the laser-induced plasma plume mentioned above. Another important part of our work is the thermodynamic aspect of forming different phases of Ni nanocrystals via PLAL. Three important thermodynamic parametersdensity of ablated species, temperature, and pressureof the laserinduced plasma plume could contribute to the phase transformations. The density of the ablated species is inversely proportional to the expansion volume of the laser-induced plasma plume, which is related to the specific heat of solvents. Given that a solvent with lower specific heat produces a plasma plume with a larger volume, the density of the ablated species in the plasma plume decreased as expected, in the following order: DI water, methanol, hexane, and acetonitrile. For example, in the case of acetonitrile, having the lowest specific heat among the solvents used in this work, the adiabatic expansion of the laser-induced plasma cools the confined volume quickly, as previously mentioned, forming metastable hcp Ni nanocrystals. According to the discussion above, the size of Ni nanoparticles depends commonly on the density of ablated species during the nucleation and growth processes and on the temperature. This is in agreement with the previous results that the size of hcp Ni nanostructures is smaller than that of fcc Ni,4,11,12 as shown in Table 1. Thus, as this study shows, the stable phase (fcc) of Ni nanocrystals is likely to form at higher density of ablated species. However, a more detailed study of phase transformation in the Ni nanocrytals depending on the solvent used in PLAL still needs to be investigated.

methanol, hexane, and acetonitrile, respectively. The XRD patterns of samples obtained from DI water and methanol were unchanged after secondary ablation. However, the phase of the Ni nanoparticles prepared from secondary ablation in hexane and acetonitrile was selectively transformed to pure fcc and hcp structure, respectively. Given that the only modification in the process is the change of solvents, the XRD patterns clearly provide conclusive evidence for a specific phase transformation of Ni nanocrystals due to solvents. Until now, there have been a few reports on the specific phase transformation of Ni nanoparticles using surfactants. For example, pure hcp Ni nanoparticles have been synthesized with protective agents, such as PCP or PEG, where the protective agents function as capping agents for inhibiting further growth of Ni nucleus.41 However, this paper is the first to report on the generation of specific phase transformation of Ni nanocrystals using different solvents in PLAL. Unfortunately, the formation mechanism of a specific crystalline phase with solvent dependences is not fully understood and certainly needs to be investigated. It is generally thought that the growth temperature and time are important parameters for the control of the structure and shape of the nanocrystals.42 For the case of fcc and hcp cobalt nanocrystals, Park et al. showed that the structural change of cobalt nanoparticles is a delicate balance between kinetic and thermodynamic growth time.43 For the PLA process in liquid, it is generally thought that the nucleation and growth of nanocrystals in the plasma plume are achieved in a short quenching time when short pulsed lasers are employed.15,16,44−48 Consistent with these studies, in our previous paper,49 we found direct evidence for the production of Al3+ ions out of the plasma plume from the aluminum plate for the first time, suggesting the production of nucleation ions in the first phase (plasma plume) and the growth of aluminum and alumina nanoparticles in the second phase (post plasma plume). The experimental results and discussions mentioned above support a phenomenon that the cooling of the plasma plume experienced by Ni nanoparticles is sufficiently different from that in different solvents to give rise to quite different phase Ni nanoparticles. During the cycle of ultrasonic and adiabatic expansion and cooling of the plasma plume, specific heat of solvents could contribute to the growth of nuclei and coalescence of nanocrytals. Specifically, the laser-induced plasma plume of high temperature and pressure is uniquely produced at the liquid−solid interface. Subsequently, the plasma plume condensates at different cooling rates in the confining solvents due to the confined effect of liquid to induce Ni nanocrystals with different phases. It is likely that the thermodynamic state with high temperature (∼6000 K) and pressure (∼10 GPa) is obviously favorable for the formation of the metastable phases that are in the high-temperature and high-pressure region on their thermodynamic equilibrium phase diagram. If this is so, the cooling efficiency of the confining liquid on the laser-induced plasma should enhance the formation of the metastable structures generated in the plasma transformation. In other words, some metastable phases can be frozen during the transition from metastable to stable due to the fast cooling time of the plasma plume in the confining liquid. In our work, we observed the stable fcc Ni in the synthesis of Ni nanocrystals by the PLA of the Ni target in water and methanol and some mixture of fcc and hcp Ni in hexane and acetonitrile. However, pure metastable hcp Ni nanoparticles were formed by secondary ablation of the mixture

4. CONCLUSIONS In this study, we present a simple and selective generation of fcc and hcp Ni nanoparticles synthesized in different solvents via a pulsed laser ablation in liquid (PLAL). By changing solvents to have different specific heats, different phases of Ni nanocrystals were formed via the PLAL processes. In primary ablation, a pure fcc phase was fabricated in methanol; however, mixed phases of fcc and hcp Ni nanocrystals were fabricated in hexane and acetonitrile. In secondary ablation, pure fcc and hcp Ni nanoparticles were selectively generated in hexane and acetonitrile, respectively. We think that the specific heat of solvents plays an important role in the cooling process of plasma plume kinetically and thermodynamically. Solvents with a low specific heat cool the plasma plume more effectively so that fast cooling in the formation of nanoparticles is preferred for metastable hcp Ni nanoparticles. We hope this simple and cost-effective method shed light on the formation mechanism of nanoparticles in the PLAL processes and selective preparation of specific phase nanoparticles. 14652



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ASSOCIATED CONTENT

S Supporting Information *

Expanded XRD patterns of the Ni nanoparticles produced in hexane and acetonitrile, temperature-dependent XRD patterns of the Ni nanoparticles prepared in hexane under the vacuum and atmosphere, temperature-dependent XRD patterns of the Ni nanoparticles prepared in methanol under the atmosphere, and N 1s core level XPS spectrum of the Ni nanoparticles prepared in acetonitrile. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82 55 772 1492, E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (KRF) grant funded by the Korea government (MEST) (2012R1A2A2A02013289) and Korea Ministry of Environment as “GAIA Project” (2012000550026). This work was supported by the Gyeongsang National University Fund for Professors on Sabbatical Leave, 2013.

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REFERENCES

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Specific Solvent Produces Specific Phase Ni Nanoparticles: A Pulsed

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