Article pubs.acs.org/IECR
Controlled Syntheses of Various Palladium Alloy Nanoparticles Dispersed in Single-Walled Carbon Nanohorns by One-Step Formation Using an Arc Discharge Method Noriaki Sano,*,† Tatporn Suntornlohanakul,†,‡,§ Chantamanee Poonjarernsilp,†,∥ Hajime Tamon,† and Tawatchai Charinpanitkul‡ †
Department of Chemical Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Center of Excellence in Particle Technology, Department of Chemical Engineering, Faculty of Engineering, and §International School of Engineering, Faculty of Engineering, Chulalongkorn University, Patumwan, Bangkok 10330, Thailand ∥ Department of Chemical Engineering, Faculty of Engineering, Rajamangala University of Technology Krungthep, 2 Nanglinchee Road, Sathorn, Bangkok 10120, Thailand ‡
ABSTRACT: The present study examined the formation of single-walled carbon nanohorns (SWCNHs) dispersed with nanoparticles of a Pd alloy that is composed of one of nine elementsAu, Pt, Cu, Fe, Ni, Ti, Mo, W, and Nbby a modified gas-injected arc-in-water (GI-AIW) method incorporating a hollow graphite anode into which wires of Pd and an alloying component were inserted to generate arc discharge. The following requirement to realize the formation of Pd-alloy nanoparticles dispersed in SWCNHs was recognized: the boiling points of the alloying component must be below the 5000 K arc plasma temperature. When this requirement is satisfied, the boiling point of this component could be used as a threshold to judge whether Pd should be enriched or diluted in the products formed by the arc discharge process. Particle size analysis revealed that nanoparticles of Pd alloy containing Au show significantly high dispersion. In contrast, alloys with Cu produce relatively large alloy nanoparticles. The tendency of the average size of the alloy nanoparticles produced by this method can be correlated by a simple equation, taking into account the ratio of boiling point to melting point, surface tension, and gas diffusivity of the alloying components. one-step gas-injected arc-in-water (GI-AIW) method.24 This was intended to reveal fundamental rules in the formation of the alloy nanoparticles, which should prove indispensable for the further development of novel SWCNH-based materials. For this observation, we prepared Pd alloys using nine species from among the metals that are most commonly utilized: Cu, Ni, Fe, Ti, Mo, Pt, Au, W, and Nb.
1. INTRODUCTION Single-walled carbon nanohorns (SWCNHs)1 are categorized as part of the nanotube family of materials,2−6 and both their structures and physical properties have been intensively investigated to explore potential applications.7−10 So far, SWCNHs have been found to be superior for some applications, such as fuel cell electrodes,11−14 drug delivery systems,15 and gas fuel storage.16,17 In some cases, however, SWCNHs must be first hybridized with metallic nanoparticles before they are suitable for use.11−14 Because such alloying in many cases enhances the useful characteristics of metallic nanoparticles,18−21 SWCNHs dispersed with alloy nanoparticles with controlled compositions can be quite promising functional materials. Recently, one specific example was reported of a gas sensor using SWCNHs dispersed with Pd− Ni alloy nanoparticles exhibiting a higher sensitivity to the detection of H2 than SWCNHs dispersed with pristine Pd nanoparticles, thus showing the significance of alloying.18 Synthesis of SWCNHs dispersed with metallic nanoparticles can be performed by one of two means: the first is a two-step method in which SWCNHs are first synthesized and then loaded with metallic nanoparticles,11−14 and the other is a onestep method in which as-grown SWCNHs are dispersed with metallic nanoparticles.18,22,23 The one-step method is superior in terms of simplicity, but controllability of structures and physical properties of products produced by this method still need to be investigated. In the present study, SWCNHs dispersed with Pd alloy nanoparticles were synthesized by a © 2014 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. GI-AIW Method To Synthesize Alloy-Dispersing SWCNHs. Detailed information pertaining to the setup of the GI-AIW method is provided in a previous report.22 Figure 1 shows the structure and dimensions of the electrodes used in the current study. A cathode consisting of a hollow graphite rod was supported vertically in distilled water (3 L), and a graphite anode (diameter = 3.05 mm) was inserted into the cathode hole (diameter = 12 mm, depth = 25 mm) from below at a constant speed of 1.5 mm s−1 to generate arc discharge therein. N2 gas was supplied to the arc discharge zone from the top of the cathode through four channels, at a flow rate of 11 L min−1. In this system, the carbon vapor emitted from the anode by the arc discharge can be quenched in the cathode hole by the N2 gas flow, so that SWCNHs are formed by self-assembly.25 In Received: Revised: Accepted: Published: 4732
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Figure 1. Dimensions and structures of the electrodes used to synthesize SWCNHs dispersed with Pd-alloy nanoparticles by the GIAIW method.
the present study, the requisite dc voltage to generate arc discharge was supplied by a welding power supply (Shindaiwa, STW200A) connected to the electrodes, and the discharge current was set to 80 A. In order to produce nanoparticles of Pd alloyed with an additional metallic component, wires (diameter = 0.3 mm, length = 60 mm) of Pd and the alloying component were inserted in a hole (diameter = 1 mm, depth = 60 mm) drilled along the axis of the graphite anode. The two wires were twisted together before insertion and were subsequently evaporated together with the graphite anode when the arc discharge was generated. By this simultaneous evaporation, SWCNHs dispersed with alloy nanoparticles can be synthesized in a one-step process. To ensure this simultaneous evaporation during the duration of the arc discharge, discharge time was limited so that the graphite anode was consumed by 20 mm. Under such conditions the graphite anode was always evaporated along with the metallic components, even when the metallic wire evaporated faster than the graphite anode by virtue of having a lower boiling point than that of graphite. 2.2. Analyses of the Product Structures. The microscopic structures of the products were directly observed by using a transmission electron microscope (TEM; JEOL, JEM1010), with an electron energy of 100 keV. The particle size distribution of the alloy nanoparticles was analyzed based on this TEM observation. The component weight fraction in the alloy nanoparticles was analyzed by an energy-dispersed X-ray analyzer (EDX; Technex Lab Co., Tiny-EDX(LE)-α), mounted on a scanning electron microscope (SEM; Technex Lab Co., Tiny-SEM 1710) with an electron energy of 17 keV. Raman spectroscopy (Ramda Vision, MicroRAM-3000L) was used to observe the influence of the reaction conditions on the carbonaceous structures in the products.
Figure 2. TEM images of (a) Pd−Pt/SWCNHs and (b) Pd−Au/ SWCNHs synthesized by the GI-AIW method. The inset of (a) resolves the horn structures of SWCNHs.
shown in Figure 2b. The horn structures are resolved in an expanded image shown in the inset of Figure 2a. Similar images were observed in other products synthesized using W, Fe, Mo, Nb, Ti, Ni, and Cu. Raman spectra were obtained for all the products synthesized with Pt, Au, W, Fe, Mo, Nb, Ti, Ni, and Cu, as shown in Figure 3. Particular attention should be paid to two peaks: the socalled graphite peak at about 1600 cm−1 and the disorder peak at about 1350 cm−1.26 Comparison of the Raman spectra shown in Figure 3 shows that there is no significant difference in the balance between the intensities of the graphite peak and the disorder peak. In all specimens, the disorder peak is slightly higher than the graphite peak. This feature can be seen in SWCNHs without alloy-nanoparticle inclusion.11,18,22 The results obtained from both the TEM observation and the Raman analysis suggest that the structure of the carbonaceous part in SWCNHs including Pd alloy nanoparticles is independent of the alloying component added to Pd given the present synthesis conditions. 3.2. Structures and Component Fractions of Pd-Alloy Nanoparticles. X-ray diffraction (XRD) analysis was conducted on all as-grown products synthesized with Pt, Au, W, Fe, Mo, Nb, Ti, Ni, and Cu. An example of one of the XRD patterns is shown in Figure 4. Here, the result for Pd−Fe/ SWCNHs is shown together with the XRD patterns of pure Pd and Fe for reference. It can be seen that the peak from the
3. RESULTS AND DISCUSSION 3.1. Structures of the Carbonaceous Component. Examples of the TEM analysis of the products are shown in Figure 2. Here, the structures of SWCNHs, including nanoparticles of Pd−Pt and Pd−Au alloys (Pd−Pt/SWCNHs and Pd−Au/SWCNHs), are shown. Nearly spherical dark spots, corresponding to metallic nanoparticles, are seen to be dispersed in SWCNHs. SWCNHs are considered to be aggregates of closed horn structures, and appear as gray particles 20−50 nm in diameter in the moderate magnification 4733
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observed to be 40.0 and 39.8°, respectively. The slight shift in peak positions of Ti and Mo only barely seen here indicates that Ti and Mo could form alloys with Pd. The peak position of pure Pt is equivalent to that of Pd, and the alloying state in Pt could not be judged by XRD. Nevertheless, EDX analysis instead of XRD does indicate that Pt coexists with Pd in the products. It must be emphasized that the aforementioned trend was not seen in the case of synthesis with Nb and W. The peak positions in XRD patterns of Nb and W should be 38.5 and 40.2°; however, the XRD patterns from the synthesized products only exhibited a peak corresponding to pure Pd. In addition, EDX analysis did not detect Nb or W. The XRD and EDX results suggest that neither Nb nor W was present in the products, and thus, the metallic nanoparticles seen in the TEM observations were composed of pure Pd, not Pd alloys. A threshold can be found in the boiling points of the alloying components when compared with the temperature of the arc plasma in the present reaction field. The temperature of the arc plasma in the GI-AIW method has been previously investigated and found to be approximately 5000 K.29 The boiling points of Nb and W are 5015 and 5930 K, respectively, which are comparatively higher than the arc plasma temperature. Thus, although the wires of W and Nb can be melted by the arc discharge, the vapor pressure of W and Nb would be too low to form nanoparticles containing W or Nb. In our observation, Nb and W remained in the anode hole as a bulky resolidified form when the arc discharge was terminated. Such bulky resolidified metals were not observed in the case of other alloying components. One important rule in the synthesis of alloy nanoparticles in SWCNHs can be derived from these results: the boiling point of the alloying components must be lower than the arc plasma temperature. In the case of Pd alloying with Cu, Ni, Fe, Ti, Mo, Pt, and Au, the component fractions in the metallic nanoparticles were determined by EDX analysis. Figure 5 shows an example of the
Figure 3. Raman spectra of the products synthesized by the GI-AIW method using Pd with nine alloying components: Pt, Au, W, Fe, Mo, Nb, Ti, Ni, and Cu. The small peak around 1450 cm−1 is from the silicon substrate (third-order optical phonon mode of silicon) on which specimen powders are placed for Raman analysis.
Figure 4. XRD patterns of Pd−Fe/SWCNHs, with pure Pd and Fe references.
product appears at a position (2θ = 41.0°) between the peaks of pure Pd (2θ = 39.8°) and Fe (2θ = 42.1°). The shift of the peak position of Fe toward Pd in Pd−Fe/SWCNHs suggests that Fe in the products is alloyed with Pd.27,28 Note that Pd− Fe/SWCNHs exhibited a prominent peak at 41.0° but not at 39.8 or 42.1°, indicating that all Pd and Fe in the product were present as alloy components. A similar logical trend was clearly observed in the cases of the syntheses with Cu, Ni, and Au. The peak positions for the Pd− Cu/SWCNHs, Pd−Ni/SWCNHs, and Pd−Au/SWCNHs were found to be 41.5, 41.9, and 38.8°, respectively, a result of shifting of the peak positions of Cu (43.3°), Ni (44.4°), and Au (38.1°), respectively, toward Pd. In all cases, no peak at pure Pd was observed. It proved rather difficult to confirm the alloying state of Pd when Ti and Mo were used as the alloying components, because the peak position of pure Pd (39.8°) is fairly close to that of pure Ti (38.9°) and Mo (40.5°). The peak positions in Pd−Ti/SWCNHs and Pd−Mo/SWCNHs were
Figure 5. EDX spectrum of Pd−Pt/SWCNHs.
EDX spectra, which was obtained from Pd−Pt/SWCNHs. The peak indicating carbon in this EDX spectrum came from the carbonaceous part of SWCNHs. The EDX spectra of the products with other alloying components were similar to that shown in Figure 5. The concentrations of Pd or Pd alloy in metal-dispersed SWCNHs investigated here were measured by EDX to be 80− 90 wt % for Cu, Fe, Ni, Au, and Pt, and to be 90−95 wt % for Ti, Mo, W, and Nb. The first group includes the components which exhibit relatively high enrichment factors (mostly higher than 1.0) as explained later. The second group includes the components which exhibit low enrichment factors (Ti and Mo), and the ones which are not alloyed with Pd in the present 4734
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were tested to correlate α, but these properties did not show such a significant relationship as is seen with the boiling point. The tendencies observed in this study are important when the component fraction in alloy nanoparticles must be controlled in order to achieve the requisite properties for a specific application. The tendencies of α seen in Figure 6 can be explained by the evaporation rate of the alloying components from their wire forms. The average evaporation rate of the wire during arc discharge was calculated from the consumed length of the wire divided by the arc discharge duration time. The wire evaporation rate is shown in Figure 7, as a function of the
conditions (Nb and W). Note that the values of the metal concentrations estimated by EDX may indicate only qualitative features in the present conditions because the peak of carbon in the EDX spectra is unbalancingly high. If one wishes to obtain an accurate metal concentration in metal-dispersed SWCNHs, direct measurement of the weight of the metallic part after thorough oxidation to remove carbon followed by reduction in H2 should be preferable. However, a large amount of the specimen is necessary for this analysis, but such an amount was not produced in this study as it is outside the scope. In this study, weight fractions among alloying metallic components obtained by EDX should be focused. An enrichment factor of each alloying component, α,was defined by eq 1 based on this EDX analysis. α=
(walloy /wPd)1 (walloy /wPd)0
(1)
where walloy and wPd correspond to the weight fractions of alloying component and Pd, respectively. The subscripts “0” and “1” correspond to the initial wires and the products of SWCNHs including alloy nanoparticles, respectively. The value of (walloy/wPd)0 was calculated from the weight ratio of the wires inserted in the anode hole, and (walloy/wPd)1 was obtained by the quantitative EDX analysis on the products. It must be recognized that the products should be enriched with the alloying component when they are formed using wires when α is higher than 1, from the definition by eq 1; otherwise, the increase of the Pd fraction is higher. Figure 6 shows α as a function of the boiling points of the alloying components. In Figure 6, one can see a tendency for α
Figure 7. Influence of the boiling points of alloying components on their evaporation rates.
boiling points of the alloying components. It can be noted that the wire evaporation rate is higher when the boiling point is lower, and it follows that a higher evaporation rate will lead to a higher concentration of the evaporated component in the plasma reaction field. A higher concentration of the alloying component should, in turn, result in a larger fraction of this component being present in the nanoparticles. 3.3. Particle Size of Pd-Alloy Nanoparticles. As already shown in Figure 2, Pd-alloy nanoparticles were highly dispersed in SWCNHs within the products synthesized by the present method. Particle size distributions were obtained by measurement of their diameters on the TEM images, and three examples are shown in Figure 8. Pd−Au/SWCNTs exhibited the smallest particle size of Pd alloy among the products synthesized with Au, Pt, Fe, Mo, Ti, Ni, and Cu, which could form dispersed Pd-alloy nanoparticles in SWCNHs. In Figure 8a, the peak in the diameter distribution for Pd−Au/SWCNHs appears to be located in the range 2−4 nm. In contrast, Pd− Cu/SWCNHs exhibited the largest alloy nanoparticles, as shown in Figure 8c, in which the peak is located in the diameter range 4−6 nm, with a much broader distribution than the others. The dispersion of nanoparticles containing other alloying components resulted in between the cases of Au and Cu.
Figure 6. Enrichment factor for each alloying component, as defined by eq 1. The range α > 1 (blue zone) indicates that the alloying component is enriched; otherwise, Pd is enriched (red zone).
to decrease with an increase in the boiling point of the alloying component. Note that α values of Cu, Ni, Fe, and Au, whose boiling points are lower than that of Pd, turned out to be above 1. On the other hand, α values of Ti, Pt, and Mo, whose boiling points are higher than that of Pd, were lower than 1.0. These results show that the boiling point of the alloying component can provide a useful estimate of how much the alloying component content can be increased by the arc discharge process, and can provide a threshold for predicting whether the product will be enriched in Pd or the alloying component. It should be noted that some other physical properties of the alloying components such as the melting point and density 4735
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nanoparticles, d [nm], based on some fundamental properties of the alloying components as per eq 2. d1/3 = k
σ 0.7 ⎛ Tb ⎞ ⎜ ⎟ ratomρ0.3 ⎝ Tm ⎠
2
(2)
where σ, ratom, ρ, Tb, and Tm are the surface tension at the melting point, atomic radius, density at standard conditions, boiling point, and melting point of the alloying components, respectively. The parameters used in this calculation are tabulated in Table 1. The coefficient k is determined as 5.097 × 10−1 by normalizing d to the average diameter of Pd particles synthesized without the addition of alloying components. Table 1. Physical Properties of Alloying Components Used in Eq 2 Au Pt Fe Mo Ti Ni Cu
ratom [pm]
ρ [kg m−3]
Tm [K]
Tb [K]
σ [J m−2]
144 136 126 139 147 124 128
19320 21450 7874 10220 4540 8902 8960
1337 2041 1811 2890 1941 1728 1357
3080 4100 3023 4885 3560 3005 2840
1.138 1.860 1.830 2.284 1.500 1.838 1.310
Prior to discussion of this empirical equation, the phenomena in the reaction zone speculated in a previous study must be revisited.29 It was speculated that three zones would be modeled in the cathode hole: (1) an arc plasma zone where the anode can be evaporated, (2) a quenching zone where vaporized components can become nanoparticles, and (3) a downstream zone where water vapor can be involved in the reaction with carbon. Here, it can be considered that the anode components (metallic components and carbon) are evaporated at the hot arc plasma zone, and SWCNHs dispersed with metallic nanoparticles are formed in the quenching zone before arriving at the cold downstream zone. It can be considered that the growth of Pd alloy nanoparticles takes place when metallic components emitted from the anode become nanosized liquid droplets in the quenching zone. There, nanoparticles would tend to be larger when the lifetime of the liquid droplet is longer. A higher boiling point, and lower melting point, should lead to an enlargement of the quenching zone containing metallic liquid droplets. This is because a higher boiling point should quicken the liquidization at the upstream zone inside the cathode hole closest to the hot plasma zone, and the lower melting point may retard solidification during quenching of the liquid droplet. Thus, when the ratio of boiling point to melting point, Tb/Tm, is higher, the lifetime of the liquid droplet should become longer, thereby resulting in larger nanoparticles. The mass transfer rate of metallic vapor by diffusion is also important to grow nanoparticles in a limited time. It is commonly known that the diffusion coefficient tends to become larger when the atomic size and atomic weight of the diffusing component are smaller.30 In addition, the larger surface tension should tend to prefer a smaller curvature of the liquid droplet surface before the nanoparticles are solidified, leading to a larger particle diameter. Equation 2 summarizes these physical tendencies. The exponent numbers 1/3, 0.7, 0.3, and 2 on d, σ, ρ, and (Tb/Tm) in eq 2 were determined by fitting of the calculated values with experimental ones.
Figure 8. Particle diameter distributions of Pd-alloy nanoparticles in Pd−Au/SWCNHs, Pd−Mo/SWCNHs, and Pd−Cu/SWCNHs.
Based on the particle size distributions obtained for all the Pd-alloy nanoparticles, their average diameters were obtained for comparison and are plotted in Figure 9. This shows that the
Figure 9. Average diameters of alloy nanoparticles dispersed in SWCNHs, as observed by TEM and calculated by eq 2
addition of noble metals, Au and Pt, results in relatively small particle diameters, whereas Cu produced the largest diameter. The sequence of average particle diameter in Figure 9 is therefore given as follows: Au < Pt < Fe < Mo < Ti < Ni < Cu. Two components, Nb and W, are excluded because they cannot form alloys with Pd under the present method of synthesis. It is desirable to propose empirical equations to predict the average particle diameter of Pd-alloy nanoparticles, as synthesized by the present method. In this study, we propose an equation to predict the average diameter of the Pd-alloy 4736
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One can notice in Figure 9 that the particle diameters calculated by eq 2 match those determined experimentally fairly well. Because the important properties of nanoparticles, including electromagnetic properties and chemical reactivity, should depend on the particle’s size, this ability to predict average size is important. Nevertheless, efforts to generalize eq 2 should be continued into the future, as the coefficient k and exponent numbers used in the present study may need to be modified when Pd is replaced by other components, or changes are made to the operating parameters of synthesis.
4. CONCLUSIONS The syntheses of SWCNHs dispersed with nanoparticles of Pd alloys including nine components, Au, Pt, Cu, Fe, Ni, Ti, Mo, W, or Nb, using the GI-AIW method were investigated. To realize these syntheses, wires of Pd and an alloying component were inserted in a hollow graphite anode. It was found that SWCNHs containing Pd-alloy nanoparticles can be synthesized only when the boiling point of the alloying component is below the temperature of the hot arc plasma zone, 5000 K. The weight fraction of Pd in the alloy nanoparticles tended to increase when the boiling point of the alloying component was higher, and this boiling point can be used as a threshold to judge whether Pd content is increased or decreased in the products produced by the arc discharge process. These tendencies can be qualitatively explained by considering the evaporation rate of the alloying components. The order of average particle diameter of the nanoparticles as per the alloying component was found to be Au < Pt < Fe < Mo < Ti < Ni < Cu. The average particles size of the Pd-alloy nanoparticles can be predicted by an equation incorporating some fundamental physical properties of the alloying components.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by JSPS KAKENHI Grant 24360327. A JSPS Postdoctoral Fellowship for Foreign Researchers FY 2012 is also acknowledged for supporting C.P.’s involvement in the research. We thank Mr. Norimoto Tatsuhiko for assistance for valuable XRD analysis.
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