Letter pubs.acs.org/NanoLett
Growth of Pt Nanowires by Atomic Layer Deposition on Highly Ordered Pyrolytic Graphite Han-Bo-Ram Lee,† Sung Hyeon Baeck,†,‡ Thomas F. Jaramillo,† and Stacey F. Bent*,† †
Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States School of Chemical Engineering, Inha University, Incheon, 402-751, Korea
‡
S Supporting Information *
ABSTRACT: The formation of Pt nanowires (NWs) by atomic layer deposition on highly ordered pyrolytic graphite (HOPG) is investigated. Pt is deposited only at the step edges of HOPG and not on the basal planes, leading to the formation of laterally aligned Pt NWs. A growth model involving a morphological transition from 0-D to 1-D structures via coalescence is presented. The width of the NWs grows at a rate greater than twice the vertical growth rate. This asymmetry is ascribed to the wetting properties of Pt on HOPG as influenced by the formation of graphene oxide. A difference in Pt growth kinetics based on crystallographic orientation may also contribute. KEYWORDS: Atomic layer deposition, Pt, nanowires, highly ordered pyrolytic graphite, nucleation, graphene
P
and size remains challenging.9,10,22−24 Also, the long multi-day reaction times and complex processes during the synthesis are obstacles to commercialization of Pt NW catalysts.9,22,23 In this work, we show that atomic layer deposition (ALD) can deposit Pt NWs of controllable diameter on a carbon support, and we identify the key nucleation steps leading to NW formation. ALD is a thin film deposition technique that uses self-limiting surface reactions induced by alternating exposure of vaporized precursors in a cyclic manner to deposit materials with atomic level control.25,26 In addition to continuous thin films, ALD can also be used to fabricate discrete 0-D and 1-D nanostructures by controlling the substrate properties. Because of its combination of good conformality and thickness controllability, ALD promises to be an important fabrication method for nanostructured Pt catalysts on complex supports. However, only a few reports of Pt ALD on carbon and carbide surfaces have been published to date.27−30 These earlier studies reported Pt NP growth but did not investigate Pt NW growth. Moreover, while they investigated ALD to fabricate Pt NP catalysts, the studies did not focus on the nucleation and formation of ALD Pt on carbon surfaces. In the current study, we investigate the growth characteristics of Pt ALD on highly ordered pyrolitic graphite (HOPG)
t has attracted significant interest for numerous catalyst applications, such as fuel cells and hydrogen technologies.1−5 Due to the high cost of Pt, nanostructuring of Pt into zero-dimensional (0-D) nanoparticles (NPs) and one-dimensional (1-D) nanowires (NWs) is a key strategy for increasing its catalytic mass activity, for example, for fuel cell applications.6,7 The higher surface-to-volume ratio of nanostructures allows for reduction of Pt loading, which may translate to reduced material costs. Pt NPs on carbon supports have been used for the oxygen reduction reaction at the cathode of proton exchange membrane fuel cells. Due to electrochemical corrosion of the carbon support during operation, however, the Pt NPs migrate, coalesce, and become detached, resulting in degradation of catalytic performance.8 Use of 1-D Pt NWs instead of NPs is a way to overcome the degradation because of the inherent structural stability of NWs.9,10 Pt NWs have been synthesized in a variety of ways, such as solution methods,11−14 nanotemplating,15−20 and electrodeposition.21 Moreover, Pt NWs have been directly synthesized on various carbon substrates for fuel cell applications, where carbon nanotubes,22 carbon spheres,9,23 carbon paper,24 and carbon cables10 have all been used as the carbon surface. However, although Pt NW/C systems have shown promising performance enhancements in catalytic activity for fuel cells, there is still significant opportunity for improvements in these systems. Because most reported Pt NWs were synthesized using a beaker-scale, wet chemical process, scaling to mass production with well-controlled shape © 2013 American Chemical Society
Received: October 13, 2012 Revised: December 8, 2012 Published: January 14, 2013 457
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deposited along certain lines, leading to a laterally aligned NW shape. Moreover, upon increasing the number of ALD cycles (Figure 1c−f), no additional formation of Pt nuclei occurs on the regions where Pt did not exist during the previous cycles. The only apparent change is an increase in the average width of the Pt NWs. The morphology of Pt resembles a beaded necklace rather than a smooth wire. Most Pt NWs form parallel lines across the HOPG surface, but some NWs are not parallel and are merged at some points, as shown in the bottom of Figure 1e. The Pt NW sample deposited for 600 cycles, shown in Figure 1f, was chosen for further Auger electron spectroscopy (AES) areal mapping analysis. Figure 2 parts a, b, and c are the SEM
surfaces as a model carbon surface. Since HOPG is composed of stacked graphene layers that are ideal single carbon sheets, it is often used to model graphene. We demonstrate the formation of laterally aligned Pt NWs on HOPG for which the diameter can be controlled by changing the number of ALD cycles. We show that ALD Pt forms only on the step edges where defects exist, and not at the basal planes which are more chemically inert. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) results establish that the Pt nucleates on the step edges as NPs then transforms into NWs when the NPs coalesce. Our results illustrate a powerful method for fabricating nanostructured Pt catalysts. We also provide important insight into the surface properties of graphene, and identify nucleation mechanisms driving NW formation at these surfaces. Finally, we show that the Pt NWs deposited by ALD on HOPG exhibit excellent stability in electrochemical environment and upon mechanical force. Figure 1a and b show plan−view scanning electron microscopy images of Pt on SiO2 surfaces after 300 and 400
Figure 2. (a) SEM image and AES areal mapping images of (b) Pt and (c) C signals.
image, Pt signal, and C signal mapping images, respectively, taken from the same region. A brighter color indicates a higher intensity of each element. The brightly colored region in Figure 2b is identical to the NW shape in Figure 2a, indicating that no Pt is deposited on the HOPG surface except for the regions where NWs form. Also, the C signal map (Figure 2c) shows an inverted image relative to the Pt signal map. Based on these observations from SEM and AES, we conclude that Pt is selectively deposited at specific regions of the HOPG surface, and that with increasing ALD cycles, additional Pt growth occurs only where Pt has already been deposited. According to the reaction mechanism that has been proposed for Pt ALD,36,37 Pt precursor molecules chemisorb on a surface through reaction with surface oxygen species that have formed during the prior oxygen pulse. Therefore, the formation of chemisorbed oxygen is required to initiate Pt nucleation. In other words, a surface rich with chemisorbed oxygen should produce a large number of Pt nuclei. In terms of chemical reactivity, the HOPG surface has two domains: the very inert
Figure 1. Plan-view SEM images of ALD Pt on SiO2 and HOPG.
ALD cycles, respectively. The surface morphology is not continuous even after 300 cycles, and at least 400 ALD cycles are needed to deposit a continuous Pt film on the SiO2 surface. In our previous report, a similar morphological transition from separate islands to films was observed on the SiO2 surface.31 Island growth is commonly seen in metal ALD systems due to the low wettability and low nucleation rate of metal films at nonmetal surfaces.27,31−35 In contrast, ALD Pt on HOPG shows a dramatically different morphology. Figure 1c and d are SEM images of Pt deposited for 300 and 400 ALD cycles, respectively, comparable exposures to those in Figure 1a and b. Pt is selectively 458
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Figure 3. (a) Pt NW width on HOPG and (b) Pt film thickness on SiO2 versus ALD cycle number. Schematic drawings of NW cross-section on HOPG when wettability of Pt NW is (c) low and (d) high. (e) A plot of the ratio of NW width to NW height (width growth rate to film thickness growth rate) as a function of the contact angle between Pt NW and HOPG surface.
basal planes where the carbon atoms are sp2 bonded,38 and step edges that have chemically active dangling bonds, instead of fully coordinated sp2 bonds.38 The active step edge sites are more easily oxidized than the basal plane;39,40 therefore, the step edge becomes a favorable site for Pt precursor adsorption after oxygen exposure, leading to selective Pt nucleation only at the step edges. Once Pt nuclei are formed on the edge sites, Pt growth on these nuclei continues because of the presence of oxygen on their surfaces. Figure 3a and b shows plots of the Pt NW width and Pt film thickness, measured by SEM, versus ALD cycle number, respectively. The data in Figure 3a were obtained on HOPG and those in Figure 3b obtained on SiO2-covered Si. Both plots show a linear increase as a function of ALD cycle number after an initial nucleation delay. A thin film growth rate of 0.5 Å/ cycle is measured on SiO2, consistent with that typically reported for ALD Pt film growth using the same precursors in the steady state growth regime.31,36,37,41 This same value of 0.5 Å/cycle is expected for the rate of Pt deposition on the surface of a growing NW in the direction normal to the surface, since both rates measure the steady state growth of Pt on Pt. This expectation is confirmed by transmission electron microscopy data (Figure 4f, vide infra) in which the height of a NW grown for 400 cycles on HOPG is measured to be 20 nm. The rate at which the width of the Pt NWs increases with ALD cycle number, on the other hand, will be a function of the shape of the NW. For a hemispherical NW, the width would be expected to grow at twice the rate of the height, since Pt can deposit on both sides of the wire. The NW width was measured from the plan-view SEM images, and the resulting data are compiled in Figure 3a. Interestingly, the growth rate of the NW width extracted from a linear fit to the data in Figure 3a is 1.3 Å/cycle, a value that is somewhat larger than twice the 0.5 Å/ cycle growth rate of the Pt film on SiO2 or the vertical growth
rate of the NW. Here we explain the difference in vertical and horizontal NW growth rates using a model of surface wetting. For Pt NWs lying flat, the width of the NW can be correlated with the contact angle between a Pt NW on the HOPG surface, as illustrated in Figure 3c and d. For example, if the Pt does not wet the HOPG surface well, the contact angle (α) will be high, and the cross-section of the NW will show a more circular shape (Figure 3c). In contrast, if the wettability of Pt on HOPG is high, the contact angle will be small, and the cross-section will exhibit the shape of a circular segment (Figure 3d). The dependence of the horizontal-to-vertical growth rate ratio on the surface contact angle can be plotted using the following analysis. During steady state growth, the height of a NW (h) can be defined by gh × t, where t is the number of ALD cycles and gh is the growth rate of the NW height, that is, gh = 0.5 Å/cycle. Similarly, the width of a NW (w) can be defined by gw × t, where gw is the growth rate of the Pt NW width. The width (w) is allowed to vary for a fixed height to satisfy the shape described by a given contact angle (α) which defines the wettability. Figure 3e shows the resulting plot of NW width-toheight ratio (or ratio of horizontal-to-vertical growth rates) versus the contact angle (see Figure S1 in the Supporting Information for the detailed derivation). The ratio rapidly decreases with contact angle in the low contact angle regime and reaches a saturation value, 0.5, in the high contact angle regime. From the plot, we can use the experimentally determined values of gw and gt to infer the contact angle of the Pt NWs on HOPG. With the experimental growth rates of NW width and height of 1.3 Å/cycle and 0.5 Å/cycle, respectively, a contact angle of 75° can be extracted from Figure 3e. Cross-sectional SEM data (see Figure S2) show an average contact angle between the Pt NWs and the HOPG of approximately 75°, in excellent agreement with our analysis based on the relative horizontal and vertical growth rates. 459
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seen to be segmented along the axial direction of the NW due to a chain of Pt grains, each of which has a rounded outer surface. Figure 4e shows another TEM image taken of Pt NWs deposited for 400 cycles, where TEM samples were prepared by a relatively nondestructive method different from the scraping method used for the samples in Figure 4a−d. Many Pt NWs are still left intact on graphene layers that were peeled off from the HOPG substrate using this method. The average width of the NWs in Figure 4e is about 50 nm, and the surface morphology is rough, again consistent with SEM results. A single NW denoted by the white arrow in Figure 4e is magnified in Figure 4f. Because the NW is situated on the edge of folded graphene layers, it was possible to take a cross-sectional TEM image of this NW. The surface morphology at the interface between the graphite surface and the NW is clearly smoother than the morphology of the NW top surface. It is noted that the height of this NW is about 20 nmequal to the thickness of a Pt film deposited for the same number of cycles (400 cycles), confirming that the height of the Pt NW grows at a rate of 0.5 Å/cycle. Here we consider the unusual morphology of the Pt NWs in the context of the nucleation and growth process. Because HOPG has dangling bonds on the step edge, Pt more easily nucleates on step edge sites than on the more chemically inert basal planes (vide supra). The step height measured by atomic force microscopy (AFM) (see Figure S3) on the HOPG substrates is 3.3 Å, which is consistent with the interplanar distance between single graphite planes, indicating that a step edge in our HOPG samples is composed of a single step. Consequently, the potential nucleation sites can be considered as running along 1-D lines. Moreover, it is known that Pt ALD typically behaves according to an island-growth mechanism during the initial growth stage.31,35,42 Therefore, it follows that Pt nuclei with a spherical cap shape will be formed along the 1D lines of nucleation sites during the initial growth stage. With increasing number of ALD cycles, these spherical caps will grow and eventually coalesce, resulting in the formation of 1-D NWs. The suggested growth model is illustrated in Figure 4g. When Pt nuclei along the HOPG step edge contact each other, grain boundaries are formed between the nuclei. This growth mechanism will lead to Pt NWs with a segmented profile along the axial direction, as observed in the plan-view microscopy images. We also observe that once a continuous NW forms, the axial lengths of individual Pt grains in the NW are almost the same irrespective of the number of ALD cycles (see Figure S4). The constant length of Pt grains in the NWs indicates that the grain boundaries do not move significantly once they form, suggesting that Pt grains grow evenly without coarsening. This effect is likely due to lower surface and grain boundary diffusion rates than the Pt growth rate.43,44 In addition to the NW morphology, the high degree of selectivity for Pt growth at the HOPG step edges provides further insight into the Pt nucleation mechanism on HOPG. The selective deposition of ALD Pt on HOPG only at the step edges and not on the basal planes was observed up to 2000 ALD cycles. This selectivity indicates that nucleation on the basal plane during Pt ALD is difficult due to the lack of chemically active sites. Since the probable nucleation sites reside on the HOPG step edges along a 1-D line, it would be expected that additional Pt will grow on the Pt nuclei but will have little bonding interaction with adjacent basal plane sites due to their chemical inertness. Such a model would result in
Figure 4. TEM images of Pt NWs deposited for (a,c,e,f) 400 cycles and (b,d) 600 cycles. The NW TEM samples in a−d were fabricated by the scraping method, and e−f were fabricated by the direct transfer method (see Supporting Information for a detailed process). (g) A visualization of the nucleation and formation mechanism of Pt NWs.
Interestingly, the higher growth rate of the NW width than that of the NW height may therefore be associated with a relatively high wettability of Pt on the graphene surface. TEM analysis was also employed to obtain morphological and microstructural information on the Pt NWs (Figure 4). The average widths of NWs in the TEM images are similar to those from the SEM images, and the surface morphology of the NWs is uneven. The bending of NWs in Figure 4a and b is likely due to mechanical damage caused during the TEM sample preparation process. Individual Pt NWs are composed of small segments along the axial direction of the NW, as evident in low magnification TEM images (Figure 4a and b). Figure 4c and d show magnified TEM images taken at the interface between each segment. Only one kind of lattice fringe is observed in each segment. In addition, fast Fourier transform images taken from single segments (Figure 4c and d, insets) show spot patterns, indicating that each segment is a single crystal grain. In Figure 4c and d, the bumpy surface profile is 460
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oxidation reaction experiments. Our results show no significant change in either catalytic activity or morphology of the Pt NWs after as many as 500 cycles of cyclic voltammetry (CV) (Figure S6). Despite the harsh electrochemical conditions, the Pt NWs were not significantly degraded, indicating strong interactions between the segmented Pt NWs and the HOPG surface. Pt NWs/HOPG may thus serve as a robust system for potential catalytic applications. In summary, we show that Pt is deposited by ALD only at the step edges of HOPG and not at the basal planes, leading to the formation of laterally aligned Pt NWs. The Pt NW height and width can be controlled by the number of ALD cycles. The growth rate of the NW width is found to be greater than two times higher than that of the NW height, leading to a crosssectional NW shape that is flatter than a perfect semicircle. In TEM results, each Pt NW is comprised of a line of single crystal grains, with the number of grains remaining constant with increasing ALD cycles. A growth model, which has a morphological transition from 0-D Pt nuclei to 1-D NWs through coalescence, is indicated. In addition, the flattened shape of the NWs is described in the context of a relatively high wettability of ALD Pt on HOPG. We suggest that a localized oxidation of the graphene catalyzed by Pt that lowers the interface energy between Pt and graphene is responsible for the NW shape. The thermodynamic and kinetic effect of specific crystallographic orientations of the Pt NW must also be considered. These results provide insight into initial growth during metal ALD on graphene surfaces and possible extension of Pt NW fabrication to various applications. A custom-made ALD reactor controlled by LabVIEW software was used for the study. A showerhead inlet and vacuum pumping lines were connected to the top and bottom of the reactor, respectively, and the substrate was placed on a 4in. diameter substrate heater. Methylcyclopentadienyltrimethylplatinum (MeCpPtMe3) and air were used for the Pt precursor and coreactant, respectively. The Pt precursor was contained in a glass bubbler, and its temperature was kept at 50 °C to obtain a proper vapor pressure. N2 was used for the carrier gas and purging gas, and flow rates controlled by a mass flow controller were fixed at 30 sccm for both purposes. The substrate temperature for most Pt ALD experiments was 300 °C. Further information on the chamber configurations and Pt ALD process can be found elsewhere.31 HOPG substrates (10 mm × 10 mm × 1.2 mm, ZYB quality) were purchased from K-TEK Nanotechnology. Prior to Pt ALD, a fresh graphite surface was generated by exfoliation using Scotch Tape, which is a well-known method to obtain graphene layers from graphite.51,52 After the exfoliation, the HOPG substrate was loaded into the chamber without additional cleaning. Si(001) substrates with native oxide were used for control studies. The Si(001) substrates were cleaned by piranha solution to achieve a higher nucleation rate of Pt.31 Field emission SEM and AFM were used for surface morphology analysis. Chemical composition analysis was performed by AES. The microstructure of Pt NWs was analyzed by TEM. For TEM sample preparation, flakes were scraped off of the HOPG substrate using sharp tweezers and dispersed in ethanol solvent using sonication. The ethanol solution containing graphite flakes and Pt NWs was dropped onto a holey carbon TEM grid. To minimize mechanical damage, Pt NWs were also directly transferred to a TEM grid without scraping in some studies. In these studies, a small amount of an adhesive was applied on the HOPG surface and
very little wetting of the HOPG surface by the Pt NW, that is, a very high contact angle. However, our observations are somewhat inconsistent with this expectation. The contact angle between the Pt NWs and HOPG was determined to be 75° by both a growth rate analysis and direct SEM measurement; this value indicates that the Pt is slightly wetting the HOPG surface. One possible explanation for this observation is the formation of graphene oxide within the region of the HOPG surface in contact with the NWs. It is known that the HOPG surface can be oxidized by oxygen at temperatures above 600 °C, leading to formation of defect sites on the basal plane. Oxidized defect sites on HOPG are chemically active and can lead to Pt nucleation by ALD. Similarly, it was reported that O3 produced additional defect sites on HOPG basal planes, leading to nucleation of an Al2O3 film on HOPG by ALD.45 Although the substrate temperature employed here is well below 600 °C, it is possible that the Pt, which is a well-known catalyst, can facilitate oxidation of the HOPG surface in its vicinity, leading to oxidized defects near the Pt NW that in turn result in higher wetting of Pt on the HOPG.46−48 We were unable to directly analyze the presence of oxygen near the Pt NWs using AES areal mapping because the lateral resolution of AES is too low to distinguish the oxygen in oxidized graphite from the surface oxygen on adjacent Pt NWs. However, TEM data does provide indirect evidence for the possible presence of oxidized graphite in regions near the NWs. When graphene layers are oxidized, the interplanar distance is increased due to lattice expansion.49,50 In TEM, a difference in contrast was observed near the interface between the NW and the HOPG surface in the image of Figure 4f. Further, in a higher magnification image of the interface between the NW and HOPG (Figure S5), the interplanar distance in the outer surface of HOPG that directly contacts the Pt NW is 3.7 Å 10% larger than the 3.3 Å interplanar distance of nonoxidized graphene. Hence, the TEM images are suggestive of different crystallographic properties, possible due to oxidized HOPG, in the regions directly adjacent to the Pt NW. Therefore, we suggest that Pt NWs may exhibit higher wetting properties on the HOPG surface during Pt ALD because of the formation of graphene oxide adjacent to the Pt NWs, enabled by the catalytic effect of Pt. We note that, while the contact angle analysis provides an explanation for the observed NW shape and growth rates, the analysis is based upon the thermodynamic properties of the NW/surface interaction for a circular NW. The shape may also be affected by the thermodynamics and the growth kinetics of certain crystallographic orientations in the Pt NW. It has been reported that the microstructure of ALD Pt has a (111) preferred in-plane orientation, with the ⟨111⟩ direction oriented along the surface normal.36 If the sides of the NWs consist of crystallographic planes that are more reactive than the top, the lateral growth rate may be higher than the vertical growth rate. Further studies should be performed to delineate the crystallographic orientations of the Pt ALD NWs on HOPG. Interestingly, the segmented Pt NWs deposited on HOPG are stable according to both mechanical and electrochemical testing. As described earlier, the Pt NWs/HOPG remained intact after TEM sample preparation that was performed by mechanical scraping, indicating that the bonding between the Pt NWs and HOPG is durable. In addition, the catalytic activity and stability of the Pt NWs were examined via methanol 461
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contacted to a TEM grid. After hardening of the adhesive, the TEM grid was torn off from the HOPG substrate with the top graphite portion (made up of graphene layers) containing Pt NWs still attached.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed calculation process, SEM, AFM, Pt grain size plot, and TEM data. This material is available free of charge via the Internet at http://pubs.acs.org.
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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 The sample preparation and surface characterization was supported by the U.S. Department of Energy Hydrogen, Fuel Cells, and Infrastructure Program through the National Renewable Energy Laboratory under Contract No. DE-AC3608-GO28308. The modeling and analysis of the nucleation mechanisms were supported by the Department of Energy under Award Number DE-SC0004782.
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dx.doi.org/10.1021/nl303803p | Nano Lett. 2013, 13, 457−463
