Formation of Continuous Pt Films on the Graphite ... - ACS Publications

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Formation of Continuous Pt Films on the Graphite Surface by Atomic Layer Deposition with Reactive O3 Han-Bo-Ram Lee†,‡ and Stacey F. Bent*,† †

Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States Department of Materials Engineering, Incheon National University, Incheon 22012, Korea

‡

S Supporting Information *

ABSTRACT: Because graphite surfaces are chemically stable, it is difficult to form a uniform layer on graphite by atomic layer deposition (ALD), which is a surface reaction-based deposition method. In this work, reactive O3 is employed for Pt ALD as a counter reactant, and a continuous Pt film is achieved on the graphite surface. The growth morphology of the O3-based Pt ALD process differs significantly from that using an O2 reactant, in which selective growth occurs on step edges of graphite. Pretreatment of the graphite with O3 prior to Pt ALD using an O2 reactant shows a continuous Pt film morphology similar to that obtained from the full O3-based ALD process. The analysis indicates that O3 etches the graphite surface and generates pits containing additional step edges, resulting in an increase in the extent of Pt nucleation. The nucleation of Pt is less active at lower deposition temperatures because the generation of additional step edges is dependent on temperature. This Pt ALD process using a reactive O3 reactant can be an effective route for fabricating a uniform and continuous Pt catalyst on three-dimensional carbon electrodes for highly efficient fuel cells.

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INTRODUCTION Pt is an essential catalyst for fuel cell technologies, such as proton exchange membrane fuel cells (PEMFCs).1−5 Because Pt is one of the most expensive materials, even more costly than Au, fuel cell researchers have made many efforts to reduce Pt loading and increase catalytic activity.3,4,6,7 The Pt/C electrode system, composed of Pt nanoparticle catalysts loaded on a porous carbon electrode, has been widely used.6−8 Because in this design high-surface-to-volume-ratio Pt nanoparticles are formed on a porous carbon electrode that itself has a large surface area, the Pt/C electrode system is advantageous with regard to the reduced use of the expensive Pt catalyst. Recently, however, the approach for reducing Pt loading by shaping it into nanoparticles is rapidly reaching physical limitations.5,9 As an alternative, some researchers have turned to a strategy in which the specific activity of Pt is increased by fabricating Pt thin films instead of nanoparticles. Because a Pt film has a specific activity much higher than that of Pt nanoparticles, the total gain in catalytic activity can be positive despite a loss in the surface-to-volume ratio from the film shape. In a successful example, a nanostructured thin film (NSTF) catalyst/electrode that is composed of Pt thin films deposited on threedimensional (3D) carbon whisker electrodes has been intensively investigated.5,10−13 In that design, sputtering was employed to deposit Pt thin films; however, there were challenges in conformality because the step coverage of sputtering can be poor on 3D nanostructures.14,15 A fabrication technique that can deposit Pt thin films inside complex, porous, © XXXX American Chemical Society

3D carbon electrodes could be advantageous. For this purpose, atomic layer deposition (ALD) is suitable because it provides excellent conformality inside 3D nanostructures as a result of its self-saturating surface reactions.14,15 Because ALD is a surface sensitive deposition method, a fundamental understanding of nucleation during Pt ALD on the carbon surface is required to successfully apply Pt ALD to catalyst fabrication on carbon electrodes. Generally, the graphite surface is chemically stable, so that it does not easily initiate nucleation of ALD, resulting in the formation of a nonuniform layer. For instance, in ALD of metal oxides, such as Al2O3 and HfO2, on graphite surfaces, selective deposition takes place only at certain surface sites, resulting in a discontinuous layer instead of a uniform oxide film.16,17 In fact, we found similar results for ALD of Pt on graphite surfaces using trimethyl(methylcyclopentadienyl)platinum(IV) and oxygen. Because graphite has a chemically nonuniform surface in which both active step edges and stable basal planes coexist, ALD Pt selectively nucleates at the step edges, resulting in the formation of laterally aligned Pt nanowires, instead of a uniform Pt layer.18 The step edge sites are more chemically active than the basal planes because of the existence of dangling bonds, so that Pt nucleation is more easily initiated on the step edge sites, leading to selective nucleation. However, for current Received: August 9, 2015 Revised: September 13, 2015

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DOI: 10.1021/acs.chemmater.5b03076 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. SEM plan view images of ALD Pt using (a) O2 and (b) O3 on the bare HOPG substrate deposited for 400 cycles and (c) using O3 for 100 cycles. (d) SEM plan view image of ALD Pt using O2 deposited for 400 cycles on HOPG pretreated with O3 for 600 s. All of the depositions were performed at 300 °C.

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approaches to the Pt/C electrodes in which continuous Pt thin films must coat 3D carbon structures, uniform formation of Pt by ALD across the carbon surface is still necessary. Because the reactivity of the Pt precursor and O2 counter reactant on the basal planes was so low, it was very hard to achieve the homogeneous adsorption of the precursor and reactant on the graphite surface needed for the uniform nucleation of Pt. Therefore, in the research presented here, we utilize O3 as a counter reactant because it is more reactive than the conventional counter reactant, O2. In several other studies, Pt ALD using the O3 counter reactant has shown a higher growth rate and a higher nucleation rate.19−22 In an earlier report from our group investigating growth on SiO2, the O3 counter reactant changed the surface properties to be more active toward Pt nucleation, resulting in an increase in growth rate and surface coverage.19 However, the effect of the ozone counter reactant on Pt ALD on graphite surfaces has not yet been studied, and continuous metal ALD films on graphite have not been achieved until now. Hence, in this work, we employ the highly reactive O3 species to deposit Pt on graphite and investigate the effects of O3 on Pt ALD on a carbon surface. By using a reactive O3 reactant, we show that a continuous and uniform Pt layer can be achieved on the graphite surface. We provide a mechanistic explanation in which the graphite surface is effectively modified to one that is active to Pt nucleation via interactions of O3 with carbon. We also show that the same uniform Pt layer can be obtained even with the conventional O2 reactant just by using an initial, one-time O3 pretreatment.

EXPERIMENTAL DETAILS

A custom-made ALD reactor controlled by LabVIEW software was used for this study. A showerhead inlet and vacuum pumping lines were connected to the top and bottom of the chamber, respectively, and the substrate was placed on a 4 in. diameter substrate heater. Trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPtMe3) was used for the Pt precursor. The Pt precursor was contained in a glass bubbler, and its temperature was kept at 50 °C to obtain a proper vapor pressure. O2 and O3 were used as the counter reactant. O2 was supplied from a 99.99% O2 cylinder. O3 was generated in situ during the ALD process from an O3 generator (IN USA OG 5000 Series) fed by pure O2 from the cylinder. The O3 concentration was routinely set to 11%. 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. Further information about the chamber configurations and Pt ALD process can be found elsewhere.23,24 Highly ordered pyrolitic graphite (HOPG) substrates were used for a carbon substrate and were purchased from K-TEK Nanotechnology (10 mm × 10 mm × 1.2 mm, ZYB quality). Prior to Pt ALD, a fresh graphite surface was generated by exfoliation using Scotch Tape, which is a well-known method for obtaining graphene layers from graphite.25 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. Field emission scanning electron microscopy (FE-SEM) (FEI Magellan) and atomic force microscopy (AFM) (Park Science) were used for surface morphology analysis. Chemical composition analysis was performed by X-ray photoelectron spectroscopy (XPS) (PHI VersaProbe Scanning XPS Microprobe). The apparent coverage was computed using ImageJ64 software from SEM plan view images, and the detailed process can be found elsewhere.23,24 The growth of Pt on HOPG was analyzed by transmission electron microscopy (TEM). For TEM sample preparation, flakes were scraped off of the HOPG B


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Figure 2. (a) XPS spectrum of HOPG substrates with and without the O3 pretreatment at 300 °C for 600 s. AFM images of (b) the bare HOPG substrate and (c) the pretreated HOPG substrate. The range of the AFM scale bar is 2 nm in height. substrate after Pt ALD and transferred to TEM grids using an ethanol solvent. In particular, for the cross sectional samples, a TEM grid with adhesive was placed on the ALD Pt-deposited HOPG and peeled off the TEM grid to obtain several graphene layers containing ALD Pt. On the TEM grid, the interface between HOPG and ALD Pt was investigated at a folding edge of the graphene layers in a cross sectional view. The detailed processes for TEM sample preparation can also be found elsewhere.18

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and mechanical scratching by tweezers, similar to ALD Pt using O2. Figure 1c shows a plan view SEM image of ALD Pt using O3 deposited for a smaller number (100) of cycles. Pt forms primarily along lines that are tentatively assigned to step edges. Interestingly, Pt nuclei are also formed between the lines, which are likely the basal plane areas. In previous reports, we found that Pt nucleation is difficult on the basal planes because of the lack of chemically active nucleation sites.18 Therefore, the formation of Pt on the basal plane using O3 indicates that the surface properties of the basal planes have become active to nucleation of ALD Pt. Because all the experimental conditions were the same for Pt ALD except the counter reactants, O2 and O3, the change in surface property must be solely related to the O3 counter reactant. Consequently, the surface property changes induced by O3 were investigated with the following experiment. The HOPG was exposed to O3 for 10 min, and subsequently, ALD Pt was performed using the O2 counter reactant. Interestingly, the surface morphology of ALD Pt using O2 on the O3pretreated HOPG is dramatically changed as shown in Figure 1d. There is no formation of Pt nanowires on HOPG anymore. Instead, a continuous Pt film occurs even when using the O2 counter reactant. Indeed, the surface morphology in Figure 1d is quite similar to that in Figure 1b. This result indicates that the exposure of the O3 to the original HOPG surface changes

RESULTS AND DISCUSSION

Panels a and b of Figure 1 show plan view SEM images of ALD Pt deposited on HOPG at 300 °C using O2 and O3 reactants, respectively. In Figure 1a, Pt has a nanowire morphology when O2 is used for the reactant, consistent with our previous results.18 Because the step edges that inherently exist on the HOPG surface with a parallel alignment are the only active nucleation sites for Pt ALD, selective deposition of Pt at these sites results in the formation of Pt nanowires.18 In contrast to the O2 counter reactant, a continuous film-like morphology across the HOPG surface is observed even with the same number of ALD cycles when O3 is used, as shown in Figure 1b. Some regions are more closely packed than other regions, and the film regions are aligned as were the Pt nanowires presented in Figure 1a. In addition, the adhesion between ALD Pt using O3 and HOPG was good enough to pass the Scotch Tape test C


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Figure 3. SEM plan view images of ALD Pt using O2 deposited for 100 cycles on HOPG pretreated with O3 for various times: (a) 0, (b) 100, (c) 300, (d) 600, (e) 1000, and (f) 1500 s. (g) Plots of Pt surface coverage vs O3 pretreatment time extracted from the SEM images. (h) Plots of carbon and oxygen contents vs O3 pretreatment time extracted from XPS spectra.

basis of earlier reports, it is known that Pt ALD nucleates much more readily at step edge sites than on the basal planes of graphite.18,29 It is also known that the oxygen reactant more easily adsorbs on the step edges than on the basal planes because of their chemically active dangling bonds, resulting in the formation of surface oxygen.30,31 Therefore, in the experiment described here, O3 appears to generate pits that contain new step edges on the basal planes, resulting in an increase in the level of surface oxygen, as detected by XPS analysis. To better understand the effect of the pretreatment, the HOPG surfaces were exposed to O3 for various lengths of time, and ALD Pt was subsequently deposited using O2 for 100 cycles. Without the pretreatment, ALD Pt was formed only along the intrinsic linear step edges, as shown in Figure 3a. Although the ALD cycle number was fixed at 100 cycles, the number of nuclei and the surface coverage increased as the pretreatment time was increased, as seen in Figure 3b−f. The Pt surface coverages were extracted from the SEM images and are plotted in Figure 3g. The coverage linearly increases with an increase in the level of pretreatment until it reaches a plateau at ∼600 s. We note that, the amounts of carbon and oxygen were

the surface properties of the basal planes, leading to nucleation of Pt. The surface of O3-pretreated HOPG was analyzed by XPS and AFM. The XPS spectrum in Figure 2a of the bare HOPG shows only a carbon peak at 284.6 eV, while the O3-pretreated HOPG sample shows an additional peak at 533 eV that corresponds to oxygen.26 AFM results show a different surface morphology of HOPG before and after the O3 pretreatment. The bare HOPG shows a smooth morphology on the basal planes with a sharp change in height at the step edges, as shown in Figure 2b. After the O3 pretreatment, however, the morphology of the basal planes (see Figure 2c) is dramatically changed. A rough surface morphology with many pits is observed on the basal planes, and the formerly sharp step edge lines are also roughened. It has been reported that O3 and O2 can etch a graphite surface and generate pits on the surface.27,28 However, whereas for O2, a high temperature of >600 °C is required to etch the graphite surface, O3 can generate pits by etching at ∼300 °C because of its high reactivity. The formation of pits on the HOPG surface is accompanied by the generation of rounded cuts into the otherwise inherently linear step edges. On the D


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Figure 4. Cross sectional TEM images of ALD Pt using (a and b) O2 deposited for 400 cycles and (c and d) O3 for 100 cycles at 300 °C. The white arrows are for guidance to distinguish interfaces.

Figure S1 of the Supporting Information). This behavior is consistent with coalescence of Pt islands.24 Because Pt nucleates along the step edges of pits, under the condition of fixed ALD cycle numbers, the coalescence of ALD Pt can be attributed to the coalescence of pits. TEM results support this explanation by revealing evidence of the formation of ALD Pt on the additional edges generated by the O3-formed pits. When the O2 reactant is used, the ALD Pt is formed only on the intrinsic step edges, which are typically just a single or double atomic plane in height.29 Therefore, the interface between ALD Pt and HOPG is sharp as shown in Figure 4a. In the magnified TEM image in Figure 4b, the interface line denoted by the white arrow is continuous from the Pt-free region to the Pt-deposited region because the Pt grows on the inherently flat HOPG surface. In contrast, as we observed from AFM images in Figure 2, the surface of HOPG is roughened when O3 is used for the reactant, leading to growth of Pt in deeper pits as shown in Figure 4c. It appears that the O3-generated pits extend deeper into the HOPG than the intrinsic step edges, resulting in the type of interface denoted by two white arrows in Figure 4d in which the Pt appears partially buried in the HOPG in cross sectional TEM images. In addition, the small particles below the interface in Figure 4c are ALD Pt nuclei viewed in a plan view due to the TEM sample preparation method (see Experimental Details). One of the advantages of performing Pt ALD using O3 is that it provides for a large range of deposition temperatures. In our

quantified from XPS data; the results are plotted in Figure 3h. Similar to the Pt coverage behavior of Figure 3g, the oxygen content increases rapidly for up to 600 s of the pretreatment time and reaches a plateau at times of >600 s. Because carbon and oxygen were the only two species detected in the XPS analysis, the carbon plots show a trend that is the opposite of that of oxygen. We provide the following explanation for the observed behavior. When O2 is employed as the reactant, ALD Pt deposits at only the intrinsic step edges because O2 cannot generate additional sites on the basal planes. In contrast, O3 can generate additional step edges via pit formation in which surface oxygen adsorbs and initiates Pt nucleation. The longer the pretreatment time, the more step edge sites can be generated by the O3 reactant, leading to more extensive nucleation of Pt. The tapering off of this effect, as seen in the plateau of the curves for surface Pt and oxygen coverage at times longer than 600 s, is probably due to the limited surface area of the substrate. Once the concentration of pits reaches a maximum on the surface of HOPG, new pits are not generated but rather pits are coalesced. Thus, because the amounts of adsorbed oxygen and deposited Pt are proportional to the length of step edges, no further increase is observed. In addition, the coalescence of pits was confirmed by an increase in the average size of Pt nuclei and a decrease in number density of the Pt nuclei at O3 pretreatment times of >600 s, which were computationally extracted from SEM data (see E


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nanowires at the step edges is seen to decrease with a decreasing deposition temperature. The decrease in nanowire width can be understood by a change in the growth rate at different temperatures; a previous report observed that the growth rate of ALD Pt using O3 on a SiO2 surface decreased from 0.70 Å/cycle at 300 °C to 0.38 Å/cycle at 150 °C and to 0.28 Å/cycle at 100 °C.19 However, the different deposition behavior of Pt on the basal planes at different temperatures cannot simply be explained by the growth rate changes, because the magnitude of the decrease observed in the number of Pt nuclei on the basal planes between 300 and 100 °C is too large. The effect of O3 exposure on HOPG as a function of temperature was further investigated by a series of pretreatment studies. HOPG substrates were pretreated for 600 s at 100, 150, and 300 °C and analyzed by XPS. The carbon intensity from the bare HOPG surface shows the highest value compared to those of the pretreated samples, and the intensity of the carbon signal decreases with an increasing pretreatment temperature, as shown in Figure 6a. For all of the samples, the carbon peaks were deconvoluted into two peaks at 284.8 and 284.3 eV, which correspond to C−O and C−C bonding, respectively.26 Although the intensities of both C−C and C−O peaks decreased with increasing pretreatment temperature, the intensity of the C−C component decreased more significantly than that of the C−O component. Consistently, oxygen peaks relevant to the C−O bonding were observed, as well, but the intensities were not high enough to resolve the peaks as shown in Figure 6b. Using the XPS data, the ratio of oxygen to carbon is plotted as a function of pretreatment temperature in Figure 6c. An increase in this ratio is clearly observed with increasing temperature. Because the basal planes, which constitute the largest part of the HOPG surface, are not chemically active, the oxygen detected by XPS is assigned to surface oxygen adsorbed at the edges of steps and defects such as etch pits. Therefore, the increase in the level of oxygen (relative to the underlying carbon) with increasing pretreatment temperature can be attributed to an increase in the number of edge sites. At higher temperatures, Pt can grow on both the step edges and the initial basal planes because the O3 reactant can generate additional step edges. However, the O3 does not generate pits with step edges as readily at lower temperatures, so that Pt is mostly formed on the intrinsic step edges. The results are consistent with a model in which O3 helps produce active sites on HOPG where Pt nucleation can occur. Because the ALD process needs an active surface species for precursor adsorption and subsequent nucleation, ALD Pt using an O2 counter reactant occurs selectively only on the chemically active step edges but not on the inert basal planes. The more reactive O3, on the other hand, etches out the graphite surface, leading to formation of pits on the basal planes accompanied by the generation of additional step edges. Because these step edges are active nucleation sites for Pt ALD, the O3 reactant allows Pt to form across the whole graphite surface. However, this etch process is temperature-dependent. At the lower temperatures, O3 does not actively generate pits, so that only the selective formation of ALD Pt on the intrinsic step edges occurs, similar to the case of ALD Pt using the O2 reactant. At higher temperatures and at higher O3 exposures, many etch pits form and a nearly continuous Pt film can be deposited by ALD across the HOPG surface. If we deposit Pt using O3 at temperatures above 300 °C, a continuous Pt film will form more rapidly because of the formation of many pits as well as CVD-like reactions by thermal decomposition of the Pt

previous report, metallic Pt could be formed using O3 at temperatures as low as 125 °C and partially oxidized Pt could be deposited at 100 °C, whereas Pt could not be deposited at temperatures below 200 °C using the O2 reactant.19 To assess the effect of temperature on the O3-based ALD of Pt on HOPG, the process was studied as a function of deposition temperature from 100 to 300 °C. Pt was deposited using O3 on HOPG for 400 cycles at 100, 150, and 300 °C. As shown in Figure 5a, the surface of HOPG is almost fully covered with

Figure 5. SEM plan view images of ALD Pt using the O3 counter reactant for 400 cycles at (a) 300, (b) 150, and (c) 100 °C.

ALD Pt irrespective of the step edges and basal planes when deposition is performed at 300 °C. However, at a deposition temperature of 150 °C, Pt mostly deposits along the step edges, and only a small number of Pt nuclei are observed on the basal planes. Interestingly, at 100 °C, Pt forms only on the step edges, and no growth on the basal planes is seen even when using the O3 reactant. In addition, the width of the Pt F



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CONCLUSIONS The uniform growth of a Pt layer on a graphite surface by ALD was demonstrated for the first time by using a reactive O3 counter reactant. ALD Pt was performed on an HOPG substrate using the commercially available MeCpPtMe3 precursor and an O3 counter reactant and compared with that using an O2 counter reactant. ALD Pt using O2 is selectively deposited on the intrinsic step edges because the step edge sites are much more active for Pt nucleation than the basal planes. In contrast to O2, a continuous Pt film is obtained on the HOPG surface when O3 is used. Moreover, on HOPG pretreated with O3, Pt can be deposited uniformly even with an O2 reactant. The surface coverage of ALD Pt after a fixed number of ALD cycles increased with an increasing level of O3 pretreatment until it reached a plateau. XPS, AFM, and TEM results showed that the O3 etches the HOPG surface by the reaction with carbon and generates pits, and that Pt is deposited by ALD on the additional step edges generated by pit formation. The process is affected by deposition temperature. At a lower deposition temperature, the O3 did not generate many pits on the basal plane, so that the formation of Pt on the basal planes did not actively occur. At higher temperatures, a nearly continuous Pt film could be formed. This Pt ALD process using an active O3 reactant can be an important process for fabricating a uniform Pt catalyst on complex 3D electrodes for next-generation fuel cells.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03076. Number density and average size of ALD Pt nuclei with increasing O3 pretreatment time (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 6. XPS spectra of HOPG pretreated with O3 for 600 s at different temperatures in (a) the C(1s) binding energy range and (b) the O(1s) binding energy range. (c) Plot of the O/C ratio determined by XPS vs pretreatment temperature.

ACKNOWLEDGMENTS The studies and analysis of nucleation and growth mechanisms were supported by the Department of Energy under Award DESC0004782. The development of the ozone Pt ALD process was supported by the U.S. Department of Energy Hydrogen, Fuel Cells, and Infrastructure Program through the National Renewable Energy Laboratory under Contract DE-AC36-08GO28308.

precursor; however, there is no selective deposition by the selfsaturated ALD reactions under these conditions. Hence, the O3 reactant is advantageous for the fabrication of continuous Pt films on graphite by its active, in situ formation of nucleation sites on the otherwise inert surface. Although the O3 reactant etches out the carbon surface during Pt ALD, which leads to loss of carbon, the amount of carbon etched out is not significant compared to the total amount of carbon used in fuel cell electrodes. This process may provide a novel route to fabricating a highly efficient Pt catalyst on a complex 3D carbon electrode. In addition, by controlling the nucleation site density through variation in deposition or O3 exposure temperature, catalyst fabrication of varying particle shape and density on 3D electrodes can be achieved.

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