ARTICLE pubs.acs.org/JPCC
Atomic Layer Deposition of Pt on Tungsten Monocarbide (WC) for the Oxygen Reduction Reaction Irene J. Hsu,† Danielle A. Hansgen,† Brian E. McCandless,‡ Brian G. Willis,*,§ and Jingguang G. Chen*,† †
Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, United States Institute of Energy Conversion, University of Delaware, Newark, Delaware 19716, United States § Department of Chemical, Materials, and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut 06269, United States ‡
ABSTRACT: Atomic layer deposition (ALD) was utilized as a synthesis method to deposit monolayers of Pt onto WC substrates for applications as oxygen reduction reaction electrocatalysts. Samples utilizing various Pt ALD cycles were characterized using surface analytical methods and scanning electron microscopy, whereas cyclic voltammetry was used to determine whether the oxygen reduction reaction takes place on the catalyst surface. ALD Pt was found to deposit onto WC substrates following an island growth mechanism. When few Pt ALD cycles are used, discrete Pt particles first formed and dispersed over the WC substrate, but at least 100 ALD cycles is required for the WC substrate to be covered with Pt. Whereas Pt monolayers are not obtained, ALD Pt on WC still shows activity for the oxygen reduction reaction. Cyclic voltammetry conducted in an O2-saturated 0.5 M H2SO4 electrolyte indicate that as few as 20 Pt ALD cycles on WC is needed to produce oxygen reduction reaction activity that is comparable to Pt bulk. Efforts to make Pt films that are even thinner and more monolayer-like are desired, and potential approaches are discussed.
1. INTRODUCTION One of the most promising clean power generation technologies is the polymer electrolyte membrane (PEM) fuel cell, but it faces some technical hurdles in large-scale commercialization. One is the high material costs arising from expensive catalytic materials. For the oxygen reduction reaction (ORR), which takes place on the cathode side of the PEM fuel cell, Pt supported on high surface area carbon is the leading catalyst. Pt shows good ORR activity, but its high cost and limited supply drives up the overall cost of the fuel cell, making it difficult to compete with conventional technologies. In addition, the slow kinetics, small exchange current densities, and high overpotentials associated with the ORR make it difficult to find a catalyst that is both effective and inexpensive.1 To make fuel cells more commercially viable, alternative catalyst materials are desired to reduce the Pt loading. To date, a wide variety of materials have been investigated, including noble metal alloys,2,3 carbon materials and derivatives,4 and transition-metal complexes.4 One strategy to reduce the Pt loading is to use one monolayer of Pt supported on a metallic substrate. A collection of studies pertaining to Pt-modified surfaces in both traditional catalysis and electrochemistry are reported in the literature.5 For the ORR, one monolayer of Pt on a variety of metal substrates have shown promise as ORR catalysts.6-9 For example, Zhang et al. compared a monolayer of Pt on a variety of single crystal surfaces using both electrochemical experiments and DFT calculations. The authors found that the oxygen binding energy was a good r 2011 American Chemical Society
descriptor for ORR activity. Oxygen binding energies that are too weak impede O-O bond cleavage and inhibit additional reactions from occurring. However, binding energies that are too strong inhibit the formation of O-H bonds and instead facilitate H2O2 formation, an undesired side product that competes with the complete reduction of oxygen. The surfaces that had the best ORR activities had an intermediate oxygen binding energy. The authors determined that one monolayer of Pt on Pd(111) was found to have an optimal oxygen binding energy, and the calculations were confirmed with experiments. Because its electronic properties are similar to Pt, tungsten monocarbide (WC) has been found to be another good catalytic substrate for supporting Pt monolayers.10-12 WC is a promising material for fuel cell applications because it is inexpensive, electrically conductive, and stable in acidic environments over large potential ranges.12 For example, one monolayer of Pt on WC has been shown to exhibit activity for the hydrogen evolution reaction (HER) that is equivalent to that of bulk Pt, allowing a significant reduction of Pt in the catalyst without sacrificing HER activity.10 For the ORR, several studies have demonstrated improved ORR activity using Pt and tungsten carbide catalysts compared with the standard Pt/C.13-18 However, for most prior studies, the tungsten carbide is a phase mixture of WC and W2C and the Pt loadings are at least 10 wt %, Received: November 23, 2010 Published: February 14, 2011 3709
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The Journal of Physical Chemistry C which is comparable to Pt loading in typical Pt/C electrocatalysts. While the current literature shows promise to combine WC with Pt in ORR, the activity of lower Pt loadings on phase-pure WC has yet to be analyzed. The objectives of the current paper is to evaluate the hypothesis that monolayers of Pt on WC may provide activity to the ORR activity comparable to traditional Pt/C catalysts. One challenge to making monolayer catalysts is finding a suitable method to synthesize them. Previous surface science studies have used physical vapor deposition (PVD) to make model systems with Pt monolayers on planar single crystal or polycrystalline surfaces.10,11 However, PVD is a line-of-sight technique and cannot be extended to 3D powder catalysts. Another approach has been to use the galvanic displacement of a Cu underpotential deposited adlayer to make the Pt monolayers on supporting substrates. The WC substrate, however, is inert to Cu underpotential deposition19 and cannot form the Cu adlayer necessary to deposit Pt. Recently, atomic layer deposition (ALD) has been utilized to make uniform nanoparticle catalysts, mostly on oxide supports.20-23 ALD relies on a series of gassolid interactions, and nucleation and growth of some metallic particles on a substrate are dependent on the precursor adsorption onto the substrate. Because of this, ALD can successfully deposit catalytic materials onto a variety of substrate geometries in a highly controlled fashion. It is therefore predicted that the deposition of low loadings of Pt can be achieved on both planar and powder samples using this method. In this work, we investigate Pt-modified WC as an ORR catalyst, using ALD to deposit Pt monolayers on the WC substrates. Because Pt ALD on WC has not yet been demonstrated, the goal of the current paper is to determine the feasibility of depositing Pt onto planar WC foils as a model system. A range of Pt loadings on WC foil substrates was studied. Materials and electrochemical characterization were carried out to determine how the overlayer structure of Pt-WC changes with increasing Pt deposition, and how it affects the ORR activity.
2. EXPERIMENTAL AND THEORETICAL METHODS WC thin films were made by carburizing a W foil (Alfa Aesar, 0.1 mm thick, 99.9% pure) as previously described elsewhere.24 WC thin-film samples were subsequently placed in a flowthrough type ALD reactor.25 Prior to ALD growth, the WC thin films were dipped in 0.3 M NaOH solution to dissolve surface oxides.26 Samples were also rinsed with acetone and deionized water and dried prior to inserting into the ALD reactor. For Pt ALD, the precursor was (trimethyl)-methylcyclopentadienylplatinum(IV) (MeCpPtMe3) (Strem Chemicals, 99%), which is widely used for Pt ALD.20-22,27 The deposition temperature was 300 °C, while the bubbler holding the precursor was kept at 50 °C to volatize the precursor. Gas lines were held at 80100 °C to avoid precursor condensation. High-purity O2 was used as the coreactant, and high-purity N2 was used as both the carrier and purge gases. The pressure was maintained at 1 ( 0.1 Torr using a feed-back loop. The typical cycle times for each growth experiment were the following: 10 s Pt pulse, 30 s N2 purge, 2 s O2 pulse, and 30 s N2 purge. These relatively long precursor exposure times were used to ensure surface saturation. Auger electron spectroscopy (AES) data were recorded ex situ with a PHI 10-155 spectrometer with electron energy of 3 keV. X-ray photoelectron spectroscopy (XPS) measurements were also recorded ex situ with a PHI-5600 system, which was
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equipped with an Al KR X-ray source (1486.6 eV) and a hemispherical detector. Pass energy was set to 24.7 eV. Scanning electron microscopy (SEM) images were taken with a JSM 7400f high-resolution microscope using an accelerating voltage of 3 kV. Glancing incidence X-ray diffraction (GI-XRD) measurements were carried out on a Rigaku D/Max 2200 diffractometer with a 18.5 cm radius two-circle goniometer and fitted with Soller slit thin-film attachment and graphite monochromator for Cu KR radiation (λ = 0.154 nm). A source tube with stationary anode source was operated at 40 kV and 40 mA. Scans were acquired at an incident angle of 1° over a 2θ range of 5-50° with a step size of 0.02° and a scan rate of 0.2°/min. The sampling depth of the measurements, calculated using atomic scattering factors and mass absorption coefficients,28 is found to be 55 nm for WC. Interplanar d-spacings were calculated using Bragg’s law after smoothing and removal of Cu KR2 to spectrally purify the intensity pattern for Cu KR1 (λ = 0.15405 nm). The resulting peak intensities were tabulated and indexed by comparison with standard data found in the International Committee for Diffraction Data (ICDD) files to determine the crystalline phases. In selected cases, line profile analysis was carried out by fitting the diffraction peak to a Pearson VII function. The diffraction domain size was calculated by determining the excess integral breadth introduced by the sample and evaluating this using the Scherrer formula. Cyclic voltammetry (CV) measurements were carried out using a three-electrode cell with a standard calomel reference electrode (SCE) and Pt gauze as the counter electrode. All CV results are reported against normal hydrogen electrode (NHE). Measurements were performed using a Princeton Applied Research Potentiostat/Galvanostat Model 263A. The electrolyte solution was 0.5 M H2SO4 (Fischer Scientific, 96.9%) made with deionized water purified to ∼18 MΩ 3 cm-1 conductivity by a Barnstead NANOpure filtration system and subsequently purged with high-purity N2 (Keen, 99.99%) for 1 hour, followed by 30 min purge with high-purity O2 (Keen, 99.99%). The headspace was filled with O2 during the duration of the experiment. The thin-film electrodes were first dipped in 0.3 M NaOH solution to remove W-oxides, then thoroughly rinsed with deionized water and dried in air. Upon submerging in electrolyte solution, the working electrode samples were cycled between 0 to 0.8 V versus NHE at 10 mV/s for 10 cycles to clean the electrode surface, after which all subsequent experiments took place. Cu stripping experiments were performed in an electrolyte solution containing 0.1 M H2SO4 and 2 mM CuSO4. To deposit one atomic layer of Cu on Pt, a potential of 0.3 V versus NHE was applied to the working electrode sample for 120 s, followed by a linear sweep voltammogram (LSV) from 0.3 to 1.2 V versus NHE at a scan rate of 100 mV/s. Background LSVs were measured in 0.1 M H2SO4 using the same scan parameters. The difference curve between the Cu stripping voltammogram and background scan was then integrated to determine the charge transferred during stripping, QM, and surface area was determined using eq 1: QM A¼ s ð1Þ QM where QMs is 440 μC/cm2, the commonly accepted value for 2-electron charge on polycrystalline Pt.29 Binding energies of atomic oxygen were calculated using the Vienna ab initio Simulation Package (VASP) version 4.6.30,31 The PW-91 functional32 was used with ultrasoft Vanderbilt pseudopotentials33 and an energy cut off of 396 eV. All 3710
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The Journal of Physical Chemistry C calculations were performed using a 3 3 unit cell with over 16 Å of vacuum separating consecutive metal slabs. A 5 5 1 Monkhorst pack k-point grid was used for all slab calculations.34 For the Pt surface, the previously optimized lattice constant value of 4.01 Å was used.35 The oxygen binding energies were calculated on the (111) face of Pt with a total of four metal layers, where the bottom two were frozen to represent Pt bulk. Bulk lattice constants were optimized for the hexagonal closed packed (hcp) form of WC and were found to be a = 2.92 and c = 2.84. Binding energies were calculated on the tungsten terminated WC(0001) surface with three layers each of alternating tungsten and carbon. The topmost tungsten layer and carbon layer were relaxed, while the remaining layers were frozen. The Pt was added to the most stable site on the WC(0001) surface and allowed to relax in the calculations. All binding energies were calculated spin polarized and have been extrapolated to kBT = 0 eV. The binding energies were calculated through the following equation: ð2Þ ΔEO ¼ Eðslab þ OÞ - EðslabÞ - EðOÞ
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Table 1. Oxygen Binding Energies of Pt, WC, and 1 ML Pt on WC Using 1/9 ML Coverage of Atomic Oxygen surface
oxygen binding energy (kcal/mol)
Pt(111)
91.7
1 ML Pt on WC(0001) WC(0001)
92.9 173.2
Where E(slab) is the energy of the clean slab, E(O) is the energy of an oxygen atom in vacuum, and E(slabþO) is the energy of the slab with an oxygen atom at 1/9 ML coverage.
3. RESULTS 3.1. DFT Calculations. Results of the oxygen binding energy calculations for Pt(111), WC(0001), and one monolayer of Pt on WC(0001) are displayed in Table 1. The oxygen binding energy on WC(0001) is relatively high, predicting a poor ORR activity due to the strong bonding of atomic oxygen. However, the binding energy changes considerably once one monolayer of Pt is added, and the value is close to that of Pt(111). A previous calculation showed that when a monolayer of Pt is deposited on WC(0001), the surface d-band density of states is shifted to be similar to Pt(111).10 This trend is also observed in Table 1, which shows that the oxygen binding energy is similar between a monolayer Pt on WC(0001) and Pt(111). This similarity suggests that a monolayer of Pt on WC could be as effective of an ORR catalyst as bulk Pt. 3.2. ALD Pt-WC Experiments. Pt ALD experiments were performed using a range of ALD cycles on WC thin-film substrates. The surface compositions of these samples are shown in part a of Figure 1. These were calculated from ex situ AES using the general equation for composition, eq 3. IA SA ð3Þ CA ¼ P In =Sn n
The AES results show a linear increase in Pt composition with the number of ALD cycles. Corresponding XPS measurements show that the deposited Pt is metallic in nature based on the characteristic Pt peak positions (part b of Figure 1). The W and C compositions steadily decrease with increasing ALD cycles. The O content remains relatively constant with all samples, although it lowers slightly after full monolayers of Pt are produced, such as on the 100-cycle sample. An XPS scan (part c of Figure 1) of the W4f region shows peaks at 31.5 and 33.9 eV, representing the W4f7/2 and W4f5/2 peaks respectively, characteristic of WC. W-oxide peaks are not present in the spectra, suggesting that the WC substrate is stable under deposition conditions. Therefore, the oxygen measured by AES is not believed to be associated with W-oxides formed during the O2
Figure 1. (a) Elemental composition determined from ex situ Auger spectroscopy for 3, 10, 30, 50, and 100 Pt ALD cycles on WC foils, and ex situ XPS spectra for (b) W4f and (c) Pt4f regions.
ALD pulses. Rather, the oxygen is likely surface contamination from air exposure. The carbon atomic weight percentages in Figure 1 include both carbidic carbon associated with WC and carbonaceous carbon, which is an undesirable contaminant. Because carbon is needed to form WC, some residual carbonaceous carbon is present on the WC thin film prior to ALD. The Pt overlayer also adsorbs C and O when exposed to atmosphere. The C depletion seen in part a of Figure 1 is largely due to depletion of carbidic C, and the majority of the carbonaceous C and O present on the sample is believed to be derived from exposure to atmosphere. GIXRD measurements for the same samples are shown in Figure 2 with a corresponding list of phases and d-spacings shown in Table 2. The measurements were taken at an incident angle of 1°, which equates to a sampling depth of 55 nm calculated based on X-ray absorption properties. Peaks 3 and 4 represent Pt(111) and Pt(200) and are detected only for the 100cycle sample, although a small signal can be seen in the 50-cycle sample. This does not mean that Pt is not present on the other samples, but the Pt loadings are so low that they cannot be detected. Scans were also taken at 0.5° incidence angle but the Pt peaks were still undetected in the 10, 30, and 50 cycle samples. GIXRD confirms that the substrate is phase-pure WC with no indication of other carbide phases such as W2C or WC1-x. The WC thin film is thick enough so that there is no signal from the underlying W foil. A Scherrer analysis of the Pt peaks (peak numbers 3 and 4) for the 100 cycle sample found that the 3711
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The Journal of Physical Chemistry C
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Figure 2. Glancing incidence X-ray diffractractograms for Pt-WC thin films after 10, 30, 50, and 100 Pt ALD cycles.
Table 2. Peak Identification and Theoretical d-Spacings for Corresponding GIXRD Data in Figure 3 peak no.
2θ (deg)
measured d-spacing (A)
identified phase
1
31.550
2.833
WC(0001)
2
35.800
2.506
WC(1010)
3
39.950
2.255
Pt(111)
4
46.550
1.949
Pt(200)
5
48.450
1.877
WC(1011)
diffraction domain size is ∼10 nm, which is in agreement with measurements taken from the SEM images. The linear increase in Pt at% seen in Figure 1 implies that a relatively consistent amount of Pt is deposited after each cycle. However, SEM images (Figure 3) of Pt-WC samples of varying Pt deposition cycles, after background subtraction, show that the deposited Pt form disperse particles instead of layer-by-layer growth. Previous studies have observed that the thin-film growth rate increases after a Pt seed is formed.36 This growth rate increase has been attributed to a higher affinity for MeCpPtMe3 adsorption and decomposition on Pt sites.37,38 The reactivity difference between WC and Pt sites leads to a preference for particles to grow rather than nucleate new particles, as observed in Figure 3. For the Pt/WC sample after 10 ALD cycles (part a of Figure 3); the particles are small (
