Hydrogen Adsorption, Absorption, and Desorption at Palladium

Article pubs.acs.org/JPCC

Hydrogen Adsorption, Absorption, and Desorption at Palladium Nanofilms formed on Au(111) by Electrochemical Atomic Layer Deposition (E-ALD): Studies using Voltammetry and In Situ Scanning Tunneling Microscopy Leah B. Sheridan,† Youn-Geun Kim,† Brian R. Perdue,† Kaushik Jagannathan,† John L. Stickney,*,† and David B. Robinson‡ †

Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States Energy Nanomaterials Department, Sandia National Laboratories, Livermore, California 94550, United States

‡

ABSTRACT: Pd nanofilms were grown on Au(111) using the electrochemical form of atomic layer deposition (E-ALD). Deposits were formed by repeated cycles of surfacelimited redox replacement (SLRR). Each cycle produced an atomic layer of Pd, allowing the reproducible formation of Pd nanofilms, with thicknesses proportional to the number of cycles performed. Pd deposits were formed with up to 30 cycles, in the present study, and used as a platform for studies of hydrogen sorption/desorption as a function of thickness. The SLRR cycle involved the initial formation of an atomic layer of Cu by underpotential deposition, followed by its galvanic exchange with PdCl42− ions at open circuit. The first three cycles were studied using in situ electrochemical scanning tunneling microscopy (ECSTM), which showed a consistent morphology from cycle to cycle and the monatomic steps indicative of layer-by-layer growth. Cyclic voltammetry was used to study the hydrogen sorption/desorption properties as a function of thickness in 0.1 M H2SO4. The results indicated that the underlying Au structure greatly influenced hydrogen adsorption, as did film thickness for deposits formed with fewer than five cycles. No hydrogen absorption occurred for the thinnest films, although it increased linearly for thicker films, producing an average H/Pd molar ratio of 0.6. Electrochemical annealing was shown to improve surface order, producing CVs that strongly resembled those characteristic of bulk Pd(111).

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Semiconductor and superlattice nanofilms,1,5 nanoclusters,1,5 and nanowires have all been formed using E-ALD. The development of surface-limited redox replacement (SLRR) by Brankovic et al.,6 Mrozek et al.,7 Vasilic and Dimitrov,8 and Kim9 made it possible to extend E-ALD to the formation of metal nanofilms. The cycle for an SLRR begins with the formation of an atomic layer of a “sacrificial” metal via UPD. The UPD precursor solution is then replaced with one containing a precursor for a more noble metal, allowing the sacrificial atomic layer to be spontaneously exchanged for the more noble metal by redox replacement at open circuit. The extent of the reaction is limited by the number of atoms in the sacrificial atomic layer and the stoichiometry of the exchange reaction. A nanofilm is formed by repeating this cycle any number of times: the thickness is proportional to the number of cycles.10−12 To date, SLRR has been used to form nanofilms of Cu,10,11,13−16 Ag,17 Ru,12 Pt,6,7,18,19 Pt/Ru,20 Pd,21,22 and Au.23 Pd deposition using an SLRR was first reported by Brankovic et al.,6 who observed that a single replacement of CuUPD produced a nearly uniform Pd ML on Au(111). Soon after,

INTRODUCTION

The interactions of hydrogen with Pd nanofilms and nanostructures are of continued significance, given their importance for hydrogen storage, sensing, and catalysis. Nanofilms or structures of Pd allow electrochemical differentiation of surface adsorption versus absorption or bulk hydride formation. The electrochemical analog of atomic layer deposition (ALD), E-ALD, uses solution based electrochemical surfacelimited reactions (SLRs) to deposit conformal nanofilms.1 SLRs, those where the reaction stops once the surface is covered, are used in a cycle to grow films one atomic layer at a time (layer-by-layer). Most SLRs result in the deposition of an atomic layer, one that is no more than one atom thick with a coverage of a monolayer (ML) or less. A ML is defined here as one depositing atom for every substrate surface atom. Electrochemical SLRs typically involves underpotential deposition (UPD). UPD is the electrodeposition of one element on another at a potential prior to, or “under,” that needed to deposit the element on itself and results from the ΔG°f of a surface compound or alloy.2−4 In E-ALD, the formation of an atomic layer takes place in a cycle, and the more cycles performed, the thicker the deposit. © 2013 American Chemical Society

Received: May 13, 2013 Revised: July 1, 2013 Published: July 8, 2013 15728

dx.doi.org/10.1021/jp404723a | J. Phys. Chem. C 2013, 117, 15728−15740

The Journal of Physical Chemistry C

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Figure 1. Schematic of an E-ALD cycle using SLRR to form a Pd nanofilm on Au(111). Note that CuUPD deposited directly on Au(111) is replaced by Pd ions to form the first Pd atomic layer.

Weaver and coworkers7 reported the formation of Pd SERSactive substrates by depositing Pd MLs on roughened Au using SLRR of either PbUPD or CuUPD. A Pd E-ALD cycle has been developed and optimized by this group on polycrystalline Au, via SLRR of CuUPD, and is described elsewhere.21,22 Those reports indicate an SLRR mechanism where electrons are indirectly transferred from the sacrificial metal to the depositing noble-metal precursor ion by way of the electrode rather than one where electrons were transferred directly between the sacrificial metal and the depositing metal ion. This nonlocal mechanism suggests that inhomogeneous deposition across the electrode surface can occur, depending on the rate of solution exchange between the sacrificial metal ion and the noble-metal precursor solution. The addition of sufficient Cl− ions to complex the Pd2+ ions has been shown to effectively reduce the exchange rate relative to the time scale for solution introduction, promoting deposit homogeneity. Those results also showed 96% exchange efficiency for electrons from the sacrificial Cu atomic layer to Pd2+ ions.22 Single-crystal substrates are of interest for fundamental studies of Pd nanofilm formation because well-defined surfaces allow investigations of deposit structure and the energetics of hydrogen adsorption as a function of crystallographic orientation. A clear demonstration of this comes from the distinct features in cyclic voltammetry (CV). CVs can provide atomic scale characterization of the relationship between adsorption and potential. The addition of in situ electrochemical scanning tunneling microscopy (EC-STM) allows investigation of the structure reactivity relationship. Over the last few decades the electrodeposition of Pd on a variety of single-crystal noble metal substrates has been reported.24−28 Specifically, Pd electrodeposition on Au(111) from Cl−containing solutions has been extensively studied, providing an excellent comparison with Pd films formed using E-ALD. Kolb et al.29,30 reported the formation of epitaxial Pd overlayers on Au(111) with a well-ordered structure, although the epitaxial deposition persisted for only the first 2 MLs and roughness and defect formation increased for thicker deposits. Both Uosaki et al.31,32 and Kolb et al.30 imaged Pd overlayers on Au(111) with EC-STM and found no signs of significant surface alloying. They also described the role of a distorted hexagonal adlayer of PdCl42− in promoting layer-by-layer growth.

In the present study, E-ALD of Pd nanofilms on Au(111) was investigated using CV and EC-STM. EC-STM was used to characterize the growth mode of the first three cycles of Pd deposits formed using SLRR and CuUPD sacrificial layers. CVs were used to examine films formed with up to 30 cycles. The effects of deposit thickness on both surface and bulk hydride formation are discussed. In addition, an “electrochemical annealing” procedure was developed and found to improve film surface order.

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EXPERIMENTAL SECTION EC-STM experiments were performed on facets of a Au bead single-crystal surface prepared using a variation of the Clavilier method.33,34 The flame-annealed bead was quenched in hydrogen-saturated 18 MΩ-cm water and quickly transferred with a protective water film through air into the EC-STM cell. The electrochemical cell used for EC-STM studies allowed flow and exchange of solutions over the Au bead working electrode and included downstream reference and auxiliary electrodes to avoid contamination. A single peristaltic pump (0.9 mL/min) was used to maintain equivalent flow rates in and out of the cell, ensuring a constant solution level, as previously described.9,35 The supporting electrolyte contained 0.05 M doubly distilled H2SO4 (Aldrich Chemicals, Milwaukee, WI) in 18 MΩ-cm water. The Pd solution consisted of 0.1 mM ultrapure-grade PdCl2 (Aldrich Chemicals) in 50 mM HCl. The Cu solution was 1 mM ultrapure-grade CuSO4 (Aldrich Chemicals) in 0.05 M H2SO4. All solutions were prepared using 18 MΩ-cm water and were sparged using N2 to minimize dissolved O2 prior to electrochemical studies. The electrochemical cell included a Au-wire auxiliary electrode and a Ag/ AgCl reference electrode (3 M KCl) (Bioanalytical Systems), against which all potentials were reported. A Nanoscope III (Digital Instruments, Santa Barbara, CA) equipped with W tips electrochemically etched (15 V, AC in 1 M KOH) from a 0.25 mm wire, were used in all EC-STM measurements. Transparent nail polish was used to coat the tip and minimize Faradaic currents. Wide-angle and atom-resolved EC-STM images in pure electrolyte were obtained prior to each experiment to confirm the presence of a well-ordered Au(111)-(1 × 1) structure. For voltammetric studies, Pd films were deposited on a Au(111) single-crystal disk in the cell of the automated electrochemical flow deposition system1,36,37 (Electrochemical 15729



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Figure 2. Time−potential−current plot of one complete Pd E-ALD cycle. CuUPD was formed at 0.15 V; Pd solution was flowed in and allowed to replace Cu until the OCP reached 0.4 V. A 60 s blank rinse, while still at OCP, completed the cycle.

Figure 3. Wide-area (300 × 300 nm) EC-STM images of Pd deposited on Au(111) after one (A), two (B), and three (C) replacement cycles. Images were obtained at +0.45 V. Bias voltage: −390 mV, tunneling current: 5 nA.

parallel to the working electrode but on the opposite side of the flow cell. The cell also contained a Ag/AgCl (3 M KCl) reference electrode (Bioanalytical Systems), against which all potentials are reported. The Au(111) disk had an exposed area

ALD L.C., Athens, GA) consisting of a variable speed pump, five valves, five solution reservoirs, a potentiostat, and E-ALD “Sequencer” software. The electrochemical flow cell (volume ≈ 0.15 mL) included an imbedded Au wire auxiliary electrode, 15730



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of 0.283 cm2, defined by the hardware used to hold it in the flow cell. Prior to loading, the disk was sonicated in acetone, rinsed three times with 18 MΩ-cm water (Milli-Q Advantage A10 water filtration system), sonicated in concentrated HNO3, rinsed three more times in 18 MΩ-cm water, and finally flameannealed in a H2 flame with subsequent cooling in N2. CVs were then performed from −0.2 to 0.9 V in 0.1 M H2SO4 and −0.2 to 0.7 V in 50 mM HCl to ensure crystallinity. X-ray analysis was performed on a PANanalytical X’PERT Pro with an open Eulerian cradle utilizing a 1.54 nm Cu Kα1 source.

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RESULTS AND DISCUSSION A schematic of the SLRR mechanism is depicted in Figure 1 and a current−potential time plot for the fourth E-ALD cycle of a five-cycle Pd nanofilm is shown in Figure 2. The cycle began by flowing the Cu2+ solution for 18 s at 17 mL/min at an applied potential of 0.15 V to deposit CuUPD. The charge passed for the sacrificial Cu layer was 463 μC/cm2, equivalent to 0.95 ML versus Pd(111). Okada et al.38 reported the formation of a complete (1 × 1) ML of CuUPD on bulk Pd(111), while Cuesta et al.39 reported a slightly lower coverage of 0.82 ML but concluded that error due to doublelayer corrections and possible anion coadsorption could account for the lower value. The auxiliary electrode allowed establishment of the open circuit potential (OCP), and the Pd2+ ion solution was introduced, allowing exchange between CuUPD and Pd2+. The OCP shifted positively to 0.4 V, the “stop potential”. Control was programed to monitor the OCP, and when the stop potential was achieved it triggered a blank rinse to remove excess Pd2+ ions and any Cu2+ ions formed. OCP continued to drift during the rinse, reaching ∼0.5 V, near the onset of Pd oxidation. This cycle was then repeated between 1 and 30 times. EC-STM Measurements. A CV of Cu UPD on Au(111) in 1 mM CuSO4 and 50 mM H2SO4 and the corresponding STM image were taken (not shown here). The CV displayed two well-known UPD features representing the 2/3 ML (√3×√3)R30° structure and completion of the pseudomorphic (1 × 1) Cu ML in agreement with the literature.9,40−42 The STM image confirmed the (√3×√3)R30° “honeycomb” structure formed at 0.25 V.9,43−45 A similar UPD layer of Cu, formed at 0.15 V, was replaced by Pd at open circuit, followed by a blank rinse. Wide-area EC-STM images (300 × 300 nm) taken at 0.45 V after one, two, and three replacement cycles (Figure 3) show evenly dispersed monoatomically high islands, as determined by section analysis. Qualitatively, these images illustrate consistent morphology from cycle to cycle. Highresolution images were also obtained (Figure 4); however, atomic resolution was not possible. After the second replacement cycle, the height profile, obtained from section analysis, showed an average step height of 0.21 ± 0.01 nm (Figure 4), a value indicative of Pd monatomic steps, which provide strong evidence of layer-by-layer growth (Frank−van der Merwe).46 Metal surface ordering due to the adsorption of various electrolyte anions is well known.47,48 Iodine adsorption on single-crystal Au, Cu, Pd, and Pt and its promotion of surface mobility has been extensively studied using UHV-EC methods and EC-STM.49−54 Room-temperature iodine adsorption on disordered Pd(111) and (100) surfaces has been shown to promote spontaneous reordering via surface reconstruction.9,49,55 Studies of halide adsorption on metal surfaces

Figure 4. High-resolution EC-STM image and section analysis of Pd deposited on Au(111) after two replacement cycles. Height profile extracted along the black line is presented; average step height is 0.21 ± 0.01 nm. Image was obtained at +0.45 V. Bias voltage: −390 mV, tunneling current: 10 nA.

indicate that close-packed hexagonal adlayers are frequently formed and show high surface mobility. Halide atoms can be thought of as forming a single bond with the substrate and display a size consistent with their van der Waals diameter. These halide layers act as a blanket, covering the surface conformally. Figure 5 compares EC-STM images of three-cycle films of Pd before and after iodine adsorption at 0.45 V. Note that the iodine atomic layer is imaged in Figure 5D−F, not the underlying Pd atoms, although the iodine layer is conformal on the underlying Pd atoms. The increased surface mobility of the Pd atoms appears to be induced by the adsorbed iodine and the use of potentials close to Pd oxidation, producing a kind of electrochemical annealing.56,57 Increases in terrace sizes and surface order are evident in Figure 5E,F. The height profile in a high-resolution image (Figure 6) shows an average step height of 0.22 ± 0.01 nm, indicating preservation of the original Pd monatomic terraces. X-ray Diffraction Measurements. A 60 E-ALD cycle Pd film was formed and characterized via X-ray diffraction. Pole figures were collected to investigate the orientation of the Pd nanofilm with respect to the underlying Au(111) substrate. A θ−θ scan was initially collected and indicated that the Pd nanofilm had a preferred (111) orientation, suggesting the collection of a pole figure focused on Pd(111), displayed in Figure 7. The Au(111) substrate pole figure is depicted in Figure 7A and shows three peaks at ψ of 70° (red line) due to diffraction from Au(1̅11), (11̅1), and (111̅) planes. Figure 7B is the pole figure for the Pd nanofilm, which displays two sets of 15731



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Figure 5. EC-STM images of three cycles of Pd SLRR on Au(111). (A−C) As-deposited Pd. (D−F) Acquired after iodine adsorption. Images were obtained at +0.45 V. Bias voltage: −390 mV, tunneling current: 5 nA.

same orientation as the Au substrate, suggesting an epitaxial deposit. The next series of major peaks occurs at ψ = 55°, the angle corresponding to (100) planes. When examining FCC crystals with Fm3̅m symmetry, such as Au and Pd, the angular difference in ϕ between the (111) scattering vector and the (100) is 60°, equivalent to that displayed in Figure 7B. These peaks show that Pd(111) domains are present on Au(100) facets. The three small peaks, innermost in Figure 7B (ψ = 35°), are at positions corresponding to (110) planes, indicating that some Pd(111) domains exist on Au(110) facets. These results are consistent with a preference for Pd nucleation and growth with a (111) orientation in Pd E-ALD films. Voltammetric Measurements: Hydrogen Adsorption. Pd films of varying thickness were formed on Au(111) using between 1 and 30 E-ALD cycles. The amount of Pd deposited was determined in two ways: by summing the charges for CuUPD each cycle and by stripping the Pd film anodically in 50 mM HCl by scanning at 5 mV/s from 0.15 to 0.70 V. Coverages were calculated with respect to the Au(111) surface where 1 ML was equivalent to 448 μC/cm2. Figure 8 is a graph of Pd coverage versus the number of E-ALD cycles and closely resembles that previously reported for E-ALD of Pd films on polycrystalline Au.22 The relationship is linear and passes through the origin, an important indicator of an ALD process. These results along with the EC-STM image support a layer-bylayer growth mechanism. In addition, the charge for Cu UPD remained constant each cycle, indicating no surface roughening, in contrast with Pd films formed by direct electrodeposition at constant potential from a Cl− containing solution, which showed signs of 3D growth after only 3 MLs.29 Figure 9 compares CVs in 0.1 M H2SO4, between −0.25 and 0.2 V, of Pd films formed with one (solid line) and two (dotdash line) cycles, as well as for the clean Au(111) substrate (dotted line). Scanning negatively, peaks were evident at −0.03 and −0.08 V, for both of the deposits, and are ascribed to hydrogen adsorption and desorption on the Pd surface because

Figure 6. High-resolution EC-STM image of three cycles of Pd SLRR on Au(111) after iodine adsorption. The corresponding height profile shows an average step height of 0.22 ± 0.01 nm. Images were obtained at +0.45 V. Bias voltage: −390 mV, tunneling current: 10 nA.

three major peaks, each three-fold symmetric, consistent with (111) symmetry. The peaks at ψ = 70° are superimposable with the peaks in Figure 7A, showing the Pd nanofilm to have the 15732



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Figure 7. 2.5d pole figure taken in the Pd(111) θ−θ configuration for a 60 cycle Pd SLRR film (A) and a pole figure taken in the Au(111) θ−θ configuration for the Au(111) substrate (B).

Figure 8. Plot of Pd coverage (i.e., film thickness) versus the number of E-ALD cycles performed, indicating a linear relationship.

they are completely absent from CVs of bare Au(111).58,59 At potentials negative of −0.17 V, hydrogen absorption becomes important, which will be discussed later. Reversal of the scan at −0.25 V resulted in three oxidative features. The broad peak at −0.074 V and a sharper peak (dotted-dashed line) at 0.015 V represent desorption of the Pd surface hydride. Figure 10 displays CVs for 2-, 3-, 5-, and 10-cycle films between −0.2 and 0.15 V. The potential of the main peak for reductive hydrogen adsorption shifts negative by 5 mV for films of three cycles or more, while the potential for the primary hydrogen desorption peak shifts negative by 15 mV. The CV of the three-cycle film (Figure 10, dashed line) suggests a transition between the twocycle and five-cycle films, as the primary hydrogen desorption peak is a doublet. For more than five cycles, the hydrogen adsorption and desorption peaks display more reversibility and overlap. Similar results have been reported by Baldauf and Kolb29 and Kibler et al.,60 although for Pd deposited at constant potential on Au(111) from Cl− containing solutions. In those studies, it was suggested that the more positive peak represents hydrogen adsorbed on and desorbed from Pd island edge sites. As the Pd islands coalesce with two more cycles, to form uniform terraces,

the hydrogen adsorption/desorption peaks shift negative as their surface coordination decreases. Pd deposited at constant potential on Au(111) has been shown, using surface X-ray scattering (SXS), to undergo a transition from a strained pseudomorphic ML to a deposit closely resembling bulk Pd(111) as it thickens.61 The shifts observed in the present study (Figure 10) might also be explained by relaxation of the second layer of Pd atoms, which then more closely resembles bulk Pd. The CVs in Figure 10 for 3-, 5-, and 10-cycle Pd deposits closely resemble those in the literature for bulk Pd(111).58,62,63 Adsorption energies (Eads) were recently calculated, using density functional theory (DFT), for Pd on Au(111).64−68 Assuming that the position of the adsorption/desorption peaks is a direct measure of the stability of the surface hydride, those DFT calculations may help explain the present results. It was suggested that strain, typically found in thin deposits, causes the d-band center to shift to higher energies, leading to stronger adsorbate interactions with the surface, possibly resulting from geometric or electronic effects due to the underlying Au.68 Kibler et al.64 suggest that a pseudomorphic Pd ML on Au(111) experiences an upshift in the d-band center compared 15733



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Figure 9. CVs of Au(111) without Pd (0x) compared with one and two E-ALD cycles of Pd on Au(111) in 0.1 M H2SO4 at 5 mV/s.

Figure 10. CVs of Pd nanofilms on Au(111), ranging from 2 to 10 E-ALD cycles in thickness, in 0.1 M H2SO4 at 10 mV/s.

while Alvarez et al.69 suggested the coadsorbed species was HSO4−, based on IR spectra of Pd overlayers on Pt(111). In addition, the CO displacement method, introduced by Clavilier et al.,71 has been used to correct the charge for the coadsorbed species.69,70 However, in contrast with the above-mentioned studies, Duncan and Lasia72 studied Pd on Au(111) in both HClO4 and H2SO4 and reported little difference in charge for the hydrogen adsorption/desorption peaks as a function of anion. Given the limited anionic dependence, shown by Duncan and Lasia, the charges measured here will be assigned to hydrogen alone for simplicity. Figure 11 indicates the maximum hydrogen adsorption for three-cycle films (2.5 ML, Figure 8), after which the charges diminish slightly over the next seven cycles. Similar trends were reported by Baldauf and Kolb29 for Pd films deposited at

with Pd(111), which would correspond to a positive shift in the adsorption potential (more negative Eads). A second report by Kibler65 and previous reports by Roudgar et al.66,67 indicate that the largest Eads occurs for two pseudomorphic MLs and decreases with increasing thickness, becoming equivalent to bulk Pd(111). Those calculated values for Eads clearly correlate with the observed peak shifts seen in Figures 9 and 10 for deposits formed using E-ALD. Figure 11 is a plot of the hydrogen adsorption charges, between −0.12 and 0.08 V, from Figures 9 and 10. It has been reported that desorption (adsorption) of hydrogen on Pd in sulfuric acid occurs simultaneously with HSO 4 − /SO 4 2− adsorption (desorption).58,63,69,70 Kim et al.59 and Wan et al.58 studied Pd(111) using STM and suggested that sulfate adsorption was accompanied by water molecule adsorption, 15734



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Figure 11. Plot of the H-adsorption charge versus the number of E-ALD cycles performed (i.e., thickness). H-adsorption charges were obtained from CVs of Pd nanofilms taken in 0.1 M H2SO4.

Figure 12. CVs of repeated cycling of a 23-cycle E-ALD Pd film in 0.1 M H2SO4 at 10 mV/s.

constant potential on Au(111). They suggested the charge decreased due to island coalescence and film smoothening with the thicker deposits. The plateau in Figure 11 at 254 μC/cm2 can be compared with the 240 μC/cm2 expected for a hydrogen ML on Pd(111). The Pd coverage for the first cycle was 0.6 ML (Figure 8), while the hydrogen coverage was 0.68 ML, suggesting that some hydrogen may be adsorbed on island edges. After two cycles the Pd coverage was 1.5 ML and the hydrogen coverage was 1.1 ML, indicating that Pd covered the whole surface. These coverages are also consistent with the STM images in Figures 3 and 4 and suggest that some hydrogen is adsorbing at step and island edges. A recent computational study predicted a

Pd(211) surface would be expected to have more than 1 ML (up to ∼1.3 MLs) of adsorbed hydrogen.73 Voltammetric Measurements: Hydrogen Absorption. Hydrogen absorption starts to become significant just below −0.1 V (Figure 10). Rapid hydrogen desorption is evident upon potential reversal at −0.2 V. The charges for absorption and desorption both increased with increasing numbers of cycles. Figure 9 indicates the presence of hydrogen absorption and desorption after the first cycle, where the Pd coverage is only 0.6 ML, although previous reports indicate that hydrogen absorption occurs only for Pd films greater than 2 MLs.29,74,75 It is possible that there were many Pd islands three atomic 15735



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Figure 13. CVs of an as-deposited 15-cycle E-ALD Pd film before and after EC annealing in 0.1 M H2SO4 at 10 mV/s. Scans 1, 2, and 5 were recorded after EC annealing.

thickness, as similar results were observed for 3- and 10-cycle films. The trend was to more reversible voltammetry, suggesting a more ordered film. Analogously, Duncan and Lasia72 reported similar behavior for a 0.8 ML Pd film on Au(111) and attributed the shift to a decrease in steps due to coalescence of islands. This behavior can be considered electrochemical annealing. An electrochemical annealing program for Pd E-ALD films was developed to improve surface order. Electrochemical annealing is generally performed at potentials near the start of surface reactions such as oxidation or the HER, where exchange currents are high and surface mobility promoted, producing results analogous to those generally observed with the increase in surface diffusion accompanying an increase in temperature for surfaces in vacuum.83,84 In addition, adsorbates can promote electrochemical annealing by coordinating with surface atoms, resulting in significant surface and electronic changes.85−88 Initially, 20 CVs in 0.1 M H2SO4 at 10 mV/s between 0.1 and 0.5 V, where no significant voltammetric features were observed, were employed on a 15 E-ALD cycle Pd film. Figure 13 displays CVs taken into the hydrogen sorption region (0.15 to −0.2 V) before and after this EC annealing program (i.e., 20 CVs). The hydrogen adsorption/ desorption peaks for the as-deposited film were broad and split from each other by 0.06 V (short dashed line). Scan 1 (long dashed line) was performed immediately following the EC annealing, between 0.1 and 0.5 V, and the adsorption peaks shifted positive by 0.02 V, while the desorption peak shifted by 0.005 V, for a CV which appeared sharper and more symmetric. In the second scan after EC annealing, Scan 2 (solid line), the adsorption/desorption peak splitting was reduced further, to 0.01 V, and both the adsorption and desorption peaks were sharper. The positions and shapes of the peaks stabilize after Scan 5 (dotted line) between 0.2 and −0.15 V. The appearance and positions of the stabilized peaks agree closely with those reported for CVs of flame-annealed Pd(111) electrodes, indicating excellent surface order.58,63,70,89 This type of EC annealing is similar to using adsorbed iodine to promote surface

layers thick on the surface, although there was no indication of them from EC-STM (Figures 3 and 4). More likely, the features below −0.175 V (Figure 9) are due to the hydrogen evolution reaction (HER) and then oxidation of some of the resulting H2 after potential reversal. This theory was tested by simply increasing the solution flow rate upon potential reversal. If the oxidation feature were due to oxidation of H2 gas produced by HER, then increasing the flow rate should rinse the H2 gas away before it can be oxidized. Indeed, the feature in question was diminished by increasing the flow rate, indicating that it was due to H2 oxidation, not hydrogen desorption. This agrees with studies of Pd overlayers on Au(111) as catalysts for the HER, in which increased catalytic activity was reported for Pd submonolayers, thought to result from mixed Pd/Au step sites.76−78 Hydrogen to Pd molar ratios (H/Pd) were obtained in 0.1 M H2SO4 by stepping to −0.25 V long enough to saturate the Pd film with hydrogen. A subsequent scan to 0.15 V was used to quantify the absorbed hydrogen via oxidative coulometry. The charge for desorption was used to quantify the adsorbed hydrogen, as some of the reductive charge below −0.15 V was due to the HER. To determine the amount of Pd present, we subsequently stripped the films by scanning from 0.15 to 0.7 V in 50 mM HCl. The average H/Pd ratio was 0.6 for 3-, 5-, 10-, and 30-cycle films, in good agreement with values reported in the literature (0.59 to 0.73).79−82 Voltammetric Measurements: EC Annealing. Figures 9 and 10 demonstrate that hydrogen adsorption and desorption peak morphologies are sensitive to Pd coverages. Repetitive potential cycling of the hydrogen region resulted in changes in the shapes and positions of the peaks, even while avoiding Pd surface oxidation. For example, CVs of a 23 -cycle Pd film in the hydrogen sorption region are plotted in Figure 12. The arrows in Figure 12 indicate the increases and decreases in peak height with increasing numbers of CVs. These shifts were most evident for hydrogen adsorption, where the peak at −0.04 V began to diminish as a shoulder formed at −0.02 V, eventually forming a new peak. This behavior was not a function of film 15736



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directly using an overpotential. However, when more than a couple of MLs were required, direct electrodeposition resulted in roughening, while the E-ALD deposits remained epitaxial. In the formation of nanofilms, the ability to control the amounts deposited at the atomic level is invaluable. It is one thing to electrodeposit homogeneous layers under well-controlled conditions on a perfectly flat single crystal using an overpotential and another to deposit a conformal monolayer on complex surface structures, such as catalysts and powders, with the atomic layer control provided by an ALD method.

ordering, as was described in the EC-STM results (Figure 5). The use of iodine in the CV studies was problematic, however, because the iodine had to be removed before CVs in the hydrogen adsorption/desorption region could be performed on the Pd surface. Schimpf et al.90 reported a procedure for regenerating clean and ordered Pd(100) using iodine adsorption and desorption. A similar study of the effect of iodine on E-ALD Pd nanofilms would be worthwhile. Returning to Figure 12, note that the final peak positions appear somewhere between the as-deposited and post-EC annealed (scan 5) CVs shown in Figure 13. It is probable that the peaks in Figure 12 would have continued improving with prolonged cycling in the hydrogen sorption region. However, repeated hydrogen absorption and desorption would likely induce stress and could cause plastic deformation of the film due to Pd lattice expansion and contraction upon insertion and removal of hydrogen.91 For this reason the EC annealing program should include 20 CVs between 0.1 to 0.5 V, followed by only five scans to −0.2 V in the hydrogen sorption region.

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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 We acknowledge the support of the National Science Foundation, Division of Materials Research #1006747 as well as the Laboratory Directed Research and Development program at Sandia National Laboratories, a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

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CONCLUSIONS E-ALD Pd nanofilms were formed on Au(111) using a SLRR where UPD Cu was exchanged for Pd. EC-STM images after one, two, and three cycles revealed monatomic steps and consistent morphology from cycle to cycle, indicating layer-bylayer growth. The linear relationship between Pd coverage and cycle number for nanofilms formed with up to 30 cycles, the maximum thickness studied in this report, demonstrated this layer-by-layer growth. These results were contrasted with reports of Pd deposited by conventional electrodeposition, in which Kolb et al. observed 3D growth after the electrodeposition of 3 MLs of Pd at an overpotential on Au(111)29 and Takahasi et al. reported an increase of surface roughness for higher Pd coverages on Au(111).92 The unique control over deposit thickness afforded by EALD proved useful for studying the effect of Pd film thickness on the hydrogen adsorption and absorption features, in part because for thicker Pd films hydrogen absorption masks voltammetry for adsorption. CVs of nanofilms deposited using 1 to 30 E-ALD cycles were performed in 0.1 M H2SO4. The underlying Au displayed a significant influence on the positions for hydrogen adsorption/desorption features in deposits formed with fewer than five cycles. Hydrogen absorption occurred for films formed with more than two cycles and produced an average H/Pd ratio of 0.6, in agreement with the literature. All Pd nanofilms deposited displayed features corresponding to the adsorption of hydrogen, even those formed with only one or two cycles. An EC annealing procedure in halide-free H2SO4 was also developed. CVs indicated improved surface ordering evidenced by sharper peaks and increased reversibility for adsorption/ desorption, producing a surface closely resembling an ideal Pd(111). Characterization of a 60-cycle Pd deposit using X-ray diffraction pole figures showed the Pd crystal planes to be epitaxial in orientation with the Au substrate. In addition, it was shown that Pd(111) domains were formed on all the low index planes of Au, suggesting that nucleation and growth of Pd was preferentially (111). Overall, the use of E-ALD has been shown to facilitate a range of experiments where the amounts and morphology of Pd deposits must be known and controlled on the atomic scale. In several cases the present results simply agreed with studies in the literature where monolayers of Pd were electrodeposited

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