Article pubs.acs.org/Langmuir
PtRu Nanofilm Formation by Electrochemical Atomic Layer Deposition (E-ALD) Nagarajan Jayaraju, Dhego Banga, Chandru Thambidurai, Xuehai Liang, Youn-Guen Kim, and John L. Stickney* Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States
ABSTRACT: The high CO tolerance of PtRu electrocatalysis, compared with pure Pt and other Pt-based alloys, makes it interesting as an anode material in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). This report describes the formation of bimetallic PtRu nanofilms using the electrochemical form of atomic layer deposition (E-ALD). Metal nanofilm formation using E-ALD is facilitated by use of surface-limited redox replacement (SLRR), where an atomic layer (AL) of a sacrificial metal is first formed by UPD. The AL is then spontaneously exchanged for a more noble metal at the opencircuit potential (OCP). In the present study, PtRu nanofilms were formed using SLRR for Pt and Ru, and Pb UPD was used to form the sacrificial layers. The PtRu E-ALD cycle consisted of Pb UPD at −0.19 V, followed by replacement using Pt(IV) ions at OCP, rinsing with blank, then Pb UPD at −0.19 V, followed by replacement using Ru(III) ions at OCP. PtRu nanofilm thickness was controlled by the number of times the cycle was repeated. PtRu nanofilms with atomic proportions of 70/30, 82/18, and 50/ 50 Pt/Ru were formed on Au on glass slides using related E-ALD cycles. The charge for Pb UPD and changes in the OCP during replacement were monitored during the deposition process. The PtRu films were then characterized by CO adsorption and electrooxidation to determine their overpotentials. The 50/50 PtRu nanofilms displayed the lowest CO electrooxidation overpotentials as well as the highest currents, compared with the other alloy compositions, pure Pt, and pure Ru. In addition, CO electrooxidation studies of the terminating AL on the 50/50 PtRu nanostructured alloy were investigated by deposition of one or two SLRR of Pt, Ru, or PtRu on top.
■
Petrii8 pioneered the study of PtRu for methanol oxidation and reported higher activity for PtRu compared to Pt electrodes. Watanabe and Motoo9 showed that the presence of a second alloying element, in addition to platinum, facilitated removal of adsorbed intermediates from the catalyst surfaces. The superior catalytic activity of PtRu alloys has been explained as a ligand effect (electronic effect) or a bifunctional mechanism. In the ligand effect,10,11 addition of Ru to Pt modifies the electronic properties of Pt, thereby weakening the adsorption of CO to Pt. In the bifunctional mechanism12,7 it is proposed that Ru adsorbs water at comparably negative potentials, providing the oxygen needed for conversion of CO to CO2. Although CO stripping can be used as a probe to
INTRODUCTION
Fuel cells are possible environmentally friendly power sources. Proton exchange membrane fuel cells (PEMFC) presently use porous carbon electrodes coated with Pt or Pt-based alloy particles which act as the catalytic sites. Hydrogen works well in PEMFC, unless contaminated with CO, a poison to Pt sites. Larger fuel molecules (containing carbon) produce CO as an intermediate or byproduct, rapidly contaminating Pt.1,2 It has been shown that some Pt alloys lower overpotentials for CO electrooxidation. Binary, tertiary, and quaternary alloys including PtRu, PtCo, PtSn, PtMo, PtRuCo, and PtRuNiZr3−5 have been investigated as anode materials for fuel cells. Among those alloys, PtRu has shown the highest catalytic activity and cell performance and is frequently used as an anode material in PEMFC and direct methanol fuel cells (DMFC) due to its increased CO tolerance.6,7 © 2014 American Chemical Society
Received: September 8, 2013 Revised: February 21, 2014 Published: February 25, 2014 3254
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
Though monometallic nanofilms of Cu, Pt, Pd, and Ru have been formed using SLRR, bimetallic nanofilms have not yet been reported formed using E-ALD and SLRR cycles. Adzic et al.4647 studied monolayer electrodeposition of Pt−Ru on a carbon support using the SLRR method to increase the activity for methanol oxidation. Recently, Tumaini et al.48 studied the deposition of bimetallic PtRu nanoclusters by SLRR of Cu on glassy carbon substrates. In the present study, PtRu nanofilms were formed by E-ALD cycles using Pb sacrificial atomic layers onto Au on glass substrates. Pt4+ and Ru3+ ions were used as precursors for Pt and Ru deposition, respectively. Overpotentials for CO oxidation on the PtRu deposits were determined by cyclic voltammetry of CO electrooxidation in sulfuric acid. That behavior was then compared to elemental Pt and Ru electrodes. CO electrooxidation studies were performed using the same flow cell deposition system in which the deposits were formed, without removing them from the cell.
determine the catalytic activity and electrochemical surface area, Cu UPD is the preferred method for determining the surface area of PtRu electrodes, because of the similarity of the atomic radii of the metals13 (Cu, 0.128 nm; Pt, 0.138 nm; Ru, 0.134 nm). Numerous studies8,9,12 have been performed to develop PtRu electrocatalysts with a higher tolerance for CO, as their performance is directly related to the fuel cell efficiency. Methods for preparing PtRu alloy electrocatalysts have involved thermal decomposition of precursors,14 spray pyrolysis,15 sol−gel technology,16 ionic liquids,17 chemical deposition,18 and electrodeposition.19,20 Electrodeposition is a simple, cost-effective, low-temperature process for growing thin films. PtRu electrodeposition on carbon supports and their catalytic activity toward methanol oxidation has been well studied.21,22 Recently, Gavrilov et al.20 electrodeposited submicrometer PtRu from chloride electrolytes, at different deposition potentials, and studied their stability and electrocatalytic activity for methanol oxidation. However, Pt and Ru electrodeposition has proven difficult due to slow kinetics, surface roughening, and 3-D growth. Atomic layer deposition (ALD) is a methodology for formation of conformal nanofilms, based on the use of surface-limited reactions to form deposits one atomic layer at a time. The electrochemical form of ALD (E-ALD) was introduced by this group, though it was initially referred to as electrochemical atomic layer epitaxy (EC-ALE).23,24 Most electrochemical surface-limited reactions are referred to as underpotential deposition (UPD),25,26 a thermodynamic process where one element deposits on a second because of a larger interaction energy between the elements and then the elements with themselves. UPD involves formation of an atomic layer of one element on a second at a potential prior to (under) that needed to deposit the element on itself. An atomic layer (AL) refers to a deposit which is only one atom thick. The corresponding coverage in terms of monolayers (ML) is generally less than one, depending on the conditions used. A ML is defined in this paper as one adsorbate atom for every substrate surface atom. In the present report, the Au on glass substrates was approximated as Au(111) single-crystal surfaces, for the purpose of determining coverage, so that 1 ML corresponds to 1.35 × 1015 atoms/cm2, or 217 μC/cm2, for a one-electron process, per surface atom. E-ALD of metal nanofilms was not possible until the work of Brankovic, Wang, and Adzic27,28 and Weaver.29 Brankovic et al. reported this work as monolayer galvanic displacement, while Weaver referred to it as surface-limited redox replacement (SLRR).29 It involved electrodeposition of an AL of a sacrificial metal by UPD and its subsequent replacement by exposure to a solution containing precursor ions of a more noble metal at open-circuit potential (OCP). The replacement reaction should be limited by the coverage of the sacrificial metal, generally less than 1 ML. Deposition of single monolayers of Pt and Pd were some of the first examples of SLRR,27,28 and atomic level control by SLRR was revealed using scanning tunneling microscopy (STM).27,30 The Pt deposit was formed by exchanging Cu UPD sacrificial layers in a solution containing Pt4+ ions. Studies of noble metal deposition by galvanic displacement have been extended by Vasilic and Dimitrov,31−34 Mroek and Weaver,29 Adzic,35−37 Brankovic,38 and Kim and Stickney.30,39,40 The author’s group has studied the use of SLRR as a cycle for E-ALD formation of Cu,41 Ru,42 Pt,3043 and Pd44,45 on gold substrates.
■
EXPERIMENTAL SECTION
Electrodeposition of PtRu nanofilms was carried out in an automated electrochemical flow cell system (Electrochemical ALD L.C., Athens, GA) consisting of peristaltic pumps, Teflon solenoid valves, an electrochemical flow cell, and a potentiostat, all controlled using “Sequencer 3”, a program designed to grow nanofilms using EALD.44,49 A gold wire embedded in the Plexiglas flow cell was used as the auxiliary electrode. An Ag/AgCl (3 M NaCl) (Bioanalytical systems, Inc., West Lafayette, IN) electrode was used as the reference electrode; however, all potentials are reported with respect to a reversible hydrogen electrode (RHE). The flow cell (4 cm × 1 cm × 0.1 cm) consisted of a planar substrate held away from the counter electrode by a 1 mm thick silicone rubber gasket. The deposition area was 3.77 cm2. The solution flow rate was 9 mL/min. Solutions used were 1 mM Pb(ClO4)2, 0.1 mM RuCl3, and 0.1 mM H2PtCl6. The Pb solution was made with 0.5 M NaClO4 (pH 4.5). The Ru solution was made with 50 mM HCl (pH 1.5), and the Pt solution was made with 50 mM HClO4 (pH 1.3). The blank solution was 0.5 M NaClO4 (pH 4.5). CVs were performed in 0.5 M H2SO4. CO adsorption was carried in CO-saturated 0.5 M H2SO4. All solutions were prepared using water from a Nanopure water filtration system (Barnstead, Dubuque, IA) (18 MΩ) attached to the house DI water system. Chemicals were obtained from Sigma-Aldrich or Alfa Aesar. Solutions bottles were contained inside a Plexiglas box and deaerated by bubbling with dry nitrogen. The substrates were Au on glass, formed by initial deposition of a 5 nm Ti adhesion layer on a glass microscopy slide, followed by 300 nm of Au at 280 °C. The resulting substrates were annealed in vacuum at 400 °C for 12 h, producing a prominent (111) growth habit. Au substrates were cleaned in nitric acid for 2 min and then by electrochemical cycling between 1.600 and 0.010 V in 0.5 M sulfuric acid, at 10 mV/s, prior to deposition. The basic E-ALD cycle used to deposit PtRu alloy films consisted of deposition of an atomic layer of Pt followed by an atomic layer of Ru. Other Pt/Ru ratios were formed by modifying the E-ALD, resulting in 70/30, 82/18, and 50/50 Pt/Ru structured alloy deposits. PtRu electrode catalytic activity was studied by CO stripping voltammetry in 0.5 M H2SO4 at a flow rate of 4 mL/min. Deposits were coated with CO by flowing a CO-saturated 0.5 M H2SO4 solution over the surface at 0.300 V for 3 min. CO electrooxidation was performed after replacing the CO solution with pure 0.5 M H2SO4. The scan was initiated in the negative direction to ensure the absence of hydrogen adsorption and desorption. The electrode was then scanned between +0.05 and +0.86 V to oxidize the adsorbed CO. CO oxidation studies were carried out in situ on the as-deposited PtRu nanofilms without taking them out of the electrochemical cell, thus avoiding their exposure to ambient and maintaining potential control. 3255
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
Figure 1. Cartoon representation of the E-ALD cycle for PtRu nanofilm deposition. For simplicity, only the CO electrooxidation features have been shown in some figures. Deposits were initially inspected using a Jenavert metallo-graphic microscope at 1000×. Electron probe microanalysis (EPMA) was performed using a Joel 8600 wavelength dispersive scanning electron microprobe for elemental analysis.
■
RESULTS AND DISCUSSIONS The basic E-ALD cycle involved UPD of an AL of Pb at −0.19 V. The PtCl62− ion precursor solution was then introduced to the cell at open-circuit potential (OCP) for the exchange. The Pb atomic layer oxidized, and PtCl62− ions were reduced, producing an AL of Pt in place of the Pb UPD. During the exchange, the OCP shifted from that corresponding to the sacrificial element, Pb, to that corresponding to Pt. The solution was then exchanged for the blank. Similar steps were performed to deposit Ru, again starting with Pb UPD, which was then exchanged Ru, using Ru3+ ions as precursors. Sequential deposition of one AL of Pt and one AL of Ru is referred to here as one PtRu E-ALD cycle (Figure 1). Choice of the sacrificial metal is important for SLRR. Previous studies by this group42 showed that Ru could not be deposited using Cu UPD, as the difference in Ru and Cu formal potentials was too small, limiting the amount of Cu that could be removed each cycle and producing deposits contaminated with Cu. Thus, Pb UPD was chosen as its formal potential was significantly more negative than Cu. The potential for Pb UPD was determined using cyclic voltammetry. Figure 2A displays a window opening CV sequence, performed to successively more negative potentials. The scans all started at 0.5 V and displayed a nearly constant current from about 0.3 to 0.0 V, at which point peak A was evident and consistent with Pb UPD on Au(111).50 Bulk Pb deposition, peak B, began near −0.21 V and displayed a small hysteresis loop, indicating nucleation. Bulk Pb stripped in a
Figure 2. (A) Cyclic voltammogram of Au electrode in 1 mM Pb(ClO4)2 in 0.5 M NaClO4; pH ≈ 4.5. Scan rate: 10 mV s−1. (B) Time−current−potential graph for one PtRu E-ALD cycle.
sharp oxidation peak B1, at −0.2 V, in the subsequent positivegoing scan. There was also a large oxidation peak “B2” at −0.1 V, consistent with dealloying of Pb from the Au substrate.50 The Pb UPD stripping peak A1 shows indications of a doublet at 0.05 V. The potential −0.19 V was selected for Pb UPD in 3256
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
potential, no time was wasted waiting for completion of the exchange. The stop potential step results in rinsing with blank for 11 s: the first 6 s are at OCP, so that any Pt4+ or Ru3+ ions are flushed from the cell and not deposited, and the next 5 s of blank rinsing was at 0.31 V to make sure all Pb was oxidized and to keep the OCP from increasing to potentials where the deposit could oxidize. The dashed curve in Figure 2B (the potential−time trace) shows the application of the stop potential step: 6 s rinse at OCP = 0.21 V and then 5 s at 0.31 V with rinsing. Note that in the exchange forming Pt the OCP gradient was steeper than that forming Ru. This could, at least in part, be the result of the larger difference in formal potentials between Pb and Pt and then for Pb and Ru. As a result, after the 6 s of rinsing at OCP, the potential for Pt was higher than 0.31 V and had to drop while that for Ru was lower than 0.31 V and had to increase (Figure 2B). The charge for Pb2+ reduction to Pb UPD is displayed as ML of Pb UPD vs number of cycles in Figure 3A. The Pb ML are displayed for both Pt deposition (squares) and Ru deposition (triangle). The Pb ML for Pt and Ru exchange both increased over the 25 cycles, indicating slight increases in surface area, or surface roughening. Possible reasons for roughening during Pt SLRR are noted below.43 In addition, significant differences in Pb coverages, MLs, are evident for the Ru- vs Pt-terminated surfaces. The amounts of Pt or Ru deposited should be controlled by the Pb coverages, as they determine the resulting number of electrons available to reduce Pt4+ or Ru3+. Assuming 100% exchange efficiency, the resulting Pt coverages should be no more than one-half the Pb MLs, since a Pt4+ species was used, while the resulting Ru coverages should be no more than two-thirds, since a Ru3+ species was used. The maximum Pt and Ru MLs, based on the Pb coverages in Figure 3A, are shown in Figure 3B. Given Figure 3B, a 50% Pt and 50% Ru deposit would be expected. EPMA, however, indicates the deposits were 70% Pt and 30% Ru, rather than 50/50. A previous report by this group describes the observations that at positive potentials, such as 0.31 V, PtCl62− ion adsorbs and can result in deposition of excess Pt upon stepping the potential down to −0.19 V for Pb UPD.43 Increases in roughness were observed from this overpotential deposition of Pt as well. The solution was to rinse longer to fully remove the adsorbed PtCl62− ions before the negative potential step to deposit Pb UPD. This mechanism was probably at work in the present study. Unfortunately, this issue was not understood at the time the present work was performed.43 In order to investigate the effect of the Pt/Ru ratio on the electrocatalytic activity for CO oxidation, the E-ALD cycle was modified by changing the number of SLRR for Pt and Ru; for instance, both 17 cycles PtPtRu and 17 cycles PtRuRu deposits were formed and characterized. EPMA suggested that the PtPtRu deposits were 82% Pt and 18% Ru, whereas the PtRuRu deposits were 50% Pt and 50% Ru. EPMA assumed the deposits were homogeneous and not thin film on an Au substrate; however, the Pt and Ru atomic percents from EPMA are a relative measure of the nanofilm stoichiometry. Seventeen cycles were performed so that the total number of AL formed was the same as for the 25 cycle PtRu deposits. Steady state CVs for the Pt/Ru 70/30, 82/18, and 50/50 nanofilms in 0.5 M H2SO4 are displayed in Figure 4 and show only indistinct currents for hydrogen adsorption and desorption between 0.3 V and 0.06 V, consistent with well-intermixed PtRu samples. On the other hand, Ru-decorated Pt surfaces or
Figure 3. (A) Coverage for Pb UPD during Pt and Ru deposition, and (B) maximum coverage for Pt and Ru calculated from reaction stoichiometry.
order to avoid bulk Pb deposition and minimize alloy formation. The program used for these studies consisted of the following steps: pumping the Pb2+ ion solution through the cell for Pb UPD at 9 mL/min, for 10 s, at −0.19 V. The auxiliary electrode was then disconnected, going open circuit, and the PtCl62− ion solution was pumped in to facilitate exchange of Pb UPD for a Pt AL. The open-circuit potential was monitored during the exchange, and once 0.21 V was reached (the “stop potential”), the cell was rinsed with blank for 6 s at OCP and then five more seconds at 0.31 V. The second half of the E-ALD cycle began the same way, pumping Pb2+ ion solution to form Pb UPD at −0.19 V, for 10 s, followed by the Ru3+ ion solution at OCP, allowing exchange of the Pb UPD for a Ru AL. When the stop potential was reached, OCP = 0.21 V, the cell was rinsed with the blank for 6 s at OCP and then 5 s at 0.31 V. These steps were intended to sequentially deposit ALs of Pt and Ru: one PtRu E-ALD cycle. This cycle was repeated 25 times to form PtRu nanofilms. The current−time (solid line) and potential−time (dashed line) plots for one and one-half E-ALD cycle are shown in Figure 2B. A majority of previous studies of SLRR were performed using a constant exchange time; 60 s for instance was used by Adzic to form Pt atomic layers from Cu UPD.27,28 The time needed for exchange, however, can vary depending on the metal being deposited (Pt, Cu, Ru, etc.), which cycle it is and the compositions of the solutions. As noted above, in the present study, cycles were programmed with a “stop potential” of 0.21 V, suggested by previous E-ALD studies using Pb UPD, as all Pb appeared to have oxidized by that potential.43 By monitoring the OCP during exchange and using the stop 3257
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
Figure 4. Cyclic voltammogram of PtRu electrode: (A) 70/30 Pt/Ru, (B) 82/18 Pt/Ru, and (C) 50/50 Pt/Ru in 0.5 M H2SO4. Scan rate: 10 mV s−1.
Figure 5. Cartoon scheme of (A) sputtered (70/30 Pt/Ru) and E-ALD prepared PtRu [(B) 70/30Pt/Ru, (C) 82/18 Pt/Ru, and (D) 50/50 Pt/Ru] nanofilm electrodes.
3258
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
Figure 6. CO stripping voltammetry on PtRu electrode (70/30 Pt/Ru) in 0.5 M H2SO4 with (A) first, (B) second, and (C) third continuous CO adsorption and oxidation cycles. Scan rate: 10 mV s−1. () Stripping of CO in the first positive-going sweep; (----) second positive-going sweep during each cycle.
Figure 7. CO stripping voltammetry on various PtRu alloy electrodes in 0.5 M H2SO4. Scan rate: 10 mV s−1.
annealed PtRu alloys show double-peak voltammograms.51−53 The current in the double-layer region is larger than that
expected for Pt but is typical for a PtRu alloy.20 The voltammograms should be limited to potentials less than 1.1 3259
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
Figure 8. CO stripping voltammetry on pure Pt, pure Ru, 50/50 Pt/Ru alloy, Ru-modified Pt, and Pt-modified Ru electrodes in 0.5 M H2SO4. Scan rate: 10 mV s−1. Table shows a summary of the onset potentials and peak potentials of CO electrooxidation for electrodes prepared by E-ALD.
hydrogen adsorption and desorption transform into hydrogen waves, typical for Pt electrodes. Schematic representations a 70/30 Pt/Ru alloy prepared by sputtering and 70/30, 82/18, and 50/50 Pt/Ru deposits prepared by E-ALD are shown in Figure 5. The PtRu alloy deposited by sputtering (Figure 5A) is shown as a random distribution of Pt and Ru atoms in a 70/30 proportion, both in the bulk at the surface. However, annealing of a sputtered PtRu deposit usually results in surface segregation of Pt. Gasteiger et al. found that for a PtRu alloy (70/30 Pt/Ru) annealing resulted in 92% Pt at the surface.54 In addition, PtRu deposits subjected to multiple cycles in a fuel cell resulted in oxidative dissolution of Ru.55,56 It is proposed here that the PtRu electrodes prepared by E-ALD had structures similar to those depicted in Figure 5B−D. From the discussion above of the deposits formed with an E-ALD cycle consisting of one SLRR for Pt and one for Ru (Figure 3B), each cycle should produce around 1/2 ML of Pt and somewhat less Ru, so a superlattice structure consisting of alternate full atomic layers of Pt and Ru would not occur. The structure in Figure 5B suggests small 2D islands might be formed of each element, each cycle, forming
Figure 9. Stability test performed on 50/50 Pt/Ru alloy by subjecting to 20 continuous CO adsorption and stripping voltammetry in 0.5 M H2SO4. Scan rate: 10 mV s−1.
V to retain the alloy composition. Above 1.1 V, Ru dissolution from the PtRu alloy is observed; the broad features for
Figure 10. CO stripping voltammetry on 50/50 Pt/Ru (base alloy) electrode with various surface-ending layers in 0.5 M H2SO4. Scan rate: 10 mV s−1. 3260
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
This group has previously reported E-ALD formation of Ru nanofilms Au,42 and the role of Ru as a promoter in CO electrooxidation has previously been investigated by other workers.52,53,59 Ad-atom-modified electrodes have been used by a number of investigators as model electrodes.60 The Ru-modified Pt deposit (Figure 8) was formed with 25 Pt cycles, followed by 2 Ru cycles, while the Pt-modified Ru deposit (Figure 8) was 25 Ru cycles and 2 Pt cycles. As seen in Figure 8, CO electrooxidation starts at about 0.67 V and 0.43 V, respectively, on pure Pt and pure Ru, the overpotential for CO electrooxidation on pure Ru being 0.24 V lower than that on pure Pt. The 50/50 PtRu alloy displayed a still lower overpotential and much increased current. The 0.26 V decrease in overpotential for 50/50 PtRu compared with pure Pt demonstrates the importance of the alloy. The Ptmodified Ru surface showed overpotential nearly as low as the 50/50 PtRu alloy, though with greatly decreased currents. Rumodified Pt shows an decrease in the CO electrooxidation current and a 0.12 V increase in overpotential relative to the 50/50 PtRu alloy. The onset and peak potentials for CO electrooxidation on the various deposits prepared by E-ALD are given in the inset of Figure 8. The CO oxidation peak potentials of 0.78 V on Pt, 0.59 V on Ru, 0.65 V on Ru-modified Pt, 0.53 V on Pt-modified Ru, and 0.56 V on 50/50 PtRu alloy prepared by E-ALD is consistent with previously reported results of about 0.80 V on Pt, 0.57 V on Ru, 0.70 V on Ru-modified Pt, 0.52 V on Pt-modified Ru, and 0.50 V on 50/50 PtRu alloy.7,52,53 PtRu nanofilms are significant because of the need for a catalyst with the lowest loading and highest activity and stability. Figure 9 displays a stability test for the 50/50 PtRu alloy. The as-prepared deposit was subjected to 20 continuous CO adsorption and electrooxidation cycles, and the CO electrooxidation peaks are displayed in Figure 9. It can be seen that after the first two or three cycles, where surface cleaning might take place, a stable peak profile for CO electrooxidation is established and maintained for the rest of the cycles. This suggests minimal change in the surface composition the 50/50 Pt/Ru structured alloy, even in 0.5 M H2SO4. Since the surface is most important in determining catalytic activity for CO electrooxidation, the effect of the terminal AL was investigated on the 50/50 PtRu alloy, as it displayed the highest activity for electrooxidation of CO (Figure 7). The 50/ 50 alloy was prepared using 17 PtRuRu E-ALD cycles and then one of the following cycles: a Pt cycle, a PtRu cycle, a RuRu cycle, or a PtPt cycle. The 50/50 PtRu deposit is referred to as the “base alloy” here, which normally ends with two Ru ALs. Figure 10 displays CO electrooxidation peaks for the base alloy and the base alloy with different terminating layers. Although the onset potentials remain essentially the same with different terminating layers, peak potentials vary somewhat. The currents for the bass alloy with one Pt AL and one Ru AL, +PtRu showed the highest CO electrooxidation currents.
what might be called a nanostructured alloy. Deposit structures such as those in Figure 5B−D might allow replenishment of lost Ru more readily than a random alloy (Figure 5A). The overpotentials for electrooxidation of adsorbed CO were studied by stripping voltammetry. As-deposited PtRu, without their removal from the electrochemical cell and exposure to ambient and loss of potential control, were exposed to a COsaturated 0.5 M H2SO4 solution in the cell for 3 min at 0.31 V. The cell was then rinsed with CO-free 0.5 M H2SO4 for 3 min, while the potential was maintained at 0.31 V. Adsorbed CO was then oxidatively stripped by cycling in the CO-free 0.5 M H2SO4. Figure 6 displays three sequential CO adsorption and electrooxidation stripping cycles (CO adsorption followed by two CVs between 0.06 and 0.86 V) on an as-formed PtRu deposit (70/30 Pt/Ru) (Figure 6A−C). In each of the three CO stripping cycles in Figure 6 the first CV (solid line) starts by scanning negatively so as to show the absence of hydrogen adsorption, the surface being blocked by adsorbed CO. The subsequent positive-going scan results in complete electrooxidation of the adsorbed CO, while the second positive scan (dashed line) was used to verify that all adsorbed CO had been oxidized. CO electrooxidation was initiated around 0.45 V and peaked at 0.53 V, although in the first cycle (Figure 6A) the CO electrooxidation peak was broader. Hayden et al. suggested that the first scan results in cleaning of some contamination on the surface, which disrupts the arrays of adsorbed CO.4 The second CO adsorption and electooxidation cycle (Figure 6B) resulted in a sharper peak, consistent with a more ordered CO layer.57 It is known that nearly all organic functional groups adsorb on Pt,58 so the need to clean the surface of the as-deposited PtRu deposit is understandable. However, it is interesting that the scan in Figure 6A looks a lot like a Ru electrode, with nearly the same overpotential and peak shape. It may be that Pt sites were contaminated before the first cycle, and most of the CO was adsorbed on Ru sites (Figure 6A). In addition, surface Ru may have been removed during the first CO cycle, resulting in an increased Pt/Ru surface ratio which promoted the sharper CO electrooxidation peaks seen in Figure 6B and 6C, which were more characteristic of a PtRu alloy. Similar studies for CO adsorption and electrooxidation were carried out using 82/18 and 50/50 Pt/Ru deposits. The electrooxidative stripping peaks (from the third cycle) for adsorbed CO on the three different alloy electrodes are shown in Figure 7. The 50/50 Pt/Ru deposit showed the lowest overpotential and highest currents. For comparison with the PtRu structured alloy deposits shown in Figure 7, the following deposits were formed: pure Pt, pure Ru, and Ru-modified Pt and Pt-modified Ru. Again, three consecutive CO adsorption electrooxidation cycles were performed on each deposit, and the CO electrooxidation peaks from the third cycle for each deposit are displayed in Figure 8, along with that for the Pt/Ru 50/50 deposit. Pt nanofilm deposition using E-ALD and SLRR on Au by the author’s group was recently reported,30,43 and the cycle used here to deposit Pt was nearly the same: 10 s of Pb UPD at −0.19 V, followed by exchange with a PtCl62− ion solution at OCP to form a Pt AL. As with the cycles for PtRu alloys above, a stop potential of 0.21 V was used with 6 s OCP rinse and the 5 s rinse at 0.31 V. The Pt deposit used for CO electrooxidation in Figure 8 was formed using 25 cycles. The cycle used to form the 25-cycle Ru deposit (Figure 8) was also the same as that used in the PtRu cycles described above in this report: 10 s Pb UPD at −0.19 V, exchange for the Ru3+ ion solution at OCP, and a stop potential of 0.21 V with rinsing at OCP and 0.31 V.
■
CONCLUSIONS E-ALD formation of PtRu-structured alloy deposits via SLRR is reported. PtRu nanofilms with bulk compositions of 70/30, 82/ 18, and 50/50 were deposited. It is proposed that these deposits were nanostructured alloys composed of layers of atomically high 2D elemental islands, rather than a random alloy that might be formed by sputtering or an annealed deposit 3261
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
(10) Krausa, M.; Vielstich, W. Study of the electrocatalytic influence of Pt/Ru and Ru on the oxidation of residues of small organic molecules. J. Electroanal. Chem. 1994, 379 (1−2), 307−314. (11) Tong; Kim, H. S.; Babu, P. K.; Waszczuk, P.; Wieckowski, A.; Oldfield, E. An NMR Investigation of CO Tolerance in a Pt/Ru Fuel Cell Catalyst. J. Am. Chem. Soc. 2001, 124 (3), 468−473. (12) Watanabe, M.; Motoo, S. Electrocatalysis by ad-atoms: Part II. Enhancement of the oxidation of methanol on platinum by ruthenium ad-atoms. J. Electroanal. Chem. 1975, 60 (3), 267−273. (13) Green, C. L.; Kucernak, A. Determination of the platinum and ruthenium surface areas in platinum-ruthenium alloy electrocatalysts by underpotential deposition of copper. I. Unsupported catalysts. J. Phys. Chem. B 2002, 106 (5), 1036−1047. (14) Sivakumar, P.; Ishak, R.; Tricoli, V. Novel Pt-Ru nanoparticles formed by vapour deposition as efficient electrocatalyst for methanol oxidation: Part I. Preparation and physical characterization. Electrochim. Acta 2005, 50 (16−17), 3312−3319. (15) Xue, X.; Liu, C.; Xing, W.; Lu, T. Physical and Electrochemical Characterizations of PtRu/C Catalysts by Spray Pyrolysis for Electrocatalytic Oxidation of Methanol. J. Electrochem. Soc. 2006, 153 (5), E79−E84. (16) Kim, J. Y.; Yang, Z. G.; Chang, C. C.; Valdez, T. I.; Narayanan, S. R.; Kumta, P. N. A Sol-Gel-Based Approach to Synthesize HighSurface-Area Pt-Ru Catalysts as Anodes for DMFCs. J. Electrochem. Soc. 2003, 150 (11), A1421−A1431. (17) Xue, X.; Lu, T.; Liu, C.; Xu, W.; Su, Y.; Lv, Y.; Xing, W. Novel preparation method of Pt-Ru/C catalyst using imidazolium ionic liquid as solvent. Electrochim. Acta 2005, 50 (16−17), 3470−3478. (18) Yan, S.; Sun, G.; Tian, J.; Jiang, L.; Qi, J.; Xin, Q. Polyol synthesis of highly active PtRu/C catalyst with high metal loading. Electrochim. Acta 2006, 52 (4), 1692−1696. (19) Coutanceau, C.; Rakotondrainibé, A. F.; Lima, A.; Garnier, E.; Pronier, S.; Léger, J. M.; Lamy, C. Preparation of Pt−Ru bimetallic anodes by galvanostatic pulse electrodeposition: characterization and application to the direct methanol fuel cell. J. Appl. Electrochem. 2004, 34 (1), 61−66. (20) Gavrilov, A. N.; Petrii, O. A.; Mukovnin, A. A.; Smirnova, N. V.; Levchenko, T. V.; Tsirlina, G. A. Pt-Ru electrodeposited on gold from chloride electrolytes. Electrochim. Acta 2007, 52 (8), 2775−2784. (21) Gavrilov, A. N.; Savinova, E. R.; Simonov, P. A.; Zaikovskii, V. I.; Cherepanova, S. V.; Tsirlina, G. A.; Parmon, V. N. On the influence of the metal loading on the structure of carbon-supported PtRu catalysts and their electrocatalytic activities in CO and methanol electrooxidation. Phys. Chem. Chem. Phys. 2007, 9 (40), 5476−5489. (22) Löffler, M. S.; Natter, H.; Hempelmann, R.; Wippermann, K. Preparation and characterisation of Pt-Ru model electrodes for the direct methanol fuel cell. Electrochim. Acta 2003, 48 (20−22), 3047− 3051. (23) Gregory, B. W.; Stickney, J. L. Electrochemical atomic layer epitaxy (ECALE). J. Electroanal. Chem. 1991, 300 (1−2), 543−561. (24) Stickney, J. L.; Wade, T. L.; Flowers, B. H. Electrodeposition of compound semiconductors using atomic layer epitaxy. Book of Abstracts, 217th ACS National Meeting, Anaheim, CA, Mar 21− 25,1999; American Chemical Society: Washington, DC, 1999; ANYL147. (25) Kolb, D. M. Physical and Electrochemcial Properties of Metal Monolayers on Metallic Substrates. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; John Wiley: New York, 1978; Vol. 11, p 125. (26) Adzic, R. R. Electrocatalytic Properties of the Surfaces Modified by Foreign Metal Ad Atoms. In Advances in Electrochemistry and Electrochemical Engineering; Gerishcher, H., Tobias, C. W., Eds.; WileyInterscience: New York, 1984; Vol. 13, p 159. (27) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Metal monolayer deposition by replacement of metal adlayers on electrode surfaces. Surf. Sci. 2001, 474 (1−3), L173−L179. (28) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. New methods of controlled monolayer-to-multilayer deposition of Pt for designing
which might result in surface segregation. CVs of the asdeposited nanofilms displayed indistinct hydrogen adsorption regions, which did suggest hydrogen adsorption but with no evidence of peaks. The CVs were typical of PtRu alloys. The PtRu alloys had 0.25 V lower overpotentials for CO electrooxidation than pure Pt nanofilms as well as significantly higher currents. The 50/50 Pt/Ru deposit showed the lowest overpotential and highest currents for CO electrooxidation, compared to Pt, Ru, Ru-modified Pt, and Pt-modified Ru deposits. It is logical that sustaining catalytic activity is a function not only of the surface composition but also of the structure and composition of the underlying material, especially over long periods of time. The E-ALD programs, such as those employed above, can be used to optimize both the bulk nanofilm composition as well as the surface composition. In the present study, under the conditions investigated, the 50/50 PtRu alloy terminated with one PtRu cycle exhibited the lowest overpotential and highest CO electrooxidation currents. The use of the automated electrochemical flow cell facilitated these studies by allowing substrate cleaning, film growth, film termination, and studies of CO electrooxidation all without removing the deposit from the cell and loss of potential control and exposure to ambient that would have resulted. Within the limit of detection, the EPMA data showed no evidence for the presence of Pb in the deposits.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: 7065421967. Fax: 7065429454. E-mail: stickney@uga. edu. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Support from the National Science Foundation, DMR1006747, as well as the Department of Energy is acknowledged and greatly appreciated.
■
REFERENCES
(1) Léger, J. M. Mechanistic aspects of methanol oxidation on platinum-based electrocatalysts. J. Appl. Electrochem. 2001, 31 (7), 767−771. (2) Kamarudin, S. K.; Achmad, F.; Daud, W. R. W. Overview on the application of direct methanol fuel cell (DMFC) for portable electronic devices. Int. J. Hydrogen Energy 2009, 34 (16), 6902−6916. (3) Petrii, O. A. Pt-Ru electrocatalysts for fuel cells: a representative review. J. Solid State Electrochem. 2008, 12 (5), 609−642. (4) Hayden, B. E.; Rendall, M. E.; South, O. Electro-oxidation of Carbon Monoxide on Well-Ordered Pt(111)/Sn Surface Alloys. J. Am. Chem. Soc. 2003, 125 (25), 7738−7742. (5) Samjeské, G.; Wang, H.; Löffler, T.; Baltruschat, H. CO and methanol oxidation at Pt-electrodes modified by Mo. Electrochim. Acta 2002, 47 (22−23), 3681−3692. (6) Liu, H.; Song, C.; Zhang, L.; Zhang, J.; Wang, H.; Wilkinson, D. P. A review of anode catalysis in the direct methanol fuel cell. J. Power Sources 2006, 155 (2), 95−110. (7) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. Methanol electrooxidation on well-characterized platinum-ruthenium bulk alloys. The J. Phys. Chem. 1993, 97 (46), 12020−12029. (8) Petrii, O. A. Activity of electrolytically deposited platinum and ruthenium by the electrooxidation of methanol. Dokl. Akad. Nauk SSSR 1965, 160 (4), 871−874. (9) Watanabe, M.; Motoo, S. Electrocatalysis by ad-atoms: Part III. Enhancement of the oxidation of carbon monoxide on platinum by ruthenium ad-atoms. J. Electroanal. Chem. 1975, 60 (3), 275−283. 3262
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
electrocatalysts at an atomic level. J. Serb. Chem. Soc 2001, 66 (11− 12), 887−898. (29) Mrozek, M. F.; Xie, Y.; Weaver, M. J. Surface-Enhanced Raman Scattering on Uniform Platinum-Group Overlayers: Preparation by Redox Replacement of Underpotential-Deposited Metals on Gold. Anal. Chem. 2001, 73 (24), 5953−5960. (30) Kim, Y.-G.; Kim, J. Y.; Vairavapandian, D.; Stickney, J. L. Platinum Nanofilm Formation by EC-ALE via Redox Replacement of UPD Copper: Studies Using in-Situ Scanning Tunneling Microscopy. J. Phys. Chem. B 2006, 110 (36), 17998−18006. (31) Vasilic, R.; Dimitrov, N. Epitaxial growth by monolayerrestricted galvanic displacement. Electrochem. Solid State Lett. 2005, 8 (11), C173−C176. (32) Vasilic, R.; Viyannalage, L. T.; Dimitrov, N. Epitaxial growth of Ag on Au(111) by galvanic displacement of Pb and Tl monolayers. J. Electrochem. Soc. 2006, 153 (9), C648−C655. (33) Mitchell, C.; Fayette, M.; Dimitrov, N. Homo- and heteroepitaxial deposition of Au by surface limited redox replacement of Pb underpotentially deposited layer in one-cell configuration. Electrochim. Acta 2012, 85 (0), 450−458. (34) Fayette, M.; Nutariya, J.; Vasiljevic, N.; Dimitrov, N. A Study of Pt Dissolution during Formic Acid Oxidation. ACS Catalysis 2013, 1709−1718. (35) Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F. Platinum Monolayer Fuel Cell Electrocatalysts. Top. Catal. 2007, 46 (3/4), 249−262. (36) Zhou, W. P.; Yang, X. F.; Vukmirovic, M. B.; Koel, B. E.; Jiao, J.; Peng, G. W.; Mavrikakis, M.; Adzic, R. R. Improving Electrocatalysts for O-2 Reduction by Fine-Tuning the Pt-Support Interaction: Pt Monolayer on the Surfaces of a Pd3Fe(111) Single-Crystal Alloy. J. Am. Chem. Soc. 2009, 131 (35), 12755−12762. (37) Zhang, J. L.; Vukmirovic, M. B.; Sasaki, K.; Nilekar, A. U.; Mavrikakis, M.; Adzic, R. R. Mixed-metal Pt monolayer electrocatalysts for enhanced oxygen reduction kinetics. J. Am. Chem. Soc. 2005, 127 (36), 12480−12481. (38) Gokcen, D.; Bae, S. E.; Brankovic, S. R. Stoichiometry of Pt Submonolayer Deposition via Surface-Limited Redox Replacement Reaction. J. Electrochem. Soc. 2010, 157 (11), D582−D587. (39) Kim, J. Y.; Kim, Y. G.; Stickney, J. L. Copper nanofilm formation by electrochemical atomic layer deposition - Ultrahigh-vacuum electrochemical and in situ STM studies. J. Electrochem. Soc. 2007, 154 (4), D260−D266. (40) Kim, J. Y.; Kim, Y. G.; Stickney, J. L. Cu nanofilm formation by electrochemical atomic layer deposition (ALD) in the presence of chloride ions. J. Electroanal. Chem. 2008, 621 (2), 205−213. (41) Thambidurai, C.; Kim, Y. G.; Jayaraju, N.; Venkatasamy, V.; Stickney, J. L. Copper Nanofilm Formation by Electrochemical ALD. J. Electrochem. Soc. 2009, 156 (8), D261−D268. (42) Thambidurai, C.; Kim, Y.-G.; Stickney, J. L. Electrodeposition of Ru by atomic layer deposition (ALD). Electrochim. Acta 2008, 53 (21), 6157−6164. (43) Jayaraju, N.; Vairavapandian, D.; Kim, Y. G.; Banga, D.; Stickney, J. L. Electrochemical Atomic Layer Deposition (E-ALD) of Pt Nanofilms using SLRR cycles. J. Electrochem. Soc. 2012. (44) Sheridan, L. B.; Czerwiniski, J.; Jayaraju, N.; Gebregziabiher, D. K.; Stickney, J. L.; Robinson, D. B.; Soriaga, M. P. Electrochemical Atomic Layer Deposition (E-ALD) of Palladium Nanofilms by Surface Limited Redox Replacement (SLRR), with EDTA Complexation. Electrocatalysis 2012, 3 (2), 96−107. (45) Sheridan, L. B.; Kim, Y.-G.; Perdue, B. R.; Jagannathan, K.; Stickney, J. L.; Robinson, D. B. 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. J. Phys. Chem. C 2013. (46) Sasaki, K.; Adzic, R. R. Monolayer-Level Ru- and NbO[sub 2]Supported Platinum Electrocatalysts for Methanol Oxidation. J. Electrochem. Soc. 2008, 155 (2), B180−B186.
(47) Ando, Y.; Sasaki, K.; Adzic, R. Electrocatalysts for methanol oxidation with ultra low content of Pt and Ru. Electrochem. Commun. 2009, 11 (6), 1135−1138. (48) Mkwizu, T. S.; Mathe, M. K.; Cukrowski, I. Electrodeposition of Multilayered Bimetallic Nanoclusters of Ruthenium and Platinum via Surface-Limited Redox−Replacement Reactions for Electrocatalytic Applications. Langmuir 2009, 26 (1), 570−580. (49) Stickney, J. L. Electrochemical Atomic Layer Epitaxy (EC-ALE): Nanoscale Control in the Electrodeposition of Compound Semiconductors. In Advances in Electrochemical Science and Engineering; Alkire, R. C., Kolb, D. M., Eds.; Wiley-VCH: Weinheim, 2002; Vol. 7, pp 1−105. (50) Green, M. P.; Hanson, K. J.; Scherson, D. A.; Xing, X.; Richter, M.; Ross, P. N.; Carr, R.; Lindau, I. Insitu Scanning Tunneling Microscopy Studies of the Underpotential Deposition of Lead on Au(111). J. Phys. Chem. 1989, 93 (6), 2181−2184. (51) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. Carbon monoxide electrooxidation on well-characterized platinum-ruthenium alloys. The J. Phys. Chem. 1994, 98 (2), 617−625. (52) Maillard, F.; Lu, G. Q.; Wieckowski, A.; Stimming, U. Rudecorated Pt surfaces as model fuel cell electrocatalysts for CO electrooxidation. J. Phys. Chem. B 2005, 109 (34), 16230−16243. (53) Davies, J. C.; Hayden, B. E.; Pegg, D. J.; Rendall, M. E. The electro-oxidation of carbon monoxide on ruthenium modified Pt(111). Surf. Sci. 2002, 496 (1−2), 110−120. (54) Gasteiger, H. A.; Ross, P. N., Jr.; Cairns, E. J. LEIS and AES on sputtered and annealed polycrystalline Pt-Ru bulk alloys. Surf. Sci. 1993, 293 (1−2), 67−80. (55) Cha, H.-C.; Chen, C.-Y.; Shiu, J.-Y. Investigation on the durability of direct methanol fuel cells. J. Power Sources 2009, 192 (2), 451−456. (56) Wang, Z.-B.; Wang, X.-P.; Zuo, P.-J.; Yang, B.-Q.; Yin, G.-P.; Feng, X.-P. Investigation of the performance decay of anodic PtRu catalyst with working time of direct methanol fuel cells. J. Power Sources 2008, 181 (1), 93−100. (57) Davies, J. C.; Hayden, B. E.; Pegg, D. J. The electrooxidation of carbon monoxide on ruthenium modified Pt(110). Electrochim. Acta 1998, 44 (6−7), 1181−1190. (58) Stickney, J. L.; Soriaga, M. P.; Hubbard, A. T.; Anderson, S. E. A survey of factors influencing the stability of organic functional groups attached to platinum electrodes. JEC 1981, 125, 73. (59) Lu, G. Q.; Waszczuk, P.; Wieckowski, A. Oxidation of CO adsorbed from CO saturated solutions on the Pt(111)/Ru electrode. J. Electroanal. Chem. 2002, 532 (1−2), 49−55. (60) Kuk, S. T.; Wieckowski, A. Methanol electrooxidation on platinum spontaneously deposited on unsupported and carbonsupported ruthenium nanoparticles. J. Power Sources 2005, 141 (1), 1−7.
3263
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
PtRu Nanofilm Formation by Electrochemical Atomic Layer Deposition (E-ALD) Nagarajan Jayaraju, Dhego Banga, Chandru Thambidurai, Xuehai Liang, Youn-Guen Kim, and John L. Stickney* Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States
ABSTRACT: The high CO tolerance of PtRu electrocatalysis, compared with pure Pt and other Pt-based alloys, makes it interesting as an anode material in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). This report describes the formation of bimetallic PtRu nanofilms using the electrochemical form of atomic layer deposition (E-ALD). Metal nanofilm formation using E-ALD is facilitated by use of surface-limited redox replacement (SLRR), where an atomic layer (AL) of a sacrificial metal is first formed by UPD. The AL is then spontaneously exchanged for a more noble metal at the opencircuit potential (OCP). In the present study, PtRu nanofilms were formed using SLRR for Pt and Ru, and Pb UPD was used to form the sacrificial layers. The PtRu E-ALD cycle consisted of Pb UPD at −0.19 V, followed by replacement using Pt(IV) ions at OCP, rinsing with blank, then Pb UPD at −0.19 V, followed by replacement using Ru(III) ions at OCP. PtRu nanofilm thickness was controlled by the number of times the cycle was repeated. PtRu nanofilms with atomic proportions of 70/30, 82/18, and 50/ 50 Pt/Ru were formed on Au on glass slides using related E-ALD cycles. The charge for Pb UPD and changes in the OCP during replacement were monitored during the deposition process. The PtRu films were then characterized by CO adsorption and electrooxidation to determine their overpotentials. The 50/50 PtRu nanofilms displayed the lowest CO electrooxidation overpotentials as well as the highest currents, compared with the other alloy compositions, pure Pt, and pure Ru. In addition, CO electrooxidation studies of the terminating AL on the 50/50 PtRu nanostructured alloy were investigated by deposition of one or two SLRR of Pt, Ru, or PtRu on top.
■
Petrii8 pioneered the study of PtRu for methanol oxidation and reported higher activity for PtRu compared to Pt electrodes. Watanabe and Motoo9 showed that the presence of a second alloying element, in addition to platinum, facilitated removal of adsorbed intermediates from the catalyst surfaces. The superior catalytic activity of PtRu alloys has been explained as a ligand effect (electronic effect) or a bifunctional mechanism. In the ligand effect,10,11 addition of Ru to Pt modifies the electronic properties of Pt, thereby weakening the adsorption of CO to Pt. In the bifunctional mechanism12,7 it is proposed that Ru adsorbs water at comparably negative potentials, providing the oxygen needed for conversion of CO to CO2. Although CO stripping can be used as a probe to
INTRODUCTION
Fuel cells are possible environmentally friendly power sources. Proton exchange membrane fuel cells (PEMFC) presently use porous carbon electrodes coated with Pt or Pt-based alloy particles which act as the catalytic sites. Hydrogen works well in PEMFC, unless contaminated with CO, a poison to Pt sites. Larger fuel molecules (containing carbon) produce CO as an intermediate or byproduct, rapidly contaminating Pt.1,2 It has been shown that some Pt alloys lower overpotentials for CO electrooxidation. Binary, tertiary, and quaternary alloys including PtRu, PtCo, PtSn, PtMo, PtRuCo, and PtRuNiZr3−5 have been investigated as anode materials for fuel cells. Among those alloys, PtRu has shown the highest catalytic activity and cell performance and is frequently used as an anode material in PEMFC and direct methanol fuel cells (DMFC) due to its increased CO tolerance.6,7 © 2014 American Chemical Society
Received: September 8, 2013 Revised: February 21, 2014 Published: February 25, 2014 3254
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
Though monometallic nanofilms of Cu, Pt, Pd, and Ru have been formed using SLRR, bimetallic nanofilms have not yet been reported formed using E-ALD and SLRR cycles. Adzic et al.4647 studied monolayer electrodeposition of Pt−Ru on a carbon support using the SLRR method to increase the activity for methanol oxidation. Recently, Tumaini et al.48 studied the deposition of bimetallic PtRu nanoclusters by SLRR of Cu on glassy carbon substrates. In the present study, PtRu nanofilms were formed by E-ALD cycles using Pb sacrificial atomic layers onto Au on glass substrates. Pt4+ and Ru3+ ions were used as precursors for Pt and Ru deposition, respectively. Overpotentials for CO oxidation on the PtRu deposits were determined by cyclic voltammetry of CO electrooxidation in sulfuric acid. That behavior was then compared to elemental Pt and Ru electrodes. CO electrooxidation studies were performed using the same flow cell deposition system in which the deposits were formed, without removing them from the cell.
determine the catalytic activity and electrochemical surface area, Cu UPD is the preferred method for determining the surface area of PtRu electrodes, because of the similarity of the atomic radii of the metals13 (Cu, 0.128 nm; Pt, 0.138 nm; Ru, 0.134 nm). Numerous studies8,9,12 have been performed to develop PtRu electrocatalysts with a higher tolerance for CO, as their performance is directly related to the fuel cell efficiency. Methods for preparing PtRu alloy electrocatalysts have involved thermal decomposition of precursors,14 spray pyrolysis,15 sol−gel technology,16 ionic liquids,17 chemical deposition,18 and electrodeposition.19,20 Electrodeposition is a simple, cost-effective, low-temperature process for growing thin films. PtRu electrodeposition on carbon supports and their catalytic activity toward methanol oxidation has been well studied.21,22 Recently, Gavrilov et al.20 electrodeposited submicrometer PtRu from chloride electrolytes, at different deposition potentials, and studied their stability and electrocatalytic activity for methanol oxidation. However, Pt and Ru electrodeposition has proven difficult due to slow kinetics, surface roughening, and 3-D growth. Atomic layer deposition (ALD) is a methodology for formation of conformal nanofilms, based on the use of surface-limited reactions to form deposits one atomic layer at a time. The electrochemical form of ALD (E-ALD) was introduced by this group, though it was initially referred to as electrochemical atomic layer epitaxy (EC-ALE).23,24 Most electrochemical surface-limited reactions are referred to as underpotential deposition (UPD),25,26 a thermodynamic process where one element deposits on a second because of a larger interaction energy between the elements and then the elements with themselves. UPD involves formation of an atomic layer of one element on a second at a potential prior to (under) that needed to deposit the element on itself. An atomic layer (AL) refers to a deposit which is only one atom thick. The corresponding coverage in terms of monolayers (ML) is generally less than one, depending on the conditions used. A ML is defined in this paper as one adsorbate atom for every substrate surface atom. In the present report, the Au on glass substrates was approximated as Au(111) single-crystal surfaces, for the purpose of determining coverage, so that 1 ML corresponds to 1.35 × 1015 atoms/cm2, or 217 μC/cm2, for a one-electron process, per surface atom. E-ALD of metal nanofilms was not possible until the work of Brankovic, Wang, and Adzic27,28 and Weaver.29 Brankovic et al. reported this work as monolayer galvanic displacement, while Weaver referred to it as surface-limited redox replacement (SLRR).29 It involved electrodeposition of an AL of a sacrificial metal by UPD and its subsequent replacement by exposure to a solution containing precursor ions of a more noble metal at open-circuit potential (OCP). The replacement reaction should be limited by the coverage of the sacrificial metal, generally less than 1 ML. Deposition of single monolayers of Pt and Pd were some of the first examples of SLRR,27,28 and atomic level control by SLRR was revealed using scanning tunneling microscopy (STM).27,30 The Pt deposit was formed by exchanging Cu UPD sacrificial layers in a solution containing Pt4+ ions. Studies of noble metal deposition by galvanic displacement have been extended by Vasilic and Dimitrov,31−34 Mroek and Weaver,29 Adzic,35−37 Brankovic,38 and Kim and Stickney.30,39,40 The author’s group has studied the use of SLRR as a cycle for E-ALD formation of Cu,41 Ru,42 Pt,3043 and Pd44,45 on gold substrates.
■
EXPERIMENTAL SECTION
Electrodeposition of PtRu nanofilms was carried out in an automated electrochemical flow cell system (Electrochemical ALD L.C., Athens, GA) consisting of peristaltic pumps, Teflon solenoid valves, an electrochemical flow cell, and a potentiostat, all controlled using “Sequencer 3”, a program designed to grow nanofilms using EALD.44,49 A gold wire embedded in the Plexiglas flow cell was used as the auxiliary electrode. An Ag/AgCl (3 M NaCl) (Bioanalytical systems, Inc., West Lafayette, IN) electrode was used as the reference electrode; however, all potentials are reported with respect to a reversible hydrogen electrode (RHE). The flow cell (4 cm × 1 cm × 0.1 cm) consisted of a planar substrate held away from the counter electrode by a 1 mm thick silicone rubber gasket. The deposition area was 3.77 cm2. The solution flow rate was 9 mL/min. Solutions used were 1 mM Pb(ClO4)2, 0.1 mM RuCl3, and 0.1 mM H2PtCl6. The Pb solution was made with 0.5 M NaClO4 (pH 4.5). The Ru solution was made with 50 mM HCl (pH 1.5), and the Pt solution was made with 50 mM HClO4 (pH 1.3). The blank solution was 0.5 M NaClO4 (pH 4.5). CVs were performed in 0.5 M H2SO4. CO adsorption was carried in CO-saturated 0.5 M H2SO4. All solutions were prepared using water from a Nanopure water filtration system (Barnstead, Dubuque, IA) (18 MΩ) attached to the house DI water system. Chemicals were obtained from Sigma-Aldrich or Alfa Aesar. Solutions bottles were contained inside a Plexiglas box and deaerated by bubbling with dry nitrogen. The substrates were Au on glass, formed by initial deposition of a 5 nm Ti adhesion layer on a glass microscopy slide, followed by 300 nm of Au at 280 °C. The resulting substrates were annealed in vacuum at 400 °C for 12 h, producing a prominent (111) growth habit. Au substrates were cleaned in nitric acid for 2 min and then by electrochemical cycling between 1.600 and 0.010 V in 0.5 M sulfuric acid, at 10 mV/s, prior to deposition. The basic E-ALD cycle used to deposit PtRu alloy films consisted of deposition of an atomic layer of Pt followed by an atomic layer of Ru. Other Pt/Ru ratios were formed by modifying the E-ALD, resulting in 70/30, 82/18, and 50/50 Pt/Ru structured alloy deposits. PtRu electrode catalytic activity was studied by CO stripping voltammetry in 0.5 M H2SO4 at a flow rate of 4 mL/min. Deposits were coated with CO by flowing a CO-saturated 0.5 M H2SO4 solution over the surface at 0.300 V for 3 min. CO electrooxidation was performed after replacing the CO solution with pure 0.5 M H2SO4. The scan was initiated in the negative direction to ensure the absence of hydrogen adsorption and desorption. The electrode was then scanned between +0.05 and +0.86 V to oxidize the adsorbed CO. CO oxidation studies were carried out in situ on the as-deposited PtRu nanofilms without taking them out of the electrochemical cell, thus avoiding their exposure to ambient and maintaining potential control. 3255
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
Figure 1. Cartoon representation of the E-ALD cycle for PtRu nanofilm deposition. For simplicity, only the CO electrooxidation features have been shown in some figures. Deposits were initially inspected using a Jenavert metallo-graphic microscope at 1000×. Electron probe microanalysis (EPMA) was performed using a Joel 8600 wavelength dispersive scanning electron microprobe for elemental analysis.
■
RESULTS AND DISCUSSIONS The basic E-ALD cycle involved UPD of an AL of Pb at −0.19 V. The PtCl62− ion precursor solution was then introduced to the cell at open-circuit potential (OCP) for the exchange. The Pb atomic layer oxidized, and PtCl62− ions were reduced, producing an AL of Pt in place of the Pb UPD. During the exchange, the OCP shifted from that corresponding to the sacrificial element, Pb, to that corresponding to Pt. The solution was then exchanged for the blank. Similar steps were performed to deposit Ru, again starting with Pb UPD, which was then exchanged Ru, using Ru3+ ions as precursors. Sequential deposition of one AL of Pt and one AL of Ru is referred to here as one PtRu E-ALD cycle (Figure 1). Choice of the sacrificial metal is important for SLRR. Previous studies by this group42 showed that Ru could not be deposited using Cu UPD, as the difference in Ru and Cu formal potentials was too small, limiting the amount of Cu that could be removed each cycle and producing deposits contaminated with Cu. Thus, Pb UPD was chosen as its formal potential was significantly more negative than Cu. The potential for Pb UPD was determined using cyclic voltammetry. Figure 2A displays a window opening CV sequence, performed to successively more negative potentials. The scans all started at 0.5 V and displayed a nearly constant current from about 0.3 to 0.0 V, at which point peak A was evident and consistent with Pb UPD on Au(111).50 Bulk Pb deposition, peak B, began near −0.21 V and displayed a small hysteresis loop, indicating nucleation. Bulk Pb stripped in a
Figure 2. (A) Cyclic voltammogram of Au electrode in 1 mM Pb(ClO4)2 in 0.5 M NaClO4; pH ≈ 4.5. Scan rate: 10 mV s−1. (B) Time−current−potential graph for one PtRu E-ALD cycle.
sharp oxidation peak B1, at −0.2 V, in the subsequent positivegoing scan. There was also a large oxidation peak “B2” at −0.1 V, consistent with dealloying of Pb from the Au substrate.50 The Pb UPD stripping peak A1 shows indications of a doublet at 0.05 V. The potential −0.19 V was selected for Pb UPD in 3256
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
potential, no time was wasted waiting for completion of the exchange. The stop potential step results in rinsing with blank for 11 s: the first 6 s are at OCP, so that any Pt4+ or Ru3+ ions are flushed from the cell and not deposited, and the next 5 s of blank rinsing was at 0.31 V to make sure all Pb was oxidized and to keep the OCP from increasing to potentials where the deposit could oxidize. The dashed curve in Figure 2B (the potential−time trace) shows the application of the stop potential step: 6 s rinse at OCP = 0.21 V and then 5 s at 0.31 V with rinsing. Note that in the exchange forming Pt the OCP gradient was steeper than that forming Ru. This could, at least in part, be the result of the larger difference in formal potentials between Pb and Pt and then for Pb and Ru. As a result, after the 6 s of rinsing at OCP, the potential for Pt was higher than 0.31 V and had to drop while that for Ru was lower than 0.31 V and had to increase (Figure 2B). The charge for Pb2+ reduction to Pb UPD is displayed as ML of Pb UPD vs number of cycles in Figure 3A. The Pb ML are displayed for both Pt deposition (squares) and Ru deposition (triangle). The Pb ML for Pt and Ru exchange both increased over the 25 cycles, indicating slight increases in surface area, or surface roughening. Possible reasons for roughening during Pt SLRR are noted below.43 In addition, significant differences in Pb coverages, MLs, are evident for the Ru- vs Pt-terminated surfaces. The amounts of Pt or Ru deposited should be controlled by the Pb coverages, as they determine the resulting number of electrons available to reduce Pt4+ or Ru3+. Assuming 100% exchange efficiency, the resulting Pt coverages should be no more than one-half the Pb MLs, since a Pt4+ species was used, while the resulting Ru coverages should be no more than two-thirds, since a Ru3+ species was used. The maximum Pt and Ru MLs, based on the Pb coverages in Figure 3A, are shown in Figure 3B. Given Figure 3B, a 50% Pt and 50% Ru deposit would be expected. EPMA, however, indicates the deposits were 70% Pt and 30% Ru, rather than 50/50. A previous report by this group describes the observations that at positive potentials, such as 0.31 V, PtCl62− ion adsorbs and can result in deposition of excess Pt upon stepping the potential down to −0.19 V for Pb UPD.43 Increases in roughness were observed from this overpotential deposition of Pt as well. The solution was to rinse longer to fully remove the adsorbed PtCl62− ions before the negative potential step to deposit Pb UPD. This mechanism was probably at work in the present study. Unfortunately, this issue was not understood at the time the present work was performed.43 In order to investigate the effect of the Pt/Ru ratio on the electrocatalytic activity for CO oxidation, the E-ALD cycle was modified by changing the number of SLRR for Pt and Ru; for instance, both 17 cycles PtPtRu and 17 cycles PtRuRu deposits were formed and characterized. EPMA suggested that the PtPtRu deposits were 82% Pt and 18% Ru, whereas the PtRuRu deposits were 50% Pt and 50% Ru. EPMA assumed the deposits were homogeneous and not thin film on an Au substrate; however, the Pt and Ru atomic percents from EPMA are a relative measure of the nanofilm stoichiometry. Seventeen cycles were performed so that the total number of AL formed was the same as for the 25 cycle PtRu deposits. Steady state CVs for the Pt/Ru 70/30, 82/18, and 50/50 nanofilms in 0.5 M H2SO4 are displayed in Figure 4 and show only indistinct currents for hydrogen adsorption and desorption between 0.3 V and 0.06 V, consistent with well-intermixed PtRu samples. On the other hand, Ru-decorated Pt surfaces or
Figure 3. (A) Coverage for Pb UPD during Pt and Ru deposition, and (B) maximum coverage for Pt and Ru calculated from reaction stoichiometry.
order to avoid bulk Pb deposition and minimize alloy formation. The program used for these studies consisted of the following steps: pumping the Pb2+ ion solution through the cell for Pb UPD at 9 mL/min, for 10 s, at −0.19 V. The auxiliary electrode was then disconnected, going open circuit, and the PtCl62− ion solution was pumped in to facilitate exchange of Pb UPD for a Pt AL. The open-circuit potential was monitored during the exchange, and once 0.21 V was reached (the “stop potential”), the cell was rinsed with blank for 6 s at OCP and then five more seconds at 0.31 V. The second half of the E-ALD cycle began the same way, pumping Pb2+ ion solution to form Pb UPD at −0.19 V, for 10 s, followed by the Ru3+ ion solution at OCP, allowing exchange of the Pb UPD for a Ru AL. When the stop potential was reached, OCP = 0.21 V, the cell was rinsed with the blank for 6 s at OCP and then 5 s at 0.31 V. These steps were intended to sequentially deposit ALs of Pt and Ru: one PtRu E-ALD cycle. This cycle was repeated 25 times to form PtRu nanofilms. The current−time (solid line) and potential−time (dashed line) plots for one and one-half E-ALD cycle are shown in Figure 2B. A majority of previous studies of SLRR were performed using a constant exchange time; 60 s for instance was used by Adzic to form Pt atomic layers from Cu UPD.27,28 The time needed for exchange, however, can vary depending on the metal being deposited (Pt, Cu, Ru, etc.), which cycle it is and the compositions of the solutions. As noted above, in the present study, cycles were programmed with a “stop potential” of 0.21 V, suggested by previous E-ALD studies using Pb UPD, as all Pb appeared to have oxidized by that potential.43 By monitoring the OCP during exchange and using the stop 3257
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
Figure 4. Cyclic voltammogram of PtRu electrode: (A) 70/30 Pt/Ru, (B) 82/18 Pt/Ru, and (C) 50/50 Pt/Ru in 0.5 M H2SO4. Scan rate: 10 mV s−1.
Figure 5. Cartoon scheme of (A) sputtered (70/30 Pt/Ru) and E-ALD prepared PtRu [(B) 70/30Pt/Ru, (C) 82/18 Pt/Ru, and (D) 50/50 Pt/Ru] nanofilm electrodes.
3258
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
Figure 6. CO stripping voltammetry on PtRu electrode (70/30 Pt/Ru) in 0.5 M H2SO4 with (A) first, (B) second, and (C) third continuous CO adsorption and oxidation cycles. Scan rate: 10 mV s−1. () Stripping of CO in the first positive-going sweep; (----) second positive-going sweep during each cycle.
Figure 7. CO stripping voltammetry on various PtRu alloy electrodes in 0.5 M H2SO4. Scan rate: 10 mV s−1.
annealed PtRu alloys show double-peak voltammograms.51−53 The current in the double-layer region is larger than that
expected for Pt but is typical for a PtRu alloy.20 The voltammograms should be limited to potentials less than 1.1 3259
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
Figure 8. CO stripping voltammetry on pure Pt, pure Ru, 50/50 Pt/Ru alloy, Ru-modified Pt, and Pt-modified Ru electrodes in 0.5 M H2SO4. Scan rate: 10 mV s−1. Table shows a summary of the onset potentials and peak potentials of CO electrooxidation for electrodes prepared by E-ALD.
hydrogen adsorption and desorption transform into hydrogen waves, typical for Pt electrodes. Schematic representations a 70/30 Pt/Ru alloy prepared by sputtering and 70/30, 82/18, and 50/50 Pt/Ru deposits prepared by E-ALD are shown in Figure 5. The PtRu alloy deposited by sputtering (Figure 5A) is shown as a random distribution of Pt and Ru atoms in a 70/30 proportion, both in the bulk at the surface. However, annealing of a sputtered PtRu deposit usually results in surface segregation of Pt. Gasteiger et al. found that for a PtRu alloy (70/30 Pt/Ru) annealing resulted in 92% Pt at the surface.54 In addition, PtRu deposits subjected to multiple cycles in a fuel cell resulted in oxidative dissolution of Ru.55,56 It is proposed here that the PtRu electrodes prepared by E-ALD had structures similar to those depicted in Figure 5B−D. From the discussion above of the deposits formed with an E-ALD cycle consisting of one SLRR for Pt and one for Ru (Figure 3B), each cycle should produce around 1/2 ML of Pt and somewhat less Ru, so a superlattice structure consisting of alternate full atomic layers of Pt and Ru would not occur. The structure in Figure 5B suggests small 2D islands might be formed of each element, each cycle, forming
Figure 9. Stability test performed on 50/50 Pt/Ru alloy by subjecting to 20 continuous CO adsorption and stripping voltammetry in 0.5 M H2SO4. Scan rate: 10 mV s−1.
V to retain the alloy composition. Above 1.1 V, Ru dissolution from the PtRu alloy is observed; the broad features for
Figure 10. CO stripping voltammetry on 50/50 Pt/Ru (base alloy) electrode with various surface-ending layers in 0.5 M H2SO4. Scan rate: 10 mV s−1. 3260
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
This group has previously reported E-ALD formation of Ru nanofilms Au,42 and the role of Ru as a promoter in CO electrooxidation has previously been investigated by other workers.52,53,59 Ad-atom-modified electrodes have been used by a number of investigators as model electrodes.60 The Ru-modified Pt deposit (Figure 8) was formed with 25 Pt cycles, followed by 2 Ru cycles, while the Pt-modified Ru deposit (Figure 8) was 25 Ru cycles and 2 Pt cycles. As seen in Figure 8, CO electrooxidation starts at about 0.67 V and 0.43 V, respectively, on pure Pt and pure Ru, the overpotential for CO electrooxidation on pure Ru being 0.24 V lower than that on pure Pt. The 50/50 PtRu alloy displayed a still lower overpotential and much increased current. The 0.26 V decrease in overpotential for 50/50 PtRu compared with pure Pt demonstrates the importance of the alloy. The Ptmodified Ru surface showed overpotential nearly as low as the 50/50 PtRu alloy, though with greatly decreased currents. Rumodified Pt shows an decrease in the CO electrooxidation current and a 0.12 V increase in overpotential relative to the 50/50 PtRu alloy. The onset and peak potentials for CO electrooxidation on the various deposits prepared by E-ALD are given in the inset of Figure 8. The CO oxidation peak potentials of 0.78 V on Pt, 0.59 V on Ru, 0.65 V on Ru-modified Pt, 0.53 V on Pt-modified Ru, and 0.56 V on 50/50 PtRu alloy prepared by E-ALD is consistent with previously reported results of about 0.80 V on Pt, 0.57 V on Ru, 0.70 V on Ru-modified Pt, 0.52 V on Pt-modified Ru, and 0.50 V on 50/50 PtRu alloy.7,52,53 PtRu nanofilms are significant because of the need for a catalyst with the lowest loading and highest activity and stability. Figure 9 displays a stability test for the 50/50 PtRu alloy. The as-prepared deposit was subjected to 20 continuous CO adsorption and electrooxidation cycles, and the CO electrooxidation peaks are displayed in Figure 9. It can be seen that after the first two or three cycles, where surface cleaning might take place, a stable peak profile for CO electrooxidation is established and maintained for the rest of the cycles. This suggests minimal change in the surface composition the 50/50 Pt/Ru structured alloy, even in 0.5 M H2SO4. Since the surface is most important in determining catalytic activity for CO electrooxidation, the effect of the terminal AL was investigated on the 50/50 PtRu alloy, as it displayed the highest activity for electrooxidation of CO (Figure 7). The 50/ 50 alloy was prepared using 17 PtRuRu E-ALD cycles and then one of the following cycles: a Pt cycle, a PtRu cycle, a RuRu cycle, or a PtPt cycle. The 50/50 PtRu deposit is referred to as the “base alloy” here, which normally ends with two Ru ALs. Figure 10 displays CO electrooxidation peaks for the base alloy and the base alloy with different terminating layers. Although the onset potentials remain essentially the same with different terminating layers, peak potentials vary somewhat. The currents for the bass alloy with one Pt AL and one Ru AL, +PtRu showed the highest CO electrooxidation currents.
what might be called a nanostructured alloy. Deposit structures such as those in Figure 5B−D might allow replenishment of lost Ru more readily than a random alloy (Figure 5A). The overpotentials for electrooxidation of adsorbed CO were studied by stripping voltammetry. As-deposited PtRu, without their removal from the electrochemical cell and exposure to ambient and loss of potential control, were exposed to a COsaturated 0.5 M H2SO4 solution in the cell for 3 min at 0.31 V. The cell was then rinsed with CO-free 0.5 M H2SO4 for 3 min, while the potential was maintained at 0.31 V. Adsorbed CO was then oxidatively stripped by cycling in the CO-free 0.5 M H2SO4. Figure 6 displays three sequential CO adsorption and electrooxidation stripping cycles (CO adsorption followed by two CVs between 0.06 and 0.86 V) on an as-formed PtRu deposit (70/30 Pt/Ru) (Figure 6A−C). In each of the three CO stripping cycles in Figure 6 the first CV (solid line) starts by scanning negatively so as to show the absence of hydrogen adsorption, the surface being blocked by adsorbed CO. The subsequent positive-going scan results in complete electrooxidation of the adsorbed CO, while the second positive scan (dashed line) was used to verify that all adsorbed CO had been oxidized. CO electrooxidation was initiated around 0.45 V and peaked at 0.53 V, although in the first cycle (Figure 6A) the CO electrooxidation peak was broader. Hayden et al. suggested that the first scan results in cleaning of some contamination on the surface, which disrupts the arrays of adsorbed CO.4 The second CO adsorption and electooxidation cycle (Figure 6B) resulted in a sharper peak, consistent with a more ordered CO layer.57 It is known that nearly all organic functional groups adsorb on Pt,58 so the need to clean the surface of the as-deposited PtRu deposit is understandable. However, it is interesting that the scan in Figure 6A looks a lot like a Ru electrode, with nearly the same overpotential and peak shape. It may be that Pt sites were contaminated before the first cycle, and most of the CO was adsorbed on Ru sites (Figure 6A). In addition, surface Ru may have been removed during the first CO cycle, resulting in an increased Pt/Ru surface ratio which promoted the sharper CO electrooxidation peaks seen in Figure 6B and 6C, which were more characteristic of a PtRu alloy. Similar studies for CO adsorption and electrooxidation were carried out using 82/18 and 50/50 Pt/Ru deposits. The electrooxidative stripping peaks (from the third cycle) for adsorbed CO on the three different alloy electrodes are shown in Figure 7. The 50/50 Pt/Ru deposit showed the lowest overpotential and highest currents. For comparison with the PtRu structured alloy deposits shown in Figure 7, the following deposits were formed: pure Pt, pure Ru, and Ru-modified Pt and Pt-modified Ru. Again, three consecutive CO adsorption electrooxidation cycles were performed on each deposit, and the CO electrooxidation peaks from the third cycle for each deposit are displayed in Figure 8, along with that for the Pt/Ru 50/50 deposit. Pt nanofilm deposition using E-ALD and SLRR on Au by the author’s group was recently reported,30,43 and the cycle used here to deposit Pt was nearly the same: 10 s of Pb UPD at −0.19 V, followed by exchange with a PtCl62− ion solution at OCP to form a Pt AL. As with the cycles for PtRu alloys above, a stop potential of 0.21 V was used with 6 s OCP rinse and the 5 s rinse at 0.31 V. The Pt deposit used for CO electrooxidation in Figure 8 was formed using 25 cycles. The cycle used to form the 25-cycle Ru deposit (Figure 8) was also the same as that used in the PtRu cycles described above in this report: 10 s Pb UPD at −0.19 V, exchange for the Ru3+ ion solution at OCP, and a stop potential of 0.21 V with rinsing at OCP and 0.31 V.
■
CONCLUSIONS E-ALD formation of PtRu-structured alloy deposits via SLRR is reported. PtRu nanofilms with bulk compositions of 70/30, 82/ 18, and 50/50 were deposited. It is proposed that these deposits were nanostructured alloys composed of layers of atomically high 2D elemental islands, rather than a random alloy that might be formed by sputtering or an annealed deposit 3261
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
(10) Krausa, M.; Vielstich, W. Study of the electrocatalytic influence of Pt/Ru and Ru on the oxidation of residues of small organic molecules. J. Electroanal. Chem. 1994, 379 (1−2), 307−314. (11) Tong; Kim, H. S.; Babu, P. K.; Waszczuk, P.; Wieckowski, A.; Oldfield, E. An NMR Investigation of CO Tolerance in a Pt/Ru Fuel Cell Catalyst. J. Am. Chem. Soc. 2001, 124 (3), 468−473. (12) Watanabe, M.; Motoo, S. Electrocatalysis by ad-atoms: Part II. Enhancement of the oxidation of methanol on platinum by ruthenium ad-atoms. J. Electroanal. Chem. 1975, 60 (3), 267−273. (13) Green, C. L.; Kucernak, A. Determination of the platinum and ruthenium surface areas in platinum-ruthenium alloy electrocatalysts by underpotential deposition of copper. I. Unsupported catalysts. J. Phys. Chem. B 2002, 106 (5), 1036−1047. (14) Sivakumar, P.; Ishak, R.; Tricoli, V. Novel Pt-Ru nanoparticles formed by vapour deposition as efficient electrocatalyst for methanol oxidation: Part I. Preparation and physical characterization. Electrochim. Acta 2005, 50 (16−17), 3312−3319. (15) Xue, X.; Liu, C.; Xing, W.; Lu, T. Physical and Electrochemical Characterizations of PtRu/C Catalysts by Spray Pyrolysis for Electrocatalytic Oxidation of Methanol. J. Electrochem. Soc. 2006, 153 (5), E79−E84. (16) Kim, J. Y.; Yang, Z. G.; Chang, C. C.; Valdez, T. I.; Narayanan, S. R.; Kumta, P. N. A Sol-Gel-Based Approach to Synthesize HighSurface-Area Pt-Ru Catalysts as Anodes for DMFCs. J. Electrochem. Soc. 2003, 150 (11), A1421−A1431. (17) Xue, X.; Lu, T.; Liu, C.; Xu, W.; Su, Y.; Lv, Y.; Xing, W. Novel preparation method of Pt-Ru/C catalyst using imidazolium ionic liquid as solvent. Electrochim. Acta 2005, 50 (16−17), 3470−3478. (18) Yan, S.; Sun, G.; Tian, J.; Jiang, L.; Qi, J.; Xin, Q. Polyol synthesis of highly active PtRu/C catalyst with high metal loading. Electrochim. Acta 2006, 52 (4), 1692−1696. (19) Coutanceau, C.; Rakotondrainibé, A. F.; Lima, A.; Garnier, E.; Pronier, S.; Léger, J. M.; Lamy, C. Preparation of Pt−Ru bimetallic anodes by galvanostatic pulse electrodeposition: characterization and application to the direct methanol fuel cell. J. Appl. Electrochem. 2004, 34 (1), 61−66. (20) Gavrilov, A. N.; Petrii, O. A.; Mukovnin, A. A.; Smirnova, N. V.; Levchenko, T. V.; Tsirlina, G. A. Pt-Ru electrodeposited on gold from chloride electrolytes. Electrochim. Acta 2007, 52 (8), 2775−2784. (21) Gavrilov, A. N.; Savinova, E. R.; Simonov, P. A.; Zaikovskii, V. I.; Cherepanova, S. V.; Tsirlina, G. A.; Parmon, V. N. On the influence of the metal loading on the structure of carbon-supported PtRu catalysts and their electrocatalytic activities in CO and methanol electrooxidation. Phys. Chem. Chem. Phys. 2007, 9 (40), 5476−5489. (22) Löffler, M. S.; Natter, H.; Hempelmann, R.; Wippermann, K. Preparation and characterisation of Pt-Ru model electrodes for the direct methanol fuel cell. Electrochim. Acta 2003, 48 (20−22), 3047− 3051. (23) Gregory, B. W.; Stickney, J. L. Electrochemical atomic layer epitaxy (ECALE). J. Electroanal. Chem. 1991, 300 (1−2), 543−561. (24) Stickney, J. L.; Wade, T. L.; Flowers, B. H. Electrodeposition of compound semiconductors using atomic layer epitaxy. Book of Abstracts, 217th ACS National Meeting, Anaheim, CA, Mar 21− 25,1999; American Chemical Society: Washington, DC, 1999; ANYL147. (25) Kolb, D. M. Physical and Electrochemcial Properties of Metal Monolayers on Metallic Substrates. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; John Wiley: New York, 1978; Vol. 11, p 125. (26) Adzic, R. R. Electrocatalytic Properties of the Surfaces Modified by Foreign Metal Ad Atoms. In Advances in Electrochemistry and Electrochemical Engineering; Gerishcher, H., Tobias, C. W., Eds.; WileyInterscience: New York, 1984; Vol. 13, p 159. (27) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Metal monolayer deposition by replacement of metal adlayers on electrode surfaces. Surf. Sci. 2001, 474 (1−3), L173−L179. (28) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. New methods of controlled monolayer-to-multilayer deposition of Pt for designing
which might result in surface segregation. CVs of the asdeposited nanofilms displayed indistinct hydrogen adsorption regions, which did suggest hydrogen adsorption but with no evidence of peaks. The CVs were typical of PtRu alloys. The PtRu alloys had 0.25 V lower overpotentials for CO electrooxidation than pure Pt nanofilms as well as significantly higher currents. The 50/50 Pt/Ru deposit showed the lowest overpotential and highest currents for CO electrooxidation, compared to Pt, Ru, Ru-modified Pt, and Pt-modified Ru deposits. It is logical that sustaining catalytic activity is a function not only of the surface composition but also of the structure and composition of the underlying material, especially over long periods of time. The E-ALD programs, such as those employed above, can be used to optimize both the bulk nanofilm composition as well as the surface composition. In the present study, under the conditions investigated, the 50/50 PtRu alloy terminated with one PtRu cycle exhibited the lowest overpotential and highest CO electrooxidation currents. The use of the automated electrochemical flow cell facilitated these studies by allowing substrate cleaning, film growth, film termination, and studies of CO electrooxidation all without removing the deposit from the cell and loss of potential control and exposure to ambient that would have resulted. Within the limit of detection, the EPMA data showed no evidence for the presence of Pb in the deposits.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: 7065421967. Fax: 7065429454. E-mail: stickney@uga. edu. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Support from the National Science Foundation, DMR1006747, as well as the Department of Energy is acknowledged and greatly appreciated.
■
REFERENCES
(1) Léger, J. M. Mechanistic aspects of methanol oxidation on platinum-based electrocatalysts. J. Appl. Electrochem. 2001, 31 (7), 767−771. (2) Kamarudin, S. K.; Achmad, F.; Daud, W. R. W. Overview on the application of direct methanol fuel cell (DMFC) for portable electronic devices. Int. J. Hydrogen Energy 2009, 34 (16), 6902−6916. (3) Petrii, O. A. Pt-Ru electrocatalysts for fuel cells: a representative review. J. Solid State Electrochem. 2008, 12 (5), 609−642. (4) Hayden, B. E.; Rendall, M. E.; South, O. Electro-oxidation of Carbon Monoxide on Well-Ordered Pt(111)/Sn Surface Alloys. J. Am. Chem. Soc. 2003, 125 (25), 7738−7742. (5) Samjeské, G.; Wang, H.; Löffler, T.; Baltruschat, H. CO and methanol oxidation at Pt-electrodes modified by Mo. Electrochim. Acta 2002, 47 (22−23), 3681−3692. (6) Liu, H.; Song, C.; Zhang, L.; Zhang, J.; Wang, H.; Wilkinson, D. P. A review of anode catalysis in the direct methanol fuel cell. J. Power Sources 2006, 155 (2), 95−110. (7) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. Methanol electrooxidation on well-characterized platinum-ruthenium bulk alloys. The J. Phys. Chem. 1993, 97 (46), 12020−12029. (8) Petrii, O. A. Activity of electrolytically deposited platinum and ruthenium by the electrooxidation of methanol. Dokl. Akad. Nauk SSSR 1965, 160 (4), 871−874. (9) Watanabe, M.; Motoo, S. Electrocatalysis by ad-atoms: Part III. Enhancement of the oxidation of carbon monoxide on platinum by ruthenium ad-atoms. J. Electroanal. Chem. 1975, 60 (3), 275−283. 3262
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
Langmuir
Article
electrocatalysts at an atomic level. J. Serb. Chem. Soc 2001, 66 (11− 12), 887−898. (29) Mrozek, M. F.; Xie, Y.; Weaver, M. J. Surface-Enhanced Raman Scattering on Uniform Platinum-Group Overlayers: Preparation by Redox Replacement of Underpotential-Deposited Metals on Gold. Anal. Chem. 2001, 73 (24), 5953−5960. (30) Kim, Y.-G.; Kim, J. Y.; Vairavapandian, D.; Stickney, J. L. Platinum Nanofilm Formation by EC-ALE via Redox Replacement of UPD Copper: Studies Using in-Situ Scanning Tunneling Microscopy. J. Phys. Chem. B 2006, 110 (36), 17998−18006. (31) Vasilic, R.; Dimitrov, N. Epitaxial growth by monolayerrestricted galvanic displacement. Electrochem. Solid State Lett. 2005, 8 (11), C173−C176. (32) Vasilic, R.; Viyannalage, L. T.; Dimitrov, N. Epitaxial growth of Ag on Au(111) by galvanic displacement of Pb and Tl monolayers. J. Electrochem. Soc. 2006, 153 (9), C648−C655. (33) Mitchell, C.; Fayette, M.; Dimitrov, N. Homo- and heteroepitaxial deposition of Au by surface limited redox replacement of Pb underpotentially deposited layer in one-cell configuration. Electrochim. Acta 2012, 85 (0), 450−458. (34) Fayette, M.; Nutariya, J.; Vasiljevic, N.; Dimitrov, N. A Study of Pt Dissolution during Formic Acid Oxidation. ACS Catalysis 2013, 1709−1718. (35) Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F. Platinum Monolayer Fuel Cell Electrocatalysts. Top. Catal. 2007, 46 (3/4), 249−262. (36) Zhou, W. P.; Yang, X. F.; Vukmirovic, M. B.; Koel, B. E.; Jiao, J.; Peng, G. W.; Mavrikakis, M.; Adzic, R. R. Improving Electrocatalysts for O-2 Reduction by Fine-Tuning the Pt-Support Interaction: Pt Monolayer on the Surfaces of a Pd3Fe(111) Single-Crystal Alloy. J. Am. Chem. Soc. 2009, 131 (35), 12755−12762. (37) Zhang, J. L.; Vukmirovic, M. B.; Sasaki, K.; Nilekar, A. U.; Mavrikakis, M.; Adzic, R. R. Mixed-metal Pt monolayer electrocatalysts for enhanced oxygen reduction kinetics. J. Am. Chem. Soc. 2005, 127 (36), 12480−12481. (38) Gokcen, D.; Bae, S. E.; Brankovic, S. R. Stoichiometry of Pt Submonolayer Deposition via Surface-Limited Redox Replacement Reaction. J. Electrochem. Soc. 2010, 157 (11), D582−D587. (39) Kim, J. Y.; Kim, Y. G.; Stickney, J. L. Copper nanofilm formation by electrochemical atomic layer deposition - Ultrahigh-vacuum electrochemical and in situ STM studies. J. Electrochem. Soc. 2007, 154 (4), D260−D266. (40) Kim, J. Y.; Kim, Y. G.; Stickney, J. L. Cu nanofilm formation by electrochemical atomic layer deposition (ALD) in the presence of chloride ions. J. Electroanal. Chem. 2008, 621 (2), 205−213. (41) Thambidurai, C.; Kim, Y. G.; Jayaraju, N.; Venkatasamy, V.; Stickney, J. L. Copper Nanofilm Formation by Electrochemical ALD. J. Electrochem. Soc. 2009, 156 (8), D261−D268. (42) Thambidurai, C.; Kim, Y.-G.; Stickney, J. L. Electrodeposition of Ru by atomic layer deposition (ALD). Electrochim. Acta 2008, 53 (21), 6157−6164. (43) Jayaraju, N.; Vairavapandian, D.; Kim, Y. G.; Banga, D.; Stickney, J. L. Electrochemical Atomic Layer Deposition (E-ALD) of Pt Nanofilms using SLRR cycles. J. Electrochem. Soc. 2012. (44) Sheridan, L. B.; Czerwiniski, J.; Jayaraju, N.; Gebregziabiher, D. K.; Stickney, J. L.; Robinson, D. B.; Soriaga, M. P. Electrochemical Atomic Layer Deposition (E-ALD) of Palladium Nanofilms by Surface Limited Redox Replacement (SLRR), with EDTA Complexation. Electrocatalysis 2012, 3 (2), 96−107. (45) Sheridan, L. B.; Kim, Y.-G.; Perdue, B. R.; Jagannathan, K.; Stickney, J. L.; Robinson, D. B. 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. J. Phys. Chem. C 2013. (46) Sasaki, K.; Adzic, R. R. Monolayer-Level Ru- and NbO[sub 2]Supported Platinum Electrocatalysts for Methanol Oxidation. J. Electrochem. Soc. 2008, 155 (2), B180−B186.
(47) Ando, Y.; Sasaki, K.; Adzic, R. Electrocatalysts for methanol oxidation with ultra low content of Pt and Ru. Electrochem. Commun. 2009, 11 (6), 1135−1138. (48) Mkwizu, T. S.; Mathe, M. K.; Cukrowski, I. Electrodeposition of Multilayered Bimetallic Nanoclusters of Ruthenium and Platinum via Surface-Limited Redox−Replacement Reactions for Electrocatalytic Applications. Langmuir 2009, 26 (1), 570−580. (49) Stickney, J. L. Electrochemical Atomic Layer Epitaxy (EC-ALE): Nanoscale Control in the Electrodeposition of Compound Semiconductors. In Advances in Electrochemical Science and Engineering; Alkire, R. C., Kolb, D. M., Eds.; Wiley-VCH: Weinheim, 2002; Vol. 7, pp 1−105. (50) Green, M. P.; Hanson, K. J.; Scherson, D. A.; Xing, X.; Richter, M.; Ross, P. N.; Carr, R.; Lindau, I. Insitu Scanning Tunneling Microscopy Studies of the Underpotential Deposition of Lead on Au(111). J. Phys. Chem. 1989, 93 (6), 2181−2184. (51) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. Carbon monoxide electrooxidation on well-characterized platinum-ruthenium alloys. The J. Phys. Chem. 1994, 98 (2), 617−625. (52) Maillard, F.; Lu, G. Q.; Wieckowski, A.; Stimming, U. Rudecorated Pt surfaces as model fuel cell electrocatalysts for CO electrooxidation. J. Phys. Chem. B 2005, 109 (34), 16230−16243. (53) Davies, J. C.; Hayden, B. E.; Pegg, D. J.; Rendall, M. E. The electro-oxidation of carbon monoxide on ruthenium modified Pt(111). Surf. Sci. 2002, 496 (1−2), 110−120. (54) Gasteiger, H. A.; Ross, P. N., Jr.; Cairns, E. J. LEIS and AES on sputtered and annealed polycrystalline Pt-Ru bulk alloys. Surf. Sci. 1993, 293 (1−2), 67−80. (55) Cha, H.-C.; Chen, C.-Y.; Shiu, J.-Y. Investigation on the durability of direct methanol fuel cells. J. Power Sources 2009, 192 (2), 451−456. (56) Wang, Z.-B.; Wang, X.-P.; Zuo, P.-J.; Yang, B.-Q.; Yin, G.-P.; Feng, X.-P. Investigation of the performance decay of anodic PtRu catalyst with working time of direct methanol fuel cells. J. Power Sources 2008, 181 (1), 93−100. (57) Davies, J. C.; Hayden, B. E.; Pegg, D. J. The electrooxidation of carbon monoxide on ruthenium modified Pt(110). Electrochim. Acta 1998, 44 (6−7), 1181−1190. (58) Stickney, J. L.; Soriaga, M. P.; Hubbard, A. T.; Anderson, S. E. A survey of factors influencing the stability of organic functional groups attached to platinum electrodes. JEC 1981, 125, 73. (59) Lu, G. Q.; Waszczuk, P.; Wieckowski, A. Oxidation of CO adsorbed from CO saturated solutions on the Pt(111)/Ru electrode. J. Electroanal. Chem. 2002, 532 (1−2), 49−55. (60) Kuk, S. T.; Wieckowski, A. Methanol electrooxidation on platinum spontaneously deposited on unsupported and carbonsupported ruthenium nanoparticles. J. Power Sources 2005, 141 (1), 1−7.
3263
dx.doi.org/10.1021/la403018v | Langmuir 2014, 30, 3254−3263
