Electrochemical Atomic Layer Deposition of Pd Ultrathin Films by

Feb 7, 2017 - This work illustrates the application of a surface limited redox replacement (SLRR) protocol for the electrochemical atomic layer deposi...
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Electrochemical Atomic Layer Deposition of Pd Ultrathin Films by Surface Limited Redox Replacement of Underpotentially Deposited H in a Single Cell Innocent Achari, Stephen Ambrozik, and Nikolay Dimitrov* Department of Chemistry, Binghamton University SUNY, Binghamton, New York 13902, United States ABSTRACT: This work illustrates the application of a surface limited redox replacement (SLRR) protocol for the electrochemical atomic layer deposition (E-ALD) of Pd ultrathin films on Au in a single cell using underpotentially deposited H (HUPD) as the sacrificial mediator. The facile deposition approach presented herein requires neither the use of sophisticated instrumentation nor the presence of metal ions other than the Pd(II) chloride complex. This provides for maximum growth efficiency and eliminates potential contamination of the deposit thereby rendering the Pd deposition a “greener” process. The growth progress is monitored by open circuit chronopotentiometry while the roughness evolution of accordingly deposited Pd films is assessed by cyclic voltammetry (CV) of HUPD and CuUPD. The CV results indicate the deposition of smooth Pd films occurring for up to 30 SLRR cycles (20 equiv monolayers) followed by a rapid transition to dendritic growth at higher thickness. The quasi-2D growth, resulting in smooth and uniform Pd-film morphology has also been confirmed up to 20 SLRR cycles by in situ scanning tunneling microscopy. Analysis of results from Pd-film stripping experiments corroborate these findings. The comparison of charges obtained by the stripping of Pd films of different thickness with generic growth models suggests that not only adsorbed but also absorbed HUPD participates in redox exchange with [PdCl4]2− complex in 2:1 stoichiometric ratio. The presented facile SLRR approach for contamination-free Pd deposition offers unsurpassed simplicity and can also be extended to other conductive substrates featuring strong H affinity.

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INTRODUCTION The search for alternative energy sources in order to address potential causes of climate change and minimize stress on crude oil1 has made green renewable and greener conventional energy sources a focal point of today’s materials science research. Developments associated with palladium have been a key focus of this research in particular, because this metal has unique properties such as its ability to selectively and reversibly absorb very large volumes of hydrogen at room temperature and pressure.2 As a consequence of this Pd thin films have been used in H storage devices, as sensors for H2 gas,3,4 and as fuel cell catalysts.3,5 Platinum has been the primary focus of catalytic materials for fuel cell electrocatalysis but due to its high cost and low abundance relative to other metals, alternatives like Pd must be pursued. That is why research is now aiming at either completely replacing Pt with Pd6,7 as an electrocatalyst or at alloying Pt with other metals of lower cost and higher abundance, yet preserving the high catalytic activity. The use of Pt or Pd8,9 in thin film configuration also minimizes the loading of these Platinum Group Metals as electrocatalysts consequently facilitating the commercialization of fuel cells.10 Ultrathin films of noble metals, such as Pd adlayers, exhibit properties unique to the film with respect to bulk counterparts. Such properties including but not limited to catalytic, electrocatalytic, electronic, and magnetic properties11 are crucial to electrocatlysis. In addition, these properties can be altered drastically based on the thickness of the film, for © XXXX American Chemical Society

instance, when studying the hydrogen evolution reaction Kibler et al.12 demonstrated that the activity of Pd overlayers depends distinctly on their thickness featuring minimum at two monolayers (2 ML < 1 ML < bulk Pd). These advantageous properties necessitate a technique that enables strictly controlled deposition of smooth Pd films in a layer by layer fashion. Based on this sample of advantages of metal thin-films, many techniques have been developed for performing the atomic layer deposition (ALD) of Pt-group metal ultrathin films.13 These methods include, physical vapor deposition,14 molecular beam epitaxy,15−17 chemical vapor deposition,18,19 metalorganic chemical vapor deposition,20 magnetron sputtering,21,22 and electrodeposition.23−26 Many of these techniques involve ultrahigh vacuum which renders the deposition process very expensive and unsustainable. Alternatively, electrochemical deposition can be carried out under standard conditions, and thus it is more economical and convenient.25 The electrochemical atomic layer deposition (E-ALD) of (quasi) epitaxial and conformal thin metal films has been achieved via a surface limited redox replacement (SLRR)27,28 process. This technique involves the underpotential deposition (UPD) of a sacrificial layer (SL) of less noble element (Cu, Pb, Tl, or H) on a Received: December 20, 2016 Revised: February 6, 2017 Published: February 7, 2017 A

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to prevent surface contamination. In the electrochemical cells, the electrodes are immersed and held in a hanging meniscus configuration.40 All electrochemical experiments were performed in three electrode setup using solutions made with Barnsted Nanopure water and ultrahigh purity grade chemicals as received from the vendors. A saturated mercury-mercurous sulfate electrode (MSE) is used as reference electrode in most experiments except those stated otherwise in the text. Also, a Pt wire has served as counter electrode (CE) in all experiments. All potentials in the manuscript are presented versus MSE (except PbUPD and CuUPD characterization experiments), and all current densities are normalized with respect to the geometric area of the electrode. All electrolytes used in deposition and characterization routines in this work are purged with ultrapure N2 gas for at least 30 min before the experiments. Characterization of Au Electrodes before Pd Deposition. Lead underpotential deposition (PbUPD) CV was performed on the electrodes to determine the quality of the Au surface and the electrochemical surface area (ECSA)41 development. The PbUPD solution contained 0.1 M NaClO4 (Sigma, 99.95%), 0.01 M HClO4 (GFS Chemical, 70% redistilled), and 0.003 M Pb(ClO4)2 (Aldrich, 99.995%). The measurements were performed using Model AFCBP Bipontentiostat (Pine Instruments) interfaced with a PC through the PineChem 2.80 software. Pb wire was used as a pseudoreference electrode (PRE). Pd Ultrathin Film Growth. The deposition of Pd thin films on Au surface is conducted by SLRR of HUPD in a single cell. The growth solution containing 0.3 mM PdCl2 (Aldrich, 99.99%), 30 mM HCl (J.T. Baker, 36.5−38%), and 0.1 M H2SO4 (GFS Chemical, redistilled) was purged with ultrapure N2 for at least 30 min. The SLRR protocol consists of repeated steps of HUPD formation by a 1 s potential pulse to −0.600 V followed by a galvanic replacement step occurring at open circuit potential (OCP).36 These Pd growth experiments were monitored by OCCP carried out with a Princeton Applied Research (PAR) Model 273 Potentiostat/Galvanostat coupled operated by Corrware Software. The potential transient was continuously collected on a PC by analog-to-digital DrDAQ Data Logger controlled by PicoLog software. Electrochemical Testing and Characterization. HUPD and CuUPD were used to characterize electrochemically the quality of the deposited Pd thin films. HUPD is conducted by CV in 0.5 M H2SO4 (GFS Chemical, redistilled) over a potential range of 0.10 to −0.67 V (vs MSE) at a sweep rate of 50 mV.s−1. CuUPD was conducted by CV in 3 mM CuSO4 (J.T. Baker, 99.8%), 0.1 M H2SO4 solution using Cu PRE at a sweep rate of 20 mV.s−1. The characterization of Pd films was administered and monitored using identical setup to those employed for Au electrode assessment as described earlier in this section. Pd Anodic Stripping. After characterization, the as-grown Pd thin-films were anodically stripped by running a linear sweep voltammetry (LSV) in 0.1 M HCl(J.T. Baker,36.5−38%) solution over a potential range −0.3 to +0.4 V with MSE as a reference electrode. The linear sweep rate was 3 mV.s−1 and was controlled by Model AFCBP Bipontentiostat (Pine Instruments) interfaced with a PC through the Af termath Data Organizer software. In Situ STM Monitoring and Characterization. In situ STM was carried out utilizing an Agilent 4500 SPM microscope, a 2100 controller, and PicoScan software in an analogous solution to that described in the deposition protocol.

conductive substrate followed by the redox replacement of the SL layer at open circuit potential (OCP) with a more noble depositing metal (Cu, Ag, Au, Pd, or Pt) as detailed in a recent review article.29 A repetition of this building block reaction results in the deposition of a noble metal film with a thickness proportional to the number of SLRR cycles performed. The growth of Pd thin films particularly, via SLRR, has been intensely studied in a flow-cell system by Stickney et al.3,30−32 These authors have shown that best results for conformal Pd growth via the displacement of CuUPD are obtained in highly chloride complexed Pd(II) solutions.30 The group’s best results include the contamination-free SLRR deposition of ultrathin films of up to 8 MLs of Pd on Pt (111) via Cu UPD.33 Moreover, it was observed that 3D growth starts from the eighth ML11 constituting a thickness greater than the 2−3 ML of coverage obtained via bulk deposition.34 The growth of Pd via Cu UPD was optimized by complexing the Pd ions using addition of EDTA (less effective)30 or chloride (most effective at [Cl−]/[Pd2+] = 500).35 The latter approach reduces the preferential displacement associated with a concentration gradient across the electrode in a flow cell and thus promotes uniform epitaxial growth. The authors claim up to 75 cycles of conformal Pd growth on polycrystalline Au with 95% SLRR efficiency.35 The growth of Pt group metals via HUPD as a SL is an approach that uses no other metals than the ions of the depositing one and therefore results in non contaminated SLRR grown thin films.36 The method is also environmentally benign since H is provided by the supporting electrolyte.36 While work on depositing Pt by SLRR of HUPD in different setups has been published recently,36−38 no reports have been found on accordingly handled deposition of Pd. Also, no SLRR work of any kind has been done for the growth of Pd films in a single cell, also known as one cell configuration.39 In this work, we report on the development of a facile and greener E-ALD method whereby the SLRR growth of Pd nanofilms via HUPD in a single cell has been exploited. The growth of Pd is conducted and studied by potential pulse routines and open circuit chronopotentiometry (OCCP). The film roughness is assessed by cyclic voltammetry (CV) in HUPD and CuUPD potential ranges. In situ scanning tunneling microscopy (STM) imaging and stripping analysis are also performed to derive further information on the Pd film growth with the number of SLRR cycles.

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EXPERIMENTAL SECTION Electrode Preparation. The working electrodes used for all electrochemical and morphological characterization experiments in this work are polycrystalline gold, Au (poly), discs (0.9999 purity) of 6 mm diameter, and single crystal Au (111) (Monocrystal Company) of 1 cm diameter. Both discs are of 2 mm thickness. These electrodes are prepared by mechanical polishing down to 0.3 μm (0.05 μm for single crystal Au (111)) using water-based, deagglomerated alumina slurry (Buehler). After polishing, the electrodes are immersed in warm concentrated HNO3 to remove any trace contaminants before thorough rinsing with Barnsted Nanopure water (>18 MΩ). The Au (111) single crystal for in situ STM morphological characterization is further electropolished following the procedure described elsewhere.37 The crystals are then annealed to red-hot in a propane torch for 5 min before being cooled rapidly in ultrapure nitrogen atmosphere, and finally, covered with a droplet of the Barnstead Nanopure water B

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The Journal of Physical Chemistry C The potential was controlled by an Agilent 300S Pico Bipotentiostat, and a Pt ring and Pd wire were used as counter and reference electrodes, respectively. STM tips were fabricated by etching Pt0.9Ir0.1 wire in 1.6 M CaCl2 utilizing an AC voltage of 25−30 V, followed by insulation with Apiezon wax to minimize the tip exposure to solution.

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RESULTS SLRR Deposition of Pd Thin Films on Au (poly) Substrates. Polycrystalline Au electrodes were used as substrates for the deposition of Pd thin films. Before Pd thinfilm growth, all of the electrodes were first characterized by a surface structure sensitive technique which involves reactions whose response is dependent on the surface geometry of the substrate.42 For both Au single- and Poly crystal electrodes PbUPD is a distinctly surface sensitive process which is known to produce characteristic peaks on the voltammetry curves for different site symmetries.41,42 Figure 1 shows a CV curve

Figure 2. Potential transient depicting growth of Pd on Au(poly) using 10 SLRR cycles of HUPD: Potential range: potential pulse to −0.600 V (for HUPD layer formation) and OCCP to 0.050 V (triggering point for next potential pulse).

−0.600 V for 1 s. The choice of potential limits, as also explained elsewhere,39 relies on a selection of a negative limit whereby H evolution is thermodynamically forbidden and only HUPD and overpotential deposition (OPD) of Pd are enabled, and occur concurrently. However, it is noteworthy that owing to the very low Pd solution concentration the Pd OPD rate over the course of HUPD layer formation (1 s) does not exceed 1 ML per minute.44 On releasing the potential control, a redox replacement of both adsorbed and, at higher Pd layer thicknesses absorbed hydrogen, by [PdCl4]2− ions occurs at OCP. The presented OCP transients depict a replacement rate that slows down from the first to the third SLRR cycle and then gradually levels off with increase of the Pd film thickness as a diffusion controlled deposition process becomes dominant. From the OCP transient graph, on analyzing the first six pulses and their transient times, it was observed that it took roughly 9, 13, 15, 18, 18, and again 18 s, respectively. Characterization of SLRR Deposited Pd Films on Au (poly) by H and Cu UPD. Cyclic voltammetry was performed to characterize the Pd thin film morphology using adsorption and desorption of H (HUPD) initially and the results were corroborated by Cu UPD characterization experiments. Figure 3A represents CV curves of HUPD on Pd films of different thickness, deposited by a different number of SLRR cycles. It has been reported that H adsorption takes place on Pd films of up to two MLs34 with insignificant absorption into the bulk. However, with greater Pd layer thicknesses, an additional amount of absorbed H atoms contributes to the overall H coverage in the UPD range (−0.400 to −0.670 V). In addition, it has been shown that the absorption process accounts for a maximum of 0.6 MLs of H absorbed for every Pd ML in acidic solution34 at Pd film thickness exceeding two MLs. That is why, the overall HUPD charge increases steadily with the number of SLRR cycles owing only to an increase of the charge associated with the absorbed H atoms (mostly contributing in the range negative to −0.600 V vs MSE). At more positive potentials the charge results predominantly from adsorbed H and as such this charge can be used as a measure of the Pd film’s roughness. Given that, the CV curves analysis in Figure 3A suggests virtually lack of surface area evolution, i.e., the deposition of smooth Pd films on Au up to 30 SLRR cycles. Apparently, substantial roughening is seen on Pd films deposited by 40

Figure 1. Typical PbUPD CV curve on a polycrystalline Au substrate before Pd deposition. The sweep rate is 20 mV s−1.

demonstrating PbUPD behavior of representative Au (poly) electrodes, used in this work. The peak structure depicting predominantly (111) surface orientation, emphasizes the domination of a peak doublet present at 0.2 V vs Pb/Pb2+ as well as the presence of Pb surface alloying characteristic for the (111) facet and evidenced by the anodic peak at 0.6 V. There is also some additional contribution of (100) and (110) shown by the other broad peaks. The CV curves provide in general information on the relative presence of grains with different orientation on the Au surface based on the comparison of the peaks associated with different low-index Au faces. Also, the shape of those peaks (height, width) attests for the overall surface quality of the Au substrate for Pd film deposition. While Figure 1 presents a typical curve for illustration of this characterization routine, Pb UPD CV experiments were performed on all Au substrates for surface characterization prior Pd deposition. The growth of Pd thin films on Au substrates was conducted via SLRRs utilizing the formation of HUPD layer and its subsequent displacement by [PdCl4]2− ions in the course of SLRR deposition in a three-electrode cell.43 Figure 2 shows a representative potential transient for a set of 10 “building block” SLRR cycles performed on Au (poly) electrodes. The growth procedure is initiated from the OCP of the substrate, followed by the application of a negative pulse to a potential of C

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Figure 3. CV curves on Au (poly) electrodes coated by the E-ALD approach developed in this work after various SLRR cycles; (A) shows HUPD CV curves at a scan rate of 50 mV s−1 and (B) presents CuUPD CV curves at a scan rate of 20 mV s−1.

Figure 4. In situ STM micrographs showing the morphology of Pd films grown by SLRR of HUPD on Au (111) electrodes. (A) Bare Au surface taken at OCP (0.17 V vs Pd/Pd2+), (B) partial coverage of PdUPD layer taken at 0.05 V vs Pd/Pd2+, (C) full coverage of a PdUPD layer taken at 0.00 V vs Pd/Pd2+, (D) Pd film deposited by one SLRR cycle, (E) Pd film deposited by three SLRR cycles, and (F) Pd film deposited by 20 SLRR cycles. The frame size on all images is (0.18 × 0.18) μm2.

Therefore, Figure 3B also corroborates the trends elucidated from the analysis of Figure 3A and specifically confirms the relatively smooth growth of Pd by up to 30 SLRR cycles and the transition to 3D growth at greater thickness of the Pd film. In Situ STM Monitoring of SLRR Deposition of Pd on Au (111). The morphology evolution over the course of SLRR deposition of Pd on a Au (111) surface was studied via in situ scanning tunneling microscopy as demonstrated for up to 20 SLRR cycles in Figure 4. The Pd deposition begins with the formation of a Pd UPD layer on bare Au (Figure 4A) as previously demonstrated by Kibler et al.46 The layer nucleates uniformly on terraces and step edges followed by island growth until a continuous monatomic layer is deposited, blanketing the entire surface and thus reproducing the morphology of the substrate, as evidenced by Figure 4B,C, respectively. Overall,

SLRR cycles, as evidenced by the respective CV curve demonstrating significantly higher HUPD formation/stripping charge in the −0.400 to −0.600 V range and therefore a higher surface area of the Pd film, respectively. It needs to be noted that the adsorption and absorption processes taking place simultaneously on Pd films of thickness greater than 2 MLs constitute a limitation of HUPD technique especially if it was used as the only surface roughness characterization tool.45 That is why, in this work, independent CV experiments have been performed on accordingly deposited Pd films in CuSO4 solution. Figure 3B shows the UPD of Cu on Pd that in this work serves as an alternative Pd surface area characterization tool. Given that Cu does not absorb into Pd,45 the curves presented in Figure 3B can be associated only with the surface area evolution of the herein assessed Pd films. D

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characterization also provides clear evidence for an abrupt roughness increase occurring between the 30th and 40th SLRR cycles manifested by an apparent increase of the UPD formation/stripping charges by a factor of 1.5 to 1.7 compared to 1.1 times increase in the range of 4 to 30 SLRR cycles. Such an abrupt change in the roughness factor indicates a transition from quasi-2D to 3D growth after 30 cycles. Further information on the deposition dynamics and efficiency of the Pd growth process with an increasing number of SLRR cycles can be obtained by a quantitative analysis of the stripping results (Figure 5) and their comparison with experimental growth trends (Figure 2), and generic growth models developed herein to account for factors contributing to the thickening of accordingly deposited Pd films. The potential transient presented in Figure 2 suggests that a faster SLRR reaction takes place in the first three deposition cycles. This can be understood by taking into account that the deposition process is initiated on a bare Au surface that would likely affect the SLRR kinetics until the electrode is covered with two Pd layers.12,46,47 In that case, according to the set of STM results in Figure 4A−C, the deposition of a full Pd UPD layer takes place in the first SLRR cycle. This finding along with the time trend in the SLRR cycle duration seen in the potential transient in Figure 2 one can speculate that it would take two more SLRR cycles (three in total) to complete the second Pd layer on the Au surface. This will be true only if we assume (i) 100% deposition efficiency and (ii) perfect layer by layer growth conditions both leading to the deposition of 0.5 ML of Pd per one layer of redox-replaced HUPD per SLRR cycle. At higher thickness (than two Pd layers), the contribution of not only adsorbed but also absorbed HUPD atoms will need to be considered. Also, the proposed model stipulates no absorption of H atoms into the first two Pd layers throughout the entire growth process. Under the above assumptions, the progress of Pd film thickening after the third SLRR cycle can be presented by the separate contribution of both adsorbed and absorbed HUPD atoms to the replacement process according to the equation below:

this layer is believed to be deposited simultaneously during the initial potential application aiding in the formation of the first HUPD layer. Following the Pd UPD layer completion, a HUPD layer is created during the first SLRR cycle which after displacement by Pd results in a uniform distribution of small Pd clusters (Figure 4D). Upon subsequent SLRR deposition cycles the clusters grow and merge into linear features with a long axis angle 60 degrees incident to the step direction, suggesting deposit epitaxy with the underlying substrate. At high Pd thicknesses (Figure 4E,F) deposits develop a quality similar to that of deposited Pt via SLRR of Pb39 and H36 and superior to that of Pd deposited via bulk deposition.46 STM results at thicknesses greater than approximately 15 MLs of deposited Pd have not been presented because of a rapid deterioration of the image quality taking place with the increasing layer thickness and related decrease of the average Pd cluster size along with cluster density increase. Similar trends were observed also in the course of Pt deposition by SLRR of HUPD layers.36 Anodic Stripping of SLRR Deposited Pd Ultrathin Films on Au (poly). Further information on the SLRR deposition of Pd was obtained by the stripping of accordingly grown layers by anodic linear sweep voltammetry. This allows for quantitative assessment of the deposition efficiency of Pd with an increasing number of SLRR cycles.3 Figure 5 shows a

Δi + 1 = 2 + 0.5 × (i − 2) + 0.3 × (Δi − 2)

(1)

Equation 1 considers the number of deposited Pd MLs (Δ) after the third SLRR cycle, i.e., i ≥ 3. Therefore, the first term (2) accounts for the number of deposited Pd MLs in the first 3 cycles, the second term, (0.5 × (i − 2)) is associated with the contribution of only adsorbed HUPD atoms, and the third term, (0.3 × (Δi − 2)) reflects the contribution of the absorbed portion of HUPD during growth (the coefficient 0.3 is a result of multiplying the Pd:H stoichiometric exchange ratio, 0.5 by the H:Pd absorption ratio, 0.634). Thus, after the fourth SLRR cycle, Δ4 = 2.5 as adsorption HUPD only contributes due to the lack of enough Pd layers to support absorption (the third term is 0). However, in the next cycle Δ5 = 3.15 instead of 3.0, that would be a result of SLRR of adsorbed H only, because 0.15 MLs are deposited due to the absorption of HUPD atoms in the 2.5 layers deposited previously. Proceeding with calculated thickness as a function of the number of SLRR cycles one can fill two columns of Table 1 where the anticipated Pd coverage is calculated for 10, 20, 30, and 40 SLRR cycles assuming replacement of adsorbed-only and adsorbed + absorbed (as needed) HUPD atoms. The third column in Table 1 presents the experimentally obtained thickness of Pd thin films by integration of all stripping curves in Figure 5 and normalization of the calculated charge density with that of a monolayer of Pd

Figure 5. Anodic stripping curves in 0.1 M HCl for Pd ultrathin films of different thickness deposited by SLRR on Au (poly). The sweep rate is 3 mV s−1.

plot of stripping curves obtained by LSV for ultrathin Pd deposits of different thickness. As a result, a set of anodic stripping curves is registered within the potential range of Pd dissolution (oxidation) in Cl− ions containing solution. It is clearly seen that the stripping charge, which is directly proportional to the amount of oxidized Pd (deposited earlier via SLRR), increases with the number of administered SLRR cycles. The gradual increase in peak height (area under the curve) appears to be proportional up to 30 SLRR cycles; a deviation from this trend peak area is observed with the Pd stripping after 40 SLRR deposition cycles.

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DISCUSSION The analysis of H and Cu UPD characterization results (Figure 3A,B) along with the in situ STM imaging clearly indicate that no significant roughening of the deposited Pd layer occurs for up to 30 successive SLRR cycles. In addition, the CV UPD E

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weakly bonded to the substrate Pd atoms from the dendritic “fingers”.49 Transition from quasi 2D to dendritic growth has also been shown to occur in the SLRR and surfactant mediated growth of Au on Pt surfaces.50 Overall, that growth mode change could be enabled by a gradual establishment of strictly diffusion limited growth that inevitably occurs (under the circumstance) with the increasing number of SLRR cycles. This type of transition was also demonstrated by a recently drafted model describing the kinetics of successive SLRR cycle in one cell configuration.51 The establishment of diffusion limited regime in the system of interest to this work is further promoted by the rapidly increasing contribution of absorbed H (see Table 1 and Figure 6) that leads to faster depletion of [PdCl4]2− complex in the electrode vicinity in comparison with the SLRR growth of other metals.

Table 1. Thickness of Pd Films Deposited by SLRR of HUPD Calculated by eq 1 and by Integration of the Stripping Curves in Figure 5 no. SLRR cycles

Δ(ML) − ADS only

Δ(ML) − ADS + ABS

Δ (ML) − experiment

10 20 30 40

5.5 10.5 15.5 20.5

6.7 13.9 21.6 29.3

5.8 14.0 22.5 41.8

on Au (poly) (approximately 400 μC/cm2).48 To further detail the obtained information, in Figure 6 calculated results by the

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CONCLUSIONS In conclusion, in this work Pd ultrathin films have been deposited on polycrystalline Au substrates via the SLRR of a HUPD layer in a single cell. The accordingly grown films have been characterized for surface roughness evolution using cyclic voltammetry of HUPD and CuUPD. Additional analysis along these lines has been performed by stripping experiments and modeling the progress of the SLRR deposition process. It has been found that during the growth of Pd films greater than 2 MLs, the SLRR is powered by the galvanic displacement of both adsorbed and absorbed HUPD atoms. It has also been concluded based on said results that the Pd film grows in quasi2D mode for up to 30 SLRR cycles, thus forming a conformal thin film with roughness factor of about 1.05. A transition from 2D to 3D (likely dendritic) growth has been considered as reason for a drastic increase of the roughness factor to about 1.60 in the SLRR range of 30 to 40. In situ STM imaging also provided additional evidence of relatively smooth and conformal growth via HUPD for up to 20 SLRR cycles. Overall, the proposed strategy enables a facile deposition on Au electrodes of contamination-free Pd films with superior quality to that of bulk deposited Pd for up to 20 deposited equivalent MLs. The most exciting aspect of the herein developed method is its simplicity, allowing for the manual generation of nearly perfect Pd films by employing only a potentiostat and oxygenevacuated, single electrochemical cell. Finally, this approach is extendable to other conductive substrates that can be terminated with a stable H layer prior to the Pd deposition and can be used also for deposition of important Pd alloys with other H adsorbing metals like Pt, Ru, Rh, etc.

Figure 6. Calculated and experimentally obtained thicknesses of Pd ultrathin film (in #Equivalent MLs) as a function of the number of SLRR cycles.

proposed generic model are plotted along with those determined experimentally (third column in Table 1) as a function of the number of applied SLRR cycles. It can be clearly seen that up to a total of 30 SLRR cycles the experimental results follow the trend reasonably well of those calculated by accounting for adsorbed + absorbed HUPD. This match suggests that absorbed HUPD undoubtedly contributes to the SLRR deposition of Pd. Also, alike with implications from the electrochemical and STM roughness characterization/monitoring in this growth range, the overall Pd film roughening (of about 1.05) is quite small and is thus similar to that reported previously for Pt (grown by SLRR of both HUPD and PbUPD)26,33 as well as for Pd (grown in flow cell by SLRR of CuUPD).3 At more than 30 SLRR cycles a substantial thickness increase manifested by a significant departure of the experimentally obtained results from theoretically calculated ones is observed in Figure 6. This observation implying a roughness factor increase to 1.6 and higher corroborates the roughness characterization results obtained by the analysis of H and Cu UPD CV curves in Figure 3. This serves as solid argument in support of transition from quasi-2D to 3D growth. Taking into account the abrupt increase of the roughness factor, we can speculatively associate this transition with dendrite nucleation and growth. This is because with any other type of surface roughening the trend would be noticeably more gradual. Another argument supporting the growth of dendrites is the shoulder observed in the stripping curve of Pd film deposited by 40 SLRR cycles (Figure 5). Such shoulder is most likely due to earlier oxidation of low-coordinated and thus

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1 (607) 777-4271. Fax: +1 (607) 777-4478. E-mail: [email protected]. ORCID

Nikolay Dimitrov: 0000-0003-1787-4575 Author Contributions

I.A. led the planning and conducting of Pd thin film growth, the electrochemical characterization, stripping experiments, and the paper drafting; S.A. led the planning and conducting of all in situ STM work; N.D. led the result analysis and interpretation. All authors edited and reviewed the manuscript. Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS The authors acknowledge the financial support by the National Science Foundation, Division of Chemistry, CHE-1310297.

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