F OCUS I SSUE ON E LECTROCHEMICAL D EPOSITION AS S URFACE C ONTROLLED P HENOMENON The Spontaneous Deposition of Au on Pt (111) and Polycrystalline Pt

Gold is epitaxially grown by spontaneous deposition (SD) on Pt(111) and Pt[poly]. It has been demonstrated that in the system of interest the SD takes place as a hybrid process, consisting of the potential-controlled reduction of an adsorbed [AuCl 4 ] − complex along with an electroless reduction of the same complex coupled with Pt surface oxidation over the course of adsorption. This hybrid deposition process is shown to promote an outstanding layer-by-layer growth mode on Pt(111) that involves the formation of smooth ﬁlms with virtually no surface roughness increase. This ﬁnding is seconded by in-situ STM experiments demonstrating that the SD of Au on Au(111) leads to similar growth results. In addition, an adlattice structure of √ 7 × √ 7 r19.1 ◦ is determined for the [AuCl 4 ] − adsorbate on Au (111) suggesting a theoretical deposited amount of about 15% Au surface coverage per SD cycle. It has been shown that a Pt(111) substrate is completely masked after ﬁve cycles of SD followed by layer densiﬁcation in continuity development. It has also been shown that the Au amount deposited speciﬁcally through the electroless route can be controlled by the amount of initial Pt oxidation, the concentration of dissolved O 2 , and the duration of [AuCl 4 ] − adsorption process. ©

The deposition of thin noble metal films is an area of significant interest for the fundamental and applied communities alike. Noble metal monolayer, to multilayer structures on a sacrificial substrate allows for the utilization of the beneficial properties of electropositive metals or alloys such as; corrosion resistance and catalytic activity, while minimizing the metals use, thus promoting cost effectiveness. In addition it has been shown that monolayer to bilayer metal films on foreign substrates exhibit different electronic properties and chemisorption trends when compared to the bulk material. This is explained through the alteration of the d-band center and orbital mixing resulting from epitaxial strain and electron ligand effects. [1][2][3][4] Overall, these unique properties can ultimately enhance a material's catalytic usefulness. 5,6 In addition to this it has also been shown that if a metal substrate is decorated by a sub-monolayer of a foreign metal in a controllable way, a synergism can result between the substrate and the foreign metal. This synergy results in enhanced properties, that in some cases are accompanied by the stabilization of the underlying substrate. [7][8][9][10][11] Adzic et al. have demonstrated that the electrodepostion of 0.26 to 0.33 layers of Au on an underlying Pt substrate not only results in steady oxygen reduction reaction activity but also, after 30,000 cycles, the accordingly decorated catalyst is proven significantly more durable than the Pt catalyst on its own. 10 Hazzazi et al. have also shown that Au deposited via forced deposition on Pt with coverages up to 0.73 layers improve a catalysts activity toward the ethanol oxidation reaction. 12 The above listed sample of advantages of ultrathin epitaxial and continuous noble metal films motivate the development of adequate deposition strategies that address the challenges of growing, epitaxially, a new phase in layer-by-layer (2D) mode. [13][14][15] More specifically, in a vast majority of systems the overpotential (bulk) growth takes place in what is known as Volmer-Weber (3D) growth mode, which involves preferential deposition at step edges, instead. 16,17 Therefore, complete coverage of the substrate can be ensured, only by the deposition of films of significant thicknesses that, in turn, can be controlled solely by charge calculations over the course of deposition. Because of this, strategies such as surface limited redox replacement (SLRR), 6,[18][19][20][21][22][23][24][25][26][27][28][29][30] Defect-Mediated Growth (DMG), 31 Surfactant Mediated Growth (SMG), 32 Self-Terminating Growth (STG), 33,34 Forced Deposition (FD) 12,[35][36][37] and Spontaneous Deposition (SD) 11,[37][38][39][40][41][42][43][44][45][46][47][48][49] have been developed in order to facilitate thin film deposition with monolayer level control. Each of the aforementioned methods involves specific protocols that either limit or control the deposition of the metal of interest via surfactant limitation 32 surface termination 33,34 or limited ion adsorption on the substrate. [18][19][20][21][22][23][24][25][26]28,29,50,51 These factors ultimately result in the operation of a cyclic process that allows for the deposition of a strict amount of material (usually limited to a ML) per cycle rendering the deposit thickness finely controllable. An additional benefit of these methods is associated with the naturally existing short period of deposition inactivity between cycles that prevents the establishment steady mass transport controlled growth. 52,53 Also, in some systems this relaxation period provides additional time for surface diffusion resulting in the formation of low surface energy 2-D structures. Along with this, the cycle succession enables outstanding control of the amount of material deposited, ultimately overcoming the limitations provided by conventional overpotential deposition. 24,32,33 In an attempt to take advantage of all of the previously described reasons on the onset of ultrathin film growth the SD of Au on Pt will be explored in this paper. The term Spontaneous Deposition has been used for a phenomenon that involves certain systems, in which an ordered adlayer of growing metal complex is formed on the substrate upon immersion at open circuit potential. 40,45 This adsorption is so strong that the substrate-adsorbate complex is then rinsed to remove any excess complex and the ordered adlayer is retained. 40,45 The substrate-adsorbate complex is then immersed in a solution of background electrolyte and a negative potential is applied in order to facilitate adsorbate reduction, thus completing one cycle (event) of SD. 40 This cycle can then be repeated over multiple iterations in order to deposit films of desired thicknesses. The amount of material deposited during one SD event is limited to the amount of complex strongly adsorbed to the substrate which allows for the deposition amount to be controlled at a minimum of this level. 45 To date SD has been shown to occur in systems of Pd, 38 Ir 41 and Pt 39,41 on Ru; Sn, 42 Pt, 43 Pd, 44 Ru and Os 45,47 on Au; and Pd, 37 Ru, 11,46,48,49 and Os 45 on Pt. Previously in this group, Mitchell et al. reported the indirect discovery of Au SD occurring on Pt at open circuit potential (OCP). 30 In this work, the spontaneous deposition of Au on Pt will be demonstrated and studied systematically as a way to decorate Pt surfaces with sub-monolayer to several layers of Au. It will be shown that in the system of interest the SD takes place as a hybrid process in which the conventional reduction of adsorbed [AuCl 4 ] − complex is accompanied by an electroless reduction of the same complex coupled with Pt surface oxidation over the course of adsorption. The overall D3002 Journal of The Electrochemical Society, 163 (12) D3001-D3007 (2016) deposition process will be examined and controlled electrochemically utilizing cyclic voltammetry (CV). The deposit structure and morphology will also be characterized utilizing characteristic CV curves of H-and Pb underpotential deposition (UPD) and in situ scanning tunneling microscopy (STM).

Experimental
Electrode preparation.-Polycrystalline and single crystal Pt (111) disks (d = 1.0 cm) of 2 mm thickness were used as working electrodes in all electrochemical and morphology characterizing experiments. The electrodes were first polished down to 0.05 μm with de-agglomerated alumina slurry (Buehler). They were then anodized in a 3:2:1 solution of ethylene glycol, hydrochloric acid and glacial acetic acid at a DC current density of 3.5-4 A cm −2 for 3-5 5 s pulses utilizing a Ag wire as cathode, and eventually terminated by immersion in HCl. This was followed by immersion in concentrated HNO 3 to remove any polishing debris and thorough rinsing with Barnstead Nanopure water (18.2 M .cm). Samples were then annealed until red-hot with a propane torch for 5 minutes, and then cooled in ultra high purity N 2 in order to avoid surface oxidation. The annealed surface was terminated with a water droplet to avoid contamination and the electrodes were immersed in a hanging meniscus configuration in a three electrode cell. 54 Also, single crystalline Au (111) (1.0 cm in diameter and 2 mm thickness) used in the morphology characterization in situ STM experiments were prepared following a procedure described in detail elsewhere. 55 Unless otherwise noted all potentials herein will be reported vs. MSE. The Au complex adsorption was carried out over numerous time intervals, and the OCP was observed throughout the process duration. Following adlayer formation the electrode was rinsed with Barnstead Nanopure water and then immersed in a reduction solution in which a potential of −0.830 V was applied for a duration of 1s. Such a negative potential was chosen to ensure a reduction of the presumably adsorbed Au complex that is to occur in a Au-free solution at a potential positive enough to avoid any complications associated with an interfering hydrogen evolution reaction. In the experiments where Au deposition specifically via the electroless route (powered by Pt oxidation) was studied, the thorough rinsing step was preceded by treatment in concentrated HNO 3 for thorough removal of any [AuCl 4 ] − adsorbate. All growth and electrochemical characterization experiments were carried out utilizing a model AFCBP Bipotentiostat (Pine Instruments) coupled with a PC through the Pinechem software (version 2.80).
Electrochemical characterization.-Prior to and following deposition Pt substrates and films were characterized electrochemically utilizing hydrogen ad-desorption. A solution of 0.1 M H 2 SO 4 (GFS Chemical, redistilled 95-98%) was utilized in order to monitor the decay of Pt surface area over a number of spontaneous deposition iterations. CV curves were registered from 0.400 V to −0.680 V at a scan rate of 50 mV s −1 . Films were also characterized utilizing Pb UPD in order to monitor the growth of Au surface area accompanying the decay of active Pt surface area. Solutions of 3 × 10 −3 M Pb(ClO 4 ) 2 (Aldrich, 99.995%), 0.1 M NaClO 4 and 1 × 10 −4 M HClO 4 were used for Pb UPD characterization. All electrochemical characterization experiments were carried out following the completion of all of the desired number of spontaneous deposition cycles, no characterization was taken prior to the completion of a desired deposit generation. All solutions were purged thoroughly with ultra-high purity N 2 . (111) and Au (111) single crystals were utilized as substrates for STM growth and characterization experiments. In situ STM experiments were carried out in identical complex adsorption and reduction solutions as described previously. Instead of the electrode being moved to different solutions, for in situ STM experiments solution flow and removal were carried out utilizing a custom flow setup. All in situ experiments were carried out in an oxygen free ultra-high purity N 2 purged environment. For ex situ STM experiments identical growing conditions were used as to those previously described. Both in situ and ex situ STM experiments were carried out utilizing an Agilent Technologies 4500 SPM microscope, with a Pico Scan 2100 controller and Pico View software (version 1.14). STM tips were generated by etching Pt 90 Ir 10 in 1.6 M CaCl 2 at 27 V (AC). Tips to be used in situ were subsequently insulated with Apiezon wax to minimize leakage currents.  30 In order to demonstrate this phenomenon at a fundamental level in the present work the spontaneous deposition of Au on Pt was first studied, electrochemically, by carrying out depositions after varied immersion times in the [AuCl 4 ] − complex containing solution. Figure 1 demonstrates the decay of H UPD charge 56 with increased immersion time as well as the growth of the fraction of Au terminated surface with increased immersion time as evidenced by the characteristic Au related signal in the Pb UPD. 57 Figure 1C is a plot that shows, based on characteristic Pb and H UPD ad/desorption charges, how the portion of Au terminated surface as well as the portion of Pt terminated surface respectively change as immersion time is increased. Additional examination of the plots in Figures 1A and 1B suggests that as the adsorption time is increased, up to 60 minutes, there is an apparent increase in the amount of deposited Au, although after 5 minutes the overall deposition rate drastically decreases. This result is far from anticipated given the finite amount of Au (∼0.15 ML) that can be reduced from a rapidly formed complete ML of adsorbed Au complex, per cycle of spontaneous deposition. 30 The additional amount of reduced Au in this case was hypothesized to be associated with the electroless reduction of adsorbed [AuCl 4 ] − complex enabled solely by the Pt substrate. In more detail, the Pt is susceptible to oxidation in a potential range positive to 0.1V (MSE) whereby the Au complex can be reduced through a bulk deposition process. This enables electroless deposition in which the Pt is likely being oxidized to Pt oxide while Au complex is reduced to elemental Au. A similar, electroless deposition growth mode (called at that time "spontaneous") was demonstrated by Brankovic et al. for Pt and Pd on Ru substrates. 38,39 In order to further investigate the electroless route in this work Pt electrodes were immersed in a solution containing Au complex, thoroughly rinsed and immersed in nitric acid in order to remove any traces of Au complex adsorbate. Electrodes were then characterized electrochemically, similarly to standard SD studied samples.

Results and Discussion
Ideally, based on the ability of nitric acid treatment to remove virtually all adsorbates and even metal deposits from Au and Pt surfaces 30,58-60 one would expect, that in the absence of side processes accompanying the adsorption step, no Au of any kind should remain on the Pt surface after the treatment. In Figure 2 it can be seen that, although any adsorbate is believed to be removed from the crystals surface, a portion of Au terminated surface has still been developed. More specifically, Figure 2A demonstrates that multiple applications of this treatment results in an increase of the Au coverage, consistently, which plateaus following the fourth repetition. Also, Figure 2B shows for comparison the result of (i) an adsorption only treatment along with (ii) the latter followed by a reduction step that completes a cycle of conventional spontaneous deposition. The results presented in Figure 2 indicate that some reduction activity had taken place in order to produce a Au deposit solely during the adsorption stage. Under these circumstances, and specifically, in the absence of externally powered reduction steps the only source of reduction power could be the earlier hypothesized electroless scenario whereby the Pt provides the reduction power for Au deposition, shifting the OCP of the system positively. In corroboration of this hypothesis, in the adsorption experiments carried out on Pt electrodes characterized later on by Pb UPD CV (Figure 2), the OCP was seen to start at a value of ∼0.400 V and over the time of immersion to slowly drift positively and eventually stabilize at ∼0.550 V, a potential well positive of the Pt oxidation potential.
Subsequent deposition stages.-In order to further investigate the overall development of SD phenomenon deposits of multiple thicknesses were produced utilizing adsorption times of three and ten minutes and the Pb UPD characterization results are presented in Figure 3. In this figure the comparison of Figures 3A and 3B suggests that over the course of many deposition cycles the Au representative surface area is only significantly different between these two samples for the first cycle. This implies that during the first one to two deposition cycles, in addition to the expected Au from the SD cycle, deposition proceeds as previously described via an electroless route. Should this happen, over time, during the first few immersions a portion of the electrode surface becomes Pt-O terminated while the rest becomes covered in Au, with [AuCl 4 ] − spontaneously adsorbed  on it, as schematically presented in Figure 4. This in turn results in the electroless Au deposition ultimately ceasing when no more surface exposed Pt is present. Support for this analysis is provided by Pb UPD results presented in Figure 1. In this figure it can be seen that as adsorption time is increased, Au surface termination concurrently increases until approximately 5 minutes elapses, following this Au deposition slows drastically. However, even following sixty minute adsorption time Pt surface coverage is clearly still present, evidenced in the H UPD. This is a result of the, previously oxidized during the adsorption step, Pt-O being reduced during the deposition step and initial CV cycles. Based on results not presented in this paper, the amount of Au deposited during these first few cycles is dependent upon (i) the solution oxygen concentration, as oxygen will compete for Pt-O reduction power, (ii) the initial degree of Pt oxidation, evidenced by differences in the amount of Au deposited over a specific time period given different pretreatments to remove oxidation and (iii) the adsorption time, especially for short time periods (Figure 1).
Following the initial deposition cycles, the surface becomes primarily Au terminated which leads predominantly to a conventional SD mode to be realized. This is furthered by the EQCM results obtained by Mitchell et al. of the spontaneous deposition of Au on Au, in which a rapid mass gain is demonstrated over the course of 5 seconds which is followed by a steady mass state. 30 This steady state suggests that, on Au, complex adsorption occurs over the course of the initial five seconds and then adsorption, and deposition activity ceases. Based on the Pb UPD CV curves presented in Figure 3 it can be seen that the Pt surface becomes completely masked by Au after 5 deposition cycles. The Au deposit develops quite smoothly over the course of 10 cycles with little to no roughness evolved, evidenced by Pb UPD. Overall, the SD of Au on Pt substrates is found to be a complex phenomenon in which both electroless (in the initial state) and controlled potential reduction (throughout the deposition process) of adsorbed [AuCl 4 ] − contribute to the quasi perfect 2D growth of Au. Moreover, this is the first system in which the SD is shown to exhibit the complexity of a hybrid mechanism involving an electroless deposition route along with conventional spontaneous deposition.

Structural and morphological characterization.-Adlayer
structure.-Due to the dynamics of the electroless deposition mode realized when a Pt surface is present, the [AuCl 4 ] − adlayer structure could not be resolved on the Pt (111) substrate by the in situ STM experiments carried out in the present work. However, because the electrode surface becomes Au coated following initial stages of SD, the [AuCl 4 ] − adlayer was investigated on a Au (111) single crystal by in situ STM. Figure 4B shows the resolved [AuCl 4 ] − adlayer that  Deposits on Pt (111).-Following adlattice structure determination, ex situ STM was carried out on Au deposits on Pt (111) in order to observe how the deposition proceeds with increasing number of cycles. Deposits were generated utilizing an adsorption time of three minutes similar to the Au films of which Pb UPD characterization was presented in Figure 3A. A three minute adsorption time was chosen because carrying out adsorption longer than this only allows for minimal deposition to proceed, based on the earlier discussed electroless Au deposition route. In addition, in subsequent cycles, once the substrate is Au terminated, complex adsorption is rapid, leaving little time required for the adsorption stage. From the images presented in Figure 5 it can be seen that the first deposition stage involves island nucleation on Pt terraces and steps. Nucleation is uniform on step edges and terraces demonstrating an improvement, from a morphological stand point, when considering bulk electrodeposition as a counterpart for comparison. 62 Initial islands cover approximately 50-60% of the substrate surface. This coverage is significantly greater than that shown in many other SD systems, 40,45,47,48 but lower than those systems demonstrated to operate solely via the electroless route of spontaneously depositing Pt and Pd on a Ru substrate. 38,39 The significant coverage demonstrated over the course of one SD cycle seconds the previously obtained results emphasizing the elec-troless route contribution to the overall deposition process. While, as mentioned earlier, this coverage is significantly greater than any previously shown over the course of a SD cycle 38,39,45,47 there are ways to control, and if needed to minimize it to negligible quantities. For example, limiting the initial time of [AuCl 4 ] − adsorption (known as a fast process 30 ) to less than 3 minutes. Another means of control would be to keep the [AuCl 4 ] − solution naturally aerated so that the ORR could compete with the electroless [AuCl 4 ] − reduction thus minimizing the impact of the latter on the overall Au deposition yield. Finally, utilizing Pt substrates with varying degrees of oxidation can also result in the complete limitation or development of a desired amount of Au growth per deposition stage.
Following initial nucleation, surface islands grow laterally ( Figure  5B) until a continuous layer of Au is completely formed on the Pt surface ( Figure 5C). It is clear that, after islands are deposited, the SD proceeds via a nearly perfect layer by layer growth mode with deposits featuring average island diameter and height of 50 nm and 1.5 nm, respectively following ten deposition steps ( Figure 5C). In Figure 5C there is also evidence of a, hexagonally organized, potential induced reconstruction, shown by Sibert et al. to occur on bilayer, but not monolayer, Au films deposited on Pt (111). 63 This suggests that the Au films deposited via 10 SD cycles are at minimum two layers thick, but likely of even greater thicknesses.
Deposits on Au (111).-In order to confirm the layer by layer growth mode demonstrated after initial nucleation stages the SD of Au was also carried out on Au (111) and investigated by in situ STM and the results are presented in Figure 6. Based on these results it can be seen that unlike in the case of when Pt (111) is used as a substrate, no clusters of reduced Au atoms are present on the surface following any of the reduction cycles. This implies that, most likely, any reduced Au is incorporated into steps, kinks and other surface imperfections. Thus, no nuclei are created on terraces, but instead it can be seen ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 207.241.231.81 Downloaded on 2018-07-22 to IP that the step front consistently proceeds and morphs over the course of multiple depositions. In order to reinforce this point a specific feature that shows the most obvious evidence of deposition has been highlighted in Figure 6. Overall, the homoepitaxial growth of Au over three SD cycles presented in Figure 6 seconds the result observed on Pt, thus suggesting generally a remarkably uniform layer by layer growth mode that operates in the system of our interest following initial nucleation.

Conclusions
It was demonstrated that the SD of Au on Pt can be realized by the adsorption of a [AuCl 4 ] − complex at OCP followed by a subsequent reduction step. In the first study of its kind, a hybrid SD growth model is demonstrated in which, initial deposition cycles on a bare Pt surface were proven to involve, unconventionally, an electroless reduction of adsorbed [AuCl 4 ] − complex paired with Pt oxide formation. It was also shown that the latter can be controlled either by limiting the time of the adsorption step or by varying the concentration of O 2 dissolved in the [AuCl 4 ] − complex containing solution. It was determined that the [AuCl 4 ] − forms a √ 7 × √ 7 r19.1 • adlattice on Au (111) that was associated with theoretical limit of Au deposition at 15% coverage per cycle. It was also demonstrated that the growth operates via island formation followed by layer by layer Au growth resulting in nearly perfect overlayers. STM and CV results evidenced that over the course of five cycles the Pt substrate becomes completely masked by Au and no signal from the underlying Pt is observed. Overall, the SD is proven, in this work, an incredibly effective method for Pt surface decoration with Au as well as for the formation of epitaxial and continuous thin overlayers of Au on Pt. Extending this deposition to thicknesses greater than those demonstrated herein may prove cumbersome, because of the nature of the procedure, however at the thicknesses studied and achieved in this work the method is undoubtedly effective.