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Self-Terminating Growth of Platinum Films by Electrochemical Deposition

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Science  07 Dec 2012:
Vol. 338, Issue 6112, pp. 1327-1330
DOI: 10.1126/science.1228925

Abstract

A self-terminating rapid electrodeposition process for controlled growth of platinum (Pt) monolayer films from a K2PtCl4-NaCl electrolyte has been developed that is tantamount to wet atomic layer deposition. Despite the deposition overpotential being in excess of 1 volt, Pt deposition was quenched at potentials just negative of proton reduction by an alteration of the double-layer structure induced by a saturated surface coverage of underpotential deposited H (Hupd). The surface was reactivated for further Pt deposition by stepping the potential to more positive values, where Hupd is oxidized and fresh sites for the adsorption of PtCl42– become available. Periodic pulsing of the potential enables sequential deposition of two-dimensional Pt layers to fabricate films of desired thickness, relevant to a range of advanced technologies.

Platinum (Pt) is a key constituent in a wide range of heterogeneous catalysts, but its high cost constrains the development of important alternative energy conversion systems such as low-temperature fuel cells (13). Strategies for enhancing catalyst performance and minimizing Pt loadings include alloying and nanoscale engineering of core/shell and related architectures that typically involve spontaneous processes, such as dealloying and segregation, to form Pt-rich surface layers (4, 5).

The deposition of two-dimensional (2D) Pt layers, which are also of interest in thin-film electronics and magnetic materials, is nontrivial because of the step-edge barrier to interlayer transport that results in roughening or 3D mound formation (6). In situ scanning tunneling microscopy (STM) of Pt electrodeposition at moderate overpotentials reveals that metal nucleation and growth on Au single-crystal surfaces proceeds by the formation of 3D clusters at defect sites (7). At small overpotentials, x-ray scattering indicates that smooth Pt monolayers can be electrodeposited on Au(111), although a long growth time of 2000 s is required (8). Voltammetric studies show a potential dependent transition between 2D island versus 3D multilayer growth, although it is only possible to obtain a partial Pt monolayer coverage in the 2D growth regime (9).

To circumvent these difficulties, surface-limited place-exchange reactions are being explored. For example, galvanic displacement of an underpotential deposited (upd) metal monolayer, typically Cu, occurs by the desired Pt-group metal, with the exchange resulting in a submonolayer coverage of the noble metal (10, 11). The process can be repeated to form multiple layers by means of a variant known as electrochemical atomic layer epitaxy (12). The multistep process typically requires an exchange of electrolytes and some care to control (or avoid) the trapping of the less-noble metal as a minor alloying constituent within the film. The reversible nature of many upd reactions makes it difficult to control deposition processes, especially subnanometer-scale films. Robust additive fabrication schemes are facilitated by irreversible processes analogous to vapor-phase deposition of thin films at low temperatures, although kinetic factors often constrain the quality of the resulting films (6).

Prior analytical studies of Pt deposition have largely limited the applied potential to values positive of underpotential deposited H (Hupd) and proton reduction. One intriguing exception is Pt deposition from a pH 10, Pt(NH3)2(H2O)22+-NaHPO4 electrolyte, in which inhibition of the reaction was evident as the potential was scanned into the Hupd region, although the magnitude and thus importance of the effect were not examined (13). Here, we show that the formation of a saturated Hupd layer exerts a quenching or self-terminating effect on Pt deposition, restricting it to a high coverage of 2D Pt islands. When repeated, by using a pulsed potential waveform to periodically oxidize the Hupd layer, sequential deposition of discrete Pt layers can be achieved. The process is thus analogous to atomic layer deposition (ALD), but with a rapid potential cycle replacing the time-consuming displacement and replacement of the ambient reactant.

We focus on Pt deposition experiments performed at room temperature in aqueous solutions consisting of 0.5 mol/liter NaCl and 3 mmol/liter K2PtCl4, with pH values ranging between 2.5 and 4 (14). Apart from this particular electrolyte, self-terminating Pt deposition was observed over a wide range of pH and Cl concentrations and was not dependent on the oxidation state (2+, 4+) of the Pt halide precursors. To isolate the partial current associated only with the growth process, an electrochemical quartz crystal microbalance (EQCM) was used to track Pt deposition on a Au electrode as the potential was swept in the negative direction. Voltammetry in Fig. 1A shows the onset of Pt deposition at 0.25 V versus a sodium-saturated calomel reference electrode (VSSCE), followed by a substantial current rise to a maximum at –0.32 VSSCE that is close to diffusion-limited PtCl42– reduction. Beyond the peak, the deposition rate decreased smoothly as the mass transfer boundary layer thickness expanded. A sharp drop in the current occurred when the potential moved negative of –0.5 VSSCE, eventually reaching a minimum near –0.7 VSSCE, followed by an increase caused by H evolution from water. The gravimetrically determined (with the EQCM) metal deposition rate revealed that the sharp drop below -0.5 VSSCE corresponded to the complete quenching of metal deposition. This remarkable self-termination or passivation process occurred despite the large applied overpotential (>1 V) available for driving the deposition reaction.

Fig. 1

Gravimetric (A) and voltammetric (B to D) measurements (2 mV/s) of Pt deposition from a NaCl-PtCl42– solution using either a static EQCM or an RDE (400 rpm). The insets in (A) are optical images of Pt films grown on 1–cm-wide Au-coated Si(100) wafers for 500 s at the indicated potentials. Voltammetry reveals the effect of pH on the background reactions (on a Pt RDE) and on PtCl42– reduction (on a Au RDE).

The gravimetric data were used to reconstruct the partial voltammogram for Pt deposition: a two-electron process. Good agreement with the measured voltammogram indicates that the current efficiency of Pt deposition is nearly 100% as the potential is swept toward the diffusion-limited value. When the current peak was approached, an apparent loss in efficiency was observed, apparently because nonuniform deposition developed as the PtCl42– depletion gradient set up a convective flow field that spanned the electrode. In contrast to the EQCM results, voltammetry with a rotating disk electrode (RDE) provided uniform mass transport that yielded a more symmetric peak (Fig. 1B). The contribution of the proton reduction reaction was isolated by performing voltammetry in the absence of the Pt complex. Merging of the respective voltammograms at negative potentials indicates that quenching of the metal deposition reaction was coincident with the onset of the H2 evolution reaction. The overlap of the diffusion-limited proton reduction current also indicates the absence of substantial homogeneous reaction between the generated H2 and PtCl42–, excluding this reaction as an explanation for the quenching of the Pt deposition reaction.

The two-electron reduction of PtCl42– to Pt was not expected to depend on pH, and the onset of appreciable Pt deposition from PtCl42– at 0.0 VSSCE shown in Fig. 1D supports this contention. In contrast, sharp acceleration of the deposition rate below –0.2 VSSCE was pH-dependent and correlated with the onset of Hupd evident in PtCl42–-free voltammetry (Fig. 1C). Chronocoulometry studies indicate that the transition between a halide and a H-covered Pt surface occurred in the same regime (15). The metal deposition rate increased with Hupd coverage and reached a peak value that was independent of pH, whereas the peak potential shifted by –0.059 V/pH, reflecting the importance of H surface chemistry in controlling the Pt deposition process. The onset of proton reduction in the absence of PtCl42–, marked by the dotted line in Fig. 1, B and C, occurred at essentially the same potential. Thus, the peak deposition rate occurred at the H reversible potential.

Moving to more-negative potentials, the metal deposition rate declined rapidly, and within 0.1 V of its peak value, the current merged with that attributable solely to diffusion-limited proton reduction, indicating complete quenching of the Pt deposition reaction.Transient studies of Hads on Pt indicate that the coverage did not reach saturation at the reversible H potential but rather occurred 0.1 V below the reversible value (16). This potential regime is precisely the one in which the metal deposition reaction is fully quenched. Cyclic voltammetry revealed that the passivation process was reversible, with reactivation being coincident with the onset of Hupd oxidation (fig. S1 and supplementary text). Self-termination of the metal deposition reaction arose from perturbation of the double-layer structure that accompanies Hads saturation of the Pt surface. Recent theoretical work indicates that the water structure adjacent to a H-covered Pt(111) surface is altered; the centroid of the O atoms within the first water layer is displaced by more than 0.1 nm from the metal surface as the water-water interactions in the first layer become stronger (17). In a related development, an EQCM study of Pt in sulfuric acid identified a “potential of minimal mass” near the reversible potential of H reactions (18). The gravimetric measurements reflect the impact of Hupd on the adjacent water structure, which leads to a minimum in coupling between the electrode and electrolyte, consistent with the recent theoretical result. In addition to Hupd perturbation of the water structure, the quenching of the metal deposition reaction occurred at potentials negative of the Pt point of zero charge (pzc), where anions would have been desorbed (15). The above combination exerts a remarkable effect whereby PtCl42– reduction is completely quenched while diffusion-limited proton reduction continues unabated.

Self-terminating Pt deposition was also examined under potentiostatic conditions. Optical micrographs of a selection of films after 500 s of deposition at various potentials are shown as insets in Fig. 1A. Only the lower half of a Au-coated Si(100) wafer was immersed in solution, where differences in reflectivity and color indicate the anomalous dependence of deposition on potential; specifically, a 33–nm-thick Pt film was deposited at –0.4 VSSCE while a nearly invisible much thinner layer was grown at –0.8 VSSCE.

X-ray photoelectron spectroscopy (XPS) further quantified the composition and thickness of Pt grown as a function of deposition time and potential on (111) textured Au (14). For films deposited at –0.8 VSSCE, a representative spectrum with the 4f doublets for the metallic states of Au and Pt is shown in Fig. 2 (inset). The ratio of the Pt and Au peak areas was used to calculate the Pt thickness, assuming it forms a uniform overlayer (14, 19). For deposition times up to 1000 s, the measured thickness varied between 0.21 and 0.25 nm, congruent with the deposition of a Pt monolayer with a thickness comparable to the (111) d-spacing of Pt. Monolayer formation was complete within the first second of stepping the potential to –0.8 VSSCE, and the absence of further growth confirmed the self-terminating nature of the deposition reaction. Beyond 1000 s, an additional increment of Pt deposition was evident. Inspection of the surface with scanning electron microscopy revealed a sparse coverage of spherically shaped Pt particles on the surface attributable to H2-induced precipitation, a process requiring some heterogeneity and extended incubation to nucleate. Particle formation can be avoided by using shorter deposition times.

Fig. 2

XPS-derived thickness (red squares) of Pt films as a function of deposition time at –0.8 VSSCE on Au-coated Si wafers from a pH 4 solution. The Au and Pt lines correspond to the (111) d-spacing of the respective bulk metals.

STM was used to directly observe the morphology of the Pt overlayer (14). Analysis was facilitated by using a flame-annealed Au(111) surface with isolated surface steps, 0.24 ± 0.02 nm in height, that served as fiduciary markers (Fig. 3A). Pt deposition resulted in three distinct levels of contrast that reflect the surface height, with the lowest level being the original Au terraces (Fig. 3B). The same three-level structure was observed independently of deposition time up to 500 s (Fig. 3C). The middle contrast level corresponds to a high density of Pt islands that covered ~85% of the Au surface, with a step height of ~0.24 nm, consistent with XPS results. Inspection with a higher rendering contrast revealed a ~10% coverage of a second layer of small Pt islands with a step height ranging between 0.23 and 0.26 nm (Fig. 3D). Step positions associated with the flame-annealed substrate were preserved, with negligible expansion or overgrowth of the 2D Pt islands occurring beyond the original step edge. The lateral span of the Pt islands was 2.02 ± 0.38 nm, corresponding to an area of 4.23 ± 1.97 nm2. Incipient coalescence of the islands was constrained by surrounding (dark) narrow channels, 2.1 ± 0.25 nm wide, that account for the remaining Pt-free portion of the first layer. The reentrant channels correspond to open Au terrace sites that were surrounded by adjacent Pt islands in what amounted to a huge increase in step density relative to the original substrate, the net geometric or electronic effect of which was to block further Pt deposition. The chemical nature of the inter-island region was assayed by exploiting the distinctive voltammetry of Pt and Au with respect to Hupd and oxide formation and reduction (fig. S2 and supplementary text).

Fig. 3

(A) STM images of representative Au(111) surface with monoatomic steps. (B and C) 2D Pt layers obtained after (B) 5 s and (C) 500 s of deposition at –0.8 VSSCE. (D) High-contrast image of 2D Pt layer morphology on Au(111). (E) Linear defects in a Pt layer associated with lifting of the reconstructed Au substrate. Inset, lower-magnification image. (F) A schematic of Hupd-terminated Pt deposition on Au(111).

Similar three-level Pt overlayers have been observed for monolayer films produced by molecular beam epitaxy (MBE) deposition at 0.05 monolayers/min (20). Pt-Au intermixing driven by the decrease in surface energy that accompanies Au surface segregation was evident. In the present work, Pt monolayer formation was effectively complete within 1 s, giving a growth rate three orders of magnitude greater than in the MBE-STM study. Exchange of the deposited Pt with the underlying Au substrate was expected to be less developed. However, intermixing and possible chemical contrast (i.e., the ligand effect) were evident on limited sections of the surface that were correlated with the original faulted geometry of the partially reconstructed Au surface. Upon lifting of the reconstruction, the excess Au atoms expelled mark the original fault location as linear 1D surface defects in the Pt overlayer (Fig. 3E). A simplified schematic of the self-terminating Pt deposition process in Fig. 3F describes how the Hupd accompanying incremental expansion of the 2D Pt islands can hinder the development of a second Pt layer, presumably by perturbation of the overlying water structure (17). This rapid process resulted in a much higher Pt island coverage than has been obtained by other methods, such as galvanic exchange reactions.

Because the saturated Hupd coverage is the agent of termination, reactivation for further Pt deposition was possible by removing the upd layer by sweeping or stepping the potential to positive values, e.g., >+0.2 VSSCE, where negligible Pt deposition occurs. Sequential pulsing between +0.4 VSSCE and –0.8 VSSCE enabled Pt monolayer deposition to be controlled in a digital manner. EQCM was used to track the mass gain, showing two net increments per cycle (Fig. 4A). We attributed the mass gain to a combination of Pt deposition [486 ng/cm2 for a monolayer of Pt(111)], anion adsorption and desorption (41 ng/cm2 for 7 × 1014 Cl ion/cm2, 117 ng/cm2 for a 0.14 fractional coverage of PtCl42–) (7, 21), and coupling to other double-layer components such as water. The anionic mass increments were expected to be asymmetric for the first cycle on the Au surface, but once it was covered, subsequent cycles only involved Pt surface chemistry. After correcting for the electroactive surface area of the Au electrode (Areal/Ageometric = 1.2, derived from reductive desorption of Au oxide in perchloric acid), the net mass gain for each cycle indicates that a near-pseudomorphic layer of Pt was deposited. XPS analysis of Pt films grown in various deposition cycles gave remarkably good agreement with EQCM data (Fig. 4B). The ability to rapidly manipulate potential and double-layer structure, as opposed to the exchange of reactants, offers simplicity, substantially improved process efficiency, and far greater process speed than other surface-limited deposition methods.

Fig. 4

Sequential deposition of Pt monoatomic layers by pulsed deposition in a pH 4 solution. (A) Mass change accompanying each potential pulse. (B) EQCM mass increase is converted to thickness and compared with XPS measurements. XPS analysis of the EQCM specimen (blue diamond) and a series of Pt films deposited on Au-coated Si wafers (black squares) are shown.

Supplementary Materials

www.sciencemag.org/cgi/content/full/338/6112/1327/DC1

Materials and Methods

Supplementary Text

Figs. S1 and S2

References (2226)

References and Notes

  1. Identification of commercial products in this paper was done to specify the experimental procedure. In no case does this imply endorsement or recommendation by NIST.
  2. Acknowledgements: We thank J. J. Mallett for his early observations on the difficulty of electrodepositing Pt films at negative potentials. This work was supported by NIST–Material Measurement Laboratory programs. The x-ray photoelectron spectrometer was provided by NIST–American Recovery and Reinvestment Act funds. Y.L. thanks the NIST–National Research Council Postdoctoral Fellowship Program for support. NIST has filed a provisional patent application (Atomic Layer Deposition of Pt from Aqueous Solutions) based on this work.
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