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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Depth-dependent Scanning Photoelectron Microspectroscopy unravels the Mechanism of Dynamic Pattern Formation in Alloy Electrodeposition Benedetto Bozzini, Matteo Amati, Tsvetina Dobrovolska, Luca Gregoratti, Ivan Krastev, Ivonne Sgura, Antonietta Taurino, and Maya Kiskinova J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01267 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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The Journal of Physical Chemistry

Depth-dependent Scanning Photoelectron Microspectroscopy unravels the

Mechanism

of

Dynamic

Pattern

Formation

in

Alloy

Electrodeposition

Benedetto Bozzini*,1, Matteo Amati2, Tsvetina Dobrovolska3, Luca Gregoratti2, Ivan Krastev3, Ivonne Sgura4, Antonietta Taurino4 5 and Maya Kiskinova2.

1

Dipartimento di Ingegneria dell’Innovazione, Università del Salento, v. Monteroni, 73100 Lecce ,

Italy 2

Elettra - Sinctrotrone Trieste S.C.p.A. S.S. 14, km 163.5 in Area Science Park, 34149 Trieste-

Basovizza, Italy 3

Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, 1113, Bulgaria¶

4

Dipartimento di Matematica e Fisica ‘E. De Giorgi’, Università del Salento, v. Arnesano, 73100

Lecce, Italy 45

Institute for Microelectronics and Microsystems, IMM-CNR, via Monteroni, 73100 Lecce, Italy

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Fascinating spatio-temporal patterns forming during the electrodeposition of some alloys have attracted the interest of the scientific communities dealing with electrochemical materials science and dynamic processes. Notwithstanding extensive experimental work and recently achieved theoretical insights, several aspects of the physical chemistry of these dynamic structures are still elusive. In particular, the analytical methods employed so far to characterize these structures, invariably failed to pinpoint any chemical or structural patterns correlated to those perceived by the naked eye or with a light microscope. In this work, we have made systematic use of the extreme surface sensitivity provided by synchrotron-based scanning photoelectron microspectroscopy, combined with progressive erosion by precisely controlled Ar+ sputtering, to achieve quantitative 3D understanding of the compositional and chemical-state distribution of an Ag-In electrodeposited layer, following the key elements Ag, In and O. The results revealed that the pattern is present only in the topmost region (ca. 100 nm) of the layer and exhibits a regular distribution of the alloying elements in certain chemical states. Specifically, pattern formation in Ag-In electrodeposits is crucially controlled by the space distribution of surface In3+ oxi-/hydroxides, deriving from reaction-diffusion processes taking place during alloy growth and this pattern disappears in depth because of the delayed reduction of In3+ present in this film to elemental In, followed by intermetallic formation.


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1) INTRODUCTION For more than half a century, the formation of spatio-temporal structures during alloy electrodeposition has been extensively studied experimentally, starting from Raub’s pioneering paper published in 19381, and recently it has been modelled mathematically in terms of dynamicsystem theory (see Section S5 of the Supporting Information). Notwithstanding the large corpus of systems investigated, recently reviewed in Bozzini et al.2 and Krastev et al.3, still the mechanisms governing the spatio-temporal organization during electrochemical phase formation are poorly understood. Electrodeposition studies (for a comprehensive list of references, see Bozzini et al.2) have initially dealt with a range of Ag alloys (Ag-Sb, Ag-Sn, Ag-In, Ag-Bi, Ag-Cd), but it has been recently shown that similar phenomena also occur with Bi, In and Sb alloys not involving Ag: Ni– P–W–Bi2, Au-In4, Co-In5,6, Pd-In7, Cu-Sb8. Moreover, alloys with Pt-group metals, as Ir-Ru9 and Pt-Ir10, electrodeposited from molten salts, have been found to exhibit the same type of dynamic behaviour. Ag-In is the alloy that has been most intensively investigated11-24 and for this reason we chose this system for the present study. Notwithstanding the relatively vast literature on electrochemical pattern formation, the general problem of correlating the electrodeposited patterns with specific alloy chemistry and plating conditions, as well as issues regarding sensitivity, reproducibility and scalability, have not been faced systematically yet. Recent reviews of experimental results2,3 and parametric mathematical modelling (see Section S5 of the Supporting Information) have paved the way of this prospective investigation. In any case, spatial patterns in electrodeposited alloys have recently been show to correlate with a two-phase structure including an intermetallic (see Bozzini et al.25 and references therein). It is worth noting here that, at present, spontaneous electrochemical patterning, though undoubtedly having a potential impact on functional properties, cannot be controlled in a manner that could compete with deliberate patterning approaches, such as lithography, atomic layer deposition and STM-based writing methods. Nevertheless, better fundamental understanding of the pattern formation mechanism based on studies that are presently at the initial stage, such as predictive mathematical modelling26 3 Environment ACS Paragon Plus


and parametric control27 -, should provide the capabilities for robust and reliable design for the growth of desired spatially pattered assemblies.

Most of the studies on pattern development in electrochemical phase formation have been essentially experimental, but some of them were also complemented by the presentation of a mathematical framework, chiefly mutuated from electrocatalysis and without specific terms addressing electrodeposition (for more details, see Bozzini et al.25 and references therein). In a series of recent papers, some of the authors (see e.g. Lacitignola et al.28 and Sgura et al.29), have introduced explicitly electrocrystallization information by coupling morphological and surface chemical dynamics (for more details, see Section S5 of the Supporting Information).

The present paper addresses explicitly the physico-chemical mechanism of spatio-temporal pattern formation in alloy electrodeposition. With the present study, we overcome some limits of analytical methods employed so far that did not possess all desired contrast capabilities to follow the structural and compositional features. In fact, the patterns that are so evident with an optical microscope, or even with naked eye, practically become invisible with methods that imply some degree of in-depth averaging, such as Scanning Electron Microscopy (SEM), Electron-beam induced Dispersive X-ray spectrometry (EDX), X-ray diffractometry (XRD) and Glow-Discharge Optical Emission Spectroscopy (GDOES). The results of the publications based on these approaches suggest that the patterns should be something extremely thin, exhibiting a high optical contrast only in the visible, which calls for highly surface sensitive chemical-state probes with adequate lateral resolution. As far as providing variable depth information with access to embedded surfaces is concerned, synchrotron-based XPS using hard-X rays30 could be an option, but the path of photoelectrons from the innermost region of the sample through the analysed layer would result in an averaging effect, ultimately similar to that of EDX. Moreover, no microscopes implementing this technique are currently available. Furthermore, X-ray standing wave (XSW) microscopy31 can in principle be 4 Environment ACS Paragon Plus

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considered in order to achieve depth resolution, though again at the cost of averaging effects. However, it is worth noting that spatially-resolved XSW characterization of materials is usually performed on flat surfaces where periodic structures can provide the necessary spectral information: this is the case, for instance, of PhotoEmission Electron Microscopy (PEEM) measurements coupled with XSW32,33. Unfortunately, the natural topography of electrodeposited samples is not ideal for XSW experiments. On the basis of our recent experience in the investigation of electrodeposited Co-In films by synchrotron-based Scanning Photoelectron Microscopy (SPEM)25, we are convinced that this approach exhibits the unique combination of chemical surface sensitivity and submicrometer spatial resolution, required to access the mechanistically crucial information. In our previous Co-In paper, we restricted our study to the surface of the spiral, and this was sufficient to understand the role of intermetallics in pattern formation. The radical novelty of the present paper is that we have also followed the patterns in-depth by rigorously controlled ion-beam sputtering, ensuring that no artefacts are generated by the erosion process. Using SPEM we have investigated an electrodeposited Ag-In alloy, that has been fully characterized with conventional methods (optical microscopy (OM), SEM, atomic-force microscopy (AFM), EDX, XRD, electrochemical methods). The detailed SPEM analysis of a single spiral was performed monitoring the evolution of the Ag, In and O core levels down to depths at which the pattern was found to disappear. This compositional and chemical-state distribution of the electrodeposited Ag-In spiral allowed us to propose a general model for electrochemical pattern formation.

2) EXPERIMENTAL The Ag-In pattern was obtained by controlled electrodeposition. The composition of the electrolyte used for the growth process is given in Table 1. The electrolyte was prepared using chemicals of pro analysi purity and distilled water and a pure copper foil (2×1 cm2) was used as the cathode



substrate. The pretreatment of the copper substrate includes electrochemical degreasing followed by pickling in a 20 vol% solution of sulphuric acid. In order to avoid the displacement-deposition of silver, the cathode was immersed into the electrolyte under current. Electrodeposition was carried out at room temperature in a 2-electrode cell with a platinized titanium anode. The spirals were obtained at a current density of 1 mA/cm2 with a deposition time of 2.5 h, yielding a coating thickness of about 30 µm. The electrodeposition conditions ensuring spiral formation from the employed electrolyte have been established and described in our previous work11-24. Theoretical information regarding the waiting time required for the formation of spirals is provided in Section S6 of the Supporting Information. During electrodeposition a snapshot of the growing layer was taken every 30 s, as detailed in Section S6.2 of the Supporting Information. A time-lapse movie of the whole pattern-formation process is available from the Journal website. The sample examined in this work (Figure 1) shows a single spiral with 23 turns and diameter of ca. 1 cm: Panel C shows a micrograph recorded in situ in the electrodeposition bath at the end of the growth process.

Photoemission experiments were performed with the Scanning Photoelectron Microscope (SPEM) operated at the ESCAmicroscopy beamline of Elettra Sincrotrone Trieste (Italy). The photon energy used for the photoemission measurements was 651.75 eV, providing an overall energy resolution of about 0.4 and of 0.3 eV for the imaging and micro-spectroscopy modes, respectively. The SPEM employs a Fresnel zone-plate as photon-focusing optics, providing a probe diameter less than 150 nm and a hemispherical electron analyser equipped with a 48-channel electron detector34. High surface sensitivity is intrinsic to soft-X ray photoelectron spectroscopy, enhanced by the grazing acceptance detection of SPEM, thus the effective probing depth in the present measurements is limited to 1 nm, with dominating contribution from the top two atomic layers. Spatially resolved maps of the elemental and chemical-state distributions were acquired by tuning the electron analyser to collect photoelectrons within a selected kinetic energy window while scanning the sample with respect to the microprobe. Furthermore, a detailed characterization of the local surface 6 Environment ACS Paragon Plus

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composition and chemical state was obtained with the microspectroscopy mode. Details on the image processing procedure are reported in Section S4 of the Supporting Information. Following literature procedures35, we mounted a razor-blade on the SPEM holder hosting the Ag-In alloy film and slid it after each sputtering step (more details can be found in Section S4 of the Supporting Information) in order to be able to keep precise track of the changes in sample morphology induced by the sputtering process. After introduction into the ultrahigh vacuum environment of the samplepreparation section of the SPEM, the samples were cleaned and eroded for depth-profiling purposes by Ar+ bombardment (1.5×10-5 mbar Ar, 1-1.5 kV, sample current of 0.8 µA) with Ar+ beam delivered at 45° with respect to the sample. The sputtering cycles (step (1)) start with 15 min at 1 kV, for light cleaning, leaving the pristine surface conditions essentially unaltered. The first “cleaning step” removed the usual surface contaminants, predominantly C-containing species that are derived from the fabrication route and ambient exposure. Usually these species are not affecting the status of the pristine alloy surface and a satisfactory protocol has been already defined and published25. The following sputtering steps were determined heuristically for monitoring progressive depth variations in the spiral pattern. Then stepwise removal of the top layers was carried out changing the sputtering time and applied voltage to increase the efficiency: (step 2) 15 min at 1 kV; (step 3) 30 min at 1 kV; (step 4) 30 min at 1.5 kV and (step 5) 120 min at 1.5 kV. We are aware that sputtering might lead to a range of artefacts, potentially affecting quantitation (see e.g. Chapters 3-5 of Czanderna et. al.36), but the scope of the present investigation is to follow the relative variations in the composition as a function of lateral position and depth, not the precise quantitative elemental analysis (see Section 3.2). For an indicative assessment of the progress of erosion of our intrinsically micrometrically irregular samples, the sputtering parameters reported above, together with material properties, have been fed into standard models for the estimation of sputtering depth (see e.g. Briggs at al.37 and references therein: in particular we used Eq. (4.2) on page 149), yielding the following nominal values for the successive sputtering steps: (1) 8 nm; (2) 17 nm; (3) 34 nm; (4) 54 nm and (5) 133 nm. Possible sputter-induced roughening of the original 7 Environment ACS Paragon Plus


surface topography was monitored by AFM (see Section 3.1). The AFM results confirmed that the changes are negligible and therefore have no impact on the measured variations in the composition. In fact, the evident chemical changes taking place in depth (see Section 3.2) indicate that intermixing and redeposition of sputtered material should be negligible.

Scanning Electron Microscopy (SEM) analyses were performed by using a dual-beam NVISION 40 Focused Ion Beam instrument, equipped with a high resolution SEM GEMINI column and a Field Emission electron gun. X-ray diffractograms were recorded in the interval 20–120° (2θ) with a Philips PW1050 diffractometer, equipped with Cu–Ka tube and scintillation detector.

3) RESULTS 3.1) OM, SEM and XRD of the pristine sample Before starting the SPEM characterization applying Ar sputtering cycles, the pristine structure of the sample was characterized by OM (Figure 1-A) and SEM (Figure 2). Previous relevant studies3,5,38 have already shown the formation of spatial structures during alloy electrodeposition. These structures can be imaged accurately by OM, but exhibit a surprisingly poor contrast in SEM, confirmed by the present study as well. Using the standard Everhart Thornley secondary electron (SE) detector (results not shown for brevity) we did not observe any contrast, whereas some contrast was gained by using the in-lens SE detector, as demonstrated by the top-view SEM image reported in Figure 2-A. Additional topographic contrast was obtained by tilting the sample (Panels (B)-(F)), which indicates roughness variations along the spiral pattern branches. Micrographs on the micron-scale show a classical electrodeposit morphology with cauliflower-type crystallites39, regardless of the position sampled within the pattern. Closer inspection shows that the dimensions of the crystallites aggregating into cauliflower-shaped globules correspond to a roughness change. 8 Environment ACS Paragon Plus

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Panels (C) and (D) show, with increasing magnification, the transition region between two spiral branches, having different topographic features. In particular, crystallite size and roughness are measurably larger on the Ag-rich side of the spiral branches (Panels (C), (E)) compared to the Inrich one (Panels (D), (F)). This formation of better-developed crystallites is compatible with faster deposition of nobler Ag. Moreover, the presence of poorly conducting patches in the In-rich zones – pinpointed by clear charging effects (that are more evident in Panel (E)) – are coherent with the formation of In3+ oxi-/hydroxide films (for more details, see Section 3.2). In view of our in-depth analysis, it is worth noting here that with deliberately compositionally heterogeneous electrodeposited samples - exhibiting a complex roughness such as the 3D cauliflower morphology of our alloys -, there is no real point in attempting a precise quantification of the sputtering depth. The sought-after observables are composition and shape changes of the spiral as a function of depth, indicative of the pattern growth electrochemistry, and for this purpose a semiquantitative estimate of the sampled depth is perfectly adequate. In fact, an accurate erosion depth calibration can be achieved only with compositionally and morphologically homogeneous samples. As evidenced by SEM results in Figure 2 and, still more accurately by AFM results (Figure 3), the amount of material removed by sputtering does not sensibly affect the sample topography. Thus we can safely assume that the sputtering-induced roughness should exert negligible influence on the elemental and chemical-state distributions derived from our SPEM analyses.

Electrodeposited Ag-In alloys are multi-phase systems consisting of α Ag-rich terminal solid solution and a range of intermetallics, depending on the precise plating conditions. As far as the formation of patterned coatings is concerned, XRD investigations have shown that γ-Ag3In is found invariably in electrodeposited alloys exhibiting spirals and other kinds of well-arranged structures19, were also formation of ε-In4Ag9 and φ-AgIn2 occurs in samples aged at room temperature for periods ranging from 1 to 20 hours16. In particular, XRD analyses of the investigated samples (Figure 4) have shown a 3-phase structure exhibiting α Ag-rich terminal solid solution (marked 9 Environment ACS Paragon Plus


with red dots) and the Ag3In and AgIn2 intermetallics (see PDF Cards No: 29-0677 and No: 651552, respectively).

3.2) SPEM imaging and microspectroscopy As noted above, this study is focused on exploring the 3D space distribution of Ag and In chemical states in electrodeposited patterns. A series of SPEM 2D maps and micro-XPS in characteristic regions of the electrodeposited patterns were collected after successive periods of sputtering, that add the third, in-depth, dimension of the distribution. Since the information provided by SPEM measurements is dominated by the signal coming from the top two atomic levels, in-depth averaging effects are avoided. The in-plane compositional inhomogeneity of a typical spiral-shaped feature was investigated in three representative regions, highlighted with white boxes in Figure 1-B. These regions represent: the neighbourhood of the spiral core (α); the Ag-rich part of a spiral branch (β) and the inner part of the Ag-depleted zone of a spiral branch (γ).

Insight into the in-plane and in-depth spatial distribution of Ag, In and O was gained by acquiring after each of the sequential sputtering steps the Ag 3d, In 3d and O 1s spectra from a microspot within the representative regions highlighted in Figure 1-B. The corresponding valence-band spectra were used as a reference for the binding energies (BE), along with the spectra of Ag and In metallic standards (see Figures S1 and S2).

Figure 5 reports representative Ag 3d5/2 spectra measured after the first “cleaning” sputtering step in regions 1 and 2. For the sake of comparison, we also show (Panel (A)) the spectrum of a pure metallic Ag sample, that appears more symmetric and slightly narrower. As illustrated in Section S3 of the Supporting Information, the spectra of Figure 5 represent one of the spin-orbit components of the Ag 3d spectrum, where metallic Ag has also a characteristic electron loss feature at the higher


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BE side. When binding to other elements, the Ag 3d peaks usually undergo BE shifts, become broader and the loss features disappear. It is also worth noting that upon oxidation, the Ag 3d spectrum shifts anomalously to lower BEs40-42, while the present study shows only slight shifts to higher BE and variable degrees of peak broadening in different locations and at different depths. Apparently, there is no spectral evidence for presence of oxidized Ag in any location. In principle, if one excludes oxide formation, the Ag 3d shifts to higher BEs by a few tenths of eV with respect to the metallic state and broadening can be due to Ag-In intermetallic formation, but also certain contributions of the reduced dimensionality cannot be excluded, on the basis of spectral information alone. In fact SEM and XRD results evidence the presence of microcrystallite and intermetallic formation, in agreement with previous XRD16,19 and Anodic Linear Sweep Voltammetry (ALSV)16 studies as well. The formation of an Ag-In intermetallic phase is also supported by the In 3d spectra (see below), so the variable Ag 3d spectral broadening can be attributed to the formation of random alloys where chemically equivalent atoms are located in different environments, as it is the case with the coexistence of solid solution and intermetallic phases43. Using Shirley background, the Ag 3d spectra can be fitted with the weighted sum of two Doniach Dunjic lineshapes44, each convoluted with a gaussian45, peaked at 368.3 and 368.6 eV. These components can be attributed to elemental or solid solution and intermetallic Ag, respectively. All the Ag 3d spectra measured in this study can be fitted with these two components, the only difference being the relative contribution of the components, that indicates lateral chemical inhomogeneity. This compositional inhomogeneity is well represented by the contrast variation of the SPEM map in Figure 5, obtained by dividing the measured maps by selecting the parts of the energy window representing each of the two components (for details, see Section S4 of the Supporting Information).

The lateral compositional inhomogeneity evidenced by the Ag 3d spectra is fully supported by the In 3d spectra measured in these locations. The “depth” evolution of the In 3d3/2 spectra recorded in the same two regions representing high- and low-intermetallic content of the pristine sample (1 and 11 Environment ACS Paragon Plus


2 indicated in Figure 1-B) are reported in. Figure 6. When forming compounds, In 3d spectra generally exhibit only slight energy shifts, but characteristic broadening. The compounds of interest for this study are oxides and hydroxides, since it has been reported that typically electrolysis is not capable of fully reducing In at the cathode surface even at high cathodic overvoltages46: specifically both In3+ oxides and hydroxides have been found at the surface of electrochemically deposited In films47. To our knowledge, no in-depth data have been reported yet.

The literature relevant to XPS at the In 3d core level is briefly surveyed in Section S1 of the Supporting Information: the speciation proposed below is based on the discussion contained therein. Using Shirley background, the In 3d5/2 micro-XPS spectra can be fitted with the weighted sum of one Doniach Dunjic lineshape convoluted with a gaussian, peaking at 443.9 eV and two gaussians centred at 444.5 and 445.2 eV. According to the literature, these three components can be attributed to intermetallic In, In2O3 and In(OH)3, respectively, in fair agreement with the results reported in Nguyen at al.48. From the selection of depth-dependent spectra reported in Figure 6, the only differences are the relative amounts of the three components. The general trend with increasing sputtering time is that the intermetallic component (blue plot) increases and the sum of the In3+ ones (green for In2O3 and red for In(OH)3) decreases, with a higher persistence of the In2O3 one.

The spectral information obtained from the two representative regions, showing the presence of different Ag and In chemical states of variable local concentration, provides basic input for selecting the proper detector channels in view of outlining the lateral compositional inhomogeneity and its depth distribution (see data-processing details in Section S4 of the Supporting Information). Figure 7 reports a representative selection of SPEM Ag 3d5/2 and In 3d5/2 SPEM images of a typical electrodeposited spiral, obtained after the indicated successive sputtering cycles. The maps reported in Panels (A) and (C) are based on the total intensity of the Ag and In 3d5/2 core level spectra, reflecting the lateral variations of the Ag and In surface concentrations. Panels (B), (D) show the Ag 12 Environment ACS Paragon Plus

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and In chemical images obtained by selecting the spectral regions representative of different chemical states. In the case of Ag (Panel (B)) the contrast can be ascribed to the combination of intermetallic- and solid solution-type of the metallic Ag state. For In (Panel (D)) the relevant contrast is between the Ag-In intermetallic and In3+, either as oxide or hydroxide. From Figure 7 it is apparent that, as a function of depth, the elemental content contrast (Panels (A), (C)) tends to fade rapidly, while chemical-state contrast (Panels (B), (D)) is clearly more persistent. Moreover, the Ag and In elemental distributions (Panels (A), (C)), as well as the chemical-state ones (Panels (B), (D)), are anticorrelated. This result is coherent with the depth-dependent total distribution of Ag and In depicted in Panel (E). More details about the quantification of the Ag/In ratio can be obtained by spatial averaging, as discussed in Section S4 of the Supporting Information. The same behaviour of the spectral components is found also in the other points we considered. Cross-sectional analyses (Figure 8) confirm that the In chemical state in the compact sections of the bulk (i.e. away from the cavities that are typical of cauliflower-type electrodeposits) is metallic. It is worth noting here that the compositional heterogeneities that are typical of the cross-section of electrodeposited alloys (see, e.g. Bozzini et al.49-51) are elemental, rather than chemical-state variations, they involve metals in the zero-valent state and derive from mass-transport related instabilities that are totally different from the Turing-Hopf ones that originate spirals. As hinted at in Section S1 of the Supporting Information, the presence of In3+ oxides and hydroxides in electrodeposited In is expected: here we provide a direct proof that the oxide and hydroxide are present in the top layers while in-depth the non-oxidized intermetallic one prevails. Correspondingly, the differences between the high- and low-intermetallic regions decrease as a function of depth, coherently with the loss of contrast highlighted in the SPEM maps of Figure 7.

For the sake of completeness, we also measured the evolution of the O 1s spectra in the same positions where Ag and In spectra were acquired. Figure 9 shows three typical O 1s spectra corresponding to Point 2 of Figure 1-B. The spectral scenario is very clear-cut: three contributions 13 Environment ACS Paragon Plus


are present in the ranges 529.0÷530.7 eV, 531.5÷532.5 eV and ca. 533 eV. According to the literature surveyed in Section S2 of the Supporting Information, these features can be assigned to In2O3, In(OH)3. and H2Oads, respectively. The total O 1s spectrum intensity decreases with depth, but the spatial variations resemble those of In3+ indicating the space correlation with this species. From the sequence of spectra of Figure 9, it is evident that the In(OH)3 component systematically decreases with depth, while the In2O3, though decreasing in intensity, is the most persistent oxide form in the film.

4) DISCUSSION The combination of high lateral-resolution SPEM mapping and high spectral-resolution micro-XPS have allowed to unravel the physico-chemical nature of electrodeposited Ag-In spatio-temporal structures. Patterns such as the spirals shown in Figure 7 can be associated with intermetallics25, but the presented results have revealed a very remarkable correlation between intermetallic and In3+ oxi-/hydroxides (see Figure 6, Panel (D) of Figure 7 and Figure 9). This finding entitles us to link the 3D chemical state distribution with classical electrochemistry of the In3+/In redox couple as well as with the mathematical modelling of electrochemical pattern formation recently developed in our group (see Bozzini et al.2, Lacitignola et al.28,52 and Section S5 of the Supporting Information).

First of all, we have demonstrated that the pattern is present only close to the surface of the electrodeposit and that it disappears at a depth of ca. 100 nm. As proved by combining AFM and SEM observations with SPEM, with a very critical eye on averaging and smearing-out effects that can result from sputtering, we can confidently state that the progressive loss of compositional contrast with depth is not a topographical artefact, but it is indeed a property of the analysed film. Moreover, this property can be straightforwardly related to the physical chemistry of In electrodeposition. In fact, it is well known that on the one hand, electroreduction of In to the 14 Environment ACS Paragon Plus

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metallic state is not complete at the surface of In electrodeposits and that residual In3+ oxi/hydroxides can be detected by surface-sensitive methods, such as electrochemical measurements and surface analyses (see e.g. Detweiler et al.46 and Metikos-Hukovic53). On the other hand, it has been recently shown that such In3+ oxi-/hydroxides can be reduced if cathodic polarization is applied in an electrolyte different from the In (or In-alloy) electrodeposition bath47. In fact, if electrodeposition is simply continued and surface-sensitive analyses are performed, there is no way to understand if the surface film with In3+ species can be reduced yielding a metallic layer below it. Instead, polarization in an In3+-free electrolyte, that cannot regenerate the In3+ film, has been shown to be able to bring about such reduction. This scenario is fully compatible with our data, as schematically represented in Figure 10. Essentially, in the compositional range relevant to our study, In electrodeposition in the presence of Ag, leads to the formation of Ag-In intermetallics, in a way that can be represented as a two-step process. At first (Panel(A)), both Ag+ and In3+ are reduced to the metallic state yielding either adatoms or nuclei (an exact assignment of the intermediate elemental species is irrelevant for the present discussion) that form a stable two-phase system consisting of the α-Ag terminal solid solution and a range (two in our case) of Ag-In intermetallics. In addition, the precipitation of basic In3+ salts takes place owing to the well-described local alkalinization of the catholyte54. Once a basic salt film is formed, two electroreduction processes can run together, the one just described, together with that sketched in Panel (B), that implies the reduction of the newly-formed intermediate (i.e. reduction of In3+ present in the adsorbed film, in addition to that of the electroactive In3+ complexes present in the solution). This two-step scheme straightforwardly explains the reason why In3+ oxi-hydroxides are found at the electrodeposit surface, but they disappear in the bulk of the coating. The last point needing explanation is the formation of a spatio-temporal pattern: mathematical modelling work on electrochemical pattern formation carried out in our group (DIB model: the acronym derives from the initials of the authors’ names), combined with the compositional and chemical state distributions demonstrated by SPEM, provide the key for the rationalization of this phenomenon. Information regarding the model, its 15 Environment ACS Paragon Plus


relevance for the alloy discussed in this work and its generality for alloy electrodeposition have been summarized in the Supporting Information (Section S5). In particular, a group of papers of ours (among which, Bozzini et al.2 offers the most straightforward link with materials-science problems) has definitively highlighted that the coupling of electrodeposit morphology with adsorption of an electroactive intermediate, under appropriate conditions, can give rise to the formation of dynamic patterns, among which spirals28. Moreover, in Lacitignola et al.52 we have shown that intermediate electroactive basic salt films are a classic case of surface chemistry potentially leading to pattern formation. It is worth noting that, comparing the SPEM maps with the SEM micrographs, it is evident that the spirals are compositional patches, accompanied by regions of specific roughness dictated by the local electrochemistry, that span a large number of crystallites. The fact that SEM imaging with a tilt angle (Figure 2, Panels (A) and (B)) exhibits better contrast than in-plane imaging, shows that morphochemical coupling is indeed involved in pattern formation, but the relevant morphology is not the more straightforward cauliflower crystallite structure on the micron-scale, but rather the subtler roughness distribution that prevails on the scale of a few hundred micron. Of course, in the present case, we have positive spectroscopic evidence of the presence of adsorbed In3+ electroactive intermediates and of their organization in a typical pattern. The present results are thus another, independent validation of the DIB model, in particular considering the region of the parameter space in which spiral patters are found28. We employed the map-based parameter identification (Map Identification Problem: MIP) method developed by us in Sgura et al.29 to locate the optimal parameter estimates and we found a remarkably good matching between the experimental SPEM map and the solution of the DIB model with optimized parameters (Figure 11). Finally, the space correlations between intermetallic and In3+ distributions as a function of depth are coherent with the scheme of Figure 10-B. Specifically, Panels (A), (B) and (C) of Figure 7 show that the elemental distributions are anticorrelated in space and that the distribution of In correlates with that of the intermetallic. In fact, quite obviously (see sketch of Figure 10-A) the positions where a large amount of Ag is present are those rich in the α-Ag phase, while the In-rich 16 Environment ACS Paragon Plus

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ones are those in which higher contents of the Ag-In intermetallic (the only form in which elemental In is present on our material) are present. The delayed process of Figure 10-B instead explains why α-Ag and In3+ are positively correlated: free-Ag is in fact able to react with electrochemically reduced In deriving from the adsorbate film: thus fresh Ag-In forms in regions were the initial surface concentration of Ag-In is low. For the same reason, the α-Ag and Ag-In distributions even out as the reduction process runs for prolonged times, the reaction product being encoded in the chemically homogeneous bulk of the film. In summary: close to the electrodeposit surface a pattern is found, essentially due to the precipitation of In3+ oxi-/hydroxides, correspondingly the surface Ag-In distribution shows a correlated pattern. Such correlated patterns have been pinpointed in Bozzini et al.25 exclusively for an Co-In surface, while the present study has clarified its depthdependence for an Ag-In electrodeposit. In the bulk, instead, α-Ag and Ag-In have a homogeneous distribution because excess Ag reacts with In0 produced by reduction of In3+-containing adsorbates. This process also explains the reason why electrodeposited patterns of this type exhibit a very poor contrast when analysed with methods, such as SEM or EDX, that average information (either mainly topographic, as in SEM or strictly compositional, as in EDX) as a result of a penetration depth extending beyond the thickness of the pattern5,33,55,56.

5) CONCLUSIONS With the aim of disclosing the physico-chemical nature and origin of the spatio-temporal patterns forming during the electrodeposition of Ag-In alloys, we have mapped systematically the 3D space distribution of the chemical state of Ag and In, by Scanning PhotoElectron Microscopy with high spatial and spectral resolution. The microspot Ag 3d spectra, measured across the spiral at different depths, are qualitatively the same and can be interpreted in terms of a combination of a Ag-rich terminal solid solution and an intermetallic, coherently with the crystal structure of the alloy as assessed by XRD. The quantitative differences boil down to different fractions of the two forms of 17 Environment ACS Paragon Plus


elemental Ag that are organized in space as a result of the details of In3+ oxi-/hydroxide film formation. The In 3d spectra evidenced the coexistence of intermetallic In, In2O3 and In(OH)3, with spatial variations in the In3+ chemical state and its systematic decrease with depth. O 1s microspot spectra confirm the existence of In2O3 and In(OH)3 and their presence and lateral distribution match accurately those of the In 3d spectra. This chemical-state scenario correlates naturally with the chemistry of In electrodeposition from aqueous solutions.

The evidenced elemental and chemical-state lateral inhomogeneity for both Ag and In evolve differently with depth: the elemental form disappears at a thickness of ca. 40 nm, while chemicalstate contrast is preserved deeper in the film, down to ca. 100 nm. The results also evidence that the elemental and chemical-state distributions of Ag (intermetallic vs. solid solution) and In (intermetallic vs. oxidized) are both anticorrelated.

In summary, the two important findings in the microspectroscopic scenario that emerge from this study are: (i) the pattern is present only at the surface and in the near-surface layers of the electrodeposit and disappears at a depth of ca. 100 nm and (ii) there is an apparent correlation between the 3D spatial distribution of Ag-In intermetallics and In3+ oxi-/hydroxides. This finds a very natural explanation if the classical electrochemistry of In electrodeposition from aqueous solutions is combined with the mathematical model of electrochemical pattern formation that we have recently developed, as detailed above in Section 4. Briefly, at the electrode/electrolyte interface both Ag+ and In3+ are reduced to the metallic state forming α-Ag and Ag-In, alongside with the precipitation of In3+ oxi-/hydroxides. These species, in turn, can be reduced at the interface between the In3+-film and the metallic α-Ag/Ag-In interface. This two-step reductive phaseformation process on the one hand explains why In3+ is found at the electrodeposit surface, but it disappears in the bulk of the coating, and on the other hand is compatible with electrochemical pattern formation via the presence of electroactive adsorbed layer in the source term of a 18 Environment ACS Paragon Plus

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morphochemical reaction-diffusion system, such as the DIB model. It is worth emphasizing that both the pattern formation mechanism and the model describing its surface persistence are general for alloy electrodeposition and can also be extended to other electrochemical reactions exhibiting two-step phase-formation, characterized by the precipitation of a reducible intermediate layer.

SUPPORTING INFORMATION DESCRIPTION The Supporting Information reports: (i) a review of the literature on XPS at the In 3d and O 1s core levels (Sections S1 and S2); (ii) additional micro-XPS material (Section S3); (iii) a detailed description of the data processing method adopted for producing SPEM maps with appropriate elemental and chemical-state contrast (Section S4); (iv) an overview of the mathematical model of pattern formation during electrochemical phase formation (Section S5) and (v) theoretical results in support of the interpretation of dynamic spiral formation (Section S6). Finally, a time-lapse movie of an in situ OM observation of the pattern forming during electrodeposition is available on the Journal website.

ACKNOWLEDGMENTS This research was performed without a dedicated financial support.



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[33] Kronast, F.; Ovsyannikov, R.; Kaiser, A.; Wiemann, C.; Yang, S.-H.; Bürgler, D. E.; Schreiber, R.; Salmassi, F.; Fischer, P.; Dűrr, H. A.; et al.. Depth-Resolved soft X-ray Photoelectron Emission Microscopy in Nanostructures via Standing-Wave Excited Photoemission. Appl. Phys. Lett. 2008, 93, 243116. [34] Abyaneh, M.; Gregoratti, L.; Amati, M.; Dalmiglio, M.; Kiskinova, M. Scanning Photoelectron Microscopy: a Powerful Technique for Probing Micro and Nano-Structures. e-Jour. of Surf. Sc. and Nanotech. 2011, 9, 158-162 . [35] Bozzini, B. Effect of Sputter-Induced and other Roughness on Auger Electron Intensities. Il Vuoto 1997, 26, I,18-29. [36] Czanderna, A. W.; Madey, T. E.; Powell, C. J. Beam Effects, Surface Topography, and Depth Profiling in Surface Analysis; Kluwer, N.Y., 2002, pp. 97-412. [37] Briggs, D.; Seah, M. P. Practical Surface Analysis, Vol. 1, Auger and X-ray Photoelectron Spectroscopy; Wiley, Chichester (UK), 1990, Chapter 4.3.1. [38] Krastev, I.; Valkova, T.; Zielonka, A. Structure and Properties of Electrodeposited Silver– Bismuth Alloys. J. Appl. Electrochem. 2004, 34, 79-85. [39] Popov, K. I.; Djokic, S. S.; Grgur, B.N. Fundamental Aspects of Electrometallurgy; Kluwer, N.Y., 2002, pp. 57-59. [40] Boronin, A. I.; Koscheev, S. V.; Zhidomirov, G. M. XPS and UPS Study of Oxygen States on Silver. J. El. Spec. Rel. Phen. 1998, 96, 43-51. [41] Gao, X.-Y.; Wang, S.-Y.; Li, J.; Zheng, Y. X.; Zhang, R.-J.; Zhou, P.; Yang, Y.-M.; Chen, L.Y. Study of Structure and Optical Properties of Silver Oxide Films by Ellipsometry, XRD and XPS Methods. Thin Solid Films 2004, 455-456, 438-442. [42] Grönbeck, H.; Klacar, S.; Martin, N. M.; Hellman, A.; Lundgren, E.; Andersen, J. N. Mechanism for Reversed Photoemission Core-Level Shifts of Oxidized Ag. Phys. Rev. B 2012, 85, 115445.



[43] Olovsson, W.; Göransson, C.; Marten, T.; Abrikosov, I. A. Core-Level Shifts in Complex Metallic Systems from First Principle. Phys. Status Solidi (b) 2006, 243, 2447-2464. [44] Doniach, S.; Sunjic, M. Many-Electron Singularities in X-ray Photoemission and X-ray Line Spectra from Metals. J. Phys. C 1970, 3, 285-291. [45] Tougaard, S. Quantitative Analysis of the Inelastic Background in Surface Electron Spectroscopy. Surf. Interface Anal. 1988, 11, 453-472. [46] Detweiler, Z. M.; White, J. L.; Bernasek, S .L.; Bocarsly, A. B. Anodized Indium Metal Electrodes for Enhanced Carbon Dioxide Reduction in Aqueous Electrolyte. Langmuir 2014, 30, 7593-7600. [47] Larrazábal, G. O.; Martín, A. J.; Mitchell, S.; Hauert, R.; Pérez-Ramírez, J. Synergistic Effects in Silver–Indium Electrocatalysts for Carbon Dioxide Reduction. J. Catal. 2016, 343, 266-277. [48] Nguyen, T. P.; Le Rendu, P.; de Vos, S. A. An X-ray Photoelectron Spectroscopy Investigation into the Interface formed between poly(2-methoxy-5-(20-ethyl-hexyloxyl)-p-phenylene vinylene) and Indium Tin Oxide. Synth. Met. 2003, 138, 113-117. [49] Bozzini, B.; Giovannelli, G.; Cavallotti, P.L. An Investigation into the Electrodeposition of Au–Cu-matrix Particulate Composites. J. Appl. Electrochem. 1999, 29, 685-692. [50] Bozzini, B.; Brevaglieri, B.; Cavallotti, P.L.; Giovannelli, G.; Natali, S. Hydrodynamic Problems related to the Electrodeposition of AuCu/B4C Composites. Electrochim. Acta 2000, 45, 3431-3438. [51] Bozzini, B.; Cavallotti, P.L., Giovannelli, G. Electrokinetic behavior of Gold Alloy and Composite Plating Baths. Metal Finishing 2002, 100, 50-60. [52] Lacitignola, D.; Bozzini, B.; Frittelli, M.; Sgura, I. Turing Pattern Formation on the Sphere for a Morphochemical Reaction-Diffusion Model for Electrodeposition. Commun. Nonlinear Sci. Numer. Simulat. 2017, 48, 484-508 .


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FIGURES AND TABLE

Figure 1 – The spiral pattern present in the electrodeposited Ag-In alloy selected for depthdependent SPEM and micro-XPS analysis. (A) Light microscopy image (metallographic optical microscope). (B) Ag 3d SPEM map of the same region: contaminant-free pristine surface after the first, “cleaning” sputtering. Contrast mode selected for emphasis on Ag content after correction for reducing topographical contrast (see details in Section S4 of the Supporting Information). (C) Final snapshot from a sequence of light macrographs, recorded in situ in the electrodeposition bath. Highlighted in white in Panel (B) are the regions, as well as two reference spots, in which we have performed high-resolution SPEM and micro-XPS.


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Figure 2 – SEM micrographs of the Ag-In electrodeposit (pristine conditions) exhibiting spiralshaped patterns. Panel (A) is a top-view image of the sample obtained by using the in-lens secondary electron detector. Panels (B)-(F) have been obtained by tilting the sample holder by 57° in order to emphasize the 3D crystallite morphology. Each rectangular frame of a given Panel delimits the region shown at higher magnification in the successive Panel, as indicated by the arrow.



Figure 3 –AFM analyses of the electrodeposited Ag-In film studied by SPEM in pristine conditions and after the last sputtering step (#5).


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Figure 4 – XRD of the Ag-In electrodeposited sample analysed by SPEM, showing a 3-phase structure featuring an α-Ag terminal solid solution and two Ag-In intermetallics, in addition to Cu reflections from the substrate.



Figure 5 –g 3d5/2 micro-XPS spectra. (A) Metallic Ag standard; (B, C) representative measurements in the two different locations of the electrodeposited Ag-In alloy shown in region β of Figure 1-B exhibiting high and low intermetallic contents -, after the first “cleaning” sputtering cycle and their fits. High lateral-resolution SPEM maps of the analysed region are shown at the bottom.


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Figure 6 – Representative In 3d5/2 micro-XPS spectra and their fits (see Section 3.2 for details) for typical locations of the electrodeposited Ag-In alloy exhibiting high (Point 1 of Figure 1-B) and low (Point 2 of Figure 1-B) intermetallic contents, as a function of sputtering time.



Figure 7 –Ag 3d5/2 ((A), (B)) and In 3d5/2 ((C), (D)) SPEM elemental (A), (C) and chemical (B), (D) maps, recorded after each of the 5 sputtering steps. All maps are corrected for removing topography contributions. The Ag/In ratio map (E) outlines the compositional variations in depth. For computational details, see Supporting Information, Section S4 and refer to the equations mentioned below. ((A), (C)) Ag and In elemental maps (contrast: spectrum/background, Eq.s (S1) and (S3)); (B) Ag chemical-state map (contrast: intermetallic/solid solution, Eq. (S2)); (D) In chemical-state map (contrast: oxide-hydroxide/intermetallic, Eq. (S4)); (E) ratio of the Ag and In elemental maps (A) and (C), Eq. (SI.5): the colour bar encodes the Ag/In elemental ratio linearly. We have 32 Environment ACS Paragon Plus

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positively assessed that the choice of the most sensitive scale bars in this Panel is the best graphical way to convey information about the progressive loss of compositional contrast leaving the whole dynamic range of the images: any other choice would have either artificially enhanced the contrast of would have caused the premature disappearance of spiral traces after the first sputtering step.



Figure 8 – In metallic / In oxide ratio map of the cross-section of the investigated sample (left) and XPS spectra (right) at the In 3d5/2 core-level. The right Panel compares a typical spectrum extracted from the compact zones of the map of the cross section (red plot) with a high-resolution micro-XPS spectrum measured after the 5th sputtering step (black plot, see Figure 6) and with a spectrum extracted from the SPEM map recorded in the same conditions (blue plot, see Figure 7, rightmost maps of Panels (C) and (D)). The presence of oxidized zones in the bulk originates mainly from the porosities of the cauliflower structure of the electrodeposit, but also from the cross-sectioning procedure, that was carried out under ambient conditions.


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Figure 9 – Representative O 1s micro-XPS normalized spectra (see Section 3.2 for details) measured in Point 2 of Figure 1-B after the indicated sputtering times (Panel (A), steps #1,2 and 5; Panel (B), step #5).



Figure 10 – Electrodeposition reaction schemes explaining the formation of an In3+ oxi-/hydroxide film - in addition to α-Ag and Ag-In intermetallics - at the growing electrode/electrolyte interface (A), followed by the reduction of the In3+ film consuming part of the α-Ag phase and yielding more Ag-In (B).


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Figure 11 – Comparison between: (A) measured SPEM map (In chemical-state map, sputtering step #1, see Figure 7) and computed morphochemical distribution from the DIB model28, obtained by MIP parameter identification29 (normalized experimental data and η variable, α=0.5; γ=0.2; k2=2.5; k3=1.5; A1=10; A2=30; B=22, C=2.3, asymptotic temporal behaviour. Details on the physical meaning of the parameters and on computational issues are provided Sections S5 and S6 of the Supporting Information).

Electrolyte composition

Concentration g dm-3

mol dm-3

In as InCl3 /Alfa Aesar/

11

0.1

Ag as KAg(CN)2 /Degussa/

3

0.03

D(+)-Glucose / Fluka/

20

0.1

KCN /Merck/

34

0.5

Table 1 – Composition of the electrolyte for deposition of alloy coatings.



TOC GRAPHIC


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Figure 1 33x13mm (300 x 300 DPI)

ACS Paragon Plus Environment




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Figure 4 279x215mm (150 x 150 DPI)


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Figure 10 25x17mm (600 x 600 DPI)


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Figure 11 26x12mm (600 x 600 DPI)



Table of Contents image 26x15mm (600 x 600 DPI)


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Figure S1 279x215mm (150 x 150 DPI)





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Figure S10 246x210mm (144 x 144 DPI)


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Depth-dependent Scanning Photoelectron ... - ACS Publications

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