Mechanism of Accelerated Zinc Electrodeposition in Confined

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Mechanism of Accelerated Zinc Electrodeposition in Confined Nanopores, Revealed by X-ray Absorption Fine Structure Spectroscopy Álvaro Muñoz-Noval, Kazuhiro Fukami, Akira Koyama, Takuya Kuruma, Atsushi Kitada, Kuniaki Murase, Takeshi Abe, Tetsuo Sakka, and Shinjiro Hayakawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05994 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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The Journal of Physical Chemistry

Mechanism of Accelerated Zinc Electrodeposition in Confined Nanopores,

Revealed

by

X-ray

Absorption

Fine

Structure

Spectroscopy Álvaro Muñoz-Noval,1,* Kazuhiro Fukami,2,* Akira Koyama,2 Takuya Kuruma,1 Atsushi Kitada,2 Kuniaki Murase,2 Takeshi Abe,3 Tetsuo Sakka,3 Shinjiro Hayakawa1 1

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Hiroshima 739-8527, Japan

2

Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan

3

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan

Corresponding authors (A.M.N.) [email protected], (K.F.) [email protected]

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Abstract Recent studies have revealed that electrochemistry at the nanometer scale is profoundly different from its conventional framework. We reported that the combination of a hydrophobic nanoporous electrode and low-charge-density metal ions resulted in a drastic acceleration in the electrodeposition reaction. In the present study, we analyzed Zn embedded in nanoporous silicon by X-ray absorption fine structure spectroscopy. As a precursor to Zn electrodeposition, Zn(II)-chelate was used in different pH conditions. The spectroscopy results clearly suggest that the accumulation of Zn(II)-chelate occurred at pH conditions where the Zn(II)-chelate had zero charge. The accumulation resulted in the promotion of Zn electrodeposition within confined nanopores. Based on this

spectroscopic investigation,

we

propose

a model for the accelerated

electrodeposition of Zn in confined nanopores.


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Introduction There is increasing demand for the fabrication of structures at the single-digit nanometer scale; therefore, interest in electrodeposition has shifted from bulk plating to nanoscale electrodeposition. From the conventional viewpoint of electrodeposition in aqueous baths, the most-accepted strategy to enhance electrodeposition on substrates is hydrophilic treatment of the substrates prior to electrodeposition.1 At the single-digit nanometer scale, recent research has revealed that the conventional strategy employing hydrophilic treatment is no longer the optimum method. Because of interactions with the solvent (water molecules in an aqueous solution) in deposition baths is not negligible at the nanometer scale, some studies, including our previous report, have shown that the solvation properties (hydrophobicity in an aqueous solution, solvophobicity in non-aqueous solution, and ionophobicity in an ionic liquid) of the solute and surface play significant roles in enhancing electrochemical properties such as in electrodeposition and electric double layer capacitors.2-12 As

an

example

of

nanoscale

electrodeposition,

we

have

studied

electrodeposition within nanopores in porous silicon electrodes.7-11 Porous silicon is a porous electrode that is easily prepared by anodization of a silicon wafer in an HF solution, and its pore size is normally tunable from macropores to micropores. Our study focused on the utilization of microporous silicon whose pore size was ~3 nm. The surface of porous silicon walls is terminated with Si-H bonds, and thus the micropores of as-prepared porous silicon are strongly hydrophobic. Using such hydrophobic porous silicon, metal electrodeposition is strongly enhanced by adopting a complex of metal ions with a low charge density. Because low charge density metal complexes behave like hydrophobic solutes, the emergence of hydrophobic interactions with microporous silicon is expected. In such conditions, the number density of the metal complex in confined nanopores becomes quite high. In a particular case, nanopores are filled with the highly concentrated solution, and we call this phenomenon surface-induced phase transition (SIFT).10,

13, 14

Theoretical analysis based on integral equation theory for

molecular liquids, a statistical-mechanical theory, suggested that the number density of metal complexes can be orders of magnitude higher with the occurrence of the SIFT in nanopores.8,

9

There exists a strong relationship between the occurrence of SIFT in



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nanopores and the local enhancement of electrodeposition. However, we have not obtained clear and direct evidence of the accumulation by SIFT. Recently, we studied the electrodeposition of zinc in microporous silicon as an example of highly efficient and high-rate electrodeposition in nanopores, which is applicable for the zinc anodes of rechargeable batteries.11 Highly efficient and high-rate electrodeposition of zinc was achieved by tuning zinc complexation using polycarboxylate anions so that SIFT in the nanopores of porous silicon was expected. We demonstrated how the penetration rate and filling homogeneity were controlled by tuning the hydration properties of the Zn(II) complex by choosing different complex sizes and geometries. The complexation degree of the Zn(II) chelate with polycarboxylate anions was controlled by the solution pH, resulting in a Zn(II) complex speciation equilibrium. As a consequence, the size and charge of the complex determined the hydration properties of the complex. In the present paper, we study the zinc deposition system further in order to detect evidence of the occurrence of SIFT of Zn(II) complexes in nanopores. The main approach in the present study was the analysis of zinc embedded porous silicon by Xray absorption fine structure spectroscopy (XAFS). There are some experimental difficulties in the characterization of metal embedded in a porous silicon matrix. For instance, porous silicon is a porous electrode with a huge specific surface area (ca. 104 m2/cm3) and there are, in some cases, only dilute quantities of Zn buried in the pores. Therefore, we chose XAFS for studying the deposition mechanism in this system. The hard X-ray range employed (corresponding to approximately the Zn K-edge) and the sensitivity of the technique to small traces and diluted samples make it a suitable analytical tool. XAFS is also an element-selective analytical technique that gives chemical state and local structure information on the element of interest. Results obtained by XAFS suggest that high-density adsorption of zero-charge Zn(II) complex on the pore wall surface enhances the electrodeposition. The high density of Zn deposition in the nanoporous electrode was confirmed with the occurrence of the SIFT for the Zn(II) complex in the nanometer-size pores. We believe that the present strategy to achieve highly-efficient electrodeposition within nanopores of porous silicon can be applicable far beyond the particular system under study (Zn electrodeposition), such as embedment of magnetic and catalytic metals.15-17 4 ACS Paragon Plus Environment

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Experimental Sample preparation Microporous silicon electrodes (PSi) with a pore diameter of 2 nm and a porous layer thickness of 2 µm was prepared starting from an n-Si (100), 10-20 Ωcm substrate. Anodization of the silicon wafer in an electrolyte solution (22 wt% HF solution) was carried out with a current density of 2.0 mA cm–2 and the duration of anodization was ~20 min. Zinc deposition was performed at a current density of –10 mA cm–2 using different zinc complexes, starting with 0.1 M ZnSO4 and adding 0.1 M disodium malonate (adjusting pH to 2.0, 3.0, 4.0, and 5.0 with small amounts of H2SO4) or 0.3 M mixture of citric acid and trisodium citrate (adjusting pH to 1.8, 3.0, and 4.8 by tuning the mixing ratio of them). Durations of electrodeposition were selected to be between 20 and 120 s. The samples were rinsed with pure water and dried after electrodeposition.

XAFS experiments As discussed in the introduction, the use of a porous electrode as the deposition substrate implies several experimental difficulties. However, by using XAFS these issues can become advantages. Because XAFS is bulk sensitive and the Si matrix is almost transparent to X-rays in the range of energies used to analyze the Zn K-edge, the result was that the measurable density of Zn was much larger than that measured using a flat substrate. Figure 1 illustrates this situation. The first stages of the deposition corresponded to the moment when the Zn started to cover the surface of the silicon. Because the high surface area of the PSi (about 104 m2/cm3) compared with the flat surface, the area to be covered during the deposition was greatly increased with respect to a standard case using a flat electrode. Therefore, in this scenario, for equivalent deposition times, there exists the possibility of observing much earlier stages with a porous electrode. This possibility may be inaccessible when studying flat substrates but would require deposition times on the order of 10–3 shorter. This aspect will be discussed properly later. Preliminary XAFS measurements were carried out at BM25A-SpLine, the Spanish CRG at the ESRF (Grenoble, France) in the Zn K-edge (9600 eV) in fluorescence yield mode, including the spectra of pristine and annealed samples 5 ACS Paragon Plus Environment


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presented in the Supporting Info (SI). XAFS measurements presented in the present study were obtained at BL01 at SPring-8 (Hyogo, Japan). For each condition (pH, time, substrate and chelate), 2 or 3 samples were measured and analyzed. A single spectrum for each sample point was enough to obtain high-quality spectra. In this sense, we also tested sample homogeneity by analyzing different points of several set of samples. The spectra were acquired in fluorescence yield mode, and three samples for each condition of pH, substrate type, and deposition time were measured to ensure repeatability. Several scans were obtained for each sample to ensure the chemical homogeneity of the surfaces. Spectra were acquired at wavenumber values of 15 Å–1. For calculating the Fourier transformation (FT) of the EXAFS signal, a k range from 2.5 to 11 Å–1 was analyzed for all the spectra. To reduce the EXAFS data, standard procedures using the Demeter package were employed.18 For fitting, k3-weighted signals were analyzed and adjusted in r and k spaces using theoretical functions from the FEFF8.4 code.19 Ab initio calculations of the Zn-malonate and citrate spectra were performed by modifying the crystallographic structures of CuC6H10O1020 and of Zn3C12H14O16,21 treated with CRYSTALFFREV22 to obtain adequate FEFF input data files. The first Zn-O shells of the ZnO phase and Zn-chelate cannot be resolved individually. The experimental resolution for the EXAFS was limited by the range of the kinetic energy from the photoelectron used to measure. This could be calculated from the relationship between the shell radial distance and the maximum value of the photoelectron wave-vector using:

δR = ଶ௞

గ

[2]

೘ೌೣ

For data analysis, the maximum k value that we have included in the FT was 11 Å–1, which meant a minimum path distance of ca. 0.14 Å for being able to resolve them. This meant the two paths could not be resolved if they were closer than this lower limit. However, the structural parameters for these paths might be adequately constrained and bonded to other paths of the corresponding phase in order to build a consistent physical model. For instance, the amplitude of Zn-Zn path, included in the model, correlates to the amplitude of the Zn-O path of the same phase. The Zn-chelate phases here


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considered do not present any path between the Zn-O and the upper limit of the fitting range. Hence, the correlated intensity of the Zn-Zn (ZnO) and Zn-O with respect of the Zn-O from the chelates gives a robust semi quantification of the phase composition in the deposits, as the independent semi quantification given from the XANES spectra fitting demonstrates. The respective paths of each phase shared an EXAFS amplitude parameter (S02) related to the coordination number or number of atoms per shell (N), but had individual Debye-Waller (DW) factors and shell radius, depending on the possibility of resolving them individually or not. Considering the non-structural EXAFS parameter, the energy shift ∆E0, was set free for convergence reasons but strongly constrained. This parameter was kept as a global fitting parameter for all the paths in a calculation. In any case, the spectra were reduced and aligned carefully and we applied criteria not to allow ∆E0 to exceed a value of 7 eV.23

Results XAFS in Figure 2 provides clear evidence about the differences in chemical and structural states of Zn at early deposition times (20 s) from Zn(II)-malonate baths. XANES and EXAFS spectra of Zn deposited on PSi from pH = 3, 4, and 5 baths were depicted with the corresponding reference spectra of ZnO (powder) and Zn (foil) in the XANES and EXAFS region of the spectrum. Before starting to explain the results, we should note that the filling fraction of Zn within the nanopore is different among the pH conditions. This was evaluated in our previous paper.11 Briefly, the pores are mostly filled with electrodeposited Zn under the condition of pH=5. On the other hand, the filling fraction was not high at the low pH conditions. The XANES spectra showed, approximately, that the chemical state of the deposits at the early stages could not be clearly ascribed to ZnO or Zn at any of the studied pH conditions. Similar evidence was observed when comparing the EXAFS spectra as a function of the photoelectron wavevector (k). In this case the dissimilarities were clearer among the pH conditions, regarding the low-k region of the spectra, which were somehow related to a different chemical environment for each of them (both a different structure at the medium-long EXAFS range and different elements in the local environment). Zn deposited on PSi was analyzed at different stages of deposition, namely after short (20 s), intermediate (60 s), and long (120 s) deposition periods. XANES at the 7 ACS Paragon Plus Environment


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long period deposition at the studied range of pH conditions (in SI) showed that the deposits in all cases reached an almost completely metallic state. However, we performed more-detailed chemical and structural analysis by fitting the EXAFS of the different conditions and deposition times. To begin discussing the results of the deposition in PSi, Fourier transformation (FT) of the EXAFS spectra for the different deposition conditions is displayed in Figure 3a with the corresponding fitting. The short period conditions in all the pH conditions showed an intense peak at ca. 2 Å (please note the phase shifts have not been corrected in the r-space, thus this distance corresponded to ca 1.6 Å in the figure). This peak corresponded to the first Zn-O shell, which was assigned as a combination of the Zn-O first shell of ZnO and Zn(II)malonate species. The intensity of this shell clearly decreased as the deposition time became longer, with this evolution clearer in the case of higher pH conditions. We performed a control experiment by depositing Zn from a ZnSO4 bath for 120 s (results shown in SI.2). In this case, the fitting results were very satisfactory using a model including Zn and ZnO with about 70±5% of Zn and 30±5% of ZnO. This result also pointed out the difficulty of achieving the metallic deposition of Zn using nonchelate baths. In any case, the fitting of the experimental spectra by considering a ZnO/metallic-Zn model has been applied first to the Zn deposited from Zn(II)-malonate baths, but we could not achieve satisfactory fitting using this two-phase model. Therefore, we included a third phase with Zn(II)-malonate in the model. Because the Zn-O distances in the first coordination shell of ZnO and of Zn-malonate cannot be distinguished, we were not able to resolve the relative phase amplitude by only fitting the first shell. Hence, we used a three atomic-shell model to resolve the phase composition with a reasonable accuracy. The phase composition and EXAFS parameters from the fitting results, including the fitting in the r-space, are summarized in Figure 3b and Table 1. The metallic fraction of Zn increased steadily with deposition time for all the conditions. However, only in the case of higher pH did it reach almost 100% after deposition for 120 s. Regarding the ZnO content, it was large with short deposition times in the case of low pH baths and its content decreased with pH, whereas it was inappreciable at pH = 5. In all cases, the ZnO fraction was smaller than the non-chelate bath. The most interesting point was the high content of Zn malonate detected after a short deposition 8 ACS Paragon Plus Environment

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period in pH = 5 baths, which is as large as ca. 20%, being the amount of ZnO that was negligible at this condition.

Deposition on a flat substrate We compared the chemical speciation and structure of Zn deposited in PSi with that deposited on flat Si in identical conditions. Here, we focused on the structural and chemical differences for the boundary cases (short and long deposition times, i.e. 20 and 120 s) with different pH conditions. The results on the phase composition determination are summarized in Table 2. The particular case of Zn deposited on flat substrates from a pH = 5 bath is presented in the SI. Table 2 summarizes the results of the best fitting analyses. In this case we observed clear differences with respect to the porous substrates for all pH conditions. The short deposition presented lower metallic content for pH = 3 and 4 than that for pH = 5, almost the same in the content of ZnO and a noticeable higher content in malonate. Differences in pH = 3 and pH = 4 conditions were hardly distinguishable within the experimental accuracy. However a qualitative difference is very clear at the pH = 5 condition compared to the low and intermediate pH conditions. Though the ZnO content is almost constant along the pH range, the main differences are observed in the contribution of Zn(II)-malonate, which diminished abruptly at pH = 5. Differences were mostly observed for the Zn(II)-malonate and Zn metal total content, but within a quite homogeneous composition, without a clear correlation with the pH of the parental solution. Thus, our conclusion was that the deposition on flat substrates even at 20 s was at an advanced stage and between 20 and 60 s reached an equivalent situation to that of the long deposition period (over 120 s) with PSi.

XANES The absorption near edge (XANES) is very sensitive to slight geometrical changes in the atomic surrounding of the scattered atom. Any change in the electronic structure coming from a change in the chemical state, coordination, or even in the geometry of the bonding caused shape changes of the XANES spectrum. We focused our attention on the high pH and short deposition condition. In Figure 4, we present the XANES ZnK edge spectrum of Zn at the early stage of deposition in a porous substrate from the pH = 5 bath. There existed clear differences in the spectra profiles. Considering the EXAFS 9 ACS Paragon Plus Environment


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results, we may consider the presence of a high amount of Zn in the form of Zn(II)malonate. Thus, we performed full multiple-scattering calculations considering a low crystalline Zn(II)-malonate structure. This calculation is included in the figure, pointing out that there was a considerable fraction of Zn(II)-malonate in the structure. The similarities in the experimental spectra with the calculated one were even clearer when comparing the derivative of the absorption edge, as shown in the central panel of the figure. According to the main spectral features that correspond to identifiable maxima and minima in the derivative, the experimental spectrum of the short time deposition at pH = 5 clearly coincided with the Zn(II)-malonate reference. At this pH, the temporal evolution of the XANES (Figure 4c) also demonstrated how Zn evolves into the metallic phase. We employed a theoretical spectrum of Zn(II)-malonate with experimental Zn references to estimate the chemical speciation of Zn in the deposits independently from the EXAFS calculations. We carried out least-squares linear combination fitting of the XANES data to fit the experimental spectra of the Zn deposits obtained in the different conditions investigated. However, this method has a high-limiting bias because of the fact that we do not know the precise chemical state and structure of the Zn(II)-malonate phase in the deposits. Thus, the current spectra may be distorted with respect to the real situation. Another source of bias was the fact that the theoretical spectrum lacked experimental uncertainties, such as energy resolution limited by the monochromator, etc. In summary, the phase estimation obtained by this method should be regarded with caution and maybe only general trends can be compared. The results of the phase composition determined by this method are included in Table 3. Four main conclusions can be extracted from the data: i) the general trend coincided with the EXAFS calculations; ii) the metallic phase increased with deposition time for all the pH conditions and substrates; iii) the highest pH presented high amounts of malonate and the highest metallic fraction and iv) the chemical speciation of Zn deposited on flat substrates for deposition times beyond 20 s only has appreciable variations (about 10%) when using low pH baths.

Zn deposited from Zn(II)-citrate complexes


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A similar study has been performed for Zn deposited from Zn-citrate complex solutions at selected pH conditions such as pH = 1.8, 3, and 4.8. Zn-citrate has three pKa values at 2.77, 4.11, and 5.3. In this case we only have boundary time conditions for the three pHs. Thus, only the 20 s and 120 s deposition times (short and long) are presented. First of all, the XANES experimental data for the short deposition using the three different baths are shown with corresponding spectra of the Zn and ZnO references in Figure 5. Following the ideas developed in the previous section using malonate, we also show an ab initio theoretical spectrum of Zn(II)-citrate, in which a large structural disorder has been included in the calculation. The comparison among short deposition samples from different baths clearly showed that the condition at pH = 3 gives Zn deposits with a large amount of Zn(II)-citrate as a chemical composition. There was clear evidence that supported this supposition regarding the derivative of the spectra, where the sample formed at pH = 3 shares its main features with the Zn(II)-citrate theoretical spectrum, but those format at pH = 1.8 and 4.8 shared more features with the metallic sample spectrum. However, the pH = 3 spectrum seemed to have also common features with the metallic Zn spectrum and to a lesser extent with that for ZnO. Thus, the XANES data pointed out that the earliest Zn deposits had a mixture of Zn chemical species, with citrate as the main contributions in the case of the pH =3 condition. In Figure 5c, we also show the comparison between the short- and long-deposition period spectra for the different pH conditions. The highest pH condition did not increase the metallic phase with deposition time significantly. A similar situation was observed for the pH = 1.8 condition. Because of the considerably larger amount of metal from the short deposition period, the metallic phase did not improve significantly in view of the evolution of the spectrum. In the case of pH = 3 where the largest changes were observed, but also where the best quality Zn was obtained after a long deposition period, if we compare with the metallic spectrum. The chemical phase composition for Zn was also quantified using EXAFS. Figure 6 illustrates the EXAFS spectra of PSi and flat Si after depositing Zn and the best-fit model using a combination of Zn, ZnO, and Zn(II)-citrate phases, whose results are included in Tables 4 and 5. Time evolution of spectra measured using the samples prepared in different pH conditions is shown. The clearest feature was the absence of a peak associated with the Zn-O shell originating from Zn(II)-citrate / ZnO on the flat 11 ACS Paragon Plus Environment


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substrates, whereas it was clearly observed when using PSi. Within the PSi substrates, a decrease in Zn-O shell contribution with deposition time was observed with all the pH conditions tested, but this decrease was much clearer at pH = 3. The difference has been semi-quantified in Table 5, where the phase composition obtained from the EXAFS fitting is summarized. The highest Zn content in the deposits after long deposition period was observed with the pH = 3 condition. This was in good agreement with the qualitative data observed in the XANES analysis, presented in Figure 6 and discussed below. Other notable results were i) the low variation in the phase composition with the pH = 1.8 and 4.8 conditions with time and ii) the high oxide content with the pH = 4.8 condition, which decreased with deposition time. When comparing the results on flat substrates, we obtained quite different results. The phase composition was quite homogeneous among the pH conditions, and it did not change drastically with deposition time. Fitting of the XANES spectra was performed using the reference experimental spectra of Zn, ZnO, and the theoretical spectrum calculated for the Zn(II)-citrate phase. The results of the fittings both for PSi and for flat substrates are shown in Table 6. Regarding the pH = 3 condition in PSi, the composition with the short deposition period was largely dominated by the Zn(II)-citrate phase, and it decreased drastically after a 120 s deposition period. An opposing situation was observed for the other conditions where the Zn(II)-citrate contribution is notably lower than in the pH=3 condition. The quantitative differences in the phase composition led us to consider the EXAFS model we employed, because it may underestimate/overestimate the Zn(II)-citrate contribution in the cases of large/low amounts of this phase in the deposits.

Discussion We will first discuss Zn deposition in PSi in the Zn(II)-malonate/citrate baths. Special attention was paid to the pH = 5 condition with the Zn(II)-malonate baths as well as the pH = 3 condition with the Zn(II)-citrate bath. In these conditions, larger amounts of the Zn(II)-malonate or Zn(II)-citrate phases were detected after a short deposition period. The high proportion of Zn(II) complexes suggested that the complexes were highly accumulated within the nanopores of PSi. This accumulation was predicted in our previous study.11 In that paper, we argued that Zn complexes with zero charge were 12 ACS Paragon Plus Environment

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formed in conditions at pH = 5 for malonate and at pH = 3 for citrate. When the charge of the complex was zero, the interaction between the complex and water molecules became weaker. As a result, a hydrophobic interaction came into play. When the pore diameter was very small and the concentration of the Zn complex was high, the nanopores were abruptly filled with the second phase where the concentration of the zero-charge complex was much higher than in the bulk solution. This was so-called SIFT. We think that the difference detected by the XAFS measurements reflects the occurrence of SIFT in the nanopores. A second important observation was the evolution of the Zn-Zn distance obtained from local structural parameters. For Zn-malonate, Figure 7a shows the timedependency in Zn-Zn shell distance obtained in the three different pH conditions with Zn(II)-malonate. At pH = 3, the Zn-Zn distance maintained a constant value from the shortest to the longest deposition time. However, a large deviation of Zn-Zn shell distance from the corresponding metallic Zn values was detected at pH = 4 and 5. The deviation was more pronounced and progressive with the deposition process at pH = 5. That meant that the deviation was large after the shortest deposition time, and as the deposition time was prolonged, the deviation became smaller. However, a nonnegligible deviation of Zn-Zn distance was still observed after 120 s of deposition in pH = 5 conditions with the malonate bath. To understand the relationship between the deviation in Zn-Zn distance and the accelerated Zn deposition within the nanopores, we proposed a model that could be expected with the occurrence of SIFT. Figure 7b schematically explains the accelerated electrodeposition of Zn in the surface of the PSi wall. During electrodeposition of Zn from the Zn(II)-malonate solution at pH = 5, Zn(II)-malonate is attracted to the Si surface. The Zn-malonate is first anchored to the Si surface, keeping the chelate group attached, at least partially. This means Zn(II)malonate forms an intermediate surface state at the PSi/solution interface where the incoming chelated complexes will attach and the Zn(II)-malonate previously attached will be subsequently reduced to metallic Zn. This process may develop a metallic layer at the PSi/solution interface with a high-density of molecular Zn(II)-chelate layer at the frontier of Zn growth. Because the pH = 5 conditions resulted in the formation of zero charge Zn(II)-malonate complexes in the deposition bath, the electrostatic repulsion between the Zn(II)-malonate complexes attached to the surface would not be significant. 13 ACS Paragon Plus Environment


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Therefore, high accumulation of Zn(II)-malonate at the silicon/solution interface becomes possible. As a result, the attached Zn(II)-malonates can promote further Zn deposition. Evidence for our model comes from the Zn-Zn distance expansion with short deposition times in conditions where zero-charge complexes are formed. Our model proposes that it is due to a distorted Zn structure in the PSi/chelate interface with the slightly longer Zn-Zn distance. The trend was observed in Figure 7a. This effect is observed under the high-pH condition in Zn(II)-malonate. Additionally, the XANES results demonstrated that there are large fractions of Zn(II)-malonate in the deposits, especially after short deposition periods. The possible mechanism for the relatively large contribution of ZnO at low pH conditions should be explained. The contribution of ZnO is prominent in the cases of Zn-malonate with pH=3 and 4. However, it is not prominent with pH=5. This difference corresponds to the occurrence of SIFT. Without the occurrence of SIFT for Zn-malonate at low pH conditions, other coexisting ions may cause the SIFT in the nanopores. We reported that such situation resulted in the change in pH locally within nanopores.12 Slight increase in pH within the nanopores may cause the precipitation of ZnO in the nanopores. The case of Zn(II)-citrate was different and more difficult to extrapolate. We may hypothesize that it follows a similar mechanism to Zn(II)-malonate. The Zn-Zn shell distance seemed to contract slightly with deposition time for the pH = 3 condition. However, in this case, even the variations are clear we only have two conditions to extrapolate a general trend. The relative values of the dissociation constant of Zn(II)-chelate may play a key role in the formation of the surface state, chelate release, and subsequent growth of Zn structures. During the deposition process, Zn(II)-malonate loses its malonate group in a competitive process when a new Zn(II)-malonate molecule reaches it. This sequence may be repeated along the deposition front, resulting in the development of the metallic Zn deposit on the Si surface. Differences in the Zn phase composition of malonate and citrate can be understood in terms of either the differences in the affinity constant of Zn(II) chelate or the relative size of the complexes with respect to the pores. We discuss here the most relevant cases, pH = 5 for Zn-malonate and pH = 3 for Zn-citrate. For instance, a key 14 ACS Paragon Plus Environment

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aspect for explaining the higher presence of the Zn(II)-chelate phase in Zn deposited on the porous electrodes originated from the affinity constant in Zn(II)-chelate. In the case of Zn(II)-malonate, the dissociation constant at pH = 5 was 2.64, which is slightly lower than the value of 2.94 for Zn(II)-citrate at pH = 3. This difference meant that the concentration of unbounded Zn(II) cation to malonate would be almost twice compared with that of unbounded Zn(II) cation to citrate. Hence, citrate is much less likely to release the Zn(II) cation for reduction. This may explain the largest amount of residual Zn(II)-citrate, even after a long deposition period. In the case of flat substrates, the remnant Zn(II)-citrate contribution not only decreased but also increased with deposition time from 20 s to 60 s for all the pH conditions investigated. This was of relevance in obtaining metallic Zn in the deposition layer. Thus, small changes in the dissociation constant may have a non-negligible effect on the deposition mechanism. Regarding the case of citrate, the deposition on PSi may favor the accumulation of Zn(II)-citrate by a confinement effect in the nanocavities. This was supported by the fact that traces of citrate were detected on the flat substrates.

Conclusions Zn electrodeposition in PSi was studied by XAFS spectroscopy. XAFS spectra clearly suggested that Zn(II)-chelating with zero charge tended to accumulate in the hydrophobic nanopores of PSi. This was direct evidence of the occurrence of SIFT in nanopores of PSi. Analysis of the spectra showed that the Zn-Zn distance with SIFT was slightly larger than that of the metallic Zn-Zn bonds. This deviation was caused by the formation of an intermediate surface state due to the high adsorption of Zn(II)chelate on the surface of the PSi wall. In the present study, we succeeded in detecting the occurrence of SIFT in a nanoporous electrode. This study gives an important experimental evidence of the occurrence of SIFT in confined nanopores. We believe that the design of nanoporous electrode systems in terms of the SIFT effect is of great importance for the development of electrochemical devices using nanoporous electrodes.

Supporting Information The supporting information is available free of charge on the ACS Publication website.



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Discrimination of the Zn-O first shell contribution from ZnO/Zn-malonate phases, analysis of Zn deposited from a non-chelate electrochemical bath, comparison between the Zn deposition in a flat and a porous substrate using a zero-valent Zn(II)chelate, and the basal ZnO contribution.

Acknowledgments Synchrotron radiation experiments were performed at the BL01B1 beam line of SPring8 with the approval of the Japan Synchrotron Radiation Research Institute (Project numbers of 2015B1398 and 2016A1297). We thank Dr T. Ina for his technical support during XAFS measurements at SPring-8. This work was supported by JSPS Grants-inAid for Scientific Researches (B) (No. 15H03877: K. F.), and (A) (16H02411, to K. M.), and by the Core Research for Evolutional Science and Technology (CREST) program of JST (T. A.).

References (1) Dixit, P.; Miao, J. Aspect-Ratio-Dependent Copper Electrodeposition Technique for very High Aspect-Ratio through-Hole Plating. J. Electrochem. Soc. 2006, 153, G552G559. (2) Kondrat, S.; Wu, P.; Qiao, R.; Kornyshev, A. A. Accelerating Charging Dynamics in Subnanometre Pores. Nat. Mater. 2014, 13, 387-393. (3) Merlet, C.; Pean, C.; Rotenberg, B.; Madden, P. A.; Daffos, B.; Taberna, P. -.; Simon, P.; Salanne, M. Highly Confined Ions Store Charge More Efficiently in Supercapacitors. Nat. Commun. 2013, 4, 2701. (4) Chaban, V. V.; Prezhdo, O. V. Nanoscale Carbon Greatly Enhances Mobility of a Highly Viscous Ionic Liquid. ACS Nano 2014, 8, 8190-8197. (5) Liu, K.; Wu, J. Boosting the Performance of Ionic-Liquid-Based Supercapacitors with Polar Additives. J. Phys. Chem. C 2016, 120, 24041-24047. (6) Rochester, C. C.; Kondrat, S.; Pruessner, G.; Kornyshev, A. A. Charging Ultrananoporous Electrodes with Size-Asymmetric Ions Assisted by Apolar Solvent. J. Phys. Chem. C 2016, 120, 16042-16050. (7) Fukami, K.; Koda, R.; Sakka, T.; Urata, T.; Amano, K.; Takaya, H.; Nakamura, M.; Ogata, Y.; Kinoshita, M. Platinum Electrodeposition in Porous Silicon: The Influence 16 ACS Paragon Plus Environment

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of Surface Solvation Effects on a Chemical Reaction in a Nanospace. Chem. Phys. Lett. 2012, 542, 99-105. (8) Fukami, K.; Koda, R.; Sakka, T.; Ogata, Y.; Kinoshita, M. Electrochemical Deposition of Platinum within Nanopores on Silicon: Drastic Acceleration Originating from Surface-Induced Phase Transition. J. Chem. Phys. 2013, 138, 094702. (9) Koda, R.; Koyama, A.; Fukami, K.; Nishi, N.; Sakka, T.; Abe, T.; Kitada, A.; Murase, K.; Kinoshita, M. Effect of Cation Species on Surface-Induced Phase Transition Observed for Platinum Complex Anions in Platinum Electrodeposition using Nanoporous Silicon. J. Chem. Phys. 2014, 141, 074701. (10) Koyama, A.; Fukami, K.; Sakka, T.; Abe, T.; Kitada, A.; Murase, K.; Kinoshita, M. Penetration of Platinum Complex Anions into Porous Silicon: Anomalous Behavior Caused by Surface-Induced Phase Transition. J. Phys. Chem. C 2015, 119, 1910519116. (11) Koyama, A.; Fukami, K.; Suzuki, Y.; Kitada, A.; Sakka, T.; Abe, T.; Murase, K.; Kinoshita, M. High-Rate Charging of Zinc Anodes Achieved by Tuning Hydration Properties of Zinc Complexes in Water Confined within Nanopores. J. Phys. Chem. C 2016, 120, 24112-24120. (12) Koyama, A.; Fukami, K.; Imaoka, Y.; Kitada, A.; Sakka, T.; Abe, T.; Murase, K.; Kinoshita, M. Dynamic Manipulation of Local pH within Nanopore Triggered by Surface-Induced Phase Transition. Phys. Chem. Chem. Phys. 2017, 19, 16323-16328. (13) Kinoshita, M. Effects of a Trace Amount of Hydrophobic Molecules on Phase Transition for Water Confined between Hydrophobic Surfaces: Theoretical Results for Simple Models. Chem. Phys. Lett. 2000, 326, 551-557. (14) Kinoshita, M. Water Structure and Phase Transition Near a Surface. J. Solution Chem. 2004, 33, 661-687. (15) Granitzer, P.; Rumpf, K.; Poelt, P.; Reichmann, A.; Krenn, H. Self-Assembled Mesoporous Silicon in the Crossover between Irregular and Regular Arrangement Applicable for Ni Filling. Physica E 2007, 38, 205-210. (16) Granitzer, P.; Rumpf, K.; Venkatesan, M.; Roca, A. G.; Cabrera, L.; Morales, M. P.; Poelt, P.; Albu, M. Magnetic Study of Fe3O4 Nanoparticles Incorporated within Mesoporous Silicon. J. Electrochem. Soc. 2010, 157, K145-K151.



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(17) Polisski, S.; Goller, B.; Heck, S. C.; Maier, S. A.; Fujii, M.; Kovalev, D. Formation of Metal Nanoparticles in Silicon Nanopores: Plasmon Resonance Studies. Appl. Phys. Lett. 2011, 98, 011912. (18) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537541. (19) Ankudinov, A.; Ravel, B.; Rehr, J.; Conradson, S. Real-Space Multiple-Scattering Calculation and Interpretation of x-Ray-Absorption Near-Edge Structure. Phys. Rev. B 1998, 58, 7565-7576. (20) Lenstra, A.; Kataeva, O. Structures of Copper(II) and Manganese(II) Di(Hydrogen Malonate) Dihydrate; Effects of Intensity Profile Truncation and Background Modelling on Structure Models. Acta Crystallogr. , Sect. B: Struct. Sci 2001, 57, 497-506. (21) Li, X.; Chen, W.; Wang, E. A Second Modification of Poly[Diaquadi-MuCitrato(3-)-Trizinc(II)]. Acta Crystallogr. , Sect. E: Struct. Rep. Online 2009, 65, M183U626. (22) Alain, M.; Jacques, M.; Diane, M.; Karine, P. MAX: Multiplatform Applications for XAFS. J. Phys. Conf. Ser. 2009, 190, 012034. (23) Kelly, S. D.; Ravel, B. EXAFS Analysis with Self-Consistent Atomic Potentials. AIP Conf. Proc. 2007, 882, 135.


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Figure Captions

Figure 1. Cartoon comparing on the use of X-ray to analyze the deposition of a sample on a porous electrode (A) and a flat electrode (B). Because the X-ray penetrates in the matrix, the volume density of material is much larger in the porous substrate.

Figure 2. XAFS spectra of the Zn deposited from Zn(II)-malonate baths with different pH for 20 s in PSi substrates compared with the ZnO and Zn spectra at the XANES (a) and EXAFS (b).

Figure 3. a) Module of the FT of the EXAFS spectra and fit of the Zn-PSi samples obtained for deposition from Zn(II)-malonate solutions at different deposition time and pH conditions of the solution; b) Time dependency of phase content in Zn-PSi for the studied range of pH calculated from EXAFS fittings (green at pH = 5, blue at pH = 4, and red at pH = 3). Zn phases considered in the model are metallic Zn, ZnO and Znmalonate.

Figure 4. XANES Zn K-edge spectra: (a) Experimental spectra (‘exp’) of the Zn deposited from pH=5 bath for 20 s, ZnO and Zn with ab initio calculation of the theoretical (‘th’) Zn(II)-malonate spectrum; (b) derivative of the spectra shown in left panel; (c) time evolution of the chemical state of the Zn in the deposits from pH=5 bath shown by XANES, compared to Zn metallic spectrum.

Figure 5. XANES Zn K-edge spectra: (a) Experimental spectra (‘exp’) of the Zn deposited from Zn(II)-citrate baths for 20 s, ZnO, Zn and ab initio calculation of the theoretical (‘th’) Zn-citrate spectrum; (b) derivative of the spectra shown in left panel; (c) time evolution of the chemical state of the Zn in the deposits from Zn(II)-citrate baths. 19 ACS Paragon Plus Environment


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Figure 6. Module of the FT of EXAFS spectra and fits of Zn deposited on PSi (a) and on flat substrates (b) from the Zn(II)-citrate complex solution at different pH conditions.

Figure 7. (a) Time dependence of EXAFS structural parameter Zn-Zn shells distance in the Zn(II)-malonate system. (b) The proposed electrochemical deposition mechanism for the Zn by using Zn-chelate solutions in case of using Zn-malonate complex at high pH conditions.


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Table 1. EXAFS parameters, s corresponds to the phase content calculated from the EXAFS amplitude factor S02 normalizing to the photoelectron amplitude factor obtained by fitting the bulk references. The maximum experimental error of the parameters are δs=0.05, δ∆R=0.001Å, δDW=0.002 Å2 and δE0=2 eV. Zn pH

Time

s

(s)

ZnO

∆R

DW

(Å)

[Zn-Zn]

s

∆R

DW

(Å)

[Zn-O]

2

4

5

s

∆R

DW

E0

(Å)

[Zn-C]

(eV)

2

(Å ) 3

Zn(II)-malonate r-f

2

(Å )

(Å )

20

0.75

0.009

0.0122

0.38

-0.012*

0.0079

0.00

n/a

n/a

5

0.03

60

0.91

0.009

0.0125

0.29

-0.012*

0.0085

0.00

n/a

n/a

3

0.02

120

1.05

0.008

0.0130

0.16

-0.012*

0.0079

0.00

n/a

n/a

5

002

20

1.00

0.014

0.0170

0.21

-0.012*

0.0038

0.20

0.008

0.025

4

0.01

60

1.15

0.009

0.0132

0.18

-0.012*

0.0058

0.00

n/a

n/a

6

0.03

120

1.19

0.009

0.0131

0.13

-0.012*

0.0063

0.00

n/a

n/a

6

0.03

20

1.12

0.018

0.0140

0.06

-0.012*

0.0056

0.30

0.010

0.028

4

0.03

60

1.21

0.016

0.0126

0.06

-0.012*

0.0067

0.14

0.010

0.026

3

0.02

120

1.14

0.012

0.0116

0.05

-0.012*

0.0068

0.05

0.010

0.026

3

0.03

(*) set parameter



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Table 2. Composition of Zn species in the total Zn deposited on the flat substrates from the Zn(II)-malonate solutions at different pH conditions. Sample condition pH 3 4 5

Phase ZnO

Time (s) 20 120 20 120 20 120

Zn

0.06 0.06 0.07 0.07 0.06 0.05

Zn(II)malonate 0.73 0.83 0.67 0.79 0.82 0.83


0.21 0.10 0.26 0.14 0.11 0.12

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Table 3. Results of the XANES fitting using least square linear combination using reference spectra of Zn, ZnO and Zn(II)-malonate. Substrate Sample condition Phase proportion PSi ZnO Zn Zn(II)-malonate pH Time (s) 14±5 66±5 20±5 3 20 13±4 66±5 21±5 60 11±5 74±5 14± 5 120* 5±3 43± 5 52±10 4 20 7±5 84±2 9±5 60 7±5 86±2 7±5 120 0±2 66±3 34±5 5 20 0±3 83±2 17±5 60 0±1 88±1 12±1 120 16±10 84±10 0±10 Flat 3 20 5±5 94±5 1±5 60 6±5 94±5 0±5 120 15±4 85±5 0±5 4 20 4±5 86±5 10±5 60 4±5 91±5 5±5 120 2±2 88± 2 10±5 5 20 0±2 71±10 29±10 60 0±2 91±5 9±5 120


R2

0.02 0.03 0.03 0.1 0.02 0.02 0.02 0.01 0.01 0.08 0.04 0.02 0.05 0.05 0.05 0.04 0.1 0.04


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Table 4. EXAFS parameters obtained from the fittings. In the table, s corresponds to the phase fraction calculated form the EXAFS amplitude factor S02 normalizing to the photoelectron amplitude factor obtained by fitting the bulk references. The maximum experimental error of the parameters are δs=0.05, δ∆R=0.001Å, δDW=0.002 Å2 and δE0=2 eV. Zn s

ZnO s ∆R (Å)

p H

Tim e (s)

1.8

20

1.0

0.01 7

DW [ZnZn] (Å2) 0.010 5

120

1.0

0.01 2

0.010 1

0.1 5

20

0.7 8 1.0

0.02 2 0.01 0

0.014 0 0.010 8

0.2 3 0.1 0

0.8 2 0.9 6

0.01 7 0.01 4

0.011 6 0.011 0

0.3 6 0.3 2

3

120

4.8

20 120

∆R (Å)

0.2 0

0.01 1 0.01 7 0.00 0 0.01 1 -0.01 0.01 6

DW [ZnO] (Å2) 0.006 8

Zn(II)-citrate s ∆R DW (Å) (Å2)

E0 (eV )

R2

0.1 0

0.01 0

0.005 2

6

0.0 6

0.012 0

0.0 5

0.00 8

0.003 9

7

0.0 6

0.006 1 0.010 6

0.1 8 0.0 4

0.00 6 0.01 0

0.010 6 0.004 7

6

0.0 2 0.0 2

0.008 3 0.009 8

0.0 0 0.0 0

n/a

n/a

4

n/a

n/a

5


6

0.0 5 0.0 4

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Table 5. Relative composition of Zn species in the total Zn deposited on PSi and flat substrates from the Zn(II)-citrate solutions at different pH conditions. Substrate PSi

Sample condition

Phase proportion ZnO Zn

Zn(II)citrate

pH

Time (s) 20 120* 3 20 120 4.8 20 120 Flat 1.8 20 120 3 20 120 4.8 20 120 (*) in this sample the measured time condition was 60 s 1.8

0.15 0.15 0.1 0.10 0.31 0.25 0.09 0.10 0.14 0.14 0.17 0.14

(†) below the uncertainty limit.


0.75 0.77 0.66 0.86 0.69 0.75 0.76 0.72 0.77 0.76 0.73 0.73

0.10 0.08 0.15 0.04† 0.00† 0.00† 0.16 0.18 0.09 0.10 0.10 0.13


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Table 6. Results of the XANES fitting using least square linear combination using reference spectra of Zn, ZnO and Zn(II)-citrate. Substrate Sample condition PSi pH Time (s) 1.8 20 120* 3 20 120 4.8 20 120 Flat 1.8 20 120 3 20 120 4.8 20 120

Phase proportion R2 ZnO Zn Zn(II)-citrate 0.003 0.001 0.18± 2 0.74± 1 0.08± 3 0.003 0.09±2 0.86± 1 0.05± 3 0.002 0.25±3 0.30± 2 0.45± 1 0.008 0.07± 1 0.003 0.11±3 0.82± 1 0.30±4 0.57± 2 0.13± 1 0.001 0.24± 4 0.67± 2 0.09± 1 0.001 0.06± 1 0.93± 1 0.01± 1 0.001 0.01± 1 0.001 0.07± 1 0.93± 1 0.05± 1 0.91± 1 0.04± 1 0.001 0.03± 1 0.96± 1 0.01± 1 0.001 0.06± 1 0.94± 1 ------0.001 0.02± 1 0.001 0.03± 1 0.95± 1


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Figure 1. Cartoon comparing on the use of X-ray to analyze the deposition of a sample on a porous electrode (A) and a flat electrode (B). Because the X-ray penetrates in the matrix, the volume density of material is much larger in the porous substrate. 255x72mm (300 x 300 DPI)

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Figure 2. XAFS spectra of the Zn deposited from Zn(II)-malonate baths with different pH for 20 s in PSi substrates compared with the ZnO and Zn spectra at the XANES (a) and EXAFS (b). 175x131mm (150 x 150 DPI)


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Figure 3. a) Module of the FT of the EXAFS spectra and fit of the Zn-PSi samples obtained for deposition from Zn(II)-malonate solutions at different deposition time and pH conditions of the solution; b) Time dependency of phase content in Zn-PSi for the studied range of pH calculated from EXAFS fittings (green at pH = 5, blue at pH = 4, and red at pH = 3). Zn phases considered in the model are metallic Zn, ZnO and Zn-malonate. 170x180mm (150 x 150 DPI)



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Figure 4. XANES Zn K-edge spectra: (a) Experimental spectra (‘exp’) of the Zn deposited from pH=5 bath for 20 s, ZnO and Zn with ab initio calculation of the theoretical (‘th’) Zn(II)-malonate spectrum; (b) derivative of the spectra shown in left panel; (c) time evolution of the chemical state of the Zn in the deposits from pH=5 bath shown by XANES, compared to Zn metallic spectrum. 177x132mm (150 x 150 DPI)


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Figure 5. XANES Zn K-edge spectra: (a) Experimental spectra (‘exp’) of the Zn deposited from Zn(II)-citrate baths for 20 s, ZnO, Zn and ab initio calculation of the theoretical (‘th’) Zn-citrate spectrum; (b) derivative of the spectra shown in left panel; (c) time evolution of the chemical state of the Zn in the deposits from Zn(II)-citrate baths. 175x132mm (150 x 150 DPI)



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Figure 6 Module of the FT of EXAFS spectra and fits of Zn deposited on PSi (a) and on flat substrates (b) from the Zn(II)-citrate complex solution at different pH conditions. 160x167mm (150 x 150 DPI)


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Figure 7. (a) Time dependence of EXAFS structural parameter Zn-Zn shells distance in the Zn(II)-malonate system. (b) The proposed electrochemical deposition mechanism for the Zn by using Zn-chelate solutions in case of using Zn-malonate complex at high pH conditions. 328x480mm (300 x 300 DPI)



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TOC graphic 85x47mm (300 x 300 DPI)


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Mechanism of Accelerated Zinc Electrodeposition in Confined

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