Deposition of gold and silver on porous silicon and inside the pores

    Deposition of gold and silver on porous silicon and inside the pores Einat Nativ-Roth, Katya Rechav, Ze’ev Porat PII: DOI: Reference:

S0040-6090(16)00033-X doi: 10.1016/j.tsf.2016.01.020 TSF 34959

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

28 May 2015 12 January 2016 12 January 2016

Please cite this article as: Einat Nativ-Roth, Katya Rechav, Ze’ev Porat, Deposition of gold and silver on porous silicon and inside the pores, Thin Solid Films (2016), doi: 10.1016/j.tsf.2016.01.020

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ACCEPTED MANUSCRIPT Deposition of gold and silver on porous silicon and inside the pores Einat Nativ-Roth(1), Katya Rechav(2) and Ze'ev Porat(3, 4)*

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(1) Ilse Katz Institute of Nanoscale Science and Technology, Ben-Gurion University of the Negev, Be'er Sheva 84105, Israel. (2) Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel. (3) Division of Chemistry, Nuclear Research Center–Negev, Be'er Sheva 84190, Israel. (4) Institutes of Applied research, Ben-Gurion University of the Negev, Be'er Sheva 84105, Israel.

Abstract

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Silver and gold were spontaneously deposited on porous silicon (PSi) by immersion-plating. Each metal formed crystallites of typical shapes on top of the PSi layer. Deposition of these metals inside the pores could be achieved by performing the immersion-plating in an ultrasonic bath. Top view and cross-section Scanning electron microscope images show slight penetration of silver into the pores but massive filling of gold, to depth of several hundreds of nanometers. Keywords: porous silicon, metal deposition, immersion-plating, ultrasonic-aided deposition, SEM.

1. Introduction

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* Corresponding author: [email protected]

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Porous silicon (PSi) acts as a modest reducing agent and can reduce metallic ions that have positive reduction potentials, with respect to hydrogen, when immersed in their aqueous solutions (immersion-plating). Thus, gold, silver, platinum and copper can be spontaneously deposited onto

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PSi, whereas cobalt and nickel can only be deposited by electroless plating in solutions that contains reducing agents or by applying an external voltage (electrodeposition). The reductive activity of PSi was explained by Ogata et al. [1] as a reaction coupled to the surface oxidation of Si atoms and Si-H by water to form Si-O-Si. They proposed that these reactions do not necessarily occur at the same sites, thus allowing nucleation and growth of metal islands at various locations on the surface. The morphology of the deposited metal, namely its density and the average particlesize, depends, according to the literature, on two main factors: a) the structure of the underlying PSi substrate, which is determined by the etching conditions [2] and b) the deposition conditions, mainly the concentration of the metallic solution used [2-4] and the deposition duration. The large surface area of the PSi layers offers a high density of suitable nucleation sites [5, 6] or hydride reducing equivalents [7]. Many of the reports about deposition of gold and especially silver were aimed at creating active substrates for surface enhancement Raman scattering (SERS) [2-5, 7, 8], whereas the attempts to deposit metals within the pores were aimed at improving the conductivity of PSi [9].

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ACCEPTED MANUSCRIPT Reductive deposition of copper on PSi was reported by Sham et al. [10]. They dipped as-prepared PSi samples in 1-100 mM aqueous solutions of CuSO4 and observed the formation of thin layers of metallic particles, accompanied by gas evolution. The reduction was found to be controlled by the availability of PSi active sited and by the concentrations of the Cu2+ ions. These workers reported

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also the deposition of gold nano-clusters on PSi [11]. Immersion of freshly etched p-type PSi in 1

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mM aqueous Na(AuCl4) yielded nearly spherical clusters, whose size depends on the deposition time: After 15 seconds clusters of a few nanometers were observed by scanning electron

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microscope (SEM), whereas after 1 to 5 minutes the clusters were in the sizes range of 10-40 nm. Cross-section SEM images showed only minor penetration of the deposited gold into the pores, with respect to the entire thickness of the PSi layer. This was explained by the trapped gas bubbles

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in the pores. Evolution of H2 bubbles was observed during the deposition process. Silver deposits are suitable to serve as SERS active substrates. Lin et al. [7] immersed freshly etched PSi samples in solutions of 10 mM AgNO3 in 1:1 water/ethanol and observed mostly

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feather-like Ag dendrites on the surface, ca.10 µm long. Only minor amounts of silver were found inside the pores. Deposition of Ag nano-particles onto PSi from 1 mM silver solutions [6, 13-15] particles of larger average diameters and a broader size-dispersion, up to 220 nm, were obtained

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after 1 h immersion. Increasing the concentrations of the deposition solutions resulted in formation

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of larger particles [3]. Andsager et al. [16] tried immersion plating of PSi in solutions of 13 metal salts, but only Au, Ag and Cu caused the quenching of the PSi photoluminescence due to metal

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deposition. Auger electron spectroscopy depth profiling revealed that metals have penetrated deeply into the pores, but with low atomic concentrations. No data about the pores’ size was given. Ogata et al. [17] studied the immersion plating of copper on PSi and Si wafer from aqueous and non-aqueous solutions. They observed that in aqueous solutions of divalent copper (CuSO4)

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containing more than 0.2 M chloride ions, copper deposition was inhibited, whereas in monovalent copper (CuCl) solutions containing 0.5 M Cl-, deposition could occur. This behavior was explained in terms of different complexing states of copper, leading to different deposition rest-potentials. In MeOH and Me2SO solutions of CuSO4, deposition of copper was detected. In another work, this group investigated the immersion plating of several metals on PSi from aqueous and non-aqueous solutions [1]. Ag and Cu but not Ni were deposited (separately) from both aqueous and MeOH solutions, but not from MeCN. This was attributed to the different redox potentials of each metal in the various solvents. Deposition of metals within the pores is a bigger challenge because of two factors: a) Penetration of the deposition solution into the pores is restricted due to the trapped hydrogen gas in the pores after anodization, the hydrophobic nature of Si /PSi and the surface tension of water. b) The reduction rate of the metal-ions is larger than their diffusion rate, and thus they are mostly reduced on the surface of the PSi or near the pores’ openings before they can diffuse inside. Nevertheless, there are a few reports about deposition of metals inside the pores. Jeske et al. [9] performed 2

ACCEPTED MANUSCRIPT cathodic deposition of gold, copper and nickel on PSi. X-ray photoelectron spectroscopy measurements indicated that the whole pore volume was filled with the metals. No data was given on the size of the pores. Steiner et al. [18] also employed electroplating in an attempt to fill the pores with indium and aluminum. They analyzed the results by Rutherford backscattering

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spectroscopy and concluded that deposition occurs mainly at the bottom of the PSi film. Andersson

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et al. [19] tried the electroless method for deposition of nickel into p-type PSi with large pores, 4µm wide. Deposition was obtained only with hypophosphite as the reducing agent, forming

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complete coverage of the pore-walls with nickel, as could be observed by cross-section SEM micrographs. Fukami et al. [20] employed electrodeposition for inserting metallic platinum, gold and palladium into the pores of macroporous PSi. They found that the supporting electrolyte and the pore depth were key factors in the preferred location of the deposition within the pores, i.e. at

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the bottom or at the opening. Cross-section SEM micrographs showed clearly the filling of the pores with the various metals. Rumpf et al. electrodeposited nickel within the pores of PSi, to

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achieve a hybride material with magnetic properties. They observed the formation of nanoparticles, 2-5 nm in size, covering the pore-walls [21] or filling them [22]. Chan et al. [23] reported a six-step procedure for coating the pores' inner surface with silver, without filling the voids themselves, thus

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creating very large area of Ag surfaces for SERS effect. The maximal enhancement obtained for

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Rhodamine 6G signals was estimated to be about ten orders of magnitude, comparable to that obtained from colloidal silver solutions.

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In the present work we describe the spontaneous immersion-plating of silver and gold on porous silicon and inside the pores from solutions of AgNO3 and HAuCl4 of various concentrations. Deposition of metals inside the pores was possible only when the immersion-plating was performed in an ultrasonic cleaning bath (ultrasonic-aided deposition). We attribute this either to the combined

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effect of the induced vibrations on gas release from the pores, insertion of the ethanolic solution into the pores and enhanced mobility of the metal ions, or to the sonochemical effect of preferential insertion of in-situ formed nanoparticles into small size nanometric pores. The results were examined and analyzed by top-view and cross-section scanning electron microscopy.

2. Materials and methods Chemicals: Hydrofluoric acid 40%, technical grade (Riedel-de Haen) was diluted with absolute ethanol (AR 99.9% supplied by "Gadot" chemicals, Israel) to make the desired concentrations of the etching solutions (8%-24% HF). AgNO3 (Aldrich, 99.9999%) was dissolved in purified water (Nanopure Water Purification Systems, 18.2 MΩ-cm) to prepare solutions of 0.05 M and 0.5 M, from which deposition solutions of other concentrations were prepared (1, 5, 20, 50, 100, 250 and 500 mM.) Si wafers: Polished prime p-type, B-doped silicon wafers (100) of typical resistivity 0.002-0.005 Ωcm and thickness of 500-550 µm (Cemal Silicon S.A). 3

ACCEPTED MANUSCRIPT Electrochemical etching: PSi layers were prepared by controlled current anodization, performed with a programmable dc power supply (Motech, Israel). A one-compartment Teflon cell served for the etching reaction. The Si wafer sample (2×2 cm2) was laid on an aluminum anode, coated with a gold foil, at the bottom of the cell. It was pressed against the anode by a cylindrical Teflon tube,

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which was fastened by the screw-cap, exposing a round area of 1.3 cm2 at the center of the silicon

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sample. The inner volume of the tube was filled with 2 mL of the HF/ethanol etching solution. The cathode was made of a platinum flat spiral, suspended 5 mm above the polished side of the silicon

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wafer. The PSi samples in this study were prepared in solutions of HF concentrations between 816% at room temperature and current densities of 4-15 mA/cm2. The etching duration of each sample was 10 min. These conditions were chosen to produce larger pores [24] so that diffusion of the metal ions into pores would be easier. The specific anodization conditions (HF concentration

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and current density) for each sample are indicated. The resulting layers were ca. 2 µm thick and of 34-43% porosity. The average pores’ diameters were ca. 16-19 nm and the distances between them

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were smaller than that.

Equipment: Programmable dc power supply (Motech, Taiwan) served for the anodic etching. Ultrasonic treatment of the PSi was done with Ultrasonic Cleaner model MRC D80.

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Scanning electron microscope (SEM) images were obtained using a high resolution JEOL JSM-

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7400F SEM equipped with a field emission electron gun. Samples were imaged as received, without coating the surface. Secondary electron images were taken at 3kV and back scattered

3. Results

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images at 10kV.

3.1. Deposition of silver and gold on PSi

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Immersion plating of silver onto PSi samples was studied in the following manner: the freshly etched samples were rinsed with water and immersed for 30 minutes in Petri dishes containing 10 mL of aqueous AgNO3 solutions of various concentrations. Visual inspection of the color changes of the surfaces indicated the build-up of silver layers, at rates which depended on the concentrations of the silver ions: in a 1 mM solution, silver deposition was slow; in a 5 mM solution it was faster and at concentrations above 50 mM full coverage of the surface was instantaneous. The effect of the solution concentration on the morphology of the deposited silver layers was examined by SEM. For each concentration, micrographs at two magnifications are presented in Fig. 1, exhibiting both the appearance of the layer and the shapes of the individual particles. A general view of the silver layer obtained in 1mM AgNO3 solution shows that it is composed of uniformly distributed particles that cover the surface (Fig. 1A). A closer look (Fig. 1B) shows small particles, some in the size-order of the pores, spread between much larger silver particles which have been merged together. These provide only partial coverage of the surface (ca. 75%), forming a fractal-like network of large islands of silver. This morphology suggests that 4

ACCEPTED MANUSCRIPT nucleation of the silver particles has started at numerous sites nearly simultaneously. However, the existence of the small particles in addition to the large ones indicates that the nucleation and growth process continues to occur also at later stages.

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Fig. 1: SEM micrographs of silver particles, formed on PSi by immersion deposited for 30 min. in AgNO3 solutions of three concentrations. Each kind is shown at two magnifications. Anodization conditions: 16% HF, 15 mA/cm2.

Deposition of silver from a 5mM solution yielded a dense structure of micrometer-size particles. The lower-resolution SEM images (Fig. 1C) demonstrate that the majority of the particles are within a relatively narrow range of size-distribution, ca. 1-3µm, forming a rather uniform and dense layer. Among the larger particles much smaller ones can be observed in the highermagnification image (Fig. 1D). Examination of the larger particles reveals that they are composed of agglomerates of smaller sub-micron particles that have merged together. A similar but denser structure was obtained in a 50 mM silver solution, constituted of large coalesced 5

ACCEPTED MANUSCRIPT micron-size particles. The facetted forms of the particles (Figs. 1E and 1F) indicates that a crystallization-like process dominates their growth, and their size is determined by the availability of diffusing Ag+ ions near the surface. Thus, increasing the concentration of the deposition solutions leads to the formation of a highly uniform layer, composed of larger

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crystalline silver particles with increased degree of coalescence. Similar structures were

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observed with 100 mM solutions; here the facetted large particles can be well observed, lying on top of the smaller particles (Fig. 2B). When 250 and 500 mM solutions were used, the deposition

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of silver was very rapid, forming thick and continuous layers that started to peel off.the PSi. In most cases, the deposited silver layers had the microstructure of the kind presented in Fig. 1. However, the reductive crystallization of silver produced also particles of other shapes, examples

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of which are shown in Fig. 2. These include large facetted crystals (2A), a large circular shape which seems to be formed by coalescence of three crystals (2B), a polyhedral crystal (2C) and

composed of pure silver (Fig. 3 inset).

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longitudinal clusters (2D). EDS analysis of such a single particle indicates that it is indeed

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Fig. 2: A) Large facetted Ag crystals lying on top of smaller ones. Deposition: 100 mM AgNO3, 30 min, scale bar: 1 μm. B) A large circular crystal. scale bar: 10 μm. C) A symmetrical polygonal crystal. Deposition: 5 mM AgNO3, 30 min. scale bar: 100 nm. D) Column-like crystals, scale bar 1 μm. Deposition as in (C). Inset: EDS analysis of a single Ag particle. Anodization conditions of all samples: 16% HF, 15 mA/cm2.

Cross-section SEM micrographs of these samples were taken after cleavage of the Ag-coated Si wafers, and are presented in Fig. 3. They provide a side view of the silver layer, showing the small crystals on the PSi surface, underneath and between the large ones. The directions of the 6

ACCEPTED MANUSCRIPT crystals growth are random, and the facetted and polygonal shapes of the crystals can be observed. This perspective demonstrates the enormous surface area of the deposited silver coatings, which makes them suitable as substrates for surface-enhanced Raman spectroscopy (SERS). Fig. 3B shows a side view of the type of rising crystals shown in Fig. 2D. It reveals that

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the thickness of the silver layer is comparable to that of the PSi layer. These micrographs show

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no penetration of silver into the pores.

Fig. 3: Cross-section SEM micrographs of silver crystals on PSi, deposited spontaneously from a

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5M AgNO3 solution for 30 min. Anodization: 16% HF, 15 mA/cm2.

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Deposition of gold on PSi was done in the same manner as silver, from aqueous solutions of HAuCl4. Here, too, the color change of the PSi indicated the formation of a gold layer. This was

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accompanied by evolution of purple color in the solution above the PSi, indicating the formation of gold colloids. These could be gold nanoparticles which were produced on the surface but not deposited or gold ions that have been reduced in the solution by the released hydrogen. The gold particles are in the form of interconnected flakes: at low concentration of the deposition solutions

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(0.1 M, Fig. 4A), a single aggregate of gold particles is observed among many scattered particles. No full coverage of the surface was formed even after 7.5 hours. At a higher concentration (0.5 mM) a denser layer of similar particles was formed after 1 hour (Fig. 4B).

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Fig. 4: Immersion plating of gold on PSi. A) 7.5 hours in 0.1 mM aqueous HAuCl4. Anodization conditions: 8% HF/ethanol solution, 4 mA/cm2. B) 1 h in 0.5 mM aqueous HAuCl4 Anodization conditions: 10% HF/ethanol solution, 15 mA/cm2. Scale bars: 100 nm. 7

ACCEPTED MANUSCRIPT 3.2. Deposition of silver and gold inside the pores As mentioned before, deposition of metals within the pores is more difficult due to several reasons: entrapped hydrogen gas inside the pores, the surface tension of water, the hydrophobic nature of the Si/PSi and the fast rate of the metal-ions reduction, compared to their diffusion rate.

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Therefore, immersion plating occurs preferentially on the surface of PSi and not within the

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pores. Several steps have been taken in attempt to overcome these obstacles and deposit silver and gold inside the pores. First, we prepared PSi samples with large pores, by controlling the

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anodization conditions. It was reported that at lower HF concentrations and higher current densities, larger pores are formed [25, 26]. In our previous work with low-resistance Si, the largest pores (ca. 21 nm in diameter) were obtained under etching conditions of 10% HF and 15

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mA/cm2 [26]. Second, the PSi samples were soaked in ethanol for 1 hour right after the anodization, under mild sonication, to release the entrapped gas bubbles from the pores and fill them with ethanol. Third, in order to reduce the surface tension of the deposition solution and

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facilitate its penetration into the pores, we prepared concentrated solutions of AgNO3 or HAuCl4 in water and diluted them to the desired concentrations with ethanol, so that the final composition of the solvent was 95% ethanol and 5% water. Furthermore, the metal

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concentrations in these solutions were low, to ensure slow deposition rate that would compete

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with the penetration rate. Finally, deposition of the metals was performed in an ultrasonic cleaning bath; under ultrasonic vibrations, the combined effect of gas release and enhanced

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solution penetration into the pores could yield the desired result. The results of rest immersion-plating of silver on a PSi sample with large pores, after 3 hours in 95% ethanolic solution of low Ag+ concentration, are shown in Fig. 5. Two locations on the same sample are presented, exhibiting scattered particles. Some of them are in the size-order of the

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pores and some have been merged to form larger particles. No filling of the pores can be observed, which means that the combination of larger pores, ethanolic solution of low Ag+ concentration and long immersion time are not sufficient conditions for in-pore deposition.

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Fig. 5: Two locations on a PSi sample after 3h immersion plating at rest in ethanolic solution of 0.5 mM AgNO3. Anodization conditions: 10% HF/ethanol solution, 15 mA/cm2. Scale bars: 100 nm. 8

ACCEPTED MANUSCRIPT The following experiment was performed on a fragment of the same PSi sample and in the same deposition solution, but in an ultrasonic bath. No prior ultrasonic treatment in ethanol was done. The results after 30 minutes of immersion and sonication are presented in Fig. 6. A dense layer of silver particles was formed, in spite of the low Ag+ concentration (0.5 mM). As in the

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experiment of rest-deposition at high Ag+ concentrations (Fig. 2A), large facetted crystals were

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lying on top of the much smaller ones, covering the surface almost entirely. It can be seen that the edges of the crystals are smoother. Enlargement of the uncovered area at the lower left corner

silver inside.

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of Fig. 6A shows the pores in the underlying PSi layer, none of which seems to contain any

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Fig. 6: A) SEM image of silver crystals deposited on PSi for 30 minutes from 0.5 mM AgNO3 solution in an ultrasonic cleaning bath. B) Magnification of the lower left corner of A. Anodization conditions as in Fig. 5. Scale bars: 100 nm.

Similar PSi samples were prepared as before and treated for 10 min. in pure ethanol inside the

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ultrasonic bath, prior to 10 min. immersion-plating in the ethanolic solution of Ag+, performed in the ultrasonic bath. SEM images (Fig. 7) show some small crystallytes of comparable size to the pores, among larger ones. Some of them appear to be located within the boundaries of the pores and filling them entirely. Nevertheless, most of the pores appear dark in the micrographs, which mean that even if silver deposition inside the pores is possible, it occurs only to a small extent. Cross-section images reveal the following: At one site (Fig. 8A) the silver crystals appear to be on the surface only, whereas no silver could be observed inside the pores. Back-scattered electrons (BSE) image of the same site is shown in Fig. 8B. This mode provides composition contrast, so heavier elements appear much brighter in the micrographs. Thus, the same silver crystals (marked) seen in the secondary electron (SE) image (8A) appear much brighter in the BSE image. No bright spots can be seen inside the PSi bulk on this site. However, at different sites on this sample, shown in Figs. 8C and 8D, the crystals appear to be embedded in the pores, ca. 50-100 nm in depth, and their “roots” seem rather uniform in thickness, in accordance to the pores’ size. Above the PSi surface line much larger crystals are observed. The marked crystal in Fig. 8C must 9

ACCEPTED MANUSCRIPT be one that fell off the surface during the cleavage of the PSi sample. It can thus be concluded that two steps are required for the deposition of silver within the pores: a) Ultrasonic pretreatment in ethanol, to release entrapped gas bubbles. b) Immersion plating under sonication.

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Fig. 7: SEM micrographs of two sites on a PSi sample, on which silver was deposited for 10 minutes from a 0.5 mM AgNO3 solution in an ultrasonic cleaning bath. Anodization conditions as in Fig. 5. The arrows point at some sites where silver seems to be within the pores.

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Fig. 8: Cross-section SEM images of three sites on the Ag-coated PSi sample of Fig. 7. A) SE image of the longitudal pores and the silver crystals on the surface. B) BSE image of the same site. The corresponding crystals on the surface are marked. C, D) BSE images of two other sites on the same sample, showing slight penetration of metallic silver into the pores. 10

ACCEPTED MANUSCRIPT Immersion-plating of gold under ultrasonic conditions was done in the following manner: A freshly prepared PSi sample (etched at 10% HF, 20 mA/cm2) was inserted into a 50 mL beaker that contained 0.5mM HAuCl4 and sonicated in the cleaning bath for 30 minutes. Top view SEM micrographs (Fig. 9) show that most of the pores are filled. BSE image (Fig. 9B) of the same

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area shows clearly separate bright spots with clear boundaries. Among them, a few dark spots are

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seen, which are pores that have not been filled with gold. Some larger gold crystals on the

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surface are marked on both pictures for orientation.

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Fig. 9: Top view SEM micrographs of gold deposited into the pores of PSi under ultrasonic conditions. SE image (A) and BSE image (B) of the same section. The larger crystals are marked on both images. Anodization conditions: 10% HF/ethanol solution, 15 mA/cm2.

SE and BSE cross-section images of that sample are presented in Figs 10A and 10B, showing that the pores are densely filled with gold to depth of a few hundreds of nm. Above the surface,

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the gold coating layer can be seen. To confirm these results, another PSi sample was prepared under similar anodization conditions and then underwent immersion-plating in the same manner. The cross-section micrographs of these samples (Fig. 10C, D) show again densely-filled pores. Note that in this sample less gold is seen on the surface, with respect to the pores, probably due to the shorter deposition time. The shapes of the gold particles on the surface are similar to the flakes obtained by rest-deposition (Fig. 4B). One of the pores in this sample (Fig. 10E, F) has been filled with gold to a much greater depth (ca. 800 nm) than the rest, demonstrating the uniform longitudal dimensions of the pores. In these sets of experiments, the gold deposition was done without prior treatment in the ultrasonic bath. Eventually, performing the deposition in the sonication bath fulfilled also the other purposes of that treatment, i.e. releasing the entrapped H2 and filling the pores with ethanol. A question that can be raised is whether rest immersion in a HAuCl4 solution after ultrasonic pre-treatment would yield gold deposition inside the pores. The results of such an experiment are presented in Fig. 11 for two sections of a PSi sample which were immersed for 11

ACCEPTED MANUSCRIPT 20 and 60 minutes. For both, gold deposition was observed only on the surface, leaving all the pores unfilled even after 60 minutes. These results prove that the ultrasonic enhancement of the gold-ions motion is essential for inserting them into the pores, whereas diffusion solely is not

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Fig. 10: Cross-section images of PSi samples on which gold deposited .A, B) SE and BSE images of the same site, showing the dense gold filling of the pores. Deposition time: 30 min. C, D) SE and BSE of another sample, deposition time: 10 min. E, F) SE and BSE images of another site on the latter sample. Anodization conditions: 10% HF/ethanol solution, 15 mA/cm2.

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Fig. 11: Top-view images of two sections of a PSi sample which were pre-treated in ethanol/ultrasonic bath and then underwent rest-deposition in 0.5 mM HAuCl4 with no sonication. A) 20 min. deposition. B) 60 min. deposition. Anodization conditions: 10% HF/ethanol solution, 15 mA/cm2.

4. Discussion

In this work we demonstrated that silver and gold can be spontaneously deposited not only on

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the surface of porous silicon but also inside the pores. As described in the Introduction, in-pore

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metal deposition has been done only with the aid of an additional reductant, chemical or electrochemical. The spontaneous immersion-deposition presented here was achieved by taking

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several steps that were aimed at facilitating the penetration of the metal ions into the pores, before being reduced on the upper surface of the PSi samples. These included: a) Reducing the surface tension of the metals solutions by making them 95% ethanolic rather than pristine aqueous. b) Using our PSi samples with the larger pores. c) Performing the immersion deposition

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in an ultrasonic cleaning bath, in order to release the entrapped hydrogen from the pores, to insert the ethanolic solution instead and to enhance the mobility of the metal ions. This combination of steps was found to be effective in reaching the goal of in-pore deposition.

To isolate the effect of the ultrasonic-aided deposition, we did some experiments in which PSi samples were pre-treated by sonication in ethanol in order to achieve the other two effects: releasing the entrapped hydrogen and filling the pores with ethanol. Following that, the samples were immersed for 20 and 60 minutes in a low-concentration ethanolic solution of gold (0.5 mM HAuCl4) at rest. These conditions were aimed at avoiding fast deposition on the surface while allowing diffusion of the AuCl4- ions into the pores. However, it was found that deposition occurred only on the surface but not inside the pores. This result indicates that the diffusion rate of the AuCl4- ions into the pores is slower than their heterogeneous reduction rate. Examination of the two images reveals that after 20 minutes of immersion only slight deposition is observed at nucleation sites which are positioned on the boundaries between the pores. After 60 minutes a 13

ACCEPTED MANUSCRIPT denser layer was formed by further growth of those crystallites. In general, the small crystallites observed in Fig. 11A, which were developed during 20 min. immersion, are of comparable size to the pores openings. This means that the deposition rate of gold was slow enough to allow diffusion of AuCl4- ions also into the pores and start nucleation. Eventually, that amount was not

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sufficient for the growth of apparent crystals, which could occur only with the aid of sonication.

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The ultrasonic-aided deposition can be also attributed to the possibility that even at mild

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ultrasonic conditions (low-power sonication) acoustic bubbles are formed and collapse. According to Mason and Lorimer, such collapse at or near a solid-liquid interface is nonsymmetric and produces powerful microjets directed towards the surface. Thus the advantage of ultrasonic cleaning is that it can reach crevices which are not accessible otherwise [27].

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Gedanken and coworkers used sonication for the deposition of catalysts into mesoporous materials [28], deposition of nanoparticles inside millimeter-size hollow tubings [29] and for the insertion of WS2 nanoparticles into mesoporous silica [30]. Their proposed mechanism was that

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the particles are pushed into the pores by the acoustic pressure created by the propagation of ultrasonic waves and the shock waves created by the collapsing bubbles [30]. Indeed, they used high-intensity Ti horn in their experiments, however, in his book on Sonochemistry Mason

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claims that H2O2 can be detected even in a 1W sonication bath [31]. Moreover, Vinatoru and

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coworkers [32-34] demonstrated that there is a distinct sonochemical effect in reaction conducted in an ultrasonic bath. In other words, acoustic bubbles can be formed and collapse

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even under such mild conditions, causing not just enhanced but also directed material transport. A prominent difference has been observed between the deposition tendencies of silver and gold inside the pores, unlike the instantaneous deposition of both metals on the surface. Silver showed

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only partial in-pore deposition in the top-view SEM images and rather shallow depth (50-100 nm). Contrary, gold showed almost full coverage in the top-view images and deeper penetration in the cross-section images (300-700 nm). Two factors may influence the extent of in-pore deposition: a) the reduction potential of the metal ions that affects the reaction rate. b) The hydration radius of the ion that affects its chances to enter the pores before being reduced outside them. The reasons for the different depths of depositions of the two metals need further research. Moreover, it is possible that modifications of the deposition conditions, such as the concentrations of the metal solutions, the deposition time or the sonication intensity can yield deeper filling of the pores.

5. Conclusions Deposition of silver and gold on samples of porous silicon, immersed in aqueous solutions of these metals, occurs spontaneously on the surface of the PSi. It was found that the rate of the immersion-plating depends on the concentrations of the deposition solutions. No metal 14

ACCEPTED MANUSCRIPT deposition was detected within the pores, due to several interfering factors: entrapped hydrogen gas within the pores, the surface tension of water, the hydrophobic nature of the surface of Si/PSi and the fast rate of the metal-ions reduction, compared to their diffusion rate. In order to facilitate deposition within the pores, a combination of several step had to be taken: Formation of

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PSi layers with larger pores, ultrasonic pre-treatment of the PSi layers in ethanol, using ethanolic

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deposition solutions of low metal-concentrations and performing the deposition in an ultrasonic cleaning bath. It was found that the last step was essential, i.e. immersion-deposition inside the

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pores occurred only under sonication. Both gold and silver were deposited inside the pores under similar conditions, however gold deposition was more prominent and deeper. More experiments are needed to find the proper conditions for deeper filling of the pores.

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ACCEPTED MANUSCRIPT Deposition of gold and silver on porous silicon and inside the pores Einat Nativ-Roth(1), Katya Rechav(2) and Ze'ev Porat(3, 4)*

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Spontaneous reduction of Ag and Au ions occurs on porous silicon (immersion plating). Such deposition inside the pores is prevented by entrapped hydrogen bubbles. Immersion plating under sonication releases the gas and enhances ions diffusion. Silver and gold were deposited within the pores by ultrasonic-aided deposition.

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