Photoassisted Immersion Deposition of Cu Clusters onto Porous

Article pubs.acs.org/JPCC

Photoassisted Immersion Deposition of Cu Clusters onto Porous Silicon: A Langmuir−Hill Ligand−Locus Model Applied to the Growth Kinetics Gonzalo Recio,†,‡ Dario Gallach,† Miguel Manso Silván,† Kazuhiro Fukami,§ Raúl José Martín Palma,† Germán Rafael Castro,∥,⊥ and Á lvaro Muñoz-Noval*,†,∥,⊥ †

Departamento de Física Aplicada, Universidad Autonoma de Madrid, Madrid 28049, Spain Departamento de Ciencias Matemáticas y Física, Universidad Católica de Temuco, 48-13302, Temuco, Chile § Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan ∥ Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid 28049, Spain ⊥ Spanish CRG, European Sinchrotron Radiation Facility, Grenoble 38000, France ‡

S Supporting Information *

ABSTRACT: Cu−porous silicon (Cu−PS) composite materials consisting of nanosized Cu clusters preferentially grown on the surface of PS were fabricated by photoassisted deposition of Cu nanoparticles onto PS. Structural and chemical characterization of the Cu particles grown in the PS matrix has been carried out by synchrotron X-ray absorption spectroscopy, from which different reaction stages have been identified within the photoassisted reaction. In particular, it was found that the reduction of Cu occurs in three main phases: (a) Cu nucleates homogeneously in a few seconds over the surface of PS by a coupled red-ox reaction; (b) clusters grow by new reduced ions, which tend to oxidize the previously deposited Cu atoms making increasingly heterogeneous Cu clusters; and (c) a competitive process between nucleation of new clusters and cluster coalescence gives rise to a bulklike Cu thin film. It was determined that the structures formed in the first two phases display surface plasmon resonance, with band intensity and broadening consistent with the increasing heterogeneity of the clusters. The growth kinetics has been fitted to a Langmuir−Hill model. Following these results, a reaction model has been proposed to explain the mechanisms involved in the first stages of Cu clustering.

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INTRODUCTION The continuous demand for increased performance for materials for electronics and photonics requires the development of new material combinations integrating functionality and affordability.1,2 In the particular case of optical biosensors, this evolution has been very fast and devices have been created based on optical phenomena in thin films and nanomaterials, including surface plasmon resonance (SPR), surface enhanced Raman spectroscopy (SERS), etc. One key feature to achieve suitable and inexpensive industrial applications lies in the simplicity of the fabrication process. Following these considerations, the photoassisted immersion deposition adds up the simplicity of the synthesis process to the low-cost materials and processes. One key aspect in this process is the adequate selection of a suitable substrate. In this sense, porous silicon (PS) not only offers compatibility with this electrochemical technique but also displays other outstanding features (i.e., semiconducting nature, tunable porosity, photonic properties, etc.) that make it a good candidate for use as composite material3−5 in contrast to other porous substrates.6,7 © 2014 American Chemical Society

PS has been exploited during the past decades as a very suitable substrate for metal electrodeposition, not only due to its special physicochemical features but also to its structural characteristics as a versatile porous semiconductor. Many works have explored the formation of several metal layers and templated nanostructures, including Au,8,9 Pt,10,11 Cu,12,13 etc. In the case of Cu deposition into PS, there exists an extensive literature that explores the mechanisms and strategies to accomplish such deposition.14,15 One of the most widely used methods takes advantage of the small energy difference between the reduction potential of Cu ions and the oxidation potential of Si in the surface of PS to promote, by means of charge carrier formation by light excitation, the reduction of Cu over the surface of PS by a coupled reaction (i.e., photoexcited electrodeposition). Noteworthingly, the reaction only takes place with the incidence of light in the PS substrate. Received: February 28, 2014 Revised: June 11, 2014 Published: June 17, 2014 14905

dx.doi.org/10.1021/jp502108b | J. Phys. Chem. C 2014, 118, 14905−14912

The Journal of Physical Chemistry C

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pore diameter. The type of Si, as will be illustrated below, can play a crucial role in the chemical reaction with the ions in the solution. Photodriven Reaction. The photoreduction of Cu is mediated by the PS surface following a displacement reaction that is summarized in the following scheme:25

Nevertheless, the red-ox reaction could take place spontaneously due to the proximity of the re-dox potentials of Cu and Si/SiH, yielding a small deposition rate, far below that corresponding to the photoassisted yield. This unlighted process could be mediated by the charge accumulation in the surface, by local thermal effects in the solution or other stochastic charge generation effects.16−18 None of these mechanisms gives rise to a significant rate of Cu reduction in the interface compared to the photoactivated mechanism. The Langmuir Hill model was first proposed by Hill in 1910 to explain the dissociation curves of hemoglobin. He attributed the observed experimental results to cooperative dynamics in which the adsorption on the molecules could be enhanced or inhibited by third-part elements.19,20 The application of this dynamic model has been successfully applied to a large number of biochemical processes. Out of the biochemical and biological spheres, few authors have employed the cooperative binding dynamics of the Langmuir Hill equation to describe several processes in which the formation and reaction of nanostructured materials are involved. For example, Bastidas et al. used Hill isotherms, among other models, to study the adsorption mechanism of benzotriazol onto Cu surface in acid solutions.21 Other utilizations of this model have been applied to describe surface adsorption dynamics in biochemicalbased devices.22 In the present study we aim at understanding the nucleation, clustering, and plating mechanisms of Cu on PS. For this analysis, we apply the cooperative ligand−locus model to the immersion deposition of Cu on a reactive substrate. We describe these results in terms of the cooperative degree determined by the photoexcited electrons, created in the substrate and migrated toward the clusters surface, acting as binding sites for the Cu ions present in the solution. The PS structures used for the present study were fabricated from highly doped p-type silicon, providing a mesoporous23 network with columnar pore shape in conditions that can eventually provide protection against the oxidation of the Cu metal clusters.24

Si + 2H 2O → SiO2 + 4H+ + 4e− SiHx + 2H 2O → SiO2 + (4 + x)H+ + (4 + x)e− Cu+2 + 2e− → Cu

In the first step of the displacement reaction, Si atoms present in the PS structure are oxidized and meanwhile the water molecule is dissociated. This reaction is catalyzed by light since photons create electron−hole pairs in the p-doped Si, favoring the hydrolysis process. These electrons are available to react in the coupled red-ox reaction in which Si is oxidized and Cu is reduced. Both Si and the as-formed SiH can react with the surrounding water. Due to the high reduction potential of Cu2+ with respect to the oxidation potential of Si, this reaction is favorable and Cu nucleates on the solution/PS interface. It should be noted that such a reaction scheme is influenced by the pH and the energy band structure of silicon. In the present study, we use a deposition bath whose pH is 2.9. According to the Pourbaix diagram, the deposition potential of Cu is independent of the pH under such acidic conditions.26 Thus, the potential of Cu2+/Cu is located around 0.3 V vs SHE. Since the valence band of silicon is located at 0.6 V vs SHE, Cu deposition via hole-injection to silicon is not expected.27 In contrast, Cu can be deposited via the conduction band of silicon under illumination. In this way, Cu forms seeds on the interface that grow as long as the substrate supplies electrons to the Cu+ ions for the reduction reaction. In the present work, samples were labeled according to the time of photoactivated deposition of Cu on PS. As an example, CuPS10 indicates Cu deposited for 10 s onto PS. Characterization of the Structures. Scanning electron microscopy (SEM) images were acquired with a Hitachi S3000N scanning electron microscope. Semiquantitative chemical information was retrieved by energy dispersive X-ray analysis (EDX). Field emission scanning electron microscopy (FESEM) images were obtained in a XL 30S-FEG (Philips). No metallization was required to characterize the samples. Synchrotron X-ray absorption spectroscopy (XAS) experiments were carried out in the K edge of Cu (8979 eV corresponding to s → p transitions) at line BM25A (Spline) in the European Synchrotron Radiation Facility (Grenoble, France). Both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were obtained for each sample. An INCA 13-elements X-ray detector was used to measure in fluorescence mode. Fluorescence spectra were collected in fluorescence mode placing the surface of the samples at 45° with respect to incident beam. Data were normalized and processed applying the same normalization parameters for all the spectra by means of the Athena Software.28 For the EXAFS fitting and analysis, the Artemis software was employed. EXAFS analyses were performed by Fourier analysis of the oscillations weighted in k2 (square wavenumber of the photon). The analysis in the k space was made within the range of the 2−9 Å−1 with a window of Kaiser−Bessel type of step 0.5 Å−1. The parameters obtained by the fitting following the EXAFS equation were the mean

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EXPERIMENTAL SECTION Dissolution Process and Reaction Cell. The PS substrates were fabricated following a standard method described in previous works (see, for instance, ref 23). The PS structures were obtained from high conductivity (0.01−0.02 Ω cm), p-type, Si wafers. The PS network is formed by the anodization in 1:2 (volume) HF:ethanol solutions (from commercial HF 48% (w/v) in water, Sigma-Aldrich) and was carried out in a homemade Teflon electrochemical cell, with a Pt reference electrode. The Si wafers pieced in 1.5 × 1.5 cm2 were galvanostatically etched assisted by a 100 W halogen lamp. Single-layer PS was obtained at 100 mA/cm2 for 100 s, resulting in thicknesses of about 10 μm. The Cu deposition over the PS matrices was carried out by immersing the PS substrates in aqueous Cu solutions ((CuSO4) 50 mM and H2SO4 1 mM to stabilize the pH, Sigma-Aldrich) at RT. The reaction was activated by turning on a halogen Xe lamp (100 W) during the required exposition time. Once the reaction is finished, the sample is removed from the solution and rinsed subsequently twice in water and twice in ethanol and dried with N2. It should be noted that the porosity and type of Si are two of the major parameters in this kind of process. Porosity can limit the inner cluster size grown inside the porous matrix by the 14906



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cluster. For larger deposition times, CuPS320 for instance, an almost continuous Cu thin film is grown by cluster coalescence, which can be observed with the naked eye as a reddish film. The reflectance spectra of PS and different CuPS cluster samples have been measured and are depicted in Figure 1e for short-time Cu deposition. A small absorption shoulder located at around 580 nm is observed as expected from the localized SPR signal of Cu clusters. It can be noted that the intensity of the optical absorption is very weak for shorter Cu deposition times (10 s) and increases significantly for increasing deposition times up to 60 s. For 60 s deposition time, the absorption peak presents a slight red shift, which can be interpreted in three different ways. First, it suggests that the absorption can be the result of a combination of the SPR and the intraband electronic transitions in Cu.29,30 Second, this red shift can be due to the transition between the localized SPR and the extended SPR conditions. And third, since the shoulder band is not totally resolved, this red shift could be also interpreted as a widening of the SPR band. These results give further support to the SEM observations of the Cu clusters growth since larger metallic clusters present red shifts in their SPR bands and cluster heterogeneity leads to wider SPR absorptions in the spectra. Cu has been found not only over the PS surface but also inside the pores of the porous structure. Given the typically large surface area of PS, metals can react all over the PS− solution interface (Figure 2a). For this reason, the in-depth chemical composition along the cross section (CS) was studied by EDX (Figure 2b). From the experimental results, the relative Cu concentration was determined by integration over 0.5 × 0.5 μm2 areas in the CS. Figure 2c depicts the weight percent of Cu, Si, and O as a function of depth (distance from the external surface). The concentration−depth chart for Cu suggests the presence of a diffusion profile, as expected for this kind of reduction mechanism in the absence of external electrical potentials. In this case, though a noticeable amount of Cu can be detected at depths of 5 μm, the concentration is already less than one-fourth its value on the surface. It can be estimated that the quantity of Cu deposited within the first micrometer of PS is more than within the remaining depth of PS, which reflects that surface nucleation dominates over the diffusion. In the case of oxygen, the concentration describes rather low fluctuations due to the oxidation of PS by the reduction reaction and by the contact of the sample with the atmosphere prior to analysis, which could have preoxidized the PS surface due to the high reactivity of Si and SiH. This feature is clearly observed in the FTIR spectrum of freshly made PS,5,31−33 which presents an incipient O−Si−O band in the 950−1200 cm−1 range centered at 1085 cm−1 (Figure 3). In any case, the absorption peaks of Si−Si bonds (615 cm−1) and passivated Si−H2 (910 cm−1) are the most relevant active bonds in the PS substrate. As Cu nucleates in metallic form following the electron displacement reaction, the initially dominant Si−Si and Si−H bonds vanish and new contributions appear due to Si−O−Si bonds (notable absorption increase in the wide band between 950 and 1200 cm−1) and surface Ox−Si−Hy at 885 cm−1. The chemistry and electron structure of Cu atoms reduced in the Cu−PS system at different stages of the photoassisted deposition are studied by XAS. The XAS spectrum of bulk Cu and CuO references obtained in transmission mode are displayed with the fluorescence spectra of the CuPS samples fabricated at different deposition times (Figure 4a). Regarding the XANES zone of the CuPS composites, displayed in Figure 4b, it can be noticed that the absorption edge in the CuPS10

coordination number (N), the mean distance of the shell to the scattering atom (R), the Debye−Waller factor (DW), and the energy potential shift (E0). In all the fittings the error was below 2% in N, 1% in R and DW, and 5% in E0. Optical characterization of the CuPS structures was carried out using a UV−vis reflectance measurements (Jasco V-560) equipped with a Hamamatsu R928 photomultiplier. All measurements were performed in the range between 350 and 900 nm with a 2 nm interval and with 1 s integration time.

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RESULTS AND DISCUSSION From a fresh PS layer with mean pore size of 50 nm (see SEM image in Figure 1a), the deposition process is carried out by the

Figure 1. FESEM images corresponding to (a) As-formed PS, (b) CuPS10, (c) CuPS20, and (d) CuPS60. (e) UV−vis−NIR reflectance spectra showing different Cu deposition times on the PS; the SPR band corresponding to the Cu nanoclusters is marked with a dashed line.

immersion of PS in the Cu solution at RT lighting up the surfaces for controlled periods of time. Once the illumination of the PS surface begins, the red-ox reaction is promoted on the PS surface. As shown in Figure 1b, corresponding to 10 s illumination, in the first stages Cu nanoparticles nucleate randomly all over the surface. It is also observed that these nucleate preferentially on the edges of the pores and partially obstruct the pores. As the irradiation time increases to 20 s, the presence of small Cu clusters of ca. 50 nm with pseudospherical shape becomes patent (Figure 1c). When the illumination time reaches 60 s, a dense population of clusters is found over the surface (Figure 1d), with a wide size distribution presenting a maximum size of around 200 nm in diameter. Such distribution of particles suggests that a competitive process exists between nucleation of a cluster and growth of a previously nucleated 14907



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Figure 2. (a) SEM image of the cross section of samples CuPS20. (b) EDX spectrum at the region pointed to in image (a). (c) Relative concentration profile of the cross section in (a) obtained by EDX. (d) Cross section image of a standard PS sample of 1 μm thickness.

shows a fluorescence spectrum with proper features of autoabsorption. In this case, the autoabsorption not only artificially shifts the absorption edge to lower energies but also damps the edge and postedge features of a given spectrum.34−36 Accordingly, in spite of damping the spectrum at the white line, two peaks just behind the edge, characteristic of fcc metals,37−39 can be identified. Thus, in view of the autoabsorption, it is reasonable to consider that Cu in this long deposition structures presents a metallic bulk state. The shape of the Cu edge and the white line in the samples obtained for shorter deposition times do not present these characteristic features of double peak. In part this can be due to the absence of a longterm range order in Cu, which is the typical case of atoms in an isolated or nanostructured form. However, pre-edge peaks, one of the features traditionally used to indentify nanosized Cu, since they are indicative of partially hybridized 3d orbitals,40 are observed in our samples. The first prepeak on the edge (inset of Figure 4b) presents differences between the CuPS samples. A gradual increment of the prepeak intensity for those obtained at increasing deposition time can be observed. This prepeak is representative of 1s → 4s transitions in 3d transition metals with hybridized sp levels.41 The intensity and energy position of this peak could also be related to the coordination number, symmetry, oxidation state, and electronegativity of the bonded atoms. In fact, when atomic Cu (electronic configuration [Ar]3d104s1) is placed in a nanosized metallic aggregate, the electronic structure is changed to a structure with an hybridized electronic state represented as [Ar]3d10−x4sp1+x,40,41 in which x represents the f raction of hybridization with respect to the bulk metallic state. In case of nanostructured particles, such a hybridization can arise from the volume-to-surface ratio of the particles (size effects) and by shape effects.42,43Thus, in our process, as Cu atoms form the clusters, d electrons hybridize with the sp orbital, populating the 4sp orbital over the normal state. When the Cu cluster grows, the hybridization of the d level decreases and the virtual sp population decreases; this causes the prepeak intensity to rise for increasing Cu deposition time. As commented above, the shape of the white line in the CuPS10−60 systems differs from that observed for Cu, but also from CuO. The reason for these differences is not only the lack of a well-defined fcc structure due to its nanostructure, but also to the presence of other coordination elements other than Cu, such as O in our particular case. This is a potential reason for the breaking of the Td symmetry inducing the pre-edge peak spectral fault.

Figure 3. FTIR spectra of a typical PS substrate (black line) and samples CuPS20 (red line) in the region of main interest. The molecular assignments to the main bands of the spectra are shown for both samples.

Figure 4. XAS and XANES spectra of the Cu structures in the Cu Kedge: (a) XAS spectra of the studied CuPS structures, CuO, and bulk Cu; (b) XANES region of the spectra; (b, inset ): detail of the prepeak evolution observed in the selected structures.

shifts to higher energies (i.e., E0 is at 1 eV) than that corresponding to the foil (bulk Cu), indicating a slight oxidation of Cu present in this sample. Sample CuPS20 does not shift significantly with respect to CuPS10. The shift increases by 1.8 eV for CuPS60 samples, suggesting an increase of the Cu oxidation state. Sample grown for 320 s (CuPS320) 14908



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same range of distances. The results of the fitting of the main EXAFS parameters provide the spectra shown in Figure 5b. With respect to the fit of the bulk structure, the first shell provided a mean neighborhood of 12 Cu atoms and a distance of 2.55 Å. In the case of the CuPS samples, the first neighbors should be O atoms at 1.60 Å in average. The distance of the first Cu neighbor is apparently not modified and remains at 2.55 Å. The quantitative analysis gives some keys to understand the reaction mechanism during deposition if we compare the RDF corresponding to the clustered samples. The fitting of the RDF provides some differences between the structures formed at different times. First, the mean number of neighbors decreases for CuPS samples presenting smaller clusters, which correspond to shorter deposition times. Additionally, the intensity ratio between the first and second neighbor oscillations is larger for shorter deposition times (decreasing cluster size) than for bulk or CuPS deposited for longer times. The first of these differences is a common feature usually observed in several nanosized materials since, when the size decreases, the surface to volume ratio increases. The second difference has been related, as pointed above, to the existence of an oxidized phase of Cu. This oxidation may occur during the red-ox process as a consequence of the coupled reduction of the Cu ions. As mentioned, the fittings to the EXAFS spectra give an O distance to Cu of ca. 1.6 Å, slightly below the 1.7 Å tabulated for the first O in CuO.44 In this case, we have attributed it to the contraction of the Cu−O bonds in the cluster surface due to the increase of the cohesion energy in structures at the nanoscale.42,43 Following these results, and complementing the SEM observations, the relative mean size of Cu clusters in each system can be estimated using EXAFS by considering the mean number of neighbors. By this approximation, one can estimate that the clusters deposited for 20 s are around 33% larger on average than the structures obtained after 10 s deposition. For CuPS60 and CuPS320 structures, the relative mean size reaches 44% and 50% larger than for CuPS10. Thus, it is possible to estimate that the cluster growth rate is not linear with time and the reactions occur very fast in the first seconds, reaching a steady velocity for times larger than 60 s. One can use, considering the sequence of PS/metallic-Cu/Cu-ions interfaces, an analogy with the cooperative binding in a ligand−molecule locus system. This analogy to a Langmuir Hill model could help in explaining the mechanisms involved in the Cu cluster growth. This growth is produced by the coupled Cu reduction/ Si oxidation in the Cu−PS system, which is mediated via electronic transport in the cluster up to the Cu cluster/Cu solution interface. Langmuir Hill models have been largely used in biochemistry, pharmacology, or enzimology,19,20 and more recently in the field of biomaterials to describe the mechanism involved in some kinds of biofunctionalized nanotube-based vapor sensors22 or to explain the surface adsorption mechanism of antibodies on functionalized Si surfaces.46 In the present approach, one can suppose that the Cu adsorbed on the PS surface acts as a ligand catalyst that enhances the subsequent deposition of new Cu ions. This enhancement is reached by the intrinsic property of surface metal atoms to efficiently exchange charge carriers through their surface. Such kinetics can be well described by the Langmuir−Hill equation. Cluster-growth kinetics can be modeled considering that the reaction of Cu metallization on Cu surfaces is enhanced by the transfer of electrons from the metalized Cu surface, which are ejected from

Beyond the XANES analysis, the EXAFS spectra of the studied samples are represented in Figure 5a in the k2χ(k)

Figure 5. EXAFS spectra obtained for the studied Cu systems: (a) EXAFS oscillations represented in the k2χ(k) space; (b) FT of the spectra represented in (a) and EXAFS fits of the Cu bulk and the studied deposition conditions of the CuPS systems.

space. Comparing the oscillations, subtle differences can be noted in the different spectra. Nevertheless, a slight displacement of the first oscillation can be observed in the three CuPS samples prepared at shorter times with respect to bulk Cu. This behavior can be interpreted as a sign of the existence of different environments in the short deposition (i.e., cluster structure) with respect to the long deposition (i.e., resembling bulk structure). The differences become more evident with respect to CuO, where the position of the oscillations in the k space differs completely from the CuPS samples. Once the Fourier transform (FT) of the EXAFS region is performed, a radial distribution function (RDF) with the short-range chemical structure around the Cu atoms is obtained. By comparing these different RDFs of the CuPS samples, it can be concluded that they all present a first neighbor at distance of approximately 1.6 Å, which can be attributed to O first neighbors considering a partial Cu oxidation.44 However, the highest intensity in the RDF of the CuPS samples corresponds to the second peak attributed to the first Cu neighbor of Cu in metallic form at ca. 2.5 Å.45 Notably, the relative intensity of the 1.6 Å peak with respect to the one found at 2.5 Å increases notably for samples CuPS60 and decreases for shorter deposition times. Additionally, at shorter deposition times, a decrease of intensity at the oscillation maxima for longer distances with respect to bulk Cu can be observed as indication of the long-range order reduction in the nanostructured samples. By performing quantitative EXAFS analysis, the fitting of the oscillations in the k space has provided the mean number and distances of neighbors, among other parameters (refer to the Supporting Information for the complete list of fit parameters). As XAS provides a convolution of the contributions of the main phases present in the material involving the target element, for the EXAFS fitting of the different CuPS samples we have considered a model which combines a Cu oxide structure (CuO) with a metallic Cu structure. Thus, the fitting process for the bulk metallic Cu has been carried out considering a three-shell model, composed by three Cu−Cu shells in the range of distances between 1 and 5 Å. In the case of the CuPS systems, a four-shell model with the first shell consisting of an O atom and three additional Cu shells has been assumed in the 14909



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the PS substrate by means of light excitation. Under such circumstances, the Cu surface coverage can be approached by S = S0

Lα Lα + K α

where S0 represents the active surface of the system, L is the concentration of active sites for Cu deposition, K is the concentration of interface free electrons that gives the half occupation of the system, and α is a phenomenological exponential parameter (known as Hill parameter), which aims at identifying whether the adsorption is a cooperative (if larger than 1), a noncooperative (if equal to 1), or a negatively cooperative (if below 1) process. In our model we can interpret this parameter as related to the affinity of the Cu ions to be reduced on PS or on the Cu cluster. It can be assumed that there should be several parameters that take part in this reaction (i.e., the pH, the relative redox potential of the Cu to the Si, the mean size of the Cu clusters, the surface chemistry of the clusters) and have a direct effect on this α parameter. For the current process this equation can be rewritten to express the covered fraction of the surface:

η=

Lα L + Kα α

In our particular case, the occupation fraction is timedependent, depending on the amount of interface free (conducting) electrons. It can be considered that, within the range of deposition times studied, light excitation generates a flow of electrons from the substrate, which is linear with time (L(t) = Qnet, where ne represents the number of charge carriers and Q is a phenomenological parameter that takes into account the efficiency of charge transfer process and other stochastic variables). Thus, a new equation to describe the occupation fraction results: η(t ) =

Figure 6. Results of the fitting of the EXAFS spectra: (a) relative cluster size derived from the relative number of neighbors and the fitting to an asymptotic Hill law; (b) relative number of Cu first neighbors, considering the first Cu and O shell; (c) O/Cu ratio of mean neighbors in the two first shells obtained by the fittings and corresponding to (b).

tα tα = t α + (K /Qne)α t α + kα

state. By taking the N(Cu)/N(O) ratio of the first Cu and O shells, respectively, in each condition it can be noticed that this ratio decreases for decreasing deposition times. In this sense, the Cu/O neighbor ratio depicted in Figure 6c can give evidence of the compositional change of the deposited Cu (mean oxidation). As follows from the figure, the Cu/O ratio increases from the shorter deposition time (at this point, Cu is reduced mainly by the oxidation of PS, so that the Cu/O ratio reaches a minimum). Once the deposition time increases, the oxidation in the coupled red-ox reaction does not occur exclusively on the PS surface, but also on the deposited metallic Cu with an oxidation potential (Cu to form CuO) of 0.52 V. It is worth noting that this potential is close to the Cu2+ to Cu reduction potential so that a competitive Cu oxidation−reduction process is initiated. Nevertheless, the oxidation mechanism at the first stages of these photoassisted immersion deposition reactions may be much more complicated, and their deep study requires further investigation.

The fitting of the experimental values to this equation gives a good fitting, as can be observed in Figure 6a. In this fit the Hill coefficient results in a value of 2.1. This means that the Cu deposition over PS can be interpreted as a cooperative growth process, interpreting the surface charge carriers (interface free electrons) as ligands. The k parameter represents a time constant that gives the time value for which the half occupation is reached. In this case, the value of k is close to 7 s but may be expected to vary significantly with the experimental conditions (i.e., temperature, light intensity, type of Si, etc.). Finally, the dependence of the mean first neighboring of the Cu atoms on the deposition time is depicted in Figure 6b. The first Cu and O neighbors of the Cu atoms corresponding to the two first shells are represented for the four different Cu deposition times and for bulk Cu. The reduction in the mean number of neighbors with respect to the bulk reference is clear even for the quasi-bulk sample CuPS320. The XAS results bring to light that this system, in spite of behaving as bulk, has a thin film structure and is formed by an aggregation of Cu clusters. In all the cases, apart from the unresolved XANES of the CuPS320, the increase of oxide content deduced from EXAFS in the CuPS systems is in coincidence with the increase of the oxidation state of the Cu observed by the energy shift in the XANES. The relative amount of O can be semiquantified in every sample to provide an estimation of the mean oxidation

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CONCLUSIONS Photoassisted deposition of Cu in porous silicon (PS) has been utilized to fabricate Cu−PS composite materials formed dominantly by surface nanosized Cu clusters on PS and, to a minor extent, by Cu diffused through the PS network. The 14910



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Figure 7. Sequence of the photoassisted immersion deposition reaction of Cu deposition on PS. Once the PS substrate is placed into the Cu solution (a), the light is switched on to illuminate the substrate and the photons start creating the electron−hole pairs in the substrate, which migrate to the interface (b); once the electrons are available in the interface, the high redox potential difference between the Cu2+ and the PS allows the direct reduction of the first on the surface, forming the cluster seeds (c); the process carries on and the Cu nucleates forming metallic clusters on the surface (and partially inside the porous matrix), where the formation of new clusters competes with the growth of former ones. In this stage, the Cu reduction may be coupled by the oxidation of surface Cu (d); the Cu photoassisted deposition goes on upon the coalescence of the clusters in a continuous Cu film over the PS substrate (e).

under project MICROSERES. K.F. expresses his thanks to KAKENHI for young scientists B (25810135).

structural and chemical characterization of the Cu structures grown in the PS matrix has been characterized by a number of techniques including FESEM, XAS, and optical spectroscopy. From the study of several samples, different stages in the photoassisted deposition reaction have been identified. The reduction of Cu occurs in at least three phases as illustrated in Figure 7: first, Cu nucleates over the surface by a coupled redox reaction resulting in parallel in the oxidation of PS. In the second stage, new clusters are formed, but the new reduced ions tend to deposit over the cluster surface, minimally oxidizing the Cu surface. In the third phase the Cu clusters grow and coalesce so that the coating behaves as a continuous Cu film. It has been observed that the structures formed during the first two stages display surface plasmon resonance, which is identified through an absorption band at around 580 nm. The kinetics of Cu deposition over the PS surface has been successfully fitted using a Langmuir−Hill model. This model allows assuming that the Cu deposition in the first stages follows a cooperative process, in which the availability of surface electrons triggers the deposition of more Cu ions. This model could eventually be generalized to explain the photoassisted deposition of other transition metals over electron donor substrates. At this point, though electron extraction from previously deposited Cu may explain the partial oxidation of the Cu nanoparticles surface, new investigations should be performed to take a deeper view into the oxidation mechanism of Cu in this kind of photoassisted immersion deposition reaction on PS.

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ASSOCIATED CONTENT

S Supporting Information *

Fitting parameters for the EXAFS spectra of the CuPS systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Spanish Beamline at the ESRF (BM25-Spline) staff for the scientific and technical support. This work was partially supported by Comunidad de Madrid 14911



Article

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Photoassisted Immersion Deposition of Cu Clusters onto Porous

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