Direct Writing on Copper Ion Doped Silica Films by Electrogeneration

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Direct Writing on Copper Ion Doped Silica Films by Electrogeneration of Metallic Microstructures Ahmed Kandory,† Hélène Cattey,‡ Lucien Saviot,§ Tijani Gharbi,† Jackie Vigneron,∥ Mathieu Frégnaux,∥ Arnaud Etcheberry,∥ and Guillaume Herlem*,† †

Laboratoire de NanoMedécine, Imagerie et Thérapeutique EA 4662, UFR Sciences & Techniques, CHU J. Minjoz, Université de Bourgogne Franche-Comté, 16 route de Gray, 25030 Besançon, Cedex, France ‡ Institut de Chimie Moléculaire, UMR-CNRS 6302, Université de Bourgogne Franche-Comté, 9 avenue A. Savary, 21078 Dijon, Cedex, France § Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303, CNRS−Université de Bourgogne Franche-Comté, 9 avenue A. Savary, BP 47870, F-21078 Dijon, Cedex, France ∥ UMR CNRS 8180, Institut Lavoisier, 45 av. Des Etats-Unis, 78035 Versailles, Cedex, France S Supporting Information *

ABSTRACT: A facile and rapid localized electrochemical reduction of colloid copper particles is proposed using the scanning electrochemical microscope (SECM) technique. In this purpose, thin films of composite silica glass containing copper salts were prepared by the sol−gel method via the dip coating technique. Acid-catalyzed tetraethylorthosilane (TEOS) solutions charged with copper nitrate were used as precursors. This one-pot experiment can be performed in mild conditions. The localized generation of copper metallic nanostructures on the silica film has been performed by electroreduction of methyl viologen on an ultramicroelectrode (UME). The UME generates reducing species, which in turn diffuse toward the silica matrix and reduce the metal ions. The diameter of the working electrode and the electrolysis period were taken into account to study the size of the generated dotted micropatterns. The compositions of the modified silica films were characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), optical microscopy, and vibrational (IR-ATR and Raman) and X-ray photoelectron spectroscopies (XPS).

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ibility,24 and high intensity colored doped glasses for optical filters or antireflective purposes.25 Synthesis of silica coating film doped with metal particles can be achieved by one of the following treatments:26−28 (i) heating the doped gel in air, (ii) exposing the doped gel to UV radiation, and (iii) reduction of the precipitated metal oxide by heating it in a reducing gas. Another alternative comes from using chemical reducing agents like hydrazine29 and glycerol (used for production of copper nanoparticles),30 ascorbic acid as reducing agent for preparation of ultrafine silver powder,31 and the citrate method (was introduced by Faraday for synthesis of gold nanoparticles).32 During the past decade, great effort has been made for surface modification with either conventional methods allowing micropatterning substrates in lift-off process20 or novel alternative technologies such as direct writing.33,34 Indeed, photolithography has several drawbacks such as complex and

INTRODUCTION

Among material science technologies of coatings, the sol−gel process is a wet chemistry synthesis route and is certainly one of the simplest methods for preparing films by dipping, spraying, or spinning techniques. Compared to other processes such as CVD, sputtering, or evaporation, sol−gel film formation requires less equipment and is low-cost. However, the main advantage of the sol−gel method is the possibility to control precisely the reaction, the structure, and the doping of the deposited coating. The sol−gel process has been widely used to prepare silica films doped with various kinds of metal nanoparticles such as gold,1,2 copper,3,4 platinum,5 silver,6,7 or palladium. Silica/metal nanoparticles composites have been the subject of numerous studies because these kinds of materials are of wide interest in various applications, for instance in optics,8−11 chemical catalysis,12−15 magnetic materials,16 and optoelectronics.3,17−20 The dispersion of metal ions into the glass matrix enhances mechanical, chemical, electrical, electronic, and magnetic properties,21−23 optical third-order nonlinear suscept© 2016 American Chemical Society

Received: September 30, 2016 Revised: December 22, 2016 Published: December 27, 2016 1129

DOI: 10.1021/acs.jpcc.6b09913 J. Phys. Chem. C 2017, 121, 1129−1139

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The Journal of Physical Chemistry C

with deionized water and dried at 110 °C for 1 h in order to remove the residual alcohol produced during silica film heat treatment. SECM Measurements. The SECM consisted of a computer-controlled ElProScan PG 340 electrochemical probe scanner (HEKA Elektronik, Germany). All experiments were carried out in a conventional three-electrode cell which was located in a Faraday cage at ambient temperature (25 °C). Homemade disc-shaped Pt microelectrodes (10 and 50 μm diameter) were used as working electrode, a silver reference electrode (SRE) wire served as a quasi-reference electrode, and a Pt wire served as a counter electrode. All the potentials are mentioned versus SRE. X-ray Diffraction Analysis. X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advance diffractometer and Cu Kα radiation with wavelength of 0.154 06 nm produced at 40 kV, and 40 mA was used as the radiation source. XRD data were collected over the 2θ range from 30° to 100° at every 0.02° with a scan speed of 0.5 s per step. Spectroscopic Characterizations. Raman spectra were measured using a Renishaw inVia micro-Raman spectrometer with a laser excitation provided by the 633 nm line of a 20 mW helium−neon laser. A reduced laser power (0.125 mW on the sample) was required to avoid heating the samples. An 50× microscope objective was used and the acquisition time was 10 s. Low-frequency Raman measurements were obtained using a Renishaw inVia setup with 532 nm laser excitation and BragGrate notch filters from OptiGrate enabling to record the Stokes and anti-Stokes Raman scattering simultaneously down to about 10 cm−1 with high throughput. A grating with 3000 grooves/mm at diffraction order −1 was used so as to have about 0.4 cm−1 per CCD element in the low-frequency range. IR-ATR spectra were performed on Spectrum65 PerkinElmer equipped with ATR module using a Ge crystal. SEM. The morphologies of the different glass surfaces were studied by using scanning electron microscope where SEM images were acquired by a FEI QUANTA 450W. On the other hand, an optical microscope (Leica DM8000 M, Germany) was also used to study the silica surfaces. X-ray Photoelectron Spectroscopy. The measurements of X-ray photoelectron spectroscopy were performed on a KAlpha spectrometer (Thermo Electron) to confirm the reduction of incorporated metal ions. This technique may provide precision information on the thickness of deposited film and the oxidation state of the elements for samples understudying. AFM Scans. The topography scans of the localized cathodic reduction of the copper salt trapped in the sol−gel matrix were performed with a Nano Observer (CSInstruments, France) in tapping mode with an aluminum-coated silicon tip. Theoretical Calculations. Density functional theory (DFT) calculations on amorphous metal ion-doped silica glass were performed using the PBE level of theory for both correlation and GGA functionals as implemented in the Crystal14 code. Consistent Gaussian basis sets of triple-ζ valence plus polarization (TZVP) functions were used for H, O, Si, Cl, K, and Cu atoms.39 It is not straightforward to obtain a suitable structure since it is amorphous. There is no unique unit cell, and the structure inherently bears a large degree of randomness. Instead, we considered first coesite as the starting structure of the representative amorphous silicon dioxide material to be modified.40 This unit cell is interesting because

costly mask fabrication, pattern transfer, and mask removal compared to pioneering techniques producing patterns in a single step. In the context of innovative techniques, scanning probe microscopies (SPM) such as scanning tunneling microscopy (STM), atomic force microscopy (AFM), and scanning electrochemical microscopy (SECM) have been exploited as direct technique for local micromodification of surfaces. The SECM was developed as a powerful tool for direct writing on surfaces through deposition of several metals such as Cu, Ag, Pd, and Au,35 whereas this technique was applied to deposit many structures: Ni(OH)2,36 polyaniline on gold, platinum, and carbon surfaces.37 The SECM offers a relatively simple and fast method for the production of fine metal particles at specific locations on doped silica films.20 In order to use the feedback mode of the SECM for reducing the incorporated copper ions, the UME tip must immerse in the solution containing the mediator molecule. The mediator of choice must have a reversible behavior to support the chemical feedback. The goal of the present work is to describe an original way to reduce metal ions on the surface of a silica matrix embedding these ions trapped during the sol−gel process. In this purpose, copper ion doped silica films were tested for direct writing by SECM as a proof of concept. Once the sol−gel film preparation is achieved, the reduction of the metal ions on the silica surface is performed by using methyl viologen as redox mediator cycling between the SECM tip and the glass substrate. To the best of our knowledge, this is the first time a simple and efficient sol−gel process trapping copper ions is used for direct writing via the SECM technique. This represents and advance toward green and sustainable electrochemistry.38

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MATERIALS AND METHODS Chemicals. All chemicals of reagent grade quality were purchased from Sigma-Aldrich (France) and used without further purification: tetraethylorthosilane (TEOS), copper nitrate trihydrate, hydrogen peroxide, sulfuric acid, methyl viologen dichloride, potassium chloride, hydrochloric acid, absolute ethanol, and glass microscope slides. All solutions have been prepared by using ultrapure water (18 MΩ, Milli-Q, Millipore). Synthesis of Copper-Doped Silica Matrix. The main step in the silica film synthesis doped with copper ion nanoparticles is the hydrolysis of TEOS in the presence of the metal salt. A mixture containing 5 mL of TEOS, 10 mL of ethanol, and 1 mL of water was prepared, and 2 mg of copper nitrate were added as the source of copper ions. A few drops of concentrated hydrochloric acid were added as a catalyst to the mixture followed by stirring for 5 h. The dip coating process has been used to prepare a coating film on a glass slide surface. The precleaned microscopy slides were dipped into the prepared solutions, withdrawn vertically at different speeds, and then rinsed with deionized water abundantly. The precleaning treatment consisted in removing first organic impurities through sequential immersion in acetone and then ethanol and rinsed with deionized water. Next, the slides are submerged in a freshly prepared piranha solution (3 volumes of concentrated H2SO4 + 1 volume of H2O2) for surface activation (generation of OH groups on the slide surface) in order to ensure a good adherence of copper-doped silica matrix on the slide surface. The coated surfaces are rinsed 1130


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Figure 1. (a) Current−distance curve for a 10 μm Pt UME tip approaching to silica matrix surface. The tip was biased at −0.9 V vs SRE in an aqueous solution containing 5 mM of methyl viologen in 0.1 M KCl. (b) Cyclic voltammogram of 5 mM MV2+ in 0.1 M KCl at a 10 μm diameter Pt electrode. Scan rate: 50 mV/s. The normalized distance L is the ratio between the UME tip−substrate gap and the tip radius while the normalized current is calculated vs the tip current at infinite distance to the surface.

of its large size parameters (a = c = 7.148 Å, b = 12.334 Å) and contains 48 atoms (16 Si and 32 O atoms), allowing good coordination if dopant ions are placed in interstitial cavities. In a second step, the symmetry of the structure was broken by placing one copper and two nitrate ions in three different cavities of the unit cell for further geometry optimization (atomic positions and cell parameters). This lattice corresponds to our copper salt-doped glass model. The amorphous glass lattice was obtained by removing copper and nitrate ions to the previous geometry and fully relaxed (ion positions plus cell parameters). To do this, a quasi-Newton optimization scheme is used, and analytical gradients for atomic positions and cell parameters are computed. The accuracy of the calculation of the bielectronic Coulomb and exchange series is controlled by five parameters according to overlap criteria between two atomic orbitals (AOs). The corresponding integral is disregarded when the value of each parameter (TOLINTEG 7 7 7 9 30) is smaller than 10−parameter or evaluated in a less precise way. Integration grid and numerical accuracy control is based on the generation of grid points on an atomic partition method, originally developed by Becke.41,42 Another selfconsistent field (SCF) control is the threshold for convergence on total energy and set to 10−7 (TOLDEE 7). The sampling of the irreducible Brillouin zone (IBZ) depends on two shrinking factors (SHRINK 2 2). The first one concerns the grid of k points in reciprocal space, relying on the Pack−Monkhorst method,43 while the second one is the sampling of k points in the “Gilat net”44 and used for the calculation of the density matrix and Fermi energy. Their values are identical since the system is insulating. Two tools for accelerating convergence were used for modifying the percent of Fock/KS matrices mixing (FMIXING 40) and the eigenvalue level shifting technique (LEVSHIFT 10 0). This negative energy shift to the diagonal Fock/KS matrix elements (in the Crystalline Orbital basis) of the occupied orbitals reduces their coupling to the “unoccupied” set and is removed after diagonalization. Vibrational frequencies and IR spectra of the systems have been calculated as the eigenvalues obtained by diagonalizing the weighted Hessian matrix at the Γ point of the IBZ within the harmonic approximation. The mass-weighted Hessian matrix is obtained by numerical differentiation of the analytical first derivative, calculated at the

modified 3N nuclear coordinates by a small displacement with respect to equilibrium geometry. The values of the infrared intensities for each normal mode are computed through the Berry phase approach. These are achieved by evaluating the dipole moment variation along the normal mode adopting the Wannier set of localized functions.45 The Born tensor is normalized to fulfill the sum rule.

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RESULTS AND DISCUSSION Electrochemical Reduction of Incorporated Cupric Ions into the Sol−Gel Matrix. The SECM was used to reduce metal ions trapped into the silica matrix described in the Materials and Methods section. The aqueous solution consisted of methyl viologen at 5 mM (acting as redox mediator) in KCl at 0.1 M (as supporting electrolyte). At first, the SECM tip (UME, Pt, 10 μm diameter) is brought close to the doped-silica matrix surface. The gap between the tip and the substrate is about 2 μm as estimated by the approach curve method as shown in Figure 1a. The efficient reduction potential that should be applied at the tip of UME was determined by cyclic voltammetry of methyl viologen dichloride performed on a 10 μm diameter Pt UME as shown in Figure 1b. From the cyclic voltammogram, the reduction potential of methyl viologen is at about −0.75 V. The electrochemical behavior of methyl viologen in aqueous solution was the topic of several studied as it gives two redox waves.46,47 The first one is quasi-reversible redox wave corresponding to a single electron transfer to MV2+ cation to produce the blue species corresponding to the MV•+ radical with subsequent reoxidation to colorless MV2+. The first redox wave is reversible and can be cycled many times without significant side reaction. The second one is less reversible (at about −1.1 V) and accompanied by adsorption of uncharged species (MV) at the electrode surface. Therefore, the SECM tip was biased at −0.90 V, and two different biasing times (2 and 5 min) were used for copper salt reduction on the surface. Methyl viologen dichloride (MV2+) is the oxidized form of a redox couple existing initially in the solution. This specie diffuses toward the surface of the UME tip where it can be converted back to its reduced form (MV•+). The reactions at the UME tip and at the surface of the copper doped-silica film are as follows: at the UME tip: 1131

2MV2 + + 2e− → 2MV •+ DOI: 10.1021/acs.jpcc.6b09913 J. Phys. Chem. C 2017, 121, 1129−1139

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Figure 2. Images of locally reduced copper salt trapped in glass by (a) optical and (b) electronic microscopy with (c) details of this area. (d) Observation by SEM of spots obtained in cathodic reduction at two different electrolysis periods. Increasing the electrolysis time to 10 min yields (e) full coverage by metallic clusters and (f) is the direct writing of the V and I letters performed at constant height. AFM topography of (g) locally reduced copper salt on sol−gel matrix. All scale bars are 50 μm except (c): 10 μm.

at silica surface:

2MV •+ + Cu 2 + → Cu 0 + 2MV2 +

cycling in cathodic reduction near the doped sol−gel surface, the SECM tip was moved up at a distance of 40 μm above the spot. Then an approach curve method was performed again on the same position. A positive feedback current is now observed.

Prior to different characterizations of the modified silica surfaces, the electrogeneration process of metallic clusters in the glass matrix generation was observed by SECM. After 1132


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Figure 3. XRD patterns of silica films doped with copper nanoclusters after electroreduction of doped ions near an UME tip (10 μm diameter) of the SECM instrument.

the sol−gel synthesis, the metallic clusters are electrogenerated locally in a short time without long-range diffusion of copper ion. Moreover, the AFM study (Figure 2g) confirms the presence of microstructures on the surface of the sol−gel matrix deposited on the glass substrate. The strokes (horizontal scars) visible on the image are due to stick−slipping of microparticles on the AFM tip. X-ray diffraction pattern of reduced copper trapped in the silica film (and kept from air in a desiccator) is shown in Figure 3. The diffractogram indicates the presence of crystalline metal as evidenced by the presence of peaks. Two peaks can be indexed and are characteristic of (111) and (200) planes of a face-centered cubic (fcc) metallic copper.52 The diameters of the SECM spots observed in Figure 2d reveal metallic clusters produced by the UME tip. These spots are greater than the UME diameter because of the mediator diffusion effect. The electrogenerated reducing agents on the tip diffuse radially in the solution to a certain distance before taking part in the reduction reaction at the silica matrix surface. Moreover, the circle shape of the electrogenerated copper pattern can be attributed to the mass transport by spherical diffusion toward a disc-shaped UME. This is more complex than that to hemispherical one because the disc is not a uniformly accessible surface. Thus, the outer edge of the disc is preferentially used during the electrolysis and blocks the access of the reactant to the inner surface area.53 Increasing the electrolysis time to 10 min leads to circle patterns of fully reduced metallic clusters (Figure 2e). The direct writing of the V and I letters on the copper ion doped sol−gel matrix deposited on a glass substrate was carried out by the SECM as a proof of concept and shown in Figure 2f. To achieve this patterning, a 10 μm diameter Pt UME was used and biased at −0.9 V with a scan rate of 0.5 μm/s at constant height. On the other hand, the chemical reduction process is slower than the electrochemical one. For example, the XRD pattern of a copper-doped silica film immersed in an aqueous solution containing 0.5 M ascorbic acid as reducing agent for 15 min is poorly defined. The XRD analysis of the sample reduced chemically is very noisy with one small peak as shown in the inset of Figure 3. Compared to silver or gold suspensions,

This is an introductory evidence for generating metallic particles on silica film. The modified silica surfaces were characterized by different techniques such as XRD, optical and electronic (SEM) microscopies, and X-ray photoelectron spectroscopy (XPS). From optical observation, uniform, transparent, blue copperdoped silica films are achieved according to the sol−gel process.48 The electrogeneration of reduced methyl viologen (MV•+) species in cathodic reduction promoted efficient reducing agents. Indeed, the localized reduction of copper ions yielded micropatterns on the doped-silica surface. Red microstructures are observed (Figure 2a). The incorporated cupric ions have been successfully reduced locally by means of a mediator which is reduced at the tip of the electrode. Characterizations. Copper ion doped silica matrix modified by near field electrochemistry also confirmed the presence of metallic nanoparticles as shown by SEM in Figure 2b and details in Figure 2c. The amount of copper particles reduced cathodically in the silica matrix is easily evidenced by changing the diameter of the SECM tip used as the working electrode and by using different biasing times. The electrolysis for 5 min produced metallic clusters (Figure 2d) larger than those obtained after 2 min of electrolysis. However, the electrolysis for long period causes the aggregation of metal particles, improves their size distribution, and generates randomly shaped agglomerates. For longer biasing time (20 min), nano- and microcrystals can be observed on the copper salt-doped glass surface. Under the electric field generated during the cathodic reduction process, diffusion of copper ions took place on the glass surface. In fact, there is no impurity in the modeled sol−gel glass as it is the case with commercial glasses, and copper ions can diffuse more easily.49 So far, diffusion coefficients and solubilities of copper and metal ions in silica are investigated by bias temperature stress (BTS), especially for microelectronic devices.50,51 In the literature, diffusion coefficients of copper ions at about 10−11 cm2/s (at 450 °C) have been measured.50 In our experimental approach, the local cathodic reduction by SECM of Cu2+ trapped in amorphous silica shows microclusters on the glass surface, which have not been reported previously in the literature. Because of the high concentration of copper salt used during 1133


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The Journal of Physical Chemistry C copper colloids may have instability. Here, no trace of CuO and Cu2O phases in the amorphous glass matrix was observed by XRD measurements. The width of XRD peaks provides information on the crystallite size of metallic nanoclusters. The broad Bragg peaks are in favor of small size particles. According to Scherrer’s equation,54,55 and broadening of the resulting diffraction peaks, the crystallite size of the electrogenerated copper metallic nanoclusters has been calculated. A mean diameter estimate of copper particles is about 50 nm for 5 min biased time at −0.90 V. The chemical environment of reduced copper in silica glass matrix or as nanoclusters electroreduced on glass surface was examined by XPS, and the results are shown in Figure 4. The survey spectrum (Figure 4a) reveals the presence of silicon, oxygen, copper, and carbon. The peak at 284.65 eV corresponds to C−C bond from unreacted TEOS in the silica matrix. The main peak at 533 eV confirms the presence of O 1s assigned to Si−O−Si bond.56 The Si 2p and Si 2s core levels show peaks (Figure 4b) centered at 103.5 and 155.3 eV (Figure 4a), respectively, which are associated with Si−O−Si bonds and Si−OH groups present in sol−gel matrix.57 Si 2p at 103.5 eV underlines the +4 oxidation state of silicon, with no residual suboxide present. The peaks at 933.7 and 953.7 eV (Figure 4c) with satellites at about 944.8 and 964.0 eV are indications of copper as CuO which is formed after reoxidation at air.12,52,58 The chemistry of the glass matrix before and after copper complexation was analyzed by vibrational spectroscopy. First, Raman measurements on glass, copper salt-doped glass, and cathodically reduced copper on glass surface are gathered in Figure 5. The spectrum of the glass matrix obtained by the sol− gel process is shown in Figure 5a and serves as reference. There are broad peaks at about 491, 788, 982, and 1057 cm−1 corresponding to Si−O−Si vibrations in the silica network. The peaks between 850 and 1300 cm−1 are related to the fraction Qn of the different silicon units, where n is the number of bridging oxygens on the tetrahedral unit.59 In this range, the peaks from Figure 5a correspond to Q2 and Q3 contributions only. The starting glass and doped glass models presenting Q4 contributions have been corrected to better fit the experimental spectra. These modifications consisted in removing one oxygen atom to all the Q4, fully relaxing the lattices, and computing new spectra (16 Si and 20 O atoms). The calculated Raman spectrum (Figure 5b) of the glass structure has peaks with a satisfactory agreement compared to experimental ones despite differences of intensity in the Qn region (Figure 5a). The differences come mainly from the fact that a comparison is made between one model having 36 atoms and the sampling of many conformations by the spectrophotometer. The addition of Cu2+ and NO3− in the glassy framework (Figure 5c) alters completely the shape, intensities, and positions of the Si−O−Si peaks. Raman spectroscopy is by far very sensitive to metal environment. The blue Cu2+ ions are visible at 482 cm−1 with a weak shoulder at 580 cm−1 and an intense and sharp peak at 1047 cm−1, in excellent agreement with the literature.60 The presence of nitrate ions does not result in Q4 contribution despite oxygen of nitrate bridging Si. In our model, there is a Si···ONO2 distance at about 1.65 Å instead of 1.60 Å, which is the mean Si−O distance in silica, but does not generate Q4 contribution. In the corrected model structure (16 Si, 26 O, 2 N, and 1 Cu atom), we can observe a distortion of the nitrate trigonal geometry in the vicinity of the tetrahedral silicon due to crystal field effect. The resulting

Figure 4. XPS spectra of electrochemically reduced copper nitrate trapped in sol−gel glass matrix (a) survey, (b) Si 2p area, and (c) copper 2p area.

theoretical spectrum (Figure 5d) is quite satisfactory in the Qn area. The normal modes corresponding to 1125, 1050, 980, and 886 cm−1 are related to N−O stretching, Si−O stretching, and Si−O plus Si−O−Si···ONO2, respectively. The peak at 1452 cm−1 in Figure 5c is found in Figure 5d (inset) at 1482 cm−1 and corresponds to N−O vibration out of the plane. After cathodic reduction process by SECM on the copper nitrate doped glass surface (and likely reoxidation of copper at air as observed by XPS), the Raman spectrum is displayed Figure 4c. Notice that the Raman laser spot was carefully focalized on the SECM spot to avoid a mix with the signal from 1134


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Figure 5. Raman spectra of (a) pure sol−gel silica matrix, (b) the calculated one for a given structure, (c) experimental spectra of copper nitrate trapped in the glass matrix, reduced electrochemically and oxidized at air, and (d) calculated Raman spectrum of copper nitrate trapped in amorphous glass. (e) Low-frequency Raman (Stokes) measurements of copper nitrate in glass and (f) the calculated one.

surrounding Cu2+ ions trapped in the glass matrix. In relation with XRD measurements, no clear presence of CuO and Cu2O crystalline phase is visible (at 220, 300, 345, and 630 cm−1) although this possibility cannot be excluded due to Cu0 reoxidation at air.60 The narrow peak observed at 623 cm−1 could be related to Cu0 in glassy matrix but does not correspond to copper oxide signature. For large amount of copper salt in sol−gel silicate, there is segregation of copper oxides showing no Cu2O and CuO Raman peaks.61 Low-frequency Raman scattering measurements (Figure 5e) clearly show two well-resolved peaks at about 45 and 70 cm−1 (Stokes). They could in principle be assigned to confined acoustic vibrations in copper nanoparticles having diameters about 1 nm.62 However, we discard this interpretation because no such very small nanoparticles have been observed in these

samples. Instead, we investigated the low-frequency vibrations of copper ions complexed by oxygen atoms from silica matrix. From theoretical calculation on the copper-doped glass model, the Raman activity (Figure 5f) at low frequency is related to copper ion chelated by O bridging and corresponds to Cu···O stretching at 61, 91, 165, and 177 cm−1. These values are the closest from experiment, but positions and intensities are strongly dependent on the model and disorder from amorphous materials.63,64 IR-ATR measurements as well as theoretical spectra of pure glass and copper doped glass from sol−gel process are shown in Figure 6. The broad band between 3250 and 3500 cm−1 on all experimental spectra illustrates the Si−OH stretching modes and corresponds to silane terminations on the surface of the glass material (Figure 6a). The bands at 2980, 2930, 1620, and 1135


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Figure 6. IR-ATR measurements of (a) glass matrix from sol−gel process compared to (b) theoretical IR spectrum in reflectance mode at the PBE/ TZVP level of theory. (c) IR-ATR spectrum of copper salt-doped glass film compared to (d) theoretical IR spectrum in reflectance mode. (e) IRATR spectrum of cathodically reduced copper in a glass matrix. 1136



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1377 cm−1 are related to the stretching modes of −CH2 and evidence the presence of unreacted TEOS in the silica matrix in Figure 6a. The characteristic bands found at 1030 and 782 cm−1 are attributed to the asymmetric and symmetric stretching modes of Si−O−Si, respectively. The same corrected amorphous lattices that were used previously for Raman calculations (by removing oxygen atom to Q4 units and optimized again) served for IR calculations. The calculated IR spectrum (Figure 6b) from our amorphous glass model displays only Si−O normal modes since it is silica bulk without Si−OH termination. The difference between theoretical and experimental frequency values (Figure 6a,b) can be used as a scaling factor. The trapping of copper nitrate in the glass matrix (Figure 6c) adds new peaks at 1630 and 1311 cm−1 and a shoulder at 1400 cm−1 and in the range of overtones of the silicon−oxygen skeleton (1630 cm−1) to the ones from glass spectrum (Figure 6a). These peaks can be attributed mainly to N−O stretching modes and interfere with vibrations of water (1635−1630 cm−1) and coming from the copper salt. It is hard to determine by vibrational spectroscopy at high frequencies how copper ions are chemically bound to the silica matrix.65 From our calculations, copper is involved in normal modes with intense activity below 200 cm−1, which can then not be observed experimentally. Nevertheless and on the basis of our amorphous cell model, DFT calculation contribution to spectra interpretation is extremely useful. Indeed, Si−O stretching modes of oxygen chelating copper are identified at about 1000 cm−1 (Figure 6d). The normal modes associated with these frequencies are Si−O···Cu stretching modes, but with a weak intensity masked by Si−O−Si bands. On the same grounds, the reduced copper in glass matrix causes only a slight modification of some band intensities (Figure 6e) compared to the Cu2+-doped glass spectrum (Figure 6c). Mainly, the N−O bands are more intense at about 1630 cm−1.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09913. Coordinates of the relaxed structures of amorphous glass and copper-doped glass; lattice views along a, b, and c axes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(G.H.) E-mail: [email protected]. ORCID

Lucien Saviot: 0000-0002-1249-2730 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Iraqi Ministry of Higher Education and the Scientific Research and Cultural Bureau/ Embassy of the Republic of Iraq in Paris, France, and Campus France for financial support. Computations have been performed on the supercomputer facilities of the “Mesocentre de calcul de FrancheComte”, France. This work was also partly supported by the French RENATECH network. We are also grateful to E. Lepleux (Scientec, France) and L. Pacheco (CS Instruments, France) for their kind assistance with AFM measurements.

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REFERENCES

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CONCLUSIONS Composite materials formed by copper ions embedded in a silica glass matrix have been prepared by the sol−gel technique under acidic conditions. Then, the doped metal ions were successfully reduced locally by the SECM technique via the feedback mode to demonstrate direct writing feasibility on a silica matrix. Copper ions were reduced electrochemically by generating reducing agents at the UME tip, which reacts in turn with the metal ions incorporated in the silica film via a chemical redox process. This methodology has several interesting advantages, such as simple operation, mild working conditions, and the short time required for the reduction of the metal ions compared to other methods. On the other hand, size and shape control of generated metallic spots can be achieved by controlling the active area of the microelectrode and electrolysis period. Several techniques were used for characterizing the glass matrix before and after electrochemical reduction: IR-ATR, micro-Raman, XRD, SEM, and optical microscopy, HR XPS, and DFT calculations. The modification of copper salt-doped glass by SECM represents a fundamental prerequisite for its use in direct writing applications. A comprehensive analysis of experimental data was achieved on the basis of DFT calculations of an amorphous glass model charged or not with copper nitrate. These calculations also show that the modelization of amorphous glass without the need for extremely large unit cells offered good agreement between experimental and theoretical results. 1137


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Direct Writing on Copper Ion Doped Silica Films by Electrogeneration

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