Chemical Deposition and Stabilization of Plasmonic Copper

Article pubs.acs.org/cm

Chemical Deposition and Stabilization of Plasmonic Copper Nanoparticle Films on Transparent Substrates Mariano D. Susman,§ Yishay Feldman,† Alexander Vaskevich,*,§ and Israel Rubinstein*,§ §

Department of Materials and Interfaces, †Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel S Supporting Information *

ABSTRACT: Preparation of supported copper nanostructures has been scarce, compared to the more noble metals Ag and Au, mainly due to the lower stability of Cu toward corrosion in aqueous solutions and oxidation in air, either during or after preparation. Still, as a markedly inexpensive metal, Cu might present an attractive substance, if suitable Cu nanoparticle (NP) deposition and stabilization methods could be developed. Here, we present the first case of glass substrates coated with Cu or Cu2O NPs using wet chemical deposition (CD), performed under well-defined conditions optimized for obtaining each of the two nanoparticulate deposits. Cu NP films were also obtained by chemical reduction of the Cu2O NP films, thereby achieving improved size uniformity. The Cu NP films display a prominent surface plasmon (SP) band in the visible range. The dependence of the SP absorbance on the local dielectric environment is shown to provide a useful tool for monitoring Cu NP corrosion processes and their inhibition. Stabilization of the Cu NP films by treatment with the corrosion inhibitor benzotriazole (BTAH), shown here for the first time, enabled study of the films’ plasmonic properties, such as their refractive index sensitivity (RIS), a basic property in sensing applications. The measured RIS values are similar to those of typical gold NP films. Introduction of an effective, low-cost, and scalable method for the preparation of stable supported Cu and Cu2O NP films may open the way to a variety of plasmonic and other applications. KEYWORDS: Cu, Cu2O, nanocrystals, localized plasmon, localized surface plasmon resonance (LSPR), benzotriazole, corrosion inhibition, electroless deposition

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INTRODUCTION Preparation of supported plasmonic nanostructures has been largely limited to the use of gold and silver.1−4 However, the high cost of these metals may prevent large-scale commercial use of plasmonic-based technologies such as localized surface plasmon resonance (LSPR) sensors, plasmonic solar cells, plasmon-enhanced photocatalysts, metal enhanced fluorescence (MEF) displays, etc., especially when large areas are required. Copper, which is ca. 6500 times and 120 times cheaper than Au and Ag, respectively, while displaying a localized plasmon band in the visible part of the spectrum, is a promising alternative metal. Cu nanoparticle (NP) films were prepared using nanosphere lithography,5 solvent evaporation under vacuum from colloidal dispersions,6 sputtering,7 and thermal evaporation.8,9 However, these approaches suffer from certain drawbacks, such as a long series of preparative steps that may endanger the films integrity, NP films with poor adhesion to © 2012 American Chemical Society

the substrate, or high manufacturing cost and difficult scalability. Chemical (electroless) deposition (CD) is a well-established, inexpensive, and easily scalable method for the preparation of metal deposits.10 An early communication11 demonstrated the possibility of preparing Au, Ag, and Cu nanoparticulate films supporting LSPR excitation using the CD approach; however, film morphology and stability issues were not addressed. It is noteworthy that essentially all other reports on CD of Cu films concerned the preparation of continuous, conductive films, primarily for microelectronic applications (printed circuit boards; interconnects in integrated circuits), while the early, sub-percolation stages of deposition, involving the formation of Received: March 4, 2012 Revised: May 13, 2012 Published: June 18, 2012 2501

dx.doi.org/10.1021/cm300699f | Chem. Mater. 2012, 24, 2501−2508

Chemistry of Materials

Article

the deposition solutions used in the present study were varied in the ranges 8−80 mM CuSO4 and 8−200 mM HCHO. The slides were immersed in open vials containing the final deposition mixture for different time periods without stirring. After deposition, the slides were rinsed with water three times and dried under a N2 stream. Reduction of Cu2O NPs to Form Cu NPs. The reaction was carried out by immersing CD Cu2O NP films in 5 mM aqueous NaBH4 for 2 min, followed by washing in water and drying under a N2 stream. BTAH Adsorption. Cu NP films were reduced with 5 mM aqueous NaBH4 for 2 min to remove traces of oxide on the Cu surface, and immediately immersed in a purged (by 30 min N2 bubbling) 1% w/v aqueous BTAH solution (pH = 5) for 10 h, followed by rinsing with copious amounts of water and drying under a N2 stream. Refractive Index Sensitivity (RIS) Measurements. The refractive index (RI) of the bulk medium was varied using water (RI = 1.331), glycerol (RI = 1.471), and water−glycerol mixtures (0, 25, 50, 75, and 100% v/v). The RI of the solutions was determined using an Abbe refractometer (Fisher Scientific Co. LR45302). Transmission UV−vis spectra were recorded at 0.2 nm resolution and 1 s/point integration time, using a cuvette with the respective solvent as baseline. Slides were rinsed and dried before changing solutions. The slides were sequentially measured in water−glycerol solutions with increasing and decreasing RI values, and were measured in air before and after each set of measurements, to establish the stability and reproducibility of the results. Characterization Methods. Transmission UV−vis spectra were obtained with a Varian CARY 50 spectrophotometer with a wavelength resolution of 1 or 2 nm and an average acquisition time per point of 0.2 s. Measurements in air were performed using a special slide holder ensuring reproducible position of the sample (spot size: 1 mm2) with air taken as baseline. Measurements in liquid media were performed in 1 cm path-length cuvettes with a special slide holder and a Teflon cap allowing air access when desired. For measurements in liquid the baseline was taken in solution prior to inserting the slide. Grazing-incidence X-ray diffraction (GIXRD) measurements were carried out at an incidence angle of 2−3° using a TTRAX III diffractometer (Rigaku), equipped with a rotating Cu anode operating at 50 kV, 200 mA and with a multilayered mirror (CBO) forming a nearly parallel X-ray beam. High-resolution scanning electron microscopy (HRSEM) images were acquired with an ULTRA 55 FEG ZEISS HRSEM using the SE and in-lens detectors, a voltage of 1.5−3 kV, and a working distance of 3−5 mm. Slides were mounted on carbon tape loaded stubs and partially painted with carbon paste to increase the electrical conductivity and minimize charging. Tilted-angle images were obtained at ca. 45° from the surface normal. Analysis of HRSEM images was performed using the software ImageJ 1.42q (National Institutes of Health (NIH), U.S.A.), measuring more than 100 particles per image. The adhesion of NP films to the substrates was evaluated qualitatively using the adhesive-tape test: A piece of clear Scotch tape (3M) was pressed against the film and pulled away. Detachment of poorly adherent films is seen by the naked eye.

isolated NPs sustaining SP excitation, have remained essentially unexplored. A major difficulty in the use of Cu-based plasmonic nanostructures, and the reason for the very few reports on plasmonic Cu films, is their tendency to corrode under ambient conditions and in common aqueous solutions.5,6,12−14 Corrosion inhibition was attempted in Cu colloids and thermally evaporated NP films by using various types of adsorbates (thiols,15,16 polymers17−20) or coatings (silica,21 graphene,22 silver23), with limited success. In the present work, we systematically studied for the first time the formation of nanoparticulate copper films on glass substrates using the CD approach. Conditions allowing deposition of Cu and Cu2O NP films were elucidated. All the films show good adhesion to the substrate, seen as excellent stability during rinsing and drying, not commonly observed with films produced by adsorption of colloidal NPs. The Cu NP films exhibit a well-defined surface plasmon (SP) band at ca. 600 nm. LSPR spectroscopy is shown to provide a sensitive and convenient tool for monitoring surface corrosion processes in Cu NPs and their inhibition. Adsorption of the corrosion inhibitor benzotriazole (BTAH, C6H5N3)24 effectively stabilizes the optical response of the Cu NP films, enabling determination of critical optical parameters of the plasmonic system.

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EXPERIMENTAL SECTION

Materials. Hydrogen peroxide solution (30%, Frutarom), sulfuric acid, AR (93−98%, Gadot), hydrochloric acid (32%, Frutarom), ammonium hydroxide (CP, 24−27%, Gadot), 3-aminopropyl trimethoxysilane (APTS) (97%, Aldrich), sodium tetrachloroaurate (III) (99.99%, Alfa Aesar), sodium borohydride (≥96%, Merck), paraformaldehyde powder (95%, Aldrich), copper(II) sulfate pentahydrate (>99.0%, Merck), sodium hydroxide pellets (>99.0%, Merck), potassium sodium tartrate (NaKT) tetrahydrate (99.0%, Aldrich), 1Hbenzotriazole (BTAH) (≥96%, Fluka), glycerol (>99.5%, BDH Analar), and methanol (absolute, Biolab) were used as received. Water was triply distilled. Samples were dried using N2 flow obtained from a liquid N2 household source. Solution pH was measured using nonbleeding pH-indicator strips, pH 0−14 (Merck, Germany). Glass microscope coverslips (Schott AG borosilicate glass D263T N° 3, 22 × 22 × 0.3 mm, supplied by Menzel-Gläser) were cut with a diamond pen to 22 × 9 mm2 slides. Substrate Preparation. Microscope cover glass slides (No. 3, Menzel-Gläser, Germany) cut to 22 × 9 mm, were cleaned by immersion in freshly prepared hot “piranha” solution (1:3 H2O2:H2SO4) for 1 h (Caution: piranha solution reacts violently with organic matter and should be handled with extreme care), thorough rinsing with water, immersion in “RCA SC1” solution (1:1:5 H2O2:NH4OH:H2O) for 1 h at 70 °C, rinsing with water and with methanol three times in an ultrasonic bath (Cole-Parmer 8890), and drying under a N2 stream. The slides were then silanized by immersion in 1% v/v APTS methanolic solution overnight, followed by thorough rinsing with methanol and drying under a N2 stream. Gold seeds were formed by immersing the amine-terminated slides in 5 mM aqueous NaAuCl4 solution (pH = 2) for 2 min to bind chloroaurate ions, rinsing with copious amounts of water, immersing in 50 mM aqueous NaBH4 for 2 min to reduce the Au(III) ions to metallic Au clusters, rinsing thoroughly with water, and drying under a N2 stream. Chemical Deposition (CD). Three aqueous solutions were prepared: 0.64 M CuSO4·5H2O (16% w/v) (denoted Sol-1); NaKT tetrahydrate (10% w/v) + NaOH (4% w/v) (Sol-2); and 3.2 M formaldehyde (HCHO) prepared from paraformaldehyde (Sol-3). Deposition solutions were prepared by mixing solutions 1−3 in different proportions, as follows: First, Sol-1 was added to Sol-2 with continuous mixing. After complete dissolution of the initially formed copper hydroxide, water and Sol-3 were added. The concentrations in

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RESULTS AND DISCUSSION

Chemical deposition (CD) of Cu and Cu2O NP films. CD presents an autocatalytic heterogeneous process that normally involves the immersion of an activated surface into a metastable solution containing a depositing metal salt, a reducing agent, a pH regulator, and (if required) a complexing agent. Here, the respective reagents were CuSO4, formaldehyde (HCHO), NaOH, and sodium potassium tartrate (NaKT), commonly used for copper CD.10,25 The glass substrates were activated by Au seeding (see the Experimental Section) and immersed in the CD solutions 2502



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Figure 1. In-situ transmission UV−vis spectra of Au-seeded glass slides under CD conditions using solutions containing 0.5 M NaOH, 0.177 M NaKT and variable CuSO4 and HCHO concentrations (indicated). Spectra were taken every 1 min for a total of 7 min.

have been suggested.25,29 A discussion of the mechanisms is beyond the scope of the present work. It can be derived from the stoichiometry of eqs 1 and 2 that increasing the Cu(II) concentration in the reaction mixture and/or decreasing the formaldehyde concentration will render Cu2O deposition thermodynamically more favorable with respect to Cu deposition. This conclusion, based on simple thermodynamic considerations, corresponds well to the experimental results (Figure 1), where a decrease in the Cu(II) concentration and an increase in the formaldehyde concentration both lead to a gradual shift of the product from Cu2O to Cu. Based on the results in Figure 1, appropriate conditions for obtaining single-phase Cu or Cu2O NP films were identified and used in all the experiments below. Preparation of Cu NP films was carried out in 8 mM Cu2+ + 200 mM HCHO solution (Figure 1, panel a3), providing moderate rates of Cu deposition and H2 evolution. Cu2O NP films were obtained by increasing the Cu2+ concentration to 80 mM and decreasing the HCHO concentration to 40 mM (Figure 1, panel b1), while maintaining relatively short deposition times. In both cases the deposition solution also contained 0.5 M NaOH and 0.177 M NaKT. The morphology of Cu and Cu2O NP films on glass slides was analyzed by HRSEM imaging (Figure 2), showing formation of NPs that grow in size with deposition time. Although the Au clusters30 are apparently smaller than the microscope resolution (∼2 nm), the spatial distributions of the deposited NPs on the surface suggest that the seeding produces rather homogenously distributed Au nuclei.31 Particle size distributions (Figure S1, Supporting Information) could be determined for Cu NP films up to ca. 9 min deposition, after which NP overlap and aggregate formation becomes dominant (Figure 2, panels a1−a5), although the films still show a welldefined LSPR band (see below). Cu2O NPs are more isolated (Figure 2, panels b1−b5) and size distributions could be analyzed for longer deposition times (Figure S1, Supporting Information). At the beginning of the deposition, the surface density of Cu NPs is ca. 10 times larger than that of Cu2O NPs (Figure 2;

under normal laboratory atmosphere for various time periods at room temperature (22−23 °C) without stirring. To study the effect of the deposition conditions, concentrations of the various components in the deposition solutions were varied in a wide range, showing substantial variability in the deposition rate as well as in the morphology and composition of the deposited NPs. In situ transmission UV− vis spectra covering the relevant range of parameters are shown in Figure 1. Two distinct spectral features are clearly identified, that is, the characteristic extinction of Cu2O in the range 400− 550 nm and the localized surface plasmon (SP) band of nanoparticulate Cu around 600 nm.12,26 Under certain deposition conditions, a shift of the phase composition with time is evident. Figure 1, panel a1, shows predominant formation of Cu2O NPs, but as deposition progresses, a band is developed at longer wavelengths (ca. 650 nm), indicating the appearance of Cu NPs. Similarly, under the conditions of Figure 1, panel a2, the initial Cu2O spectrum is gradually overtaken by the spectrum of Cu NPs. Formation of Cu and Cu2O NPs is in line with previous reports on the phase composition of continuous copper based films prepared by CD.10,25,27,28 The overall reaction for Cu deposition involves hydrogen evolution and can be written as [CuT2]2 − + 2HCHO + 4OH− → Cu°(s) + H 2(g) + 2H 2O + 2HCOO− + 2T2 − (1) 2−

where T represents the tartrate anion. In analogy, the overall reaction resulting in Cu2O deposition can be written as25 2[CuT2]2 − + 5OH− + HCHO → Cu 2O(s) + HCOO− + 3H 2O + 4T2 −

(2)

The detailed mechanisms of the reactions leading to Cu or Cu2O deposition are rather complex. Production of Cu by disproportionation of Cu2O to Cu + Cu(II), as well as production of Cu2O by comproportionation of Cu + Cu(II), 2503



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overlapping and leading to SP band damping.6,34 A relatively efficient separation of these bands with the consequent observation of a well-defined SP band in Cu NP films may depend on the NP morphology, as observed in the present work, as well as in previous reports.6,11,35,36 The spectra corresponding to Cu2O NPs (Figure 3b) are characterized by the gradually increasing oxide absorption around 350−550 nm. The slides’ color correspondingly changes from light yellow to yellow-brown (Figure S3, Supporting Information). An increasing brownish tint seen after ca. 15 min Cu2O deposition is attributed to increasing Cu NP codeposition, evidenced by the growth of a band at ca. 580−600 nm, as discussed. In-situ spectroscopic measurements showed that deposition did not occur on bare glass slides or on APTS-modified slides (Figure S4, Supporting Information). Hence, Au seeding is essential for the deposition of both Cu and Cu2O NP films. Nucleation and NP growth in the bulk solution was not detected in the relevant time range. While the drying of adsorbed NPs commonly induces aggregation and major spectral changes,37 comparison of in situ and ex-situ measurements in the present system (compare Figure 3a, b and Figure 1, panels a3 and b1) indicates that water rinsing and drying under N2 does not effect substantial spectral (and hence structural) changes. Both Cu and Cu2O NP films show strong adhesion to the glass substrates, successfully passing the Scotch tape test, in line with previous reports on the adhesion of thin continuous CD Cu films on aminosilanefunctionalized Si/SiO2.38 The phase composition of the NP films deposited on seeded glass was verified using GIXRD measurements (Figure 3c, d). The peak positions in the diffractograms are in agreement with literature values for fcc Cu and fcc Cu2O phases,39,40 while no diffraction associated with the Au seeds is seen. The results for Cu NPs (Figure 3c) indicate formation of a pure metallic Cu phase throughout the deposition. CD of Cu2O NPs (Figure 3d) starts with the formation of a pure fcc Cu2O phase; after ca. 20 min evidence of Cu codeposition is observed, and after 25 min the GIXRD pattern is dominated by the Cu phase. The increasing formation of Cu is consistent with the optical and HRSEM data, as discussed. The size and spatial distributions of the CD Cu2O NPs (Figure 2, panels b1−b5) are superior to those of the Cu NPs (Figure 2, panels a1−a5). It was therefore hypothesized that chemical reduction of Cu2O NP films may lead to Cu NP films with an improved size dispersion. Figure 4 presents the spectral and morphological changes associated with chemical reduction (in 5 mM NaBH4) of a Cu2O NP film. The UV−vis spectrum after reduction (Figure 4a) shows a broad, red-shifted Cu SP band, indicating conversion of Cu2O NPs to Cu NPs. The resultant Cu NP film is reminiscent of the Cu2O precursor film (Figure 4b), showing a size distribution substantially narrower than that of directly deposited Cu NPs of a similar average particle size (Figure S5, Supporting Information). The decrease in average particle diameter from 177 to 164 nm is smaller than that expected for the change in molar density from Cu2O to Cu (0.084 to 0.14 mol Cu/cm3), and it is attributed to the considerable porosity of the Cu NPs. The shape41 and surface density42 of metal NPs have a major effect on the spectral properties of immobilized NP layers. Evolution of the spectra of metallic Cu NPs with deposition time (Figure 3a), showing red-shift and broadening of the SP band, is characteristic of plasmon coupling, resulting from the

Figure 2. HRSEM images of Cu films (a1−a5) and Cu2O NP films (b1−b5), prepared by CD on Au-seeded glass slides for increasing deposition times (indicated), using the SE detector (scale bars: 200 nm). Tilted-angle HRSEM images of a 15 min Cu NP film (c) and a 15 min Cu2O NP film (d), respectively, using the in-lens (c) and SE (d) detectors (scale bars: ∼100 nm).

Figure S1b, Supporting Information), leading to a much faster process of particle overlap and broadening of the size distribution, while the largely separated Cu2O NPs show only a slight broadening with deposition time (Figure S1, Supporting Information). The Cu2O films consist mostly of well-defined truncated octahedral NPs (Figure 2; Figure S2a, Supporting Information), formed when the growth is nearly isotropic; that is, the growth rates in the ⟨100⟩ and ⟨111⟩ directions are virtually equal.32 At later stages of Cu2O deposition (more than ca. 15 min), the formation of large, rough particles is evident (Figure S2b, Supporting Information), accompanied by visible gas evolution. Therefore, these features may represent Cu particles, but their identification requires further study. Tilted-angle HRSEM imaging reveals differences in morphology between CD metallic and oxide NPs. Cu NPs appear as rough hemispheres with no identifiable crystallographic planes (Figure 2c), while Cu2O NPs appear mostly as half truncated octahedra (Figure 2d). Ex-situ LSPR spectra of Cu NP films prepared using increasing deposition times (Figure 3a) exhibit a well-defined SP extinction band, continuously increasing in intensity and red-shifting from ca. 585 to ca. 610 nm. The peak full width at half-maximum (fwhm) grows with time from ca. 50 nm to ca. 100 nm. The slides’ color changes from gray to bright brown (Figure S3, Supporting Information), the characteristic color of bulk Cu metal. In gold, and more frequently in copper, interband transitions from filled d to empty sp conduction bands are present in a spectral region close to that of the SP polaritons,33 sometimes 2504



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Figure 3. Ex-situ transmission UV−vis spectra of CD Cu (a) and Cu2O (b) NP films, on Au-seeded glass, prepared using increasing deposition times (indicated). GIXRD patterns of the Cu (c) and Cu2O (d) NP films are presented; reference diffraction patterns of pure Cu and Cu2O phases are shown at the bottom.

dependence of the SP extinction for different Cu NP layers is beyond the scope of the present work. Stabilization of Cu NPs. Use of Cu nanostructures in plasmonic and other applications is impeded primarily by their tendency to oxidize under ambient conditions and corrode in aqueous solutions. Exposure of Cu NP films to laboratory air for several hours results in red-shift and intensity increase of the SP band43 (Figure S6, Supporting Information), indicating progressive NP surface oxidation. The latter may be reversed by reduction with NaBH4, observed as a blue-shift and intensity decrease of the SP band (Figure 5a). As the Cu NP films are prepared without any surfactants or capping agents, the reduction treatment provides clean and well-defined Cu NP surface conditions for further corrosion and inhibition experiments. To evaluate the corrosion behavior, a 9 min Cu CD film immersed in triply distilled water with air access was tested using in situ LSPR measurements. During the 8 h experiment, the sample showed a continuous red-shift and decrease of the SP band intensity, indicating rapid corrosion6,43,44 (Figure 5c; Figure S7, Supporting Information). The latter practically precludes study or application of the plasmonic properties of the Cu NP films. Adsorption of benzotriazole (BTAH) is effective in Cu corrosion prevention, mainly at neutral to alkaline pH.24 The kinetics of BTAH adsorption onto CD Cu NP films were monitored in situ by recording the change in the SP extinction intensity at a constant wavelength (620 nm) vs adsorption time, in a closed cuvette containing a deaerated BTAH solution (Figure 5b). The lowest BTAH concentration used to stabilize macroscopic Cu surfaces is in the ppm range;24 in the present study, a 1% aqueous BTAH solution was used. Complete

Figure 4. (a) Transmission UV−vis spectra corresponding to chemical reduction (in 5 mM NaBH4) of a 15 min CD Cu2O NP film on Auseeded glass. (b) Respective HRSEM images (scale bars: 200 nm).

decrease in interparticle distance as the deposition progresses (Figure 2, panels a1−a5). For Cu NP films obtained by chemical reduction of Cu2O NP films, the SP band is substantially red-shifted and wider compared to that of Cu NP films prepared by direct CD (Figures 4a and 3a, respectively). These differences are attributed primarily to structural features of the individual NPs (pores, internal voids, etc.) rather than to SP band modification by plasmon coupling, as the latter is assumed to be negligible in the well-separated, reduced oxide NPs (Figure 4b). Further analysis of the shape 2505



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In-situ corrosion measurements in water with air access, carried out with a BTAH-protected Cu NP sample, showed no SP damping in the measured time frame (Figure 5c). This result demonstrates the effective stabilizing effect of BTAH protection of Cu NP films. Refractive Index Sensitivity (RIS) of Cu NP Films. The RIS, a basic property of LSPR transducers, is defined as change in the SP band (intensity, wavelength, or other parameters) effected by unit change of the refractive index (RI) of the bulk medium. To determine the RIS of BTAH-stabilized Cu NP films, the RI was varied by using water (RI = 1.331), glycerol (RI = 1.471), and water−glycerol mixtures. An increase in the RI of the medium shows the expected red-shift and intensity increase of the SP band (Figure 6a, b). The stability of the transducers was confirmed by carrying out the measurements sequentially in air, in water−glycerol mixtures of increasing and then decreasing RI values, and finally in air again, showing excellent reproducibility. The BTAH-treated Cu NP transducers are therefore resistant to oxidative corrosion in the time

Figure 5. (a) Spectral changes (measured in air) of a 6 min CD Cu NP film undergoing 2 min reduction in 5 mM NaBH4 followed by 10 h BTAH adsorption. (b) In-situ adsorption kinetics of BTAH, measured as change of the extinction intensity at 620 nm with time, for a freshly reduced 9 min CD Cu NP film immersed in 1% BTAH solution. (c) The effect of corrosion in water (open to air) on unprotected and on BTAH-protected 9 min CD Cu NP films (freshly reduced), presented as change in the maximum extinction of the SP band with time.

adsorption, represented by stabilization of the SP signal, is achieved within ca. 3 h. BTAH adsorption, carried out for 10 h to ensure complete surface coverage, exhibits characteristic redshift and intensity increase (Figure 5a). Our data are different from those of Kapoor et al.,45 where addition of aqueous BTAH to a Cu colloid did not result in change of the SP band position; the discrepancy may be due to the presence of gelatin in the colloid reaction medium. The structure of the BTAH layer formed on Cu surfaces has been extensively discussed, but it remains poorly understood. The protective layer may not necessarily be a BTAH monolayer on Cu, but rather a Cu(I)−BTAH multilayered complex whose thickness may depend on the treatment conditions (BTAH concentration, pH, temperature, etc.). In the present case, the change of ca. 0.2. a.u. accompanying BTAH adsorption (Figure 5a) suggests a layer that is much thicker than a BTAH monolayer, but a detailed analysis is beyond the scope of the present work.

Figure 6. Refractive index sensitivity (RISλ) measurements of BTAHprotected Cu NP transducers. Transmission UV−vis spectra in water− glycerol mixtures (the first and last measurements were carried out in air) for (a) a 9 min CD Cu NP film and (b) a 15 min CD Cu2O NP film, chemically reduced to a Cu NP film as in Figure 4. Note that in (a) and (b) the initial and final measurements in air overlap. (c) Wavelength of the extinction maximum vs. RI of the medium; the RISλ is calculated as the slope of the best-fit lines. 2506



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frame of the experiments, allowing reliable determination of their optical properties. Values of the RIS for wavelength shift (RISλ) calculated from Figure 6c (the experimental points in air deviate from the straight line and were not included in the calculation) are 48, 76, and 131 nm/RIU for Cu NP transducers prepared by 6 and 9 min Cu CD and by reduction of a 15 min CD Cu2O film, respectively. The corresponding figures-of-merit (FOM) values (FOM = RISλ/fwhm)46,47 are 0.7, 1.0, and 0.7, respectively. These RISλ and FOM values are comparable to typical values for nanostructured Au films.48,49 The Cu NP transducers show the same trend as Au nanoisland films, that is, increase in the RISλ with red-shift of the SP band maximum.49 Previous reports on the sensitivity of plasmonic Cu NP films, carried out without stabilization, gave RISλ values of ca. 77 nm/RIU11 and 68 nm/ RIU,9 within the range of values measured in the present work. Further modification of the Cu NP morphology may lead to higher RIS values.

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ASSOCIATED CONTENT

S Supporting Information *

Particle size distributions of supported Cu, Cu2O, and chemically reduced Cu2O NPs; HRSEM images of a Cu2O nanocrystal and codeposited Cu particles; pictures of the slides; in situ blank measurements in CD; and air oxidation and corrosion of Cu NP films observed by LSPR. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], israel. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of this work by the Israel Science Foundation, Grant No. 1251/11, and by the Minerva Foundation, with funding from the Federal German Ministry for Education and Research, is gratefully acknowledged. The electron microscopy studies were conducted at the Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging, Weizmann Institute. This research is made possible in part by the historic generosity of the Harold Perlman family.

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CONCLUSIONS Cu and Cu2O nanoparticle (NP) films were prepared by chemical deposition (CD) on Au-seeded glass substrates. The copper oxidation state in the deposited films is determined by the solution composition, allowing preparation of single-phase metal or oxide NP films. Cu2O NP films exhibit better crystallinity, a narrower size distribution, and a larger separation between the NPs compared to the Cu NP films. These properties were exploited for attaining Cu NP films with improved size and spatial distribution by chemical reduction of Cu2O NP films. CD Cu NP films display a well-defined LSPR extinction band around 600 nm, red-shifting with increasing average particle size. Oxidation and corrosion, largely precluding practical application of Cu NP transducers, could be monitored effectively using LSPR measurements. Stabilization of the Cu NPs by means of BTAH adsorption enabled reliable determination of the RIS of Cu NP transducers; the measured values, in the range 48−131 nm/RIU, are comparable to those reported for evaporated Au nanoisland films of similar NP sizes. The present results indicate that Cu NP films may provide stable and sensitive optical transducers that could successfully compete with considerably more expensive Au and Ag based transducers. It is interesting to compare the plasmonic properties of Cu and Au in terms of performance per weight of the metal. Approximating the shape of the Cu NPs to half-spheres and considering their surface density, the amount of Cu in a NP film can be estimated and compared to Au NP films. This leads to 13 and 36 μg Cu cm−2 for films prepared by 6 and 9 min CD, with respective SP extinctions of 0.51 and 0.95 au. Au nanoisland films of 5 and 10 nm nominal thickness, prepared by thermal evaporation and annealing,48,49 present corresponding values of 10 and 19 μg Au cm−2, showing extinctions of 0.25 and 0.53 au, respectively. Cu NP films therefore exhibit a comparable performance in terms of extinction per amount of metal, but they are far superior to gold in terms of price. Cu NP films are of particular interest for applications such as plasmonic solar cells,50 where a large surface area of the NP film is required for effective energy conversion. In such applications the inexpensive Cu metal and the low manufacturing cost and scalability associated with CD may prove crucial for practical use.

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REFERENCES

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