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New Strategy for Preparing Thin Gold Films on Modified Glass Surfaces by Electroless Deposition Sabahudin Hrapovic,† Yali Liu,† Gary Enright,‡ Farid Bensebaa,§ and John H. T. Luong*,† Biotechnology Research Institute, National Research Council Canada, Montreal, Quebec, Canada H4P 2R2, Steacie Institute of Molecular Sciences, National Research Council Canada, Ottawa, Ontario, Canada K1A 0R6, and Institute of Chemical Process and Environment Technology, National Research Council Canada, Ottawa, Ontario, Canada K1A 0R6 Received November 27, 2002. In Final Form: February 24, 2003
This paper describes a new strategy for fabricating continuous gold films based on the self-assembly of the gold colloid monolayer on a poly(diallyldimethylammonium chloride)-modified glass slide, followed by electroless plating. Hydroxylamine-mediated reduction was proven as an excellent route to enlargement of immobilized nanoparticles on polymer-coated glass substrates in comparison to formaldehyde-mediated reduction. Au colloidal surface-catalyzed reduction of Au3+ by hydroxylamine exhibited very fast kinetics as monitored and confirmed by UV-vis spectroscopy in real time. The nanoscale morphology of the gold film was dependent on the initial coverage of gold nanoparticles and thermal annealing. Atomic force micrographs further revealed that enlarged particles were neither spherical nor cyclindrical, but highly complex in shape. The gold film thickness and its corresponding surface roughness could be easily controlled by setting the electroless deposition time. X-ray diffraction certified uniformity of deposits with the Au(111) crystallographic structure as the predominant one. No organic contamination during the course of electroless plating was observed as confirmed by both X-ray photoelectron spectroscopy and contact angle measurements. The stable and continuous gold films were used as electrodes for electrochemical experiments.
Introduction Electroless deposition (ELD) of metals is based on the deposition and reduction of metallic ions from a solution to a surface without applying an electrical potential.1 A target surface is immersed in a plating bath consisting of complexed metal ions and a reducing agent, for example, formaldehyde,2-3 hydrazine,4 or hydroxylamine.5 In this autocatalytic redox process, the noncatalytic surface must be preactivated with a metal catalyst, for example, palladium, before the metallization can occur.6 The key requirement of the electroless deposition is also to arrange the chemistry such that the kinetics of homogeneous electron transfer from the reducing agent to the metal ion is slow.7 Otherwise, the metal ion would simply be reduced in the bulk solution. To effect the reduction only at the target surface, a catalyst that accelerates the rate of metal * Corresponding author. Phone: 514-496-6175. Fax: 514-4966265. E-mail:
[email protected]. † Biotechnology Research Institute. ‡ Steacie Institute of Molecular Sciences. § Institute of Chemical Process and Environment Technology. (1) Electroless Plating: Fundamentals and Applications; Mallory, G. O., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990; Chapter 1, pp 29-33. (2) Hou, Z.; Dante, S.; Abbott, N.; Stroeve, P. Langmuir 1999, 15, 3011-3014. (3) Hidber, P. C.; Nealey, P. F.; Helbig, W.; Whitesides, G. M. Langmuir 1996, 12, 5209-5215. (4) Vaskelis, A. In Coating Technology Handbook; Satas, D., Tracton, A. A., Eds.; Marcel Dekker: New York, 2001; p 213. (5) (a) Meltzer, S.; Resch, R.; Koel, B. E.; Thompson, M. E.; Madhukar, A.; Requicha, A. A. G.; Will, P. Langmuir 2001, 17, 1713-1718. (b) Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726-728. (6) van de Putten, A. M. T.; de Bakker, J. W. G.; Fokkink, L. G. J. J. Electrochem. Soc. 1992, 139, 3475-3480. (7) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920-1928.
reduction must be applied to the surface to be coated. Because of the ability to create uniform coatings over small selected areas, ELD has been widely used for the deposition of metals such as copper, silver, gold, nickel, rhodium, and cobalt for the production of fine metal patterns for numerous applications in template-directed formation of micro- and nanoscale metallic structures.8 In particular, electroless plating on insulators such as plastics, glass, and ceramics has attracted much attention owing to its applications in printed circuit boards.9 The conventional procedure used for electroless plating of gold consists of several steps.7,10-11 First, Sn2+ must be adsorbed onto the plating surface which is then immersed in an aqueous solution of ammoniacal silver nitrate so that the surface becomes coated with nanoscopic silver particles. The Ag-coated surface is immersed in a gold plating solution for several hours to achieve a good deposition. In this step, the Ag particles are galvanically displaced by Au since gold is a more noble metal and the resulting Au particles are excellent sites for the oxidation of formaldehyde and concurrent reduction of Au(I) to Au(0). Finally, the Au-coated surface is immersed in nitric acid to dissolve any residual Sn or Ag that might be strongly adsorbed on the surface. This paper describes a simple electroless procedure for deposition of gold from solution onto glass surfaces precoated with a thin layer of poly(diallyldimethylammonium chloride), PDDA. The adhesive layer exhibits (8) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375-1380. (9) Hajdu, J. B. Plat. Surf. Finish. 1996, 83 (Sept), 29-33. (10) Hou, Z.; Abbott, N. L.; Stroeve, P. Langmuir 1998, 14, 32873297. (11) Hilmi, A.; Luong, J. H. T. Anal. Chem. 2000, 72, 4677-4682.
10.1021/la0269199 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/02/2003
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chemisorption with colloidal gold via electrostatic interactions, and the surface-confined colloidal gold nanoparticles are then grown into larger particles by treatment with formaldehyde/Au3+ or hydroxylamine/Au3+. This completely solution based procedure is cost-effective and can be performed in any laboratory. Atomic force microscopy (AFM), cyclic voltammetry (CV), and UV-vis spectroscopy are used to examine the morphology as well as the progress of electroless deposition. X-ray diffraction (XRD) was performed in order to verify the crystallographic structure and uniformity of deposits. By X-ray photoelectron spectroscopy (XPS), we elucidated the surface chemical compositions of Au layers and verified the presence of any contaminants during the course of electroless plating. Contact angle measurements were performed in order to reconfirm any potential organic contaminations. Experimental Section Immobilization of Au Nanoparticles on the PDDACoated Glass Surface. All glassware was cleaned in aqua regia (concentrated nitric acid/hydrochloric acid, 1/3). To effect the removal of organic contamination which in turn improves the adhesion of the polymer layer, glass microscope slides (Fisher Scientific, Fair Lawn, NJ) were carefully cleaned using a multistep procedure. First, glass slides were degreased in the vapor of ethanol in a Soxhlet extractor for 2 h followed by rinsing in deionized water. After soaking in a solution of nitric acid/ water (50/50) for 20 min, rinsed slides were immersed in freshly prepared piranha solution (concentrated sulfuric acid/hydrogen peroxide, 70:30%) for 30 min and then extensively rinsed with methanol and warm water (60 °C). (Safety: Piranha solution reacts violently with several organic compounds, and it should be handled with extreme caution.) All solvents were reagent quality and used without further purification. Cleaned glass slides were dried under a nitrogen stream and immersed in 0.01 M PDDA (Mw ) 200 000-300 000; Aldrich, Milwaukee, WI) for 2 h. The resulting glass slides were thoroughly rinsed with deionized water and dried under a nitrogen atmosphere. The polymer-coated slides were subsequently incubated in colloidal gold solution (5, 10, and 20 nm in diameter; Sigma, St. Louis, MO) for self-assembly of the gold nanoparticles. The incubation time exhibited a pronounced effect on the gold film as addressed later. The gold monolayers were rinsed with water and used immediately for Au electroless deposition. Electroless Deposition with Formaldehyde and Hydroxylamine. The glass slides with immobilized gold particles were immersed in a freshly prepared solution containing 0.625 M formaldehyde/0.127 M Na3Au(SO3)2 or 0.01% HAuCl4/0.4 mM hydroxylamine hydrochloride. With or without continuous stirring, the reaction was allowed to proceed for 24 h and the resulting gold layer was monitored during the course of electroless deposition. In these experiments, all electrolytes were prepared from high-purity chemicals. X-ray Diffractometry Analysis. The X-ray diffraction patterns from the gold films were obtained on a Scintag X2 powder diffractometer (Cupertino, CA) using graphite-monochromatized Cu K radiation (1.5405 Å) with a θ-θ scan mode. In this geometry, the sample was fixed and the scattering vector was normal to the surface of the film. X-ray Photoelectron Spectroscopy. XPS data were obtained using a KRATOS AXIS Ultra (Kratos, Manchester, U.K.) equipped with a hemispherical analyzer and an eight-channel detector. All data were acquired using monochromated Al KR X-rays and processed with Vision 2 software. The sample was fixed on a bar holder. The bar holder itself was mounted on an automated sample stage to allow automated variable angle measurement. XPS spectra were recorded at seven different takeoff angles of 0, 20, 40, 40, 60 70, and 80°, measured from the surface normal. The size of the analyzed area was about 1 mm2. Survey and high-resolution XPS spectra were collected using 160 and 40 eV pass energy, respectively. The pressure in the analyzer chamber was around 10-9 Torr. An electron flood gun was used to neutralize the charge during the experiment. Binding
Langmuir, Vol. 19, No. 9, 2003 3959 energy (BE) data were referenced to the main carbon peak set to 285.0 eV. Contact Angle Measurements. Following the procedure of Hou et al.,10 the samples were immersed in a ∼1 mM ethanol solution of hexadecylthiol (Aldrich) for 24 h. This was followed by thorough rinsing with ethanol and drying under a stream of nitrogen. The contact angle measurements were then performed on a Tantec model CAM-Micro X12 manual contact angle meter (Lunderskov, Germany) using hexadecane as a solvent. The measurements were made on static droplets in contact with the syringe, and six measurements were made at various locations on each treated surface. Atomic Force Microscopy. AFM images were obtained using a Nanoscope IV (Nanoscope IV, Digital Instruments-Veeco, Santa Barbara, CA) with a silicon tip operated in tapping mode to characterize the surface morphology. The images obtained by AFM were not manipulated, altered, or enhanced in any way. Cyclic Voltammmetry. All measurements were performed in aqueous 0.1 M H2SO4 using a three-electrode potentiostat. The reference electrode was an Ag/AgCl electrode, and the counter electrode was platinum. The electrical potential of the gold was cycled between -0.2 and +1.4 V for cleaning gold substrates or between +0.1 and +1.4 V for data acquisition at 0.1 V/s. UV-Vis Spectroscopy. UV-vis spectroscopic measurements were performed using a Beckman spectrophotometer (DU-640, Beckman, Fullerton, CA) at room temperature in the range of 300-800 nm with a 1 cm optimal length cuvette. A small piece of glass slide coated with gold particles was placed perpendicular to the light beam inside the cuvette and exposed to plating without agitation.
Results and Discussion Immobilization of Gold Nanoparticles. Colloid gold particles were immobilized by establishing an adhesive polymer layer on the glass substrate, followed by chemisorption of colloid gold with monodispersity. It was reasoned that PDDA was firmly adsorbed on glass surfaces via ionic interactions between the negatively charged SiOH of glass and the quaternary ammonium groups of the polymer. Indeed, it has been reported that immersion of a substrate (glass, quartz, silica wafer, gold, silver, and even Teflon) into an aqueous 1% solution of this positively charged compound results in the strong adsorption of a nanolayer (1.6 nm) of PDDA on the substrate.12 Gold nanoparticles were then able to interact strongly with the anion Cl- of the adsorbed polymer on the glass surface since anions such as Cl-, Br-, and I- strongly adsorb to gold to decorate the gold surface with excess negative charges. As a result of electrostatic attraction, a thin bilayer formed on the surface of the glass substrate. After 2-3 h into the experiment, a submonolayer of physically isolated particles was formed due to chemisorption of colloidal gold. The adhesion of gold nanoparticles (20 nm in diameter) on glass precoated with PDDA was confirmed by AFM as shown in Figure 1A. There were no aggregates of ill-defined dimensions on the surface, and most of the particles were of the same size. Such results implied that interactions between the polymer and gold nanoparticles reduced the surface mobility of the nanoparticles and effectively circumvented the spontaneous coalescence of gold particles to form aggregates. Tip convolution resulted in a distorted, enlarged view of the true particle size in the x-y plane. However, the particle heights were accurate since the average height of adhered particles was estimated to be 21 ( 4.9 nm, agreeable to the known size of the gold nanoparticles (20 nm). Similar behavior was also observed (12) (a) Kotov, N. A.; Harazsti, T.; Turi, L.; Zavala, G.; Geer, R. E.; Dekany, I.; Fendler, J. H. J. Am. Chem. Soc. 1997, 119 (29), 68216823. (b) Moriguchi, I.; Teraoka, Y.; Kagawa, S.; Fendler, J. H. Chem. Mater. 1999, 11, 1603-1608.
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Figure 1. (A) AFM analysis (phase image, 1 µm × 0.5 µm, top; line scans, bottom) displays the adhesion of 20 nm Au nanoparticles immobilized on a PDDA-modified glass surface after 2 h of incubation. (B) A closely packed colloid monolayer obtained after 18 h of immobilization in Au solution containing 10 nm Au nanoparticles (phase image, 1 µm × 0.5 µm, top; line scans, bottom).
when 5 or 10 nm diameter gold colloid was used to seed the PDDA-coated glass slide. The surface roughness of the gold film was dependent mainly on the roughness of the polymer-coated glass slide, which could be reduced by annealing. AFM images and line scans further revealed that a closely packed colloid monolayer was obtained after 18 h of immobilization using either 5, 10, or 20 nm Au nanoparticles. As an example, Figure 1B displays an atomic force micrograph for the adhesion of 10 nm gold nanoparticles on the polymer-coated glass surface. The particles retained their spherical character with an
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average height of 10.7 ( 1.8 nm, an indicator of the formation of a monolayer. AFM tip convolution prevented estimation of the particle ellipticity G (the ratio of major/ minor axes; G ) 1 for a sphere). However, none of the high aspect ratio rod-shaped particles were noted and such a result was verified by numerous images and line scans. Colloid immobilization was not a sedimentation reaction since a polymer-coated glass substrate suspended upside down in solution yielded colloidal surfaces very similar to those obtained by complete immersion. The results obtained thus proved that PDDA could be an alternative approach for conveniently seeding gold colloid on glass substrates in addition to popular silanization procedures using (3-mercaptopropyl)trimethoxysilane (MPTMS) or (3-aminopropyl)trimethoxysilane (APTMS).13-15 Furthermore, the choice of PDDA was justified by its ability to form a uniform monolayer on glass in a short period of time comparing to 12 h of soaking time with APTMS14 or 1 day of soaking time in MPTMS.15 AFM image analysis was performed to check the surface coverage and the uniformity of the monolayer of Au nanoparticles created using different incubation times and concentrations of PDDA. The optimum concentration of PDDA was 0.01 M with a soaking time of 2 h. The roughness measurements of the PDDA layer on glass performed by AFM image analysis revealed no morphological feature of PDDA/glass, and the PDDAmodified glass looked basically the same as a highly clean glass surface. The maximum measured value for the mean roughness (Ra) of the PDDA layer on glass was 1.049 nm (16 h of soaking time from a 0.1 M PDDA solution). The average mean roughness value for clean glass microscope slides was 1.031 nm. Electroless Deposition. In the plating method with formaldehyde or hydroxylamine, the reduction was known to drastically accelerate at gold surfaces since the resulting Au particles immobilized on polymer-coated glass were excellent sites for the oxidation of formaldehyde or hydroxylamine and the concurrent reduction of Au(I) to Au(0). Therefore, with a monolayer of colloidal gold on the polymer-coated glass surface, no new nucleation occurred and Au3+ ions would participate in the production of larger particles. In brief, gold plating continued on the gold particles, and the reaction with formaldehyde can be described as 2AuI + HCHO + 3OH- f 2Au0 + HCOO+ 2H2O.7,11 Hydroxylamine has been known to be oxidized to nitrite with the four-electron oxidation (NH2OH + H2O f HNO2 + 4e- + 4H+).16 Sequentially, nitrite was oxidized to nitrate with a concurrent reduction of Au(I) to Au(0). As confirmed by AFM measurements, electroless plating with formaldehyde on the surface with a submonolayer of colloid gold (5 nm) allowed the production of significantly larger gold nanoparticles at only sites where initial gold nanoparticles were already anchored. As clearly shown in Figure 2A, enlarged particles (14.5-25 nm in diameter) were neither spherical nor cyclindrical, but highly complex in shape. Agitation of the plating solution was essential for controlling the morphology of the plated gold film and significantly accelerated the rate of electroless deposition. Without agitation of the plating solution, the surface was still not closely packed after 16 h of electroless deposition (Figure 2A). In contrast, the surface was close packed with gold nanoparticles and relatively homogeneous (13) Brown, K. R.; Lyon, L. A.; Fox, A. P.; Reiss, B. D.; Natan, M. J. Chem. Mater. 2000, 12, 314-323. (14) Jin, Y.; Kang, X.; Song, Y.; Zhang, B.; Cheng, G.; Dong, S. Anal. Chem. 2001, 73, 2843-2848. (15) Cheng, W.; Dong, S.; Wang, E. Langmuir 2002, 18, 9947-9952. (16) Prince, R. C.; George, G. N. Nat. Struct. Biol. 1997, 4, 247-250.
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Figure 2. AFM analysis (height image and lines scans, 1 µm × 0.5 µm) of a gold film prepared with a submonolayer of 5 nm Au nanoparticles on a PDDA-modified glass slide after Au electroless plating in formaldehyde: (A) 16 h of electroless deposition without agitation; (B) 2 h of plating with agitation; (C) 10 min of plating with agitation in hydroxylamine.
within only 2 h into the deposition with agitation. The resulting nanoparticles, however, were appreciably smaller with the diameter ranging from 8.4 to 11 nm (Figure 2B). It was further observed that hydroxylamine-mediated reduction of Au3+ was an excellent route to enlargement of immobilized gold particles. When hydroxylamine was used instead of formaldehyde, the deposition time was drastically reduced to 12 min with a surface almost completely packed with significantly larger nanoparticles; some of them were above 55 nm (Figure 2C). It was concluded that the Au particles immobilized on polymercoated glass served as excellent sites for the current reduction of Au(I) to Au(0). The result also confirmed that hydroxylamine-mediated reduction of Au3+ mostly occurred at gold surfaces with no new particle nucleation, that is, all Au3+ participated in production of larger particles at sites where colloidal gold was present. Upon further growth, the particles coalesced and formed a coherent gold layer during the course of electroless deposition. However, there was still the presence of scattered holes in the plated film (Figure 2C), a result of incomplete coalescence of large particles due to very wide initial interparticle spacings. Preparation of Continuous Gold Films. On the basis of the above results, a series of experiments was carried out to prepare a homogeneous gold film with a controllable thickness and nanoscopic surface roughness via hydroxylamine-mediated reduction of Au3+. Accordingly, only the glass substrate with a high-coverage monolayer of colloid
gold was used for gold plating. As expected, the initial nanoparticles grew into larger particles or films and one big particle consisted of a few small particles as confirmed by atomic force micrographs with an enlarged scale (Figure 3). AFM images were performed to follow the time course of Au3+/NH2OH-mediated enlargement of immobilized Au nanoparticles (Figure 3A-D). The gold film was continuous with a close-packed arrangement of particles with different sizes. The plated gold layer was up to 68 nm in height after 8 min of electroless deposition, and the deposition rate appeared linear (Figure 4, dashed line). Interestingly, the optimal thickness for a gold film used in surface plasmon resonance (SPR) measurements is about 45-60 nm.17 During the course of plating, the surface roughness increased from 3.38 to 8.73 nm after 2 min of electroless plating (Figure 4, solid line). The surface roughness then reached a maximum value (12.29 nm) after 6 min into the experiment and then decreased to 7.11 nm. The surface obtained in this study was fundamentally different from the one prepared by vacuum deposition. As gold has been known to weakly bind to glass, the gold atoms initially accumulate as islands during vacuum deposition. Consequently, it is very difficult to prepare a gold film with a suitable thickness and surface roughness by this conventional procedure. For certain biosensing applications such as SPR biosensors, it is necessary to have gold films with about 50 nm thickness and a few spots of nanoscopic surface roughness to achieve (17) Raether, H. Surface Plasmons; Springer-Verlag: Berlin, 1988.
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Figure 3. AFM analysis (phase image, 1 µm × 1 µm) of a gold film prepared with a high-coverage monolayer of 10 nm Au nanoparticles on a PDDA-modified glass slide after Au electroless plating in hydroxylamine with stirring: (A) highcoverage monolayer of 10 nm Au nanoparticles before plating, (B) after 2 min of Au plating, (C) 6 min, and (D) 10 min.
Figure 4. Influence of the deposition time on the surface roughness (solid line) and the thickness of the plated gold layer (dashed line with a slope ) 6.88 nm/min, i.e., the average rate of deposition, R2 ) 0.981) of Au electroless deposits on a highcoverage monolayer of 10 nm Au nanoparticles. Plating was performed with agitation in hydroxylamine.
good response. Consequently, not all gold films with the same thickness prepared by vacuum deposition exhibit similar SPR responses. Therefore, the procedure described here could be a potential approach for preparing SPR-active substrates instead of conventional vacuum deposition with the aforementioned several shortcomings. In this study, the approach for the initial nanoparticles growing into larger particles or films was determined mainly by the initial particle diameter. No attempt was made to increase the amount of Au3+ since according to the Gibbs-Thomson equation, the surface free energy will decrease as the dimensions of particles increase.18 There-
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fore, the deposition rate is somewhat slow, regardless of a change of concentration during plating. We have also investigated the effect of the particle size of colloidal gold used to fabricate a high-coverage Au colloid monolayer before plating. The main factor influencing the quality of deposits was the particle distribution, not their size. Only a uniform and dense layer before plating led to the formation of smooth gold electroless deposits. In the case of a submonolayer of Au nanoparticles, particles with smaller diameter exhibited a lower roughness and a corresponding low coverage regardless of whether formaldehyde or hydroxylamine was used for lengthy electroless plating. The effects of thermal treatment on the nanostructure and mechanical properties of electrodeposited gold films were evaluated. The samples after Au electroless deposition were dried under nitrogen atmosphere and annealed at 250 °C in air atmosphere for 2 h. The roughness value only decreased about 1 nm after annealing with no indication of any cracking of the gold layer. Comparison of the structures A and B as shown in Figure 5, however, shows that the shape of the gold features changed significantly during the annealing process. The particles coalesced and became bigger (up to 65 nm) with noticeably rounded shapes compared to those of the untreated sample (31-50 nm). Adhesion Test. The so-called “Scotch tape”19 qualitative test was also performed to assess the adhesion of Au electroless deposits on the polymer-modified glass. The test was performed only on uniform gold layers (no thermal annealing) with the reflective gold color appearance and thicknesses between 50 and 60 nm. The samples were stored in air for 1 week before the adhesion test. The Scotch tape was attached to the previously cleaned and dry Au/ glass surface very firmly, and the tape was then peeled off at a constant rate, 0.5 cm‚s-1. The tape was analyzed under an optical microscope, and the surface area, which was eventually stripped off, was calculated. In the case of performing the tape test on the edge of the sample, 3-5% of the surface was peeled off at 0.5 cm‚s-1. This behavior was not completely unexpected because of a nonuniformity of the Au layer on the edges due to the concentration and diffusion effects during the course of electroless deposition. If the tape test was performed in the middle of the Au/glass slide, only less than 1% of treated surface was peeled off even when the tape was stripped off at 1 cm‚s-1. UV-Vis Spectra. In the visible range, the optical spectrum of spherical gold particles with an average size of 3.4 nm or higher is generally dominated by the plasmon band, a peak at around 520 nm caused by the excitation of surface plasmons.20 When gold nanoparticles aggregate, their surface plasmon resonance shifts to a longer wavelength, centered between 600 and 800 nm, depending on the size and shape of particles and aggregate as well as the interparticle distance within the aggregate. The intensity and position of λmax reflect the extent of aggregation, resulting from coupling of surface plasmons between closely spaced particles. Metal nanoparticles embedded in glass have also been shown to introduce thirdorder optical nonlinearities in the composite at wavelengths close to that of the characteristic surface plas(18) (a) Cramp, J. H. W.; Hillson, P. J. J. Photogr. Sci. 1976, 24, 25-28. (b) Konstantinov, I.; Malinowaki, J. J. Photogr. Sci. 1975, 23, 1-5. (19) http://www.cchem.berkeley.edu/rmgrp/mag.pdf. (20) Kreibig, U.; Vollmer, M. Optimal Properties of Metal Clusters; Springer Series in Materials Science, Vol. 25; Springer: Heidelberg, 1995.
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Figure 5. Atomic force micrographs (images, top; lines scans, bottom) of electroless gold deposits obtained with tapping-mode AFM imaging (1 µm × 1 µm). (A) Before annealing, roughness Ra ) 10.50 nm. (B) After thermal annealing, roughness Ra ) 9.7 nm.
mon resonance of the embedded metal nanoparticles.21 Therefore, a series of experiments were performed to monitor surface evolution of colloidal gold film by UV-vis spectroscopy in real time. This study confirmed that when spherical nanogold particles adhered to the glass surface coated with PDDA, the resulting spectrum of this substrate in the visible range exhibited an absorption peak at 520 nm and a shoulder at 600 nm. During the course of electroless deposition using formaldehyde, both the peak and its neighbor shoulder increased and slightly shifted to longer wavelengths (Figure 6A). The absorbances at 400 and 800 nm also increased as expected; however, the overall shape of the curve did not change noticeably. In other words, no evidence for particle aggregation was observed with UV-vis spectroscopy, in agreement with AFM measurements as described earlier. Rapid kinetics of NH2OH-mediated reduction of Au3+ was illustrated in Figure 6B. After 12 min of electroless plating, the sample had an absorbance of about 0.55 in comparison to 0.09 obtained with formaldehyde plating. The absorption band obtained with hydroxylamine plating also broadened considerably with λmax of 560 nm instead of 520 nm. X-ray Diffractometry. Samples for XRD, XPS, and contact angle analysis were prepared by immersion of cleaned and previously cut (10 × 30 mm) microscope slides in 0.01 M PDDA for 2 h followed by soaking of 10 nm Sigma Au nanoparticles for 18 h and the Au electroless (hydroxylamine-based) procedure for 12 min. The final Au deposits were uniform with a typical gold-yellow reflective surface. No thermal annealing or other postplating surface modification was performed. XPS and XRD analyses were performed within 24 and 48 h of sample preparation, respectively. A typical diffraction pattern is shown in Figure 7A. The pattern was very similar to the (21) Porstendorfer, J.; Berg, K.-J.; Berg, G. J. Quant. Spectrosc. Radiat. Transfer 1999, 63, 479-486.
result of Hou et al.10 for electroless gold plating on highindex glass and clearly showed the preferred orientation of the Au(111) at 38.34° plane parallel to the surface and a second orientation, Au(200) at 44.52°. Other orientations such as Au(220), Au(311), and Au(222) at 64.84°, 77.90°, and 81.94°, respectively, exhibited very small intensities comparing to the Au(111) peak. Furthermore, the orientation Au(222) at 81.94° resulted from the second-order diffraction of Au(111) in agreement with the work of Hou et al.10 X-ray Photoelectron Spectroscopy. Figure 7B shows survey spectra of the Au deposit confirming the presence of a significant amount of gold, carbon, oxygen, and silicon. Only trace amounts of sodium, nitrogen, and chlorine were detected (table not shown). A carbon atom bonded to hydrogen and/or another carbon atom was the dominant contributor to the XPS carbon peak, although a contribution from a weak oxidized carbon has also been observed. In the case of nitrogen, three oxidation states were observed, assigned to nitrate, nitrite, and ammonium. The trace amounts of chlorine observed on the samples were probably from the precursors used to prepare the gold particles. On the basis of the chemical structure of PDDA, a ratio of 4 for the C/N concentration was anticipated. A ratio of 16 was measured for the analyzed sample, and the discrepancy between the expected and measured carbon-to-nitrogen ratio was due to carbon contamination physisorbed onto the top layer. Contact Angle Measurements. Because the analysis was performed within 2 weeks of sample preparation, some signs of sample aging on certain areas were noticeable, which could affect the results. The samples were prepared without any postplating treatment such as thermal annealing and were stored at room temperature and in air atmosphere. However, even without any postplating procedure, the results showed contact angles between 35.6° and 40.5° ((3) confirming the formation of a self-assembled
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Figure 6. (A) Optical spectra of a gold nanoparticle monolayer on PDDA-modified glass as prepared (a) and after electroless plating with formaldehyde for 2 h (b), 4 h (c), 8 h (d), 10 h (e), and 12 h (f). (B) Optical spectra of a gold nanoparticle monolayer on PDDA-modified glass as prepared (a) and after electroless plating with hydroxylamine for 2 min (b), 4 min (c), 6 min (d), 8 min (e), and 10 min (f).
monolayer (SAM), which was in agreement with the work of Hou et al.10 This value was slight smaller than the value measured on a SAM on evaporated gold (47°). Without any postplating treatment, contact angles of hexadecylthiol as reported by Hou et al.10 were smaller than 10°. Our new strategy of gold electroless deposition, avoiding the use of a titanium underlying layer and the presence of surface sensing (SnCl2) followed by Ag deposition, led to the formation of clean and organized Au surfaces comparable to evaporated Au layers.2 Cyclic Voltammetry. On the basis of the atomic force micrographs, it was reasonable that the Au-modified glass could serve as a working electrode in cyclic voltammetric experiments. Therefore, CV could be conducted to electrochemically examine the growth and progress of gold deposition using the Au-modified glass slide as a working electrode. A titanium wire (0.5 mm diameter, Alfa Aesar, Ward Hill, MA) was carefully attached to the Au/glass slide with a plastic clamp to establish the electrical connection between Au/glass and the potentiostat. Only the tip of the Ti wire was exposed to the electrolyte. The remaining surface was electrically insulated using liquid electrical tape (Star Brite, Ft. Lauderdale, FL). Note that pure Ti is passive and electrochemically inactive over a very large potential region. Figure 8, curve a, shows cyclic voltammetry of Ti wire attached on a 5 nm Au nanoparticle/PDDA-modified glass slide. A submonolayer of nano-
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Figure 7. (A) XRD pattern of electroless Au prepared by hydroxylamine-mediated reduction on a uniform monolayer of 10 nm Au nanoparticles. The XRD pattern reveals the dominant Au(111) orientation at 38.34°. Other orientations are Au(200), Au(220), Au(311), and Au(222) at 44.52°, 64.84°, 77.90°, and 81.94°, respectively, and exhibited much smaller relative intensities compared to Au(111). (B) XPS survey spectra of the Au electroless deposit. Significant amounts of gold and oxygen were detected. Silica, carbon, and sodium were also observed.
gold particles was formed by 2 h immersion of a PDDA/ glass slide in the nanogold solution. In this case, CV obtained did not show any typical Au features. Only the region of Ti passivation followed by oxygen evolution reaction (starting at +1.5 V vs Ag/AgCl) was observed. Subsequently, the Ti wire was disconnected and the electroless deposition of Au was performed using hydroxylamine solution for 5 min. After thorough rinsing, the Ti wire was reconnected to the Au/glass slide with very cautious manipulation so that Ti was always attached to the glass at the center of the slide. As shown in Figure 8 (curve b), the gold layer was already established after only 5 min of electroless Au deposition and CV displayed typical Au characteristics (a peak of oxide formation during the anodic scan and its reduction during the cathodic scan). The high double layer charging current, broad and nonwell-defined peaks indicated the breakdowns and nonuniformity of the gold layer after only 5 min of electroless deposition (curve b). Curve c shows further growth of the Au layer after 10 min of Au electroless deposition with sharp and well-distinguished Au redox peaks. Curves d and e show the cyclic voltammograms after 20 and 30
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Langmuir, Vol. 19, No. 9, 2003 3965
Conclusions
Figure 8. Electrochemical monitoring of the formation and growth of the Au layer during gold electroless deposition. Cyclic voltammetry of a Ti/Au electrode in 1 M H2SO4 with a scan rate of 100 mV‚s-1. (a) Ti attached to a 5 nm Au/PDDA-modified glass slide. (b) After 5 min of Au electroless deposition with hydroxylamine on a 5 nm Au/PDDA-modified glass slide, (c) 10 min, (d) 20 min, and (e) 30 min.
min, respectively, with increases of peak heights as expected to confirm the enlargement of the Au layer.
The paper presents a simple, inexpensive, and highly reproducible method to fabricate a homogeneous gold film, based on the self-assembly of a gold colloid layer on a glass surface precoated with a thin layer of poly(diallyldimethylammonium chloride), followed by electroless gold plating. Compared to formaldehyde-mediated reduction, the kinetics of hydroxylamine reduction of adsorbed Au3+ is very rapid and the nanoscale morphology is critically dependent on the initial coverage of gold nanoparticles. The prepared gold films are very stable and continuous and can be used as electrodes in electrochemical experiments or as SPR substrates. Accordingly, electroless plating of high-coverage Au colloid monolayers can be an alternative to conventional vacuum deposition for preparation of thin gold films. This method should also be applicable to the electroless deposition of other metals such as copper, nickel, and platinum. Work is in progress toward this objective. Acknowledgment. The authors thank Dr. Pierre Desjardin of the Steacie Institute of Molecular Sciences, National Research Council Canada, for performing the contact angle measurements. LA0269199
