Fabrication of Stable, Highly Flat Gold Film Electrodes with an

Anal. Chem. 2007, 79, 6851-6856

Fabrication of Stable, Highly Flat Gold Film Electrodes with an Effective Thickness on the Order of 10 nm Shin-ya Kishioka,*,† Junichi Nishino,†,‡ and Hiroshi Sakaguchi§,|

Department of Chemistry, Faculty of Engineering, Nagaoka University of Technology, Kamitomioka, Nagaoka, Niigata 940-2188, Japan, Research Institute of Electronics, Shizuoka University, Johoku, Hamamatsu, Shizuoka 432-8011, Japan, and Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan

Stable gold film electrodes were fabricated by a combination of sputtered gold and an epoxy adhesion layer on a glass substrate using the template-stripped technique. An XRR scan analysis indicated the thickness and roughness of the gold layer to be on the order of 10 nm and subnanometer, respectively. In spite of their small thickness, the electrode had sufficient conductivity and stuck to the subject during the electrochemical measurements as did the usual gold working electrodes. Thin noble metal films, especially gold and platinum prepared by various techniques, such as sputtering, vapor deposition, and electroless plating, have been widely employed as working electrodes in electrochemistry.1 Gold films are quite useful not only in electrochemical measurements but also in different fields, e.g., surface plasmon resonance,2 substrates for self-assembled monolayer,3 and so on. Since these films are directly fabricated on glass and SiO2 substrates and show less adhesion to the substrate, it is necessary to first make underlayers of transitionmetal (Cr, W, Ti) or silane compounds in order to enhance the adhesion to the substrate.4,5 A Cr underlayer for gold films could enhance the stability to various treatments such as cleaning in strongly oxidizing solutions, in ultrasonic baths, and potential cycling during voltammetric experiments if there was no pinhole defect. However, it seems difficult to make thin metal films without any defects. Underlayers using Cr and other metals indicate another problem of interdiffusion to the noble metal surface.6-9 * Corresponding author. E-mail: [email protected]. † Nagaoka University of Technology. ‡ Present address: Department of Chemistry and Biology Engineering, Fukui National College of Technology, Sabae, Fukui 916-8507, Japan. § Shizuoka University. | Precursory Research for Embryonic Science and Technology (PRESTO). (1) Anderson, J. L.; Winograd, N. Laboratory Techniques in Electroanalytical Chemistry; Marcel Dekker, Inc.: New York, 1996. (2) Hanken, D. G.; Jordan, C. E.; Frey, B. L.; Corn, R. M. Electroanal. Chem. 1998, 20, 141-225. (3) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (4) Goss, C. A.; Charych, D. H.; Majda, M. Anal. Chem. 1991, 63, 85-88. (5) Wanunu, M.; Vaskevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2004, 126, 5569-5576. (6) Josowicz, M.; Janata, J.; Levy, M. J. Electrochem. Soc. 1988, 135, 112115. (7) Tisone, T. C.; Drovek, J. J. Vac. Sci. Technol. 1972, 9, 271-275. 10.1021/ac070603u CCC: $37.00 Published on Web 08/04/2007

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This phenomenon is known to change the surface property of the metal electrodes.10 In order to avoid the formation of an alloy, the thickness of the noble metals needs to be least ∼200 nm.4 Such an electrode shows no optical transparency in the visible region. The properties required of an ideal metal film electrode for electroanalytical chemistry are as follows: (1) flatness, (2) small thickness to transmit visible light, and (3) high stability to various treatments and sufficient conductivity for electrochemical measurements. To resolve these difficulties, we used the templatestripped (TS) method.11,12 This technique was proposed and demonstrated for the first time by Semenza and co-workers to establish an ultraflat surface for scanning tunneling microscope (STM) measurements in the biological field. Subsequently, it has become a popular technique to make a flat surface for an experimental scanning probe microscopy technique;13-19 however, it has been rarely employed in preparing working electrodes for electrochemistry. The TS method in this study contains optically transferred epoxy adhesion layers instead of transition-metal or silane compound underlayers. Specular X-ray reflectometry (XRR) and UV/vis spectrometry showed that thin gold film electrodes prepared by the TS technique were quite flat and had a good optical transparency and a purely metallic surface property.

(8) Ashwell, G. W. B.; Heckingbottom, R. J. Electrochem. Soc. 1981, 128, 649654. (9) Holloway, P. H. Gold Bull. 1978, 12, 99-106. (10) Cohen, R.; Janata, J. J. Electroanal. Chem. 1983, 151, 33-39. (11) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 39-46. (12) Wagner, P.; Hegner, M.; Gu ¨ ntherodt, H.-J.; Semenza, G. Langmuir 1995, 11, 3867-3875. (13) He, L.; Robertson, J. W. F.; Ka¨rcher, J. L. I.; Schiller, S. M.; Knoll, W.; Naumann, R. Langmuir 2005, 21, 11666-11672. (14) Brayshaw, D. J.; Berry, M.; McMaster, T. J. Nanotechnology 2004, 15, 1391-1396. (15) Ragan, R.; Ohlberg, D.; Blackstock, J. J.; Kim, S.; Williams, R. S. J. Phys. Chem. B 2004, 108, 20187-20192. (16) Blackstock, J. J.; Li, Z.; Freeman, M. R.; Stewart, D. R. Surf. Sci. 2003, 546, 87-96. (17) Naumann, R.; Schiller, S. M.; Giess, F.; Grohe, B.; Hartman, K. B.; Ka¨rcher, I.; Ko ¨per, I.; Lu ¨ bben, J.; Vasilerv, K.; Knoll, W. Langmuir 2003, 19, 54355443. (18) Unal, K.; Aronsson, B.-O.; Mugnier, Y.; Descouts, P. Surf. Interface Anal. 2002, 34, 490-493. (19) Ederth, T. Phys. Rev. A 2000, 62, 062104.

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EXPERIMENTAL SECTION Materials and Chemicals. Gold was sputtered on Si(100) wafer pieces (17 mm × 23 mm, Nilaco Co., Ltd.), which were first cleaned in piranha solution (a 1:3 by volume mixture of 30% hydrogen peroxide and concentrated sulfuric acid) for 15 min, to remove any organic contamination. (Caution: piranha solution is a powerful oxidizing agent and reacts violently with organic compounds. It should be handled with extreme care.) Gold on Si was glued face down using epoxy glue (Epo-tek 301-2, Epoxy Technologies) on a glass coverslip (16 mm × 22 mm, thickness 0.5 mm, Matsunami Glass) or quartz substrate (10 nm × 10 mm, thickness 0.5 mm). The reagents used for the electrochemical measurements were H2SO4 (Wako Pure Chemical), K4Fe(CN)6‚ 3H2O (Wako Pure Chemical), and KCl (Wako Pure Chemical). All chemicals were reagent grade and employed without further purification, and all solutions were prepared with Milli-Q water. Apparatus and Procedures. The thin gold film was prepared by ion sputtering at 1.3 × 10-1 Pa under flowing Ar (VPS-020, ULVAC ). The XRR system is a Mac Science XRD system (M03XHF22) operating with a Cu KR X-ray source and a goniometer with a 0.0004° resolution. The XRR scans from the gold/ epoxy/glass (or quartz) samples were analyzed using the Rigaku GXRR software package. The optical absorption spectrum was measured using a spectrophotometer (Ocean Optics, USB2000). The STM measurements were carried under the currentconstant mode using a commercial-based instrument (Molecular Imaging, PicoSPM) in air at room temperature. All STM images were obtained at a tip bias of -0.2 V with a constant current of 5 pA. An electrochemically etched Pt-Ir (90:10) wire was used as the tip. The resistivity of gold/epoxy/glass substrates was measured with a four-point probe technique at ambient temperature by means of a microvolt ammeter (Keithley model 2000 multimeter). The electrochemical measurements were performed using the conventional three-electrode cell configuration. A Pt wire and an Ag/AgCl (3 mol dm-3 (M) KCl) electrode were the auxiliary and reference electrodes, respectively. The working gold film and polished polycrystalline Au disk (1.6-mm diameter, BAS) electrodes were sonicated in deionized water for 15 min prior to each measurement. The exposed film electrode area using an O-ring was 0.502 cm2. The voltammetric curves were measured using a function generator (HB-111, Hokuto Denko Co., Ltd.) and a potentiostat (HA-502, Hokuto Denko) with an IR compensation unit (HR-203, Hokuto Denko), which generated a small alternative current signal (1 kHz, 35 mV), and it was superposed upon the output of the function generator. The results were recorded on an X-Y recorder (D72BP, Riken Denshi Co., Ltd.). Impedance measurements were performed by applying a 10-mV peak-to-peak ac signal using a frequency response analyzer (S-5720C, NF Corp.) and a potentiostat (HA-501G, Hokuto Denko), which were both controlled by a personal computer (PC-9801, NEC). The dc bias was set at the formal potential. The ac modulation frequency range from 0.1 to (20) ISO 2409, Paints and varnishes -Cross-cut test, 1991. (21) Bontempi, E. Recent Res. Dev. Chem. Phys. 2004, 5, 461-488. (22) Kiessig, H. Ann. Phys. 1931, 10, 769-788.

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Figure 1. Preparation of gold/epoxy/glass electrode.

50 000 Hz was used. All experiments were carried out at room temperature (25 ( 1 °C). RESULTS AND DISCUSSION Fabrication and Characterization of Gold Electrodes. After the gold/Si was stuck to the glass substrate using an epoxy glue (5 µL cm-2), it was cured by heating on a hot plate at 80 °C for 3 h. The Si/gold/epoxy/glass substrates were soaked in THF and then the Si plates were stripped (Figure 1). The resulting gold/ epoxy/glass substrates were subjected to sonication in hot water for 15 min; however, any stripping of gold from epoxy/glass was not found. A semiquantitative, the so-called cross-cut test, was used to examine the adhesion of the gold films to the epoxy layers, which is a modified and simplified version of ISO2409.20 After a right-angled lattice pattern was cut into the surface of the gold films (1-mm interval in 1-cm2 area), a piece of adhesive tape (cellophane tape, Nichiban) was firmly attached over the gold films and then removed. We observed that only the tape was removed and no transfer to the tape had occurred. This phenomenon indicates that the gold films and epoxy layers are firmly bonded. XRR is a nondestructive method that provides thickness, density, and interface information from fairly complex film stacks irrespective of their physical state, whether that be amorphous, polycrystalline, or single crystal.21 This technique was used to analyze the gold/epoxy hybrid materials. In Figure 2, the solid line shows a typical XRR scan obtained from a gold film attached on epoxy/glass. The normal output of an XRR experiment exhibits a regime of nominal total external reflection when the specular angle is less than the critical angle, θc:

θc ) x2δ ) xλ2roF/π

(1)

where δ is the real part of the X-ray refractive index expressed in terms of the classical electron radius ro, the X-ray wavelength λ, and the electron density F (electrons nm-3). Above the critical angle, the intensity decreases with an inverse fourth-power

Figure 2. Specular XRR scan from gold layer glued on glass using epoxy. Sputtering time of gold was 2 min. The simulation is denoted by the circles.

Figure 3. Transmission UV/vis spectra of gold/epoxy/quartz substrates. Sputtering times of gold films were (a) 5, (b) 4, (c) 3, and (d) 2 min.

Table 1. Thickness, Density, and Roughness of Gold Film Glued on Glass Using Epoxy. sputtering time/min

thickness/nm

density/g cm-3

roughness/Å

2 5

12.92 33.00

19.3 19.3

2.80 5.00

dependence on the scattering angle. Simultaneously, the intensity of X-rays reflected by thin film or layer stack varies periodically in response to changes of either the angle of incidence or the X-ray wavelength. These periodic variations arise from the interference of radiation reflected by each of the interlayer interfaces (Kiessig fringe).22 At first, it is possible to calculate the density of gold from the analysis of θc in Figure 2. Next, the thickness of the gold film can be determined from the amplitude of oscillation, Kiessig fringe, in Figure 2, whose decay rate infers the small surface roughness. Subsequently, a model structure consisting of (a) an epoxy substrate and (b) the gold metal was set up as the initially assumed structure for the calculation of a trial reflectometry curve using the X-ray reflectometry theory. The best-fit calculated XRR profile was indicated in Figure 2 by the circles. Table 1 summarizes the thickness, density, and roughness of the thick and thin gold films surface on epoxy/glass. The density value agrees with that of gold (19.3 g cm-3), indicating that the surface layer is composed of gold and the roughness of the gold surface is quite low, almost on a subnanometer order. The transmission UV/vis spectra of the gold/epoxy/quartz substrates are shown in Figure 3. Spectral transmissions of the epoxy layer used in this study are 94% at 320 nm and 99% at 4001200 nm.23 As a blank sample, both epoxy/quartz and quartz alone were used; however, there were no difference at 300-800 nm. Therefore, quartz plate was exclusively employed as a background. Figure 4 shows the relation between the absorbance of the gold film layers at 520 nm and sputtering time, which was superimposed on the thickness of the gold film obtained from the XRR results. This plot indicates that the absorbance linearly depends on the sputtering time; therefore, the thickness of the gold films can be estimated from the absorbance data. In order to obtain the information about the surface morphology, an STM measurement was carried out. Figure 5 compares

the upper surface and vertical sectional view of the gold/epoxy/ glass in which the gold films were deposited at different sputtering times. Particles of relatively homogeneous size were observed in Figure 5A and B, whereas apparent domains were identified in the images of the thinner film, Figure 5C and D, though a flat area was dominant. If the adhesion layer of Cr was used between the thin gold film and the glass substrate, and the electrolyte was immersed in the domain of the gold films, then the anodic dissolution of Cr must occur during the electrochemical measurements. However, the epoxy adhesion layer is completely inert to potential scanning in the electrolyte and thus can maintain a strong adhesion to the gold. We were unfortunately unable to construct a gold/epoxy/glass electrode using a thinner gold film, which was deposited by a sputtering time less than 90 s. It was difficult to remove the Si from the gold/epoxy/glass, presumably because of the defective domain filled by the epoxy glue, which directly bound the Si to the glass after the curing process. Electrical and Electrochemical Properties of Gold Electrodes. The resistivities of thick and thin gold/epoxy/glass surface at room temperature were (4.1 ( 0.5) × 10-6 Ω cm (sputtering time 5 min) and (7.0 ( 0.5) × 10-6 Ω cm (sputtering time 2 min). These values were greater than that of bulk Au (2.4 × 10-6 Ω cm) and agreed closely with those reported.24 The thinner gold/epoxy/glass had sufficient electronic conductivity though there are defective domains on its surface shown in STM

(23) EPO-TEK 301-2 Technical Data Sheet, Epoxy Technology, Inc., 2005, Rev. VIII.

(24) Chopra, K. L; Bobb, L. C.; Francombe, M. H. J. Appl. Phys. 1963, 34, 16991702.

Figure 4. Relationship between absorbance at 520 nm and sputtering times of gold/epoxy/glass substrates. Thickness of gold films were obtained from XRR analysis.

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Figure 5. Typical STM images and vertical sectional views of the gold/epoxy/glass surfaces. Sputtering time of gold were (A,B) 5 and (C,D) 2 min.

image of Figure 5C. This small resistivity allows us to carry out electrochemical measurements. Figure 6 is a typical cyclic voltammetric curve of the gold/epoxy/glass in a 0.05 M H2SO4 aqueous solution. The CV shape shows a peak at ∼1.0 V, which is representative of the surface oxide formation on Au in a H2SO4 6854

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solution.25 Even after repeated potential cycling over 5 h, the CV trace did not change its response for the thick Au film electrode for which the sputtering time was more than 5 min. For a thinner (25) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429-453.

Figure 6. Cyclic voltammogram of gold/epoxy/glass electrode in 0.05 M H2SO4 aqueous solution. Sputtering time of gold was 2 min. The sweep rate was 0.1 V s-1.

Figure 8. Complex-plane plots of the faradaic impedance for K4Fe(CN)6 at the formal potential (E°′ ) 255 mV vs Ag/AgCl). The plots were obtained in 0.5 M KCl aqueous solution at Au polycrystalline disk (O) and gold/epoxy/ glass (b) electrodes. RΩ values at 50 kHz were 1.1 (O) and 6.2 (b) Ω cm2. The double layer differential capacitance values were 5.9 × 10-5 (O) and 2.4 × 10-5 (b) F cm-2.

resistance, RΩ, which is the intercept of the real axis in complexplane plot, consists of Rm and Rs in series with the electrode impedance, Zel.26

RΩ ) R m + R s

Figure 7. Cyclic voltammograms for 5 mM K4Fe(CN)6 in 0.5 M KCl aqueous solution at gold/epoxy/glass electrodes. Sputtering times of gold films were (a solid) 5, (b dot) 4, (c short dash) 3, and (d dash and solid, e dash) 2 min. Curve e was recorded in the absence of i-R compensation. The sweep rate was 0.2 V s-1.

film electrode, the CV response was kept during several tens of minutes. Next, the effects of the gold/epoxy/glass electrodes on the electrode reaction for the reversible redox couple were investigated. Figure 7 shows the CV response of the gold/epoxy/ glass electrodes which have a different gold film thickness. The voltammograms were obtained in a 0.5 M KCl aqueous solution containing 5 mM K4Fe(CN)6 and i-R drop was compensated (ad). Each voltammogram shows almost the same traces and potential peak separation, ∼56 mV, which is an index of the reversible electrode reaction irrespective of the thickness of the gold film layers. The effect of i-R compensation on the potential application was indicated in Figure 7e. Potential shift due to an uncompensated i-R drop was only 45 mV even in thinner gold film electrode case and gold/epoxy/glass electrodes can be an effective working electrode for electrode reaction. The electric current conducted through the metal of the working electrode by transport of electrons gives rise to the metal resistance, Rm. This component is added to the solution resistance, Rs. In order to separate Rm of gold films and Rs, electrochemical impedance measurements were carried out. Figure 8 shows complex-plane plots of the faradic impedance for 10 mM K4Fe(CN)6 at the formal potential (E°′). In this case, we determined E°′ as the midpoint of the anodic and cathodic peak potentials for cyclic voltammograms in Figure 7 on the assumption that the redox couple has a common diffusion coefficient. The plots were obtained in 0.5 M KCl aqueous solution at gold/epoxy/glass (b) and Au polycrystalline disk (O) electrodes as a reference. Ohmic

(2)

The RΩ value for Au polycrystalline disk electrode was 1.1 Ω cm2. If the Rm value of bulk Au could be negligibly small compared with that of Au film, the Rs value is close to 1.1 Ω cm2. On the other hand, the RΩ value for gold/epoxy/glass (sputtering time 2 min) was 6.2 Ω cm2 and the resulting Rm value was 5.1 Ω cm2. These results indicate that the Au film is more resistive than the bulk Au; however, the resistivity is not so critical for an electrontransfer reaction crossing a solid/solution interface. The assumption of Randles’ equivalent circuit enables us to obtain the kinetic parameter concerning the heterogeneous electron-transfer process for K4Fe(CN)6.27 Each plot of Figure 8 in the low-frequency region can be taken to have a slope of 45°, which indicates that the process is in the semi-infinite linear diffusion-controlled regime. The charge-transfer resistances, Rct, evaluated from the diameter of the semicircle were 2.68 (O) and 4.97 (b) Ω cm2, which corresponded to 9.8 × 10-3 and 5.3 × 10-3 cm s-1 as the apparent formal rate constants k°′. Though these values are less than 2 orders of magnitude compared with those at treated Pt electrodes ever reported,28 the difference of the values obtained at Au bulk and film electrodes substantiates the results of electrical resistivity and Rm measurements for gold/epoxy/glass. In conclusion, gold/epoxy/glass electrodes prepared by the TS technique provide (1) a very flat surface whose roughness is less than 1 nm, (2) a very thin gold film, ∼10 nm, which allows a sufficient optical transparency in the visible region, and (3) a high stability and a sufficient electronic conductivity to electrochemical measurements. The TS technique is applicable to any conducting or semiconducting material such as platinum15,16 and metal oxides,18 which will play important roles in electrochemistry and related analytical fields. (26) Sluyters-Rehbach, M. Pure Appl. Chem. 1994, 66, 1831-1891. (27) Randles, J. E. B. Discuss. Faraday Soc. 1947, 1, 11-19. (28) Huang, W.; McCreery, R. J. Electroanal. Chem. 1992, 326, 1-12.

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Application of this method to optically transparent electrodes in spectroelectrochemistry, a surface plasmon resonance sensor that works without the Cr layer,17,29 and an electrochemical QCM apparatus is currently underway in our laboratory.

Kato from NUT for heating apparatus. This research was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science.

ACKNOWLEDGMENT The authors thank Professor Hidetoshi Saitoh from NUT for the XRR measurements and Professors Keizo Uematsu and Zenji

Received for review March 27, 2007. Accepted June 24, 2007.

(29) Roy, D. Appl. Spectrosc. 2001, 55, 1046-1052.

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