Compositional and Electrochemical Characterization of Noble Metal

May 10, 2007 - microscopy, atomic force microscopy, Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy,. Raman spectroscopy, and...
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Langmuir 2007, 23, 6812-6818

Compositional and Electrochemical Characterization of Noble Metal-Diamondlike Carbon Nanocomposite Thin Films Nicola Menegazzo,† Chunming Jin,‡ Roger J. Narayan,‡ and Boris Mizaikoff*,† School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, and Department of Biomedical Engineering, UniVersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7575 ReceiVed September 3, 2006. In Final Form: January 30, 2007 A detailed characterization of platinum- and gold-diamondlike carbon (DLC) nanocomposite films deposited onto silicon substrates is presented. A modified pulsed laser deposition (PLD) technique was used to incorporate noble metal nanoclusters into hydrogen-free DLC films. Several analytical techniques, including transmission electron microscopy, atomic force microscopy, Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and nanoindentation testing, were used to investigate these thin films in an effort to determine their physical and electrochemical properties. Rutherford backscattering spectroscopy indicated that the gold- and platinum-DLC films contain metal concentrations between three and 36 atomic percent. Cross-sectional transmission electron microscopy revealed that metal is present as arrays of noble metal islands embedded within the DLC matrix, while atomic force microscopy provided evidence of target splashing. In addition, due to the inclusion of metal, metal-DLC thin films exhibited greater conductivity than their metal-free counterparts. The electrochemical properties were studied using quasi-reversible redox couples and correlated to the metal concentration. Finally, the influence of the layer’s composition on the electron-transfer kinetics and the achievable working potential window is discussed. The results discussed herein suggest that metal-DLC thin films grown by pulsed laser deposition present a promising alternative electrode material for electrochemistry.

1. Introduction Carbon is a unique element that is able to form linear, trigonal, or tetrahedral bond coordination.1 Diamond and graphite are the principal crystalline allotropes of carbon. In diamond, each carbon atom is covalently bonded to four neighboring carbon atoms, which is referred to as sp3 hybridization. Diamond is the hardest known material (80-104 GPa), and it exhibits the largest bulk modulus of any solid. Diamond also demonstrates excellent friction and wear properties; the dangling carbon bonds at the surface of diamond may form hydrocarbon or graphite lubrication films. In addition, a large optical band gap provides transparency from the ultraviolet to the infrared ranges. Diamond is chemically inert, and is essentially nonreactive in oxidative environments up to temperatures of 800 °C. However, synthetic polycrystalline diamond films grown by chemical vapor deposition (CVD) from gaseous hydrocarbons require high substrate temperatures (∼800 °C), precluding growth on temperature-sensitive materials, such as polymers and optical elements. Films grown at lower temperatures contain hydrogen and graphite impurities. Furthermore, considerable thermal expansion mismatch between diamond and most substrate materials leads to poor adhesion.2-7 Carbon is also existent in an amorphous form, which contains a mixture of sp2- and sp3-hybridized carbon atoms exhibiting * Corresponding author: [email protected]. † Georgia Institute of Technology. ‡ University of North Carolina at Chapel Hill. (1) Narayan, R. J. Int. Mater. ReV. 2006, 51 (1), 1-17. (2) Michler, J.; Mermoux, M.; von Kaenel, Y.; Haouni, A.; Lucazeau, G.; Blank, E. Thin Solid Films 1999, 357 (2), 189-201. (3) Diniz, A. V.; Ferreira, N. G.; Corat, E. J.; Trava-Airoldi, V. J. Mater. Res. (Brazil) 2003, 6 (1), 57-61. (4) Peng, X. L.; Tsui, Y. C.; Clyne, T. W. Diamond Relat. Mater. 1997, 6 (11), 1612-1621. (5) Chalker, P. R.; Jones, A. M.; Johnston, C.; Buckley-Golder, I. M., Surf. Coat. Technol. 1991, 47 (1-3), 365-74. (6) Guo, H.; Alam, M. Thin Solid Films 1992, 212 (1-2), 173-9. (7) Schwarzbach, D.; Haubner, R.; Lux, B. Diamond Relat. Mater. 1994, 3 (4-6), 757-64.

properties intermediate between those of graphite and diamond.8-12 Such materials are denominated “diamondlike,” indicating that some material properties are similar to those of diamond. One approach to depositing diamondlike carbon (DLC) films involves the ablation of a carbon source with laser pulses. Pulsed laser deposition (PLD) of DLC is a straightforward process performed at temperatures ranging 20-75 °C.13 The carbon source utilized will dictate the type of DLC layer obtained: hydrogen-free carbon sources (e.g., high purity graphite) provide hydrogen-free DLC films, whereas hydrocarbon sources provide DLC films with significant hydrogen and/or hydrocarbon incorporation. The reported growth rate of DLC films deposited using a krypton fluoride (λ ) 248 nm) excimer laser are on the order of 0.01 nm/ pulse.13-15 Similar to diamond, DLC is chemically inert below 800 °C. For example, Srividya et al.16 deposited DLC thin films onto aluminum and stainless steel substrates, and demonstrated low corrosion rates in sodium chloride solutions. Janotta et al. utilized 30-200 nm thick DLC films deposited directly onto zinc selenide infrared attenuated total reflectance (IR-ATR) waveguides, and demonstrated real-time quantitative monitoring of highly oxidative hydrogen peroxide, peracetic acid, and peroxydisulfuric acid (8) Sharma, A. K.; Narayan, R. J.; Narayan, J.; Jagannadham, K. Mater. Sci. Eng., B 2000, 77 (2), 139-143. (9) Narayan, R. J. Appl. Surf. Sci. 2005, 245 (1-4), 420-430. (10) Wei, Q.; Sankar, J.; Sharma, A. K.; Oktyabrsky, S.; Narayan, J.; Narayan, R. J. J. Mater. Res. 2000, 15 (3), 633-641. (11) Wei, Q.; Narayan, R. J.; Sharma, A. K.; Sankar, J.; Narayan, J. J. Vac. Sci. Technol., A 1999, 17 (6), 3406-3414. (12) Wei, Q.; Narayan, R. J.; Narayan, J.; Sankar, J.; Sharma, A. K. Mater. Sci. Eng., B 1998, ;53 (3), 262-266. (13) Voevodin, A. A.; Donley, M. S. Surf. Coat. Technol. 1996, 82 (3), 199213. (14) Voevodin, A. A.; Rebholz, C.; Matthews, A. Tribol. Trans. 1995, 38 (4), 829-36. (15) Voevodin, A. A.; Phelps, A. W.; Zabinski, J. S.; Donley, M. S. Diamond Relat. Mater. 1996, 5 (11), 1264-1269. (16) Srividya, C.; Sunkara, M.; Babu, S. V. J. Mater. Eng. Perform. 1997, 6 (5), 586-590.

10.1021/la062582p CCC: $37.00 © 2007 American Chemical Society Published on Web 05/10/2007

Carbon Nanocomposite Thin Films

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Figure 1. (a) Schematic of pulsed laser deposition system. (b) Schematic of multicomponent rotating target used in pulsed laser deposition of metal-diamondlike carbon nanocomposite films.

solutions using IR-ATR spectroscopy without degradation of the waveguide substrate due to DLC protection.17,18 In addition, DLC films have demonstrated more than 1 order of magnitude lower intrinsic stresses than polycrystalline diamond films, suggesting that DLC-based electrodes may provide greater stability under harsh electrochemical conditions.19,20 Several researchers have shown the incorporation of modifying elements within diamondlike carbon films using a conventional pulsed laser deposition system. For example, dopant element targets and graphite targets can be co-ablated during the pulsed laser deposition process. Several modifying elements, including gold, silicon, oxygen, tungsten, iron, platinum, niobium, fluorine, nitrogen, titanium, and molybdenum, have been incorporated within DLC thin films.21-26 The electrochemical properties of modified DLC thin films have been examined by several investigators.27-34 Hydrogenated DLC films displayed working potential windows comparable to those obtainable at boron(17) Janotta, M.; Rudolph, D.; Kueng, A.; Kranz, C.; Voraberger, H.-S.; Waldhauser, W.; Mizaikoff, B. Langmuir 2004, 20 (20), 8634-8640. (18) Janotta, M.; Vogt, F.; Voraberger, H.-S.; Waldhauser, W.; Lackner Jurgen, M.; Stotter, C.; Beutl, M.; Mizaikoff, B. Anal. Chem. 2004, 76 (2), 384-91. (19) Kumar, S.; Sarangi, D.; Dixit, P. N.; Panwar, O. S.; Bhattacharyya, R. Thin Solid Films 1999, 346 (1,2), 130-137. (20) Ferrari, A. C.; Kleinsorge, B.; Morrison, N. A.; Hart, A.; Stolojan, V.; Robertson, J. J. Appl. Phys. 1999, 85 (10), 7191-7197. (21) Schiffmann, K. I.; Fryda, M.; Goerigk, G.; Lauer, R.; Hinze, P.; Bulack, A. Thin Solid Films 1999, 347 (1,2), 60-71. (22) Amir, O.; Kalish, R., J. Appl. Phys. 1991, 70 (9), 4958-62. (23) Donnet, C.; Fontaine, J.; Grill, A.; Patel, V.; Jahnes, C.; Belin, M. Surf. Coat. Technol. 1997, 94-95 (1-3), 531-536. (24) Corbella, C.; Oncins, G.; Gomez, M. A.; Polo, M. C.; Pascual, E.; GarciaCespedes, J.; Andujar, J. L.; Bertran, E. Diamond Relat. Mater. 2005, 14 (3-7), 1103-1107. (25) Grill, A.; Patel, V.; Jahnes, C., J. Electrochem. Soc. 1998, 145 (5), 16491653. (26) Grischke, M.; Hieke, A.; Morgenweck, F.; Dimigen, H. Diamond Relat. Mater. 1998, 7 (2-5), 454-458. (27) Pleskov, Y. V.; Evstefeeva, Y. E.; Baranov, A. M. Russ. J. Electrochem. 2001, 37 (6), 644-646. (28) Pleskov, Y. V.; Evstefeeva, Y. E.; Baranov, A. M. Diamond Relat. Mater. 2002, 11 (8), 1518-1522. (29) Pleskov, Y. V.; Krotova, M. D.; Polyakov, V. I.; Khomich, A. V.; Rukovishnikov, A. I.; Druz, B. L.; Zaritskii, I. M. Russ. J. Electrochem. 2000, 36 (9), 1008-1013. (30) Pleskov, Y. V.; Krotova, M. D.; Polyakov, V. I.; Khomich, A. V.; Rukovishnikov, A. I.; Druz, B. L.; Zaritskiy, I. J. Electroanal. Chem. 2002, 519 (1-2), 60-64. (31) Moon, J.-M.; Park, S.; Lee, Y.-K.; Sook Bang, G.; Hong, Y.-K.; Park, C.; Cheol Jeon, I. J. Electroanal. Chem. 1999, 464 (2), 230-237. (32) Lagrini, A.; Deslouis, C.; Cachet, H.; Benlahsen, M.; Charvet, S. Electrochem. Commun. 2004, 6 (3), 245-248. (33) Adamopoulos, G.; Godet, C.; Deslouis, C.; Cachet, H.; Lagrini, A.; Saidani, B., Diamond Relat. Mater. 2003, 12 (3-7), 613-617. (34) Cachet, H.; Deslouis, C.; Chouiki, M.; Saidani, B.; Conway, N.; Godet, C. Effect of Nitrogen Concentration on the Electrochemical Behavior of Amorphous Hydrogenated Carbon a-C:H:N Films. Proc. Electrochem. Soc. 2004, 38-47.

doped diamond (BDD) films.33 However, the peak separation of common redox mediators (e.g., Fe(CN)63-/4-) measured with hydrogenated nitrogen-doped or platinum-DLC thin films demonstrated hindered heterogeneous electron transfers.27,30,33 In contrast, an almost ideal electrochemical behavior, with peak separations of 70 mV and below, have been reported for hydrogenfree nitrogen-doped DLC films.32,35 Narayan et al. developed a variation of the conventional pulsed laser deposition process for incorporating modifying elements into DLC films.36-38 Briefly, a single multicomponent target is loaded into the pulsed laser deposition chamber. This target contains pure graphite, which is covered by a segment of the desired modifying element target. The focused laser beam sequentially ablates the graphite target component and the modifying element target component to form composite layers. Adherent, 1 µm thick titanium-DLC and silver-DLC films have been demonstrated using this technique. In the present study, platinum- and gold-DLC nanocomposite films prepared using the modified pulsed laser deposition process were investigated with transmission electron microscopy, atomic force microscopy, Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and nanoindentation testing. The results indicate that hydrogen-free noble metal-DLC thin films are robust electrode materials with electrochemical activities directly influenced by the metal content. 2. Experimental The depositions were performed in a stainless steel high vacuum chamber, which was maintained at a pressure of ∼ 5 × 10-6 Torr during film deposition (Figure 1a). A Compex 205 KrF excimer laser (Coherent Inc., Fort Lauderdale, FL) operating at λ ) 248 nm was used for target ablation. The laser was operated with a pulse duration of 25 ns and a frequency of 10 Hz. The output of ∼200 mJ and a spot size of ∼0.05 cm2 provided an energy density of ∼4-5 J/cm2. The target to substrate distance was maintained at 4.5 cm. The total time for each deposition was 10 min. A high-purity 4 cm diameter graphite pellet was used as the target for pulsed laser deposition of metal-free DLC thin films. Metal-DLC nanocomposite films were prepared by partially covering the graphite pellet surface with a segment of gold or platinum (Figure 1b). The target was (35) Zeng, A.; Liu, E.; Tan, S. N.; Zhang, S.; Gao, J. Electroanalysis 2002, 14 (15-16), 1110-1115. (36) Morrison, M. L.; Buchanan, R. A.; Liaw, P. K.; Berry, C. J.; Brigmon, R. L.; Riester, L.; Abernathy, H.; Jin, C.; Narayan, R. J. Diamond Relat. Mater. 2006, 15 (1), 138-146. (37) Narayan, R. J.; Abernathy, H.; Riester, L.; Berry, C. J.; Brigmon, R. J. Mater. Eng. Perform. 2005, 14 (4), 435-440. (38) Narayan, R. J. Diamond Relat. Mater. 2005, 14 (8), 1319-1330.

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Figure 2. (a) Cross-sectional low-resolution transmission electron micrograph of platinum-diamondlike carbon film on a silicon substrate. (b) High-resolution cross-sectional transmission electron micrograph of platinum-diamondlike carbon film on a silicon substrate. Platinum nanoclusters are indicated by the white arrows. rotated at a speed of 5 rpm, and the focused laser beam sequentially ablated graphite and metal portions of the multicomponent target. Cross-sectional Z-contrast images of the metal-DLC nanocomposite films were obtained using a 2010 F scanning transmission electron microscope (JEOL, Tokyo, Japan) equipped with Gatan Image Filter attachment. In this TEM/STEM system, a 1.6 Å probe is scanned across the sample. A high angle annular detector collects electrons that are scattered at large angles (75-150 mrad). Contrast is proportional to the atomic number (Z) squared. A PicoPlus/4500 atomic force microscope (Agilent Technologies, Tempe, AZ) equipped with a 10 × 10 µm range multipurpose scanner was used to record the surface topography of metal-free and metalDLC films in air. The atomic force microscopy studies were performed in contact mode with ultra-sharp tips (MikroMasch, Wilsonville, OR) showing a nominal curvature radius of less than 10 nm. The obtained images were processed using the Scanning Probe Image Processor (SPIP) software package (Image Metrology, Lyngby, Denmark). Rutherford backscattering spectroscopy (RBS) data were collected on a 1.7 MV Tandetron particle accelerator (General Ionex, Newburyport, MA) using a 2.25 MeV He2+ ion beam. The diameter of the beam spot size was 1.5 mm. Spectral simulations were performed with the RUMP software package (Genplot, Cortland, OH). The X-ray photoelectron spectroscopy (XPS) data was acquired on an SSX-100 ESCA spectrometer (Surface Science Labs, Mountain View, CA) equipped with a small spot monochromatic Al KR source (1486.6 eV). A general survey spectrum (0-1100 eV, 1 scan at 1 eV steps, 800 µm spot size, 150 eV pass energy) was recorded to verify surface constituents. High-resolution spectra (10 scans at 0.1 eV steps, 400 µm spot size, 50 eV pass energy) were recorded for the carbon 1s singlet, gold 4f, and platinum 4f doublet peaks. Deconvolution of the carbon 1s peak was performed using the peak fitting functions of the Analysis 2000 software (Service Physics Inc., Bend, OR). By utilizing high-energy X-rays sources - such as in the reported study - the peak intensity for core-level electrons is solely dependent on atomic factors, and not on the chemical environment.39 Therefore, the determination of sp2- and sp3-carbon hybridization states is facilitated, as the same sensitivity factor can be used for both species.40 Film thicknesses were measured with a Dektak3 ST stylus profilometer (Veeco/ Sloan Technologies, Santa Barbara, CA). Nanohardness and Young’s modulus values were determined using an MTS Nanoindenter XP instrument (MTS Instruments, Oak Ridge, TN). The samples were examined using an ultralow load DCM indentation head and a three-sided diamond pyramid (Berkovitch) tip. Indentations were performed using a trapezoidal loading curve. The maximum load was varied between 1 and 60 mN. Nanohardness and Young’s modulus values were measured as a function of indentation depth, and determined using the Oliver-Pharr model.38

The tip was calibrated following the partial unloading method and was cleaned with isopropanol between indentations. Electrochemical characterization was performed in a custombuilt three-electrode Teflon cell using a model 660A potentiostat (CH Instruments, Austin, TX). An Ag|AgCl reference electrode was used for the redox mediator studies, and a Hg|HgSO4 electrode for determining the potential window of the electrodes. Platinum wires were used as the counter electrodes, while the DLC films (0.068 cm2 exposed area) were used as the working electrodes. Additionally, 1 mm platinum, 1 mm gold, and 3 mm glassy carbon disk electrodes were used for comparison of the working potential windows. The two redox mediator solutions contained 0.1 M potassium chloride as supporting electrolyte (Aldrich, St. Louis, MA) and 5 mM hexaammineruthenium(III) trichloride and hexaammineruthenium(II) dichloride (Ru(NH3)63+/2+) or 10 mM potassium hexacyanoferrate(II) trihydrate and potassium hexacyanoferrate(III) (Fe(CN)63-/4-). A 0.5 M solution of sulfuric acid (Fisher Scientific, Fair Lawn, NJ) was used for the determination of the working potential windows. In order to minimize effects from solution resistance, which resemble systems with finite heterogeneous kinetics,41 automatic compensation was utilized for all voltammetric measurements via the potentiostat software interface. In addition, due to the semiconducting nature of DLC,42 all electrochemical experiments were performed excluding ambient light, in order to minimize any photoconductive effects. Solutions were prepared using deionized water with a resistance of 18.2 MΩ·cm at 25 °C provided by a water purification system (Millipore Milli-Q, Billerica, MA), and were sparged with argon (Airgas, Marietta, GA) for at least 15 min prior to the analysis. All chemicals were used as received without additional purification. Heterogeneous electron-transfer rate constants (ks) and chargetransfer coefficients (R) were obtained with a fitting procedure from the experimental data using the DigiElch software package.43,44

3. Results and Discussion 3.1. Transmission Electron Microscopy Studies. A lowresolution transmission electron micrograph of a platinum-DLC thin film is shown in Figure 2a, and a high-resolution transmission electron micrograph of a platinum-diamondlike carbon thin film is shown in Figure 2b. Metals that do not chemically bond with (39) Himpsel, F. J.; McFeely, F. R.; Taleb-Ibrahimi, A.; Yarmoff, J. A.; Hollinger, G. Phys. ReV. B 1988, 38 (9), 6084-96. (40) Speranza, G.; Laidani, N. Diamond Relat. Mater. 2004, 13 (3), 445-450. (41) Saveant, J. M.; Imbeaux, J. C., J. Electroanal. Chem. Interfacial Electrochem. 1970, 28 (2), 327-40. (42) Robertson, J. Semicond. Sci. Technol. 2003, 18 (3), S12-S19. (43) Rudolph, M. J. Electroanal. Chem. 2003, 543 (1), 23-39. (44) Rudolph, M. J. Electroanal. Chem. 2004, 571 (2), 289-307.

Carbon Nanocomposite Thin Films

Langmuir, Vol. 23, No. 12, 2007 6815 Table 1. Properties of Metal-Diamondlike Carbon Nanocomposite Films as Determined by Rutherford Backscattering Spectroscopy (RBS), X-Ray Photoelectron Spectroscopy (XPS), and Profilometry target composition metal sample (percent metal) concentration DLC Au #1 Au #2 Au #3 Pt #1 Pt #2 Pt #3

Figure 3. AFM topographical image of (a) platinum-DLC nanocomposite and (b) metal-free diamondlike carbon films. The images taken were of 2.5 µm × 2.5 µm areas with identical height (z-) scale. Curves c and d represent the height profiles of the layers as indicated by the white arrows; please note the different units between the x- and z-axis.

carbon, including gold and platinum, formed self-assembled arrays of nearly spherical metal clusters within the DLC matrix. Atomically sharp boundaries between a metal nanoparticles and the hard carbon matrix were observed. An average metal nanoparticle size of 3-7 nm was observed in the platinumDLC film, which is in good agreement with values reported in the literature.36-38 The driving force for the clustering of metal within these metal-DLC films is a reduction in surface energy. It can be concluded that the formation of nanoparticles primarily occurs during ablation. Ostwald ripening is prevented, and arrays of metal nanoparticles are formed within the DLC matrix. 3.2. Atomic Force Microscopy and Nanoindentation Studies. Figure 3a shows the topography of platinum-DLC obtained by contact mode atomic force microscopy (AFM) imaging. A topographic image of a metal-free DLC thin film is shown in Figure 3b for comparison. The surface of the platinum-DLC film is characterized by several protrusions of different sizes and heights, which is attributed to metal particulate formed during film deposition. In contrast to the previously discussed transmission electron microscopy results, metal particles measured at the surface of metal-DLC nanocomposites are disc-shaped with heights of up to 40 nm. Microscopy studies performed on metal thin films grown with the PLD technique routinely show the presence of surface features similar to those observed herein.45 These particulates are attributed to a “splashing” mechanism occurring during the PLD process. Splashing takes place in materials through subsurface boiling or shock wave ejection of particulates. As a result, the highly energetic plume originating from the ablated target may contain droplets of molten metal, which give rise to these comparatively large features. The size of the droplets formed may be minimized by carefully repolishing the metal component of the target and by performing the deposition at higher laser fluences.46-48 Nanohardness values for platinum-DLC thin films on silicon (100) substrates were 16.9 ( 0.8 GPa, and Young’s modulus (45) Faehler, S.; Stoermer, M.; Krebs, H. U. Appl. Surf. Sci. 1997, 109/110, 433-436. (46) van de Riet, E.; Nillesen, C. J. C. M.; Dieleman, J. J. Appl. Phys. 1993, 74 (3), 2008-12. (47) Yoshitake, T.; Nagayama, K. Vacuum 2004, 74 (3-4), 515-520. (48) Nakata, Y.; Gunji, S.; Okada, T.; Maeda, M. Appl. Phys. A 2004, A79 (4-6), 1279-1282.

0% 50% 25% 10% 50% 30% 25%

N/A 36.0% 11.0% 3.0% 11.5% 5.7% 4.0%

concentration of sp3-hybridized atoms (n ) 4)

thickness (nm)

46 ( 2% 41 ( 11% 38 ( 2% 42 ( 6% 38 ( 4% 39 ( 12% 40 ( 8%

50 105 67 63 58 38 50

values for these films were determined at 181.0 ( 4.0 GPa. These values were somewhat greater than those observed in a-C:H-copper coatings prepared using plasma-enhanced chemical vapor deposition (PECVD) or hybrid microwave plasma-assisted chemical vapor deposition/ sputtering techniques (∼10 GPa).49-54 However, these values were significantly lower than the values observed for metal-free DLC thin films on silicon (100) substrates, which were 42.1 ( 1.4 GPa and 379.0 ( 42.9 GPa, respectively. It is believed that these differences reflect the fact that the metalDLC films possess a somewhat lower concentration of sp3hybridized carbon atoms. In addition, the low modulus metal particles contribute to reduced mechanical property values. 3.3. Rutherford Backscattering, X-Ray Photoelectron, and Raman Spectroscopic Studies. Rutherford backscattering spectroscopy (RBS) was used to provide compositional information on the metal-DLC thin films. Since the target materials are not ablated simultaneously, depth-wise sample homogeneity may not be ideal. RBS is insensitive to both the sample’s chemistry, matrix effects, as well as morphology, making it ideal for metal quantification.55 The compositional data derived from spectral simulations, along with relevant physical properties for each analyzed layer, is summarized in Table 1. The metal content in these nanocomposite materials varies proportionally with the extent of metal masking the ablation target. Utilizing the PLD approach described, gold-DLC layers containing approximately 3-36% gold, and platinum-DLC containing approximately 4-11.5% platinum were deposited. In addition to the percentage of metal masking the graphite target, the amount of metal embedded into the DLC matrix is also dependent upon the physical properties of the metal itself, such as the ablation rate and optical reflectivity.56 Information about the hybridization state of carbon atoms residing near the surface (within ∼5 nm) is obtainable via X-ray photoelectron spectroscopy (XPS). Contributions from each constituent at DLC surfaces is commonly determined by mathematically deconvoluting the carbon 1s photoemission peak into sp2-hybridized carbon (BE ) 284.4 ( 0.1 eV), sp3-hybridized (49) Pauleau, Y.; Thiery, F. Surf. Coat. Technol. 2004, 180-181, 313-322. (50) Sheeja, D.; Tay, B. K.; Sze, J. Y.; Yu, L. J.; Lau, S. P. Diamond Relat. Mater. 2003, 12 (10-11), 2032-2036. (51) Strondl, C.; Carvalho, N. M.; De Hosson, J. T. M.; van der Kolk, G. J. Surf. Coat. Technol. 2003, 162 (2-3), 288-293. (52) Voevodin, A. A.; Zabinski, J. S. Thin Solid Films 2000, 370 (1,2), 223231. (53) Rusli; Yoon, S. F.; Yang, H.; Ahn, J.; Huang, Q. F.; Zhang, Q.; Guo, Y. P.; Yang, C. Y.; Teo, E. J.; Wee, A. T. S.; Huan, A. C. H. Thin Solid Films 1999, 355-356, 174-178. (54) Rusli, H.; Yoon, S. F.; Huang, Q. F.; Ahn, J.; Zhang, Q.; Yang, H.; Wu, Y. S.; Teo, E. J.; Osipowicz, T.; Watt, F. Diamond Relat. Mater. 2001, 10 (2), 132-138. (55) Czanderna, A. W. Methods Surf. Charact. 1991, 2, 1-44. (56) Chrisey, D. B.; Hubler, G. K. Pulsed Laser Deposition of Thin Films; John Wiley & Sons, Inc.: New York, 1994; p 613.

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carbon (BE ) 285.2 ( 0.1 eV), and carbon bonded to adventitious oxygen (BE ) 286.5 eV).57,58 Quantification of sp2- and sp3-hybridized carbon at the surface is especially important in DLC films, as both mechanical13 and electrochemical59 properties can be affected by the relative percentage of each constituent. Ideally, a high sp3-carbon content is indicative that DLC films will exhibit properties similar to those of diamond. The deconvoluted carbon spectrum for the metal-free DLC film reveals that approximately 46% of the film surface is composed of sp3-hybridized carbon (Table 1). The metal-DLC films contain slightly lower amounts of sp3hybridized carbon atoms. The decreased sp3-fraction is attributed to a reduction in stress via the formation of thin graphitic (sp2-) “shells” surrounding the metal clusters embedded in the carbon matrix.60 Hence, increasing the metal concentration should correspond to a decrease in sp3-hybridized carbon atoms. However, the data obtained by XPS shows no such correlation, contrasting previous findings for ruthenium-61 and chromiumDLC62 films, where the expected trend was reported. This apparent discrepancy is most likely related to the analytical method used for quantifying the fraction of sp3-hybridized carbon atoms. Ruthenium- and chromium-DLC films were investigated using electron energy loss spectroscopy (EELS), which determines the average fraction of sp3-hybridized carbon atoms throughout the thickness of an electron transparent region of the film. DLC surfaces are inherently damaged due to collisions with impinging species, resulting in higher-than-anticipated sp2-content.63,64 Since XPS is highly selective to surface composition, it is expected that the sp2-carbon content measured is influenced by the collisional damage as well as the metal content. In addition to verifying the hybridization state of carbon, XPS can be used to determine the elemental composition of surface. Survey scans of metal-free and metal-DLC layers indicate that the only elements measurable at the surface are carbon, the metal modifier (for metal-DLC) and adventitious oxygen (Figure 4). Similarly to EELS, Raman spectroscopy is capable of probing the entire thickness of the film and can provide further insight into the bonding characteristics of the DLC matrix.65 The Raman spectrum of the metal-free DLC film is characterized by a broad asymmetric peak, which is comprised of a dominating segment centered in the 1510-1557 cm-1 region (G-band), and a small shoulder at 1350 cm-1 (D-band), as shown in Figure 5. The G-band results from the E2g stretching mode of sp2-hybridized carbon atoms, and the D-band results from the A1g breathing mode of six-membered carbon rings.66,67 The Raman spectra of metal-DLC films with larger metal concentrations exhibited increased D-band intensities and higher G-band positions, suggesting that larger sp2-hybridized carbon clusters are formed (57) Diaz, J.; Paolicelli, G.; Ferrer, S.; Comin, F. Phys. ReV. B 1996, 54 (11), 8064-8069. (58) Merel, P.; Tabbal, M.; Chaker, M.; Moisa, S.; Margot, J. Appl. Surf. Sci. 1998, 136 (1/2), 105-110. (59) Niwa, O.; Jia, J.; Sato, Y.; Kato, D.; Kurita, R.; Maruyama, K.; Suzuki, K.; Hirono, S. J. Am. Chem. Soc. 2006, 128 (22), 7144-7145. (60) Bewilogua, K.; Wittorf, R.; Thomsen, H.; Weber, M. Thin Solid Films 2004, 447-448, 142-147. (61) Lian, G. D.; Dickey, E. C.; Ueno, M.; Sunkara, M. K. Diamond Relat. Mater. 2002, 11 (12), 1890-1896. (62) Fan, X.; Dickey, E. C.; Pennycook, S. J.; Sunkara, M. K. Appl. Phys. Lett. 1999, 75 (18), 2740-2742. (63) Steffen, H. J.; Roux, C. D.; Marton, D.; Rabalais, J. W. Phys. ReV. B Condens. Matter Mater. Phys. 1991, 44 (8), 3981-90. (64) Arena, C.; Kleinsorge, B.; Robertson, J.; Milne, W. I.; Welland, M. E. J. Appl. Phys. 1999, 85 (3), 1609-1615. (65) Wagner, J.; Ramsteiner, M.; Wild, C.; Koidl, P. Phys. ReV. B 1989, 40 (3), 1817-24. (66) Wu, W.-y.; Ting, J.-m. Thin Solid Films 2002, 420-421, 166-171. (67) Bozhko, A.; Takagi, T.; Takeno, T.; Shupegin, M. J. Phys. Condens. Matter 2004, 16 (46), 8447-8458.

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Figure 4. XPS spectra illustrating the typical surface composition of Au-, Pt-, and metal-free DLC. The only peaks observed are those pertaining to carbon, oxygen and the appropriate metal modifier (only the 4f peaks have been labeled for clarity).

Figure 5. Raman spectra of 36% gold-diamondlike carbon, 11% gold-diamondlike carbon, 3% gold-diamondlike carbon, and metalfree diamondlike carbon films.

in metal-DLC compared to unmodified DLC.68,69 Furthermore, smaller G-band FWHM in films with higher metal concentrations indicate that stress within these films is released. Similarly to the ruthenium- and chromium-DLC layers previously mentioned, the combined spectral changes recorded indicate that the fraction of sp2-carbon in the DLC layers increases with metal content. 3.4. Electrochemical Characterization. Electronic conduction in DLC films containing metal clusters is not only dependent upon by the type of metal, but also on its ability to react with the DLC matrix forming carbide bonds.70 In general, with increasing metal content, the conductivity of a layer will increase accordingly, approaching that of the pure metal at very high concentrations (typically >60 at. %).70 The electron-transfer mechanism in these nanocomposite-type materials has yet to be fully elucidated, although Pleskov et al. indicated that electrons may be shuttled via two distinct pathways: (1) homogeneously through a hopping mechanism between sp2-hybridized carbon clusters, and (2) heterogeneously through the metal clusters, which function as catalytic centers facilitating the electron transfer at the electrode/electrolyte interface.27,28 The primary objective of this study was to evaluate the performance of platinum- and gold-DLC thin films as electrode (68) Ferrari, A. C.; Robertson, J. Phys. ReV. B 2000, 61 (20), 14095-14107. (69) Chhowalla, M.; Ferrari, A. C.; Robertson, J.; Amaratunga, G. A. J. Appl. Phys. Lett. 2000, 76 (11), 1419-1421. (70) Benndorf, C.; Boettger, E.; Fryda, M.; Haubold, H. G.; Klages, C. P.; Koeberle, H. Synth. Met. 1991, 43 (3), 4055-8.

Carbon Nanocomposite Thin Films

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Figure 6. Working potential windows (a) of glassy carbon, platinum, and platinum-diamondlike carbon films and (b) of glassy carbon, gold, and gold-diamondlike carbon films in 0.5 M H2SO4 sparged with argon. The y-axis reflects the measured current density. Table 2. Peak Separation of Ru(NH3)63+/2+ and Fe(CN)63-/4Redox Couple with Respect to the Metal Concentration at 100 mV/s (n ) 3) sample

∆E (Ru(NH3)63+/2+) (mV)

∆E (Fe(CN)63-/4-) (mV)

Au #1 Au #2 Au #3 Pt #1 Pt #2 Pt #3

104 ( 11 140 ( 14 164 ( 29 124 ( 8 172 ( 7 179 (7

122 ( 5 159 ( 20 244 ( 25 147 ( 6 201 ( 14 204 ( 40

materials. Thus, initial experiments included measuring the potential working window, herein defined as the measured current density of 1 mA/cm2. The potential window was determined to be approximately 2.0 V for the platinum-DLC film and approximately 2.3 V for the gold-DLC film. These values are smaller than those reported for hydrogen-free nitrogen-doped DLC layers, where potential windows larger than 3 V have been measured.35,71 Reactive species responsible for gas evolution from aqueous solutions require adsorption of intermediates onto electrode surfaces. Metal-DLC nanocomposite surfaces contain considerable amounts of sp2-hybridized carbon, which provide adsorption sites facilitating oxygen gas evolution.72 In addition, the formation of hydrogen gas is catalyzed by the presence of metal at the surface,73 further decreasing the cathodic potential range compared to DLC layers that do not contain metals. Although this may be perceived as a negative attribute from an electroanalytical viewpoint, the electrocatalytic activity observed at metal-DLC surfaces is considered desirable for several electrochemical systems.74-76 For comparative purposes, Figure 6 illustrates the working potential windows that are obtained with metal-DLC electrodes, glassy carbon electrodes (entirely sp2-hybridized), and the respective pristine metals. It is evident that the electrochemical window of metal-DLC films lies in between the useful potential range provided by the two main constituents, which implies that metal-DLC electrodes provide a wider electrochemical window than the pure metal, while offering the mechanical and chemical stability of an inert ceramic. Two redox mediators, Ru(NH3)63+/2+ and Fe(CN)63-/4-, were used to determine the electrochemical performance of the metalDLC electrodes. The diffusion coefficients (in 0.1 M KCl) used in the digital simulations were 7.6 × 10-6 cm/s2 and 6.5 × 10-6 (71) Yoo, K.; Miller, B.; Kalish, R.; Shi, X. Electrochem. Solid-State Lett. 1999, 2 (5), 233-235. (72) Martin, H. B.; Argoitia, A.; Landau, U.; Anderson, A. B.; Angus, J. C. J. Electrochem. Soc. 1996, 143 (6), L133-L136. (73) Norskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U. J. Electrochem. Soc. 2005, 152 (3), J23-J26. (74) You, T.; Niwa, O.; Horiuchi, T.; Tomita, M.; Iwasaki, Y.; Ueno, Y.; Hirono, S. Chem. Mater. 2002, 14 (11), 4796-4799. (75) Rajalakshmi, N.; Ryu, H.; Dhathathreyan, K. S. Chem. Eng. J. 2004, 102 (3), 241-247. (76) Gomez de la Fuente, J. L.; Rojas, S.; Martinez-Huerta, M. V.; Terreros, P.; Pena, M. A.; Fierro, J. L. G. Carbon 2006, 44 (10), 1919-1929.

Table 3. Kinetic Parameters for the Metal-DLC Layers Derived from Digital Simulations (n ) 3) sample

Ru(NH3)63+/2+ ks ( 10-3cm/s)

Fe(CN)64-/3ks ( 10-3cm/s)

Ru(NH3)63+ R

Fe(CN)641-R

Au #1 Au #2 Au #3 Pt #1 Pt #2 Pt #3

6.1 ( 1.6 3.5 ( 0.9 2.4 ( 0.9 4.3 ( 0.6 2.3 ( 0.2 2.3 ( 0.8

3.5 ( 0.7 2.3 ( 0.8 1.0 ( 0.1 2.8 ( 0.5 1.3 ( 0.1 1.3 ( 0.3

0.50 ( 0.03 0.51 ( 0.01 0.51 ( 0.01 0.52 ( 0.01 0.52 ( 0.01 0.52 ( 0.02

0.50 ( 0.01 0.46 ( 0.02 0.48 ( 0.03 0.49 ( 0.02 0.48 ( 0.01 0.48 ( 0.02

cm/s2 for Fe(CN)63- and Fe(CN)64-, respectively.77 The diffusion coefficient for Ru(NH3)62+ in the electrolyte utilized is not readily available from the literature, therefore it was determined experimentally by cyclic voltammetry and calculated using the Randles-Sevcˇik equation.77 The value obtained was 5.5 ( 0.5 × 10-6 cm/s2. Similarly, a diffusion coefficient of 9.2 ( 0.7 × 10-6 cm/s2 was measured for Ru(NH3)63+, which is identical to values reported elsewhere.78 The separation between the anodic and cathodic peaks (∆Ep) can be used as a measure of the facility with which heterogeneous electron transfers occur.77 The ∆Ep values measured at the nanocomposite layers correlate to the amount of embedded metal, with peak separations ranging from approximately 100 mV for layers with the highest metal content, to 250 mV for layers with the lowest metal content (for ν ) 100 mV/s, see Table 2). In combination with the nearly featureless voltammograms obtained at metal-free DLC, the relationship between ∆Ep and the metal content suggests that heterogeneous electron transfer is facilitated by the presence of metal sites at the surface. This is agreement with the hypothesis of Pleskov et al.,27,28 which indicates that the electrochemical activity observed at metal-DLC layers results from metal-catalyzed electron-transfer mechanisms. Furthermore, due to labile surface carbon-oxygen moieties, the electrochemical response to several redox couples at entirely sp2-carbon electrodes is affected by the nature of the electrolyte, and any surface pretreatment prior to the electrochemical analysis.79-81 Conversely, electrons in metal-DLC films are primarily transferred heterogeneously at metal sites, hence, changes in electrolyte and oxidation/reduction of sp2-carbon at the nanocomposite films are less likely to affect the measured electrochemical activity, as typically observed for pure sp2-carbon electrodes. Heterogeneous electron-transfer rate constants (ks) extracted from the experimental CVs, are summarized in Table 3. The ks (77) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Application, 2nd ed.; John Wiley & Sons, Inc.: New York, 2001. (78) Marken, F.; Eklund, J. C.; Compton, R. G. J. Electroanal. Chem. 1995, 395 (1-2), 335-8. (79) Ilangovan, G.; Chandarasekara Pillai, K. J. Electroanal. Chem. 1997, 431 (1), 11-14. (80) Engstrom, R. C. Anal. Chem. 1982, 54 (13), 2310-14. (81) McCreery, R. L. Carbon Electrodes: Structural Effects on Electron Transfer Kinetics; Marcel Dekker, Inc.: New York, 1991; Vol. 17, pp 221-374.

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values calculated for both redox mediators are 1-2 orders of magnitude smaller than those determined for the same redox systems at other carbon-based electrodes, such as, e.g., glassy carbon82-84 and doped diamond electrodes.85,86 Heterogeneous electron-transfer rates of DLC-based electrodes,28,30,35,87-89 which can be estimated from reported ∆Ep values,90 span over several orders of magnitude, with the values determined in the present study located towards the upper end.

4. Conclusions Metal-free and noble metal-diamondlike carbon thin films were deposited onto silicon substrates by pulsed laser deposition, and their physical and electrochemical properties for application as electrode materials have been studied. XPS studies revealed (82) Chen, P.; Fryling, M. A.; McCreery, R. L. Anal. Chem. 1995, 67 (18), 3115-22. (83) Chen, P.; McCreery, R. L. Anal. Chem. 1996, 68 (22), 3958-3965. (84) Chen, Q.; Swain, G. M. Langmuir 1998, 14 (24), 7017-7026. (85) Hupert, M.; Muck, A.; Wang, J.; Stotter, J.; Cvackova, Z.; Haymond, S.; Show, Y.; Swain, G. M. Diamond Relat. Mater. 2003, 12 (10-11), 1940-1949. (86) Chen, Q.; Gruen, D. M.; Krauss, A. R.; Corrigan, T. D.; Witek, M.; Swain, G. M. J. Electrochem. Soc. 2001, 148 (1), E44-E51. (87) Cachet, H.; Debiemme-Chouvy, C.; Deslouis, C.; Lagrini, A.; Vivier, V. Surf. Interface Anal. 2006, 38 (4), 719-722. (88) Sunkara, M. K.; Chandrasekaran, H.; Koduri, P. New Diamond Front. Carbon Technol. 1999, 9 (6), 407-415. (89) Yee, N. C.; Shi, Q.; Cai, W.-B.; Scherson, D. A.; Miller, B. Electrochem. Solid-State Lett. 2001, 4 (10), E42-E44. (90) Nicholson, R. S. Anal. Chem. 1965, 37 (11), 1351-5.

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that surface sp2-carbon content is nearly constant, regardless of the type of metal and content. In contrast, visible Raman spectroscopy clearly demonstrates that inclusion of metal induces an increase in bulk sp2-carbon content, and a concomitant reordering of sp2 sites. As electrode materials, the layers presented provide working potential windows that are intermediate between those of the pristine metal and fully sp2-carbon electrodes. The cathodic range is especially limited due to hydrogen gas evolution catalyzed by the metal present at the surface. Finally, additional electrochemical characterization with quasi-reversible redox mediators revealed near-Nernstian electrochemical performance for those layers with the highest metal content, which has not been previously documented for metal-diamondlike carbon films. Acknowledgment. K. Scammon of the Advanced Materials Processing and Analysis Center (AMPAC) at the University of Central Florida is gratefully acknowledged for support with the Rutherford backscattering spectroscopy experiments. J. Wiedemair and C. Kranz are thanked for support in AFM imaging and electrochemical analysis. H. Abernathy and M. Liu are thanked for their assistance with Raman spectroscopy. This work was in part supported by NSF Grant #0216368 (BM) and NIH Grant #R21 EB003090-01 (RJN). LA062582P