Electrochemical and XPS Characterization of Glassy Carbon

Dec 1, 1996 - (VI)-modified glassy carbon electrode (GCE) in H2SO4 is studied. ... and cathodically reduced GCE surface groups is indicated by the for...
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Langmuir 1997, 13, 566-575

Electrochemical and XPS Characterization of Glassy Carbon Electrode Surface Effects on the Preparation of a Monomeric Molybdate(VI)-Modified Electrode G. Ilangovan and K. Chandrasekara Pillai* Department of Physical Chemistry, University of Madras, Guindy Campus, Madras 600 025, India Received January 17, 1996X Evaluation of electrochemical surface pretreatment methods on the preparation of monomeric molybdate(VI)-modified glassy carbon electrode (GCE) in H2SO4 is studied. The results show that the cathodically treated GCE is unusually highly activated toward Mo(VI) adsorption, but the anodized electrode does not respond, unlike the previous reports for the other adsorbed systems. The analyses of the voltammograms of Mo(VI) on a cathodized electrode show three pairs of reversible peaks with a surface excess on the order of =1 × 10-10 mol cm-2 for each of the peaks. The interaction parameters for the three processes are 1.14 × 109, 1.38 × 109, and -5.50 × 109 cm2 mol-1, respectively. Strong interaction between the Mo(VI) ion and cathodically reduced GCE surface groups is indicated by the formation of Mo(V) (detected by x-ray photoelectron spectroscopy (XPS) and electrochemical polarization measurements) during the course of Mo(VI) adsorption at open circuit conditions. The Mo(VI) adsorption via the mixed potential mechanism has been formulated involving Mo(VI) reduction to Mo(V) coupled with surface alcohol group oxidation to hydroperoxide. XPS analyses show that the cathodized surface contains higher amounts of >C-Osurface groups. The quantities of >C-O- surface groups correlate with the observed differences in the response of the pretreated GCE surfaces toward Mo(VI) adsorption.

1. Introduction Electrochemical studies relating to isopoly- and heteropoly-molybdates have become an important area of investigation in recent years, especially because of their several possible applications1-3 such as electrocatalysis, molecular electronics, and electrochromic devices. An interesting feature of these compounds is that they are attractive agents for modifying bare electrode surfaces3,4 and also for entrapping them into polymer matrices.1,2 It is not known whether a similar phenomenon is possible in more acidic solutions of pH less than 2, in which hexavalent molybdenum exists predominantly as monomeric species.5-7 In strong acidic solutions electroreduction of Mo(VI) ion has long been established to lead to a product which is recognized as a convenient, powerful homogeneous redox catalyst toward the reduction of NH2OH, NO3-, ClO4-, ClO3-, etc.,8 which is used advantageously as a sensitive analytical method in estimating millimolar quantities of Mo(VI)/anions in solutions.9 Furthermore, Mo(VI) adsorption on carbon surfaces in solutions of higher mineral acid concentrations has a greater relevance in the analytical estimation of Mo(VI).10 We have thus attempted to prepare a monomeric molybdate(VI)-modified glassy carbon (GC) composite electrode * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Keita, B.; Bouaziz, D.; Nadjo, L.; Deronzier, A. J. Electroanal. Chem. 1990, 279, 187-203. (2) Dong, S.; Song, F.; Wang, B.; Liu, B. Electroanalysis 1992, 4, 643-646. (3) Ingersoll, D.; Kulesza, D. P. J.; Faulkner, L. R. J. Electrochem. Soc. 1994, 141, 140-147. (4) Dong, S.; Wang, B. Electrochim. Acta 1992, 37, 11-16. (5) Pope, M. T. Heteropoly and Homopoly Oxomettalates; Springer Verlag: Berlin, 1983. (6) Cruwagen, J. J.; Heyns, J. B. B.; Rohwer, E. F. C. H. J. Inorg. Nucl. Chem. 1976, 38, 2033-2036. (7) Cruwagen, J. J. Personnal communication, 1994. (8) Kolthaff, I. M.; Hodara, I. J. Electroanal. Chem. 1963, 5, 2-16. (9) Edmonds, T. E. Anal. Chim. Acta 1980, 116, 323-333. (10) Benzo, Z.; Araujo, P.; Sierralta, S.; Ruette, F. Anal. Chem. 1993, 65, 1107-1113.

in 0.1 M H2SO4, and the results regarding preparation and characterization are presented in this paper. To obtain a stable monomeric molybdate(VI) coating on GCE, the surface pretreatment method of GCE has to be formulated in the first place. Several pretreatment procedures specific for specific electrochemical reactions in solution11-35 and adsorption and electron-transfer (11) Blaedel, W. J.; Jenkins, R. A. (A) Anal. Chem. 1974, 46, 19521945; (B) Anal. Chem. 1975, 47, 1337-1343. (12) Gonon, F. G.; Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 53, 1386-1389. (13) Engstrom, R. C. Anal. Chem. 1982, 54, 2310-2314. (14) Falat, L.; Cheng, H. Y. J. Electroanal. Chem. 1983, 157, 393397. (15) Cenas, N.; Rozgaite, J.; Pocius, A.; Kulys, J. J. Electroanal. Chem. 1983, 154, 121-128. (16) Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984, 56, 136141. (17) Cabaniss, G. E.; Diamantis, A. A.; Murphy, W. R., Jr.; Linton, R. W.; Meyer, T. J. J. Am. Chem. Soc. 1985, 107, 1845-1853. (18) Wang, J.; Hutchins, L. D. Anal. Chim. Acta 1985, 167, 325-334. (19) Nagaoka, T.; Yoshino, T. Anal. Chem. 1986, 58, 1037-1042. (20) Kovach, P. M.; Deakin, M. R.; Wightman, R. M. J. Phys. Chem. 1986, 90, 4612-4617. (21) Nagaoka, T.; Fukunaga, T.; Yoshino, T.; Watanabe, I.; Nakayama, T.; Okazaki, S. Anal. Chem. 1988, 60, 2766-2769. (22) Kepley, L. J.; Bard, A. J. Anal. Chem. 1988, 60, 1459-1467. (23) Bowling, R.; Packard, R. T.; McCreery, R. L. Langmuir 1989, 5, 683-688. (24) Bowers, M. L.; Yenser, B. A. Anal. Chim. Acta 1991, 243, 43-53. (25) Wightman, R. M.; Deakin, M. R.; Kovach, P. M.; Kuhr, W. G.; Stutts, K. J. J. Electrochem. Soc. 1984, 131, 1578-1583. (26) Hu, I. F.; Karweik, D. H.; Kuwana, T. J. Electroanal. Chem. 1985, 188, 59-72. (27) Kamau, G. N.; Willis, W. S.; Rusling, J. F. Anal. Chem. 1985, 57, 545-551. (28) Deakin, M. R.; Stutts, K. J.; Wightman, R. M. J. Electroanal. Chem. 1985, 182, 113-122. (29) Fagan, D. T.; Hu, I. F.; Kuwana, T. Anal. Chem. 1985, 57, 27592763. (30) Poon, M.; McCreery, R. L.; Engstrom, R. C. Anal. Chem. 1988, 60, 1725-30. (31) Rice, R. J.; Pontikos, N. M.; McCreery, R. L. J. Am. Chem. Soc. 1990, 112, 4617-4622. (32) Pontikos, N. M.; McCreery, R. L. J. Electroanal. Chem. 1992, 324, 229-242. (33) McDermott, M. T.; Kneten, K.; McCreery, R. L. J. Phys. Chem. 1992, 96, 3124-3130. (34) Allred, C. D.; McCreery, R. L. Anal. Chem. 1992, 64, 444-448. (35) Zhang, H.; Coury, L. A., Jr. Anal. Chem. 1993, 65, 1552-1558.

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processes of adsorbable substrates33,34,36-40 have been proposed. These include electrochemical methods,11-24 nonelectrochemical methods such as polishing,25-27 vacuum heat treatment,25,28,29,38 laser irradiation,30-32 fracturing,31-34 and high-intensity ultrasonic treatment.35 Amongst these procedures for electrode activation, the electrochemical methods of pretreatment have been used extensively since they are convenient, inexpensive, and quick and with the additional advantage that they can be performed in situ within the electrolyte. Long-duration preanodization followed by short-duration cathodization,13,16,17,19,21 application of square-wave pulses several times between two extreme potentials,12,14,18,20 method of cycling the potential several times over a wide potential range between two prestated voltages,11,15,22,24,39 etc., have proved effective as electrochemical pretreatment procedures. In the present work, we have chosen to study the most common electrochemical pretreatment methods such as static anodization, static cathodization, and potential cycling. Studies have been carried out with polished GCE for comparison. The results show that monomeric molybdate(VI) adsorption is remarkably higher on the electrochemically reduced GCE surface, but there is no adsorption on the anodically pretreated electrode, unlike the other redox systems studied until now,20,22,33,34,36-40 which showed enhanced adsorption on electrochemically oxidized carbon electrodes. In the present work, surface effects of GCE, arising due to different electrochemical surface pretreatment methods, on the preparation of monomeric molybdate(VI)/GCE composite electrodes are analyzed by X-ray photoelectron spectroscopy (XPS), which is a surface-sensitive analytical technique,16,17,27,41-43 by following changes in the surface elemental composition of GCE. 2. Experimental Details 2.1. Materials. (NH4)6Mo7O24‚4H2O was of analytical grade obtained from BDH and used as received without further purification. The Mo(VI) ion concentration in bulk solution was determined by gravimetry as PbMoO4.44 Solutions were made using double-distilled water, which was earlier purified by passing it through an activated charcoal column. 2.2. Apparatus. Electrochemical experiments were carried out with a Wenking potentiostat (ST72) and scan generator (VSG83) coupled with a Graphtech X-Y recorder (WX 2300). The experimental cell was of the usual H type. Solutions were deaerated by purging purified N2. A 3 mm diameter GCE pressfitted into a Teflon shroud was used as the working electrode. The potentials were referred to with respect to SCE unless otherwise stated. A Planix digital planimeter (Tamaya) was used for integration of voltammetric peaks. 2.3. Electrode Pretreatment Methods. The electrodes were first hand-polished to mirror finish by successively fine grades of polishing papers (2/0, 3/0, 4/0 Winning Emergy India), degreased with trichloroethylene, and washed with doubledistilled water. The electrodes were then pretreated by four different methods and designated accordingly as follows: (i) (36) Vasquez, R. E.; Imai, H. Bioelectrochem. Bioenerg. 1985, 14, 389-403. (37) Vasquez, R. E.; Hono, M.; Kitani, A.; Sasaki, K. J. Electroanal. Chem. 1985, 196, 397-415. (38) Hance, G. W.; Kuwana, T. Anal. Chem. 1987, 59, 131-134. (39) Barbero, C.; Silber, J. J.; Sereno, L. J. Electroanal. Chem. 1988, 248, 321-340. (40) Bodalbhai, L.; Brajter-Toth, A. Anal. Chem. 1988, 60, 25572561. (41) Pillai, K. C.; Young, V. Y.; Bockris, J. O’M. Appl. Surf. Sci. 1983, 16, 322-344. (42) Pillai, K. C.; Young, V. Y.; Bockris, J. O’M. J. Colloid Interface Sci. 1985, 103, 145-153. (43) Bowers, M. L.; Hefter, J.; Dugger, D. L.; Wilson, R. Anal. Chim. Acta 1991, 248, 127-142. (44) Chandrasekara Pillai, K.; Ilangovan, G. Bull. Electrochem. 1993, 9, 592-595.

Langmuir, Vol. 13, No. 3, 1997 567 electrodes used as such without any further electrochemical treatment (polished electrode); (ii) electrochemically cycled 20 times at a scan rate of 20 mV s-1 between 0.7 and -1.0 V and termined at 0.7 V (cycled electrode); (iii) polarized for 2 min at -1.5 V (cathodized electrode); (iv) polarized for 2 min at 1.5 V (anodized electrode). A 0.1 M H2SO4 solution was used as the electrolyte for electrode pretreatment. 2.4. X-ray Photoelectron Spectroscopy. GC electrodes (5 mm diameter) treated by any of the specific methods were washed, thoroughly, in a plastic glovebox kept under N2 purging, fixed to Ni sample holders by means of silver epoxy, and loaded into the spectrometer. XPS were obtained with a VG Scientific ESCALAB MK II spectrometer. Mg KR radiation was used as the X-ray source (1252.6 eV) with a pass energy of 50 eV. The pressure inside the analyzer was maintained at 10-9 Torr. For each sample, initially a survey scan was recorded followed by high-resolution spectra of individual elements. Resolution of the spectrometer was 0.1 eV. Acquired data were fed back to a IBM PC for analysis. Data reduction was carried out by deconvoluting each of the high-resolution composite XPS peaks into the peaks of the individual species of different oxidation states. This was done using a nonlinear regression analysis.45 Regression models were constructed from the appropriate number of Gaussian peaks with base-line sigmoidal correction. Peak position, its intensity, and its width for each individual Gaussian peaks were used as input parameters in deconvoluting the measured spectra. The criteria for the best fit were the good agreement between experimental points and calculated spectra and the minimum in the standard deviation. The final result consisted of the area under each of the resolved peaks for the individual oxidation states, the total area under the raw data, and the total area under the convoluted curve, besides the parameters concerning the resolved peaks.

3. Results 3.1. Electrochemistry. 3.1.1. Preparation of Monomeric Molybdate(VI)-Modified GC Electrode by the Potential Cycling Method. Mo(VI) monomer modified electrodes are obtained by cycling the potential of GCE at potential scan rate (v) 200 mV s-1 for 20 times between +0.5 and -0.2 V in a freshly prepared solution of Mo(VI) ion (derived from Mo7O246-) in 0.1 M H2SO4. The cyclic voltammogram of the first cycle during preparation in a 0.3 mM Mo(VI) solution is illustrated in Figure 1, curve A, for a bare cathodized electrode. There are three peaks C1, C2, and C3 during cathodic sweep at potentials 0.210, 0.110, and -0.120 V, and during anodic sweep three peaks A1, A2, and A3 appear at 0.230, 0.130, and -0.090 V. A1, A2, and A3 are anodic counterparts of C1, C2, and C3. The three pairs of peaks C1/A1, C2/A2, and C3/A3 correspond to various forms of Mo(VI) in solution.46 The detailed electrochemical reduction mechanism of monomeric molybdate(VI) shows that they correspond to 1e,1H+ (C1/A1), 1e,1H+ (C2/A2), and 2e,2H+ (C3/A3) processes, and the details will be described elsewhere.46 The electrode is taken out after 20 cycles and rinsed thoroughly with a 0.1 M H2SO4 electrolyte solution, and its electrochemistry is examined in pure supporting electrolyte 0.1 M H2SO4. 3.1.2. Characterization of a Modified Composite Electrode in 0.1 M H2SO4. The voltammogram of Mo(VI) monomer modified GCE recorded in Mo(VI) ion free 0.1 M H2SO4 in the same potential range gives the results shown in Figure 1, curve B. As can be observed, the voltammogram presents the same features described for the solution containing Mo(VI) ion. These voltammograms do not change upon cycling or when the electrode is left in the electrolyte solution for prolonged periods (>48 h). (45) Chandramouli, G. V. R.; Lalitha, S.; Manoharan, P. T. Comput. Chem. 1990, 14, 257-258. (46) Ilangovan, G.; Chandrasekara Pillai, K. In preparation.

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Figure 1. (A) Cyclic voltammogram of the cathodically pretreated GCE in 0.3 mM Mo(VI) in 0.1 M H2SO4. (B) Molybdate(VI)-modified electrode taken from A after 20 cycles and recorded in 0.1 M H2SO4. Scan rate ) 200 mV s-1.

Magnitudes of peak currents for all the cathodic and anodic peaks on modified GCE are almost equal to their respective peaks for the solution analogue Mo(VI) on bare GCE, indicating the strength and irreversible nature of adsorption of the redox components. It is noteworthy that the 0′ ((Epa + Epc)/2) on modified surface formal potential Esur GCE for C1/A1, C2/A2, and C3/A3 redox couples was the same as that for the solution analogue Mo(VI) on bare GCE. This feature is in accord with the first-order predictability of formal potentials generally observed for immobilized molecules.47 The three pairs of peaks C1/A1, C2/A2, and C3/A3 show the characteristics of reversible surface redox electrochemistry. Typically, a plot of cathodic (ipc) as well as anodic (ipa) peak current as a function of v is linear up to 300 mV s-1 scan rate with zero intercept. The peak potentials (Epc and Epa) do not vary with v; the ratios of ipc/ipa are maintained equal to unity at all scan rates (for C3/A3 the ratio is 0.7). However, the peak potential separation (Epa - Epc) is cycled > polished > anodized electrodes. Indeed, the anodically polarized electrode fails to show any voltam-

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Figure 4. Cyclic voltammetry of 2.0 mM Mo(VI) in a 0.1 M H2SO4 solution at different pretreated GC electrodes: (a) polished; (b) cycled; (c) cathodized; (d) anodized (see text). Scan rate ) 100 mV s-1.

metric peaks of the Mo(VI)/Mo(V) redox species. Instead, it gives a large but otherwise featureless capacitive-like current in the presence of Mo(VI) ions in solution (Figure 4, trace d). The Mo(VI) monolayer films fabricated on different pretreated GC electrodes are further examined in a 0.1 M H2SO4 solution devoid of dissolved Mo(VI) ion. Each of the modified electrodes continues to give voltammetric patterns resembling their solution analogue Mo(VI) on bare GC electrodes. It must be emphasized that the adsorption of Mo(VI) ion is totally absent on the anodized 0′ is the same, and the electrode. At all other surfaces Esur peak to peak separation (Epa - Epc) is CdO, -C-O-C-, chemisorbed water, or stoichiometric hydroxide. b Due to >C-OH, adsorbed CO, adsorbed O , bound H O. 2 2 O(R) of -O(β)sCdO(R).

Table 2. Relative Quantities (%) of Different Levels for Various Pretreated GCE Samples (BE Values Given Are Averaged Values) sample energy level C 1s graphic (284.6) alcohol, phenolic, peroxide, or ether (286.3) carbonyl (287.8) carboxyl and ester (289.2) oxidized C/graphitic C alcohol C/oxidized C O 1s a (532.0) b (533.7) c (535.6) O(533.7)/O(total) atomic ratio NO1s/NC1s c

polished

cycled

cathodized

anodized

67.99 17.45 9.34 5.22 0.47 0.55

66.63 21.32 6.81 5.24 0.50 0.64

62.63 26.08 11.29

56.93 27.84 15.23

0.60 0.70

0.76 0.64

82.67 17.33

80.50 19.50

65.52 34.88

0.17 0.21

0.20 0.27

0.35 0.19

77.52 18.85 3.61 0.18 0.54

a Due to >CdO, -C-O-C-, chemisorbed water, or stoichiometric hydroxide. b Due to >C-OH, adsorbed CO, adsorbed O , bound H O. 2 2 O(R) of -O(β)-CdO(R).

sample. This latter observation provides a useful clue to the enhanced activation of the cathodically treated GC electrode to Mo(VI) ion adsorption (Figures 4 and 5). Presumably, formation of a specific surface functional group corresponding to the 286.3 eV binding energy C 1s level on cathodic treatment facilitates the adsorption of the Mo(VI) system. Features of the O 1s Spectrum. The fitted O 1s spectra of the four samples are shown in Figure 10. Two oxygen species are noticed with the polished electrode (Figure 10a) at BE ) 532.0 and 533.7 eV. The lower BE signal (532.0 eV) could be assigned to oxygen-containing species such as >CdO,55 -C-O-C-,41 chemisorbed water, or stoichiometric hydroxide (mostly hydroxide group attached to metal centers).56 The BE ) 533.7 eV could be due to >C-O-, e.g., >C-OH,55 CO, some form of adsorbed oxygen, or bound water.41,42 With cycled electrode (Figure 10b) and cathodized electrode (Figure 10c), two peaks are observed, but anodically oxidized electrode shows an additional peak at higher BE ) 535.6 eV (Figure 10d). Evidence for an O 1s peak at 535.5 eV has been previously reported to correspond to adsorbed water,55 and this peak is also characteristic of O(R) in compounds containing the functional group -O(β)-CdO(R).41,57 (The O(β) has a binding energy of 533.0 eV.) The energy levels of different oxygen species observed with various electrodes are given in Table 1. The relative (55) Kozlowski, K.; Sherwood, P. M. A. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2745-2756. (56) Wagner, C. D.; Zatko, D. A.; Raymond, R. H. Anal. Chem. 1980, 52, 1445-1451. (57) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. In Handbook of X-ray photoelectron spectroscopy; Chastain, J., Ed.; Physical Electronics Division, Perkin-Elmer: Eden Prairie, MN, 1979; p 42.

intensities are listed in Table 2. In all spectra the peak at the lowest BE (532.0 eV) has the greater intensity, although its intensity does vary with pretreatment. With the cathodized sample the binding energy component associated with >C-O- species (BE ) 533.7 eV) increases dramatically. The relative peak area associated with this level is maximum at 0.35 for the cathodized electrode, indicating that >C-O- containing species are produced to a greater extent on cathodization. It may be relevant to note that the O(533.7)/O(total) ratio and alcohol C/oxidized C ratio (Table 2) parallel each other. Both these ratios, having the highest value for the cathodically treated electrode, indicate a direct relation between the presence of >C-O- containing functional groups on the cathodized electrode surface and its greater electrode activity toward Mo(VI) ion adsorption and its reduction. An elemental ratio of total oxygen to total carbon is employed to estimate the amount of oxygenated functional groups on pretreated surfaces. The O/C atomic ratio (NO1s/ NC1s) is evaluated from the corresponding XPS intensities, window widths, and photoionization cross sections for Mg KR.58 The values for pretreated electrodes are listed in Table 2. The O/C atomic ratio is lowest for the cathodically reduced sample (0.19) and it is highest for the anodically oxidized sample (0.54). 3.2.2. XPS of Molybdate(VI) Dip-Coated GCE. The Mo 3d level XPS spectrum of a Mo(VI) dip-coated GCE (prepared as in section 3.1.4) is presented in Figure 11, which also includes a Mo 3d level spectrum of a pressed pellet of (NH4)6Mo7O24‚4H2O for comparison. The standard substance gives rise to two deconvoluted peaks at BE ) 233.4 and 236.5 eV, respectively. These values are consistent with spin orbit splitting of the Mo 3d level of (58) Yeh, J. J.; Lindu, T. At. Data Nucl. Data Tables 1985, 32, 1-155.

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Table 3. Binding Energy Values (eV) of Mo 3d Energy Level for (NH4)6Mo7O24‚4H2O and Mo(VI) Dip-Coated GC Electrode (fwhm Values Given in Parentheses) binding energy (eV) Mo 3d5/2 Mo 3d3/2

sample (NH4)6Mo7O24‚4H2O Mo(VI) dip-coated GCE

233.4 (3.2) 233.0 (3.0) 232.0 (2.7)

236.4 (3.7) 236.2 (3.9) 235.1 (2.5)

binding energy difference (eV)

intensity ratio Mo 3d5/2:Mo 3d3/2

Mo oxidation state

3.0 3.2 3.1

3:2.4 3:2.4 3:2.0

VI VI V

Figure 11. XPS spectra of Mo 3d energy level for (A) (NH4)6Mo7O24‚4H2O; (B) Mo(VI) dip-coated GC electrode. Numbers 5 and 6 are the assigned oxidation states of Mo.

Figure 10. XPS spectra of O 1s energy level for GC electrode pretreated by different methods: (a) polished; (b) cycled; (c) cathodized; (d) anodized.

Mo in the oxidation state +659 (Table 3). During XPS measurements, the ammonium molybdate exhibits photochromism as reported previously by Winograd et al.60 The intensity ratio for 3d5/2:3d3/2 levels is 3:2.4 instead of the theoretical value of 3:2,59 and the small deviation could be related to this effect. The BE values of heptamolybdate are in good agreement with the recent reports of Clayton and Lu,61 although these authors have not reported photochromic behavior of their samples during XPS measurements. The Mo 3d spectrum of a Mo(VI) dip-coated electrode sample is shifted to the lower BE side, indicating the presence of lower oxidation states of Mo. The spectrum can be deconvoluted into four peaks. The two peaks at BE ) 231.9 and 235.0 eV can be assigned to Mo in oxidation state +5, while the other two peaks at BE ) 233.0 and 236.2 eV correspond to Mo in oxidation state +6.59-61 4. Discussion 4.1. Background Process. The background current for carbon electrodes varies greatly with surface history and nearly always contains components from the double(59) Grunert, W.; Stakheev, A. Y.; Feldhaus, R.; Anders, K.; Shpire, E. S.; Minachev, K. M. J. Phys. Chem. 1991, 95, 1323-1328. (60) Kim, K. S.; Baitinger, W. I.; Amy, J. W.; Winograd, N. J. Electron Spectrosc. 1974, 5, 351-367. (61) Clayton, C. R.; Lu, Y. C. Surf. Interface Anal. 1989, 14, 66-70.

layer charging process and faradaic processes. The electrochemical results of Figure 7 show in our case background voltammograms are mostly flat and featureless with all the pretreated electrodes. Even the anodized electrode presents similar voltammograms in contrast to generally reported broad anodic and cathodic surface waves for anodized GC electrodes.13,16,19,21,39 The anodic treatment used in our case is relatively mild (Eapplied ) 1.5 V (SCE) only for 2 min), whereas aggressive conditions like anodization at higher potentials and longer exposure timings, mostly anodic followed by (short-duration) cathodic treatment, etc., have been used in previous studies.13,16,19,21,39 This may account for the lack of creation, on the electrode surface, of highly reactive oxygencontaining surface functional groups and oxidized surface carbons following mild anodization in our case; however, there are other possible explanations. These possibilities include the formation of oxygen surface sites with redox transitions which are quite slow and spread over a wide potential range.11 The significantly lowered background current for a cathodically treated electrode (Figure 7) clearly demonstrates that the processes suggested above for an anodically pretreated electrode are present to a negligible extent. Indeed, all previous reports on GCE background currents and the associated processes have been mainly concerned with anodically pretreated electrodes. No information, either electrochemical or spectroscopic, is available regarding the background processes of GCE pretreated following exclusively the cathodic polarization. In order to obtain a qualitative view of the general process governing the background current, we have utilized our CV and XPS measurements made with pretreated bare electrodes. Expressing the background current as the surface charge (QGC) under the entire cathodic scan in the potential

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Table 4. Atomic Ratio from XPS and Surface Charge from CV Normalized with Respect to Cathodized Electrode for Various Pretreated Samples XPS sample

NO1s/NC1s

polished cycled cathodized anodized

0.21 0.27 0.19 0.54

CV ratio

1a

1.10 1.42 1.00 2.84

QGC (µC)

ratio 2b

41.6 30.9 17.9 121.6

2.32 1.73 1.00 6.79

a Ratio 1: (N O1s/NC1s)x/(NO1s/NC1s)cathodized sample. x denotes specific pretreated sample. b Ratio 2: (QGC)x/(QGC)cathodized sample. x denotes specific pretreated sample.

region +500 to -200 mV (SCE), the calculated values for each of the pretreated bare GC electrodes are given in Table 4. Assuming that QGC represents the amount of carbon-oxygen functional groups and carbon oxides along with the double-layer charging component, the normalized parameters

(QGC)x/(QGC)ref

(2)

(NO1s/NC1s)x/(NO1s/NC1s)ref

(3)

from CV and

from XPS can be considered to represent the relative quantities of surface functional groups caused by pretreatment x relative to the reference sample. The polished sample could not be used as the reference, for reasons to be explained later. Since QGC and NO1s/NC1s are the lowest for the cathodized electrode, it is taken as the reference, and the ratios calculated for the other electrodes are listed in Table 4. The agreement between the two parameters is good for cycled electrode (and, of course, for cathodized electrode), whereas the charge ratio values are consistently higher by around 55% than the atomic ratio values for the other two samples. The vast disagreement in the case of anodized electrode indicates abundant quantities of O and C containing surface species of GCE, generated by anodic pretreatment, which are able to contribute to QGC but escaped detection by XPS. As the XPS technique, with the takeoff angle (θ) equal to 8° used in our studies, is able to detect the O and C that are present only to a depth (λ3) of 1 Å,62,63 a higher value for QGC can be expected if a porous surface layer of considerable thickness has formed on anodic pretreatment. The penetration of the electrolyte ions along with H2O molecules deep into the porous hydrated film can give rise to higher background currents. In the case of a cathodically treated electrode, the surface region modified by cathodization can be regarded as quite small and therefore, approximately, the entire region is sampled by XPS. The behavior of the cycled electrode is quite consistent because the repeated oxidation and reduction due to potential cycling between the anodic and cathodic regions during pretreatment can allow the creation of surface oxygen species in small amounts (NO1s/NC1s ) 0.27). Along with this the formation of the surface porous film is probably thin enough that most of it is sampled by XPS. Note that the two ratios differ by 20% only for the cycled electrode. (62) λ3, the effective mean escape depth for glassy carbon, has been calculated from the expression λ3 ) λ sin θ, where λ is the mean free path of the photoelectron in GCE. λ for GCE has been taken as 7 Å, the value reported in ref 63 for carbon materials. (63) Riggs, W. M.; Parker, M. J. Surface Analysis by X-Ray Photoelectron Spectroscopy. In Methods of Surface Analysis; Czanderna, A. W. Ed.; Elsevier: New York, 1975.

The 55% difference between the two ratios for the polished electrode (Table 4) demonstrates that porous film of considerable thickness is present even on the polished electrode. This observation is consistent with Auger depth profile analysis of extensively polished GCE, which has revealed that polishing could introduce oxygen to a depth of 200 Å or more.27 It is precisely for this reason the polished sample is not used as the reference in calculating the parameters of Table 4. 4.2. Adsorption and Electroreduction of Monomeric Molybdate(VI) Ion at Treated GC Electrodes. The electrochemical results indicate that monomeric molybdate(VI) investigated in this study is strongly adsorbed on GCE when the electrode comes in contact with the Mo(VI) containing H2SO4 solutions. Although equilibrium is reached after ca. 5 min, the adsorption is practically irreversible. Figures 6 and 7 indicate that the adsorption of monomeric molybdate(VI) is highest at cathodically pretreated GCE and completely absent at the anodized GC electrode. Extensive surface oxidation thus seems detrimental for Mo(VI) ion adsorption. The inactivity of the anodized GCE is probably because it is known that, at the anodized GC surface, electron transfer to the anionic redox system is slowed down and anion adsorption is greatly reduced presumably due to carboxylate groups in the oxide film (ion-exchange mechanism).20,34 However, this is unlikely because there appear to be no carboxylic groups present on the anodized GCE surface as can be evidenced from the high-resolution C 1s level XPS spectrum (Figure 9d). Besides, the three redox peaks of Figures 1 and 4 are associated with electroreduction of monomeric Mo(VI) species of different forms, anionic and neutral.46 As oxidized GCE surface responds to all three redox pairs of peaks in the same way, this indicates that the charge-specific mechanism is not operative. The absence of activation of the anodized electrode to Mo(VI) adsorption and its reduction implies that those such activating factors, which are normally attributed to oxidative electrochemical pretreatment, are not effective as far as Mo(VI) ion electrochemistry at GCE is concerned. Thus, one can speculate that Mo(VI) ion adsorption at GCE does not follow the mechanisms: (i) catalytic effects due to surface functional groups (redox mediation,12,15,16 proton transfer mediation17) or (ii) exposure of different lattice sites.23,30,31 Furthermore, since an increase in the background current correlates well with background capacitance and the microscopic surface area, as reported by many17,19,22,26 and our XPS and QGC results discussed in the previous section, the fact that Mo(VI) ion adsorption is not favored by higher background currents (Figures 6 and 10) rules out any mechanism wherein microscopic surface area determines the behavior of this adsorbing species. The activation of cathodically treated GC electrode may arise due to impurity desorption and the associated surface cleaning as a result of vigorous reductive treatment. The enhancement of hydrophilicity induced by reductive treatment, as proposed by some authors,16,17,28 may be applicable to activation of the GC electrode to Mo(VI) adsorption in the present case. However, these interpretations fail to account for the Mo(V) formation in the adsorbed layer on cathodized GCE during Mo(VI) adsorption (Figures 6 and 11B). It seems to be necessary to consider another mechanism. For the formation of Mo(V) in the adsorbed layer on GCE at open circuit potential, we propose that the formation of Mo(V) in the adsorption layer, during Mo(VI) adsorption, consists of both reduction of Mo(VI) to Mo(V) and oxidation of some surface functional groups

+

+

Characterization of GCE Surface Effects

Langmuir, Vol. 13, No. 3, 1997 575

on the reduced GCE by a mixed potential mechanism. A survey of the redox groups on the GCE surface from ref 64A-D indicates that their redox potentials (E1/2) are as follows: carboxylic to alkane (1.74-2.04 V (NHE));64B carbonyl oxidation (0.5-1.4 V (NHE));64C phenol to quinone (1.0-1.2 V (NHE));64C alcohol to carbonyl (2.1-3.0 V (NHE));64D alcohol to peroxide (-0.63 to -0.20 V (NHE)).64C Out of these groups, the redox potential of the alcohol to hydroperoxide group matches to form the oxidation reaction

2>C-OH f >C-O-O-C< + 2H+ + 2e

(I)

(E1/2 ) -0.87 to -0.44 V (SCE)) coupled to the reduction of Mo(VI) to Mo(V) 0′ ) 0.24 V (SCE)) (II) Mo(VI) + e f Mo(V) (Esur

The open circuit potential (Eocp) of GCE in Mo(VI) containing 0.1 M H2SO4 is observed to be 200 mV (SCE) (Figure 6). This is in between E0′ values of the above two processes (reactions I and II) and therefore fulfills the primary requirement for the mixed potential mechanism being operative.65 The Eocp ) 200 mV (SCE) is close to 0′ Esur of the Mo(VI)/Mo(V) redox system (240 mV (SCE)), and this implies that the Mo(VI) reduction to Mo(V) should be highly reversible compared to the coupled reaction (reaction I) in accordance with the mixed potential mechanism.65 Two experimental observations justify this requirement. First, it is clear from Figures 1 and 4 that reaction II (C1/A1 redox transition) is indeed kinetically facile. Second, the lack of a detectable pair of surface redox peaks in the voltammograms of the bare cathodized electrode in the blank solution (Figure 7) is a clear indication of the slowness of the surface group oxidation. It is due to this vast difference in the degree of reversibility of the two reactions I and II65 that the peak A2 that results when the electrode is anodically polarized from its Eocp (64) (A) Randlin, C. P. Carbon. In Encyclopedia of Electrochemistry of Elements; Bard, A. J., Lund, H., Eds.; Dekker: New York, 1976; Vol. VII, Chapter 1. (B) Eberson, L.; Nyberg, K. Carboxylic Acids, Esters and Anhydrides. In Encyclopedia of Electrochemistry of Elements; Bard, A. J., Lund, H., Eds.; Dekker: New York, 1980; Vol. XII, Chapter 2. (C) Evans, D. H. Carbonyl Compounds. In Encyclopedia of Electrochemistry of Elements; Bard, A. J., Lund, H., Eds.; Dekker: New York, 1980; Vol. XII, Chapter 1. (D) Parker, V. D.; Suntholm, G.; Svanholm, U.; Ronlan, A.; Hammerich, O.; Hydroxyl Compounds. In Encyclopedia of Electrochemistry of Elements; Bard, A. J., Lund, H., Eds.; Dekker: New York, 1980; Vol. XII, Chapter 2. (65) Shreir, L. L. Corrosion; Butterworth: Oxford, U.K., 1994.

(Figure 6) can be considered to correspond to the reverse of reaction II, namely, Mo(V) oxidation, with insignificant contribution from the surface group oxidation reaction (reaction I). The Mo(VI) adsorption on GCE via a mixed potential mechanism is consistent with the behavior of various electrochemically pretreated GC electrodes. The XPS data provide a more detailed proof of this effect. It is clear from Table 2 that for the cathodized electrode the amount of the specific >C-O- containing functional groups is highest. As the >C-OH group is essential for the adsorption of Mo(VI) ion via a mixed potential mechanism, lower quantities of this surface compound introduced by the other treatments can decrease the response of these pretreated electrodes, as observed experimentally (Figure 4). It is of interest to note that the polished electrode shows a better response than the anodized electrode in spite of the comparable quantity of >C-O- groups on these two samples (O(535.7)/O(total) = 0.17). This must be related to the lesser oxidation of the polished surface (oxidized C/graphitic C ) 0.47) compared to the anodically oxidized sample (0.76). Indeed, it is this higher surface oxidation which renders the anodized electrode totally inactive to Mo(VI) adsorption, although moderate quantities of >C-O- containing groups are present on its surface. These studies thus show that, as far as Mo(VI) adsorption on GCE is concerned, the spontaneity of the coupled reactions I and II seems to be the prime activating factor of cathodized GCE. Other factors like the surface cleaning by impurity removal and enhanced hydrophilicity on account of surface reduced groups notably alcoholic introduced by the cathodic pretreatment appear to assist additionally the activation of the cathodized GCE, at least during the initial stage of adsorption. It is indeed wellknown that the introduction of a hydroxy group renders a molecule more hydrophilic.66 A positive confirmation of the quantity of surface hydroxyl groups on pretreated GCE surfaces by the suggested derivatization method67 is in progress, though at present we have no data to report. Acknowledgment. The authors are grateful to the Council of Scientific and Industrial Research, New Delhi, India, for awarding both JRF and SRF to G.I. Acknowledgments are also due to Regional Sophisticated Instrumentation Centre, IIT, Madras, India, for providing the XPS instrumental facility and BANDFIT program. LA960053N (66) Espenscheid, M. W.; Ghataic-Roy, A. R.; Moore, R. B., III; Renner, R. M.; Szentirmay, M. N.; Martin, C. R. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1051-1070. (67) Collier, W. G.; Tougas, T. T. Anal. Chem. 1987, 55, 396-399.