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Langmuir 1998, 14, 4802-4808
Selenolates as Alternatives to Thiolates for Self-Assembled Monolayers: A SERS Study Frank K. Huang, Ronald C. Horton, Jr., David C. Myles, and Robin L. Garrell* Department of Chemistry and Biochemistry, University of California, Los Angeles Los Angeles, California 90095-1569 Received March 4, 1998 To determine whether selenolates are viable alternatives to thiolates for self-assembled monolayers (SAMs), the formation and oxidative stability of monolayers made from diphenyl diselenide (DPDSe) solution were assessed by surface-enhanced Raman spectroscopy. Upon adsorption, the diselenide bond is cleaved to form benzeneselenolate, analogous to formation of benzenethiolate monolayers from diphenyl disulfide (DPDS). DPDSe displaces benzenethiolate from gold, but DPDS does not displace benzeneselenolate. Competitive adsorption experiments show that adsorption of DPDSe is more favorable by ∼0.7 kcal/mol. Unlike benzenethiolate, the benzeneselenolate monolayer is unstable both in air and to UV light. Long-term exposure to air results in oxidation to protonated and deprotonated benzeneseleninic acid. Exposure to UV results in C-Se bond cleavage (analogous to C-S bond cleavage in benzenethiolate) and formation of SeO2 and SeO32-. The higher adsorptivity of benzeneselenolate and its similar oxidative behavior to benzenethiolate suggests that selenolates are an attractive alternative to thiolates for building SAMs.
Introduction Alkanethiolates have been widely used in model studies of the structure, wettability, and stability of SAMs. One of the reasons for this is that the Au-S bond in alkanethiolate SAMs is very stable, estimated at ∼40 kcal/ mol.1-4 Strategies for the design of novel SAMs on gold take advantage of this strong thiolate-gold interaction and focus on modifying the tail group functionality.1,3,5-8 Such thiolate SAMs have been incorporated into biomimetic films, sensors, protective coatings, and lubricants. Despite their strong affinity for gold, adsorbed thiolates are unstable toward ozone treatment, air oxidation, and photooxidation.9-17 The instability of thiolate SAMs makes them less than optimal for use in air or harsher environments. We therefore need to consider alternative adsorbates that can provide a more stable and stronger * To whom correspondence should be addressed. (1) Nuzzo, R. G.; Segarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (2) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (3) Ulman, A. Chem. Rev. 1996, 96, 1533. (4) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (5) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371. (6) Evans, S. D.; Goppert-Berarducci, K. E.; Urankar, E.; Gerenser, L. J.; Ulman, A. Langmuir 1991, 7, 2700. (7) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121. (8) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (9) Dishner, M. H.; Feher, F. J.; Hemminger, J. C. Chem. Commun. 1996, 1971. (10) Garrell, R. L.; Chadwick, J. E.; Severence, D. L.; McDonald, N. A.; Myles, D. C. J. Am. Chem. Soc. 1995, 117, 11563. (11) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342. (12) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657. (13) Lewis, M.; Tarlov, M. J.; Carron, K. J. Am. Chem. Soc. 1995, 117, 9574. (14) Worley, C. G.; Linton, R. W. J. Vac. Sci. Technol. 1995, 13, 2281. (15) Teuscher, J. H.; Huang, F. K.; Yeager, L. J.; Garrell, R. L. J. Am. Chem. Soc., submitted. (16) Li, Y.; Huang, J.; Robert T. McIver, J.; Hemminger, J. C. J. Am. Chem. Soc. 1992, 114, 2428. (17) Huang, F. K. Ph.D. Thesis, University of California, Los Angeles, 1998.
adsorbate-surface interaction. Selenols represent one such alternative. There has been little work on organoselenolate monolayers. Samant et al. were the first to characterize a selenolate monolayer.18 Using X-ray diffraction, they determined the two-dimensional structure of the docosaneselenolate SAM on gold. They found that it was distorted ∼3% from the typical (v3 × v3)R30° structure of a thiolate SAM. They determined that the molecular tilt angle was 15 ( 1° off-perpendicular.18 This value is only slightly larger than the 12° tilt angle for the analogous thiolate SAM. Although the structure of the alkanethiolate and alkaneselenolate SAMs are similar, the strength of the adsorbate-surface interactions were postulated to be different. On the basis of literature values for Au-Se and Au-S bond strengths,19 Samant et al. proposed that the thiolate chemisorbs more strongly than the selenolate. Although they presented no experimental evidence to support this hypothesis, they may have discouraged others from using organoselenolates to fabricate self-assembled monolayers. A more recent study of organoselenium monolayers was completed by Dishner et al. In this work, scanning tunneling microscopy (STM) revealed that the twodimensional structure of benzeneselenolate monolayers on gold follows a (x3 × x3)R30° unit cell.20 The STM images also revealed gold islands, which they attributed to the effects of chemisorbing benzeneselenol on gold. After thermal annealing, the gold surface undergoes Ostwald ripening, forming hexagonal stepped terraces that are stabilized by adsorbed benzeneselenolate.20 The fact that STM images of well-ordered benzeneselenolate monolayers could be obtained, while STM images of benzenethiolate monolayers cannot,21 suggests that the selenium(18) Samant, M. G.; Brown, C. A.; Gordon, J. G., II. Langmuir 1992, 8, 1615. (19) Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: New York, 1997. (20) Dishner, M. A.; Hemminger, J. C.; Feher, F. J. Langmuir 1997, 13, 4788. (21) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319.
S0743-7463(98)00263-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/30/1998
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gold interaction is stronger than the sulfur-gold interaction. The stronger interaction results in reduced mobility and improved imaging of the adsorbed selenolate. The goal of the present work is to determine whether selenolates form more robust monolayers than thiolates. Two characteristics are compared: the adsorptivities of thiols vs selenols and the stability of the monolayers to air and UV radiation. Diphenyl disulfide (DPDS) and diphenyl diselenide (DPDSe) were selected as model compounds. Surface-enhanced Raman (SER) spectroscopy was used to determine the compositions of DPDS and DPDSe mixed monolayers formed by displacement and competitive adsorption. The compositions of the monolayers allowed us to determine the relative adsorptivities of DPDS and DPDSe. The stability of DPDSe monolayers to photooxidation and air oxidation was characterized by SER spectroscopy. The results demonstrate that selenolates are attractive alternatives to thiolates for fabricating SAMs on gold. Methods Experiments. Chemicals. Diphenyl disulfide (Aldrich, 99%) and diphenyl diselenide (Acro˜s, 99%) were recrystallized in ethanol. Absolute ethanol (Quantum Chemical Corp.) was used as the solvent for the disulfide/diselenide solutions. Electrode Polishing and Roughening. A rotating disk gold electrode (Pine Instruments, Grove City, PA) was polished with 5 µm silicon carbide and subsequently with 1 µm alumina. The electrode was then roughened in an electrochemical cell containing N2-purged 0.1 M KCl, a Pt auxiliary electrode, and a SCE reference electrode. The roughening procedure consisted of 20 oxidation-reduction cycles between -0.6 and +1.2 V at a rate of 0.5 V/s, pausing at -0.6 for 8 s and at 1.2 V for 1.2 s, and finally holding the potential at -0.6 V for 5 min. The roughened electrode was rinsed with purified water and air-dried prior to immersion in diselenide or disulfide solution. Monolayer Preparation. Aromatic thiolate and selenolate monolayers were prepared by immersing the gold surface (electrode or evaporated gold film) in 2 mM diphenyl diselenide or diphenyl disulfide in ethanol solution for at least 1 h. The monolayers were rinsed thoroughly with ethanol. Raman Spectroscopy. Raman spectra were obtained with a Jobin-Yvon HR 640 single monochromator with a 1200 groove/ mm holographic grating, SPEX liquid N2-cooled CCD detector, and Kaiser supernotch holographic prefilter. The monochromator and data acquisition were controlled by SPEX Prism software on a personal computer. A Lexel 3500 laser (Ar+, all lines) pumped a Lexel 479 cw Ti/sapphire laser to provide 70-80 mW of 735 nm radiation, focused to a diameter of ca. 400 µm, for the SER experiments. The scattered light was collected at a 90° angle to the excitation beam with a Canon 50 mm f/1.4 redenhanced lens. The slit width and height were 30 µm and 20 mm, respectively, for all spectra, giving a spectral band-pass of 0.7 cm-1 at 735 nm. The 1061 cm-1 band in the SER spectrum of benzeneselenolate and the 1074 cm-1 band in the SER spectrum of benzenethiolate were used for qualitative and quantitative characterization of the monolayers. These bands have different SER cross sections, which need to be taken into account in calculating monolayer composition. This was done by determining the relative SER cross sections of the two bands, calculated by ratioing the areas of the two bands in the SER spectra of the pure monolayers. The pure monolayers were formed by placing ∼0.25 µL of a 2 mM ethanolic solution of the diphenyl diselenide or diphenyl disulfide on separate halves of the same electrode. After the ethanol evaporated, the monolayers were rinsed thoroughly with ethanol and air-dried. Spectra of several spots on several different surfaces were obtained. The 1061 and 1074 cm-1 band areas were calculated using the multipeak fitting utility in the IGOR program. To compare data obtained from different surfaces, the areas of the 1074 and 1061 cm-1 bands were normalized against a band at 734 cm-1 in the Raman spectrum of Teflon (surrounding the SER-active area of the electrode) obtained immediately after each SER spectrum. The SER cross section of the 1074 cm-1
Figure 1. Raman spectrum of solid diphenyl diselenide (a) and the SER spectrum of benzeneselenolate (formed by adsorbing DPDSe) (b) and benzenethiolate (formed by adsorbing DPDS) (c) on roughened gold. band of the thiolate was determined to be 1.2((0.1) times that of the 1061 cm-1 band of the selenolate; this is the relative SER cross section factor. Displacement experiments were performed by first immersing a roughened gold electrode for 2 h in a 2 mM ethanolic DPDS or DPDSe solution. Displacement was then evaluated by immersing the monolayer-coated surface in a 2 mM ethanolic solution of the second adsorbate. After a specified immersion period, the electrode was rinsed with ethanol and dried in air. SER spectra were obtained of the monolayer as prepared, and after the specified immersion times in the second solution. The competitive adsorption experiments were performed by immersing the polished, roughened gold electrode for 1 h in an ethanolic solution containing a mixture of DPDS and DPDSe. The mole ratios ranged from 0.10:0.90 to 0.99:0.01 (DPDS/ DPDSe), with a total adsorbate concentration of 1 mM. After the electrode was rinsed with ethanol, SER spectra of three surface locations were obtained for each mixed monolayer. The areas of the 1061 and 1074 cm-1 bands were calculated and the monolayer composition was determined by dividing the ratio of the DPDS/DPDSe band areas by 1.2, the relative SER cross section factor. The resulting value was then converted to mole fraction DPDS and DPDSe. Calculations. Ab initio calculations were performed using the Spartan program package.22 Geometry optimization of benzeneselenolate was performed using Hartree-Fock (HF) theory with a 3-21G(*) basis set (HF/3-21G(*)). Molecular orbital energies and the electrostatic charges were also calculated using the Spartan program package.
Results and Discussion Homogeneous Monolayers of Selenolates and Thiolates. Figure 1 shows a Raman spectrum of solid DPDSe (a) and the SER spectrum of a gold electrode surface immersed in an ethanolic DPDSe solution (b) and DPDS solution (c). In Figure 1a, the 260 cm-1 band at arises from the ν(SeSe) vibration and the 310 and 664 cm-1 bands have contributions from the ν(CSe) vibration.23,24 The 997, 1018, 1061, and 1571 cm-1 bands are assigned to in-plane phenyl ring modes.23,24 A SER spectrum of benzenethiolate (2) is shown Figure 1c. The (22) “Spartan 3.0.1”, Wave function Inc., 1994. (23) Rasheed, F. S.; Kimmel, H. S. Spectrosc. Lett. 1977, 10, 791. (24) Green, W. H.; Harvey, A. B. J. Chem. Phys. 1968, 49, 3586.
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bands at 419 and 694 cm-1 have contributions from ν(CS). The bands at 997, 1018, and 1074 are assigned to phenyl ring modes. The structure of the monolayer formed from DPDSe was analyzed by comparing its spectrum to that of a benzenethiolate monolayer formed from DPDS (Figure 1). The band at 260 cm-1 does not appear in the spectrum, indicating the absence of the Se-Se bond in the monolayer. We propose that the Se-Se bond cleaves upon adsorption onto gold, forming benzeneselenolate (1). This is analogous to the behavior of diphenyl disulfide, in which the S-S bond cleaves upon adsorption to form benzenethiolate (2).25 To determine qualitatively the orientation of benzeneselenolate, we examined differences in relative intensities in the Raman (Figure 1a) and SER spectrum (Figure 1b). Bands that decrease in relative intensity from the Raman to the SER spectrum involve changes in polarizabilities that are more parallel to the surface. Bands that increase in relative intensity arise from modes involving changes in polarizabilities that are more perpendicular to the surface. The bands at 304 and 664 cm-1 decrease in relative intensity in the SER spectrum, suggesting that the orientation of the C-Se bond must be near parallel to the surface. Bands that have contributions from in-plane phenyl ring modes (997, 1018, and 1061 cm-1) have high relative intensities in the SER spectrum of benzeneselenolate. This suggests a near perpendicular orientation of the phenyl ring relative to the surface. The deduced orientations of the C-Se bond and the phenyl ring lead to a logical adsorbate orientation in which the C-Se bond is slanted and the phenyl ring is upright relative to the surface.
Displacement Experiments. Previous work has shown that adsorbates that can interact strongly with gold can displace adsorbates that interact more weakly.10,26,27 The feasibility of using SER spectroscopy to monitor such displacement reactions has been demonstrated for aromatic thiolates and sulfinates.10,27,28 For example, SER spectra of a benzenesulfinate monolayer on gold after immersion in benzenethiolate solution show that the sulfinate is completely displaced by thiolate.10,27 We used SERS to determine whether benzeneselenolate has a stronger or weaker interaction with gold than benzenethiolate. Figure 2b shows a plot of the mole fraction benzenethiolate in the monolayer as a function of immersion time for a pure benzeneselenolate monolayer immersed in a 2 mM ethanolic DPDS (disulfide) solution. The mole fraction benzenethiolate reaches 0.20 after 5 min immersion and plateaus at 0.30 after 3 h. At 15 h, the mole fraction reaches 0.31; this corresponds to a ratio of ∼30:70 benzenethiolate/benzeneselenolate. The fact that a plateau is reached at this ratio suggests that it is the equilibrium monolayer composition. Figure 2a shows the decrease in benzenethiolate mole fraction when a (25) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir 1998, 14, 3570. Szafranski, C. A. Ph.D. Thesis, University of California, Los Angeles, 1994. (26) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398. (27) Garrell, R. L.; Chadwick, J. E. Colloids Surf. A 1994, 93, 59. (28) Chadwick, J. E.; Myles, D. C.; Garrell, R. L. J. Am. Chem. Soc. 1993, 115, 10364.
Huang et al.
Figure 2. Mole fraction of benzenethiolate as a function of time for the (a) immersion of benzeneselenolate in DPDS solution (2) and (b) the immersion of benzenethiolate in DPDSe solution (b).
benzenethiolate monolayer is immersed in 2 mM ethanolic DPDSe solution. The rate of decrease in thiolate mole fraction when displaced by selenolate appears to be much slower than the rate of growth in thiolate mole fraction when it displaces selenolate. At 5 min, the monolayer mole fraction is ∼0.97. It does not decline to 0.72 until 7 h and continues to decrease even after 15 h. At 50 h, the monolayer mole fraction reaches 0.37, quite close to the plateau value of 0.31 reached in the reverse displacement experiment. The similarity between these values at long immersion times suggests that a monolayer mole ratio of 30:70 represents the equilibrium value for the mixed monolayer composition at room temperature. In the immersion of a pure benzeneselenolate monolayer in DPDS (Figure 2a), the monolayer mole fraction exhibits a high rate of growth from 0 to 5 min and plateaus within 3 h. The high rate of growth implies that DPDS easily intercalates within the benzeneselenolate monolayer without displacement. The different packing structure of benzeneselenolate could explain why DPDS intercalates within the monolayer. Previous work on selenolate monolayers has shown that alkaneselenolates pack differently from alkanethiolates.18 Figure 3 provides evidence to support our hypothesis that DPDS intercalates within a benzeneselenolate monolayer without displacing benzeneselenolate. Figure 3 shows normalized areas of the 1061 cm-1 selenolate and 1074 cm-1 thiolate bands as a function of immersion time for the immersion of benzeneselenolate in DPDS solution (Figure 3a), and the immersion of benzenethiolate in DPDSe solution (Figure 3b). In Figure 3a, the area of the benzeneselenolate band remains fairly constant while the area of the benzenethiolate band increases as a function of immersion time. We interpret this as an increase in benzenethiolate surface coverage without detectable loss of benzeneselenolate. Benzenethiolate intercalating within the benzeneselenolate monolayer could be the result of lower initial surface coverage in the benzeneselenolate monolayer. We therefore attempted to improve the benzeneselenolate surface coverage by immersing the roughened electrode in a 20 mM solution of DPDSe for 2 h and subsequently immersing it in a 2 mM DPDS solution. The results of this experiment are presented in Figure 4. Despite the higher concentration of DPDSe used to form the pure selenolate monolayer, benzenethiolate is still incorporated
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Figure 5. Monolayer mole fraction of benzenethiolate as a function of solution mole fraction of DPDS: data points (2), average for each solution mole fraction (O), and Langmuir isotherm fit (- - -). Figure 3. Plot of normalized intensities of the SER identification bands for benzeneselenolate (b) and benzenethiolate (2) in the immersion of benzeneselenolate in DPDS solution (a) and the immersion of benzenethiolate in DPDSe solution (b).
Figure 4. Mole fraction of benzenethiolate as a function of time for the immersion of a 20 mM DPDSe benzeneselenolate monolayer into DPDS solution.
into the monolayer. It appears that a 10-fold increase in the solution concentration does not increase surface coverage to inhibit incorporation of benzenethiolate. This either suggests that maximum surface coverage is achieved for the immersion of gold into 2 mM DPDSe or that further treatment of the monolayer may be required to achieve greater surface coverage. In the immersion of benzenethiolate in DPDSe solution (Figure 2b), the mole fraction of benzenethiolate decreases slowly up to and after 15 h, suggesting that benzeneselenolate displaces benzenethiolate. As shown in Figure 3b, the intensity of the benzenethiolate band decreases while the intensity of the benzeneselenolate band increases as the pure benzenethiolate monolayer is immersed in DPDSe, indicating that benzeneselenolate displaces ben-
zenethiolate. When a benzenethiolate monolayer is immersed in DPDSe solution, benzeneselenolate displaces benzenethiolate until benzeneselenolate molecules occupy as many sites as allowed for a pure benzeneselenolate monolayer. At this stage, some benzenethiolate molecules remain, and the monolayer composition reaches equilibrium at the ∼30:70 ratio. The displacement experiments provided a measure of the relative chemisorption strengths of benzeneselenolate and benzenethiolate, as well as insights into surface coverage in the pure monolayers. Benzeneselenolate adsorbs more readily that benzenethiolate, contrary to the prediction of Samant et al.18 This suggests that selenolates can serve as alternatives to thiolates, when stronger adsorbate-surface interactions are desirable. The structure of the benzeneselenolate monolayer appears to allow benzenethiolate to intercalate without displacing the selenolate. While we have offered evidence for the intercalation, a more direct method is required to characterize the surface coverage in the monolayers. Only then can we establish when and why benzenethiolate intercalates into benzeneselenolate monolayers. Competitive Adsorption Experiments. We performed competitive adsorption experiments to determine the relative adsorption equilibrium constants and the relative free energies of adsorption of benzenethiolate and benzeneselenolate. The experiments were conducted by immersing the roughened gold electrode into a solution containing both DPDS and DPDSe in a known ratio. We obtained SER spectra of the mixed monolayers made from DPDS/DPDSe solutions of known composition. The spectra were analyzed to obtain the compositions of the mixed monolayers. Figure 5 shows a plot of the mole fraction of benzenethiolate in the monolayer as a function of mole fraction DPDS in the immersion solution. In a 50:50 DPDS/DPDSe solution mole ratio, the monolayer mole ratio reaches ∼20: 80. For a 90:10 solution ratio, the monolayer ratio is 40: 60. It is not until the solution composition reaches 99.9: 0.1 that the monolayer becomes essentially pure benzenethiolate. The plot of monolayer vs solution composition (Figure 5) shows that benzeneselenolate adsorbs more readily than benzenethiolate, but not overwhelmingly. We were concerned about the scatter in the monolayer composition data obtained in the 0.4-0.8 DPDS solution
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mole fraction range (e.g., range of 0.1-0.3 monolayer mole fraction for 0.6 solution mole fraction). It is possible that a 1 h adsorption time results in submonolayer coverage, which could lead to nonequilibrium monolayer compositions. To resolve this problem, we monitored the monolayer composition as a function of immersion time for a bare gold electrode immersed in a 50:50 DPDS/DPDSe solution, and for another electrode immersed in a 95:5 DPDS/DPDSe solution. The results (not shown) reveal that the monolayer composition reaches equilibrium, and the SER signal maximizes within 30 min. A 1 h immersion was therefore sufficient to obtain maximum surface coverage. Other possible causes for the scatter in the data would include poorer peak fits for weak spectral bands or errors of stock solution volumes. Both possibilities can be ruled out. The band intensities in the 0.4-0.8 mole fraction range are relatively high, and the signal-to-noise ratio in the spectra is very good, so the peak fits should be reliable. The precision in measuring aliquots of stock solution is better than 1%. The source of the scatter remains undetermined. The difference in the free energies of adsorption (∆Gads) of benzenethiolate and benzeneselenolate was determined from adsorption Keq values of each component, obtained by fitting the data in Figure 5 to a competitive adsorption isotherm model. We used a variation of the Langmiur analytical isotherm for a two-component adsorption process.29
θA ) θB )
(KAχA)R 1 + (KAχA)R + (KBχB)β (KBχB)β 1 + (KBχB)β + (KAχA)R
(1)
(2)
In this model, θA is the monolayer mole fraction and χA is the solution mole fraction of component A. R and β are constants representing the component-specific nonidealities, which can range from 0 to 1.29 A value of 1 follows the ideal Langmuir isotherm, in which all active sites have equivalent adsorption energies.30,31 A value less than 1 shows the degree of deviation from the Langmuir isotherm, with 0 indicating the largest distribution of active-site adsorption energies.30,31 KA is the equilibrium constant, defined by
KA )
[Aads] χA[S]
(3)
[Aads] is the surface concentration of one component and [S] is the concentration of bare sites.32 For our purposes, we substituted χB with 1 - χA and fitted eq 1 to the data in Figure 5 with θA designated as the monolayer mole fraction. The fitting involved calculating Keq, R, and β to obtain the best fit. We calculated Keq values of 3.1 × 104 and 1.0 × 105 for benzenethiolate and benzeneselenolate, respectively. For the nonideality terms (R and β), we obtained values of 0.5 (benzenethiolate) and 0.6 (benzeneselenolate), indicating the deviation from the pure Langmuir isotherm for competitive adsorption. The greater Keq value for benzeneselenolate is consistent with (29) Koopal, L. K.; Riemsdijk, W. H. v.; Wit, J. C. M. D.; Benedetti, M. F. J. Colloid Interface Sci. 1994, 166, 51. (30) Sips, R. J. Chem. Phys. 1950, 18, 1024. (31) Sips, R. J. Chem. Phys. 1948, 16, 490. (32) Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces; John Wiley & Sons: New York, 1996.
Figure 6. SER spectra of benzeneselenolate monolayer on gold, as prepared and after 5 and 20 days exposure to air, and the SER spectrum of benzeneseleninate on gold.
our experimental observations that benzeneselenolate adsorbs more readily than benzenethiolate. We use the relationship between Keq and ∆Gads, shown in eq 4, to calculate the ∆Gads for each monolayer component:
∆Gads ) -RT ln K
(4)
The difference in the free energies of adsorption for DPDS and DPDSe, ∆∆Gads was calculated to be 0.7 kcal/ mol, favoring DPDSe adsorption. This small difference is sufficient to favor benzeneselenolate adsorption over benzenethiolate adsorption and is consistent with the similar chemical structure of the two compounds. Factors Contributing to Relative Adsorptivity. Chadwick et al. have shown that the relative HOMO energies can be used to predict the relative adsorptivities of adsorbates.10,27,28 Charge-transfer interactions between the surface and the adsorbate are most strongly influenced by overlap between the HOMO of the adsorbate and the LUMO of the surface.28 Since we used gold as the surface for all of the monolayers, the LUMO remains constant, and the level of interaction depends only on the HOMO energy of the adsorbate. We performed a geometry optimization of benzeneselenolate at the HF/3-21G(*) level. The HOMO of benzeneselenolate was calculated to be -1.70 eV. The HOMO of benzenethiolate has been previously calculated to be -1.94 eV.10 The HOMO energies between these two adsorbates predict that if charge-transfer interactions governed the adsorptivities, benzeneselenolate would adsorb more readily than benzenethiolate, consistent with our displacement and competitive adsorption experimental results. We also calculated the electrostatic charges using the Mulliken population analysis. Because the effective charge on the gold surface is electropositive, the headgroup (-S- or -Se-) that possesses a more negative charge will be more strongly attracted to the surface.10,27,28 We obtained values of -0.54 for benzenethiolate and -0.63 for benzeneselenolate. The more negative charge on the selenium may contribute to the greater adsorptivity of the selenolate compared with the thiolate. Air Oxidation of Benzeneselenolate Monolayers. We evaluated the long-term stability of benzeneselenolate on gold under ambient conditions. Figure 6 displays the SER spectra of benzeneselenolate on gold immediately after monolayer formation and after 5 and 20 days of air
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Figure 7. Magnified SER spectra of air-oxidized benzeneselenolate and the SER spectrum of benzeneseleninate.
Figure 9. SER spectra of benzeneselenolate on gold UV irradiated for 10 min (a), 30 min (b), and 2 h (c) and then rinsed with triply distilled water (d).
Figure 8. Raman spectrum of solid benzeneseleninic acid (a) and the SER spectrum of benzeneseleninate on gold (b).
exposure. After 5 days of air exposure, we observed the appearance of bands at 675, 838, 1282, 1580, and 1598 cm-1. The 675 cm-1 band is not easily seen. Figure 7 shows a magnified SER spectrum after 5 days of air exposure and confirms the existence of the 675 cm-1 band. Initially, we suspected that these bands might be due to benzeneseleninate (3).
For reference, we obtained a Raman spectrum of solid benzeneseleninic acid and an SER spectrum of benzeneseleninate monolayer on gold, shown in Figure 8. The benzeneseleninate monolayer was formed by immersing a roughened gold electrode in 2 mM aqueous benzeneseleninic acid. The 390 cm-1 band in the Raman spectrum (33) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley & Sons: New York, 1986; p 248. (34) Klayman, D. L.; Gu¨nther, W. H. H. Organic Selenium Compounds: Their Chemistry and Biology; John Wiley & Sons: New York, 1973.
of benzeneseleninic acid (Figure 8a) is assigned to δ(SeO2-), and the 786 and 851 cm-1 bands are assigned to νas(SeO2-) and νs(SeO2-).33-37 In the SER spectrum of benzeneseleninate, the 404 cm-1 band is assigned to δ(SeO2-), based on its correlation with the band at 390 cm-1 from the Raman spectrum. The bands at 775 and 838 cm-1 (magnified in Figure 7) are attributed to νas(SeO2-)and νs(SeO2-). These bands are red shifted compared with their corresponding bands in the Raman spectrum (Figure 8a). These shifts indicate bidentate O-coordination of benzeneseleninate to gold.28 The SER spectrum of benzeneselenolate air oxidized for 5 days (Figure 6) has bands at 1282, 1580, and 1598 cm-1 that were not present in the SER spectrum at 0 days. These bands are also present in the SER spectrum of benzeneseleninate (Figure 8b), supporting the hypothesis that benzeneselenolate air oxidizes to benzeneseleninate. The νs(SeO2-) band at 838 cm-1 also appears in the SER spectrum of air-oxidized benzeneselenolate, offering further support for benzeneseleninate formation; however, the normally weaker band at 775 cm-1, assigned to νas(SeO2-), is too weak to be observed and may be buried in the background. The δ(SeO2-) band at 404 cm-1 could not be identified in the spectrum of the oxidized monolayer, because it overlaps a band at 404 cm-1 in the SER spectrum of benzeneselenolate. We do not have a solid assignment for the band at 675 cm-1 but tentatively assign it to ν(Se-OH).37 In doing so, we suggest that the product formed by oxidation and exposure to humidity may be benzeneseleninic acid rather than benzeneseleninate. Photooxidation of Benzeneselenolate Monolayers. Figure 9 shows the SER spectra of benzeneselenolate irradiated with UV light for (a) 10 min, (b) 30 min, and (c) 2 h. At 10 min irradiation, the 775, 838, 1282, 1580, and 1598 cm-1 bands appear, indicating formation of benzeneseleninate. After 30 min irradiation, the in-plane phenyl ring modes at 997, 1018, 1061, and 1571 cm-1 have significantly decreased in intensity, while the bands at (35) Falk, M.; Gigue`re, P. E. Can. J. Chem. 1958, 36, 1680. (36) Gigue`re, P. A.; Falk, M. Spectrochim. Acta 1960, 16, 1. (37) Paetzold, R. Z. Chem. 1964, 4, 321.
4808 Langmuir, Vol. 14, No. 17, 1998
310 and 661 cm-1, assigned to ν(CSe), have nearly disappeared from the spectrum. These changes clearly indicate that the C-Se bond has cleaved, resulting in loss of phenyl. (Presumably, the desorbed species are phenol or biphenyl.) We observed similar behavior when benzenethiolate on roughened gold was UV irradiated in air: the C-S bond cleaves, and the phenyl ring desorbs.15,17 Benzenesulfinate, however, was not detected. In the UV irradiation of benzeneselenolate, benzeneseleninate was detected after only 10 min of irradiation. The SER spectrum in Figure 9b contains bands that could not be assigned to benzeneselenolate or benzeneseleninate. The broad shoulder at 424 cm-1 has contributions from both δ(SeO2-) and δ(SeO32-). The 471 and 538 cm-1 bands could not be assigned. The 773 and 856 cm-1 bands have contributions from the νas(SeO32-) and νs(SeO32-) modes of adsorbed SeO32-,33,35 and the 803 and 909 cm-1 bands have contributions from the νas(SeO2) and νs(SeO2) modes of adsorbed SeO2.33,36 The formation of the two selenium oxide species, SeO2 and SeO32-, is consistent with C-Se bond cleavage. The photooxidation of benzeneselenolate is quite analogous to that of benzenethiolate on roughened gold. After 2 h of UV irradiation (Figure 9c), the bands assigned to the phenyl ring decrease in intensity compared to 30 min of UV irradiation, showing continued loss of phenyl. We performed an aqueous rinse on the surface after 2 h of UV irradiation and obtained a SER spectrum of the rinsed surface. The resulting spectrum, shown in Figure 9d, is essentially that of a bare gold surface, showing that the selenium oxide species are readily removed by water. The reactivity of benzeneselenolate to UV radiation has (38) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (39) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (40) Bandyopadhyay, K.; Vijayamohanan, K. Langmuir 1998, 14, 625.
Huang et al.
promise for applications in photopatterning monolayers, as has been done for thiolate monolayers on gold.38,39 The benzeneselenolate monolayer can be masked and UV irradiated to convert unmasked areas of benzeneselenolate to SeO2 or SeO32-. These compounds can then be rinsed off to reveal the mask pattern of bare gold. Conclusions We have examined two factors that determine whether selenolates can act as alternatives to thiolates for fabricating self-assembled monolayers on gold: adsorptivity and oxidative stability. Our results demonstrate that benzeneselenolate possesses greater adsorptivity than benzenethiolate. This implies that selenolates could be used as alternatives to thiolates when stronger adsorbatesurface interactions are desired. Their disadvantage is that they may not form monolayers with as high a surface coverage as can be attained with thiolates. Because of this, selenolates may be less useful as protective coatings. Benzeneselenolate appears to be less stable to air oxidation than benzenethiolate. Both adsorbates photooxidize upon UV irradiation. Their mechanisms of degradation appear to be similar: both undergo C-Y bond cleavage and form the respective selenium or sulfur oxides. The fact that benzeneselenolate monolayers oxidize in air makes them unreliable for use in air for long periods of time. Acknowledgment. The authors thank Dr. Tonya Herne at the National Institutes of Standards and Technology for valuable discussions. Note Added in Proof: The recent cyclic voltammetry, electrical impedance, and quartz microbalance measurements of Bandyopadhyay and Vijayamohanan show that diphenyl diselenide forms a self-assembled monolayer on polycrystalline gold with 99% surface coverage.40 LA980263V
