gold (111) interface

339

1. EkctroanaL Chem., 324 (1992) 339-358 Elsevier Sequoia S.A., Lausanne

JEC 01873

Adsorption of benzonitrile (111) interface Jocelyn Richer

l, Anna

at the aqueous solution/gold

Iannelli and Jacek Lipkowski

Guelph-Waterloo Centre for Graduate Studies in Chemistry, Guelph Campus, Universiry of Guelph, Guelph, Ontario NlG 2Wl (Canada) (Received 7 March 1991; in revised form 24 September 1991)

Abstract The adsorption of benzonitrile (BN) at a gold (111) single crystal electrode has been investigated quantitatively using chronocoulometry. The procedure described in this paper differs slightly from previous work in that experiments were performed not only in neutral pH solutions but also at pH 2.4 in order to shift the potential corresponding to the oxidation of the metal surface toward more positive values and hence extend the double layer region effectively. The following adsorption parameters: film pressure, Gibbs surface excess, Gibbs energy of adsorption and electrosorption valency, were determined as functions of the electrode potential and the charge density for concentrations of BN up to 0.03 M (close to saturation). The values of the relative Gibbs excess indicate that adsorbed BN molecules are oriented parallel to the electrode surface at negative and moderately positive potentials and low bulk concentrations. The Gibbs energy at maximum adsorption is equal to -27 kJ mol-’ which suggests that BN is chemisorbed weakly on Auflll). The results presented here are compared with those obtained from other studies of BN adsorption on gold and mercury.

INTRODUCTION

This work is part of a project devoted to the study of the influence of the surface crystallography on the adsorption of organic molecules at gold electrodes. The energetics of the adsorption of benzonitrile (BN) at a gold (111) single crystal electrode were investigated by chronocoulometry and the quantitative results obtained are presented in this paper. Systematic studies of the thermodynamics of adsorption of BN conducted on a family of gold single crystals will be presented soon, followed by a spectroelectrochemical investigation of the molecular phenomena taking place at the same interfaces.

Present address: IBM Almaden Research Center, 650 Harry Road, K33/80 San Jose, CA 95120, USA.

l

0022-0728/92/$05.00

0 1992 - Elsevier Sequoia S.A. All rights reserved

340

BN was chosen as a model adsorbate in this project because it can be studied easily by IR spectroscopy since the characteristic stretching modes of nitriles are well out of the spectral region where water absorbs infrared radiations. Blomgren et al. investigated the adsorption of BN at the aqueous solution/mercury interface using the electrocapillary method and determined the maximum surface excess and the Gibbs energy of adsorption [l]. Double layer capacity measurements were also performed on this same system by Laitinen and Mosier [2]. De Battisti and Trasatti studied the adsorption of a number of nitriles on mercury [3]. Wergert and BaumgHrtel investigated the adsorption of BN and its substituted compounds on a mercury electrode from non-aqueous acetonitrile and derived their Gibbs energy of adsorption [4]. The adsorption of BN was studied on polycrystalline gold in aqueous solutions by spectroelectrochemistry. Gao and Weaver probed the BN-gold bonding by surface enhanced Raman spectroscopy (SERS) [5,6] and Bewick and Pons investigated this same system using electrochemically modulated infrared spectroscopy (EMIRS) [7]. The adsorption of BN at solid metals from the gas phase was also studied. Solomon et al. investigated recently the adsorption of BN at the A~(1001 surface [8]. Kishi et al. examined the adsorption of acetonitrile and BN on the nickel (111) surface [9] and studied the adsorption of BN and alkyl cyanides on evaporated nickel and palladium films [lo] by X-ray photoelectron spectroscopy (XPS). Recently, Kordesch et al. investigated the adsorption and reaction of BN and other nitriles on clean and oxygen-covered copper (111) using high resolution electron energy loss spectroscopy (HREELS) [11,121. EXPERIMENTAL.

The experimental procedure is similar to that already described in previous communications [13-151. However, certain modifications have been introduced in this work in order to improve the accuracy and precision of the data; these modifications are discussed below. Water and chemicals The solutions were prepared from Mill&Q water (Millipore Corp.) with a resistivity higher than 17 MR cm. KClO, (ACS Certified from Fisher Scientific) was calcined at 300°C recrystallized twice and dried before use. Perchloric acid (Suprapur from E. Merck) was used without further purification to prepare the acidic solutions. BN (HPLC grade 99.9% pure from Aldrich Chemical Co.) was also used without further purification. Electrodes A 4 mm diameter single crystal electrode was grown inductively in a graphite crucible from 99.99% gold nuggets (Engelhard Corp.) according to the Bridgeman technique [16]. The rod was etched slightly in aqua regia to verify the macroscopic quality of the crystal. The walls were then polished carefully with 600 grit paper

341

and electropolished to a mirror-like finish under potential control in a mixture of HCl, ethanol and glycerol [17]. Afterwards, the crystal was oriented by Back-Latie X-ray diffraction, cut and polished with successively finer grades of alumina powder down to 0.05 pm. It was then cleaned thoroughly in an ultrasonic bath and rinsed several times with pure water before annealing at atmospheric pressure in a muffle furnace for 24 h at 800°C. It was found that by cleaning the rod carefully prior to annealing, problems such as deposition of alumina and other impurities at the single crystal surface were avoided. The electrode was then welded to a gold wire, dipped in a solution of 0.1 M HCIO, and cleaned electrochemically by cycling the potential at about 2 mV s-i between the hydrogen evolution and the gold oxidation regions overnight. A gold coil was used as the counter electrode and an external saturated calomel electrode (SCE) served as reference. Experimental procedure The solution contained in the cell was deaerated with argon for about 20 min before each experiment. The working electrode was conditioned by rinsing with boiling Mill&Q water, flaming in an air + natural gas flame [18], quenching with boiling water, dipping in the investigated solution and cycling a dozen times between - 0.80 and 1.30 V(SCE) in neutral medium (or - 0.40 and 1.40 V(SCE) in acidic medium) while bubbling argon and stirring. The experiments were performed using the hanging electrolyte technique [19]. Argon was allowed to flow over the solution at all times. The temperature was 22 f 1°C. Previous investigations [13-15,20-241 have shown that the maximum adsorption of organic species on gold usually occurs in the vicinity of the potential of zero charge (pzc) and very often at more positive potentials. Given that the pzc for the (111) face of gold is found at 0.27 V(SCE), and that a thorough analysis of the adsorption data might be hindered by the incipient oxidation of the metallic surface which occurs at a potential only slightly more positive than the pzc in neutral electrolyte, it was decided to run experiments in both neutral and acidic solutions. A series of experiments were first performed in 0.10 M KCIO,: the BN concentration was increased systematically from 5 X 10m5 M to 0.03 M by spiking the base electrolyte contained in the electrochemical cell with pure BN or with a concentrated solution of BN using glass and Teflon syringes (Gastight from Hamilton Co.). Nine concentrations of BN were studied using this procedure. This same procedure was then repeated in a KClO, solution acidified with HClO, to pH 2.4 while carefully maintaining the overall perchlorate ion concentration at 0.10 M. The charge density data acquired at both pH values were overlapped following the method described by Pezzatini et al. [25] and in that way it was possible to extend the so-called double layer region effectively by about 200 mV in the positive direction. The different steps involved in the data acquisition have been described elsewhere [13-151. These steps include the qualitative characterization of the interface by cyclic voltammetry, the determination of the pzc by differential capacity and the acquisition of the current transients under computer control.

342

Cyclic voltammograms were recorded in order to assess the purity of water and the chemicals used to ensure that the solution in the cell was well deoxygenated, to verify that the solution was not creeping up the walls of the electrode and finally, to determine the potential range in which the electrode behaved as an ideally polarizable electrode. The voltammograms were recorded between - 0.80 and 1.30 V(SCE) in neutral solutions and between -0.40 and 1.40 V(SCE) in pH 2.4 solutions; the sweep rate was equal to 10 mV s- ‘. The differential capacity was determined from ac impedance measurements for each concentration of BN investigated. The sweep rate used in these measurements was equal to 10 mV s-‘; the amplitude of the ac signal was 5 mV rms and its frequency was equal to 25 Hz. Potential step experiments were performed over the double layer region in order to determine the electrode charge density ((~~1. Very similar experiments were conducted in neutral pH and pH 2.4. In both cases, the electrode was held at potential E > E, (E, corresponds to the negative limit of potentials explored in the given series of measurements) for 30 s to ensure that adsorption of BN reached equilibrium and then the potential was stepped to E,. Current transients were acquired simultaneously on a microcomputer over a 205 ms time window and these chronoamperometric curves were subsequently stored on floppy disks. The procedure was repeated for E ranging from -0.75 to 0.60 WSCE) in neutral solutions (where E, = - 0.80 V) and for E between -0.35 and 0.80 WSCE) at pH 2.4 (where E, = -0.40 V); the potential increment between each step was equal to 50 mV. The current transients were then integrated digitally to yield the chronocoulometric curves. The primary piece of information, the difference between charge density at E and E,, AqM, was obtained by extrapolation of the linear portion of the chronocoulometric curves to t = 0. The absolute charge densities, (TV, were then calculated from AuM with the help of the value of the pzc determined independently from differential capacity curves as described previously [13-151. Instrzunentation The instrumentation has already been described in previous papers [13-151. In this work, an IBM XT computer equipped with an ISC-16 interface (RC Electronics, CA) was used for the digital data acquisition; the interface has its own memory buffer and allows for the acquisition of transients at an A/D conversion frequency of up to 1 MHz with a resolution of 1 mV. Errors The estimation of the error propagation in the data processing is based on a recent paper by Oldham [26]. The accuracy of the current integration procedure determined with the help of a dummy cell is 0.005 PC. When the data are reported as charge densities for an electrode whose area is 0.1 cm’, the accuracy of the charge density is 0.05 PC cme2. However, the precision of the charge densities determined in the electrochemical experiment is lower, equal to 0.3 KC cmv2. The precision is primarily

343

determined by the errors in reproducing the contact area between the single crystal surface and the electrolyte solution, and by the uncertainty of the charge overlap procedure used to combine together the two sets of data determined in the neutral and in the acidified electrolytes. The film pressure was calculated using the trapezoidal rule to approximate the integral of charge with respect to potential. The error of the integration can be estimated from the formula [26]: 6~ = S, AE/N’j2

(1)

where S, is the standard deviation of the charge density, AE is the potential range over which the integration was performed, and N is the number of charge density points taken in the integration. The error in r amounts to 0.25, 0.63 and 0.92 mN m-l for AE equal to 0.1, 0.5 and 1 V respectively. The Gibbs surface excess was calculated by differentiation of the film pressure with respect to RTlnc, where c is the bulk BN concentration. The error in the derivative can be estimated from 1261: 6r = 0.71SJA

(2)

where S, is the standard deviation of the film pressure and A is the increment of RTlnc. Taking S, as equal to 0.2 mN m-l (for small values of I) and 0.6 kJ mol-’ for A, the error 6I can be estimated as equal to 0.25 X lo-” mol cme2. The r versus RTlnc plots are linear in the plateau region of the isotherm, hence the standard deviation for I,,,,, equal to 0.5 x lo-” mol cmv2, was determined from linear regression analysis. The Gibbs energy of adsorption was calculated from the Henry coefficients and the coefficients were determined from the initial slopes of the film pressure versus bulk concentration of BN plots. For this procedure the error, S AGad:, is given by the formula:

(3) where &r/r and SI’_/I_ are the relative errors for the film pressure and the maximum surface concentration of BN, respectively. For r values equal to or lower than 1 mN m-l, the relative error is on the order of 30%. Since the relative error of I,,,, is lo%, the uncertainty of S AG,,, is equal to 1 kJ mol-‘. In addition to the indeterminate errors estimated above all, the intensive quantities such as i, C, uM, n‘ and I can be affected by a determinate error of the effective electrode area. All the calculations performed in this work were made assuming that the effective electrode area is equal to the geometric area (roughness factor equal to one).

344 RESULTS AND DISCUSSION

Characterization of the interface Cyclic voltammetry

The voltammograms presented in Fig. la were recorded in solutions containing only the supporting electrolyte at two different pH’s; their features are characteristic of the (111) single crystal surface of gold and are thus very similar to the voltammograms found in the literature [16]. The very small values of current density in the double layer region of the voltammograms indicate that the solutions were free of oxygen and that the walls of the electrode were dry. The evolution of hydrogen starts below - 0.80 V(SCE) in the neutral solution and at about - 0.40 V(SCE) at pH 2.4. The gold oxide is not formed before 0.60 V(SCE) in the neutral solution; this limit lies beyond 0.80 V(SCE) in the acidic medium. Hence, the double layer region can be extended from -0.80 to 0.80 V(SCE) by changing the pH of the solution. The cyclic voltammograms shown in Fig. lb were recorded in solutions containing the base electrolyte and 0.0256 M BN in both neutral pH and pH 2.4. The dramatic decrease of the anodic current density in the region where gold normally oxidizes indicates that BN can inhibit the oxidation of the gold surface. This observation suggests that BN must interact relatively strongly with the gold surface at the positive electrode potentials. A broad hump can be seen at E approximately equal to 0.5 V on the CV recorded in neutral solution. The position of the hump

N

-0.9

-0.3

0.3 E / V(SCE)

0.9

1.5

-0.9

-0.3

03

“9

1.1,

E / V(SCE)

Fig. 1. Cyclic voltammograms recorded at 10 mV s-l on an Au(ll1) solutions: (a) pure supporting electrolyte, (b) 0.026 M BN solution. ((pH = 2.4) solutions.

electrode in 0.10 M KCIO, ) Neutral, (- - - - - -1 acidified

345

shifts by about 200 mV in the positive direction when the pH of the electrolyte lowered to 2.4.

is

Differential capacity The potential of zero charge was determined from the position of the diffuse layer minimum on a differential capacity curve measured in a 0.01 M solution of KCIO,. The pzc was found to be equal to 0.27 + 0.10 WSCE) which is close to the values reported elsewhere [27]. Figure 2 shows the differential capacity curves determined in neutral and acidified solutions. The values of the differential capacity between -0.4 V(SCE) and 0.4 V(SCE) obtained in the neutral solution overlap well with the curves determined at pH = 2.4 (Fig. 2). The potential range in which the capacity is determined is extended in the negative direction when the measurements are performed in the neutral solution and in the positive direction when experiments are performed in the acidified electrolyte. At the negative potentials, adsorptiondesorption peaks are observed; these grow taller and shift towards more negative potentials as the concentration of BN increases. Simultaneously, the value of the differential capacity around the pzc decreases systematically as the solution becomes more concentrated in BN. The curves merge at potentials lower than -0.70 WSCE) which indicates that BN is totally desorbed when E < - 0.70 V(SCE).

101"'."""""""

-900

-600

-300

E / Fig. 2. Differential

0

300

600

900

mV(SCE)

capacity curves recorded on an Au(ll1) electrode in neutral solutions of 0.10 M KCIO, (open symbols) and 0.1 M KClO., acidified to pH = 2.4 (filled symbols) containing different amountsofBN:~O~O;~v~9.1X1O~4;~0~2.5X10~3;~~~6.7x10~3;~~~1.8x10-2;(n)3.0x10-2M. The capacity was determined using a 5 mV (rms) sine wave modulated at 25 Hz and a 10 mV s-l sweep .. rate. A series RC circuit was assumed in the calculations.

346

r------7

-a3

-19

-7

-4

Fig. 3. Relative charge density vs. In [BN] determined potentials indicated in the figure in mV (SCE).

by chronocoulometry

at different electrode

The maxima observed at the positive potentials also have features characteristic for the anodic adsorption-desorption peaks. They shift in the positive direction and their heights increase with the bulk concentration of BN. Their positions on the potential axis almost coincide with the positions of the humps observed earlier on CVs shown in Fig. lb. However, the heights of the capacity peaks are equal to about one fifth of the pseudocapacities corresponding to the humps. In addition significant inhibition of the oxide formation by benzonitrile at potentials as positive as 1.4 V is difficult to reconcile with the assumption that desorption of the BN molecules starts already at potentials a few hundred millivolts more negative. In summary, the behaviour of BN molecules at positive polarizations is complex. For this reason further data analysis was restricted to the potential range between - 0.8 and + 0.5 V(SCE). Electrode charge density Because the determination of the adsorption parameters relies on small differences in charge density, Aa, values have to be as accurate as possible. The Au, values were therefore plotted as a function of the logarithm of the organic concentration, in Fig. 3, for each potential investigated and a best fit line was drawn through the points with the standard deviation typically equal to 0.3 PC cm-*. The relative charge densities were read off these smoothed curves and used in further calculations of the adsorption parameters. The absolute charge densities were calculated from the Aa, values. Figure 4 shows plots of uM versus E for pure electrolyte and for five selected BN concentrations. The curves corresponding to solutions of BN intersect the curve

347 10

0

c-4

I

E s Y.

-10

\ z

-20

/’

-30 -900

1

-600

*

-300

E /

I

0

.

I

300

mV(SCE)

Fig. 4. Absolute charge density vs. electrode potential determined on an A&11) electrode in 0.10 M KClO, solutions over the whole range of soiubility of BN. The curves contained betweenthe plots for 0 and 0.030 M BN solutions correspond to 0.018, 6.7X 10e3, 2.5 X lo-’ and 9.1 X 10e4 M concentration of benzonitrile. For the sake of clarity only the curves determined for the five most concentrated solutions of BN are included in this figure. (0) Data acquired in the neutral solution, (0) data acquired at pH = 2.4.

for the pure electrolyte between 0.27 and 0.35 V(SCE); uM varies in this domain between 0 and 6 PC cm-*. These values correspond to the potential and the charge of maximum adsorption (E,, and a,,,,), respectively. All curves merge when E < -0.60 V(SCE), indicating that BN molecules are desorbed from the A&11) surface at this negative polarization.

Film pressure The film pressure (rr = ‘yccO- ‘yc,where T,=~ and y= are the interfacial tensions in the absence and presence of BN, respectively), was calculated by back integration of the (TV vs. E curves. Typical T versus E plots are shown in Fig. 5a. In addition the data analysis was carried out using charge as the independent electrical variable. Parsons’ function [28], defined as 5 = y + oM E, was calculated and the surface pressure of adsorbed BN, @ = ,&, - &, was determined. The @ versus gr,, plots are shown in Fig. 5b. The r and

348 50

50

a 40

40

‘;

‘;

30

Is E

E \ 8

\ c

30

E z

2o

10

10

0 cl -500

20

0 -300

-100

100

E / mV(SCE)

300

500

-25

-20

-15

-10

-5

0

5

10

oM / @me2

Fig. 5. (a) Film pressure vs. electrode potential and (b) surface pressure vs. charge density determined on an Au(ll1) single-crystal electrode in 0.10 M KCIO, solutions of different BN concentrations: (a) 1.2X 10m4; (b) 3.4X 10M4; (c) 9.1 X 10m4; (d) 2.5 x 10v3; (e) 6.7X 10e3; (0 0.018; (B) 0.030 M.

addition, ,$ is calculated from absolute charge densities, hence the accuracy of @ depends on the uncertainty in the determination of the pzc. In contrast, rr is determined from a difference between relative values of charge and is therefore independent of the error of the pzc. Adsorption isotherms The values of the fihn pressure and surface pressure were plotted against the logarithm of the bulk BN concentration at constant values of E and cM, respectively. The relative Gibbs surface excesses were then calculated by differentiation of these plots. The mole fraction corresponding to the highest concentration of BN was equal to 7 x 10e4; this value is small enough to assume that the activity of BN depends linearly on its concentration according to Henry’s Law [29,30]. Thus concentrations instead of activities were used in the determination of surface excesses. The adsorption isotherms determined with respect to E are presented in Figs. 6a and 6b. The graphs are three-dimensional. Figure 6a represents raw data whereas Fig. 6b shows the data smoothed with the help of a least-squares procedure. The black points in Fig. 6b show the position of the pzc. Clearly, for the low bulk concentrations of BN, adsorption is almost symmetric with respect to the pzc. All adsorption isotherms display a characteristic sigmoidal shape. The well-defined plateau, observed at higher bulk BN concentrations, corresponds to the maximum surface excess of BN which is equal to 4.5 X lo-” mol cm-‘. At

349

Fig. 6. Three-dimensional plots representing the adsorption isotherms for BN adsorbed at an Au(M) surface; (a) the raw data obtained from the analysis performed using potential as the independent electrical variable; (b) the data from (a) smoothed with the help of a least-square procedure. Tire black dots correspond to the position of the pm; Cc) the isotherms determined using charge as the independent electrical variable.

350

-700

-400

-100

200

500

E / mV(SCE)

Fig. 7. Gibbs energy of adsorption vs. electrode potential.

the most positive potentials and the highest bulk benxonitrile concentrations the surface excess rises, suggesting that the BN molecules may change their orientation under these conditions. Figure 6c shows the adsorption isotherms determined using charge as the independent electrical variable. The range of positive charge densities has been restricted so that only regular sigmoidal isotherms are observed, similar in appearance to these presented earlier in Fig. 6a. The limiting surface concentration observed on these curves corresponds to r,, equal to 4.5 x lo-” mol cm2 which is in excellent agreement with the value determined using E as the independent electrical variable. Gibbs energy of adsorption The Gibbs energies of adsorption were determined

from the initial slopes of the T and 0 versus mole fraction of BN plots using the Henry isotherm as described h our previous contributions [21-241. Figure 7 shows a plot of AG” versus the electrode potential. The standard state corresponds to unit mole fraction of the organic species in the bulk and monolayer coverage by non-interacting adsorbates (unsymmetrical choice of the standard state [31,32]). The AG” values have been calculated assuming that monolayer coverage corresponds to r,, = 4.5 X lo- lo mol cme2. The estimated error corresponds to 1 kJ mol-’ for potentials close to E max and has a somewhat higher value for the most negative potentials investigated. The AG” versus E plot has a quasi-parabolic shape with a maximum absolute value observed at E,, = 0.27V.

351

Fig. 8. Gibbs energy of adsorption vs. electrode charge density.

Figure 8 shows a plot of AG” against the electrode charge density. The dependence of the Gibbs energy on uM also has a quasi-parabolic character with the maximum of its absolute value observed at a,, = 0, where urnax is the charge of maximum adsorption. is equal to -27 k.I mol-‘, which is The value of AGO, at E,, or a,, characteristic for weak chemisorption. Further thermodynamic correlations The first derivative of AG” versus E is equal to the electrosorption valency, y’. Independently, the electrosorption valency can also be determined from the dependence of the charge density on the surface concentration of BN at constant electrode potential using the following relationship [33,34]:

(4) Thus, the slope of the aM versus I plot can be compared with the first derivative of the AG” versus E plot to verify the consistency of the results. Figure 9 shows plots of ur,, versus I for various electrode potentials. The plots are fairly linear in the whole range of the electrode potentials investigated. The electrosorption valencies determined from the slopes of these plots are shown in Fig. 10. Independently, the AG” versus E data were fitted by a polynomial and differentiated numerically. The electrosorption valencies determined from the Gibbs energy of adsorption data are also plotted in Fig. 10. The agreement

352

15

10

I

’

’

-?”

’

~ ’

’

---b.

0

350 5

c\I I

E " y

-.,.-

.------a-

t

0

?

.4200

__-*----

-0

-5

\ b

>

-10

-15

-20

-251 0

’

’

’

’

1

2

3

4

70

10

r / mol.cm

-2

Fig. 9. Charge density vs. surface excess of BN.determined at constant electrode potential. The values of E/mV are indicated in the figure. The straight lines were drawn through the points corresponding to low coverages.

E /

mV(SCE)

Fig. 10. Dependence of the eIectrosorption valency on the electrode potential for the adsorption of BN on an Au(ll1) electrode. y’ was determined from the first derivative of the free energy of adsorption vs. potential presented in Fig. 7 and from the initial slope of the plots of the charge density vs. surface excess shown in Fig. 9.

353 I

400

.

0-0-0

I

’

-2

pm

7.5

’

I

o

1

0------o-

2.5

0

-

200

G z

0

Y

\ w

-200

0 -x.

l

0 -,o.o

i

o-15. 0 0 =-

l

0

o

l

-,2.soo

-

%

o

0

‘..

‘%

-400

;,.s

l

‘0

E

.

.

II Job. ‘,

l

6.

17.5

0,20.0

‘! -22.5 \ -600 0

I

t

1

2

.

I

1

t .c

3

10’ Or / mol.cm

. 5

-2

Fig. 11. Electrode potential vs. surface excess of BN determined at constant charge density on a Au(ll1) electrode. The values of charge are given in the figure. The straight lines were drawn through the points corresponding to low coverages.

between the two sets of data is very good, which indicates that our results are self consistent. The consistency of the results analyzed using charge as the independent variable can be verified on the basis of the following relationship. (:).,=(z)r

(5)

According to this expression the first derivative of the Gibbs energy of adsorption with respect to the charge density should be equal to the slope of the electrode potential versus the surface excess plot taken at constant charge density. Figure 11 shows the dependence of the electrode potential on the surface excess taken at constant gM. The plots are non-linear, hence the slope of only the initial sections of these curves was calculated and plotted against uM in Fig. 12. In addition, the AG” versus cM data were fitted to a polynomial and differentiated. The result is also plotted in Fig. 12. Once again, the agreement between the two sets of data is satisfactory and shows that our results are self-consistent. DISCUSSION

Quantitative data for BN adsorption onto an Au(ll1) surface have been presented. The data were analyzed using both the potential and charge density as

354 0.5

0.5

G a s 0.0

2 1” \ L

3 o_ I -0.5

c ::

.-t

3

co

I.2 Y -1.0

__

-20

-1.0 -15

-10

-5

crM /

0

5

10

pccm-2

Fig. 12. Comparison of the dependence of @E/X),_

and (a AC?/&)

on the electrode charge density.

TABLE 1 Adsorption parameters for BN adsorbed on the A~(1111 electrode Parameter

Value

ummax /PC cm-’ &,,x /V E, /V io*“rm, /mol cm-’

variable between -O~Ccm-* when f’+Oand6~Ccm-2 when F+F,,,, variable between 0.27 V when F + 0 and 0.35 V when F -D F,,,, _ -0.1 v 4.5 for not too positive potentials and not too large concentrations _ 8 for very positive potentials and bulk concentrations close to saturation -27

A Co,,,, /kJ mol-’

the independent electrical variables. We do not claim any preference with respect to the choice of the electrical variable. The important adsorption parameters characterizing BN adsorption onto the Au(ll1) surface are summarized in Table 1. Orientation of adsorbed BN molecules

Figure 13 shows four possible adsorption states of BN as suggested by Nakayama et al. DOI. Adsorption states III and IV require significant rehybridization of the CN group to form $(C, N) species (state III) or T~(C, N) species (state IV). Such

(I)

cn,

(m)

(N)

Fig. 13. Possible adsorption states of benzonitrile [lo].

355

rehybridization can be induced only by a strong interaction of the adsorbed molecules with the metal surface. In fact, states III and IV have been reported for d metals such as Ni and Pd at which BN molecules are known to chemisorb strongly [lo]. For the Au(ll1) surface the maximum Gibbs energy of adsorption is equal to - 27 kJ mol-‘; this value is characteristic for a weak adsorbate-substrate interaction. Consequently, it is unlikely that the BN molecule will adsorb in states III and IV at the Au(ll1) electrode surface. This conclusion is supported by a recent high resolution electron energy loss spectroscopy (HREEIS) study [S] which demonstrated that BN adsorption at the Au(100) surface from the gas phase takes place without a change of the CN bond order. States I and II correspond to the two extreme situations i.e. to the vertical N-bonded and the parallel orientations, respectively. Maximum packing densities calculated from the inverse of the cross-sectional areas of a space filling model of BN are equal to 3.3 X 10-i’ mol cmm2 and 8.2 x 10-i’ mol cmd2 for the parallel and the vertical orientations respectively [l]. The value of I,,, corresponding to the first plateau on the adsorption isotherm is close to, although somewhat higher than, the maximum packing density calculated for the parallel orientation. The magnitude of I,, suggests that adsorbed BN molecules are oriented parallel to the electrode surface at not too positive potentials and not too large bulk concentrations. However, the experimental values of I can be affected by relatively large errors made in the measurements of the electrode area. Consequently, an assignment of the adsorbate orientation based on packing densities must be taken with caution. Fortunately, additional information about the orientation of the adsorbed molecule is provided by the shift of the pzc caused by displacement of the interfacial water by the organic species. Figure 11 shows a plot of E at gM = 0 as a function of I. BN adsorption at low coverages, apparently, does not affect the position of the pzc. Only for surface coverages higher than one half of a monolayer does the pzc shift slowly in the negative direction, giving a final value of E, approximately equal to - 100 mV. Assuming no partial charge transfer between adsorbed BN molecules and the metal surface, the shift of the pzc can be expressed as: AE,,, = I(pL/E)

(6)

where i;i is the effective dipole moment equal to: F;j=g-nr

(7)

Here, ii is the component of the dipole moment perpendicular to the surface, org and w stand for the organic and water molecules, respectively, and E is the permittivity of the inner layer. Clearly a small shift of the pzc indicates that jZ is small. It is reasonable to assume that at zero charge density the contribution of iZ” to jZ is small and hence, the small value of p indicates that the adsorbed BN molecules assume an orientation in which the component of its dipole moment in the direction normal to the surface is small. An isolated BN molecule in the gas

356

phase has a large dipole moment of about 4 D along the C, axis. Consequently, a small value of ii for the adsorbed molecule strongly suggests that BN is adsorbed flat with its aromatic ring oriented parallel to the metal surface. The fact that the pzc does not change on BN adsorption at low coverages and shifts slowly toward negative values at higher coverages may suggest that the adsorbed molecule assumes an ideal flat orientation at small I and a tilted orientation at I’ approaching I,,. This conclusion is consistent with the results of Solomun et al. [8] on BN adsorption at the Au(100) surface from the gas phase. The HREEL spectra of BN adsorbed at the Au(lOO) surface in a submonolayer amount, reported by these authors, show a total absence of the CN-stretching band and quite intense bands corresponding to C-H out of plane vibrations, indicating that the BN molecules are oriented parallel to the metal surface. Solomun et al. suggested that the interaction of BN with the gold surface is essentially dominated by the electrostatic interaction between r-electron clouds and the delocalized polarizable free electrons in the metal. They ruled out strong localized interactions of metal d states with distinct nitrile orbit&. Consequently, they predicted weak site specificity in BN adsorption at Au surfaces. In fact, this prediction is confirmed by additonal investigations of BN adsorption on the two other low index surfaces of gold, performed recently in our laboratory [35]. Our new results show that the effect of surface crystallograbhy on BN adsorption at the Au/solution interface is weak indeed. It is also supported by the apparent similarity between BN adsorption at Au(lll)/solution and Hg/solution interfaces. The values of AGO,,,, and I,, given in Table 1 for the Au(lll)/solution interface are very close to the values of AG” at the pzc equal to -25 kJ mol-’ and I,, = 3.7 x lo-” mol cmP2 determined by Blomgren et al. [l] for BN adsorption at the Hg electrode. In general, all the adsorption parameters reported in this paper have values typically observed for aromatic molecules adsorbed flat at the mercury electrode surface [36]. The values of I? observed at very positive charges and high bulk BN concentrations approach the maximum packing density calculated for the vertical orientation. The vertical orientation has neither been observed in the studies of BN adsorption at Au(lOO) from the gas phase nor at the Hg electrode from an electrolyte solution. It has been observed, however, for BN adsorption from a solution at an Au polycrystalline electrode by IR [7] and Raman spectroscopies [5,6]. However, the present evidence for the vertical orientation of BN molecules should be taken with caution as it comes from the surface excess data which were obtained at the positive limit of potentials where Au-solvent interactions become important and where the effects of ionic co-adsorption cannot be ruled out. Therefore, additional spectroscopic data are needed to reach a firm conclusion about the properties of BN molecules at the positive end of explored potentials. CONCLUSION

BN is weakly chemisorbed on the Au(ll1) surface. The BN molecules assume a flat orientation on a negatively charged surface, and in the vicinity of the pzc. The

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interactions between the adsorbed molecule and the metal surface are probably dominated by electrostatic interactions between r-electron clouds and the delocalized polarizable free electrons in the metal. The surface excess data suggest that at more positive charge densities and higher bulk concentrations, BN molecules reorient toward a vertical state. Since the determination of surface excess at the positive potentials is susceptible to experimental errors, this last conclusion should be viewed as tentative. It requires confirmation by independent spectroscopic experiments. ACKNOWLEDGEMENTS

This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. J.R. wishes to acknowledge the support of NSERC in the form of a Fellowship. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

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