J. Phys. Chem. 1995, 99, 16011-16017
16011
Surface-Bound Carbonyl Compounds as Lewis Acids. Photoluminescence as a Probe for the Binding of Ketones and Aldehydes to Cadmium Sulfide and Cadmium Selenide Surfaces Keith D. Kepler: George C. Lisensky,*v$Manish Patel: L. Anna Sigworth,*and Arthur B. Ellis*J Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, and Department of Chemistry, Beloit College, Beloit, Wisconsin 5351 1 Received: May 3, 1995; In Final Form: July 5, 1995@
The band gap photoluminescence (PL) intensity of n-CdSe and Te-doped n-CdS (CdS:Te) single crystals is reversibly quenched by adsorption of more than two dozen aldehydes and ketones from cyclohexane solution onto the (0001) surfaces of these CdS(e) substrates. The adsorbate-induced quenching of PL intensity is consistent with the adsorbates acting as Lewis acids toward these surfaces. For a representative selection of aldehydes and ketones, the decrease in PL intensity is well fit by a dead-layer model for adsorption onto CdSe, allowing estimation of the adduct-induced expansion in depletion width ranging from several hundred to as much as -1000 A. Temporal PL results suggest that the change in surface recombination velocity accompanying adsorption is modest and a less important contributor to steady-state PL quenching than the band-bending increase inferred from the fit to the dead-layer model. Adduct formation constants, estimated from fits of the concentration-dependent PL changes to the Langmuir adsorption isotherm model, vary over 4 orders of magnitude, from -10’ to lo5M-I. Hammett plots of adduct formation constants of several families of derivatives show that the formation of CdS:Te surface adducts can be stabilized by electron-withdrawing substituents, consistent with a Lewis acidic interaction with the surface. Adduct stability is enhanced by resonance effects: Substitution of phenyl substitutents for methyl substituents can lead to order-of-magnitude increases in binding constants. a-Diketones and quinones exhibit the largest binding constants, presumably due to the ability of the second carbonyl group to stabilize the negative charge transferred from the semiconductor. A model of orbital interactions for the carbonyl functional group with the CdS(e) surface is proposed. Evidence that steric factors contribute to surface coverage and thus to the magnitude of PL quenching is presented.
Introduction We and others have shown that surface-adsorbate adduct formation can be monitored by changes in the steady-state photoluminescence (PL) intensity emitted from II-VI semiconductors like CdS and CdSe [CdS(e)].i-3 In cases where adsorption and desorption of molecular species are sufficiently rapid, the reversibility of the PL changes may permit use of this effect for on-line chemical ~ e n s i n g . ~ For single-crystal substrates, we have modeled these reversible interactions as being characteristic of a weak charge-transfer complex for which the adduct-induced PL changes reflect the nature of the adsorbing molecule’s interaction with the surface: Adsorbing Lewis acids, which draw electrons from the semiconductor bulk to surface states, quench PL intensity, and adsorbing Lewis bases, which push electrons from the surface states back into the bulk semiconductor, are observed to enhance the PL intensity.’ These effects on PL intensity can be modeled as resulting from a change in the depletion width of the semiconductor caused by the shifts in electron distribution that occur during adduct formation: Photogenerated electron-hole pairs are separated by the electric field characterizing the depletion width, resulting in a nonemissive zone or “dead-layer’’ whose thickness is roughly on the order of the depletion width.5 The concentration dependence of PL changes has been used to estimate adduct binding constants using the Langmuir adsorption isotherm model.6
T Authors to whom correspondence should be addressed. ’
University of Wisconsin-Madison.
* Beloit College.
‘Abstract published in Aduunce ACS Abstracts, September 15, 1995.
A change in surface recombination velocity, like a change in dead-layer thickness, can alter the PL inten~ity.~Timeresolved PL decay measurements provide a complementary technique for examining surface adduct formation, using the kinetics of electron-hole pair recombination as a probe. Both PL experiments are in situ, nondestructive methods for studying surface-adsorbate adducts. In previous solution studies, we have used families of compounds like aliphatic and aromatic amines to explore the steric and electronic factors that govem adduct formation, typically using nonpolar solvents like cyclohexane and toluene.’.* For example, we have found evidence for surface chelation by diamines such as ethylenediamine and o-phenylenediamine and for electronic control over changes in deadlayer thickness using a series of substituted aniline derivatives that yielded linear Hammett plots. The rich surface chemistry of the amines led us to look for other functional groups that might interact with CdS(e) surfaces and thus to this study of compounds with carbonyl functional groups. The reactivity of organic carbonyl compounds with various 11-VI surfaces has been reported. For example, aldehydes have been shown to react with zinc oxide surfaces under ultrahigh vacuum conditions; the reactions are generally characterized by nucleophilic attack of surface oxygen and subsequent decomposition of the adsorbing compound^.^ Adsorption of benzoic acid derivatives onto CdTe and CdSe surfaces and the resulting effect on the semiconductor’s work function have been studied in air; the infrared spectra of these adsorbates suggest that bridging carboxylate species form on the semiconductor surfaces, and contact potential difference
0022-3654/95/2099-16011$09.00/0 0 1995 American Chemical Society
Kepler et al.
16012 J. Phys. Chem., Vol. 99, No. 43, 1995
measurements indicate that adsorbate-induced changes in semiconductor band bending are less than 50 mV.I0 In this paper we extend our methodology to embrace several dozen ketones and aldehydes in order to investigate the interaction of the carbonyl functional group with CdS(e) surfaces. Specifically, we demonstrate that these molecules reversibly quench the PL intensity of CdS(e) single crystals when adsorbed from cyclohexane solutions, consistent with the adsorbates acting as Lewis acids toward these surfaces. For CdSe, these PL changes can be fit by the aforementioned dead-layer model for a representative selection of the carbonyl compounds examined. Collectively, our steady-state PL data provide evidence for a significant steric contribution to the degree of PL quenching observed. Time-resolved PL experiments conducted with several of these compounds suggest that adduct-induced changes in surface recombination velocity are modest under our experimental conditions. We find that binding constants for the compounds studied span roughly 4 orders of magnitude at room temperature. The trends observed are consistent with ligation of the carbonyl group to surface chalcogen atoms and are best correlated with the ability of the adsorbate to impart stabilization through inductive and resonance effects. Experimental Section Materials and Methods. Single-crystal, vapor-grown, cplates of 10 ppm, Te-doped n-CdS (CdS:Te) and of n-CdSe, resistivity -2 SZ cm, were obtained from Cleveland Crystals, Inc. from several different lots. The crystals were etched in either a BrzNeOH solution (1:30 v/v for 15 s) or a concentrated HCl solution (10 s) and rinsed in MeOH. No significant difference in response was observed for the two treatments. The shiny Cd-rich (0001) face was used in all PL experiments.”.I2 The carbonyl compounds used included the following: acetophenone and derivatives (p-CH3, p-C1, m-CF3, and 2,2,2trifluoro); benzaldehyde and derivatives (p-CH3, p-C1, p-Br, and m-CF3); benzophenone and disubstituted derivatives (p-CH3, p-C1, and m-CS); benzil and disubstituted derivatives (p-CH3, p-Br, and decafluoro); 2,3-butanedione; 1,4-dibrom0-2,3-butanedione; 1-phenyl-1,2-propanedione;2,4-pentanedione; 1-phenyl1,3-butanedione; 2,5-hexanedione; 1,4-diphenyl-1,Cbutanedione; 1,6-diphenyl- 1,6-hexanedione; 1,4-benzoquinone; phenanthrenequinone; 3,3,5,5-tetramethyl-l,2-~yclopentanedione; acetone; hexachloroacetone; acetaldehyde; t-butylbenzene; nbutylbenzene. All of these compounds were obtained from Aldrich except benzaldehyde, acquired from Fisher, and 4,4‘dimethylbenzophenone, obtained from Kodak. All were at least 98% pure as purchased. The compounds were purified by recrystallization, distillation, or sublimation prior to use. Cyclohexane was dried over CaH2 and distilled under Nz. Solutions were made and stored in a NZ glovebag and used within 48 h. Steady-State Optical Measurements. A small plate of the CdS(e) crystal was epoxied to a glass rod that was held in a glass cell that has previously been described.’ The crystal samples were mounted with the Cd-rich face exposed to the incident laser beam. A teflon stopcock at the bottom of the cell permitted drainage of the cyclohexane solvent or cyclohexane solutions. Glass syringes were used to inject the solutions through a sidearm against positive N? pressure. Samples of CdSe were excited with a Coherent Innova 90-5 Ar’ laser (457.9 and 514.5 nm) or a Melles-Griot Model 80 He-Ne laser (632.8 nm), as previously described.’ PL intensity was continuously monitored at the band maximum of -7 12 nm as different cyclohexane solutions and cyclohexane solvent (the
reference ambient) were injected. A fiber optic cable picked up the emitted light, which was detected by either a monochromator/R928 photomultiplier tube or an Oriel Instaspec I1 diode array setup and monitored with a recorder in time-base mode. Under the low-resolution (3 nm) conditions employed, there was no change in the PL spectral distribution upon adsorption for any of the solutions examined, allowing the PL to be monitored at a single wavelength, the band maximum. Samples of CdS:Te were excited by a Xe arc lamp at 450 nm (10 nm slit width) in a Shimadzu RF9000 spectrophotofluorimeter. The band edge PL was monitored at 507 nm ( 5 nm slit width). The general procedure followed was to establish a PL intensity baseline with the semiconductor sample in cyclohexane and then to inject the most concentrated cyclohexane solution of the adsorbate to be studied. After a rinse to return to the cyclohexane baseline, a series of solutions was injected in order of increasing concentration. In a few cases solvent was reinjected between solution injections, restoring the PL baseline intensity, but desorption was sufficiently slow in many cases that the return to solvent baseline was only performed at the end of the series of injections. A method that was occasionally used to speed the return of the PL intensity back to its original value involved exposing the sample to a 1 mM solution of the Lewis base, o-phenylenediamine, followed by rinsing with cyclohexane. We have previously shown that this diamine has a strong binding constant (-lo4 M-I) yet fast desorption kinetics8 These characteristics allow it to displace many of the carbonyl compounds from the surface, facilitating restoration of the PL intensity baseline value. All of the carbonyl compounds were tested 2-5 times, using both CdS:Te and CdSe substrates, except for benzaldehyde and acetophenone and their derivatives on CdSe, where only one experiment was done. The reported changes in PL intensity and in equilibrium binding constants, K , are averages of these runs. The excitation wavelength was 450 nm for CdS:Te and 458 nm for CdSe unless otherwise noted. The incident laser light intensities for CdSe were -10 mW/cm? and -50 mW1 cm2when the photomultiplier tube detector and diode array were used, respectively. The intensity of the Xe arc lamp used for exciting CdS samples was -1 mW/cm2. Time-Resolved Optical Measurements. A sample cell was made from a 1.O cm path length square quartz cuvette, using a design similar to that of the steady-state cell. Excitation was provided by a Coherent Antares Nd:YAG laser system, as previously described.I3 Rhodamine 6G dye, used to provide 583 nm excitation, produced an average power of about 150 mW at 3.8 MHz with a pulse duration of -2 ps. A maximum output of -1020 photons/cm3/pulse was achieved with a beam diameter of -1 mm. A time-correlated, single photon counting detection system was used to measure the PL decay pr0fi1es.I~ The instrument response function was -70 ps. The multiexponential decay profiles were modeled by the Kohlrausch equation, using a modification of the program CURFIT by Bevington. I s The PL decay plots were iteratively reconvoluted from the instrument response function. Results and Discussion PL Quenching. Tables 1 and 2 present a summary of the 32 aldehydes and ketones used in this study, grouped in several cases by families of derivatives. When each of these compounds except acetone is added to cyclohexane, the band edge PL intensity of CdS(e) single crystals is quenched relative to the intensity in cyclohexane. Figure 1 shows a typical trace of the quenching of band edge PL intensity due to adsorption of the carbonyl compound. In this case, CdS:Te was exposed to
J. Phys. Chem., Vol. 99, No. 43, 1995 16013
Surface-Bound Carbonyl Compounds as Lewis Acids
TABLE 1: Binding Constants and PL Intensity Quenching for Monocarbonyl Compound Adsorption onto CdS:Te and CdSe compound (monocarbonyls) acetaldehyde acetone acetophenone
0
II CH3-C-H 0
II
R3C-C-CR3
0 Re I - C H 3
benzophenone
0
II
RQcQR
benzaldehyde
0
K, M-'
% PL quench" CdS:Ts CdSe
R=H R = C1 R = p-CH3 R=H R = p-C1 R = m-CF3 2,2,2-trifluoro R = p-CH3 R=H R = p-C1 R = m-CF3 R = p-CH3 R=H R = p-Br R = p-C1 R = m-CF3
CdS:Te
CdSe
45
70
100
90
20 45 35 35 40 40 35 40 35 40 35 60 45 40 55 50
-50 45 30 35 45 30
50 200 1000 2000 1000 2000 1000 2000 2000 2000 4000 300 600 2000 2000 2000
10 2000 400 1000 600 1000 200 2000 900 2000 3000 200 1000 800 2000 1000
40 35
40 30 45 70
50 65
80 IO
The maximum percentage by which the PL intensities of CdSe and CdS:Te are reduced from their baseline values in pure cyclohexane when the indicated compound is added to cyclohexane. Acetone on CdSe enhanced the PL intensity. The excitation wavelength was 458 nm for CdSe and 450 nm for CdS:Te. Several runs were made except for benzaldehyde and acetophenone derivatives on CdSe. The error in a single measurement was less than 5%. The standard deviation between runs was typically &lo%. Equilibrium binding constants for adsorption of the indicated compound onto CdSe and CdS:Te, obtained from fits to the Langmuir adsorption isotherm model (eq 3). Several runs were conducted for all systems except for benzaldehyde and acetophenone derivatives on CdSe. The excitation wavelength was 458 nm for CdSe and 450 nm for CdS:Te. The error in the Langmuir fit for a single run was generally less than 10%. The standard deviation of the average of several runs was &25%.
TABLE 2: Binding Constants and PL Intensity Quenching for Dicarbonyl Compound Adsorption onto CdS:Te and CdSe % PL quench" CdS:Te CdSe
compound (dicarbonvls)
K, M-' CdS:Te
CdSe
60 IO 50
10000 30000 300000 200 000
20000 30000 200000
20000 100000 30000
30000 100000 100000
50 30
15 80 55 20 20 60 20
100 3000 90 3 000
200 2000 100 3000
15
15
3000
2000
3,3,5,5-tetramethyl- 1,2-~yclopentanedione
30
50
20000
30000
1,4-benzoquinone
50
55
40000
30000
phenanthrenequinone
40
60
200000
200000
benzil
R = p-CH, R=H R = p-Br
-FIO 2,3-butanedione
RlrR2 = CH3 RI,Rz = CH2Br RI = Ph; R2 = CH3
2,4-pentanedione
R = CH3 R = Ph R = CH3 R = Ph
2.5-hexanedione
a
40 35 30 25 35 50 25 15 5
See Table 1. See Table 1.
benzaldehyde solutions over the range 1-200 mM. The kinetics of desorption are slow for these compounds, as illustrated by the slow return to the cyclohexane baseline PL intensity; repeated rinsing with solvent was generally necessary to reattain this value. As described above, PL quenching is consistent with the adsorbate acting as a Lewis acid toward the surface. Figure 2 presents an orbital diagram showing how the lowest unoccupied molecular orbital (LUMO) of an adsorbing carbonyl compound
can stabilize the surface states of the semiconductor, trapping additional charge at the surface and expanding the depletion region. All of the carbonyl compounds examined quenched the PL intensity of both CdS:Te and CdSe, with the exception of acetone adsorbing onto CdSe. Tables 1 and 2 reveal that the maximum amount of quenching, where PL quenching has saturated with concentration, ranged from about 5 to 80%, with values of -40% being typical. From the analysis of the PL
Kepler et al.
16014 J. Phys. Chem., Vol. 99, No. 43, 1995
I
633 nml
oC-yJ n
I
c)
10 min
0
200
resulting from exposure of a HC1-etched CdS:Te sample to cyclohexane (initial and final response) and sequential exposure to cyclohexane solutions of benzaldehyde at the indicated concentrations. The band edge PL spectral peak is superimposed in the lower right corner to show the baseline position. Upward spikes are caused by the draining of the sample cell when changing solutions. The sample was excited with 450 nm light.
I
---_ - -----E f
-1 -I n-Semiconductor Interface LewisAcid Figure 2. Perturbation of surface electronic states at the n-type CdS(e) semiconductor surfaces by the LUMO of an adsorbing Lewis acid. The interaction at the interface stabilizes the surface electronic states of n-CdS(e), moving them toward the valence band edge, and destabilizes the LUMO of the adsorbate. Additional electrons move from the semiconductor bulk to fill the stabilized surface states up to the Fermi level, Ef,resulting in the expansion of the depletion layer (center part of figure). The shaded region indicates the extent of the depletion region before and after adduct formation.
quenching data of Tables 1 and 2, no correlation was found for the collection of compounds studied with 13C NMR carbonyl carbon chemical shifts, carbonyl infrared stretching frequencies, molecular dipole moment, ionization potential, or reduction potential.16 Likewise, within a family of derivatives, no correlation was found between substituent Hammett parameters17 and the amount of PL quenching. We will provide evidence below that steric effects may make an important contribution to PL quenching: The binding sites and/or surface coverage may be sufficiently different among these compounds that correlations with the magnitude of PL quenching cannot be made. The increase in PL intensity found for acetone adsorbing onto CdSe may reflect its very weak binding constant; a different binding mode may result from the high solution concentrations required to observe PL changes for this system. Dead-Layer Model. For adsorption of several of the carbonyl compounds onto CdSe, PL quenching was analyzed using the dead-layer model.5 As described above, the model assumes that a region of thickness, D, on the order of the depletion width, is nonemissive. Adsorption-induced changes in D, assumed to correspond roughly to changes in the depletion width, W , are estimated by analysis of changes in PL intensity using eq 1:
600
800
1000
1200
AJI (A>
Time Figure 1. Relative changes in PL intensity, monitored at 507 nm,
400
Figure 3. Maximum values of the expansion of the dead-layer thickness, AD, caused by the adsorption of benzaldehyde and its p-CH3 and p-C1 derivatives, benzophenone, benzil, and 2,3-butanedione onto the Cd-rich (0001) face of an etched, single-crystal n-CdSe sample. Values of AD were calculated from PL quenching ratios using eq 1 in the text. The concentrations used to produce these maximum changes in dead-layer thickness are 0.20 M for benzaldehyde, 0.042 M for p-CH3-benzaldehyde, 0.026 M for benzophenone, 0.0094 M for p-C1benzaldehyde, 0.0019 M for benzil, and 0.0014 M for 2,3-butanedione. (The minimum concentrations needed to obtain saturation of PL intensity for these compounds may be less than the concentrations used.) Excitation wavelengths used are indicated in the figure.
Here, P L f is the PL intensity in a cyclohexane reference solvent, PL, is the PL intensity in a cyclohexane solution of the carbonyl compound, a ' = (a p) is the sum of the semiconductor absorptivities for the incident and emitted light, respectively, and AD = (Dref - a,) is the difference in the dead-layer thickness in passing from pure solvent to carbonyl solution. Adsorbate-induced changes in surface recombination velocity are ignored in this simple model (vide infra). Equation 1 predicts that different PL ratios will be observed, but AD will remain constant, as the excitation wavelength is varied. A representative selection of compounds, shown in Figure 3, was used to test the general applicability of the deadlayer model for carbonyl compound adsorbates. Absorptivities for accessible excitation wavelengths vary by a factor of nearly 3 for CdSe and provide a good test of the model. The change in dead-layer thickness, AD, obtained for each of the test compounds was found to be constant within experimental error for the three excitation wavelengths employed. Dead-layer thicknesses were found to increase from -200 to 1000 A on exposure to the solutions of the six test carbonyl compounds. The absorptivitiesof CdS vary less with wavelengths accessible to us, and this substrate does not provide as demanding a test of the dead-layer model. Time-Resolved Data. The general expression for PL intensity from which the dead-layer model is derived also includes the surface recombination velocity, The dead-layer model assumes that either S is insensitive to adsorption or is relatively large both before and after adsorption (S >> v z p and G 2 / z p ,where Lp is the minority carrier (hole) diffusion length and zpis the hole lifetime). This assumption can be investigated by making PL decay measurements before and after adsorption. The PL decay curves of n-CdSe are nonexponential and can be well fit by the "stretched exponential", two-parameter Kohlrausch equation, eq 2. l3
+
The parameter p [0 < p < 11 has been related to a distribution of exponential decay times that are serially linked @? increases as the distribution of decay times becomes more homogeneous),
Surface-Bound Carbonyl Compounds as Lewis Acids
h
U
0.8
-- -
-
U
-.- 0.6 -
J. Phys. Chem., Vol. 99, No. 43, 1995 16015
m Cyclohexane Benzaldehyde solution
P)
N
Eb
z
0.4
-
0.2
1 0 -
82
0-
l
0
2
I
I
.
I
I
I
.
l
l
6
4
l
l
l
,
8
.
0.03
.
10
Time (ns)
Figure 4. Luminescence decay from an etched CdSe single crystal in
I
cyclohexane and in a 0.2 M benzaldehyde solution. The sample was excited with 587 nm light at an estimated power of -2 mW. Fits to the Kohlrausch equation (eq 2) yield values for p of 0.34 4~0.02 and for r of 250 f 5 ps in the solvent and for /?of 0.36 f 0.02 and for t of 240 f 5 ps in solution.
and the parameter t, the lifetime, represents the peak of the distribution. The good fit of the semiconductor decay time profiles to this equation may reflect the notion that our surfaces have multiple recombination sites with different decay kinetics. Cyclohexane solutions of the compounds benzaldehyde, 2,3butanedione, and 2,4-pentanedione were used in the timeresolved PL experiment with CdSe single crystals. The concentration of the solutions corresponded to saturation of the steady-state PL quenching; a comparable reduction in PL intensity was observable in the time-resolved experiments in passing from solvent to carbonyl solution. Figure 4 shows the decay profile obtained for cyclohexane and for a 0.2 M cyclohexane solution of benzaldehyde. The t and ,8 values were nearly identical for solvent and solution (250 ps and 0.34 for the solvent; 240 ps and 0.36 for the benzaldehyde solution) for this experiment and for the two dione solutions tested. It is noteworthy that these values are also similar to those we reported for p-methoxyaniline adsorption onto CdSe.7 We conclude from our steady-state and temporal PL measurements that adsorbateinduced changes in surface recombination kinetics appear to be a less important contributor to the steady-state PL changes observed than are changes in band bending caused by adsorption. Adduct Formation Constants. Concentration-dependent PL quenching like that shown in Figure 1 can be used to estimate equilibrium binding constants for adsorption of the carbonyl compounds onto CdS(e) surfaces using the Langmuir adsorption isotherm The quantitative form of the model is given by eq 3:
0 = KCI(1
+ KC)
or 1/0 = 1
+ (l/KC)
(3)
Here, 0 is the fractional surface coverage, K is the equilibrium constant, and C is the solution concentration of the adsorbate. The PL intensity is used as a rough measure of 0:Saturation of PL intensity defines 0 = 1, and the PL intensity observed in pure solvent is 0 = 0. At intermediate coverages 0 is estimated using the fractional change in dead-layer thickness, eq 4: (4) Double reciprocal plots of 0-' vs C-' give an estimate of the binding constant K. Figure 5 shows two adsorption plots and their corresponding fits to the Langmuir model. Values for K of -30 000 M-I and -1400 M-' are estimated for the a-diketone benzil and for
4
0.4 0.2
I
30000 6000
o
0.001 25
0.0025
M
Figure 5. Plots of the fractional surface coverage 0, defined by eq 4, as a function of (a) benzophenone and (b) benzil concentration for adsorption onto n-CdSe. The binding constants, K, extracted from the double-reciprocal plots shown as insets (eq 3), are 1400 f 25 M-' for benzophenone and 30000 f 2500 M-' for benzil. The linearity of the plots implies a good fit to the Langmuir adsorption isotherm model. The excitation wavelength is 457.9 nm in both cases.
1
2
3
4
5
CdS log K
Figure 6. Plot of log K for adsorption onto CdSe vs log K for adsorption onto CdS:Te (denoted "CdS" in the plot) for the carbonyl compounds studied. The slope of the best fit line to the data (shown in graph) is 1.02.
benzophenone, respectively. Values of log K obtained in like manner for the 32 carbonyl compounds adsorbed onto both CdS: Te and CdSe are summarized in Tables 1 and 2. These equilibrium constants span a range of nearly 4 orders of magnitude, from -10' to lo5 M-I, Before the analysis of the molecular dependence of K, it is worth emphasizing that the results in Tables 1 and 2 were obtained for adsorption onto the Cd-rich, (0001) face of the CdS(e) single-crystal substrates. As illustrated in Figure 6, we see little difference between CdS:Te and CdSe; K values obtained for the two surfaces for a given aldehyde or ketone were, except in one case, never different by more than a factor of 5 and usually within a factor of 2. These results suggest that the adsorbate-surface interaction is similar for the two surfaces. Factors Affecting Binding Constants. Inductive Effects. The compounds studied included several families of carbonyl
Kepler et al.
16016 J. Phys. Chem., Vol. 99, No. 43, I995 6
ac-cQ 5
< 0'
4
Y
33
-1 ; 2 1
n
0
0
'
0 4
R'
'R'
Figure 8. Plot of log K for adducts formed on CdSe and CdS:Te with mono- and dicarbonyl compounds. K
-0.4
-0.2
0.0
0.2
0.4
0.8
0.6
Hammett Parameter,
-
1,000 M-1
K
- 30,000
M-1
K
- 200,000
M-1
1.0
G
Figure 7. Plots of log(K/Ko), where KO is the binding constant for the unsubstituted compound, vs Hammett parameter for the (a) benzil, (b) benzaldehyde, (c) benzophenone, and (d) acetophenone families of derivatives adsorbed onto CdS:Te. The substituents are p-CH3, H, p-C1, p-Br, and m-CF3 with o values of -0.17, 0.00, 0.23, 0.23, and 0.43, respectively. For the disubstituted benzil and benzophenone families, the Hammett parameters were assumed to be additive, although it is unlikely that the benzil surface adduct is symmetrical. In this case, the contribution of the second substituent is probably overestimated in the benzil Hammett plot.
derivativeswhose binding constants provide insight into the role of inductive effects on the stability of surface adducts formed with CdS(e). Figure 7 presents plots of log(KIK0) for CdS:Te, where KOis the binding constant of the unsubstituted "reference" compound for a family of derivatives, vs the Hammett parameters of the substituents for derivatives of the compounds benzil, benzaldehyde, benzophenone, and acetophenone. The Hammett parameters are assumed to be additive in the case of disubstituted derivatives. The plots are roughly linear, with slopes that range from strongly positive to nearly zero. Positive slopes indicate that more electron-withdrawing substituents stabilize binding to the semiconductor surface through inductive effects, as expected for Lewis acidic adsorbates. The difference in the Hammett slopes, p, among the four families of derivatives examined is intriguing and may reflect different ligating geometries that affect charge transfer to varying extents across each family of compounds. Resonance E'ects. Figure 8 provides evidence for the importance of charge delocalization via resonance in the formation and stability of these surface adducts. Collectively, monocarbonyl compounds and p-, y-, and edicarbonyl compounds exhibit similar trends in K values: When no phenyl groups are present, K is -lo2 M-' and increases by roughly an order of magnitude when one phenyl group is introduced. This presumably reflects the ability of the phenyl ring to delocalize
Figure 9. Idealized possible binding schemes of the carbonyl functional group to chalcogen sites on the Cd-rich surface of CdS(e), emphasizing stabilization of the adduct through resonance effects. Adduct formation constants are indicated above the compounds.
the charge transferred from the surface. From acetophenone to benzophenone, the addition of the second phenyl ring to the same carbonyl group provides little additional stabilization, perhaps reflecting a balance between electronic and steric effects on K. The a-diketones display approximately an order-of-magnitude larger binding constant than the aromatic monocarbonyls and p-, y-, and E-dicarbonyl compounds, which we attribute to the ability of the second carbonyl group to accommodate negative charge. Phenanthrenequinone, which has extensive resonance stabilization, also has an exceptionally large binding constant in the range of lo5 M-I. On the basis of our PL results, we can envision several possible bonding models for ligation of these carbonyl compounds, as illustrated in Figure 9. Given the acidic nature of the interaction, we believe that the carbonyl compounds bind to basic chalcogen sites on the CdS(e) surfaces, including oxygen, which may be present as part of an oxide phase.4 From
Surface-Bound Carbonyl Compounds as Lewis Acids their similar binding constants, we suspect that /?-,y-, and E dicarbonyl compounds bind in like manner to the monocarbonyl compounds, possibly through their carbonyl carbon atom, as shown for benzaldehyde in the figure, or through the carbonyl n system. In both cases, electron density would become localized on the oxygen atom. The compounds n-butylbenzene and t-butylbenzene were also tested and showed little effect on PL intensity when adsorbed onto CdSe, suggesting that chargetransfer interactions between the phenyl ring and surface are minimal. For the a-diketones and benzoquinone, in contrast, we suspect that ligation occurs through the carbonyl oxygen atom (Figure 9) in a manner that parallels reactions of these compounds with certain alkyl phosphites, wherein electron transfer from the phosphorus atom to the diketone leads to the formation of a dipolar ion or sometimes an oxyphosphorane.'* Steric constraints in a-diketones, such as phenanthrenequinone and 3,3,5,5-tetramethyl-1,2-~yclopentanedione,suggest that binding through the carbonyl carbon atom or through x interactions is unlikely. The large Hammett slope of the benzil derivatives may be a result of this different mode of binding. There is the possibility that the a-diketones can chelate to surface atoms as they do in some cases to phosphites to form stable oxyphosphoranes.'* However, the equally large binding constant found for benzoquinone, which can stabilize charge through resonance but not chelate, suggests that this is an unlikely possibility. We conclude that the stability of carbonyl surface adducts appears to be determined by the extent of inductive and resonance stabilization of the electron density transferred from the surface. Steric Effects. We speculated above that the PL quenching values for the carbonyl compounds might reflect steric factors that result in different binding sites and/or surface coverages. Several pairs of compounds were compared to examine the effect that phenyl ring substitution has on the quenching of PL intensity: acetaldehyde vs benzaldehyde; benzaldehyde derivatives vs benzophenone derivatives; 2,3-butanedione vs 1-phenyl1,2-propanedione; 2,4-pentanedione vs 1-phenyl-1,3-butanedione; 2,5-hexanedione vs 1,4-diphenyl-1,Cbutanedione. In most cases, the substitution of one or two bulky phenyl rings at the carbonyl carbon atom for a methyl group or hydrogen atom causes K to increase while PL quenching either decreases or remains the same within the error of the measurement. The increase in K, which is almost an order of magnitude in most cases, presumably reflects the resonance stabilization afforded by the phenyl ring. We believe that the smaller amount of PL quenching for the aromatic partner of the pairs is indicative of fewer molecules binding to the surface; assuming that neither the type of surface bond nor the number of adsorbed molecules changes with the addition of the phenyl ring, we would have expected PL quenching to increase, since the additional phenyl
J. Phys. Chem., Vol. 99, No. 43, I995 16017 substituents would allow more electron density to be transferred from the surface to the molecule. In contrast, in all cases either no change or a decrease in PL intensity was observed, suggesting that fewer molecules are bound to the surface, consistent with the introduction of sterically demanding phenyl rings. Acknowledgment. We thank Professors Robert McMahon, Clark Landis, David Cahen, and Dr. Merlin Bruening for helpful comments. We are grateful to the Research Corporation for a William and Flora Hewlett Foundation Grant and to the donors of the Petroleum Research Fund administered by the American Chemical Society for support of this research. References and Notes (1) See, for example, Zhang, J. 2.;Geselbracht, M. J.; Ellis, A. B. J. Am. Chem. SOC.1993, 115, 7789, and references therein. (2) Dannhauser, T.; O'Neil, M.; Johansson, K.; Whitten, D.; McLendon, G. J. Phys. Chem. 1986, 90, 6074. (3) Chandler, R. R.; Coffer, J. L. J. Phys. Chem. 1991, 95, 4. (4) Winder, E. J.; Moore, D. E.; Neu, D. R.; Ellis, A. B.; Geisz, J. F.: Kuech, T. F. J. Cryst. Growth 1995, 148, 63. (5) (a) Mettler, K. Appl. Phys. 1977, 12, 75. (b) Burk, A. A., Jr.; Johnson, P. B.; Hobson, W. S.; Ellis, A. B. J. Appl. Phys. 1986, 59, 1621. (c) Geisz, J. F.; Kuech, T. F.; Ellis, A. B. J. Appl. Phys. 1995, 77, 1233. (6) Atkins, P. W. Physical Chemistry, 4th ed.; Freeman: New York, 1990; p 885. (7) Murphy, C. J.; Lisensky, G. C.; Leung, L. K.; Kowach, G. R.; Ellis, A. B. J. Am. Chem. SOC.1990, 112, 8344. (8) Lisensky, G.C.; Penn, R. L.; Murphy, C. J.: Ellis, A. B. Science (Washington, D.C.) 1990, 248, 840. (9) Vohs, J. M.; Barteau, M. A. Langmuir 1989, 5, 965. (10) (a) Moons, E.; Bruening, M.; Burstein, L.; Libman, J.; Shanzer, A.; Cahen, D. J m J. A d . Phys. 1993.32, 730. (b) Bruening, M.; Moons, E.; Yaron-Marcovich, D:; Cahen, D.; Libman, J.; Shanzer, A.-J. Am. Chem. SOC.1994, 116, 2972. (11) Warekois, E. P.; Lavine, M. C.; Mariano, A. N.; Gatos, H. C. J. Appl. Phys. 1962, 33, 690. (12) Ellis, A. B.; Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S. J. Am. Chem. SOC.1977, 99, 2839. (13) Leung, L. K.; Meyer, G. J.; Lisensky, G. C.; Ellis, A. B. J. Phys. Chem. 1990, 94, 1214. (14) O'Connor, D. Z.; Phillips, D. In Time-Correlated Single Photon Counting; Academic Press: New York, 1985. (15) Bevington, P. R. In Data Reduction and Error Analysis for Physical Sciences; McGraw-Hill: New York, 1969. (16) (a) The Sadtler Standard Carbon-I3 NMR Spectra Collection; Sadtler Research Laboratories, 1980. (b) The Sadtler Standard Infrared Grating Spectra Collection; Sadtler Research Laboratories: Philadelphia, PA, 1980. (c) McClellan, A. L. Tables of Experimental Dipole Moments; W. H. Freeman: El Cemto, CA 1989; Vol. 3. (d) Ionization potentials were obtained for the compounds benzaldehyde, acetophenone, benzoquinone, and phenanthrenequinone from the Beilstein chemical reference. (e) Reduction potentials for radical ion formation of the compounds benzil, benzaldehyde, benzophenone, and acetophenone in DMF were obtained from Bard, A. J.; Lund, H. Encyclopedia of Electrochemistry of the Elements V. XI& Marcel Dekker: New York, 1978. (17) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper and Row: New York, 1987, p 144. (18) (a) Ramirez, F. Acc. Chem. Res. 1968, 1, 168. (b) Fauduet, H.; Burgada, R. Synthesis 1980, 642.
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