Spectroscopic studies of the structure of the adsorption layer of

Department of Mining and Minerals Engineering, Virginia Polytechnic Institute and ... X-ray photoelectron spectroscopy (XPS) in order to elucidate the...
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Langmuir 1991, 7, 101-108

101

Spectroscopic Studies of the Structure of the Adsorption Layer of Thionocarbamate. 2. On Cuprous Sulfide J. A. Mielczarski' and R. H. Yoon Department of Mining and Minerals Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 -0239 Received January 19, 1990. In Final Form: May 14, 1990 The spontaneousadsorption of 0-isobutyl-N-(ethoxycarbony1)thionocarbamat.e (ClHgOSCNHCOOCzH5, TC) on a semiconductor (CuzS)has been studied by infrared external reflection spectroscopy (ERS)and X-ray photoelectron spectroscopy (XPS) in order to elucidate the molecular orientation and coordination of the thionocarbamate-metal complex in the adsorption layer. It has been observed that since cuprous sulfide is a low-absorption material in the infrared region, a set of surface selection rules different from those applicable to metal substrates has to be used. Both positive and negative absorption bands were observed in the recorded spectra of an adsorption layer, depending on the angle of incidence and the polarization of the incident radiation. The intensity of an infrared absorption band in the reflected radiation was calculated for a three-phase adsorption system by using assumed optical constants for the adsorption layer, at various angles of incidence. Both p- and s-polarizations have been considered, and calculations have been made for all three electric vector components at the interface. Generally good agreement was obtained between experimental and theoretically calculated absorbance values for the system investigated. It has been shown that the infrared reflection method can be used to determine both the chemical nature and the orientation of individual molecular groups of adsorbed specieson low-absorption material at mono- and multilayer coverages. Application of the reflection method, supported by theoretical calculations, for determining the thickness and optical properties of the adsorption layer will also be discussed. Both the infrared and XPS results showed that the thionocarbamate molecules are oriented in the adsorption layer with their aliphatic chains toward the solution at monolayer coverage. The sixmembered complex Cu(TC') (TC' = C~H~OSCN-OOC~HS), with dissociation of the N-H bonding in the thionocarbamate molecule, forms the adsorption layer. As the surface coverage increases, the position of the adsorbed molecules changes gradually, and they become randomly oriented.

Introduction The adsorption of 0-isobutyl-N-(ethoxycarbonyl) thionocarbamate on copper metal and copper, Cu(II),activated zinc sulfide has been investigated by infrared reflectionabsorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS), as described in part 1.' Changes in the orientation of the adsorbed species with increasing coverage have been reported. In the present work, adsorption studies on a semiconductor, cuprous sulfide, were performed by using the external infrared reflection method. The infrared external reflection technique has been used recently to determine the structure of the spontaneously adsorbed layers formed by ionic2 and nonionic3 surfactants on low-absorption solids (k < 1). For such solids, different optical phenomena have been observed as compared to those observed with metal substrates (k > 1). Therefore, the optical behavior of Cu2S coated with various adsorption layers is discussed in the present work using optical theory. The external reflection technique has been used to characterize nonmetallic surfaces with deposits of thin This paper describes layers in only a few recent the use of the reflection method to study spontaneously adsorbed anisotropic layers at mono- and multilayer coverages. Application of the reflection technique supported by theoretical calculations makes it possible to (1) Mielczarski, J. A.; Yoon, R. H. J. Colloid Interface Sci. 1989,131, 423. (2) Mielczarski, J. A.; Yoon, R. H. J. Phys. Chem. 1989, 93, 2034. (3) Mielczarski, J. A.; Yordan, J. L.; Yoon, R. H. Preprints of Papers;

presented at the 196th ACS National Meeting, Loe Angeles,CA, September 26-30,1988; Vol. 33, No. 4, pp 729-733. (4) Porter, M. D.; Bright, T. B.; Allara,D. L.; Kuwana, T. Anal. Chem. 1986,58, 2461. (5) Ishino, Y.; Iehida, H. Langmuir 1988, 4, 1341. (6) Wong, J. 5.;Yen, Y . 4 . Appl. Spectrosc. 1988,42, 598.

0743-7463I91I 2407-0101$02.50,I O I

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understand the mechanisms by which the thionocarbamate interacts with the substrate.

Experimental Section Preparation of the Adsorption Layer. The CuzS slab (12 X 25 mm) was mineralchalcocite from Messina,Transvaal,South Africa. It was cleaned with emery paper and then polished successively with 1-,0.3-, and 0.05-wm alumina. The polished sample was washed with alcohol in an ultrasonic bath, followed by an HzS04 solution (pH 2) and distilled water. The sample was then immersed in 100 mL of 1.8 X 10-4M thionocarbamate solution at a pH of 6.0 i 0.2 for a period of 5 min to 43 h. The reagent used in the present work was 0-isobutyl-N-(ethoxycarbony1)thionocarbamate(>95% pure) obtained from American Cyanamid Co. After contacting the reagent solution, the sample was immediately placed in an FTIR or XPS spectrometer to record the spectra. Since the surface of the sample became sufficientlyhydrophobicafter the treatment, it was not necessary to wash and dry it before taking the spectra. Measurement. The infrared spectra were recorded by using an FTIR spectrometer,Perkin-Elmer Model 1710,equipped with an MCT detectorand an externalreflection attachment (Spectra Tech, Inc.) designed for recording a single reflection. A wire grid polarizer (Harrick Scientific Corp.) was placed before the reflection attachment and provided polarization selection. The spectrometer ?as purged with nitrogen gas to minimize the amount of water vapor present. The spectra were recorded at a 4-cm-l resolution by co-adding 100-200 scans in the 4000-500-cm-l region. The absorbance was defined as -log (R/ Ro) where Ro and R are the reflectivitiesof the sample before and after contacting the thionocarbamate solution, respectively. The XPS spectra were collected on a Kratos XSAM 800 spectrometer with Ka excitation from a magnesium anode operated at 10 kV and 10 mA. The pressure in the analyzer chamber was nearly 10-9Pa. The energy scale was calibrated by using the Fermi edge and the 4f7,~line of gold (BE = 84.0 eV) and the 2p3p line of copper (BE = 932.6 eV). The measurements were performed with a take-off angle close to 90'. The C Is, S 0 1991 American Chemical Society

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102 Langmuir, Vol. 7, No. I, 1991

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"' F i g u r e 1. Transmission spectra of a thin film of thionocarbamate (a) and the precipitated 1:l complex of cuprous thionocarbamate, Cu(TC') (b), 1800

1600

1203

14W

1

,Elll,l,

Table I. Infrared Frequencies of the Strongest Absorption Band and Assignments. TC thin film 3262 2965 2937 2900 2875 1772 1520

2963 2937 2901 2874 1722

assignment uNH vaCH3 Y~CHZ

~0-h

&HZ uC=O, uCN (com vib) YCN,6NH (com vib) vCN 6CHz 6CHi YCO,YCN(com vib) YCN YCO,YCS,YCN(com vib)

1545 1493 1380 1245 1201 1050

1380 1256 1171 1057 a Y,

Cu(TC') precipitate

stretch; 6, bend; com vib, complex vibration.

2p, and 0 1s lines were fitted by a curve-fitting program with a Gaussian peak shape. The polarization studies were made with an automatic ellipsometer (Rudolph Instruments, Inc., Research Model 444A3, equipped with an automatic rotating analyzer and a monochromator. The optical data were recorded and processed with an IBM Model 502 computer, equipped with an Epson FX1050 plotter. Precipitation of Copper-Thionocarbamate Complex. The copper-thionocarbamate precipitate was obtained by mixing CuSO4 and thionocarbamate (TC) solutions. Ascorbic acid was added to reduce the cupric ions to cuprous ions. The precipitate was filtered, washed with water, and dried in a vacuum desiccator. The transmission spectrum of the precipitate is presented in Figure 1 and described in Table I. For the reasons given in our previous work,' the spectrum may suggest a six-membered chelate, formed as in which the N-H bond is shown to dissociate. C4H&CNHCOC2H~+ CU+

II II s o TC

-C

C,H,OC.+

I:

s.

N *.COC,H,

:I

+ H+

.o

%u' Cu(TC')

Unfortunately, there exist no reliable assignments of (ethoxycarbony1)thionocarbamate and its copper complex in the literature, making the assignments in Table I slightly uncertain. These assignments have been made on the basis of the discussions of the infrared spectra of similar compounds presented earlier.'*'-11 (7) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman & Hall: London, 1975; Vol. 1, Chapters 12-14, 22. ( 8 ) Bogdanov, 0. S.;Vainshenker, I. A.; Podnek, A. K.; Ryaboi, V. I.; Yanis, N. A. Tsuetn. Met. 1976,17 (4), 79. (9) Rao, C. N. R.; Venkataraghavan,R.; Kaaturi, T. R. Can. J.Chem. 1964,42,36. (10) Tarantelli, T.; Furlani, C. J. Chem. SOC.A 1971,1213. (11)Furer, A. L.;Furer, V. L.; Maklakov, L. I. Zhur. Prikl. Spektr. 1982,36 (l),55.

0

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F i g u r e 2. Calculated absorbance components (All, AJ for a hypothetical monolayer of thionocarbamate (nz= 1.5, kt = 0.37 at 1245 cm-l, dz = ol+' m) on cuprous sulfide (ns = 6.27, ks = 0.19). Phase 1air ( n l = 1,kl= 0). Insert shows the experimental configuration. An elemental analysis of the precipitate shows the following (inwt%): C,35.36;H,5.26;0,17.60;N,5.09;S,12.15;Cu,24.31. These values are in good agreement with the theoretical values (C, 32.86;H, 5.92;0,18.68; N, 5.48; S, 12.53;Cu, 24.84) calculated for Cu(TC'), a 1:l complex. Thisfindingdiffersfromapreviously reported work,lZin which the precipitate was considered to have the structure of Cu(TC)&l. The XPS results also support the finding that the precipitate is a 1:l cuprous-thionocarbamate complex (Table 111). The intensity ratio of the oxygen line is somewhat lower than expected from the theoretical calculation, but the differences are within acceptable limits.

Calculation of Absorbance Theoretical calculations of the intensity of the absorption bands i n the reflection spectra were performed b y using a three-phase model and the exact equations of classical electromagnetic theory.13 The calculations were made b y using the following data: phase I (air), nl = 1.0 and kl = 0; phase 2 (adsorption layer), n2 = 1.5 and k2 = 0.37 at 1201 and 1253 cm-'. The k2 values were assumed on the basis of the transmission spectra of the precipitated Cu(TC') complex. As has been f o u n d i n the present work, i t is the only adsorption product of thionocarbamate on cuprous sulfide. The n2 value was assumed as typical of organic compounds. On the basis of previous results' and this work, it is known that thionocarbamate molecules are oriented in the first layer; therefore, the thickness of the adsorbed monolayer can be assumed to be 10-g m. The optical constants of cuprous sulfide (phase 3) are n3 = 6.27 and k3 = 0.19 at 1000 cm-1.2 Results of the calculations conducted for monolayer coverage are shown i n Figure 2 as a function of the angle of incidence (e) for both s-polarization (Al) and p-polarization (All). Similar results were obtained for the cuprous sulfide and ethyl xanthate systema2 The calculations show that for p-polarization the absorbance reaches a m a x i m u m (positive absorbance) at 0 somewhat below the (12) Leppinen, J. 0.;Basilio, C. I.; Yoon,R. H.Colloid Surf. 1988,32, 113. (13) Hansen, W. N. J. Opt. SOC.Am. 1968,58, 380.

Structure of Adsorption Layer of Thionocarbamate

-.00044 0

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-.0004

30

EO

0

90

30

Figure 3. Calculated absorbance components (Ail,A , ) for the system as in Figure 2 for various K2 values: (a) 0.37; (b)0.6; (c) 0.8; (d) 1.0; (e) 1.2. Brewster angle, 8B = 81' and a minimum (negative absorbance) at 8 somewhat above it. For s-polarization,the negative absorbance reaches a maximum when the incident radiation is vertical to the interface. Because of the limitations in the optic geometry and the angular dispersion of the incident beam, the following conditions were chosen to obtain maximum sensitivity: for p-polarization, 8 = 70'; for s-polarization, 8 = 25'. Under theseconditions, the recorded reflection spectra of an isotropic adsorption layer are expected to show the absorbance for s-polarization (negative) reversed with respect to p-polarization (positive), and for p-polarization the intensities of the absorption will be 1 order of magnitude higher than those for s-polarization. Since the values of the optical constants for the adsorption layer used in the calculations are only approximations, it would be of interest to see the effect of changing the kz and nzvalues. It is well-known that optical properties of a thin film change with thickness, especially at close to monolayer ~0verage.l~ Figure 3 shows the All and A L values for different values of kz and nz = 1.5. At = 70°, All increases initially with increasing kz, reaches a maximum when kz is between 0.6 and 0.8, and then decreases at higher kz values. As for Al, the negative absorbance increases steadily with increasing kz. It is shown that under the chosen experimental conditions the relative changes in absorbance with kz are greater for s-polarization than for p-polarization. The opposite behavior is observed when n2 is varied and kz is held constant (Figure 4). The A A component is almost independent of nz. On the other hand, the All component decreases with increasing values of nz. Since it is difficult to determine the exact values of nz and kz,and the values are expected to change with coverage, the results of the calculations shown in Figures 3 and 4 may be helpful. Depending on which values of n2 or kz are more accurate, the fitting of the experimental and theoretically calculated data should be based on the Al or All components, respectively. In the adsorption layer, absorption of the incident radiation results from interaction between the electric dipole moments of the molecular groups and the electric (14)Jovancicevic, V.; Yang, B.; Bockris, J. O'M.J . Electrochem. SOC. 1988,135,94.

EO

90

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ANCU OF INCIDENCE

Figure 4. Calculated absorbance components (Ail, A , ) for the system as in Figure 2 for various n2 values: (a) 1.3; (b) 1.5; (c) 1.7; (d) 1.9; (e) 2.1.

/ ", 0

30

80

I

ANGLE OF INCIDENCE Figure 5. Electric field intensities (E2) for three phases. For phase 1 ( E x l ,E y l , E Z l )and phase 2 (Exz,EYz,E,t) at the first interface (air-adsorption layer), and for phase 3 (ErstEy3, Ez3) at the second interface (adsorptionlayer-cuprous sulfide). Insert shows the calculation points.

field vector ( E ) . Figure 5 shows the calculated intensities in three of the mean square electric field vectors (E2) directions for a three-phase system. The calculation was made by using the exact equations13 for points at the boundary of the first interface in phases 1 and 2 and at the second interface in phase 3 (Figure 5, insert). The results show that the intensities of the ( E y 2 )and (E,Z) field components remain nearly constant across the ) adsorption layer, while the normal component (E Z 2jumps (i.e., it is not a continuous function) when crossing the phase boundaries, from large values in the first phase to almost zero in the third phase. Note, however, that the ( E Z 2 values ) are practically constant in phase 2. The results show that at higher 0 values the absorbance in the isotropic adsorption layer is caused mainly by the (E,Z) component, while for lower angles of incidence the ( E y 2 )and ( E x 2 )components dominate. Therefore, it is of interest to consider the two absorbance components (All, and All,) separately. The theoretical calculation of these components (ALy, A , I , , ~AllZ, , and All,) has been conducted by using the approximate equations in the same

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104 Langmuir, Vol. 7, No. 1, 1991 .woB i -Dolarita

t ion

.wo+ h

r2 a

\ v

-

W

I

0

-.om4

I I Jo 60 90

ANGLE OF INCIDENCE

Figure 6. Calculatedabsorbancecomponentsfor the data shown in Figure 2: obtained by using exact equations, Al and Ail; approximate equations? AA,, AllzJ,Allz, Allz. way as recently reported,2 and the results are shown in Figure 6. Also shown in this figure are the calculated results ( A , and All)obtained by using the exact eq~ati0ns.l~ Small differences between these two sets of calculated data occur at angles close to OB, indicating that very good approximations may be obtained by simple equatiom2 According to the results presented in Figure 6, when the molecules in the adsorption layer are oriented, the positive as well as negative absorption bands are expected for p-polarization at an angle of incidence below OB. The molecular groups of the adsorbed species having transition dipole moments normal to the interface will appear as positive absorbance bands (All,) in the reflection spectra, while those oriented parallel will appear as negative absorbance bands (Allx). These phenomena have only recently been experimentally observeda2v5In the above discussion of intensity variations, an isotropic adsorption layer was considered. For the case of an anisotropic adsorption layer, the absorption bands will show a 3-fold increase in the observed intensities. From this discussion, it is apparent that spectral differences may be a result of structural and chemical changes in the adsorption layer and optical effects as well. Therefore, a knowledge of the scope of the changes in these optical effects is vitally important for interpretation of the reflection spectra. Furthermore, the theoretical examination makes possible the prediction of the optimum experimentalconditions under which maximum sensitivity can be obtained.

Results and Discussion Infrared Study. Figure 7 shows the reflection spectra of the spontaneously adsorbed layer after different treatment times. These spectra were recorded at 70° for p-polarization and 25O for s-polarization, assuring that the optimum sensitivity was as predicted on the basis of the theoretical calculation. In order to form a thick adsorption layer, the sample was contacted with a 4 X M thionocarbamate solution for up to 48 h (Figure 7e and 7g). In the spectra recorded for p-polarization, both positive and negative absorption bands were observed, indicating that the adsorbed molecules are oriented at the surface. Comparison of the results for p-polarization and the

moo 1400 1000 em Figure 7. Reflection spectra of the adsorption layer of thionocarbamate on cuprous sulfide after various treatment times: (a) 5 min; (b) 15 min; (c) 80 min; (d) 5 h; (e) 48 h (all for p-polarization);( f ) 5 h; ( 9 ) 48 h (for s-polarization). The spectra after 48 h of treatment were recorded for adsorption from a M. solution of 4 X reflection spectra of the thionocarbamate adsorbed on copper' shows certain similarities; the band at 3262 cm-l due to the N-H stretching mode has disappeared after adsorption (not shown in Figure 7), suggesting that the hydrogen from N-H group dissociates during the formation of the copper-thionocarbamate complex. Although the absorption bands are positive and negative, their positions (Figure 7a-e) are similar to those observed for the precipitated Cu(TC') complex (Figure 1and Table I) and for the adsorption layer on copper.' This finding suggests that the adsorption product on the chalcocite is the sixmembered complex Cu(TC'). Note that the C=O band has been shifted to a lower frequency, while the C-N band has been shifted to a higher frequency from the thionocarbamate spectrum (Figure l),which supports the above conclusion. The negative absorption bands at 1545 and 1201 cm-' and the shoulder at 1243cm-' (Figure 7b-d) are due to the stretching vibration of the C-N group. This indicates, on the basis of the discussion above, that the position of the C-N groups in the adsorbed molecules is parallel to the interface. The strong positive bands at 1736 and 1253 cm-1 (Figure 7b-d) show that the positions of the C-0 and C-0 molecular groups are perpendicular to the interface. The positive band at 1372 cm-' due to the symmetric bending of the CHBgroup suggests that the position of the aliphatic chains is perpendicular to the

Structure of Adsorption Layer of Thionocarbamate

/.

p%7+ '\

+

cu,s

+

///////////d Figure 8. Proposed molecular orientation of thionocarbamate

on cuprous sulfide.

surface, as was also postulated re~ent1y.l~ Therefore, a molecular arrangement of the adsorbed molecule on cuprous sulfide can be proposed, as shown in Figure 8. After shorter (Figure 7a) and longer (Figure 7e) adsorption times, only positive absorption bands are observed in the recorded spectra for p-polarization. This indicates that orientation of the adsorbed molecules changes with the coverage. At lower coverages (Figure 7a),the molecules are likely to be inclined to the surface, while at multilayer coverages (Figure 7e) the majority of the adsorbed molecules are disoriented. Note, however, that the recorded spectrum (Figure 7e) differs (hasdifferent relative ratios of absorption bands) from that of the precipitated Cu(TC') complex (Figure 1). This may suggest that orientation of the molecules changes gradually in the adsorption layer, and the randomly oriented molecules of the Cu(TC') complex are adsorbed on the top of those that are well-oriented. As has been predicted for s-polarization (Figure 6), only negative absorption bands are observed in the reflection spectra of the sample contacted with the thionocarbamate solution (Figure 7f). The absorption bands are characteristic for those molecular groups that have transition moment components parallel to the interface. The spectra obtained after shorter contact times are similar, except that the bands are at much lower intensities; these have therefore been omitted. Close inspection of the spectra for both polarizations of the sample treated for 5 h (Figure 7d and 7f) shows that the negative bands have the same positions and that they can all be assigned to the same C-N molecular group with the exception of the band at 1050 cm-l, which will be discussed later. I t should be noted that in general the absorption bands are due to vibration of more than one molecular group in thionocarbamate, with perhaps the exception of the band at 1201 cm-l, which is due to the C-N stretching mode. Even the band near 1730 cm-' (Figure 7d) is due to a complex vibration of the C=O and C-N modes. The splitting of this band can, therefore, be explained as a result of mutual cancellation between the positive C=O band and the negative C-N band, which have transition moments perpendicular and parallel to the interface, respectively. The absence of the negative bands near 1050 cm-' in the p-polarization spectrum (Figure 7d) suggests that they are due to a complex vibration of different molecular groups. These bands can be assigned to the stretching vibration of the C-N, C=S, and C-C groups. A t certain (15) Rabolt, J. F.; Burns, F. C.; Schlotter,N. E.; Swalen,J. 0.J . Chem. Phys. 1983, 78, 946. (16) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52.

Langmuir, Vol. 7, No. I, 1991 105

positions of these molecular groups relative to the interface, the positive (C=S, C-C) and negative (C-N) bands can mutually cancel each other out, resulting in a disappearance of the absorption bands. The band at 1050 cm-' is observed in the reflection spectrum of thick adsorption layer (Figure 7e) in which moleculesare randomly oriented or at the lower angle of incidence (Figure 9) when the All, to All, ratio decreases (Figure 6). These findings support the proposed molecular orientation of thionocarbamate on chalcocite presented in Figure 8. Although the thionocarbamate spectrum does not show pure absorption bands for individual molecular groups having transition momenta perpendicular to each other in the adsorbed molecule,the intensity ratio of the 1253-cm-l (C-0) and 1201-cm-' (C-N) bands for p-polarization (Figure 7) can still be used as a measure of orientation. After 5 min and 48 h of adsorption, the majority of the molecules in the adsorption layer are randomly oriented, as is evident by the positive band at 1201cm-l. However, after shorter adsorption times, the adsorbed species are more or less oriented. The orientation factors are 1.3,1.6, 1.7, and 2.6 for adsorption times of 15,30,80,and 300 min, respectively. These results may indicate that the adsorption layers show the most ordered structure after 15 min of contact time. The thickness of the adsorption layer after 15 min of adsorption (Figure 7b) was estimated by ellipsometry to be one to two monolayers. Note that the p-polarization spectra show both positive and negative absorption bands, and some bands disappear depending on the recording conditions. On the contrary, the reflection spectra of the oriented layer of thionocarbamates on copper did not show the majority of the bands observed with s-polarization in the present work (Figure 7f). The reason is that with copper the molecular groups having transition moments parallel to the interface do not appear in the adsorption spectrum because of the surface selection rule. These results again support the conclusion that adsorbed molecules are oriented on both of the substrates. Figure 7g, representing an s-polarization spectrum of an adsorption layer of approximately 40-nm thickness, shows a band distortion which is caused by anomalous dispersion of the refractive index. The band distortion was first observed with the 1724-cm-l band in the s-polarization spectrum when the thickness of the adsorption layer reached approximately 15 nm. The distortion of the carbonyl band was also reported for the thin films of poly(methy1methacrylate) on glassy ~ a r b o nThus, .~ the results obtained in the present work show that thick adsorption layers are more amenable to band distortions. Also, s-polarization spectra are more readily distorted than the p-polarization spectra. The reflection spectra of the adsorption layer of thionocarbamate on cuprous sulfide were also recorded at various angles of incidencewith both s- and p-polarizations, Selected examples of the spectra after a 1-h adsorption time are shown in Figure 9. As predicted by theoretical analysis (Figures 5 and 6), the p-polarization spectra (Figure 9b,c) are dominated by the absorption bands that have transition moments perpendicular to the interface when '6 is somewhat below OB. At lower 6 values, however, those bands that are due to the molecular groups having transition moments parallel to the interface dominate the spectrum. In this case, the recorded spectrum (Figure 9a) becomes similar to the s-polarization spectrum (Figure 9d), where only negative absorption bands are observed. By comparison of the spectra recorded at various angles

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106 Langmuir, Vol. 7, No. 1, 1991 s-polarization

T

I3

I

I

I 1' p-polarization I I,

d

I I

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[

6r10-5

1600

1200

cm-

Figure 9. Reflection spectra of the adsorption layer of thionocarbamate on Cu2S,after 1 h of adsorption time, for p- and s-polarization,at various angles of incidence: (a) 21'; (b) 50'; (c) 70'; (d) 21'.

of incidence and polarization with results of the theoretical absorbance calculation, the thickness of the adsorption layer can be estimated. One must consider, however, several assumptions made in the theoretical calculations. Firstly, the calculations were made for a flat and smooth isotropic adsorption layer of a finite thickness, and the interfaces (air/adsorption layer and adsorption layer/ substrate) were considered parallel to each other. Secondly, the optical constants of the adsorption layer were estimated on the basis of the transmission spectrum of the precipitated Cu(TC') complex. In reality, however, the adsorption layer is anisotropic, and, in spite of a careful polishing the roughness factor may be significantly greater than 1. Furthermore, the optical constants for the adsorption layer of near monolayer coverages may be significantly different from those of the precipitate. For example, the monolayer of l-octyne on iron has a k value 4 times higher and an n value 25% higher than those of the bulk pre~ipitate.'~ Therefore, a correlation between the theoretical and experimental results is much more complex than would be apparent from the initial calculation. Figure 10 compares the results of the theoretical absorbance calculations and the experimental results (some of these data are from Figure 9). The calculations were made for the AIY,Allx,and All, components using various kz values. As discussed earlier, it is assumed that the molecules in the adsorption layer are oriented and that the absorption bands at 1253and 1201 cm-'are due to the transition moments of the molecular groups that are perpendicular and parallel to the interface, respectively. The experimental results cannot be matched to the three calculated absorbance values (ALy,Allz,Allz),on the basis of the optical constant kz, obtained for the precipitate Cu(TC') complex. Higher absorbances for the AI, and All, components or lower absorbances for the All, component are obtained. However, good agreement between the experimental and calculated results obtained for

Figure 10. Calculated absorbancecomponentsfor a hypothetical isotropic monolayer of thionocarbamate on CuzS (optical data as in Figure 2) at various optical constants for the adsorption layer. All,for various k0 (at 1253 cm-'1: (a) 0.37;(b) 1.0;(c) 1.2; (d) 1.4. A,,, and Al, for various k2 (at 1201 cm-'1: (e) 0.37;( f ) 1.0; (g) 1.2; (h) 1.4. Experimental results (note 12 X higher ababsorbance at 1253cm-l,p-polarization;(0) sorbancescale): (0) absorbance at 1201cm-l, p-polarization;(A)absorbance at 1201 cm-l, s-polarization. various angles of incidence and for both polarizations was found when nz was equal to 1.5 and the kz value was near 1.0, for an absorbance scale 12 times higher (Figure 10). The absorption bands at 1253 and 1201 cm-l show a 3-fold increase in the observed intensities due to the anisotropy of adsorption layer. This phenomenon was also recently reported for a thin anisotropic film.I0 Hence, from an observation of the difference between experimental data after 1 h of adsorption and calculated absorbances, coverage of nearly four monolayers is expected. The ellipsometric studies show that the coverage is also nearly four monolayers. This good agreement may be coincidental because of effects resulting from changes in the value of nz and the surface roughness on the observed absorbance, and such possibilities should also be considered. Any increase in the value of n2 involves a decrease in the All,component in comparison toAl1, and A Ly (Figure 4). However, the increase in roughness of the interfaces involves an increase in absorbance, but it is not clear if the proportionality of this increase is the same for the three absorbance components. Hence, both of the effects are compensated to a certain degree. A t lower coverage (Figure 7b), the intensity ratio of the absorption band at 1253 and 1201 cm-' shows a value of 1.3. This indicates that, for this coverage, increases in k p and nz values can be considered. It should also be kept in mind that similar changes can be expected when (C-0-C) groups are inclined to the plane of the interface. In conclusion, if the assignment of the absorption bands is correct and the structure of the adsorption layer is wellknown, the recorded reflection spectra could also provide important quantitative information about the adsorption layer, even though the optical properties of the adsorption layer are not precisely known. Moreover, certain data about the optical properties of the adsorption layer could also be obtained. Certainly, this approach ignores certain differences between the model calculation and the experimental data obtained and, therefore, is obviouslycrude. XPS Study. The results obtained are presented in Tables I1 and 111. On freshly exposed cuprous sulfide,

Structure of Adsorption Layer of Thionocarbamate

Langmuir, Vol. 7, No. 1, 1991 107

Table 11. Binding Energy (eV) Observed for S, C, N, 0, and Cu Atoms on Chalcocite after Various Treatments

core level

~~~~~

161.6 161.6 161.7 161.7

284.6 0.5 162.8 284.9 286.3 287.6 289.3 1.3 162.8 284.9 286.4 287.7 289.3 5 162.9 284.9 286.4 287.7 289.3 43 162.9 284.9 286.3 287.6 289.2 Cu(TC')a 162.9 284.9 286.3 287.7 289.3 a Precipitated 1:l complex of cuprous thionocarbamate. 0

532.4

399.0 399.1 399.0 399.1 399.1

531.9 531.8 531.9 531.8 531.7

533.4 533.6 533.6 533.5 533.5

932.6 932.8 933.0 933.0 933.1 933.0

75.1 75.3 75.5 75.4 75.5 75.5

336.1 336.2 336.3 336.4

338.3 338.3 338.4 338.5 338.5

Table 111. Relative Intensities of the Measured Lines of Chalcocite before and after Thionocarbamate Addition treatment s 2P c 1s 0 1s time, h Scua Sw C-C C-0 C=S C-0 Nls 0" CEO C-0 C U ~ R S / Zcu3D

before 0 0.5 1.3 5 43 Cu(TC)O Cu(TC)b CuzSb a

-161.7

162.9

284.9

1

0.97 0.74 0.63 0.00

1 1 1 1 1 1

3.48 2.08 2.30 1.61 2.12 1.87

-286.3 1.29 1.16 0.92 1.01 0.75 0.99 0.94

-287.6

-289.3

-399.1

-531.8

-533.5

-933.0 -75.5 1.28 2.70 0.71 1.66 1.41 3.88 0.65 1.40 1.39 3.08 0.59 1.38 1.31 2.89 0.73 1.48 1.13 1.47 0.63 1.24 1.06 1.42 0.80 1.60 1.07 1.51 2.13 2.98 Scofield atomic ionization cross sections.14 0.36

0.59 0.44 0.50 0.41 0.48 0.47

0.54 0.47 0.45 0.40 0.51 0.47

0.66 0.58 0.65 0.67 0.61 0.66

1

Precipitated 1:l complex of cuprous thionocarbamate. Calculated on the basis of

certain preadsorbed species were observed. The positions of the recorded lines for Cu2S and the preadsorbed species (Table 11) are similar to those reported and discussed p r e v i o u ~ l y for ~ ~synthetic J~ cuprous sulfide. The intensity ratio of the Cu 2p3p line to the S 2p line is much lower than was expected for cuprous sulfide from theoretical calculation (Table 111). However, the intensity ratio of the Cu 3p to the S 2p lines is close to the expected value. Since an electron escape depth from the Cu 3p level is higher than that desired from the Cu 2 ~ 3 1 2level, the difference in the relative intensity ratio of these lines indicates that preadsorbed species (oxygen, water, and OH-groups adsorbed, carbon contamination) are situated on the top of the freshly exposed sulfide surface. After thionocarbamate adsorption, the new characteristic photoelectron and Auger lines appeared (Table 11). The S 2p3p line at 162.9 eV, four components of the C 1s line (C-C at 284.9 eV, C-0 at 286.3 eV, N-C=S at 287.7 eV, and N-C=O at 289.3 eV), the N 1s line at 399.1 eV, two components of the 0 1s line (C=O at 531.8 eV and C-0 at 533.5 eV), the Cu 2p3/2 line at 933.0, the Cu 3p line at 75.5, and the Cu LMM Auger line at 338.5 eV are characteristic of the adsorbed layer. The positions of the lines are similar to those observed for precipitated Cu(TC') and for the adsorbed layer of the thionocarbamate on copper.' This supports the explanation that on the cuprous sulfide surface the 1:l complex of cuprous thionocarbamate is formed with a six-membered ring, where the copper atom is coordinated with thionocarbamate molecules through the sulfur and oxygen atoms, as is shown in Figure 8. The formation of a 1:l complex in the adsorption layer is confirmed by the fact that the relative intensity ratios between the Cu 2~312,N ls, and S 2p lines for the adsorption layer after 43 h of adsorption time (when no signal from the substrate is observed) are very similar to the values obtained for a precipitated Cu(TC') complex and from theoretical calculation (Table 111). Also, the relative intensities of the C 1s and 0 1s components are approximately equal to those expected for the 1:l complex. More complex XPS spectra are observed after shorter adsorption times, where signals from the substrate, pread(17) Mielczarski,J. A.; Suoninen,E. Surf. Interface Anal. 1984,6,34. (18) Mielczarski, J. A. J. Colloid Interface Sci. 1987, 120, 201.

sorbed species, and adsorption layer of thionocarbamatecopper complexes are expected (Tables I1 and 111). The signal from the substrate is easy to distinguish from the adsorption layer of thionocarbamate by using the S 2p line, where a difference of 1.2 eV is found between the positions of the sulfur doublets from cuprous sulfide and from adsorbed molecules. Moreover, the Cu LMM Auger line shows the difference in position of about 2.1 eV for copper bonded in the substrate and with thionocarbamate. Close inspection of the signal intensities from the substrate and the adsorption layer (Table 111)shows that after 5 h of adsorption time a strong signal from the substrate is still being recorded. This suggests that there is uneven coverage of the substrate by the adsorption layer. Analysis of the relative intensities of the C 1s and 0 1s components shows that these values at first decrease with increasing adsorption time and then show irregular changes (Table 111). The changes may be explained as a result of the difference in position of the adsorbed molecules in the surface layer with increased coverage. After a shorter treatment time, the relative intensity ratio of the signals from aliphatic carbon atoms and oxygen atoms bonded with these carbons is much higher than would be expected for a randomly oriented adsorption layer (Table 111).This indicates that, at lower coverage, the CH3 groups of the thionocarbamate are on the top of the adsorption layer, since the photoemission from the carbon atoms is not attenuated in comparison to the emissions from carbon bound with oxygen ((2-0)and particularly from carbon bound in C=O and C=S groups. These results indicate that the position of the adsorbed molecules is close to that presented in Figure 8. It should be noted that the differences in relative intensities observed after shorter adsorption times are probably due in part to the orientation of the adsorbed molecules and partly to the adsorption of hydrocarbon contamination (C 1s line nearly 284.6 eV) on a freshly exposed surface. The contribution of hydrocarbon contamination to the adsorbed species is rather small, as has been recently reported with regard to adsorption of xan-

108 Langmuir, Vol. 7, No. 1, 1991

thate homologues on copper and cuprous sulfide.17J9 The position of the C 1s line characteristic for hydrocarbon contamination on cuprous sulfide is shifted by 0.3 eV in comparison to the C 1s position of the C-C group of the adsorbed thionocarbamate molecules, which makes it easier to monitor changes in the amount of contamination. The irregular changes in the relative intensity with increased coverage (Table 111)can be explained as a result of reverse orientation of the thionocarbamate molecules on the top of one another in the initial monolayers. The changes in position of adsorbed molecules and uneven coverage can explain the apparent discrepancy between the infrared and XPS results for multilayer coverages. The infrared results show that, even after 43 h of adsorption time, a certain number of the adsorbed molecules have positions close to vertical with respect to the substrate (Figure 7e). However, in the XPS spectra for about 1 h of adsorption time, the relative intensity ratios are similar to those which are expected for randomly oriented molecules (Table 111). It should be kept in mind that the infrared reflection spectrum is characteristic of all of the adsorption layer while the XPS spectrum is sensitive to the top 2-3 nm of the adsorption layer. In the case of XPS studies, it is important to know whether the adsorption layer is stable under the experimental conditions used for analysis. Figure 11shows the infrared reflection spectra of the same sample recorded before and after XPS studies. The differences between the spectra indicate that the amount of adsorbate does not decrease strongly during the XPS measurement; however, the structure of the adsorption layer does change. The changes in the intensity ratios between positive and negative absorption bands indicate that the positions of the adsorbed molecules change with respect to the interface during the spectroscopic analysis. Conclusions Infrared external reflection spectroscopy supported by the theoreticalcalculation of the absorbance values in three (19) Mielczarski, J. A.; Suoninen, E.; Johansson, L. S.;Laajalehto, K. Int. J . Miner. Process. 1989,26, 81. (20) Scofield, J. M. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129.

Mielczarski and Yoon

r

8 2000

1600

1200

CIP

Figure 11. Reflection spectra of adsorption layer of thionocarbamate (after 48 h of adsorption) recorded for p-polarization before (a) and after (b) XPS studies.

directions at the interface makes it possible to determine the chemical nature of the adsorbed species and the orientation of the individual molecular groups of the adsorbed species at the interface. Information concerning the thickness and optical properties of the adsorption layer can also be obtained. The results obtained by using the XPS and ellipsometry methods confirmed the conclusions drawn from the infrared studies. The combined spectroscopic results allow one to propose the mechanism of growth of the adsorption layer of thionocarbamate on a semiconductor substrate even at near monolayer coverages. Acknowledgment. We thank Dr. Cesar Basilio for his assistance with the precipitation of the copper-thionocarbamate complex. We also acknowledge the American Cyanamid Co. for providingthe reagents and for financially supporting this work. We are grateful to Beth Howell for her efforts in the preparation of the manuscript.