J. Phys. Chem. 1996, 100, 7181-7184
7181
Qualitative and Quantitative Evaluation of Heterogeneous Adsorbed Monolayers on Semiconductor Electrode by Infrared Reflection Spectroscopy J. A. Mielczarski,* Z. Xu,† and J. M. Cases Laboratoire “LEM” UA 235 CNRS, INPL-ENSG, B. P. 40, 54501 VandoeuVre-les-Nancy, Cedex, France ReceiVed: NoVember 9, 1995; In Final Form: February 3, 1996X
In situ infrared reflection spectroscopy combined with spectral simulation was applied to the study of the supramolecular structure of ethyl xanthate (C2H5OCS2-) species on cuprous sulfide. The surface layer was produced electrochemically from aqueous solution. Two adsorption products are observed at potentials above 350 mV (SHE): the cuprous ethyl xanthate complex (C2H5OCS2Cu) and liquidlike form of dixanthogen (C2H5OCS2)2. The detailed compositional and structural characterization is made on the basis of the comparison of simulated and experimental spectroscopic results. The quantitative evaluation of the two-component heterogeneous layer is discussed in detail. The in situ cell configuration used in this study was tailored specifically for the investigated system.
Introduction Thiol surfactants are widely used to modify the properties of solids. Although the spontaneous assembly of some thiol molecules at metallic surface represents a simple and powerful method of constructing an interface with well-defined structure and composition, as was shown in numerous papers for gold and some thiol systems,1 this process is more complex for nonmetallic substrates, as for example sulfides. Hence, understanding how the interfacial structure influences the chemistry and physics of the interface, with the aim of learning how to control the interfacial processes, is of both scientific and technological interest. Here we report the use of infrared reflection spectroscopy for in situ characterization of the multicomponent surface product formed at a cuprous sulfide electrode. In this paper we expand our previous in situ study of xanthate adsorption on cuprous sulfide in which simulation was performed for a one-component homogeneous adsorbed layer.2 In that work we found that two major adsorbed products are present on the surface of cuprous sulfide if the potential of substrate is high enough. Many studies on nonmetallic substrates2-5 have demonstrated that the obtained infrared reflection spectra supported by computational techniques are an excellent source of detailed information on the composition and structure of the adsorbed surface layer. Hitherto, the spectral simulation of adsorbed layer was limited to one-component isotropic or anisotropic surface structure. In this work we will present a quantitative evaluation of the two-component adsorbed layer. This kind of complexity of the adsorption layer, where each surface component has different optical properties, causes difficulties in the use of a simple ellipsometry for the quantitative evaluation. It is shown that in situ infrared reflection spectroscopy combined with spectral simulation offers a good ability to obtain chemical and structural order information, as well as quantitative evaluation of the multicomponent adsorption layer as a function of substrate potential. Experimental Section Materials. The cuprous sulfide, Cu2S, was a natural mineral (chalcocite) obtained from Ward’s Natural Science. Potassium ethyl xanthate was synthesized from ethanol, CS2, and KOH in † Permanent address: Department of Mining and Metallurgical Engineering, McGill University, Montreal, Canada. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
0022-3654/96/20100-7181$12.00/0
Figure 1. Schematic diagram of multilayer system with two-component adsorption layer. Air-window interface was omitted for convenience (see text for explanation). Directions of p- and s-polarized electric field vectors are marked on incident beam.
a standard way. The ethyl dixanthogen solid phase was obtained by oxidation of ethyl xanthate by iodine in aqueous solution. Dixanthogen has a melting point about 28 °C; therefore, the presence of small impurities in the sample results in lack of ability to crystalize at room temperature. Other details about materials can be found in the previous paper.2 Preparation and Characterization of Adsorbed Layers. The optical scheme of the system used is shown in Figure 1. The transparent window was a ZnSe reflection element with a fixed position. The experimentally determined mean thickness of the aqueous solution layer in our in situ cell was about 1 µm. A three-electrode system and a Tacussel PRT 20-2X potentiostat were used in the electrochemical experiments. The cuprous sulfide slab sample was the working electrode, while a platinum wire mesh, separated from the xanthate solution by a glass frit, served as the counter electrode. An Ag/AgCl electrode served as the reference electrode. All potentials are reported against the standard hydrogen electrode (SHE). In situ infrared reflection spectra were recorded on a Bruker IFS88 FTIR spectrometer equipped with an MCT detector and reflection attachments. A wire-grid polarizer was placed before the sample and provided p- or s-polarized light. The accessories were supplied by Harrick Scientific Co. The reflection spectra of the adsorption layers were obtained using the polarized light. The absorbance was defined as -log(R/R0), where R0 and R are the reflectivities of the systems without and with the investigated medium (i.e., two-component adsorption layer), respectively. Both sample and reference spectra are the average © 1996 American Chemical Society
7182 J. Phys. Chem., Vol. 100, No. 17, 1996
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of the same number of scans, from 200 to 1000 scans, depending on energy throughput. Other experimental details about the adsorption layer formation and its spectroscopic characterization can be found elsewhere.2 Experimental and Simulation Results In our previous in situ spectroelectrochemical studies2 of the ethyl xanthate-cuprous sulfide system, it was shown that at potentials below 350 mV, the major adsorption product is an isotropic multilayer of cuprous ethyl xanthate, as was deduced from comparison of the experimental and simulated reflection spectra. As was demonstrated in that work, the simulation contributes significantly to the detailed, on the molecular level, qualitative and quantitative characterization of the adsorbed xanthate structure. Because of optical effects, the recorded bands intensities and positions did not display any simple relationship to the nature and structure of the adsorbed xanthate layer. It is also important to mention that the in situ cell configuration used in this study was tailored specifically for the investigated system.2 This means that in the designing of the in situ cell the optimum of experimental sensitivity and, equally important, the maximum confidence in the interpretation of experimental results were considered. Spectral simulation of different in situ cell configurations enabled the determination of the experimental conditions under which optical effects are less dependent on certain parameters, which are difficult to precisely control in real experimental systems (uncollimated incident beam, surface roughness, real thickness of solution layer). This significantly increases the confidence of the performed quantitative evaluation. The spectral simulations were made for adsorbed two-component isotropic layer similarly to that presented recently for one-component adsorption layer.2 The simulations were limited to the frequency region of the most characteristic vibration of the adsorbed ethyl xanthate molecules, i.e., between 1400 and 950 cm-1 where very specific absorbance bands from each of the two different adsorbed components (cuprous ethyl xanthate and ethyl dixanthogen) could be easily distinguished and identified. The CH stretching vibration region, which is commonly used for the simulation spectra of organic adsorbed molecules,4,5 cannot be employed in these studies in order to distinguish the two adsorbed components. The bands at about 1200, 1124, and 1036 cm-1 are characteristic for cuprous ethyl xanthate complex while the bands at about 1262, 1242, 1110, and 1024 cm-1 are due to ethyl dixanthogen (Figure 2a). The detailed discussion of simulated and experimental results with only one-component surface layer, i.e., cuprous xanthate was reported recently.2 The experimental spectrum of the adsorbed layer obtained after 20 min holding of cuprous sulfide electrode in xanthate solution at a potential of 370 mV is shown in Figure 2a. Optical System. The simulations were performed for a fivephase system: window-aqueous solution-adsorbed layer containing two components-cuprous sulfide (Figure 1). Since it is convenient to discuss the optical effects in terms of changes in the incident angle at window-solution interface, the airwindow interface was not included in this calculation. Moreover, we decided to maintain the incident angle normal to the plane of air-window interface in our experimental cell which, in fact, eliminates the necessity of including the air phase in the calculation. The optical consideration presented in the work was limited to the simulation of absorbance, the value measured in experimental studies. In this theoretical consideration the previously determined2 optimal optical configuration with a ZnSe window and incident angle of 45° are used. Optical Constants of Multilayer Components. The optical constants of cuprous ethyl xanthate complex (C2H2OCS2Cu),
Figure 2. Reflection spectra for p-polarization: (a) experimental in situ spectrum of cuprous sulfide electrode in ethyl xanthate solution, concentration of 1.6 × 10-4 M, at a potential of 370 mV; (b) simulated spectrum of 5.2 nm thick isotropic layer of cuprous ethyl xanthate and 6.9 nm thick layer of liquid form of dixanthogen on cuprous sulfide for 1 µm aqueous solution in in situ cell.
Figure 3. Optical constants, refractive index, n, and absorption coefficient, k, of ethyl dixanthogen. Solid form of dixanthogen (dashdotted line), liquid form of dixanthogen (solid line). These optical constants were determined at a temperature of 22 ( 2 °C.
the cuprous sulfide, ZnSe, and water are the same as those used in the previous work.2 The optical constants of the second adsorption product, dixanthogen, were determined for the solid and liquid forms by the reflection method described before.6 It is noteworthy that this method does not require a priori any optical information (such as n∞ for Kramers-Kronig analysis) and can be used for any type of sample. The obtained data are presented in Figure 3. There are striking differences between the optical properties of crystalline and liquid forms of dixanthogen. This observation will be utilized to determine the structure of the adsorbed dixanthogen. In the first step, the spectral simulation was performed using the optical constants of solid dixanthogen and different thicknesses of each adsorption layer components. The cuprous xanthate was assumed as the first adsorbed product followed by the formation of dixanthogen. This is in an agreement with the previous experimental observations.2 The example of
Heterogeneous Adsorbed Monolayers
J. Phys. Chem., Vol. 100, No. 17, 1996 7183
Figure 4. Reflection spectrum of two-component adsorption layer of isotropic cuprous ethyl xanthate and ethyl dixanthogen, each of them 1 nm thick layer, on cuprous sulfide simulated for 1 µm in situ cell with ZnSe window. Simulation was performed for p-polarization and incident angle of 45°: (a) solid form of dixanthogen; (b) liquid form of dixanthogen.
simulated reflection spectra of the adsorbed layer containing the two components, each forming a 1 nm thick isotropic layer, on cuprous sulfide electrode in an in situ cell are presented in Figure 4a. The simulated spectrum was obtained by a straightforward combination of the independent spectra of each component. The results obtained are very different from those observed experimentally (Figure 2a) not only in respect to magnitude of absorbances. More importantly, the position and shape of certain bands are very different from these observed experimentally. It is interesting to note that the simulation performed for reverse position of the two surface components results in the same simulated spectra at the given layer thicknesses. This important observation indicates that the infrared spectra give a nondisturbed quantitative information about all of the adsorbed species at monolayer quantities. It is only when adsorbed components form thick coverages that the attenuation of the signal from the species placed closer to the substrate surface will be significant. This phenomenon will be discussed later in this work. Close inspection of the reflection spectra presented in Figures 4a and 2a discloses that the major differences are in the frequency region where dixanthogen has its absorbance bands. To understand this observation other spectral simulations were performed using the optical constants of a liquid form of dixanthogen that yield a spectrum presented in Figure 4b. The simulated spectrum clearly shows a much better agreement with the experimental result (Figure 2a). The major difference comes from relative absorbance intensities, whereas the position and shape of the absorbance bands are almost the same. These observations indicate that dixanthogen in the adsorption layer appears in the liquidlike form. Therefore, the optical constants of dixanthogen in the liquid form were used in all following optical considerations. Theoretical Consideration of Possible Changes during Adsorbed Layer Formation. There are three major variables which can be changed in the experimental configuration of an in situ infrared cell during the formation of a two-component adsorption layer. They are (i) the thickness of the aqueous solution, (ii) the amount of each of the two components in the surface layer, and (iii) the structure of the adsorption layer. These three parameters were investigated using the spectral simulation. Thickness of Aqueous Solution. The simulation was conducted for the solution layer thickness from 0 to 2 µm, and the
Figure 5. Simulated spectra of 1 nm thick layer of cuprous ethyl xanthate and 2 nm thick layer of liquid form of dixanthogen on cuprous sulfide for p-polarization in an in situ cell for four solution layer thicknesses: (a) 0 µm; (b) 0.5 µm; (c) 1 µm; (d) 2 µm.
results are presented in Figure 5. The strong decrease in the absorbance intensity with an increasing of solution layer thickness is observed. For example, the intensity of the bands predicted for the in situ cell with a 0.5 µm solution layer (Figure 5b) is about 17 times higher than that for a 2 µm solution layer (Figure 5d). Therefore, in a real system where some roughness of the electrode and window surfaces is expected, the recorded spectrum will represent mainly the composition and structure of the electrode surface which is at closest contact with the window. It is also possible that the adsorption products formed on cuprous sulfide could readsorb on ZnSe window (it can happen particularly for dixanthogen, which has a water solubility of 1.2 × 10-5 M), or direct contact between adsorption layer and the window could take place because of some variations from the ideal parallel position between them. Obviously the species being in direct contact with window will produce an extremely high intensity which could disturb significantly the quantitative evaluation. Fortunately, as shown in Figure 5a, the species will produce a strongly disturbed, by optical effects, very different spectrum. Hence, if the unwanted close contact of the adsorption product with the window will take place, it can be evaluated by the strong changes in shape of the group of absorbance bands at about 1250 and 1050 cm-1 (Figure 5a,bd). The experimental spectrum (Figure 2a) and other spectra recorded in separate experiments (not shown) did not display the predicted modification of reflection spectrum as is shown in Figure 5a. This clearly indicates that any transfer of the adsorption products from electrode to window surface did not take place at least to noticeable extend. Adsorption Layer Thickness. The reflection spectra were simulated for the two-component adsorption layer as a function of thickness of each component. The results show that there are linear relationships between the amount adsorbed and the absorbance intensity in the range of absorbance below 0.03, which is related to the thickness of the cuprous ethyl xanthate and ethyl dixanthogen layer below 50 and 90 nm, respectively. Moreover, the observed position and shape of bands are identical
7184 J. Phys. Chem., Vol. 100, No. 17, 1996
Figure 6. Simulated spectra of thick layers of cuprous ethyl xanthate and liquid form of dixanthogen on cuprous sulfide for p-polarization in an in situ cell as a function of layer thickness and relative position of components in adsorption layer: (a) electrode: cuprous xanthate (100 nm)-dixanthogen (200 nm); (b) electrode: cuprous xanthate (250 nm)-dixanthogen (500 nm); (c) electrode: dixanthogen (200 nm)cuprous xanthate (100 nm); (d) electrode: dixanthogen (500 nm)cuprous xanthate (250 nm).
in all the spectra calculated below these thickness limits for each component. Assuming that the thickness of ordered monolayer is about 1 nm, this observation indicates that even for relatively thick two-component adsorption layer the linear relationship exists. For much thicker two-component adsorption layers the full spectral simulation is required to evaluate quantitatively the amount of the adsorbed components (see also next section). Examples of nonlinear relationships between the absorbance and the thickness of the surface layer for higher than 0.03 absorbance are presented in Figure 6. Structural Effect. The two-component adsorption layer can form a well separated layer-by-layer structure or produce a wellmixed structure. Most probably the obtained structure will be somewhere between these two extreme limits. Therefore, the spectral simulation was also carried out for the system with the reverse position of the two components in the adsorption layer. It was found that all spectra simulated for the thicknesses of the adsorption layers in the linear range of absorbance (below 50 and 90 nm for cuprous ethyl xanthate and ethyl dixanthogen, respectively) are identical, independently of the absolute position of the two components at the surface of cuprous sulfide. All the simulated spectra are similar to the one presented in Figure 4b, the only difference is the intensity of the absorbance bands results from different thickness assumed. The relative absorbance between the bands remains the same. This observation indicates that the same spectra are expected for these two components independently of their spacial distribution within the adsorption layer. The latter conclusion is valid for the real system if we assume that there is a very similar interaction between the two types of the adsorbed species in the mixed and separate systems, or the interaction does not noticeably change the optical properties of each independent component in the mixed system. Moreover, from this simulation it is evident that the possible attenuation effect of the signal from the bottom adsorbed layer caused by the upper adsorption layer is negligible in this range of thickness of the adsorbed layer produced experimentally. However, for a relatively thick adsorption layer the arrangement of the two components in the adsorbed layer can be closely monitored. The simulated spectra presented in Figure 6 show the important differences, in the relative intensity and the band
Mielczarski et al. positions and shapes, predicted for the adsorption layer with different component order. Close inspection of the spectra in Figures 6a and b and Figures 6c and d clearly shows that the component on the top of the adsorption layer is better “visible” than that below. Obviously, for mixed adsorption layer this effect disappears, and only optical effects will disturb the reflection spectrum of thick adsorption layer. Evaluation of Experimental Results. The theoretical consideration shows that the heterogeneous adsorption layer with two different adsorption products will produce a reflection spectrum which is a simple combination of the spectra of two separate components if their observed absorbance is lower than the 0.03 absorbance unit. This value is far above the absorbances observed experimentally in this work. In Figure 2 the experimental and simulated spectra are presented in the same absorbance scale. Taking into account the complexity of the system under investigation and the real experimental conditions (for detailed discussion see also ref 2) the observed agreement between both of the results is considered as very good. Since the reflection spectrum presented in Figure 2b is calculated for an isotropic layer, this agreement also indicates that both adsorption products observed experimentally form isotropic surface products. This finding allows quantitative evaluation of the amount of the adsorbed products on the basis of direct comparison of the experimental and calculated spectra. It was determined that they are equal to 5.2 and 6.9 nm thick layers of cuprous ethyl xanthate and dixanthogen, respectively. The observed very small difference between the experimental and simulated spectra could indicate some lateral interaction between different adsorbed molecules; hence, most probably there is no ideal separation (layer-by-layer) within the surface structure obtained in the electrochemically controlled adsorption conditions. Conclusions It has been shown that infrared reflection spectroscopy combined with spectral simulation are excellent sources of detailed information about the nature and structure of a twocomponent surface layer. It was shown that although the infrared reflection spectra recorded for nonmetallic substrates are much more complex and their interpretation is not as straightforward as those for metals, they can provide much more details about the characterized multicomponent surface layer. Spectral simulation was applied intensively to evaluate the nature and structure of a two-component surface layer produced during the adsorption of ethyl xanthate on cuprous sulfide electrode under controlled potential conditions. The recorded in situ reflection spectra allow to determine the formation of isotropic adsorption layer containing heterogeneously distributed cuprous ethyl xanthate and liquidlike ethyl dixanthogen surface species. The amount of each of the produced surface components was determined. Acknowledgment. Part of this work was financially supported by European Community (project BRE2-CT94-0606). References and Notes (1) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87 and references therein. Ulman, A. An Introduction to Ultra-Thin Organic Films From Langmuir-Bloggett to Self-Assembly; Academic Press: Boston, 1991. (2) Mielczarski, J. A.; Mielczarski, E.; Zachwieja, J.; Cases, J. M. Langmuir 1995, 11, 2787-2799. (3) Reference 2 and references therein. (4) Hoffman, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 13041312. (5) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927-945. (6) Mielczarski, J. A.; Milosevic, M.; Berets, S. L. Appl. Spectrosc. 1992, 46, 1040-1044.
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