Reactivity of Lead Chalcogenide (001) Surfaces Interacting with H2S

Oct 30, 2007 - Lada V. Yashina , Alexander S. Zyubin , Ralph Püttner , Tatiana S. Zyubina , Vera S. Neudachina , Petar Stojanov , John Riley , Sergey...
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J. Phys. Chem. C 2007, 111, 17297-17304

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Reactivity of Lead Chalcogenide (001) Surfaces Interacting with H2S Lada V. Yashina,*,† Ralph Pu1 ttner,‡ Tatiana S. Zyubina,§ Maxim Poygin,‡ Vladimir I. Shtanov,† Vera S. Neudachina,†,§ Sergey L. Molodtsov,⊥ and Yury A. Dobrovolsky§ Department of Chemistry, Moscow State UniVersity, Leninskie gory, 119992 Moscow, Russia, Institut fu¨r Experimentalphysik, Freie UniVersita¨t Berlin, Arnimallee 14, D-14195 Berlin-Dahlem, Germany, Institute of Problems of Chemical Physics RAS, 142432 ChernogoloVka, Moscow Region, Russia, and Institut fu¨r Festko¨rperphysik, Technische UniVersita¨t Dresden, D-01062 Dresden, Germany ReceiVed: March 27, 2007; In Final Form: August 9, 2007

The interaction of the semiconductor compounds PbS, PbSe, and PbTe with H2S was studied using X-ray photoemission spectroscopy performed at the synchrotron radiation facility BESSY II. For all surfaces sulfidation subsequent to dissociation of H2S was observed. For PbS, the sulfur atoms occupy the surface anion vacancies. In the case of PbSe, either a similar interaction or an anion substitution at the surface takes place; besides, H2S adsorption leads to the agglomeration of additional sulfur atoms forming Sn2-. For the reaction of H2S with PbTe surfaces we suggest a mechanism which includes at least three steps. The first step leads to a formation of Pb-S-Te bonds, which is also supported by the results of quantum chemical calculations performed in the framework of cluster approach. The subsequent steps include further gradual oxidation of the tellurium atoms and result finally in the formation of quite stable thiotellurite species.

1. Introduction The well-known characteristics of lead chalcogenides, such as small band gaps with positive temperature coefficients, high static dielectric constants, and large carrier mobility, are unique for compound semiconductors; for this reason, they have been subjected to many theoretical and experimental investigations.1 Considerable efforts have been taken to understand the physical and electronic properties of these materials, as well as the underlying structure of the (001) termination.2-7 The studies performed for PbS and PbTe indicate remarkable relaxation along the (001) direction with an oscillatory contraction and expansion of interlayer spacing;2-4 however, they disagree in the magnitude of the surface rumpling. Thus, the geometrical and electronic properties of clean PbX (X ) S, Te) surfaces remain a controversial topic. The use of lead chalcogenides as working electrodes in solidstate electrochemical sensors for the detection of H2S can be a future application8 since this type of sensor offers numerous advantages in comparison to common electrochemical and semiconductor sensors used nowadays. These sensors possess, e.g., high selectivity andsin contrast to semiconductor sensorss the possibility to operate at ambient temperature. They function as an electrochemical cell that consists of a working electrode, a solid electrolyte, and a reference electrode.8 If a constant potential is applied at the reference electrode the EMF (electromotive force) of the cell depends on the concentration of the gas to which the working electrode is exposed. This working electrode is usually composed of PbS deposited from an aqueous solution. It is also possible to compose working electrodes of PbSe or PbTe; however, this leads to a longer response time. * Corresponding author. Tel.: +7 495 939 4665. Fax: +7 495 939 0998. E-mail: [email protected]. † Moscow State University. ‡ Freie Universita ¨ t Berlin. § Institute of Problems of Chemical Physics RAS. ⊥ Technische Universita ¨ t Dresden.

To optimize the sensor properties, the surface chemical changes induced by the H2S adsorption, and hence the sensor action mechanism, have to be understood at the atomic scale. To get insight into the mechanism of the sensor activity, as mentioned above, it is necessary to investigate the fundamental chemical processes that occur on the surface of the working electrodes as a consequence of the H2S exposure. The comprehensive analysis of such processes is highly complicated; it is, therefore, natural to simplify the system, i.e., to study model reactions. In a detailed study of the reactivity of the working electrodes composed by the different lead chalcogenides, the following issues should be addressed: (i) Investigation of the clean surfaces of PbX (X ) S, Se, Te) single crystals. (ii) The interaction of clean PbX surfaces with H2S. (iii) The oxidation of the PbX surfaces with subsequent adsorption of H2S: the latter issue can be considered as a simple model for the working electrode surface behavior. Previously, the clean PbX surfaces were studied both experimentally and theoretically, especially in the case of lead sulfide.2-7 Although some aspects of the clean surfaces, such as the differential relaxation and the surface core-level shift, are still under discussion, they are beyond the scope of this publication. The present report addresses the second issue mentioned above, with PbS, PbSe, and PbTe (001) surfaces being the object of the investigations. There are different possibilities for the behavior of H2S at the PbX (X ) S, Se, Te) surfaces: molecular adsorption/ physisorption; dissociative adsorption accompanied by Pb-SH and X-H bond formation; surface sulfidation with Pb-S bonds formation and H2 elimination; anion substitution by sulfur with H2Se/H2Te elimination. For other compound semiconductors it was observed that the dominating adsorption mechanism depends on the temperature. For example, for A3B5 semiconductors the molecular adsorption prevails at low temperatures, at T = 170 K the dissociative adsorption is realized, and at higher temperatures a surface sulfidation is observed.9-11

10.1021/jp0724119 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/30/2007

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In the case of PbTe it turned out that the H2S adsorption is a complex process which includes several steps; to clarify this issue, we present quantum chemical calculations for the interaction of H2S molecule with the surface of PbTe. 2. Experimental Details The X-ray photoelectron spectra were recorded at the synchrotron radiation facility BESSY II in Berlin, Germany, using the Russian-German beam line (RGBL).12 The spectra were taken with an experimental setup that consists of a preparation chamber and an analysis chamber. The latter one is equipped with a CLAM 4 (Thermo VG Scientific) electron energy analyzer. After loading the samples the entire experimental setup was baked for approximately 12 h at 150 °C in order to obtain a base pressure in the low 10-10 mbar range. PbS, PbSe, and PbTe single crystals were obtained by the vapor-liquid-solid (VLS) method which is described in detail in ref 13. The PbTe crystals were p-type samples, whereas the PbSe and PbS crystals possessed electron conductivity. For all crystals the typical dislocation density was in the range of 4 × 104 to 2.5 × 105 cm-2. Clean surfaces were obtained by cleaving the crystals in the preparation chamber, and their quality was confirmed with sharp LEED pattern. For cleaving and during the measurements (if not stated otherwise) the sample holder was cooled with liquid nitrogen (since this leads to better cleavage conditions and a decrease of the phonon broadening in the photoemission spectra). The Pb 5d, Se 3d, and Te 4d spectra were recorded at hν ) 125 eV, and the S 2p spectra were obtained at hν ) 205 eV. For all these spectra an analyzer pass energy of 2.5 eV was used in order to obtain an overall instrumental (monochromator and analyzer) energy resolution of 35 and 50 meV full width at half-maximum (fwhm) for the photon energy of 125 and 205 eV, respectively. To ensure a high surface purity, complementary O 1s and C 1s spectra were recorded at hν ) 600 eV. The absolute energy scale is limited by the mechanics of the monochromator and is in the order of ≈0.2 eV. The detection angle was ≈80°. The surface size of the crystal was =5 × 5 mm2 while the diameter of the X-ray spot was estimated to be in the order of 100 µm; its exact position at the sample surface was not precisely controlled. In the preparation chamber the samples were exposed at room temperature to H2S (99.5 vol %). Subsequently, they were transferred to the analysis chamber where they were cooled with liquid nitrogen. For the data analysis, the spectra were fitted by the GaussianLorentzian convolution functions with simultaneous optimization of the background parameters. The background was described by the equation

U(E) ) a + sS(E) + tT(E)

(1)

where S(E) is a Shirley background and T(E) is a Tougaard background with the parameter C ) 1643 eV2.14,15 To determine the parameter t in eq 1, spectra were recorded in a sufficiently wide kinetic energy range. Additional details on the fitting procedure are given further below. 3. Results and Discussion 3.1. H2S Adsorption on PbS and PbSe Surfaces. Figure 1 shows the S 2p spectra of a clean PbS(001) surface as well as of the PbSe(001) and PbTe(001) surfaces subsequent to the H2S exposure. For a better comparison of the different spectra presented in Figure 1, the S 2p3/2 (S 2p1/2) components of all features are indicated by solid (dashed) vertical lines.

Figure 1. S 2p spectra of (a) a clean PbS(100) surface as well as (b) a PbSe(100) and (c) a PbTe(100) surface exposed to H2S. The S 2p3/2 (S 2p1/2) components of the spectral features S I-S VI are indicated with solid (dashed) vertical lines. S I is assigned to a Pb-S bond, S II to a Pb-S-Te bond, S III and S IV to Pb-(S-S)2- or Pb-S-S-H (for details, see text), and S V to S0.

Figure 2. S 2p spectrum for a PbSe surfaces after an H2S exposure. The solid line through the data points represents the fit result, and the different subspectra are the contributions of the spectral features S I and S III-S V.

The S 2p spectrum of a clean PbS(001) surface (Figure 1a) consists of one spin-orbit split feature which is identified as S2- in a Pb-S bond formation and labeled S I. We observed that the Pb 5d/S 2p intensity ratio is much higher for a PbS(001) surface at room temperature than for a surface freshly cut at liquid nitrogen temperature. This agrees well with the observation that a PbS(001) surface at room temperature and ultrahigh vacuum (UHV) conditions possesses vacancies in the anion sublattice, i.e., it is Pb-terminated,16 and let us conjecture that the crystal surface was Pb-terminated prior to the H2S exposure. Both the PbS(001) and the PbSe(001) surfaces were exposed to 8 × 105 langmuir (L) of H2S, however, using different exposure times/pressure combination. In detail, the PbS surface was exposed to a partial pressure of 2.4 × 10-4 mbar and the PbSe surface to 2 × 10-3 mbar. These exposures did not lead to observable changes in the line shapes of the peaks corre-

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TABLE 1: Summary of the Results for the S 2p Features Observed for PbSe after H2S Exposure (Compare to Figure 2)a feature

assignment

ChS, meV

relative intensities

I III IV V

S2S-Pb bond in Sn2S-S bond in Sn2S0

0 1040 ( 100 2606 ( 100 3220 ( 30

0.14 0.13 0.15 0.58

a

Given are the assignment, the chemical shifts, ChS, as well as the relative intensities for the different exposures.

sponding to the elements forming the sample, i.e., in Pb 5d and S 2p lines in the case of PbS and Pb 5d and Se 3d lines in the case of PbSe. Nevertheless, for PbS a decrease of the Pb/S intensity was found indicating that the adsorption of H2S at the clean Pb-terminated PbS surface leads to an increase of sulfur atoms concentration without changes in their binding energies. For PbSe surfaces, sulfur was found at the surface subsequent to exposure. The corresponding S 2p spectrum is shown in Figure 1b and in more detail in Figure 2. It consists of four spin-orbit split features labeled S I, S III, S IV, and S V. The chemical shifts for these features relative to S I are summarized in Table 1. Spectral feature S I can readily be assigned to S2in a Pb-S bond formation. This has no influence on the corresponding Pb 5d spectrum of PbSe since the difference in the binding energies for the Pb 5d lines in PbS and PbSe is less than 100 meV and cannot be resolved. The dominant feature S V can be assigned to the formation of S0 state in accordance with refs 17 and 18. The features S III and S IV arise in the spectrum with comparable intensities, and their origin is not fully understood. However, following refs 19-21, we suppose that these features with a chemical shift of =1.1 and =2.6 eV relative to feature S I are related to two different sulfur atoms in oligosulfide particles like (S-S)n2-. On the basis of this assumption, feature S III can be explained either by a sulfur atom in a nearest-neighbor or in a next-nearest-neighbor position to a lead atom and feature S IV by sulfur atoms further away from lead. By taking all the above-described spectral features into account, we conclude that in the case of PbS the H2S molecules dissociate at the surface, thus filling the anion vacancies:

[...-Pb-S-Pb- [ ] -Pb-S-...] + H2S f [...-Pb-S-Pb -S- Pb-S-...] + H2 (2) For PbSe, we can assume that a reaction similar to 2 can take place if the surface is Pb-terminated (i.e., Se vacancies are present). Alternatively (or in parallel), an anion substitution reaction according to the scheme

PbSe + H2S ) PbS + H2Sev

(3)

may occur. Furthermore, for PbSe the presence of features S III-V can be explained by the agglomeration of sulfur atoms at the surface once all vacancies are occupied. The reaction can be described by the following scheme

Figure 3. Pb 5d spectra for (a) a clean PbTe surface, (b) a PbTe surface after H2S exposure I, and (c) a PbTe surface after H2S exposure II. The solid lines through the data points represent the fit results, and the different subspectra are the contributions of the spectral features Pb I and Pb II.

which explains the appearance of additional features S III-V with a binding energy typical for a S-S bond. 3.2. H2S Adsorption on the PbTe Surface. Figures 3-5 display the Pb 5d, Te 4d, and S 2p spectra for clean PbTe(001) surfaces and those chemically modified by H2S. The S 2p spectrum is also shown in Figure 1c, and the fit results are summarized in the Tables 2 and 3. Two different PbTe samples prepared from the same crystal were exposed to 8 × 105 L of H2S. For these experiments two different H2S partial pressures, namely, 2 × 10-3 mbar (exposure I) and 2.4 × 10-4 mbar (exposure II), were used. The spectra indicate that exposure II induced a stronger interaction between the surface and the gas than exposure I (see below); however, in order to clarify this behavior, complimentary investigations with different partial pressures and exposure times have to be performed. Figure 3a-c shows the Pb 5d spectrum of a clean PbTe surface as well as those after exposures I and II. The spectrum of the clean surface consists of a spin-orbit split feature Pb I which can readily be assigned to Pb atoms forming Pb-Te bonds. The peaks measured after exposure I (Figure 3b) show a slight asymmetry toward higher binding energies and indicate the presence of at least two spectral features. The chemical shift for Pb II relative to Pb I is 320 ( 70 meV; this agrees well with the difference in the Pb 5d binding energy of 350 meV between the vacuum-cleaved PbTe and PbS, i.e., feature Pb II

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Figure 4. Te 4d spectra for (a) a clean surface PbTe, (b) a PbTe surface after H2S exposure I, (c) a PbTe surface after H2S exposure II, and (d) a PbTe surface also after exposure II, however, additionally exposed to synchrotron radiation before the spectrum was recorded.

can be explained by the formation of Pb-S bonds. After exposure II (Figure 3c) the spectral feature Pb II is more intense than the feature Pb I. This clearly indicates a higher percentage of sulfur atoms on the surface, i.e., a more pronounced surface modification by H2S than for exposure I. The Te 4d spectrum of a clean PbTe(001) surface is shown in Figure 4a and consists of two spin-orbit split peaks. As described in detail in ref 22, these peaks can be decomposed into a surface and a bulk component with a surface core-level shift of -0.3 eV. The spectrum after exposure I is displayed in Figure 4b and consists of the spectral features Te I and Te II, which are split by 1082 ( 30 meV. Based on the analysis presented in ref 22, we can assign the feature Te I unambiguously to the bulk state of the clean surface (Figure 4a) and feature Te II to the Te0 state, in accordance with available literature data.23 The Te I/Te II intensity ratio in Figure 4b is similar to the bulk/surface intensity ratio in Figure 4a. The high relative intensity for feature Te II agrees well with the value expected for the uppermost layer (0.5 ( 0.05, estimated using the TTP-2m formula24) so that we conclude that all Te atoms in the uppermost surface layer are modified. The corresponding S 2p spectrum displayed in Figure 5a consists of two almost equally intense features which are both related to S2-, however, in a surrounding with different electron negativity. The chemical shift between these two components amounts to 627 ( 20 meV. Feature S I at lower binding energies is consistent with a Pb-S bond formation and corresponds to feature Pb II (Figure 3b). Feature S II is exclusively observed in the spectra of the PbTe surfaces (see Figure 1c) and can be explained with sulfur atoms in positions where they are bonded either to tellurium atoms or to both lead and tellurium atoms. We suggest that the Pb-S-Te bonds are formed, which is also supported by the results of our quantum chemical modeling

presented below. These explanations for features S I and S II are consistent with the higher binding energy for feature Te II since (i) a Pb-S bond formation (feature S I) as a result of complete substitution of Te decreases the electron density on the Te site, resulting in a neutral tellurium state and (ii) a TeS/Pb-Te-S bond formation (feature S II) decreases the electron density at the tellurium site as well. Exposure II led to a more pronounced tellurium oxidation forming the additional features Te III and Te IV which are displayed in Figure 4c. The chemical shift for feature Te IV amounts to 3.35 ( 0.07 eV relative to the Te2- lines of a clean PbTe surface, and a comparison with values for different tellurium compounds lets us conclude that this feature corresponds to the Te4+ state.23,25 The chemical shift of 1.4 ( 0.1 eV for feature Te III indicates some intermediate states of tellurium oxidation. It should be noted that the Te 4d spectra changed during measurements. Namely, we observed that the relative intensity of feature Te IV decreased during the exposure to synchrotron radiation (hν ) 125 eV). This can be seen, e.g., in Figure 4d which refers to the same sample and gas exposure as displayed in Figure 4c; however, the spectrum was recorded after a 20 min exposure to synchrotron radiation. In the S 2p spectra shown in Figure 5, parts b and c, exposure II induced the additional feature S III. This feature is shifted by 860 ( 70 meV relative to feature S I and evidently corresponds to Te4+ state in Te 4d spectra (feature Te IV in Figure 4c). This correlation is confirmed by the time-dependent changes in the intensity ratios for the different components in both the Te 4d and S 2p photoemission spectra, i.e., Te IV and S III intensities decrease after 20 min of sample exposure to synchrotron radiation in vacuum. Thus, we can assume that the H2S exposure results in a formation of a surface compound, which includes Te4+, Pb2+, and S2- species. Compounds which consist of Te4+ and sulfur,

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Figure 6. Optimized geometry of the (PbTe)34 cluster used for H2S adsorption modeling (structure a).

Figure 5. S 2p spectra of a PbTe surface (a) after H2S exposure I, (b) after H2S exposure II, and (c) also after H2S exposure II, however, exposed to synchrotron radiation before the spectrum was recorded. The solid lines through the data points represent the fit results, and the different subspectra are the contributions of the spectral features S I-S III.

e.g., the thiotellurite anions (TeS32-) and their salts, are wellknown from literature,26 and the formation of a lead thiotellurite species can explain all additional spectral features arising after exposure II. This is to our knowledge the first time the formation of lead thiotellurite at a surface is reported. In summary, we propose the following mechanism of the surface reaction which consists of the following steps:

Reactions 6 and 7 supposedly occur once all surface Pb-Te bonds are replaced by Pb-S ones. Contrary to PbS and PbSe,

we do not suggest major spectral contributions caused by the filling of anion vacancies since we observed for PbTe that the Pb 5d/Te 4d intensity ratio is independent of the sample temperature and the time spent by the sample in UHV after cleaving and concluded that this surface is not Pb-terminated at room temperature under UHV conditions. In summary, the H2S exposure causes for PbTescontrary to PbS and PbSespronounced changes in the spectra of both components, i.e., the lead and the chalcogenide. In addition, the corresponding S 2p spectra exhibit a multicomponent structure. To understand the observed effects and to get further insight into the reaction mechanism on its atomic scale, a quantum chemical modeling for the H2S adsorption at a PbTe(001) surface was performed. A comprehensive description of the computational procedure and the results will be reported elsewhere.27 In the present paper we will only summarize the major aspects. The present quantum chemical results are restricted to the interaction of a single H2S molecule with the surface, i.e., the interaction between H2S molecules adsorbed at different (neighbor) sites is neglected. Therefore, the results describe the initial steps of adsorption, and we apply them only to exposure I. The geometry optimization and chemical shift calculations were performed with the hybrid density functional B3LYP method using Gaussian-03 package. The chemical shifts were estimated for each atom of interest in the initial state approximation as the difference in the electric potential (EP) at the atom center before and after adsorption. These results are compared with the values obtained experimentally. In this way the atomic geometry of the absorption species can be deduced, and the reaction mechanism can be described comprehensively on an atomic scale. The calculated chemical shifts and the corresponding chemisorption enthalpies are given in Table 4. The PbTe(001) surface was modeled as a cluster cut from the bulk structure. For this we used the (PbTe)34 25/25/9/9 cluster displayed in Figure 6 (in the following referred to as structure a), with nine central surface and subsurface atom positions (9/9) being optimized. It was found that a further increase of the cluster size to (PbTe)52 does not lead to significant changes in the results (see Table 4, structure e). Six different adsorption geometries reflecting possible adsorption mechanisms described in the Introduction to the present paper were studied; the central fragments, i.e., adsorption sites of these different (PbTe)34-H2S clusters, are depicted in Figure 7 together with the corresponding calculated photoemission spectra. For the simulation of these spectra we assumed that the surface monolayer is fully modified, i.e., the intensity for the additional component comprises 50%. Our calculations show that the adsorption of H2S at the surface of a (PbTe)34 cluster is energetically unfavorable for

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TABLE 2: Summary of the Results for the Te 4d Features Observed in the PbTe Spectra after H2S Exposurea relative intensities featureb

assignment

ChS, meV

exposure I

exposure II

exposure II + 20 min

I II III IV

Te2--Pb bulk Te-S Te-S TeS32-

0 1082 ( 30 1406 ( 100 3347 ( 70

0.53 0.47 0 0

0.06 0.06 0.46 0.42

0.08 0.15 0.41 0.36

a Given are the assignments, the chemical shifts, ChS, and relative intensities of the different features in the Te 4d spectra obtained after different exposures to H2S. b For details of exposures I and II, see text. “+ 20 min” indicates that the sample was exposed to synchrotron radiation for 20 min before these spectra were recorded, see also text.

TABLE 3: Summary of the Results for the S 2p Features Observed in the PbTe Spectra after H2S Exposurea relative intensities exposure II featureb assignment ChS, meV exposure I exposure II + 20 minc I II III

S2--Pb Pb-S-Te Pb-S-Te

0 626 ( 20 856 ( 70

0.40 0.60

0.20 0.46 0.34

0.35 0.44 0.21

a Given are the assignments, the chemical shifts, ChS, and relative intensities of the different features in the S 2p spectra obtained after different exposures to H2S. b For details of exposures I and II, see text. c “+ 20 min” indicates that the sample was exposed for 20 min to synchrotron radiation before the spectra were recorded, see also text.

TABLE 4: Summary of the Theoretical Results Performed on the (LanL2DZ*, 6-31G** +PP) Calculation Level structure

∆E, kcal/mola atom ∆EP, eVb Z*, ec

(PbTe)34 (a)

Pb Te

(PbTe)34 + H2S in infinity (PbTe)34* 2H2S (b)

0 -3.70d

(PbTe)34*SH *H (c) the hydrogen is bonded with tellurium, the sulfur (SH) is bonded with surface lead (PbTe)34*S (d) + H2 the sulfur is bonded with surface lead

(PbTe)34*S (e) + H2 the sulfur is bonded with tellurium and partly with lead (PbTe)52*S (e) + H2 the sulfur is bonded with tellurium and partly with lead

0.6 -0.6 -0.03 -0.03 1.55 -0.27

0.7 -0.7

22.3

Pb Te S Pb

56.6

Te S Pb

1.07 -0.07 0.63

-0.3 -0.4 0.7

Te S

0 0

-0.4

30.7

Pb

0.07

0.8

25.2

Te S Pb

1.10 0.26 0.06

-0.4 -0.5 0.8

Te S Pb Te S Pb Te S

1.01 0.26 0.44 0.61 0.88 0 2.01 -0.03

-0.4 -0.5 0.7 -0.3 -0.8 0.8 0.1 -1

Pb34Te33S*Te (f) + H2

32.0

Pb34Te33S*TeH2 (g)

30.7

0.8

a For different clusters modeling the H S adsorption the energies, 2 ∆E, are given relative to the value for the (PbTe)34 cluster with an H2S molecule at infinite distance. b The changes of the electric potential, ∆EP, are presented relative to the value for the (PbTe)34 cluster for Te, Pb, and relative to the S-Pb bond in structure d for S. c Mulliken atomic charges. d Relative to (PbTe)34 + 2H2S.

the formation of most of the above-mentioned structures. Only the formation of the physisorption structure b is an exothermic

process, although its energy (-3.7 kcal/mol or 80 meV per one molecule) is rather low. However, the calculations do not take into account nonlocalized effects like the interaction of the adsorbed species with distant adsorbed species. In addition, the calculated values correspond to T ) 0 K although it is known from literature9,10 that H2S does not dissociate at the surface of other semiconductors below 170 K. Independent from these adsorption enthalpies, for the interpretation of the core-level spectra only the chemical shifts induced by the adsorption are relevant. These chemical shifts are determined by the local electron density, which is expected to be more accurately described by the present calculations than the above-mentioned adsorption enthalpies. Therefore, the calculated chemical shifts for the Pb 5d, Te 4d, and S 2p levels in the structures b-g are correlated to the experimental values obtained for PbTe surface. In our further discussion, the chemical shifts for the Pb 5d and the Te 4d levels are given relative to the value of clean surfaces (spectral features Pb I and Te I) represented by structure a. The chemical shifts for the S 2p level are given relative to sulfur atoms in a Pb-S bond formation; this corresponds to structure d and spectral feature S I. By considering the calculated spectra depicted in Figure 7 we conclude that none of the individually considered structures can describe accurately the photoemission spectra of all elements simultaneously. First of all, each structure gives only one spinorbit split doublet in the S 2p spectra, which is due to the fact that only one H2S molecule is considered. Consequently, a combination of at least two different structures has to be taken into account to simulate the experimental curves. In the following we will show that the structures b, c, d, and g are not in agreement with the experimental results so that they can be omitted in the combinations of different structures. Structure b modeling the H2S physisorption leads to practically no changes in the Pb 5d and Te 4d spectra, and the chemical shift of 1.55 eV for the S 2p level does not correspond to our experimental findings. For dissociative adsorption (structure c) the Pb 5d and Te 4d spectra seem to fit visibly the experimental curves; however, the calculations give a small negative chemical shift of -0.26 eV (-0.07 eV) for the Pb 5d (S 2p) level, which also contradicts our experimental observations. Structure d models surface sulfidation with a sulfur site directly on top of lead atoms and can be identified with feature S I. The EP for the lead atom in such a Pb-S bond formation is higher by 0.63 eV than in a “clean” cluster (structure a), which is almost twice higher than in our experiments. Finally, the exchange of tellurium and sulfur modeled by structure g gives rise to an essentially higher chemical shift of 2.01 eV for the Te 4d level. Note that the H2Te particles obtained in structure g will probably desorb. In this case we find a situation similar to the filling of anion vacancies at the surface as proposed for PbS and PbSe. This results in theoretical chemical shifts of 0 eV for Pb and -0.03 eV for S, which reflect the experimental observations. However, as discussed above, we assume that the

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Figure 7. Optimized geometry of the (PbTe)34 cluster with adsorbed H2S molecule and the corresponding calculated photoemission spectra of Pb 5d, Te 4d, and S 2p lines. The top row refers to the experimental spectra registered after exposure I. Structure b models the physisorption of two H2S molecules forming a hydrogen bridge bond, c corresponds to dissociative adsorption with -H+ and -SH- particles formation, structures d-f refer to surface sulfidation with H2 desorption, and g represents the substitution reaction PbTe + H2S f PbS + H2Te. The last row shows a combination of calculated spectra {e + f} taken with intensity ratio e/f ) 70:30. All distances are given in angstroms.

PbTe surface is not Pb-terminated so that a direct filling of the vacancies is rather unlikely. For a possible substitution of

tellurium by sulfur with subsequent desorption of H2Te we expect an increase of the Pb 5d/Te 4d intensity ratio. Contrary

17304 J. Phys. Chem. C, Vol. 111, No. 46, 2007 to this expectation, we observe a decrease of this ratio which may serve as an indication for sulfur atoms above lead atoms, which is an additional argument to exclude structure g from future consideration. Note that a pure S-Te bond, i.e., a S atom bonded directly to one Te atom, does not correspond to an energy minimum of the surface geometry. An optimization of this geometry results in structure e. Thus, a combination of calculated spectra corresponding to structures e and f should be considered to find an explanation for our experimental results. The calculated spectra for the {e + f} combination are presented at the bottom of Figure 7; the resulting spectra were obtained using the e/f intensity ratio of 70:30, which allowed us to obtain a pictorial rendition. All these structures refer to surface sulfidation accompanied by H2 desorption. Structure e illustrates surface sulfidation where the sulfur atom can be found in a bridge position. In this case electron density is transferred from both Pb and Te to the S atom; the binding energy of the Pb 5d level is almost identical to the value for the initial PbTe cluster. The calculated chemical shift for Te 4d is in agreement with experimental data. For structure f (where sulfur “pushes” the neighbor Te atoms to the surface, thus itself penetrating into the center of the cluster) the calculated chemical shift for the Te 4d level is somewhat lower than observed in the experiment, and the Pb 5d chemical shift is in line with our experimental findings. For the S 2p level, the difference in binding energy between the two structures (0.62 eV) is very close to the experimental value. Note that the combination of spectra considered here does not fully describe the experimental data; therefore, we cannot exclude a small admixture of the other structures (b-g). Therefore, we can conclude that the spectral features observed after exposure I can be explained by the Pb-S-Te bonds formation (structures e and f). To reproduce the higher positive chemical shifts as they were observed in the S 2p and Te 4d spectra after exposure II, more complicated structures with several H2S molecules adsorbed simultaneously at one site have to be considered in the calculations. Such calculations are, however, extremely complicated and time-consuming since a large number of different geometries has to be taken into account. For this reason, they are not performed at present so that it is impossible to draw firm conclusions on the reaction mechanism of thiotellurite species formation expressed by eq 7 at an atomic scale. 4. Conclusions In summary, we found that the mechanism of the interaction of the semiconductor compounds PbS, PbSe, and PbTe with H2S shows distinct differences. For PbS, the sulfur atoms from decomposed H2S occupy the anion vacancies at the surface. For PbSe, first either a similar mechanism or anion substitution is assumed. Furthermore, an intensive peak related to S-S bonds is observed in the S 2p spectra. The reaction between H2S and the PbTe surface first leads to the formation of Pb-S-Te bonds, which is also supported by the results of our quantum chemical calculations. The second stage includes further tellurium oxidation, which finally leads to the formation of quite stable thiotellurite species.

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