ong>Acetaldehydeong> on Si(111)-7×7 J. Phys. Chem. B, Vol. 101, No. 10, 1997 1873TABLE 1: Vibrational Frequencies (in meV) ong>ofong> Gaseous/Liquid ong>Acetaldehydeong> ong>andong> ong>Acetaldehydeong> on Si(111)-7×7 aCH dCH 3(D 3)CH(D)Omode no. gaseous b liquid c 3(D 3)CH(D)Oon Si(111) at 120 KCH 3 stret ν 1 373 (281) 373ν 2 362 (264) 380 (263)CH stret ν 3 350 (256) 373 (257)CO stret ν 4 216 213 (212) 212 (∼200)CH 3 deform ν 5 179 (130) 177 (135) 185 (140)ν 12 168 (128) 167 (127)CH bend ν 6 174 (138) 173 (138)ν 14 95 (83) 95 (83) 98 (88)CC stret ν 8 138 (129,143) 138 (134) 144 (∼135)CH 3 rock ν 9 114 (93) 113 (93) 124 (∼90)CCO deform ν 10 65 (54) 64 (52) 58a1 meV ) 8.066 cm -1 . b References 16, 17, 19, ong>andong> 20. c References16-19. d This work.Figure 1. HREELS ong>ofong> CH 3CHO on Si(111)-7×7. The surface wasexposed to CH 3CHO at 120 K ong>andong> then heated at the indicatedtemperatures. All spectra were recorded at 120 K.heated at T S = 1500 K repeatedly until no C, O, N, or otherimpurities could be detected by XPS, AES, ong>andong> HREELS. Thesamples prepared in this manner also showed sharp 7 × 7 LEEDpatterns.The high-purity acetaldehyde-h 4 ,-d 1 , ong>andong> -d 4 samples fromAldrich were purified by vacuum distillation. FTIR spectroscopywas used to check D enrichment for CH 3 CDO ong>andong> CD 3 -CDO samples. On the bais ong>ofong> the FTIR measurement, we foundthat the D enrichment is >95% in CH 3 CDO ong>andong> CD 3 CDO. Theacetaldehyde was introduced into the UHV chamber through1/8 in. o.d. stainless steel tubing which had a flared tip with ani.d. ong>ofong> ∼1/4 in. to achieve even coverage when the surface wasmoved close for TPD dosing. After the dosing line wasrepeatedly purged with acetaldehyde, the surface was movedin front ong>ofong> the doser. The dosages were estimated according tothe pressure reading from an ion gauge.ResultsFigure 1 shows HREELS spectra ong>ofong> CH 3 CHO on Si(111)7×7.After the surface was exposed to 1 langmuir (L) ong>ofong> CH 3 CHOat 120 K, HREELS showed peaks at 98, 124, 144, 185, ong>andong>380 meV. By analogy to IR/Raman results for gaseous/liquidCH 3 CHO, 16-20 listed in Table 1, we assigned these peaks toCH bending, CH 3 rocking, C-C stretching ong>andong> CH 3 symmetricdeformation, ong>andong> CH stretching vibrations, respectively. Inaddition, a rather weak hump at ∼58 meV was noted. Thishump could be attributed to the CCO deformation vibrationalmode. The CO stretching mode, expected at 217 meV, wasnot well-resolved in the spectrum. However, it was observedin the corresponding CD 3 CDO HREELS spectrum, where theadjacent, more intense CD 3 ong>andong> CD vibrational modes shiftedtoward lower frequencies.When the sample was heated to 250 K, there were no obviouschanges in the CH 3 CHO HREELS spectrum. In contrast, thefollowing changes were noted at 350 K: (1) The 144 meV peakdue to the C-C stretching ong>andong> CH 3 symmetric deformationmodes virtually disappeared; (2) the 185 meV peak becameweaker; (3) an intense peak appeared at ∼100 meV, ong>andong> theFigure 2. HREELS ong>ofong> CH 3CHO (bottom), CH 3CDO, ong>andong> CD 3CDO(top) on Si(111)-7×7 at 120 K. The spectra were prepared ong>andong> recordedunder the same conditions as those in Figure 1. Vibrational assignmentsare given in Table 1 ong>andong> discussed in the text.CO stretching mode at 208 meV became noticeable. At 550K, the 100 meV peak vanished, ong>andong> a weak mode at 270 meVdue to the Si-H stretching vibration appeared in the spectrum.Further heating the sample at 700 K caused no significantchanges except the decrease in the intensity ong>ofong> the CH x -relatedmodes ong>andong> the concomitant increase in the Si-H mode. Finally,at 1150 K, the HREELS spectrum was dominated by a peak at∼110 meV, indicating the formation ong>ofong> silicon carbide. 9,15The 120 K HREELS spectra taken from CH 3 CHO, CH 3 CDOong>andong> CD 3 CDO on Si(111)7×7 are shown in Figure 2. Theexpected peak shifts upon deuteration were observed, ong>andong> theassignments ong>ofong> the vibrational modes are listed in Table 1together with the IR/Raman results for gaseous/liquidacetaldehyde. 13-20 In comparison with the 120 K CH 3 CHOHREELS spectrum, the two CH bending modes for CH 3 CDOshifted from 98 ong>andong> 185 meV to 88 ong>andong> ∼140 meV, respectively.The latter peak overlapped with the CC stretching ong>andong> CH 3symmetric deformation modes to give a broad peak centered at∼138 meV. Meanwhile, the relative intensity ong>ofong> the 185 meVpeak became weaker, ong>andong> a corresponding CD stretchingvibrational peak appeared at 263 meV. In the CD 3 CDOspectrum, the rocking, asymmetric deformation, ong>andong> stretchingpeaks ong>ofong> the methyl group shifted from 124, 185, ong>andong> 370 meV
1874 J. Phys. Chem. B, Vol. 101, No. 10, 1997 Bu et al.abFigure 3. XPS spectra ong>ofong> C 1s (a) ong>andong> O 1s (b), using an Al X-ray source.The Si(111) surface was dosed ong>andong> heated under the same experimentalconditions as those in Figure 1.to 88, 130, ong>andong> 280 meV, respectively. Accompanying thesechanges, the CO stretching mode became better resolved becauseong>ofong> the red shift ong>ofong> the CH 3 deformation mode at 185 meV. TheCD 3 deformation mode expected at 130 meV overlapped withthe CD bending ong>andong> CC stretching vibrational modes.The thermal effect on the CH 3 CDO ong>andong> CD 3 CDO HREELSspectra was less clear, because the CC stretching modeoverlapped with some ong>ofong> the CD-related vibrations. Heatingthe sample at 250 K caused no obvious change in either CH 3 -CDO or CD 3 CDO spectra. At 350 K, a slight decrease in the∼130 meV peak intensity was noted. Above 500 K, the lowerfrequency part (
ong>Acetaldehydeong> on Si(111)-7×7 J. Phys. Chem. B, Vol. 101, No. 10, 1997 1875baFigure 5. (a) TPD for m/e ) 48 (CD 3CDO) at the indicated dosingtime. The relative coverage is discussed in the text. (b) TPD ong>ofong> CH 3-CDO on Si(111)7×7 for m/e ) 3, 4, 28, ong>andong> 44. The Si(111) surfacewas exposed to CH 3CDO at 120 K for 60 s.dependence would predict. We speculate that this is due to aninitial consumption ong>ofong> parent molecules by the dosing line asthey first enter the vacuum chamber. The parent mass (m/e )48) showed a single peak at ∼320 K. At lower coverages, thereis a shoulder at ∼350 K.At higher temperatures, other fragments were observed asshown in Figure 5b. For CH 3 CDO, m/e ) 3 (DH) ong>andong> m/e )4(D 2 ) showed single peaks at 750 K due to the desorption ong>ofong>the surface hydrogen. In the corresponding CD 3 CDO experiment,m/e ) 4 also showed a single peak at 750 K with a muchsmaller peak at ∼300 K, presumably caused by the cracking ong>ofong>the parent molecules in the RGA. For m/e ) 44, two majorcomponents at ∼320 ong>andong> ∼1000 K were evident in the spectrum.DiscussionThe thermal stability ong>ofong> acetaldehyde adsorbed on Si(111)7×7was studied by using HREELS, XPS, UPS, ong>andong> TPD techniques.The molecular adsorption ong>ofong> acetaldehyde on the Si(111) surfaceat 120 K was clearly evident in HREELS ong>andong> UPS. In the CH 3 -CHO HREEL spectrum, all the molecular vibrations wereobserved ong>andong> compared with the IR/Raman results for thegaseous/liquid CH 3 CHO (see Table 1). Some ong>ofong> the modes werenot resolved in the spectrum with the resolution used in thisstudy. However, with the aid ong>ofong> the spectra taken from CH 3 -CDO ong>andong> CD 3 CDO, the assignment ong>ofong> each mode is fairlystraightforward. For example, the 185 meV peak for CH 3 CHOis composed ong>ofong> CH 3 deformation ong>andong> CH bending modes. Inthe CH 3 CDO spectrum, the relative intensity ong>ofong> the 185 meVpeak decreased ong>andong> the 140 meV peak became broader becausethe CH bending mode shifted from 185 to ∼140 meV. In thecorresponding CD 3 CDO spectrum, the 185 meV peak vanisheddue to the shift ong>ofong> CH deformation mode from ∼180 to 130meV.In the C 1s XPS spectrum, the C 1s signal showed two peaks at285.0 ong>andong> 287.8 eV. They are from the carbon atoms in CH 3ong>andong> C(H)dO groups, respectively. Similarly, two C 1s XPSpeaks were observed for the acetaldehyde on Fe(100) ong>andong> Cu-(110) surfaces. In the case ong>ofong> CH 3 CHO/Fe(100), 1 the lowerBE peak was more intense than the higher BE one, ong>andong> thepeak intensity difference was attributed to the partial dissociationong>ofong> the molecules upon adsorption. In the present study ong>ofong> CH 3 -CHO on Si(111)7×7, we also noticed that the two C 1s XPSpeak intensities were different. This may indicate a partialdissociative adsorption ong>ofong> acetaldehyde on Si(111). The argumentis consistent with the high-temperature XPS result, e.g.,at 350 K, where the C 1s XPS shows a single broad peak at 284.5eV due to the breaking ong>ofong> the C-C bond (see later discussion).Furthermore, the O 1s XPS peak shows some contribution fromthe 530.8 eV component, which becomes the dominant featureat temperatures above 350 K. On the other hong>andong>, the differencein C 1s XPS peak intensity at
1876 J. Phys. Chem. B, Vol. 101, No. 10, 1997 Bu et al.Figure 6. He I (21.2 eV) UPS results ong>ofong> CH 3CHO on Si(111)-7×7.The surface was dosed at 120 K as indicated. The S 1 ong>andong> S 2 surfacestates decreased as the surface was exposed to increasing amounts ong>ofong>CH 3CHO molecules.from 531.7 to 530.8 eV. In HREELS, the C-C stretching modeat 144 meV virtually disappeared, while an intense peak at ∼100meV appeared in the spectrum. A peak at 105 meV wasobserved in our earlier study ong>ofong> TMIn on Si(111)7×7 whenthe surface was heated to 520 K to break the In-C bonds. 22The peak was assigned to the CH 3 deformation mode for theCH 3 group on Si(111)7×7, which was discussed in detail inref 22. Another intense peak at 128 meV is likely due to theSi-O bond, which persisted up to 700 K, while a small humpappeared at ∼208 meV, which is red-shifted by more than 10meV as compared to a normal carbonyl CO double bond. Thisred shift, coupled with the 128 meV Si-O stretching mode ong>andong>the observed C 1s XPS peak shifting, indicates the formation ong>ofong>the Si-O bond ong>andong> thus the weakening ong>ofong> the CO bond. Inthe corresponding UPS spectra, the two peaks at 5.7 ong>andong> 10.0eV due to 10a′ ong>andong> 1a′′ molecular orbitals attenuated significantly,ong>andong> a new peak at ∼6.5 eV was noted. The 5.7 eVpeak is associated with the O lone electron pairs; its peakintensity decrease suggests the formation ong>ofong> an Si-O bond,which is consistent with the HREELS results discussed above.The 10.0 eV peak has the character ong>ofong> a π CH 3 bond, ong>andong> itspeak intensity attenuation is likely due to the C-C bondbreaking ong>andong> the migration ong>ofong> the CH 3 group from CH 3 CHOonto the substrate. The appearance ong>ofong> the new 6.5 eV peak isattributable to the formation ong>ofong> the Si-C bond (i.e., Si-CH 3species) ong>andong> the Si-O bond. The aforementioned changes inboth HREELS ong>andong> UPS indicate the break ong>ofong> the C-C bondong>andong> the formation ong>ofong> Si-CH 3 species at 350 K.Further heating the sample to 550 K caused the disappearanceong>ofong> the ∼100 meV HREELS peak ong>andong> the appearance ong>ofong> a newpeak at ∼270 meV attributable to the Si-H stretching mode.These simultaneous changes indicate the cracking ong>ofong> the CHbond ong>andong> the formation ong>ofong> the Si-H species. The Si-Hstretching mode at ∼270 meV, being a little higher than that ong>ofong>∼260 meV for H on a clean Si surface, is probably caused bythe Si atoms back-bonded to O atoms. At higher temperatures,CH x -related features decreased in intensity, indicating a furtherbreaking ong>ofong> the CH bond. Above 800 K, the HREELS spectrafor all acetaldehyde isotopes are dominated by a single peak at110 meV, which can be ascribed to the formation ong>ofong> SiC, 22 whilethe Si-O stretching mode at 128 meV vanished. In thecorresponding XPS results, the O 1s XPS peak virtually disappeared,ong>andong> the C 1s XPS peak shifted to 282.8 eV at >800 K.In the UPS spectra, the intense peak at ∼6.5 eV diminishedong>andong> the spectrum at 1150 K was similar to that ong>ofong> the cleansurface, except for the presence ong>ofong> a weak Si-C feature at ∼7eV. In the TPD measurement, two peaks were noted for them/e ) 44 at ∼320 ong>andong> ∼1000 K. The lower temperature one(∼320 K) follows the parent prong>ofong>ile, while the higher temperatureone (∼1000 K) is attributed to the SiO species. Thisobservation is consistent with our surface analysis results, wherethe O-related features, e.g., O 1s XPS peak ong>andong> the 128 meVpeak in HREELS, practically vanished above 1000 K. We alsoexamined the possible product CO (m/e ) 28), but the resultswere not conclusive. As can be seen in Figure 5b, the dominantpeak at ∼320 K again follows the parent prong>ofong>ile, while a smallhump at ∼680 K was not observed in the parent spectrum.However, a similar hump was evident in the m/e ) 44 (SiO)spectrum; this hump may have resulted from Si + ion fragment.The decomposition process described above can be summarizedas follows:120-250 KCH 3 CHO(g)98 CH3 CHO(a)∼350 K98 CH3 (a) + CHO(a) + CH 3 CHO(g)∼450-700 K98 CHx (a) + HO(a) + O(a)∼1150 K98 C(a)ConclusionThe adsorption ong>andong> thermal decomposition ong>ofong> CH 3 CHO, CH 3 -CDO, ong>andong> CD 3 CDO on Si(111)7×7 have been studied withHREELS, XPS, TPD, ong>andong> UPS. ong>Acetaldehydeong> was found toadsorb molecularly on the surface at 120 K. However, somedissociative adsorption may also have occurred according toour XPS results. No obvious changes were noted when thesample was heated to 250 K. At 350 K, on the other hong>andong>, thebreaking ong>ofong> the C-C bond, the formation ong>ofong> the Si-O bond,ong>andong> the CH 3 groups on the surface were indicated by HREELSong>andong> UPS spectra. A partial desorption ong>ofong> the parent moleculewas also revealed by the decrease ong>ofong> the C ong>andong> O XPS peakintensity ong>andong> the appearance ong>ofong> a desorption peak at ∼320 Kfor the parent molecule. Further heating the sample to 700 Kcaused the decomposition ong>ofong> the CH 3 adspecies ong>andong> theformation ong>ofong> Si-H species. Finally, at 1150 K, O-containingspecies ong>andong> H adspecies desorbed from the surface, while Cremained on the substrate to form silicon carbide.Acknowledgment. The authors gratefully acknowledge thesupport ong>ofong> this work by the Office ong>ofong> Naval Research.References ong>andong> Notes(1) Benziger, J. B.; Madix, R. J. J. Catal. 1982, 74, 55.(2) Prabhakaran, K.; Rao, C. N. R. Appl. Surf. 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