Adsorption and Thermal Reaction of Short-Chain Alcohols on Ge (100)

Article pubs.acs.org/JPCC

Adsorption and Thermal Reaction of Short-Chain Alcohols on Ge(100) Tsung-Hsiang Lin, Bo-Yu Lin, Ting Hao, Hsiu-Yun Chien, Jeng-Han Wang,* and Wei-Hsiu Hung* Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan S Supporting Information *

ABSTRACT: The adsorption and thermal decomposition of alcohols (CH3OH, C2H5OH, and C4H9OH) on Ge(100) were investigated with temperature-programmed desorption and Xray photoelectron spectra. At 105 K, CH3OH adsorbs both molecularly and dissociatively on Ge(100). Chemisorbed CH3OH molecules dissociate to form surface CH3O and hydrogen in a temperature range 150−300 K. Surface CH3O can dehydrogenate to yield CH2O as two desorption features, which depend on coverage. At small coverage, surface CH3O undergoes mainly α-hydrogen elimination to desorb CH2O at 490 K. At large coverage, another desorption of CH2O occurs predominantly at 525 K, which is initiated by a recombinative desorption of CH3OH. A calculation with density functional theory at the B3LYP/6-311+G** level shows that the dissociation of the O−H bond has a much smaller barrier (150 kJ/mol). Desorption of CH2O results from the moderate barriers (∼110 kJ/mol) for cleavage of the C−H bond of surface CH3O and weak adsorption energy of CH2O (−56 kJ/mol). The recombination of surface CH3O with H occurs at large coverage with an energy barrier 127−140 kJ/mol. Similarly to CH3OH, C2H5OH and C4H9OH undergo the mechanism of thermal reactions through formation of alkoxyl intermediates. The longer-chain alkoxyl decomposes to desorb aldehyde at lower temperature because the interaction of its alkoxyl chain with the surface is stronger. On annealing to ∼570 K, all alkoxyl groups are completely removed from the surface via dehydrogenation and recombination to desorb aldehyde and alcohol, respectively. At a large coverage, the longer-chain alkoxyl undergoes dehydrogenation to a larger extent than recombinative desorption.

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CH3O−H bond.11,12 The CH3O group might be oriented either parallel or perpendicular to the surface.12 At a large coverage of methanol, chain-like arrays of H−Ge−Ge−OCH3 are produced by successive adsorption along the dimer-row direction.11 Infrared spectra and DFT calculations indicated also that ethanol (C2H5OH) prefers to undergo the cleavage of the O−H bond on Ge(100) at 310 K, rather than the C−O bond.13 The preceding authors focused mainly on the structural geometry and configuration of alkoxyl groups upon adsorption of CH3OH or C2H5OH on Ge(100), but thermal reactions of alkoxyl groups on a Ge surface have been studied much less than for Si and other metallic surfaces.11 The measurements of X-ray photoelectron spectra (XPS) showed that CH3O groups undergo a partial fragmentation to form surface −CHx species on Si(100) near 300 K at a large exposure.14 Thermal desorption spectra and theoretical calculations revealed further that alkoxyl groups undergo reaction channels involving elimination of β-hydrogen and other competing decompositions.15,16 The major desorption products included hydrogen molecules, aldehydes, alkenes, and SiO 2 , whereas the

INTRODUCTION Germanium (Ge) is a semiconductor material for highperformance integrated circuits because both electrons and holes possess great mobility. Having a direct transition of which the energy is only slightly greater than the indirect band gap, Ge has a greater absorption coefficient than silicon, making Ge desirable in many optoelectronic and photovoltaic applications. Much effort has been directed to introduce organic functional groups on Ge surfaces and to stabilize their electronic and optical properties.1−7 A detailed understanding of adsorption and thermal reactions of organic compounds on Ge surfaces is hence important both from fundamental and technologic points of view. Buriak and Loscutoff and Bent reviewed comprehensively the chemical reactions of organic compounds involved in functionalization of the Ge surface.8,9 On the basis of highresolution electron energy-loss spectra (HREELS), methanol (CH3OH) was proposed to undergo the scission of the C−OH bond near 300 K and to form methyl radical and hydroxyl species on Ge(100).10 In contrast, the data from a scanning tunneling microscope (SEM) and calculations based on density functional theory (DFT) indicated that CH3OH undergoes dissociative adsorption to form surface methoxyl (CH3O) and hydrogen on a Ge−Ge dimer through the cleavage of the © 2013 American Chemical Society

Received: September 10, 2012 Revised: January 10, 2013 Published: January 16, 2013 2760

dx.doi.org/10.1021/jp308990x | J. Phys. Chem. C 2013, 117, 2760−2768

The Journal of Physical Chemistry C

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which was a skimmer with an entrance aperture (diameter 2.8 mm). For TPD measurement, the sample surface was placed about 2 mm before the aperture and in line of sight of the ionizer of the mass spectrometer; TPD scans were recorded on ramping the sample at a linear rate, ∼1.5 K/s. DFT calculations on the cluster model were performed with Gaussian03.18 A Ge15H16 cluster that contains two surface dimers in the same row served as a model of the Ge(100) surface. This cluster, which has been widely applied in the study of interfacial reactions on Ge(100) surface, shows a benefit for the investigation of interdimer reactions and, with sufficient accuracy, is computationally more efficient than clusters with more dimers or two parallel dimers in adjacent rows.8,9,13,19 All dangling bonds of the subsurface Ge atoms terminated with H atoms. The computation at the B3LYP/6-311+G** level was employed for geometrical optimization and calculation of the energies of reaction intermediates (local minima) and transition states without constraining degrees of freedom. The hybrid Hartree−Fock/DFT method, B3LYP, includes Becke’s threeparameter nonlocal-exchange function with the correlation function of Lee, Yang, and Parr.20−22 The standard all-electron split-valence basis set (6-311++G**) includes a polarization dfunction on non-hydrogen atoms.23−26 All potential energies were calculated with unscaled corrections for zero-point energy (ZPE) at the B3LYP/6-311+G** level.

desorption of the parent molecular alcohol was not observed. These results indicated that the thermal reaction of the alkoxyl group involved the cleavage of C−O and C−H bonds on Si(100). A preceding DFT calculation also predicted that CH3OH can form surface CH3O on an epoxidized C(100) surface.17 The reactions of alcohols on Ge(100) are expected to differ somewhat from those on Si(100) and C(100) because of subtle differences in their chemical reactivity and electronic structures. We report here an investigation of the adsorption and thermal reaction of alcohols on a Ge(100) surface, using temperature-programmed desorption (TPD) and XPS. The mechanism of thermal decomposition of CH3OH was systematically investigated with DFT calculation and correlated with the experimental results. The length of the alkyl chain is expected to have a significant influence on the reaction mechanism because of the interaction between the Ge surface and the alkyl chain. To examine the thermal reaction as a function of alkyl chain, we undertook a comparison of experimental data for alcohols of varied chain length (i.e., ROH with R = CH3, C2H5, and C4H9).

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EXPERIMENTS AND CALCULATIONS The experiments were conducted in an ultrahigh vacuum (UHV) chamber with a base pressure of 2 × 10−10 Torr. This system was equipped with a quadrupole mass filter (EPIC, Hiden), a low-energy electron-diffraction (LEED) apparatus, and an electron-energy analyzer (HA100, VSW). The Ge(100) samples (Sb-doped n-type, 1−10 Ω cm) for our work had a thickness of 0.3 mm. The Ge sample was mounted on an Si sample of the same dimensions. A Ta strip (thickness 0.025 mm) was uniformly pressed between Ge and Si samples with Ta foils at both ends, which were in turn mounted on a copper block. The sample could be cooled to 105 K with liquid nitrogen via conduction through the copper block and heated with resistive heating of the Ta strip. The sample temperature was monitored with a thermocouple (type K) spot-welded onto a thin Ta foil inserted between the Ge and Si samples. The surface was cleaned through sputtering with Ar+ ions and annealed at 870 K; according to LEED, the surface then exhibited c(4 × 2) patterns.10,11,16 The cleanliness of the Ge surface was verified with XPS measurements. CH3OH (99.9%, Acros), C2H5OH (absolute, Acros), and C4H9OH (>99%, Acros) were purified with several freeze−pump−thaw cycles. During dosing, the partial pressure of alcohols was controlled at 1 × 10−9 Torr. The sample was placed ∼3 cm before the doser to minimize the contamination of the UHV system by the dosed alcohols. XPS were measured at the HSGM and WR beamlines of National Synchrotron Radiation Research Center (NSRRC), Taiwan; the angle of incidence of photons was 45° from the surface normal. Emitted photoelectrons were collected with an electron analyzer at an angle 10° from the surface normal in an angle-integrated mode. Collected spectra were numerically fitted with Voigt functions after Shirley background subtraction with a third-order polynomial to each side of the feature. The onset of photoemission from an Au foil attached to the sample holder served as the Fermi level, corresponding to zero binding energy. The photon energies used to collect XPS spectra were 200 eV for Ge 3d, 380 eV for C 1s, and 610 eV for O 1s. A quadrupole mass filter served for analysis of desorption products in the TPD measurement; this mass analyzer was enclosed in a differentially pumped cylinder, at the end of

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RESULTS AND DISCUSSION The chemical identity of a surface species on the Ge surface was characterized with XPS. Figure 1 shows XPS data of C and O 1s

Figure 1. (a) XPS spectra of C and O 1s for a Ge(100) surface at 105 K exposed to CH3OH for various durations. Dots represent data collected after background subtraction, solid lines are fitted curves, and various components are shown with dashed lines. The photon energies used to collect these spectra are 380 eV for C 1s and 610 eV for O 1s.

recorded for a Ge(100) surface exposed at 105 K to CH3OH for varied durations. The O 1s spectra for all exposures were deconvoluted with two components at 531.5 and 533.1 eV, corresponding to two adsorption features. The latter component is assigned to adsorbed CH3OH molecules, whereas the former component is attributed to surface CH3O that is bound to the Ge surface via the O atom, as observed on other surfaces.27,28 The surface CH3O is produced from the dissociation of a CH3O−H bond, CH3OH(g) → CH3O(a) + H(a). Like the O 1s spectrum, the C 1s spectrum contains two features at 285.5 and 286.3 eV; the latter component is 2761



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attributed to adsorbed CH3OH molecules, and the former component is assigned to surface CH3O. The XPS data show hence that CH3OH can adsorb both molecularly and dissociatively on the Ge surface at 105 K. The intensities of O 1s and C 1s assigned to surface CH3O gradually attenuated with the duration of exposure beyond ∼30 s, but the intensity of O 1s and C 1s due to adsorbed CH3OH increased with exposure without saturation up to 80 s. The surface sites for chemisorption hence became saturated, and further CH3OH molecules physisorbed on the surface. Figure 2 shows a comparison of Ge 3d spectra for a Ge(100) surface at 105 K before and after exposure to CH3OH. The

Figure 3. Composite TPD spectra collected from Ge(100) at 105 K exposed to CH3OH for 60 s.

This signal is thus attributed to the fragment CH3 produced in the ionized fragmentation of desorbed CH3OH. According to TPD data, H2, CH2O, and CH3OH are the major products of desorption. The previous TPD work indicated that the surface CH3 is desorbed at 725 K, and surface OH is desorbed in the form of GeO at 710 K.31,32 Direct desorption of CH3 (m/z = 15) and GeO (m/z = 90) are absent from the decomposition of CH3OH. These facts indicate that the decomposition of CH3OH occurs without breakage of the C−O bond, CH3OH(a) → CH3(a) + OH(a). Figure 4 depicts TPD scans for m/z = 2, 29, and 32 as a function of duration of exposure to CH3OH. The desorption of

Figure 2. XPS spectra of Ge 3d for a Ge(100) surface at 105 K exposed to CH3OH for various durations. The photon energy used to collect these spectra was 200 eV.

spectra of 3d for the clean Ge surface were thus deconvoluted into two d5/2 features at 29.4 and 29.0 eV, which are attributed to bulk and surface dimeric Ge, respectively.29 Upon adsorption of CH3OH, the intensity of Ge 3d5/2 assigned to surface Ge decreased, and a new 3d5/2 feature appeared at 29.9 eV, attributed to surface Ge bonded to chemisorbed CH3OH and CH3O. In addition to CH3O, surface H formed due to dissociative adsorption of CH3OH was bound to surface Ge. In fitting the Ge 3d region, we assumed that the signal due to surface Ge bound to H overlapped that of bulk Ge because their binding energies are similar.30 Several fragments were recorded to detect the possible products evolved during thermal decomposition of CH3OH on Ge(100) surface. Figure 3 shows composite TPD scans for the surface with CH3OH exposure for 60 s. The intensity at m/z = 29 is about 1.5 times that at m/z = 30, and the ratio of their intensities is near that obtained from CH2O. Accordingly, both signals are attributed to desorption of CH2O, instead of another possible species, C2H6, which has a signal at m/z = 30 much less than that of m/z = 29. The signal at m/z = 15 concurs with desorption of CH3OH (m/z = 32), and their relative intensity (0.17) of m/z = 15 to m/z = 32 is the same as that observed for an ionized fragmentation of CH3OH in the mass spectrometer.

Figure 4. TPD spectra of H2 (m/z = 2), CH2O (m/z = 29), and CH3OH (m/z = 32) collected from Ge(100) at 105 K exposed to CH3OH for exposure durations (a) 5 s, (b) 10 s, (c) 20 s, (d) 30 s, and (e) 60 s.

CH3OH (m/z = 32) is absent at a small exposure, indicating that all chemisorbed CH3OH undergoes decomposition. At large exposure, the signal of CH3OH is observed with maxima at 155, 285, and 525 K. The first feature at 155 K, observed only for an exposure duration greater than 30 s, is attributed to originate from a physisorbed CH3OH multilayer, in accordance with TPD work on CH3OH on other surfaces.33 The second feature corresponds to molecular desorption from a small portion of chemisorbed CH3OH. The feature at 525 K is 2762



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attributed to originate from the recombination of surface CH3O and H adatom, CH3O(a) + H(a) → CH3OH(g). The desorption temperature of CH3OH decreased slightly with increased exposure, indicating second-order kinetics of recombinative desorption (Supporting Information, Figure S1). Three desorption features at m/z = 29 are observed with signal maxima at ∼155, 490, and 525 K. The first feature was observed only at exposure of duration greater than ∼30 s; as its maximum temperature is identical to that of physisorbed CH3OH, its signal is due to the fragmentation of CH3OH desorbed from the physisorbed multilayer at 155 K. The features at 490 and 525 K are attributed to the desorption of CH2O produced from dehydrogenation of CH3O. The formation of CH2O is analogous to the desorption of ethene from β-hydrogen elimination of surface-bonded ethyl groups.34−37 Accordingly, the desorption temperature of CH2O was nearly unchanged with the exposure of CH3OH, indicating first-order kinetics (Supporting Information, Figure S1). As depicted in a reported DFT calculation, dehydrogenation of CH3O might be facilitated by the interaction between the surface and α-hydrogen (α denotes the position of hydrogen with respect to the O atom).34 The feature at 155 K is the major desorption at small exposures, whereas the feature at 525 K gradually becomes predominant at large exposures. At large exposure, desorption of CH2O and CH3OH concurs in exhibiting signal maxima at 525 K. This condition indicates that CH3O undergoes both a recombinative reaction with surface hydrogen to desorb CH3OH and dehydrogenation. To estimate the relative amount of desorbed CH2O and CH3OH, we assumed both molecules to have the same ionization crosssection in the mass spectrometer.38 According to the TPD data, ∼75% of the CH3O intermediate underwent dehydrogenation to form CH2O at 490 and 525 K rather than recombination to form CH3OH at 525 K at the saturation coverage. The desorption intensities of CH2O and CH3OH attain maxima at durations greater than 30 s because the chemisorption of CH3OH becomes saturated, consistent with XPS data as shown in Figure 1. A desorption state of H2 is observed with a signal maximum at 620 K. The desorption temperature resembles that obtained for adsorption of hydrogen on a Ge(100) surface.39,40 This desorption is hence attributed to the combination of surface hydrogen (2H(a) → H2(g)), which originates from dehydrogenation of CH3OH and CH3O as described above. We applied the thermal evolution of XPS spectra to characterize the variation of surface composition during thermal decomposition of CH3OH and to correlate with TPD results for the elucidation of the reaction intermediates. Figure 5 shows C and O 1s spectra for a Ge(100) surface exposed to CH3OH at 105 K for 5 s and subsequently annealed to various temperatures. All XPS spectra were recorded for samples at 105 K after being heated to a desired temperature. The intensities of C 1s at 286.3 eV and O 1s at 533.1 eV gradually attenuated on annealing the sample to 150 K and completely disappeared at 300 K. In contrast, the intensities of features of C 1s at 285.5 eV and O 1s at 531.5 eV increased, revealing that chemisorbed CH3OH further dissociated to form additional surface CH3O and H in the temperature range 150−300 K. Upon further annealing to 470 K, the maxima of C and O 1s shift to smaller binding energies, 285.1 and 530.5 eV, respectively, and their intensities are significantly attenuated. These downward shifts of C and O 1s are attributed to surface CH2O formed from

Figure 5. XPS spectra of C and O 1s for a Ge(100) surface at 105 K exposed to CH3OH for 5 s and subsequently heated to indicated temperatures.

dehydrogenation of CH3O. The surface CH2O intermediate subsequently desorbed as a gaseous product, as shown in TPD data. Hence, desorption of CH2O was maximum at 470 K corresponding to the decrease of C 1s and O 1s intensities. The maxima of C 1s and O 1s completely disappeared at 550 K, indicating that no residual C- and O-containing species was left on the surface. A second desorption of CH2O gradually appeared at high temperature with increased exposure for thermal decomposition of CH3OH. For comparison, the thermal evolutions of C and O 1s spectra were recorded for Ge(100) exposed to CH3OH for 60 s, as shown in Figure 6. The initial surface was saturated with molecular CH3OH and dissociative CH3O. The thermal evolutions of C and O 1s spectra were similar to the case of small exposure. On annealing to 470 K, the intensities of

Figure 6. XPS spectra of C and O 1s for a Ge(100) surface at 105 K exposed to CH3OH for 60 s and subsequently heated to indicated temperatures. 2763



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Figure 7. Potential energy surfaces for adsorption and reactions of CH3OH on Ge(100). The numbers in parentheses specify the potential energies (kJ/mol) of reaction intermediates and transition states. All potential energies are referred to the CH3OH molecule separated from the cluster surface.

C 1s and O 1s decreased to a smaller extent than that observed at a small exposure. This condition indicates that a larger portion of CH3O is present on the surface at 470 K at saturated coverage. The signals of C and O 1s completely disappeared on further annealing to 590 K because of the desorption of CH2O and CH3OH. According to the TPD data, two desorption features of CH2O are observed from dehydrogenation of CH3O at 490 and 525 K; the feature at 115 K was observed only at small exposures, whereas the feature at 525 K was predominant at a large exposure and accompanied desorption of CH3OH. CH3O can undergo dehydrogenation to form surface CH2O that subsequently desorbs. Dehydrogenation of CH3O might be facilitated by the interaction between the surface and αhydrogen, similar to β-hydrogen elimination for an alkyl group on the surface.34−37 Dehydrogenation of CH3O essentially requires neighboring empty adsorption sites to accommodate dissociative hydrogen and therefore occurred preferentially at small exposures. Conversely, the formation of CH2O was inhibited at large exposures because most adsorption sites were occupied by CH3O and H. This argument is supported by the thermal evolution of XPS spectra shown in Figure 6. At a saturated coverage, much CH3O is still present on the surface at 470 K, but the recombination of CH3O with surface H is activated at 525 K, resulting in desorption of CH3OH as shown in Figure 4. Desorption of CH3OH yields empty adsorption sites that allow other neighboring CH3O to undergo hydrogenation forming CH2O. As a result, the desorption of CH3OH was accompanied with desorption of CH2O at 525 K. On the basis of the TPD and XPS data, we propose the adsorption and decomposition of CH3OH according to the following reactions:

CH3OH(g) → CH3OH(a) or CH3O(a) + H(a)

105 K (1)

CH3OH(a) → CH3O(a) + H(a)

CH3O(a) → CH 2O(a) + H(a) CH 2O(a) → CH 2O(g)

150−300 K

450−500 K

450−550 K

(2) (3) (4)

CH3O(a) + H(a) → CH3OH(g) 470−550 K (at large coverage)

2H(a) → H 2(g)

580−650 K

(5) (6)

Although Ge(100) and Si(100) exhibit similarities of surface structure and reconstruction, the thermal reaction of CH3OH molecules on these surfaces shows a subtle distinction.37,41 On both surfaces, CH3OH molecules dissociate to form surface CH3O and H by cleavage of the O−H bond. The XPS data show no residual C and O on the Ge(100) surface after further thermal reaction of CH3O. This condition indicates that all surface CH3O either recombines to desorb molecular CH3OH or dehydrogenates to CH2O without breaking the C−O bond on Ge(100). Some CH3O, however, undergoes fragmentation to produce surface CHx species at large exposure through the rupture of the C−O bond on Si(100).14 This condition indicates that the thermal decomposition of CH3O on Ge(100) occurs less readily than on Si(100). To examine further the proposed mechanism, we performed DFT calculations for the reaction paths of CH3OH on the dimer of a Ge15H16 cluster, at the B3LYP/6-311+G** level. Figure 7 depicts the potential energy surface for the adsorption and reaction of CH3OH on Ge(100); the potential energy is referred to the reactants, CH3OH + Ge(100). The reaction 2764



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are also illustrated in Figure S2 and Tables S1 and S2, which are available in Supporting Information. CH3OH initially adsorbs exothermically on a dimeric Ge atom through the O atom with adsorption energy 80 kJ/mol. The adsorbed CH3OH(a)1 readily undergoes scission of the O−H bond through two possible transition structures, TS1 and TS2, with energy barriers of heights 35 and 40 kJ/mol, respectively. The reaction energies of both paths are −105 and −87 kJ/mol, corresponding to products CH3O(a)1 + H(a)2 and CH3O(a)1 + H(a)3, respectively (the subscripts denote the dimeric Ge atoms bonded to the adspecies, as shown in Figure 8). The reaction products, CH3O(a) + H(a), are bonded to two separate Ge atoms of the same dimer or the neighboring dimer. Alternatively, adsorbed CH3OH(a)1 can proceed to cleavage of the C−O bond to produce OH(a)1 + CH3(a)2 or OH(a)1 + CH3(a)3. Their reaction energies are −176 and −161 kJ/mol, respectively, which are more exothermic than those for the scissions of the O−H bond, but their corresponding activation energies (154 and 178 kJ/mol) are greater than the desorption energy for CH3OH(a)1. These results indicate that adsorbed CH3OH(a)1 prefers to cleave the O−H bond or to desorb intact from Ge(100), rather than undergoing scission of the C− O bond, consistent with experimental observation. Conversely, CH3 species is observed neither on the surface nor as a desorption product during the decomposition of CH3OH on Ge(100). Adsorbed CH3OH molecules undergo a preferential scission of the C−H bond also on the Si(100) surface.16,42−45 The CH3O(a)1 intermediates can undergo further dehydrogenation by the scission of the C−H bond. Among the possible paths, two exhibit smaller energy barriers (107 and 111 kJ/mol) via transition states TS3 and TS4, which produce the surface CH2O(a)1 + H(a)2 + H(a)3. Direct desorption of CH2O from dehydrogenation, CH3O(a)1 → CH2O(g) + H(a)1, is also considered via two possible transition states, TS5 and TS6, in our calculation, but these reactions exhibit greater activation barriers (243 and 219 kJ/mol) because of a strong repulsive interaction in the four-membered ring (Ge−O−C−H) as shown in Figure S2, Supporting Information. Similarly, the scission of the C−H bond, resulting in the formation of surface CH2O, is the most favorable path for CH3O(a) on the Si(100) surface. Also, the direct desorption of CH2O exhibits significantly high barriers.15 Another possible reaction for CH3O(a)1 is to cleave the C− O bond, resulting in formation of CH3(a) and O(a). The rupture of the C−O bond requires an activation energy >310 kJ/mol and is hence kinetically unfavorable. The DFT data thus show that the formation of surface CH2O(a)1 is a preferred path for further thermal reaction of the CH3O(a)1 intermediate, consistent with the TPD and XPS experiments in which no formation of CH3 was observed. The surface CH2O(a)1 resulting from dehydrogenation of CH3O(a) is singly bound to a dimeric Ge atom and might transform into a more stable configuration of di-σ bonded CH2O(a)12 through two steps with transition structures TS7 and TS8. The first step corresponds to the migration of H between the neighboring Ge dimers and produces an empty adsorption site neighboring to CH2O(a)1. In the following step, CH2O(a)1 forms a second σ-bond to the empty Ge site and exhibits a configuration of di-σ bonded CH2O(a)12. The activation barrier of transformation is mainly determined by the first step of H migration (113 kJ/mol), which is much greater than the desorption energy of CH2O(a) (56 kJ/mol). CH2O(a)1 thus prefers to desorb directly upon annealing to

energies (ΔE) and activation energies (Ea) for the possible elementary steps are listed in Table 1. Figure 8 shows the Table 1. Reaction Energies (ΔE) and Activation Energies (Ea) of Elementary Steps for the CH3OH/Ge15H16 System Calculated at the B3LPY/6-311++G** Level with ZPE Correction; the Subscripts Indicate the Adsorption Positions of Surface Ge Atoms As Shown in Figure 8 elementary steps CH3OH(g) → CH3OH(a)1 CH3OH(a)1 → TS1 → CH3O(a)1 + H(a)2 CH3OH(a)1 → TS2 → CH3O(a)1 + H(a)3 CH3O(a)1 +H(a)2 → TS3 → CH2O(a)1 + H(a)2 + H(a)3 CH3O(a)1 + H(a)3 → TS4 → CH2O(a)1 + H(a)2 +H(a)3 CH3O(a)1 + H(a)2 → TS5 → CH2O(g) + H(a)1 + H(a)2 CH3O(a)1 + H(a)3 → TS6 → CH2O(g) + H(a)1 + H(a)3 CH2O(a)1 + H(a)2 + H(a)3 → TS7 → CH2O(a)1 + H(a)3 + H(a)4 CH2O(a)1 + H(a)3 + H(a)4 → TS8 → CH2O(a)12 + H(a)3 + H(a)4 CH3OH(a)1 → TSa → OH(a)1 + CH3(a)2 CH3OH(a)1 → TSb → OH(a)1 + CH3(a)3 CH3O(a)1 + H(a)2 → TSc → O(a)1 + H(a)2 + CH3(a)3 CH3O(a)1 + H(a)3 → TSd → O(a)1 + CH3(a)2 + H(a)3

ΔE (kJ/mol)

Ea (kJ/mol)

−80 −105 −87 92

35 40 107

74

111

75

243

78

219

−47

113

−52

18

−176 −161 98

154 178 311

83

312

Figure 8. Schematics of reaction intermediates and transition states involved in the most possible route of thermal decomposition of CH3OH on Ge(100).

schematics of the structures of intermediates and transition states involved in the most possible route of thermal decomposition of CH3OH. The structures, atomic coordinates, and potential energies of all intermediates and transition states 2765



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525 K, rather than forming di-σ bonded CH2O(a)12 before desorption. According to the DFT calculation, the possible elementary steps for the decomposition of CH3OH are schematically summarized in Figure 9. To correlate with the experimental

Figure 10. TPD spectra of H2 (m/z = 2), C4H8O (m/z = 72), and C4H9OH (m/z = 56) collected from Ge(100) at 105 K exposed to C4H9OH for exposure durations (a) 5 s, (b) 10 s, (c) 20 s, (d) 35 s, and (e) 70 s. Figure 9. Schematic of the reaction mechanism for the thermal reaction of CH3OH on Ge(100) surface. The reaction energy (ΔE) and activation energy (Ea), in kJ/mol, of each reaction are referred to Table 1. The red solid arrows indicate the reaction paths that are energetically favorable.

desorption features of C4H9OH were observed at 190 and 495 K. The former, observed only at large exposures, is due to the desorption of physisorbed C4H9OH. The decomposition of C4H9OH produces two desorption features of aldehyde (C4H8O) at 460 and 495 K. The former feature is predominant at small exposures. The intensity of the feature at 495 K increased with increasing exposure and became nearly exclusive at a saturated coverage. The desorption of C4H8O at 495 K concurs with desorption of C4H9OH at 495 K. Thermal decomposition of C2H5OH produces desorption of H2, C2H4O, and C2H5OH and also shows characteristics of TPD curves similar to CH3OH (Supporting Information, Figure S3). CH3OH, C2H5OH, and C4H9OH thus undergo similar mechanisms of thermal reactions on Ge(100). Based on the TPD and XPS data, we propose that the thermal reactions of n-alkyl alcohols (CnH2n+1OH) produce surface alkoxyls (CnH2n+1O) on Ge(100) via cleavage of the O−H bond. Alkoxyls can undergo thermal dehydrogenation to desorb aldehydes in two manners that depend on coverage. In the first manner, alkoxyl dehydrogenates to form surface aldehyde that subsequently desorbs; as this reaction requires empty sites to accommodate dissociative H, it prefers to occur at a small coverage. The second manner occurs at a greater temperature and gradually becomes the major reaction path with increasing coverage. At a large coverage, the desorption of aldehyde is initiated by the recombinative desorption of alcohol. A comparison of TPD data for CH3OH, C2H5OH, and C4H9OH reveals that the decomposition temperature is inversely proportional to the length of the carbon chain. As proposed above, the dehydrogenation of alkoxyl results in desorption of aldehyde through elimination of α-hydrogen. Preceding work indicates that the longer alkyl chain exhibits a greater tendency to maximize the interaction with alignment along the surface.36,47−49 α-Hydrogen elimination of the longerchain alkoxyl is hence facilitated by the surface to a greater degree and occurs at a lower temperature. At a large coverage, surface alkoxyl can undergo both recombination with hydrogen to desorb alcohol and dehydrogenation to desorb aldehyde. About 25% of CH3O undergoes recombination to desorb CH3OH, whereas only a small amount of recombinative

observations, the feasible reaction paths are indicated with solid arrows; dashed arrows indicate the reaction steps that are energetically unfavorable to occur. At adsorption temperature 105 K, a portion of adsorbed CH3OH dissociates to form CH3O and H because of the small energy barrier for the dissociation of O−H. The CH3O(a) undergoes dehydrogenation to form surface CH2O(a) with energy barriers, 107−111 kJ/mol, that are smaller than those, 127−140 kJ/mol, of the recombination to desorb CH3OH. TPD data thus show that CH2O is the main desorption product at a small coverage, exhibiting a desorption maximum at 490 K, but dehydrogenation that requires additional empty sites to accommodate dissociated H is inhibited at a large coverage. An alternative reaction path of CH3O is a direct desorption of CH2O, CH3O(a) → CH2O(g) + H(a), without formation of intermediate CH2O(a); the corresponding reaction barrier, 219−243 kJ/mol, is greater than that of recombination. CH3O(a) accordingly undergoes recombination with surface H to desorb molecular CH3OH at 525 K. The small difference (35 K) between desorption temperatures of dehydrogenation and recombination agrees satisfactorily with a small difference in their reaction barriers. The recombinative desorption of CH3OH produces empty sites that allow neighboring CH3O to undergo dehydrogenation; the TPD data thus show a desorption maximum of CH2O at 525 K at a large coverage, accompanied with the desorption of CH3OH. To understand the effect of the length of the alkyl chain on the reactions of alcohols on Ge(100), we tested the adsorption and thermal decomposition of C2H5OH and C4H9OH. Our TPD measurements indicate that H2, C4H8O, and C4H9OH are the desorption products from the thermal decomposition of C4H9OH. Figure 10 shows the TPD scans of H2 (m/z = 2), C4H8O (m/z = 72), and C4H9OH (m/z = 56) for a Ge surface at 105 K exposed to C4H9OH for varied duration. The desorptions of C4H8O and C4H9OH were recorded with the scans at m/z = 72 and m/z = 56, respectively.46 Three 2766



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Functionalization, and Langmuir−Blodgett Assembly. J. Am. Chem. Soc. 2005, 127, 11871−11875. (4) Collins, G.; Holmes, J. D. Chemical Functionalisation of Silicon and Germanium Nanowires. J. Mater. Chem. 2011, 21, 11052−11069. (5) Knapp, D.; Brunschwig, B. S.; Lewis, N. S. Chemical, Electronic, and Electrical Properties of Alkylated Ge(111) Surfaces. J. Phys. Chem. C 2011, 114, 12300−12307. (6) Hanrath, T.; Korgel, B. A. Chemical Surface Passivation of Ge Nanowires. J. Am. Chem. Soc. 2004, 126, 15466−15472. (7) Sharp, I. D.; Schoell, S. J.; Hoeb, M.; Brandt, M. S.; Stutzmann, M. Electronic Properties of Self-Assembled Alkyl Monolayers on Ge Surfaces. Appl. Phys. Lett. 2008, 92, 223306. (8) Buriak, J. M. Organometallic Chemistry on Silicon and Germanium Surfaces. Chem. Rev. 2002, 102, 1271−1308. (9) Loscutoff, P. W.; Bent, S. F. Reactivity of the Germanium Surface: Chemical Passivation and Functionalization. Annu. Rev. Phys. Chem. 2006, 57, 467−495. (10) Lim, C. W.; Soon, J. M.; Ma, N. L.; Chen, W.; Loh, K. P. High Resolution Electron Energy Loss Spectroscopy Study of Clean, AirExposed and Methanol-Dosed Ge(100) Surface. Surf. Sci. 2005, 575, 51−59. (11) Bae, S.-S.; Kim, D. H.; Kim, A.; Jung, S. J.; Hong, S.; Kim, S. Dissociative Chemisorption of Methanol on Ge(100). J. Phys. Chem. C 2007, 111, 15013−15019. (12) Kim, D. H.; Bae, S.-S.; Hong, S.; Kim, S. Atomic and Electronic Structure of Methanol on Ge(100). Surf. Sci. 2010, 604, 129−135. (13) Kachian, J. S.; Bent, S. F. Sulfur versus Oxygen Reactivity of Organic Molecules at the Ge(100)-2 × 1 Surface. J. Am. Chem. Soc. 2009, 131, 7005−7015. (14) Casaletto, M. P.; Zanoni, R.; Carbone, M.; Piancastelli, M. N.; Aballe, L.; Weiss, K.; Horn, K. Methanol Adsorption on Si(100)-2 × 1 Investigated by High-Resolution Photoemission. Surf. Sci. 2002, 505, 251−259. (15) Zhang, L.; Carman, A. J.; Casey, S. M. Adsorption and Thermal Decomposition Chemistry of 1-Propanol and Other Primary Alcohols on the Si(100) Surface. J. Phys. Chem. B 2003, 107, 8424−8432. (16) Cho, J.; Choi, C. H. Thermal Decomposition Mechanisms of Methanol, Ethanol, and 1-Propanol on the Si(100)-2 × 1 Surface. J. Phys. Chem. C 2008, 112, 6907−6913. (17) Xu, Y.-J.; Zhang, Y.-F.; Li, J.-Q. Predicting Facile Epoxidation of the Diamond (100) Surface by Dioxiranes and Subsequent RingOpening Reactions with Nucleophiles. J. Phys. Chem. B 2006, 110, 6148−6153. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; et al. Gaussian 03, revision E.01; Gaussian, Inc.: Wallingford, CT, 2003. (19) Xu, Y.-J.; Zhang, Y.-F.; Li, J.-Q. Organic Functionalization of the Si(100) and Ge(100) Surfaces by Cycloadditions of Carbenes and Nitrenes: A Theoretical Prediction. J. Phys. Chem. B 2006, 110, 3197− 3205. (20) Becke, A. D. Density-Functional Thermochemistry. I. The Effect of the Exchange-Only Gradient Correction. J. Chem. Phys. 1992, 96, 2155−2160. (21) Becke, A. D. Density-Functional Thermochemistry. II. The Effect of the Perdew−Wang Generalized-Gradient Correlation Correction. J. Chem. Phys. 1992, 97, 9173−9177. (22) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (23) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry; Scheafer, H. F., Ed.; Plenum: New York, 1977. (24) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270−283. (25) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for K to Au Including the Outermost Core Orbitals. J. Chem. Phys. 1985, 82, 299−310.

desorption is observed for C4H9OH. This condition indicates that the C4 alkoxyl prefers dehydrogenation to recombination, consistent with the argument that the interaction of the alkoxyl chain with the surface facilitates dehydrogenation.

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CONCLUSIONS Our TPD and XPS data elucidate the mechanism of adsorption and thermal reaction of short-chain alcohols on Ge(100), combined with DFT calculation. CH3OH adsorbs molecularly and dissociatively on Ge(100) at 105 K; the chemisorbed CH3OH readily dissociates to form surface CH3O and H on annealing the surface to 300 K. Two desorption features of CH2O are observed, which depend on coverage. At a small coverage, surface CH3O prefers to undergo dehydrogenation to desorb CH2O at 490 K via the surface mediation. According to DFT calculation, CH2O uni-σ bonded to a dimeric Ge atom is the intermediate before desorption. At a large coverage, CH3O can undergo recombination with surface H to desorb CH3OH at 525 K, which subsequently initiates neighboring CH3O to undergo dehydrogenation. C2H5OH and C4H9OH also decompose to desorb aldehydes via formation of alkoxyl intermediates as observed for CH3OH. The dehydrogenation of the longer-chain alkoxyl occurs at lower temperature because of the stronger interaction of the alkoxyl chain with the surface. The greater fraction of longer-chain alkoxyl undergoes dehydrogenation rather than recombinative reaction. On annealing the sample to ∼500 K, the decomposition of alcohol results eventually in desorption of aldehyde or alcohol molecules without deposition of surface C and O.

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ASSOCIATED CONTENT

* Supporting Information S

Optimized geometric structures, bond distances, potential energies of intermediates, and transition structures for the reaction on Ge(100), and TPD scans as a function of C2H5OH exposure. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(W.-H.H.) E-mail: [email protected]. Phone: +886-277346125. (J.-H.W.) E-mail: [email protected]. Phone: +886-2-77346123. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS National Science Council of Taiwan, under Grants NSC 982113-M-003-004-MY3 and 101-2113-M-003-006-MY3, supported this research. We thank NSRRC for the provision of beam time and NCHC for the CPU time.

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REFERENCES

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Adsorption and Thermal Reaction of Short-Chain Alcohols on Ge (100)

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