Functionalization of β-Ga2O3 Nanoribbons: A Combined

Oct 24, 2008 - V. M. Bermudez*. Electronics Science and Technology Division, Naval Research Laboratory, Washington, D.C. 20375-5347. Langmuir , 2008 ...
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Langmuir 2008, 24, 12943-12952

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Functionalization of β-Ga2O3 Nanoribbons: A Combined Computational and Infrared Spectroscopic Study V. M. Bermudez* Electronics Science and Technology DiVision, NaVal Research Laboratory, Washington, D.C. 20375-5347 ReceiVed July 17, 2008. ReVised Manuscript ReceiVed September 15, 2008 The adsorption of H2O, alcohols (CH3OH and 1-octanol), and carboxylic acids (formic, acetic, and pentanoic) on β-Ga2O3 nanoribbons has been studied using infrared reflection-absorption spectroscopy (IRRAS) and/or ab initio computational modeling. Adsorption energies and geometries are sensitive to surface structure, and hydrogen bonding plays a significant role in stabilizing adsorbed species. On the more stable (100)-B surface, computation shows that the physisorption of H2O or CH3OH is weakly exothermic whereas chemisorption via O-H bond dissociation is weakly endothermic. Experiment finds that a large fraction of a saturation coverage of adsorbed 1-octanol is displaced by exposure to acetic acid vapor. This is consistent with computational results showing that acids adsorb more strongly than methanol on this surface. The remaining alcohol, not displaced by acetic acid, suggests the presence of defects and/or (100)-A regions because computation shows that this less-stable surface adsorbs methanol more strongly than does the (100)-B. The ν(C-H) modes of adsorbed 1-octanol are easily detected whereas no adsorbed H2O is observed even though H2O and CH3OH exhibit similar adsorption energies. It is inferred from this that the failure to detect H2O on the dominant (100)-B surface results from the orientation of the physisorbed H2O essentially parallel to the surface. Computation shows that this configuration is stabilized by H bonding. For chemisorbed formic acid, computation shows that a bridging carboxylate structure is favored over a bidentate or monodentate configuration. Computation also shows that chemisorption is favored on the (100)-A surface but physisorption is favored on the more stable (100)-B. Analysis of IRRAS data for acetic and pentanoic acids finds evidence for both types of adsorption. The carboxylate resists displacement by H2O vapor, which suggests that carboxylic acids may be useful for functionalizing β-Ga2O3 surfaces. The results provide insight into the interplay between surface structure and reactivity on an oxide surface and about the importance of hydrogen bonding in determining adsorbate structure.

1. Introduction The purpose of this work is to explore chemical reactions of potential use in functionalizing β-Ga2O3 surfaces. Single crystals or crystalline thin films of this material are generally unavailable. Ultrathin films have been prepared by the oxidation of CoGa (100) surfaces.1 These are well ordered but exhibit an electronic structure (e.g., a surface state in the band gap) and a vibrational spectrum that are not representative of the bulk material. This behavior may be typical of such ultrathin oxide films.2 Highsurface-area (HSA) Ga2O3 powders and polycrystalline thin films, however, have been widely studied3-19 in the context of surface chemistry, but the surface structure in this case is not well defined, which can complicate the interpretation of results in terms of microscopic models. The present work makes use of β-Ga2O3 nanoribbons (NRs). These are well-defined single crystals having the (100) plane as the wide face of the ribbon20,21 and can be used in lieu of bulk single crystals for studies of adsorption phenomena. There is considerable interest in functionalizing β-Ga2O3 surfaces, either in thin-film or NR form. This material, which can be made n-type semiconducting either by doping or by O vacancy (VO) formation, is of value as a transparent conducting oxide22 (TCO) to replace indium-doped tin oxide (ITO) in electrooptic devices such as organic light-emitting diodes (OLEDs). β-Ga2O3 has superior optical properties in the near-ultraviolet and is easier than ITO to fabricate reproducibly. Functionalizing is a useful approach to passivating the TCO surface and to controlling the interface with the active OLED material. A second * Corresponding author. E-mail: [email protected]. Tel: +1202-767-6728. Fax: +1-202-767-1165. (1) Schmitz, G.; Gassmann, P.; Franchy, R. J. Appl. Phys. 1998, 83, 2533. (2) Freysoldt, C.; Rinke, P.; Scheffler, M. Phys. ReV. Lett. 2007, 99, 086101.

issue relates to the recent development23 of chemical sensors in which β-Ga2O3 NRs serve as the dielectric material in a capacitor. Adsorption of an analyte changes the capacitance, and functionalizing the surface to promote the adsorption of a particular species could then make such devices highly selective as chemical sensors. Functionalization is also of interest as a method to promote the “solubility” of NRs in organic media, which can be (3) Bozon-Verduraz, F.; Potvin, C. J. Chim. Phys. 1976, 73, 43. (4) Pohle, R.; Fleischer, M.; Meixner, H. Sens. Acutators, B 2000, 68, 151. (5) Collins, S. E.; Briand, L. E.; Gambaro, L. A.; Baltana´s, M. A.; Bonivardi, A. L. J. Phys. Chem. C 2008, 112, 14988. (6) Branda, M. M.; Collins, S. E.; Castellani, N. J.; Baltana´s, M. A.; Bonivardi, A. L. J. Phys. Chem. B 2006, 110, 11847. (7) Collins, S. E.; Baltana´s, M. A.; Bonivardi, A. L. J. Phys. Chem. B 2006, 110, 5498. (8) Collins, S. E.; Baltana´s, M. A.; Bonivardi, A. L. Langmuir 2005, 21, 962. (9) (a) Collins, S. E.; Baltana´s, M. A.; Bonivardi, A. L. J. Catal. 2004, 226, 410. (b) Collins, S. E.; Baltana´s, M. A.; Bonivardi, A. L. Appl. Catal., A 2005, 295, 126. (10) Rodrı´guez Delgado, M.; Otero Area´n, C. Mater. Lett. 2003, 57, 2292. (11) Rodrı´guez Delgado, M.; Otero Area´n, C. Z. Anorg. Allg. Chem. 2005, 631, 2115. (12) Vimont, A.; Lavalley, J. C.; Sahibed-Dine, A.; Otero Area´n, C.; Rodrı´guez Delgado, M.; Daturi, M. J. Phys. Chem. B 2005, 109, 9656. (13) Lavalley, J. C.; Daturi, M.; Montouillout, V.; Clet, G.; Otero Area´n, C.; Rodrı´guez Delgado, M.; Sahibed-dine, A. Phys. Chem. Chem. Phys. 2003, 5, 1301. (14) (a) Rodrı´guez Delgado, M.; Morterra, C.; Cerrato, G.; Magnacca, G.; Otero Area´n, C. Langmuir 2002, 18, 10255. (b) Otero Area´n, C.; Lo´pez Bellan, A.; Pen˜arroya Mentruit, M.; Rodrı´guez Delgado, M.; Turnes Palomino, G. Microporous Mesoporous Mater. 2000, 40, 35. (15) Meriaudeau, P.; Primet, M. J. Mol. Catal. 1990, 61, 227. (16) Ivanov, A. V.; Koklin, A. E.; Uvarova, E. B.; Kustov, L. M. Phys. Chem. Chem. Phys. 2003, 5, 4718. (17) Kazansky, V. B.; Subbotina, I. R.; Pronin, A. A.; Schlo¨gl, R.; Jentoft, F. C. J. Phys. Chem. B 2006, 110, 7975. (18) (a) Becker, F.; Krummel, Ch.; Freiling, A.; Fleischer, M.; Kohl, C. Fresenius’ J. Anal. Chem. 1997, 358, 187. (b) Kohl, D.; Ochs, Th.; Geyer, W.; Fleischer, M.; Meixner, H. Sens. Actuators, B 1999, 59, 140. (19) Josepovits, V. K.; Krafcsik, O.; Kiss, G.; Perczel, I. V. Sens. Actuators, B 1998, 48, 373.

10.1021/la8022979 CCC: $40.75  2008 American Chemical Society Published on Web 10/25/2008

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Figure 1. Views along the surface normal and in the surface plane (edge-wise, along the [010] axis) showing the (1 × 1) and (2 × 1) unit cells derived from the β-Ga2O3 (100)-A and -B surface structures.24 The various inequivalent atom types are labeled as in ref 24 (Ga(I) ) Ga(Td) and Ga(II) ) Ga(Oh)). The small dashed and large solid rectangles show the (1 × 1) and (2 × 1) cells, respectively. The small solid circles represent adsorbed atoms (e.g., H) and illustrate the different packing densities for the (1 × 1) and (2 × 1) unit cells. The diagrams show ideal terminations, neglecting the small displacements that result from relaxation. In the (100)-A (2 × 1) cell, for a given choice of Ga(II) site there are two inequivalent choices (a or b) for the O(II) site. In the edge-wise views, only the outermost plane of each atom type is shown.

useful in manipulating NRs for device fabrication. In another context, the interaction of organic species with Ga2O3 surfaces is relevant to the use of this material as a catalyst support, which has motivated much of the work on HSA powders. The bulk β-Ga2O3 lattice24 involves both tetrahedral Ga(Td) (or Ga(I)) and octahedral Ga(Oh) (or Ga(II)) cation sites as well as three inequivalent anion sites, conventionally labeled O(I), O(II), and O(III). The Ga(Td) site is slightly distorted and the Ga(Oh) site is more strongly distorted from the respective ideal configuration. The O(I) and O(II) atoms are 3-fold-coordinated to Ga atoms whereas the O(III) atom is 4-fold-coordinated. The so-called (100)-B surface,24 which has the lowest surface energy (0.68 J m-2), is almost ideally terminated and undergoes little relaxation in comparison to other, less stable surfaces. Because of this high degree of stability, the (100)-B surface is thought to form the wide face of the NR and, therefore, to be the major contributor to the total exposed surface area. On this surface (Figure 1b), Ga(Oh) is missing one of six O bulk nearest neighbors, and O(III) is missing one of four Ga nearest neighbors. All other sites, including the Ga(Td), are fully coordinated. (20) Wang, Z. L. Annu. ReV. Phys. Chem. 2004, 55, 159. (21) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 902. (22) Ohta, H.; Nomura, K.; Hiramatsu, H.; Ueda, K.; Kamiya, T.; Hirano, M.; Hosono, H. Solid-State Electron. 2003, 47, 2261. (23) Prokes, S. M. (Naval Research Laboratory); unpublished work. (24) Bermudez, V. M. Chem. Phys. 2006, 323, 193.

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An alternative (100) surface termination, the (100)-A, has Ga(Oh) and O(II) as the only unsaturated sites (Figure 1a). The (100)-A surface energy, 1.13 J m-2 after relaxation, is higher than that of the (100)-B but lower than that of other surfaces. The higher surface energy for (100)-A is thought to result from the fact that the coordinatively unsaturated surface O(II) is missing one of three bulk Ga nearest neighbors, whereas on the (100)-B, the unsaturated O(III) is missing only one of four Ga neighbors. Another difference between the (100)-A and -B surfaces can be seen by examining Figure 1. On (100)-A, the plane occupied by the unsaturated O(II) sites lies above the plane of the Ga(II) (or Ga(Oh)) sites, whereas on (100)-B, the unsaturated Ga(Oh) and O(III) sites are essentially coplanar. Furthermore, the distance between unsaturated Ga and O sites is greater on (100)-A than on (100)-B. These differences, although subtle, will be seen to have significant effects on adsorption. For either (100) termination, the Ga(Td) is fully coordinated, hence in the absence of VO defects, the (100) surface is expected to be a relatively weak Lewis acid in comparison to typical Ga2O3 HSA powders that involve unsaturated Ga(Td) sites. This view is supported by experimental data25 for the exposure of NRs to pyridine. The high degree of stability exhibited by the β-Ga2O3 (100)-B surface presents a problem in functionalization. The standard approach to functionalizing an oxide is to react OH groups with species such as an alkyl methoxysilane (e.g., R-Si(OCH3)3, where R is an alkyl group). The reaction is quantitative and forms Si-O-M bonds, where M is a substrate metal atom, with the release of CH3OH. The OH groups occur naturally on air-exposed surfaces or can be prepared by plasma processing or by damaging the surface by ion bombardment and then reacting with H2O vapor. However, the NR surface resists reaction with H2O under normal conditions (see below). Promoting hydrolysis by energetic methods, without also incurring damage, is difficult because of the nanoscale dimensions of the individual NRs. This work uses infrared reflection-absorption spectroscopy (IRRAS) to study the effects of exposing β-Ga2O3 NRs to H2O, alcohols, and carboxylic acids. An advantage afforded by the use of NRs is that they can be deposited as a thin layer (see below) on a metallic substrate. The resulting “buried metal layer” structure,25 which would be difficult to fabricate beginning with a bulk single crystal, then provides high sensitivity to adsorbate vibrational modes. Ab initio quantum chemical modeling of the molecular and dissociative adsorption of these reagents is used to interpret quantitatively the observed reactivity or lack thereof.

2. Experimental Details All of the relevant details concerning sample preparation and characterization, the ultrahigh vacuum (UHV) chamber, and the data acquisition have been given previously.25 Briefly, a suspension of NRs in CH3OH was prepared by ultrasonic agitation, and a layer of the material was then deposited by spraying this suspension onto a heated substrate using an airbrush. The substrate was an optically thick film of Au on a thin mica wafer.26 Prior to each set of experiments the sample was cleaned in situ by outgassing at ∼400 °C, heating for 1 h at 500 °C in 8 × 10-7 Torr O2, and cooling to 200 °C in 5 × 10-6 Torr O2. The O2 flowed continuously through the chamber while being pumped by a turbo pump. The IRRAS measurements were performed at an angle of incidence of 82°. Typically, 2000 scans were averaged at a resolution of 8 cm-1, and triangle apodization and 2-fold zero filling were applied to the interferogram before Fourier transformation. A narrow-band Hg1 - xCdxTe (MCT-A) detector was used with a KRS-5 wire-grid (25) Bermudez, V. M.; Prokes, S. M. Langmuir 2007, 23, 12566. (26) Molecular Imaging Corp.; now available from Structure Probe Inc. (SPI) http://www.2spi.com/catalog/gold-sub.shtml.

Functionalization of β-Ga2O3 Nanoribbons polarizer, which gave a useable signal down to 600 cm-1. However, strong absorption by Ga2O3 prevented the detection of weak adsorbate modes below 800 cm-1. The data were obtained with static p-polarized radiation (i.e., with no polarization modulation) because the experiments were done under high-vacuum conditions with essentially no gas-phase species present. The data are given in the form of (δR/R)p, the fractional change in p-polarized reflectance caused by adsorption, and baselines have been subtracted where appropriate. As discussed elsewhere,25 the present IRRAS experiment is governed by the metal-surface selection rule,27 hence adsorbate vibrational modes are detectable only in p polarization. As a check, data were also obtained in s polarization to verify the absence of any features due to surface species. “Blank” experiments were also done (see below) using a bare substrate in order to rule out contributions due to adsorption on the Au itself. The chemicals used (H2O, 1-octanol, pentanoic acid, and glacial acetic acid) were all reagent grade. Each was contained in a glass reservoir attached to a separate stainless steel gas-handling manifold. All reagents were used as received, after degassing via repeated freeze-pump-thaw cycles with freezing being done in a dry ice/ acetone bath. The purity of each reagent was checked by drawing a sample of the vapor from the manifold into a 5 cm gas cell with KBr windows. The IR transmission spectrum (at 4 cm-1 resolution) was then compared with reference data.28 No foreign species were detected except in the case of pentanoic acid, which showed evidence of an aldehyde impurity (Supporting Information). The reagent vapor pressure (VP) can be an issue when using a molecular beam doser. (See below.) 1-Octanol has a room-temperature VP29 of 83 mTorr. The VP of pentanoic (or valeric) acid, CH3(CH2)3CO2H, has been measured30 in the range of 130 to 179 °C. Extrapolation to room temperature of the Antoine equation fitted to these data gives a VP of 1.2 Torr; however, the (uncalibrated) capacitance manometer on the manifold indicated a VP of only ∼340 mTorr. Acetic acid is known31 to attack oxidized Cu surfaces, and presumably the same is true for pentanoic acid. In both cases, the liquid in the reservoir became discolored, presumably because of the reaction of the vapor with the Cu gasket at the flange connecting the reservoir to the leak valve. However, the IR spectra of the acid vapors used in the present work showed no evidence of volatile species formed by reaction with Cu gaskets or valve seals, and Auger electron spectroscopy gave no indication of foreign metals on the sample surface. The sample was exposed to the reagent vapor using a calibrated pinhole doser32 while the chamber was pumped continuously by a throttled turbo pump. Maintaining a continuous flow of reagent through the chamber reduces the buildup of impurities displaced from internal surfaces. All of the reagents used here are strongly retained by the internal surfaces of the UHV chamber, and care was taken to avoid complications due to displacement of one species by another.

3. Computational Details Ab initio calculations were performed using the CRYSTAL 06 code.33,34 The 2D periodic slab (2-DPS) models of the (100) surfaces were the same as those used previously24,25 to investigate the bare (27) Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211. (28) National Institute of Standards and Technology (NIST) Chemistry WebBook (NIST Standard Reference Database No. 69; June, 2005) http:// webbook.nist.gov/chemistry/. (29) Nasirzadeh, K.; Neueder, R.; Kunz, W. J. Chem. Eng. Data 2006, 51, 7. (30) Clifford, S. L.; Ramjugernath, D.; Raal, J. D. J. Chem. Eng. Data 2004, 49, 1189. The Antoine equation is given in this reference as ln(P) ) A- B/((T/C) + C) where P is in kPa and A, B, and C are constants. The T/C term should be T/°C and means T in units of °C. Hence, the expression to be used is ln(P) ) A- B/(T + C). (31) Gil, H.; Leygraf, C. J. Electrochem. Soc. 2007, 154, C272. (32) Campbell, C. T.; Valone, S. M. J. Vac. Sci. Technol., A 1985, 3, 408. (33) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; Zicovich-Wilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, Ph.; Llunell, M. CRYSTAL 06 User’s Manual 2007:Theoretical Chemistry Group, University of Turin. The manual and basis sets may be obtained at http:// www.crystal.unito.it/. (34) Pisani, C.; Dovesi, R.; Roetti, C. Hartree-Fock Ab Initio Treatment of Crystalline Systems; Springer: Berlin, 1988.

Langmuir, Vol. 24, No. 22, 2008 12945 surfaces and the adsorption of H atoms. The slab unit cell had a thickness equivalent to that of four primitive unit cells (40 atomic layers), which is sufficient to preclude any significant interaction between the top and bottom surfaces. The necessity for so thick a slab will become apparent in the subsequent discussion. An -OH (or -OCH3) group was placed on a surface Ga(Oh) site, and an H atom was placed on an unsaturated surface anion site (O(III) for (100)-B and O(II) for (100)-A) to represent dissociatively adsorbed H2O (or CH3OH). As noted above, all other types of sites are fully coordinated on the defect-free (100) surface and are, therefore, assumed to be nonreactive in adsorption. CH3OH is employed here as a model for the larger 1-octanol molecule used in the IRRAS experiments. Likewise formic acid, HCO2H, is used in computational studies (described in more detail below) as a surrogate for pentanoic acid. Large adsorbates require the use of correspondingly large surface unit cells in order to avoid severe steric effects, which can make the 2-DPS calculations intractable. For H2O adsorption, (1 × 1) and (2 × 1) surface unit cells were used, whereas for CH3OH and HCO2H, only the larger cell was used. The different surface unit cells are shown in Figure 1. For the (1 × 1) cell, the surface is completely saturated, with an adsorbate at every Ga(Oh) and unsaturated O site. For the (2 × 1), equivalent sites adjacent to an adsorbate are left vacant. The bare slab unit cell is centrosymmetric, and adsorbates are placed on both surfaces so as to maintain this symmetry and avoid generating a dipole potential across the slab. As described previously,24,25 the 2-DPS (either with or without adsorbates) was first optimized using a restricted Hartree-Fock (RHF) treatment with Durand-Barthelat effective-core pseudopotentials (ECPs) for the lattice atoms. These are large-core ECPs in which the Ga 3d shallow core level is treated as part of the core rather than as a valence orbital. For H, C, and O in the molecules, 6-311G(d,p) basis sets were used.35 These are of triple-ζ quality with a single d polarization shell for C and O and a single p shell for H. The lattice constants were kept fixed at the values determined by optimizing the bulk lattice, but all atoms in the slab and the molecule were unconstrained. A single-point calculation for the optimized structure was then done using density functional theory (DFT) with the B3LYP hybrid functional and all-electron Gaussian basis sets for all atoms. This procedure was tested (see below) by comparing the results with those obtained for optimizing using DFT with all-electron basis sets for the lattice atoms. The Ga and lattice-O all-electron basis sets are designated as 86-411d41G and 8-411d1G, respectively, and the exponents of the outer shells were reoptimized24 for Ga2O3. These basis sets are thus of triple-ζ quality with either one (d1) or two (d41) polarization shells. The basis sets for H, C, and O in the molecules were the same as those cited above. An (8 × 8 × 1) grid was used for k-point sampling, and an extra-large (75 974) grid was used for DFT integration. In CRYSTAL, truncation of the sums of Coulomb and exchange terms in the Fock matrix is determined by five overlap criteria33,34 (T1-T5). These were set at 10-7 for T1-T4 and 10-14 for T5. In the multipolar expansion zone,33,34 a maximum order of L ) 6 was used. For the dissociative adsorption of X-H (X ) HO or CH3O), the adsorption energy is given by

∆Eads ) 1/2{E(2X + 2H + slab) - E(slab) 2E(X-H) + ∆EBSSE} (1) where E(slab) and E(2X + 2H + slab) are the DFT total energies per slab unit cell of the bare and adsorbate-covered slabs (after relaxation), respectively, and E(X-H) is the DFT energy of a free X-H molecule. Recall that there are two X-H adsorbates per unit cell, one on the top and one on the bottom surface. For consistency, the same basis sets, computational procedures, and parameters were (35) H, C, and O basis sets were obtained from the Extensible Computational Chemistry Environment Basis Set Database, version 02/02/06, developed and distributed by the Molecular Science Computing Facility, Environmental and Molecular Sciences Laboratory, which is part of the Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352.

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used in obtaining all energies to be combined in deriving ∆Eads. Specifically, E(X-H) was obtained by first optimizing the structure at the RHF/6-311G(d,p) level and then doing a single-point calculation at the B3LYP/6-311G(d,p) level. The correction for the basis set superposition error, ∆EBSSE, was obtained using the counterpoise method. Here the energy of the slab is obtained with the adsorbate atoms removed and again with these atoms replaced by “ghost” atoms. Likewise, the total energy of the adsorbate species is obtained with the slab atoms removed and again with the slab replaced by ghosts. From these four quantities, ∆EBSSE is obtained. In the present sign convention, a negative ∆Eads indicates an exothermic adsorption process. An entirely analogous procedure is followed in obtaining ∆Eads for nondissociative molecular adsorption.

4. Results and Discussion 4.1. H2O. The adsorption of H2O on Ga2O3 HSA powders has been studied previously.3,4 Surfaces from which OH has been removed by heating in vacuo at 500 °C are seen3 to dissociate H2O vapor at room temperature, leading to the reappearance of chemisorbed OH groups that can also form H bonds to molecular H2O. The IR absorption bands due to the ν(O-H) stretching and δ(O-H) bending modes of chemisorbed OH are considerably weaker than bands seen for other adsorbates (e.g., pyridine) on the same HSA samples.3 Recently it has been reported36 that nanocrystalline β-Ga2O3 powder can be converted to the oxyhydroxide GaO(OH) by reaction with H2O superheated to 200 °C in an autoclave. In the present case, no evidence of adsorption was seen in IRRAS after exposure to H2O vapor, either at room temperature or at 200 °C, for doses as high as ∼4 × 1016 H2O/cm2. These conditions are of course much milder than autoclaving. The IR spectrum of GaO(OH), for reference, has been given elsewhere.37-39 This lack of reactivity is consistent with computational results given below for the adsorption of H2O on the (100)-B surface. It is noted, however, that previous work25 found evidence for GaO(OH) on NRs following prolonged exposure to room air. 4.1.1. Computational Modeling of Adsorbed H2O. We first consider the case of dissociative adsorption on the (100)-B surface. These results, shown in Figure 2 and in Table 1, will be described in some detail so as to provide background for subsequent discussions of CH3OH and HCO2H, which will then be brief. 4.1.1.1. Dissociative Adsorption on (100)-B. For the (1 × 1) surface unit cell, an O-H bond length of 0.962 Å was found for H adsorbed on the O(III) lattice site. For OH adsorbed on the Ga(Oh) site, Ga-O and O-H distances of 1.820 and 0.942 Å and a Ga-O-H angle of 117° were determined. A ∆Eads of +0.17 eV per H2O was found. For the (2 × 1) surface unit cell, an O-H bond length of 0.979 Å was found for adsorbed H. For adsorbed OH, Ga-O and O-H distances of 1.847 and 0.939 Å and a Ga-O-H angle of 116° were determined. A ∆E ads of +0.27 eV per H2O was found. Thus, the dissociative adsorption of H2O is unfavorable on the pristine β-Ga2O3 (100)-B surface and slightly more so in a (2 × 1) than in a (1 × 1) unit cell. Because the ∆Eads values are small, the accuracy of the calculation is an issue. As noted above, geometry optimization was performed in an RHF calculation with ECPs for the lattice atoms, followed by a single-point DFT calculation of the total energy using all-electron basis sets. As a test, an all-DFT calculation for the dissociative adsorption of H2O in a (1 × 1) surface unit cell was done in which optimization was performed (36) Zhang, Y. C.; Wu, X.; Hu, X. Y.; Shi, Q. F. Mater. Lett. 2007, 61, 1497. (37) Huang, C.-C.; Yeh, C.-S.; Ho, C.-J. J. Phys. Chem. B 2004, 108, 4940. (38) Sato, T.; Nakamura, T. Thermochim. Acta 1982, 53, 281. (39) Zhao, Y.; Frost, R. L.; Yang, J.; Martens, W. N. J. Phys. Chem. C 2008, 112, 3568.

Figure 2. Optimized structures for H2O dissociatively adsorbed on the (100)-B surface in (a) (1 × 1) and (b) (2 × 1) surface unit cells and for (c) molecular H2O adsorbed in a (2 × 1) surface unit cell. Ga, O, and H are shown in green, red, and gray, respectively. Some of the different types of Ga and O sites are labeled in structure a. The heavy lines in these and in subsequent structures show H-bonds, identified as such on the basis of the H · · · O distances. Note the obvious distortion of the Ga(Oh) octahedra. Different structures are shown from different perspectives. The (x, y, z) axes, with x parallel to the Ga(Oh) row, are shown in structure c for reference in the text. Table 1. Summary of ∆EadsResultsa molecule water methanol formic acid

adsorptionb

(100)-A

(100)-B

physi chemi physi chemi physi chemi

-1.07 -0.95c,d -0.97 -0.76c -0.88 -1.34

-0.35 +0.27d -0.29 +0.36 -0.72e -0.49e

a

All energies are in eV per molecule and were computed, as described in the text, for a (2 × 1) surface cell. All energies have been corrected for BSSE. b “physi” refers to physisorption (or molecular adsorption) of the intact molecule. “chemi” refers to chemisorption (or dissociative adsorption) involving the dissociation of the O-H bond. c For chemisorption of H2O or CH3OH on the (100)-A surface, the H atom was placed on the O(II) site (labeled a in Figure 1a), which is closer to the Ga-OH or Ga-OCH3 site. For H2O, placing the H on site b gave ∆Eads ) -0.28 eV per H2O. d For chemisorption of H2O in a (1 × 1) surface cell, the (100)-A surface gave ∆Eads ) -0.81 eV per H2O, and the (100)-B gave +0.17 eV per H2O. e Results for the adsorption of acetic acid on (100)-B were essentially the same as those for formic acid. (See the text.)

using DFT with the all-electron basis sets described above. The resulting ∆Eads was only 18 meV per H2O smaller in magnitude that the value of +0.17 eV per H2O given above. Another test, involving a thicker 2-DPS, is described below. The higher packing density of dissociated H2O on the (1 × 1) surface is somewhat less unfavorable than that of the (2 × 1) structure. This can be understood in terms of H bonding between H adsorbed at the O(III) site and the O of the Ga-OH site (Figure 2). Two such bonds can form per O(III)-H for the (1 × 1) surface, but only one can form for the (2 × 1). These results can be compared with those for the adsorption of hydrogen on the same surface.25 Adsorption of atomic H gives ∆Eads ) -1.96 eV per H for the (1 × 1) unit cell and -1.93 eV per H for the (2 × 1). In this case, the Ga-H and O-H groups are too far apart, even on the (1 × 1) surface, for a significant H-bonding interaction, and both surface unit cells give essentially the same

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result. Another comparison is with ∆Eads for the dissociative adsorption of H2O on the R-Al2O3 (0001) surface,40 which consists of 3-fold-coordinated Al(Oh) ions (missing three bulk nearestneighbor O atoms) and 3-fold-coordinated O ions (missing one bulk Al nearest neighbor). Here ∆Eads ≈ -1.5 eV per H2O is found, with the exact value depending on the details of the calculation. The slightly endothermic value for the β-Ga2O3 (100)-B surface, however, is consistent with its relatively low degree of coordinative unsaturation and its correspondingly high degree of stability. For example, a surface energy of 1.54 J m-2 has been computed41 for the relaxed R-Al2O3 (0001) surface versus 0.68 J m-2 for the relaxed β-Ga2O3 (100)-B surface.24 Dissociative adsorption of H2O leads to distortion in the vicinity of the active sites. This is difficult to see clearly in Figure 2a,b but will be described and quantified here. Some of this is simply a reversal of the displacements that occur24 when atoms on the clean surface relax from the ideally terminated positions. Some distortion of the nominally octahedral Ga(Oh) site is an intrinsic property of the β-Ga2O3 lattice and is not related to the surface structure. Dissociative adsorption leads to further distortion in the form of a displacement of the adsorbed OH toward the O(III)-H site, accompanied by lateral and vertical displacements of the Ga(Oh) and O(III) sites. In the (1 × 1) cell, the Ga(Oh) surface site is essentially 6-fold-coordinated with two O(I), one O(II), and two O(III) nearest-neighbors together with the adsorbed OH. The two Ga(Oh)-O(III) distances (2.193 and 2.221 Å) are, however, larger than the value of 1.974 Å for the clean relaxed surface. In the (2 × 1) cell, relaxation increases the distance to O(III)-H even further, to 2.565 Å, whereas the distance to the other O(III) (with no adsorbed H) decreases to 1.963 Å. This makes the Ga(Oh)-OH site effectively 5-fold-coordinated on the (2 × 1) surface. These distortions have been quantified by computing the displacements in each layer of the relaxed, adsorbate-covered slab relative to those in the relaxed clean slab. For the (2 × 1), dissociative adsorption of H2O causes a lateral displacement of 0.38 Å and an outward displacement of 0.14 Å for the O(III) site. For the Ga(Oh) site, the displacements are 0.24 Å laterally and 0.27 Å outward. For the purpose of discussion, a displacement of 0.02 Å (i.e., ∼1% of a Ga-O nearest-neighbor distance) is arbitrarily defined as small. For the (2 × 1) cell with adsorbed H2O, the displacements do not become small until about layer 15 (with the surface as layer 1). The results are comparable for the (1 × 1) cell, although the displacements at the surface O(III) site are smaller than for the (2 × 1). Because the midplane of the slab lies between layers 20 and 21, it is useful to determine whether increasing the number of layers would significantly affect the results. Hence, a calculation was done for a 60-layer 2-DPS with a (2 × 1) surface unit cell. Here, the midplane is between layers 30 and 31. The result was ∆E ads ) +0.33 eV versus +0.27 eV per H2O for the 40-layer slab, using the same ∆E BSSE for both calculations. It is not clear why ∆E ads is slightly more endothermic for the thicker slab, but the difference is not large enough to affect the present analysis. 4.1.1.2. Molecular Adsorption on (100)-B. Physisorption of molecular H2O via a Ga(Oh) · · · OH2 bond (Figure 2c) was analyzed using the 40-layer slab with a (2 × 1) surface unit cell. An adsorption bond length of 2.154 Å was found, which is somewhat longer than the Ga(Oh)-O distances in the β-Ga2O3 lattice (1.93-2.08 Å). There is also an indication of H bonding to two surface O atoms with similar r(O · · · H) distances of 2.220

and 2.234 Å. An exothermic ∆Eads of -0.35 eV per H2O was found for the adsorption energy. The adsorption-induced displacements of the surface atoms are e0.1 Å in magnitude, significantly smaller than in the case of dissociative adsorption. The conclusion at this point is that H bonding is important in stabilizing either molecularly or dissociatively adsorbed H2O. In the latter case, the surface distortions needed to accommodate the H bonding make the process energetically unfavorable. The BSSEs encountered here are all relatively large in comparison to ∆Eads. The values found are about 0.40 (0.54) eV per H2O for molecular (dissociative) adsorption, and similar values occur for the other adsorbates. The larger contribution (about two-thirds) is derived from the effect on the total energy of the Ga2O3 slab of the added variational freedom provided by the adsorbate. Damin et al.42 have analyzed a similar situation for CO adsorbed on MgO (100) and attributed the effect to the incomplete screening of negative charge on surface oxygen ions that “penetrates” the wave function of the adsorbed CO. However, when all ∆Eads values have been corrected for BSSE, the CO/ MgO (100) results are reasonably independent of the basis set, which indicates that the ∆EBSSE correction is effective. Another issue, also discussed by Damin et al.,42 concerns the importance of dispersion forces in describing weak physisorption bonds. These can be treated properly using second-order Møller-Plesset (MP2) theory or DFT with special functionals,43 neither of which was available in the present work. For CO/MgO (100), DFT with the B3LYP functional underestimates ∆Eads. However, this would have no serious effect on the present analysis because a proper treatment of dispersion would then result in an even greater stabilization of molecularly versus dissociatively adsorbed H2O. The absence of detectable physisorbed H2O in IRRAS can be understood by examining Figures 1b and 2c. Most (but not all) NRs lie essentially flat on the Au substrate on which they are deposited (Supporting Information). Hence the z axis, which is the Ga2O3 surface normal, lies parallel to the p-polarized electric field of the IR radiation (Supporting Information). The presence of the (y, z) mirror plane means that z-polarized radiation can excite only those modes that are invariant to reflection in this plane.44 The νas(O-H) asymmetric stretch is not invariant and is therefore symmetry-forbidden in the type of experiment done here. The νs(O-H) symmetric stretch and the δ(H-O-H) bending modes are allowed but are mainly y-polarized, and for an allowed mode to be detectable there must also be a finite projection of the dynamic dipole moment in the zdirection. The H2O normal modes, even the two that are symmetry-allowed, are therefore undetectable. Stated differently, if the H2O in Figure 2c were perpendicular, rather than parallel, to the surface, then νas(O-H) would remain x-polarized and therefore forbidden. However, νs(O-H) and δ(H-O-H) would change from being y- to z-polarized and would be detectable if the oscillator strengths were sufficiently large. 4.1.1.3. Adsorption on (100)-A. Calculations were done for adsorption in a (2 × 1) unit cell on the (100)-A surface in order to determine whether reaction is more favorable on a somewhat less stable surface. In β-Ga2O3 HSA powders, one might expect the presence of both types of (100) surfaces, among other (hkl) planes. An examination of Figure 1 shows the structural differences between the (100)-A and -B planes. For (100)-A, the Ga(Oh) and O(II) adsorption sites are farther apart, both horizontally and vertically, than are the Ga(Oh) and O(III) sites on (100)-B. Furthermore, in the (100)-A (2 × 1) cell, the two

(40) Moskaleva, L. V.; Nasluzov, V. A.; Chen, Z.-X.; Ro¨sch, N. Phys. Chem. Chem. Phys. 2004, 6, 4505. (41) Pinto, H. P.; Nieminen, R. M.; Elliott, S. D. Phys. ReV. B 2004, 70, 125402.

(42) Damin, A.; Dovesi, R.; Zecchina, A.; Ugliengo, P. Surf. Sci. 2001, 479, 255. (43) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157. (44) Fan, J.; Trenary, M. Langmuir 1994, 10, 3649.

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Figure 3. Similar to Figure 2 but showing optimized structures for (a) chemisorbed and (b) physisorbed H2O on the (100)-A surface in (2 × 1) unit cells. In structure a, the H atom is placed on the closer of the two possible O(II) sites, labeled a in Figure 1. The two structures are shown from different perspectives.

O(II) sites are not equivalent with respect to the single occupied Ga(Oh) site. The results for the (100)-A surface, summarized in Table 1 and in Figure 3, show pronounced differences from those for the (100)-B. For (100)-A, both modes of adsorption give exothermic ∆Eads values of comparable magnitude. Furthermore, dissociative adsorption in a (1 × 1) cell is slightly less favorable (-0.81 eV per H2O) than in a (2 × 1) cell (-0.95 eV per H2O), reversing the pattern noted above for (100)-B. The H bonds are shorter on (100)-A, being 1.664 and 1.736 Å for dissociative and molecular adsorption, respectively. The corresponding values for (100)-B, given above, are 1.683 and 2.230 Å (with the latter being an average of two slightly different values). The adsorption-induced displacements are generally smaller than on the (100)-B, with the largest being a lateral displacement of the active O(II) site by 0.21 (0.25) Å for dissociative (molecular) adsorption. Finally, the orientation of the physisorbed H2O differs for the two surfaces, as can be seen by comparing Figures 2c and 3b. H bonding has a significant effect on adsorption. If, in dissociative adsorption in the (2 × 1) cell, the H atom is placed on the farther of the two O(II) sites (labeled b in Figure 1a), then H bonding is not feasible because the distance, in the relaxed structure, from the H to the Ga-OH oxygen atom increases to 3.93 Å. In this case, ∆Eads decreases from -0.95 to -0.28 eV per H2O. All of these differences result from the difference in surface stability, as reflected in the surface energies, and from the different relative positions of the unsaturated Ga and O sites on the two surfaces. For the structures shown in Figure 3, all adsorbate normal modes are symmetry-allowed (because all bonds lie in the (y, z) mirror plane). Except for νas(OH) of H2O (which is mainly y-polarized), these would be detectable if (100)-A regions made up a large fraction of the total surface and if the oscillator strengths are sufficiently large. 4.2. 1-Octanol. The adsorption of CH3OH and CH3CH2OH on β-Ga2O3 has been studied experimentally4-6 and computationally.6,45 Both molecular and dissociative adsorption were observed in IR experiments on hydroxylated material (i.e., powders containing surface OH groups). Chemisorption and decomposition of the resulting CH3O-Ga species increase at (45) Branda, M. M.; Garda, G. R.; Rodriquez, H. A.; Castellani, N. J. Appl. Surf. Sci. 2007, 254, 120.

Bermudez

Figure 4. IRRAS data for the exposure of β-Ga2O3 NRs to 1-octanol vapor, followed by exposure to acetic acid vapor. (a-c) Total 1-octanol doses are ∼0.19 × 1015, 1.4 × 1015, and 22 × 1015 molecules/cm2 respectively. (d) Acetic acid dose of ∼1.4 × 1017 molecules/cm2 following exposure c.

higher temperatures. Theoretical modeling has suggested that surfaces without O vacancies are more reactive than those with vacancies. In the present IRRAS experiments, a long-chain species (1-octanol) was used in order to increase the sensitivity to the C-H stretching modes. Figure 4 shows data obtained for exposure to 1-octanol that exhibit clear C-H stretching modes. δ(C-H) bending modes were also observed in the 1380-1480 cm-1 range (not shown) but were relatively weak, as in the spectrum of the gas-phase molecule. Exposure of the alcohol-saturated surface to acetic acid vapor displaced a large fraction of the alcohol, indicating that most of the alcohol is weakly adsorbed. This can be seen in Figure 4 by noting the changes in the intensities of the 2930 cm-1 νas(CH2) asymmetric and 2858 cm-1 νs(CH2) symmetric stretching modes that can arise only from CH2 groups in 1-octanol. The increased intensities of the 2962 cm-1 νas(CH3) and 2882 cm-1 νs(CH3) modes relative to those of the ν(CH2) modes, in Figure 4d, result from the superposition of the acetate (see below) and 1-octanol ν(CH3) modes. The assignments and polarizations of the C-H stretching modes in n-alkyl chains are discussed in refs 46 and 47 and in the Supporting Information. However, the reverse experiment (not shown), in which a surface saturated with preadsorbed acetic acid was exposed to 1-octanol vapor, led to no significant displacement of the acid as seen in the intensity of the acetate modes. The residual, strongly adsorbed 1-octanol, which is not displaced by acetic acid, is discussed below. 4.2.1. Computational Modeling of Adsorbed Methanol. For the dissociative adsorption of CH3OH, geometry optimization with the (2 × 1) unit cell (Figure 5a) gave an O-H distance of 0.978 Å for the O(III)-H bond. For the Ga(Oh)-OCH3 site, the Ga-O and O-CH3 distances were 1.852 and 1.389 Å, respectively, and the Ga-O-C angle was 126°. As was seen for dissociative adsorption of H2O, displacements occur at the adsorption sites to allow the formation of a single short H-bond (r(O-H) ) 1.698 Å) between O(III)-H and the methoxy O atom. Quantitatively, these displacements are essentially the (46) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (47) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927.

Functionalization of β-Ga2O3 Nanoribbons

Figure 5. Similar to Figure 2 but showing optimized structures for (a) dissociative and (b) molecular adsorption of CH3OH on the (100)-B surface in a (2 × 1) surface unit cell. The heavy lines indicate H-bonding interactions with (a) r(O · · · H) ) 1.690 Å and (b) r(O · · · H) ) 2.181 Å.

same as those given above for H2O. A single-point DFT treatment then gave an endothermic ∆Eads of +0.36 eV per CH3OH, which is close to the value of +0.27 eV per H2O given above for the dissociative adsorption of H2O in a (2 × 1) structure. The CH3OH results differ considerably from the exothermic values obtained previously.6,45 There, a different model was used for the (100) surface, one that terminates in a layer of 1- and 2-fold-coordinated O atoms and may be representative of a defective surface more appropriate to HSA powders than to NRs. The physisorption of molecular CH3OH, via a Ga(Oh) · · · O bond to the molecular O atom was also analyzed using the (2 × 1) surface unit cell. The optimized Ga(Oh) · · · O bond length (Figure 5b) was 2.169 Å with a ∆Eads of -0.29 eV per CH3OH, which is close to the corresponding value for H2O (-0.35 eV per H2O). There is also an indication of an H-bonding interaction between the molecular OH and the nearest surface O atom, with r(O · · · H) ) 2.171 Å. The results are thus consistent with the detection of predominantly weakly adsorbed 1-octanol in IRRAS. Adsorption of CH3OH on the (100)-A surface was also analyzed computationally, and the results are summarized in Table 1. The ∆Eads values are similar to those for H2O on the same surface and are thus different from those for CH3OH on the (100)-B. Both forms of adsorption are again exothermic on the (100)-A surface. The optimized structures are similar to those in Figure 3 with OCH3 replacing OH. As in the case of H2O, the H-bond lengths are shorter than for the (100)-B surfaces. These are 1.494 and 1.525 Å, respectively, for chemisorption and physisorption versus 1.698 and 2.171 Å for CH3OH adsorbed on the (100)-B surface. Computationally, H2O and CH3OH behave in a quantitatively similar manner with regard to interaction with the (100)-B surface. The two reagents also interact similarly with the (100)-A surface. Given that physisorbed 1-octantol is clearly detectable in IRRAS, it is reasonable to conclude that H2O also physisorbs but is not easily observed for the reasons given above. A fraction (∼25%) of the total 1-octanol coverage at saturation resists displacement by acetic acid vapor and is therefore strongly adsorbed (Figures 4c,d). A similar effect was seen previously25 for exposure to pyridine vapor. In that case, adsorption was studied under steadystate conditions, at ambient pressures of up to ∼5 Torr, using polarization-modulated IRRAS. Most of the adsorbed pyridine

Langmuir, Vol. 24, No. 22, 2008 12949

Figure 6. IRRAS results, in the carboxylate bond-stretching region, for exposure to acetic acid vapor. (a-c) Total doses of ∼0.18 × 1016, 3.4 × 1016, and 23 × 1016 molecules/cm2 respectively. Weak structure in the ∼1500-1700 cm-1 range is due to H2O vapor in the optical path.

was removed when the gas-phase reagent was evacuated, indicating weakly bound physisorbed species. However, a fraction remained and exhibited spectroscopic features indicative of chemisorption. This was suggested to result from the presence of VO defects that would expose more strongly Lewis acidic Ga(Td) sites. In the case of 1-octanol, the same explanation might apply or, alternatively, chemisorption may indicate the presence of (100)-A regions on the predominantly (100)-B surface. Another possible type of reactive site would be step edges that have been observed48,49 on the (100) surface of bulk β-Ga2O3 single crystals and homoepitaxial films. However, the present NR samples have not been investigated in this context. 4.3. Acetic and Pentanoic Acids. The adsorption of acetic50-52 and other small carboxylic53-55 acids on Al2O3 HSA powders has been investigated using IR spectroscopy. Chemisorption is observed, leading to the formation of carboxylate species. To our knowledge, there has been no previous study of carboxylic acid adsorption on any form of Ga2O3. However, surface formate species have been observed5,9 in the synthesis of CH3OH from CO2 and H2 (and in the decomposition of CH3OH) on β-Ga2O3 and Pd/β-Ga2O3 HSA powders. Figure 6 shows IRRAS data for exposure to acetic acid. The ν(CH3) stretching modes of the CH3 group (not shown) are too weak to be useful. A similar observation has been made in the case of acetic acid adsorption on Al2O3 HSA powders,50,52,53 for which the ν(CH3) modes are undetectable. Strong bands are seen at about 1583 and 1443 cm-1 that correspond in frequency to the νas(OCO) and νs(OCO) stretching modes of an acetate. The separation of ∼140 cm-1 suggests a bridging species,56 one in (48) Oshima, T.; Arai, N.; Suzuki, N.; Ohira, S.; Fujita, S. Thin Solid Films 2008, 516, 5768. (49) Ohira, S.; Arai, N.; Oshima, T.; Fujita, S. Appl. Surf. Sci. 2008, 254, 7838. (50) Shimizu, K.; Kawabata, H.; Satsuma, A.; Hattori, T. J. Phys. Chem. B 1999, 103, 5240. (51) Ferri, D.; Bu¨rgi, T.; Baiker, A. HelV. Chem. Acta 2002, 85, 3639. (52) Carlos-Cuellar, S.; Li, P.; Christensen, A. P.; Krueger, B. J.; Burrichter, C.; Grassian, V. H. J. Phys. Chem. A 2003, 107, 4250. (53) Touwslager, F. J.; Sondag, A. H. M. Langmuir 1994, 10, 1028. (54) (a) Dobson, K. D.; McQuillan, A. J. Spectrochim. Acta A 1999, 55, 1395. (b) Dobson, K. D.; McQuillan, A. J. Spectrochim. Acta A 2000, 56, 557. (55) van den Brand, J.; Blajiev, O.; Beentjes, P. C. J.; Terryn, H.; de Wit, J. H. W. Langmuir 2004, 20, 6308. (56) Tackett, J. E. Appl. Spectrosc. 1989, 43, 483.

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Figure 7. IRRAS results, in the C-H stretching region, for exposure to pentanoic acid vapor. (a-d) Total doses of ∼0.14 × 1016, 0.60 × 1016, 1.5 × 1016, and 25 × 1016 molecules/cm2.

which the two O atoms are bonded to different Ga sites. These modes are seen5,9 at about 1580 and 1372 cm-1 for bridging formate on β-Ga2O3 HSA powder. For acetate on Al2O3, these modes appear at about 1550-1590 and 1465 cm-1 (ref 50) or at about 1569 and 1414 cm-1 (ref 51). Ga acetate in an aqueous 0.6 M NaCl solution forms57 an acetate structure bridging two edge-sharing Ga octahedra for which νas(OCO) and νs(OCO) occur at 1560 and 1455 cm-1, respectively. A further discussion of this assignment is given below. The weak mode near 1335 cm-1 is the acetate δs(CH3) symmetric deformation.58,59 The 1200 cm-1 mode may be the ν(C-O) stretch of the physisorbed acid which, in the gas-phase acetic acid monomer, is seen60 at 1178 cm-1. At saturation (Figure 6c), a weak shoulder appears at ∼1710 cm-1 and is assigned to the ν(CdO) mode of the physisorbed acid. This mode is strong in the vapor, where it appears60 at 1788 cm-1 for the monomer. As noted above, the IRRAS intensity of a normal mode depends strongly on the orientation of the dynamic dipole moment with respect to the p-polarized electric field of the IR radiation27 (Supporting Information). Hence, it is difficult to assess the relative coverages of physisorbed and chemisorbed species simply from the respective peak intensities. Exposure to H2O (∼3.0 × 1016 H2O/cm2) following a saturation dose of acetic acid (not shown) had little or no effect, indicating that H2O does not displace or hydrolyze the adsorbed species under these conditions. Figures 7 and 8 show similar results for pentanoic acid. The spectra are similar to those for acetic acid except that the ν(C-H) stretching modes can be seen clearly. This part of the spectrum is similar to that of the gas-phase molecule (Supporting Information). The lower-energy part of the spectrum (Figure 8) is again dominated by strong features, suggesting bridging carboxylate νas(OCO) and νs(OCO) modes at about 1575 and 1440 cm-1. The latter shows additional structure that is ascribed to the δas(CH2) asymmetric deformation mode typically occurring in the 1430-1480 cm-1 range. As in the case of acetic acid, exposure to ∼7.5 × 1016 H2O/cm2 following a saturation dose of pentanoic acid had no apparent effect on the carboxylate ¨ hman, L.-O.; Kubicki, J. D.; Persson, P. J. Chem. Soc., (57) Clause´n, M.; O Dalton Trans. 2002, 2559. (58) Quile`s, F.; Burneau, A. Vib. Spectrosc. 1998, 16, 105. (59) Rotzinger, F. P.; Kesselman-Truttmann, J. M.; Hug, S. J.; Shklover, V.; Gra¨tzel, M. J. Phys. Chem. B 2004, 108, 5004. (60) Mare´chal, Y. J. Chem. Phys. 1987, 87, 6344.

Bermudez

Figure 8. Similar to Figure 7 but showing the low-energy part of the spectrum. The doses are the same as in Figure 7.

species. A shoulder at about 1710 cm-1, which is not significantly affected by exposure to H2O, is assigned to the physisorbed acid as in the case of acetic acid. Additional weak structure, which is entirely reproducible but difficult to assign conclusively, is seen below 1400 cm-1. 4.3.1. Computational Modeling of Adsorbed Carboxylic Acids. The chemisorption of carboxylic acids was modeled computationally using formic acid in a (2 × 1) unit cell. The results are summarized in Table 1 and Figure 9a,b. Formic acid was used in order to minimize the size of the adsorbate and, thus, the size of the surface unit cell needed to avoid severe steric interaction. The procedure was essentially identical to that described above for H2O and CH3OH. The formate group was placed in the bridging configuration shown in Figure 9a, and the H atom released in adsorption was placed on an O(III) site. The optimized structure (Figure 9a,b) gave ∆E ads ) -0.49 eV per molecule. This is a fairly weak interaction, but it is nevertheless more exothermic than that for chemisorbed or physisorbed H2O or CH3OH. The distance from the H atom to either of the formate O atoms is 2.351 Å, which may permit a weak H-bonding interaction. A calculation was also done beginning with a bidentate structure (not shown) in which both molecular O atoms are bonded to the same Ga. During geometry optimization, this relaxed to the structure shown in Figure 9a,b, after passing through an intermediate monodentate phase with only one Ga-O bond per molecule. These results indicate that the bridging structure is the most stable carboxylate configuration. The distortions of the two Ga(Oh) sites and the O(III) site are similar to, but somewhat smaller than, those found above for the dissociative adsorption of H2O and CH3OH. The Ga(Oh) surface sites are essentially 6-fold-coordinated, although the distance to the O(III)-H site is elongated to 2.308 versus 1.974 Å on the bare surface. Physisorption of molecular formic acid on the (100)-B surface was also investigated, resulting in the optimized structure shown in Figure 9c. The Ga-O and O · · · H distances are 2.147 and 1.616 Å, respectively, and the adsorption energy is -0.72 eV per molecule. The (100)-A surface is more reactive than the (100)-B toward formic acid, as was seen for H2O and CH3OH. However, in this case, chemisorption is energetically favored over physisorption (Table 1). The optimized structures (not shown) are similar to those for the (100)-B surface except that, in the case of physisorption, the H bond is to the coordinatively unsaturated O(II) atom as in Figure 3b. Finally, results were also

Functionalization of β-Ga2O3 Nanoribbons

Figure 9. (a, b) Optimized structure for the bridging formate species on the (100)-B surface. In (b), the y axis points out of the plane of the page. (c) Optimized structure for physisorbed formic acid.

obtained for acetic acid adsorption on the (100)-B surface. The CH3 group is sufficiently small that the (2 × 1) unit cell can be used. This was done to determine if the close agreement between the results for H2O and CH3OH also applies to the case of HCO2H and CH3CO2H. The results were essentially the same for both species. Using the same ∆EBSSE as for formic acid, ∆Eads was 12 (61) meV/molecule more negative for chemisorbed (physisorbed) acetic acid. The optimized structures were also analogous to those shown in Figure 9. 4.3.2. Interpretation of the Carboxylate Vibrational Modes. The assignment of the features at about 1580 and 1440 cm-1 to the νas(OCO) and νs(OCO) modes of a bridging carboxylate appears to be well founded. However, two observations suggest the need for further consideration. First, physisorption, not chemisorption, is computed to be energetically favored on the (100)-B surface. Second and more serious is the fact that for the bridging carboxylate geometry in Figure 9a,b the νas(OCO) mode is symmetry-forbidden,44 in the type of IRRAS experiment performed here, for either the (100)-A or -B surface. This results from the fact that νas(OCO) is not invariant to reflection in the (y, z) mirror plane and is therefore x-polarized. Any effect of the alkyl group, in chemisorbed acetic or pentanoic acid, on the mirror plane is neglected here. A clear example of this selection rule is seen61 for bridging formate on Cu(110) where a strong νs(OCO) mode is found with no detectable νas(OCO). However, for randomly oriented acetates in solution56-58 or on HSA Al2O3 powders,50 νas(OCO) is somewhat more intense than νs(OCO). (61) Hayden, B. E.; Prince, K.; Woodruff, D. P.; Bradshaw, A. M. Surf. Sci. 1983, 133, 589.

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The observation of a carboxylate is thus at variance with the computational result, assuming the (100)-B to be the dominant surface type. However, the difference in adsorption energy between physisorption (-0.72 eV/molecule) and chemisorption (-0.49 eV/molecule) is fairly small, and the adsorption may be controlled by kinetic, rather than thermodynamic, factors. Regarding the apparent violation of the selection rule, two explanations are proposed. One is that there is evidence that not all NRs lie flat on the Au substrate (Supporting Information). In this case, the surface normal of the ribbon (i.e., the z axis in Figure 9b) is no longer parallel to the p-polarized electric field of the IR radiation, and modes such as νas(OCO) that are polarized in the plane of the NR surface can then become allowed in IRRAS. In principle, the same mechanism would allow a finite IRRAS intensity for the vibrational modes of H2O molecularly adsorbed on the (100)-B surface. For a flat-lying ribbon, these are either symmetry-forbidden or allowed but weak for reasons discussed above. However, as noted above, IR data for β-Ga2O3 HSA powders3 (on which adsorbates are randomly oriented) show that these modes are weak in comparison to those of other adsorbates. The results of Shimizu et al.50 for the adsorption of acetic acid on γ-Al2O3 HSA powders are also noteworthy in this regard. Here, the acetate ν(OCO) modes are about an order of magnitude stronger than the ν(O-H) modes removed by chemisorption. The second factor involves adsorption at VO defects that expose Ga(Td) sites. This has already been implicated in the appearance of strongly adsorbed pyridine, as noted above. Referring to Figure 1b, one notes that a carboxylate bridging a Ga(I) and a Ga(II) site (i.e., a Ga(Td) and a Ga(Oh)) would have no symmetry elements and would be inclined relative to the surface normal as a result of the fact that the Ga(I) plane lies below the Ga(II) plane. Either of the effects described here would remove the symmetry restriction forbidding the appearance of the νas(OCO) mode in IRRAS and would yield a finite component of the νas(OCO) dynamic dipole in the direction of the p-polarized electric field. 4.4. Adsorption on the Bare Au Substrate. As noted above, the NR samples are prepared by spraying a suspension in CH3OH onto a heated Au substrate in room air. The resulting layer is not completely continuous (Supporting Information), hence attention must be paid to possible effects from adsorption directly onto the Au substrate. A blank Au/mica sample was prepared using a procedure nominally identical to that followed for the actual samples. This included spraying the heated substrate in room air with pure CH3OH and in situ annealing in O2 as described above. The data, which are shown and discussed in more detail in the Supporting Information, are summarized here. The ν(C-H) stretching modes seen in IRRAS following exposure to H atoms are a useful measure of the level of chemically accessible impurity C contamination. As noted previously,25 the present cleaning method is not sufficient for the complete removal of C contamination from Au. Thus the Au substrate, following exposure to H atoms, shows a clear feature due to ν(C-H) stretching modes in hydrogenated amorphous C. For the NR/Au sample, this feature is significantly weaker. Bearing in mind that the NR layer is loosely packed, with a finite internal surface area, this suggests that cleaning removes most of the C from the NRs and that most of the Au surface is in intimate contact with NRs and therefore is inaccessible to H atoms. If either of these conditions was not satisfied, then the ν(C-H) intensity for the NR sample would be comparable to or even larger than that for C-contaminated Au. As noted previously,25 no Ga-H or O-H modes were detected in IRRAS, following the exposure of NRs

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to H atoms as a result of the energetically favorable recombinative desorption of H2. Exposing bare Au to 1-octanol leads to a strong ν(C-H) spectrum that is quite different from that seen for an NR/Au sample. The data indicate that the alkyl chain lies essentially flat on and interacts strongly with the Au surface. The qualitative appearance of the spectrum and its response to subsequent acetic acid exposure are distinctly different from what is seen for similar treatments of the NR/Au sample. Exposing the Au surface to acetic or pentanoic acid vapor resulted in no detectable carboxylate features in IRRAS, in clear contrast to the NR results. The ν(C-H) stretching modes were clearly observed for pentanoic acid; however, as in the case of 1-octanol, the spectrum was very different from that observed for adsorption on NRs and suggested an alkyl chain strongly perturbed by interaction with Au. These results indicate that the data shown above for 1-octanol and for the organic acids represent the interaction of these reagents with the NRs and not with the Au substrate.

5. Summary The adsorption of H2O, alcohols, and carboxylic acids on β-Ga2O3 (100) surfaces has been studied experimentally using IRRAS and computationally using 2D periodic slab models. The results provide insight into the interplay between surface structure and reactivity on an oxide surface and the importance of hydrogen bonding in determining adsorbate structure. The specific findings are as follows. (1) Adsorption is very sensitive to the β-Ga2O3 surface structure. This is seen by comparing computational results (Table 1) for two (100) surface terminations. The less stable (100)-A surface is the more reactive for both physisorption and chemisorption, as expected, and the energy difference between the two modes of adsorption depends on the surface and on the adsorbate. This indicates the difficulty in computational modeling of the adsorption behavior of heterogeneous β-Ga2O3 HSA powders where several different (hkl) planes are expected. (2) For the molecules studied here, hydrogen bonding plays a significant role in stabilizing adsorbed species. This is seen most clearly in the computational results for the dissociative adsorption of H2O in a (2 × 1) cell on the (100)-A surface. Placing the H atom on a surface O site that is too far from the associated Ga-OH site to permit H bonding decreases ∆Eads from -0.95 to -0.28 eV per H2O. For geometric reasons, hydrogen bonds form more easily (i.e., are shorter) on the (100)-A than on the (100)-B surface, which contributes to an increased ∆Eads on the former. (3) On the more stable (100)-B surface, computation shows that the physisorption of H2O and alcohols is weakly exothermic whereas chemisorption via O-H bond dissociation is weakly endothermic. Experiment finds that a large fraction of a saturation coverage of 1-octanol is displaced by exposure to acetic acid vapor. This is consistent with computational results showing that a carboxylic acid adsorbs more strongly than an alcohol on this surface. The remaining alcohol, not displaced by acetic acid, suggests that chemisorption occurs in addition to physisorption. The 1-octanol data are comparable to previous results25 for the

Bermudez

adsorption of pyridine, which show weak physisorption as the dominant effect but with a significant contribution from chemisorption. The pyridine data were interpreted in terms of VO defects on the (100)-B surface that would expose Ga(Td) Lewis acid sites. A additional factor, in the case of 1-octanol, might be the presence of (100)-A regions on the predominantly (100)-B surface because computation indicates that the (100)-A surface interacts more strongly with an alcohol than does the (100)-B. (4) No adsorbed H2O was observed in IRRAS whereas the ν(C-H) modes of adsorbed 1-octanol were easily detected. For either the (100)-A or -B surface, H2O and CH3OH behave similarly with regard to the computed adsorption energies (cf. Table 1). It is inferred from this that H2O also adsorbs molecularly and that the failure to detect H2O on the dominant (100)-B surface results from the orientation of the physisorbed molecule essentially parallel to the surface. Computation shows that this configuration (Figure 2c) is stabilized by H bonding. In this orientation, the internal modes of H2O have little if any projection on the surface normal and are therefore weak as a result of the metal-surface selection rule27 that governs the present experiment. (5) For chemisorbed formic acid, computation shows that a bridging carboxylate structure is energetically favored over a bidentate or monodentate configuration. Computation also shows that chemisorption is favored on the (100)-A surface but physisorption is favored on the more stable (100)-B. An analysis of IRRAS data for acetic and pentanoic acids finds evidence for both types of adsorption. This observation is thus at variance with the computational result, assuming the (100)-B to be the dominant surface type. However, the difference in adsorption energy between physisorption (-0.72 eV/molecule) and chemisorption (-0.49 eV/molecule) is small, and the adsorption may be controlled by kinetic rather than thermodynamic factors. The IRRAS data show both the νs(OCO) and νas(OCO) modes of the bridging carboxylate, even though the latter is symmetryforbidden for ideal surfaces. Two explanations are proposed for this anomaly. One is that not all NRs lie flat on the Au substrate, and the other involves adsorption at defects. Either of these would allow the appearance of the νas(OCO) mode in IRRAS. (6) The strong chemisorption of carboxylic acids and the resistance of the resulting carboxylate to displacement by subsequent exposure to H2O vapor suggest that this may be a viable approach to the functionalization of β-Ga2O3 NR surfaces. Acknowledgment. S. M. Prokes is thanked for providing the nanoribbon material. This work was supported by the Office of Naval Research. Computer facilities were provided by the DOD High-Performance Computing Modernization Program at the AFRL-MSRC, Wright-Patterson AFB, Ohio. Supporting Information Available: Sample morphology, IR transmission spectrum of pentanoic acid vapor, IRRAS data for bare Au and NR/Au samples after exposure to atomic H, IR data for 1-octanol vapor and liquid and for 1-octanol adsorbed on bare Au and on Ga2O3, IR data for pentanoic acid vapor and for pentanoic acid adsorbed on bare Au and on Ga2O3, and nanoribbons not lying flat on the Au substrate. This material is available free of charge via the Internet at http://pubs.acs.org. LA8022979