4162
J. Phys. Chem. 1995,99, 4162-4169
Stabilization of NiO(ll1) Thin Films by Surface Hydroxyls M. A. Langell* and M. H. Nassir Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304 Received: July 13, 1994; In Final Form: September 21, 1994@ The stabilization of NiO( 111)-oriented thin films by surface hydroxylation has been studied by XPS, LEED, AES, and HREELS as a function of substrate temperature. The Ni2+ polar, crystalline films were grown on Ni(100) under low pressures of oxygen gas to a limiting thickness of approximately 3 monolayer, and a substantial fraction of the surface nickel sites were found to be terminated by adsorbed hydroxyls. The hydroxyl concentration was estimated to be 0.63-0.84 monolayers relative to the surface nickel sites, with the concentration somewhat variable from film to film. The hydroxyls stabilize the polar NiO( 11 1) surface by partial compensation of repulsive cation-cation interactions, and their removal triggers a phase transition to the more stable NiO(100) orientation. While some hydroxyls are displaced at lower temperatures, the majority desorb at 593 K, leaving behind no detectable OH,d, in either XPS or HREELS and creating a concurrent transition from (1 11) to (100) symmetry in LEED.
1. Introduction The oxidation of nickel metal at low pressures of 0 2 eventually leads to oxide nucleation and the subsequent slow growth of three-dimensional nickel monoxide films. Despite the significant mismatch in lattice parameters between the metal and its oxide, crystalline NiO(100) films have been grown successfully on Ni( 100),2-15 Ni( 111),2-4716917 and Ni( 110).2-4718,19 NiO(100) forms a natural cleavage plane for single crystal substrates,20and the nickel oxide (100) thin films have many surface properties comparable to those observed for single crystal NiO( 100) surfaces. The nonpolar (100) orientation of the monoxide most effectively balances electrostatic interactions among the surface lattice ions and is the energetically favored surface configuration of ionic rock salt materials. A metastable NiO( 111) thin film has been reported in the oxidation of Ni( 100),411J3,21922 and Ni( 111),4J4J9*23 although conditions favoring the formation of nickel oxide films oriented along (1 11) typically require substrate temperatures of 5 300 K. The NiO( 111) film converts readily into NiO(100) upon annealing and is postulated to result from kinetic limitations that preclude the growth of the thermodynamically more stable NiO( 100).13,22The ready accessibility of Ni2+ gas adsorption sites" indicates that a substantial fraction of the NiO(ll1) surface is Ni2+ polar. NiO( 111) thin film studies that utilize X-ray photoelectron spectroscopy (XPS)2,3y1 inevitably reveal the presence of an 0 1s feature at 531.2 eV, in addition to that of the dominant nickel oxide 02-state at 529.4 eV. The assignment of the 531.2 eV binding energy species to surface hydroxyls has been confirmed by high-resolution electron energy loss spectroscopy (HREELS).' Cappus et al.l4 propose that the hydroxyls form by dissociative adsorption of ambient H20 at NiO(ll1) defect sites. Langell et al." suggest that their presence serves to stabilize the thermodynamically disfavored (1 11) orientation. Through comparison of the various literature XPS data, it appears that their concentration is somewhat variable, depending upon the conditions under which the thin films are fabricated. The present study investigates the stabilization of NiO( 111) thin films by surface hydroxyls. It has long been realized that oxide surfaces are generally hydroxylated to varying degrees 1-15~17324
* Author to whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, March 1, 1995.
0022-3654/95/2099-4 162$09.00/0
Figure 1. LEED pattern obtained from the NiO( 111) thin film taken at 80 eV primary beam energy.
under ambient condition^.^^^^^ Upon annealing under vacuum, the oxides typically dehydroxylate over a range of temperatures as a function of site heterogeneity and the relative proximity of adjacent hydroxyl groups. While some dehydroxylation of the nickel oxide thin film can be accommodated, the primary effect of dehydroxylation found in the present study is to destabilize the NiO( 111) orientation. At 593 K, a complete desorption of surface hydroxyls results in a phase transition to the NiO( 100) thin film.
2. Experimental Section NiO( 111) crystalline films were grown on Ni( loo), obtained from the Goodfellow Corp. (99.999%, f0.5").Procedures for their fabrication have been previously cited in the literature.' 1-13 The nickel crystal was suspended between two tantalum wires used for resistive heating, and substrate temperatures were measured with a chromel-alumel thermocouple spot-welded to the back of the Ni(100) sample. The metal substrate was first cleaned by repeated cycles of Ar+ bombardment (2 keV, 10 ,uamp/cm2 s for 15 min), oxygen anneal (1 x Torr, 623 K for 15 min) and vacuum anneal ( 5 3 x Torr, 973 K for 15 min) until no impurities could be detected by Auger electron spectroscopy (AES). The clean Ni( 100) surface was Torr 0 2 at 300 K for 20 min (600 then exposed to 5 x Langmuirs), conditions that lead to saturation coverages of NiO0 1995 American Chemical Society
Stabilization of NiO( 111) Thin Films
J. Phys. Chem., Vol. 99, No. 12, 1995 4163
0
100
I
I
I
24)
300
400
Ni
I
,
500
Ba)
E
I
I
1
700
0m
800
m (ev)
Figure 2. Auger spectrum obtained from the NiO(ll1) thin film grown on a Ni(100) single crystal substrate. The spectrum is taken in the differential mode with 2 eV modulation energy, a 0.1 s time constant, and a scan rate of 5 eV/s. The Auger excitation beam is 2 keV.
(111) and the best quality crystalline films as judged by XPS, AES, low-energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (HREELS). The analyses are described in the text below. The ultrahigh vacuum (UHV) system and its components have been described previ~usly.~'Auger and X P spectroscopies were obtained with a Physical Electronics (@) 15-255 G doublepass cylindrical mirror analyzer (CMA). AES employed a 2 keV primary beam from an electron gun concentric with the analyzer. The AES a(NE)/aE data were acquired in lock-in mode with a 2 eV modulation energy, a 0.1 s time constant, and a scan rate of 5 eV/s. XPS data were taken with a Mg K a source (hv = 1253.6 eV). Energy analysis was performed in a constant-pass energy mode using either 25 eV pass energy for 500 scans or 50 eV pass energy for 100 scans, as noted in the figure captions. The spectra were referenced to the nickel oxide 0 1s transition at 529.4 eV, which has been shown to yield a constant binding energy over a wide range of nonstoichiometric surface condit i o n ~ . ~Curve-fitting ~,~~ procedures30 used in XPS analysis assume a double Gaussian peak shape. Low-energy electron diffraction was performed with CP 15120 four-grid LEED optics for two dimensional surface crystallography. High-resolution electron energy loss spectroscopic data were obtained in the specular mode at 60" relative to the surface normal using a single-pass 127" sector electron energy analyzer and a primary scattering beam energy of 2 eV. The spectrometer is described in detail e l ~ e w h e r e . ~ ~
tions, the oxide thin film gave a characteristic NiO( 111) LEED pattern (Figure 1) with no trace of p(2 x 2), c(2 x 2), or NiO(100) diffraction features. The symmetry of the LEED pattern can be explained as a superposition resulting from two sets of hexagonal NiO( 111) domains, one in which the NiO(l11) (710) and Ni(100) (010) are coincident and the other, rotated by 90", in which the oxide (710) and metal substrate (001) align. Comparison of intensityvoltage characteristicsof the diffraction features with dynamical LEED calculations21 has indicated that each of the two hexagonal orientations actually can be formed in two distinct fcc stacking sequences in which the second layers differ from each other by a rotation of 60". Differences in stacking sequence are not apparent in simple symmetry considerations of the LEED diffraction pattern in Figure 1. A typical Auger spectrum obtained from the NiO( 111) thin film is shown in Figure 2. The average Auger intensity ratio, taken as the 0 510 eV/Ni 848 eV peak-to-peak height, was found to be Io/l~i= 1.12. An estimate of the NiO(ll1) thickness can be made by assuming a uniform coverage of the nickel substrate by the oxide film and an exponential attenuation of the Auger intensity with depth into the surface:
3. Description of Data 3.1. NiO(ll1) Thin Film. The NiO(ll1) thin film is metastable and has been proposed to form on Ni(100) upon exposure to low pressures of 0 2 under kinetically constrained conditions. For Ni( 100) substrate temperatures 1300 K, NiO(111) thin film formation is preceded by Ni(100)-0,dS p(2 x 2) and c(2 x 2) submonolayer structures. At long 0 2 exposures, (1 11) domains are often found to be mixed with the thermodynamically more stable NiO( 100) orientation. In the present experiments, the best quality NiO( 111) film was produced by exposure to 600 langmuirs of 0 2 at 5 x lo-' Torr and a 300 K Ni( 100) substrate temperature. Under these experimental condi-
where the nickel AES signal can be separated into contributions from the oxide and from the nickel metal substrate:
m
4164 J. Phys. Chem., Vol. 99, No. 12, 1995
Langell and Nassir TABLE 1: XPS Binding Energies in eV for the NiO(ll1) Thin Film Grown on a Ni(100) Single Crystal Substrate Ni 2pm
Ni 2p3/2
NiO(ll1) Ni(100) OHds
3
I
1
I
4
880
870
8&l
850
Blndlng Energy (ev)
6
I
I
1
534
532
530
528
526
e
!
Blndlng Energy (ev) Figure 3. XPS of the NiO( 111) thin film on a Ni( 100) single crystal substrate, taken at 25 eV pass energy for (a) the Ni 2p region and (b) the 0 1s region. Spectral assignments are made in the text.
By approximating surface lattice spacings and densities for the thin film by those observed in the bulk, the concentrations of oxygen and nickel in the oxide layer are c0,ox= cNi,Ox = 0.1:3 ions/A2 and the distance between layers is nox = 2.41 A. Correspondin quantities for the metal substrate are C~i,,,,~d = 0.161 atoms/.f2 and xmetd= 1.76 A. Values for the electron mean free path are 20 = 9.5 for the 0 510 eV KLzLz electron,' &ox = 10.8 A for the Ni 848 eV L3M4,5M4,5 electron in the and = 10.2 A for the Ni 848 eV electron in the metal s u b ~ t r a t e .The ~ ~ quantity cos 0 corrects for the CMA collection angle. The relative AES sensitivity factor is obtained from single crystal NiO( 100) Auger data taken under similar spectroscopic ~ o n d i t i o n s , 2and ~ ~is~measured ~~~~ to be SdSNi = 2.54." Using the above information, the NiO(111) film thickness m is calculated to be between 3 and 4 monolayers (ML), in good agreement with previous estimates.6,' 1.13 XP spectra obtained from the NiO( 111) surface are shown in Figure 3 for the Ni 2p and 0 1s regions, and the corresponding spectral assignments are summarized in Table 1. The nickel
854.0 855.7 852.5
Ni 2 ~ 1 1 2
satellite 861.1
Ni 2p1,2 872.0
satellite 879.4
obscured
870.0
obscured
0 1s
529.4 531.2
2p spectrum contains contributions from zero valent metal in the substrate, with binding energies at 852.5 and 870.0 eV for 2~312and 2~112,respectively, and from Ni2+ in the oxide thin film. The Ni2+ shows resolvable multiplet splitting of the 2~312 transition, with binding energies of 854.0 and 855.7 eV, and a characteristic satellite peak at 861.1 eV. The NiZ+2~112binding energy is measured at 872.0 eV with a satellite at 879.4 eV. The Ni and NiO spectral features are comparable to those reported in the l i t e r a t ~ r e . ~ ~ The oxygen 1s XP spectrum (Figure 3b) is predominantly comprised of an oxide peak at 529.4 eV but also contains a shoulder at 531.2 eV. The relative intensity of the shoulder was found to vary slightly among the different films fabricated and, in the present series of studies, spanned the range of 2535% total 0 1s intensity. The shoulder is always observed in XPS measurements of NiO( 111) thin films23391'-153'7924and is attributed to a surface hydroxyl species. Assuming the hydroxyls are primarily found at the outermost surface of the NiO(111) thin film, that the film is 3 ML thick, and that the escape depth of the 0 1s electron from the oxide is l o , = 11 A,34 comparison of 1531.2/1529.4 X P S 0 1s intensities leads to an estimate of surface hydroxyl concentrations of OOH = 0.63 0.84 ML. The assignment to a surface hydroxyl species is confirmed in the HREEL spectrum (Figure 4) by the VOH adsorbate vibrational loss at 471 meV (3768 cm-l). The relatively low XPS OH,d, 0 1s binding energy and the high HREELS VOH loss energy indicate that the hydroxyl possesses substantialionic character. Comparison with literature IR data11$26s35 suggests the species is singly coordinated to a Ni2+ surface site and sits primarily in an atop, non-hydrogen bonding configuration. In addition to the VOH mode at 471 meV, the HREEL spectrum (Figure 4) contains an intense loss at 69.6 meV (557 cm-') from the excitation of Fuchs-Kliewer surface lattice phonon^.^^.^^ The NiO( 111) thin film phonon energy is slightly less than the 70.5 meV (564 cm-l) loss observed for stoichiometric surfaces of macroscopically thick NiO crystals' 1,27,38 but is comparable to that reported elsewhere for NiO( 100)" and NiO(111)11323thin films grown on nickel metal. It is also within the range of loss energies reported for nonstoichiometric NiO(100) single crystal surfaces that have been deliberately reduced by heating under lop7 Torr of HZ gas. Ar+ bombardment of NiO( 100) single crystal substrates does not induce comparable shifts from 70.5 meV to detectably lower loss energies despite the fact that the bombardment creates isolated F, oxygen vacancies and introduces disorder into the near-surface region. Based on the results of the H2 reduction and A r ' bombardment studies,38it was postulated that the phonon loss energy is not so much sensitive to nonstoichiometry and disorder per se, but rather is a function of ordered domain size. Characteristic of ionic surfaces, the phonon is essentially a transverse optical lattice vibration in which cations and anions oscillate with opposite phase. The substantial dipole moment associated with the phonon motion produces a large scattering cross section and thus a very intense electron energy loss signature. Due to non-negligible probability for multiple scattering, surfaces of bulk oxides typically produce a series of
J. Phys. Chem., Vol. 99, No. 12, 1995 4165
Stabilization of NiO( 111) Thin Films
471 meV
I
0.2
0.2
0
0.4
0.6
Electron Energy Loss (mew Figure 4. High-resolution electron energy loss spectrum of the N i O ( l l 1 ) thin film on Ni(100). The spectrum is taken at a scattering beam energy of 2 eV in the specular mode and shows losses due to Fuchs-Kliewer phonons at integral multiples of 69.6 meV, as well as the hydroxyl stretching loss at 471 meV. 529.4 eV
529,4 eV
773 K
603 K 523 K 593 K
473 K 583 K
423 K
573 K
\ 'tl,
373 K 300K
1
1
1
I
1
I
I
534
532
530
528
526
824
Blndlng Energy (ev)
i
I
I
I
534
532
530
528
526
524
Blndlng Energy (ev)
Figure 5. XP 0 1s spectral region for the NiO( 111) thin film as a function of substrate temperature. Data were acquired at a 50 eV pass energy.
Fuchs-Kliewer phonon excitations at integral multiples of the single loss peak. The NiO( 111) HREEL spectrum shows some structure related to double scattering at 139 meV (11 12 cm-'). However the double loss is broad and considerably reduced in intensity relative to that observed for macroscopically thick NiO substrates due to the high defect density of the NiO( 111) thin film, the small average domain size, and the close proximity of the underlying Ni( 100) substrate conduction band.
3.2. Thermal Stability of the Surface Hydroxyl Group. It has long be known that oxide surfaces tend to be hydroxylated as a result of their interaction with ambient water vapor and that annealing the oxide under vacuum to progressively higher temperatures decreases the surface hydroxyl concentration proportionately.26 To some extent, NiO( 111) thin films also exhibit this behavior. The NiO( 111) surface described above was annealed at substrate temperatures ranging from 300 to 800
4166 J. Phys. Chem., Vol. 99,No. 12, 1995
Langell and Nassir
3.5 I
2.0
c
\
c
\
,
0
200 0.4 I
QO
1
400
500
800
700
I
69 1 0.2
q 0
\
a
Dehydroxylation of the NiO( 111) surface produces related changes in the HREEL spectrum of the thin film (Figure 7). At 300 K, the 471 meV VOH mode and the 69.6 meV FuchsKliewer lattice phonon are apparent, as is some broad, ill-defined structure centered at about 139 meV and associated with multiple phonon losses. As the substrate is heated, the phonon spectrum actually improves slightly as a result of increased surface diffusion and other annealing effects which eliminate defects and increase NiO( 111) domain size in the thin film. The single-loss phonon becomes more intense and the multiple-loss phonons sharpen to produce a distinct double-loss feature. However, no significant changes occur in VOH until substrate annealing temperatures 2573 K are reached, at which point the intensity of the hydroxyl stretching mode begins to decrease. The VOH loss energy remains constant to within the limit of the HREELS measurement. For temperatures 2593 K, the hydroxyl is no longer detectable in HREELS. The two-dimensional symmetry of the NiO( 111) surface is initially also improved by the annealing process (Figure 8), but at ~ 5 8 K, 3 the LEED pattern exhibits streaking, indicating that the NiO( 111) structure is becoming disrupted. At 593 K the pattem undergoes an abrupt change to square Ni0(100), with the NiO( 111)hexagonal domains no longer apparent. Annealing at higher temperatures initially improves the NiO( 100) LEED pattern, but by 778 K, weak c(2 x 2) oxygen adsorbate features are also detected. Thus, at substrate temperatures between 593 and 603 K, a phase transition from NiO(ll1) to NiO( 100) thin films occurs concurrently with the disappearance of hydroxyls in the XP and HREEL spectra. Cooling the substrate to 300 K neither regenerates hydroxyls by adsorption from the background nor causes a reverse phase transition to the NiO( 111) surface. 4. Discussion
\ 300 eo0 700 B
O200
400
500
Temporaturo (K) Figure 6. (a) X P 0 1s spectral intensity of the 531.2 eV hydroxyl and the 529.4 eV oxide peaks and (b) the ratio of hydroxyVoxide 0 1s peak intensities as a function of substrate temperature.
K for 5 min, and XP 0 1s spectra were taken after the intervals shown in Figure 5. Initial changes in the spectrum (1473 K), are not very dramatic for either the oxide 529.4 eV or the hydroxyl 531.2 eV peak although both decrease slightly in intensity with anneal temperature. At approximately 523 K, the hydroxyl 0 1s peak begins a substantial decrease in intensity relative to the oxide peak and can no longer be detected for temperatures 2603 K. The 529.4 eV also decreases in intensity as the substrate is heated, with the effect being most pronounced for substrate temperatures 2 603 K. The intensity changes are quantified in Figure 6, in which XPS peak areas are plotted as a function of substrate temperature. For both 531.2 and 529.4 eV peaks, a net decrease in absolute 0 1s intensities is now readily apparent (Figure 6a). However, the hydroxyl peak decreases in intensity more rapidly than does the oxide peak as the substrate is heated to progressively higher temperatures. As can be seen in Figure 6b, in which the ratio of 0 1s hydroxylloxide is plotted as a function of substrate temperature, the effect is particularly pronounced for substrate temperatures greater than 523 K. The NiO( 111) thin film becomes increasingly dehydroxylated until at 603 K no detectable hydroxyls remain. Cooling the surface to 300 K does not regenerate the 53 1.2 eV hydroxyl peak and only causes slight increases (520%) in the intensity of the 529.4 eV NiO lattice oxygen peak. Thus, the decrease in 0 1s intensity represents a permanent loss in near-surface oxygen.
HREELS and XPS data indicate that the X3 ML NiO( 111) thin films grown under UHV conditions on Ni(100) are inevitably contaminated with surface hydroxyls resulting from dissociative adsorption of ambient H20. The HREELS VOH stretching energy of 471 meV correlates with an isolated, ionic hydroxyl species, and comparison with theoretically calculated Gaussian 88/90 stretching energies for MgO-OH,d, clusters34 suggests the hydroxyls are singly coordinated to a surface nickel in an atop adsorption site. At 531.2 eV, the XPS 0 1s binding energy is slightly less than the average hydroxyl 0 1s binding energy ( ~ 5 3 1 . 5eV33,39),consistent with the ionic nature of the surface hydroxyl. In the present set of experiments, the concentration of surface hydroxyls is estimated to be 0.63-0.84 ML relative to the surface nickel concentration. The XPS 531.2529.4 eV 0 1s intensities are in good agreement with those from spectra previously reported in the literature,11~13~14J7,z4 as are the estimates of surface layer coverage based on these data. Acetic acid adsorption studies on the hydroxylated NiO( 111) thin film have previously shown' that acetate formation occurs through condensation with hydroxylated Ni2+ and thus establish the presence of accessible surface Ni2+ adsorption sites. In the acetic acid adsorption studies, the assumption that hydroxylation occurs primarily at the outermost surface layer leads to an estimate of 6OH = 0.65 ML from 0 1s XPS intensities. For the related surface of a NiO( 111) grown on Ni( 111),14 studies using similar methods of estimating hydroxyl surface coverage reported 6OH = 0.4 ML relative to the total surface ion concentration. For this surface ISS measurements determined that the outermost layer is approximately 70 at. % Ni, giving 6OH = 0.57 relative to the surface nickel sites, in reasonable agreement with the values obtained in the present studies. The
J. Phys. Chem., Vol. 99, No. 12, 1995 4167
Stabilization of NiO( 111) Thin Films 8
8
x10
5
*0
4
-
4
423 K
ii
l3
5
471 meV
138 meV
III
2
II I
‘0
E
i3
373 K
A
471 meV
J+
-
173 K
2
323 K
Ill I I
1
I
1,
1
300 K
-473K
I
Clean Surface
0
I
0
200
I
Clean Surface
I
0
800
400
523K
200
0
Electron Energy Lor8 (mew
400
800
Electron Energy Lorr (mew
x1e
69.6 meV
139 meV
773 K
I 0
Clean Surface 200
400
600
Electron Energy Lorr (mew Figure 7. HREEL spectrum of the NiO(ll1)thin film as a function of substrate temperature. The HREEL spectra were taken with a 2 eV primary beam energy.
hydroxyl concentration is somewhat variable, depending upon the thin film fabrication conditions, and while some hydroxyls
may be occluded in lower layers, both the accessibility of hydroxylated Ni2+ sites in the acetic acid adsorption studies”
4168 J. Phys. Chem., Vol. 99, No. 12, 1995
Langell and Nassir
Cooled to
300 K
773 K
603 K
Figure 8. Low-energy electron diffraction pattern from the oxide thin film surface as a function of substrate temperature. The LEED pattern was taken at 80 eV.
and angularly resolved XPS/UPS measurements in the NiO(1 1 1) thin film studies14provide direct evidence that a substantial amount of hydroxylation occurs at the outermost surface layer.
Upon heating the hydroxylated NiO( 1 1 1) thin film to 593 K, an irreversible transition to the NiO( 100) orientation occurs concurrently with complete removal of surface hydroxyls from the film. Bulk-terminated NiO( 1 1 1) is not an energetically
Stabilization of NiO( 111) Thin Films favorable orientation for the nickel oxide surface and has been proposed to be kinetically selected over the thermodynamically stable NiO( 100) at low substrate temperatures (1300 K).13,22 The present studies indicate that surface hydroxylation through adsorption of residual H20 from the UHV ambient is an important factor in the creation of this otherwise metastable surface. It is not possible to maintain the (1 11) orientation after the surface has been dehydroxylated. While it was readily possible to form Ni(lOO)-O,d,,ll NiO(100) thin films on Ni(loo)," and (100) orientations of bulk nickel oxide single crystal^^^,^^,^* using the present experimental setup at a comparable base pressure, a NiO(ll1) thin film could not be fabricated without hydroxyl features detectable in the X P and HREEL spectra. All other NiO( 111) thin film studies that report the relevant XP11,13-15,17,24 and HREEL11,14,23 spectral regions show features attributable to the hydroxyl species. It is, therefore, concluded that the hydroxyls stabilize the NiO( 111) surface and are necessary for the formation of the (111) orientation of nickel oxide thin films. Unlike the nonpolar (100) orientation, which effectively balances out cationic and anionic electrostatic interactions, a bulklike termination of the NiO(ll1) thin film creates net repulsive interactions among the cations in the outermost surface layer. For the Ni2+ polar surface, these repulsive interactions can at least be partially compensated by the presence of the hydroxyl adsorbates. The hydroxyls have significant ionic character and, from the viewpoint of the surface Ni2+, appear similar in nature to lattice oxygen. Removal of the surface hydroxyl removes the stabilization of the ionic repulsive forces. The NiO( 111) surface, therefore, reconstructs to the thermodynamically favored NiO( 100) orientation when the surface is sufficiently dehydroxylated. Estimates of hydroxyl concentrations just prior to the NiO(1 11) to NiO( 100) thin film transition indicate that approximately one-half of the Ni2+ surface sites must be hydroxylated to stabilize NiO( 111) thin film. These concentrations are too high merely to be associated with surface defect sites, as was suggested previously,1oand they are thus attributed to hydroxyls adsorbed at regular surface Ni2+lattice sites. Since no fractional order diffraction features are detected in LEED, the hydroxyls must be randomly distributed among the Ni2+ sites which may partially explain the high background typically associated with NiO( 111) thin films.
5. Conclusion NiO( 111) thin films fabricated by oxidizing Ni( 100) under 5 x Torr of 0 2 grow to approximately 3 ML and show many of the XPS, HREELS and LEED characteristics expected for surfaces on bulk nickel oxide crystals. The films are predominantly Ni2+polar with 63-84% of the outermost nickel sites terminated by hydroxyl adsorbates. The hydroxyls, which are ionic in character, provide the Ni2+ with an environment similar to that of the lattice oxygens and help stabilize the repulsive electrostatic interactions among the surface Ni2+.The hydroxyls desorb at 593 K, causing the metastable NiO( 111) thin film to convert to the thermodynamically favored NiO(100) orientation. Acknowledgment. The authors are grateful for support from NSF grant CHE-9220341 and from the University of Nebraska Center for Materials Research and Analysis. References and Notes (1) Holloway, P. H.; Hudson, J. B. Surf:Sei. 1974, 43, 123.
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