ZnxNi1-xO Rocksalt Oxide Surfaces: Novel Environment for Zn2+ and

13912

J. Phys. Chem. C 2007, 111, 13912-13921

ZnxNi1-xO Rocksalt Oxide Surfaces: Novel Environment for Zn2+ and Its Effect on the NiO Band Structure K. J. Gaskell, A. Starace, and M. A. Langell* Department of Chemistry, UniVersity of Nebraska, Lincoln, Nebraska 68588-0304 ReceiVed: May 10, 2007; In Final Form: June 5, 2007

Rocksalt ZnxNi1-xO solid solutions, which form at the nickel-rich end of the phase diagram, can be prepared over a wide compositional range and are homogeneous up to Zn0.32Ni0.68O when prepared by thermal dissolution in air. Auger parameter analysis indicates that the zinc is octahedrally coordinated, an unusual environment for solid-state Zn2+. The surface composition, as determined by Auger and X-ray photoelectron spectroscopies shows comparable surface and bulk concentrations until phase separation occurs, at which point the surface becomes enriched in wurtzite ZnO. The charge-transfer nature of the NiO electronic structure remains largely intact as zinc is added to the lattice. In ab initio Hartree-Fock calculations of the ZnxNi1-xO valence band, zinc interacts with the lattice oxygen primarily through the Zn 4s orbitals and is less covalent than in wurtzite ZnO. A detectable shift of O 2p character away from the valence band maximum to more strongly bonding energies accompanies increased zinc concentration as a result of Zn 4s hybridization with O 2p states. However, the zinc serves primarily to dilute nickel-nickel coupling interactions in the electronic structure of the solid solution, and Ni 3d-O 2p hybridization is not strongly affected, despite the fact that the Ni 2p photoemission changes measurably with increased zinc content. Changes in the Ni 2p XPS satellite structure as zinc is added to the rocksalt lattice are a result of attenuation in nonlocal screening due to decreased adjacent nickel site occupancy.

Introduction Mixed-metal oxides present the opportunity to tailor chemical, electronic, and magnetic properties and to design new systems with unique materials characteristics. One deceptively simple mixed-metal oxide system, ZnxNi1-xO (0 e x < ∼0.3), is based on the very stable rocksalt NiO lattice and contains Zn2+ substituting for Ni2+ at up to approximately 30-35% zinc relative to the total metal concentration.1-4 Both metals are formally M2+, their stable oxidation state in solid-state oxides, but the octahedral coordination of the rocksalt structure is unusual for zinc, which is typically tetrahedral as, for example, is found in ZnO.5 The unique chemical environment of the zinc in the nickel oxide lattice can potentially result in new and interesting properties. NiO adopts a rocksalt crystal structure with a unit cell lattice parameter of ao ) 4.17777 Å at 300 K.6 The material is antiferromagnetic, with a Neél temperature of 523 K,7 and the alignment of the unpaired electron spins in alternating (111) planes causes a slight distortion to rhombohedral symmetry of approximately 1% along the 〈111〉 direction.8,9 Although Ni2+ is 3d8 pure NiO is an insulator with a band gap of 4.2 eV10 resulting from the strong Coulomb interactions among the highly correlated d-electrons and compounded by localization of these electrons through hybridization with adjacent oxygen 2p levels. The valence band structure of the pure NiO material has been extensively investigated both experimentally11-21 and computationally22-32 and is described in greater detail below. The ability of zinc to dissolve in the NiO rocksalt lattice has been recognized for some time and has been investigated primarily through X-ray diffraction studies of the bulk ZnO* Author to whom correspondence should be addressed. Phone: +1 (402) 472-2702. Fax: +1 (402) 472-9402. E-mail: [email protected].

NiO system.1-4 The solid solution is known to be nominally face-centered cubic (fcc) rocksalt for zinc concentrations up to the solubility limit although, as is found with the parent NiO structure, a slight rhombohedral distortion lowers the symmetry to the R3hm space group.33 The solid solution follows Vegard’s law,34 in which the unit cell parameter increases linearly with zinc concentration from ao ) 4.1777 Å at x ) 0 to 4.2107 Å at x ) 0.3, indicating a homogeneous solution in which Zn2+ ions randomly substitute for Ni2+ in bulk cation lattice positions. Since zinc is 3d10 substituting Zn2+ for Ni2+ can be anticipated to affect both the magnetic and the electronic structures of the parent NiO lattice. Magnetic studies33 have been performed on the ZnxNi1-xO solid solution and show that, although diluted by the zinc atoms, the antiferromagnetic structure persists throughout the range 0 e x e 0.3, with the Neél temperature decreasing with increasing zinc content. Very few other studies have been carried out on solid solutions of ZnxNi1-xO in the rocksalt phase. To our knowledge the surface of this material has not previously been characterized nor has there been any attempt to determine the effect that increasing zinc concentration has on the rocksalt NiO band structure. Using the Auger parameter formalism35 to analyze zinc coordination in ZnxNi1-xO, we present X-ray photoelectron spectra that show the zinc is, indeed, in a unique chemical environment in the octahedral lattice of this solid solution. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are used to measure surface composition for comparison to bulk zinc concentrations. Satellite structure from Ni 2p XPS is also analyzed and is used in conjunction with CRYSTAL98 periodic ab initio linear combination of atomic orbital (LCAO) unrestricted Hartree-Fock calculations to elucidate the effect of the zinc on the oxide band structure.

10.1021/jp073590x CCC: $37.00 © 2007 American Chemical Society Published on Web 08/28/2007

ZnxNi1-xO Rocksalt Oxide Surfaces Experimental Details Polycrystalline solid solutions of nickel-zinc oxides were synthesized by calcination over the range of 0.05 e Zn e 0.3 by preparing stoichiometric mixtures of NiO (Alfa Aesar, 99%) and ZnO (JT Baker, 99%), grinding them with an agate mortar and pestle for approximately 30 min to mix intimately. The mixture was then pressed into pellets 12 mm in diameter and approximately 1 mm in thickness at a pressure of 15,000 psi for 10 min. The pellets, which were initially gray in color, were annealed in air at 1273 K for 1 week, during which time they turned green. After removal from the furnace, the pellets were allowed to cool in a desiccator and stored there until mounted in the ultrahigh-vacuum (UHV) chamber on a goniometer carousel capable of holding up to 12 samples. Samples transferred to the goniometer showed minimal surface contamination, and the data acquired here were obtained without in situ UHV cleaning. AES and XPS were used to determine the surface composition of the ZnxNi1-xO samples. The UHV chamber operated at a pressure of 1 × 10-8 Pa or lower. The chamber was equipped with a Physical Electronics double-pass cylindrical mirror analyzer, model 15-255G, which could be operated in lock-in or pulse count mode. AES spectra were collected in the derivative mode with lock-in detection at a modulation energy of 2 eV, a primary beam voltage of 3 kV, and the scan taken at 1 eV increments at a rate of 1 eV/s. The AES spectra were signal-averaged over 10 scans. XPS data were collected in the pulse count mode using Mg KR (1253.6 eV) X-ray radiation at 300 W, a pass energy of 25 eV, a step size of 0.1 eV, and a count time of 0.5 s, and were signal-averaged over 100 scans. XPS binding energies were calibrated to the O 1s lattice oxygen peak at 529.5 eV, as described previously,36,37 to compensate for the spectrometer work function and surface charging. In addition to the XPS studies performed on polycrystalline ZnxNi1-xO samples as a function of zinc content, XPS temperature studies were also performed and were carried out on the (100) face of an NiO single-crystal sample. Achromatic Mg KR (1253.6 eV) X-ray radiation at 300 W was used, and the photoemission was analyzed with a 125 mm Omnicron (EA 125) hemispherical analyzer with seven-channel detection capabilities. Data were collected at a pass energy of 25 eV, using a step size of 0.05 eV and a count time of 0.1 s, and were signalaveraged over 10 scans. Curve-resolved O 1s and Ni 2p spectra were fit after removal of a Shirley background38,39 with peaks of a 60%/40% Gaussian/ Lorentzian sum. XPS surface concentrations were calculated from the peak areas of the Ni 2p, Zn 2p3/2, and O 1s regions, and relative sensitivity factors were calculated for each peak from the data for pure NiO and ZnO. Surface concentrations were also obtained from the Auger data, but because nickel and zinc transitions overlap, intensities were obtained by integrating the derivative spectra to obtain absolute integrated intensities of the overlapped zinc-nickel peak and using the spectra of the pure oxides to determine the relative contributions of zinc and nickel in the peak. The procedure has previously been described in greater detail.37 The bulk ZnxNi1-xO material was analyzed for homogeneity by powder X-ray diffraction (XRD) using a Rigaku D/Max-B diffractometer (Cu KR radiation). Solid solutions of the polycrystalline material were obtained for x e 0.3 with an average crystallite size of 0.2-0.3 µm as measured by XRD Rietveld analysis. Samples of higher zinc content (0.40 e Zn e 0.90) were also prepared but were found to phase-separate into ZnO and ZnxNi1-xO for x > 0.3. The lattice parameters were

J. Phys. Chem. C, Vol. 111, No. 37, 2007 13913 calculated via use of the TOPAZ program employing crystallographic information obtained from the International Centre for Diffraction Data (ICDD) PDF-2 database in the form of cards 36-1451 and 47-1049 for ZnO and NiO, respectively. The reference XRD data were used to fit the ZnxNi1-xO spectra, allowing the software to optimize crystallite size, lattice parameter, and specimen displacement to attain the best fit. Calculation Methods As an aid to the interpretation of experimental data, periodic ab initio LCAO unrestricted Hartree-Fock (UHF) calculations were carried out using the program CRYSTAL98.40-42 Extended Gaussian basis sets composed of 27 atomic orbitals for nickel and 13 orbitals for oxygen were employed, as has been previously shown to model the NiO band structure accurately.23 An equivalent basis set for zinc with 31 atomic orbitals was adopted from a previous study elucidating wurtzite to rocksalt phase changes in ZnO.43 Within the crystal program, the numerical accuracy of the two-electron Coulomb and exchange series are controlled via the use of five “cutoff” parameters and the values used here are 10-6, 10-8, 10-6, 10-7, and 10-14, ensuring a high degree of numerical accuracy. In Crystal calculations, the primitive lattice of paramagnetic, cubic NiO can be described by just two atoms (1 Ni and 1 O), in a “supercell” construct, which, when repeated periodically within user-specified symmetry constraints, describes the infinite lattice. The use of larger supercells is required to reproduce the more complex nature of both the antiferromagnetic properties of NiO and the ZnxNi1-xO solid. Computations for ferromagnetic and antiferromagnetic NiO were carried out at supercell sizes of 4, 8, 16, and 32 atoms to evaluate the stability of the supercell method. The total energy in hartrees per formula unit of NiO was found to be constant to five decimal places, the same accuracy reported in a previous study investigating the stability of supercells of MgO.44 Electronic properties of pure NiO were in agreement with previous Crystal calculations for this material.23 Optimized lattice parameters have been found to be comparable for ferromagnetic and antiferromagnetic NiO,23 and since calculations based on the ferromagnetic system are computationally much less expensive than the corresponding antiferromagnetic system, lattice parameter optimizations were carried out on ferromagnetic supercells. Values were calculated for ZnxNi1-xO with the compositions of x ) 0.125, 0.25, and 0.375, using supercells containing 16 atoms for concentrations of x ) 0.125 and 0.375 and 8 atoms for x ) 0.25. Solid solutions were formed via the direct substitution of zinc for nickel ions in the ideal rocksalt position according to the required stoichiometry of the solid solution. The geometry of each solid solution was minimized, with respect to the lattice parameter, ao, whereby the total energy was recorded as a function of the unit cell dimension at a minimum of 20 different values and fit to a second-order polynomial, the minimum taken to be the optimal value of ao. Optimized lattice parameters derived from calculations involving an a posteriori correction for correlation energy, employed a Perdew-Wang generalized gradient approximation (GGA) functional.45 The electronic properties of the system were based on antiferromagnetically ordered NiO. Density of state (DOS) plots were calculated for NiO and Zn0.25Ni0.75O based on the antiferromagnetic AF2 structure of the pure nickel oxide. A supercell containing 4 atoms was used for the calculation of antiferromagnetic nickel oxide and a 16 atom supercell for the Zn0.25Ni0.75O solid solution. The AF2 structure was maintained in the calculation of the solid solution by the placing zinc ions

13914 J. Phys. Chem. C, Vol. 111, No. 37, 2007

Figure 1. Powder X-ray diffraction patterns of ZnxNi1-xO containing various percentages of zinc oxide, including pure nickel oxide (x ) 0) and zinc oxide (x ) 1). Phase separation is first evident in these data at 40% ZnO by the appearance of the wurtzite ZnO XRD pattern.

Figure 2. Lattice constants derived from experimental data (filled triangles) compared to those predicted by ab initio methods, Hartree Fock (HF) (empty circles), and HF corrected by a posteriori correction for correlation energy (filled circles). The experimental data point at 1.0 mole fraction is taken from ref 43.

in both a spin up and a spin down position in the supercell, allowing the overall spin of zero to be maintained. Results and Discussion Powder XRD data for a series of ZnxNi1-xO samples are shown in Figure 1. Diffraction features solely due to the rocksalt crystal structure appear for samples with x e 0.3 (e30% ZnO), indicating that at these concentrations single-phase solid solutions of cubic ZnxNi1-xO have formed. For XRD of ZnxNi1-xO samples fabricated with x g 0.4 (40% ZnO), diffraction features due to both the rocksalt ZnxNi1-xO and wurtzite ZnO crystal structures are observed, as a result of phase separation into ZnxNi1-xO (x ≈ 0.3) and ZnO. The maximum zinc concentration that could be obtained under the present synthetic conditions for single-phase ZnxNi1-xO, without any indication of phase separation, is Zn0.32Ni0.68O, and this range of solid solution formation is compatible with previously reported data.1-3 The ZnxNi1-xO rocksalt unit cell parameter, ao, was obtained from experimental XRD data through Rietveld analysis with TOPAZ software and, over the range at which the solid solution forms, is found to increase linearly with x (Figure 2) as the larger Zn2+ is substituted for the smaller Ni2+ in the NiO lattice. This results in an increase in unit cell length with increased zinc concentration of 4.1777 Å for NiO to 4.2107 Å for

Gaskell et al. Zn0.3Ni0.7O with a linear least-square fit slope of 0.1026 Å/x. The linear nature of the plot indicates that the solid solution follows Vegard’s law,34 an empirical relation in which the lattice unit cell length varies linearly from that of one pure constituent at x ) 0 (rocksalt NiO) to that of the other pure constituent at x ) 1 (rocksalt ZnO) and indicates an ideal mixing of the two components. Experimental data for phase-separated samples show that the lattice parameter, ao, no longer increases above x ) 0.3, indicating that above 30% zinc concentration the samples are phase-separated into wurtzite ZnO and a rocksalt solid solution of constant composition, nominally Zn0.3Ni0.7O. Deviations from Vegard’s law can either be negative, resulting from an increase in attractive interactions within the solid solution, or positive due to more repulsive interactions in the solid solution relative to that of the pure oxide materials. Linear extrapolation of the ZnxNi1-xO XRD data over the range of solid solution formation (0 e x e 0.3) to that of rocksalt ZnO at x ) 1 is shown in Figure 2. While pure rocksalt zinc monoxide is only thermodynamically stable at high pressure, lower ambient pressure unit cell parameters can been calculated from highpressure data46-48 by extrapolation to low-pressure values to yield a ZnO rocksalt unit cell parameter of ao ) 4.2800 Å.46 The XRD data show that ZnxNi1-xO closely follows Vegard’s law, with neither positive nor negative deviations, over the range of solid solution formation. This indicates a random distribution of zinc within the NiO lattice to form an ideal, homogeneous solid solution. To model the solid solution behavior computationally, lattice parameters for rocksalt ZnxNi1-xO were calculated over the entire range of 0 e x e 1 by ab initio Hartree-Fock methods using CRYSTAL98. The calculated values correctly predict that Vegard’s law is obeyed and show a linear, positive slope for ao with increasing zinc concentration. However, the slope is underestimated by slightly more than a factor of 3. Calculated unit cell parameters are typically overestimated for oxide calculations, and at x ) 0 the calculation leads to a 2% overestimation of the unit cell parameter for the NiO lattice. This level of accuracy is consistent with previous investigations of NiO49 and of the related system MgO44,50 by comparable computational methods. At x ) 1, the calculation models pure rocksalt ZnO and leads to a smaller overestimation of the ZnO lattice parameter of only 0.3%. The agreement between experimental and calculated parameters is directly proportional to the zinc content, giving rise to the underestimated slope. Overestimation of lattice parameters can be expected when using HF methods to calculate bulk transition metal oxide properties due to the neglect of explicit treatment of electron correlation in the Hartree-Fock Hamiltonian. Since electron correlation effects are more substantial for the 3d8 NiO system than for the 3d10 filled-shell ZnO, it is perhaps unsurprising that the calculation better predicts bulk lattice parameter values for ZnO than for NiO. However, it is possible to correct Hartree-Fock energy minimization in an a posteriori inclusion of electron correlation in which corrections are determined by density functional methods.41 When such corrections are included, lattice parameters for both oxides are now somewhat underestimated for the rocksalt ZnxNi1-xO lattice (Figure 2), for NiO by approximately 1%, but for cubic ZnO by 2%. Regardless of whether a posteriori corrections are included, the slope of the Vegard’s law plot remains essentially constant, at 0.0310 Å/x for HF and 0.0307 Å/x for HF with a posteriori inclusion of electron correlation. The error in the slope is, therefore, inherent in the method of calculation used to obtain lattice parameters to sufficient accuracy needed to follow the

ZnxNi1-xO Rocksalt Oxide Surfaces

J. Phys. Chem. C, Vol. 111, No. 37, 2007 13915 TABLE 1: Auger Parameter Data: Zn L3M4,5M4,5 Auger Kinetic Energies, Zn 2p3/2 Photoelectron Binding Energies, and Zinc Auger Parameters. ZnxNi1-xO x value

Zn L3M4,5M4,5 kinetic energy (eV)a

Zn 2p3/2 binding energy (eV)a

Auger parameter (eV)

0.05 0.1 0.2 0.3 1.0

989.5 990.0 989.6 989.4 988.6

1021.7 1021.2 1021.6 1021.6 1021.5

2011.2 2011.2 2011.2 2011.0 2010.0

a XPS energies are calibrated to the rocksalt lattice O 1s binding energy of 529.5 eV.

Figure 3. Zinc photoemission for various ZnxNi1-xO solid solutions and zinc oxide corresponding to (a) 2p3/2 XPS and (b) Zn L3M4,5M4,5 Auger transition XPS. Intensities have been scaled to allow spectra to be displayed with approximately the same peak heights.

small increase in lattice parameter over the range of the solid solution variability. Both XRD Reitveld analysis and HF calculations have been performed assuming ideal rocksalt coordination, and the effect of small distortions from the rocksalt structure was not investigated. The trends in lattice parameters are correct, however, and the differences in computational versus experimental lattice parameters are in line with current computational accuracies for both HF and DFT methods. The chemical environment in the NiO rocksalt lattice is unusual for Zn2+, which prefers tetrahedral coordination in the solid state with a valence band that is largely composed of less strongly electron-correlated 4s and 4p orbitals. Auger parameter analysis has been shown to be particularly sensitive to the chemical environment for solid-state zinc compounds.51 The modified Auger parameter,52 R′, is obtained through XPS measurements and is defined as

R′ ) KE(Auger) - KE(photoelectron) + hυ

(1)

where KE(Auger) is the kinetic energy of an Auger transition, KE(photoelectron) is the kinetic energy of a core level photoelectron, and hυ is the photoexitation energy. For zinc, the L3M4,5M4,5 Auger electron and the Zn 2p3/2 core photoemission transitions are convenient choices. In the absence of final state effects, the Auger parameter can be related to the chemical environment of the metal and is a good measure of the effect that the surrounding lattice oxygen has on the cation. XPS of the Zn 2p3/2 and the Zn L3M4,5M4,5 Auger transition are shown in Figure 3 for a range of ZnxNi1-xO samples, and the Auger parameters calculated from these data are given in

Figure 4. AES spectra for various ZnxNi1-xO solid solutions. In addition to oxygen, nickel, and zinc Auger transitions, a small amount of carbon contamination (e15% monolayer) can be seen at 273 eV.

Table 1. The XPS and Auger photoemission data are plotted as a function of binding energy (BE)

BE ) hν - KE - eφ

(2)

where eφ is the work function of the sample. In obtaining Auger parameters, the work function difference contributes to both photoelectron and electron measurements and, to a first approximation, cancels in the Auger parameter calculation. Charging effects and other systematic experimental errors also cancel, and the Auger parameters listed in Table 1 have been obtained on uncorrected data, with no calibration or shift to the as-acquired experimental energies. Absolute values for the Zn 2p3/2 and the Zn L3M4,5M4,5 reported in Table 1, however, have been calibrated to correct for work function and charging effects by setting the lattice O 1s binding energy to 529.5 eV36,37 for comparison of these absolute energies with literature data. For Zn2+ in ZnxNi1-xO solid solutions, R′ was found to be constant over the range of homogeneous solid solution formation and was measured to be 2011.1 ( 0.3 eV, as can be compared to literature values of 2009.8 eV for ZnO52,53 and 2013.8 eV for zinc metal.52 This is a significant shift for a metal that is formally Zn2+ in both ZnO and ZnxNi1-xO materials and is a result of the zinc octahedral environment in the ZnxNi1-xO solid solution. Auger electron spectra taken in differential mode are shown in Figure 4 for the ZnxNi1-xO solid solutions, and apart from a small amount of carbon contamination, all samples are free from impurities to within the detection limit. The spectra can be used to obtain quantitative information on the surface concentrations, and the nickel L3M4,5M4,5 and zinc L3M4,5M4,5 Auger transitions were used to determine the metal composition


xs )

(IZn/SZn) INi IZn + SNi SZn

(

)

Gaskell et al.

(3)

where xs is the fractional concentration of zinc at the solid solution surface, Si are the relative sensitivity factors, and Ii are integrated Auger intensities for the particular element, i, and Auger transition. Equation 3 assumes a uniform concentration distribution within the Auger sampling depth and sensitivity factors that contain corrections for the spectrometer transmission function. The relative sensitivity factors SNi (1.480) and SZn (1.534) were obtained from NiO and ZnO standard samples relative to the area of the O KL2L2 peak (SO ) 1.000) to obtain Si metal values relative to unit oxygen concentration. In the AES spectra of the solid solutions, the Zn L3M2,3M2,3 peak overlaps with that of the Ni L3M4,5M4,5 transition, and the absolute intensity due to zinc in the overlapping peak was removed by measuring the Zn L3M4,5M4,5 integrated intensity and calibrating the Zn L3M2,3M2,3/Zn L3M4,5M4,5 ratio with intensities obtained from the pure ZnO reference sample. Surface concentrations are summarized in Table 2 and are plotted as a function of bulk concentration in Figure 5. XPS data can be used to obtain surface composition in a similar manner if the corresponding sensitivity factors are substituted into eq 3. Sensitivity factors used in the analysis were obtained by calibration with NiO and ZnO reference samples and are reported relative to the O 1s intensity as SNi/ SO ) 6.792 and SZn/SO ) 7.249. These values are plotted as a function of bulk x concentrations in Figure 5, along with the surface concentration calculated from AES data. The data indicate that the surface is stoichiometric to slightly enriched in zinc concentration over the stable solubility range for samples prepared under the presently employed thermal diffusion synthesis conditions but that as the solubility limit of x ≈ 0.30 is exceeded there is a pronounced enrichment of surface zinc concentration upon phase separation. Due to the greater surface sensitivity of the XPS measurement under the present spectroscopic conditions, XPS shows greater enhancement in zinc than does the corresponding AES measurement. The O 1s XPS spectra are shown in Figure 6 for a series of ZnxNi1-xO samples, and depending upon the zinc concentration, the samples can contain up to three different resolvable oxygen species, detectable as distinctly separate peaks in the O 1s spectrum. O 1s spectra for NiO and for solid solutions of ZnxNi1-xO are similar and show two different species, a lattice oxygen peak at 529.5 eV and a hydroxyl peak at 531.4 eV, and these values are in agreement with previously reported literature binding energies for NiO.54 The O 1s spectrum of ZnO can also be fit to two peaks, the zinc oxide lattice peak at 530.1 eV and a hydroxyl species at 531.4 eV, again in good agreement with literature values.55 After phase separation, good fits to the data can, however, only be achieved with three peaks from the NiO/ ZnxNi1-xO lattice oxygen, the ZnO lattice oxygen, and a combined ZnxNi1-xO/ZnO hydroxyl peak that is not uniquely resolvable into ZnxNi1-xO-hydroxyl and ZnO-hydroxyl contributions. In the region where phase separation has occurred, lattice peaks observed for the NiO (peak 1) and ZnO (peak 2) spectra have been used to fit the ZnxNi1-xO data along with that of a common hydroxyl peak 3. The relative intensity of ZnO peak 2 can be taken to be proportional to the ZnO that has phase-separated from the ZnxNi1-xO solid solution. The fit to the O 1s region implies that even at the bulk homogeneous concentration of x ) 0.3 some phase separation has begun to appear at the surface. This result is in agreement with XPS and

TABLE 2: Bulk vs Surface x Values for the ZnxNi1-xO Samples Derived from Auger and XPS Data ZnxNi1-xO (bulk x)

Auger surface composition (surface x)

XPS surface composition (surface x)

0 0.05 0.1 0.2 0.3 0.4a 0.5a

0.00 0.12 0.088 0.295 0.348 0.444 0.501

0.00 0.132 0.161 0.340 0.590 0.645 0.726

a

Phase separation is observed for samples with x > 0.3.

Figure 5. Surface zinc mole fraction calculated from AES spectra and Ni 2p, Zn 2p, O 1s XPS spectra for ZnxNi1-xO vs the net bulk zinc composition. Note that bulk compositions for x g 0.4 have phaseseparated.

AES data that indicate preferential migration of zinc to create a zinc-enriched surface and leads to the conclusion that phase separation occurs somewhat earlier at the surface than in the bulk. The effect of the zinc on ZnxNi1-xO electronic properties can be investigated by UHF calculations of the band structure coupled with analysis of the Ni 2p XPS satellite structure (Figure 7), which has been shown to be sensitive to the band structure of rocksalt NiO.28,56-61 There is a long-standing and continuing controversy over the exact nature of the nickel oxide band structure, with Mott-Hubbard62-66 and charge-transfer insulator22,67-69 models representing two theories describing the insulating nature of the partially filled Ni 3d8 valence band. In the Mott-Hubbard model, the band gap results from Coulomb interactions among the strongly correlated Ni 3d electrons, opening up a band gap within the 3d levels. In the chargetransfer insulator model, the 3d nickel valence band structure shows admixture with oxygen 2p levels

Ψvalence ) R3dn + β3dn+1L + γ3dn+2L2

(4)

to produce a localized band with significant amounts of O 2p character at the top of the valence band. Here, R, β, and γ are mixing parameters, and L represents a hole in the O 2p band. A more accurate electronic structure might better be described as a combination30,70 of Mott-Hubbard and “pure” charge transfer, as defined by eq 4, since a band gap is clearly present within the Ni 3d levels but is not solely due to d-d Coulomb interactions. The amount of O 2p character in the top of the valence band and the nature of the band gap is still under active debate. To illustrate the nature of the NiO electronic structure and the effect that dissolution of zinc has on the ZnxNi1-xO electronic


J. Phys. Chem. C, Vol. 111, No. 37, 2007 13917

Figure 6. O 1s spectra for various compositions of ZnxNi1-xO fit with (1) ZnxNi1-xO “NiO”-like lattice peak, (2) wurtzite ZnO lattice peak, and (3) combined ZnxNi1-xO + ZnO hydroxyl peak.

Figure 7. Curve fit Ni 2p XPS spectra for various compositions of ZnxNi1-xO, where peaks labeled 1, 2, and 3 are associated with 2p53d9L, 2p53d9L′, and 2p53d.8 Note that the 50% ZnO spectrum results from a sample that is phase-separated into wurtzite ZnO and Zn0.3Ni0.7O and shows satellite structure equivalent to the ZnO 30% sample.

properties, the total (DOS) and projected (PDOS) densities of states are shown for NiO and Zn0.25Ni0.75O in Figure 8. The NiO band structure has previously been calculated by UHF periodic methods,23 and the DOS and PDOS for NiO shown in Figure 8a are in agreement with these results. The top of the valence band comprises predominantly O 2p states with a smaller contribution from Ni 3d, the bottom of the band is primarily nickel 3d in nature, and the center shows both O 2p and Ni 3d character. The bottom of the conduction band is composed mainly of Ni 3d eg states. The DOS thus shows a band gap has formed within the Ni 3d levels but with substantial O 2p charge-transfer character at the top of the valence band. Mu¨lliken population analysis yields cation/anion charges of (1.876 close to the formal q ) (2, and the slightly negative

net bond overlap population of QNi-O ) -0.012 indicates crystal stabilization to have a large electrostatic component. The chargetransfer “hybridization” should, therefore, be evaluated with consideration of the substantial ionic nature of the crystal structure. The effect on the band structure of placing a d10 zinc ion into the NiO rocksalt lattice is shown in Figure 8b for Zn0.25Ni0.75O. The nickel 3d band structure appears relatively unaffected by the presence of the zinc and the Ni 3d PDOS is comparable for NiO and Zn0.25Ni0.75O, with the exception of a slight sharpening of the 3d eg conduction band that implies a proportionately greater localization of this state. The Zn 3d electrons are found in a tight, localized structure at the bottom edge of the NiO valence band. The calculation predicts insignificant splitting due to the crystal field, as expected for a d10 metal, which does not gain in energy from crystal field stabilization. The Zn 3d levels do not participate in bonding within the material to any significant extent, and no Zn 3d character is found above the band gap. The zinc does interact with the lattice oxygen, however, as can be seen in the O 2p PDOS, which sharpens in the Zn0.25Ni0.75O valence band and shifts some of the weight in intensity found in NiO down from the valence band maximum to overlap in energy with a portion of the zinc valence orbitals. Zinc interaction is primarily through Zn 4s, although a large fraction of the 4s states and essentially all of the 4p states are unoccupied and are found in high-lying structure above the unoccupied Ni 3d levels at the top of the conduction band. The more substantial interaction of zinc valence 4s, over that of the 4p levels, is a direct consequence of the octahedral environment of the rocksalt lattice, since 4p orbitals have incorrect symmetry to overlap with the O 2p in the all-bonding octahedral arrangement of the ground state. Mu¨lliken charge analysis is summarized in Table 3 for 16 and 32 ion supercells. The 16 ion supercell construct for Zn0.25Ni0.75O contains 2 zinc ions and 6 nickel ions and allows for five different ion arrangements that preserve the AF2 antiferromagnetic structure of the bulk material. The energy for all five supercell arrangements are within 5 meV per Zn0.25Ni0.75O formula unit of each other and are essentially equal to within the accuracy of the computational method. This is as expected for a random, ideal solid solution. Depending upon the number of next-nearest-neighbor zinc ions, the Mu¨lliken charge calculated for nickel in Zn0.25Ni0.75O is qNi ) 1.8691.872, close to that observed in NiO. However, the charge found for the zinc ion, qZn ) 1.670-1.710, is considerably more ionic than that in the tetrahedrally coordinate wurtzite ZnO, where


Gaskell et al.

Figure 8. Total and projected density of states plots showing the valence and conduction bands for (a) NiO and (b) Zn0.25Ni0.75O calculated by ab initio methods.

Mu¨lliken analysis by similar HF computational methods yielded a much more covalent charge of qZn ) 1.26.43 Overlap populations remain relatively low for all species, as would be expected for predominantly ionic compounds. While there is a slight variation that depends upon the next-nearestneighbor metal distribution, the values remain comparable for the different ion distribution and size of supercell, with Ni-O overlap remaining small and slightly negative and Zn-O remaining small and slightly positive throughout. The positive zinc-oxygen overlap population, QZn-O ) 0.012-0.019, indicates that the nearest neighbor Zn-O interactions are net bonding in character. Zn-O bonding is more covalent than Ni-O nearest-neighbor interactions which, at QNi-O ) -0.009 to -0.014, have comparable overlap populations to that calculated for NiO. The slight variation in Q results from the various possible numbers of zinc next-nearest neighbors, as is indicated in Table 3. The charge-transfer nature of the valence band states (eq 4) leads to satellite structure in the Ni 2p core photoemission spectra22,61,69 due to two general mechanisms

2p6Ψvalence + hν f 2p53d8 + e-

(5a)

f 2p53d9L + e-

(5b)

where eq 5b is a direct result of the O 2p f Ni 3d chargetransfer character and places a hole in a neighboring lattice oxygen O 2p level. The satellite structure varies quantitatively as a function of x for the ZnxNi1-xO solid solutions (Figure 7) and the Ni 2p3/2 component can be fit with a minimum of three peaks, a main feature composed of a doublet at approximately 854 and 856 eV (peaks 1 and 2) and a broader, less intense

peak at approximately 863 eV (peak 3). This pattern is repeated in the 2p1/2 spectral feature, and each spectrum in Figure 8 has been fit to six peaks using a 40%/60% Gaussian/Lorentzian sum after the removal of the Shirley background.38,39 Quantitative plots of the satellite peak intensities as a function of zinc concentration are given in Figure 9. Both peaks 2 and 3 decrease with zinc concentration. Peaks 1 and 2 have been assigned20-22,71-74 to photoemission through eq 5b to produce a 2p53d9L final state and collectively are often designated as the “main” peak in the Ni 2p3/2 spectrum. The broad satellite at ∼863 eV (peak 3) has previously been associated in the literature with the more poorly screened 2p53d8 mechanism of eq 5a, but the decrease in intensity clearly points to a contribution from a second mechanism. The band structure calculations (Figure 8) show substantial Ni 3d-O 2p hybridization at the top of the valence band and DOS with primarily Ni 3d character at about 6-8 eV higher binding energy and remain consistent with this assignment. Although a small shift in oxygen intensity away from the valence band edge occurs with zinc hybridization in the ZnxNi1-xO solid solution, the reduced concentration of nickel cations requires proportionately fewer oxygen states for hybridization, and the Ni 3d-O 2p interactions are comparable in ZnxNi1-xO and NiO as demonstrated by the Mu¨lliken charge and nearest-neighbor overlap populations. The doublet structure of the main peak can be attributed to the presence of two different photoemission channels leading to the 2p53d9L final state, one involving a single nickel site screened by a shift of electron density from the surrounding nearest-neighbor oxygen 2p to the nickel 3d level (peak 1) and the second involving a nonlocal effect that requires adjacent


J. Phys. Chem. C, Vol. 111, No. 37, 2007 13919

TABLE 3: Mu1 lliken Charges and Overlap Populations from Ab Initio Calculations for Antiferromagnetic Zn0.25Ni0.75O and NiO ion (no. of ions in equivalent environmenta) Zn (2) Ni (6) O (2) O (6) Zn (2) Ni (4) Ni (2) O (4) O (4)

ion charge (e)

nearest neighbors

next nearest neighbors

Zn0.25Ni0.75O 16-Ion Supercell (Lowest Energy)b 1.670 6O 12 Ni 1.872 6O 8 Ni/4 Zn -1.867 6 Ni 12 O -1.807 2 Zn 4 Ni 12 O Zn0.25Ni0.75O 16-Ion Supercell (Higher Energy Cluster)c 1.719 6O 10 Ni /2 Zn 1.872 6O 8 Ni/4 Zn 1.869 6O 10 Ni /2 Zn -1.845 1 Zn 5 Ni 12 O -1.821 2 Zn 4 Ni 12 O

O (2)

Zn0.25Ni0.75O 32-Ion Supercelld 1.703 6O 1.727 6O 1.874 6O 1.871 6O 1.873 6O -1.849 1 Zn 5 Ni -1.826 2 Zn 4 Ni -1.786 3 Zn 3 Ni -1.867 6 Ni

Ni (2) O (1)

1.876 -1.876

Zn (2) Zn (2) Ni (4) Ni (2) Ni (6) O (6) O (6) O (2)

NiO 4-Ion Supercell 6O 6 Ni

10 Ni/2 Zn 9 Ni/3 Zn 8 Ni/2 Zn 10 Ni/2 Zn 9 Ni/3 Zn 12 O

nearest neighbor overlap population 0.019 -0.011, -0.014 -0.011 0.019-0.014 0.012, 0.013 -0.012, -0.014 -0.009, -0.011 0.012-0.012 0.013-0.014

12 O

0.009, 0.026 0.010, 0.012 -0.011 to -0.016 -0.010, -0.012 -0.010 to -0.013 0.007-0.010 -0.010 to -0.013 0.008-0.026 -0.011 to -0.014 0.028, 0.012 -0.012, -0.016 -0.011, -0.012

12 Ni 12 O

-0.012 -0.012

12 O 12 O

a Equivalent environment refers to ions with equivalent numbers of nearest and next-nearest neighbors. b There are five possible ways to arrange 2 Zn ions in a 16-ion unit cell while maintaining the antiferromagnetic nature; this structure represents the lowest-energy structure. c Lies above the lowest-energy structure by 5 meV per Zn0.25Ni0.75O formula unit. d The 32 atom supercell was chosen to show the largest variation in next-nearestneighbor distributions within a supercell and is one of many possible arrangements.

Figure 9. Ni 2p peak composition versus the mole fraction of zinc (x) as detailed in the legend. Peaks 1, 2, and 3 are labeled consistently in Figure 7 and in the text. Peak 3 contains contribution from both local and nonlocal effects, and no attempts were made to fit peak 3 to the individual contributions.

nickel cation occupancy (peak 2). Thus, although the zinc only indirectly affects the nickel electronic structure, it has a measurable effect on the nickel 2p photoemission by diluting the nickel concentration in the rocksalt lattice and decreasing the number of nickel-occupied sites adjacent to the cation undergoing photoemission. Nonlocal interactions can be totally eliminated in the nickel impurity limit, and doping MgO with Ni2+ to produce a very dilute solid solution, NixMg1-xO with x ≈ 0.03,60,61 has been shown to effectively eliminate the doublet structure to produce a single main peak in the Ni 2p3/2 spectrum. The data presented here show that the effect of diluting neighboring nickel site occupancy is equally dramatic in nickel

2p photoemission at the high nickel concentration limit and sets limits on the nonlocal channel to require a mechanism in which neither magnesium nor zinc can effectively participate. Several magnetic coupling schemes15,75-77 have been proposed to account for the doublet structure, including one that assigns peak 2 to the same 2p53d9L final state as peak 1 but accompanied by dipole-forbidden (∆S ) 1) triplet to singlet d-d transitions made possible by Ni-Ni antiferromagnetic coupling. Within the context of this model, an increase of zinc serves to dilute the antiferromagnetic coupling in the ZnxNi1-xO, and a decrease of intensity would be expected for photoemission features that depend upon antiferromagnetic interactions in the material. Furthermore, the 1.8 eV separation between peaks 1 and 2 is compatible with magnetically allowed 3A2g f 1Eg and 3A 1 2g f T2g transitions that are observed in NiO optical spectra.78,79 Similar energetics have been observed in resonant inelastic X-ray scattering studies,15 which postulate a doublespin flip between adjacent antiferromagnetically coupled nickel cations in the NiO lattice. However, long-range antiferromagnetic ordering does not appear to be a factor in the Ni 2p peak shape. As shown in Figure 10, the relative intensity of peak 2 to peak 1 appears insensitive to antiferromagnetic ordering in nickel oxide, and the doublet peak structure persists well above the Neél temperature. Similar insensitivity to antiferromagnetic ordering has been found for the NiO valence band spectrum.19 An additional complication is that a similar decrease in peak intensity is observed with increasing zinc concentration for the 863 eV satellite peak (peak 3), which is generally attributed to the inherently local 2p53d8 final state mechanism. However, Ni 2p photoemission is considerably more complex than this simple assignment implies, and peak shape features could potentially include contributions of varying importance from d-d transi-


Gaskell et al.

Figure 10. Curve fit Ni 2p XPS spectra of the NiO (100) single-crystal surface taken at a series of temperatures, from 298 K (well below the Neél temperature) to 640 K (well above the Neél temperature). In these spectra, the peak 2/peak 3 intensity ratio is constant at 0.35 ( 0.05 regardless of substrate temperature.

tions, nonlocal screening mechanisms, and 2p53d10L2 final states. In the latter case, Coulomb repulsion in the L2 final state can be expected to at least partially offset the extra Ni 3d10 screening and could potentially shift the energy of the feature associated with this state into the satellite structure to yield overlapping peaks. That peaks 2 and 3 decrease comparably in intensity as a function of zinc concentration (Figure 9) points to a common channel in the photoemission mechanism that is eliminated as zinc replaces nickel in adjacent cation sites. Configurations with full charge-transfer character,19,20,22,61 5 2p 3d10L2, can be expected to yield photoemission features that overlap in energy with those from 2p53d8 states in the broad satellite structure of the Ni 2p spectrum. Furthermore, cluster calculations61 that localize holes on different oxygen lattice sites to simulate the photoemission process indicate that two channels can be envisioned leading to the charge transfer of a single hole. The first, local channel, given by eq 5b, involves charge transfer between oxygen in immediate octahedral coordination to the nickel directly undergoing photoemission to give the 2p53d9L state of peak 1. NiO6 cluster calculations, containing only a single metal, show evidence of this channel and a single-peak structure in the main Ni 2p3/2 transition. The second, nonlocal channel60,61,80-84 results from charge transfer from oxygen to nickel in an octahedral cluster adjacent to the photoemitting cation

2p6ΨValence + hν f 2p53d9L′ + e-

(6)

where 3d9L′ refers to charge transfer in a neighboring octahedral cluster to the nickel with the 2p5 core hole. The energetics of this channel agree well with the peak 2 binding energy, and simulated photoemission from clusters containing more than one nickel result in the familiar main peak doublet structure. The requirement for a common photoemission channel in peaks 2 and 3, along with the predicted energies of the 2p53d8, 2p53d9L, and 2p53d10L2 features in the Ni 2p spectrum and the two possible channels in the 2p53d9L photoemission mechanism suggest that peak 3 is composed of at least two significant

contributions, one resulting from the unscreened mechanism leading to the 2p53d8 final state and a second resulting in a doubly screened process to yield a 2p5(3d9L)(3d9L′) state with both local and nonlocal oxygen screening character. The 2p53d8 state that is commonly associated with the satellite feature must be present to agree with the single metal site character that gives the satellite intensity even for dilute nickel solutions in MgO;60 the nonlocal 3d9L′ character must be present since peak 2 and peak 3 are both quenched with increasing zinc concentration in ZnxNi1-xO and thus must both have nonlocal characteristics. The decrease in nonlocal screening with increased zinc content can be attributed to dilution of nickel in the ZnxNi1-xO solid solution, resulting in a decreased probability of any given NiO6 cluster having NiO6 clusters as neighbors. In the PDOS for Zn 3d, the band structure due to Zn 3d eg and t2g orbitals appear just below -10 eV as two intense, closely spaced but discrete states. Zinc 3d10 states are filled and are thus unsuitable for participation in charge-transfer mechanisms that shift electron density onto the metal site, and they are too low in energy to overlap efficiently with O 2p states. The unoccupied 4s and 4p orbitals are too high in energy and do not appear to be able efficiently participate in charge transfer through overlap with O 2p occupied states. Thus, no likely mechanism exists for transfer of a hole to the oxygen, and the next nearest neighboring ZnO6 clusters do not contribute to nonlocal screening. Conclusions XRD results show that homogeneous ZnxNi1-xO rocksalt solid solutions in concentrations up to Zn0.32Ni0.68O have been formed in this nickel-rich portion of the ZnO-NiO phase diagram. The rocksalt lattice parameter increases linearly with zinc content in agreement with Vegard’s law until bulk-phase separation occurs to produce a mixture of wurtzite ZnO and a solid solution with a rocksalt lattice parameter of ∼Zn0.3Ni0.7O. The metal ratio at the surface of the solid solution is comparable to that of the bulk up to the point of bulk-phase separation, although a slight preferential migration of zinc to the surface appears to result in premature surface phase separation prior to the 32% ZnO composition observed in the bulk. After bulk-phase separation, the surface enrichment of wurtzite ZnO becomes substantial. Auger parameter analysis indicates the zinc to have octahedral coordination, expected in a rocksalt structure. This coordination is unusual for Zn2+, which prefers tetrahedral coordination in the solid state. Hartree-Fock studies of the ground state DOS and Mu¨lliken population analysis comparing NiO and Zn0.25Ni0.75O show an increase in oxygen admixture and covalent bonding at the valence band maximum due to overlap of O 2p and Zn 4s orbitals. Very little hybridization is observed in the PDOS of the Zn 4p states, which have incorrect symmetry for octahedral bonding in the ground-state wave function, and the zinc is less covalent in the ZnxNi1-xO solid solution than found in wurtzite ZnO. The Zn 4s-O 2p hybridization results in a detectable shift of O 2p character away from the valence band maximum to more strongly bonding energies. Ab intitio UHF DOS calculations and Mu¨lliken population analysis show that the Ni 3d-O 2p interactions are largely unaffected by the dissolution of zinc into the rocksalt NiO lattice. Zinc serves primarily to dilute nickel-nickel coupling interactions, however, and has a significant effect on the Ni 2p XPS peak shape. The intensity of the leading feature of the main peak, assigned to a locally screened 2p53d9L mechanism, remains proportional to the nickel concentration. However, the

ZnxNi1-xO Rocksalt Oxide Surfaces shoulder peak in the main feature (3d9L′) and the broad satellite at ∼863 eV decrease in intensity in direct proportion to zinc dilution of nonlocal screening effects. The latter peak has been assigned to a combination of unscreened 2p53d8 and nonlocally screened 2p5(3d9L)(3d9L′) character. Acknowledgment. We gratefully acknowledge support from the National Science Foundation through Grant No. CHE0213320. References and Notes (1) Navrotsky, A.; Muan, A. J. Inorg. Nucl. Chem. 1971, 33, 35. (2) Hahn, W. C.; Muan, A. J. Phys. Chem. Solids 1961, 19, 338. (3) Kedesdy, H.; Drukalsky, A. J. Am. Chem. Soc. 1954, 76, 5941. (4) Rigamonti, R. Gazz. Chim. Ital. 1946, 76, 974. (5) Vainsthtein, B. K.; Fridkin, V. M.; Indenbom, V. L. Modern Crystallography, 3rd ed.; Springer: New York, 2000; Vol. 2, p 156. (6) Smart, J. S.; Greenwald, S. Phys. ReV. 1951, 82, 113. (7) Marynowski, M.; Franzen, W.; El-Batanouny, M.; Staemmler, V. Phys. ReV. B 1999, 60, 6053. (8) Jauch, W.; Reehuis, M. Phys. ReV. B 2004, 70, 195121. (9) Springthorpe, A. J. Phys. Status Solidi 1967, 24, K3. (10) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Phys. ReV. B 1998, 57, 1505. (11) Nesbitt, H. W.; Lengrand, D.; Bancroft, G. M. Phys. Chem. Miner. 2000, 27, 357. (12) Hufner, S. Solid-State Sciences: Photoelecton Spectroscopy, 2nd ed.; Springer: Heidelberg, Germany, 1996; p 185. (13) Matsubara, M.; Uozumi, T.; Kotani, A.; Parlebas, J. C. J. Phys. Soc. Jpn. 2005, 74, 2052. (14) Magnuson, M.; Butorin, A. A.; Nordgren, J. J. Phys.: Condens. Matter 2002, 14, 3669. (15) Duca, L.-C.; Schmidt, T.; Magnuson, M.; Forsberg, J.; Olsson, A.; Nordgren, J.; Okada, K.; Kotani, A. Phys. ReV. Lett. 2006, 96, 067402. (16) Hu¨fner, S.; Schmidt, S.; Mu¨ller, F. Phys. ReV. Lett. 2006, 97, 269701. (17) Duca, L.-C.; Schmidt, T.; Magnuson, M.; Forsberg, J.; Olsson, A.; Nordgren, J.; Okada, K.; Kotani, A. Phys. ReV. Lett. 2006, 96, 269702. (18) Nakajima, N.; Kato, H.; Sakisaka, Y. J. Electron Spectrosc. Relat. Phenom. 2005, 873, 144-147. (19) Tjernberg, O.; Soederholm, S.; Rogelet, T.; Karlsson, U. O.; Qvarford, M.; Lindau, I.; Almbladh, C.-O.; Hellbom, L. J. Vacuum 1995, 46, 1215. (20) McKay, J. M.; Henrich, V. E. Phys. ReV. Lett. 1984, 53, 2343. (21) Sawatzky, G. A.; Allen, J. W. Phys. ReV. Lett. 1984, 53, 2339. (22) Fujimori, A.; Minami, F. Phys. ReV. B 1984, 30, 957. (23) Towler, M. D.; Allan, N. L.; Harrison, N. M.; Saunders, V. R.; Mackrodt, W. C.; Apra`, E. Phys. ReV. B 1994, 50, 5041. (24) Takahashi, H; Munakata, F.; Yamanaka, Y. J. Phys.: Condens. Matter 1995, 7, 1583. (25) Noguera, C.; Mackrodt, W. J. Phys.: Condens. Matter 2000 12, 2163. (26) Kuzmin, A.; Purans, J.; Kalendarev, R. Phys. Status Solidi C 2005, 2, 665. (27) de P. R. Moreira, I.; Illas, F.; Martin, R. L. Phys. ReV. B 2002, 65, 155102. (28) Ren, X.; Leonov, I.; Keller, G.; Kollar, M.; Nekrasov, I.; Vollhardt, D. Phys. ReV. B 2006, 74, 195114. (29) Faleev, S. V.; Van Schilfgaarde, M.; Kotani, T. Phys. ReV. Lett. 2004, 93, 126406. (30) Zhang, W.-B.; Hu, Y.-L.; Han, K.-L.; Tang, B.-Y. Phys. ReV. B 2006, 74, 054421. (31) Patterson, C. H. Int. J. Quantum Chem. 2006, 106, 3383. (32) Ferrari A. M.; Pisani C. J. Phys. Chem. B 2006, 110, 7909. (33) Rodic, D.; Spasojevic, V.; Krusigerski, V.; Tellgren, R.; Rundlof, H. Phys. Status Solidi B 2000, 218, 527. (34) Denton, A.; Ashcroft, N. Phys. ReV. A 1991, 43, 3161. (35) Wagner, C.; Joshi, A. J. Electron Spectrosc. Relat. Phenom. 1998, 47, 283. (36) Petitto, S. C.; Marsh, E. M.; Langell, M. J. Phys. Chem. B 2006, 110, 1309. (37) Nydegger, M.; Couderc G.; Langell, M. Appl. Surf. Sci. 1999, 147, 58. (38) Shirley, D. Phys. ReV. B 1972, 5, 4709. (39) Vegh, J. J. Electron Spectrosc. Relat. Phenom. 2006, 151, 159.

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ZnxNi1-xO Rocksalt Oxide Surfaces: Novel Environment for Zn2+ and

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