Hydration of MgO(100) Surface Promoted at 011 Steps - American

Mar 31, 2015 - Akira Sasahara,* Tatsuya Murakami, and Masahiko Tomitori. Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923-1292, ...
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Hydration of MgO(100) Surface Promoted at ⟨011⟩ Steps Akira Sasahara,* Tatsuya Murakami, and Masahiko Tomitori Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923-1292, Japan ABSTRACT: Dissolution of magnesium oxide (MgO)(100) surfaces in water was examined by an atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) techniques. MgO(100) surfaces annealed at 1273 K in air for 288 h showed rectangular terraces surrounded by multiatomic height steps along the ⟨011⟩ directions. After immersion in water, bank-like structures appeared along the ⟨011⟩ steps: the end of the terraces remained smooth, while the inner regions of the terraces were roughened and subsided with respect to the end. Epitaxial growth of Mg(OH)2 from the ⟨011⟩ steps toward the inner regions of the terraces is proposed.

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INTRODUCTION Dissolution in water has been an intensely studied topic in the research of magnesium oxide (MgO).1−3 One major motivation for the dissolution studies is the importance of the MgO as a passivation layer on Mg. Detailed understanding of the dissolution property may lead to an effective use of the MgO passivation layer. A representative application of Mg and Mg alloys is in the structural components of portable electronic devices, cars, airplanes, and so on, where their lightweight, high specific stiffness, and easy machinability are exploited.4−6 Such Mg-based components have to maintain mechanical properties in humid or aqueous environments, and their life duration depends on the water resistance of the surface MgO layer. An application of the Mg and Mg alloys actively using the water solubility of the surface MgO layer is for safe implants.7,8 The Mg-based implants are required to be degraded in the body in a proper period to avoid explantation surgery. Thus, water solubility of the MgO is a key property that determines the performance of the Mg-based products. Applying surface-sensitive techniques on a single crystal surface is beneficial in gaining a detailed insight into the properties of oxide surfaces.9 Among the low index surfaces of MgO, the (100) surface with the lowest surface energy10 has been most widely studied by surface science methods. Komiyama et al. first reported an observation of dissolution of polished MgO(100) surfaces in water, where growth of etch pits with irregular shapes was confirmed by in situ atomic force microscope (AFM) imaging.11 Jordan et al. estimated the dissolution rate of cleaved MgO(100) surfaces in HCl aqueous solution by surface subsidence analysis using the AFM.12 Formation of Mg(OH)2-like surface layer was concluded on the basis of the fact that the dissolution rate of the surfaces was comparable to that of Mg(OH)2. Dependence of the dissolution rates in the HCl aqueous solution on crystallographic planes was examined by Suárez et al.13 Polished (110) and (111) surfaces were found to dissolve forming ridges and © 2015 American Chemical Society

etch pits composed of (100) sidewalls, respectively. Thereby dissolution rates of the two surfaces were comparable to that of the (100) surface. In the dissolution experiment of the polished MgO(100) surfaces in HNO3 aqueous solution by Mejias et al., inverse pyramidal etch pits with {01l} (1 ≤ l ≤ 4) sidewalls, different from a thermodynamically more preferable hydroxylated (111) surface, appeared on the surface.14 These authors concluded that the surface morphology was under kinetic control. The effect of pH of the aqueous solution was examined by Thissen et al. on MgO(100) surfaces with a step-terrace structure prepared by annealing in air.15 Formation of the etch pits was promoted in an acidic pH range, which was interpreted as the lowering of chemical potential by the cooperative effect of the etch pits, H adatoms, and H2O admolecules. Giner et al. found that dissolution of the air-annealed MgO(100) surface in NaOH aqueous solution was promoted by adding NaCl in the solution. It was proposed that the chloride ions were coordinated to the detached Mg ions and hindered the formation of the Mg(OH)2 layer that suppressed the dissolution by trapping the eluted ions.16 The MgO(100) surfaces used in the past studies dissolved independently of the surface nanostructures, though there remains a possibility that point defects such as atom vacancies, which are smaller than the spatial resolution of the AFM, triggered the dissolution. This paper reports the role of steps in the dissolution of the MgO(100) surface. One finding is that the MgO(100) surface showed rectangular terraces separated by steps along the orthogonal [011] and the [01̅1] directions after long-time annealing. The [011] and the [01̅1] of the MgO with a rock salt structure are equivalent and denoted as ⟨011⟩. The ⟨011⟩ steps were attributed to the stabilization of the {011} surfaces by hydroxylation. Another finding is that bankReceived: February 21, 2015 Revised: March 31, 2015 Published: March 31, 2015 8250

DOI: 10.1021/acs.jpcc.5b01759 J. Phys. Chem. C 2015, 119, 8250−8257

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The Journal of Physical Chemistry C like structures appeared along the ⟨011⟩ steps after immersion in water. It was proposed that the ⟨011⟩ steps provided {111} surfaces at their corners and that the {111} surfaces promoted the epitaxial growth of Mg(OH)2 with lower water solubility.

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EXPERIMENTAL METHODS Samples were prepared under laboratory air conditions, where the temperature was around 293 K and the humidity was around 50%. Mirror-polished MgO(100) wafers (orientation accuracy: ±0.3°, 10 × 10 × 0.5 mm3, Shinkosha) were first degreased by successive sonication in acetone, ethanol, and Milli-Q water. The wafers were then heated in a sapphire tube for a tube-type electric furnace at 1273 K for 96 h and cooled for 6 h to room temperature. The 96 h annealing was repeated three times to attain annealing of 288 h. The sapphire tube was free from vaporizing and was more suitable for long-time annealing than quartz tube that evaporates SiOx.17 The dissolution test was performed in 100 mL of nonpurged Milli-Q water at room temperature. The pH of the Milli-Q water was reduced from 6.8 to 6.1 in 2 h due to the dissolution of CO2 and remained 6.1 for a day. Contact mode AFM images were obtained in air using a commercial multipurpose scanning probe microscope (SPM 5500, Agilent technologies). Fresh silicon nitride cantilevers with a spring constant of 0.6 N/m (OMCL-TR800PSA-1, OLYMPUS) were used in each measurement. The applied force was estimated to be ∼10−8 N. The images were presented without filtering, and the cross sections and the root-meansquare surface roughness were measured after smoothing with a nine-point median filter. X-ray photoelectron spectroscopy (XPS) analysis was performed using a commercial system (Axis Ultra DLD, Kratos) with a base pressure of 1 × 10−7 Pa. Monochromatic Al Kα was used as an excitation source. The photoelectron emission angle with respect to surface normal, θ, was set to 0°. The pass energy of the analyzer and the energy step were 160 and 1.0 eV for wide scans, respectively, and 20 and 0.1 eV for narrow scans, respectively. Charging of the sample surfaces was compensated by a neutralizer. The spectra were deconvoluted into mixed Gaussian−Lorentzian curves (70:30) after Shirleytype background subtraction. Mg 2p peaks with a small spin− orbit splitting18 were fitted by single curves. Binding energies of the spectra were calibrated so that the binding energy of the O 1s peak of lattice O of MgO became 532.0 eV. The calibration was based on the O 1s spectrum of a mixed powder of MgO (99.9%, Wako Pure Chemical Industries) and rutile TiO2 (>99.99%, Kojundo Chemical Laboratory), where the binding energy of the O 1s peak of lattice O of TiO2 was assumed to be 530.3 eV.19 Optics for LEED (SPECTALEED, Omicron) were installed in a homemade ultrahigh vacuum chamber with a base pressure of 2 × 10−8 Pa.

Figure 1. (a−c) AFM images of the MgO(100) surfaces (1000 × 1000 nm2). (a) As-received surface, (b) surface annealed for 96 h, and (c) surface annealed for 288 h. The inset in the image (b) shows a wide scan image of the surface (3000 × 3000 nm2). (d) Cross sections along the solid lines in the images (a−c). (e) A LEED pattern of the surface annealed for 288 h. Incident electron energy was 133 eV. (f) A narrow-scan AFM image of the 288 h annealed surface (100 × 100 nm2). (g) A cross section along the solid line in the image (f).

straight along the ⟨011⟩ directions, and the remaining 65% were curved. The heights of the steps were integral multiples of the height of the monatomic step of the MgO along the [100] axis, 0.21 nm. The steps along the cross section (2) in Figure 1d had the heights of 0.82, 1.05, 0.81, and 2.06 nm, which correspond respectively to four-, five-, four-, and ten-atomic heights. Particles with heights of 20−60 nm and diameters of 50−200 nm were nonuniformly dispersed on the surface as shown in the inset wide scan image. The particles are probably oxides of intrinsic impurities of the MgO wafer. Calcium, a typical impurity in MgO, segregated onto the surface to form oxide particles when the MgO was annealed in air.20 The density of the impurity particles was 1.5 × 10−6 nm−2. The topography of the surface shown in Figure 1b drastically changed after the 96 h annealing was repeated twice more. The

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RESULTS AND DISCUSSION Figure 1a−c shows the AFM topography images of the MgO(100) surfaces. The as-received surface showed a rough and irregular appearance as shown in Figure 1a. Fluctuation in height reached 0.5 nm as shown in the cross section (1) in Figure 1d, and the surface roughness was 0.8 nm. A step-terrace structure appeared on the surface after annealing for 96 h as shown in Figure 1b. The surface roughness on the terraces decreased to 0.3 nm, and the density of the steps recognized in Figure 1b was 2 × 10−2 nm−1. A total of 35% of the steps were 8251

DOI: 10.1021/acs.jpcc.5b01759 J. Phys. Chem. C 2015, 119, 8250−8257

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The Journal of Physical Chemistry C surface exhibited rectangular terraces separated by the ⟨011⟩ steps with heights more than 2 nm (10 atomic height) as shown in Figure 1c. The edges of the pits on the terraces were also parallel to the ⟨011⟩ directions. Curved steps of one-, two-, and three-atomic heights were occasionally observed in the terraces. Such curved steps were indicated by the arrows in Figure 1c. The cross section (3) in Figure 1d includes four ⟨011⟩ steps and two curved steps. The ratio of the ⟨011⟩ steps rose to 85%, while the density of the steps decreased to 1.3 × 10−2 nm−1. The terraces appeared uniformly flat and smooth, and the roughness on the terraces further decreased to 0.2 nm. The (1 × 1) LEED pattern with streaks shown in Figure 1e indicated that the surface atoms were largely arranged with the bulk-terminated periodicity but were locally disordered. The density of the impurity particles slightly increased to 3.0 × 10−6 nm−2. The surface topography shown in Figure 1c, consisting of the rectangular terraces surrounded by the ⟨011⟩ steps, differs from the topography previously reported for the MgO(100) surfaces annealed in oxidative atmospheres. The surface annealed at 1273 K in pure O2 of 1 atm exhibited the terraces separated by steps with irregular edges.21 The steps had one or two atomic heights and were arranged roughly parallel with the [011] direction. A similar step-terrace structure was observed also on the surface annealed at 1373 K in air.22 The nearly parallel steps with irregular edges probably reflect the deviation of the surface orientation from the [100] axis. Benedetti et al. succeeded to fabricate such parallel steps by controlling the miscut angle to 2°.23 Ota et al. found that the ⟨011⟩ steps were formed around the CaO particles.20 However, the ⟨011⟩ steps in Figure 1b,c are obviously independent of the impurity particles. Emergence of the ⟨011⟩ steps with multiatomic heights indicates that stable {011} surfaces were formed between two terraces. Surface energy of the nonreconstructed (011) surface is predicted to be more than twice as high as that of the nonreconstructed (100) surface.10,24 De Leeuw et al. showed in their Born model calculation that the surface energy of the (011) surface almost halved by faceting to form ridges consisting of the {100} sidewalls with the height of 0.59 nm.24 Faceting to form the {100} surfaces was concluded in the LEED analysis of a clean MgO(111) surface.25,26 De Leeuw et al. also reported that the hydroxylation further reduced the surface energy of the MgO surfaces.24 When fully hydroxylated, surface energy of the faceted (011) surface decreased to less than that of the nonreconstructed (100) surface. Unfortunately, zigzag morphology with widths less than 1 nm as a result of the faceting, as well as the surface hydroxyl (OH) groups, are below the spatial resolution of the AFM. Figure 1f shows the narrow scan image around the ⟨011⟩ step. A sawtooth-like geometry with the scale of 20 nm, which is indicated by the arrow, was resolved, but more fine structures were not recognized along the steps. Bunching of low steps might have also contributed to the formation of the ⟨011⟩ steps with multiatomic heights. The narrow terrace with a width of ∼10 nm was resolved as a lessbright rectangular region in Figure 1f. The narrow terrace was recognized also as a slight change in the slope of the cross section shown in Figure 1g, where both ends of the terrace are indicated by the arrowheads. Narrower terraces and accompanying steps, even if included in the narrow terrace with 10 nm width, are unlikely to be resolved. The surfaces with the ⟨011⟩ steps exhibited unique topography after immersion in water. Figure 2a shows the

Figure 2. (a−c) AFM images of the MgO(100) surfaces with the ⟨011⟩ steps immersed in water for (a) 1, (b) 3, and (c) 24 h (1000 × 1000 nm2). (d) Cross sections along the solid lines in the images. (e) An AFM image of as-received MgO(100) surfaces immersed in water for 24 h (1000 × 1000 nm2). (f) A model for the estimation of the width of bank w from its width along the cross sections in AFM images L. The tip modeled by a sphere with a radius R passes the bank with the heights h1 and h2.

AFM topography image of the surface immersed in water for 1 h. The end of the terraces and the edges of the pits remained smooth in a belt with a width of ∼40 nm, in contrast to the inner regions of the terraces that were roughened to form bumps with diameters of 20−50 nm. The inner regions were lower than the ends by 0.4−0.8 nm as shown in the cross section (1) in Figure 2d and had a roughness of 0.22 nm. Smooth patches as indicated by the arrows were interspersed in the terraces. Roughening and subsidence of the inner regions 8252

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The Journal of Physical Chemistry C indicate that they dissolved faster than the end. The difference in the subsidence depth among the terraces probably reflects the difference in formation rates of the “belts”, because the inner regions of the terraces are expected to have more uniform structures. Dissolution of the surface progressed after an additional 2 h immersion as shown in Figure 2b. The roughness of the inner regions of the terraces increased to 1.2 nm, and their subsidence became 1.3−2.3 nm as shown in the cross section (2) in Figure 2d. The belts appeared like banks due to the increase of the subsidence of the inner regions, and the widths of the “banks” increased to ∼60 nm. Figure 2c shows the surface topography after further immersion for 21 h. The width of the banks, roughness in the inner regions, and subsidence of the inner regions reached ∼90, 2.7, and 7−11 nm, respectively. The banks were not observed on the as-received surface immersed in water as shown in Figure 2e, where the surface was covered by particles with diameters of 20−40 nm and heights of 5−30 nm. Here we consider the growth of the banks in the lateral direction. Lateral size of a protrusion in the AFM image is determined by a convolution of the geometry of the protrusion and the tip. The increase of the widths of the banks does not necessarily mean growth in the lateral direction, because the heights of the banks changed with the immersion time. In the model shown in Figure 2f, the spherical tip of a radius R moving from left to right along the scan line lifts up from the terrace A when the periphery of the sphere touches the bank with height h1. The tip passes the bank with width w and lands on the terrace B lower than the bank by h2 when the periphery of the sphere is separated from the bank. The bank is presented as a hillock with a width of L in the cross section along the scan line. L= =

2 r1 +

2w +

2 r2

⎡ ⎤ ⎛ R − h1 ⎞2 ⎛ R − h 2 ⎞2 ⎥ ⎟ +w+R 1−⎜ ⎟ 2 ⎢R 1 − ⎜ ⎢ ⎝ R ⎠ ⎝ R ⎠ ⎥⎦ ⎣ (1) Figure 3. (a) Wide-scan XPS spectrum of the MgO(100) surface annealed for 288 h. (b) Narrow-scan XPS spectra of the MgO(100) surfaces. Component peaks with thin lines were superimposed on the spectra. The arrowheads in O 1s spectrum of the mixed powder of MgO + TiO2 indicate the peaks from TiO2.

By applying the R of 15 nm, a typical radius of curvature of the cantilever tip, and the h1 and h2 along the cross sections in Figure 2d, ws are estimated to be 15, 28, and 35 nm for the surfaces immersed for 1, 3, and 24 h, respectively. The increase of the widths of the banks in the AFM images reflected their growth in the lateral direction. Change of the chemical composition and structures of the MgO surfaces induced by the dissolution was examined by XPS. Figure 3a shows a wide scan spectrum of the 288 h annealed surface. The O 1s, Mg 2p, and Mg 2s core level peaks were observed at around 530, 90, and 50 eV, respectively.27 The peaks in the range of 300−380 eV and at around 240 eV are Mg Auger peaks.28 In addition to the Mg and O peaks, C, N, Ca, F, Fe, and Ba peaks were observed. Carbon was detected on all the wafers and is from adventitious hydrocarbons. Other trace impurities were detected on some wafers while not on others and are probably intrinsic impurities of the wafers. The topography of the MgO surfaces was independent of the presence of the impurities. Figure 3b shows narrow scan spectra of O 1s and Mg 2p regions of the MgO(100) surfaces. The spectra taken from a mixed powder of MgO and TiO2 are also shown in the bottommost panel for comparison. The O 1s spectrum of the

as-received MgO surface exhibited two peaks. The major peak was from the lattice O of MgO, which is referred to hereafter as OMgO. In order to deconvolute the minor components at the higher binding energy side, the O 1s spectrum of the mixed powder is considered. The two peaks indicated by the arrowheads are from TiO2. The difference in binding energies, height ratio, and width ratios of the peaks was determined from a separate measurement of pure TiO2 powder. The binding energy of the intense peak calibrated to 530.3 eV is from the lattice O of TiO2,26 and the minor peak at 531.7 eV is from the OH groups formed on the TiO2 surface.29 Three additional peaks were necessary, at minimum, to fit the spectrum. The major peak at 532.0 eV is associated with the OMgO. Other two peaks at 533.9 and 534.9 eV were associated with hydroxide and carbonate species, respectively, by referring adsorption studies of H2O30−32 and CO225,33 on MgO(100) surfaces. Hydration to Mg(OH)2 and subsequent carbonation to 8253

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The Journal of Physical Chemistry C Table 1. Intensity Ratios of XPS Peaks on MgO(110) Surfaces as-received surface surface annealed for 96 h surface annealed for 288 h 288 h annealed surface immersed in H2O as-received surface immersed in H2O

OOH/OMgO (Oadd/OMgO)

OMg(OH)2/OMgO

OMgCO3/OMgO

MgMg(OH)2/MgMgO

0.024 0.23 0.15 0.30 0.065

0.36 0.067 0.061 0.59 0.28

0.071 0.034 0.032 0.19 0.067

0.18 0.033 0.029 0.29 0.15

MgCO3 are common for MgO stored in air.34 The O of the Mg(OH)2 and MgCO3 are referred to as OMg(OH)2 and OMgCO3, respectively. The Mg 2p spectra of the mixed powder was fitted by three peaks at 51.6, 52.3, and 53.2 eV. The most intense peak at 51.6 eV is from the lattice Mg of MgO, and the Mg is referred to as MgMgO. The difference in binding energy between the MgMgO and the OMgO peaks was 480.4 eV and was within ±0.3 eV from the differences in the previous studies.30,35 The peaks at 52.3 and 53.2 eV were associated with Mg of Mg(OH)2 and MgCO3, respectively, so that the order of the binding energies were consistent with that in the previous study.22 The Mg of Mg(OH)2 and MgCO3 are referred to as MgMg(OH)2 and MgMgCO3, respectively. The binding energies of the MgMg(OH)2 and MgMgCO3 peaks are higher than that of the MgMgO peak by 0.7 and 1.6 eV, respectively. The binding energy difference between the MgMgO and MgMg(OH)2 peaks of 0.7 eV agrees with that reported by Newberg et al.30 On the other hand, Aswal et al. reported that the binding energies of the MgMg(OH)2 and MgMgCO3 peaks were higher than that of the MgMgO peak by 0.2 and 1.0 eV, respectively.22 Small binding energy differences among MgMgO, MgMg(OH)2, and MgMgCO3 component peaks make the deconvolution of the Mg 2p peak difficult. To return to the as-received surface, the O 1s spectrum was fitted by the OMgO, OMg(OH)2, and OMgCO3 peaks and another weak peak at 533.0 eV. The O species giving the peak at 533.0 eV is temporarily referred to as Oadd and is considered later. The intensity ratios of the Oadd, OMg(OH)2, and OMgCO3 peaks with respect to the OMgO peak were 0.024, 0.36, and 0.071, respectively. The Mg 2p spectrum was well fitted by the MgMgO, MgMg(OH)2, and MgMgCO3 peaks, and the intensity ratios of the MgMg(OH)2 and MgMgCO3 peaks with respect to the MgMgO peak were 0.18 and 0.025, respectively. The peak intensity ratios are listed in Table 1. Formation of Mg(OH)2 and MgCO3 had proceeded on the as-received surface as well as on the MgO powder. The Mg/O peak intensity ratios for MgO, Mg(OH)2, and MgCO3 were 0.20, 0.11, and 0.071, respectively. After 96 h annealing, the Oadd/OMgO peak intensity ratio increased to 0.23, about 10 times of that on the as-received surface. Instead, the OMg(OH)2/OMgO and the OMgCO3/OMgO ratios decreased to 0.067 and 0.034, respectively. In the Mg 2p spectrum, the MgMgO peak became a dominant component. The Mg/O ratios for MgO, Mg(OH)2, and MgCO3 were 0.20, 0.10, and 0.068, respectively, and agreed with those on the asreceived surface. This indicates that the stoichiometry of the MgO, Mg(OH)2, and MgCO3 was maintained and that the Mg(OH)2 and MgCO3 were decomposed to MgO during the annealing. On the 288 h annealed surface, the Oadd/OMgO ratio decreased to 0.15, while the OMg(OH)2/OMgO and the OMgCO3/ OMgO ratios were 0.061 and 0.032, respectively, and were

Mg

MgCO3/MgMgO

0.025 0.012 0.011 0.064 0.022

comparable to those on the 96 h annealed surface. The maintenance of the stoichiometry of the MgO, Mg(OH)2, and MgCO3 were confirmed by the Mg/O ratios of 0.19, 0.093, and 0.068, respectively. Thus, the Oadd decreased with the annealing time, while the amount of the Mg(OH)2, and MgCO3 were independent of the annealing time. The Mg(OH)2 and MgCO3 on the annealed surfaces were probably formed during the cooling period. The most probable assignment of the Oadd is the O of the surface OH group. Dissociative adsorption of H2O molecules on the MgO(100) surface generates two kinds of surface OH groups36 that are indistinguishable by XPS.32 ‐Mg‐O‐ + H 2O → ‐Mg(‐OH)‐OH‐

The O atom of the surface OH group is coordinated to lattice ions, and therefore its ionicity is likely to be between those of the ionic OMgO and the covalent OMg(OH)2. In that case, the binding energy of the Oadd peak is between that of the OMgO and OMg(OH)2 peaks.37 The H2O molecules and monovalent O anions, which yielded binding energies higher than that of OMgO by ∼330 and 2.3 eV,38 respectively, are not suitable for the Oadd. The Oadd is assigned to the O of the surface OH group and is referred to as OOH. Here, we estimate the coverage of the surface OH groups. The intensity of the O 1s peak I on the MgO(100) surface is expressed by the following equation: ∞

I∝

⎡ (n − 1)d ⎤ ⎥ λ cos θ ⎦

∑ Nn exp⎢⎣− n=1

(2)

where n, Nn, d, and λ are, respectively, the number of atom layers along the [001] axis, density of O atoms in each layer, the distance between the adjacent layers, and the inelastic mean free path of O 1s photoelectrons in MgO.32 By ignoring the screening effect of the OH groups coordinated to the surface Mg atoms, the ratio of the surface OH groups, x, with respect to the O atoms in the atom layers is related to the intensity ratio of the OOH and OMgO peaks as follows. IOH 2x = IMgO 1 exp( ( d / − − λ cos θ ))]−1 − x [

(3)

Using λ of 2.35 nm by the TPP-2 M formula, x was calculated to be 0.79 for the 288 h annealed surface. The coverage of 0.79 matches with the coverage of the surface OH groups, 0.75, that was predicted to minimize the surface energy of the MgO(100) surface by de Leeuw et al.24 Higher density of the surface OH groups on the 96 h annealed surface, which was expected from the larger OOH/OMgO peak intensity ratio, reflects high surface area indicated by the greater surface roughness. Figure 4 shows XPS spectra of the surface immersed in water for 24 h. In the wide scan spectrum, Figure 4a, C and F were detected as impurities. Other impurities in the surface layers would have been removed with the corrosion of the surface 39

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Figure 4. (a) Wide-scan XPS spectrum of the 288 h annealed MgO(100) surface after immersion in water for 24 h. (b) Narrow-scan XPS spectra of the 288 h annealed MgO(100) surface. Component peaks with thin lines were superimposed on the spectra.

layers. The O OH/OMgO , O Mg(OH)2/O MgO , OMgCO 3/OMgO , MgMg(OH)2/MgMgO, and MgMgCO3/MgMgO peak intensity ratios obtained from the narrow scan spectra in Figure 4b, respectively, were 0.30, 0.59, 0.19, 0.29, and 0.064 and are listed in Table 1. The increase of Mg(OH)2 and MgCO3 on the 288 h annealed surface is highlighted in comparison with the asreceived surface that exhibited comparable OMg(OH)2/OMgO and OMgCO3/OMgO ratios before and after immersion in water. The additional peak at 536.0 eV is associated with H2O molecules,30 which might have been strongly trapped in cavities and pores formed by the dissolution. The most likely interpretation of the banks in Figure 2a-c is that less soluble Mg(OH)2 was formed along the ⟨011⟩ steps. Vermilyea reported that the dissolution rate of well crystallized Mg(OH)2 was less than one-hundredth of that of MgO in pH 6 to 9.40 According to molecular dynamics simulation on the hydroxylated MgO(100) surface by Jug et al., Mg(OH)2 epitaxially grew on the MgO(111) sidewall of the valley that was formed by the detaching of the subsurface Mg atoms.41,42 The {111} surfaces appear at the ⟨011⟩ steps by detaching the corner ions as shown in Figure 5a. Fast detaching of the lowcoordinated ions is very likely. The detached ions are incorporated to the Mg(OH)2 epitaxially grown on the hydroxylated {111} surfaces, as shown in Figure 5b.41,42 Such Mg(OH)2 is well crystallized and therefore less soluble than the MgO terraces. The Mg(OH)2 crystal extends from the ⟨011⟩ steps toward the inner region of the terraces as shown in Figure

Figure 5. Model of the growth of the Mg(OH)2 at the ⟨011⟩ step. Midsize gray balls, large black balls, and the smallest white balls represent Mg, O, and H atoms, respectively.

5c. The smooth patches in Figure 2a are probably well crystallized Mg(OH)2 that was casually formed on the terrace. The growth of the Mg(OH)2 crystal is expected to be limited on the (100) terraces, because wide {111} surfaces cannot grow on the terraces due to irregular detachment of the surface ions. The detached ions trapped in the surface layer were released into bulk water before the Mg(OH)2 crystal was formed. The growth of the banks was faster than the dissolution of the inner regions of the terraces in the initial 3 h. The average rates of the growth of the bank were 15 and 6.5 nm h−1 in the first 1 h and the second 2 h immersions, respectively. The average rates of the subsidence of the terraces in the two immersion periods were estimated to be 0.53 and 0.41 nm h−1, respectively, from the cross sections in Figure 2d. According to the growth process of the bank proposed above, the height of the banks is expected to decrease toward the inner regions of 8255

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The Journal of Physical Chemistry C

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the terraces because the inner regions subside as a source of ions. Such lowering of the banks is indeed observed as the gentle slopes indicated by the arrowheads in the cross section (2) in Figure 2d. In the last 21 h immersion, the average bank growth rates decreased to 0.33 nm h−1, while the average subsidence rates of the terraces remained largely unchanged at 0.45 nm h−1. When the bank growth rate becomes smaller than the subsidence rate, the growth of the bank would stop. However, the banks appear almost symmetric in the cross section (3) shown in Figure 2d. This suggests growth of the Mg(OH)2 at the bottom of the inner region sides of the banks. The Mg(OH)2 hanging over the MgO might have suppressed the dissolution of the subjacent MgO to form the {111} surface.

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CONCLUSION The ex situ analysis of MgO(100) surfaces by the AFM and XPS techniques revealed that bank-like structures were formed along the ⟨011⟩ steps by immersion in water. The banks were assigned to less soluble Mg(OH)2, and the ⟨011⟩ steps were expected to provide well-ordered {111} surfaces for the epitaxial growth of the Mg(OH)2. The pH dependence of the dissolution of the surface is the future subject to reinforce the assignment of the banks. If the assignment to Mg(OH)2 is correct, the banks are formed in aqueous solutions of pH 6−9. The Mg(OH)2 formed at the steps dissolves with a dissociation rate equivalent to the MgO terrace at pH below 6.40 At pH above 9, dissolution of MgO as well as Mg(OH)2 is suppressed by the common ion effect of OH−.40 The growth process and the crystalline structure of the banks can be examined by an in situ noncontact mode AFM (NC-AFM) technique. The NCAFM regulates the probe-sample distance without touching the sample surface and therefore provides atomic scale surface topography in liquid without perturbing the surface structures.17,43

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-761-51-1503. Fax: +81761-51-1149. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research from the Japanese Society for the Promotion of Science (JSPS) KAKENHI Grant Numbers 24246014, 26600024, and 26630330.

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DOI: 10.1021/acs.jpcc.5b01759 J. Phys. Chem. C 2015, 119, 8250−8257

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DOI: 10.1021/acs.jpcc.5b01759 J. Phys. Chem. C 2015, 119, 8250−8257