Size Effects in MgO Cube Dissolution - Langmuir (ACS Publications)

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Size Effects in MgO Cube Dissolution Stefan O. Baumann, Johannes Schneider, Andreas Sternig, Daniel Thomele, Slavica Stankic, Thomas Berger, Henrik Grönbeck, and Oliver Diwald Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504651v • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Size Effects in MgO Cube Dissolution

Stefan O. Baumann1, Johannes Schneider1,2, Andreas Sternig1, Daniel Thomele1,2, Slavica Stankic3,4, Thomas Berger2, Henrik Grönbeck5, and Oliver Diwald2* 1

Institute of Particle Technology, Friedrich-Alexander Universität Erlangen-Nürnberg, Cauerstrasse 4, 91058 Erlangen (Germany) 2

Department of Materials Science and Physics, University of Salzburg, Hellbrunnerstrasse 34/ III, A-5020 Salzburg (Austria)

3

CNRS, Institut des Nanosciences de Paris, UMR7588, 4 place Jussieu, 75252 Paris Cedex 05 (France) 4

UPMC – Université Paris 06, INSP, UMR 7588, 4 place Jussieu, 75252 Paris Cedex 05 (France)

5

Department of Applied Physics and Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, (Sweden)

Email: [email protected] 1 ACS Paragon Plus Environment

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Abstract

Stability parameters and dissolution behavior of engineered nanomaterials in aqueous systems are critical to assess their functionality and fate under environmental conditions. Using Scanning Electron Microscopy, Transmission Electron Microscopy and X-ray diffraction we investigated the stability of cubic MgO particles in water. MgO dissolution proceeding via water dissociation at the oxide surface, disintegration of Mg2+-O2- surface elements and their subsequent solvation ultimately leads to precipitation of Mg(OH)2 nanosheets. At a pH ≥ 10 MgO nanocubes with a size distribution below 10 nm quantitatively dissolve within few minutes and convert into Mg(OH)2 nanosheets. This effect is different from MgO cubes originating from magnesium combustion in air. With a size distribution in the range 10 nm ≤ d ≤ 1000 nm they dissolve with a significantly smaller dissolution rate in water. On these particles water induced etching generates (110) faces which – above a certain face area – dissolve at a rate equal to that of (100) planes.[Geysermans et al. 1] The delayed solubility of microcrystalline MgO is attributed to surface hydroxide induced self-inhibition effects occurring at the (100) and (110) microplanes. The present work underlines the importance of morphology evolution and surface faceting of engineered nanomaterials particles during their dissolution.

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Introduction

It has become a major objective in the development of engineered nanomaterials to characterize particle stability in different chemical environments and to determine the preservation of functional properties during materials processing and storage.2-8 In particular, the issue of water mediated degradation of nanoparticle based materials has drawn increasing attention.4 In this respect MgO dissolution and transformation into Mg(OH)2 brucite represents a prominent system for related investigations since respective insights are also of broader interests for very different fields like the application of refractory materials9, passive water treatment10, geochemistry and – last but not least – the design of nanomaterials and catalysts.11-16 MgO dissolution processes on well-defined single crystal surfaces have been studied experimentally17-19 and theoretically.20-22 A recent surface science study on the interaction of water with MgO(001) films revealed the emergence of disordered hydroxide surface layers that inhibit the dissolution process at higher pH values.15 In comparison to surface science studies on extended 2-dimensional crystal faces, less work has been devoted to model studies on MgO nanoparticles of defined shape.13,23,24 For MgO smoke particles with edge lengths of several hundreds of nanometers it could have been shown that their cubic morphologies can transform into those of truncated octahedra.1,25-28 Related shape evolutions have been attributed to the preferential dissolution of high energy corner and edge features in liquid water. The observed water-induced (110) cuts of the MgO cubes were explained theoretically via Wulff equilibrium shapes which are constrained and arise from the slower kinetics related to the formation of (110) facets rather the (100) facets.1 Environmental Transmission Electron microscopy (ETEM) studies on MgO smoke particles revealed that MgO (100) surfaces are extremely resistant to dissociative water adsorption under water partial pressures up to 10 3 ACS Paragon Plus Environment

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Torr.29 Electron irradiation, however, initiates MgO cube hydroxylation and drives Mg(OH)2 formation upon substantial volume expansion. Another recent study aimed at surface energies of much smaller anhydrous MgO nanoparticles and addressed their interaction with water vapor.30 Representative high resolution transmission electron micrographs indicate the formation of MgO - Mg(OH)2 core shell structures upon progressive hydration via the gasphase. Literature reports on the macroscopic solubility of polycrystalline MgO in liquid water are very variable. While some authors find practically no dependence of the dissolution rate on sample morphology and particle size31,32 others claim substantial effects of the MgO calcination procedure on the solubility of the resulting coarse grained materials.9 Ultimately, commercial MgO suppliers specify their product as “practically insoluble” in water.33 In conclusion, size effects associated with the water induced transformation of MgO particles into corresponding hydroxides have remained essentially un-addressed, although related insights are critical for the above mentioned activity fields. Here, we employ powders of cubic MgO particles, which were synthesized by different gas phase synthesis approaches, as particulate model systems. The absence of solvents during particle nucleation and growth prevents substantial particle aggregation and sintering during annealing and for this reason provides a - in terms of microstructure - welldefined starting situation34 to study the transformation of nanomaterials in liquids. Potential mass transfer limitations which may arise from the availability of internal surface area and porosity can be excluded. Moreover, both particle systems employed, i.e. MgO nanocubes synthesized by chemical vapor synthesis at reduced pressures and larger MgO cubes which were collected by Mg combustion in air are well-defined in terms of morphology. As a third measure for studies on model systems we annealed all particle powders in high vacuum as well as pure oxygen atmosphere in order to desorb and decompose adsorbed surface 4 ACS Paragon Plus Environment

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contaminants, respectively, and to eliminate related interferences with MgO dissolution in a bulk liquid. This study on model systems provides insights into size effects during metal oxide dissolution processes and clearly underlines that particles must be sufficiently large in order to establish a protective surface layer which delays or even inhibits dissolution. Experimental Material synthesis and vacuum annealing We use chemical vapor synthesis for the production of MgO nanocubes.35 Stable process conditions are guaranteed by the spatial separation of the evaporation and oxidation zone. For this purpose, a reactor consisting of two quartz glass tubes, which are placed inside a cylindrical furnace, is used. The inner tube hosts ceramic ships with Mg pieces (99.98%, Aldrich). Heating to T = 913 K guarantees a metal vapor pressure of 1 mm Hg column (1.33 mbar). An argon stream (Ar 5.0) transports the metal vapor away from the evaporation zone to the end of the inner glass tube. At this position, the argon/ metal vapor mixture meets molecular oxygen arriving through the outer glass tube. The exothermic oxidation reaction gives rise to a bright flame in the reactor and MgO nanoparticles are formed as a result of homogeneous nucleation in the gas phase. Continuous pumping keeps the residence time of resulting nuclei within the flame short and prevents substantial coarsening and coalescence. Further details are provided in Ref. 35. The combustion of Mg ribbons (99.98%, Neyco) in air is employed for the synthesis of MgO smoke and was initiated by a thin Ni–Cr wire held in contact with an extremity of the ribbon and that could be resistively heated. A glass-plate was kept at a constant height (~10 cm) above the sampling point, allowing collecting particles near the generation zone. The general particle properties correspond to those reported for smoke particles in literature.1,26-30


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After production, the two types of MgO powders – ensembles of MgO nanocubes and and those of larger sized MgO cubes − are transferred into quartz glass cells, which allow thermal powder activation of the powders in defined gas atmospheres. For both types of samples, the as-obtained MgO powders are cleaned of organic contaminants by heating to 1123 K at a rate of 10 K min-1 in high vacuum (p < 10-5 mbar) and exposing to molecular oxygen at this temperature. Then, at pressures p < 5 · 10-6 mbar the sample temperature was raised to 1173 K and kept for 1 h at this temperature until full dehydroxylation of the sample surface was achieved. Specific surface areas were determined from Nitrogen sorption isotherms acquired at 77 K (Micromeritics ASAP 2020). The BET specific surface areas for MgO nanocubes and MgO cubes correspond to 300 ± 30 m2 g-1 and 4.8 ± 0.5 m2·g-1, respectively. Materials characterization X-ray diffraction (XRD) measurements were performed on a Bruker AXS D8 Advance diffractometer using Cu Kα radiation (λ = 154 pm). For structure (XRD) and morphology characterization (EM) a quantity of 100 mg of the vacuum annealed MgO powders was dispersed in 50 ml high grade water (Millipore Simplicity M 185). Scanning electron microscopy (SEM) measurements were performed on a Zeiss Gemini Ultra 55 microscope operating between 5 kV and 10 kV. The transmission electron microscopy (TEM) investigations were performed on a Phillips CM300 UT operated at 300 kV. For microscopic measurements, small amounts of the metal oxide powders were cast on a carbon grid. Transformation experiments Dissolution kinetics were studied by means of time dependent powder XRD measurements. For measurements under static conditions (in situ experiments), MgO powder was deposited on a silicon wafer and covered with drops of high grade water (1 mg powder in 6 ACS Paragon Plus Environment

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50 µl water) (Figure S1, Supporting Information). Diffractograms were taken in time intervals of 540 s (~ 9 minutes). For time-dependent experiments under convective conditions (ex situ experiments, Figure S1), Argon flushing was applied to the aqueous particle dispersions in order to i) exclude CO2 uptake by the aqueous phase and ii) to provide sufficient convective motion inside the dispersion to reduce or even eliminate mass transfer limitations. Typically, these experiments are organized in a sequence of five steps: i) powder immersion into the liquid (50 mg in 25 ml H2O), ii) a specified time of contact with liquid water, iii) centrifugation for 450 s (~ 8 minutes), iv) removal of the supernatant solution and drying under vacuum conditions down to pressure p < 10-5 mbar (1800 seconds) and v) XRD measurements followed by phase analysis. Phase analysis using the Rietveld method revealed the relative periclase-to-brucite phase compositions as a function of MgO exposure time to water. In contrast to electron microscopy, related experimental results are statistically significant, because sufficiently large quantities of particles (20 – 100 mg) were sampled for each experiment.36 Results Gas phase synthesis of MgO particles followed by thermal annealing in high vacuum and oxygen yields nanocube agglomerates which are exempt from solid-solid interfaces.37,38 Electron microscopy reveals that these micrometer sized agglomerates correspond to ensembles of MgO nanocubes (Figure 1 and 2a) with edge lengths below 10 nm (Figure 1a). An average cubes size of 6 nm (Figure 1a) is perfectly consistent with the crystallite domain size as determined by evaluation of the diffraction line broadening using the Scherrer equation.37


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Figure 1. Transmission electron micrographs and particle size distribution plots of a) MgO nanocubes produced by chemical vapor synthesis and b) MgO cubes produced by magnesium combustion in air. MgO cubes which were obtained via magnesium combustion in air are considerably larger in size. Corresponding size distribution functions are in the range between 10 nm and 1000 nm (Figure 1b). After immersion of the MgO powders into liquid water, the resulting dispersions were subjected to convective mixing by flushing with Argon gas for 30 minutes (Supporting Information). For both series of experiments the pH values of the supernatant solutions increased from pH = 6 to pH = 10 within the first minutes and stayed constant thereafter. This guarantees an effective buffering of the solution. After subsequent water removal and vacuum drying at room temperature the sample’s microstructure was analyzed by scanning electron microscopy. In case of MgO nanocubes the microstructure has been transformed into nest-like aggregates of thin nanosheets with a few nanometers in diameter (Figure 2c). After an identical procedure of H2O admission and removal, most MgO cubes (Figure 1b, originating


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from Mg combustion in air) were found to retain their shape (Figure 2d). In addition, also particles with a needle like habit (see arrows) were observed at some places of the SEM.

Figure 2. Scanning electron micrographs of a) MgO nanocubes produced by chemical vapor synthesis and b) MgO cubes produced by magnesium combustion in air. c) MgO nanocube dispersion in water produces lamellar Mg(OH)2 structures (see below). d) Upon dispersion of MgO cubes in water, the majority of particles retain their morphology. In addition, elongated structures emerge. Insets in Figure 2a, c and d show the corresponding TEM images. Transmission electron microscopy shows further details of the particle morphologies after MgO dispersion in water. Lamellar structures as well as particles with needle-like crystal habit having a diameter of few nanometers were observed as a result of MgO nanocube dissolution (Figure 3 a and c).39 After exposure to liquid water MgO cubes show preferential erosion of corner and edge sites (SI, Figure S2) which is consistent with previously reported 9 ACS Paragon Plus Environment

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observations.1,25-28 In addition, elongated structures of a few hundred nanometers in length are observed and attributed to scrolled-up Mg(OH)2 structures (Figures 3b, c and d).

Figure 3. Transmission electron micrographs of solid residues collected after exposure of MgO nanocubes (a, b) and MgO cubes (c, d) in water and subsequent drying at room temperature and in vacuum. X-ray diffraction patterns of MgO nanocubes before dispersion in water (a) as well as MgO nanocubes (b) and cubes (c) after dispersion in water are shown in Figure 4 (see also Supporting Information, Figure S3). The diffraction patterns reveal the quantitative transformation of MgO nanocubes into brucite. After applying an identical procedure, the MgO cubes related diffractrogram points only to a minor fraction of the brucite phase (Supporting Information, Figure S3). Using the basal (001) and non-basal (110) reflections of the MgO nanocubes derived brucite at Bragg angles 2Θ = 18.8° and 58.9°, respectively, the 10 ACS Paragon Plus Environment

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crystallite sizes along the x001 (a axis) and x110 (c axis) were estimated using the Scherrer equation. The obtained values x001 = 3 nm and x110 = 20 nm are consistent with the sheet-like morphology of the brucite nanocrystals (Figure 2). The estimated domain size of 3 nm − which is at the threshold of reliability with respect to smaller sizes 40 – would correspond to a stack of 6 layers assuming an interlayer spacing of 472 pm along the c axis. An intensity ratio I001/I110, related to the (001) and (110) diffraction lines, would exceed a value of 1 for the sheet-like brucite nanoparticles in case of pronounced orientation effects.41 In the present case, we find an intensity ratio of I001/I110 = 1.1. This rules out oriented crystal growth effects42 and is consistent with the entangled coexistence of different nanosheets.43

Figure 4. XRD patterns of MgO nanocubes (a) before and (b) after dispersion in water in comparison to MgO cubes after dispersion in water (c). The reaction of MgO nanocubes and MgO cubes with water in the absence of convective motion was subjected to time-dependent XRD measurements. Figure 5 plots the 2Θ region of the periclase (002) and brucite (011) reflections. After 9 min the Mg(OH)2 (011) reflection is observed in case of MgO nanocube dissolution, while the MgO (002) diffraction feature is 11 ACS Paragon Plus Environment

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quantitatively extinguished after 1 h. For MgO cubes, however, no significant amount of the brucite phase is detected by means of X-ray diffraction after 2 hours. Consistent with electron microscopy measurements (Figures 2d, 3c–d), only a negligible fraction of smaller MgO particles, which does not significantly contribute to the diffraction pattern, is transformed into Mg(OH)2 in the time given.


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Figure 5. Time-dependent XRD pattern evolution upon MgO nanocube (left upper panel) and MgO cubes (right upper panel) exposure to liquid water in static environment. Patterns (a) in both upper panels correspond to the initial MgO sample prior to water exposure. Patterns (b), (c), (d), (e) and (f) correspond to XRD measurements which were started at t = 0 min, 27 min, 54 min, 81 min and 108 min after first contact with liquid water, respectively. The process of brucite formation (lower panel) was tracked via the intensity of the (011) reflection and, at the same time, the extinction of the periclase (002) diffraction feature for Mg nanocubes (a) and MgO cubes (b). Complementary MgO nanocube and cube dissolution experiments were carried out under convective conditions (Ar flushing of the aqueous particle dispersions, Figure 6). Limited by the centrifugation step, the shortest feasible time of contact between MgO and liquid H2O corresponds to 450 seconds. After this period we observed a complete conversion of MgO nanocubes into Mg(OH)2. Taking 450 s as an upper limit for the conversion of MgO nanocubes with an average edge length of 6 nm we can derive an apparent dissolution rate constant kDiss ≥ 3.6·10-6 mol·m-2·s-1 (Figure 6a). The calculation procedure used to translate the time-dependent decrease of a characteristic nanocube length d into the dissolution rate constant kDiss accounts for the differences in the specific surface area associated with the approximated monodisperse MgO cube ensembles of different cube size. The procedure is outlined in detail in the Supporting Information.


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Figure 6: Relative number of undissolved Mg2+-O2- ions for different ensembles of monodisperse MgO cubes as a function of cube size and time of exposure to liquid water. While the solid curves show predicted values, the open squares are experimental data points related to MgO cube ensembles and determined by phase analysis of the XRD data. Applying this value for monodisperse MgO nanocube ensembles leads to the predicted solid curves which are plotted in Figure 6 as traces a, b, c and d. Comparison of these curves with the experimentally obtained data points of MgO cubes (squares in Figure 6), which exhibit a broad size distribution in the range between 10 nm ≤ d ≤ 1000 nm (Figure 1), points to clear size effects in the dissolution behavior. Apparently, the dissolution of MgO cubes is delayed by orders of magnitude. If that was not the case, the cube sizes would have to be significantly larger (edge length of d = 7 µm). On the other hand, one can derive for MgO cube ensembles with characteristic edge lengths of d = 100 nm or d = 1000 nm values of kDiss = 6.7·10-8 mol·m-2·s-1 or kDiss = 6.7·10-7 mol·m-2·s-1 for the dissolution rate constants assuming that the dissolution processes are completed within a period of 5·105 s (Figure 6).


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Discussion Equilibrium thermodynamics state that the room temperature reaction of bulk MgO with water to Mg(OH)2 is clearly favored. The Gibbs free energies for reaction with water vapor and liquid water correspond to -35.6 kJ·mol-1 and -27.1 kJ·mol-1, respectively.44 Moreover, the room temperature solubility of MgO and Mg(OH)2 correspond to 86 mg·L-1 and 12 mg·L-1, respectively. Therefore, for an equilibrium situation that is exempt from any kinetic limitation one would expect the quantitative transformation of solid MgO into precipitated Mg(OH)2. One aspect with MgO nanocubes is the reduced Madelung potential. As the first step in MgO dissolution should be faceting of the cubes and, hence, the removal of edge atoms (or MgO units) a reduced Madelung potential could facilitate this step. In order to test such a hypothesis, a set of DFT calculations was performed for MgO cubes with edge lengths of 0.6, 1.0 and 1.5 nm.45 In particular, the energy penalty for creation of a MgO double vacancy at the corner of the cube was calculated for the three cases. It was, however, found that the difference in vacancy formation was small already in this size regime. For the 1.0 and 1.5 nm cubes, it differed by only 0.03 eV. This result indicates that the differences in dissolution should not be related to differences in the chemical bond (or electrostatics) of the nanocubes. Initial and apparent chemical rate constants for MgO dissolution are critically dependent on the pH.17,46-49 While proton attack strongly enhances the dissolution in acidic solutions, the dissolution rates are significantly smaller in the alkaline regime. Related values are reported for MgO films to be in the range between 10-5 and 10-6 mol·m-2·s-1

15

and 10-7 to

10-9 mol·m-2·s-1 for coarse grained MgO particles and single crystal surfaces, respectively.46 The complex leaching behavior of MgO polycrystals proceeds via different stages. As shown for MgO nanocubes in liquid water, a change in phase composition - i.e. the ratio of periclase


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to brucite (Figure 5a) - is completed after few minutes (Figure 6a) or one hour of contact time (Figure 5c) in the presence or absence of convective mass transfer, respectively. For understanding the substantially slower dissolution observed for larger sized MgO cubes it is worthy to also address changes of the particle morphology. In recent work Jupille and coworkers analyzed series of TEM images that were taken from MgO smoke crystallites and explored the shape transformation in great detail. 1,27,28 As a result, it was found that cube edge cuts parallel to the MgO (110) faces appear first upon dissolution in water. An induction period - designated here as phase I and indicated in the schematic of Figure 7 by the horizontal evolution of a blue square and its truncated derivatives - corresponds to the accelerated process of edge element disintegration. After completion of phase I both (100) and (110) faces dissolve at a similar and shape-determining rate (Figure 7).1 Moreover, it has been clearly demonstrated that after several days of water exposure the h110/h100 ratio levels at a constant value of 1.19 (Figure 7). This leads to a stationary crystal shape1 that is retained during during Phase II. Related observation is perfectly consistent with electron microscopic observations on MgO cubes performed in our lab (see Figure S2 in Supporting Information for further information).50


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Figure 7: Scheme describing the postulated two phases of MgO cube dissolution. While in Phase I low coordinated surface elements such as MgO cube corners and edges become preferentially depleted and dissolved upon formation of (110) microplanes a second Phase II applies only for larger particles with extended face areas. While the lengths of the arrows correspond to the respective dissolution rates, the light blue squares should serve as a guide to the eye and indicate the original size of the MgO cubes with an edge length of 100 nm. The question arises why and in what way the dissolution process depends on the presence and evolution of extended surface planes. Recent surface science work on atomically clean MgO (100) films revealed for high pH values that the dissolution process is initially fast, slows down with time and ultimately stops after the elimination of approximately 15 monolayers.15 Complementary LEED measurements point to substantial surface reconstructions leading to a strongly disordered surface which inhibits further dissolution via mass transfer limitations


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through the apparently stable, disordered and cohesive Mg(OH)2 layer on top of the MgO face. While larger MgO cubes in fact display extended (100) and (110) faces (Figures 1 and 2), the situation is apparently very different for MgO nanocubes with sizes below 10 nm (represented by red squares in Figure 7).51 In such a case, the particle volume is too small to feature surface planes of sufficient size that can inhibit further particle dissolution at higher pH values.15 Before a certain face size for the (110) and (100) planes is reached in the course of preferential erosion of edge and corner sites (Figure 7, Phase II), the nanoparticles have been already disintegrated and, therefore, do not undergo Phase II. The novel observation that the dissolution behavior of metal oxide nanoparticles shows a dependence on particle size and, thus, depends on the extension of specific surface planes, was obtained in the course of experiments with particulate model systems. These are well-characterized in terms of their morphology and particle size distribution. It is only in these conditions that the metal oxide surface chemistry can rigorously be controlled and studied. Related insights are central to aqueous reactivity and the environmental fate of this class of materials.7 Although the high pH value, the system arrives at during the very first phase of dissolution, appears to be far off from conditions typically found in environmental systems, we believe that our study has substantial implications for the behavior of nanomaterials in the ambient. One must not forget, that in humid environments thin-film water is ubiquitous.52 Nanomaterials are high surface area materials and become instantaneously coated with water films with thicknesses ranging between one molecular layer to few nanometers.53 These films provide a less discussed reaction medium54,55 with an essentially unexplored surface and solvent chemistry52,53 that can be determining for functionality and fate of metal oxide nanomaterials in the ambient. Moreover, our findings are


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of direct relevance for nanomaterials processing in aqueous media and for maintaining the functionality of metal oxide nanoparticle based devices under ambient conditions.56 Conclusions Vapor phase grown MgO cubes exhibiting different size distributions have been utilized as particulate model systems to study particle size effects in dissolution processes. By means of electron microscopy in conjunction with X-ray diffraction we studied the dissolution of MgO nanocubes with edge lengths below d = 10 nm as well as the formation of Mg(OH)2 nanosheets and determined that the apparent rate constant for dissolution is equal or larger to kDiss = 3.6·10-6 mol·m-2·s-1. MgO cubes with edge lengths in the range of 100 nm - 1000 nm actually exhibit a dissolution dependent shape transformation with (100) and (110) planes as the most abundant particle faces which are subject to delayed dissolution. With reference to related surface science studies the size effects observed for the dissolution of MgO cubes must be attributed to passivating surface hydroxide layers in alkaline solution which slow down or even stop the conversion into Mg(OH)2. Acknowledgments We acknowledge support from the Deutsche Forschungsgemeinschaft project DI 1613/2-1 as well as support in the framework of COST Action (CM1104) “Reducible oxide chemistry, structure and functions”. Supporting Information Available Details about the set-up of the time-dependent XRD experiments, the derivation of the rate constant kDiss from the time-dependent decrease of a characteristic length d, and additional electron microscopy and XRD data are provided in the Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org/.


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References (1) Geysermans, P., Finocchi, F., Goniakowski, J., Hacquart, R., Jupille, J. Combination of (100), (110) and (111) facets in MgO crystals shapes from dry to wet environment. Phys. Chem. Chem. Phys. 2009, 11, 2228–2233. (2) Narayanan, R., Tabor, C., El-Sayed, M. A. Can the Observed Changes in the Size or Shape of a Colloidal Nanocatalyst Reveal the Nanocatalysis Mechanism Type: Homogeneous or Heterogeneous? Top. Catal. 2008, 48, 60–74. (3) Meldrum, F. C., Cölfen, H. Controlling mineral morphologies and structures in biological and synthetic systems. Chem. Rev. 2008, 108, 4332–4432. (4) Tso, C.-P., Zhung, C.-M., Shih, Y.-h., Tseng, Y.-M., Wu, S.-C., Doong, R.-A. Stability of metal oxide nanoparticles in aqueous solutions. Water Sci. Technol. 2010, 61, 127– 133. (5) Schürer, B., Elser, M. J., Sternig, A., Peukert, W., Diwald, O. Delamination and dissolution of titanate nanowires: A combined structure and in situ second harmonic generation study. J. Phys. Chem. C 2011, 115, 12381–12387. (6) Levard, C., Hotze, E. M., Lowry, G. V., Brown, G. E. Environmental transformations of silver nanoparticles: impact on stability and toxicity. Environ. Sci. Technol. 2012, 46, 6900–6914. (7) Batley, G. E., Kirby, J. K., McLaughlin, M. J. Fate and risks of nanomaterials in aquatic and terrestrial environments. Acc. Chem. Res. 2013, 46, 854–862. (8) Gheisi, A., Sternig, A., Rangus, M., Redhammer, G., Hartmann, M., Diwald, O. Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions. Cryst. Growth Des. 2014, 14, 4236–4239. (9) Thomas, J. J., Musso, S., Prestini, I., Troczynski, T. Kinetics and Activation Energy of Magnesium Oxide Hydration. J. Am. Ceram. Soc. 2014, 97, 275–282. (10) Rötting, T. S., Ayora, C., Carrera, J. Improved Passive Treatment of High Zn and Mn Concentrations Using Caustic Magnesia (MgO): Particle Size Effects. Environ. Sci. Technol. 2008, 42, 9370–9377. (11) Lucas, E., Decker, S., Khaleel, A., Seitz, A., Fultz, S., Ponce, A., Li, W., Carnes, C., Klabunde, K. J. Nanocrystalline metal oxides as unique chemical reagents/sorbents. Chem. Eur. J. 2001, 7, 2505–2510. (12) Jeevanandam, P., Mulukutla, R. S., Yang, Z., Kwen, H., Klabunde, K. J. Nanocrystals to Nanorods: A Precursor Approach for the Synthesis of Magnesium Hydroxide Nanorods from Magnesium Oxychloride Nanorods Starting from Nanocrystalline Magnesium Oxide. Chem. Mater. 2007, 19, 5395–5403. (13) Cadigan, C. A., Corpuz, A. R., Lin, F., Caskey, C. M., Finch, K. B. H., Wang, X., Richards, R. M. Nanoscale (111) faceted rock-salt metal oxides in catalysis. Catal. Sci. Technol. 2013, 3, 900. (14) Vidic, J., Stankic, S., Haque, F., Ciric, D., Le Goffic, R., Vidy, A., Jupille, J., Delmas, B. Selective antibacterial effects of mixed ZnMgO nanoparticles. J. Nanopart. Res.2013, 15. (15) Ringleb, F., Sterrer, M., Freund, H.-J. Preparation of Pd–MgO model catalysts by deposition of Pd from aqueous precursor solutions onto Ag(001) - supported MgO(001) thin films. B. Appl. Catal. A: General 2014, 474, 186–193. (16) Carrasco, E., Aumer, A., Gomes, J. F., Fujimori, Y., Sterrer, M. Vibrational spectroscopic observation of ice dewetting on MgO(001). Chem. Commun. 2013, 49, 4355–4357. (17) Wogelius, R. A., Refson, K., Fraser, D. G., Grime, G. W., Goff, J. P. Periclase surface hydroxylation during dissolution. Geochim. Cosmochim. Acta 1995, 59, 1875–1881. (18) Suárez, M. F., Compton, R. G. Dissolution of magnesium oxide in aqueous acid: An atomic force microscopy study. J. Phys. Chem. B 1998, 102, 7156–7162. 21 ACS Paragon Plus Environment

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(38) McKenna, K. P., Koller, D., Sternig, A., Siedl, N., Govind, N., Sushko, P. V., Diwald, O. Optical properties of nanocrystal interfaces in compressed MgO nanopowders. ACS Nano 2011, 5, 3003–3009. (39) Frühwirth, O., Herzog, G. W., Hollerer, I., Rachetti, A. Dissolution and hydration kinetics of MgO. Surf. Techn. 1985, 24, 301–317. (40) Weidenthaler, C. Pitfalls in the characterization of nanoporous and nanosized materials. Nanoscale 2011, 3, 792–810. (41) Henrist, C., Mathieu, J.-P., Vogels, C., Rulmont, A., Cloots, R. Morphological study of magnesium hydroxide nanoparticles precipitated in dilute aqueous solution. J. Cryst. Growth 2003, 249, 321–330. (42) Matos, Charlene R S, Xavier, M. J., Barreto, L. S., Costa, N. B., Gimenez, I. F. Principal component analysis of X-ray diffraction patterns to yield morphological classification of brucite particles. Anal. Chem. 2007, 79, 2091–2095. (43) Baumann, S. O., Liu, C., Elser, M. J., Sternig, A., Siedl, N., Berger, T., Diwald, O. On the entangled growth of NaTaO3 cubes and Na2Ti3O7 wires in sodium hydroxide solution. Chem. Eur. J. 2013, 19, 10235–10243. (44) Haynes W. M., Lide D. R., Bruno T. J. CRC Handbook of Chemistry and Physics: A ready-reference book of chemical and physical data 2013-2014, 94th ed.; CRC Press: Boca Raton (Fla.), London, New York, op. 2013. (45) Density Functional Theory (DFT) was used with the local density approximation according to Perdew and Wang. In particular the dmol program was used as in Ref. [Sternig et al. J. Phys. Chem. C, 2011,115 15853–15861]. (46) Giner, I. , Ozcan, O., Grundmeier, G.; In situ AFM studies of the stability of MgO(100) in aqueous electrolytes, Corros. Sci. 2014, 87, 51-59. (47) Mejias, J. A., Berry, A. J., Refson, K., Fraser, D. G. The kinetics and mechanism of MgO dissolution. Chem. Phys. Lett. 1999, 314, 558–563. (48) Raschman, P., Fedoročková, A. Dissolution kinetics of periclase in dilute hydrochloric acid. Chem. Eng. Sci. 2008, 63, 576–586. (49) Fedoročková, A., Raschman, P. Effects of pH and acid anions on the dissolution kinetics of MgO. Chem. Eng. J. 2008, 143, 265–272. (50) In addition to the predominant fraction of particles with (110) cuts, we observed a variety of - in terms of particle morphology - less regular structures. So far, we did not observe truncated octahedra as discussed in Reference 1 in the final state of dissolution. Significant parts of remaining structures - which to some extent corresponds to the hydroxide - emerge upon swelling of the originally cubic particles and result from the significant volume change upon transformation of the oxide into the hydroxide.(Ref. 29) Despite the fact that we use the approximation of cubic particle shapes rather than accounting for (110) and (111) truncations, the way we predicted the volume decrease dependence on dissolution time represents a sufficient first approximation for the present purpose. (51) Technically, it is impossible to track particle morphology changes with high resolution transmission electron microscopy due to their small size as well as due to materials specific charging effects. (52) Newberg, J. T., McIntire, T. M., Hemminger, J. C. Reaction of bromide with bromate in thin-film water. J. Phys. Chem. A 2010, 114, 9480–9485. (53) Garde, S., Schlossman, M. L. Water at functional interfaces. MRS Bull. 2014, 39, 1051– 1053. (54) Zhang, H., Li, Y., Liu, P., Li, Y., Yang, D., Yang, H., Zhao, H. A new vapor-phase hydrothermal method to concurrently grow ZnO nanotube and nanorod array films on different sides of a zinc foil substrate. Chem.-Eur. J., 2012, 18, 5165–5169. 23 ACS Paragon Plus Environment

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(55) Liu, P., Zhang, H., Liu, H., Wang, Y., Yao, X., Zhu, G., Zhang, S., Zhao, H. A facile vapor-phase hydrothermal method for direct growth of titanate nanotubes on a Titanium substrate via a distinctive nanosheet roll-up Mechanism. J. Am. Chem. Soc. 2011, 133, 19032–19035. (56) Siedl, N., Gügel, P., Diwald, O. Synthesis and aggregation of In2O3 nanoparticles: Impact of process parameters on stoichiometry changes and optical properties. Langmuir 2013, 29, 6077–6083.


Size Effects in MgO Cube Dissolution - Langmuir (ACS Publications)

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