Factors Governing MgO(111) Faceting in the Thermal Decomposition

Feb 9, 2018 - In a molten NaNO3:KNO3 medium, the formation of liquid eutectic mixtures facilitate H2O retention and ionic mobility, from which well-de...
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Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Factors Governing MgO(111) Faceting in the Thermal Decomposition of Oxide Precursors Mariano D. Susman,§ Hien N. Pham,† Abhaya K. Datye,† Sivadinarayana Chinta,# and Jeffrey D. Rimer*,§ §

Department of Chemical and Biomolecular Engineering, University of Houston, 4726 Calhoun Road, Houston, Texas 77204, United States † Department of Chemical and Biological Engineering and Center for Microengineered Materials, University of New Mexico, Albuquerque, New Mexico 87131-0001, United States # SABIC Technology Center, 1600 Industrial Blvd., Sugar Land, Houston, Texas 77478, United States S Supporting Information *

ABSTRACT: The preparation of metal oxide particles, such as MgO, NiO, and ZnO, exposing polar facets via the decomposition of suitable precursors in air, molten salts, ionic liquids, or other media has been reported; however, the main driving force(s) and factors that determine their morphology have largely remained elusive. For instance, the adsorption of ions within a molten medium on a growing oxide surface has been proposed to stabilize MgO{111} facets by the so-called molten salt route (MSR). In this article, we examine the thermal decomposition of MgO precursors to assess the influence of precursor decomposition pathways, including the physical state of the reaction intermediates and the possibility of dissolution−recrystallization processes in the formation of octahedral MgO particles. We found that solid-to-solid conversions and recrystallizations in molten nitrates or chlorides are usually incapable of producing well-defined MgO{111} facets, indicating that ion adsorption on MgO may not be the main morphological driving force. Here, we show for Mg(NO3)2· 6H2O, the most suited MgO(111) precursor, that its decomposition trajectory is crucial in determining the exposure of {111} facets. The decomposition trajectory may be regulated by using controlled heating profiles, regulated atmospheres, or by incorporating particular salts in the reaction mixture. Our findings indicate that, for neat Mg(NO3)2·6H2O decomposition in air, MgO{111} facets are promoted via magnesium nitrate intermediates in the molten state and in the presence of residual water. These conditions can be achieved by heating the hexahydrate precursor at high rates. On the contrary, decomposition of anhydrous Mg(NO3)2 results in ill-defined MgO morphologies. In a molten NaNO3:KNO3 medium, the formation of liquid eutectic mixtures facilitate H2O retention and ionic mobility, from which well-defined octahedral MgO crystals form, thereby emphasizing the crucial role of water in Mg(II)-nitrate systems. In a NaCl:KCl ionic medium, precursor decomposition occurs via a K3NaMgCl6 intermediate, which melts before converting to the oxide. MgO(111) forms under local melting of the intermediate (before NaCl:KCl melts) and when the H2O content is negligible. In summary, the formation of polar MgO(111) particles is facilitated in molten salt media when MgO is generated via a liquid-to-solid reaction (with intermediates in the molten state). The presence of residual water and ions impact MgO(111) crystallization in ways that still remain elusive, but are not necessarily governed by adsorption−stabilization processes.

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INTRODUCTION

agents or other additives, or by modifying reaction conditions, such as solvent selection or applied electrochemical potentials. Alternatively, doping may result in simultaneous changes in both morphology and crystal structure.7,8 The focus of this study is magnesium oxide (MgO), which is a rock-salt oxide insulator with high ionic character that has found applications as a catalyst (and catalyst support) for many base-catalyzed organic reactions.9 Examples include the

The (photo)catalytic activity of several metals and metal oxides has been found to depend on the crystal facets exposed for a variety of chemical reactions.1−6 In general, low-index facets, which in most cases are thermodynamically more stable, show lower activities than high-index facets. Exposed facets with specific ionic spacing(s), as well as particular surface defects (e.g., under coordinated bonds), can enhance reactivity. The ability to control particle morphology and maximize the exposure of specific crystal planes is of high interest in many applications. For example, the preferential exposure of specific facets can be achieved by using growth modifiers, capping © XXXX American Chemical Society

Received: December 21, 2017 Revised: February 8, 2018 Published: February 9, 2018 A

DOI: 10.1021/acs.chemmater.7b05302 Chem. Mater. XXXX, XXX, XXX−XXX

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the physical state of the system, it can modify the degree of hydration during decomposition, and/or it can chemically interact with the precursor and its reaction intermediates. Alternatively, the medium may adsorb to and stabilize specific crystal facets in situ, or favor dissolution−recrystallization processes. Moreover, it is feasible that nonclassical crystallization pathways,35 if relevant, may be affected by the MSR. Here, we study the decomposition of different MgO precursors and the synthetic parameters determining the formation of {111} crystal facets. We evaluate the influence of precursors, the reaction medium, synthesis temperature, and decomposition profiles on the morphological outcomes. Our findings elucidate the role of electrostatic stabilization in the production of high-energy facets by exogenous ions. Moreover, we identify the physical state of the intermediates and the presence of residual water in the system as key properties responsible for the formation of MgO{111} faceted crystals.

following: Claisen−Schmidt, Michael, Wittig, Knoevenagel, Cannizzaro, Tischenko, Henry, alcohol dehydrogenation and coupling, H2 transfer, aldol condensation, cross-condensation, transesterification, and isomerization reactions.9 Despite significant research in the design and engineering of MgO crystals, simple methods for the controlled preparation of high surface energy MgO facets and studies of facet-dependent properties using this material are scarce. Because of their low surface polarity, MgO {100} faces are believed to be the most stable in air. Crystals exhibiting exclusively these facets can be produced by gas phase methods. For instance, the production of well-defined cubic MgO smoke particles was demonstrated more than 70 years ago by burning metallic Mg in the presence of oxygen.10,11 More recently, MgO nanocubes (ca. 3−10 nm) were produced by chemical vapor deposition (CVD),12 whereas Klabunde et al. prepared high-surface-area MgO nanocube aggregates by an aerogel method.13−15 The preparation of MgO(110) supports and thin films is known;16,17 however, MgO crystals exposing {110} facets are far less common. Because of its polarity, the MgO{111} facet is considered to be the least thermodynamically stable in air and is putatively stabilized by hydroxylation in water or alcohol-based solvents.18 Richards et al.19−21 produced MgO(111) nanoplates in a methanolic growth medium by decomposing metastable Mg(OH)(OCH3) intermediates in the presence of benzyl alcohol. Catalytic studies have shown that these particles are more active than MgO particles composed of {110} and {100} facets for the ozonation of 4-chlorophenol.22 Partial or full exposure of {111} facets has also been achieved by the molten salt route and by gel combustion synthesis, using Mg(NO3)2· 6H2O as the Mg(II) source.23−25 To our knowledge, there are no other direct bulk synthesis methods reporting the production of well-defined MgO(111) octahedral particles. The molten salt route (MSR, or salt melt synthesis, SMS)26 and methods utilizing ionic liquids (ILs)27−29 are nonaqueous alternatives for the preparation of thermally stable inorganic materials. Salts, hydroxides, deep eutectic mixtures, and ILs can potentially be recycled, whereas common salts are economical (i.e., expendable) reaction media. Another advantage of using molten salts is the reported colloidal stability of dispersed metal oxide particles in these media,30 which potentially allows for the recrystallization of nanoparticles to expose desired facets. Du et al. demonstrated the feasibility of obtaining MgO in the nanosize range by MSR; however, particles showed irregular morphologies under their synthesis conditions.31 Xu et al.25,32 showed that the decomposition of Mg(NO3)2·6H2O in molten LiNO3 results in octahedral MgO particles with average crystallite size of ca. 1 μm. They hypothesized that strong electrostatic interactions occur between the free ions in the molten medium and the polar (high surface energy) facets of the oxide product. During crystal growth, these interactions presumably lower the surface energies of MgO{111} facets, which results in slower growth rates and a concomitant increase in the relative area of the high-energy facets in the oxide particles.25,33,34 The resulting particles can lead to enhanced catalytic performance. For example, MgO octahedra were shown to be more active than commercial MgO powder in Claisen−Schmidt condensation reactions.32,34 In order to select a suitable MSR strategy, it is desirable to first understand the main driving force(s) that control the crystal habit. In MSR, the medium can have diverse functions; it can behave as a purely dispersive (diluting) phase, it can form eutectic mixtures with the precursor affecting the evolution of

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RESULTS AND DISCUSSION The thermal decomposition of most common MgO precursors results in MgO formation at temperatures in the range of 300− 600 °C (TMgO in Table S1). Molten nitrate media are stable as liquids in the 120−600 °C range, whereas molten chlorides exist at 600−1400 °C (Table S2). Due to their relatively low melting point and low cost, these media are advantageous for MgO preparation compared to other molten salts (MSs). In the MSR, product formation should ideally occur at a temperature between the melting point (MP) and the boiling point (BP) of the salt medium (or alternatively at less than its decomposition temperature, Tdec, if the medium decomposes below the BP).26 As most precursors decompose to MgO below 600 °C, nitrates are a class of materials with low MP, which make them suitable for MSR-based syntheses. In chloride media, MgO formation is likely to occur before melting of the medium; however, after melting, it is possible that the initial MgO formation is followed by dissolution−recrystallization processes which may expose {111} facets. Indeed, this may occur in any molten medium. Here, we studied the morphological outcomes of calcining different MgO precursors in molten nitrate and chloride media in order to determine whether adsorption−stabilization and dissolution−recrystallization processes are effective for maximizing MgO(111) faceting. Xu et al. used LiNO3 as a MS medium, as it is stable in the liquid state in the range of 250−600 °C; however, it is an increasingly expensive salt. Because of its small ionic radius, Li+ can also be incorporated into several solid lattices and may be difficult to remove after synthesis;36 therefore, less expensive alternative media would be preferred. To replace this medium, we have initially studied NaCl:KCl (1:1 wt %) and NaNO3:KNO3 (3:2 wt %) eutectic mixtures, noting that the solubility of MgO in NaCl:KCl has been shown to be negligible.37 A variety of MgO precursors were used in this study: Mg(NO3)2·6H2O, Mg(OH)2 prepared by precipitation and by hydrothermal methods, and Mg5(CO3)(OH)4·4H2O (hydromagnesite) particles. MgO smoke cubic particles were used as a blank to evaluate the tendency of MgO{100} to recrystallize. As discussed in the Supporting Information, effective formation of MgO{111} facets is only attained from Mg(NO3)2·6H2O and MgCl2·6H2O (see below), whereas solid precursors which do not melt before decomposition do not effectively produce high-energy facets by the MSR. Our findings indicate three points. (1) Dissolution−recrystallization processes are operative after the formation of MgO, but are not B

DOI: 10.1021/acs.chemmater.7b05302 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials effective in maximizing {111} surface area. In a NaCl:KCl medium, MgO cubes partially transform from {100} surfaces to {111} facets (Figure S1, E3 and E4), whereas an extensive thermal treatment in this MS does not alter the octahedral facets (Figure S1, A3 and A4), thus indicating that dissolution− recrystallization is not particularly limited by ionic diffusion (or viscosity of the medium). (2) Adsorption−stabilization of ions on forming MgO surfaces does not fully explain the experimental results. (3) Alternative factors seem critical for dictating the final particle morphology, which may include the dynamics of MgO formation via liquid Mg(II) intermediate species. In order to gain a deeper understanding of this synthesis method, we explored the properties of Mg(NO3)2·6H2O that make it a suitable precursor for obtaining octahedral crystals. It should be noted that Mg(NO3)2·6H2O and MgCl2·6H2O, in combination with different nitrate media, both produce octahedral MgO (Figure 1). These inorganic salts are known

Figure 2. TEM characterization of MgO octahedral particles produced in LiNO3. (A) Wide-angle TEM image of an octahedral particle oriented along the [111] direction and (B) corresponding SAED. (C) High-resolution TEM image showing (111) lattice spacings and (D) corresponding FFT of the image.

Thermal Decomposition of Mg(NO3)2·6H2O in Air. Thermogravimetric analysis (TGA), thermal differential analysis (TDA), and differential scanning calorimetry (DSC) have been widely employed to study the decomposition of a variety of MgO precursors.38 Partial understanding of decomposition mechanisms point to the potential role of reaction conditions (e.g., heating profile, sample size, and local vapor/gas pressures, among others) in the decomposition pathway, the final MgO morphology, and the physical state (i.e., solid or liquid) of the system; however, the latter is difficult to directly assess and is often inferred from TGA-TDA/DSC data. Representative TGA-TDA/DSC profiles reported in the literature for the precursors employed in this article, i.e., Mg(NO3)2·6H2O,38,39 MgCl2·6H2O,40 Mg(AcO)2·4H2O,41 Mg(OH)2,42 and hydromagnesite,43 as well as their typical interpretations, are presented in Figure S4. In most cases, thermal decomposition is accompanied by a series of processes that include dehydration, hydrolysis, melting, and denitration/ dechlorination/decarbonation steps. For the Mg(NO3)2·6H2O precursor, it has been reported that anhydrous Mg(NO3)2 cannot be produced thermally, as the final dehydration stages occur on the onset of Mg(NO3)2 decomposition;39 therefore, Mg(NO3)2 is not usually considered a stable intermediate. This has been concluded from the lack of a well-defined plateau at 300−400 °C, prior to MgO formation, in previously reported TGA plots (Figure S5). We performed both TGA and DSC measurements on the Mg(NO3)2.6H2O precursor using a heating rate of 2.5 °C min−1. The results of these measurements (Figure 3) show similarities and differences compared to previous studies. The TGA profile exhibits a well-defined plateau at 300−425 °C, corresponding to a weight loss of 56%. This value matches the theoretically predicted 58% loss for anhydrous Mg(NO3)2 formation (within experimental error). In the DSC profile, we observe a weak endothermic signal at 344−355 °C (labeled 4) that is not accompanied by a significant change in sample weight. Such behavior was previously reported39,44 for this system without explanation; however, we attribute this peak to

Figure 1. SEM of MgO produced by thermal decomposition of 10 wt % precursor mixtures at 500 °C where letters (columns) refer to precursors and numbers (rows) refer to the salt media: (A) Mg(NO3)2·6H2O and (B) MgCl2·6H2O in (1) LiNO3 for 5 h, and in (2) NaNO3:KNO3 (3:2) for 10 h. Heating rates: 2.5 °C min−1.

to produce common eutectic mixtures with alkali nitrates and chlorides. Transmission electron microscopy (TEM) of samples produced in LiNO3 confirmed their octahedral morphology (Figure 2). The hexagonal contour of an appropriately aligned isolated particle, together with the associated selected area electron diffraction (SAED) pattern, indicate that the exposed triangular facet is parallel to the MgO(111) plane (Figure 2A,B). Additional indexing of the (111) facet was carried out by high-resolution TEM imaging and fast Fourier transform (FFT) analysis (Figure 2C,D). Before evaluating the relevance of the molten media, it is useful to understand how Mg(NO3)2·6H2O decomposes in air and to identify the relevant parameter(s) during this process that alter the shape of oxide products. Octahedral MgO particles produced by the MSR frequently appear as polydisperse aggregates instead of as isolated octahedral crystals. C

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(e.g., spheroidal). At intermediate rates, the morphology is between those extreme cases (Figures 4C and image A2 in Figure S1). Collectively, these results indicate that the heating rate impacts MgO crystal habit. In order to assess the sample phase composition (i.e., decomposition pathway) as a function of temperature under different heating rates, samples were partially calcined to increasing temperatures (Figure 4A, dashed traces and symbols). Detailed descriptions of the temperature profiles are reported in Figure S6. Differences in the temperature profiles of partial and full heat treatments were assumed to be negligible. Sample compositions were estimated from powder X-ray diffraction (PXRD) patterns in Figure 5, taken immediately after cooling to room temperature (RT). The presence of amorphous phases was found to be negligible in all samples. PXRD patterns of samples sequentially withdrawn from the furnace at increasing temperatures, treated under air and a heating rate of 2.5 °C min−1, are shown in Figure 5A. It should be emphasized that samples were measured after cooling to RT; therefore, the corresponding phase composition at high temperature may differ, but the relative water contents in the samples are unchanged. In situ PXRD measurements at high temperatures are experimentally restricted due to the evolution of corrosive gases (NO2, HCl, Cl2, etc.) and the reactivity of the MSs. The phase changes observed by PXRD are qualitatively consistent with the measured TGA profile in Figure 3. In the absence of heating, Mg(NO3)2·6H2O is the predominant phase. Increasing the temperature to 250 °C leads to the partially dehydrated form, Mg(NO 3) 2 ·2H 2 O, prior to complete conversion to an anhydrous phase. At 350 °C, the predominant phase is rhombohedral Mg(NO3)2,45 whereas a cubic Mg(NO3)2 phase is detected at 400 °C. Further decomposition of the latter leads to the formation of MgO. These findings indicate a polymorphic transition39 from rhombohedral to cubic anhydrous Mg(NO3)2 at ∼350 °C. This transformation may follow the Ostwald rule of stages,46 assuming that the cubic polymorph is the more thermodynamically stable phase. At 450 °C, denitration proceeds and the corresponding PXRD pattern contains both cubic Mg(NO3)2 and MgO phases, whereas complete transformation to the latter occurs at T ≥ 500 °C. The physical state of the sample right after withdrawal from the furnace was noted by visual inspection (Figure S7). At a heating rate of 2.5 °C min−1, the synthesis mixture remains a liquid up to 250 °C, but it shifts to a solid or glassy state at higher temperatures. We posit that PXRD patterns measured at RT are representative of the phase composition(s) at higher temperatures. PXRD patterns of samples produced at heating rates of 20 and 0.5 °C min−1 by partial treatments are presented in Figure S8. Several changes occur when Mg(NO3)2·6H2O is heated at a fast rate (Figure S8A). First, chemical transitions are shifted to higher temperatures (by approximately 50 °C), as commonly observed in TGA wherein the transitions become less defined (Figure S5). Second, the temperature range at which hydrous phases are stable is extended, indicating that water removal is less effective. For instance, diffraction peaks corresponding to Mg(NO3)2·2H2O span from 300 to 500 °C, whereas at 2.5 °C min−1 this phase is present at 250−350 °C. At higher heating rates, MgO forms in mixtures still containing water (e.g., H2O is detected at 500 °C in Figure S8A). Third, the physical state of the system is liquid at temperatures as high

Figure 3. Decomposition profiles of Mg(NO3)2.6H2O in air (100 mL/ min, 2.5 °C min−1) measured by TGA (black line; left axis) and DSC (blue line; right axis). Sample weights for TGA and DSC are 26 and 12.6 mg, respectively. Decomposition steps include: (1) melting of Mg(NO3)2.6H2O, (2) dehydration to Mg(NO3)2.2H2O and (3) to Mg(NO3)2, (4) a polymorphic conversion, and (5) denitration and MgO formation.

a polymorphic transition of anhydrous magnesium nitrates, as discussed later. Given the relevance of the heating rates in TGA analyses, we evaluated the morphology of MgO products obtained under different decomposition rates (Figure 4A, solid traces). Samples were heated from 25 to 550 °C at rates of 20, 2.5, and 0.5 °C min−1. At the fastest heating rate, SEM images reveal relatively monodisperse MgO crystals (average size = 1.5 μm) with an octahedral-like morphology and surfaces presenting wide layered steps (Figure 4B) and textured corners. At slower heating rates (Figure 4D), the particle morphology is ill-defined

Figure 4. (A) Heating profiles used for the thermal decomposition of Mg(NO3)2·6H2O in air. Solid traces indicate direct treatments, whereas dashed traces indicate stepwise (partial) treatments for different heating profiles (details are provided in Figure S6). (B−D) SEM images of MgO products produced by single profile treatments to 550 °C at different heating rates: (B) 20, (C) 2.5, and (D) 0.5 °C min−1. Scale bars: 500 nm. D

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Figure 5. PXRD patterns of Mg(NO3)2·6H2O calcined (stepwise) to increasing temperatures (details are provided in Figure 3A and Figure S6 of the Supporting Information) at a nominal rate 2.5 °C min−1, (A) under air and (B) in LiNO3 (30 wt % of Mg(NO3)2·6H2O), after cooling to RT. Calcination temperatures are indicated. Reference diffraction patterns correspond to (1) Mg(NO3)2·6H2O (ICDD no. 00-014-0101), (2) Mg(NO3)2·2H2O (ICDD no. 01-076-0930), (3) Mg3(OH)4(NO3)2 (ICDD no. 00-026-1221), (4) rhombohedral Mg(NO3)2 calculated from single crystalline diffraction data,45 (5) cubic Mg(NO3)2 (ICDD no. 00-019-0765), and (6) MgO (ICDD no. 01-087-0653). (C) Phase transitions observed from PXRD patterns measured at RT, tracking the thermal decomposition of the Mg(NO3)2·6H2O precursor. Each space group and corresponding crystal system is indicated.

as 500 °C (Figure S7). This implies that fully anhydrous Mg(NO3)2 cannot be formed at fast heating rates, and thus, the system largely decomposes in a liquid state. Slow heating rates (Figure S8B) shift the TGA profile in the opposite direction; i.e., Mg(NO3)2·2H2O is stable in a shorter temperature range (250−300 °C), the reaction mixture is solid at T > 250 °C, and complete MgO formation occurs at T < 450 °C (while at 2.5 °C min−1 this occurs at 450 < T < 500 °C). As a consequence, MgO forms via a solid-state reaction where the ionic mobility is comparatively low. Moreover, we do not observe evidence of melting of the purely anhydrous Mg(NO3)2 phase at 390−420 °C, as previously reported.47 To substantiate the effect of water, Mg(NO3)2·6H2O was calcined using a tubular furnace where H2O-free air was flown during calcination (Figure 6) and compared to the same thermal treatment performed under lab (wet) air. Samples were heated at 20 °C min−1, with an intermediate 4-h period of constant heating at 300 °C to dry the samples (Figure 6A), after which the samples were analyzed for their hydration state (Figure 6B). Thermal treatment was continued for several samples until MgO formation at 550 °C, followed by SEM analysis of crystal habit (Figure 6C,D). According to PXRD analysis (Figure 6B), the sample treated under H2O-free air flow retained less water than the sample treated in lab air, as expected. A higher proportion of

Mg(NO3)2 was obtained in the former procedure, whereas significant fractions of Mg3(OH)4(NO3)2 and Mg(NO3)2· 6H2O appeared in wet air. Interestingly, the morphology of MgO crystals can be correlated with the water content in the synthesis mixture; textured octahedral particles formed under high water content (Figure 6C), whereas ill-defined morphologies formed at a lesser degree of hydration (Figure 6D). The stages observed when treating samples at different heating rates are summarized in Figure 5C, showing the intermediates formed and their crystal structure at RT, respectively. During the thermal decomposition of Mg(NO 3 )2 ·6H 2O, other hydroxo-nitrates have been reported. Examples include Mg2(OH)3NO3 (12 h at 330 °C),48 which is claimed to undergo a topotactic decomposition to MgO;49 however, we did not observe the formation of this phase, nor Mg(OH)NO3· nH2O intermediates,50,51 in our samples. We believe the texture (i.e., stepped faces and rough corners) of the octahedral particles produced at high heating rates may result from step bunching taking place during crystal growth at high supersaturations, or may be attributed to nonclassical growth modes. There is no definitive evidence of the latter mechanism in this study; however, the visual appearance of octahedral-like particles lacking well-defined facets (Figure 3B), rough surface features (Figure 4C), and “fused” crystals (e.g., Figure 4D) are akin to nonclassical pathways and thus suggest E

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(based on phases identified by PXRD). The WRC decreases as Mg2+ ions are depleted with the concomitant formation of MgO crystals. These findings are consistent with prior reports indicating other MS systems exhibit high WRC under similar synthesis conditions.54,55 Attempts to increase the initial Mg:H2O ratio by H2O (l) addition did not significantly alter the crystal morphology. This seems to indicate that the WRC is limited by the Mg(II) content. Excess H2O is likely released from the system before MgO formation. It should be noted that when using a heating profile that enhances water removal from the mixture prior to nitrate decomposition, rhombic (or rhom) Mg(NO3)2 was observed in PXRD patterns (Figure S10), thus confirming lower degrees of hydration during MgO formation can be achieved in these systems. This phase was not observed when heating the mixture with a linear ramp (Figures 4A, 5B and S9). Octahedral particles formed upon further heating to 550 °C (not shown), indicating that relatively low water levels in molten nitrate media are sufficient to stabilize octahedral facets. According to Jupille et al.,56,57 cubic (or cub) MgO particles can be selectively etched in water to preferentially expose MgO{111} facets, resulting in truncated octahedra. They concluded that under wet conditions, hydroxylated MgO surface energies Γ(hkl) follow the order Γ(111) < Γ(100) < Γ(110), which is the inverse of O-terminated surfaces produced in air (i.e., Γ(100) < Γ(111)). Moreover, hydroxylated MgO(111) has been shown to be more stable than reconstructed MgO(111) exposing (100) microfacets.58,59 It could be hypothesized that in Mg(II)-nitrate systems chemisorption of H2O and/or OH− during MgO formation may render MgO{111} facets more thermodynamically stable; however, during the later stages of calcination in MS media the water content is significantly reduced, which lessens the likelihood of this putative mechanism. Alternatively, kinetic factors may play a role in the formation of {111} facets. Notably, H2O may affect the coordination sphere of Mg(II) during MgO formation, thus altering the nature of the growth unit(s) and their mode of incorporation in MgO particles. Elucidating the exact mechanism(s) requires further study. MgO octahedra prepared in molten LiNO3 by Xu et al. and characterized by TEM show what appear to be grain boundaries (or oriented crystal domains).25 These images along with cursory evidence in our study (Figures 4B−D) are surprisingly similar to images of materials that form via nonclassical crystallization mechanisms.35 This observation is merely suggestive in that such pathways are possible during the formation of MgO. Further studies to acquire definitive proof are required to confirm the detailed mechanism of crystallization. Thermal Decomposition of Mg(NO 3 ) 2 ·6H 2 O in NaCl:KCl (1:1). When octahedral MgO crystals are produced in less expensive media, such as NaCl:KCl, drastic changes in the decomposition pathway take place (Figure S1, image A3). To study the decomposition pathway in NaCl:KCl (1:1), a 30 wt % Mg(NO3)2·6H2O mixture was ground, thermally treated to 350 °C, cooled, and measured by PXRD (Figure 7A). KMgCl3.6H2O (carnallite) readily formed during grinding by a mechanochemical reaction with KCl.62 After calcination above 350 °C, K3NaMgCl6 was the only intermediate that was identified.60,61 It formed at this temperature under three separate growth environments: static (wet) air, H2O-free air flow, and N2 flow. While the reactions leading to the formation of this intermediate are not clear, its pathway may potentially

Figure 6. Mg(NO3)2·6H2O was calcined according to the heating ramp in (A), either under H2O-free air flow (120 mL/min) or under static laboratory air. After reaching the full and partial treatment, samples were analyzed by (B) PXRD and (C and D) SEM. Electron micrographs correspond to MgO produced by full treatment under (C) lab air and (D) H2O-free air flow. Scale bars: 500 nm.

the possibility for crystallization by particle attachment or processes involving (nearly) oriented attachment.35 Thermal Decomposition of Mg(NO3)2·6H2O in Molten Nitrates. The stepwise decomposition of Mg(NO3)2·6H2O in LiNO3 at 2.5 °C min−1 is presented in Figure 5B. In this study, we used 30 wt % Mg(NO3)2·6H2O to increase the PXRD signal of the reaction intermediates. Similar to samples prepared under high heating rates in air (Figure S8A), a high water content persists throughout the series of decompositions. A high water retention capacity (WRC) was also demonstrated for the Mg(NO3)2-NaNO3-KNO3 system (Figure S9). When alkali nitrates are included in the reaction medium, eutectic mixtures of relatively low melting point (≤250 °C) are formed (see Table S2).52 In the Mg(NO3)2-LiNO3-H2O and Mg(NO3)2-NaNO3-KNO3-H2O eutectic systems, the presence of a liquid reaction mixture during most of the Mg(NO3)2· 6H2 O decomposition process is ensured (Figure S7), particularly at high temperatures during the formation of MgO.53 These conditions yield octahedral MgO crystals wherein we postulate that the high mobility of Mg2+ ions (in the presence of residual water) for eutectic mixtures, similar to molten Mg(NO3)2·nH2O (n < 2), likely facilitates the formation of MgO{111} facets. The hygroscopic nature of alkali nitrates increases as the ionic radius of the alkali cation decreases; therefore, LiNO3 is expected to have a higher WRC than NaNO3-KNO3. Nevertheless, the WRC of these mixtures is largely determined by the presence of Mg2+ ions, as H2O is preferentially associated with Mg(II) phases rather than to the alkali ions F

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Chemistry of Materials

judging by the visible appearance of a powder inside the crucible within the furnace. Nevertheless, the formation of octahedral MgO was confirmed after purification and SEM imaging (Figure S11), even after enhancing the sample drying by a prolonged pretreatment at 300 °C before full decomposition (this was intended to avoid any possible MgO(111) stabilization by H2O/OH−, potentially present due to incomplete drying). Zhang et al. reported that K3NaMgCl6 melts in the range 461−473 °C. Using synthesized K3NaMgCl6, we confirmed the system is fully molten at T ≥ 350 °C, and the formation of MgO with Cl2 bubbling in the liquid occurs at T ≥ 525 °C. By SEM, we confirmed that the MgO crystals are terminated with {111} facets. Therefore, in NaCl:KCl (1:1), while bulk melting of the mixture does not occur, the K3NaMgCl6 intermediate might locally melt, conferring high mobility to the Mg(II) species. Given the negligible WRC of this system, MgO(111) stabilization may take place via the adsorption of K+/Cl− ions on {111} facets or due to the addition of specific growth units (e.g., Mg(II)-chloride complexes). A proposed pathway for MgO synthesis in the presence of NaCl:KCl (1:1) is presented in Scheme S3. It should be noted that in other Mg(II)-chloride systems, the WRC may be higher because of the formation of hydrated or hydroxylated species such as KMgCl3·6H2O (carnallite),64 Mg(OH)Cl, LiCl·H2O, and NaOH·H2O. The role of these species are not well understood and are worthy of further investigation. Although more experiments or computational modeling may be required to explore the effect of MgO−anion interactions on the interfacial energy of crystals, as well as the formation of Mg(II)anion complexes and their potential effect on growth mechanisms, we postulate that the increased ionic mobility of the liquid intermediates is key for understanding the formation of high-surface-energy facets in metal oxides produced by thermal decomposition. Finally, it worth highlighting that while the exact surface structure of the final particles produced is currently unknown, {111} facets may potentially exhibit different surface terminations, i.e., chloride-, hydroxide-59,65 or O-terminated. After salt removal by water rinsing, surfaces may become hydroxylated, and we believe these (and potentially chlorinated) surfaces would be unstable toward dehydration/ oxidation at high temperatures in air, resulting in particles exposing O-terminations. As illustrated in Figure 8, the ionic arrangements of MgO(100) and MgO(111) surfaces are

Figure 7. PXRD patterns of (A) a 30 wt % reaction mixture of Mg(NO3)2·6H2O in NaCl:KCl (1:1) partially decomposed at 350 °C (1 h, 2.5 °C min−1) with the corresponding NaCl:KCl (1:1) blank pattern. (B) Patterns of synthesized K3NaMgCl6 and the resulting decomposition mixture after calcination at 550 °C (2 h, 2.5 C min−1) under (dry) air flow. XRD reference patterns correspond to NaCl (ICDD no. 00-005-0628), KCl (no. 01-073-0389 and 01-076-3366), KNO3 (no. 00-005-0377), KMgCl3·6H2O (no. 00-024-0869), and MgO (no. 01-087-0653). The K3NaMgCl6 reference pattern is taken from literature.60,61

involve the generation of potassium or sodium oxides or nitrates; however, none of these phases could be confirmed by PXRD after calcination at 350 °C. The thermal decomposition of K3NaMgCl6 intermediate was carefully studied. The latter was synthesized following the method of Zhang et al.60 with some modifications. In brief, a Mg(NO3)2·6H2O:KCl:NaCl:NH4Cl mixture with 1:3:1:3 mol ratios was calcined for 1 h at 400 °C (5 °C min−1) under N2 flow to avoid oxidation. The synthesized material was confirmed to be composed mainly of K3NaMgCl6 (Figure 7B). The PXRD pattern of thermally decomposed K3NaMgCl6 exposed to H2O-free air at 550 °C shows its direct conversion to MgO. At high temperatures, Cl− anions are oxidized to Cl2 during the formation of MgO (eq 1), with KNO3 or O2 acting as a Lux-Flood base. Indeed, the formation of Cl2 has previously been reported in the oxidation of magnesium chloride in air at high temperatures63 and was confirmed by its characteristic odor and by the formation of a blue complex when bubbling the released gases in a KI/starch solution. 1 K3NaMgCl6(l) + O2 2 → MgO(s) + Cl 2(g ) + NaCl + 3KCl (1)

Figure 8. Proposed surface structures of (A) cubic particles where nonpolar {100} facets exhibit both Mg(II) (gray) and O(II) (red) ions, and of (B) octahedral particles whose polar {111} facets are terminated exclusively by O(II). Insets show surface planes viewed in the and directions (planes are indicated in light blue on the three-dimensional structures).

The formation of MgO occurs at temperatures less than 550 °C, which is considerably lower than the MP of a NaCl:KCl (1:1) eutectic mixture (ca. 658 °C). Samples removed from the furnace at 550 °C did not show any evidence of melting, G

DOI: 10.1021/acs.chemmater.7b05302 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

When evaluating the formation of reaction intermediates, samples were removed from the furnace, left to cool naturally to room temperature, and immediately characterized by PXRD. Synthesis of the K3NaMgCl6 Intermediate. The protocol described by Zhang et al. was used with slight modifications.60 Briefly, a solid mixture composed of Mg(NO3)2·6H2O: KCl: NaCl: NH4Cl in molar proportions 1:3:1:3 was mortared, transferred to an alumina crucible, and calcined for 1 h at 400 °C (5 °C min−1) under N2 flow (120 cc min−1). The compact product was ground in ambient conditions and stored. Decomposition of this precursor was studied by calcination under air flow (120 mL/min) at different temperatures. Characterization of Materials. Scanning electron microscopy (SEM) images were acquired with a LEO FEI 1525 SEM microscope and Inlens detector using an accelerating voltage of 3 kV and a working distance of 3−5 mm. Samples were prepared by placing the powder on an Al stub supporting carbon tape and coating the samples with a thin layer of carbon (ca. 15−30 nm) to improve the sample conductivity. Powder X-ray diffraction (PXRD) was performed using a SmartLab Rigaku diffractometer with a Cu Kα source (λ = 1.5406 Å). Reference PXRD patterns were selected from the ICDD PDF-2 2013 database, unless otherwise mentioned. The rhombohedral Mg(NO3)2 PXRD pattern was calculated from single crystalline diffraction data using the program Diamond version 3.2k. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained in a JEOL 2010F 200 kV electron microscope equipped with a cool snap camera for wide-angle, low magnification images and a MultiScan CCD camera (MSC) for high resolution images in Gatan imaging filter (GIF) mode. Thermogravimetric analysis (TGA) was performed with a 2050 TGA V5.3C thermogravimetric analyzer (TA Instruments) using a heating rate of 2.5 °C min−1 and 100 mL min−1 air flow. Differential scanning calorimetry (DSC) was carried out using a Mettler Toledo gas controller DSC system. The sample was placed in an alumina crucible and heated to 600 °C.

markedly different. The ability to alter the chemical landscape of MgO crystal surfaces (i.e., binding sites for adsorbates) impacts their performance in applications such as catalysis where it is expected that O-terminated {111} facets exhibit different catalytic activities.

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CONCLUSIONS The thermal decomposition of metal nitrates involves an intricate series of phase transformations and decomposition steps, which depend on the reaction medium. Here, we have shown in the case of Mg(II) that the decomposition of intermediates in the liquid state is critical for the formation of MgO with {111} facets. In air, these facets on MgO exhibit higher surface energy than more conventionally observed {100} facets. Our findings highlight the advantages of using MSs in the preparation of octahedral MgO crystals with {111} facets to ensure that pathways involving liquid intermediates are undertaken. To our knowledge, the relevance of these liquid intermediates in the morphological control of oxide products produced by thermal decompositions of pure precursor or under different reaction media has not previously been reported. Given the similarity of metal oxide syntheses, it is likely that the pathways observed in this study for MgO apply to other oxide systems (e.g., NiO, ZnO) for the generation of new morphologies and surface terminations. Indeed, the ability to tailor metal oxides with predetermined facets opens possibilities to rationally design functional materials with improved properties for catalysis, gas adsorption, and other commercially relevant applications.

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EXPERIMENTAL SECTION

Materials. Mg(NO3)2·6H2O (≥99%), MgCl2·6H2O (≥99%), NaOH (Macron Chemicals), NaCl (≥99.5%), KCl (≥99.0%), NH4Cl (≥99.5%, Mallinckrodt), LiNO3 (≥99.0%), NaNO3 (J. T. Baker, 99.6%), KNO3 (J. T. Baker, 99.4%), basic magnesium carbonate (purum), and magnesium chips (≥99.98%) were acquired from SigmaAldrich unless otherwise mentioned. All reagents were used as received without purification. Deionized (DI) water was provided by an Aqua Solutions RODI-C-12A purification system (18.2 MΩ). Preparation of Oxide Precursors. MgO smoke particles were produced by burning Mg chips in air and collecting the smoke particles with an inverted glass funnel. Mg(OH)2 particles were prepared by two methods: (1) Precipitation method. A 2.0 M NaOH solution was rapidly added to a 10 mL solution of 1.0 M MgCl2·6H2O under magnetic stirring in a polypropylene (PP) vial. The reaction mixture was aged for 1.5 h, and the precipitate was separated by centrifugation (5000 rpm, 5 min) and purified by three cycles of washing (∼30 mL DI water) and centrifugation. The final product was suspended in ethanol, dried overnight at 50 °C, and ground with a mortar and pestle. (2) Hydrothermal method. Approximately 10 mL of the reaction mixture obtained after 1.5 h aging was transferred to a 25 mL stainlesssteel Teflon-lined autoclave (Parr Instruments) and hydrothermally treated at 180 °C in a Thermo Fisher Scientific Precision 3050 Heratherm gravity convection oven. The mixture was removed after 16 h and cooled to room temperature by immersion in a water bath. The resulting suspension was purified as previously described in the precipitation method. Calcination in Air and in Molten Salts. All calcinations were performed in alumina crucibles in a Thermolyne muffle furnace in lab (wet) air, or in a Lindberg Blue M tubular furnace (Thermo Fischer Scientific) under the flow of H2O-free air (