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. 2021 Jan 11;6(3):2312–2317. doi: 10.1021/acsomega.0c05619

Electrochemical Growth of Mg(OH)x Layered Films Stacked Parallel to the Substrates and Their Thermal Conversion to (111)-Oriented Nanoporous MgO Films

Tsutomu Shinagawa †,‡,*, Masaya Chigane †, Masanobu Izaki ‡
PMCID: PMC7841930  PMID: 33521469

Abstract

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Stacking layered metal hydroxide films parallel to a substrate is challenging. Here, we demonstrate a simple and rapid electrodeposition method for stacking magnesium hydroxide layered films. Room-temperature cathodic electrolysis (40 mA cm–2) in a Mg(NO3)2 aqueous solution induces the deposition of ⟨001⟩-oriented Mg(OH)x layered films stacked parallel to the substrate at the deposition rate of ∼2 μm min–1. The obtained Mg(OH)x layered films undergo an overall oriented transformation by heat treatment to form ⟨111⟩-oriented nanoporous MgO films.

Introduction

Layered metal hydroxides (LMHs), including layered hydroxide salts (LHSs, MxII(OH)2x−myAym–·nH2O) and layered double hydroxides (LDHs, M1–xIIM’xIII(OH)2(Am–)x/m·nH2O), where typically MII = Mg, Fe, Co, Ni, and Zn, M′III = Al, Cr, Fe, Co, and In, Am– = Cl–, NO3–, SO42–, and CO32–, have attracted increasing attention owing to their potential applications such as in catalysts, capacitors, adsorbents, chemical sensors, and photoelectrodes.1−6 LMHs also act as a valuable precursor to yield unique nanostructured materials, e.g., two-dimensional (2D) hydroxide thin films by exfoliation and nanoporous oxides by thermal decomposition.7−10

Typically, LMHs are synthesized in a powder form by precipitation or hydrothermal methods.11 However, the application of LMHs and the corresponding nanoporous oxides as electrodes requires the deposition of LMH films onto substrates. Since LMHs consist of hydroxides and hydrates, the vapor-phase deposition methods (e.g., sputtering) are not applicable. Therefore, a facile and controllable deposition method for LMHs is desired. In particular, oriented film formation, in which the LMH layer is stacked parallel to the substrate, remains a major challenge. The oriented LMH layered films stacked on a centimeter-size substrate will offer new applications as a unique functional 2D material. Further, when the thermal decomposition of the oriented LMHs proceeds topotactically, the formation of highly oriented nanoporous oxides can be expected.12,13

In this paper, we report an electrochemical method for easy stacking of ⟨001⟩-oriented layered magnesium hydroxide on substrates in an aqueous solution and their thermal decomposition to ⟨111⟩-oriented nanoporous magnesium oxide (Scheme 1).

Scheme 1. Schematic of the Formation of Oriented Mg(OH)x and MgO Films.

Scheme 1

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Magnesium hydroxide Mg(OH)2, known as “brucite”, is one of the fundamental LMHs that does not contain an anion Am– or hydrate water. In the Mg(OH)2 crystal with a hexagonal structure, edge-sharing octahedral Mg–(OH)6 forms 2D sheets, which are parallel to the ab plane and are stacked in the c-axis direction via weak interlayer forces with a distance of 4.78 Å (Figure 1a). The 2D M–(OH)6 sheet structure is a key framework that is common to most LMHs that contain various metal and anionic species.14 Magnesium hydroxide decomposes at temperatures above ∼350 °C to generate pseudomorphic magnesium oxide MgO with a cubic rock salt structure (Figure 1b).15 The crystallography of the thermal decomposition process has been intensively studied using transmission electron microscopy (TEM) and reported to proceed topotactically with a crystallographic orientation relationship of Mg(OH)2(001)[110]//MgO(111)[11̅0].16−20 Magnesium oxide is a dielectric with a band gap energy of 7.8 eV.21 Due to the high thermal and chemical stability, MgO calcined at high temperatures above 1800 °C is used as a refractory material, while small MgO particles calcined at low temperatures (400–600 °C) exhibit high catalytic activity and adsorption properties.22−24

Figure 1.

Figure 1

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Crystal structure of (a) Mg(OH)2 (brucite, ICDD no. 44-1482) and (b) MgO (periclase, ICDD no. 45-0946), which were produced using VESTA version 3.38 software.25

To grow magnesium hydroxide on a substrate, we used the electrodeposition technique, which is one of the few deposition methods that allows for controlled deposition of LMHs. Thus far, electrochemical deposition of LMHs in an aqueous solution has been reported mainly for LHSs,26 including Mg(OH)2.27−31 Recently, electrodeposition of MgO/Mg(OH)2 mixtures has been reported using a magnesium nitrate melt.32,33 An advantage of the electrodeposition is that by applying a cathodic current to a conductive substrate, OH– ions can be generated on the substrate surface. The following three cathodic reactions 1–3 are typically used to generate OH– ions26,34,35

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The generated OH– ions react with metal ions to precipitate LHS species on the substrate. A precipitation reaction can be formally expressed as follows13,36

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The generation rate of OH– ions can be directly controlled by the applied current (or electric potential), which affects the deposition rate of LHSs. The amount of deposition (film thickness) can also be adjusted by the total electric charge applied to the substrate.

Results and Discussion

Electrodeposition of Mg(OH)x was carried out using a 50 mM (M = mol dm–3) Mg(NO3)2 aqueous solution by applying cathodic current at room temperature. Figure 2 shows cross-sectional field-emission scanning electron microscopy (FESEM) images of deposits obtained on Sn:In2O3-coated glass (ITO) substrates at various constant current densities ranging from 0.5 to 60.0 mA cm–2. At the low current density of 0.5 mA cm–2, leaf-like thin layers densely grew perpendicular to the substrate. The growth direction and morphology of Mg(OH)x deposits changed with an increase in the current density. While similar tendency can be seen in the literature,30 the cross-sectional morphology is unknown due to surface observation only. Above 20.0 mA cm–2, thin layers grew parallel to the substrate after a slight deposition of vertical layers near the substrate interface, resulting in layered films with a thickness of 0.4–0.55 μm. Because the total charge density supplied to the substrates was maintained at the constant value of 0.5 C cm–2, the deposition time was 25.0, 12.5, and 8.3 s when the current density was 20.0, 40.0, and 60.0 mA cm–2, respectively. Thus, layered films parallel to the substrate were formed at the deposition rate of 1.2–3.2 μm min–1. Further studies are underway in our laboratory on the deposition mechanism by which increased OH– generation rates give the layered film structure and the applicability of this technique to other metal species.

Figure 2.

Figure 2

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Cross-sectional FESEM images of Mg(OH)x deposits electrodeposited on ITO substrates in a 50 mM Mg(NO3)2 aqueous solution at the current density of (a) 0.5, (b) 1.0, (c) 5.0, (d) 10.0, (e) 20.0, (f) 40.0, and (g) 60.0 mA cm–2.

To characterize Mg(OH)x layered films in detail, single-crystal n-Si(100) substrates were used because they avoid overlapping X-ray diffraction (XRD) peaks between Mg(OH)x and the substrate and can be heat-treated at temperatures above 600 °C. Figure 3 shows FESEM images of Mg(OH)x electrodeposited on the Si substrate in a 50 mM Mg(NO3)2 aqueous solution at the current density of 40.0 mA cm–2. Similar to the ITO substrate, a stacking structure was observed, suggesting that this method is substrate independent. We also confirmed that similar Mg(OH)x layered films were obtained on the Si substrate at current densities of 20.0 and 60.0 mA cm–2. Magnified cross-sectional FESEM images revealed that slightly wavy thin layers (15–20 nm thickness) grew parallel to the substrate, resulting in 0.4–0.5-μm-thick layered films. The surface images showed that unfaceted plate-like grains with diameters of approximately 100 nm coalesce to give a continuous film.

Figure 3.

Figure 3

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(a, b) Cross-sectional and (c, d) surface FESEM images of as-deposited Mg(OH)x films electrodeposited on Si(100) substrates in a 50 mM Mg(NO3)2 aqueous solution at the current density of 40.0 mA cm–2.

The θ–2θ XRD pattern of the obtained Mg(OH)x layered film showed a broad peak at 2θ = ∼18.5°, which was assignable to 001 Mg(OH)2 (2θ = 18.53°; ICDD no. 44-1482), and no other peaks, except for the Si substrate, were observed (Figure 4a). In the attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectrum (Figure 4b), a characteristic absorption band corresponding to OH stretching was observed at 3693 cm–1. This value was in agreement with that for Mg(OH)2 reported in the literature.37,38 At 1318 cm–1, a slight peak assignable to a tridentate carbonate ion was observed,24 which is likely due to the uptake of dissolved CO2 during electrodeposition. The constituent elements of the deposits were confirmed by X-ray photoelectron spectroscopy (XPS) to be only Mg and O (Figure 4c). These results indicate that low-crystalline Mg(OH)x layers were deposited with a preferred ⟨001⟩ orientation to form the stacking structure. This means that the Mg(OH)2 (001) plane consisting of the 2D Mg–(OH)6 sheet is parallel to the substrate.

Figure 4.

Figure 4

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(a) θ–2θ XRD pattern, the (b) ATR-FTIR spectrum, and the (c) XPS survey spectrum of as-deposited Mg(OH)x films electrodeposited on Si(100) substrates in a 50 mM Mg(NO3)2 aqueous solution at the current density of 40.0 mA cm–2. The ICDD data of Mg(OH)2(#44-1482) are also shown in (a).

According to the topotactic relationship of Mg(OH)2(001) [110]//MgO(111)[11̅0] between the Mg(OH)2 precursor and the thermal decomposition product MgO,16−20 generation of MgO with a ⟨111⟩ preferred orientation can be expected by heating the ⟨001⟩-oriented Mg(OH)x films. The electrodeposited Mg(OH)x layered films were heat-treated at 800 °C for 1 h in air. As shown in Figure 5, the layered structure changed to a nanoporous structure comprising aggregates of faceted nanocrystals with diameters of ∼60 nm. The nanopores were connected from the surface to the substrate. Although heat treatment reduced the thickness to approximately 4/5 (∼350 nm), cracks were not observed on the surface. Similar nanoporous structures were observed for the heat-treated Mg(OH)x layered films electrodeposited at current densities of 20.0 and 60.0 mA cm–2 (Figure S1 in the Supporting Information).

Figure 5.

Figure 5

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(a, b) Cross-sectional and (c, d) surface FESEM images of MgO on Si(100) substrates heated at 800 °C for 1 h in air.

In the θ–2θ XRD pattern after heating, sharp peaks were observed at 2θ = 36.9 and 78.6°, which were assigned to 111 and 222 diffractions of MgO with a cubic structure (2θ = 36.94 and 78.63°; ICDD no. 45-0946), respectively, and no other peaks were observed, except for those attributed to the Si substrate (Figure 6a). In-plane XRD measurements were performed to further confirm the growth orientation. As shown in Figure 6b, a peak was observed only at 2θ = 62.2°, which corresponded to the 220 diffraction of MgO (2θ = 62.30°; ICDD no. 45-0946). Because the plane angle between the (111) and (11̅0) planes in the cubic system is 90°, these XRD results clearly indicate that the ⟨001⟩-oriented Mg(OH)x layered films were thermally converted to ⟨111⟩-oriented MgO films. XRD patterns for the heat-treated Mg(OH)x layered films electrodeposited at current densities of 20.0 and 60.0 mA cm–2 are shown in Figure S2 in the Supporting Information, where the formation of ⟨111⟩-oriented MgO films was confirmed. In the FTIR spectrum (Figure 6c), the OH peak completely disappeared, while a weak broad peak likely owing to CO2 adsorption was observed at 1406 cm–1.

Figure 6.

Figure 6

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(a) θ–2θ and (b) in-plane XRD patterns, and the (c) ATR-FTIR spectrum of MgO on Si(100) substrates heated at 800 °C for 1 h in air. The ICDD data of MgO (#45-0946) are also shown in (a).

The MgO(111) plane has attracted attention due to its polarity and high surface energy, and the surface structure and chemical activity of the (111) plane have been the subject of research both theoretically and experimentally.24,39−46 The surface of ⟨111⟩-oriented MgO films obtained from the ⟨001⟩-oriented Mg(OH)x layered films is apparently flat in the FESEM observation (Figure 5), and no obvious formation of neutral (100) facets are observed. Therefore, the surface of the obtained ⟨111⟩-MgO films may be stabilized by atomic-scale reconstruction39 and/or termination by CO2 adsorption.46 In fact, Mutch et al.24 reported that the (111) plane of MgO has a higher CO2 adsorption capacity than the (100) plane and shows an improved adsorption capacity by sintering at 800 °C, demonstrating the importance of controlling the specific facet of oxides.

The transformation from ⟨001⟩-Mg(OH)2 to ⟨111⟩-MgO has been previously observed by electron diffraction on TEM from half a century ago16,18 but was limited to an isolated nano- or micro-Mg(OH)2 crystal particle. To our knowledge, this is the first macroscopic observation of the transformation from ⟨001⟩-Mg(OH)x to ⟨111⟩-MgO on a centimeter scale. As shown in Figure 1, when the interlayer distance of the Mg(OH)2 is shortened and the 2D Mg–(OH)6 layers are in close contact with each other, the crystal structure becomes almost the same as that of MgO, which may allow thermal conversion while maintaining the Mg–O arrangement, i.e., topotactic transformation. However, thermal conversion involving dehydration, Mg(OH)2 → MgO + H2O, and the loss of the interlayer distance causes volume shrinkage. The theoretical shrinkage rates in the horizontal and vertical directions of ⟨001⟩-Mg(OH)2 are 5.7 and 96.3%, respectively; for the Mg(OH)2 lattice, the nearest Mg–Mg distances in the (001) plane and in the ⟨001⟩ direction are 3.15 and 4.77 Å, while those in the (111) plane and in the ⟨111⟩ direction for the MgO lattice are 2.98 and 2.43 Å, respectively. Thus, shrinkage occurs mainly in the vertical direction of the layered film, so the film thickness decreases while forming fine pores, but no noticeable cracks occur on the surface.

Conclusions

In summary, we have demonstrated an electrochemical method affording the formation of ⟨001⟩-oriented Mg(OH)x layered films stacked parallel to the substrate and their thermal conversion to ⟨111⟩-oriented nanoporous MgO. The oriented transformation took place throughout the film and was macroscopic enough to be detected by conventional XRD measurements. Using the electrochemical method, the preparation of various layered metal hydroxides stacked parallel to the substrate and the development of their thermal decomposed products, i.e., oriented nanoporous oxides are expected.

Experimental Section

Electrodeposition of Mg(OH)x Films

Aqueous solutions were prepared using reagent-grade chemicals and deionized water (>10 MΩ cm) purified by the Millipore Elix Advantage5 system. Sn-doped In2O3-coated glass (ITO, Geomatec Co., Ltd., ≤10 Ω sq–1) and n-type single-crystal Si(100) wafer (Canosis Co., Ltd., 525-μm-thick Sb-doped Si, ≤0.02 Ω cm) were used as a substrate. Prior to the electrodeposition, ITO and Si substrates (10 mm × 30 mm) were treated with a UV–ozone cleaner and then rinsed with deionized water. The electrodeposition of Mg(OH)x on the ITO substrate was carried out galvanostatically in an aqueous solution containing 50 mM (M = mol dm–3) Mg(NO3)2 (Wako Pure Chemical Industries) at room temperature with a potentio/galvanostat (Hokuto Denko, HABF5001) by applying different current densities of 0.5, 1.0, 5.0, 10.0, 20.0, 40.0, and 60.0 mA cm–2 at the total electrical charge of 0.5 C cm–2.

Electrodeposition of Mg(OH)x Layered Films and Thermal Conversion to MgO

Mg(OH)x layered films were electrodeposited on the Si substrate (10 mm × 30 mm) in a 50 mM Mg(NO3)2 aqueous solution by applying a cathodic current of 40 mA cm–2 at the total electrical charge of 0.5 C cm–2. After deposition, the deposits were rinsed with deionized water and dried in air. Thermal conversion from the obtained Mg(OH)x layered films to MgO films was performed by calcination at 800 °C for 1 h (increased rate of 10 °C min–1) in air using an electric furnace.

Characterization of Films

The morphological and structural characterizations of obtained films were carried out with field-emission scanning electron microscopy (FESEM, JEOL JSM6700F and JSM7800F) and X-ray diffractometry using Cu Kα radiation (Rigaku SmartLab) using the out-of-plane θ–2θ scan and in-plane scan modes. Chemical characterization was carried out by Fourier transform infrared spectroscopy with a diamond attenuated total reflection crystal (ATR-FTIR, ThermoNicolet 4700-DuraSampleIRII) and X-ray photoelectron spectroscopy (XPS, Kratos AXIS-Ultra DLD) using monochromated Al Kα radiation.

Acknowledgments

This work was supported by a Grant-in-Aid for Scientific Research (C) (No. 18K05310) from JSPS.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c05619.

  • FESEM images and XRD patterns of heated films obtained at different current densities (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c05619_si_001.pdf (328.9KB, pdf)

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