Mn-Promoted Growth of Mg-Based Spinel and Pyroxene Nanostructures.

In this work, we demonstrate Mn-promoted growth of oxide nanostructures standing on a single-crystal MgAl2O4 substrate after heat treatment. Unlike the short truncated spinel pyramids under Au seeds, the addition of Mn produces spinel nanopillars with lengths of 100-300 nm and pyroxene nanowires up to 10 μm. Compared to Au seeds, Au/Mn seeds have different adsorption behavior and therefore provide an additional mass transfer path along seed surfaces that promotes the growth of nanostructures. This vaporization approach has a potential of being applicable to a wide range of complicated oxides.

Introduction

Vertically aligned oxide nanowires on substrates have attracted extensive attention because of their potential applications such as solar cells,[1,2] light-emitting devices,[3,4] field-emission devices,[5−7] and piezoelectric nanogenerators.[8] Up to now, a variety of synthesis techniques have been developed for the growth of well-aligned oxide nanowires including vapor deposition,[9,10] thermal oxidation,[11,12] laser ablation,[13] and solvothermal method,[14,15] most of which focus on simple metal oxides such as ZnO, TiO2, Fe2O3, and In2O3. Recently, a new method has been developed and successfully applied to TiO2 and ZnO.[16] This method utilizes vaporization products of oxide substrates, which can be adsorbed and transported along seed surface, to grow single-crystal oxide nanowires. With high quality in nanowire crystallinity and simplicity in experiment settings, this technique shows promise in fabricating various standing oxide nanowires.

In this study, we extend this method to a multicomponent oxide, MgAl2O4. Compared to TiO2 nanowires (length > 5 μm), the MgAl2O4 substrate grows into pyramid-shaped MgAl2O4 bases, with a height typically less than 20 nm under identical growth temperature and Au seeds.[17,18] It is worth noting that no adsorbate is observed at seed surfaces in the case of MgAl2O4 with pure Au seeds;[18,19] in contrast, thin adsorbate layers appear at seed surfaces for TiO2 and ZnO nanowires.[16] This work is motivated by promoting the growth of MgAl2O4 through tailoring the characteristics of seed surface. Another seed element is added according to its phase diagram with Au and its solubility in MgAl2O4. Therefore, Mn is chosen in order to (i) form seed particles with adsorption behavior differing from pure gold and (ii) have a controllable influence on oxide structure. Consequently, two types of longer nanostructures are obtained with Au/Mn seeds. Nanopillars 100–300 nm in length inherit the spinel structure of the substrate, while, another type of nanowire with a few microns in length is identified as pyroxene MgSiO3, which is an important substance for petrogenetic indicators[20] and silicate glasses.[21] Although the introduction of Si from the quartz tube is not anticipated, the growth mechanism of the above two nanostructures is of particular interest and will be discussed here.

Results and Discussion

Morphology and Composition of Samples

The synthesis of oxide nanostructures was facilitated via heat treatment of Au-coated MgAl2O4 substrates in inert gas with Mn powder added to the other end of a closed quartz tube. The epitaxial regrowth of the MgAl2O4 substrate under pure Au seeds is depicted in Figure c. Interestingly, with Mn added to the system, significant growth of nanostructures is observed after heat treatment at identical experiment settings. As shown in the SEM micrograph in Figure a and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) micrograph in Figure b, two types of nanostructures are distributed on substrate surface. The shorter type is denoted as type A nanopillars, and the other type with a length of a few microns is referred to as type B nanowires. Both types have similar diameters of less than 200 nm, constrained by the size of distinct seeds on the top. A gradual change in the diameter of nanostructures is observed. For type A, there is a cap-like region adjacent to the seed, whereas type B nanowire has a necking region at the bottom (see the HAADF image in Figure f).

Figure 1

Morphology and compositions of nanostructures grown on the substrate. (a) Side-view SEM micrograph of MgAl2O4 substrate after heating at 1100 °C for 60 min with Mn added. (b) HAADF-STEM micrograph of type A nanopillars and type B nanowires. (c) MgAl2O4 truncated pyramid after heating at 1100 °C for 60 min without Mn. (d–f) EDS mappings of Al, Mg, Mn, O, Si, and Au in nanopillar (d), nanowire body (i), and nanowire bottom (f) with corresponding HAADF micrographs. (g) Quantified atom ratios of elements in ∼20 nanowires and nanopillars. Scale bars: 500 nm in panels (a,c), 250 nm in panel (b), and 30 nm in panels (d–f).

From energy-dispersive X-ray spectroscopy (EDS) mappings shown in Figure d–f, all the seeds are composed of Au, Mn, and trace Mg, Al, and Si; while the body of nanostructures consists of Mg, Al, Mn, Si, and O. In particular, unlike Al-rich nanopillars, the nanowires are deficient in Al except the necking area. In order to obtain precise composition of the nanostructures, elemental quantification is performed using Bruker Esprit software, and the results are presented in Figure g. According to the relative atomic ratios between elements, namely, Mn/Au and X/Mg (X = Al, Mn, Si, O), most particles are AuMn and a few of them are AuMn2. The bottom of nanopillars and the necking region of nanowires have similar compositions with a Mg:Al:O ratio that is approximately equal to 1:2:4 and trace Si and Mn. Specially, the body region of type A nanowires is close to Mg:Si:O = 1:1:3 with a minority of Al and Mn, whereas in nanopillars, the cap region has a higher concentration of Si than in the bottom. It should be noted that the quantified atom percentage of oxygen may not be fully reliable in EDS. Except the quartz tube (99.99% SiO2), no other source of Si exists in the current system that only contains high-purity Mn powder and single-crystal MgAl2O4. The unexpected Si probably originates from the quartz tube at high temperature, likely associated with the existence of manganese because Si is not detected in the grown pyramids in understudied systems without manganese (i.e., the pyramids shown in Figure c).

Crystal Structures of the Nanopillars and Nanowires

The detailed structures of the two types are carefully examined with a transmission electron microscope (TEM) as seen from Figures and 3. As shown in the diffraction pattern (DP) and lattice fringes in high-resolution TEM (HRTEM) micrographs in Figure b,c, the nanopillars have a cubic spinel structure, which is the same as bulk spinel substrates, and grow along the <111> direction. Thus, the spinel nanopillars probably have an epitaxial orientation relationship with the (111)-oriented MgAl2O4 substrate. In addition, X-ray diffraction results indicate only the (111)-oriented spinel. It should be noted that the weak signals from nanopillars can be submerged in the strong substrate signal, leading to nondetectable orientations of nanopillars in XRD.

Figure 2

Type A nanopillar. (a) TEM micrograph of single nanopillar (scale bar is 50 nm). (b) Indexed DP of the nanopillar. (c) HRTEM micrograph of the crystal. The d spacings measured in the lattice fringe correspond to {111} spinel (0.461 nm) and {220} spinel (0.286 nm). (d,e) High-loss EELS spectra of O-K and Mn-L2,3.

Figure 3

Nanowire. (a,b) DP and HRTEM images of corresponding nanowires at the [010] and [100] zone axes, respectively. The d spacings measured correspond to the (100) and (010) of the orthopyroxene phase, and the HRTEM simulations inserted are calculated from the orthopyroxene structure with a multislice approach. The defocus value and sample thickness are 77 and 50.58 nm in panel (a) and 107 and 31.08 nm in panel (b). The scale bar in TEM micrographs is 20 nm. (c) The [010] projections of the three polymorphs of MgSiO3 generated by Vesta[28] including ICSD 9328, ICSD 30808, and ICSD 30893.

In addition, the interface between the particle and the grown oxide is not atomic sharp, forming a transition region that overlaps the two parts. Another feature is the asymmetric shape of the cap, indicating an unbalanced growth along the axis that may account for the deviation from substrate normal as observed in Figure a. In order to reveal the variations in structures, energy loss electron spectra (EELS) are acquired across the interface with O-K and Mn-L2,3 edges recorded simultaneously in Figure d,e. The O-K edges far away from the interface can be fingerprinted to MgAl2O4 spinel.[22,23] As it approaches the interface, the shoulder feature of the predominant peak remains until in the cap region where the shoulder becomes a sharp peak with a reducing intensity. This new feature mimics the shape of fourfold coordinated Si in quartz and some silicates such as forsterite,[24−26] indicating a structure change over this region. On the other hand, no visible change except the intensity is observed for Mn-L2,3 edge at 640 and 650 eV. Notably, the highest counts of Mn-L2,3 signals are recorded near the interface, implying an enrichment of Mn at the growth front.

Figure presents experimental DPs and HRTEM micrographs of type B nanowires along different zone axes (another two sets of DPs and corresponding DP simulations can be found in Figure S1). After examining the DP simulations of all stable phases (>20) reported in the Inorganic Crystal Structure Database (ICSD) that contain Mg, Si, Mn, Al, and O, we believe that the crystal structure of type B is orthopyroxene-MgSiO3 (more details in the Supporting Information and Figure S2). This is further verified by the good agreement between experimental and simulated HRTEM micrographs inserted in Figure a,b. It is worth noting that the (100) planar defects can be observed in most nanowires viewed along the [010] zone axis, leading to the existence of non-integer reflections and streaks parallel to (100) in the DPs. Such stacking faults have been previously reported during phase transformations between polymorphs of MgSiO3, likely the transition from protoenstatite (Pbcn, 1300–1850 K) to orthoenstatite (Pbca, 400–1300 K) or low-clinoenstatite (P21/c)[27] considering the pressure and temperature conditions in our case. Based on the cation distribution reported in the pyroxene group (general formula XYZ2O6, where X represents the distorted M2 site, Y represents octahedral sites, and Z represents tetrahedral sites),[20] we believe that Si occupies tetrahedral sites, Mn prefers M2 sites, and Mg and Al can be located in both M1 and M2 sites accompanying cation disorder.

According to the diffraction patterns of multiple nanowires (>20), type B nanowires grow along the [001] direction. The angle between the type B nanowires and the substrate is approximately 82° (see the measurement from SEM images in Figure S3 and the scheme in Figure b). Although type B nanowires have a preferential growth direction with respect to the spinel bases, no clear evidence regarding lattice matching exists between orthopyroxene-MgSiO3 and MgAl2O4 lattices (see the atomic models inserted in Figure b).

Figure 4

Scheme of growth models in (a) Au-coated pure MgAl2O4 system and (b) Au-MgAl2O4 with Mn added in the tube. The size of circles in the atmosphere represents the relative amount of corresponding gaseous species. In the atomic models of spinel and orthopyroxene, red, cyan, orange, and blue spheres are O, Al, Mg, and Si, respectively.

Growth Mechanism of the Oxide Nanostructures

Taken together, type A nanopillars have a spinel structure with higher content of Si in the cap region, and type B nanowires are orthopyroxene MgSiO3 with a spinel bottom that is close to MgAl2O4 (EDS quantification errors are 5–10% for O and 2–5% for the remaining elements). Within this closed tube, the source materials for these nanostructures can be (i) solid MgAl2O4 substrate, (ii) manganese powder, and (iii) the quartz tube (99.99% SiO2). With pure Au seeds, the formation of a short truncated pyramid base is proposed to result from surface diffusion of the substrate at elevated temperatures (see Figure a).[17,29] According to the shape differences appearing at the roots of the nanostructures (see Figure S5), the above bases also form at the beginning growth stage of the present two nanostructures. In addition, thin oxide layers are observed at the surface of nanoseeds and are alike for both nanostructures (see quantified mappings in Figure S4). With the significant O layer and weak sparse signals of Mg, Al, and Si, particularly in the regions close to the interface, mass transfer of oxide clusters along the seed surface is believed to play an important role in the growth of multicomponent oxide nanopillars and nanowires.

At growth temperature (1373 K), the vaporization products are primarily Mg, Mn, O, SiO, Au, and a smaller amount of Al with corresponding equilibrium pressures listed in Table . The oxidation species of AlO and Al2O are negligible here because they are a few orders of magnitude less than Al and O2 species,[33,34] and the vapor of Au is not considered here due to its limited contribution to the growth of nanostructures. Due to the incongruent vaporization of MgAl2O4, the concentration of Al in an argon atmosphere is 3 orders less than those of Mg and SiO, which leads to Al-deficient MgSiO3 nanowires. This phase is connected to the spinel base through the cap region, which is likely to be a mixture of orthopyroxene and spinel structure. The major composition of this cap region, Mg-Al-Si-O, favors the formation of both aluminous orthopyroxene[35] and Si-doped MgAl2O4 spinel that has been reported by Hashishin et al.[36] despite the lack of thermodynamic data. As a result, micron-scale pyroxene nanowires will continuously grow under some of the seeds if pyroxene nucleates at the droplet–cap interface.

Table 1

Vaporization of (Solid) Phases at 1375 K in the System

initial phase		high-temperature species	partial pressure in vacuum (bar)	ref
MgAl2O4 (s)	MgO	Mg (g), O2 (g)	∼10–11, ∼10–11	(30)
	Al2O3	Al (g), O2 (g)	∼10–15, ∼10–15
Mn (s)		Mn (g)	1.6 × 10–4	(31)
SiO2 (s)		O2 (g), SiO (g)	∼10–11, ∼10–11	(30,32)
Au (s)		Au (g)	5.3 × 10–8	(31)

Conclusions

Two types of nanostructures grow simultaneously on the MgAl2O4 substrate promoted by additive Mn. The shorter nanopillars are 100–300 nm in length with a structure of cubic spinel, and the micron-scale nanowires are determined as Mn- and Al-doped orthoenstatite MgSiO3. The formation of Au/Mn seeds makes it possible to collect the vaporization products via surface adsorption and mass transfer. This thermal vaporization route opens up possibilities for synthesizing multicomponent oxide nanowires.

Experimental Section

Chemicals

Single-crystal (111) MgAl2O4 substrates were purchased from MTI Corp.

Synthesis of Oxide Nanostructures

The synthesis of oxide nanostructures was facilitated via heat treatment in inert gas. In detail, a Au film was deposited with a thickness of 10 nm. The as-prepared substrate was then transferred to the center of a quartz tube (one end closed, 15 cm in length and 1 cm in diameter) in which an excess of Mn powder (0.1–0.5 grams, analytical grade) was placed at the closed end. After filling with Ar at a pressure of 200 torr, the open end of the tube was sealed with a flame. Subsequently, this enclosed vessel was loaded to the isothermal region of a tube furnace and heated to 1100 °C at a rate of 10 °C/min. With increasing temperature, dewetted Au particles formed and further mixed with Mn. After maintaining at 1100 °C for 60 min, the quartz tube was slowly cooled to room temperature.

Characterization Techniques

Scanning electron micrographs were obtained with a Zeiss Ultra Plus field-emission scanning electron microscope (SEM) operated at 5–15 kV. Prior to SEM observation, a 10 nm carbon layer was coated on the surface of spinel substrates using an ion sputter and carbon coating unit (E-1045, Hitachi, Japan). Transmission electron micrographs were acquired using an FEI Talos F200X operated at 200 kV. Chemical analysis of prepared nanostructures was obtained using a Super-X EDS system with four SDD detectors. The electron energy loss spectra (EELS) were measured using a spherical-aberration-corrected FEI Titan 80-300 HB. For TEM characterization, nanostructures were scraped from the substrate with TEM copper grids.