Topochemical Deintercalation of Al from MoAlB: Stepwise Etching

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Topochemical Deintercalation of Al from MoAlB: Stepwise Etching Pathway, Layered Intergrowth Structures, and Two-Dimensional MBene Lucas T. Alameda, Parivash Moradifar, Zachary Metzger, Nasim Alem, and Raymond E. Schaak J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04705 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 16, 2018

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Journal of the American Chemical Society

Topochemical Deintercalation of Al from MoAlB: Stepwise Etching Pathway, Layered Intergrowth Structures, and TwoDimensional MBene Lucas T. Alameda,‡,1 Parivash Moradifar,‡,2 Zachary P. Metzger,1 Nasim Alem,*,2,3 and Raymond E. Schaak*,1,3 1

Department of Chemistry, 2 Department of Materials Science and Engineering, and 3 Materials Research Institute, The Pennsylvania State University, University Park, PA 16802

ABSTRACT: The synthesis of refractory materials usually relies on high-temperature conditions to drive diffusion-limited solidstate reactions. These reactions result in thermodynamically-stable products that are rarely amenable to low-temperature topochemical transformations that post-synthetically modify subtle structural features. Here, we show that topochemical deintercalation of Al from MoAlB single crystals, achieved by room-temperature reaction with NaOH, occurs in a stepwise manner to produce several metastable Mo-Al-B intergrowth phases and a two-dimensional MoB (MBene) monolayer, which is a boride analog to graphenelike MXene carbides and nitrides. A high-resolution microscopic investigation reveals that stacking faults form in MoAlB as Al is deintercalated and that the stacking faulty density increases as more Al is removed. Within nanoscale regions containing high densities of stacking faults, four previously unreported Mo-Al-B (MAB) intergrowth phases were identified, including Mo2AlB2, Mo3Al2B3, Mo4Al3B4, and Mo6Al5B6. One of these deintercalation products, Mo2AlB2, is identified as the likely MAB-phase precursor that is needed to achieve a high-yield synthesis of 2-D MoB, a highly targeted two-dimensional MBene. Microscopic evidence of an isolated MoB monolayer is shown, demonstrating the feasibility of using room-temperature metastable phase engineering and deintercalation to access two-dimensional MBenes.

INTRODUCTION The rate-limiting step of most solid-state inorganic reactions is the diffusion of atoms. High temperatures are typically required to move reactants through a solid-state matrix that is comprised of microscale grains, which are characteristic of bulk-scale syntheses. Nucleation and crystallization of targeted phases can only occur when all atoms are fully mixed at the atomic level. However, the consequence of using heat to drive solid-state diffusion is that upon nucleation, there is a large excess of thermal energy relative to that which is needed to crystallize most phases of a given composition. This limits the scope of accessible products to phases that are thermodynamically stable at high temperatures, as metastable and lowtemperature phases cannot withstand such conditions. Motivated by the possibility of discovering new or improved properties in phases that may otherwise be missed by high-temperature methods, several lower-temperature “soft chemistry” approaches have been developed.1 Such approaches bypass diffusion barriers, thereby lowering reaction temperatures and shifting the rate-limiting step from diffusion to nucleation.2 Using this strategy, many metastable and lowtemperature phases have been synthesized. Key principles of soft chemistry include achieving atomic-level mixing through low-temperature processes, such as solution-phase dissolution and precipitation, or utilizing topochemical transformations where structural motifs of the reactant are preserved in the product after low-temperature manipulations.3 This structural

retention decreases the extent of atomic rearrangement needed to convert the reactants into products. It is therefore possible to selectively target more-reactive components of a structure at lower temperatures while leaving intact less-reactive components, which require higher temperatures to manipulate. Such strategies are analogous to the different reactivity of various functional groups in the synthesis of organic molecules. Topochemical deintercalation reactions are particularly interesting, because they can transform stable layered compounds that can be synthesized at high temperatures into new and otherwise inaccessible layered or non-layered analogs under mild conditions that preserve key structural features. For example, treatment of b-CaGe2 with HCl at -40 °C results in the deintercalation of Ca2+ to form a CaCl2 byproduct while leaving behind crystals of a new layered GeH phase that retains the Ge layering motif present in b-CaGe2.4 Similarly, dehydration of acidic layered Ruddlesden-Popper perovskite oxides, H2[An-1BnO3n+1] (A = alkali, alkaline earth, or rare earth metal, B = transition metal), below 500 °C results in deintercalation of interlayer protons and oxygen (as H2O) to form cubic, A-site defective structural analogs.5 These and other examples of topochemical deintercalation demonstrate how the primary layering motif of the material can be established using thermodynamic control (i.e. high temperatures), and then subtle changes to the structure can be achieved using kinetic control (low temperatures).

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MAX phases,6 which have the formula Mn+1AXn (M = early transition metal, A = Al or Si, and X = carbon or nitrogen) and space group P63/mmc, are refractory layered compounds that undergo topochemical deintercalation of Al or Si upon reaction with HF and related etchants under mild conditions.7,8 Subsequent swelling of the deintercalated MAX phases leads to exfoliation into 2-D nanosheets of MXenes, which are metal carbide and nitride analogues of graphene that are widely investigated for applications in supercapacitors, batteries, and catalysis.9 Their boride analogues, referred to as MAB phases and MBenes (M = transition metal; A = aluminum; B = boron), are emerging as potentially important alternatives to MAX phases and MXenes because additional compositional and structural variety offered by MAB phases may enable new or improved properties in their MBene derivatives relative to MXenes.10,11 Additionally, MBenes may be accessible using alternative, safer etchants than the fluoride-based etchants required for MXene synthesis.12 Unlike MXenes, however, MBenes have yet to be synthesized. Extending the topochemical deintercalation chemistry that has been widely demonstrated on MAX phases to MAB phases would open the door to a new class of MXene-like 2D metal borides that may show distinct or enhanced properties relative to MXenes.

Figure 1. Crystal structures of representative MAX phases (left, Ti3AlC2 [100]), MAlB-type MAB phases (middle, MoAlB [001]), and M2AlB2-type MAB phases (right, Fe2AlB2 [100]).

Despite their lack of experimental realization, multiple studies have predicted that MBenes should be accessible by topochemical deintercalation of Al from MAB phases.10,11 Partial etching of Al from a prototype MAB phase, MoAlB, was achieved using NaOH as an etchant.12 In contrast to deintercalation of Al from MAX phases, which goes to completion and forms MXenes, Al deintercalates from MoAlB in layers spaced every 50 – 300 nm, resulting in MoAlB slabs of nanoscale thickness. This partial, periodic Al removal observed in MoAlB is not observed during the etching of MAX phases, where Al is instead removed from every layer of the structure. Interestingly, the Al substructures in the MAX and MAB phases are different, which may contribute to their different behavior during topochemical deintercalation. Whereas all MAX phases contain a planar single layer of Al, some MAB

phases, including MoAlB (space group Cmcm),10 instead contain a zigzag double layer (Figure 1). These differences in both structure and reactivity underscore the need for an atomiclevel understanding of the deintercalation process in MoAlB as a model MAB phase. Such understanding may help to identify and eventually overcome the roadblocks that currently preclude the synthesis of MBenes from MAB phases. More broadly, such insights may also help to facilitate the development of topochemical deintercalation routes to metastable and low-temperature MAB phases that would be inaccessible using the high-temperature routes needed to synthesize them directly. Accordingly, here we present a microscopic investigation into the topochemical deintercalation of Al from MoAlB single crystals upon treatment with either NaOH or a solution of LiF in HCl – two commonly used etchants for MAX and MAB phases – by using annular dark-field scanning transmission electron microscopy (ADF-STEM). We find that deintercalation of Al from MoAlB is accompanied by the formation of a high density of (0k0) stacking faults, which helps to rationalize the observed differences in etching periodicity in MAX vs MAB phases and points to alternative approaches for achieving exfoliation of MAB phases into MBenes. We observe that the metastable phase Mo2AlB2 (space goup Cmmm)10 emerges during topochemical deintercalation of Al from MoAlB. Mo2AlB2, which is structurally related to the MAX phases that undergo complete Al deintercalation to form MXenes, has not been experimentally realized in MAB phases, but may be the necessary precursor for the synthesis of 2D MBenes. Additionally, we observe that the high density of stacking faults that are generated during the topochemical deintercalation of Al from MoAlB leads to the formation of several previously unidentified MAB intergrowth phases as nanoscale grains embedded in MoAlB, including Mo2AlB2, Mo3Al2B3, Mo4Al3B4, and Mo6Al5B6. Identification of these MAB phases indicates that low-temperature soft chemistry routes are useful in refractory boride systems for navigating subtle structural differences that would not be tractable using traditional high-temperature reactions. Also, the microscopic insights into topochemical deintercalation of MoAlB indicate that exfoliation of MAB phases to form MBenes, which has remained synthetically elusive, may be feasible through routes that differ from those used to generate MXenes.

EXPERIMENTAL SECTION Synthesis. MoAlB single crystals were prepared using a modification of a reported procedure.13 Briefly, Mo powder (Sigma Aldrich 99.99%, B powder (Alfa Aesar 98%), and Al chips (NOAH Technologies 99.99%) were added to a 2-mL alumina crucible (Coorstek) in a Mo:Al:B molar ratio of 1:53.3:1. A total of 2 grams of reactants were added to the crucible, which was then placed in a mullite tube furnace under an argon flow. The tube was flushed with argon for 10 minutes before the temperature was raised to 1400 °C. The sample was held at this temperature for 10 hours before cooling to 900 °C at a rate of 1 °C/min, and then cooled to room temperature at a rate of 3 °C/min. The excess flux media was digested in 3M HCl at room temperature to yield MoAlB single crystals. Etching. MoAlB single crystals were etched with an aqueous solution of 10% NaOH or a solution of 2 M LiF in 6 M HCl at room temperature for 2 hours and 24 hours. The


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Journal of the American Chemical Society NaOH-etched crystals were then separated from the etchant with vacuum filtration and rinsed with deionized water. For the LiF/HCl samples, the solution was decanted and the vial was refilled with deionized water several times until ~ pH = 7. The crystals were then separated from the solution with vacuum filtration and rinsed with deionized water. (Caution: An acidic solution of LiF will generate dilute hydrofluoric acid (HF), and therefore should be treated with the same safety precautions used for HF.) Characterization. ADF-STEM images were collected on a FEI Titan G2 aberration corrected high-resolution scanning/transmission electron microscope at an accelerating voltage of 200 kV. Additional imaging details are included in the Supporting Information. TEM sample preparation was done by focused ion beam (FIB) using a FEI Helios NanoLab 660FIB/FESEM. Figure S2c shows a SEM image of the crystal during the FIB process. A thin slice of the crystal was cut from the bulk sample at the (100) crystal facet and perpendicular to the [001] crystallographic direction, and then lifted out of the crystal. It was attached to a half copper TEM grid and thinned until it was electron transparent. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (SEMEDS) were performed on a FEI NOVA NanoSEM 630 at an accelerating voltage of 5 kV for imaging and 20 kV for EDS, and a working distance of 3 mm for imaging and 5 mm for EDS. Samples were prepared for EDS by coating an SEM stub with carbon tape (TED PELLA) upon which MoAlB single crystals were placed with the basal plane perpendicular to the SEM stub and the [100] orientation facing up. For imaging of stacking-fault-band streaking, the crystals were mounted in epoxy (Loctite) with the basal plane perpendicular to the SEM stub and the [100] orientation facing up. X-ray diffraction was performed by first grinding multiple crystals with an agate mortar and pestle to prepare a powder of MoAlB. XRD patterns were then collected on a Bruker D8 Advance powder Xray diffractometer.

high-contrast lines proceeding straight down from the crystal surface. (Note that the contrast observed at stacking faults is a result of stacking-fault-induced strain. Thus, changes in contrast can be used to identify stacking fault distributions in lowmagnification ADF-STEM images where atoms are not atomically resolved.) At a stacking fault, the zigzag Al double layer of MoAlB is replaced with a single layer of Al in a linear configuration, which is analogous to the interlayer Al in the M2AlB2 structure type. The difference between unfaulted and faulted MoAlB is shown in Figures 2b and 2c, respectively. The distance between each stacking fault ranges from tens to hundreds of nanometers.

RESULTS AND DISCUSSION Topochemical Deintercalation of Aluminum. Topochemical deintercalation reactions, including for MAX and MAB phases, are typically carried out using polycrystalline samples. For this study, phase-pure MoAlB single crystals (Fig. S1) were used so that we could control the extent of Al deintercalation, focus on the surface of the crystal, orient the crystal so that the crystallographic direction was known, and use FIB to extract slices of known crystallographic orientation. MoAlB single crystals were treated with 10% NaOH to etch Al (Fig. 2). The ADF-STEM images in Figure 2, focus on regions within a few micrometers of the crystal surface. The images were collected in the [001] crystallographic direction, which is parallel to the (0k0) planes, therefore providing an edge-on view of both the MoB layers and Al layers. The insets of Figures 2a and 2d show a schematic representation of the etching process at different time points. Figure 2a shows that prior to etching, (0k0) stacking faults are sparsely distributed throughout the surface region of the MoAlB crystal. The formation of sparsely distributed stacking faults in MoAlB has been reported for flux growth synthesis and high-temperature powder synthesis.14,15 Hultman and coworkers hypothesized that stacking faults form as a result of stepwise intercalation of Al into MoB during the formation of MoAlB.14 At low magnification, the stacking faults appear as

Figure 2. ADF-STEM images of MoAlB at various stages of etching with 10% NaOH. (a) As-synthesized MoAlB showing a few stacking faults as high-contrast lines vertically proceeding



down from the crystal surface along the crystal lattice planes. The inset shows a schematic of the crystal and stacking faults. (b) Atomic-scale-resolution ADF-STEM image of pristine MoAlB. (c) Atomic-scale-resolution image of MoAlB showing a single stacking fault in the vertical direction indicated by an arrow below the image. (d) MoAlB after 2 hours of etching showing the formation of several stacking fault bands. The inset shows a schematic of the crystal and stacking fault bands. (e) Highermagnification image of the sample with a high density of stacking faults. (f) Higher-magnification image of two stacking fault bands separated by a region of mostly unperturbed MoAlB with no stacking faults.

After 2 hours of treatment with 10% NaOH, regions of high stacking fault density (“stacking fault bands”) appear (Fig. 2d). These regions are ~100 nm wide and separated from each other by < 100 nm of mostly unperturbed MoAlB. In the defect-rich regions, stacking faults occur within nanometers of each other, such that the frequency of stacking faults (“Al single layers”) and normal stacking sites (“Al double layers”) is comparable (Fig 2e). In the regions of unperturbed MoAlB, there are no observable high-contrast lines that are attributed to stacking faults (Fig 2f). An SEM image of the crystal edges after 2 hours of treatment shows that microscale streaking occurs that is in the same plane as the stacking fault bands, and is also consistent with the size and periodicity of the bands, suggesting that the bands are observable by SEM (Figure S2 of the Supporting Information). The streaks do not exist prior to etching and they are present over the entire crystal edge (which is on the order of millimeters), confirming that the stacking fault bands observed by ADF-STEM occur everywhere that the crystal edge was exposed to NaOH. Collectively, these images demonstrate that upon initial NaOH treatment, Al partially deintercalates from MoAlB in a manner that was unexpected. As atoms are removed from an Al double layer, the remaining Al atoms redistribute evenly between the two adjacent MoB layers. Thus, instead of forming vacant channels between MoB layers upon Al deintercalation, as is observed upon Al deintercalation of MAX phases to form MXenes, the remaining atoms spread out to form an Al single layer that holds together the boride layers. The stacking faults assemble as bands, similar to the stacking fault behavior observed in many other materials, including MAX phases. Ordered Stacking Variants Induced by Topochemical Deintercalation. With further NaOH etching beyond 2 hours, the stacking fault bands become continuous as Al deintercalates from the unperturbed areas of MoAlB and the stacking fault density concomitantly increases. At this point, stacking faults appear to order into regions containing nanoscale domains of various other MoB-Al stacking motifs (Figure 3). Four unique stacking variants emerged, which ranged in composition from Mo6Al5B6 to Mo2AlB2 and follow the stoichiometry MoxAlx1Bx. With the exception of Mo2AlB2, for which the unit cell and lattice parameters have already been determined using density functional theory calculations,10 the proposed chemical formula for each intergrowth structure was determined by a simple analysis based on the structures observed by ADFSTEM imaging and the change in the number of Al layers relative to MoB layers. These ordered regions were observed repeatedly in multiple regions of the crystal, and were also observed when LiF/HCl was used as the etchant. A rough

phase fraction of the four intergrowth structures was determined by counting the number of times each phase appeared in 49 different imaged regions (85 total data points). Mo2AlB2 was the most abundant (44%), followed by Mo4Al3B4 (35%), Mo6Al5B6 (14%), and Mo3Al2B3 (7%). Surrounding each ordered nanoscale domain are regions containing random distributions of stacking faults. Lattice defects have been observed previously in MAB phases, including (0k0) stacking faults in MoAlB,15 twist boundaries in Cr2AlB2 due to 90° rotation about the b-axis, and [001] tilt boundaries in Fe2AlB2.14 However, to our knowledge, there have not been reports of defect ordering or defect engineering, yielding regions of other ordered phases, in MAB compounds. Stacking faults are also well known in MAX phases.16-18 They are caused by the insertion of extra MX layers between the A layers. For example, Wang and coworkers showed that TaC slabs insert into Ta2AlC to yield Ta4AlC3 and Ta6AlC5.16 The insertion of a TaC slab caused the number of Ta layers in the carbide substructure to increase from 2 to 4 for Ta4AlC3 or 2 to 6 for Ta6AlC5. These intergrowth structures can be described by the formula M2nAX2n-1, where n represents the number of TaC layers in each carbide substructure and is equal to 1, 2, and 3 for Ta2AlC, Ta4AlC3, and Ta6AlC5, respectively. The M2nAX2n-1 tantalum aluminum carbides can be roughly considered as MAX-phase structural analogs to RuddlesdenPopper layered perovskites, (A2[A’n-1BnO3n+1]), which are periodic intergrowths of rocksalt AO layers and perovskite ABO3 layers. The Al substructure in M2nAX2n-1 remains unchanged across each phase and is analogous to the alkaline earth metal (A) in Ruddlesden-Popper phases. In contrast, the stacking behavior observed in etched MoAlB is fundamentally different from that of Ruddlesden-Popper and Ta-Al-C phases. The MoB substructure remains unchanged across all phases. Instead, the Al substructure changes between single-layer or double-layer states. Thus, the stacking variants that form during the topochemical deintercalation of Al from MoAlB represent unique structural motifs for ternary layered borides and carbides. To understand the structural relationships between the stacking variants, the layering can be understood in terms of the sequence of Al layers rather than the sequence of MoB layers. Accordingly, each Al substructure in Figure 3 is labeled with the number of Al layers present, where 1 represents a linear single layer and 2 represents a zigzag double layer. Each intergrowth phase then contains a unique stacking sequence of single-layer (1) and double-layer (2) motifs: [1]n, [211]n, [12]n, and [122]n for Mo2AlB2, Mo3Al2B3, Mo4Al3B4, and Mo6Al5B6, respectively. For comparison, MoAlB, which contains only double layers, would be denoted as [2]n Figure 3 presents potential deintercalation pathways to the four different intergrowth phases, starting with MoAlB as an entryway. Considering only the molecular formulas, it is reasonable to guess that each phase could be accessed by sequentially decreasing the Al content, starting with the conversion of MoAlB to Mo6Al5B6, followed by the formation of Mo4Al3B4, then Mo3Al2B3, and finally, Mo2AlB2. However, careful inspection of the stacking sequences shows that two of the conversions, Mo6Al5B6 à Mo4Al3B4 and Mo4Al3B4 àMo3Al2B3, are not possible. In these two cases, there is no way to convert an Al double layer to a single layer and obtain the product phase. We assume Al dissolution is irreversible under the conditions of the Al deintercalation reaction, and therefore Aldeficient phases cannot act as intermediates to Al-rich phases. (Al would have to be inserted, which is not consistent with the


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Figure 3. ADF-STEM images of stacking variants and their corresponding crystal structures observed after etching MoAlB with 10% NaOH or LiF/HCl for 24 hours. Arrows show possible reaction pathways between phases assuming irreversible dissolution of Al. ‘1’ represents a linear Al single layer, and ‘2’ represents a zigzag Al double layer. Note that the Al atoms in Mo2AlB2 and Mo3Al2B3 are not visible due to a higher level of induced microstructural strain that results in more lattice distortion. Scale bars are 1 nm.

reaction conditions that favor Al removal.) This prohibits the formation of Mo4Al3B4 from Mo2AlB2, suggesting that Mo4Al3B4 must form either directly from MoAlB, or from an additional phase that has not yet been observed. It should be emphasized that these are proposed pathways based on exsitu analysis, and may not be a true representation of the actual reaction mechanism. In addition to the structural differences between MAXphase stacking faults and MoAlB stacking faults, there are important synthetic differences between the two. The high density of MoAlB stacking faults reported in this study are due to the post-synthetic, intentional removal of Al using “soft chemistry,” rather than unintentional insertion of MoB during high temperature synthesis. Thus, there is potential for the development of kinetically-controlled topochemical deintercalation reactions to selectively target different stacking variants in the Mo-Al-B system and isolate phase pure bulk samples, particularly if applied to polycrystalline samples having smaller grain sizes and diffusion distances. Such capabilities in refractory borides are not well established. We anticipate that low-temperature structural engineering will result in modulation of the properties that are influenced by layering motifs, including electrical and thermal conductivi-

ty, oxidation resistance, and the various mechanical properties that are being extensively studied in MAB phases.19-21 Formation of Etched Cavities. After further treatment of MoAlB with 10% NaOH for 24 hours (Fig. 4), etched cavities form. The cavities are ~10 nm wide at the crystal surface and are distributed periodically, occurring once every 100 – 200 nm. This observation is consistent with previous SEM images showing similar periodic etching features.12 The cross-sectional view of etched MoAlB reveals that the cavities have a range of depths spanning from less than 100 nm, as is shown in Figure 4a, to over several micrometers. Notably, a very high density of stacking faults forms after etching for 24 hours. In the region near the crystal surface, there are no remaining areas of unperturbed MoAlB; ADF-STEM images from all areas of the crystal surface exhibit significantly increased contrast due to strain from extensive stacking faults. Careful inspection of atomic-resolution images reveals that the density of stacking faults becomes greater than the density of Al double layers after the 24 hour treatment (Fig. 4c). Of the 15 Al layers shown in Figure 4c, 9 are faulted single layers and 6 are unperturbed double layers. Similar ratios were observed consistently over large regions



of the sample. The stacking faults extend a few micrometers below the surface of the crystal before terminating, at which point unperturbed MoAlB is again observed. SEM-EDS maps of the crystal edge show that the chemical homogeneity of Mo and Al is preserved through the etching process (Figure S3 of the Supporting Information).

Figure 4. (a) ADF-STEM image of MoAlB after 24 hours of etching with 10% NaOH showing large etched cavities and stacking fault saturation. (b) ADF-STEM image of the tip of an etched cavity showing extensive strain and deformation of the atomic rows at the crack tip, which terminates at a stacking fault. (c) High resolution ADF-STEM image of a representative surface region indicating a high density of stacking faults.

Nearly identical behavior to the NaOH treatment was observed when the crystals were etched with a solution of 2M LiF in 6M HCl (Figure S4 of the Supporting Information), which is a commonly used Al etchant for MAX phases. After 2 hours of treatment with LiF/HCl, there was moderate stacking fault formation, and after 24 hours of treatment a high density of stacking faults formed along with etched cavities. The one significant difference between the NaOH and LiF/HCl etchants was that LiF/HCl caused extensive corrosion of the boride layer in addition to Al deintercalation. With LiF/HCl, etched “branches” were observed to propagate in [010], corroding through MoB layers to form a network of cavities (Fig. 5). This behavior suggests that the MoB layer slowly dissolves in the presence of HF at room temperature and therefore that LiF/HCl is not suitable for Al deintercalation from MoAlB, as competing corrosion pathways dominate. Consistent with this microscopic observation, hollowing was reported previously for HF-treated MoAlB crystals.12 The sequential formation of stacking faults followed by etched cavities suggests that the cavities may be dependent on stacking faults. This relationship is reasonable if one assumes that cavities only form when both Al layers between two adjacent MoB layers are removed. In that case, a possible pathway for Al removal could be a stepwise deintercalation of the two Al layers, where a stacking fault would be

described as a reaction intermediate. Further evidence of the stacking fault dependency on cavity formation is evident in Figure 4b, which shows the point where a cavity terminates. In this image, the tip of the cavity appears to preferentially track a stacking fault over an adjacent Al double layer. It should be noted, though, that due to the high level of strain present at the tips of cavities, only a few sites could be atomically resolved. Thus, we cannot yet generalize this behavior to all cavities. Alternatively, etched cavities may form due to strain caused by contraction of the crystal upon Al deintercalation. Strain from the newly formed etched cavities may then contribute to additional stacking fault density in the neighboring regions.

Figure 5. (a) ADF-STEM image of MoAlB after 24 hours of etching with a solution of LiF in HCl showing etched cavities, etched “branches,” and high density of stacking faults. As this image indicates, etching preferentially occurs along [100] and [010] crystallographic orientations.

From a rough contrast analysis of the images presented in Figures 2a, 2d, and 4a, we propose the following reaction pathway for partial etching of Al from MoAlB (Fig. 6). (1) Al atoms topochemically deintercalate to convert Al double layers into stacking faults containing Al single layers, which occurs in bands that extend a few micrometers into the bulk of the crystal. (2) The stacking fault bands eventually reach a point of saturation, where the entire surface region is uniformly faulted with a defect density greater than 50%. At this point, the stoichiometry of the surface region is closer to that of Mo2AlB2, a phase that cannot be made by direct hightemperature synthesis, than it is to MoAlB. (3) Al single layers are then removed from stacking faults, completely removing the Al between two adjacent MoB layers and resulting in an etched cavity that then swells to ~10 nm wide. Alternatively, step 3 may occur in reverse order where cavities form due to strain, which is then followed by dissolution of the Al atoms that line the inside of the cavity. Implications for the Synthesis of MBenes. Previous attempts to exfoliate MoAlB into 2D MoB, a MBene, instead resulted in the periodic formation of etched cavities, which was attributed to periodic etching of Al. This microscopic study now reveals that Al is actually partially etched from the majority of the Al double layers of MoAlB. In most cases, only one layer of Al is removed from the double layer, resulting in the formation of a stacking fault having a single Al layer rather than an etched cavity. Fully exfoliating a MAB phase to form a MBene requires full etching of Al between the MoB layers, and therefore pushing the topo-


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Figure 6. Combined reaction scheme representing the topochemical deintercalation of Al from MoAlB upon treatment with 10% NaOH.

Figure 7. Left: ADF-STEM image of two isolated, delaminated MoB (MBene) sheets inside an etched cavity. Middle: Highermagnification and contrast-enhanced image of the cavity containing the MBene sheets, as indicated by the blue outline. Right: Proposed, idealized structure of the delaminated region of the MBene sheets.

chemical deintercalation of Al to completion across the entire crystal surface. While it is not yet understood why complete etching of an Al double layer doesn’t occur across the entire crystal surface, one possible explanation is that the formation of certain stacking sequences, i.e. some of the intermediate stacking variants described above, causes deintercalation to cease. From this microscopic study, we now know that one of the stacking variants formed upon partial Al deintercalation of MoAlB adopts the Mo2AlB2 structure. Considering that MAX phases are structurally more similar to Mo2AlB2 (single-layer Al) than MoAlB (double-layer Al), we propose that the synthesis of 2D MoB (MBene) requires Mo2AlB2 as the starting material, because the intermediate stacking variants described above, which may cause deintercalation to cease, cannot form from Mo2AlB2. The synthetically-accessible MoAlB phase is therefore unlikely to produce MBenes in high yield, which motivates the development of synthetic routes to phase-pure Mo2AlB2. Consistent with this hypothesis, Sun and coworkers recently reported DFT studies predicting that 2D molybdenum boride is indeed accessible via HF treatment of Mo2AlB2.10 Upon HF treatment, the DFT study predicted that AlF3 and

H2 would form, analogous to the process that etches Al from MAX phases using HF. Several MAB-phase structure types are known to form as phase-pure compounds, including M4AlB6, M3AlB4, M2AlB2, and MAlB, where M = transition metal.22 However, each structure type can only accommodate certain metals. To our knowledge, there were no experimental reports of the Mo-containing Mo2AlB2 phase on which the DFT study was based. However, the results of our microscopic study suggest that Mo2AlB2 may indeed be synthetically accessible using low-temperature soft chemistry pathways. Preliminary experimental evidence supporting Sun and coworkers’ conclusion that 2D MBenes are accessible, stable, and can be delaminated into single sheets is shown in Figure 7. In this image, two MoB monolayers split apart as the stacking fault holding them together is etched. The monolayers are partially delaminated from the inner wall of an etched cavity, and the longer of the two (left side) extends approximately 10 nm before leaving the field of view. Although it has not been observed or achieved on a large scale, this result closes the loop between the microscopic insights and experimental implications and suggests that the formation of stable 2D MoB is feasible using the knowledge gained from this study.

CONCLUSIONS In conclusion, ADF-STEM imaging revealed important insights into the pathway by which Al is deintercalated from MoAlB, a prototype MAB phase that is of interest as a precursor for the formation of 2D MBenes. Deintercalation of Al occurs in a stepwise manner, involving the formation of stacking fault bands that grow over time until a high density of stacking faults is reached. Aluminum then is removed from stacking faults to form etched cavities. Considering that cavity formation is preceded by the formation of a high density of stacking faults, where the composition of the crystal and the local structure more closely resemble Mo2AlB2 than MoAlB, Mo2AlB2 is proposed as a viable MAB-phase precursor that will enable the high-yield synthesis of 2D MoB (MBene). Preliminary progress towards the synthesis of Mo2AlB2, as well as the other intergrowth phases Mo3Al2B3, Mo4Al3B4, and Mo6Al5B6, was demonstrated through the observation of regions where ordering of stacking faults occurred upon topochemical deintercalation of Al in MoAlB. Additionally, an isolated 2D MoB monolayer was observed, suggesting that the synthesis and exfoliation of MBenes is indeed feasible. The observation that diverse stacking variants and layered intergrowth compounds form through “soft chemistry” pathways has important implications for achiev-



ing synthetic control over subtle structural features in refractory boride systems, which typically require hightemperature reactions and therefore are limited to phases that are stable at high temperatures. Optimization, expansion, and generalization of this topochemical deintercalation approach beyond MAB-phase compounds would provide a powerful strategy for transcending thermodynamic limitations to the phase accessibility of refractory materials.

ASSOCIATED CONTENT Supporting Information XRD data, additional electron microscopy images, and details of imaging conditions. The Supporting Information is available free of charge on the ACS Publications website. PDF file

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Author Contributions ‡ These authors contributed equally.

ACKNOWLEDGMENT L.T.A., Z.P.M., and R.E.S. were supported by the U.S. National Science Foundation (NSF) Center for Chemical Innovation in Solar Fuels (CHE-1305124). P.M. and N.A. were supported by the Penn State Center for Nanoscale Science, a NSF-sponsored Materials Science and Engineering Center under award number NSF-DMR 1420620. The authors thank Tom Mallouk for helpful discussions and Ke Wang for electron microscopy support.

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Topochemical Deintercalation of Al from MoAlB: Stepwise Etching

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