Partial Etching of Al from MoAlB Single Crystals To Expose

Oct 16, 2017 - Partial Etching of Al from MoAlB Single Crystals To Expose Catalytically Active Basal Planes for the Hydrogen Evolution Reaction...
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Communication Cite This: Chem. Mater. 2017, 29, 8953-8957

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Partial Etching of Al from MoAlB Single Crystals To Expose Catalytically Active Basal Planes for the Hydrogen Evolution Reaction Lucas T. Alameda, Cameron F. Holder, Julie L. Fenton, and Raymond E. Schaak* Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

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wo-dimensional (2D) materials such as graphene,1,2 transition metal dichalcogenides (TMDs),3 hexagonal boron nitride,4 and various other “beyond-graphene” materials continue to attract significant attention because they exhibit unique or enhanced size-dependent properties that differ from their three-dimensional (3D) bulk counterparts. For example, bulk MoS2 is catalytically inert for the hydrogen evolution reaction (HER) that facilitates the production of molecular hydrogen from water electrolysis, but 2D MoS2 is an active HER catalyst. The catalytic activity of 2D MoS2 is attributed to the edge sites, as the basal planes are inactive.5 The edge site activity of MoS2 and related layered materials represents a challenging problem in catalyst design, as basal plane exposure is thermodynamically favored. Elaborate schemes have been developed to increase edge site density,6,7 but this can come at the expense of practical utility. Therefore, the identification of 2D materials with active sites exposed on the basal planes represents a powerful alternative. MXenes,8,9 a growing family of 2D metal carbides and nitrides of interest for applications as supercapacitors10 and batteries,11 are emerging as promising 2D HER catalysts.12 MXenes are derived by chemically etching and exfoliating 3D MAX phases, which comprise a large family of Mn+1AXn compounds that typically contain an early transition metal (M), Al or Si (A), and carbon or nitrogen (X).13 Unlike the TMD catalysts having HER-active edge sites, several MXene carbides have been computationally predicted to have HERactive basal planes.12 Complementary experimental studies of polycrystalline Mo2CTx (T = surface functional group) MXene powders with micron-scale platelet grains showed HER activity as predicted, although both basal planes and edge sites were exposed. MXenes are therefore potentially promising 2D HER catalysts, but are limited to carbide, nitride, and carbonitride phases due to compositional limitations of the parent MAX phases. In addition to metal carbides, metal borides also have been gaining attention as HER catalysts,14−18 especially molybdenum borides that include α-MoB, β-MoB, Mo2B, and MoB2.19,20 However, efforts to access high surface area metal boride materials are challenging, particularly for the molybdenum-based systems that are of greatest interest for their high activity and stability. The ability to access MXene-like 2D borides through etching boride analogues of MAX phases would significantly expand the diversity of available MXene materials.21 By analogy to other classes of 2D materials, this may lead to the discovery of materials having new or enhanced optical, electronic, mechanical, and catalytic properties. © 2017 American Chemical Society

MoAlB, a member of a class of layered ternary borides (MAB phases) that are structurally similar to MAX phases (Figure 1a),22 has been gaining attention recently due to its flexural strength, compressive strength, oxidation resistance, metallic conductivity, and high thermal conductivity.23,24 MoAlB is orthorhombic with MoB bilayers that alternate with two Al

Figure 1. (a) Crystal structure of MoAlB and powder XRD patterns of a MoAlB single crystal laid flat on the sample holder, MoAlB powder (ground from multiple single crystals), and a simulated XRD pattern for reference.26 SEM images of (b) a MoAlB single crystal and (c) an enlarged section of the crystal edge showing faceting. Received: June 17, 2017 Revised: October 12, 2017 Published: October 16, 2017 8953

DOI: 10.1021/acs.chemmater.7b02511 Chem. Mater. 2017, 29, 8953−8957

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Chemistry of Materials layers, resulting in a layered crystal structure. Similar to some MAX phases, first-principles calculations suggest that the bonding between the transition metal and interleaved aluminum layer is metallic and weaker than the covalent and ionic bonding present between the transition metal and boron.25 Much like in MAX phases, this makes the Al layer of MoAlB an attractive target for chemical etching and suggests that 2D metal boride analogues of MXenes may be accessible through the exfoliation of MoAlB and other structurally related compounds. Accordingly, we show here that MoAlB is amenable to interlayer etching through reaction with NaOH. Furthermore, we demonstrate using single crystals that the basal plane of MoAlB is catalytically active for the HER in acidic media and that interlayer etching increases the surface area, and concomitantly the HER activity, by exposing previously buried basal planes. Single crystals of MoAlB were synthesized using an aluminum flux, as reported previously27 and described in the Supporting Information. After dissolving the aluminum flux with 3 M HCl, rectangular plate-like MoAlB single crystals 20− 100 μm in thickness and 1−5 mm in length were produced, as shown in the SEM image in Figure 1b. The XRD pattern of a collection of single crystals ground into a powder matches well with that expected for MoAlB (Figure 1a). SEM-EDS analysis indicates an even distribution of Mo and Al colocalized in the crystals and confirms the expected 1:1 Mo:Al ratio (Figure S1 and Table S1 of the Supporting Information). XRD analysis of a MoAlB single crystal reveals that the basal plane is perpendicular to [010], as only (0k0) reflections are observed when the basal plane is parallel to the XRD sample holder (Figure 1a). The morphology and structural anisotropy of the layered MoAlB single crystals leads to the basal plane being capped by MoB layers, whereas the crystal edges have exposed Al layers that alternate with MoB layers. The Al layer of MoAlB was partially etched by soaking the crystals and/or powders, which were prepared by grinding a collection of MoAlB single crystals with a mortar and pestle because phase-pure MoAlB powders have not been directly accessible, in 10% NaOH at room temperature for 24 h. Figure 2a,b shows SEM images indicating that the etching treatment yielded MoAlB slabs having thicknesses that range from 50− 300 nm. The partially etched MoAlB single crystal now exhibits periodic gaps with spacings of 10−100 nm. Because fully etched MXenes are known to restack into multilayers of thickness >10 nm (especially in the presence of cations), further characterization beyond SEM is necessary to discriminate between periodic and full etching.8 Accordingly, powder XRD data show that the material retains its crystal structure after the etching treatment (Figure S2 of the Supporting Information), and compositional analysis by SEMEDS reveals a Mo:Al atomic ratio of 57:43 (Table S1 of the Supporting Information). This indicates that the Al content has decreased measurably relative to the as-synthesized MoAlB, which has a Mo:Al atomic ratio of 51:49. If full etching of Al were achieved, (0k0) reflections would be expected to broaden and shift to lower angles, all other reflections would be expected to weaken significantly or disappear completely, and Al content would be expected to approach zero. If partial, but not periodic, etching of Al occurred (i.e., some Al removed from every layer), the corresponding diffraction pattern would be expected to largely resemble that of bulk MoAlB, with the exception of new shoulder peaks around (0k0) reflections. Therefore, we can conclude that neither full Al etching nor

Figure 2. SEM images showing etched MoAlB single crystals after (a) treatment with 10% NaOH, (b) HER catalysis and fragmentation, and (c) treatment with 10% NaOH followed by intercalation with 25% urea.

partial etching of small amounts of Al from every layer occurred. Instead, these results indicate that the reaction of MoAlB with NaOH resulted in partial, periodic etching of the Al to yield MoAlB slabs with maximum thicknesses of 50−300 nm, which are thinner than can be accessed through typical high-temperature powder syntheses. Significantly, the demonstration of partial Al etching from MoAlB represents an important step toward chemical delamination of a MAB phase, which is an important goal in emerging 2-D materials research that has not yet been realized.21 Though mechanical stress has been shown to lead to partial delamination in a few ternary boride systems,28 such approaches do not remove aluminum from the structure, nor do they have the potential to lead to complete delamination of single-layer metal borides, which is anticipated to be possible upon further development and optimization of this chemical etching strategy. To date, only fluoride-based reagents, most commonly HF, have been used to unambiguously etch MAX phases and obtain the resulting single-layer or few-layer MXenes.8 Safer alternatives, including strong bases, have been explored with limited success.29 When treated with HF, the Al layer in MoAlB showed evidence of etching, but significant corrosion was also observed (Figure S3 of the Supporting Information). Treatment with 5% and 10% HF for 24 or 48 h led to the formation of surface coatings and significant dissolution. The surface coatings and dissolution occurred selectively in the [001] direction, as (001) and (100) planes were identified by crosssectional XRD of oriented single crystals. No etching was observed upon treatment with 48% HF. Instead, significant reaction with the surface of the crystal and formation of a fluorine-containing coating occurred. NaOH (10%, room 8954

DOI: 10.1021/acs.chemmater.7b02511 Chem. Mater. 2017, 29, 8953−8957

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Chemistry of Materials

cover the metal wire and the edges of the plate-like single crystals, leaving the basal plane opposite the wire as the only exposed surface of the crystal; photographs of the electrodes are provided in Figure S4 of the Supporting Information. Indeed, MoAlB catalyzes the HER in 0.50 M H2SO4, exhibiting an overpotential of 361 mV at the operationally relevant current density of 10 mA/cm2 (Figure 4). Because only the basal plane

temperature) is therefore preferred over HF as an etchant for MoAlB, which contrasts the chemical etching behavior of MAX phases and therefore points to alternative pathways for achieving delamination in MAB phases. In addition to HF, more aggressive etching treatments were attempted, including 30% NaOH (room temperature, 24 h), 10% NaOH (room temperature, 72 h), and 10% NaOH (70 °C, 24 h). As shown in Figure S2 of the Supporting Information, increasing the NaOH concentration or soaking time did not result in increased etching, and etching at 70 °C resulted in significant corrosion. MoAlB sheets were obtained from MoAlB powders after partial etching in NaOH followed by intercalation with urea (Figure 2c), as described in the Supporting Information. (Note that MoAlB powders synthesized directly through hightemperature reactions of the constituent elements tend to have impurities, as do the bulk powders from which HER-active MXenes such as Mo2CTx are derived;30 in contrast, the MoAlB powders derived from the single crystals are phase pure.) The powder XRD pattern of a drop-cast sample of the dispersion of the MoAlB sheets showed significant preferred orientation (Figure S2 of the Supporting Information), with the exclusive presence of strong (0k0) reflections, suggesting that they retain the morphology of the slabs obtained by etching. TEM, HRTEM, SAED, and EDS further confirm the morphology, composition, and structure of the sheets. The TEM images in Figure 3 show that the MoAlB sheets have lateral dimensions

Figure 4. Polarization data for HER catalysis in 0.5 M H2SO4 for (green) as-synthesized (nonetched) MoAlB single crystals with only the basal plane exposed, (blue) as-synthesized MoAlB single crystals with both the basal plane and nonetched edges exposed, and (red) etched MoAlB single crystals, as well as (purple) Cu wire and (black) Pt wire for comparison.

of the MoAlB single crystal is exposed during catalysis, the catalytic activity can be unambiguously attributed to the basal plane. Seven MoAlB crystals were independently analyzed, and all showed HER activity with overpotentials at 10 mA/cm2 in the range of approximately 360−390 mV. When both the basal plane and the edges of nonetched, as-synthesized MoAlB single crystal are exposed, the geometric area-normalized overpotential increased to 400 mV at 10 mA/cm2. This increase in overpotential therefore suggests that the edges are less active than the basal planes for HER catalysis, as exposing them increases the surface area but does not concomitantly add a correspondingly higher active site density. Consistent with this observation, the Tafel slope increases upon exposure of the crystal edge (Table S1 of the Supporting Information). In order to best compare the difference in activity between the basal plane and edges, single-crystal electrodes of similar facet sizes were used for comparison of facet-specific activity (Table S1 of the Supporting Information). The overpotential of 361 mV at 10 mA/cm2 for the MoAlB basal plane represents moderate activity compared to some of the most highly active nonprecious metal HER catalysts. However, the single-crystal MoAlB electrode was determined to have a nearly atomically flat surface by atomic force microscopy (Figure S5 of the Supporting Information), with a RMS surface roughness of 3.5 Å, and therefore exceptionally low surface area relative to powders or nanostructured materials. It follows that increasing the exposure of MoAlB basal planes will increase the number of exposed active sites, and therefore decrease the overpotential normalized to geometric surface area. Accordingly, we evaluated interlayer-etched MoAlB single crystals for HER activity in 0.50 M H2SO4 by first soaking the NaOHtreated crystals in 6 M HCl for 24 h, which is anticipated to reduce the extent of −OH termination. Electrodes of etched

Figure 3. (a) TEM and (b) HRTEM images of freestanding MoAlB sheets, along with the corresponding SAED pattern in panel c. (d) HAADF-STEM image and corresponding EDS element maps of a single MoAlB sheet showing the even distribution and colocalization of the Mo-L and Al-K signals.

ranging from a few hundred nanometers to a few micrometers. The varied thickness of the sheets reflects the varied thickness of the slabs obtained during etching. EDS mapping indicates an even distribution of Mo and Al across the sheets (Figure 3), and EDS analysis yields a ∼1:1 ratio of Mo to Al (Table S1 of the Supporting Information). Lattice spacings corresponding to the (200), (100), and (002) planes of MoAlB were observed by HRTEM. The corresponding SAED pattern was indexed as the [010] direction of MoAlB, confirming both the MoAlB crystal structure and the preferred orientation of the sheets. To evaluate the HER activity of MoAlB in strongly acidic aqueous solutions where MoS2, Mo2CTx, and other 2D HER catalysts are known to be active, electrodes were prepared using the MoAlB single crystals in order to exclusively target the MoB-capped basal plane. MoAlB single crystal electrodes were constructed by adhering a Sn-plated Cu wire to one of the basal planes using silver paste. Two-part epoxy was then used to 8955

DOI: 10.1021/acs.chemmater.7b02511 Chem. Mater. 2017, 29, 8953−8957

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MoAlB single crystals were constructed by leaving both the basal plane and edge sites exposed (Figure S4), which includes some previously buried basal planes that are now accessible because of the Al etching. Both the nonetched and the etched MoAlB crystals maintain catalytic activity under acidic conditions for at least 24 h (Figure S6 of the Supporting Information). The overpotential of 301 mV at a current density of 10 mA/cm2 for the etched MoAlB single crystals represents a significant decrease of 99 mV relative to the overpotential of 400 mV for the comparable nonetched crystals that had both basal planes and nonetched edges exposed (Figure 4). The increase in HER activity of MoAlB after etching is attributed to the increase in the surface area of exposed catalytically active basal planes achieved by selective leaching of Al from the layered crystal structure. Minor fracturing of the etched electrodes is observed after prolonged catalytic testing, and this is attributed to interlayer bubble formation during hydrogen evolution (Figure 2b and Figure S7 of the Supporting Information). Normalizing the polarization data to the electrochemically active surface area (ECSA) provides further support that the basal planes are primarily responsible for the observed catalytic activity. The ECSA of the etched MoAlB crystal exposing both basal planes and all edges (Figure S8), which also expose some previously buried basal planes, is approximately 5.8 times greater than the ECSA of the nonetched MoAlB crystal that exposes only the top basal plane. This increase is consistent with the increase in surface area expected given the exposure of an additional basal plane along with the periodicity of etching along the edges, which correlates with the number of additional exposed basal planes that were previously buried. Accordingly, a slight increase in ECSA-normalized overpotential is observed in etched MoAlB relative to the nonetched crystal (Figure S9), because the ECSA of the etched crystal includes both basal planes, which appear to be highly active, and edges, which do not appear to have high catalytic activity. In conclusion, the Al layer in MoAlB single crystals has been partially etched using NaOH to yield crystals with separated MoAlB slabs of nanoscale thickness, and the slabs have been released to isolate freestanding MoAlB sheets. Furthermore, the basal plane of MoAlB showed moderate activity for the HER in acidic media, and the activity was improved significantly by partially etching Al from MoAlB to expose some of the buried basal planes. Optimization and extension of this approach to other MAB phases, including WAlB and the structurally related Fe2AlB2-type compounds Fe2AlB2, Mn2AlB2, and Cr2AlB2 would allow access to a new class of 2D borides that may exhibit enhanced or unique properties relative to their bulk 3D counterparts.21

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Julie L. Fenton: 0000-0002-6485-0405 Raymond E. Schaak: 0000-0002-7468-8181 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation (NSF) Center for Chemical Innovation in Solar Fuels (CHE-1305124). The authors thank W. Adam Phelan and the Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM): a National Science Foundation Materials Innovation Platform (NSF DMR-1539918) for helpful discussions, as well as Tom Mallouk for help with crystal growth, Julie Anderson for help collecting SEM images, and Tim Tighe for help collecting AFM images.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02511.

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Experimental details and additional characterization data (PDF)

AUTHOR INFORMATION

Corresponding Author

*R. E. Schaak. E-mail: [email protected]. 8956

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DOI: 10.1021/acs.chemmater.7b02511 Chem. Mater. 2017, 29, 8953−8957