Communication Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Concentric Advancing Front Corrugations and Multiple Ordered Growth of 2D Mo2C Crystals Lin Li,† Min Gao,‡ and Dong Shi*,† †
School of Optoelectronic Science and Engineering and ‡School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
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S Supporting Information *
ABSTRACT: Despite a prolific family of transition metal carbides having been obtained via the chemical vapor deposition method into the 2D world looking for large-area ultrathin functional crystals, understanding of the growth mechanism has not grown at the same pace as the lack of investigation into the wealth of imperfect results. Here in this work, we conducted ill-controlled growth of 2D Mo2C, yielding multiple concentric hexagonal architectures. We deduce the in situ occurrence corrugations surrounding Mo2C formed upon co-solidification of the advancing Mo front with C atoms populated in the furnace atmosphere. Thus, the deduced mechanism scheme encodes predictions that are consistent with recent experimental results. Formation of 6-fold ordered dendrites exactly as predicted further corroborates our deduction, thereby projecting the scheme into the growth scenario of functional 2D transition metal carbides.
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predicted for monolayer TMCs years ahead.11 However, it was experimentally confirmed via recent advances on large-area high-quality ultrathin Mo2C crystals that the novel 2D limited electronic properties in such systems are obviously thickness dependent.11,14 Therefore, sizable monolayer 2D TMC crystals that enable comprehensive physical characterizations remain the ultimate to reach. Unfortunately, such an ideal material platform remains unachievable in the 2D TMCs community. A very encouraging effort in synthesizing monolayer 2D TMCs was tried by Naguib and co-workers by selective extraction from ternary TMCs.8,12 Unfortunately, the crystal sheets they obtained were seriously affected by defect abundance and surface terminations. Very recently, large-area high-quality ultrathin 2D TMCs have been achieved via chemical vapor deposition (CVD),11,14 a method which now stands at full maturity. The thickness, ∼3 nm, which translates into a handful of atomic layers, once again inspires confidence to tackle the ultimate challenge of ideal monolayer TMCs using the theoretically reinforced CVD methods. While increasing efforts are being devoted toward CVDprocessed monolayer TMCs,15−18 fundamental understanding of the growing mechanism is not progressing at the same pace. Even the simplest framework illustrating routinely induced growth pathways has never been and could never be outlined with no information about the malformed architectures beyond individual ultrathin sheets. Seemingly, a wealth of imperfect
n modern advanced optoelectronic technologies in pursuit of ideal (generally refers to large-area, defect-free, ultrathin, or even single-layer) functional crystals, researchers have created a prolific family of diverse 2D materials demonstrating a variety of fascinating properties.1−3 To a large extent, currently the unprecedented flourishing prospects in the 2D materials’ world have benefitted from the valuable experience accumulated in the past decade throughout the intensive wellconducted research activities on graphene, the banner 2D material of monolayer carbon sheets. In view of the amazing 2D-limited exceptional functionalities as demonstrated from graphene and other 2D materials made soon after,4−6 modern functional 2D materials are thus highly expected. Among the most remarkable research advances in this context, large-area and high-quality ultrathin transition metal carbides (TMCs)7−10 have been attracting increasing interest in extraordinary superconductivity,11 an electronic property which is, so far, still much less achievable than dielectricity, ferroelectricity, semiconductivity, and so on in the 2D materials’ world. Noted following the breakthroughs in synthesis of the monolayer Ti3C2 in 2011,12 2D TMCs now represent a subgroup of materials in the prolific family of 2D Mxenes (where M denotes transition metals and X denotes carbon or nitrogen). Among the uniqueness of 2D TMCs is the combination of outstanding electronic properties and excellent mechanical properties, basically due to their mixed covalent− ionic bonding between metal and carbon atoms.13,14 Some of the unique properties being characterized in the very recent advances on nanometer-thick ultrathin TMCs sheets are, in fact, experimental affirmation of what had been theoretically © XXXX American Chemical Society
Received: March 11, 2019 Revised: April 25, 2019 Published: May 1, 2019 A
DOI: 10.1021/acs.cgd.9b00314 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 1. Characterization of Mo2C architecture. (a) Optical image of the as-grown Mo2C structure on liquid Cu surface, showing pyramid mode. (b) XPS survey spectrum of Mo2C. (c) AFM image of multiple concentric Mo2C hexagon. (d and e) Mo and C peaks of the Mo2C. (f−h) STEM images of pyramid Mo2C, showing staircase-like and aggregation of Mo atoms.
We analyzed the elemental composition of the multiple concentric hexagons by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The charge corrected survey spectrum shows sharp peaks (Figure 1b) signaling Mo 3d and C 1s as amplified in Figure 1d and e. Systematic deconvolution of Mo 3d and C 1s peaks into summarization of Gaussian− Lorentzian curves yields an atomic ratio of Mo/C ∼ 2/1, indicative of the overall expected conversion of atomic Mo and C into Mo2C crystals.19−21 The split in the Mo 3d peak is attributable to the spin−orbital coupling that corresponds to Mo 3d5/2 and Mo 3d7/2 levels (Figure 1d), respectively. Despite everything looking perfectly the same with recent reports, a notable protuberance (marked by the red circle in Figure 1d) in the left-side tail of the Mo 3d5/2 curve indicates the presence of metallic Mo. In addition, the extraordinarily bright central spot in the multiple concentric hexagonal Mo2C (Figure 1f−h) is likely another characteristic phenomenon of chemically unreacted metallic Mo. Moreover, Raman measurement confirmed the existence of Mo2C (Figure S3), whereas the typical peak at 650 cm−1 corresponding the Mo−C
and radically different results had been buried in the laboratory archives, leaving no more than an implicit expression as quoted from a group of investigators that “it was difficult to fabricate individual thick crystals as large as thinner crystals because of high nucleation density at high growth temperature”. We were thus “provoked” to explore the buried wealth because we believe they are more than worthy of investigation and helpful for fundamental understanding. In this study, we investigated “ill-controlled’’ CVD growth of 2D Mo2C (Figure S1), a very representative functional 2D TMC of intensive focus at present. While identically the same material source and furnace step for large-area high-quality ultrathin 2D Mo2C crystals were used, the deliberately controlled overheated metallic substrate and overconcentrated CH4 carbon source drive our efforts into the intended imperfections. Instead of ultrathin crystal sheets, we obtained a kind of complex yet uniform architecture as shown in Figure 1, i.e., multiple concentric hexagons demonstrating staircaselike height gradient (Figure 1a and c, Figure S2). B
DOI: 10.1021/acs.cgd.9b00314 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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stretching mode was detected.22 To further study the crystal structure and determine the phase, the samples were transferred onto TEM grids (Figure S4a). The corresponding selected area electron diffraction (SAED) patterns are shown in Figure S4b. The lattice parameters of the crystal derived from the SAED results are a = 4.77 Å, b = 5.98 Å, and c = 5.21 Å. These data are strikingly in agreement with those of orthorhombic Mo2C,23 confirming the phase of α-Mo2C. The first question in our mind is whether the central condensation of atomic Mo in the multiple hexagonal architecture is a surface or volume phenomenon. We examined a few dozens of recently published microscopic images within the 2D Mo2C material frame, and have found similar observations and attribution of atomic Mo nanoparticle deposition on the surface of ultrathin Mo2C crystals formed upon slight deviation from the most optimal growth condition details (Figure S5). Hereby, one might also attribute the central condensation of atomic Mo observed in the present study to a surface phenomenon. However, had the investigators examined a little bit more on the microscopic image, they would have distinguished immediately two specific local areas throughout the entire image regions, i.e., the right central point and the periphery of the ultrathin hexagonal Mo2C crystals, both of which are featured by bigger/denser Mo nanoparticles compared with the rest. In view of this, we intuitively attribute the central condensation of atomic Mo in the multiple concentric hexagonal Mo2C architectures to a volume phenomenon. We sketch our reasons with a deduced schematic diagram as shown in Figure 2.
exposed to the hot atmosphere abundant in atomic C as thermally dissociated from CH4. Then, unavoidable continuous collision of C atoms with the periphery of the atomic Mo convex array leads to the formation of the thermally target compound, Mo2C, having an inherent hexagonal closely packed lattice structure. This process is marked in Figure 2. While more and more Mo atoms are forced to gush throughout upon prolonged heating in the thermally static furnace, the periphery of the initially nucleated Mo convex array is solidified the moment when Mo2C is formed. Therefore, this constrains lateral growth of the Mo center. In the meantime, regarding the conic Mo convex arrangement, we emphasize the temperature gradient, a general feature as a consequence of heat loss (across and then out to the environment) as qualitatively marked with arrows in Figure 2b. We can therefore reasonably assume that along with the upward progression of the conic convex arrangement, the embodied Mo atoms will cool down at some point below the critical temperature (Tc) for the formation of α-Mo2C crystal. Prolonged cooling over the critical point leads to concentration and finally condensation down to solid metallic Mo centers, following the vertical propagation of the Mo flux. A statement recently made by a group of investigators that “it is difficult to fabricate individual thick α-Mo2C crystals at high growth temperature” is consistent with our assumption on one point of view, i.e., the overheated condition yields thick central crystal plates regardless of morphologies. With the constraints from the condensed solid metallic Mo center as well as the formed solid α-Mo2C crystal (Figure 2b), the constantly gushing Mo flux has to expand beneath the αMo2C crystal. Of course, the α-Mo2C crystal also keeps growing. As long as there is more than enough carbon source throughout the whole process, homogeneous crystal epitaxy along equivalent lateral directions in terms of the intrinsic sixfold lattice symmetry will be always guaranteed. Consequently, the uniform hexagonal architecture is explained. The fact, as jointly concluded throughout recent studies, that diverse α-Mo2C morphologies lose the inherent sixfold symmetry form in the presence of very low concentration of carbon source corroborates our explanation. Then, things become interesting when we turn to the competition between the two processes, i.e., crystal epitaxial growth and the lateral propaganda of the atomic Mo flux (Figure 2b). According to the aforementioned assumption, the lateral frontier Mo atoms remain hot enough for cocrystallization as they propagate to a critical distance away from the gushing center. This together with the sufficient atomic carbon source jointly maintains the two processes in a balanced way, thereby yielding a smooth and clear crystal surface. The moment the frontier Mo atoms reach the critical point (Figure 2c) at which the temperature decreases to Tc, the atomic Mo becomes unreactive to form Mo2C and starts to accumulate there before condensation. Should no further cocrystallization be allowed with the condensation of the frontier Mo flux, a thick hexagonal α-Mo2C crystal carrying the dense periphery of metallic Mo will be formed (Figure 2d). This speculation can be viewed as an augmentation of the very recent experimental observation mentioned in the previous paragraph, i.e., even grown at the most optimal temperature (∼50 °C lower than used in this study) for ultrathin crystal sheets, any delayed termination of the atomic Mo diffusion at a slow cooling rate will cause accumulation of extra atomic Mo at the lower periphery of the hexagonal α-Mo2C crystals.
Figure 2. Schematic showing the formation process of Mo2C architecture. (a-d) Gradual evolution of Mo2C architecture driven by convex formation of Mo atoms. (e and f) Side- and top-view of the Mo2C architecture.
Recently, it was jointly concluded that only with the diffusion of Mo atoms from the in situ alloyed Cu−Mo intersectional layer up to the surface of melt Cu top layer could Mo2C crystal be formed in the atomic C abundant furnace atmosphere. Accordingly, we sketch in Figure 2a the formation of reactive atomic Mo center nucleated prior to conversion into Mo2C upon of collision with C atoms. Under overheated conditions in the present study, a dense array Mo atoms gushes throughout the melt Cu layer and instantaneously nucleates into a conical convex arrangement of reactive Mo center C
DOI: 10.1021/acs.cgd.9b00314 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 3. Experimental evidence of the proposed mechanism for formation of Mo2C architecture. (a and b) Optical images of Mo2C structure after gently shaking. (c) Raman mapping image of Mo2C structure. (d) Optical image of symmetrically dendritic Mo2C architectures.
Figure 3 shows direct experimental evidence supporting this formulated growth mechanism scheme which concludes the existence of metallic kinks (physical segregation without covalent chemical bonds) between the stepwise formed Mo2C fragments in the multiple hexagonal entirety. The fact, as experimentally observed, that it is very easy to disentangle the hexagons from the entirety simply by gentle shaking (Figure 3a and b), affirms of the physical nature of the hexagonal segregation kink rings. Such physical segregation by metallic Mo was further corroborated by Raman mapping image which demonstrates clear hexagonal segregation traces as displayed in Figure 3c. Generally, the segregation across the corners of the multiple concentric hexagons is weaker than the case across the center of the hexagon edges, indicating less condensation of metallic Mo at the corner. This together with the uniform hexagonal fragment and shapes jointly concludes that the lateral crystal growth at the corners has the highest priority in this study; otherwise, the uniform hexagonal shapes could not be guaranteed. Intuitively, the crystallization dynamics hereby is probably governed by a set of complex factors which are far beyond the focus of the present study. Later, thus formulated crystal growth mechanism scheme was further corroborated by experimental affirmation of the precisely predicted resultant architectures upon changing the growth condition. What can be predicted from the above formulated essentials is that upon lowering the concentration of the carbon source to some insufficient level at which the metallic Mo frontier starts to compete to react with C atoms and form Mo2C, the deduced growth priority takes effect on the sixfold hexagon corners, thereby forming the snowflake-like
Nevertheless, crystallization restarts after such an interval of condensation in the present study. Otherwise, individual thick instead of multiple concentric hexagons will be obtained in the end. Apparently, the driving force lies in the higher growth temperature which could be interpreted in terms of relatively strengthened buildup of hot (above Tc) Mo atom beneath the solidified periphery as depicted in Figure 2d. Once the buildup exceeds the constraint limit thereof, excessive hot metallic Mo breaks through along the periphery of the already formed central hexagon (Metallic Mo central axis + Mo2C + metallic periphery), thereby restarting the crystallization of Mo2C, which in turn continues until the frontier temperature of the new wave of progressing metallic Mo reaches Tc again. In such circulation, multiple concentric hexagonal Mo2C crystals will be formed in the far end until no excess buildup of metallic Mo could be realized beneath the distanced ending periphery (Figure 2e and f). We emphasize that regardless of any temperature disturbance in the thermostatic furnace, the driving force which sends atomic Mo onto the melt Cu surface becomes more and more diluted along with more and more distanced and enlarged periphery. Therefore, each restarted growth yields thinner Mo2C than the previous stage. This explains the height profile of the multiple concentric hexagonal Mo2C crystals (Figure 1). To clarify the crystalline planes and growth direction, STEM measurement has been conducted. The edge step is clearly observed at STEM images (Figure S6), showing a verified growth direction from inside to outside. The atomic structure of the edge step confirmed our proposed growth mechanism for the hierarchical Mo2C crystals. D
DOI: 10.1021/acs.cgd.9b00314 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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dendrites on the initially formed hexagonal cores. In situ experimental monitoring by lowering the carbon source concentration while keeping all the rest parameters unchanged does yield such symmetrically dendritic Mo2C architectures (Figure 3d and Figure S7). In summary, our results seem to stray in the opposite direction from the predominant research thrust toward imperfection-free ultrathin sheet of TMCs, but the mechanistic scheme we deduced may open a gate into the scenario of complete understanding and control over the functional 2D TMCs.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Lin Li: 0000-0001-6526-3395 Min Gao: 0000-0003-3899-2933 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China under Grant No. 51872038.
METHODS
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CVD Growth of Hierarchical Mo2C Architecture. One big piece of Cu foil (Alfa Aesar, 99.8% purity, 50 μm thick) was cut into 10 × 10 mm2 pieces. After that, the small piece was put on top of a metal Mo foil (Alfa Aesar, 99.95% purity, 50 μm thick), which served as supporting substrate and Mo source in the CVD reaction. A quartz tube with outer diameter 25 mm, inner diameter 22 mm, and length 1220 mm was used. Then, a horizontal tube furnace (Lindberg Blue M, TF55030C) was set at a temperature of 1100 °C and gradually heated under H2 with a flow rate of 40 sccm. Subsequently, an annealing process was conducted with a time of 30 min to active the surface of Cu. 5 sccm CH4 was then introduced into the tube as carbon source to initiate the growth. The hierarchical Mo2C crystals in a large-area are grown with a growth time of several minutes. After the CVD process, the as-grown Mo2C samples were quickly removed from the high temperature zone under H2 gas, and then rapidly cooled to room temperature. Transfer of Hierarchical Mo2C Architecture. The chemical etching method was employed with assistance of poly(methyl methacrylate) (PMMA) to transfer the as-grown samples on Cu surface. First, a spin-coated process at 6000 rpm for 3 min was performed and then PMMA (weight-averaged molecular mass Mw = 600 000, 4 wt % in ethyl lactate) was covered onto the surface of Mo2C samples. After that, the substrates were cured at 170 °C for 5 min to strengthen the bond between Mo2C and PMMA. Then, the PMMA-coated samples were put into a 1 m (NH4)2S2O8 aqueous solution, whereas the underlying Mo and Cu substrates could be etched off. Meanwhile, the PMMA/Mo2C thin film layer was detached from the substrate. After cleaning with pure water, the PMMA/Mo2C layer was stamped onto the SiO2/Si or TEM grids for further measurements. Finally, the PMMA was removed by warm acetone, with clean Mo2C architecture left behind. Characterization of Hierarchical Mo2C Architecture. Olympus BX51 microscope was used to obtain the optical images. Atomic force microscopy (AFM) images were collected by a Bruker Dimension FastScan atomic force microscope. The working mode was set in the tapping mode. An Omicron EAC2000−125 analyzer was employed to conduct the XPS elemental analysis. The base pressure was fixed at 10−9 Torr in the analysis process. An Al K α monochromatized radiation (hν = 1486.6 eV) was employed as the Xray source. A WITec Raman microscope with laser excitation at 532 nm was used at ambient conditions. Bright-field TEM images of the Mo2C crystals were performed on Philips CM30 TEM with a voltage of 300 kV. High-resolution STEM-ADF imaging was performed at an aberration-corrected Nion UltraSTEM-100, in which a cold field emission gun was equipped. The operating voltage was set at 100 kV.
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Communication
REFERENCES
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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.cgd.9b00314. Procedures and additional data (PDF) E
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of van der Waals heterostructures of graphene and two-dimensional superconducting Mo2C. Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 95, 201403. (16) Deng, R. X.; Zhang, H. R.; Zhang, Y. H.; Chen, Z. Y.; Sui, Y. P.; Ge, X. M.; Liang, Y. J.; Hu, S. K.; Yu, G. H.; Jiang, D. Graphene/ Mo2C heterostructure directly grown by chemical vapor deposition. Chin. Phys. B 2017, 26, 067901. (17) Xu, C.; Song, S.; Liu, Z.; Chen, L.; Wang, L.; Fan, D.; Kang, N.; Ma, X.; Cheng, H. M.; Ren, W. Strongly coupled high-quality graphene/2D superconducting Mo2C vertical heterostructures with aligned orientation. ACS Nano 2017, 11, 5906−5914. (18) Geng, D.; Zhao, X.; Chen, Z.; Sun, W.; Fu, W.; Chen, J.; Liu, W.; Zhou, W.; Loh, K. P. Direct synthesis of large-area 2D Mo2C on in situ grown graphene. Adv. Mater. 2017, 29, 1700072. (19) Ç akır, D.; Sevik, C.; Gülseren, O.; Peeters, F. M. Mo2C as a high capacity anode material: a first-principles study. J. Mater. Chem. A 2016, 4, 6029−6035. (20) Liu, Y.; Yu, G.; Li, G.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. Coupling Mo2C with nitrogen-rich nanocarbon leads to efficient hydrogen-evolution electrocatalytic sites. Angew. Chem., Int. Ed. 2015, 54, 10752−10757. (21) Anasori, B.; Lukatskaya, M. R.; Gogotsi, Y. 2D metal carbides and nitrides (MXenes) for energy storage. Nat. Rev. Mater. 2017, 2, No. e16098. (22) Xiao, T. C.; York, A. P. E.; Al-Megren, H.; Williams, C. V.; Wang, H. T.; Green, M. L. H. Preparation and characterization of bimetallic cobalt and molybdenum carbides. J. Catal. 2001, 202, 100− 109. (23) Parthé, E.; Sadogopan, V. The structure of dimolybdenum carbide by neutron diffraction technique. Acta Crystallogr. 1963, 16, 202−205.
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