Article pubs.acs.org/JPCC
Growth Mechanism for Single- and Multi-Layer MoS2 Nanocrystals Lars P. Hansen,† Erik Johnson,‡ Michael Brorson,† and Stig Helveg*,† †
Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Kgs. Lyngby, Denmark Nano-Science Center, Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
‡
S Supporting Information *
ABSTRACT: Transmission electron microscopy (TEM) is used to study growth of MoS2 nanocrystals in situ. The nanocrystals are formed from a submonolayer molybdenum oxide dispersed on an oxide support by sulfidation in an H2S/ H2 atmosphere. From series of time-resolved TEM images, it is revealed that single-layer MoS2 nanocrystals form preferentially and that multi-layer nanocrystals form late in the sulfidation process. The TEM images pinpoint that step sites in the support can act as nucleation centers for single-layer nanocrystals and that single-layer nanocrystals grow along the support surface. Moreover, the TEM images reveal that multi-layer MoS2 nanocrystals form in a layer-by-layer mode by the homogeneous nucleation of additional MoS2 layers onto already formed single-layer MoS2 nanocrystals. Hereby, the atomic-scale observations suggest that the formation of multi-layer MoS2 nanocrystals is an energetically more activated process than growth of singlelayers. These findings explain why process parameters, such as temperature, can tune the relative fraction of single- to multi-layer MoS2 nanocrystals, which is important for their use in, e.g., hydrotreating catalysis.
1. INTRODUCTION Transition metal dichalcogenide (TMD) materials have unique physicochemical properties that are strongly structure-sensitive.1−8 For instance, extended and atomically thin TMD crystals are considered for electronic and optoelectronic applications,2,3 and nanometer-sized TMD crystals are employed as catalysts in oil refinement, hydrogen evolution or photooxidation reactions.4−8 A key research theme is therefore the development of reliable and scalable synthesis procedures for well-defined TMD crystals. A variety of methods have been considered including “top-down” procedures, such as exfoliation of bulk crystals, and “bottom-up” procedures, such as physical or chemical transformations of a precursor material.2,3 A prominent example of the latter type of synthesis is the annual conversion of ∼25000 tons of MoO3 (dispersed on aluminum oxide) into MoS2 nanocrystals, which serve as hydrotreating catalysts in oil refineries for the removal of environmentally harmful sulfur from ∼2500 million tons of mineral oil each year.9 Here, we focus on the “bottom-up” synthesis of MoS2 nanocrystals on an oxide support in relation to hydrodesulfurization catalysis (Figure 1a). These crystallites consist of S−Mo−S slabs, which are layers of hexagonally arranged Mo atoms coordinated each to six S atoms in a trigonal prismatic geometry, and which are stacked to various degrees by van der Waals interactions. The MoS2 nanocrystals have catalytic properties that are strongly structure-sensitive and uniquely related to their edges.4,7 Hereby, the size, shape, stacking and epitaxy of the MoS2 slabs become decisive for the S-containing reactants to reach the catalytic active sites.4−8 As the MoS2 slabs emerge during sulfidation of an oxidic precursor dispersed on the support, information about the sulfidation process is © 2014 American Chemical Society
important to retrieve in order to improve the understanding of how to control the evolution of specific MoS2 structures and, thus, to guide further optimization of the MoS2-based catalyst properties. During the sulfidation of the oxide precursor, two overall reactions, i.e., the reduction Mo(VI) → Mo(IV) and the sulfidation MoO3 → MoSx, can simultaneously be at play.10−16 A description of these processes and the resulting MoS2 structures is developing.16−28 Such studies benefit in particular from high-resolution transmission electron microscopy (TEM) due to its capability for directly resolving the size, shape and stacking of the MoS2 slabs.29−36 Up to now, TEM has mainly been performed ex situ as post-mortem or abrupted growth studies. Complementary in situ studies providing direct and time-resolved observations of the nucleation and growth of MoS2 nanocrystals have, however, not been available. The present study remedies this situation by providing the first in situ TEM observations of the growth of MoS2 nanocrystals. With recent advances, electron microscopes have been made available for the acquisition of consecutive image series of solid materials with subsecond temporal and atomic-scale spatial resolution during exposure to reactive gas environments.37−39 Such microscopes have provided unprecedented new insight into the formation of semiconductor, metallic, oxidic and carbonaceous nanostructures.40−43 However, similar observations of transition metal sulfide materials represent a challenge because sulfiding gases corrode the microscope. In the present study, this problem is overcome by Received: July 11, 2014 Revised: September 2, 2014 Published: September 3, 2014 22768
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2. EXPERIMENTAL SECTION In the present experiments, the formation of MoS2 nanocrystals was studied in situ in a transmission electron microscope. Herein, the nanocrystals grew on a high-surface area MgAl2O4 support material, consisting of 5−50 nm wide crystallites with faceted morphology (Figure S1, Supporting Information). The MgAl2O4 support was incipient wetness impregnated with an aqueous ammonium heptamolybdate solution and subsequently calcined for 2 h at 450 °C in air. The nominal Mo coverage corresponded to a submonolayer of ∼3 Mo atoms/nm2. The resulting MoO3/MgAl2O4 precursor was dispersed onto Protochips E-chips to facilitate sulfidation inside the electron microscope (Supporting Information). The catalyst precursor was sulfided by exposure to 0.8 mbar of 10% H2S in H2 (Air Liquide, nominal cleanness H2 99.999%, H2S 98%) at 690 °C for up to 5 h, to mimic sulfiding conditions used in laboratory studies of refinery catalysts.12−15,19−25 The in situ sulfidation experiments were conducted using a CM300 FEG-ST (Philips/FEI Company) transmission electron microscope equipped with a differentially pumped vacuum system. To overcome the problem by corrosion of materials in the microscope and cross-contamination of sulfur-free in situ experiments, the electron microscope was dedicated to the present purpose. The microscope was exposed to the H2S/H2 gas mixture over prolonged period of time until saturation by sulfur established a constant gas composition (Figure S5). The microscope was operated with an electron energy of 300 keV and with an effective pixel size of 0.07 nm of the chargedcoupled device camera (Tietz F114). The high spatial resolution allows the MoS2 (001) spacing of d = 0.615 nm to be resolved, and hereby the present work sets apart from ref 44. The present experiments were conducted using a low electron dose-rate in order to ensure a noninvasive effect of the electron illumination on the dynamic observations.45−47 Specifically, the time-resolved TEM images were recorded using a low electron dose-rate of 100 e−/Å2s and an
Figure 1. (a) Illustrations of mechanisms for sulfidation of molybdenum oxide (MoO3) into MoS2 nanocrystals, consisting of one (1L) or two (2L) MoS2 slabs. Slabs viewed along the (001) plane are represented by black lines. (b, c) TEM images of the same area (57 × 67 nm2) of a Mo-loaded MgAl2O4 specimen prior to and after sulfidation at 1 mbar of H2S:H2 = 1:9 and 720 °C for 300 min. In part c, a 1L nanocrystal is indicated (full arrowhead) and a 2L nanocrystal (open arrowhead) demonstrates the MoS2 (001) lattice spacing of 0.62 nm.
the use of a dedicated high-resolution electron microscope. Using this microscope, time-resolved TEM image series are obtained in situ of the growth of MoS2 nanocrystals during sulfidation of a submonolayer molybdenum oxide dispersed on a MgAl2O4 support in an H2S/H2 atmosphere. From these dynamic observations, information about the growth mechanism for single- and multi-layer MoS2 nanocrystals is reported.
Figure 2. Time-resolved in situ TEM image series of the growth of nanocrystals consisting of (a) 1L, (b) 2L and (c) 3L MoS2 slabs. Growth conditions: 0.8 mbar of H2S:H2 =1:9 at 690 °C. The images represent times of (a) 122 min, 138 min, 152 min, (b) 225 min, 243 min, 257 min, and (c) 178 min, 192 min, 208 min, relative to the time t = 0 min of reaching the temperature 690 °C (Figure S4). Sketches are included to guide the eye in identifying the MoS2 slabs. The frame sizes are 17.3 × 7.9 nm2. 22769
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accumulated electron dose of up to 5.4 × 105 e−/Å2 (Figure S4). Under these electron illumination conditions, MoS2 nanocrystals observed in situ in the same area during the sulfidation and under the electron illumination conditions as well as post mortem in areas after the sulfidation and without prior exposure to the electron beam have similar distributions in the length and stacking (Figure S11). This finding demonstrates that the electron illumination had a negligible effect on the nanocrystal growth.
3. RESULTS AND DISCUSSION Electron micrographs of the as-prepared precursor material show a dark, speckled contrast across the support grains corresponding to a high and uniform dispersion of MoO3 (Figures 1b and S2). Consecutive TEM images acquired during the sulfidation reaction of a specific area of the catalyst materials show that the speckled MoO3 contrast coarsens and eventually disappears, while areas emerge with a brighter and uniform contrast, similar to the unimpregnated MgAl2O4 support (movie, Supporting Information). Moreover, additional highly anisotropic elongated dark contrast features appear (Figure 1c), with increasing areal density, length, and stacking in the course of time (movie, Supporting Information). In similar experiments, employing 0.8 mbar H2 at 690 °C (but no H2S), faint contrast changes of the precursor are observed and the dark elongated features are absent (Figure S13), indicating a reduction of MoO3. Exposing the precursor to 0.10 mbar H2S at 690 °C (but no H2), however, resulted in dark elongated features (Figure S13). The elongated features are therefore attributed to sulfur-derived structures. The elongated features aligned in pairs (2L, Figure 1c) are separated by 0.62 nm, corresponding to the MoS2 (001) spacing. The dark features are therefore attributed to MoS2 nanocrystals containing one (1L), two (2L), or three (3L) MoS2 slabs oriented with the (001) basal plane along the electron beam direction. The orientation of the MoS2 nanocrystals with respect to the electron beam has a strong impact on the TEM image contrast.29−32 As long as the supported MoS2 nanocrystals have the (001) basal plane approximately ±9 degrees within the electron beam direction, a substantial image contrast is retained (Figure S3). For larger tilts, the contrast weakens and the slabs become indistinguishable from the support material.31 The dark line features may therefore only represent a part of warped MoS2 nanocrystals. Moreover, the present TEM images reveal only the length and stacking height of the MoS2 slabs, which is sufficient to derive dynamic information on the growth of the nanocrystals. Complementary information about the shape and atom arrangement of the nanocrystals orthogonal to ⟨001⟩ can currently be obtained ex situ using single-atom sensitive electron microscopy.33,34 First, the formation of single-layer (1L) MoS2 nanocrystals is addressed (Figure 2a). The TEM images show that once a 1L nanocrystal has formed, it subsequently grows in length as the sulfidation reaction proceeds. The 1L nanocrystals appear with an areal density, which is rapidly increasing in the beginning of the experiment, and, which subsequently tends to stabilize (Figure 3). The nanocrystals have a narrowly distributed length with an average of approximately 2.3 nm early in the experiment and obtain a broader length distribution with an average of approximately 4.1 nm as the reaction proceeds (Figures 3 and S7). This growth mode can be explained by the conversion of the molybdenum oxide into mobile MoOxSy species in the H2S/H2 environment, and the subsequent
Figure 3. (a) Number of 1L, 2L and 3L MoS2 nanocrystals as a function of sulfidation time. The observations are obtained from a ∼15000 nm2 area covered in four contiguous TEM images, including the area in the Supporting Information movie. (b) Average projected length of the MoS2 nanocrystals from the same areas. The error bars for 1L nanocrystals denote the standard deviation of the length distribution (Figure S10). The distributions include only nanocrystals longer than or equal to 1.0 nm, because shorter ones cannot unambiguously be distinguished from the support under the applied electron optical settings.
attachment of these species into a nucleus or a growing MoS2 slab.16,44 Such hypothetical species are likely too small or too fast to be observed in the present images, but molybdenum redistribution can explain the disappearance of the dark speckled contrast of the oxide particles (Figure 1b,c).16 Moreover, the epitaxial relationship between the MoS 2 nanocrystals and the support surface has been much debated.29,30,35,36 Here, TEM images obtained from the edges of the MgAl2O4 grains show that the MoS2 nanocrystals are oriented with their basal plane along the oxide surface contouring the oxide morphology (Figures 4 and S12). The alignment is attributed to the interaction of the MoS2 basal plane with the oxide surface. Remarkably, the 1L nanocrystals remain rather immobile during growth even at the elevated temperatures (Figure 2).17,44 The immobility was further confirmed by the
Figure 4. High-resolution TEM image of a 1L MoS2 nanocrystal protruding from a monatomic step in the MgAl2O4 support with its (001) basal plane oriented parallel to the MgAl2O4 (111) surface. The TEM image is obtained ex situ after sulfidation of the catalyst precursor in a tube furnace providing MoS2 nanocrystals with size- and stackingdistributions comparable to the in situ sulfidation experiments in the electron microscope (Figure S10). 22770
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2L nanocrystals form with a projected length similar to the supporting 1L nanocrystal (Figure 1c and 2b,c), the growth of the additional slabs does not seem to be limited by the availability of Mo species (diffusion-limited growth). Instead, it is proposed that under the present conditions the growth of a next slab is affected by the actual reaction steps (reactionlimited growth). To form an additional slab, the mobile molybdenum species have to move from sites on the MgAl2O4 surface to the basal plane of the single-layer MoS2. This reaction step must be energetically costly because of the weak bonding to the MoS2 basal plane. Concerning nucleation on the existing 1L MoS2 nanocrystal, it is noted that the partially covering second slabs were located at both center and end positions of the supporting MoS2 layer (Figure 2b and Tables S1 and S2), suggesting that the additional slabs tend to nucleate homogeneously anywhere on the underlying MoS2 slab. Homogeneous nucleation is generally associated with a higher activation energy barrier than heterogeneous nucleation due to a higher abundance of coordinative unsaturated sites at the nucleus edges. The destabilization of the mobile molybdenum species on the MoS2 basal plane and the homogeneous nucleation of the next slab suggest that a higher activation barrier is associated with formation of multi-layer MoS2 nanocrystals. These kinetic effects tend to lower the fraction of multi-layer nanocrystals as well as to impede their appearance in the sulfidation process compared to 1L nanocrystals, as actually observed (Figures 3 and S7). Furthermore, the lower fractions of 3L and 4L compared to 2L nanocrystals (Figure 3a) are likely reflecting an increasing barrier for transferring Mo species from the support to the basal plane sites of nanocrystals with increasing thickness. The present findings of MoS2 nanocrystal growth help rationalizing previous studies of the activation of heterogeneous oil refining catalysts as well as the preparation of MoS2 thin films. For example, a higher overall or spatially inhomogeneous loading of molybdenum oxide was reported to facilitate the formation of multi-layer MoS2 nanocrystals.18,44 With the present growth mechanism, the higher abundance of multi-layer MoS2 nanocrystals is explained by the higher local loading that entails a higher local concentration of mobile molybdenum species on the support and, thus, a higher attempt rate for such species to reach sites for nucleation on the basal planes of MoS2 nanocrystals. Moreover, the increased activation for transfer of molybdenum species from sites on the oxide support to the MoS2 basal plane can also explain the previous finding that multi-layer nanocrystals form at higher sulfidation temperature.57 The observed growth mechanism is, however, different from that reported for the growth of MS2 (M = Mo, W) fullerene and nanotube structures by the phase transformation of bulk-like MOx particles.10,11 In this process, the migration of molybdenum species across an inert support is absent and instead a gradual O−S anion exchange reaction within the solid phase takes place. Such an exchange mechanism was not resolved in the present study. The anion exchange may, however, contribute in case of high Mo-loading on the support and should thus be regarded as complementary to the present growth mechanism involving a submonolayer Mo loading.
preservation of slabs in images acquired during several hours of sulfidation and in images recorded during and after in situ observations (Figure S8). Whereas a successive reduction of their length was not observed, occasionally, however, slabs disappeared entirely in consecutive images (Figure S8). A sudden disappearance is likely due to a tilt of the support grain caused by thermal drift of the TEM specimen. Immobility of MoS2 slabs may be attributed to the formation of chemical linkages to the support.16,44 Such linkages are believed to be located at the edges of the MoS2 nanocrystals that possess chemical reactivity contrary to the weakly interacting sites on the basal plane.48−51 In fact, MoS2 nanocrystals can be observed to protrude from step sites in the support (Figure 4, Figure S12), indicating that edge-bonding of a MoS2 slab to a MgAl2O4 step represents a linkage type. In the analogous case of layered carbon structures, growth was previously observed to preferentially proceed from step sites on Ni surfaces.43,52 It is possible that the step sites on MgAl2O4 play a similar role for heterogeneous nucleation and growth of 1L MoS2 nanocrystals. Next, the formation of MoS2 nanocrystals consisting of multiple slabs is addressed. Such nanocrystals appear late in the sulfidation reaction, contain up to four slabs with an average length similar to or larger than that of the 1L nanocrystals (Figure 3 and S10), and constitute less than approximately 10% of all observed MoS2 nanocrystals (Figure 3 and S7). These findings suggest that the formation of multi-layer nanocrystals is impeded compared to single-layer growth. The observation that 1L nanocrystals remain immobile indicates that the multilayer MoS2 nanocrystals do not emerge by migration and coalescence of 1L MoS2 nanocrystals (layer-on-layer growth, Figure 1a). Instead, the time-resolved images reveal that multilayer nanocrystals form by a successive growth of additional layers (layer-by-layer growth, Figure 1a). Figure 2b shows the formation of a 2L MoS2 nanocrystal. The process involves an intermediate step with a partial coverage of basal plane sites on an existing 1L nanocrystal. The intermediate slab subsequently grows to match the size of the underlying slab. A similar growth mode is shown for a 3L nanocrystal (Figure 2c), for which the second and third slab forms successively on top of an existing 1L nanocrystal. That the intermediate states with partially covering second or third slabs are not resolved in the timeseries in Figure 2c is likely due to a combination of the short slab length and a close match of the growth and image acquisition rates. In a few cases, the multiple MoS2 layers in a nanocrystal appear to grow synchronously (Table S1), suggesting that the layer-by-layer growth mode contributes in parallel (Figure 1a). It cannot be excluded that these growth events are also mediated by the more dominating layer-by-layer growth mode and that the applied frame acquisition rate was too short to resolve the characteristic intermediate steps (e.g., as in Figure 2b). The MoS2 slabs can stack in different polytypes among which the 2H stacking sequence is the thermodynamical more stable phase.53,54 The driving force toward the bulk phase, however, appears to be offset for the present system, because nanocrystals with an increasing stacking height form with decreasing abundance (Figure 3). The effect is likely due to the support surface. On one hand, the surface energy of MoS2 (001) (0.28 J/m2) is far lower than the surface energy of the most stable surfaces (100, 110, 111) of MgAl2O4 (2.27−3.07 J/ m2),55,56 which favors wetting of MgAl2O4 by MoS2. On the other hand, the time-resolved TEM images pinpoint kinetic inhibition of multi-layer growth: As the majority (84%) of the
4. CONCLUSIONS By means of a dedicated high-resolution electron microscope, atomic-scale observations are obtained in situ of the formation of MoS2 nanocrystals by the gaseous sulfidation of a submonolayer molybdenum oxide precursor dispersed on an 22771
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inert oxide support. The time-resolved observations hereby reveal elementary processes involved in the formation of singleand multi-layer MoS2 nanocrystals. The observations can be rationalized by a growth mechanism including heterogeneous nucleation and step-flow growth of single-layer and homogeneous nucleation and layer-by-layer growth of multi-layer MoS2 nanocrystals. These findings enable an explanation of activation phenomena in hydrotreating catalysts.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, characterization data, image analyses and a movie of time-lapse TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(S.H.) E-mail:
[email protected]. Author Contributions
All authors contributed equally. Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENTS L.P.H. was financially supported by The Danish Council for Technology and Innovation (08-044837). We acknowledge financial support from The Danish Council for Strategic Research (grant Cat-C). Dr. Maria J. L. Østergård, Dr. Ying-Shi Pan, Dr. Ramchandra R. Tiruvalam, and Sven Ullmann, Haldor Topsøe A/S, are acknowledged for fruitful discussions and support. The article is dedicated to the legacy of Dr. Haldor Topsøe (1913−2013) and his dedication to science and catalysis.
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REFERENCES
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The Journal of Physical Chemistry C
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