Experiments and Consideration about Surface Nonstoichiometry of

Sep 14, 2015 - The sulfate species are anchored to the active sites of the (013) planes. The thickness of the surface coating was found to vary as a f...
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Experiments and Consideration about Surface Nonstoichiometry of Few-Layer MoS2 Prepared by Chemical Vapor Deposition Inga G. Vasilyeva,† Igor P. Asanov,*,†,‡ and Leonid M. Kulikov§ †

Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, 3, Acad. Lavrentieva Pr., Novosibirsk 630090, Russia ‡ Novosibirsk State University, 2 Pirogova Str., Novosibirsk 630090, Russia § Frantsevich Institute for Problems of Materials Science, Ukrainian National Academy of Sciences, Akademika Krzhyzhanovs’koho St., 3, 03680 Kyiv, Ukraine ABSTRACT: A detailed chemical and structural characterization of the 2H-MoS2 prepared by chemical vapor-phase method is presented. The nanosized MoS2 (five to eight layers and the lateral planes area from 17 to 55 nm) was confirmed by atomic force microscopy, X-ray powder diffraction, and Raman spectroscopy. Here the main attention was given to active edge sites and vacancy defects in the nanosized MoS2. To settle the problem, we pioneered the application of an effective combination of adequate techniques such as highresolution differential dissolution, X-ray photoelectron spectroscopy with different excitation energies, and IR spectroscopy coupled to thermal gravimetric analysis. The results indicate that the sulfur vacancies at the topmost surface layers are created by the incongruent MoS2 evaporation in vacuum at a temperature above 1000 K and the active chemically undercoordinated Mo atoms on the (110) plane react with residual oxygen to produce the surface-oxidized layers reconstructed relative to the perfect MoS2 lattice. The structure of the surface-oxidized layers on the MoS2 particles is presented by Mo with a varying number of nearest neighboring oxygen atoms. The sulfate species are anchored to the active sites of the (013) planes. The thickness of the surface coating was found to vary as a function of the sintering temperature, and the conditions to control composition and thickness of the altered top layers are given.

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INTRODUCTION Single- and few-layer 2H-MoS2 has various unique electronic, optical, mechanical, and chemical properties, promising its potential applications in electronic device, transistor, energystorage device, and catalysis and in the area of biomedicine. All of this illustrates good reviews considering in-detail synthesis, characterization, and unique properties of the nanosized MoS2.1−5 These specific properties make MoS2 particularly attractive not only for applications but also for studies to probe fundamental science. Here of prime importance is to develop a quantitative understanding of the chemistry of the material because application effects of 2D MoS2, in general, are associated with edge states forming during synthesis where environmental influence plays a crucial role. Being synthesized by different methods, 2D MoS2 possesses different width, edge states, and defects that give rise to new properties; however, only lately has the role of defects in the property formation come to be considered more seriously.6 The problems are not in the number of techniques studying the structural reconstruction of nanosized MoS 2 , but surprisingly, that the detailed study of surface chemistry of the 2D MoS2 still remains to be fully covered because the most common techniques used in the studies give no comprehensive chemical information.1−6 So, energy-dispersive analysis (EDS) © XXXX American Chemical Society

determines only the elemental composition with accuracy inadequate to that desired. X-ray photoelectron spectroscopy (XPS) examining the composition of the top few layers would not do to determine spatial stoichiometry because of the preferential sulfur losses by sputtering. The ability of infrared (IR) technique to control the impurity elements at grain boundaries is limited mainly to quantitative data. Together with Rutherford backscattering spectroscopy (RBS) and electron probe microanalysis (EPMA), all of these techniques contribute little if any to understanding of the surface chemistry of nanoscale MoS2. To achieve an exhaustive view of altered surface state and the chemical reactivity of the MoS2-functionalized nanosheets, we used the effective combination of adequate techniques. Here a unique ability of a new stoichiographic method of differential dissolution (DD) to separate complex mixture into individual phases and to identify them according to their stoichiometric formulas was used. Separation takes place for dissolution in the specially induced dynamic regime with progressively increasing concentration of solvent, and it is an effective method realizing Received: August 3, 2015 Revised: September 5, 2015

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DOI: 10.1021/acs.jpcc.5b07485 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Size and Structural Parameters of As-Made S1−S4 Samples S1 S2 S3 S4

sample

D(013) nm and number of layers

at 1070 K at 1100 K at1120 K at 1123 K

4.2(2) 4.7(2) 3.9(2) ≤1 μm

5 5−6 4−5 >1000

D(110)

a (nm)

c (nm)

17(1) nm 43(3) nm 53(3) nm 10−50 μm

0.31621(4) 0.31600(5) 0.31622(5) 0.31601(1)

1.2294(7) 1.2255(7) 1.2254(8) 1.2298(5)

of the concentrated acids with water, (HCl/HNO3)/H2O, heated from 20 to 75 °C. Being in the reactor, the sample is dissolved layer-by-layer and the solution formed is fed by peristaltic pumps to the analyzer detector, which is an inductively coupled plasma atomic spectrometer (ICP AES). Both elements, Mo and S, are determined simultaneously with a sensitivity of 10−3 μg/mL and an error of 1 at %. The kinetic curves of the element dissolution and their atomic ratio Mo/S stoichiogram were automatically recorded based on the analysis of 450−600 portions of the solution entering the analyzer with a frequency of 1 s. The constant and strongly linear stoichiogram was an indicator of the homogeneous dissolution based on the invariance principle of the dissolution stoichiometry that the stoichiograms remain the same during dissolution in any solvent under different conditions. The variable and nonmonotonic linear stoichiogram indicated any chemical inhomogeneity interpretation of which is performed using a set of special processing programs.7 In general, the DD method might characterize the complex samples, providing information on number, content and composition of phases, going from the surface through the entire bulk of the sample. There are numerous examples of the DD application for precise examination of various high-tech materials in the form of crystals, thin films, and nanoscale powders.7,8 The surface layers of the samples were studied by XPS with a Phoibos 150 (SPECS) spectrometer using monochromatic Al− Kα (1486.6 eV) and Ag−Lα (2984.3 eV) lines. The binding energy was calibrated relative to the surface C 1s signal at 284.5 eV. The atomic concentrations were calculated from the areas of lines taking into account the values of photoionization crosssection and transmission functions of spectrometer. The cross sections for the Ag−Lα excitation were determined using data.11 The S 2p core level spectrum was decomposed with the parameters of the spin−orbit splitting ΔE (2p1/2 − 2p3/2) = 1.18 eV and areas ratio 1:2. The Mo 3d core level spectra present a set of Mo 3d3/2, 5/2 doublets with the separation equal to 3.13 eV, the intensity ratio of 2:3, and the contribution from the S 2s line. The intensity ratios of S 2s components for sulfide and sulfate groups were taken from the S 2p spectrum, and their typical separation was ca. 6.8 eV. Two techniques, the IR and Raman spectroscopy, were employed to identify the impurity surface species. The Raman spectra were recorded with a SPEX 1877 triple spectrometer using a low-power Ar laser (488 nm, P < 1 mW) to avoid laserheating effects. The IR spectra were recorded with VERTEX 80 and UV-3101 PC Shimadzu spectrometers using a KBr pellet technique. To transform IR data into quantitative values, we measured thermal characteristics by a DTA-TG technique with a Q-1500D apparatus recording parameters in a temperature range of 300−1000 κ at a heating rate of 10 κ/min.

functions of both bulk and local analysis simultaneously providing chemical information due to identification of compounds by their primary attribute, that is, the elemental stoichiometry.7,8 XPS with different excitation energies allowed nondestructive determination of the altered layer composition, its extent to a depth, and finding of number and origin of nearest-neighboring atoms of Mo. IR spectroscopy being coupled to thermal gravimetric analysis may provide the quantitative characterization of absorbates. The nanosized MoS2 prepared by a special chemical vapor deposition was the subject of the study.9,10 This synthesis allowed preparation of the MoS2 nanosheets with controllable size and with the high yield. The latter factor was a radically crucial because a low yield is one of the problems seriously hindering the application of the material. Therefore, intimate knowledge of sizes and surface state of this type of MoS2 opens a way to assume features of its properties and areas of application.

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EXPERIMENTAL METHODS 2H-MoS2 nanosheets were synthesized from initial elements in evacuated quartz ampules (∼10−6 mbar) at 650−670 K under the control of the sulfur vapor pressure. The chemical deposition from the vapor phase was carried out by the vapor−liquid−solid mechanism using the autofluctuating temperature regime that provides a limited self-assembly of the S−Mo−S layers. The final size of the samples marked as S1, S2, and S3 was a function of the sintering temperature at 1070, 1100, and 1120 K, respectively, for 12 h. These samples were studied structurally and chemically in detail together with the bulk MoS2 crystals to observe major distinctions existing between them. This comparison allowed a better understanding of the nanosized product features. The bulk crystals marked as S4 were prepared via special chemical transport with sulfur. It was the sublimation transport from 920 to 850 °C under sulfur pressure higher than the dissociation pressure of MoS2. The XRD study was performed with a powder diffractometer HZG-4A (Cu Kα 0.154185 nm radiation) using the full-profile analysis and WinCSD program package for structure computations. The average grain sizes were calculated from the most intense diffraction peaks [013] and [110] with the Scherrer formula, Δ2Θ = 0.94λ/L·cos 2Θ, after subtracting the instrumental and sample contributions to the peak profile using the Stocks formula. Diameter and thicknesses were measured for at least 900 particles for each S1−S3 sample from AFM images. The data were used for drawing up size distributions. Samples for AFM were prepared by drop casting the MoS2 suspension in dimethylformamide on freshly cleaved Si surfaces and dried under room temperature. The precise surface and bulk stoichiometry of the samples was determined by DD with a spatial resolution of ∼5 Å/cm2 to get reliable interpretation of the surface composition.7,8 The procedure includes dissolution of a scotched flat sample in the regime of progressively increasing concentration of solvent starting from H2O and going to molar in a one-to-one mixture

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RESULTS The absence of the low-power (002) and (004) reflections in the XRD patterns of the samples S1, S2, and S3 is characteristic of the few-layer crystalline MoS2, whereas the (013) and (110) B

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Figure 1. AFM images and associated height profiles of 2H-MoS2: (A) sample S1, (B) sample S2, and (C) sample S3.

reflections identical to the reported data of JCPDS cards (no. 37-1492) are indexed readily hexagonal MoS2. Structural parameters (in the brackets errors of measurement are given) and coherent scattering regions being dimensional characteristics for the samples S1−S4 are listed in Table 1 together with their sintering temperature. According to Table 1, all of the samples, irrespective of their dispersion, are described by the same structural type with minor changes in the number of layers and with the size of lateral plane (110) varying from 17 to 53 nm. One can see sharp distinctions in size between S1−S3 and bulk S4. No strong certainty between the lattice parameters and the crystallite dimensions was observed because other sources (may be

extended defects or bending layers) can contribute to the lattice parameters. The structural results demonstrate the oriented growth of the MoS2 nanoparticles faceting preferably by the planes (013) and (110) having different properties. The faces (110) are known to be smooth and fairly inactive because the sulfur is defect-free. The faces (013) are inclined and rough, with active sites like steps, kinks, and undercoordinated atoms. The AFM information characterizes the texture of S1−S3, including size, shape, and arrangement of the initial crystallites in nanoparticles and agglomerates. As one can see, quite irregular spheroidal particles filled fully with small initial crystallites, which are packed parallel and form C

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maximum of 2.7 and 3.1 cm−1, respectively. Turning into S1− S3, the Raman spectra show a progressive shifting of both the A1g and 1E2g bands by ca. 0.3 and 1 cm−1; this suggests that the dimensions of all of these samples are really in nanoregime. The observed frequencies of vibrations are characteristic phonon modes for samples with the number of layers varying from 4 to 7.3 Another feature of the spectra is broadening of the 1E2g band from 3.4 to 3.9 cm−1 and from 5.1 to 5.2 cm−1 when going from S4 (bulk) to S1, respectively, which produces a change in the force coefficient on the layer. The observed broadening and shifting of the Raman peaks relative to the bulk sample may be also characteristic of small sizes of the lateral area. This fact was supported by the structural and AFM results performed in addition to the Raman measurements which, as known, could depend on the laser power.12 Note that profiles of S1 and S2 look rather like only qualitatively, but broadening and shifting of their peaks show in reality slightly different values. The occurrence of the low-intensity band at 286 cm−1 being inplane vibrations forbidden in experiments with backward scattering for the flat S−Mo−S layer in the spectra of S1−S3 may be explained by bending of the layers.13 All the more, the change in the intensity ratio 1E2g/A1g when going from a micron-size powder to nanosheet sample is also well correlated with the intensity of the 286 cm−1 band. A small band at ∼450 cm−1 can be considered as a superposition of the second-order processes involving LA (M) modes in point M; this maximum is interpreted as the oxidized state of MoS2 without any specific data about its nature.14 Overall, all of the above-listed methods gave similar information concerning the S1−S4 sizes. It seems that the particles retain generally original sizes after ultrasonication, although following AFM images they increased slightly, which may be due to physisorption of the solvent used to prepare stable dispersions of S1−S3. To extend the range of the characterization of the nanosized S1−S3, we also used other type of techniques. A quantitative study of the surface and bulk stoichiometry was made by DD. Figure 3 displays the kinetic curves for the Mo and S element dissolution, which were recorded as a function of time over the full dissolution of solid S1−S3 in a solvent varying from H2O, 20 °C to a hot mixture of concentrated acids with water, (HCl/ HNO3)/H2O. Following Figure 3A−C, dissolution of the sole phase with the molar ratio of S/Mo close to 2 without any impurity phases takes place for S1−S3. In Figure 3D, the kinetic dissolution of the nanosized MoS2 is given compared with that of the bulk crystals to show a high chemical activity of the former. The 4−6 mg specimens of S1−S3 completely dissolved for ∼5−7 min in the hot acid solvent, whereas only

thin slot-like micropores, are typical of S1 (Figure 1A). The distribution function for 900 particles can be presented in the following form: 500 nm as 1%, 350 nm as 2%, 250 nm as 6%, 200 nm as 10.5%, 100 nm as 27.5%, and S2 > S1. The initial oxygen partial pressure may be taken as ∼5 × 10−6 mbar; then, the po2/ps2 ratio is more than enough to realize the Mo/MoO3 equilibrium for all of the samples. It seems that the total oxygen content was also sufficient to transform all of the Mo active ions into the MoO3 state in S1−S3. Note that the results of DD, XPS, and IR study demonstrate that it was readily achieved, but there are two contradictions required to be explained. According to DD, XPS, and IR data, S1 with the lowest content of the active Mo ions demonstrates the highest content of the oxide state, whereas for S2 and S3 the situation is quite the contrary. If for S2 and S3 the higher sintering temperatures stimulate the MoO3 losses due to its intensive sublimation from the liquid state (its Tm.p. = 1074 K), for S1 other factors were influential. In this case only a part of oxygen was taken to form MoO3 and then in a reducing oxygen environment; as might be anticipated with reference to the equilibrium phase relationship for the Mo−O binary system, the formation of other oxides becomes possible.27 Indeed, another Mo6+Ox oxide in coordination lower than octahedral, even close to four oxygen atoms, was detected by XPS and IR spectroscopy. We believe that the exchange of active surface sulfur atoms at the edges of the MoS2 (110) plane with the gaseous oxygen leads to the formation of such oxide. The nature of the reduced oxide is unusual and its hypothetical tetrahedral structure remains unknown. Because its tetrahedral slab appears as one of the three building blocks of the γ-Mo4O11 unit cell, this oxide may be considered as a precursor for the known γ-Mo4O11 oxide intermediate between MoO3 and MoO2.15 Because of the MoOx obtained by the exchange reaction in addition to MoO3 originating from the Mo active ions oxidation, the total content of the oxides for S1 is the highest. For S2 and S3, no reduced oxide was observed by

Figure 8. Formation of oxidized states on surface layers of nanosized MoS2.

arise on lateral planes of MoS2 and how two oxidized states different in structure and depth distribution occur. It should be emphasized that in any case the compositional variety of the surface lateral planes begins from the MoS2 nonstoichiometry, and fortunately it is an absolutely controllable characteristic, which is good for the creation of this materials with predetermined properties. The stoichiometric samples with the flat inactive physical (110) plane may be of interest for future biosensor applications. Creating the active Mo sites by the sulfur vacancies in MoS2 may be useful for catalytic reactions. It is likely that occurrence of the surface oxidation states imparts strength to the MoS2 nanoparticles, improving their lubricating properties compared with bulk MoS2, which was unaffected by the oxidation according our XPS experiment. Besides, textures of the test samples with a variety of poor sizes may provide new types of absorbents. Therefore, our work opens a new avenue in the development of another type of 2HMoS2 as 2D nanomaterials. On the contrary, it may bring up negative effects because of a higher ability to participate in undesirable chemical reactions with oxygen, although after this study it became clear how to restrict this negative process.

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CONCLUSIONS A more sophisticated characterization of the four- to fivelayered MoS2 particles faced by (013) planes and extended (110) lateral planes was performed to detect and to study the surface nonstoichiometry and nanostructure of top layers. It was found that functionalization of the (110) planes of MoS2 is created by the oxidation of chemically undercoordinated Mo atoms occurring after sulfur removal during incongruent evaporation of MoS2 in vacuum and the exchange of active edge sulfur ions for oxygen ions. The MoO3 layers are created H

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inside the surface slabs of MoS2, whereas MoOx clusters are located at the top layer of the slabs. The (013) planes are active to create sulfate-containing surface, providing specific texture of the nanosized samples. The critical parameters were found for how to prepare the stoichiometric nanosized MoS2 with smooth and inactive lateral planes or to create MoS2 with the oxidized surface of desired composition and structure. Having original texture, specific surface composition, and nanostructure, this 2D nanomaterial may be attractive for nanocatalysis, gas nanosensors, and effective solid lubricant.

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AUTHOR INFORMATION

Corresponding Author

*Tel: + 7 (383) 316-53-41. Fax: + 7 (383) 330-94-89. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We express appreciation to Dr. R. Nikolaev, Dr. S. Artemkina, and Dr. P. Gevko (IIC) and also Dr. H. Kenig-Ettel (IPMS) and Dr. L. Akselrud (Livov National University) for their assistance in part experiments.

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DOI: 10.1021/acs.jpcc.5b07485 J. Phys. Chem. C XXXX, XXX, XXX−XXX