Carbon Nanotube Composite - American

ARTICLE pubs.acs.org/JPCC

Charge Transfer in the MoS2/Carbon Nanotube Composite V.O. Koroteev,*,† L.G. Bulusheva,†,‡ I.P. Asanov,†,‡ E.V. Shlyakhova,† D.V. Vyalikh,§ and A.V. Okotrub†,‡ †

Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Acad. Lavrentiev ave., Novosibirsk 630090, Russia Novosibirsk State Technical University, 20 K. Marx ave., Novosibirsk 630092, Russia § Institute of Solid State Physics, Dresden University of Technology, D-01062 Dresden, Germany ‡

ABSTRACT: Composite MoS2/carbon nanotube material has been produced by hydrothermal decomposition of a mixture of multiwall carbon nanotubes (CNTs) and a water solution of ammonium molybdate and thiourea. Transmission electron microscopy and Raman spectroscopy showed formation of MoS2 layers on the CNT surface and MoS2 flakes. X-ray photoelectron spectroscopy revealed a downshift of C 1s peak of the composite as compared to the pristine CNT sample that was related to charge transfer between the components. This fact was confirmed by near-edge X-ray absorption fine structure spectroscopy which detected a decrease of intensity of π* resonance in the C K-edge spectrum after the MoS2 deposition. Quantum-chemical calculations of a CNT@MoS2 model showed a positive charging of the CNT surface. Comparison of field emission characteristics of CNTs and the composite indicated lowering of the voltage threshold in the latter sample.

’ INTRODUCTION Coating of carbon nanotubes (CNTs) with inorganic materials is a promising tool for realizing various applications of CNTs in nanodevices.1 Formation of continuous crystallized layers on the CNT surface is achieved when there is commensurability between an inorganic material lattice and a graphite lattice. In particular, CNTs have been coated with insulator BN layers2 and semiconducting layers of MoS2 and WS2.3,4 These inorganic solids have a two-dimensional layered structure with hexagonal packing of atoms in a layer. In transition metal (Mo, W) disulfides, the hexagonal layers of metal atoms are sandwiched between two layers of sulfur, and as it has been recently demonstrated these triple-atomic sheets can be separated from a crystal by dispersion of a bulk compound in a solvent.5 Nowadays, the methods for obtaining the coaxial heterogeneous structures from MoS2 nanotubes and CNTs are most developed.3,6 9 The CNTs serve as templates for confining and directing the growth of tubular MoS2 layers resulting from thermolysis of Mo(VI) compounds in the reduction atmosphere3,7 or hydrothermal reaction between molybdenum and sulfur sources. 8,9 It is expected that the MoS2 /CNT composites should have special chemical, physical, and mechanical properties as compared to the individual components. The electrodes from MoS2 overlayers supported on coaxial multiwall CNTs have demonstrated highly reversible capacity (approaching 400 mAh/g) and excellent cyclability in lithium storage/release process that was attributed to a unique synergy at the nanoscale between the CNT core and MoS2 sheets. 10 Since the properties of nanocomposite are mainly determined by the interface electronic states, in the present work we have studied the electronic structure of MoS2/CNT nanomaterial produced by a hydrothermal route. Effect of MoS2 covering on r 2011 American Chemical Society

the electronic state of CNTs has been revealed using X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, field electron emission measurements, and results from the quantum-chemical calculation of a CNT@MoS2 model.

’ EXPERIMENTAL METHOD Synthesis. Multiwall CNTs were synthesized by a catalytic chemical vapor deposition (CCVD) method using a Kepleratetype molecule [H4Mo72Fe30O254(CH3COO)10{Mo2O7(H2O)}{H2Mo2O8(H2O)}3(H2O)87] 3 80H2O as a catalyst source.11 The molecules were anchoring to MgO support from a water solution and then annealed in air for 10 min at 700 °C to produce clusters made of Fe and Mo oxides.12 The supported metal oxides were introduced in a horizontal tubular reactor and reduced in H2 flow (50 sccm) for 10 min at 900 °C to form metal catalyst. The CCVD growth of CNTs was performed with C2H4 (150 sccm) for 30 min at 900 °C. The product was washed with HCl to remove MgO and metal impurities located at the outer walls of CNTs. The MoS2/CNT composite was prepared by the hydrothermal route as follows. (NH4)2MoO4 (57.5 mg) and (NH2)2CS (57.5 mg) were dissolved in 10 mL of distilled water, and then CNTs (18.4 mg) were added into the solution and ultrasonically dispersed in it. The mixture was autoclaved at 235 °C for 72 h. After cooling to room temperature the resulting precipitate was collected by filtration through a 10-nm-pore membrane and washed with distilled water. A ratio between amounts of CNTs and reagents used for MoS2 formation were calculated based on a specific surface of the Received: June 24, 2011 Revised: September 13, 2011 Published: September 28, 2011 21199

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The Journal of Physical Chemistry C CNTs and according to reaction 3(NH4)6Mo7O24 + 49(NH2)2CS + 40H2O f 21MoS2 + 7SO2 + 49CO2 + 116NH3. Characterization. The structure of the composite material was studied using transmission electron microscopy (TEM) with a Jeol 2010 microscope and Raman spectroscopy with a Spex triple spectrometer using a 488 nm Ar+ laser. The XPS and NEXAFS experiments were performed at the Berliner Elektronenspeicherring f€ur Synchrotronstrahlung (BESSY) using radiation from the Russian-German beamline and the MUSTANG experimental station. The XPS spectra were collected using a hemispherical analyzer VG CLAM-4. The overall XPS spectrum as well as C 1s, S 2p, and Mo 3d lines were measured using monochromatized synchrotron radiation at 800 eV. The energy resolution of the lines was better than 0.4 eV (full width at a half-maximum (fwhm)). For spectrum analysis, the background signal was subtracted by Shirley’s method.13 NEXAFS spectra near the C K-edge of CNT and composite samples were acquired in the total-electron yield mode. The spectra were normalized to the primary photon current from a gold-covered grid recorded simultaneously. The monochromatization of the incident radiation was ∼80 meV, fwhm. The electron emission properties of the CNT and composite samples were studied in a diode regime using a handmade setup.14 The measurements were performed at room temperature in a vacuum of 5  10 4 Pa. An investigated sample was put onto carbon scotch tape, which was fixed on the surface of a stainless steel cathode. The value of the tunneling current as a function of the electric field strength was measured applying sawtooth voltage with a frequency of 0.1 Hz. The data were obtained by averaging over the result of 10 measurements for a sample.

’ CALCULATION METHOD A CNT@MoS2 model was composed from an armchair (4,4) CNT core sheathed with an armchair (8,8) MoS2 tube (Figure 1). The dangling bonds at the edges of the CNT were saturated by hydrogen atoms. The composition of the model was C80H16@Mo24S48. The distance between CNT surface and inner sulfur layer of the Mo24S48 tube was ∼3.5 Å, which equals the sum of the carbon and sulfur van der Waals radius. The electronic structure of the model was calculated using a semiempirical AM1 method15 included in the Openmopac code.16 The method is parametrized for carbon, sulfur, and molybdenum, and it is time conserving as compared to the ab initio approaches. Furthermore, in the AM1 method the function describing repulsion between the atomic cores is tuned for accounting long-distance interactions,15 which is important for a system with van der Waals interactions. ’ RESULTS AND DISCUSSION TEM analysis showed the product of hydrothermal synthesis contains mainly CNTs with an admixture of thin MoS2 flacks and ribbons (shown by arrows in Figure 2a). CNTs were 10 20 nm in the outer diameter and composed of 5 25 walls. Since both molybdenum and sulfur atoms have the larger weight than the carbon atom the images of the MoS2 layers are highly contrasted. From high-resolution TEM (HR TEM) images we estimated that only ∼40% of CNTs are sheathed within MoS2 nanotubes (parts b and c of Figure 2). The number of MoS2 layers in these nanotubes is varied from 1 to 3 walls parallel to the basal planes of the CNTs. The adjacent coating layers are typically separated by 6.2 6.6 Å (Figure 2b), and the values are close to 6.15 Å; the

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Figure 1. Atomic structure of a CNT@MoS2 model with C80H16@Mo24S48 composition. The inner tube is armchair (4,4) carbon tube with boundary saturated with hydrogen atoms. In the outer MoS2 tube metal atoms (darker) are sandwiched between two layers of sulfur atoms.

interlayer distance in the hexagonal 2H-MoS2 crystal.17 An increase of the interlayer MoS2 distance is caused by a discontinuity of the outer deposited layers (shown by circle in Figure 2c). The fact that some of the CNTs were not coated by MoS2 could be related to existence of defects in graphitic walls. Actually, one can see that MoS2 layers do not cover the ends of CNTs (Figure 2b), and they are disrupted at the CNT kinks (Figure 2c). Since in the result of synthesis the CNT coverage was incomplete, the extra MoS2 species produced the flacks, which are also formed after hydrothermal reactions without a template.18 Probably the sheathing of the whole outer surface of multiwall CNTs requires a high temperature annealing of pristine CNTs as it has been done by Wang et al.10 Existence of many defects in the walls of pristine CNTs is supported by Raman spectrum exhibiting a D band at 1355 cm 1 (Figure 3b), which is attributed to the disordering in the graphitic layers or an amorphous carbon.19,20 The G band (in-plane stretching, E2g mode) is located at 1585 cm 1. As the CNT sample was not contaminated with the amorphous carbon, high ratio of intensities of D and G bands (ID/IG = 0.62) should be related with defects, particularly, dislocation of graphitic layers, missing of carbon atoms, topological defects at kinks and nanotube ends, sp3 carbon atoms bonded to functional surface groups. To gain insight to the effect of MoS2 deposition on the defectness of CNTs the Raman spectra of CNT, and composite samples were normalized to the intensity of the G band. We were surprised that the ID/IG ratio was lowered to 0.57 in the composite spectrum. Heating of a mixture of ammonium molybdate and thiourea yields ammonia, which could initiate a decarboxylation reaction clearing the CNT surface from oxygencontaining groups formed during laboratory conditions storage of the sample. Actually, the overall XPS spectrum measured for CNT sample heated at 200 °C in a thiourea water solution in autoclave exhibited no oxygen signal suggesting that 21200

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Figure 3. Raman spectra of pristine CNT sample (1) and MoS2/CNT composite prepared by hydrothermal route (2).

Figure 2. (a) TEM image of the composite sample contained multiwall CNTs and thin MoS2 flacks (shown by arrows). (b, c) HR TEM images of CNTs. The dark fringes correspond to the MoS2 layers.

hydrothermal process can be used for purification of CNTs. Downshift of the G band by 4 cm 1 could be due to change in the electronic structure of CNTs sheathing with MoS2 nanotubes. In the low-energy region the Raman spectrum of composite exhibits peaks at 285, 382, 405, and 454 cm 1 (Figure 3a). The two most intense peaks at 382 and 405 cm 1 correspond to the E12g and A1g modes of hexagonal MoS2.21 E12g and A1g modes

involve, respectively, in-layer displacements of molybdenum and sulfur atoms and out-of-layer symmetric displacements of sulfur atoms along the c axis.22 The sharpness of these peaks indicates a high crystallinity of the formed MoS2 layers. Red-shift of the E12g peak and blue-shift of the A1g peak and their broadening compared to the bulk MoS2 is due to small size of MoS2 flakes and MoS2 nanotubes.23,24 The peak at 285 cm 1 is assigned to the E1g phonon mode, which is forbidden in backscattering experiments on a basal MoS2 plane.25 Small intensity of the peak in the Raman spectrum of the composite could be caused by a curvature of MoS2 layers. An asymmetric peak arisen at ∼454 cm 1 was previously considered as a superposition of a second-order process involving the longitudinal acoustic phonon scattering and Raman inactive A2u mode, which is activated by the strong resonance effect.21 Alternatively, this peak could be related with the oxidized state of molybdenum.8 The overall XPS spectrum measured for the MoS2/CNT composite showed signals of carbon, molybdenum, sulfur, and oxygen. As discussed before, the amount of oxygen-containing groups on the CNT surface is negligible. Hence, the oxygen should bind with sulfur and/or molybdenum. Sulfur species, occurred in the composite sample, were determined from the high-resolution XPS S 2p spectrum (Figure 4a). The spectrum was fitted by three 2p3/2 2p1/2 spin orbit doublets separated by ∼1.2 eV with intensity ratio 2:1. The components of the main doublet are located at binding energies of 163.00 and 161.85 eV corresponded to the S 2p3/2 and S 2p1/2 lines of MoS2.26 Doublet being the next in energy is composed of components at 164.09 and 162.90 eV, which could be assigned to disulfides S22 or polysulfides Sn2 .27 The simulation of the HR TEM images of nanostructures MoS2 showed that sulfur dimers can decorate the layer edges.28 The high-energy components at 169.44 and 168.26 eV are assigned to the S 2p3/2 and S 2p1/2 lines of S4+ species more likely realized as SO32 groups. Since after the synthesis the soluble salts were washed away of the sample, these groups could be located at the edges of MoS2 layers. As compared to the main MoS2 product amount of the S22‑ and S4+ species was estimated from the relative areas of corresponding S 2p3/2 2p1/2 doublets to be ca. 21 and 11%, respectively. The high-resolution XPS in the Mo 3d range of the composite sample showed a peak at 226.17 eV corresponding to the S 2s binding energy of MoS2 and two doublets composed of 3d5/2 and 21201

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Table 1. Binding Energy, FWHM, Area, and Assignment of Components Derived from XPS Mo 3d Spectrum of MoS2/CNT Composite Sample Shown in Figure 4b identity

binding energy, eV

fwhm, eV

area, %

Mo 3d5/2

229.10

0.79

16.35

Mo 3d3/2

232.25

0.92

10.81

Mo 3d5/2

232.44

2.54

3.78

assignment MoS226 MoO326,30

Mo 3d3/2

235.58

2.54

2.5

S 2s S 2s

226.17 227.22

1.88 1.88

45.26 7.70

MoS2 di- and polysulfides

S 2s

232.56

2.88

13.26

S4+ state

3d3/2 components separated by ∼3.1 eV with intensity ratio ∼3:2 (Figure 4b). The intense Mo 3d5/2 and Mo 3d3/2 components located at 229.10 and 232.25 eV are characteristic of MoS2, while a high-energy doublet corresponds to molybdenum oxides.29 The oxidized molybdenum forms, such as MoO3 and MoO42 , are generally detected in MoS2 samples,26 and partial or complete oxygen removal is achieved with vacuum annealing of sample.30 Taking into account that the S 2p spectrum indicated three forms of sulfur, additional S 2s components with binding energies of 227.22 and 232.56 eV corresponding to the polysuldes and S4+ species were selected in the considered spectral region. The binding energy and area of all components are collected in Table 1. Previously it has been shown that the MoSx stoichiometry can be determined using position of Mo 3d5/2 peak and a difference between Mo 3d5/2 and S 2p3/2 binding energies.30 On the basis of linear dependencies presented in that paper, composition of molybdenum sulfide in the synthesized composite sample was determined to be MoS1.96 (0.10. A ratio between areas of the low-energy Mo 3d doublet and the S 2s peak gives a value x = 1.67 and only accounting the S 2s component corresponding to polysulfides allows obtaining the required composition MoS1.95. This result supports proposition that sulfur atoms form dimers or even chains on the edges of MoS2 layers. The XPS C 1s spectrum of CNTs exhibits a single peak located at ∼284.4 eV (Figure 5a) and is attributed to graphitelike carbon atoms.31 The spectrum of the MoS2/CNT composite has the same shape implying the chemical state of carbon atoms was not markedly changed after the CNT coating. However, position of the C 1s line of composite is downshifted by ∼0.15 eV as compared to the pristine CNTs. The similar shifts have been previously observed for single-walled CNTs treated with nitric acid32 or sonicated in o-dichlorobenzene.33 The shift of the C 1s peak toward a low binding energy is related to the decrease in the Fermi level energy due to the p-doping of CNTs. Whereas XPS differentiates among and quantifies various types of chemical states, NEXAFS spectroscopy gives information on the partial density of unoccupied electronic states.34 The NEXAFS spectra measured near the C K-edge of pristine CNTs and MoS2/CNT composite are compared in Figure 5b. The spectra were normalized to the intensity at 330 eV. Both the spectra show two main intense peaks located at 285.4 and 291.8 eV and corresponding to the π* and σ* resonances. Sharpness of the σ* resonance having exitonic nature35 is evident of the high degree of CNT layer graphitization. After the threshold the spectrum of composite closely resembles the CNT spectrum in the shape, particularly, the features at ∼304.5 and 308.0 eV match in both spectra. This fact indicates

Figure 4. XPS S 2p (a) and Mo 3d (b) spectra of MoS2/CNT composite. The spectra were fitted to components corresponding to the different states of sulfur (MoS2, disulfides S22 and oxidized sulfur S4+ species) and molybdenum (MoS2, MoO3).

that the atomic structure of CNT walls was unchanged as a result of hydrothermal reaction. The π* resonance in the composite spectrum has slightly lowered intensity that could be due to a depletion of π-electronic states in the CNTs sheathing within MoS2 nanotubes. Transfer of electrons from the CNT core to the MoS2 covering was confirmed by quantum-chemical calculation of CNT@MoS2 model displayed in Figure 1. It was found that total Coulson charge for the outer Mo24S48 and inner C80H16 tube has negative and positive value, respectively. Each carbon atom donates ca. 0.027e to the MoS2 nanotube that well agrees with a small shift of the XPS C 1s line and decrease of the π* resonance near C K-edge for the CNTs in the composite sample (Figure 5). The transferred electrons are located on sulfur atoms. To monitor a shift of the Fermi level of CNTs with the MoS2 deposition we measured field electron emission characteristics for the pristine CNT and MoS2/CNT composite samples. Figure 6 depicts the typical curves of an emission current density (J) as a function of an applied electric field (E) measured for the CNT and MoS2/CNT composite samples. The applied field is given as the macroscopic electric field defined by a ratio of the applied voltage to the interelectrode distance. The turn-on field is defined as the electric field that required achieving a current density of 1  10 8 A/cm2. From the measurements, the turn-on field value was found to be 0.55 and 0.60 V/μm for the composite and CNTs, respectively. Examination of the field electron 21202

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factor, and B is a constant. Since the TEM analysis of composite sample revealed that the tips of CNTs were not covered by MoS2 layers we speculate that the enhancement factor determined by aspect ratio of CNTs should has the same value for both investigated samples. Thus, the smaller line slope observed at the initial stage of electron emission from the composite sample corresponds to the lower value of workfunction of the sheathed CNTs as compared with the pure CNTs. As the workfunction of multiwall CNTs possessing metallic conductivity is closely related to their Fermi energy the obtained result evidence the lowering of the Fermi level of CNTs coated with 1 3 MoS2 layers. A “plateau” in the FN plot of the composite represents saturation of emission current. For higher fields, the plots for the CNT and MoS2/CNT composite samples have the same slope that could be attributed to the emission from CNTs cleaned of the MoS2 coating as well as surface adsorbed molecules.37

Figure 5. XPS C 1s spectra (a) and NEXAFS C K-edge spectra (b) of pristine CNT sample (1) and MoS2/CNTs composite (2).

’ CONCLUSION Decomposition of ammonium molybdate and thiourea under hydrothermal conditions produced MoS2, which deposited on the surface of multiwall CNTs forming a tubular sheath. HR TEM analysis showed that the number of walls in MoS2 nanotubes is varied from 1 to 3 and the CNT ends and kinks are free from the MoS2 coating. Taking into account this observation we speculate that nonsheathing CNTs occurred in the product have too defect surface that prevents growth of hexagonal MoS2 layers. Shifting of the XPS C 1s peak to the lower binding energy and reduction of the intensity of π*-resonance in the CK-edge NEXAFS spectrum of the MoS2/CNT composite as compared with the pristine CNT sample were related to charge transfer from CNT core to MoS2 sheath. This experimental finding was supported by quantum-chemical calculation of a CNT@MoS2 model indicated that every carbon atom donates ca. 0.027e to the outer MoS2 tube. As the result of the π-electron system depletion the Fermi level of CNTs goes down providing appearance of field electron emission from the coated CNTs at the lower applied voltage. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

Figure 6. Field emission current density vs macroscopic electric field curves measured for pristine CNTs (1) and MoS2/ CNT composite (2). The insert shows the curves in the Fowler Nordheim coordinates.

emission property of MoS2 flowerlike nanostructures revealed the turn-on field of 4.5 V/μm,36 which is considerably larger than the value for the MoS2/CNT composite. Hence, electron emission from the composite recorded at low fields is generated by the CNT constituents. Reduction of the turn-on field value for the MoS2/CNT composite relative to the pure CNTs should be attributed to the MoS2 coating. The Fowler Nordheim (FN) plots of ln(J/E2) vs 1/E are shown in the inset of Figure 6. The plot for the pure CNTs can be fitted by two straight lines, while three lines are required in the case of the MoS2/CNT composite. At low fields the FN plot slope is proportional to (Bj3/2/β), where j is the workfunction of the emitter material, β is the enhancement

’ ACKNOWLEDGMENT The work was financially supported by the Russian Foundation for Basic Research (grant 10-03-00696-a), the Ministry of Education and Science of Russian Federation (contract 2.1.2/ 9444), and the bilateral Program “Russian-German Laboratory at BESSY. We are grateful to Mr. A. V. Ischenko for the TEM measurements. ’ REFERENCES (1) Katayama, M.; Honda, S.-I.; Ikuno, T.; Lee, K.-Y.; Kishida, M.; Murata, Y.; Oura, K. J. Surf. Sci. Nanotech. 2004, 2, 244–255. (2) Golberg, D.; Dorozhkin, P. S.; Bando, Y.; Dong, Z.-C.; Grobert, N.; Reyes-Reyes, M.; Terrones, H.; Terrones, M. Appl. Phys. Lett. 2003, 82, 1275–1277. (3) Hsu, W. K.; Zhu, Y. Q.; Kroto, H. W.; Walton, D. R. M.; Kamalakaran, R.; Terrones, M. Appl. Phys. Lett. 2000, 77, 4130–4132. 21203

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