Toward Exploring the Structure of Monolayer to Few-layer TaS2 by Efficient Ultrasound-free Exfoliation
© The Author(s). 2018
Received: 26 November 2017
Accepted: 6 January 2018
Published: 15 January 2018
Tantalum disulfide nanosheets have attracted great interest due to its electronic properties and device applications. Traditional solution-ased ultrasonic process is limited by ultrasound which may cause the disintegration into submicron-sized flake. Here, an efficient multi-step intercalation and ultrasound-free process has been successfully used to exfoliate 1T-TaS2. The obtained TaS2 nanosheets reveal an average thickness of 3 nm and several micrometers in size. The formation of few-layer TaS2 nanosheets as well as monolayer TaS2 sheets is further confirmed by atomic force microscopy images. The few-layer TaS2 nanosheets remain the 1T structure, whereas monolayer TaS2 sheets show lattice distortion and may adopt the 1H-like structure with trigonal prism coordination.
Layered transition metal dichalcogenides (TMDs) have attracted great interest as two-dimensional materials due to their unique structural features and outstanding performances in the field of energy [1–9] and electricity [10–14]. Because of their lamellar structures, TMDs usually serve as the hosts for the intercalation of a wide variety of electron-donating species ranging from Lewis bases to alkali metals and can be exfoliated into nanosheets using organolithium  or alkali metal naphthalene  reduction chemistry and ultrasound physics [5, 9, 11, 15–19]. The exfoliated TMDs usually show enhanced properties in the field of electric [11, 14], catalyst [5, 6, 8, 9], and energy storage [2–4, 7].
As one of the typical TMD isomorphism, tantalum disulfide (TaS2) shows a unique combination of valuable structural, mechanical, chemical, and electronic properties, which has been studied for decades [10, 12, 15, 20–22]. Its lamellar structure and electronic properties are beneficial for high-power applications such as micro-electromechanical system, device applications [18, 21, 23], and superconductor [10, 12]. 1T and 2H phases are the most representative structures. The distinction between two phases can be the S-Ta-S coordinated situations, which are recognized as octahedral coordination (1T) and prismatic coordination (2H). The two phases are quite different in electronic structures and other properties. For example, bulk 1T-TaS2 shows charge density wave (CDW) transition and such a transition is found to vanish measured at (001) oriented single-crystal nanoflake (< 5 nm thickness) [18, 21, 23]. Whereas, the 2H phase shows superconductivity and its critical temperature can be raised by intercalation [24–26].
TMDs including TaS2 can be successfully exfoliated by mechanical method  or ultrasound-assisted intercalation process [1, 4–9, 14, 15, 19]. The exfoliated TaS2 obtained by mechanical method is only used to explore its microscopic properties [20, 23, 27]. All the other methods have not got rid of the physical ultrasound assistance [15–17, 19], which may suffer the size of the exfoliated nanoflakes  and limits their wide applications. Therefore, it is necessary for us to explore an efficient alternative exfoliation route.
Our search leads us to a multi-step intercalation method using alkali metal-ammonia as the first step intercalation reaction. In sodium liquid ammonia solution, for example, the metal transfers an electron to the solution to produce a radical solvated electron. The reaction introduces both alkali metal and ammonia molecule into the interlayer of TaS2. Moreover, the ammonia can be replaced by hydroxyl-contained water . Hydroxyl-contained glucose was ever used to intercalate perovskite [29, 30] which can produce very large layer spacing. It is interesting to consider whether the hydroxyl-contained glucose can get into the TaS2 layer just like water and achieve the exfoliation of TaS2. And water-organic mixed solvent was ever employed to exfoliate high-quantity two-dimensional material [31, 32].
Herein, using a multi-step ultrasound-free intercalation process, we report efficient exfoliation of 1T-TaS2. The exfoliated TaS2 nanosheets show 1T structure with several micrometers in size and an average thickness of about 3 nm. Monolayer TaS2 can also be obtained, and some work was done to explore its structure features.
Ta, Na, and S power are available from Aladdin reagent. All reagents were used as received without further purification.
First, 1T-TaS2 was prepared by sealing a stoichiometric amount of Ta and S in vacuum quartz tube, then quenching the tube after heating to 900 °C for 1 day.
The exfoliation mechanism can be attributed to a multi-step intercalation process. Na x (NH3) y TaS2 was synthesized through liquid ammonia procedure. First, a sealed reaction bulb with certain amounts of sodium was placed under − 80 °C to obtain the ammonia solution of sodium. Then, the TaS2 powder was added into the deep-blue solution with a molar ratio of TaS2:Na being 1:0.8. After a 10–20-min reaction, liquid ammonia evaporated and Na x (NH3) y TaS2 was available.
Fifty milligrams of Na x (NH3) y TaS2 was exposed in air to transfer to Na x (H2O) y TaS2, after this, the Na x (H2O) y TaS2 was ground and added to 20 ml 0.2 g/ml aqueous glucose solution. The mixing solution was stirred for 5 min to disperse in the air and then bubbled with nitrogen to expel the oxygen for avoiding the oxidation. Na x (C6H12O6) y TaS2 can be separated from the reaction solution after about 2 h. The above dispersion was sealed and kept stirring for 6 h. Then, the solution was centrifuged at 12000 rpm for 30 min to obtain the product (marked as sample A) for further characterization. For contrast, part of the solution was standing for 1 h and the supernatant solution was centrifuged (marked as sample B). The water bubbled with nitrogen for more than 30 min (anaerobic water) could be utilized to wash off glucose and then alcohol was used to wash the sample to move water.
X-ray diffraction (XRD) patterns were recorded on a Philips X’pert Pro Super diffractometer with Cu Kα radiation at room temperature. XRD pattern of Na x (NH3) y TaS2 was measured in air-free capillary tube, differently, and all the other intercalated samples were measured in air. The scanning electron microscopy (SEM) images were taken on a JEOL JSM-6700F field emission scanning electron microscope (FESEM, 20 kV). Transmission electron microscopy (TEM) images were obtained on a JEOL-2010 with an accelerating voltage of 200 kV. Atomic force microscopy (AFM, DI Innova Multimode SPM platform) was used for detecting the thickness. Microscopic structure investigations were carried via high-angle annular dark field (HAADF) imaging mode in an advanced JEM-ARM 200F aberration-corrected high-resolution TEM facility.
Results and Discussion
It is found that full dispersal of Na x (H2O) y TaS2 in glucose solution is a key factor to exfoliate TaS2. Additional file 1: Figure S2 shows the disperse status of Na x (H2O) y TaS2 in aerobic water, i.e., operating in air, and anaerobic water, i.e., dispelling the oxygen of water, respectively. If oxygen is bubbled before adding Na x (H2O) y TaS2, it will never be dispersed in aqueous glucose solution and exfoliated TaS2 cannot be obtained. In contrary, Na x (H2O) y TaS2 can be fully dispersed in aerobic water, which may be due to the hydroxylation process of the layer edge . Then, the full dispersal leads to the effectively exfoliation. All products can be centrifuged and used for the characterization.
Figure 2b (A) shows the XRD pattern of sample A. The 001, 002, 100, 101, and 003 peaks are completely in accordance with 1T-TaS2. The sharp 100 peak, and wide and strong 00 l reflections indicate the ab extensional orientation of the nanosheets. The average thickness is estimated to be 3 nm by Scherrer equation based on the FWHM of 001 peak. Besides, The XRD pattern (Fig. 2b (B)) of sample B shows that the 001 peak vanishes completely, i.e., the period of c-axis disappears, meaning the formation TaS2 monolayers. Besides, as the same structure, 1T-TiS2 can also be exfoliated by our method. The XRD pattern of exfoliated TiS2 is shown in Additional file 1: Figure S3. The average thickness of exfoliated TiS2 is estimated to be 4 nm by Scherrer equation. This shows the potential of our method to expand into other TMDs.
Additional file 1: Figure S4 shows the Raman spectra of bulk TaS2 and exfoliated nanosheets. There are two Raman features that can be determined, which are E1g and A1g modes [35, 36]. From bulk to nanosheet, the E1g and A1g modes show some shifts. These shifts are probably attributed to the decrease of the force constant resulted from the weakening of the interlayer Van der Waals force between layers. There is also a new mode of vibration which may come from structure changes. The FWHM of exfoliated TaS2 is larger than that of bulk, which also indicates the decrease of thickness
Our approach can produce micro-sized sheets (maximum lateral size is 5 μm) and monolayer TaS2 (minimum thickness is 0.45 nm). Ultrasound is not necessary and may suffer the size of the exfoliated nanoflakes. More details about the differences between different exfoliation techniques are shown in Additional file 1: Table S1.
When we further characterized the monolayer nanosheet, an interesting phenomenon is found that the atomic distance between two Ta atoms from three directions are 3.4, 3.45, and 3.53 Å, respectively (Fig. 6b). The corresponding FFT pattern shows a distorted hexagon (2.93, 2.98, and 3.07 Å, respectively) which indicates the lattice distort in ab plane (Fig. 6b). The intensity profile recorded along CD line (Fig. 6e) differs from the case of few-layer TaS2 (Fig. 6c). The contrasts contributed by S atoms distribute asymmetrically along the CD line, and its relative intensities are much stronger than that in Fig. 6c, which means another S-Ta-S coordination in the monolayer structure. Note that there are two S-Ta-S coordinated situations in TaS2, the results are consistent with the 1H single layer structure with trigonal prism coordination of S-Ta-S (Additional file 1: Figure S6b) . In this case, the S atoms are asymmetrically distributed around Ta atom along the CD line, and the overlap of two S atoms along c-axis (the electron beam direction) result in the enhancement of the signal from the S atoms (Fig. 6f). Therefore, 1H structure with trigonal prism coordination of S-Ta-S in monolayer TaS2 sheet is proposed. Such a single layer structure may be unstable, and the lattice distortion would probably occur. This structure may be responsible for the new vibration mode in Raman result. Of course, the essence of lattice distortion in monolayer TaS2 should be further explored. Meanwhile, different structural phases of TaS2 sheets may be exploited for different device applications.
In summary, we have developed an efficient exfoliation method for the exfoliation of tantalum disulfide and obtained micron-sized monolayer to few-layer sheet. This is a pure chemical exfoliation method in which ultrasound is not necessary. Monolayer and few-layer sample have all been characterized in detail, and coordination change from octahedron to triangular prism is found on monolayer TaS2. This really warns everyone to pay attention to the point of property variation caused by structural changes in exploring the finite-size effect, while few-layer TaS2 still keeps the 1T structure.
Briefly, the present work not only provides an efficient ultrasound-free strategy for synthesizing high-exfoliated functional nanosheets but also explores the structure of monolayer and few-layer TaS2. The present work will provide insights to better understanding 2D materials and their physical and chemical properties, and offer more opportunities for their applications.
This work was supported by the National Natural Science Foundation of China (no. 21671182).
National Natural Science Foundation of China (no. 21671182).
Availability of Data and Materials
All data are fully available without restriction.
The idea come from YH and YH carried out the sample preparation and the measurements. The manuscript was written by YH. QH and BZ helped with the experiment. BL and ZG improved the manuscript. YW and KT participated to the theoretical analysis. All the authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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