Synthesis And Properties Study Of New Intercalation Compounds Of Layered Transition Metal Dichalcogenides

The materials with a lamellar structure, which feature their abundance and wide applications, play important roles in both the nature and fields of synthetic compounds. For instance, layered transition metal dichalcogenides hold unique advantages in terms of their diverse polymorphs and intriguing physical properties, covering insulators, semiconductors, half metals, metals and superconductors. Due to the connection by weak van der Waals forces between the layers, it is possible to modify both the structure and properties of the layered hosts by overcoming the van der Waals forces and introducing guest species into the interlayer gallery under certain reaction conditions.In this dissertation, we aim to investigate the synthesis and properties study of new layered materials by improving the synthetic techniques and modifying the type of layered hosts. The alkali metal intercalation compounds of nonstoichiometric transition metal dichalcogenides were primarily prepared through the solution-based method at room temperature and solid-state reaction at high temperatures, and their structures, compositions, electrical transport and magnetic properties being characterized. In addition, series of derivatives of the intercalation compounds were obtained via soft chemistry strategy by tuning the content of ions in the interlayer gallery, and systemic study on their structure and properties were carried out.The dissertation mainly consists of five chapters:1. The background of the dissertation is introduced in this chapter, including a brief review of synthetic techniques of modifying both the structures and properties of different layered materials, such as out-of-plane intercalation, in-plane doping and physical approaches. Meanwhile, the experimental methods and applications were summarized in terms of two-dimensional layered transition metal dichalcogenide nanomaterials.2. In this chapter, series of alkali metal intercalation compounds of 3R-NbS2 were synthesized using solution-based method at room temperature, which were represented by the formula AxNbS2 (A=Li, Na). The structures, compositions, electrical transport and magnetic properties were systematically investigated for the representative samples. Both the LixNbS2 and NaxNbS2 remained the 3R-polytype of the host, differing from the previous reports which led to the 2H-polytype of sodium intercalated compounds prepared through one-step processes based on solid-state reaction at high temperatures. All the investigated samples exhibited paramagnetic property, and a metal-semiconductor transition was observed in the low temperature region as a result of the carrier localization. Afterwards, the electrochemical performance of as-prepared compounds Li0.90NbS2 and Na0.90 NbS2 was characterized as cathodes for rechargeable lithium and sodium batteries respectively. Li0.90NbS2 exhibited poor rate performance and cycle stability. The eletrode material was found to suffer from pulverization after 40 cycles, which was estimated to be a leading cause of the capacity degradation. Nao.9oNbS2 delivered a initial discharge capacity of 65 mAh g-1 at 0.2 C which was retained after 100 cycles. The bulk materials were exfoliated into nanoplates according to the morphology of the eletrode material after 40 cycles, which was favorable for the insertion-deinsertion of sodium ions. The current work investigated the electrochemical performance of 3R-NaxNbS2 as cathode for rechargeable sodium batteries for the first time.3. In this chapter, we synthesized the potassium intercalation compounds of 3R-and 2H-NbS2 using solution-based method at room temperature and solid-state reaction at high temperatures. For 2H-NbS2, the intercalation compounds obtained through solution-based method remained the polymorph of the host. While a structural transformation occurred for 3R-NbS2 obtained through both two methods, which was evidenced by the characterization of X-ray diffraction pattern and High-resolution transmission electron microscope. Distinguishing from the initial three-layer stacking arrangement (ABCABCABC) of the NbS2 layers, the crystal lattice changed into a primitive hexagonal cell with period stacking of one NbS2 layer (AAA) upon intercalation. The space group of the intercalation compound K0.77Nb1.1S2 was determined to be P-6m2 according to the structure of its acidic product. A structural model was then proposed which turned out to match well with the experimental XRD data. Furthermore, soft chemistry strategy was employed by tuning the contents of K+ ions in the interlayer gallery to modify both the structure and physical properties of the new intercalation compound. We investigated the structures, electrical transport and magnetic properties for the series of hydrated derivatives KxNb1.1S2·yH2O. For the higher doped samples KxNb1.1S2·yH2O (x=0.48, 0.35.0.28 and 0.16), a metal-semiconductor transtion was observed in the low temperature region as a result of the carrier localization. While for the lower doped samples KxNb1.1S2·yH2O (x=0.14,0.13,0.12 and 0.07), superconducting transition at 4 K was observed which is almost independent on the doping level. To the best of our knowledge, the potassium intercalation compound of 3R-NbS2 was synthesized by the solution-based method for the first time. In addition, the superconducting derivatives are new members in the NbS2-based system.4. In this chapter, we synthesized the alkali metal intercalation compounds of 1 T-TiS2 using solution-based method at room temperature. For the guest of Li, the host structure was kept upon intercalation, while a semiconductor-metal transition occurred as a result of the accommodation of the lithium ions. Note that a metal-semiconductor transtion was observed in the low temperature range for LiTiS2, which was estimated to be caused by the nonstoichiometric composition of the host. For the guests of Na and K, a structural transformation occurred from the period stacking of one TiS2 layer per unit cell (AAA) into three-layer stacking arrangement (ABCABCABC). Afterwards, the organic-inorganic hybrids (C3-TiS2 and C6-TiS2) were obtained by two-step method. Firstly, LiTiS2 was exfoliated into HxTiS2, then the protonated product underwent acid-base reaction with amine at room temperature. Note that the obtained C3-TiS2 nanoplates remained the integrity of the host structure.5. In this chapter, the alkali metal intercalation compounds of FeOCl were synthesized using solution-based method at room temperature. For the intercalation process of lithium, naphthalene was co-intercalated with lithium ions. And the host was found to be slightly exfoliated in the solution of naphthalene/THF. For the intercalation process of sodium and potassium, the host structure was broken in terms of the combination of partial Cl atoms with alkali metal ions during the intercalation process, which led to the formation of ACl (A=Na, K). In addition, the intercalation compounds have a strong hygroscopic behavior resulting in the expansion of the interlayer gallery to 9.0～13.7 A. The current work offers evidence for the influence of the guest type on the structure and composition of intercalation products.