A New Potassium Intercalation Compound of 3R ... - Wiley Online Libraryhttps://www.researchgate.net/...A...Synthesized.../A-New-Potassium-Intercalation-Compo...

DOI: 10.1002/slct.201600324

Full Papers

z Inorganic Chemistry

A New Potassium Intercalation Compound of 3R-Nb1.1S2 and its Superconducting Hydrated Derivative Synthesized via Soft Chemistry Strategy Qiao-Yan Hao,[a] Bai-Chuan Zhu,[a] Da-Ke Wang,[a] Su-Yuan Zeng,[b] Zhan Gao,[a] Yi-Wei Hu,[a] Yan Wang,[a] Yong-Kun Wang,[a] and Kai-Bin Tang*[a] A new polymorph of potassium intercalation compound of 3RNb1.1S2 with chemical composition K0.77Nb1.1S2 was synthesized for the first time by a facile solution-phase method. Its structure was comprehensively characterized by powder XRD and high-resolution TEM. Furthermore, its hydrated derivatives Kx Nb1.1S2* yH2O were synthesized via soft chemistry strategy using etching agents of ethanol, water and an I2/CH3CN solution,

Introduction Since the discovery of an increased superconducting transition temperature (Tc) for the host 2H-TaS2 upon intercalation of small organic molecules, intercalation chemistry has been extensively developed in layered transition metal dichalcogenides (TMDs) family for more than four decades.[1–6] Appealing properties including optical, electrical or magnetic were realized by intercalating alkali metals, transition metals and ammonia-ion into the layers of TMD materials.[7–9] Furthermore, soft chemistry offers an effective tool to design new low-dimensional structural compounds which is associated with reversible reactions including redox processes and acido-basic reactions.[10] In recent years, TMD materials possessing versatile properties have been prepared by soft chemistry routes on the platform of intercalation chemistry, motivating us to explore new layered compounds with respect to crystal structure and properties via facile chemical methods.[11–13] As a veteran of TMDs family, niobium disulphide (NbS2) was originally synthesized 50 years ago.[14] It crystallizes into two principal polymorphs, namely 2H (P63/mmc) and 3R (R3m).[15, 16] For 2H phase, the stacking arrangement repeats every two adjacent NbS2 layers, while there are three NbS2 layers per unit cell for 3R phase. Both polymorphs feature the trigonal prismatic configuration within the triple layers. Another 1T phase

[a] Dr. Q.-Y. Hao, Dr. B.-C. Zhu, Dr. D.-K. Wang, Dr. Z. Gao, Dr. Y.-W. Hu, Dr. Y. Wang, Dr. Y.-K. Wang, Prof. K.-B. Tang Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. E-mail: [email protected] [b] Dr. S.-Y. Zeng College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, Shandong 252059, P. R. China.

respectively. Superconducting transition at 4.0 K was observed for the product with the doping content x = 0.12. While superconductivity disappeared in products KxNb1.1S2* yH2O (x = 0.48 and 0.35) which displayed a metal-to-semiconductor transition at low temperature. In addition, the XPS analysis revealed an upward shift in the Fermi level in K0.12Nb1.1S2* yH2O compared with the host or K0.77Nb1.1S2. was also reported which is composed of only one NbS2 layer per unit cell and can form octahedral configuration.[17] Up to now, the traditional solid-state synthetic routes led to the synthesis of alkali metal intercalation compounds represented by AxNbS2 (A=Li, Na, K, Rb and Cs), all of which are the derivatives of 2H phase.[18–20] Electrochemical method was applied to investigate the intercalation/deintercalation behavior of Li/Na ions in NbS2 crystal lattice.[21] Compared with the above synthetic techniques, solution-phase reaction has an advantage in the facile and mild processes, including the employment of n-butyllithium solution, alkali-metal/liquid ammonia and alkali-metal/naphthalenide solution.[13, 22, 23] More recently, alkali metal intercalation compounds 3R-AxNbS2 (A=Li, Na) were prepared by reacting 3R-NbS2 with alkali-metal/naphthalenide solution, generating a new polymorph of sodium intercalation compound in 3R phase.[24] It is desirable to investigate the influence of larger alkali metal ions intercalation on the structure and properties of the 3R host. Herein, we primarily describe the intercalation behavior of 3R-Nb1.1S2 with potassium through a facile solution-phase method. The structural and magnetic properties were comprehensively characterized for the as-prepared sample K0.77Nb1.1S2. Also, solid-state reaction was performed by directly reacting K piece with bulk 3R-Nb1.1S2 which led to a multiphase mixture. Furthermore, soft chemistry strategy was designed to synthesize the hydrated derivatives by partial deintercalation of K + ions from the interlayer gallery. Superconducting transition at 4.0 K was observed in the sample K0.12Nb1.1S2* yH2O. The XPS analysis revealed a rise in the Fermi level in the superconducting phase compared with the host or the fresh phase.

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/slct.201600324

ChemistrySelect 2016, 1, 2610 – 2616

2610

2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers Results and Discussion 1. Characterization of K0.77Nb1.1S2 synthesized by the solution–phase reaction. 1.1 Powder XRD and HRTEM analyses. Powder XRD patterns (Figure S1) confirmed the phase purity of the hosts (3R and 2H). Their lattice parameters were indexed to be a = 3.327(3) , c = 3 3 5.961(2) and a = 3.318(6) , c = 2 3 5.969(7) based on the space group R3m and P63/mmc respectively, being consistent with the previous reports.[25] It is worth to mention that pure 3R phase could only be obtained when the initial molar ratio of Nb:S was around 1:2, and excess un-reacted yellow sulfur was condensed in one end of the tube after complete reaction. Once the amounts of S powder were increased gradually, the product was a mixture of both 3R and 2H phases. The above results demonstrate that 3R phase is nonstoichiometric as previously discussed.[15] Its average composition was determined to be Nb1.1S2 according to the results of elemental analysis listed in Table S1. Figure 1 A shows the

Figure 1. A. Powder XRD patterns measured at ambient conditions: (a) the fresh phase K0.77Nb1.1S2; (b-c) after exposure of sample-a to the air for 2 days and 6 days, respectively. B. (a-b) SAED pattern and HRTEM image corresponding to the ab-plane of K0.77Nb1.1S2 crystals; (c-d) HRTEM image and SAED pattern along the c axis of K0.77Nb1.1S2 crystals.

XRD patterns for the fresh potassium intercalation compound (hereafter referred to as “the fresh phase”) regarding 3R-Nb1.1S2 and its time-lapsed derivatives in air. The lattice parameters of the fresh phase were determined to be a = 3.351(9) and c = 7.794(0) by indexing the reflections based on a hexagonal unit cell (see the comparison of d values between observed and calculated ones in Table S2). The systematic absence for the space group R3m has been demolished as evidenced by the appearance of reflection 100. It indicates that the crystal lattice changed into a cell with periodic stacking of one layer of NbS2 from the initial three-layer stacking per unit cell as a result of potassium intercalation. Moreover, considerable shift of the 00l reflections towards lower angles was observed in the XRD patterns of its 2-day and 6-day aged products, corresponding to the c-axis length (d001) of 8.442 and 8.883 , respectively. The evolution of interlayer distance should be contributed to the hydration of the actively fresh phase depending on the ambient atmosphere, which is a common phenomenon in the alkali metal intercalated TMD compounds and partially accomChemistrySelect 2016, 1, 2610 – 2616

panied with a decrease of the intercalate loading.[26, 27] The chemical composition for the fresh phase was determined to be K0.77Nb1.1S2 according to the K/Nb atomic ratios based on the ICP-OES analysis (details listed in Table S3) as well as the elemental analysis of S. For comparison, the potassium intercalation compound of 2H-NbS2 was synthesized and characterized by XRD (Figure S2). The reflections were indexed based on the space group P63/mmc, which is consistent with the case of KNbS2 (PDF# 74–0636, Figure S3a) as reported in Omloo et al.’s work and can be distinguished from that of K0.77Nb1.1 S2.[18] Note that the XRD pattern of K0.77Nb1.1S2 is similar to another reported compound K0.67NbS2 (PDF# 89–3996, P-6m2) as illustrated in Figure S3b, we also tried to index it by a hexagonal unit cell with a = 3.351(9) and c = 2x7.794(0) . However, the reflections of 101, 103 and 105 etc. originated from the structure of periodic stacking with two NbS2 layers per unit cell were absent even though the collection time of the XRD pattern was prolonged, suggesting the structure of the present compound is different from the reported one. To verify the results of XRD analysis, the microscopic structure of K0.77Nb1.1S2 was probed further by HRTEM analysis as illustrated in Figure 1B(a-d). The hexagonal phase was revealed by the ordered hexagonal array of electron diffraction pattern and clear lattice fringes in HRTEM image of the ab-plane (Figure 1a-b). The d-spacing calculated from the lattice fringes was about 2.91 , which corresponded to the (010) and (100) planes. In addition, the lattice fringes along the c-axis (Figure 1c) were carefully observed along the curly edge of the plate-like crystals. An average interlayer spacing of 7.90 was determined, very close to the value of d001 in the XRD pattern. What’s more, the spot standing by the centre-point in the corresponding SAED pattern (Figure 1d) was assigned to the (001) plane, which led to a lattice constant of c = 7.9 . It should be pointed out that if the present compound adopts the structure of K0.67NbS2 (P-6m2), its non-extinct 001 reflection with d = 2 3 7.9 would appear. Therefore, according to XRD and HRTEM analyses K0.77Nb1.1S2 can be confirmed to adopt the structure of periodic stacking with one NbS2 layer per unit cell, which is a new polymorph of NbS2-based intercalation compounds. 1.2. Heat treatment of the fresh phase. The structural evolution on heating at various temperatures was illustrated in Figure 2. The high background of the profiles was due to scattering from the amorphous capillary tube. Two main conclusions can be drawn from analysis of the XRD data. Firstly, the characteristic reflections 00l exhibited a slight shift towards lower angles, corresponding interlayer distances expanded from the value of 7.9 (100˚ C) to 8.1 (300˚ C). Compared with that of the fresh phase (7.8 ), the subtle difference in lattice parameter c was caused by modifying the distribution of K + ions in the interlayer gallery. In fact, an asymmetric shape for the fresh phase was observed for 00l reflections implying an imperfect periodicity along the c-axis, which may be caused by the inhomogeneous distribution of K + ions in the interlayer gallery or incomplete intercalation. And the annealing process led to more symmetric and sharper peaks as highlighted in the 2611


Full Papers charge number for these two elements. In view of the above analysis, the structural model of Figure S3c (P-6m2) is the best candidate for K0.77Nb1.1S2. Rietveld refinement was carried out using GSAS program. Based on the space group of P-6m2, the interlayer K and excess Nb occupy the 1b and 1 f trigonal prismatic sites, respectively. Figure 3 displays the Rietveld refinement patterns, and Table 1

Figure 2. Powder XRD patterns of post-annealed products of the fresh phase (left); The local regions corresponding to 102 reflections are highlighted for the fresh phase and products annealed at 100–300˚ C (right).

regions of 102 reflections, indicating that the structural periodicity became more elegant. Secondly, when increasing the temperature to 300 ˚ C, a new reflection located at 31.5 ˚ appeared and gradually increased in intensity as the temperature was increased further. Indexing the reflection suggested that it arised either from KNbS2 (PDF# 74–0636) or K0.67NbS2 (PDF# 89–3996), along with other three ones marked with upper triangle shape in Figure 2.[18] This suggests that the fresh phase underwent a structural phase transition above the temperature of 300˚ C. Note that additional five reflections were detected in the product annealed at 500˚ C, which were ascribed to the hydrated derivative of the fresh phase resulting from brief exposure to the air during the sealing process for the glass capillary. Based upon the above analysis, the product annealed at 200˚ C was selected to collect an eligible XRD data to perform Rietveld Refinement further. 1.3. Rietveld refinement. On the basis of the above conclusion that there is only one NbS2 layer per unit cell, K0.77Nb1.1S2 has two possible structural models as illustrated in Figure S3c and d. To search for more evidence, we investigated the structure of the product obtained by exchanging K + ions in K0.77Nb1.1S2 by protons in acidic condition.[28] As shown in Figure S4, all the reflections of the obtained HxNb1.1S2 were indexable based on the structure of 3R host (PDF #41-1318) with lattice parameters a = 3.331(8) and c = 17.925(8) , indicating that the host structure was regained after complete removal of K + ions from the interlayer gallery. According to the above experimental result, the intralayer Nb atoms in K0.77Nb1.1S2 can be deduced to adopt trigonal prismatic coordination (P-6m2) like 3R host rather than octahedral (P-3m1). In addition, if K0.77Nb1.1S2 crystallizes in a structure with space group P-3m1, both the interlayer Nb and K would occupy the same position of 1b octahedral site, which is unreasonable due to the striking difference in ionic size and ChemistrySelect 2016, 1, 2610 – 2616

Figure 3. Rietveld refinement results for the fresh phase. The experimental XRD pattern is represented by solid black line, and the calculated pattern by the red dotted line. Their difference is represented by the blue curve at the bottom. The green vertical bars indicate the positions of allowed Bragg reflections. The inset shows the structural modification upon potassium intercalation.

Table 1. Structural parameters for K0.77Nb1.1S2. atom

site

x

y

z

S.O.F.

U(2)

K Nb S Nb1

1b 1a 2h 1f

0 0 1/3 2/3

0 0 2/3 1/3

0.5 0 0.1852(3) 0.5

0.77 1 1 0.1

0.10 0.01 0.01 0.01

Space group: P-6m2, a = 3.3523(4) , c = 8.0682(6) , V = 78.5251(5) 3; Rwp = 3.10 %, Rp = 1.97 %, c2 = 3.38. The isotropic displacement parameter (U) was fixed at 0.01 to avoid obtaining unreasonable values for Nb and S. The problem is possibly caused by the existence of impurity (such as water or organic compounds) in the crystal lattice, which was limited by the solution-phase reaction.

lists the final structural parameters. The obtained R factors were fairly low, indicating the validity of our proposed structural model. For the sake of comparison, Rietveld refinement was also performed based on the P-3m1 structural model (Figure S5 and Table S4). Actually, the R factors are much larger and the goodness-of-fit (c2) of 6.48 is almost twice the value of 3.38 obtained from the P-6m2 structural model, which provides evidence to support the conclusion that the present compound adopts the space group P-6m2.[29]

2612


Full Papers 1.4. Magnetic properties of K0.77Nb1.1S2 and its air–aged product. Magnetic susceptibility of the fresh phase displayed a paramagnetic behavior over the temperature range 2–300 K as shown in Figure 4. The fluctuation in susceptibility above 100 K

Figure 5. XRD patterns of KxNb1.1S2* yH2O (left); The local regions corresponding to the position of 100 and 101 reflections are highlighted to illustrate a shrinkage of a-axis due to a decrease in the K + loading (right). The corresponding lattice parameters were determined with the least-square method based on the observed reflections between 2q values of 58 and 608.

Figure 4. Temperature dependence of magnetic susceptibility (c-T) for K0.77 Nb1.1S2 measured in a field cooling (FC) mode with H = 100 Oe. The c-T and c 1-T curves for its aged product exposed in air for 2 days are shown as the inset.

was estimated to be contributed by the defects in the crystal lattice introduced during the intercalation process. After exposure to the air for 2 days, it exhibited similar Curie-Weiss behavior (inset of Figure 4) in the temperature range 150–300 K like the 3R host.[24] During the aging procedure, the distribution of K + ions became more homogeneous as demonstrated by the XRD patterns, and intrinsic magnetic property of the compound was magnified. Attempts to measure the electrical resistivity of the fresh phase failed as the pellet collapsed within a few minutes in air.

2. Characterization of the hydrated derivatives KxNb1.1S2* yH2 O (x = 0.48, 0.35 and 0.12) synthesized via soft chemistry strategy. 2.1 Powder XRD analysis. The hydrated derivatives with chemical composition KxNb1.1S2* yH2O (x = 0.48, 0.35 and 0.12) were synthesized by partial deintercalation of K + ions from the interlayer gallery as described in the experimental section (see Supporting Information). Their corresponding XRD patterns were compared in Figure 5. All the observed reflections were indexable on the basis of space group P-6m2, indicating that the hydrated derivatives were isostructural with the fresh phase. In addition, the 00l reflections gradually shifted towards lower angles from x = 0.48 to 0.12, corresponding to an expanded interlayer gallery. The changes can be explained by the relaxation of Coulomb forces between K + ions and negatively charged NbS2 layers due to an increase in K + loss. Compared with that of the host, the interlayer disChemistrySelect 2016, 1, 2610 – 2616

tances for the hydrated derivatives were expanded by ca. 3.82.4 , corresponding to a monolayer arrangement of K + ions and H2O molecules in the galleries.[30] Note that a second-stage phase or interstratification of two layer types with different interlayer distances may exist in the sample K0.12Nb1.1S2* yH2O, as indicated by its very broad 00l reflections. Similar results have been discussed previously in the literature which pointed out that two-phase regions existed below the doping level x = 0.27.[6] On the other hand, the lattice parameter a for KxNb1.1S2* yH2O exhibited a slight shrinkage compared with the fresh phase as highlighted in the position of 100 reflections, which was contributed to the raise of average oxidation state of Nb due to deintercalation of considerable amounts of K + ions from the crystal lattice. Further, potassium intercalation compound synthesized by direct reaction of 3R-Nb1.1S2 with K piece as well as its hydrated derivative was characterized using ICP-OES and XRD. Their chemical compositions were determined to be K0.81Nb1.1S2 and K0.13Nb1.1S2* yH2O, respectively. As shown in Figure S6a, solidstate reaction yielded a multiphase mixture compared with solution-phase method. Analysis of XRD pattern revealed that three phases coexisted in the product, including two intercalated phases with different interlayer distances and the un-reacted host. The intercalated phases can be indexed based on the space group of P-6m2, as in the case of sample K0.77Nb1.1S2. In addition, 6-hour aging process in air led to expansion of the layer distance and enhanced reflection intensity as displayed in Figure S6b, resembling the behavior of K0.77Nb1.1S2. After the deintercalation treatment with an I2/CH3CN solution, the hydrated derivative containing at least two different phases were obtained as shown in Figure S6c, whose interlayer distances were about 9.27 and 7.51 , respectively. The above result demonstrates that compared with the solid-state reaction, the room-temperature reaction in solution phase has the advantage of preparing more homogeneous samples. The disparity was estimated to be caused by the stronger reducing ability of

2613


Full Papers K naphthalenide adduct (K + C10H8 ) than pure metal K or a faster diffusion rate in solution-phase. 2.2 Electrical and magnetic properties. The electrical transport and magnetic properties of the hydrated derivatives were investigated. It is necessary to measure the resistivity within one day once the samples were prepared to eliminate the effect of the ambient atmosphere. Note that all the investigated samples were fabricated into pellets for the resistivity measurement under the same conditions without post annealing treatment due to their structural instability at elevated temperatures. No superconducting transition was observed down to 2 K for KxNb1.1S2* yH2O (x = 0.48 and 0.35). While superconducting transition at 4 K occurred for K0.12Nb1.1 S2* yH2O (referred to as “the SC phase”) and K0.13Nb1.1S2* yH2O, which were obtained from the starting materials K0.77Nb1.1S2 and K0.81Nb1.1S2, respectively. As shown in Figure 6a, the normalized resistivity of samples KxNb1.1S2* yH2O (x = 0.48 and 0.35) underwent a metal-to-semi-

Figure 7. (a) 1-T curve for K0.12Nb1.1S2* yH2O at 0 Oe; (b) M-T curve for K0.12 Nb1.1S2* yH2O under conditions of ZFC and FC at 50 Oe.

Figure 6. (a) Temperature dependence of the normalized electrical resistivity for KxNb1.1S2* yH2O (x = 0.48 and 0.35); (b) –ln (1) vs. T 1/4 plot of the VRH model and (c) ln (1/T) vs. T 1 plot of the SPH model for K0.48Nb1.1S2* yH2O.

conductor transition at low temperature, with a turning point temperature (Tmin) at 46 K and 35 K, respectively. The resistivity data in the temperature range 4 K-Tmin were fitted with the two common models to describe the semiconducting behavior, namely the small polaron hopping (SPH) model and the variable range hopping (VRH) model.[31, 32] Both samples matched with the VRH model, and the results for K0.48Nb1.1S2* yH2O are illustrated in Figure 6b and c. The powders of K0.12Nb1.1S2* yH2O were metallic blue. Figure 7a shows the temperature dependence of zero-field resistivity (1) for it. The sample exhibited metal behavior in the ChemistrySelect 2016, 1, 2610 – 2616

electrical conductivity above 25 K. At temperatures below 4 K, the resistivity underwent a sharp drop, and then vanished around 3 K as shown in the inset of Figure 7a. Figure 7b displays magnetization (M) as a function of temperature from 2 K to 20 K for K0.12Nb1.1S2* yH2O under magnetic field of 50 Oe. An obvious diamagnetic signal indicated in the ZFC curve decreased to zero at about 4 K, confirming the occurrence of superconductivity. Furthermore, characterization of K0.13Nb1.1S2* yH2O was carried out, and superconducting transition at about 4 K was confirmed as illustrated in Figure S7. Note that there was a clear divergence between the FC and ZFC curves below 18 K, which was estimated to be contributed to magnetic impurity, such as paramagnetic or antiferromagnetic components. Taking account of the conductivity behavior of samples KxNb1.1 S2* yH2O (x = 0.48, 0.35 and 0.12), we presume that the intercalated phase with larger interlayer distance (9.27 ) in K0.13Nb1.1 S2* yH2O contributes to the superconductivity, while that with smaller interlayer distance (7.51 ) is responsible for the divergence between the ZFC and FC curves. The Tc value in the current work is higher than that of previous results of A0.5NbS2 and A0.5NbS2* yH2O (A = alkali metals) in the range of 1–3 K.[33, 34] Compared with the hydrated alkali metal intercalation compouds of 2H-TaS2, the Tc value for sample K0.12Nb1.1S2* yH2O is comparable with Sr0.15TaS2* yH2O.[35] Whereas in the case of Lix NbS2 prepared by high-temperature reaction, the Tc of samples crystallizing in both 2H and 3R phases was in the range of 2.03.2 K for 0.1 x 0.2, which is lower than that of sample K0.12 Nb1.1S2* yH2O.[36] Our work provides evidence that the physical 2614


Full Papers properties of intercalated layered materials are correlated to complex factors, such as polymorph, stoichiometry and the doping level. 3. XPS study of the host, fresh and SC phases. XPS analysis was carried out to determine oxidation states of the elements for the host (3R-Nb1.1S2), the fresh phase (K0.77Nb1.1 S2) and the SC phase (K0.12Nb1.1S2* yH2O). Their corresponding Nb3d, S2p, O1 s and K2p spectra are presented in Figure 8. The

Figure 8. XPS spectra of the host, fresh and SC phases: (a) Nb 3d, (b) S 2p, (c) O 1 s and (d) K 2p.

observed Nb3d spectra of the host (Figure 8a) were fitted with three pairs of peaks, including one peak at 210.1 eV (3d3/2 of Nb5 + ) and other two peaks located at 206.4 eV (3d3/2 of Nb3 + ) and 205.6 eV (3d3/2 of Nb2 + ).[37, 38] The Nb3d3/2 and Nb3d5/2 lines were separated by 2.7 eV due to spin orbit splitting, with a peak area ratio of approximately 2:3 (see the details in Table S5). For the SC phase, in spite of a similar oxidation state distribution, the binding energies of all the Nb3d peaks shifted to higher values by 0.3 ~ 0.4 eV compared with that of the host, which can be interpreted as an upward shift in the Fermi level.[39] For the fresh phase, Nb5 + signal was absent, the binding energies at 209.0, 206.4 and 205.6 eV were therefore attributed to 3d3/2 peaks of Nb4 + , Nb3 + and Nb2 + , respectively. Considerable shift of binding energy (1.1 eV) from 210.1 to 209.0 eV was ascribed to the reduction of Nb5 + to balance the number of electrons transferred from K to the host lattice. Figure 8b shows the S2p spectra of the investigated samples. The S2p peaks were located at 161.4 eV and 160.3 eV for the host, corresponding to the binding energies of 2p1/2 and 2p3/2 of S2 .[37] For the SC phase, the two lines shifted by 0.4 eV towards higher binding energy. The peak at 169.1 eV should be associated with the formation of sulfate due to the quasi-planar configuration of the hydrated derivative which suffered oxidation easily. For the fresh phase, the S2p bands exhibited a low


binding energy shoulder associated with charge transfer from the intercalate. Similar behavior has been reported in WS2.[39] The O1s spectra for the three phases were illustrated in Figure 8c which revealed the interaction of air/water with the samples. Parallel features appeared for the host and the SC phase. Both presented three peaks at 532.9, 532.0 0.2 and 530.7 0.2 eV, which were assigned to adsorbed/intercalated molecular water (H2Oad/H2Oin), adsorbed water/hydroxyl group (H2Ohy) and adsorbed hydroxyl (OH ).[40–42] Compared with that of 37 % in the host, the two components (H2Oad/H2Oin and H2Ohy) contribution in the SC phase was increased to 59 % according to the ratios of the integrals of the three peaks, which is consistent with the fact that the SC phase was more hygroscopic and accommodated water molecules in the interlayer. On the other hand, the O1s spectral profiles showed evident difference for the fresh phase, with peaks located at 533.0, 531.3 and 529.6 eV. It is reasonable to assign the highest binding energy to adsorbed molecular water. While the middle binding energy (531.3 eV) constrained due to the spectra shape was mainly contributed to both the water/hydroxyl group and adsorbed hydroxyl. The lowest binding energy was presumed to be assigned to oxidation product (O2 ) formed by the surface K + ions with water vapor. Note that the contribution of water/hydroxyl group and adsorbed hydroxyl was 74 %, indicating high-affinity for water molecules of the fresh phase due to the coverage of K + ions on the crystal surface which led to the formation of potassium hydroxide. Figure 8d shows the K2p binding energy. There was no signal from K species in the host. The two symmetric peaks for the fresh phase were ascribed to the 2p1/2 (295.5 eV) and 2p3/2 (292.7 eV) of K + . For the SC phase, the peak intensity decreased significantly due to large loss of K + ions. Moreover, the K2p lines shifted to higher binding energy by 0.5 eV as compared to the fresh phase, which is analogous to the drifts of Nb3d and S2p lines between the SC and host phases and indicates that the SC phase is more electron-rich than the host or the fresh phase.

Conclusions A structural modification was induced upon intercalation of potassium into the crystal lattice of 3R-Nb1.1S2 by a facile solutionphase reaction. The XRD and HRTEM analysis revealed a hexagonal cell with periodic stacking of one-layer NbS2 for as-prepared intercalated compound K0.77Nb1.1S2. A structural model based on the space group of P-6m2 was proposed, and detailed crystallographic parameters were calculated. Magnetic susceptibility confirmed the paramagnetic behavior over the temperature range 2–300 K for the fresh phase. In addition, its hydrated derivatives with chemical composition KxNb1.1S2* yH2 O (x = 0.48, 0.35 and 0.12) were synthesized via soft chemistry strategy. A metal-to-semiconductor transition occurred at low temperature in KxNb1.1S2* yH2O (x = 0.48 and 0.35), while reproducible superconductivity was observed at about 4 K for K0.12Nb1.1S2* yH2O. Potassium intercalation of 3R-Nb1.1S2 was also performed by solid-state reaction which led to a multiphase mixture, whose hydrated derivative also displayed super2615


Full Papers conducting transition around 4 K. In addition, the elements in the SC phase exhibited a shift of 0.3-0.5 eV towards higher binding energy compared with the host or the fresh phase, indicating an upward shift in the Fermi level. The current work provides substantial experimental evidence for intercalation chemistry of 3R-Nb1.1S2 and demonstrates the complexity of both structural and physical properties of TMDs family. Supporting Information: Experimental Section, supplementary characterizations, the four structural models mentioned in the text and details of the ICP-OES and XPS analyses.

Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 21171158) and National Basic Research Program of China (2010CB934700). Keywords: Niobium disulfide · Intercalation · Superconductor · Polymorph · XPS analysis [1] a) F. R. Gamble, F. J. Disalvo, R. A. Klemm, T. H. Geballe, Science 1970, 168, 568–570; b) F. R. Gamble, J. H. Osiecki, M. Cais, R. Pisharod, Science 1971, 174, 493–497. [2] a) M. S. Whittingham, R. R. Chianelli, J. Chem. Educ. 1980, 57, 569–574; b) M. S. Whittingham, A. J. Jacobson, Intercalation Chemistry, Academic Press: New York, 1982, pp. 595. [3] Intercalated Layered Materials (Ed.: F. Levy), D. Reidel: Dordrecht, 1979, pp. 578. [4] a) R. Schollhorn, Angew. Chem. 1980, 92, 1015; b) R. Schollhorn, Angew. Chem., Int. Ed. Engl. 1980, 19, 983–1003; c) R. Schollhorn, Inclusion Compounds (Ed.: J. L. Atwood, J. E. D. Davies, D. D. Macnicol), Academic Press: London, 1984, pp. 78. [5] R. H. Friend, A. D. Yoffe, Adv. Phys. 1987, 36, 1–94. [6] A. Lerf, Dalton Trans. 2014, 43, 10276–10291. [7] R. B. Somoano, V. Hadek, A. Rembaum, J. Chem. Phys. 1973, 58, 697–701. [8] E. Morosan, H. W. Zandbergen, B. S. Dennis, J. W. G. Bos, Y. Onose, T. Klimczuk, A. P. Ramirez, N. P. Ong, R. J. Cava, Nat. Phys. 2006, 2, 544–550. [9] Q. Liu, X. Li, Z. Xiao, Y. Zhou, H. Chen, A. Khalil, T. Xiang, J. Xu, W. Chu, X. Wu, J. Yang, C. Wang, Y. Xiong, C. Jin, P. M. Ajayan, L. Song, Adv. Mater. 2015, 27, 4837–4844. [10] a) J. Rouxel, Acs Symp. Ser. 1992, 499, 88–113; b) J. Rouxel, M. Tournoux, Solid State Ionics 1996, 84, 141–149. [11] C. W. Lin, X. J. Zhu, J. Feng, C. Z. Wu, S. L. Hu, J. Peng, Y. Q. Guo, L. L. Peng, J. Y. Zhao, J. L. Huang, J. L. Yang, Y. Xie, J. Am. Chem. Soc. 2013, 135, 5144–5151. [12] M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. S. Li, S. Jin, J. Am. Chem. Soc. 2013, 135, 10274–10277. [13] J. Zheng, H. Zhang, S. Dong, Y. Liu, C. T. Nai, H. S. Shin, H. Y. Jeong, B. Liu, K. P. Loh, Nat. Commun. 2014, 5, 2995–3001. [14] F. Jellinek, G. Brauer, H. Muller, Nature 1960, 185, 376–377.


[15] W. G. Fisher, M. J. Sienko, Inorg. Chem. 1980, 19, 39–43. [16] W. G. Mcmullan, J. C. Irwin, Phys. Stat. Sol. B. 1985, 129, 465–469. [17] C. J. Carmalt, T. D. Manning, I. P. Parkin, E. S. Peters, A. L. Hector, Thin Solid Films 2004, 495, 469–470. [18] W. P. F. Omloo, F. Jellinek, J. Less-Common Met. 1970, 20, 121–129. [19] R. Schollhorn, A. Lerf, J. Less-Common Met. 1975, 42, 89–100. [20] a) J. E. Gareh, M. G. Barker, M. J. Begley, Mater. Res. Bull. 1995, 30, 57–63; b) J. E. Gareh, M. G. Barker, M. J. Begley, A. S. Batsanov, J. Chem. Soc., Dalton Trans. 1995, 3825–3830. [21] a) Y. H. Liao, K. -S. Park, P. Singh, W. S. Li, J. B. Goodenough, J. Power Sources 2014, 245, 27–32; b) Y. H. Liao, K. -S. Park, P. H. Xiao, G. Henkelman, W. S. Li, J. B. Goodenough, Chem. Mater. 2013, 25, 1699–1705. [22] D. W. Murphy, F. J. Disalvo, G. W. Hull, J. V. Waszczak, Inorg. Chem. 1976, 15, 17–21. [23] a) T. P. Ying, X. L. Chen, G. Wang, S. F. Jin, T. T. Zhou, X. F. Lai, H. Zhang, W. Y. Wang, Sci. Rep. 2012, 2, 426; b) T. P. Ying, X. L. Chen, G. Wang, S. F. Jin, X. F. Lai, T. T. Zhou, H. Zhang, S. J. Shen, W. Y. Wang, J. Am. Chem. Soc. 2013, 135, 2951–2954. [24] Q. Y. Hao, D. K. Wang, B. C. Zhu, S. Y. Zeng, Z. Gao, Y. Wang, B. Li, Y. K. Wang, Z. K. Tang, K. B. Tang, J. Alloy Compd. 2016, 663, 225–229. [25] A. Niazi, A. K. Rastogi, J. Phys.: Condens. Matter 2001, 13, 6787–6796. [26] R. Schollhorn, A. Weiss, Z Naturforsch. 1973, 28b, 711–715. [27] A. Zak, Y. Feldman, V. Lyakhovitskaya, G. Leitus, R. Popovitz-Biro, E. Wachtel, H. Cohen, S. Reich, R. Tenne, J. Am. Chem. Soc. 2002, 124, 4747–4758. [28] K. Izawa, S. Ida, U. Unal, T. Yamaguchi, J. -H. Kang, J. -H. Choy, Y. Matsumoto, J. Solid State Chem. 2008, 181, 319–324. [29] L. B. Mccusker, R. B. Von Dreele, D. E. Cox, D. Louer, P. Scardi, J. Appl. Cryst. 1999, 32, 36–50. [30] A. Lerf, R. Schollhorn, Inorg. Chem. 1977, 16, 2950–2956. [31] N. F. Mott, Conduction in glasses containing transition metal ions, J. Non-Cryst. Solids 1968, pp. 1–17. [32] N. F. Mott, E. A. Davies, Electron Processes in Non-crystalline Materials, Clarendon, Oxford, 1979, vol. 152. [33] A. Lerf, F. Sernetz, W. Biberacher, R. Schollhorn, Mater. Res. Bull. 1979, 14, 797–805. [34] B. H. Chen, B. Eichhorn, J. L. Peng, R. L. Greene, J. Solid State Chem. 1993, 103, 307–313. [35] F. Sernetz, A. Lerf, R. Schollhorn, Mater. Res. Bull. 1974, 9, 1597–1602. [36] C. S. McEwen, D. J. St. Julien, P. P. Edwards, M. J. Sienko, Inorg. Chem. 1985, 24, 1656–1660. [37] J. F. Moulder, W. F. Stickle, P. E. Sobol, K. D. Bomben, Handbook of X-ray Photoelectron Spectroscopy. [38] V. V. Atuchin, I. E. Kalabin, V. G. Kesler, N. V. Pervukhina, J. Electron. Spectrosc. Relat. Phenom. 2005, 142, 129–134. [39] F. Kopnov, Y. Feldman, R. Popovitz-Biro, A. Vilan, H. Cohen, A. Zak, R. Tenne, Chem. Mater. 2008, 20, 4099–4105. [40] J. H. Linn, W. E. Swartz, Appl. Surf. Sci.1984, 20, 154–166. [41] H. W. Nesbitt, I. J. Muir, Geochim. Cosmochim. Acta 1994, 58, 4667–4679. [42] S. W. Knipe, J. R. Mycroft, A. R. Pratt, H. W. Nesbitt, G. M. Bancroft, Geochim. Cosmochim. Acta 1995, 59, 1079–1090.

Submitted: April 5, 2016 Accepted: June 23, 2016

2616
