Bulk Fabrication of WS2 Nanoplates: Investigation on the Morphology

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Bulk Fabrication of WS2 Nanoplates: Investigation on Morphology Evolution and Electrochemical Performance Jingwen Qian, Zhijian Peng, Peilun Wang, and Xiuli Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04601 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016

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ACS Applied Materials & Interfaces

Bulk Fabrication of WS2 Nanoplates: Investigation on Morphology Evolution and Electrochemical Performance Jingwen Qian,†,‡ Zhijian Peng, *,† Peilun Wang,† and Xiuli Fu*,‡ †

School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China

‡

State Key Laboratory of Information Photonics and Optical Communications, and School of Science, Beijing

University of Posts and Telecommunications, Beijing 100876, P. R. China

ABSTRACT: Two-dimensional layered chalcogenide WS2, similar to graphene, is considered very interesting for materials scientists. But to make it a useful material platform, it is necessary to develop sophisticated synthesis methods to control its morphology. In this paper, we present a simple approach to prepare various morphologies of WS2 nanostructures by direct thermal evaporation of WO3 and S powders onto Si substrates sputtered with W film without using any nanostructured W-contained precursors and highly toxic sulfides gases. This method can produce bulk quantities of pure hexagonal, horizontally-grown WS2 nanoplates, vertically-grown nanoplates and nanoplatesformed flowers simply by tuning the distance between the substrate and source powders. The synthesis mechanism and morphology evolution model were proposed. Moreover, when employed as thin film anode materials, the lithium ion battery with the as-prepared vertically-grown WS2 nanoplates presented a rechargeable performance between 3 and 0.01 V with a discharge capacity of about 773 mAh/cm3 after recycling for three times, much better than its already-reported counterparts with the randomly-distributed WS2 nanosheets electrodes, but the battery with the horizontally-grown WS2 nanoplates could not show any charge-discharge cycling property, which could be attributed to the different structures of WS2 anodes for Li+ ions intercalation or de-intercalation.

KEYWORDS: WS2 nanoplates; Thermal evaporation; Crystal growth; Microstructure; Electrochemical property

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1. INTRODUCTION The most famous two-dimensional layered material (2DLM) is graphene, a spectacular success that has pushed 2DLM to the frontier of modern materials science. But now the interest in 2DLM nanostructures has been extended well beyond the territory of graphene.1 For example, WS2 and MoS2 are also of graphene-like structure, consisting of strong covalently bonded layers but holding together by weak van der Waals forces.2 Each layer in them has a trigonal prismatic structure, formed by stacking sandwiches consisting of one sheet of transition-metal atoms between two sheets of sulfur atoms. Their stable poly-type phase at ambient conditions is 2H with a hexagonal packing of layers in an AbA BaB sequence.3 Because of the so-called graphene-like layered structure, WS2 and MoS2 have showed excellent solid lubricating ability

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and tribological properties.5 But the richer variety in

composition and structure can provide graphene-like WS2 and MoS2 nanostructures more fascinating new phenomena, particularly striking optical and electrical properties.6-8 For instance, unlike graphene, WS2 and MoS2 bulk crystals are indirect band-gap semiconductors, but their monolayers own direct band-gaps corresponding to the visible to near infrared photon energies,6 making them more suitable than graphene for applications in light-emitting and functional transistor. More recently, WS2 has been extensively explored for optoelectronics such as light-emitting diodes,9-12 photodetectors,13 solar cells,9 and photocatalysts.14 As for developing optoelectronic devices for physical, chemical and biochemical applications, Peimyoo and Gutierrez et al. reported that, compared with multilayer WS2, monolayer (1L) WS2 possesses more remarkable physical and optical properties.15,16 And Jung et al. even synthesized horizontally- and vertically-grown WS2, indicating that the two morphologies of WS2 can present unique properties suitable for specific applications, such as horizontally-grown WS2 for optoelectronics and vertically-grown WS2 for electrochemical reactions.17 In a word, the already reported studies show that the structure and morphology would have a significant impact on the optoelectronic performance of WS2. As for solar cells, the crystal structure and morphology of WS2 would also affect the photosensitive property. For example, thin stoichiometric layers of MS2 (M=W, Mo) crystallized in 2H-MS2 structure, were not photosensitive because of the poor orientation of the microcrystallites and their small sizes, while WS2 films which have the basal (002) plane parallel to the substrate can achieve good photoconversion efficiencies.18 Moreover, when applied as photocatalysts and electrocatalysts, the metallic octahedral 1T-WS2 buckling nanostructure was demonstrated an efficient hydrogen evolution


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electrocatalyst, whereas the usually semiconducting prismatic 2H-WS2 nanoplates would act as a visible light sensitizer,14,19,20 implying that such performance was also determined by the structure and morphology of WS2. To explore these properties, various synthesis approaches to prepare WS2 nanostructures with different structures and morphologies have been developed, such as a microwave plasma process on the basis of W(CO)6 and H2S,21 the thermolysis of tetraalkylammonium thiotungstates,22 the heating of amorphous W with a H2S–N2H2 gaseous mixture,23 the heating of W and S powder mixture,5 molecular beam epitaxy,24 pulsed-laser deposition,25 and a sonochemical technique.26 Vapor deposition, especially thermal evaporation, stands as a particularly appealing and versatile synthesis strategy in fabricating 2D heterostructures with clean and sharp interfaces, being more controllable, safe, environmentally friendly and reproducible. However, the development of controlled synthesis of WS2 nanostructures by vapor deposition techniques still needs a better understanding on the fundamentals of the morphology-controlled growth and growth mechanism. While the techniques of vapor deposition have been extensively studied in the growth of thin films and nanomaterials such as nanowires, nanotubes and graphene, knowledge obtained from these materials may not be simply applied in the growth of 2D nanostructure.3 Thus, a fundamental understanding on the growth mechanism and influential factors would pave the way for the controlled growth of WS2 or more other 2DLM nanomaterials by vapor deposition. Furthermore, the investigation on the relation between different morphologies is also crucial for realizing morphology-controlled synthesis. Herein, in this study, a simple and facile strategy was developed, which can controllably synthesize a large quantity of WS2 nanoplates with different morphologies (pure hexagonal, horizontally-grown WS2 nanoplates, vertically-grown nanoplates and nanoplates-formed flowers) simply by thermal evaporation of WO3 and S onto a Si substrate sputtered with W film. A model for the growth mechanism was also proposed, elucidating how the morphology transformed. Moreover, the horizontally- and vertically-grown nanoplates, which have completely opposite surface morphology profiles, were selected as thin film anode materials for lithium ion batteries, and the influence of WS2 nanoplates morphology on their electrochemical performances was investigated.

2. EXPERIMENTAL SECTION 2.1 Setup and raw materials. The reported products were synthesized by a high-temperature thermal evaporation process with a horizontal quartz tube furnace (Figure 1). In the quartz tube, the temperature decreases gradually along the downstream, giving two temperature regions (high temperature, HT, and low temperature, LT), which can effectively control the temperature gradient in the tube and synthesis temperature on the substrates.


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Commercially bought WO3 and S powders of analytical grade were applied as the thermal evaporation sources, and silicon wafers sputtered with W films were used as the substrates for growing the WS2 nanoplates. 2.2 Synthesis of WS2 nanoplates. In a typical synthesis process, a quartz boat with 1 g WO3 was placed exactly at the center of HT region, while another boat with 1 g S powder was located at 10 cm apart from the WO3 powder on the upstream of the furnace. During the processing, three W-coated single-crystal silicon substrates (I, II, and III) in a size of 1 cm × 1 cm were positioned at LT zone, approximately 16, 14 and 12 cm away from the WO3 source on the downstream, respectively. The thickness of the W film was about 30 nm. Before heating, the quartz tube was evacuated and flushed repeatedly with Ar gas for several times. Then the furnace was heated up to 1323.15 K in the HT region and 773.15 K in the LT one, and then held there for 1 h. The simulated temperatures of substrates I, II, and III as shown in Figure 1, which were calculated by the P1 radiation model in Fluent 14.0, are approximately 862.89, 1016.26 and 1169.63 K, respectively. Throughout the processing, Ar gas flow was kept with a flowing rate of 200 sccm. After that, the furnace was cooled down naturally to room temperature. Finally, a purple layer of products was formed on the surface of the silicon substrate.

Figure 1. Schematic of the experimental setup for growing the WS2 nanoplates.

2.3 Product characterizations. The phase composition of the as-synthesized products was identified by grazing incidence

X-ray diffraction

(GI-XRD, D/max-RB, Cu Kα radiation, and λ=1.5418 Å)

in

continuous

scanning mode. The scanning rate was 6 °/min and the incidence angle of X-ray was 1°. The morphology of the


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products was examined by a field emission scanning electron microscope (FE-SEM, S4800). The mean thickness of the obtained nanoplates and their distribution were evaluated from a few randomly chosen areas in the SEM images where each area contained about 100 nanoplates. The products were also characterized by transmission electron microscopy (TEM, Tecnai G2 F20 U-TWIN) and high-resolution transmission electron microscopy (HRTEM). Their chemical composition was measured by the energy-dispersive X-ray (EDX) spectroscopy attached to the TEM. In addition, the thickness of the nanoplates thin film anodes was measured by a vertically scanning white-light interfering profilometer (Micro XAM-3D). 2.4 Electrochemical characterization. The electrochemical characterization was performed by using a threeelectrodes experimental cell. The anode was the WS2 nanoplats grown on n-type Si substrates in a size of 1 cm × 1 cm, while the counter and reference electrodes were Li metal. The thickness of typical vertically-grown WS2 nanoplates anode was 1.2 µm with an apparent electrode density of 0.75 g/cm3, and the thickness of the horizontallygrown WS2 nanoplates anode was 0.2 µm with a density of 2.66 g/cm3. The electrolyte was 1 mol/L LiPF6 dissolved in a 50/50 vol% mixture of ethylene carbonate and diethyl carbonate. The test cells were assembled in an argonfilled glove box. The cells were galvanostatically charged and discharged at room temperature in a voltage range of 0.01-3.0 V on a LAND battery testing system (CT-2001A battery cycler) to measure the electrochemical response of the designed batteries. The current density used was 5 µA/cm2.

3. RESULTS AND DISCUSSION 3.1 Morphology evolution of WS2 nanoplates. The products were examined first by XRD. It was found that the products grown on the three substrates present the same XRD pattern. Figure 2 shows typical XRD pattern of the as-prepared products, in which the diffraction lines can be readily indexed to those of the hexagonal phase of 2HWS2 (JCPD No. 08-0237). There are no additional diffraction lines related to WO3 and W in the patterns, confirming the complete conversion of the W films on the substrates to WS2, and implying the high purity of the obtained WS2 products. Moreover, the intensity of (002) plane could be expected the strongest, at least ten times stronger than those of the other diffraction peaks, indicating that this plane is the dominant growth direction or preferential orientation of the grains during the formation of the as-proposed WS2 products under the desgined synthesis conditions. And the intensive sharpness of the (002) diffraction peak indicates the good crystallinity of the prepared products.


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Figure 2. Typical XRD pattern of the obtained products. All the diffraction peaks in the pattern can be assigned to pure 2H (hexagonal) WS2 phase.

The morphology of the products was then examined by FE-SEM. Typical SEM images of the obtained nanoplates are presented in Figure 3. From the low-magnification SEM images as shown in Figure 3(a-c), it can be seen that a large quantity of nanoplates were successfully obtained on all the three substrates set at different distances away from the source powders. The detailed formation process of the WS2 nanostructures was carefully investigated, and a clear distance-dependent morphology evolution could be observed. From the substrate I, which was settled 16 cm away from the WO3 source, the longest distance among the three substrates, horizontally-grown (flat) WS2 nanoplates could be obtained (Figure 3a and 3d). It could be observed that many of the nanoplates were slightly tilted with the substrate, having one side jointed with it, but only very few of them stood up vertically. In addition, with a closer view of such nanoplates, it could be found that they were of hexagonal-like structure. As the distance between the source powders and substrate decreased, from the substrate II, where the distance decreased to 14 cm, vertically-grown WS2 nanoplates were obtained (Figure 3b and 3e). Most of these plates were vertically standing up, layer upon layer, with only few of them remaining flat. When coming to the substare III, which was set at the closest position (12 cm) away from the WO3 source powder among the three substrates, flowers of nanoplates (nanoplates-formed flowers) were obtained (Figure 3c and 3f). Moreover, the thickness of the obtained nanoplates is also distance-dependent. By examining about a hundred of nanoplates in each sample from the SEM images, the calculated thicknesses of the nanoplates on the substrates I, II, and III were 25±10, 40±25 and 45±20 nm, respectively (please see the control Experiments 1, 2, and 3 in Table 1). This observation indicates that under the designed conditions, the shorter the distance was, the larger the thickness of the nanoplates would be.


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(a)

(b)

(c)

(d)

(e)

(f)

Figure 3. (a, b, c) Low-magnification and (d, e, f) high-magnification SEM images of the nanoplates on the substrates with different distances away from the WO3 source powder: I (16 cm), II (14 cm) and III (12 cm). (b)

(a)

(c)

(d)

Figure 4. (a) Typical TEM image of the WS2 nanoplates grown on substrate I. (b) Typical TEM image of the WS2 nanoplates collected from substrates II or III, and the inset shows a single nanoplate prepared after a sufficiently long time of ultrasonic dispersing. (c) Typical EDX spectrum of the WS2 nanoplates, in which the Cu and C peaks are raised from the TEM grid to support the samples. (d) Typical HRTEM of a WS2 nanosheet.

To provide further insight into the obtained WS2 nanoplates, TEM investigation was also performed. Typical TEM image of the nanostructures grown on substrate I is displayed in Figure 4a, revealing that they are plates with sharp edges, which is in accordance with the observation from the SEM images as shown in Figure 3d. Figure 4b


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presents a typical TEM image of the nanoplates collected from the substrates II or III, which also exhibits the crosslinked feature of such nanoplates as shown in Figure 3e and 3f. After the samples were ultrasonically dispersed in ethanol solution for a sufficiently long time, single nanoplate could be observed in TEM examination, which is shown in the inset of this figure. It was found that the nanoplates grown on the substrates II and III were nearly spherical, no longer hexagonal. This result might imply that besides the morphology the crystalline structure of the nanoplates might also be influenced by the distance. Typical EDX spectrum of the WS2 nanoplates is illustrated in Figure 4c, which demonstrates that, except for Cu and C elements from the supporting grid, only W and S could be detected, and the molar ratio of W to S is 1:1.82, very close to the W/S atomic ratio of stoichiometrical WS2. Typical HRTEM image of a WS2 nanosheet as shown in Figure 4d reveals a honeycomb-like structure of the plain view projection, and the calculated spacings of the observed lattice planes from the well-resolved periodic lattice fringe are approximately 0.248, 0.242 and 0.257 nm, which can be indexed to those of the (010), (100) and (210) planes of hexagonal WS2, respectively. All these results further confirmed that the prepared products are crystalline WS2 nanoplates. Considering that in the XRD patterns the (002) diffraction peak of the samples was the strongest, all these results imply that the nanoplates are single crystals with (002) plane as the dominant growth direction, the one perpendicular to the plate. Although the morphology evolution of the present WS2 nanostructures seems to be distance-dependent during the present processes, it acutally relies on the substrate temparture and amount of deposition materials from the sources, because the change in the distance from the sources varies intrinsically the substrate temperature and amount of the conveyed materials by the transfer gas. To control the morphology of the WS2 nanoplates, the synergistic effects of the main deposition parameters including substrate temperature, Ar gas flow rate and heating time were investigated. The results on the control experiments are listed in Table 1. As discussed above, in the Experiments 1, 2 and 3 (also see Figure 3), when the distance away from the WO3 source power decreased from 16 to 12 cm, intrinsically, the substrate temperature increased from 862.89 to 1169.63 K (see Figure 1) and more materials were conveyed by the transfer gas onto the substrates; as a result, the morphology of the obtained WS2 nanostructures was changed from horizontally-grown nanoplates, vertically-grown nanoplates to nanoplates-formed flowers, and the thicknesses of the nanoplates were increased. Because the amount of materials conveyed by the transfer gas can also be manipulated by the Ar gas flow rate, in order to investigate its role on the morphology of the WS2 nanoplates, while the distances from the sources was kept as that of Experiment 2 (14 cm), the nanostructures


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were grown at different Ar gas flow rates as presented in the Experiments 4 and 5. It is seen that, when the Ar gas flow rate decreased to 180 and 150 sccm, both of the nanoplates were vertically-standing on the substrates, very similar to those of Experiments 2, but the thicknesses of the obtained nanoplates decreased to 20±10 and 15±10 nm, respectively. Compared with Experiment 2, the substrate temperatures in these two cases were not changed and the increase in Ar gas flow rate would only bring more source materials onto the substrate. Therefore, from the above results it can be concluded that the morphology of the nanoplates was controlled by the substrate temperature, and almost independent on the Ar gas flow rate in the designed range. And because the more deposition materials were conveyed by the transfer gas onto the substrate, the nanoplates grew thicker. Moreover, when the Ar gas flow rate was higher than 220 sccm, no nanoplates but nanorods were observed. The SEM-EDX measurement on the nanorods (see Figure S1) revealed that they consisted of tungsten oxide together with tungsten sulfide, indicating that at high Ar gas flow rate, W film will mainly turn into tungsten oxide nanorods. And when the Ar gas flow rate was lower than 100 sccm, irregularly shaped particles were resulted in. The SEM-EDX surface scanning on the nanoparticles (see Figure S2) revealed a spatial distribution of W and O elements as well as the presence of S throughout the image, indicating that these particles are tungsten oxide and at low Ar gas flow rate the W film will turn into tungsten oxide crystals but not completely grow into WS2 ones. In addition, when the distance between substrate and WO3 source powder was kept at 14 cm, dozens of control experiments as shown in Figure S3 were carried out, which revealed the limit conditions of growing the vertically- or horizontally-grown nanoplates. The results prove that it mainly depends on the temperature whether the nanoplates grow vertically or horizontally. And the amount of materials conveyed by the transfer gas mainly influences the nanoplates thickness. In order to study the growth process of the nanoplates, the control experiments were performed for a shorter time, while all the other preparation parameters were kept as those of Experiment 3. On the substrate III, which is 1169.63 K, when the heating time was 45 min (in the Experiment 6 as listed in Table 1), there are numbers of vertically-grown nanoplates standing on the substrate, which is similar with the case shown in Experiment 2 or Figure 3b (on substrate II with a temperature of 1016.26 and heating for 1 h). And it was shown that the nanoplatesformed flowers were grown from the vertically-stood nanoplates. But when the heating time was 30 min as shown in the Experiment 7, there were only some particles but no WS2 nanoplates. EDX analysis (as shown in the Figure S4) revealed that the particles were Si, which might be formed by the melted Si from the substrate. The reason for this phenomenon might be that no sufficient WO3 vapor reached the substrate in a short period of heating time to


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complete the nucleation and growth of WS2 nanoplates. Moreover, it was demonstrated by the comparative Experiment 8 (also at a temperature as that on Substrate III) that, when the WO3 powder in quartz boat was covered by the substrates, keeping the side with W film facing to but about 1 mm off the WO3 powder and heating for 30 min, nanoplates tilted with the substrate was found but with a mean thickness of 55±25 nm. In this case the nanoplates possessed the same morphology as but much larger thickness than those in the case in Experiment 1 or Figure 3a (on substrate I at a temperature of 862.89 K and heating for 1 h). The morphologies of the nanoplates in the Experiments 6 and 8 are very similar to those of the Experiments 2 and 1, respectively, indicating that growth process of the WS2 nanoplates were as follows: firstly, the nanoplates were horizontally-grown; then, the nanoplates were vertically-standing; and when more and more nanoplates grew up, flowers were formed by the nanoplates. And when the substrate was set very close to the WO3 source powder as shown in the Experiment 8, the nanoplates were almost horizontally-grown but the thickness of the nanoplates was the largest among all the control experiments, showing that the source WO3 vapor mainly influenced the nanoplates thickness, which agrees with the results of Experiment 4 and 5. Table 1 Growth conditions, morphology and thickness of the products in control experiments Heating time (min)

Substrate temperature (K)

Ar gas flow rate (sccm)

1

60

862.89

200

25±10

2

60

1016.26

200

40±25

3

60

1169.63

200

45±20

4

60

1016.26

180

20±10

Exp

Position of source powders and substrate

Products morpholgy

Nanoplates thickness (nm)


10

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5

60

1016.26

150

15±10

6

45

1169.63

200

35±10

7

30

1169.63

200

-

8

30

1169.63

200

50±25

9

60

1169.63

200

-

10

60

1169.63

200

-

Note:

In addition, to clarify the role of the source powder WO3 in the growth of WS2 nanoplates, a control Experiment 9 without WO3 powder was designed, while all the other parameters were kept as those of Experiment 3. The SEM image in combination with EDX examination (see Figure S5) shows that there were no WS2 nanoplates but Si particles on the substrate, which was the melted Si from the substrate, revealing that the vapor of the source powder WO3 participated in the nucleation and growth of the present WS2 nanoplates. Similarly, a control Experiment 10, in which the substrate was Si wafer without W film while all the other parameters were kept as those of Experiment 3, was designed to clarify the role of the W film on Si substrate in the growth of WS2 nanoplates. The SEM image together with EDX examination (see Figure S6) indicates that there were no WS2 nanoplates but WOx particles on


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the substrate, revealing that the W film on the substrate might act as self-catalyst and also participate in the nucleation and growth of the present WS2 nanoplates. 3.2 Growth mechanism of WS2 nanoplates. The chemical reactions during the large-scale synthesis of the present WS2 nanoplates are supposed as following. Briefly, when the substrate temperature increased to a proper temperature, the metallic W film on the substrate would react with the residual oxygen in the system, forming WO3 crystals: 2W 3O WO .

(1)

Then, WO3 crystals might grow up with the evaporation of the source powder WO3, and were transformed into WS2 by thermally sulfurating at elevated temperature. 2WO 7S 2WS 3SO .

(2)

Figure 5. Schematic of the growth process of WS2 nanoplates, corresponding to the observed SEM images.

A schematic is presented in Figure 5 for the formation process of the proposed WS2 nanoplates during the thermal evaporation of WO3 and S onto a Si substrate sputtered with W film. As discussed above, on the substrates with different distances away from the source powders, WS2 nanoplates of different surface morphologies were observed. On substrate I, which has the lowest temperature of 862.89 K and least deposition materials, horizontallygrown small nanoplates that have one side jointed with the metalic W film were observed. With decreasing distance (on substate II at a temperature of 1016.26 K and somewhat more deposition materials), when the number of the small nanoplates increased, a large quantity of vertically stood WS2 nanoplates formed and these nanoplates might still grow thicker. On substrate III (at the highest temperature of 1169.63 K and most abundant deposition materials), the nanostructures might even transform into cross-linked nanoplates-formed flowers. Corresponding to the observed SEM images, it can be infered that the nanoplates are formed through a VS mechanism by the oxidation and sulfuration of the W film on the silicon substracte. In the first stage, it turns into crystalline WO3 by its


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reaction with the residual oxygen in the system as shown in Reaction (1) during the temperature increasing process. Then the WO3 crystals grow up by absorbing the WO3 vapor evaporated from the source powder. In the second step, as the WO3 layer grows thicker, surface diffusion and surface roughness of the WO3 layer might act some parts. The strain in the WO3 layer grows with its increasing thickness, because of the lattice mismatch between the crystallized WO3 layer and Si substrate, and the volume shrinkage.27 As a result, cracks appear in the WO3 layer and some sections of the WO3 layer separate from the substrate layer. With the time proceeding, WO3 is partially reduced by sulfur vapor to form volatile suboxide species WO3-x, which would be further sulfurized, leading to the formation of small WS2 nanoplates.28 When more W and S atoms join in, the nanoplates grow up, and when a new metallic W interface is exposed to the oxidation atomsphere, the oxidation and sulfuration are carried out to the deeper layer of the applied W film, resulting in the pressing out of the orginally synthesized WS2 nanoplates by the newly formed nanoplates, giving out the image that the nanoplates grow up vertically (Step 3). On the other hand, the newly formed WS2 nanoplates may act as seeds for the WS2 growth because of the diffusion of S and W atoms which evaporate from the source powders. And as confirmed by the recorded XRD pattern in Figure 2, the (002) plane of the nanoplates is highly oriented, which would promote them become thicker by the growth process along the (002) direction. The far the substrate set away from the source powders, the less S and W atoms would reach on the WS2 nanoplates seeds, which is why the nanoplates grown on the substrate II (14 cm away from the source) are much thicker than those on the substrate I (16 cm away from the source). Then with closer distance on substrate III (12 cm away from the source), the reaction proceeded more quickly and more nanoplates formed. A large scale of nanoplates would grow up, crowding with each other and accumulating on the top of each other. Then the shape of a flower formed (Step 4). In addition, as for the shape change from hexagon with six sharp angle tips of the nanoplates on the substrate I (as seen in Figure 3d and 4a) to nearly circle with rounded corners of those nanoplates on the substrates II and III (see the inset of Figure 4b), it is because the nanoplates grow under a thermodynamically controlled process, which tends to form regular shapes enwrapped by low-energy crystal facets. At higher temperature, the growth rate might become higher. Due to the higher energy of six sharp angle tips, the hexagonal nanoplates (formed at a lower temperature on substrate I) would gradually grow bigger and meanwhile, their shapes change to more stable circle forms (formed at a higher temperature on substrates II or III).29


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3.3 Electrochemical performance of WS2 nanoplates. A key feature of WS2 is the existence of van der Waals force across the gap between the S–W–S sheets and a large interlayer spacing between the S–W–S sheets along the c-axis (d = 6.18 Å for (002) planes), which can provide the space for Li+ ions (with a diameter of 2.12 Å) insertion/de-insertion.30,31 Thus, WS2 could be developed as an intercalation host to form a promising electrode material in high energy density batteries. For lithium ion batteries, it was reported in literature that the morphology of electrode materials had a great influence on their electrochemical properties.32 To investigate the electrochemical performance of our WS2 nanoplates, the vertically- and horizontally-grown WS2 nanoplates were selected as thin film anodes in lithium ion battery because they are completely of vis-a-vis in surface morphology profile, and their electrochemical characterizations were conducted. Figure 6a shows the galvanostatic discharge-charge profiles of the lithium ion batteries with the as-prepared vertically- and horizontally-grown WS2 nanoplates as anode materials in the first three circles at a current density of 5 µA/cm2. In the first discharge process (Li+ insertion), the verticallygrown WS2 nanoplates electrode delivered a lithium insertion (discharge) capacity of about 5266 mAh/cm3. A slope starting from 0.75 down to 0.5 V as well as a lithium insertion plateaus at about 0.5 V were observed in the first discharge curve, which could be attributed to the subsequent conversion reaction of Li+ ions with WS2 and the formation of a solid electrolyte interlayer (SEI).33 During the first charge process, the charge voltage could be recovered to 3 V with a reversible capacity of 1606 mAh/cm3, implying a low Coulombic efficiency of about 30.5%. In the second and third discharge curves, the potential plateau of the battery with the vertically-grown WS2 nanoplates electrode at approximately 0.5 V as presented in the first discharge disappeared, indicating an improved reversibility of lithiation and de-lithiation with cycling. After the first decay of capacity between the discharge and charge processes, the WS2 nanoplates exhibited a sustaining improvement in capacity recovery with recycling, presenting a 72% Coulombic efficiency and a reversible charge capacity of 560 mAh/cm3 in the third cycle. The charge-discharge behavior of the battery with the as-prepared vertically-grown WS2 nanoplates electrode is similar to that with randomly-distributed WS2 nanosheets electrodes reported in other works,30,34 but the battery with the present vertically-grown WS2 nanoplates electrode exhibited a much higher discharge capacity (about 773 mAh/cm3, or 1030 mAh/g) than those in literatures (about 600-800 mAh/g)31-34, due to the highly ordered structure of our WS2 nanoplates accommodating for the Li+ ions flow. On the other hand, for the battery with the present horizontally-grown WS2 nanoplates, in the first discharge, the curve also presented a slope from 0.75 down to 0.5 V,


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and a steep voltage drop was observed. But its voltage remained unchanged thereafter, and the charge process disapeared (i.e. without Li+ ion exsertion process).

(a)

(b)

Figure 6. (a) Discharge-charge profiles of the lithium ion batteries with the vertically- and horizontally-grown WS2 nanoplates as thin film anode materials at a current density of 5 µA/cm2, and (b) schematic of the charge-discharge processes in the verticallyand horizontally-grown WS2 nanoplates in view along b axis.

The different electrochemical performances of the batteries with the vertically- and horizontally-grown WS2 nanoplates can be attributed to their different structures of WS2 anodes. As shown in Figure 6b, for the anode with the vertically-grown WS2 nanoplates, the nanoplates are almost standing alignedly onto the substrate with interlayer spacing of (002) planes paralleled to the direction of Li+ ions flow, indicating that it is easy for for Li+ ions to intercalate or de-intercalate. But for the anode with the horizontally-grown WS2 nanoplates, the interlayer spacing of (002) planes are perpendicular to the direction of Li+ ions flow, implying that it will be very difficult for Li+ ions to intercalate or de-intercalate. The intercalation reactions for WS2 nanoplates could be given as following.33,35 At the counter electrode the intercalation would be, Li ⇌ Li e ,

(3)

at the working electrode the intercalation would be, xLi xe WS ⇌ Li WS ,

(4)

and the conversion reaction at the working electrode would be, Li WS 4Li 4e ⇌ W 2Li S xLi,

(5)


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where, x represents the number of moles of intercalating Li+ ions (or corresponding electrons) in the host materials. In the first discharge process, Li+ ions intercalate into the WS2 defect sites and interlayer sites. After these sites are saturated with Li+ ions, Li+ ions will then diffuse into the WS2 structure to form LixWS2 compounds.32 During the first charge process, Li+ ions de-intercalate from LixWS2, returning back to free Li+ ions. It should be noted that the irreversible capacity (3660 mAh/cm3) in the first cycle of the battery with the vertically-grown WS2 nanoplate is very high, which may be caused by a fraction of Li+ ions that were trapped in the defect sites,36 the conversion reaction of Li+ ions with LixWS2,33 and the continuous formation of a SEI on WS2 surfaces.37 Then after the first cycle, the intercalation reaction would dominate the electrochemical processes. Thus the Coulombic efficiency increased in the later cycles. For the anode with the horizontally-grown WS2 nanoplates, because it is difficult for Li+ ions to intercalate into the interlayer spacing of (002) planes in WS2 nanoplates, the initial discharge capacity is generally ascribed to the irreversible Li insertion into the WS2 nanoplates defects, as well as the production of the SEI layer on the surface of the electrode owing to electrolyte decomposition. Since there are little Li+ ions deintercalating from the horizontally-grown WS2 nanoplates, so it will present no charge process.

4. CONCLUSIONS This work presents a facile approach to the large-scale synthesis of pure horizontally-grown WS2 nanoplates, vertically-grown nanoplates and nanoplates-formed flowers simply by thermal evaporation of WO3 and S powders onto a Si substrate sputtered with W film without using any nanostructured W-contained precursors and highly toxic sulfides gases. The as-synthesized WS2 nanoplates are of hexagonal phase WS2, and the morphology of the WS2 nanoplates can be manipulated by tuning the distance between the substrate and source powders. The nanoplates were supposed to grow through the oxidation and sulfuration of W films in a vapor-solid mechanism. When employed as thin film anode materials, the lithium ion battery with the as-prepared vertically-grown WS2 nanoplates presented a rechargeable performance between 3 and 0.01 V with a discharge capacity of about 773 mAh/cm3 after recycling for three times, much better than its already-reported counterparts with the randomly-distributed WS2 nanosheets electrodes, but the battery with the horizontally-grown WS2 nanoplates could not show any chargedischarge cycling performance, which could be attributed to the different structures of WS2 anodes for Li+ ions intercalation or de-intercalation. Such performance influenced by nanostructure ushers a new possible direction to further improve the electrochemical performance of electrochemically active 2D nanomaterials.

ASSOCIATED CONTENT


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○ S Supporting Information

Additional results, including EDX spectra of the samples prepared under controlled conditions (both figures and tables), and scattering plot of gas flow rate versus temperature for growing the horizontally and vertically WS2 nanoplates. These materials are available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION

Corresponding author †

Telephone: 86-10-82320255. Fax: 86-10-82322624. E-mail: [email protected] (ZJ Peng).

‡

Telephone: 86-10-62282242. Fax: 86-10-62282242. E-mail: [email protected] (XL Fu).

Author Contributions J.W.Q. performed the experiments with technical support from the coauthors and obtained data representation. P.L.W. carried out the temperature modeling of the furnace. Z.J.P. and X.L.F. supervised the whole work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support for this work from the National Natural Science Foundation of China (grant nos. 61274015, 11274052 and 51172030), Excellent Adviser Foundation in China University of Geosciences from the Fundamental Research Funds for the Central Universities, and Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications) is gratefully acknowledged. And the authors would also like to thank Professor Lizhen Fan and Dr Dan Zhou from Beijing University of Science and Technology for their helps in electrochemical characterizations of our materials.

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Table of Contents


Bulk Fabrication of WS2 Nanoplates: Investigation on the Morphology

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