Flowerlike WSe2 and WS2 microspheres: one-pot synthesis, formation mechanism and application in heavy metal ion sequestration

Layered transition metal dichalcogenides (TMDs), MX2 where M is a transition metal (such as Mo, W, and Nb) and X is a chalcogen (S, Se and Te), have received intensive research interest due to their unique electronic, optical, mechanical and chemical properties. Among them, WSe2 and WS2 have been extensively studied for a variety of applications including valleytronics, batteries, electrocatalysis, photocatalysis, bioimaging labels, and electronic devices. WSe2 and WS2 have a layered structure in the form of Se–W–Se or S–W–S with a W atomic layer sandwiched by two hexagonal chalcogen atomic layers. The interlayers are coupled by weak van der Waals forces and the intralayer W–Se/S bonding is covalent. Most reports focus on the fabrication of nanosheets/ nanotubes of WSe2 and WS2 by chemical vapor deposition (CVD), electrochemical exfoliation and hot-injection approaches. These methods require high temperature, expensive precursors or toxic H2S/H2Se, and harsh conditions that give low yields. The growth mechanisms of TMDs have been rarely described, although the formation mechanisms of similar lamellar bismuth chalcogenide materials have been explored. Moreover, there are few reports on the construction of three-dimensional (3D) hierarchical nanosheet-assembled flowerlike architectures of WSe2, although 3D flowerlike MoS2 and metal oxides or hydroxides such as FeOOH and AlOOH have been widely studied. WS2 nanoflowers were produced by using CVD with low yields. 9 Hierarchical nanosheets assembled into flowerlike structures normally show superior performance in batteries and water treatment. Therefore, it is highly desirable to develop hierarchical WSe2 and WS2 using a facile, cost-effective and scalable approach. As a new class of heavy metal ion scavenger, metal chalcogenides have been used in heavy metal ion sequestration for water treatment, including ZnS, K2xMnxSn3 xS6, (NH4)4In12Se20, H2xMnxSn3 xS6 and HxNayInSz, 11 due to the strong affinity of the chalcogen to heavy metal ions. In this study, flowerlike WSe2 and WS2 microspheres consisting of their corresponding nanosheets were synthesized by a simple, low-cost and high-yield one-pot template-free solvothermal approach. Their growth mechanism was elucidated by in situ synchrotron radiation X-ray diffraction (SR-XRD). WSe2 and WS2 were used in heavy metal ion sequestration for the first time and showed exceptional uptake capacities for Pb and Hg, making them ideal candidates for heavy metal remediation in practical water purification. WSe2 and WS2 were prepared in high yields (43 g) by a solvothermal method from the reaction of W(CO)6 and Se or S powders in p-xylene (see details in ESI†). Fig. 1 shows the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the flowerlike WSe2. The WSe2 sample displays a hierarchical flowerlike micro/nanostructure (Fig. 1). The flowerlike WSe2 is composed of many self-assembled petals that are curly nanosheets (Fig. 1c). These WSe2 nanosheets are 5 nm in thickness and 300 nm in width, and are assembled together to form 3D WSe2 flowers with diameters ranging from 500 nm to 1 mm. Fig. 1d demonstrates that the thin WSe2 nanosheets consist of a few layers (o10 layers) stacking together and the interlayer spacing of the nanosheets is 0.689 nm, corresponding to the (002) plane of the hexagonal WSe2, with slight expansion due to the strain from the layer curvature. The lattice spacing of 0.285 nm corresponds to the (100) plane of the 2H–WSe2 phase. The X-ray diffraction a CSIRO Manufacturing, The Commonwealth Scientific and Industrial Research Organization (CSIRO), Clayton South, Victoria, 3169, Australia b Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Parkville, Victoria, 3010, Australia. E-mail: rcaruso@unimelb.edu.au c School of Engineering and Information Technology, Murdoch University, Murdoch, Western Australia, 6150, Australia d Department of Materials Science and Engineering, Monash University, Clayton, Victoria, 3800, Australia e Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cc00577b Received 21st January 2016, Accepted 25th February 2016

hierarchical nanosheet-assembled flowerlike architectures of WSe 2 , although 3D flowerlike MoS 2 and metal oxides or hydroxides such as FeOOH and AlOOH have been widely studied. 8 WS 2 nanoflowers were produced by using CVD with low yields. 9 Hierarchical nanosheets assembled into flowerlike structures normally show superior performance in batteries and water treatment. 8,10 Therefore, it is highly desirable to develop hierarchical WSe 2 and WS 2 using a facile, cost-effective and scalable approach.
As a new class of heavy metal ion scavenger, metal chalcogenides have been used in heavy metal ion sequestration for water treatment, including ZnS, K 2x Mn x Sn 3Àx S 6 , (NH 4 ) 4 In 12 Se 20 , H 2x Mn x Sn 3Àx S 6 and H x Na y InS z , 11 due to the strong affinity of the chalcogen to heavy metal ions. In this study, flowerlike WSe 2 and WS 2 microspheres consisting of their corresponding nanosheets were synthesized by a simple, low-cost and high-yield one-pot template-free solvothermal approach. Their growth mechanism was elucidated by in situ synchrotron radiation X-ray diffraction (SR-XRD). WSe 2 and WS 2 were used in heavy metal ion sequestration for the first time and showed exceptional uptake capacities for Pb 2+ and Hg 2+ , making them ideal candidates for heavy metal remediation in practical water purification. WSe 2 and WS 2 were prepared in high yields (43 g) by a solvothermal method from the reaction of W(CO) 6 and Se or S powders in p-xylene (see details in ESI †). Fig. 1 shows the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the flowerlike WSe 2 . The WSe 2 sample displays a hierarchical flowerlike micro/nanostructure (Fig. 1). The flowerlike WSe 2 is composed of many self-assembled petals that are curly nanosheets (Fig. 1c). These WSe 2 nanosheets are 5 nm in thickness and 300 nm in width, and are assembled together to form 3D WSe 2 flowers with diameters ranging from 500 nm to 1 mm. Fig. 1d demonstrates that the thin WSe 2 nanosheets consist of a few layers (o10 layers) stacking together and the interlayer spacing of the nanosheets is 0.689 nm, corresponding to the (002) plane of the hexagonal WSe 2 , with slight expansion due to the strain from the layer curvature. 3b The lattice spacing of 0.285 nm corresponds to the (100) plane of the 2H-WSe 2 phase. The X-ray diffraction (XRD) pattern (Fig. S1a, ESI †) is indexed to the hexagonal 2H-WSe 2 structure (JCPDS Card No. 38-1388). The energy dispersive X-ray (EDX) spectroscopy confirms the composition of WSe 2 with a W/Se atomic ratio of 0.51 (Fig. S1b, ESI †), in good agreement with the result (W/Se atomic ratio = 0.54) from inductively coupled plasma optical emission spectroscopy (ICP-OES). The X-ray photoelectron spectroscopy (XPS, Fig. S2, ESI †) shows that the W 4f 7/2 (31.9 eV) and W 4f 5/2 (34.0 eV) peaks are consistent with previous studies of WSe 2 samples. 4a,12 The small peaks at 35.6 eV and 37.8 eV are assigned to W 4f 7/2 and W 4f 5/2 from WO 3 , likely due to oxidation because of exposure to air during sample preparation and transfer for XPS characterization, and were observed in previous WSe 2 reports. 4a,12 The Se 3d peak was located at 54.4 eV which could be deconvoluted into 3d 5/2 and 3d 3/2 peaks.
A similar solvothermal approach using sulfur powder instead of selenium produced a WS 2 sample. The product is composed of microspheres with an average diameter of 1 mm (Fig. 2a). The WS 2 microsphere is comprised of curly nanosheets ( Fig. 2b and c), yet with smaller width (several tens of nanometers) compared to that of the WSe 2 nanosheets. The WS 2 nanosheets consist of 5-10 layers and the interlayer distance is 0.647 nm corresponding to the (002) plane of the hexagonal WS 2 (Fig. 2d). The XRD pattern (Fig. S3a, ESI †) can be assigned to the hexagonal 2H-WS 2 structure (JCPDS Card No. 84-1398), yet with a small shift in (002) peak towards low angle, which is probably caused by the layer curvature. 3b Both the EDX spectroscopy (Fig. S3b, ESI †) and ICP-OES results show the presence of W and S in the WS 2 microspheres with a W/S atomic ratio close to 0.5. The XPS spectra (Fig. S4, ESI †) display the W 4f 7/2 , W 4f 5/2 and W 5p 5/2 peaks as well as S 2p peak consistent with previous reported WS 2 samples, corroborating that the sample is pure WS 2 without oxidation. 4b,c,5b These results demonstrate that the solvothermal method is efficient in producing nanosheet self-assembled 3D WSe 2 and WS 2 micro/nanostructures in high yields. The presence of nanosheets in this architecture enhances the surface-to-volume ratio and offers more accessible interfaces for sequestration, while the entire microstructure enables fast and easy sedimentation and separation from water. 8a-c In order to unravel the growth mechanism of flowerlike WSe 2 and WS 2 microspheres, the synthesis conditions, including temperature and reaction time, were varied and the samples were characterized by SEM and ex situ XRD.
For WSe 2 , the optimal temperature is 250 1C, which is above the melting point (m.p.) of Se (221 1C). Below 250 1C, there was residual Se in the product ( Fig. S5 & S6, ESI †). WS 2 microspheres formed at 150 1C and crystallized upon rising temperature and the morphology evolved from irregular microspheres with smooth surfaces to relatively uniform microspheres with a well-defined nanosheet assembly (Fig. S7, ESI †). At 250 1C, the smooth WSe 2 microspheres gradually developed into coarse spheres with improved crystallinity and eventually crystalline flowerlike WSe 2 formed (Fig. S8, ESI †), as the reaction time was prolonged. A similar process also occurred in the growth of WS 2 microspheres except that the lateral size of the WS 2 nanosheets was smaller than that of the WSe 2 nanosheets (Fig. S9, ESI †). Therefore, for both WSe 2 and WS 2 , smooth microspheres initially formed and then the nanosheets were developed and crystallized on the surface of microspheres under the solvothermal conditions. Duphil et al.
reported an ambient solution method to produce merely amorphous WSe 2 and WS 2 irregular nanoparticles using W(CO) 6 , Se and S in p-xylene. 13 Pol et al. previously reported a high-temperature solid reaction between W(CO) 6 and Se to prepare WSe 2 nanoparticles. 14 They both proposed a two-step reaction mechanism involving first the complete decomposition of W(CO) 6 into W and subsequent reaction of W with Se/S without giving direct evidence. In the absence of Se or S, W(CO) 6 was heated at varying temperatures in p-xylene in a Teflon-lined autoclave. The resultant products  To track the phase evolution as a function of reaction time, timeresolved in situ SR-XRD experiments were performed. The in situ SR-XRD is able to provide valuable real-time phase evolution during the reaction, 1e,15 as illustrated in Scheme S1, ESI. † The in situ SR-XRD patterns of WSe 2 (Fig. 3) demonstrate that the solvothermal process involves dissolution of W(CO) 6 , melting of Se and growth of flowerlike WSe 2 . The in situ SR-XRD patterns (Fig. 2a) of the starting precursors are readily indexed to the orthorhombic phase of W(CO) 6 (JCPDS Card No. 40-0752) and hexagonal Se (JCPDS Card No. 06-0362), consistent with ex situ XRD patterns (Fig. S6, ESI †).
Upon heating in p-xylene, the peaks of W(CO) 6 continuously decreased in intensity and vanished after about 9 min at 118 1C (Fig. 3b-d). This temperature was much lower than the decomposition/melting point of W(CO) 6 (B170 1C) indicating that W(CO) 6 was neither melted nor completely decomposed into elemental tungsten. W(CO) 6 was likely dissolved in p-xylene. The peaks of the Se precursor remained until the temperature reached 236 1C (above the melting point of Se, 221 1C), when the Se precursor peak disappeared instantly. WSe 2 formed rapidly after the melting of Se. The elemental W intermediate arising from the decomposition of W(CO) 6 proposed in the early literature was not detected. 13,14 The formation mechanism determined from the in situ SR-XRD result agrees with the aforementioned ex situ SEM and XRD characterization. The heating rate governs the rate of Se melting and hence the whole reaction rate. Therefore, the reaction temperature plays a critical role in the formation of WSe 2 . Since evidence for the formation of W was not found in either ex situ or in situ experiments, we do not deem that the reaction between dissolved W(CO) 6 and melted Se in p-xylene in this case followed the twostep process as proposed in the literature. 13,14 The reaction mechanism might be a straightforward one-step reaction, or a modified two-step process initially involving the partial dissociation of W(CO) 6 into W(CO) 6Àx , 16 and the subsequent oxidation by Se/S atoms. The underlying mechanism will be investigated in the future through advanced in situ techniques (e.g. mass spectrometry). Besides, agglomerate WSe 2 with poor crystallinity (Fig. S11, ESI †) was obtained when W(CO) 6 reacted with Se without p-xylene, highlighting the importance of the p-xylene solvent in inducing the formation of flowerlike structures.
The growth process of WS 2 was also explored by the in situ SR-XRD (Fig. S12, ESI †). Sulfur (m.p. 115 1C) and W(CO) 6 were dissolved at 83 1C and 116 1C, respectively. Then WS 2 was gradually formed and crystallized upon heating. Both precursors vanished below their melting or decomposition point and no W intermediate was detected, consistent with the results of WSe 2 .
The synthesis of WTe 2 at 250 1C with the solvothermal method using W(CO) 6 and Te (m.p. 450 1C) was attempted. However, the product was only a mixture of Te and TeO 2 with W(CO) 6 , which was removed by rinsing with acetone (Fig. S13, ESI †).
Based on the above ex situ and in situ experimental results, the growth of WS 2 and WSe 2 began with the dissolution of S and W(CO) 6 , melting of Se and then both precursors reacted quickly to produce crystalline WS 2 and WSe 2 under the solvothermal condition. The absence of complete dissociation of W(CO) 6 into W in this work appears to defy the commonly accepted two-step reaction mechanism. 13,14 The knowledge gained about the growth mechanism of tungsten sulfide and selenide by the time-resolved in situ SR-XRD technique will inspire new studies of various layered transition metal dichalcogenides.
These as-synthesized flowerlike WSe 2 and WS 2 microspheres possess many nanosheets and abundant chalcogen ligands with innate reactivity towards soft heavy metal ions (Hg 2+ , Pb 2+ , etc.) and structural rigidity. These features are desirable for the sequestration of heavy metal ions from water. The flowerlike WSe 2 and WS 2 microspheres were used to sequester various heavy metal ions, including As(V), As(III), Cd 2+ , Pb 2+ and Hg 2+ , from water. They did not remove As(V), As(III) and Cd 2+ , whereas they exhibited high uptake capacities for Hg 2+ and Pb 2+ . This is due to the innate selectivity of metal chalcogenides for soft heavy metal ions (i.e. Hg 2+ and Pb 2+ ). 11 Fig. 4 exhibits the variation in Pb 2+ and Fig. 3 (a) Time-resolved in situ SR-XRD patterns showing evolution from W(CO) 6 and Se precursors to WSe 2 , (b) temperature ramp rate (20 1C min À1 ), (c and d) characteristic SR-XRD patterns of in situ products at the noted reaction time. Note that the reaction rate in the micro-reactor is much faster than that of the bulk solvothermal reaction in the autoclave. Hg 2+ concentrations before and after sequestration by WSe 2 and WS 2 samples at 0.5 g L À1 . The WSe 2 and WS 2 were able to reduce the concentration of Pb 2+ from 52.30 mg L À1 to 0.76 and 0.42 mg L À1 (Fig. 4a), respectively, giving a Pb 2+ uptake capacity of ca. 103 mg g À1 . The highest Pb 2+ uptake capacities of WSe 2 and WS 2 are 288 and 386 mg g À1 when treating 1048 mg L À1 of lead ions in water, which are substantially higher than those of nanostructured adsorbents with abundant hydroxyl groups reported previously, such as urchin-like FeOOH (80 mg g À1 ), 8a flowerlike zinc silicate (210 mg g À1 ) and flowerlike AlOOH (124.2 mg g À1 ) under similar conditions (Table S1, ESI †). 8c,10 Particularly, WSe 2 and WS 2 show extremely high uptake capacities for Hg 2+ (Fig. 4b). WSe 2 and WS 2 could decrease the concentration of Hg 2+ from 978 mg L À1 to 222 mg L À1 and 0.71 mg L À1 resulting in impressive capacities of 1512 mg g À1 and 1954 mg g À1 , respectively. Their capacities for Hg 2+ are significantly higher than those of conventional metal oxide and carbon-based adsorbents (Table S1, ESI †) and those of metal chalcogenides (Table S2, ESI †). 11 Flowerlike WSe 2 was characterized after removal of Pb 2+ and Hg 2+ (Fig. S14 & S15, ESI †). PbWO 4 polyhedron nanoparticles are embedded in the WSe 2 nanosheets suggesting that Pb 2+ ions were removed via a chemical reaction. When treating Hg 2+ ions in water, the WSe 2 nanosheets were almost fully covered by Hg 2 Cl 2 and Hg 3 Se 2 Cl 2 nanoparticles, suggesting that WSe 2 is a reducing agent that has high reactivity towards Hg 2+ leading to the high Hg 2+ uptake capacity. Likewise, PbWO 4 nanoparticles are immobilized on WS 2 nanosheets ( Fig. S16 & S17, ESI †). WS 2 microspheres are enclosed by as-formed Hg 3 S 2 Cl 2 and Hg 2 Cl 2 particles, indicating extremely high reactivity and reducibility of WS 2 towards Hg 2+ ions, which resulted in such a high Hg 2+ sequestration capacity. The WS 2 microsphere is superior to the flowerlike WSe 2 in terms of uptake capacities for Pb 2+ and Hg 2+ , possibly because sulfide has higher reactivity towards Pb 2+ and Hg 2+ than selenide. This mechanism is different from the direct cation exchange mechanism of other metal chalcogenides for heavy metal ion sequestration. 11 In summary, flowerlike WSe 2 and WS 2 microspheres assembled of nanosheets were synthesized by a facile and high-yield solvothermal method. The in situ SR-XRD gave insights into the growth mechanism of the flowerlike WSe 2 and WS 2 , which is the reaction between dissolved W(CO) 6 and dissolved S or melted Se without complete decomposition of W(CO) 6 into a W intermediate. Flowerlike WSe 2 and WS 2 microspheres were used for heavy metal ion sequestration and exhibited remarkable uptake capacities for Pb 2+ (288 mg g À1 for WSe 2 and 386 mg g À1 for WS 2 ) and Hg 2+ (1512 mg g À1 for WSe 2 and 1954 mg g À1 for WS 2 ), showing great potential in heavy metal remediation. The synthesis method and SR-XRD characterization in this work offer novel approaches towards rational design and fabrication of hierarchical transition metal dichalcogenides and understanding of their formation mechanism.
This project received financial support from Joint Research Project funding (GJHZ1224) from the Chinese Academy of Sciences and CSIRO. We acknowledge the Australian Synchrotron for the Powder Diffraction beamline access, and CSIRO Minerals for laboratory in situ XRD access. The CSIRO Office of the Chief Executive (OCE) Postdoctoral and Science Leader Schemes are acknowledged for supporting this work. R. A. C. acknowledges the Australian Research Council for a Future Fellowship (FT0990583).