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Cite this: DOI: 10.1039/c4nr03220a

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One-step gas–solid reaction synthesis of W@WS2 nanorattles and their novel catalytic activity Yan Wen,a Haijun Zhangb and Shaowei Zhang*a W@WS2 nanorattles were synthesised by heating a mixture of WO3 nanoparticles and S at a relatively low temperature between 750 and 950 °C in H2/Ar. In addition to the temperature, the competition between the H2 reduction and the S sulphidation sub-reactions dominated the reaction mechanisms and the mor-

Received 11th June 2014, Accepted 28th August 2014 DOI: 10.1039/c4nr03220a www.rsc.org/nanoscale

phologies of final products. Based on this, several types of nanostructures, including WS2 nanoflakes, inorganic fullerene-like nanoparticles and W@WS2 nanorattles (with desirable core size and shell thickness), could be selectively prepared by simply tailoring the processing parameters. Moreover, it was found for the first time that as-prepared W@WS2 nanorattles exhibited excellent catalytic activities which were close to or even better than their much more costive Au-based counterparts.

Introduction Nanorattles, nanoparticles with a core-in-hollow-shell structure, have received a great deal of attention because of their unique properties and great application potential. For example, metal-cored nanorattles could exhibit excellent catalytic activities, arising from their unique structure in which the nanoparticle cores are effectively isolated, avoiding their agglomeration and exposing more active surfaces. Consequently, they could perform much better than their naked or conventionally coated nanoparticle counterparts. To synthesize nanorattle materials, several techniques and methodologies have been suggested, among which the template synthesis is the most commonly used because of its good reliability and controllability. With this technique, a core–shell structured precursor was initially coated with another material, forming a three-layered egg-like composite particle (analogous to ‘yolk, egg white, and eggshell’) which was further subjected to chemical etching or calcination to remove the ‘egg white’ part, creating a nanorattle with a core inherited from the precursor and a shell from the coating. With this technique, several types of nanorattles with different core/shell sizes and thicknesses were prepared, e.g. Au@ZrO2 nanorattles from an Au–SiO2–ZrO2 composite precursor,1 hybrid Au–Pt@α-Fe2O3 from an Au–Pt-polyelectrolyte multilayer (PEM)-α-FeOOH precursor,2 Pt@hollow porous carbon (hmC) from a Pt-SiO2phenol-formaldehyde resin (PF) precursor,3 or Pt-TiO2-phenol precursor,4 and SiO2@TiO2 from a SiO2-polystyrene-sulfonated a

College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, EX4 4QF, UK. E-mail: [email protected] b The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 43081, China

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polystyrene-TiO2 composite.5 In spite of these, this technique still suffered from several drawbacks including the requirement of a core–shell structured precursor produced by a complex multi-step process using hazardous etching agents, thus it was not applicable to large-scale production. Another modified template synthesis technique was the socalled preshell–postcore technique.6 Hollow nanospheres were used as nanoreactors into which reactant precursors diffused and then reacted.7–10 By using this method, Cu@SiO2 7 and Ag@polypyrrole-chitosan (PPy-CS) nanorattles10 were prepared. Although this modified route could partially overcome the drawbacks of the conventional template synthesis route stated above and improve the product purity, the complex reaction processes used and the involvement of ultraviolet irradiation made it also difficult to be used for large-scale production. Apart from the template-based synthesis routes, interdiffusion processes based on the Kirkendall effect and the Ostwald ripening mechanism were used. However, they were only applicable to some special cases such as Co@CoSe11 and ZnS nanorattles.12 To overcome the drawbacks of the techniques reported so far, a scalable one-step synthesis technique by simply heating oxide precursors and S at a low temperature in H2/Ar was developed by the authors, and used to produce several types of nanorattles. In this paper, the work on the synthesis of W@WS2 nanorattles is reported. The thicknesses of WS2 shells and sizes of W cores were tailored by altering the gas flow rate. Based on the results, a reaction mechanism was proposed. In addition, to explore potential applications of as-prepared nanorattles, their catalytic performance was investigated as an example using the conversion reaction from 2-nitroaniline (2-NA) to o-phenylenediamine (OPD) as a working tool. This conversion reaction was usually catalysed by highly costive

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catalysts such as gold nanoparticles.13 The present work demonstrated that as-prepared nanorattles showed much better catalytic activity and thus could be potentially used to replace those costive catalysts.

Experimental section Preparation of W@WS2 nanorattles WO3 nanoparticles (650 °C, WO3 nanoparticles were initially reduced by H2 to WO3−x containing many O vacancies (eqn (2)) before the sulphidation by S (eqn (1)). The O vacancies in WO3−x facilitated the diffusion of S during the further reaction.15 The synergy (competition) between the H2 reduction and the S sulphidation determined the morphologies of the final products. When the concentrations of S and H2 were appropriate, a continuous WS2 shell would be firstly formed on the surface of WO3−x (eqn (1)) (Fig. 5a), preventing effectively the sintering between WO3−x nanoparticles, which otherwise would lead to the formation of relatively large tungsten oxide precursors. As reported before, if the sintered WO3−x precursors were larger than 200 nm, WS2 IF nanoparticles, even if formed, would not be stable and thus tend to convert quickly into 2H flakes.19 Therefore, the initially formed WS2 shell played a vital role in restricting the precursor’s size to below the critical value. When the H2 flow rate was appropriately low, the relative concentration of S in the atmosphere surrounding WO3−x was high, so there would be sufficient amounts of S which diffused to the interface of WS2

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and WO3−x and reacted with the latter to form WS2 IF nanoparticles (eqn (1)) (Fig. 4b). On the other hand, if the H2 flow rate was high, the S concentration would be low. In this case, it would become much more difficult for the sulphidation reaction (eqn (1)) to be completed. Consequently, intermediate WS2–WO3−x core–shell nanoparticles would be formed (Fig. 5a). Because WS2 is stable in H2 at 750 °C, H2 diffused through the initially formed WS2 shell to reduce the WO3−x core to W. Since W has a higher density (19.25 g cm−3) than WO3 (7.16 g cm−3), the volume of shrinkage occurred with the reduction process, and as a result, W@WS2 nanorattles were formed (Fig. 5c). Depending on the flow rates of H2, WS2 shell thicknesses and the corresponding core sizes varied. A high H2 flow rate resulted in a thin shell and a large core, and vice versa (Fig. 4b–d). This was because with increasing the flow rate of H2, the concentration of S in the local atmosphere surrounding the precursor particles decreased. Consequently, less WO3 was converted to WS2, leaving WS2–WO3 nanoparticles with thinner shells. Upon the depletion of S, the thicknesses of WS2 shells were fixed and would not change in the successive H2 reduction process. Finally, W@WS2 nanorattles with thin shells were formed. Catalytic activity of as-prepared nanorattles

ð6Þ

The as-prepared nanorattles were used as a catalyst to catalyse several important reactions. Here, as an example, their catalytic performance in the case of 2-NA to OPD conversion is presented. To convert 2-NA to OPD via hydrogenation, NaBH4 water solution was used as the H2 source (eqn (6)). 2-NA almost did not react with NaBH4 when a catalyst was absent, even after a long period, e.g., two days.20 Therefore, an appropriate type and amount of catalyst must be used to accelerate the reaction. Fig. 6 illustrates the evolution of UV-vis spectrum of 2-NA and NaBH4 reaction mixture with reaction time when as-pre-

Fig. 6 (a) Time-dependent UV-vis spectral changes of the reaction mixture catalysed by W@WS2 nanorattles. The inset picture shows together the solutions before and after 28 min reaction. (b) Plot of ln(Ct/C0) versus time.

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pared nanorattles were used as the catalyst. 2-NA solution had two UV absorption peaks centred at around 280 nm and 410 nm.21 Upon addition of W@WS2 nanorattles, the peak at 410 nm reduced quickly with the time. Meanwhile, the peak centred at 280 nm shifted gradually towards 290 nm, the characteristic peak of OPD.19 After 28 min reaction, only the 290 nm peak appeared, indicating that 2-NA had been completely converted into OPD. The ratio of 2-NA concentration at time t (Ct) to that at time 0 (C0) was determined from the relative intensity ratio of the respective absorbance, At/A0, at 410 nm. A linear relationship between ln(Ct/C0) and time was obtained (Fig. 6b), indicating that the reaction was first-order. From the slope of the straight line in Fig. 6b, the reaction rate constant when using W@WS2 nanorattle as the catalyst was determined as 1.85 × 10−3 s−1. The nanorattles could be recycled by simply using centrifugation. To test their stability as a catalyst, they were repeatedly used to catalyse the reaction for 3 cycles. The UV-vis spectrum from each cycle is shown in Fig. 7. Only the characteristic peak of OPD at 290 nm appeared in each cycle, indicating that 2-NA had been completely converted into OPD, which further indicated that even after 3 cycles the nanorattles still exhibited

catalytic activity. Nevertheless, the time to complete the reaction was increased with the cycle, being 37, 52 and 70 min for cycles 1, 2 and 3, respectively, indicating the gradually decreased catalytic activity with the cycle. Interestingly, after 3 cycles, the nanorattles still retained their morphologies (Fig. 8). These results suggested that as-synthesised nanorattles not only had excellent structural stability but also good catalytic stability. The catalytic performance of as-prepared nanorattles was close to or slightly better than that of the popular Au@SiO2 nanorattle catalysts (the rate constant was 1.69 × 10−3 s−1 (ref. 22)) for the conversion reaction, but much better than that of immobilized Au nanoparticles (the rate constant was 1.27 × 10−3 s−1).13 This, along with the lower cost of as-prepared nanorattles, suggested that they could be potentially used to replace the expensive gold-based catalysts currently used. Furthermore, it is believed that they also could be potentially used to catalyse other important reactions, e.g. H2 generation reactions (the work is ongoing and will be reported in another future publication). Nevertheless, future work is still needed to clarify the catalytic mechanism and further optimise the sizes of the catalysts to further improve their catalytic performance.

Conclusions

Fig. 7 UV-vis absorption spectra of the reaction solutions catalysed by W@WS2 nanorattles, after (a) 1 cycle, (b) 2 cycles and (c) 3 cycles.

W@WS2 nanorattles were prepared via a novel one-step sulphidation process, i.e., by heating WO3 nanopowder and S at a temperature between 750 and 950 °C under a H2/Ar atmosphere. By varying the H2 flow rate, WS2 nanoflakes, IF nanoparticles and W@WS2 nanorattles with different shell thicknesses could be readily produced. Based on the results, the reaction mechanism was proposed. When H2 was used, WO3−x–WS2 core–shell nanoparticles were initially formed. After the depletion of S, the WO3−x core could not be converted into WS2 but reduced by H2 to W gradually, leading to the formation of W@WS2 nanorattles. The catalytic activities of asprepared W@WS2 nanorattles were tested using the conversion reaction from 2-NA to OPD as a working tool. They showed better catalytic performance than Au-based nanocatalysts currently used or proposed. It is believed that the synthesis strategy used in this work could be readily extended to fabricate other transitional metal nanorattles such as Mo@MoS2 and Ti@TiS2. Furthermore, the as-prepared nanorattles could be similarly used to catalyse other important reactions.

Notes and references

Fig. 8

TEM image of as-prepared nanorattles after 3 catalytic cycles.

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One-step gas-solid reaction synthesis of W@WS2 nanorattles and their novel catalytic activity.

W@WS2 nanorattles were synthesised by heating a mixture of WO3 nanoparticles and S at a relatively low temperature between 750 and 950 °C in H2/Ar. In...
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