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Many faces of Ni3Bi2S2: tunable nanoparticle morphology via microwave-assisted nanocrystal-conversion Maria Roslova, Wouter Van den Broek, Anna Isaeva, Thomas Doert, and Michael Ruck Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01647 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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Crystal Growth & Design

Figure 1. (a) XRD patterns of as-prepared Bi2S3 nanoparticles and Ni3Bi2S2 nanoparticles obtained after a treatment of these nanoparticles in the solution of nickel acetate at 240 °C within 30 min. Blue ticks denote positions of the Bragg reflections of Bi2S3 (structural data from37) and Ni3Bi2S2 (structural data from29). Green triangles show the position of Bi impurity reflections (CSD 616519); (b, d) SEM images of Ni3Bi2S2 grown on the Bi2S3 precursor; (c) SEM image of Bi2S3 submicron rods. 84x84mm (300 x 300 DPI)

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. (a) XRD patterns of as-prepared NiBi nanoparticles and Ni3Bi2S2 nanoparticles obtained after the treatment of NiBi in thiosemicarbazide solution at 240 °C for 30 min. Green triangles indicate the reflections of the Bi impurity (CSD 616519). Blue ticks denote positions of the Bragg reflections for NiBi (structural data from38) and Ni3Bi2S2 (structural data from29); (b) An SEM image of the presynthesized NiBi; (c) An SEM image of Ni3Bi2S2 converted from the NiBi precursor. 30x11mm (300 x 300 DPI)


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Figure 3. A putative scheme of the structural transformation of NiBi (left) into Ni3Bi2S2 (right). Sulfur atoms are omitted in the fragment of the Ni3Bi2S2 structure framed by the red rectangle. Only Ni−S and Ni−Bi contacts with the length less or equal to 3 Å are shown. The unit cells are outlined in black. 207x78mm (300 x 300 DPI)



Figure 4. XRD patterns collected using CuKα1 radiation after 2, 3, 5, 15 and 40 minutes of microwave reflux of a Bi(NO3)3, Ni(OAc)2 and thiosemicarbazide mixture in ethylene glycol. Blue ticks denote the positions of the Bragg reflections for Ni3Bi2S2, red diamonds and green triangles indicate the reflections of NiBi and Bi, correspondingly. 84x84mm (300 x 300 DPI)


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Figure 5. (a-b) SEM and (c-d) TEM micrographs of Ni3Bi2S2 obtained at 240 °C starting from Bi(NO3)3 and Ni(CH3COO)2 solutions at pH ≈ 4. 84x84mm (300 x 300 DPI)



Figure 6. SAED patterns viewed along the main zone-axis of Ni3Bi2S2. The reflections were assigned by comparison with the simulated diffraction patterns based on the crystal structure of Ni3Bi2S2 in the space group C2/m 29. 84x84mm (600 x 600 DPI)


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Figure 7. Schematic representation of a possible microwave-assisted nanocrystal-conversion polyol route. 84x31mm (300 x 300 DPI)



Figure 8. Reduction kinetics for Bi(NO3)3∙5H2O and Ni(CH3COO)2∙4H2O salts dissolved in EG/NaOH. The individual 0.02 M solutions of precursor salts were refluxed at a constant power of 300 W. 84x84mm (600 x 600 DPI)


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Figure 9. (a) An SEM micrograph of Ni3Bi2S2 obtained at 240 °C starting from Bi(NO3)3 and Ni(CH3COO)2 solutions at pH ≈ 12; (b−d) TEM images of a typical Ni3Bi2S2 sub-micron particle hosting 50 nm wide NiS nanoparticles. 84x84mm (300 x 300 DPI)



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Many faces of Ni3Bi2S2: tunable nanoparticle morphology via microwave-assisted nanocrystalconversion Maria Roslova*†, Wouter Van den Broek§, Anna Isaeva†, Thomas Doert† and Michael Ruck† †Department of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany §Institute of Physics, Humboldt University Berlin, 12489, Berlin, Germany


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ABSTRACT

Several combinations of the microwave-assisted polyol route and the conversion chemistry techniques were exploited to access the bimetallic sulfide Ni3Bi2S2 with a variety of morphological features. First, Bi2S3 microstructures can be converted into Ni3Bi2S2 at 240 °C; the precursor's rod-like shape and size pertain to the final product. Second, round Ni3Bi2S2 particles can be obtained directly from a presynthesized NiBi intermetallic precursor; the resultant submicron size particles agglomerate and thus differ from the starting alloy's shape. Third, microwave reflux of bismuth nitrate and nickel acetate solution in ethylene glycol in the presence of thiosemicarbazide can be employed to produce Ni3Bi2S2 with a peculiar flower-like morphology. The presence and the decisive role of the in situ generated NiBi intermediate are unraveled, confirming that the reaction proceeds via transformation of solid rather than via a solution-dissolution process. NiBi nanoparticles pre-configure the Ni3Bi2S2 product morphology in the wide range of pH values. In its turn, pH value is found to be a key factor that determines the type of impurities accompanying the Ni3Bi2S2 ternary phase. At pH ≈ 4 bismuth precipitates as a main side-phase, while pH ≈ 12 favors for the formation of NiS impurity.

INTRODUCTION

It has been widely demonstrated that inorganic nanoparticles can be converted into more complex nanocrystalline solids through chemical reactions.1−3 Although chemical conversion often leads to morphology transformation of nanocrystals due to changes in composition and/or crystal structure, also many reaction paths are reported preserving the original morphology. Lowtemperature synthesis methods are especially advantageous for such conversion reactions, since only the selective movement of particular atoms is facilitated, while the most of the parent lattice


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remains intact. Thus, a partial reorganization of the host solid favors the growth of a new phase comprising entire structural fragments of the parent compound.4−7 Nanocrystal-conversion-chemistry is applicable to diverse classes of solids, including metals, oxides, pnictides, intermetallics (recently reviewed in Ref. 1) and hybrid nanostructures.8,9 For example, the key precedent for the synthesis of nanostructured mixed metal sulfides is the conversion of Co nanoparticles into hollow Co3S4 and Co9S8 nanospheres while maintaining their overall shape.10,11 A similar reaction with selenium yields hollow CoSe nanocrystals.10 Hollow nanostructures of metal sulfides attract attention nowadays owing to their envisioned applications for lithium-ion batteries and hybrid supercapacitors.12 The conversion chemistry approach can be employed for intermetallic systems as well. For instance, AuCuSn2 nanocrystals were produced starting with a reaction of gold nanoparticles from tin to yield intermetallic AuSn nanocrystals. The latter, in turn, reacted with additional Sn and Cu in solution to form the final product, AuCuSn2.13 Use of microwave energy provides a particular advantage in conversion reactions, namely synthesis acceleration and enhanced nucleation of a selected phase under favorable conditions, rather than any specific influence on crystal growth. The microwave-assisted polyol route has been proven to be highly efficient for all material classes listed above.14,15 Beyond that elemental metals16−19 and intermetallics 20−23 are especially straightforward to manufacture because of the polyols' ability to act as an absorber of microwave radiation and as a reducing agent simultaneously. Both classes, elemental metals and intermetallics can be employed either for in situ or consequent nanocrystal-conversion reactions, thus promoting the formation of structurally and compositionally more complex systems. In this work, we explore and rationalize the combined microwave-assisted nanocrystal-


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conversion method for the first time. For that we combine a polyol reduction process to generate metallic or intermetallic nanoparticles with a conversion reaction to chemically transform the pre-formed nanoscaled templates into derivative solids. The application of this synthesis is exemplarily shown on the ternary metal-rich sulfide Ni3Bi2S2. In general, synthesis of nanosized ternary mixed metal chalcogenides by wet-chemistry routes remains largely unexplored, notwithstanding the fact that downscaling exerts notable influence on their physical properties. For example, finite-size effects were recently demonstrated in superconducting Pd3Bi2Se2 nanoparticles, as significant critical-field enhancement originating from the submicron nature of the samples was observed in comparison to the bulk metallic properties.24 The structural family of materials related to Pd3Bi2Se2 is abundant and is commonly referred to under the general notion of “parkerites” after the name given to the Ni3(Bi,Pb)2S2 mineral.25 The title compound, Ni3Bi2S2, has a parkerite-type structure and is intrinsically a quasi-2D material that demonstrates superconducting properties.26 It crystalizes in the monoclinic space group C2/m and consists of quasi-2D slabs running parallel to the [001] direction and formed by the Ni–Bi and Ni–S bonds. 27−29

Nanostructured Ni3Bi2S2 was obtained for the first time by treating Bi2S3 nanoribbons with

a nickel salt in ethylene glycol under alkaline conditions.30 Reduced nickel diffusion into the Bi2S3 crystal structure was initially considered as a driving force of the reaction.30 The same reaction mechanism was proposed for Ni3Pb2S2, too.31 Later investigations of the reaction mechanism proved its complexity: it was assumed that formation of NiBi and Bi intermediates plays an important role in the reaction pathway.32,33 The two-step morphological templating was proposed for the synthesis of Ni3Bi2S2 starting from bismuth, 34, however, the retention of the starting material size and shape was observed only in the case when morphology of the precursor was spherical. In contrast, the synthetic approach employed in 30 allows preserving of the Bi2S3


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nanoribbons shape despite the different Bi/S ratio in the binary and the ternary phases as well as the formation of elementary Bi observed in diffraction patterns.30 Another example of a chemical transformation with a morphology retention in the Ni−Bi−S system is the conversion from Bi8Ni8SI2 into the intermetallic subsulfide Bi8Ni8S by reductive pseudomorphosis.35 The microwave-assisted nanocrystal-conversion method reported herein employs a polyol process generating metallic or intermetallic nanoparticles and a conversion reaction using these pre-formed nanoparticles as templates for chemical transformation into derivative solids. This method enables facile, high-yield, and scalable fabrication of Ni3Bi2S2 with tunable morphology in a wide pH range. Development of the particles' morphology, compositional and structural evolution in the course of the microwave-assisted nanocrystal-conversion reactions were studied by means of scanning (SEM) and transmission (TEM) electron microscopy, energy dispersive spectroscopy (EDX) and X-ray diffraction (XRD). The presented method can be implemented either as a two-step process using either presynthesized NiBi or Bi2S3, or as a one-step, one-pot process starting from soluble nickel and bismuth salts. We convincingly prove that the latter process is driven by in situ generation of NiBi. A skillful choice of the precursor stabilizes the "2D" morphology favorable for the parkerite-type layered monoclinic structure. To the best of our knowledge, this habitus has never been reported for parkerites in the literature before. EXPERIMENTAL Reagents Bi(NO3)3∙5H2O (Riedel-de Haen, 98.5 %), Ni(CH3COO)2∙4H2O (Merck, 99 %), thiosemicarbazide (Sigma-Aldrich, 99 %), thiourea (ABCR, 99 %) NaOH (Grüssing, 99 %). All reagents were used as purchased. Ethylene glycol (EG, Fluka, 99 %) was dried under vacuum at


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100 °C before synthesis in order to prevent it from premature or inhomogeneous boiling at the chosen reaction conditions. Synthesis of nanoscale crystals Microwave syntheses were performed in a CEM Discover System operating at 2.45 GHz and 300 W, unless stated otherwise. Bi2S3 microstructures were produced following the method described in Ref. 30, which was adapted for microwave heating. In a typical synthesis, a solution containing 0.096 g (0.2 mmol) of Bi(NO3)3∙5H2O and 0.023 g (0.3 mmol) of thiourea in 15 ml EG was filled in a 30 ml reaction vessel. The vessel was then placed in a microwave oven and stirred until complete dissolution of the reagents. Then the stirring was stopped and the solution was heated to 197 °C in 3 min and refluxed at that temperature for 30 min in a dynamic-mode. NiBi particles were synthesized by first dissolving 0.096 g (0.2 mmol) of Bi(NO3)3∙5H2O and 0.050 g (0.2 mmol) of Ni(CH3COO)2∙4H2O in 15 ml EG. The solution was heated to 220 °C in 4 min in a dynamic-mode and held at this temperature for 30 minutes. To access the Ni3Bi2S2 product, the freshly prepared NiBi or Bi2S3 nanoparticles were resuspended in 15 ml EG by sonication. Then 0.018 g (0.2 mmol) of thiosemicarbazide and 0.075 mg (0.3 mmol) of Ni(CH3COO)2∙4H2O in 2 ml EG was added to NiBi and Bi2S3, respectively. The solutions were heated to 240°C for 10 minutes in a constant power mode at the operating power of 100 W and then held for 30 minutes under an intense stirring. For the one-step, one pot synthesis of Ni3Bi2S2 a mixture of Bi(NO3)3∙5H2O, 0.075 g (0.3 mmol) of Ni(CH3COO)2∙4H2O and 0.018 g (0.2 mmol) of thiosemicarbazide together with 15 ml EG


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were placed in a 20 ml microwave vessel. The mixture was heated to 240°C in 4 min in a dynamic-mode and kept at this temperature for 10−30 minutes. In some cases NaOH was used for the pH adjustment. All reactions were quenched by cooling down to room temperature with a high-pressurized air flow. The final products were collected by centrifugation (~3900g rcf for 5−10 minutes) after washing several times with ethanol and distilled water. Characterization X-ray powder diffraction (XRD) patterns were collected using a PANalytical X’Pert Pro MPD diffractometer with CuKα1 radiation (λ = 1.54056 Å, Ge-monochromator, step size 0.026°, Bragg angular range 10–90°) operating at 40 kV and 40 mA at room temperature. The standard grinding procedure was employed for samples preparation.36 The Jana2006 software was used for the further data analysis.37 Particle size and shape were determined with a scanning electron microscope, Hitachi SU8020. Energy dispersive X-ray spectra were collected using an Oxford Silicon Drift Detector X-MaxN. Samples for the TEM analyses were prepared by immersion of nanopowders in ethanol and dispersion by sonication. The resulted suspension was dropped onto a carbon coated copper grid and dried on air. High-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) experiments were conducted on a CM20 microscope (Philips) operating at 200 kV. For image acquisition, a 2k x 2k Slow-Scan CCD-Camera (Gatan) was used. Diffraction pattern simulations were done with the JEMS software.38 The composition of the TEM sample was measured alongside the image acquisition by EDX on a built-in setup (Oxford-Instruments). RESULTS AND DISCUSSION Rod-shaped Ni3Bi2S2 microstructures from presynthesized Bi2S3


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Bi2S3 prepared by the microwave-assisted polyol method forms rods with lengths of 1−3 µm and diameters of 20−50 nm. Full conversion of Bi2S3 into Ni3Bi2S2 occurs after the 30-minute microwave treatment in nickel-acetate solution and is accompanied by the formation of a metallic-bismuth side phase, as revealed by XRD phase analysis (Figure 1, a). SEM shows that the dimensions of the Bi2S3 template are well preserved, while the surface of the rods becomes more eroded. Alongside the rods, smaller particles with a non-uniform shape appear after the treatment. EDX mapping reveals their enhanced bismuth content (up to ~35 at. %), so that they most likely correspond to the elemental-bismuth impurities found in XRD.

Figure 1. (a) XRD patterns of as-prepared Bi2S3 and Ni3Bi2S2 microstructures obtained after a treatment of these nanoparticles in the solution of nickel acetate at 240 °C within 30 min. Blue ticks denote positions of the Bragg reflections of Bi2S3 (structural data from

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) and Ni3Bi2S2

(structural data from29). Green triangles show the positions of Bi impurity reflections (CSD 616519); (b, d) SEM images of Ni3Bi2S2 grown on the Bi2S3 precursor; (c) SEM image of Bi2S3


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submicron rods. Despite the certain differences in the relative intensities for Ni3Bi2S2 prepared in this work and the single crystal dataset given in 29, the structural model proposed for the bulk in the literature is valid for the microstructures as well, when absorption and roughness corrections for the Bragg-Brentano geometry are taken into account. Round Ni3Bi2S2 nanoparticles from presynthesized NiBi Rommel et al.32,33 assumed that NiBi plays a key role as an intermediate in the transformation of Bi2S3 into the ternary compound Ni3Bi2S2. It was argued that NiBi may appear as a result of nickel intercalation into elemental bismuth that is obtained via Bi2S3 reduction in ethylene glycol under alkaline conditions. In our case, the ethylene glycol redox potential would suffice, even under the acidic conditions with pH ≈ 4, to trigger reductive conversion of Bi2S3. The latter may serve as a physical scaffold for the formation of the NiBi intermediate. In both cases, the experiments were monitored by means of XRD; we have also collected data for the reactions quenched at various timings. However, the presence of crystalline NiBi in any form was not observed in any reaction system. A possible explanation could be that the conversion takes place on a nanometer scale and does not include dissolution and re-assembling of an entire nanoparticle, although the finite size of the nanoparticles enables facile re-organization and recrystallization. In the following we put forward and test a hypothesis that NiBi is present as an intermediate in the reaction and can be further transformed into Ni3Bi2S2 nanostructures that are morphologically similar to the pristine precursor. In order to gain further insight, we monitored a reaction of the presynthesized NiBi with thiosemicarbazide in ethylene glycol. The microwave-assisted polyol process can be easily adapted to synthesis of intermetallic nanoparticles starting from soluble


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bismuth and nickel salts at 220 °C and pH ≈ 4 for 30 minutes according to the reactions: HOCH2CH2OH → CH3CHO + H2O 3Bi3+ + 3Ni2+ + 10CH3CHO → 3NiBi + 5CH3COCOCH3+10H+ It is worth noting that the optimal reaction temperature lies between 220 °C and 240 °C. As long as the reaction temperature is lower than 220 °C, only bismuth reduction occurs in the solution; whereas at temperatures higher than 240 °C a significant portion of NiBi3 appears. The XRD pattern of the freshly prepared NiBi concerts well with the previously reported data.40−42 SEM confirmed that the nanoparticles have maintained the nearly spherical morphology and have a broad particle-size distribution in the range between 30 and 150 nm (Figure 2). The treatment of the freshly prepared intermetallic nanoparticles at 240 °C for 30 min by thiosemicarbazide, which decomposes at this temperature generating N2H4 and H2S, results in the conversion of NiBi to Ni3Bi2S2. A significant seed growth was disclosed by SEM. As a result of agglomeration, the product particles did not retain the precursor original morphology. In contrast to Ni3Bi2S2 grown from the Bi2S3, no separately lying bismuth nanoparticles were found in this case. Therefore, the excessive Bi from the reaction might have formed a thin shell around the Ni3Bi2S2 nanoparticles, but due to relatively small particles size this shell cannot be seen by EDX. To summarize, NiBi can be transformed into the ternary sulfide despite the fact that there is no obvious structural correspondence between the starting and product phases, as opposed to topotactic conversions.43 However, a certain structural similarity between the structures of NiBi and Ni3Bi2S2 can be traced (Figure 3). The former can be regarded as a hcp arrangement of Bi atoms with a largely ordered distribution of Ni atoms in both octahedral and trigonal-bipyramidal


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voids,40 whereas the latter was described as a distorted ccp arrangement of both Bi and S atoms, in which the Ni atoms occupy a half of the octahedral voids along all main crystallographic directions.29 Moreover, the parkerite-type structure features rather spacious "pathways" propagating along the [110] direction that grant it a pseudo-layered character (Figure 3). Based on this, we might speculate that a proverbial hcp ↔ ccp martensitic transformation that proceeds by the dislocation movements on adjacent close-packed plane may be relevant for the observed conversion reaction. Presumably, the "pathways" within the Ni3Bi2S2 structure facilitate the shear of the above mentioned Bi planes within the parent NiBi structure and the intake of further atoms. Just like in the above described synthesis from the Bi2S3 precursor, it is a solid-state process, rather than a solution-dissolution one. Significant lattice mismatches between the structures of NiBi and Ni3Bi2S2 lead to further distortions of the Ni substructure. The Ni−Ni distance increases from 2.68 Å in NiBi to 2.76 Å in Ni3Bi2S2, whereas the distance between the two adjacent nickel atomic planes (see Figure 3) increases by 1.64 Å.

Figure 2. (a) XRD patterns of as-prepared NiBi nanoparticles and Ni3Bi2S2 nanoparticles obtained after the treatment of NiBi in thiosemicarbazide solution at 240 °C for 30 min. Green triangles indicate the reflections of the Bi impurity (ICSD 616519). Blue ticks denote positions


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of the Bragg reflections for NiBi (structural data from 40) and Ni3Bi2S2 (structural data from 29); (b) An SEM image of the presynthesized NiBi; (c) An SEM image of Ni3Bi2S2 converted from the NiBi precursor.

Figure 3. A putative scheme of the structural transformation of NiBi (left) into Ni3Bi2S2 (right). Sulfur atoms are omitted in the fragment of the Ni3Bi2S2 structure framed by the red rectangle. Only Ni−S and Ni−Bi contacts with the length less or equal to 3 Å are shown. The unit cells are outlined in black. Flower-shaped Ni3Bi2S2 nanoparticles from in situ generated NiBi In situ generation of NiBi may promote a distinctive morphology. In the microwave process, metal particles generated in the reaction solution are regarded centers of local overheating. They can serve as "hot spots" for the ensuing reactions and thus give rise to a new, specific morphology. We found that the polyol reduction of Bi(NO3)3 and Ni(OAc)2 even in the presence of thiosemicarbazide can cause in situ generation of NiBi, as evidenced by XRD. The XRD patterns of the products observed for the reaction quenched after 2, 3, 5, 10 and 40 minutes of microwave reflux are given in Figure 4. The precipitation of NiBi occurs after 2 min under


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microwave irradiation and its conversion to Ni3Bi2S2 proceeds simultaneously. After 10−15 minutes of reflux this phase completely disappears while Ni3Bi2S2 and only minor amount of bismuth side phase are present in the reaction mixture. The bismuth is accumulated over time and becomes more crystalline, so that after 40 minutes of irradiation approximately equal amounts of phases form in the reaction (52.52 at% of Ni3Bi2S2 and 47.48 at% of Bi).

Figure 4. XRD patterns collected using CuKα1 radiation after 2, 3, 5, 10 and 40 minutes of microwave reflux of a Bi(NO3)3, Ni(OAc)2 and thiosemicarbazide mixture in ethylene glycol. Blue ticks denote the positions of the Bragg reflections for Ni3Bi2S2, red diamonds and green triangles indicate the reflections of NiBi and Bi, correspondingly. Figure 5 shows SEM and low-magnification TEM images of the typical Ni3Bi2S2 microstructures resembling flowers. The sheet-like (petal-like) morphology may be preferable for Ni3Bi2S2 in view of its pseudo-layered monoclinic structure.27,29 The presence of the ethylene glycol in some


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cases might be also favorable for the layered morphology formation as it was shown in 44. Submicron sizes Ni3Bi2S2 particles were not observed even for shorter reaction times. However, the conversion is fast and complete; the product yield is usually 90−95 %. EDX of a typical particle also shows that the atomic ratio of Ni to Bi to S is approximately 3:2:2 (see Figure S1).

Figure 5. (a-b) SEM and (c-d) TEM micrographs of Ni3Bi2S2 obtained at 240 °C starting from Bi(NO3)3 and Ni(CH3COO)2 solutions at pH ≈ 4. The same selected area electron diffraction (SAED) pattern was reproducibly found for several flat oriented "petals" of various "flowers" (Figure 6, a). It can be assigned to the main [110] zone of the monoclinic Ni3Bi2S2 lattice proposed by 29 and is consistent with the reflection conditions of the C2/m space group. Moreover, an auxiliary [120] zone (Figure 6, c) could be reached by tilting a flat "petal" from the [110] zone while keeping the (00l) series of reflections, that remained unchanged. The "petals" oriented almost perpendicularly to the viewing direction


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obscured each other, so only one instance of an ED pattern was feasible (Figure 6, b). It corresponds to the [010] zone of Ni3Bi2S2 and is contributed by at least two individual crystallites showing a slight misalignment. Comparison with Figure 3, right suggests that the largest facet of the flakes in this orientation is built by almost planar quasi-2D slabs propagating in the ab plane. All collected ED patterns and reflection conditions therein accord with the structure solution reported in 29 and do not support the structural model from27. In general, very sharp diffraction spots corroborate the formation of well-developed singlecrystalline material in the fast, strongly kinetically-driven microwave-assisted process.

Figure 6. SAED patterns viewed along the main zone-axis of Ni3Bi2S2. The reflections were assigned by comparison with the simulated diffraction patterns based on the crystal structure of Ni3Bi2S2 in the space group C2/m29. To summarize, we proved that the NiBi intermediate appears in the reaction of polyol reduction of nickel and bismuth salts, and that the nanoparticles of the intermetallics serve as "hot spots"


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for the further conversion into Ni3Bi2S2. A general scheme of the employed microwave-assisted nanocrystal-conversion process is given in Figure 7.

Figure 7. Schematic representation of a possible microwave-assisted nanocrystal-conversion polyol route. The unrevealed mechanism of formation of ternary compounds by a combination of the microwave-assisted polyol route and the nanocrystal-conversion approach let us retrospectively rationalize some earlier literature data. For example, in situ generated intermetallic intermediates may obviously have played an important role in the synthesis of Pd3Bi2X2 (X = S, Se) from palladium acetate, bismuth nitrate and a sulfur / selenium source24. Interaction of reduced metallic nanoparticles with the surrounding solution and with each other may encourage formation of ternary phases especially for those metals that are easily reducible in polyols (e.g. in ethylene glycol). Building on this realization, an alternative formation mechanism may be formulated for the Ag−Bi−S45, Cu−Bi−S46 and Cu−Sb−S47 systems. It is worth to mention that for systems containing less reducible elements such as tin or antimony the investment of the product formation enthalpy as a reaction driving force should be also taken into account48.


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The role of pH A key factor promoting reduction of Ni2+ and Bi3+ to M0 nanoparticles is the pH value. The hydroxide helps to deprotonate the ethylene glycol, which increases the reducing power.49 We examined the kinetics of reduction for the individual precursor salts in the microwave process to gain some insight into the reaction pathway. Figure 8 shows the time-dependence of the reduction rate (i. e. the OH-/Mn+ ratio in the reaction system) at the operating power of 300 W. The reduction of Ni2+ is strongly pH-dependent, whereas the onset of Bi3+ reduction is less susceptible to pH. However, at a 40x excess of NaOH both cations undergo reduction at comparable rates. Based on these data, we can speculate that the bismuth-based intermediates (Bi2S3, BiS, Bi) start to form first in a non-alkaline solution, because of the strong affinity of sulfur to metal ions. These phases are potentially conversable in the course of the reaction. On the other hand, alkaline media favors the formation of nickel sulfides which are anticipated to not be engaged in any further reactions under the given conditions. The reduction of the Bi(NO3)3, Ni(CH3COO)2 in the presence of thiosemicarbazide presumably occurs in several steps and wherefore the curve for the mixture does not resemble the pHdependencies of the precursors reduction rates. The corresponding graphs are provided in the Supporting Information (Figure S2).


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Figure 8 Reduction kinetics for Bi(NO3)3∙5H2O and Ni(CH3COO)2∙4H2O salts dissolved in EG/NaOH. The individual 0.1 mmol solutions of precursor salts were refluxed at a constant power of 300 W. In a reference experiment, when the pH value of the precursor solution was adjusted to 12 using 40x excess of NaOH, a side phase appeared in addition to the main Ni3Bi2S2 phase. Based on the XRD analysis, the impurity reflections were assigned to NiS (see Figure S3). Figure 9 depicts low-magnification SEM and TEM images of Ni3Bi2S2 decorated with NiS nanoparticles. The latter have a shape of round black dots of ca. 50 nm in diameter and are uniformly distributed over the surface of the Ni3Bi2S2 "flowers", as can be clearly seen in Figure 9. EDX further confirms that the contaminated areas always exhibit Bi-deficiency in comparison with the ideal Ni3Bi2S2 composition.


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Figure 9. (a) An SEM micrograph of Ni3Bi2S2 obtained at 240 °C starting from Bi(NO3)3 and Ni(CH3COO)2 solutions at pH ≈ 12; (b−d) TEM images of a typical Ni3Bi2S2 sub-micron particle hosting 50 nm wide NiS nanoparticles. Thus, the correlation between the pH value and the impurity type was revealed and rationalized. At pH ≈ 4 elemental bismuth appears as a main side phase, while pH ≈ 12 favors the formation of NiS impurity. Remaining bismuth impurities in Ni3Bi2S2 are not expected to have a detrimental effect on the superconducting and magnetic properties of the latter, since bismuth is strongly diamagnetic even in the nanostructured form.50 The NiS nanoparticles, however, could contribute weak ferromagnetism51, which might deteriorate the superconducting properties of the Ni3Bi2S2 matrix. Thus, a more detailed study of the physical properties of the presented nanostructures can be foreseen as a valuable extension to this work. CONCLUSIONS


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Morphology-defining synthesis and architectural control may become crucial for design and fabrication of nanostructures tailored for particular applications. In this work, we have developed a facile, high-yield synthetic approach to ternary metal-rich nanostructured Ni3Bi2S2 compound in a wide range of pH starting from unsophisticated precursors under mild conditions. We have also demonstrated how the size, the shape and the morphology of the product can be tailored by controlling the formation conditions of the NiBi intermediate that is being converted into the target compound Ni3Bi2S2 subsequently. In the case of in situ generated NiBi, a peculiar flowerlike morphology can be obtained, whereas the use of presynthesized Bi2S3 or NiBi nanoparticles results in the product morphology resembling the original one of the pristine precursor. The overall morphology is pertained when the Bi2S3 nanoparticles are converted into Ni3Bi2S2, while significant seed growth and merging particles are observed in the case of NiBi conversion. Due to the change in the Ni/Bi ratio and significant structural rearrangement upon conversion of NiBi into Ni3Bi2S2, impurity phases inevitably appear in the reaction. We showed that the type of impurity can be strongly influenced by addition of an alkali base. At pH ≈ 4 bismuth appears as a main side phase, while pH ≈ 12 favors the formation of NiS impurity. The 50-nm-wide nanoparticles of the latter are pinned on the “petals” of the Ni3Bi2S2 "flowers" in contrast to the randomly distributed bismuth side phase. A combination of the nanocrystal-conversion approach and the microwave-assisted polyol process, which we have exemplified by synthesis of morphologically controlled Ni3Bi2S2, can potentially develop into a versatile tool able to fulfill the increasingly stringent and sophisticated requirements for the design of functional nanomaterials.


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ASSOCIATED CONTENT Supporting Information Time- and temperature dependencies of the reduction rates against pH for the mixture of Bi(NO3)3, Ni(CH3COO)2 and thiosemicarbazide, typical EDX and XRD data for as-prepared Ni3Bi2S2 (PDF). AUTHOR INFORMATION Corresponding Author *e-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Maria-Reiche-Programme to support academic careers of female post-doc researchers (TU Dresden). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT M.R. acknowledges Maria Reiche Support Programme of Technical University Dresden. W. V. B. acknowledges the DFG project BR 5095/2-1. We thank Andrea Brünner and Michaela Münch for lab assistance and technical support. We are indebted to Prof. Dr. Ute Kaiser (University of


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Ulm) and Prof. Dr. C. T. Koch (Humboldt-University Berlin) for granting access to the microscopy facilities at the University of Ulm.


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For Table of Contents Use Only Many faces of Ni3Bi2S2: tunable nanoparticle morphology via microwave-assisted nanocrystalconversion Maria Roslova*†, Wouter Van den Broek§, Anna Isaeva†, Thomas Doert† and Michael Ruck†

A combination of the microwave-assisted polyol route and the conversion chemistry techniques was employed to access Ni3Bi2S2 starting from Bi2S3, NiBi and soluble salts of nickel and bismuth. The essential role of presynthesized or in situ generated NiBi as a reaction intermediate was revealed and rationalized. Round, rod-shaped and flower-like Ni3Bi2S2 nanoparticles were obtained.


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