Article pubs.acs.org/IC
Solution Synthesis of Nonequilibrium Zincblende MnS Nanowires Li Zhang,†,‡,§,∥ Su You,†,‡,§,∥ Ming Zuo,† and Qing Yang*,†,‡,§,∥ †
Hefei National Laboratory of Physical Sciences at the Microscale (HFNL), ‡Department of Chemistry, §Laboratory of Nanomaterials for Energy Conversion (LNEC), ∥Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei 230026, Anhui, People’s Republic of China S Supporting Information *
ABSTRACT: Uniform four-coordinate nonequilibrium MnS nanowires mainly in zincblende structure, other than the stable rock-salt phase, are reported for the first time. The MnS nanowires are grown via a solution−solid−solid model from the reaction of a Mn(II) source with dibenzyl disulfide in oleylamine at 180−200 °C catalyzed by Ag2S nanocrystals in a body-centered cubic (bcc) fast-ionic phase transformed from their low-temperature monoclinic form. Investigations show that most of the zincblende MnS nanowires are grown along the ⟨112⟩ zone axis but a small proportion grow along the ⟨111⟩ZB/⟨0001⟩Wur axis with zincblende/defect-section and/or wurtzite/defect-section superlattices connected with the stems along the ⟨112⟩ direction. The nanowires have a tendency to grow straight at relatively low reaction temperature for short reaction times but twist at high temperature for long reaction times. Meanwhile, relatively high temperatures and long times favor the transition of the MnS nanowires in the zincblende phase to the corresponding thermodynamic ones in rock-salt form. Interestingly, even small increases in reaction pressure (1−2 atm) sensitively influence the growth of the MnS nanowires from zincblende to wurtzite form in the present catalytic system although low-pressure changes commonly do not have an obvious effect on condensed matter. In addition, the optical and magnetic properties of the zincblende MnS nanowires were studied, and they are varied largely from the bulk.
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conditions, mainly via a solvothermal process.11 Typically, the nanostructured fast-ionic conductor catalyst of body-centered cubic (bcc)-phase Ag 2 Se with a high density of Ag + vacancies12−14 could promote the epitaxial growth of 1D nanostructures at relatively low temperatures.11 This SSS growth model has been adopted and improved by Hu and co-workers for the growth of ternary ZnCdS nanorods15 and by Pradhan and co-workers for 1D CdSe−AgInSe2, CdSe− AgGaSe2, ZnSe−AgInSe2, and ZnSe−AgGaSe2 heteronanostructures.16 Very recently, this regime has been intensively explored for growing metastable zincblende MnSe nanowires kinetically;17 however, nonequilibrium zincblende MnS nanowires have not been reported to date, since it is predicted that it would be more difficult to obtain MnS in zincblende phase, compared with MnSe, according to the early theoretical
INTRODUCTION Until now, there has been considerable progress made for the synthesis of semiconducting nanowires via solution-phase strategies based on different growth mechanisms, typically including templating,1,2 anisotropic3,4 and oriented attachment growth models.5,6 Comparably, the catalytic growth regime in solution phase has been proven to be versatile, supporting the growth of a variety of nanowires via different catalysts mainly on the basis of a solution−liquid−solid (SLS) growth mechanism,7,8 even though the catalytic growth of onedimensional (1D) wires was initially demonstrated via a vapor−liquid−solid (VLS) model in the case of Si catalyzed by liquid Au alloyed droplets.9,10 Recently, a solution−solid−solid (SSS) mechanism has been revealed for the growth of semiconducting nanowires representatively including the ZnSe, CdSe, ZnS and CdS nanowires in addition to their heteronanostructures catalyzed by Ag2Se, Ag2S, and/or Cu2S nanocrystals that are present as fast-ionic (superionic) phase under solution-phase reaction © 2017 American Chemical Society
Received: January 30, 2017 Published: June 29, 2017 7679
DOI: 10.1021/acs.inorgchem.7b00247 Inorg. Chem. 2017, 56, 7679−7686
Article
Inorganic Chemistry
Characterization. The samples were characterized by powder Xray diffraction (XRD, performed using a Philips X’pert PRO X-ray diffractometer, Cu Kα, λ = 1.54182 Å), transmission electron microscopy (TEM, using Hitachi, Model H-7650), high-resolution transmission electron microscopy (HRTEM, using a JEOL Model JEM-ARF200F TEM/STEM instrument with a spherical aberration corrector) and energy-dispersive X-ray spectroscopy (EDX, using the JEOL ModelJEM-ARF200F TEM/STEM instrument with a spherical aberration corrector). Differential scanning calorimetry (DSC) measurements were performed using a Perkin Elmer, Model DSC 8500 system, in the range of 20−200 °C, with a heating/cooling rate of 5 °C min−1. Raman spectra acquired from the as-synthesized MnS nanowires were measured at room temperature in air with the 785 nm (1.58 eV) line of an argon laser on an inVia micro-Raman spectrometer. The optical absorption spectra of the samples dispersed in hexane were recorded using a spectrophotometer (UV-Vis-NIR system, Model DUV-3700) to collect absorbance from 300 nm to 1600 nm at room temperature. A vibrating sample magnetometer (VSM) (performed on a Quantum Design, Model MPMS XL-7 system) was used to characterize the magnetic properties of the samples enclosed in a medical capsule.
investigations on the tetrahedrally bonded compound semiconductors.18,19 In the present work, we report the synthesis of uniform, fourcoordinate nonequilibrium zincblende MnS nanowires via an alternative facile colloidal route from the reaction of varied Mn(II) sources with dibenzyl disulfide ((PhCH2)2S2) in the media of oleylamine (OAm) at 180−200 °C over the catalysis of Ag2S nanocrystals in a body-centered cubic (bcc) fast-ionic phase, transformed from monoclinic phase under the reaction conditions, regardless of what type of Ag source was introduced into stock solutions. The nonequilibrium MnS and monoclinic Ag2S heterostructured nanowires are also first reported in the same route. Investigations show that the zincblende MnS nanowires are typically grown along the metastable ⟨112⟩ zone axis with a very small proportion along the ⟨111⟩ axis connected with the stems along the ⟨112⟩ direction, and they have a trend to grow straight at relatively low reaction temperature for short reaction times but twist at high temperature during long reaction times. It is importantly noted that nanowires grown in the ⟨112⟩ direction have been rarely reported to date for a four-coordinate diamond-type bonding solid. Interestingly, studies suggest that an increased reaction pressure, even in a small scale (1−2 atm), could lead to a transition of the MnS nanowires from zincblende to wurtzite phase, and the pressure is also favorable for the stabilization of the wurtzite MnS nanowires. The optical and magnetic properties of the MnS nanowires were studied, and they have a large optical band gap (Eg) of 3.1 eV but a small Néel transition temperature at 50 K that is largely different from that of the bulk MnS in a stable rock-salt structure.
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RESULTS AND DISCUSSION As shown in Figure 1, as well as Figure S2 in the Supporting Information, both MnS and Ag2S can be detected in the room-
EXPERIMENTAL SECTION
Materials. Silver nitrate (AgNO3, >99.95%), hexane (97%), and ethanol (99.7%) were purchased from Shanghai Reagent Company, PRC. Manganese chloride tetrahydrate (MnCl2·4H2O, 99.9%), manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O, 99.9%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Dibenzyl disulfide ((PhCH2)2S2, 98.0%), manganese(II) acetylacetonate (Mn(acac)2, 99.99%), oleic acid (OA, tech. 90%) and oleylamine (OAm, 70%, Batch No. STBD9442 V) were purchased from Aldrich. Dibenzylamine (DBA, 98%) and 1-octadecene (ODE, tech. 90%) were purchased from Alfa Aesar, and 1-hexadecylamine (HDA, 90%) was purchased from ACROS. All chemicals were used as purchased without further purification. Synthesis of Ag2S−Nanocrystal Stock Solution. A quantity of 3.5 mL of OAm in a 100 mL three-necked flask was degassed at 150 °C for 30 min under argon flow. A solution containing 0.5 mL of AgNO3 ethanol solution (6 mmol/L), 0.0025 g of (PhCH2)2S2, and 0.5 mL of OAm then was quickly injected into the flask. After that, the flask was kept at 150 °C for another 30 min, and Ag2S nanocrystals were prepared in the stock solution (Figure S1 in the Supporting Information). Synthesis of Nonequilibrium MnS Nanowires via Catalysis of Ag2S Nanocrystals. The above stock solution with Ag2S nanocrystals in the flask was further heated to 190 °C for several minutes. A solution of 0.0396 g MnCl2·4H2O mixed with 0.0246 g (PhCH2)2S2 in 1 mL of OAm then was swiftly injected into the flask and reacted for 10 min at that temperature. For the synthesis, the molar ratio of Ag/Mn was chosen to be 1.5%, with one exception, which will be notified later. After reaction, the products were collected by centrifugation and washed several times with hexane. The asobtained samples were dispersed in hexane for further characterization. Additional control syntheses were performed under different conditions, including variations of reaction temperature and reaction time, and changes of Ag and Mn sources, and solvents in stock solution, which was not described here in detail.
Figure 1. XRD pattern of the samples synthesized using a reaction carried out at 190 °C for 10 min.
temperature X-ray diffraction (XRD) pattern for the samples synthesized by reaction at 190 °C carried out for 10 min using the Mn-precursor of manganese chloride tetrahydrate in the presence of the early grown Ag2S nanocrystals (see Figure S1 in the Supporting Information). Typically, MnS is indexed to the zincblende (ZB) phase (JCPDS File No. 89-4953) with a small proportion of wurtzite (Wur) form (JCPDS File No. 89-4089) based on Rietveld refinement of the XRD pattern (Figure S3 in the Supporting Information), and Ag2S is indexed to the monoclinic phase at room temperature (marked with a blue asterisk (*) in Figures S1 and S2; see JCPDS File No. 893840). The proportion of zincblende phase in the MnS nanowires can be improved from ∼75% (Figure S3 in the Supporting Information) to 95% (Figure S4 in the Supporting Information) by removing some impurities from solution. The XRD characterization suggests that the four-coordinate nonequilibrium zincblende MnS nanowires can be synthesized and well-controlled in the present route, and they are generated by the catalysis of bcc-Ag2S nanocrystals at reaction temperatures 7680
DOI: 10.1021/acs.inorgchem.7b00247 Inorg. Chem. 2017, 56, 7679−7686
Article
Inorganic Chemistry of 180−200 °C over the transition temperature from the monoclinic Ag2S phase to the bcc Ag2S phase (171−177 °C; see Figure S5 in the Supporting Information), via the solution− solid−solid (SSS) growth model recently established.11,17 Raman spectra confirmed the formation of the fourcoordinate MnS in high quality. In Figure 2, the scattering
Figure 2. Room-temperature Raman spectra of the MnS nanowires synthesized at 190 °C for 10 min. Figure 3. TEM images of the MnS nanowires and/or Ag2S−MnS heterostructured nanowires synthesized at 190 °C for 10 min with (a, b) low magnification and (c−f) high magnification from different areas (the stems of the nanowires are shown in panels (c) and (d), and panels (e) and (f) show that the ends of the nanowires are terminated with Ag2S tips.
peaks at 286 and 371 cm−1 are attributed to the transverse optical (TO) and longitudinal optical (LO) phonon modes, respectively, for the zincblende MnS nanowires. The result is consistent with the previous report on zincblende ZnS.20−22 The peak at 316 cm−1 is regarded as a disorder-induced firstorder Raman spectrum representing the density of phonon states along X−W−L, of which X, L and W are reported as the three most important high-symmetry critical points at the Brillouin Zone (BZ) boundary.21 Meanwhile, the peak at 316 cm−1 may also be assigned to the TO mode for the existence of small proportion of MnS in wurtzite form.23 With regard to the second-order Raman features, the peak at 475 cm−1 in the intermediate-frequency region can be assigned to the combination of the LA (longitudinal acoustic) and LO while the LA overtone (104 cm−1)24 is absent from our measurement. The highest mode frequency at 655 cm−1 can be ascribed to the combination of TO and LO modes.20,21,25 Figures 3a and 3b shows two TEM images of the samples taken from different areas. The samples are present as 1D MnS nanowires and/or Ag2S−MnS heteronanowires, which are composed of MnS nanowires as stems/trunks adjacent to Ag2S nanocrystals as end tips (Figures 3c−f), confirming the SSS growth of the MnS nanowires catalyzed by Ag2S. The nanowires generally are straight with a uniform diameter (Figure 3), and their diameter is centered at ∼9.5 nm, consistent with the value in the diameter distribution histogram (Figure S6 in the Supporting Information). Figure 4a shows the high-angle annular dark-field imaging in the scanning TEM (HAADF-STEM) image for the nanowires, and it is confirmed that the nanowires are straight with a narrow distribution in diameter, supporting the above characterization (see Figure 3, as well as Figure S6). Figures 4b−d show the STEM energy-dispersive X-ray spectroscopic (STEM-EDX) elemental mappings of the nanowires, revealing that both Mn and S are evenly distributed in the nanowires (Figures 4b and 4c) but Ag signal is comparatively weak in the
Figure 4. HAADF-STEM image (a) and (b−d) STEM-EDX elemental mappings of Mn (panel (b)), S (panel (c)), and Ag (panel (d)) for the nanowires in the stem part. All scale bars = 10 nm.
stems (Figure 4d). Meanwhile, the EDX spectrum of the nanowires reveals that the average Mn/S atomic ratio is 0.518:0.479 (Figure S7 in the Supporting Information), which is consistent with the stoichiometric molar ratio of MnS, while Ag is just negligible in the sample. Figure 5 shows two typical HRTEM images and their corresponding electron diffraction (ED) patterns for the straight MnS nanowires, and it is found that the MnS nanowires are crystallized in zincblende phase with single crystal nature over the catalysis of Ag2S, as seen in the terminated end (Figure S8 in the Supporting Information). In detail, there are two-dimensional (2D) atomic lattice fringes with a spacing of ∼4.02 Å that are in good agreement with the 7681
DOI: 10.1021/acs.inorgchem.7b00247 Inorg. Chem. 2017, 56, 7679−7686
Article
Inorganic Chemistry
Figure 5. (a, c) HRTEM images and (b, d) corresponding ED patterns for two typical individual straight MnS nanowires grown along the ⟨112⟩ direction.
d-spacing of {110} planes for the zincblende MnS (Figures 5a and 5c). The electron diffraction (ED) patterns, projected along the [111̅ ] axis, demonstrate a set of 6-fold symmetric spots for the {110} planes with parts exposed laterally (Figures 5b and 5d), which confirms that the straight zincblende MnS nanowires grow along the metastable ⟨112⟩ direction rather than the ⟨111⟩ direction, as commonly observed for representative four-coordinate Grimm−Sommerfeld (diamond-type) bonding solids. The growth direction of the MnS nanowires along the ⟨112⟩ direction (see Figure 5, as well as Figure S8) is also different from the solvothermal growth of metastable zincblende MnSe nanowires along the ⟨110⟩ direction, which has been reported very recently.17 Moreover, the nanowires are defect-free in most cases, which is consistent with the early investigation reported by Korgel group on the growth of GaAs and GaP nanowires.26−28 Based on their investigations, narrow diameter nanowires (
