Article pubs.acs.org/cm
Cite This: Chem. Mater. 2018, 30, 1701−1709
Synthesis, Formation Mechanism, and Magnetic Properties of Monodisperse Semiconducting Spinel CdCr2S4 Nanocrystals via a Facile “Seed-Mediated” Growth Method Chao Pang,†,‡ Ling Gao,† Amit Vikram Singh,‡ Hanjiao Chen,§ Michael K. Bowman,§ Ningzhong Bao,† Liming Shen,*,† and Arunava Gupta*,‡ †
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing, Jiangsu 210009, P. R. China ‡ Center for Materials for Information Technology (MINT), The University of Alabama, Tuscaloosa, Alabama 35487, United States § Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States S Supporting Information *
ABSTRACT: The chalcogenide spinel CdCr2S4 is a well-established ferromagnetic semiconductor that exhibits unique properties and is a promising candidate for spintronic applications. With band gap in the visible wavelength region, CdCr2S4 nanocrystals offer the exciting possibility of tailoring both the optical and magnetic properties with precise morphology control. However, the synthesis of monodisperse CdCr2S4 nanocrystals is challenging and has not been reported. A unique “seed-mediated” growth process has been developed for the synthesis that involves using cubic-phase CdS, which has a face-centered cubic crystal structure similar to that of CdCr2S4, as a “seed” to react with CrCl3·6H2O in a solution mixture of 1-dodecanethiol and 1-octadecene. Remarkably, hexagonal-phase CdS is ineffective as a “seed” for formation of the desired product. A mechanism for formation of monodisperse CdCr2S4 nanocrystals by the selective growth process is proposed, and the structural and magnetic properties of the synthesized nanocrystals are presented. The novel synthetic strategy can be exploited for the controlled formation of other complex magnetic chalcogenides.
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INTRODUCTION Magnetic spinels with the general formula AB2X4 (A, B = metal, X = oxygen, chalcogen) have attracted broad interest because of their diverse properties with applications in electronics, magnetism, catalysis, and electrochemical technologies.1−4 They are traditionally synthesized through solid-state methods, chemical transport reaction, vapor deposition, electrochemical deposition, or chemical bath deposition. These synthesis procedures usually result in the formation of large particles or films with irregular shape and with broad size distribution that influence their physicochemical properties.5−9 During the past two decades, much effort has been devoted to developing chemical synthesis of colloidal magnetic spinel nanostructures with tunable shapes and sizes.10−14 There have also been numerous studies related to understanding the formation mechanism to help in controlling the morphology of the nanostructures and developing new synthesis routes. Most of these investigations have focused on magnetic spinel oxide nanomaterials.15−17 However, studies of magnetic spinel chalcogenide nanomaterials, in particular chromium-based chalcogenide spinels (chalcospinels) ACr2X4 (A = Cu, Cd, Hg, Fe, Co, etc.; X = S, Se, and Te), have been limited. This is in part because of the difficulties associated with solution-based © 2018 American Chemical Society
synthesis of the chalcospinels, which are composed of transition metal elements that commonly exist in variable oxidation states with limited source of suitable precursors.18,19 In bulk form, the chromium-based chalcospinels exhibit unusual magnetic properties resulting from exchange and superexchange interactions between the Cr3+ ions having a halffilled t2g ground state (S = 3/2) with no charge and orbital degrees of freedom.20,21 Moreover, depending on the nature of A-site cation(s), they display metallic, semiconducting, or insulating characteristics resulting from strong electronic correlation combined with a strong coupling of the structural and electronic degrees of freedom.21−23 In recent years, there have been a few reports on the synthesis of metallic ternary and quaternary chromium-based chalcospinel nanomaterials, such as CuCr2Se4, CuCr2S4, CuCr2Te 4, CoxCu1−xCr2S 4, and CuCr2S4‑xSex, with tunable size, shape, and magnetic properties.24−27 However, chromium-based chalcospinel nanomaterials exhibiting both ferromagnetic and semiconducting characteristics remain largely unexplored. CdCr2S4, a prototypical Received: December 18, 2017 Revised: February 5, 2018 Published: February 6, 2018 1701
DOI: 10.1021/acs.chemmater.7b05227 Chem. Mater. 2018, 30, 1701−1709
Article
Chemistry of Materials
formed when hexagonal-phase CdS is used as “seed”. We propose a mechanism for the formation of CdCr2S4 by the unique seed-mediated process and provide details regarding the structural and magnetic properties of the nanocrystals.
chromium-based chalcospinel ferromagnetic semiconductor, has been widely studied in the bulk because of its unique properties such as colossal magnetoresistance, magnetic fieldinduced structural transformation, spin and orbital frustration, and relaxor ferroelectricity coupled with colossal magnetocapacitive coupling.28−31 Of particular interest are colossal magnetocapacitance and magnetoresistance effects, which makes CdCr2S4 an interesting candidate for spintronic applications and also a potential multiferroic material.32 Besides, the discovery of large magnetic entropy change over a wide temperature span suggests that CdCr2S4 is a promising material for magnetic refrigeration technology.33 Because of its semiconducting characteristics, with band gap in the visible wavelength range, CdCr2S4 in nanocrystalline form offers the exciting possibility of tailoring both the optical and magnetic properties with precise shape and size control, which is promising for optical magnetic field sensors.22 However, preparing homogeneous nanocrystalline CdCr2S4 spinels under mild synthesis conditions remains a challenge (Table S1, Supporting Information). We have previously reported on the colloidal syntheses of different semiconducting ternary (CuCr2S(Se)4, CuSbS2, and CuInS2) and quaternary (CulnxGa1−xS(Se)2 and Cu2ZnSnS4) chalcogenide nanocrystals with various shapes and sizes.24−27,34−36 These syntheses are carried out in an inertgas-protected open system and involve thermal decomposition and reaction of suitable metal and chalcogenide precursors. Despite considerable efforts, we have been unsuccessful in extending this hot-injection method to the synthesis of CdCr2S4 nanocrystals. However, we recently succeeded in synthesizing CdCr2S4 nanocrystals using a high-temperature solvothermal method,37 but this reaction requires elevated temperature and prolonged reaction time. We observed that both the hot-injection method and low reaction temperature conditions in the solvothermal process overwhelmingly favor formation of hexagonal CdS instead of the desired spinel CdCr2S4 phase. The two common crystalline phases of CdS are hexagonal wurtzite (h-CdS; space group P63mc) and cubic zinc blende (c-CdS; space group F4̅3m), with the hexagonal phase being thermodynamically more stable than the cubic variant.38 Based on our earlier work on phase-controlled synthesis of CdS nanocrystals,39 we envisioned that the crystal structure may play an important role in the formation of spinel CdCr2S4 nanocrystals (Table 1). To confirm this hypothesis and advance
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Chemicals. All chemicals were used as received. 1-Dodecanethiol (98.0%), 1-octadecene (95.0%), and octadecylamine (80−90%) were purchased from Aladdin Industrial Corporation. Chromium chloride hexahydrate (CrCl3·6H2O, ≥99%) was obtained from Alfa Aesar. Synthesis of c-CdS and h-CdS followed the procedure reported by Bao et al. from thermolysis of Cd-thiourea complex.39 Synthesis of CdCr2S4 Nanocrystals. In a typical synthesis under optimal conditions, cubic zinc blende structure CdS (c-CdS, 36 mg, 0.25 mmol) and chromium chloride hexahydrate (CrCl3·6H2O, 134 mg, 0.5 mmol) were mixed in an autoclave (20 mL) with 1-octadecene (ODE, 12 mL) and 1-dodecanethiol (1-DDT, 2 mL). The autoclave was then placed in a furnace and heated to 355 °C at a rate of 5 °C· min−1 and maintained for 15 h. The nanocrystals were separated from the reaction mixture by precipitation using a mixture of hexane (15 mL) and ethanol (5 mL). The black precipitate was isolated via centrifugation (7000 rpm) and washed three times before drying. Characterization Methods. XRD patterns were recorded on a Rigaku-Smart Lab Advance system equipped with Cu Kα radiation source operated as a rotating anode at 40 kV and 100 mA. Transmission electron microscopy (TEM) analysis was performed using a JEOL JEM-2010 UHR and an aberration-corrected TEM (Titan 80-300). TEM image nonlinear processing was carried out using Gatan digital micrograph version 3.9. Scanning electron microscopy (SEM) analysis was performed using a HITACHI S4800 FESEM equipped with EDX detector. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo Scientific KAlpha spectrometer. Magnetic measurements were performed using a Quantum Design Dynacool Physical Properties Measurement System (PPMS) equipped with a Vibrating Sample Magnetometer (VSM). Continuous wave (CW) EPR spectra were recorded in the temperature range of 20−150 K on a Bruker ELEXYS E540 X-band spectrometer equipped with an ER 4102 ST resonator (BrukerBiospin, Billerica, MA) and an ER 4111VT variable temperature unit (Bruker). CW simulation data processing was performed using MATLAB EasySpin program.
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RESULTS AND DISCUSSION “Seed-Mediated” Growth. The reaction process for the synthesis of CdCr2S4 nanocrystals is schematically illustrated in Figure 1. We established that the use of c-CdS as cadmium source is essential for synthesizing the desired spinel phase compound using the seed-mediated growth process. This procedure also reduces the required formation temperature and reaction time as compared to using CdCl2 as the precursor (375 °C, 72 h).37 We conducted a series of temperaturedependent and time-dependent experiments using the “seed” process to investigate the formation mechanism of CdCr2S4 nanocrystals. The phase purity and crystallinity of the products are established by X-ray powder diffraction (XRD) measurements. XRD patterns of the CdCr2S4 and impurity products, together with standard diffraction patterns of bulk CdCr2S4, cubic zinc blende, and hexagonal wurtzite CdS, are shown in Figure 2. Figure 2a shows XRD patterns of products obtained from reaction of c-CdS and CrCl3·6H2O at different temperatures from 310 to 375 °C for a fixed reaction time of 15 h. When the reaction temperature is below the boiling point of ODE solvent (∼315 °C), c-CdS does not react with CrCl3· 6H2O, and the presence of 1-DDT results in its transformation to h-CdS. Moreover, no Cr-containing solid product is
Table 1. Lattice Parameters of h-CdS, c-CdS, and CdCr2S4 Stoichiometry
h-CdS
c-CdS
CdCr2S4
crystal phase crystal system space group lattice parameters [Å, deg] unit cell volume [Å3]
wurtzite hexagonal P63mc (186) a = 4.15, b = 4.15, c = 6.74; α = 90, β = 90, γ = 120 99.8
zinc blende cubic F4̅3m (216) a = 5.82, b = 5.82, c = 5.82; α = 90, β = 90, γ = 90 196.9
spinel cubic Fd3̅m (227) a = 10.24, b = 10.24, c = 10.24; α = 90, β = 90, γ = 90 1063.4
EXPERIMENTAL SECTION
the synthesis of chalcospinel nanocrystals, herein we report on the synthesis of monodisperse spinel CdCr2S4 nanocrystals via a facile “seed-mediated” growth process. In this solution-based approach, we use c-CdS, which has a face-centered cubic crystal structure similar to that of spinel CdCr2S4, as “seed” to react with CrCl3·6H2O in a solution mixture of 1-dodecanethiol (1DDT) and 1-octadecene (ODE). Interestingly, CdCr2S4 is not 1702
DOI: 10.1021/acs.chemmater.7b05227 Chem. Mater. 2018, 30, 1701−1709
Article
Chemistry of Materials
Figure 1. Schematic illustration of the reaction process for the synthesis of CdCr2S4 nanocrystals.
Figure 2. XRD patterns of as-synthesized nanocrystals prepared by the reaction of c-CdS and CrCl3·6H2O: (a) at different temperatures from 310 to 375 °C for 15 h, and (b) with different reaction times from 3 to 24 h at 355 °C. The chromium sulfide and chromium oxide impurity phases are indexed by the symbols # and *, respectively. Standard XRD patterns for bulk CdCr2S4 (JCPDS #16−0506), h-CdS (JCPDS #75−1545), and c-CdS (JCPDS #10−0454) are also provided.
355 °C are shown in Figure 2b. As can be seen, some h-CdS is formed along with CdCr2S4 for reaction times of 3−9 h, but its concentration decreases for longer reaction times. The appearance of h-CdS for shorter reaction times is associated with the cool down procedure as mentioned above. Since the pressure inside the autoclave remains high when taken out from the furnace, it cannot be opened quickly to retrieve the products for characterization. During this period, which usually takes several hours, some unreacted c-CdS converts to h-CdS. Based on the XRD results, we estimate that it takes several hours to reach vapor−liquid equilibrium inside the autoclave. With subsequent increase of temperature and pressure the cCdS gradually reacts with CrCl3·6H2O and 1-DDT to form CdCr2S4 nanocrystals. No significant differences in the peak intensities are observed for the XRD results of 3, 5, and 7 h at 355 °C, confirming that h-CdS is not actually produced during the reaction at this temperature. When the reaction time is more than 12 h, the h-CdS phase is completely absent and phase-pure spinel CdCr2S4 is obtained. With increase in the reaction time to 18 h and above, the presence of a small amount of chromium oxide impurity phase is observed. To summarize the findings, higher temperature and pressure enhance the formation of spinel CdCr2S4. However, h-CdS is ineffective for the formation of spinel CdCr2S4. To validate the above statement, we used h-CdS instead of cCdS to react with CrCl3·6H2O and 1-DDT under the same conditions. The results show that h-CdS is ineffective for the reaction, with no evidence of any reaction (Figure S1, Supporting Information). Furthermore, we find that as
obtained, which is consistent with the results obtained from hot-injection and low-temperature solvothermal methods.37 With increasing reaction temperature, the spinel phase CdCr2S4 gradually begins to appear and eventually dominates in comparison with conversion to the h-CdS polymorph. Since the solvent has a high boiling point, it takes a sufficiently long time during the heating cycle to achieve vapor−liquid equilibrium inside the autoclave and subsequently attain the set temperature, particularly for reaction temperatures below 350 °C. Thus, below the critical reaction condition for the formation of CdCr2S4 nanocrystals, the lower temperature inside the autoclave leaves most of the c-CdS to be unreacted. After removal from the furnace, the cooling step takes several hours to release the pressure inside the autoclave before it can be safely opened. During the time that the pressure decreases, the unreacted c-CdS can be converted to form h-CdS. We found the reaction temperature range of 350−355 °C to be optimal for the formation of phase-pure spinel CdCr2S4, with all the diffraction peaks indexed with bulk spinel phase CdCr2S4 (face-centered cubic lattice; space group Fd3̅m), matching very well with the standard JCPDS pattern #16−0506. The calculated unit cell lattice parameter is 10.27 ± 0.15 Å, which is close to the reported bulk value of 10.24 Å.31 With increasing the reaction temperature from 355 to 375 °C, small amounts of chromium sulfide and chromium oxide impurities are also observed, which can be attributed to thermolysis of unreacted Cr-complex in the solution. A series of XRD patterns of products prepared by reaction of c-CdS and CrCl3·6H2O at different reaction times from 3 to 24 h at a fixed temperature of 1703
DOI: 10.1021/acs.chemmater.7b05227 Chem. Mater. 2018, 30, 1701−1709
Article
Chemistry of Materials
this reaction is lower than that for transformation to h-CdS. We believe that the similar face-centered cubic lattice of c-CdS and CdCr2S4 renders the reaction to be kinetically favored. Formation Mechanism. The zinc blende unit cell of c-CdS (a = 5.82 Å), consisting of four formula units, can be viewed as a face-centered cubic lattice of S2− anions with half of the eight available tetrahedral voids being occupied by Cd2+ cations, and the four octahedral voids remain vacant. Similarly, the normal spinel CdCr2S4 (a = 10.24 Å, which is close to double the lattice parameter of c-CdS) consists of eight formula units and can be considered as a face-centered cubic lattice of S2−, with the Cd2+ ions occupying 1/8 of the 64 available tetrahedral voids, while the Cr3+ occupy half of the 32 octahedral voids. The cubic unit cell thus contains 32 S2− anions, eight Cd2+ cations, and 16 Cr3+cations, with the basic structural component being a S2− bonded to one Cd2+ and three Cr3+. As shown in Figure 3, the formation of a unit cell of CdCr2S4
compared to the high-temperature solvothermal method using CdCl2 as cadmium source, the spinel CdCr2S4 formation temperature using c-CdS seed-mediated growth process is reduced from 375 to as low as 325 °C, and the reaction time decreased from 72 to 15 h.37 Such reaction temperatures can also be attained for colloidal hot-injection synthesis, so we carried out a series of reactions from 325 to 355 °C involving hot injection of 1-DDT into a coordinating solvent containing c-CdS and CrCl3·6H2O. However, all attempts yielded only hCdS (Figure S2, Supporting Information). This suggests that high-pressure is essential in the seed-mediated growth process to aid in the formation of spinel CdCr2S4. The coordination compound CrCl3·6H2O exists as several distinct hydrated isomers, with the most common and commercially purchased variant being [Cr(H2O)4Cl2]Cl·2H2O, which has hydrogenbonded “cage” water molecules with chloride ions in chains.40 [Cr(H2O)4Cl2]Cl·2H2O has a melting point around 83 °C and decomposes on further heating. Based on our thermogravimetric analysis (Figure S3, Supporting Information) and other reported literature results,41 [Cr(H2O)4Cl2]Cl·2H2O decomposes with the evolution of hydrogen chloride gas, which also helps in increasing the pressure during the solvothermal reaction. The final pyrolysis product is Cr2O3, and this may explain its presence as an impurity phase after an extended reaction period. Thermodynamic and Kinetic Analysis. The standard enthalpy of formation of bulk spinel CdCr2S4 has been estimated based on empirical equations to be −513.0 kJ mol−1.42 In comparison, the experimentally determined value of ΔHθf (298 K) for h-CdS is −161.9 kJ mol−1,43 but comparable thermodynamic data for c-CdS is not available in the literature. Nevertheless, based on first-principles calculations, h-CdS is predicted to be thermodynamically more stable, but the energy difference between h-CdS and c-CdS is only about 1.1 meV/ atom (∼105 J/mol).44 With decreasing size, the relative stability of the two phases will increasingly be dominated by surface energy, which is also influenced by the particle shape. Indeed, the formation of nanocrystalline c-CdS is favored for small sizes (
