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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05227 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018

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Chemistry of Materials

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, AL 35487, United States. ┴

Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487, United States.

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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 1dodecanethiol 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 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 by 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 But 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 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 3 ACS Paragon Plus Environment


reports on the synthesis of metallic ternary and quaternary chromium-based chalcospinel nanomaterials, such as CuCr2Se4, CuCr2S4, CuCr2Te4, CoxCu1-xCr2S4, 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 chromium-based chalcospinel ferromagnetic semiconductor - has been widely studied in the bulk because of its unique properties such as colossal magnetoresistance, magnetic field-induced 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 homogenous nanocrystalline CdCr2S4 spinels under mild synthesis conditions remains a challenge (Table S1, Supplementary 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 inert-gas-protected open system that 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.


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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 43 ), 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 the synthesis of chalcospinel nanocrystals, herein we report on the synthesis of monodisperse spinel CdCr2S4 nanocrystals via a facile and mild “seedmediated” growth process. In this solution-based approach, we use c-CdS, which has a facecentered cubic crystal structure similar to that of spinel CdCr2S4, as “seed” to react with CrCl3·6H2O in a solution mixture of 1-dodecanethiol (1-DDT) and 1-octadecene (ODE). Interestingly, CdCr2S4 is not 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. Table 1. Lattice parameters of h-CdS, c-CdS, and CdCr2S4. Stoichiometry

h-CdS

c-CdS

CdCr2S4

Crystal Phase

Wurtzite

Zinc Blende

Spinel

Crystal System

Hexagonal

Cubic

Cubic

Space Group

P63mc (186)

F43m (216)

Fd3m (227)

Lattice Parameters

a=4.15, b=4.15,

a=5.82, b=5.82,

a=10.24, b=10.24,

[Å, ˚]

c=6.74;

c=5.82;

c=10.24;



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α=90, β=90, γ=120

α=90, β=90, γ=90

α=90, β=90, γ=90

99.8

196.9

1063.4

Unit Cell Volume [Å3]

EXPERIMENTAL SECTION 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 Cdthiourea complex.39 Synthesis of CdCr2S4 nanocrystals. In a typical reaction, cubic zinc blende structure CdS (cCdS, 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 (1DDT, 2 mL). The autoclave was then placed in a furnace and heated to 355 ºC at a rate of 5 ºC·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 non-linear processing was carried out using Gatan digital micrograph version 3.9. Scanning electron microscopy (SEM) analysis was performed using a HITACHI S-4800 FESEM equipped with EDX detector. X-ray 6 ACS Paragon Plus Environment

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photoelectron spectroscopy (XPS) analysis was carried out on a Thermo Scientific K-Alpha 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 (Bruker-Biospin, Billerica, MA) and an ER 4111VT variable temperature unit (Bruker). CW simulations data processing was performed using MATLAB EasySpin program. RESULTS AND DISSCUSSION “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 ºC, 72 h).37 We conducted a series of temperature-dependent 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.



Figure 1. Schematic illustration of the reaction process for the synthesis of CdCr2S4 nanocrystals. 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 310-375 ºC for a fixed reaction time of 15 h. When the reaction temperature is below the boiling point of ODE solvent (~315 ºC), c-CdS does not react with CrCl3·6H2O, and the presence of 1-DDT results in its transformation to h-CdS. Moreover, no Crcontaining solid product is obtained, which is consistent with the results obtained from hotinjection 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 boing 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 ºC 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 Fd 3 m), 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 ºC to 375 ºC, 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 3-24 h at a fixed temperature of 355 ºC 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 8 ACS Paragon Plus Environment

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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 c-CdS 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 3h, 5h, and 7h at 355 ºC, confirming that hCdS 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 generally enhances the formation of spinel CdCr2S4. Lower reaction temperature and time results in transformation of c-CdS to h-CdS that does not lead to the formation of spinel CdCr2S4.

Figure 2. XRD patterns of as-synthesized nanocrystals prepared by the reaction of c-CdS and CrCl3·6H2O: (a) at different temperatures from 310-375 ºC for 15 h, and (b) with different reaction times from 3-24 h at 355 ºC. 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. 9 ACS Paragon Plus Environment


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To validate the above statement, we used h-CdS instead of c-CdS 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, Supplementary Information). Furthermore, we find that as 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 ºC to 325 ºC and the reaction time decreased from 72 h 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 ºC to 355 ºC involving hot injection of 1-DDT into a coordinating solvent containing c-CdS and CrCl3·6H2O. However, all attempts yielded only hCdS (Figure S2, Supplementary 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 hydrogen-bonded “cage” water molecules with chloride ions in chains.40 [Cr(H2O)4Cl2]Cl·2H2O has a melting point around 83 ºC and decomposes on further heating. Based on our thermogravimetric analysis (Figure

S3,

Supplementary

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 Δ H 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 first10 ACS Paragon Plus Environment

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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, formation of nanocrystals c-CdS is favored for small sizes (< 4.5 nm) and even for large sizes (~ 15 nm) with suitable choice of capping ligand.45-46 Pressure-induced phase transformation of h-CdS, in both bulk and nanostructured form, has been widely investigated and is found to transform to a rock salt phase.47 The phase transformation in the bulk occurs only at pressures above 2 GPa,48 but the value is not available for colloidal particles. Nevertheless, it is reasonable to assume that the phase transformation of CdS nanocrystals is also influenced by pressure. Based on the above considerations, we can conclude that under suitable temperature and pressure conditions it is thermodynamically favorable for c-CdS to be either transformed to hCdS or to react in the presence of a Cr precursor to form the desired spinel-phase CdCr2S4. The activation energy for cubic to hexagonal transformation for bulk CdS powder has been experimentally determined to be 280 kJ mol−1, with the transformation considered to be a linear growth process where the cubic-hexagonal interface moves progressively along the [111] direction of the cubic form.49-50 With decrease in particle size the activation barrier can be significantly reduced, with phase transformation from c-CdS to h-CdS nanocrystals reported to occur at temperatures as low as 200-300 °C.39 Our observation of the facile formation of CdCr2S4 by reaction of c-CdS “seed” with Cr precursor by solvothermal synthesis suggests that the activation energy for 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 8 available tetrahedral voids being occupied by Cd2+ cations and the 4 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, 8 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 can be considered starting from a stack of 8 c-CdS unit cells, with removal of 24 Cd2+ from the tetrahedral sites (3.5 and 2.5 from the A and B blocks, respectively) and incorporation of 16 Cr 3+ ions into the octahedral sites (4 in each A block). During the conversion reaction, Cr(III) ion, which favors octahedral coordination as a d3 metal ion with a lowest-lying half-filled t2g orbital, serves as a solute to enhance the movement and rearrangement of cations in the c-CdS lattice. The stabilization of the A subunits is achieved by the favorable ionic radius of Cr(III) (0.76 Å) that can fit into octahedral sites formed by Cd(II) ion vacancies inducing greater thermodynamic stability to the Cr(III) center. Further experiments are underway to understand the detailed mechanism of the conversion. It should be noted that unlike c-CdS, a facile conversion pathway is not available for h-CdS to spinel CdCr2S4. Even though the nearest-neighbor atomic coordination is the same (four) for both polymorphs, significant rearrangement of the ions is needed for conversion from h-CdS. This may explain the inability to synthesize CdCr2S4 by reaction of Cr precursor with h-CdS.


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Figure 3. Schematic illustrating the conversion mechanism of cubic zinc blende CdS to spinel CdCr2S4. The CdCr2S4 spinel unit cell, consisting of 8 formula units, can be considered as stacking of the illustrated A and B blocks. The Cr(III) ions (blue balls) occupy the octahedral sites and Cd(II) ions (pink balls) occupy the tetrahedral sites. Morphology and structural analysis. The morphology and structure of the synthesized CdCr2S4 nanocrystals have been investigated by transmission electron microscopy (TEM). As shown in Figure 4a, the “seed” c-CdS nanocrystals have a uniform size of ~5 nm. Figure 4b and 4c show TEM images of monodisperse CdCr2S4 nanocrystals with size of ~20.0 nm synthesized at two different temperatures, in good agreement with the average size of ~16.0-19.0 nm calculated using the Scherrer formula from the XRD peak widths. The size distribution histogram of the CdCr2S4 nanocrystals is presented in the Supplementary Information. (Figure S4, Supplementary Information) The size of the synthesized CdCr2S4 nanocrystals are expected to be larger than those produced using CdCl2 as precursor because of the initial size of the c-CdS “seed”.37 High-resolution TEM (HRTEM) image (Figure 4d) shows lattice fringes with measured d spacing’s of 0.382 nm and 0.329 nm, corresponding to the (220) and (311) planes of spinel CdCr2S4, respectively. The corresponding fast Fourier transform (FFT) pattern (Figure 4e) of 13 ACS Paragon Plus Environment


Figure 4d confirms a well-ordered structure and reveals the cubic close packing of face-centered cubic CdCr2S4 nanocrystals. Highly periodic spots in the FFT image indicate the collective zone axis to be [111] and can be indexed to the (220), (311), (222), (422), (511), and (440) planes of standard bulk CdCr2S4 (JCPDS #16-0506). Thus, combining Figure 4d and 4e, the orientation of face-centered cubic CdCr2S4 nanocrystals is reconstructed in Figure 4f.

Figure 4. TEM images of (a) “seed” c-CdS and synthesized CdCr2S4 nanocrystals formed by reaction at (b) 350 ºC and (c) 355 ºC. (d) HRTEM image of a CdCr2S4 nanocrystal. (e) FFT pattern extracted from the image (d); and (f) 3D reconstruction of a CdCr2S4 nanocrystal (Cd, pink; Cr, green; S, yellow) based on (d) and (e). X-ray photoelectron spectroscopy (XPS) further confirms the formation of CdCr2S4 nanocrystals (Figure 5). The survey spectrum shows the presence Cd, Cr, and S, with higher resolution spectra of Cd 3d, Cr 2p, and S 2p being acquired to determine the oxidation state of


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the constituent elements. In the Cd 3d spectrum, peaks at 405.2 and 412.0 eV are ascribed to Cd 3d5/2 and Cd 3d3/2, which can be assigned to Cd (II) with a peak splitting of 6.8 eV.51 The Cr 2p spectrum exhibits a spin-orbit separation of 9.8 eV between the Cr 2p3/2 and Cr 2p1/2 states, indicative of Cr (III).52 The Cr 2p3/2 itself is split into two peaks at 575.1 and 576.1 eV. The peak separation with binding energy difference ∆E of about 1 eV is typical of the 3d elements. Such a value of ∆E has previously been reported for other chalcogenide spinels.53 The S 2p spectrum binding energies of 161.6 and 162.8 eV, with a doublet separation of 1.2 eV, can be attributed to the presence of S2-.54 We further confirmed the composition of the nanocrystals using energy dispersive X-ray analysis (EDX), which shows that the Cd:Cr:S concentration is very close to the expected 1:2:4 ratio (Figure S5, Supplementary Information). The optical absorption characteristics of the nanocrystals dispersed in ethanol has been investigated using UV-Vis spectroscopy (Figure S6, Supplementary Information). Based on Tauc plot, the band gap is estimated to be ~2.5 eV, which is consistent with the reported bulk value.55



Figure 5. (a) XPS survey spectrum of the synthesized CdCr2S4 nanocrystals. High-resolution XPS spectra of (b) Cd 3d, (c) Cr 2p, and (d) S 2p of the CdCr2S4 nanocrystals. Magnetic properties. The magnetic properties of the synthesized CdCr2S4 nanocrystals have been measured using a Quantum Design Dynacool Physical Properties Measurement System (PPMS) equipped with a Vibrating Sample Magnetometer (VSM). We have carried out a series of zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements, as a function of temperature for different applied magnetic fields, to determine the Curie temperature (TC). The results in Figure 6a inset shows the TC, defined as the peak in dM/dT, to be ~71 K as deduced from the low-field (50 Oe) measurement, which is close to the reported bulk value of ~80 K.31 The ZFC and FC curves measured at higher magnetic fields of 1 and 5 kOe completely overlap,


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unlike those at lower fields (50 and 100 Oe) that are distinct at lower temperatures but merge together at higher temperatures. This indicates a typical characteristic of superparamagnetism originating from the nano-scale crystallite size.24 The blocking temperature deduced from low field (50 Oe) measurement is ~50 K. The magnetization of the CdCr2S4 nanocrystals has also been measured as a function of applied magnetic field at temperatures of 5 and 300 K (Figure 6b). As indicated, the CdCr2S4 nanocrystals exhibit paramagnetic behaviour at room temperature and ferromagnetic behaviour at 5 K with a very small coercivity and hysteresis. The magnetization value at 5 K is ~70.8 emu g-1 at a field of 90 kOe, which is a somewhat smaller than the bulk value of 97 emu g-1.56 This decrease is likely due to reduced dimensions of nanoscale materials, with increase in the number of surface atoms introducing surface disorder, defects, dangling bonds and vacancies at anion and/or cation sites. These surface effects will reduce spin ordering, leading to a decrease in the net magnetization.57 Additionally, nonsaturation of the magnetization curve even at high magnetic fields suggests presence of surface spin disorder resulting in spin-glass-like behaviour. Nonetheless, the magnetization values are much higher than those obtained for nanocrystals synthesized by the high-temperature solvothermal method (19 emu g-1). This is not surprising considering the improved crystallinity of the CdCr2S4 nanocrystals and reduced carbonization of organic ligands and solvent at lower synthesis temperatures.



Figure 6. (a) Zero-field-cooled (ZFC) and field-cooled (FC) magnetization (M) versus temperature (T) plots of the synthesized CdCr2S4 nanocrystals measured at different magnetic fields. Inset shows plots of the slope, dM/dT, calculated from (a). (b) Magnetization as a function of applied magnetic field measured at 5 K and 300 K. Inset displays a portion of the hysteresis loop measured at 5 K on an enlarged field scale. (c) Magnetization as a function of applied magnetic field (0 to 50 kOe) for the synthesized CdCr2S4 nanocrystals measured from 22 to 203 K. (d) Magnetic entropy changes (∆Sm) extracted from (c). The observed magnetization behavior of the CdCr2S4 nanocrystals suggests that they are suitable for probing the magnetocaloric properties. For this purpose, magnetization isotherms of the CdCr2S4 nanocrystals have been measured from 22 to 203 K in field ranges up to 50 kOe


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(during the field increase cycle) with temperature steps of 3 K, as shown in Figure 6c. The magnetic entropy change ∆Sm is then calculated using the Maxwell relation:

∆S T, H = ∂M/ ∂T dH.

(1)

The entropy changes are plotted as a function of temperature in the field range of 0-50 kOe (Figure 6d) The maximum entropy change is observed around TC (~86 K), with -∆Sm = 0.71 J kg1

K-1 at Hmax=50 kOe. Interestingly, ∆Sm spans a broad temperature range, with the full width at

half maximum (δTFWHM) approaching ~87.8 K in the magnetic field range of 0 to 50 kOe. The refrigerant capacity (RC) based on the magnetic entropy change has a reasonably high value of 58.1 J kg-1 at 50 kOe, as calculated from $

RC = − $ % ∆ !∆" #!, &

(2)

where T1 and T2 are the temperatures at the two ends of δTFWHM.58 For comparison, the reported value for 10 nm Pr0.5Ca0.5MnO3 nanoparticles with a similar TC (~83 K) is only 7.01 J kg-1.59 This makes the CdCr2S4 nanocrystals attractive for magnetic refrigeration applications in the liquid nitrogen temperature range.



Figure 7. (a) The differential EPR spectra of synthesized CdCr2S4 nanocrystals from 20 to 150 K. (b) EPR spectrum at 90 K fitted by a convolution of Gaussian and Lorentzian line shapes. (c) Plot of the extracted g value as a function of temperature. (d) Double integral of CW EPR spectra characterizing the spin susceptibility of the CdCr2S4 nanocrystals. To further investigate the dynamic magnetic properties of CdCr2S4 nanocrystals, electron paramagnetic resonance (EPR) spectroscopy has been used to probe the coupling interactions. The EPR signal is produced by the localized 3d electrons of Cr3+ ions. Figure 7a shows the EPR spectra at temperatures from 20 to 150 K, with steps of 5 K in the Tc region and 10 K at higher and lower temperatures. The resonance field shifts to lower values with decreasing temperature.


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This is evidence of ferromagnetism in the sample. At the Curie temperature, around ~80K, the first-derivative EPR signal is sharp and most intense. As the temperature decreases below Tc, the first-derivative scans become weaker as they broaden, even though the total integrated EPR absorption signal increases. For temperatures above Tc, the scans become shaper but less intense. The EPR spectra are fitted well by a convolution of Lorentzian and Gaussian line shapes below and around Tc. The fitting at Tc is shown in Figure 7b. However, at temperatures above Tc, the spectra exhibit simple Lorentzian line shapes instead of a convolution of line shapes, indicating rapid spin relaxation and rapidly fluctuating internal fields above Tc. The g values obtained from fitting with the equation '=

() *+ "

,

(3)

which are close to 2.0 above Tc. The center of the EPR signal shifts to lower fields as the temperature decreases and the determined apparent g value increases, as shown in Figure 7c. The internal ferromagnetic exchange field adds to the applied external magnetic field, H, thus the EPR signal appears at lower values of applied field as the sample is magnetized. The temperature dependence of the double integral of the EPR spectra is shown in Figure 7d. The double integral is proportional to the spin susceptibility of the CdCr2S4 nanocrystals. The spin susceptibility exhibits ferromagnetic behavior, increasing much faster than 1/T as the temperature is lowered through Tc. The linewidths at the temperatures from the fitting are shown in the Supporting Information (Figure S7, Supplementary Information).



CONCLUSION In summary, monodisperse semiconducting ferromagnetic spinel CdCr2S4 nanocrystals have been synthesized via a facile and mild “seed-mediated” growth procedure under solvothermal conditions by reaction of cubic zinc blende structured CdS with CrCl3·6H2O in a solution mixture of 1-dodecanethiol and 1-octadecene. A series of temperaturedependent and time-dependent experiments have been conducted to investigate the formation mechanism of the CdCr2S4 nanocrystals. The zinc blende structured CdS, which has a similar face-centred cubic crystal structure as spinel CdCr2S4, is energetically favoured as “seed” to form cubic spinel CdCr2S4. In contrast, the more stable hexagonal wurtzite structured CdS is ineffective for the reaction and leads to the formation of undesired products. Use of high pressure during synthesis aids in the selectivity for reaction of cubic CdS with the Cr precursor to yield the spinel CdCr2S4 phase. The synthesized monodisperse CdCr2S4 nanocrystals are characterized by XRD, XPS, TEM, EDS, PPMS VSM, and CW EPR. As a ferromagnetic semiconductor, the unique nanosize-dependent magnetic properties make CdCr2S4 attractive for potential applications in spintronics and magnetic refrigeration applications at liquid nitrogen temperatures. This work also offers a novel strategy and guiding reaction mechanism for the synthesis of a variety of other magnetic chalcogenides for different applications.


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ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org: list of publications describing different routes for the synthesis of CdCr2S4, XRD results, TGA curves, EDX spectrum and elemental composition, UV-Vis spectrum, and fitting parameters for CW EPR spectra. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L. Shen) *E-mail: [email protected] (A. Gupta) ORCID Chao Pang: 0000-0002-1633-7009 Amit Vikram Singh: 0000-0002-3687-9171 Michael K. Bowman: 0000-0003-3464-9409 Arunava Gupta: 0000-0002-1785-7209 Notes The authors declare no competing financial interests. Acknowledgements This research was supported by the US National Science Foundation under Grant No. CHE1508259 the Natural Science Foundation of China (No. 51772150), the Natural Science



Foundation of Jiangsu Province (BK20160093), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors are grateful to Dr. Ziyou Zhou and Dr. Qiubo Zhang for help with the TEM analysis. As a joint Ph.D. student, Chao Pang was partially supported by the China Scholarship Council.


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