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Preparation by a facile method and characterization of amorphous and crystalline nickel sulfide nanophases
Journal of Alloys and Compounds 582 (2014) 447–456
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Preparation by a facile method and characterization of amorphous and crystalline nickel sulfide nanophases S. Nagaveena ⇑, C.K. Mahadevan Physics Research Centre, S.T. Hindu College, M.S. University, Nagercoil 629 002, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 17 March 2013 Received in revised form 2 August 2013 Accepted 5 August 2013 Available online 21 August 2013 Keywords: Nickel sulfides Microwave synthesis Optical Electrical and magnetic properties
a b s t r a c t A simple solvothermal route using a domestic microwave oven has been developed to prepare the prominent nickel sulfide nanophases (amorphous NiS, and crystalline NiS1.03, b-NiS and a-NiS). The prepared nanophases have been characterized chemically, structurally, optically, electrically, and magnetically by the available methods like thermogravimetric and differential thermal analyses, X-ray powder diffraction analysis, scanning electron microscopic, and transmission electron microscopic analyses, energy dispersive X-ray spectroscopic, Fourier transform-infrared spectral, UV–Vis spectral and photoluminescence spectral analyses, AC and DC electrical measurements at various temperatures in the range 40–150 °C, and vibrating sample magnetometric measurements. The average particle sizes obtained through transmission electron microscopic analysis are 15, 17, 18, 20 nm respectively for the amorphous NiS, NiS1.03, bNiS and a-NiS nanophases. Results obtained in the present study indicates that the method adopted is found to be an effective and economical one for preparing these nanophases with high purity, reduced size, homogeneity, and useful optical, electrical and magnetic properties. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured nickel sulfides have stimulated great interest because of their technological utilization such as hydrogenation catalysis, magnetic storage media, as possible transformation toughen in window glass, due to their extremely small size, large surface area, large anisotropy and perfect crystallinity [1–3]. Nickel sulfide exhibits complicated compositional, structural and magnetic phase behavior. Depending on the synthetic process, a variety of compositions can be obtained. The prominent phases exhibited by NiS are two stoichiometric phases: the low temperature rhombohedral (b-NiS millerite) and high temperature hexagonal (a-NiS) crystal structures and one non-stoichiometric phase hexagonal (NiS1.03) crystal structure [4]. The high temperature phase is antiferromagnetic and exhibits a metal-insulator transition [5,6]. At present, requirement of renewable clean energy sources is at a higher level. In this respect, it is understood that lithium ion battery technology could be the most promising approach. So, low cost combined with high energy density, better cycling stability, and non- or less toxic and more environmentally friendly materials as electrodes for lithium ion batteries has become important. Very recently, Hu et al. [7] have constructed electrodes based on Bi2S3 nanostructures and studied their electrochemical hydrogen storage behavior and the morphology-dependence of the ⇑ Corresponding author. Tel.: +91 9600242255. E-mail address: [email protected] (S. Nagaveena). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.08.031
capacity. Nickel sulfide is one of the most promising cathode materials. NiS has a high theoretical capacity of 590 mA h g1 with a good electronic conductivity [8]. Han et al. [9] have indicated that the total charge–discharge mechanism is represented as: NiS + 2Li M Ni + Li2S. Traditionally, NiS nanoparticles was prepared by high temperature solid state reaction and vapor phase reaction [10,11]. Later, elemental reaction in liquid ammonia, molecular precursor method and homogenous sulfide precipitation followed by a sulfiding procedure were used to synthesize NiS [12–15]. Meng et al. [16] have reported the synthesis of a-NiS and b-NiS nanophases by solvothermal reduction route using Teflon-lined stainless steel autoclaves by two different solvents: ethanol for a-NiS and pyridine for b-NiS, but no strictly amorphous NiS and non-stoichiometric NiS1.03. Sun [17] has reported the synthesis of NiS nanocrystals with hexagonal layered structure (a-NiS and NiS1.03 mixed) using hydrothermal method. However, they could not obtain a-NiS and NiS1.03 phases in the pure form. Idris et al. [18] have reported the synthesis of nanocrystalline a-NiS–b-NiS (mixed) powder by a hydrothermal microwave autoclave but not the individual nanophases in the pure form. Ubale and Bargal [19] have prepared and characterized (NiS)x(CdS)1x thin films deposited onto glass substrates. The NiS component was found to be b-NiS. Now, the important question is how to prepare all the four prominent nanophases of NiS (amorphous NiS and crystalline NiS1.03, b-NiS and a-NiS) in the pure form by a simple and economical method. Recently, novel solvothermal and hydrothermal
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methods have been used to prepare high quality ZnS, CdS, SnS, etc., quantum dots [20–24]. We made an attempt, in the present work, to prepare all the four nanophases in the pure form by a simple microwave assisted solvothermal route using a domestic microwave oven. The calcination temperatures were determined with the help of thermogravimetric and differential thermal analyses (TG–DTA). With this simple route, we were able to obtain the above nanophases in the pure form for the first time. We report here the results obtained.
(Ca) was also measured. The dielectric constant (er) of the pellet was determined using the relation er = Cp/Ca, where Cp is the capacitance of the pelletized sample. The AC conductivity (rac) was determined using the relation
rac ¼ eo er x tan d
ð2Þ 12
2
1
2
where eo is the permittivity of free space (8.85 10 C N m ) and x is the angular frequency (x = 2gf, f is the applied frequency). The DC resistance of the pellet was measured at various temperatures ranging from 40 to 150 °C by the conventional two-probe technique using a million megohmmeter. The DC electrical conductivity (rdc) was calculated using the relation
rdc ¼ d=ðRAÞ
ð3Þ
where R is the measured resistance, d is the thickness of the sample and A is the area of the face in contact with the electrode.
2. Experimental procedures 2.1. Synthesis Analytical Reagent (AR) grade nickel chloride hexahydrate (NiCl26H2O) and thiourea (NH2CSNH2) taken in 1:3 molar ratio were mixed and dissolved in 100 ml ethylene glycol and kept in a domestic microwave oven (IFB model number 17PG1S operated with frequency 2.45 GHz and power 800 W). Microwave irradiation was carried out until the solvent got evaporated. In this microwave synthesis, ethylene glycol and microwave irradiation play the key role for the preparation of NiS nanoparticles. Ethylene glycol acts as both reaction media and dispersion media which can effectively absorb and stabilize the surface of the particles, thus producing NiS nanoparticles with smaller size. In the reaction mechanism, first, the strong complexion between nickel chloride hexahydrate and thiourea leads to the formation of nickel–thiourea complex in the microwave synthesis, which prevents the production of a large number of free S2 in the formation of the products. Secondly, the nickel–thiourea complexes undergo thermal decomposition under microwave irradiation to produce NiS nanoparticles. The reaction can be expressed as follows. Ethylene glycol
NiCl2 6H2 O þ NH2 CSNH2 !½NiðCN2 H4 Þ2 Cl2 þ 6H2 O Stirring
Decomposition under
½NiðCN2 H4 Þ2 Cl2 þ 6H2 O ! NiS # þ 3H2 S " þ CO2 " þ 3CH4 " þ 2HCl microwave heating
" þ 4N2 " þ 4H2 O The product (in the form of colloidal precipitate) obtained was cooled to room temperature naturally and washed several times with double distilled water and then with acetone to remove the organic impurities present, if any. Finally, the product was filtered and dried in atmospheric air and collected as the yield. The thermal characteristic of the as-prepared sample was understood by carrying out thermogravimetric (TG–DTA) measurements using Netzsch STA 409C analyser at a heating rate of 10 °C/min from 30 to 900 °C in air. In order to understand the formation of various phases of NiS, the as-prepared sample was calcined at different temperatures, viz. 300, 500, and 700 °C for 1 h in air in each case and then brought to room temperature naturally. 2.2. Characterizations All the four samples including the as-prepared sample was characterized structurally, chemically, optically, electrically and magnetically. X-ray diffraction (XRD) patterns were obtained using an automated diffractometer (X’PERT PRO PANalyti0 cal) with monochromatic Cu Ka radiation (k = 1.54056 A Å). The average grain (particle) size (D) was estimated from the peak width using the Scherrer’s formula:
D ¼ Kk=ðb cos hÞ
ð1Þ
where k is the X-ray wavelength, b is the full width at half maximum of a diffraction peak, h is the Bragg angle, and K is the Scherrer’s constant (0.96). The FTIR spectra were recorded using Perkin Elmer SPECTRUM RS1 spectrophotometer. The morphology, microstructures and chemical composition were recorded by JEOL-JSM-5600LV SEM and HITACHI H-800 TEM instruments. The UV– Vis–NIR spectra were recorded using Perkin Elmer k35 spectrophotometer. Photoluminescence (PL) measurements were carried out using a Perkin Elmer LS55 model luminescence spectrometer with pulsed xenon lamp as the excitation source (excited at 320 nm). The magnetic measurements were made at room temperature by using a vibrating sample magnetometer (VSM VT-7410, Lakeshore). The samples prepared were pelletized using a hydraulic press (with a pressure of about 4 tons) and used for the electrical measurements. The flat surfaces of the cylindrical pellets were coated with good quality graphite to obtain a good conductive surface layer. A traveling microscope (Least count = 0.001 cm) was used to measure the dimensions of the pellet. Change of grain size due to agglomeration on pelletising the sample was not considered. The capacitance (Cp) and dielectric loss factor (tan d) were measured at various temperatures ranging from 40 to 150 °C using an LCR meter (Agilent 4284 A) by the parallel plate capacitor method with five different frequencies, viz. 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz. The sample was annealed in the holder assembly at 160 °C before making observations. The observations were made while cooling the sample. Temperature was controlled to an accuracy of ±0.5 °C. Air capacitance
3. Results and discussion 3.1. Structure, morphology and phase purity The prepared nanophases can be represented as amorphous NiS (the as-prepared), NiS1.03 (calcined at 300 °C), b-NiS (calcined at 500 °C) and a-NiS (calcined at 700 °C). The as-prepared and calcined samples were found to be black in color. The TG–DTA curve (Fig. 1) indicates that the sample becomes thermally more stable beyond 270 °C, and a total weight loss of 26.15% within 30–870 °C. The major weight loss can be attributed to the desorption of physically adsorbed water molecules which is also evidenced by the exothermic peak observed in the DTA curve. The small exothermic peaks can be attributed to the crystal structure change with increasing temperature. The X-ray diffraction (XRD) patterns observed are shown in Fig. 2a. From the XRD pattern of the as-prepared sample (Amorphous NiS), it can be noted that exactly no peak is clearly detected which reveals that the as-prepared sample is mostly in amorphous state with high level of disorder. This amorphous state is considered to be metastable and can be transformed to a more stable crystalline state by calcining the sample at higher temperatures. The XRD patterns of the samples calcined at 300, 500, and 700 °C show respectively the formation of nanocrystalline NiS1.03, b-NiS and a-NiS phases. The patterns match well with those available in the literature (JCPDS Card Nos.: 2-1273, 12-41 and 2-1280 respectively). No peak of other phases or impurities was detected above the equipment limit. The lattice parameters and the particle sizes obtained using the Scherrer formula are provided in Table 1. The Fourier transform infrared (FTIR) spectra observed are shown in Fig. 2b. The strong absorption bands observed at <450 cm1 could be due to Ni–S vibrational modes. It can be noted that exactly no other peak is clearly detected which reveals the absence of organic impurities above the equipment limit. Absence of peak related to adsorbed water molecules is expected in the case of crystalline nanoparticles as these were obtained by annealing the as-prepared sample at higher temperatures. Presence of adsorbed water molecules (evidenced by thermogravimetric analysis) is not indicated clearly by the FTIR spectrum for the as-prepared sample. This may lead to an understanding that the adsorbed water molecules present may not be sufficient enough to exhibit a peak at significant level. Salavati-Niasari et al. [25] have reported the preparation of a-NiS nanoparticles of average size 20 nm from bis(salicylidene)nickel(II) by a chemical process in oleylamine. The FTIR spectrum of this showed two weak stretch vibrations at 2920 and 2885 cm1 (attributed to the C–H stretching modes of the oleylamine carbon chain) which indicated that oleylamine molecules were observed on the surface of nickel sulfide nanoparticles. Thus, it can be understood that the nanoparticles (all the four phases) prepared in the present study are of high purity. The energy dispersive X-ray (EDX) spectral analysis of samples (see Fig. 3A for the EDX spectra and Table 1 for the chemical compositions) indicate the presence of Ni and S elements alone (in
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Fig. 1. TG–DTA curve for the as-prepared NiS sample.
Fig. 2. (a) The XRD patterns and (b) the FTIR spectra for amorphous NiS, NiS1.03, b-NiS and a-NiS nanoparticles.
Table 1 The observed lattice parameters, particle sizes and chemical compositions. System
Lattice parameters from Present study
Amorphous NiS NiS1.03 b-NiS a-NiS
Particle from
Sizes (nm)
Chemical composition (at.%)
JCPDS files
a (Å)
c (Å)
a (Å)
c (Å)
XRD
TEM
Ni
S
Ni:S ratio
_ 3.425 9.619 3.411
_ 5.328 3.154 5.315
_ 3.438 9.616 3.438
_ 5.348 3.152 5.348
_ 17.8 18.6 20.7
15 17 18 20
49.25 49.31 49.85 49.95
50.75 50.69 50.15 50.05
1:1.030 1:1.028 1:1.006 1:1.002
approximately 1:1 ratio) and no other impurities in the samples. The Ni:S ratios (see Table 1) for the as-prepared and calcined at 300 °C samples are 1:1.030 and 1:1.028 respectively. This indicates that these two samples may be considered as NiS1.03 in the amorphous and crystalline states. Thus the samples prepared are of high phase purity. The scanning electron microscopic (SEM) images (see Fig. 3B) shows that the nanoparticles formed are of uniformly spherical shaped, may be caused by the interface action between
ethylene glycol and the precursors (reactants). It is believed that ethylene glycol (extensively used in solvothermal methods), due to its good capability of controlling nucleation and growth [26], has played a critical role in the formation of NiS nanoparticles. The transmission electron microscopic (TEM) images (Fig. 4) shows that the nanoparticles (all the four phases) prepared are agglomerative and spherical in shape. In addition, the morphology is observed to be homogeneous. The average particle sizes
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Fig. 3. (A) The EDX spectra and (B) the SEM images for (a) amorphous NiS, (b) NiS1.03, (c) b-NiS and (d) a-NiS nanoparticles.
obtained (see Table 1) are well comparable to that obtained from XRD data estimated by using the Scherrer formula (see Table 1, no value for as-prepared NiS as it is amorphous). The small difference observed may be attributed to the difference in the method of estimation. 3.2. Optical properties The optical absorption spectra observed (for the samples dispersed in distilled water) are shown in Fig. 5a. The optical absorption is found to increase with the increase in calcination temperature. The spectra reveal a peak centered at around 440 nm. This can be attributed to the metal–ligand charge transfer which is a dp–p transition. The optical bandgap energy (Eg) values estimated from the absorption edges are given in Table 2. This indicates that the UV–Vis absorption peaks are blue – shifted compared to that of bulk NiS (Eg = 2.1 eV) [27] as well as exhibiting the grain size dependence. The decrement of particle size which increased the optical bandgap energy of the nanoparticles indicates the presence of quantum confinement effect, consistent with the theoretical argument made by Brus [28]. In addition, the steep absorption edges observed indicate the uniform particle size and morphology with fairly good crystallinity [29]. Panday [30] has observed that the UV–Vis absorption peaks are blue- shifted compared to that of bulk NiS. Liu et al. [31] have reported an Eg value of 5.56 eV for NiS coated onto the P(St-co-AM) assigned to the optical transition of the first excited state of the NiS nanoparticles. Panday [30] has explained, as the size of the as-pre-
pared NiS rods is considerably larger than the excitonic Bohr radius, that their as-prepared samples having their edge tip exhibit quantum confinement. In semiconductors the excitons are weakly bound Mott-Wannier excitons, for which the electron–hole (e–h) distance is large in comparison with the lattice constant. The Mott-Wannier exciton resembles a hydrogen atom and is characterized by the exciton Bohr radius (aB) and the exciton Rydberg energy (Ry ). Absolute values of aB for the common semiconductors range in the interval 1–10 nm and the Ry takes values of approximately 1–100 meV [32]. The photoluminescence (PL) spectra observed at room temperature at an excitation wavelength of 320 nm are shown in Fig. 5b. The PL spectra show two major emission peaks centered at 324 nm (UV) and 652 nm (red) and a minor emission peak centered at 384 nm (UV). The result indicates that the spectra observed for all the four nanophases are with similarly positioned peaks but with different peak intensities. The peak intensity (the yield) increases with the increase in calcination temperature, i.e., the yield is maximum for a-NiS and minimum for the amorphous phase. Dong et al. [33] have observed similar spectral pattern for the NiS phase in the petal and sphere like microcrystals. They have observed ultraviolet emission peaks centered at 316 and 386 nm. These two peaks are in agreement with those peaks observed in the present study for all the four nanophases centered at 324 and 384 nm. Pan et al. [34] have observed red emission centered at 648 nm for NiS nanoparticles (frequency-doubling peak). This is in agreement with the peak observed in the present work centered at 652 nm.
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Fig. 3 (continued)
Fig. 4. The TEM images for (a) amorphous NiS, (b) NiS1.03, (c) b-NiS and (d) a-NiS nanoparticles.
3.3. Electrical properties The dielectric measurements were carried out for pelletized samples and the dielectric parameters, viz. dielectric constant (er), dielectric loss factor (tan d) and AC electrical conductivity
(rac) were determined for five different frequencies, viz., 100 Hz, 1 kHz, 10 kHz, 100 kHz, and 1 MHz at various temperatures in the range 40–150 °C. The temperature dependence of these three parameters observed for all the four nanophases is shown in Fig. 6. The dielectric parameters are found to increase with the
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Fig. 5. (a) UV–Vis–NIR absorption spectra and (b) Photoluminescence spectra for Amorphous NiS, NiS1.03, b-NiS and a-NiS nanoparticles. Table 2 The observed bandgap energies and magnetic parameters. System
Eg (eV)
Hc (Oe)
Mr (emu/g)
Ms (emu/g)
Amorphous NiS NiS1.03 b-NiS a-NiS
4.52 4.35 4.01 3.81
409 413 466 820
0.06 0.09 0.13 0.16
0.08 0.19 0.33 0.41
increase in temperature. Also, these parameters increase with the increase in calcination temperature of the as-prepared (amorphous) nanophase. The er and the tan d values decrease and the rac value increases with the increase in frequency.
The dielectric behavior, as per the theory, of nanostructured materials is primarily due to different types of polarizations present in the material [35]. Nanocrystalline materials possess enormous number of interfaces, and the large number of defects present at these interfaces can cause a change of positive or negative space charge distribution. On applying an electric field these space charges move and are trapped by these defects, resulting in the formation of dipole moments (space charge polarization). Interfaces in nanostructured nickel sulfides may possess excess sulfur atoms which are equivalent to negative charges giving dipole moments. These dipoles rotate when exposed to an electric field giving a resultant dipole moment in the direction of the applied field (rotation of polarization direction) [36]. These polarizations occur up to frequencies of around 1 kHz. The high er value
Fig. 6. The dielectric parameters for (a) amorphous NiS, (b) NiS1.03, (c) b-NiS and (d) a-NiS nanoparticles.
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453
Fig. 6 (continued)
observed at low frequencies is mainly due to the space charge polarization and rotation of polarization direction [37]. As the temperature increases, more and more dipoles will be oriented [38], resulting in an increase in the value of the dipole moment. The observed increase of dielectric parameters with the increase of temperature can be explained as due to the above reason. Up to 100 kHz frequency, the ionic polarization may exist. Beyond that, only electronic polarization may exist. The observed decrease of er value with the increase of frequency can be explained as due to this. The er values obtained for the NiS nanocrystals in the present study are considered to be smaller. The low er values observed for the nanocrystals indicate that the polarization mechanism in the nanocrystals considered is mainly due to the space charge polarization. So, it can be understood that there seems the occurrence of nanoconfined states in the case of all the amorphous and crystalline nanophases considered in the present study which may substantially contribute to the electrical properties [39]. Thus, the space charge contribution plays an important role in charge transport process and polarizability in the case of all the amorphous and crystalline nanophases considered in the present study. The AC conductivity, in the present work, was estimated depending on the dielectric measurements. It could be seen that the AC conductivity obeys the empirical formula of the frequency dependence given by the AC power law:
rac ðx; TÞ ¼ Bxn
ð4Þ
where B and n are constants depending on the temperature and material and x is the angular frequency; n is dimensionless and B has the electrical conductivity units.
The exponent n was calculated at various temperatures for all the four nanoparticles prepared by plotting ln rac versus ln x according to Eq. (4) as shown in Fig. 7. The plots represent nearly straight lines with slope equal to the exponent n and intercept parts equal to ln B on vertical axis at ln x = 0. It is known that the value of n ranges between 0 and 1; when n = 0, the electrical conduction is frequency independent or DC conduction, but for 0 < n 6 1, the conduction is frequency dependent or AC conduction. In the present study, the n value lies in the range 0.75–0.83 which suggests that the conduction phenomenon in all the four nanophases studied is AC conduction which is due to hopping of charges. The DC electrical conductivity (rdc) observed for the pelletized samples at various temperatures in the range 40–150 °C are shown in Fig. 8a. The rdc value increases with the increase in temperature. Also, it increases with the increase in calcination temperature of the as-prepared (amorphous) nanophase. In addition, the rac is found to be more than rdc which is normally expected. It is generally accepted that smaller the particle size higher the lattice defects. There are reports suggesting that lattice defects form acceptor or donor like levels in the forbidden energy gap and act as trapping centres for charge carriers which affect the electrical behavior [40]. The electrical resistivity of nanocrystalline material is higher than that of both conventional coarse grained polycrystalline material and alloys. The magnitude of electrical resistivity and hence the electrical conductivity can be changed by altering the size of the electrically conducting component of the nanocrystalline material. The rdc values observed in the present study are found to be very small (i.e. the resistivities are very large). Also, the grain sizes observed for all the phases are found to be within 20 nm. When the grain size is smaller than the electron mean free path, the grain boundary scattering dominates and hence the elec-
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Fig. 6 (continued)
Fig. 7. The plots between ln x and ln rac for (a) amorphous NiS, (b) NiS1.03, (c) b-NiS and (d) a-NiS nanoparticles.
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455
Fig. 8. (a) The DC electrical conductivities (108 mho/m) and (b) the magnetization curves for amorphous NiS, NiS1.03, b-NiS and a-NiS nanoparticles.
trical resistivity is increased. The higher electrical resistivity observed in the present study indicates that the grain boundary scattering may also play an important role in the charge transport process. 3.4. Magnetic properties Fig. 8b shows the magnetization curves (M–H plots) obtained from the magnetic measurements made at room temperature by using a vibrating sample magnetometer with magnetic fields up to 20 kOe. The observed coercivity (Hc), retentivity (Mr), and saturation magnetization (Ms) values are given in Table 2. The results indicate that the magnetic parameters (Hc, Mr and Ms) increase with increase in calcination temperature. Neel [41] suggested that fine particles of an antiferromagnetic material should exhibit magnetic properties such as superparamagnetism and weak ferromagnetism. The 20 nm nanoparticle is usually a single domain state with all spins align in one direction [42,43]. Ferromagnetism is supposed to be limited to infinite magnets, because thermal excitations in finite magnets cause the net moment to fluctuate between opposite directions. However, it is difficult to distinguish the magnetism of particles larger than about 1 nm from true ferromagnetism, because interatomic exchange ensures well developed ferromagnetic correlations inside the particles [42]. The nanomagnetism strongly depends on the surfaces and interfaces. The magnetic parameters are normally influenced by the particle size and shape, dipole–dipole and exchange interactions and microstructure of the particles. The dipole–dipole and exchange interactions change the magnetic properties of magnetic materials and the effective anisotropy of the nanoparticles may be increased by these interactions. This leads to an enhancement of coercivity [44]. The particles turn from multidomain to single domain when the size is decreased and may lead to become superparamagnetic due to the small size [45]. In a single- domain cluster the atomic magnetic moments are coupled via exchange interactions to form a large net cluster moment. Thus, for a given superparamagnetic nanocluster there are rotational barriers to spin alignment arising from magnetocrystalline, magnetoelastic, and shape anisotropies [46]. The M–H plots observed in the present study show ferromagnetic behavior with reasonable hysteretic parameters. This can be explained as due to an increased concentration of Ni vacancies. Results obtained through EDX analysis indicate the presence of Ni vacancies (see Table 1). It has been shown that the magnetic prop-
erties of nanograined oxides critically depend on the presence of defects like intergranular boundaries [47,48]. Straumal et al. [48] have investigated the structure of grain boundaries in pure nanograined ZnO and found that most probably the proper combination of interpenetrating crystalline and amorphous phases is the condition for the ferromagnetism in pure ZnO. Recently, Srikrishna Ramya and Mahadevan [49] have observed superparamagnetism in the case of as-prepared (amorphous) nanophase and ferromagnetism with reasonable hysteretic parameters in the case of other nanophases (e-, b-, and a-) of Fe2O3. They have explained this magnetic behavior as due to an increased concentration of oxygen vacancies near the surface. Their nanoparticles contain both a very developed free surface as well as some grain boundaries with extremely high specific area. Anamolous magnetic properties were observed on CoO and NiO nanoparticles [50,51]. Wang et al. [52] have reported that the NiS2 nanotubes with 200 nm diameter and about 20 nm thickness of tube-wall exhibit ferromagnetic interactions and even tending to superparamagnetic property at room temperature. Zhang et al. [53] have reported that a-NiS and Ni3S4 nanocrystals are no longer antiferromagnetic, both of them show paramagnetism due to the size effect. Tang et al. [54] have explained the ferromagnetic property of flower-like a-NiS nanostructures as due to the increase of the uncompensated moments at the nanocrystal surface resulting from the reduced coordination of the surface spins. Salavathi-Niasari et al. [25] have reported that both of the a-NiS and Ni3S4 nanoparticles show paramagnetism due to size effect. Wang et al. [55] have observed that the magnetic properties of a-Fe2O3 can be changed, by controlling the morphology, from superparamagnetic behavior to ferromagnetic behavior which are different from the behavior of bulk material. Spherical nanoparticles (with average size around 60 nm) show a superparamagnetic behavior (with coercivity almost zero) and hollow microspheres show a normal (strong) ferromagnetic behavior (with coercivity around 2528 Oe) at room temperature, However, a good ferromagnetic order has been observed for nickel sulfide nanospheres in the present study which is not in agreement with that observed for nanospheres by Wang et al. [55]. Mitra et al. [56] have reported the coercivity dependence on the shape of a-Fe2O3 nanocrystals:Hc for nanocube is 330 Oe, Hc for nanospindle is 390 Oe, and Hc for nanorhombohedral is 1250 Oe. Normally, the coercivity increases with the increase of aspect ratio (shape anisotropy). The aspect ratio for spherical shaped nanocrystal is 1 (no shape anisotropy). In particles with aspect ratio higher than one, the magnetic spins are preferentially aligned along the long axes. Their reversal to the
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opposite direction requires higher energies when compared to spherical particles [57]. The high coercivities observed in the present study, at room temperature, for nickel sulfide nanophases unambiguously indicate that the observed ferromagnetism does not come from any experimental artifact, but an intrinsic property of the nanophases considered. So, increase in coercivity observed for the nickel sulfide nanophases in the present study can be explained as due to the creation of Ni vacancies, magneto-crystalline anisotropic effects due to some grain boundaries and uncompensated moments at the nanocrystal surface resulting from the reduced coordination of the surface spins. Further, we note that a clear understanding of the magnetic properties of the nickel sulfide nanophases requires additional investigations to be carried out. Magnetic material used in the storage media requires that it should have a large hysteresis loop with large coercivity which allows permanent and stable storage. Coercivity between 350 and 4000 Oe is often desirable in typical magnetic storage medias. The observed higher coercivities make the nickel sulfide nanophases considered in the present study promising candidates for these applications. 4. Conclusions The four nanophases of nickel sulfide, viz., amorphous NiS(asprepared) and NiS1.03, b-NiS, and a-NiS in the crystalline state have been successfully prepared by the proposed simple solvothermal method using a domestic microwave oven along with required calcination (300 °C for NiS1.03, 500 °C for b-NiS and 700 °C for a-NiS). The phases were identified by XRD analysis. The SEM and TEM images show the morphology and homogeneity of the clusters. EDX and FTIR spectral analyses confirm the high phase purity. The observed optical bandgap energies range from 3.81 to 4.52 eV. The PL spectral analysis indicates a maximum yield for a-NiS. Results of electrical measurements indicate that space charge contribution plays an important role in charge transport process and polarizability. The magnetic measurement indicates ferromagnetism for all the four phases considered in the present study. References [1] J.T. Sparks, T. Komoto, J. Appl. Phys. 34 (1963) 1191–1192. [2] A.M. Fernandez, M.T.S. Nair, P.K. Nair, Mater. Manuf. Process. 8 (1993) 535– 548. [3] W.M. Kriven, Mater. Sci. Eng. A 127 (1990) 249–255. [4] G. Kullerud, R.A. Yund, J. Petrol. 3 (1962) 126–131. [5] J.T. Sparks, T. Komoto, Phys. Lett. A 25 (1967) 398–399. [6] J. Trahan, R.G. Goodrich, S.F. Watkins, Phys. Rev. B 2 (1970) 2859–2863. [7] P. Hu, Y. Cao, B. Lu, Mater. Lett. 106 (2013) 297–300. [8] S.C. Han, H.S. Kim, M.S. Song, J.H. Kim, H.J. Ahn, J.Y. Lee, J. Alloys Comp. 351 (2003) 273–278. [9] S.C. Han, K.W. Kim, H.J. Ahn, J.H. Ahn, J.Y. Lee, J. Alloys Comp. 361 (2003) 247– 251. [10] R. Coustal, J. Chim. Phys. 38 (1958) 277. [11] A. Wold, K. Ddwight, J. Solid State Chem. 96 (1992) 53–58.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Preparation by a facile method and characterization of amorphous and crystalline nickel sulfide nanophases S. Nagaveena ⇑, C.K. Mahadevan Physics Research Centre, S.T. Hindu College, M.S. University, Nagercoil 629 002, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 17 March 2013 Received in revised form 2 August 2013 Accepted 5 August 2013 Available online 21 August 2013 Keywords: Nickel sulfides Microwave synthesis Optical Electrical and magnetic properties
a b s t r a c t A simple solvothermal route using a domestic microwave oven has been developed to prepare the prominent nickel sulfide nanophases (amorphous NiS, and crystalline NiS1.03, b-NiS and a-NiS). The prepared nanophases have been characterized chemically, structurally, optically, electrically, and magnetically by the available methods like thermogravimetric and differential thermal analyses, X-ray powder diffraction analysis, scanning electron microscopic, and transmission electron microscopic analyses, energy dispersive X-ray spectroscopic, Fourier transform-infrared spectral, UV–Vis spectral and photoluminescence spectral analyses, AC and DC electrical measurements at various temperatures in the range 40–150 °C, and vibrating sample magnetometric measurements. The average particle sizes obtained through transmission electron microscopic analysis are 15, 17, 18, 20 nm respectively for the amorphous NiS, NiS1.03, bNiS and a-NiS nanophases. Results obtained in the present study indicates that the method adopted is found to be an effective and economical one for preparing these nanophases with high purity, reduced size, homogeneity, and useful optical, electrical and magnetic properties. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured nickel sulfides have stimulated great interest because of their technological utilization such as hydrogenation catalysis, magnetic storage media, as possible transformation toughen in window glass, due to their extremely small size, large surface area, large anisotropy and perfect crystallinity [1–3]. Nickel sulfide exhibits complicated compositional, structural and magnetic phase behavior. Depending on the synthetic process, a variety of compositions can be obtained. The prominent phases exhibited by NiS are two stoichiometric phases: the low temperature rhombohedral (b-NiS millerite) and high temperature hexagonal (a-NiS) crystal structures and one non-stoichiometric phase hexagonal (NiS1.03) crystal structure [4]. The high temperature phase is antiferromagnetic and exhibits a metal-insulator transition [5,6]. At present, requirement of renewable clean energy sources is at a higher level. In this respect, it is understood that lithium ion battery technology could be the most promising approach. So, low cost combined with high energy density, better cycling stability, and non- or less toxic and more environmentally friendly materials as electrodes for lithium ion batteries has become important. Very recently, Hu et al. [7] have constructed electrodes based on Bi2S3 nanostructures and studied their electrochemical hydrogen storage behavior and the morphology-dependence of the ⇑ Corresponding author. Tel.: +91 9600242255. E-mail address: [email protected] (S. Nagaveena). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.08.031
capacity. Nickel sulfide is one of the most promising cathode materials. NiS has a high theoretical capacity of 590 mA h g1 with a good electronic conductivity [8]. Han et al. [9] have indicated that the total charge–discharge mechanism is represented as: NiS + 2Li M Ni + Li2S. Traditionally, NiS nanoparticles was prepared by high temperature solid state reaction and vapor phase reaction [10,11]. Later, elemental reaction in liquid ammonia, molecular precursor method and homogenous sulfide precipitation followed by a sulfiding procedure were used to synthesize NiS [12–15]. Meng et al. [16] have reported the synthesis of a-NiS and b-NiS nanophases by solvothermal reduction route using Teflon-lined stainless steel autoclaves by two different solvents: ethanol for a-NiS and pyridine for b-NiS, but no strictly amorphous NiS and non-stoichiometric NiS1.03. Sun [17] has reported the synthesis of NiS nanocrystals with hexagonal layered structure (a-NiS and NiS1.03 mixed) using hydrothermal method. However, they could not obtain a-NiS and NiS1.03 phases in the pure form. Idris et al. [18] have reported the synthesis of nanocrystalline a-NiS–b-NiS (mixed) powder by a hydrothermal microwave autoclave but not the individual nanophases in the pure form. Ubale and Bargal [19] have prepared and characterized (NiS)x(CdS)1x thin films deposited onto glass substrates. The NiS component was found to be b-NiS. Now, the important question is how to prepare all the four prominent nanophases of NiS (amorphous NiS and crystalline NiS1.03, b-NiS and a-NiS) in the pure form by a simple and economical method. Recently, novel solvothermal and hydrothermal
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methods have been used to prepare high quality ZnS, CdS, SnS, etc., quantum dots [20–24]. We made an attempt, in the present work, to prepare all the four nanophases in the pure form by a simple microwave assisted solvothermal route using a domestic microwave oven. The calcination temperatures were determined with the help of thermogravimetric and differential thermal analyses (TG–DTA). With this simple route, we were able to obtain the above nanophases in the pure form for the first time. We report here the results obtained.
(Ca) was also measured. The dielectric constant (er) of the pellet was determined using the relation er = Cp/Ca, where Cp is the capacitance of the pelletized sample. The AC conductivity (rac) was determined using the relation
rac ¼ eo er x tan d
ð2Þ 12
2
1
2
where eo is the permittivity of free space (8.85 10 C N m ) and x is the angular frequency (x = 2gf, f is the applied frequency). The DC resistance of the pellet was measured at various temperatures ranging from 40 to 150 °C by the conventional two-probe technique using a million megohmmeter. The DC electrical conductivity (rdc) was calculated using the relation
rdc ¼ d=ðRAÞ
ð3Þ
where R is the measured resistance, d is the thickness of the sample and A is the area of the face in contact with the electrode.
2. Experimental procedures 2.1. Synthesis Analytical Reagent (AR) grade nickel chloride hexahydrate (NiCl26H2O) and thiourea (NH2CSNH2) taken in 1:3 molar ratio were mixed and dissolved in 100 ml ethylene glycol and kept in a domestic microwave oven (IFB model number 17PG1S operated with frequency 2.45 GHz and power 800 W). Microwave irradiation was carried out until the solvent got evaporated. In this microwave synthesis, ethylene glycol and microwave irradiation play the key role for the preparation of NiS nanoparticles. Ethylene glycol acts as both reaction media and dispersion media which can effectively absorb and stabilize the surface of the particles, thus producing NiS nanoparticles with smaller size. In the reaction mechanism, first, the strong complexion between nickel chloride hexahydrate and thiourea leads to the formation of nickel–thiourea complex in the microwave synthesis, which prevents the production of a large number of free S2 in the formation of the products. Secondly, the nickel–thiourea complexes undergo thermal decomposition under microwave irradiation to produce NiS nanoparticles. The reaction can be expressed as follows. Ethylene glycol
NiCl2 6H2 O þ NH2 CSNH2 !½NiðCN2 H4 Þ2 Cl2 þ 6H2 O Stirring
Decomposition under
½NiðCN2 H4 Þ2 Cl2 þ 6H2 O ! NiS # þ 3H2 S " þ CO2 " þ 3CH4 " þ 2HCl microwave heating
" þ 4N2 " þ 4H2 O The product (in the form of colloidal precipitate) obtained was cooled to room temperature naturally and washed several times with double distilled water and then with acetone to remove the organic impurities present, if any. Finally, the product was filtered and dried in atmospheric air and collected as the yield. The thermal characteristic of the as-prepared sample was understood by carrying out thermogravimetric (TG–DTA) measurements using Netzsch STA 409C analyser at a heating rate of 10 °C/min from 30 to 900 °C in air. In order to understand the formation of various phases of NiS, the as-prepared sample was calcined at different temperatures, viz. 300, 500, and 700 °C for 1 h in air in each case and then brought to room temperature naturally. 2.2. Characterizations All the four samples including the as-prepared sample was characterized structurally, chemically, optically, electrically and magnetically. X-ray diffraction (XRD) patterns were obtained using an automated diffractometer (X’PERT PRO PANalyti0 cal) with monochromatic Cu Ka radiation (k = 1.54056 A Å). The average grain (particle) size (D) was estimated from the peak width using the Scherrer’s formula:
D ¼ Kk=ðb cos hÞ
ð1Þ
where k is the X-ray wavelength, b is the full width at half maximum of a diffraction peak, h is the Bragg angle, and K is the Scherrer’s constant (0.96). The FTIR spectra were recorded using Perkin Elmer SPECTRUM RS1 spectrophotometer. The morphology, microstructures and chemical composition were recorded by JEOL-JSM-5600LV SEM and HITACHI H-800 TEM instruments. The UV– Vis–NIR spectra were recorded using Perkin Elmer k35 spectrophotometer. Photoluminescence (PL) measurements were carried out using a Perkin Elmer LS55 model luminescence spectrometer with pulsed xenon lamp as the excitation source (excited at 320 nm). The magnetic measurements were made at room temperature by using a vibrating sample magnetometer (VSM VT-7410, Lakeshore). The samples prepared were pelletized using a hydraulic press (with a pressure of about 4 tons) and used for the electrical measurements. The flat surfaces of the cylindrical pellets were coated with good quality graphite to obtain a good conductive surface layer. A traveling microscope (Least count = 0.001 cm) was used to measure the dimensions of the pellet. Change of grain size due to agglomeration on pelletising the sample was not considered. The capacitance (Cp) and dielectric loss factor (tan d) were measured at various temperatures ranging from 40 to 150 °C using an LCR meter (Agilent 4284 A) by the parallel plate capacitor method with five different frequencies, viz. 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz. The sample was annealed in the holder assembly at 160 °C before making observations. The observations were made while cooling the sample. Temperature was controlled to an accuracy of ±0.5 °C. Air capacitance
3. Results and discussion 3.1. Structure, morphology and phase purity The prepared nanophases can be represented as amorphous NiS (the as-prepared), NiS1.03 (calcined at 300 °C), b-NiS (calcined at 500 °C) and a-NiS (calcined at 700 °C). The as-prepared and calcined samples were found to be black in color. The TG–DTA curve (Fig. 1) indicates that the sample becomes thermally more stable beyond 270 °C, and a total weight loss of 26.15% within 30–870 °C. The major weight loss can be attributed to the desorption of physically adsorbed water molecules which is also evidenced by the exothermic peak observed in the DTA curve. The small exothermic peaks can be attributed to the crystal structure change with increasing temperature. The X-ray diffraction (XRD) patterns observed are shown in Fig. 2a. From the XRD pattern of the as-prepared sample (Amorphous NiS), it can be noted that exactly no peak is clearly detected which reveals that the as-prepared sample is mostly in amorphous state with high level of disorder. This amorphous state is considered to be metastable and can be transformed to a more stable crystalline state by calcining the sample at higher temperatures. The XRD patterns of the samples calcined at 300, 500, and 700 °C show respectively the formation of nanocrystalline NiS1.03, b-NiS and a-NiS phases. The patterns match well with those available in the literature (JCPDS Card Nos.: 2-1273, 12-41 and 2-1280 respectively). No peak of other phases or impurities was detected above the equipment limit. The lattice parameters and the particle sizes obtained using the Scherrer formula are provided in Table 1. The Fourier transform infrared (FTIR) spectra observed are shown in Fig. 2b. The strong absorption bands observed at <450 cm1 could be due to Ni–S vibrational modes. It can be noted that exactly no other peak is clearly detected which reveals the absence of organic impurities above the equipment limit. Absence of peak related to adsorbed water molecules is expected in the case of crystalline nanoparticles as these were obtained by annealing the as-prepared sample at higher temperatures. Presence of adsorbed water molecules (evidenced by thermogravimetric analysis) is not indicated clearly by the FTIR spectrum for the as-prepared sample. This may lead to an understanding that the adsorbed water molecules present may not be sufficient enough to exhibit a peak at significant level. Salavati-Niasari et al. [25] have reported the preparation of a-NiS nanoparticles of average size 20 nm from bis(salicylidene)nickel(II) by a chemical process in oleylamine. The FTIR spectrum of this showed two weak stretch vibrations at 2920 and 2885 cm1 (attributed to the C–H stretching modes of the oleylamine carbon chain) which indicated that oleylamine molecules were observed on the surface of nickel sulfide nanoparticles. Thus, it can be understood that the nanoparticles (all the four phases) prepared in the present study are of high purity. The energy dispersive X-ray (EDX) spectral analysis of samples (see Fig. 3A for the EDX spectra and Table 1 for the chemical compositions) indicate the presence of Ni and S elements alone (in
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Fig. 1. TG–DTA curve for the as-prepared NiS sample.
Fig. 2. (a) The XRD patterns and (b) the FTIR spectra for amorphous NiS, NiS1.03, b-NiS and a-NiS nanoparticles.
Table 1 The observed lattice parameters, particle sizes and chemical compositions. System
Lattice parameters from Present study
Amorphous NiS NiS1.03 b-NiS a-NiS
Particle from
Sizes (nm)
Chemical composition (at.%)
JCPDS files
a (Å)
c (Å)
a (Å)
c (Å)
XRD
TEM
Ni
S
Ni:S ratio
_ 3.425 9.619 3.411
_ 5.328 3.154 5.315
_ 3.438 9.616 3.438
_ 5.348 3.152 5.348
_ 17.8 18.6 20.7
15 17 18 20
49.25 49.31 49.85 49.95
50.75 50.69 50.15 50.05
1:1.030 1:1.028 1:1.006 1:1.002
approximately 1:1 ratio) and no other impurities in the samples. The Ni:S ratios (see Table 1) for the as-prepared and calcined at 300 °C samples are 1:1.030 and 1:1.028 respectively. This indicates that these two samples may be considered as NiS1.03 in the amorphous and crystalline states. Thus the samples prepared are of high phase purity. The scanning electron microscopic (SEM) images (see Fig. 3B) shows that the nanoparticles formed are of uniformly spherical shaped, may be caused by the interface action between
ethylene glycol and the precursors (reactants). It is believed that ethylene glycol (extensively used in solvothermal methods), due to its good capability of controlling nucleation and growth [26], has played a critical role in the formation of NiS nanoparticles. The transmission electron microscopic (TEM) images (Fig. 4) shows that the nanoparticles (all the four phases) prepared are agglomerative and spherical in shape. In addition, the morphology is observed to be homogeneous. The average particle sizes
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Fig. 3. (A) The EDX spectra and (B) the SEM images for (a) amorphous NiS, (b) NiS1.03, (c) b-NiS and (d) a-NiS nanoparticles.
obtained (see Table 1) are well comparable to that obtained from XRD data estimated by using the Scherrer formula (see Table 1, no value for as-prepared NiS as it is amorphous). The small difference observed may be attributed to the difference in the method of estimation. 3.2. Optical properties The optical absorption spectra observed (for the samples dispersed in distilled water) are shown in Fig. 5a. The optical absorption is found to increase with the increase in calcination temperature. The spectra reveal a peak centered at around 440 nm. This can be attributed to the metal–ligand charge transfer which is a dp–p transition. The optical bandgap energy (Eg) values estimated from the absorption edges are given in Table 2. This indicates that the UV–Vis absorption peaks are blue – shifted compared to that of bulk NiS (Eg = 2.1 eV) [27] as well as exhibiting the grain size dependence. The decrement of particle size which increased the optical bandgap energy of the nanoparticles indicates the presence of quantum confinement effect, consistent with the theoretical argument made by Brus [28]. In addition, the steep absorption edges observed indicate the uniform particle size and morphology with fairly good crystallinity [29]. Panday [30] has observed that the UV–Vis absorption peaks are blue- shifted compared to that of bulk NiS. Liu et al. [31] have reported an Eg value of 5.56 eV for NiS coated onto the P(St-co-AM) assigned to the optical transition of the first excited state of the NiS nanoparticles. Panday [30] has explained, as the size of the as-pre-
pared NiS rods is considerably larger than the excitonic Bohr radius, that their as-prepared samples having their edge tip exhibit quantum confinement. In semiconductors the excitons are weakly bound Mott-Wannier excitons, for which the electron–hole (e–h) distance is large in comparison with the lattice constant. The Mott-Wannier exciton resembles a hydrogen atom and is characterized by the exciton Bohr radius (aB) and the exciton Rydberg energy (Ry ). Absolute values of aB for the common semiconductors range in the interval 1–10 nm and the Ry takes values of approximately 1–100 meV [32]. The photoluminescence (PL) spectra observed at room temperature at an excitation wavelength of 320 nm are shown in Fig. 5b. The PL spectra show two major emission peaks centered at 324 nm (UV) and 652 nm (red) and a minor emission peak centered at 384 nm (UV). The result indicates that the spectra observed for all the four nanophases are with similarly positioned peaks but with different peak intensities. The peak intensity (the yield) increases with the increase in calcination temperature, i.e., the yield is maximum for a-NiS and minimum for the amorphous phase. Dong et al. [33] have observed similar spectral pattern for the NiS phase in the petal and sphere like microcrystals. They have observed ultraviolet emission peaks centered at 316 and 386 nm. These two peaks are in agreement with those peaks observed in the present study for all the four nanophases centered at 324 and 384 nm. Pan et al. [34] have observed red emission centered at 648 nm for NiS nanoparticles (frequency-doubling peak). This is in agreement with the peak observed in the present work centered at 652 nm.
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451
Fig. 3 (continued)
Fig. 4. The TEM images for (a) amorphous NiS, (b) NiS1.03, (c) b-NiS and (d) a-NiS nanoparticles.
3.3. Electrical properties The dielectric measurements were carried out for pelletized samples and the dielectric parameters, viz. dielectric constant (er), dielectric loss factor (tan d) and AC electrical conductivity
(rac) were determined for five different frequencies, viz., 100 Hz, 1 kHz, 10 kHz, 100 kHz, and 1 MHz at various temperatures in the range 40–150 °C. The temperature dependence of these three parameters observed for all the four nanophases is shown in Fig. 6. The dielectric parameters are found to increase with the
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Fig. 5. (a) UV–Vis–NIR absorption spectra and (b) Photoluminescence spectra for Amorphous NiS, NiS1.03, b-NiS and a-NiS nanoparticles. Table 2 The observed bandgap energies and magnetic parameters. System
Eg (eV)
Hc (Oe)
Mr (emu/g)
Ms (emu/g)
Amorphous NiS NiS1.03 b-NiS a-NiS
4.52 4.35 4.01 3.81
409 413 466 820
0.06 0.09 0.13 0.16
0.08 0.19 0.33 0.41
increase in temperature. Also, these parameters increase with the increase in calcination temperature of the as-prepared (amorphous) nanophase. The er and the tan d values decrease and the rac value increases with the increase in frequency.
The dielectric behavior, as per the theory, of nanostructured materials is primarily due to different types of polarizations present in the material [35]. Nanocrystalline materials possess enormous number of interfaces, and the large number of defects present at these interfaces can cause a change of positive or negative space charge distribution. On applying an electric field these space charges move and are trapped by these defects, resulting in the formation of dipole moments (space charge polarization). Interfaces in nanostructured nickel sulfides may possess excess sulfur atoms which are equivalent to negative charges giving dipole moments. These dipoles rotate when exposed to an electric field giving a resultant dipole moment in the direction of the applied field (rotation of polarization direction) [36]. These polarizations occur up to frequencies of around 1 kHz. The high er value
Fig. 6. The dielectric parameters for (a) amorphous NiS, (b) NiS1.03, (c) b-NiS and (d) a-NiS nanoparticles.
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453
Fig. 6 (continued)
observed at low frequencies is mainly due to the space charge polarization and rotation of polarization direction [37]. As the temperature increases, more and more dipoles will be oriented [38], resulting in an increase in the value of the dipole moment. The observed increase of dielectric parameters with the increase of temperature can be explained as due to the above reason. Up to 100 kHz frequency, the ionic polarization may exist. Beyond that, only electronic polarization may exist. The observed decrease of er value with the increase of frequency can be explained as due to this. The er values obtained for the NiS nanocrystals in the present study are considered to be smaller. The low er values observed for the nanocrystals indicate that the polarization mechanism in the nanocrystals considered is mainly due to the space charge polarization. So, it can be understood that there seems the occurrence of nanoconfined states in the case of all the amorphous and crystalline nanophases considered in the present study which may substantially contribute to the electrical properties [39]. Thus, the space charge contribution plays an important role in charge transport process and polarizability in the case of all the amorphous and crystalline nanophases considered in the present study. The AC conductivity, in the present work, was estimated depending on the dielectric measurements. It could be seen that the AC conductivity obeys the empirical formula of the frequency dependence given by the AC power law:
rac ðx; TÞ ¼ Bxn
ð4Þ
where B and n are constants depending on the temperature and material and x is the angular frequency; n is dimensionless and B has the electrical conductivity units.
The exponent n was calculated at various temperatures for all the four nanoparticles prepared by plotting ln rac versus ln x according to Eq. (4) as shown in Fig. 7. The plots represent nearly straight lines with slope equal to the exponent n and intercept parts equal to ln B on vertical axis at ln x = 0. It is known that the value of n ranges between 0 and 1; when n = 0, the electrical conduction is frequency independent or DC conduction, but for 0 < n 6 1, the conduction is frequency dependent or AC conduction. In the present study, the n value lies in the range 0.75–0.83 which suggests that the conduction phenomenon in all the four nanophases studied is AC conduction which is due to hopping of charges. The DC electrical conductivity (rdc) observed for the pelletized samples at various temperatures in the range 40–150 °C are shown in Fig. 8a. The rdc value increases with the increase in temperature. Also, it increases with the increase in calcination temperature of the as-prepared (amorphous) nanophase. In addition, the rac is found to be more than rdc which is normally expected. It is generally accepted that smaller the particle size higher the lattice defects. There are reports suggesting that lattice defects form acceptor or donor like levels in the forbidden energy gap and act as trapping centres for charge carriers which affect the electrical behavior [40]. The electrical resistivity of nanocrystalline material is higher than that of both conventional coarse grained polycrystalline material and alloys. The magnitude of electrical resistivity and hence the electrical conductivity can be changed by altering the size of the electrically conducting component of the nanocrystalline material. The rdc values observed in the present study are found to be very small (i.e. the resistivities are very large). Also, the grain sizes observed for all the phases are found to be within 20 nm. When the grain size is smaller than the electron mean free path, the grain boundary scattering dominates and hence the elec-
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Fig. 6 (continued)
Fig. 7. The plots between ln x and ln rac for (a) amorphous NiS, (b) NiS1.03, (c) b-NiS and (d) a-NiS nanoparticles.
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Fig. 8. (a) The DC electrical conductivities (108 mho/m) and (b) the magnetization curves for amorphous NiS, NiS1.03, b-NiS and a-NiS nanoparticles.
trical resistivity is increased. The higher electrical resistivity observed in the present study indicates that the grain boundary scattering may also play an important role in the charge transport process. 3.4. Magnetic properties Fig. 8b shows the magnetization curves (M–H plots) obtained from the magnetic measurements made at room temperature by using a vibrating sample magnetometer with magnetic fields up to 20 kOe. The observed coercivity (Hc), retentivity (Mr), and saturation magnetization (Ms) values are given in Table 2. The results indicate that the magnetic parameters (Hc, Mr and Ms) increase with increase in calcination temperature. Neel [41] suggested that fine particles of an antiferromagnetic material should exhibit magnetic properties such as superparamagnetism and weak ferromagnetism. The 20 nm nanoparticle is usually a single domain state with all spins align in one direction [42,43]. Ferromagnetism is supposed to be limited to infinite magnets, because thermal excitations in finite magnets cause the net moment to fluctuate between opposite directions. However, it is difficult to distinguish the magnetism of particles larger than about 1 nm from true ferromagnetism, because interatomic exchange ensures well developed ferromagnetic correlations inside the particles [42]. The nanomagnetism strongly depends on the surfaces and interfaces. The magnetic parameters are normally influenced by the particle size and shape, dipole–dipole and exchange interactions and microstructure of the particles. The dipole–dipole and exchange interactions change the magnetic properties of magnetic materials and the effective anisotropy of the nanoparticles may be increased by these interactions. This leads to an enhancement of coercivity [44]. The particles turn from multidomain to single domain when the size is decreased and may lead to become superparamagnetic due to the small size [45]. In a single- domain cluster the atomic magnetic moments are coupled via exchange interactions to form a large net cluster moment. Thus, for a given superparamagnetic nanocluster there are rotational barriers to spin alignment arising from magnetocrystalline, magnetoelastic, and shape anisotropies [46]. The M–H plots observed in the present study show ferromagnetic behavior with reasonable hysteretic parameters. This can be explained as due to an increased concentration of Ni vacancies. Results obtained through EDX analysis indicate the presence of Ni vacancies (see Table 1). It has been shown that the magnetic prop-
erties of nanograined oxides critically depend on the presence of defects like intergranular boundaries [47,48]. Straumal et al. [48] have investigated the structure of grain boundaries in pure nanograined ZnO and found that most probably the proper combination of interpenetrating crystalline and amorphous phases is the condition for the ferromagnetism in pure ZnO. Recently, Srikrishna Ramya and Mahadevan [49] have observed superparamagnetism in the case of as-prepared (amorphous) nanophase and ferromagnetism with reasonable hysteretic parameters in the case of other nanophases (e-, b-, and a-) of Fe2O3. They have explained this magnetic behavior as due to an increased concentration of oxygen vacancies near the surface. Their nanoparticles contain both a very developed free surface as well as some grain boundaries with extremely high specific area. Anamolous magnetic properties were observed on CoO and NiO nanoparticles [50,51]. Wang et al. [52] have reported that the NiS2 nanotubes with 200 nm diameter and about 20 nm thickness of tube-wall exhibit ferromagnetic interactions and even tending to superparamagnetic property at room temperature. Zhang et al. [53] have reported that a-NiS and Ni3S4 nanocrystals are no longer antiferromagnetic, both of them show paramagnetism due to the size effect. Tang et al. [54] have explained the ferromagnetic property of flower-like a-NiS nanostructures as due to the increase of the uncompensated moments at the nanocrystal surface resulting from the reduced coordination of the surface spins. Salavathi-Niasari et al. [25] have reported that both of the a-NiS and Ni3S4 nanoparticles show paramagnetism due to size effect. Wang et al. [55] have observed that the magnetic properties of a-Fe2O3 can be changed, by controlling the morphology, from superparamagnetic behavior to ferromagnetic behavior which are different from the behavior of bulk material. Spherical nanoparticles (with average size around 60 nm) show a superparamagnetic behavior (with coercivity almost zero) and hollow microspheres show a normal (strong) ferromagnetic behavior (with coercivity around 2528 Oe) at room temperature, However, a good ferromagnetic order has been observed for nickel sulfide nanospheres in the present study which is not in agreement with that observed for nanospheres by Wang et al. [55]. Mitra et al. [56] have reported the coercivity dependence on the shape of a-Fe2O3 nanocrystals:Hc for nanocube is 330 Oe, Hc for nanospindle is 390 Oe, and Hc for nanorhombohedral is 1250 Oe. Normally, the coercivity increases with the increase of aspect ratio (shape anisotropy). The aspect ratio for spherical shaped nanocrystal is 1 (no shape anisotropy). In particles with aspect ratio higher than one, the magnetic spins are preferentially aligned along the long axes. Their reversal to the
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opposite direction requires higher energies when compared to spherical particles [57]. The high coercivities observed in the present study, at room temperature, for nickel sulfide nanophases unambiguously indicate that the observed ferromagnetism does not come from any experimental artifact, but an intrinsic property of the nanophases considered. So, increase in coercivity observed for the nickel sulfide nanophases in the present study can be explained as due to the creation of Ni vacancies, magneto-crystalline anisotropic effects due to some grain boundaries and uncompensated moments at the nanocrystal surface resulting from the reduced coordination of the surface spins. Further, we note that a clear understanding of the magnetic properties of the nickel sulfide nanophases requires additional investigations to be carried out. Magnetic material used in the storage media requires that it should have a large hysteresis loop with large coercivity which allows permanent and stable storage. Coercivity between 350 and 4000 Oe is often desirable in typical magnetic storage medias. The observed higher coercivities make the nickel sulfide nanophases considered in the present study promising candidates for these applications. 4. Conclusions The four nanophases of nickel sulfide, viz., amorphous NiS(asprepared) and NiS1.03, b-NiS, and a-NiS in the crystalline state have been successfully prepared by the proposed simple solvothermal method using a domestic microwave oven along with required calcination (300 °C for NiS1.03, 500 °C for b-NiS and 700 °C for a-NiS). The phases were identified by XRD analysis. The SEM and TEM images show the morphology and homogeneity of the clusters. EDX and FTIR spectral analyses confirm the high phase purity. The observed optical bandgap energies range from 3.81 to 4.52 eV. The PL spectral analysis indicates a maximum yield for a-NiS. Results of electrical measurements indicate that space charge contribution plays an important role in charge transport process and polarizability. The magnetic measurement indicates ferromagnetism for all the four phases considered in the present study. References [1] J.T. Sparks, T. Komoto, J. Appl. Phys. 34 (1963) 1191–1192. [2] A.M. Fernandez, M.T.S. Nair, P.K. Nair, Mater. Manuf. Process. 8 (1993) 535– 548. [3] W.M. Kriven, Mater. Sci. Eng. A 127 (1990) 249–255. [4] G. Kullerud, R.A. Yund, J. Petrol. 3 (1962) 126–131. [5] J.T. Sparks, T. Komoto, Phys. Lett. A 25 (1967) 398–399. [6] J. Trahan, R.G. Goodrich, S.F. Watkins, Phys. Rev. B 2 (1970) 2859–2863. [7] P. Hu, Y. Cao, B. Lu, Mater. Lett. 106 (2013) 297–300. [8] S.C. Han, H.S. Kim, M.S. Song, J.H. Kim, H.J. Ahn, J.Y. Lee, J. Alloys Comp. 351 (2003) 273–278. [9] S.C. Han, K.W. Kim, H.J. Ahn, J.H. Ahn, J.Y. Lee, J. Alloys Comp. 361 (2003) 247– 251. [10] R. Coustal, J. Chim. Phys. 38 (1958) 277. [11] A. Wold, K. Ddwight, J. Solid State Chem. 96 (1992) 53–58.
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