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Investigation of structural, optical, dielectric and magnetic properties of Sno2 nanorods and nanospheres
Journal Pre-proof Investigation of Structural, optical, Dielectric and Magnetic Properties of Sno2 Nanorods and Nanospheres
Naveed Hussain, Zulfiqar, Tahirzeb Khan, Rajwali Khan, Shaukat Ali Khattak, Shahid Ali, Gulzar Khan PII:
S0254-0584(19)31197-6
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122382
Reference:
MAC 122382
To appear in:
Materials Chemistry and Physics
Received Date:
01 March 2019
Accepted Date:
28 October 2019
Please cite this article as: Naveed Hussain, Zulfiqar, Tahirzeb Khan, Rajwali Khan, Shaukat Ali Khattak, Shahid Ali, Gulzar Khan, Investigation of Structural, optical, Dielectric and Magnetic Properties of Sno2 Nanorods and Nanospheres, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122382
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INVESTIGATION OF STRUCTURAL, OPTICAL, DIELECTRIC AND MAGNETIC PROPERTIES OF SnO2 NANORODS AND NANOSPHERES Naveed Hussain1, Zulfiqar1,3, Tahirzeb Khan1*, Rajwali Khan1, Shaukat Ali Khattak1, Shahid Ali2 Gulzar Khan1, 1
Department of Physics, Abdul Wali Khan University, Mardan, KPK 23200, Pakistan
2 Department
of Physics, University of Peshawar, KP 25120, Pakistan
3Department
of Material Science & Engineering, Zhejiang University, Hangzhou, 310027 China
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Abstract We report structural, dielectric, optical and magnetic properties of SnO2 nano-rods and nanospheres, synthesized by hydrothermal route. X-ray diffraction (XRD) confirms that both SnO2 nanorods and nanospheres have a rutile tetragonal structure with an average crystallite size of 10.8410 ± 1.7832 nm and 16.1080 ± 3.8384, respectively. Energy-dispersive X-ray (EDX) spectroscopy suggests that both samples, of nanorods and nanospheres, are composed of Sn and O elements. The broader and more intense photoluminescence (PL) peak of the nanorods than that of the nanospheres is due to the presence of a greater number of defects in the former than in the latter. The high dielectric constant in the case of nanorods is due to small size with more grain boundaries and high defect density. The conductivity of spheres is due to the large volume of conducting grains while the conductivity of rods is due to the increased charge density (charges detached from vacancies and conduction charge carriers). Both samples show ferromagnetic behavior, where the saturation magnetization (Ms), remanent magnetization (Mr) and coercivity in SnO2 nanorods are greater than in the spheres. It is due to the presence of a large concentration of oxygen vacancies (defects).
Keywords: Hydrothermal, Grain boundaries, Magnetic properties, Tin oxide, Defects, Dispersion
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1. Introduction Nanomaterials have attracted great attention due to their intriguing structural, optical, magnetic and dielectric properties, which are different from those of the bulk materials [1]. Among the nanomaterials, metal oxides are of great importance with a significant role in several areas of chemistry, physics, and materials science. Technologically, metal oxides have applications in the fabrication of microelectronic circuits, fuel cells [2, 3], piezoelectric devices, sensors, piezoelectric devices, coatings for the passivation of surfaces against and corrosion [4, 5]. Oxide nanoparticles exhibit unique physical and chemical properties owing to their limited size and high density of corner or edge surface sites as compared to bulk [6, 7]. Metal oxides including Al2O3, MgO, ZrO2, CeO2, Fe2O3, ZnO, TiO2, In2O3 and SnO2, have attracted considerable attention because of their unique properties and potential applications in various fields of nanotechnology [8]. Among all these metal oxides, SnO2 is the most important semiconductor with rutile type tetragonal crystal structure [8,9]. The SnO2 is an n-type wide bandgap (3.6 eV at 300 K) semiconductor, having exciton binding energy (∼130 meV), suitable for various applications [10]. SnO2 nanostructures have a large surface to volume ratio due to which it has various morphological structures such as nanowires, nanotubes, hollow spheres, nanoflowers, nanorods, solid nanospheres and mesoporous structure [11]. Among all these morphologies of SnO2, one dimensional (1D) nanorods and three dimensional (3D) solid nanospheres have attracted great attention due to their novel applications in the various technological fields [9, 10]. The SnO2 nanorods and nanospheres have unique magnetic, electrical, optical, structural and dielectric properties [11, 12]. Because of the diluted magnetic semiconducting properties, SnO2 is used in magnetic data storage and magnetic resonance imaging [10]. SnO2 nanorods and nanospheres are used as photocatalyst for the photodegradation of organic compounds, polishing powder, sometimes in mixtures with lead oxide, for polishing glass, jewelry and marble. Silver SnO2 coatings can be exploited, using chemical vapor deposition, to coat glass bottles with a thin layer (< 0.1 μm) of SnO2, which helps to adhere to a subsequent protective polymer coating such as polyethylene to the glass [11, 13]. SnO2 nanorods and nanospheres have a high specific surface area, good stability and low density due to which they are widely used in sensing applications, including smoke sensors, humidity sensors, and gas sensors to improve their response time and sensitivity [12, 14-15]. Due to its large number of active sites, high surface response, and strong
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radiation absorption enhance the photocatalytic removal of dyes. The transparent electrical conduction property of SnO2 nanorods and spheres are mostly used in transparent ovens and liquid crystal displays [16]. A number of methods, such as wet chemical methods, rapid oxidation, chemical vapor deposition (CVD), thermal evaporation, sol-gel, solvothermal, co-precipitation, colloidal, spray pyrolysis,
solid-state,
template
method,
green
synthesis
methods,
sonochemical,
mechanochemical, microwave and hydrothermal techniques, are used for synthesis of SnO2 nanostructures [17, 18]. In this work, we synthesized SnO2 nanorods and nanospheres by the hydrothermal method. Structural, optical, magnetic and dielectric properties of SnO2 nanorods and nanospheres are investigated and compared with each other and are reported for the first time [19, 20].
2. Materials and Methods Chemicals with high purity (≥ 98%), such as tin chloride (SnCl2.2H2O), sodium hydroxide (NaOH) and deionized water, were purchased from Sigma-Aldrich Company and used without further purification. SnO2 nanorods and nanospheres were synthesized by hydrothermal method, where 1.52 g and 2.10 g of SnCl2.2H2O and 1.4 g of NaOH were dissolved in 80 ml deionized water for the synthesis of nanospheres and nanorods respectively: SnCl2.2H2O + 2NaOH+ 2H2O
Sn(OH)2 + 2NaCl + 4H2O
(1)
The solution was stirred for ~ 30 minutes at room temperature and then transferred into 100 ml stainless steel Teflon autoclave and kept at 180 oC for 16 hours for nanorods and at 200 oC for 24 hours for nanospheres respectively. The precipitates were washed thoroughly (seven times) with deionized water and ethanol by centrifugation. This was followed by drying the precipitates in an oven at 70 °C. The product was ground into fine powder by using mortar and pestle. 2.1 Materials Measurements The synthesized samples were characterized by X-ray diffraction (XRD) using the XRD Empyrean 200895 (Netherland) with Cu Kα radiation with a wavelength λ=1.5418 Å at 2θ values between 10° and 80°. The morphologies of the products were investigated by a scanning electron microscope (SEM S-4800) with an accelerating voltage of 5 kV. The chemical
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composition was analyzed by using an energy dispersive x-ray (EDX) spectroscopy attached to the SEM. The UV-visible (UV–vis) diffuse reflectance spectrum (DRS) was obtained from a UV–Vis-NIR spectrophotometer UV-3600 model. Fourier transform infrared spectroscopy (FTIR) spectra were obtained by using the FTIR TENSOR 27 system. Raman Spectra is recorded using the LabRamHRUV system. The dielectric properties and conductivity as a function of frequency were measured on centered Gold electrode pellet with a typical Impedance analyzer (Agilent 4292) in frequency range 40 Hz to 15 MHz. Dc magnetization (M(H)) measurements were carried out using a Quantum Design magnetic properties measurement system (MPMS) at room temperature.
3. Results and Discussion 3.1 XRD Analysis Fig. 1 shows XRD patterns of SnO2 nanorods and nanospheres prepared by the hydrothermal method. All the diffraction peaks are indexed to the tetragonal rutile phase of SnO2 according to the Joint Committee on Powder Diffraction Standards (JCPDS) file no. 41-1445 and no other peaks corresponding to impurity phases were observed confirming the formation of SnO2 nanorods and spheres. The intense and narrow XRD peaks indicate the crystalline nature of SnO2 nanorods and nanospheres. A careful observation suggests that the peaks of nanorods are broader and less intense than that of nanospheres, which is most probably due to small grain size and large structure disorder in nanorods. The average crystallite size (𝒕) is calculated from XRD using the Debye Scherer formula kλ
𝒕 = β Cos (θ)
(2)
Where k is the correction factor (0.9), λ is the X-ray wavelength, β is the full width at half maximum (FWHM) of the peak and θ is the Bragg diffraction angle. The average crystallite sizes are 10.8410±1.7832 nm and 16.1080±3.8384 nm of nanorods and nanospheres, respectively. The Rietveld refinement (weighted profile factor RWP = 9.51%, and the goodness-of-fit χ2 = 2.673) gives the lattice constants of nanosphere having as a = 4.7320 and c =3.1942 [13,14] while for nanorods the lattice parameters are a = b = 4.7606 Å and c = 3.1442 Å as shown in Fig 1(a) and (b). The (hkl) values, “d(hkl)” spacing, lattice volume, sample density, X-rays density, porosity, and specific surface area are all given in Table.1 [9, 21].
40
(321)
(202)
(220) (002) (310) (301)
(111)
Intensity (a.u)
20
Standard Nano rods Nanospheres
(211)
(101)
(200)
(110)
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60
80
2Degree
Figure 1. XRD spectra of SnO2 nanorods (red) and nanospheres (blue) in comparison with the standard spectra (black). Table. 1 XRD Parameters
Parameters
SnO2 nanorods
SnO2 nanospheres
(hkl)
(110)
(101)
(211)
(110)
(101)
(211)
d(hkl) (Å)
3.3663
2.6435
1.7629
3.3460
2.6423
1.7644
Crystallite size. t (nm)
9.3335
11.722
11.466
19.2226
16.728
12.3735
5
9
Average crystallite size 𝑡 10.8410 ± 1.7832
9 16.1080 ± 3.8384
± 𝜎(nm) Lattice parameter a = b # 4.7606
3.1442
4.7320
c(Å) Lattice volume V (Å)
71.2597
Sample density ρ(M)= 1.5726 m
71.5239 30.3600
(g/cm3)
π𝑟2h
X-rays ρ(X) =𝑁
density 3.5150 M 𝐴𝑎
2
𝑐
3.5020
(g/cm3) ρ(M)
Porosity=1 ― ρ(X)
0.9037
0.7199
3.1942
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Specific surface area
19.6810
13.2954
6 𝑚2 = ( ) 𝑡ρ(X) g 3.2 FTIR Spectroscopy Fig. 2 shows FTIR spectra for the samples in the range of 250-4500 cm-1. The peak appearing at 503 cm−1 is due to the terminal oxygen vibration of Sn–O. The presence of SnO2 as a crystalline phase is confirmed by the peak at 640 cm−1 that corresponds to the O–Sn–O bridge functional groups of SnO2. This is in agreement with the results of XRD analysis (Fig. 1). The bands at 1636 cm−1 and 3411 cm−1 are attributed to O-H bending mode and O–H hydroxyl stretching vibrational mode due to absorbed water on the sample surface respectively. Alkyl CH deforming and stretching modes occur at 1109 cm-1 and 2914 cm-1, respectively. A band at 1370 cm-1 is assigned to the anti-symmetric stretching vibration of ON ions, arisen from the nitrates adsorbed on the surfaces of samples from the atmosphere. It is suggested that the high surface area of these nanostructured materials results in the rapid absorption of water from the atmosphere because the FTIR samples were kept in open-air [22, 23]. In the case of nanospheres, the peak at 530 cm-1 is attributed to the vibrational mode of Sn-O. The band at 709 cm-1 is assigned to the vibration of the O-Sn-O band in SnO2.
The
absorption peak at 1629 cm-1 is attributed to OH hydroxyl
bending mode and 3408 cm-1 band originates from the vibration of hydroxyl (OH) stretching mode. The broad and strong at 3408 cm-1 describes that there are several O-H bonds in the sample, suggesting the surfaces of the particles that are covered with hydroxyl species. Alkyl CH deforming and stretching modes occur at 1079 cm-1 and 2974 cm-1, respectively. A band at 1393 cm-1 is assigned to the anti-symmetric stretching vibration of ON- ions, arising from the nitrates adsorbed on the surface of samples from the atmosphere [24, 25].
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1.0 SnO2 nanospheres SnO2 nanorods
0.6
-CH-ON-
0.4
0
500
O-H
O-Sn-O
0.0
Sn-O
0.2
C-H
-OH-
Transmission (a.u)
0.8
1000 1500 2000 2500 3000 3500 4000 4500 -1
Wavenumber (cm ) Fig. 2 FTIR spectra of SnO2 nanorods and nanospheres.
3.3 Field Emission Scanning Electron Microscopy and Energy Dispersive X-Rays Spectroscopy Figs. 3 (a,b) and (c) show the FE-SEM images of the SnO2 solid nanospheres and nanorods respectively. The nanorods are uniformly distributed, while the spheres are non-uniformly distributed also confirmed somewhere else [25-27] 3.4 EDX Figs. 3 (d) and (e) depicts the composition of SnO2 samples, characterized by EDX that suggests that the samples are composed of Sn and O [9]. Italso shows the higher concentration of the tin element than the oxygen, which is in strong agreement with XRD results. In the adjacent table corresponds to nanospheres the atomic % of O in the nanorods is smaller than that of the nanospheres, suggesting that the former contain large numbers of vacancies of oxygen than the latter. The nanorods have a large surface area to accommodate the higher concentration of O vacancies. The atomic % of Sn is smaller in nanospheres than in the nanorods, showing that the number of vacancies of Sn is greater in nanospheres than in the nanorods [26].
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Fig.3. (a)- (c) show SEM images of SnO2 nanorods and nanospheres while (d) & (e) sow the EDX spectra.
There are also other peaks ranging from 1 to 3 keV, which is the Si and Pt that are due to the background of the sample holder. 3.5 Raman Spectroscopy The formation of tetragonal rutile SnO2 nanostructures was further characterized by Raman spectra. It is used to investigate the crystalline structure and to evaluate the grain size of SnO2 nanorods and nanospheres. Figure 4 presents the typical Raman spectra of the nanostructures in the 150–1000cm−1 region at room temperature. There are three Raman peaks, centered at 450, 622 and 789 cm-1 for the nanorods while at 460, 631 and 820 cm-1 for the nanospheres corresponding to the vibrational modes of Eg, A1g and B2g, respectively. These peaks correspond to rutile tetragonal SnO2 nanostructure. The Eg vibrational mode is sensitive to oxygen vacancies and its intensity increases with increasing concentration of oxygen vacancies. Careful observation of Fig. 4 shows that the intensity of Eg vibrational mode is larger in nanorods than in the nanospheres, revealing a greater number of oxygen vacancies in nanorods than in the nanospheres. The A1g vibrational mode is sensitive to crystallinity and its intensity increases with
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improvement in crystallization. The intensity of A1g is greater for nanospheres than for the nanorods. The peak at 789 and 820 cm−1 are assigned to B2g vibrational mode [15]. From Raman spectra, it is inferred that nanorods have a greater number of defects with the large specific surface area than the nanospheres. The results obtained from Raman spectra are congruent with that of EDX. SnO2 Nanorods
Intensity ( a.u.)
80
SnO2 Nanospheres
Eg A1g
60 40
B2g
20 0 200
400 600 800 -1 Raman Shift ( cm )
1000
Fig. 4 Room-temperature Raman spectra of SnO2 nanorods and nanospheres.
3.6 UV-Vis Spectroscopy UV-vis spectra are measured for understanding the optical properties of the nanostructures. The theory of optical absorption gives the relationship between the absorbance [F(R)] and reflectance R and is given by the so-called Kubelka–Munk function [9] expressed as, F(R) =
(1 ― 𝑅)2 2𝑅
.
(3)
Fig. 5 depicts the UV-vis spectra of SnO2 nanorods and spheres. The calculated bandgap values of nanorods and nanospheres are 3.88 eV and 3.66 eV respectively. The bandgap of SnO2 nanorods is blue shifted, due to its smaller size. In addition [22, 28, 29]. The larger bandgap of nanorods is due to the smaller size (a large number of oxygen vacancies) while the sharp rise of
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the absorption edge in nanospheres demonstrates the high crystallinity with less number of defects in the nanospheres.
SnO2 nanorods, Eg=3.87eV
24
SnO2 nanospheres, Eg=3.66 eV
(F(r)xE)
2
20 16 12 8 4 0
0
1
2
3
4
5
6
7
Enargy (eV) Fig. 5 Optical bandgap energy estimation of the SnO2 nanorods and nanospheres.
3.7 Photoluminescence (PL) Spectroscopy Fig. 6 shows the photoluminescence (PL) spectra of the SnO2 nanorods and nanospheres in the range of 400-800 nm at room temperature. For both morphologies, visible light emission is observed: for nanorods at 3.07 eV and the nanospheres at 2.9 eV. A broad green emission band with a central wavelength at 2.1 eV is observed for both nanorods and nanospheres. The emission of visible light for both samples is most probably due to crystal defects (cation and oxygen vacancies). Two other peaks are observed at 1.88 eV and 1.82 eV for nanorods and nanospheres, respectively [30]. The emission intensities increase with increasing defects densities (oxygen vacancies). The emission intensities for nanorods are higher than for nanospheres, indicating a large number of defects in nanorods. The smaller size of nanorods with a larger surface area accommodates a high concentration of oxygen vacancies. These vacancies act as color centers, emitting high visible light intensities in the visible region.
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SnO2 Nanospheres SnO2 Nanorods
2400
Intensity (a.u.)
2000 1600 1200 800 400 0
400
500
600
700
800
Wavelength (nm) Fig. 6 PL spectra of SnO2 nanorods and nanospheres.
3.8 Dielectric Constant Fig. 7(a) depicts the dependence of dielectric constant (εr) on frequency for the nanostructures at room temperature. Both morphologies exhibit the dispersion behavior, explained by the Maxwell-Wagner model. This model suggests that dielectric material contained well-conducting grain partitioned by insulating (poorly conducting) boundaries When the external field is applied, the charge carriers can easily move through the volume of conducting grains along field direction till they reach the poorly conducting grain boundaries. At these boundaries, the accumulation of a large number of charge carriers takes place, resulting in large polarization. Small size (large surface area with a large number of defects) contributes to polarization by trapping free charge carriers producing a large number of electric dipoles resulting in large polarization. At higher frequencies, due to fast switching of the direction of the applied field, the charge carriers are not able to reach grain boundaries and thus dielectric constant decreases. At high frequencies, frequency-independent behavior of dielectric constant is observed. The large values of dielectric constant (εr) of SnO2 nanorods as compared to SnO2 nanospheres are observed (This is due to a smaller size with a large surface area containing a large number of defects) [21, 22].
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3.9 Dielectric Loss (εʹʹ) Dielectric loss occurs when charge carriers are not able to follow the variations of the applied field. Dielectric loss (εʹʹ) of SnO2 nanorods and nanospheres are shown in Fig. 7(b). Both samples exhibited the dispersion behavior, i.e. high dielectric loss at low frequencies and frequency-independent behavior at high frequencies. The possible reasons for dielectric loss at low frequencies are hopping of ions, conduction process due to migration of ions and ionic polarization [31, 34].
SnO2 Nanospheres SnO2 Nanorods
600
10
Dielectric loss ()
Dielectric Constant (r)
700
500 400 300 200
8 6 4 2
100
(a)
0
3
10
4
10
5
10
6
10
7
10
0
8
10
Conductivity ac( cm)-1
SnO2 nanospheres SnO2 nanorods
160
tan(
120 80 40
(c) 3
10
4
10
5
6
10 10 log (rad/s)
7
10
8
10
(b) 3
4
10
log (rad/s) 200
0
SnO2 nanospheres SnO2 nanorods
10
5
6
10 10 log (rad/s)
7
10
8
10
2.5
SnO2 nanospheres SnO2 nanorods
2.0 1.5 1.0 0.5 0.0
(d) 3
10
4
10
5
10
6
10
7
10
log rad/s)
Fig. 7 (a) Variation of dielectric constant εt, (b) dielectric loss, (c) tangent loss (𝑡𝑎𝑛 𝛿) and (d) conductivity with frequencies for SnO2 nanorods and nanospheres.
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3.10 Dielectric Loss Factor (δ) Loss tangent (𝑡𝑎𝑛 𝛿) represents the dissipated energy in the dielectric. The variation of loss tangent of SnO2 nanorods and nanospheres are shown in Fig. 7(c). The term (𝑡𝑎𝑛𝛿) shows similar behavior as the dielectric loss (εʹʹ) due to direct proportionality between the two as shown in equation (4) [32]. 𝑡𝑎𝑛 𝛿 =
εʹʹ εʹ
(4)
The large (𝑡𝑎𝑛𝛿) values at low frequencies are due to space charge polarization. At large frequencies, charge carriers are not able to follow the electric field and as a result, 𝑡𝑎𝑛𝛿 shows no variations with frequencies. The nanorods show larger dielectric loss due to the large surface area accommodating a large number of defects. So a large number of defects in the nanorods leads to greater space-charge polarization than the nanospheres [33, 34], as confirmed by XRD and EDX. No relaxation peak is observed in the case of both samples. 3.11 Conductivity (σac) The frequency dependence of conductivity is shown for nanostructures at room temperature in Fig. 7(d). Initially, the conductivity increases slowly with increasing frequency but increases sharply at higher frequencies. At low frequencies of the applied field, transport of charge carriers takes place through infinite paths that result in a low magnitude of conductivity. At high frequencies, charge carriers start moving with the applied field which increases the conductivity. Enough energy is, at high frequencies, provided to charge carriers resulting in their motion along the field. Besides, the energy of the applied field is so high to liberate charge carriers from defects (traps, oxygen vacancies) to increase charge density (conduction charge carriers along with liberated charge carriers from defects) resulting in a sharp increase in conductivity. The conductivity of SnO2 nanospheres is higher than that of nanorods which is most possibly due to the larger grain size of the SnO2 nanospheres as compared to that of nanorods. Large grain size results in large conducting grain volume and the small number of resistive grain boundaries. Consequently, charge carriers can easily move through large conducting grain volume, resulting in high conductivity [21]. The sharp increase in conductivity of nanorods at large enough frequencies is because the field provides enough energy to bound charge carriers to set them free
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from defects in which charge carriers are trapped. These liberated charge carriers along with other conduction charge carriers result in a sharp increase of conductivity. 3.12 Magnetic Hysteresis (M-H) loops The ferromagnetism in any material depends on temperature, domain size, and impurities. Fig. 8 shows magnetic hysteresis (M–H) loops of SnO2 nanorods and spheres, measured at room temperature. The observed ferromagnetism in SnO2 nanorods and nanospheres are due to the presence of large numbers of oxygen vacancies, Sn interstitials, Si and Pt impurities, confirmed by EDX analysis. Similar results have been reported elsewhere. [34]. The saturation magnetization (Ms) was observed up to 0.0485 emu/g for nanorods, while 0.0315 emu/g for the nanospheres. The remnant magnetization (Mr) of 0.0074 emu/g with a coercive field (Hc) of 104 Oe was observed in the nanorods, while nanospheres have remnant magnetization (Mr) of 0.0048 emu/g and coercive field of 93 Oe. The saturation magnetization (Ms), remnant magnetization (Mr) and coercivity in SnO2 nanorods are greater than that of nanospheres [35-40]. The nanorods have a large surface area and more chances of defects due to which they have a high value of saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc) as compared to nanospheres [41, 42, 43].
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0.08
M(emu/g)
0.04
SnO2 nanospheres SnO2 nanorods
0.00
-0.04
-0.08 -6000 -4000 -2000
0
2000
4000
6000
H (Oe) Fig. 8 Room temperature (M-H) loops of SnO2 nanorods and nanospheres.
Conclusion SnO2 nanorods and nanospheres were synthesized by hydrothermal route. XRD confirmed the rutile tetragonal structure of SnO2 nanorods and nanospheres. The XRD peaks of nanorods are broader than that of nanospheres, which is due to small grain size and large defect density of the nanorods. The average crystallite size of SnO2 nanorods and nanospheres are 10.8410 ± 1.7832 and 16.1080 ± 3.8384, while porosity is 0.9437 and 0.7199 and surface area is 19.6810 m2g-1 and 13.2954 m2g-1, respectively. SEM showed the formation of SnO2 nanorods and spheres. EDX results revealed that the nanostructures contain only Sn and O and there are no impurity atoms and phases. There is lesser content of oxygen in the case of nanorods than in the spheres,
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showing that there are more oxygen-deficient regions (oxygen vacancies) in nanorods than in the nanospheres. In Raman spectra, A1g showed the improvement in crystallization. The intensity of the A1g peak is enhanced in the case of nanospheres as compared to nanorods. The intensity of the E1g peak is sensitive to oxygen vacancies and an increase in its intensity reveals more oxygen-deficient regions in nanorods. The intensity of E1g is high for nanorods, revealing that there are more oxygen vacancies. Raman spectra are in agreement with XRD, FTIR, and EDX. The estimated bandgap energy of nanorods and nanospheres are 3.89 eV and 3.67 eV, respectively, where the large bandgap of nanorods is consistent with their small size. The enhancement of PL peak intensities of nanorods is due to a large number of defects (oxygen vacancies). A large number of grains boundaries and smaller crystallite size with a high concentration of vacancies were caused due to high polarization. This high concentration of oxygen vacancies are available for trapping of conduction charge carriers, producing RDP and SCP resulting in an increased dielectric constant. The conductivity of nanospheres is slightly greater than that of nanorods, due to large conducting volume of grains of the former: the charge carriers can move freely through grains. In nanorods, at higher frequencies, the applied field provides enough energy to trapped charge carriers to set them free. These charge carriers along with conduction charge carriers enhance the conductivity of nanorods. The spin-spin interaction between the bound charge carriers and Sn ions provides ferromagnetic behavior both in nanorods and nanospheres, which is stronger in the case of nanorods. The saturation magnetization (Ms) is 0.0485 emu/g and 0.0315 emu/g; the remanent magnetization (Mr) is 0.0074 emu/g and 0.0048 emu/g and coercivity is 104 Oe and 93 Oe for nanorods and nanospheres, respectively.
Acknowledgments
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The first two authors Naveed Hussain and Zulfiqar have equal contribution in this paper. This work is financially supported by the Higher Education Research Endowment Fund (NO.PMU122/HEREF/2014- 15/Vol-111/) Khyber Pakhtunkhwa (KPK) Pakistan, Higher Education Commission under START-UP RESEARCH GRANT PROGRAM (Grant No: 21-1525/SRGP/R&D/HEC/2017), (Grant No: 21-1732/SRGP/ R&D/HEC/2017), the Fundamental Research Funds for the Higher Education Commission (HEC) Pakistan.
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9. Han X et al., Multilayered Nanocrystalline SnO2 Hollow Microspheres Synthesized by Chemically Induced Self-Assembly in the Hydrothermal Environment. Journal of Physics and Chemistry (2007) 14067-14071. 10. G. Madhumitha et al., Devastated crops: multifunctional efficacy for the production of nanoparticles. Journal of Nanomaterials (2013) 03-05. 11. A. A. Firooz et al., Preparation of SnO2 nanoparticles and nanorods by using a hydrothermal method at low temperature. Materials Letters 62 (2008) 1789-1792. 12. X. Liu et al., Visible light photocatalytic degradation of methylene blue by SnO2 quantum dots prepared via microwave-assisted method. Catalysis Science & Technology 3 (2013) 1805-1809. 13. A. Kar et al., Rapid thermal annealing effects on tin oxide nanowires prepared by vapor– liquid–solid technique. Nanotechnology 20 (2009) 065704-065707. 14. M. Batzill et al., The surface and materials science of SnO2. Progress in Surface Science 79 (2005) 47-154. 15. D.F. Zhang et al., Size-controllable one-dimensional SnO2 nanocrystals: synthesis, growth mechanism, and gas sensing property. Physical Chemistry Chemical Physics 8 (2006) 48744880. 16. K. Manseki et al., Microwave synthesis of size-controllable SnO2 nanocrystals for dyesensitized solar cells. New Journal of Chemistry 38 (2014) 598-603. 17. Y. Masuda et al., Superhydrophilic SnO2 nanosheet-assembled film. Thin Solid Films 544 (2013) 567-570. 18. O. Lupan et al., Synthesis of one-dimensional SnO2 nanorods via a hydrothermal technique. Physica E: Low-dimensional Systems and Nanostructures 41(2009) 533-536. 19. J. Hu et al., Laser‐ablation growth and optical properties of wide and long single‐crystal SnO2 ribbons. Advanced Functional Materials 13 (2003) 493-496. 20. G. Cheng et al., Controlled growth of oxygen-deficient tin oxide nanostructures via a solvothermal approach in mixed solvents and their optical properties. Nanotechnology 18 (2007) 355604-355607. 21. H. Wang et al., Synthesis and gas sensitivities of SnO2 nanorods and hollow microspheres. Journal of Solid State Chemistry 181 (2008) 122-129.
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22. S. Das et al., Optical properties of SnO2 nanoparticles and nanorods synthesized by solvothermal process. Journal of applied physics 99 (2006) 114303. 23. R. Khan et al., Variation of structural, optical, dielectric and magnetic properties of SnO2 nanoparticles. Journal of Materials Science: Materials in Electronics 28 (2017) 4625-4636. 24. A. A. Firooz et al., Preparation of SnO2 nanoparticles and nanorods by using a hydrothermal method at low temperature. Materials Letters 62 (2008) 1789-1792. 25. L. Tan et al., Hydrothermal synthesis of SnO2 nanostructures with different morphologies and their optical properties. Journal of Nanomaterials (2011) 23. 26. F. Gu et al., Synthesis and luminescence properties of SnO2 nanoparticles. Chemical Physics Letters 372 (2003) 451-454. 27. L. Shi et al., Facile fabrication and optical property of hollow SnO2 spheres and their application in water treatment. Langmuir 26 (2010) 18718-18722. 28. H. Wang et al., Synthesis and gas sensitivities of SnO2 nanorods and hollow microspheres. Journal of Solid State Chemistry 181 (2008) 122-129. 29. D. Singh et al., Structural, morphological and gas sensing study of zinc doped tin oxide nanoparticles synthesized via hydrothermal technique. Journal of Molecular Structure 1115 (2016) 250-257. 30. H. W. Kim et al., Synthesis and characteristics of SnO2 nanorods on Pd-coated substrates. Brazilian journal of physics 35 (2005) 1006-1009. 31. N. Ahmad et al., Effect of (Mn-Co) co-doping on the structural, morphological, optical, photoluminescence and electrical properties of SnO2. Journal of Alloys and Compounds 720 (2017) 502-509. 32. G. Patil et al., Spray pyrolysis deposition of nanostructured tin oxide thin films. ISRN Nanotechnology (2012). 33. H. Chen et al., A novel hydrothermal route to synthesize solid SnO2 nanospheres and their photoluminescence property. Applied Physics A 97 (2009) 581. 34. M. A. Iqbal et al., High-frequency dielectric properties of Eu+ 3-substituted Li–Mg ferrites synthesized by the sol–gel auto-combustion method. Journal of Alloys and Compounds 586 (2014) 404-410. 35. C. Koops et al., On the dispersion of resistivity and dielectric constant of some semiconductors at audio frequencies. Physical Review 83 (1951) 121.
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36. G. Ali et al., Effect of Co substitution on the structural, electrical, and magnetic properties of Bi 0.9 La 0.1 FeO3 by sol-gel synthesis. International Journal of Minerals, Metallurgy, and Materials 20 (2013) 166-171. 37. Zulfiqar et al., Structural, dielectric and ferromagnetic behavior of (Zn,Co) co-doped SnO2 nanoparticles. J. Ceramics International 42 (2016) 17128-17136. 38. R. Khan et al., Effects of Ni co-doping concentrations on dielectric and magnetic properties of (Co, Ni) co-doped SnO2 nanoparticles. Journal of Materials Science: Materials in Electronics 27 (2016) 7725-7730. 39. R. Khan et al., Magnetic and dielectric properties of (Co, Zn) co-doped SnO2 diluted magnetic semiconducting nanoparticles. Journal of Materials Science: Materials in Electronics 27 (2016) 5960-5966. 40. R. Khan et al., Dielectric and magnetic properties of (Zn, Co) co-doped SnO2 nanoparticles. Chinese Physics B 24 (2015) 127803. 41. R. Khan et al., Effect of annealing on structural, dielectric, transport and magnetic properties of (Zn, Co) co-doped SnO2 nanoparticles. Journal of Materials Science: Materials in Electronics 27 (2016) 4003-4010. 42. R. Khan et al., Effect of annealing temperature on the dielectric and magnetic response of (Co, Zn) co-doped SnO2 nanoparticles. Journal of Materials Science: Materials in Electronics 28 (2017) 2673-2679. 43. M. Hanesch et al., Raman spectroscopy of iron oxides and (oxy) hydroxides at low laser power and possible applications in environmental magnetic studies. Geophysical Journal International 177 (2009) 941-948.
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Highlights
Successfully synthesized SnO2 nanorods and nanospheres by hydrothermal route. Nanorods have larger surface area and defect density than spheres. Emission intensities are enhanced in the case of nanorods due to large number of defects. Both dielectric properties and ferromagnetism are enhanced in the case of nanorods. Role of defects is of vital importance in tailoring the, dielectric and magnetic properties.
Naveed Hussain, Zulfiqar, Tahirzeb Khan, Rajwali Khan, Shaukat Ali Khattak, Shahid Ali, Gulzar Khan PII:
S0254-0584(19)31197-6
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122382
Reference:
MAC 122382
To appear in:
Materials Chemistry and Physics
Received Date:
01 March 2019
Accepted Date:
28 October 2019
Please cite this article as: Naveed Hussain, Zulfiqar, Tahirzeb Khan, Rajwali Khan, Shaukat Ali Khattak, Shahid Ali, Gulzar Khan, Investigation of Structural, optical, Dielectric and Magnetic Properties of Sno2 Nanorods and Nanospheres, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122382
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INVESTIGATION OF STRUCTURAL, OPTICAL, DIELECTRIC AND MAGNETIC PROPERTIES OF SnO2 NANORODS AND NANOSPHERES Naveed Hussain1, Zulfiqar1,3, Tahirzeb Khan1*, Rajwali Khan1, Shaukat Ali Khattak1, Shahid Ali2 Gulzar Khan1, 1
Department of Physics, Abdul Wali Khan University, Mardan, KPK 23200, Pakistan
2 Department
of Physics, University of Peshawar, KP 25120, Pakistan
3Department
of Material Science & Engineering, Zhejiang University, Hangzhou, 310027 China
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Abstract We report structural, dielectric, optical and magnetic properties of SnO2 nano-rods and nanospheres, synthesized by hydrothermal route. X-ray diffraction (XRD) confirms that both SnO2 nanorods and nanospheres have a rutile tetragonal structure with an average crystallite size of 10.8410 ± 1.7832 nm and 16.1080 ± 3.8384, respectively. Energy-dispersive X-ray (EDX) spectroscopy suggests that both samples, of nanorods and nanospheres, are composed of Sn and O elements. The broader and more intense photoluminescence (PL) peak of the nanorods than that of the nanospheres is due to the presence of a greater number of defects in the former than in the latter. The high dielectric constant in the case of nanorods is due to small size with more grain boundaries and high defect density. The conductivity of spheres is due to the large volume of conducting grains while the conductivity of rods is due to the increased charge density (charges detached from vacancies and conduction charge carriers). Both samples show ferromagnetic behavior, where the saturation magnetization (Ms), remanent magnetization (Mr) and coercivity in SnO2 nanorods are greater than in the spheres. It is due to the presence of a large concentration of oxygen vacancies (defects).
Keywords: Hydrothermal, Grain boundaries, Magnetic properties, Tin oxide, Defects, Dispersion
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1. Introduction Nanomaterials have attracted great attention due to their intriguing structural, optical, magnetic and dielectric properties, which are different from those of the bulk materials [1]. Among the nanomaterials, metal oxides are of great importance with a significant role in several areas of chemistry, physics, and materials science. Technologically, metal oxides have applications in the fabrication of microelectronic circuits, fuel cells [2, 3], piezoelectric devices, sensors, piezoelectric devices, coatings for the passivation of surfaces against and corrosion [4, 5]. Oxide nanoparticles exhibit unique physical and chemical properties owing to their limited size and high density of corner or edge surface sites as compared to bulk [6, 7]. Metal oxides including Al2O3, MgO, ZrO2, CeO2, Fe2O3, ZnO, TiO2, In2O3 and SnO2, have attracted considerable attention because of their unique properties and potential applications in various fields of nanotechnology [8]. Among all these metal oxides, SnO2 is the most important semiconductor with rutile type tetragonal crystal structure [8,9]. The SnO2 is an n-type wide bandgap (3.6 eV at 300 K) semiconductor, having exciton binding energy (∼130 meV), suitable for various applications [10]. SnO2 nanostructures have a large surface to volume ratio due to which it has various morphological structures such as nanowires, nanotubes, hollow spheres, nanoflowers, nanorods, solid nanospheres and mesoporous structure [11]. Among all these morphologies of SnO2, one dimensional (1D) nanorods and three dimensional (3D) solid nanospheres have attracted great attention due to their novel applications in the various technological fields [9, 10]. The SnO2 nanorods and nanospheres have unique magnetic, electrical, optical, structural and dielectric properties [11, 12]. Because of the diluted magnetic semiconducting properties, SnO2 is used in magnetic data storage and magnetic resonance imaging [10]. SnO2 nanorods and nanospheres are used as photocatalyst for the photodegradation of organic compounds, polishing powder, sometimes in mixtures with lead oxide, for polishing glass, jewelry and marble. Silver SnO2 coatings can be exploited, using chemical vapor deposition, to coat glass bottles with a thin layer (< 0.1 μm) of SnO2, which helps to adhere to a subsequent protective polymer coating such as polyethylene to the glass [11, 13]. SnO2 nanorods and nanospheres have a high specific surface area, good stability and low density due to which they are widely used in sensing applications, including smoke sensors, humidity sensors, and gas sensors to improve their response time and sensitivity [12, 14-15]. Due to its large number of active sites, high surface response, and strong
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radiation absorption enhance the photocatalytic removal of dyes. The transparent electrical conduction property of SnO2 nanorods and spheres are mostly used in transparent ovens and liquid crystal displays [16]. A number of methods, such as wet chemical methods, rapid oxidation, chemical vapor deposition (CVD), thermal evaporation, sol-gel, solvothermal, co-precipitation, colloidal, spray pyrolysis,
solid-state,
template
method,
green
synthesis
methods,
sonochemical,
mechanochemical, microwave and hydrothermal techniques, are used for synthesis of SnO2 nanostructures [17, 18]. In this work, we synthesized SnO2 nanorods and nanospheres by the hydrothermal method. Structural, optical, magnetic and dielectric properties of SnO2 nanorods and nanospheres are investigated and compared with each other and are reported for the first time [19, 20].
2. Materials and Methods Chemicals with high purity (≥ 98%), such as tin chloride (SnCl2.2H2O), sodium hydroxide (NaOH) and deionized water, were purchased from Sigma-Aldrich Company and used without further purification. SnO2 nanorods and nanospheres were synthesized by hydrothermal method, where 1.52 g and 2.10 g of SnCl2.2H2O and 1.4 g of NaOH were dissolved in 80 ml deionized water for the synthesis of nanospheres and nanorods respectively: SnCl2.2H2O + 2NaOH+ 2H2O
Sn(OH)2 + 2NaCl + 4H2O
(1)
The solution was stirred for ~ 30 minutes at room temperature and then transferred into 100 ml stainless steel Teflon autoclave and kept at 180 oC for 16 hours for nanorods and at 200 oC for 24 hours for nanospheres respectively. The precipitates were washed thoroughly (seven times) with deionized water and ethanol by centrifugation. This was followed by drying the precipitates in an oven at 70 °C. The product was ground into fine powder by using mortar and pestle. 2.1 Materials Measurements The synthesized samples were characterized by X-ray diffraction (XRD) using the XRD Empyrean 200895 (Netherland) with Cu Kα radiation with a wavelength λ=1.5418 Å at 2θ values between 10° and 80°. The morphologies of the products were investigated by a scanning electron microscope (SEM S-4800) with an accelerating voltage of 5 kV. The chemical
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composition was analyzed by using an energy dispersive x-ray (EDX) spectroscopy attached to the SEM. The UV-visible (UV–vis) diffuse reflectance spectrum (DRS) was obtained from a UV–Vis-NIR spectrophotometer UV-3600 model. Fourier transform infrared spectroscopy (FTIR) spectra were obtained by using the FTIR TENSOR 27 system. Raman Spectra is recorded using the LabRamHRUV system. The dielectric properties and conductivity as a function of frequency were measured on centered Gold electrode pellet with a typical Impedance analyzer (Agilent 4292) in frequency range 40 Hz to 15 MHz. Dc magnetization (M(H)) measurements were carried out using a Quantum Design magnetic properties measurement system (MPMS) at room temperature.
3. Results and Discussion 3.1 XRD Analysis Fig. 1 shows XRD patterns of SnO2 nanorods and nanospheres prepared by the hydrothermal method. All the diffraction peaks are indexed to the tetragonal rutile phase of SnO2 according to the Joint Committee on Powder Diffraction Standards (JCPDS) file no. 41-1445 and no other peaks corresponding to impurity phases were observed confirming the formation of SnO2 nanorods and spheres. The intense and narrow XRD peaks indicate the crystalline nature of SnO2 nanorods and nanospheres. A careful observation suggests that the peaks of nanorods are broader and less intense than that of nanospheres, which is most probably due to small grain size and large structure disorder in nanorods. The average crystallite size (𝒕) is calculated from XRD using the Debye Scherer formula kλ
𝒕 = β Cos (θ)
(2)
Where k is the correction factor (0.9), λ is the X-ray wavelength, β is the full width at half maximum (FWHM) of the peak and θ is the Bragg diffraction angle. The average crystallite sizes are 10.8410±1.7832 nm and 16.1080±3.8384 nm of nanorods and nanospheres, respectively. The Rietveld refinement (weighted profile factor RWP = 9.51%, and the goodness-of-fit χ2 = 2.673) gives the lattice constants of nanosphere having as a = 4.7320 and c =3.1942 [13,14] while for nanorods the lattice parameters are a = b = 4.7606 Å and c = 3.1442 Å as shown in Fig 1(a) and (b). The (hkl) values, “d(hkl)” spacing, lattice volume, sample density, X-rays density, porosity, and specific surface area are all given in Table.1 [9, 21].
40
(321)
(202)
(220) (002) (310) (301)
(111)
Intensity (a.u)
20
Standard Nano rods Nanospheres
(211)
(101)
(200)
(110)
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60
80
2Degree
Figure 1. XRD spectra of SnO2 nanorods (red) and nanospheres (blue) in comparison with the standard spectra (black). Table. 1 XRD Parameters
Parameters
SnO2 nanorods
SnO2 nanospheres
(hkl)
(110)
(101)
(211)
(110)
(101)
(211)
d(hkl) (Å)
3.3663
2.6435
1.7629
3.3460
2.6423
1.7644
Crystallite size. t (nm)
9.3335
11.722
11.466
19.2226
16.728
12.3735
5
9
Average crystallite size 𝑡 10.8410 ± 1.7832
9 16.1080 ± 3.8384
± 𝜎(nm) Lattice parameter a = b # 4.7606
3.1442
4.7320
c(Å) Lattice volume V (Å)
71.2597
Sample density ρ(M)= 1.5726 m
71.5239 30.3600
(g/cm3)
π𝑟2h
X-rays ρ(X) =𝑁
density 3.5150 M 𝐴𝑎
2
𝑐
3.5020
(g/cm3) ρ(M)
Porosity=1 ― ρ(X)
0.9037
0.7199
3.1942
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Specific surface area
19.6810
13.2954
6 𝑚2 = ( ) 𝑡ρ(X) g 3.2 FTIR Spectroscopy Fig. 2 shows FTIR spectra for the samples in the range of 250-4500 cm-1. The peak appearing at 503 cm−1 is due to the terminal oxygen vibration of Sn–O. The presence of SnO2 as a crystalline phase is confirmed by the peak at 640 cm−1 that corresponds to the O–Sn–O bridge functional groups of SnO2. This is in agreement with the results of XRD analysis (Fig. 1). The bands at 1636 cm−1 and 3411 cm−1 are attributed to O-H bending mode and O–H hydroxyl stretching vibrational mode due to absorbed water on the sample surface respectively. Alkyl CH deforming and stretching modes occur at 1109 cm-1 and 2914 cm-1, respectively. A band at 1370 cm-1 is assigned to the anti-symmetric stretching vibration of ON ions, arisen from the nitrates adsorbed on the surfaces of samples from the atmosphere. It is suggested that the high surface area of these nanostructured materials results in the rapid absorption of water from the atmosphere because the FTIR samples were kept in open-air [22, 23]. In the case of nanospheres, the peak at 530 cm-1 is attributed to the vibrational mode of Sn-O. The band at 709 cm-1 is assigned to the vibration of the O-Sn-O band in SnO2.
The
absorption peak at 1629 cm-1 is attributed to OH hydroxyl
bending mode and 3408 cm-1 band originates from the vibration of hydroxyl (OH) stretching mode. The broad and strong at 3408 cm-1 describes that there are several O-H bonds in the sample, suggesting the surfaces of the particles that are covered with hydroxyl species. Alkyl CH deforming and stretching modes occur at 1079 cm-1 and 2974 cm-1, respectively. A band at 1393 cm-1 is assigned to the anti-symmetric stretching vibration of ON- ions, arising from the nitrates adsorbed on the surface of samples from the atmosphere [24, 25].
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1.0 SnO2 nanospheres SnO2 nanorods
0.6
-CH-ON-
0.4
0
500
O-H
O-Sn-O
0.0
Sn-O
0.2
C-H
-OH-
Transmission (a.u)
0.8
1000 1500 2000 2500 3000 3500 4000 4500 -1
Wavenumber (cm ) Fig. 2 FTIR spectra of SnO2 nanorods and nanospheres.
3.3 Field Emission Scanning Electron Microscopy and Energy Dispersive X-Rays Spectroscopy Figs. 3 (a,b) and (c) show the FE-SEM images of the SnO2 solid nanospheres and nanorods respectively. The nanorods are uniformly distributed, while the spheres are non-uniformly distributed also confirmed somewhere else [25-27] 3.4 EDX Figs. 3 (d) and (e) depicts the composition of SnO2 samples, characterized by EDX that suggests that the samples are composed of Sn and O [9]. Italso shows the higher concentration of the tin element than the oxygen, which is in strong agreement with XRD results. In the adjacent table corresponds to nanospheres the atomic % of O in the nanorods is smaller than that of the nanospheres, suggesting that the former contain large numbers of vacancies of oxygen than the latter. The nanorods have a large surface area to accommodate the higher concentration of O vacancies. The atomic % of Sn is smaller in nanospheres than in the nanorods, showing that the number of vacancies of Sn is greater in nanospheres than in the nanorods [26].
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Fig.3. (a)- (c) show SEM images of SnO2 nanorods and nanospheres while (d) & (e) sow the EDX spectra.
There are also other peaks ranging from 1 to 3 keV, which is the Si and Pt that are due to the background of the sample holder. 3.5 Raman Spectroscopy The formation of tetragonal rutile SnO2 nanostructures was further characterized by Raman spectra. It is used to investigate the crystalline structure and to evaluate the grain size of SnO2 nanorods and nanospheres. Figure 4 presents the typical Raman spectra of the nanostructures in the 150–1000cm−1 region at room temperature. There are three Raman peaks, centered at 450, 622 and 789 cm-1 for the nanorods while at 460, 631 and 820 cm-1 for the nanospheres corresponding to the vibrational modes of Eg, A1g and B2g, respectively. These peaks correspond to rutile tetragonal SnO2 nanostructure. The Eg vibrational mode is sensitive to oxygen vacancies and its intensity increases with increasing concentration of oxygen vacancies. Careful observation of Fig. 4 shows that the intensity of Eg vibrational mode is larger in nanorods than in the nanospheres, revealing a greater number of oxygen vacancies in nanorods than in the nanospheres. The A1g vibrational mode is sensitive to crystallinity and its intensity increases with
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improvement in crystallization. The intensity of A1g is greater for nanospheres than for the nanorods. The peak at 789 and 820 cm−1 are assigned to B2g vibrational mode [15]. From Raman spectra, it is inferred that nanorods have a greater number of defects with the large specific surface area than the nanospheres. The results obtained from Raman spectra are congruent with that of EDX. SnO2 Nanorods
Intensity ( a.u.)
80
SnO2 Nanospheres
Eg A1g
60 40
B2g
20 0 200
400 600 800 -1 Raman Shift ( cm )
1000
Fig. 4 Room-temperature Raman spectra of SnO2 nanorods and nanospheres.
3.6 UV-Vis Spectroscopy UV-vis spectra are measured for understanding the optical properties of the nanostructures. The theory of optical absorption gives the relationship between the absorbance [F(R)] and reflectance R and is given by the so-called Kubelka–Munk function [9] expressed as, F(R) =
(1 ― 𝑅)2 2𝑅
.
(3)
Fig. 5 depicts the UV-vis spectra of SnO2 nanorods and spheres. The calculated bandgap values of nanorods and nanospheres are 3.88 eV and 3.66 eV respectively. The bandgap of SnO2 nanorods is blue shifted, due to its smaller size. In addition [22, 28, 29]. The larger bandgap of nanorods is due to the smaller size (a large number of oxygen vacancies) while the sharp rise of
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the absorption edge in nanospheres demonstrates the high crystallinity with less number of defects in the nanospheres.
SnO2 nanorods, Eg=3.87eV
24
SnO2 nanospheres, Eg=3.66 eV
(F(r)xE)
2
20 16 12 8 4 0
0
1
2
3
4
5
6
7
Enargy (eV) Fig. 5 Optical bandgap energy estimation of the SnO2 nanorods and nanospheres.
3.7 Photoluminescence (PL) Spectroscopy Fig. 6 shows the photoluminescence (PL) spectra of the SnO2 nanorods and nanospheres in the range of 400-800 nm at room temperature. For both morphologies, visible light emission is observed: for nanorods at 3.07 eV and the nanospheres at 2.9 eV. A broad green emission band with a central wavelength at 2.1 eV is observed for both nanorods and nanospheres. The emission of visible light for both samples is most probably due to crystal defects (cation and oxygen vacancies). Two other peaks are observed at 1.88 eV and 1.82 eV for nanorods and nanospheres, respectively [30]. The emission intensities increase with increasing defects densities (oxygen vacancies). The emission intensities for nanorods are higher than for nanospheres, indicating a large number of defects in nanorods. The smaller size of nanorods with a larger surface area accommodates a high concentration of oxygen vacancies. These vacancies act as color centers, emitting high visible light intensities in the visible region.
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SnO2 Nanospheres SnO2 Nanorods
2400
Intensity (a.u.)
2000 1600 1200 800 400 0
400
500
600
700
800
Wavelength (nm) Fig. 6 PL spectra of SnO2 nanorods and nanospheres.
3.8 Dielectric Constant Fig. 7(a) depicts the dependence of dielectric constant (εr) on frequency for the nanostructures at room temperature. Both morphologies exhibit the dispersion behavior, explained by the Maxwell-Wagner model. This model suggests that dielectric material contained well-conducting grain partitioned by insulating (poorly conducting) boundaries When the external field is applied, the charge carriers can easily move through the volume of conducting grains along field direction till they reach the poorly conducting grain boundaries. At these boundaries, the accumulation of a large number of charge carriers takes place, resulting in large polarization. Small size (large surface area with a large number of defects) contributes to polarization by trapping free charge carriers producing a large number of electric dipoles resulting in large polarization. At higher frequencies, due to fast switching of the direction of the applied field, the charge carriers are not able to reach grain boundaries and thus dielectric constant decreases. At high frequencies, frequency-independent behavior of dielectric constant is observed. The large values of dielectric constant (εr) of SnO2 nanorods as compared to SnO2 nanospheres are observed (This is due to a smaller size with a large surface area containing a large number of defects) [21, 22].
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3.9 Dielectric Loss (εʹʹ) Dielectric loss occurs when charge carriers are not able to follow the variations of the applied field. Dielectric loss (εʹʹ) of SnO2 nanorods and nanospheres are shown in Fig. 7(b). Both samples exhibited the dispersion behavior, i.e. high dielectric loss at low frequencies and frequency-independent behavior at high frequencies. The possible reasons for dielectric loss at low frequencies are hopping of ions, conduction process due to migration of ions and ionic polarization [31, 34].
SnO2 Nanospheres SnO2 Nanorods
600
10
Dielectric loss ()
Dielectric Constant (r)
700
500 400 300 200
8 6 4 2
100
(a)
0
3
10
4
10
5
10
6
10
7
10
0
8
10
Conductivity ac( cm)-1
SnO2 nanospheres SnO2 nanorods
160
tan(
120 80 40
(c) 3
10
4
10
5
6
10 10 log (rad/s)
7
10
8
10
(b) 3
4
10
log (rad/s) 200
0
SnO2 nanospheres SnO2 nanorods
10
5
6
10 10 log (rad/s)
7
10
8
10
2.5
SnO2 nanospheres SnO2 nanorods
2.0 1.5 1.0 0.5 0.0
(d) 3
10
4
10
5
10
6
10
7
10
log rad/s)
Fig. 7 (a) Variation of dielectric constant εt, (b) dielectric loss, (c) tangent loss (𝑡𝑎𝑛 𝛿) and (d) conductivity with frequencies for SnO2 nanorods and nanospheres.
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3.10 Dielectric Loss Factor (δ) Loss tangent (𝑡𝑎𝑛 𝛿) represents the dissipated energy in the dielectric. The variation of loss tangent of SnO2 nanorods and nanospheres are shown in Fig. 7(c). The term (𝑡𝑎𝑛𝛿) shows similar behavior as the dielectric loss (εʹʹ) due to direct proportionality between the two as shown in equation (4) [32]. 𝑡𝑎𝑛 𝛿 =
εʹʹ εʹ
(4)
The large (𝑡𝑎𝑛𝛿) values at low frequencies are due to space charge polarization. At large frequencies, charge carriers are not able to follow the electric field and as a result, 𝑡𝑎𝑛𝛿 shows no variations with frequencies. The nanorods show larger dielectric loss due to the large surface area accommodating a large number of defects. So a large number of defects in the nanorods leads to greater space-charge polarization than the nanospheres [33, 34], as confirmed by XRD and EDX. No relaxation peak is observed in the case of both samples. 3.11 Conductivity (σac) The frequency dependence of conductivity is shown for nanostructures at room temperature in Fig. 7(d). Initially, the conductivity increases slowly with increasing frequency but increases sharply at higher frequencies. At low frequencies of the applied field, transport of charge carriers takes place through infinite paths that result in a low magnitude of conductivity. At high frequencies, charge carriers start moving with the applied field which increases the conductivity. Enough energy is, at high frequencies, provided to charge carriers resulting in their motion along the field. Besides, the energy of the applied field is so high to liberate charge carriers from defects (traps, oxygen vacancies) to increase charge density (conduction charge carriers along with liberated charge carriers from defects) resulting in a sharp increase in conductivity. The conductivity of SnO2 nanospheres is higher than that of nanorods which is most possibly due to the larger grain size of the SnO2 nanospheres as compared to that of nanorods. Large grain size results in large conducting grain volume and the small number of resistive grain boundaries. Consequently, charge carriers can easily move through large conducting grain volume, resulting in high conductivity [21]. The sharp increase in conductivity of nanorods at large enough frequencies is because the field provides enough energy to bound charge carriers to set them free
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from defects in which charge carriers are trapped. These liberated charge carriers along with other conduction charge carriers result in a sharp increase of conductivity. 3.12 Magnetic Hysteresis (M-H) loops The ferromagnetism in any material depends on temperature, domain size, and impurities. Fig. 8 shows magnetic hysteresis (M–H) loops of SnO2 nanorods and spheres, measured at room temperature. The observed ferromagnetism in SnO2 nanorods and nanospheres are due to the presence of large numbers of oxygen vacancies, Sn interstitials, Si and Pt impurities, confirmed by EDX analysis. Similar results have been reported elsewhere. [34]. The saturation magnetization (Ms) was observed up to 0.0485 emu/g for nanorods, while 0.0315 emu/g for the nanospheres. The remnant magnetization (Mr) of 0.0074 emu/g with a coercive field (Hc) of 104 Oe was observed in the nanorods, while nanospheres have remnant magnetization (Mr) of 0.0048 emu/g and coercive field of 93 Oe. The saturation magnetization (Ms), remnant magnetization (Mr) and coercivity in SnO2 nanorods are greater than that of nanospheres [35-40]. The nanorods have a large surface area and more chances of defects due to which they have a high value of saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc) as compared to nanospheres [41, 42, 43].
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0.08
M(emu/g)
0.04
SnO2 nanospheres SnO2 nanorods
0.00
-0.04
-0.08 -6000 -4000 -2000
0
2000
4000
6000
H (Oe) Fig. 8 Room temperature (M-H) loops of SnO2 nanorods and nanospheres.
Conclusion SnO2 nanorods and nanospheres were synthesized by hydrothermal route. XRD confirmed the rutile tetragonal structure of SnO2 nanorods and nanospheres. The XRD peaks of nanorods are broader than that of nanospheres, which is due to small grain size and large defect density of the nanorods. The average crystallite size of SnO2 nanorods and nanospheres are 10.8410 ± 1.7832 and 16.1080 ± 3.8384, while porosity is 0.9437 and 0.7199 and surface area is 19.6810 m2g-1 and 13.2954 m2g-1, respectively. SEM showed the formation of SnO2 nanorods and spheres. EDX results revealed that the nanostructures contain only Sn and O and there are no impurity atoms and phases. There is lesser content of oxygen in the case of nanorods than in the spheres,
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showing that there are more oxygen-deficient regions (oxygen vacancies) in nanorods than in the nanospheres. In Raman spectra, A1g showed the improvement in crystallization. The intensity of the A1g peak is enhanced in the case of nanospheres as compared to nanorods. The intensity of the E1g peak is sensitive to oxygen vacancies and an increase in its intensity reveals more oxygen-deficient regions in nanorods. The intensity of E1g is high for nanorods, revealing that there are more oxygen vacancies. Raman spectra are in agreement with XRD, FTIR, and EDX. The estimated bandgap energy of nanorods and nanospheres are 3.89 eV and 3.67 eV, respectively, where the large bandgap of nanorods is consistent with their small size. The enhancement of PL peak intensities of nanorods is due to a large number of defects (oxygen vacancies). A large number of grains boundaries and smaller crystallite size with a high concentration of vacancies were caused due to high polarization. This high concentration of oxygen vacancies are available for trapping of conduction charge carriers, producing RDP and SCP resulting in an increased dielectric constant. The conductivity of nanospheres is slightly greater than that of nanorods, due to large conducting volume of grains of the former: the charge carriers can move freely through grains. In nanorods, at higher frequencies, the applied field provides enough energy to trapped charge carriers to set them free. These charge carriers along with conduction charge carriers enhance the conductivity of nanorods. The spin-spin interaction between the bound charge carriers and Sn ions provides ferromagnetic behavior both in nanorods and nanospheres, which is stronger in the case of nanorods. The saturation magnetization (Ms) is 0.0485 emu/g and 0.0315 emu/g; the remanent magnetization (Mr) is 0.0074 emu/g and 0.0048 emu/g and coercivity is 104 Oe and 93 Oe for nanorods and nanospheres, respectively.
Acknowledgments
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The first two authors Naveed Hussain and Zulfiqar have equal contribution in this paper. This work is financially supported by the Higher Education Research Endowment Fund (NO.PMU122/HEREF/2014- 15/Vol-111/) Khyber Pakhtunkhwa (KPK) Pakistan, Higher Education Commission under START-UP RESEARCH GRANT PROGRAM (Grant No: 21-1525/SRGP/R&D/HEC/2017), (Grant No: 21-1732/SRGP/ R&D/HEC/2017), the Fundamental Research Funds for the Higher Education Commission (HEC) Pakistan.
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Highlights
Successfully synthesized SnO2 nanorods and nanospheres by hydrothermal route. Nanorods have larger surface area and defect density than spheres. Emission intensities are enhanced in the case of nanorods due to large number of defects. Both dielectric properties and ferromagnetism are enhanced in the case of nanorods. Role of defects is of vital importance in tailoring the, dielectric and magnetic properties.