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Photocatalytic and electrochemical performance of hydrothermally synthesized cubic Cd2SnO4 nanoparticles
Materials Science and Engineering B 214 (2016) 37–45
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Photocatalytic and electrochemical performance of hydrothermally synthesized cubic Cd2SnO4 nanoparticles S. Dinesh a, M. Anandan a, V.K. Premkumar a, S. Barathan a,⇑, G. Sivakumar b, N. Anandhan c a
Department of Physics, Annamalai University, Annamalainagar, Chidambaram, Tamilnadu 608002, India CISL, Department of Physics, Annamalai University, Annamalainagar, Chidambaram, Tamilnadu 608002, India c School of Physics, Alagappa University, Science Block, Karaikudi, Tamilnadu 630004, India b
a r t i c l e
i n f o
Article history: Received 25 June 2016 Received in revised form 10 August 2016 Accepted 30 August 2016
Keywords: Photocatalysis Hydrothermal method Cd2SnO4 Cyclic voltammetry
a b s t r a c t Herein, we report the synthesis of cubic Cd2SnO4 nanoparticles through a simple hydrothermal method. Effect of annealing temperature on phase transition of cubic Cd2SnO4 nanoparticles was observed from Xray diffraction analysis and their structural parameters were determined. Results of thermo gravimetric/ differential thermal analyses indicate two peaks, one at 674 °C correspond to cubic phase and other one at 820 °C which confirms the phase transition of the prepared nanoparticles from cubic to orthorhombic. FESEM and TEM images illustrate the cubic shaped morphology of the samples with their size around 25 nm. From the optical measurements bandgap value of the prepared sample was estimated to be 2.59 eV. Photocatalytic performance of prepared cubic Cd2SnO4 nanoparticles were studied by degrading methyl green and methylene blue dyes under UV-light irradiation for a time of 90 min. Using cyclic voltammetry analysis electrochemical performance of the studied sample was investigated, which gave a capacitance value of 112 F g1. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Tailoring the microstructure and electronic structure of semiconductor photocatalysts via nano-engineering is currently a very attractive area of research. Photocatalysis is a highly engaging process with multiple applications, especially in the fields of photo electrochemistry, pollution removal and fuel production [1–4]. Metal oxide nanostructured materials have attracted much attention due to their prominent properties and potential applications in heterogeneous catalysis, optoelectronics, metallurgy, thin film coating, fine ceramic composites and photocatalysts [5]. ZnO, CuO, TiO2, CeO2, SnO2 and ZrO2 are some of the semiconductor oxide materials having attractive application in photocatalysis due to their environmental sustainability and high catalytic efficiency in degradation of various environmental pollutants such as pesticides, detergents, dyes and volatile organic compounds [6–11]. In recent past, ternary oxide materials like Zn2SnO4, Mn2SnO4, Mg2SnO4, Co2SnO4 and Cd2SnO4 attract many researchers due to their multifunctional properties. Among these materials Zn2SnO4 is widely studied due to its attractive properties, such as wide bandgap (3.6 eV), high-electron mobility, high-electrical ⇑ Corresponding author. E-mail address: [email protected] (S. Barathan). http://dx.doi.org/10.1016/j.mseb.2016.08.006 0921-5107/Ó 2016 Elsevier B.V. All rights reserved.
conductivity and low visible light absorption. And, these impressive properties make way to some important applications such as lithium ion batteries, gas sensors, solar cells and photocatalysis [12–15]. Recently we reported on the photocatalytic activity of hydrothermally synthesized Zn2SnO4 nanoparticles by degradation of methyl green dye under UV-light irradiation [15] and Qin et al. investigated the photocatalytic performance of Mg2SnO4/SnO2 heterostructures by degrading methylene blue dye and concluded that the red shift of band gap, oxygen vacancies, and higher specific surface areas of polyhedral Mg2SnO4/SnO2 heterostructures contribute to the high photocatalytic performance [16]. But, photocatalytic studies on other materials were still at the early stage. Cadmium stannate (Cd2SnO4) is one such material, which is widely studied as a transparent conducting oxide (TCO) material due to its unique optical and electronic properties [17,18]. Cd2SnO4 has an indirect bandgap of 2.5 eV, electrical conductivity of (1 10 X1 cm1) and mobility of (10 100 cm2 V1 s1), which makes this as a promising material for photovoltaic applications, optoelectronic devices and a potential photoanode material for solar water splitting [19–21]. In recent times, many techniques have been adopted for the synthesis of Cd2SnO4 materials such as, sputtering, spray pyrolysis, sol–gel method, co-precipitation, thermal combustion method and hydrothermal method [22–27]. However, it is hard to synthesize phase pure Cd2SnO4 because the formation of Cd2SnO4 generally accompanied with the
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impurities like SnO2, CdO, or CdSnO3 [28,29]. Among the above mentioned methods, hydrothermal method holds an advantage of low reaction temperature and also provides an efficient way to prepare well-crystallized and phase-pure nanoparticles [30]. Though research on application of Cd2SnO4 as a TCO material for photovoltaic applications and gas sensing are impressive, reports on photocatalytic application of Cd2SnO4 are very limited. Recently, Zhang et al. reported the photocatalytic efficiency of CdS, Cd2SnO4 and CdS/Cd2SnO4 nanoparticles in the degradation of RhB dye under visible light irradiation. They observed that CdS/Cd2SnO4 nanoparticles showed enhanced photocatalytic efficiency than CdS and Cd2SnO4 nanoparticles [27]. In this study, we successfully synthesized the cubic Cd2SnO4 nanoparticles using hydrothermal method and subsequently investigated the structural properties of as-synthesized and calcined Cd2SnO4 nanoparticles, followed by the morphological and optical studies of Cd2SnO4 nanoparticles. Photocatalytic activity of the prepared Cd2SnO4 nanoparticles was examined by the degradation of methyl green (MG) dye and methylene blue (MB) dye under UV-light illumination. Finally, electro chemical performance of the prepared cubic Cd2SnO4 nanoparticles was investigated. To the best of our knowledge, this is the first report on the photocatalytic performance of cubic Cd2SnO4 nanoparticles using MG dye.
2. Experimental Cd2SnO4 nanoparticles with cubic morphology were synthesized by a simple hydrothermal method. Cadmium acetate dihydrate and tin chloride pentahydrate purchased from Merck, India, were taken as the precursor materials. All the chemicals used were of analytic grade and used without further purification. In a typical procedure, 0.06 M of cadmium acetate dihydrate and 0.03 M of tin chloride pentahydrate were dissolved in 50 ml of deionized water respectively under constant stirring. Then the two solutions were mixed together under vigorous stirring; successively 1 M of KOH solution was added dropwise until a cloudy color mixed solution was formed. Then the solution was transferred into a 200 ml Teflon coated stainless steel autoclave and kept in a hot air oven at 180 °C for 12 h. After the reaction, autoclave was allowed to cool at room temperature and the final product was filtered and then washed with ethanol and deionized water several times. Finally, the product was dried in air at 80 °C for 3 h and used for further characterizations. Fig. 1 represents the graphical illustration of synthesis procedure of cubic Cd2SnO4 nanoparticles. 2.1. Characterization The crystal structure and phase purity of the prepared samples were analyzed by X-ray diffraction using a PAN analytical X’PERTPRO diffractometer employed with Cu Ka radiation (k = 1.5406 Å) at a scan step size of 0.05 degree per second and covering the angle range of 10° to 80°. Morphological aspect and the size of the prepared samples were characterized by field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HR-TEM). For FESEM studies ZEISS supra 40VP model was used with an acceleration voltage of 5 kV and the same instrument was used for the energy-dispersive X-ray (EDX) measurements, whereas for HR-TEM analysis JEOL JEM-2000EX microscope was used with an accelerating voltage of 200 kV. Optical properties of the prepared samples were investigated by a SHIMADZU 1800 UV–vis spectrometer analyzed over the range of 200–800 nm. Thermo gravimetric and differential thermal analyses (TG/DTA) were performed by SDT Q600 20 thermal analyzer.
2.2. Photocatalysis A multi-lamp photoreactor with UV lamp (365 nm) was used to perform the photocatalytic reaction under ambient temperature (303 K). In a typical experiment, 40 ml aqueous suspensions of dye (1 105 M) and 0.20 g of the photocatalyst were loaded in reaction tubes. In order to establish an adsorption–desorption equilibrium, the suspension was magnetically stirred for half an hour in the dark before irradiation and the suspension was maintained under constant air-equilibrated conditions. Then the degradation of dye at different irradiation time intervals was examined with the help of UV–vis spectrometer. 2.3. Electrochemical analysis Cyclic voltammetry analysis was carried out to investigate the electrochemical properties of prepared cubic Cd2SnO4 nanoparticles, and was performed by an electrochemical work station (Model CHI 660) at different scan rates 10, 20, 30, 40 and 50 mV s1 operated between 1.5 V and +1.6 V at room temperature. Further, the electro chemical measurements were observed by 0.2 M tetra butyl ammonium per chlorate electrolyte with a standard three electrode system comprising of a working electrode where the sample will be placed, an Ag/AgCl electrode which acts as a reference electrode and a high platinum wire which is used as a counter electrode. 3. Results and discussion 3.1. Structural studies In order to obtain the desired cubic Cd2SnO4 nanostructures, the as-synthesized nanoparticles were calcined at different temperatures (500 °C, 700 °C and 900 °C). The crystal structure and phase purity of the as-synthesized and calcined samples were investigated by XRD method. Fig. 2 shows the typical XRD patterns of as-synthesized and calcined samples. It is evident that synthesized nanoparticles undergone phase changes at different calcination temperatures. As-synthesized sample has a mixed phase with impurities of CdO and SnO2, when the sample undergone heating at 500 °C for 3 h the peaks of impurities like CdO and SnO2 gets reduced which might be the indicative of cubic phase formation. Further, the annealing temperature was increased to 700 °C which exhibit a pure cubic phase nature. Fig. 2c shows the XRD pattern of Cd2SnO4 nanoparticles calcined at 700 °C for 3 h, the peaks at 27.56°, 32.35°, 33.81°, 39.32°, 48.60°, 51.80°, 66.96° and 67.79° corresponding to (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (5 3 3) and (6 2 2) planes could be well indexed with the standard JCPDS (#80–1468) which confirms the cubic phase without any other impurities. When the calcination temperature is further increased to 900 °C a drastic change in phase was observed, with major peak positioned at 31.56° corresponding to (1 3 0) plane and all the other peaks in Fig. 2d were in good agreement with JCPDS (#80– 1466) which indicates the orthorhombic structure of the sample [31]. In the present study, as the sample calcined at 700 °C possess the desired phase cubic structure further characterization were carried out for this sample alone. The average crystallite size of the samples was calculated using Scherrer’s formula [32],
D ¼ kk=b cos h
ð1Þ
where, ‘D’ is the average crystallite size, ‘k’ is the wavelength of radiation (1.5406 Å), k is a constant(0.94), b is the FWHM and h is the Bragg’s angle. Other structural parameters such as dislocation
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Fig. 1. Graphical illustration of Cd2SnO4 nanoparticles synthesis procedure.
By calculating the texture coefficient (Tc), the quantitative information regarding preferential crystal orientation could be determined [36]. Texture coefficient can be determined by the formula,
T c ðhklÞ ¼
ðN1 Þ½
IðhklÞ=I0 ðhklÞ X IðhklÞ=I0 ðhklÞ
ð5Þ
n
Fig. 2. (a–d) XRD pattern of as-synthesized and calcined (500 °C, 700 °C and 900 °C) Cd2SnO4 nanoparticles.
density (d), micro strain (e) and stacking fault (SF) were also determined from the XRD data by following equations [33–35],
d¼
e¼
1
ð2Þ
D2 bcosh 4 "
SF ¼
ð3Þ 2p
#
2
45ðtanhÞ
1=2
b
ð4Þ
If all the above mentioned factors were minimum, then the crystallinity of the prepared sample should be better. It is evident from Table 1 that the values of dislocation density (d), micro strain (e) and stacking fault (SF) were small and indicates the high crystallinity of synthesized Cd2SnO4 nanoparticles.
where, Tc (hkl) represents the texture coefficient of the (hkl) plane, i is the measured intensity, I0 is standard intensity from JCPDS and N is the number of diffraction peaks considered. For textured coefficient, it is known that value of Tc will be close to unity for a randomly distributed crystallites and value of Tc will be greater than 1 if the (hkl) plane of the sample is preferentially oriented. Table 1 show the calculated Tc value for cubic Cd2SnO4 nanoparticles and indicating that prepared sample is highly oriented towards (3 1 1) plane. If the size of the particles were in the range of below 100 nm, broadening of X-ray diffraction lines might occur. This strain induced broadening caused by crystal imperfections and distortion are correlated by e = bs/tanh and can be investigated by Williamson and Hall methods (W-H method). W-H analysis is a simplified integral breadth method where size-induced and strain-induced broadening is de-convoluted by considering the peak width as a function of 2h [37]. Unlike Scherrer’s method which depends on 1/cosh, the W-H method varies with tanh. This dependency of h on both effects set the foundation for the separation of size and strain broadening in the analysis of Williamson and Hall method. By considering Scherrer’s equation and e = bs/tanh, following equation can be written
bhkl cosh ¼
kk þ 4etanh D
ð6Þ
where, D is the crystallite size, h is bragg angle, b is the full width half maximum (FWHM), e is the lattice strain and k is the shape factor. Eq. (6) represents the uniform deformation model (UDM) of W-H method. The value of bcosh was plotted against 4sinh for the preferred orientation peaks of cubic Cd2SnO4 nanoparticles and was linearly fitted. The slope of the fitted line gives the strain,
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Table 1 Various structural parameters calculated from XRD pattern. Sample name
dhkl (nm)
Dislocation density (d) 1015
Micro strain (e) 103
Lattice strain (g)
Stacking fault (SF)
Texture coefficient
Particle size (nm) Scherrer’s formula
W-H plot
CTO-7
0.276
1.11
1.0158
0.037
1.134
1.034
30
29
Fig. 4. TG/DTA graph of Cd2SnO4 nanoparticles. Fig. 3. W-H plot of Cd2SnO4 nanoparticles calcined at 700 °C.
whereas y-intercept gives the particle size. Fig. 3 illustrates the W-H plot cubic Cd2SnO4 nanoparticles and the calculated values of strain and particle size were tabulated in Table 1. It is observed that the calculated particle size (29 nm) using W-H plot is in good agreement with the size calculated by using Scherrer’s method (30 nm). 3.2. Thermal analysis Thermal stability of the synthesized nanoparticles was analyzed by the thermo gravimetric and differential thermal analyses (TG/ DTA). Fig. 4 shows the TG/DTA curves of the cubic Cd2SnO4 nanoparticles, from the figure it is visible that TG exhibits three steps of weight loss. The first weight loss in the range between 105 and 200 °C could be ascribed to the sudden removal of water content in the sample, during the second weight loss between 200 and 340 °C all the organic components might decomposes and finally the third weight loss above 400 °C which is the important one where the crystallization of the synthesized material gets initiated. At 580 °C a total weight loss of 39% was occurred, above this no weight loss recurs. From this point decomposition gets stopped and the phase formation occurs, which is evident by the DTA curve. One endothermic peak followed by two exothermic peaks was observed from the DTA curve, endothermic peak at 170 °C is due to the degradation of water content in the sample. Exothermic peak at 674 °C reveals the cubic phase formation of the synthesized nanoparticles and the other peak at 820 °C might be the indicative for the occurrence of phase transition from cubic to orthorhombic. This phase transition from cubic to orthorhombic was observed in XRD patterns of calcined samples.
tions, from the images it is evident that synthesized Cd2SnO4 nanoparticles does have cubic structures with some irregular cubic shaped nanoparticles. Elemental composition of the prepared sample was investigated by EDX measurements; Fig. 5c represents the EDX graph of the cubic Cd2SnO4 nanoparticles. From the figure it is evident that prepared sample contains Cd, Sn and O without any major impurities other than the carbon peak which might be due to the carbon tape used in the sample holder during the analysis. To know the structure of the prepared cubic Cd2SnO4 nanoparticles in detail, TEM analysis was carried out. Fig. 6a & b illustrates the TEM images, where Fig. 6a clearly shows the presence of cubic shaped nanoparticles with their size ranges between 20 and 25 nm. Average particle size of the TEM images were in accordance with the values calculated from the XRD measurements in Table 1. Fig. 6b shows the lattice fringes of the synthesized nanoparticles with an accurate ‘d’ spacing value of 0.27 nm, which indicates the structural characteristics of cubic spinel Cd2SnO4 nanoparticles is along the (3 1 1) plane and the inset image showcase the corresponding SAED pattern which is in agreement with the XRD pattern of cubic Cd2SnO4 nanoparticles. 3.4. Optical analysis Optical properties of the synthesized cubic spinel Cd2SnO4 nanoparticles were analyzed using a UV–visible spectrometer. Fig. 7a shows the absorption spectra of Cd2SnO4 nanoparticles calcined at 700 °C, from figure it is seen that maximum absorption was observed around 300 nm. Similar results were observed by other researchers [38,27]. Tauc’s formula [39] was used to determine the optical absorption band gap. It is given by,
3.3. Morphological studies
ðahv Þ
Morphological studies of the prepared cubic Cd2SnO4 nanoparticles were investigated by FESEM. Fig. 5a & b shows the FESEM images of the cubic Cd2SnO4 nanoparticles at different magnifica-
where, a is absorption coefficient, hv is photon energy, K is a constant corresponds to the material, and n depends on the nature of transition in a semiconductor. For allowed direct transition n = 1/2
1=n
¼ Kðhv Eg Þ
ð7Þ
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Fig. 5. (a & b) represents the FESEM images of cubic Cd2SnO4 nanoparticles at different magnifications and (c) represents the corresponding EDX spectra.
Fig. 6. (a & b) TEM images of cubic Cd2SnO4 nanoparticles and the inset illustrates the corresponding SAED pattern.
and for indirect transition n = 2, it is known that Cd2SnO4 is a direct semiconductor and hence, by plotting (ahv)2 vs hv the optical bandgap, Eg can be determined. Fig. 7b illustrates the tauc plot of synthesized Cd2SnO4 nanoparticles, from the graph the bandgap (Eg) value is estimated to be 2.59 eV by intersecting the extrapolated linear portion with (hv) x-axis. 3.5. Surface area analysis Fig. 8 represents the N2 adsorption – desorption isotherms of cubic Cd2SnO4 nanoparticles, whereas inset indicates the corresponding pore size distribution curve. From the figures it is apparent that Cd2SnO4 nanoparticles display type IV isotherm with the hysteresis appears in the range of 0.4–1.0 P/P0 which implies the mesoporous nature of the synthesized samples [40]. Inset of
Fig. 8 shows the plot of pore volume vs pore diameter, from the figure average pore size can be estimated using BarrettJoyner-Halenda (BJH) method, for cubic Cd2SnO4 sample pore size was calculated as 4.78 nm with a pore volume of 0.263 cm3 g1, which further confirms the mesoporous nature of the synthesized samples. The specific surface area of the cubic Cd2SnO4 nanoparticles was estimated as 32.27 m2/g using Brunauer-Emmett-Teller (BET) method. 3.6. Photocatalytic activity Photodegradation of MG and MB dyes were used to analyze the photocatalytic properties of synthesized cubic Cd2SnO4 nanoparticles using a multilamp photoreactor with UV lamp (365 nm). Decrease in absorption intensity is used as an indicator for the
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Fig. 7. (a) UV–vis absorption spectra of cubic Cd2SnO4 nanoparticles and (b) Tauc’s Plot for bandgap determination of hydrothermally synthesized Cd2SnO4 nanoparticles.
in Fig. 10. In general, organic dyes follow apparent first order kinetics which is in agreement with general Langmuir-Hinshelwood mechanism. If C is considered to be tiny then the equation can be given as [41];
lnðC0 =CÞ ¼ kKt kapp t
Fig. 8. Nitrogen adsorption–desorption isotherms of Cd2SnO4 nanoparticles and inset shows its pore size distribution plot.
degradation of respective dyes; Fig. 9(a & b) shows the absorption spectra of MG and MB dye at different time intervals in the presence of cubic Cd2SnO4 photocatalyst. Before irradiation the UV–vis absorption spectra of MG dye and MB dye showed intense peaks at 626 nm and 661 nm, respectively. Under dark lighting conditions in the presence of Cd2SnO4 photocatalyst (0.20 g/L) prominent peaks of both dyes remained stable. Once the irradiation process gets started absorption intensity of both MG and MB dye gets decreased periodically with an increase in time exposure. This indicates the degradation of methyl green and methylene blue dyes in the presence of Cd2SnO4 photocatalyst. Fig. 9c illustrates the % degradation rate of MG dye at different time intervals, which is defined as:
% degradation ¼ ðC0 C=C0 Þ 100
ð8Þ
where, C0 denotes the initial absorbance whereas ‘C’ represents the absorbance at time, t. From Fig. 9c it is clear that MG dye underwent a maximum degradation of 82% for 90 min of UV light illumination in the presence of Cd2SnO4 catalyst whereas MB dye had 86% of degradation over 90 min of UV-light irradiation. Furthermore, the kinetic study of the MG dye and MB degradation was illustrated
ð9Þ
where, C0 is the initial concentration, C is the concentration corresponding to different time intervals t And kapp Is the rate constant. From the ln(C0/C) vs time plot, rate constant (kapp) for MG and MB dye is calculated as 0.87 min1 and 0.92 min1, respectively. Photocatalytic activity of cubic Cd2SnO4 nanoparticles against organic dye degradation can be ascribed to the presence of large number of oxygen vacancies in the synthesized photocatalyst. Formation of oxygen vacancies will leads to the higher density of reactive OH, which then facilliates the photocatalytic reaction. When the sample is subject to UV light irradiation, the particles yield high-energy electron–hole pairs. These electron–hole pairs react with water and DISSOLVED oxygen to produce OH free radicals with high chemical activity, and react with the dyes molecule adsorbed on the surface of photocatalyst. Then the oxidation–reduction reaction takes place which in turn initiate the photodegradation of dye molecules [42]. 3.7. Electrochemical studies The electrochemical properties of the prepared cubic Cd2SnO4 nanoparticles were investigated by cyclic voltammetry (CV) analysis. Fig. 11 shows the CV patterns of the synthesized sample at different scan rates. From Fig. 11 it is evident that shape of the patterns was different from conventional electric double layer capacitance (EDLC), where the shape of the CV patterns must be close to a rectangular shape. Hence, the difference of shape in CV might be due to the pseudo capacitive properties of the synthesized nanoparticles. The presence of pseudo capacitive peaks in cyclic voltagrams further confirms the pseudo capacitive nature of the material, which is due to the reversible electrochemical reactions. Pseudo capacitance holds the advantage of high energy transfer during the faradaic reaction and leads to the better storage than the conventional EDLC’s [43]. The specific capacitance (Cs) can be estimated from the following equation [44],
Cs ¼
Q Dv :m
ð10Þ
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Fig. 9. (a & b) UV–vis absorption spectra of MG and MB dye solution at different time intervals in the presence of cubic Cd2SnO4Cd2SnO4 nanoparticles as photocatalyst and (c) extent of degradation rate of methyl green and methylene blue dyes at different time intervals.
Fig. 10. Kinetics study of methyl green dye (a) and methylene blue dye (b).
where, Q is the average charge during anodic and cathodic scan, m is the mass of the active material used in (g) and Dv is the scan rate (mV/s). The calculated specific capacitance values for different scan rates have been tabulated in Table 2. From the table it is clear that synthesized cubic Cd2SnO4 nanoparticles calcined at 700 °C
exhibited good specific capacitance of 112 F g1 at the scan rate of 10 mV s1. Furthermore, when the scan rate increases the values of specific capacitance gets decreased steeply and attained a specific capacitance value of 8 F g1 at 50 mV s1. From the present study it is observed that maximum specific capacitance value was obtained
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Facility (SAIF), Cochin, Kerala, India for providing their analytical instrument facilities. References
Fig. 11. Cyclic voltagrams of cubic Cd2SnO4 nanoparticles at different scan rates.
Table 2 Shows the specific capacitance values corresponding to different scan rates. Scan rate (mV s1)
Specific capacitance (F g1)
10 20 30 40 50
112 60 23 13 8
at low scan rate, which might arise from the low faradaic reaction. When the samples scanned at higher scan rates, occurrence of the ionic diffusion will arise from the outer regions of the surface alone, on the other hand at low scan rates, both inner and outer surfaces are responsible for tuning the specific capacitance and this leads to the higher values of capacitance at low scan rates [45].
4. Conclusion Cubic Cd2SnO4 nanoparticles have been successfully synthesized via simple hydrothermal method. XRD results showed that the sample calcined at 700 °C exhibited cubic phase which is further confirmed by TG/DTA results. FESEM and TEM reveal the cubic morphology of the synthesized cubic Cd2SnO4 nanoparticles and EDX spectra confirms the presence of Cd, Sn and O elements without any impurities. Average particle size from TEM image ranges between 20 and 25 nm which agrees with the average crystallite size 30 nm calculated by Scherrer’s method from XRD pattern. The prepared cubic Cd2SnO4 nanoparticles showed good photocatalytic performance against MG and MB dyes; however cubic Cd2SnO4 nanoparticles calcined at 700 °C exhibited better performance for MB dye under UV-light irradiation by degrading 86% of dye solution over a time period of 90 min. From cyclic voltammetry analysis, specific capacitance value of 112 F g1 at a scan rate of 10 mV s1 was observed. From the observed results, it is concluded that the synthesized cubic Cd2SnO4 nanoparticles could be a potential candidate for super capacitor applications. Acknowledgements The authors wish to thank Centralized Instrumentation and Services Laboratory (CISL), Annamalai University, Annamalai nagar, Tamilnadu, India and Sophisticated Analytical Instrumentation
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Photocatalytic and electrochemical performance of hydrothermally synthesized cubic Cd2SnO4 nanoparticles S. Dinesh a, M. Anandan a, V.K. Premkumar a, S. Barathan a,⇑, G. Sivakumar b, N. Anandhan c a
Department of Physics, Annamalai University, Annamalainagar, Chidambaram, Tamilnadu 608002, India CISL, Department of Physics, Annamalai University, Annamalainagar, Chidambaram, Tamilnadu 608002, India c School of Physics, Alagappa University, Science Block, Karaikudi, Tamilnadu 630004, India b
a r t i c l e
i n f o
Article history: Received 25 June 2016 Received in revised form 10 August 2016 Accepted 30 August 2016
Keywords: Photocatalysis Hydrothermal method Cd2SnO4 Cyclic voltammetry
a b s t r a c t Herein, we report the synthesis of cubic Cd2SnO4 nanoparticles through a simple hydrothermal method. Effect of annealing temperature on phase transition of cubic Cd2SnO4 nanoparticles was observed from Xray diffraction analysis and their structural parameters were determined. Results of thermo gravimetric/ differential thermal analyses indicate two peaks, one at 674 °C correspond to cubic phase and other one at 820 °C which confirms the phase transition of the prepared nanoparticles from cubic to orthorhombic. FESEM and TEM images illustrate the cubic shaped morphology of the samples with their size around 25 nm. From the optical measurements bandgap value of the prepared sample was estimated to be 2.59 eV. Photocatalytic performance of prepared cubic Cd2SnO4 nanoparticles were studied by degrading methyl green and methylene blue dyes under UV-light irradiation for a time of 90 min. Using cyclic voltammetry analysis electrochemical performance of the studied sample was investigated, which gave a capacitance value of 112 F g1. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Tailoring the microstructure and electronic structure of semiconductor photocatalysts via nano-engineering is currently a very attractive area of research. Photocatalysis is a highly engaging process with multiple applications, especially in the fields of photo electrochemistry, pollution removal and fuel production [1–4]. Metal oxide nanostructured materials have attracted much attention due to their prominent properties and potential applications in heterogeneous catalysis, optoelectronics, metallurgy, thin film coating, fine ceramic composites and photocatalysts [5]. ZnO, CuO, TiO2, CeO2, SnO2 and ZrO2 are some of the semiconductor oxide materials having attractive application in photocatalysis due to their environmental sustainability and high catalytic efficiency in degradation of various environmental pollutants such as pesticides, detergents, dyes and volatile organic compounds [6–11]. In recent past, ternary oxide materials like Zn2SnO4, Mn2SnO4, Mg2SnO4, Co2SnO4 and Cd2SnO4 attract many researchers due to their multifunctional properties. Among these materials Zn2SnO4 is widely studied due to its attractive properties, such as wide bandgap (3.6 eV), high-electron mobility, high-electrical ⇑ Corresponding author. E-mail address: [email protected] (S. Barathan). http://dx.doi.org/10.1016/j.mseb.2016.08.006 0921-5107/Ó 2016 Elsevier B.V. All rights reserved.
conductivity and low visible light absorption. And, these impressive properties make way to some important applications such as lithium ion batteries, gas sensors, solar cells and photocatalysis [12–15]. Recently we reported on the photocatalytic activity of hydrothermally synthesized Zn2SnO4 nanoparticles by degradation of methyl green dye under UV-light irradiation [15] and Qin et al. investigated the photocatalytic performance of Mg2SnO4/SnO2 heterostructures by degrading methylene blue dye and concluded that the red shift of band gap, oxygen vacancies, and higher specific surface areas of polyhedral Mg2SnO4/SnO2 heterostructures contribute to the high photocatalytic performance [16]. But, photocatalytic studies on other materials were still at the early stage. Cadmium stannate (Cd2SnO4) is one such material, which is widely studied as a transparent conducting oxide (TCO) material due to its unique optical and electronic properties [17,18]. Cd2SnO4 has an indirect bandgap of 2.5 eV, electrical conductivity of (1 10 X1 cm1) and mobility of (10 100 cm2 V1 s1), which makes this as a promising material for photovoltaic applications, optoelectronic devices and a potential photoanode material for solar water splitting [19–21]. In recent times, many techniques have been adopted for the synthesis of Cd2SnO4 materials such as, sputtering, spray pyrolysis, sol–gel method, co-precipitation, thermal combustion method and hydrothermal method [22–27]. However, it is hard to synthesize phase pure Cd2SnO4 because the formation of Cd2SnO4 generally accompanied with the
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impurities like SnO2, CdO, or CdSnO3 [28,29]. Among the above mentioned methods, hydrothermal method holds an advantage of low reaction temperature and also provides an efficient way to prepare well-crystallized and phase-pure nanoparticles [30]. Though research on application of Cd2SnO4 as a TCO material for photovoltaic applications and gas sensing are impressive, reports on photocatalytic application of Cd2SnO4 are very limited. Recently, Zhang et al. reported the photocatalytic efficiency of CdS, Cd2SnO4 and CdS/Cd2SnO4 nanoparticles in the degradation of RhB dye under visible light irradiation. They observed that CdS/Cd2SnO4 nanoparticles showed enhanced photocatalytic efficiency than CdS and Cd2SnO4 nanoparticles [27]. In this study, we successfully synthesized the cubic Cd2SnO4 nanoparticles using hydrothermal method and subsequently investigated the structural properties of as-synthesized and calcined Cd2SnO4 nanoparticles, followed by the morphological and optical studies of Cd2SnO4 nanoparticles. Photocatalytic activity of the prepared Cd2SnO4 nanoparticles was examined by the degradation of methyl green (MG) dye and methylene blue (MB) dye under UV-light illumination. Finally, electro chemical performance of the prepared cubic Cd2SnO4 nanoparticles was investigated. To the best of our knowledge, this is the first report on the photocatalytic performance of cubic Cd2SnO4 nanoparticles using MG dye.
2. Experimental Cd2SnO4 nanoparticles with cubic morphology were synthesized by a simple hydrothermal method. Cadmium acetate dihydrate and tin chloride pentahydrate purchased from Merck, India, were taken as the precursor materials. All the chemicals used were of analytic grade and used without further purification. In a typical procedure, 0.06 M of cadmium acetate dihydrate and 0.03 M of tin chloride pentahydrate were dissolved in 50 ml of deionized water respectively under constant stirring. Then the two solutions were mixed together under vigorous stirring; successively 1 M of KOH solution was added dropwise until a cloudy color mixed solution was formed. Then the solution was transferred into a 200 ml Teflon coated stainless steel autoclave and kept in a hot air oven at 180 °C for 12 h. After the reaction, autoclave was allowed to cool at room temperature and the final product was filtered and then washed with ethanol and deionized water several times. Finally, the product was dried in air at 80 °C for 3 h and used for further characterizations. Fig. 1 represents the graphical illustration of synthesis procedure of cubic Cd2SnO4 nanoparticles. 2.1. Characterization The crystal structure and phase purity of the prepared samples were analyzed by X-ray diffraction using a PAN analytical X’PERTPRO diffractometer employed with Cu Ka radiation (k = 1.5406 Å) at a scan step size of 0.05 degree per second and covering the angle range of 10° to 80°. Morphological aspect and the size of the prepared samples were characterized by field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HR-TEM). For FESEM studies ZEISS supra 40VP model was used with an acceleration voltage of 5 kV and the same instrument was used for the energy-dispersive X-ray (EDX) measurements, whereas for HR-TEM analysis JEOL JEM-2000EX microscope was used with an accelerating voltage of 200 kV. Optical properties of the prepared samples were investigated by a SHIMADZU 1800 UV–vis spectrometer analyzed over the range of 200–800 nm. Thermo gravimetric and differential thermal analyses (TG/DTA) were performed by SDT Q600 20 thermal analyzer.
2.2. Photocatalysis A multi-lamp photoreactor with UV lamp (365 nm) was used to perform the photocatalytic reaction under ambient temperature (303 K). In a typical experiment, 40 ml aqueous suspensions of dye (1 105 M) and 0.20 g of the photocatalyst were loaded in reaction tubes. In order to establish an adsorption–desorption equilibrium, the suspension was magnetically stirred for half an hour in the dark before irradiation and the suspension was maintained under constant air-equilibrated conditions. Then the degradation of dye at different irradiation time intervals was examined with the help of UV–vis spectrometer. 2.3. Electrochemical analysis Cyclic voltammetry analysis was carried out to investigate the electrochemical properties of prepared cubic Cd2SnO4 nanoparticles, and was performed by an electrochemical work station (Model CHI 660) at different scan rates 10, 20, 30, 40 and 50 mV s1 operated between 1.5 V and +1.6 V at room temperature. Further, the electro chemical measurements were observed by 0.2 M tetra butyl ammonium per chlorate electrolyte with a standard three electrode system comprising of a working electrode where the sample will be placed, an Ag/AgCl electrode which acts as a reference electrode and a high platinum wire which is used as a counter electrode. 3. Results and discussion 3.1. Structural studies In order to obtain the desired cubic Cd2SnO4 nanostructures, the as-synthesized nanoparticles were calcined at different temperatures (500 °C, 700 °C and 900 °C). The crystal structure and phase purity of the as-synthesized and calcined samples were investigated by XRD method. Fig. 2 shows the typical XRD patterns of as-synthesized and calcined samples. It is evident that synthesized nanoparticles undergone phase changes at different calcination temperatures. As-synthesized sample has a mixed phase with impurities of CdO and SnO2, when the sample undergone heating at 500 °C for 3 h the peaks of impurities like CdO and SnO2 gets reduced which might be the indicative of cubic phase formation. Further, the annealing temperature was increased to 700 °C which exhibit a pure cubic phase nature. Fig. 2c shows the XRD pattern of Cd2SnO4 nanoparticles calcined at 700 °C for 3 h, the peaks at 27.56°, 32.35°, 33.81°, 39.32°, 48.60°, 51.80°, 66.96° and 67.79° corresponding to (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (5 3 3) and (6 2 2) planes could be well indexed with the standard JCPDS (#80–1468) which confirms the cubic phase without any other impurities. When the calcination temperature is further increased to 900 °C a drastic change in phase was observed, with major peak positioned at 31.56° corresponding to (1 3 0) plane and all the other peaks in Fig. 2d were in good agreement with JCPDS (#80– 1466) which indicates the orthorhombic structure of the sample [31]. In the present study, as the sample calcined at 700 °C possess the desired phase cubic structure further characterization were carried out for this sample alone. The average crystallite size of the samples was calculated using Scherrer’s formula [32],
D ¼ kk=b cos h
ð1Þ
where, ‘D’ is the average crystallite size, ‘k’ is the wavelength of radiation (1.5406 Å), k is a constant(0.94), b is the FWHM and h is the Bragg’s angle. Other structural parameters such as dislocation
S. Dinesh et al. / Materials Science and Engineering B 214 (2016) 37–45
39
Fig. 1. Graphical illustration of Cd2SnO4 nanoparticles synthesis procedure.
By calculating the texture coefficient (Tc), the quantitative information regarding preferential crystal orientation could be determined [36]. Texture coefficient can be determined by the formula,
T c ðhklÞ ¼
ðN1 Þ½
IðhklÞ=I0 ðhklÞ X IðhklÞ=I0 ðhklÞ
ð5Þ
n
Fig. 2. (a–d) XRD pattern of as-synthesized and calcined (500 °C, 700 °C and 900 °C) Cd2SnO4 nanoparticles.
density (d), micro strain (e) and stacking fault (SF) were also determined from the XRD data by following equations [33–35],
d¼
e¼
1
ð2Þ
D2 bcosh 4 "
SF ¼
ð3Þ 2p
#
2
45ðtanhÞ
1=2
b
ð4Þ
If all the above mentioned factors were minimum, then the crystallinity of the prepared sample should be better. It is evident from Table 1 that the values of dislocation density (d), micro strain (e) and stacking fault (SF) were small and indicates the high crystallinity of synthesized Cd2SnO4 nanoparticles.
where, Tc (hkl) represents the texture coefficient of the (hkl) plane, i is the measured intensity, I0 is standard intensity from JCPDS and N is the number of diffraction peaks considered. For textured coefficient, it is known that value of Tc will be close to unity for a randomly distributed crystallites and value of Tc will be greater than 1 if the (hkl) plane of the sample is preferentially oriented. Table 1 show the calculated Tc value for cubic Cd2SnO4 nanoparticles and indicating that prepared sample is highly oriented towards (3 1 1) plane. If the size of the particles were in the range of below 100 nm, broadening of X-ray diffraction lines might occur. This strain induced broadening caused by crystal imperfections and distortion are correlated by e = bs/tanh and can be investigated by Williamson and Hall methods (W-H method). W-H analysis is a simplified integral breadth method where size-induced and strain-induced broadening is de-convoluted by considering the peak width as a function of 2h [37]. Unlike Scherrer’s method which depends on 1/cosh, the W-H method varies with tanh. This dependency of h on both effects set the foundation for the separation of size and strain broadening in the analysis of Williamson and Hall method. By considering Scherrer’s equation and e = bs/tanh, following equation can be written
bhkl cosh ¼
kk þ 4etanh D
ð6Þ
where, D is the crystallite size, h is bragg angle, b is the full width half maximum (FWHM), e is the lattice strain and k is the shape factor. Eq. (6) represents the uniform deformation model (UDM) of W-H method. The value of bcosh was plotted against 4sinh for the preferred orientation peaks of cubic Cd2SnO4 nanoparticles and was linearly fitted. The slope of the fitted line gives the strain,
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Table 1 Various structural parameters calculated from XRD pattern. Sample name
dhkl (nm)
Dislocation density (d) 1015
Micro strain (e) 103
Lattice strain (g)
Stacking fault (SF)
Texture coefficient
Particle size (nm) Scherrer’s formula
W-H plot
CTO-7
0.276
1.11
1.0158
0.037
1.134
1.034
30
29
Fig. 4. TG/DTA graph of Cd2SnO4 nanoparticles. Fig. 3. W-H plot of Cd2SnO4 nanoparticles calcined at 700 °C.
whereas y-intercept gives the particle size. Fig. 3 illustrates the W-H plot cubic Cd2SnO4 nanoparticles and the calculated values of strain and particle size were tabulated in Table 1. It is observed that the calculated particle size (29 nm) using W-H plot is in good agreement with the size calculated by using Scherrer’s method (30 nm). 3.2. Thermal analysis Thermal stability of the synthesized nanoparticles was analyzed by the thermo gravimetric and differential thermal analyses (TG/ DTA). Fig. 4 shows the TG/DTA curves of the cubic Cd2SnO4 nanoparticles, from the figure it is visible that TG exhibits three steps of weight loss. The first weight loss in the range between 105 and 200 °C could be ascribed to the sudden removal of water content in the sample, during the second weight loss between 200 and 340 °C all the organic components might decomposes and finally the third weight loss above 400 °C which is the important one where the crystallization of the synthesized material gets initiated. At 580 °C a total weight loss of 39% was occurred, above this no weight loss recurs. From this point decomposition gets stopped and the phase formation occurs, which is evident by the DTA curve. One endothermic peak followed by two exothermic peaks was observed from the DTA curve, endothermic peak at 170 °C is due to the degradation of water content in the sample. Exothermic peak at 674 °C reveals the cubic phase formation of the synthesized nanoparticles and the other peak at 820 °C might be the indicative for the occurrence of phase transition from cubic to orthorhombic. This phase transition from cubic to orthorhombic was observed in XRD patterns of calcined samples.
tions, from the images it is evident that synthesized Cd2SnO4 nanoparticles does have cubic structures with some irregular cubic shaped nanoparticles. Elemental composition of the prepared sample was investigated by EDX measurements; Fig. 5c represents the EDX graph of the cubic Cd2SnO4 nanoparticles. From the figure it is evident that prepared sample contains Cd, Sn and O without any major impurities other than the carbon peak which might be due to the carbon tape used in the sample holder during the analysis. To know the structure of the prepared cubic Cd2SnO4 nanoparticles in detail, TEM analysis was carried out. Fig. 6a & b illustrates the TEM images, where Fig. 6a clearly shows the presence of cubic shaped nanoparticles with their size ranges between 20 and 25 nm. Average particle size of the TEM images were in accordance with the values calculated from the XRD measurements in Table 1. Fig. 6b shows the lattice fringes of the synthesized nanoparticles with an accurate ‘d’ spacing value of 0.27 nm, which indicates the structural characteristics of cubic spinel Cd2SnO4 nanoparticles is along the (3 1 1) plane and the inset image showcase the corresponding SAED pattern which is in agreement with the XRD pattern of cubic Cd2SnO4 nanoparticles. 3.4. Optical analysis Optical properties of the synthesized cubic spinel Cd2SnO4 nanoparticles were analyzed using a UV–visible spectrometer. Fig. 7a shows the absorption spectra of Cd2SnO4 nanoparticles calcined at 700 °C, from figure it is seen that maximum absorption was observed around 300 nm. Similar results were observed by other researchers [38,27]. Tauc’s formula [39] was used to determine the optical absorption band gap. It is given by,
3.3. Morphological studies
ðahv Þ
Morphological studies of the prepared cubic Cd2SnO4 nanoparticles were investigated by FESEM. Fig. 5a & b shows the FESEM images of the cubic Cd2SnO4 nanoparticles at different magnifica-
where, a is absorption coefficient, hv is photon energy, K is a constant corresponds to the material, and n depends on the nature of transition in a semiconductor. For allowed direct transition n = 1/2
1=n
¼ Kðhv Eg Þ
ð7Þ
S. Dinesh et al. / Materials Science and Engineering B 214 (2016) 37–45
41
Fig. 5. (a & b) represents the FESEM images of cubic Cd2SnO4 nanoparticles at different magnifications and (c) represents the corresponding EDX spectra.
Fig. 6. (a & b) TEM images of cubic Cd2SnO4 nanoparticles and the inset illustrates the corresponding SAED pattern.
and for indirect transition n = 2, it is known that Cd2SnO4 is a direct semiconductor and hence, by plotting (ahv)2 vs hv the optical bandgap, Eg can be determined. Fig. 7b illustrates the tauc plot of synthesized Cd2SnO4 nanoparticles, from the graph the bandgap (Eg) value is estimated to be 2.59 eV by intersecting the extrapolated linear portion with (hv) x-axis. 3.5. Surface area analysis Fig. 8 represents the N2 adsorption – desorption isotherms of cubic Cd2SnO4 nanoparticles, whereas inset indicates the corresponding pore size distribution curve. From the figures it is apparent that Cd2SnO4 nanoparticles display type IV isotherm with the hysteresis appears in the range of 0.4–1.0 P/P0 which implies the mesoporous nature of the synthesized samples [40]. Inset of
Fig. 8 shows the plot of pore volume vs pore diameter, from the figure average pore size can be estimated using BarrettJoyner-Halenda (BJH) method, for cubic Cd2SnO4 sample pore size was calculated as 4.78 nm with a pore volume of 0.263 cm3 g1, which further confirms the mesoporous nature of the synthesized samples. The specific surface area of the cubic Cd2SnO4 nanoparticles was estimated as 32.27 m2/g using Brunauer-Emmett-Teller (BET) method. 3.6. Photocatalytic activity Photodegradation of MG and MB dyes were used to analyze the photocatalytic properties of synthesized cubic Cd2SnO4 nanoparticles using a multilamp photoreactor with UV lamp (365 nm). Decrease in absorption intensity is used as an indicator for the
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Fig. 7. (a) UV–vis absorption spectra of cubic Cd2SnO4 nanoparticles and (b) Tauc’s Plot for bandgap determination of hydrothermally synthesized Cd2SnO4 nanoparticles.
in Fig. 10. In general, organic dyes follow apparent first order kinetics which is in agreement with general Langmuir-Hinshelwood mechanism. If C is considered to be tiny then the equation can be given as [41];
lnðC0 =CÞ ¼ kKt kapp t
Fig. 8. Nitrogen adsorption–desorption isotherms of Cd2SnO4 nanoparticles and inset shows its pore size distribution plot.
degradation of respective dyes; Fig. 9(a & b) shows the absorption spectra of MG and MB dye at different time intervals in the presence of cubic Cd2SnO4 photocatalyst. Before irradiation the UV–vis absorption spectra of MG dye and MB dye showed intense peaks at 626 nm and 661 nm, respectively. Under dark lighting conditions in the presence of Cd2SnO4 photocatalyst (0.20 g/L) prominent peaks of both dyes remained stable. Once the irradiation process gets started absorption intensity of both MG and MB dye gets decreased periodically with an increase in time exposure. This indicates the degradation of methyl green and methylene blue dyes in the presence of Cd2SnO4 photocatalyst. Fig. 9c illustrates the % degradation rate of MG dye at different time intervals, which is defined as:
% degradation ¼ ðC0 C=C0 Þ 100
ð8Þ
where, C0 denotes the initial absorbance whereas ‘C’ represents the absorbance at time, t. From Fig. 9c it is clear that MG dye underwent a maximum degradation of 82% for 90 min of UV light illumination in the presence of Cd2SnO4 catalyst whereas MB dye had 86% of degradation over 90 min of UV-light irradiation. Furthermore, the kinetic study of the MG dye and MB degradation was illustrated
ð9Þ
where, C0 is the initial concentration, C is the concentration corresponding to different time intervals t And kapp Is the rate constant. From the ln(C0/C) vs time plot, rate constant (kapp) for MG and MB dye is calculated as 0.87 min1 and 0.92 min1, respectively. Photocatalytic activity of cubic Cd2SnO4 nanoparticles against organic dye degradation can be ascribed to the presence of large number of oxygen vacancies in the synthesized photocatalyst. Formation of oxygen vacancies will leads to the higher density of reactive OH, which then facilliates the photocatalytic reaction. When the sample is subject to UV light irradiation, the particles yield high-energy electron–hole pairs. These electron–hole pairs react with water and DISSOLVED oxygen to produce OH free radicals with high chemical activity, and react with the dyes molecule adsorbed on the surface of photocatalyst. Then the oxidation–reduction reaction takes place which in turn initiate the photodegradation of dye molecules [42]. 3.7. Electrochemical studies The electrochemical properties of the prepared cubic Cd2SnO4 nanoparticles were investigated by cyclic voltammetry (CV) analysis. Fig. 11 shows the CV patterns of the synthesized sample at different scan rates. From Fig. 11 it is evident that shape of the patterns was different from conventional electric double layer capacitance (EDLC), where the shape of the CV patterns must be close to a rectangular shape. Hence, the difference of shape in CV might be due to the pseudo capacitive properties of the synthesized nanoparticles. The presence of pseudo capacitive peaks in cyclic voltagrams further confirms the pseudo capacitive nature of the material, which is due to the reversible electrochemical reactions. Pseudo capacitance holds the advantage of high energy transfer during the faradaic reaction and leads to the better storage than the conventional EDLC’s [43]. The specific capacitance (Cs) can be estimated from the following equation [44],
Cs ¼
Q Dv :m
ð10Þ
S. Dinesh et al. / Materials Science and Engineering B 214 (2016) 37–45
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Fig. 9. (a & b) UV–vis absorption spectra of MG and MB dye solution at different time intervals in the presence of cubic Cd2SnO4Cd2SnO4 nanoparticles as photocatalyst and (c) extent of degradation rate of methyl green and methylene blue dyes at different time intervals.
Fig. 10. Kinetics study of methyl green dye (a) and methylene blue dye (b).
where, Q is the average charge during anodic and cathodic scan, m is the mass of the active material used in (g) and Dv is the scan rate (mV/s). The calculated specific capacitance values for different scan rates have been tabulated in Table 2. From the table it is clear that synthesized cubic Cd2SnO4 nanoparticles calcined at 700 °C
exhibited good specific capacitance of 112 F g1 at the scan rate of 10 mV s1. Furthermore, when the scan rate increases the values of specific capacitance gets decreased steeply and attained a specific capacitance value of 8 F g1 at 50 mV s1. From the present study it is observed that maximum specific capacitance value was obtained
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Facility (SAIF), Cochin, Kerala, India for providing their analytical instrument facilities. References
Fig. 11. Cyclic voltagrams of cubic Cd2SnO4 nanoparticles at different scan rates.
Table 2 Shows the specific capacitance values corresponding to different scan rates. Scan rate (mV s1)
Specific capacitance (F g1)
10 20 30 40 50
112 60 23 13 8
at low scan rate, which might arise from the low faradaic reaction. When the samples scanned at higher scan rates, occurrence of the ionic diffusion will arise from the outer regions of the surface alone, on the other hand at low scan rates, both inner and outer surfaces are responsible for tuning the specific capacitance and this leads to the higher values of capacitance at low scan rates [45].
4. Conclusion Cubic Cd2SnO4 nanoparticles have been successfully synthesized via simple hydrothermal method. XRD results showed that the sample calcined at 700 °C exhibited cubic phase which is further confirmed by TG/DTA results. FESEM and TEM reveal the cubic morphology of the synthesized cubic Cd2SnO4 nanoparticles and EDX spectra confirms the presence of Cd, Sn and O elements without any impurities. Average particle size from TEM image ranges between 20 and 25 nm which agrees with the average crystallite size 30 nm calculated by Scherrer’s method from XRD pattern. The prepared cubic Cd2SnO4 nanoparticles showed good photocatalytic performance against MG and MB dyes; however cubic Cd2SnO4 nanoparticles calcined at 700 °C exhibited better performance for MB dye under UV-light irradiation by degrading 86% of dye solution over a time period of 90 min. From cyclic voltammetry analysis, specific capacitance value of 112 F g1 at a scan rate of 10 mV s1 was observed. From the observed results, it is concluded that the synthesized cubic Cd2SnO4 nanoparticles could be a potential candidate for super capacitor applications. Acknowledgements The authors wish to thank Centralized Instrumentation and Services Laboratory (CISL), Annamalai University, Annamalai nagar, Tamilnadu, India and Sophisticated Analytical Instrumentation
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