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Multifunctional performance of nanocrystalline tin oxide
Journal of Alloys and Compounds 723 (2017) 201e207
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Multifunctional performance of nanocrystalline tin oxide Rasmita Barik, Nishu Devi, Debkumar Nandi, Samarjeet Siwal, Sarit Kumar Ghosh, Kaushik Mallick* Department of Chemistry, University of Johannesburg, P.O. Box: 524, Auckland Park, 2006, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 May 2017 Received in revised form 15 June 2017 Accepted 16 June 2017 Available online 22 June 2017
Crystalline tin oxide, with nanometre size range in diameter, was synthesized using a solution phase approach. The oxide material showed the photocatalytic performance for the reduction of a xanthene based dye in presence of ultra-violet irradiation, where the conduction band electron is responsible for the reduction mechanism. In addition to that the calculated power density and energy density values of the synthesized tin oxide nanoparticles, in combination with graphitic carbon, is comparable with the electrochemical supercapacitor like materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: Crystalline tin oxide Solution phase synthesis Conduction band electron Electrochemical supercapacitor
1. Introduction Nanotechnology has emerged as one of the most exciting fields in material sciences and the synthesis of nanocrystalline materials is becoming one of the most fascinating techniques that has been applied in most of the research areas in science and engineering. Nanosized semiconductor metal oxides are known to possess distinctive functionalities due to their high density surface sites and have emerged as an important class of materials with the great potential for diverse applications in electronics [1], optics [2] and catalysis [3]. Size of the material has a profound effect on a number of intrinsic properties in systems including the band gap [4] and at the same time the surface to volume ratio can also have the consequence on the electronic [5], magnetic [6] and optical [7] properties of the system. Among the various properties of nanomaterials, semiconductor nanocrystals with quantum confinement effect leads to spatial enclosure of the electronic charge carriers within the crystal dimension and thus the transport properties of the system are largely affected by the size and geometry of the materials [8,9]. Tin dioxide, SnO2, is an n-type semiconductor with a wide-band gap of 3.6 eV [10], is considered as one of the smart materials, among the various semiconductor metal oxides, due to their good stability, nontoxicity and low cost which allow for its diverse
* Corresponding author. E-mail address: [email protected] (K. Mallick). http://dx.doi.org/10.1016/j.jallcom.2017.06.180 0925-8388/© 2017 Elsevier B.V. All rights reserved.
application. The physicochemical properties of SnO2 are closely linked to its size and shape. Report has been published for the synthesis of mesoporous SnO2, with large surface area, using a hydrothermal synthesis route for the photo-degradation of organic dyes [11]. The sensing property of the large pore sized SnO2-based thick films were investigated and reported for the detection of very low concentration of hydrogen sulphide [12]. A hydrothermal synthesis route was introduced for the synthesis of graphene-tin oxide composite and was applied as a catalyst for the electrochemical detection of dopamine [13,14]. A solution-based method has been employed for the synthesis of SnO2-MnO2 composites for the supercapacitor application [15]. SnO2 and carbon cloth flexible composite system showed excellent performance as an anode material in lithium-ion battery application [16]. Micro structured SnO2, has been reported [17] by a hydrothermal synthesis route, exhibit excellent photocatalytic performance with potential applications in waste water purification. Highly aligned nanorods of SnO2 were synthesized by a hydrothermal method on a graphene matrix and has been reported [18] for a gas sensing application with improved performances. Excellent organic vapour sensing property with long-term stability has been obtained from the interconnected SnO2 and Fe2O3 based nanotubes synthesized by combining the single nozzle electrospinning and thermal treatment methods [19]. In this work, crystalline SnO2 particles, with the diameter ranging from 5 to 25 nm, have been prepared using a solution phase approach. The synthesized material was used as a photocatalyst for
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the reduction of a xanthene based dye. In addition to that, a detailed study indicates that the crystalline SnO2 particles have the potential as an electrochemical supercapacitor when combined with graphitic carbon. The synthesized nanomaterial was also characterized using different optical, microscopic and surface characterization techniques. 2. Experimental 2.1. Materials All the chemicals used for this study including stannous chloride, citric Acid, sodium hydroxide and xanthene based dye were purchased from Merck. The chemicals and solvents were used as received without further purification. All the solutions were prepared using mili-Q water. 2.2. Synthesis of nanostructured SnO2 In a typical synthesis method, 40 mL of SnCl2 (101 mol dm3) was taken in a three-neck flask equipped with a condenser and thermometer. A water solution of citric acid (10 mL, 102 mol. dm3) was added dropwise to the stannous chloride solution under continuous stirring condition under the room temperature. After the complete addition, the system was heated at 60 C and water solution of sodium hydroxide (101 mol. dm3) was added dropwise to the above mixture solution until the complete precipitation was formed and later allowed to cool at the room temperature. The solution was centrifuged and washed with ethanol, acetone and water for several times. The solid material was collected and dried in oven. The dried material was further calcined at 500 C for 4 h. Finally, the synthesized powder material was characterized using microscopic and spectroscopic techniques and also applied as a catalyst for a photochemical reaction. The synthesized material, in combination with graphitic carbon, was also studied for the electrochemical charge-discharge performance for the supercapacitor application. 2.3. Photocatalytic experiments Initially, 20 mg of the synthesized material was suspended in 100 mL of xanthene dye (104 mol dm3) in a conical flask and placed inside the dark room, under the continuous starring condition for the period of 30 min, to allow an adsorption-desorption equilibrium. After that the conical flask was placed under the Philips UV-C (TUV T8) (germicidal) lamp with optical intensity value of ~40 mW/cm2 adjacent to the solution. For a control experiment, the second set was exposed under daylight with optical intensity value of 3.5 mW/cm2 adjacent to the solution. At a particular time interval, for the period of 3 h, 2.5 mL of the solution, from both the experiments, were collected and the intensity of absorption was monitored using a spectrophotometer.
LaB6 source. The TEM samples were prepared by depositing small amount of synthesized material onto a 200 mesh size Cu grid. The X-ray diffraction (XRD) patterns were recorded on a Shimadzu XD3A performed over a diffraction angle range of 2q ¼ 20 e80 . X-ray photoelectron spectra (XPS) were collected in a UHV chamber attached to a physical electronics 560 ESCA/SAM instrument. The UVevis spectra were measured using a Shimadzu UV-1800 spectrophotometer using a quartz cuvette. Electrochemical studies were carried out with a Bio-logic SP-200 potentiostat connected to a data controller. A three-electrode system was used in the experiment with a glassy carbon electrode (GCE) as the working electrode. Ag/AgCl electrode (saturated KCl) and a Pt-electrode were used as the reference and counter electrodes, respectively.
3. Results and discussion The X-ray diffraction study was done for the synthesized nanomaterial, which is indexed to the formation of rutile tetragonal phase of SnO2 (Fig. 1A). All the crystalline peaks are well matched with the reference (JCPDS 00-041-1445) and no other impurity was observed [20]. Furthermore, the height and sharpness of the peaks suggest that the product is well crystallized. From the XRD pattern it is evident that the preferred orientation of the crystal is towards the direction of (110) plane. The detail formation mechanism of SnO2 is mentioned in the supporting document. To identify the chemical state of the tin (Sn), X-ray photoelectron spectroscopy (XPS) measurements were performed. Fig. 1B shows the high resolution XPS spectrum of the SnO2 nanocrystals, where two peaks, separated by 8.75 eV, with the binding energy values of 486.8 and 495.3 eV were observed due to the spin-orbit splitting of the Sn 3d level, namely Sn 3d5/2 and Sn 3d3/2 electrons, respectively, indicates the presence of Sn(IV) component [21]. The TEM image (Fig. 2) shows the wide size distribution, within the range between 5 and 25 nm, of the synthesized material. The image also indicates the collection of the SnO2 particles with prominent grain boundaries for individual the particles. The study of the pore structure of the solids is connected with interpretation of adsorption-desorption isotherm. Fig. 3A indicate the present tin oxide sample can be categorized as the type-IV isotherms with the combination of H-1 and H-2 types of hysteresis loop, as per IUPAC recommendation, where the agglomeration of spheroidal and corpuscular types of
2.4. Electrochemical experiments The incorporation of graphitic carbon to the synthesized materials was done through a milling process for the period of 3 h by maintaining the ratio of 1:10, respectively. The electrochemical properties show the evidences for the successful incorporation and the composite formation. 2.5. Characterization Transmission electron microscopy (TEM) studies were performed using a Philips CM200 TEM instrument equipped with a
Fig. 1. (A) X-ray diffraction pattern of the tetragonal phase of SnO2 nanocrystalline particles with the preferred orientation towards the direction of (110) plane. (B) High resolution XPS spectrum of the SnO2 nanocrystals with the binding energy values of 486.8 and 495.3 eV represent the spin-orbit splitting of Sn 3d level and indicates the presence of Sn (IV) component.
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Fig. 2. The TEM image shows the SnO2 particles, within the size range of 5e25 nm, with prominent grain boundaries.
Fig. 3. (A) Nitrogen adsorption and desorption isotherms and (B) pore size distribution (inset) of the synthesized SnO2 particles.
Fig. 4. (A) UVevisible spectrum of the dispersed SnO2 nano-crystals in aqueous solution and (B) Tauc-Mott plot (in-set) for calculating the band gap energy of the nanocrystals.
3.1. Performance of tin oxide as a photocatalyst particles are involved. The isotherm was also used to investigate the surface area and pore size distribution of the material using Brunauer, Emmett, and Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. The calculated surface area of the synthesized SnO2 was found to be 44.95 m2g1 and a wide distribution of the pore radius of the material was noticed in Fig. 3B along with the pore volume of 0.0018 cm3g1. The result was attributed to the interstitial spaces between the adjacent nanoparticles or the mesoporosity was developed through the cluster formation of nanocrystallites. In this current study, we have measured the UVevisible absorption spectrum of the synthesized oxide material where the absorption peak was observed at 308 nm (Fig. 4A) and the spectrum was also used to determine the bandgap energy using Tauc plot (Fig. 4B). The bandgap energy was estimated from the extrapolated line and the calculated bandgap value of SnO2 was 3.52 eV. To find the photo-corrosion effect of SnO2, the sample was exposed under the ultra-violet source for 2 h and a similar bandgap (3.52 eV) values were obtained before and after the irradiation indicates the photo tolerance of the materials.
Tin oxide is a functional metal oxide, with unique optical and electrical properties, and has been applied in various applications, such as, gas sensor, catalysts for esterification, bio-diesel production, dye-sensitized in solar cells and electrode material for lithium batteries [22,23]. Apart from that, reports has also been published on the degradation of the common pollutants, more specifically organic dyes, where tin oxide act as a photocatalyst [24,25]. In this experiment, we have used the nanocrystalline tin oxide as a photocatalyst for the reduction of a dialkylamino group functionalized cationic xanthene dye. Fig. 5 shows the comparative study through spectrophotometric evidence for the reduction process of dye under the daylight and UV-irradiation condition at different time intervals at the room temperature in presence of the tin oxide catalyst. The characteristic spectra of the dye with absorption peak at the wavelength of 554 nm was observed in the spectrophotometer. A little quenching of the peak intensity was observed when the reaction was performed under day-light condition for the period of 3 h (Fig. 5A). Fig. 5B represents the
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The photon energy from the ultraviolet source, deep UV (UV-C), is within the range of 6.53e4.43 eV (considering the wavelength between 190 and 280 nm) and which is the sufficient amount of energy to transfer an electrons from the valence band to the conduction band of SnO2 nanocrystal. The conduction band electron interact with the p-electron of carbon atom (at 9-position) and that induced the electronic rearrangement to gain the aromaticity of ring (I) of the ‘xanthene segment’ of the dye (Scheme 1). The electron-pair present in C-9 atom abstract the proton from the solvent and lowers the free energy of the transition state which lead to produce reduced form of the dye molecule. The extended conjugation of parent molecule is lost in the reduced form and which caused the quenching of the spectra. 3.2. Nanocrystalline tin oxide with an electrochemical capacitor behaviour
Fig. 5. (A) UVevis spectra of the xanthene dye (104 mol dm3) in the presence of SnO2 at different time intervals under the exposure of day-light. (B) the magnified image of the spectra within the box. (C) UVevis spectra of the xanthene dye (104 mol dm3) in the presence of SnO2 at different time intervals under the exposure of Philips UV-C (TUV T8) (germicidal) lamp. Complete quenching of the absorbance peak, at the wavelength of 554 nm, of the dye took 120 min. (D) Absorption of the dye (Ct/C0, where, Ct is the absorption maxima at different time intervals and C0 is the initial absorption maxima) as a function of time.
magnified image of the section within the box in Fig. 5A. Fig. 5C shows the complete quenching of the peak at the wavelength of 554 nm within the period of 2 h. Fig. 5D shows the graphical representation of absorbance (C/C0) as a function of time (min) for the quenching of the absorption peak at 554 nm of the dye, in the presence of UV-irradiation, with the rate constant value of 0.0157 min1. The gradual quenching of the spectra indicates the chemical transformation or reduction of the dye. The little quenching effect of the dye, as observed in Fig. 5 (A and B) under day-light condition, can be explained on the basis of prolonged adsorption-desorption dynamics between dye and the catalyst. The complete quenching phenomenon of the peak at the wavelength of 554 nm under UV-irradiation condition, in Fig. 5C, can be explained as follows. In this study, SnO2 was also used as the active photocatalyst and the reaction was carried out under the exposure of UV-irradiation.
Electrochemical capacitors also known as supercapacitors, an energy storage devices, has been attracted by the scientists due to their higher energy density, longer cycle-life, comprehensive application temperature range [26]. In the present study, we report the supercapacitive performance of the synthesized nanocrystalline SnO2, as a base material, in terms of specific capacitance, energy and power densities and electrochemical stability. To determine the electrochemical characteristics of the sample the cyclic voltammetry technique was employed. Fig. 6A shows the voltammogram of the SnO2 sample in KOH (1.0 mol dm3) at a potential sweep rate of 25 mV S1 and the maximum current density value 0.003 A g1 has been obtained at 0.5 V. To increase the conductivity of the SnO2, we have applied graphitic carbon (Cg) in the ratio of 8:2 (SnO2 and Cg). Fig. 6B shows the voltammogram of the Cg modified working electrode and the current density value 0.028 A g1 has been obtained at 0.5 V. Fig. 6C shows the voltammogram of the Cg-SnO2 modified working electrode in KOH at a potential sweep rate of 25 mV S1 and a substantial improvement on the current response, 2.65 A g1, has been noticed. The synergetic effect in the current density value of the Cg-SnO2 modified electrode could be due to the creation of the electronic and structural heterogeneity of the composite. Now the Cg-SnO2 composite has been used as the working material for rest of the studies and a steady improvement of the current density values, as evidenced by the cyclic voltammograms, has been noticed with increasing scan rate from 5 to 100 mV S1, curves (a)-(f), respectively, for the material (Fig. 7). Fig. 8 (A and B) shows the galvanostatic charge-discharge curves for the Cg-SnO2 composite modified electrodes, measured within the potential range from 0.0 V to þ0.5 V, using a series of current density values from 3.0 to 0.1 Ag1 in KOH. The specific capacitance values, obtained from charge-discharge cycles, of the corresponding material were 40, 43, 47, 53, 68, 80 and 98 F g1 for the current densities values of 3.0, 2.5, 2.0,1.0, 0.5, 0.25 and 0.1 A. g1, respectively. The graphical representation, bar-chart (Fig. 9A), indicate, on the basis of specific capacitance (CP) calculation, the maximum value of specific capacitance has been obtained when the device was performed under the minimum in-put current condition. A comparative table (supplementary data, Table S1) shows the specific capacitance values for the tin based materials, synthesized using different techniques. Stability of the material is an important parameter for the endurance of the device. We have used the galvanostatic charge-discharge measurement to evaluate the durability of the Cg-SnO2 working material. Fig. 9B shows that Cg-SnO2 composite retains about 91% of its original capacitance after 1000 cycles, charging and discharging, indicates the high stability of the material. Fig. 10A shows the graphical representation of energy density and power density of the device as a function of current density. With decreasing the current density values, the energy density values
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Scheme 1. Photocatalytic reduction mechanism for the xanthene dye in presence of SnO2 under UV-irradiation condition.
Fig. 6. Cyclic voltammograms of the (A) SnO2, (B) gC and (C) gC-SnO2 in 1.0 M KOH at the scan rate of 25 mV. S1.
Fig. 7. CV curves, (aef), for the gC-SnO2 modified electrode at scan rates between 5 and 100 mVs1 in 0.1 M KOH using a three-electrode system.
are inversely proportional (Fig. 10B). increases and the power density values decreases. The nonlinear nature of the curve B (energy density vs. current density) is due to the time dimension, a variable factor that changes with current input, associated with the energy density value. It is evident from the graphical representation that energy density and power density
4. Conclusion In summary, a solution phase approach has been applied for the synthesis of crystalline tin oxide where the size distribution of the
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Fig. 8. (A) The current density dependant (from 3.0 to 0.1 A g1) galvanostatic chargedischarge curves of the gC-SnO2 modified electrode measured in the potential range from 0.0 V to þ0.5 V in 1.0 M KOH.
Fig. 10. (A) The graphical relation between the energy density and power density as the function of current density and (B) graphical representation for the relation between energy density and power density for Cg-SnO2 material, in the potential range of 2.5e0.1 V.
Acknowledgement The authors acknowledge financial support from the University of Johannesburg and the National Research Foundation, South Africa. RB, NH, DN, SS, further acknowledge financial support from the Global Excellence and Stature fellowship from the University of Johannesburg. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.06.180. References
Fig. 9. (A) The bar chart shows the specific capacitance values of gC-SnO2 modified electrode materials within current density ranges from 3.0 to 0.1 A g1. (B) Chargedischarge cycling performance of gC-SnO2 at current density of 2.5A.g1 for 1000 cycles (inset).
particles are within the range between 5 and 25 nm, as evidenced by the transmission electron microscopy. The synthesized tin oxide nanoparticles shows the catalytic performance for the photoreduction (the conduction band electron is responsible in presence of UV-irradiation) of the xanthene based dye with the reduction rate constant value of 0.0157 min1. The present report also demonstrate the supercapacitive behaviour of the synthesized tin oxide particles in presence of graphitic carbon. The synergistic improvement of the device performance, by the addition of graphitic carbon to the tin oxide system, is due to the creation of electronic and structural heterogeneity with in the composite. The 91% retention of specific capacitance value of the Cg-SnO2 composite after 1000 cycles (charging and discharging) indicates a good stability of the material for the purpose of practical use as the energy storage devices.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Multifunctional performance of nanocrystalline tin oxide Rasmita Barik, Nishu Devi, Debkumar Nandi, Samarjeet Siwal, Sarit Kumar Ghosh, Kaushik Mallick* Department of Chemistry, University of Johannesburg, P.O. Box: 524, Auckland Park, 2006, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 May 2017 Received in revised form 15 June 2017 Accepted 16 June 2017 Available online 22 June 2017
Crystalline tin oxide, with nanometre size range in diameter, was synthesized using a solution phase approach. The oxide material showed the photocatalytic performance for the reduction of a xanthene based dye in presence of ultra-violet irradiation, where the conduction band electron is responsible for the reduction mechanism. In addition to that the calculated power density and energy density values of the synthesized tin oxide nanoparticles, in combination with graphitic carbon, is comparable with the electrochemical supercapacitor like materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: Crystalline tin oxide Solution phase synthesis Conduction band electron Electrochemical supercapacitor
1. Introduction Nanotechnology has emerged as one of the most exciting fields in material sciences and the synthesis of nanocrystalline materials is becoming one of the most fascinating techniques that has been applied in most of the research areas in science and engineering. Nanosized semiconductor metal oxides are known to possess distinctive functionalities due to their high density surface sites and have emerged as an important class of materials with the great potential for diverse applications in electronics [1], optics [2] and catalysis [3]. Size of the material has a profound effect on a number of intrinsic properties in systems including the band gap [4] and at the same time the surface to volume ratio can also have the consequence on the electronic [5], magnetic [6] and optical [7] properties of the system. Among the various properties of nanomaterials, semiconductor nanocrystals with quantum confinement effect leads to spatial enclosure of the electronic charge carriers within the crystal dimension and thus the transport properties of the system are largely affected by the size and geometry of the materials [8,9]. Tin dioxide, SnO2, is an n-type semiconductor with a wide-band gap of 3.6 eV [10], is considered as one of the smart materials, among the various semiconductor metal oxides, due to their good stability, nontoxicity and low cost which allow for its diverse
* Corresponding author. E-mail address: [email protected] (K. Mallick). http://dx.doi.org/10.1016/j.jallcom.2017.06.180 0925-8388/© 2017 Elsevier B.V. All rights reserved.
application. The physicochemical properties of SnO2 are closely linked to its size and shape. Report has been published for the synthesis of mesoporous SnO2, with large surface area, using a hydrothermal synthesis route for the photo-degradation of organic dyes [11]. The sensing property of the large pore sized SnO2-based thick films were investigated and reported for the detection of very low concentration of hydrogen sulphide [12]. A hydrothermal synthesis route was introduced for the synthesis of graphene-tin oxide composite and was applied as a catalyst for the electrochemical detection of dopamine [13,14]. A solution-based method has been employed for the synthesis of SnO2-MnO2 composites for the supercapacitor application [15]. SnO2 and carbon cloth flexible composite system showed excellent performance as an anode material in lithium-ion battery application [16]. Micro structured SnO2, has been reported [17] by a hydrothermal synthesis route, exhibit excellent photocatalytic performance with potential applications in waste water purification. Highly aligned nanorods of SnO2 were synthesized by a hydrothermal method on a graphene matrix and has been reported [18] for a gas sensing application with improved performances. Excellent organic vapour sensing property with long-term stability has been obtained from the interconnected SnO2 and Fe2O3 based nanotubes synthesized by combining the single nozzle electrospinning and thermal treatment methods [19]. In this work, crystalline SnO2 particles, with the diameter ranging from 5 to 25 nm, have been prepared using a solution phase approach. The synthesized material was used as a photocatalyst for
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the reduction of a xanthene based dye. In addition to that, a detailed study indicates that the crystalline SnO2 particles have the potential as an electrochemical supercapacitor when combined with graphitic carbon. The synthesized nanomaterial was also characterized using different optical, microscopic and surface characterization techniques. 2. Experimental 2.1. Materials All the chemicals used for this study including stannous chloride, citric Acid, sodium hydroxide and xanthene based dye were purchased from Merck. The chemicals and solvents were used as received without further purification. All the solutions were prepared using mili-Q water. 2.2. Synthesis of nanostructured SnO2 In a typical synthesis method, 40 mL of SnCl2 (101 mol dm3) was taken in a three-neck flask equipped with a condenser and thermometer. A water solution of citric acid (10 mL, 102 mol. dm3) was added dropwise to the stannous chloride solution under continuous stirring condition under the room temperature. After the complete addition, the system was heated at 60 C and water solution of sodium hydroxide (101 mol. dm3) was added dropwise to the above mixture solution until the complete precipitation was formed and later allowed to cool at the room temperature. The solution was centrifuged and washed with ethanol, acetone and water for several times. The solid material was collected and dried in oven. The dried material was further calcined at 500 C for 4 h. Finally, the synthesized powder material was characterized using microscopic and spectroscopic techniques and also applied as a catalyst for a photochemical reaction. The synthesized material, in combination with graphitic carbon, was also studied for the electrochemical charge-discharge performance for the supercapacitor application. 2.3. Photocatalytic experiments Initially, 20 mg of the synthesized material was suspended in 100 mL of xanthene dye (104 mol dm3) in a conical flask and placed inside the dark room, under the continuous starring condition for the period of 30 min, to allow an adsorption-desorption equilibrium. After that the conical flask was placed under the Philips UV-C (TUV T8) (germicidal) lamp with optical intensity value of ~40 mW/cm2 adjacent to the solution. For a control experiment, the second set was exposed under daylight with optical intensity value of 3.5 mW/cm2 adjacent to the solution. At a particular time interval, for the period of 3 h, 2.5 mL of the solution, from both the experiments, were collected and the intensity of absorption was monitored using a spectrophotometer.
LaB6 source. The TEM samples were prepared by depositing small amount of synthesized material onto a 200 mesh size Cu grid. The X-ray diffraction (XRD) patterns were recorded on a Shimadzu XD3A performed over a diffraction angle range of 2q ¼ 20 e80 . X-ray photoelectron spectra (XPS) were collected in a UHV chamber attached to a physical electronics 560 ESCA/SAM instrument. The UVevis spectra were measured using a Shimadzu UV-1800 spectrophotometer using a quartz cuvette. Electrochemical studies were carried out with a Bio-logic SP-200 potentiostat connected to a data controller. A three-electrode system was used in the experiment with a glassy carbon electrode (GCE) as the working electrode. Ag/AgCl electrode (saturated KCl) and a Pt-electrode were used as the reference and counter electrodes, respectively.
3. Results and discussion The X-ray diffraction study was done for the synthesized nanomaterial, which is indexed to the formation of rutile tetragonal phase of SnO2 (Fig. 1A). All the crystalline peaks are well matched with the reference (JCPDS 00-041-1445) and no other impurity was observed [20]. Furthermore, the height and sharpness of the peaks suggest that the product is well crystallized. From the XRD pattern it is evident that the preferred orientation of the crystal is towards the direction of (110) plane. The detail formation mechanism of SnO2 is mentioned in the supporting document. To identify the chemical state of the tin (Sn), X-ray photoelectron spectroscopy (XPS) measurements were performed. Fig. 1B shows the high resolution XPS spectrum of the SnO2 nanocrystals, where two peaks, separated by 8.75 eV, with the binding energy values of 486.8 and 495.3 eV were observed due to the spin-orbit splitting of the Sn 3d level, namely Sn 3d5/2 and Sn 3d3/2 electrons, respectively, indicates the presence of Sn(IV) component [21]. The TEM image (Fig. 2) shows the wide size distribution, within the range between 5 and 25 nm, of the synthesized material. The image also indicates the collection of the SnO2 particles with prominent grain boundaries for individual the particles. The study of the pore structure of the solids is connected with interpretation of adsorption-desorption isotherm. Fig. 3A indicate the present tin oxide sample can be categorized as the type-IV isotherms with the combination of H-1 and H-2 types of hysteresis loop, as per IUPAC recommendation, where the agglomeration of spheroidal and corpuscular types of
2.4. Electrochemical experiments The incorporation of graphitic carbon to the synthesized materials was done through a milling process for the period of 3 h by maintaining the ratio of 1:10, respectively. The electrochemical properties show the evidences for the successful incorporation and the composite formation. 2.5. Characterization Transmission electron microscopy (TEM) studies were performed using a Philips CM200 TEM instrument equipped with a
Fig. 1. (A) X-ray diffraction pattern of the tetragonal phase of SnO2 nanocrystalline particles with the preferred orientation towards the direction of (110) plane. (B) High resolution XPS spectrum of the SnO2 nanocrystals with the binding energy values of 486.8 and 495.3 eV represent the spin-orbit splitting of Sn 3d level and indicates the presence of Sn (IV) component.
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Fig. 2. The TEM image shows the SnO2 particles, within the size range of 5e25 nm, with prominent grain boundaries.
Fig. 3. (A) Nitrogen adsorption and desorption isotherms and (B) pore size distribution (inset) of the synthesized SnO2 particles.
Fig. 4. (A) UVevisible spectrum of the dispersed SnO2 nano-crystals in aqueous solution and (B) Tauc-Mott plot (in-set) for calculating the band gap energy of the nanocrystals.
3.1. Performance of tin oxide as a photocatalyst particles are involved. The isotherm was also used to investigate the surface area and pore size distribution of the material using Brunauer, Emmett, and Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. The calculated surface area of the synthesized SnO2 was found to be 44.95 m2g1 and a wide distribution of the pore radius of the material was noticed in Fig. 3B along with the pore volume of 0.0018 cm3g1. The result was attributed to the interstitial spaces between the adjacent nanoparticles or the mesoporosity was developed through the cluster formation of nanocrystallites. In this current study, we have measured the UVevisible absorption spectrum of the synthesized oxide material where the absorption peak was observed at 308 nm (Fig. 4A) and the spectrum was also used to determine the bandgap energy using Tauc plot (Fig. 4B). The bandgap energy was estimated from the extrapolated line and the calculated bandgap value of SnO2 was 3.52 eV. To find the photo-corrosion effect of SnO2, the sample was exposed under the ultra-violet source for 2 h and a similar bandgap (3.52 eV) values were obtained before and after the irradiation indicates the photo tolerance of the materials.
Tin oxide is a functional metal oxide, with unique optical and electrical properties, and has been applied in various applications, such as, gas sensor, catalysts for esterification, bio-diesel production, dye-sensitized in solar cells and electrode material for lithium batteries [22,23]. Apart from that, reports has also been published on the degradation of the common pollutants, more specifically organic dyes, where tin oxide act as a photocatalyst [24,25]. In this experiment, we have used the nanocrystalline tin oxide as a photocatalyst for the reduction of a dialkylamino group functionalized cationic xanthene dye. Fig. 5 shows the comparative study through spectrophotometric evidence for the reduction process of dye under the daylight and UV-irradiation condition at different time intervals at the room temperature in presence of the tin oxide catalyst. The characteristic spectra of the dye with absorption peak at the wavelength of 554 nm was observed in the spectrophotometer. A little quenching of the peak intensity was observed when the reaction was performed under day-light condition for the period of 3 h (Fig. 5A). Fig. 5B represents the
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The photon energy from the ultraviolet source, deep UV (UV-C), is within the range of 6.53e4.43 eV (considering the wavelength between 190 and 280 nm) and which is the sufficient amount of energy to transfer an electrons from the valence band to the conduction band of SnO2 nanocrystal. The conduction band electron interact with the p-electron of carbon atom (at 9-position) and that induced the electronic rearrangement to gain the aromaticity of ring (I) of the ‘xanthene segment’ of the dye (Scheme 1). The electron-pair present in C-9 atom abstract the proton from the solvent and lowers the free energy of the transition state which lead to produce reduced form of the dye molecule. The extended conjugation of parent molecule is lost in the reduced form and which caused the quenching of the spectra. 3.2. Nanocrystalline tin oxide with an electrochemical capacitor behaviour
Fig. 5. (A) UVevis spectra of the xanthene dye (104 mol dm3) in the presence of SnO2 at different time intervals under the exposure of day-light. (B) the magnified image of the spectra within the box. (C) UVevis spectra of the xanthene dye (104 mol dm3) in the presence of SnO2 at different time intervals under the exposure of Philips UV-C (TUV T8) (germicidal) lamp. Complete quenching of the absorbance peak, at the wavelength of 554 nm, of the dye took 120 min. (D) Absorption of the dye (Ct/C0, where, Ct is the absorption maxima at different time intervals and C0 is the initial absorption maxima) as a function of time.
magnified image of the section within the box in Fig. 5A. Fig. 5C shows the complete quenching of the peak at the wavelength of 554 nm within the period of 2 h. Fig. 5D shows the graphical representation of absorbance (C/C0) as a function of time (min) for the quenching of the absorption peak at 554 nm of the dye, in the presence of UV-irradiation, with the rate constant value of 0.0157 min1. The gradual quenching of the spectra indicates the chemical transformation or reduction of the dye. The little quenching effect of the dye, as observed in Fig. 5 (A and B) under day-light condition, can be explained on the basis of prolonged adsorption-desorption dynamics between dye and the catalyst. The complete quenching phenomenon of the peak at the wavelength of 554 nm under UV-irradiation condition, in Fig. 5C, can be explained as follows. In this study, SnO2 was also used as the active photocatalyst and the reaction was carried out under the exposure of UV-irradiation.
Electrochemical capacitors also known as supercapacitors, an energy storage devices, has been attracted by the scientists due to their higher energy density, longer cycle-life, comprehensive application temperature range [26]. In the present study, we report the supercapacitive performance of the synthesized nanocrystalline SnO2, as a base material, in terms of specific capacitance, energy and power densities and electrochemical stability. To determine the electrochemical characteristics of the sample the cyclic voltammetry technique was employed. Fig. 6A shows the voltammogram of the SnO2 sample in KOH (1.0 mol dm3) at a potential sweep rate of 25 mV S1 and the maximum current density value 0.003 A g1 has been obtained at 0.5 V. To increase the conductivity of the SnO2, we have applied graphitic carbon (Cg) in the ratio of 8:2 (SnO2 and Cg). Fig. 6B shows the voltammogram of the Cg modified working electrode and the current density value 0.028 A g1 has been obtained at 0.5 V. Fig. 6C shows the voltammogram of the Cg-SnO2 modified working electrode in KOH at a potential sweep rate of 25 mV S1 and a substantial improvement on the current response, 2.65 A g1, has been noticed. The synergetic effect in the current density value of the Cg-SnO2 modified electrode could be due to the creation of the electronic and structural heterogeneity of the composite. Now the Cg-SnO2 composite has been used as the working material for rest of the studies and a steady improvement of the current density values, as evidenced by the cyclic voltammograms, has been noticed with increasing scan rate from 5 to 100 mV S1, curves (a)-(f), respectively, for the material (Fig. 7). Fig. 8 (A and B) shows the galvanostatic charge-discharge curves for the Cg-SnO2 composite modified electrodes, measured within the potential range from 0.0 V to þ0.5 V, using a series of current density values from 3.0 to 0.1 Ag1 in KOH. The specific capacitance values, obtained from charge-discharge cycles, of the corresponding material were 40, 43, 47, 53, 68, 80 and 98 F g1 for the current densities values of 3.0, 2.5, 2.0,1.0, 0.5, 0.25 and 0.1 A. g1, respectively. The graphical representation, bar-chart (Fig. 9A), indicate, on the basis of specific capacitance (CP) calculation, the maximum value of specific capacitance has been obtained when the device was performed under the minimum in-put current condition. A comparative table (supplementary data, Table S1) shows the specific capacitance values for the tin based materials, synthesized using different techniques. Stability of the material is an important parameter for the endurance of the device. We have used the galvanostatic charge-discharge measurement to evaluate the durability of the Cg-SnO2 working material. Fig. 9B shows that Cg-SnO2 composite retains about 91% of its original capacitance after 1000 cycles, charging and discharging, indicates the high stability of the material. Fig. 10A shows the graphical representation of energy density and power density of the device as a function of current density. With decreasing the current density values, the energy density values
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Scheme 1. Photocatalytic reduction mechanism for the xanthene dye in presence of SnO2 under UV-irradiation condition.
Fig. 6. Cyclic voltammograms of the (A) SnO2, (B) gC and (C) gC-SnO2 in 1.0 M KOH at the scan rate of 25 mV. S1.
Fig. 7. CV curves, (aef), for the gC-SnO2 modified electrode at scan rates between 5 and 100 mVs1 in 0.1 M KOH using a three-electrode system.
are inversely proportional (Fig. 10B). increases and the power density values decreases. The nonlinear nature of the curve B (energy density vs. current density) is due to the time dimension, a variable factor that changes with current input, associated with the energy density value. It is evident from the graphical representation that energy density and power density
4. Conclusion In summary, a solution phase approach has been applied for the synthesis of crystalline tin oxide where the size distribution of the
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Fig. 8. (A) The current density dependant (from 3.0 to 0.1 A g1) galvanostatic chargedischarge curves of the gC-SnO2 modified electrode measured in the potential range from 0.0 V to þ0.5 V in 1.0 M KOH.
Fig. 10. (A) The graphical relation between the energy density and power density as the function of current density and (B) graphical representation for the relation between energy density and power density for Cg-SnO2 material, in the potential range of 2.5e0.1 V.
Acknowledgement The authors acknowledge financial support from the University of Johannesburg and the National Research Foundation, South Africa. RB, NH, DN, SS, further acknowledge financial support from the Global Excellence and Stature fellowship from the University of Johannesburg. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.06.180. References
Fig. 9. (A) The bar chart shows the specific capacitance values of gC-SnO2 modified electrode materials within current density ranges from 3.0 to 0.1 A g1. (B) Chargedischarge cycling performance of gC-SnO2 at current density of 2.5A.g1 for 1000 cycles (inset).
particles are within the range between 5 and 25 nm, as evidenced by the transmission electron microscopy. The synthesized tin oxide nanoparticles shows the catalytic performance for the photoreduction (the conduction band electron is responsible in presence of UV-irradiation) of the xanthene based dye with the reduction rate constant value of 0.0157 min1. The present report also demonstrate the supercapacitive behaviour of the synthesized tin oxide particles in presence of graphitic carbon. The synergistic improvement of the device performance, by the addition of graphitic carbon to the tin oxide system, is due to the creation of electronic and structural heterogeneity with in the composite. The 91% retention of specific capacitance value of the Cg-SnO2 composite after 1000 cycles (charging and discharging) indicates a good stability of the material for the purpose of practical use as the energy storage devices.
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