Electrochemical performance of SnO 2 hexagonal nanoplates

Transcription

1 DOI /s y ORIGINAL PAPER Electrochemical performance of SnO 2 hexagonal nanoplates D. Vasanth Raj & N. Ponpandian & D. Mangalaraj & A. Balamurugan & C. Viswanathan Received: 20 June 2013 /Revised: 6 August 2013 /Accepted: 14 August 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract This study reports the electrochemical performance of SnO 2 hexagonal nanoplates (SnO 2 -NPs) coated on copper substrate by electrodeposition method in different aqueous electrolytes. The influence of deposition voltage on the morphology of the nanoplates was investigated by scanning electron microscopy. The synthesized SnO 2 was characterized using SEM, X-ray diffraction, Raman, FTIR and UV visible absorption spectrum. The results clearly have shown that with the increase in deposition voltage at constant deposition time, the thickness of the plate decreased. The obtained nanoplates were of several hundred nanometers in planar dimension and about nm in thickness. The electrochemical reaction of SnO 2 -NPs with lithium ions were investigated by cyclic voltammetry in LiOH H 2 O, Li 2 CO 3, LiNO 3 and Li 2 SO 4 aqueous solution. The SnO 2 hexagonal nanoplates deposited on copper substrate can be an ideal anode material for aqueous rechargeable lithium-ion battery. Keywords Batteries. Electrochemical characterizations. Electron microscopies. FTIR. XRD Introduction Rapid growth in industrialization has resulted in increased demand for energy and has also lead to serious environment issues. So the research focus has now shifted intensely towards the development of reliable energy resources which will have negligible adverse effects on environment. Lithium ion batteries (LIBs) are one of the most promising power D. V. Raj: N. Ponpandian : D. Mangalaraj : A. Balamurugan : C. Viswanathan (*) Department of Nanoscience and technology, Bharathiar University, Coimbatore , Tamilnadu, India viswanathan@buc.edu.in resources for energy storage applications such as portable electronic devices, electric and hybrid electric vehicles with high energy density and long life. But the issues related with LIBs still persists, such as safety issues arising from toxic and flammable electrolytes, its high cost, operating voltage and precise cell assembly conditions. Researches have been focused on developing aqueous rechargeable lithium ion batteries (ARLBs) to resolve the above mentioned drawbacks since 1990 s[1] and also developing a material which can be easily, effectively recyclable and with less gas evolution [2]. Nanomaterials have gained tremendous interest in secondary batteries due to its nanoscale phenomena and unique morphology. The storage capacity and recyclability of aqueous LIBs are limited by the size, shape, and morphology and crystal structure of such nanomaterials. And their capacity can be effectively enhanced through the controlled synthesis of electrode materials with a specific structure and morphology. The growth of nanoscale active materials directly on current collectors is a means by which the performance of aqueous lithium ion battery can be increased. The electrochemical performance of nanomaterials is strongly dependent on the electrode fabrication process. The electrodeposition method is one of the methods for the synthesis of nanostructures on flexible substrates. It has many advantages such as simple, low cost, environmental friendly, high growth rate and easier control of shape and size [3 5]. Tin oxide (SnO 2 ) is one of the most important transition metal oxides; and has received enormous attention in lithium ion battery system, because of its numerous advantages such as low electrical resistivity ( Ωcm), high theoretical capacity (784 mah/g) and high rate of charge and discharge performance. In the past decade, tin oxide (SnO 2 ) with different morphologies has been applied as anode materials for nonaqueous lithium ion batteries due to their high specific capacity relative to graphite [6 9]. Recently different SnO 2 nanostructures, cube-like arrangement [10], porous network

2 structure [11], uniform mesopores [12], nanotubes [13], have been prepared by electrodeposition method. In our work, a two electrode electrodeposition set up is proposed for the controlled preparation of SnO 2 -NPs, using the anodized copper substrates in H 2 SO 4 solution. Here, we optimized the deposition conditions for the size-controlled electrochemical Fig. 1 SEM images of SnO 2 -NPs with different applied potentials a) 3.6 V, b) 3.8 V, c) 4.0 V, d) 4.2 V. The highly magnified images are shown on the right of each SEM image, e) SEM image of Cu foil f) Cross sectional view of SnO 2 -NPs deposited at 4.2 V. (g) EDX spectrum of SnO 2 deposited at 4.2 V

3 (g) cps / ev Cu Sn O Sn Cu Fig. 1 (continued) kev synthesis of 2D SnO 2 -NPs on metal substrates. The thickness of the plates was controlled by adjusting deposition voltage and time. SnO 2 nanoplates of 50 nm thickness were achieved at higher (4.2 V) deposition voltage. The morphology and crystal structure of electrodeposited SnO 2 -NPs were examined using scanning electron microscope (SEM) and X-ray diffraction pattern (XRD). Cyclic voltammetry (CV) was carried out to analyze the alloying/de-alloying reaction of lithium ions with SnO 2 -NPs in different aqueous electrolyte solutions. Experimental procedure Electrochemical Anodization Anodization is an electrolytic process for protection or decoration of metal surfaces. It increases the thickness of the natural oxide layer on the surface of metals, and also provides better adhesion. Copper foil with the thickness of 0.1 mm and surface area of 2x2 cm 2 wasusedasthe substrate. It was ultrasonically cleaned in acetone, 0.1 M HCl aqueous solution and finally with de-ionized water to remove surface impurities. Well cleaned copper substrates were used as an anode and platinum wire was chosen as cathode during the anodization. Finally the copper substrate was anodized by 0.1 M aqueous H 2 SO 4 solution by applying 5 V for 30 min. Electrodeposition The Solutions for electrodeposition were composed of 25 mm SnCl 2. 2H 2 O and 75 mm of Conc.HNO 3. Oxygen is blown into the mixed solutions for 1 h to oxidize the stannous ion (Sn 2+ ) into the stannic ion (Sn 4+ ). The cell for deposition was a two-electrode cell, in which a platinum wire was used as counter electrode with

4 Fig. 2 XRD patterns of SnO 2 hexagonal nanoplates obtained at different applied potentials a) 3.6 V, c) 3.8 V, e) 4.0 V, g) 4.2 V separation from the working electrode copper by 1 cm. The exposed area of working electrode is 2 2cm 2.Deposition voltages of 3.6, 3.8, 4.0 and 4.2 V were applied to the substrate for 3 min using a potentiostat. The ph of the electrolyte solution was maintained at 1.8. After electrodeposition, the deposits were washed and immersed in deionized water to remove the chloride contaminants and dried in air at ambient conditions for subsequent analyses. The general morphology of the products was examined using scanning electron microscopy SEM (JEOL 6210). X- ray diffraction measurements were carried (PANalytical XPERT PRO with Cu-Kα radiation (λ= nm)) for analyzing crystal structure. Fourier transform infrared (FT- IR) spectra of the samples were recorded on a BRUKER TENSOR 27 IR spectrophotometer. The Raman measurements were carried out on a Horiba Jobin-Yvon HR800 using Fig. 3 FTIR spectrum of SnO 2 nanoplates deposited at different deposition volt

5 Fig. 4 (a) Raman spectrum of SnO 2 nanoplates deposited at 4.2 V (b) UV vis spectra of SnO 2 nanoplates deposited at 4.2 V the 514 nm of an Ar + laser as the excitation source. UV-visible absorption spectra of the samples were studied in the wavelength range of nm to determine the band gap using Jasco V-650 spectrophotometer. The electrochemical performances of the SnO 2 -NPs were measured at room temperatures using three-electrode cells with platinum wire as the counter electrode and Ag/AgCl as reference electrode. The asprepared thin films were used as the working electrode. The electrolytes were 0.1 M LiOH H 2 O, Li 2 CO 3,Li 2 SO 4 and LiNO 3. The potential range for cyclic voltammetry (CV) measurements was -0.8 V to 0.6 V at a scan rate of 20 mvs -1.The chronoamperometry measurements were taken at a constant voltage of +0.1 V, and the current change was monitored for 60 s at room temperature. All electrochemical measurements were controlled via a Bio logic science instruments (Model- SP-50) potentiostat-galvanostat. Results and discussion Figure 1a to d shows the SEM images of the SnO 2 -NPs obtained by passing a different deposition potential from 3.6

6 Fig. 5 SEM image of SnO 2 nanoplates deposited at 3.6 V at 5mins to 4.2 V at constant deposition time of 3 min on anodized copper substrates. Figure 1e shows the SEM image of Cu foil before deposition and Fig. 1f shows the cross sectional view of SnO 2 -NPs deposited at 4.2 V. The as-prepared samples with yellow color were distributed uniformly and adhered firmly onto the Cu substrate. SnO 2 nanoplates with different shapes and sizes can be seen in the Fig. 1. The lowest deposition potential (3.6 V, Fig. 1a) produced a number of overlapping nanoplates with micrometer size and hexagonal shape. And also the nanoplates seemed to be deposited by smaller nanoparticles in ordered assembly. The small particles diffuse and re-crystallize into fine hexagonal nanoplates. Intense SnO 2 nanoplates was obtained at 3.8 V (Fig. 1b) but here the deposit consists of nanoplates with smaller thickness. By increasing the potential to 4.0 V, plate-like nanostructures (Fig. 1c) started to appear with a lesser thickness when compared with nanoplates observed previously at 3.8 V (Fig. 1b). When the applied potential was increased to 4.2 V, the deposit became more uniform and was fully constituted by the hexagonal shaped nanoplates (Fig. 1d). But the length of the plates increased while its thickness dramatically decreased. The thickness of the nanoplates ranged from 50 nm to 300 nm estimated from the SEM image, and most of the nanoplates were arranged perpendicular to the substrate. The small variation in the electrochemical potential gives rise to the variation in thickness and length of the plates. The increases in applied voltage lead to the enhancement of interconnection between particles. Because at higher voltage, the strength of the electric field between two electrodes is high, therefore the particles may interconnect with each other and results in larger particles. Preusambly, it could be the reason for the increase in length of the SnO 2 nanoplates with the increase in deposition voltage. As can be seen from the FESEM images, plates has grown almost vertical, i.e. perpendicular to the substrate surface and separated from one another and which helped easy diffusion of electroactive species, leading to reduced internal resistance [14]. And there was no collapse and fracture in the 2D nanoplates. Figure 1g shows the EDX spectrum of SnO 2 - NPs deposited at 4.2 V. The signals of Sn, O and Cu were readily detected by EDX spectrum on SnO 2 -NPs. The components of materials are Sn and O. The Cu peaks arise from the copper foil used as the substrate. The weight percentage of O is about This indicates that the type of oxide prepared is SnO 2 [10]. No impurities were observed. In order to understand the information about crystallographic structure of electrodeposited material, XRD analyses were conducted. Figure 2 shows the XRD pattern of electrodeposited SnO 2 nanoplates at different deposition voltages. The diffraction patterns show well defined sharp peaks, indicating crystallinity of the synthesized products. The diffraction peaks from the SnO 2 at 2θ=(25.97 ), (31.78 ), (33.86 ), (36.57 ), (38.49 ), (43.61 ), (47.61 ), (50.49 ), (53.69 ), (56.57 ), (59.12 ) and (61.52 ) matches with the (111), (021), (101), (111), (210), (124), (117), (112), (223), (028), (135) and (310) diffraction planes respectively. The major reflections can be indexed to orthorhombic unit cell of SnO 2 [labeled O] which was in good agreement with the standard data from JCPDS card (no. # ). However it seemed that some reflections can also be assigned to the tetragonal unit cell [labeled T and JCPDS card no. # ]. The above results proved that the as prepared samples had a mixture of orthorhombic and tetragonal SnO 2. No impure phases were detected. The broadening of the XRD peaks of nanoplates indicated that their sizes were very small. The size of crystalline materials can be estimated by the Scherer formula and it was found to be 23 nm. The deposition voltage hadn t changed the phase of SnO 2. The peak corresponding to Cu foil substrate was also observed and was indexed. Local molecular structure and bonding of the component atoms can be detected by FTIR analysis. FTIR spectrum of SnO 2 nanoplates deposited at different voltages is shown in Fig. 3. The bands observed at 3459 cm -1 and 1344 cm -1 indicated the presence of O-H bonds on the SnO 2 nanoplates. Thebandat1417cm -1 was assigned to the vibrational harmonics of Sn-O-Sn bridges obtained by condensation of hydroxyl groups and the band observed at 916 cm -1 was attributed to Sn-OH vibrations [15]. The Sn O and Sn O Sn vibrations appeared in the range of cm 1. The Sn-

7 Fig. 6 (a) Cyclic voltammetry (1 st cycle) of SnO 2 nanoplates in aqueous LiOH H 2 O solution. (b) Cyclic voltammetry (15 th cycle) of SnO 2 nanoplates in aqueous LiOH H 2 O solution O-Sn vibration appeared at 752 cm -1. The peak at 679 cm -1 was attributed to the Sn-O-Sn asymmetric vibrations. And the band at 533 cm -1 is correlated to Sn-O vibration [16]. The corresponding bands are observed for all the SnO 2 -NPs prepared at different deposition voltages. Figure 4a shows the Raman spectra of SnO 2 nanoplates deposited at 4.2 V. The spectra exhibits well resolved Raman lines. The Raman signals observed at 474, 632, and 774 cm -1 in the Fig. 4a is related to the E g, A lg,andb 2g vibration modes of SnO 2 respectively with reference to the fundamental Raman peaks for SnO 2 [17]. The IR modes will become weakly active when the structural changes are induced by disorders, size effects and oxygen deficiencies. The oxygen deficiencies gives rise to a non stoichiometric SnO x at the surface and these oxygen deficiencies could also be responsible for producing IR active-modes. Therefore the Raman peaks located at 252 and 292 cm -1 corresponds to IR-active E u 2TOmode[18]. Figure 4b shows the UV-visible spectrum of SnO 2 -NPs deposited at 4.2 V. Powder of SnO 2 is scratched easily from SnO 2 coated Cu substrate, grounded and used for UV vis analysis. From the absorption spectrum, it was observed that the lower cut-off wavelength for the electrodeposited SnO 2

8 was about 264 nm which corresponds to the blue shift. Also, the spectrum hasn t shown any absorption peak in the wavelength range between nm. The optical band gap energy of the SnO 2 -NPs was calculated from the absorption data by plotting (αhν) 2 versus hν and extrapolating the linear portion of the curve to (αhν) 2 =0, where α is the absorption coefficient and hν is the photon energy which was found to be 4.2 ev larger than the value of bulk SnO 2 (3.6 ev). The increase in the band gap may be due to the association of the quantum size effect of the present SnO 2 -NPs [19]. SEM image of the SnO 2 hexagonal plates deposited on Cu substrate is shown in Fig. 5 for deposition time of 5 mins. When the deposition time was shorter (3 min), SnO 2 hexagonal nanoplates with the thickness of 50 nm to 300 nm were obtained. Moreover, the size of the nanoplates increased with the increase in deposition time. Increase in thickness of nanoplates to an average size of 600 nm to 1 μm with larger separation between the plates is noticed in Fig. 5. But the samples deposited at lower deposition time were more densely packed together. The morphology of the SnO 2 nanoplates was highly influenced by the applied deposition. The growth and reduction rate of Sn 4+ ions increased when the deposition time was increased to 5 mins, which lead to larger average diameter of the SnO 2 nanoplates. Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions and thermodynamics of redox processes. Courtney et.al rationalized the reaction of SnO 2 with lithium by SnO 2 þ 4Li þ þ 4e 2Li 2 O þ Sn Sn þ xli þ Li x Sn ð1þ ð2þ Where the equation (1) is an irreversible reaction and equation (2) is an alloying/de-alloying of Sn with Li + [20]. Electrochemical processes of electrodes in aqueous solutions are much more complicated than those in the non-aqueous solutions. The first reduction peak that appeared in the CV of SnO 2 corresponds to the irreversible reduction reaction of SnO 2 into metallic Sn in non-aqueous solution. In case of aqueous solution the reduction of SnO 2 into metallic Sn was observed reversibly in multiple cycles. Figure 6a and b shows the 1 st and 15 th cyclic voltammetric curves of SnO 2 -NPs in LiOH H 2 O solution at a potential range between -0.8 V to 0.6 V (vs. Ag/AgCl) at a scan rate of 20 mvs -1. It can be seen that the SnO 2 -NPs exhibits anodic and cathodic peaks at V and 0.48 V, 0.31 V, V, V respectively in first cycle. The first cathodic peaks at 0.48 V and 0.31 V indicate the reduction of SnO 2 into metallic tin. The existence of other cathodic peaks and V indicates the alloying of Sn with Li + ions. The anodic peak at V suggests the de-alloying reaction of Li x Sn. The O 2 evolution was observed at 0.8 V and H 2 evolution was observed at -1.0 V; therefore we restricted the potential window between -0.8 V and 0.6 V to suppress the gas evolution. At the 15 th cycle samples exhibit anodic and cathodic peaks at V, V and 0.47 V, 0.32 V, V and V respectively. The anodic and cathodic peaks were not identical to each other in both the cycles; the peak separation has arisen due to the polarization of charge transfer reaction. The current density of the electrode increased when the number of cycles increased which indicated the stability of the electrode. Moreover, it can be clearly observed that the redox current of SnO 2 -NPs deposited at 4.2 V was much higher than that of other SnO 2 -NPs. These results indicated that SnO 2 NPs deposited at 4.2 V had Fig. 7 Cyclic voltammetry of SnO 2 nanoplates in aqueous Li 2 CO 3 solution

9 Fig. 8 Cyclic voltammetry of SnO 2 nanoplates in aqueous LiNO 3 solution faster kinetics towards Li + ions. Figure 7 shows the cyclic voltammetric curves of SnO 2 -NPs in Li 2 CO 3 solution at a potential range of -0.8 V to 0.6 V (vs. Ag/AgCl) at a scan rate of 20 mvs -1. The CV curve clearly shows the oxidation and reduction peaks. SnO 2 -NPs deposited at 4.0 and 4.2 V exhibited nearly identical cathodic and anodic peaks approximately at the same potential positions (observed at 0.04 V, V and V). And the samples deposited at 3.6 and 3.8 V exhibited peaks at same potential positions (observed at 0.02 V and V). SnO 2 -NPs deposited at higher potentials had higher current density. Increase in surface area of SnO 2 - NPs at higher deposition potential favoured faster and easier Li-ion alloying and de-alloying. The electrochemical properties of SnO 2 -NPs have been investigated in LiNO 3 solution. The CV curves of SnO 2 -NPs in LiNO 3 solution at a scan rate of 20 mvs 1 in a potential window between -0.8 V to 0 V (vs. Ag/AgCl) are depicted in Fig. 8. Some broad shoulders were observed at -0.2 V and V in oxidation curve. And a sharp and broad reduction peaks were observed at V, V and -0.2 V. These Fig. 9 Cyclic voltammetry of SnO 2 nanoplates in aqueous Li 2 SO 4 solution

10 Fig. 10 Cyclic voltammetry of SnO 2 nanoplates deposited at 4.2 V in Na 2 SO 3 solution reduction peaks corresponds to reduction of SnO 2 into metallic tin. The potential window was restricted between -0.8 V and 0 V in LiNO 3 aqueous solution because the cathodic oxygen evolution was observed at 0.01 V. The high anodic hydrogen and cathodic oxygen evolution has reduced the energy density [2]. The redox current of SnO 2 -NPs in LiNO 3 solution was found to be less when compared with LiOH H 2 O and Li 2 CO 3 aqueous solution. Figure 9 shows the CV curve of SnO 2 -NPs in Li 2 SO 4 solution. The shape of the CV curves shows a capacitance characteristic, which is close to an ideal rectangular shape. Therefore the SnO 2 -NPs exhibit ideal capacitance behaviour in a 0.1 M Li 2 SO 4 solution. The capacitance of the SnO 2 -NPs electrode in Li 2 SO 4 which resulted from a redox reaction by the lithium ion alloying and dealloying process was further checked by carrying out the CV for SnO 2 -NPs in 0.5 M of Na 2 SO 3 solution. Figure 10 shows the CV curve of SnO 2 -NP deposited at 4.2 V in Na 2 SO 3 solution at a scan rate of 5 mv/s. There was no peak in the curve, showing no oxidation and reduction reactions occurring in Na 2 SO 3 solution. The redox mechanism of SnO 2 -NPs Fig. 11 Chronoamperometry of SnO 2 hexagonal nanoplates

11 Fig. 12 Cyclic voltammetry of SnO 2 nanoplates deposited at 3.6 V at 5 mins in aqueous LiOH H 2 O solution in the Na 2 SO 3 complies with that of SnO 2 -NPs in Li 2 SO 3 solution. The specific capacitance (Fg -1 ) was calculated from the CV curve of Na 2 SO 3 according to the following equation: C sp ¼ I=S m Where, I is the current density (A), S is the scan rate (mvs -1 ) and m the mass of the electrode active material. The obtained specific capacitance value for SnO 2 -NPs electrodeposited at 4.2Vwas57Fg -1 at a scan rate of 5 mvs -1. The potential of the working electrode was stepped and the resulting current from faradic process occurring at the electrode was monitored as a function of time in chronoamperometry analysis. Figure 11 shows the chronoamperometry curves of SnO 2 hexagonal nanoplates prepared at different deposition voltages. The working electrodes was primarily biased at -0.1 V with an Ag/AgCl as a reference electrode for 60 s to allow for lithium alloying. The polarity was then switched immediately to +0.1 V to initiate lithium de-alloying and the variation in the current response was recorded. A gradual decrease in current density was observed for all the electrodes per unit time which indicate of good electrode activity and high stability [21]. When compared with other three electrodes, SnO 2 -NPs deposited at 3.6 V showed slower current decay. SnO 2 -NPs deposited at voltages higher than 3.6 V showed improved and faster lithium-ion diffusion rate. Typical CV of the SnO 2 -NPs deposited at 5 mins in LiOH H 2 O (0.1 M) aqueous solution is presented in Fig. 12. Three cathodic peaks were observed at 0.4 V, V and -0.7 V. The broad cathodic peak observed at 0.4 V was assigned to the SnO 2 reduction into metallic tin and Li 2 O (Eq(1)). Upon scanning towards more negative direction, the second cathodic peaks emerge at V and -0.7 V. Such peaks repeatedly appear at same potential for all the cycles which indicated the reversible alloying reaction of Li ions with Sn metals. The anodic peaks observed at V and V were assigned to the de-alloying reaction of Sn and Li-ions. But in the case of SnO 2 -NP electrode prepared at 5 mins the redox current intensities of the electrode apparently decreased with increasing number of cycles. The decrease in current density affects the stability of the electrode in LiOH H 2 O aqueous solution. The thickness of the plates affects the charge transfer reaction; and it lead to the decrease in current density of the electrode. Conclusion We successfully prepared SnO 2 hexagonal nanoplates on Cu substrates by a simple and relatively low temperature electrochemical deposition method. The SEM analysis revealed that applied potential influences the morphology of the nanoplates formed on the copper substrate. X-ray diffraction patterns have confirmed that the SnO 2 -NPs obtained were of mixed phase and comprised of orthorhombic and tetragonal crystal structure. SnO 2 hexagonal nanoplates showed reversible alloying and dealloying reactions with lithium ions in LiOH H 2 OandLi 2 CO 3 electrolyte solutions and it corresponds to battery behavior of an electrode. In aqueous Li 2 SO 4 solution, SnO 2 -NPs exhibited supercapacitance characteristics. SnO 2 -NPs displayed improved redox current density in aqueous LiOH H 2 O solutions, as evidenced by the cyclic voltammetry investigations. The

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