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Tetragonal pencil-like VO2(R) as electrode materials for high-performance redox activities
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 6121–6128 www.elsevier.com/locate/ceramint
Tetragonal pencil-like VO2(R) as electrode materials for high-performance redox activities I. Mjejria, N. Etteyeba, S. Somrania, F. Sediria,b,n a
Laboratory of Condensed Matter Chemistry, IPEIT, University of Tunis 2, Jawaher Lel Nehru 1008, BP 229 Montfleury, Tunisia b Chemistry Department, Sciences Faculty of Tunis, Tunis El Manar University, 2092 El Manar, Tunisia Received 14 December 2015; received in revised form 24 December 2015; accepted 28 December 2015 Available online 7 January 2016
Abstract Pencil-like tetragonal vanadium dioxide has been synthesized via one-step hydrothermal treatment. The compounds were analyzed through Xray powder diffraction; scanning electron microscope and X-ray photoelectron spectroscopy (XPS). The optical properties of the as-synthesized material were studied by UV–visible diffuse reflection spectroscopy and room temperature photoluminescence. Thin films of VO2(R) deposited on ITO substrates were electrochemically characterized by cyclic voltammetry (CV). The voltammograms show a reversible redox behavior with a doping/dedoping process corresponding to reversible cation intercalation/de-intercalation into the crystal lattice of the pencil. This process is easier in propylene carbonate than in aqueous solvent. It is also easier for the small Li þ cation than larger ones, Na þ and K þ . This is attributed to a probable presence of one tunnel cavities in the structure of VO2(R). The good electrochemical property of the VO2(R) is attributed to its unique ultralong nanopencils structure with a good structural stability. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Spectroscopy; C. Chemical properties; E. Electrodes; Hydrothermal synthesis
1. Introduction The synthesis of inorganic nanoscale materials with special morphologies has been of great interest in recent years [1–3]. Onedimensional (1D) nanostructures such as nanotubes, nanoneedles, nanowires and nanorods, have attracted increasing attention due to their physical and chemical properties which are strongly linked to their shape and size in comparison to dense materials [4–6]. Vanadium oxides, is one group of important inorganic functional materials, have received considerable attention due to their richness in structural and compositional variants, along with their extensive applications, such as lithium batteries [7–10], electrochromic devices [11,12], thermochromic [13], and catalysts [14–16]. Among these oxides, nanostructured VO2 is of particular interest because they have unique optical and electrochemical properties, which lead to a wide variety of potential applications inclun
Corresponding author. Tel.: +216 71336641; fax: +216 71337323. E-mail addresses: [email protected], [email protected] (F. Sediri). http://dx.doi.org/10.1016/j.ceramint.2015.12.173 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
ding lithium-ion batteries [17,18], and optical switching devices [19–21]. Since the discovery of the metal-semi-Conductor transition in VO2 by Morin [22], this new frontier lures the attention of many researchers [23,24]. Vanadium dioxide (VO2), exists in different polymorphic forms [25–27]. The different polymorphic phases depend on thermodynamic conditions, which mainly include system pressure, temperature and chemical circumstance [26]. The polymorphic varieties in this system include stable phases and metastable phases [25–27]. Among the stable phases, VO2(R) with stable tetragonal structure is an attractive material for various applications especially as thermochromic devices [28,29]. It has not been studied widely until now. In view of these important applications of nanostructured vanadium oxides, various methodologies have been developed to synthesize different nanostructures ranging from nanotubes, nanorods, and nanoribbons to nanosheets [30–32]. As one of the solution methods, the hydrothermal treatment is a simple and feasible method to prepare onedimensional nanoscaled materials. This paper deals with the hydrothermal synthesis of pencil-like nanocrystalline VO2(R) of the first time by using V2O5 as a
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vanadium source and triethylenglycol which, acts as a reducing and structure-directing agent. The electrochemical properties of the VO2(R) films deposited on indium-tin-oxide (ITO)-coated glass has been investigated using cyclic voltammetry (CV) in the presence of Li þ , Na þ and K þ respectively, in propylene carbonate electrolytic solution and in an aqueous electrolytic one. 2. Experimental 2.1. Hydrothermal synthesis All of the chemical reagents were purchased from Acros Organic and used without further purification. Vanadium (V) oxide was used as vanadium source. The organics reagents, triethylenglycol (C6H14O4), have been used as templates for the first time. VO2(R) pencil-like were hydrothermally synthesized, from a mixture of V2O5, triethylenglycol (C6H14O4) and distilled water (5 mL) in a molar ratio 1:1:415. Reactants were introduced in this order and stirred a few minutes before introducing the resulting suspension in a Teflon-lined steel autoclave and the temperature set at 220 1C for 2 days. The pH of the solution remains close to pH ¼ 7 during the whole synthesis.
2.2. Characterization techniques The crystallographic structures of the products were characterized by a powder X-ray diffraction system (X'Pert Pro Panalytical diffractometer) with CuKα radiation (λ¼ 1.5418 Å) and graphite monochromator. The XRD measurements were carried out by applying a step scanning method (2θ range from 81 to 801), the scanning rate is 0.0171 s 1 and the scanning step lasted 1 s. The scanning electron microscopy (SEM) study was recorded with a SEI instrument (operating at 5 kV) microscope. X-ray photoelectron spectroscopy (XPS) experiments were performed using a Shimadzu ESCALAB at room temperature. The optical parameters of the sample were calculated from the optical absorbance data recorded in the wavelength range from 200 to 800 nm using a UV–visible spectrophotometer Shimadzu-3101PC. Photoluminescence (PL) spectroscopy was performed to investigate the optical properties of the samples using a 250 mm Jobin Yvon luminescence spectrometer. 2.3. Electrochemical measurements The electrochemical measurements were carried out using one compartment cell and a BioLogic SP150 potentiostat/galvanostat
Fig. 1. Powder X-ray diffraction patterns of the resulting products synthesized at 220 1C for 2 days.
Fig. 2. SEM micrographs of the resulting products synthesized at 220 1C for 2 days.
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Fig. 4. UV–visible absorption spectrum of VO2(R) nanopencils.
apparatus. Ag/AgCl electrode and a stainless steel grid were used as reference and counter-electrode electrode, respectively. The working electrode is a film of VO2(R) deposited on a plate of indium tin oxide (ITO). The electrode was obtained: 0.5 mg of VO2(R) nanobelts was dispersed in 1 mL of water by ultrasonic treatment to obtain a suspension. 100 mL of this suspension were deposited on a 1 cm2 area ITO-coated glass plate then the water evaporated. The layer obtained was about 1 mm thickness and was covered with 10 mL of a Nafion solution (obtained by dissolving in ethanol a commercial Nafion solution 9/1 V/V) and dried. The final working electrode is an ITO-coated glass plate covered by a thin layer of vanadium oxide nanomaterial protected by Nafion membrane order to prevent the degradation of the material into the solution. The operating voltage was controlled between 0.5 V and 0.3 V at different scan rates. 1 M lithium perchlorate (LiClO4) in propylene carbonate (PC) was used as electrolytic solution. All measurements were performed at room temperature.
3. Results and discussion 3.1. X-ray diffraction
Fig. 3. XPS survey spectrum (a), high-resolution XPS spectra of V2p and O1s (c) of the as-synthesized VO2(R) nanopencils.
Powder X-ray diffraction pattern of the resulting sample is shown in Fig. 1. The diffraction peaks of the material appear at the same positions and all of these peaks can be indexed to the stable tetragonal phase VO2(R) (space group: P42/mnm) according to (JCPDS 79-1655), suggesting that the V5 þ ions have been reduced to V4 þ ions by polyethylenglycol. No peak of any other phase or impurity was detected from the XRD pattern. This shows that the VO2(R) with high-purity can be obtained via the hydrothermal treatment at 220 1C. Indeed, strong and sharp diffraction peaks also indicate good crystallinity of the hydrothermal product. The average crystallite size of the as-synthesized
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materials was calculated using Scherrer's formula [33]: D¼
0:9λ L cos θ
where D is the average crystallite size, λ¼ 0.15418 nm, L is the half-maximum peak width and θ is the diffraction angle in degrees. The average crystallite size value, calculated from XRD patterns, was about 90 nm. 3.2. Scanning electronic microscopy The morphology of synthesized materials was studied using the scanning electron microscopy (SEM). Fig. 2 shows SEM images of the as-synthesized material. It is clearly observed in Fig. 2a and b that the as-obtained VO2(R) is made of a homogenous phase with particles regularly sized and which display pencil-like morphology, with a width ranging from 80 nm to 120 nm. This result is good agreement with the average crystallite size extracted from X-ray diffraction patterns.
3.4. Optical properties The optical properties of as-obtained VO2(R) were investigated by UV–visible and photoluminescence spectroscopy. The absorption spectrum of the nanopencils is shown in Fig. 4. It is well-known that the band maxima of the charge transfer transition of O-Vn þ depends on the number of O atoms surrounding the central vanadium ion [41,42]. Therefore, V5 þ in tetrahedral coordination absorbs in the range 240–350 nm, in square pyramidal coordination at 350–450 nm and in octahedral coordination at 450–600 nm [41,42]. In fact, the spectra exhibit an absorption band at around 240 nm, which can be due to lower energy charge transfer to V4 þ species in tetrahedral coordination [43]. However, the absorption band located at around 345 nm can be attributed to the n–π* transition centered on the V ¼ O group in square pyramidal coordination [44]. The band gap energy for these bands is calculated using the equation [45]: Eg ðeVÞ ¼ hν ¼ hc=λ
3.3. X-ray photoelectron spectroscopy (XPS) The composition and vanadium valence state of the surface of the as-obtained VO2(R) pencil-like were further investigated by Xray photoelectron spectroscopy (XPS), as shown in (Fig. 3a), and the fitted-curves about V2p3/2, V2p1/2 and O1s are illustrated in (Fig. 3b and c). The XPS survey spectrum (Fig. 3a) reveals that the sample only consists of vanadium and oxygen (the C1s peak appeared, which could be due to some CO2 absorbed on the surface of the sample). Fig. 3b and c shown three peaks centered at 518.76 eV, 526.25 eV and 531.69 eV are assigned to the V2p3/2, V2p1/2 and O1s, respectively [34–36]. In fact, the V2p3/2 peak of the sample is divided into two peaks at the binding energies of 517.32 and 518.13 eV, assigned to V4 þ and V5 þ , respectively (Fig. 3b) [36]. However, The O1s core peak was divided into two peaks with the binding energies at 529.64 and 532.88 eV. This is attributed to O2 and O species, respectively [37–39]. According to the analysis proposed by Chao-jun Cui et al. [40], superoxide ions O is associated with the presence of V4 þ ions.
Fig. 5. Room temperature photoluminescence spectrum of VO2(R) nanopencils.
Fig. 6. The first 1000 cycles of CV curves between 0.5 V and 0.3 V vs. Ag/ AgCl at 50 mV/s in the solution containing PC and 1 M LiClO4.
Fig. 7. Performance coulombic efficiency of VO2(R) nanopencils.
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where Eg is the band gap energy in eV, h is the Planck's constant, c is the speed of light in vacuum and λ is the absorption wavelength in nm. The values of Eg for VO2(R) are found to be 2.53 eV. The optical band gap of the sample can be attributed to direct transition from occupied 2p bands of oxygen to unoccupied 3d bands of vanadium in VO2(R). Photoluminescence (PL) property can explain the nature of the intrinsic defect in VO2(R) because the energy levels associate with the defects populating the large band gap of the material and producing radiative emissions at different wavelengths. Fig. 5 shows the PL spectra of the VO2(R) under the excitation of the 325 nm Xe lamp. The spectra reveal that the samples consist of several bands with a very broad band in the region 350–650 nm. Indeed, the peak at 361 nm may be assigned to free-excition emission [46,47]. However, the luminescent mechanism at 452 and 460 nm was attributed to the electric charge transfer, corresponding to the weak energy of V ¼ O bond [48]. Besides, the shoulder peaks at 527 nm may be caused by defect energy gap [46–48].
4. Electrochemical properties
Fig. 8. Cyclic voltammetric curves of the 1st cycle of VO2(R) nanopencils recorded at different scan rates (10, 20, 30,40, 50 mV/s) in 1 M LiClO4 dissolved in propylene carbonate solution.
Fig. 10. CV curves of VO2(R) nanopencils deposited on ITO-coated glass, recorded at 50 mV/s in propylene carbonate solution of 1 M LiClO4, 1 M NaClO4 or 1 M KClO4.
The thin film electrochemical stability is one of the key parameters to be taken into account when aiming at applications. The repetitive cycling of VO2 thin film in in propylene carbonate 1 M LiClO4 at a scan rate of 50 mV/s shows good reversibility, cyclability and stability in terms of capacity up to 1000 cycles (Fig. 6). The voltammograms show a large redox current wave located at cathodic potentials peak to the appears at around 0.29 V corresponding to the reduction of V5 þ , whereas during the positive scan a large current peak appears at about 0.1 V indicating the oxidation of V4 þ cation. In agreement with a clear reversible process, the charges deduced from the surface of cyclic voltammetric in reduction and in oxidation are equal (Qred/Qox ¼ 95%) (Fig. 7). The insertion process of Li þ ions intercalation/deintercalation in tetragonal VO2 thin films can be represented by the following equation: VO2 þ xLi þ þ xe⇌Lix VO2
Fig. 9. Plot of the peak current vs. the square root of scan rate (V1/2).
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Fig. 11. Cyclic voltammetric curves of the 1st cycle of VO2(R) nanopencils recorded at 50 mV/s in PC containing 1 M LiClO4 and aqueous solution containing 1 M LiClO4.
After 1000 cycles, the intercalation/deintercalation process remains reversible. This amount is stable upon 1000 cycles. Furthermore, a slight decrease in capacity is observed upon cycling, with a loss of 1% after 1000 cycles. VO2(R) shows good reversibility, cyclability and stability in terms of capacity up to 1000 cycles. When the scan rate has been increased to 50 mV/s (Fig. 8), the cathodic and anodic peak currents increased simultaneously. The system's voltammetric characteristic features are more evident, thus indicating the importance of dynamics of lithium intercalation/deintercalation occurring in the material to provide electroneutrality. Fig. 9 displays the relationship between the current peak density j (ja and jc) and the root of scan rate V1/2. The peak current is proportionate to V1/2, which may indicate a diffusion-controlled process [49]. The diffusion coefficient D of lithium ions (Li þ ) was calculated using Randles–Sevcik equation [50–52]: j ¼ 2:69 10 n
5 3=2
AD
1=2
Cv
V4 þ in VO2(R) under-goes also one-electron transfer process. The presence of only one redox wave for each cation indicates that the dynamics of intercalation/deintercalation is the same and occurs mainly in the large cavities. To evaluate how the solvent will affect the electrochemical activities of VO2(R), The superposition of the first cycle responses of VO2(R) nanopencils deposited on ITO-covered glass slide recorded at 50 mV/s in PC containing 1 M LiClO4 and aqueous solution containing 1 M LiClO4, is shown in Fig. 11. In both cases, the CV curves indicate the presence of only two peaks: a reduction current peak and an oxidation current peak. This confirms alone tunnel in the structure of VO2(R). In fact, in the aqueous medium there is a decrease in the current which is interpreted by the slower diffusion of lithium in comparison with the organic medium. In the aqueous medium the Li þ ions are solvated by water molecules. It is clear that the organic medium facilitates Li-ion diffusion through the crystal structure, being effective in facilitating the electrochemical reaction. 5. Conclusion In this study, a stable VO2(R) pencil-like were successfully synthesized using a simple hydrothermal route. The synthesized VO2(R) is highly crystalline and exhibits a pencil cross-section. Electrochemical measurements carried out on thin films of the nanopencils reveal more reversible redox processes when PC is used as a solvent instead of water. These redox processes are also more reversible for small cations than for larger ones. VO2(R) shows good reversibility, cyclability and stability in terms of capacity up to 1000 cycles. Such a highly favorable electrochemical property is attributed to the intrinsic properties of crystalline VO2(R) nanopencils. These results might renew interest in designing other metal oxide electrodes in the future. Acknowledgments
1=2
where j is the anodic peak current density at oxidation state, n is the number of electrons transferred, A is the electrode area (cm2), D is the diffusion coefficient (cm2/s), C is the concentration of the electrolyte (mol/cm3), and v is the voltage scan rate (V/s). The linear relation is expected for a diffusion-controlled process allowing the determination of the diffusion coefficient, þ DLi ¼ 9.58*10 10 cm2 s 1. In our system, because the tunnel size of VO2(R) is much larger than that of Li-ion, apparently, the existence of VO2(R) tunnel will accelerate the diffusion of the Li-ions, and thus leads to promotion of the electrochemical reaction. Fig. 10 shows the 1st cycle of the CV curves recorded in propylene carbonate solutions (LiClO4, NaClO4 and KClO4). The CV curves recorded in aqueous medium show the presence of a single redox process. The redox processes are more difficult for larger cations than for small ones. The repeated voltammograms for each of the cations are stable indicating that vanadium dioxide nanopencils are stable whatever the cation. For the large cations (rK þ 4 rNa þ 4 rLi þ )
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 6121–6128 www.elsevier.com/locate/ceramint
Tetragonal pencil-like VO2(R) as electrode materials for high-performance redox activities I. Mjejria, N. Etteyeba, S. Somrania, F. Sediria,b,n a
Laboratory of Condensed Matter Chemistry, IPEIT, University of Tunis 2, Jawaher Lel Nehru 1008, BP 229 Montfleury, Tunisia b Chemistry Department, Sciences Faculty of Tunis, Tunis El Manar University, 2092 El Manar, Tunisia Received 14 December 2015; received in revised form 24 December 2015; accepted 28 December 2015 Available online 7 January 2016
Abstract Pencil-like tetragonal vanadium dioxide has been synthesized via one-step hydrothermal treatment. The compounds were analyzed through Xray powder diffraction; scanning electron microscope and X-ray photoelectron spectroscopy (XPS). The optical properties of the as-synthesized material were studied by UV–visible diffuse reflection spectroscopy and room temperature photoluminescence. Thin films of VO2(R) deposited on ITO substrates were electrochemically characterized by cyclic voltammetry (CV). The voltammograms show a reversible redox behavior with a doping/dedoping process corresponding to reversible cation intercalation/de-intercalation into the crystal lattice of the pencil. This process is easier in propylene carbonate than in aqueous solvent. It is also easier for the small Li þ cation than larger ones, Na þ and K þ . This is attributed to a probable presence of one tunnel cavities in the structure of VO2(R). The good electrochemical property of the VO2(R) is attributed to its unique ultralong nanopencils structure with a good structural stability. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Spectroscopy; C. Chemical properties; E. Electrodes; Hydrothermal synthesis
1. Introduction The synthesis of inorganic nanoscale materials with special morphologies has been of great interest in recent years [1–3]. Onedimensional (1D) nanostructures such as nanotubes, nanoneedles, nanowires and nanorods, have attracted increasing attention due to their physical and chemical properties which are strongly linked to their shape and size in comparison to dense materials [4–6]. Vanadium oxides, is one group of important inorganic functional materials, have received considerable attention due to their richness in structural and compositional variants, along with their extensive applications, such as lithium batteries [7–10], electrochromic devices [11,12], thermochromic [13], and catalysts [14–16]. Among these oxides, nanostructured VO2 is of particular interest because they have unique optical and electrochemical properties, which lead to a wide variety of potential applications inclun
Corresponding author. Tel.: +216 71336641; fax: +216 71337323. E-mail addresses: [email protected], [email protected] (F. Sediri). http://dx.doi.org/10.1016/j.ceramint.2015.12.173 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
ding lithium-ion batteries [17,18], and optical switching devices [19–21]. Since the discovery of the metal-semi-Conductor transition in VO2 by Morin [22], this new frontier lures the attention of many researchers [23,24]. Vanadium dioxide (VO2), exists in different polymorphic forms [25–27]. The different polymorphic phases depend on thermodynamic conditions, which mainly include system pressure, temperature and chemical circumstance [26]. The polymorphic varieties in this system include stable phases and metastable phases [25–27]. Among the stable phases, VO2(R) with stable tetragonal structure is an attractive material for various applications especially as thermochromic devices [28,29]. It has not been studied widely until now. In view of these important applications of nanostructured vanadium oxides, various methodologies have been developed to synthesize different nanostructures ranging from nanotubes, nanorods, and nanoribbons to nanosheets [30–32]. As one of the solution methods, the hydrothermal treatment is a simple and feasible method to prepare onedimensional nanoscaled materials. This paper deals with the hydrothermal synthesis of pencil-like nanocrystalline VO2(R) of the first time by using V2O5 as a
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vanadium source and triethylenglycol which, acts as a reducing and structure-directing agent. The electrochemical properties of the VO2(R) films deposited on indium-tin-oxide (ITO)-coated glass has been investigated using cyclic voltammetry (CV) in the presence of Li þ , Na þ and K þ respectively, in propylene carbonate electrolytic solution and in an aqueous electrolytic one. 2. Experimental 2.1. Hydrothermal synthesis All of the chemical reagents were purchased from Acros Organic and used without further purification. Vanadium (V) oxide was used as vanadium source. The organics reagents, triethylenglycol (C6H14O4), have been used as templates for the first time. VO2(R) pencil-like were hydrothermally synthesized, from a mixture of V2O5, triethylenglycol (C6H14O4) and distilled water (5 mL) in a molar ratio 1:1:415. Reactants were introduced in this order and stirred a few minutes before introducing the resulting suspension in a Teflon-lined steel autoclave and the temperature set at 220 1C for 2 days. The pH of the solution remains close to pH ¼ 7 during the whole synthesis.
2.2. Characterization techniques The crystallographic structures of the products were characterized by a powder X-ray diffraction system (X'Pert Pro Panalytical diffractometer) with CuKα radiation (λ¼ 1.5418 Å) and graphite monochromator. The XRD measurements were carried out by applying a step scanning method (2θ range from 81 to 801), the scanning rate is 0.0171 s 1 and the scanning step lasted 1 s. The scanning electron microscopy (SEM) study was recorded with a SEI instrument (operating at 5 kV) microscope. X-ray photoelectron spectroscopy (XPS) experiments were performed using a Shimadzu ESCALAB at room temperature. The optical parameters of the sample were calculated from the optical absorbance data recorded in the wavelength range from 200 to 800 nm using a UV–visible spectrophotometer Shimadzu-3101PC. Photoluminescence (PL) spectroscopy was performed to investigate the optical properties of the samples using a 250 mm Jobin Yvon luminescence spectrometer. 2.3. Electrochemical measurements The electrochemical measurements were carried out using one compartment cell and a BioLogic SP150 potentiostat/galvanostat
Fig. 1. Powder X-ray diffraction patterns of the resulting products synthesized at 220 1C for 2 days.
Fig. 2. SEM micrographs of the resulting products synthesized at 220 1C for 2 days.
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Fig. 4. UV–visible absorption spectrum of VO2(R) nanopencils.
apparatus. Ag/AgCl electrode and a stainless steel grid were used as reference and counter-electrode electrode, respectively. The working electrode is a film of VO2(R) deposited on a plate of indium tin oxide (ITO). The electrode was obtained: 0.5 mg of VO2(R) nanobelts was dispersed in 1 mL of water by ultrasonic treatment to obtain a suspension. 100 mL of this suspension were deposited on a 1 cm2 area ITO-coated glass plate then the water evaporated. The layer obtained was about 1 mm thickness and was covered with 10 mL of a Nafion solution (obtained by dissolving in ethanol a commercial Nafion solution 9/1 V/V) and dried. The final working electrode is an ITO-coated glass plate covered by a thin layer of vanadium oxide nanomaterial protected by Nafion membrane order to prevent the degradation of the material into the solution. The operating voltage was controlled between 0.5 V and 0.3 V at different scan rates. 1 M lithium perchlorate (LiClO4) in propylene carbonate (PC) was used as electrolytic solution. All measurements were performed at room temperature.
3. Results and discussion 3.1. X-ray diffraction
Fig. 3. XPS survey spectrum (a), high-resolution XPS spectra of V2p and O1s (c) of the as-synthesized VO2(R) nanopencils.
Powder X-ray diffraction pattern of the resulting sample is shown in Fig. 1. The diffraction peaks of the material appear at the same positions and all of these peaks can be indexed to the stable tetragonal phase VO2(R) (space group: P42/mnm) according to (JCPDS 79-1655), suggesting that the V5 þ ions have been reduced to V4 þ ions by polyethylenglycol. No peak of any other phase or impurity was detected from the XRD pattern. This shows that the VO2(R) with high-purity can be obtained via the hydrothermal treatment at 220 1C. Indeed, strong and sharp diffraction peaks also indicate good crystallinity of the hydrothermal product. The average crystallite size of the as-synthesized
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materials was calculated using Scherrer's formula [33]: D¼
0:9λ L cos θ
where D is the average crystallite size, λ¼ 0.15418 nm, L is the half-maximum peak width and θ is the diffraction angle in degrees. The average crystallite size value, calculated from XRD patterns, was about 90 nm. 3.2. Scanning electronic microscopy The morphology of synthesized materials was studied using the scanning electron microscopy (SEM). Fig. 2 shows SEM images of the as-synthesized material. It is clearly observed in Fig. 2a and b that the as-obtained VO2(R) is made of a homogenous phase with particles regularly sized and which display pencil-like morphology, with a width ranging from 80 nm to 120 nm. This result is good agreement with the average crystallite size extracted from X-ray diffraction patterns.
3.4. Optical properties The optical properties of as-obtained VO2(R) were investigated by UV–visible and photoluminescence spectroscopy. The absorption spectrum of the nanopencils is shown in Fig. 4. It is well-known that the band maxima of the charge transfer transition of O-Vn þ depends on the number of O atoms surrounding the central vanadium ion [41,42]. Therefore, V5 þ in tetrahedral coordination absorbs in the range 240–350 nm, in square pyramidal coordination at 350–450 nm and in octahedral coordination at 450–600 nm [41,42]. In fact, the spectra exhibit an absorption band at around 240 nm, which can be due to lower energy charge transfer to V4 þ species in tetrahedral coordination [43]. However, the absorption band located at around 345 nm can be attributed to the n–π* transition centered on the V ¼ O group in square pyramidal coordination [44]. The band gap energy for these bands is calculated using the equation [45]: Eg ðeVÞ ¼ hν ¼ hc=λ
3.3. X-ray photoelectron spectroscopy (XPS) The composition and vanadium valence state of the surface of the as-obtained VO2(R) pencil-like were further investigated by Xray photoelectron spectroscopy (XPS), as shown in (Fig. 3a), and the fitted-curves about V2p3/2, V2p1/2 and O1s are illustrated in (Fig. 3b and c). The XPS survey spectrum (Fig. 3a) reveals that the sample only consists of vanadium and oxygen (the C1s peak appeared, which could be due to some CO2 absorbed on the surface of the sample). Fig. 3b and c shown three peaks centered at 518.76 eV, 526.25 eV and 531.69 eV are assigned to the V2p3/2, V2p1/2 and O1s, respectively [34–36]. In fact, the V2p3/2 peak of the sample is divided into two peaks at the binding energies of 517.32 and 518.13 eV, assigned to V4 þ and V5 þ , respectively (Fig. 3b) [36]. However, The O1s core peak was divided into two peaks with the binding energies at 529.64 and 532.88 eV. This is attributed to O2 and O species, respectively [37–39]. According to the analysis proposed by Chao-jun Cui et al. [40], superoxide ions O is associated with the presence of V4 þ ions.
Fig. 5. Room temperature photoluminescence spectrum of VO2(R) nanopencils.
Fig. 6. The first 1000 cycles of CV curves between 0.5 V and 0.3 V vs. Ag/ AgCl at 50 mV/s in the solution containing PC and 1 M LiClO4.
Fig. 7. Performance coulombic efficiency of VO2(R) nanopencils.
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where Eg is the band gap energy in eV, h is the Planck's constant, c is the speed of light in vacuum and λ is the absorption wavelength in nm. The values of Eg for VO2(R) are found to be 2.53 eV. The optical band gap of the sample can be attributed to direct transition from occupied 2p bands of oxygen to unoccupied 3d bands of vanadium in VO2(R). Photoluminescence (PL) property can explain the nature of the intrinsic defect in VO2(R) because the energy levels associate with the defects populating the large band gap of the material and producing radiative emissions at different wavelengths. Fig. 5 shows the PL spectra of the VO2(R) under the excitation of the 325 nm Xe lamp. The spectra reveal that the samples consist of several bands with a very broad band in the region 350–650 nm. Indeed, the peak at 361 nm may be assigned to free-excition emission [46,47]. However, the luminescent mechanism at 452 and 460 nm was attributed to the electric charge transfer, corresponding to the weak energy of V ¼ O bond [48]. Besides, the shoulder peaks at 527 nm may be caused by defect energy gap [46–48].
4. Electrochemical properties
Fig. 8. Cyclic voltammetric curves of the 1st cycle of VO2(R) nanopencils recorded at different scan rates (10, 20, 30,40, 50 mV/s) in 1 M LiClO4 dissolved in propylene carbonate solution.
Fig. 10. CV curves of VO2(R) nanopencils deposited on ITO-coated glass, recorded at 50 mV/s in propylene carbonate solution of 1 M LiClO4, 1 M NaClO4 or 1 M KClO4.
The thin film electrochemical stability is one of the key parameters to be taken into account when aiming at applications. The repetitive cycling of VO2 thin film in in propylene carbonate 1 M LiClO4 at a scan rate of 50 mV/s shows good reversibility, cyclability and stability in terms of capacity up to 1000 cycles (Fig. 6). The voltammograms show a large redox current wave located at cathodic potentials peak to the appears at around 0.29 V corresponding to the reduction of V5 þ , whereas during the positive scan a large current peak appears at about 0.1 V indicating the oxidation of V4 þ cation. In agreement with a clear reversible process, the charges deduced from the surface of cyclic voltammetric in reduction and in oxidation are equal (Qred/Qox ¼ 95%) (Fig. 7). The insertion process of Li þ ions intercalation/deintercalation in tetragonal VO2 thin films can be represented by the following equation: VO2 þ xLi þ þ xe⇌Lix VO2
Fig. 9. Plot of the peak current vs. the square root of scan rate (V1/2).
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Fig. 11. Cyclic voltammetric curves of the 1st cycle of VO2(R) nanopencils recorded at 50 mV/s in PC containing 1 M LiClO4 and aqueous solution containing 1 M LiClO4.
After 1000 cycles, the intercalation/deintercalation process remains reversible. This amount is stable upon 1000 cycles. Furthermore, a slight decrease in capacity is observed upon cycling, with a loss of 1% after 1000 cycles. VO2(R) shows good reversibility, cyclability and stability in terms of capacity up to 1000 cycles. When the scan rate has been increased to 50 mV/s (Fig. 8), the cathodic and anodic peak currents increased simultaneously. The system's voltammetric characteristic features are more evident, thus indicating the importance of dynamics of lithium intercalation/deintercalation occurring in the material to provide electroneutrality. Fig. 9 displays the relationship between the current peak density j (ja and jc) and the root of scan rate V1/2. The peak current is proportionate to V1/2, which may indicate a diffusion-controlled process [49]. The diffusion coefficient D of lithium ions (Li þ ) was calculated using Randles–Sevcik equation [50–52]: j ¼ 2:69 10 n
5 3=2
AD
1=2
Cv
V4 þ in VO2(R) under-goes also one-electron transfer process. The presence of only one redox wave for each cation indicates that the dynamics of intercalation/deintercalation is the same and occurs mainly in the large cavities. To evaluate how the solvent will affect the electrochemical activities of VO2(R), The superposition of the first cycle responses of VO2(R) nanopencils deposited on ITO-covered glass slide recorded at 50 mV/s in PC containing 1 M LiClO4 and aqueous solution containing 1 M LiClO4, is shown in Fig. 11. In both cases, the CV curves indicate the presence of only two peaks: a reduction current peak and an oxidation current peak. This confirms alone tunnel in the structure of VO2(R). In fact, in the aqueous medium there is a decrease in the current which is interpreted by the slower diffusion of lithium in comparison with the organic medium. In the aqueous medium the Li þ ions are solvated by water molecules. It is clear that the organic medium facilitates Li-ion diffusion through the crystal structure, being effective in facilitating the electrochemical reaction. 5. Conclusion In this study, a stable VO2(R) pencil-like were successfully synthesized using a simple hydrothermal route. The synthesized VO2(R) is highly crystalline and exhibits a pencil cross-section. Electrochemical measurements carried out on thin films of the nanopencils reveal more reversible redox processes when PC is used as a solvent instead of water. These redox processes are also more reversible for small cations than for larger ones. VO2(R) shows good reversibility, cyclability and stability in terms of capacity up to 1000 cycles. Such a highly favorable electrochemical property is attributed to the intrinsic properties of crystalline VO2(R) nanopencils. These results might renew interest in designing other metal oxide electrodes in the future. Acknowledgments
1=2
where j is the anodic peak current density at oxidation state, n is the number of electrons transferred, A is the electrode area (cm2), D is the diffusion coefficient (cm2/s), C is the concentration of the electrolyte (mol/cm3), and v is the voltage scan rate (V/s). The linear relation is expected for a diffusion-controlled process allowing the determination of the diffusion coefficient, þ DLi ¼ 9.58*10 10 cm2 s 1. In our system, because the tunnel size of VO2(R) is much larger than that of Li-ion, apparently, the existence of VO2(R) tunnel will accelerate the diffusion of the Li-ions, and thus leads to promotion of the electrochemical reaction. Fig. 10 shows the 1st cycle of the CV curves recorded in propylene carbonate solutions (LiClO4, NaClO4 and KClO4). The CV curves recorded in aqueous medium show the presence of a single redox process. The redox processes are more difficult for larger cations than for small ones. The repeated voltammograms for each of the cations are stable indicating that vanadium dioxide nanopencils are stable whatever the cation. For the large cations (rK þ 4 rNa þ 4 rLi þ )
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