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Enhanced electrochemical performance of TiO2 nanotube array electrodes by controlling the introduction of substoichiometric titanium oxides
Journal of Alloys and Compounds 680 (2016) 538e543
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced electrochemical performance of TiO2 nanotube array electrodes by controlling the introduction of substoichiometric titanium oxides Shupei Sun, Xiaoming Liao*, Guangfu Yin, Yadong Yao, Zhongbing Huang, Ximing Pu College of Materials Science and Engineering, Sichuan University, Chengdu, Sichuan, 610065, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2016 Received in revised form 21 March 2016 Accepted 18 April 2016 Available online 21 April 2016
Although anodized titania nanotubes (TNTs) possess unique advantages as an electrode material, the poor electrical conductivity limits their practical application. We herein report a facile route to enhance electrical conductivity and improve electrochemical behavior by controlling the introduction of substoichiometric titanium oxides (TinO2n-1), which can facilitate charge propagation in TNTs and improve kinetics of ions and electron transport in the electrode. Specific capacitance is as high as 3.77 mF cm2, which is about 47 times higher than that of the untreated samples (0.08 mF cm2). Galvanostatic chargedischarge results show TNT arrays-based electrode stable capacitance behavior with excellent capacitance retention even after 1000 continuous charge-discharge cycles at a current density of 50 mA cm2 in Li2SO4 electrolyte. The ease of synthesis and the superior electrochemical performance suggest a promising application for the TNT arrays-based material in energy storage field. © 2016 Elsevier B.V. All rights reserved.
Keywords: Titania nanotubes Heat treatment Substoichiometric titanium oxides Electrochemical properties Electrode material
1. Introduction Since the anodized titania nanotubes (TNTs) were firstly reported in 1984 [1]. TNTs have been extensively studied as a novel material in various applications including photocatalytic degradation of environment pollutants [2], solar cells [3], energy storage device [4], drug delivery [5] and sensors [6]. In addition, the excellent chemical stability and unique tube structure have led to TNTs new development as a promising supercapacitor electrode material [7e12]. The hollow structure and open space between nanotubes contribute to accessible surface area and promote the speedy permeation of electrolyte ions. Furthermore, the wellaligned nanotubes vertically oriented from the surface of Ti substrate, as current collector, do not need polymer binder and conductive additives [13]. However, the pristine TiO2 presents semiconducting nature of wide band gap (3.2 eV for anatase and 3.0 eV for rutile) [14] with poor electrical conductivity, which restricts its wide application in electrochemical capacitors (also called supercapacitors). Therefore, any methods which can solve above-mentioned problems will be welcome. Recently, non-stoichiometric structure of TiO2 has been
* Corresponding author. E-mail address: [email protected] (X. Liao). http://dx.doi.org/10.1016/j.jallcom.2016.04.171 0925-8388/© 2016 Elsevier B.V. All rights reserved.
extensively studied due to enhanced electrical conductivity and improved electrochemical activity resulted from reduction of full valence Ti4þ and production of point defect (oxygen vacancies and Ti interstitials) [15e21]. Zhou et al. introduced oxygen vacancies into TNTs by electrochemical doping process, increasing charge carrier densities and enhancing conductivity. The specific capacitance was increased to 1.84 mF cm2 [11]. Lu et al. discovered that controlling introduction of oxygen vacancy (Ti3þ sites) states in nanotube array via thermal treatment in hydrogen atmosphere at 400 C could achieve specific capacitance of 3.24 mF cm2 and excellent cycling stability [12]. Xiao et al. indicated that the TNTs calcined in N2 possessed lower electrical resistance and higher photocurrent density than those calcined in air and argon [15]. All in all, the electrochemical behaviors of TNTs are strongly affected by their non-stoichiometric structure. li phases of titanium oxides, a series of Among them, Magne electrically conducting compounds with the generic formula TinO2n-1 (3 n 10) [22e25], have been widely used as electrocatalyst supporting materials [23], photocatalysts [26], and durable electrode materials [22] because of their high electrical conductivity, superior chemical stability and excellent corrosion resistant ability. In our study, a facile heat treatment regime was employed to introduce sub-stoichiometric structure within TNTs to improve electrochemical properties. Various microstructure
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characterizations and electrochemical measurements were conducted to investigate the crystal phase, surface morphology, surface state, electrical conductivity, and electrochemical properties of the obtained TNTs. 2. Experimental procedure 2.1. Fabrication of TNTs After being abrased with metallographic abrasive paper in different grades, all the Ti sheets (TA2, 10 10 1 mm3) were cleaned by sequential ultrasonication in pure acetone (Analytic Reagent, AR, KeLong Chemical, Chengdu), ethanol (AR, KeLong Chemical, Chengdu) and deionized water respectively for 10 min to remove surface impurities and pollutants. Self-assembled TNTs were fabricated by one-step anodization of Ti sheets in a twoelectrode system, using ethylene glycerol (AR, KeLong Chemical, Chengdu) containing 0.3 wt% NH4F (AR, KeLong Chemical, Chengdu) and 6 vol% deionized H2O. The anodization process was performed under a constant voltage of 50 V for 30 min. The asprepared samples were cleaned by ultrasonication in ethanol and deionized water for 2 min respectively, and then dried at 70 C for 2 h in air. Afterwards, TiO2 samples were annealed at 450 C in N2 atmosphere for different periods of time: 1 h, 3 h, and 5 h (referred as TiO2-1, TiO2-3, and TiO2-5, respectively). We also studied the effect of annealing temperatures on surface micrograph, and electrochemical performance of TNTs and concluded that the sample annealed at 450 C obtained largest specific capacitance (supporting information). As a result, we performed our experiment at 450 C to investigate the effect of annealing time on the surface morphology, structure and electrochemical performance of TNT arrays. 2.2. Materials characterization and electrochemical measurements The surface morphology, surface wetting ability, and microstructure of the fabricated TNTs were investigated by scanning electron microscopy (SEM, Hitachi S4800, Japan), contact angle instrument (JC2000C1, China), X-ray diffraction (XRD, DX-1000, China), respectively. Electrochemical measurements for TiO2 electrodes were performed in a 2 M Li2SO4 aqueous solution with an electrochemical workstation (CHI660E, China). Cyclic voltammetry (CV) tests were conducted over a potential voltage range from 0 to 0.9 V (vs Ag/AgCl) at different scan rates (from 20 to 1000 mV s1). The galvanostatic charge-discharge (CD) testing was performed under different current densities (from 0.05 to 0.8 mA cm2). Motto-Schottky plots were measured at a frequency of 1 KHz. Electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range of 0.1 Hze100 kHz at a constant potential of 0.1 V and an AC voltage amplitude of 5 mV. The cycling stability of the samples was tested by galvanostatic CD measurement performed up to 1000 cycles at a current density of 50 mA cm2. 3. Results and discussion 3.1. Morphology and structure analyses Fig. 1a shows the XRD patterns obtained over 2q range of 20 e60 from TNTs annealed at 450 C under N2 atmosphere for different time. The peaks centered at ~25.4 , 30.8 and 38.5 are the characteristic peaks of anatase TiO2 (JCPDF 21-1272), brooktie TiO2 (JCPDF 29-1360) [27] and Ti (JCPDF 44-1294), respectively. From Fig. 1a it can be seen that anatase is the dominant phase of the prepared samples. According to Fig. 1b, the peaks with 2q of ~28.6 ,
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~29.3 , ~30.4 and ~32.1 are assigned to Ti6O11 (JCPDF 50-0788) [28], Ti8O15 (JCPDF 50-0790) [23] and Ti3O5 (JCPDF 40-0806 [29], JCPDF 23-0606), respectively. The results confirm the existence of substoichiometric titanium oxides (TinO2n-1). From Fig. 1b we can see that the sample TiO2-1 has the peaks of Ti6O11, Ti8O15 and Ti3O5, whose diffraction intensity is higher than that of the corresponding peaks for the sample TiO2-5. However, there are only peaks of Ti6O11 and Ti8O15 for the sample TiO2-3. It is reported that the structure of TiO2 is sensitive to ambient oxygen pressure [9,30]. Annealing in neutral or hydrogen atmosphere leads to the formation of nonstoichiometric nanostructure attributed to partial reduction of tetravalent titanium cations [9,10,12,31]. In this paper, we observe the diffraction peaks of li phase titanium oxides via XRD, which have high electrical Magne conductivity and excellent chemical stability. The electrical conductivities of Ti3O5, Ti6O11, and Ti8O5 are 630 S cm1, 63 S cm1, 25 S cm1, respectively [32]. We can anticipate that TNTs with different structures of TinO2n-1 exhibit various electrical conductivity and electrochemical behavior. Fig. 2 shows SEM micrographs of the prepared TiO2 nanotubes. The anodized TNTs have well-defined open-top and uninterrupted nanotube structure, which provide large surface area for electrochemical reactions and unique electron transport pathways. It can be observed that the prolonged annealing time does not have detrimental effect on surface micrograph from SEM images. In our study, the water contact angles of TiO2-1, TiO2-3, and TiO2-5 are 53 , 77, and 42 respectively, indicating hydrophilic surface of samples. Hydrophilic performance enables to enhance the close contact of electrolyte with active material surface, increasing the electrochemical reaction sites. 3.2. Electrochemical characterizations Electrical properties of TNTs were subsequently investigated by electrochemical impedance measurements. Mott-Schottky plots were drawn based on capacitances derived from the imaginary part of the impedance at each potential with 1 kHz frequency. According to the Mott-Schottky theory, the space charge capacitance Csc of a semiconductor is expressed as
1 2 K T U Ufb B ¼ 2 q Csc εε0 qND
(1)
where ε is the dielectric constant, ε0 the permittivity of vacuum, q the electronic charge, ND the donor concentration (for an n-type semiconductor), U the applied potential, Ufb the flat band potential, kB the Boltzmann constant, and T the temperature [33]. The donor density can be calculated from the slope. Fig. 3a shows that all the samples exhibit positive slopes, indicating n-type semiconductor nature. The calculated carrier densities of TiO2-1, TiO2-3, and TiO2-5 are 7.36 1022/cm3, 3.79 1021/cm3, 2.48 1022/cm3, respectively. Thermal treatment in inert atmosphere leads to oxygen loss on the surface of nanotube arrays, resulting in the generation of oxygen deficiency and reduction of Ti4þ. In our study, the generation of substoichiometric titanium oxides also can be attributed to the oxygen depletion inside TiO2 network. Bartholomew and Fankl li investigated systematically the electrical properties of Magne phase titanium oxides and discovered that electrical resistivity decreases with the increase in the oxygen vacancies which are known to be electron donors [34,35]. Salari pointed out that annealing for longer time led to atom reorganization, resulting in lower concentration of oxygen vacancies [31]. Hence, annealing for too long time leads to the decrease of electrical conductivity to some extent. It has been stated that sub-oxides TinO2n-1 have various electrical conductivity with different “n” value. The sample
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Fig. 1. XRD patterns of the samples annealed for different time (a), the enlarged XRD patterns of the samples (b).
Fig. 2. SEM images of samples: TiO2-1 (a, b), TiO2-3 (c, d), TiO2-5 (e, f).
TiO2-3 just has the diffraction peaks of Ti6O11 and Ti8O15 whose electrical conductivities are lower than that of Ti3O5. Compared to TiO2-1, TiO2-5 has weak diffraction intensity of TinO2n-1, and the peak centered at 32.1 is negligible. In conclusion, annealing for 1 h is more likely to produce high charge donor concentration and excellent electrical conductivity. To investigate the electrochemical properties of TiO2-1, TiO2-3 and TiO2-5, electrochemical measurements were conducted in a three-electrode system. Fig. 3b shows CV curves of TiO2-1, TiO2-3
and TiO2-5 electrodes collected at a scan rate of 100 mV s1. All the plots exhibit quasi-rectangular shapes without evident redox peaks, which are a typical character of double-layer capacitance. Furthermore, the area under CV curve for TiO2-1 is larger than that of other samples, implying enhanced electrochemical properties. The CV curves of TiO2-1 obtained at various scan rates are shown in Fig. 3c. The shapes of CV curves retain analogous rectangle as the scan rates increase from 20 to 1000 mV s1, indicating good capacitive behavior and high-rate capability. The rate capability of
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Fig. 3. The Mott-Schottky plots (a) and CV plots at a scan rate of 100 mV s1 (b) for TiO2-1, TiO2-3, TiO2-5, and the CV plots at different scan rates for TiO2-1 (c), areal capacitances of the samples measured as a function of scan rate (d).
electrode material is dependent on the rate of ions diffusion in the electrode materials and electrode conductivity [12]. As all the samples have the highly ordered and open-top nanotube structure and all the tests are conducted in the same electrolyte, the electrolyte ions have similar diffusion rates, and so rate capability is correlated to its improved electrical conductivity. The specific capacitances are calculated by the following equation
Cs ¼ C=S ¼ I=½ðdV=dtÞ S
(2)
Where I is the charge-discharge current, dV/dt the scan rate, and S the surface area. Fig. 3d shows the calculated areal capacitance of these electrodes as a function of scan rate. The areal capacitances of TiO2-1, TiO2-3 and TiO2-5 are 3.77 mF cm2, 1.68 mF cm2, and 2.90 mF cm2 at a scan rate of 100 mV s1, respectively. The specific capacitance for TiO2-1 is almost 47 times higher than that of the unannealed samples (0.08 mF cm2, Supporting Information), which is also higher than the values reported in the published work (0.911e2.6 mF cm2) [9e11,31,36]. The enhanced capacitance is ascribed to the improved electrical conductivity. The decrease of specific capacitance with increasing scan rates is due to active sites inaccessible to electrolyte ions at high scan rates. To the best of our knowledge, electrical conductivity and accessible surface area play important roles in the electrochemical behavior of TNTs. The substoichiometric titanium oxides have high electrical conductivity comparable to that of graphitized carbon [22], thereby facilitating charge propagation in TNTs and improving kinetics of ions and electron transport in the electrode. The highlyordered and uninterrupted structure provides large surface area and direct pathways, reducing the disturbance of interparticle connection and promoting the ions transport rather than disordered nanoarrrays [9]. The hydrophilic surface of TNTs could ensure the efficient contact between the surface of electroactive materials with electrolyte, increasing the electrochemical utilization of electrode materials [37]. In conclusion, TiO2 nanotubes with li phase titanium oxides can achieve excellent Magne
electrochemical properties. Literature reported that harsh thermal treatment was required li phase titanium oxides, for example to fabricate pure Magne thermal reduction above 800 C in hydrogen gas [23,38,39]. In our work, we synthesized the substoichiometric titanium oxides by heat treatment at 450 C in N2 atmosphere. There are two possible reasons for the existence of TinO2n-1 at low temperature. One is that annealing in N2 atmosphere appears to be more favorable to promote the reduction of Ti4þ [15] and enhance the formation of TinO2n-1 than annealing in Ar. Another possible reason is that substoichiometric titanium oxides have already existed in the process of anodization [40], which can retain its structure under proper thermal treatment. With regard to the formation mechanism of TinO2n-1, further research is needed. The electrochemical performance of TiO2 samples was further investigated by galvanostatic CD measurements. Fig. 4a and b present the galvanostatic CD plots of TNTs subjected to annealing for 1 h under N2 atmosphere. The curves keep linear and symmetric at high current densities, which typically are the characteristic of double layer capacitance. The deviation of linear profiles at low current densities indicates the existence of Faradic reactions which are not observed from CV plots. Similar phenomena are also reported in other literature [9,31], which is attributed to oxygen depletion derived from thermal treatment under oxygen-free atmosphere. The specific capacitance is calculated by the following equation.
Cs ¼ ½I Dt=½S DV
(3)
Where I is discharge current, Dt discharge time, S the surface area, DV the potential window. The areal capacitance of TiO2-1 is 3.33 mF cm2 at a current density of 50 mA cm2. The specific capacitances obtained from galvanostatic CD test at various current densities are shown in Fig. 4c. The sample TiO2-1 possesses the largest specific capacitance than that of others, which is agreed with the results of CV tests.
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Fig. 4. The galvanostatic charge-discharge curves of TiO2-1 achieved from 50 mA cm2 to 200 mA cm2 (a) and from 300 mA cm2 to 800 mA cm2 (b), the average areal capacitance at different current densities (c), Nyquist plots of TiO2-1, TiO2-3 and TiO2-5 (d).
Electrochemical impedance spectroscopy (EIS) was subsequently used to understand charge transfer and ion diffusion process, evaluating the internal resistance of the electrode material and the resistance between the electrode and the electrolyte. Fig. 4d shows the Nyquist plots of TiO2-1, TiO2-3 and TiO2-5, which are carried out over a frequency range of 0.01 Hze100 kHz. The high frequency region of the spectra is shown as the inset. The semicircle characteristic is caused by charge transfer process. The absence of semicircular arc for TiO2-1 indicates excellent charge transport capability. This observation agrees well with the MottoSchottky data in the previous section. The curve in low frequency is caused by ion diffusion process. For the pure supercapacitor electrode, it exhibits almost vertical line on Nyquist plots. From the experimental data, the deviation of vertical line is ascribed to pseudocapacitance influence resulted from the substoichiometric nanostructure [31]. Excellent cycling capability is one of important factors for highperformance supercapacitor. The electrodes were measured at the current density of 50 mA cm2 for 1000 cycles. As shown in Fig. 5, the overall capacitance reduction of TNTs annealed for 1 h is about 12% after 1000 cycles, exhibiting good stability in a cyclic chargedischarge process. The first 10 and last 10 cycles are also shown as insets. Thereby, the high electrochemical capacitance performance and good cycling stability indicate that the modified TNTs are a promising electrode material. 4. Conclusion Here we systematically study the effect of heat-treatment regime on electrochemical properties of TNTs. Upon annealing in N2 atmosphere, we obtain substoichiometric titanium dioxides into TiO2 network without losing its nanotube-type structure, which increase carrier density resulted from depletion of oxygen atoms and enhance electrochemical properties. Mott-Schottky plots and li phase titanium oxides EIS tests confirm the contribution of Magne and enhancement in charge transfer within electrode material. It
Fig. 5. Cyclic performance of TNTs electrode annealed for 1 h at a current density of 50 mA cm2.
has been shown that different structures of TinO2n-1 lead to different electrochemical performance. Such a substoichiometric structure could enhance areal capacitance. The sample annealed at 450 C for 1 h has largest specific capacitance of 3.77 mF cm2 at a scan rate of 100 mV s1. The results suggest that these nanotube arrays can be used as an alternative candidate in energy storage devices. Acknowledgments The authors are very much grateful to the National Engineering Research Center for Biomaterials, Sichuan University for the
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assistance with the microscopy work. S.P. Sun thanks to Mr. Yan for his encouragement and support during the research. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.04.171. References [1] M. Assefpour-Dezfuly, C. Vlachos, E.H. Andrews, Oxide morphology and adhesive bonding on titanium surfaces, J. Mater. Sci. 19 (1984) 3626e3639. [2] Q. Zheng, H.-J. Lee, J. Lee, W. Choi, N.-B. Park, C. Lee, Electrochromic titania nanotube arrays for the enhanced photocatalytic degradation of phenol and pharmaceutical compounds, Chem. Eng. J. 249 (2014) 285e292. [3] Q.Y. Wang, S. Li, J.L. Qiao, R.C. Jin, Y.F. Yu, S.M. Gao, CdS-CdSe (CdTe) core-shell quantum dots sensitized TiO2 nanotube array solar cells, Sol. Energy Mater. Sol. Cells 132 (2015) 650e654. [4] A. Lamberti, N. Garino, A. Sacco, S. Bianco, A. Chiodoni, C. Gerbaldi, As-grown vertically aligned amorphous TiO2 nanotube arrays as high-rate Li-based micro-battery anodes with improved long-term performance, Electrochim. Acta 151 (2015) 222e229. [5] L.H. Shi, H. Xu, X.M. Liao, G.F. Yin, Y.D. Yao, Z.B. Huang, X.C. Chen, X.M. Pu, Fabrication of two-layer nanotubes with the pear-like structure by an insitu voltage up anodization and the application as a drug delivery platform, J. Alloy. Compd. 647 (2015) 590e595. [6] J.J. Liao, S.W. Lin, Y. Yang, K. Liu, W.C. Du, Highly selective and sensitive glucose sensors based on organic electrochemical transistors using TiO2 nanotube arrays-based gate electrodes, Sens. Actuators B Chem. 208 (2015) 457e463. [7] H.C. He, Y.H. Zhang, P. Xiao, Y.N. Yang, Q. Lou, F. Yang, Preparation of Ni nanoparticles-TiO2 nanotube arrays composite and its application for electrochemical capacitor, Bull. Korean Chem. Soc. 33 (2012) 1613e1616. [8] R.S. Ray, B. Sarma, A.L. Jurovitzki, M. Misra, Fabrication and characterization of titania nanotube/cobalt sulfide supercapacitor electrode in various electrolytes, Chem. Eng. J. 260 (2015) 671e683. [9] M. Salari, K. Konstantinov, H.K. Liu, Enhancement of the capacitance in TiO2 nanotubes through controlled introduction of oxygen vacancies, J. Mater. Chem. 21 (2011) 5128e5133. [10] M. Salari, S.H. Aboutalebi, A.T. Chidembo, K. Konstantinov, H.K. Liu, Surface engineering of self-assembled TiO2 nanotube arrays: a practical route towards energy storage applications, J. Alloy. Compd. 586 (2014) 197e201. [11] H. Zhou, Y.R. Zhang, Electrochemically self-doped TiO2 nanotube arrays for supercapacitors, J. Phys. Chem. C 118 (2014) 5626e5636. [12] X.H. Lu, G.M. Wang, T. Zhai, M.H. Yu, J.Y. Gan, Y.X. Tong, Y. Li, Hydrogenated TiO2 nanotube arrays for supercapacitors, Nano Lett. 12 (2012) 1690e1696. [13] C.Z. Yuan, L. Yang, L.R. Hou, L.F. Shen, X.G. Zhang, X.W. David Lou, Growth of ultrathin mesoporous Co3O4 nanosheet arrays on Ni foam for highperformance electrochemical capacitors, Energy Environ. Sci. 5 (2012) 7883e7887. [14] A. Bendavid, P.J. Martin, A. Jamting, H. Takikawa, Structural and optical properties of titanium oxide thin films deposited by filtered arc deposition, Thin Solid Films 355 (1999) 6e11. [15] P. Xiao, D.W. Liu, B.B. Garcia, S. Sepehri, Y.H. Zhang, G.Z. Cao, Electrochemical and photoelectrical properties of titania nanotube arrays annealed in different gases, Sens. Actuators B Chem. 134 (2008) 367e372. [16] F. Zuo, K. Bozhilov, R.J. Dillon, L. Wang, P. Smith, X. Zhao, C. Bardeen, P.Y. Feng, Active facets on titanium(III)-doped TiO2: an effective strategy to improve the visible-light photocatalytic activity, Angew. Chem. Int. Ed. 51 (2012) 6223e6226. [17] R. Ren, Z.H. Wen, S.M. Cui, Y. Hou, X.R. Guo, J.H. Chen, Controllable synthesis and tunable photocatalytic properties of Ti3þ-doped TiO2, Sci. Rep. 5 (2015) 10714.
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Enhanced electrochemical performance of TiO2 nanotube array electrodes by controlling the introduction of substoichiometric titanium oxides Shupei Sun, Xiaoming Liao*, Guangfu Yin, Yadong Yao, Zhongbing Huang, Ximing Pu College of Materials Science and Engineering, Sichuan University, Chengdu, Sichuan, 610065, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2016 Received in revised form 21 March 2016 Accepted 18 April 2016 Available online 21 April 2016
Although anodized titania nanotubes (TNTs) possess unique advantages as an electrode material, the poor electrical conductivity limits their practical application. We herein report a facile route to enhance electrical conductivity and improve electrochemical behavior by controlling the introduction of substoichiometric titanium oxides (TinO2n-1), which can facilitate charge propagation in TNTs and improve kinetics of ions and electron transport in the electrode. Specific capacitance is as high as 3.77 mF cm2, which is about 47 times higher than that of the untreated samples (0.08 mF cm2). Galvanostatic chargedischarge results show TNT arrays-based electrode stable capacitance behavior with excellent capacitance retention even after 1000 continuous charge-discharge cycles at a current density of 50 mA cm2 in Li2SO4 electrolyte. The ease of synthesis and the superior electrochemical performance suggest a promising application for the TNT arrays-based material in energy storage field. © 2016 Elsevier B.V. All rights reserved.
Keywords: Titania nanotubes Heat treatment Substoichiometric titanium oxides Electrochemical properties Electrode material
1. Introduction Since the anodized titania nanotubes (TNTs) were firstly reported in 1984 [1]. TNTs have been extensively studied as a novel material in various applications including photocatalytic degradation of environment pollutants [2], solar cells [3], energy storage device [4], drug delivery [5] and sensors [6]. In addition, the excellent chemical stability and unique tube structure have led to TNTs new development as a promising supercapacitor electrode material [7e12]. The hollow structure and open space between nanotubes contribute to accessible surface area and promote the speedy permeation of electrolyte ions. Furthermore, the wellaligned nanotubes vertically oriented from the surface of Ti substrate, as current collector, do not need polymer binder and conductive additives [13]. However, the pristine TiO2 presents semiconducting nature of wide band gap (3.2 eV for anatase and 3.0 eV for rutile) [14] with poor electrical conductivity, which restricts its wide application in electrochemical capacitors (also called supercapacitors). Therefore, any methods which can solve above-mentioned problems will be welcome. Recently, non-stoichiometric structure of TiO2 has been
* Corresponding author. E-mail address: [email protected] (X. Liao). http://dx.doi.org/10.1016/j.jallcom.2016.04.171 0925-8388/© 2016 Elsevier B.V. All rights reserved.
extensively studied due to enhanced electrical conductivity and improved electrochemical activity resulted from reduction of full valence Ti4þ and production of point defect (oxygen vacancies and Ti interstitials) [15e21]. Zhou et al. introduced oxygen vacancies into TNTs by electrochemical doping process, increasing charge carrier densities and enhancing conductivity. The specific capacitance was increased to 1.84 mF cm2 [11]. Lu et al. discovered that controlling introduction of oxygen vacancy (Ti3þ sites) states in nanotube array via thermal treatment in hydrogen atmosphere at 400 C could achieve specific capacitance of 3.24 mF cm2 and excellent cycling stability [12]. Xiao et al. indicated that the TNTs calcined in N2 possessed lower electrical resistance and higher photocurrent density than those calcined in air and argon [15]. All in all, the electrochemical behaviors of TNTs are strongly affected by their non-stoichiometric structure. li phases of titanium oxides, a series of Among them, Magne electrically conducting compounds with the generic formula TinO2n-1 (3 n 10) [22e25], have been widely used as electrocatalyst supporting materials [23], photocatalysts [26], and durable electrode materials [22] because of their high electrical conductivity, superior chemical stability and excellent corrosion resistant ability. In our study, a facile heat treatment regime was employed to introduce sub-stoichiometric structure within TNTs to improve electrochemical properties. Various microstructure
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characterizations and electrochemical measurements were conducted to investigate the crystal phase, surface morphology, surface state, electrical conductivity, and electrochemical properties of the obtained TNTs. 2. Experimental procedure 2.1. Fabrication of TNTs After being abrased with metallographic abrasive paper in different grades, all the Ti sheets (TA2, 10 10 1 mm3) were cleaned by sequential ultrasonication in pure acetone (Analytic Reagent, AR, KeLong Chemical, Chengdu), ethanol (AR, KeLong Chemical, Chengdu) and deionized water respectively for 10 min to remove surface impurities and pollutants. Self-assembled TNTs were fabricated by one-step anodization of Ti sheets in a twoelectrode system, using ethylene glycerol (AR, KeLong Chemical, Chengdu) containing 0.3 wt% NH4F (AR, KeLong Chemical, Chengdu) and 6 vol% deionized H2O. The anodization process was performed under a constant voltage of 50 V for 30 min. The asprepared samples were cleaned by ultrasonication in ethanol and deionized water for 2 min respectively, and then dried at 70 C for 2 h in air. Afterwards, TiO2 samples were annealed at 450 C in N2 atmosphere for different periods of time: 1 h, 3 h, and 5 h (referred as TiO2-1, TiO2-3, and TiO2-5, respectively). We also studied the effect of annealing temperatures on surface micrograph, and electrochemical performance of TNTs and concluded that the sample annealed at 450 C obtained largest specific capacitance (supporting information). As a result, we performed our experiment at 450 C to investigate the effect of annealing time on the surface morphology, structure and electrochemical performance of TNT arrays. 2.2. Materials characterization and electrochemical measurements The surface morphology, surface wetting ability, and microstructure of the fabricated TNTs were investigated by scanning electron microscopy (SEM, Hitachi S4800, Japan), contact angle instrument (JC2000C1, China), X-ray diffraction (XRD, DX-1000, China), respectively. Electrochemical measurements for TiO2 electrodes were performed in a 2 M Li2SO4 aqueous solution with an electrochemical workstation (CHI660E, China). Cyclic voltammetry (CV) tests were conducted over a potential voltage range from 0 to 0.9 V (vs Ag/AgCl) at different scan rates (from 20 to 1000 mV s1). The galvanostatic charge-discharge (CD) testing was performed under different current densities (from 0.05 to 0.8 mA cm2). Motto-Schottky plots were measured at a frequency of 1 KHz. Electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range of 0.1 Hze100 kHz at a constant potential of 0.1 V and an AC voltage amplitude of 5 mV. The cycling stability of the samples was tested by galvanostatic CD measurement performed up to 1000 cycles at a current density of 50 mA cm2. 3. Results and discussion 3.1. Morphology and structure analyses Fig. 1a shows the XRD patterns obtained over 2q range of 20 e60 from TNTs annealed at 450 C under N2 atmosphere for different time. The peaks centered at ~25.4 , 30.8 and 38.5 are the characteristic peaks of anatase TiO2 (JCPDF 21-1272), brooktie TiO2 (JCPDF 29-1360) [27] and Ti (JCPDF 44-1294), respectively. From Fig. 1a it can be seen that anatase is the dominant phase of the prepared samples. According to Fig. 1b, the peaks with 2q of ~28.6 ,
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~29.3 , ~30.4 and ~32.1 are assigned to Ti6O11 (JCPDF 50-0788) [28], Ti8O15 (JCPDF 50-0790) [23] and Ti3O5 (JCPDF 40-0806 [29], JCPDF 23-0606), respectively. The results confirm the existence of substoichiometric titanium oxides (TinO2n-1). From Fig. 1b we can see that the sample TiO2-1 has the peaks of Ti6O11, Ti8O15 and Ti3O5, whose diffraction intensity is higher than that of the corresponding peaks for the sample TiO2-5. However, there are only peaks of Ti6O11 and Ti8O15 for the sample TiO2-3. It is reported that the structure of TiO2 is sensitive to ambient oxygen pressure [9,30]. Annealing in neutral or hydrogen atmosphere leads to the formation of nonstoichiometric nanostructure attributed to partial reduction of tetravalent titanium cations [9,10,12,31]. In this paper, we observe the diffraction peaks of li phase titanium oxides via XRD, which have high electrical Magne conductivity and excellent chemical stability. The electrical conductivities of Ti3O5, Ti6O11, and Ti8O5 are 630 S cm1, 63 S cm1, 25 S cm1, respectively [32]. We can anticipate that TNTs with different structures of TinO2n-1 exhibit various electrical conductivity and electrochemical behavior. Fig. 2 shows SEM micrographs of the prepared TiO2 nanotubes. The anodized TNTs have well-defined open-top and uninterrupted nanotube structure, which provide large surface area for electrochemical reactions and unique electron transport pathways. It can be observed that the prolonged annealing time does not have detrimental effect on surface micrograph from SEM images. In our study, the water contact angles of TiO2-1, TiO2-3, and TiO2-5 are 53 , 77, and 42 respectively, indicating hydrophilic surface of samples. Hydrophilic performance enables to enhance the close contact of electrolyte with active material surface, increasing the electrochemical reaction sites. 3.2. Electrochemical characterizations Electrical properties of TNTs were subsequently investigated by electrochemical impedance measurements. Mott-Schottky plots were drawn based on capacitances derived from the imaginary part of the impedance at each potential with 1 kHz frequency. According to the Mott-Schottky theory, the space charge capacitance Csc of a semiconductor is expressed as
1 2 K T U Ufb B ¼ 2 q Csc εε0 qND
(1)
where ε is the dielectric constant, ε0 the permittivity of vacuum, q the electronic charge, ND the donor concentration (for an n-type semiconductor), U the applied potential, Ufb the flat band potential, kB the Boltzmann constant, and T the temperature [33]. The donor density can be calculated from the slope. Fig. 3a shows that all the samples exhibit positive slopes, indicating n-type semiconductor nature. The calculated carrier densities of TiO2-1, TiO2-3, and TiO2-5 are 7.36 1022/cm3, 3.79 1021/cm3, 2.48 1022/cm3, respectively. Thermal treatment in inert atmosphere leads to oxygen loss on the surface of nanotube arrays, resulting in the generation of oxygen deficiency and reduction of Ti4þ. In our study, the generation of substoichiometric titanium oxides also can be attributed to the oxygen depletion inside TiO2 network. Bartholomew and Fankl li investigated systematically the electrical properties of Magne phase titanium oxides and discovered that electrical resistivity decreases with the increase in the oxygen vacancies which are known to be electron donors [34,35]. Salari pointed out that annealing for longer time led to atom reorganization, resulting in lower concentration of oxygen vacancies [31]. Hence, annealing for too long time leads to the decrease of electrical conductivity to some extent. It has been stated that sub-oxides TinO2n-1 have various electrical conductivity with different “n” value. The sample
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Fig. 1. XRD patterns of the samples annealed for different time (a), the enlarged XRD patterns of the samples (b).
Fig. 2. SEM images of samples: TiO2-1 (a, b), TiO2-3 (c, d), TiO2-5 (e, f).
TiO2-3 just has the diffraction peaks of Ti6O11 and Ti8O15 whose electrical conductivities are lower than that of Ti3O5. Compared to TiO2-1, TiO2-5 has weak diffraction intensity of TinO2n-1, and the peak centered at 32.1 is negligible. In conclusion, annealing for 1 h is more likely to produce high charge donor concentration and excellent electrical conductivity. To investigate the electrochemical properties of TiO2-1, TiO2-3 and TiO2-5, electrochemical measurements were conducted in a three-electrode system. Fig. 3b shows CV curves of TiO2-1, TiO2-3
and TiO2-5 electrodes collected at a scan rate of 100 mV s1. All the plots exhibit quasi-rectangular shapes without evident redox peaks, which are a typical character of double-layer capacitance. Furthermore, the area under CV curve for TiO2-1 is larger than that of other samples, implying enhanced electrochemical properties. The CV curves of TiO2-1 obtained at various scan rates are shown in Fig. 3c. The shapes of CV curves retain analogous rectangle as the scan rates increase from 20 to 1000 mV s1, indicating good capacitive behavior and high-rate capability. The rate capability of
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Fig. 3. The Mott-Schottky plots (a) and CV plots at a scan rate of 100 mV s1 (b) for TiO2-1, TiO2-3, TiO2-5, and the CV plots at different scan rates for TiO2-1 (c), areal capacitances of the samples measured as a function of scan rate (d).
electrode material is dependent on the rate of ions diffusion in the electrode materials and electrode conductivity [12]. As all the samples have the highly ordered and open-top nanotube structure and all the tests are conducted in the same electrolyte, the electrolyte ions have similar diffusion rates, and so rate capability is correlated to its improved electrical conductivity. The specific capacitances are calculated by the following equation
Cs ¼ C=S ¼ I=½ðdV=dtÞ S
(2)
Where I is the charge-discharge current, dV/dt the scan rate, and S the surface area. Fig. 3d shows the calculated areal capacitance of these electrodes as a function of scan rate. The areal capacitances of TiO2-1, TiO2-3 and TiO2-5 are 3.77 mF cm2, 1.68 mF cm2, and 2.90 mF cm2 at a scan rate of 100 mV s1, respectively. The specific capacitance for TiO2-1 is almost 47 times higher than that of the unannealed samples (0.08 mF cm2, Supporting Information), which is also higher than the values reported in the published work (0.911e2.6 mF cm2) [9e11,31,36]. The enhanced capacitance is ascribed to the improved electrical conductivity. The decrease of specific capacitance with increasing scan rates is due to active sites inaccessible to electrolyte ions at high scan rates. To the best of our knowledge, electrical conductivity and accessible surface area play important roles in the electrochemical behavior of TNTs. The substoichiometric titanium oxides have high electrical conductivity comparable to that of graphitized carbon [22], thereby facilitating charge propagation in TNTs and improving kinetics of ions and electron transport in the electrode. The highlyordered and uninterrupted structure provides large surface area and direct pathways, reducing the disturbance of interparticle connection and promoting the ions transport rather than disordered nanoarrrays [9]. The hydrophilic surface of TNTs could ensure the efficient contact between the surface of electroactive materials with electrolyte, increasing the electrochemical utilization of electrode materials [37]. In conclusion, TiO2 nanotubes with li phase titanium oxides can achieve excellent Magne
electrochemical properties. Literature reported that harsh thermal treatment was required li phase titanium oxides, for example to fabricate pure Magne thermal reduction above 800 C in hydrogen gas [23,38,39]. In our work, we synthesized the substoichiometric titanium oxides by heat treatment at 450 C in N2 atmosphere. There are two possible reasons for the existence of TinO2n-1 at low temperature. One is that annealing in N2 atmosphere appears to be more favorable to promote the reduction of Ti4þ [15] and enhance the formation of TinO2n-1 than annealing in Ar. Another possible reason is that substoichiometric titanium oxides have already existed in the process of anodization [40], which can retain its structure under proper thermal treatment. With regard to the formation mechanism of TinO2n-1, further research is needed. The electrochemical performance of TiO2 samples was further investigated by galvanostatic CD measurements. Fig. 4a and b present the galvanostatic CD plots of TNTs subjected to annealing for 1 h under N2 atmosphere. The curves keep linear and symmetric at high current densities, which typically are the characteristic of double layer capacitance. The deviation of linear profiles at low current densities indicates the existence of Faradic reactions which are not observed from CV plots. Similar phenomena are also reported in other literature [9,31], which is attributed to oxygen depletion derived from thermal treatment under oxygen-free atmosphere. The specific capacitance is calculated by the following equation.
Cs ¼ ½I Dt=½S DV
(3)
Where I is discharge current, Dt discharge time, S the surface area, DV the potential window. The areal capacitance of TiO2-1 is 3.33 mF cm2 at a current density of 50 mA cm2. The specific capacitances obtained from galvanostatic CD test at various current densities are shown in Fig. 4c. The sample TiO2-1 possesses the largest specific capacitance than that of others, which is agreed with the results of CV tests.
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Fig. 4. The galvanostatic charge-discharge curves of TiO2-1 achieved from 50 mA cm2 to 200 mA cm2 (a) and from 300 mA cm2 to 800 mA cm2 (b), the average areal capacitance at different current densities (c), Nyquist plots of TiO2-1, TiO2-3 and TiO2-5 (d).
Electrochemical impedance spectroscopy (EIS) was subsequently used to understand charge transfer and ion diffusion process, evaluating the internal resistance of the electrode material and the resistance between the electrode and the electrolyte. Fig. 4d shows the Nyquist plots of TiO2-1, TiO2-3 and TiO2-5, which are carried out over a frequency range of 0.01 Hze100 kHz. The high frequency region of the spectra is shown as the inset. The semicircle characteristic is caused by charge transfer process. The absence of semicircular arc for TiO2-1 indicates excellent charge transport capability. This observation agrees well with the MottoSchottky data in the previous section. The curve in low frequency is caused by ion diffusion process. For the pure supercapacitor electrode, it exhibits almost vertical line on Nyquist plots. From the experimental data, the deviation of vertical line is ascribed to pseudocapacitance influence resulted from the substoichiometric nanostructure [31]. Excellent cycling capability is one of important factors for highperformance supercapacitor. The electrodes were measured at the current density of 50 mA cm2 for 1000 cycles. As shown in Fig. 5, the overall capacitance reduction of TNTs annealed for 1 h is about 12% after 1000 cycles, exhibiting good stability in a cyclic chargedischarge process. The first 10 and last 10 cycles are also shown as insets. Thereby, the high electrochemical capacitance performance and good cycling stability indicate that the modified TNTs are a promising electrode material. 4. Conclusion Here we systematically study the effect of heat-treatment regime on electrochemical properties of TNTs. Upon annealing in N2 atmosphere, we obtain substoichiometric titanium dioxides into TiO2 network without losing its nanotube-type structure, which increase carrier density resulted from depletion of oxygen atoms and enhance electrochemical properties. Mott-Schottky plots and li phase titanium oxides EIS tests confirm the contribution of Magne and enhancement in charge transfer within electrode material. It
Fig. 5. Cyclic performance of TNTs electrode annealed for 1 h at a current density of 50 mA cm2.
has been shown that different structures of TinO2n-1 lead to different electrochemical performance. Such a substoichiometric structure could enhance areal capacitance. The sample annealed at 450 C for 1 h has largest specific capacitance of 3.77 mF cm2 at a scan rate of 100 mV s1. The results suggest that these nanotube arrays can be used as an alternative candidate in energy storage devices. Acknowledgments The authors are very much grateful to the National Engineering Research Center for Biomaterials, Sichuan University for the
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assistance with the microscopy work. S.P. Sun thanks to Mr. Yan for his encouragement and support during the research. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.04.171. References [1] M. Assefpour-Dezfuly, C. Vlachos, E.H. Andrews, Oxide morphology and adhesive bonding on titanium surfaces, J. Mater. Sci. 19 (1984) 3626e3639. [2] Q. Zheng, H.-J. Lee, J. Lee, W. Choi, N.-B. Park, C. Lee, Electrochromic titania nanotube arrays for the enhanced photocatalytic degradation of phenol and pharmaceutical compounds, Chem. Eng. J. 249 (2014) 285e292. [3] Q.Y. Wang, S. Li, J.L. Qiao, R.C. Jin, Y.F. Yu, S.M. Gao, CdS-CdSe (CdTe) core-shell quantum dots sensitized TiO2 nanotube array solar cells, Sol. Energy Mater. Sol. Cells 132 (2015) 650e654. [4] A. Lamberti, N. Garino, A. Sacco, S. Bianco, A. Chiodoni, C. Gerbaldi, As-grown vertically aligned amorphous TiO2 nanotube arrays as high-rate Li-based micro-battery anodes with improved long-term performance, Electrochim. Acta 151 (2015) 222e229. [5] L.H. Shi, H. Xu, X.M. Liao, G.F. Yin, Y.D. Yao, Z.B. Huang, X.C. Chen, X.M. Pu, Fabrication of two-layer nanotubes with the pear-like structure by an insitu voltage up anodization and the application as a drug delivery platform, J. Alloy. Compd. 647 (2015) 590e595. [6] J.J. Liao, S.W. Lin, Y. Yang, K. Liu, W.C. Du, Highly selective and sensitive glucose sensors based on organic electrochemical transistors using TiO2 nanotube arrays-based gate electrodes, Sens. Actuators B Chem. 208 (2015) 457e463. [7] H.C. He, Y.H. Zhang, P. Xiao, Y.N. Yang, Q. Lou, F. Yang, Preparation of Ni nanoparticles-TiO2 nanotube arrays composite and its application for electrochemical capacitor, Bull. Korean Chem. Soc. 33 (2012) 1613e1616. [8] R.S. Ray, B. Sarma, A.L. Jurovitzki, M. Misra, Fabrication and characterization of titania nanotube/cobalt sulfide supercapacitor electrode in various electrolytes, Chem. Eng. J. 260 (2015) 671e683. [9] M. Salari, K. Konstantinov, H.K. Liu, Enhancement of the capacitance in TiO2 nanotubes through controlled introduction of oxygen vacancies, J. Mater. Chem. 21 (2011) 5128e5133. [10] M. Salari, S.H. Aboutalebi, A.T. Chidembo, K. Konstantinov, H.K. Liu, Surface engineering of self-assembled TiO2 nanotube arrays: a practical route towards energy storage applications, J. Alloy. Compd. 586 (2014) 197e201. [11] H. Zhou, Y.R. Zhang, Electrochemically self-doped TiO2 nanotube arrays for supercapacitors, J. Phys. Chem. C 118 (2014) 5626e5636. [12] X.H. Lu, G.M. Wang, T. Zhai, M.H. Yu, J.Y. Gan, Y.X. Tong, Y. Li, Hydrogenated TiO2 nanotube arrays for supercapacitors, Nano Lett. 12 (2012) 1690e1696. [13] C.Z. Yuan, L. Yang, L.R. Hou, L.F. Shen, X.G. Zhang, X.W. David Lou, Growth of ultrathin mesoporous Co3O4 nanosheet arrays on Ni foam for highperformance electrochemical capacitors, Energy Environ. Sci. 5 (2012) 7883e7887. [14] A. Bendavid, P.J. Martin, A. Jamting, H. Takikawa, Structural and optical properties of titanium oxide thin films deposited by filtered arc deposition, Thin Solid Films 355 (1999) 6e11. [15] P. Xiao, D.W. Liu, B.B. Garcia, S. Sepehri, Y.H. Zhang, G.Z. Cao, Electrochemical and photoelectrical properties of titania nanotube arrays annealed in different gases, Sens. Actuators B Chem. 134 (2008) 367e372. [16] F. Zuo, K. Bozhilov, R.J. Dillon, L. Wang, P. Smith, X. Zhao, C. Bardeen, P.Y. Feng, Active facets on titanium(III)-doped TiO2: an effective strategy to improve the visible-light photocatalytic activity, Angew. Chem. Int. Ed. 51 (2012) 6223e6226. [17] R. Ren, Z.H. Wen, S.M. Cui, Y. Hou, X.R. Guo, J.H. Chen, Controllable synthesis and tunable photocatalytic properties of Ti3þ-doped TiO2, Sci. Rep. 5 (2015) 10714.
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