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Preparation and capacitance properties of Mn-doped TiO2 nanotube arrays by anodisation of Ti–Mn alloy
Journal of Alloys and Compounds 658 (2016) 177e182
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation and capacitance properties of Mn-doped TiO2 nanotube arrays by anodisation of TieMn alloy Xuewen Ning, Xixin Wang, Xiaofei Yu, Jiaxin Li, Jianling Zhao* School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2015 Received in revised form 21 October 2015 Accepted 22 October 2015 Available online 27 October 2015
TieMn (Mn: 3.0, 7.0, 10.0 wt%) alloys were prepared by a power metallurgical method. Mn-doped TiO2 nanotube arrays have been achieved directly by anodising the TieMn alloys. Morphologies, crystal structure, chemical composition, supercapacitor performances of samples were characterised by scanning electron microscopy, X-ray diffraction, inductively coupled plasma optical emission spectrometry, and electrochemical analysis, respectively. The effects of manganese content, scan rate, current density, and annealing temperature on the capacitive properties were studied. Results show that doping Mn into TiO2 nanotube arrays obviously enhanced their capacitive properties. The nanotube arrays prepared from TieMn alloy (Mn: 7.0 wt%) exhibited the best capacitive performance and after annealed at 300 C, the areal capacitance reached up to 1662.5 mF/cm2 at a current density of 5.0 mA/cm2. The retention of capacitance was 89.0% when the current density changed from 5.0 to 12.5 mA/cm2 and capacitance retention was 84.6% after 2000 cycles. The factors that contributed to the superior areal capacitance of the film have also been discussed. © 2015 Elsevier B.V. All rights reserved.
Keywords: Anodisation Mn-doped TiO2 Nanotube arrays Capacitance properties
1. Introduction Titanium dioxide (TiO2) is an important function material with good chemical stability, low cost, low toxicity, natural abundance and environmentally friendly nature [1e3]. It is currently being intensively studied for various applications in environment and energy areas, such as, photocatalysts, pollutant cleansers, lithium ion batteries, supercapacitors, gas sensors, solar energy cells, and so forth [4,5]. The materials' morphology has a great influence on their performance [6]. TiO2 nanotube arrays fabricated by anodisation, with highly ordered structures and large specific surface areas, are of enhanced or new properties compared to other morphologies. For example, the TiO2 nanotube arrays showed significantly higher charge-collection efficiencies and light-harvesting efficiencies than those of the nanoparticles [7]. Especially, TiO2 nanotube arrays can be employed directly as supercapacitors because of the backcontacted nanotube layers on the substrate [4,8e13]. Modified TiO2 nanotube arrays, with better performances, have a wider
* Corresponding author. E-mail address: [email protected] (J. Zhao). http://dx.doi.org/10.1016/j.jallcom.2015.10.204 0925-8388/© 2015 Elsevier B.V. All rights reserved.
range of applications, such as dye sensitized solar cell [14], catalyst [15], etc. In order to further improve the capacitive properties, numerous research efforts concerning modification of TiO2 nanotube arrays have been made. Previously reported modification methods included annealing the TiO2 nanotube arrays in Ar [10], NH3 [16], or H2 atmosphere [17], electrochemical hydrogenation treatment [1] and combination the nanotube arrays with other oxides [18e20]. As MnO2 is one of the most promising pseudocapacitive materials with high theoretical specific capacitance and it is suitable for a pseudocapacitive electrode in hybrid system with other active electrode materials like TiO2 [21]. For example, the composites of TiO2 nanotube arrays with MnO2 exhibited a remarkable specific capacitance [22,23]. Metal ions doping is one of the effective approaches to improve the properties of TiO2 nanotube arrays [24e26]. Whereas, to the best of our knowledge, there are few reports on the capacitance properties of Mnedoped TiO2 nanotube arrays. In this research, Mnedoped TiO2 nanotube arrays were prepared by the anodisation of TieMn alloys and their morphologies, structures and capacitance were discussed in detail. In addition, as a comparison, the TiO2 nanotube arrays were also prepared and characterized.
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2. Experimental TieMn (99.9% purity, Mn: 3.0, 7.0, 10.0 wt%) alloys prepared by a power metallurgical method were cut into 20 mm 40 mm 2 mm foils, and then polished with metallographic abrasive paper, ultrasonically washed in twice-distilled water and ethanol before use. Anodisation was carried out using a programcontrolled DC source (Dahua Coop., Beijing, China). The anodisation set-up consisted of a two-electrode configuration with a TieMn alloy foil as the anodic electrode and a platinum foil (30 mm 50 mm 0.1 mm) as the cathodic electrode. The electrolyte used in this process was an ethylene glycol solution containing 0.25 wt% NH4F and water. All chemical regents were analytical grade. The anodisation experiments were conducted at 40 V for 3 h. After anodisation, the samples were rinsed in deionised water, air-dried, and then characterized. For brevity, the Mndoped TiO2 nanotube arrays prepared from TieMn alloys with 3.0 wt% Mn, 7.0 wt% Mn and 10.0 wt% Mn were denoted by TiO2eMn (3%), TiO2eMn (7%), and TiO2eMn (10%), respectively. In addition, as a comparison, the TiO2 nanotube arrays were prepared by the same method (denoted by TiO2). Morphologies, structures, and chemical composition of the samples were investigated using a scanning electron microscope (SEM, Hitachi, S-4800), an X-ray diffractometer (XRD, Rigaku, D/ MAX-2500), and an inductively coupled plasma optical emission spectrometer (ICP-OES, Leemon, Prodigy Xp), respectively. The electrochemical properties of the samples were investigated by cyclic voltammetry (CV) and galvanostatic chargeedischarge (GCD) testing using an LK2005 electrochemical analyser (Lanlika, China). The electrochemical measurements were conducted in a standard three-electrode configuration composed of a sample as the working electrode, a platinum foil as the counter-electrode, a saturated calomel electrode (SCE) as the reference electrode, and 0.5 M Na2SO4 as the electrolyte. The analysis of the capacitive performance of each sample was based on the geometric area of the working electrode [17,27]. 3. Results and discussion Previous studies showed that TiO2 and ion-doped TiO2 nanotube arrays could be prepared in ethylene glycol electrolyte [28e30]. In this work, by changing experiment conditions, the TiO2 and Mndoped TiO2 nanotube arrays with similar morphologies were successfully prepared respectively and the SEM images are shown in Figs. 1 and 2, the diameter being about 110 nm and the length about 3.5 mm. The preparation conditions of nanotube arrays were shown in Table 1, it can be seen that the preparation conditions changed along with alloy composition. When the voltage, oxidation time and NH4F concentration was constant, the water concentration and reaction temperature decreased with the increasing manganese contents. Lower water concentration and reaction temperature resulted in lower dissolution ability of the electrolyte [29e31]. The solubility of the oxides increased with the increasing manganese contents. Therefore, only by reducing the dissolution ability of the electrolyte, nanotube arrays with similar morphology could be obtained. ICP-OES analysis indicated that the mass ratios of Mn/(Mn þ Ti) in TiO2eMn (3%), TiO2eMn (7%), and TiO2eMn (10%) were 2.05%, 3.02%, and 5.44%, respectively, which were significantly lower than those in the TieMn alloys. The reason might be that manganese oxide was more easily to be dissolved in the electrolytes than that of titanium oxide. To study the effect of the manganese contents on the capacitive properties, the CV curves of TiO2 and Mn-doped TiO2 nanotube
arrays film annealed at 300 C with a scan rate of 10 mV/s were tested and the results are shown in Fig. 3. According to Fig. 3, it could be calculated that the areal capacitance values of TiO2, TiO2eMn (3%), TiO2eMn (7%) and TiO2eMn (10%) were 141.9, 389.7, 415.8, 324.1 mF/cm2, respectively. The areal capacitance of Mndoped TiO2 nanotube arrays was obviously higher than that of TiO2 nanotube arrays. With the increasing manganese contents, the areal capacitance first increased and then decreased. The areal capacitance of TiO2eMn (7%) reached up the highest value. Manganese oxides were promising pseudocapacitive materials with high theoretical specific capacitance. Doping manganese into TiO2 materials could obviously enhance their capacitive properties [32,33]. The electrical conductivity of TiO2, TiO2eMn (3%), TiO2eMn (7%) and TiO2eMn (10%), tested under same conditions, were 0.674, 0.631, 0.629 and 0.532 mS/cm, respectively. Apparently, doping manganese would introduce defects into oxide lattice. For TiO2eMn (3%) and TiO2eMn (7%), the doping content of manganese was relatively low. The small number of defects existed in Mn-doped TiO2 nanotube arrays would have little effect on electrical conductivity compared with that of TiO2. Therefore, the areal capacitance would increase with the increasing manganese content. However, for TiO2eMn (10%), the doping content of manganese was high, too much defects would lead to reduced electrical conductivity and decreased areal capacitance consequently. The annealing temperature exerted a significant influence on the capacitive performance, CV curves of the TiO2 and TiO2eMn (7%) samples annealed under different temperatures ranging from 100 to 500 C are shown in Fig. 4. According to Fig. 4, it could be calculated that the areal capacitance of TiO2 sample annealed at 100, 300, and 500 C were 93.1, 141.9 and 26.3 mF/cm2, respectively. The areal capacitance of TiO2eMn (7%) sample annealed at 100, 300, and 500 C were 257.8, 415.8 and 235.3 mF/cm2, respectively. The areal capacitance of Mn-doped TiO2 was obviously higher than that of TiO2 nanotube arrays. Both of the TiO2eMn (7%) and TiO2 nanotube arrays film obtained the highest areal capacitance when annealed at 300 C. XRD analysis of TiO2 and TiO2eMn (7%) samples annealed at 100, 300, and 500 C were conducted and the results are shown in Fig. 5. When the samples were annealed at 100 C, no diffraction peaks was detected, indicating that the oxides were amorphous. When the samples were annealed at 300 C and 500 C, the diffraction peaks of both TiO2 and TiO2eMn (7%) were indexed only to anatase TiO2 (pdf Card no. 65-5714). The intensity of the diffraction peak increased at 500 C. At 300 C, the calculated grain size of TiO2 and TiO2eMn (7%) were 19.7 nm and 20.8 nm; at 500 C, the grain size increased to 31.5 nm and 24.2 nm respectively. No characteristic peaks of manganese oxide have been detected in Mn-doped TiO2 nanotube arrays, which indicated that element manganese was doped into the crystal structure of TiO2 because of its high dispersion and lower manganese concentration. The influence of annealing temperature has two aspects. Along with the increase in annealing temperature, on the one hand, the conductivity of the oxides increased with the improved crystallinity and degree of structure order, thus the capacitive performance increased accordingly. On the other hand, the hydrophilicity would decrease with the decreased amount of surface hydroxyl groups [31], thus the capacitive performance decreased accordingly. The film annealed at 300 C would be more conducive to balancing the conductivity and hydrophilicity. Therefore, the most suitable annealing temperature was 300 C, and thus the film was annealed at 300 C for subsequent measurements. Fig. 6A shows the CV curves of TiO2eMn (7%) sample annealed
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Fig. 1. Surface SEM images of nanotube arrays: (a) TiO2; (b) TiO2eMn (3%); (c) TiO2eMn (7%); (d) TiO2eMn (10%).
Fig. 2. Cross-sectional SEM images of nanotube arrays: (a) TiO2; (b) TiO2eMn (3%); (c) TiO2eMn (7%); (d) TiO2eMn (10%).
Table 1 Preparation conditions of the nanotube arrays. Nanotube arrays
Voltage (V)
NH4F (wt%)
t (h)
H2O (wt%)
T ( C)
TiO2 TiO2eMn (3%) TiO2eMn (7%) TiO2eMn (10%)
40 40 40 40
0.25 0.25 0.25 0.25
3 3 3 3
15 10 5 5
40 40 40 30
at 300 C with different scan rates. Obviously, the chargeedischarge current increased congruously along with the increasing potential scan rate. It could be calculated that the areal capacitance of
TiO2eMn (7%) sample were 718.0, 540.5, 415.8 and 367.0 mF/cm2 at scan rates of 5.0, 7.5, 10.0, and 12.5 mV/s, respectively. The GCD measurement results are shown in Fig. 6B. The areal capacitance of TiO2eMn (7%) sample were 1662.5, 1631.3, 1525.0 and 1480.0 mF/ cm2 at a current density of 5.0, 7.5, 10.0 and 12.5 mA/cm2, respectively. In Fig. 6, the areal capacitance values of TiO2 were 187.5 mF/ cm2 at a scan rate of 12.5 mV/s and 118.6 mF/cm2 at a current density of 12.5 mA/cm2, respectively. As shown in Fig. 6A, compared the CV curves of TiO2eMn (7%) to that of TiO2, the chargeedischarge current of TiO2eMn (7%) increased obviously at the positive scan potential larger than 0.6 V
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Fig. 3. CV curves of samples annealed at 300 C with scan rate of 10 mV/s.
and at the negative scan potential smaller than 0.5 V. In Fig. 6B, the chargeedischarge curve of TiO2 presented a typical linear relationship with time, indicating that the predominant storage mechanism of TiO2 was electric double layer capacitance behaviour. In case of TiO2eMn (7%), the potential increased slowly during the charge process and exhibited a short charging plateau at about 0.6 V. Correspondingly, during the discharge process the voltage decreased slowly and exhibited a short discharging plateau at about 0.5 V. The obvious chargeedischarge plateau for TiO2eMn (7%) nanotube arrays displayed a behaviour between an electric double layer capacitance and a pseudocapacitance. Pseudocapacitance behaviour of TiO2eMn (7%) sample was attributed to reversible redox transitions involving the exchange of protons and/or cations with electrolyte, as well as the transition between different oxidation states of manganese oxide within the electrode potential window of 0e0.8 V [21,22]. In this system, the main redox reaction equation was:
MnO2 þ xNaþ þ xe #MnOONax
(1)
Besides high specific capacitance, good cycling performance is also one of the most important characteristics for highperformance supercapacitors. The long-term stability of TiO2eMn (7%) was examined by GCD cycling at a current density of 12.5 mA/ cm2 for 2000 cycles. Fig. 7 displays the capacitance retention as a function of cycle number. The capacitance dropped quickly during
Fig. 5. XRD patterns of samples (a) TiO2, 500 C; (b) TiO2eMn (7%), 500 C; (c) TiO2, 300 C; (d) TiO2eMn (7%), 300 C; (e) TiO2, 100 C; and (f) TiO2eMn (7%), 100 C.
the first 400 cycles, then the areal capacitance tended to be stable gradually. The capacitance retention was 84.6% after the 2000 cycles, displaying the excellent long-term cycling stability, which might have been ascribed to the outstanding structural stability and perfect contact between the nanotube arrays and substrate alloy. EIS measurement is commonly used with the help of frequency to know the charge transportation kinetics between electrode surface and electrolyte. The equivalent circuits used to describe the charge storage process and Nyquist plots of TiO2 and TiO2eMn (7%) were given in Fig. 8. These plots were tested in the range of frequency from 0.01 Hz to 100 kHz at open potential and an alternating current (AC) voltage amplitude of 5 mV. Noteworthy, there is no semicircle observed in samples and the experimental data agreed well with the simulation data according to the equivalent circuits. The solution series resistance (Rs) values for TiO2 (2.4 U) and TiO2eMn (7%) (2.3 U), obtained from simulation, were almost the same, which meant that the two samples had the same combination resistance of electrolyte, intrinsic resistance of active
Fig. 4. CV curves of the samples at different annealing temperatures (scan rate: 10 mV/s) (A) TiO2; (B) TiO2eMn (7%).
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Fig. 6. CV curves of samples at different scan rates (A) and GCD curves at different current densities (B).
from the contribution of Faradic pseudo-capacitance (Cf) [9,23]. The Warburg resistance (Zw) admittance coefficient (Y0) of TiO2 and TiO2eMn (7%) were 0.33 and 0.74 mS$s0.5/cm2, respectively, which indicated that the lower Warburg resistance and faster ion diffusion rates of TiO2eMn (7%) than that of TiO2. The charge transfer resistance (Rct) of TiO2eMn (7%) was 6 U, obtained from simulation, whereas for TiO2 the Rct was 11 U, which indicated that the faster electrochemical reaction rates and better conductivity of TiO2eMn (7%) than that of TiO2. 4. Conclusions
Fig. 7. Cycling performance of TiO2eMn (7%) at a current density of 12.5 mA/cm2 for 2000 cycles.
materials, and contact resistance at the active material/current collector interface [29]. For TiO2eMn (7%), Nyquist plot was more vertical than TiO2 toward the low frequency side, which resulted
In summary, Mn-doped TiO2 nanotube arrays film was successfully prepared by a simple anodisation method. Doping manganese into TiO2 nanotube arrays obviously enhanced their capacitive properties. The annealing temperature and manganese contents exerted significant influences on the capacitive performance of the nanotube arrays film. When manganese content of nanotube arrays was 3.02 wt% and annealing temperature was 300 C, the areal capacitance reached up to the highest value of 1662.5 mF/cm2 at a current density of 5.0 mA/cm2. The retention of capacitance was 89.0% when the current density changed from 5.0 to 12.5 mA/cm2. Moreover, the capacitance retention was 84.6% after 2000 cycles. The enhanced electrochemical properties of the film could be mainly ascribed to the appropriate oxide composition and structure, which provided a larger effective contact area, more active sites, higher conductivity and hydrophilicity.
Fig. 8. Impedance Nyquist plot of samples and the fit results using a ZVIEW electrochemical software.
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Acknowledgements This work is supported by National Natural Science Foundation of China (51272064), Natural Science Foundation of Hebei Province of China (E2013202032), the Talent Training Project of Hebei Province (2013) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13060). References [1] H. Wu, D. Li, X. Zhu, C. Yang, D. Liu, X. Chen, Y. Song, L. Lu, High-performance and renewable supercapacitors based on TiO2 nanotube array electrodes treated by an electrochemical doping approach, Electrochim. Acta 116 (2014) 129e136. [2] S. Liu, Z. Wang, C. Yu, H.B. Wu, G. Wang, Q. Dong, J. Qiu, A. Eychmüller, X.W. David Lou, A flexible TiO2(B)-based battery electrode with superior power rate and ultralong cycle life, Adv. Mater. 25 (2013) 3462e3467. [3] P. Roy, S. Berger, P. Schmuki, TiO2 nanotubes: synthesis and applications, Angew. Chem. Int. 50 (2011) 2904e2939. [4] H. Li, Z. Chen, C.K. Tsang, Z. Li, X. Ran, C. Lee, B. Nie, L. Zheng, T. Hung, J. Lu, B. Pan, Y.Y. Li, Electrochemical doping of anatase TiO2 in organic electrolytes for high-performance supercapacitors and photocatalysts, J. Mater. Chem. A 2 (2014) 229e236. [5] M.Z. Lin, H. Chen, W.F. Chen, A. Nakaruk, P. Koshy, C.C. Sorrell, Effect of singlecation doping and codoping with Mn and Fe on the photocatalytic performance of TiO2 thin films, Int. J. Hydrogen Energy 39 (2014) 21500e21511. [6] Y. Yang, G. Ruan, C. Xiang, G. Wang, J.M. Tour, Flexible three-dimensional nanoporous metal-based energy devices, J. Am. Chem. Soc. 136 (2014) 6187e6190. [7] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays, Nano Lett. 7 (2007) 69e74. [8] M. Salari, S.H. Aboutalebi, K. Konstantinov, H.K. Liu, A highly ordered titania nanotube array as a supercapacitor electrode, Phys. Chem. Chem. Phys. 13 (2011) 5038. [9] A. Al-Osta, V.V. Jadhav, N.A. Saad, R.S. Mane, M. Naushad, K.N. Hui, S.-H. Han, Diameter-dependent electrochemical supercapacitive properties of anodized titanium oxide nanotubes, Scr. Mater. 104 (2015) 60e63. [10] 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) 5128. [11] M. Zhou, A.M. Glushenkov, O. Kartachova, Y. Li, Y. Chen, Titanium dioxide nanotube films for electrochemical supercapacitors: biocompatibility and operation in an electrolyte based on a physiological fluid, J. Electrochem. Soc. 162 (2015) A5065eA5069. [12] Y.B. Xie, Preparation and capacitance properties of titania nanotube arrays, Adv. Mater. Res. 148 (2011) 912e915. [13] X. Xia, J. Luo, Z. Zeng, C. Guan, Y. Zhang, J. Tu, H. Zhang, H.J. Fan, Integrated photoelectrochemical energy storage: solar hydrogen generation and supercapacitor, Sci. Rep. U. K. 2 (2012). [14] H. Wang, H. Li, J. Wang, J. Wu, D. Li, M. Liu, P. Su, Nitrogen-doped TiO2 nanoparticles better TiO2 nanotube array photo-anodes for dye sensitized solar cells, Electrochim. Acta 137 (2014) 744e750.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation and capacitance properties of Mn-doped TiO2 nanotube arrays by anodisation of TieMn alloy Xuewen Ning, Xixin Wang, Xiaofei Yu, Jiaxin Li, Jianling Zhao* School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2015 Received in revised form 21 October 2015 Accepted 22 October 2015 Available online 27 October 2015
TieMn (Mn: 3.0, 7.0, 10.0 wt%) alloys were prepared by a power metallurgical method. Mn-doped TiO2 nanotube arrays have been achieved directly by anodising the TieMn alloys. Morphologies, crystal structure, chemical composition, supercapacitor performances of samples were characterised by scanning electron microscopy, X-ray diffraction, inductively coupled plasma optical emission spectrometry, and electrochemical analysis, respectively. The effects of manganese content, scan rate, current density, and annealing temperature on the capacitive properties were studied. Results show that doping Mn into TiO2 nanotube arrays obviously enhanced their capacitive properties. The nanotube arrays prepared from TieMn alloy (Mn: 7.0 wt%) exhibited the best capacitive performance and after annealed at 300 C, the areal capacitance reached up to 1662.5 mF/cm2 at a current density of 5.0 mA/cm2. The retention of capacitance was 89.0% when the current density changed from 5.0 to 12.5 mA/cm2 and capacitance retention was 84.6% after 2000 cycles. The factors that contributed to the superior areal capacitance of the film have also been discussed. © 2015 Elsevier B.V. All rights reserved.
Keywords: Anodisation Mn-doped TiO2 Nanotube arrays Capacitance properties
1. Introduction Titanium dioxide (TiO2) is an important function material with good chemical stability, low cost, low toxicity, natural abundance and environmentally friendly nature [1e3]. It is currently being intensively studied for various applications in environment and energy areas, such as, photocatalysts, pollutant cleansers, lithium ion batteries, supercapacitors, gas sensors, solar energy cells, and so forth [4,5]. The materials' morphology has a great influence on their performance [6]. TiO2 nanotube arrays fabricated by anodisation, with highly ordered structures and large specific surface areas, are of enhanced or new properties compared to other morphologies. For example, the TiO2 nanotube arrays showed significantly higher charge-collection efficiencies and light-harvesting efficiencies than those of the nanoparticles [7]. Especially, TiO2 nanotube arrays can be employed directly as supercapacitors because of the backcontacted nanotube layers on the substrate [4,8e13]. Modified TiO2 nanotube arrays, with better performances, have a wider
* Corresponding author. E-mail address: [email protected] (J. Zhao). http://dx.doi.org/10.1016/j.jallcom.2015.10.204 0925-8388/© 2015 Elsevier B.V. All rights reserved.
range of applications, such as dye sensitized solar cell [14], catalyst [15], etc. In order to further improve the capacitive properties, numerous research efforts concerning modification of TiO2 nanotube arrays have been made. Previously reported modification methods included annealing the TiO2 nanotube arrays in Ar [10], NH3 [16], or H2 atmosphere [17], electrochemical hydrogenation treatment [1] and combination the nanotube arrays with other oxides [18e20]. As MnO2 is one of the most promising pseudocapacitive materials with high theoretical specific capacitance and it is suitable for a pseudocapacitive electrode in hybrid system with other active electrode materials like TiO2 [21]. For example, the composites of TiO2 nanotube arrays with MnO2 exhibited a remarkable specific capacitance [22,23]. Metal ions doping is one of the effective approaches to improve the properties of TiO2 nanotube arrays [24e26]. Whereas, to the best of our knowledge, there are few reports on the capacitance properties of Mnedoped TiO2 nanotube arrays. In this research, Mnedoped TiO2 nanotube arrays were prepared by the anodisation of TieMn alloys and their morphologies, structures and capacitance were discussed in detail. In addition, as a comparison, the TiO2 nanotube arrays were also prepared and characterized.
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2. Experimental TieMn (99.9% purity, Mn: 3.0, 7.0, 10.0 wt%) alloys prepared by a power metallurgical method were cut into 20 mm 40 mm 2 mm foils, and then polished with metallographic abrasive paper, ultrasonically washed in twice-distilled water and ethanol before use. Anodisation was carried out using a programcontrolled DC source (Dahua Coop., Beijing, China). The anodisation set-up consisted of a two-electrode configuration with a TieMn alloy foil as the anodic electrode and a platinum foil (30 mm 50 mm 0.1 mm) as the cathodic electrode. The electrolyte used in this process was an ethylene glycol solution containing 0.25 wt% NH4F and water. All chemical regents were analytical grade. The anodisation experiments were conducted at 40 V for 3 h. After anodisation, the samples were rinsed in deionised water, air-dried, and then characterized. For brevity, the Mndoped TiO2 nanotube arrays prepared from TieMn alloys with 3.0 wt% Mn, 7.0 wt% Mn and 10.0 wt% Mn were denoted by TiO2eMn (3%), TiO2eMn (7%), and TiO2eMn (10%), respectively. In addition, as a comparison, the TiO2 nanotube arrays were prepared by the same method (denoted by TiO2). Morphologies, structures, and chemical composition of the samples were investigated using a scanning electron microscope (SEM, Hitachi, S-4800), an X-ray diffractometer (XRD, Rigaku, D/ MAX-2500), and an inductively coupled plasma optical emission spectrometer (ICP-OES, Leemon, Prodigy Xp), respectively. The electrochemical properties of the samples were investigated by cyclic voltammetry (CV) and galvanostatic chargeedischarge (GCD) testing using an LK2005 electrochemical analyser (Lanlika, China). The electrochemical measurements were conducted in a standard three-electrode configuration composed of a sample as the working electrode, a platinum foil as the counter-electrode, a saturated calomel electrode (SCE) as the reference electrode, and 0.5 M Na2SO4 as the electrolyte. The analysis of the capacitive performance of each sample was based on the geometric area of the working electrode [17,27]. 3. Results and discussion Previous studies showed that TiO2 and ion-doped TiO2 nanotube arrays could be prepared in ethylene glycol electrolyte [28e30]. In this work, by changing experiment conditions, the TiO2 and Mndoped TiO2 nanotube arrays with similar morphologies were successfully prepared respectively and the SEM images are shown in Figs. 1 and 2, the diameter being about 110 nm and the length about 3.5 mm. The preparation conditions of nanotube arrays were shown in Table 1, it can be seen that the preparation conditions changed along with alloy composition. When the voltage, oxidation time and NH4F concentration was constant, the water concentration and reaction temperature decreased with the increasing manganese contents. Lower water concentration and reaction temperature resulted in lower dissolution ability of the electrolyte [29e31]. The solubility of the oxides increased with the increasing manganese contents. Therefore, only by reducing the dissolution ability of the electrolyte, nanotube arrays with similar morphology could be obtained. ICP-OES analysis indicated that the mass ratios of Mn/(Mn þ Ti) in TiO2eMn (3%), TiO2eMn (7%), and TiO2eMn (10%) were 2.05%, 3.02%, and 5.44%, respectively, which were significantly lower than those in the TieMn alloys. The reason might be that manganese oxide was more easily to be dissolved in the electrolytes than that of titanium oxide. To study the effect of the manganese contents on the capacitive properties, the CV curves of TiO2 and Mn-doped TiO2 nanotube
arrays film annealed at 300 C with a scan rate of 10 mV/s were tested and the results are shown in Fig. 3. According to Fig. 3, it could be calculated that the areal capacitance values of TiO2, TiO2eMn (3%), TiO2eMn (7%) and TiO2eMn (10%) were 141.9, 389.7, 415.8, 324.1 mF/cm2, respectively. The areal capacitance of Mndoped TiO2 nanotube arrays was obviously higher than that of TiO2 nanotube arrays. With the increasing manganese contents, the areal capacitance first increased and then decreased. The areal capacitance of TiO2eMn (7%) reached up the highest value. Manganese oxides were promising pseudocapacitive materials with high theoretical specific capacitance. Doping manganese into TiO2 materials could obviously enhance their capacitive properties [32,33]. The electrical conductivity of TiO2, TiO2eMn (3%), TiO2eMn (7%) and TiO2eMn (10%), tested under same conditions, were 0.674, 0.631, 0.629 and 0.532 mS/cm, respectively. Apparently, doping manganese would introduce defects into oxide lattice. For TiO2eMn (3%) and TiO2eMn (7%), the doping content of manganese was relatively low. The small number of defects existed in Mn-doped TiO2 nanotube arrays would have little effect on electrical conductivity compared with that of TiO2. Therefore, the areal capacitance would increase with the increasing manganese content. However, for TiO2eMn (10%), the doping content of manganese was high, too much defects would lead to reduced electrical conductivity and decreased areal capacitance consequently. The annealing temperature exerted a significant influence on the capacitive performance, CV curves of the TiO2 and TiO2eMn (7%) samples annealed under different temperatures ranging from 100 to 500 C are shown in Fig. 4. According to Fig. 4, it could be calculated that the areal capacitance of TiO2 sample annealed at 100, 300, and 500 C were 93.1, 141.9 and 26.3 mF/cm2, respectively. The areal capacitance of TiO2eMn (7%) sample annealed at 100, 300, and 500 C were 257.8, 415.8 and 235.3 mF/cm2, respectively. The areal capacitance of Mn-doped TiO2 was obviously higher than that of TiO2 nanotube arrays. Both of the TiO2eMn (7%) and TiO2 nanotube arrays film obtained the highest areal capacitance when annealed at 300 C. XRD analysis of TiO2 and TiO2eMn (7%) samples annealed at 100, 300, and 500 C were conducted and the results are shown in Fig. 5. When the samples were annealed at 100 C, no diffraction peaks was detected, indicating that the oxides were amorphous. When the samples were annealed at 300 C and 500 C, the diffraction peaks of both TiO2 and TiO2eMn (7%) were indexed only to anatase TiO2 (pdf Card no. 65-5714). The intensity of the diffraction peak increased at 500 C. At 300 C, the calculated grain size of TiO2 and TiO2eMn (7%) were 19.7 nm and 20.8 nm; at 500 C, the grain size increased to 31.5 nm and 24.2 nm respectively. No characteristic peaks of manganese oxide have been detected in Mn-doped TiO2 nanotube arrays, which indicated that element manganese was doped into the crystal structure of TiO2 because of its high dispersion and lower manganese concentration. The influence of annealing temperature has two aspects. Along with the increase in annealing temperature, on the one hand, the conductivity of the oxides increased with the improved crystallinity and degree of structure order, thus the capacitive performance increased accordingly. On the other hand, the hydrophilicity would decrease with the decreased amount of surface hydroxyl groups [31], thus the capacitive performance decreased accordingly. The film annealed at 300 C would be more conducive to balancing the conductivity and hydrophilicity. Therefore, the most suitable annealing temperature was 300 C, and thus the film was annealed at 300 C for subsequent measurements. Fig. 6A shows the CV curves of TiO2eMn (7%) sample annealed
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Fig. 1. Surface SEM images of nanotube arrays: (a) TiO2; (b) TiO2eMn (3%); (c) TiO2eMn (7%); (d) TiO2eMn (10%).
Fig. 2. Cross-sectional SEM images of nanotube arrays: (a) TiO2; (b) TiO2eMn (3%); (c) TiO2eMn (7%); (d) TiO2eMn (10%).
Table 1 Preparation conditions of the nanotube arrays. Nanotube arrays
Voltage (V)
NH4F (wt%)
t (h)
H2O (wt%)
T ( C)
TiO2 TiO2eMn (3%) TiO2eMn (7%) TiO2eMn (10%)
40 40 40 40
0.25 0.25 0.25 0.25
3 3 3 3
15 10 5 5
40 40 40 30
at 300 C with different scan rates. Obviously, the chargeedischarge current increased congruously along with the increasing potential scan rate. It could be calculated that the areal capacitance of
TiO2eMn (7%) sample were 718.0, 540.5, 415.8 and 367.0 mF/cm2 at scan rates of 5.0, 7.5, 10.0, and 12.5 mV/s, respectively. The GCD measurement results are shown in Fig. 6B. The areal capacitance of TiO2eMn (7%) sample were 1662.5, 1631.3, 1525.0 and 1480.0 mF/ cm2 at a current density of 5.0, 7.5, 10.0 and 12.5 mA/cm2, respectively. In Fig. 6, the areal capacitance values of TiO2 were 187.5 mF/ cm2 at a scan rate of 12.5 mV/s and 118.6 mF/cm2 at a current density of 12.5 mA/cm2, respectively. As shown in Fig. 6A, compared the CV curves of TiO2eMn (7%) to that of TiO2, the chargeedischarge current of TiO2eMn (7%) increased obviously at the positive scan potential larger than 0.6 V
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Fig. 3. CV curves of samples annealed at 300 C with scan rate of 10 mV/s.
and at the negative scan potential smaller than 0.5 V. In Fig. 6B, the chargeedischarge curve of TiO2 presented a typical linear relationship with time, indicating that the predominant storage mechanism of TiO2 was electric double layer capacitance behaviour. In case of TiO2eMn (7%), the potential increased slowly during the charge process and exhibited a short charging plateau at about 0.6 V. Correspondingly, during the discharge process the voltage decreased slowly and exhibited a short discharging plateau at about 0.5 V. The obvious chargeedischarge plateau for TiO2eMn (7%) nanotube arrays displayed a behaviour between an electric double layer capacitance and a pseudocapacitance. Pseudocapacitance behaviour of TiO2eMn (7%) sample was attributed to reversible redox transitions involving the exchange of protons and/or cations with electrolyte, as well as the transition between different oxidation states of manganese oxide within the electrode potential window of 0e0.8 V [21,22]. In this system, the main redox reaction equation was:
MnO2 þ xNaþ þ xe #MnOONax
(1)
Besides high specific capacitance, good cycling performance is also one of the most important characteristics for highperformance supercapacitors. The long-term stability of TiO2eMn (7%) was examined by GCD cycling at a current density of 12.5 mA/ cm2 for 2000 cycles. Fig. 7 displays the capacitance retention as a function of cycle number. The capacitance dropped quickly during
Fig. 5. XRD patterns of samples (a) TiO2, 500 C; (b) TiO2eMn (7%), 500 C; (c) TiO2, 300 C; (d) TiO2eMn (7%), 300 C; (e) TiO2, 100 C; and (f) TiO2eMn (7%), 100 C.
the first 400 cycles, then the areal capacitance tended to be stable gradually. The capacitance retention was 84.6% after the 2000 cycles, displaying the excellent long-term cycling stability, which might have been ascribed to the outstanding structural stability and perfect contact between the nanotube arrays and substrate alloy. EIS measurement is commonly used with the help of frequency to know the charge transportation kinetics between electrode surface and electrolyte. The equivalent circuits used to describe the charge storage process and Nyquist plots of TiO2 and TiO2eMn (7%) were given in Fig. 8. These plots were tested in the range of frequency from 0.01 Hz to 100 kHz at open potential and an alternating current (AC) voltage amplitude of 5 mV. Noteworthy, there is no semicircle observed in samples and the experimental data agreed well with the simulation data according to the equivalent circuits. The solution series resistance (Rs) values for TiO2 (2.4 U) and TiO2eMn (7%) (2.3 U), obtained from simulation, were almost the same, which meant that the two samples had the same combination resistance of electrolyte, intrinsic resistance of active
Fig. 4. CV curves of the samples at different annealing temperatures (scan rate: 10 mV/s) (A) TiO2; (B) TiO2eMn (7%).
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Fig. 6. CV curves of samples at different scan rates (A) and GCD curves at different current densities (B).
from the contribution of Faradic pseudo-capacitance (Cf) [9,23]. The Warburg resistance (Zw) admittance coefficient (Y0) of TiO2 and TiO2eMn (7%) were 0.33 and 0.74 mS$s0.5/cm2, respectively, which indicated that the lower Warburg resistance and faster ion diffusion rates of TiO2eMn (7%) than that of TiO2. The charge transfer resistance (Rct) of TiO2eMn (7%) was 6 U, obtained from simulation, whereas for TiO2 the Rct was 11 U, which indicated that the faster electrochemical reaction rates and better conductivity of TiO2eMn (7%) than that of TiO2. 4. Conclusions
Fig. 7. Cycling performance of TiO2eMn (7%) at a current density of 12.5 mA/cm2 for 2000 cycles.
materials, and contact resistance at the active material/current collector interface [29]. For TiO2eMn (7%), Nyquist plot was more vertical than TiO2 toward the low frequency side, which resulted
In summary, Mn-doped TiO2 nanotube arrays film was successfully prepared by a simple anodisation method. Doping manganese into TiO2 nanotube arrays obviously enhanced their capacitive properties. The annealing temperature and manganese contents exerted significant influences on the capacitive performance of the nanotube arrays film. When manganese content of nanotube arrays was 3.02 wt% and annealing temperature was 300 C, the areal capacitance reached up to the highest value of 1662.5 mF/cm2 at a current density of 5.0 mA/cm2. The retention of capacitance was 89.0% when the current density changed from 5.0 to 12.5 mA/cm2. Moreover, the capacitance retention was 84.6% after 2000 cycles. The enhanced electrochemical properties of the film could be mainly ascribed to the appropriate oxide composition and structure, which provided a larger effective contact area, more active sites, higher conductivity and hydrophilicity.
Fig. 8. Impedance Nyquist plot of samples and the fit results using a ZVIEW electrochemical software.
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