Electrochemically Self-Doped TiO2 Nanotube Arrays for Supercapacitors

Mar 3, 2014 - capacitors and batteries/fuel cells.1−3 The supercapacitors have been ...... A.; Salvador, P. Decoupling of Transport, Charge Storage,...
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Electrochemically Self-Doped TiO2 Nanotube Arrays for Supercapacitors He Zhou and Yanrong Zhang* Environmental Science Research Institute, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China

J. Phys. Chem. C 2014.118:5626-5636. Downloaded from pubs.acs.org by LINKOPING UNIV on 01/07/19. For personal use only.

S Supporting Information *

ABSTRACT: The application of highly ordered TiO2 nanotube arrays (NTAs) for energy storage devices such as supercapacitors has been attractive and of great interest owing to their large surface area and greatly improved charge-transfer pathways compared to those of nonoriented structures. Modification of the semiconductor nature of TiO2 is important for its application in constructing high-performance supercapacitors. Hence, the present study demonstrates a novel method involving fabrication of self-doped TiO2 NTAs by a simple cathodic polarization treatment on the pristine TiO2 NTAs to achieve improved conductivity and capacitive properties of TiO2. The self-doped TiO2 NTAs at −1.4 V (vs SCE) exhibited 5 orders of magnitude improvement on carrier density and 39 times enhancement in capacitance compared to those of the pristine TiO2 NTAs. Impedance analysis based on a proposed simplified transmission line model proved that the enhanced capacitive behavior of the self-doped TiO2 NTAs was due to a decrease of charge-transport resistance through the solid material. Moreover, the MnO2 species was introduced onto the TiO2 NTAs by an impregnation−electrodeposition method, and the optimal specific capacitance achieved (1232 F g−1) clearly confirmed the suitability of self-doped TiO2 NTAs as effective current collector materials for supercapacitors.

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real specific capacitance normally less than 500 F g−110,11 because of a relatively low electrical conductivity of 10−5−10−6 S cm−1.12 To overcome this issue, incorporating nanosized pseudocapacitive materials into electrically conductive frameworks (acting as current collectors), such as carbon nanotubes,13 carbon nanofibers,14 graphene,15 TiN nanotubes,16 ZnO nanorods,17 and Zn2SnO4 nanorods18 has been proven to be a wise and promising approach. Hence, the design of nanostructured current collectors with high surface area and enhanced conductivity is strongly required for supercapacitors. Highly ordered TiO2 nanotube arrays (NTAs) that are being widely used in the field of photocatalysis have attracted much attention in recent years because of their high regulation, large surface area, excellent controllability, stability, and simple fabrication method.19−21 Thus, using TiO2 NTAs as an alternative to current collector materials in supercapacitors has become an interesting area to be explored.22−25 However, the semiconductor nature of TiO2 often leads to low electrochemical activity and poor conductivity thereby restricting its applications in the construction of highperformance supercapacitors. It has been reported that the semiconducting behavior of TiO2 NTAs can be significantly altered by a thermal treatment that involves an introduction of oxygen vacancies (Ti3+ sites) or surface disorders to achieve improved capacitive behavior.24,26,27 Because the introduction

INTRODUCTION Supercapacitors, which generally represent a class of energy storage devices, are of great interest nowadays owing to their high power density, long life cycle, and bridging function for the power and energy gap between traditional dielectric capacitors and batteries/fuel cells.1−3 The supercapacitors have been classified as electrical double-layer capacitors (EDLCs) and pseudocapacitors based on factors such as charge storage mechanism and active materials used.4 In the case of EDLCs, which are normally based on electrostatic charge diffusion and accumulation at the electrode−electrolyte interface, carbon-based active materials are being used as electrodes because of their high surface area, electrical conductivity, and extraordinary chemical stability.5−8 However, the limited charge accumulation in the electrical double layer restricts the specific capacitance of EDLCs to a range of relatively small values (less than 250 F g−1).5−8 Transitionmetal oxides and electrically conducting polymers are used as electrode materials in pseudocapacitors, where fast and reversible surface or near-surface reactions for charge storage are predominant.4,9 Because of the higher specific capacitance compared to EDLCs using double-layer charge storage, pseudocapacitors have the potential to meet the higher requirements of future electrical energy storage systems. However, poor electrical conductivity and irreversible faradaic reaction behavior of the electrode material would lead to gradual loss in pseudocapacitance behavior. For example, though MnO2 is a promising pseudocapacitive material with high theoretical specific capacitance of 1400 F g−1, it exhibits a © 2014 American Chemical Society

Received: August 19, 2013 Revised: March 1, 2014 Published: March 3, 2014 5626

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Figure 1. (a) Schematic diagram showing the fabrication of electrochemically self-doped TiO2 NTAs where “E” is the potential used in the polarization process. With respect to the cathodic potential applied, two procedures can be distinguished: ① completely reversible reduction and oxidation of the TiO2 NTAs; ② some Ti3+ remains upon the postoxidation, leading to the self-doped TiO2 NTAs. (b) CV curve of the TiO2 NTAs in 0.5 M Na2SO4 at a scan rate of 100 mV s−1; the inset shows an enlarged view of the region contained within the dashed lined rectangle. (c) FE-SEM image of the −1.4 V TiO2 sample with an inset showing a side view.

workstation (CS310, CorrTest, China) at room temperature using the pristine TiO2 NTAs, saturated calomel electrode (SCE), and Pt mesh as working, reference, and counter electrodes, respectively. The supporting electrolyte used was 0.5 M Na2SO4 aqueous solution. The potentiostatic cathodic reduction was performed at different potentials of −1.2, −1.4, −1.6, and −1.8 V for the polarization period of 1, 5, 10, 15, and 20 min, respectively. The samples were finally dried in air at 80 °C for 30 min. The self-doped TiO2 NTAs prepared by cathodic polarization for 10 min at potentials of −1.2, −1.4, −1.6, and −1.8 V are denoted as −1.2 V TiO2, −1.4 V TiO2, −1.6 V TiO2, and −1.8 V TiO2, respectively. Preparation of MnO2/TiO2 Composite. The uniform deposition of MnO2 along the surface of the −1.4 V TiO2 was achieved using an impregnation−electrodepositon method. Initially, the −1.4 V TiO2 was soaked in 0.1 M manganese acetate aqueous solution for 12 h to accumulate Mn2+ in the inner side of the nanotubes. After receiving a mild washing with deionized water, the electrode was anodized at 0.8 V (vs SCE) in 0.5 M Na2SO 4 aqueous electrolyte solution under potentiostatic mode for a period of 2 min. After the electrodeposition process, the electrode sample was thoroughly washed with deionized water and dried at 80 °C. Because MnO2 was hardly formed on the pristine TiO2 electrode under potentiostatic mode at 0.8 V, galvanostatic mode was employed to deposit MnO2 on the pristine TiO2 and self-doped TiO2 for further comparative studies (details provided as Supporting Information). Material Characterization. The morphology and microstructure of the synthesized products were characterized by field emission scanning electron microscopy (FE-SEM; NANOSEM 450, FEI), transmission electron microscopy

of oxygen vacancies into the lattice of TiO2 was possible by an annealing process under oxygen-deficient atmosphere24,26,28 or by using a reductant,29 it was believed that self-doping of TiO2 NTAs (Ti3+ doped) by electrochemical reduction could be a wise approach to overcome the limitations. Hence, the present work is focused on fabricating self-doped TiO2 NTAs via a facile electrochemical method to meet the requirements of using it as an effective current collector in supercapacitor devices.

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EXPERIMENTAL METHODS

Preparation of TiO2 NTAs. Titanium foil (99.7% purity, 0.1 mm thickness) used in anodization experiments was thoroughly cleaned by sequential ultrasonication in acetone and ethanol followed by a chemical polishing using a solution containing HF/HNO3/H2O with a volume ratio of 1:3:6 for a period of 1 min. It was then rinsed with deionized water and dried in a nitrogen stream. The anodization process was performed in a two-electrode system consisting of Ti foil and platium mesh electrode materials as anode and cathode, respectively, using ethylene glycol containing 0.5 wt % NH4F and 10 wt % H2O as electrolyte at a constant potential of 20 V for 2 h. The distance between the electrodes was fixed at 2.5 cm, and the electrolyte temperature was constantly maintained as 25 °C using a thermostat. The as-formed samples were thoroughly washed with deionized water, dried at 100 °C, and calcined at 450 °C at ambient atmosphere for 2 h. The asprepared TiO2 NTAs are denoted as pristine TiO2. Preparation of Electrochemically Self-Doped TiO2 NTAs. Electrochemical reduction technique was adopted for fabricating self-doped TiO2 NTAs. The experiment was carried out in a three-electrode system with an electrochemical 5627

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(TEM; Tecnai G20 U-Twin) and N2 adsorption at −196 °C with an ASAP 2020 apparatus (Micromeritics Instrument Ltd. Corp.). The phase and elemental composition of the samples were investigated using X-ray diffraction technique (XRD; PANalytical PW3040/60) with Cu Kα radiation (λ = 1.54056 Å), X-ray photoelectron spectroscopy (XPS; VG Multilab 2000) with 300 W Al Kα X-ray (hν = 1486.6 eV) at 15 KV, and Raman spectroscopy performed on a Laser Micro-Raman Spectrometer (HORIBA Jobin Yvon LabRAM) at room temperature with Ar+ laser of 514.5 nm excitation. The specific surface area of the samples was calculated from the Brunauer− Emmett−Teller (BET) analysis and converted to real surface area with respect to geometrical area (details provided as Supporting Information). In the XPS spectral studies, the binding energy was calibrated keeping the C 1s photoelectron peak as a reference. The electrochemical response of the prepared samples was investigated by cyclic voltammetry (CV), galvanostatic charge− discharge tests, and electrochemical impedance spectroscopic (EIS) studies in a conventional three-electrode cell employing an electrochemical workstation (CS310, CorrTest, China). Mott−Schottky plots were measured at a frequency of 1 Hz. EIS measurements were performed between 100 kHz and 0.01 Hz with a 5 mV rms sinusoidal modulation under three different applied DC potentials of −0.1, 0.4, and 0.9 V (vs SCE). The stability of the samples (pristine TiO2, −1.2 V TiO2, −1.4 V TiO2, −1.6 V TiO2, and −1.8 V TiO2) was investigated by CV measurement performed up to 2000 cycles at a scan rate of 100 mV s−1. The electrochemical studies described above were carried out in a 0.5 M Na2SO4 aqueous solution at room temperature.

would be reflected as a cathodic capacitance peak followed by an increase of the negative current density in the voltammetry pattern under the following circumstances.33,38 (1) The traps are energetically localized.33 In this case, the traps will capture free electrons when they pass through and will not release them at moderate potential level. As a result of trap filling, a cathodic capacitance peak would be formed at around the flatband potential (Vfb) becuase of a depletion of conduction band electrons in TiO2 nanomaterials at applied potentials positive of Vfb, after which the negative current density gets increased by the enhancement of conductivity. (2) The processes occuring at the semiconductor−solution interface are controlled by trapping−detrapping kinetics.38 The main reactions that take place in the system are trapping of electrons from the conduction band and detrapping, charge transfer of electrons at the semiconductor−solution interface from the traps and conduction band. If charge transfer from the traps is slow enough compared to the velocity of trap charging, electrons will accumulate in trap sites when the voltammetry scan is carried out in a cathodic direction; thus, a cathodic capacitance peak induced by filling of the traps could be observed, followed either by the faradic current from the conduction band or by the capacitive current. Interestingly, depending on the rate of detrapping with respect to the velocity of trap charging, a symmetric (or quasi-symmetric) anodic capacitance peak could be formed in the anodic scan or not. It seems that case (1) can also be explained by this trapping−detrapping kinetics assuming that both the detrapping and charge transfer from the traps are sufficiently slow with respect to the trap charging velocity. In both cases, the cathodic capacitance peak assigned as filling of the trap should appear at around Vfb potential because traps capture free electrons from the conduction band. Therefore, it can be understood that the peak at ca. +0.1 V followed by an increase of negative current at ca. −0.2 V as shown in Figure 1b is due to the filling of trap sites because the measurement of Vfb provides a value of +0.17 V for the pristine TiO2 NTAs (detailed below), and the peak appearing at ca. −0.9 V could be attributed to the reduction of Ti4+ to Ti3+ along with the proton intercalation because the intercalation of protons or other small cations in nanostructured TiO2 takes place at potentials significantly negative of Vfb.33,39 Typically, up to ca. 1% of the Ti4+ in a TiO2 layer could be reduced to Ti3+ using the electrochemical reduction approach.30,32,40 The high density of Ti3+ dopant states leads to an almost metallic behavior of the material and an explicit color change from very light gray to brown or black, which determines the maximum utilization of the electrochemical reduction process only for filling of the TiO2 NTAs and electrochromic switching.30,31,41 It is noteworthy that the Ti3+ species are extremely sensitive to undergoing oxidation, and the electrochemical reduction reaction was observed to be reversible for the reported applications (cathodic polarization was applied for a brief period of several seconds).30−32,41 In these studies, the oxidation of Ti3+ to Ti4+ occurs as the potential changes into less negative values, and the entire Ti3+ can be converted at a potential positive of approximately −0.5 V (vs Ag/AgCl). However, in our previous study,42 the reduction of TiO2 NTAs was observed not to be completely reversible under a condition of enough cathodic polarization overpotential and process duration. As illustrated in the scheme (Figure 1a), two different procedures were seen with respect to the cathodic potential applied. At both ranges of applied potential, reduction of the Ti4+ to Ti3+ along with proton

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RESULTS AND DISCUSSION The self-doped TiO2 NTAs were fabricated by a two-step approach, as illustrated in Figure 1a. Initially, the fabrication of TiO2 NTAs was accomplished by anodization of Ti foil at potentiostatic mode in an electrolyte containing F− ions followed by a thermal treatment at 450 °C under ambient atmospheric condition to attain anatase crystal. The as-prepared arrays underwent a cathodic polarization process at suitable negative potential using inert aqueous electrolyte to achieve self-doped TiO2 NTAs. Figure 1b shows a cyclic voltammetry curve for the TiO2 NTAs recorded in 0.5 M Na2SO4 aqueous solution. The apparent peak at ca. −0.9 V (vs SCE) actually results from the reduction of Ti4+ to Ti3+ accompanied by charge compensation via proton (or other small cations such as Li+, Na+, and K+, if present) intercalation (Ti4+ + e− + H+ → Ti3+H+), and the further increase at more negative voltage of ca. −1.2 V is due to hydrogen evolution (2H+ + 2e− → H2).30−32 It should be noted that, besides the proton injection theory, the trap theory describes the charging−discharging of TiO2 in terms of a trapping−detrapping process in TiO2 bulk or surface.33−36 According to the trap theory, changing the electrochemical potential of electron reservoirs will cause not only an accommodation of the field by excess charges but also a change of Fermi level position of the electrons with respect to the conduction band, with associated variation of both free electron density in the conduction band and localized electron density in trap states.34 Thus, the charging−discharging behavior of TiO2 was depicted as a trapping−detrapping process of electrons by trap sites and conduction band and was found to be dominated by the former.34,37 According to this theory, the general existence of traps (such as surface states), 5628

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Figure 2. (a) XRD spectra, (b) Ti 2p XPS spectra, (c) Raman spectra, and (d) Mott−Schottky plots of the samples with the inset showing the plots of the −1.2 V TiO2, −1.4 V TiO2, −1.6 V TiO2, and −1.8 V TiO2 samples separately. The red solid line in (d) depicts the linear fit to the Mott− Schottky curve for the pristine TiO2.

−1.8 V TiO2. It confirms the growth of a thinner TiO2 film on the surface of Ti in the case of −1.8 V TiO2, which was further confirmed by N2 adsorption analysis that demonstrated a loss of real surface area per geometrical area for the −1.8 V TiO2 compared to the other samples (Figure S2 and Table S1 of Supporting Information). From the transmission electron microscopic analysis, the inner diameter and thickness of the wall were observed to be about 65 and 8 nm (Figure S3 of Supporting Information), respectively, and the presence of anatase structure, having (101) plane, was confirmed by highresolution lattice fringes at 0.35 nm. X-ray photoelectron spectroscopic and Raman spectroscopic studies were performed to elucidate the chemical composition and oxidation state of TiO2 NTAs. Figure 2b shows the Ti 2p XPS spectra of the samples in which two broad peaks centered at about 458.9 and 464.2 eV, corresponding to the characteristic Ti 2p1/2 and Ti 2p3/2 peaks of Ti4+, respectively, are observed for all the samples.24 In a comparison with the pristine TiO2, the peaks of every self-doped TiO2 NTAs show a negative shift in binding energy, suggesting the presence of Ti3+ state in the self-doped TiO2 NTAs as the characteristic Ti 2p1/2 and Ti 2p3/2 peaks of Ti3+ locate at slightly negative sites of ca. 458.0 and 459.5 eV.24,29,43 The peaks for the −1.4 V TiO2 sample shift toward a largest value of about 0.6 eV that indicates the larger number of oxygen vacancies present. As shown in Figure S4 of Supporting Information, a comparison of survey and O 1s XPS spectra was made and no changes were observed in the characteristic peaks. The O 1s peak observed for every sample could be split into two peaks with similar intensity centered at 529.8 and 531.5 eV, which correspond to characteristic peaks of Ti−O−Ti and the surface Ti−OH species, respectively.24 The findings from XPS characterization suggest oxygen vacancies are introduced into the lattice of TiO2 in the process of cathodic polarization while the surface of TiO2 shows no significant change. The formation of oxygen vacancies was further confirmed by Raman spectra studies (Figure 2c).

intercalation takes place that turns out a change of NTAs color from light gray to black. However, the electrochemical reduction approach was reversible only to a certain extent (not completely) at potentials more negative than −1.2 V that normally yield self-doped TiO2 NTAs upon postoxidation, whereas a completely reversible redox process could be possible at potentials between −0.9 and −1.2 V. The oxygen vacancies that formed in the self-doped TiO2 NTAs at the more negative potentials could be attributed to the much stronger expansion of the TiO2 lattice that occurs because of H+ intercalation and H2 evolution, which eventually results in probable cleavage of Ti−O bonds (formation of oxygen vacancies) or even results in the destruction of nanotubes, as in the case of −1.8 V TiO2 (discussed below). In this study, after the electrochemical reduction process, the material underwent a drying process under 80 °C and atmospheric conditions to oxidize the Ti3+ and acquire stability of the samples. Finally, self-doped NTAs with light blue color were obtained. The as-prepared TiO2 NTAs excluding the one prepared at −1.8 V consist of highly ordered tubes with inner diameter of 40−70 nm, wall thickness of 7−15 nm, and tube length of ∼1.6 μm (Figure 1c and Figure S1 of Supporting Information). The scanning electron microscopic studies revealed that there were no prominent changes in the morphology upon polarization of the TiO2 NTAs at −1.2, −1.4, and −1.6 V (vs SCE). But a further increase in the negative potential to −1.8 V (vs SCE) led to destruction of the TiO2 tubes (Figure S1d of Supporting Information), which was confirmed by the X-ray diffraction and N2 adsorption analyses. Figure 2a shows the XRD pattern of the samples; the peaks centered at ∼25.3° and ∼38.4° are the characteristic peaks of anatase TiO2 (JCPDF 21-1272) and Ti (JCPDF 44-1294), respectively. Among the prepared samples, the spectrum data for −1.8 V TiO2 shows a much weaker intensity ratio of these two peaks. Unlike other samples, an unimpressive peak located at ∼35.1° corresponding to the (100) plane of Ti can be clearly observed in the spectrum of the 5629

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Figure 3. (a) CV curves of the samples obtained at a scan rate of 100 mV s−1. (b) Areal capacitances of the samples measured as a function of scan rate. (c) Galvanostatic charge−discharge curves of the samples collected at a current density of 50 μA cm−2. (d) Cycle performance of the samples measured at a scan rate of 100 mV s−1 for 2000 cycles.

The peaks located at ∼398, ∼517, and ∼633 cm−1 for the samples could be attributed to the anatase phase.44 The intensities of the peaks for the −1.8 V TiO2 are much weaker than the peaks of other samples, again confirming the destruction of nanotubes with the potential applied in the cathodic polarization process. Compared to the pristine TiO2, Raman peaks of the other TiO2 samples show asymmetrical broadening and a slight negative shift, suggesting the increased amount of oxygen vacancies.24,26,45 Thus, from the instrumental characterizations of XPS and Raman spectroscopy, it can be concluded that the potential of −1.4 V applied in the cathodic polarization process was optimal for introducing oxygen vacancies into the TiO2 NTAs. The findings were further confirmed by an alteration of transient currents during the process of cathodic polarization as a function of time (Figure S5 of Supporting Information). The process could be categorized into two stages. In the first stage of 0 s to about 60 s, the cathodic currents decrease with the increasing time, which can be attributed to the gradual completion of the self-doping. In the second stage, i.e., the latter part of the process, the current remained stable for −1.2 or −1.4 V while it increased to give rise to a peak for −1.6 or −1.8 V, which suggests a possible structural change of the nanotubes as in the case of −1.8 V TiO2 sample’s destruction. As shown in Figure S5 of Supporting Information, a cathodic current passed on the tubes during the cathodic process and it dramatically increased with the increase of cathodic potential. At higher negative potentials, such as −1.6 and −1.8 V, the intercalation of H+ and H2 evolution were highly accelerated, expanding the TiO2 lattice to such an extent that nanotubes were destroyed because of cracks generated at weak spots of the nanotubes. Thus, a moderate potential of −1.4 V, rather than more negative potentials of −1.6 and −1.8 V, proved to be sufficient for introducing oxygen vacancies. Similar results have been observed when a reductant was used to introduce oxygen vacancies into TiO2 nanoparticles.29 The effect of self-doping toward the electrical properties of TiO2 NTAs was investigated by performing potential-depend-

ent capacity measurements at a frequency of 1 Hz. Mott− Schottky plots were drawn based on the capacitances that were derived from the imaginary part of the impedance obtained at each potential and the real surface area calculated from the BET analysis (Figure S2 and Table S1 of Supporting Information). According to the Mott−Schottky theory, the space charge capacitance Csc of a semiconductor is expressed as46 k T⎞ 1 2 ⎛ = ⎜U − Ufb − B ⎟ 2 εε0qND ⎝ q ⎠ CSC

where q is the electronic charge, ND the donor concentration (for an n-type semiconductor), ε0 the permittivity of vacuum, ε the dielectric constant, U the applied potential, Ufb the flat band potential, T the temperature, and kB the Boltzmann constant. As shown in Figure 2d, both the pristine TiO2 and −1.2 V TiO2 electrode systems exhibit a strong capacity dependence on the voltage which is typical for space charge layer controlled capacity of an n-type semiconductor exposed in electrolyte.47 The flatband potentials of the pristine TiO2 and −1.2 V TiO2 were evaluated to be +0.17 V and −0.39 V, respectively. In contrast, the −1.4 V TiO2, −1.6 V TiO2, and −1.8 V TiO2 electrode systems show only a weak dependence of capacitance on the applied voltage, delivering a metallic behavior feature with a dependence of capacitance solely on the Helmholtz layer at the solid−liquid interface.47 The more negative Vfb of the doped TiO2 (−0.92 V for −1.4 V TiO2, −1.00 V for −1.6 V TiO2, and −0.92 V for −1.8 V TiO2) compared to that of the pristine TiO2 indicates a shift of conduction band to more positive potentials,48 which leads to enhanced electrical conductivity of these samples. To find out the impact of selfdoping in terms of electrical conductivity quantitatively, carrier density for both the pristine and self-doped TiO2 was calculated using the Mott−Schottky equation. As expected, the pristine TiO2 exhibited the lowest carrier density (1.55 × 1016 cm−3) because of its well-defined semiconductor characteristic behavior. Significantly, a maximum of 5 orders magnitude enhancement in carrier density was achieved by self-doping of 5630

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Figure 4. Nyquist plots of (a) the samples measured at an applied DC potential of −0.1 V, (b) pristine TiO2, and (c) −1.4 V TiO2 measured at three different applied DC potentials of −0.1, 0.4, and 0.9 V with insets showing the Bode plots and an enlargement of the high-frequency regions. The scattered dots and solid lines are the experimental and fit impedance data, respectively. The fit data are based on the model circuit in panel d, which shows the proposed equivalent circuit for all the samples.

the TiO2 NTAs (2.94 × 1020, 2.14 × 1021, 1.93 × 1021, and 1.93 × 1021 cm−3 for the −1.2 V TiO2, −1.4 V TiO2, −1.6 V TiO2, and −1.8 V TiO2, respectively). As elucidated in the XPS and Raman spectroscopic characterizations, the increased carrier densities could be attributed to the increased oxygen vacancy states, which act as electron donors for TiO2 semiconductor.49 To evaluate the effect of self-doping on the electrochemical properties of TiO2 NTAs, electrochemical measurements were conducted in a conventional three-electrode cell with a Pt mesh counter electrode and an SCE reference electrode in 0.5 M Na2SO4 aqueous solution. Figure 3a shows the CV curve of the samples recorded at a scan rate of 100 mV s−1. The curves of the pristine TiO2 and −1.2 V TiO2 show a strong dependence of current density on potential because of the n-type semiconductor properties of these samples as identified by the Mott−Schottky measurements. The higher resistances at more positive potentials for these samples led to the decrease of current densities, forming a sloping shape in the curves. Compared to the pristine TiO2 and −1.2 V TiO2, the −1.4 V TiO2, −1.6 V TiO2, and −1.8 V TiO2 samples deliver obvious capacitive characteristic curves close to ideal rectangular shapes.50 Furthermore, the CV curves of the −1.4 V TiO2 exhibit unaltered rectangular shapes as the scan rate increases from 5 to 500 mV s−1 (Figure S6a of Supporting Information), showing good capacitive behavior and high-rate capability though semiconductor characteristic of the −1.4 V TiO2 sample was still slightly preserved from the dependence of current density on potential. Because there was no evidence for any faradic reactions on the rectangular CV curves, it could be deduced that the self-doped TiO2 NTAs act as pure doublelayer capacitors. Figure 3b shows the calculated capacitance of the samples as a function of scan rate (see Supporting Information for detailed calculations). The capacitances of selfdoped TiO2 NTAs are significantly higher than that of the

pristine TiO2 NTAs. Because of the highest content of oxygen vacancies introduced, the −1.4 V TiO2 generates the largest capacitance as expected, which achieves a capacitance of 1.84 mF cm−2 at a scan rate of 5 mV s−1, a 39-fold enhancement compared to that of the pristine TiO2 (0.047 mF cm−2). Moreover, the self-doped TiO2 NTAs show excellent high-rate capacitance. With an increase in scan rate from 5 to 500 mV s−1, the capacitances of the −1.2 V TiO2, −1.4 V TiO2, −1.6 V TiO2, and −1.8 V TiO2 keep high retentions of 92.6%, 90.4%, 84.1%, and 89.4%, respectively. For pure double-layer capacitors, the gradual decrease of the capacitance response with increasing scan rate was due to the limitation of ion accessibility to the inner region of the porous structure on the relevant time scale.51,52 The electrochemical properties of the samples were further studied by galvanostatic charge−discharge measurements. Figure 3c shows the charge−discharge curves of different TiO2 electrodes collected at a current density of 50 μA cm−2. The charge−discharge curves of the self-doped TiO2 NTAs are symmetric and substantially extended over the pristine TiO2, revealing excellent and improved capacitive behavior. Furthermore, the self-doped TiO2 NTAs show much smaller IR drops (0.145 V for −1.2 V TiO2, 0.018 V for −1.4 V TiO2, 0.024 V for −1.6 V TiO2, and 0.041 V for −1.8 V TiO2), again confirming the superior electrical conductivity accomplished by self-doping. The slightly nonlinear sloping potential profile of the −1.2 V TiO2 was attributed to the strong dependence of conductivity on potential as illustrated by the Mott−Schottky plots, which was consistent with the CV results as well. For the −1.4 V TiO2 sample, the relationship between potential and charging−discharging time always keeps good linearity when current density varies from 10 to 200 μA cm−2 (Figure S6b of Supporting Information), confirming the nonfaradic capacitive behavior and high-rate capability. Capacitances of the samples 5631

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Table 1. Calculated Value of the Equivalent Circuit Elements for the Samples sample pristine TiO2

−1.2 V TiO2

−1.4 V TiO2

−1.6 V TiO2

−1.8 V TiO2

DC potential (V)

Rs (Ω cm−2)

−0.1 0.4 0.9 −0.1 0.4 0.9 −0.1 0.4 0.9 −0.1 0.4 0.9 −0.1 0.4 0.9

7.355 7.760 7.734 6.851 7.126 7.257 6.798 6.807 6.797 6.146 6.102 6.106 7.087 7.097 7.094

R1 (Ω cm−2)

CPE1−T (mF sp‑1 cm−2)

CPE1−p

R2 (Ω cm−2)

× × × × × × × × × × × × × × ×

0.115 0.011 0.009 0.635 0.170 0.035 1.183 0.871 0.706 0.981 0.783 0.659 0.545 0.406 0.342

0.882 0.946 0.953 0.999 0.979 0.871 0.979 0.996 0.997 0.976 0.975 0.978 0.944 0.958 0.956

8.89 86868 109700 3.57 6.95 238 6.24 8.24 8.63 4.30 4.68 5.18 4.34 4.75 4.73

5.54 6.77 7.11 1.92 8.75 9.20 6.31 7.24 9.56 2.90 9.91 8.12 4.77 8.06 1.85

105 107 107 105 106 107 106 106 106 105 106 106 105 107 106

R3 (Ω cm−2)

CPE2−T (mF sp‑1 cm−2)

CPE2−p

× × × × × × × × × × × × × × ×

0.132 0.012 0.002 0.623 0.178 0.008 1.145 0.867 0.696 1.016 0.791 0.653 0.531 0.436 0.367

0.955 0.724 0.990 0.962 0.965 0.871 0.953 0.962 0.964 0.954 0.956 0.952 0.950 0.947 0.945

1.45 8.24 1.95 5.09 6.02 6.91 3.30 4.86 2.83 2.65 8.88 1.41 2.63 5.00 1.24

105 107 107 105 106 108 105 106 107 105 106 107 105 107 106

interface, and a capacitive element related to charging the porous matrix.55−57 The self-doped TiO2 NTAs differs with the pristine TiO2 electrode in terms of exhibiting typical doublelayer capacitive behavior with a couple of plots producing impedance lines starting with 45° and reaching almost a vertical line at higher frequencies and lower frequencies, respectively.58−60 Moreover, the strong dependence of conductivity on potential was further confirmed for the pristine TiO2 and −1.2 V TiO2 (Figure 4b and Figure S9a of Supporting Information), whereas the influence of potential on conductivity was very weak for the −1.4 V TiO2, −1.6 V TiO2, and −1.8 V TiO2 electrode systems (Figure 4c and Figure S9b,c of Supporting Information). Because the transmission line model is complicated to be used for calculation, the model has been simplified as shown in Figure 4d. In this equivalent circuit, Rs is the solution resistance and R1 and R3 are the charge-transfer resistances at the solid− electrolyte interface; R2 is the charge-transport resistance through the solid material, and CPE1 and CPE2 are constant phase elements (defined as Z = [T(iω)p]−1) generally used to depict real double-layer capacitors.60 The simplification of the transmission line model was conducted via a consolidation of the distributed charge-transport resistance elements, chargetransfer resistance elements, and capacitive elements, respectively, which claims to be proper because of the following reasons: (1) No process related to the conducting behavior of the NTAs was omitted during the simplification. (2) The essence of the transmission line model and the simplified model was consistent. Charging−discharging of the top region of the NTAs must overcome the charge-transport resistance through the solid material. Thus, a decrease of both the real part and imaginary part of impedance at high frequencies (compared to that at low frequencies) was associated with the TiO2 layer over the Ti foil in contact with the electrolyte at the near-bottom region of the nanotubes because of the insulation of the top region by the much larger resistance of the solid material compared to that of the conductive CPE elements at high frequencies. This essence has been well-retained in the simplified circuit (the R1 and CPE1 values and R3 and CPE2 values are used to simulate the charge-transfer resistances and capacitances of the near-bottom and near-top regions of nanotubes, respectively). (3) Certainly, the simplified circuit fits

derived from the charge−discharge curves measured at different current densities are given in Figure S7 of Supporting Information, from which are obtained results that are the same as those of the CV measurements, such as the result that the −1.4 V TiO2 sample yields the highest capacitance. To test the cycling stability of the samples, which is one of the most important characteristics for high-performance supercapacitors, CV measurements were carried out at a scan rate of 100 mV s−1 up to 2000 cycles. As shown in Figure 3d, all the TiO2 samples exhibit excellent long-term capacitive stability with the retention of capacitances over 90% after 2000 cycles. Additionally, the shape of the CV curves measured for the −1.4 V TiO2 sample always keeps close to an ideal rectangle during the 2000 cycles (Figure S8 of Supporting Information). Because an insufficient amount of oxygen vacancies in TiO2 nanotubes will lead to a strong dependence of current density on potential (like the CV curves of the −1.2 V TiO2 sample), the unchanged rectangular shape suggests that there were no significant changes in the content of oxygen vacancies with progressive scanning cycles. In other words, the high cycling stability of the self-doped TiO2 NTAs justifies the stability of the introduced oxygen vacancies. The above-mentioned results confirm that the electrochemical performance of TiO2 NTAs is strongly correlated to their electrical conductivity. To gain a better understanding of the conducting behavior of the porous TiO2 structure, electrochemical impedance spectroscopy was carried out because it is an excellent and well-established technique for characterization of electrochemical systems with series coupled processes.53,54 EIS measurements were performed between 100 kHz and 0.01 Hz with a 5 mV rms sinusoidal modulation under three different applied DC potentials of −0.1, 0.4, and 0.9 V (vs SCE) to clarify the effect of potential on the conductivity of the samples. Figure 4a shows the comparative Nyquist plots of the samples measured at −0.1 V DC potential. For the pristine TiO2, the plots start with a 45° impedance line at the highfrequency region and then approach a depressed semicircular arc at the low-frequency region. These phenomena are completely consistent with the transmission line model previously developed, in which the equivalent circuit of the TiO2 nanotube electrode system is made of a distributed combination of electron-transport resistance through the solid material, charge-transfer resistance at the solid−electrolyte 5632

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well with the experimental data (Figure 4b,c and Figure S9 of Supporting Information). The values calculated for the equivalent circuit elements of the samples are summarized in Table 1. Consistent with the Mott−Schottky measurements, the charge-transport resistance R2 is strongly dependent on the DC potential with a higher value at more positive applied potential only for the pristine TiO2 and −1.2 V TiO2 but not for the −1.4 V TiO2, −1.6 V TiO2, and −1.8 V TiO2. It could also be noted that the Rs values of pristine TiO2 and −1.2 V TiO2 depend on applied DC potential unlike those of −1.4 V TiO2, −1.6 V TiO2, and −1.8 V TiO2. This has been attributed to the blending of chargetransport resistance of the near-bottom region into Rs (thus the dependence of charge-transport resistance on potential will result in the dependence of Rs on potential). After doping, the solid TiO2 NTAs are much more conductive with a dramatic decrease of R2 values compared to the those of pristine TiO2, particularly at higher DC potentials. Especially for the welldoped −1.4 V TiO2, −1.6 V TiO2, and −1.8 V TiO2 samples, with significantly decreased charge-transport resistances through the solid state, the entire nanotube contributes to impedance in the same way (little Fermi level differences are expected from the beginning of the tube to the end); thus, the calculated data for R1−CPE1 and R3−CPE2 are interchangeable (the difference between R1 and R3 can be omitted because of the substantially higher values of R1 and R3 compared to the impedance from CPE1 and CPE2 for these samples, which determines that the R1 and R3 make almost no contribution to the impedance of the R−CPE pairs). From the calculated results, a quantitative comparison of capacitance between the samples could also be made. The values of most of the CPE1−p and CPE2−p are close to 1; thus, the CPE elements can be considered as pure capacitors. In comparison to the pristine TiO2, the self-doped samples exhibit greatly enhanced capacitances (sum of the CPE1−T and CPE2−T), which is in good agreement with the results of CV and galvanostatic charge−discharge measurements. It should be noted that all the samples exhibit a higher capacitance at lower applied DC potential, suggesting the self-doped TiO2 NTAs still remain in more or less semiconductor characteristic behavior as mentioned above. As for the charge-transfer resistances R1 and R3, the calculated values show no regularity and no important variation, suggesting there is no significant modification of the charge-transfer resistance as the surface of TiO2 remains unchanged upon self-doping as seen in XPS studies. Taking into account the EIS study, it has been concluded that the enhanced capacitive behavior of the selfdoped TiO2 NTAs can be attributed to the decrease of the charge-transport resistance through the solid material, which was accomplished by the introduction of oxygen vacancy states. Because the electric conductivity and capacitive performance of TiO2 NTAs are very sensitive to the content of introduced oxygen vacancies, the effect of polarization time was studied and is considered to be another important parameter affecting self-doping. As shown in Figure 5, the dependence of capacitance for the polarization period of 1, 5, 10, 15, and 20 min was investigated at different applied cathodic potentials. With a relative lower potential of −1.2 V or −1.4 V applied in the polarization process, the obtained capacitance of the TiO2 NTAs initially increases and keeps stable as the polarization time is prolonged over 10 min. Moreover, the CV curves of the −1.4 V TiO2 always maintain close to a rectangular shape (Figure S10 of Supporting Information), indicating oxygen

Figure 5. Areal capacitance of the TiO2 NTAs that underwent polarization at different potentials for different periods of time. The value of capacitance was derived from the CV curves measured at 100 mV s−1.

vacancies are introduced and are stable with the prolonging polarization time. In contrast, a gradual loss of the capacitance was observed at −1.8 V with increasing polarization time. This was attributed to the destruction of the TiO2 NTAs, as mentioned above. In the case of −1.6 V, the resulting capacitance was always constant with the polarization time varying from 1 to 20 min, suggesting that the structure and composition of the TiO2 NTAs are unchanged beyond 1 min at this potential. However, the final capacitance of −1.6 V TiO2 was less than that of −1.4 V TiO2 that resulted from the damage of the TiO2 nanotubes during the initial 60 s period of cathodic polarization. For a further confirmation of the suitability of self-doped TiO2 NTAs as current collectors for supercapacitors, the −1.4 V TiO2 was loaded with high capacitive MnO2. As shown in Figure 6a, a thin layer of MnO2 nanowires were formed along the NTAs using the impregnation−electrodeposition method. The spectrum of Mn 2p XPS reveals two peaks located at 642.2 and 654.1 eV (Figure 6b), which are consistent with the characteristic Mn 2p3/2 and Mn 2p1/2 binding energies of MnO2, indicating that the Mn4+ ions are dominant in the sample.61,62 CV curves were recorded at different scan rates of 5 to 200 mV s−1 to study the capacitive performance of the MnO2/−1.4 V TiO2 composite, and the results are presented in Figure 6c. This nanostructured composite shows a nearly ideal capacitive CV shape with only small distortions even at a high scan rate of 200 mV s−1, which results in capacitance retention of 58.8% compared with that measured at 5 mV s−1 (14.8 mF cm−1 at 5 mV s−1 and 8.7 mF cm−1 at 200 mV s−1; Figure 6d). In addition to the remarkable rate capability, the MnO2/−1.4 V TiO2 composite achieves a splendid specific capacitance of 1232 F g−1 (based on the mass of MnO2; for detailed calculations see Supporting Information and Figure S11) at the scan rate of 5 mV s−1, which is close to the theoretical value of MnO2 (1400 F g−1). To the best of our knowledge, the observed value is higher than that recently reported for MnO2/ H−TiO2 composite (912 F g−1, based on the mass of MnO2),24 which was known to be the best ever achieved by a MnO2/ TiO2 composite. To clarify the impact of self-doping of the TiO2 NTAs on the capacitive performance of MnO2/TiO2 composite, a galvanostatic electrodeposition method was employed to deposit MnO2 onto the pristine TiO2 and selfdoped TiO2 (detailed in Supporting Information). Similarly, high-rate capability, remarkable specific capacitance, and improved capacitive properties were found to be exhibited by the self-doped TiO2. These results suggest that the composite of MnO2 prepared on the −1.4 V TiO2 by both methods has 5633

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Figure 6. (a) FE-SEM image, (b) Mn 2p XPS spectrum, (c) CV curves recorded at different scan rates, and (d) specific capacitances and areal capacitances measured as a function of scan rate of the MnO2/−1.4 V TiO2 composite synthesized by the impregnation−electrodeposition method.

good electrical conductivity and charge separation and transportation in these highly ordered tubular structures are efficient, justifying the suitability of using the self-doped TiO2 NTAs as substrates for high capacitive materials.

composite and analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

CONCLUSIONS The electrochemical response of the self-doped TiO2 NTAs, as an electrode material for supercapacitors, was found to be improved to a great extent. TiO2 NTAs, polarized cathodically at −1.4 V for the period of 10 min, exhibited the highest capacitance of 1.84 mF cm−2 at a scan rate of 5 mV s−1 (39 times higher than that of the pristine TiO2) as well as excellent long-term stability with capacitance retention of 93.1% after 2000 cycles. The enhancement in capacitance has been attributed to the significant decrease in charge-transport resistance through the solid material which was accomplished by introducing oxygen vacancy sites. Furthermore, the selfdoped TiO2 NTAs were verified to be excellent current collectors for other capacitive active materials such as MnO2. The MnO2/−1.4 V TiO2 composite achieves the highest specific capacitance of 1232 F g−1 at the scan rate of 5 mV s−1, which is close to the theoretical value of MnO2. New opportunities of TiO2 materials for constructing high-performance supercapacitors as well as other energy storage devices could be explored based on the findings established in the present work.

Corresponding Author

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AUTHOR INFORMATION

*Environmental Science Research Institute, Huazhong University of Science and Technology, Luoyu Road #1037, Wuhan 430074, China. E-mail: [email protected]. Tel.: +86 027 87792101. Fax: +86 027 87792101. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by International Science & Technology Cooperation Program of China (2013DFG50150), Ministry of Science and Technology of China (2010DFA22770), and the National Natural Science Foundation of China (51079056/E090301). The authors thank the Analytical and Testing Center of HUST for the use of SEM, XPS, and Raman spectroscopy equipment.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

SEM of the pristine TiO2, −1.6 V TiO2, and −1.8 V TiO2; N2 adsorption−desorption isotherms and BET analysis; TEM images of the −1.4 V TiO2; survey and O 1s XPS spectra; current transients recorded in the cathodic polarization process; CV and charge−discharge curves; Nyquist plots of the −1.2 V TiO2, −1.6 V TiO2, and −1.8 V TiO2; capacitive equations and calculations; and galvanostatic fabrication of MnO2/TiO2 5634

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