Surface-Engineered TiO2 for High-Performance Flexible Supercapacitor Applications

Titanium dioxide (TiO2) shows excellent pseudocapacitive properties. However, the low internal conductivity of TiO2 limits its use in supercapacitor applications. Therefore, an efficient surface engineering process was developed to enhance the overall pseudocapacitive performance of rutile TiO2 nanorods. Specifically, surface-engineered TiO2 nanorod arrays coordinated on carbon cloth were established through the Kapton tape-assisted hydrothermal route. X-ray diffraction analysis confirmed the formation of a tetragonal TiO2 rutile phase. Morphological analysis revealed the formation of uniform nanorods with an apparent high surface-to-volume aspect ratio. X-ray photoelectron spectroscopy analysis showed that the TiO2 synthesized in the presence of Kapton tape and annealed under air had high content of hydroxyl groups and Ti3+, which is favorable for supercapacitor performance. Surface treatment of the samples led to significantly enhanced conductivity and electrochemical behavior of TiO2. The surface-engineered TiO2 nanorod arrays show specific capacitance of about 57.62 mF/cm2 at 10 mV/s in 2 M KOH, with excellent rate capability of about 83% at 200 mV/s, and also exhibit long cycle life, retaining 91% of their original capacitance after 10,000 charge/discharge cycles, which is among the highest values reported for TiO2-based supercapacitors.


Introduction
Nowadays, our lives are strongly dependent on portable and wearable devices, including mobile phones, laptops, and electrical-powered transportation utilities. In addition, fossil fuels are being rapidly depleted, contributing to severe environmental pollution. 1,2 Thus, researchers are ramping up their efforts to develop efficient energy storage and conversion devices, such as metal ion batteries and supercapacitors. Supercapacitors are unique due to their large power density and outstanding long-life stability. Similar to metal ion batteries, electrochemical capacitors consist of two electrochemical active electrodes separated by an electrolyte. 3 Based on the charge storage mechanism, supercapacitor electrode materials can be divided into three types: (i) The electric double-layer capacitor (EDLC) electrode, in which the charge can be non-faradaic stored such as graphene, carbon nanotubes, and activated carbon. The charge is electrostatically adsorbed at the electrode-electrolyte interface. (ii) Pseudocapacitive electrodes, for instance, MnO 2 , RuO 2 , and conductive polymers. The charge in this type can be stored through fast and reversible faradaic reactions at the surface or subsurface of the active electrode material for metal oxides or through doping or de-doping when the polymer is in use. In both EDLC and pseudocapacitive electrodes, the charge is directly proportional to the potential difference and the cyclic voltammetry curves, having either a rectangular or semi-rectangular shape. Moreover, they show high power density. (iii) Battery-type electrodes such as metal hydroxides, metal oxides, metal sulfides, and metal phosphides, in which the charge can be stored through faradaic reaction. The cycle voltammetry and charge/discharge curves are distinguished by pairs of peaks and plateaus, respectively, associated with the redox reactions involved, and thus it can supply large capacity. [4][5][6][7][8] Battery-type electrodes such as Ni(OH) 2 , Co 3 O 4 , and NiCo 2 O 4 work in basic electrolytes, such as KOH. In addition, these electrode materials face many problems such as aggregation, lack of charge transfer, and difficulties in electrolyte ion diffusion. Thus, it is important to introduce backbone materials with suitable morphology and electrochemical properties that can work in basic electrolytes to enhance the performance of battery-type electrodes.
Among different metal oxides titanium dioxide (TiO 2 ) has received considerable attention in a wide range of applications due to its unique properties. For instance, TiO 2 has been used in solar energy conversion, memristors, water treatment, and photocatalytic water spitting due to its high reflective index and ability to absorb ultraviolet (UV) light. [9][10][11][12] However, there are three different phases-rutile, anatase, and brookite-for TiO 2 . Anatase TiO 2 has been widely used in energy storage applications including metal ion batteries. [13][14][15] TiO 2 shows low specific capacitance of about 330 mAh/g and low volume expansion through a lithiation and delithiation process, which leads to enhanced rate capability and cycle life stability, and thus it has been used as an anode in lithium-ion batteries. 16 TiO 2 is further used in supercapacitors, where engineering of the surface charge of TiO 2 nanotube arrays on Ti foil provided high specific capacitance of about 1.6 mF/cm 2 at a scan rate of 1 mV/s in 1 M NaOH, with 97% capacitance retention after 1000 cycles. This improved performance can be attributed to the presence of Ti 3+ , which enhances the electrical conductivity of TiO 2 . 17 Furthermore, controlling the phase transition of TiO 2 leads to enhanced electrochemical performance, as reported by Salari et al., 18 who found that the change from anatase to rutile could overcome the charge transfer resistance. Thus, the capacitive behavior was improved, with specific capacitance of about 2.6 mF/cm 2 at a scan rate of 1 mV/s in 1 M NaOH. To further improve the capacitive behavior of TiO 2 , Zhou and his research team annealed TiO 2 under hydrogen. They found that dealloying was induced in the presence of a hydrogen atmosphere, leading to the formation of Ti 3+ , which improved conductivity. The hydrogenated TiO 2 showed specific capacitance of 1.2 mF/cm 2 at 2 mV/s in 0.5 M Na 2 SO 4 with rate capability of 58% at 1000 mV/s. 19 Meanwhile, the surface modification of TiO 2 can be used as a backbone in a hierarchical core-shell structure. For instance, Ke et al. 20 fabricated a surface-charged hydrogenated TiO2@Ni(OH) 2 core-shell heterostructure. They reported good enhancement of the supercapacitive performance compared to Ni(OH) 2 alone. The specific capacity was around 306 mAh/g at 1 mV/s in 6 M KOH, which is higher than that of bare Ni(OH) 2 , (~200 mAh/g) with 65% rate capability at 100 mV/s. They attributed such performance to the negative surface charge of TiO 2 which helps in the growth of TiO 2 thin nanosheets; thus, more active sites participated in the electrochemical performance. Core-shell or clustered nanostructures of TiO 2 /RuO 2 was reported by Park and his team. 21 Due to the synergistic effect between TiO 2 and RuO 2 , they achieved high specific capacitance of around 1200 F/g at 1A/g in 1 M H 2 SO 4 , which is twice as high as that of bare RuO 2 .
Based on the results discussed above, TiO 2 nanomaterials show promising properties in supercapacitor applications by modifying their surface charge and/or changing the morphology, leading to enhanced conductivity and surface area, which are key factors for high-performance supercapacitors. Thus, we have designed a low-cost process to modify the surface charge of TiO 2 . The process involves the growth of TiO 2 nanorod arrays on carbon cloth with the assistance of Kapton tape, followed by annealing at 550°C. This process aims to (1) modify the surface of TiO 2 by increasing the density of hydroxyl groups on the nanorod arrays after the annealing process, and (2) create a higher amount of Ti 3+ on the surface of the nanorods, thus enhancing the conductivity of the electrode materials. Specifically, the TiO 2 with tape annealed under air showed higher Ti 3+ content and capacitive improvement of 160% compared to the sample annealed under a nitrogen atmosphere, with 91% retention after 10,000 cycles. This pseudocapacitive performance represents one of the best foundations for TiO 2 -based electrode materials. Moreover, the new routes for synthesizing surface-engineered TiO 2 nanorod arrays can be applied to a wide range of energy storage materials.

Preparation of Self-Supported Surface-Engineered TiO 2
Surface engineering of a material can be performed either by depositing a thin film with the required characteristics or by chemical modification of the material. This treatment is aimed at modifying the microstructure and/or the composition of the region near the surface. In this work, the growth of TiO 2 nanorod arrays on carbon cloth (CC) was achieved according to a previous report. 20 Surface modification was performed by chemical treatment. Specifically, 15 ml of HCl at 37% concentration was added to 15 ml deionized (DI) water, and then 1.5 ml of titanium butoxide was added to the acid solution under continuous stirring. The mixture was subsequently transferred into a 50 ml polytetrafluoroethylene (PTFE)-lined stainless steel autoclave, and the hydrothermal process was performed at 150°C for 9 h. After cooling to room temperature, the obtained TiO 2 nanorod arrays were collected, washed with DI water, and dried at 65°C in the oven overnight. For the annealing process, the dried film was placed in a tube furnace and annealed at 550°C for 2 h under an air/N 2 atmosphere. 22

Characterization
Various characterization techniques were employed to investigate the different properties of TiO 2 nanorod arrays grown on carbon cloth (CC). XRD (Bruker D8 Advance, Cu Kα, λ = 0.154 nm) was used to check the phase formation, and the morphological properties were assessed using scanning electron microscopy (SEM; Zeiss Supra 40) and transmission electron microscopy (TEM; JEOL 2010F at 200 kV). Analysis of the oxidation states and surface properties was established by x-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra delay-line detector [DLD]), and the spectra were calibrated using a carbon spectrum as a reference).

Electrochemical Performance Measurements
A 1 cm × 1 cm piece of TiO 2 @CC was used as the working electrode, while the reference and counter electrodes were Ag/AgCl and Pt, respectively; 2 M KOH aqueous solution was used as an electrolyte. The three-electrode cell configuration was used to investigate the pseudocapacitive performance of different samples in terms of cyclic voltammetry (CV), galvanostatic charge-discharge measurement, cyclic stability, and electrochemical impedance spectroscopy (EIS) (the frequency for the impedance test was varied from 10 6 to 0.1 Hz, 10 mV). All measurements were carried out with the Solartron 1470E and 1400A system.
The specific capacitance was calculated based on the total mass of the electrode, according to the following equation: where C is the specific capacitance of the electrode in the three-electrode configuration, (ΔQ/ΔV) is the slope of the discharge curve after the voltage drop, and A is the area of the electrode.

Results and Discussion
The fabrication process of the self-supported TiO 2 nanorod arrays on carbon cloth is schematically displayed in Fig. 1a. Essentially, TiO 2 nanorods are grown on one side of carbon cloth while the other side is covered with Kapton tape, using the hydrothermal method, and annealing of different samples is then carried out under air or N 2 atmosphere for different samples at 550 °C for 2 h (as described in the experimental section).  . 00-021-1276). Additionally, the diffraction peak detected around 26° is clearly attributable to the carbon cloth. 23,24 However, the pattern for the sample annealed under nitrogen shows very weak diffraction peaks in the 2θ range from 60° to 75°, which may be attributed to the lack of oxygen atoms, as will be later assessed by XPS data. The pattern for the sample annealed under air shows strong peaks characteristic of the rutile phase. Generally, the peaks are stronger than those of other samples, which could indicate high crystallinity of this sample in these annealing conditions.
The morphological properties of different TiO 2 -based samples were investigated using SEM. TiO 2 nanorod arrays were grown uniformly on the entire surface of carbon cloth fibers, as indicated in the low-magnification SEM images of different TiO 2 -based samples, which are shown in the insets of Fig. 2a, b, and c. In addition, the SEM micrographs indicate that the samples possess a high surface-to-volume aspect ratio. The high-magnification SEM images together with the TEM image shown in Figure S1 reveal that the rods are several micrometers in length and about 150-400 nm in diameter. The structural details of the different samples were further established by high-resolution TEM as shown in Fig. 2d, e, and f. The detected interplanar spacing of about 0.32 nm belongs to the (110) plane of rutile TiO 2 (PDF Card No. 00-021-1276).
XPS was carried out to clarify the influence of surface treatment in the presence of Kapton tape and the annealing conditions on the surface composition and to assign the balance state of the different TiO 2 samples (Fig. 3). Figure 3a shows the Ti 2p spectra of different TiO 2 -based samples. There are two main peaks located at 464.9 and 459 eV on average, corresponding to Ti 2p 3/2 and Ti 2p 1/2 , together with the separation in binding energy between the two main peaks found to be 5.7 eV corresponding to TiO 2 . Even more, there is a small shift towards lower binding energy for the sample of TiO 2 with tape annealed under air, indicating the presence of a small fraction of Ti 3+ on the surface of TiO 2 nanorod. 25-27 XPS spectra of O 1s are shown in Fig. 3b. The peak around 530.26 eV arises from the formation of metal-oxygen linkage (Ti-O-Ti) and the peak around 531.7 eV can be attributed to the presence of the hydroxyl group as Ti-OH, indicating the surface functionalization by the OH group. The Ti-OH to Ti-O-Ti peaks ratio is found to be 23%, 8%, 20%, and 10% for TiO 2 with tape before annealing, TiO 2 without tape annealed under air, TiO 2 with tape annealed under N 2 , and TiO 2 with tape annealed under air, respectively, compared to O 1s. Furthermore, it is noted that for a sample of TiO 2 with tape annealed under air, the Ti-O-Ti bonding peak is shifted to lower binding energy at 529.7 eV, which suggests the existence of Ti 3+ . The surface modification with the hydroxyl group and the presence of Ti 3+ is beneficial for pseudocapacitive behavior. 21,28,29 The oxidation state of N in all samples is displayed in Fig. 3c. The N 1s is only detected in TiO 2 with tape before annealing and TiO 2 with tape annealed under N 2 which may respectively come from the desorption of polyamide and carbonized polyamide on the surface of TiO 2 nanorods in the two samples. Additionally, the absence of N 1s in the sample of TiO 2 with tape annealed under air may lead to the formation of oxygen vacancies. 30 The influence of Kapton tape and annealing conditions on the electrochemical performance of TiO 2 as a cathode is examined using the three-electrode configuration with 2 M KOH as an electrolyte, as depicted in Fig. 4 and Figures S2-S4. The cyclic voltammetry at different scan rates of TiO 2 with tape under air is shown in Fig. 4a, in which the presence of the semi-rectangular shape of CV curves confirms the pseudocapacitive behavior of TiO 2 . Figure 4b displays the charge and discharge curves of TiO 2 with tape under air at different current densities. Figure 4b reveals the nearly symmetrical and linear behavior suggesting the high coulombic efficiency and rate capability, which is favorable for supercapacitors. Additionally, one can distinguish between a supercapacitor and a battery as follows: (1) the charge and discharge potentials show a plateau with time because of the formation of different phases for batteries, while they are linear time-dependent for a supercapacitor; (2) batteries show redox peaks with potential difference separation in the cyclic voltammetry curve, whereas the CV curves appear as rectangular or semi-rectangular for a supercapacitor. 31 A comparison of the electrochemical behavior of TiO 2 with tape before annealing, TiO 2 with tape under N 2 , TiO 2 with tape under air, and TiO 2 with tape under air was investigated. As shown in Fig. 5a, the CV curves of different samples at a scan rate of 200 mV/s reveal that the TiO 2 with tape and annealed under air shows the highest current density among the samples, suggesting an improvement in the energy storage capability. Additionally, the symmetrical charge-discharge curves with small IR drops. The TiO 2 sample with tape under air is still able to charge/discharge even at higher current density of about 20 mA/cm 2 compared with other samples, indicating improved ionic/electronic conductivity and enhanced electroactive properties of TiO 2 , as shown in Fig. 5b. Moreover, the energy storage in TiO 2 may result from the cation intercalation and de-intercalation processes according to the following mechanism: Through the charge/discharge process, the H 3 O + ions from the electrolyte can intercalate/de-intercalate into TiO 2 nanorods, which contributes to the electrochemical performance. The engineered surface of TiO 2 is another factor that enhances its capacitive properties, due to surface roughness that improves the surface wettability of the electrode and enhances the adsorption of electrolyte ions into the TiO 2 .
The specific capacitance as a function of scan rate for different samples is shown in Fig. 5c. The TiO 2 with tape under air shows specific areal capacitance of about 57.6 mF/ cm 2 at a scan rate of 10 mV/s, whereas specific capacitance values of about 4.4 mF/cm 2 , 13.9 mF/cm 2 , and 3.6 mF/cm 2 are reported for TiO 2 with tape before annealing, TiO 2 without tape under air, and TiO 2 with tape under N 2 at the same scan rate, respectively. Moreover, the TiO 2 with tape under air can maintain 60% of its original specific capacitance even at a faster scan rate of 1000 mV/s. Among the different samples, TiO 2 with tape under air exhibits the lowest resistivity, as shown in the Nyquist plot in Fig. 5d. For further explanation, the equivalent resistance (R S ), which is the sum of the electrolyte ionic resistance, the electronic resistance of the electrode materials, and interface resistance, can be calculated from the interception with the real axis. Another important resistance is the charge transfer resistance (R CT ), which indicates the electrode/electrolyte interface resistance. The measured electrochemical impedance for different samples was fitted based on the equivalent circuit, as shown in Fig. 5d, where R S is equivalent resistance, C DL is double-layer capacitance, W o is the Warburg diffusion element, R CT is charge transfer resistance, and C F is the faradaic capacitance. The obtained values are summarized in Table I Fig. 5e, the TiO 2 with tape under air preserved 90% of its original specific capacitance after 10,000 charge/discharge cycles. This may be attributed to the enhancement in its conductivity, charge transfer, and ion diffusion.
TiO 2 with tape under air demonstrated significantly better electrochemical performance when compared with other TiO 2 -based active materials, which is reported as a backbone for supercapacitor electrode materials, as shown in Table II. For a detailed comparison, Fornasini et al. 32 fabricated TiO 2 nanoparticles with reduced graphene oxide in situ using the LightScribe ® technique. Their micro-supercapacitor achieved specific capacitance of about 10 mF/ cm 2 at current density of 5 µA/cm 2 in polyvinyl alcohol/ H 2 SO 4 , and retained 72% of its original capacitance after 3000 charge/discharge cycles. Blue TiO 2 nanosheets demonstrated specific areal capacitance of ~ 19 mF/cm 2 at 0.5 mA/cm 2 in 1 M Na 2 SO 4 , with rate capability of 39% at 10 mA/cm 2 , 33 whereas TiO2 nanotube arrays grown on Ti foil demonstrated areal capacitance of 200 mF/cm 2 at current density of 1 mA/cm 2 in 1 M HCl, 90% of which was retained after 5000 cycles at 1 mA/cm 2 . 28 Zhang et al. 34 fabricated MoO 3 /titania nanotube composites on Ti sheets. They reported areal capacitance of around 3.14 mF/cm 2 for TiO 2 nanotube arrays at a scan rate of 0.6 mA/cm 2 in 2 M Li 2 SO 4 . Nitridation of a hollow TiO 2 multi-shell at 900°C supplied 7 mF/cm 2 at a scan rate of 50 mV/s, while the monolayer of nitrided titania showed only 0.8 mF/cm 2 . 35 K-doped mixed-phase TiO 2 nanofibers supported on Ti foil exhibited 21.45 mF/cm 2 at a scan rate of 500 mV/s in 1 M KOH, with 91% retention of its capacitance at 0.5 mA/cm 2 after 2500 cycles. 29 The TiO 2 with tape under air designed in this work showed areal capacitance of about 27.8 mF/ cm 2 at current density of 1 mA/cm 2 . Such enhancement in the electrochemical behavior can be attributed to the surface engineering of TiO 2 nanorods by surface functionalization via the hydroxyl group or induced Ti 3+ through the oxygen voids.

Conclusions
In summary, we have designed a low-cost strategy to solve the internal resistance and hence the overall electrochemical performance of TiO 2 , using a Kapton tape-assisted hydrothermal process. We found that after annealing, the surface of the TiO 2 was modified by increasing the number of hydroxyl groups and Ti 3+ on the surface. Moreover, the derived surface-engineered TiO 2 @CC provides high specific areal capacitance of about 27.8mF/cm 2 at 1 mA/cm 2 , with excellent rate capability and long cycle life stability, retaining over 91% of its initial specific capacitance after 10,000 cycles. The overall high pseudocapacitive behavior suggests that the strategy of surface-engineered TiO 2 provides new opportunities for the use of TiO 2 as a backbone in highperformance supercapacitor devices.