**3. Physicochemical characterization of the blue TiO2 nanosheets**

The blue TiO2 (blue titanium oxide) nanosheets were prepared using layered TiB2 and HF as starting materials via hydrothermal process in acidic medium [31]. The mechanism for the formation of layer like blue titanium oxide from the precursor TiB2 is due to the topochemical reaction as seen in the preparation of siloxenes and MXenes under acidic medium [19, 33]. The TiB2 is a layered material with titanium and boride arranged in alternating layers. In this case the TiB2 reacts with the HF, which topochemically dissolves boron with the release of hydrogen gas. This generated hydrogen gas creates the oxygen vacancies via H2 reduction of Ti4+ on the surface [31, 34]. The digital micrograph of the precursor and the final product is depicted in the **Figure 1(A)**, which shows the change in from gray TiB2 to blue colored TiO2. The X-ray diffraction pattern of the precursor TiB2 and the prepared TiO2 were provided in **Figure 1(B)**, which confirms the complete transformation of TiB2 to TiO2 and matched with the anatase TiO2 (JCPDS No: 021-1272) [35]. The implication of the bule colored TiO2 can be corelated with the vacancy of oxygen, disordered surface and deficiency of oxygen which can be confirmed using the spectroscopic analysis such as electron spin resonance spectroscopy (ESR), UV–visible (UV–vis) and photoluminescence (PL) respectively. The X-band electron spin resonance spectrum of the blue titanium oxide was obtained under the temperature of 77 K with the frequency of 9364 GHz and provided in **Figure 1(C)**. The presence of Ti3+ centers in the g- region (ranging from 1.87 to 1.99) which is associated to the resonation of Ti3+ species [36]. In addition, the presence of a number of partially overlapping signals in the ESR spectrum is matched well to the different types of Ti3+ centers present in the blue titanium oxide. The obtained ESR results is in agreement with the previous finding on the partially reduced TiO2 [31]. **Figure 1(D)** shows the comparative UV–vis spectra of commercial TiO2 and blue titanium oxide and the peak observed at 354 nm corresponds to the intrinsic absorption edge of TiO2 which is red-shifted for the blue titanium oxide to that of the commercial TiO2 confirming the increase in electron concentration on the surface of blue titanium

#### **Figure 1.**

*(A) Digital micrograph of titanium boride and blue titanium oxide. (B) X-ray diffraction pattern of the titanium boride and blue titanium oxide. (C) Electron spin resonance spectrum of blue titanium oxide. (D) UV–visible spectrum of commercial anatase titanium oxide and blue titanium oxide. (E) Photoluminescence spectrum of commercial anatase TiO2 and blue titanium oxide. (F) Raman spectrum of blue titanium oxide. (G) XPS survey spectrum of TiB2 and blue titanium oxide. XPS core level spectrum of(H) Ti 2p and (I) O 1 s state in blue titanium oxide.*

oxide. Furthermore, the blue titanium oxide shows increased absorbance compared to the commercial TiO2 over 400–800 nm which is due to the absorbance of lowenergy-photon and/or thermal-excitations of trapped electrons in localized states of defects [37–40]. **Figure 1(E)** shows the comparative PL spectra of commercial and prepared TiO2. The PL spectra depicts the presence of peak over 510–540 nm for the blue titanium oxide compared to that of the commercial TiO2 which shows a broad band over the region of 400–800 nm [41, 42]. The blue titanium oxide shows higher luminescence intensity to that of the commercial TiO2 suggesting the low electron–hole recombination rate of blue titanium oxide [43, 44]. These studies confirming the higher electronic conductive states of blue titanium oxide compared to the commercial TiO2. The laser Raman spectrum of the blue TiO2 is presented in **Figure 1(F)** which discloses the occurrence of a major vibration band at 148.25 cm−1 corresponds to the Eg mode of anatase TiO2 [45]. In addition to the major vibrational band, the occurrence of the three minor vibrational bands occurred at 396, 517, and 640 cm−1 which correspond to the B1g, A1g, and Eg vibrational modes of anatase TiO2, respectively [38, 46].

The comparative X-ray photoelectron survey spectrum of TiB2 and blue titanium oxide is provided in **Figure 1(G)**, which shows the presence of Ti 2p, B 1 s, C 1 s and O 1 s states [47]. The existence of oxygen and carbon peaks in the spectrum is due to atmospheric interaction with the sample. From the comparative XPS spectra of TiB2 and blue titanium oxide, exhibits the complete removal of the boron from TiB2 which is evident from the XPS spectrum. This study proves the compete dissolution of the boron during the hydrothermal reaction [47]. The XPS spectrum of Ti and O

*Energy Storage Properties of Topochemically Synthesized Blue TiO2 Nanostructures in Aqueous… DOI: http://dx.doi.org/10.5772/intechopen.102186*

**Figure 2.**

oxidation states of elements present in the blue titanium oxide nanostructures were provided in **Figure 1(H** and **I)**. The XPS core level spectrum of Ti 2p state reveals the existence of the 2p3/2 and 2p1/2 at 458, and 464 eV respectively. In addition, the observance of small-satellite peak at 472.2 eV corresponding to the defect of oxygen in the blue titanium oxide [38, 48]. Overall, the core-level spectrum of the titanium highlights the Ti4+ oxidation state with littleTi3+ state due to oxygen deficiency in blue titanium oxide [49]. **Figure 1(I)** represents the O 1 s states of blue titanium oxide at 530 eV respectively [38, 48]. The calculation of the O/Ti atomic ratio of blue titanium oxide nanostructures is found to be 2.47 from XPS analysis which is in agreement with the literature [50].

To analyze the surface morphology of blue titanium oxide nanostructures field emission scanning electron micrograph (FE-SEM) with energy dispersive spectra (EDS) and high-resolution transmission electron microscope (HR-TEM) were performed. The FE-SEM of blue titanium oxide is provided in **Figure 2(A)**; reveals the presences of nano-sheets/plate-like blue titanium oxide as result of topochemical reaction. The elemental-mapping of Ti & O in the blue titanium oxide is shown in **Figure 2(B** and **C)** and EDS spectrum (**Figure 2(D)**) indicated the homogeneous distributions of Ti and O throughout the blue titanium oxide sheet/plate. With an O/Ti atomic ratio of about 3.92 present in the blue titanium oxide nanostructures [50, 51]. The HR-TEM micrographs of blue titanium oxide obtained under various levels of magnifications are shown in **Figure 2(E**–**H)**. The HR-TEM micrograph of blue titanium oxide (**Figure 2(E**–**F)** revealed the presence of sheet-like structures with the lateral size ranging from 200 nm in length and 250 nm in breadth. The magnified portion of **Figure 2(F)** is provided in **Figure 2(G)** reveals the presence of thinner blue titanium oxide sheets with few-layers. The lattice fringes (**Figure 2(H)**) of blue titanium oxide sheets exhibits d = 0.35 nm (an interplanar-spacing) corresponds to (101) plane and SAED pattern (inset of **Figure 2(H)**) reveals the clear diffraction spots signifying the crystallinity-nature of blue titanium oxide [37].

## **4. Electrochemical characterization of blue titanium oxide electrode**

#### **4.1 Preparation of electrode for electrochemical characterization**

The working electrode was prepared by mixing the active material (blue titanium oxide) with carbon black and PVDF in an appropriate ratio of 80:15:5 using NMP as solvent. The slurry was spin coated on stainless steel substrate for the three electrode and two electrode characterizations. For the fabrication of symmetric supercapacitor, CR2032 coin cell configuration is utilized and the TiO2-coated stainless-steel substrates with an area of 1.86 cm2 as electrodes separated by a Celgard membrane and 1 M TEABF4 in acetonitrile as the electrolyte. All the electrochemical measurements were performed using an Autolab PGSTAT302N electrochemical workstation.

#### **4.2 Half-cell characterization of blue titanium oxide electrode**

The electrochemical properties of the blue titanium oxide electrodes in aqueous electrolyte (half-cell) were inspected using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge (CD) analysis, respectively. The half-cell electrochemical characterization of the blue titanium oxide electrode was measured in 1 M sodium sulfate electrolyte and provided in **Figure 3**. The cyclic voltammogram provided in **Figure 3(A)** exhibits the existence of the quasi-rectangular behavior suggesting the pseudocapacitive nature of charge-storage in the blue titanium oxide electrode [52, 53]. The indication of the pseudocapacitance nature in blue titanium oxide due to ion-intercalation/de-intercalation phenomenon in addition to the electric double layer/surface capacitance, we quantify the contribution on the overall capacitance using Dunn's method [54]. The plot ofslope of log (i) versus log (ν) obtained from the power law is provided in **Figure 3(B)** [55, 56]:

$$\dot{\mathbf{a}} = \mathbf{a} \,\nu^b \tag{1}$$

where, *"v"*,*"a"* and *"b"* are scan rate (mV s−1), adjustable parameters, respectively. The value of *"b"* is calculated from the slope of log i versus log ν. From the obtained b-value, the nature of the capacitance is evaluated as when b value is 0.5, reveals the diffusion mediated ion-intercalation/de-intercalation whereas the b is 1 represents the surface capacitive nature in the electrode. The contribution of the capacitance related to the surface and diffusion process can be quantified using the relation [54]:

$$\mathbf{I}\{\mathbf{V}\} = \mathbf{k}\_1 \boldsymbol{\nu} + \mathbf{k}\_2 \boldsymbol{\nu}^{1/2} \tag{2}$$

Were "k1 ν" and "k2 ν 1/2" related to the contributions from surface- and diffusion-mediated intercalation/de-intercalation process, respectively. The slope and intercept of the plot between I(V)/v1/2 and v1/2, provides the value of k1 and k2 to determine the capacitance contributions [54]. The overall charge stored and their contribution related to the diffusion and surface is calculated using the CV curve in **Figure 3(C)** and the contribution plot is provided in **Figure 3(D)**. From the **Figure 3(D)**, the diffusion capacitance is increasing from 42.10 to 77.29% when the scan rate is decreased from 50 to 5 mV s−1. The diffusion capacitance is higher at the low scan rate which is due to the fact that at high scan rates, the electrolyte ions faces time constraints which limits the diffusion of electrolyte ions whereas low scan rates provide sufficient time for the electrolyte ions to diffuse into the interior surface of the blue titanium oxide electrode [57]. The charge discharge profile shows sloppy symmetric type curves which is evident from **Figure 3(E)** and delivered a high gravimetric specific capacitance of 19 F g-1 and areal capacitance of 19 mF cm−2 obtained at a constant current of 0.5 mA as seen in **Figure 3(F)**. The excellent electrochemical performance of the blue titanium oxide in the neutral electrolyte is due to the reason such as sheetlike structure, enhanced electrical conductivity with more oxygen vacant sites in blue titanium oxide.

*Energy Storage Properties of Topochemically Synthesized Blue TiO2 Nanostructures in Aqueous… DOI: http://dx.doi.org/10.5772/intechopen.102186*

#### **Figure 3.**

*Three electrode characterization of the blue titanium oxide electrode (A) cyclic voltammetry, (B) plot of log (I) vs. scan rate. The inset in (B) shows the plot of b-values vs. potential. (C) CV profile of blue titanium oxide electrode with the shaded part revealing the capacitive current. (D) Capacitance contribution plot. (E) Galvanostatic charge discharge analysis and the (F) plot of specific capacitance of the blue titanium oxide electrode.*

#### **4.3 Full-cell characterization of blue titanium oxide symmetric supercapacitor**

In the view point of the practical application, a symmetric supercapacitor is fabricated using blue titanium oxide as electrode and organic liquid (TEABF4) is used as electrolyte. The electrochemical performance of the fabricated blue titanium oxide SSC was tested for the electrochemical stability of the operating potential window by measuring the CV profiles over the increasing voltage from 0 to 3 3.0 V at a scan rate of 100 mV s−1 as shown in **Figure 4**, which shows the distorted rectangular shaped curves and also suggested that the blue titanium oxide SSC device can operate at a window of 3.0 V without any sign of evolution. The cyclic voltammogram of the fabricated SSC displayed typical rectangular shaped curves for all the CV scans at different rates ranging from 5 to 1000 mV s−1 (**Figure 5(A** and **B)**) suggesting the pseudocapacitive nature of charge-storage in blue titanium oxide via intercalation/de-intercalation phenomenon [58, 59]. The plot of the scan rate verses specific capacitance is provided in **Figure 5(C)**, depicts a high capacitance of 6.67 F g−1 measured at a scan rate of 5 mV s−1. The EIS analysis of the blue titanium oxide SSC is provided in **Figure 5(D**–**F)** and represented in the form of Nyquist and Bode plots [60]. The high- and mid- frequency region can be used to determine the solution resistance (Rs) or equivalent series resistance (ESR) whereas the low- frequency region directly related to the frequency dependent ion diffusion kinetics of electrolyte ions to the electrode [19]. The values of Rs and Rct from the Nyquist plot are determined to be 1.60 and 15.4 Ω, respectively. The presence of the Warburg element in the model circuit related to the frequency dependent ion diffusion kinetics of electrolyte ions to the electroactive surface as evident from **Figure 5(D)** [61]. **Figure 5(E)** shows the Bode phase angle plot of blue titanium oxide SSCs which tails the phase angle at the low- frequency region (0.01 Hz) is about 64.94°, thus, suggesting the pseudocapacitive nature of the blue titanium oxide SSCs [33]. **Figure 5(F)** presents the dependence of specific capacitance as a function of applied frequency which is about 3.24 F g−1 for the blue titanium oxide SSCs, and the capacitance decreases with respect to an increase in frequency [62].

#### **Figure 4.**

*CV profile ofblue titanium oxide symmetric supercapacitor measured with increasing in voltage window.*

**Figure 5.**

*Full cell characterization of the blue titanium oxide SSC. (A and B) CV profiles and (C) plot of specific capacitance of blue titanium oxide SSC. (D) Nyquist plot and the enlarged portion is provided in the inset; (E) bode phase angle plot and (F) effect of frequency on capacitance blue titanium oxide SSC.*

The charge–discharge profiles of the fabricated SSC measured using various constant currents (0.5 to 10 mA) is provided in **Figure 6(A)**. The profiles indicate the existence of the quasi symmetric profiles suggesting the pseudocapacitive behavior of the blue titanium oxide SSCs [63] and the dependance of current on the capacitance is given in **Figure 6(B)**. A high specific capacitance of the 4.80 F g−1 is obtained from the GCD profile measured at a constant current of 0.5 mA. The rate capability of the blue titanium oxide SSC device is provided in **Figure 6(C)** which indicating the better capacitance retention when switched from low- to high- current suggesting superior rate capability [19]. The energy and power density plot is represented in the form of Ragone plot (**Figure 6(D)**) which holds a high-energydensity of 6.0 Wh kg−1 with a power of about 750 W kg−1, respectively. The long tern cyclic life of the blue titanium oxide SSC depicts the capacitance retention of about 90.2% after 10,000 cycles (**Figure 6(E)**), highlighting the better cycle

*Energy Storage Properties of Topochemically Synthesized Blue TiO2 Nanostructures in Aqueous… DOI: http://dx.doi.org/10.5772/intechopen.102186*

#### **Figure 6.**

*Full cell characterization of the blue titanium oxide SSC. (A) CD profiles; (B) plot of specific capacitance; (C) rate capability; (D) Ragone plot; (E) cycling stability; the inset in (E) shows the Nyquist plot before and after 10000 cycles of blue titanium oxide SSC device. (F) Real time application of the blue titanium oxide SSC device.*

life [25]. The impedance analysis after stability shows the increase in the ESR and charge transfer resistance such as 1.60 to 4.17 Ω and 15.4 to 24.4 Ω. **Figure 6(F)** shows the practical applications of the blue titanium oxide SSC glowing a blue lamp over 10 seconds after charged upto 3 V using a constant current of 0.5 mA.

#### **5. Conclusion**

In this book chapter we have demonstrated the preparation of the blue titanium oxide via topochemical reduction of titanium boride in hydrofluoric acid via hydrothermal process. In this we have discussed the formation of 2D layered titanium oxide with oxygen deficiency and the utilization in the energy storage sector. The electrochemical energy storage properties of the blue titanium oxide is tested via symmetric configuration type supercapacitor operated over a wide potential range of 3.0 V with good electrochemical stability and higher rate. The prepared blue titanium oxide shows higher output than those of other 2D electrode materials-based supercapacitors. From overall studies, the prepared materials shows better electrochemical performance which paves the way as a promising electrode material for next-generation smart energy storage sectors.

#### **Acknowledgements**

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2019R1A2C3009747, 2020R1A2C2007366, 2021R1I1A1A01049635 & 2021R1A4A2000934).

#### **Conflict of interest**

"The authors declare no conflict of interest."
