*3.1. Synthesis and Characterization of TiO2 Nanosheets*

Hierarchical TiO2 nanosheets composed of anatase and bronze nanocrystallites were synthesized through a solvothermal method, followed by controlled heat treatment. Ethylene glycol was used as solvent to facilitate nanosheet formation through the interaction of OH groups of glycol with Ti-OH groups [48,49]. This resulted in the free energy change of the TiO2 crystallographic planes and an associated anisotropic crystal growth [29,49]. Selection of a less reactive precursor TiCl3 was aimed at controlled hydrolysis and condensation, which is necessary for the nanosheet formation [50]. The uniform size/shape distributions and ultrathin nature of the sheets were also assisted by the high viscosity of ethylene glycol [9]. Reaction between TiCl3 and ethylene glycol resulted in the formation of Ti-glycolate complex during the hydrothermal reaction. Further hydrolysis and condensation formed TiO2 bronze nanosheets, which was transformed to anatase-bronze hybrid nanosheets upon controlled heat treatment. Scanning electron microscopy (SEM) images of the TiO2 nanosheets demonstrated their 2D morphology and (Figure 2a) hierarchical flower like microstructure, composed of numerous petals. High-resolution transmission electron microscopy (TEM) images (Figure 2b,c) of individual nanosheets verified the existence of nanograins of 7 ± 2 nm size. Lattice spacings of 0.34 and 0.62 nm represent the (101) and (110) planes of anatase and bronze phases, respectively. The presence of well-defined anatase-bronze nanograin boundaries is also evident from these images [9]. This was anticipated, due to the mismatch between anatase and bronze crystal structures. Nanograin boundaries present in metal oxides often result in unique physical/ chemical properties, including excellent catalytic and electrochemical performance [51]. Nanointerfaces present in these hierarchical TiO2 nanosheets can also act as additional sites for Na-ion storage. Our previous study confirmed the advantages of such nanointerfaces for enhancing pseudocapacitive type Na-ion storage [9].

**Figure 2.** (**a**) SEM image, (**b**,**c**) TEM images, (**d**) X-ray diffraction pattern, (**e**) Raman spectrum, (**f**) N2 adsorption–desorption isotherm, and pore size distribution (inset) of TiO2 nanosheets.

The X-ray diffraction pattern of the TiO2 nanosheets (Figure 2d) also confirmed the coexistence of anatase and bronze polymorphs in the sample. Peaks designated as (101), (004), (200), (105), (211), (204) corresponded to anatase phase (JCPDS 21–1272). Whereas, (001), (110), (002), (310), (103), (003), and (204) signals represented bronze phase (JCPDS 46-1237) [9]. The anatase and bronze content, calculated from the relative intensity of the (310) bronze peak and (004) anatase peak, was found to be 83% and 17%, respectively. Particle size calculation, using a Debye–Scherrer equation also revealed the existence of

7 ± 2 nm sized crystallites, which was in good agreement with the TEM results. A more surface sensitive technique, Raman spectroscopy was used to further confirm the phase purity and uniform nanoscale distribution of the anatase and bronze polymorphs (Figure 2e). Raman active modes were 144, 197, 399, 514, and 639 cm−<sup>1</sup> for anatase, and 123, 145, 161, 172, 196, 201, and 259 cm−<sup>1</sup> for bronze polymorphs [52]. These results and the quantification of anatase/bronze content from high-resolution Raman spectra (Figure S1) were also in line with the XRD results, confirming the coexistence and nanoscale distribution of anatase and bronze nanocrystallites [52]. The textural property investigation of TiO2 nanosheets was performed through N2 adsorption–desorption analysis (Figure 2f). This active material exhibited a high surface area of 106 m2/g. Type IV isotherms and H3 type hysteresis, characteristic of a mesoporous structure were identified in this case [13]. Mesoporosity was also evidenced by the higher steepness of the isotherm at high relative pressure (P/P0 = 0.4–1.0) [13]. Barrett-Joyner-Halenda (BJH) method pore size distribution measurements (Figure 2f inset) further confirmed the mesoporosity of the TiO2 nanosheets. Such high surface area and mesoporosity of the active material are advantageous for improved Na-ion storage due to the superior contact with the electrolyte solution, and the possibility of pore/defect assisted pseudocapacitive type ion storage [49]. Thus, it can be summarized that controlled hydrolysis and condensation of TiCl3 in ethanol water mixture followed by calcination resulted in the formation of mesoporous hierarchical anatase–bronze hybrid TiO2 nanosheets.
