*3.4.2 TEM analysis*

The bright field TEM images of MTO nanoparticles milled for 20 and 35 hrs are shown in **Figure 7(a)** and **(b)** respectively. From TEM images it was evident that the powders milled for 20 hrs does not exhibit that much distinct particles (clustering of MTO nanoparticles), but for 35 hrs milled powders a clear nanosized *Synthesis of Nano-Composites Mg2TiO4 Powders via Mechanical Alloying Method… DOI: http://dx.doi.org/10.5772/intechopen.94275*

**Figure 7.** *Bright field-TEM micrographs of (a) 20 and (b) 35 hrs milled MTO powder.*

particle is observed. The size of the parent oxides is about 2 μm. As the milling time increases the size of the initial particles decreases and for 35 hrs milled nanocrystalline MTO powders the average particle size is found to be around 60–120 nm. This crystallite size is nearly consistent with the calculated data by Williamson-Hall plot method.

The selected area electron diffraction (SAED) pattern of the 35 hrs milled powders is shown in **Figure 8(a)**. The SAED ring pattern indicates that the phase of MTO- nanoparticles was polycrystalline in structure and the distance between crystalline planes (i.e., inter planer spacing or *d*-spacing) was consistent with the standard pattern for a spinel MTO crystal structure. To analyze the lattice fringes for the 35 hrs milled powders, high resolution- TEM (HR-TEM) study was carried out, (as shown in **Figure 8(b)**). However, the clear lattice fringes showed that each particle has single crystalline structure. For 35 hrs milled powders, the distance between crystalline planes is evaluated and is found to 2.968 Å, which indicates the preferable crystal growth plane is (220). Thus, it could be concluded that during the high energy mechanical alloying a solid-state reaction between MgO and TiO2 took place at room temperature.

#### **Figure 8.**

*(a) The selected area electron diffraction (SAED) pattern and (b) HR-TEM for 35 hrs milled powder (Adapted with permission from Bhuyan et al., 2020, @ Springer [19]).*

**Figure 9.**

*Room temperature UV – Visible spectra of pure and milled MTO powders, inset: Variation of bandgap with milling time (adapted with permission from Bhuyan et al., 2020, @ Springer [19]).*

## *3.4.3 Optical properties*

Room temperature UV – Visible spectra were taken for all the as-prepared samples in order to see the effect of milling duration on the optical band gap of the mechanically alloyed MTO nano-powders, and are illustrated in **Figure 9**. A strong absorption peak at around 356 nm is observed for un-milled MTO powders, while with increase in milling duration, the peak slightly shifted to 352 nm for 35 hrs milled powders. It shows that there is a clear sign of blue-shift in the absorption peak with decrease in average crystallite size. This indicates that with decrease in particle size the band-gap increases. However, the enhanced absorption in mechanically alloyed MTO nanoparticles can be attributed to a large surface to volume ratio and enhanced oscillator strength with decrease in average particle size.

Tauc relation [40], is employed to estimate the optical band gap of all the milled samples. According to this relation, *αhυ = β (hν –E*g*) n ,* where, *hυ* is the photon energy*, β* is a constant which measures the crystalline order of the samples and *n* = 1/2 for direct bandgap structure (As MTO belongs to cubic structures and exhibiting direct band gap). The variation of bandgap with milling time is plotted and shown in inset of **Figure 9**. The plot indicates that with increasing milling time from 0 to 35 hrs the bandgap enhanced from 3.26 eV - 3.78 eV. This result is consistent with the previously reported results [40–42]. The optical bandgap (*E*g) of all the milled powders are determined by the extrapolation of the best linear fit between (*αhυ)*<sup>2</sup> and *hυ* to intercept the *hυ* axis (α = 0), (taken along x-axis). The figure shows the dependence of the absorption coefficients (*αhυ)*<sup>2</sup> with photon energy. The position and slope of the optical absorption edge makes this material as a suitable UV light absorber.

*Synthesis of Nano-Composites Mg2TiO4 Powders via Mechanical Alloying Method… DOI: http://dx.doi.org/10.5772/intechopen.94275*
