3.1. Surface morphology of La2O3 nanocrystals
The micrographs of La2O3, captured from a scanning electron microscope (Fig. 2. (a-b)) depicts the rod-like morphology. The synthesized La2O3 nanocrystals are uniform in size and shape, showing its homogeneous formation during the synthesis. The micrographs at different locations present the uniform morphology with dimensions varying in few hundreds of nanometers.
The diameter of the La2O3 nanocrystals is in the range of 5 to 30 nm and length is 100 to 300 nm (Fig. 2.(a & b)). Fig. 2c shows the TEM micrograph of the La2O3 nanocrystals, corresponding high-resolution image (Fig. 2d) and SAED ring pattern (Fig. 2e). The TEM analysis shows the crystalline structure of La2O3. The interplanar spacing is 0.334 nm (from Fig. 2d). The ring pattern with intense spot in Fig. 2e, the La2O3 nanocrystals showed the intense diffraction spots suggesting the particles formed.
3.2. Crystal structure of La2O3 using XRD
The XRD of hydrothermally synthesized La2O3 nanocrystals is shown in Fig. 3. The synthesized La2O3 nanocrystals are of high purity and pattern indexed with hexagonal phase (space group P-3m1, ICDD No. 83-1344) .
The sharp diffraction peaks at respective Bragg angles indicate that the high crystallinity achieved at considerably low-temperatures. Thus, both morphological and structural analysis concludes the quality of the synthesized nanocrystals. Also, the broad peaks with large FWHM depict the nanocrystalline nature, which is in good agreement with high-resolution TEM studies presented in Fig. 2 (c-d).
3.3. Chemical composition of La2O3 nanocrystals using XPS
Further, the La2O3 sample is subjected for XPS study to examine the composition. All the binding energy data of La2O3 sample obtained from the XPS analysis is corrected according to the standard referencing C 1s peak (284.7 eV). From Fig. 4a, the XPS survey spectrum shows only the presence of two metal elements, lanthanum and oxygen. The survey also shows that there is no presence of other metal elements on the surface of La2O3 sample. The presence of minor C 1s peak (Fig. 4a) is due to the surface adsorbed carbon atoms/molecules during the hydrothermal synthesis. The binding energy at 833.6 and 850.1 eV are indexed to the presence of La 3d5/2 and La 3d3/2, respectively, as shown in Fig. 4b. The binding energy peak at 529.5 eV, in Fig. 4c, is indexed to the O2− in the La2O3 crystal. It is also seen that the O 1s profile is asymmetric indicating the presence of two oxygen species in the nearby region.
3.4. BET surface area analysis
Brunauer–Emmett–Teller (BET) nitrogen gas adsorption-desorption measurements are used to find out the specific surface area of the La2O3 nanocrystals. The isotherm shows that the particles are porous (Fig. 5). The specific surface area of La2O3 from the BET apparatus is measured to be 72.33 m2/g. The value is predominant compared to already published literatures   .
3.5. Identification of chemical bonding by FTIR
The FTIR spectrum is recorded to show the functional groups of the La2O3 nanocrystals (as shown in Fig. 6). The stretching vibration of O-H bond at 3427 cm-1 and the bending vibration of H-O-H absorption peak at 1631 cm-1 are due to the presence of moisture in La2O3 sample . The absorption bond at 3608 cm-1 is assigned to the presence of bond tension in hydroxyl groups of lanthanum oxide. Further, the bands at 1483 cm-1 and 1440 cm-1 are attributed to asymmetric stretching mode of the C-O bond . The absorption bands at 858 and 657 cm-1 are assigned to bending out of plane vibrations and La-O stretching vibration, respectively .
3.6. Output characteristics of triboelectric nanogenerators
To evaluate the maximum power generated by the device, TENG device is connected to an electrical load (resistor)s . The obtained voltage is as shown in the Fig. 7. The resistance value is swept from 0 to 50 MΩ. Respective voltage and current produced by the La2O3-TENG device are plotted against the external load resistance. The product of the same (i.e., voltage and current) gives the power value as is found to be maximum at the point where current and voltage intersect each other at 30 MΩ.
The current amplitude reduces with growing external load resistance owing to resistive loss, during which the voltage increases. Oscilloscope is used to record the voltage and current generated by La2O3-TENG device. The performance of the device is tested by tapping the TENG using the motorized fixture (Fig. 1). The phenomenon of chemisorptions on the surface of teflon and La2O3 film surface of molecular oxygen species results in resistivity changes of triboelectric material [29,30]. When Teflon and La2O3 nanorods film come into contact, spontaneous polarization occurs . This result in the dipole moments on teflon film and La2O3 surface and thus voltage generates. The open-circuit voltage and short circuit produced by the La2O3-TENG device is 120 V and 23.7 μA. The device yields a maximum power of 2.85 mW at an external load resistance of 30 MΩ (Fig. 7). The corresponding power density of the La2O3 TENG device is calculated to be 7.125 W/m2.