# Facile Synthesis of Highly Conductive Vanadium-Doped NiO Film for Transparent Conductive Oxide
## 3 Results
### Study of Material Properties

Table 1: Measured values of structural parameters for NiO and V:NiO. Figure 1: (a) XRD spectra of pristine and 3 at.%V:NiO nanoparticles (NPs) and (b) enlarged XRD spectra of pristine NiO and 3 at.%V:NiO NPs. Figure 2: (a) FTIR spectra of NiO and 3 at.%V:NiO with precursor materials, (b) EDS spectra of 3 at.%V:NiO NPs, and (c) EDS elemental mapping of the corresponding elements of Ni, O, and V (scale bar, 2 \(\upmu\)m). The Raman spectra of NiO and 3 at.%V:NiO are given in Figure 3a. From the analysis, four characteristic Raman peaks were positioned in both samples, corresponding to the shaking peaks of NiO. The peaks at 324 and 479 cm\({}^{-1}\) are the first-order transverse optical (TO) and longitudinal optical (LO) phonon modes of NiO NPs, respectively. The peaks at 546 and 686 cm\({}^{-1}\) are because of the combination of 2 TO and 2 LO. Meanwhile, an extra peak observed at 760 cm\({}^{-1}\) can be associated with the V-O stretching mode in the 3 at.%V:NiO sample. This result shows that vanadium can be incorporated with oxygen. Figure 3b, c depicts the results of ultraviolet-visible spectroscopy studies of ethanol-dispersed pristine and doped NiO samples. A gradual red shift in the absorption spectra with increasing V concentration is observed from Figure 3b. This shift is consistent with the incorporation of V\({}^{5+}\) into the NiO lattice. This indicates that the band gap energy in 3 at.%V:NiO is lower than that in pristine NiO. Meanwhile, the optical band gap estimated for pristine NiO NPs by Tauc plot (Figure 3c) was around 3.65 eV, which is well matched with the previously reported values [2; 3]. In NiO NPs doped with different percentages of V, it was clear that after introducing vanadium content into the NiO crystal lattice, the optical band gap was slightly reduced to 3.62 eV for 1.5 at.%V:NiO and 3.59 eV for 3 at.%V:NiO. It is important to study the optical transmission spectra of pristine NiO and V:NiO films, shown in Figure 3d. Both NiO and V:NiO films coated on an FTO substrate still exhibited high transmission (>80%) in the visible region. A small variation in transmission was observed for both 1.5 and 3 at.%V:NiO films when compared to that for the pristine NiO. However, we also noticed that the optical transmittance decreased with increasing V doping percentage. NiO film with a higher doping percentage (5 at.%V:NiO) exhibited low transmission as compared to the pristine NiO film, while  
Figure 3: (**a**) Raman spectra of NiO and 3 at.%V:NiO NP powder samples; (**b**) UV-absorption spectra of diluted NiO and V:NiO NP solutions with different atomic percentages of V; (**c**) Corresponding Tauc plots of the absorption spectra; (**d**) Optical transmission spectra of NiO and V:NiO films with different atomic percentages of V.  
V:NiO film with 3 at.% V doping content exhibited transmittance higher than 80% for the visible range. Therefore, the optimized V content of 3 at.% for NiO NPs was used for further studies. FESEM micrographs with higher-resolution images of pristine NiO and 3 at.%V:NiO powder samples are shown in Figure 4a,b. The samples exhibit similar morphology with ultra-small-sized spherical structures that are agglomerated. TEM images showing the same sample morphology are displayed in Figure 4c,d. The particle diameters are summarized in Figure 4g. The statistical distributions of the nanoparticle diameters of pristine NiO and 3 at.%V:NiO were measured to be \(\sim\)3.5 \(\pm\) 0.5 nm and \(\sim\)2.5 \(\pm\) 0.5 nm, respectively. This result was consistent with the result of XRD. The particle size variations in the TEM analysis of both samples may be due to the influence of large Ni ions (ionic radius, 0.83 A\({}^{\circ}\)) replaced by small V ions (ionic radius, 0.72 A\({}^{\circ}\)). Meanwhile, the HRTEM results revealed that the interplanar distance of both samples in Figure 4e,f was 0.21 nm. Figure 4: (a) FESEM images of **(a)** NiO and **(b)** 3 at.%V:NiO and TEM patterns of **(c)** NiO and **(d)** 3 at.%V:NiO.