Effect of pH, Surfactant, and Temperature on Mixed-Phase Structure and Band Gap Properties of BiNbO₄ Nanoparticles Prepared Using Different Routes

α-and β-BiNbO₄ nanoparticles were successfully prepared using three different routes, namely co-precipitation, hydrothermal, and citrate precursor methods. Structural characterization has been analyzed using powder X-ray diffraction where results confirmed the mixed-phase nature of the prepared powders. When co-precipitation method is applied, BiNbO₄ prepared at pH 2 and calcined at 750^◦ C shows predominately the α-BiNbO₄ owning the narrowest band gap of 2.86 eV, whereas an increase in the pH value resulted in the β-BiNbO₄ phase being the predominant phase. On the other hand, samples prepared using citrate precursor method show co-existence of both α-and β-BiNbO₄ at all pH values under study; however, at pH 10 the β-BiNbO₄ was the dominant phase exhibiting a higher bandgap energy value of 3.16 eV. When applying the hydrothermal approach, BiNbO₄ prepared at all pH ranges of the present study show a band gap >3.2 eV indicating their activity in the ultraviolet region of the spectrum. Moreover, increasing calcination temperature to 900^◦ C, a gradual transformation of α-to β was observed. Lastly, it was noticed that as the lattice volume increases, the band gap of prepared BiNbO₄ decreases.