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

alpha- and beta-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 degrees C shows predominately the alpha-BiNbO4 owning the narrowest band gap of 2.86 eV, whereas an increase in the pH value resulted in the beta-BiNbO4 phase being the predominant phase. On the other hand, samples prepared using citrate precursor method show co-existence of both alpha- and beta-BiNbO4 at all pH values under study; however, at pH 10 the beta-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 degrees C, a gradual transformation of alpha- to beta was observed. Lastly, it was noticed that as the lattice volume increases, the band gap of prepared BiNbO4 decreases.