Novel graphitic carbon nitride (g-C$_3$N$_4$)/potassium bismuth ferrite (KBiFe$_2$O$_5$) composite photocatalysts were synthesized using facile grinding and calcination method. X-ray diffraction, transmission electron microscopy, scanning electron microscopy and UV–visible diffuse reflectance spectroscopy analysis were carried out to investigate structural,morphological and optical properties of the prepared samples. The photocatalytic properties of the samples were studied by photocatalytic degradation of methylene blue under visible light irradiation. The composite showed enhanced photocatalytic efficiency compared to both pristine g-C$_3$N$_4$ and KBiFe$_2$O$_5$. Out of four composites with different weight percentagesof KBiFe$_2$O$_5$, one with 30 wt% showed the maximum photocatalytic efficiency. Samples with a higher content of KBiFe$_2$O$_5$ showed decreased photocatalytic efficiency indicating 30 wt% as the optimum composition. The increase in the photocatalytic efficiency is mainly due to efficient charge separation of photo-generated electron–hole pairs in the composites.The possible mechanism for the photo-catalysis of g-C$_3$N$_4$/KBiFe$_2$O$_5$ composites was also proposed.

Polycrystalline samples of potassium bismuth ferrite (KBiFe$_2$O$_5$) were synthesized using sol–gel technique employing different complexing agents, like citric acid, maleic acid, sucrose and tartaric acid. Investigation onthe influence of different complexing agents on the magnetic, optical and photocatalytic properties of KBiFe$_2$O$_5$ (KBFO) were carried out. X-ray diffraction studies revealed that the pure phase KBiFe2O5 was formed at thecalcination temperature of 600$^{\circ}$C for complexing agent, sucrose and tartaric acid, whereas in case of the sample prepared using citric acid and maleic acid as complexing agent some impurity peaks were observed. The samplesprepared using different complexing agents at 700$^{\circ}$C gave pure KBFO polycrystalline samples. So 600–700$^{\circ}$C is the suitable calcination temperature for the formation of single-phase KBFO. These results are also confirmed from the thermo-gravimetric and differential scanning calorimetry analysis carried out on the KBFO precursors. It was also observed that the complexing agent has significant influence on the magnetic properties of the material. Maximum induced magnetization of 0.605 emu g$^{-1}$ has been observed for the sample prepared using tartaric acid at the calcination temperature of 600$^{\circ}$C and also magnetization saturated for the maximum applied field of 15 kOe, whereas sample prepared with sucrose as the complexing agent under similar condition did not show saturation in the magnetization and gave magnetization value of 0.096 emu g$^{-1}$ for the applied filed of 15 kOe. This difference is attributed to the particle size difference obtained with different complexing agents. For the singlephase KBFO samples prepared using citric acid, maleic acid, sucrose and tartaric acid at calcination temperature of 700$^{\circ}$C, the magnetization values are 0.08, 0.0561, 0.044 and 0.0471 emu g$^{-1}$, respectively. Optical properties of the samples were studied using diffuse reflectance spectroscopy. All the samples exhibited strong absorption in the visible region of the solar spectrum and featured a bandgap of 1.75 eV. Evaluation of the visible lightinduced photocatalytic activity of pure-phase KBFO was also carried out by degradation of methylene blue as a model pollutant.