Tuning chemical composition and structural properties of bismuth ferrite based thin films by reactive bipolar HiPIMS

Chromium doped bismuth ferrite (BFCO) based thin films with controllable chemical composition and structural properties were grown by bipolar HiPIMS in Ar:O-2 mixt gas environment. The energy, amount and type of the accelerated ions were adjusted by varying the positive discharge pulse amplitude, duration and delay from the initial negative pulse. Bi/(Fe + Cr) compositional ratio obtained from EDX decreased as the positive pulse amplitude and duration were augmented and the delay between negative and positive pulses was diminished. XRD measurements revealed the presence of bismuth oxide secondary phases for the films exhibiting Bi over-stoichiometry. The diffraction pattern of the sample with Bi/(Fe + Cr) of 1 showed peaks corresponding only to the BiFeO3 hexagonal structure. This film presented a lower piezoresponse (d(33) = 40 p.m./V) than the BFCO sample obtained by monopolar HiPIMS where no positive discharge pulse was used and for which the EDX results revealed a Bi/(Fe + Cr) of 2.1 (d(33) = 60 p.m./V). The photo-piezocatalytic properties of both these BFCO coatings for MB dye degradation were studied. The results revealed that BFCO layer deposited on FTO substrate by monopolar HiPIMS (Bi/(Fe + Cr) = 2.1) under ultrasonic vibrations exhibited a remarkable piezocatalytic performance, with 75 % degradation of MB dye within 120 min (reaction rate constant k value 0.0124 min(-1)). Additionally, when both visible light and mechanical oscillations were used simultaneously, the degradation efficiency was further increased up to 80 %, with k value seven-fold higher than that of photocatalysis. The proposed approach provides an environmentally friendly and innovative way to synthesize high efficiency piezophotocatalyst for wastewater treatment and energy applications.