Band engineering via grain boundary defect states for large scale tuning of photoconductivity in Bi1−<i>x</i>Ca<i>x</i>Fe1−<i>y</i>Ti<i>y</i>O3−δ

Spark plasma sintered Bi1−xCaxFe1−yTiyO3−δ (BCFTO) (x = y = 0.05 and 0.1) nanoparticle ceramics are studied for photoconductivity properties. As-prepared (AP) BCFTO hosts a large concentration of grain boundary (GB) oxygen vacancies (OV), whereas air annealed (AA) BCFTO have significantly suppressed GB OV. X-ray absorption near edge spectroscopy study confirms that Fe and Ti remain in 3+ and 4+ oxidation states, respectively. Thus, lattice OV created when only Ca2+ is substituted in BiFeO3 are charge compensated in Ca and Ti codoped BiFeO3. This ascertains that BCFTO is devoid of lattice OV. Photoconductivity studies show four orders of more photocurrent arising from GB OV contributions in BCFTO-AP compared to that in BCFTO-AA samples. A large increase in the activation energy for the AA samples (0.4 eV to 1.6 eV) compared to that for the AP samples (0.06 eV to 0.5 eV) is obtained from ln ω vs 1/T Arrhenius plots. This further substantiates the suppression of GB OV resulting in poor photoconductivity. Diffuse band edges observed in Kubelka-Munk plots of BCFTO-AP samples are a consequence of OV defect states occupying the bulk bandgap. In the absence of OV defect states, band edge becomes sharper. Density functional theory (DFT) calculations further support the experimental observations. DFT study shows that the presence of Ca and Ti does not enhance the photocurrent as these codopants do not produce mid-bandgap states. The mid-bandgap defect states are attributed only to the unsaturated bonds and OV at the GB in BCFTO. These studies manifest a critical role of OV residing at the GB in tuning the photoconductivity and, hence, the photoresponse of BCFTO.

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