Improving the ferroelectric and photovoltaic properties of Nd-modified bismuth titanate ceramics via cobalt doping and oxygen vacancy regulation

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Original abstract

This study investigates effects of cobalt (Co) substitution and oxygen-vacancy manipulation on the ferrophotovoltaic efficacy of Nd-modified bismuth titanate ceramics, Bi 3.25 Nd 0.75 Ti 3−x Co x O 12 (BNdT-Cox). Cobalt ions were included at the titanium sites to alter the electronic structure and defect chemistry, while oxygen vacancies were systematically adjusted via post-annealing in an oxygen-rich environment for varying durations (0, 4, 8, and 12 h). Rietveld refinement indicated notable octahedral distortions, especially in the Ti(2)O 6 octahedra, implying a preferential incorporation of transition-metal ions and the existence of pronounced octahedral tilting and rotation. The structural alterations, along with the creation of oxygen vacancies, decreased the bandgap of pure BNdT from 3.22 eV to 1.88 eV; however, they also diminished the remanent polarization, so constraining the ferrophotovoltaic response. Subsequent oxygen annealing significantly alleviated this trade-off by elevating the bandgap from 1.88 eV to 2.6 eV, concurrently augmenting the remanent polarization from 0.5 to 8 µC/cm² and diminishing the oxygen-vacancy concentration from around 30% to 10%. The findings indicate that the synergistic management of transition-metal doping and oxygen-vacancy concentration constitutes an effective approach for engineering Aurivillius-type photoferroelectrics with elevated remanent polarization (P r ) and reduced bandgap (E g ), both of which are critical for enhancing ferrophotovoltaic energy-conversion efficiency. Enhancing the Ferrophotovoltaic Efficacy of Co-doped BNdT Ceramics. The figure illustrates the influence of oxygen oxidation on the bandgap, remanent polarization, and oxygen vacancy density within Nd-modified bismuth titanate (BIT) ceramics. Prior to oxidation (left), the material exhibits a substantial oxygen vacancy concentration (30%), a diminished remanent polarization (P r ), and a reduced bandgap (E g ). Conversely, following oxidation (right), the material’s bandgap expands from 1.88 eV to 2.6 eV, 2P r experiences a notable enhancement from 0.5 µC/cm² to 8 µC/cm², and the oxygen vacancy concentration diminishes from 30% to 10%. Consequently, these transformations culminate in an optimized ferrophotovoltaic material, thereby improving energy conversion efficiency.

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