In 2023, the world is facing problems such as global warming, hazardous chemicals, non-destructible plasticizers, similar to viruses. All of these issues are major concerns for our environment. To address these issues, there is a strong need for a multifunctional material that serves as a catalyst, which is stable, efficient, selective, and reusable. The conventional catalyst based on photoinduced charge carriers is insufficient. The intrinsic properties of a material, such as ferroelectricity, piezoelectricity, and magnetism, can play a role in the catalysis process and enhance the catalyst's activity by driving forces such as spontaneous polarization, piezoelectric coefficient, and ferromagnetism at room temperature.

These qualities are found and cherished in the classic ceramic semiconductor Bismuth Ferrite (BiFeO₃; BFO), commonly known as the "The drosophila of multiferroics." BFO exhibits ferroic orderings at room temperature. For catalytic purposes, a large surface area is required to have a high number of active sites. Therefore, nanosized BFO particles are chosen. Although BFO nanoparticles (NPs) absorb visible light, they have smaller ferroelectric polarization than bulk BFO due to reduced crystal asymmetry, and they also possess weak ferromagnetic properties. To achieve a larger surface area while maintaining increased ferroelectric and magnetic properties, ionic engineering at Bi and Fe sites is utilized. The dopants not only further reduce the particle size of BFO NPs but also influence the shape of NPs, resulting in different morphologies. The macroscopic and local crystal structure of doped BFO NPs is examined using several techniques. The alteration in Bi-O and Fe-O bond parameters and the increase in microstrain of the BFO crystal structure due to doping are major factors responsible for altered ferroic properties.

Doping magnetic ions at the Fe site of BFO NPs leads to an increase in total magnetization mainly through superexchange interaction, disrupting the spin cycloid. Further co-doping at the Bi site results in even higher magnetization due to changes in the Fe-O-Fe bond angle and the presence of uncompensated spins on the surface of small-sized NPs. Doping also influences the coercive field and anisotropy of the NPs, making them suitable for magnetic hyperthermia. Doped BFO NPs exhibit efficiency in converting magnetic energy into heat when exposed to an alternating magnetic field, showing promise for applications in magnetic hyperthermia for cancer treatment. The local ferroelectricity and piezoelectric response in doped BFO NPs are found to be higher than in pristine BFO NPs due to crystal structure distortion caused by asymmetrical Bi-O bonds in the BiO₁₂ cuboctahedron and resulting alterations in the FeO6 octahedron. Doping Mn at the Fe site does not significantly influence the ferroelectricity of BFO NPs. However, further doping of divalent cations (Ca²⁺ or Ba²⁺) at the Bi site in BiFe_0.95Mn_0.05O₃ (BFM) NPs reduces the local ferroelectricity, while doping of trivalent (Dy³⁺) and monovalent (Ag⁺) cations into BFM NPs increases the ferroelectric properties of BFM NPs approximately three times.

This study finds that single doping at the Fe site has a substantial influence on the band gap compared to doping at the Bi site, primarily due to the indirect effect on the electronic structure of BFO NPs. The band gap of BFO, which is 2.2 eV, can be reduced to 1.6 eV with the appropriate ratio of Ba and Mn co-doping. This provides the maximum utilization of the solar spectrum to generate photoinduced charge carriers for catalytic purposes. Dopants at the Bi site have a slight impact on the light absorption properties of the BFO NPs, creating oxygen vacancies or defect states in the band gap.

The catalytic activity of the doped BFO NPs for the degradation of organic pollutants under visible and UV light illumination is compared. Ba-doped BFM NPs exhibit enhanced photocatalytic efficiency compared to pristine BFM and BFO NPs. 1 mol% Ba-doped BFM NPs degrade rhodamine B and methyl orange dyes within 60 and 25 minutes under UV and visible illumination, respectively. The increased photocatalytic efficiency in Ba-doped BFM NPs is attributed to a cooperative effect of factors such as increased light absorption ability, large surface area, active surface, reduced recombination of charge carriers, and spontaneous polarization to mitigate photoinduced charge carrier recombination. In piezo-photocatalysis, Dy-BFM and Ag-BFM NPs show the best photocatalytic activity under ultrasonication conditions. The increase in spontaneous polarization by mono- and trivalent doping is one of the major factors in enhancing the photocatalytic performance, along with stronger light absorption in the visible range, low recombination rate of charge carriers, and larger surface area of NPs.

Dibutyl phthalate (DBP) is notorious as an endocrine disruptor, making it a significant threat to both human health and the environment. Successful photodegradation of DBP is achieved under UV light irradiation within 2.5 hours using Ag-doped BFM NPs supported on graphene oxide (GO) nanosheets. Gas chromatography and mass spectrometry (GCMS) show that Ag-BFM NPs supported on GO have a higher photodegradation efficiency than the pristine free-standing BFO NPs alone. The photodegradation process of DBP generates various intermediate products, such as phthalic acid, benzoic acid, benzaldehyde, and 3-methyl butyric acid, before achieving complete mineralization into carbon dioxide and water.

The investigation delves into the functionality exhibited by doped BFM NPs concerning the hydrogen evolution reaction (HER). The electrocatalytic activity of BFM NPs undergoes a transformative shift as a consequence of mono-, di-, and trivalent cation substitutions. Notably, strategic engineering of doping at the Bi site within BFM NPs yields a remarkable outcome, namely the reduction of the kinetic overpotential prerequisite for HER. This diminished overpotential in doped BFM NPs arises from the confluence of multifarious factors: diminished charge transfer resistance, augmented specific surface area, a discernible distribution of pore sizes ranging from narrow to broad, particles endowed with a shape boasting abundant active facets, and the integration of dopants as novel active sites upon the surface. Furthermore, the presence of surface defects, oxygen vacancies, and amplified microstrain within doped BFM NPs contributes to the reduced overpotential.

This study underscores the successful synthesis of phase-pure doped and undoped BFO NPs with diverse morphologies using different methods and the profound impact of doping on their magnetic, electrical, optical, and catalytic properties. The findings highlight the tremendous multifunctional potential of doped BFO NPs in a wide range of applications. These doped BFM NPs encapsulate immense potential to revolutionize the realm of HER in the photoelectrochemical domain, owing to their profound light absorption capabilities and aptitude for catalysis.