Exploring the Role of Nitrogen Doping in Enhancing the Catalytic Performance of Bismuth Ferrite Nanoparticles
Bismuth ferrite (BFO) nanoparticles have emerged as a promising non-toxic catalyst with remarkable potential for the photodegradation of various environmental pollutants. Notably, this study achieved enhanced catalytic performance through anionic substitution, where replacing oxygen atoms with nitrogen introduces spin-polarized defect states within the BFO’s energy gap, resulting in a notable reduction in the energy band gap.
Nitrogen doping of bismuth ferrite yields a novel material with exceptional capabilities for the photodegradation of methylene blue dye and the reduction of 4-nitrophenol. Comprehensive characterization, including X-ray diffraction, Fourier-transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, has unequivocally confirmed the successful incorporation of nitrogen into the BFO nanoparticle lattice. Interestingly, field emission scanning electron microscopy analysis revealed no significant alteration in nanoparticle size after nitrogen doping.
Meanwhile, UV-diffuse reflectance spectroscopy unveiled a distinct decrease in the energy gap upon nitrogen incorporation. The observed improvements in catalytic activities can be attributed to nitrogen ions, introduced as substitutes, effectively occupying the oxygen defects within the sample, thereby diminishing recombination centers for photogenerated charge carriers and decreasing recombination rates.
Additionally, adsorption kinetics studies underscore the efficacy of the catalyst surface in adsorbing methylene blue and/or 4-nitrophenol, conforming to the Ho pseudo-second-order model. This study not only highlights the exciting potential of nitrogen-doped bismuth ferrite nanoparticles in environmental remediation but also sheds light on the intricate interplay between anionic substitution, band structure modification, and catalytic performance enhancement.
Multiferroic Materials: Unlocking the Potential of Bismuth Ferrite
Multiferroic materials have garnered significant attention, not just for their simultaneous display of ferromagnetism and ferroelectricity but also due to the remarkable magnetoelectric coupling they offer. In this intriguing realm of materials, where compounds like BiFeO3 (BFO), BiMnO3, and YMnO3 have been thoroughly explored, BFO emerges as a standout candidate with exceptional promise for diverse practical applications.
BFO exhibits a unique blend of room-temperature ferroelectric and antiferromagnetic orders, a narrow energy band gap, robust chemical stability, and cost-effectiveness. It has potential applications ranging from spintronics, memory devices, and data storage to ferroelectric random-access memory devices, sensors, digital recording, microwave and satellite communications, photovoltaics, and even the eco-friendly photodegradation of organic pollutants.
Although the magnetoelectric coupling in BFO may be relatively weak, it showcases substantially intensified multiferroic properties. BFO is a ferroelectric and an antiferromagnetic material with a Curie temperature Tc of 1103 K and a Neel temperature TN of 643 K. The structural arrangement of BFO is rhombohedrally distorted perovskite, adhering to the R3c space group, with lattice parameters of a = 5.58 Å and c = 13.9 Å.
The ferroelectric behavior in BFO is primarily attributed to the presence of Bi3+ ions, while its G-type antiferromagnetic characteristics are associated with Fe3+ ions. This antiferromagnetic spin structure exhibits a modulation period of 620 Å, resulting in a spiral modulated spin structure. At the nanoscale, BFO exhibits intriguing magnetic, electrical, and optical properties due to size effects that distinguish it from its bulk counterparts.
Harnessing the Photocatalytic Potential of Bismuth Ferrite Nanoparticles
Numerous investigations have highlighted the potential of BFO as a formidable material for the photodegradation of organic dyes. The narrow energy gap of BFO, typically ranging from 2.1 to 2.7 eV, enables its activation in the presence of visible light and leads to the generation of electron–hole pairs. Additionally, the inherent ferroelectric behavior of BFO, characterized by spontaneous polarization, induces band bending, directing electron–hole pairs in opposite directions.
These photo-generated holes play a pivotal role in oxidizing various organic pollutants, effectively transforming BFO into a photocatalyst under visible light conditions. Nevertheless, the rapid recombination of these photogenerated electron–hole pairs limits the catalytic efficacy of BFO in practical applications.
Various strategies have been explored thus far to enhance the effective separation of electron–hole pairs and augment the catalytic performance of BFO, including modifications to its structural parameters, such as particle size and surface morphology, elemental doping with both metals and non-metals, the creation of hetero and homo junctions with materials possessing low band gaps, and the introduction of noble metal dopants.
Addressing the Limitations of Metal-Doped Bismuth Ferrite
Doping BFO with rare earth and transition metals significantly enhances its catalytic performance. However, the leaching of metals and potential toxicity concerns impose limitations on the utilization of metal-doped BFO in applications related to water purification or drinking water treatment.
Accordingly, in our current study, we have introduced nitrogen ions to explore its impact on the catalytic behavior of BFO nanoparticles. The adoption of nitrogen doping has been deemed productive and advantageous due to its atomic size, which closely approximates that of oxygen within the ABO3 structure.
The incorporation of nitrogen brings about alterations in the crystal structure and/or suppresses the recombination rate of photogenerated electron–hole pairs, resulting in a notable enhancement in catalytic efficiency compared to pure BFO nanoparticles. The mode of nitrogen doping can be either substitutional, interstitial, or both.
Substitutional doping replaces oxygen ions, and the surface of the nanoparticles is modified by nitrogen bonding via interactive forces such as dipole–dipole interaction, electrostatic interaction, van der Waals interaction, or London forces. The introduction of nitrogen into the interstitial position, on the other hand, alters the lattice parameters or crystal structure of the nanoparticles.
Notably, substitutional doping reduces the energy band gap to a relatively lesser extent than interstitial doping. While doping at cation sites has been extensively reported, there is notably limited research on doping at oxygen sites. Recently, Jia et al. synthesized nitrogen-doped bismuth ferrite (N-BFO) using melamine as the nitrogen precursor and found enhanced degradation of bisphenol A under visible light. However, no studies have yet addressed the photodegradation of organic dyes in the presence of N-BFO.
Unveiling the Synthesis and Characterization of Nitrogen-Doped Bismuth Ferrite
The main purpose of this work was to employ an economical and energy-efficient auto-combustion method to synthesize stoichiometrically pure BFO nanoparticles at low temperatures. Subsequently, we aimed to introduce nitrogen into these BFO nanoparticles (xN-BFO) and investigate their impact on catalytic activity.
The role of nitrogen in suppressing oxygen vacancies of BFO is systematically investigated by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and UV–Vis diffuse reflectance spectroscopy (DRS) techniques. Specifically, we evaluated their effectiveness in the photodegradation of methylene blue (MB). Additionally, we studied their potential in the reduction of 4-nitrophenol (4-NP), a toxic and environmentally significant organic pollutant, into aminophenol (4-AP).
The auto-combustion method was employed to synthesize nano-sized BFO particles at low temperature, using bismuth nitrate and iron nitrate as Bi and Fe precursors, with ascorbic acid as the chelating agent. To introduce nitrogen, ammonium chloride, dissolved in methanol, served as the nitrogen source, and the mixture was heated at 60°C for 24 hours.
Different levels of nitrogen-doped BFO nanoparticles were produced by varying the amount of ammonium chloride, denoted as xN-BFO (x = 0, 0.01 g, 0.25 g, 1 g). The pristine and nitrogen-doped BFO nanoparticles underwent comprehensive characterization through a combination of microscopic and spectroscopic techniques, including XRD, FTIR, FESEM, EDS, and XPS.
Unveiling the Optical and Structural Properties of Nitrogen-Doped Bismuth Ferrite
The XRD pattern for both pure BFO and xN-BFO confirms the excellent crystalline nature of the synthesized nanoparticles, with all prominent peaks attributed to the distorted rhombohedral structure of BFO with the R3C space group. Notably, the sharp peak at approximately 32° signifies that the BFO nanoparticles are oriented along the (110) direction, and the peak splitting along the (104) plane confirms the distorted rhombohedral structure.
Interestingly, in contrast to the typical behavior observed in doping, which often results in the merging of the (110) and (104) peaks into a single peak, nitrogen doping does not result in the merging of these two peaks, suggesting that the addition of nitrogen has no discernible impact on the rhombohedral structure of BFO.
The observed shift toward higher 2θ values for the (012) diffraction peak with increasing nitrogen concentration is likely a result of nitrogen ion substitution at the oxygen sites within the BFO lattice. Additionally, besides the characteristic peaks, several impurity phase peaks of Bi2O3 are also present, and their intensity increases with higher nitrogen concentrations.
FTIR spectroscopy analysis confirms the presence of Bi-O and Fe–O bonds in the samples, while the peaks observed within the range of 1046–1107 cm−1 serve as confirmation of the existence of C-O and C-N bonds. As the nitrogen concentration increases, there is a noticeable rise in the intensity of the peak in this region, and the peak also shifts toward the higher wavenumber side, suggesting the elevation in nitrogen ion concentration and the substitution of a greater number of oxygen ions by nitrogen ions.
The FESEM images reveal a consistent and uniform nature of the samples, with the presence of substantial structures that could potentially signify the presence of agglomerated particles. The EDS analysis confirms the stoichiometric ratio and the presence of Bi, Fe, O, and N in the synthesized samples, with the atomic percentage of N ions increasing proportionally with the nitrogen precursor concentration.
Unveiling the Optical Properties of Nitrogen-Doped Bismuth Ferrite
The UV–Vis DRS reveals that both BFO and nitrogen-doped BFO exhibit light absorption across both the UV and visible spectral regions, with nitrogen-doped BFO nanoparticles exhibiting lower light reflectivity compared to pure BFO, indicating a higher potential for light absorption in this spectral range.
Furthermore, as the nitrogen level rises, so does the strength of the absorption. By analyzing the Tauc plot, it becomes evident that the energy gap (Eg) values exhibit a decline with increasing nitrogen concentration, suggesting the potential for enhanced absorption efficiency.
The reduction in energy gap may result from multiple factors such as structural defects on the surface, distortion of FeO6 octahedral (spin polarization), formation of impurity bands just above the valence band of BFO, or lattice modifications, such as oxygen ion substitution with nitrogen ions or interstitial nitrogen doping or rearrangement of molecular orbitals upon doping.
The XPS analysis confirms the presence of distinct peaks corresponding to Bi, Fe, O, N, and C, with the N 1s spectra in 1N-BFO revealing two pronounced peaks, indicating the existence of multiple oxidation states of nitrogen. The chemical shifts toward lower binding energy regions in the Bi-4f, Fe-2p, and O-1s states provide compelling evidence for the partial breaking of Bi-O and Fe–O bonds and the formation of Bi-N and Fe–N bonds, primarily driven by the slightly lower electronegativity of nitrogen (3.04) in contrast to oxygen (3.44).
Unveiling the Catalytic Performance of Nitrogen-Doped Bismuth Ferrite
The investigation into the photocatalytic performance of both pure BFO and 1N-BFO in the degradation of methylene blue dye under natural sunlight revealed that the incorporation of nitrogen led to a notable enhancement in photocatalytic activity, reaching up to 63% degradation efficiency within 105 minutes, compared to 53.6% for pure BFO.
The kinetics of the degradation reaction followed a pseudo-first-order Langmuir–Hinshelwood reaction model, with the determined rate constants for pure BFO and 1N-BFO being 0.00595/min and 0.00945/min, respectively. These findings clearly indicate a substantial enhancement in photocatalytic activity upon the nitrogen doping of pure BFO.
This improvement can be attributed to the governing factors of nanoparticle photocatalytic activity, particularly light absorption efficiency and reduction in the rate of recombination of photogenerated electron–hole pairs. The incorporation of nitrogen in 1N-BFO led to a significant reduction in the energy band gap, which increased the visible light activity, and may have also decreased the oxygen vacancies, which serve as electron–hole pair recombination centers.
The catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) was also investigated in the presence of sodium borohydride (NaBH4), with the results revealing a notable increase in the rate of reaction with an increase in nitrogen concentration. The rate constants for pure BFO and xN-BFO were determined using the pseudo-first-order Langmuir–Hinshelwood model, further confirming the enhanced catalytic performance of nitrogen-doped BFO.
The adsorption kinetics of both methylene blue and 4-nitrophenol onto the surfaces of BFO and xN-BFO were investigated using the Lagergren pseudo-first-order model and the Ho pseudo-second-order model. The analysis revealed that the adsorption of both organic pollutants followed the pseudo-second-order kinetics, highlighting the effectiveness of these materials in pollutant removal processes.
Unveiling the Recyclability and Stability of Nitrogen-Doped Bismuth Ferrite
The recyclability study confirmed the stability of both BFO and nitrogen-doped BFO, with no significant performance loss after five cycles. The reduction efficiency remained nearly constant, with 97.5 to 97.3% for BFO and 99.3 to 99.1% for 1N-BFO from the 1st to the 5th cycle, respectively.
This indicates that there was no significant loss in catalytic activity during the reduction process, and BFO and 1N-BFO demonstrate excellent stability and can be considered viable catalysts for the reduction of organic pollutants, suitable for practical applications.
Conclusion: Unlocking the Potential of Nitrogen-Doped Bismuth Ferrite in Environmental Remediation
In summary, this study not only highlights the exciting potential of nitrogen-doped bismuth ferrite nanoparticles in environmental remediation but also sheds light on the intricate interplay between anionic substitution, band structure modification, and catalytic performance enhancement.
The incorporation of nitrogen into the BFO lattice led to a notable reduction in the energy band gap, which enhanced the visible light activity, and also decreased the oxygen vacancies, thereby diminishing the recombination centers for photogenerated charge carriers.
The nitrogen-doped BFO nanoparticles exhibited superior catalytic performance in both the photodegradation of methylene blue and the reduction of 4-nitrophenol, with the adsorption kinetics studies further underscoring the effectiveness of these materials in pollutant removal processes.
The recyclability study confirmed the excellent stability of the nitrogen-doped BFO catalyst, making it a viable option for practical applications in environmental remediation. This study not only advances the understanding of the role of anionic substitution in modulating the properties of bismuth ferrite but also paves the way for the development of highly efficient and eco-friendly catalysts for addressing pressing environmental challenges.
