Photoactive Brownmillerite Multiferroic KBiFe2O5 and Its Potential Application in Sunlight-Driven Photocatalysis.

KBiFe2O5 (KBFO) is an upcoming promising brownmillerite-structured multiferroic photoactive material for next-generation photovoltaic and photocatalytic applications. In the present work, KBFO has been developed using multistep thermal treatment method to reduce the volatility of constituent elements and improve the stability of compound. The band gap of KBFO (found to be ∼1.68 eV) extends to the near-infrared region compared to traditional perovskite-structured multiferroics. The magnetic and dielectric transitions occur in the same temperature range (740 K-800 K), reflecting the existence of magneto-dielectric effect in the as-synthesized sample. It also shows promising photocatalytic activity by degrading organic effluents under natural sunlight compared to regular perovskite BiFeO3 photocatalyst (operating under visible light). A new application of brownmillerite multiferroic KBFO photocatalyst in environmental and energy applications has been explored by integrating the structural, optical, magnetic, and dielectric properties of the same.

Introduction

In recent years, multiferroic materials have gained enormous attention in the field of next-generation photovoltaics (PV). We come across various multiferroics with higher photovoltages (attributed to their band gap) justifying their potential application in photovoltaic and photocatalytic applications.[1] Multiferroic compounds are advantageous over ferroelectric compounds due to their lower band gap, which is an important requirement for PV applications. However, most of these ferroelectric materials, e.g., PbTiO3, PbZrO3, PbZr1–TiO3, etc., being lead (Pb)-based perovskites, are hazardous to the environment. Few of the lead-free ferroelectric materials, e.g., BaTiO3, KNbO3, and NaTaO3, have wide band gaps[2−4] and hence fail to utilize a wider range of solar energy. Overall, these perovskite ferroelectric materials have large band gap above 3 eV and can absorb only a small range of solar spectra. Multiferroics have relatively lower band gaps compared to ferroelectric materials due to the influence of magnetic ordering on electron–electron interaction resulting in a lower band gap as well as improved photocurrents.[5] One of the lead (Pb)-free perovskite multiferroic compounds, which has attracted enormous interest to date, is BiFeO3 with a band gap of ∼2.6 eV.[6] BiFeO3 has a lower band gap compared to the typical perovskite ferroelectric material due to the coexistence of both ferroelectric and ferromagnetic characteristics. However, its theoretical photovoltaic efficiency is limited to about 7%,[7] which can be overcome by decreasing the band gap to IR region. A lower band gap facilitates improved photocatalytic and photovoltaic performance of such compounds. Brownmillerites, which are derived from perovskite structures (ABO3) and have an empirical formula A2B2O5, boast of lower band gaps compared to many perovskites. Typical perovskite-structured multiferroic materials with corner-sharing BO6 octahedra lose their dielectric and magnetic properties at higher temperatures.[7−10] Brownmillerite-structured (A2B2O5) compounds with their oxygen-deficient nature need to be explored as an alternative. A2B2O5-like structures possess oxygen deficiency along the [110] direction and corner-sharing BO6 octahedra alternate with rows of corner-sharing BO4 tetrahedra.[11−14] Typically, Fe-based brownmillerite compounds contain FeO4 tetrahedra leading to the occurrence of lower band gaps unlike Fe-based perovskites, which consist of FeO6 octahedra only.[15] The lower band gap of Fe-based brownmillerite compounds is attributed to the presence of shorter Fe–O bond lengths and high covalence in FeO4 tetrahedron compared to FeO6 octahedron.[16,17] Oxygen deficiency in the brownmillerite structure also leads to high Curie temperature (Tc) and lower band gap (Eg), which is a major advantage over typical ABO3 perovskite structures.[18,19] Two well-known brownmillerite compounds, Sr2Fe2O5 and Ca2Fe2O5, too exhibit high Tc (Curie temperature) and lower band gap.[20,21]

Perovskite- and brownmillerite-structured metal oxides are also potential materials for catalytic applications.[22] The catalytic activity in perovskite-type metal oxides is attributed mostly to the presence of B-site transition-metal cation.[23] The catalytic properties of these metal oxides have already been related to the number of d-electrons and occupancy of the antibonding orbitals of M–OH.[24] Enhanced catalytic activity of perovskites with oxygen vacancies[22,25] motivates the fact that brownmillerites (with ordered oxygen vacancies) can act as a potential catalytic material. These brownmillerites exhibit smaller band gap compared to regular perovskites, rendering an additional advantage to the material to absorb the broad spectrum of sunlight. A thorough investigation of the role of brownmillerite KBiFe2O5 (KBFO) in sunlight-driven photocatalytic degradation of organics dyes can help us to confirm the same.

KBiFe2O5 is one of the recently discovered novel brownmillerite compounds having high Tc (∼780 K). It consists of tetrahedral Fe3+ in a Fe2O3 block that alternates with a [(K,Bi)O2] block. KBiFe2O5 synthesized by hydrothermal method has shown a band gap of 1.59 eV.[7] However, the yield from hydrothermal synthesis is low and the time taken to form KBFO crystalline sediment is as high as 72 h. Moreover, the crystalline sediments synthesized by this method are not suitable to fabricate nano/microstructured thin films for solar cell application. KBFO has shown promising magnetic and dielectric properties and reported a significant photovoltaic effect.[4,12,20] In the present work, we explore the synthesis of KBFO by multistep thermal treatment unlike one-step thermal treatment reported by Zhai et al.[26] Multistep thermal treatment synthesis can reduce the synthesis temperature to 650 °C compared to previous reports.[15,26]

The compound gains importance due to the fact that multiferroic KBFO exhibits spontaneous polarization upon illumination by visible light irradiation, causing an imbalance of charges, thereby inducing a strong internal electric field leading to separation of charges.[7] These charges are responsible for redox reactions involved in photocatalysis. The possibility of enhanced charge separation in KBFO upon irradiation by light has motivated us to explore natural sunlight-driven photocatalytic responses by degrading organic dyes such as methylene blue (MB) and Congo red (CR). The photocatalytic properties of brownmillerite multiferroics have been overlooked by many researchers. Hence, the present work forms a crucial part of primary investigations for any future exploration of KBFO as a potential candidate for environmental and energy applications.

Results and Discussion

The structural properties of as-synthesized samples were analyzed by room-temperature X-ray diffraction (XRD) (Figure a). In earlier reports, secondary phases have been observed in KBFO samples synthesized at 650 °C by one-step thermal treatment method.[15,26] Bismuth trioxide (Bi2O3) is one of the primary precursors in the synthesis of KBFO. Bi2O3 exists in six different phases such as monoclinic (α-Bi2O3), body-centered cubic (γ-Bi2O3), face-centered cubic (fcc) (δ-Bi2O3), tetragonal (β-Bi2O3), triclinic (ω-Bi2O3), and orthorhombic (ε-Bi2O3).[27−33] Among them, α-phase is the most stable at room temperature, whereas the other five forms are unstable crystal modifications formed at high temperatures. Hence, if the synthesis temperature goes beyond the phase-transition temperature (729 °C) of Bi2O3, it transforms into δ-Bi2O3 phase, which is stable at high temperature.[34] Existence of fcc (δ-Bi2O3) phase and other polymorphs of Bi2O3 may lead to the formation of pyrochlore phases, which were observed in KBFO synthesized at higher temperatures by one-step thermal treatment method.[15,26] In the present work, unlike the one-step thermal treatment method, samples were heated multiple times at lower temperatures. Initially, hand-milled precursors were calcined at 550 °C (K5) for 4 h at a heating rate of 5 °C/min. The sample was cooled down to room temperature. However, XRD and X-ray photoelectron spectroscopy (XPS) studies confirmed that single-phase KBFO phase could not be achieved at this temperature. Hence, K5 samples were further calcined at 650 °C (K6) at the same heating rate leading to single-phase KBFO. Further calcination of K6 samples at 750 °C (K7) led to the occurrence of pyrochlore phase in XRD pattern. Hence, KBFO synthesis temperature was optimized to 650 °C, which is lower compared to earlier reports. This multistep thermal treatment is advantageous over the one-step thermal treatment method as it reduces the final synthesis temperature as well as the volatility of elements and improves the stability of synthesized compound.

Figure 1

(a) XRD patterns of KBFO samples synthesized at different temperatures K5, K6, and K7 and scanning electron microscopy (SEM) images of as-synthesized polycrystalline KBFO synthesized at different temperatures (b) K5, (c) K6, and (d) K7.

Table gives a clear account of KBFO synthesis methods, growth parameters, and the corresponding band gaps reported to date. Figure a–d shows the XRD pattern and SEM images of the samples calcined at different temperatures. As is evident from the XRD pattern in Figure a in the present case, the single-phase compound could be obtained at 650 °C (K6) by multistep thermal treatment method.[26,35] Brownmillerite characteristic peak of (110) was found to be absent for 550 °C synthesized sample (K5), whereas it was observed for both K6 and K7 (750 °C) samples.[11] Morphology of the as-prepared samples was investigated by SEM. The corresponding image of K5 sample (non-brownmillerite phase) showed irregular morphology with no defined structures. As we move to K6 samples with a single phase, regular interconnected grains start to appear. Upon increasing the calcination temperature to 750 °C for K7, larger platelike morphology was observed (Figure b–d). The morphology of K7 (with a secondary pyrochlore phase) is different and seemingly provides larger grains with a low surface area/porosity, which is undesirable for catalytic activity.

Table 1

As-Reported KBFO Synthesis Methods, Growth Parameters, and Corresponding Band Gaps

s. no		synthesis route	synthesis temperature	band gap (eV)	references
1	polycrystalline KBFO	solid-state reaction (multistep thermal treatment)	first step 550 °C	1.68	this work
			second step 650 °C
2	crystalline KBFO	hydrothermal	220 °C/72 h	1.59	(7)
3	polycrystalline KBFO	solid-state reaction	800 °C	1.76	(15)
5	polycrystalline KBFO	solid-state reaction (one-step thermal treatment)	850 °C	1.65	(26)
4	KBFO thin films	sol–gel by spin coating	750 °C	2.54	(35)
2	polycrystalline KBFO	sol–gel method	700 °C	1.63	(36)

XPS was employed to determine the elemental composition and electronic state of constituent elements shown in Figure a–c. The spin–orbit splitting values of Fe 2p components for K5, K6, and K7 samples were found to be ∼13.2 eV. A satellite peak appeared at about 8 eV above the binding energy of Fe 2p3/2 and Fe 2p1/2 shown in Figure a. The Fe 2p peak could be fitted (Lorentzian–Gaussian fitting) to two peaks indicating the presence of both octahedral and tetrahedral species in Fe3+ oxidation state, typical for brownmillerite compounds. Brownmillerite compounds have FeO6 (octahedra) and FeO4 (tetrahedra) species in alternative layers of crystal structure with ordered oxygen vacancies along the 110 direction.[11−14] For ideal Fe-based brownmillerite compounds, the Fe3+ (tet)/Fe3+ (oct) ratio is estimated to be 1.[37] For K5 samples, the Fe 2p3/2 and Fe 2p1/2 could be fitted to a single peak at 711.8 eV and 725.3 eV, respectively, which indicates the presence of Fe3+ (oct) only. Both XRD and XPS studies confirm that calcination at 550 °C does not lead to brownmillerite KBFO phase. Since the powder XRD patterns of K6 and K7 samples appear similar, finding traces of Fe oxide and confirming brownmillerite KBFO phase from room-temperature XRD pattern is difficult. However, as is clear from the XPS fitting of Fe 2p spin–orbit, the peaks for K6 as well as K7 samples could be fitted to two peaks. In K6, Fe 2p3/2 is fitted to two peaks at 711.4 and 713.8 eV, and Fe 2p1/2 spin–orbit peak is fitted to 724.9 and 727.7 eV, which correspond to the Fe3+ in octahedra (FeO6) and tetrahedra (FeO4) coordinations. Similar to that in K7, Fe 2p3/2 fitted to two peaks at 710.4 and 712.8 eV and Fe 2p1/2 spin–orbit is fitted to 723.9 and 726.7 eV, which correspond to the Fe3+ in octahedra (FeO6) and tetrahedra (FeO4) coordination. The Fe3+ (tet) to Fe3+ (oct) ratio was slightly higher in K6 (0.35) compared to K7 (0.34), which indicates that K6 possesses more Fe3+ (tet) coordination compared to K7, a significant feature of brownmillerite compound.[37,38] The binding energy of Bi–O peaks, as obtained from the Bi 4f spin–orbit studies for sample K5 (Figure b), point toward the absence of brownmillerite phase of KBFO. It is also revealed that the Bi 4f7/2 and Bi 4f5/2 peaks shift toward lower energy in K7 pointing toward the weakening of Bi–O bonds in KBiFe2O5. When Bi–O bonds are weakened, it alters the bond angles and bond lengths of neighboring Fe atoms[39] as is clear from the Fe 2p spin–orbit peak shifting toward lower energy (Figure b). Hence, a detailed analysis of XPS data shows that a growth temperature of 650 °C is the optimized growth temperature for KBFO in the present case.

Figure 2

X-ray photoelectron spectra and the corresponding fits belonging to elements Fe, Bi, and O for K5, K6, and K7 samples: (a) Fe 2p, (b) Bi 4f, and (c) O 1s.

It is likely that the magnetic ordering in KBFO influences the electron–electron interaction in the compound, which leads to a lower band gap compared to ferroelectric compounds.[15] Magnetism in such compounds could be an additional advantage in applications such as photocatalysis, to retrieve the catalyst after effluent treatment by applying an external magnetic field. Considering the above option, the magnetic properties of KBFO have been investigated in detail. The M–H curves of the as-synthesized KBFO samples were recorded at room temperature with a maximum magnetic field of 20 kOe, as shown in Figure a. S-type hysteresis loop is observed for all samples. K5 sample showed higher coercivity compared to K6 and K7 (Figure a). It also reveals weak ferromagnetism, which can be attributed to the presence of traces of Fe oxide. The absence of brownmillerite KBFO phase in K5 sample has already been confirmed by both XRD and XPS studies. K6 shows the highest magnetization value ∼1.91 emu/g at a magnetic field of 20 kOe along with a coexistence of weak ferromagnetic and antiferromagnetic behavior, which was revealed in temperature-dependent magnetization (M–T) curve with two transitions (Figure b). Upon increasing the synthesis temperature from 650 to 750 °C, the magnetization value decreased from 1.91 emu/g to 0.6 emu/g (Figure c). The M–T curve of K7 samples shows a dominant antiferromagnetic nature, and the magnetization decreases from (1.91 emu/g) K6 to (0.6 emu/g) K7 sample. From XPS studies, it was clear that the peaks corresponding to Bi 4f and O 2p shift toward lower energy for K7 indicating weakening of the Bi–O bond. Since Bi is a nonmagnetic element, the weakening of Bi–O bonds may not affect the magnetic properties of KBFO directly. However, it affects the bond length and angles of neighboring magnetic element (Fe), thereby altering Fe binding energy leading to lower values of magnetization.[39] Thus, the decrease in magnetization in K7 sample could be attributed to the weakening of Bi–O bonds, as observed by XPS studies.

Figure 3

(a) M–H data of polycrystalline KBFO with different synthesis temperatures. (b) Magnetization of KBFO as a function of temperature (M–T). (c) Magnetization values at 20 kOe versus synthesis temperatures for polycrystalline KBFO.

The phenomenon of spontaneous polarization in KBFO is expected to assist the photovoltaic and photocatalytic phenomena. Hence, the dielectric properties of K6 were investigated at different frequencies and temperatures (Figure a,b). Temperature-dependent dielectric constant (ε) and dielectric loss (tan δ) were measured at 10 Hz, 100 Hz, 1 kHz, 10 kHz, and 100 kHz by varying temperature from 303 to 800 K.

Figure 4

(a) Dielectric constant of KBFO as a function of temperature measured at 101–5 Hz; (b) dielectric loss tangent as a function of temperature.

The dielectric constant (ε) of sample K6 decreases rapidly with increase in frequency, as shown in Figure a. This is a typical phenomenon interpreted by the Maxwell–Wagner model. The higher values of ε at lower frequencies are due to the contribution of various polarizations such as space charges, ionic defects, and permanent dipoles and induced atomic and ionic dipoles.[15,40] With the increase in frequency, few of these contributions to dielectric constant (ε) decrease. For example, the space charges cannot follow the reversal of fields at higher frequencies, and then undergo relaxation due to their large effective mass.[15] From the ε vs T curve (Figure a), a maximum ε was observed at about 750–760 K, which is close to the reported Curie temperature (Tc = 780 K) of KBFO.[7] The antiferromagnetic transition for K6 in the M vs T plot (Figure b) also sets in at about the same temperature. It is worth noting that the magnetic and dielectric transitions with respect to temperature occur around the same temperature range and reflects the magneto-dielectric effect in the as-synthesized sample.

Optical absorption study of K6 samples (Figure a) reveals absorption in the visible to NIR region with a band gap of ∼1.68 eV. The single-phase sample (K6) was further investigated for photodegradation and potential photocatalysis application. The band gap of KBFO has an excellent spectral match with solar spectra compared to some of existing multiferroics and semiconductors for PV and photocatalytic applications. It is different from the conventional semiconductor photocatalysts in the sense that separation of charges occur in the presence of electric field. Light irradiation leads to charge imbalance in domains of polycrystalline KBFO due to spontaneous polarization. The coexistence of both ferroelectric and magnetic properties enhances the photocatalytic performance of KBFO. A schematic of the mechanism of dye degradation is presented in Figure b. The ferroelectric nature of KBFO is responsible for inducing redox reactions upon solar light illumination. The magnetic properties of KBFO are an additional asset to photocatalytic process. In addition to lower band gap, the magnetic behavior of such samples can also be used for separation of KBFO particles from the dye suspension by applying strong external magnetic field after photodegradation, providing an economical and natural method. Tuning of magnetic properties will enable us to obtain a magnetically retrievable photocatalyst, as a smart photocatalyst for next-generation photodegradation applications.

Figure 5

(a) UV–visible absorption spectra of K6 sample; the inset shows the plot of (αhυ)2 versus photon energy (hυ). (b) Schematic representation of spontaneous polarization-assisted photocatalysis in multiferroics.

Utilization of KBFO for remediation of polluted water was explored by the photodegradation activity of Methylene blue (MB) and Congo red (CR) organic dyes under direct sunlight irradiation. The initial dye concentration was kept at Co: 1 × 10–5 mol/L. The catalyst-loaded dye solution (10 mg/10 mL) was thoroughly blended using ultrasonication under dark condition. The experiment was conducted for different time intervals under natural sunlight, and the dyes were found to degrade eventually. The sunlight illuminance profile throughout the experiment was recorded, and is shown in Figure S1.

A possible degradation pathway for MB is shown in Figure . During the dissolution of MB in water, Cl– ion separates from MB core structure. During photodegradation of MB in the presence of catalyst, active species such as OH• and HO2• are generated. These active species break the N–CH3 bond and then oxidize −CH3 groups, which are connected to the terminal of core structure. They also break the S–N and C–N bonds and produce unstable organic byproducts. The oxidization reactions continue until the MB degrades completely into smaller inorganic products such as H2O, CO2, NO3, SO42–, and Cl–.[41,42][41,42]

Figure 6

Possible degradation pathway of MB.

The sunlight-driven degradations of both MB and CR were recorded at regular intervals using UV–visible absorption spectroscopy. After 150 min of sunlight exposure, MB and CR were degraded by 97 and 96.7% of their initial concentration, respectively. The degradation profile, C/Co ratio graph, and photocatalytic reaction kinetics of MB using KBFO (K6) are also evaluated as shown in Figure a,b. The degradation rate of MB in the presence of KBFO is determined to be 0.0252 min–1, which is approximately 106 times higher compared to the degradation of bare MB (0.00023 min–1). The photodegradation studies of CR are discussed in Supporting Information Figure S2.

Figure 7

(a) Degradation profile of MB in the presence of K6. (b) C/Co for MB under natural sunlight; the inset shows the photocatalytic reaction kinetics.

Reusability of photocatalyst determines its role in practical applications. To verify the reusability of KBFO photocatalyst, repeated photocatalytic tests were performed for three cycles, as shown in Figure . For first two cycles, there was no significant change in photodegradation performance of KBFO. In the third cycle, a slight reduction in photodegradation efficiency was observed, which could be attributed to low solar light intensity compared to the first two cycles. XRD results on KBFO, before and after photocatalysis, are shown in Figure S3 revealing the stability of KBFO after three cycles of degradation.

Figure 8

Reusability of KBFO for the degradation MB for three cycles.

The photocatalytic mechanism of KBFO under the influence of various scavengers [AgNO3, ethylenediaminetetraacetic acid (EDTA), and isopropyl alcohol (IPA)] is summarized in Figure a,b. KBFO, without any scavengers, showed a photodegradation rate (k) of 0.0252 min–1. Addition of AgNO3 (1 mmol) as electron (e–) scavenger to the dye–catalyst solution has no significant effect on photodegradation performance (k = 0.026 min–1). Similarly, when hydroxyl radical (OH•) species are trapped by the addition of IPA (1 mmol) during photocatalysis, no significant change in the photocatalytic degradation was observed (k = 0.0226 min–1). This suggests that electrons (e–) and OH• radicals are not primary active species for MB degradation. In the presence of hole (h+) scavenger EDTA (1 mmol), the photodegradation performance slowed down (k = 0.0062 min–1), which was lower compared to KBFO in the absence of any scavenger (k = 0.0252 min–1). Therefore, active species trapping experiments point toward the fact that the photocatalytic performance of KBFO is mainly governed by holes (h+).

Figure 9

(a, b) Comparison of photocatalytic activities of KBFO catalyst for the degradation of MB with or without adding various scavengers.

The band gap of KBFO is closer to the infrared region with a higher tendency to generate photoinduced charge carriers compared to UV-operated compounds. The UV-operated photocatalyst can absorb only 5% of sunlight coming to earth.[43] For a better understanding of the present work, an elaborate comparison of the photocatalytic activity of KBFO with perovskite BiFeO3 photocatalyst is provided in Table S1. Since BiFeO3 is one such perovskite that has been widely reported for its photocatalytic activities, this comparison gives us the state of the art when it comes to perovskites. The results discussed here are encouraging for an emerging photocatalyst as a pollutant remedial measure.

Conclusions

Brownmillerite-structured multifunctional KBiFe2O5 was synthesized by multistep thermal treatment method and reduced synthesis temperature to avoid any phase transition. The presence of a magneto–dielectric coupling in as-synthesized KBFO enables light irradiation to produce charge imbalance in domains of polycrystalline KBFO due to spontaneous polarization, thus enhancing the photodegradation activity by KBFO. Repeated photodegradation of Methylene blue (MB) and Congo red (CR) is conducted under sunlight for three cycles, verifying the reusability of photocatalyst for degradation of MB. Active species trapping experiments showed that the photocatalytic performance of KBFO is mainly governed by holes (h+) with partial contribution from OH• radical species. The photodegradation performance of KBFO was compared to the already existing perovskite BiFeO3 in the visible region. In this regard, KBiFe2O5 would be an efficient multifunctional solar energy-harvesting material for energy and environmental applications.

Experimental Section

Preparation and Characterization of Materials

Highly pure precursors of Fe2O3, Bi2O3, and K2CO3 are mixed in a stoichiometric ratio in a mortar, under thorough wet grinding, until a uniform mixture is obtained. The fine powder is then calcined at 550–750 °C for 4 h in a muffle furnace in multiple steps at a heating rate of 5 °C/min. The samples will henceforth be referred to as K5 (calcined at 550 °C), K6 (calcined at 650 °C), and K7 (calcined at 750 °C). The morphological, structural, magnetic, dielectric, optical, and spectroscopic studies have been performed by a high-resolution scanning electron microscope (FEI Quanta FEG 200), a X-ray diffractometer (Bruker S4 pioneer), a vibrating sample magnetometer (VSM EZ9, Microsense Inc.), a UV–visible spectrophotometer (Jasco, V-730), and an X-ray photoelectron spectroscopy (SPECS GmbH, Germany).

Photocatalytic Measurements

The photocatalytic performance of KBiFe2O5 was analyzed by degrading organic dyes MB and CR under natural sunlight. The initial concentration of MB and CR was 5 × 10–5 mol/L, and catalyst concentration was 10 mg/10 mL. Prior to sunlight irradiation, the suspension was ultrasonicated in the dark for 30 min to achieve the adsorption–desorption equilibrium. At every 30 min time interval, 10 mL of suspension was collected and catalyst was separated by centrifugation. The concentration of MB and CR was then detected by measuring the maximum absorbance at 664 and 498 nm, respectively, using UV–visible absorption spectroscopy (Jasco, V-730). The collected catalyst powder was dried before another photodegradation test. The reusability of catalyst was tested for three cycles.