Facile Synthesis, Characterization, and Photocatalytic Performance of BiOF/BiFeO 3 Hybrid Heterojunction for Benzylamine Coupling under Simulated Light Irradiation

: Under simulated light irradiation, the aerobic oxidation of benzylamine to N , N -benzylidene benzylamine was carried out as a model reaction to investigate the photocatalytic activity of a hy-drothermally prepared composite based on BiOF and BiFeO 3 materials. The prepared photocatalysts were characterized using several spectroscopic techniques, such as powder X-ray diffraction (PXRD), diffuse reﬂectance spectroscopy (DRS), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectroscopy (FTIR). Band gap analysis showed that the composite exhibits a band gap that lies in the UV region (3.5 eV). Nonetheless, pristine BiOF and BiFeO 3 exhibited band gaps of 3.8 eV and 2.15 eV, respectively. N , N -benzylidenebenzylamine was selectively achieved with a high conversion yield of ~80% under atmospheric conditions in which the product was conﬁrmed using 1 H-NMR, 13 C-NMR, and FTIR spectroscopic techniques. Various control experiments were conducted to further conﬁrm the enhanced photocatalytic performance of the reported composite.


Introduction
Since its discovery in 1960, bismuth ferrite has garnered considerable interest due to its rarity as one of the few multiferroics with the simultaneous presence of ferroelectric and antiferromagnetic order characteristics in a perovskite structure [1,2]. An inorganic semiconductor, bismuth ferrite (BiFeO 3 , BFO) is a promising multifunctional material with a wide range of intriguing applications, including spintronics [3,4], sensors [5,6], photocatalysis [7][8][9], optical devices [10,11], and data storage [12]. Due to the narrow band gap energy of BFO and the potential for internal polarization suppression of the electron-hole recombination rate, it has unique advantages as a heterogenous photocatalyst [13,14]. Compared to the commonly used TiO 2 photocatalyst [15], which absorbs in the UV region due to its broad band gap, BFO narrow band gap enables the greatest and most efficient exploitation of visible light from solar radiation energy [16,17]. In photocatalytic applications, the behavior of BFO in the presence of visible light is of particular interest due to the number of distinctive properties it holds, including a low cost [18], nontoxicity [19,20], chemical stability [18], discrete crystalline structure [18], and special electro-optical properties [21].
BFO perovskites can only stabilize over a limited range of temperatures, and, until now, the problem of obtaining single-phase pure BFO crystallite remains challenging. Nonetheless [8], a range of wet chemical techniques have been employed to create pure single-phase BiFeO 3 . A hydrothermal procedure can create pure single-phase BiFeO 3 at low temperatures, but pressure is necessary [22]. These procedures include the microemulsion Photochem 2023, 3 188 technique [23], the ferrioxalate precursor method [24], the solution combustion method [25], and the citrate method [26], and using sol-gel techniques based on ethylene glycol [27].
It has been proven in many previous studies that metal-oxides-based photocatalysts perform better when heterojunctions are formed; thus, attention to the assembly of various heterojunction structures has grown in the past few years. For example, heterojunctions such as BiOF/BiOI [28], MoS 2 /BiVO 4 [29], BiOI/TiO 2 [30], NiS/CdS [31], and SnO 2 /TiO 2 [32] were used as photocatalysts for the removal of diclofenac potassium, the photodegradation of methylene blue, the photodegradation of methyl orange, photocatalytic H 2 -production, and the decolorization of rhodamine-B, respectively.
In this study, the coprecipitation method was used to generate pure single-phase BFO crystallites at a low temperature of about 70 • C. This method had the ability to create BFO powders that were well crystallized, had regulated morphology, and had a limited dispersion of particle size. Notably, all previous studies on BFO materials have focused on magnetic and dielectric properties. Nevertheless, our present study focuses on the photocatalytic aspect of this material and a newly prepared heterojunction based on BiFeO 3 and BiOF. The photocatalytic activity has been investigated using the photocatalytic benzyl amine coupling reaction as a model reaction.

Materials and General Procedures
Bismuth nitrate pentahydrate (BiNO 3 ·5H 2 O), iron nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O), ammonium fluoride (NH 4 F), deuterated chloroform (CDCl 3 ), acetonitrile (CH 3 CN, ACN), glacial acetic acid (CH 3 COOH), and benzylamine (C 7 H 9 N) were purchased from Sigma-Aldrich (St. Louis Missouri, MO, USA) and used as received. The FTIR spectrum was obtained using ATR-IR spectroscopy (Agilent Technologies Cary 600 Series FTIR Spectrometer, Santa Clara, CA, USA) across a range of 4000 to 500 cm −1 (512 scan average), in which the background was first collected using potassium bromide where the oily product was sandwiched between two KBr disks. Using a benchtop Rigaku MiniFlex X-ray diffractometer (Neu-Isenburg, Germany) with a CuKα radiation tube (λ = 1.542 Å) operating at 40 kV across a range of 3-50 • (2θ) and a rate of 2 • C min −1 , powder X-ray diffraction (PXRD) (Neu-Isenburg, Germany) measurements were made. The sample surface morphology and elemental analysis were analyzed using Quattro ESEM instrument scanning electron microscopy (SEM) (Waltham, MA, USA), equipped with an energy-dispersive X-ray (EDX) detector, that was operated at high vacuum with 30 kV accelerating voltage. Diffuse reflectance (DRS) of BiFeO 3 , BiOF, and the composite was measured using a Shimadzu UV-3600 spectrophotometer (Kyoto, Japan) over wavelength ranges from 200 nm to 800 nm after a baseline measurement using barium sulfate. Proton nuclear magnetic resonance, 1 H-NMR, and 13 C-NMR spectra were acquired using a Varian 400 MHz (Palo Alto, CA, USA) in d-chloroform as the solvent to validate the identity of the product. Nitrogen sorption experiments at 77 K were used to assess the surface area and porosity of the photocatalysts.

Preparation of BiFeO 3
Bismuth ferrite was synthesized using the coprecipitation method. Bismuth nitrate and iron nitrate were dissolved separately in 2-methoxyethanol and left under stirring until completely dissolved. In detail, 6.18 × 10 −3 moles of bismuth nitrate and iron nitrate equivalent to 3 g and 2.5 g were used, respectively. A total of 30 mL of 2-methoxyethanol was used to dissolve bismuth nitrate and an additional 5 mL of ethylene glycol was added to enhance the solubility of bismuth nitrate. For iron nitrate, 8 mL of 2-methoxyethanol was needed. The two solutions were mixed, and the pH was set to 5 using acetic acid and ammonium hydroxide. The solution was then left under continuous stirring and heating at 70 • C for around 7 h. The product was then collected and washed with DI water and ethanol several times. It was subsequently dried for 2 h at 80 • C, then calcined at 600 • C for 4 h (Scheme 1). was used to dissolve bismuth nitrate and an additional 5 mL of ethylene glycol was added to enhance the solubility of bismuth nitrate. For iron nitrate, 8 mL of 2-methoxyethanol was needed. The two solutions were mixed, and the pH was set to 5 using acetic acid and ammonium hydroxide. The solution was then left under continuous stirring and heating at 70 °C for around 7 h. The product was then collected and washed with DI water and ethanol several times. It was subsequently dried for 2 h at 80 °C, then calcined at 600 °C for 4 h (Scheme 1). Scheme 1. Schematic syntheses of the hydrothermal method for preparing BiFeO 3 , BiOF, and BiOF/BiFeO 3 composite using hydrothermal methods.

Preparation of BiOF
According to previously reported procedure, BiOF was synthesized using simple coprecipitation method [28]. Bismuth nitrate and ammonium fluoride were utilized in quantities of 0.002 mols each, or 970 mg and 74.08 mg, respectively. Bismuth nitrate was dissolved in 20 mL of 4 M acetic acid with constant stirring and heating at 80 • C. Then, ammonium fluoride was dissolved in 2 mL of deionized water (DI) and added to bismuth nitrate solution. The mixture was left under continuous stirring and heating at 80 • C for 7 h. The product was then collected and washed with DI water and ethanol several times. It was subsequently dried for 2 h at 80 • C and calcined for 4 h at 400 • C (Scheme 1).

Preparation of BiFeO 3 /BiOF Composite
BiFeO 3 /BiOF composite was synthesized using the simple hydrothermal method. A molar ratio of 1:1 was used from each of BiFeO 3 and BiOF, which was added to a 25 mL Teflon line autoclave with 15 mL of DI water. The mixture was sonicated for 1 h before heating at 140 • C for 72 h (Scheme 1).

Photocatalytic Activity
In a 25 mL round-bottom flask connected to a condenser, the photocatalytic reaction was conducted under aerobic conditions. An optimum of 5.0 mg (0.16, 0.02, 0.009 mmol for BiOF, BiFeO 3 , and BiOF/BiFeO 3 composite, respectively) of each sample was added to 51 µL (0.50 mmol) of benzyl amine in 2 mL of acetonitrile (ACN). The reaction was then irradiated by a 400 W halogen lamp for 24 h. The photocatalysts were washed with ACN and separated from the product by syringe filtration. Product purification was completed by simple column chromatography using silica gel and a mixture of ethyl acetate and hexane (1:2) to remove any possible impurities or byproducts. To isolate the product (Nbenzylidenebenzylamine) that was dissolved in ACN, the solvent was evaporated using a rotary evaporator. Finally, 1 H-NMR, 13 C-NMR, and FTIR techniques were used to confirm the identity of the product. All control experiments followed the same procedure with required changes in some variables, as outlined in Table 1. BiOF/BiFeO3 composite using hydrothermal methods.

Preparation of BiOF
According to previously reported procedure, BiOF was synthesized using simple coprecipitation method [28]. Bismuth nitrate and ammonium fluoride were utilized in quantities of 0.002 mols each, or 970 mg and 74.08 mg, respectively. Bismuth nitrate was dissolved in 20 mL of 4M acetic acid with constant stirring and heating at 80 °C. Then, ammonium fluoride was dissolved in 2 mL of deionized water (DI) and added to bismuth nitrate solution. The mixture was left under continuous stirring and heating at 80 °C for 7 h. The product was then collected and washed with DI water and ethanol several times. It was subsequently dried for 2 h at 80 °C and calcined for 4 h at 400 °C (Scheme 1).

Preparation of BiFeO3/BiOF Composite
BiFeO3/BiOF composite was synthesized using the simple hydrothermal method. A molar ratio of 1:1 was used from each of BiFeO3 and BiOF, which was added to a 25 mL Teflon line autoclave with 15 mL of DI water. The mixture was sonicated for 1 h before heating at 140 °C for 72 h (Scheme 1).

Photocatalytic Activity
In a 25 mL round-bottom flask connected to a condenser, the photocatalytic reaction was conducted under aerobic conditions. An optimum of 5.0 mg (0.16, 0.02, 0.009 mmol for BiOF, BiFeO3, and BiOF/BiFeO3 composite, respectively) of each sample was added to 51 µL (0.50 mmol) of benzyl amine in 2 mL of acetonitrile (ACN). The reaction was then irradiated by a 400 W halogen lamp for 24 h. The photocatalysts were washed with ACN and separated from the product by syringe filtration. Product purification was completed by simple column chromatography using silica gel and a mixture of ethyl acetate and hexane (1:2) to remove any possible impurities or byproducts. To isolate the product (N-benzylidenebenzylamine) that was dissolved in ACN, the solvent was evaporated using a rotary evaporator. Finally, 1 H-NMR, 13 C-NMR, and FTIR techniques were used to confirm the identity of the product. All control experiments followed the same procedure with required changes in some variables, as outlined in Table 1.

Characterization of the Photocatalysts
The purity and phase structure of prepared BiFeO3 and BiOF were confirmed by the PXRD diffraction pattern and compared to the standard PXRD database (JCPDS file No. 01-073-0548 and 73-1595 for BiFeO3 and BiOF, respectively), as shown in Figures S1 and S2 in the Supporting Information. All diffraction peaks were matched using Match!3

Characterization of the Photocatalysts
The purity and phase structure of prepared BiFeO 3 and BiOF were confirmed by the PXRD diffraction pattern and compared to the standard PXRD database (JCPDS file No. 01-073-0548 and 73-1595 for BiFeO 3 and BiOF, respectively), as shown in Figures S1 and S2 in the Supporting Information. All diffraction peaks were matched using Match!3 software (version 3.0) to the pure trigonal and tetragonal phases of BiFeO 3   The band gaps of BiFeO3, BiOF, and BiOF/BiFeO3 composite were measured using the Tauc method [33], with which conduction band positions and band potential measurements could be estimated as previously reported [34,35]. When plotting (αhν) vs. photon energy (hν), as shown in Figure 2, the BiFeO3 exhibited a band of 2.15 eV lying in the visible region, and BiOF showed a larger band gap of 3.8 eV in the UV region. The BiOF/BiFeO3 composite exhibited a band gap of 3.5 eV lying in the UV light region compared to pure BiOF, indicating that the synthesized composite forms a heterojunction interface.  (122), which appeared at 2θ of 19.34°, 29.2°, 31.93°, 37.21°, 39.62°, 45.93°, 51.5°, and 55.56°, respectively. Moreover, upon comparing the PXRD pattern of the prepared photocatalyst BiOF/BiFeO3 composite with both mixed metal oxides (Figure 1), the obtained patterns showed a combination of similar peaks from both mixed materials, which confirmed the preparation of the composite. The band gaps of BiFeO3, BiOF, and BiOF/BiFeO3 composite were measured using the Tauc method [33], with which conduction band positions and band potential measurements could be estimated as previously reported [34,35]. When plotting (αhν) vs. photon energy (hν), as shown in Figure 2, the BiFeO3 exhibited a band of 2.15 eV lying in the visible region, and BiOF showed a larger band gap of 3.8 eV in the UV region. The BiOF/BiFeO3 composite exhibited a band gap of 3.5 eV lying in the UV light region compared to pure BiOF, indicating that the synthesized composite forms a heterojunction interface.
The band gaps of BiFeO 3 , BiOF, and BiOF/BiFeO 3 composite were measured using the Tauc method [33], with which conduction band positions and band potential measurements could be estimated as previously reported [34,35]. When plotting (αhν) vs. photon energy (hν), as shown in Figure 2, the BiFeO 3 exhibited a band of 2.15 eV lying in the visible region, and BiOF showed a larger band gap of 3.8 eV in the UV region. The BiOF/BiFeO 3 composite exhibited a band gap of 3.5 eV lying in the UV light region compared to pure BiOF, indicating that the synthesized composite forms a heterojunction interface. SEM images of pure BiFeO3, BiOF, and composite BiOF/BiFeO3 photocatalysts are shown in Figure 3. BiOF showed agglomerated small particles on an irregular surface, whereas BiFeO3 possessed a spherical particle shape. As for the BiOF/BiFeO3 composite, there is no specific adopted morphology from either material, but the composite has an irregular surface with agglomerated particles. The EDX data ( Figures S3-S5, Supporting Information) were used to confirm the elemental composition of the synthesized photocatalyst. Atomic percent confirmed the presence of Bi, Fe, F, and O in the photocatalyst with a higher presence in Bi and O compared to both BiFeO3 and BiOF (Tables S1-S3, SEM images of pure BiFeO 3 , BiOF, and composite BiOF/BiFeO 3 photocatalysts are shown in Figure 3. BiOF showed agglomerated small particles on an irregular surface, whereas BiFeO 3 possessed a spherical particle shape. As for the BiOF/BiFeO 3 compos-ite, there is no specific adopted morphology from either material, but the composite has an irregular surface with agglomerated particles. The EDX data ( Figures S3-S5, Supporting Information) were used to confirm the elemental composition of the synthesized photocatalyst. Atomic percent confirmed the presence of Bi, Fe, F, and O in the photocatalyst with a higher presence in Bi and O compared to both BiFeO 3 and BiOF (Tables S1-S3, Supporting Information). SEM images of pure BiFeO3, BiOF, and composite BiOF/BiFeO3 photocatalysts are shown in Figure 3. BiOF showed agglomerated small particles on an irregular surface, whereas BiFeO3 possessed a spherical particle shape. As for the BiOF/BiFeO3 composite, there is no specific adopted morphology from either material, but the composite has an irregular surface with agglomerated particles. The EDX data ( Figures S3-S5, Supporting Information) were used to confirm the elemental composition of the synthesized photocatalyst. Atomic percent confirmed the presence of Bi, Fe, F, and O in the photocatalyst with a higher presence in Bi and O compared to both BiFeO3 and BiOF (Tables S1-S3, Supporting Information).

Photocatalytic Activities
Based on their band gap values, the photocatalytic performance of the reported photocatalysts was tested using the photocatalytic oxidative coupling reaction of benzylamine as a model photocatalytic reaction. The photocatalytic experiments were carried out using 51 µL of benzyl amine placed in a 25 mL round-bottom flask irradiated from the side (10 cm from the radiation source) using light from a 400 W halogen lamp with a wavelength Figure 4. FTIR spectra of the prepared photocatalysts with corresponding band assignments.

Photocatalytic Activities
Based on their band gap values, the photocatalytic performance of the reported photocatalysts was tested using the photocatalytic oxidative coupling reaction of benzylamine as a model photocatalytic reaction. The photocatalytic experiments were carried out using 51 µL of benzyl amine placed in a 25 mL round-bottom flask irradiated from the side (10 cm from the radiation source) using light from a 400 W halogen lamp with a wavelength range of 340-850 nm. The round-bottom flask was attached to a condenser open at the top to the atmosphere. A pale-yellow oily product of N-benzylidenebenzylamine was formed and confirmed using 1 H-NMR and 13 C-NMR ( Figure S6, Supporting Information). Peak shifts were as follows: 1 δ 162, 139.23, 136.09, 130.78, 128.61, 128.49, 128.27, 127.98, 126.99, 65.06. Because of the newly appeared double bond formed after the coupling of another benzyl amine, the peak at 8.4 ppm was the most deshielded signal. The 1 H-NMR spectrum of the starting material benzyl amine was compared to the products in which the signal at 3.9 ppm (-CH 2 protons) was downfielded to 4.9 in N-benzylidenebenzylamine, which further confirms the coupling step. In agreement with this finding, 13 C-NMR data showed the highest shift at δ C = 162 ppm, which belongs to the characteristic peak of the N=C bond of the products. Having the expected shifts along with the integration of each peak, the conversion percent yields using the three photocatalysts were calculated to be 18, 39, and 80% for BiFeO 3 , BiOF, and BiOF/BiFeO 3 composite, respectively, with no starting material or any other byproduct detected (Table 1). Due to rapid charge/hole recombination at lower band gaps, BiFeO 3 shows low conversion, while the composite had the highest conversion because the recombination was suppressed.
The enhanced yield of formation under UV light irradiation for the prepared BiOF/BiFeO 3 composite heterojunction in comparison to pure BiOF or BFO is mainly explained by the fact that a heterojunction formed by adding BiOF could absorb more photons and act as an efficient transfer of excited electrons from the BiOF conduction band to the conduction band of BFO particles; moreover, a transfer from the valence band of BiOF to the valence band of BFO suppresses recombination and allows greater electron transfer to benzyl amine.
To provide more evidence regarding the completion of the coupling reaction, FTIR spectra were recorded for the starting material and compared to the product as well. Upon coupling, the characteristic band of -NH stretch in benzyl amine (3200 and 3500 cm −1 ) disappeared, and a new band at 1643 cm −1 appeared, which corresponded to the C=N of the product. Moreover, the bands of the benzene ring were obvious and appeared between 1650 and 1800 cm −1 ( Figure S7, Supporting Information).
Different control experiments have been carried out to investigate other reaction conditions. As illustrated in Table 2, using the BiOF/BiFeO 3 composite photocatalyst showed the highest conversion. The percent yields were calculated as previously reported [33]. The highest yields of N-benzylidenebenzylamine products were obtained in an open-air and acetonitrile environment, while 0, 7, and 13% were obtained using BiFeO 3 , BiOF, and BiOF/BiFeO 3 composite, respectively, in a solvent-free condition (Entries II-IV), which confirms that our photocatalysts work best in the solvent environment. Many other control experiments have shown no conversion yields in the absence of either light (Entries V-VII) or photocatalysts (entries VIII), which strongly supports the high photocatalytic activity of the reported photocatalysts under visible or UV irradiation.
A plausible mechanism of the oxidative coupling of benzylamine is explained in Scheme 2. The process happens through an O 2 mediated pathway when the photoexcited electron of the photocatalyst first rises to the conduction band (CB) upon radiation. It then reacts with atmospheric molecular oxygen to produce an O 2 − radical, which was previously confirmed using electron spin resonance (EPR) measurements [36][37][38]. An aminium radical cation intermediate is then formed by oxidized benzylamine by the photoinduced hole in the valence band (VB). Hydrogen-atom abstraction from the O 2 − radical then occurs at this radical cation, producing phenylmethanamine (Ph-CH 2 -NH 2 ), and, thus, forming Ph-CH=NH and H 2 O 2 [36]. H 2 O 2 is then dissociated to · OH radicals, which act as a supporting material for benzylamine oxidation to the produced imine that finally reacts with another free benzylamine to produce the N-benzylidenebenzylamine product, liberating ammonia. Scheme 2. Proposed mechanism of benzylamine coupling over reported photocatalyst.

Conclusions
In this study, benzylamine was photocatalytically coupled to N-benzylidenebenzylamine using the newly synthesized BiOF/BiFeO3 composite photocatalyst, which showed a high conversion yield of 80% compared to other materials. The composite's construction was proven by multiple characterization methods, including XRD, DRS, SEM, EDX, and FT-IR. According to the measured band gaps, the BiOF/BiFeO3 composite exhibited a lower band gap compared to BiOF (3.5 eV vs. 3.8 eV), which can be successfully employed as a photocatalyst in the UV region. The reported synthesis procedure provides an easy, eco-friendly, and low-cost route for developing various active materials for photocatalytic applications.

Conclusions
In this study, benzylamine was photocatalytically coupled to N-benzylidenebenzylamine using the newly synthesized BiOF/BiFeO 3 composite photocatalyst, which showed a high conversion yield of 80% compared to other materials. The composite's construction was proven by multiple characterization methods, including XRD, DRS, SEM, EDX, and FT-IR. According to the measured band gaps, the BiOF/BiFeO 3 composite exhibited a lower band gap compared to BiOF (3.5 eV vs. 3.8 eV), which can be successfully employed as a photocatalyst in the UV region. The reported synthesis procedure provides an easy, eco-friendly, and low-cost route for developing various active materials for photocatalytic applications.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/photochem3010012/s1, Figure S1: Matched X-ray spectroscopy of the BiOF using Match!3 software (version 3.0). Figure S2: Matched X-ray spectroscopy of the BiFeO 3 using Match!3 software (version 3.0); Figure S3: EDX elemental analysis of BiOF; Figure S4: EDX elemental analysis of BiFeO 3 ; Figure S5: EDX elemental analysis of BiOF/ BiFeO 3 composite; Table S1: Weight and atomic percentages of BiOF; Table S2: Weight and atomic percentages of BiFeO 3 ; Table S3: Weight and atomic percentages of BiOF/ BiFeO 3 composite; Figure S6: Spectra of N-benzylidenebenzylamine product using 1 H-NMR (A) and 13  Data Availability Statement: The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.