Crystal Plane Impact of ZnFe₂O₄–Ag Nanoparticles Influencing Photocatalytical and Antibacterial Properties: Experimental and Theoretical Studies

Abstract

This paper describes the crystal interphase impact of ZnFe₂O₄–Ag in the photodegradation of Rhodamine B. Prepared ZnFe₂O₄ nanoparticles (NPs) were deposited with Ag NPs to offer ZnFe₂O₄–Ag (0–2.5%). An X-ray diffraction peak corresponding to the Ag NPs was detected if the particle content reached about 2.0%, observing multiple crystalline interphases in HR-TEM. Magnetic saturation (Ms) was increased ∼160% times for ZnFe₂O₄–Ag (7.25 to 18.71 emu/g) and ZnFe₂O₄ (9.62 to 25.09 emu/g) if the temperature is lowered from 298 to 5.0 K; while for Fe₃O₄ (91.09 to 96.19 emu/g), the Ms increment was just about 5.6%. After analyzing the DFT–Density of State, a decrease of bandgap energy for ZnFe₂O₄–Ag₆ from the influence of the size of Ag cluster was seen. Quantum yield (Φ) was 0.60 for ZnFe₂O₄, 0.25 for ZnFe₂O₄–Ag (1.0%), 0.70 for ZnFe₂O₄–Ag (1.5%), 0.66 for ZnFe₂O₄–Ag (2.0%), and 0.66 for ZnFe₂O₄–Ag (2.5%), showing that the disposition of Ag NPs (1.5–2.5%) increases the Φ to >0.60. The samples were used to photo-oxidize RhB under visible light assisted by photopowered Langmuir adsorption. The degradation follows first-order kinetics (k = 5.5 × 10^–3 min^–1), resulting in a greater k = 2.0 × 10^–3 min^–1 for ZnFe₂O₄–Ag than for ZnFe₂O₄ (or Fe₃O₄, k = 1.1 × 10^–3 min^–1). DFT-total energy was used to analyze the intermediates formed from the RhB oxidation. Finally, the ZnFe₂O₄–Ag exhibits good antibacterial behavior because of the presence of Zn and the Ag components.

Introduction

The study of iron oxides (Fe₂O₃ and Fe₃O₄) is a growing research topic because of their good superparamagnetic behavior and suitable biocompatibility in several biological applications such as cell imaging and drug delivery.¹ As is known, the factors like size, crystalline nature, internal structural texture, and unbalanced surface spins significantly influence the magnetic properties. Nonetheless, the air sensitivity of iron oxides is one of the main obstacles to its use as it tends to react quickly with water or acids; moreover, if Fe₃O₄ NPs are present in the solution, the aggregation of the particles is extremely quick due to the high magnetic nature.² It is known that nanoparticles (size, 1.0–100 nm) behave exceptionally because of the quantum confinement effect, and reveal unique electronic, magnetic, and optical properties.³ The improvement of the above properties can be achieved if the iron oxide is doped or deposited by different metals i.e., MFe₂O₄ (M = Mn, Co, Ni, Zn, Mg, etc.). Thus, a high magnetization saturation was obtained for MnFe₂O₄ NPs, while for CoFe₂O₄ NPs, a hard-magnet property is observed as compared to Fe₃O₄.⁴ This means that the structural change in the two cation centers {(M²⁺_1–cFe_c³⁺)_A[M_c²⁺ Fe_2–c³⁺]_BO₄ (c = inversion parameter) impacts significantly the electronic and magnetic properties. The distribution degree in the inversion character (i.e., occupational disorder factor c) influences the fractional number, improving the magnetic and catalytic properties.⁵ This means that the structural change in the zinc ferrites can trap electrons inside the quasi-stable energy states (active sites) to reduce the recombination effect (e^–/h⁺).

Although a narrow band gap energy (E_g = 1.92 eV) was obtained for ZnFe₂O₄, supporting visible light absorption,^6,7 it suffers largely from a high recombination effect, affecting the photocatalytic efficiency.⁷ This is the reason why the ferrite system has to be modified to use as effective photocatalysts.⁸

Namely, the electrochemical potential of the valence band (VB) is estimated to be 0.38 V for ZnFe₂O₄, while for its conduction band (CB), it has been determined to be −1.54 V vs NHE.⁶ There are several reports on MFe₂O₄ (M = Mn, Co, Ni, Zn, Mg, etc.); however, ZnFe₂O₄ is more interesting for the geometrical change associated with electronic, magnetic, and catalytic properties. Zn²⁺ ions prefer to occupy a tetrahedral geometry in the spinel structure {(M²⁺_1–cFe_c³⁺)_A[M_c²⁺ Fe_2–c³⁺]_BO₄ (c = inversion parameter) impacting significantly the electronic, magnetic, and catalytic behaviors.⁹ ZnFe₂O₄ exhibits an antiferromagnetic property at low temperature (10 K) in a normal spinel structure, but it behaves in a paramagnetic nature at room temperature. This occurs because Zn²⁺ (nonmagnetic) occupies strongly at tetrahedral A sites, forcing all Fe³⁺ ions to reside in the octahedral B sites, and as a result, the negative superexchange interaction among the Fe³⁺ in the octahedral occurs leading to antiferromagnetism at low temperature. For the nanosize of ZnFe₂O₄, a ferromagnetic order was seen even at room temperature due to the redistribution of cations between both A and B sites, where some Zn²⁺ ions (positioned in A site) are transferred to B site, in the same way, some of Fe³⁺ (B site) has been moved to A site. These movements impact considerably on the electronic, magnetic, and catalytic characteristics. Zn²⁺ ion is nontoxic, and a biologically essential element, exhibiting a narrow band gap energy (E_g = 1.92 eV), which favors visible light absorption. Furthermore, the spinel structure of ZnFe₂O₄ gives additional catalytic sites by virtue of the crystal lattice enhancing the photodegradation capabilities. The electronic inert nature of Zn²⁺ (d¹⁰ configuration) offers high photochemical stability for ZnFe₂O₄, giving the advantage for the recovery and reuse in the photodegradation. ZnFe₂O₄–Ag (2.5%) performs as an effective photocatalyst since Ag NPs can absorb visible light because of the SPR effect, inducing the flow of plasmon electrons from Ag NPs to the CB of ZnFe₂O₄. As a result, the charge separation can be enhanced. At the same time, a portion of the CB electrons of ZnFe₂O₄ can be shifted toward the Ag⁺ ions, reducing some Ag⁺ ions to Ag NPs; in addition, it has been shown to be an antibacterial agent more than other elements. So, the present work is focused on the deposition of Ag NPs on ZnFe₂O₄ at different proportions, and the influence of electronic and magnetic properties on the photocatalytic and antibacterial behavior was analyzed. Comprehensively, for ZnFe₂O₄–Ag NPs, the band gap energy determined by diffuse reflectance spectra (DRS) is corroborated with that derived from DFT-density of states (DOS). To the best of our knowledge, the theoretical and experimental studies of ZnFe₂O₄–Ag have not been reported in the literature. The photocatalytic performance of ZnFe₂O₄–Ag NPs was also studied for RhB and phenol oxidations as the former can act as photosensitizer. The visible light-powered ZnFe₂O₄–Ag (2.5%) favored the RhB-adsorption (Langmuir isotherm model) and then it involved the degradation. Furthermore, the intermediate formation from the RhB oxidation was analyzed by the DFT total energy calculation.

Results and Discussion

X-ray Diffraction

All the samples were analyzed by powder X-ray diffraction (PXRD) using Cu Kα radiation at 2θ in the range of 20°–80° (see Figure 1, Table S1), and compared to those reported in crystallographic data index No. 0002576F AMCSD (American Mineralogist Crystal Structure Database). The crystalline parameters such as a = b = c = 8.3515 Å obtained for both ZnFe₂O₄ and ZnFe₂O₄–Ag coincided with the cubic phase (JCPDS Card No. 89-1010, and JCPDS 19-0629 for Fe₃O₄). Because the peaks originating from spinel ZnFe₂O₄ at 2θ are 30.0, 35.00, 37.00, 43.00, 53.00, 57.00, and 62.00, representing the crystal planes of [220], [311], [222], [400], [422], [511], and [440], respectively,¹⁰ observing a clean diffraction pattern without additional peaks, it confirms the existence of a single-phase structure of fcc with Fd3m,¹¹ where Zn²⁺ is occupied both octahedral and tetrahedral sites, and Fe²⁺ is located at the tetrahedral geometry (Table S2). In the formation of zinc ferrites, a partial quenching of the peak intensity (100) corresponding to ZnO was detected during the addition of Fe³⁺ at different concentrations. If the ratio of Fe³⁺ ions was increased, the peaks turned to be sharper with high intensity related to the crystallinity of ZnFe₂O₄ NPs. The major lattice planes (100), (002), and (101) originating from ZnO are shifted as observed from the position of I_max of the diffraction signal. This phenomenon can be explained since the formation of ZnFe₂O₄ (without incorporation of Fe³⁺) that shifts signals to higher angles as the ionic radius of Zn²⁺ (0.074 nm) is greater than that of Fe³⁺ (0.064 nm). The crystallite size calculated by the Debye–Scherrer’s equation is almost the same as 35.46 nm for Fe₃O₄, 35.22 nm for ZnFe₂O₄, and 35.23 nm for ZnFe₂O₄–Ag (2.5%) NPs. Typically, the high peak intensity (311) at 35.0° is characterized as a good degree of crystallinity; yet, a small peak shift observed for ZnFe₂O₄–Ag (2%) was due to the influence of Ag in the crystalline phase. For example, the crystalline peaks are shifted in the right direction by the presence of a strained lattice. The compositional change in the sample would certainly influence the cell dimensions; as a result, the peak has been shifted in the right direction. Generally, the residual stress shifts the peaks in one direction, as it is required to balance at grain boundaries to satisfy the constraint strain tensor. This means that the tensile stress has been compensated by the compressive stress as the peak shift is associated with the hkl parameters.

Figure 1.

XRD analysis of zinc ferrites.

In the spinel Fe₃O₄ (Fe³⁺_Td Fe³⁺Fe²⁺_OhO₄) or in the inverse spinel like Fe₃O₄ (Fe³⁺(A) Fe²⁺ Fe³⁺(B)O₄·) (A site, Fe³⁺ with d⁵ high-spin; B site, Fe³⁺ with d⁵ high-spin and Fe²⁺ with d⁶ high-spin), the substitution of Fe³⁺ in the B-site by other metal ions causes a structural change, improving the electrical and magnetic properties. In the case of ZnFe₂O₄, the Fe³⁺ ion is occupied in an octahedral geometry, while both Fe²⁺ and Zn²⁺ ions are shared competitively in a tetrahedral geometry. Some of the Fe²⁺ ions have been replaced by Zn²⁺ in the tetrahedral sites due to its suitable electronic configuration. This observation is consistent with the previous studies that the occupation of Zn²⁺ in both tetrahedral and octahedral sites was proven by the X-ray-absorption fine structure (EXAFS).¹²

Fourier Transform Infrared (FT-IR) Spectra

FT-IR spectra (4000 and 400 cm^–1, Figure S1) were recorded for the samples and the vibrational signals were analyzed. Metal-O stretching primarily originated from the different distances that result in Fe³⁺–O^2–, as the metal ion is occupied in both octahedral and tetrahedral sites. Thus, a typical vibrational peak (≈480 cm^–1) from the Zn_Oh–O–Fe bond (octahedral Zn ion), and another peak (≈660 cm^–1) from its tetrahedral position are observed as reported.^13,14 The Zn–O bonding in Zn–O–Fe, namely, 590 cm^–1 (v_1, M_tetra–O) and 418 cm^–1 (v₂, M_octa–O) corresponding to tetrahedral and octahedral Zn–O vibrations was observed, respectively. In addition, a minor peak splitting between 1600 and 800 cm^–1 was noticed for Zn–O–Fe (bond-stretching) and establishes the existence of the tetrahedral building units.¹⁵ Moreover, Zn²⁺ is preferred to localize in the tetrahedral sites than in other sites, forming a covalent bond through the sp³ hybridization. Nonetheless, other signals like ≈1100 cm¹ and 1200 cm^–1 appeared for the metal sulfate which was used for the preparation of zinc ferrite. Similarly, the peaks (2924 and 2851 cm^–1) indicate the presence of ZnFe₂O₄–Ag (2.5%) even though the signal corresponding to Ag NPs has not been detected because of its small amount in zinc ferrite. The present results are corroborated with other compounds like NiFe₂O₄, CoFe₂O₄, and ZnFe₂O₄, observing two typical bands, one at high-frequency (600–550 cm^–1) corresponding to tetrahedral and another at low-frequency peak (450–400 cm^–1) representing the octahedral sites.¹⁶ The signals at 592 cm^–1 (Ni²⁺-O) for NiFe₂O₄, 572 cm^–1 (Co²⁺-O) for CoFe₂O₄, and 546 cm^–1 (Zn²⁺-O) for ZnFe₂O₄ grow from the tetrahedral sites,¹⁷ while the peaks such as 446, 452, and 438 cm^–1 corresponding to the respective metal ions originate from the octahedral sites.¹⁸ Commonly, the appearance of bands around 1090 and 1600 cm^–1 can be characterized as O–H bonds (adsorbed H₂O).

SEM Analysis

TEM and SEM analyses were performed and the morphological characteristics of the samples (facets 100 and 111, Figure 2) were studied. The results show the presence of an octahedral morphology for ZnFe₂O₄ NPs, consistent with the previous studies.¹⁹ The deposition of Ag NPs on ZnFe₂O₄ has been seen clearly in the SEM images, and distributed randomly on the surface.

Figure 2.

Scanning electron microscopy (SEM) analysis: (a) Fe₃O₄, (b) ZnFe₂O₄, (c) ZnFe₂O₄–Ag (1.0%), (d) ZnFe₂O₄–Ag (2.0%), and (e) ZnFe₂O₄–Ag (2.5%).

A few Ag NPs are dispersed on the surface of ZnFe₂O₄ if the Ag content is low (1.0%) (Figure 2c–e); in contrast, a greater distribution of Ag NPs was observed for the higher content of 2.5%. The particle size determined by SEM images is approximately consistent with those estimated by XRD.

TEM and HRTEM Studies

The size and morphology of Fe₃O₄, ZnFe₂O₄, and ZnFe₂O₄–Ag NPs were also studied by TEM (Figure 3), observing octahedral cubic structure for ZnFe₂O₄ NPs. The size of Ag NPs on the surface of zinc ferrite is found to be ∼16–25 nm, suggesting that nonmagnetic Ag NPs are offered the smallest size (10 to 15 nm) and distributed randomly in the ZnFe₂O₄ as observed in the SEM. Yet, the size of ZnFe₂O₄ NPs (70–80 nm) and Fe₃O₄ NPs (∼120 nm) is relatively greater because of their magnetic properties which encourage particle agglomeration (Figure 3a,b). Since the metal oxide acts as a nucleation center, it controls the size of Ag deposition (average size, ∼25 nm, Figure 3c,d), and also it alters the epitaxial growth of Ag atoms on the ZnFe₂O₄ surface. Furthermore, with the EDS analysis (Table S3), the elemental composition of the samples was determined, and the results reveal the existence of Zn, Fe, Ag, and O. The elemental percentage is nearly close to the theoretical values.²⁰ The atomic ratio for Fe/Zn resulted to be 2.13 for ZnFe₂O₄ and 2.36 for ZnFe₂O₄–Ag (2.5%); however, the latter ratio is slightly greater than the former because of the existence of Ag NPs.

Figure 3.

TEM in darkfield for zinc ferrites (top): (a) Fe₃O₄, (b) ZnFe₂O₄ NPs, and (c) ZnFe₂O₄–Ag NPs, and (d) Ag NPs. HR-TEM (middle): (e) ZnFe₂O₄–Ag interface and (f) FFT for HR-TEM of ZnFe₂O₄. (g) Lattice indexation of ZnFe₂O₄–Ag interface. XPS (bottom): (h) Fe 2p; (i) Ag 3d; (j) Zn 2p.

In the HR-TEM image (Figure 3), the round-shaped Ag NPs (>20 nm) are dispersed over ZnFe₂O₄, giving a high crystallinity for ZnFe₂O₄–Ag NPs (Figure 3e) that can improve the electronic and catalytic properties.^13,21 Predominantly, ZnFe₂O₄ presents in the fcc cubic octahedral structure, where the d- spacing has resulted to 4.36 Å and 0.298 nm in the lattice fringes for the plane (111) and (220), respectively, agreeing with the previous report.²² The presence of crystal planes (222), (111), and (220) are also seen in the ZnFe₂O₄–Ag NPs by the existence of spinel-type structures. The Ag atom is positioned in the vicinity of the zinc ferrite plane (020) (Figure 3f), showing its interaction with metal oxide. This implies that the interface of zinc ferrite dictates the Ag nucleation at the preferential crystalline plane (111) to offer a directional epitaxial growth for ZnFe₂O₄–Ag NPs. The higher degree angle was seen for ZnFe₂O₄–Ag NPs due to a relatively greater ionic radius of Zn²⁺ (0.074 nm) than for Fe³⁺ (0.064 nm).²³

X-ray Photoelectron-Spectroscopy (XPS)

XPS was carried out for the ferrite samples to confirm the oxidation state of metal ions Fe²⁺, Fe³⁺, Zn²⁺, and Ag⁰ (see Table 1, Figure S2), showing the existence of peaks corresponding to Zn, Fe, Ag, and O for ZnFe₂O₄–Ag NPs (Figure 3h–j). In particular, two signature peaks (1022.08 eV, fwhm = 4.36 eV for Zn 2p_3/2 and ∼1044 eV, fwhm = 3.21 eV for Zn 2p_1/2) representing Zn species are detected; the first signal at 1022.08 eV originates from Zn²⁺ that is located in the tetrahedral A sites, while the later signal emerges from Zn²⁺ occupying the octahedral B sites.^24,25 This means that Zn²⁺ is present in both A and B sites within the ferrite crystal. In the spectra, the range 730–705 eV is assigned to Fe 2p, especially, two typical signals (∼711 eV for Fe 2p_3/2 from Fe²⁺ and ∼725 eV for 2p_1/2 from Fe³⁺) and the energy difference between those peaks (14.0 eV)²⁶ confirm the presence of Fe₃O₄ in ZnFe₂O₄ matrix. The Fe 2p_3/2 signal can be deconvoluted into two peaks (712.08 and 711.08 eV) assigned to Fe³⁺ in the octahedral and tetrahedral geometry, respectively. Moreover, the satellite peak appearing around 725 eV between Fe 2p_3/2 and 2p_1/2 suggests the absence of the Fe₃O₄ phase in the composite. The peak originating from Fe 2p_3/2 is also fitted into two peaks (∼711.08 and ∼710.0 eV), indicating Fe³⁺ is presented in both tetrahedral (A site) and octahedral (B sites) for ZnFe₂O₄. This shows that the ZnFe₂O₄ is presented in a partially inverse spinel structure, offering a mixture of two oxidation states (Fe²⁺ and Fe³⁺) in contrast to Fe³⁺ in Fe₂O₃.²⁷ The binding energy observed for Fe 2p_3/2 (712.08 and 711.08 eV) is consistent with its tetrahedral (A site) and octahedral (B site) structures, respectively. Another signal detected at 530.08 eV represents the O atom associated with the bonding of Fe³⁺ to O (Fe³⁺–O). A broad peak ∼530.08 eV corresponds to O 1s and is fitted into three peaks (∼528.0, 529.0, and 531.0 eV) assigned to the lattice oxygen O^2– from Zn–O and Fe–O linkages. The other two peaks (531.0 and 530.0 eV) are characterized as the surface-absorbed oxygen. The peaks of Ag 3d_5/2 and Ag 3d_3/2 (369 and 375 eV) correspond to the Ag⁰ atoms present in ZnFe₂O₄–Ag. However, the broadening of the Ag-peak is highly related to the Ag cluster epitaxial growth on the metal oxide. The high electronegativity of Ag (χ = 2.4) facilitates the interaction between the Ag atom and ferrite, promoting its epitaxial growth on ZnFe₂O₄. Thus, the electronic transfer can occur from the conduction band (CB) to Ag NPs in the ZnFe₂O₄ array.²⁸ Some minor peaks associated with impurities corresponding to Na, C, and N as sodium citrate and sodium bicarbonate were used for the preparations of Ag NPs and zinc ferrite, despite the possibility of the XPS chamber contamination (Table S4).

Table 1. XPS Data of the Nanocomposites.

	ZnFe2O4–Ag(2.5%)		ZnFe2O4		Fe3O4
Transition	EB (eV)	fwhm	EB (eV)	fwhm	EB (eV)	fwhm
Zn 2p1/2	1044.08	3.2	1045.03	1.54	-	-
Zn 2p3/2	1022.08	1.0	1022.08	1.02	-	-
Fe 2p1/2	725.08	2.9	726.08	3.7	725.08	3.9
Fe 2p3/2	711.08	5.3	712.08	4.1	711.08	4.3
Fe(II)Fe 2p1/2 Satellite	-	-	-	-	736.08	5.0
Fe(III)Fe 2p1/2 Satellite	734.08	3.4	733.08	3.1	733.08	2.4
O 1s	530.08	2.62	531.08	3.21	531.08	3.17
Ag 3d3/2	375.08	2.43	-	-	-	-
Ag 3d5/2	369.08	2.11	-	-	-	-

Magnetic Studies

The magnetic properties of the samples were analyzed on a Vibrating sample magnetometer (VSM) by applying the external magnetic fields at 298 and 5.0 K. In an isothermal magnetization (M-H), by plotting of magnetization (M) against the applied magnetic field (H), hysteresis loops were obtained; This reveals the formation of a single domain in the randomly oriented uniaxial spherical particles.²⁹ Since the magnetic values are highly dependent on size, the nature of morphology, and composition of the samples, a lowest magnetic saturation (Ms, 7.25 emu/g) was observed for ZnFe₂O₄–Ag than for ZnFe₂O₄ (Ms, 9.61 emu/g). For magnetite (Fe₃O₄), as anticipated, the value was greater than that observed for other samples (91.10 emu/g, see Table 2, Figure 4). However, the Ms value was increased if the temperature was lowered from 298 to 5.0 K for ZnFe₂O₄–Ag (7.25 to 18.71 emu/g), an increase of about 160% times greater than that observed at room temperature. The same behavior was observed for ZnFe₂O₄ (9.62 to 25.09 emu/g). While for Fe₃O₄, the increase was found to be just 5.0% (91.09 to 96.19 emu/g). This means that the magnetization of both ZnFe₂O₄ and ZnFe₂O₄–Ag is highly temperature sensitive, agreeing with the squareness ratio (SQ) and Hc values. For example, the squareness ratio (SQ = Mr/Ms), which is associated with the randomly oriented-uniaxial grains (small single-domain particles), results to a smaller value for ZnFe₂O₄–Ag (0.008 emu/g) than for ZnFe₂O₄ (0.054 emu/g) or Fe₃O₄ (0.055 emu/g). The magnetic behaviors are related to the nonequilibrium distribution of Fe³⁺ in the spinel A and B sites, causing significant crystal defects. Thus, a lower coercivity (Hc) for ZnFe₂O₄ (0.14 kOe) and ZnFe₂O₄–Ag (2.5%) (0.13 kOe) has resulted as compared to that for Fe₃O₄ (0.21 kOe). The cationic distribution between tetrahedral and octahedral sites can increase the cation inversion parameter, improving the magnetization; mostly, it depends on the manner in which the Fe³⁺ is being distributed in the geometries.³⁰ A decrease in the surface anisotropy is attributed to the deposition of Ag NPs.

Table 2. Magnetic Parameters of Ferrite Samples.

	T = 298 K				T = 5 K
	Ms	Mr	SQ	Hc	Ms	Mr	SQ	Hc
material	emu/g			kOe	emu/g			kOe
Fe3O4	91.09	5.02	0.055	0.21	96.19	6.01	0.062	0.13
ZnFe2O4	9.62	0.52	0.054	0.14	25.09	4.67	0.186	0.46
ZnFe2O4–Ag(2.5%)	7.25	0.06	0.008	0.13	18.71	3.71	0.198	0.46

Figure 4.

Magnetic properties: (a) full VSM spectra; (b) VSM intersection at 298 K, and (c) VSM at 5K.

Diffusion Reflectance Spectra (DRS)

For all the samples, the DRS (Figure S3) was recorded, observing a broad-band around 298 nm. The peak intensity was diminished for ZnFe₂O₄–Ag NPs at 300 nm as compared to other samples. Moreover, if the deposition amount of Ag NPs on ZnFe₂O₄ is increased, the Ag NPs become nonplasmonic (>0.75 atom %). The bandgap energy was determined by using the DRS. (i) Tauc method: α = A(hv – E_g)²/λ for direct (allowed) and α = A(hv – E_g)^1/2/λ for indirect (allowed) (α = absorption coefficient; A = absorption constant for indirect transitions depending on the transition probability). The extrapolation of the horizontal y-axis³¹ against the x-axis (photon energy, hv) intercepts the bandgap energy. (ii) In the Kubelka–Munk function F(R),³² the diffuse absorption spectra were converted into the reflectance spectra through which the band gap energy was calculated as indicated in eqs S3 and S4.³³ In the present work_, the Tauc’s plot was obtained by extrapolating (αhv)^1/2 against the photon energy, calculating the indirect bandgap energy as 2.00 eV for Fe₃O₄, 1.9 eV for ZnFe₂O₃, and 1.76 for ZnFe₂O₄–Ag. The results show a decrease in the bandgap energy for ZnFe₂O₄–Ag, which assists the hot electron injection from Ag NPs to the CBs; consequently, it increases the half-life of photoinduced electrons as described in Scheme 1.³⁴ This is consistent with DFT-DOS spectra, where the Fermi energy level of Ag NPs has been positioned just below the conduction band (CB), producing a Schottky barrier in the interface of ZnFe₂O₄. It is known that ligand metal charge transfer (LMCT) is accounted for the color of iron oxide (Fe₃O₄, Fe³⁺, 3d⁵) because of the forbidding electronic transitions (both spin and d–d transitions). Yet, if the magnetic coupling has occurred appropriately between next-nearest-neighbor Fe³⁺ ions, then these d–d transitions (⁶A₁ → ⁴T₁, ⁶A₁ →⁴T₂, and ⁶A₁ → ⁴E) are allowed as it depends upon the Fe-to-Fe distance. The quantum yields (Φ) were determined by measuring the transmitted energy (γ) at 400 nm as reported³⁵ as 0.60 for ZnFe₂O₄, 0.25 for ZnFe₂O₄–Ag (1.0%), 0.70 ZnFe₂O₄–Ag (1.5%), 0.66 for ZnFe₂O₄–Ag (2.0%), and 0.66 for ZnFe₂O₄–Ag (2.5%), showing that the disposition of Ag NPs (1.5–2.5%) gives a good quantum yield of Φ > 0.60.

Scheme 1. (a) Proposed Mechanism for the RhB Oxidation, (b) Radical Formation Detected by EPR.

Adsorption Studies

The adsorption behavior of RhB over ZnFe₂O₄–Ag NPs (2.5%) was studied (Figure 5a) by measuring its concentration at 553 nm, and then plotted against the time to obtain an equilibrium constant (q_e). The adsorption data were analyzed by fitting in the following models in Langmuir (red), Freundlich (green), and Redlich–Peterson (blue) equations.³⁶ The results show that the Freundlich and Redlich-Peterson models allowed a good fit as compared to the Langmuir model for ZnFe₂O₄–Ag (2.5%) and Fe₃O₄. The maximum adsorption constant results in q_m = 206 mol/g for ZnFe₂O₄–Ag (2.5%), rather than for ZnFe₂O₄ (2.36 × 10^–3 mol/g) or for Fe₃O₄ (2.43 mol/g)_, respectively. Notably, ZnFe₂O₄–Ag NPs follow the Redlich-Peterson model, resembling Langmuir adsorption at low concentrations, but ZnFe₂O₄, yielded a poor plot (see Figure S4).

Figure 5.

(a) RhB adsorption: Nonlinear fit for Redlich-Peterson, Freundlich, and Langmuir, isotherms for ZnFe₂O₄–Ag (2.5%). (b) Plot of (C/C₀) vs time under UV light; (c) Plot of (C/C₀) vs time under visible light. (d) Plot of (−ln C/C₀) vs time under visible light; (e) Evaluation of stability and reusability of ZnFe₂O₄–Ag (2.5%).

Photocatalytic Degradation of RhB

The photocatalytic properties of ZnFe₂O₄Ag (2.5%), ZnFe₂O₄, and Fe₃O₄ were analyzed under UV or visible lights using RhB as a model pollutant. The substrate concentration was measured through absorbance intensity at 553 nm and plotted against the time (Figure 5b,c). A considerable change of color (pink to colorless) was observed, and it was reflected in the absorption maximum by shifting λ₀ = 553 to λ_f = 538 nm during the oxidation (Figure S4a,b). The degradation follows a first-order kinetics, and the rate constant (k) was calculated using the equation ln[C] = −kt + ln[C₀] (Figure 5d). The results reveal a greater value for ZnFe₂O₄–Ag (2.5%) (k = 2.6 × 10^–3 min^–1) than for ZnFe₂O₄ (k = 5.0 × 10^–4 min^–1) or for Fe₃O₄ (k = 4.1 × 10^–4 min^–1) under UV light. The oxidation of RhB was about 74.5% for ZnFe₂O₄–Ag (2.5%), 34.2.3% for ZnFe₂O₄, and 27.2% for Fe₃O₄ under UV light. While it was 92.2% for ZnFe₂O₄–Ag (2.5%), 52.4% for ZnFe₂O₄, and 40.8% for Fe₃O₄ under visible light observing a greater oxidation rate for ZnFe₂O₄–Ag (2.5%) (k = 5.5 × 10^–3 min^–1) as opposed to others (k = 2.0 × 10^–3 min^–1 for ZnFe₂O₄ and k = 1.1 × 10^–3 min^–1 for Fe₃O₄). The present results are consistent with the reported work, that is, for bare ZnFe₂O₄ and Bi₂WO₆, the RhB photodegradation was just 28% and 44% under visible light, respectively; however, the percentage was increased to 76% when it was modified to Bi₂WO₆/ZnFe₂O₄.³⁷ For ZnFe₂O₄, it was reported around 60%,³⁸ and the value was relatively higher in the other studies.^39,40 This indicates that without any catalyst, the oxidation of RhB was found to be less than 5.0% even after 3 h of irradiation, and it was increased to 31% after 3 h of irradiation in the presence of ZnFe₂O₄ (Zn_xFe₂O₄). Nonetheless, a significant increase in degradation (93%) was reported if the Zn_xFe₂O₄ was decorated with Au.⁴¹ It is known that the photocatalytic activity of pure ZnFe₂O₄ is relatively poor because of the rapid recombination effect^40,42 which was reduced considerably for ZnO/ZnFe₂O₄ or Ag-ZnFe₂O₄@rGO composites.^39,43 The removal of RhB was about 90% in the present work, and some studies have reported more than 90%; this is probably due to the experimental conditions adopted for the oxidation, including the concentration of H₂O₂^44,45 and the magnetization associated agglomeration. Thus, ZnFe₂O₄–Ag (2.5%) is shown as an effective photocatalyst because the plasmonic metal deposition on ZnFe₂O₄ promotes favorable active sites, supporting the photoactivation process. In the literature, the removal of pollutants by ZnFe₂O₄-based composites was reported.^39,40 The reusability and stability of ZnFe₂O₄–Ag (2.5%) was recovered magnetically and reused up to five times for the degradation of RhB. The results (Figure 5e) show an excellent performance in reusability with high efficacy during the recycles, and the composite was considerably stable even after five consecutive cycles.

Since the RhB can act as a photosensitizer altering the efficiency of the oxidation, we have also used phenol as a colorless pollutant to assess the catalytic degradation efficiency. The phenol oxidation was about 64.5% in the presence of ZnFe₂O₄–Ag as compared to 39.4% for ZnFe₂O₄ and 28.5% for Fe₃O₄ under visible light (Figures S5a and S6c). So, if ZnFe₂O₄ and ZnFe₂O₄–Ag NPs are excited by photons under visible light, it generates electron–hole pairs; consequently, the photogenerated electrons can be transferred from the CB of ZnFe₂O₄ (−1.54 V vs NHE)⁴³ to the surface of Ag NPs (E_Ag+/Ag, 0.80 V vs NHE) since Ag NPs can absorb visible light because of the SPR effect. The plasmon-induced electrons of Ag NPs can also flow easily to CB of ZnFe₂O₄; as a result, the charge separation can be enhanced. Moreover, a portion of the CB electrons of ZnFe₂O₄ can be shifted toward the Ag⁺ ions, reducing some Ag⁺ ions to Ag NPs. However, other electrons would be scavenged by O₂ molecules in water to produce superoxide free radicals (*O^2–, reactive active species). Additionally, the reactive holes in VB could oxidize the pollutants efficiently due to the more negative potential of the VB (ZnFe₂O₄, 0.38 V vs NHE)^43,46 relative to that of H₂O (E_OH/H2O = 2.87 V vs NHE).^43,47 The EPR measurements (Scheme 1b) were carried out in a quartz tube at room temperature in aqueous suspensions using a Jeol JES-TE300 spectrometer operating in X-Band fashions at 100 kHz modulation frequency. The external calibration of the magnetic field was performed using a Jeol ES-FC5 precision gaussmeter with a 5350B HP microwave frequency counter. Spectral acquisition and manipulations were performed using ES-IPRITS-TE software. EPR spectra recorded show that there are formation radicals that are oxidizing the substrate. The mismatching of the heterostructure of ZnFe₂O₄ (cationic valence and coordination number, Zn²⁺ for tetrahedral, Fe³⁺ for octahedral in Zn–O–Fe) induces the polarizing capacity of Fe³⁺ over Zn²⁺ contributing more surface charge on ZnFe₂O₄–Ag. The DFT-E_g determined shows that the value is lower for ZnFe₂O₄/Ag when compared to that for ZnFe₂O₄ (see computational section). Previous reports⁴⁸ establish the formation of prominent intermediates for the oxidation of RhB, and the oxidation of RhB by different ferrite and nonferrite materials is summarized in Table 3.

Table 3. Different Ferrite and Nonferrite Materials as Photocatalysts for the RhB.

Nanomaterial	Size (nm)	Reaction time (min)	Light source	Removal (%)	ref
TiO2/GO nanocompositesa	50	20	UV	90.56	(49)
Fe3O4/ZnO/GOb	5.0	50	UV	100	(50)
MWCNT–CuNiFe2O4b	3–14	30	UV	100	(51)
MWCNTs/CoFe2O4b	32	60	UV	95.2	(52)
ZnFe2O4@g-C3N4	5.0	30	Visible	96.2	(53)
ZnFe2O4	20	300	Visible	100	(54)
ZnS–ZnFe2O4	20	90	UV	97.7	(55)
NixZnyFe2O4	30	180	Sunlight	94.0	(56)
Co0.7Zn0.3Fe2O4	20	40	Sunlight	∼20.0	(57)
ZnFe2O4	19	30	Visible	60.0	(38)
ZnFe2O4	12	60	Sunlight	94.0	(58)
ZnFe2O4/TiO2/flake graphite	100–1000	30	UV	99.0	(59)
ZnO	500–400	180	Solar	100	(60)
TiO2/C	20–50	4	Visible	100	(61)
TiO2 P-25	20	30	UV	55.0	(62)
CQD/ZnO	10–30	120	UV	80.0	(63)
CQD/Cu/N/Ag3PO4	200–300	60	LED	95.5	(64)
Ag3PO4/SAPO-34 (SAPO-34= silicon aluminum phosphate)	100–200	30	Visible	90.8	(44)
CQD/CdS	10–50	180	Visible	90.0	(65)
Cu2-xSe/CdS	-	180	Visible	∼95	(45)
CQDs/Bi2MoO6	20	120	Visible	100	(66)
CQDs/BiOCl/BiOBr	40	60	Visible	100	(67)
Sr/Ce/AC	50	120	Solar	91.0	(68)
ZnO/Au	30	180	UV	95.0	(69)
ZnFe2O4Ag (2.5%)	20 nm	420	UV	74.5	Present work
ZnFe2O4,	10–15	420	UV	34.2	Present work
Fe3O4	10–15	420	UV	27.2	Present work
ZnFe2O4Ag (2.5%)	20	360	Visible	92.4	Present work
ZnFe2O4	10–15	360	Visible	52.3	Present work
Fe3O4	10–15	360	Visible	46.3	Present work

^aDegradation of acid orange 7 dye.

^bDegradation of acid blue 113 dye.

This is consistent with the DFT studies, where the total energy of the intermediates shows a feasible cleavage of the diphenyl group by OH radicals (Scheme 2).

Scheme 2.

(a) Illustration of RhB oxidation; (b) total energy of intermediates modeled by DFT at 6-311G/BB95K.

1

2

3

4

5

6

7

8

Point of zero charge (PZC) was determined by applying the pH drift method for ZnFe₂O₄ and ZnFe₂O₄–Ag (2.5%). The results (see Figure S7) show 7.9 for ZnFe₂O₄. For the addition of Ag on ZnFe₂O₄, the PZC value gradually decreases to 7.15 for ZnFe₂O₄–Ag (2.0) and 7.6 for ZnFe₂O₄–Ag (1.0). A higher rate constant results at low pH in contrast to the high pHs since the production of OH radicals is more feasible at low pH than at high pH where the presence of OH^– ions is prominent.

Density Functional Theory (DFT)

We explained the oxidation of RhB through the DFT studies.⁷⁰ The structure of intermediates was optimized, and the total energy with respect to the RhB+2^•OH system was determined (see Scheme 2). The photo-oxidation occurred through the attack of two OH radicals that cleaved the benzoic group (RhB structure), as it required about +148 kcal/mol to form the first adduct (Step 1, nonspontaneous process). In step 2, the OH radical is attacked at the tertiary imine undergoing easy oxidation to produce ethanoic acid because of low total energy (−347 kcal/mol). In step 3, ^•OH radicals attack at the tertiary amine attached to the aromatic center, yielding acetic acid as an endothermic process (+118 net kcal/mol) with respect to the starting system; yet, it is more feasible than the first step. The last step is highly exothermic (−734 kcal/mol) and it involves the formation of two products (xanthene core to splitting) by taking three OH radicals.

Computational Modeling of ZnFe₂O₄–Ag

The structural and electronic properties of ZnFe₂O₄–Ag are analyzed, and the energy level of the valence (VB) and conduction (CB) bands are determined. DFT structural optimization of ZnFe₂O₄–Ag was also performed using periodic functional HSEh1PBE with the LANL2DZ basis set. First, the structure of ZnFe₂O₄ was constructed after considering its spinel fcc geometry consisting of 56 atoms (8 Zn, 16 Fe, and 32 O). The total volume resulting was 567.93 ų (a = 8.281 Å, b = 8.20 Å, and c = 8.283 Å). Similarly, the structure of the Ag₇ cluster was also optimized to attach to the surface of ZnFe₂O₄ yielding ZnFe₂O₄–(Ag₇)_n (Figures S8, S9, and S10). After analyzing the electronic properties, it was noticed that CB was formed by mixing empty d orbitals from both Zn²⁺ and Fe²⁺ (ZnFe₂O₄); while for VB, only empty d orbitals from Fe³⁺ were involved. Apparently, oxygen 3p orbitals have not contributed significantly to the formation of either CB or CV band. The overall electron density of unoccupied states was increased if the Ag atoms adhered to ZnFe₂O₄ due to the participation of 4d orbitals in CB (Ag cluster). As a result, new sets of electronic states, holding either e^– or h⁺, are formed; the formation of charged pairs has been seen through the excitons, reducing the recombination effect, and it has enhanced the photon harvesting capability. This is consistent with ZnO, Fe₃O₄, TiO₂, and ZnS systems.⁷¹

The metallic bonds present in ZnFe₂O₄ are involved in the delocalization associated with the Ag clusters as stated in the jellium model⁷² and the resulting electronic shell has resembled that in other studies.⁷³ The total energy (E_t) of the systems was defined by the interaction of ZnFe₂O₄ (E_zf) with a number (n) of (Ag₇)_n clusters (eq 9):

9

The interface between Ag5 d and Fe²⁺ 3 d influences the degree of interaction in the electronic states. If the energy difference is yielded as ΔE ≥ 0, then the Ag cluster has interacted preferably on the ZnFe₂O₄ system, producing a synergistic effect. On the other hand, if the energy difference is ΔE ≤ 0, it would not generate an energetically positive impact on the interaction of the Ag clusters with ZnFe₂O₄. For ZnFe₂O₄–(Ag₇)_n (n = 1,2), a high stability results, increasing the electron density at the edge of occupied states (VB), and the same notation was observed in the edge of unfilled states (CB). As a result, the contribution of electron density from Ag4 d to the bandgap energy is significantly reduced to 1.11 for ZnFe₂O₄–(Ag₇)₁ and to 1.02 eV for ZnFe₂O₄–(Ag₇)₂ (Figure S11). In contrast, for ZnFe₂O₄–(Ag₇)₃ and ZnFe₂O₄–(Ag₇)₄, an unstable geometry was obtained, as Ag d states are located at the band edges, increasing the band gap energy.

For the deposition of Ag on ZnFe₂O₄ such as ZnFe₂O₄–(Ag₇)_n (n = 5,6), a decrease in total energy was seen. If the size of the cluster is augmented, the net bandgap energy is 2.55 for (Ag₇)₅ and 2.46 eV for (Ag₇)₆ (Figure S8). Notably, the calculated bandgap energy is usually greater as compared to the experimental values due to the quantization of states present in small-scale systems.⁷⁴ For ZnFe₂O₄, the experimental bandgap energy (1.9 eV)⁷⁵ is relatively smaller than the calculation values (2.7–3.0 eV) owing to the quantization of orbitals. If the Ag₇ cluster was placed on ZnFe₂O₄, a new set of unoccupied energy states has been generated by lying between CB and VB right above the Fermi level (energy state of Ag d, E ≈ −0.4 ± 0.1 eV). It forms an e^–/h⁺ pair excitons occurring between VB and Ag d, which facilitates the “hot electron injection” into CB by increasing the electron density (Figure 6c). The filled shells contribute considerably to the stability of ZnFe₂O₄–Ag. However, a specific ratio of ZnFe₂O₄:Ag can enhance the photocatalytic properties, particularly, the amount of Ag NPs deposition (i.e., a certain percentage) allows the surface modification for ZnFe₂O₄.⁷⁶ Yet, a marginal contribution from the Ag cluster was detected in the full DOS spectra (Figure 6a,b).

Figure 6.

DOS for (a) bare ZnFe₂O₄ and (b) ZnFe₂O₄–(Ag₇)₆ denoting valence bands (red area), conduction bands (green area), and Ag_4d states; DOS of Ag₇ is presented as a blue dashed line. (c) Proposed band energy for ZnFe₂O₄ and ZnFe₂O₄–(Ag₇)₆ based on jellium exciton approach.

Antibacterial Studies

The antibacterial properties of ZnFe₂O₄–Ag (2.5%), ZnFe₂O₄, and Fe₃O₄ were also studied against different pathogens (Staphylococcus aureus, Bacillus cereus, Escherichia coli, and Salmonella typhi) (Figure 7). The results show a significant bacterial growth inhibition in the presence of both ZnFe₂O₄ and ZnFe₂O₄–Ag NPs (Table S5). The existence of Ag atoms on ZnFe₂O₄ (ZnFe₂O₄–Ag NPs) enhances further antimicrobial activities, resulting in the lowest MIC value for B. cereus as 62.22 μg/mL for ZnFe₂O₄–Ag NPs, ∼82.96 μg/mL for ZnFe₂O₄, and ∼414.82 μg/mL for Fe₃O₄ NPs. The same behavior was observed for other pathogens (Table 4, Figure 6b) due to Ag⁺ or Ag NPs which react with an organic functional group (thioglucoside, carboxyl, hydroxyl), inhibiting the bacterial growth through altering the cellular functions. Thus, the entrance of NPs into the bacterial cell affects the electron transfer system in the enzyme/protein gene expression.⁷⁷

Figure 7.

Bacterial inhibition: (a) proposed mechanism of action; (b) MIC data of antibacterial studies.

Table 4. Antibacterial Data of Zinc Ferrites.

	Minimal inhibitory concentration [uM]
	Gram-Positive		Gram-negative
Inhibitor	Bacillus cereus	Lactobacillus acidophilus	Escherichia coli	Salmonella typhi
ZnFe2O4	82.96	82.96	62.22	82.96
Fe3O4	414.82	414.82	207.41	82.96
ZnFe2O4–Ag	62.22	62.22	41.48	41.48

It is still unclear how a specific pathway is being adopted for the bacterial inhibition; however, the release of Ag⁺ from ZnFe₂O₄–Ag NPs is expected, and it generates ROS (H₂O₂, ¹O₂, and OH^•) which can interact with cells to inhibit the DNA replication.⁷⁸ The generation of radicals from the sample was proven by the EPR studies at room temperature in aqueous suspensions (see Scheme 1b), showing that there is a formation of free radicals, consisting of the reported studies.⁷⁹ It is also coherent with other reports that ZnO NPs can produce a variety of ROS (H₂O₂, ^•OH, and ¹O₂) in aqueous solution, and it was also further proven by the EPR technique.⁸⁰ So, ZnO in Fe₂O₃ plays a decisive role in the generation of ionic signals between cells and intercellular cytoplasmic organoids.⁸¹ The external oxidative species can be easily diffused into a single-layer cell membrane (Gram-Positive: Lactobacillus acidophilus (Bacillus cereus) as compared to a three-layer membrane configuration of E. coli and S. Typhi (Gram-negative type). Small size particles can improve the particle–cell contacts to enhance the intrinsic toxicity, and it suggests that the size of ZnFe₂O₄ and ZnFe₂O₄–Ag NPs (<50 nm) encourages the inhibition of cell growth. We have also studied the behavior of S. cerevisiae cells with ZnFe₂O₄–Ag (2.5%) in confocal fluorescence microscopy, showing the increasing diffusion of ZnFe₂O₄–Ag NPs (2.5%) into the cell as it emits blue light at 405 nm.

Experimental Sections

Chemical, Materials, And Characterization Details

Zn(CH₃COO)₂, AgNO₃, NaBH₄, Na₂CO₃, MeOH, CH₃CN, and polyethylene glycol (Sigma-Aldrich) were used as received. For the inoculation, cysteine Tryptic Agar (CTA, BIOXON), Mueller Hinton Agar (MHA), and Nutritive Broth (DIBICO) were employed. For cell culture, Petri dishes (90 × 15 mm) and 96-wells plates were considered.

Fe₃O₄ NPs

KOH (1.0 M, 3.57 g, 50 mL) dissolved in water was added to an aqueous solution of FeCl₂ (15 mM, 50 mL), and the resulting mixture was stirred for 25 min before it was transferred to a Teflon (100 mL) autoclave, which was then heated at 180 °C for 18 h. After the mixture slowly cooled, a dark-colored product was obtained which was washed with ethanol three times, followed by cold acetone. Finally, after deionized water was used to remove excess KOH, the product was treated thermally in a furnace at 75 °C overnight. Yield: 23.5%.

ZnFe₂O₄ NPs

As reported previously,⁸² FeSO₄·7H₂O (1.0 g, 3.59 mmol) and Zn(OAc)₂·2H₂O (0.43 g, 1.95 mmol) were dissolved in an aqueous solution (40 mL) containing N₂H₄·H₂O (10 M, 0.05 mmol). The resulting mixture was transferred to a Teflon-contained autoclave for heating for 14 h at 180 °C. A brownish sample was obtained after the mixture was centrifuged (6000 rpm for 15 min), and it was washed with different ethanol/water mixtures (25:75; 50:50), followed by pure EtOH). The product was dried in an oven at 60 °C for 13 h, yielding 19.1% (Scheme 3).

Scheme 3. Preparation of Zinc Ferrite.

Ag Doping of Fe₃O₄ and ZnFe₂O₄ NPs

Using the modified procedure,⁸³ a nanocomposite was prepared. Briefly, ZnFe₂O₄ NPs (40 mg) were suspended in 50 mL of ethanol, and the mixture was stirred at 4 °C under dark conditions. To this suspension, AgNO₃ solution (2.36 mL, 2.0 mM) was slowly added under stirring, and then NaBH₄ solution (3.1 mL, 10 mM) was added. After the reaction, the sample was separated magnetically and dried at 60 °C for 12 h. Yield: ca. 35 mg (47.29% Fe and 21.41% Zn). The same procedure was adopted for the deposition of Ag at different proportions (1.18 mL, 1.0%; 1.77 mL, 1.5%; 2.36 mL, 2%; 2.95 mL, 2.5%).

Adsorption Studies

The adsorption behavior of ZnFe₂O₄–Ag (2.5%), ZnFe₂O₄, and Fe₃O₄ with RhB dye was analyzed. Typically, the adsorbent (10.0 mg) was added to the RhB solution (10 mL, 1.0 × 10^–6 M to 7.5 × 10^–6 M) in an amber bottle, stirred for 15 h in the dark room, and then centrifuged at 15 000 rpm for 10 min to collect the supernatant. The equilibrium concentration of RhB was calculated by measuring the absorbance at λ_max, RhB = 554 nm, and corroborated with the standard plot. The equilibrium capacity of adsorption q_e (mol/g) was determined by employing the equation q_e = (C₀ – C_e) V/m, where C₀ (mol/L) = [RhB] at time 0; C_e = [RhB] at equilibrium time, V (mL) = volume of the dye solution, and m (mg) = adsorbent mass.

Photocatalytic Studies

Photocatalytic degradation of RhB dye was studied using ZnFe₂O₄–Ag (2.5%), ZnFe₂O₄, and Fe₃O₄ under UV or visible light. The reactor was equipped with a xenon lamp (UV irradiation, Newport 361, 1000 W) having a Pyrex glass filter (λ = 390 nm). The catalytic sample (15 mg) was mixed with RhB (20 mL, 2.0 μM) and allowed to degrade under the photoreactor. The concentration of RhB was determined at different time intervals (60, 120, 180, 240, and 300 min) at λ_max, RhB. Similarly, the degradation of an aqueous solution of RhB (5.0 mL, 10 μM) with the sample (Fe₃O₄, ZnFe₂O₄, or ZnFe₂O₄–Ag (2.5%, 0.2 mg/mL) was observed under visible light (halogen lamp, 1.0 m, 239 W/m²). Hydrogen peroxide (0.3%) was used for the activation. Separately, the photocatalytic oxidation of phenol (5 mL, 100 μM) was studied under visible light (halogen lamp, 1.0 m, 239 W/m²) in the presence of Fe₃O₄, ZnFe₂O₄, or ZnFe₂O₄–Ag (2.5%, 0.2 mg/mL). The concentration of phenol was determined through the 4-amino antipyrine method. Phenol solution (1.0 mL) was first mixed with NH₄OH (0.5 N, 25 μL) to adjust the pH of the solution (7.9 ± 0.1), and then to which 4-aminoantipyrine (10 μL) was added, followed by K₃[Fe(CN)₆] (10 μL). The resulting solution was stirred for 15.0 min in the phosphate buffer. The solution turned red, facilitating the measurement of the substrate concentration in the visible region in the spectrophotometer.

Antibacterial Activity

Antibacterial properties of the samples ZnFe₂O₄–Ag (2.5%), ZnFe₂O₄, and Fe₃O₄ were studied using different bacterial strains (two strains of Gram-positive (Staphylococcus aureus and Bacillus cereus) and two of Gram-negative strains (Escherichia coli and Salmonella typhi), which were obtained from WFCC/WDCM-100, Faculty of Chemistry, UNAM, Mexico, and maintained in Cysteine Tryptic Agar (CTA, 4 °C). The characterization of strains was followed as indicated in ref (84). (see Supporting Information).

Minimal inhibitory concentration (MIC) was determined against bacteria as reported previously.⁸³ The strain standardization was performed in the suspension of Mueller-Hinton nutritive agar to reach 1.0 × 10⁶ CFU mL^–1. A solution of Fe₃O₄, ZnFe₂O₄, or ZnFe₂O₄–Ag was first prepared as a stock dilution and then diluted from 5 to 200 μg·mL^–1.

Bacterial qualitative assays were evaluated with the assistance of the disk diffusion method as described elsewhere.^84,85,26,86 Mueller Hinton Agar plates were first inoculated with previously standardized inoculum, then the sterile paper discs (Whatman No.1, 6.0 mm in diameter) were placed on the surface of the plates. After the solution of Fe₃O₄, ZnFe₂O₄, or ZnFe₂O₄–Ag NPs (20 μL, 30.0 mM) were dispersed on the surface of each disc, the plates were then incubated for 24 h at 35 °C under visible light. During the measurement of diameters (mm) of cell growth inhibition zones, an aqueous solution of p-iodonitrotetrazolium chloride (10 μL, 10 mM) was added as an indicator.

Computational Procedure

The structural optimization of ZnFe₂O₄ and ZnFe₂O₄–Ag atoms was performed by Density Functional Theory (DFT) using a periodic functional like the HSEh1PBE LANL2DZ basis set. Single crystal structural data of ZnFe₂O₄ were obtained from the Chem Tube 3D database, University of Liverpool, for optimization. The model consists of a spinel fcc cubic structure (56 atoms: 8 Zn, 16 Fe, and 32 O), and a = 8.281 Å, b = 8.20 Å, and c = 8.283 Å to result to a total volume of 567.93 ų. Silver clusters (Ag₇)_n (n = 1–7) were placed over ZnFe₂O₄ and optimized the cell structure. The clusters were distributed evenly on each of the crystal unit facets (Figure S7). To determine the electronic structure, ultrasoft pseudopotentials (USP) were used, and for the solvent effect, an IEF-PCM was employed.

Physical Measurements

X-ray diffraction (XRD) analysis was carried out by using a diffractometer (Rigaku RU300) with Cu Kα radiation (λ = 1.5406 nm) at 2θ between 20° and 80°. The obtained XRD data for the samples were compared to those reported in the crystallographic data index No. 0002576F AMCSD (American Mineralogist Crystal Structure Database). The observed crystalline structure for ZnFe₂O₄ and ZnFe₂O₄–Ag corresponds to fcc, for normal spinel (fd3m) where Zn²⁺ occupies the octahedral and tetrahedral geometries while Fe²⁺ is seated at the tetrahedral site. The crystal size was calculated using the following equations

10

β_obs = fwhm of refection; β₀ = instrumental fwhm minimum; λ = wavelength

11

where τ = mean size of the crystal lattice; λ = X-ray wavelength; β = fwhm in radians, and θ = Bragg angle. Lattice constant a can be calculated from the Miller indices (h, k, l) using the relation of a = d(h² + k² + l²)^1/2, where d (interplanar spacing) is calculated by using the Bragg law. Crystalline parameters: a = b = c = 8.3515 Å (a 25 °C) agree with the cubic phase. FT-IR spectrum (model 100/100N FT-IR, PerkinElmer) and a UV–vis spectrophotometer (PerkinElmer 25, range of 190 to 1100 nm) were used to record the vibrational and optical absorption spectra of the samples, respectively. The scanning electron microscope (JEOL JSM-5600LV) is equipped with an energy dispersive X-ray spectrometer (EDS microanalyzer) to obtain the SEM images. The samples were prepared by placing a drop of the NPs solution on a 300 mesh copper grid and collecting the data in 15 different spots. The size and nature of crystalline NPs were studied by a transmission electron microscope (TEM, JEOL 2010) with an accelerating voltage of 100 kV and high-resolution transmission electron microscopy in a JEOL 2010 FE-TEM. An X-ray photoelectron spectroscope (XPS, K Alpha Thermo-Fisher Scientific Instruments), where the chamber pressure was 10^–9 mBar, equipped with Al κα X-ray (1486.6 eV) was employed to analyze the oxidation state of elements in the samples. The survey scan (160 eV) with high resolution (40 eV) was used to elucidate the presence of the components and their oxidation states. The magnetic measurement was carried out in a vibrating sample magnetometer (VSM, Quantum Device PPMS Evercool II, LPCNO, INSA, Toulouse) at room temperature which identified and detailed the magnetic and coercitivity properties. The magnetic moment per formula unit (μ_fu) was calculated as follows:

12

M = magnetization in emu/g; W = molecular weight; μ_B = Bohr magneton (9.274 × 10^–24 J/T); N_A = Avogadro number. Molecular weight was determined using EDS data. The Ag content in ZnFe₂O₄–Ag was found to be approximately 2.0% w/w.

Conclusions

AgNPs were deposited on ZnFe₂O₄ NPs to yield ZnFe₂O₄–Ag NPs. The signal for Ag was not detected in XRD until its content reached 2.5%; however, the silver particle presence in ZnFe₂O₄–Ag was established by TEM, HRTEM, and XPS. The existence of multiple interphases in ZnFe₂O₄–AgNPs is confirmed by HR-TEM, generating synergistic effects. The enhancement of the Ms values (160% times greater) for ZnFe₂O₄–Ag (from 7.25 to 18.71 emu/g is observed at 5.0 K as compared to 298 K. In contrast, the improvement is just about 5.0% for Fe₃O₄ (91.09 to 96.19 emu/g), illustrating that both ZnFe₂O₄ and ZnFe₂O₄–Ag are highly temperature sensitive to the magnetization. This means that these catalytic materials can be separated magnetically from the reaction medium and can be reused. A reduction of bandgap energy for ZnFe₂O₃–Ag is observed by the DRS spectra and it is corroborated with DOS. ZnFe₂O₃–Ag (2.5%) photocatalytically degrades rhodamine B (k = 5.5 × 10^–3 min^–1) under visible light and follows first-order kinetics associated with photopowered adsorption (nonlinear Langmuir isotherm model). The degradation rate for ZnFe₂O₄ and Fe₃O₄ resulted as k = 2.0 × 10^–3 min^–1 and k = 1.1 × 10^–3 min^–1, respectively. A greater antibacterial property for ZnFe₂O₄–Ag is seen by the deposition of Ag NPs on ZnFe₂O₄ as compared to other samples.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c03153.

• Powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR) spectra, X-ray photoelectron-spectroscopy (XPS), antibacterial activity parameters, adsorption and degradation of RhB, pH effect in the photodegradation and computational studies of ZnFe₂O₄–Ag (PDF)

The authors declare no competing financial interest.