Rational Design of Peroxymonosulfate Activation and Photoinduced

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Article Cite This: ACS Omega 2019, 4, 4113−4128

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Rational Design of Peroxymonosulfate Activation and Photoinduced Catalysis Tandem Systems for Artificial Conversion of Solar Light to Chemical Energy Shiying Fan,† Xinyong Li,*,†,‡ Meichun Qin,† Jincheng Mu,† Liang Wang,† Guoqiang Gan,† Xinyang Wang,† and Aicheng Chen*,‡ †

ACS Omega 2019.4:4113-4128. Downloaded from pubs.acs.org by WEBSTER UNIV on 03/03/19. For personal use only.

State Key Laboratory of Fine Chemicals, Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China ‡ Electrochemical Technology Centre, Department of Chemistry, University of Guelph, 50 Stone Rd E, Guelph, Ontario N1G 2W1, Canada S Supporting Information *

ABSTRACT: It has been known that dedicated photoinduced catalysis over artificial functional nanostructures and/or combined with effective peroxymonosulfate (PMS) activations toward highly effective greener synthesis and/ or environmental remediation have been well recognized to be one of the best options for efficiently exploiting solar light to convert into chemical fuel and energy. Novel ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) hierarchical yolk−shell hollow nanomicrospheres/PMS/Vis (ZCF/PMS/Vis) tandem systems with remarkable light harvesting capability and superior environmental catalytic activity toward elimination of antibiotic enrofloxacin up to 90.5% removal efficiency have been successfully designed and fabricated via combined solvothermal strategy and morphologically conserved sintering in this current work. The physicochemical characteristics, diverse structures including the spinel crystal structures, hierarchical nano-microstructure, and internal correlations of structuredependent properties, and the catalytic reaction mechanism in terms of the synergetic effect between instant PMS activation and dedicated photoinitiated catalysis have been methodically inspected and thoroughly illustrated by a variety of in/ex situ physicochemical ways, and the diverse microstructures including solid microspheres with villiform surfaces, YSHMs composed of exquisite particles, and YSHM alternative formation of diverse microstructures could be ultimately tailored and formed. ZCF YSHMs exhibit higher efficiency of both dedicated catalysis and spatial charge separations owing to their physicochemical characteristics and surface structures, namely, more surface oxygen vacancies, highest specific surface area, and interior structures. Specifically, as confirmed by primary combined characterizations, namely, room-temperature Mössbauer, in situ spin-trap EPR, SPV, ns-TAS, and in situ Raman and sequential investigations, the primary reactive oxygen species were deemed to be SO4·− and ·OH radicals, which generated instantly and simultaneously through surface covalent Cu2+ ions. Sequential-derived Cu(II)/Cu(III)/Cu(II) redox cycling initiated PMS activations and photoinitiated catalysis, and the superior catalytic performance as derived from ZCF HYSHM hierarchical-structured spinels could be primarily attributed to the diverse bulk and surface structures, highly efficient photonic energy harvesting, spatial charge separations and surface−interfacial transfers, more surface oxygen vacancies, and crucial reactive species including SO4·− and ·OH generations with long radical lifetimes up to 14.57 μs. The work could bring a brand new and deep insight into further understanding of both the intrinsic spinel structural influence factors governing the catalytic properties and the synergetic effect between instant PMS activation and simultaneous photoinitiated catalysis at the molecular level, which would be very beneficial for mimicking the natural photosynthetic solar energy harness system with marvelous featured properties in both environmental elimination and solar energy conversions.

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INTRODUCTION

spectrum and its conversion to electricity and/or chemical energy through biomimetic photosynthetic transduction, e.g., solar-mediated photocatalysis, have garnered tremendous attention.2b,3 In fact, tremendous progress has been achieved

Nowadays, the world has to face critical challenges including energy crisis and environmental pollutions.1 Photosynthesis motivates a bunch of research work on the natural process and incites attempts to imitate it in the laboratory, which made it available to develop artificial nanoscale assemblies, which fulfill many of the multifunctions of the natural process.2 Prospects for the efficient harness and utilization of the fully solar light © 2019 American Chemical Society

Received: November 4, 2018 Accepted: February 11, 2019 Published: February 25, 2019 4113

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lifetime, and strong oxidation ability. SO4·− radicals have been identified to have reduction potential (2.5 to 3.1 V) equivalent and/or higher than that of ·OH species (1.8−2.7 V vs NHE), which makes them serve possibly as one of the alternatives to hydroxyl radicals, probably owing to their pH value and superior oxidizing ability and capability.13 Jo et al. first reported highly effective peroxymonosulfate (PMS) activations over light-excited TiO2 semiconductors through a charged complex pathway, which has been proven that the supplementations of PMS species lead to highly efficient dechlorination of dichloroacetate and 4-chlorophenol over visible-light-irradiated TiO2 catalysts.14 Deng et al. have successfully investigated sulfite-boosted visible-light-initiated photo degradation of methyl orange over BiOBr plasmonic materials and indicated that the degradation efficiency of methyl orange was greatly enhanced by sulfite.15 A bunch of previous studies have revealed that some transition metals including cobalt ions could play a crucial role for effective PMS activations.16 Meanwhile, serving as one of the most toxic heavy metals, the presence of cobalt in reaction solutions could cause serious health concerns and/or problems. Hence, taking into account the concerns of future applications, it is crucial to invent novel catalytic materials possessing higher activity toward efficient PMS activations and visible light irradiation. Compared to other good photocatalytic materials and/or highly effective catalytic materials for PMS activations, for example,TiO2,17 halide ions,18 Fe(III)-doped g-C3N4,13a cobalt-based materials,19 the spinel structure (AB2O4) has been well recognized to possess several advantages including a relatively narrow band gap, even being excited by visible light without any modification, easy to be separated, and efficient activation of PMS, which makes it superior in environmental remediation and solar light harvesting and conversion.13c,d,20 It can be revealed from previous studies that both surfacecovalent Fe(III) and Cu(II) species exhibited a boosted synergistic influence upon sulfite activations, providing potential solutions for the heterogeneous catalysis of sulfite.9b,21 Wang et al. developed magnetic carbon nanospheres supported on oxide or manganese or cobalt oxide with yolk−shell structures, which were accordingly identified to be very active in heterogeneous PMS activation toward phenol degradation.22 Li and his colleagues have developed novel yolk−shell Fe0@Fe3O4 composites by a facile one-step thermal-induced simultaneous oxidation strategy, which proved to be more effective activations for persulfate toward beneficial oxidation of dibutyl phthalate under neutral reaction conditions.23 Huang et al. investigated the atrazine degradation properties through photocatalyzing sulfite over zinc copper based ferrites, for example, ZnxCu1−xFe2O4, under stimulated solar light irradiations, which proved that zinc copper ferrite composites could serve as efficient catalytic materials to activate sulfite.21 The general mechanism of instant PMS activation and simultaneous photocatalysis over AB2O4 spinel tandem systems is illustrated in Scheme 1; both of the photogenerated electrons and some of the metal ions in the spinel structures could stimulate PMS to generate SO4·− radicals. Meanwhile, the photogenerated holes could react with H2O or −OH group to produce ·OH radicals, which would eliminate pollution by working together with sulfate radicals. Hence, both of these pioneering investigations indicated that the multinary spinel structure could serve as an effective catalytic material toward PMS instant activation and simultaneous artificial light−energy conversion.

toward artificial conversion of solar energy to electricity and/or chemical energy via photoelectrochemistry and environmental photocatalytic remediation,4 and the common intrinsic characteristics for the aforementioned process are the high generation and utilization of photoinduced charge carriers as inspired by the solar-mediated redox process and conversion of excitation energy to chemical potential over photocatalysis.5 Obviously, highly artificial harvesting of solar energy followed by direct conversion to either electricity and/or chemical energy is of scientific significance, and environmental remediation via photoinduced catalysis has been well recognized to be one of the best options for efficient utilization and conversion of solar light to chemical energy.6 Following up this issue, numerous research works regarding environmental catalytic remediation of various pollutants have effectively been widely attempted and extensively explored.6 As a concern of energetic and environmental scientists, trace toxic organic substances (abbreviated as TTOSs), including pharmaceuticals, personal care products, pesticides and herbicides in water, and principal organic chemicals, have been popularly applied in industry and agriculture and mostly are recalcitrant to natural degradation and extremely poisonous and long-lived in nature with features of accumulating in living organisms, leading to a variety of terrible consequences such as cancer and chronic toxicity, and causing serious issues to the environment.1,7 Obviously, deep investigations of TTOS eliminations and successful development of effectual, cost effective, and environmentally benign water treatment technologies are undoubtedly of significant importance. Considering the well-recognized fact that the increasingly serious energy and environmental crises have encountered our world nowadays, the adoptions of sustainable energy sources developed by green chemistry have currently received enormous attention from all over the world. Heterogeneous photocatalytic redox (HPCR) technology taking advantage of functional materials has turned out to be one of the most effective strategies for extremely making the most of solar energy for simultaneous environmental purification and converting into eco-friendly artificial energy with the primary features of low-cost, environmentally friendly, and sustainable treatment. However, the removal efficiency of TTOSs is still unsatisfied by a single HPCR strategy in terms of future technological applications; for example, target pollutants and some intermediates cannot be completely eliminated, and the intermediates produced in the degradations might become another important environmental source of toxicity. Further enhancing the reaction efficiency of highly reactive radical (RR) productions toward highly effective removal of TTOSs simultaneously is currently facing critical challenges. By referring to recent numerous reports,8 it can be deduced that facial and flexible design of a joint reaction coupling system with a specific feature of the HPCR process with other dedicated technologies including photoelectrocatalysis,9 photoFenton,10 sono-photocatalysis,8b,11 and microwave photocatalysis8c,12 could incur the synergistic effects of various technologies, greatly improving the generation of RRs, solar light conversion, and the decontamination efficiency of pollutants. Recently, sulfate radical (SO4·−)-boosted photocatalytic processes have attained tremendous attention from scientists for their potential applications in wastewater treatment due to numerous advantages of possessing top safety and stability, higher redox potentials of SO4·− species with much longer 4114

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architectures.24c Overall, these aforementioned research outcomes fully revealed that the multinary spinel structure could serve as an efficient catalyst not only for efficient PMS activation but also for artificial light−energy conversion and environmental catalytic remediation. Meanwhile, the primary scientific issues including the correlations between diverse structure and activity, the synergetic mechanism of the instant PMS activation, and simultaneous photoinitiated catalysis as well as the details of the light-induced interfacial electron transfer and redox process are still unclear and deserve more systematic and deeply investigations. We herein successfully invented and constructed novel ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) hierarchical yolk−shell hollow nano-microspheres/PMS/Vis (ZCF/PMS/Vis) tandem systems with superior catalytic properties for efficient fluoroquinolone antibiotic elimination [enrofloxacin (ENR)] by an adaptable self-sacrificing template approach followed by methodical research on their potential utilization in artificial light−energy conversions and environmental remediation for the ultimate purpose of designing nanostructured materials for efficient artificial light harvesting and energy conversion with excellent activity, stability, and recyclability. Furthermore, various in/ex situ physicochemical techniques, namely, HRTEM/TEM mapping, Mö ssbauer , in situ electron paramagnetic resonance (EPR), surface photovoltage (SPV), in situ transient absorption spectrometer (TAS), X-ray photoelectron spectroscopy (XPS), and in situ Raman investigations, have been accordingly employed to explore and elucidate the correlations of structures and properties, the synergetic effect between instant PMS activation and simultaneous photocatalysis, and the light-induced redox process over the tandem systems. Furthermore, the density functional theory (abbreviated as DFT) was utilized for deep exploration of the band gap energies and the electronic and crystal structures of the nanocomposites at the atomic and electronic structure level. This work would therefore bring an innovative insight into further understanding both of the intrinsic structural influence factors governing the properties of the spinel-structure materials and the catalysis mechanism at the molecular level, which would inevitably afford an alternative approach for facial fabrication of functional catalysts for PMS activation toward enhancing the efficiency of environmental catalysis and solar power utilizations. Moreover, this work will bring us to further elucidate and interpret the synergistic effect as derived from the ZCF/PMS/Vis tandem systems on TTOS eliminations with sustainable solar-energy conversion, which is of environmental and energy significance.

Scheme 1. Representative Synergistic Mechanism of Spinel Photocatalysis and PMS Activation Tandem Fenton-like Systems

In fact, besides high activity for PMS activations, AB2O4 spinels also have amazing properties and wide applications in electronics, catalysis, magnetism, and electrochemical technologies due to their diverse structures. On the other hand, with the development of the design and synthesis of specific crystal-, micro-, and interfacial- structures, particularly nanostructured composites, the correlation between the diverse structures with definite chemical compositions and their properties currently receive much scientific attention in recent years. This is due to the major scientific significance for the further clarification of the intrinsic influencing factors that govern the properties of these materials at molecular and/or even atomic levels.24 Notably, there is pioneering work that focuses on the structure-dependent properties over dedicated ferrite spinels with nanostructures that possessed diverse chemical compositions.25 Wang and his colleagues successfully developed CoFe 2O 4 hollow spheres with multishelled structures possessing adjustable numbers of layers ranging from 1 to 4 serving as well-performed anode materials for supercapacitors through a one-step strategy employing sequential thermal treatments. The intrinsic features of the diverse structures, that is, the number and porosity of shells, were successfully fabricated by tuning the preparation parameters to generate hollow spheres with better durability and capacity. Hence, this strategy could therefore act as a straightforward and general way toward tailing of metal oxide nanocomposites with specific hollow-structured microspheres and adjustable numbers of shells. The aforementioned outcomes revealed that the CoFe2O4 hollow-structured spheres showed boosted electrochemical performance as referred to the previous literature owing to their specific structures, that is, porous hollow multishelled structures, which would not only shorten the diffusion pathway of the electrolyte ions but also offer abundant active sites toward redox reaction and ultimately incur the increase of the electrochemical performance.26 Liu et al. exploited a facial solvothermal strategy to fabricate dual-shelled ZnFe2O4 hollow-structured spheres and shifted the reaction rate to exactly regulate the intrinsic structures along with the sequential thermal treatment process. While checking the catalytic oxidation of odichlorobenzene gaseous species over dual-shelled spinelstructured ZnFe2O4, the superior catalytic performance could be attributed to the features of diversified structures, including possessing more active sites, namely, adsorption, activation, and desorption sites, and much higher surface area (126.7 m2 g−1) as well as more efficient light harvestings due to the multiple light scattering derived from dual-shelled interior

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RESULTS AND DISCUSSION Microstructure Characterizations. The ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) hierarchical yolk−shell hollow nano-microspheres (HYSHMs) were tailored and prepared through combined one-pot facial solvothermal approach and morphologically conserved sinterings.24c The morphologies of the novel ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) hierarchical yolk−shell hollow nano-microspheres (HYSHMs) were observed using extrahigh-resolution SEM and TEM/HRTEM/mapping. Uniform precursor ZnxCu1−xFe hybrid nanospheres were first constructed by a one-pot solvothermal strategy, which is unveiled in Figure S1. The general micromorphologies of the precursor ZnxCu1−xFe hybrids were regular solid microspheres with diameters of approximately 750, 600, and 500 nm for x values equal to 0, 1, and 0.5, respectively. A bunch of villiform was 4115

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catalysis efficiency. Importantly, as depicted in Figure 1c,f,i, the intrinsic structures of these composites are also greatly influenced by different component distributions of spinel structures. Interestingly, the diverse structures turned out to be formed from solid nano-microspheres (SMs) (while x equals 0) to yolk−shell hollow-structured microspheres (YSHMs) (while x equals 1 or 0.5). Furthermore, the diameters of the interior yolk ranged from ∼300 (x = 1) to ∼250 nm (x = 0.5) as indicated in Figure 2. We therefore suppose that the facile

highly dispersed over the exterior surfaces of the CuFe hybrid and Zn0.5Cu0.5Fe hybrid precursors, while the surfaces of the ZnFe hybrid precursors presented nanoparticles. Figure 1

Figure 1. SEM images (a,d,j), SEM spectra at high magnification (b,e,j), and TEM images of CuFe2O4, ZnFe2O4, and Zn0.5Cu0.5Fe2O4 hierarchical yolk−shell hollow nano-microspheres (c,f,i)

illustrates the surface morphologies and interior microstructures of the ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) HYSHMs by TEM and FESEM, respectively. The regular spheres manifested nonapparent aggregations as observed from Figure 1 and possessed average diameters of ∼500, ∼450, and ∼420 nm relative to the numerical x values of chemical compositions varying from 0 to 1, with the spherical shape of the precursors being fantastically retained (Figure S1). Additionally, it could be obviously noticed that the exterior structure of the aforementioned microspheres differed with various x values. When x is 0, the surface was covered with villiform states; as for x equals 1, the surface was changed to exquisite particles, which were smaller than those when x is 0.5 for dedicated Zn0.5Cu0.5Fe2O4 HYSHMs. Rough surface nanoparticles would favor to produce mesoporous materials ultimately, which coincide well with the outcomes of BET analysis. As clearly revealed in Figure S2 and Table 1, the afore prepared ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) HYSHMs exhibited classic type IV isotherms with the features of stretched and slim hysteresis loops at certain pressures. The integrated arrangements manifest the pore size to be 7.92, 12.09, and 12.22 nm for ZnFe2O4 (ZF), CuFe2O4 (CF), and Zn0.5Cu0.5Fe2O4 (ZCF), respectively, indicating the formation of mesoporous structures. Moreover, based on the Brunauer−Emmett−Teller analysis, the multiple spinel-structured Zn 0.5 Cu 0.5 Fe 2 O 4 HYSHMs exhibit the highest value of 75.87 m2 g−1 compared with those of ZF (63.07 m2 g−1) and CF (47.24 m2 g−1), suggesting multiple ZCF spinel HYSHMs could provide more active sites in response to contaminants, incurring higher

Figure 2. TEM images (a,d,g), HRTEM images (b,e,h), and SAED patterns (e,f,i) of CuFe2O4, ZnFe2O4, and Zn0.5Cu0.5Fe2O4 yolk−shell structures; HAADF-STEM images of Fe, Zn, Cu, and O.

solvothermal strategy can be used to fabricate spinel ferrite hierarchical nanostructures. CF prefers to form solid microspheres with villiform surfaces, ZF develops YSHMs composed of exquisite particles, while partially displacing another transition metal into the A-site of the ferrites, and multinary ZCF spinels tend to form YSHMs with an obvious core and a higher surface nanoparticle size as illustrated in Scheme 2. The Scheme 2. Schematic Diagram of the Synthesis of ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) Multinary Spinel with Various Hierarchical Yolk−Shell Nano-Microsphere Structures

Table 1. Physicochemical Properties of ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) HYSHMs sample

XPS Cu/Zn/Fe

EDX Cu/Zn/Fe

Ov (%)

BET (m2/g)

pore size (nm)

ΔHPP (Gauss)

g-value

Zn0.5Cu0.5Fe2O4 ZnFe2O4 CuFe2O4

5.4/5.6/21.5 13.4/27.4 10.5/22.5

4.63/4.82/17.54 10.52/19.63 9.34/18.52

14.6 9.4 9.6

75.87 63.07 47.24

12.22 7.92 12.1

1346 752 1499

2.129 2.037 2.072

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Figure 3. XRD patterns (a), FTIR images (b), EPR spectra (c), and Mössbauer spectra of ZnFe2O4 (d), CuFe2O4 (e), and Zn0.5Cu0.5Fe2O4 (f) HYSHMs.

Figure 4. XPS full spectra of spinel samples (a), and the individual XPS spectra of Zn 2p (b), Fe 2p (c), Cu 2p (d), and O 1s (e); magnetic hysteresis loops (f) of ZnxCu1−xFe2O4 (0 ≤ x ≤ 1).

structured CF, ZF, and ZCF samples, respectively (Figure 2). What is more, the distinct patterns of selected area electron diffractions unveiled that all the HYSHMs have polycrystalline structures, while multinary spinel ZnxCu1−xFe2O4 (x = 0.5) HYSHMs present much higher crystallization. The elemental mapping images ultimately indicated that the Cu, Zn, Fe, and O elements were highly dispersed spanning over the ZnxCu1−xFe2O4 (x = 0.5) HYSHMs. The aforementioned analysis indicated that diverse microstructures including hierarchical nano-microspheres could be tailored via a facial

observed difference in surface states and intrinsic morphology between ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) HYSHMs as observed by SEM and TEM could be attributed to the lattice strain and molecular structural disorder, which would lead to the variable ionic radii and possible agglomerations of the nanoparticles as well as different site-occupation priorities of the Zn, Cu, and Fe elements within the crystal lattice in the course of the crystallization process.27 It is apparent to learn that the different lattice fringes at 0.249, 0.265, and 0.57 nm accord well with the (400), (311), and (511) planes of spinel4117

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width (ΔHPP) and Lande’s splitting factor (g-value) as well as the chemical and physical characteristics such as anisotropy, dipolar interactions, paramagnetic species, and superparamagnetic properties over ZnxCu1−xFe2O4 HYSHMs spanning over the formations of ferrite nanocrystallites (Figure 3c).33 The broad EPR peak centered at g ∼2.00 could be accordingly assigned to Fe3+ ions with a high spin state of octahedral sites in the spinels, which reveals the superparamagnetic behavior of ZnxCu1−xFe2O4 HYSHMs.34 It is worthy to mention that the peak-to-peak amplitude, the point of minimum derivative, and the point of maximum derivative shift toward the higher field for the multinary ferrite than that for zinc ferrite and lower than that for copper ferrite (Table 1). This variation is possible due to the strengthening dipolar interaction among cations through oxygen.35 Additionally, EPR techniques could perceive some thermally stimulated surface electrons of spin-structured materials with an extended lifetime similar to previous work.36 The multinary spinels could therefore alternatively bolster the photoinduced e−−h+ separation efficiency as unveiled by referring to the high EPR signal intensity spanning across the spinel-structured ZCF HYSHMs. On the other hand, the Mössbauer spectroscopy has been well recognized to be one of the most efficient tools to investigate the specific compositions, magnetic properties, and the chemical states of tetrahedral and octahedral Fe ions of the aforementioned spinels.37 Herein, Figure 3d−f sequentially shows the Mössbauer spectra of the aforementioned spinel samples as taken at room temperature (RT). The doublet features as observed from the RT Mössbauer spectra of ZnFe2O4 (ZF) samples in Figure 3d alternatively indicate the superparamagnetic relaxation occurred among the spinel structures. In fact, the Mössbauer spectra of ZF composites have been well fitted into two specific doublets, namely, D1 and D2 peaks, which ultimately manifested the superparamagnetic relaxation of tetrahedral and octahedral Fe ions and accorded well with that of the integrated FTIR outcomes as shown in Figure 3b. The Mössbauer spectra of CF samples were accordingly integrated with fitting one quadrupole doublet and three sextets. The fitted sextets could be assigned to Fe3+ ions that occupied the octahedral and tetrahedral sites and revealed the presence of Fe3+ ions with different coordination environments and different hyperfine field values. Together with the aforementioned sextets, the weak doublet was also distinctly observed and could be assigned from the partial contributions of superparamagnetic phase fractions in the spinels. Interestingly, for ZF and ZCF samples, similar patterns originating from their corresponding Mössbauer spectra distinctly indicated that the value of the sextet quadrupole splitting approximately approaches zero, which could be due to the so-called paramagnetic behaviors where the Fe3+ ions possibly occupy the positions of the unit cells’ Bsites. The magnetic splitting as appeared in the Mössbauer spectrum of ZCF firmly verified the existence of ferrimagnetic phase, indicating ZCF possesses higher magnetic performance than ZF. Both of the research outcomes as derived from Mössbauer and EPR measurements accorded well with the speculations and elucidations of the mixed magnetic states. X-ray photoelectron spectroscopy has been widely used to investigate the chemical compositions, chemical state, and electronic state of the materials. Herein, the total XPS spectra of Fe 2p, Zn 2p, O 1s, and Cu 2p and the ZnxCu1−xFe2O4 HYSHMs were accordingly were recorded and are depicted in Figure 4a, which distinctly reveal the existence of Fe, Cu, O,

solvothermal and sequential morphologically conserved thermal-incurred transformation strategy, which would present a higher catalytic efficiency due to their physicochemical characteristics and surface structures, namely, more surface oxygen vacancies, highest specific surface area, and interior structures. Physicochemical Characterization. The diverse crystal structures and the surface−interfacial structures together with the chemical compositions and states were identified by various physiochemical techniques including X-ray diffraction (XRD), XPS, FTIR, EPR, VSM EDX, and Mö ssbauer spectroscopy, as shown in Table 1 and Figures 3 and 4. The Cu/Zn and Cu/Zn/Fe atomic ratios of synthesized HYSHMs were approximately 1.0 and 1/1/2 as measured from EDX and XPS analysis, which coincide well with the chemical compositions of the ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) spinels. As shown in Figure 3a, the XRD diffraction peaks could be sequentially and correspondingly indexed to the face-centered cubic crystallographic planes of spinel-structured Zn0.5Cu0.5Fe2O4 (PDF 01-077-0010).28 Additionally, none of the full shifting and splitting of the peaks for all of the ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) spinel-structured samples have ever been observed, which implied that their crystal structures corresponded to the dedicated single-phase cubic structure with the Fd3m space group. It can be readily revealed from the XRD patterns of ZF and CF nanocomposites that wellcrystallized phase structures were formed with fine crystallites derived from peak broadening, whereas that of ZCF samples exhibited higher XRD peak intensities spanning from different chemical compositions due to the higher crystallinity and rational cation distributions in the spinel lattice.29 Additionally, one can readily notice that the degree of endothermic reactions as derived from the chemical formation processes of multiple spinel-structured ferrites was much higher than that of those ferrites with a single phase. It has been well recognized that surface temperature is a crucial factor governing the molecular concentration affecting the preferred surface-oriented growth of the crystal,27 and therefore appropriate heats attributed from the endothermic reactions of the aforementioned ZCF samples could be accordingly released consequently increasing the crystallinity of the surface crystal, which would ultimately lead to the multiple spinel-structured samples possessing superior photoinduced catalytic performances. The FTIR spectra of ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) HYSHMs spanning across 4000−400 cm−1 are shown in Figure 3b.The broad FTIR bands centered at 3400 and 3800 cm−1 could be generally assigned to the hydrogen-bonded O−H vibration mode of stretching arising from the surface-bonded hydroxyl groups over the nanostructures.30 Additionally, the FTIR spectra revealed the specific bands spanning across ranges of 400−490 and 600−540 cm−1 could be accordingly attributed to the metal−oxygen vibration mode of stretchings stemming from the octahedral and tetrahedral sites of the spinel structures, respectively. Especially, the FTIR absorption band of ZF samples centered around 700 cm−1 can be elucidated and interpreted in terms of the cation-exchange effects among the tetrahedral and octahedral occupational sites of the zinc ferrites.31 Obviously, the aforeintegrated FTIR featured results that fully verify the successful formation of the spinelstructured ferrites, which accorded well with those of the XRD interpreted characterizations.32 EPR spectroscopy enables us to check and determine the values of various parameters including the peak-to-peak line 4118

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Figure 5. (a) UV−vis diffuse reflectance spectroscopy (DRS) and (b) Tauc’s curve for ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) HYSHMs.

oxygen vacancies, and surface defects could alternatively lead to tuning of their magnetic properties.41 The magnetic hysteresis loops of ZnxCu1−xFe2O4 HYSHMs are shown in Figure 4f. In fact, the relevant information of saturation magnetization (Ms), coercivity (Hc), and the remnant magnetization (Mr) can be accordingly determined using these loop curves as measured via the vibrating sample magnetometer (VSM). It can be easily observed from the magnetization curve that the saturation magnetization value of ZCF HYSHMs showed the highest saturation magnetization of 30.27 emu/g, while those of ZF and CF ferrite HYSHMs were approximately 15.83 and 9.31 emu/g, respectively. The higher saturation magnetization for multinary spinels could be generally ascribed to the stronger magnetic superexchange interactions between octahedral (O-sites) and tetrahedral (Tsites) sites, the spin orientation over the surface−interface,35 and the α-Fe2O3 phase absence, which would ultimately lower their saturation magnetizations.21 The high magnetic property indicated that ZCF HYSHMs can be considered as efficient recyclable catalysts that could be conveniently regathered via an external applied magnetic field after catalytic reactions. Optical Properties. To deeply inspect and elucidate the optical absorption properties of the as-prepared ZnxCu1−xFe2O4 multiple spinel HYSHMs, both of the ultraviolet−visible spectroscopy and first-principles calculations were intensively conducted. As manifested in Figure 5, the assessed Eg of ZF, CF, and ZCF can be estimated to be 2.15, 1.71, and 1.65 eV, respectively. Notably, the UV−vis spectra of ZCF showed superior light absorption and narrow band gap as measured by Tauc equations, which could be attributed to their different electronegativities and radii [Cu2+ (0.72Å), Zn2+ (0.74Å)] as well as specific diverse crystal and electronic structures, indicating the spinel could be effectively activated under simulated light irradiation and has excellent artificial light harvesting capability.42 It has been well recognized that the diverse structures, namely, the crystal, micro-, and electronic structures, could generally govern and tailor their catalytic performance, which could be further investigated and interpreted with the first principles. Hence, the electronic structure and density of states (DOS) of ZnxCu1−xFe2O4 multiple spinel HYSHMs were therefore explored by employing density functional theory (DFT) calculations with the Cu and Fe 3d states treated with on-site correction for Coulomb interaction (DFT+U) to deeply comprehend the intrinsic electronic and optical characteristics. As previous experimental studies demonstrated that ZnFe2O4 has a normal spinel structure and CuFe2O4 usually possesses an inverse spinel structure,43 in this work, the

and Zn elements. Figure 4b−e illustrates the Zn 2p, Fe 2p, Cu 2p, and O 1s individual high-resolution XPS spectra of ZnxCu1−xFe2O4 HYSHMs, and it can be deduced from the XPS spectra of Zn 2p1/2 and Zn 2p3/2 that the binding energies of two apparent XPS peaks centered at 1044.4−1044.6 and 1021.2−1021.3 eV, respectively, manifested the oxidation state of Zn (+2) in ZnxCu1−xFe2O4 HYSHM multinary spinel composites. A clear 2p3/2 main line centered at 711.1−711.2 eV was observed due to the predominant Fe3+. Meanwhile, the Cu 2p3/2 XPS spectrum presents one dominant peak centered at binding energy of 934.2−934.3 eV and a satellite peak at approximately 942 eV, verifying the chemical state of Cu2+ species. As distinctly revealed in Figure 4e, for the O 1s XPS spectra, the peaks with the binding energies spanning across 529.9−530.1 and 531.4−532.5 eV could be assigned to the lattice oxygen and O2− ions in the oxygen-deficient regions (oxygen vacancies) within ZnxCu1−xFe2O4 nanocomposites.24c,38 The ratios of Ov species on the surface of ZnxCu1−xFe2O4 composites were 14.6, 9.6, and 9.4% for ZCF, CF, and ZF, respectively, indicating that the ZCF had a bunch of surface oxygen vacancies. With regard to the role of surface oxygen vacancies in the catalytic reactions, it has been well recognized that surface oxygen vacancies could be beneficial to heterogeneous catalytic reactions through highly efficient adsorption and catalytic dissociation of adsorbates. Additionally, surface oxygen vacancies could tune the semiconductor band bending favoring the spatial electron−hole pair separation and interfacial charge transfers, which alternatively incur the enhancement of photocatalytic performance.39 Also, it could be observed that the binding energies of Cu 2p3/2 and Zn 2p3/2 of ZCF nanocomposites exhibited negative shifting up to ∼0.1 eV in comparison with those of the ZF and CF composites. The binding energies of Fe 2p3/2 and O 1s spectra for CF exhibit a positive shift to ∼0.1. Compared with the dedicated single spinel, the binding energy shifting of the Zn 2p, Cu 2p, Fe 2p, and O 1s spectra within the multinary spinel clearly indicated that the Cu, Zn, Fe, and O elements’ chemical state and environment have been accordingly tuned, which implied the successful formation of multinary zinc copper ferrite spinels. Notably, the crystal defects, crystal lattice strain, state relaxations, and charge carrier wave functions spanning across the diverse microstructure might ultimately lead to charge redistribution and finally change the binding energies, which are crucial for favoring the photoinduced interfacial charge transfers, greatly enhancing the catalysis performance.38a,40 It is worth mentioning that a slight change in the surface and interface structure including the surface compositions, surface 4119

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Figure 6. Electronic band structures and state densities of ZnFe2O4 (a,d), CuFe2O4 (b,e), and Zn0.5Cu0.5Fe2O4 (c,f) HYSHM multinary spinel structures.

Figure 7. (a) ENR degradation by ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) under different conditions (I0 = 30 mW cm−2, C0 = 20 mg L−1). (b) Roomtemperature PL spectra. (c) SPV spectra and (d) TAS characteristic kinetic curves of ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) multinary spinel structures.

unit cells Zn8Fe16O32 and (Fe8)(Cu8Fe8)O32 were used to model ZnFe2O4 and CuFe2O4 (Figure S4a,b). Considering the combined results of XRD, XPS, EPR, and Mössbauer spectra, the octahedral B sites of unit cells were occupied by Fe3+ ions in the Zn0.5Cu0.5Fe2O4 multinary spinel structures. The model of (Zn4Cu4)(Fe16)O32 was constructed for Zn0.5Cu0.5Fe2O4 by locating Cu atoms to substitute half Zn atoms of perfect Zn8Fe16O32 (Figure S4c). Figure 6 gives the band structures of Zn8Fe16O32, (Fe8)(Cu8Fe8)O32, and (Zn4Cu4)(Fe16)O32, and

all of them were spin-polarized semiconductors. As revealed in Figure 6a, the normal-spinel-structured ZnFe2O4 could serve as a direct-gap semiconductor with a band gap energy of 2.087 eV. Meanwhile, the indirect band gap energy of the inversespinel-structured CuFe2O4 was estimated to be 1.712 eV (Figure 6b), which coincides well with the value of 1.71 eV derived from the experiments (Figure 5b). It is worth noting that, for the aforementioned Zn0.5Cu0.5Fe2O4 multinary spinel structures, the cation substitutions of Cu species could alter the 4120

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active radicals (e.g., SO4·−) through the instant adsorptions and simultaneous PMS activations as intrinsically derived from spinels. As indicated by the previous literature published by Croue et al.,20a the proposed Cu(II) as well as the derived Cu(II)/Cu(III)/Cu(II) redox cycle plays a crucial role in PMS activations by the CuFe2O4 spinel followed by very highly efficient generation of SO4·− radical active species, and the reaction pathway of the sulfate radical generated from Cu(I) and Fe(II) ignited activations of PMS species is probably not the primary channel, which are attributed to that the general reduction of either Fe(III) or Cu(II) over PMS is insufficient and thermodynamically difficult due to the consideration of HSO5− (1.8 V) possessing higher redox potential than those of Cu(II) (0.15 V) and Fe(III) (0.77 V) (for solid-phase reactions, the reductive potential of Cu(III)/Cu(II) could approach as high as 2.3 V).44 Following up this scientific point, the spinel-structured CuFe2O4 sample taken in this current work would therefore have the higher ability for PMS activations than that of Zn0.5Cu0.5Fe2O4 due to the large percentage of surface Cu(II) involved in the efficient sulfate radical generations, and the lower PMS catalytic activity of ZF could be ascribed to the poor catalytic PMS activation ability of Fe(III) and Zn(II).13c,20b For comparison, the spinel/PMS/ Vis tandem systems were also investigated, and it can be clearly seen that in the photocatalysis−PMS oxidation coupled systems, the ENR degradation rate could attain more than 90.5% over ZCF multinary spinel HYSHMs, which indicated ZCF HYSHMs possess the excellent artificial light-conversion ability and catalytic efficiency. Interestingly, as also shown in Figure 7a, the photoinduced catalytic activity of CF is identical or even higher than that of the ZCF during the first 100 min of the reactions, which might be generally ascribed to the dual synergetic interactions between instant PMS activations and simultaneous photoinduced dedicated catalysis as derived from spinel-structured CF and ZCF samples under visible light irradiations and mild conditions. Additionally, to better reveal the different catalytic activities, the harsher reaction conditions (i.e., with a higher concentration of the ENR) for ENR catalytic degradation over spinels including CF, ZF, and ZCF were adopted and elucidated accordingly (Figure S3). It could be obviously seen from Figure S3 that the catalytic activity decreased rapidly with increasing ENR concentrations from 20 mg L−1 over ZF/PMS/Vis, CF/PMS/Vis, and ZCF/PMS/Vis tandem systems under the optimum reaction conditions, which possibly reveal that further increasing the concentration of ENR might simultaneously induce a bunch of ENR molecules to compete for the adsorption sites and active sites resulting in lower catalytic performance ultimately. Additionally, one can also notice that the catalytic degradation rate did not decrease drastically while at relatively lower ENR concentrations spanning from 10 up to 20 mg L−1 compared with those higher concentrations, and the catalytic activity checked under the reaction conditions, that is, the ENR concentration ranging from 10 up to 50 mg L−1, I0 = 30 mW cm−2, λ ≥ 420 nm, T = 298 K, P = 1 atm, sequentially followed the order ZCF/PMS/ Vis > CF/PMS/Vis > ZF/PMS/Vis, which could be initially ascribed to the fact that the spinel tandem systems could generate a plentiful amount of active radicals (e.g., ·OH and SO4·−) through the aforementioned synergetic effect between instant PMS activations and simultaneous photoinduced dedicated catalysis as derived from spinels removing low concentrated ENR molecules more easily under the optimum reaction conditions. Herein, ZCF multinary spinel HYSHMs

electronic properties of normal-spinel-structured ZnFe2O4 with a band gap of 1.643 eV (Figure 6c). Notably, the state densities including the total density of states (DOS) and partial DOS (PDOS) projected on Cu, Fe, Zn, and O atoms for various ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) multinary spinels are plotted by the spin-polarized DFT+U method in Figure 6d,e, which shows that they are spin-polarized semiconductors. In the normal spinel ZnFe2O4 structure, the valence bands mainly consist of the chemical states of Zn 3d, Fe 3d, and O 2p. The conduction bands generally originate from the combined Fe 3d and O 2p states. As for the inverse spinel CuFe2O4, it is worth mentioning that the occupied chemical state below the Fermi level is primarily attributed to simultaneous couplings between the d-electrons from Fe and Cu dual atoms and p-electrons from O individual atoms, while the unoccupied states just above the Fermi level are mainly contributed by p-orbitals of O atoms. Furthermore, it could be perceived from the spinelstructure Zn0.5Cu0.5Fe2O4 that the hybrid orbitals of Zn 2p, Fe 3d, Cu 3d, and O 2p were the primary VBM components, whereas Fe 3d generally made contributions to the deeper CBM and the density of states above the Fermi level could be primarily contributed by O 2p of oxygen atom. Catalytic Performance. The effect of various ZnxCu1−xFe2O4 multinary spinel HYSHMs obtained by the solvothermal strategy with diverse microstructures on the catalytic performance has been systematically evaluated and explored accordingly. The catalytic activities of the assynthesized HYSHMs were evaluated by ENR degradation with or without PMS under visible light irradiation (Figure 7a, Figure S3, and Table S1). From Figure S3a and Table S1, it could be clearly observed that there are slight differences among the spinels and ZCF HYSHMs exhibited relatively higher adsorption ability (9.4%) than CF HYSHMs (9.0%) and ZF HYSHMs (9.3%), which is in accordance with the result of BET analysis. Therefore, it can be speculated that the adsorption of ENR on these samples might be attributed to Brunauer−Emmett−Teller specific surface areas of abundant surface−interfacial structures including surface oxygen vacancies and distorted crystal and surface defects, which would offer more adsorption, activation, and reaction sites for the preadsorbed ENR contaminated species, greatly favoring the positive catalytic degradations.39c It is worth noting that the simultaneous and swift uptake of ENR molecules could approach equilibrium within a relatively short time of 30 min. Therefore, before the following activation reactions, we mixed both of the catalysts and ENR solution in the dark for 30 min to reach equilibrium. The activities of PMS/Vis systems (i.e., without catalyst) were explored in a parallel manner, and approximately 10% efficiencies toward ENR decompositions were accordingly achieved, which initially indicated that the coexistence of ZCF multinary spinel HYSHMs is one of the crucial factors for getting higher ENR removal efficiency. In order to check and reveal the PMS activation and sequential activated PMS-initiated catalytic ability of various as-synthesized spinel HYSHMs, we therefore characterized the catalytic performances of ZF, CF, and ZCF HYSHMs under dark conditions in the presence of PMS species (Figure S3a and Table S1). It can be seen from Figure S3a and Table S1 that CF HYSHMs exhibited the highest performance for the activated PMS-initiated catalysis (39.5%), which is higher than those of ZF HYSHMs (9.4%) and ZCF HYSHMs (35.3%). This might be ascribed to the fact that the spinel tandem systems could generate a plentiful amount of 4121

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held both a strong magnetic property and the highest catalytic performance for environmental remediation through artificial utilization of solar energy, and thus it might be one of the best options for the activation of PMS owing to its effective magnetic separation, efficient PMS activation, and excellent catalytic capability. Kinetic Behaviors. The kinetic behaviors of the photogenerated charge carriers over the ZnxCu1−xFe2O4 multinary spinel HYSHMs were detected by surface photoluminescence (PL) and surface photovoltage spectroscopy (SPV). It is worth noting that the PL outcomes could evaluate and indicate the higher efficiency of light-induced spatial charge separations (Figure 7b). The composites were illuminated at 325 nm. The peak around 300−350 nm can be observed in the spectra of CF and ZF, while two apparent peaks can be detected in the spectra of ZCF around 320 and 440 nm. The peaks around 300−350 nm could be ascribed to the band edge emission in the spinel-structured ferrites, and the peak centered at 440 nm could be accordingly attributed to the neutral or charged oxygen active species, that is, thermally activated surface defects toward surface-derived oxygen vacancies and/or electrons in the spinel structures.36,45 It is also learned that the lower PL intensities of the ZCF spinel-structured HYSHMs show relative intensities in comparison with those of ZF and CF binary spinels spinel-structured HYSHMs, which probably manifested that the ZCF HYSHMs have lower spatial charge recombination capability and higher catalytic performances accordingly. The SPV spectra of the ZCF multinary spinel as seen in Figure 7c clearly revealed very broad active and sensitive response SPV peaks in the spectra. Notably, as indicated from Figure 7c, all of the HYSHMs present apparent photoelectric signals spanning from 300 to 500 nm, which could be basically assigned to the photogenerated electron transitions starting from the valence band to the conduction band. In comparison to ZF and CF, the SPV signals of ZCF were more distinguished and their corresponding catalytic performance were much higher while the external light possessing the light wavelengths spanning from 300 to 500 nm or even longer wavelengths, that is, the visible region above 500 nm. The SPV signals that arose were therefore due to the changes of surface potential barriers derived from the spatial separation of photogenerated charge carriers in a space charge region of a semiconductor upon light irradiations, which generally depend on processes of light absorption, photogeneration, charge separation, charge transport, recombination, and trapping and emission of charge carriers. Hence, the current SPV investigations further revealed that the ZCF HYSHMs present an outstanding simulated solar light harvesting and highly artificial energy conversion ability. The specific influence of spatial charge carrier behavior, the correlations between the diverse microstructures, and the intrinsic catalytic performance were further investigated and elucidated by TAS technology. The TAS investigations were conducted in C2H5OH solvent under 266 nm UV laser shining over ZnxCu1−xFe2O4 multinary spinel HYSHM dispersions. The transient kinetics of ZnxCu1−xFe2O4 multinary spinel HYSHMs derived from 350 nm laser shining has been accordingly well fitted with a multiexponential function program (Figure 7d) for delivering the time constants (τx) and the corresponding contributed proportions (ax %)46 (Table 2). It is worth mentioning that the spatial charge separation derived from laser irradiations could lead to photoinduced charge carrier with different lifetimes of

Table 2. Exponential Decay Components of Fractional Emission Amplitudes of ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) HYSHMs sample

τ1 (ns)

a1 (%)

τ2 (μs)

a2 (%)

τ (μs)

CF ZF ZCF

53.3 59.4 77.0

1.67 10.13 2.37

12.0 14.01 14.9

98.33 89.87 97.63

11.8 12.6 14.5

τ1 and τ2.36 The mean lifetime (τ) of the separation time over ZCF HYSHMs was estimated to be approximately 14.5 μs, while those for ZF and CF were measured to be 12.6 and 11.8 μs, respectively. Notably, the primary reason why ZCF multiple spinel HYSHMs have superior catalytic capability was therefore attributed to the lowest recombination rate of excited longer lifetime electrons and holes.47 Sulfate Radical Generation Pathways. With the aim of deeply evaluating and exploring the detailed mechanism of PMS activation at the atomic level, various combined physicochemical ways including EPR radical capture, FTIR, and in situ Raman spectra characterizations were conducted to detect the active species and the oxides’ surface species during catalytic activation of PMS. Notably, both of the SO4·− and · OH radicals were well recognized to be the crucial reactive species during activation of PMS processes.13c,19a,20b To have a deep insight into the reaction mechanism and determine the main oxidant species, in situ EPR DMPO trappings were conducted to systematically investigate both of the ·OH and SO4· active radicals during the catalytic reaction under various conditions. The in situ EPR spectra of four various simulated samples as measured by the DMPO spin trapping in ENR aqueous solutions are shown in Figure 8a. It is worth mentioning that none of the DMPO spin-trapping signals were certainly detected while DMPO was introduced into the aforementioned ENR antibiotic aqueous solutions mixed with the suspended ZCF nanocomposites, and this clearly indicates that none of the active radicals could be formed provided the PMS absence in the reaction systems. Herein, it can be further revealed that after PMS species were accordingly added into the ENR aqueous solution with simultaneous visible light irradiations, the EPR signals with ratio 1:2:2:1 could be identified and assigned to DMPO-OH spin adducts, and the generation of ·OH radicals resulting from both light irradiation and HSO5− involved critical hydrolysis progress (HSO5− + H2O ↔ H2O2 + HSO4−; H2O2 →2·OH).15 Furthermore, the addition of Zn0.5Cu0.5Fe2O4 HYSHMs could greatly favor the increase of the EPR spin-trapped signal intensity, and this might partially reveal their superior catalytic ability for PMS activations. The EPR spin-trap signals derived from the adducts of DMPO-·OH and DMPO-SO4·− obviously manifest that the dual reactive radicals, namely, SO4·− and HO· radical species, could be produced through efficient PMS activation by Zn0.5Cu0.5Fe2O4 HYSHMs. Interestingly, it might be noticed that while visible light was applied to the aforementioned systems, the EPR intensities assigned to ·OH and SO4·− signals decreased and simultaneously increased, respectively, which might suggest the pregenerated ·OH could effectively activate PMS, ultimately forming SO4·− radicals (HSO5− + ·OH → SO4·− + OH−). Combined with the results of higher degradation rate for the catalysis with PMS under visiblelight-irradiated coupled systems, we can therefore reveal that both of the photoinduced electrons and ZCF composites could 4122

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Figure 8. In situ EPR DMPO-capture spectra (a), FTIR (b), and in situ Raman (c) of fresh and used Zn0.5Cu0.5Fe2O4 HYSHMs.

efficiently activate PMS simultaneous with generation of SO4·− active radicals. To further identify the intermediates originating from PMS decomposition, we compared the FTIR spectra of ZCF before and after reaction without any rinse process. According to Figure 8b, after reaction with PMS, the newly emerged peaks centered around 1100 and 1250 cm−1 in the FTIR spectra of ZCF could be accordingly attributed to the S−O stretching vibration mode of HSO5 − 20a anions. Additionally, as mentioned above, the broad FTIR peaks centered at 3400 cm−1 fully indicate the existence of surface −OH groups.30 Compared with the fresh ZCF FTIR spectra, after reaction with PMS, the peak around 3400 cm−1 became clear and some were red-shifted, which reveals that the surface −OH group might be substituted followed by binding with metal sites of the aforementioned structured samples, during PMS decomposition process.48 It has been previously reported that highvalent X(n+1)+, a minor ionic radius compared to Xn+, would possess much higher binding ability and capability with specific ligands.49 Obviously, the tailing and variation of the chemical redox state of the surface-attached metal will generally lead to the relevant FTIR peak shiftings of surface hydroxyl groups (−OH) derived from the dissociation of surface-absorbed H2O over the aforementioned oxide complex. The FTIR peak redshift of the surface −OH band demonstrated that partial shifting of the electron density to the surface active sites occurs simultaneously due to multiple surface−interface chemical interactions. In the in situ Raman spectra (Figure 8c), it is worth noting that the brand new peak centered at approximately 835 cm−1 was distinctively observed at the process of HSO5− catalytic dissociations. Additionally, one can clearly notice that tremendous gas bubbles desorbed from the surface of the oxide complex were generated spanning across the catalytic dissociations of HSO5− anions under the specific conditions of heavy concentrations (Figure S5). Based on a number of studies, the peak around 835 cm−1 can be assigned to the emerged peroxo species bound to the surface sites and ultimately isolated from the surface as O250 species. It is unveiled in Scheme 3 that the mechanism was proposed on the basis of numerous previous studies and reports. For the sake of achieving the great artificial solar energy harvesting and conversion intrinsic trait, ZCF HYSHMs could inevitably serve as alternative environmentally benign PMS activation catalysts for pollutant removal by effective harvesting and conversion of simulated solar light. The aforementioned active species including SO4·− and ·OH radicals could be generated instantly and simultaneously through effective PMS activations and photoinduced catalysis over the combined ZCF/PMS/Vis

Scheme 3. Proposed Pathways of Photoinduced Catalysis Coupling with Activations of PMS for ENR Eliminations in the ZCF/PMS/Vis Tandem Systems

systems. It can be assumed that the Cu(II)-HSO5− complex was first formed and the surface-attached −OH group could bond with a possible higher transient valence, that is,  Cu(III) formed a Cu(III)−−OH new surface group generating a sulfate radical (see reaction 1) ultimately. After HSO5− was oxidized to SO5·− by Cu(III), the adjacent SO5·− anions over the surface sites could interact with each other producing the surface-bonded peroxo species (eventually O2) and SO4·− radicals ultimately (see reaction 2). While visible light was being exposed and irradiated on the systems, a large amount of photoinduced electrons could be stimulated from VB to CB and the correlative holes were left in VB simultaneously (reaction 3). The photoinduced holes can generate ·OH (reaction 4) and SO5·− (reaction 5) through reacting with surface-adsorbed water or OH− and PMS species, respectively. On the other hand, the plentiful photoinduced electrons could also stimulate PMS to be activated to produce SO4·− (reaction 6) species followed by more effective e−−h+ spatial separation efficiency. The synergistic effects between visible light, ZCF multinary spinel HYSHMs, and PMS efficiently enhanced the ENR degradation activity. Both ·OH and SO4·− are the main dominant species involved in ENR pollutant degradation and even mineralization (reaction 7). Cu(II)−OH + HSO5− → Cu(II)HSO5− + OH− → Cu(III)−OH + SO4·− 4123

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each other, producing the surface-bonded peroxo species (eventually O2) and SO4·− radicals ultimately. This work could provide new insights into the evolutionary relationship of diverse structure-dependent activity, the intrinsic mechanisms of sulfate radical generation, and the synergistic effect among the instant PMS oxidation combined with simultaneous photoinduced catalysis over ZCF HYSHM tandem spinels, which offers a novel design strategy for high artificial solar energy utilization and conversion to chemical energy by multiple spinel diverse nanostructures.

2Cu(III)−OH + 2HSO5− → 2Cu(II)·SO5− + 2H 2O → 2Cu(II)−OH + O2 ↑ + 2SO4·− + 2H+

■

(2)

ZCF + hν → ZCF(e− + h+)

(3)

ZCF(h+) + H 2O or OH− → ·OH

(4)

HSO5− + hν → SO5·− + ·OH

(5)

ZCF(e−) + HSO5− → SO4·− + OH−

(6)

ZCF(h+)/·OH/SO4·− + ENR → products

(7)

■

EXPERIMENTAL SECTION ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) Hierarchical Yolk−Shell Nano-Microsphere Synthesis. All chemical reagents and solvents were accordingly purchased from a commercial company and directly used without any further purification. In a typical synthesis of the ZnFe2O4 yolk−shell structure, 0.5 mmol of Fe(NO3)3·9H2O and 0.25 mmol of Zn(NO3)2·6H2O were dissolved in a mixture of glycerol (16 mL) and isopropanol (80 mL) to produce clear solutions via vigorous stirring for approximately 20 min. The obtained solutions were therefore put into a stainless steel autoclave and kept at constant 180 °C for the extended autoclaving time of 6 h. After cooling to room temperature, the formed ZnxCu1−xFe2O4 glycerate precursor spheres (GPSs) were then collected by centrifugation and washed thoroughly with deionized water and absolute ethanol at least three times. The yolk−shell ZnxCu1−xFe2O4 oxide particles could be formed via thermal annealing of Cu-Zn-Fe GPSs in air at a calcination temperature of 400 °C for approximately 120 min with a constant annealing rate of 10 °C/min. Zn0.5Cu0.5Fe2O4 and CuFe2O4 were synthesized using a similar method by adjusting the amount of nitrate. In a typical synthesis of the Zn0.5Cu0.5Fe2O4 yolk− shell structure, 0.125 mmol of Zn(NO3)2·6H2O, 0.125 mmol of Cu(NO3)2·3H2O, and 0.5 mmol of Fe(NO3)3·9H2O were used, and for CuFe2O4, 0.5 mmol of Fe(NO3)3·9H2O and 0.25 mmol of Cu(NO3)2·3H2O were used. Instruments for Characterization. X-ray diffraction (XRD) patterns of samples were recorded and collected from 20.0 to 80.0° (2θ) on a Rigaku D/max-2400 diffractometer using CuKα radiation. The microstructures were screened and explored by a Hitachi SU8010 field emission scanning electron microscope (FESEM). The energydispersive X-ray spectroscope (EDX) attached to the FESEM instrument was employed to identify and analyze the chemical compositions. The diverse microstructures of spinels were determined by a Tecnai F30 TEM with an applied acceleration voltage of 300 kV. A Thermo ESCALAB 250XI X-ray photoelectron spectroscopy (XPS) was employed to characterize the chemical states and compositions of the samples. The NOVA 4200e Quantachrome was used to characterize the physicochemical characteristics including the BET specific surface area, nitrogen adsorption/desorption, and pore size distributions. The EPR spin-trap experiments were conducted by a Bruker ECS106 X-Band electron paramagnetic resonance spectrometer (Bruker A200, Germany). The UV2300 II UV− vis spectrometer was used to characterize the optical properties of the samples in diffusion reflection spectroscopy mode (UV− vis DRS) spanning across the wavelength ranges of 200−700 nm. The photoluminescence (PL) properties of the electrode surface were investigated by a Hitachi F-4500 fluorescence spectrophotometer (excited at λ = 280 nm). The nanosecond transient laser flash photolysis technique (LP920, Edinburgh, 2

CONCLUSIONS It is always desirable to fabricate a facile, eco-friendly, and great productive oxidation process for high conversion sustainable energy to chemical energy as well as efficient elimination of toxic organic pollutants. The novel PMS activation and photoinduced catalysis tandem systems for efficient artificial conversion of solar to chemical energy over ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) hierarchical yolk−shell nano-microspheres possessing superior catalytic properties for enrofloxacin (ENR) fluoroquinolone antibiotic elimination with brilliant artificial solar energy conversion capability have been first successfully rationally designed and tailored by a facial solvothermal and sequential morphologically conserved thermal treatment strategy. The alternative formation of diverse microstructures was found to be in connection with the partial A-site replacements of the spinels; for example, the CuFe2O4 spinel prefer to form solid microspheres with villiform surfaces, the ZnFe2O4 spinel develops YSHMs composed of exquisite particles, and multiple Zn0.5Cu0.5Fe2O4 (ZCF) spinel tends to form YSHMs with an obvious core and a higher surface area, in which the Fe3+ cations occupy the B sites of the spinel structures with higher superparamagnetic properties. Notably, the multiple Zn0.5Cu0.5Fe2O4 HYSHM tandem spinel exhibits not only an outstanding visible light harvesting and spatial charge separation ability with an average lifetime (τ) of a recombination time of 14.57 μs (those for ZnFe2O4 and CuFe2O4 spinels were only 12.65 and 11.82 μs, respectively) but also a catalysis efficiency of nearly 90.5% for ENR eliminations resulting from both PMS activation and visiblelight-induced photocatalysis (in comparison with those of 86.7% for CuFe2O4 and 65.2% for ZnFe2O4 YSYHMs), which could be ascribed to their physicochemical characteristics and surface structures, namely, more surface oxygen vacancies, highest specific surface area, and interior structures. Notably, in the ZCF/PMS/Vis tandem systems, ·OH and SO4·− were both detected and acted as the primary reactive species in the reaction that generated instantly and simultaneously through effective PMS activations and photoinduced catalysis over the ZCF HYSHM spinel; that is, in addition to the common generation process of ·OH and SO4·− through photoinitiated catalysis and PMS activations, the surface covalent Cu2+ ions in the ZCF could also take the role of facilitating the formation of the Cu2+−HSO5− complex, and the specific surface −OH group linked to a higher transient metal valence (Cu3+) would thus form a new Cu3+−−OH surface group, generating a sulfate radical. After HSO5− was oxidized to SO5·− by Cu3+, the adjacent SO5·− anions over the surface sites could interact with 4124

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ns lowest detection limit, with excitation wavelength of 266 nm, an illumination spot area of 1 cm2) was employed to measure and investigate the transient optical absorptions. The photoinduced redox behavior including interfacial charge separation and transfers was explored by using a surface photovoltage (SPV) system, which was assembled by a lock-in amplifier (model SR830-DSP) with an optical chopper (model SR540) running at a frequency of 20 Hz and a monochromator (model Omni-λ 3005). The molecular vibration properties were investigated by using a Bruker Vertex 70 FTIR spectrometer in the wavelength region of 4000−400 cm−1. An in situ microscopic Raman spectrometer (B&W TEK Opto-electronics, BWS465-785H) equipped with a 785 nm laser light irradiation was employed to explore the surface molecular behavior in the presence and absence of PMS species. The aforementioned oxide composites were generally pressed into slices with 1 mm in thickness and 13 mm in diameter, and Milli-Q water or PMS solution (20 mM) was sequentially dropped onto the slice. Afterward, the rinsed slice was accordingly scanned spanning from 800 to 1100 cm−1 with a resolution of 2 cm−1. Computational Methods and Models. All of the quantum computations in this work were performed by the CASTEP module of Materials Studio 6.0. To illustrate the correlated 3d orbital of Cu and Fe atoms, quantum calculations were generally performed at the DFT+U level by the Perdew− Burke−Ernzerhof functional (PBE) in the generalized gradient approximation (GGA) scheme, and BFG methods were employed to optimize the geometries. Herein, the U term of 2.5 and 5.0 eV were generally used to describe the 3d states of Fe and Cu atoms, respectively. A cutoff energy of 440 eV was set for the extension of the wave functions in plane wave basis. Catalytic Evaluation. The catalytic degradation of ENR pollutants was selected to evaluate the capability of the innovatively fabricated ZnxCu1−xFe2O4 (0 ≤ x ≤ 1)/PMS/Vis tandem systems with diverse structures. The visible-lightignited photocatalytic reactions over the aforementioned spinel-structured materials were performed using a 500 W high power xenon lamp equipped with a 420 nm cutoff filter, which gave an irradiation of 30 mW cm−2 for visible light irradiation. All the experiments were conducted in a 150 mL quartz reaction cell under fixed rotation speed. First, the ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) HYSHMs (50 mg L−1) were homogeneously dispersed into the ENR solutions with concentrations of 20 mg L−1 for 0.5 h to reach the adsorption/desorption equilibrium. The catalytic degradation reactions were therefore ignited by PMS species with a dose of 0.01 g under visible light illumination (λ ≥ 420 nm, T = 275 K, P = 1 atm) simultaneously. All the aforementioned catalytic experiments were performed at mild conditions. Specifically, approximately 1 mL of reactants was withdrawn from the solutions at certain time intervals followed by analysis with a UV−vis spectrophotometer (UV2300 II, Japan) to simultaneously screen and investigate the intensity variation of optical absorption peaks centered at a wavelength of 275 nm in comparison with the original ENR aqueous solution.

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Detailed ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) hierarchical yolk− shell nano-microspheres synthesis, characterization and computational method, photocatalytic reaction evaluations, SEM and particle size distribution of the afore prepared ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) glycerate precursors; the isotherms of N2 adsorption/desorption and pore size distribution of ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) hierarchical yolk−shell nanomicrospheres; photos of the ZnxCu1−xFe2O4 hierarchical yolk−shell nano-microspheres with or without PMS solution; theoretical model of ZnxCu1−xFe2O4 (0 ≤ x ≤ 1) hierarchical yolk−shell nano-microspheres (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.L.). *E-mail: [email protected] (A.C.). ORCID

Xinyong Li: 0000-0002-3182-9626 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Major Program of the National Natural Science Foundation of China (No. 21590813), the Key Project of the National Ministry of Science and Technology (No. 2016YFC0204204), the National Natural Science Foundation of China (No. 21577012), the Program of Introducing Talents of Discipline to Universities (B13012), and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education.

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DOI: 10.1021/acsomega.8b03071 ACS Omega 2019, 4, 4113−4128