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Jan 26, 2018 - •S Supporting Information. ABSTRACT: A .... detectors, etc.44−46 In MIS nanojunction, upon light irradiation, ..... this confirms t...
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Article Cite This: ACS Omega 2018, 3, 1090−1101

Rare-Earth-Based MIS Type Core−Shell Nanospheres with Visible-Light-Driven Photocatalytic Activity through an Electron Hopping−Trapping Mechanism Suganya Josephine G. A. and Sivasamy Arumugam* Chemical Engineering Area, CSIR-Central Leather Research Institute, Adyar, Chennai, India, 600020 S Supporting Information *

ABSTRACT: A bilayered rare-earth-based metal−insulator−semiconductor, Dy2O3@SiO2@ZnO core−shell nanospheres, was synthesized by a stepwise synthesis for enhanced visible photocatalytic activity. The prepared material was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, ultraviolet− visible diffuse reflectance spectroscopy, field-emission scanning electron microscopy, energy-dispersive spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, Brunauer−Emmett−Teller, and electron paramagnetic resonance techniques. Dy2O3@SiO2@ZnO core−shell nanospheres were found be in a spherically arranged cauliflower-like morphology (40−60 nm). The highresolution transmission electron microscopy analysis proved the core−shell morphology of the prepared material with a single Dy2O3 core and two shells comprising SiO2 and ZnO. The material possessed a surface roughness of 4. 98 nm (2 × 2 μm area) and a band gap energy of 2.82 eV. The in situ generation of OH radicals was confirmed by electron paramagnetic resonance. Electron hopping through the SiO2 layer from ZnO to Dy2O3 played a major role in trapping electrons in the f-shells of lanthanides, thus, preventing the recombination of electron−hole pair. X-ray photoelectron spectroscopy studies proved the band alignment of the material. Brunauer−Emmett−Teller analysis further showed the core− shell surface area was 14 m2/g. The visible photocatalytic activity was tested against 2,4-D (2,4-dichlorophenoxyacetic acid), an endocrine disruptor. The kinetic studies showed that the photocatalytic degradation process followed a pseudo-first-order pathway. The photocatalyst was found to be reusable even up to the third cycle.

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INTRODUCTION Nanocrystalline semiconductors are often employed as suitable materials for light-induced photocatalytic processes.1,2 Metal oxides, such as ZnO, TiO2, SnO2, Bi2O3, WO3, CeO2, etc., are widely studied for their photocatalytic activity in the UV and visible regions.3−7 Although, a number of nanomaterials are active in the UV region, advancements toward the development of modified materials for visible-light-induced photocatalysis have gained importance since the discovery of a TiO2-based photocatalyst for water splitting reactions by Fujishima and Honda in 1972.8 Because of the consciousness of energy crisis in developed and developing countries, a material active in the visible region could be employed widely as a solar photocatalyst. The solar energy spectrum consists of nearly 46% visible, 5−7% ultraviolet, and 47% infrared radiation.9 Hence, band gap energy (Eg) of the semiconductor materials need to be improvised or modified to absorb visible light for photocatalytic applications.10−12 Researchers across the globe are in search of alternative techniques to modify the Eg of semiconductor metal oxide nanoparticles. These modifications could be carried out by doping of metals,13−15 nonmetals,16,17 and rare-earth materials18−21 and preparation of mixed metal oxides,22,23 graphenebased materials,24 and core−shell nanocomposites.25−28 Although TiO2-based materials have been widely employed for © 2018 American Chemical Society

photocatalysis, certain weaknesses, such as stability, cost, and quantum efficiency, have led to ZnO emerging as a suitable alternative.29 Core−shell nanomaterials are nanocomposites that consist of a core material and a material that surrounds the core creating a shell-like structure. Nobel metals, such as Au, Ag, Pd,30−32 and metal oxides, such as Fe2O3 or ZnO,33,34 are often employed as the core, and SiO2, TiO2, or polymers35−37 are often employed as shell materials. Noble metals easily form colloidal solution, facilitating the preparation of core, and the organic precursors of SiO2 and TiO2 are common and could easily surround the core, forming the shell material.35,36 Some of the examples of visible-lightactive core−shell nanocomposites are N-doped ZnO/g-C3N4,38 BiVO4@Bi2O3,39 CdS@TaON,27 and Ag@Fe3O4@SiO2@ TiO2.40 Recently, it has been shown by various research groups that the incorporation of rare-earth materials in semiconductors enhance the photocatalytic activity of the material because of the presence of f-orbitals in lanthanides.41 Lanthanide materials are well-known insulators, but the presence of f-shells contributes toward the trapping of electrons on irradiation when developed in combination with other semiconductor materials.41−43 Received: October 20, 2017 Accepted: January 12, 2018 Published: January 26, 2018 1090

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core−shell nanosphere for the degradation of an endocrine disruptor 2,4-D. The prepared photocatalyst were characterized by FT-IR, XRD, UV−vis DRS, FE-SEM, EDAX, HR-TEM, SAED, XPS, BET, and EPR techniques. The photocatalytic activity of the prepared DSZ for the degradation of 2,4-D under visible light irradiation were carried out. The preliminary studies on the effect of pH, catalyst dosage, variation of initial 2,4-D concentrations, and the kinetics of photocatalytic degradation with respect to time were conducted. The reaction followed pseudo-first-order kinetics. The degradation processes were monitored by UV−visible absorbance measurement and decrease in COD of the solution. Recyclability studies were conducted, and the core−shell nanosphere was found to be reusable even up to the third cycle of usage. The efficiency of the prepared DSZ was compared with pristine ZnO.

Thus, leaving the holes in the valence band of the semiconductor to react with water molecules for the production of hydroxyl radicals to initiate the photodegradation reaction of the target molecules. Doping of lanthanides on ZnO, TiO2, WO3, etc., have been widely attempted, also very recently doping of ZnO on lanthanide material have been carried out by our research group.41 Hence, the present study accounts for the first report employing rare-earth material as a core material for investigations on photocatalytic activity. ZnO was chosen as a shell material as ZnO is a widely applied semiconductor for photocatalytic application. But, the stability of ZnO is a challenging factor; hence, enhancing the stability by forming a core−shell material is investigated. Also, SiO2 has been employed widely as a core−shell material because of the ease of preparation and cost.35 Hence, in this Article, to enhance the activity of ZnO, we have synthesized Dy2O3 as core and formed SiO2 and ZnO as the subsequent shell layers. The proposed preparation of a rare-earthbased MIS (rare-earth metal/insulator/semiconductor) (REMIS) sequence is a similar structural sequel to MIS (metal−insulator− semiconductor) nanojunction widely applied for hydrogen evolution in photoelectrochemical cells, photoelectrodes, photodetectors, etc.44−46 In MIS nanojunction, upon light irradiation, the excited electron from the conduction band of the semiconductor passes through the thin layer of insulator to the metal layer by electron tunneling via an electron hopping mechanism.46 The insulator layer often employed is SiO2; because of the presence of defect sites in SiO2,47 the excited electron from the semiconductor layer are transported to the metal layer by hopping of electrons through SiO2. Hence, in the present study, the rare-earth metal@insulator@semiconductor (REMIS) prepared may act through a similar mechanism for enhancing the photocatalytic activity. Semiconductor photocatalysts have a variety of applications, which include solar cells, sensors, luminescence material, upconversion agents, oxidation/reduction catalysts, photocatalytic degradation of organics, photocatalytic water splitting, etc.48−54 Because there is an enormous increase in the rate of environmental pollution, in particular water pollution, there is a global alarm in the enforcement of improved technologies for the treatment of emerging contaminants. Contamination of water bodies is often caused by discharges from chemical, pharmaceutical, tanneries, and textile industries and from the agricultural sector due to water runoff.55 The most important effluent that threaten mankind are dyes and endocrine-disrupting agents because they are often toxic and carcinogenic.55 2,4-D (2,4-dichlorophenoxyacetic acid) is a commonly employed herbicide in the agricultural sector, which contributes to the contamination of water bodies due to runoff from farmlands sprayed with herbicides.56 2,4-D has been classified as an endocrine disrupting agent,57 and the LD50 for rat is found to be 100−500 mg/kg.58 The toxicity of 2,4-D in humans has been studied by various scientific communities and is known to cause a disruption in energy production by the depletion of the primary energy molecule ATP (adenosine triphosphate).59,60 2,4-D is also known to interfere with the functions of serotonin and dopamine (neurotransmitters); mostly in young organisms, this exposure would result in the delay of brain development and abnormal behavioral patterns, for example, immobility, tremor, apathy, and repetitive movements. Studies suggested that females are more prone to be affected than males.61,62 Therefore, the present investigation is mainly focused on the synthesis of a highly visible active Dy2O3@SiO2@ZnO (DSZ)

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RESULTS AND DISCUSSION Characterization of the Prepared Core−Shell Nanosphere by FT-IR and XRD. The prepared DSZ core−shell nanosphere, pristine Dy2O3, and ZnO were characterized by FT-IR analysis, and the results are shown in Figure S1. A band at 492 cm−1 appeared due to the presence of Zn−O stretching in the prepared ZnO (Figure S1i). Dy2O3 showed bands around 567 and 473 cm−1 due to the presence of Dy−O in the prepared material (Figure S1ii). Figure S1iii shows the presence of stretching vibrations due to the presence of Zn−O and Dy−O at 557 and 470 cm−1, respectively. A band at 800 and 1094 cm−1 were attributed to the presence of Si−O stretch, which confirms the presence of SiO2 in the prepared DSZ nanosphere. In addition to these, a broad peak at 3400 cm−1 were observed in all the spectra, which corresponds to the O−H stretching vibrations from the surface hydroxyl groups. The results were corroborated with the literature,63−65 which confirms the presence of Zn, Dy, and Si as oxides in the prepared DSZ core−shell nanosphere. The crystallinity of the prepared DSZ nanosphere were examined by XRD, and the results are shown in Figure 1a. The peaks were found to be sharp and narrow. The hump at 2θ of 10−20° confirmed the presence of silica in the prepared material. Peaks in the XRD pattern at (211), (222), (400), (440), and (622) are attributed to the characteristic planes of Dy2O3 and at (100), (002), (101), (102), (110), (103), (004), (112), and (201) are characteristic planes for ZnO, respectively. The diffraction peaks were compared with the standard patterns (ZnO, JCPDS 36-1451; Dy2O3, JCPDS 01-079-1722).41 Hence, the presence of Dy2O3, ZnO, and SiO2 are further confirmed by XRD analysis. The crystallite size of the prepared material was calculated using the Scherer eq 2 D = kλ /(β cos θ )

(2)

where k is 0.94 for spherical samples, λ is the wavelength of radiation corresponding to 0.154 nm, β is the full width at halfmaximum, and θ is half the diffraction angle. The crystallite size was found to be in the nano range of 50−60 nm. Determination of Band Gap Energy. The absorption of light energy photons are an important phenomena in the determination of photocatalytic activity of any catalyst. The energy required for the transportation of an electron from the valence band to the conduction band is determined by the band gap energy of the material. Hence, the UV−visible DRS results would positively predict the photocatalytic activities of the prepared catalyst. The absorbance spectrum of the prepared DSZ core−shell nanosphere are displayed in Figure 1b. The results show that the maximum absorption occurred around 390−400 nm; in addition 1091

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EDAX spectrum, and the ratio of atomic dysprosium/silica was found to be 1:2, respectively (Figure S3). The elemental mapping of the prepared DS nanosphere is displayed in Figure 2b−d. The mapping studies show the presence of the Dy, Si, and O on the prepared DS nanosphere. Figure 3a and b shows the morphology of the prepared DSZ to be spherical in a cauliflowerlike arrangement. The size of the particles varied from 40−60 nm. The EDAX patterns in Figure S4 shows the presence of Dy, Si, Zn, and O in the sample. The elemental mapping further confirms the distribution of the elements in the prepared DSZ photocatalyst Figure 3c−f. High-Resolution Transmission Electron Microscopy. Figure 4 shows the HR-TEM images and SAED patterns of the prepared DSZ core−shell nanosphere. The core−shell morphology of the prepared DSZ was confirmed from the HR-TEM images as shown in Figure 4a. The size of the DS core was measured to be ∼38 nm (20 nm Dy2O3 and a 9 nm thick SiO2 shell layer) and the size of DS core, along with ZnO shell, was found to be 55 nm. Therefore, the thickness of the ZnO shell layer is ∼8.5 nm. The core and two shells could be clearly seen in the image. Since the material has a cauliflower like morphology the circles of the outer particles clearly show the core−shell morphology. One of the outer spheres shows the distinct Dy2O3 core and subsequent SiO2 and ZnO shells in the prepared DSZ photocatalyst (inset of Figure 4a). The core−shell morphology could be clearly evidenced from the HR-TEM analysis. The SAED patterns of DSZ are shown in Figure 4b. Analysis of Surface Roughness, Surface Area, and Band Alignment. The atomic force microscopy images of the prepared DSZ are shown in Figure 4c and d. The results prove the prepared material to be nanosized and spherical. The surface roughness profile (Figure S5) shows a number of peaks and troughs on the surface of the core−shell photocatalyst, which confirms the roughness on the surface, which would facilitate the absorption of organic moieties thereby initiating the reaction process. The surface roughness was found to be 4.98 nm for a 2 × 2 μm area. The BET surface area of the prepared DSZ were analyzed by the N2 adsorption−desorption isotherm (Figure S6). The surface area were calculated to be 14.008 m2/g with an average pore diameter of 32.9 nm. The X-ray photoelectron spectroscopy results of the prepared DSZ core−shell photocatalyst is shown in Figure 5. The XPS survey spectrum shows the emission lines due to the presence of Dy, Zn, Si, and O in the prepared DSZ core−shell along with a weak C line. Figures 5 and S7 also shows the individual elemental XPS spectra of all the elements present in the prepared DSZ core− shell nanosphere. The line at 156.55 eV corresponds to the presence of 4d5/2 orbital of Dy and the line at 103.95 eV is assigned to the presence of the oxide form of Si 2p in the material. The presence of Zn in the sample is confirmed by the emission lines at 1021.1 7 and 1044.23 eV due to the 2p3/2 and 2p1/2 orbitals of Zn, respectively. A sharp peak centered at around 531.19 eV was observed, which is due to the presence metal−oxygen bonds present in the prepared DSZ core−shell nanosphere. The emission line at 285.05 was attributed to the weak C 1s line. All the results were corroborated with literature.41,66 Hence, XPS analysis serves as a support to the presence of Dy, Si, and Zn as oxides in the prepared core−shell nanomaterial. Electron Paramagnetic Resonance Spectroscopy. A photocatalytic degradation process is generally initiated by a radical mediated mechanism either in the presence of OH radical or O2•− radical. The generation of radical species in the

Figure 1. (a) XRD patterns of Dy2O3, ZnO, and DSZ and (b) UV−vis DRS spectra of the prepared ZnO, Dy2O3, and DSZ.

to this, a few peaks were observed around 700−800 nm, which are the characteristic peaks of rare-earth elements. The f−f transitions of the lanthanide material Dy2O3 present in the sample contributes to this absorption phenomena. From the UV−vis absorption studies, the band gap energy of the material was calculate using eq 3 A = [k × hv − Eg1/2]/hv

(3)

where v is the frequency, h is the Planck’s constant, and k equals a constant. The band gap was calculated by plotting (Ahv)2 versus hv. From the plot, a tangent was drawn, and the intercept of the two lines was taken as the band gap of the photocatalyst. The band gap energy was found to be 2.82, 2.98, and 4.63 eV for DSZ, pristine ZnO, and pristine Dy2O3, respectively (Figure S2). The band gap energy of the prepared DSZ core−shell photocatalyst was found to decrease after the modifications conducted. From these results, we can conclude that the band gap energy of the material is modified effectively toward the formation of a lower band gap material, which could possibly exhibit better photocatalytic activity. Surface Morphology by Field Emission Scanning Electron Microscopy. The morphologies of the prepared DS and DSZ are shown in Figures 2 and 3. Both DS and DSZ were found to have spherical morphology. Figure 2a shows the DS to be spherical with particle size ranging from 30 to 40 nm, consisting of the elements dysprosium, silica, and oxygen from 1092

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Figure 2. (a−d) FE-SEM image of the prepared DS and its elemental mapping showing Dy, Si, and O.

photoreaction system were detected by EPR analysis in the presence of the spin-trapping agent DMPO. Required amount of DSZ, water, and DMPO were taken in a glass tube and irradiated under visible light (15 min). Then, the mixture was filtered immediately and transferred to a EPR flat cell, and the spectrum was recorded. The results are shown in Figure 6. Characteristic peaks appeared at 3412.41, 3427.14, 3442.37, and 3457.45 in the intensity ratio of 1:2:2:1, corresponding to the DMPO spin-trapped OH radical. The g factor was found to be 2.0037 eV, which is equal to that of a free electron. Hence, this confirms the in situ generation of OH radicals in the reaction system. Photocatalytic Activity of Core−Shell Nanosphere. From the characterization results, we can conclude that the prepared DSZ photocatalyst is crystalline, nanosized, and in a cauliflower-like core−shell morphology consisting of Dy2O3 core and subsequent shells of SiO2 and ZnO, with a particle size of 50−60 nm. The band gap energy was found to be 2.82 eV. All these techniques suggested the presence of Dy2O3, SiO2, and ZnO in the prepared nanomaterial with a core−shell morphology. The material exhibited an absorption in the visible region at 400 nm, in addition an absorption occurs at 700−800 nm, which is due to the presence of f−f transitions present in the f-shells of Dy2O3. Therefore, upon visible light irradiation of the DSZ core−shell nanomaterial, the electron from the valence band (VB) of ZnO outer shell (in DSZ) excites to the conduction band (CB) by absorbing photons of light, satisfying the band gap energy (2. 82 eV). The prepared material contains a similar structure to that of MIS nanomaterials,45 wherein a rareearth material (Dy2O3) is present instead of the metal and SiO2

acts as a thin insulating layer in between Dy2O3 and semiconductor material ZnO, therefore, we refer to it as REMIS nanomaterial. Hence, there would a similar possibility, as in the case of MIS material, for the transfer of electrons through the insulating layer to the surface of the rare-earth metal. Hence, the excited electron from CB of ZnO shell is transferred through the SiO2 layer via an electron-hopping mechanism46 through the defects in SiO2 present in the layer near the ZnO/SiO2 interface.47,67,68 This electron-tunneling process allows the electrons to get through the insulating SiO2 layer to the Dy2O3 core, where it is trapped in the f-shells characteristic of lanthanide elements. Once the electrons are trapped in the f-shells, the separated holes in the VB of ZnO are free to react. Thus, the process of recombination is prevented, and water molecules could easily react with the separated holes to produce OH radicals, which are active initiators of the photodegradation process. The mechanism of the photocatalytic activity is represented in Figure 7. The production of OH radicals in the in situ process serves as a support for the proposed mechanism. Hence, the prepared DSZ core−shell would be photocatalytically active under visible light. In the case of pristine ZnO, upon light irradiation, the possibilities of the exited electron in the CB to recombine with the hole in the VB is very high. Hence, the activity of ZnO would be lesser than that of the prepared DSZ core−shell. The EPR studies support this proposal because the time for OH radical production is higher for pristine ZnO (30 min). Therefore, it is worth studying the photocatalytic activity of the prepared DSZ core−shell nanosphere. Photocatalytic Degradation of a Model Pollutant 2, 4-D under Visible Light Irradiation. Photocatalytic Activity 1093

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Figure 3. (a, b) FE-SEM image of the prepared DSZ and (c−f) corresponding elemental mapping showing Dy, Si, Zn, and O.

of Core−Shell Nanosphere. Photocatalytic activities of the prepared ZnO, Dy2O3, DSZ 1:1, DSZ 1:2, DSZ 1:3, and DSZ 1:4 were compared by analyzing 10 ppm of 2,4-D in 10 mL of solution with a catalyst dosage of 10 mg, irradiated under visible light. A blank reaction without catalyst was also studied simultaneously (only irradiation). The results are shown in Figure S8a. The experimental analysis revealed the prepared DSZ 1:3 to be the best photocatalyst; hence, further studies were conducted with DSZ 1:3. Studies on the effect of pH, initial catalyst dosage, and initial 2,4-D concentrations were conducted to optimize the experimental parameters. Figure S8b shows the results on the effect of pH; 10 mg of catalyst in 10 ppm 2,4-D were taken and made up to the appropriate pH (2−12). Neutral pH showed maximum degradation (71%) with a minor decrease in other pH ranges. Hence, for ease of preparation, neutral pH was used for further studies. The catalyst dosage for the reaction mixtures was optimized by conducting the experiments with a catalyst dosages between 3 and 20 mg. Figure S8c shows 10 mg of catalyst to result in higher degradation efficiency. Catalyst dosages above 10 mg resulted in decreased degradation; this is

because upon increased catalyst dosage, the absorption of the penetrated light inside the reaction is lesser because the light entering is reflected and not efficiently absorbed by the catalyst for further reaction. Hence, 10 mg of catalyst per 10 mL of solution was fixed as the optimum catalyst dosage. Figure S8d shows the plots for the variation of initial 2,4-D concentration (5−50 ppm). As the concentration of 2,4-D increased, the percentage of degradation was found to decrease. For 50 ppm of 2,4-D, the percentage of degradation was nearly 47%. Kinetic of Photodegradation of 2,4-D. The kinetics for the photocatalytic degradation of 2,4-D under visible light irradiation with the prepared DSZ core−shell photocatalyst were conducted for 10, 15, 20, and 25 ppm 24-D concentration. The results are shown in Figure 8a. Aliquots of sample were collected at regular time intervals, and the UV−visible absorbance was measured to check the progress of the photoreaction. The percentage of degradation reached more than 99% in 960, 1200, 1320, and 1440 min, respectively, for 10, 15, 20, and 25 ppm 2,4-D concentrations, respectively. The kinetics of the reaction was found to follow a pseudo-first-order kinetics pattern. The pseudo-first-order 1094

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Figure 4. (a) HR-TEM images, (b) SAED pattern, (c) and (d) 2D and 3D AFM spectra of the prepared DSZ.

plots are shown in Figure 8b. The rate constants were calculated to be 2.56 (10 ppm), 2.20 (15 ppm), 1.86 (20 ppm), and 1.67 × 10−3 min−1 (25 ppm), respectively, and the R2 values were well above 0.998. The progress of the photoreaction were tested by the reduction in COD levels of the aliquots of sample collected (Figure 8c). The decrease in COD of the sample proves the degradation of the 2,4-D moiety present. A decrease in COD from 252 to 23 (10 ppm), 504 to 23 (15 ppm), 804 to 63 (25 ppm), and 961 to 126 mg/L (50 ppm) was observed. The decrease in COD levels depicts the destruction of organic molecules present in the solution, hence, proving the enhanced photocatalytic activity of the prepared DSZ core−shell nanosphere.69 Figure 8d shows the UV−visible absorbance spectrum of 20 ppm 2,4-D degradation. The aliquots of sample collected were scanned from 200 to 800 nm to confirm the degradation process. The decrease in absorbance of the characteristic peak of 2,4-D at 282.5 nm shows that the 2,4-D moiety has completely degraded and that the absorbance reached zero at the completion of the reaction. This proves that DSZ 1:3 is a better photocatalyst for the degradation of 2,4-D. Reusability Studies. The prepared DSZ 1:3 core−shell photocatalyst was tested for efficiency by conducting the photodegradation reaction with the same catalyst up to 3 cycles. The photocatalyst after the completion of reaction was filtered, dried, and charged for the next cycle of operation, and this was continued until the third cycle. The results are shown in Figure 9. The experiments proved that the prepared core−shell photocatalyst

retained its properties even up to the third cycle of operation. The photocatalytic degradation efficiency was above 99% in each of the cycles. The reused catalyst was analyzed for its FT-IR and FE-SEM results, and the results are shown in Figure S9. The results prove that the catalyst has not changed after the degradation reaction. No new peaks were observed in the FT-IR spectra (Figure S9a) proving the absence of adsorbed organic moieties on the surface of the material. And the spectrum was similar to that of the as-prepared catalyst. The FE-SEM analysis (Figure S9b) also showed the spherical nature of the material, showing that the catalyst is unchanged after the photocatalytic reaction. Hence, DSZ is a better catalyst for environmental remediation.

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CONCLUSION A highly visible active rare-earth-based MIS-type Dy2O3@SiO2@ ZnO core−shell nanosphere material was prepared by a simple stepwise procedure, and the prepared material was characterized by FT-IR, XRD, UV−vis DRS, FESEM, EDAX, HRTEM, AFM, XPS, and EPR techniques. The DS core−shell was prepared by a CTAB-mediated pathway. The synthesized DS was used as a precursor for the preparation of the target DSZ core− shell nanosphere. DS was found to be spherical in shape, whereas the DSZ core−shell formed a cauliflower-like arrangement. The particle size ranged from 40 to 60 nm and the surface roughness was 4.98 nm for a 2 × 2 μm area. The EDAX pattern, elemental mapping studies, and XPS spectra confirm the presence of Dy, Si, and Zn as oxides in the prepared material. The DSZ 1095

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Figure 5. (a) XPS full survey spectrum and individual spectra of (b) Dy, (c) Si. and (d) Zn.

Figure 7. Mechanism for the photocatalytic activity of the prepared DSZ photocatalyst.

in the Dy2O3 core. The transfer of electrons from the CB of ZnO to f-shells of Dy2O3 is proposed to occur through an electron-hopping mechanism through defect sites present in the insulating SiO2 layer, which is further followed by the trapping of the electrons in the f-shells of core Dy2O3. Thus, this leaves behind the separated hole free to react with water molecules to generate OH radicals. The in situ generation of OH radicals in the system was also confirmed by EPR studies. The studies on the photocatalytic degradation of a model pollutant 2,4-D confirmed the visible light photocatalytic activity. The reaction

Figure 6. EPR spectrum of the DMPO spin-trapped OH radical formed in the in situ process by the prepared DSZ.

core−shell possessed a band gap energy of 2.82 eV and was highly active under visible light irradiation. The recombination of the separated electrons and holes in the CB and VB of ZnO shell are prevented by electron trapping in the f-shells present 1096

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Figure 8. (a) Plot of kinetics, (b) pseudo-first-order rate constant, (c) reduction in COD levels, and (d) the UV−visible absorbance spectrum of the photocatalytic degradation of 2,4-D by DSZ under visible light.

followed pseudo-first-order kinetics, and degradation was confirmed by a reduction in COD level and UV−vis absorption measurement. The prepared DSZ was found to be reusable even up to the third cycle of usage without losing its activity. Hence, the prepared DSZ core−shell nanosphere is a better catalyst of environmental remediation, and this study paves way for further research on multi-layered core−shell materials utilizing rare-earth elements for visible photocatalytic activity and environmental remediation.

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EXPERIMENTAL SECTION Materials. Zinc acetate tetrahydrate (99%, Zn(OAC)2· 4H2O), zinc nitrate hexahydrate (98%, Zn(NO3)2·6H2O, dysprosium nitrate hexahydrate (99.9%, Dy(NO3)·6H2O), tetraethyl ortho silicate (99. 9%, TeOS), cetyltrimethylammonium bromide (CTAB), and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were purchased from Sigma-Aldrich, India. 2,4-D (2,4-dichlorophenoxyacetic acid) was procured from Agrochemicals Pvt., Ltd., Chennai. Ammonium carbonate, sodium carbonate, and sodium hydroxide were supplied by Sisco Research Laboratories, India. Ethanol and hydrochloric acid were procured

Figure 9. Reusability studies on DSZ for the photocatalytic degradation of 2,4-D up to 3 cycles. 1097

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Figure 10. Schematic representation for the preparation of DSZ core−shell nanosphere.

dissolved in the required amount of water and added dropwise to the above solution under constant stirring. After the addition was complete, the mixture was stirred further for 15 min. Then, the precipitate was filtered and washed with water and, finally, ethanol. The precipitates were sintered at 300 °C in a muffle furnace. The product obtained is labeled as DSZ 1:3 (weight of DS (0.4 g): weight of theoretical ZnO (1.2g)). Similarly, DSZ 1:1, DSZ 1:2, and DSZ 1:4 were prepared accordingly. The molar ratio of zinc acetate and ammonium carbonate was maintained as 1:2. The scheme for the preparation of DSZ is shown in Figure 10. Instrumental Analysis. The prepared photocatalysts were characterized with a Fourier transform infrared (FT-IR) spectrometer in a scan range of 400−4000 cm−1 (PerkinElmer). The X-ray diffraction (XRD) patterns were recorded in a PAN analytical X-ray diffractometer (Germany) with Cu Kα radiation in the 2θ scan range between 10° and 70°. An accelerating voltage of 40 kV and an emission current of 25 mA were used. The band gap of the material was analyzed in an ultraviolet visible−diffuse reflectance spectrometer (UV−vis DRS) (JASCO, model V-650). The surface morphology of the prepared material were characterized by field-emission scanning electron microscope (FE-SEM) and energy dispersive X-ray diffraction (EDAX) (Supra 55-Carl Zeiss, Germany), atomic force microscope (AFM) (NTEGRA PRIMA-NTMDT, Ireland), and high resolution−transmission electron microscope (HR-TEM) (JEOL 3010). The band alignment studies were recorded by X-ray photoelectron spectroscopy (XPS) (Omicron ESCA Probe spectrometer), and electron paramagnetic resonance (EPR) experiments were conducted in a Bruker model EMX X Band, EPR Spectrometer. Photocatalytic Experiments. The photocatalytic experiments under visible light irradiation were conducted in an annulartype photoreactor supplied by Heber Scientific Company, Ltd., Chennai, India. The reactor consisted of a tungsten filament lamp (500 W) capable of emitting visible light, which was used as the irradiation source. The photocatalytic reactor tubes were made of glass with height × diameter of 30 × 1.5 and 30 × 3.5 cm, respectively. The visible-light-emitting lamp was surrounded with a water jacketed tube, which was continuously cooled with water circulation to remove the heat produced by the lamp. Simultaneously, cooling fans were also provided inside the reactor to

from Merck, Mumbai, India. All the chemicals except DMPO were used as received without further purification. DMPO was purified and checked for purity before analysis. Methods. The synthesis of core−shell nanosphere was performed in a three-step process.70−72 First, Dy2O3 was prepared by a precipitation technique and was used for the preparation of Dy2O3@SiO2 (DS) nanosphere. Then the prepared DS was used as the starting material for the core−shell Dy2O3@SiO2@ ZnO (DSZ) core−shell. Step 1: Preparation of Dy2O3. Dy2O3 was prepared by a precipitation technique70, employing nitrate as the source of dysprosium and sodium carbonate as the precipitating agent. Dy(NO3)2· 6H2O (0.1 M)was dissolved in 100 mL of distilled water. To this solution, under vigorous stirring, 0.1 M Na2CO3 dissolved in 100 mL of distilled water was slowly added. After the addition was complete, the precipitate was stirred for a further 30 min. Then, the white precipitate was filtered, washed, and dried. The dried powder was sintered at 700 °C for 2 h in a muffle furnace. ZnO was prepared by a similar procedure, employing zinc nitrate instead of dysprosium nitrate. Step 2: Preparation of Dy2O3@SiO2 Nanospheres. Dy2O3@ SiO2 nanospheres were prepared by a typical procedure reported elsewhere.71 Absolute alcohol (250 mL), 60 mL of water, 10.2 mL of aqueous ammonia, and 1.2 mL of TEOS were injected into a 250 mL conical flask. The solution was stirred for 10 min at 40 °C. Then 1 g of Dy2O3 and 0.6 g of CTAB were sonicated (25% amplitude, 750W) in 60 mL of distilled water, which was then added to the above solution. The mixture was continuously stirred for 12 h at 40 °C. The nanospheres, consisting of silica and Dy2O3, were then separated by centrifugation (2000 rpm, 10 min). The solid material was washed with hot deionized water three times to remove the unreacted CTAB. For each washing, hot water was added to the material and sonicated for 2 min (25% amplitude, 750 W) to completely disperse the nanoparticles in water. The prepared Dy2O3@SiO2 material was named DS. Step 3: Preparation of DSZ Core−Shell Particles. Multilayered core−shell DSZ was prepared according to a modified procedure reported elsewhere.72 DS (0.4 g) was dissolved in required amount of water and dispersed ultrasonically using a sonicator at 25% amp (750 W) for 10 min. After sonication, 1.6176 g of zinc acetate dissolved in 20 mL of distilled water was added to the solution. Then, 2.83 g of ammonium carbonate was 1098

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(5) Kumar, V.; Govind, A.; Nagarajan, R. Optical and photocatalytic properties of heavily F−-doped SnO2 nanocrystals by a novel singlesource precursor approach. Inorg. Chem. 2011, 50, 5637−5645. (6) Anandan, S.; Miyauchi, M. Improved Photocatalytic Efficiency of a WO3 System by an Efficient Visible-Light Induced Hole Transfer. Chem. Commun. 2012, 48, 4323−4325. (7) Liyanage, A. D.; Perera, S. D.; Tan, K.; Chabal, Y.; Balkus, K. J. Synthesis, Characterization, and Photocatalytic Activity of Y-Doped CeO2 Nanorods. ACS Catal. 2014, 4, 577−584. (8) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (9) Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C. PhotoElectrochemical Hydrogen Generation from Water Using Solar Energy. Materials-Related Aspects. Int. J. Hydrogen Energy 2002, 27, 991−1022. (10) Hu, J.; Li, H.; Huang, C.; Liu, M.; Qiu, X. Enhanced Photocatalytic Activity of Bi2O3Under Visible Light Irradiation by Cu (II) Clusters Modification. Appl. Catal., B 2013, 142−143, 598−603. (11) Kisch, H.; Sakthivel, S.; Janczarek, M.; Mitoraj, D. A Low-Band Gap, Nitrogen-Modified Titania Visible-Light Photocatalyst. J. Phys. Chem. C 2007, 111, 11445−11449. (12) Gupta, S.; De Leon, L.; Subramanian, V. R. Mn-Modified Bi2Ti2O7Photocatalysts: Bandgap Engineered Multifunctional Photocatalysts for Hydrogen Generation. Phys. Chem. Chem. Phys. 2014, 16, 12719−12727. (13) Borgarello, E.; Kiwi, J.; Graetzel, M.; Pelizzetti, E.; Visca, M. Visible Light Induced Water Cleavage in Colloidal Solutions of Chromium-Doped Titanium Dioxide Particles. J. Am. Chem. Soc. 1982, 104, 2996−3002. (14) Ambrus, Z.; Balazs, N.; Alapi, T.; Wittmann, G.; Sipos, P.; Dombi, A.; Mogyorosi, K. Synthesis, Structure and Photocatalytic Properties ofFe (III)-Doped TiO2Prepared From TiCl3. Appl. Catal., B 2008, 81, 27−37. (15) Liyanage, A. D.; Perera, S. D.; Tan, K.; Chabal, Y.; Balkus, K. J. Synthesis, Characterization, and Photocatalytic Activity of Y-Doped CeO2Nanorods. ACS Catal. 2014, 4, 577−584. (16) Qin, H.; Li, W.; Xia, Y.; He, T. Photocatalytic Activity of Heterostructures Based on ZnOand N-Doped ZnO. ACS Appl. Mater. Interfaces 2011, 3, 3152−3156. (17) Ohno, T.; Mitsui, T.; Matsumura, M. Photocatalytic Activity of S-Doped TiO2 Photocatalyst under Visible Light. Chem. Lett. 2003, 32, 364−365. (18) Ranjit, K. T.; Willner, I.; Bossmann, S. H.; Braun, A. M. Lanthanide Oxide-Doped Titanium Dioxide Photocatalysts: Novel Photocatalysts for the Enhanced Degradation of P-Chlorophenoxyacetic Acid. Environ. Sci. Technol. 2001, 35, 1544−1549. (19) Xu, A. W.; Gao, Y.; Liu, H. Q. The Preparation, Characterization, and their Photocatalytic Activities of Rare-Earth-Doped TiO2 Nanoparticles. J. Catal. 2002, 207, 151−157. (20) Anandan, S.; Miyauchi, M. Ce-Doped ZnO (CexZn1−XO) Becomes an Efficient Visible-Light-Sensitive Photocatalyst by CoCatalyst (Cu2+) Grafting. Phys. Chem. Chem. Phys. 2011, 13, 14937− 14945. (21) Josephine, G. A. S.; Sivasamy, A. Nanocrystalline ZnO Doped on Lanthanide Oxide Dy2O3: A Novel and UV Light Active Photocatalyst For Environmental Remediation. Environ. Sci. Technol. Lett. 2014, 1, 172−178. (22) Fu, X.; Clark, L. A.; Yang, Q.; Anderson, M. A. Enhanced Photocatalytic Performance of Titania-Based Binary Metal Oxides: TiO2/SiO2and TiO2/ZrO2. Environ. Sci. Technol. 1996, 30, 647−653. (23) Lee, K.; Ruddy, D. A.; Dukovic, G.; Neale, N. R. Synthesis, Optical, and Photocatalytic Properties of Cobalt Mixed-Metal Spinel Oxides CO (Al1‑Xgax)2O4. J. Mater. Chem. A 2015, 3, 8115−8122. (24) Bai, X.; Wang, L.; Zhu, Y. Visible Photocatalytic Activity Enhancement of ZnWO4by Graphene Hybridization. ACS Catal. 2012, 2, 2769−2778. (25) Bridewell, V. L.; Alam, R.; Karwacki, C. J.; Kamat, P. V. CdSe/ CdS Nanorod Photocatalysts: Tuning the Interfacial Charge Transfer Process through Shell Length. Chem. Mater. 2015, 27, 5064−5071.

dissipate the heat produced. The reactor contained a port for mounting 12 small tubes (hexagonally arranged) and 1 large tube at the same time. The small tubes were employed to conduct the preliminary experiments with a sample volume of 10 mL made up with the appropriate pH with the required concentration of catalyst and 2,4-D. The experiments were conducted for 10 h of visible light irradiation. The kinetics of photodegradation were conducted with respect to time in a 200 mL volume of the sample in the large tube of the photoreactor. The progress of the reaction was monitored any time by the absorbance measurement of the reaction mixture after filtration. Hence, the residual concentration of the targeted compound before and after photocatalytic degradation would be easily computed from the calibration chart, and hence, the percentage degradation of the dye molecule could be calculated using eq 1. degradation efficiency (%) = C0 − Ce/C0 × 100

(1)

where C0 and Ce are the initial and final concentrations of 2,4-D in the aqueous phase, respectively.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01607. FT-IR spectrum and band gap plots of ZnO, Dy2O3, and DSZ; EDAX pattern of DS and DSZ; surface roughness profile, N2 adsorption−desorption isotherm, O 1s and C 1s XPS spectrum of DSZ; preliminary studies on the photocatalytic degradation of 2,4-D under visible light irradiation; and FT-IR and FE-SEM results of reused DSZ photocatalyst (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-04424912150. Fax: +91-044-24911589. ORCID

Sivasamy Arumugam: 0000-0003-2926-8791 Notes

The authors declare no competing financial interest. E-mail: [email protected].

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ACKNOWLEDGMENTS Authors would like thank CSIR, New Delhi, for providing a Research Associateship for (G.A.S.J.) and Director, CSIRCLRI, provided necessary facilities to carry out this work.

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REFERENCES

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