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2D heterojunction for enhanced visible-light-driven photocatalysis
Materials Science in Semiconductor Processing 107 (2020) 104834
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SnO2 quantum dots decorated NiFe2O4 nanoplates: 0D/2D heterojunction for enhanced visible-light-driven photocatalysis Bathula Babu 1, *, Ravindranadh Koutavarapu 1, Jaesool Shim ***, Kisoo Yoo ** School of Mechanical Engineering, Yeungnam University, Gyeongsan, 712-749, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: NiFe2O4 SnO2 Quantum dots Heterostructured nanocomposites Visible-light photocatalysis
Nowadays, the fabrication of visible-light-induced photocatalysts with efficient light absorption and higher charge separation with improved active sites has appeared as an effective strategy for the degradation of haz ardous pollutants. In this study, novel quantum dots (QDs)-based semiconductor photocatalyst nanocomposites (NiFe2O4/SnO2) with various amounts of SnO2 QDs (SQDs) (5, 10, and 15 mg) were successfully constructed through a simple hydrothermal synthesis. The as-prepared nanocomposites were studied using several micro scopic and spectroscopic techniques, and the results further confirmed the establishment of heterojunctions between NiFe2O4 and SQDs. The results of photocatalytic investigations showed that the synthesized hetero structured NiFe2O4/SQD photocatalysts displayed substantially improved catalytic efficiency for the degradation of rhodamine B (RhB) upon visible-light treatment. In particular, the NiFe2O4/SQD nanocomposite with 10 mg of SQDs achieved an RhB degradation of 98% upon visible-light treatment within 105 min. The improved catalytic activity of the heterostructured nanocomposite can be credited to the synergistic interactions between NiFe2O4 and SQDs. Furthermore, the p-n heterojunction between NiFe2O4 and SQDs enable the direct transfer of photoinduced electrons from NiFe2O4 to the SQDs, which could retard the recombination of electron–hole pairs and enhance the catalytic activity. A probable catalytic reaction mechanism for the improved degradation ef ficiency of RhB by NiFe2O4/SQD nanocomposites is also proposed.
1. Introduction In recent times, different kinds of organic pollutants have been released in wastewater due to industrialization and the rapid growth of the population. This has become a serious problem for environmental and health reasons, and such pollution also disrupts aquatic and animal life [1,2]. As a result, the scientific community has been focused on the establishment of novel water purification technologies for the remedi ation of pollutants. Fortunately, the development of visible-light-driven semiconductor photocatalysis has been attracting intense attention for its potential applications, and these materials offer an environmentally friendly technology for removing numerous hazardous pollutants from wastewater [3,4]. Nowadays, a large number of visible-light-activated semiconductor photocatalysts such as Ag3PO4, CdS, and Bi2WO6 have been successfully synthesized with enhanced photocatalytic activity [5–7].
Currently, magnetic semiconductors have been receiving a great deal of scientific attention as photocatalysts owing to their stability and recyclability. In particular, NiFe2O4 is a significant p-type semiconductor for the degradation of contaminants upon exposure to visible light owing to its tunable bandgap (~1.7 eV), cost effectiveness, chemical durability, high magnetic separation, and exceptional ability for solar light utilization [8]. However, pristine NiFe2O4 photocatalysts typically exhibit a weak catalytic performance upon visible light exposure owing to the rapid recombination of photoinduced charge carriers [9]. To overcome these deficits and heighten the photocatalytic efficacy of p-type NiFe2O4, various techniques have been employed, such as doping with noble metals, or forming heterojunctions with semiconductor metal oxides [10–12]. Among these strategies, the construction of semiconductor-based NiFe2O4 heterostructures is a promising way to enhance the photocatalytic activity against harmful organic pollutants. Further, the heterostructured nanocomposites could significantly
* Corresponding author.; ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected], [email protected] (B. Babu), [email protected] (J. Shim), [email protected] (K. Yoo). 1 These authors contributed equally to this work https://doi.org/10.1016/j.mssp.2019.104834 Received 19 August 2019; Received in revised form 1 October 2019; Accepted 7 November 2019 Available online 15 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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improve the charge transfer mechanism, diminish the fast reunion of charge carriers, and enhance the influence of active radical species [13]. Metal-oxide semiconductors such as tin oxide (SnO2), zinc oxide (ZnO), zirconium oxide (ZrO2), and titanium oxide (TiO2) are promising catalysts for the degradation of hazardous contaminants [14–18]. Among these, n-type semiconductor, SnO2 has attracted enormous in terest owing to its wide band gap (~3.6 eV), cost effectiveness, non-toxic nature, high stability, and environmental sustainability. In spite of its great potential, the photocatalytic activity of SnO2 is confined to the ultraviolet (UV) region due to its wide bandgap. Nevertheless, the photoinduced charge carriers in SnO2 undergo quick recombination leading to a further reduction of its photocatalytic activity [19,20]. To date, zero-dimensional nanomaterials such as quantum dots (QDs), have attracted significant attention compared to bulk materials due to their exceptional photocatalytic activity resulting from their tiny particle size and large surface area [21,22]. Moreover, the surface properties of semiconductor QDs hinder aggregation and improve the photocatalytic activity [23]. Accordingly, enormous efforts have been made to extend the light absorption efficiency of n-type SnO2 QDs (SQDs) to the visible region and to prevent the recombination of photogenerated charge carriers through various approaches. The fabrication of p-n hetero junction NiFe2O4/SnO2 QD nanocomposites might be beneficial for the detachment and transport of photoinduced charge carriers, which can boost the photocatalytic performance under visible-light irradiation [24–26]. Further, there have only been limited reports on SnO2 QD-based heterostructure nanocomposites with outstanding catalytic perfor mance under UV and visible light. Lee et al. successfully synthesized novel UV-light-activated SQD-ZnS nanocomposites using a two-step hydrazine-assisted hydrothermal technique. Their results demon strated that the strong interfacial interactions between SQD and ZnS enable the direct transfer of charge carriers, which effectively hinders the recombination of charge carriers and improves the degradation rate of rhodamine B (RhB) [27]. Babu et al. constructed an SQD/g-C3N4 heterojunction photocatalyst via a simple in-situ chemical reduction procedure for the improvement of catalytic performance upon visible light illumination. Further experiments demonstrated that the hetero junctions formed between SQDs over g-C3N4 nanolayers could improve the photocatalytic efficiency and they exhibited exceptional catalytic performance for the degradation of dye pollutants [28]. Ma et al. suc cessfully examined the catalytic activity of heterostructured SQD/SnS2 nanocomposites under visible light. Good dispersion of SQDs over SnS2 nanosheets could improve the synergistic interactions and provided a remarkably improved catalytic performance for the reduction of Cr(VI) [29]. These reports motivated us to fabricate heterostructures with various amounts (5, 10, and 15 mg) of SnO2 QDs dispersed on NiFe2O4 nanoplates using a facile hydrothermal technique, which may produce heterostructured nanocomposites with improved photocatalytic properties. Herein, we successfully demonstrate a facile and economical hy drothermal technique for the production of NiFe2O4/SnO2 QD p-n het erojunction nanocomposites with various amounts of SQDs (5, 10, and 15 mg) for the improvement of the photocatalytic degradation of RhB through visible light irradiation. The phase structures, morphologies, chemical compositions, and photocatalytic performance of the synthe sized catalysts are described specifically. Furthermore, based on the results of radical trapping experiments and photoluminescence (PL) examinations, a plausible mechanism for the improved catalytic activity for the degradation of RhB upon visible light treatment is presented. Finally, these findings suggest that the fabrication of heterostructured nanocomposites consisting of semiconductor QDs decorated onto a magnetic semiconductor is an effective strategy for the degradation of hazardous contaminants under visible-light illumination that has abundant potential applications in energy-related fields.
2. Experimental 2.1. Chemicals Nickel chloride hexahydrate (NiCl2⋅6H2O), iron chloride hexahy drate (FeCl3⋅6H2O), sodium hydroxide (NaOH), tin chloride (SnCl4⋅5H2O), hydrazine monohydrate (N2H4. H2O), ethanol, benzo quinone (BQ), isopropyl alcohol (IPA), triethanolamine (TEOA), and rhodamine B (RhB) were analytical grade and used as received without additional refinement. All the chemicals were purchased from Sigma –Aldrich Co., Ltd., Korea. 2.2. Synthesis of NiFe2O4 nanoplates First, 2 g of iron chloride hexahydrate (0.1 mol/L) was liquefied into 100 mL of deionized (DI) water followed by stirring. Second, 0.42 g of nickel chloride hexahydrate (0.05 mol/L) was distributed into the above aqueous solution of iron chloride under magnetic stirring. Using a NaOH solution, the pH was adjusted to 13.0, and a red-brown product pro gressively precipitated from the solution. Thereafter, the formed pre cipitate was collected, washed numerous times with ethanol and DI water, and then annealed at 500 � C for 3 h in air. Finally, a reddishbrown NiFe2O4 powder was obtained. 2.3. Synthesis of SnO2 QDs One gram of tin chloride was dissolved in 125 mL of DI water. The solution was stirred for 30 min to obtain a homogeneous solution. A 2.5mL hydrazine solution was added dropwise into the above mixed solu tion. The slurry thus formed was stirred for 30 min and kept at 100 � C for 18 h. Finally, SnO2 QDs were obtained via centrifugation and were rinsed several times with DI water and ethanol before drying overnight in an oven at 60 � C. 2.4. Synthesis of NiFe2O4/SQD nanocomposites Novel NiFe2O4/SQD nanocomposites with different loading amounts of SQDs (5, 10, and 15 mg) were successfully prepared by a simple hy drothermal technique, which can be described as follows. Typically, 100 mg of NiFe2O4 nanoplates were dissolved in 40 mL of ethanol and stirred for 30 min. Secondly, various amounts of SQDs (5, 10, and 15 mg) were distributed in 40 mL of ethanol under stirring for 30 min in another beaker and then added dropwise to the above solution. After stirring for 30 min, the suspension was transferred into a 100-mL Teflonlined stainless-steel autoclave and kept at 180 � C for 24 h. After cooling naturally to room temperature, the precipitates with various amounts of SQDs (hereinafter designated as NFS-5, NFS-10, and NFS-15, respec tively) were obtained via centrifugation, and were washed several times with DI water and ethanol before drying overnight in an oven at 80 � C. 2.5. Photocatalytic activity The catalytic performance of the prepared catalysts was estimated by examining the degradation of RhB in an aqueous solution. In these ex periments, 20 mg of the photocatalysts was distributed into 50 mL of RhB dye solution with a dye concentration of 10 ppm and stirred for 30 min in the dark to attain adsorption-desorption equilibrium. After wards, the solution was exposed to visible light from a 100-W solar simulator with a UV cut-off filter (λ ˃ 400 nm) to investigate the degra dation effect. During catalytic experiments, 3-mL samples were taken from the reacted RhB solution and centrifuged, then UV–vis spectro scopic analysis was used to measure the concentration of RhB at a wavelength of 552 nm. 2
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3. Results and discussion
The phase structure and physicochemical properties of the synthe sized NiFe2O4/SQD nanocomposites were examined by powder X-ray diffraction (XRD), and the obtained patterns are presented in Fig. 1. The diffraction pattern of pure NiFe2O4 exhibited characteristic peaks at 30.29� , 35.66� , 37.35� , 43.31� , 53.80� , 57.36� , and 62.98� which can be attributed to the (220), (311), (222), (400), (442), (511), and (440) planes of the spinel structure of NiFe2O4 with no impurity phases (JCPDS# 00-054-0964) [30]. From the XRD pattern of pure SQDs (Fig. 1 (b)), the major diffraction peaks at 26.15� , 33.69� , 52.12� , and 64.72� correspond to the (110), (101), (211), and (112) crystalline planes of the tetragonal phase of SnO2, respectively (JCPDS #00-041-1445) [31]. Furthermore, the XRD patterns of the NiFe2O4/SQD nanocomposites (Fig. 1(c)) clearly exhibit two sets of characteristic peaks that can be accredited to the single-phase spinel structure of NiFe2O4 (#), and tetragonal phase of SnO2 (*), suggesting that both NiFe2O4 and SQDs are effectively coupled in the synthesis processes. Nevertheless, the XRD peaks of the SQDs were not as prominent as the NiFe2O4 XRD peaks. This can be due to the huge variation in the peak intensities of NiFe2O4 and SQD, and also the fact that very small amounts of SQDs were used for the construction of the NiFe2O4/SQD nanocomposites. The magnified XRD pattern of the prepared nanocomposites is presented in Fig. S1, which provides a clearer information of the XRD peak positions of the SQDs. Moreover, with the increasing loading of the SQDs, the diffraction peak intensities of NiFe2O4 gradually decrease, indicating the successful interaction between NiFe2O4 and the SQDs. Thus, the XRD results demonstrate the high crystallinity of the as-prepared nanocomposites with a binary system, and no other impurity phases were detected.
495.53 eV, compatible with the Sn3d5/2 and Sn3d3/2 states, respectively. Additionally, the spin-orbital splitting between Sn 3d5/2 and Sn 3d3/2 is around 8.48 eV, which can be attributed to the binding energy of Sn4þ ions in the SQDs [32]. Further, the two deconvoluted XPS peaks at the binding energies of 489.34 and 491.02 eV should be attributed to the existence of Sn in Sn4þ and Sn2þ oxidation states, respectively [33]. The high-resolution XPS spectrum of the O 1s state for NFS-10 is presented in Fig. S2, and it can be deconvoluted into three peaks with binding en ergies of 529.38, 530.81, and 533.69 eV. The peak at 529.38 eV is attributed to the metal-oxygen bond, the peak at 530.81 eV is associated to Ni-O-H, while the peak at 533.69 eV is attributed to absorbed water [34–36]. The high-resolution Fe 2p XPS spectrum (Fig. 2(c)) can be deconvoluted into eight peaks centered at 710.33, 713.18, 715.82, 719.68, 723.81, 726.89, 729.54, and 732.13 eV. The peaks at 713.18 and 726.89 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively [10]. Four peaks can be assigned to Fe2þ (710.33 and 723.81 eV) and Fe3þ (715.82 and 729.54 eV) ions in the hematite phase [37], whereas the peaks at 719.68 and 732.13 eV could be attributed to the satellite peaks [38]. The observed binding energies for Fe 2p are well-matched with the reported data [39–41]. The high-resolution Ni 2p XPS spectrum shown in Fig. 2(d) exhibits two binding energy peaks situated at 856.70 and 874.55 eV, which are consistent with Ni 2p3/2 and Ni 2p1/2, respectively, with a binding energy difference of 17.85 eV, which confirms that Ni is in the 2þ oxidation state [42]. In addition, the Ni 2p3/2 peak can be deconvoluted into two energy bands positioned at 854.17 and 858.89 eV, respectively, which could be attributed to the existence of the Ni2þ and Ni3þ oxidation states [43]. Further, the Ni 2p scan spectrum also contains two satellite peaks at 863.36 and 880.46 eV with signifi cant intensities [44]. The XPS analysis evidently confirms the formation of heterojunctions between NiFe2O4 and SQDs, and is in good agreement with the XRD results.
3.2. XPS analysis
3.3. Morphology studies
X-ray photoelectron spectroscopy (XPS) analysis was applied to further investigate the chemical composition and the interactions be tween the components of the synthesized NiFe2O4/SQD nano composites. The XPS analysis was carried out with the optimized sample of the prepared nanocomposites, i.e., 10 mg of SnO2 QDs in 100 mg of NiFe2O4 nanoplates and it is represented as NFS-10 nanocomposite. The XPS survey scan of the NFS-10 nanocomposite shows the existence of Sn, O, Fe, and Ni elements, and is presented in Fig. 2(a). The high-resolution XPS spectrum of the Sn 3d state is presented in Fig. 2(b). It shows two asymmetric peaks at the higher binding energies of 487.05 and
The surface morphology and the interfacial interactions of the asprepared nanocomposites were examined by using field-emission scan ning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) studies. The FESEM image of pure NiFe2O4 is presented in Fig. 3(a) and it indicates that the NiFe2O4 exhibit an irregular shaped plate-like nanostructures with non–uniform grain sizes. In addition, the surface of the NiFe2O4 nanoplates are clean and smooth. In Fig. 3(c), the FESEM image of NFS-10 nanocomposite show the morphology of the NiFe2O4 nanoplates after SQDs were deposited. Tiny SQDs decorate the NiFe2O4 nanoplates and the
3.1. Phase structures
Fig. 1. XRD patterns of (a) pure NiFe2O4, (b) pure SnO2 QDs, and (c) NFS nanocomposites. 3
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Fig. 2. High-resolution XPS spectra of the NFS-10 nanocomposite.
Fig. 3. FESEM and EDS analysis of (a, b) NiFe2O4 nanoplates, and (c, d) NFS-10 nanocomposite.
morphology of the NiFe2O4 nanoplates was unaffected by loading them with SQDs. The purity and elemental composition of the NiFe2O4 nanoplates and NFS-10 nanocomposite were investigated using energydispersive X-ray spectroscopy (EDS), and the results are presented in Fig. 3 (b & d). It was confirmed that the NFS-10 nanocomposite con sisted of Ni, Fe, Sn, and O. Additionally, EDS analysis provided the weight% and atomic% values of the elements in the NiFe2O4, and NFS10 nanocomposite. Further, Fig. 4 presents TEM images of pristine NiFe2O4, pristine SQDs, and the NFS-5, NFS-10, and NFS-15 nano composites. As observed in Fig. 4(a), the TEM image of pristine NiFe2O4 displays nanoplate-like morphology, whereas the pure SQDs (Fig. 4(b)) show a spherical structured morphology. Fig. 4(c–e) provides the TEM images of NFS-5, NFS-10, and NFS-15 nanocomposites, clearly showing that the SQDs were successfully dispersed on the surface of the NiFe2O4
nanoplates. Furthermore, it was evident that the deposition of the SQDs over the surface of NiFe2O4 nanoplates progressively increased with the increasing loading of the SQDs. The HRTEM technique was used to further confirm the fabrication of heterostructured NiFe2O4/SQD nanocomposites and the images are shown in Fig. 5. The HRTEM image (Fig. 5(a)) clearly demonstrates the successful deposition of SQDs over the surface of NiFe2O4 nanoplates. The lattice fringe pattern of the heterostructured NFS-10 nanocomposite is presented in Fig. 5(b). From Fig. 5(b), the measured lattice fringe patterns with spacings of 0.246 nm and 0.339 nm could be accredited to the (311) plane of NiFe2O4 and the (110) plane of SnO2, respectively [45,46]. Nevertheless, the selected area electron diffraction (SAED) pattern presented in the inset of Fig. 5(b) shows a number of discrete concentric rings with superimposed bright spots, confirming the 4
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Fig. 4. TEM images of (a) pure NiFe2O4, (b) pure SQDs, and the (c) NFS-5, (d) NFS-10, and (e) NFS-15 nanocomposites.
Fig. 5. (a) HRTEM image and (b) lattice fringe pattern of the NiFe2O4/SQD nanocomposite (NFS-10) in the HRTEM image.
development of NiFe2O4/SQD nanocomposites. These outcomes evidently signify the formation of heterojunctions between NiFe2O4 and SQDs. In addition, Fig. 6 illustrates the elemental color mapping of the NFS-10 nanocomposite, which further confirms the dispersion of the SQDs over NiFe2O4. The obtained results strongly agree with the FESEM, TEM and XRD analysis presented above. The NiFe2O4/SQD hetero junctions thus formed could enhance the photocatalytic activity.
energies of pure SQDs, NiFe2O4, and NiFe2O4/SQD nanocomposites can be estimated using the equation Eg ¼ 1240/λ, where Eg is the bandgap energy and λ is the absorption wavelength. The measured bandgap en ergies were 3.463, 1.675, and 1.754 eV for pure SQDs, pure NiFe2O4, and NFS-10 nanocomposite, respectively and is presented in Fig. 7(b) using the Tauc plots. This demonstrates that the development of heter ojunctions can significantly hinder the reunion of charge carriers and subsequently shift the light response ability towards the visible region. Additionally, the valence band (VB) and conduction band (CB) po tentials of the SQDs and NiFe2O4 can be measured using the empirical formulae [49]:
3.4. Optical absorption study The optical absorption study is an efficient approach to examine the visible light absorption ability of the as-prepared nanocomposites, and the recorded optical diffuse reflectance spectra (DRS) of pristine SQDs, pristine NiFe2O4, and the NFS-5, NFS-10, and NFS-15 nanocomposites are illustrated in Fig. 7(a). The pure SQDs exhibited only UV absorption with an absorption band edge of about 358 nm, which is due to the free excitonic absorption of SQDs [47]. The pristine NiFe2O4 has a strong fundamental absorption edge in the visible region located at around 740 nm [48]. In the case of the NiFe2O4/SQD nanocomposites, the DRS spectra contain both UV and visible absorption bands, which signifies the development of heterojunctions between NiFe2O4 and SQDs. Furthermore, the light harvesting capability of the NiFe2O4/SQD nanocomposites was shifted into the visible region compared to pure SQDs with absorption edges ranging around 689–707 nm. The bandgap
EVB ¼ χ – Ee þ (0.5) Eg ECB ¼ EVB – Eg where EVB and ECB are the VB and CB potentials, respectively. χ is the electronegativity of the semiconductor; for SQD and NiFe2O4 the χ values are 6.217 and 4.655 eV, respectively. Eg and Ee refer to the bandgap energy and energy of free electrons on the hydrogen scale (~4.5 eV), respectively. Based on the above data, the VB and CB posi tions of SQDs were measured to be þ3.449 and 0.014 eV, while those for NiFe2O4 were found to be þ0.993 and 0.682 eV, respectively. PL spectroscopy has been extensively used to examine the transfer mechanism and separation effectiveness of photoinduced electron–hole pairs, which is the key factor for the improvement of catalytic activity. 5
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Fig. 6. EDS elemental color mapping of the NFS-10 nanocomposite. (a) Mapping region, (b) Ni Ka1, (c) Fe Ka1, (d) O Ka1, and (e) Sn La1. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article).
Fig. 7. (a) UV–vis DRS, and (b) Tauc plots of pure SnO2 QDs, NiFe2O4, NFS-5, NFS-10, and NFS-15 nanocomposites.
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Fig. 8. PL spectra of pure SQDs, pure NiFe2O4, NFS-5, NFS-10, and NFS-15 nanocomposites.
The PL spectra of SQDs, NiFe2O4, and NiFe2O4/SQD nanocomposites are presented in Fig. 8. For pure SQDs, a small peak at around 452 nm can be ascribed to the radiative reunion of photoinduced charge carriers occupying oxygen vacancies on the SQD surface [50]. The strongest emission peak around 592 nm originates from crystalline defects which materialize during the growth process [51]. Further, the PL spectra of NiFe2O4 exhibited two emission peaks positioned at 458 nm and 584 nm that can be ascribed to the defect levels generated by oxygen vacancies in NiFe2O4 [52,53]. Moreover, the NiFe2O4/SQD nanocomposites
showed significantly reduced emission intensities compared with the pure samples. The emission intensities were found to decrease in the order of NFS-10 ˂ NFS-15 ˂ NFS-5 ˂ NiFe2O4 ˂ SQDs, demonstrating that the recombination of charge carriers was progressively weakened in the NFS-10 nanocomposite. These outcomes suggest that the synergistic interactions between NiFe2O4 and SQDs significantly lead to the reduction of the recombination of photoinduced charge carriers, which in turn lead to a substantial catalytic enrichment in the visible region.
Fig. 9. (a) Absorption spectra of the NFS-10 nanocomposite for the photocatalytic degradation of RhB, (b) photodegradation analysis, (c) kinetics of RhB photo catalytic degradation: [ln(C0/C)] vs. the irradiation time for each catalyst, and (d) plot of the k values of all the samples. 7
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3.5. Photocatalytic activity of NiFe2O4/SQD nanocomposites for RhB degradation
be described using a pseudo-first-order kinetics model, ln (C/C0) ¼ kt, where k is the kinetic rate constant of the degradation process. The ki netics of RhB degradation with all prepared samples are represented in Fig. 9(c). The calculated k values are 0.0019, 0.0052, 0.0102, 0.0242, and 0.0145 min 1 for SQDs, NiFe2O4, NFS-5, NFS-10, and NFS-15, respectively (Fig. 9(d)). Remarkably, the NFS-10 nanocomposite exhibited a considerably superior k value compared to the remaining samples and generated a rate constant for RhB degradation ~4.66 times higher than that of pure NiFe2O4. The effect of catalyst loading (15, 20, and 25 mg) on the degradation of RhB over the NFS-10 photocatalyst is presented in Fig. 10(a). The NFS-10 nanocomposite showed the highest catalytic activity at 20 mg loading. The degradation efficiency progressively decreased with increasing catalyst loading. This can be due to the agglomeration of particles and the depletion of active sites [57]. Hence, overloading of catalyst may negatively affect the rate of RhB degradation. In addition, the stability of the prepared catalysts is an essential parameter for their practical application. To confirm stability, the reusability of the NFS-10 nanocomposite was investigated by three cycling tests measuring the degradation of RhB under the same conditions, and the consistent results are demonstrated in Fig. 10(b). It was clearly observed that the NFS-10 photocatalyst exhibited only a minor reduction in photocatalytic activ ity rather than a considerable loss after three cycles, and more than 90% of the photocatalytic efficacy was retained, which reveals the stability and reusability of the synthesized nanocomposites. To further explore the catalytic reaction mechanism of NiFe2O4/SQD heterostructures, scavenger experiments were executed with various radicals over the NFS-10 nanocomposite to confirm the contributions of the active species in the degradation process. It has been widely recognized that scavengers such as superoxide radicals (�O2 ), holes (hþ), and hydroxyl radicals (�OH) play a key role in photocatalytic degradation and activate the oxidation of organic pollutants. In this study, controlled experiments were conducted by using scavengers such as BQ, TEOA, and IPA, to capture �O2 , hþ, and �OH radicals, respec tively. Fig. 10(c) illustrates the effect of various scavengers on the degradation of RhB by NFS-10 photocatalyst under visible light. The catalytic activity of the NFS-10 photocatalyst was remarkably inhibited upon the addition of IPA and TEOA as �OH and hþ scavengers, resulting
The improved catalytic performance of the as-prepared hetero structured NiFe2O4/SQD nanocomposites was determined by measuring the degradation of the organic dye RhB upon visible-light treatment. The time-dependent optical absorption spectra for the degradation of RhB upon visible illumination in the presence of the NFS-10 photocatalyst is presented in Fig. 9(a). The RhB dye exhibited a single absorption peak at 552 nm in the visible region, which decreased progressively and finally disappeared after 105 min of visible-light illumination. For a more quantitative understanding of the photocatalytic activity, we have examined the change in RhB concentration (C) after photodegradation, relative to the initial concentration (C0) of RhB dye [54]. Fig. 9(b) presents the change in the RhB concentration (C/C0) as a function of time using pristine SQDs, NiFe2O4, NFS-5, NFS-10, and NFS-15 nano composites under visible-light treatment. In the absence of photocatalyst (dark), no considerable degradation was detected. It was observed that the RhB concentration decreased by 16% and 41% in presence of pris tine SQDs and NiFe2O4, respectively. However, the heterostructured NiFe2O4/SQD nanocomposites possess photocatalytic activity signifi cantly greater than that of pristine SQDs or NiFe2O4. The observed visible-light-induced degradations of RhB for the NFS-5, NFS-10, and NFS-15 nanocomposites were 70%, 98%, and 82%, respectively, for a duration of 105 min. This could be ascribed to the close interfacial contact between p-type NiFe2O4 and the n-type SQDs, which enhanced the absorption of visible light. Thus, n-type SQDs act as an electron tank to receive the photogenerated electrons from p-type NiFe2O4, due to the development of strong p-n heterojunction between NiFe2O4 and SQDs. In this way, the recombination of charge carriers is inhibited and the photocatalytic performance is enhanced [3]. It was observed that among the synthesized nanocomposites, the NFS-10 photocatalyst exhibited the maximum efficiency for the degradation of RhB, which is in accordance with the PL results. The excess deposition of SQDs over NiFe2O4 can decrease the effective contact area, hindering photon absorption and the production of active sites, which reduces the photocatalytic activity [55, 56]. Further, the catalytic activity of the constructed photocatalysts can
Fig. 10. (a) Effect of catalyst loading, (b) recycling test, and (c, d) radical trapping experiment for the degradation of RhB over the NFS-10 nanocomposite in the absence and presence of scavengers. 8
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Fig. 11. Proposed mechanism for charge transfer and photocatalytic degradation of RhB in the NiFe2O4/SQD nanocomposite.
in a decline from 98% to 56% and 64%, respectively (Fig. 10(d)). In contrast, the degradation efficiency was only reduced slightly to 91% with the introduction of BQ as an �O2 scavenger. These observations suggest that photocatalysis can be influenced by the �OH and hþ radicals in the degradation of RhB on the surface of the NFS-10 nanocomposite. Based on the above experimental outcomes, an acceptable mecha nism for the improved catalytic activity of the NiFe2O4/SQD photo catalyst upon visible-light illumination is proposed, and schematically represented in Fig. 11. NiFe2O4 is a p-type semiconductor whose Fermi energy level (Ef p) is located close to the VB, while SQD is a typical ntype semiconductor whose Fermi energy level (Ef n) lies close to CB. When p-type NiFe2O4 is in contact with n-type SQD, the p-type NiFe2O4 and n-type SQD could form the p–n heterojunction, and the electrons diffuse from n-SQD into p-NiFe2O4. As a result, negative charges accu mulated in the p-NiFe2O4 region near the junction. Mean-while, holes diffuse from the p-NiFe2O4 region to the n-SQD region near the junction, creating a positive section in the n-SQD region in the vicinity of the junction. When the Fermi levels of NiFe2O4 and SQD reach equilibration, the internal electric field directed from n-SQD to p-NiFe2O4 is simulta neously established to stop the charge diffusion from n-SQD to pNiFe2O4. Meanwhile, the energy bands of NiFe2O4 shift upward along the Fermi level (Ef p) and SQD shift downward along the Fermi level (Ef n) near the interface (Fig. 11), which are similar to the reported heterojunction photocatalysts [44,58,59]. Under visible-light irradia tion, NiFe2O4 can be easily excited and electron–hole pairs are produced on its surface. In the SQDs, excitation by visible light is impossible owing to its wide bandgap. Thus, the electrons produced in the CB of p-type NiFe2O4 would be rapidly transported to the CB of the n-type SQDs owing to the strong heterojunction formation, as the CB potential of p-type NiFe2O4 ( 0.682 eV) is more negative than that of n-type SQDs ( 0.014 eV). Conversely, the photoinduced holes in the VB of p-type NiFe2O4 cannot migrate to the VB of the n-type SQDs, because the VB potential of p-type NiFe2O4 (þ0.993 eV) is less positive than that of n-type SQDs (þ3.449 eV). Thus, the excited electrons and holes can be effectively separated, removing the possibility of charge carrier recom bination. The CB potential of the n-type SQDs is less negative than Eθ(O2/�O2 ) ¼ 0.046 eV, thus �OH radicals can be produced from the electrons in the CB through a two-electron oxidation pathway [60]. In contrast, the VB potential of p-type NiFe2O4 is less positive than Eθ(�OH/H2O) ¼ þ2.27 eV, demonstrating that the photoinduced holes of p-type NiFe2O4 cannot oxidize H2O to form �OH radicals, and they directly degrade the organic pollutant. Based on our results, the photoinduced active radicals �OH and hþ are beneficial for the degra dation of RhB and the mechanism can be summarized as follows:
H2O2 þ �O2 → �OH þ OH þ O2
NiFe2O4/SQD þ hυ → NiFe2O4(e–CB þ hþVB)/SQD
Acknowledgements
–
NiFe2O4(e SQD(e
–
CB)
CB
þ
hþVB)/SQD
→
�OH þ RhB dye → Degradation products (major contribution) �O2 þ RhB dye → Degradation products (minor contribution) NiFe2O4(hþVB) þ RhB dye → Degradation products (major contribution) Thus, these outcomes suggest that the strong interfacial interactions in the heterostructured nanocomposite would improve the electrontransfer mechanism and hinder the recombination of photogenerated charge carriers, which remarkably enhances the catalytic activity of the NiFe2O4/SQD nanocomposite upon visible light illumination. 4. Conclusions In summary, the SQDs were deposited in different amounts over NiFe2O4 to construct novel p-n heterojunction NiFe2O4/SQD nano composites through a simple hydrothermal technique for the degrada tion of organic pollutants from wastewater upon visible light irradiation. The structure, composition, and morphology of the prepared NiFe2O4/ SQD nanocomposites were confirmed by XRD, TEM, HRTEM, and XPS studies. The structural investigations confirmed the spinel structure of NiFe2O4 and tetragonal phase of SnO2. A good dispersion of the SQDs over the NiFe2O4 nanoplates was clearly demonstrated from TEM and HRTEM studies. Furthermore, optical and PL studies indicated improved visible-light absorption with a functional interface between NiFe2O4 and SQDs, where the photoinduced electron–hole pairs are efficiently separated, which considerably enhanced the catalytic activity. More over, the SQDs dispersed on the surface of NiFe2O4 nanoplates played a significant role as an electron-conduction bridge, allowing electrons to migrate towards the SQDs and increasing the effectiveness of the sepa ration of photoinduced electron–hole pairs in NiFe2O4. In addition, the formation of p-n heterojunction between p-type NiFe2O4 and n-type SQDs can significantly improves the photocatalytic performance. The resul tant NFS-10 photocatalyst exhibited an exceptional visible-light-induced catalytic activity for the degradation of RhB (98%) within 105 min. In addition, radical trapping experiments confirmed that the �OH and hþ species play a leading role in the degradation of RhB over the NFS-10 nanocomposites. These findings suggest that the fabricated NiFe2O4/ SQD nanocomposites can provide potential avenues for visible-lightdriven photocatalysis for energy-related applications. Declaration of competing interest The authors declare no conflict of interest.
NiFe2O4(hþVB)/SQD(e–CB)
This work was supported by a National Research Foundation of Korea grant funded by the Korean government (2018R1D1A1B07050330, 2017R1C1B2001990, and 2017R1A4A1015581).
þ O2 → �O2
2SQD(e–CB) þ O2 þ 2Hþ → H2O2
9
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Appendix A. Supplementary data
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
SnO2 quantum dots decorated NiFe2O4 nanoplates: 0D/2D heterojunction for enhanced visible-light-driven photocatalysis Bathula Babu 1, *, Ravindranadh Koutavarapu 1, Jaesool Shim ***, Kisoo Yoo ** School of Mechanical Engineering, Yeungnam University, Gyeongsan, 712-749, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: NiFe2O4 SnO2 Quantum dots Heterostructured nanocomposites Visible-light photocatalysis
Nowadays, the fabrication of visible-light-induced photocatalysts with efficient light absorption and higher charge separation with improved active sites has appeared as an effective strategy for the degradation of haz ardous pollutants. In this study, novel quantum dots (QDs)-based semiconductor photocatalyst nanocomposites (NiFe2O4/SnO2) with various amounts of SnO2 QDs (SQDs) (5, 10, and 15 mg) were successfully constructed through a simple hydrothermal synthesis. The as-prepared nanocomposites were studied using several micro scopic and spectroscopic techniques, and the results further confirmed the establishment of heterojunctions between NiFe2O4 and SQDs. The results of photocatalytic investigations showed that the synthesized hetero structured NiFe2O4/SQD photocatalysts displayed substantially improved catalytic efficiency for the degradation of rhodamine B (RhB) upon visible-light treatment. In particular, the NiFe2O4/SQD nanocomposite with 10 mg of SQDs achieved an RhB degradation of 98% upon visible-light treatment within 105 min. The improved catalytic activity of the heterostructured nanocomposite can be credited to the synergistic interactions between NiFe2O4 and SQDs. Furthermore, the p-n heterojunction between NiFe2O4 and SQDs enable the direct transfer of photoinduced electrons from NiFe2O4 to the SQDs, which could retard the recombination of electron–hole pairs and enhance the catalytic activity. A probable catalytic reaction mechanism for the improved degradation ef ficiency of RhB by NiFe2O4/SQD nanocomposites is also proposed.
1. Introduction In recent times, different kinds of organic pollutants have been released in wastewater due to industrialization and the rapid growth of the population. This has become a serious problem for environmental and health reasons, and such pollution also disrupts aquatic and animal life [1,2]. As a result, the scientific community has been focused on the establishment of novel water purification technologies for the remedi ation of pollutants. Fortunately, the development of visible-light-driven semiconductor photocatalysis has been attracting intense attention for its potential applications, and these materials offer an environmentally friendly technology for removing numerous hazardous pollutants from wastewater [3,4]. Nowadays, a large number of visible-light-activated semiconductor photocatalysts such as Ag3PO4, CdS, and Bi2WO6 have been successfully synthesized with enhanced photocatalytic activity [5–7].
Currently, magnetic semiconductors have been receiving a great deal of scientific attention as photocatalysts owing to their stability and recyclability. In particular, NiFe2O4 is a significant p-type semiconductor for the degradation of contaminants upon exposure to visible light owing to its tunable bandgap (~1.7 eV), cost effectiveness, chemical durability, high magnetic separation, and exceptional ability for solar light utilization [8]. However, pristine NiFe2O4 photocatalysts typically exhibit a weak catalytic performance upon visible light exposure owing to the rapid recombination of photoinduced charge carriers [9]. To overcome these deficits and heighten the photocatalytic efficacy of p-type NiFe2O4, various techniques have been employed, such as doping with noble metals, or forming heterojunctions with semiconductor metal oxides [10–12]. Among these strategies, the construction of semiconductor-based NiFe2O4 heterostructures is a promising way to enhance the photocatalytic activity against harmful organic pollutants. Further, the heterostructured nanocomposites could significantly
* Corresponding author.; ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected], [email protected] (B. Babu), [email protected] (J. Shim), [email protected] (K. Yoo). 1 These authors contributed equally to this work https://doi.org/10.1016/j.mssp.2019.104834 Received 19 August 2019; Received in revised form 1 October 2019; Accepted 7 November 2019 Available online 15 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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improve the charge transfer mechanism, diminish the fast reunion of charge carriers, and enhance the influence of active radical species [13]. Metal-oxide semiconductors such as tin oxide (SnO2), zinc oxide (ZnO), zirconium oxide (ZrO2), and titanium oxide (TiO2) are promising catalysts for the degradation of hazardous contaminants [14–18]. Among these, n-type semiconductor, SnO2 has attracted enormous in terest owing to its wide band gap (~3.6 eV), cost effectiveness, non-toxic nature, high stability, and environmental sustainability. In spite of its great potential, the photocatalytic activity of SnO2 is confined to the ultraviolet (UV) region due to its wide bandgap. Nevertheless, the photoinduced charge carriers in SnO2 undergo quick recombination leading to a further reduction of its photocatalytic activity [19,20]. To date, zero-dimensional nanomaterials such as quantum dots (QDs), have attracted significant attention compared to bulk materials due to their exceptional photocatalytic activity resulting from their tiny particle size and large surface area [21,22]. Moreover, the surface properties of semiconductor QDs hinder aggregation and improve the photocatalytic activity [23]. Accordingly, enormous efforts have been made to extend the light absorption efficiency of n-type SnO2 QDs (SQDs) to the visible region and to prevent the recombination of photogenerated charge carriers through various approaches. The fabrication of p-n hetero junction NiFe2O4/SnO2 QD nanocomposites might be beneficial for the detachment and transport of photoinduced charge carriers, which can boost the photocatalytic performance under visible-light irradiation [24–26]. Further, there have only been limited reports on SnO2 QD-based heterostructure nanocomposites with outstanding catalytic perfor mance under UV and visible light. Lee et al. successfully synthesized novel UV-light-activated SQD-ZnS nanocomposites using a two-step hydrazine-assisted hydrothermal technique. Their results demon strated that the strong interfacial interactions between SQD and ZnS enable the direct transfer of charge carriers, which effectively hinders the recombination of charge carriers and improves the degradation rate of rhodamine B (RhB) [27]. Babu et al. constructed an SQD/g-C3N4 heterojunction photocatalyst via a simple in-situ chemical reduction procedure for the improvement of catalytic performance upon visible light illumination. Further experiments demonstrated that the hetero junctions formed between SQDs over g-C3N4 nanolayers could improve the photocatalytic efficiency and they exhibited exceptional catalytic performance for the degradation of dye pollutants [28]. Ma et al. suc cessfully examined the catalytic activity of heterostructured SQD/SnS2 nanocomposites under visible light. Good dispersion of SQDs over SnS2 nanosheets could improve the synergistic interactions and provided a remarkably improved catalytic performance for the reduction of Cr(VI) [29]. These reports motivated us to fabricate heterostructures with various amounts (5, 10, and 15 mg) of SnO2 QDs dispersed on NiFe2O4 nanoplates using a facile hydrothermal technique, which may produce heterostructured nanocomposites with improved photocatalytic properties. Herein, we successfully demonstrate a facile and economical hy drothermal technique for the production of NiFe2O4/SnO2 QD p-n het erojunction nanocomposites with various amounts of SQDs (5, 10, and 15 mg) for the improvement of the photocatalytic degradation of RhB through visible light irradiation. The phase structures, morphologies, chemical compositions, and photocatalytic performance of the synthe sized catalysts are described specifically. Furthermore, based on the results of radical trapping experiments and photoluminescence (PL) examinations, a plausible mechanism for the improved catalytic activity for the degradation of RhB upon visible light treatment is presented. Finally, these findings suggest that the fabrication of heterostructured nanocomposites consisting of semiconductor QDs decorated onto a magnetic semiconductor is an effective strategy for the degradation of hazardous contaminants under visible-light illumination that has abundant potential applications in energy-related fields.
2. Experimental 2.1. Chemicals Nickel chloride hexahydrate (NiCl2⋅6H2O), iron chloride hexahy drate (FeCl3⋅6H2O), sodium hydroxide (NaOH), tin chloride (SnCl4⋅5H2O), hydrazine monohydrate (N2H4. H2O), ethanol, benzo quinone (BQ), isopropyl alcohol (IPA), triethanolamine (TEOA), and rhodamine B (RhB) were analytical grade and used as received without additional refinement. All the chemicals were purchased from Sigma –Aldrich Co., Ltd., Korea. 2.2. Synthesis of NiFe2O4 nanoplates First, 2 g of iron chloride hexahydrate (0.1 mol/L) was liquefied into 100 mL of deionized (DI) water followed by stirring. Second, 0.42 g of nickel chloride hexahydrate (0.05 mol/L) was distributed into the above aqueous solution of iron chloride under magnetic stirring. Using a NaOH solution, the pH was adjusted to 13.0, and a red-brown product pro gressively precipitated from the solution. Thereafter, the formed pre cipitate was collected, washed numerous times with ethanol and DI water, and then annealed at 500 � C for 3 h in air. Finally, a reddishbrown NiFe2O4 powder was obtained. 2.3. Synthesis of SnO2 QDs One gram of tin chloride was dissolved in 125 mL of DI water. The solution was stirred for 30 min to obtain a homogeneous solution. A 2.5mL hydrazine solution was added dropwise into the above mixed solu tion. The slurry thus formed was stirred for 30 min and kept at 100 � C for 18 h. Finally, SnO2 QDs were obtained via centrifugation and were rinsed several times with DI water and ethanol before drying overnight in an oven at 60 � C. 2.4. Synthesis of NiFe2O4/SQD nanocomposites Novel NiFe2O4/SQD nanocomposites with different loading amounts of SQDs (5, 10, and 15 mg) were successfully prepared by a simple hy drothermal technique, which can be described as follows. Typically, 100 mg of NiFe2O4 nanoplates were dissolved in 40 mL of ethanol and stirred for 30 min. Secondly, various amounts of SQDs (5, 10, and 15 mg) were distributed in 40 mL of ethanol under stirring for 30 min in another beaker and then added dropwise to the above solution. After stirring for 30 min, the suspension was transferred into a 100-mL Teflonlined stainless-steel autoclave and kept at 180 � C for 24 h. After cooling naturally to room temperature, the precipitates with various amounts of SQDs (hereinafter designated as NFS-5, NFS-10, and NFS-15, respec tively) were obtained via centrifugation, and were washed several times with DI water and ethanol before drying overnight in an oven at 80 � C. 2.5. Photocatalytic activity The catalytic performance of the prepared catalysts was estimated by examining the degradation of RhB in an aqueous solution. In these ex periments, 20 mg of the photocatalysts was distributed into 50 mL of RhB dye solution with a dye concentration of 10 ppm and stirred for 30 min in the dark to attain adsorption-desorption equilibrium. After wards, the solution was exposed to visible light from a 100-W solar simulator with a UV cut-off filter (λ ˃ 400 nm) to investigate the degra dation effect. During catalytic experiments, 3-mL samples were taken from the reacted RhB solution and centrifuged, then UV–vis spectro scopic analysis was used to measure the concentration of RhB at a wavelength of 552 nm. 2
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3. Results and discussion
The phase structure and physicochemical properties of the synthe sized NiFe2O4/SQD nanocomposites were examined by powder X-ray diffraction (XRD), and the obtained patterns are presented in Fig. 1. The diffraction pattern of pure NiFe2O4 exhibited characteristic peaks at 30.29� , 35.66� , 37.35� , 43.31� , 53.80� , 57.36� , and 62.98� which can be attributed to the (220), (311), (222), (400), (442), (511), and (440) planes of the spinel structure of NiFe2O4 with no impurity phases (JCPDS# 00-054-0964) [30]. From the XRD pattern of pure SQDs (Fig. 1 (b)), the major diffraction peaks at 26.15� , 33.69� , 52.12� , and 64.72� correspond to the (110), (101), (211), and (112) crystalline planes of the tetragonal phase of SnO2, respectively (JCPDS #00-041-1445) [31]. Furthermore, the XRD patterns of the NiFe2O4/SQD nanocomposites (Fig. 1(c)) clearly exhibit two sets of characteristic peaks that can be accredited to the single-phase spinel structure of NiFe2O4 (#), and tetragonal phase of SnO2 (*), suggesting that both NiFe2O4 and SQDs are effectively coupled in the synthesis processes. Nevertheless, the XRD peaks of the SQDs were not as prominent as the NiFe2O4 XRD peaks. This can be due to the huge variation in the peak intensities of NiFe2O4 and SQD, and also the fact that very small amounts of SQDs were used for the construction of the NiFe2O4/SQD nanocomposites. The magnified XRD pattern of the prepared nanocomposites is presented in Fig. S1, which provides a clearer information of the XRD peak positions of the SQDs. Moreover, with the increasing loading of the SQDs, the diffraction peak intensities of NiFe2O4 gradually decrease, indicating the successful interaction between NiFe2O4 and the SQDs. Thus, the XRD results demonstrate the high crystallinity of the as-prepared nanocomposites with a binary system, and no other impurity phases were detected.
495.53 eV, compatible with the Sn3d5/2 and Sn3d3/2 states, respectively. Additionally, the spin-orbital splitting between Sn 3d5/2 and Sn 3d3/2 is around 8.48 eV, which can be attributed to the binding energy of Sn4þ ions in the SQDs [32]. Further, the two deconvoluted XPS peaks at the binding energies of 489.34 and 491.02 eV should be attributed to the existence of Sn in Sn4þ and Sn2þ oxidation states, respectively [33]. The high-resolution XPS spectrum of the O 1s state for NFS-10 is presented in Fig. S2, and it can be deconvoluted into three peaks with binding en ergies of 529.38, 530.81, and 533.69 eV. The peak at 529.38 eV is attributed to the metal-oxygen bond, the peak at 530.81 eV is associated to Ni-O-H, while the peak at 533.69 eV is attributed to absorbed water [34–36]. The high-resolution Fe 2p XPS spectrum (Fig. 2(c)) can be deconvoluted into eight peaks centered at 710.33, 713.18, 715.82, 719.68, 723.81, 726.89, 729.54, and 732.13 eV. The peaks at 713.18 and 726.89 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively [10]. Four peaks can be assigned to Fe2þ (710.33 and 723.81 eV) and Fe3þ (715.82 and 729.54 eV) ions in the hematite phase [37], whereas the peaks at 719.68 and 732.13 eV could be attributed to the satellite peaks [38]. The observed binding energies for Fe 2p are well-matched with the reported data [39–41]. The high-resolution Ni 2p XPS spectrum shown in Fig. 2(d) exhibits two binding energy peaks situated at 856.70 and 874.55 eV, which are consistent with Ni 2p3/2 and Ni 2p1/2, respectively, with a binding energy difference of 17.85 eV, which confirms that Ni is in the 2þ oxidation state [42]. In addition, the Ni 2p3/2 peak can be deconvoluted into two energy bands positioned at 854.17 and 858.89 eV, respectively, which could be attributed to the existence of the Ni2þ and Ni3þ oxidation states [43]. Further, the Ni 2p scan spectrum also contains two satellite peaks at 863.36 and 880.46 eV with signifi cant intensities [44]. The XPS analysis evidently confirms the formation of heterojunctions between NiFe2O4 and SQDs, and is in good agreement with the XRD results.
3.2. XPS analysis
3.3. Morphology studies
X-ray photoelectron spectroscopy (XPS) analysis was applied to further investigate the chemical composition and the interactions be tween the components of the synthesized NiFe2O4/SQD nano composites. The XPS analysis was carried out with the optimized sample of the prepared nanocomposites, i.e., 10 mg of SnO2 QDs in 100 mg of NiFe2O4 nanoplates and it is represented as NFS-10 nanocomposite. The XPS survey scan of the NFS-10 nanocomposite shows the existence of Sn, O, Fe, and Ni elements, and is presented in Fig. 2(a). The high-resolution XPS spectrum of the Sn 3d state is presented in Fig. 2(b). It shows two asymmetric peaks at the higher binding energies of 487.05 and
The surface morphology and the interfacial interactions of the asprepared nanocomposites were examined by using field-emission scan ning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) studies. The FESEM image of pure NiFe2O4 is presented in Fig. 3(a) and it indicates that the NiFe2O4 exhibit an irregular shaped plate-like nanostructures with non–uniform grain sizes. In addition, the surface of the NiFe2O4 nanoplates are clean and smooth. In Fig. 3(c), the FESEM image of NFS-10 nanocomposite show the morphology of the NiFe2O4 nanoplates after SQDs were deposited. Tiny SQDs decorate the NiFe2O4 nanoplates and the
3.1. Phase structures
Fig. 1. XRD patterns of (a) pure NiFe2O4, (b) pure SnO2 QDs, and (c) NFS nanocomposites. 3
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Fig. 2. High-resolution XPS spectra of the NFS-10 nanocomposite.
Fig. 3. FESEM and EDS analysis of (a, b) NiFe2O4 nanoplates, and (c, d) NFS-10 nanocomposite.
morphology of the NiFe2O4 nanoplates was unaffected by loading them with SQDs. The purity and elemental composition of the NiFe2O4 nanoplates and NFS-10 nanocomposite were investigated using energydispersive X-ray spectroscopy (EDS), and the results are presented in Fig. 3 (b & d). It was confirmed that the NFS-10 nanocomposite con sisted of Ni, Fe, Sn, and O. Additionally, EDS analysis provided the weight% and atomic% values of the elements in the NiFe2O4, and NFS10 nanocomposite. Further, Fig. 4 presents TEM images of pristine NiFe2O4, pristine SQDs, and the NFS-5, NFS-10, and NFS-15 nano composites. As observed in Fig. 4(a), the TEM image of pristine NiFe2O4 displays nanoplate-like morphology, whereas the pure SQDs (Fig. 4(b)) show a spherical structured morphology. Fig. 4(c–e) provides the TEM images of NFS-5, NFS-10, and NFS-15 nanocomposites, clearly showing that the SQDs were successfully dispersed on the surface of the NiFe2O4
nanoplates. Furthermore, it was evident that the deposition of the SQDs over the surface of NiFe2O4 nanoplates progressively increased with the increasing loading of the SQDs. The HRTEM technique was used to further confirm the fabrication of heterostructured NiFe2O4/SQD nanocomposites and the images are shown in Fig. 5. The HRTEM image (Fig. 5(a)) clearly demonstrates the successful deposition of SQDs over the surface of NiFe2O4 nanoplates. The lattice fringe pattern of the heterostructured NFS-10 nanocomposite is presented in Fig. 5(b). From Fig. 5(b), the measured lattice fringe patterns with spacings of 0.246 nm and 0.339 nm could be accredited to the (311) plane of NiFe2O4 and the (110) plane of SnO2, respectively [45,46]. Nevertheless, the selected area electron diffraction (SAED) pattern presented in the inset of Fig. 5(b) shows a number of discrete concentric rings with superimposed bright spots, confirming the 4
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Fig. 4. TEM images of (a) pure NiFe2O4, (b) pure SQDs, and the (c) NFS-5, (d) NFS-10, and (e) NFS-15 nanocomposites.
Fig. 5. (a) HRTEM image and (b) lattice fringe pattern of the NiFe2O4/SQD nanocomposite (NFS-10) in the HRTEM image.
development of NiFe2O4/SQD nanocomposites. These outcomes evidently signify the formation of heterojunctions between NiFe2O4 and SQDs. In addition, Fig. 6 illustrates the elemental color mapping of the NFS-10 nanocomposite, which further confirms the dispersion of the SQDs over NiFe2O4. The obtained results strongly agree with the FESEM, TEM and XRD analysis presented above. The NiFe2O4/SQD hetero junctions thus formed could enhance the photocatalytic activity.
energies of pure SQDs, NiFe2O4, and NiFe2O4/SQD nanocomposites can be estimated using the equation Eg ¼ 1240/λ, where Eg is the bandgap energy and λ is the absorption wavelength. The measured bandgap en ergies were 3.463, 1.675, and 1.754 eV for pure SQDs, pure NiFe2O4, and NFS-10 nanocomposite, respectively and is presented in Fig. 7(b) using the Tauc plots. This demonstrates that the development of heter ojunctions can significantly hinder the reunion of charge carriers and subsequently shift the light response ability towards the visible region. Additionally, the valence band (VB) and conduction band (CB) po tentials of the SQDs and NiFe2O4 can be measured using the empirical formulae [49]:
3.4. Optical absorption study The optical absorption study is an efficient approach to examine the visible light absorption ability of the as-prepared nanocomposites, and the recorded optical diffuse reflectance spectra (DRS) of pristine SQDs, pristine NiFe2O4, and the NFS-5, NFS-10, and NFS-15 nanocomposites are illustrated in Fig. 7(a). The pure SQDs exhibited only UV absorption with an absorption band edge of about 358 nm, which is due to the free excitonic absorption of SQDs [47]. The pristine NiFe2O4 has a strong fundamental absorption edge in the visible region located at around 740 nm [48]. In the case of the NiFe2O4/SQD nanocomposites, the DRS spectra contain both UV and visible absorption bands, which signifies the development of heterojunctions between NiFe2O4 and SQDs. Furthermore, the light harvesting capability of the NiFe2O4/SQD nanocomposites was shifted into the visible region compared to pure SQDs with absorption edges ranging around 689–707 nm. The bandgap
EVB ¼ χ – Ee þ (0.5) Eg ECB ¼ EVB – Eg where EVB and ECB are the VB and CB potentials, respectively. χ is the electronegativity of the semiconductor; for SQD and NiFe2O4 the χ values are 6.217 and 4.655 eV, respectively. Eg and Ee refer to the bandgap energy and energy of free electrons on the hydrogen scale (~4.5 eV), respectively. Based on the above data, the VB and CB posi tions of SQDs were measured to be þ3.449 and 0.014 eV, while those for NiFe2O4 were found to be þ0.993 and 0.682 eV, respectively. PL spectroscopy has been extensively used to examine the transfer mechanism and separation effectiveness of photoinduced electron–hole pairs, which is the key factor for the improvement of catalytic activity. 5
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Fig. 6. EDS elemental color mapping of the NFS-10 nanocomposite. (a) Mapping region, (b) Ni Ka1, (c) Fe Ka1, (d) O Ka1, and (e) Sn La1. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article).
Fig. 7. (a) UV–vis DRS, and (b) Tauc plots of pure SnO2 QDs, NiFe2O4, NFS-5, NFS-10, and NFS-15 nanocomposites.
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Fig. 8. PL spectra of pure SQDs, pure NiFe2O4, NFS-5, NFS-10, and NFS-15 nanocomposites.
The PL spectra of SQDs, NiFe2O4, and NiFe2O4/SQD nanocomposites are presented in Fig. 8. For pure SQDs, a small peak at around 452 nm can be ascribed to the radiative reunion of photoinduced charge carriers occupying oxygen vacancies on the SQD surface [50]. The strongest emission peak around 592 nm originates from crystalline defects which materialize during the growth process [51]. Further, the PL spectra of NiFe2O4 exhibited two emission peaks positioned at 458 nm and 584 nm that can be ascribed to the defect levels generated by oxygen vacancies in NiFe2O4 [52,53]. Moreover, the NiFe2O4/SQD nanocomposites
showed significantly reduced emission intensities compared with the pure samples. The emission intensities were found to decrease in the order of NFS-10 ˂ NFS-15 ˂ NFS-5 ˂ NiFe2O4 ˂ SQDs, demonstrating that the recombination of charge carriers was progressively weakened in the NFS-10 nanocomposite. These outcomes suggest that the synergistic interactions between NiFe2O4 and SQDs significantly lead to the reduction of the recombination of photoinduced charge carriers, which in turn lead to a substantial catalytic enrichment in the visible region.
Fig. 9. (a) Absorption spectra of the NFS-10 nanocomposite for the photocatalytic degradation of RhB, (b) photodegradation analysis, (c) kinetics of RhB photo catalytic degradation: [ln(C0/C)] vs. the irradiation time for each catalyst, and (d) plot of the k values of all the samples. 7
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3.5. Photocatalytic activity of NiFe2O4/SQD nanocomposites for RhB degradation
be described using a pseudo-first-order kinetics model, ln (C/C0) ¼ kt, where k is the kinetic rate constant of the degradation process. The ki netics of RhB degradation with all prepared samples are represented in Fig. 9(c). The calculated k values are 0.0019, 0.0052, 0.0102, 0.0242, and 0.0145 min 1 for SQDs, NiFe2O4, NFS-5, NFS-10, and NFS-15, respectively (Fig. 9(d)). Remarkably, the NFS-10 nanocomposite exhibited a considerably superior k value compared to the remaining samples and generated a rate constant for RhB degradation ~4.66 times higher than that of pure NiFe2O4. The effect of catalyst loading (15, 20, and 25 mg) on the degradation of RhB over the NFS-10 photocatalyst is presented in Fig. 10(a). The NFS-10 nanocomposite showed the highest catalytic activity at 20 mg loading. The degradation efficiency progressively decreased with increasing catalyst loading. This can be due to the agglomeration of particles and the depletion of active sites [57]. Hence, overloading of catalyst may negatively affect the rate of RhB degradation. In addition, the stability of the prepared catalysts is an essential parameter for their practical application. To confirm stability, the reusability of the NFS-10 nanocomposite was investigated by three cycling tests measuring the degradation of RhB under the same conditions, and the consistent results are demonstrated in Fig. 10(b). It was clearly observed that the NFS-10 photocatalyst exhibited only a minor reduction in photocatalytic activ ity rather than a considerable loss after three cycles, and more than 90% of the photocatalytic efficacy was retained, which reveals the stability and reusability of the synthesized nanocomposites. To further explore the catalytic reaction mechanism of NiFe2O4/SQD heterostructures, scavenger experiments were executed with various radicals over the NFS-10 nanocomposite to confirm the contributions of the active species in the degradation process. It has been widely recognized that scavengers such as superoxide radicals (�O2 ), holes (hþ), and hydroxyl radicals (�OH) play a key role in photocatalytic degradation and activate the oxidation of organic pollutants. In this study, controlled experiments were conducted by using scavengers such as BQ, TEOA, and IPA, to capture �O2 , hþ, and �OH radicals, respec tively. Fig. 10(c) illustrates the effect of various scavengers on the degradation of RhB by NFS-10 photocatalyst under visible light. The catalytic activity of the NFS-10 photocatalyst was remarkably inhibited upon the addition of IPA and TEOA as �OH and hþ scavengers, resulting
The improved catalytic performance of the as-prepared hetero structured NiFe2O4/SQD nanocomposites was determined by measuring the degradation of the organic dye RhB upon visible-light treatment. The time-dependent optical absorption spectra for the degradation of RhB upon visible illumination in the presence of the NFS-10 photocatalyst is presented in Fig. 9(a). The RhB dye exhibited a single absorption peak at 552 nm in the visible region, which decreased progressively and finally disappeared after 105 min of visible-light illumination. For a more quantitative understanding of the photocatalytic activity, we have examined the change in RhB concentration (C) after photodegradation, relative to the initial concentration (C0) of RhB dye [54]. Fig. 9(b) presents the change in the RhB concentration (C/C0) as a function of time using pristine SQDs, NiFe2O4, NFS-5, NFS-10, and NFS-15 nano composites under visible-light treatment. In the absence of photocatalyst (dark), no considerable degradation was detected. It was observed that the RhB concentration decreased by 16% and 41% in presence of pris tine SQDs and NiFe2O4, respectively. However, the heterostructured NiFe2O4/SQD nanocomposites possess photocatalytic activity signifi cantly greater than that of pristine SQDs or NiFe2O4. The observed visible-light-induced degradations of RhB for the NFS-5, NFS-10, and NFS-15 nanocomposites were 70%, 98%, and 82%, respectively, for a duration of 105 min. This could be ascribed to the close interfacial contact between p-type NiFe2O4 and the n-type SQDs, which enhanced the absorption of visible light. Thus, n-type SQDs act as an electron tank to receive the photogenerated electrons from p-type NiFe2O4, due to the development of strong p-n heterojunction between NiFe2O4 and SQDs. In this way, the recombination of charge carriers is inhibited and the photocatalytic performance is enhanced [3]. It was observed that among the synthesized nanocomposites, the NFS-10 photocatalyst exhibited the maximum efficiency for the degradation of RhB, which is in accordance with the PL results. The excess deposition of SQDs over NiFe2O4 can decrease the effective contact area, hindering photon absorption and the production of active sites, which reduces the photocatalytic activity [55, 56]. Further, the catalytic activity of the constructed photocatalysts can
Fig. 10. (a) Effect of catalyst loading, (b) recycling test, and (c, d) radical trapping experiment for the degradation of RhB over the NFS-10 nanocomposite in the absence and presence of scavengers. 8
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Fig. 11. Proposed mechanism for charge transfer and photocatalytic degradation of RhB in the NiFe2O4/SQD nanocomposite.
in a decline from 98% to 56% and 64%, respectively (Fig. 10(d)). In contrast, the degradation efficiency was only reduced slightly to 91% with the introduction of BQ as an �O2 scavenger. These observations suggest that photocatalysis can be influenced by the �OH and hþ radicals in the degradation of RhB on the surface of the NFS-10 nanocomposite. Based on the above experimental outcomes, an acceptable mecha nism for the improved catalytic activity of the NiFe2O4/SQD photo catalyst upon visible-light illumination is proposed, and schematically represented in Fig. 11. NiFe2O4 is a p-type semiconductor whose Fermi energy level (Ef p) is located close to the VB, while SQD is a typical ntype semiconductor whose Fermi energy level (Ef n) lies close to CB. When p-type NiFe2O4 is in contact with n-type SQD, the p-type NiFe2O4 and n-type SQD could form the p–n heterojunction, and the electrons diffuse from n-SQD into p-NiFe2O4. As a result, negative charges accu mulated in the p-NiFe2O4 region near the junction. Mean-while, holes diffuse from the p-NiFe2O4 region to the n-SQD region near the junction, creating a positive section in the n-SQD region in the vicinity of the junction. When the Fermi levels of NiFe2O4 and SQD reach equilibration, the internal electric field directed from n-SQD to p-NiFe2O4 is simulta neously established to stop the charge diffusion from n-SQD to pNiFe2O4. Meanwhile, the energy bands of NiFe2O4 shift upward along the Fermi level (Ef p) and SQD shift downward along the Fermi level (Ef n) near the interface (Fig. 11), which are similar to the reported heterojunction photocatalysts [44,58,59]. Under visible-light irradia tion, NiFe2O4 can be easily excited and electron–hole pairs are produced on its surface. In the SQDs, excitation by visible light is impossible owing to its wide bandgap. Thus, the electrons produced in the CB of p-type NiFe2O4 would be rapidly transported to the CB of the n-type SQDs owing to the strong heterojunction formation, as the CB potential of p-type NiFe2O4 ( 0.682 eV) is more negative than that of n-type SQDs ( 0.014 eV). Conversely, the photoinduced holes in the VB of p-type NiFe2O4 cannot migrate to the VB of the n-type SQDs, because the VB potential of p-type NiFe2O4 (þ0.993 eV) is less positive than that of n-type SQDs (þ3.449 eV). Thus, the excited electrons and holes can be effectively separated, removing the possibility of charge carrier recom bination. The CB potential of the n-type SQDs is less negative than Eθ(O2/�O2 ) ¼ 0.046 eV, thus �OH radicals can be produced from the electrons in the CB through a two-electron oxidation pathway [60]. In contrast, the VB potential of p-type NiFe2O4 is less positive than Eθ(�OH/H2O) ¼ þ2.27 eV, demonstrating that the photoinduced holes of p-type NiFe2O4 cannot oxidize H2O to form �OH radicals, and they directly degrade the organic pollutant. Based on our results, the photoinduced active radicals �OH and hþ are beneficial for the degra dation of RhB and the mechanism can be summarized as follows:
H2O2 þ �O2 → �OH þ OH þ O2
NiFe2O4/SQD þ hυ → NiFe2O4(e–CB þ hþVB)/SQD
Acknowledgements
–
NiFe2O4(e SQD(e
–
CB)
CB
þ
hþVB)/SQD
→
�OH þ RhB dye → Degradation products (major contribution) �O2 þ RhB dye → Degradation products (minor contribution) NiFe2O4(hþVB) þ RhB dye → Degradation products (major contribution) Thus, these outcomes suggest that the strong interfacial interactions in the heterostructured nanocomposite would improve the electrontransfer mechanism and hinder the recombination of photogenerated charge carriers, which remarkably enhances the catalytic activity of the NiFe2O4/SQD nanocomposite upon visible light illumination. 4. Conclusions In summary, the SQDs were deposited in different amounts over NiFe2O4 to construct novel p-n heterojunction NiFe2O4/SQD nano composites through a simple hydrothermal technique for the degrada tion of organic pollutants from wastewater upon visible light irradiation. The structure, composition, and morphology of the prepared NiFe2O4/ SQD nanocomposites were confirmed by XRD, TEM, HRTEM, and XPS studies. The structural investigations confirmed the spinel structure of NiFe2O4 and tetragonal phase of SnO2. A good dispersion of the SQDs over the NiFe2O4 nanoplates was clearly demonstrated from TEM and HRTEM studies. Furthermore, optical and PL studies indicated improved visible-light absorption with a functional interface between NiFe2O4 and SQDs, where the photoinduced electron–hole pairs are efficiently separated, which considerably enhanced the catalytic activity. More over, the SQDs dispersed on the surface of NiFe2O4 nanoplates played a significant role as an electron-conduction bridge, allowing electrons to migrate towards the SQDs and increasing the effectiveness of the sepa ration of photoinduced electron–hole pairs in NiFe2O4. In addition, the formation of p-n heterojunction between p-type NiFe2O4 and n-type SQDs can significantly improves the photocatalytic performance. The resul tant NFS-10 photocatalyst exhibited an exceptional visible-light-induced catalytic activity for the degradation of RhB (98%) within 105 min. In addition, radical trapping experiments confirmed that the �OH and hþ species play a leading role in the degradation of RhB over the NFS-10 nanocomposites. These findings suggest that the fabricated NiFe2O4/ SQD nanocomposites can provide potential avenues for visible-lightdriven photocatalysis for energy-related applications. Declaration of competing interest The authors declare no conflict of interest.
NiFe2O4(hþVB)/SQD(e–CB)
This work was supported by a National Research Foundation of Korea grant funded by the Korean government (2018R1D1A1B07050330, 2017R1C1B2001990, and 2017R1A4A1015581).
þ O2 → �O2
2SQD(e–CB) þ O2 þ 2Hþ → H2O2
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Appendix A. Supplementary data
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