Heterogeneous Semiconductor Shells ... - ACS Publications

Feb 6, 2017 - been proven to be one of the most efficient UC materials.14−16 ..... deposition, the particle surface became pretty rough and concave,...
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Heterogeneous Semiconductor Shells Sequentially Coated on Upconversion Nanoplates for NIR-Light Enhanced Photocatalysis Cao Cui,† Meijie Tou,† Mohua Li,† Zhenguo Luo, Lingbo Xiao, Song Bai, and Zhengquan Li* Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Zhejiang Normal University, Jinhua, Zhejiang 321004, P. R. China S Supporting Information *

ABSTRACT: Combination of upconversion nanocrystals (UCNs) with CeO2 is a decent choice to construct NIR-activated photocatalysts for utilizing the NIR light in the solar spectrum. Herein we present a facile approach to deposit a CeO2 layer with controllable thickness on the plate-shaped NaYF4:Yb,Tm UCNs. The developed core−shell nanocomposites display obvious photocatalytic activity under the NIR light and exhibit enhanced activity under the full solar spectrum. For enhancing the separation of photogenerated electrons and holes on the CeO2 surface, we sequentially coat a ZnO shell on the nanocomposites so as to form a heterojunction structure for achieving a better activity. The developed hybrid photocatalysts have been characterized with TEM, SEM, PL, etc., and the working mechanism of such UCN-semiconductor heterojunction photocatalysts has been proposed.

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light.22−24 Furthermore, CeO2 is highly biocompatible and environmentally benign, favoring the application for the environmental remedy.20,21 All of the above features make CeO2 a good candidate to couple with the UCNs for achieving a hybrid UCN-semiconductor photocatalyst. In the past years, hexagonal NaYF4 NCs codoped with Yb3+ and Tm3+ ions has been proven to be one of the most efficient UC materials.14−16,25−29 Therefore, developing facile approaches to prepare core−shell nanocomposites consisting of NaYF4:Yb,Tm (shorten as NYF) and CeO2 and exploring their NIR-activated photocatalytic activity are of great importance. It is known that photocatalytic activity of semiconductors also strongly depends on the separation of photogenerated electrons (e−) and holes (h+) besides the efficiency in light absorption.30−32 Like other semiconductors, the photocatalytic activity of CeO2 NCs is also severely restrained by the surface e−-h+ recombination to some extent. In order to suppress such recombination and enhance the separation of photogenerated e− and h+, creation of CeO2-based heterojunction structure with another semiconductor is a good choice.32 For the NIRactivated hybrid photocatalysts, because the UCNs generally possess a relatively low quantum efficiency compared to traditional down-conversion (or down-shifting) phosphors, efficiently converting the NIR light to separated e− and h+ is very crucial.28,29 Specifically, the separation of photogenerated e− and h+ is a decisive step for the overall photocatalytic activity of such kinds of hybrid photocatalysts when the upconverted emissions from UCNs have been absorbed. To our knowledge, development of a shell-on-shell heterojunction structure on the

INTRODUCTION Environmental pollution and energy shortage are two big global challenges in the long term development of human society.1−4 One decent solution for both challenges is to develop efficient semiconductor photocatalysts which not only can utilize clean solar energy but also produce active species for the environmental remedy. In the past decades, many new and novel photocatalysts have been developed, showing a promising future for this technology.5−8 However, one big limitation of semiconductor photocatalysis is the narrow absorption band of semiconductors which can only make use of ultraviolet (UV) or visible (vis) light. Although many strategies have been managed to broaden the absorption range of semiconductors such as doping, sensitization, and coupling with metals or other semiconductors, most semiconductors are still unavailable to utilize the near-infrared (NIR) light despite that the NIR light accounts for around 48% in the solar energy.9−13 To overcome this limitation, the use of upconversion nanocrystals (UCNs) as a light-converter for semiconductors has been regarded as one of the promising solutions.14−17 For this purpose, synthesis of hybrid UCN-semiconductor photocatalysts capable of efficiently utilizing the NIR light is thus highly demanded. As cerium is the most abundant element in the rare earth family, ceria (CeO2) has played an important role in both academic and industrial communities.18,19 Owing to its oxygen storage capacity, high thermal stability, and electrical conductivity, CeO2 nanocrystals (NCs) show widespread applications ranging from catalysis to fuel cells, oxygen sensor, polishing material, UV absorbent, and antioxidant medicine.20−22 Serving as a semiconductor photocatalyst, CeO2 has a strong absorption in the UV region with high refractive index and diffusivity, and thus it can efficiently harvest the UV © 2017 American Chemical Society

Received: December 19, 2016 Published: February 6, 2017 2328

DOI: 10.1021/acs.inorgchem.6b03079 Inorg. Chem. 2017, 56, 2328−2336

Article

Inorganic Chemistry UCNs aiming for enhanced e−-h+ separation and photocatalytic activity is still rarely explored. In this work, we demonstrate a facile method to coat a CeO2 layer on the UCNs with a uniform core−shell structure. Upon surface-modification of high-quality but hydrophobic upconversion (UC) NYF nanoplates, a CeO2 layer composed of small NCs has been successfully deposited under a mild aqueous solution. The developed NYF@CeO2 nanoparticles (NPs) show obvious photocatalytic activity under the NIR light and enhanced activity under the full solar spectrum. The thickness of the CeO2 layer can be tuned by the concentration of cerium precursor, providing a convenient means to tune their photocatalytic activity. Furthermore, a ZnO shell is sequentially coated on the NYF@CeO2 NPs so as to form a heterojunction structure on the UCNs. After the growth of ZnO shell, the hybrid NYF@CeO2/ZnO NPs exhibit significantly enhanced activity under the NIR light due to the efficient separation of photogenerated e− and h+. This work may provide a new strategy for the design and synthesis of NIR-activated hybrid photocatalysts.

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solution. After stirring for 10 min, the solution was heated to 95 °C and maintained for 3 h. After cooling to room-temperature, the products were collected from the solution by centrifuging at a speed of 5 000 rpm. The products were then washed with ethanol and DI water thrice, respectively, and finally dispersed in DI water for further use. Photocatalytic Measurement. Photocatalytic activities of samples were evaluated by the decolouration of a model dye pollutant (methyl orange, MO). The simulated sunlight was supplied by a Xe lamp (PLS-SXE300, 100 mW/cm2) which was equipped with various filters for providing different irradiation bands. In a typical experiment, 50 mL of MO solution (10−5 M) was loaded in a 100 mL beaker. Then, a suitable amount of photocatalysts (∼50 mg) was added in the solution and stirred for 1 h in the dark. After that, the solution was exposed to the irradiation of Xe lamp for a given period of time. The real-time concentration of MO molecules in solution was measured by the UV−vis spectrometer through monitoring the absorption peak centered at 475 nm. Characterizations. Fourier transform infrared (FTIR) spectra were carried out on a Nicolet 8700 FTIR spectrometer in a KBr pellet. Transmission electron microscopy (TEM) and energy dispersive X-ray (EDS) spectroscopy was performed on a JEOL 2010F TEM. Scanning electron microscopy (SEM) was conducted on a Hitachi S-4800 scanning electron microscopy. Powder X-ray diffraction (XRD) was carried out on a Philips X’ Pert Pro X-ray diffractometer equipped with a Cu Kα radiation. Photoluminescent (PL) spectra were acquired on a Hitachi F-7000 spectrometer equipped with a commercial 980 nm NIR laser. Luminescence decay curves were acquired on an Edinburg FLS980 spectrometer with a 980 nm NIR pulsed laser. UV−vis absorption spectra were obtained on a Shimazhu UV-2450 UV−vis spectrometer. The photocurrent and Mott−Schottky measurements were performed on a CHI 660D electrochemical workstation using the sample-coated ITO glass as a photoelectrode, a Pt foil as counter electrode, and a Ag/AgCl electrode as reference electrode. X-ray photoelectron spectra (XPS) were collected on an ESCALab 250 Xray photoelectron spectrometer.

EXPERIMENTAL SECTION

Preparation of Water-Dispersible β-Phase NaYF 4 :Yb(20%),Tm(0.5%) Nanoplates. Uniform β-phase NYF nanoplates with oleic acid (OA)-capped surface were prepared with a user-friendly protocol we previously developed.25 To make the NYF nanoplates water-dispersible, diluted HCl was used to strip off the OA molecules through a protonation process.33 In a typical process, 1 mmol of NYF nanoplates was added in a 20 mL flask containing 10 mL of deionized (DI) water. Under magnetic stirring, diluted HCl (0.1 M) was slowly dropped in the solution until the pH value reached 4. Then, the mixed solution was aged at room-temperature (20 °C) for 2 h. During this period, the pH value of solution slowly increased and a few drops of HCl solutions were intermittently added to the solution for maintaining the pH value. When the pH value did not change any more, the NYF nanoplates were further aged in solution for another 30 min. After that, 10 mL of diethyl ether was poured into the solution, and the water-phase was separated from the solution with a separating funnel. OA-free NYF nanoplates were then collected from the waterphase by centrifuging. The collected sample was washed with acetone and DI water twice, respectively, and finally dispersed in 10 mL of DI water. Synthesis of Core−Shell NYF@CeO2 NPs. The OA-free NYF nanoplates were used as seeds for the deposition of a CeO2 layer on their surface. In a typical synthesis, 0.2 mL of OA-free NYF particle solution (0.1 M) was added in a mixed solution of 10 mL of ethanol and 10 mL of DI water. Then, 0.2 mL of aqueous Ce(NO3)3 solution (0.1 M) and 0.4 mL of aqueous hexamethylenetetramine (HMTA) solution (0.1 M) were sequentially added to the solution and stirred for 10 min. After that, the solution was slowly heated to 60 °C and maintained for 2 h. After cooling to room-temperature, the core−shell particles were collected by centrifuging at a speed of 6 000 rpm. The collected sample was then washed with ethanol and DI water thrice, respectively, and finally dispersed in 2 mL of DI water. The NYF@CeO2 NPs with different CeO2 thickness were also prepared through adjusting the addition volumes of Ce(NO3)3 and HMTA solutions. To avoid producing free CeO2 NPs, the volume ratio of Ce(NO3)3 to HMTA was kept at 1:2. Four samples (termed as S1, S2, S3, and S4) were prepared according to the following volumes of Ce(NO3)3/HMTA: 0.02 mL/0.04 mL, 0.1 mL/0.2 mL, 0.2 mL/0.4, and 0.4 mL/0.8 mL. Synthesis of NYF@CeO2/ZnO NPs. The deposition of ZnO shell on the NYF@CeO2 NPs is similar to the protocol for CeO2 deposition. In a 25 mL flask, 1 mL of NYF@CeO2 particle solution (0.01 M) and 0.6 g of polyvinylpyrrolidone (PVP, K30) was dispersed in 20 mL of DI water. Then, 0.1 mL of HMTA solution (0.1 M) and 0.1 mL of Zn(NO3)2 solution (0.1 M) were sequentially added in the

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RESULTS AND DISCUSSION Synthetic Strategy of the NYF@CeO2 NPs. It is known that CeO2 NCs can be facilely formed in basic condition with aqueous cerium salts under a mild condition (below 100 °C).21,34 For depositing a CeO2 layer, the NYF nanoplates are required to be water-dispersible because the high-quality NYF nanoplates prepared from organic solvents naturally possess a hydrophobic surface. To this end, our first task is to choose a suitable surface-modification method for producing waterdispersible NYF nanoplates. Although many surface-modification methods are developed (e.g., ligand exchange, covering surfactants and silica-coating), these methods generally produce an organic/inorganic layer on these fluorescent NCs, leading to notable PL decrease due to the light scattering and absorption effect.35−37 In contrast, surface-striping of hydrophobic ligands from the NCs is a better choice, because it produces waterdispersible NCs with a bare surface. Furthermore, such bare surface can provide a direct contact between these NCs and the shell materials to be coated, facilitating energy transfer between the core−shell components. In this case, we adopted an OAstripping method to produce water-dispersible NYF nanoplates with a bare surface (see Scheme 1). Upon the addition of diluted HCl solution, the OA molecules around NYF nanoplates could be gradually protonated and leave the particles surface. Therefore, colloidal NYF nanoplates with good monodispersity were obtained. The elimination of OA molecules from the NYF nanoplates can be confirmed by the FTIR spectra (see Figure S1, in the Supporting Information). After the preparation of water-dispersible NYF nanoplates, the next task is to create a favorable growing condition for the 2329

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The thickness of the CeO2 layer is estimated to be 25 nm, judging from the increased diameter of these nanoplates. Figure 1D,E shows magnified HRTEM images of the deposited CeO2 NCs, from which clear lattice fringes are observed. The distance between the two fringes is about 0.31 nm, matching well with the (111) plane of CeO2 crystal. We also noticed that the crystal orientation of these deposited CeO2 NCs is not along the same direction, suggesting that these small NCs might be randomly attached on the nanoplates surface. SEM images have also been used to characterize the samples during different preparation stages. Figure 2A,B shows SEM

Scheme 1. Schematic Illustration of the Formation Process of NYF@CeO2 NPs

CeO2 layer. Considering that the growth rate of CeO2 NCs is very fast in conventional basic media (e.g., NaOH solution or ammonia), a weak and stable basic condition is thus preferred. Here we employed HMTA as a basic source because it not only provides a weak basic condition but also can complex with the Ce3+ ions.38 As such, the release rate of Ce3+ ions in the solution can be well controlled and the growth rate of CeO2 NCs is thus regulated. For the same reason, HMTA was also employed for the sequential growth of ZnO shell on the surface of NYF@CeO2 NPs. Morphologies, Phases, and Compositions of the Prepared Samples. Figure 1A displays a typical TEM image

Figure 2. SEM images of samples collected from different preparation stages: (A) NYF nanoplates; (B) NYF nanoplates after OA stripping; (C, D) NYF@CeO2 NPs with different magnifications.

images of NYF nanoplates before and after stripping off the OA molecules. Both samples display uniform sizes and regular shapes. There is no obvious difference between these two samples, indicating that the OA-stripping step only removed a monolayer of molecules from the NYF nanoplates and it did not affect their sizes and shapes. After the deposition of a CeO2 layer, lots of small NCs can be observed on the particle surface (see Figure 2C and 2D), confirming that a heterogeneous CeO2 layer has been formed. XRD patterns of the NYF nanoplates are shown in Figure 3A. All peaks from the sample can be clearly indexed to pure βphase NaYF4 crystal (JCPDS No. 16-0334). After the deposition of the CeO2 layer, it was found that another set of XRD peaks appeared besides those from the NYF nanoplates (Figure 3B). The new peaks located at 28.55°, 33.08°, 47.47°, and 56.33° (2θ degree) are consistent with the (111), (200), (220), and (311) planes of cubic-phase CeO2 crystal with a fluorite structure (JCPDS No. 34-0394). Evidently, two sets of XRD peaks clearly confirm the formation of pure CeO2 NCs on the NYF nanoplates. Without using NYF nanoplates as the seeds, we also obtained some pure CeO2 NCs under the identical condition (see TEM in Figure S2, SI, and XRD in Figure 3C), proving that the chosen condition is favorable for the growth of small CeO2 NCs with a well crystalline nature.

Figure 1. TEM images of NYF nanoplates before and after CeO2 deposition: (A) NYF nanoplates; (B, C) NYF@CeO2 NPs; (D, E) HRTEM of CeO2 NCs on the particle surface.

of the NYF nanoplates synthesized from organic solvents. The prepared NCs exhibit a uniform size and regular shape, selfassembled on the copper grid. Each NC shows a hexagonal plate-like structure, which is a characteristic shape of β-phase NYF NCs.25 Average diameter of these nanoplates is about 280 nm (diagonal length) and their thickness is around 60 nm. After depositing a CeO2 layer, a rough and concave surface was observed on these nanoplates (see Figure 1B). Under highresolution TEM (HRTEM), one can observe a thin layer consisting of many small NCs (Figure 1C), implying that a heterogeneous layer has been covered on the NYF nanoplates. 2330

DOI: 10.1021/acs.inorgchem.6b03079 Inorg. Chem. 2017, 56, 2328−2336

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Inorganic Chemistry

Based on above observations, the formation process of the CeO2 layer can be interpreted as follows (see Scheme 1). At the beginning, tiny CeO2 nuclei were formed in solution and then migrate to the surface of NYF nanoplates. For lowering their surface energy, these CeO2 nuclei prefer to be attached on the nanoplates surface and thus being deposited on the big NYF nanoplates. As more and more CeO2 nuclei were produced in solution, the newly formed CeO2 nuclei would be sequentially attached on the predeposited CeO2 NCs. As a result, a rough CeO2 layer composed of numerous small NCs was formed on the NYF nanoplates. UC Emissions of NYF@CeO2 NPs. Figure 5A shows the PL spectrum of the NYF nanoplates under the excitation of NIR light, in which two characteristic UV peaks and two vis peaks are displayed. The UV emissions centered at 349 and 362 nm are attributed to the 1I6 → 3F4 and 1D2 → 3H6 transitions of Tm3+ ions, while the two vis emissions located at 450 and 476 nm can be assigned to the 1D2 → 3F4 and 1G4 → 3H6 transitions, respectively.14−16,28,29,39 In comparison with the absorption spectrum of pure CeO2 NCs, one can find that CeO2 NCs have strong absorption in the UV region which matches well with the UV emissions from the NYF nanoplates. Figure 5B gives UC spectra of the NYF nanoplates before and after the CeO2 deposition. As expected, upconverted UV peaks from the NYF nanoplates have been significantly absorbed by the CeO2 layer. In contrast, the blue peaks only have a slight decrease, owing to the relatively weak absorption of CeO2 NCs in this region. The strong absorption of NIR-to-UV peaks suggests that the CeO2 layer can efficiently utilize the UV emissions from the NYF nanoplates under the NIR irradiation. Owing to the direct contact between the core and the shell, a fluorescence resonance energy transfer (FRET) process is preferred in the sample (see Figure S3, SI), which greatly benefits the energy transfer between the NYF core and the CeO2 layer. Photocatalytic Activities of NYF@CeO2 NPs. Photocatalytic activities of the prepared NYF@CeO2 NPs were evaluated by the decolouration of MO solution. Specifically, the samples with different CeO 2 thickness (S1∼S4) were investigated and their photocatalytic performance were displayed in Figure 6A. With a few CeO2 NCs decorated (e.g., S1 and S2), the samples show increasing activities along with the amount of deposited CeO2 NCs. Sample S3 reaches the highest activity when the thickness of the CeO2 layer is around 25 nm under the given photocatalytic condition. Further increasing the amount of CeO2 NCs, the activity of the sample (e.g., S4) becomes decreased along with the CeO2 thickness, suggesting that a suitable thickness is crucial to the optimal activities of the NYF@CeO2 NPs. Such shell thicknessdependent activity can be understood as follows (also see Scheme S1, SI): (1) when sufficient UC emissions are supplied by the NYF nanoplates, the increased thickness in CeO2 NCs can provide more catalytic sites and make full use of the upconverted emissions. (2) When the CeO2 layer becomes thicker, the scattering of incident light will increase and the migration distance of photogenerated e− and h+ from the inner layer to the particle surface becomes longer, leading to some inevitable e − -h + recombination. As a result of these competitions, a suitable thickness of CeO2 layer is preferred for the best photocatalytic performance of the hybrid photocatalysts. The activity of NYF@CeO2 NPs was also assessed with different irradiation bands of the simulated sunlight (Xe lamp),

Figure 3. XRD patterns of (A) NYF nanoplates, (B) NYF@CeO2 NPs, and (C) CeO2 NCs.

Depositing CeO2 Layer with Different Thickness. Through controlling the addition volume of Ce(NO3)3 and HMTA solutions, the growth of the CeO2 layer on the NYF nanoplates can be controlled. Figure 4 displays four typical

Figure 4. Representative TEM image of NYF@CeO2 NP prepared with different volumes of Ce3+/HMTA solutions: (A) 0.02 mL/0.04 mL; (B) 0.1 mL/0.2 mL; (C) 0.2 mL/0.4 mL; (D) 0.4 mL/0.8 mL. Each inset is the corresponding magnified TEM image.

TEM images of single NYF@CeO2 NP obtained with different volumes of Ce(NO3)3 and HMTA solutions (both 0.1 M). As a small volume of solutions were added (0.02 mL/0.04 mL), only a few tiny CeO2 NCs were produced on the particle surface and most of the nanoplates surface is yet bare (Figure 4A). Along with the increase in addition volume, the bare surfaces of NYF nanoplates were gradually covered with CeO2 NCs (Figure 4B). Further increasing the addition volume, the CeO2 layer became thick and compact (Figure 4C,D). After CeO 2 deposition, the particle surface became pretty rough and concave, due to the fact that the CeO2 layer was formed through random attachment of small CeO2 NCs rather than epitaxial growth. 2331

DOI: 10.1021/acs.inorgchem.6b03079 Inorg. Chem. 2017, 56, 2328−2336

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Figure 5. (A) UC spectrum of NYF nanoplates (blue line) and absorption spectrum of CeO2 NCs (red line); (B) UC spectra of NYF nanoplates before and after CeO2 deposition.

Figure 6. Comparison of photocatalytic activities of different samples: (A) S1∼S4 under NIR irradiation; (B) S3 under different irradiation bands; (C) CeO2 NCs and NYF@CeO2 NPs under full solar spectrum; (D) activities of the NYF@CeO2 NPs in recycling experiment.

catalysts also exhibit good recycling activity and can be repeatedly used for at least five rounds (Figure 6D). Enhancing Photocatalytic Activity of NYF@CeO2 NPs by ZnO Coating. For enhancing the photocatalytic activity of the NYF@CeO2 NPs, we have successfully coated a ZnO shell on their surface. The coating of ZnO shell could be sequentially conducted after the CeO2 deposition in the HMTA solution with Zn(NO3)2 as precursor. Under low-magnification TEM, the image contrast of ZnO shell is pretty low and only plate-like particles are observed (Figure 7A,B). However, an obvious shell about 15 nm can be found on the NYF@CeO2 NPs under the HRTEM mode (see Figure7C). To identify the compositions of this new shell, XRD patterns and EDS analysis were performed (see Figures S5 and S6, SI). The XRD patterns shows that a new set of XRD peaks appear, matching well with the hexagonal ZnO crystal (JPCDS No. 36-1451). At the same time, clear Zn signal can be detected in the EDS spectrum along with other elements in the NYF@CeO2 NPs. Both results confirm that the sequentially deposited shell is pure ZnO. Thickness of the ZnO shell can also be increased by adding

using sample S3 as an example (Figure 6B). Under the UV band (300−400 nm), this sample exhibits a pretty high activity, implying that the CeO2 layer can be solely activated by the UV light. Furthermore, notable activity is also observed under the vis band (400−780 nm), due to the absorption edge of CeO2 NCs having some extension to the vis region. Such vis activity of sample can also be applied to the decolouration of toluene solution (see Figure S4, SI), to exclude the possible sensitization effect of MO solution. Accompanying with the NIR band (>780 nm), obviously, the sample displays enhanced activity under the vis-NIR light (400−2500 nm) or full solar spectrum (300−2500 nm) in comparison with that under the vis or UV band, respectively. These results suggest that the hybrid NYF@CeO2 NPs not only are capable of working under individual irradiation band but also can enhance the activity through combining different irradiation bands. The activity of pure CeO2 NCs (see TEM image in Figure S2, SI) was also evaluated under the full solar spectrum, which is obviously lower than the NYF@CeO2 NPs (Figure 6C). Above results confirm the vital contribution of the NYF nanoplates in the hybrid photocatalysts. At the same time, the hybrid photo2332

DOI: 10.1021/acs.inorgchem.6b03079 Inorg. Chem. 2017, 56, 2328−2336

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Strikingly, the data from the NYF@CeO2/ZnO NPs is about 10 times higher than that from NYF@CeO2 NPs. At the same time, we observed that PL spectra of the NYF@CeO2 NPs significantly decreased after the ZnO coating (see Figure S7, SI). These results imply that the sequential deposition of ZnO shell can significantly enhance the spatial separation of photogenerated e− and h+. Figure 8C gives UV−vis diffuse reflectance spectra of the prepared NYF@CeO2 NPs and NYF@CeO2/ZnO NPs, as well as pure ZnO NCs and CeO2 NCs which were prepared under the identical condition (see Figure S2, SI). It is observed that the CeO2 NCs have a stronger and broader absorption in the UV region than the ZnO NCs. After a layer of CeO2 or CeO2/ ZnO was deposited on the NYF nanoplates, strong UV absorption was also exhibited in both samples. In particular, it is worth noting that the absorption edge of NYF@CeO2/ZnO NPs is obviously oblique and a little extended to the Vis region compared with that of the NYF@CeO2 NPs. Based on the Kubelka−Munk (K-M) equation, the optical bandgaps of above samples were estimated (see Figure 8D). The ZnO NCs and CeO2 NCs have a bandgap of 3.22 and 2.89 eV, respectively, close to the values of their bulk counterparts.40,41 Since the NYF nanoplates are insulator materials the NYF@CeO2 NPs have a similar data to that of the pure CeO2 NCs. In the case of CeO2/ZnO double shell codeposited on the NYF nanoplates, the sample displays an absorption edge of 2.78 eV, implying that there is a strong coupling between the CeO2 and ZnO shell. Specifically, the CeO2/ZnO double shell have formed a semiconductor-semiconductor heterojunction on the NYF nanoplates, which results in significantly enhanced separation of photogenerated e− and h+ and a little broadening of light absorption. Possible Photocatalytic Mechanism of the NYF@ CeO2/ZnO NPs. To analyze the mechanism of the above photocatalytic results, the energy band structures of CeO2 NCs and ZnO NCs were explored by XPS valence band spectra and electrochemical Mott−Schottky experiments,42,43 respectively

Figure 7. (A, B) TEM images and (C) HRTEM image of NYF@ CeO2/ZnO NPs at different magnifications; (D) HRTEM image of sample with a thicker ZnO shell.

more HMTA and Zn(NO3)2 solutions during the synthesis (see Figure 7D). Photocatalytic activities of the NYF@CeO2 NPs before and after ZnO deposition were shown in Figure 8A. Obviously, the NYF@CeO2/ZnO NPs display a better activity than the NYF@ CeO2 NPs. In particular, the NYF@CeO2/ZnO NPs show a very fast decolouration rate at the first 40 min during which most of the dye molecules have been decolorized. To elucidate the enhancing mechanism, we performed photocurrent measurements on both samples under the NIR irradiation.

Figure 8. (A) Photocatalytic activities and (B) photocurrent measurements of NYF@CeO2 NPs before and after ZnO deposition; (C) UV−vis spectra and (D) optical bandgaps of CeO2 NCs, ZnO NCs, NYF@CeO2 NPs, and NYF@CeO2/ZnO NPs. 2333

DOI: 10.1021/acs.inorgchem.6b03079 Inorg. Chem. 2017, 56, 2328−2336

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(see Table S1, SI). First, from the XPS data it is clear that the ZnO NCs have a more positive valence band maximum (VBM) than that of the CeO2 NCs. Second, the flat-band potentials (Efb) of both semiconductor NCs could be determined from the Mott−Schottky experiments, which are about −0.21 and −0.15 eV for CeO2 and ZnO, respectively. Since both samples are undoped n-type semiconductors, the conduction band minimum (CBM) for CeO2 and ZnO are estimated to be −0.51 and −0.45 eV. Suggested by these results, it is estimated that a heterojunction could be established owing to the staggered band alignment of both semiconductors.44 To investigate the main reactive species generated from the NYF@CeO2/ZnO photocatalysts, we employed several scavengers such as ethylene diamine teraacetic acid (EDTA), tertiary butanol (t-BuOH) and benzoquinone (BQ) for the detection of h+, •OH and •O2−, respectively (see Figure S8, SI). With the introduction of EDTA and t-BuOH, the decolouration rate of MO solution has just slightly restrained, compared to the control experiment without scavengers. This result indicates that h+ and •OH are not the main reactive species. When BQ was added instead, however, the decolouration rate is extremely restrained, revealing that •O2− is the dominant reactive species. According to the above experimental results, possible working mechanism of the NIR-activated NYF@CeO2/ZnO photocatalysts is illustrated in Scheme 2. Being exposed under

CONCLUSIONS In summary, we have developed a facile approach to deposit the CeO2 layer on the NYF nanoplates. Through striping off the OA molecules, hydrophobic NYF nanoplates became waterdispersible and served as a good substrate for the deposition of CeO2 NCs on their surface under a mild aqueous condition. The prepared NYF@CeO2 NPs showed a uniform morphology and their phases and compositions have been characterized. Photocatalytic activities of the NYF@CeO2 NPs have suggested that these hybrid photocatalysts exhibit obvious activity under the NIR light and enhanced activity under the solar spectrum. Furthermore, we have succeeded in sequentially coating a ZnO shell on the NYF@CeO2 NPs for building a semiconductor heterojunction structure on the UCNs. The developed NYF@ CeO2/ZnO NPs showed significantly enhanced separation of the photogenerated e− and h+ and thus have distinctly improved their photocatalytic activity under the NIR light. A working mechanism of such UCN-semiconductor heterojunction photocatalysts has also been proposed.

Scheme 2. Schematic Illustration of the Working Mechanism of NYF@CeO2/ZnO NPs under the NIR Light

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03079. Additional figures and a table as discussed in the text. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhengquan Li: 0000-0002-0084-5113 Author Contributions †

Equal contribution from C.C., M.T., and M.L.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from National Natural Science Foundation of China (Nos 21273203 and 21603191) and Natural Science Foundation of Zhejiang Province (Nos LR15B010001 and LQ16B010001).

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the NIR light, the NYF nanoplates emit strong UV and vis emissions. Most of these emissions can be absorbed by the CeO2/ZnO shell surrounded, leading to the production of photogenerated e− and h+. Because the CBM of CeO2 layer is more negative than the ZnO shell and its VBM is more positive, a type II heterojunction is thus formed between two deposited layers. The formation of CeO2/ZnO heterojunction can enable an efficient spatial separation of photogenerated e− and h+. As a result, the photogenerated e− from the CeO2 layer will easily migrate to the surface of the ZnO shell and react with oxygen molecules to form •O2− radicals, serving as the main ROS species for the decolouration of MO solution.45 In contrast, some photogenerated h+ from the ZnO may migrate to the CeO2 layer and did not directly participate in the photocatalytic reaction. The spatial separation of photogenerated e− and h+ plays an important role for supplying a steady flow of electrons on the surface of the hybrid photocatalysts and reducing the possibility of surface e−-h+ recombination.

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