Controlled Growth of Metal–Organic Framework on Upconversion

Jan 12, 2017 - ... (UCNPs) with MOF to build a NIR-responsive composite photocatalyst. The MOF .... Metal–organic frameworks for solar energy conver...
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Controlled Growth of Metal−Organic Framework on Upconversion Nanocrystals for NIR-Enhanced Photocatalysis Mohua Li, Zhenjian Zheng, Yangqiong Zheng, Cao Cui, Chunxia Li,* and Zhengquan Li* Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, People’s Republic of China S Supporting Information *

ABSTRACT: Development of MOF-based photocatalysts is intriguing research due to their structural flexibility and tremendous catalytic sites, whereas most MOFs only can take use of UV/visible light and lack of response to NIR light. Herein, we present a facile approach to integrate upconversion nanoparticles (UCNPs) with MOF to build a NIR-responsive composite photocatalyst. The MOF shell with controllable thickness can be grown on the UCNPs, thus exhibiting tunable photocatalytic activities under NIR irradiation. Furthermore, we extend visible absorption of the MOF shell by adding −NH2 groups so that the composite photocatalysts have a better utilization of UC emissions and sunlight to improve their activities. The developed composite photocatalysts have been characterized by XRD, TEM, PL, etc., and their photocatalytic performances were systematically explored. The formation and working mechanism of the composite photocatalysts were also elucidated. KEYWORDS: photocatalysts, upconversion, core−shell structure, MOF, MIL-53(Fe)

1. INTRODUCTION Metal−organic frameworks (MOFs) are a class of crystalline nanoporous materials with well-defined pore structures and tunable chemical properties.1 Their unique properties such as high surface area and structural flexibility have endowed them a widespread application ranging from gas storage, separation, sensors, drug delivery, to catalysis.2−5 Recently, increasing interest has been focused on exploring the MOF materials as a new type of photocatalysts.6−8 In contrast with traditional inorganic photocatalysts, tremendous catalytically active sites (metals and/or linkers) exist throughout the MOF materials, and they can be reached via their open channels. Furthermore, many physical properties for the photocatalysts such as lightharvesting, porosity, adsorption, and transportation of guest molecules can be tailored in the MOFs materials. Because of their excellent structural features and tunable properties, development of MOF-based photocatalysts is a hot topic of research fields in materials’ communities in the past few years.9−11 Among diverse developed photocatalysts, Fe-based MOFs have aroused extreme interest because iron is an earthabundant element and these MOFs have lots of merits such as nontoxicity, stability, intrinsically optical absorbance, and low cost.12 In particular, MIL-53(Fe) has a one-dimensional porous structure composed of infinite FeO4(OH)2 octahedra and bisbidentate terephthalate linkers.13 The existence of small and well-dispersive Fe(III)−O clusters in the structure can effectively limit the recombination of photogenerated charge carriers and thus enable it superior photoactivity.14 Besides, the highly abundant Fe(III)−O clusters can naturally respond to © XXXX American Chemical Society

the short-wavelength visible light (ca. 470 nm), making the MIL-53(Fe) a promising visible-light-driven photocatalyst.15 However, like most of the inorganic photocatalysts, most MOF photocatalysts are still unable to take use of the near-infrared (NIR) light for photocatalysis.16−18 Design and synthesis of NIR-responsive MOF-based photocatalysts will thus be a significant and challenging work because the NIR light possesses a big percentage (∼48%) of the solar energy. Upconversion nanoparticles (UCNPs) are a special kind of optical materials, which can absorb NIR photons and emit ultraviolet (UV) and visible emissions.19,20 Because of their unique optical properties, the UCNPs have been intensively used in cutting-edge biomedical research as fluorescent labels, imaging agents, and biomedicines.21−23 Because the MOF photocatalysts can be activated by UV/visible photons, integrating UCNPs with MOF photocatalysts will be a good choice to create composite photocatalysts, which will be able to utilize both UV−visible and NIR lights. Very recently, several UCNP/MOF composites such as UCNP/ZIF-8 have been reported.24 However, it should be noted that these composites are generally used as bifunctional materials in which the UCNPs and MOFs were employed as biolabels and porous drug containers, limiting their application in catalysis.25,26 On the other hand, for constructing a UCNP/MOF photocatalyst, it requires that both UCNPs and MOFs synergistically work together, severing as the light harvest part and catalytic sites, Received: December 8, 2016 Accepted: January 6, 2017

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DOI: 10.1021/acsami.6b15792 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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grown, the intermediate products were collected from the solution by centrifugation with a speed of 7000 rpm for 8 min. To increase the shell thickness of MIL-53(Fe), the above growing process was repeated according to the same protocol until a desirable thickness was obtained. Finally, the products were collected from the solution by centrifuge and washed with ethanol and water, respectively. 2.4. Synthesis of Core−Shell NaYF4:Yb,Tm@NH2-MIL-53(Fe) NPs. The protocol for the preparation of NaYF4:Yb,Tm@NH2-MIL53(Fe) NPs was similar to that of NaYF4:Yb,Tm@MIL-53(Fe) NPs except that 2-aminoterephthalic acid (2-NH2-TPA) instead of TPA was used as organic linker. The shell thickness was also tuned by the layer-by-layer growing method. 2.5. Photocatalytic Measurement. Photocatalytic activities of samples were evaluated by the degradation of three models of dye pollutions (i.e., rhodamine B (RhB), methylene blue (MB), and phenol). The simulated sunlight was supplied by a Xe lamp (Perfect Light PLS-SXE300, power density of 100 mW/cm2), which was equipped with various filters for providing different irradiation bands. In a typical experiment, 50 mL of dye solution (5 × 10−5 M) was loaded in a 100 mL breaker put on a circulating water system. Next, 20 mg of photocatalysts was added to the solution and stirred for 30 min in the dark to reach an adsorption−desorption equilibrium. After that, the solution was exposed to the irradiation of Xe lamp for a given period of time. The real-time concentration of dye molecules in solution was monitored by the UV−vis spectrometer. 2.6. Characterizations. Transmission electron microscopy (TEM) and energy dispersive X-ray (EDS) spectroscopy were performed on a JEOL 2010F TEM operated at 220 keV. Powder Xray diffraction (XRD) was carried out on a Philips X’Pert Pro X-ray diffractometer equipped with Cu Kα radiation. Photoluminescent (PL) spectra were acquired on a Hitachi F-7000 spectrometer equipped with a commercial 980 nm NIR laser. UV−vis absorption spectra were obtained on a Shimazhu UV-2450 UV−vis spectrometer. Fourier transform infrared spectra (FTIR) were acquired on a Nicolet NEXUS670 infrared spectrometer using KBr pellet. The photocurrent measurements were performed on a CHI 660D electrochemical workstation with a three-electrode system using the sample-coated ITO glass as a photoelectrode, a Pt foil as counter electrode, and an Ag/AgCl electrode as reference electrode.

respectively. To our knowledge, very few efforts have been paid to the development of NIR-responsive MOF photocatalysts. In this work, we present a facile approach to synthesize NaYF4:Yb,Tm/MIL-53(Fe) photocatalysts with a uniform core−shell structure. The NaYF4:Yb,Tm can be used as a typical UCNP to simultaneously emit UV and blue emissions upon the NIR excitation, and these emissions can be utilized to activate the MIL-53(Fe) for photocatalysis. Consequently, the developed NaYF4:Yb,Tm/MIL-53(Fe) photocatalyst is able to utilize the NIR light besides the UV−visible light. Furthermore, the core−shell configuration of photocatalysts ensures the stability of composite particles, and the MOF shell enables a high adsorption to the catalytic reagents. Moreover, through using amino-modified organic linkers, we also developed UCNP/NH2-MIL-53(Fe) photocatalysts that exhibit a significantly wide absorption in the visible region. The modified MOF shell can take a better utilization of the UC emissions and sunlight and thus greatly improve their photocatalytic activity. We believe that the rational combination of UCNPs and MOF is a feasible way to develop NIR-responsive photocatalyst, and it will shed some new insight for the design and synthesis of new composite photocatalysts.

2. EXPERIMENTAL SECTION 2.1. Preparation of β-NaYF4:Yb(20%),Tm(0.5%) Nanoplates. Uniform β-phase NaYF4:Yb(20%),Tm(0.5%) nanoplates caped with oleic acid (OA) were prepared with a user-friendly protocol.27 In a typical synthesis, 0.795 mmol of YCl3, 0.20 mmol of YbCl3, and 0.005 mmol of TmCl3 were put into 16 mL of octadecene and 4 mL of OA in a flask. Upon heating to 160 °C, a homogeneous solution was formed, and the solution was then cooled to room temperature. Subsequently, 10 mL of methanol solution containing 4 mmol of NH4F and 2.5 mmol of NaOH was added into the flask and heated to 60−80 °C to evaporate methanol. After that, the solution was degassed at 100 °C for 10 min and then heated to 300 °C under Ar protection. After being kept at 300 °C for 1.5 h, the solution was naturally cooled to room temperature. The product (NaYF4:Yb,Tm nanoplates) was precipitated from the solution with ethanol and then washed with ethanol and cyclohexance twice. 2.2. Surface Modification of NaYF4:Yb,Tm Nanoplates. To create a suitable surface for the growth of MIL-53(Fe), the ligands on the NaYF4:Yb,Tm nanoplates were removed and then coated with a PVP layer. For removing OA ligands, diluted HCl was used as a soft stripping agent through the protonation process.28,29 In a typical process, 0.15 mmol of the OA-capped NaYF4:Yb,Tm UCNPs was dispersed in 10 mL of ethanol, which contains 1.2 mL of HCl (35 wt %) (pH = 1). The UCNPs then were ultrasonically treated in solution for 30 min. After that, the UCNPs were collected from the solution by centrifuge with a speed of 7000 rpm for 8 min. To purify the particle surface, these nanoplates were put in another acidic ethanol solution (pH = 4) and ultrasonically treated for 5 min twice. Washed with ethanol and distilled water, respectively, these nanoplates were disperse in 10 mL of ethanol. For coating a PVP layer, the particle solution was added dropwise into 30 mL of ethanol solution in which 0.030 g of PVP had been dispersed (Mw = 40 000). The solution was then magnetically stirred at room- temperature overnight (12 h). Finally, the products were collected from the solution by centrifuge and washed with ethanol to remove excessive PVP molecules. 2.3. Synthesis of Core−Shell NaYF4:Yb,Tm@MIL-53(Fe) NPs. A layer-by-layer growing method was employed for the creation of MIL-53(Fe) shell on NaYF4:Yb,Tm nanoplates. In a typical procedure, 0.15 mmol of PVP-modified UCNPs was dispersed in 30 mL of absolute ethanol, and then 0.05 mmol of FeCl3 was added. The solution was stirred for 30 min for absorbing Fe3+ on the particle surface. After that, 0.05 mmol of terephthalic acid (TPA) was added to the solution, and the solution was heated to 40 °C and maintained at this temperature for 40 min. After a thin layer of MIL-53(Fe) was

3. RESULTS AND DISCUSSION 3.1. Synthetic Strategy. The synthetic process for the core−shell UCNP@MIL-53(Fe) NPs was illustrated in Scheme 1. At the beginning, uniform NaYF4:Yb,Tm nanoplates were Scheme 1. Schematic Illustration of the Formation Process of UCNP@MIL-53(Fe) NPs

synthesized from the organic solvents. Because these nanoplates were naturally covered with OA ligands after synthesis, they cannot be dispersed in polar solvents, but are required for the sequential growth of MIL-53(Fe) shell. To remove these OA ligands, diluted HCl solution was used to protonate the carboxylate groups of OA molecules and make them leave the particle surface.28 This step can produce nanoplates with a B

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ACS Applied Materials & Interfaces naked surface, which can be directly dispersed in polar solvents. To initiate the MOF growth, we sequentially coated a PVP layer on the naked nanoplates. The PVP layer is able of adsorbing metal ions around the particle surface because of the strong coordination capability of pyrrolyl groups.29,30 When Fe3+ ions are enriched on the particle surface, the nucleation of MIL-53(Fe) will prefer to start on the nanoplates surface after adding organic TPA linkers. As such, the nanoplates can serve as a depositing substrate for the MOF growth, avoiding homogeneous nucleation in the solution. Once a thin MOF layer was formed, continuous growth of the MOF shell can be realized through the layer-by-layer growth, which can prevent the formation of free MOF particles. Different thicknesses of MOF shell can be accomplished by repeating the addition of Fe3+ ions and TPA linkers. 3.2. Sample Characterizations. Figure 1 gives the XRD patterns of the prepared NaYF4:Yb,Tm nanoplates before and

Figure 2. TEM images of (A) OA-capped UCNPs, (B) ligand-free UCNPs, (C) PVP-modified UCNPs, (D,E) UCNP@MIL-53(Fe) NPs at different magnifications, and (F) elemental mappings of particles in (E).

nanoplates remained unchanged (Figure 2B), suggesting that this step did not affect the size or shape of the NaYF4:Yb,Tm nanoplates. Further coated with a PVP layer, uniform morphology of these nanoplates is still preserved (Figure 2C), and they can be well dispersed in polar solvents such as ethanol with a good size-distribution (see Figure S1). When a MOF shell was grown on these nanoplates, the particles surface became obviously coarse and concave (Figure 2D). When magnifying the size, one can see that a heterogeneous shell has been created on the particles (Figure 2E). The MOF shell is about 30 nm in thickness. To confirm the compositions of the prepared core−shell particles, elemental mapping technique was further applied on the prepared particles. As suggested by the results (Figure 2F), elemental Y and F are mainly distributed at the core part, while Fe and O mainly lie around the particles. This result clear reveals that the prepared core− shell NPs consist of NaYF4:Yb,Tm cores and MIL-53(Fe) shells. The removal of OA ligand and sequential addition of PVP on the NaYF4:Yb,Tm nanoplates can be confirmed by the FTIR spectra (Figure S2). Prior to modification, FTIR peaks at 2930 and 2865 cm−1 (−CH2−) and peaks at 1565 and 1463 cm−1 (−COOH) were clearly observed (Figure S2A), indicating the existence of OA ligands on the particle surface.28 After HCl treatment, these peaks has been significantly reduced (Figure S2B), suggesting that most of the OA ligands have been removed from the particle surface. When the naked NaYF4:Yb,Tm nanoplates were mixed with PVP polymer in solution and aged for hours, several characteristic peaks from the CO and −CH2− groups simultaneously emerged (Figure S2C).30 This result implies that a layer of PVP has been attached on the particle surface, providing the particle a new surface for dispersing in polar solvents and absorbing Fe3+ ions on the surface. 3.3. Controlling MOF Shell Thickness. To probe the growing process of the MIL-53(Fe) shell, a series of control samples were collected and measured by TEM after each round of adding Fe3+ ions and TPA linkers. After the first round, it is found that a few small heterogeneous NPs were created on the nanoplates (Figure 3A), showing the nucleation of MOF crystals on the particles surface. After the second round, more heterogeneous NPs were formed and fully covered the particle surface, leading to the formation of a MOF shell of around 16

Figure 1. XRD patterns of the prepared (A) UCNPs, (B) UCNP@ MIL-53(Fe) NPs, and (C) UCNP@NH2-MIL-53(Fe) NPs. The blue asterisks refer to the XRD peaks from MIL-53(Fe).

after coating the MOF shell. All of the diffraction peaks from primitive NaYF4:Yb,Tm nanoplates (Figure 1A) can be clearly indexed to pure β-phase NaYF4 crystal (JCPDS no. 16-0334). The peaks at 17.2°, 30.1°, 30.7°, 43.5°, and 53.7° (2θ degree) are consistent with the (100), (110), (101), (201), and (211) planes of NaYF4 crystal. After the growth of a MIL-53(Fe) shell, the sample exhibits another set of XRD peaks (marked with blue asterisks) in addition to those from the NaYF4:Yb,Tm nanoplates (Figure 1B). The peaks at 9.2°, 12.6°, 17.2°, and 25.5° agreed well with the XRD patterns of MIL-53(Fe) documented in the literature.31 No impurities peaks were observed in the XRD pattern, showing that the MOF shell is also pure in phase. When substituting 2-NH2-TPA for TPA during the MOF growth, the prepared sample exhibits an XRD pattern similar to those of the UCNP@MIL-53(Fe) (Figure 1C), implying that the NH2-MIL-53(Fe) shell has the same crystal structure as that of MIL-53(Fe). This result is reasonable because these two MOFs generally possess the same crystal structure in their bulk samples.15 The morphologies of the samples prepared at different stages are characterized by the TEM and are shown in Figure 2. The OA-coated NaYF4:Yb,Tm NCs (Figure 2A) display an obvious hexagonal plate-like appearance, which is a characteristic morphology of NaYF4 crystal in the β-phase.27 These nanoplates are uniform in size, and the average diameter of them is about 240 nm. When the OA ligands on their surface were stripped off by HCl, the size and morphology of these C

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Figure 4. (A,B) TEM images of UCNP@NH2-MIL-53(Fe) NPs; and (C,D) FTIR spectra and DRS spectra of different prepared samples.

Figure 3. (A−D) TEM images of UCNP@MIL-53(Fe) NPs prepared after 1−4 addition rounds of Fe3+ ions and TPA linkers. (The scale bar in the inset is 50 nm.)

given in Figure 4C. In comparison with pure MIL-53(Fe), both samples have FTIR peaks similar to those of pure MIL-53(Fe). The difference between UCNP@NH 2 -MIL-53(Fe) and UCNP@MIL-53(Fe) is that there are two weak peaks around 3480 and 3380 cm−1, which should be ascribed to the stretching vibrations from −NH2 groups.15 Although there are many similarities that exist in the UCNP@MIL-53(Fe) NPs before and after −NH2 modification, their DRS curves are much different (Figure 4D). Specifically, the main absorption of UCNP@MIL-53(Fe) is in the range of 200−470 nm due to the transitions in the Fe(III)−O clusters, and a sharp peak at 450 nm originated from the transition of (6A1g → 4A1g + 4Eg(G)) in Fe(III).32,33 After modification with −NH2 groups, the absorption of the UCNP@NH2-MIL-53(Fe) NPs has been significantly enhanced, and the absorption edge was extended to 700 nm. This is because the −NH2 groups could serve as an antenna to absorb additional light and transfer energy to the Fe(III)−O clusters.12 The enhanced light absorption in the UCNP@NH2-MIL-53(Fe) NPs should benefit their NIR-driven photocatalytic activity because it has a wide response to the UC emissions from the UCNP cores. 3.6. UC Spectra and Photocurrent. Figure 5A shows the UC spectra of the prepared UCNPs, UCNP@MIL-53(Fe), and UCNP@NH2-MIL-53(Fe) under the excitation of a 980 nm NIR diode laser. The UCNPs give two UV emissions at 347 and 362 nm and two blue emissions at 452 and 476 nm, due to the transitions of 1I6 → 3F4, 1D2 → 3H6, 1D2 → 3F4, and 1G4 → 3 H6 in Tm3+ ions, respectively.34,35 After being coated with a MIL-53(Fe) shell, two UV emissions were almost completely absorbed by the MOF shell, while two blue emissions were only partially absorbed. When the NH2-MIL-53(Fe) shell was coated, both the UV and the blue emissions were significantly quenched. This result indicates that both MOF shells can take use of the UC emissions from the UCNP cores, and the NH2MIL-53(Fe) shell has a wider response toward the visible region. When the MOF shells were activated by the UC emissions, the capability of generating photogenerated (PG) electrons (e−) and holes (h+) is a key process for their photocatalytic activity. To determine the production of PG charge carries, transient photocurrents of the prepared samples were also measured under NIR irradiation (Figure 5B). Undoubtedly, the photocurrent curves of samples are strongly

nm (Figure 3B). Further repeating the addition of Fe3+ and TPA, a compact MOF shell of 30 nm could be formed after the third round, and the shell continuously increased to 51 nm after the fourth round (Figure 3C and D). The above results indicate that the formation of the MIL-53(Fe) shell undergoes a nucleation, growth, and coalescence process for building a compact MOF shell, other than epitaxial growth. At the same time, the thickness of the MIL-53(Fe) shell can be facilely regulated by the cycling addition of Fe3+ ions and TPA linkers. 3.4. Role of PVP Polymer. During the synthesis, the PVP polymer plays a very important role in the formation of uniform and compact MOF shell. Using naked NaYF4:Yb,Tm nanoplates as seeds (without PVP modification), no obvious nucleation of MIL-53(Fe) on the particles was observed even after two rounds of addition of Fe3+ and TPA (Figure S3A and B). This phenomenon confirms that the PVP layer can serve as nucleation sites for the MOF growth, due to the fact that the Fe3+ ions can be absorbed by PVP and then nucleate on the polymer upon TPA addition. Otherwise, homogeneous nucleation of MOF would happen in the solution rather than heterogeneous nucleation on the particle surface. Some irregular MOF crystals on the nanoplates only could be found after the third addition round, and they could not form a compact shell (Figure S3C and D). The MOF crystals might be formed in solution via homogeneous nucleation and then attached on the nanoplates. The above results show that PVP can favor the nucleation and growth of the MIL-53(Fe) on the nanoplates and finally produce the compact MOF shell. 3.5. Modifying MOF with −NH2 Groups. According to the same synthetic protocol, core−shell UCNP@NH2-MIL53(Fe) NPs were also synthesized by replacing TPA with 2NH2-TPA. Figure 4A and B shows the TEM images of UCNP@NH2-MIL-53(Fe) NPs prepared after three addition rounds. Obviously, the core−shell morphology of this product is very similar to that of UCNP@MIL-53(Fe), and their thickness is also around 30 nm. Accompanied by the XRD results (Figure 1C), the results suggest that the introduction of −NH2 groups in the MOF shell affects neither their morphology nor their phase. FTIR spectra of the UCNP@ MIL-53(Fe) NPs with or without −NH2 modification were also D

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Figure 6. (A) Degradation profiles of RhB solution catalyzed by UCNP@MIL-53(Fe) NPs with different shell thickness; (B) comparison of samples’ activities under the NIR light; (C) comparison of samples’ activities under the full solar spectrum; and (D) histogram of samples’ activities over different dye pollutions (green, UCNP@ MIL-53(Fe); red, UCNP@NH2-MIL-53(Fe)). Figure 5. (A) UC spectra of the prepared UCNPs, UCNP@MIL53(Fe), and UCNP@NH2-MIL-53(Fe) under the NIR excitation; and (B) transient photocurrent response of samples under the NIR irradiation.

Considering that the MOF shells can individually function under the UV−visible light, the activities of the prepared samples were also evaluated under the simulated solar spectrum. In comparison with pure MOF samples, the prepared UCNP@MOFs NPs exhibit an enhanced activity, due to the synergistic utilization of both NIR light and UV−visible light (Figures 6C and S4). Besides RhB, the activities of UCNP@ MIL-Fe(53) and UCNP@NH2-MIL-Fe(53) were also assessed by other dye pollutions such as MB and phenol (Figure 6D). It is found that the prepared UCNP@MOF NPs can also effectively degrade these dyes, showing a general activity toward all of these dyes. Among both samples, the UCNP@NH2-MILFe(53) NPs display a better activity than the UCNP@MILFe(53) NPs. This is due to the fact that the −NH2-modified MOF shell has a better utilization of UC emissions under the NIR light and broadening absorption to visible light under the full solar spectrum. 3.8. Photocatalytic Mechanism. To understand photocatalytic mechanism of the prepared UCNP@MOF NPs, reactive species generated from the samples in the dye degradation were identified. Three scavengers, ethylenediamineteraacetic acid (EDTA), isopropyl alcohol (IPA), and benzoquinone (BQ), were used for the detection of PG h+, • OH, and • O2−, respectively (Figure S5).36 With the introduction of IPA, the degradation rate of RhB just slightly reduced, indicating that •OH is not the main reactive specie. When BQ or EDTA was added, the degradation rate of dye was significantly restrained, revealing that •O2− and h+ are the main reactive species. According to the absorption spectrum, the optical band gap of the UCNP@MIL-53(Fe) was calculated to be 2.72 eV (Figure S6), which is consistent with the data of pure MIL53(Fe) reported elsewhere.37 This result implies that the band structure of the UCNP@MOF is mainly determined by the MOF shell. As documented in the literature, valence band (VB) and conduction band (CB) of MIL-53(Fe) are −0.4 and 2.32 eV, respectively. Comparing the CB and VB bands with the redox potentials of RhB (1.43 V vs NHE), •OH/OH− (2.38 V

correlated with the light irradiation, confirming that the MOF shells can efficiently produce PG e− and h+ by utilizing the UC emissions. Furthermore, the photocurrent curves from both samples are very stable during the switching on/off operation, implying that both samples have a stable photoelectrical performance, which should benefit their photocatalytic stability. 3.7. Photocatalytic Activities. Photocatalytic properties of the prepared samples were evaluated by the degradation of different dye pollutions. Using RhB as an example, we first evaluated the thickness effect of MOF shell on its photocatalytic activity. As shown in Figure 6A, the activity of UCNP@MIL-53(Fe) strongly depends on the MOF thickness, and it gradually increases along with the increase of MOF layer and then decreases. This shell-dependent activity can be understood by the competition of catalytic sites and the light scattering effect. As the shell increases, more catalytic sites are supplied by the MOF shell, and more UC emissions can be utilized. However, notable light scattering will happen if the shell is pretty thick, and the incident NIR light will become weakened for the excitation of UCNP cores. As such, the UCNP@MIL-53(Fe) NPs with a 30 nm thickness (after threeround preparation) show the best photocatalytic activity among these samples. With the same amount of MOF content (molar weight), we also evaluated the photocatalytic activities of pure MIL-Fe(53) and NH2-MIL-53(Fe) (Figure 6B). Unlike their UCNP@ MOFs counterparts, pure MOFs did not show obvious activity under the NIR light, suggesting that both the UCNP cores and the MOF shells are indispensable for the NIR-activity of the core−shell samples. Prior to light irradiation, however, all four samples show obvious adsorption to the dye molecules, due to the high surface of MOF structures. This feature should benefit their photocatalytic properties because the adsorption of reagents is one of the key steps for heterogeneous catalysis. E

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vs NHE), and •O2−/O2 (−0.33 V vs NHE),38 it can be inferred that PG h+ will directly decolorize the RhB molecules. For the PG e−, it will produce •O2− radicals as another main reactive species rather than forming •OH radicals, which requires a more negative VB band. On the basis of all of the collected experimental results, a photocatalytic mechanism for the UCNP@MIL-53(Fe) NPs was proposed in Scheme 2. Under the excitation of 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/acsami.6b15792. Size distribution of UCNPs, FTIR spectra of samples, TEM images of control samples, activity of samples under UV−visible band, ROS determination, band gap of MIL-53(Fe), and luminescence decay curves of samples (PDF)

Scheme 2. Schematic Illustration of the Photocatalytic Mechanism of the Prepared UCNP@MIL-53(Fe) NPs

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhengquan Li: 0000-0002-0084-5113 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.L. acknowledges financial support from the National Nature Science Foundation of China (no. 21273203) and the Natural Science Foundation of Zhejiang Province (no. R15B010001). C.L. is thankful for financial support from the National Nature Science Foundation of China (nos. 51422209 and 51572258).

the UCNPs absorb the NIR photons, and the electrons in Tm3+ ions are excited to high energy levels. On the basis of timeresolved fluorescence dynamic curves (Figure S7), one can find that the relaxation of excited electrons in the UCNPs has been greatly accelerated after coating with a MOF shell. It suggests that most of the excited electrons may transfer to the surrounding MOF shell via an efficient fluorescent resonance energy transfer (FRET) process.39 This ET transfer process can better utilize the excited electrons from UCNPs than the traditional radiation−reabsorption process. The MIL-53(Fe) shell, specifically the Fe(III)−O clusters, then is activated by the excited UCNPs and sequentially produces PG e− and h+. When the PG e− migrate to the particle surface, they will react with surrounding O2 molecules and produce •O2− radicals, which can serve as a typical oxidant for the dye degradation. On the other hand, when the PG h+ migrates to the particle surface, they will directly work as oxidant to the dyes, which has a redox potential lower than 2.32 eV. Besides working under the NIR light, the MOF shell is able to be activated by the UV and visible lights. Therefore, the UCNP@MOFs composites will show an enhanced activity under the full solar spectrum.

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REFERENCES

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4. CONCLUSIONS In summary, we have developed a facile approach to synthesize UCNP@MOF photocatalysts through surface modification of the UCNPs and sequential coating of PVP polymer. Uniform core−shell composite photocatalysts have been developed via this approach, and the samples were characterized by various techniques. The developed photocatalysts exhibit an enhanced activity under the solar spectrum due to their response to the NIR light. Through controlling the MOF thickness, the NIRresponsive activity of samples can be regulated. At the same time, their activity has been further improved by modifying the MOF shell with −NH2 groups. The working mechanism of the UCNP@MOF photocatalysts has also been elucidated through the PL, absorption, photocurrent, and scavenger experiments. F

DOI: 10.1021/acsami.6b15792 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b15792 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX