Efficient UV–Vis-NIR Responsive Upconversion and Plasmonic

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Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10889-10899

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Efficient UV−Vis-NIR Responsive Upconversion and PlasmonicEnhanced Photocatalyst Based on Lanthanide-Doped NaYF4/SnO2/ Ag Qingyong Tian,†,∥ Weijing Yao,†,∥ Wei Wu,*,†,‡ Jun Liu,† Zhaohui Wu,§ Li Liu,† Zhigao Dai,*,† and Changzhong Jiang*,†

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School of Printing and Packaging and School of Physics and Technology, Wuhan University, 299 Bayi Road, Wuchang District, Wuhan 430072, Hubei Province, P. R. China ‡ Suzhou Research Institute of Wuhan University, 399 Linquan Street, Wuzhong District, Suzhou 215000, Jiangsu Province, P. R. China § Hunan Key Laboratory of Applied Environmental Photocatalysis, Changsha University, 98 Hongshan Road, Kaifu District, Changsha 410005, Hunan Province, P. R. China S Supporting Information *

ABSTRACT: Efficiently reclaiming the utilization of solar light in a photocatalysis system remains very challenging. By integrating full advantage of upconversion material, plasmonic metals, and narrow bandgap semiconductors, β-NaYF4:18%Yb3+ and 2%Tm3+@SnO2@Ag nanoparticles (denoted as NaYF4@SnO2@Ag NPs) are designed and successfully synthesized as a wide-spectral (UV−vis-NIR) responsive upconversion and plasmonic-enhanced photocatalyst. The as-obtained NaYF4@SnO2@ Ag NPs present broadband optical absorption dimension, excellent photocatalytic efficiency, and good stability for the degradation of organic dyes. The enhanced photocatalytic performance of NaYF4@SnO2@Ag NPs can be attributed to the synergistic effects of the components composed in this core/shell architecture that result in higher photocarriers yield and favor the efficient transfer of photocarriers and energy. This work gives insight to guidance of fabricating efficient, multicomponent upconversion catalysts and proposes a potential in the field of high-efficiency environmental and energy-related applications. KEYWORDS: Upconversion materials, Plasmonic photocatalyst, Core/shell structure, Energy transfer, FDTD calculation

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successfully employed in the photocatalyst system. 7,12,13 Particularly, loading of noble metals is a fairly practical approach to effectively increase the absorption of visible light because of its located surface plasmon resonance (LSPR) effect, which arises from collective oscillation of free electrons under appropriate incident light.14−16 Utilization of LSPR will induce critical absorption of visible light to expand the light absorption dimension. Therefore, solar energy conversion can be improved efficiently through plasmonic energy-transfer enhancement.11 During the plasmonic energy-transfer process, the LSPR-excited hot electrons would transfer from the metal to the intimately contacted semiconductors, facilitating efficient charges separation in the metal/semiconductor heterostructures.17,18 Furthermore, LSPR-induced strong localized surface electromagnetic fields can considerably excite the surrounding semiconductors to further yield electron−hole pairs.19 It is noteworthy that all the

INTRODUCTION

Currently, worsening environmental pollution, a global overwhelming crisis, threatens the health of human beings and sustainable development of natural ecosystems. Thus, environmental cleaning and remediation is an urgent task.1 Photocatalysis based on semiconductors is considered a valuable and promising technique for environmental protection and remediation of contaminants. The semiconductor-based photocatalysts, which can use the energy of natural sunlight directly and convert it to chemical energy, have been widely applied in photodecomposition or photo-oxidization of hazardous organic molecules.2−5 Common semiconductors, such as TiO2, ZnO, SnO2 etc., have been widely investigated and applied for being cost-effective and pollution-free.6−9 However, a problem is that no more than 5% of the total solar energy, only the ultraviolet (UV) light, can be harvested due to wide bandgaps (>3 eV).10,11 To solve these issues, many efforts were devoted to efficiently reclaim the utilization of solar energy in the past decades. By the coupling of narrow bandgap semiconductors, loading noble metals, and energy band engineering, visible light has been © 2017 American Chemical Society

Received: August 15, 2017 Revised: September 11, 2017 Published: September 21, 2017 10889

DOI: 10.1021/acssuschemeng.7b02806 ACS Sustainable Chem. Eng. 2017, 5, 10889−10899

ACS Sustainable Chemistry & Engineering

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available features of noble metals loading are drastically beneficial for the photocatalytic performance of plasmonic metal/semiconductor photocatalysts. Actually, metal/semiconductor photocatalytic heterostructures have gained increasing interest and attention in the application of photocatalysis. For example, αFe2O3/Ag/SnO2 nanocomposites exhibit superior photocatalytic activity compared to monocomponents.6 However, despite 5% UV and 43% visible light available to utilize through LSPRenhanced metal/semiconductor heterostructures, more than half of the sunlight (52%) in the near-infrared (NIR) region has still not been exploited. Therefore, it must be of significance to take the NIR light into account and exploit it in the photocatalysis system. Rare earth (RE) ions-doped upconversion materials possess a nonlinear optical feature to absorb long wavelength photons (low energy) and emit shorter wavelength photons (high energy) through multiple photon absorption or energy transfer processes and have paved a way for their application in biomedical, solar-energy conversion, and anticounterfeiting.10−25 The hexagonal phase NaYF4, occupying the advantage of wide energy bandgap (∼8 eV)26 and a very low phonon energy (∼360 cm−1 ≈45 meV),27,28 is considered as one of the most efficient upconversion matrixes now. Doping with different RE ions in the NaYF4 matrix will induce different upconversion emissions under the irradiation of NIR light. To overcome the low utilization of solar energy of semiconductor-based photocatalysts, hexagonal phase NaYF4 has been combined with common semiconductors to expand the absorption threshold to the NIR region. For instance, upconversion-based NIR-driven photocatalysts NaYF4:Yb3+, and Tm3+@TiO2 have been prepared for the degradation organic dyes with higher photocatalytic activity.29−34 Xu et al.35 further decorated NaYF4:Yb3+, Er3+, and Tm3+@porous-TiO2 core@shell microspheres with Au nanoparticles (denoted as NYF@TiO2 -Au) to fabricate plasmonic and upconversion-enhanced photocatalysts and tailored the photocatalytic performances with varying Au loading rate. Furthermore, numerous experiments have shown that the upconversion luminescence can be improved through coupling with noble nanoparticles.36,37 Therefore, it will be a wonderful proposition to combine upconversion materials with metal/ semiconductor heterostructures, serving as an efficient agriculture of photocatalysts. The conventional semiconductors, like ZnO and CdS, combined with upconversion materials as photocatalysts have been reported.38,39 Although SnO2, one of the most alternative n-type semiconductor materials with a wide bandgap of 3.6 eV and high electrical conductivity,40,41 has no literature reported heretofore. In this work, we coated SnO2 on the surface of Yb3+- and Tm3+doped hexagonal NaYF4 and further decorated it with Ag NPs to design a novel UV−vis-NIR responsive upconversion and plasmonic-enhanced photocatalyst. The as-designed NaYF4@ SnO2@Ag NPs show broadband absorption, exceptional photocatalytic performance, and good stability for the degradation of RhB dyes. The theoretical finite-difference time-domain (FDTD) calculations further support our interpretations of the plasmonic-enhanced catalytic mechanisms. The involved photocatalysis mechanisms of carriers and energy transfer are proposed to keep in-depth understanding of the enhancement. This work will be particularly useful in fabricating plasmonic-enhanced and UV−vis-NIR responsive photocatalysts.

Research Article

EXPERIMENTAL SECTION

Chemicals. Yttrium chloride hexahydrate (YCl3·6H20), ytterbium(III) chloride hexahydrate (YbCl3·6H20), thulium(III) chloride hexahydrate (TmCl3·6H20), 1-octadecene (ODE, C18H36), oleic acid (OA, C18H34O2), polyvinylpyrrolidone (PVP, MW = 10000), silver nitrate (AgNO3), D(+)-glucose (C6H12O6), and Rhodamine B (RhB) were purchased from Aladdin Reagents Co., Ltd. Sodium tetrafluoroborate (K2SnO3) and urea (CON2H4) were purchased from Sigmaaldrich Reagents Co., Ltd. Sodium fluoride (NaF), hydrochloric acid (HCl), methanol (CH4O), ammonia (NH3·H2O), cyclohexane (C6H12), and ethanol (C2H6O2) were purchased from Sinopharm Chemical Reagent Co., Ltd. Preparation of Hexagonal β-NaYF4:18% Yb3+, 2% Tm3+ Upconversion Nanoplates (denoted as NaYF4 UCNPs). The synthesis of hexagonal-phase NaYF4 UCNPs was carried out by referring to Zhang’s work and with modification.42 Here, 1 mmol of ReCl3·6H20 (Re = Y, Yb, Tm, Y: Yb: Tm = 80:18:2) was added into a 50 mL threenecked flask containing 10 mL of OA and 10 mL of ODE. The solution was heated to 160 °C after stirring for 30 min in an argon atmosphere. Then, the mixture was stirred constantly at this temperature for 60 min to form a homogeneous solution. When the reaction finished and naturally cooled to 50 °C, 10 mL of 0.5 M NaF methanol solution was injected and stirred at this temperature for 30 min to consume the fluoride completely. Subsequently, the mixed solution was heated to 110 °C for evaporating the methanol. Finally, the mixed solution was heated to 300 °C and maintained for 90 min to obtain the NaYF4 UCNPs. Under the protection of argon gas, the mixture was cooled naturally. The products were precipitated by adding 20 mL of ethanol and washed with ethanol/cyclohexane four times. Modification of NaYF4 UCNPs. The as-prepared NaYF4 UCNPs were dispersed into an HCl solution with molarity of 0.1 M and ultrasonicated for 5 min to eliminate OA molecules. Then, the products were centrifuged and redispersed into a PVP solution (0.25 g of PVP in 30 mL of water/ethanol, Vwater/ Vethanol = 1:1) and stirred for 2h. Then, the centrifuged products could be well dispersed in deionized water or ethanol. Preparation β-NaYF4:18% Yb3+, 2% Tm3+ @SnO2 Core/Shell Nanoparticles (denoted as NaYF4@SnO2 CSNPs). The synthesis method of the SnO2 shell was according to our previous work.7 Typically, 50 mg of as-prepared NaYF4 UCNPs were put in an ultrapure water (UPW)/ethanol solution (VUPW:Vethanol = 15.625:9.625 mL). A certain amount of urea and K2SnO3 (murea:mK2SnO3 = 5:1) was added and stirred for 10 min. Finally, the slurry was transferred into a 50 mL Teflon-lined stainless-steel autoclave and maintained at 170 °C for 12 h. The precipitates were centrifuged and washed when the reaction finished before drying at 50 °C for 720 min. Preparation of Ag NPs-Decorated β-NaYF4:18% Yb3+, 2% Tm3+ @SnO2 Core/Shell Nanoparticles (denoted as NaYF4@ SnO2@Ag NPs). Subsequently, the Ag nanoparticles were decorated on the surface of SnO2 by reducing silver nitrate with glucose. Briefly, 20 mg of NaYF4@SnO2 CSNPs, 20 μL of ammonia, and 5.5 mg of silver nitrate was mixed with 20 mL of UPW. After stirring at 30 °C for 1 h, 10 mL of 0.03 mM glucose was added dropwise with a constant pressure funnel and stirred at 30 °C for further 30 min. Then, the precipitates were centrifuged and washed to remove the redundant glucose when the reaction finished before drying under vacuum at 50 °C for 720 min. Characterization. The nanostructures and morphology of the products were characterized by field emission scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM and HR-TEM, JEOL JEM-2010 (HT)), and powder X-ray diffraction (XRD, PANalytical X’Pert PRO). X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Scientific ESCALAB 250Xi system. The UV−vis absorption spectrum was recorded using a Shimadzu 2550 spectrophotometer. The upconversion emission spectra of the products was carried out using a Hitachi F-4600 spectrophotometer of 5J2-0004 equipped with an external CNI (2W) 980 nm IR fiber-coupled laser system (Changchun New Industries Optoelectronics Tech. Co., Ltd.). Photocatalytic Tests. The photocatalytic performances of the products were investigated by monitoring the photodegradation of RhB 10890

DOI: 10.1021/acssuschemeng.7b02806 ACS Sustainable Chem. Eng. 2017, 5, 10889−10899

Research Article

ACS Sustainable Chemistry & Engineering dyes. Briefly, 5 mg of the as-obtained samples are dispersed into 10 mL of the RhB solution (10 mg·L−1). Then, the mixture was placed in a dark environment for 30 min with stirring for reaching the absorption/ desorption equilibrium. Then, the mixture was irradiated under a different light source. A 300 W Xe lamp was the simulated solar light source. UV photocatalytic experiments were tested with a UV optical filter (λ < 420 nm). Visible light photocatalytic experiments were tested with a vis optical filter (λ > 420 nm). NIR photocatalytic experiments were tested with a vis 2 W 980 nm diode laser equipped with a beam expander. A self-constructed cooling water circulator was used to keep the reaction temperature constant. The solution is sampled at varied irradiation time intervals, and the corresponding UV−vis spectra are recorded to monitor the progress of the reaction by using a Shimadzu 2550 spectrophotometer. The photocatalytic stability of NaYF4@ SnO2@Ag NPs was further investigated by recycling the photocatalyst for RhB degradation.

Subsequently, the coating of SnO2 shells and decorating of Ag nanoparticles on the surface of NaYF4 UCNPs were fabricated through simple and reproducible synthesis strategies. The schematic illustration of this process is demonstrated in Figure 2a. First, treated-NaYF4 UCNPs were coated by a SnO2 layer via

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RESULTS AND DISCUSSION Structural and Morphological Characterization. The synthesis of hexagonal-phase NaYF4:Yb3+ and Tm3+ UCNPs was carried out referring to the Zhang’s work and with some modification.42 Herein, NH4F was substituted by NaF as the F source for the synthesis of UCNPs. The as-prepared NaYF4 UCNPs appear to agglomerate together with poor dispersion in Figure 1a, for the OA was used as a capping ligand. Subsequently,

Figure 2. (a) Schematic illustration of the preparation process of NaYF4@SnO2@Ag NPs. (b) SEM, (c) TEM, and (d) HR-TEM of NaYF4@SnO2 CSNPs and (e) SEM, (f) TEM, and (g) HR-TEM of NaYF4@SnO2@Ag NPs. Insets in panels b and e show the single architecture of NaYF4@SnO2 CSNPs and NaYF4@SnO2@Ag NPs. The scalebar is 100 nm.

a facile hydrothermal method. The formed core−shell structures of NaYF4 UCNPs with a porous SnO2 shell are observed in Figure 2b. Furthermore, the porous SnO2 shell layer contributes to the roughness of the prepared core−shell structure, which also is demonstrated by the TEM image (Figure 2c). This porous structure of the SnO2 shell provides a large surface, resulting in the favor of loading of noble metals. Then, Ag NPs are decorated on the surface of NaYF4@SnO2 CSNPs via a polyol method by using glucose as the reducing agent. The Ag NPs with average sizes of ∼22.5 nm (Figure S2) are evenly distributed on the surface of NaYF4@SnO2 CSNPs (Figure 2e; inset is the single architecture of NaYF4@SnO2@Ag NPs). Furthermore, the decorated Ag NPs, marked with blue, are clearly presented on the surface of NaYF4@SnO2 CSNPs (Figure 2f), indicating the successful synthesis of NaYF4@SnO2 CSNPs. Meanwhile, the interplanar distances illustrated in the HR-TEM images are 0.334 nm for the (110) planes of rutile SnO2 (Figure 2d) and 0.236 nm for the (111) planes of cubic Ag (Figure 2g). The crystallographic phase of the NaYF4 UCNPs, NaYF4@ SnO2 CSNPs, and NaYF4@SnO2@Ag NPs are recorded by XRD patterns (Figure 3). Figure 3a presents the XRD diffraction of NaYF4 UCNPs; all positions and intensities could be well corresponded to the hexagonal phase of β-NaYF4 (P63/m, a = 55.96 A, c = 53.53 A), as reported in JCPDS card no. 28-1192. The narrow peaks with high intensity indicate the good crystallinity of hexagonal NaYF4. No other diffraction peaks are observed, indicating the successful synthesis of NaYF4 UCNPs via our modified method. The doped Yb3+ and Tm3+ ions

Figure 1. Morphology of NaYF4 UCNPs: (a) SEM image of OA-coated NaYF4 UCNPs, (b) SEM, (c) TEM, and (d) HR-TEM image of treatedNaYF4 UCNPs (inset: SEAD pattern).

the OA molecules coated on the surface of UCNPs were removed by ultrasonic treatment with HCl solution and further modified with PVP. Then, the treated-NaYF4 UCNPs can well disperse in aqueous or ethanol. According to the high-resolution SEM image of treated-NaYF4 UCNPs (Figure 1b), the hexagonal UCNPs with average edge length of ∼104 nm and thickness of ∼108 nm (Figure S1, Supporting Information) are well observed. Furthermore, the regular hexagonal nanoplate architectures consisted of flat hexagonal top surfaces, and rectangular side surfaces are also observed in the corresponding TEM image (Figure 1c). Additionally, well-defined crystal lattices of NaYF4 UCNPs are also performed in the HR-TEM image (Figure 1d). The clearly marked lattice distance is 0.517 and 0.298 nm, corresponding to the (100) and (110) faces of hexagonal NaYF4 UCNPs. The inset SEAD pattern further demonstrates the perfect hexagonal crystal structure and uniformity of the obtained UCNPs. 10891

DOI: 10.1021/acssuschemeng.7b02806 ACS Sustainable Chem. Eng. 2017, 5, 10889−10899

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ACS Sustainable Chemistry & Engineering

The high-resolutions of Y 3d, Sn 3d, and Ad 3d with regard to the samples are revealed in detail using C 1s (Figure S3) as the internal reference coordinate. The spectrum of Y 3d in Figure 4b shows two peaks located at 159.11 and 161.11 eV (ΔEg = 2 eV) attributed to Y 3d5/2 and Y 3d3/2. The Sn 3d spectrum (Figure 4c) presents two signal peaks at 486.21 and 494.61 eV (ΔEg = 8.4 eV), ascribed to Sn 3d5/2 and Sn 3d3/2 of Sn4+, respectively. After being decorated with Ag nanoparticles, the XPS signal of Ag 3d orbitals (Figure 4d) are observed at 367.21 and 373.21 eV (ΔEg = 6 eV), in accordance with metallic silver.44 Optical Properties Characterization. The UV−vis absorption spectra of NaYF4 UCNPs, NaYF4@SnO2 CSNPs, and NaYF4@SnO2@Ag NPs are examined and compared in Figure 5a. The pure NaYF4 UCNPs have no absorption during the range from 200 to 1000 nm, while a sharp peak arises from 400 nm to the ultraviolet region when examining the NaYF4@ SnO2 CSNPs, corresponding to the intrinsic absorption of SnO2 (≈ 390 nm, 3.2 eV). Notably, a strong and broad absorption ranging in the visible region is performed when using NaYF4@ SnO2@Ag NPs. An absorption peak value is located around 470 nm, attributed to the LSPR effect of Ag NPs.45,46 The broadband photoresponse and efficient photon-harvesting ability of NaYF4@SnO2@Ag NPs shows its potential application in the field of photoenergy. The upconversion luminescence spectra of the as-obtained products are performed under 980 nm laser irradiation in Figure 5b. In the inset of the photograph of NaYF4 UCNPs colloids, a blue color for the UCNPs solution is presented. For the NaYF4 UCNPs colloids, two emission peaks are located in the ultraviolent region at 347 and 362 nm, two blue emission peaks at 452, 475, and two red emissions centered at 646 and 697 nm. The emission fluorescence could be induced by the electronic transitions of Tm3+ consecutively: 1I6 → 3F4, 1D2 → 3H6, 1D2 → 3F4, 1G4 → 3H6, 1G4 → 3F4, and 3F3 → 3H6, respectively.24 After coating with the SnO2 shell, the intensity of

Figure 3. XRD patterns of (a) NaYF4 UCNPs, (b) NaYF4@SnO2 CSNPs, and (c) NaYF4@SnO2@Ag NPs. (Inset: standard JCPDS cards of β-NaYF4: 28-1192, SnO2: 41-1443, and Ag: 04-0783).

randomly occupied in the cation sites offered by an ordered array of F− ions.43 After coating with the SnO2 shell, additional diffraction peaks at 26.61°, 33.89°, and 51.78° (as indicated by the arrows) assigned to the (110), (101), and (211) crystal faces of the rutile phase of SnO2 (JCPDS card no. 41-1443), respectively, are presented in Figure 3b. In contrast, two more diffraction peaks at 38.12° and 44.28° are observed in the XRD pattern of Ag-decorated NaYF4@SnO2 (Figure 3c), which correspond to the (111) and (200) crystal faces of cubic silver (JCPDS card no. 04-0783). The XRD patterns indicate the successful preparation of NaYF4@SnO2@Ag NPs. The composition and valence states of elements on the surface of NaYF4@SnO2 CSNPs and NaYF4@SnO2@Ag NPs are characterized by XPS spectra. The full-scale spectrum depicts the presence of C, O, Na, F, Y, Sn, and Ag elements in Figure 4a.

Figure 4. XPS spectra of NaYF4@SnO2 CSNPs (red line) and NaYF4@SnO2@Ag NPs (blue line): (a) full-scale and high-resolution of (b) Y 3d, (c) Sn 3d, and (d) Ag 3d. 10892

DOI: 10.1021/acssuschemeng.7b02806 ACS Sustainable Chem. Eng. 2017, 5, 10889−10899

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ACS Sustainable Chemistry & Engineering

Figure 5. (a) UV−vis absorption spectra of the products. (b) Upconversion luminescence spectra of the prepared products under 980 nm excitation (inset: photograph of the corresponding nanoparticle colloids).

Figure 6. (a) Energy level diagrams of Tm3+ and Yb3+ dopant ions upconversion mechanisms under 980 nm laser diode excitation. The solid, dotted, and curly arrows represent the emission, energy transfer, and multiphonon relaxation processes, respectively. The power dependence of the upconversion emissions are in (b) NaYF4@SnO2 CSNPs and (c) NaYF4@SnO2@Ag NPs. (d) Longevity of the NaYF4 UCNPs.

The corresponding energy level diagrams of the Tm3+ and Yb3+ dopant ions upconversion mechanisms under 980 nm laser diode excitation are schematically illustrated in Figure 6a. Yb3+ ions act as the sensitizer, and Tm3+ ions act as activator. The pump photons can be absorbed by the Yb3+ ions and excited from ground state 2F7/2 to the excited state 2F5/2. The transited energy would successively transfer from Yb3+ ions to populate the 3H5, 3 F2, and 1G4 levels of Tm3+. The populating of 1D2 should be attributed to a cross-relaxation process between Tm3+ ions as illustrated.30 Finally, the excited Tm3+ ions relax nonradiatively or radiatively to the ground state of Tm3+, inducing the upconversion emission: 1I6 → 3F4 (347 nm), 1D2 → 3H6 (362 nm), 1D2 → 3F4 (452 nm), 1G4 → 3H6 (475 nm), 1G4 → 3F4 (646 nm), and 3F3 → 3H6 (697 nm). As is well known, the upconversion emission is a two-photon or multiphoton upconversion process. The emission luminescence intensity (Iem) and the excitation power (Iex) follows the proportionated nth power law (eq 1):

the emission peaks demonstrated strong quenching in the ultraviolent regions. The reason is probably caused by the shielding effect of the SnO2 shell that concealed the emission of NaYF4 UCNPs and reabsorbed the upconverted emissions in the ultraviolent region.47,48 With further decoration of Ag NPs, the emissions in the integrated blue region are quenched. The intensity of λpeak = 475 nm has dramatically decreased (722.5 to 35.6), indicating the effective excited LSPR effect of Ag nanoparticles.35 Comparing with the UV−vis spectrum of NaYF4@SnO2@Ag nanoparticles in Figure 5a, where the products show intensive absorption around 470 nm, the quenching should also be attributed to the reabsorption of upconverted emissions by Ag nanoparticles. In other words, it indicates the presence of a fluorescent resonance energy transfer (FRET) from the NaYF4 UCNPs to the surrounding SnO2 shell and Ag NPs.48,49 The designed architecture realizes the NIR excitation to “indirectly” excite SnO2 to improve photocatalytic activity. 10893

DOI: 10.1021/acssuschemeng.7b02806 ACS Sustainable Chem. Eng. 2017, 5, 10889−10899

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ACS Sustainable Chemistry & Engineering

Figure 7. Photocatalytic degradation of RhB aqueous solutions and kinetics rates in the presence of various samples under (a, a′) UV, (b, b′) visible, (c, c′) NIR, and (d, d′) simulated solar irradiation. (e) Comparison of normalized photocatalytic degradation rate of RhB in the presence of NaYF4 UCNPs, NaYF4@SnO2 CSNPs, and NaYF4@SnO2@Ag NPs under UV, visible, NIR, and simulated solar irradiation. (f) Degradability of six cycling runs in the presence of NaYF4@SnO2@Ag NPs photocatalysts under simulated solar light irradiation.

Iem ∝ (Iex)n

logarithmic plot at 475, 646, and 697 nm were 1.77, 1.77, and 1.29 (Figure 6b), respectively, indicating that the upconversion transition procese is a two-photon-involved energy transfer process. Actually, the energy level diagrams in Figure 6a indicate

(1)

where n represents the number of sequential photons required to excite the upconversion process.24 The slopes of the double 10894

DOI: 10.1021/acssuschemeng.7b02806 ACS Sustainable Chem. Eng. 2017, 5, 10889−10899

Research Article

ACS Sustainable Chemistry & Engineering that the upconversion process for the blue emission (λpeak = 475 nm) involved three near-infrared photons. The n value is significantly lower than expected. The reason could be the competition mechanism between efficient upconversion processes and linear decay for depletion of intermediate excited states in the β-NaYF4: Yb3+, Tm3+ UCNPs.50 The n values of NaYF4@SnO2 at 475, 646, and 697 nm are calculated to 1.89, 1.87, and 1.28 (Figure 6c), respectively. The increased n values could be attributed to the linear decay of intermediate excited states to the ground state that experience an improvement. The population of these intermediate states become lower.50,51 Furthermore, the longevity of the NaYF4 UCNPs is tested for 160 days at different intervals. Figure 6d records the intensity of the emission at 475 nm, which has no obvious decrease after 160 days, indicating the stable fluorescent property and good longevity of as-prepared NaYF4 UCNPs. Photocatalytic Performance. The photocatalytic performances of the NaYF4 UCNPs, NaYF4@SnO2 CSNPs and NaYF4@ SnO2@Ag NPs are assessed by monitoring the photodegradation of RhB dyes under separated irradiation of UV, visible, NIR irradiation, and simulated solar light (Figure 7). Apparently, the designed NaYF4@SnO2@Ag NPs photocatalysts show the highest photocatalytic performance under irradiation of a different light source. For instance, prior to UV irradiation (Figure 7a), the RhB can be completely photodegraded within 75 min when NaYF4@SnO2@Ag NPs are used as the photocatalysts. The NaYF4@SnO2 CSNPs show slightly weaker photocatalytic activity, while there is no apparent degradation response when examining the NaYF4 UCNPs due to its disabled capture ability of the UV photons. Under visible light irradiation (Figure 7b), the NaYF4@SnO2@Ag NPs can completely degrade of RhB within 120 min, while the NaYF4 CUNPs and NaYF4@ SnO2 CSNPs show a slight degradation ability to RhB dyes because of unexploitable visible light. The photocatalytic degradation under NIR light is tested to investigate the photocatalytic property of designed upconversion photocatalysts (Figure 7c). The NaYF4@SnO2 CSNPs show slight better photodegradation ability than NaYF4 UCNPs. After being decorated with Ag NPs, the NaYF4@SnO2@Ag NPs represent significantly improved photodegradation ability. Although the photodegradation ability under NIR light is much slower than that under UV and visible irradiation, the observations indeed indicate the NIR photoactivity of the designed NaYF4@SnO2@ Ag NPs. Finally, the photocatalysis under simulated solar light is investigated for practical applications. Interestingly, the designed NaYF4@SnO2@Ag NPs show efficient photocatalytic activity to fully degrade the RhB dyes within 10 min in Figure 7d. Corresponding photodegradation and kinetics rates are presented in Figure S4 and Figure 7a′−d′. The comparisons of normalized photocatalytic degradation rates under UV, visible, NIR, and simulated solar irradiation are displayed in Figure 7e and Table 1. The kinetics rate follows a pseudo-first-order expression (eq 2): −ln(C /C0) = kt

Table 1. Corresponding Normalized Photocatalytic Degradation Rates of Bare RhB and in the Presence of NaYF4 UCNPs, NaYF4@SnO2 CSNPs, and NaYF4@SnO2@Ag NPs under UV, Visible, NIR, and Simulated Solar Irradiation Degradation rate (×10−2) Samples

UV (min−1)

Vis (min−1)

NIR (h−1)

Solar (min−1)

RhB NaYF4 NaYF4@SnO2 NaYF4@SnO2@Ag

0.08 0.12 3.25 3.42

0.02 0.11 0.20 2.29

0.52 3.68 10.97 57.57

0.58 0.67 11.80 27.00

by recycling the photocatalysis of RhB dyes under simulated solar light irradiation. The designed photocatalysts performed good photocatalytic activity after six cycles with a duration irradiation of 10 min per cycle (Figure 7f). Taking comprehensive consideration of the enhancement mechanisms, a schematic photocatalysis mechanism under UV− vis-NIR irradiation of NaYF4@SnO2@Ag NPs is proposed in Figure 8. Prior to UV light irradiation, the NaYF4 UCNPs and

Figure 8. Schematic illustration of NaYF4@SnO2@Ag NPs photocatalysis mechanisms under UV−vis-NIR irradiation.

LSPR effect of Ag NPs cannot be excited by photons in the UV region. The narrow bandgap semiconductor SnO2 plays a critical role in the photocatalytic degradation in the NaYF4@SnO2 CSNPs and NaYF4@SnO2@Ag NPs photocatalysts. Reductive electrons and the same amount of highly oxidative holes can be significantly excited at the bandgap of SnO2. When the noble metal Ag NPs contact with the SnO2, an ohmic contact formed at the interface of Ag-SnO2. Because the work functions of Ag and the SnO2 crystal are 4.26 and 4.7 eV, the photoexcited electrons in the conduction band of SnO2 will be injected downhill to the Ag NPs when they get the same Femi level and finally improve the electron−hole separation.52 With exposure to visible light, the LSPR effect of Ag NPs plays a dominant role in the visible photoactivity of NaYF4@SnO2@Ag NPs. The LSPR energy posited higher than that of CB of SnO2, which will facilitate the smooth transfer of excited hot electrons to the CB of SnO2 to yield free superoxide anion radicals (•O2−) for dyes oxidation, while the holes retained in the Ag NPs can directly attack RhB molecules or combine with hydroxyl (H2O/OH−) to form hydroxyl radicals (•OH) for the degradation.53 It has been reported that the probability of energy transfer can be maximally enhanced when the size of the spherical metal particles is around 20 nm.54 Here, the average sizes of Ag NPs is about 22.5 nm; the plasmonic resonance energy transfer from Ag to surrounding SnO2 will be unhindered in our designed photocatalyst. When the NaYF4@SnO2@Ag NPs are exposed under NIR irradiation, the NaYF4 core tends to predominate within the NIR

(2)

where k is the apparent rate constant. The degradation efficiency of RhB decreases with the following order: NaYF4@SnO2@Ag NPs > NaYF4@SnO2 CSNPs > NaYF4 UCNPs > bare RhB. For different light irradiation, the degradation efficiency is in the following order: Solar > UV > Vis. All the photocatalysts performed the lowest efficiency under NIR irradiation with the low power density of the laser source (2 W). The stability of NaYF4@SnO2@Ag NPs photocatalysts are further investigated 10895

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Figure 9. FDTD calculated local electric field enhancement (log |E|) of NaYF4@SnO2 CSNPs and NaYF4@SnO2@Ag NPs at the XY plane at different incident wavelengths of (a, b) 375, (c, d) 475, and (e, f) 980 nm, respectively.

SnO2@Ag core/shell nanoparticles demonstrate efficient photocatalytic performance. To further reveal the origin of the photocatalytic response, the spatial distributions of the electromagnetic field intensity for NaYF4@SnO2 CSNPs and NaYF4@SnO2@Ag NPs are simulated by the FDTD method. The image based on FDTD modeling is presented in Figure S5. The k and E vectors are the incident direction of laser light and the polarization direction, respectively. Different incident wavelengths at 375, 475, and 980 nm simulate the UV, vis and NIR light sources, respectively. Comparatively, a general enhancement of an electric field for NaYF4@SnO2@Ag NPs is observed after being decorated with Ag NPs (Figure 9). The electric field presented the strongest intensity at the junction of the SnO2 shell and Ag NPs, which is induced by the LSPR effect.55,56 The electromagnetic enhancement is |E| = 101.867 = 73.62-fold, where |E| = |Elocal/Ein| and Elocal and Ein are the local and incident electric fields, respectively. With the strong localized plasmonic electromagnetic field enhancement at the junction of SnO2/Ag, the generation rate of electron−hole pairs would be increased substantially.57 Thus, a rising number of photoinduced carriers are generated at the junction of SnO2 and Ag NPs. In addition, the polarized electric field of the plasmonic Ag NPs can further improve the separation for photogenerated carriers in SnO2.52,58 Accordingly, the excited LSPR is responsible for the enhanced photocatalytic performance under vis light irradiation. The local |E| intensity for NaYF4@SnO2@Ag NPs under NIR light irradiation (Figure 9f) is enhanced 101.450 = 28.18 times. Comparatively, the LSPR effect of Ag NPs induce the enhancement of upconversion intensity.36,37

photoactivity. NaYF4 UCNPs can be excited by lasers excitation via the anti-Stokes photoluminescence process to convert lowenergy NIR excitation (980 nm) to ultraviolet (347 and 362 nm) and visible emissions (452, 475, 646, and 697 nm), as shown in the upconversion luminescence spectra in Figure 5b. The ultraviolet emissions can be directly reabsorbed by the SnO2 shell to generate electron−hole pairs, and the visible emissions could efficiently excite the LSPR effect of Ag NPs to product hot electrons and further excite the SnO2 shell. Finally, the carriers migrate to the surface and react to yield free radicals and then degrade RhB molecules. For practical applications, the photocatalytic performances of NaYF4 UCNPs, NaYF4@SnO2 CSNPs, and NaYF4@SnO2@Ag NPs are performed under simulated solar light irradiation. Obvious enhancement in photocatalytic performance is anticipated in the presence of as-designed NaYF4@SnO2@Ag NPs. The enhancement contributed the combination of ternary components and the synergism effect between them. In summary, the wide-gap semiconductor SnO2 shell harvests the UV light and generates electron−hole pairs directly. The decorated Ag nanoparticles will take full advantage of the LSPR effect to employ light in the visible region and transfer the resonance energy to the SnO2 shell. Furthermore, the strong localized surface electric field of Ag NPs could further excite the surrounding semiconductor SnO2 shell to produce more photogenerated charge carriers.19 At the same time, the upconversion emissions excited from the NaYF4 core under NIR irradiation can be reabsorbed by surrounding the SnO2 shell and Ag NPs, which further enhances the excitation of SnO2 and the LSPR effect of Ag NPs, respectively. The synergism effect results in a high yielding rate of the carriers. The core−shell architecture even benefited for the efficient transportation and separation of generated carriers. Finally, the designed fullspectral-responsive upconversion photocatalysts NaYF4@

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CONCLUSIONS In summary, we designed and successfully synthesized a novel UV−vis-NIR responsive upconversion and plasmonic-enhanced photocatalyst NaYF4@SnO2@Ag NPs. Lanthanide ions Yb3+10896

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and Tm3+-doped hexagonal NaYF4 UCNPs with uniform nanoplates are synthesized via a chemical-modified method and then composited with a common semiconductor SnO2 and noble metal Ag NPs to fabricate the multicomponent NaYF4@ SnO2@Ag NPs photocatalyst. The as-designed NaYF4@SnO2@ Ag NPs photocatalysts take full advantage of intrinsic absorption of SnO2, LSPR effect of Ag NPs, localized surface electromagnetic field, upconversion emissions, and reabsorption of upconversion emissions, thus improving the utilization efficiency of solar light extensively. The synergistic effects of the components result in higher yield of photocarriers. Furthermore, this core/shell architecture is beneficial for the generation and separation of the electron−hole pairs. The NaYF4@SnO2@Ag NPs photocatalysts performed superior photocatalytic efficiencies for the complete degradation of RhB dyes within 10 min and performed good photocatalytic activity after six cycles. FDTD simulation has further revealed the plasmonic-enhanced photocatalytic mechanism. Efficient photocatalytic activity and good stability indicate the further application in the environmentaland photoenergy-related fields. This work represents the first demonstration of combining upconversion material with SnO2 and plasmonic Ag NPs for designing an efficient UV−vis-NIR responsive and well-stabilized photocatalyst.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02806. Size distribution of NaYF4 UCNPs and Ag NPs, XPS spectra, photocatalytic activity, and FTDT simulated model. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-27-68778529. Fax: +86-27-68778433. E-mail: [email protected] (W. Wu). *Tel: +86-27-68778529. Fax: +86-27-68778433. E-mail: [email protected] (Z. Dai). *Tel: +86-27-68778529. Fax: +86-27-68778433. E-mail: [email protected] (C. Jiang). ORCID

Wei Wu: 0000-0002-7672-7965 Author Contributions ∥

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S Supporting Information *

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Research Article

Qingyong Tian and Weijing Yao contributions are equal.

Author Contributions

All authors contributed during the preparation of the manuscript. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (51471121), Basic Research Plan Program of Shenzhen City (JCYJ20160517104459444), Natural Science Foundation of Jiangsu Province (BK20160383), and Wuhan University. 10897

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