Efficient UVâVis-NIR Responsive Upconversion and Plasmonic

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An efficient UV-Vis-NIR responsive upconversion and plasmonic enhanced 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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02806 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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ACS Sustainable Chemistry & Engineering

An efficient UV-Vis-NIR responsive upconversion and plasmonic enhanced photocatalyst based on lanthanide-doped NaYF4/SnO2/Ag

Qingyong Tian,a, # Weijing Yao,a, # Wei Wu,a, b* Jun Liu,a Zhaohui Wu,c Li Liu,a Zhigao Dai,a* Changzhong Jianga* a

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 b

Suzhou Research Institute of Wuhan University, 399 Linquan street, Wuzhong District, Suzhou 215000, Jiangsu Province, P. R. China

c

Hunan Key Laboratory of Applied Environmental Photocatalysis, Changsha

University, 98 Hongshan Road, Kaifu District, Changsha 410005, Hunan Province, P. R. China

# Contributions are equal

Corresponding authors email address:

[email protected] (W. Wu),

*

To whom correspondence should be addressed. Tel: +86-27-68778529. Fax: +86-27-68778433. E-mail: [email protected] (W. Wu), [email protected] (Z. Dai) and [email protected] (C. Jiang).

ACS Paragon Plus Environment


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[email protected] (Z. G. Dai)

[email protected] (C. Z. Jiang)

Abstract Efficiently reclaiming the utilization of solar light in the photocatalysis system remains very challenging. Integrating full advantage of upconversion material, plasmonic

metal

and

narrow

bandgap

semiconductor,

β-NaYF4:18%Yb3+,

2%Tm3+@SnO2@Ag nanoparticles (denoted as NaYF4@SnO2@Ag NPs) are designed and successfully synthesized as 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 will give insight guidance of fabricating efficient, multi-component upconversion catalysts, and propose the potential in the field of high-efficiency environmental and energy-related applications. Key words: upconversion materials, plasmonic photocatalyst, core/shell structure, energy transfer, FDTD calculation. Introduction Currently, the worsening environmental pollution, a global overwhelming crisis,


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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 as 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 converse it to the chemical energy, have been widely applied in photo-decomposition or photo-oxidization of hazardous organic molecule.2-5 The common semiconductors, such as TiO2, ZnO, SnO2 etc., have been widely investigated and applied for their cost-effective and pollution-free.6-9 While, the problem is that no more than 5% of the total solar energy, only the ultraviolet (UV) light can be harvested due to their wide bandgap (> 3 eV).10-11 To solve these issues, many efforts were devoted to efficient reclaim the utilization of solar energy in the past decades. By coupling of narrow bandgap semiconductors, loading noble metals and energy band engineering, the visible light has been 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 its located surface plasmon resonance (LSPR) effect, which arise from collective oscillation of free electrons under appropriate incident light.14-16 Utilization of LSPR will induce critical absorption of visible light to expand 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 intimate contacted



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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 available features of noble metals loading are drastic 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, the α-Fe2O3/Ag/SnO2 nanocomposites exhibit superior photocatalytic activity compared to mono-component.6 However, despite 5% UV and 43% Vis light are available to utilize through LSPR-enhanced metal/semiconductor heterostructures, more than half of the sunlight (52%) in near-infrared (NIR) region has still not been exploited yet. Therefore, it must be of significance to take the NIR light into account and exploit in the photocatalysis system. Rare earth (RE) ions doped upconversion materials, possess the nonlinear optical feature to absorb long wavelength photons (low energy) and emit shorter wavelength photons (high energy) through multiple photon absorption or energy transfer process, have paved a way for the application in biomedical, solar-energy conversion, and anti-counterfeiting.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 matrix now. Doping with different RE ions in the NaYF4 matrix will induce different upconversion


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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+, Tm3+@TiO2 have been prepared for the degradation the organic dyes with higher photocatalytic activity.29-34 Xu et al.35 further decorated NaYF4:Yb3+, Er3+, Tm3+@porous-TiO2 core@shell microspheres with Au nanoparticles (denoted as NYF@TiO2-Au) to fabricate a plasmonic and upconversion enhanced photocatalysts, and tailored the photocatalytic performances with varying Au loading rate. Furthermore, numerous experiments have performed 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 While SnO2, one of the most alternative n-type semiconductor materials with the 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 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



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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-deep understanding of the enhancement. This work will be particularly useful in fabricating of plasmonic-enhanced and UV-Vis-NIR responsive photocatalysts. Experimental 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) are purchased from Sigma-aldrich 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 the Zhang’s work and with modification.42 1 mmol of ReCl3·6H20 (Re = Y, Yb, Tm, Y: Yb: Tm =80: 18: 2) was added into a 50 mL three-necked flask contain with 10 mL of OA and 10 mL of ODE. The solution was heated to 160 ℃ after stirring for 30 min in


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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 down to 50 ℃, 10 mL of 0.5 M NaF methanol solution was injected and starred at this temperature for 30 min to consume the fluoride completely. Subsequently, the mixed solution was heated to 110 ℃ for evaporating the methanol. Finally, the mixed solution was heated to 300 ℃, maintaining 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 for four times. The modification of NaYF4 UCNPs The as-prepared NaYF4 UCNPs were dispersed into the HCl solution with molarity of 0.1 M and ultra-sonicated for 5 min to eliminate the OA molecules. Then the products were centrifuged and re-dispersed into the PVP solution (0.25 g of PVP in 30 mL of water/ ethanol, Vwater/ Vethanol = 1: 1) and stirring for 2h. Then the centrifuged products can be well dispersed in deionized water or ethanol. The preparation β-NaYF4:18% Yb3+, 2% Tm3+ @SnO2 core/ shell nanoparticles (denoted as NaYF4@SnO2 CSNPs) The synthesis method of 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). 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 of Teflon-lined stainless-steel autoclave, and maintained at



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170 °C for 12 h. The precipitates were centrifuged and washed when the reaction finished before drying at 50 °C for 720 min. The 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 was decorated on the surface of SnO2 by reducing of 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 drop-wisely with a constant pressure funnel, and stirring 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 up-conversion emission spectra of the products were 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).


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Photocatalytic Tests The photocatalytic performances of the products were investigated by monitoring the photodegradation of RhB dyes. Briefly, 5 mg of as-obtained samples are dispersed into 10 mL of the RhB solution (10 mg·L-1). Then, the mixture was placed in the dark environment for 30 min with stirring for reaching the absorption/desorption equilibrium. Then, the mixture was irradiated under different light source. A 300 W Xe lamp was simulated solar light source. UV photocatalytic experiments were tested with UV optical filter (λ < 420 nm). Visible light photocatalytic experiments were tested with vis optical filter (λ > 420 nm). NIR photocatalytic experiments were tested with vis a 2 W 980 nm diode laser equipped with 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. Results and discussion Structural and morphological characterization The synthesis of hexagonal-phase NaYF4: Yb3+, 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 capping ligand. Subsequently, the OA molecules coated on the



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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 surface 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 nm 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. Subsequently, the coating of SnO2 shell 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. Firstly, treated-NaYF4 UCNPs were coated by SnO2 layer via a facile hydrothermal method. The formed core-shell structures of NaYF4 UCNPs with 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 SnO2 shell provide large surface, resulting in the favor of loading of noble metals. Then, the Ag NPs are decorated on


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the surface of NaYF4@SnO2 CSNPs via a polyol method by using glucose as the reducing agent. The Ag NPs with average size of ~22.5 nm (Figure S2) are evenly distributed on the surface of NaYF4@SnO2 CSNPs (Figure 2e, inset the single architecture of NaYF4@SnO2@Ag NPs). Furthermore, the decorated Ag NPs, as 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 illustrate in 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), respectively. The crystallographic phase of the NaYF4 UCNPs, NaYF4@SnO2 CSNPs and NaYF4@SnO2@Ag NPs are recorded by XRD patterns (Figure 3). The 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 randomly occupied in the cation sites offered by ordered array of F- ions.43 After coating with 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 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



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(Figure 3c), which are corresponded 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. The high-resolution of Y 3d, Sn 3d and Ad 3d with regard to the samples are revealed in detail using C 1s (Figure S3) as internal reference coordinate. The spectrum of Y 3d in Figure 4b shows two peaks located at 159.11 and 161.11 eV (△ Eg = 2 eV) should be attributed to the Y 3d5/2 and Y 3d3/2. The Sn 3d spectrum (Figure 4c) presents two signal peaks at 486.21 and 494.61 eV (△ Eg = 8.4 eV), ascribed to Sn 3d5/2 and Sn 3d3/2 of Sn4+, respectively. After decorated with Ag nanoparticles, the XPS signal of Ag 3d orbitals (Figure 4d) are observed at 367.21 and 373.21 eV (△ Eg = 6 eV) that 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 of 200 to 1000 nm. While, a sharp peak emerging 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


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around 470 nm, attributing to the LSPR effect of Ag NPs.45-46 Broadband photo-response and efficient photon-harvest ability of NaYF4@SnO2@Ag NPs, lying its potential application in the field of photo-energy. The upconversion luminescence spectra of as-obtained products are performed under 980 nm laser irradiation in Figure 5b. Inset the photograph of NaYF4 UCNPs colloids, a blue color for the UCNPs solution is presented. For the NaYF4 UCNPs colloids, two emission peaks located in ultraviolent region at 347 and 362 nm, two blue emission peaks at 452, 475, and two red emissions centered at 646 and 697 nm are emitted. 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 SnO2 shell, the intensity of the emission peaks demonstrated strongly quench in the ultraviolent regions. The reason is probably caused by the shielding effect of SnO2 shell that concealed the emission of NaYF4 UCNPs and re-absorbed the upconverted emissions in 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 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, therefore, 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



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architecture realizes the NIR excitation to “indirectly” excite SnO2 to improve photocatalytic activity. 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, respectively. 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 successive transfer from Yb3+ ions to populate the 3H5, 3F2 and 1G4 levels of Tm3+. And the populating of the 1D2 should attributed to a cross-relaxation process between Tm3+ ions as illustrated.30 Finally, the excited Tm3+ ions relax non-radiatively or radiatively to the ground state of Tm3+, inducing the upconversion emission: 1I6 → 3F4 (347 nm), 1

D2 → 3H6 (362 nm), 1D2 → 3F4 (452 nm), 1G4 → 3H6 (475 nm), 1G4 → 3F4 (646 nm)

and 3F3 → 3H6 (697 nm), respectively. As is well known, the upconversion emission is a two-photon or multi-photon upconversion process. The emission luminescence intensity (Iem) and the excitation power (Iex) follows the proportionated nth power law (eq. 1): I em ∝ ( I ex )

n

eq. 1

where n represents the number of sequential photons required to excite the upconversion process.24 The slopes of the double logarithmic plot at 475, 646 and 697 nm were 1.77, 1.77 and 1.29 (Figure 6b), indicating the upconversion transition process are two-photon involved energy transfer process, respectively. Actually, the energy level diagrams in Figure 6a indicate the upconversion process for the blue


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emission (λpeak = 475 nm) involved three near-infrared photons. The n value is significantly lower than the 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). 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 have 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 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 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 degradation of RhB within 120 min. While, the NaYF4 CUNPs and NaYF4@SnO2 CSNPs show



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slight degradation ability to RhB dyes because of unexploitable of 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 decorated with Ag NPs, the NaYF4@SnO2@Ag NPs represent significant improved photodegradation ability. Although the photodegradation ability under NIR light is much slower than that under UV and visible irradiation, but 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 rate are presented in Figure S4 and Figure 7a′-d′. The comparison of normalized photocatalytic degradation rate 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

eq. 2

where k is the apparent rate constant. The degradation efficiency of RhB decreasing with the follow order: NaYF4@SnO2@Ag NPs > NaYF4@SnO2 CSNPs > NaYF4 UCNPs > bare RhB. For the different light irradiation, the degradation efficiency is in the follow order: Solar > UV > Vis. And all the photocatalysts performed the lowest efficiency under NIR irradiation as the low power density of the laser source (2 W). The stability of NaYF4@SnO2@Ag NPs photocatalysts are further investigated by recycling the photocatalysis of RhB dyes under simulated solar light irradiation. The designed photocatalysts performed well photocatalytic activity after 6 cycles with


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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 LSPR effect of Ag NPs cannot be excited by photons in the UV region. The narrow bandgap semiconductor SnO2 plays the 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 function of Ag and SnO2 crystal are 4.26 eV 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 When exposure to the 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 the 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 combined 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 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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SnO2 will be unhindered in our designed photocatalyst. When the NaYF4@SnO2@Ag NPs exposed under the NIR irradiation, the NaYF4 core trend to predominate within the NIR photoactivity. NaYF4 UCNPs can be excited by lasers excitation via anti-Stokes photoluminescence process, to convert low-energy NIR excitation (980 nm) to ultraviolet (347 and 362 nm) and visible emissions (452, 475, 646 and 697 nm), as show in the upconversion luminescence spectra in Figure 5b. During, the ultraviolet emissions can be directly re-absorbed by SnO2 shell to generate electron-hole pairs, and the visible emissions could efficient excited the LSPR effect of Ag NPs to product hot electrons and further excited the SnO2 shell. Finally, the carriers immigrate to the surface and react to yield free radicals and then degrade RhB molecules. For the practical application, 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 harvest the UV light and generate electron-hole pairs directly. And the decorated Ag nanoparticles will take full advantage of the LSPR effect to employ the light in visible region and transfer the resonance energy to 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


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Page 19 of 45

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emissions excited from NaYF4 core under NIR irradiation can be re-absorbed by surrounding SnO2 shell and Ag NPs, which further enhance the excitation of SnO2 and LSPR effect of Ag NPs, respectively. The synergism effect results in high yielding rate of the carriers. The core-shell architecture even benefited for the efficient transportation and separation of generated carriers. Finally, the designed full-spectral-responsive upconversion photocatalysts NaYF4@SnO2@Ag core/shell nanoparticles demonstrate efficient photocatalytic performance. For 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. 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. And different incident wavelengths at 375 nm, 475 nm and 980 nm are simulated the UV, Vis and NIR light source, respectively. Comparatively, a general enhancement of electric field for NaYF4@SnO2@Ag NPs are observed after decorated with Ag NPs (Figure 9). The electric field presented the strongest intensity at the junction of SnO2 shell and Ag NPs, which 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 pair would be increased thrillingly.57 Thus, a rising number of photo-induced carriers are generated at the junction of SnO2 and Ag NPs. In addition, the polarized electric field



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Page 20 of 45

of the plasmonic Ag NPs, which can further improve the separation for photogenerated carriers in SnO2.52, 58 Accordingly, the excited LSPR is responsible for the enhanced photocatalytic performance under the 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 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+ and Tm3+ doped hexagonal NaYF4 UCNPs with uniform nanoplates are synthesized via chemical modified method. Then, composited with common semiconductor SnO2 and noble mental 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 re-absorption 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 completely degradation of RhB dyes within 10 minutes, and performed well photocatalytic activity after 6 cycles. FDTD simulation is further


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revealed the plasmonic enhanced photocatalytic mechanism. Efficient photocatalytic activity and good stability indicate the further application in the environmental and photo-energy related fields. The work represents the first demonstration of combining upconversion material with SnO2 and plasmonic Ag NPs for designing an efficient UV-Vis-NIR responsive, good stabilized photocatalyst. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. The sizes distribution of NaYF4 UCNPs and Ag NPs, XPS spectra, photocatalytic activity and FTDT simulated model. Acknowledgements 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. 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 that they have no competing interests. Reference 1.

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(13),

1612-1619

DOI:

10.1002/adfm.201202148 58. Jiang , J.; Li, H.; Zhang, L., New insight into daylight photocatalysis of AgBr@Ag: synergistic effect between semiconductor photocatalysis and plasmonic photocatalysis.

Chem.

Eur.

J.

2012,

18

(20),

10.1002/chem.201102606


6360-6369

DOI:


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Figure Captions Figure 1 Morphology of NaYF4 UCNPs. (a) SEM image of OA-coated NaYF4 UCNPs, (b) SEM, (c) TEM and (d) HR-TEM image of treated-NaYF4 UCNPs (inset the SEAD pattern) Figure 2 (a) The schematic illustration of the preparation process of NaYF4@SnO2@Ag NPs. (b) SEM, (c) TEM and (d) HR-TEM of NaYF4@SnO2 CSNPs, (e) SEM, (c) TEM and (d) HR-TEM of NaYF4@SnO2@Ag NPs. The insets in

(b,

e)

show

the

single

architecture

of

NaYF4@SnO2

CSNPs

and

NaYF4@SnO2@Ag NPs, the scalebar is 100 nm. Figure 3 The XRD patterns of (a) NaYF4 UCNPs, (b) NaYF4@SnO2 CSNPs and (c) NaYF4@SnO2@Ag NPs. (inset the standard JCPDS card of β-NaYF4: 28-1192, SnO2: 41-1443 and Ag: 04-0783). Figure

4

The

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. Figure 5 The (a) UV-vis absorption spectra of the products and (b) Upconversion luminescence spectra of the prepared products under 980 nm excitation (inset the photograph of the corresponding nanoparticle colloids). Figure 6 (a) The energy level diagrams of the 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 multi-phonon relaxation processes, respectively. The power dependence of the upconversion emissions in (b)


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NaYF4@SnO2 CSNPs and (c) NaYF4@SnO2@Ag NPs. (d) the longevity of the NaYF4 UCNPs. Figure 7. Photocatalytic degradation of RhB aqueous solution 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 and (f) the degradability of 6 cycling runs in the presence of NaYF4@SnO2@Ag NPs photocatalysts under simulated solar light irradiation. Figure 8 A schematic illustration of the NaYF4@SnO2@Ag NPs photocatalysis mechanisms under UV-Vis-NIR irradiation. Figure 9 The 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.



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Figure 1.


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Figure 2.



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Figure 3.


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Figure 4.



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Figure 5.


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Figure 6.



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Figure 7.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment


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Figure 8.


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Figure 9.



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Tables Table 1 The 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

0.08

0.02

0.52

0.58

NaYF4

0.12

0.11

3.68

0.67

NaYF4@SnO2

3.25

0.20

10.97

11.80

NaYF4@SnO2@Ag

3.42

2.29

57.57

27.00


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TOC-Synopsis

•

Integrating full advantage of upconversion material, plasmonic metal and wide bandgap semiconductor, NaYF4@SnO2@Ag NPs are designed and successfully synthesized as UV-Vis-NIR responsive upconversion and plasmonic enhanced photocatalyst.


Efficient UVâVis-NIR Responsive Upconversion and Plasmonic

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