Decoration of upconversion nanoparticles@mSiO₂ core–shell nanostructures with CdS nanocrystals for excellent infrared light triggered photocatalysis

Abstract

In this work, we demonstrate a facile process to synthesize upconversion nanoparticles (UCNPs)@mesoporous SiO₂ (mSiO₂) core–shell nanoparticles decorated with CdS nanoparticles. The size and morphology of the as-prepared products have been investigated carefully; which show that the UCNPs and CdS nanoparticles are ca. 50 and 5 nm in diameter, respectively, and the mesoporous silica layer is 10 nm in thickness. The fluorescence spectra of the as-prepared UCNPs@mSiO₂/CdS nanoparticles show that the fluorescence emissions (380, 451 and 470 nm) have been greatly quenched via energy transitions from the UCNPs nanoparticles (donors) to the CdS nanoparticles (acceptors). CdS nanoparticles were activated by the UCNPs to produce photo-generated ˙OH radicals under irradiation of infrared (IR) light. Photodegradation towards RhB dyes was studied to demonstrate the photocatalytic properties for the as-prepared UCNPs@mSiO₂/CdS nanoparticles under irradiation of near infrared light. The as-designed nanostructures of UCNPs@mSiO₂/CdS nanoparticles show excellent photocatalytic performance on photodegradation towards RhB under irradiation of infrared light. This kind of nanostructure may find potential applications in photodynamic therapy of cancer cells, for use as nanotransducers, in dyed sensitized solar cells etc.

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1. Introduction

Multifunctional nanostructures with improved physical and chemical properties have attracted worldwide attention.¹ Infrared (IR) light driven photocatalysis is of great scientific and technological interest owing to 53% of incoming solar energy being IR light.² However, most of the photocatalysts including TiO₂,³ ZnO,⁴ CdSe⁵ are driven by UV light. Upconversion nanoparticles (UCNPs) are known as important nanotransducers; which can efficiently absorb two or more two near-infrared (NIR) photons to give UV-Vis-NIR fluorescence emissions.^6,7 In the past two decades, UCNPs have been extensively studied owing to their unique optical properties and used as nanotransducers for wide application in energy transfer,⁸ photocatalysis,⁹ photodynamic therapy¹⁰ etc. To date, much effort has been made to design fluorescence resonance energy transfer (FRET) configurations incorporating UCNPs (donors) and semiconductors (acceptors), for instance, NaYF₄:Yb,Er/CdSe,¹¹ NaYF₄:Yb,Tm(Er)/CdTe,¹² YF₃:Yb,Tm@TiO₂,^13,14 NaYF₄:Yb,Tm@TiO₂,¹⁵ YF₃:Tb,Tm@TiO₂,¹⁶ YF₃:Yb,Tm/TiO₂/graphene,^17,18 NaYF₄:Yb,Tm/Er@SiO₂@TiO₂,¹⁹ BiVO₄/CaF₂:Er,Tm,Yb,²⁰ NaYF₄:Yb,Er/CdSe/ZnS,²¹ and NaYF₄:Yb,Tm/ZnO.^22,23 However, most of the semiconductors including TiO₂, ZnO only can be activated using UV light; which is the obstacle for enhanced the energy transfer efficiency using UCNPs as nanotransducers. In order to improve near infrared (NIR) photo-response and energy transfer efficiency, visible light photo-responded semiconductors have been selected and used as acceptors for energy transfer. Hence, some core–shell nanostructures including NaYF₄:Yb,Er/C–TiO₂,²⁴ NaYF₄:Er,Yb/Bi₂MoO₆,²⁵ NaYF₄:Yb,Er,Tm@porous TiO₂/Au²⁶ and BiOI/ZnWO₄:Er,Tm,Yb,²⁷ Y₂O₃:Yb, Er/Bi₂S₃ (ref. 28) and CaTiO₃/CaF₂/TiO₂:Yb,Er,Tm²⁹ have been achieved for enhanced photocatalytic performance.

Cadmium sulfide (CdS) with a narrow band gap energy of 2.42 eV has been recognized as alternative and important candidate for wide applications in dye-sensitized solar cells,^30,31 fluorescence probes,³² photocatalysts^33,34 and optoelectronic devices^35,36 owing to absorbing most of the visible light in the solar spectrum. Recently, Guo and Qin et al. demonstrated a two-step hydrothermal method to synthesize NaYF₄:Yb,Tm/CdS/TiO₂ composites with a heterojunction structure; which exhibited enhanced IR energy transfer efficiency and photocatalytic performance under irradiation of NIR light.³⁷ Li and Yu et al. developed a solution process to assemble CdS nanoparticles on the surface of NaYF₄:Yb,Tm microrods and to prepare a novel near infrared photocatalyst of NaYF₄:Yb,Tm/CdS nanocomposites; which also showed excellent photocatalytic performance towards degradation on RhB dyes and methylene blue.³⁸

In this work, a facile process has been developed to fabricate UCNPs@mesoporous silica (mSiO₂)/CdS nanocomposites; in which a thin intermediate layer of mesoporous silica was controlled in close proximity (<10 nm) and CdS nanoparticles are controlled with an average size of 5 nm in diameter. The stead-state and dynamic fluorescence of the as-prepared products have been investigated carefully to illustrate the energy transfer mechanism. Upconversion nanoparticles have the potential to absorb the near-infrared (NIR) light in solar energy and improve the photocatalytic performance. Moreover, thin mesoporous silica layer can adsorb and remove lots of dye molecules in wastewater, which has been widely application in catalysis,^39,40 drug delivery,^41,42 separations,^43,44 cell targeting⁴⁵ and etc. In particular, CdS nanoparticles on the surface of mesoporous silica can be activated by UCNPs via energy transfer process under irradiation of infrared light. As expected, the as-prepared UCNPs@mSiO₂/CdS nanoparticles show excellent adsorption ability and photodegradation towards organic dyes (RhB) under irradiation of IR light.

2. Experimental

2.1 Reagents

Sodium hydroxide (NaOH, 96%), ammonium fluoride (NH₄F, 98%), YCl₃·6H₂O (99.99%), YbCl₃·6H₂O (99.99%), TmCl₃·6H₂O (99.99%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), tetraethoxysilane (TEOS, 99%), octadecyltrimethoxysilane (C18TMS, 90%), polyoxyethylene (5) nonylphenylether (Igepal CO-520), terephthalic acid, rhodamine B (RhB, 99%) and cadmium acetate were purchased from Aladdin Chemical Reagent Corporation. Methanol, cyclohexane, hydrazine hydrate, thiourea, ammonia solution and ethanol were purchased from Sinopharm Chemical Reagent Corporation. NaYF₄:Yb(30%),Tm(0.5%)@NaYF₄ core–shell nanoparticles giving strong UV emission have been synthesized via a sequential growth process and dispersed in cyclohexane solution.⁴⁶ All chemicals are of analytical grade and used without further purification.

2.2 Synthesis of NaYF₄:Yb/Tm@NaYF₄@SiO₂ core–shell nanoparticles

NaYF₄:Yb/Tm@NaYF₄@SiO₂ core–shell nanoparticles have been synthesized via a modified protocol.⁴⁷ In a typical procedure, 1.36 g Igepal CO-520 was added into a glass bottle (20 mL in total volume) and dissolved using 5 mL cyclohexane to form clear solution. And then, 5 mL of 0.02 M NaYF₄:Yb/Tm@NaYF₄ nanoparticles cyclohexane solution was added into the above solution to form a transparent solution. Subsequently, 200 μL ammonia hydroxide solution (25–28%, wt%) was added into the previous solution and dispersed by ultrasonication for 6 min. Finally, 50 μL TEOS and 20 μL C18TMS were added into the previous solution and kept at room temperature for 24 h by shaking. The nanoparticles were precipitated using ethanol and collected by centrifugation, and then air dried in oven at 60 °C for 3 h.

2.3 Synthesis of NaYF₄:Yb/Tm@NaYF₄@mesoporous silica (mSiO₂) core–shell nanoparticles decorated with CdS nanocrystals

0.16 mmol of cadmium acetate was added into a 10 mL phial containing 2 mL of ethanol and 1.7 mL of hydrazine hydrate to form a clear solution, and then 0.045 g NaYF₄:Yb/Tm@NaYF₄@SiO₂ nanoparticles were added into the solution with vigorous stirring. Subsequently, 1 mL ethanol solution containing 0.32 mmol thiourea was added dropwise into the previous suspension solution with magnetic stirring; which was kept at room temperature for 7 h. The product was collected by centrifugation and washed with ethanol and distilled water for three times; respectively. The NaYF₄:Yb/Tm@NaYF₄@SiO₂/CdS core–shell nanoparticles have been achieved and dried at 60 °C for 5 h. The as-synthesized NaYF₄:Yb/Tm@NaYF₄@SiO₂/CdS nanoparticles was placed into the Muffle furnace and then calcined at 500 °C for 2 h with a heating rate of 1 °C min⁻¹ to form the NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdO nanoparticles. Finally, the NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles were obtained by a previous sulfidization process.⁴⁸

2.4 Photocatalytic activity

30 mg NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles were dispersed in 30 mL of RhB solution (1 mg L⁻¹) and stirred in the dark for 30 min to reach adsorption/desorption equilibrium. After that, the RhB solution was under irradiation of a 1500 mW cm⁻² xenon lamp with wavelength of 320–1100 nm equipped with (or without) an infrared filter under stirring. For comparison, the photocatalytic experiments were also carried out under irradiation of a continuous wave (CW) laser of 980 nm. The distance between of beaker and the Xe lamp or laser is 5 cm.

2.5 Characterization

The as-prepared samples were well dispersed in ethanol solution and deposited on carbon coated copper grids for transmission electron microscope (TEM) measurements; which were carried out on a JEOL-2010 transmission microscopy with an acceleration voltage of 200 kV. The phase and chemical composition of the as-prepared products have been investigated by X-ray powder diffraction (XRD) and X-ray photoelectron spectra (XPS, ESCALab 250Xi); respectively. X-ray powder diffraction (XRD) was carried out on a Siemens D5005 X-ray powder diffractometer equipped with Cu K_α radiation (λ = 1.78897 Å) with a scan speed of 8 s per step and the step size was 0.02°. Upconversion fluorescent spectra were measured on an Edinburgh FLS980 fluorescence spectrometer using an external CW laser of 980 nm. UV-Vis absorption spectra were obtained using a Hitachi U-5100 spectrophotometer (Hitachi High-Technology Corporation, Japan) and calibrated using the aqueous solution.

3. Results and discussion

3.1 Synthesis and characterization of NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles

The synthetic process for upconversion nanoparticles@mesoporous silica (UCNPs@mSiO₂) core–shell nanoparticles decorated with CdS nanocrystals was illustrated in Fig. 1a. As shown in Fig. 1a, a thin layer of amorphous silica was coated on the surface of UCNPs via micro-emulsion process using TEOS and C18TMS as mesoporous silica source.⁴⁹ Subsequently, CdS nanocrystals with several nanometers in diameter were decorated on the surface of silica nanoparticles to fabricate UCNPs@SiO₂/CdS nanostructures according to our previously reported protocol.⁴⁸ The UCNPs@mSiO₂/CdS nanoparticles can be achieved via calcination and sulfidation process. In the present study, NaYF₄:Yb/Tm@NaYF₄ core–shell nanocrystals (Fig. S1, in the ESI†) with 50 nm in diameter have been achieved via a sequential growth process and used as nanotransducers for energy transfer, which give strong UV and blue emissions under excitation of near infrared light (980 nm). Fig. 1b shows the TEM image of NaYF₄:Yb/Tm@NaYF₄@SiO₂ core–shell nanoparticles and the uniform and thin layer of silica is 8 nm in thickness. The silica layer was controlled thinner than 10 nm, which has been prove to be a appropriated distance for efficient FRET configurations incorporating UCNPs and quantum dots for spectroscopy and imaging-based biosensing diagnostics,⁵⁰ and cellular imaging,⁵¹ sensing,^52,53 photocatalysis,⁵⁴ and etc. As shown in Fig. 1c and d, a lot of small nanoparticles with ca. 5 nm in diameter were coated on the surface of NaYF₄:Yb/Tm@NaYF₄@mSiO₂ core–shell nanoparticles. Moreover, the small nanoparticles are crystalline with a clear lattice fringes with 0.21 nm; which can be indexed to the (110) plane of hexagonal phase of CdS (Fig. S2, in the ESI†). The X-rays diffraction pattern (XRD) of the as-prepared final product has been shown in Fig. S3 (in the ESI†); in which the strong diffraction peaks are ascribed to the hexagonal phase of NaYF₄ (JCPDS no. 16-0344) and the weak diffraction peaks located at 24.81, 26.51 and 28.20° can be indexed to the crystal planes (100), (002) and (101) of hexagonal CdS (JCPDS no. 41-1049).⁴⁸ Fig. 1e–h show that the scanning transmission electron microscopy image (STEM) and elemental mapping images of F, Si, Cd. As shown in Fig. 1i, the merge image of the elemental mapping for F, Si, Cd demonstrate that the elements of Si and Cd are located outside of the NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS core–shell nanoparticles. Moreover, the chemical composition of the as-prepared NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles has been investigated by energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectra (XPS) (Fig. S4 and S5 and Table S1 in the ESI†). All these approved that the NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanospheres have been synthesized successfully.

Fig. 1 (a) Schematic illustration the synthetic process for UCNPs@mSiO₂/CdS nanospheres. (b) TEM image of amorphous silica coated NaYF₄:Yb/Tm@NaYF₄ core–shell nanoparticles. (c and d) TEM images of NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS core–shell nanoparticles. (e–i) Scanning transmission electron microscopy image and elemental mapping images of F, Si, Cd and merge image; respectively.

3.2 Optical properties of NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles

Fig. 2a shows the upconversion fluorescence spectrum of NaYF₄:Yb/Tm@NaYF₄ nanoparticles and the UV-Vis absorption spectrum of the as-prepared NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles. A wide shoulder peak located at approximately 480 nm for the quantum dots and nanoparticles of CdS was observed;⁵⁵ which overlaps very well with the blue emissions of the as-prepared NaYF₄:Yb/Tm@NaYF₄ nanoparticles. The large spectral overlap indicates that the upconversion blue emission can be strongly quenched by energy transfer. As shown in Fig. 2b, four strong emission peaks located at 349, 362, 450 and 474 nm are attributed to the transitions of ¹I₆ → ³F₄, ¹D₂ → ³H₆, ¹D₂ → ³F₄ and ¹G₄ → ³H₆ for Tm³⁺, respectively. It's clearly observed that these upconversion fluorescent emissions are efficiently quenched for the as-prepared NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles compared to that's of the up-conversion nanoparticles. These fluorescence peaks for the as-prepared NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles with thin thickness of silica of 3 nm and 7 nm (Fig. S6, in the ESI†) are greatly quenched, illustrating the samples with thin layers of silica with high energy transfer efficiency. In particular, the emissions for Tm³⁺ (under excitation of 980 nm) at 349, 362, 450 and 474 nm can excite the CdS nanoparticles.^56,57 Therefore, the NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles loading more nanoparticles of CdS shows weak upconversion fluorescence emissions (Fig. S6c and S7, in the ESI†). Previous studies revealed that the fluorescent transitions of Tm³⁺ are efficiently quenched due to the near infrared (NIR) photon energy transferred to the nearby CdS nanoparticles via the irradiative energy transfer (IET) and FRET process.^37,58 The fluorescence mechanism and the energy transfer process has been summarized in Fig. 2c.

Fig. 2 (a) UV-Vis absorbance spectra of the as-prepared NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles and fluorescence spectra of Yb³⁺ and Tm³⁺ co-doped NaYF₄:Yb/Tm@NaYF₄ nanoparticles, respectively. (b) Fluorescence spectra of the as-prepared NaYF₄:Yb/Tm@NaYF₄@mSiO₂ and NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles with different silica shell thickness. (c) Irradiative energy transfer (IET) and fluorescence resonance energy transfer (FRET) between NaYF₄:Yb/Tm@NaYF₄ and CdS.

The dynamic fluorescence spectra of the as-prepared products have been investigated carefully to prove the possible energy transfer mechanism. As shown in Fig. 3, it's clearly observed that the luminescence decays of the excited state levels of ¹I₆ → ³F₄, ¹D₂ → ³H₆, ¹D₂ → ³F₄ and ¹G₄ → ³H₆ for Tm³⁺ show a little shift to short life; indicating the energy transfer partially via FRET process. In addition, the luminescence decays of the excited state levels for the sample with thinner thickness of SiO₂ layer become short; which indicated that the energy transfer is achieved more efficiently via FRET process.

Fig. 3 The luminescence decays of the excited state levels of Tm³⁺ for NaYF₄:Yb/Tm@NaYF₄@mSiO₂ and the NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles of different silica shell thicknesses at 349 nm (a); 362 nm (b); 450 nm (c) and 474 nm (d), respectively.

3.3 Photodegradation towards RhB using NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles

It's well known that the semiconductor (CdS) can be excited by the up-converting luminescence from UCNPs and the activated CdS nanoparticles will produce electrons and holes in the conduction band (CB) and the valence band (VB). In particular, these electron–hole pairs might migrate from the inner region to the surfaces; which will oxidize H₂O to ˙OH for degradation towards RhB dyes. Fig. 4a shows the fluorescence spectra of 2-hydroxy-terephthalic acid (TAOH) under irradiation of NIR light (250 W Xe lamp equipped with an IR filter) in presence of NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles, indicating ˙OH produced from the activated CdS nanoparticles and increased with the extension of irradiation time.⁵⁹ As shown in Fig. 4b, 85% of RhB molecules have been decomposed in 20 min under irradiation of a 250 W Xe lamp. In addition, 47% of the RhB molecules can be adsorbed and removed after stirred in dark condition, owing to the NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles with an intermediate layer of mesoporous silica. Fig. 4c shows that more than 90% of the RhB dyes can be photocatalytic degradation in 300 min under irradiation of IR light (a 1500 mW cm⁻² xenon lamp equipped with an IR filter); showing better photocatalytic ability driven by IR light than UCNPs/TiO₂ nanofibers photocatalyst.⁶⁰ Fig. 4d shows that the organic dyes have been efficiently decomposed y in presence of as-prepared NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles under irradiation of a 1500 mW cm⁻² xenon lamp equipped with an IR filter. In addition, the as-prepared NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles with thin thickness of silica shell indicated better photocatalytic properties; which can be ascribed to its higher energy transfer efficiency. As shown in Fig. 4e, the as-prepared nanostructures incorporating mesoporous silica, UCNPs and CdS show enhanced photocatalytic properties under irradiation of simulation solar light. It's possible attributed to the intermediate layer of mesoporous silica for the designed nanostructure; which can adsorb the dye molecules and facilitate the separation of photoelectrons and photo-holes derived from activated CdS nanoparticles. Additionally, the as-prepared NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles exhibit excellent chemical ability under irradiation of IR light (Fig. 4f).

Fig. 4 (a) Fluorescence spectra of 2-hydroxy-terephthalic acid (TAOH) in presence of NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles under irradiation of IR light. (b and c) Time-dependent absorption spectra of Rhodamine B solution (1 mg mL⁻¹, 30 mL) in presence of NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles under irradiation of a 250 W Xe lamp with and without a IR filter; respectively. (d) Kinetic curves of the degradation of RhB molecules (1 mg mL⁻¹, 30 mL) in presence of different photocatalysts under excitation of IR light. (e) Kinetic curves of the degradation of RhB molecules (10 mg mL⁻¹, 30 mL) under excitation of a 250 W Xe lamp. (f) The recycling activity for the as-prepared NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles under excitation of IR light.

4. Conclusions

In summary, we have synthesized NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles successfully via a layer by layer process. The as-designed NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles are with an intermediate layer of mesoporous silica; which was controlled in 10 nm in thickness for efficiently energy transfer. CdS nanoparticles with average size of 5 nm have been decorated on the surface of the NaYF₄:Yb/Tm@NaYF₄@mSiO₂ core–shell nanoparticles via a facile solution process. The upconversion fluorescence emissions for the as-prepared NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles have been greatly quenched, which indicates that the near infrared photons can be efficiently transferred to CdS nanoparticles from NaYF₄:Yb/Tm@NaYF₄ via FRET and IET. In addition, CdS nanoparticles can be activated under irradiation of IR light to produce photo-generated ˙OH radicals. Photocatalytic experiments demonstrated that the RhB molecules can be decomposed efficiently in presence of the as-prepared core–shell nanoparticles of NaYF₄:Yb/Tm@NaYF₄@mSiO₂/CdS nanoparticles under irradiation of near infrared light. This kind of nanostructures incorporated with mesoporous silica and UCNPs may find potential applications in chemotherapy, photodynamic therapy of cancer cells and etc.

Acknowledgements

This work is supported from the National Natural Science Foundation of China (Grants No. 21471043, 21101140, 51372276, 51403195), and Beijing Municipal Science and Technology Project (Z131100005213007), International Science & Technology Cooperation Program of China (2013DFA50920) and the Key Project of Anhui Provincial Educational Department (JZ2014AJZR0113).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09261f

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