Constructing near-infrared-light-mediated core-shell nanostructures incorporating upconversion nanoparticles (UCNPs) and semiconductors is of great importance for potential applications in photocatalysis, nano-biomedical engineering, solar cell, etc. In this work, we have demonstrated a two-step solution process to synthesize UCNPs@Zn0.5Cd0.5S core-shell nanoparticles (CSN). Firstly, a layer of AA-Zn(Cd)[OH]4− composites was coated on UCNPs to form UCNPs@AA-Zn(Cd)[OH]4− composites, which has been converted to UCNPs@Zn0.5Cd0.5S CSN via sulfidation reaction process using thioacetamide (TAA) as the sulfur source. Moreover, the UCNPs@Zn0.5Cd0.5S yolk-shell nanoparticles (YSN) have been obtained from the UCNPs@Zn0.5Cd0.5S CSN after calcination at 400°C, which show significantly photocatalytic activity for reduction of Cr(VI) under near-infrared light. All these can be attributed to the enhanced crystallization degree, resulting in enhanced energy transfer efficiency and separation efficiency of the photogenerated electrons and holes. An alternative strategy is provided in this study for fabrication of UCNP/semiconductor composites for various applications.

1. Introduction

The photocatalytic technology driven by solar light has become more and more attractive because solar energy is one of the most abundant resources [15]. Heterogeneous photocatalytic techniques based on semiconductor materials are widely used in photocatalytic decomposition of water, CO2 reduction, and organic reactions [68]. Semiconductor materials have their unique electronic band structure, which will produce highly reactive electrons and holes when the electron of the valence band transits to the conduction band inspired by the light of appropriate wavelength. Moreover, the photogenerated electrons can reduce Cr(VI) under sunlight or simulated sunlight irradiation [912]. However, most of the semiconductor photocatalysts can only be activated with UV or visible light. In particular, near-infrared (NIR) light occupies more than 40% of the incoming solar energy, which makes it more significant to fabricate and develop photocatalysts driven by NIR light [1316]. Despite this, the traditional plasmonic nanoparticles which can be absorbed in the NIR region are used in catalysis, bioimaging, and so on [1719]. However, lanthanide ions doped upconversion nanoparticles (UCNPs), which can convert infrared energy to UV–Vis energy and excite semiconductors to broaden the scope of the spectral response and raise the utilization ratio of solar energy. Up to date, much effort has been devoted to fabricate NIR-light-mediated UCNP-based nanocomposites to fully utilize solar energy [2026], including UCNPs@CuS [2730], UCNPs@ZnO [3133], UCNPs@TiO2 [3437], UCNPs@Bi2WO6 [38, 39], and UCNPs@Bi2MoO6 [4042]. As for the UCNP/semiconductor composites, the fluorescence energy can be passed from UCNPs (donors) to semiconductors (acceptors) to broaden the scope of the spectral response of the semiconductor effectively. These composite materials exhibit encouraging therapeutic efficacy of photodynamic therapy and the photochemical oxidation of organic matter under irradiation of NIR light [43, 44]. Many synthetic strategies including the epitaxial growth method, chemical assembly method, hydrothermal method, and electrospinning technique have been pioneered to fabricate UCNP-based Förster resonance energy transfer (FRET) nanostructures [4549]. However, the large lattice mismatch makes it difficult for coating directly a layer of semiconductors on the surface of UCNPs.

Herein, a facile chemical solution process has been proposed to synthesize UCNPs@Zn0.5Cd0.5S core-shell nanoparticles (CSN), in which UCNPs@AA-Zn(Cd)[OH]4− composites are firstly synthesized and subsequently converted to UCNPs@Zn0.5Cd0.5S CSN by liquid sulfidation, as shown in Figure 1(a). UCNPs@Zn0.5Cd0.5S yolk-shell nanoparticles (YSN) can be derived from the as-obtained core-shell nanoparticles of UCNPs@Zn0.5Cd0.5S after calcination owing to the UCNPs@Zn0.5Cd0.5S CSN incorporated by the surfactant. The optical properties of the samples have been studied carefully, and photochemical reduction tests of Cr(VI) ions are demonstrated.

2. Experimental

All the chemicals are of analytic grade and used as received. Hydrophilic NaYF4:Yb(30%)/Tm(0.5%)@NaYF4 (denoted as UCNPs) has been fabricated according to previously reported protocol [50, 51].

2.1. Synthesis of UCNPs@AA-Zn(Cd)[OH]4− Composites

In a typical process, 262.32 mg hexadecyltrimethylammonium bromide (CTAB) and 4.2 mg l-ascorbic acid (AA) were added into a 100 mL flask and dissolved using 60 mL deionized water to form clear solution. 2 mg as-prepared hydrophilic UCNPs, 25.24 mg hexamethylenetetramine (HMTA), 26.78 mg Zn(NO3)2·6H2O, and 18.66 mg Cd(CH3COO)2·2H2O with an equal mole ratio of Zn/Cd were added into the previous solution with stirring. Subsequently, the mixed solution was heated to 85°C and maintained for 10 h. Finally, the solution was centrifuged after cooling to room temperature at a speed of 9500 rpm to collect the products, then washed with ethanol and deionized water for three times, and dried naturally. The yield of the UCNPs@AA-Zn(Cd)[OH]4− composites is about 90%.

2.2. Synthesis of UCNPs@Zn0.5Cd0.5S Core-Shell and Yolk-Shell Nanoparticles

Typically, 20 mg previously prepared UCNPs@AA-Zn(Cd)[OH]4− composites and 3.76 mg TAA were added into a flask, which was dispersed using 15 mL deionized water and stirred for 12 h, then the temperature was raised to 95°C and kept for 2 h. The product in yellow was centrifugally separated (9500 rpm, 5 min) and the precipitate washed with deionized water and ethanol to remove the redundant ions. The as-synthesized UCNPs@Zn0.5Cd0.5S core-shell nanoparticles were placed into a muffle furnace and then calcined at 400°C for 1 h to form UCNPs@Zn0.5Cd0.5S yolk-shell nanoparticles (UCNPs@Zn0.5Cd0.5S YSN). For comparison, UCNPs@ZnS CSN and UCNPs@ZnS YSN have been synthesized according to the same protocol. The yield of products at this stage is up to more than 95%.

2.3. Photocatalytic Activity

In a typical process, 5 mg of the photocatalyst was added under stirring into 50 mL of Cr(VI) solution (20 mg L−1) which were prepared by dissolving K2Cr2O7 into distilled water. The mixture was magnetically stirred for 1 h in the dark till adsorption-desorption equilibrium before irradiation. The solution was under irradiation of a 1500 mW cm−2 xenon lamp (PLX-300D, Beijing Precise Technology Co. Ltd.) with an emission wavelength of 320 nm–2500 nm. The NIR band (780–2500 nm) was obtained by equipping with an UV–Vis filter. The photocatalyst was centrifugally separated (9500 rpm, 2 min), and the Cr(VI) concentration was determined by diphenylcarbazide (DPC) method.

3. Results and Discussion

In this work, UCNPs with ca. 42 nm in diameter have been prepared and used as NIR light nanotransducers (Figure S1a, in the Supporting Information) [50]. The UCNPs@AA-Zn(Cd)[OH]4− core-shell composites with 20 nm in shell thickness were synthesized by a modified process [33], as revealed by TEM image shown in Figure S1b (in the Supporting Information). The FESEM and TEM images of the sample, which were obtained from UCNPs@AA-Zn(Cd)[OH]4− composites and thioacetamide (TAA) at 95°C for 2 h have been shown in Figures 2(a)2(c), showing the sample consisted of core-shell nanoparticles. The elemental images shown in Figure S2 (in the Supporting Information) confirm the chemical composition of the as-obtained sample. It is interesting to note that the UCNPs@ZnxCd1-xS CSN has been converted to the yolk-shell-like nanoparticles obtained after annealing at 400°C for 1 h and the yolk-shell nanoparticles are ca. 80 nm in total diameter and 20 nm in shell thickness (Figures 2(d) and 2(e)). The lattice spacing of 3.6 and 2.5 Å from the shell edge is shown in Figure 2(f), which is a little smaller than that of the (100) and (102) crystal plane of CdS nanostructures [43], confirming that the shell component is alloyed ZnxCd1-xS. The STEM and elemental mapping images including Zn, F, Cd, and Y are shown in Figures 2(g)2(l), which further confirm that the yolk-shell nanoparticles with a shell consisted of elements of Zn, Cd, and S. X-ray diffraction (XRD) has been used to characterize the phase of the samples. All the sharper diffraction peaks (Figure 3(a)) coincide perfectly with the β-NaYF4 (JCPDS no. 28–1192). On the side, the broad diffraction peak at 27° shown in Figure 3(b) can be derived to three peaks located at 25.6, 27.0, and 28.2° by Gaussian curve, which obviously shifted to higher diffraction angles of CdS (JCPDS no. 41–1049) compared to the pure CdS diffraction peaks at 24.8, 26.5, and 28.1° [52]. The EDX spectrum (Figure S3, in the Supporting Information) and XPS of UCNPs@ZnxCd1-xS YSN (Figure S4, in the Supporting Information) are also operated to decide the chemical composition of UCNPs@ZnxCd1-xS YSN. The accurate chemical composition of Zn/Cd in UCNPs@ZnxCd1-xS is 0.51/0.49, which has been calculated using atomic absorption spectroscopy (AAS). Therefore, all the above analyses demonstrate successive synthesis of the CSN and YSN of UCNPs@Zn0.5Cd0.5S. Herein, the UCNPs@Zn0.5Cd0.5S CSN can be transferred to yolk-shell nanostructures because the shell (Zn0.5Cd0.5S) is comprised of lots of small nanoparticles stabilized by l-ascorbic acid (AA) molecules, as revealed by the FTIR spectra (Figure S5, in the Supporting Information). Some organic molecules or adsorbed functional group will be removed to form a void between the core and shell components during the annealing process whereas the small nanoparticles of Zn0.5Cd0.5S would be aggregated and the crystallization degree of Zn0.5Cd0.5S will be improved. As expected, the UCNPs@Zn0.5Cd0.5S YSN show better photocatalytic ability than UCNPs@Zn0.5Cd0.5S CSN.

The fluorescence spectra (Edinburgh FLS980, UK) and UV–visible spectra (Hitachi U-5100, Japan) have been used to study the optical properties of the samples. It is worthy to note that the fluorescence emissions at 342, 361, 451, and 476 nm (Figure 4(a)) for UCNPs@Zn0.5Cd0.5S YSN have been quenched and the quenching efficiency is up to 99.90% for all the emissions (342, 361, 476, and 476 nm), confirming that the yolk-shell nanoparticles show efficiently optical absorption and energy transfer between the yolk and shell. Figure 4(b) shows that the UCNPs@Zn0.5Cd0.5S YSN exhibit stronger UV–Vis absorption at 400–600 nm, which overlap well with the fluorescence emissions of the UCNPs and will result in an efficient FRET process occurring between UCNPs and Zn0.5Cd0.5S. Thus, the as-prepared UCNPs@Zn0.5Cd0.5S YSN will efficiently produce a large amount of reactive oxygen species including hydroxyl-free radicals (OH) and photogenerated electrons under irradiation of NIR light or visible light.

In this work, the UCNPs can give ultraviolet–visible light emissions under excitation of NIR light, which can activate the shell component (Zn0.5Cd0.5S) via irradiation energy transfer (IET) or FRET process. The electron from the valence band of the excited shell can transit to the conduction band under the appropriate wavelength excitation, then a large amount of photogenerated electrons (e) is produced from the conduction band and positive hole (h+) from the valence band [46, 53]. After that, the positive hole (h+) can react with Cr(VI), which results in reduction of Cr(VI) to Cr(III). A rationalization mechanism for the photochemical reduction of Cr(VI) has been previously demonstrated as follows [54, 55]:

The terephthalic acid (TA) was used to generate the fluorescent product for detecting OH radicals in solution [56, 57]. The fluorescence intensity of 2-hydroxy-terephthalic acid (TAOH) produced from terephthalic acid (TA) with OH shows a dramatic enhancement with time (Figure S6, in the Supporting Information) in the presence of UCNPs@Zn0.5Cd0.5S YSN under irradiation of NIR light, which verified that the YSN has the great ability to produce OH and electrons. Thus, the as-prepared UCNPs@Zn0.5Cd0.5S YSN would exhibit good NIR-mediated photochemical reduction or oxidation performance. In the present study, 50 mL (20 μg/mL) Cr(VI) aqueous solution was irradiated using infrared light (IR) or simulated solar light. Before irradiation, mixtures of the as-obtained photocatalysts and K2Cr2O7 aqueous solution were stirred in the dark for 1 h to reach adsorption equilibrium. Diphenylcarbazide (DPC) method has been widely used to analyze the concentration of Cr(VI) [9]. As shown in Figure 5, there is no obvious change in the Cr(VI) concentration in the absence of the photocatalyst after irradiation for 1 h, whereas nearly 38.1% and 98.7% of Cr(VI) have been reduced to Cr(III) in 30 min for UCNPs@Zn0.5Cd0.5S YSN under irradiation of a Xe lamp equipped with or without an UV–Vis filter. UCNPs@Zn0.5Cd0.5S YSN has better photocatalytic activity property than UCNPs@Zn0.5Cd0.5S CSN because of better crystallization during the annealing process and has the highest photocatalytic activity among all of the samples no matter with (Figure 5(b)) or without (Figure 5(c)) UV–Vis filter, indicating that the import of CdS can enhance the energy transfer efficiency between UCNPs with the shell. Additionally, in order to investigate the repeatability and chemical stability of photocatalyst, three uninterrupted cycling tests have been operated and shown in Figure 5(d). Therefore, the as-prepared UCNPs@Zn0.5Cd0.5S YSN shows good chemical stability, which has a strong ability for producing OH and electrons for photochemical reduction of Cr(VI). The photocatalytic mechanism has been illustrated and summarized in Scheme 1. The synthetic method described here has demonstrated that it will provide an important strategy for constructing nanocomposite-incorporated UCNPs and semiconductors for NIR-mediated photochemical process.

4. Conclusions

In summary, we have synthesized UCNPs@Zn0.5Cd0.5S core-shell and yolk-shell nanoparticles by a facile epitaxial growth process. UCNPs@AA-[Zn0.5Cd0.5(OH)4]2− nanocomposites are firstly synthesized via a modified process, which can be converted to UCNPs@Zn0.5Cd0.5S CSN via a sulfidation reaction. The UCNPs@Zn0.5Cd0.5S YSN can be achieved after calcination of UCNPs@Zn0.5Cd0.5S CSN at 400°C. In addition, the crystallization degree for the final UCNPs@Zn0.5Cd0.5S YSN has been enhanced during the annealing process. The fluorescence emissions for Tm3+ in UCNPs@Zn0.5Cd0.5S yolk-shell nanoparticles have been quenched greatly, demonstrating efficient energy transfer between the achieved two components of UCNPs and Zn0.5Cd0.5S. The as-obtained UCNPs@Zn0.5Cd0.5S YSN exhibit superior photocatalytic activity for reduction of Cr(VI) under no matter simulated solar light or IR light. Therefore, this work should inspire further exploration for alterative nanoparticles with high potential for potential applications in nanomedicine, photocatalytic, energy conversion, etc.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

There are no conflicts of interest.

Authors’ Contributions

W. Wang, F. Zhang, and W. Dong contributed equally to this work.


We acknowledge the funding supports from the National Natural Science Foundation of China (Grants 21471043 and 31501576).

Supplementary Materials

Figure S1: (a) transmission electron microscopy (TEM) image of NaYF4:Yb/Tm@NaYF4 nanoparticles. (b) Transmission electron microscopy (TEM) image of UCNPs@AA-Zn[(OH)4]2− nanoparticles. Figure S2: STEM image and elemental imaging of UCNPs@Zn0.5Cd0.5S CSN. Figure S3: (a) energy-dispersive X-ray spectra of the as-prepared product shown in Figure 1; (b) element composition of the as-prepared UCNPs@Zn0.5Cd0.5S YSN from EDX analyses. Figure S4: X-ray photoelectron spectra of the as-prepared yolk-shell-like nanoparticles of UCNPs@Zn0.5Cd0.5S YSN: (a) a general survey; (b) Zn 2p; (c) Cd 3d; (d) S 2p; (e) Na 1s; (f) Y 3d; (g) Yb 4d; and (h)Tm 4d. Figure S5: Fourier transform infrared (FT-IR) spectrum of the as-prepared UCNPs@Zn0.5Cd0.5S CSN and UCNPs@Zn0.5Cd0.5S YSN. Figure S6: (a) fluorescence spectra of 2-hydroxy-terephthalic acid (TAOH) with the addition of UCNPs@Zn0.5Cd0.5S YSN excited by a 980 nm CW laser. (b) Photocatalytic reduction of Cr(VI) in the presence of UCNPs@Zn0.5Cd0.5S YSN at given irradiation times under irradiation of a 1500 mW Xe lamp with a UV–Vis filter. (Supplementary Materials)