Near-infrared light-driven photocatalytic NaYF₄:Yb,Tm@ZnO core/shell nanomaterials and their performance

Abstract

Herein, the infrared-responsive photocatalyst NaYF₄:Yb,Tm@ZnO has been successfully synthesized by combining semiconductor ZnO with an upconversion material, NaYF₄:Yb,Tm. In this composite, NaYF₄:Yb,Tm emits intense ultraviolet and blue upconversion luminescence upon excitation by a 980 nm laser and provides the necessary energy of ultraviolet light to ZnO. The photocatalytic activity of NaYF₄:Yb,Tm@ZnO composites has been studied using methylene blue by irradiation with a 980 nm laser, and the results indicate that the NaYF₄:Yb,Tm@ZnO composite is an advanced near-infrared-driven photocatalyst; this study presents a promising strategy to utilize the near-infrared-responsive upconversion materials for photocatalytic applications.

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Introduction

In recent years, environmental pollution has become a very serious problem,¹ and the application of photocatalysis for controlling environmental pollution has attracted significant attention of researchers from all over the world.² Ever since Fujishima and Honda discovered photocatalysis on the TiO₂ electrode for hydrogen production phenomenon in 1972,³ TiO₂ photocatalysts have attracted the attention of researchers due to their oxidation ability, environmentally friendly nature and chemical stability under the irradiation of ultraviolet (UV) light;^4–9 however, UV radiation of the solar energy occupies only ca. 5%, whereas the energy in the visible light (ca. 48%) and near-infrared (NIR) light (ca. 47%) range is not used for photocatalysis.^10,11 To fully use the solar energy in the photocatalysis of semiconductors, several methods, such as semiconductor coupling^12–14 and impurity doping, have been proposed for extending the absorption range.^15–17 The semiconductor photocatalysis can be adjusted to the visible light region using these methods.^18–20 However, the efficiency of using long-wavelength light in the field of photocatalysis is still a challenge due to the poor absorption of photocatalytic materials.

Moreover, nanomaterials with upconversion (UC) luminescence have received extensive attention owing to their potential application in bio-analysis, medical therapy, optical devices, and photovoltaic cells.²¹ To date, fluoride crystals have been regarded as one of the best UC matrices due to their low phonon energy and high chemical stability. Note that a higher UC efficiency can be expected in rare earth ion (RE³⁺)-doped NaYF₄ crystals due to their unique optical properties arising from the intra 4f transitions.²²

In the last decade, the focused NaYF₄:Yb,Er/Tm agents have been successfully applied to advanced assembly materials. For example, the preparation of the NaYF₄:Yb,Tm/CdS composite has been reported, and the NIR photocatalytic activity has been investigated by degrading Rhodamine B and methylene blue.²³ The NaYF₄:Yb,Tm@TiO₂ core@shell photocatalyst has been successfully fabricated by a simple hydrothermal method followed by hydrolysis of titanium isopropoxide (TTIP) with polyvinylpyrrolidone K-30 (PVP) as the coupling agent; this study suggests a promising system to utilize the NIR energy of sunlight for photochemical and photoelectrical applications based on TiO₂.^24,25 The NaYF₄:Yb,Tm/g-C₃N₄ (NYT/C₃N₄) photocatalyst has been successfully fabricated by a stepwise method to utilize the NIR energy of sunlight for photochemical and photoelectrical applications based on g-C₃N₄.²⁶ We have recently proposed the assembly of composites formed by NaYF₄:Yb,Tm as a core and Cu₂O as a shell that presents an interesting high performance for organic pollutant degradation under NIR irradiation.¹¹ The infrared-responsive photocatalysts of NaYF₄:Yb,Tm@ZnO were synthesized by a two-step high-temperature thermolysis method.²⁷ In another method, the NaYF₄:Yb, Tm@ZnO heterogeneous nanoparticles with an epitaxial interface have been prepared by carefully controlling the temperature. This heterogeneous core–shell structure facilitates the enhancement in both the upconversion luminescence and the ultrahigh photocurrent response by providing efficient energy transfer channels.²⁸ However, the photocatalytic activity of these prepared materials has not been comprehensively investigated.

Inspired by the abovementioned investigation, herein, a core–shell nanoparticle consisting of the upconversion luminescent NaYF₄:Yb,Tm and the n-type semiconductor ZnO for NIR responsive photocatalysis was synthesized. Compared with those reported in the previous study, the photocatalytic activities of the NaYF₄:Yb,Tm@ZnO composite were studied by the degradation of methylene blue in an aqueous solution upon NIR irradiation, and the degradation rate reached 68.7%. An important consequence was found that the NaYF₄:Yb,Tm@ZnO composite showed a significantly improved catalytic activity under NIR irradiation as compared to ZnO and the physical mixture of NaYF₄:Yb,Tm and ZnO.

Experimental

YCl₃·6H₂O (99.999%), YbCl₃ (99.998%), TmCl₃ (99.995%), NaOH, NH₄F, oleic acid (OA), 1-octadecene (ODE) and zinc(II) acetylacetonate (Zn(acac)₂) were purchased from the Sigma-Aldrich (St. Louis, MO, USA) company and used without further purification.

NaYF₄:20%Yb,2%Tm nanoparticles were synthesized by a solvothermal method. Typically, 1 mmol YCl₃, YbCl₃, and TmCl₃ in a ratio of 78 : 20 : 2 were added to the mixture of 15 mL ODE and 6 mL OA in a 50 mL two-necked flask followed by heating to 160 °C to form a homogeneous solution. After being cooled down to room temperature, a 10 mL methanol solution containing NaOH (0.01 g) and NH₄F (0.148 g) was slowly added to the two-necked flask, and the mixture was stirred for 30 min and then heated slowly to 100 °C for another 30 min to remove water, oxygen and methanol. Subsequently, the solution was quickly heated up to 300 °C and maintained at this temperature for 1.5 h under an argon atmosphere. After the solution was cooled down naturally, samples were obtained by centrifugation and washed three times with ethanol.

ZnO-coated NaYF₄:Yb,Tm nanocomposites were prepared as follows: 0.1 mmol NaYF₄:Yb,Tm was mixed with benzyl ether (5 mL) in a round-bottom flask under stirring. After this, 0.02636 g (0.1 mmol) Zn(acac)₂ was mixed with a mixture of benzyl ether (10 mL) in a 50 mL round-bottom flask under stirring; then, the mixture was heated to 150 °C, maintained at this temperature for 1.5 h and then slowly added to the flask that contained NaYF₄:Yb,Tm; then, the mixed solution was stirred for 30 min. After this, the solution was heated to 200 °C and maintained at this temperature for 1 h under an argon atmosphere; then, the mixture was heated to 250 °C and maintained at this temperature for 30 min under an argon atmosphere. After the solution was cooled down naturally, nanocrystals were obtained from the solution with ethanol and water by centrifugation, washed three times, dried at 60 °C for 12 h, and annealed at 400 °C for 2 h under an argon atmosphere.

Pure ZnO nanocrystals were prepared by the abovementioned procedures without the addition of NaYF₄:Yb,Tm, and the NaYF₄:Yb,Tm nanocrystals were also prepared by following the abovementioned procedures. Then, they were mixed mechanically at the same molar ratio as used in the preparation of the NaYF4 : Yb,Tm@ZnO composites.

Photocatalysis of the products was monitored by means of degradation of methyl blue (MB). Then, 0.5 mg of the NaYF₄:Yb,Tm@ZnO nanocomposites were dispersed in a cuvette containing a MB aqueous solution (0.5 mL 15 mg L⁻¹). Prior to irradiation, the aqueous solution was kept in dark for 5 h for establishing an adsorption–desorption equilibrium of MB on the surface of NaYF₄:Yb,Tm@ZnO nanocomposites before irradiation. A diode laser of 980 nm with a power of 10 W cm⁻² was used as the irradiation source. After irradiation for 2 h, 0.3 mL of MB aqueous solution was taken out for UV-vis absorbance measurement and then put back into the quartz corvette.

The X-ray powder diffraction (XRD) analyses were performed using a Bruker D8 Advance X-ray diffractometer (Billerica, MA, USA) with Cu Kα1 irradiation (λ = 1.54056 Å). The transmission electron microscopy (TEM) images were obtained using a JEOL JEM-1400 transmission electron microscope (Tokyo, Japan). The upconversion (UC) luminescence spectra were obtained using a ZOLIX fluorescence spectrometer system, and the excitation source used was an external 980 nm semiconductor laser (ZOLIX INSTRUMENTS CO. LTD) with an optic fiber accessory. The absorption of water was measured by a UV-3101PC UV-VIS-NIR scanning spectrophotometer (Shimadzu, Kyoto, Japan).

Results and discussion

To determine the crystal structure of the as-synthesized products, the hexagonal phase NaYF₄:Yb,Tm NCs and NaYF₄:Yb,Tm@ZnO composite NCs were examined by XRD. Fig. 1a shows the XRD patterns of the prepared NCs. The diffraction peaks of the NaYF₄:Yb,Tm NCs could be indexed to a pure hexagonal phase NaYF₄ structure (JCPDS 16-0334) very well, and no other phase can be identified. In addition to the XRD pattern of the hexagonal phase of NaYF₄, the characteristic diffraction peaks (100), (101) and (110) of ZnO (JCPDS 79-0208) can be observed in Fig. 1b. It should be concluded preliminarily that the composites of NaYF₄ and ZnO have been synthesized.

Fig. 1 XRD patterns of NaYF₄:Yb,Tm nanocrystals (a) and with ZnO coating (b). Standard XRD patterns of JCPDS 16-0334(NaYF₄) and 79-0208 (ZnO) are also shown.

The NaYF₄:Yb,Tm NCs and NaYF₄:Yb,Tm@ZnO composite products prepared by TEM are displayed in Fig. 2. In Fig. 2a, NaYF₄:Yb,Tm NCs show hexagonal appearance with a uniform size of 28 nm. After surface modification by ZnO, the average size of the NaYF₄:Yb,Tm@ZnO composites increased to 31 nm, as shown in Fig. 2b. The thickness of the shell ZnO is about 5 nm.

Fig. 2 TEM images of NaYF₄:Yb,Tm nanocrystals (a) and core–shell NaYF₄:Yb,Tm@ZnO nanoparticles (b).

NaYF₄:Yb,Tm is a typical upconversion luminescent material for converting NIR excitation light to higher energies via successive energy transfer processes. Scheme in Fig. 3a illustrates the processes of upconversion luminescence in a Yb³⁺–Tm³⁺ codoped system. Yb³⁺ ions absorb the light of 980 nm, and some energy transfers from the excited Yb³⁺ ions to the neighboring Tm³⁺ ions, which are excited and then provide emissions.²⁹ The upconversion fluorescence spectra of NaYF₄:Yb,Tm and photoluminescence emission spectra of NaYF₄:Yb,Tm@ZnO composites under 980 nm excitation are shown in Fig. 3b. In the spectrum of NaYF₄:Yb,Tm, three UV emission peaks located at 291 nm, 345 nm, and 361 nm, blue emission peaks at 451 nm and 476 nm and a red emission peak at 645 nm are attributed to the transitions of Tm³⁺ ions corresponding to ¹I₆ → ³H₆, ¹I₆ → ³F₄, and ¹D₂ → ³H₆, ¹D₂ → ³F₄ and ¹G₄ → ³H₆ and ¹G₄ → ³F₄. The peaks of the upconversion luminescence are same for two samples; this means that all emissions are from Tm³⁺ ions. However, after the coating of ZnO, the upconversion intensity of NaYF₄:Yb,Tm@ZnO is weaker than that of pure NaYF₄:Yb,Tm obviously. Especially, upon coating with a ZnO layer, the intensities of the emissions at 291, 345 and 361 nm decrease significantly when compared with those of 451 and 476 nm that change less. Because the photon energies of 291, 345, and 361 nm are higher than the band gap of ZnO, the spectral change may be relevant to the presence of ZnO around the upconversion NaYF₄:Yb,Tm particles according to the different decreasing ratios.

Fig. 3 (a) The upconversion luminescence processes in a Yb³⁺–Tm³⁺ codoped system and working mechanism of NaYF₄:Yb,Tm@ZnO nanoparticle under excitation at 980 nm. (b) Photoluminescence (PL) spectra of NaYF₄:Yb,Tm and NaYF₄:Yb,Tm@ZnO composites under 980 nm excitation at room temperature.

To ascertain the origin of photoluminescence spectral changes, the UV-vis absorption spectrum of NaYF₄:Yb,Tm@ZnO was measured, as shown in Fig. 4. In the absorption band in the UV region, a peak starting at about 387 nm, corresponding to the band gap (3.2 eV) of ZnO semiconductor, overlaps with the emission spectra of 291, 345 and 361 nm. Therefore, we can speculate that the emission spectra of 291, 345 and 361 nm generated via the upconversion process of NaYF₄:Yb,Tm has been absorbed partially by ZnO. Therefore, it is possible to realize the photocatalysis under near-infrared light using NaYF₄:Yb,Tm@ZnO nanocomposites, as shown in Fig. 3a.

Fig. 4 UV-Vis absorbance spectra of NaYF₄:Yb,Tm@ZnO composites.

To judge the photocatalytic activity of different particles under NIR irradiation of 980 nm, MB was used as a model pollutant. Herein, 0.5 mg NaYF₄:Yb,Tm@ZnO particles were dispersed in a 0.5 mL MB aqueous solution (15 mg L⁻¹). Under NIR irradiation for 2 hours, 0.4 mL of MB aqueous solution was extracted for absorbance measurement. Photocatalytic activities of pure NaYF₄:Yb,Tm, ZnO and their physical mixture of NaYF₄:Yb,Tm and ZnO (NaYF₄:Yb,Tm/ZnO) were also tested in this way. Fig. 5 shows the photocatalytic activities of the as-prepared samples for the degradation of MB under NIR irradiation of 980 nm. As shown in Fig. 5(a), in the presence of the NaYF₄:Yb,Tm@ZnO composites, the absorption spectra of MB with a peak at 664 nm decreases gradually with an increase in irradiation time; this obviously indicates the degradation of MB.

Fig. 5 (a) Absorbance spectra of MB catalyzed by the core–shell photocatalyst at different irradiation times under NIR excitation; (b) the time-dependent ratios of C/C₀ in the presence of NaYF₄:Yb,Tm@ZnO core–shell composite and NaYF₄:Yb,Tm/ZnO physical mixture and with photocatalysts in the dark.

To investigate the origin of the photocatalytic activity of the NaYF₄:Yb,Tm@ZnO composites, five parallel experiments were designed as follows: (1) the MB solution in the presence of ZnO was kept in the dark. (2) The pure MB solution was irradiated with NIR light of 980 nm. (3) The MB solution was irradiated with NIR light of 980 nm in the presence of NaYF₄:Yb,Tm. (4) The MB solution was irradiated with NIR light of 980 nm in the presence of the mixture of NaYF₄:Yb,Tm/ZnO. (5) The MB solution was irradiated with NIR light of 980 nm in the presence of the NaYF₄:Yb,Tm@ZnO composites. Then, the time-dependent rates of C/C₀ were detected for different samples, as shown in Fig. 5b. The C₀ is the primary concentration of MB and the C is the concentration of MB irradiated with the irradiation of 980 nm laser or in the dark at the time t; the first experiment indicates a very small variation of the concentration of MB under the dark condition, as shown in Fig. 5b. It means that almost no degradation of MB occurred in the solution with ZnO in the dark. The same result was observed in the pure MB solution, and only 6% of MB was decomposed in the solution with NaYF₄:Yb,Tm irritated by 980 nm for 10 h according to the change in concentration. As is known, NIR irradiation and nonradiative relaxations of excited Yb³⁺ and Tm³⁺ ions can generate thermal energy. The results obtained in the abovementioned experiments indicate that the thermal energy generated by NIR irradiation or under the dark condition is not high enough to induce thermal degradation of MB appreciably. The NaYF₄:Yb,Tm@ZnO composite and NaYF₄:Yb,Tm/ZnO mixture exhibit strong photocatalytic activity, resulting in 68.7% and 37% degradation of MB after irradiation for 10 h, respectively. This could be explained by their energy migration routes, as has been discussed in the photoluminescence section. The upconversion luminescence of NaYF₄:Yb,Tm@ZnO decreases significantly when compared with that of the NaYF₄:Yb,Tm/ZnO mixture. As NaYF₄:Yb,Tm and ZnO attach closely to each other and form compact interfaces, the core-shell-structured particles benefit FRET processes, whereas the NaYF₄:Yb,Tm/ZnO mixture has no contact interfaces between NaYF₄:Yb,Tm and ZnO particles, and then, ZnO can be excited mainly via radiation-reabsorption. Therefore, more NIR photon energy in the core–shell structure has been used to activate ZnO, and this results in high photocatalytic activity.

To assess the repeatability of near-infrared light-driven NaYF₄:Yb,Tm@ZnO, four consecutive photocatalytic experiments for the degradation of MB solution irradiated by NIR light of 980 nm for 12 h were studied. After each photocatalytic reaction, NaYF₄:Yb,Tm@ZnO was separated and washed with deionized water and ethanol several times for next run. Then, the time-dependent rates of C/C₀ were detected for different runs. As shown in Fig. 6, about 68.7% of MB could be degraded for the first run and about 43% of MB is degraded in the fourth run; this indicated the good repeatability of the near-infrared light-driven photocatalytic activity of NaYF₄:Yb,Tm@ZnO.

Fig. 6 Recyclability of the NaYF₄:Yb,Tm@ZnO core–shell composite under NIR light of 980 nm.

Conclusions

In this study, near-infrared core–shell structured NaYF₄:Yb,Tm@ZnO photocatalysts were prepared by a two-step high-temperature thermolysis method. XRD, TEM, the UV-vis spectra, and the PL spectra were used to characterize the photocatalytic material. The degradation of MB exposed to NaYF₄:Yb,Tm@ZnO core–shell particles upon NIR radiation demonstrated the NIR-driven photocatalytic capability. Compared with NaYF₄:Yb,Tm, ZnO, and the mixture of NaYF₄:Yb,Tm and ZnO, the NaYF₄:Yb,Tm@ZnO composite showed a significantly improved catalytic activity in the degradation of MB under 980 nm irradiation. This study suggests a promising system to research NIR photocatalysis, which may have an important implication on the utilization of solar energy.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the National Natural Science Foundation of China under Grant No. 61575019, 51272022 and 11474018.

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