1 Introduction

Multi-component heterostructure nanocomposites，which composed of two or more nanomaterials，can inherit the unique properties and overcome the limitation of single component ^［1-2］. At the same time，the multi-component heterostructure can endow many new properties，such as magnetism，light，electricity and etc. ^［3-6］. Photon upconversion（UC）has the ability to converting near infrared（NIR）photons into visible or ultraviolet（UV）radiation，it is a unique imaging technique that utilizes low photon energy ^［7-8］. The UC nanomaterials are composed of matrix doped with rare earth ions used as sensitizer and activators. And modification of luminescence can be achieved by changing the type of rare earth ions. Owing to the low photon energy，high rare-earth ion solubility and good thermal stability，the rare-earth fluorides（NaREF₄）are the most effective matrix. NaGdF₄ is as candidate own to lower vibrational phonon energy and matching lattice with doped ions ^［9-10］. Besides，the hexagonal phase NaGdF₄ can improve the luminous efficiency of UC，and Gd³⁺ has a large energy level interval，it can also pass sensitization ion（Yb³⁺）and activation ion（Er³⁺，Tm³⁺ and Ho³⁺）achieves high-efficiency UC luminescence. Er³⁺ has fluorescence response of blue，green and red light，and the emission could be adjusted by tuning structure. Therefore，NaGdF₄：Yb，Er has the advantages of low excitation energy，high luminous efficiency and good thermal stability，so it has potential application prospects in biological imaging，drug delivery，tumor treatment，display equipment，solar energy conversion，etc.^［11-12］. However，NaGdF₄：Yb，Er has a tendency to aggregate and quench，which greatly reduces its photoluminescence（PL）performance，and the oil phase system limits its application. Meanwhile，realizing the tuning of light is expected to broaden the application range of NaGdF₄：Yb，Er. In order to make up for its shortcomings，many strategies had been reported，such as forming core-shell structure，tuning size or morphology and constructing the composites^［13-15］. Among many methods，construction heterostructure by compounding with other materials can not only improve the stability and compatibility of NaGdF₄：Yb，Er，but also tuning optical properties.

Metal-organic frameworks（MOFs）are a class of carrier materials with unprecedented chemical and structural tunability. Their synthetic controllability and structural design properties make MOFs as ideal platforms for identifying design features for advanced functional materials^［16］. Meanwhile，MOFs are considered as promising energy transmission（ET）platform to achieve collaborative molecular level functions and promote efficient energy transfer due to the highly accessible and spatially discrete linkers^［17］. Li et al. integrated UC nanoparticles and MOF to construct a composite photocatalyst that has adjustable photocatalytic activity^［18］. Chen et al. prepared core-shell UC nanoparticle@MOF nanoprobes for luminescent/magnetic dual-mode targeted imaging^［13］. In our work，MIL-101/NaGdF₄：Yb，Er were prepared by the two-step method，the photocatalyst displayed higher photocurrent and better degradation ability for Rhodamine B owing to synergistic effect^［14］. Those works make full use of the adsorption and good bio-compatibility of MOFs. Zeolitic imidazolate frameworks（ZIFs）is a nitrogen-containing MOFs material obtained by compounding an imidazole or purine organic ligand with a transition metal ion. In particular，ZIF-67 is a regular dodecahedral structure material containing Co²⁺，it has the advantages of high stability，high porosity，and large specific surface area^［19］. The multiple energy levels contained in Co²⁺ can meet the energy exchange requirements with rare earth ions^［20］. Therefore，construction heterostructure containing ZIF-67 is an effective strategy to broaden the application prospect of NaGdF₄：Yb，Er while achieving light tuning.

Here in，we have prepared multi-component heterostructure ZIF-67/NaGdF₄：Yb，Er by a stepwise synthesis method. The NaGdF₄：Yb，Er nanorods were loaded on the surface of ZIF-67，which was used as a carrier. The heterostructure ZIF-67/NaGdF₄：Yb，Er，which avoided agglomeration and quench of UC nanoparticles，display better stability. Meanwhile，the composites improved the compatibility of alcohol，so that it broke through the constraints of oil phase systems. ZIF-67 is also used as an ET platform to achieve UC PL tuning of NaGdF₄：Yb，Er nanorods by energy transition. Compared with NaGdF₄：Yb，Er nanorods under 980 nm laser excitation，the PL performance of heterostructure ZIF-67/NaGdF₄：Yb，Er has converted from green light to red light that owing to the synergistic effect between individual components. The incorporation of ZIF-67 alter the PL performance of NaGdF₄：Yb，Er nanorods and lead to a heterostructure with good stability，which broadens the application and promotes the progress of key technologies in the field of photon UC nanomaterials.

1 Experiments

1.1　Synthesis of NaGdF₄:Yb,Er upconversion nanoparticles

In a typical preparation，1.2 g NaOH was dissolved in 2 ml deionized water and ultrasonically dispersed. After the heat released，the solution was heating and stirring in a 50 ℃ water bath. Next，8 ml alcohol and 20 ml oleic acid were added to the above solution under stirring 20 min to be a transparent solution. 1 mmol of Ln（NO₃）₃·6H₂O（Ln：78% Gd；20% Yb；2% Er）aqueous solution were added under vigorous stirring. Then，0.8 g PVP dissolved in 3 ml of ethanol and add to above solution. Subsequently，8 ml of NaF aqueous solution（1 mol/L）was added dropwise to the solution. Keep stirring to give it had a good dispersion to form a translucent colloidal solution. Finally，the mixed solution was transferred into reaction kettle and heated at 180 ℃ for 18 h. After reaction was completed，the system was cooled to room temperature naturally. The prepared samples were separated and washed used deionized water and ethanol by centrifugation to remove oleic acid and other remnants，and was stored in cyclohexane solvent.

1.2　Synthesis of ZIF-67

0.2911g of Co（NO₃）₂·6H₂O and 0.3284 g of 2-methylimidazole was dissolved in 25 ml of methanol and ultrasound for 10 min，respectively. Then the dissolved 2-methylimidazole solution was added dropwise to the Co（NO₃）₂·6H₂O solution，and stirred at room temperature for 3 h. Finally，the samples were separated to remove other remnants and stored in methanol.

1.3　Synthesis of the heterostructure ZIF-67/NaGdF₄:Yb,Er

Briefly，the prepared NaGdF₄：Yb，Er nanorods and ZIF-67 were mixed together. And then some PVP was added as dispersant，the mixture were stirred at 50 ℃ for 24 h. Finally，the samples were separated and washed to remove other remnants and stored in methanol.

2 Results and discussions

As showed in Fig. 1，firstly，the NaGdF₄：Yb，Er nanorods was prepared at 180℃ and react for 18 h by hydrothermal treatment，which experience three-stage is nucleation，growth and regrowth during the reaction. And then，NaGdF₄：Yb，Er nanorods were added into ZIF-67，which prepared at room temperature for 3 h. Meanwhile，some PVP was added in the mixture as dispersant to prevent NaGdF₄：Yb，Er nanorods from aggregation. Finally，this mixture was string at 50℃ for 24 h to obtained heterostructure ZIF-67/NaGdF₄：Yb，Er. The detailed information of experiment could be found from Supplementary material

图 1. 异质结构ZIF-67/NaGdF4：Yb，Er的制备流程图

Fig. 1. Illustration for the preparation of heterostructure ZIF-67/NaGdF₄: Yb, Er

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The morphology of samples are characterized via transmission electron microscopy（TEM）. As shown in Fig. 2（a），the length of prepared NaGdF₄：Yb，Er nanorods are around 50 nm. And the corresponding high-resolution transmission electron microscopy（HRTEM）image（inset of Fig. 2（a））reveals clear lattice fringes with an inter-planar spacing of 0.49 nm，which is ascribed to the（101）plane of β-NaGdF₄：Yb，Er. The result was consistent with hexagonal phase（PDF：27-0699），and determined by X-ray diffraction（XRD）（Fig. 3（a））. The size of ZIF-67 was approximately 500 nm，and it exhibited a typical and regular dodecahedron（Fig. 2（b））. As shown in Fig. 2（c-d），the NaGdF₄：Yb，Er nanorods are uniformly loaded on the surface of ZIF-67，and it was confirmed to be NaGdF₄：Yb，Er nanorods by HRTEM（Inset of Fig. 2（d））. Furthermore，as shown in energy dispersive X-ray spectroscopy（EDS），the Co element（from ZIF-67）was mainly distributed in the inside，and the F，Gd，Yb and Er elements（from NaGdF₄：Yb，Er nanorods）were homogeneously distributed throughout the heterostructure（Fig. 2（e））. And the Fourier transform infrared（FT-IR）spectra（Fig. 3（b））evidenced the formation of heterostructure in ZIF-67/NaGdF₄：Yb，Er，this is consistent with the TEM and EDS results. From above analysis，the heterostructure ZIF-67/NaGdF₄：Yb，Er was prepared successfully.

图 2. 异质结构ZIF-67/NaGdF4：Yb，Er的形貌表征（a）NaGdF4：Yb，Er纳米棒的透射电镜图，插图是其高分辨透射电镜图，（b）ZIF-67的透射电镜图；（c）异质结构ZIF-67/NaGdF4：Yb，Er的透射电镜图和它的3D模型（插图），（d）异质结构ZIF-67/NaGdF4：Yb，Er的局部放大投射电镜图和高分辨透射电镜图（插图），（e）ZIF-67/NaGdF4：Yb，Er的元素分布

Fig. 2. The morphological characterization of heterostructure ZIF-67/NaGdF₄: Yb, Er (a) TEM image of NaGdF₄: Yb, Er nanorods, the inset is HRTEM image taken from (a) , (b) TEM image of ZIF-67, (c) TEM image of heterostructure ZIF-67/NaGdF₄: Yb, Er, the inset is 3D model of (c) , (d) The partial enlargement of (c) , the inset is HRTEM image taken from (c) , (e) EDS elemental mapping of heterostructure ZIF-67/NaGdF₄: Yb, Er.

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图 3. 异质结构ZIF-67/NaGdF4：Yb，Er的X射线衍射和红外谱

Fig. 3. The XRD and FT-IR spectra of heterostructure ZIF-67/NaGdF₄: Yb, Er

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The UC PL properties and conversion mechanism of heterostructure ZIF-67/NaGdF₄：Yb，Er were measured. As shown in Fig. 4（a），compared with PL performance of NaGdF₄：Yb，Er nanorods under 980 nm laser excitation，the green light（541 nm）of heterostructure ZIF-67/NaGdF₄：Yb，Er was drastically reduced and the red light（658 nm）was enhanced largely. And compared with ZIF-67，the red light has a significant enhanced，indicates that the PL tuning was attributed to the formation of the heterostructure. The absorption of heterostructure composites were further explored via UV-vis spectroscopy（Fig. 4（b））. The heterostructure ZIF-67/NaGdF₄：Yb，Er exhibited a broader absorption range and higher absorption strength compared with each component. Meanwhile，a new absorption peak appeared in the green region（510 nm-630 nm）compared with NaGdF₄：Yb，Er nanorods，which was contributed by ZIF-67. As shown in Fig. 4（c），the heterostructure ZIF-67/NaGdF₄：Yb，Er emitted red light（658 nm）under 541 nm laser excitation owing to the synergistic effect between ZIF-67 and NaGdF₄：Yb，Er nanorods. The photon energy of 541 nm was absorbed by ZIF-67 and emitted as red light（635 nm）through energy level transition，then transited to the energy level of Er³⁺ through photon relaxation，finally appeared as 658 nm emission of Er³⁺. This mechanism will be further discussed in Fig. 6. Benefiting from the construction of heterostructure，this ZIF-67/NaGdF₄：Yb，Er achieved PL tuning and improved intensity of luminescence，which broadens its scope of application.

图 4. 异质结构ZIF-67/NaGdF4：Yb，Er荧光性能的表征和对比（a）980 nm激发光下样品的上转换荧光性能，（b）样品的紫外吸收光谱，（c）样品的光致发光激发谱对比，（d）使用不同含量的NaGdF4：Yb，Er纳米棒溶液制备得到的异质结构ZIF-67/NaGdF4：Yb，Er的上转换荧光性能

Fig. 4. Characterization and comparison of fluorescence properties of heterostructure ZIF-67/NaGdF₄: Yb, Er (a) UC PL spectra under 980 nm laser excitation, (b) UV-vis spectra, (c) PL emission spectra of heterostructure ZIF-67/NaGdF₄: Yb, Er, the inset is PL emission spectra of NaGdF₄: Yb, Er nanorods and ZIF-67, (d) UC PL spectra of heterostructure ZIF-67/NaGdF₄: Yb, Er with different concentration of NaGdF₄: Yb, Er nanorods, the inset is the variation of PL intensity

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图 5. 使用不同含量NaGdF4：Yb，Er纳米棒溶液制备得到的异质结构ZIF-67/NaGdF4：Yb，Er的形貌

Fig. 5. TEM images of heterostructure ZIF-67/NaGdF₄: Yb, Er with different NaGdF₄: Yb, Er nanorods concentration

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图 6. 异质结构ZIF-67/NaGdF4：Yb，Er中上转换机理的能级跃迁示意图

Fig. 6. Schematic energy level diagram showing the UC process mechanism of heterostructure ZIF-67/NaGdF₄: Yb, Er

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More interestingly，it found that the intensity of PL was related to the ratio of NaGdF₄：Yb，Er and ZIF-67 as shown in Fig. 4 d. With the amount of NaGdF₄：Yb，Er increased，the intensity of red light（658 nm）enhanced linearly and remains unchanged after reaching the maximum. This is due to saturation of NaGdF₄：Yb，Er loading on ZIF-67，which could be further verified by TEM（Fig. 5）. And the aggregated NaGdF₄：Yb，Er nanorods could be found when the amount of NaGdF₄：Yb，Er was increased after saturation（30%）. Therefore，the heterostructure ZIF-67/NaGdF₄：Yb，Er not only tune PL emission，but also improved stability.

As can be seen from Fig. 4，the emission peak of the heterostructure ZIF-67/NaGdF₄：Yb，Er appeared at 658 nm is corresponding to the energy level transition of Er³⁺. Without ZIF-67，the energy transfer process of NaGdF₄：Yb，Er is as follows^［15］：the electrons in the ²F_7/2 ground state of Yb³⁺ are excited to the ²F_5/2 excited states after absorbing the 980 nm excitation. And then the energy is transferred to ²I_11/2，⁴F_9/2，and ⁴F_7/2 level of Er³⁺ by radiation ^［21］. Subsequently，the energy transferred to the ⁴I_11/2 level of Er³⁺ causes the electrons in this level jumped to the ⁴F_7/2 state. And then the electrons relax rapidly to the ²H_11/2 and the ⁴S_3/2 states through multiphonon relaxation steps，leading to the green emission bands. The electrons in the ⁴F_9/2 level of Er³⁺ was following nonradiative transition process to the ground state ⁴I_15/2 of Er³⁺ by absorbing the additional excitation energy migrated from Yb³⁺ and producing the red emission ^［22-23］. The green light emitted at 524 nm and 541 nm，which were mainly derived from ²H_11/2 to ⁴I_15/2，⁴S_3/2 to ⁴I_15/2 energy transfer，respectively ^［24］. And the red light emitted at 658 nm is mainly derived from ⁴F_9/2 to ⁴I_15/2 energy transfer. For heterostructure ZIF-67/NaGdF₄：Yb，Er，the new energy level is introduced from ZIF-67^［20］. Just as the result revealed in Fig. 3（b）and 3（c），the energy at the ²H_11/2 and ⁴S_3/2 of Er³⁺ is transferred to S1 of Co²⁺（form ZIF-67）. And then energy at S1 of Co²⁺ transferred to ⁴F_9/2 of Er³⁺，so that the PL intensity enhanced linearly. New energy transfer takes place in the heterostructure ZIF-67/NaGdF₄：Yb，Er，which is the key to the PL tuning and enhancement.

3 Conclusions

In summary，the heterostructure ZIF-67/NaGdF₄：Yb，Er was prepared by a facile stepwise synthesis method，the NaGdF₄：Yb，Er nanorods are uniformly loaded on the surface of ZIF-67. And the heterostructure overcame the shortcomings of NaGdF₄：Yb，Er nanorods in agglomeration and quench. Under the 980 nm laser excitation，the energy transfer takes place in the heterostructure ZIF-67/NaGdF₄：Yb，Er. And controllable PL tuning was realized by construction heterostructure the enhanced emission was converted from green light to red light. This strategy greatly enhances the applicability of heterostructure ZIF-67/NaGdF₄：Yb，Er，break through the limitation of oil phase system of NaGdF₄：Yb，Er nanorods，making it promising for biological imaging，bio-molecular detection and bio-sensor.