Facile synthesis of flower-shaped Au/GdVO₄:Eu core/shell nanoparticles by using citrate as stabilizer and complexing agent

Abstract

Metal/rare-earth core/shell hetero-nanostructures combine the optical properties of metal cores and rare-earth shells, which are widely reported and expected to be used in multitask applications. However, there is still no facile and efficient strategy to directly prepare such materials. Herein, we present a facile hydrothermal method for directly coating rare-earth vanadate shells onto the Au nanoparticle (AuNP) cores. Citrate plays a fascinating and critical role in the whole synthesis process, and not only acts as the capping agent to stabilize the as-prepared AuNP cores and the final products but also serves as the complexing agent to assist the nucleation and growth of rare-earth vanadate shells. Interestingly, the grown Au/GdVO₄:Eu core/shell NPs have a flower-like shape with tunable plasmon resonance and bright fluorescence. The morphology and crystallinity as well as the growth mechanism and tunable optical properties of the Au/GdVO₄:Eu are investigated. The method developed here could be extended for preparing other metal/rare-earth hybrids and the multifunctional products with specific morphology have the potential in the photocatalytic and biomedical application.

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1. Introduction

Hetero-nanostructures combine the unique properties of each component and also introduce new functional characters because of the inter-material interaction at the interface.^1–5 Surface plasmon resonance (SPR) of metallic nanocrystals could largely enhance the local field and confine the light at the nanoscale.^6–8 These properties confer plasmon the ability to manipulate the behaviors of photons and photon–electron interaction. Many efforts have been devoted to the synthesis and investigation of plasmon-based hetero-nanostructures.^9,10 Especially, the core/shell type hybrid nanostructures could maximize the interface area of the two materials, which have drawn intense attention and been applied in plentiful applications of photovoltaics,^4,9,10 photocatalysis,^11–17 optoelectronics,¹⁸ nanolaser,¹⁹ sensing,²⁰ and biomedicine.^21,22

Rare-earth nanocrystals have excellent luminescence properties including multicolor emission through ion doping, high-efficiency upconversion luminescence, sharp and featured emission line-shape, high photostability, etc.^23–28 Rare-earth vanadate (especially GdVO₄) based materials show wonderful multifunctional characteristics and exhibit great potential in many applications.²⁹ GdVO₄ plays an important part as a host matrix for luminescent lanthanide ions (Ln³⁺) because of the equal valence and similar ionic radii. Owing to the strong absorption of the VO₄³⁻ group under UV radiation, doped GdVO₄ is used as a multicolor phosphor (doped with Eu³⁺, Dy³⁺, Sm³⁺)^30,31 and upconversion material (doped with Yb³⁺/Er³⁺),³² and is widely used in the fields of fluorescence bio-imaging, LED, display, and laser gain materials.³³ Gd-containing nanocrystals also have been shown as an excellent contrast agent for MR and CT imaging.³⁴

Interestingly, optical manipulation and emission enhancement of rare-earth nanocrystals by plasmonic interaction are widely reported in metal/rare-earth hetero-nanostructures and expected to be used in biolabeling, imaging, light display, LED, and solar cell.^35,36 For core/shell typed metal/rare-earth hetero-nanostructures, an urgent problem need to be solved is that growing rare-earth nanocrystal shells on metallic cores and forming well-defined morphology are very difficult. The reason is there normally have large lattice mismatch and interfacial energy between these two materials. Many reported products of metal/rare-earth hybrids have inhomogeneous shape and poor dispersion or utilize silica shell as a platform to achieve uniform samples.^37,38 Similar problem has been met by metal/semiconductor core/shell hetero-nanostructures. Many methods have been introduced to break down this barrier, for instance, nonepitaxial growth method through ion exchange,^39,40 introducing wetting intermediate layer,⁴¹ forming molecule–metal cation complex to assist nucleation and crystal growth,⁴² and so on. It is expected that all these methods could be applied in the preparation of core/shell metal/rare-earth hetero-nanostructures. Recently, utilizing specific chelating agent to grow rare-earth oxide and fluoride onto the gold nanorods are reported.^42,43 Synthesis of gold/GdVO₄ core/shell nanorods have been reported via a conversion method from oxide to vanadate.⁴ However, considering the violent reaction conditions such as high temperature and high pH value for the growth of crystalline rare-earth compounds, challenges still exist for directly depositing well-defined and uniform rare-earth shells onto metallic nanoparticles, which is rarely reported in literatures.

In this paper, we report a very facile method to prepare Au/GdVO₄:Eu core/shell NPs. In the presence of citrate, the only surfactant or capping agent molecule used in the present work, GdVO₄:Eu shells could be directly grown onto the pre-prepared wild citrate-stabilized gold NPs (citrate-AuNPs). It's an amazing coincidence that citrate acts as not only the surfactant to stabilize gold nanoparticles but also the complexing agent molecules to combine rare-earth ions. At this condition, the citrate–ion complexes intrinsically anchor at the surface of gold nanoparticles and assist the nucleation and growth of rare-earth shells with perfect morphology. The method could be extended to synthesize other rare-earth vanadates. The prepared Au/GdVO₄:Eu NPs are flower-type shape with large surface-area-to-volume ratio. The Au/GdVO₄:Eu nanoflowers integrate plasmon resonances and fluorescence and both these two properties could be tuned by adjusting the shell thickness and crystallinity.

2. Experimental section

2.1. Materials

Chloroauric acid (HAuCl₄·4H₂O, 99.99%), trisodium citrate dihydrate (C₆H₅Na₃O₇·2H₂O, ≥99.5%) and trisodium tetraoxovanadate dodecahydrate (Na₃VO₄·12H₂O, 99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Gadolinium(III) nitrate hexahydrate (Gd(NO₃)₃·6H₂O, 99.99%) and europium(III) nitrate hexahydrate (Eu(NO₃)₃·6H₂O, 99.99%) were purchased from Aladdin Industrial Inc. All of the chemicals were used without further purification. Ultrapure water with a resistivity of approximately 18.25 MΩ cm was used in all aqueous solutions.

2.2. Synthesis of citrate-stabilized gold nanoparticles

The citrate-AuNPs were produced via a classical method with some modification.⁴⁴ First, 270 μL of 0.1 M trisodium citrate was added to 100 mL ultrapure water. The solution was heated to boil and vigorously stirred using magnetic bar. Then 500 μL of 50 mM HAuCl₄ was quickly added to the boiling aqueous solution. After continuous stirring and boiling for 30 min, the solution was allowed to cool to room temperature. Eventually, the color of the solution changed from colorless to wine-red, indicating that gold nanoparticles were formed. Finally, the solution was centrifuged at 10 000 rpm for 10 min and resuspended in ultrapure water. The products are citrate-AuNPs with sizes up to 25 nm.

2.3. Synthesis of Au/GdVO₄:Eu core/shell nanoparticles

For the synthesis of Au/GdVO₄:Eu core/shell NPs, 0.6 mL of 0.1 M trisodium citrate aqueous solution was added to the mixed solution of 5 mL as-synthesized citrate-AuNPs and 5 mL ultrapure water under continuous stirring. 200 μL of 0.01 M Gd(NO₃)₃ aqueous solution containing 5% Eu(NO₃)₃ and 100 μL of 0.1 M Na₃VO₄ aqueous solution were injected into the former mixture solution. Finally, after the solution was mixed well, the as-obtained mixture was transferred into a Teflon bottle (20 mL in capacity) held in a stainless steel autoclave. The solution was sealed and maintained in the oven at 180 °C for 12 h. After the autoclave was cooled to the room temperature, the final products were collected by centrifuged at 8000 rpm for 8 min.

2.4. Characterizations

Transmission electron microscopy (TEM) images were performed on a JEM 2010 HT transmission electron microscope at 200 kV and high-resolution TEM (HRTEM) images were taken on a JEOL 2100F transmission electron microscope at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) was observed using an S-4800 high resolution field emission scanning electron microscope at an accelerating voltage of 2.0 kV. Energy dispersive spectrometer (EDS) analysis was performed on an EDS equipped on a FEG SEM Sirion 200 operated at 20.0 kV. X-ray diffraction (XRD) patterns were obtained on a Bruker D8 advance X-ray diffractometer using Cu-Kα radiation (λ = 0.15418 nm). The absorption spectra were recorded on UV-Vis-NIR spectrophotometer (Varian Cary 5000 and TU-1810). Photoluminescence emission spectra were acquired on a Hitachi F-4500 fluorescence spectrophotometer with a Xe lamp as the excitation source.

3. Results and discussion

3.1. Morphology and structure of Au/GdVO₄:Eu nanoflowers

The monodisperse citrate-AuNPs were prepared in advance. The as-obtained citrate-AuNPs with size of 25 nm are quasi-spherical shape and relatively uniform (Fig. S1†). The morphology of the final products of Au/GdVO₄:Eu NPs is exhibited in Fig. 1. The flower-like nanostructures are well arranged, which seem like a geometry constituted by a lot slices of nanoflakes. The average size of the NPs is approximately 100 nm, and the thickness of the nanoflake at outside is approximately 20 nm. As the TEM image shown in Fig. 1b, 25 nm AuNP cores are well-packaged by the GdVO₄:Eu shells with thickness of 40 nm. The inset TEM image of a single particle displays that the Au/GdVO₄:Eu core/shell NPs are multangular and flower-like, which is consistent with the SEM result.

Fig. 1 Morphology of Au/GdVO₄:Eu core/shell flower-like nanoparticles is shown in SEM image (a) and TEM image (b).

To investigate the formation process of the rare-earth vanadate shells, we first studied the products generated in different reaction time. As shown in Fig. 2, the GdVO₄:Eu shells become thicken when the reaction time is prolonged from 0.5 h to 12 h. The corresponding shell thickness of the samples are 12 ± 2 nm (0.5 h), 21 ± 1 nm (1 h), 27.9 ± 2 nm (5 h) and 29 ± 1 nm (12 h), respectively. According to the observations from the TEM images, we could have a general idea of the formation process of the rare-earth vanadate shells. In the first half hour during the reaction, the vanadate began to form on the surface of AuNPs, but massive dissociative floccules could also be seen in the image. After an hour, the relatively complete vanadate shells were formed basically, and the dissociative samples were disappeared. When the reaction time was prolonged to 5 h, more compact vanadate shells were obtained and the thickness was increased. The morphology and thickness of vanadate shells changed slightly when the reaction time was further extended to 12 h. Moreover, comparing the TEM images of samples collected at different reaction time, we can see that the relative contrast of vanadate shells becomes darker. It implies the crystallinity of GdVO₄:Eu shell has changed during the whole growth process.

Fig. 2 TEM images of Au/GdVO₄:Eu core/shell NPs with different reaction time: (a) 0.5 h, (b) 1 h, (c) 5 h, and (d) 12 h. The volume of 0.1 M Gd(NO₃)₃ aqueous solution is 250 μL.

As the XRD patterns shown in Fig. 3a, the diffraction peaks can be indexed by two sorts of crystalline phase, the cubic face-centered Au phase with cell constant a = 4.08 Å (JCPDS card no. 04-0784) in red lines, and the tetragonal phase of GdVO₄ with cell parameters a = 7.2126 Å and c = 6.3483 Å (JCPDS card no. 17-0260) in green lines. All the diffraction peaks are well assigned to standard cards. Compared to the characteristic peaks of Au crystals, the intensity of diffraction peak located at 24.7° corresponding to the (200) facets of GdVO₄ increases along with the reaction time. In addition, the peak of GdVO₄ (200) facets is unobvious in the XRD patterns of sample at 0.5 h and 1 h, while the shells can be observed in the TEM images in Fig. 2a and b. The increasing intensity demonstrates that the GdVO₄ is gradually generated and the crystallinity of GdVO₄:Eu shell is gradually improved during the growth.

Fig. 3 (a) XRD patterns of Au/GdVO₄:Eu NPs with different reaction time. (b) EDS spectrum of Au/GdVO₄:Eu NPs. (c) HRTEM image of single Au/GdVO₄:Eu core/shell NP, (d) HRTEM image corresponding to the region marked by the white square in (c).

Further characterizations were performed to exam the detailed structure and composition of Au/GdVO₄:Eu core/shell NPs. The EDS analysis in Fig. 3b indicates the presence of Au, V, Eu and Gd in the core/shell nanostructures. According to the HRTEM images (Fig. 3c and d), the GdVO₄ shell is polycrystalline and the lattice fringes with a spacing of 0.36 nm in the shell region correspond to the (200) plane of the tetragonal GdVO₄.

3.2. Growth mechanism

It is reported that the growth of a heterogeneous shell on a nanoparticle is strong dependent on the surface free energy,⁴⁵ which is a barrier for preparing the well-defined hetero-nanostructures. The morphology of nanocrystals also depends on the relative growth rates of different crystal facets, which results in quite different shapes. In a typical liquid phase synthesis, organic additives usually act as surfactants, linking agents or capping agents.^46,47 Owing to the various interactions among organic additives and metal ions or crystal facets, various novel products generated.^48,49

The primary growth procedure of GdVO₄:Eu shell on the citrate-AuNPs is illustrated in Scheme 1. In our case, the Cit³⁻ is introduced into the reaction, which plays a critical role in the whole synthesis process.^50,51 Firstly, Cit³⁻ is employed as capping agent to modify the certain crystal surface for stabilizing the nanocrystals. On the other hand, the Cit³⁻ is a strong chelating agent with four binding sites, including three carboxyl group and one hydroxyl group, among which three sites can be bound with rare-earth ions (Ln³⁺). It coordinates with Ln³⁺ to form stable Ln³⁺–Cit³⁻ complex. Under hydrothermal conditions, an anion-exchange reaction between VO₄³⁻ and Cit³⁻ would take place in the high temperature and pressure, and the vanadate nuclei are formed on the Au surface. Therefore, the Cit³⁻ acts as an intermediate to link core and shell, overcome the huge barrier induced by the large crystalline mismatch.

Scheme 1 Schematic illustration for growing Au/GdVO₄:Eu core/shell nanoparticles.

According to LaMer^'s model,^50,52 the formation of Ln³⁺–Cit³⁻ complexes could also control the concentration of free Ln³⁺ ions in solution, and help to control the speed of nucleation and growth. Then, the shell growth could be regulated to achieve a uniform and well-defined morphology. On the other hand, the Cit³⁻ possibly acts as soft template which binds selectively on the specific active facets, thus the crystals would prefer to grow along with these active facets and finally form the structures which seem like flowers compounded by nanoplates. It can be obtained that the array orientation of vanadate shell is random (Fig. 3c and d). Due to the Cit³⁻ binding selectively on the specific active facets, the crystals grow preferentially along with these active facets. Meanwhile, the growing orientation is varied, resulting in small crystallite with random orientation. Furthermore, because the synthesis is accomplished in the autoclave at 180 °C, the products are high crystalline with few defects.

To verify the influence of Cit³⁻ on the growth evolution of rare-earth vanadate shell, we tried to apply the CTAB-stabilized AuNPs (CTAB-AuNPs) in our synthesis. However, it was failed to prepare the Au/GdVO₄:Eu core/shell NPs (Fig. S2a†). We also attempted to alter the concentration of the Cit³⁻ in the reaction, while the other conditions were kept unchanged. As shown in Fig. S2b,† in the absence of/with a small amount of additional Cit³⁻, AuNPs were packaged in the GdVO₄ and no monodisperse core/shell NPs were observed. This result is attributed to the insufficient chelating capability of Cit³⁻ under this condition. The insufficient chelation makes the nucleation and growth of GdVO₄ nuclei so fast that GdVO₄ entangles with AuNPs and precipitates from the solution. While introducing moderate amount of Cit³⁻, the well-packaged Au/GdVO₄:Eu core/shell NPs were prepared (Fig. S2c†). Whereas, once the amount of Cit³⁻ was excessive, no well-defined core/shell NPs was formed and most of the AuNPs were aggregated in the products (Fig. S2d†). The excessive Cit³⁻ and the strong chelation of Cit³⁻ to Gd³⁺ suppress the anion exchange reaction of VO₄³⁻ and Cit³⁻.

Therefore, three critical roles of Cit³⁻ are observed in the reaction of preparing well-defined Au/GdVO₄:Eu core/shell flower-like NPs: (a) linker and chelating agent, (b) stabilizer, (c) soft template. Cit³⁻ is a favorable and compatible ligand for stabilizing the products and tuning the interface between two dissimilar materials simultaneously. After having a full understanding of the multiple roles of Cit³⁻, the citrate-assisted method could be extended to the synthesis of various metal/vanadate hetero-nanostructures and hybrids composed of other rare-earth compounds.

3.3. Tunable plasmon resonance and bright fluorescence

Localized SPR is a distinct character of noble metal NPs and their hetero-nanocomplex.^53–55 In the absorption spectra of Au/GdVO₄:Eu NPs obtained form 0.5 h to 12 h (Fig. 4a), the SPR bands are varied from 530 nm to 558 nm, which is attribute to the increase of local refractive index caused by the growth of GdVO₄ shell. The absorption peak appeared at 274 nm is attributed to VO₄³⁻. As the reaction carried on, the shells become thicker and the crystallinity is improved. We found that the increasing trend of SPR band is consistent with that of the shell thickness (Fig. S3a†). Eu³⁺ doped gadolinium vanadate is a common bio-phosphor.²⁹ Bright red emission can be observed in the aqueous solution of Au/GdVO₄:Eu NPs under UV light irradiation, and the emission spectrum excited at 300 nm is shown in Fig. 4b. The multi-emission peaks located at approximately 594, 617, 650 and 697 nm come from the electric dipole transitions from 5D₀ to 7F_J (J = 1, 2, 3, 4) of Eu³⁺, whereas the weak peak at 538 nm belongs to the transition of 5D₁–7F₁. The most intense emission located at 617 nm is ascribed to the transition of 5D₀–7F₂, a forced electric-dipole transition of the Eu³⁺ ions in GdVO₄. Generally, the whole excitation and emission process of GdVO₄:Eu³⁺ under UV radiation can be described as follows: the VO₄³⁻ groups absorb UV radiation and are excited to the excited states, then the excited energy subsequently transfers to Eu³⁺ ions after a thermally activated energy migration through the vanadate sublattice, and finally the excited Eu³⁺ ions produce strong red emissions in the de-excitation process. From the spectra in Fig. 4b and the plotted curves in Fig. S3b,† it can be seen that the Eu³⁺ fluorescence intensity is enhanced as the reaction time is increased, which is probably related to the thickness and crystalline of the vanadate shells.

Fig. 4 (a) Absorption and (b) fluorescence spectra of Au/GdVO₄:Eu NPs with different reaction time. The insets are photographs of the sample under white light (left) and UV light (right) irradiation.

The thickness of rare-earth vanadate shells is also successfully adjusted via controlling the addition of rare-earth nitrates in the reaction. As shown in Fig. 5, the shell thickness of Au/GdVO₄:Eu NPs increases when the volume of 0.01 M Gd(NO₃)₃ increases from 50 μL (Fig. 5a) to 250 μL (Fig. 5c). Meanwhile, the SPR band is red-shifted from 542 nm to 562 nm (Fig. 5d). The corresponding emission spectra have also been measured and shown in Fig. 5e. It is obvious that the fluorescence intensity becomes stronger as the shell become thicker. Similar results are observed when the concentration of AuNPs is increased while keeping the addition of Gd(NO₃)₃ and Na₃VO₄ unchanged. The blue-shift of SPR band (Fig. S5a†) indicates the shell thickness is decreased as the concentration of AuNPs is increased. As a consequence, the fluorescence intensity is gradually decreased (Fig. S5b†).

Fig. 5 TEM images of Au/GdVO₄:Eu NPs with different amount of 0.1 M Gd(NO₃)₃ aqueous solution: (a) 50 μL, (b) 100 μL, (c) 250 μL. (d) Absorption and (e) fluorescence spectra of the corresponding samples.

In the preceding reaction of different time, we observe that the thickness and the crystallinity of vanadate shell are increased. These two factors are both contributed to the plasmon shift and fluorescence enhancement. As the reaction time is set to 12 h, the shell thickness varies with the amount of added Gd(NO₃)₃ and AuNPs and the fluorescence intensity of these samples could be adjusted in consequence. In the present samples, the fluorescence properties are determined by the thickness and the crystallinity of vanadate shell as well as the plasmon interaction between the core and the shell, while the specific details are needed to be further studied.

4. Conclusions

In summary, we have developed a facile and efficient approach for coating rare-earth vanadate directly on AuNPs to synthesize multifunctional core/shell nanostructures. We have also demonstrated that Cit³⁻ plays a key role in the formation of well-defined rare-earth vanadate shells on AuNPs. Cit³⁻ acts as a favorable and compatible ligand for stabilizing the products and tuning the interface between two dissimilar materials simultaneously. This Cit³⁻-assisted synthetic methodology is simple and convenient, because no other steps will be necessary, such as surfactant exchange or transformation of the shell composition from oxide to vanadate. Given this, we may apply this method into the synthesis of various metal/vanadate hetero-nanostructures. The prepared Au/GdVO₄:Eu core/shell NPs have a well-defined flower-like morphology. This characteristic shape has a large surface-area-to-volume ratio and possesses potential capability in the application of photocatalysis because vanadate has been shown as a photocatalytic materials and the plasmon interaction could enhance this ability. Moreover, we have successfully adjusted the thickness of rare-earth vanadate shells via controlling the addition of rare-earth nitrates or the concentration of AuNPs in the reaction. The plasmon band and the bright red emission at 617 nm of Au/GdVO₄:Eu NPs is tunable and can be adjusted by the shell thickness. These results show that the size-controllable and crystalline Au/GdVO₄:Eu core/shell NPs combining intense plasmon resonance and strong fluorescence have potential in the application of biomedical labeling/imaging and photothermal therapy.

Acknowledgements

The authors thank Ms Jia Yue for the technique help. This work was supported by the National Program on Key Science Research of China (2011CB922201), and the National Natural Science Foundation of China (11374236, 11174229, and 51372175).

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Footnote

† Electronic supplementary information (ESI) available: TEM images of initial Au nanoparticles, TEM images of hybrids synthesized using CTAB-stabilized Au NPs and different amounts of sodium citrate, absorption and fluorescence evolution of Au/GdVO₄:Eu core/shell nanoparticles prepared at different growth condition. See DOI: 10.1039/c5ra23958c

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