Trans. Nonferrous Met. Soc. China 30(2020) 1031-1037

Effect of Eu³⁺-doping on morphology and fluorescent properties of neodymium vanadate nanorod-arrays

Li TIAN^1,2,3,4, Shan-min CHEN^1,4, Qiang LIU^1,4, Jie-ling WU^1,4, Rui-ni ZHAO³, Shan LI¹, Li-juan CHEN^1,4

1. School of Materials Science and Engineering, Hunan University of Science and Technology, Xiangtan 411201, China;

2. Hunan Provincial Key Defense Laboratory of High Temperature Wear-resisting Materials and Preparation Technology, Xiangtan 411201, China;

3. Hunan Provincial Key Laboratory of Controllable Preparation and Functional Application of Fine Polymers, Xiangtan 411201, China;

4. Hunan Provincial Key Laboratory of Advanced Materials for New Energy Storage and Conversion, Xiangtan 411201, China

Received 24 May 2019; accepted 10 March 2020

Abstract: Tetragonal structural (t-NdVO₄) nanorod-arrays were fabricated by simple one-pot hydrothermal method. The phase, morphology and microstructure of NdVO₄ were characterized by X-ray diffractometer, scanning electron microscope (SEM), transmission electron microscope (TEM), dispersive X-ray spectrometer (EDS) and selected area electron diffraction (SAED) techniques. t-NdVO₄ nanorods are single-crystalline with a length of 100 nm and a diameter of 25 nm, which grow orientally along the direction of (112) crystalline plane and self-assemble to form nanorod-arrays. The results show that Eu³⁺-doping interrupts the formation of NdVO₄ nanorod-arrays, and then leads to the red-shift of the strongest luminescence emission of Nd³⁺ transition from ⁴D_3/2 state to ⁴I_11/2 and decreases its intensity of the fluorescence emission at 400 nm sharply. The research results have some reference values to optimize the photoluminescence performance of rare earth vanadates.

Key words: Eu³⁺-doping; morphology; fluorescent properties; neodymium vanadate; nanorod-arrays; hydrothermal method

1 Introduction

Rare earth vanadate nanomaterials possess the excellent physical and chemical properties owing to the unique 4f electronic structure. A variety of micro-topographies further expend their application range. At present, they have been extensively exploited in the fields of light, electricity, magnetism, catalysis, sensing and even infiltrated into other fields such as medical image science, biological probes and biological tags, showing broad application prospects in our daily lives [1,2]. The neodymium orthovanadate (NdVO₄) is an important member in rare earth vanadate functional materials, which has attracted a lot of attention from the international researchers recently because of its distinctive properties and potential application [3-6].

Having great novel photoelectric performances, NdVO₄ nanocrystals are suggested to use in the fields of imaging, biological labeling, detecting, and lasers [7-10]. It is well known that the fabrication methods of inorganic functional materials play an important role in achieving the particular nanostuctures and novel morphology [11,12]. By far, there are different fabrication techniques to obtain NdVO₄ nanoparticles, for example, sonochemical operation, sol-gel preparation and microwave method [13-20]. YUVARAJ and KALAI [18] prepared NdVO₄ nanoparticles by sonochemical synthesis and studied the structural, magnetic and grain size dependent electrical properties. WU et al [19] obtained single-crystalline NdVO₄ nanorods by two-step stategy, and researched their emissions in the ultraviolet. FAN et al [20] synthesized the single-crystalline lanthanide orthovanadate nanorods by hydrothermal operation. However, the as-synthesized NdVO₄ nanorods are of different sizes and no NdVO₄ nanorod-arrays can be observed.

It is a challenge to obtain monodisperse NdVO₄ nanorods by simple preparation method. And NdVO₄ nanoarrays composed of monodisperse nanorods have not been reported so far. In this work, we prepared highly oriented t-NdVO₄ nanorod-arrays via a simple and controllable hydrothermal reaction process, particularly without hard or soft template. The synthesis method is reproducible. The crystalline phase, morphology and photoluminescence performance of the Eu³⁺-doping NdVO₄ nanoarrays were studied. The effect of Eu³⁺-doping on the crystalline phase and morphology of NdVO₄ nanoarrays was studied, showing that Eu³⁺-doping interrupts the oriented-growth and self-assembly of NdVO₄ nanorods. And Eu³⁺-doping leads to the red-shift of the strongest luminescence emission of Nd³⁺ transition from ⁴D_3/2 state to ⁴I_11/2 and decreases its intensity of the fluorescence emission at 400 nm sharply.

2 Experimental

2.1 Sample preparation

All chemicals including Nd(NO₃)₃·6H₂O, NH₄VO₃, NaOH and EDTA-2Na with analytic purity were not further purified before they were used. Typically, 0.428 g EDTA-2Na, 0.443 g Nd(NO₃)₃·6H₂O and 5.0 mL distilled water were added into a beaker under vigorous stirring to form transparent solution. At the same time, 0.117 g NH₄VO₃ (1.0 mmol) was dissolved with 5.0 mL NaOH aqueous solution in another glass container. The resulting two kinds of solutions were poured into a Teflon-lined stainless steel autoclave of 20 mL capacity, and the pH value of the mixture was adjusted to 10 with NaOH aqueous solution. And then, the hydrothermal reaction temperature was kept constant at 180 °C for 24 h. The autoclave was cooled in air to room temperature after hydrothermal reaction. The precipitates were centrifuged, washed with deionized water and ethanol several times, and then dried in a vacuum oven. All Eu³⁺-doping samples were prepared by the hydrothermal method under the same conditions. The Eu³⁺ contents were chosen to be 0, 1, 3, 5 and 7 wt.% in the NdVO4:Eu³⁺ samples, respectively.

2.2 Characterization

Powder X-ray diffractometer (XRD, Bucker D8 Advance) with Cu K_α radiation (λ=0.1541 nm) was used to characterize the phase of the products. The graphite monochromator was operated at 40 kV, 40 mA and a scanning speed of 10 (°)/min from 10 to 80 °C. The morphology and crystal structures of the NdVO₄ nanocrystals were characterized by scanning electron microscopy (SEM) images taken on a JEOL JSM-6330F field emission scanning electron microscope. The samples were gold-coated prior to the SEM analysis. The microstructure of NdVO₄ nanoarrays was further characterized by a JEM-2010HR transmission electron microscope (TEM). The accelerating operated voltage is 200 kV. The energy dispersive X-ray spectrum (EDS) was collected on an Oxford ISIS-300 energy dispersive X-ray spectrometer. Photoluminescence properties at room temperature were studied using PL spectrum with fluorescence spectrophotometer (F-4500). The products were excited by ultraviolet light obtained from xenon lamp with the excitation wavelength of 250 nm.

3 Results and discussion

3.1 Phase and crystal structures

The phase purity and crystal structures of the samples were characterized by powder X-ray diffractometry (XRD). Figure 1(a) shows the XRD pattern of as-prepared NdVO₄ nanorod-arrays. All the peaks can be well indexed to the tetragonal zircon-type structure of neodymium vanadium oxide (NdVO₄, JCPDS No. 15-0769) which has better photoluminescence performance than the samples with monazite-type structure [21], and the standard XRD pattern is shown in Fig. 1(b). As shown in Figs. 1(a) and (b), there are no other peaks found, indicating that highly-pure NdVO₄ can be achieved. Those sharp peaks in Fig. 1(a) indicate good crystallinity of the products. Compared with the standard card, the (200) and (112) diffraction peaks are observed to be different, revealing the orientation of crystal growth. In Fig. 1(a) the diffraction peak of (112) crystalline plane is higher and the diffraction peak of (200) crystalline plane is lower. Maybe it is related to the nanorod morphology and indicates the growth direction of (112) crystalline plane of NdVO₄ nanoarrays. The mean crystallite size of the as-synthesized of NdVO₄ nanoparticles is estimated by using the Scherrer equation to be 25 nm based on the FWHM (full-width at half maximum) of the (220) peak.

Fig. 1 XRD patterns of as-prepared NdVO₄ nanoparticles (a), standard sample of NdVO₄ (JCPDS No. 15-0769) (b) and as-prepared NdVO₄:Eu³⁺ with different contents of Eu³⁺ (c)

Ion-doping experiments were carried out to testify the effect of Eu³⁺-doping on the crystalline phase, the morphology, the microstructure and the fluorescence of neodymium vanadate nanorod- arrays, keeping Eu³⁺ contents of NdVO₄:Eu³⁺ samples at 1, 3, 5 and 7 wt.%, respectively. The XRD patterns of the products are shown in Fig. 1(c). All the diffraction peaks shown in Fig. 1(c) are in agreement with the standard crystallographic data (JCPDS No. 15-0769) of tetragonal structural neodymium vanadium oxide (NdVO₄, space group I41/amd). That is to say, NdVO₄ can be gained and Eu³⁺-doping has no distinct effect on the formation of tetragonal structural NdVO₄.

3.2 Morphology and microstructure

The morphology and microstructure of as-prepared NdVO₄ nanopaticles were examined by scanning electron microscopy. Figures 2(a-c) present the SEM images of as-obtained NdVO₄ nanorod-arrays with different magnifications. It is obvious that the NdVO₄ nanorods self-assemble to yield uniform nanoarrays. The higher magnification SEM images of neodymium vanadate nanoparticles shown in Figs. 2(b) and (c) indicate that NdVO₄ nanoarrays are composed of massive nanorods. The diameter of the nanorods is about 25 nm and the length is 100 nm or so. It is much smaller compared with the NdVO₄ nanorods synthesized by WU et al [19], which have rectangular cross-sections from about 30 nm × 30 nm to 100 nm × 200 nm and the length from 400 to 700 nm. This would be of great significance because of the possible novel properties induced by the reduced dimensionality and monodispersity.

Figure 2(d) gives the TEM image of NdVO₄ nanorod-arrays with ultrasonic treatment in need of TEM test, showing the self-assembly of NdVO₄ nanorods. It is clear that the nanorods still orient with a fine order. The SEAD pattern shown in Fig. 2(e) is obtained from the area circled in Fig. 2(d), indicating the growth direction of (112) crystalline plane and single-crystalline character of NdVO₄ nanorods.

Fig. 2 SEM images with different magnifications (a, b, c) and TEM image (d) of as-prepared NdVO₄ nanorod-arrays, and SAED pattern of NdVO₄ nanorod (e) obtained from area circled in Fig. 2(d)

Fig. 3 SEM images of NdVO₄ nanoparticles prepared with different Eu³⁺-doping contents at different magnifications

Figure 3 displays the morphologies of the products prepared with different contents of Eu³⁺ being 1, 3, 5 and 7 wt.%, respectively. When

Eu³⁺ is doped into the reaction system, it is clear that no NdVO₄ nanorod-arrays are found. And the NdVO₄ nanoparticles in Fig. 3 are all in a mess and out of order. It is indicated that Eu³⁺-doping interrupts the self-assembly of NdVO₄ nanorods and is unfavorable for the formation of NdVO₄ nanorod-arrays. The destructive behavior of Eu³⁺-doping to the morphology of NdVO₄ nanorod- arrays maybe influence their photoluminescence (PL) performance.

Figure 4 shows the EDS spectra of NdVO₄ nanoparticles without Eu³⁺-doping and with the Eu³⁺-doping contents of 3 and 7 wt.%, respectively. Without Eu³⁺-doping, the detected Nd/V/O mole ratio of NdVO₄ nanorod-arrays is about 1/1/4. When Eu³⁺ is doped into the reaction system, no oriented NdVO₄ nanorod-arrays are found as shown in Fig. 3, showing the effect of Eu³⁺-doping on the formation and the morphology of NdVO₄ nanocrystals. In the EDS patterns of the samples synthesized with Eu³⁺-doping, there is no obvious Eu signal detected. But from the EDS data, we find the Eu³⁺ mole fraction of 0.38% in the product prepared with Eu³⁺-doping content of 7 wt.%. It is suggested that lower content of Eu³⁺ cannot be detected by EDS in this work.

3.3 Photoluminescence performance

Fig. 4 EDS spectra of as-prepared NdVO₄ nanorod- arrays without Eu³⁺-doping (a) and with Eu³⁺-doping contents of 3 wt.% (b) and 7 wt.% (c)

The photoluminescence (PL) properties of NdVO₄ nanoparticles are recorded by fluorescence spectrometer at room temperature. Figure 5 shows the PL spectra of the samples without Eu³⁺-doping and with different Eu³⁺-doping amounts of 1, 3, 5 and 7 wt.%. As shown in Fig. 5(a), a number of sharp emission peaks occur in the wavelength region from 525 to 725 nm, with the excitation wavelength of 310 nm. The emssion bands of NdVO₄:Eu³⁺ are associated with the f-f transitions (around 540 and 559 nm) and ⁵D₀-⁷F₁ (around 595 nm), ⁵D₀-⁷F₂ (around 620 nm), ⁵D₀-⁷F₃ (around 702 nm) transitions of Eu³⁺ [22]. From the results, the variation trend of the samples with different Eu³⁺-doping amounts is the same. The integrated intensity increases gradually with the Eu³⁺-doping content increasing to 7 wt.%. This indicates that more Eu³⁺ ions are incorporated into the host lattice at a higher Eu³⁺-doping content (7 wt.%) and prominent energy migration between the Eu³⁺ ions takes place. The mission intensity of ⁵D₀→⁷F_j depends on the amount of Eu³⁺ [23].

Fig. 5 Photoluminescence spectra of as-prepared NdVO₄:Eu³⁺ without Eu³⁺-doping and with different Eu³⁺-doping contents

To identify the effect of Eu³⁺-doping on the photoluminescence performance of NdVO₄, Fig. 5(b) shows the emission spectra of the samples in the wavelength region from 280 to 480 nm excited by 250 nm. There are two distinct luminescence emission peaks observed, one weak peak at 300 nm and the other strong peak at 400 nm, if no Eu³⁺ ions are doped in the reaction system. Because NdVO₄ nanocrystals are composed of Nd³⁺ and , the weak peak at 300 nm (4.2 eV) is most likely a result of the electron transition in , which corresponds to electron transition from O 2p nonbonding state to V 3d and O 2p antibonding states. The strong emission peak at 400 nm can be attributed to Nd³⁺ transition from the ⁴D_3/2 state to ⁴I_11/2.

At the same time, two extraordinary weak luminescence emission peaks can been detected at 352 and 467 nm which are assigned to Nd³⁺ transitions from the ⁴D_3/2 and ⁴G_11/2 states to ⁴I_9/2, respectively. When Eu³⁺ ions are doped in the reaction system and interrupt the formation of NdVO₄ nanorods, the intensity of the strongest emission peak decreases sharply, indicating the quench effect of Eu³⁺ on the fluorescence properties of NdVO₄ nanocrystals. In addition, it can be found that the strongest emission peak of 400 nm has some red-shift to 412 nm, showing the change of Nd³⁺ transition from the ⁴D_3/2 state to ⁴I_11/2, owing to the doping of Eu³⁺. As we all know, the size of Eu³⁺ is smaller than that of Nd³⁺. When NdVO₄ nanocrystals are doped by Eu³⁺ with smaller size, the lattice constant decreases and the intensity of crystal field increases. Generally, the photo- luminescence emission peak of rare earth vanadates has some red-shift due to the increasing intensity of crystal field [24]. The study on the detailed relation between crystal structure and photoluminescence performance is under way.

4 Conclusions

(1) Tetragonal structural NdVO₄ nanoarrays composed of monodisperse nanorods were prepared by simple and reproducible hydrothermal methods.

(2) NdVO₄ nanoparticles are testified to grow orientally along the direction of (112) crystalline plane to form tetragonal structural NdVO₄ nanorods with the mean length of 100 nm and the diameter of 25 nm. Tetragonal structural NdVO₄ nanorods show single-crystalline character and self-assemble into NdVO₄ nanoarrays.

(3) Eu³⁺-doping has no effect on the crystalline phase of NdVO₄ nanoarrays, but interrupts the oriented-growth and self-assembly of NdVO₄ nanorods. NdVO₄ nanorod arrays prepared exhibit strong luminescence emission at 400 nm attributed to Nd³⁺ transition from the ⁴D_3/2 state to ⁴I_11/2. The strongest luminescence emission at 400 nm has red-shift to 412 nm with sharp decrease of the fluorescence emission intensity due to the Eu³⁺-doping. It is indicated an interruption of Eu³⁺-doping to the morphology of NdVO₄ nanoarrays and a quench effect of Eu³⁺ on the fluorescence property of NdVO₄ nanoarrays, which is of a certain significance to optimize the photo- luminescence performance of rare earth vanadates.

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Eu³⁺掺杂对钒酸钕纳米棒阵列形貌和荧光性能的影响

田 俐^1,2,3,4，陈善民^1,4，刘 强^1,4，吴杰灵^1,4，赵瑞妮³，黎 珊¹，陈丽娟^1,4

1. 湖南科技大学 材料科学与工程学院，湘潭 411201；

2. 湖南省高温耐磨材料与制备技术重点防御实验室，湘潭 411201；

3. 湖南省精细聚合物可控制备与功能应用重点实验室，湘潭 411201；

4. 新能源储存与转换先进材料湖南省重点实验室，湘潭 411201

摘 要：采用简单一锅水热法制备具有四方晶相的NdVO₄(t-NdVO₄)纳米棒阵列。通过X射线粉末衍射仪(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、X射线能谱仪(EDS)和选区电子衍射(SAED)技术对t-NdVO₄纳米棒阵列的物相、形貌和显微组织进行表征。所制备的t-NdVO₄纳米棒为单晶，长度约为100 nm，直径为25 nm，并且沿(112)晶面方向定向生长、自组装而成纳米棒阵列。研究表明，Eu^{3 +}掺杂会影响NdVO₄纳米棒阵列的形成，并导致Nd³⁺从⁴D_3/2状态到⁴I_11/2的最强光发射红移，并且在400 nm处急剧降低其荧光发射强度。研究结果对优化稀土钒酸盐的光致发光性能具有一定的参考价值。

关键词：Eu³⁺掺杂；形貌；荧光性能；钒酸铵；纳米棒阵列；水热合成

(Edited by Wei-ping CHEN)

Foundation item: Project (51202066) supported by the National Natural Science Foundation of China; Project (NCET-13-0784) supported by the Program for New Century Excellent Talents of the Education Ministry, China

Corresponding author: Li TIAN; Tel: +86-18627323439; E-mail: 1060072@hnust.edu.cn

DOI: 10.1016/S1003-6326(20)65274-8