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Abstract
LaVO4:Cu,Eu nanorod array films have been prepared on Cu substrates by the hydrothermal epitaxy method. Most of the LaVO4 crystallizes into the tetragonal (t-) LaVO4 phase, with a small amount of monoclinic (m-) LaVO4. Cu2+ ions enter the LaVO4 matrix together with Eu3+ ions. An epitaxial layer composed of tightly packed LaVO4:Cu,Eu nanoparticles grows on Cu substrate and the nanorod array is then assembled by LaVO4:Cu,Eu square nanorods growing on the epitaxial layer. The side length of nanorods is tunable from 20∼300 nm by adjusting the reactants concentration. The LaVO4:Cu,Eu nanorods of the array are single crystals and grow along the (101) plane. A lattice rotation occurs at the growth of the LaVO4:Cu,Eu nanorods due to the lattice mismatch between LaVO4 (112) and Cu (110). Based on the experimental results, a formation mechanism including the surface dissolution of the Cu substrate, the epitaxial layer and the self-assembly of the nanorods is proposed for the LaVO4:Cu,Eu nanorod array film. Finally, the LaVO4:Cu,Eu nanorod array films exhibit red emissions, relatively high quantum efficiencies and long lifetimes.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, the development of films with nano-array structures has attracted great interest owing to their special optical and electrical characteristics [1]. The nanostructures uauslly affect their intrinsic chemical and physical properties. Well aligned architectures of ordered materials with control and feature size may have many potential applications such as gas sensors, photocatalytic devices and solar cells [2]. It is expected that luminescent thin films with nano-array structure will show potential advantages of enhanced intensity, uniformity and excellent adhesion to substrates [3,4].
Generally, the microstructure of host crystals has crucial impact on its luminescent properties. Owing to an efficient energy transfer from the VO43− group to the dopants, lanthanide orthovanadate (LaVO4:Ln3+) has been widely studied [5]. Because of the large ionic radius of La3+ ion, it is different from other lanthanide orthorvanadate that LaVO4 crystals have two common polymorphs: metastable tetragonal phase (t-) and stable monoclinic phase (m-). Many studies confirm that metastable t-LaVO4 is a better host crystal for its luminescent properties [6]. Hydrothermal method is an effective way for the synthesis of metastable substance. So far, t-LaVO4 crystal powders with various shapes and sizes have been obtained with the presence of EDTA at a wide pH range of 3∼10 [7–9]. However, few research has been reported on the preparation of t-LaVO4 thin films by single hydrothermal method. The t-LaVO4 thin films have been prepared by pulsed laser deposition [10], magnetron sputtering [11] and sol-gel process [12]. Spin coating and solid phase epitaxy are two common technologies among these methods. LaVO4 thin films obtained by spin coating process commonly show weak luminescence, and additional heat treatment is required to increase the crystalline quality of the films and enhance their emission intensities [12,13]. Solid phase epitaxy technology needs expensive devices and is unsuitable to industry production. Hydrothermal epitaxy is a prospect preparation technology for thin films with high crystalline quality. In addition, hydrothermal technology enables directed self-assembly to organize particulate units (e.g., molecules or nanoparticles) into high ordered structures, which is beneficial to obtain nano-array structures [14,15]. The difficulty in preparing of the films by hydrothermal epitaxy has been attributed to the small mismatch between films and substrates. Cu is a cubic structure with the lattice surface of (110), (100) and (111) and the interplanar distance of (110) plane (0.256 nm) is close to the t-LaVO4 (112) plane (0.278 nm) with a small lattice mismatch of 7.91%. Therefore, the preparation of t-LaVO4 nano-array films by hydrothermal epitaxy is feasible but still a challenge.
In this work, a new design for the preparation of LaVO4:Cu,Eu nanorod array films is proposed. Copper sheet is selected as a substrate, and the LaVO4:Cu,Eu nanorod array films are assembled with EDTA by a single hydrothermal epitaxial process. By means of XRD, SEM, HRTEM and Raman spectroscopy, the microstructures of the LaVO4:Cu,Eu nano-array films are illustrated to investigate the growth process. Furthermore, the luminescent properties of the LaVO4:Cu,Eu nano-array films are studied. The preparation of LaVO4:Cu,Eu nano-array films would promote the development of other photoelectric functional films, which is of great value to the further understanding of the epitaxial films and 1D nano-array assembly.
2. Experimental
2.1 Materials and synthesis
The copper sheet was cut to the size of 22 mm×13 mm. Before placed into the initial solution, the copper sheet was polished with water abrasive paper and the oxide layer was removed by ultrasonic wave in 3% hydrochloric acid solution.
The hydrothermal process was as follows: 2 mL of 0.10 mol/L Na2EDTA aqueous solution was added into 2 mL of 0.10 mol/L LaCl3 and EuCl3 mixed solution with the La/Eu molar ratio of 95:5, and 2 mL of 0.10 mol/L NH4VO3 aqueous solution was slowly added to the above solution. Appropriate amount aqueous solution was added with the total volume of about 16 mL. The pH value was adjusted to 10 with NaOH solution. All above experimental process was carried out under stirring. The initial solution was transferred into a teflon-lined autoclave. At the same time, the copper sheet was placed to lean against the inner wall of teflon lining. The reaction occurred at 200 °C for 48 h. After the reaction was over, the copper sheet was removed from the solution and washed by alcohol and water for several times. Finally, the copper sheet was dried in the air.
In order to investigate the controllable synthesis of the t-LaVO4:Cu,Eu nanorods, similar experiments were carried out by adjusting the molar concentration of all the reactants (including LaCl3 and EuCl3 mixed solution, NH4VO3, Na2EDTA) from 0.10 mol/L to 0.05 and 0.20 mol/L with other experimental conditions fixed.
2.2 Measurements and characterization
Powder X-ray diffraction (XRD) patterns were performed on a Rigaku Ultima IV diffractometer. The surface and sectional morphologies of the films were observed on a Hitachi S-4800 field-emission scanning electron microscopy (SEM). The TEM and HRTEM images of the film were taken on a JEOL-2100 transmission electron microscopy measurement with the selected electron diffraction (SAED). The Raman spectra were measured by a HORIBA Lab Ram HR Evolution Raman system. The Uv-vis diffuse reflectance spectra were characterized on a Shimadzu 2550 spectrophotometer. The luminescence spectra, quantum efficiencies and lifetimes were determined on a HORIBA FluoroMax-3 spectrophotometer.
3. Results and discussion
3.1 Microstructures of the LaVO4:Cu,Eu films
Figure 1 shows the SEM images of the films prepared from different concentrations of the reactants, and the inserts depict the corresponding magnified images. Figure 1(a1) displays the surface image of the film prepared from 0.05 mol/L of reactants. Orderly square nanorods can be observed with a side length of 200∼300 nm. Figure 1(a2) displays the cross-section image of the film. It is clear that a layer on Cu substrate is composed of nanoparticles with the diameter of 30∼100 nm and a nanorod array with the length of about 1.2 µm grows on the nanoparticles. Figure 1(b1) illustrates the surface image of the film prepared from 0.10 mol/L of reactants. It can be seen that orderly nanorods are square in cross-section with a side length of 60∼120 nm. The sectional view in Fig. 1(b2) displays that the film is composed of a thin layer of compact nanoparticles on Cu substrate and a nanorod array aligned by ordered nanorods with a height of about 1.2 µm. Figure 1(c1) and 1(c2) display the surface and cross-section images of the film obtained from the concentration of 0.20 mol/L. The nanorod array is observed by orderly aligned nanorods with a side length of 20∼40 nm. It can be observed that the layer on Cu substrate is constructed by compact nanoparticles with a diameter of less than 20 nm and the nanorod array has an average height of 1.1 µm. Undoubtedly, the obtained LaVO4:Cu, Eu nanorod array films are constructed by a layer of nanoparticles and a nanorod array, and the size of nanorods is well controlled by adjusting the concentration of the reactants.
Fig. 1. The surface (1) and cross-section (2) SEM images of the films prepared from different concentrations of the reactants: (a) 0.05 mol/L; (b) 0.10 mol/L; (c) 0.20 mol/L.
Figure 2(a) shows the XRD patterns of Cu substrate and the films prepared from different concentrations of the reactants. The pattern of Cu substrate displays the intense characteristic peaks at 43.5°, 74.2° and 50.5°, corresponding to (111), (200) and (220) planes of JCPDS NO. 03-1005, respectively. All the patterns of the films also display the intense characteristic peaks of Cu substrate and it is noted that the (220) diffraction peak of Cu relatively increased compared with Cu substrate. Except for the peaks of Cu, the peaks around 17.9°, 23.8°, 29.9°, 32.0°, 33.8°, 36.4° and 38.5° are well indexed to t-LaVO4 phase (JCPDS NO.32-0504). For the films prepared from 0.05 and 0.10 mol/L of the reactants, the weak peaks around 21.4°, 26.5° and 28.1° are indexed to monoclinic LaVO4, respectively corresponding to (101), (200) and (120) planes of JCPDS 70-0216. From the pattern of the film from 0.20 mol/L of the reactants, m-LaVO4 can hardly be observed and the sharp peaks indicate the high crystallinity of t-LaVO4 nanocrystals. With the increase of the concentration of the reactants, the diffractiom intensities of the tetragonal phase are obviously enhanced, while those of the monoclinic phase are decreased. It means that the composition of the films is mainly t-LaVO4 with a small amount of m-LaVO4. Compared with the standard pattern, the relative reflection intensities of (101) and (112) are obviously strengthened, indicating the preferentially orientation of (101) and (112) facets [16]. No other phases are examined. The film obtained from 0.20 mol/L of the reactants has the strongest peaks compared to the films prepared from 0.05 and 0.10 mol/L of the reactants. It means that the film obtained from 0.20 mol/L of the reactants has high crystalline quality. Based on the diffraction data of the films obtained from different concentrations of the reactants, the refined lattice parameters are listed in Tab. 1. The cell parameters are close for variation concentrations of the reactants. The lattice parameters are a little larger than the standard values of JCPDS 32-0504 and the previous reports [17]. The increase of the cell parameters may be deduced that the Cu2+ ions with smaller ionic radius enter t-LaVO4 matrix together with Eu3+ ions. Figure 2(b) shows the Raman spectra of t-LaVO4:Cu,Eu nanorod array film. The peaks at around 264, 380, 757, 789, 799, 852, 865 and 902 cm−1 confirm the existence of the vanadate ions [18]. Except the peak of 757 cm−1, all above peaks are associated with the internal vibration of vanadate groups from t-LaVO4. The peak around 757 cm−1 is corresponding to the (Ag) vibration of the vanadate ions in m-LaVO4. The peaks at 162 and 215 cm−1 come from the external modes (rotation/translation) of VO43− group [19]. The translation of La atoms occurs at 126 and 466 cm−1. A weak peak at 579 cm−1 is attributed to the characteristic vibrational mode of Cu-O bond. There is no CuO phase in the films in term of XRD analysis, and the divalent Cu2+ ions should substitute La3+ sites in the LaVO4 lattice. So the analysis of Raman spectra also confirms that the films are composed by t-LaVO4:Cu,Eu and a small amount of m-LaVO4:Cu,Eu.
Fig. 2. XRD patterns and Raman spectra of the films prepared from different concentrations of the reactants.
Table 1. Lattice parameters of the LaVO4:Cu,Eu films prepared from different concentrations of the reactants
In order to further investigate the element composition of the prepared nanorod array films, the film prepared from 0.20 mol/L of the reactants was peeled off from Cu substrate and characterized by EDS and mapping graphs (as shown in Fig. 3). The EDS pattern in Fig. 3 confirms that the nanorod array film contains La, V, O, Cu and Eu elements. The molar fractions of La, Eu, Cu, V and O are 17.2%, 1.2%, 1.7%, 20.5% and 59.4%, respectively. According to the results of the mapping graphs, the elements of Cu and Eu are uniform in the nanorod array. In combination with the above analysis, it is confirmed that Cu2+ and Eu3+ ions enter the lattice of LaVO4 nanorods simultaneously, and LaVO4:Cu,Eu films have been prepared.
Figure 4 shows the TEM image, the corresponding HRTEM pattern and SAED pattern of the t-LaVO4:Cu,Eu film prepared from 0.20 mol/L of the reactants. The TEM image in Fig. 4(a) displays that the LaVO4:Cu,Eu nanorods of the nanorod array are 60∼120 nm in diameter. Figure 4(b) and 4(c) show the HRTEM and SAED patterns of the nanorods taken from the selected areas marked in Fig. 4(a). The measured interplanar spacings in Fig. 4(b) are 0.278, 0.296 and 0.494 nm, corresponding to (112), (211) and ($\left( {0\bar{1}1} \right)$) facets of t-LaVO4, respectively. A 30°-twisted rotation occurs at the growth direction of t-LaVO4:Cu,Eu nanorods due to the lattice mismatch between the (112) direction of LaVO4 and the (110) direction of Cu [20]. The SAED pattern confirms that the LaVO4:Cu,Eu nanorods of the array are single crystalline structure. This growth behavior would give rise to the main exposure of (112) facets, which gives a reasonable explanation of the enhancement of the relative reflection intensity of the (112) diffraction peak in the XRD patterns shown in Fig. 2a [21]. The results hint that the novel microstructure of LaVO4:Cu,Eu nanorod array is subordinated to a new growth mechanism.
Fig. 4. TEM image (a), the corresponding HRTEM pattern (b) and SAED pattern (c) of the LaVO4:Cu,Eu nanorod array film prepared from 0.20 mol/L of the reactants.
3.2 Formation mechanism of the LaVO4:Cu,Eu nanorod array film on the Cu substrate
Based on the above experimental results, a growth process of LaVO4:Cu,Eu nanorod array film is deduced, including the surface dissolution of Cu substrate, the epitaxial layer and the self-assembled nanorod array (shown in Fig. 5). At the initial stage of the hydrothermal process, the LaVO4 nuclei adhere to the surface of Cu (110) plane. Owing to the existence of the oxygen in the solution, copper has a tendency to oxidize and the oxide of copper dissolves in alkaline solution. The Cu atoms of other lattice oxide with the oxygen dissolved in the solution. The reaction equations are as follows:
From the XRD results, the peak of (220) plane is increased, which is probably caused by the small mismatch between (110) facets of Cu substrate and LaVO4 without suffering oxidation and dissolution. When Cu2+ and Eu3+ ions enter the LaVO4 lattice to form the LaVO4:Cu,Eu crystallines, the LaVO4:Cu,Eu crystallines on Cu (110) substrate aggregate and grow into nanoparticles. With the hydrothermal reaction prolonging, a compact layer forms by the aggregation of the LaVO4:Cu,Eu nanoparticles minimizing the total interfacial energy via Ostwald ripening process [16]. In addition, the compact nanoparticles layer prevents the surface dissolution of Cu substrate. The compact layer serves as a seed layer to attribute the formation of LaVO4:Cu,Eu nanorod array with the capping agent of EDTA [21], which subsequently leads to the self-assembly process of well aligned LaVO4:Cu,Eu nanorods [15]. According to the previous report on the nanorod-materials, the exposed high energy (112) facet of the LaVO4:Cu,Eu nanorod array film is different from that in most reported articles [22].
3.3 The luminescent properties of the LaVO4:Eu3+ nanorod array films
Figure 6 displays the UV-Vis spectra, luminescence spectra and lifetimes of LaVO4:Cu,Eu nanorod array films prepared from different concentrations of the reactants. All the spectra in Fig. 6(a) show a strong broad band in the range of 200∼340 nm. It can be seen that the strong absorption peaks at 265, 254, 308, 255 and 309 nm are observed for all the LaVO4:Cu,Eu nanorod array films, which are attributed to the charge transfer from the VO43- to rare earth vanadates [23]. There is also an absorption band in the region of 350∼600 nm due to the absorption of Cu substrate. For all films, the absorption in UV region is much stronger than the visible region, indicating that the LaVO4:Cu,Eu nanorod array films should be suitable for phosphor matrix material. Figure 6(b) displays the excitation (the insert) and emission spectra of LaVO4:Cu,Eu nanorod array film. All the excitation spectra display a broad absorption band with the maximum around 315 nm, belonging to the charge-transfer transitions from oxygen to the central vanadium atom inside VO43- ions [24]. The intensity of the broad absorption band is enhanced obviously with the increase of the concentration of the reactants. All the emission peaks are attributed to 5D0→7Fx (x = 0∼4) transitions of Eu3+ activators. Although Cu substrate has absorbance in the region of visible light, all the LaVO4:Cu,Eu nanorod array films exhibit intense 5D0→7F2 emission. The emission intensity enhances with the increase of the concentration from 0.05 to 0.20 mol/L. The LaVO4:Cu,Eu nanorods prepared from the concentration of 0.20 mol/L have the largest aspect ratio and highest crystalline quality, which leads to the improvement of luminescence intensity [25]. It is well known that there exists an efficient energy transfer from the VO43− group to the Eu ions because of the overlap of their wave functions [26]. From the lattice constant data in Table 1, it can be found that the parameters of the film obtained from 0.2 mol/L concentration are relatively small. The reduced spacing between Eu3+ ions and VO43- tetrahedrons increases overlap of their wave functions, resulting in the enhanced indirect emission. In addition, the enhancement of the red emissions may be caused by the new microstructure of LaVO4 nanorod array films. The Cu2+ ions enter the lattice of LaVO4 and create one O2- (oxygen vacancy) to compensate the charge difference between them in the LaVO4:Cu,Eu lattice, and the energy transfers can conspicuously be enhanced not only from excited states of VO43- to Eu3+, but also from O2- to Eu3+ [27]. The measured quantum efficiencies are 7.4%, 6.2% and 15.4% for the films from 0.05, 0.10 and 0.20 mol/L concentrations, respectively. The results are close to the previous reports [28]. In addition, the thickness of the films is only about 1 µm for the LaVO4:Cu,Eu nanorod array films and the UV absorption of Cu can not be subtracted. It suggests that the independent structural nanorod array films would have better quantum efficiencies. The film from 0.20 mol/L concentration has the highest quantum efficiency, which may be related to the large amount of t- LaVO4 acrodding to the result of XRD [29]. The lifetims displayed in Fig. 6(c) are 0.61, 0.65 and 0.71ms for the films from 0.05, 0.10 and 0.20 mol/L concentrations, which are similar to the literatures [13,28]. The film prepared from the concentration of 0.20 mol/L has the longest lifetime. As above mentioned, the film obtained from 0.20 mol/L concentration has the good crystallinity and fewer defects, which may be a contributor to the long lifetime [12]. The LaVO4:Cu,Eu nanorod array film with the novel microstructure will become a promising red phosphor material.
Fig. 6. Uv-vis spectra, luminescence spectra and lifetimes of the LaVO4:Cu,Eu nanorod array films prepared from different concentrations of the reactants.
4. Conclusion
In summary, LaVO4:Cu,Eu nanorod array films have been successfully synthesized on Cu substrate by hydrothermal epitaxy method. An epitaxial layer on Cu substrate is composed of LaVO4:Cu,Eu compact nanoparticles. The LaVO4:Cu,Eu nanorod array grown on the epitaxial layer is assembled by the square nanorods growing along (101) plane. A reduction in lattice misfit was achieved through a rotation between the lattices of LaVO4 films and Cu substrate. In addition, a formation mechanism including the surface dissolution of Cu substrate, epitaxial layer and the self-assembly of the nanorod arrays is a reasonable explanation for LaVO4:Cu,Eu nanorod array film. The LaVO4:Cu,Eu nanorod array films exhibit excellent luminescence properties. The LaVO4:Cu,Eu nanorod array films with high-energy facets will become promising materials with great application potentials.
Funding
National Natural Science Foundation of China (51804035); Foundation of Liaoning Educational Committee (LQ2017013).
Acknowledgments
The authors are grateful to the financial support of the Foundation of Liaoning Educational Committee (LQ2017013) and National Natural Science Foundation of China (51804035).
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