Synthesis of Er³⁺-doped perovskite nanorods with outstanding UC PL behavior

Abstract

The Er-doped perovskite single-crystal NaNbO₃ nanorods with a series of doping content (0–1 wt%) were successfully synthesized by hydrothermal method. The XRD and Raman analyses indicated that the Er³⁺ ion firstly entered into the B site (Nb⁵⁺) in the ABO₃ perovskite structure and then the A site (Na⁺) with a further increase of Er-doping content. High-resolution transmission electron microscopy (HRTEM) was employed to demonstrate the single-crystal features of NaNbO₃ nanorods. The as-synthesized Er-doped NaNbO₃ nanorods exhibited excellent up-conversion (UC) photoluminescence (PL) behavior. The strong green emission and the weak red emission of Er³⁺ were observed in the UC PL spectra. The maximum emission intensity of the UC PL spectrum was achieved in NaNbO₃ nanorods with 0.5 wt% of Er³⁺ doped. Furthermore, the UC PL spectra obtained under different laser input powers confirmed that two photons contributed to the observed up-conversion luminescence properties. Owing to these luminescent merits, the as-synthesized Er-doped single-crystal perovskite NaNbO₃ nanorods exhibit potential application in novel multifunctional devices.

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Introduction

Owing to the excellent properties in piezoelectricity, ferroelectricity and ferromagnetism, perovskite oxides have attracted considerable attention in the wide application fields.^1–4 The ion doping strategy is usually utilized to effectively realize new properties on account of the adjustable crystal structure of perovskite oxides. Certain kinds of rare earth ions, including Er³⁺, have been incorporated into the perovskite oxide host matrix to obtain the up-conversion (UC) photoluminescence (PL) materials in the previous reports.^5–12 According to the previous literature, the photoluminescence properties of the ABO₃ type perovskite oxides could be remarkably affected by the substitution site and the chemical environment of doping ions in the host matrix, which are sensitive to the doping concentrations.^13–17

As one of the most common perovskite oxides, alkaline niobates exhibit the excellent nonlinear optical, ferroelectric, piezoelectric, electrooptic, ionic conductive and photocatalytic properties.^18,19 With the development of functional devices with extremely small scale, much attention has been focused on the synthesis of nanoscale materials with one-dimensional (1D), such as nanowires and nanorods on account of their unique electronic, magnetic and optics properties. Among the approach developed for the preparation of nanoscale NaNbO₃ so far,^20–24 the hydrothermal synthesis exhibits outstanding advantages such as good chemical homogeneity, facile synthesis procedure and high purity.^25–28

In current work, the Er-doped perovskite single-crystal NaNbO₃ nanorods have been successfully synthesized by hydrothermal method. Based on the comprehensive investigations, the crystal structure, morphology, substitution mechanism, especially up-conversion photoluminescence property had been thoroughly investigated and the corresponding mechanism had been proposed. The current research demonstrated that these Er-doped NaNbO₃ nanorods as a multifunctional material would be of great reference for the fundamental study of optical-electric couplings in related application fields.

Experimental

Er-doped perovskite NaNbO₃ nanorods were synthesized by a hydrothermal method as following. The 19.2 g of NaOH (99%) was first dissolved in 40 mL of distilled water, and then 2 g of Nb₂O₅ (99.99%) was added into the NaOH solution. The different amount of Er(NO₃)₃ was incorporated in the solution as the source of Er³⁺. The weight proportions of Er in each sample were designed as 0, 0.25, 0.5, 0.75 and 1 wt% and the corresponding samples were respectively denoted as E0, E025, E050, E075 and E100 for short. After stirring for 30 min, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The hydrothermal reaction was performed at 150 °C for 4 h. The as-synthesized powders were filtered and washed with distilled water for several times, and then dried at 80 °C for 12 h. The after-dries products were further heat-treated in air at 650 °C for 8 h.

The phase contents of products were determined by X-ray diffraction analysis (XRD, D8 Advance, Bruker Co., Germany). The microstructures of products were observed by using a field-emission scanning electron microscope (FESEM, JSM-7001F, JEOL, Japan). The microstructure and crystal state of as-synthesized nanorods were detected by high-resolution transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). Raman spectra were excited using the 633 nm excitation source from He–Ne laser and collected by a micro-Raman spectrometer at different temperatures. The up-conversion photoluminescence was recorded by using a spectrophotometer (LabRAM HR Evolution) under the excitation of a 980 nm laser diode.

Results & discussion

The representative FESEM images of NaNbO₃ nanorods with different contents of Er³⁺ doped were shown in Fig. 1. Generally speaking, the microstructure of nanorods could be preserved within all the composition of Er³⁺-doped NaNbO₃. Derived from the SEM images in Fig. 1a and b, the average size for the sample E0 was around 200 nm in diameter and tens of micrometers in length. As shown in Fig. 1c and d, the morphology and size of NaNbO₃ nanorods for the sample E0 and E050 were similar. These indicated that even the Er³⁺-doping content of 0.5 wt% would hardly make distinct difference on the microstructure. As for the sample E100 in Fig. 1e and f, the nanorods with rough surface were observed and the average size in diameter was extended to around 500 nm. In a word, the doping of Er³⁺ into NaNbO₃ nanorods would result in the broadening of diameter size and the roughness of surface.

Fig. 1 The SEM images of as-synthesized NaNbO₃ nanorods with different Er³⁺ doping contents: (a and b), 0 wt%; (c and d), 0.5 wt%; (e and f), 1 wt%.

High-resolution transmission electron microscopy (HRTEM) was employed to investigate the microstructures and crystalline states of the Er³⁺-doped NaNbO₃ nanorods. The typical TEM images of nanorods in sample E0 and E050 were presented in Fig. 2a and c. Both of the observed morphologies exhibited the nanostructures with smooth surface and around 200 nm in diameter size, which were in good agreement with the results of SEM. The HRTEM images of the observed nanorods were further displayed in Fig. 2b and d. The intervals of the lattice fringes were indicated as 0.396 and 0.395 nm, respectively, which both corresponded well with the d spacing value of the (110) plane for orthorhombic NaNbO₃ (JCPDS: 74-2437). The corresponding diffraction patterns obtained by fast Fourier transform (FFT) for Fig. 2b and d in the insets evidently certified the good-crystallinity and single-crystal state of the Er³⁺-doped perovskite NaNbO₃ nanorods.

Fig. 2 (a) TEM image, (b) HRTEM image and diffraction pattern of NaNbO₃ nanorods without Er³⁺ doped. (c) TEM image, (d) HRTEM image and diffraction pattern of NaNbO₃ nanorods with 0.5 wt% of Er³⁺ doped.

Based on the SEM and TEM results, the formation of Er³⁺-doped perovskite NaNbO₃ nanorods can be rationally delineated in Fig. 3. In high temperature reaction system, Nb₂O₅ can dissolve into strong alkali solution to Nb₆O₁₉⁸⁻ ions, although Nb₂O₅ is non-water-soluble in normal reaction environment. The reaction equations occurring after the dissolution of Nb₂O₅ to form NaNbO₃ can be depicted as following, according to the process shown in Fig. 3a.

3Nb₂O₅ + 8OH⁻ = Nb₆O₁₉⁸⁻ + 4H₂O (1)

Nb₆O₁₉⁸⁻ + 34OH⁻ = 6NbO₆⁷⁻ + 17H₂O (2)

NbO₆⁷⁻ + Na⁺ + 3H₂O = NaNbO₃ + 6OH⁻ (3)

Fig. 3 The schematic illustrations for the formation of Er³⁺-doped perovskite NaNbO₃ nanorods.

In the alkali condition of hydrothermal reaction, the abundant OH⁻ ions can be attracted to the surface of newly-formed NaNbO₃ nucleus owing to the different charge attracting. Then the dissolved Nb₆O₁₉⁸⁻ ions continued to react with the OH⁻ ions on the surface of NaNbO₃ as shown in Fig. 3c. Considering the strong repulsive force between the Nb₆O₁₉⁸⁻ ions because of the high charge value, the ions preferred to combine and react with the OH⁻ in the areas far apart from each other to reduce the repulsive force. In other words, the OH⁻ ions located in the both ends of the NaNbO₃ nucleus were preferential in the reaction. After the continuous growth, the NaNbO₃ nucleus would be lengthened and the nanorods could be finally formed.

The X-ray diffraction patterns of NaNbO₃ with different doping content of Er³⁺ were shown in Fig. 4a. All these patterns could be mainly indexed with the orthorhombic perovskite NaNbO₃ (JCPDS: 74-2437). Moreover, only in the sample E100 there could find the trace of a secondary phase indicated as Na₃NbO₄ (JCPDS: 22-1391). It is generally accepted that the phase structure of NaNbO₃-based ceramics could be assessed by the amplified (200) and (020) diffraction peaks at 2θ ≈ 45–48°.^29,30 The step scanning of (002)/(200) diffraction peaks were depicted in Fig. 4b and the variation of corresponding peak position was further shown in Fig. 4d. It found that the (200) and (002) diffraction peaks of Er³⁺-doped NaNbO₃ firstly shifted to lower angle and then toward higher angle with the increase of Er³⁺-doping content, rendering a minimum inflection point in the sample E050. These XRD results here confirmed the change of lattice parameter owing to the ion substitution of Er³⁺ into perovskite structure of NaNbO₃. To demonstrate the occurrence of lattice distortions, cell-refinement operations were performed to the XRD results, as illustrated in Fig. 4c. The variation curve of the as-calculated unit-cell volume was depicted in Fig. 4d, which exhibited an opposite correlation with the variation of diffraction peak positions. The changing tendency of first decrease and then increase with the value of the unit-cell volume indicated that the Er³⁺ ions successfully entered into the crystal lattices and probably occupied different atom sites in ABO₃ perovskite structure with the different content of Er³⁺ doped. The variation of the lattice constant would be caused by the difference of ionic radius between the doped and host one. Considering of the Shannon effective ionic radii, the sequence of ionic radius for involved ions could be depicted as following: r(Nb⁵⁺) < r(Er³⁺) < r(Na⁺).^15,29 The substitution of Er³⁺ for Na⁺ at A site would cause the shrinkage of the crystal lattice and thus shift the peak to a higher angle. In contrast, the substitution of Er³⁺ for Nb⁵⁺ at the B sites could lead to the expansion of crystal lattice, and bring about the shift of diffraction peaks toward a lower angle. Based on the XRD results, Er³⁺ ions would substitute for the Nb⁵⁺ ions in B site when the doping content of Er³⁺ was below 0.5 wt%. However, the Na⁺ in A site would be substituted by Er³⁺ for the further increasing content of Er³⁺. The schematic illustrations for the unit cells of the perovskite Er³⁺-doped NaNbO₃ sample were further shown in Fig. 4e. According to the cell-refinement results above, the cell volume would gradually expand from 0.0594 nm³ for NaNbO₃ to 0.0597 nm³ for NaNbO₃–0.5 wt% Er³⁺.

Fig. 4 (a) XRD patterns. (b) Step scanning of (002) and (200) diffraction peaks. (c) Representative cell-refinement result for XRD pattern of sample E05. (d) Variation curve of the (200)/(002) diffraction peak position and the as-calculated unit-cell volume. (e) Schematic illustration for the crystal structures of Er³⁺-doped perovskite NaNbO₃.

The Raman analyses for the samples E0 and E050 within the wavenumber range of 100–1000 cm⁻¹ were performed under the excitation by laser source of 633 nm and the measured spectra were displayed in Fig. 5. Both the samples without and with Er³⁺ doped (in Fig. 5a and c) exhibited the typical Raman spectra of NaNbO₃ with perovskite structure.^31–33 As marked in Fig. 5a and c, the different bands in the range 100–1000 cm⁻¹ could be associated with the different internal vibrational modes of NbO₆ octahedron structure. Of note, no new peaks were observed in the Raman spectra for sample with 0.5 wt% Er³⁺ doped when comparing with those for pure NaNbO₃ at the same measuring temperature points, implying that the introduction of Er³⁺ may not bring about additional modes.

Fig. 5 The Raman spectra measured at different temperatures for the NaNbO₃ nanorods (a and b) without Er³⁺ doped and (c and d) with 0.5 wt% of Er³⁺ doped.

To reveal the effect of doping-Er³⁺ on the phase-transition of the as-obtained NaNbO₃ nanorods, the Raman analyses in different temperatures was also carried out for samples E0 and E050 under the excitation of 633 nm source. The shape variation of Raman spectra with the increasing temperature demonstrated the phase transformation of NaNbO₃ from orthorhombic to tetragonal and finally cubic phase at the higher temperature. Based on the distinct Raman spectra measured in different temperatures shown in Fig. 5b and d, the phase-transition temperature points T_O-T and T_T-C for the as-synthesized NaNbO₃ nanorods would stay around 200 and 400 °C in both of the sample E0 and E050. Herein, it easily reached the conclusion that the incorporation of Er³⁺ would not bring about obvious influence on the phase transition temperature of NaNbO₃ nanorods. And the results here consisted well with the XRD results of NaNbO₃ with different content of Er³⁺ doping.

In order to investigate the up-conversion photoluminescence (UC PL) properties of the as-synthesized NaNbO₃ nanorods with different content of Er³⁺ doped, the UC PL spectra obtained under infrared radiation excitation of 980 nm at 0.36 W were summarized in Fig. 6. As expected, the NaNbO₃ nanorods without the doping of Er³⁺ did not exhibit upconversion luminescence behavior. At the meantime, the UC PL spectra including green and red emission bands were clearly obtained for these samples doped with Er³⁺. The strong visible green emission at 524 and 543 nm were ascribed to the intra 4f–4f electronic transitions from the excited states ²H_11/2 and ⁴S_3/2 to the ground state ⁴I_15/2 of Er³⁺. And the weak red emission around 661 nm is caused by the relaxation process from ⁴F_9/2 to ⁴I_15/2. Here the UC emission peaks of the Er³⁺-doped NaNbO₃ nanorods in the visible region are consistent well with the characteristic UC PL processes of Er³⁺ ion in other host matrix as indicated in previous literatures.^9,34–36 In addition, there was no obvious shift of the emission peaks among the samples with different content of Er³⁺ doped. Fig. 6b depicted the variation of integrated green (505–595 nm) and red (630–700 nm) emission intensities of Er³⁺-doped NaNbO₃ nanorods. As seen, both of the intensity for the green and red emission peaks firstly got remarkable enhanced and subsequently dropped as the content of Er³⁺ further increased. The as-synthesized NaNbO₃ nanorods with the Er³⁺-doping content of 0.5 wt% exhibited the most excellent UC PL property. The presence of this inflection point was probably ascribed to the concentration-quenching effect of Er³⁺. According to the previous literatures, the concentration-quenching is mainly caused by the energy transfer among doping ions (Er³⁺ in this work).^37–40 When the content of Er³⁺ was increased, the distance between Er³⁺ ions became smaller and the probability of energy transfer among Er³⁺ ions was consequently promoted. As a result, the energy loss caused by the energy transfer between Er³⁺ ions became more and the transition energy of Er³⁺ would be accordingly weakened. Besides, the ratio of red and green emission intensity I_Red/I_Green (the inset of Fig. 6b) was firstly decreased slightly and then enhanced with the doping content of Er³⁺ increased, showing familiar inflection point for the sample E050. The variation of I_Red/I_Green was possibly ascribed to the change in site symmetry of the Er³⁺ ions induced by the increase of Er³⁺-doping content.⁵ These also implied the larger value-changes of the green emission intensity than those of red emission intensity in both of the enhancement and decrement. In other words, the green emission could be more sensitive to the doping content of Er³⁺ than the red emission. On the other hand, it implied that the ratio of green and red emission intensity could be partly adjusted by the introduction of different content of Er³⁺.

Fig. 6 (a) The UC PL spectra and (b) the variation of green and red emission intensities for the as-synthesized NaNbO₃ nanorods with different content of Er³⁺ doped.

In order to elucidate the UC PL mechanism, the dependence of emission intensity on laser power for sample E050 under 980 nm excitation was investigated and the spectra were shown in Fig. 7. The chosen laser power of 0.36, 0.67, 0.95, 1.22 W was approximately corresponding to the power density of 37.9, 70.5, 100, 128.4 W mm⁻², respectively. A continuous promotion of emission intensity was clearly observed with the increase of input power. In the up-conversion mechanism, the emission intensity I_Em is proportional to the mth power of the infrared radiation excitation intensity, wherein m is the number of the absorbed photons.^35,41 The variation curves of logarithmic emission intensities for the green & red bands against the logarithmic excitation power were shown in the inset of Fig. 7, respectively depicted in block and round dots. Clearly, the two curves both yielded straight lines and the values of the slopes obtained were calculated as 1.99 and 2.04 for the green and red bands, respectively. This result confirmed that two photons contributed to the observed up-conversion luminescence property of the as-synthesized Er-doped NaNbO₃ nanorods.

Fig. 7 The UC PL spectra for the sample E050 obtained under different laser input powers.

Conclusions

The Er³⁺-doped perovskite single-crystal NaNbO₃ nanorods with a series of doping contents (0–1 wt%) had been successfully synthesized via the hydrothermal method. The single-crystal nature of products was confirmed by the HRTEM. The comprehensive X-ray diffraction analyses indicated that Er³⁺ firstly occupied Nb⁵⁺ site in perovskite NaNbO₃ and then occupied the Na⁺ site with the increase of Er³⁺-doping content. The strong green emission and the weak red emission of Er³⁺ were observed in the UC PL spectra for Er-doped NaNbO₃ nanorods. The maximum of the emission intensity for the UC PL spectrum was achieved in NaNbO₃ nanorods with 0.5 wt% of Er³⁺ doped. The UC PL spectra obtained under different laser input powers confirmed that two photons contribute to the observed up-conversion luminescence property. In a word, the Er-doped content of 0.5 wt% was appropriate for the preparation of perovskite NaNbO₃ nanorods with excellent upconversion luminescence and ferroelectric property. The as-synthesized single-crystal NaNbO₃ nanorods showed great potential for pursing novel coupling properties in the application of multifunctional materials.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (No. 51272119 and 51302145), Beijing Institute of Technology Research Fund Program for Young Scholars, and the State Key Laboratory of New Ceramics & Fine Processing of Tsinghua University.

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