1. Introduction

Recently, the development of optoelectronic technology has focused on certain new and more efficient luminescence devices [1–3]. Rare earth (RE) ions doped oxide phosphors have got much attention due to their great potential for application in the luminescence devices such as solid-state lighting, flat plane display, and optical fiber [4,5]. CeO₂, which has a cubic fluorite crystal structure, is a favorable kind of host materials for phosphors owing to its unique properties, including high refractive index, optical transparency, and chemical stability [6,7]. More importantly, the incorporation of high concentration of RE ions into CeO₂ nanocrystals is possible owing to the analogical radius of the dopant and the host Ce⁴⁺ ions [7]. Hence, different RE ions (such as Nd³⁺, Eu³⁺, Tm³⁺, Ho³⁺ and Yb³⁺) doped CeO₂ nanocrystals have been widely investigated as functional materials [2, 3, 6–8]. However, the high active surface atoms induce aggregation of nanoparticles and consequently degradation of performance [9]. In order to control the size and distribution of the nanoparticles, they have been embedded into inert matrix materials to maintain their excellent performance.

Er³⁺ is acknowledged as an interesting activator for solid-state laser in the visible to near-infrared range owing to its characteristics of abundant energy level structure. In particular, the ⁴I_13/2→⁴I_15/2 transition emits at 1.54 µm located in the low-loss optical window for optical fiber communication [10]. Unfortunately, the excitation cross-section of Er³⁺ is as low as 10⁻²¹ cm² because of the forbidden intra-4f transition [11]. In addition, the photoluminescence (PL) from ⁴I_13/2 can be easily quenched by phonon-assisted molecule collision [12]. Fortunately, the 1.54 µm Er³⁺ emission can be enhanced by the efficient energy transfer from wide band gap materials, such as In₂O₃, ZnO [12,13].

In this work, Er³⁺-doped CeO₂-SiO₂ nanocomposite (NC) was fabricated by a sol–gel method. The enhanced 1532 nm Er³⁺ emission induced by the energy transfer from CeO₂ nanocrystals was investigated systematically. And the influence of the annealing temperature and CeO₂ content on the microstructure and PL properties of the nanocomposite was discussed in detailed.

2. Experimental

Reagent graded chemicals of tetraethylorthosilicate (TEOS), cerium nitrate (Ce(NO₃)₃), ethyl alcohol (EA, CH₃CH₂OH), erbium nitrate (Er(NO₃)₃) and deionized water (H₂O) were used as starting materials to synthesis the gel. First, TEOS were dissolved in EA. Ce(NO₃)₃ and Er(NO₃)₃ were dissolved in deionized water. Then two parts were mixed slowly, and a trace of nitric acid was added as a catalyst for the hydrolysis. The molar ratio of TEOS:CH₃CH₂OH:H₂O in the final solution was 1:4:10. After stirring for 3 hours, the obtained clear sol was poured into vessels to formed wet gels. The gels were then aged at room temperature for one week and finally dried in steps from 30 °C to 80 °C in one week to produce dry gels (DG) with compositions of xCeO₂-(99-x)SiO₂:1Er³⁺ (in mol%, x = 0, 5, 10, 15, 20, denoted at NC0, NC5, NC10, NC15 and NC20, respectively). The NC samples were achieved by annealing these DGs at 600°C, 800 °C, or 1000 °C for 2 h to precipitate the CeO₂ nanocrystals.

X-ray diffraction (XRD) analysis was carried out with a powder diffractometer (DMAX2500 RIGAKU) using Cu K_α radiation (λ = 0.154 nm). The microstructure of the sample was characterize by a transmission electron microscope (TEM, JEM-2010). The Fourier transform infrared spectra (FTIR) were recorded in the wavelength range of 400-4000 cm⁻¹ with 4 cm⁻¹ resolution by the KBr method (Spectrum One). FTIR specimen was prepared by diluting the same weight of powder sample with KBr in equal proportions, then pressing into a diameter of 13 mm, thickness of 0.05 mm sheet. The PL spectra and fluorescence decay curves were recorded on an Edinburgh Instruments FLS920 spectrofluorometer.

3. Results and discussion

To identify the crystalline phase, XRD measurement was performed on the NC10 sample annealed at different temperatures. As shown in Fig. 1(a), XRD pattern of the DG one exhibits a typical amorphous feature. After annealed at 600 °C, several diffraction peaks corresponding to cubic CeO₂ (JCPDs No. 65-2975) emerge. With increasing annealing temperature from 600 °C to 1000 °C, the peaks become sharp, indicating the growth of cubic CeO₂ nanocrystals. Based on the Scherrer equation [14], the mean crystallite sizes of CeO₂ in the sample annealed at 600 °C, 800 °C, and 1000 °C are estimated to be about 5.2, 5.6, and 10.8 nm, respectively. To further investigate the influence of the CeO₂ content on the microstructure of the obtained nanocomposite, XRD patterns of NC samples with different CeO₂ contents annealed at 1000 °C are displayed in Fig. 1(b). Their XRD diffraction peaks are basically coincident, including peak intensities and widths, implying that samples in this CeO₂ content range are microstructural stable. The high-resolution TEM (HRTEM) image of the NC10 sample annealed at 1000 °C demonstrates that some particles sized about 4-8 nm are homogeneously dispersed among the amorphous matrix, as shown in Fig. 1(c). The corresponding selected area electron diffraction (SAED) pattern in the inset further confirms the crystalline nature of CeO₂ particles.

 

Fig. 1 XRD patterns of the NC10 sample annealed at different temperatures (a) and NC samples with different CeO₂ contents annealed at 1000 °C (b). (c) HRTEM image of the NC10 sample annealed at 1000 °C; inset shows corresponding SAED pattern.

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As is well known, one of the most serious restrictions to RE doped sol-gel silica is the residual OH^- which may lead to the quenching of the RE emission [15]. IR spectra of the NC10 sample annealed at different temperatures and DG samples with different CeO₂ contents are shown in Fig. 2. The bands at 470 cm⁻¹ and 550 cm⁻¹ can be attributed to the Si-O vibration [15,16] and the peaks at 800 cm⁻¹ and 1080 cm⁻¹ are corresponding to the Si-O-Si bond [15,18]. The intensity of these peaks increases with annealing temperature, indicating the increasing number of [SiO₄]⁴⁻ tetrahedral units and Si-O-Si bonds in the NC sample [17,18]. The small peak at 1380 cm⁻¹ arises from the vibrational mode of NO₃^- units [19]. As shown in Fig. 2(b), the peak intensifies with increasing the CeO₂ content due to the introduction of Ce⁴⁺ by the form of Ce(NO₃)₃. After annealed at 600 °C, the peak disappears due to the decomposition of the group (in Fig. 2(a)). The peak at 1635 cm⁻¹ can be ascribed to the H-O-H bending vibration of the free water or the physically absorbed water [18]. Despite the intensity decreasing with the increase of annealing temperature, the band is still observed, possible owing to the existence of residual free water resided in the micro-pores of the sample. Broad peaks centered around 3465 cm⁻¹ and 954cm⁻¹ originate from the -OH symmetric stretching from the surface hydroxyl group [15]. As a result of surface dehydroxylation during the annealing process, the intensity of these peaks weakens, and finally disappears at 1000 °C, which is beneficial to the Er³⁺ emission.

 

Fig. 2 IR spectra of the NC10 sample annealed at different temperatures (a) and DG samples with different CeO₂ contents (b).

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In order to explore the energy transfer between CeO₂ nanocrystals and Er³⁺ ions in the obtained nanocomposite, the PL excitation (PLE) spectra of the NC10 sample annealed at different temperatures monitored at 1532 nm, corresponding to the Er^{3+ 4}I_13/2-⁴I_15/2 transition, is shown in Fig. 3(a). For comparison, the PLE spectrum of the sample without CeO₂ (NC0) annealed at 1000 °C is also displayed in the figure. Three excitation peaks at 490 nm, 522 nm and 654 nm are ascribed to the transitions from ⁴I_15/2 ground state to ⁴F_7/2, ²H_11/2, and ⁴F_9/2 excited states of Er³⁺ ions, respectively. Interestingly, compared with the NC0 sample, for the NC10 one an additional strong excitation band centered at 346 nm was observed, which can be attributed to the charge transfer from O^2- valence band to Ce⁴⁺ conduction band [14,20], indicating the presence of efficient energy transfer from CeO₂ nanocrystals to Er³⁺ ions. Furthermore, the excitation bands intensify with increasing the annealing temperature because of the precipitation of the CeO₂ nanocryscals and the decrease of residual OH^- in the silica matrix.

 

Fig. 3 (a) PLE spectra of NC10 samples annealed at different temperatures. (b) Normalized visible and near-infrared PL spectra of the NC10 sample annealed at 1000 °C, excited at 346 nm and 520 nm respectively. The inset shows the normalized emitted photons ratio at the ⁴I_13/2 → ⁴I_15/2 transition for two excitation wavelength. (c) PL spectra of the NC10 sample annealed at 1000 °C excited at different wavelength. For comparison, excitation and emission spectra of the NC0 sample annealed at 1000 °C are also displayed in (a) and (b).

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Normally, the energy transfer efficiency between semiconductor and RE ions is weak owing to the limit solubility of RE in the host and consequently weak interaction between them [12]. In the obtained nanocomposite, the incorporation of most Er³⁺ ions into CeO₂ nanocrystals is expected due to the similar radius between the dopant and the host Ce⁴⁺ ions, which may induce the efficient energy transfer from nanocrystals to dopants. To reveal the distribution of Er³⁺ in the nanocomposite, visible and near-infrared (NIR) PL spectra of the NC10 sample annealed at 1000 °C upon indirect excitation at 346 nm and direct excitation at 520 nm were measured. As shown in Fig. 3(b), under direct excitation at 520 nm (in this case all Er³⁺ ions might be excited), the PL spectrum shows well-resolved peaks, indicating that most of Er³⁺ ions are incorporated into CeO₂ nanocrystals by substituting Ce⁴⁺ ions due to the similar radius of the Er³⁺ ions (0.089 nm) and the host Ce⁴⁺ ions (0.087 nm). Under indirect excitation at 346 nm, the line-shape of the spectrum is different from that excited at 520 nm because only the Er³⁺ ions located in/near nano-crystals could benefit from the energy transfer [20]. In comparison, the PL spectrum of the NC0 sample annealed at 1000 °C shows a typical Er³⁺ emission in silica. Under the identical measured conditions and corrected for the excitation intensity at two excitation wavelength, the efficiency of the two excitation schemes is displayed in the inset of Fig. 3 (b) [21]. Obviously, the efficiency of the indirect excitation is enhanced with respect to the direct one. Moreover, PL spectra of the NC10 sample excited in the range from 270 nm to 510 nm are shown in Fig. 3(c). Excited within the region of the CeO₂ band-to-band transition, spectra show structured peaks, further implying the efficient energy transfer from CeO₂ to Er³⁺ ions located in/near nanocrytals.

To evaluate the influence of the annealing temperature on the PL properties, PL spectra and decay curves of the NC10 sample annealed at different temperatures are measured and exhibited in Fig. 4, under the excitation of 346 nm. The emission intensifies with increasing annealing temperature. The mean decay lifetimes (τ) of the emission at 1532 nm can be evaluated by the equation:

 

Fig. 4 (a) PL spectra and (b) decay curves of the NC10 sample annealed at different temperatures excited at 346 nm.

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Figure 5 presents PL spectra and luminescence decay curves of NC samples with different CeO₂ contents annealed at 1000 °C. The characteristic NIR emission of Er³⁺ is hardly detected in the NC0 sample (without CeO₂ content). The emission intensity increases monotonously with the CeO₂ content. Impressively, the intensity of the 1532 nm emission in the sample with 20 mol % CeO₂ is about 12 times as high as that in the sample with 5 mol% CeO₂. The mean PL lifetime evaluated from the Eq. (1) increases monotonically from 0.20 ms to 0.99 ms, when the CeO₂ content increases from 0 to 20 mol%. As the CeO₂ content adds, more Er³⁺ ions can be partitioned into the nanocrystals which defend Er³⁺ emission from the residual OH^- quenching, provide a low phonon energy environment to reduce the nonradiative recombination, and sensitize the Er³⁺ emission [20].

 

Fig. 5 (a) PL spectra and (b) decay curves of the samples with different CeO₂ contents excited at 346 nm. Inset in (a) shows the dependence of 1532 nm emission intensity on the CeO₂ content. The decay curve for the NC0 sample is excited at 520 nm.

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4. Conclusions

In summary, Er³⁺-doped CeO₂-SiO₂ composite was successfully prepared by the sol-gel process. The influence of the annealing temperature and CeO₂ content on the photoluminescence properties was systematically investigated. As the annealing temperature and the CeO₂ content increase, the amount of Er³⁺ ions locating in the CeO₂ lattices increases, resulting in the remarkable intensification of the Er³⁺ emission, which can be attributed to the effective energy transfer process due to the formation of CeO₂ nanocrystals as well as the elimination of residual OH^- in the silica matrix. It demonstrates that the CeO₂ may be one of the most effective sensitizer to improve the photoluminescence behaviors.