A convenient and efficient synthesis method to improve the emission intensity of rare earth ion doped phosphors: the synthesis and luminescent properties of novel SrO:Ce³⁺ phosphor

Abstract

Convenient, efficient synthesis methods that improve the emission intensity of rare earth ion doped phosphors are relatively rare. In this study, a simple modified solid-state reaction is proposed. This approach can greatly improve reaction temperature and overcome the requirement for harsh conditions. Its advantages come from the substitution of a solid–solid interface for a solid–gas interface. A novel Ce³⁺ doped SrO phosphor with an enhancive bright cyan emission is prepared and the photoluminescent properties of SrO:Ce³⁺ are first reported. This study will provide valuable clues for synthesizing many other ion doped functional materials besides rare earth ion doped luminescent materials.

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Introduction

Doping is the most important and efficient method of endowing hosts with various functions. For optical materials, doped rare earth ions acting as isolated active centers can emit rich and colorful lights.^1–3 For many other functional materials, such as magnetic materials, thermoelectric materials, superconductors, and electrode materials, doped ions will also greatly improve or affect their performance and properties.^4,5 There are many techniques for introducing doping ions into a host. The most common ones are ion implantation, thermal diffusion, and chemical vapor deposition.^6–8 Among these, the high temperature solid state reaction utilizing ion diffusion is the simplest and most efficient means of producing doped materials and has been used for many years. However, for hosts with relatively low physicochemical stability, high temperature reactions must be performed in an atmosphere of an inactive gas, such as nitrogen and argon, to avoid the effects of oxygen, carbon dioxide or water vapour in air. This increases production cost and complexity.

SrO is an important basic chemical raw material. Many properties of doped SrO have recently been demonstrated in the fields of optical, display and magnetic materials. For example, doped SrO crystal exhibits the properties of new candidates for transparent and half-metallic ferromagnetic DMSs (diluted magnetic semiconductors).⁹ Satoru et al. investigated the sputtering yield of SrO barrier coatings under He⁺, Ar⁺, and Kr⁺ ion beams in a plasma display panel.¹⁰ Moreover, the photoluminescence (PL) of Eu²⁺ and Eu³⁺ in SrO are reported.^11–13 Fu et al. shows long-lasting phosphorescence in Eu³⁺ and B codoped SrO.^14,15

Despite the wide potential application of doped SrO, simple and convenient methods of directly synthesizing different types of ion doped SrO are still rare, especially for rare earth ion doped SrO at high temperatures. Although the melting and boiling points of SrO are about 2600 °C and 3200 °C, respectively,¹⁶ it will practically ‘volatilize’ if heated above 1400 °C in air (as shown in Fig. 1s†). This is probably due to SrO reacting with air at high temperatures, because the thermodynamic parameters of SrO mentioned above are measured in an argon atmosphere.¹⁷ To date, several studies have investigated rare earth ion doped SrO, but their reaction temperatures were relatively low and a high temperature solid state reaction (SSR) is not reported. Mari¹³ observed photoluminescence of Eu³⁺ in SrO prepared by combustion synthesis with further heating to 1000 °C. R. W. Reynolds and L. A. Boatner¹⁸ obtained Ce³⁺ doped SrO single crystals by an arc-fusion process. Due to the representative characteristics of Ce in rare earth ions, we chose CeO₂ as our raw material. A simple and convenient method of synthesizing Ce³⁺ doped SrO is proposed. Its photoluminescent properties are studied.

Experimental

Sample preparation

In this modified SSR, SrCO₃ (99.9%), SiO₂ (99.9%), (99.9%), and CeO₂ (99.99%) were employed as raw materials. First, SrCO₃ and CeO₂ were mixed well in a stoichiometric ratio in an agate mortar for 30 min. Then, SrCO₃ and SiO₂ were mixed homogeneously in an agate mortar at a molar ratio of 3 : 1. The mixed strontium carbonate and silica powder plays a key role in this method. Doped and undoped SrO phosphors were synthesized by placing the starting materials in the middle of the mixed powder in a sandwich structure. This was placed in a corundum crucible with a lid. The crucible was then placed in a box type electric-resistance furnace with a silicon–molybdenum bar at a high temperature for a certain time. Therefore, the reaction is referred to as an interlayer solid-state reaction in this study, ISSR for short. After sintering, the products were stripped out from the middle of the sandwich and ground, preceding post-calcination under selective conditions. The SrO:Ce³⁺ sample should be annealed at 550 °C for 1 h in CO atmosphere, reducing to Ce⁴⁺. The SrO powder crystal is chemically unstable to H₂O and CO₂ and should be handled with care.

For comparison, Ce doped SrO phosphors were also prepared using a traditional solid-state reaction (SSR) without covering the mixed powder. The other treatment processes were the same.

Measurements and characterization

Sample structures were identified by powder X-ray diffraction (XRD) analysis (Bruker AXS D8), with graphite-monochromatized Cu Kα radiation (λ = 0.15405 nm) operating at 40 kV and 40 mA. The absorption spectra were obtained by a UV-visible spectrophotometer (Hitachi U4100) using BaSO₄ as a reference. The photoluminescence (PL) measurements and photoluminescence excitation (PLE) spectra were obtained using a Hitachi F7000 spectrometer equipped with a 150 W xenon lamp under a working voltage of 700 V. The excitation and emission slits were set at 1.0 nm and 2.5 nm, respectively. The luminescence decay curve was obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns; gate = 50 ns) as the excitation source (Continuum Sunlite OPO). The quantum efficiency yields were analyzed with a PL quantum-efficiency measurement system (C9920-02, Hamamatsu Photonics, Shizuoka, Japan) using a 150 W xenon lamp. All the measurements were obtained at room temperature.

Results and discussion

Synthesis mechanism

SrO has a typical crystal structure because all the normal alkaline-earth oxides with MO formula have the NaCl crystal structure, except for BeO. Although SrO has a very high melting point, synthesizing rare earth ion doped SrO using traditional SSR is difficult because the high reaction temperature and long processing time in air result in SrO ‘volatilization’. The single quotation marks mean that it is not a simple physical process. A number of complicated chemical reactions may cause melting and disappearance of SrO above 1400 °C, as shown in Scheme 1 (left). It should be mentioned that Ce³⁺ doped SrO can also be synthesized using traditional SSR, but the PL emission intensity of SrO:Ce³⁺ is quite weak using this method (shown in Fig. 6). This phenomenon may be due to the inferior Ce ion doping process at lower temperatures.

Scheme 1 The sketch map of the traditional solid state reaction (left) and ISSR method (right).

In the ISSR method, the starting materials are first heated at 1200 °C for 2 hours. At this temperature SrCO₃ can be decomposed into SrO and CO₂. The furnace is further heated to 1530 °C and kept at that temperature for 4 hours. Increasing the temperature enhances the reaction and diffusion rates between CeO₂ and SrO. This is the main cause of the higher PL emission of Ce³⁺ doped SrO samples synthesized using the ISSR method. Therefore, there are three key effects of the covering layer made from a mixture of SrCO₃ and SiO₂ in the ISSR method (Scheme 1 right). First, the coating makes a solid–solid interface instead of a solid–gas interface. The coating can efficiently exclude the active molecules in air, preventing the SrO from volatilizing. Second, the coating can protect the starting materials from thermal convection shock in the furnace. Third, the SrCO₃ and SiO₂ mixture will form a stable silicate compound at a high temperature. Reacting the target product, SrO, with this silicate compound is difficult. Thus, due to the superior doping process of Ce ions at a relatively higher temperature, a better Ce ion emission in SrO can be obtained.

In addition, reducing raw material particle size will be of great benefit to the diffusion of the ions such as in the sol–gel method,¹⁹ combustion synthesis,²⁰ and co-precipitation method.²¹ Nevertheless, a post-calcination process is still required for some synthesis methods because the main factors in the ion doping process, chemical reaction, diffusion, crystal growth and defect adjustment are all greatly affected by temperature. Therefore, the ISSR method may still have the benefits of convenient operation and low cost in synthesizing ion doped materials.

Crystal structure and emission color of Ce³⁺ doped SrO

The typical XRD patterns of undoped and Ce³⁺ doped SrO phosphors synthesized by ISSR and traditional SSR, as well as the standard SrO pattern, are shown in Fig. 1. All XRD patterns are found to agree well with cubic SrO (space group: Fm3̄m). The XRD patterns indicate that at a high temperature, the raw material SrCO₃ will be easily decomposed into SrO, and most importantly, it is shown that the special high temperature used in the ISSR did not generate any impurities or induce significant changes in the host structure or the doped Ce³⁺ ions. Therefore, it is possible that the better PL emission properties of Ce³⁺ may originate from good Ce³⁺ ion diffusion in SrO, better crystal growth and better crystal defect adjustment of SrO at a relatively high temperature. Fig. 2 shows images of Ce³⁺ doped and undoped SrO synthesized by ISSR under a 254 nm UV light. A bright cyan emission can be observed.

Fig. 1 (a) XRD patterns of as-synthesized SrO:0.01Ce³⁺, SrO and the standard SrO pattern (PDF#48-1477). (b) A schematic of the geometric structure of the SrO cubic cell.

Fig. 2 Images of SrO:Ce³⁺ (a) and SrO (b) synthesized by ISSR under 254 nm UV light. (c) CIE coordinates of SrO:Ce³⁺ and SrO phosphor in a CIE 1931 chromaticity diagram, together with commercial RGB phosphors: Y₂O₃:Eu³⁺ (R), Y₂SiO₅:Tb³⁺ (G) and Y₂SiO₅:Ce³⁺ (B).

The PL properties of Ce³⁺ doped and undoped SrO synthesized by ISSR

The PL spectra of SrO and SrO:Ce³⁺ synthesized by ISSR are presented in Fig. 3. The emission spectrum of SrO:Ce³⁺ shows a broad peak located around 460 nm under an excitation of 290 nm. The emission is attributed to the ⁵d₁ → ⁴f₁ transition of the Ce³⁺ ions. Although the ⁵d₁ state of Ce³⁺ will split into at most 5 levels, in the room temperature PL excitation spectrum only two peaks can be recognized, which are located at about 288 and 338 nm.

Fig. 3 PL (solid curve) and PLE (dash curve) spectra of SrO and SrO:0.01Ce³⁺. The inset shows the PL integrated intensity of SrO:xCe³⁺ (x = 0.005–0.05).

From the emission spectrum of SrO:Ce³⁺, the color coordinates are calculated to be (0.168, 0.216). In general, RGB (red, green and blue) tricolor phosphors are used in 3D display devices such as field emission displays (FEDs) and plasma display panels (PDPs). However, the introduction of a cyan-emitting phosphor enlarges the display gamut and makes the images of the devices more colorful and natural (Fig. 2c).^22,23 Importantly, the bright cyan emission of the sample implies that the ISSR method may be a potential synthesis method to obtain many other functional materials.

It is also interesting that the undoped SrO can also give a green emission. The PL spectrum of SrO shows a broad band peaked at 514 nm under 307 nm excitation. The color coordinates are (0.28, 0.44) (the image is shown in Fig. 2). The green SrO emission is attributed to lattice defects and deformed crystals of pure SrO, not to impurities.^12,24–27 Similar host emissions are reported in CaO, MgO and BaO.²⁶ The quantum efficiencies of Ce doped and undoped SrO are about 20% and 10%, respectively.

The inset in Fig. 3 shows emission intensity as a function of the doping content. For those samples with higher Ce³⁺ dopant content, concentration quenching was observed. The optimal Ce³⁺ dopant content was found to be 1 mol%, from which it is possible to calculate the critical distance (R_c) for concentration quenching using the following equation:²⁸

where V is the volume of the unit cell, x_c is the critical concentration of the activator ion, and N is the number of total Ce³⁺ sites per unit cell. In SrO, N = 4, V = 136.9 ų, and the critical concentration, x_c, is about 0.01 in our system. Based on eqn (1), the critical distance, R_c, is calculated to be 18.7 Å. The large R_c means a large critical concentration of dopant.

UV-visible absorption spectra are employed to investigate the samples' absorption of light. The absorption spectra of undoped and Ce³⁺ doped SrO shows that both samples have no absorption above 450 nm (Fig. 4). It is demonstrated that the absorption of Ce³⁺ ions overlapped with the host absorption at 320 nm.

Fig. 4 UV-visible absorption spectra of un-doped SrO (line 1) and Ce³⁺ doped SrO (line 2) measured at room temperature.

Fig. 5 shows the luminescence decay curves of the SrO:Ce³⁺ and SrO phosphors excited at 290 nm, which provide further insight into the luminescent centers of SrO and SrO:Ce³⁺. The luminescent lifetime of SrO is calculated on the assumption that the observed decay curves are the sum of several first order decay curves.²⁶ The decay curves of undoped SrO can be well fitted with a second-order exponential equation,²⁹

I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (2)

where I is the luminescence intensity at time t; A₁ and A₂ are constants, and τ₁ and τ₂ are defined as the rapid and slow luminescent lifetimes. The lifetime values are determined to be 1.03 μs and 0.105 μs for SrO. These values are smaller than that of SrO in Coluccia's results,²⁶ which may be due to the quenching of PL from surface absorption, because the host emission is attributed to the presence of local surface states. Considering the overlap of the host and Ce³⁺ emissions, we try to fit the decay curve of SrO:Ce³⁺ with a third-order exponential equation. The lifetime values are calculated to be 1.75 μs, 0.407 μs and 0.022 μs. We do not presume too much on the reliability of these values. However, it is really possible that the orders of magnitude of the values are reliable. Fig. 5 shows intuitively that in the initial period of the SrO:Ce curve, decay is faster than for SrO. Because the lifetime of the d–f transition of Ce³⁺ is a nanosecond, the calculated lifetime value, 0.022 μs (22 ns), of SrO:Ce may be attributed to the d–f transition of Ce³⁺. The short decay time of the order of μs can prevent cross-talk in displays (within about 2 ms). Therefore, taking into account the suitable emission color and the extremely short life time, the potential application of SrO:Ce³⁺ in 3D display devices is anticipated.

Fig. 5 The luminescence decay curves of SrO:0.01Ce³⁺ excited at 290 nm and monitored at 460 nm and SrO excited at 290 nm and monitored at 514 nm.

The emission intensity of SrO:Ce synthesized by ISSR and traditional SSR

To compare the emission intensity of SrO:Ce synthesized by different methods, the PL emission spectrum is measured (the SEM images of the samples are shown in the ESI, Fig. 2s†). Fig. 6 shows that the spectra of the samples share a similar peak type, which is due to the transitions from d level to ²F_7/2 and ²F_5/2 of Ce³⁺. However, emission intensity varies considerably. The emission intensity of SrO:Ce synthesized by ISSR is obviously stronger than that of the sample synthesized by traditional SSR, even if at a relatively lower synthesis temperature.

Fig. 6 PL emission spectra of SrO:0.01Ce³⁺ synthesized at different temperatures (1530 °C, 1450 °C, 1350 °C) by ISSR and the spectrum of the sample obtained at 1450 °C prepared by a traditional solid-state reaction.

The emission intensities of the products were strongly affected by the reaction temperatures because temperature will affect the depth and impurity concentration of doping. For the samples synthesized by traditional SSR, 1450 °C is the highest possible temperature. The emission of this sample is quite low in comparison to the sample synthesized by ISSR at 1350 °C. When the reaction is carried out at a higher temperature (e.g. 1450 °C or 1530 °C), the emission intensity increases simultaneously. When the synthesis temperature reaches 1530 °C, the emission intensity of the sample from the ISSR method is 50 percent higher than that of the sample from traditional SSR. The intensive luminescence of Ce³⁺ ions in SrO synthesized by ISSR can be attributed to good diffusion and better interaction between host and doping ions.^24,25 The reduction in crystal defects due to the higher reaction temperature may be another cause. Due to the complexity of the solid state reaction, additional experimental confirmation is required.

The stability of Ce³⁺ doped SrO synthesized by ISSR

SrO is assumed to be chemically unstable in air. We have investigated the stability of Ce³⁺ doped SrO synthesized by ISSR. The sample's temporal evolution spectrum is introduced (Fig. 7). Fig. 7 shows that the emission intensity of SrO:Ce³⁺ decreased greatly during the initial 24 hours. However, when the sample was exposed for more than 24 hours, the change in emission intensity was not marked. SrO is truly unstable in air. When exposed to air for 24 hours, the sample generally expanded. The XRD measured after 24 hours (Fig. 3s†) shows that there are some peaks that belong to Sr(OH)₂ and SrCO₃. However, in this case, the shapes of the PL luminescent spectra are not changed, which indicates that the crystal structure of the luminescent part of the sample is not changed, because a slight change in the coordination environment would cause remarkable variety in the Ce³⁺ emission spectrum.³⁰ Therefore, we deduce that the decomposer may provide a shielding environment, which prevents the core structure of the phosphor from future carbonation and hydrolysis. Therefore, deliberate attempts to modify the surface may engender a good phosphor.

Fig. 7 PL spectra of SrO:0.01Ce³⁺ exposed to air from 10 minutes to 3 months.

Conclusions

A convenient modified solid state reaction method, ISSR, for obtaining Ce³⁺ doped SrO was reported. Using the ISSR method, the reaction temperature of doping Ce³⁺ into SrO can be increased to as high as 1530 °C in air. An enhanced cyan Ce³⁺ emission is observed, which is due to better diffusion of Ce ions at a high temperature. Most importantly, the results of the investigations indicate that ISSR may provide a new concept and strategy for conveniently synthesizing other ion doped functional materials.

Acknowledgements

The authors are grateful for the financial aid from the Hong Kong, Macao and Taiwan Science and Technology Cooperation Special Project of China MOST (Grant No. 2014DFT10310), the Program of Science and Technology Development Plan of Jilin Province of China (Grant No. 20140201007GX), the National Basic Research Program of China (973 Program, Grant No. 2014CB643801), and the National Natural Science Foundation of China (Grant No. 51102229, 51402288, 21401184).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15089b

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