Co-precipitation synthesis and photoluminescence properties of BaTiF₆:Mn⁴⁺: an efficient red phosphor for warm white LEDs

Abstract

The investigation of efficient red phosphors is highly desired for the development of novel warm white light emitting diodes (WLEDs). In this paper, we report on an efficient red phosphor of Mn⁴⁺-activated BaTiF₆ by a facile co-precipitation method as a promising candidate for warm white LEDs. BaTi_1−xF₆:xMn⁴⁺ phosphors show efficient pure red emission with a high quantum yield (QY) of 44.5% under 460 nm excitation. The BaTi_1−xF₆:xMn⁴⁺ phosphor exhibits a number of advantages. Firstly, the corresponding excitation/absorption profile matches the commercial blue LED chip well. Secondly, it also exhibits appropriate CIE coordinates (x = 0.694, y = 0.306) with an activation energy of 0.603 eV. The demonstration of a blue chip combined with a blend of yellow-emitting YAG:Ce³⁺ and newly developed BaTi_0.97F₆:0.03Mn⁴⁺ red phosphor greatly improved the colour rendering index (CRI) from 69.9 to 83.5, while significantly decreasing the correlated colour temperature (CCT) from 5088 to 4213 K, thus validating their application in warm white LEDs.

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Introduction

Over the last few decades, WLEDs have been studied and widely applied in display backlighting, indoor and outdoor lighting, architectural lighting and indicator lights, due to their extraordinary luminous efficiency, low energy consumption, compactness, long operation lifetime and environmental friendliness.^1–3 The combination of a blue-emitting LED chip with yellow YAG:Ce³⁺-based phosphors is commonly used to obtain white light.^4–7 However, such WLEDs exhibit the disadvantages of low CRI, high CCT and chromaticity drifts due to the red deficiency of the white-light spectrum.⁸ To overcome these shortcomings, a red phosphor must be integrated into the packages of WLEDs, which will then enable the realisation of a warm white emission with high CRI (>80) and low CCT (<4000 K).^9–11

Currently, most commercial red-emitting phosphors are based on Eu²⁺ activated nitrides, such as CaAlSiN₃:Eu²⁺ and Sr₂Si₅N₈:Eu²⁺, which exhibit excellent thermal stability and efficient luminescence.^12–14 Nevertheless, these nitride phosphors still have inherent disadvantages: (1) a significant portion of the broadband emission is beyond the long-wavelength sensitivity limit of the human eye, which decreases the luminous efficacy (LE) of radiation; (2) the synthesis method requires extremely rigorous conditions (high temperature and a reducing atmosphere). Moreover, many researchers worldwide have been devoted to developing red-emitting phosphors, which may be suitable for white LEDs, including oxides, sulfides, organic compounds, etc.^15–22 Therefore, there is still an opportunity to develop a novel and robust red phosphor that demonstrates efficient and intense emission under blue excitation around 460 nm.

Fluorides are an ideal luminescence substrate because of their high thermal stability, structural diversity and low phonon energy.^23–25 Recently, Mn⁴⁺-activated red fluoride phosphors have gained significant research interest because of their narrow band emission from 600 to 650 nm under blue light excitation.^26,27 Various synthesis techniques have been developed to prepare Mn⁴⁺-doped alkaline/alkaline earth hexafluoride phosphors, such as A₂XF₆:Mn⁴⁺ and BaXF₆:Mn⁴⁺ (A = K, Na, Cs, Rb or NH₄; X = Si, Ge, Sn or Ti), including wet chemical etching routes, hydrothermal methods, chemical co-precipitation and cation exchange reactions.^28–43 Adachi et al. prepared BaTiF₆:Mn⁴⁺ by etching titanium sponge and BaF₂ in HF/H₂SiF₆/KMnO₄ solution for about 6 h, and studied its photo-luminescence (PL) properties.²⁹ However, an impurity phase was discovered in the as-synthesised products, which is probably due to the use of H₂SiF₆. In addition, BaTiF₆:Mn⁴⁺ was also obtained by a hydrothermal method at 120–180 °C for 5–20 h, which showed high emission efficiency.^32–37 However, this method has problems in controlling the valence states of manganese in the host. More recently, Zhu et al. firstly reported a high-efficiency red phosphor, K₂TiF₆:Mn⁴⁺, synthesised via a cation exchange method of soaking K₂TiF₆ in a HF/K₂MnF₆ mixed solution at room temperature for 20 min.⁴² Liu et al. demonstrated a simple co-precipitation method to synthesize K₂MF₆:Mn⁴⁺ (M = Ge, Si) and Na₂SiF₆:Mn⁴⁺ with K₂MnF₆ as a manganese source.^31,44 These strategies can obtain red phosphors with exceedingly high emission efficiency, high colour purity and admirable thermal stability. More importantly, the method is simple and has demonstrated good repeatability, low cost and high yield. However, to the best of our knowledge, there are few reports on Ba₂TiF₆:Mn⁴⁺ prepared by the co-precipitation method as a red phosphor for warm WLED applications.

Herein, we report an efficient co-precipitation method by using K₂MnF₆ as a manganese source for the preparation of a series of Mn⁴⁺-doped BaTiF₆ phosphors. We study their crystal structure, luminescence properties, diffuse reflectance spectra, PL decay curves and thermal quenching behaviour. We also successfully obtained WLEDs via a blend of red-emitting BaTiF₆:Mn⁴⁺ and yellow-emitting YAG:Ce³⁺ phosphors together with a blue LED chip, and investigated their optical properties. All the results demonstrate that the red-emitting BaTiF₆:Mn⁴⁺ phosphor has great potential for applications in warm WLEDs.

Experimental

Materials

All the reagents including titanium dioxide (TiO₂), Barium fluoride (BaF₂), hydrofluoric acid (HF), potassium hydrogen fluoride (KHF₂), potassium permanganate (KMnO₄) and hydrogen peroxide (H₂O₂) are of analytical grade and commercially available.

Synthesis of the K₂MnF₆ precursor and BaTiF₆:Mn⁴⁺ phosphors

K₂MnF₆ was synthesized by the previously reported method.⁴⁵ Specifically, 9 g of KHF₂ and 0.45 g of KMnO₄ were fully dissolved in 30 ml HF (40 wt%) solution. After 30 min of stirring, 0.3 ml of H₂O₂ (30 wt%) was slowly added to precipitate the K₂MnF₆ yellow powders. The obtained precipitate was filtered and washed with acetone and oven-dried at 80 °C for 2.0 h.

Powder samples of BaTi_1−xF₆:xMn⁴⁺ (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) were synthesised using the co-precipitation method at room temperature. Typically, 2.5 mmol of TiO₂ was fully dissolved in 5 ml of 40% HF solution. Subsequently, 0.075 mmol of K₂MnF₆ was put into the above solution under vigorous stirring. Then, 2.5 mmol of BaF₂ was gradually added into the mixed solution, which was kept for 30 min under continuous stirring. At last, the precipitate BaTi_0.97F₆:0.03Mn⁴⁺ was filtered, washed with methanol several times and dried at 80 °C for 2 hours. Additional samples with different Mn⁴⁺ concentrations were synthesized using similar procedures.

Fabrication of WLED devices

WLEDs were fabricated by integrating a mixture of transparent silicone resin and the phosphor blend of red-emitting BaTi_0.97F₆:0.03Mn⁴⁺ and yellow-emitting YAG:Ce³⁺ (the ratio of mass is 11 : m : 1, m = 0 or 11) on a blue LED chip (450–460 nm, 3.0–3.4 V, 350 mA, Shenzhen VANO Technology Co., Ltd). After vacuum treatment to remove air bubbles, the mixture was coated on the lead frame and subsequently heated at 100 °C for 1 h and 150 °C for 3 h.

Characterization

The phase purity of the as-prepared phosphors was measured by a Rigaku D/MAX-200 X-ray diffractometer operating at 40 kV, 26 mA with Cu Kα radiation (λ = 1.5405 Å). The morphology and elemental analysis measurements of the samples were analyzed via scanning electron microscopy (SEM, FEI Quanta 200 Thermal FE environmental scanning electron microscopy) equipped with an energy dispersive X-ray spectroscope (EDS). Diffuse reflection spectra of these samples were taken on an ultraviolet-visible–near-infrared (UV-vis-NIR) spectrophotometer (Varian Cary 5000). The photoluminescence excitation (PLE) and PL spectra within the temperature range 300–500 K together with the decay curves were measured using time-resolved and steady-state spectrometry (Edinburgh Instruments FSP920 equipped with a 450 W Xe lamp, a 60 W μF 900 μs flash lamp, TM300 excitation monochromator and double TM300 emission monochromators, and thermo-electric cooled red-sensitive PMT). For PL measurements above room temperature (RT), the sample was mounted in an Oxford OptistatDN2 nitrogen cryostat. The photoluminescence quantum yield (QY) of the sample was measured using a commercial spectrometer equipped with an integrating sphere of 3.3 inches in radius (Hamamatsu, C9920-02). The electroluminescence (EL) spectra and photoelectric properties including LE, CCT, CIE chromaticity coordinates and CRI of the fabricated WLEDs were evaluated using a Labsphere CDS2100 spectrometer and Labsphere LPS-100-0260 power supply.

Results and discussion

Structure and morphology

The powder XRD patterns of the BaTiF₆:Mn⁴⁺ phosphors are shown in Fig. 1a. The diffraction peaks of seven samples with different Mn⁴⁺ doping concentrations coincide well with the standard JCPDS card of rhombohedral BaTiF₆ (No. 76-0269, space group of R3̄m) without any impurity phase, thus indicating their high phase purity. Additionally, doping Mn⁴⁺ ions in BaTiF₆ did not cause any visible change of crystal structure evidencing the incorporation of Mn⁴⁺ in the BaTiF₆ host. A schematic diagram of the BaTiF₆ crystal structure is shown in Fig. 1b. The Ti⁴⁺ ions are located at the centre of the TiF₆ octahedra and their connection with Ba²⁺ ions is through ionic bonding. According to the database of Shannon's ionic radii, the effective ionic radii of the Mn⁴⁺ ion and Ti⁴⁺ ion in an octahedral environment are 53 pm and 60.5 pm, respectively.⁴⁶ Thus, due to their similar ionic radii and matching valency, the Mn⁴⁺ ion prefers to occupy the Ti⁴⁺ site to form the luminescence centre.

Fig. 1 (a) XRD patterns of BaTi_1−xF₆:xMn⁴⁺ phosphors synthesized with different Mn⁴⁺ concentrations. (b) Crystal structure scheme of BaTiF₆. (c) SEM image and (d) EDS spectrum of BaTi_0.97F₆:0.03Mn⁴⁺.

The SEM image and EDS spectrum of BaTi_0.97F₆:0.03Mn⁴⁺ are shown in Fig. 1c and d. The SEM image exhibits irregularly-shaped micrometer-sized particles ranging from ∼1 to 5 μm. The corresponding EDS spectrum confirms that the main chemical elements present are Ba, Ti, F and Mn. The atomic percentages of Ba, Ti, and F are about 17%, 13%, and 69%, respectively, which is close to the BaTiF₆ stoichiometric atom ratio of 1 : 1 : 6. These results further indicate that the Mn⁴⁺ ion-doped BaTiF₆ products were successfully synthesized by the co-precipitation approach.

Optical properties

The room temperature PLE and PL spectra of the BaTiF₆:Mn⁴⁺ samples are given in Fig. 2a. By monitoring the characteristic emission of Mn⁴⁺ ions at 631 nm, the PLE spectrum is observed to span a very broad spectral region from 300 to 550 nm. The two maxima exhibited at 360 nm and 460 nm are assigned to the ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g transitions of Mn⁴⁺ ions, respectively. Under 460 nm excitation, the PL spectrum is composed of a series of sharp peaks around 600–650 nm due to the ²E_g → ⁴A_2g transition of Mn⁴⁺, similar to that of BaTiF₆:Mn⁴⁺ prepared via the hydrothermal method.^33,37 Furthermore, the body colour of the phosphors is light orange under natural light in reflectance of the strong absorption of Mn⁴⁺ in the visible region (Fig. 2b and Fig. 4). Upon UV lamp illumination (365 nm), the phosphors exhibit brilliant red luminescence (Fig. 2c).

Fig. 2 (a) PLE (λ_em = 631 nm) and PL (λ_ex = 460 nm) spectra of the synthesized BaTiF₆:Mn⁴⁺ phosphor. Photographs of BaTiF₆:Mn⁴⁺ taken under illumination of (b) natural light and (c) 365 nm UV lamp.

In order to optimise the doping concentration of Mn⁴⁺ ions, Fig. 3a shows the PL spectra of the BaTi_1−xF₆:xMn⁴⁺ (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) samples excited at 460 nm, and the normalized integrated PL intensity of BaTi_1−xF₆:xMn⁴⁺ samples as a function of Mn⁴⁺ doping concentration are shown in Fig. 3b. With increasing Mn⁴⁺ content, the intensity of the emission peaks gradually increases and reaches the maximum at the Mn⁴⁺ concentration of x = 0.03, and then decreases with further increasing of the Mn⁴⁺ doping concentration due to the concentration quenching effect.⁴⁷ The internal QY of the BaTi_0.97F₆:0.03Mn⁴⁺ sample was measured to be 44.5% under 460 nm light excitation (Fig. S2, ESI†). The CIE chromaticity coordinates of the emission from BaTi_0.97F₆:0.03Mn⁴⁺ were calculated to be (0.69, 0.31), which are very close to the NTSC standard values for red (x = 0.67, y = 0.33). Meanwhile, BaTi_0.97F₆:0.03Mn⁴⁺ red phosphors were synthesized using a hydrothermal method at 150 °C for 12 h (Fig. S1, ESI†). Comparatively, the PL intensity of the obtained BaTi_0.97F₆:0.03Mn⁴⁺ phosphor using the coprecipitation method is better than that using the hydrothermal method, almost 3.6 times higher (Fig. S1, ESI†).

Fig. 3 (a) PL (λ_ex = 460 nm) spectra of BaTiF₆:xMn⁴⁺ with different Mn⁴⁺ concentrations and (b) the normalized integrated PL intensity of BaTi_1−xF₆:xMn⁴⁺ samples as a function of Mn⁴⁺ doping concentration.

The diffuse reflectance spectra of BaTi_1−xF₆:xMn⁴⁺ with different concentrations of Mn⁴⁺ are presented in Fig. 4. The results are fully consistent with the PLE spectra of BaTiF₆:Mn⁴⁺ in Fig. 2a, with two broad absorption bands in the spectral region of 300–410 nm and 410–550 nm with maxima at ∼360 and 460 nm observed, which are attributed to the ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g transitions of Mn⁴⁺, respectively. As expected, the absorption intensity of Mn⁴⁺ increased with increasing Mn⁴⁺ ion concentration.

Fig. 4 Diffuse reflection spectra of BaTi_1−xF₆:xMn⁴⁺ with different Mn⁴⁺ concentrations.

Determining the PL lifetime is an important factor for phosphors, as it depends strongly on the activator concentration. The PL decay curves of the BaTi_1−xF₆:xMn⁴⁺ phosphors (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) excited at 460 nm and monitored at 631 nm are plotted in Fig. 5. The PL decay curves were well fitted with a single-order exponential decay mode by the following equation:⁴⁸

I_t = I₀ exp(−t/τ) (1)

where I₀ and I_t are the luminescence intensities at times t₀ and t₁, respectively; τ is the decay time. The decay times were determined to be 4.6, 4.4, 4.1, 3.7, 3.5, 3.3, and 3.1 ms for the samples of BaTi_1−xF₆:xMn⁴⁺ with increasing x from 0.005, 0.01, 0.02, 0.03, 0.04, 0.05 to 0.06, respectively. Clearly, the decay lifetime decreases with increasing Mn⁴⁺ doping concentration because of the gradual increase in non-radiative transition processes among the Mn⁴⁺ ions.³⁴

Fig. 5 PL decay curves of BaTi_1−xF₆:xMn⁴⁺ phosphors with different Mn⁴⁺ concentrations upon monitoring at 631 nm with the excitation of 460 nm.

Temperature quenching properties

It is well known that thermal stability is a vital factor of phosphors for LED applications. Fig. 6a shows the temperature dependent emission spectra of the BaTi_0.97F₆:0.03Mn⁴⁺ phosphor under excitation at 460 nm, and the relative emission intensity as a function of temperature is shown in the inset of Fig. 6a. It is noticeable that the phosphor exhibits similarly-shaped emission spectra, however there are obvious differences in the relative intensities of the peaks. When the temperature increases from 300 to 500 K, the PL intensity gradually decreases due to thermal quenching. Interestingly, compared to room temperature (300 K), the phosphor at 425 K retains 46% of its initial intensity. To better understand the thermal quenching behaviour, the Arrhenius equation was applied to calculate the activation energy as follows:^49–52where I₀ and I_T are the initial emission intensity and the intensity at temperature T, A is a constant for a certain host, k is the Boltzmann constant (8.617 × 10⁵ eV K⁻¹), and E_a is the activation energy for thermal quenching. Fig. 6b shows the line-of-best-fit for the emission thermal quenching model plotted as ln[(I₀/I_T) − 1] against 1/kT for the present BaTi_0.97F₆:0.03Mn⁴⁺ phosphor. According to eqn (2), the activation energy ΔE of the BaTi_0.97F₆:0.03Mn⁴⁺ phosphor was calculated to be 0.603 ± 0.006 eV, which is very close to previously reported results for a K₂LiAlF₆:Mn⁴⁺ (∼0.62 eV) phosphor.⁴³ At the same time, the relative emission intensities of both Stokes (I_s) and anti-Stokes (I_a) vibronic transitions gradually decreases as the temperature increases and comparatively the Stokes vibronic emission decreases more greatly (Fig. S3, ESI†). The ratio of I_a to I_s shows a linear temperature dependence from 0.34 at RT to 0.52 at 500 K. The reason is that its emission is ascribed to the vibronic radiative transition around the zero phonon line (ZPL), and the intensity of vibronic emission of a Mn⁴⁺ ion usually increases as the temperature increases.³⁰

Fig. 6 (a) PL spectra (λ_ex = 460 nm) of the BaTi_0.97F₆:0.03Mn⁴⁺ phosphor measured within the temperature range of 300–500 K. The inset exhibits the normalized PL intensity as a function of temperature. (b) Monolog plot of ln[(I₀/I_T) − 1] versus 1/kT for the BaTi_0.97F₆:0.03Mn⁴⁺ phosphor. The red line represents the fitting result using eqn (2), its fitting error R² is 0.990.

EL spectra of WLEDs fabricated with BaTiF₆:Mn⁴⁺

To further validate the potential application of the BaTiF₆:Mn⁴⁺ phosphors, WLEDs were fabricated using a blue chip combined with a blend of yellow-emitting YAG:Ce³⁺ and the red-emitting optimized BaTi_0.97F₆:0.03Mn⁴⁺ phosphor operated under different driven currents. The EL spectra of the fabricated WLED under the current regulation from 20 to 200 mA are presented in Fig. 7a. The WLED spectrum is composed of three emission bands: at 445 nm due to the blue chip; at 560 nm from YAG:Ce³⁺; and at 631 nm from the BaTiF₆:Mn⁴⁺ phosphor. When increasing the forward-bias current from 20 to 200 mA, the CIE chromaticity coordinates of the WLED only shift slightly from (0.364, 0.338) to (0.356, 0.340), as illustrated in Fig. 7d. Correspondingly, the CCT and CRI values also exhibit only a small change from 4213 to 4410 K and 83.5 to 82.2, respectively, as tabulated in Table 1. Unfortunately, the luminous efficacy gradually decreases from 115 to 90.9 lm W⁻¹ with increasing forward-bias current, due to the monotonically decreasing external quantum efficiency of the blue InGaN chip.⁵² Moreover, it is obvious that the prototype device emits warm white light (Fig. 7b) under 20 mA forward-bias current (IF). Comparatively, the CIE color coordinates and CRI of the WLED with a blue chip in combination with a YAG:Ce³⁺ phosphor, whose spectrum is presented in Fig. 7c, are (0.344, 0.362) and 66.9, respectively, at a CCT of 5088 K. These results suggest that the incorporation of the BaTiF₆:Mn⁴⁺ red phosphor can effectively improve the CCT and CRI values of the WLED and make it a promising candidate for indoor lighting.

Fig. 7 (a) EL spectra of the as-prepared WLED using the commercially-available YAG:Ce³⁺ and newly developed BaTi_0.97F₆:0.03Mn⁴⁺ under various currents (20–200 mA). (b) Digital photograph of the resulting WLED operated at 20 mA. (c) EL spectrum of the WLED relying on the commercially-available YAG:Ce³⁺-based phosphor operated at 20 mA. (d) CIE chromaticity coordinates of WLEDs based on a dual phosphor combination (YAG:Ce³⁺ + BaTi_0.97F₆:0.03Mn⁴⁺) operated under different forward bias currents.

Table 1 Photoelectric properties of WLEDs based on a dual phosphor combination (YAG:Ce³⁺ + BaTi_0.97F₆:0.03Mn⁴⁺) operated at different forward bias currents. The results are compared to a single phosphor device using only YAG:Ce³⁺

Composition	IF (mA)	LE (lm W^−1)	CCT (K)	CIE-x	CIE-y	CRI
Chip + YAG	20	162	5088	0.344	0.362	66.9
Chip + YAG + BaTiF6:Mn^4+	20	115	4213	0.364	0.338	83.5
	50	110	4260	0.362	0.335	83.1
	100	103	4322	0.359	0.330	82.7
	150	96.3	4369	0.357	0.327	82.4
	200	90.9	4410	0.356	0.324	82.2

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Conclusions

In conclusion, a series of Mn⁴⁺ activated BaTiF₆ red phosphors have been successfully synthesized by using a facile co-precipitation method and examined for their use in warm WLEDs. The excitation and reflectance spectra of the BaTiF₆:Mn⁴⁺ phosphor exhibit strong broad absorption in the 300–550 nm region, which matches perfectly with the emission of the commercial blue chip. The emission intensity of the optimized BaTi_1−xF₆:xMn⁴⁺ (x = 0.03) under 460 nm excitation has appropriate CIE coordinates (x = 0.694, y = 0.306). Moreover, when partnering the red-emitting BaTiF₆:Mn⁴⁺ phosphor together with a yellow-emitting YAG:Ce³⁺ phosphor to a blue LED chip, a WLED with a low CCT of 4213 K and a higher CRI of 83.5 is achieved. These results demonstrate that BaTiF₆:Mn⁴⁺ phosphors have considerable potential for use in warm WLEDs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51772336, 51572302, 21271191 and 21661033), the “973” programs (2014CB643801), the Joint Funds of the National Natural Science Foundation of China and Guangdong Province (U1301242), Teamwork Projects of the Guangdong Natural Science Foundation (S2013030012842), Guangzhou Science & Technology Project (2013B090800019, 2015B090926011 and 2017A050501008), and the Natural Science Foundation of the Guangdong Province (2014A030313114).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tc04196a

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