Remarkably Enhancing Green-Excitation Efficiency for Solar Energy

Apr 21, 2017 - Of all types of solar spectral conversions, inorganic phosphors possess many advantages, including durability, long life, color stabili...
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Remarkably Enhancing Green-Excitation Efficiency for Solar Energy Utilization: Red Phosphors Ba2ZnS3:Eu2+, X− Co-Doped Halide Ions (X = Cl, Br, I) Tingting Luo,† Yun Du,† Zhongxian Qiu, Yanmei Li, Xiaofang Wang, Wenli Zhou, Jilin Zhang, Liping Yu, and Shixun Lian* Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research (Ministry of Education) and Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha, Hunan 410081, China S Supporting Information *

ABSTRACT: Eu2+-activated Ba2ZnS3 has been reported as a red phosphor with a broad emission band peaking at 650 nm under blue excitation for white-LED. In this study, Ba2ZnS3:Eu2+, X− (X = F, Cl, Br, I) phosphors doped with halide ions were prepared by traditional hightemperature solid-state reaction. Phase identification of powders was performed by X-ray powder diffraction analysis, confirming the existence of single-phase Ba2ZnS3 crystals without dopant. The corresponding excitation spectra showed an additional broad band in the green region peaking at 550 nm when the phosphor was halogenated except by the smallest F−. It was proved that the green-excitation efficiency successively strengthened from Cl−, to Br−, to I−, which suggested larger halide ions made a greater contribution to the further splitting of the t2g energy level of the doped Eu2+ ions in the host Ba2ZnS3, and the optimized formula Ba1.995ZnS2.82:Eu2+0.005, I−0.18 showed a potential application in solar spectral conversion for agricultural greenhouse and solar cell. Defect chemistry theory and crystal field theory provided insights into the key role of halide ions in enhancing green-excitation efficiency.

1. INTRODUCTION As the most abundant and sustainable source of energy available to humanity,1 solar energy is always attempted for fullscale application in various fields. In particular, sunlight serves as the essential energy resource for green plants’ photosynthesis, which plays a vital role in sustainable agriculture development. However, due to a poor match between the solar irradiation spectrum and the chloroplast absorption spectrum, solar spectral conversion has been considered as a way to improve the efficiency of solar energy conversion in the photosynthetic process of agricultural crops to augment their production since the early 1970s.2−4 Green plants harvest sunlight through the green pigment chlorophyll antenna complexes with accessory carotenes and xanthophylls, but these compounds only absorb blue and red light. Considering the fact that the little green component of the solar spectrum can be absorbed by chloroplast in the photosynthesis process of plants, green-to-red (GTR) conversion is highly desirable for modifications of light quality, which can realize the modulation of morphogenesis, boost the growth rate of plants, increase grain yield, alleviate the current use of pesticides, etc. Of all types of solar spectral conversions, inorganic phosphors possess many advantages, including durability, long life, color stability, and a relatively benign environmental © 2017 American Chemical Society

footprint. To our knowledge, several inorganic conversions have been recently promoted to match the absorption characteristics of chlorophyllous pigment species in the cells of photosynthetic organisms, such as (Ca,Sr)S:Eu2+,5,6 Ca0.6Sr0.4S:Cu+, Eu2+,7 CaZnOS:Eu2+,8 BaY2S4:Eu2+, Er3+,9 A3MgSi2O8:Eu2+, Mn2+ (A = Ca, Sr or Ba),10,11 etc. In particular, field tests7 and algae reactors5 both provided a convictive illustration about the Eu2+-doped alkaline earth sulfide serving for solar spectral conversion for improving the photosynthetic activity. The superior optical character of Eu2+doped sulfides makes them particularly attractive as solar spectral conversion materials. To date, Ba2ZnS3:Eu2+ has drawn attention as a novel red phosphor with a broad emission band at 650 nm for blue chipbased white light-emitting diodes (W-LED).12−14 In this work, we aimed to achieve GTR conversion in Ba 2ZnS3:Eu2+ phosphor, which has been reported to have merely NUV and blue excitation bands. As a result, halide ions are introduced into Ba2ZnS3-based red phosphors. The novel Ba2ZnS3:Eu2+, X− (X = Cl, Br, I) red phosphors are proved to possess an obviously enhanced excitation band at the visible light region, Received: February 7, 2017 Published: April 21, 2017 5720

DOI: 10.1021/acs.inorgchem.7b00335 Inorg. Chem. 2017, 56, 5720−5727

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Figure 1. Crystalline structure of Ba2ZnS3 (a), and representative XRD patterns of Ba2ZnS3 and Ba1.995ZnS3−2x:Eu2+0.005, I−2x phosphors (b).

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Composition Identification. As shown in Figure 1a, Ba2ZnS3 has an orthorhombic structure, belonging to the space group Pnam with parameters a = 12.05 Å, b = 12.65 Å, and c = 4.21 Å. Ba2ZnS3 has two crystallographically independent cation sites in a unit cell: a seven-coordinated Ba2+ site and a four-coordinated Zn2+ site. Zn−S tetrahedral chains along [001] are connected by Ba2+.15 The activator Eu2+ (1.20 Å) ions are expected to occupy the Ba2+ (1.38 Å) sites and form neutral point defects Eu×Ba owing to their similar ionic radii (eq 3), while the Zn2+ (0.60 Å) sites are too small to be replaced. When X− (X = F, Cl, Br, I) ions are doped into Ba2ZnS3-based phosphor, two entirely different cases take place due to the distinct radius percentage difference between doped ions X− and matrix ions S2− (further discussed in section 3.3). In view of an overlarge radius discrepancy, it is difficult for the smallest halide ion F− to replace the S2− site. Even if it works, F−F clusters are responsible for the charge compensation, namely, one F− takes the place of S2− to be a positive defect F•S while the other F− is inserted as an interstitial defect F′i (eq 4). However, it is feasible for the other halide ions X− (X = Cl, Br, I) to occupy the S2− site owing to an acceptable radius difference, but the defect reaction of substituting X− (X = Cl, Br, I) for S2− is different from that of F− replacing S2− (if it happens) because it is quite difficult for these larger X− ions to situate in the interstices of crystal lattice. As a result, a Zn2+ vacancy defect VZn ″ is formed for the sake of charge balance, while two X− (X = Cl, Br, or I) anions substitute two S2− to generate two positive defect X•S (eq 5).

especially a remarkable green excitation band, while the broad red emission band remains almost unchanged. To our knowledge, this is the first attempt to develop an Eu2+-doped Ba2ZnS3 phosphor with such a dramatic change in its excitation spectrum by halogenation. Owing to the high-matchable luminescent spectra, Ba2ZnS3:Eu2+, I− phosphor can serve as a novel alternative for a green-to-red conversion auxiliary in agriculture. In addition, taking eco-environmental sustainability into account, the carbon thermal reduction strategy we adopted here gets rid of the utilization or formation of toxic SO2, H2S, or CS2.

2. EXPERIMENTAL PROCEDURES 2.1. Materials and Synthesis. A series of Ba1.995ZnS3−2x:Eu2+0.005, (X = F, Cl, Br, I) phosphors was prepared by a carbon thermal reduction route. Commercial reagents, such as BaSO4 (A.R.), ZnS (A.R.), BaX2 (A.R., X = F, Cl, Br, I), and Eu2O3 (99.99%), were used without any further treatment. As illustrated in eq 1, carbon powder plays a significant part in gaining BaS from deoxidization of BaSO4. The stoichiometric amounts of BaSO4, ZnS, carbon powder, Eu2O3, and BaX2 were ground and mixed thoroughly in an agate mortar before being transferred into crucibles and fired at 900 °C for 4 h under CO-reducing atmosphere. In the end, the samples were cooled to room temperature and reground into powder for subsequent characterization and analysis. Taking the formation process of Ba1.995Eu0.005ZnS3−2xX2x as an example, the reaction proceeds as follows X−2x

BaSO4 + 4C → BaS + 4CO↑

(1)

(2 − x)BaS + x BaX 2 + ZnS + 0.0025Eu 2O3 → Ba1.995Eu 0.005ZnS3 − 2x X 2x

(2)

2.2. Materials Characterization. A Hitachi F-4500 spectrophotometer (Tokyo, Japan) equipped with a 150 W Xe lamp as the excitation source was utilized to conduct photoluminescence (PL) and photoluminescence excitation (PLE) spectra measurements. The crystalline phases and composition of the samples were determined by X-ray diffractometry with Cu Kα radiation at 40 kV and 300 mA (RigakuD/MAX-2550 X-ray diffractometer, Tokyo, Japan). Diffuse reflectance spectra were recorded on a Hitachi U-3010 spectrometer equipped with an integration sphere attachment using BaSO4 as background. The luminescence decay lifetimes were measured and obtained from an Edinburgh FLS920 Combined Fluorescence Lifetime & Steady State Spectrometer (Edinburgh Instruments Ltd., Livingston, U.K.) using a pulsed hydrogen lamp (nF900) as the excitation source. PL quantum yields (QYs) of phosphors were obtained directly by using the absolute PL quantum yield measurement system (C9920-02, Hamamatsu Photonics K. K., Japan). All characterizations were carried out at room temperature.

BaBa + Eu 2 + → Eu×Ba + Ba 2 +

(3)

2F− + SS → F•S + F′i + S2 −

(4)

2X− + ZnZn + 2SS → 2X •S + V″Zn + Zn 2 + + 2S2 −

(5)

2+

−

Thus, when Eu and I ions are codoped into Ba2ZnS3, the total defect reaction can be considered to proceed as follows 2I− + Eu 2 + + BaBa + ZnZn + 2SS → 2I•S + Eu×Ba + V″Zn + BaS + ZnS

(6)

Figure 1b shows the representative X-ray diffraction (XRD) patterns of Ba2ZnS3 host and Ba1.995ZnS3−2x:Eu2+0.005, I−2x (x = 0, 0.09) phosphors prepared by the approach mentioned above. All diffraction peaks of the as-synthesized samples can be readily indexed to PDF card no. 74-0132, which definitely 5721

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Figure 2. PLE spectra monitored at 650 nm (a) and PL spectra excited at 370 (b), 475 (c), and 550 nm (d) of Ba1.995ZnS2.82:Eu2+0.005, X−0.18 phosphors (X = F, Cl, Br, and I).

remarkable green excitation peak situated at about 550 nm, is observed in their PLE spectra. The green-excitation efficiency successively strengthens from Cl−, to Br−, to I−. The PL spectra excited at 370 nm are presented in Figure 2b with an emission band peaking at around 633 nm, which theoretically is related ″ and to the charge transfer between the native defects, e.g., VZn V•• S in Ba2ZnS3 matrix. As illustrated in Figure 2c, upon 475 nm excitation, the emission band of Ba2ZnS3:Eu2+ phosphor is situated at 650 nm, which results from the 5d → 4f transition of Eu2+. It is obvious that the emission intensity tends to increase when Ba2ZnS3:Eu2+ is halogenated by Cl−, Br−, and I−, respectively. Under 550 nm excitation, Ba1.995ZnS2.82:Eu2+0.005, X−0.18 (X = Cl, Br, I) phosphors also exhibit a strong red emission band peaking at about 650 nm as shown in Figure 2d, which demonstrates that “GTR conversion” is achieved in Ba2ZnS3:Eu2+, X− (X = Cl, Br, I) phosphors. It is concluded that I− makes the biggest contribution to boosting the efficiency of GTR conversion in Eu2+-activated Ba2ZnS3 phosphor. The red emission bands in Figure 2d are also attributed to the 5d → 4f transition of Eu2+. Further support for the effect of halogenation on green-excitation efficiency is displayed in Figure S1, which offers representative excitation spectra of some other Ba2ZnS3:Eu2+, X− phosphors whose X− ions are provided by other halides ZnX2, KX, and NH4X. Figure 3 presents the representative photoluminescence spectra of Ba1.995ZnS3−2x:Eu2+0.005, I−2x phosphors with x varying from 0% to 11.0%. As seen in Figure 3a, with increasing I− ions incorporated into the crystal lattice, a progressively enhanced excitation band at the visible light

manifests that it is advisable to develop Ba2ZnS3-based phosphors by the carbon thermal reduction method. The asprepared Ba2ZnS3 host phosphor is verified to form a pure Ba2ZnS3 phase, while trace impurities belonging to ZnS are detected in Ba1.995ZnS3−2x:Eu2+0.005, I−2x (x = 0, 0.09) phosphors. As illustrated in eq 6, the recombination of replaced S2− anions and escaped Zn2+ cations is responsible for the appearance of ZnS impurities phase. Therefore, the pure-phase phosphor Ba1.995ZnS3−2x:Eu2+0.005, I−2x should be obtained after decreasing a little stoichiometric percentage of ZnS in raw materials.16 Moreover, it is worth pointing out that the impurities are independent of the optical property of Ba2ZnS3:Eu2+, I− red phosphors. 3.2. Effect of Halogenation on Green-Excitation Efficiency in Ba2ZnS3:Eu2+, X−. The representative PLE and PL spectra of the red phosphors Ba2ZnS3:Eu2+, X− doped with halogen anions (X = F, Cl, Br, and I) are exhibited in Figure 2. As shown in Figure 2a, the PLE spectrum of assynthesized Ba1.995ZnS3:Eu2+0.005 phosphor monitored at 650 nm (the black short dash) consists of a high-energy NUV excitation band at 370 nm and a broad blue excitation band at 475 nm; the former is ascribed to the absorption of host, and the latter originates from the 4f7 → 4f65d1 transition of Eu2+, which is basically in accordance with the previous report14 and further confirms the fact that the impurity phase in Ba1.995ZnS3−2x:Eu2+0.005, I−2x phosphors makes no difference to their optical property. As Ba2ZnS3:Eu2+ phosphor is halogenated by X− (X = Cl, Br, I) ions, a noticeably enhanced excitation band at the visible light region, especially a 5722

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Figure 3. PLE spectra monitored at 650 nm (a), normalized PL spectra excited at 370 nm (b), and PL spectra excited at 475 (c) and 550 nm (d) of Ba1.995ZnS3−2x:Eu2+0.005, I−2x phosphors.

Figure 4. Diffuse reflection spectra of Ba1.995ZnS3−2x:Eu2+0.005, I−2x phosphors (a) and absorption spectra of Ba2ZnS3−2xI2x host (b).

a maximum when x is equal to 9.0%, and then keep almost the same intensity at x = 11.0%. The phenomenon is in good agreement with the evolution of PLE spectra with different I− content monitored at 650 nm. It is thought that the enhancement of emission from Eu2+ on the longer wavelength side (650 nm) contributes to the red shift of PL spectra excited at 370 nm due to an overlap of the 633 nm band and enhanced 650 nm band. A series of Ba1.995ZnS3−2x:Eu2+0.005, I−2x phosphors with x altering from 11.0% to 20.0% are prepared for a further investigation. The PLE spectra monitored at 650 nm and PL spectra excited at 475 and 550 nm are shown in Figure S2. Clearly, the intensities of the excitation and emission bands

region is detected and the optimal excitation intensity appears at x = 9.0%, which demonstrates the positive optical sensitization effect of I− ions on Ba2ZnS3:Eu2+, peculiarly a distinct sensitization on green light excitation as shown in the inset of Figure 3a, which provides a normalized PLE spectra comparison between Ba2ZnS3:Eu2+ and Ba2ZnS3:Eu2+, I− phosphors. Figure 3b displays the normalized emission spectra upon excitation at 370 nm, which reveals a red shift of peak positions from 633 to 640 nm with the increase of I− ion concentration. The PL spectra excited at 475 and 550 nm are shown in Figure 3c and 3d, respectively. Obviously, as I− content gradually increases, the intensities of the broad red emission bands peaking at 650 nm both increase first, reaching 5723

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spectrum under the same excitation conditions.23 However, there are few reports about the effect of halide ions (monovalence anions) on the excitation spectral properties. Calculations of the radius percentage difference between halide ions and matrix lattice ions are summarized in Table 1.

from Eu2+ transition are demonstrated to almost remain unchanged when I− content is increased continuously, which makes it clear that the capacity of incorporating I− into Ba2ZnS3 crystal lattice reaches the limit at x = 9.0%. The role of the halogenation by I− ions in heightening greenexcitation efficiency is further confirmed by a comparison of quantum yields (QYs) between Ba1.995ZnS3:Eu2+0.005 and Ba1.995ZnS2.82:Eu2+0.005, I−0.18 phosphors. As indicated in Table S1, the QY of Ba1.995ZnS2.82:Eu2+0.005, I−0.18 is higher than that of Ba1.995ZnS3:Eu2+0.005 whether the phosphors are excited by 370, 475, or 550 nm. In particular, the QY upon 550 nm excitation is proved to get an obvious improvement by codoping I− ions, which indicates that I− ions have a positive effect on enhancing green-excitation efficiency of Eu2+-activated Ba2ZnS3 phosphor. The sensitization of I− ion for green excitation can be also illustrated in the diffuse reflection spectra as presented in Figure 4a. It is detected that the addition of I− has a significant effect on the absorption behavior of Eu2+ in Ba2ZnS3. A more intensive absorption band at the visible light region is observed in Ba1.995ZnS3−2x:Eu2+0.005, I−2x than that in Ba1.995ZnS3:Eu2+0.005. With the increasing content of I− ions, the absorption intensity of visible light monotonously increases until a maximum absorption appears at x = 9.0%, corresponding to the results shown in Figure 4a. The phenomenon further accounts for a positive trend of optical sensitization by I− ions, especially an enhanced absorption at the green light region. The influence of I− ion on Ba2ZnS3 host is also explored. Generally speaking, the band gap of the Ba2ZnS3 host can be estimated according to the following equation17,18 [F(R ∞)hν]n = A(hν − Eg )

Table 1. Ionic Radii Difference Percentage (Dr) between Matrix Anions (S2−) and Doped Halide Anions24,25 a doped ions −

F Cl− Br− I−

[R m(CN ) − R d(CN )] R m(CN )

(6) (6) (6) (6)

+27.72 +1.63 −6.52 −19.56

There are no data of 7-CN of halide ions which are replaced with data of 6-CN of halide ions.

According to the theory of defect chemistry, all of these X− (X = F, Cl, Br, and I) ions in Table 1 have a tendency to replace the S2− site in Ba2ZnS3 crystal in high-temperature solid-state reaction. The value of Dr (+27.72) between F− ions and S2− ions is close to 30%, so it is quite difficult for F− ions to replace the site of the S2− ions. The Dr (+1.63) for Cl−, (−6.52) for Br− between Cl− or Br− ions, and S2− ions is very small, so the process of substituting Cl− or Br− ions for S2− ions is very easy to occur. For the reason that Dr (−19.56) between I− and S2− is enough big but much less than 30%, replacement of S2− ions with I− in the Ba2ZnS3 crystal lattice is able to be realized, which will result in a bigger change in the crystal field splitting of the Eu2+ in Ba2ZnS3 matrix. On the other hand, as is well known, the crystal field splitting strength of the 5d state in Eu2+ is greatly affected by its coordination surrounding.26−30 As discussed above, the activator Eu2+ ions replace the Ba2+ site, which means Eu2+ is coordinated with seven S2− anions in the form of the EuS7 polyhedron as shown in Figure 5a. Obviously, this EuS7

(7)

(8)

where R, K, and S represent the reflection, absorption, and scattering coefficient, respectively. As illustrated in Figure 4b, the Eg value was estimated to be about 3.20 eV for Ba2ZnS3 host from the linear extrapolation of [F(R∞)hv]2 = 0, which is basically in agreement with the value reported previously (Eg = 3.35 eV).19 However, the Eg value obviously decreases to 3.11 eV when I− ions are introduced into the Ba2ZnS3 host. Therefore, one of the key roles of I− ions doping is narrowing the band gap of the host Ba2ZnS3. 3.3. Possible Mechanisms. An acceptable percentage difference in ion radii between doped and substituted ions must not exceed 30%.20 On the basis of eq 921 Dr = 100 ×

0.133 0.181 0.196 0.220

Dr/%, S2−: 0.184/nm (6)

a

where hν is the photon energy, A is a proportionality constant, Eg is the value of the band gap, n = 2 represents a direct transition and n = 1/2 represents an indirect transition, and F(R∞) is the Kubelka−Munk function, defined as F(R ∞) = (1 − R )2 /R = K /S

radius/nm (CN)

Figure 5. Structural diagrams of EuS7 polyhedron (a) in Ba2ZnS3:Eu2+ and Eu(S,I)7 polyhedron (b) in Ba2ZnS3:Eu2+, I−.

(9)

polyhedron presents a capped trigonal prism structure with a capping S2− at the top, which is analogous to an octahedron. When Eu2+-activated Ba2ZnS3 phosphor is halogenated by I− ions, some S2− ions originally coordinated with Eu2+ are substituted by I− ions. In consideration of the difference between I− and S2−, the Eu(S,I)7 polyhedron in which only the capping S2− site is replaced by I− is more desirable as depicted in Figure 5b. Owing to the fact that the I− ion (0.22 nm) is much larger than the S2− (0.184 nm) ion and the charge of I− is less than that of S2−, the new Eu(S,I)7 polyhedron is supposed to be an elongated one, which contributes significantly to a

where Dr is the radius percentage discrepancy, Rm(CN) is the radius of the host ion, Rd(CN) is the radius of the doped ion, and CN is the coordination number. Alkali metal ions codoping (monovalence-cations) and the crystal cell substitution are common strategies for crystal field splitting effect to enhance the luminescent efficiency for phosphor, such as some recent reports, Li + -codoped KMg4(PO4)3:Eu2+ phosphor exhibits a 3.8-fold enhancement over Li+-free phosphors22 and substitution of Y + SiO4 for Ba + PO4 in Ba3Y(PO4)3 produces the red shift of the emission 5724

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Figure 6. Schematic energy level diagrams of Eu2+ in (a) Ba2ZnS3:Eu2+ and (b) Ba2ZnS3:Eu2+, X− (X = Cl, Br, I).

sequently, the lowest excited level gets closer to the ground state, which is responsible for the newly-emerged greenexcitation band at about 550 nm. Upon excitation at 475 and 550 nm, one 4f electron of Eu2+ is stimulated to the upper and lower t2g levels, respectively. After nonradiative relaxation, the electron finally goes back to the 4f ground state, resulting in the red emission band peaking at around 650 nm. The nephelauxetic effect and Jahn−Teller theorem also apply to the uplift of the green excitation of Eu2+ in the situation of Eu(S,X)7 (X = Cl or Br) coordination polyhedrons. Nevertheless, in the case of F− halogenation, no stable Eu(S,F)7 polyhedron exists resulting from the overlarge radius difference between F− and S2− as discussed above, which is why F− cannot contribute to formation of a green-excitation band in Eu2+doped Ba2ZnS3. To further validate the opinion mentioned above, the roomtemperature fluorescent decay curves of the Eu2+ luminescence in Ba 1.995ZnS 3:Eu 2+0.005 and Ba1.995ZnS2.82 :Eu2+0.005, I−0.18 phosphors upon excitation at 475 nm are measured, respectively. As demonstrated in Figure 7, the decay curves are well fitted with a double-exponential decay mode by the following equation16,34

coordination polyhedron (analogous octahedron) with lower symmetry. For the case of a crystal field, the crystal field splitting (Dq) of Eu2+ can be determined as obeying30 Dq =

Ze 2r 4 6R5

(10)

where Dq stands for a measure of the energy level separation, R refers to the distance from the central ion to its ligands, Z is the charge or valence of the anion, r represents the radius of the d wave function, and e is the charge of an electron. When the capping S2− ion is substituted by the larger I− ion, the elongated Eu(S,I)7 polyhedron results in a shorter average distance between Eu2+ and its ligands, which contributes to an increased magnitude of the crystal field strength. Thus, the crystal field splitting of the Eu2+ ion is reasonably increased. A similar explanation is also offered for the enhanced crystal field splitting when S2− ion is substituted by Br− , or Cl− ion. For purpose of a better understanding of the green-excitation enhancement effect from X− (X = Cl, Br, I), the schematic energy level diagrams of Eu 2+ in Ba 2 ZnS 3 :Eu 2+ and Ba2ZnS3:Eu2+, X− phosphors are given in Figure 6. As shown in Figure 6a, the five degenerate d orbitals of Eu2+ split into two sets eg and t2g in the EuS7 coordination polyhedron with an analogous octahedral structure. The electron transition from the 4f state to the t2g set gives rise to the blue excitation band at 475 nm. Correspondingly, a red emission band at 650 nm arises as the result of 4f65d1 → 4f7 electron transition. However, in Ba2ZnS3:Eu2+, I− phosphor, the 5d states of Eu2+ have a higher energy than that in Ba2ZnS3:Eu2+ due to a weaker nephelauxetic effect which results from the lower charge and larger radius of I− in the Eu(S,I)7 coordination polyhedron.30,31 On the basis of the above discussion, the five degenerate 5d states of Eu2+ suffer from an increased crystal field splitting, first splitting into two sets eg and t2g as depicted in Figure 6b. According to the Jahn− Teller theorem,32,33 the degenerate eg and t2g levels will suffer further splitting due to the increased distortion of the coordination polyhedron. As a result, with a lower symmetry and a larger distortion, the t2g set of Eu2+ in the Eu(S,I)7 coordination polyhedron further splits into two sets. Con-

I = A1 exp( − t /τ1) + A 2 exp(−t /τ2)

(11)

where I is the luminescence intensity, A1 and A2 are constants, t is time, and τ1 and τ2 represent the rapid and slow lifetime values for exponential components. On the basis of the fitting of the decay curves, the A1, A2, τ1, and τ2 values are acquired. Therefore, the average lifetimes (τ*) can be calculated by the following formula35 τ * = (A1τ12 + A 2 τ2 2)/(A1τ1 + A 2 τ2)

(12)

The calculated average lifetimes are 4.48 and 5.51 μs for Ba1.995ZnS3:Eu2+0.005 and Ba1.995ZnS2.82:Eu2+0.005, I−0.18 phosphors, respectively. Obviously, the average lifetime increases when I− ions are incorporated into Eu2+-doped Ba2ZnS3, which is considered to stem from an additional relaxation process between the upper t2g and the lower t2g levels, providing a good proof for further splitting of the t2g energy level in Ba2ZnS3:Eu2+, I− phosphors. 5725

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00335. Representative excitation spectra of Ba2ZnS3:Eu2+, X− phosphors of which X− (X = F, Cl, Br, and I) ions are provided by ZnX2, KX and NH4X; representative PLE and PL spectra of Ba1.995ZnS3−2x:Eu2+0.005, I−2x (x = 11%−20%) phosphors; quantum yields (QYs) of Ba1.995ZnS3:Eu2+0.005 and Ba1.995ZnS2.82:Eu2+0.005, I−0.18 phosphors excited at 370, 475, and 550 nm (PDF)

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AUTHOR INFORMATION

Corresponding Author Figure 7. Decay curves and corresponding lifetime values of Ba1.995ZnS3:Eu2+0.005 (the blue one) and Ba1.995ZnS2.82:Eu2+0.005, I−0.18 (the green one) upon excitation at 475 nm.

*E-mail: [email protected]. Phone/Fax: +86 731 88865345.

3.4. Potential Applications. Thanks to the positive impact of halogenation on heightening GTR conversion efficiency, Ba2ZnS3:Eu2+, X− (X = Cl, Br, I) phosphors display a potential application for solar energy utilization. Some other materials were also proposed for the same application, such as Bi-doped glasses36 and DE2 phosphors Ca0.6Sr0.4S:Cu+, Eu2+,7 etc. Although Ba2ZnS3:Eu2+, X− phosphors exhibit weaker stability than Bi glasses due to the common drawback of sensitivity to moisture just as other sulfide materials, functional agricultural films provide a good application form for these phosphors. Generally speaking, agricultural films can be used for 1 year because the plastic layers provide a relatively good protection for the solar spectral conversion agents in films. Furthermore, a permanent laminated glass device can be also developed for a long-term application as our previous work reported.6 Therefore, after further modification, e.g., improving QYs by introducing Sr2+ into Ba2+ sites,12 and achieving surface modification by a core−shell structure,37 the Ba2ZnS3:Eu2+, X− (X = Cl, Br, I) red phosphors can be developed as novel solar spectral conversion agents for agricultural greenhouse or solar cell.

Author Contributions

ORCID

Jilin Zhang: 0000-0001-7235-341X Shixun Lian: 0000-0001-6524-2703 †

T.L. and Y.D.: Both authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (Grant nos. 21571059, 21501058, 21471055, 51402105), National Key Research and Development Program (Grant no. 2016YFB0302403), and Hunan Provincial Natural Science Foundation of China (Grant nos. 2015JJ2100, 2017JJ3200).

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REFERENCES

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4. CONCLUSIONS The red-emitting phosphors Ba2ZnS3:Eu2+, X− (X = Cl, Br, I) are developed for the first time via a carbon thermal reduction route. In this study, we succeed in achieving GTR conversion in Eu2+-doped Ba2ZnS3 phosphor by incorporating halide ions into the crystal lattice of Ba2ZnS3. It is proved that the greenexcitation efficiency successively strengthens from Cl−, to Br−, to I−, while F− makes no contribution to enhance excitation in green light. A possible explanation and mechanism for the halogenation effect is that when the S2− ions are substituted by monovalent halide ions (Cl−, Br−, or I−), the crystal field splitting of 5d states in Eu2+ reasonably increases. Meanwhile, the lower symmetry of the distorted Eu(S,X)7 polyhedron (X = Cl, Br, or I) gives rise to the further splitting of the t2g energy level. The electron transition from the 4f ground state to the lower-energy t2g excited state accounts for the occurrence of the green-excitation band peaking at 550 nm. After modification, the optimized red phosphor Ba1.995ZnS2.82:Eu2+0.005, I−0.18 can serve as a brand new solar spectral conversion auxiliary for agricultural greenhouse or solar cell. 5726

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