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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Two-Step Synthesis and Surface Modification of CaZnOS:Mn2+ Phosphors and the Fabrication of a Luminescent Poly(dimethylsiloxane) Film Zihan Xu, Zhiguo Xia,* and Quanlin Liu The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China ABSTRACT: The CaZnOS:Mn2+ (CZOSM) phosphor has been extensively studied for its excellent optical performance, with a typical red emission band peaking at about 580 nm ascribed to the 4T1(4G)−6A1(6S) transition of Mn2+. Herein the CZOSM phosphor was synthesized by a novel two-step method accompanied by control of the morphology of the precursor in the first step followed by sintering in the second step, which demonstrated improved emission intensity and uniform morphology simultaneously compared to those obtained by the traditional solid-state reaction route. Thus, uniform ZnS:Mn2+ particles could be obtained by a hydrothermal method, and then a Ca(OH)2 shell was coated onto the ZnS:Mn2+ particles via a precipitation reaction. After that, these mixtures were sintered at the optimum temperature 800 °C in an argon atmosphere to prepare the CZOSM particles. Oleic acid (OA) was further used to transfer the hydrophilic CZOSM phosphors to hydrophobic ones. Finally, luminescent poly(dimethylsiloxane) (PDMS) films were fabricated by using the hydrophobic CZOSM@OA powders, and their optical performance and flexibility were evaluated. Our results provide insight into the synthesis of hydrophobic phosphor particles used in luminescent PDMS films and help to unravel their potential application for flexible optical devices.

1. INTRODUCTION Recently, piezoelectric semiconductors with advanced optical, electrical, and mechanical multifunctional properties have great potential applications in future optoelectronic devices.1 The research focused on calcium zinc sulfide (CaZnOS) has been a hot issue since it was first reported as an intermediate product in the recovery of zinc from its sulfide by carbothermal reduction in the presence of lime.2 When CaZnOS hosts are doped with different activators (Ce3+, Eu2+, Mn2+, Cu+, Sm3+, Er3+, etc.), they become interesting for their multifunctional luminescent properties, such as photoluminescence (PL)3−7 and mechanoluminescence (ML).8−12 For example, CaZnOS:Ce3+ exhibits a broad band emission in the wavelength range of 450−650 nm peaking at 506 and 564 nm.6 CaZnOS:Eu2+ reveals a broad absorption band and red emission.3−5 CaZnOS:Cu+ can be used for visualizing stress distributions in practical applications.10−12 CaZnOS:Sm3+ shows strong red emission induced by dynamic mechanical stress.1 The quaternary piezoelectric semiconductor CaZnOS:Er3+ shows both ML and up-conversion luminescence properties,7 and so on. It is found that Mn2+-doped CaZnOS (CZOSM) has received the most extensive investigations during basic theory research and practical applications. The elasticomechanoluminescence (EML) properties of CZOSM phosphors are extensively investigated by Zhang et al.,13,14 and they have also successfully realized color manipulation of © XXXX American Chemical Society

intense emission of the CZOSM phosphor from yellow to red by adjustment of the Mn2+ concentration.15 Tu et al. reported that the codoping of Li+ ions significantly enhanced the crystallinity and ML intensity of CZOSM.16 Sonwane et al. reported that EML can be induced, and consequently an intense red-light emission occurs when a UV-irradiated CZOSM/epoxy resin composite is exposed to ultrasonic waves.17 As a typical example of PL tuning, our group has also recently reported tunable luminescence properties from the codoped CaZnOS:Ce3+,Na+,Mn2+ phosphors.18 Joos et al. combined an experimental−theoretical investigation of the insulating material CaZnOS and the luminescent material CZOSM,19 and a single-particle band diagram and a manyparticle multiplet energy-level scheme of the Mn defect were constructed and discussed in order to explain the various spectral features that are important for the functional behavior of the material. Considering applications in PL- or ML-based optical materials, the fabrication of inorganic phosphor-based composites can supply such a platform compared to that of traditional inorganic powder samples. However, the irregular and aggregated powder particles and hydrophilic surface properties will seriously affect their further application as Received: December 4, 2017

A

DOI: 10.1021/acs.inorgchem.7b03060 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry organic−inorganic luminescent composites. Herein, we first proposed a novel two-step method to acquire ideal phosphor particles with small and uniform morphologies, followed by an oleic acid (OA) coating technique, which was used to modify the surface properties of the phosphors. As far as we know, this two-step method is new and also meaningful, accompanied by improved emission intensity and uniform morphology simultaneously. As shown in Scheme 1, we have comparatively

product was separated by centrifugation, washed several times with absolute ethanol and deionized water, and then dried under vacuum at 80 °C for 4 h. Coating of Ca(OH)2 onto ZnS:Mn2+ Particles. In a typical synthesis, 2 mL of a 4 mol/L CaCl2 solution and 16 mmol of ZnS:Mn2+ particles were mixed in a vial, then 2 mL of a 8 M/L NaOH solution was added dropwise to the mixture under vigorous stirring. Then, the as-obtained turbid liquid containing Ca(OH)2-coated ZnS:Mn2+ particles, also marked as ZnS:Mn2+/Ca(OH)2, was treated by the filter membrane and further washed with ethanol. The product, Ca(OH)2-coated ZnS:Mn2+, was finally separated by centrifugation, washed several times with absolute ethanol and deionized water, and at last dried under vacuum at 80 °C for 4 h. Phase Formation of CZOSM-II Phosphors. Ca(OH)2-coated ZnS:Mn2+ particles were further ground thoroughly in an agate mortar for 15−20 min. Then, the mixture was transferred to a crucible and sintered at different temperatures (400, 600, 700, 800, 900, 1000, and 1100 °C) for 4 h in an argon atmosphere. Finally, we found suitable experimental conditions and obtained the final CZOSM-II phosphors via the two-step technique mentioned above. Modification of CZOSM Phosphors. CZOSM-II phosphors obtained by the two-step technique were further treated from the hydrophilic to the hydrophobic property via the method reported in ref 20. Specifically, OA (4.3 mL, 13.5 mmol) was first dissolved in 25 mL of anhydrous absolute ethanol, and then 1 g of CZOSM powder was added and dispersed in the above solution via ultrasonication for 1 h. This mixture was loaded into a 100 mL autoclave and heated at 140 °C for 6 h. After the reaction, the mixture was allowed to cool and centrifuged at 7000 rpm for 10 min. The product was washed repeatedly with ethanol to remove excess OA and dried at 70 °C for 4 h to produce OA-passivated CZOSM (CZOSM@OA) powder. Fabrication of a Luminescent PDMS Film Containing a CZOSM Phosphor. A luminescence PDMS film containing a CZOSM phosphor was fabricated by a method similar to that in our previous work.21 In a typical synthesis, silanol-terminated PDMS (0.18 g, DMS-S21, Gelest) was dissolved in anhydrous tetrahydrofuran (0.02 g) in a 250 mL round-bottomed flask. Different ammounts of CZOSM@OA (0.02, 0.04, 0.08, and 0.16 g) were added into the flask, and the mixture was stirred with a magnetic stir bar. The solution was evacuated for 0.5 h and then dried at 80 °C overnight to form the final luminescent film for the measurement. Characterization. X-ray diffraction (XRD) data were collected using a Rigaku TTR III diffractometer (Cu Kα radiation) at 40 kV and 200 mA. A continuous scan mode was employed with a step width of 0.02° in 2θ. PL and photoluminescence excitation (PLE) spectra were obtained using a fluorescence spectrophotometer (F-4600, Hitachi, Japan) equipped with a photomultiplier tube operating at 400 V, with a 150 W xenon lamp used as the excitation source. The morphologies and energy-dispersive spectroscopy (EDS) elemental mapping of the particles were observed by using scanning electron microscopy (SEM; JEOLJSM-6510A). Fourier transform infrared (FT-IR) spectra were collected on a FTIT-AVATAR370 spectrophotometer over the wavenumber range of 4000−400 cm−1, and a standard KBr pellet technique was employed. Contact-angle tests were conducted on a JC200D contact-angle measuring device. The sessile drop method was used for the wettability measurement to determine the hydrophilic or hydrophobic characteristic. A video camera equipped with a homemade image analysis device allowed determination, from the shape of the droplet, of the contact angle between a given liquid and the surface of the sample. The reported contact angle was an average value from 5 droplets for each treatment. The measurement was performed with distilled water.

Scheme 1. Synthesis Procedure Illustration of CZOSM Powders with Different Particle Sizes through the Nominal One-Step (CZOSM-I) and Two-Step (CZOSM-II) Methods

demonstrated the traditional one-step solid-state reaction method for the phosphor powders (CZOSM-I) and the proposed two-step strategy in this paper. In such a two-step method, we first prepared the ZnS:Mn2+ particles with small and uniform size by a hydrothermal reaction and then the shell of Ca(OH)2 was coated onto the surface of the ZnS:Mn2+ particles via a precipitation reaction at room temperature. Then, these mixtures were sintered at an optimum 800 °C in an argon atmosphere to form the CaZnOS phase (CZOSM-II). Finally, through the treatment of OA, the final products CZOSM@OA became hydrophobic. Furthermore, hydrophobic CZOSM@OA were fitted well with poly(dimethylsiloxane) (PDMS) as a transparent flexible material and their optical performance and flexibility investigated.

2. EXPERIMENTAL SECTION All reagents mentioned below were of analytical grade and were used without any further purification. Synthesis of CZOSM-I Phosphors by a One-Step Solid-State Reaction. The detailed synthesis procedure is given in Scheme 1, and this one-step solid-state reaction contained mixing of the starting materials of CaCO3, ZnS, MnCO3, and Li2CO3 used as a flux with 5 wt %. The ground mixtures were transferred to a crucible and sintered at the optimum synthesis temperature, 1000 °C, for 4 h in an argon atmosphere, as reported in ref 18. Synthesis of CZOSM-II Phosphors by a Two-Step Method. The detailed synthesis procedure is also given in Scheme 1 and includes the synthesis of ZnS:Mn2+ particles and the coating of Ca(OH)2 on the surface of ZnS:Mn2+ particles in the first step and the phase formation of CZOSM-II phosphors via the sintering reaction in the second step. The experimental details are given below. Preparation of ZnS:Mn2+ Particles. ZnS:Mn2+ particles were prepared by a hydrothermal method, and a typical synthesis procedure is as follows: 3 mmol of thiourea dispersed in 10 mL of deionized distilled water was added to a solution of 3 mmol of ZnCl2 and 0.9 mmol of MnCl2 in 20 mL of deionized distilled water under stirring for 30 min. Then, 3 mmol of NaOH in 10 mL of deionized distilled water was added to the above mixture under vigorous stirring for another 30 min. The mixture was transferred into an autoclave, sealed, and kept at 200 °C for 24 h. After cooling to room temperature naturally, the final

3. RESULTS AND DISCUSSION Powder XRD patterns of ZnS:Mn 2+ , Ca(OH) 2 -coated ZnS:Mn2+, as-prepared CZOSM-II via a two-step method, and modified CZOSM samples are shown in Figure 1a. The standard data for ZnS, Ca(OH)2, and CaZnOS are also listed as a comparison. It is clearly seen that the XRD patterns of B

DOI: 10.1021/acs.inorgchem.7b03060 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (a) PLE (left) and PL (right) spectra of CZOSM samples prepared at 700, 800, 900, and 1000 °C from Ca(OH)2-coated ZnS:Mn2+ particles. (b) PLE (left) and PL (right) spectra of CZOSM samples prepared by the two-step method and conventional one-step solid state reaction method.

Figure 1. (a) XRD patterns of the as-prepared ZnS:Mn2+, ZnS:Mn2+/ Ca(OH)2, CZOSM-II obtained at 800 °C, and CZOSM-II after OA modification. (b) XRD patterns of the as-prepared ZnS:Mn2+/ Ca(OH)2 samples sintered at 400, 600, 800, 1000, and 1100 °C. The standard data for ZnS (PDF 990097), Ca(OH)2 (PDF 780315), and CaZnOS (ISCD 245309) are shown for reference.

particles. The PLE spectra of the CZOSM sample, shown in Figure 2a, consist of several peaks centering at 336 nm and some weak peaks around 370 and 430 nm, corresponding to the electronic transitions of Mn2+ from ground-level 6A1 (6S) to the 4T1(4P), 4T2 (4D), and [4A1(4G), 4E(4G)] levels.22 Besides, it can be obviously observed that the sample synthesized at 800 °C has the optimal emission intensity among these different sintering temperatures, which matches well with the results of the XRD patterns. Thus, the best sintering temperature could be determined as 800 °C in the following study. In order to clearly compare the differences in the luminescence properties of CZOSM prepared by the two different methods, the PL spectra of this series of samples with the same chemical compositions achieved by the one-step and two-step synthetic methods are comparatively displayed in Figure 2b. It can be observed that the emission intensities of the samples synthesized through the two-step method are obviously enhanced compared with those obtained by the onestep method, which should be related with the morphologies and modified surface properties of the as-obtained samples by different methods as discussed below. In general, the particle sizes and morphologies of the phosphors should be the main reason for the difference of the luminescence properties.23 Therefore, to verify whether the results of the PL spectra are credible, we have performed SEM measurement to check the difference, as shown in Figure 3. From these SEM images, we can observe that the nearly spherelike ZnS:Mn2+ particles with a uniform diameter of 1 μm can be obtained (Figure 3a). When coated with Ca(OH)2, the surfaces of the as-prepared particles become ambiguous, and some aggregation particles appears (the inset of Figure 3a). After the Ca(OH)2-coated ZnS:Mn2+ particles are sintered at different temperatures, it can be seen that the sample prepared at 700 °C has some irregular microparticles (Figure 3b), while

ZnS:Mn2+ match well with the standard data of ZnS (PDF 990097). When there is a Ca(OH)2 coating, the crystalline phase of the as-obtained sample is composed of ZnS and Ca(OH)2 (PDF 780315) patterns, indicating the coexistence of the two phases. After calcination in an argon atmosphere, wellformed polycrystalline CaZnOS can be synthesized, which matches well with the standard file of CaZnOS (ICSD 245309). Moreover, after surface modification with OA, the coated OA does not generate any impurities or induce significant changes in the host structure, apart from the intensity of two peaks around 32°, indexed as (004) and (012), which is due to the preferred orientation effect. Moreover, for the XRD patterns of CZOSM samples prepared at different selected temperatures (Figure 1b), ZnS, ZnO, and CaO act as the main phases when the Ca(OH)2-coated ZnS:Mn2+ particles are heated at 400 °C. As the temperature rises to 600 °C, the main phase becomes CaZnOS, with ZnO and CaO still remaining. When the temperature increases to 800 °C, all of the characteristic XRD peaks of the as-obtained CZOSM match quite well with the literature values and can therefore be ascribed to the pure polycrystalline phase CaZnOS. When the temperature rises higher to 1000 and 1100 °C, the peak around 32°, indexed as (004), is extremely strong because of the preferred orientation effect and irregular particles. The as-prepared CZOSM via the two-step method exhibits a typical red emission band peaking at 580 nm, which is consistent with our previous results.18 The red emission from Mn2+ is due to the admixture of parity between the 3d and 4p configurations lifting the spin selection rule and also possible electron−phonon coupling. Figure 2a demonstrates the PLE and PL spectra of CZOSM samples prepared at 700, 800, 900, and 1000 °C by sintering of the Ca(OH)2-coated ZnS:Mn2+ C

DOI: 10.1021/acs.inorgchem.7b03060 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. (a) Comparison of the FT-IR spectra of CZOSM and CZOSM@OA. Contact angles of water droplets of (b) CZOSM and (c) CZOSM@OA.

CZOSM@OA. It is worth noting that the strength of the peak from 3750 to 3100 cm−1 corresponding to −OH of OA decreases; however, the strength of the peaks at 2920 and 2850 cm−1 corresponding to the asymmetric −CH2− and symmetric −CH2− stretching of OA, respectively, and the peak at 1590 cm−1 corresponding to −COO− increase, confirming the presence and bonding of OA on the phosphor particles’ surface.25 These results are related to the loss of −OH of OA after modification. So, we believe that OH− in OA combines with one H+ on the surface of the CZOSM particle, forming H2O molecules, and then falls off from the compound, leaving the rest of the long-chain alkanes of OA to chemically adsorbed on the surface of CZOSM. Contact-angle measurements were also carried out to further characterize the effect of coverage of OA onto the surface of CZOSM, as shown in Figure 5b,c. The contact angle of CZOSM is 36.5°, while the contact angle of the modified CZOSM@OA is 100°, which shows that hydrophilic CZOSM changed to hydrophobic CZOSM by modification. In this way, the contact angle of 36.5° can also indicate that the surface of CZOSM could be gathered with −OH. Furthermore, the spectral profiles of CZOSM before and after modification are shown in Figure 6a. CZOSM@OA

Figure 3. SEM images of (a) ZnS:Mn2+, (inset) the ZnS:Mn2+/ Ca(OH)2 precursor, selected CZOSM-II samples prepared at (b) 700, (c) 800, (d) 900, and (e) 1000 °C, and (f) the CZOSM-I sample prepared by a solid-state reaction.

serious aggregation phases appear with big particle sizes when the sintering temperature is increased to 900 °C (Figure 3d) and 1000 °C (Figure 3e). As a comparison, we can find that the sample sintered at 800 °C has good crystalline properties and uniform particle sizes (Figure 3c). However, some aggregation particles are formed when we prepare the samples by the onestep solid-state reaction at the selected optimum phase formation temperature (1000 °C), as shown by the SEM image in Figure 3f. EDS elemental mapping, as shown in Figure 4, indicates that the Ca, Zn, O, S, and Mn atoms are

Figure 4. SEM image of the typical particle (a) and related SEM elemental mapping images of Ca (b), Zn (c), O (d), S (e), and Mn (f) for the CZOSM sample via the two-step method.

homogeneously distributed throughout the whole selected CZOSM-II sample prepared by the two-step method. As is known, the luminescence intensities of the phosphors are related to the crystallinities of the samples when the same concentration and distribution of a dopant are maintained among different samples; therefore, the better the crystallinity, the higher the luminescence intensity.24 The above results reveal that our proposed two-step synthetic method by control of the morphology of the precursor in the first step, followed by sintering in the second step, is a facile and efficient method, with good luminescence properties and uniform morphologies. FT-IR spectra were carried out to characterize whether OA is chemisorbed as a carboxylate onto the CZOSM samples. Figure 5a comparatively demonstrates the IR spectra of CZOSM and

Figure 6. (a) PLE (left) and PL (right) spectra of CZOSM and CZOSM@OA samples. (b) Typical SEM image of the CZOSM@OA sample.

samples show the same emission and excitation peak positions as those of pristine CZOSM, revealing that OA acting as a hydrophobic passivating layer does not alter the optical properties of the CZOSM phosphor. Anyway, the emission intensities decrease a little, which does not affect the application of the CZOSM@OA samples. Microscopically, Figure 6b D

DOI: 10.1021/acs.inorgchem.7b03060 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

4. CONCLUSION In summary, we successfully synthesized a CZOSM phosphor with both small size and good luminescent properties by a twostep method. Briefly, ZnS:Mn2+/Ca(OH)2 core/shell particles were synthesized first, and then the powders were heated at 800 °C in an argon atmosphere to obtain the CZOSM phosphor in the second step. Subsequently, OA was used to change the hydrophilic CZOSM to a hydrophobic one during the hydrothermal process. Consequently, CZOSM@OA particles are qualified with excellent luminescent properties, exhibiting a typical red emission band peaking at 580 nm and suitable morphology and hydrophobicity for further applications. Finally, the PDMS/CZOSM@OA film was fabricated with good flexibility and transparency. Additionally, the concentration of CZOSM@OA in PDMS also controlled the luminescent intensity and film transparency. It is believed that this two-step method for the preparation of CZOSM is worthwhile for promoting control of the morphology and luminescence properties of the phosphor materials, which benefit the fabrication of organic luminescence composites.

shows SEM images of the CZOSM@OA samples. Compared to the image in Figure 3c, we can clearly observe that CZOSM powders gathered after modification and the surface is very smooth, suggesting that OA is chemisorbed as a carboxylate onto the CZOSM particle surface. Following surface modification from the hydrophilic powder to the hydrophobic one, we applied the as-obtained CZOSM@ OA samples with PDMS to fabricate a kind of flexible luminescent film, as seen in Figure 7. Without modification,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.X.). ORCID

Zhiguo Xia: 0000-0002-9670-3223 Quanlin Liu: 0000-0003-3533-7140 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was supported by the National Natural Science Foundation of China (Grants 51722202, 91622125, and 51572023) and Natural Science Foundations of Beijing (Grant 2172036).

Figure 7. (a) Photographic images of the as-fabricated PDMS/ CZOSM film (i) and PDMS/CZOSM@OA samples, with different mass ratios of PDMS and CZOSM@OA, where parts ii−v represent the mass ratios of 10:1, 10:2, 10:4, and 10:8, respectively. Photographic images of PDMA/CZOSM@OA samples without (b) and with (c) stretching.

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REFERENCES

(1) Wang, W.; Peng, D.; Zhang, H.; Yang, X.; Pan, C. Mechanically induced strong red emission in samarium ions doped piezoelectric semiconductor CaZnOS for dynamic pressure sensing and imaging. Opt. Commun. 2017, 395, 24. (2) Petrova, S.; Mar’evich, V.; Zakharov, R.; Selivanov, E.; Chumarev, V.; Udoeva, Y. L. Crystal Structure of Zinc Calcium Oxysulfide. Dokl. Chem. 2003, 393, 255. (3) Budde, B.; Luo, H.; Dorenbos, P.; van der Kolk, E. Luminescent properties and energy level structure of CaZnOS: Eu2+. Opt. Mater. 2017, 69, 378. (4) Qiu, Z.; Rong, C.; Zhou, W.; Zhang, J.; Li, C.; Yu, L.; Liu, S.; Lian, S. A Strategy for Synthesizing CaZnOS:Eu2+ Phosphor and Comparison of Optical Properties with CaS:Eu2+. J. Alloys Compd. 2014, 583, 335. (5) Kuo, T. W.; Liu, W. R.; Chen, T. M. High color rendering white light-emitting-diode illuminator using the red-emitting Eu2+-activated CaZnOS phosphors excited by blue LED. Opt. Express 2010, 18, 8187. (6) Zhang, Z. J.; Feng, A.; Sun, X. Y.; Guo, K.; Man, Z. Y.; Zhao, J. T. Preparation, electronic structure and luminescence properties of Ce3+activated CaZnOS under UV and X-ray excitation. J. Alloys Compd. 2014, 592, 73. (7) Zhang, H.; Peng, D.; Wang, W.; Dong, L.; Pan, C. Mechanically induced light emission and infrared-laser-induced upconversion in the Er-doped CaZnOS multifunctional piezoelectric semiconductor for optical pressure and temperature sensing. J. Phys. Chem. C 2015, 119, 28136.

CZOSM particles tend to gather together so easily that we can still see the luminescent CZOSM particles with the naked eye under a 254 nm UV lamp (Figure 7a−i). In contrast, modified CZOSM@OA has a uniform distribution and a good luminescence performance when dispersed in PDMS. The asprepared PDMS film containing CZOSM@OA samples shows a strong red emission under irradiation of UV light [Figure 7a(ii)−(v)]. In the meantime, it is obviously found that the distribution of modified CZOSM particles is very uniform, indicating that OA can greatly improve the dispersion and lipophilicity of CZOSM. We can obviously observe that the intensity of the red light decreases when the concentration of CZOSM@OA increases, as shown in Figure 7(ii)−(v), because the transparency of the film decreases with more phosphor particles. The film is hardly transparent when the mass ratio of CZOSM@OA and PDMA is up to 8:10. A comparison on the flexibility of the film can be seen in Figure 7b,c. Before stretching, the length of the film is about 3 cm. The length becomes 2 × 6 cm after stretching, demonstrating good properties of flexibility. E

DOI: 10.1021/acs.inorgchem.7b03060 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b03060 Inorg. Chem. XXXX, XXX, XXX−XXX