A single-step direct hydrothermal synthesis of SrTiO3

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A single-step direct hydrothermal synthesis of SrTiO3 nanoparticles from crystalline P25 TiO2 powders Article  in  Journal of Materials Science · January 2016 DOI: 10.1007/s10853-015-9445-7

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A single-step direct hydrothermal synthesis of SrTiO3 nanoparticles from crystalline P25 TiO2 powders Yabing Zhang, Li Zhong & Dongping Duan

Journal of Materials Science Full Set - Includes `Journal of Materials Science Letters' ISSN 0022-2461 J Mater Sci DOI 10.1007/s10853-015-9445-7

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Author's personal copy J Mater Sci DOI 10.1007/s10853-015-9445-7

A single-step direct hydrothermal synthesis of SrTiO3 nanoparticles from crystalline P25 TiO2 powders Yabing Zhang1,2 • Li Zhong1 • Dongping Duan1,3

Received: 15 June 2015 / Accepted: 16 September 2015 Ó Springer Science+Business Media New York 2015

Abstract In the study, strontium titanium (SrTiO3) nanoparticles were successfully synthesized by a singlestep direct hydrothermal process under alkaline condition from crystalline P25 titanium dioxide (TiO2) powders and strontium hydroxide octahydrate (Sr(OH)28H2O) at 220°C. The samples obtained were characterized by X-ray diffraction (XRD), indicating that the products were highly crystalline cubic SrTiO3 nanoparticles. The lattice parameter, unit cell volume, and atomic position were refined by Highscore Plus and Maud program to determine the crystal structure parameters. The thermal field emission scanning electron microscope and energy-dispersive spectrometer (FE-SEM-EDS) showed the samples prepared were cubic SrTiO3 nanoparticles with regular morphology. The fine morphologies and structures of SrTiO3 were investigated by field emission high-resolution transmission electron microscope (HR-TEM). The specific surface areas of samples were investigated by the BET method. As a comparison, SrTiO3 nanoparticles also were synthesized by solid-state reaction. The samples synthesized by hydrothermal method have bigger specific surface areas

Electronic supplementary material The online version of this article (doi:10.1007/s10853-015-9445-7) contains supplementary material, which is available to authorized users. & Dongping Duan [email protected]; [email protected] 1

Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

2

University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

3

Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Qinghai 810008, People’s Republic of China

and smaller grain sizes than the sample synthesized by solid-state method. Big mole ratio Sr/Ti and short reaction time are helpful to produce small particles with large specific surface area. The reaction mechanism of the hydrothermal process was illustrated finally.

Introduction Perovskite-type mixed-metal oxide functional ceramic materials have been extensively researched because of their outstanding piezoelectricity, ferroelectricity, and photocatalysis properties [1–6]. Strontium titanium (SrTiO3), as an important perovskite functional ceramic material, was widely used in multilayer capacitors, solar cells, and photocatalysts [7–13]. Many methods have been reported to prepare SrTiO3 ceramic materials. Conventionally, SrTiO3 is synthesized using high-temperature solid-state reaction by employing strontium carbonate (SrCO3) or strontium oxide (SrO) to react with titanium dioxide (TiO2), which results in serious agglomeration, large particle size, considerable impurity, and irregular particle morphologies [14]. In recent years, various liquid phase methods to synthesize SrTiO3 perovskite ceramic materials have been reported to overcome these disadvantages including sol–gel [15–17], combustion method [18], solution-precipitation [19], microwave-assisted [20], solvothermal synthesis [21], ultrasonics sonochemistry [22], advanced solid state methods [23], and hydrothermal routes [9, 24]. For example, Ashiri et al. [25] successfully synthesized BaTiO3 nanoparticles by a modified, cost-effective sol–gel procedure, and the result indicated that the polymorphic transformation to tetragonal (ferroelectric characteristic) occurred at 900°C.

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Among these methods, hydrothermal synthesis is also widely used to prepare SrTiO3 perovskite ceramic materials. Hydrothermal synthesis has the advantages of producing small nanoparticles with uniform size, high crystallinity, and regular morphology. It can be easily performed to control particle size and morphology by adjusting the synthesis conditions, such as reaction time, temperature, pH value, and starting materials ratio. In addition, SrTiO3 can be prepared by hydrothermal process using many available precursors, which play an essential role in the nucleation and growth of SrTiO3 nanocrystals. Many literatures have been reported to synthesize SrTiO3 nanoparticles by employing some organic titanium precursors. For example, Chen et al. [26] prepared nanocrystalline SrTiO3 powders with a simple mode of size distribution, smaller particle size, and a lower agglomeration under a moderate condition derived from different precursors. The precursors were prepared by Sr(OH)2 nH2O and Ti(OC4H9)4 in 2-methoxyethanol media under N2 atmosphere. Jayabal et al. [8] synthesized cubic SrTiO3 nanoparticles for dye-sensitized solar cell application using Ti(CH3CH3CHO)4 and Sr(C2H3O2)2 as reactants with a facile hydrothermal synthesis. Xu et al. [27] prepared SrTiO3 nanoparticles with different particle sizes with the help of polyvinyl alcohol using Ti(OC4H9)4 as titanium precursors. Huihui Li et al. [28] synthesized FeO3-Sensitized SrTiO3 powders with small particles of about 50 nm by microwave-assisted hydrothermal reaction using Ti(CH3CH3CHO)4 and SrCl26H2O as raw materials. However, there are few literatures reported to directly synthesize SrTiO3 using inorganic titanium precursors, such as metatitanic acid, titanium tetrachloride, and crystalline titanium dioxide. Inorganic titanium precursors are inexpensive, stable, and easy to handle compared to organic titanium sources. Zhang et al. [29] prepared SrTiO3 nanoparticles using TiCl4 and SrCl26H2O as the starting materials and studied formation mechanism of SrTiO3 nanoparticles under hydrothermal conditions. It is found that the mechanisms conformed to dissolution–crystallization–aggregative growth–recrystallization mechanism model, which was seriously affected by TiO2nH2O gel produced from TiCl4. Ashiri et al. successfully synthesized pure carbonate-free SrTiO3 nanocrystals using titanium chloride and strontium chloride as the starting materials at lower temperature (about 50°C) and short reaction time (about 45 min) by ultrasound-assisted wet chemical processing method. This pathway obtained very fine SrTiO3 nanocrystals with size of 4–11 nm [30] and 7–17 nm [31], which provided a simple, fast, cost-effective synthesis route for SrTiO3 perovskite materials. Zhang et al. [32] firstly formed TiO2 nanotube arrays on Ti substrate by anodization. After that, TiO2 nanotube arrays were converted into TiO2–SrTiO3 heterostructures by controlled

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substitution of Sr under hydrothermal conditions. The TiO2–SrTiO3 composite heterostructures obtained with 1 h or less hydrothermal treatment exhibit the best photoelectrochemical performance with nearly 100 % increase in external quantum efficiency at 360 nm. Peng et al. [33] synthesized nanocrystals by a hydrothermal reaction route using anatase TiO2 nanosheets with dominant {001} facets and commercial anatase TiO2 nanoparticles as precursors. The anatase TiO2 nanosheets with dominant {001} facets were synthesized by a hydrothermal route using tetrabutyl titanate [Ti(OC4H9)4] source and hydrofluoric acid (HF) solution as solvent. The research indicated that the SrTiO3 products obtained from TiO2 with {001} facets showed large specific surface area, enhanced photocurrent density, and photocatalytic hydrogen production activity. In this study, we report a single-step direct hydrothermal synthesis of SrTiO3 nanoparticles by employing crystalline P25 titanium dioxide (TiO2) as the titanium precursor under alkaline condition without calcination. The strontium hydroxide octahydrate (Sr(OH)28H2O) was chosen as the strontium source. Sodium hydroxide (NaOH) was served as the alkaline medium. The reaction duration and mole ratio of raw materials were investigated to study their influences on the characteristics of the samples and the reaction mechanism of this hydrothermal process.

Experiment procedure Materials Titanium dioxide (P25 TiO2, 99.8 %, Degussa) and Strontium hydroxide octahydrate (Sr(OH)28H2O, 97 %, Tianjin Guangfu Institute of Fine Chemical, China) powders were used as the titanium and strontium sources, respectively. P25 TiO2 is a mixed oxide made by EVONIK-DEGUSSA using AEROSIL technology, which consist of 80 % anatase and 20 % rutile. The average grain size is about 20 nm. Sodium hydroxide (NaOH, 98 %, Xilong Chemical Co., Ltd., Swatow, Guangdong, China) was used as the alkaline medium and mineralizer. Deionized water (resistivity 18.2 MX cm) was prepared by Ulupure water purification system. Teflon-lined stainless steel autoclave was supplied by Yantai Songling Chemical Equipment Co., Ltd. Sample preparation A typical hydrothermal preparation route of SrTiO3 nanoparticles is shown Fig. 1a. An amount of P25 TiO2 powders were mixed with a determined quality of Sr(OH)28H2O powders. The mole ratio of Sr and Ti (Sr/ Ti) was set as 1 or 2 by changing the amount of TiO2.

Author's personal copy J Mater Sci Fig. 1 Flow chart for synthesizing SrTiO3 nanoparticles: a hydrothermal process, b solid-state reaction process

Maintaining agitation, 50 mL 5 mol L-1 NaOH solution was added to the mixture. Then, the suspension obtained was transferred to a 100-mL Teflon-lined stainless steel autoclave and heated to 220 °C for 20 h, 40 h, 60 h, and 120 h respectively. After finishing the hydrothermal reaction, the reactor was cooled to about 80 °C. The contents were taken out and washed with hot water several times to remove excess alkali. Subsequently, the slurry was stirred and applied ultrasonic dispersion with dilute hydrochloric acid for 10 min to eliminate unreacted hydroxides. The residual slurry was centrifuged using a TG16-WS centrifuge at 9000 rpm for 10 min and washed repeatedly by hot deionized water until the pH value of the centrifugation liquid approximated to 7. The solid product was dried in an oven for 12 h at 80 °C. Finally, the product obtained was collected and characterized. Depending on different Sr/Ti and reaction durations, the samples were denoted as H1-20, H2-20, H1-40, and H2-40, H1-60, H2-60, H1-120, and H2120 respectively. For example, H1-20 represented the sample synthesized with mole ratio of 1 and reaction duration of 20 h by hydrothermal process. As a comparison, SrTiO3 was also synthesized by a traditional hightemperature solid-state reaction of TiO2 and SrCO3 at 1000 °C for 4 h [34], as shown in Fig. 1b. Moreover, the sample prepared from the solid-state reaction was denoted as S1-4. Sample characterization The crystal structures of the powders obtained were analyzed by X-ray diffraction (XRD) using a diffractometer (X’Pert PRO MPD, PANalytical B.V., the Netherlands) with CuKa (k = 0.15406 nm) operating at 40 kV and

40 mA at room temperature to confirm the formation of the crystalline SrTiO3 cubic phase. The 2h value was typically ranged from 20° to 80°, with step sizes of 0.05° and 1 s per step. The patterns recorded were further analyzed using the Highscore Plus software and Maud program. The morphology and microstructure of the synthesized samples were characterized by thermal field emission scanning electron microscope and energy-dispersive spectrometer (SEM-EDS, 30 kV-137 eV, JSM7001F ? INCA, X-MAX, Japan, UK) and high-resolution transmission electron microscope (TEM, 200 kV, Tecnal G2 20 S-TWIN, US). In addition, the products were also examined by Brunauer, Emmett, and Teller (BET) analysis (Autosorb-1, Quantachrome, US) to investigate the specific surface area.

Results and discussion X-ray diffraction analysis To identify the purity and crystalline structure, the X-ray diffraction patterns of the samples obtained at different reaction times and Sr/Ti are shown in Fig. 2. No diffraction peaks of rutile and anatase phase of TiO2 or other impurities were observed from the XRD patterns. Enough purity is very important for the application of SrTiO3 to functional materials field and photocatalysis. All diffraction peaks for these samples were well matched with the standard pattern of cubic SrTiO3 (JCPDS No. 350734), indicating that the samples obtained were pure-phase SrTiO3 without any contaminants and the formation of carbonate. The peaks can be characterized to (100), (110), (111),

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Author's personal copy J Mater Sci Fig. 2 XRD patterns of SrTiO3 nanoparticles synthesized at different reaction durations and Sr/Ti mole ratio: (a) 20 h, (b) 40 h, (c) 60 h, (d) 120 h. a Sr/Ti = 1, b Sr/Ti = 2

(200), (210), (211), (220), (300), and (310) crystal plane respectively. The highest intensity peak corresponds to the (110) crystal plane at 2h = 32.42°. Compared to previous research of hydrothermal synthesis of strontium titanate powders [35], these sharp and narrow peaks implied that the nanoparticles were highly crystalline. The average sizes of the SrTiO3 grains were estimated using the Scherrer equation (Eq. (1)).

where D is the grain size of nanocrystals, k is the X-ray wavelength of diffractometer CuKa (0.15406 nm), B is the full width at half maximum (FWHM) intensity in radians, and h is the diffraction peak angle. The average crystalline sizes of samples estimated are displayed in Fig. 3. The results are well consistent with observation by FE-SEM. Fuentes et al. [36] synthesized SrTiO3 nanomaterial by a sol–gel-hydrothermal method. The average particle sizes calculated by Scherrer equation were between 46 and 54 nm, which are similar with our research.

The experimental, calculated, and difference plots of the Maud Rietveld refinement of the powder XRD patterns of samples are shown in Fig. S1. The differences between experimental and calculated values are very small, showing the refinement results are reliable. The lattice parameter and unit cell volume of SrTiO3 nanoparticles are refined and are displayed in Fig. 4, which are matched well with standard SrTiO3 crystal structure. However, as the increase of reaction time, the lattice parameter decreases and is ˚ of more and more close to the standard value of 3.905 A a. In addition, the lattice parameter of samples prepared with Sr/Ti of 2 is slightly larger than that of samples prepared with Sr/Ti of 2. It can be easily understood that the crystal structure slowly becomes more stable with the increase of reaction time. When the hydrothermal reaction starts, rapid nucleation results in bigger lattice distortion for Sr/Ti of 2 because vast Sr2? exists in solution compared to Sr/Ti of 1. Unit cell volume also has same effect. Table 1 presents the atomic position, occupation factor, and Biso value of samples. These refinement results are

Fig. 3 Crystalline sizes of SrTiO3 nanoparticles estimated with XRD and SEM

Fig. 4 Lattice parameter and unit cell volume of samples

D ¼ ð0:9kÞ=ðB cos hÞ

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ð1Þ

Author's personal copy J Mater Sci Table 1 Atomic position of elements of samples Samples

Atom

x

y

z

Occupancy

H1-20

Sr

0

0

0

1

Ti

0.5

0.5

0.5

1

0.055

O

0

0.5

0.5

1

0.711

Sr

0

0

0

1

0.186

Ti

0.5

0.5

0.5

1

-0.734

O

0

0.5

0.5

1

-0.754

Sr

0

0

0

1

0.725

Ti

0.5

0.5

0.5

1

-0.135

O

0

0.5

0.5

1

0.492

Sr

0

0

0

1

0.686

Ti O

0.5 0

0.5 0.5

0.5 0.5

1 1

-0.019 0.462

Sr

0

0

0

1

0.714

Ti

0.5

0.5

0.5

1

-0.134

H2-20

H1-40

H2-40

H1-60

H2-60

H1-120

H2-120

Biso 0.806

O

0

0.5

0.5

1

0.494

Sr

0

0

0

1

0.526

Ti

0.5

0.5

0.5

1

0.522

O

0

0.5

0.5

1

0.528

Sr

0

0

0

1

0.805

Ti

0.5

0.5

0.5

1

0.040

O

0

0.5

0.5

1

0.732

Sr

0

0

0

1

0.650

Ti

0.5

0.5

0.5

1

-0.011

O

0

0.5

0.5

1

0.539

well consistent with standard SrTiO3 crystal data (COD reference number 7212245). Fig. S2a shows the XRD pattern of SrTiO3 synthesized from solid-state reaction at 1000 °C for 4 h. The diffraction peaks of TiO2 and SrCO3 were found, indicating that the product had impurity. In addition, these very narrow peaks also meant the large particle size according to Scherrer equation. The morphologies analysis of samples The EDS spectrum of H1-20 is presented in Fig. S3, revealing the presence of Sr, Ti, and O. The ratio of atom is basically matched with the stoichiometry, which also shows the high purity of the sample. The sample layer is so thin that X-ray can easily penetrate and cause strong O, Sr, and Ti peaks. Figure S2b displays the SEM micrograph of SrTiO3 nanoparticles prepared by solid-state reaction. Large particles of about 100 nm are observed, and the edges of these particles are relatively smooth compared to the SrTiO3 nanoparticles synthesized by hydrothermal process.

Figure 5 shows the SEM micrographs of samples synthesized by hydrothermal process. According to the SEM micrographs, SrTiO3 nanoparticles with uniform size and regular cubic morphology were successfully synthesized by a simple single-step direct hydrothermal process. Particles with sphere or other morphologies were not found [37]. When the Sr/Ti was arranged from 1 to 2, the grain size decreased. In addition, prolonging the reaction duration from 20 to 120 h, the grain size increased. Figure 6 displays the size distribution of the SrTiO3 nanoparticles corresponding to Fig. 5. 100 particles were selected from each SEM photo to make size distribution statistics. An average crystalline size ranging from 33 to 49 nm is shown in Fig. 3, which is well consistent with the XRD results. A possible explanation is that there is vast amount of Sr2? in solution for Sr/Ti = 2 compared to Sr/Ti = 1. When the hydrothermal reaction begins, quick nucleation improves the combined opportunities of Sr2? to [Ti(OH)6]2- and produces many small particles in very short periods. As the time extends, these small particles slowly grow up and become large particles. The long reaction time is helpful to produce large particle. However, when the reaction duration was set up in 120 h, the difference between Sr/Ti = 1 and Sr/Ti = 2 was negligible because of the termination of reaction. Figure 7 shows the TEM graphs of H1-20. SrTiO3 cubes are clearly shown in Fig. 7a. Figure 7b displays the highresolution TEM and FFT micrographs of H1-20. The clear lattice stripes with spacing of 0.385 nm can be observed, which corresponds to the (100) crystal plane according to XRD standard data (JCPDS No. 350734). Figure 8 displays the TEM photos of H1-40 and H2-40. Figure 9 shows the different magnification TEM micrographs of H1-60. From these graphs, regular cubic crystals can be clearly found. Figure 10 illustrates the TEM micrographs of H1-120. Regular cubic crystals can be clearly seen in Fig. 10a. High-resolution TEM photo of H1-120 is shown in Fig. 10b. The clear lattice fringes with spacing of 0.195 nm can be observed. It has a good agreement with XRD analysis to determine (200) crystal plane. The selected area electron diffraction (SAED) pattern shows a set of bright halos in Fig. 10c, which indicates that the nanoparticles synthesized are polycrystalline material. The luminous spots show good crystallinity of the sample. The brightest four circles can be standardized as (110), (111), (200), (211), (220), and (310) crystal plane respectively by standard XRD data (JCPDS No. 350734). The (110) planes of perovskite materials have the lowest surface energy and planar growth rate [38]. The particle sizes observed from TEM micrographs are well matched with XRD and SEM results. It is confirmed that cubic SrTiO3 nanoparticles were successfully synthesized.

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Fig. 5 SEM micrographs of SrTiO3 nanoparticles: a H1-20, b H2-20, c H1-40, d H2-40, e H1-60, f H2-60, g H1-120, and h H2-120

The specific surface areas The specific surface areas of SrTiO3 nanoparticles synthesized were investigated by the BET method. Figure S4

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displays the specific surface area of samples. As the increase of mole ratio of Sr and Ti from 1 to 2, the specific surface area had an increased trend. However, it decreased with prolonging reaction duration. This is consistent with

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Fig. 6 Size distribution of SrTiO3 nanoparticles synthesized at different times: a 20 h, b 40 h, c 60 h, and d 120 h

Fig. 7 TEM micrographs of H1-20: a 100 nm, b HR-TEM and FFT

the interpretation of particle size. It is well known that small particle has large specific surface area. The specific surface area of sample synthesized by solid-state reaction is 3.109 m2 g-1, which is almost ten times smaller than that of sample synthesized by the hydrothermal process. The large specific surface area nanoparticles will have promising applications in photocatalysts, capacitors, and solar cells realm. The mechanism of hydrothermal process For this hydrothermal process, dissolution–precipitation mechanism [39] can be made a good explanation. Riman et al. [40] studied kinetics and mechanism of hydrothermal

synthesis of barium titanate. They considered that dissolution–precipitation mechanism may be the dominant mechanism throughout the barium titanate synthesis. Similarly, the synthesis of SrTiO3 also follows this mechanism. The total reaction equation of this hydrothermal synthesis is TiO2 þ SrðOHÞ2  8H2 O ¼ SrTiO3 þ 9H2 O

ð2Þ

The dissolution–precipitation mechanism mainly involves three steps. Figure 11 shows the diagrammatic sketch of hydrothermal synthesis mechanism. Crystalline TiO2 powders are so extremely stable that only strong alkalinity can break Ti–O bonds to form high activity [Ti(OH)6]2-, which can easily combine with Sr2? to

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Fig. 8 TEM micrographs of samples: a H1-40, b H2-40

Fig. 9 TEM micrographs of H1-60: a 200 nm, b 100 nm, and c 20 nm

generate SrTiO3 crystal nucleus. P25 TiO2 powders are not dissolved in the alkaline medium at room temperature until the suspension is heated to high temperature. As the TiO2 powders are gradually dissolved to soluble [Ti(OH)6]2-, SrTiO3 new phases are continuously formed by the precipitation reaction between Sr2? and [Ti(OH)6]2-. TiO2 dissolution, SrTiO3 crystalline grains formation, and growth simultaneously happen at this stage. When all of TiO2 are dissolved, the dissolution reaction will terminate. However, the crystal nucleation and growth will go on until the precipitation completes. The final stage is that SrTiO3 new nuclei produced grow up to become large grains and form nanoparticles with a uniform size and morphology. It is well known that P25 TiO2 is a mixture of anatase and rutile; however, the products obtained are uniform cubic nanoparticles and do not have anything with the starting material morphologies.

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It well confirms the correctness of the dissolution–precipitation mechanism.

Conclusions In the present research, perovskite phase cubic SrTiO3 nanoparticles with uniform sizes and regular morphologies were successfully synthesized by a simple single-step direct hydrothermal process using the crystalline P25 TiO2 powders as the titanium precursor under alkaline condition at 220 °C. Several factors were investigated to study their influences on the characteristics of the samples. The Sr/Ti and reaction duration directly influenced the size and specific surface area of particle. Small particles with large specific surface areas were easily produced under the large Sr/Ti and short reaction duration condition. However,

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Fig. 10 TEM micrographs of H1-120: a 100 nm, b HRTEM, and c SAED

Fig. 11 Diagrammatic sketch of hydrothermal synthesis mechanism

small Sr/Ti and long reaction duration were beneficial to generate large particles. The present study provides a simple single-step hydrothermal route to synthesize perovskite cubic SrTiO3 nanoparticles with a narrow size distribution, small particle size, good equity, high

crystallinity, and well-controlled morphology from crystalline P25 TiO2 powders. Acknowledgements The authors acknowledge the financial support of the National Natural Science Foundation of China (NSFC, No.

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Author's personal copy J Mater Sci 51402301) and the Qinghai Province Science and Technology Support Program (2015-GX-108A). 18.

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