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Controllable phase transition ITO nano powders and temperature-structure sensitivity
Chemical Physics Letters 742 (2020) 137174
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Controllable phase transition ITO nano powders and temperature-structure sensitivity Xiaoyu Zhaia, Yiqing Zhanga,b, Yujie Chena, Yunqian Maa, Jiaxiang Liua,
T
⁎
a
Beijing Key Laboratory of Electrochemical Process and Technology for Materials, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China b Mechanical and Electronic Engineering Department, Henan Polytechnic, Zhengzhou 450046, China
H I GH L IG H T S
flexibly controllable phase transition ITO NPs by hydrothermal method. • Preparing 150 nm single cubic phase and 20 nm single hexagonal phase ITO NPs. • Fabricating • Deeply revealing the phase transition relationship between precursors and oxides.
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrothermal method Phase transition Hexagonal indium tin oxide nano powders Fluorescence Temperature-structure sensitivity
Flexibly controllable phase transition indium tin oxide (ITO) nano powders (NPs) were prepared by an environmentally friendly hydrothermal method. The characterization results show that ITO NPs undergo a novel phase transition from cubic phase to hexagonal phase with hydrothermal temperature increasing. Single cubic and hexagonal phase ITO NPs with a diameter of 150 and 20 nm were respectively synthesized at 120 and 250 ℃ hydrothermal temperature combined with a followed 550 ℃ calcination process. The fluorescence emission intensity of hexagonal ITO (h-ITO) NPs was stronger than the one of cubic ITO (c-ITO) NPs due to more oxygen vacancies.
1. Introduction Transparent conductive indium tin oxide (ITO) is widely used in optoelectronic devices, such as liquid crystal flat panel display, thin film transistor and transparent electrode/substrate material [1–4], due to its high electrical conductivity, transparency to light and infrared reflectivity [5–7]. There are two different crystal structures of ITO, containing cubic phase and hexagonal phase. Cubic ITO (c-ITO) is a normal temperature and pressure phase, showing the same structure of Fe-Mn ore. However, hexagonal ITO (h-ITO) is a high temperature (> 800 ℃) and pressure (103-107 kPa) phase, showing the same structure of corundum. Due to the better anion layer filling structure in h-ITO, h-ITO shows some advantages compared to c-ITO, such as higher specific gravity, higher green density and more stable conductivity [8]. In addition, h-ITO shows many excellent properties such as high ethanol sensitivity [9] and high catalytic activity [10]. Owing to that the presence of water is bad for the preparation of hITO [11], most of the researches have been focused on synthesizing hITO by solvothermal method or water-solvent binary system, currently. ⁎
Gao et al. [11] have got InOOH in In(NO3)3/ethylene glycol/ethanol/ NaOH system and obtained h-In2O3 by calcining at 500 ℃ for 2 h based on solvothermal method. Tao et al. [12] have acquired InOOH in ethylene glycol/water/urea system and obtained h-In2O3 by calcining at 400 ℃ for 2 h based on solvothermal method. Moreover, Liu et al. [13] have prepared h-In2O3 in acetyl acetone indium/methanol/water/ ethanol system at 200 ℃ for 48 h by a hydrothermal method. However, the above methods have the same shortcomings due to the use of organic solvent or indium tin organic salt, such as strong toxicity, high cost and certain risk during the operation process. Thus, setting up an effective, low cost and environmentally friendly way, such as hydrothermal method without organic solvent and indium tin organic salt, to prepare h-ITO, is a prerequisite for developing high-performance optoelectronic components. Moreover, process variables will also have important influences on the morphology and properties of ITO powders [14]. In this paper, using metal In and SnCl4·5H2O as the raw materials, urea as precipitant, InOOH precursor was prepared by hydrothermal method and then h-ITO was obtained by calcining the precursor at
Corresponding author. E-mail address: [email protected] (J. Liu).
https://doi.org/10.1016/j.cplett.2020.137174 Received 20 November 2019; Received in revised form 17 January 2020; Accepted 1 February 2020 Available online 01 February 2020 0009-2614/ © 2020 Elsevier B.V. All rights reserved.
Chemical Physics Letters 742 (2020) 137174
X. Zhai, et al.
550 ℃ for 3 h. The influences from reaction temperature on the crystal structure, morphology and optical properties of ITO powders, prepared by hydrothermal growth technology, were studied systematically. Without any organic solvents, h-ITO nano powders (NPs) with good crystallinity were prepared. By tuning the hydrothermal temperature, the crystal phase transition can be easily controlled from cubic phase to hexagonal phase. This paper provides a new effective way, hydrothermal method, to prepare h-ITO NPs. Significantly, this method possesses several advantages, including low cost, non-toxic, environmentally friendly and simple preparation process. 2. Experimental 2.1. Reagents HNO3 (GR) was purchased from Beijing Tong Guang Fine Chemicals Company. Metal In (purity: 99.99%) was purchased from Liuzhou Smelting Company. Urea (GR) and SnCl4·5H2O (mass ratio: 99.99%) were purchased from Sinopharm Chemical Regent Beijing Co., Ltd. All reagents were used as received.
Fig. 1. XRD patterns of precursors prepared at different hydrothermal temperatures.
2.2. Preparation of ITO NPs 3 g indium was dissolved in concentrated nitric acid under vigorous stirring and heating at 60 ℃ to prepare In(NO3)3. In(NO3)3 solution was mixed with SnCl4·5H2O with the In2O3/SnO2 mass ratio of 9:1. Urea solution, prepared with a certain amount of urea (n (In3+): n (urea) = 1:5), was added into the above mixed solution. After being stirred for 0.5 h, the suspensions were transferred into teflon-lined stainless steel autoclaves and heated at different reaction temperatures (120, 160, 200, 240 and 250 ℃) for 6 h. The precipitates were washed by deionized water and absolute ethanol to remove chloride ions and nitrate ions and dried at room temperature for 12 h. The precursors were finally calcined at 550 ℃ for 3 h to prepare yellow ITO NPs.
Fig. 2. XRD patterns of ITO powders prepared at different hydrothermal temperatures with calcining at 550 ℃.
2.3. Characterization
different hydrothermal temperatures. The crystal structure of ITO powders transformed from cubic phase to hexagonal phase with the increase of reaction temperature, corresponding to the precursor’s transition from In(OH)3 to InOOH. When the hydrothermal temperature was 120 ℃, the product showed cubic In2O3 phase (JCPDS No. 712194), proving the successful synthesis of c-ITO. However, when the hydrothermal temperature was 250 ℃, the product showed hexagonal In2O3 phase (JCPDS No. 22-0336), meaning the successful synthesis of h-ITO. Moreover, when the hydrothermal temperature was between 120 and 250 ℃, the XRD pattern showed two phases of cubic In2O3 structure (JCPDS No.71-2194) and hexagonal In2O3 structure (JCPDS No.22-0336). Combining with the result shown in Fig. 1, it can be proved that the structure of precursor plays a key role in the structure of oxide. The corresponding relationship between indium-tin hydroxide and oxide is agreement with previous references [16,17]. In the hydrothermal reaction, a large number of OH– generated due to the decomposition of urea, making In3+ hydrolysis to form In(OH)3. With increasing reaction temperature, the obtained In(OH)3 nuclei eliminated water molecules from their core to form InOOH [18]. Cubic In2O3 could be obtained by thermal dehydration of In(OH)3, while hexagonal In2O3 was prepared by thermal dehydration of InOOH. The reactions during this process could be expressed as follows:
TG curves were recorded using a DTG-60A thermal analysis system with a heating rate of 10 ℃/min. Powder X-ray diffraction patterns (XRD) were recorded using a Rigaku D/Max-2400 diffractometer with Cu Kα radiation (1.5418 Å). Raman spectra were recorded using a Renishaw InVia confocal microprobe Raman system. Morphologies were recorded using a H-800 transmission electron microscopy (TEM). Solid-state fluorescence emission spectra were recorded on a Hitachi F7000 FL spectrophotometer. X-ray photoelectron spectroscopy (XPS) curves were recorded on a Thermo Fisher Scientific ESCALAB 250 system. 3. Results and discussion 3.1. Structural study on precursors and ITO NPs XRD pattern can help to analyze the structure of a crystal [15]. Fig. 1 shows the XRD patterns of precursors prepared at different hydrothermal temperatures. As shown in Fig. 1, the In(OH)3 precursors eventually transformed into InOOH with the increase of hydrothermal temperature. When the reaction temperature was 120 ℃, the products showed cubic In(OH)3 phase (JCPDS No. 76-1463). When the reaction temperature was 200 ℃, the products consisted of two crystal phases, orthorhombic InOOH phase (JCPDS No. 71-2283) and cubic In(OH)3 phase (JCPDS No. 76–1463). However, the XRD pattern was in good agreement with orthorhombic InOOH phase (JCPDS No. 71-2283) at 250 ℃, indicating the successful preparation of orthorhombic InOOH phase precursors. The precursors were calcined at 550 ℃ for 3 h to prepare ITO powders. Fig. 2 shows the XRD patterns of ITO powders prepared at 2
CO(NH2)2+3H2O → 2NH4++2OH–+CO2↑
(1)
In3++3OH– → In(OH)3
(2)
In(OH)3 → InOOH + H2O
(3)
2In(OH)3 → c-In2O3 + 3H2O
(4)
Chemical Physics Letters 742 (2020) 137174
X. Zhai, et al.
Γopt = 4Ag + 4Eg + 14Tg + 5Au + 5Eu + 16Tu
(6)
Ag, Eg and Tg represent the Raman active vibration modes. Au and Eu represent inactive modes. Tu represents infrared active vibration mode. Fig. 4 shows the Raman vibrational modes, located at 132, 306, 365, 495 and 627 cm−1, respectively, which are in good agreement with the ones of c-In2O3 [20]. This result proved that c-ITO could be obtained at 120 ℃ after calcining at 550 ℃. For h-ITO, the optical vibration modes are shown below [19]:
Γopt = 2A1g + 5Eg + 2A1u + 2A2u + 3A2g + 4Eu
A1u, A2u, A2g and Eu are Raman active or infrared active vibration modes. A1g and Eg are Raman active vibration modes. Fig. 4 shows the Raman vibrational modes, located at 163, 219, 269, 305, 384, 501 and 590 cm−1, respectively, which are in good agreement with the ones of h-In2O3 [21]. This also testified that h-ITO could be prepared at 250 ℃ after calcination at 550 ℃. However, ITO NPs prepared at 160 ℃ and 240 ℃ both show two kinds of vibrational modes. Significantly, the Raman spectra of h-ITO and c-ITO were similar to the ones of h-In2O3 and c-In2O3 due to the main contribution to the vibrational modes coming from the host lattice of h-In2O3 and c-In2O3. Novel phase transition between cubic phase and hexagonal phase controlled by hydrothermal temperature shows the excellent temperature-structure sensitivity, meaning a potential application of temperature-structure sensor.
Fig. 3. TG curves of the precursors prepared at different hydrothermal temperatures.
2InOOH → h-In2O3 + H2O
(7)
(5)
TG analysis was used to examine the conversion process of precursors during calcination. Fig. 3 shows the TG curves of precursors prepared at different hydrothermal temperatures. The total mass losses of three precursors were 19.07%, 9.4% and 8.11% (25 to 800 ℃) when the reaction temperatures were 120 ℃, 200 ℃ and 250 ℃, respectively. The TG curves could be divided into three stages. In the first stage, from 25 to 200 ℃, the mass loss was caused by the removal of adsorbed water and glycol in the precursors. The second stage is from 200 to 550 ℃ and the mass loss was caused by the thermal dehydration from precursor to oxide. The theoretical mass loss caused by the phase transition from In(OH)3 to c-ITO and InOOH to h-ITO were 16.28% and 6.09%, respectively. For the precursors obtained at 120 and 250 ℃, the mass losses caused by the phase transition from In(OH)3 to c-ITO and InOOH to h-ITO were 16.88% and 5.92%, respectively. The experimental mass losses were very close to the theoretical mass losses. For the precursor prepared at 200 ℃, the mass loss of 8.54% for the second stage was between the two theoretical values due to the precursor consisted of In(OH)3 and InOOH. With increasing hydrothermal temperature, the mass loss in the second stage decreased, indicating the increase of the amount of InOOH phase, which is in consistent with the XRD analysis. In the third stage from 550 to 800 ℃, TG curves tended to be horizontal lines, which meant that ITO phase fully formed when the calcination temperature was above 550 ℃. In order to study the effect of temperature on phase transition, Raman spectra were recorded. Fig. 4 shows the Raman spectra of ITO powders prepared at different hydrothermal temperatures. For c-In2O3, the optical vibration modes are shown below [19]:
3.2. Morphology of ITO NPs TEM images were recorded to give microstructures [22] of ITO NPs prepared at different hydrothermal temperatures. ITO NPs have been prepared and studied by some scholars. However, fabricating ITO submicron particles remains a challenge and has gained little attention [23]. Fig. 5 shows the TEM images of ITO powders prepared at different hydrothermal temperatures. When the reaction temperature was 120 ℃, the ITO NPs showed cubic shape with a particle size of 150 nm. Continuing to increase the hydrothermal temperature to 160 ℃, ITO NPs showed irregularly cuboid morphology. Moreover, when the reaction temperatures were 200, 240 and 250 ℃, ITO NPs transformed from cuboid to sphere and the particle sizes were 15, 18 and 20 nm, respectively. When the hydrothermal temperature increased from 120 to 200 ℃, the particle size of ITO powders reduced dramatically. High reaction temperature promoted the decomposition of urea and made the pressure in the reaction still rise. Therefore, larger particles collapsed to form small particles. An increased nucleation rate caused more fine grains to generate due to the increased solubility of reactants. And the solution reached a state of oversaturation at 200 ℃. According to Ostwald ripening theory, the newly generated grains in the supersaturated system began to grow up. The particles showed an increased size due to continuous growth with the increase of hydrothermal temperature. Significantly, particle size can be flexibly controllable by tuning the hydrothermal temperature. 3.3. Fluorescence of ITO NPs Fluorescence plays an important role in the research of semiconductor luminescence. At room temperature, bulk In2O3 material is nonluminous matter [24]. However, nano-In2O3 shows light phenomenon due to oxygen vacancy and quantum confined effect. Fig. 6 shows fluorescence emission spectra of h-ITO and c-ITO under the same excitation wavelength at room temperature. The emission spectra of h-ITO and c-ITO were similar and showed six emission peaks at 411, 424, 450, 470, 483 and 492 nm in blue light region. H-ITO showed higher fluorescence emission intensity than the one of c-ITO. ITO shows a similar mechanism of blue light emission with In2O3. The fluorescence emission peaks were not caused by quantum confinement effect due to the large diameter, which is much larger than indium
Fig. 4. Raman spectra of ITO powders prepared at different hydrothermal temperatures. 3
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Fig. 5. TEM images of ITO powders prepared at different hydrothermal temperatures: (a) 120 ℃, (b) 160 ℃, (c) 200 ℃, (d) 240 ℃ and (e) 250 ℃.
Compared with c-ITO, h-ITO showed more oxygen vacancies. Significantly, Fig. 6 showed that the blue light emission intensity of h-ITO was stronger than the one of c-ITO. This phenomenon proved that stronger blue light emission intensity was caused by more oxygen vacancies. 4. Conclusions Controllable phase transition ITO NPs were prepared by hydrothermal method without organic solvent, showing several advantages, such as green environment protection, simpler synthetic process, lower cost and nonhazardous. ITO NPs undergo a controllable phase transition from cubic phase to hexagonal phase with the increase of hydrothermal temperature. Single cubic phase and hexagonal phase ITO NPs with a diameter of 150 nm and 20 nm were respectively synthesized at 120 ℃ and 250 ℃ hydrothermal temperature combined with a followed 550 ℃ calcination process. The fluorescence emission intensity of h-ITO NPs was stronger than the one of c-ITO NPs due to more oxygen vacancies. Hydrothermal temperature plays a key role in the structure, morphology and optical properties of ITO NPs. Novel phase transition between cubic phase and hexagonal phase controlled by hydrothermal temperature shows the excellent temperature-structure sensitivity. Significantly, this work not only deeply reveals the relationship between the transition of precursors and the phase transition of ITO NPs, but also clarifies the effects from hydrothermal temperature on the phase structure, morphology and fluorescence of ITO NPs, which will promote the further research and wider application of next generation of ITO functional materials, such as temperature-structure sensor.
Fig. 6. Fluorescence emission spectra of h-ITO and c-ITO NPs.
trioxide’s critical Bohr radius of 2.14 nm [25]. Besides oxygen vacancy, the luminescence mechanism was also related to the tin ion doping concentration. Maintaining tin dopant content constant in the experiments, the probability of capturing electrons to make acceptor vacancy form exciton increased when oxygen vacancy concentration increased. Thereby, the probability of exciton radiative recombination increased [26], which was shown in Equation (8) [27]. Ultimately, the photon emission number of products increased, which was shown as an increased luminescence intensity.
V ·o + (VIn ′Vo) ′ + hν → (VIn ′Vo) x + (Vo) x
(8)
The blue light emission intensity of h-ITO was stronger than the one of c-ITO, which might be a result of more oxygen vacancies in h-ITO. Fig. 7 shows the XPS spectra of two ITO NPs and the corresponding O1s XPS spectra. According to Fig. 7a and c, two ITO NPs were both consisted of In, O and Sn without other impurities, corresponding to the XRD results. Fig. 7b and d show the O1s XPS spectra of c-ITO and h-ITO. There were three separate peaks at 529.9, 531.1 and 531.7 eV, corresponding to the oxygen of In-O, the oxygen of Sn-O and the oxygen of In-O in a defective state, respectively [28]. The oxygen vacancy content of O1s spectrum in c-ITO lattice was 17.534%. However, the oxygen vacancy content of O1s spectrum in h-ITO lattice was 36.373%.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by Beijing Natural Science Foundation (Grant No. 2192041). 4
Chemical Physics Letters 742 (2020) 137174
X. Zhai, et al.
Fig. 7. XPS spectra of ITO NPs: (a) XPS spectra of c-ITO, (b) O1s XPS spectra of c-ITO, (c) XPS spectra of h-ITO and (d) O1s XPS spectra of h-ITO.
Author Contributions: Jiaxiang Liu conceived and designed the experiments; Xiaoyu Zhai, Yiqing Zhang, Yujie Chen and Yunqian Ma performed the experiments and analyzed the data; Xiaoyu Zhai and Jiaxiang Liu offered helpful discussion in the study and co-wrote the paper.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Controllable phase transition ITO nano powders and temperature-structure sensitivity Xiaoyu Zhaia, Yiqing Zhanga,b, Yujie Chena, Yunqian Maa, Jiaxiang Liua,
T
⁎
a
Beijing Key Laboratory of Electrochemical Process and Technology for Materials, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China b Mechanical and Electronic Engineering Department, Henan Polytechnic, Zhengzhou 450046, China
H I GH L IG H T S
flexibly controllable phase transition ITO NPs by hydrothermal method. • Preparing 150 nm single cubic phase and 20 nm single hexagonal phase ITO NPs. • Fabricating • Deeply revealing the phase transition relationship between precursors and oxides.
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrothermal method Phase transition Hexagonal indium tin oxide nano powders Fluorescence Temperature-structure sensitivity
Flexibly controllable phase transition indium tin oxide (ITO) nano powders (NPs) were prepared by an environmentally friendly hydrothermal method. The characterization results show that ITO NPs undergo a novel phase transition from cubic phase to hexagonal phase with hydrothermal temperature increasing. Single cubic and hexagonal phase ITO NPs with a diameter of 150 and 20 nm were respectively synthesized at 120 and 250 ℃ hydrothermal temperature combined with a followed 550 ℃ calcination process. The fluorescence emission intensity of hexagonal ITO (h-ITO) NPs was stronger than the one of cubic ITO (c-ITO) NPs due to more oxygen vacancies.
1. Introduction Transparent conductive indium tin oxide (ITO) is widely used in optoelectronic devices, such as liquid crystal flat panel display, thin film transistor and transparent electrode/substrate material [1–4], due to its high electrical conductivity, transparency to light and infrared reflectivity [5–7]. There are two different crystal structures of ITO, containing cubic phase and hexagonal phase. Cubic ITO (c-ITO) is a normal temperature and pressure phase, showing the same structure of Fe-Mn ore. However, hexagonal ITO (h-ITO) is a high temperature (> 800 ℃) and pressure (103-107 kPa) phase, showing the same structure of corundum. Due to the better anion layer filling structure in h-ITO, h-ITO shows some advantages compared to c-ITO, such as higher specific gravity, higher green density and more stable conductivity [8]. In addition, h-ITO shows many excellent properties such as high ethanol sensitivity [9] and high catalytic activity [10]. Owing to that the presence of water is bad for the preparation of hITO [11], most of the researches have been focused on synthesizing hITO by solvothermal method or water-solvent binary system, currently. ⁎
Gao et al. [11] have got InOOH in In(NO3)3/ethylene glycol/ethanol/ NaOH system and obtained h-In2O3 by calcining at 500 ℃ for 2 h based on solvothermal method. Tao et al. [12] have acquired InOOH in ethylene glycol/water/urea system and obtained h-In2O3 by calcining at 400 ℃ for 2 h based on solvothermal method. Moreover, Liu et al. [13] have prepared h-In2O3 in acetyl acetone indium/methanol/water/ ethanol system at 200 ℃ for 48 h by a hydrothermal method. However, the above methods have the same shortcomings due to the use of organic solvent or indium tin organic salt, such as strong toxicity, high cost and certain risk during the operation process. Thus, setting up an effective, low cost and environmentally friendly way, such as hydrothermal method without organic solvent and indium tin organic salt, to prepare h-ITO, is a prerequisite for developing high-performance optoelectronic components. Moreover, process variables will also have important influences on the morphology and properties of ITO powders [14]. In this paper, using metal In and SnCl4·5H2O as the raw materials, urea as precipitant, InOOH precursor was prepared by hydrothermal method and then h-ITO was obtained by calcining the precursor at
Corresponding author. E-mail address: [email protected] (J. Liu).
https://doi.org/10.1016/j.cplett.2020.137174 Received 20 November 2019; Received in revised form 17 January 2020; Accepted 1 February 2020 Available online 01 February 2020 0009-2614/ © 2020 Elsevier B.V. All rights reserved.
Chemical Physics Letters 742 (2020) 137174
X. Zhai, et al.
550 ℃ for 3 h. The influences from reaction temperature on the crystal structure, morphology and optical properties of ITO powders, prepared by hydrothermal growth technology, were studied systematically. Without any organic solvents, h-ITO nano powders (NPs) with good crystallinity were prepared. By tuning the hydrothermal temperature, the crystal phase transition can be easily controlled from cubic phase to hexagonal phase. This paper provides a new effective way, hydrothermal method, to prepare h-ITO NPs. Significantly, this method possesses several advantages, including low cost, non-toxic, environmentally friendly and simple preparation process. 2. Experimental 2.1. Reagents HNO3 (GR) was purchased from Beijing Tong Guang Fine Chemicals Company. Metal In (purity: 99.99%) was purchased from Liuzhou Smelting Company. Urea (GR) and SnCl4·5H2O (mass ratio: 99.99%) were purchased from Sinopharm Chemical Regent Beijing Co., Ltd. All reagents were used as received.
Fig. 1. XRD patterns of precursors prepared at different hydrothermal temperatures.
2.2. Preparation of ITO NPs 3 g indium was dissolved in concentrated nitric acid under vigorous stirring and heating at 60 ℃ to prepare In(NO3)3. In(NO3)3 solution was mixed with SnCl4·5H2O with the In2O3/SnO2 mass ratio of 9:1. Urea solution, prepared with a certain amount of urea (n (In3+): n (urea) = 1:5), was added into the above mixed solution. After being stirred for 0.5 h, the suspensions were transferred into teflon-lined stainless steel autoclaves and heated at different reaction temperatures (120, 160, 200, 240 and 250 ℃) for 6 h. The precipitates were washed by deionized water and absolute ethanol to remove chloride ions and nitrate ions and dried at room temperature for 12 h. The precursors were finally calcined at 550 ℃ for 3 h to prepare yellow ITO NPs.
Fig. 2. XRD patterns of ITO powders prepared at different hydrothermal temperatures with calcining at 550 ℃.
2.3. Characterization
different hydrothermal temperatures. The crystal structure of ITO powders transformed from cubic phase to hexagonal phase with the increase of reaction temperature, corresponding to the precursor’s transition from In(OH)3 to InOOH. When the hydrothermal temperature was 120 ℃, the product showed cubic In2O3 phase (JCPDS No. 712194), proving the successful synthesis of c-ITO. However, when the hydrothermal temperature was 250 ℃, the product showed hexagonal In2O3 phase (JCPDS No. 22-0336), meaning the successful synthesis of h-ITO. Moreover, when the hydrothermal temperature was between 120 and 250 ℃, the XRD pattern showed two phases of cubic In2O3 structure (JCPDS No.71-2194) and hexagonal In2O3 structure (JCPDS No.22-0336). Combining with the result shown in Fig. 1, it can be proved that the structure of precursor plays a key role in the structure of oxide. The corresponding relationship between indium-tin hydroxide and oxide is agreement with previous references [16,17]. In the hydrothermal reaction, a large number of OH– generated due to the decomposition of urea, making In3+ hydrolysis to form In(OH)3. With increasing reaction temperature, the obtained In(OH)3 nuclei eliminated water molecules from their core to form InOOH [18]. Cubic In2O3 could be obtained by thermal dehydration of In(OH)3, while hexagonal In2O3 was prepared by thermal dehydration of InOOH. The reactions during this process could be expressed as follows:
TG curves were recorded using a DTG-60A thermal analysis system with a heating rate of 10 ℃/min. Powder X-ray diffraction patterns (XRD) were recorded using a Rigaku D/Max-2400 diffractometer with Cu Kα radiation (1.5418 Å). Raman spectra were recorded using a Renishaw InVia confocal microprobe Raman system. Morphologies were recorded using a H-800 transmission electron microscopy (TEM). Solid-state fluorescence emission spectra were recorded on a Hitachi F7000 FL spectrophotometer. X-ray photoelectron spectroscopy (XPS) curves were recorded on a Thermo Fisher Scientific ESCALAB 250 system. 3. Results and discussion 3.1. Structural study on precursors and ITO NPs XRD pattern can help to analyze the structure of a crystal [15]. Fig. 1 shows the XRD patterns of precursors prepared at different hydrothermal temperatures. As shown in Fig. 1, the In(OH)3 precursors eventually transformed into InOOH with the increase of hydrothermal temperature. When the reaction temperature was 120 ℃, the products showed cubic In(OH)3 phase (JCPDS No. 76-1463). When the reaction temperature was 200 ℃, the products consisted of two crystal phases, orthorhombic InOOH phase (JCPDS No. 71-2283) and cubic In(OH)3 phase (JCPDS No. 76–1463). However, the XRD pattern was in good agreement with orthorhombic InOOH phase (JCPDS No. 71-2283) at 250 ℃, indicating the successful preparation of orthorhombic InOOH phase precursors. The precursors were calcined at 550 ℃ for 3 h to prepare ITO powders. Fig. 2 shows the XRD patterns of ITO powders prepared at 2
CO(NH2)2+3H2O → 2NH4++2OH–+CO2↑
(1)
In3++3OH– → In(OH)3
(2)
In(OH)3 → InOOH + H2O
(3)
2In(OH)3 → c-In2O3 + 3H2O
(4)
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Γopt = 4Ag + 4Eg + 14Tg + 5Au + 5Eu + 16Tu
(6)
Ag, Eg and Tg represent the Raman active vibration modes. Au and Eu represent inactive modes. Tu represents infrared active vibration mode. Fig. 4 shows the Raman vibrational modes, located at 132, 306, 365, 495 and 627 cm−1, respectively, which are in good agreement with the ones of c-In2O3 [20]. This result proved that c-ITO could be obtained at 120 ℃ after calcining at 550 ℃. For h-ITO, the optical vibration modes are shown below [19]:
Γopt = 2A1g + 5Eg + 2A1u + 2A2u + 3A2g + 4Eu
A1u, A2u, A2g and Eu are Raman active or infrared active vibration modes. A1g and Eg are Raman active vibration modes. Fig. 4 shows the Raman vibrational modes, located at 163, 219, 269, 305, 384, 501 and 590 cm−1, respectively, which are in good agreement with the ones of h-In2O3 [21]. This also testified that h-ITO could be prepared at 250 ℃ after calcination at 550 ℃. However, ITO NPs prepared at 160 ℃ and 240 ℃ both show two kinds of vibrational modes. Significantly, the Raman spectra of h-ITO and c-ITO were similar to the ones of h-In2O3 and c-In2O3 due to the main contribution to the vibrational modes coming from the host lattice of h-In2O3 and c-In2O3. Novel phase transition between cubic phase and hexagonal phase controlled by hydrothermal temperature shows the excellent temperature-structure sensitivity, meaning a potential application of temperature-structure sensor.
Fig. 3. TG curves of the precursors prepared at different hydrothermal temperatures.
2InOOH → h-In2O3 + H2O
(7)
(5)
TG analysis was used to examine the conversion process of precursors during calcination. Fig. 3 shows the TG curves of precursors prepared at different hydrothermal temperatures. The total mass losses of three precursors were 19.07%, 9.4% and 8.11% (25 to 800 ℃) when the reaction temperatures were 120 ℃, 200 ℃ and 250 ℃, respectively. The TG curves could be divided into three stages. In the first stage, from 25 to 200 ℃, the mass loss was caused by the removal of adsorbed water and glycol in the precursors. The second stage is from 200 to 550 ℃ and the mass loss was caused by the thermal dehydration from precursor to oxide. The theoretical mass loss caused by the phase transition from In(OH)3 to c-ITO and InOOH to h-ITO were 16.28% and 6.09%, respectively. For the precursors obtained at 120 and 250 ℃, the mass losses caused by the phase transition from In(OH)3 to c-ITO and InOOH to h-ITO were 16.88% and 5.92%, respectively. The experimental mass losses were very close to the theoretical mass losses. For the precursor prepared at 200 ℃, the mass loss of 8.54% for the second stage was between the two theoretical values due to the precursor consisted of In(OH)3 and InOOH. With increasing hydrothermal temperature, the mass loss in the second stage decreased, indicating the increase of the amount of InOOH phase, which is in consistent with the XRD analysis. In the third stage from 550 to 800 ℃, TG curves tended to be horizontal lines, which meant that ITO phase fully formed when the calcination temperature was above 550 ℃. In order to study the effect of temperature on phase transition, Raman spectra were recorded. Fig. 4 shows the Raman spectra of ITO powders prepared at different hydrothermal temperatures. For c-In2O3, the optical vibration modes are shown below [19]:
3.2. Morphology of ITO NPs TEM images were recorded to give microstructures [22] of ITO NPs prepared at different hydrothermal temperatures. ITO NPs have been prepared and studied by some scholars. However, fabricating ITO submicron particles remains a challenge and has gained little attention [23]. Fig. 5 shows the TEM images of ITO powders prepared at different hydrothermal temperatures. When the reaction temperature was 120 ℃, the ITO NPs showed cubic shape with a particle size of 150 nm. Continuing to increase the hydrothermal temperature to 160 ℃, ITO NPs showed irregularly cuboid morphology. Moreover, when the reaction temperatures were 200, 240 and 250 ℃, ITO NPs transformed from cuboid to sphere and the particle sizes were 15, 18 and 20 nm, respectively. When the hydrothermal temperature increased from 120 to 200 ℃, the particle size of ITO powders reduced dramatically. High reaction temperature promoted the decomposition of urea and made the pressure in the reaction still rise. Therefore, larger particles collapsed to form small particles. An increased nucleation rate caused more fine grains to generate due to the increased solubility of reactants. And the solution reached a state of oversaturation at 200 ℃. According to Ostwald ripening theory, the newly generated grains in the supersaturated system began to grow up. The particles showed an increased size due to continuous growth with the increase of hydrothermal temperature. Significantly, particle size can be flexibly controllable by tuning the hydrothermal temperature. 3.3. Fluorescence of ITO NPs Fluorescence plays an important role in the research of semiconductor luminescence. At room temperature, bulk In2O3 material is nonluminous matter [24]. However, nano-In2O3 shows light phenomenon due to oxygen vacancy and quantum confined effect. Fig. 6 shows fluorescence emission spectra of h-ITO and c-ITO under the same excitation wavelength at room temperature. The emission spectra of h-ITO and c-ITO were similar and showed six emission peaks at 411, 424, 450, 470, 483 and 492 nm in blue light region. H-ITO showed higher fluorescence emission intensity than the one of c-ITO. ITO shows a similar mechanism of blue light emission with In2O3. The fluorescence emission peaks were not caused by quantum confinement effect due to the large diameter, which is much larger than indium
Fig. 4. Raman spectra of ITO powders prepared at different hydrothermal temperatures. 3
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Fig. 5. TEM images of ITO powders prepared at different hydrothermal temperatures: (a) 120 ℃, (b) 160 ℃, (c) 200 ℃, (d) 240 ℃ and (e) 250 ℃.
Compared with c-ITO, h-ITO showed more oxygen vacancies. Significantly, Fig. 6 showed that the blue light emission intensity of h-ITO was stronger than the one of c-ITO. This phenomenon proved that stronger blue light emission intensity was caused by more oxygen vacancies. 4. Conclusions Controllable phase transition ITO NPs were prepared by hydrothermal method without organic solvent, showing several advantages, such as green environment protection, simpler synthetic process, lower cost and nonhazardous. ITO NPs undergo a controllable phase transition from cubic phase to hexagonal phase with the increase of hydrothermal temperature. Single cubic phase and hexagonal phase ITO NPs with a diameter of 150 nm and 20 nm were respectively synthesized at 120 ℃ and 250 ℃ hydrothermal temperature combined with a followed 550 ℃ calcination process. The fluorescence emission intensity of h-ITO NPs was stronger than the one of c-ITO NPs due to more oxygen vacancies. Hydrothermal temperature plays a key role in the structure, morphology and optical properties of ITO NPs. Novel phase transition between cubic phase and hexagonal phase controlled by hydrothermal temperature shows the excellent temperature-structure sensitivity. Significantly, this work not only deeply reveals the relationship between the transition of precursors and the phase transition of ITO NPs, but also clarifies the effects from hydrothermal temperature on the phase structure, morphology and fluorescence of ITO NPs, which will promote the further research and wider application of next generation of ITO functional materials, such as temperature-structure sensor.
Fig. 6. Fluorescence emission spectra of h-ITO and c-ITO NPs.
trioxide’s critical Bohr radius of 2.14 nm [25]. Besides oxygen vacancy, the luminescence mechanism was also related to the tin ion doping concentration. Maintaining tin dopant content constant in the experiments, the probability of capturing electrons to make acceptor vacancy form exciton increased when oxygen vacancy concentration increased. Thereby, the probability of exciton radiative recombination increased [26], which was shown in Equation (8) [27]. Ultimately, the photon emission number of products increased, which was shown as an increased luminescence intensity.
V ·o + (VIn ′Vo) ′ + hν → (VIn ′Vo) x + (Vo) x
(8)
The blue light emission intensity of h-ITO was stronger than the one of c-ITO, which might be a result of more oxygen vacancies in h-ITO. Fig. 7 shows the XPS spectra of two ITO NPs and the corresponding O1s XPS spectra. According to Fig. 7a and c, two ITO NPs were both consisted of In, O and Sn without other impurities, corresponding to the XRD results. Fig. 7b and d show the O1s XPS spectra of c-ITO and h-ITO. There were three separate peaks at 529.9, 531.1 and 531.7 eV, corresponding to the oxygen of In-O, the oxygen of Sn-O and the oxygen of In-O in a defective state, respectively [28]. The oxygen vacancy content of O1s spectrum in c-ITO lattice was 17.534%. However, the oxygen vacancy content of O1s spectrum in h-ITO lattice was 36.373%.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by Beijing Natural Science Foundation (Grant No. 2192041). 4
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Fig. 7. XPS spectra of ITO NPs: (a) XPS spectra of c-ITO, (b) O1s XPS spectra of c-ITO, (c) XPS spectra of h-ITO and (d) O1s XPS spectra of h-ITO.
Author Contributions: Jiaxiang Liu conceived and designed the experiments; Xiaoyu Zhai, Yiqing Zhang, Yujie Chen and Yunqian Ma performed the experiments and analyzed the data; Xiaoyu Zhai and Jiaxiang Liu offered helpful discussion in the study and co-wrote the paper.
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