Synthesis and Characterization of Monodispersed Î²-Ga 2 O 3

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Synthesis and characterization of monodispersed #-Ga2O3 nanospheres via morphology controlled Ga4(OH)10SO4 precursors Dae Ho Yoon Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504209f • Publication Date (Web): 24 Dec 2014 Downloaded from http://pubs.acs.org on December 30, 2014

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Synthesis and characterization of monodispersed β-Ga2O3 nanospheres via morphology controlled Ga4(OH)10SO4 precursors

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Langmuir la-2014-04209f.R1 Article 10-Dec-2014 Kang, Bong Kyun; Sungkyunkwan University, School of Advanced Materials Science and Engineering Lim, Hyeong Dae; Sungkyunkwan University, School of Advanced Materials Science and Engineering Mang, Sung Ryul; Sungkyunkwan University, SKKU Advanced Institute of Nanotechnology (SAINT) Song, Keun Man; Sungkyunkwan University, School of Advanced Materials Science and Engineering Jung, Mong Kwon; Hyosung Corporation, R&D Business Labs Kim, Sang-Woo; Sungkyunkwan University, School of Advanced Materials Science and Engineering Yoon, Dae Ho; Sungkyunkwan University, School of Advanced Materials Science and Engineering

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Synthesis and characterization of monodispersed βGa2O3 nanospheres via morphology controlled Ga4(OH)10SO4 precursors Bong Kyun Kang, † Hyeong Dae Lim, † Sung Ryul Mang, ‡ Keun Man Song, † Mong Kwon Jung, § Sang-Woo Kim†,‡ and Dae Ho Yoon†,‡* †

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea,

‡

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea, §

Hyosung Corporation, R&D Business Labs, Anyang, 431-080, Republic of Korea.

ABSTRACT To our best knowledge, monodispersed β-Ga2O3 nanospheres were successfully synthesized for first time via morphology controlled Gallium precursors using the forced hydrolysis method, followed by thermal calcination processes. The morphology and particle sizes of the gallium precursors were strongly dependent on the varying (R = SO42/NO3-) concentration ratios. As R decreased, the size of the prepared gallium precursors


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decreased and morphology was altered from sphere to rod. The synthesized S2 (R = 0.33) consists of uniform and monodispersed amorphous nanospheres with diameters of about 200 nm. The monodispersed β-Ga2O3 nanospheres were synthesized using thermal calcination processes at various temperatures ranging from 500 to 1000 °C. Monodispersed β-Ga2O3 nanospheres (200 nm) consist of small particles of approximately 10 ~ 20 nm with rough surface at 1000 °C for 1 hour. The UV (375 nm) and broad blue (400 ~ 450 nm) emission indicate recombination via a self-trapped exciton and the defect band emission. Our approach described here is to show the exploration of β-Ga2O3 nanospheres as an automatically dispersion, three-dimensional support for fabrication of hierarchical materials, which is potentially important for a broad range of optoelectronic applications.

Keywords: Semiconductor, β-Ga2O3, Nanospheres, Morphology, Precursors, Forced hydrolysis method.

INTRODUCTUION In recent years, increasing interest has been given to the synthesis of metal-oxide semiconductor nanomaterials such as nanotubes, nanowires, nanoparticles, and hollow nanostructures1-4. Especially, metal-oxide semiconductor nanomaterials are attractive candidates as active elements for advanced nanoscale devices due to their unique electronic and optical properties, low effective density, high specific surface area, and shell permeability that are important in many technology applications such as catalysis, photonics, sensors, solar energy conversion, and electrochemical energy storage5-9.


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To date, many chemical methods such as template-free, surfactant assisted, and salt-mediated processes, have been developed for the synthesis of various metal-oxide semiconductor nanostructures through controlling the reaction rates and selectively altering the growth kinetics of different crystal facets using different molecular precursors, capping agents, and organic surfactants at reaction temperatures for certain growth times10-12. However, simple and effective synthesis routes for the preparation of metal-oxide nanomaterials need to be intensively reexamined to improve their unique properties with controllable morphology, size, desired chemical composition, and structural features. Among the various semiconductor nanostructures, gallium oxide (Ga2O3) with α-, β-, γ-, δ-, and ε-crystal structures, and which has a wide band-gap of 4.9 eV at room temperature, is a promising candidate material for transparent conducting oxide, electroluminescent devices, photocatalysis, and gas sensors13-15. Among these phases, the thermodynamically stable phase of the β-Ga2O3 monoclinic structure has received attention because it also has excellent chemical and thermal stability, electric conductivity, and luminescence properties that depend on the native oxygen, gallium vacancies, and gallium oxide complex vacancies16. Forced hydrolysis method provides an especially promising homogeneous chemical precipitation route for large-scale production of nanosized and well dispersed metal hydrous particles17-18. However, to our knowledge, no attempt was made in the reports to synthesize monodispersed gallium hydrate nanospheres (NSs) by changing the ratio of sulfate and nitrate ions using both the forced hydrolysis method at low temperature and the phase transition of gallium hydrate precursor to β-Ga2O3 nanostructures by heat treatment. In this study, we synthesized the monodispersed Ga4(OH)10SO4 precursors by controlling the various experiment conditions. The effect of sulfate ions was intensively studied throughout the


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forced hydrolysis method. The sulfate ions play an important role as a regulator that can mediate the nucleation and growth of the precursors. Also, the characterization of the monodispersed βGa2O3 NSs was confirmed at various firing temperatures. This study therefore suggests a synthesis strategy of monodispersed metal oxide NSs or hierarchical materials to extend their applications.

EXPERIMENTAL SECTION Synthesis of GaOOH, Ga4(OH)10SO4 precursors, and monodispersed β-Ga2O3 NSs. The mixed solution was prepared by dissolving reagent grade of Ga2(SO4)3•xH2O (Sigma-Aldrich, 99.99%), Ga(NO3)3•xH2O (Sigma-Aldrich, 99.9%), and urea powder (Sigma-Aldrich) in 100 mL deionized water. Briefly, gallium nitrate, sulfate (Ga3+ concentration of 0.01 M), and urea (0.1 M) were dissolved in deionized water. Rod-like structures of GaOOH and spherical Ga4(OH10)SO4 precursors were obtained after aging the mixture in a 98 ±1 °C oil bath for 2 hours, at varying (SO42-/NO3-) concentration ratios. The precipitate was separated by centrifugation then washed three times with deionized water and ethanol to remove excess surfactants, and then freeze dried at -110 °C. Using the forced hydrolysis method, the prepared monodispersed Ga4(OH)10SO4 precursors were calcined in air atmosphere as a function of firing temperatures from 500 to 1000 °C for 1 hour. Characterization. The surface morphology of the precursors and the monodispersed β-Ga2O3 NSs as a function of firing temperatures were observed using field-emission scanning electron microscopy (FESEM, JEOL 7500F). Scanning Transmission electron microscopy (STEM) and high-resolution transmission electron microscopy (HRTEM) was carried out using a JEM2100F with an accelerating voltage of 200 kV. The crystallinity and structure of the precursors and β-


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Ga2O3 NSs were examined by powder X-ray diffraction (PXRD, Bruker D8 FOCUS) using Cukα radiation, respectively. Fourier transform infrared (FTIR, Bruker IFS-66/S) spectroscopy was employed to analyze the chemical bonding within the materials. The thermogravimetrydifferential thermal analysis (TG-DTA, Seiko Exstar) was measured in air at a heating rate of 5 °C /min from 30 to 1000 °C to confirm the reaction of Ga4(OH)10SO4 precursors and crystallization of β-Ga2O3 NSs. The optical properties of the monodispersed β-Ga2O3 NSs were analyzed using a room temperature cathodoluminescence (MONO CL3+, GATAN) equipped with an excitation energy of 10 keV.

RESULTS AND DISCUSSION The monodispersed spherical Ga4(OH)10SO4 precursors were conveniently obtained at 98±1 °C for 2 hours before calcination. Uniform spherical precursors with average diameters around 200 nm were fabricated with gallium salts containing sulfate and nitrate ions in the presence of urea. The total concentrations of Ga3+ ions and urea were fixed at 0.01 M and 0.1 M, respectively, and the concentration ratio R = SO42-/NO3- was fixed at 0.33. The types of starting materials and various concentration ratios effect not only the morphologies but also the crystalline phase of the gallium precursors. The changed ratios of the gallium salts are listed in Table S1. The controlled morphology of the gallium precursors as a function of concentration ratios was investigated using FESEM. When only using the gallium nitrate starting materials, the precursors (sample S1) have prismatic rod structures and smooth faceted surfaces with diameters in the range of 200 ~ 400 nm and lengths of about 1.5 ~ 2 µm (Figure 1a). Figure 1b and 1c show that complete precursors (sample S2) and aggregated precursors (sample S3) were formed with


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increasing gallium sulfate. Moreover, the size of these precursors gradually increased as the concentration ratio (SO42-/NO3-) of the reactant solution increased (Figure S1). Also, these experimental results suggest that gallium sulfate played an important role in the controlled morphology, size, and dispensability of the gallium precursors. Figure 1d-1h show TEM, scanning transmission electron microscope (STEM), and energydispersive X-ray spectroscopy (EDS) mapping images as well as the EDS spectrum of the monodispersed gallium precursors (sample S2). The TEM image (Figure 1d) indicates that the synthesized S2 consists of uniform and monodispersed amorphous precursors with diameters of about 200 nm. It can be clearly seen from the element mapping in the EDS mapping images that the elements of Ga, O, and S clearly existed in the S2. The EDS mapping result shows the relative locations of Ga, O, and S within the precursors. The Ga and S atoms appear in the same positions with similar diameters, demonstrating that the Ga and S atoms were combined as Ga4(OH)10SO4 precursors. The element mapping of C is not shown because a large amount of C existed in the support film of the TEM grid. Based on the above experiments, it can be seen that the SO42--mediated precipitation process involves the formation of Ga4(OH)10SO4 precursors followed by several growth steps, as illustrated in Figure 2. Sequential hydrolysis and homogenous nucleation occurred and formed spheres similar to the precursors which are amorphous Ga(OH)3. In the case of the Ga(OH)3 precursors without SO42- ions, the amorphous Ga(OH)3 precursors such as negatively charged complexes of Ga(OH)4- were dissolved as temperatures increased up to 98 °C because the urea decomposed at around 80 °C and released uniform -OH ions into the solution. Furthermore, the increase in the pH caused enhanced gelation and increased the polymeric sizes. By prolonging the refluxing at high pH and aging time, the Ga(OH)4- aqueous solution spontaneously formed


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aggregated crystalline GaOOH rod structures19-20. However, in the case of the Ga(OH)3 precursors with SO42- ions, the amorphous Ga(OH)3 precursors were precipitated and polymerized with a spherical shape. The better coordination bond strength and higher affinity of SO42- than NO3- ions can cause a spherical composition of Ga4(OH)10SO4 and result in a favorable formation of spherical amorphous precursors. In addition, the amorphous Ga4(OH)10SO4 precursors are neutral and the compact primary particles resulted in the morphological transformation of the amorphous Ga4(OH)10SO4 and aggregated individual fine precursors (Figure S2) 21-22. The crystallinity, chemical bonds and thermal analysis of gallium precursors were further conformed by PXRD, FTIR and TG-DTA analysis (Figure S3, S4 and S5). Figure 3a shows the PXRD patterns of the β-Ga2O3 NSs after firing from 600 to 1000 °C for 1 hour. The PXRD pattern of the β-Ga2O3 NSs indicates no obvious diffraction peaks occurred because the prepared Ga4(OH)10SO4 precursors are amorphous and cannot be deformed by the crystalline β-Ga2O3 insufficient temperatures at 600 °C. The PXRD patterns at 1000 °C were easily indexed to the monoclinic crystalline β-Ga2O3 (JCPDS Card 41-1103). The PXRD patterns for the β-Ga2O3 NSs at firing temperatures of 800 °C could barely be indexed because of the residual SO4 ions in the β-Ga2O3 NSs and the low intensity peaks of β-Ga2O3 with a large full width at half maximum (FWHM). The FWHM of the PXRD peaks for the β-Ga2O3 NSs decreased with increasing firing temperatures ranging from 600 to 1000 °C. This result was attributed to the fact that the amorphous Ga4(OH)10SO4 precursors were perfectly crystallized after the firing temperatures were controlled (Figure S6). The intensities of the diffraction peaks sharply increased due to the enhanced crystallization and crystallite growth23. In addition, the


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grain sizes of β-Ga2O3 nanospheres were calculated using the Scherrer equation (1) based on the XRD patterns24. t=(0.9×λ)/B·cosθ

(1)

A calculated grain size is t and (λ= 1.54056 Å) is the X-ray wavelength. 0.9 is a constant for spherical particles, and B is the FWHM for the diffraction angle. The grain sizes of the β-Ga2O3 nanospheres hollow NSs were 20.19 nm at a firing temperature 1000 °C. Fig. 3b shows the FTIR spectra in the 4000 ~ 400 cm-1 region of the β-Ga2O3 NSs after firing from 600 to 1000 °C. The vibration modes of the β-Ga2O3 NSs are attributed to the symmetric stretching of the S=O (1106 cm-1) vibrations by the strong bond with Ga(OH)3 during the forced hydrolysis treatment25. The results suggest that the component of the prepared S2 consists of Ga4(OH)10SO4. With increasing firing temperatures, the absorption peak at 1106 cm-1 was drastically decreased. Strong absorption peaks positioned at 466 and 681 cm-1 (Ga-O stretching vibration) are identified in the β-Ga2O3 NSs obtained above 600 °C for 1 hour26. These results are in good agreement with the PXRD results and revealed the formation of crystalline β-Ga2O3 NSs using firing conditions. Furthermore, the FTIR spectra show a broad absorption band of the -OH group (1631, 3443 cm1

), which results from the remianing -OH ions or the H2O absorbed by the β-Ga2O3 NSs when the

samples were analyzed at room temperature in air27. The amorphous spherical precursor S2 can crystallize into β-Ga2O3 NSs after the calcination process at 1000 °C for 1hour in air. The prepared monodispersed β-Ga2O3 NSs consist of small particles of approximately 20 ~ 50 nm with rough surface, as shown in Figure 4a. The average particle size of the β-Ga2O3 NSs is around 200 nm. However, no necking phenomenon was observed between particles. According to the FESEM images, the porous and granular particles without necking were attributed to the decomposition of sulfate ions on the S2 at around


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1000 °C28. The HRTEM image in Figure 4b reveal the monodispersed β-Ga2O3 NSs are constructed by the good crystallinity of small particles. The interplanar spacing of about 0.2967 nm correspond to the (400) crystal plane of monoclinic β-Ga2O3, which agreed well with the PXRD patterns shown in Figure 3a. A broad blue emission band from 300 to 600 nm was observed over the 700 °C. A strong blue emission peak was centered at around 400 ~ 450 nm up to 900 °C. In addition, a UV emission at around 375 nm suddenly increased at 1000 °C, which resulted from the enhanced recombination via a self-trapped exciton29. Furthermore, the blue emission of β-Ga2O3 indicates the defect band emission due to the vacancies in the β-Ga2O3 structure, such as oxygen vacancies (donor band), gallium vacancies, and gallium-oxygen vacancy pairs (acceptor band). It was suggested that the emission peak originated from the recombination of an electron on a donor with a hole on an acceptor formed by an oxygen and gallium or gallium-oxygen vacancy pair in the β-Ga2O3 crystal structures30-32.

CONCLUSION The monodispersed β-Ga2O3 NSs were successfully synthesized by facile and reproducible by urea based forced hydrolysis method using Gallium sulfate and nitrate as starting materials. The morphology of gallium precursors were controlled by the molar ratio of sulfate and nitrate. We obtained uniform and monodispersed spherical Ga4(OH)10SO4 precursors at the R = 0.33. As R decreased, the size of the prepared gallium precursors decreased and morphology was altered from sphere to rod. The formation of uniform polycrystalline β-Ga2O3 NSs with diameters of about 200 nm at 1000 °C for 1 hour were confirmed by FESEM, STEM and HRTEM analysis. The UV and broad blue emission indicate recombination via a self-trapped exciton and the defect


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band emission due to the vacancies in the β-Ga2O3 structure, such as oxygen vacancies (donor band), gallium vacancies, and gallium-oxygen vacancy pairs (acceptor band). Further works are being carried out to explore the potential application of these monodispersed nanoparticles as photo-catalysts and optical device.

Figure 1. FESEM images of the gallium precursors prepared with different ratios of R = SO42/NO3- : (a) sample S1, (b) sample S2, and (c) sample S3. TEM image (d), EDS spectrum (e), and


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element maps showing the spatial distribution of Ga (f), O (g), and S (h) in the as-prepared sample S2.


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Figure 2. Illustration of the demonstration and morphology evolution for gallium precursors at R = 0.33.


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Figure 3. PXRD (a) patterns and FTIR (b) spectra of sample S2 as a function of calcination temperatures from 600 to 1000 °C for 1 hour in Air.


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Figure 4. FESEM image (a) inset STEM image and HRTEM images (b) inset corresponding fast Fourier transform patterns of sample S2 calcined at 1000 °C for 1 hour in Air.


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Figure 5. Cathodoluminescence (CL) spectra of the β-Ga2O3 NSs as a function of the firing temperatures ranging from 500 to 1000 °C for 1 hour in Air.


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SUPPORTING INFORMATION Tables summarizing experimental details for sample preparation, FESEM images, Illustration of the demonstration and morphology evolution, PXRD, FTIR and TG-DTA data of the gallium precursors, PXRD, FTIR, CL data and representative band-structure scheme for the UV and blue luminescence in β-Ga2O3 nanospheres. This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author *Tel: +82-31-290-7361. Fax: +82-31-290-7410. E-mail: [email protected] (D.H. Yoon) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF- 2013R1A2A2A01010027).

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23. Shang, M.; Wang, W.; Xu, H. New Bi2WO6 Nanocages with High Visible-Light-Driven Photocatalytic Activities Prepared in Refluxing EG. Cryst. Growth. Des. 2009, 9, 991-996. 24. Borchert, H.; Shevchenko, E. V.; Robert, A.; Mekis, I.; Kornowski, A.; Grübel, G.; Weller, H. Determination of Nanocrystal Sizes: A Comparison of TEM, SAXS, and XRD Studies of Highly Monodisperse CoPt3 Particles. Langmuir 2005, 21, 1931-1936. 25. Kahandal, S. S.; Kale, S. R.; Disale, S. T.; Jayaram, R. V. Sulphated yttria–zirconia as a regioselective catalyst system for the alcoholysis of epoxides. Catal. Sci. Technol. 2012, 2, 14931499. 26. Rambabu, U.; Munirathnam, N. R.; Prakash, T. L.; Vengalrao, B.; Buddhudu, S. Synthesis and characterization of morphologically different high purity gallium oxide nanopowders. J. Mater. Sci. 2007, 42, 9262-9266. 27. Xiao, H.; Pei, H.; Hu, W.; Jiang, B.; Qiu, Y. Ga2O3 nanowires grown on GaN-Ga2O3 coreshell nanoparticles using a new method: Structure, morphology, and composition. Mater. Lett. 2010, 64, 2399-2402. 28. Qui, H.; Liu, H.; Sang, Y.; Lv, Y.; Zhang, Z.; Zhang, Y.; Ohachi, T.; Wang, J. Ammonium sulfate regulation of morphology of Nd:Y2O3 precursor via urea precipitation method and its effect on the sintering properties of Nd:Y2O3 nanopowders. CrystEngComm 2012, 14, 17831789. 29. Binet, L.; Gourier, D. ORIGIN OF THE BLUE LUMINESCENCE OF β-Ga2O3. J. Phys. Chem Solids 1998, 59, 1241-1249.


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30. Yan, S.; Wan, L.; Li, Z.; Zhou, Y.; Zou, Z. Synthesis of a mesoporous single crystal Ga2O3 nanoplate with improved photoluminescence and high sensitivity in detecting CO. Chem. Comm. 2010, 46, 6388-6390. 31. Sinha, G.; Chaudhuri, S. Controlled solvothermal synthesis of β-Ga2O3 3D microstructures and their optical properties. Mater. Chem. Phys. 2009, 114, 644-649. 32. Ho, C.-H.; Tseng, C.-Y.; Tien, L.-C. Thermoreflectance characterization of β-Ga2O3 thinfilm nanostrips. Opt. Express 2010, 18, 16360-16369.

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Synthesis and Characterization of Monodispersed Î²-Ga 2 O 3

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