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Synthesis of cubic In2O3 by a liquid plasma method without chemical additives Seong-Hoon Kim, Ho-Suk Choi, and Kwang-Deog Jung Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01504 • Publication Date (Web): 05 Feb 2016 Downloaded from http://pubs.acs.org on February 13, 2016

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Crystal Growth & Design

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Synthesis of cubic In2O3 by a liquid plasma method without chemical additives Seong-Hoon Kima,b, Ho-Suk Choib and Kwang-Deog Jung*,a a

Clean Energy Research Center, Korea Institute of Science and Technology,

Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea b

Department of Chemical Engineering, Chungnam National University, 220 Gung-Dong,

Yuseong-Gu, Daejeon 305-764, Korea

ABSTRACT: Cubic In(OH)3 crystals were prepared by a liquid plasma method at 6 kV and 55 mA using an H-cell. The solution pH in the cathode increased with the reaction time up to 15 min due to proton reduction, which promoted the deprotonation of In aqua complexes. In(OH)3 was formed by the deprotonation of these complexes during proton reduction. After 15 min, the pH in the cathode cell was significantly decreased, which was ascribed to proton transfer from the anode to the cathode. pH measurements showed that a pH difference developed between the cathode and anode cells during the reaction. Nucleation and crystal growth were monitored via high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) analysis of samples prepared with different reaction times. Initially, small amorphous particles were formed by Ostwald ripening, and these particles rearranged to form rods with 50–90 nm lengths. Cubic In(OH)3 crystals formed via the oriented attachment of the rods. Finally, cubic In2O3 crystals were prepared by calcining the In(OH)3 samples. Photoluminescence emission peaks corresponding to green and orange emissions were observed for In2O3 prepared by calcining the In(OH)3 samples. Further, XPS analysis revealed oxygen defects in the In2O3 nanocubes. *e-mail: [email protected]; Tel: +822-958-5218; Fax: +822-958-5219

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Synthesis of cubic In2O3 by a liquid plasma method without chemical additives Seong-Hoon Kima,b, Ho-Suk Choib and Kwang-Deog Jung*,a a

Clean Energy Research Center, Korea Institute of Science and Technology,

Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea b

Department of Chemical Engineering, Chungnam National University, 220 Gung-Dong,

Yuseong-Gu, Daejeon 305-764, Korea

ABSTRACT: Cubic In(OH)3 crystals were prepared by a liquid plasma method at 6 kV and 55 mA using an H-cell. The solution pH in the cathode increased with the reaction time up to 15 min due to proton reduction, which promoted the deprotonation of In aqua complexes. In(OH)3 was formed by the deprotonation of these complexes during proton reduction. After 15 min, the pH in the cathode cell was significantly decreased, which was ascribed to proton transfer from the anode to the cathode. pH measurements showed that a pH difference between the cathode and anode cells developed during the reaction. Nucleation and crystal growth were monitored via high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) analysis of samples prepared with different reaction times. Initially, small amorphous particles were formed by Ostwald ripening, and these particles rearranged to form rods with 50–90 nm lengths. Cubic In(OH)3 crystals formed via the oriented attachment of the


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rods. Finally, cubic In2O3 crystals were prepared by calcining the In(OH)3 samples. Photoluminescence emission peaks corresponding to green and orange emissions were observed for the In2O3 prepared by calcining the In(OH)3 samples. Further, XPS analysis revealed oxygen defects in the In2O3 nanocubes.

Introduction Indium hydroxide (In(OH)3) and indium oxide (In2O3) have drawn considerable attention due to their semiconducting and optical properties. In(OH)3 has a wide band gap of 5.15 eV and the conductivity of its films varies in the range of 10−7 to 10−3 S/cm2, typical of such a wide band gap [1]. Nonetheless, In(OH)3 is primarily used as a precursor for preparing In2O3. In2O3, an important n-type semiconductor with a wide bandgap of about 3.0 eV, has been widely applied in ultrasensitive sensors, optoelectronic devices, photocatalysis, and solar cells [2–9]. Recently, the importance of the morphology and size of materials at the nanoscale has been in focus [10]. It was reported that catalytic materials with dendritic structures had high step-atom densities on the surface, resulting in good electrochemical activity [11]. Therefore, many attempts have been made to synthesize In(OH)3 with specific shapes such as rods, cubes, spheres, flowers, donuts, wires, sheets, corns, and nanotubes [2–6, 8, 9, 12–20]. For several decades, nanoparticles have been prepared by liquid-phase methods, including hydrothermal synthesis, sol-gel methods, solvothermal synthesis, and spray pyrolysis. The advantages of liquid phase methods for preparing nanoparticles have been reviewed [21, 22]. Recently, liquid plasma has been considered as a new liquid phase medium for the preparation of nanoparticles. Liquid plasma is generated by the high density bombardment of a solution by electrons by applying a high voltage difference between a positively charged solution and negatively charged



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metal probe. Then, the energetic electrons collide with gas molecules and create various excited species such as other electrons, ions, and radicals. The liquid plasma method has several advantages: 1) rapid preparation and 2) no additives are required except the metal precursors. Three liquid plasma methods have been reported for rapidly preparing metal nanoparticles or metal oxides at room temperature. One plasma method used a metal foil as an anode, while the plasma was generated from the cathode with Ar gas. Cations such as Ag and Au were produced from the same metal anode, and were consequently reduced to metal particles at the liquid interface by the plasma electrons from the cathode [23, 24]. Similarly, Cu2O nanoparticles were synthesized using a Cu anode [25]. In the second plasma method, a Pt anode was placed at the bottom of a solution and a Pt precursor was dissolved. Then, platinum nanoparticles were formed by the interaction of chloroplatinic acid and the plasma electrons generated with hydrogen gas [26, 27]. In the third plasma method, Pt nanoparticles were synthesized with assistance from fructose and plasma electrons without any gas input [28]. Because these liquid plasma methods used a single cell, problems from oxygen evolution such as oxygen reduction and side reactions could not be avoided. In particular, In(OH)3 particles could not be formed in the single-cell plasma method using indium metal salts. It was expected that use of metal salts would afford a versatile way to design the resulting metal particles. Therefore, we attempted to discover a plasma method that would produce metal oxides/hydroxides, using metal salts without additives. We successfully prepared In(OH)3 using a liquid plasma method that utilized an H-cell. The anode and cathode cells were separated for selective proton transfer and to prevent oxygen transfer from the anode to the cathode via the cationic membrane. To the best of our knowledge, this is the first report of the preparation of In(OH)3 by a plasma method using indium metal salts without any additives. In2O3 has been


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prepared by the calcination of In(OH)3 (the related references are listed in Table S1). The In(OH)3 crystals were found to grow by Ostwald ripening, and oriented attachment, as in the case of hydrothermal methods. Nonetheless, the photoluminescence (PL) in the resulting In2O3 crystals revealed green and orange emissions similar to those prepared via the vapor-liquid-solid (VLS) method.

Experimental Synthesis of In(OH)3 and In2O3 It was not possible to prepare In(OH)3 particles by the conventional single-cell plasma method (Figure S1), however, In(OH)3 cubes were prepared by the liquid plasma method using an Hcell. The cathode and anode were separated by a cationic membrane (Nafion 115). An indium nitrate solution (25 mL, In(NO3)3·xH2O, Aldrich) of 5, 10, 20, or 100 mM was introduced into both the cathode and anode. The term 20 mM-In is used to describe an indium nitrate solution of 20 mM. For the liquid plasma reaction, a titanium rod was used as the cathode for the plasma discharge and a platinum wire as the anode. The plasma reaction was conducted at 55 mA and 6 kV using a DC power supply. The In(OH)3 products were centrifuged and washed three times with deionized water and ethanol. The washed samples were dried at 100 °C in a vacuum oven overnight. In2O3 was prepared by calcining the In(OH)3 at 500 °C for 2 h. Three representative In2O3 samples were prepared using In(OH)3 synthesized under different reaction conditions. InO-A, InO-B, and InO-C indicate samples of In2O3 obtained from In(OH)3 prepared in 20 mM-In solution for 70 min, 20 mM-In solution for 240 min, and 100 mM-In solution for 70 min, respectively.



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Characterization X-ray analysis was performed to confirm the bulk structures of the In(OH)3 and In2O3 samples using an X-ray diffractometer (XRD, D/MAX-2500/PC, Rigaku Co., Tokyo, Japan). Cu Kα radiation (λ = 1.5406 Å) was used as the X-ray source. SEM images were obtained to study the morphologies of the In(OH)3 and In2O3 samples using a Hitachi-S4200 FE-SEM instrument. High-resolution transmission electron microscopy (HR-TEM) analysis was performed and selected area electron diffraction (SAED) patterns were obtained to elucidate the mechanism of nucleation and growth using a transmission electron microscope (TEM, Tecnai F20 G2). The oxygen vacancies in In(OH)3 were confirmed by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe) with an Al Kα X-ray source (1486.6 eV). UV-Vis diffuse reflectance spectroscopy was used to determine the band gap of the prepared In2O3, and PL spectra were measured at room temperature using a PL spectrometer (LabRAM HR-800). A He-Cd laser source of 325 nm was used for the luminescence measurements.

Results and discussion Figure 1 shows the pH changes with respect to reaction time in both the cathode and anode cells for the various indium nitrate concentrations at a fixed current of 55 mA and 6 kV using the Hcell with the Nafion membrane. The initial pH at 0 min in the cell was determined from the concentration of the indium nitrate. For all the solutions, the pH in the cathode cell began to increase as soon as the potential was applied, but subsequently decreased and approached a value around 2.0. The highest pH values for the 5, 10, 20, and 100 mM-In solutions were reached after 5, 10, 10, and 30 min, respectively. In contrast, the pH in the anode cell steadily decreased with the reaction time.


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It has been reported that there is evidence of water electrolysis by the liquid plasma at the surface of the aqueous solution [29]. Water oxidation in the anode cell occurs at the Pt electrode immersed in the solution as follows: H2O → 2H+ + 2e− + 1/2O2↑

(1)

At the liquid surface, in the case of the cathode, protons are reduced by the injected electrons, producing hydrogen gas as follows: 2 H+ + 2 e− → H2↑

(2)

The pH in the cathode increases due to the hydrogen evolution reaction (HER) during the electron injection at the plasma/liquid interface (equation (2)). Therefore, the initial pH increase in the cathode cell can be attributed to the removal of protons via reaction (2). The hydration and deprotonation of hexaaqua metal complexes make metal cations acidic [30]. Therefore, the indium cations can supply protons as follows: [In(OH2)6]3+ ⇌ [In(OH2)5(OH)]2+ + H+

(3)

[In(OH2)5(OH)]2+ ⇌ [In(OH2)4(OH)2]+ + H+

(4)

[In(OH2)4(OH)2]+ ⇌ In(OH2)3(OH)3 + H+

(5)

The low pH of the indium nitrate solution at 0 min is due to the deprotonation, which depends on the indium nitrate concentration. When the pH of the solution increases owing to proton removal due to the HER, the deprotonation proceeds further. The hexaaqua complex can also be directly converted into indium hydroxide as follows: [In(OH2)6]3+ + 3 e− → In(OH2)3(OH)3 + 1.5 H2

(6)

Assuming 100% faradic efficiency at a constant 55 mA, the In(OH)3 formation rates would be 0.011 mmol/min, using 3 electrons per molecule of In(OH)3. Then, we calculated that reaction (6) in the 5, 10, 20, and 100 mM-In solutions should be completed at 11.0, 21.9, 43.9, and 219.3



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min, respectively, because 25 mL of each solution was used for the experiments. Except in the case of the 100 mM-In solution, the pH difference between the cathode and anode cells was nearly maintained even after 250 min, indicating that a steady state was established. Much more reaction time was required to reach the steady state with the 100 mM-In solution. The pH difference at the steady state results from a balance between proton production by water electrolysis and proton reduction by the plasma electrons. Figure 2 shows the color changes in a 20 mM-In solution containing phenol red while conducting the liquid plasma reaction without stirring the solution. The yellow color in both the cathode and anode cells at 0 min indicates that the solution is acidic. The color at the plasma/liquid interface in the cathode cell changes to red at a reaction time of 1.5 min, indicating that the surface becomes basic due to the HER. The red color at the liquid interface diffuses to the bottom and then particles (most likely In(OH)3) are precipitated at a reaction time of 15 min. Then, the small In(OH)3 particles dissolve to form indium cations in the acidic solution at the bottom. Therefore, in the stirred system, In(OH)3 particles are formed at the plasma/liquid interface and are precipitated. Then, some of the precipitated small particles are dissolved to form indium cations that are again converted to In(OH)3 at the plasma/liquid interface, while others slowly increase in size. Therefore, it is suggested that Ostwald ripening may be involved in the nucleation and growth of the In(OH)3 in a liquid plasma system. The relatively high pH in the 100 mM-In solution at the cathode was maintained for 250 min, which is due to the formation of a large amount of In(OH)3 in the more concentrated solution of the cathode. Figure 3 shows the XRD patterns of the samples prepared in 5, 10, 20, and 100 mM-In solutions at 70 min. The characteristic peaks at 22.2° (200), 31.6° (220), and 51.2° (420) correspond to In(OH)3 with a body-centered cubic (bcc) structure (JCPDS card no. 008-9898). Additional


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peaks at 25.2° and 32.7° from the sample prepared in 100 mM-In solution at 70 min were also observed, which can be assigned to the InOOH phase. The InOOH phase shows a broad and weak pattern, indicating its semicrystalline nature. The peaks of the InOOH phase (25.2° and 32.7°), marked with asterisks in Figure 3(e), are shifted to lower angles as compared with those of bulk InOOH (25.8° and 33.7°). Several explanations have been given for this angle shift [19]: it might be due to lattice expansion either by the insertion of water or because of the semicrystalline nature of the InOOH phase, and disorder in the distribution of the OH groups and O ions in the bulk phase. Figure 4 shows SEM images of the samples prepared in 5, 10, 20, and 100 mM-In solutions at 70 min. About 75 nm cubes were mainly observed with the 5 mM-In solution. Many ~120 nm cubes and a few nanosheets (200~600 nm) were observed with the 10 mM-In solution. Nanocubes, nanosheets, and nanorods entangled with each other were observed with the 20 mM-In solution, as shown in the white dashed circle in Figure 4(c), and large nanosheets (~190 nm) with long nanorods were observed in the 100 mM-In solution, showing that the particles are larger at higher concentrations of indium. Nanocubes of In(OH)3 have usually been prepared by hydrothermal methods [2, 3, 5, 6, 8, 9, 12– 17, 20]. Along with nanocubes, rods and sheets were also observed in these studies. Urea [5, 13, 15], KOH [19], and NaOH [13, 16, 17] have been used as OH− sources. NaBH4 has been used as a H− source [14]. The roles of both OH− and H− can be described as the deprotonation of the hexaaqua indium complex (equations (3)–(5)). Thus, like the OH− and H− ions, the electrons in the liquid plasma reaction can facilitate the deprotonation of the hexaaqua indium complexes. SEM and TEM studies with the product from the 20 mM-In solution were carried out to examine the nucleation and crystal growth mechanism in detail. Figure 5 shows SEM images of the



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products collected at reaction times of 10, 15, 30, 70, 120, and 240 min. The SEM image at 10 min did not show any definite morphologies. Particles of In(OH)3 at 15 min appeared as rods that looked uniform in size (~70 nm lengths) in the inset image. A mixture of predominantly rods (~120 nm in length) and some square sheets (~200 nm sides) was observed at 70 min. The observation of square sheets may indicate the oriented attachment of rods of ca. 130 nm. Cubic In(OH)3 with ~300 nm side lengths (inset) was mainly observed at 240 min. Thus, the size and shape of the particles are transformed with respect to the reaction time; undefined small particles at 10 min → rods at 15 min → rods (many) and sheets (few) at 30 min → rods (negligible), sheets (many), and cubes (many) at 70 min → sheets (few) and cubes (many) at 120 min → cubes at 240 min. Figure 6 shows the TEM images and SAED patterns for the products after reaction times of 10, 15, 70, and 240 min for the 20 mM-In solution. Aggregates of amorphous small particles (Figure 6(a-1)) and bundles of rods (Figure 6(a-2)) were observed at 10 min. The crystallinity and the lengths of the rod bundles increased at 15 min, as shown in the SAED pattern. Figure 6(b) shows that the irregular particles were aligned into unidirectional bundles, as shown by the sequence of numbers 1–6. The length of the rods was uniform (70 nm) at 15 min, and increased with the reaction time up to 70 min, but did not increase thereafter (Figure S2). Sheets (Figure 6(c-2)) and rods (Figure 6(c-1) were also observed at 70 min. The SAED patterns of Figure 6(c-3) show that the cubic crystals were very well developed. Cubes (Figure 6(d-3)) and sheets (Figure 6(d-2)) were observed at 240 min. The SAED patterns of Figure 6(d) indicate that the cubes were well developed at 240 min. As mentioned in the SEM analysis discussion, most of the rods and sheets disappeared with time, becoming attached to form the cubes.


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In the hydrothermal method, Zhu et al. suggested that flower-like aggregates of In(OH)3 nanorods were formed by oriented attachment (OA) and cubes were formed by Ostwald ripening [13]. Jean and Her preferred the OA mechanism [15], although the growth of strip-like and square nanocubes might also have occurred based on the Ostwald ripening. Similarly, Klaumunzer et al. suggested OA involving three steps: 1) 1-D OA of In(OH)3 nanorods; 2) 2-D OA of parallel rods to form bundles; and (3) merging of the rod bundles into cuboids [19]. However, in this study, nucleation occurred at the basic plasma/liquid interface. It proceeded rapidly until the highest pH was attained at about 15 min in the 20 mM solution (cf. Figure 1). The small In(OH)3 particles were dissolved to form In3+ and the dissolved In3+ was redeposited to grow into particles at the plasma/liquid interface. The aggregated particles were transformed into rods by their rearrangement, as shown in the number sequence 1–6 in Figure 6(b). It is clear that the rods attached to form the sheets and then the cubes, as shown in Figures 6(c) and 6(d). Based on these experimental observations, the growth mechanism of In(OH)3 by the liquid plasma method is schematically depicted in Scheme 1. Initially, amorphous particles are produced by Ostwald ripening, and rods are formed by the rearrangement of the aggregated particles. The rods then grow into sheets and cubes by the OA mechanism. The size of the cubes is uniform as a result of the repulsive forces between charged particles. In2O3 was prepared by the dehydration of In(OH)3. The dehydration steps were monitored by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) (Figure S3). The TGA and DTA results show that the dehydration of In(OH)3 was complete at 600 °C. In2O3 was prepared by calcining In(OH)3 at 500 °C for 2 h. The structure of In2O3 was confirmed by XRD. Figure 7 shows the XRD patterns of the In2O3 samples prepared by the calcination of In(OH)3 samples at 500 °C for 2 h. Samples InO-A and InO-B, with the cubic structure of In2O3 (JCPDS



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card no. 010-3287), were successfully formed by calcining In(OH)3 samples prepared in 20 mMIn solution for 70 and 240 min, respectively. InO-C, an In2O3 sample formed from the 100 mMIn solution at 70 min, shows a rhombohedral structure (JCPDS card no. 001-8476), as well as a face-centered cubic (fcc) structure. Because rhombohedral In2O3 can be prepared from InOOH [3, 9], it is clear that the InOOH phase from the 100 mM-In solution in Figure 3-(d) was transformed into rhombohedral In2O3. Figure 8 shows the SEM images of In2O3 samples obtained by calcination of the differently prepared In(OH)3 samples at 500 °C. The SEM images of InO-A and InO-B (Figure 8(a) and (b)) are not very different from those of the In(OH)3 precursors (Figure 5(d) and (f)), whereas the shapes of InO-C (Figure 8(c)) changed into a collection of grain structures after the In(OH)3 calcination process (Figure 4(d)). The size of cubes of the InO-A is smaller than that of the In(OH)3 precursor, resulting from shrinkage during calcination. However, the overall appearance of each sample was generally retained. TEM measurements were performed to observe the shapes of the structural elements and the precise crystal structures of the In2O3. HR-TEM images and SAED patterns for InO-A, InO-B, and InO-C are exhibited in Figure 9(a), (b), and (c), respectively. The fast Fourier-transform (FFT) patterns of InO-A were also obtained, as shown in Figure 9(a-3). In the HR-TEM images, rods with cubic In2O3 structure are observed for all the In2O3, which exist as collections of small rods. The fundamental structures of the In2O3 samples (Figure 9) are not much different from those of the In(OH)3 precursors (Figure 6), although the attachment among the rods becomes looser. As described in Scheme 1, small In(OH)3 rods are formed by the rearrangement of amorphous particles, and cubic crystals of In(OH)3 are formed by OA. The HR-TEM images indicate that the small In(OH)3 rods comprising the large cubic rods are dehydrated


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independently, resulting in the loss of cohesion among the small In2O3 rods, as shown in Figure 9. In Figure 9(a-2), (b-2), and (c-2), the SAED patterns show ring shapes for InO-A, a mixture of irregular and regular spots for InO-B, and regular spots for InO-C. Additionally, the InO-C contains sheets, differently from InO-A and InO-B. Interestingly, the sheets have rhombohedral structures. Figure 10 shows the characteristic PL curves of the In2O3 samples at room temperature. Both InO-A and InO-B show PL peaks at 558 and 651 nm, while InO-C shows PL peaks at 530 and 604 nm. Generally, emission spectra can be divided into near-band-edge (NBE) and deep-level (DL) emissions. The visible emission (400–700 nm) of In2O3 corresponds to the DL emission [31]. It has been reported that the DL emission originates from defects or oxygen vacancies in the lattice sites of the In2O3 crystals. The different emission peaks can originate from the various energy levels yielded by the oxygen vacancies [32]. For example, emission peaks at 384 and 405 nm have been attributed to singly ionized oxygen vacancies [7] and a high degree of crystalline perfection [31], respectively. It was suggested that the luminescence at longer wavelengths (500– 650 nm) was due to deep oxygen vacancy energy levels of the In2O3 samples during preparation [31, 33]. The deep oxygen vacancies are related to structural defects and/or stacking faults [34, 35], which indicate that the cubic In2O3 samples prepared by the liquid plasma method contain such defects and/or stacking faults. PL is usually observed in the 400–500 nm range for In2O3 samples prepared by liquid chemical methods, or at 384 and 530 nm for In2O3 samples prepared by chemical vapor deposition [31]. In2O3 nanocubes deposited on a fluorine-doped tin oxide (FTO) glass by an electrodeposition method showed a blue emission peak at 405 nm [7]. Interestingly, In2O3 prepared by an Au-catalyzed vapor-liquid-solid (VLS) method showed emission peaks at 580–620 nm, and no emission peaks were observed at

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