Formation of Spherical, Rod- and Branch-Shaped Colloidal In₂O₃ Nanocrystals through Simple Thermolysis of an Oleate Precursor

Abstract

Synthesis of colloidal indium(III) oxide (In₂O₃) nanocrystals (NCs) by a simple non-injection method, using indium(III) oleate as an indium and oxygen source, was studied under conditions of various precursor concentrations, reaction temperatures and times. In the case of using a solution with a low precursor concentration, spherical In₂O₃ NCs that exhibited high crystallinity were successfully obtained. Their size was easily controllable in the range of 4 to 6 nm in diameter by changing the reaction temperature and time. Because the growth of the spherical NCs was developed by the precipitation of new In₂O₃ components from solution and not by Ostwald ripening, the obtained NCs exhibited a narrow size distribution and were almost monodispersed. In contrast to the case of using a solution with a low precursor concentration, rod- and triple-fork-shaped NCs were obtained in the case of using a solution with a high precursor concentration. Formation of the rod- and triple-fork-shaped NCs was attributed to the high growth rate after the nucleation because of the high initial precursor concentration.

1. Introduction

Indium oxide (In₂O₃) has been extensively studied because of its technological significance, owing to its high n-type electronic conductivity and high optical transparency in the visible region, when tin oxide (SnO₂) is doped into In₂O₃ as an impurity,¹⁾ and is now widely used as transparent electrodes in various devices, such as liquid crystal displays. Because nanocrystals (NCs) and nanostructures exhibit a unique nature, In₂O₃ NCs have also been intensively studied. In the early period, the synthesis of In₂O₃ NCs with a size >10 nm, which are applicable to electrically conductive paint, was studied.^2,3) Since Seo et al. reported the synthesis of In₂O₃ NCs with a size of 4–8 nm,⁴⁾ studies on In₂O₃ NCs with nearly monodispersed and single-nanometer size became common,⁵⁾ and the application field of In₂O₃ NCs expanded to electrically conductive inks for ink-jet printing.⁶⁾

Recently, In₂O₃-based NCs have attracted renewed attention as a material for plasmonic semiconductor nanostructures, because the localized surface plasmon resonance (LSPR) can be controlled in the region from mid-infrared to near-infrared by changing the conduction carrier density in addition to the size and surface modification of NCs, which is unlike the conventional metallic NCs.^7–9) When magnetic ions, such as the trivalent iron ion (Fe³⁺), are doped, the behavior of LSPR depends on the concentration of the Fe³⁺ ion.¹⁰⁾ The magnetic ion doped In₂O₃ NCs exhibit weak but clear ferromagnetism;^11,12) therefore, In₂O₃-based NCs are expected as a multifunctional material that exhibits unique plasmonic, magnetic, electrical and optical properties.

Because their small size, <10 nm-diameter, plays a crucial role in the unique properties of In₂O₃-based NCs, improving the synthesis method of In₂O₃-based NCs with a size of <10 nm is important to develop devices and applications using the new functions of In₂O₃-based NCs described above. However, most of the recent studies on In₂O₃-based NCs have focused on their properties. In₂O₃-based NCs in the recent reports were synthesized based on the thermolysis of indium(III) acetylacetonate in oleylamine that was first reported by Seo et al.,⁴⁾ and a few studies to develop the synthesis method of In₂O₃-based NCs were recently reported. Here, we studied the synthesis of In₂O₃ NCs and report a simple non-injection method, i.e., the simple heating of the starting solution resulting In₂O₃ colloidal solution directly without separation of any other solid phases, using indium(III) oleate (In(OA)₃) as an indium and oxygen source. The effects of the concentration of In(OA)₃ in the reaction solution on the shape of In₂O₃ NCs were studied, and crystal growth was discussed based on the variation of the size and its distribution of In₂O₃ NCs depending on the reaction temperature and time.

2. Materials and Methods

2.1 Chemicals

InCl₃, (99.99% In, Alfa Aeser), sodium oleate (>97.0%, Tokyo Chemical Industry), octadecene (ODE; >90%, Tokyo Chemical Industry), tri-n-octylphosphine (TOP; 70%, Aldrich), tri-n-octylamine (TOA; >98.0%, Wako Pure Chemical Industries), hexane (≥99.0%, Sigma-Aldrich), toluene (≥99.5%, Wako Pure Chemical Industries), ethanol (99.5%, Sigma-Aldrich), and methanol (>99.8%, Wako Pure Chemical Industries) were commercially available. All chemicals were used without further purification.

2.2 Preparation of indium oleate

Sodium oleate (3.65 g, 12 mmol) was weighed and dissolved in a mixed solvent of ultrapure water (32 mL) and ethanol (4.0 mL) at 80°C, and then cooled to ambient temperature. Indium chloride (887.7 mg, 4 mmol) was weighed and dissolved in a mixed solvent of ultrapure water (4 mL) and ethanol (1.6 mL) at ambient temperature. When the indium chloride solution was dropped into the sodium oleate solution with stirring, white In(OA)₃ was precipitated. The solution was then centrifuged to separate and extract the In(OA)₃. The In(OA)₃ powder extracted was washed with ultrapure water, and finally dried under vacuum at ambient temperature. Thermogravimetric analysis of the In(OA)₃ prepared in the present study indicated that it contained 18.2 mass% of In.

2.3 Synthesis of In₂O₃ nanocrystals

Reaction solutions (5.0 mL) with In(OA)₃ concentrations of 0.124 M and 0.05 M were prepared to study the effect of the concentration on nanocrystal formation. 1.25 or 0.25 mmol of In(OA)₃ was dissolved into the mixed solvent of TOP (3.0 mL) and ODE (2.0 mL) in a glass vial (12 mL capacity) at 140°C under argon atmosphere; then, clear and colorless solutions were obtained. The source solution in the glass vial was placed in an oil bath maintained at 260–350°C and allowed to react for 1.5–30 min under flowing argon. After the reaction, the vial was rapidly removed from the oil bath and then cooled to ambient temperature.

In₂O₃ NCs were extracted from each solution as follows. Hexane (1.5 mL) was added to the solution; then, ethanol (1–2 mL) was added to precipitate In₂O₃ NCs. Subsequently, the solution was centrifuged to separate the precipitate. The precipitate was dispersed in hexane (1 mL) again, and then TOA (1 mL) and ethanol (1–2 mL) were added to precipitate In₂O₃ NCs. After the In₂O₃ NCs were extracted by centrifugation, the In₂O₃ NCs were dispersed in hexane again. Then, methanol (1 mL) and ethanol (1–2 mL) were added to precipitate In₂O₃ NCs, and the solution was centrifuged to separate the In₂O₃ NCs and dried at ambient temperature under vacuum.

2.4 Characterization

X-ray diffraction (XRD) measurements were performed using a diffractometer (RINT2500, Rigaku, Japan) with Cu-Kα radiation to identify the phases obtained. Transmission electron microscopy (TEM) images were obtained on a transmission electron microscope (JEM-2100, JEOL, Japan) operating at 200 kV. Each sample for TEM was prepared by dispersing the NCs extracted from the solution in toluene, and then placing this toluene dispersion on a carbon-coated copper grid (EM Japan; 200 mesh), which was subsequently dried overnight under vacuum at room temperature.

3. Results and Discussion

3.1 Controlling the shape of NCs

Figure 1 shows XRD profiles of NCs synthesized at 330°C using 0.124 M and 0.05 M In(OA)₃ solutions. All diffraction peaks were identified as those of cubic bixbyite-type In₂O₃. Whereas the metastable rhombohedral form frequently appears in the syntheses of In₂O₃ NCs,^13–16) the stable bixbyite form appeared in the present synthesis. This may be a result of the comparatively high reaction temperature in the present synthesis as compared with the case where the rhombohedral form appears. Although the diffractions of the NCs obtained from the 0.124 M In(OA)₃ solution were slightly sharper than those synthesized from the 0.05 M In(OA)₃ solution, which indicated a slightly larger crystal size of NCs obtained from the 0.124 M In(OA)₃ solution than that from the 0.05 M In(OA)₃ solution owing to the large amount of indium source, no distinct difference depending on the In(OA)₃ concentration was detected in the XRD profiles of NCs obtained.

Fig. 1

XRD profiles of In₂O₃ NCs synthesized using 0.124 M ((a) and (b)) and 0.05 M ((c) and (d)) In(OA)₃ solutions. The reactions were conducted at 330°C for 3 ((a) and (c)) and 10 min ((b) and (d)). (e) Calculated powder XRD profile of cubic bixbyite-type In₂O₃ for comparison.

Figure 2 shows TEM images of In₂O₃ NCs obtained from 0.124 M and 0.05 M In(OA)₃ solutions, which indicated that the obtained NCs were highly dispersed in toluene. These images showed that the two In(OA)₃ solutions resulted in clearly different shapes of NCs. The shape of NCs synthesized from the In(OA)₃ solution with a high concentration, 0.124 M, was not spherical but was rather rod- and triple-fork-like, which were not formed by the aggregation of spherical particles (see Fig. A3 in Appendix), while that synthesized from the In(OA)₃ solution with a low concentration, 0.05 M, was almost spherical with faceted surfaces. While the formation of branch-shaped, such as triple-fork-like and tetrapod, NCs has been reported for various materials, such as CdTe, CdSe and ZnO tetrapods,^17–19) this has yet to be reported for In₂O₃ NCs. The formation of triple-fork-like shaped NCs was the result of the fast crystal growth after nucleation because of the high concentration of the indium source in the reaction solution during the crystal growth period, similar to the formation of CdTe tetrapods.¹⁷⁾ Although the rod- and triple-fork-like shapes of the NCs are very interesting, they may be difficult to apply to various devices. On the other hand, in the synthesis using the 0.05 M In(OA)₃ solution, moderate crystal growth might occur after nucleation because of the low initial concentration of the indium source in the reaction solution, which resulted in the approximately spherical NCs. The diameter of the spherical NCs was in the range of 4–5 nm, based on the TEM images. While the indium acetate is usually employed as a source material of indium and oxygen in the synthesis of In₂O₃ NCs having >10 nm particle size,²⁾ In(OA)₃ was used in the present synthesis. According to Ref. 5), to use fatty acid salts of indium such as the indium myristate (In(C₁₄H₂₇O₂)₃) is effective in order to obtain small and spherical In₂O₃ NCs. Therefore, the In(OA)₃ may be essential for the formation of In₂O₃ NCs having <10 nm in the present study; however, we unfortunately cannot go into further discussion about this because the details of the precursor complex that contains In(OA)₃ and TOP are not clear at present.

Fig. 2

TEM images of In₂O₃ NCs synthesized using 0.124 M ((a) and (b)) and 0.05 M ((c) and (d)) In(OA)₃ solutions. The reactions were conducted at 330°C for 3 ((a) and (c)) and 10 min ((b) and (d)).

3.2 Controlling the size of NCs

Figure 3 shows XRD profiles of the NCs synthesized at 330°C for various reaction times using the 0.05 M In(OA)₃ solution. In the case of a reaction time of 1.5 min (Fig. 3(a)), the diffractions were significantly broadened owing to the small crystal size, but all diffractions were identified as those of In₂O₃ with a cubic bixbyite structure. The diffractions gradually sharpened with an increasing reaction time, which indicated that the size of In₂O₃ NCs increased with an increasing reaction time. This observation indicated that the present synthesis enabled control of the size of the NCs by changing the reaction time.

Fig. 3

XRD profiles of In₂O₃ NCs synthesized at 330°C using 0.05 M In(OA)₃ solution. The reaction times are (a) 1.5, (b) 2, (c) 3, (d) 5, (e) 10 and (f) 30 min. (g) Calculated powder XRD profile of cubic bixbyite-type In₂O₃ for comparison.

Figure 4 shows TEM images and size distribution of In₂O₃ NCs synthesized at 330°C for various reaction times using the 0.05 M In(OA)₃ solution. All NCs obtained exhibited a spherical shape and were dispersed well regardless of the reaction time. The average diameter (d_avg) of NCs obtained, based on the TEM images, was 3.8 nm for the 1.5-min reaction. d_avg increased with the increasing reaction time, and was 5.3 nm for the 30-min reaction. Figure 5 shows the lattice image of the NC synthesized by a 30-min reaction as an example. The lattice fringes that corresponded to the (220) crystal plane of In₂O₃ with a cubic bixbyite structure (lattice spacing is 2.9 Å)²⁰⁾ were clearly observed, which indicated high crystallinity of NCs obtained by the present synthesis method.

Fig. 4

TEM images and the size distribution of In₂O₃ NCs synthesized at 330°C for (a) 1.5, (b) 2, (c) 3, (d) 10 and (e) 30 min using 0.05 M In(OA)₃ solution. d_avg and σ denote the average diameter and standard deviation of the particle distribution, respectively.

Fig. 5

High resolution TEM image of In₂O₃ NC synthesized at 330°C for 30 min.

Figure 6(a) and (b) respectively show the time evolution of d_avg and the volume of the particle calculated from d_avg of In₂O₃ NCs synthesized at 330°C for various reaction times using the 0.05 M In(OA)₃ solution. As seen in Fig. 6(a), the time evolution was similar to the case where coarsening of NCs was caused by the precipitation of new crystal components from the solution.²¹⁾ When the coarsening of NCs occurred by the coalescence of NCs, i.e., Ostwald ripening, the time evolution of the particle volume is described by the linear function of time;²²⁾ however, the observed data (Fig. 6(b)) poorly matched with this model (dashed line in Fig. 6(b)). Although the isolated yield of In₂O₃ NCs provides an information about the crystal growth, i.e., whether the growth was an result of the precipitation of new In₂O₃ components from solution or not, we could not determine it because the amount of the NCs extracted was very small (34 mg at maximum) and they contained organic impurities such as capping ligand, solvent and unreacted In(OA)₃. In the case where the coarsening of NCs is achieved by Ostwald ripening growth, the size distribution becomes broader with the increasing average size, that is, standard deviation, σ increases with the increasing average size;^23,24) however, σ did not depend on the reaction time at least in the case of the reaction with 1.5 ≤ t ≤ 10 min and that for the reaction with t = 30 min (σ = 0.7 nm) was slightly large but approximately the same as compared to the reaction with 1.5 ≤ t ≤ 10 min (σ = 0.6 nm). Therefore, we safely concluded that the growth of NCs observed in the present study was achieved by the precipitation of new In₂O₃ components from solution. The time evolution of the crystal growth rate (Fig. 6(d)) that corresponds to the time derivative of the solid line in Fig. 6(a) reduced with an increasing reaction time because of the reduction of In(OA)₃ concentration along with the development of the reaction. The growth rate observed was of a similar order of magnitude as other solvothermal syntheses of NCs.²¹⁾ Thus, the present synthesis is suitable to obtain nearly monodispersed In₂O₃ NCs with a desired size of several nm in diameter.

Fig. 6

Time evolution of (a) average diameter, d_avg, (b) particle volume, (c) standard deviation, σ, of the particle diameter distribution and (d) growth rate, dr/dt, of In₂O₃ NCs synthesized at 330°C using 0.05 M In(OA)₃ solution. The curve in (a) is the fitted curve using d = at^b (a and b are the fitting parameters). The dashed line in (b) indicates a linear regression line. The line in (c) is only as a guide for eyes.

Figure 7 shows the XRD profiles of In₂O₃ NCs synthesized at various temperatures from 260 to 350°C for 30 min using the 0.05 M In(OA)₃ solution. For the reaction at 260°C (Fig. 7(a)), the diffractions were significantly broadened owing to a small crystal size, but all diffractions were identified as those of In₂O₃ with a cubic bixbyite structure. The diffractions were sharpened with an increasing reaction temperature because of the increasing size of the In₂O₃ NCs obtained. Figure 8 shows TEM images and the size distribution of In₂O₃ NCs synthesized at various temperatures for 30 min using the 0.05 M In(OA)₃ solution. Because the NCs synthesized at 260°C were strongly aggregated, their size distribution could not be evaluated. In contrast, NCs synthesized at temperatures higher than 280°C were highly dispersed, which indicated that the surface states of NCs were completely different between NCs synthesized at 260°C and >280°C. The reaction solutions in the present synthesis contained two organic ligands, that is, oleate ion and TOP, which possibly behaved as capping ligands.^25,26) Moreels et al. studied surface capping ligands of PbSe NCs synthesized from the solution containing oleate ion and TOP by nuclear magnetic resonance spectroscopy,²⁷⁾ and reported that the surfaces of NCs were capped with mainly oleate ions and TOP was only a minor part of the capping ligand. Based on their report, it was inferred first that the surfaces of the present In₂O₃ NCs were capped with oleate ions and not TOP. The PbSe NCs in their report were synthesized at a maximum of 160°C;^27,28) therefore, the oleate remained in an unchanged form in the reaction solution, because the degradation of oleate begins at 150–200°C.^29,30) However, the reaction temperatures in the present synthesis were higher than the temperature that the decomposition of oleate begins, and the formation of the In₂O₃ NCs developed with the consumption of oxygen that was supplied by the decomposition of the COO-group in oleate. Therefore, the oleate does not remain in its original form at the reaction temperatures in the present syntheses.

Fig. 7

XRD profiles of In₂O₃ NCs synthesized at (a) 260, (b) 280, (c) 300, (d) 330 and (e) 350°C for 30 min using 0.05 M In(OA)₃ solution. (f) Calculated powder XRD profile of cubic bixbyite-type In₂O₃ for comparison.

Fig. 8

TEM images and the size distribution of In₂O₃ NCs synthesized at (a) 260, (b) 280, (c) 330 and (d) 350°C for 30 min using 0.05 M In(OA)₃ solution. d_avg and σ denote average diameter and standard deviation of the particle distribution, respectively.

Improving the dispersibility of NCs with an increasing reaction temperature was previously observed in the synthesis of ZnO NCs.³¹⁾ In that case, ZnO NCs were formed by hydrolysis of zinc di-butoxide; therefore, the surfaces of ZnO NCs as formed were capped with butoxide groups. When the reaction temperature was increased, the exchange of surface ligands developed from the butoxide group to oleylamine. As a result, the dispersibility of ZnO NCs significantly improved. Similar to this situation, we inferred that exchange of the surface ligands of the present In₂O₃ NCs developed at temperatures higher than 280°C. For NCs synthesized at 260°C, because the surfaces of NCs must be covered with the degradation products of the oleate having a short chain length, the NCs obtained exhibited less dispersibility. For NCs synthesized at >280°C, the surface ligands must be exchanged from the degradation products of oleate to TOP; therefore, the dispersibility of In₂O₃ NCs significantly improved. Of course, we do not have any evidence at present, so detailed spectroscopic study, such as nuclear magnetic resonance spectroscopy similar to Ref. 27) or X-ray photoemission spectroscopy that is surface sensitive, are needed to confirm the variation of the dispersibility that depends on the reaction temperature.

The average sizes of NCs synthesized at >280°C were in the range of 5 to 6 nm and slightly increased with an increasing reaction temperature. The distribution of the size was slightly broadened with the increasing reaction temperature; however, σ reached a maximum of 0.7 nm. This strongly suggested that the growth of In₂O₃ NCs was not attributed to Ostwald ripening but the precipitation of new In₂O₃ components from solution even for the reaction at 350°C.

4. Conclusions

In the present study, we studied the synthesis of In₂O₃ NCs by a simple non-injection method using In(OA)₃ as an indium and oxygen source, and successfully obtained highly crystallized In₂O₃ NCs having spherical, rod- and triple-fork-like shapes. Especially for the case of spherical NCs, the size was controllable in the range of 4–6 nm in diameter. Because the growth of spherical NCs obtained in the present conditions developed by the precipitation of new In₂O₃ components from solution and not by Ostwald ripening, the obtained NCs were almost monodispersed. This method is expected to be widely applicable to synthesize In₂O₃ NCs doped with various metallic oxides by using the solution containing the dopant metallic oleate in addition to In(OA)₃, because the preparation method of various metallic oleates has already been established.¹²⁾

Acknowledgments

This work was partly performed under the Cooperative Research Program of the “Network Joint Research Center for Materials and Devices” (No. 20171069 and 20181081) and “Dynamic Alliance for Open Innovation Bridging Human, Environment, and Materials”.

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Appendices

Appendix

Figure A1 and A2 respectively show thermogravimetric (TG) curve of the In(OA)₃ obtained under O₂ atmosphere and XRD pattern of the sample after TG analysis together with that of cubic bixbyite-type In₂O₃. From these results, we determined the In concentration in the In(OA)₃ prepared in the present study as 18.2 mass%.

Fig. A1

TG curve of In(OA)₃ used in the present study.

Fig. A2

XRD pattern of the sample after TG using In(OA)₃ as a starting sample together with that of bixbyite-type In₂O₃.

Fig. A3

TEM images of In₂O₃ NCs synthesized using 0.124 M In(OA)₃ solutions. The reactions were conducted at 330°C for (a) 3 and (b) 10 min. These images are enlarged from Fig. 2(a) and (b), respectively.