Optimization Preparation of Indium Tin Oxide Nanoparticles via Microemulsion Method Using Orthogonal Experiment

Abstract

Indium tin oxide (ITO), an experimentally friendly transparent conducting oxide (TCO), has attracted great attention in the photoelectric field due to its intrinsically low resistivity and high transparency. In this work, the experimental conditions of preparing ITO nanoparticles using the microemulsion method were optimized by an orthogonal experiment. The optimal experimental conditions were obtained: mass ratio of the surfactant (AEO-3, MOA-5), a co-surfactant (n-propyl alcohol) of 5:3, molar ratio of indium and ammonia of 1:20, calcination temperature of 700 °C and calcination time of 4 h. Subsequently, the influence from process variables on the resistivity was researched systematically. The results demonstrated that the calcination temperature had a great effect on the resistivity; the resistivity reduced from 11.28 to 2.72 Ω·cm with the increase in the calcination temperature from 500 to 700 °C. Ultimately, ITO nanoparticles were prepared and systematically characterized under the optimal experimental conditions. The particles with a size of 60 nm were attributed to the cubic ITO crystal phase and showed low resistivity of 0.3675 Ω·cm. Significantly, ITO nanoparticles with low resistivity were obtained using the microemulsion method, which has potential application in the field of ITO nanoparticle preparation.

1. Introduction

Indium tin oxide (ITO) is an n-type semiconductor oxide [1,2]. In recent years, due to low resistivity combined with high transparency to visible light [3,4], ITO films have been widely used in various technological areas, including flat panel displays [5,6], solar cells [7,8], gas sensors [9,10] and organic light emitting diodes [11,12]. ITO thin films are usually prepared by magnetron sputtering [13], using ITO targets as raw materials. It has been reported that the properties of ITO nanoparticles have a significant influence on the properties of ITO targets [14]. Therefore, it is necessary to prepare ITO nanoparticles with a high performance.

Many methods have been used to prepare ITO nanoparticles, such as the hydrothermal method [15], co-precipitation method [16], solvothermal method and so on. The hydrothermal method [17] is environmentally friendly, but the high reaction temperature leads to the hard agglomeration of nanoparticles. The co-precipitation method [18] has a simple preparation process, but the morphology and dispersibility of the nanoparticles are difficult to control. The advantage of the solvothermal method [19] is that it can change the solvent system to improve the properties of the nanoparticles, but it has requirements for equipment safety. Compared with the above methods, the microemulsion method, with a simple and safe process, could control the morphology and size of the nanoparticles. It has been used in the preparation of metal nanocrystals, metal oxides and polymers, such as high-efficiency CsPbBr₃/CsPb₂Br₅ composite [20], nanostructured CaO/CuO composites [21], hydrophobically associative polyacrylamides [22] and sub-100 nm PEG NPs [23]. However, the microemulsion method has not been applied in the preparation of ITO nanoparticles.

The microemulsion method also has preliminary application in the ITO field. Zhan et al. [24] prepared In₂O₃ nanoparticles using the inverse microemulsion method and obtained In₂O₃ nanoparticles with narrow particle size distribution and high sphericity in a range from 400 °C to 600 °C. Yang et al. [25] prepared spherical and rod-like In(OH)₃ using the microemulsion-mediated hydrothermal method and prepared spherical and rod-like In₂O₃ nanoparticles by calcination. Devi et al. [26] successfully prepared ITO nanoparticles with a high tin-doping amount at a low temperature with emulsion technology. However, the microemulsion method is not commonly used in the ITO nanoparticles preparation field.

This work aims to systematically study the preparation of ITO nanoparticles using the microemulsion method and the study of its properties, explain the mechanism of the microemulsion method, and finally promote the application of the microemulsion method in the field of ITO nanoparticles preparation. To systematically study the effect of the preparation process of the microemulsion method on ITO resistivity, we designed an orthogonal experiment and calculated the scientific range variance to obtain the best preparation process. Then, through system characterization, we sought to prove the accuracy of orthogonal experimental calculation.

In this paper, cubic ITO (c-ITO) nanoparticles with low resistivity were successfully synthesized using the microemulsion method. Firstly, the orthogonal experiment was employed to optimize the experimental conditions of ITO nanoparticles, and the influence of mass ratio of the surfactant and co-surfactant, molar ratio of indium and ammonia, calcination temperature and calcination time on resistivity was investigated, whereafter ITO nanoparticles were synthesized and characterized systematically under the optimal experimental conditions. Finally, the formation mechanism of ITO nanoparticles using the microemulsion method was proposed and analyzed to improve the performance of ITO nanoparticles. This paper provides a new method for the preparation of c-ITO nanoparticles and provides a reference for future research on ITO nanoparticles preparation.

2. Experimental Section

2.1. Materials

Metal indium (purity > 99.99%) was purchased from Liuzhou Smelting Co. Ltd. (Liuzhou, China). SnCl₄·5H₂O (A.R.) was purchased from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China). Ammonium hydroxide (A.R.) and n-propyl alcohol (A.R.) and n-hexane (A.R.) were purchased from Beijing Tongguang Fine Chemical Company (Beijing, China). Fatty alcohol-polyoxyethylene ether (MOA-5 (A.R.) and AEO-3 (A.R.)) were purchased from Shanghai Macklin Biochemical Co. Ltd. (Shanghai, China). HCl (purity: 37%) was purchased from Shanghai Macklin Biochemical Co. Ltd. (Shanghai, China).

2.2. Preparation of ITO Nanoparticles

In this experiment, ITO nanoparticles were prepared using the microemulsion method, wherein 1.5 g indium was dissolved in 3 mL hydrochloric acid, and the solution was continually stirred and heated until the metal indium was completely dissolved. InCl₃ solution was mixed with SnCl₄·5H₂O with the In₂O₃/SnO₂ mass ratio of 9:1. The mixed solution was used as the aqueous phase, with AEO-3 and MOA-5 mixed in a mass ratio of 1:1 as the surfactants, n-propanol as the co-surfactant and n-hexane as the oil phase, preparing the microemulsion in an appropriate proportion. Ammonium hydroxide was slowly added to the above microemulsion, accompanied with continuous stirring for 2 h. Then the white precipitate was centrifuged, washed with deionized water and ethanol and dried at 80 °C for 4 h. Finally, the precursor was calcined at a certain temperature in a muffle furnace, resulting in yellow ITO nanoparticles. The experimental process is shown in Figure 1.

2.3. Characterization

X-ray diffractometer (XRD, D8 Advance, Karlsruhe, German) was used to measure the phase composition of ITO nanoparticles. Mettler Toledo 2+ synchronous thermal analyzer (Columbus, OH, USA) was employed to analyze the thermogravimetric (TG) and differential thermal analysis (DTA) of ITO precursor. The morphology of ITO nanoparticles was studied using a scanning electron microscope (SEM, TESCAN MAIA3, Tescan Brno, Brno, Czech Republic), selected area electron diffraction (SAED) and high-resolution TEM (HRTEM, JEM 2100, Tokyo, Japan). A 4-Point Probes Resistivity Measurement System (RTS-9, Guangzhou, China) was adopted to measure the resistivity of ITO nanoparticles. The carrier concentration and mobility of ITO nanoparticles were measured by Hall effect measurement system (LanHai Instrument Co., Ltd., Linhai, China). Elemental states of ITO nanoparticles were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo-Scientific, Waltham, MA, USA). Diffusion reflection spectra were obtained by ultraviolet-visible spectrophotometer (UV-Vis, UV-3600, Shimadzu Corporation, Kyoto, Japan).

3. Results and Discussion

3.1. Orthogonal Experiment Optimization of ITO Nanoparticles Preparation Process

3.1.1. Orthogonal Experiment Design and Data

In the hope of obtaining the optimal preparation process of ITO nanoparticles, four factors which have important effects on the resistivity were selected to conduct the orthogonal experiment, namely the mass ratio of surfactant and co-surfactant (M_s/M_c), molar ratio of metal indium and ammonia (N_In/N_a), calcination temperature (t_c) and calcination time (t_h). The orthogonal experiment was designed based on our previous work.

Table 1. Factors and levels for orthogonal experiment.

	Factors	Ms/Mc (A)	NIn/Na (B)	tc (°C) (C)	th (h) (D)
Levels
Ⅰ		5:1	1:10	500	2
Ⅱ		5:2	1:15	600	3
Ⅲ		5:3	1:20	700	4

shows the three levels of the four factors. Assuming there is no interaction between these four factors, orthogonal array table L₉(3⁴) was used to design the orthogonal experiment. The experimental plan and results are shown in

Table 2. Orthogonal experiment plan and results.

Run	Factor				Resistivity Rik (Ω·cm)
	A	B	C	D	1	2
1	1	1	1	1	16.100	17.175
2	1	2	2	2	7.616	6.828
3	1	3	3	3	3.710	3.310
4	2	1	2	3	3.190	3.506
5	2	2	3	1	2.884	3.262
6	2	3	1	2	9.766	7.760
7	3	1	3	2	1.732	1.412
8	3	2	1	3	7.996	8.864
9	3	3	2	1	1.868	1.470

.

3.1.2. Range Analysis of the Orthogonal Experiment Results

Only using the above data in Table 2, it was impossible to obtain the optimal conditions for the experiment. Therefore, range analysis was carried out on the data in Table 2, and the results are shown in

Table 3. Range analysis of experiment data.

Run	Factor				Resistivity Rik (Ω·cm)
	A	B	C	D	1	2
1	1	1	1	1	16.100	17.175
2	1	2	2	2	7.616	6.828
3	1	3	3	3	3.710	3.310
4	2	1	2	3	3.190	3.506
5	2	2	3	1	2.884	3.262
6	2	3	1	2	9.766	7.760
7	3	1	3	2	1.732	1.412
8	3	2	1	3	7.996	8.864
9	3	3	2	1	1.868	1.470
R1	54.739	43.115	67.661	42.759
R2	30.368	37.450	24.478	35.114
R3	23.342	27.884	16.310	30.576
R	31.397	15.231	51.351	12.183

. Among them, R1, R2 and R3 respectively correspond to the sum of the resistivity at three levels of each factor. R is the difference between the maximum and minimum values of R1, R2 and R3 for each factor. The greater the value of R, the greater the influence of this factor on the experimental results [27,28]. Taking factor A as an example, the resistivity sum of the Ⅰ level is maximum (54.739), while the Ⅲ level is minimum (23.342), so the R value of factor A is 31.397. The R values of factors B, C, D were calculated using the same method.

It can be concluded from the Table 3 that R_C > R_A > R_B > R_D. This indicates that the factor impacting sequence on the resistivity of ITO nanoparticles are t_c > M_S/M_C > N_In/N_a > t_h. According to the analysis of the experimental data, the best experimental conditions are A₃B₃C₃D₃, which indicates that the best synthesis conditions are as follows: t_c = 700 °C M_s/M_c = 5/3, N_In/N_a = 1/20, t_h = 4 h.

3.1.3. Variance Analysis of Orthogonal Experiment Results

As is well known, experimental errors are inevitable. Though the influence significance of various factors could be obtained by the range analysis, the experimental error cannot be judged by the range analysis [29]. Therefore, the variance analysis was carried out to study the significance of various factors and evaluate the experimental error size of each factor.

Table 4. Variance analysis of experiment data.

Run	Factor				Resistivity Rik (Ω·cm)
	A	B	C	D	1	2
1	1	1	1	1	16.100	17.175
2	1	2	2	2	7.616	6.828
3	1	3	3	3	3.710	3.310
4	2	1	2	3	3.190	3.506
5	2	2	3	1	2.884	3.262
6	2	3	1	2	9.766	7.760
7	3	1	3	2	1.732	1.412
8	3	2	1	3	7.996	8.864
9	3	3	2	1	1.868	1.470
R1	54.739	43.115	67.661	42.759
R2	30.368	37.450	24.478	35.114
R3	23.342	27.884	16.310	30.576
R	31.397	15.231	51.351	12.183
Qj	90.5045762	19.7546652	250.213644	12.6370112	Qe = 3.6887865
fj	2	2	2	2	fe = 9
Fj	110.407742	24.0989802	305.238969	15.4160588	F0.05(2,9) = 4.26
					F0.01(2,9) = 8.02

shows all the calculation results of variance analysis. Taking factor A as an example, the analysis process was as follows [29,30]:

The sum of squared deviations for factor A(Q_A):

$$\begin{gathered} {\mathrm{Q}}_{\mathrm{A}} = {{\displaystyle \sum }}_{h=1}^r{\mathrm{n}}_h({\overline{R}}_h-\overline{R}{)}^2 ={{\displaystyle \sum }}_{h=1}^3{\mathrm{n}}_h({\overline{R}}_h-\overline{R}{)}^2 \\ =6 \times (9.123167-6.024944{)}^2+6 \times (5.061333-6.024944{)}^2+6 \times (3.890333-6.024944{)}^2=90.5045762 \end{gathered}$$

(1)

where n_h, r, h, ${\overline{R}}_h$ and $\overline{R}$ are the experiment number of h level, factor level quantity, the h level, resistivity average of h level and total resistivity average, respectively.

The freedom degrees of factor A (f_A):

f_A = 3 − 1 = 2

(2)

The sum of squared deviations generated by repeated experimental error (Q_e):

$$\mathrm{Q}{\mathrm{e}}_{ }={{\displaystyle \sum }}_{\mathrm{i}=1}^9{{\displaystyle \sum }}_{\mathrm{k}=1}^2\left(R_{ik} - {\overline{R}}_i\right)=3.6887865$$

(3)

where R_ik and ${\overline{R}}_i$ are the resistivity of experiment ik and resistivity average of experiment i.

The freedom degrees of experimental error (f_e):

$$f_{\mathrm{e}} =f_{\mathrm{T}} -{\displaystyle \sum }{f}_j=17-2 \times 4=9$$

(4)

F value of factor A:

$${\mathrm{F}}_{\mathrm{A}}=\raisebox{1ex}{$Q_{\mathrm{A}}/f_{\mathrm{A}}$}\!\left/ \!\raisebox{-1ex}{$Q_{\mathrm{e}}/f_{\mathrm{e}}$}\right.=110.407742$$

(5)

The same calculation method was employed to obtain the sum of squared deviations Q_B, Q_C and Q_D, the freedom degrees f_B, f_C and f_D and the F values F_B, F_C and F_D, in which j = A, B, C, D.

As can be seen from Table 4, the four factors have a significant influence on the resistivity of ITO nanoparticles (F_j > F0.01). The F value of each factor follows the sequence: F_C > F_A > F_B > F_D, which is consistent with the results of range analysis. It can be seen that calcination temperature is a significant effect on resistivity, followed by M_s/M_c, N_In/N_a and calcination time. Therefore, the best experimental conditions are A₃B₃C₃D₃: M_s/Mc = 5/3, N_In/N_a = 1/20, t_c = 700 °C, t_h = 4 h.

3.1.4. The Influence of Various Factors on Resistivity

The resistivity of ITO nanoparticles is related to mobility and carrier concentration, and the relationship is as follows [31,32]:

$$\rho =\frac{1}{ne\mu },$$

(6)

According to Mathison’s law, the mobility of ITO material can be expressed as [33]:

$$\frac{1}{\mu } = \frac{1}{{\mu }_g} + \frac{1}{{\mu }_I},$$

(7)

In the formula, ρ is the resistivity, μ is the mobility, n is the carrier concentration, e is the absolute value of the electron charge and μ_g, μ_I are the mobility of grain boundary scattering and the mobility of impurity ion scattering, respectively. In this experiment, the doping amount of tin was unchanged, that is, n and μ_I are fixed values. At this time, the resistivity of ITO nanoparticles was determined by the mobility μ_g of grain boundary scattering. The grain boundary scattering mainly depends on the grain size and the structure densification degree.

Figure 2 shows single-factor analysis of the orthogonal experiment, where R1, R2 and R3 represent levels 1, 2 and 3. It shows that the resistivity of ITO nanoparticles decreases continuously as the level of each factor changes from R1 to R3. It can be observed that the curves of M_S/M_C and t_c decreased dramatically, while the curves of N_In/N_a and t_h exhibited a slight bend, indicating that the effects of M_S/M_C and t_c are greater than those of N_In/N_a and t_h on the resistivity. The results are consistent with the above results of range and variance analysis.

As shown in Figure 2, the resistivity of ITO nanoparticles exhibits a continuously decreasing tendency with the decrease in M_S/M_C. The microemulsion system is more stable with the increase in co-surfactant; hence the size of ITO precursor particles is uniform, and the dispersion of particles is improved [34], resulting in a higher μ_g and lower resistivity. Similarly, the resistivity of ITO nanoparticles decreases continuously with the decrease in N_In/N_a. It has been reported that the concentration of precipitant has an influence on growth rate and formation rate of crystal [35]. Sufficient precipitant not only ensures the complete precipitation of metal ions but also makes the particle size of R3 more uniform than others, which is conducive to the decrease in resistivity. In addition, the resistivity of ITO nanoparticles decreases significantly with the increase in calcination temperature and calcination time. That could be attributed to the large grain size caused by the high calcination temperature and long calcination time. The large grain size increases the contact area between nanoparticles and decreases interface resistivity [36], leading to the decrease in resistivity.

3.2. Characterization of ITO Nanoparticles

According to the analysis results of the orthogonal experiment, ITO nanoparticles were prepared under the optimum experimental conditions. Figure 3a shows the TG-DTA curves of the ITO precursor, revealing two main thermal decomposition steps. As shown in Figure 3a, the first exothermic reaction at 0–200 °C was mainly due to the evaporation of the adsorbed water and hydroxyl groups on the surface of ITO precursor, and the mass loss was about 2.33%. The second exothermic reaction at 200–500 °C can be attributed to the decomposition of hydroxide precursors into oxides by heating. The mass loss (17.52%) at this stage is higher than the theoretical mass loss (16.28%) of the transition from In(OH)₃ to In₂O₃. That is attributed to the decomposition of the residual surfactants adsorbed by the precursor surface at 200–500 °C. When the calcination temperature was higher than 500 °C, mass loss did not reoccur, indicating that the phase transformation of ITO precursor was completed.

In order to further verify the above analysis, the XRD patterns of ITO nanoparticles and precursors are shown in Figure 3b. The diffraction peaks of the ITO precursor appeared at 2θ = 22.262°, 31.680°, 45.425°, 51.160° and 56.477°, which can be attributed to the crystal phase of In(OH)₃. Moreover, there was no diffraction peak of other impurity phases, which is in good agreement with the TG results. The diffraction peaks of ITO nanoparticles appeared at 2θ = 30.580°, 35.466°, 45.691°, 51.037° and 60.676°, which can be attributed to c-ITO. In addition, there was no diffraction peak of SnO₂ in the XRD pattern, confirming that Sn⁴⁺ was completely doped into the main lattice of In₂O₃. According to XRD diffraction patterns, grain size can be obtained by using the Debye–Scherrer formula (Equation (8)) [37,38]. The calculation results are listed in

Table 5. Grain size of different planes of ITO NPs.

Peak Position (°)	Β(FWHM)	D (nm)	D Average (nm)
30.580(222)	0.762	10.80
35.466(400)	0.696	11.99
45.691(431)	0.572	15.07	11.99
51.037(440)	0.864	10.18
60.676(622)	0.771	11.92

, and average grain size of crystalline ITO nanoparticles was 12 nm.

As the raw material of ITO films, the properties of ITO nanoparticles are closely related to the properties of ITO films. Compared with the ITO films in the literature [2,14,39], the peak positions of ITO nanoparticles prepared using the microemulsion method shifted to a large angle, and FWHM became wider. The reason may be the existence of residual stress in ITO nanoparticles. In other words, the residual stress in ITO nanoparticles is an important factor affecting the XRD peak position shift and FWHM broadening and will also affect the lattice parameters of ITO nanoparticles.

$$D=\frac{0.89\lambda }{{\beta }_{HKL}\cos \theta },$$

(8)

After the crystal phase analysis of crystalline ITO nanoparticles and the precursor, the microstructure analysis of ITO nanoparticles was carried out. Figure 4a,b show the SEM images of crystalline ITO nanoparticles. It can be seen that ITO particles agglomerate into aggregates with a length of 100 nm and a width of 60 nm. The particles are uniformly distributed, but the dispersion is poor. This can probably be attributed to the fact that the surfactant could not effectively disperse the primary particles, resulting in ITO aggregates formation. Figure 4c shows the HRTEM image of crystalline ITO nanoparticles. It can be seen that the interplanar spacing is 0.4064 nm, corresponding to the typical (211) plane of c-ITO. Figure 4d shows the selected electron diffraction (SAED) pattern of ITO nanoparticles. As shown in Figure 4d, the pattern exhibits halo rings and discrete spots, which confirms that the ITO nanoparticles have a polycrystalline structure. Furthermore, the three diffraction rings could be attributed to the (222), (521) and (622) plane of c-ITO without any indication of other oxides, which further verifies that Sn⁴⁺ was doped with In₂O₃ lattice and a single c-ITO structure was formed.

In the analysis in the previous section, we mentioned that the residual stress of ITO nanoparticles will affect its lattice parameters. Li et al. [40] prepared high-performance ITO film using the sol-gel method. In their work, different lattice plane distances are measured through the inversed FFT image: d₂₁₁ = 0.413 nm, d₂₂₂ = 0.292 nm, d₄₀₀ = 0.252 nm. Compared with the above ITO films, the d₂₁₁ = 0.4064 and d₂₂₂ = 0.2887 of ITO nanoparticles are smaller. This shows that the lattice distance of ITO nanoparticles is shifted due to the influence of residual stress.

The change of the lattice distance of ITO nanoparticles affects the lattice parameters. The change of lattice parameters causes the absorption edge of ITO nanoparticles to shift and then affect the band gap (E_g) of ITO nanoparticles.

As they are used as semiconductor materials, it is necessary to measure the electrical properties of ITO nanoparticles. Figure 5a shows the resistivity, carrier concentration and mobility of ITO nanoparticles. The resistivity of ITO nanoparticles is 0.3675 Ω·cm, the carrier concentration is 1.0107 × 10¹⁹ cm⁻³ and the mobility is 1.68445 cm²V⁻¹s⁻¹. As shown in

Table 6. Resistivity comparison of ITO NPs.

	Factors	Method	Resistivity (Ω·cm)
Article
This paper		Microemulsion method	0.3675
Ref [16].		co-precipitation method	4.900
Ref [36].		solvothermal method	1.258
Ref [48].		solvothermal method	0.64
Ref [49].		co-precipitation method	0.68

, the resistivity of ITO nanoparticles prepared in this experiment is obviously lower than those of Ref [16], Ref [36] and so on. Figure 5b shows the O1S XPS spectrum of ITO nanoparticles, and it could be deconvoluted into three peaks: 529.90 eV (In-O), 530.65 eV (Sn-O) and 531.65 eV (oxygen vacancy) [41]. This shows that the oxygen vacancy content of ITO nanoparticles is 27.39%. It has been reported that high oxygen vacancy content is beneficial to an increase in carrier concentration [42], leading to lower resistivity. In addition, the band gap of ITO nanoparticles is also related to carrier concentration [43]. Figure 5c shows the UV-Vis diffuse reflectance spectra and the corresponding absorption spectra of ITO nanoparticles. Based on the following formula conversion [44,45]:

A = (1 − R_x)²/(2R_x),

(9)

(αhv)² = A(hv − E_g),

(10)

where α, hv, E_g and A are the absorption coefficient, photon energy, optical band gap and the parameter that relates to the effective masses, respectively. The band gap can be obtained by the extrapolated values of the linear section (see Figure 5d). The band gap of c-ITO nanoparticles (3.80 eV) is higher than that (3.60 eV) of ITO nanoparticles reported in the literature [46]. According to Burstein–Moss shift theory, for doped and highly degenerate ITO nanoparticles, the higher the carrier concentration the higher the band gap [47]. Consequently, it is obvious that ITO nanoparticles with a high band gap have excellent electrical properties.

In the previous section, we mentioned the influence of residual stress on ITO particles, which changed the lattice parameters of the particles compared with ITO film. The change of lattice parameters results in the deviation of the absorption edge of ITO nanoparticles, and finally, the deviation of its band gap was compared with ITO films [2].

4. Preparation Mechanism of Crystalline ITO Nanoparticles

Though a microemulsion appears to be macroscopically homogenous, microemulsion preparation is not simple. It is important for microemulsion preparation to investigate the pseudo-ternary phase diagrams. Significantly, the pseudo-ternary phase diagrams could generally be obtained using the titration method, keeping the relative concentration of any two constituents (oil/surfactant/water/co-surfactant) fixed. In this paper, the relative concentrations of surfactant and co-surfactant were fixed, and the pseudo ternary phase diagrams were obtained for microemulsion preparation (see Figure S1). The microemulsion method takes advantage of microemulsion droplets formed with the surfactant and co-surfactant, and the ITO precursor solution is packed into microemulsion droplets, resulting in the formation of fine ITO nanoparticles with good dispersion.

Microemulsion droplets play two roles in this process; firstly, as a nanoreactor that provides a reaction place for preparing ITO precursor, and secondly, as a soft template that controls the morphology and size of the ITO precursor to a certain extent. The preparation mechanism of ITO nanoparticles using the microemulsion method is shown in Figure 6. Because the surfactant could reduce the interfacial tension between two immiscible phases [34], the addition of AEO-3 and MOA-5 as surfactants contributes to the water-in-oil (W/O) emulsion formation of indium tin solution and n-hexane. In order to further reduce the particle size and agglomeration, the n-propyl alcohol as a co-surfactant is added to the above solution. According to the negative interfacial tension theory, the addition of a co-surfactant could break the equilibrium between the two phases and make the effective interfacial tension negative, which leads to interfacial expansion and the formation of smaller droplets [50]. In microemulsion droplets, the metal ions (In³⁺ and Sn⁴⁺) react with the precipitant to produce an ITO precursor. As the calcination proceeds, the organic residues and precursor are easily decomposed, eventually forming the fine ITO nanoparticles.

5. Conclusions

In this paper, we have reported a new strategy for the preparation of ITO nanoparticles using the microemulsion method. Firstly, the orthogonal experiment was used to optimize the process conditions of preparing ITO nanoparticles using the microemulsion method. The optimum conditions are as follows: M_s/M_c = 5:3, N_In/N_a = 1:20, t_c = 700 °C, t_h = 4 h. Subsequently, under the optimal synthesis conditions, the c-ITO nanoparticles presented a low resistivity of 0.3675 Ω·cm, a carrier concentration of 1.0107 × 10¹⁹ cm⁻³, a mobility of 1.68445 cm²V⁻¹s⁻¹, an oxygen vacancy content of 27.39% and a band gap of 3.8 eV. Finally, the formation mechanism of ITO nanoparticles using the microemulsion method was systematically investigated, and the distribution ratio of each microemulsion component in the experiment was determined using a pseudo ternary phase diagram. The experimental results show that the microemulsion method can prepare high-performance ITO nanoparticles. This work provides a new direction for the preparation of ITO powder and promotes the development of ITO nanocomposites.