Vanadium Doped Antimony-Tin Oxide Nano-Sols and Their Films Produced by a Sol-Coating Method

Seong Je Jeona, Jai Joon Leeb, Jun Tae Kimc, and Sang Man Kood† Hybrid Nano Particle Laboratory, Department of Chemical Engineering, Hanyang University,

Seoul 133-791, Korea agolord33@daum.net, bljj925@naver.com, ccleanup1@empal.com and dsangman@hanyang.ac.kr

Keywords: Vanadium doped antimony tin oxide, peptization, resistivity, sol-coating, transparency. Abstract. Ethanol-based antimony-tin oxide (ATO) and vanadium-doped ATO (V-ATO) nano-sol solutions were prepared in H2O containing tetramethylammonium hydroxide by peptizing their coagulated precipitates which were synthesized by hydrothermal reaction at 170 in an autoclave. Also, the ATO and V-ATO films were formed by spin coating to be investigated with dopants (V, Sb) and solid content on their conductivity and transparency.

Introduction

Transparent conducting oxides films have been widely studied for energy storage devices, transparent electrodes, catalyst, gas sensors, and heat reflecting mirrors [1-7] because they exhibit good transparency and electrical conductivity. Indium tin oxide (ITO) has been widely used among the various TCOs. However, disadvantages to its use include the high expense of indium and its difficulty to be etched by wet chemical methods during the patterning process [8]. For these reasons, there have been many reports about the conductivity and physical properties of SnO2 thin films containing various dopants such as antimony (Sb), fluorine (F), niobium (Nb), cadmium (Cd), molybdenum (Mo), or zinc (Zn) [9-16]. In this study, we synthesized antimony-tin oxide (ATO) and vanadium-doped antimony-tin oxide

(V-ATO) nano-sols with high concentration and its homogeneous films by a sol-coating method. The influence of Sb content in ATO and another dopant of vanadium on the crystallite size and electrical conductivity of the film was discussed. Experimental

The ATO and V-ATO sols were prepared by dissolving 1 g of Sn powder, calculated amount of Sb2O3 and V2O5 in 300 mL distillated-deionized water with 30 mL nitric acid (60.0 %). The mixture was well stirred to blend homogeneously at room temperature. At that time, the vessel was opened to evolve large amounts of NO2 gas. Amounts of antimony and vanadium were varied with molar ratio of [Sb]/[Sn] and [V]/[Sn] to 0.02 - 0.2 and 0 - 0.05, respectively. After aging for 1 hour, the mixture turned into yellow suspension. This suspension was poured into 400 mL capable autoclave and heated at 170 for 12 hours. After the autoclave was cooled to the room temperature, excess nitric acid and other impurities in the resulting blue-colored ATO or V-ATO cake were removed by several

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times of filtration and washing with distillated-deionized water. The cake was replaced in 250 mL flask with 100 mL H2O. 1.0 M of aqueous tetramethylammonium hydroxide solution as peptizing agent and dispersant was slowly added into the suspension until pH value was reached to 12.0. The flask were then heated up to 60 and peptized for 4 hours. The final products were evaporated and dried at 80 under reduced pressure. And then dried ATO and V-ATO particles were redispersed in anhydrous ethanol to stable sols.

ATO and V-ATO films were prepared by spin-coating process on PET substrates with a radius of 1 inch and thickness of 0.1 using 2 mL of the sols. The spin-coating was conducted in three stages: 5000 rev./min. for 45 seconds, 7000 rev./min. for 45 seconds, and 5000 rev./min. for 45 seconds. The coated films were then dried at 80 for 6 hours in a vacuum oven.

Results and discussion

Crystallite sizes of the prepared ATO particles could be calculated from (100) and (211) peaks of

the XRD patterns by the Scherrer equation and decreased from 4.8 nm to 2.2 nm as the molar ratio of [Sb]/[Sn] was increased from 0.0 to 0.2 as shown in Figure 1 (a). However, the size reduction was almost saturated above. Therefore, the effect of vanadium doping on the crystallite size in ATO with fixed [Sb]/[Sn] ratio of 0.1 was studied. As shown in Figure 1(b), the crystallite size of V-ATO was minimized at [V]/[Sn] ratio of 0.0125 and became larger at further vanadium doping. Fig. 2 shows HR-TEM images of the ATO and V-ATO nanoparticles. The particle size was about 7 - 8 nm for the sample with [Sb]/[Sn] ratio of 0.05. When Sb doping level increased to [Sb]/[Sn] ratio of 0.1, the particle size decreased to about 5 nm. In addition, for the sample with vanadium doping ([V]/[Sn] = 0.0125) and [Sb]/[Sn] ratio of 0.10, the particle size was below 5 nm, roughly in accordance with XRD data. Doping with Sb and a small amount of vanadium to SnO2 might inhibit growth of particles, which corresponded with the result of Pyke et al. [17].

Figure 1. Crystallite size as a function of (a) [Sb]/[Sn] ratio in ATO particles and (b) [V]/[Sn] ratio at fixed [Sb]/[Sn] ratio of 0.1 in vanadium-doped ATO particles.

Electrical resistivity was measured for ATO films with various Sb doping contents as shown in

Figure 3(a). The resistivity value of the ATO film decreased with increasing antimony content of the sol and reached to a minimum value of 40.1 Ω⋅ for [Sb]/[Sn] ratio of 0.1. However, the electrical resistivity increased significantly with further addition of Sb. When vanadium dopant was also doped

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with amounts of only 0.0125 molar ratio or less, the resistivity reached lower value (17.4 Ω ⋅) as shown in Fig. 3(b). However, with the addition of vanadium above [V]/[Sn] ratio of 0.0125 in antimony ([Sb]/[Sn] ratio of 0.1) doped the SnO2 structure, the electrical resistivity increased significantly.

Figure 2. TEM images with [Sb]/[Sn] ratio of (a) 0.05, (b) 0.10, and (c) 0.20 and (d) 0.10 with vanadium doping ([V]/[Sn] = 0.0125).

Figure 3. Variations of sheet resistance and resistivity of ATO and V-ATO films ; (a) with antimony doping contents of [Sb]/[Sn] ratio of 0.02 - 0.15 and (b) with vanadium doping contents of [V]/[Sn] ratio of 0 - 0.05 at fixed antimony content of [Sb]/[Sn] ratio of 10.

Q.-H. Wu et al. [18] have shown that the XPS spectrum of V2O5 have a peak of 516.9 eV for V2p3/2. The typical binding energies of the V2p3/2 peak in VO2 (V4+) and V2O3 (V3+) were previously reported to be between 515.2-516.4 and 515.5-515.85 eV, respectively [19-21]. As found for the analyses of XPS spectra in Fig. 4, the V 2p3/2 binding energy for the V2O5 (V5+) was observed to be 517.2 eV. With increasing vanadium dopants, V2p binding energies were decreased by configuration of V 3d electrons. In the case of V 2p3/2 for the Sb and V ([Sb]/[Sn] and [V]/[Sn] ratio of 0.1 and 0.025, respectively) doped SnO2 (spectrum (a)), when compared with spectrum (b), the intensity of V5+ ionic

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species was decreased while that of V3+ ionic species increase, thereby forming of the V4+ ionic species, related with the re-oxidation of some parts of V3+ ionic species [22]. In addition, analyses of XPS for the V-ATO films clearly show that the V3+ and V4+ ionic species also increase as amounts of doped V2O5 increase due to the various multiplet configuration in the photoemission final states, related with the core hole-3d electrons interaction [19,21,23]. These results agree well with our results seen in Fig. 4(a).

Fig. 4. Analyses of XPS spectra of V2p3/2 binding energies; (a) Sb and V ([Sb]/[Sn] and [V]/[Sn] ratio of 0.1 and 0.025, respectively) doped SnO2, (b) Sb and V ([Sb]/[Sn] and [V]/[Sn] ratio of 0.1 and 0.0125, respectively) doped SnO2 films.

The substitution of Sn4+ by V5+ as well as Sb5+ leads to an electron donor site. To the contrary, if Sn4+ is replaced by V3+ an acceptor site of electron is created. A large amount of vanadium dopants leads to the increase of the resistivity for the V-ATO films due to an electron trap increase by forming of V3+ ionic species in the SnO2 matrix. These results of XPS spectra were in accordance with the results of analyses of the crystallite size and electrical resistivity. Consequently, in the case of higher doping content (for above [V]/[Sn] ratio of 0.0125) of vanadium, the V3+ ionic species in V-ATO particles was formed, they remove electrons originated from Sb5+ or V5+ and act as an electron trap. Based on these results, the ATO containing the high vanadium doping contents can adversely affect the conductive function of their films. However, while using vanadium dopants below [V]/[Sn] ratio of 0.0125, it was possible to afford smaller nano-crystallites size and lower resistivities.

The transmittance of V-ATO film was about 95% at wavelengths near 800 nm and gradually dropped at short wave lengths until it reached its value of 90% at around 400 nm. The fast decrease below 400 nm was due to the absorption of light caused by the excitation of electrons from the valence band to the conduction band of SnO2. For SnO2, generally, the optical absorption edge was calculated from a published band gap of 3.60 eV [24] to be 344 nm. The transmittance loss was below 10 % for V-ATO films in the visible range between 400 and 800nm. Therefore, it is transparent enough for various TCOs films of the V-ATO film uses.

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Fig. 5. UV-Vis transmittance spectrum of V-ATO films in the visible region.

Summary

The optimum condition for better conductivity of Sb doped tin dioxide nano-films could be obtained by the addition of the vanadium without requiring a sintering process. Important results of this study can be summarized as follows: 1. At vanadium dopants concentration in the range of [V]/[Sn] ratio of 0 – 0.0125, we found that the use of vanadium as a dopant results in the decrease of crystallite size and resistivity of antimony-tin dioxide (for the sample with [Sb]/[Sn] ratio of 0.1). 2. The vanadium content in the films has been analyzed by XPS, and we found a decreased resistivity because V5+ ion species act as electron donors in the SnO2 lattice. However, high doping contents (for above [V]/[Sn] ratio of 0.0125) lead to an increased resistivity due to the increase of V3+ ionic species causing the electron trap. 3. The lowest value of resistivity for Sb doped tin oxide layer on the PET film was 4.01×101 Ω⋅ while for the vanadium doped antimony tin oxide was 1.74×101 Ω⋅. 4. At analysis of optical properties, the transmittance loss was smaller than 10 % for V-ATO films in the visible range between 400 and 800 nm.

Acknowledgement

Authors greatly acknowledge the support from the Korea Ministry of Commerce, Industry and Energy (project no. 10017199-2005-22).

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