Formation of ultrafast-switching viologen-anchored TiO2 electrochromic device by introducing Sb-doped SnO2 nanoparticles

doi:10.1016/j.solmat.2009.05.007

Solar Energy Materials and Solar Cells

Volume 93, Issue 12, December 2009, Pages 2108-2112

https://doi.org/10.1016/j.solmat.2009.05.007 Get rights and content

Abstract

Ultrafast-switching viologen-anchored TiO₂ electrochromic device (ECD) was developed by introducing Sb-doped SnO₂ (Sb_xSn₁₋_xO₂, ATO) as counter electrode (CE), and the switching behavior of the fabricated ECD was investigated as a function of Sb-doping concentration. About 9-nm-sized Sb_xSn₁₋_xO₂ (x=0–0.3) nanoparticles were synthesized by a solvothermal reaction of tin (IV) chloride and antimony (III) chloride at 240 °C, and employed to fabricate 2.4-μm-thick transparent CE. Working electrode (WE) was formed from the 7-nm-sized TiO₂ nanoparticle by a doctor blade method, and the thickness of the nanoporous TiO₂ electrode was 4.5 μm. The phosphonated viologen, bis(2-phosphonylethyl)-4,4′-bipyridinium dibromide, was then adsorbed on the prepared films for the construction of the ECD. The response time was strongly dependent on the doping concentration of Sb in ATO, and the fastest switching response was observed at 3 mol%. At this composition, the coloration time was 5.7 ms, and the bleaching time was 14.4 ms, which is regarded as one of the best results so far reported.

Introduction

A novel electrochromic device (ECD) based on nanocrystalline titania electrode anchored with organic electrochromophores attracts great attention as a novel reflective-type display [1], [2]. This ECD demonstrates a fast-switching response, since the coloration reaction is based on the interfacial electron transfer between the nanocrystalline electrode and the anchored electrochromophore. Also the high contrast can be obtained, because the nanocrystalline TiO₂ can load a great amount of electrochromophores due to its high surface area [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16].

In general, the TiO₂-based ECD are classified as white background-type and transparent-type devices. For the white background-type ECD, metal plates are used for the counter electrode (CE) and the large rutile particles are coated on its surface to hide the background color [10]. In this ECD, rich blue color can be achieved due to the high charge capacity of the metal plate. However, long-term stability of the device is not guaranteed, since the redox reaction on the metal surface is not generally reversible. For the transparent-type ECD, SnO₂-based transparent conductive oxide (TCO) has been usually used, but the coloration efficiency is relatively inferior to that of the white background-type ECD due to the low charge capacity of SnO₂-based TCO. In order to enrich the coloration, Prussian blue, carbon, WO₃, and others, have often been used as the CE material of the viologen-anchored TiO₂ ECD [9], [10], [17], [18].

Antimony-doped tin oxide (ATO, Sb_xSn₁₋_xO₂) has been often used as electrode material, with its high electric conductivity, high charge capacity, and optical transparency [19], [20], [21]. There are several reports for the application of ATO to the CE in the viologen-anchored TiO₂ ECD, but the structure of CE or chemical composition of ATO was not systematically studied yet.

The fast-switching response is a promising advantage for the organic dye-anchored TiO₂ ECD, and further improvement of switching speed will be one of the crucial subjects for the commercial application of this device. In this work, we realized a fast-switching viologen-anchored TiO₂ ECD by tailoring the nanoporous ATO CE structure with the size- and shape-controlled ATO nanoparticles. In addition, the effect of the chemical composition of the ATO on the switching response was systematically studied.

Section snippets

Experimental details

ATO nanoparticle used for the formation of the nanoporous CE was synthesized by a solvothermal reaction in ethanol. That is, 8.5 mmol of tin (IV) chloride pentahydrate (SnCl₄·5H₂O, Aldrich) and the stoichiometric amount of antimony (III) chloride (SbCl₃, Aldrich) were dissolved in 50 ml of anhydrous ethanol. After vigorous stirring for 1 h, the solution was then neutralized by adding tetrabutyl ammonium hydroxide (TBAH, 40 wt% water solution, Aldrich). The resultant solution was transferred to a

Results and discussion

The TEM images of the ATO nanoparticles synthesized by a solvothermal reaction are shown in Fig. 1. As shown in Fig. 1a, as-prepared 3 mol% Sb-doped ATO nanoparticles are uniformly spread over an amorphous carbon-coated TEM grid with less aggregation. As shown in the high-resolution TEM image shown in Fig. 1b, the uniform fringes are found over the entire area of each nanoparticle, suggesting that the individual particles are a single crystal with a size of about 9 nm. The fringe interval of 0.33

Conclusions

It was found in this work that the fastest switching response of ECD was achieved by controlling the doping level of Sb to 3 mol% in the ATO nanoparticles, used for the formation of CE. At this composition, the coloration time was 5.7 ms, and the bleaching time was 14.4 ms, which is regarded as one of the best results so far reported. First of all, it is deduced that the fast-switching response is caused by the high surface area (86 m² g⁻¹) of the nanoporous CE derived from the 9-nm-sized ATO

Acknowledgments

This research was supported by the Korea Science and Engineering Foundation (KOSEF R01-2006-000-10956-0).

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Formation of ultrafast-switching viologen-anchored TiO2 electrochromic device by introducing Sb-doped SnO2 nanoparticles

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusions

Acknowledgments

Electrochim. Acta

Microporous Mesoporous Mater.

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Thin Solid Films

Displays

Sol. Energy Mater. Sol. Cells

Electrochim. Acta

Electrochim. Acta

Thin Solid Films

Ultrafast colour displays

Nature

Electrochromism: Fundamentals and Applications

Formation of ultrafast-switching viologen-anchored TiO₂ electrochromic device by introducing Sb-doped SnO₂ nanoparticles