Effect of Dispersants on Photochromic Behavior of Tungsten Oxide Nanoparticles in Methylcellulose

Abstract

Tungsten oxide-based photochromic films that change reversibly in air between colorless-transparent in the dark and dark blue under UV irradiation were prepared by using methylcellulose as a film matrix and various dispersants. Alpha-hydroxyl acid such as glycolic acid (GA) or glyceric acid (GlyA) is the best dispersant because it can make the film transparent by adding a small quantity much less than that of 3-hydroxypropionic acid or ethylene glycol. Fourier-transform infrared spectra and Raman spectra indicate that a strong interaction exists between WO3 and GA or GlyA. The coloration and bleaching processes of the prepared films were investigated to clarify the effect of the dispersants and the moisture contents. The bleaching rate remarkably decreased in the films containing GA or GlyA but accelerated by increasing the contact with O2. Measurements of electron-spin resonance reveals that GA and GlyA as dispersants stabilize the W5+ state. This paper shows that the coloring rate and the period for keeping the blue-colored state are tunable by changing the dispersants. The photochromic films containing α-hydroxyl acid as the dispersant have the potential for application as rewritable film on which information displayed with blue-colored state can be clearly readable for longer times compared with other dispersants.

UV Photochromism in Transition Metal Oxides and Hybrid Materials

“…WO 3 ‐based photochromic films consisting of methylcellulose and dispersants were found to adjust the rate of chromic change and reversal. [ 175 ] Alpha‐hydroxyl acid such as glycolic acid (GA) or glyceric acid (GlyA) makes the film transparent by addition of a small quantity of 3‐hydroxypropionic acid or EG. The chromic change and reversal process of the prepared films were tuned according to the type of dispersants and moisture contents.…”

Highly Efficient Photochromic Tungsten Oxide@PNIPAM Composite Spheres with a Fast Response

“…Although the photochromic property of WO 3 could be substantially improved via hybridization with a polymer, it is unclear how the organic component influences the photochromic property of tungsten oxide, thus restricting the improvement of their photochromic property. The coloration response time of composite materials with tungsten oxide (e.g., tungsten oxide/methylcellulose (WO 3 /MC) composite films , or WO 3 /PVA fibers) is usually measured in minutes, and a high content of tungsten oxide in materials is usually needed to obtain an observable color change, which inevitably influences the other properties of the materials (e.g., the mechanical properties). For example, a photochromic time of tens of seconds was reported for WO 3 /poly­(vinylpyrrolidone) (WO 3 /PVP) films that were slightly colored when the WO 3 content was 30 wt % after 120 s of UV irradiation, although the coloration processes could be accelerated by increasing the WO 3 content.…”

Enhanced Coloration/Bleaching Photochromic Performance of WO3 Based on PVP/PU Composite Matrix

“…Many methods have been used to improve the photochromic performance, such as introducing structural defects (oxygen vacancies, [8,9] impurities, [10,11] disorder degree [12,13] ), enlarging surface area (nanosized, [14][15][16][17][18] porous structure [19][20][21][22] ), combined with other transparent semiconductor [23][24][25] and using hydrogen bond rich dispersant. [26,27] However, the intrinsic drawbacks of WO 3 based photochromic smart windows still need further investigation, especially the bleaching process, which has not been studied further yet. [7] In previous study, WO 3 based photochromic foil in bleached state needs more than 24 hours to recover back to original state, or high temperature which can accelerate the migration rate of H + , or ozone atmosphere which can enhance the oxidation reaction.…”

“…To achieve this purpose, it is necessary to improve the process of charge carrier generation/transportation and ion intercalation/deintercalation. Many methods have been used to improve the photochromic performance, such as introducing structural defects (oxygen vacancies, impurities, disorder degree), enlarging surface area (nanosized, porous structure), combined with other transparent semiconductor and using hydrogen bond rich dispersant …”