SECONDMENTS

Photochromism is the reversible transformation of WO3-d species between two forms by the absorption of UV light, where the two forms have different absorption spectra in the IR. This advanced material is a key element for sensing application meaure with the mean of IR laser diode.

The objective of this mobility was to study the photochromic properties of tungsten oxide nanoparticle thin films. Two types of films were studied, WO3-d alone and also “composite films” of WO3-d and Ta2O5. Thin films, obtained by dip-coating, were irradiated by a 254nm UV light and their optical properties were studied by transmission in the range of 200nm-2000nm.
In a second step, the reversibility of the photochromic properties were studied, for that, the previously irradiated films were left in the state and a measurement of transmission were carried out every hour.

• Realisation of WO3-d suspension
• Realisation of thin film by dip-coating
• UV light irradiation at various time
• Study of photochromic reversibility
• Reproductibility of the study on different films

The films were made from suspensions of nanoparticles in ethanol and shaped by the dip-coating method. The thin films were then irradiated for several minutes, using a UV lamp of 254 nm, and the evolution of the optical transmission of the products was monitored from 200 nm to 2000 nm. Two different types of film were studied, films of WO_3-d, and films of WO_3-d covered with a layer of Ta₂O₅. A first part of the work consisted in irradiating the films at different times in order to follow the evolution of the optical properties of the films according to the irradiation time. When the WO_3-d film was irradiated, the signal evolved in the visible and near-infrared range. This evolution was very fast in the first minutes before slowing down from 25 minutes of irradiation and then stabilizing from 2h. This evolution seemed to be related to two different phenomena, a fast kinetic surface phenomenon, and a slower kinetic volume diffusion phenomenon.

Then the reversibility of photochromism was studied. It showed that the film of WO3-d alone had a kinetics of return to the initial state much slower than the film of WO3-d + Ta2O5. For the film of WO3-d alone, the reversibility was initially linear (first 10 hours) and then, from 15 hours, there was a gradual stabilization and a much slower kinetics (Figure). The reversibility kinetics of the WO3-d + Ta2O5 film was much faster and linear. The evolution of the transmission at 1200nm during the aging cycles showed that the return to 100% transmission was not obtained and even when waiting longer before resuming the measurement. These results tended to show the good behavior of the film under irradiation fp sensing application.

Figure. a) Transmission spectra of the reversibility of photochromism on film of WO3-d. b) Study of the reversibility of photochromism at 1200 nm for the three irradiation cycles.