Although photochromic behaviour first attracted attention in the late 1880s, like the sepia photographs from that era the response was monochrome. It was not until 2003 that researchers in Japan were able to elicit a multicolour response using mesoporous TiO2 embedded with silver nanoparticles, allowing potential applications in rewritable colour copy paper, electronic paper and high-density multi-wavelength optical memory.

Among those attracted by the potential of the new photochromic material was David Babonneau, CNRS Research Director of the Physics and Properties of Nanostructures at the Université de Poitiers in France, whose team showed that they could also fabricate these nanocomposite films on flexible substrates, opening up further opportunities in high-density multi-wavelength optical data storage, rewritable colour papers, plastic packaging products, and secured credit cards. But making all this potential a reality requires a much better understanding of the mechanisms and limiting factors in the photochromic process, which their latest work now addresses.

On the track of plasmons

Previous studies of the nanocomposite’s photochromic response have indicated the behaviour’s likely origins in the excitation of plasmons – resonant collective oscillations of electrons at the surface of the metal nanoparticles. The optical absorption changes only take place in the presence of oxygen, where visible light leads to redox reactions so that the silver atoms in the nanoparticles oxidize to silver ions. Exciting plasmons in the nanoparticles makes them unstable so that they release these silver ions, resulting in morphological changes in the silver nanoparticle and a change in the optical response. In contrast, exposure to UV radiation, excites electrons in the TiO2 matrix that reduce the silver ions, reversing the whole process. Yet while the initial state of the nanocomposite film, the incident light and environmental parameters seem to have a crucial influence on the process, the role of these factors is little understood.

One approach that has been successful in monitoring the growth of plasmonic nanoparticles is grazing incidence small-angle x-ray scattering (GISAXS), which can reveal changes in the electron density at surfaces and interfaces. Babonneau and colleagues at Université de Poitiers, Université de Lyon, Université Jean Monnet-Saint-Etienne and Université de Grenoble Alpes adopted the same tool to study their TiO2-Ag nanoparticle composites, combined with optical transmission to track spectral changes in response to radiation.

Correlating morphology and photochromic behaviour

With the aid of their real-time measurements, the researchers found a good correlation between the changes in the morphology of the structure and the optical behaviour. Exposing the nanocomposite to visible radiation led to the oxidation of the silver causing the smaller nanoparticles to dissolve, a process that sped up under x-ray radiation. In the absence of these nanoparticles less light is absorbed. The researchers also observed the reversal of the process under UV radiation. However they also noticed that the ionization of the silver atoms to dissolve small nanoparticles outpaced nanoparticle growth, ultimately degrading the material from one cycle to the next.

Babonneau and colleagues conclude, "The study may open up the possibility of exploring the influence of various factors that have an impact on the photochromic transformation process in Ag/TiO2 nanocomposite films (initial state of the film, excitation wavelength and irradiance, environmental conditions, etc), which is mandatory for optimizing their functional properties."

Full details are reported in Nano Futures.