Purely Visible-Light-Induced Photochromism in Ag–TiO₂ Nanoheterostructures

Abstract

We report titania nanoheterostructures decorated with silver, exhibiting tuneable photochromic properties for the first time when stimulated only by visible white light (domestic indoor lamp), with no UV wavelengths. Photochromic materials show reversible color changes under light exposure. However, all inorganic photochromic nanoparticles (NPs) require UV light to operate. Conventionally, multicolor photochromism in Ag-TiO films involves a change in color to brownish-gray during UV-light irradiation (i.e., reduction of Ag to Ag) and a (re)bleaching (i.e., (re)oxidation of Ag to colorless Ag) upon visible-light exposure. In this work, on the contrary, we demonstrate visible-light-induced photochromism (ranging from yellow to violet) of 1-10 mol % Ag-modified titania NPs using both spectroscopic and colorimetric CIEL*a*b* analyses. This is not a bleaching of the UV-induced color but a change in color itself under exposure to visible light, and it is shown to be a completely different mechanism-driven by the interfacial charge transfer of an electron from the valence band of TiO to that of the AgO clusters that surround the titania-to the usual UV-triggered photochromism reported in titania-based materials. The quantity of Ag or irradiation time dictated the magnitude and degree of tuneability of the color change, from pale yellow to dark blue, with a rapid change visible only after a few seconds, and the intensity and red shift of surface plasmon resonance induced under visible light also increased. This effect was reversible after annealing in the dark at 100 °C/15 min. Photocatalytic activity under visible light was also assessed against the abatement of nitrogen oxide pollutants, for interior use, therefore showing the coexistence of photochromism and photocatalysis-both triggered by the same wavelength-in the same material, making it a multifunctional material. Moreover, we also demonstrate and explain why X-ray photoelectron spectroscopy is an unreliable technique with such materials.

High‐Temperature Photochromism of Fe‐Doped SrTiO3 Caused by UV‐Induced Bulk Stoichiometry Changes

“…In analogy to the voltage‐induced electrochromism due to a redistribution of oxygen vacancies in Fe:STO (electrocoloration), one might thus also expect existence of light‐induced photochromism. Photochromic effects based on color centers have been frequently reported for oxides with metals (SrTiO 3 , TiO 2 , and many other oxides) . However, those photochromic experiments usually take place around room temperature or much below and the corresponding phenomena are not caused by oxygen stoichiometry changes of the bulk under illumination.…”

TiO2 Surface Hybridisation with Noble Metals (Ag and CuxO) for Solar De-NOx and VOC Removal

“…Still, such an efficient charge separation is able to increase the lifetime of the photo-generated exciton, thus improving the PC activity, cf Scheme 1c, in which the left side is supposed to dominate. Moreover, the visible component present in the solar lamp is able to sensitise TiO2 by transferring excited electrons from CuO/Ag NPs to the conduction band of TiO2 through IFCT [24,76], thus further enhancing the PC activity.…”

“…In this work we have modified the surface of TiO2 with silver and/or copper to enhance solar-light induced PC activity. Silver has been chosen because of its well-known properties in enhancing TiO2 PC activity with both UVA and visible-light sources [23,24], and antibiotic character [25]. On the other hand, copper was selected as it is able to induce visible-light absorption in titania, it has antimicrobial properties [26], yet because of it is relatively inexpensive compared to other noble metals.…”

The upconversion luminescence modulation and its enhancement in Er3+‐doped Na0.5Bi0.5TiO3 based on photochromic reaction

“…However, the reversibility of these materials is poor. [15][16][17] More importantly, no emission modulation property has been reported in these materials. Recently, another PC inorganic system, the rareearth (RE)-doped ferroelectrics Na 0.5 Bi 2.5 Nb 2 O 9 (NBN) and K 0.5 Na 0.5 NbO 3 (KNN), have been reported to show an excellent luminescence emission modulation.…”

“…have demonstrated to have PC behavior. However, the reversibility of these materials is poor . More importantly, no emission modulation property has been reported in these materials.…”