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-TiO2 films involves a change in color to brownish-gray during UV-light irradiation (i.e., reduction of Ag+ to Ag-0) and a (re)bleaching (i.e., (re)oxidation of Ag-0 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 TiO2 to that of the AgxO 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 degrees 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.

keywords

ENHANCED PHOTOCATALYTIC ACTIVITY; SILVER NANOPARTICLES; NANO-TITANIA; MULTICOLOR PHOTOCHROMISM; OPTICAL-PROPERTIES; SENSING PROPERTIES; PLASMON RESONANCE; DOPED TIO2; FILMS; AG

subject category

Chemistry; Materials Science

authors

Tobaldi, DM; Gallo, MJH; Otero-Irurueta, G; Singh, MK; Pullar, RC; Seabra, MP; Labrincha, JA

Groups

G4 - Renewable Materials and Circular Economy

Projects

ECO-SEE – ECO-innovative, Safe and Energy Efficient wall panels and materials for a healthier indoor environment (ECO-SEE)

RMNE-UA-National Network of Electron Microscopy (REDE/1509/RME/2005 )

CICECO - Aveiro Institute of Materials (UID/CTM/50011/2013)

acknowledgements

D.M.T. is grateful to the ECO-SEE project (funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no. 609234). R.C.P. acknowledges the support of FCT grant SFRH/BPD/97115/2013. G.O.-I. would like to thank FCT from Portugal for his Post-Doctoral research grant (SFRH/BPD/90562/2012). This work was developed in the scope of the project CICECO-Aveiro Institute of Materials (ref. FCT UID/CTM/50011/2013), financed by national funds through the FCT/MEC, and when applicable co-financed by FEDER under the PT2020 Partnership Agreement. Professors L. D. Carlos and R A. S. Ferreira (Physics Department and CICECO-Aveiro Institute of Materials, University of Aveiro, Portugal) are kindly acknowledged for the constructive and fruitful discussions. M. Ferro and RNME-University of Aveiro, FCT Project REDE/1509/RME/2005-are also acknowledged for HR-TEM analysis.