A Semiconducting Material Exhibiting Visible-Light Promoted Photochromism, Photoluminescent and Photocatalytic Activity

Multifunctional materials working under solely visible-light are expected to play a significant role in photo-electronics, in particular photo-switches, photo-optical sensors, smart windows, displays, optical storage memories, and self-cleaning materials.
In this work, we have modified the surface of a versatile semiconductor material (TiO2) with a noble metal (copper), and simultaneously doped its lattice with a rare-earth element (neodymium). Exploiting the ability of a semiconductor to generate an exciton upon excitation, a multifunctional material working with visible-light and showing photochromic, photoluminescent and photocatalytic activity was engineered. Advanced transmission electron microscopy techniques – aberration-corrected imaging combined with image simulation, statistical analyses and electron energy-loss spectroscopy – were used to characterise thoroughly the structure and local chemical environment as a function of the Nd3+ content, as well as to link the variation of these aspects to the evolution of the physico-chemical properties. Combined X-ray, spectroscopic and microscopy technique results showed that Nd entered the TiO2 lattice in a substitutional fashion. When at 1 mol%, neodymium atoms were ultra-dispersed inside the TiO2 (anatase) structure. On the contrary, when the neodymium mol% was higher than 1 mol%, neodymium atoms mostly clustered at the surface of the grains, forming a dense network. Furthermore, the Cu-modified / Nd-doped TiO2 exhibited faster (two times), as well as a reversible photoswitching that was remarkably more stable compared to standard Cu-TiO2 photochromic material. The same material also displayed visible-light and solar-light induced photocatalytic activity in the gas-solid phase (degradation of benzene, one of the most hazardous pollutants found indoor) superior to unmodified titania (1.35 and nearly 3.00 times higher, with a LED visible- and solar-light lamps, respectively). It was demonstrated that the optimum amounts of copper and neodymium, for both photochromic and photocatalytic performances, were 1 mol%.