Synergy of Neodymium and Copper for Fast and Reversible Visible-light Promoted Photochromism, and Photocatalysis, in Cu/Nd-TiO2 Nanoparticles

Multifunctional materials working under solely visible-light are expected to play a significant role in photoelectronics, in particular photoswitches, photo-optical sensors, smart windows, displays, optical storage memories, and self-cleaning materials. We have modified the surface of a versatile semiconductor material (TiO2) with a noble metal (copper) and simultaneously doped its structure 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 and photocatalytic activity was engineered. A combination of high-resolution (scanning) transmission electron microscopy (including image simulations), spatially resolved electron spectroscopies, and X-ray analyses were used to characterize the structure and local chemical environment of the modified material. We showed that Nd entered the TiO2 structure in a substitutional fashion, and when at 1 mol %, neodymium atoms were ultradispersed inside the TiO2 (anatase) structure. On the contrary, when the neodymium concentration 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) and definitely more stable photoswitching compared to standard Cu-TiO2 photochromic material and to switchable inorganic visible light photochromic materials reported so far. 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 visible-emitting LED and solar-light lamps, respectively). Copper acted as the chromophore (and visible-light absorber for the photocatalytic reaction). Neodymium, on the other hand, behaved itself as an electron donor to fill the deep trap states in TiO2, thus increasing the lifetime of the photogenerated exciton in titania. It was demonstrated that the optimum amounts of copper and neodymium, for both photochromic and photocatalytic reactions, were 1 mol %. The manufacture of that multifunctional material can be scaled up to make smart energy-efficient windows with self-cleaning abilities.