From Isolated Ag⁺ Ions to Aggregated Ag⁰ Nanoclusters in Silver-Exchanged Engelhard Titanosilicate (ETS-10) Molecular Sieve: Reversible Behavior

Recently, it has been shown that Engelhard titanosilicate (ETS-10), a crystalline microporous titanosilicate, is an inverse-shape-selective photocatalyst. The main drawback in the extensive use of this material was its band gap value, located in the UV, that does not allow the use of solar light. In this work, we succeed in shifting the ETS-10 light absorption down to the visible region of the electromagnetic spectrum by introducing Ag⁺ cations inside the ETS-10 channels. Thermal-, chemical-, and UV-phototreatments have been applied to Ag−ETS-10 to tune, in a controlled and progressive way, the aggregation of isolated Ag⁺ counterions into metal nanoclusters of increasing nuclearity. This is a direct means to tune the frequency of silver nanoparticle plasmon resonance and thus the light absorption properties of the material. It is further reported that transformation of isolated Ag⁺ ions into aggregated Ag⁰ nanoclusters is almost reversible. Cycles of H₂-chemical reduction/O₂-chemical oxidation and of UV-photoreduction/vis-photo-oxidation are reported. UV−vis, Ag K-edge extended X-ray absorption fine structure (EXAFS), and Fourier transform infrared (FTIR) spectroscopy of adsorbed CO are the main techniques used to monitor the evolution of the silver aggregation along the different reduction/oxidation treatments.