Structural, Optical and Chemical Properties of Few-Atom Silver Clusters Confined in Zeolites

The unique physiochemical properties of metals have contributed to several areas of extensive research relating to energy, environment, and medicine in the past few decades. Metal clusters (size < 2nm) are ensembles of two to several hundred metal atoms that have received increasing attention due to their unique size and shape dependent chemical, optical, electrical, and magnetic properties. From various metal clusters reported to date, silver clusters (AgCLs) are of special interest possessing peculiar physicochemical properties that largely depended on their unique electronic structures. To reap the benefits of AgCLs, one needs to modulate the electronic properties via controlling their size, shape and particularly stabilize them on or within a support for practical applications.

Encapsulation of AgCLs in organic and inorganic supports, like DNA,thiol, glutathione, glass, and zeolites have demonstrated promising applications in imaging, sensing, optics and catalysis. Zeolites as microporous aluminosilicate materials that confine few atoms AgCLs (Ag2-Ag6) showed unique photoluminescence (PL), fingerprint optical absorption and unprecedented catalytic activities among other supports owing to the formation of small, well-defined AgCLs with the closed electronic shells. For instance, tetrahedral Ag4 clusters in FAU (Faujasite) zeolites recently showed ~100% quantum yield (QY) with high photostability by improving the degree of AgCLs order in the FAU zeolites. Very bright AgCLs in ZSM-5 (Zeolite Socony Mobil-5), FAU, and LTA (Linde Type A) zeolites accounted for the high-efficiency photocatalytic decomposition of NOx and pesticides. Also, high catalytic activity of AgCLs in zeolites was reported for the oxidation of CO, reduction of NOX and reforming of CH4.

Fundamental understanding of the confined AgCLs structures and their local environment are of paramount importance to properly link them to those fascinating properties for further tuning and preserving them in their functional state. However, detailed understanding of their atomic scale structures as well as direct correlations between the optical and chemical properties so far has not been achieved. One reason hereof is the structural complexity of metal exchanged zeolites and the high sensitivity of the structure of the AgCLs when exposed to different chemical environments and characterization techniques. Therefore, site-specific characterizations and/or cautious methodologies should be used in concert with theoretical modeling to obtain a realistic information about the confined AgCLs, particularly in their functional state.

Within this PhD project, the detailed structure and local environment of AgCLs confined in LTA and FAU zeolites were investigated by a combination of different spectroscopic techniques; such as X-ray excited optical luminescence (XEOL), extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), UV-Visible-NIR, florescence (FL), photoluminescence (PL), Fourier transform infrared (FTIR) and electron spin resonance (ESR). These techniques allowed a direct correlation of the cluster structures to their optical and chemical properties. Particularly, the detailed luminescence, photophysics as well as chemical properties of AgCLs were studied for the first time to deeply understand the bright and dark nature of these clusters and their role in CO oxidation.

The overall objective of this dissertation is to investigate the relationship of structural with optical and chemical of AgCLs confined in LTA and FAU zeolites through spectroscopic techniques to provide the essential information for fundamental understandings of the confined AgCLs and advanced their applications.