**Abstract**

We use operando X-ray absorption fine structure (XAFS) to analyze the relation between the properties of photoluminescence (PL) and the structures of Ag clusters and Ag ions. The Ag clusters are generated by evacuation in the cavity of Ag-type zeolite-X. The Ag clusters in the zeolite cavity collapse when exposed to the atmosphere. The results reported herein indicate that the collapsing Ag cluster plays an important role in generating strong PL bands and that Ag clusters might not be a direct species of PL. Results of XAFS analysis show that the Ag cluster formed in the zeolite cavity by evacuation can be tetrahedral with four atoms. By evacuation, 9 or 10 Ag tetrahedral are formed, two of which are expected to be responsible for strong PL. This result suggests that the Ag ion position after cluster collapse plays an important role in PL band generation and that Ag clusters are not direct luminescent species of PL.

**Keywords:** zeolite, Ag cluster, photoluminescence, XAFS, operando measurement

## **1. Introduction**

Zeolites are crystalline aluminosilicates with large ordered cavities (cages) that exhibit unique properties because of their cages [1–4]. They are used widely in applications such as catalysis, ion exchange, and separation [5–9].

The fact that Ag-exchanged zeolites exhibit luminous properties despite containing no rare earth metal component indicates zeolite as a promising luminescent material for future use. In recent years, many researchers have investigated the luminescence behavior of silver-exchanged zeolites [10–18]. Many reports have described that Ag clusters formed with a zeolite framework are luminescent species. In studies using inorganic material matrices such as silica, light emission from silver clusters formed in the matrix has been confirmed [19, 20]. However, the existence

of Ag clusters in PL studies of zeolites has been confirmed only indirectly. In earlier studies, PL and XAFS measurements of zeolites were taken to confirm the relation between PL bands and the local structure of Ag clusters [21, 22]. Results confirmed that Ag clusters were formed during heat treatment processing, however, the Ag cluster decomposed when cooled to room temperature (RT). The XAFS spectra of the unheated sample and the sample cooled after heating were very similar. The Ag clusters formed during heating but collapsed after cooling.

To elucidate the PL mechanism of Ag zeolite, it is fundamentally important to elucidate the relation between the formation and collapse processes of Ag clusters and PL. Accurate PL cannot be observed by thermal excitation. Therefore, it is impossible to observe this process by heat treatment. Ag clusters can be formed not only by heat treatment but also by evacuation. In the case of evacuation, clusters can form at RT. Therefore, as described herein, Ag clusters are formed by evacuation; then the clusters are destroyed by introducing air. In these processes, the relation between cluster formation/collapse and PL is elucidated by the time-resolved operando measurement of PL and XAFS.

For these purposes, we developed an in-situ/operand measurement cell, which can be possible to measure XAFS and PL simultaneously. In this chapter, we discuss the result for time-resolved operando measurement of PL and structure change measured by Quick measurement mode-XAFS.
