Thermal-Driven Formation of Silver Clusters Inside Na/Li FAUY Zeolites for Formaldehyde Detection

Abstract

In this research, the LiY zeolite was firstly synthesized by using NaY as the parent zeolite; thereafter, the LiYAg and NaYAg zeolites created for formaldehyde gas detection were prepared with further Ag+-Li+/Na+ exchange and a mild thermal treatment at 300 °C to promote the formation of luminescent Ag CLs. The spectra experimental results indicated that Ag CLs showed stronger and blue-shifted emissions in LiYAg compared with in NaYAg, and the emission intensity of Ag CLs in both zeolites monotonously decreased when exposed to increasing formaldehyde gas content. Moreover, the linear dependence of the Ag CLs’ emission intensity variation on formaldehyde content indicated a reliable method for fast and sensitive formaldehyde detection. According to the XPS, UV–vis absorption, and N2 adsorption–desorption isotherm studies, the formaldehyde-gas-induced luminescence quenching of Ag CLs is due to the formation of Ag2O and Ag NPs, in which the higher content of Ag+/Ag0 redox couples in LiYAg and larger surface area of NaYAg benefit the precise detection of formaldehyde gas in low- and high-content ranges, respectively. Furthermore, the blue-shifted peak position and widened FWHM of Ag CLs can also be used for the indication of formaldehyde gas and the detection limit of NaYAg and LiYAg, which both meet with the standards of the WHO and OSHA.

Zeolite-Encaged Luminescent Silver Nanoclusters

“…Formaldehyde is the main indoor pollutant emitted by buildings and decorative materials and is harmful to human health [ 74 , 75 ]. Recent studies indicated that the luminescence properties of FAUY zeolite-encaged Ag NCs show a rapid and sensitive response to formaldehyde gas [ 76 , 77 ]. Figure 11 a shows the PL spectra of Ag NCs-loaded FAUY zeolites in normal and increasing concentrations of formaldehyde atmospheres from 80 to 2560 ppb.…”

“… Emission spectra of Ag−FAUY under different formaldehyde concentration ( a ), and emission intensity of Ag−FAUY at different formaldehyde concentration (I 0 : original, I 1 :at different formaldehyde concentration) ( b ) [ 76 ], and relative emission intensity of Ag NCs in Li−FAUY and Na−FAUY zeolites ( c ) [ 77 ]. …”

Luminescence Properties and Theoretical Modeling of the Ag3 Cluster Confined inside the D6r Cavity of Faujasite Zeolite: Implications for the Design of Tunable Emission Phosphor

“…15−17 It has been reported that after thermal treatment, X-ray or electron beam irradiation on the Ag + -exchanged zeolites, Ag 2 + , Ag 3 n+ , Ag 4 n+ , and Ag 6 n+ may be formed in the D6r or sodalite cages of zeolites. 4,12,18,19 The formation efficiency and luminescence properties of Ag NCs can be manipulated through the selection of zeolite topological structure, framework, and extra-framework modifications, adjustment of silver loading degree, and thermal treatment. For example, recent studies indicated that the Ag NCs confined inside Linda A (LTA) and FAU zeolites exhibit blue-shifted emission when the radius of the extra-framework cation is reduced, and show significantly higher emission intensity in the Li + -type zeolites than in the Na + /K + /Ca 2+ -type zeolites; 20−23 the peak emission wavelength of Ag NCs can be turned over 40 nm through desilication or dealumination treatment on the host FAU zeolites before Ag +exchange; 24 the Ag NCs in Li-LTA zeolites exhibit red-shifted emission from blue to green-yellow with increasing hydration level.…”

“…Metal nanoclusters are ultrasmall luminescent centers that have discrete energy structures and exhibit size-dependent luminescence properties like quantum dots, among which the Ag NCs have been mostly studied. Upon UV excitation, the Ag NCs show tunable emission throughout the visible region with quantum yields approaching that of commercial phosphors. , The extraordinary luminescence properties of Ag NCs have attracted considerable research interest in many applications, such as imaging, illumination, detection, and biosensors. − Due to the high surface energy, Ag NCs are easily agglomerated into large silver nanoparticles (Ag NPs), resulting in quenched radiative emission. Therefore, it is crucial to confine the ultrasmall Ag NCs in suitable matrixes to limit their overgrowth and meanwhile provide appropriate crystal fields and site symmetry for tunable emission.…”

“…The basic units of the zeolite framework are constructed by connecting the corner-sharing TO 4 tetrahedra (T = Si, Al) of silicon–oxygen tetrahedra [SiO 4 ] 4– and aluminum–oxygen tetrahedra [AlO 4 ] 5– , often the Al 3+ can be substituted by P 3+ or Ga 3+ . − Owing to the superior advantages of efficient ions exchange capacity that allows the easily intake of Ag + , uniform molecular-sized cage structure that benefits the confinement of functional clusters, and high chemical and thermal stability that enable long lasing applications, the zeolites have been considered as desirable hosts for Ag NCs. − It has been reported that after thermal treatment, X-ray or electron beam irradiation on the Ag + -exchanged zeolites, Ag 2 + , Ag 3 n + , Ag 4 n + , and Ag 6 n + may be formed in the D6r or sodalite cages of zeolites. ,,, The formation efficiency and luminescence properties of Ag NCs can be manipulated through the selection of zeolite topological structure, framework, and extra-framework modifications, adjustment of silver loading degree, and thermal treatment. For example, recent studies indicated that the Ag NCs confined inside Linda A (LTA) and FAU zeolites exhibit blue-shifted emission when the radius of the extra-framework cation is reduced, and show significantly higher emission intensity in the Li + -type zeolites than in the Na + /K + /Ca 2+ -type zeolites; − the peak emission wavelength of Ag NCs can be turned over 40 nm through desilication or dealumination treatment on the host FAU zeolites before Ag + -exchange; the Ag NCs in Li-LTA zeolites exhibit red-shifted emission from blue to green-yellow with increasing hydration level. , …”