The electronic structure of Cu+, Ag+, and Au+ zeolites

Zeolite-Encaged Luminescent Silver Nanoclusters

Silver nanoclusters (Ag NCs) are nanoscale aggregates that possess molecular-like discrete energy levels, resulting in electronic configuration-dependent tunable luminescence spanning the entire visible range. Benefiting from the efficient ion exchange capacity, nanometer dimensional cages, and high thermal and chemical stabilities, zeolites have been employed as desirable inorganic matrices to disperse and stabilize Ag NCs. This paper reviewed the recent research progresses on the luminescence properties, spectral manipulation, as well as the theoretical modelling of electronic structure and optical transition of Ag NCs confined inside various zeolites with different topology structures. Furthermore, potential applications of the zeolite-encaged luminescent Ag NCs in lighting, gas monitoring and sensing were presented. This review concludes with a brief comment on the possible future directions in the study of zeolite-encaged luminescent Ag NCs.

Origin of the polychromatic photoluminescence of zeolite confined Ag clusters: temperature- and co-cation-dependent luminescence

Zeolite confined silver clusters (AgCLs) have attracted extensive attention due to their remarkable luminescent properties, but the elucidation of the underlying photophysical processes and especially the excited-state dynamics remains a challenge. Herein, we investigate the bright photoluminescence of AgCLs confined in Linde Type A zeolites (LTA) by systematically varying the temperature (298-77 K) and co-cation composition (Li/Na) and examining their respective influence on the steady-state and time-resolved photoluminescence. The observed polychromatic emission of the tetrahedral Ag4(H2O) n 2+ clusters ranges from orange to violet and three distinct emitting species are identified, corresponding to three long-lived triplet states populated consecutively and separated by a small energy barrier. These long-lived species are at the origin of the polychromatic luminescence with high photoluminescence quantum yields. Furthermore, the Li-content dependence of decay times points to the importance of guest-host-guest interactions in tuning the luminescent properties with a 43% decrease of the dominating decay time by increasing Li content. Based on our findings, a simplified model for the photophysical kinetics is proposed that identifies the excited-state processes. The results outlined here pave the way for a rational design of confined metal clusters in various frames and inspire the specified applications of Ag-zeolites.

Local structure change of luminescent Ag zeolite-A and -X studied by in situ XAFS and IR spectroscopy

The in situ X-ray absorption fine structure (XAFS) for the structural changes of Ag clusters produced in the cavity of luminescent zeolites by thermal treatment of Ag zeolite-A and Ag zeolite-X has been studied. The following procedures are compared: (i) samples are heated and cooled to room temperature under atmosphere (under air); (ii) samples are heated and cooled to room temperature in a vacuum and then exposed to air. It was confirmed that the Ag clusters were broken when the Ag zeolite was exposed to air for Ag zeolite-X, which complements our previous results for Ag₁₂-A. It is suggested that the deformation of the Ag clusters plays an important role in the generation of a strong photoluminescence band, and Ag clusters may not be direct species producing the strong photoluminescence. The local structure of the Ag ions was found to be slightly different from that of the unheated species. The difference may originate from the formation and breakdown of Ag clusters in the zeolite cavity.

Luminescent Ag+ exchanged SOD zeolites with their potential applications in white LEDs

Ag+ exchanged zeolites (Ag-zeolites) combining the advantages of high stability of microporous zeolites with the unique optical properties of silver clusters are attractive as promising functional materials. In this work, we systematically studied the optical properties of Ag+ exchanged SOD zeolites, obtaining tuned emission from yellow to blue and for the first time acquired white-emitting Ag-zeolite phosphors at a mild temperature (<300 °C). This tunable emission color was found to be caused by lattice shrinkage and lattice expansion of the framework in SOD zeolites. The obtained white emitting phosphors show Commission Internationale de I'Eclairage (CIE) chromaticity coordinates of (0.33, 0.34) under 395 nm excitation. In addition, we fabricated warm white LEDs with these white emitting phosphors, which show CIE coordinates of (0.37, 0.34), with a lower correlated color temperature (CCT) of 4329 K and a higher color rendering index (CRI) of 91.

Noble Metal Particles Confined in Zeolites: Synthesis, Characterization, and Applications

Noble metal nanoparticles or subnanometric particles confined in zeolites, that is, metal@zeolite, represent an important type of functional materials with typical core-shell structure. This type of material is known for decades and recently became a research hotspot due to their emerging applications in various fields. Remarkable achievements are made dealing with the synthesis, characterization, and applications of noble metal particles confined in zeolites. Here, the most representative research progress in metal@zeolites is briefly reviewed, aiming to boost further research on this topic. For the synthesis of metal@zeolites, various strategies, such as direct synthesis from inorganic or ligand-assisted noble metal precursors, multistep postsynthesis encapsulation and ion-exchange followed by reduction, are introduced and compared. For the characterization of metal@zeolites, several most useful techniques, such as electron microscopy, X-ray based spectroscopy, infrared and fluorescence emission spectroscopy, are recommended to check the successful confinement of noble metal particles in zeolite matrix and their unique physiochemical properties. For the applications of metal@zeolites, catalysis and optics are involved with an emphasis on catalytic applications including the size-dependent catalytic properties, the sintering-resistance properties, the substrate shape-selective catalysis, and catalysis modulation by zeolite microenvironment.

Silver Zeolite Composite-Based LEDs: Origin of Electroluminescence and Charge Transport

In this contribution, we report on the first time use of silver-exchanged zeolites embedded in the nonconductive polystyrene (PS) and their use as hybrid emitters in light-emitting diodes (ZEOLEDs). The turn on voltage and EL intensity are strongly dependent on the concentration of metal clusters. It is shown that the key to optimize this technology is improving the zeolite anode contact. Such an optimized device based on cheap abundant materials could provide an alternative for the commercial phosphor converted LEDs. A ZEOLED with a voltage polarity dependent color is demonstrated.

Entropy in multiple equilibria, compounds with different sites

The influence of entropy in multiple chemical equilibria is investigated for systems with different types of sites for the condition that the binding enthalpy of the species is the same within each type of sites and independent of those species that are already bonded. This allows splitting of the free reaction enthalpy into the particle distribution term and all other contributions for each type of sites separately and, hence, to evaluate this entropy contribution to the free reaction enthalpy. The situations for which this applies can be chemically very different, e.g. acid base, ligand exchange, isomerisation, conformational change, rearrangement of a ligand, ion exchange, adsorption of a species on the surface of a particle or a dendrimer, insertion of charged or neutral species into the cavities of a microporous or mesoporous host. We provide physical insight by discussing Xrc1{n1ABn2}Xrc2 systems. The number of coordination sites A and B are n1 and n2, respectively. The indices rc1 = 1, 2,…,n1 and rc2 = 1, 2,…,n2 count the number of X bonded to sites A and sites B, respectively. An important result is that the large number of equilibrium constants needed to describe those situations can be expressed as a function of two constants only. This allows studying systems quantitatively by experimental and theoretical means which otherwise might be difficult to handle. It has also implication for theoretical studies in the sense that it is sufficient to model only two reactions instead of many in order to describe a system. The results remain valid for systems with more than two types of different sites. The description of the entropy driven development of the fractional equilibrium coverage of the sites provides a new tool for understanding adsorption and ion exchange isotherms. The fractional equilibrium coverage of the sites can be described as a linear combination of individual Langmuir isotherms despite of the fact that such a linear combination has never the shape of the original Langmuir isotherm. This is remarkable and very useful. It provides us with new tools for describing and testing isotherms based on well defined, transparent physical ideas. Explicit solution for systems with 2, 3, 4, 5, 6, and 12 coordination sites are reported. Applications to a system with 12 coordination sites serve to illustrate information that can be obtained for complex situations.

Atomic scale reversible opto-structural switching of few atom luminescent silver clusters confined in LTA zeolites

Luminescent silver clusters (AgCLs) stabilized inside partially Ag exchanged Na LTA zeolites show a remarkable reversible on-off switching of their green-yellowish luminescence that is easily tuned by a hydration and dehydration cycle, making them very promising materials for sensing applications. We have used a unique combination of photoluminescence (PL), UV-visible-NIR Diffuse Reflectance (DRS), X-ray absorption fine structure (XAFS), Fourier Transform-Infrared (FTIR) and electron spin resonance (ESR) spectroscopies to unravel the atomic-scale structural changes responsible for the reversible optical behavior of the confined AgCLs in LTA zeolites. Water coordinated, diamagnetic, tetrahedral AgCLs [Ag4(H2O)4]2+ with Ag atoms positioned along the axis of the sodalite six-membered rings are at the origin of the broad and intense green-yellowish luminescence in the hydrated sample. Upon dehydration, luminescent [Ag4(H2O)4]2+ clusters are transformed into non-luminescent (dark), diamagnetic, octahedral AgCLs [Ag6(OF)14]2+ with Ag atoms interacting strongly with zeolite framework oxygen (OF) of the sodalite four-membered rings. This highly responsive on-off switching reveals that besides quantum confinement and molecular-size, coordinated water and framework oxygen ligands strongly affect the organization of AgCLs valence electrons and play a crucial role in the opto-structural properties of AgCLs.

Entropy in multiple equilibria, theory and applications

Entropy controls the dependence of the equilibrium constants in the synthesis of host–guest composites on the occupation rc for channels of different length.

HZSM-5 zeolites containing impurity iron species for the photocatalytic reduction of CO2 with H2O

The impurity [Fe³⁺–O²⁻] species in HZSM-5 zeolite is predominantly tetrahedrally coordinated and is the photoactive centre for photocatalytic CO₂ conversion.

Visible emission from Ag(+) exchanged SOD zeolites

Broad visible emissions dominant at green or red have been observed for the thermally-treated Ag(+) exchanged SOD zeolites, determined by the Ag(+) loading contents and the excitation wavelengths. Contrary to the notable reversible green/red dominant emission evolution in the Ag(+) exchanged LTA zeolites upon hydration/dehydration in air (or water vapor)/vacuum, emission spectra of the Ag(+) exchanged SOD zeolites are insensitive to the environmental change. This is most probably due to the difficult H2O permeation in SOD zeolites in comparison with LTA zeolites. By combining the environment dependent emission spectra of the Ag(+) exchanged LTA and SOD zeolites, we proposed the following emission mechanisms for Ag(+) exchanged LTA and SOD zeolites: the green emission is due to the transition from ligand-to-metal (framework O(2-)→ Ag(+)) charge transfer state to the ground state and the red emission is due to the transition from the metal-metal (Ag(+)-Ag(+)) charge transfer state to the ground state. The insensitive environment dependent emission characteristics of Ag(+) exchanged SOD zeolites may have potential applications as robust phosphors.

Probing zeolites by vibrational spectroscopies

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Brønsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

Silver zeolite-loaded silicone elastomers: a multidisciplinary approach to synthesis and antimicrobial assessment

The fabrication of silicone elastomers containing organo-silane modified Ag-zeolites with potential for applications as antibacterial medical devices is demonstrated.

Behavior of Ag3 clusters inside a nanometer-sized space of ZSM-5 zeolite

We found from DFT calculations that Ag-Ag orbital interactions as well as Ag-O electrostatic interactions determine the structures of three silver cations inside a nanometer-sized cavity of ZSM-5 (Ag(3)-ZSM-5) in lower and higher spin states. Both interactions strongly depend on the number of Al atoms substituted for Si atoms on the ZSM-5 framework (ZSM-5(Al(n))), where n ranges from 1 to 3. In smaller n, stronger Ag-Ag orbital interactions and weaker Ag-O electrostatic interactions operate. Accordingly, there are significant dependencies of the structures of three silver cations on the number of Al atoms. In lower spin states of Ag(3)-ZSM-5(Al(1)) and Ag(3)-ZSM-5(Al(2)), D(3h)-like triangle clusters are contained inside ZSM-5 whereas their higher spin states have triangle clusters distorted significantly from the D(3h) structure. In lower spin states, the totally symmetric orbital consisting of 5s(Ag) orbitals is responsible for cluster formation, whereas in higher spin states occupation of a 5s(Ag)-based orbital with one node results in significant distortion of the triangle clusters. The distortion can be partially understood by analogies to Jahn-Teller distortion of the bare D(3h) Ag(3)(+) cluster in the triplet spin state. When n is 3, we found that three silver cations are isolated in a lower spin state and that a linear cluster consisting of two silver cations is formed in a higher spin state. Thus, we demonstrate from DFT calculations that the number of Al atoms can control the properties of three silver cations inside a ZSM-5 cavity. Since the structural and electronic features of the enclosed silver clusters can link to their catalytic properties, the DFT findings can help us to understand the catalytic activity of Ag-ZSM-5.

Probing the surfaces of heterogeneous catalysts by in situ IR spectroscopy

This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with "in situ" controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H(2), CO, NO and C(2)H(4) are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some "key studies" and trying to evidence similarities and differences among surfaces and probes (572 references).

Synthesis and Luminescence Properties of Ag2S and PbS Clusters in Zeolite A

Zeolite A provides a suitable environment to host Ag2S and PbS clusters, so that spectroscopic investigations on very small particles are possible. The Ag2S monomer is colorless and shows photoluminescence at 490 nm with a lifetime of 300 micros, while the absorption and luminescence of Ag4S2 and larger clusters are red-shifted. The properties of these Ag2S/zeolite A materials depend on the co-cations. Results for Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, and Sr2+ are reported. Excitation energy transfer between Ag2S and Ag4S2 has been studied in materials containing Ca2+ co-cations. PbS particles can be prepared by the same method as Ag2S in the cavities of zeolite A. The PbS monomers obtained are yellow and show photoluminescence at 570 nm, with a lifetime of 700 ns.

Particle Distribution in a Microporous Material: Theoretical Concept

Particle distribution and exchange equilibria in a microporous host material, built up of equivalent particle sites, which are grouped in larger subsets, are described. Simplified descriptions evolve from the exact formulae in the thermodynamic limit. We find that, for example, a single zeolite A nanocrystal consisting of about 1000 pseudo-unit-cells fixes a lower limit for the use of the approximate formula describing particle distribution. A rational selectivity coefficient, which is approximately constant over the whole exchange range, only results if a single zeolite crystal consists of one million pseudo-unit-cells or more, or if a sufficiently large number of smaller crystals is considered. On the basis of the statistical particle distribution model, a closed, simple formula for the ion-exchange isotherm is then derived, which is valid for systems involving a variable number of coupled-exchange reactions. Its similarity to the Langmuir isotherm is discussed. The theory on ion-exchange equilibria is used to derive formulae for the change of free-energy, enthalpy, and entropy occurring in coupled ion-exchange reactions. The findings, though applicable to virtually any particle exchanging system with the structural properties described above, are applied to zeolite A, since this material can be treated as a nearly ideal model. The results derived can straightforwardly be used to evaluate experimental data quantitatively, since the common inequivalence of the host sites can be taken into account.

Particle Distribution in a Microporous Material: Experiments with Zeolite A

The theory on particle distribution and exchange equilibria in a microporous material is applied to experimental ion-exchange data involving zeolite Na-A and zeolite K-A, with silver ions as the exchanging species. The presented method enables direct evaluation of the measured data and consideration of nonequivalent particle sites. The isotherms of the K+ versus Ag+ exchange in zeolite K-A rise much more steeply, at low exchange degrees, than those of the Na+ versus Ag+ exchange in zeolite Na-A. This result implies a different course of the ion-exchange reactions. Spectroscopic measurements on dehydrated, partly silver-exchanged zeolites Na-A and K-A do indeed show that in zeolite Na-A, the Na+ ions occupying four-ring positions are exchanged faster for Ag+ than the Na+ ions occupying eight- and six-ring positions, while in zeolite K-A the exchange does not start with the four-ring ion but with six-ring ions, followed by the four-ring ion. These findings are consistent with the results obtained from evaluation of the ion-exchange data. The resulting thermodynamic quantities significantly differ from published reference values, which we suggest should be revised.