The integration of metal atoms and clusters in well-defined dielectric cavities is a powerful strategy to impart new properties to them that depend on the size and geometry of the confined space as well as on metal-host electrostatic interactions. Here, we unravel the dependence of the electronic properties of metal clusters on space confinement by studying the ionization potential of silver clusters embedded in four different zeolite environments over a range of silver concentrations. Extensive characterization reveals a strong influence of silver loading and host environment on the cluster ionization potential, which is also correlated to the cluster's optical and structural properties. Through fine-tuning of the zeolite host environment, we demonstrate photoluminescence quantum yields approaching unity. This work extends our understanding of structure-property relationships of small metal clusters and applies this understanding to develop highly photoluminescent materials with potential applications in optoelectronics and bioimaging.

Silver (Ag) clusters confined in matrices possess remarkable luminescence properties, but little is known about their structural and electronic properties. We characterized the bright green luminescence of Ag clusters confined in partially exchanged Ag-Linde Type A (LTA) zeolites by means of a combination of x-ray excited optical luminescence-extended x-ray absorption fine structure, time-dependent-density functional theory calculations, and time-resolved spectroscopy. A mixture of tetrahedral Ag(HO) ( = 2 and = 4) clusters occupies the center of a fraction of the sodalite cages. Their optical properties originate from a confined two-electron superatom quantum system with hybridized Ag and water O orbitals delocalized over the cluster. Upon excitation, one electron of the s-type highest occupied molecular orbital is promoted to the p-type lowest unoccupied molecular orbitals and relaxes through enhanced intersystem crossing into long-lived triplet states.

The structure and bonding of a series of gold clusters and gold nanomaterials stabilized by ligands or confined within nanoporous alumina have been investigated using EXAFS, XANES, and WAXS. Two gold clusters stabilized by two different ligands, Au55(PPh3)12Cl6 and Au55(T8 -OSS−SH)12Cl6, were confirmed to be of face-centered cubic structure type with metal−metal distances of 2.785 and 2.794 Å, respectively, shorter than in bulk gold. Colloidal gold of 180 Å diameter stabilized by sulfonated phosphine ligands had structural and electronic properties very similar to those of bulk gold but smaller Debye−Waller factors. The cluster Au55(PPh3)12Cl6 adsorbed into nanoporous alumina membrane was found to retain its integrity inside the membrane but with slightly longer Au−Au bonds due to some aggregation. The same cluster thermally transformed into colloidal gold within the alumina membrane was found to be almost identical structurally and electronically to the bulk. Gold nanowires electrochemically grown within the nanoporous alumina were found to be composed on average of 120 Å diameter crystallites. These have the same structure as the bulk, but with smaller Debye−Waller factors, indicating either a better crystallinity or that the gold atoms are more tightly held than in the bulk. The difference of area method L3 − kL2 was used to quantify the d orbital occupancy. The two ligand-stabilized Au55 clusters both had a smaller value (2.7) than the bulk material (4.1). The nanomaterials inside the membrane also showed smaller L3 − kL2 values. The geometrical and electronic structures of these gold materials show a very clear pattern of buildup as the number of gold atoms increases from Au55 clusters through Au colloids and nanowires to the bulk metal.

The hydrothermal crystallization of CoAPO-5 molecular sieves has been studied using time-resolved in-situ SAXS/WAXS, UV-vis, Raman, and XAS. Data collected during heating to 180 degrees C allowed the observation of different steps occurring during the transformation of the amorphous gel into a crystalline material from a macroscopic and atomic perspective. Raman spectroscopy detected the initial formation of Al-O-P bonds, whereas SAXS showed that these gel particles had a broad size distribution ranging from ca. 7 to 20 nm before crystallization began. WAXS showed that this crystallization was sharp and occurred at around 160 degrees C. Analysis of the crystallization kinetics suggested a one-dimensional growth process. XAS showed that Co(2+) transformed via a two-stage process during heating involving (i) a gradual transformation of octahedral coordination into tetrahedral coordination before the appearance of Bragg peaks corresponding to AFI, suggesting progressive incorporation of Co(2+) into the poorly ordered Al-O-P network up to ca. 150 degrees C, and (ii) a rapid transformation of remaining octahedral Co(2+) at the onset of crystallization. Co(2+) was observed to retard crystallization of AFI but provided valuable information regarding the synthesis process by acting as an internal probe. A three-stage, one-dimensional crystallization mechanism is proposed: (i) an initial reaction between aluminum and phosphate units forming a primary amorphous phase, (ii) progressive condensation of linear Al-O-P chains forming a poorly ordered structure separated by template molecules up to ca. 155 degrees C, and (iii) rapid internal reorganization of the aluminophosphate network leading to crystallization of the AFI crystal structure.

The synthesis and characterization of LTA(Li)–Ag zeolites with water-responsive photoluminescence properties and high external quantum efficiencies is described in this study.

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The effects of the addition of manganese to a series of TiO 2 -supported cobalt Fischer-Tropsch (FT) catalysts prepared by different methods were studied by a combination of X-ray diffraction (XRD), temperatureprogrammed reduction (TPR), transmission electron microscopy (TEM), and in situ X-ray absorption fine structure (XAFS) spectroscopy at the Co and Mn K-edges. After calcination, the catalysts were generally composed of large Co 3 O 4 clusters in the range 15-35 nm and a MnO 2 -type phase, which existed either dispersed on the TiO 2 surface or covering the Co 3 O 4 particles. Manganese was also found to coexist with the Co 3 O 4 in the form of Co 3-x Mn x O 4 solutions, as revealed by XRD and XAFS. Characterization of the catalysts after H 2 reduction at 350°C by XAFS and TEM showed mostly the formation of very small Co 0 particles (around 2-6 nm), indicating that the cobalt phase tends to redisperse during the reduction process from Co 3 O 4 to Co 0 . The presence of manganese was found to hamper the cobalt reducibility, with this effect being more severe when Co 3-x Mn x O 4 solutions were initially present in the catalyst precursors. Moreover, the presence of manganese generally led to the formation of larger cobalt agglomerates (∼8-15 nm) upon reduction, probably as a consequence of the decrease in cobalt reducibility. The XAFS results revealed that all reduced catalysts contained manganese entirely in a Mn 2+ state, and two well-distinguished compounds could be identified: (1) a highly dispersed Ti 2 MnO 4 -type phase located at the TiO 2 surface and (2) a less dispersed MnO phase being in the proximity of the cobalt particles. Furthermore, the MnO was also found to exist partially mixed with a CoO phase in the form of rock-salt Mn 1-x Co x O-type solid solutions. The existence of the later solutions was further confirmed by scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) for a Mn-rich sample. Finally, the cobalt active site composition in the catalysts after reduction at 300 and 350°C was linked to the catalytic performances obtained under reaction conditions of 220°C, 1 bar, and H 2 /CO ) 2. The catalysts with larger Co 0 particles (∼ >5 nm) and lower Co reduction extents displayed a higher intrinsic hydrogenation activity and a longer catalyst lifetime. Interestingly, the MnO and Mn 1-x Co x O species effectively promoted these larger Co 0 particles by increasing the C 5+ selectivity and decreasing the CH 4 production, while they did not significantly influence the selectivity of the catalysts containing very small Co 0 particles.

STEM-EELS and EXAFS have been used to investigate the location and electronic state of Mn as promoter in TiO2-supported cobalt Fischer-Tropsch catalysts prepared by two different procedures. It was found that the extent of interaction between Mn and the active Co phase as well as the level of Mn dispersion over the TiO2 surface largely determine the enhancement of the selectivity in the Fischer-Tropsch synthesis at pressures of 1 bar.