Search

Thin silver chloride layers evolve oxygen under UV/vis illumination in aqueous solution under appropriate conditions. AgCl deposited on a conducting support photocatalyzes the oxidation of water to O2 in the presence of a small excess of silver ions in solution. The light sensitivity in the visible part of the spectrum is due to self-sensitization caused by reduced silver species. Anodic polarization reoxidizes the reduced silver species. Considerable improvement of sensitivity has been observed with AgBr sensitized AgCl photoanodes. To test its water splitting capability, the AgCl photoanode was combined with hydrogen-producing semiconductors, such as a platinized silicon solar cell and platinized p-GaInP2. AgCl layers were employed in the anodic part and the H2 evolving semiconductors in the cathodic part of a photoelectrochemical cell for light-assisted water splitting. The AgCl electrodes were characterized with surface photovoltage spectroscopy (SPS), which identified the transition from the valence band to silver cluster levels and by in-situ UV/vis diffuse reflectance spectroscopy, which detects the reduced and reoxidized silver species.

Silver sulfide clusters are synthesized in the cavities of sodium and calcium zeolite A microcrystals by exposing the activated Ag⁺-loaded zeolite to H2S. The growth of the silver sulfide clusters during rehydration of the samples is investigated by means of diffuse reflectance spectroscopy. Monomers of Ag2S are formed at low Ag⁺-loading through reaction of AgSH molecules, yielding colorless composites, which exhibit a characteristic blue-green photoluminescence. Yellow colored samples showing an orange-red luminescence with an average decay time of 81 μs (−160 °C) are obtained at medium silver sulfide content. Further increasing the silver sulfide loading and therefore the cluster size causes a bathochromic shift of this emission which is accompanied by a shortening of the luminescence lifetime. The samples generally exhibit large Stokes-shifts, which can be attributed to HOMO−LUMO transitions with small oscillator strengths. The Ag2S−zeolite host−guest system constitutes a three-dimensional array of silver sulfide clusters. The experimental data and results from quantum chemical calculations suggest that the optical absorption and luminescence properties of this material are mainly due to the presence of isolated silver sulfide clusters inside the zeolite cavities. The characteristics of the composites are thereby to a certain extent influenced by the cocations. Most remarkably, stronger luminescence, visible at room temperature, could be observed for silver sulfide clusters in calcium zeolite A compared to clusters in sodium zeolite A.

The orientation of the S1 ← S0 π,π* transition dipole moments of oxonine (Ox⁺), pyronine (Py⁺), and POPOP (5,5‘-diphenyl-2,2‘-p-phenylenebis(oxazole)) in the channels of zeolite L crystals was investigated by means of fluorescence microscopy and single-crystal imaging. Qualitative observations led to the result that the transition moment of POPOP is aligned along the c-axis of the hexagonal crystals whereas the fluorescence of Ox⁺ and Py⁺ is not. More detailed investigations on Ox⁺ showed a cone-shaped distribution of the transition moments with a half-cone angle of 72°. The orientation of the transition dipole moment for all of these molecules is parallel to the molecules' long axis. We found by means of space-filling arguments that POPOP, the van der Waals length of which is about 21 Å, can only be aligned along the channel axis. This is in full agreement with the observed fluorescence anisotropy. For Ox⁺ and Py⁺, geometrical arguments based on the zeolite L structure give room for only two possible arrangements of the molecules' long axis:  a half cone angle of up to 40° for Ox⁺ and up to 30° for Py⁺, and an angle of about 90° for both of them with respect to the c-axis of zeolite L. The surprising discrepancy between geometrical considerations and the results of the fluorescence measurements can be explained by assuming that Ox⁺ and Py⁺ are exposed to a considerable anisotropic electrical field in the zeolite channels.

Single-crystal or very compact AgCl materials are hardly light sensitive. In presence of adsorbed Ag⁺ ions, AgCl precipitates with their correspondingly large surface area, however, lead to the discovery of photography on paper by Henry Fox Talbot in 1834 and were recently found to act as a catalyst for sustained photocatalytic oxidation of water to O2. How large must a cluster be so that the inner atoms can be regarded as bulk? How do the surface atoms differ from inner ones? What is the difference between atoms at the corner, the edge, and the plane? What happens upon adsorbing water molecules and solvated Ag⁺ ions on the AgCl cluster surface? Of what type are the first electronic transitions of such clusters, how large is their oscillator strength, and how are they influenced by adsorbed silver cations? Cubic (AgCl)n clusters with n = 4, 32, 108, and 256 have been studied by means of MO calculations and compared with the AgCl molecule and with the infinite AgCl crystal. The Ag−Cl distance was found to increase by 0.35 Å from AgCl to (AgCl)4 and by 0.13 Å to (AgCl)32, but then the changes become small, 0.02 Å from (AgCl)108 to (AgCl)256, despite the fact that the latter still contains 58% surface atoms. The HOMO is made up of Cl lone pairs. It changes little from AgCl to (AgCl)4, then increases smoothly until no significant change is observed after (AgCl)108. The lowest unoccupied orbitals are of the 5s(Ag) type and can be identified as surface state levels (SURS) mainly localized at the corners. The next higher levels extend over the whole cluster. They correlate with the lower edge of the conduction band of the crystal. The charges of the innermost (AgCl)108 species are almost the same as those of the innermost (AgCl)256. These results lead to the conclusion that the (AgCl)108 is sufficiently large for studying the influence of adsorption of an H2O and of Ag⁺(H2O)2. The largest stabilization of H2O on (AgCl)108 is observed when it is coordinated to Ag⁺ at a corner site, which is slightly favored with respect to an Ag⁺ site at the plane. Water coordinated to Ag⁺ in the plane and on the edge has only minor influence on the SURS and no influence on the HOMO region. However, coordination at the corner shifts the SURS by about 0.5 eV to higher energy. Although the [Ag(H2O)n]⁺ (n = 2, 4, 6) species have been investigated, the most direct way to study the interaction of solvated silver ions with an (AgCl)n cluster is to choose [Ag(H2O)2]⁺. We distinguish between a silver site, a chloride site, an interstitial site, and points in between. The position with the Ag⁺ of the aquocomplex directly on top of an Ag⁺ of the cluster was found to be the most stable. The frontier orbital region of the (AgCl)108 is little affected by the adsorbed aquocomplex. However, the 5s(Ag) level shows a bonding interaction with the surface at the most stable position. It is stabilized by interacting with 5p(Ag) which derives from the cluster LUMO region and lies more than 1 eV below the SURS of (AgCl)108, thus forming a new low-lying surface state. Investigating the frontier orbital electronic dipole-allowed transitions, we found that for (AgCl)108 the HOMO-to-SURS transition is very weak and that transitions to the next higher levels are forbidden. In the case of [Ag(OH2)2]⁺ adsorbed on (AgCl)108 electronic dipole allowed transitions, corresponding to a charge transfer from the 3p(Cl)-type HOMO of the cluster to the 5s(Ag) level of the aquocomplex, were found to be responsible for the increased photochemical activity observed for such systems.

The degree of exchange of methyl viologen (MV²⁺) within the channels of zeolite L microcrystals was determined as a function of the amount of MV²⁺ added to an aqueous zeolite L suspension. Care was taken to remove molecules that have been adsorbed on the outer zeolite surface. Thus, the obtained values for the maximum occupation probability per unit cell are 0.78 for the commercial and 0.85 MV²⁺ for the self-synthesized potassium zeolite L for which the equilibrium constant was found to be in the order of 10⁴ at room temperature. Adsorption isotherms and BET results show that the self-synthesized zeolite has a larger specific surface area as well as a larger and, thus, more accessible pore volume for the MV²⁺ than the commercial sample. IR spectra of very thin layers on ZnSe in high vacuum and Raman spectra at ambient conditions of MV²⁺−L zeolite at different loading levels are presented. They are compared with a MVCl2−KBr pellet and MV²⁺−Y zeolite spectra. The MV²⁺−L zeolite spectra indicate weak interactions between the MV²⁺ and the zeolite framework. They also indicate that the two pyridyl rings of the intercalated MV²⁺ are twisted. It was found that framework vibrations of the zeolite can be used as an internal standard for fast and nondestructive determinations of the MV²⁺ loading. Raman spectra are better suited for this purpose than IR. The reason for this is that the IR intensities of the zeolite framework vibrations at about 1050 cm^-1 are much higher than those for all MV²⁺ modes, while the only strong zeolite framework Raman band at about 500 cm^-1 is narrow and well-isolated and of similar intensity to the relevant MV²⁺ signals at high loading. On the basis of Rietveld refinement of X-ray data and molecular modeling, a model of the MV²⁺ location in the channels of zeolite L is proposed. The MV²⁺ lies along the channel wall, and the angle between the main MV²⁺ axis and the c-axis of the zeolite is 27°.

The stacking of pyronine and oxonine in the channels of zeolite L microcrystals is possibly due to their high affinity for entering the channels and to the narrowness of inside the channels, which prevents the dyes from gliding past each other. This allowed us to invent experiments for observing energy migration in pyronine-loaded zeolite L microcrystals of cylinder morphology. Organic dyes have the tendency to form aggregates at relatively low concentrations which cause fast thermal relaxation of electronic excitation energy. The role of the zeolite is to prevent this aggregation even at very high concentrations and to superimpose a specific organization. Light is absorbed by a pyronine molecule located somewhere in one of the zeolite channels. The excitation energy migrates preferentially in both directions along the axis of the cylinder because of the pronounced anisotropy of the system. It is eventually trapped by an oxonine located at the front or at the back of the microcrystal. This process is called front−back trapping. The electronically excited oxonine then emits the excitation with a quantum yield of approximately one. The pronounced anisotropy of the electronic transition moments of both pyronine and oxonine can be observed in an optical fluorescence microscope by means of a polarizer. Maximum luminescence appears parallel to the longitudinal axis of the cylindrical microcrystals, extinction appears perpendicular to it, and their base always appears dark. We report experimental results for the front−back trapping efficiency of pyronine-loaded zeolite L microcrystals of different average lengths, namely 700, 1100, and 1500 nm, different pyronine occupation probability, ranging from 0.03 to 0.48; and modification at the base with oxonine as luminescent traps. Extremely fast electronic excitation energy migration along the axis of cylindrical crystals has been observed, supported by the increase of the effective excitation lifetime caused by self-absorption and re-emission of the pyronine vertical to the cylinder axis. Effective energy migration lengths of up to 166 nm upon pyronine excitation have been observed, which thus leads to the remarkable properties of this material.

Equilibria in zeolites and other microporous materials are discussed. We present an independent particles in a box approach, which leads to a thermodynamic description of internal occupation equilibria of the type ZXi-1 + ZXi+1 &rlarr2; 2ZXi, where Z denotes the framework of the material and X the particles that can interchange places. The independent particles in a box are defined by considering a crystal consisting of a finite number of unit cells or boxes each of which can be filled with a specific number of particles. All empty sites in a box have equal probability to be occupied, independent of the number of particles present, as long as sites are available. Each time a particle does fall in a box, the probability for a next one to hit this box is reduced by 1 divided by the number of sites available in an empty box. Hence, as soon as a box is filled, the probability for a particle to hit it becomes zero. The maximum number of particles in the system is equal to the maximum number of sites in a box multiplied by the number of boxes. This allows equilibrium constants and the decrease of entropy as a function of the equivalent fraction of exchanging species to be calculated. We show that the plot of the logarithm of the equilibrium constant versus the equivalent fraction of exchanging species is not linear and that the nonlinearity is caused by the decrease of entropy. On the basis of this observation, we suggest the independent particles in a box to be used as a reference for “ideal behavior” and to serve as a reference for determining activity coefficients. The generalization of the theory leads to the independent particles in boxes with different sites. It is discussed in detail with regard to two nonequivalent sites corresponding to the internal equilibria ZX1ρ1X2ρ2 &rlarr2; ZX1ρ+1X2ρ2-1 in which X1 and X2 are the same species but occupy site 1 and 2, respectively, of a box. We show the solution of this problem and explain the distribution of the particles among the different sites as a function of the average exchange degree.

Zeolite L containing the resorufin anion (Res^-) in its anionic framework can be prepared by incorporating the neutral resorufin molecule (ResH) from the gas phase and then exchanging the protons with potassium ions which leads to dark violet colored microcrystals. The reversibility of the deprotonation/protonation reaction of resorufin can be monitored by measuring the UV/vis absorption spectrum of the samples. The absorption spectra of Res^- and ResH and their solvent dependence was interpreted using the EHMO-EDiT theory. While Res^- is strongly fluorescent in solution, the fluorescence is completely quenched when Res^- is located inside the channels of potassium zeolite L. This allows the investigation of the exit kinetics of the resorufin molecules as a function of the size of the solvent molecules which enter the zeolite channels and displace the incorporated resorufin molecules irreversibly. The following series was found for the displacement rate:  water ≫ methanol > ethanol > 1-propanol ≳ 1-butanol. Three cases can be distinguished concerning the mechanism of the displacement process:  In case 1 the solvent molecules can pass a dye molecule inside the channel easily. For this case the exit kinetics can be explained by a homogeneous Markoff chain. In case 2 the solvent molecules are considerably hindered when passing a dye molecule inside the channel, and in case 3 the solvent molecules are too large to pass a dye molecule and, hence, the exit rate of dye molecules is zero. While water corresponds to case 1, 1-butanol can be assigned to case 3 and may therefore be used to remove resorufin molecules or molecules with similar properties from the outer surfaces of zeolite L without displacing the intercalated molecules.

We show that the intercalation of dye molecules, which penetrate the cylinders from the bottom and the top surface, into the linear channels of zeolite L can be observed with the help of a fluorescence microscope. By means of a polarizer, we have proved the alignment of the dye molecules in the channels, because maximum luminescence appears parallel to the longitudinal axis of the microcrystals and extinction perpendicular to it. A simple and elegant experiment for the visual proof of the energy transfer from pyronine to oxonine in zeolite is based on the observation that both dyes are intercalated from an aqueous solution within about the same time. This leads to high dye concentrations in the zeolite and therefore to short distances between the molecules, which enables energy transfer between them. This experiment also allows a simple determination of the shortest distances between neighboring dye molecules along linear channels.

A method for calculating electronic dipole-induced transitions in solids based on extended Hückel tight-binding (EHTB) wave functions is presented. The proposed computational treatment relies on the position formulation for intensity calculations. It is compared with the well-known velocity method that involves differentiation of the wave functions with respect to the electron positions. The described CEDiT (crystal electronic dipole-induced transitions) computations are applied to interpret the low-energy absorption spectrum of polyacetylene and the group VIA transition metal dichalcogenide MoS2. Good agreement between calculation and experimental absorption spectra is obtained. In the case of polyacetylene the first prominent absorption band is due to π* ← π transitions about the Z point of the irreducible Brillouin zone. The high density of states (DOS) encountered at Z can be identified as responsible for the characteristic shape of this band. For MoS2 we plot the oscillator strengths of the first four prominent electronic transitions as a function of the k vector in the irreducible Brillouin zone. The excitonic transitions A and B are due to resonance of two almost degenerate interband absorptions confined to a relatively well-defined region in k space at about kA = (²²/57, ²/57) and kB = (¹/3, ²/57), respectively. The origin in k space of the interband absorptions C and D is also discussed.

Microporous materials containing linear channels running through hexagonal microcrystals allow the formation of very concentrated monomeric and highly anisotropic oriented dye systems that support extremely fast energy migration. Energy migration can be described as a homogeneous Markoffian random walk, in which each energy transfer step is incoherent and occurs from a thermalized initial state. The dyes investigated have an electronic transition dipole moment μS1←S0 which coincides with their long axes. The individual energy transfer steps calculated based on dipole−dipole interactions occur with rate constants of up to 30 ps^-1. This fast energy migration cannot be described by a diffusive process immediately after irradiation but becomes diffusive after several tenths of a picosecond. After this time a constant diffusion coefficient D can be defined with values of up to about 0.3 cm² s^-1 for an optimized system based on, for example, cylindrical zeolite L microcrystals and oxonine. A main part of this study refers to excitation trapping on the surface of cylindrical microcrystals. We distinguish between front trapping (traps positioned on the front of the cylinders), front−back trapping (traps on the front and on the back), coat trapping (traps on the coat), axial trapping (traps located in the central channel), and point trapping (a single trap at the center of the front). In cylindrical microcrystals with a size of 50 nm containing about 33 000 chromophores and complete coverage of the outer surface by traps, a total trapping efficiency of 99.8% can be obtained. The front−back trapping efficiency is 60.4% and the coat trapping efficiency is 39.4%. The front trapping efficiencies reach 99.0% if only the front is covered by traps. In a microcrystal of 37 nm length, still containing 12 600 chromophores, point trapping efficiencies of up to 93.0% have been calculated.

A detailed study of the vibrational structure of the silasesquioxanes H8Si8O12 and H10Si10O15 based on INS, IR, and Raman spectra and on a normal-coordinate analysis is reported. The inelastic neutron scattering (INS) spectrum of crystalline H8Si8O12 is in good agreement with the optical data and allowed the assignment of optically forbidden transitions. The previously published force field, determined from IR and Raman data of H8Si8O12 and D8Si8O12, provided a good fit to the INS frequencies and intensities except in the O−Si−H bending region, which is sensitive to intermolecular interactions. The INS spectrum, a comparison of the Raman spectra of dissolved and solid H8Si8O12, and published diffraction data were used to analyse the influence of crystal packing. A modified force field was developed which is adapted to crystalline H8Si8O12 and distinguishes between O−Si−H bending force constants for the axial and the equatorial hydrogen atoms. Excellent agreement with all spectra was thus obtained. This force field was also applied to the INS spectra of H10Si10O15 and D8Si8O12. Based on a normal-coordinate analysis of the INS data, the total mean-square displacements (msd) of the hydrogen atoms of H8Si8O12 were extracted as well as limits for the total msd of the silicon atoms. The msd's and the neutron diffraction values are in good agreement, showing that no or very little static disorder is present in crystalline H8Si8O12. Based on the msd's, the lowest internal torsional frequency was estimated to be 41 ± 7 cm^-1.

The IR and Raman spectra of the monosubstituted hydrosilasesquioxanes RH7Si8O12, R = Co(CO)4, Ph, CH&dbd;CHPh, and CH2CH2Ph, have been analyzed by spectral correlation and a normal coordinate analysis. They were treated as a superposition of the spectra of the siloxane cage and the substituent. The molecules PhH7Si8O12, (PhCH&dbd;CH)H7Si8O12, and (PhCH2CH2)H7Si8O12 have allowed the investigation of the organic-substituted siloxane cage CH7Si8O12, contributing each a case of an Si−Cphenyl, Si−Cvinyl, and Si−Calkyl bond, respectively. The force constants of the different Si−C bonds were related to the Si−C bond orders and extrapolated for Si−Cacetyl of (PhC&tbd1;CH)H7Si8O12, which is not yet available. The inorganic-substituted hydrosilasesquioxane [Co(CO)4(H7Si8O12)] could be analyzed the same way as the organic-substituted compounds and showed a similar pattern for the siloxane cage vibrations. Although most spectral features of the siloxane cages could be understood assuming a local C3v symmetry, vibrations indicating a lower symmetry occurred in all monosubstituted compounds and could be attributed to vibrational coupling with modes of the substituent. Especially the totally symmetric ring-opening vibration at 456 cm^-1 in H8Si8O12 showed a specific dependence on the different substituents.

The reaction of H2S with activated Ag⁺-loaded zeolite A leads to silver sulfide zeolite A composites. The optical absorption spectra of samples with different loading densities of silver sulfide suggest the formation of small silver sulfide clusters which are stable under ambient conditions. To our knowledge, this is the first report on the preparation and the optical absorption and emission spectra of silver sulfide clusters in the cavities of a zeolite in the size regime below 15 Å. The loading densities of the investigated silver sulfide zeolite A composites are 0.05, 0.10, 0.25, 0.50, 1.00, 1.25, 1.50, 1.75, and 2.00 silver ions per pseudo unit cell of zeolite A. The samples are colorless (low loading density) or yellow-green (high loading density). The luminescence is blue-green for samples with low silver sulfide content, yellow or orange at medium silver sulfide content, and red at high silver sulfide content.