Macroscopic properties of semiconductor nanoparticle networks in functional devices strongly depend on the electronic structure of the material. Analytical methods allowing for the characterization of the electronic structure in situ, i.e., in the presence of an application-relevant medium, are therefore highly desirable. Here, we present the first spectral data obtained under Fermi level control of electrons accumulated in anatase TiO 2 electrodes in the energy range from the MIR to the UV (0.1−3.3 eV). Band gap states were electrochemically populated in mesoporous TiO 2 films in contact with an aqueous electrolyte. The combination of electrochemical and spectroscopic measurements allows us for the first time to determine both the energetic location of the electronic ground states as well as the energies of the associated optical transitions in the energetic range between the fundamental absorption threshold and the onset of lattice absorption. On the basis of our observations, we attribute spectral contributions in the vis/NIR to d−d transitions of Ti 3+ species and a broad MIR absorption, monotonically increasing toward lower wavenumbers, to a quasi-delocalization of electrons. Importantly, signal intensities in the vis/NIR and MIR are linearly correlated. Absorbance and extractable charge show the same exponential dependence on electrode potential. Our results demonstrate that signals in the vis/NIR and MIR are associated with an exponential distribution of band gap states.

The adsorption of several quadrupolar and nonpolar gases on the Metal Organic Framework Cu-BTC has been studied by combining experimental measurements and Monte Carlo simulations. Four main adsorption sites for this structure have been identified: site I close to the copper atoms, site I' in the bigger cavities, site II located in the small octahedral cages, and site III at the windows of the four open faces of the octahedral cage. Our simulations identify the octahedral cages (sites II and III) and the big cages (site I') as the preferred positions for adsorption, while site I, near the copper atoms, remains empty over the entire range of pressures analyzed due to its reduced accessibility. The occupation of the different sites for ethane and propane in Cu-BTC proceeds similarly as for methane, and shows small differences for O2 and N2 that can be attributed to the quadrupole moment of these molecules. Site II is filled predominantly for methane (the nonpolar molecule), whereas for N2, the occupation of II and I' can be considered almost equivalent. The molecular sitting for O2 shows an intermediate behavior between those observed for methane and for N2. The differences between simulated and experimental data at elevated temperatures for propane are tentatively attributed to a reversible change in the lattice parameters of Cu-BTC by dehydration and by temperature, blocking the accessibility to site III and reducing that to site I'. Adsorption parameters of the investigated molecules have been determined from the simulations.

We have extended the random walk numerical simulation (RWNS) method with exponential distribution of trap states so that disordered morphologies of metal-oxide nanostructures are taken into account. By using a stochastic cluster model we generate random packings of nanoparticles with texture parameters (porosity, roughness factor) commonly found in TiO 2 and related nanostructures. We then place electron traps according to two alternative models: in the so-called r 2 model, traps are located on the surface of the nanoparticles, whereas in the r 3 model, the traps are distributed throughout the whole material. RWNS simulations with exponential distribution of trap states are carried out for different porosities and particles sizes. It is observed that the total number of traps in the simulation sample is the key parameter that governs the behavior of the diffusion coefficient. Both models reproduce the experimental dependence on the porosity although only the r 2 model explains the increase of the diffusion coefficient with the particle size. The numerical method utilized here can be considered as a first step toward a full and realistic modelization of electron transport in nanostructured devices.

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The collection efficiency of carriers in solar cells based on nanostructured electrodes is determined for different degrees or morphological one-dimensional order. The transport process is modeled by random walk numerical simulation in a mesoporous electrode that resembles the morphology of nanostructured TiO2 electrodes typically used in dye-sensitized solar cells and related systems. By applying an energy relaxation procedure in the presence of an external potential, a preferential direction is induced in the system. It is found that the partially ordered electrode can almost double the collection efficiency with respect to the disordered electrode. However, this improvement depends strongly on the probability of recombination. For too rapid or too slow recombination, working with partially ordered electrodes will not be beneficial. The computational method utilized here makes it possible to relate the charge collection efficiency with morphology. The collection efficiency is found to reach very rapidly a saturation value, meaning that, in the region of interest, a slight degree of ordering might be sufficient to induce a large improvement in collection efficiency.

18The industry of phosphoric acid produces a calcium-rich by-product known as

Here we show the suitability of nanoindentation to study in detail the micromechanical response of silica colloidal crystals (CCs). The sensitivity to displacements smaller than the submicrometer spheres size, even resolving discrete events and superficial features, revealed particulate features with analogies to atomic crystals. Significant robustness, long-range structural deformation, and large energy dissipation were found. Easily implemented temperature/rate-dependent nanoindentation quantified the paramount role of adsorbed water endowing silica CCs with properties of wet granular materials like viscoplasticity. A novel “nongranular” CC was fabricated by substituting capillary bridges with silica necks to directly test water-independent mechanical response. Silica CCs, as specific (nanometric, ordered) wet granular assemblies with well-defined configuration, may be useful model systems for granular science and capillary cohesion at the nanoscale.

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