Separation and Recombination of Photocarriers from Color Centers and Optically Silent Trap States from 100 to 450 K: The Halide Double Photochromic Perovskite Cs2AgBiBr6.

Compared to lead-based solar cells whose power conversion efficiency is 25.2%, the highest power conversion efficiency of a halide double Cs2AgBiBr6-based perovskite solar cell is less than 3%. It was therefore relevant to unravel the inherent reason(s) for such a low efficiency in the latter that may be related to trapping/detrapping of photocarriers. Accordingly, photocoloration and photobleaching phenomena occurring in the Cs2AgBiBr6 photochromic perovskite were examined from 100 to 450 K by diffuse reflectance spectroscopy (DRS). The separation and recombination of photogenerated charge carriers implicated both color centers and optically silent trap states within the bandgap. The processes were reversible subsequent to heating after illumination at 100 K but were mostly irreversible at 290 K. DRS spectral and kinetic measurements at T = 100-450 K were carried out after visible light illumination that further revealed the nature of the various charge carrier traps in Cs2AgBiBr6. Results confirmed the separation of photogenerated electrons and holes, with formation of the color centers identified as deep electron traps. Three different photoinduced color centers were responsible for the absorption bands observed at 1.78 (ab1), 1.39 (ab2), and 1.10 eV (ab3) at 100 K. Annealing of these electron-type color centers occurred in the temperature range of 100-450 K via recombination with holes in the valence band following their thermal release from the several hole traps. Application of a first-order kinetic model to the thermoprogrammed annealing (TPA) of the color centers' spectra yielded estimates of the activation energies of hole detrapping and lifetimes of trapped holes at room temperature. The irreversibility of photocoloration at 290 K was caused by the formation of new deep hole trap states.

Water Will Be the Coal of the Future-The Untamed Dream of Jules Verne for a Solar Fuel.

This article evokes the futuristic visions of two giants, one a writer, Jules Verne, who foresaw water as the coal of the future, and the other a scientist, Giacomo Ciamician, who foresaw the utilization of solar energy as an energy source with which to drive photochemical and photocatalytic reactions for the betterment of mankind. Specifically, we examine briefly the early work of the 1960s and 1970s on the photosplitting of free water and water adsorbed on solid supports, based mostly on metal oxides, from which both hydrogen and oxygen evolve in the expected stoichiometric ratio of 2 to 1. The two oil crises of the 1970s (1973 and 1979) spurred the interest of researchers from various disciplines (photochemistry, photo-catalysis and photoelectrochemistry) in search of a Holy Grail photocatalyst, process, or strategy to achieve efficient water splitting so as to provide an energy source alternative to fossil fuels. Some approaches to the photosplitting of water adsorbed on solid insulators (high bandgap materials; Ebg ≥ 5 eV) and semiconductor photocatalysts (metal oxides) are described from which we deduce that metal oxides with bandgap energies around 5 eV (e.g., ZrO2) are more promising materials to achieve significant water splitting on the basis of quantum yields than narrower bandgap photocatalysts (e.g., TiO2; Ebg ≈ 3.0-3.2 eV), which tend to be relatively inactive by comparison. Although proof of concept of the photosplitting of water has been demonstrated repeatedly in the last four decades, much remains to be done to find the Holy Grail photocatalyst and/or strategy to achieve significant yields of hydrogen.

DFT model cluster studies of O2 adsorption on hydrogenated titania sub-nanoparticles.

In the present paper, we examine the general applicability of different TiO2 model clusters to study of local chemical events on TiO2 sub-nanoparticles. Our previous DFT study of TiO2 activation through H adsorption and following deactivation by O2 adsorption using small amorphous Ti8O16 cluster were complemented by examination of rutile-type and spherical Ti15O30 nanoclusters. The obtained results were thoroughly compared with experimental data and results of related computational studies using other TiO2 models including periodic structures. It turned out that all considered model TiO2 model systems provide qualitatively similar results. It was shown that atomic hydrogen is adsorbed with negligible activation energy on surface O atoms, which is accompanied by the appearance of reduced Ti(3+) species and corresponding localized band gap 3d-Ti states. Oxygen molecule is adsorbed on Ti(3+) sites spontaneously forming molecular O2 (-) species by capturing an extra electron of Ti(3+) ion, which results in disappearance of Ti(3+) species and corresponding band gap states. Calculated g-tensor values of Ti(3+) and O2 (-) species agree well with the results of EPR studies and do not depend on the used TiO2 model cluster. Additionally, it was shown that the various cluster calculations provide results comparable with the calculations of periodic structures with respect to the modeling of chemical processes under study. As a whole, the present study approves the validity of molecular cluster approach to study of local chemical events on TiO2 sub-nanoparticles.

Visible light absorption by various titanium dioxide specimens.

A set of heat-induced and photoinduced absorption spectra of various compositions of Degussa P25 TiO2 and different polymers has been examined. The spectra are described as the sum of overlapping absorption bands (ABs) with maxima at 2.90 eV (427 nm, AB1), 2.55 eV (486 nm, AB2), and 2.05 eV (604 nm, AB3); the spectra correlate entirely with the experimentally observed absorption spectra after the reduction of TiO2. Absorption spectra of visible-light-active TiO2 photocatalysts reported recently in the literature have also been analyzed. Relatively narrow absorption spectra are very similar and independent of the method of photocatalyst preparation. The average absorption spectrum can be described reasonably well by the sum of the two absorption bands AB1 and AB2. It is argued that visible light activation of TiO2 specimens (anion-doped or otherwise) implicates defects associated with oxygen vacancies that give rise to color centers displaying these absorption bands and not to a narrowing of the original band gap of TiO2 (EBG approximately 3.2 eV, anatase) through mixing of dopant and oxygen states, as has been suggested recently in the literature.