A novel concept for photovoltaic cells:: Clusters of titanium dioxide encapsulated within zeolites as photoactive semiconductors

Discrete clusters of TiO2 (of only a few titanium atoms) are prepared within the internal micropore space of zeolite Y (4.8 wt% Ti loading) and characterized by Roman spectroscopy (rutile- and anatase-like structures), electron microscopy combined with elemental analyses (coincident Si and Ti spatial distribution), and X-ray diffraction (minor zeolite crystallinity decrease). The parent TiO2@Y sample is modified either by adsorption of acid-organic compounds (benzoic and 4-aminobenzoic acids or catechol) or by nitrogen doping. After modification, the optical UV/Vis spectrum of the parent TiO2@Y (onset of the absorption bond at wavelengths < 300 nm and bandgap of 4.2 eV) changes, and the appearance of new bonds expanding to the visible region is observed. In contrast to the inactive zeolite Y matrix, all the zeolite-encapsulated TiO2 species exhibit a photovoltaic response. The influence of the I-2/I-3(-) concentration in the electrolyte solution on the temporal profile of the photovoltage clearly shows that I-2/I-3(-) is also a suitable carrier for the positive charge in zeolite-based photovoltaic devices. The photocurrent response and the efficiency of the photovoltaic cell based on zeolite-encapsulated TiO2 materials depend on the nature of the organic modifier and on the N-doping. The most efficient photovoltaic cell is that based on N-doped TiO2@Y, which exhibits a V-OC (voltage at open circuit) of 270 mV an I-SC of 5.8 mu A (current at short circuit), and a fill factor (FF) of 0.4. Although these values are low compared to current dye-sensitized TiO2 solar cells, our findings could open up a promise for a stimulating research on the photovoltaic activity of zeolite-based host-guest solids.