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 Raman 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 band at wavelengths < 300 nm and bandgap of 4.2 eV) changes, and the appearance of new bands 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 I₂/I₃ ^- concentration in the electrolyte solution on the temporal profile of the photovoltage clearly shows that I₂/I₃ ^- 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 μA (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.