Photoreactivity depends on the generation of holes (h⁺) and electrons (e^-) within a compound and the ability to utilise these photo-generated species to react with absorbed molecules. In general, holes and electrons are generated when metal oxide semiconductors are exposed to light supplying photon energy that excites an electron from the valence band to the conduction band by absorption. Acceptor and donor molecules can 'trap' the resulting holes and electrons leading to photocatalysis with the hole acting as an oxidising agent and the electron acting as a reducing agent. An ultimate problem in utilising the holes and electrons is the recombination process in which the electron 'falls' back and recombines with the hole. In order to optimise the reaction of photo-generated species with absorbed molecules, metal oxide semiconductors with a high surface area and small particle size are required.

One potential answer is the use of zeolites with semiconducting frameworks. Zeolites are well-defined crystal structures with a regular internal porosity and are commonly used as molecular sieves and ion exchangers. Natural zeolites are composed of aluminium oxide and silicon oxide but these days zeolites can be made of various mixed oxides including semi-conducting titanium oxides.

The discovery of zeolitic microporous titanosilicate (titanium oxide + silicon oxide) materials has attracted much research in their photocatalytic application to assist in environmental clean up by degrading toxic organic compounds into simpler and harmless substances. These titanosilicates have advantages over the conventional titanium dioxide anatase photocatalyst: regular porosity and high surface area as well as the nano-sized titanium oxide already present within the framework. This should enable the optimisation of the reaction of photo-generated holes and electrons with adsorbed molecules. The pores perform as molecular sieves, filtering organic compounds based on their size.

An enormous amount of work has been undertaken on the well-known photocatalyst anatase. Anatase is a bulk titanium oxide structure and this makes the 3-dimensional Ti-O-Ti semiconducting framework. ETS-10, on the other hand, is a titanosilicate material and consists of 1dimensional nanowire Ti-O-Ti chains. Thinking of the Ti-O-Ti chains as 1-D nanowire semiconductors is in fact somewhat misleading due to the existence of line defects which may cause breaks in this chain. Thus, the titanosilicate pharmacosiderite which possesses 3-D nanoclusters of titanium oxide, could bridge the differences between anatase and ETS-10.

ETS-10, a famous member of this titanosilicate family, has shown photoreactivity that is quite different from its anatase counterpart as observed using infrared and EPR (Electron Paramagnetic Resonance) spectroscopy. The titanium oxide semiconductor is present in ETS-10 as 1-dimensional -Ti-O-Ti- chains that run in two dimensions along the a and b direction. The structure itself consists of corner sharing linked octahedral TiO₆ that are surrounded by tetrahedral silicate SiO₄ that construct the three-dimensional pore structure. The framework carries a -2 negative charge and the electrical balance of the structure is preserved by extra framework cations. As mentioned above, the 1D chains of titanium oxides in ETS-10 are fragmented, due to defects in the structure. This does lead to interesting differences in ETS-10's photoreactivity.

When ETS-10 adsorbs organic molecules and is irradiated by UV light, e^- and h⁺ are generated. The organic molecules act as h⁺ scavengers while electrons are trapped by titanium in the framework (Ti⁴⁺ + e^- → Ti³⁺). Interestingly, the electron is transferable to oxygen gas that is introduced afterward (O₂ + e^- → O₂^{- ).}

A further example of micro-porous titanosilicate is pharmacosiderite. This framework is composed of a central cluster of four octaherdral titania units linked by a silicate tetrahedral unit. In this material the semiconducting titanium oxide exists in nanosized 3-D clusters. The pores of this structure are occupied by charge-neutralizing potassium cations and water molecules.

EPR results show that in this case an O^- radical species is formed when the sample is irradiated in UV light in the presence of O₂. A further experiment using O¹⁷ isotope enriched O₂ showed that the O^- radical species was not being formed from the molecular O₂ but must instead be formed within the framework. The question of why the nanoclusters present in pharmacosiderite behave differently from the nanowires in ETS-10 has still to be answered.