The chemical phenomena: chemiluminescence, fire, and voltaic cells, generate energy with chemical reactions. How the energy is dissipated (light, heat, or electricity, respectively) is different, but they all involve the creation of energy from chemical reactions. The chemical reactions occur because the product(s) of the reaction are lower in energy than the reactant(s). And the difference in energy between the products and reactants is given off in the form of heat, light, or electricity after the reaction.
In the Voltaic cell, electrons are lower in energy when in Cu(0) metal than in Zn(0) metal. Electrons flow from Zn(0) to Cu(2+) because of this energy difference.
In TiO2 photocatalysis, the steps are backwards. The energy--light--is added first. Without light, TiO2 can't perform photocatalysis or chemical reactions. Light energy starts the process by exciting a TiO2 particle creating a photovoltage (voltage created by light). The photovoltage is then used to induce chemical reactions on molecules in close proximity to the particle. The photovoltage is defined as the difference in energy between the electron (e-) and hole (h+) created by the electron moving from the filled to empty TiO2 orbitals.
The photovoltage of TiO2, 3.2 V, is strong enough to induce many chemical reactions. The oxidizing power of TiO2 in this state is ~1.7 V more than the oxidizing power of bleach (1.48 V)! It is strong enough to directly oxidize water creating a free-moving oxidant with an oxidation potential of 2.8 V, the hydroxyl radical (*OH). These short-lived oxidants rapidly react to reform water (H2O).
In the presence of organic pollution, hydroxyl radicals abstract a hydrogen atom to reform water. The loss of a hydrogen atom makes the organic compound significantly more reactive to oxygen (O2) in the air. Oxygen is a strong oxidant (1.23 V), but it is kinetically slow. Hydrogen abstraction with the hydroxyl radical opens up the door to reactions with oxygen that would otherwise not occur or occur very slowly. This is the heart of term, photocatalysis, TiO2 uses light and water to promote reactions between organic pollutants and oxygen.
What makes TiO2 a good photocatalyst? TiO2 is a large band gap semiconductor. Meaning: the difference between its filled and unfilled electronic states are comparatively large at 3.2 V. An example of a small band gap semiconductor is silicon, 1.14 V, and a "semiconductor" with little or no bandgap is copper, 0 V. Copper is a metallic conductor, where the electrons are free to travel between the filled and unfilled states.
Once the pollutants react with oxygen they are turned into less harmful salts or gases. This is how TiO2 cleans the environment. For example, when TiO2 breaks down methane (CH4) into CO2, a significantly worse greenhouse gas is removed from the environment. Methane has a 26 times greater global warming potential than CO2. Removing methane from the air effectively removes 26 molecules of CO2 from the air while producing one molecule of CO2 in return.
To create TiO2's photovoltage, ultraviolet (UV) photons have sufficient energy to promote electrons across TiO2's large 3.2 V band gap. UV light is absorbed by TiO2 with high very efficiency. This makes it a common ingredient in sunscreen as a UV protection agent. The TiO2 used sunscreen, and many other household products, is typically in the less photoactive rutile form and coated with a layer of silica to reduce its photochemical activity.
There are many metal oxides similar to TiO2 (titanium dioxide) that produce photovoltages. Sand for example, silicon dioxide, ......