All molecules and materials have orbitals (of different energy) that are either filled or empty with electrons (e-). Energy, such as light, can be used to move electrons from one orbital to another.
After absorbing a UV photon, TiO2 exists in an excited state. The excited state has lots of potential energy (like a bike at the top of a hill), and can transfer that energy through the electron and hole to molecules in the air like oxygen and water. The amount of potential energy TiO2 has to perform these reactions is roughly equal to the energy of the light absorbed. UV photons are high in energy allowing TiO2 to perform many useful reactions.
The reaction of methane (CH4) with hydroxyl radicals is summarized above. Note, oxygen (not superoxide) plays a key role as an oxidant in the reaction. Oxygen is a slow oxidant, however, and the role TiO2 plays is to speed up the overall reaction between pollutants and oxygen using sunlight. This is called photocatalysis. TiO2 is not consumed in the process. Water acts as a co-catalyst, cycling between water to hydroxyl radical and back to water.
Learn more about how trees and TiO2 clean the air below.
How do trees clean the air? They do it in two ways. First, trees perform photosynthesis which turns water into oxygen and carbon dioxide or CO2 into cellulose (what plants are made of). This removes CO2 from the air, the most abundant greenhouse gas in the atmosphere. But how do trees remove the air pollution that's toxic like NOx, smog, ozone, and sulfur dioxide? Plants absorb these pollutants as nutrients to help them grow. Plants need more than just carbon from CO2 to grow, and this is where fertilizers comes in and why 1% of the world's total energy consumption is devoted to making ammonia (NH3) for agriculture. These nutrients are essential for plants and it's hard for them to get them otherwise from soil or the air. Even though 78% of our air is composed of nitrogen (N2), this molecule is almost completely inert and can't be used by most plants and trees. Removing NOx from the air, trees can use the nitrogen atom to build amino acids vital to the plant's growth. In this process, plants also remove one of the most toxic gases in our environment. NOx is responsible for numerous health problems, contributes to increased ground level ozone concentrations, and is a greenhouse gas that's almost 300 times worse than CO2. Every one molecule of NOx emitted by power plants and automobiles is the equivalent of adding 300 molecules of CO2 to the atmosphere.
TiO2 Clothing: Technology
TiO2 or “titanium (Ti) dioxide (O2)” is a mineral formed in a ratio of two oxygen atoms to every one titanium atom. It has many useful properties, with air-cleaning catalysis being one of them. Driven by sunlight and water, TiO2 is able to clean the air indefinitely, breaking down pollution into harmless gasses and salts. In this process pollution is removed from the environment forever.
How it works
What makes TiO2 a great catalyst for air cleaning? First, a catalyst or “photocatalyst” is a material that uses light to perform a chemical reaction that otherwise would not occur–or would occur very slowly. On their own, sunlight and oxygen can destroy air pollution, but the reaction is very slow and inefficient. TiO2 makes the air cleaning process billions of times faster.
How does it do this? It helps start the reaction. By removing one hydrogen atom from water (H2O), TiO2 is able to start a cascade of reactions that lead to a pollutant's--like methane--break down into water and carbon dioxide (CO2). After TiO2 removes a hydrogen atom from water, the water becomes a hydroxyl radical that is very unstable and will rapidly remove a hydrogen atom from nearby pollutants such as methane (CH4) to reform water.
Methane is unstable with only three hydrogen atoms and will rapidly react with oxygen in the air causing it to eventually decompose. This works for larger molecules too that can be comprised of 1,000s of atoms, TiO2 will them break apart, little by little, and turn them into smaller molecules like water and CO2. In the process, TiO2 stays the same, waiting for another photon of light and a water molecule to start the process of air cleaning all over again. Is this a good thing? Yes! For every molecule of methane removed from the atmosphere, an equivalent of 28-36 molecules of CO2 are removed. Why? Because methane has a nearly 30 times worse global warming potential (GWP) than CO2. The small amount of CO2 made during TiO2 photocatalysis is offset by the much greater GWP of the pollutant.
TiO2 nanoparticles are ultra small particles roughly 10-20 nm in diameter. That is, they are 10,000 times thinner than a human hair! Particles on this small of a dimension have an extremely high surface area to mass ratio allowing each particle to do more with less. The goal of nanotechnology is to do more chemistry with less material (that's sustainability). TiO2 nanoparticles absorb UV light, making them useful as additives in sunscreens for UV protection. Here, photoactive TiO2 is designed differently allowing the absorbed UV energy to be transferred to water molecules to create hydroxyl radicals that will clean the air. In sunscreen, TiO2 absorbs UV and that energy is turned into heat.
The atmosphere’s detergent
Hydroxyl radicals (HO*) are oxidized water molecules (H2O). Photo-excited TiO2 nanoparticles remove an electron and proton (H+) from water to form this powerful oxidant. The electron and proton are then transferred to oxygen (O2) in the air to provide another useful oxidant: superoxide (O2H). These two ingredients are used to remove organic and inorganic material from our environment, thereby cleaning the air. This process involves the breaking down of the material into harmless gasses. Similar to how a burning candle is reduced to smoke and air, TiO2 breaks down harmful pollution into less harmful gasses like CO2, water, and inorganic salts. The hydroxyl radical and superoxide ion initiate many of the reactions with pollutants that cause them to break down.
The driving force
Sunlight comes in many forms. High energy light, also known as ultraviolet or UV, comes in three forms: A, B, and C. UV-C is absorbed mostly by the ozone layer, where it produces hydroxyl radicals to help clean air pollution from natural sources like wildfires and volcanic eruptions. UV-A and UV-B light is transmitted through the atmosphere and reaches the earth’s surface. UV-A and UV-B are known for their destructive power, causing skin cancer and fading colors on wood, fabric, and other materials. TiO2 uses UV-A and UV-B sunlight to create a large photo-voltage capable of oxidizing water. Silicon solar panels use sunlight to create a photo-voltage to generate electricity, TiO2 uses it to remove air pollution.