TiO2 nanoparticles make other chemistries react with light, but is not itself consumed. The physics is based on the semiconductor properties of TiO2: light with enough energy (i.e. UV), an charge pair can be kicked free and propagate through the crystal. When the particle is small enough the charges can reach the surface without recombining.
Having reached the surface the charges transform water and oxygen into free radicals. These radicals in turn react with organic molecules, breaking chemical bonds and causing damage to simple organisms such as bacteria, molds and algae. The effect is similar to that of sunlight exposure, but greatly accelerated by the photocatalyst material.
Key commercial applications of photocatalysis include self-cleaning building facades, air purifying pavements and anti-greening infrastructure.
The benefit of breaking down organic pollution and controlling biological growth is obvious. Doing so without consuming energy or chemicals is even better. But the photocatalyst needs exposure to UV radiation and access tothe pollution to be broken down. Any coating (including biofilm!) thick enough to block light will supress the effect underneath it, and bacteria hiding deep inside dark pores will not be attacked. For many porous surfaces, an extra trick s required for an anti-greening solution.
When most of us think of growth on concrete and stones, we think of molds, moss and algae. However, the problem often starts with cyanobacteria. These single-cell organisms found almost everywhere get their energy from photosynthesis and their nutrients from air, and can thrive almost anywhere with moisture. The big issue is not the bacteria themselves, but that they bind moisture and provide nutrients for larger organisms to survive. The flip side of this is that if we can keep the first tiny organisms in check, we may not have to deal with the larger ones.
There are at least three strategies for keeping growth in check. The oldest method is simply using herbicides and impregnations containing toxins. This works fine until the toxin is transported elsewhere and the cleaning solution becomes an ecologic problem. A second method is to delay the initial growth by making a super-hydrophobic surface that repels splashes and makes the surface dry out quicker. However, once the growth gets a foothold, it can still thrive unless manually cleaned.
The third strategy is photocatalysis, where surfaces use sunlight to keep themselves clean of organic matter. This has the potential to keep going forever, but is weak against fouling that blocks the light or hides in deep pores. Interestingly, combining the latter two strategies becomes very powerful since the strengths of photocatalysis complement those of super-hydrophobicity.
Hydrophobicity and Hydrophilicity
Hydrophobic (‘water-fearing’) and hydrophilic (‘water-loving’) surfaces are defined by the contact angle between the surface and a water droplet, also called wettability. On hydrophilic surfaces droplets spread out and can form films, while on hydrophobic surfaces they form beads. On super-hydrophobic surfaces, water does not stick but rolls off – literally like water off a duck’s back. This is often referred to as the ‘lotus effect’, even though the physical mechanisms are slightly different.
Wettability is a material property that depends on surface chemistry as well as surface structure, and can be affected by weathering, abrasion and reactions on the surface. Additionally, organic matter in the water can reduce surface tension and droplet sizes and increase wetting, and oils might not be repelled at all. So while a super-hybrophobic surface can seem completely impervious to staining, it is only a matter of time (and surface chemistry) before something sticks.
However something lasts: the waterproofing. Hydrophobic pore walls prevent liquid water from penetrating while allowing water vapour to leave. This stops waterborne fouling and nutrients from reaching the deep pores, while also giving a quicker drying surface. Protected from wear by the pores themselves, this effect can last decades.
Air purification and NO2 selectivity
Photocatalysis is not only a good solution for clean surfaces, but also for clean air. It can break down a variety of organic pollutants but is especially suitable for removing NO2 pollution from urban air by turning it into nitrates. This is in fact the standardised test case for evaluating photocatalytic materials. However, even ISO testresults do not tell you everything – and in fact may obscure one critical bit of information: NO2 selectivity.
The selectivity is crucial because the toxic gas is the NO2, but the activtiy is measured by breaking down NO gas, which generates NO2. It is then possible to claim a material as ‘highly active for de-NOx’ while in reality it is a net generator of pollution! The flaw in the test is simply that it allows presenting only one side of the coin, the real problem is agressive marketing of worthless solutions. We would advice anyone to always compare NO% and NOx% rates for active materials: NOx breakers should have a NOx/NO ratio above 80%, while ratio below 60% might be more of a problem than a solution.
We believe that photocatalytic NOx remediation has great potential, and is a great side effect of a self-cleaning façade. But the air you clean only benefits your downwind neighbours until we start seeing community scale projects. The solutions already exists, but as long as there is no clear economic benefit of improving your neighbours air, we believe the way forward is to focus on anti-greening and reduced maintenance and let the environmental benefits remain as a bonus for the common good.
Stability and transparency
Regardless of which benefit or material property we are after, it is important that nanoparticles remain as nano and don’t aggregate or grow. A good indicator of product quality is, perhaps surprisingly, simply looking at it.
True nanoparticles are too small to scatter visible light, while micron sized TiO2 has a very strong white color. If a dispersion is claimed to contain only nanoparticles it should then be transparent, while milky white indicates aggregation has taken place. Of course other components in a mix may add colour (as with Joma Hydro C-44), and a product may have so little particles in it that it may be hard to tell, but a change from transparent to milky is rarely a good sign.
If nanoparticles are sterically stabilised, the polymers can crosslink and lead to viscosity increase. If the dispersion is still transparent it means the nano properties are still intact and a good stirring or shaking may be all that’s needed to bring viscosity back down. Viscosity affect penetration into pores as well as spraying. For electrostatically stabilised particles crosslinking is not an issue, but these products are sensitive to pH change so that f.ex. diluting with water might lead to aggregation.
Is nano safe?
There is occasional public concern over safety of ‘nano’. Of course this depends on which material: there are materials such as TiO2 and carbon black that are approved for food and cosmetics and have been used widely for decades. On the other hand, structures such as quantum dots are typically made with toxic materials. The question then becomes: are nanoparticles inherently dangerous because of their size?
The answer is ‘maybe’. The skin is proven to be a good barrier, and the bioavailability (uptake to the body) of nanoparticles in the intestines is very low. The potential concern is the lungs: even aggregates up to some micrometers are small enough to enter all the way in. Nanoparticles are so small that they do not deposit easily in the lungs or anywhere else. Still, breathing large amounts of any type of fine dust can be dangerous even if the particles themselves are harmless.
Inhaling microdroplets of nano dispersions may be the largest concern. In addition to working as a vehicle for nanoparticles to enter the lungs, the dispersions often have additives or are corrosive – something that could damage your lungs far more than the ‘nano’ property. Breathing masks should always be used when spraying.
So far we have looked only at human toxicity, but there is also the question of enviormental effects and end fate. If released into the environment through spills or wear of surfaces, dispersed nano TiO2 will quickly aggregate with itself or other minerals and eventually become part of the soil like other dust and sand. Aggregated TiO2 loses the nano-properties and essentially becomes white pigment. Dispersing agents or polymer stabilisers on the particles may delay aggregation, but only temporarily.
To sum up: Dry nanopowders should be handled with care, but nanoparticles in dispersions such as Joma products are not inherently dangerous unless you get spray droplets in your lungs. Some dispersions contain other hazardous components, and safety instructions should therefore always be followed.