Nathan Lewis says:
TiO2 is an incredibly common cheap chemical. It's in toothpaste. It's the pigment in white paint, these little tiny particles.
He has to make the material cheap and embeddable into house paint and other building material and have it not increase the maintenance costs.
Here is another transcript discussion from Caltech
Here is a recent podcast interview with Dr Lewis
This is part of a larger series of podcasts on nanotechnology