# Photocatalytic Lithography
## 4 Applications of Direct and Remote Photocatalytic Lithography
### Patterning of Polymer Brushes
Key to an efficient implementation of this approach to photocatalytic lithography was the availability of a reliable procedure, based on electric field-assisted sol-gel deposition, to coat different substrates (transparent or not: glass, quartz, silicon) with a smooth, highly photoactive anatase film [41]. With such films a remarkably good resolution could be achieved, which would have been impossible to obtain with rougher coatings, such as those obtained by the sintering of pre-formed titania nanoparticles. Moreover, thanks to the high photoactivity of these films,lithographic patterns could be generated with a relatively safe 360 nm UV light and with exposition times in the order of minutes. Using these films as photoactive substrates, both positive and negative patterns of polymer brushes could be made. Positive patterns were obtained by the photocatalytic degradation of an initiator grafted on the titanium dioxide substrate, and subsequent amplification of the pattern by surface-initiated polymerization. Using a square grid as a photomask, a polymer replica of the grid was obtained. Conversely, when the photoactive substrate was first modified with a hydrophobic silane and then subjected to photocatalysis with the same kind of photomask, the exposed surfaces could be refilled with the polymerization initiator. After surface-initiated polymerization, the inverse replica, i.e., the negative of the mask (in this case, an array of squares), was obtained. The time required for the photocatalytic degradation of the silanes used was in the order of minutes. In both cases, a resolution down to 5 um could be easily achieved (Figure 4). Figure 4: Positive and negative patterns of polymer brushes obtained by direct photocatalytic lithography: (**a**) schematic representation of the experimental procedures; (**b**) Atomic force microscopy (AFM) images of the corresponding positive and negative patterns of polymer brushes. Reproduced with permission from [42], [PERSON], 2016. Figure 3: Schematic representation of (**a**) polymer brushes obtained by surface-initiated polymerization and (**b**) the process to obtain patterned polymer brushes. The yellow spheres represent a surface-anchoring functional group for the initiator (red spheres) and the blue spheres the monomer(s). If the polymerization is controlled, the polymer chains are capped with an initiation site. The direct approach is, however, restricted to the use of surfaces made of (or coated with) photocatalytic materials. Remote photocatalysis has the advantage of being applicable to any kind of surface which could be functionalized with a suitable polymerization initiator. To explore remote photocatalytic lithography, a titanium oxide film about 100 nm thick was deposited on a transparent glass slide and positioned with the photoactive side facing the polymerization initiator-functionalized surface. A gap of around 100 \(\upmu\)m was kept between the substrate and the photoactive film. A simple photomask (a TEM grid) was placed between the grafted surface and the TiO\({}_{2}\) surface. According to the previous literature, H\({}_{2}\)O\({}_{2}\) molecules are formed at the TiO\({}_{2}\) surface and migrate to the initiator-grafted surface, where, thanks again to UV radiation, they are converted into hydroxyl radicals OH\({}^{\bullet}\). These species, in turn, promptly react with the organic component of the initiator molecules, causing their degradation. The use of glass as a substrate for titania reflected in longer exposition times compared to the direct approach (irradiation energy and power being constant) because of the inner-filter effect of glass. Nevertheless, a very good lithographic resolution was achieved, of a quality comparable to that obtained with direct photocatalysis (Figure 5) [39]. Moreover, since the surfaces exposed to the photocatalytic treatment were virtually free from organic contamination, this made it possible to obtain \"clean\" patterns of polymer brushes on silicon substrates to study their electrochemical behavior [43].