# Photocatalytic Lithography
## 3 Photocatalytic Lithography  
To transfer a pattern from a mask to a surface of choice, conventional photolithography makes use of photoresists. These are photosensitive materials, typically based on polymers. When the photoresist is exposed to light of the proper wavelength (typically in the UV region), its chemical structure changes, becoming more soluble or, alternatively, insoluble (crosslinked). The first kind is called positive photoresist, while the second kind is a negative photoresist. With positive photoresists, light strategically hits the material in the areas that one wishes to remove. The exposed areas are  
Figure 1: Primary steps in the mechanism of photocatalysis: (**1**) formation of charge carriers by photon absorption; (**2**) charge carrier recombination; (**3**) trapping of a conduction-band electron at a Ti(IV) site to yield Ti(III); (**4**) trapping of a valence-band hole at a superficial Ti–OH group; (**5**) initiation of an oxidative pathway by a valence-band hole; (**6**) initiation of a reductive pathway by a conduction-band electron; (**7**) further thermal (e.g., hydrolysis or active oxygen species-mediated reactions) and photocatalytic reactions to yield mineralization products. Adapted with permission from [8], American Chemical Society, 2014.  
then washed away with a solvent, leaving the underlying material exposed. In this way, an identical copy of the photomask pattern is reproduced on the surface. The behavior of negative photoresists, when exposed to UV radiation, is exactly the opposite--instead of becoming more soluble, negative photoresists become extremely difficult to dissolve. As a result, the UV-exposed negative resist remains on the surface while the solvent removes it from the unexposed areas, leaving on the surface an inverse replica of the photomask pattern.  
Photocatalytic lithography exploits reactive oxygen species (ROS) to selectively modify and/or degrade organic and inorganic species on a target surface. These ROS are produced on the surface of a photoactive material upon irradiation with light of the proper wavelength. The activity of ROS as lithographic tools is not limited to the degradation of organic molecules but extends to the oxidation and reduction of inorganic (e.g., metallic) species as well. Since ROS can diffuse in the surrounding atmosphere for up to hundreds of micrometers, it is possible to perform photocatalytic lithography not only with a \"direct\" approach, in which the patterning is achieved directly on the photoactive surface, but also with a \"remote\" approach, which is virtually surface-independent.  
Compared to conventional photolithography, photocatalytic lithography does not require the use of polymeric photoresists, reducing the number of patterning steps and, most importantly, preventing residues from being left on the surface. This latter feature can be very important for some applications, such as the patterning of graphene and graphene-based structures. Moreover, the resolution that can be achieved is comparable, if not better, to that of conventional photolithography. Indeed, photocatalytic lithography promises to achieve feature resolution smaller than the wavelength of excitation.