# Photocatalytic Lithography
## 4 Applications of Direct and Remote Photocatalytic Lithography
### Patterning of Graphene and Graphene Oxide  
Graphene, a two-dimensional (2D) sp\({}^{2}\) carbon network, has attracted widespread attention thanks to its unique electronic, mechanical, and thermal properties [50]. Its remarkable carrier mobility, mechanical flexibility, optical transparency, and chemical stability provide great opportunities for the  
Figure 7: Photocatalytic dissolution and reshaping of triangular silver nanoprisms self-assembled on titanium dioxide upon UV irradiation (**a–c**). The prisms are transformed into spheroids (**d**,**e**), which results in (**f–i**) patterns having different SERS behavior. Reproduced with permission from [49], IOP Publishing, 2017.  
development of high-performance electronic devices [51; 52]. The main challenge of graphene-based electronics arises from the need to minimize chemical contamination, which could otherwise degrade the performance of the device. In this regard, and in contrast with conventional photolithography and electron-beam lithography, the photocatalytic approach is especially advantageous, as it does not introduce any photoresistor other than extraneous chemical species. Graphene can be produced in different ways, such as by means of chemical vapor deposition (CVD) or by the chemical exfoliation of graphite. Usually the first method is preferred to obtain defect-free graphene, while the other method is more suitable for the larger-scale production of graphene flakes. Graphene oxide, which can be conveniently obtained by the chemical oxidation and exfoliation of graphite [53], can be reduced to graphene by thermal methods or by means of reducing agents such as hydrazine, sodium borohydride, or ascorbic acid. The possibility of reducing graphene oxide by means of photocatalysis was reported by [PERSON] et al. in 2010 [54]. In that work, a layer-by-layer (LbL) assembly of poly(diallyl dimethylammonium) (PDDA), graphene oxide (GO), and titanium oxide nanosheets, with the structure (PDDA/GO/PDDA/TiOx)20 (where 20 is the nominal number of layers) was subjected to 30 min irradiation with a 300 W xenon lamp in the presence of a suitable photomask. As a result of such a photo-thermal/catalytic reduction approach, photoconductive patterns of a reduced graphene oxide (RGO)-titania composite were obtained. The authors did not provide a detailed mechanism for the reaction but demonstrated, by means of control experiments, that both photothermal and photocatalytic effects were necessary to achieve the described patterning effect (Figure 8a-c). In 2011, [PERSON] et al. reported the patterning and chemical modification of single- and few-layer graphene by means of remote photocatalytic lithography [55]. The tailoring of graphene--including ribbon cutting, generation of arbitrary patterns, layer-by-layer thinning and localized conversion to insulating graphene oxide--was achieved using a patterned titania-coated quartz photomask. Photocatalytic lithography was performed by putting the patterned titania photomask onto graphene, with the photoactive side facing the graphene, followed by irradiation with \(\leq\)410 nm UV light from a 500 W xenon lamp for 75 to 90 min under ambient conditions. Highly reactive photogenerated hydrogen peroxide and hydroxyl radicals were identified as the patterning and chemical modification agents. Notably, the structure of the photomask was found to have a strong influence on the resolution of the obtained features--when a titania photomask directly patterned on a quartz plate was used, the photodegraded graphene area was found to far exceed the feature size of the mask pattern (e.g., more than 10 \(\upmu\)m). This was attributed to the surface diffusion of H2O2 on the graphene surface and subsequent decomposition into reactive \({}^{\bullet}\)OH. In contrast, a photomask with a layered quartz/titania/patterned chromium film led to a well-controlled patterning, pointing out the importance of a proper design of the photoactive mask.