**3.2 Periodic surface texturing of thin films**

Periodic texturing also called patterning on thin film surfaces is a potential technique for the fabrication of photonic nanostructures for various optical applications [50]. It is worthy to note that the improved solar light absorption with high surface to volume ratio and enhanced light harvesting efficiency of the MOS thin films can be enhanced by periodic nanostructuring [6, 50]. There are several surface texturing techniques like conventional photolithography, nano-imprint lithography, electron-beam lithography, laser patterning, dip-pen lithography, reactive ion etching etc. available in the literatures [6] but these techniques are very costly, complicated, time consuming and also have several limitations owing to the nature of the component materials [6, 12]. Hence, versatile, simple and cost effective unconventional soft lithography is used now-days as an alternative to these conventional lithography techniques. This technique is largely used to generate periodic structures on metal oxide/mixed metal oxide including polymer based thin film surfaces [6, 12, 13]. In the next sub-sections, a special emphasis is given on solgel based soft lithography technique to perform periodic surface texturing on mixed metal oxide thin films.

#### *3.2.1 Importance of periodic surface texturing*

It is no-doubt that periodic surface texturing (pattering) is used to improve the functional properties of metal oxide thin films. The main objective of surface texturing is to effectively manage the incident light into thin film matrix. Thin films with different periodic surface structures are capable to enhance light absorption *via* light scattering and anti-reflective effects [50]. Improved light absorption ability of nanostructure thin films can enhance the performance of optoelectronic devices [6, 50]. Generally, light management in the nanostructure device based on two simple strategies- (a) anti-reflection and (b) enhancement in light absorption [50]. The enhanced light absorption occurs in surface textured thin films through multi-internal reflection which increases the light propagation length into the absorbing layer. Theoretically, it is possible to improve the light absorption up to an enhancement factor of 4n2 (Lambertian limit) where 'n' is denoted as refractive index of the material [50]. It is found that ordered three-dimensional nanostructured materials reach or exceed the Lambertian limit. Thus, it is established that the light absorption not only depends on the materials properties but also on the geometry of materials [50].

#### *3.2.2 Periodic surface texturing techniques*

As already stated in the previous sub-sections, periodic surface texturing of thin films is generally performed by conventional photolithography. Beside photolithography technique, several other techniques like nano-imprint lithography, electron-beam lithography, laser patterning, dip-pen lithography, reactive ion etching etc. are also used for the surface texturing [6, 13]. In soft lithography, one of the nonconventional lithography techniques, a soft organic material is mostly used to produce patterned structures without using light or any other high energy particles [6]. The main feature of this technique is to use a surface patterned elastomeric stamp which is generally made of polydimethyl siloxane (PDMS). This PDMS stamp can be used either as a mold to impart the patterns through physical confinement of a liquid precursor that dries to build the patterned film or as a stamp to directly transfers the precursor material to the substrate [51]. This technique mainly consists of different types such as replica molding (REM), microcontact printing (μCP), micromolding in capillaries (MIMIC), microtransfer molding (μTM). By using soft lithography techniques, it is possible to fabricate periodic surface textured films with features, ≥30 nm [51].

#### *3.2.3 Applications*

Periodic surface textured metal oxide based thin films have diverse applications in various fields like self-cleaning, photovoltaics, catalysis, energy conversion and storage, electronic devices, sensor and solar water splitting [52]. Now-a-days, surface patterned metal oxide thin films are also largely used in photovoltaic cells as active layers, photocatalysis and photoanode in photoelectrochemical (PEC) cells [4–6, 52]. Nanostructuring on the thin film surface increases the active surface area as well as photon capturing ability which are beneficial for the enhancement of photocatalytic and PEC performances [6, 13]. Thus, the PEC performance of MOS thin films can be improved by periodic surface texturing. It is worthy to note that overall PEC performance for solar water splitting depends on three fundamental factors- (i) absorption efficiency (*ƞ*abs), (ii) charge separation efficiency (*ƞ*sep) and (iii) charge transfer efficiency (*ƞ*trans). The performance for solar water splitting is expressed as ƞabs × ƞsep × ƞtrans [53]. It is very challenging to get high value of the product of ƞabs and ƞsep (i.e. ƞabs × ƞsep) because these are coupled with each other [53]. By increasing the active layer thickness, it is possible to increase ƞabs value but it reduces the ƞsep value.

As a result, the product of ƞabs and ƞsep decreases. However, the nanostructuring on MOS metal oxide thin film surfaces can mitigate the problem. Several nanostructured mixed metal oxide thin films including IO based nanostructured thin films are reported for improving PEC performances [4, 6, 53].
