The generation of laser induced periodic surface structures (LIPSS) represents a particular kind of top down approach to produce highly reproducible nano/microstructures without going for any sophisticated process of lithography. This method is much simpler and cost effective but comes along with a certain degree of randomness in the geometry of the structures. In many cases, however, the aspect of randomness can be well tolerated. Therefore, LIPSS and related structures have been used for various applications such as surface enhanced Raman spectroscopy (SERS), colorization of metals, enhancement of emission efficiency of incandescent light sources, tribology, efficient photoelectron emission, realization of efficient photovoltaic cell and LED, optical memory and controlling hydrophobic properties of materials etc.^[1–13]. Dyes are the waste products of various industries like textile, leather, plastic, pharmaceutical, or paper. These dyes generally cause serious water pollution. Currently, heterogeneous ultraviolet (UV) photocatalytic dye decomposition with semiconductor nanostructures is one of the best established methods for removal of these dyes^{[14, 15]}. Here we report on the use of LIPSS generated on Si (SiLIPSS) for enhanced photocatalytic dye decomposition activity. Discussion is given for the cause of this enhancement and its significance with respect to the already reported methods. A comparison of the performance of the grown samples was carried out with respect to the standard reference material Pilkington Activ^TM.

Photocatalytically active films were prepared by deposition of TiO₂ onto the SiLIPSS using a magnetron sputtering system. The SiLIPSS were prepared with linearly polarized femtosecond (fs) laser pulses at a central wavelength of 800 nm emitted from an amplified Ti:sapphire laser system (Spitfire, Spectra Physics). The duration and repetition rate of the laser pulses were 120 fs and 1 kHz, respectively. The samples were fixed on a computer controlled two-axis translation stage (MFA-CC stages, Newport) in the focal plane of a cylindrical lens (focal length f = 100 mm) for the laser processing. The produced line focus had an extension of 100 × 12 000 μm². The samples were moved by raster scanning with a velocity of 0.5 mm/s across this line focus to generate LIPSS in an area of 2 × 2 cm². All the experiments were carried out in air environment. The LIPSS were characterized using a field-emission scanning electron microscope (FESEM, JEOL JSM-6400F). The FESEM image of the SiLIPSS is shown in Fig. 1(a).

These SiLIPSS were generated under optimized working conditions (pulse energy 0.3 mJ, velocity 0.5 mm/s). The orientation of SiLIPSS were found to be perpendicular to the polarization of the laser beam (shown as double sided arrow in Fig. 1(a)). In order to quantitatively estimate the period of these SiLIPSS the two-dimensional fast Fourier transform (2D-FFT) analysis of their FESEM image was done by using ImageJ software. From this 2D-FFT analysis (Fig. 1(b)), the period was estimated to be around 500 nm.

The details of the growth conditions for the TiO₂ thin films were discussed in a previous publication^[16]. Briefly summarized, TiO₂ thin films were grown by sputtering material from a Ti target in two different oxygen atmospheres. In the first case, the oxygen pressure was chosen to be high (3 × 10^–2 mbar) in the sputtering chamber. In the second case, the oxygen pressure was one order of magnitude lower (2.1 × 10^–3 mbar). These types of thin films will be referred to as high pressure grown TiO₂ thin films (HPTiO₂) and low pressure grown TiO₂ thin films (LPTiO₂), respectively. The deposition time of the thin films was 90 min for the HPTiO₂ and 70 min for the LPTiO₂ to obtain nearly comparable film thicknesses of about 180 nm and 145 nm, respectively. To enable a comparison to a reference surface, TiO₂ thin films were also deposited onto Si containing no LIPSS (NoLIPSS). The details about these samples are given in Table 1.

A reactor consisting of a UV light source (Philips CLEO Professional 140W-R, UVA-power app. 21 mW/cm²) with a mounted-in magnetic stirrer system was used for photocatalytic induced dye decomposition studies. For this aqueous solution of methylene blue (MB) dye (0.01 mmol/L) was taken as a test medium. The coated samples were put in a solution filled beaker (30 ml) and closed by a quartz plate to prevent solution evaporation. The distance between the test sample and the light source was about 32 cm. Prior to light irradiation, the solution was kept for 30 min in dark condition for complete adsorption-desorption of dyes on the catalyst surface. Thereafter, the samples were irradiated with UV light for the photocatalytic reaction. The whole photoreaction (dark as well light exposure condition) was carried out under continuous slow stirring. To ensure reproducibility and comparability, the experimental conditions were kept the same for all the samples. The MB dye decomposition was measured over a whole irradiation time of four hours by using UV-visible absorption spectroscopy of the solution (Perkin Elmer Lambda 2) comparing the intensities at a wavelength of 664 nm. For comparison, the photocatalysis at the same conditions was carried out with standard Pilkington Activ^TM reference glass samples.

The influence of oxygen partial pressure during film deposition on the film structure was investigated by X-ray diffraction (XRD) measurements (Siemens D5000 diffractometer with Cu Kα radiation and a detector scan with an incidence angle of 1°)^[17]. These investigations revealed a strong dependence of the appearance of the TiO₂ phase on oxygen partial pressure. So, the XRD investigations of the TiO₂ layers deposited at low sputtering pressure identified both rutile and anatase phases but layers deposited at pressures above 4 × 10^–3 mbar showed only anatase reflection peaks. Until the critical value of about 4 × 10^–3 mbar, where rutile and anatase are formed, the XRD peak-intensities for both TiO₂ modifications decreased. But above this value, the intensity of the anatase (101) peak increased strongly with higher sputtering pressure.

Fig. 2 shows the UV photocatalytic degradation of MB dye (plot of C/C₀ versus time) with high pressure grown TiO₂ thin films on Si surface containg LIPSS (HPTiO₂-SiLIPSS) and no LIPSS (HPTiO₂-NoLIPSS). Here, C₀ is the initial concentration of aqueous solution of MB dye and C is its concentration after a certain time of photocatalytic reaction. As can be seen from this figure, the MB dye reduces to about 8% of its initial concentration after 4 h of photocatalytic activity for HPTiO₂-SiLIPSS sample whereas it is 30% for HPTiO₂-NoLIPSS samples. The corresponding values of MB dye degradation efficiency [(C₀ − C)/C₀ × 100] are 92% and 70% for HPTiO₂-SiLIPSS and HPTiO₂-NoLIPSS, respectively. This clearly indicates that the SiLIPSS played an important role in the enhancement of the photocatalytic dye decomposition activity.

Fig. 3 shows the C/C₀ versus time plot for MB dye decomposition with low pressure grown TiO₂ thin films on Si surface containing LIPSS (LPTiO₂-SiLIPSS) and no LIPSS (LPTiO₂-NoLIPSS).

As can be seen from this figure, the MB dye content reduces to about 25% of its initial concentration after 4 h of photocatalytic activity with the LPTiO₂-SiLIPSS sample whereas with LPTiO₂-NoLIPSS it is 70%. In this case, the dye decomposition efficiencies are found to be 75% and 30% respectively for LPTiO₂-SiLIPSS and LPTiO₂-NoLIPSS respectively. So, this again indicates that the SiLIPSS played an important role in enhancement of the photocatalytic dye decomposition activity.

The reaction rate constant (k) is a kind of figure of merit parameter in order to quantitatively express the photocatalytic activity of the materials. In order to know further details of the enhancement of the photocatalytic activity due to LIPSS we estimated the reaction rate constants of the various samples by following the standard procedure. In brief, to estimate this parameter first a graph between ln(C₀/C) versus photocatalysis time (t) was drawn. Thereafter considering the pseudo-first kinetics, a linear fit line obeying the equation ln(C₀/C) = kt was drawn for this graph. The slope of this linear fit line provided us the reaction constant k.

The ln(C₀/C) versus photocatalysis time graph for high pressure grown TiO₂ thin films, HPTiO₂-NoLIPSS and HPTiO₂-SiLIPSS are shown in Figs. 4(a) and 4(b), respectively. The quadratic symbols in these figures represent the experimental data and solid line represents their linear fit lines. The reaction rate constants obtained from the slope of the linear fit line of these graphs are given in Table 2. As can be seen from this table, the reaction rate constants of sample HPTiO₂-NoLIPSS and HPTiO₂-SiLIPSS are 0.26 and 0.54 h^–1, respectively.

From the ratio of these reaction rate constants, the enhancement factor of photocatalytic dye decomposition activity of high pressure grown TiO₂ thin film in presence of LIPSS is found to be 2.1 (Table 2).

Similarly, the ln(C₀/C) versus time graph for low pressure grown TiO₂ thin films, LPTiO₂-NoLIPSS and LPTiO₂-SiLIPSS are shown in Figs. 5(a) and 5(b), respectively.

The reaction rate constants obtained from the slope of the linear fit line of these graphs are given in Table 3.

As can be seen from this table, the reaction rate constants of sample LPTiO₂-NoLIPSS and HPTiO₂-SiLIPSS are 0.09 and 0.31 h^–1, respectively. From the ratio of these reaction rate constant the enhancement factor of photocatalytic dye decomposition activity of low pressure grown TiO₂ thin film in presence of LIPSS is found to be 3.3 (Table 3).

It is to note here that the enhancement factor in low pressure TiO₂ thin film is greater than high pressure TiO₂ thin film. The inferior photocatalytic property of LPTiO₂-NoLIPSS is probably the main cause for this.

The LIPSS generally increase roughness and surface area. This phenomenon has already been exploited in frame of applications like increase of friction, wettability etc.^{[4, 18]}. Kuladeep et al., have demonstrated that the SiLIPSS generated by fs laser can offer rms roughness as high as 50 nm^[19]. In the present case, the similar increase of roughness was observed in AFM study of the LIPSS (Fig. 6). The cross-sectional analysis (Fig. 6(b)) of AFM image (Fig. 6(a)) indicates that LIPSS contains the crests and valleys like structures. These crests and valleys increase the surface area of the deposited thin films. It can be assumed that this factor is responsible for an enhanced photocatalytic activity in our case.

For a reliable evaluation of the enhancement of photocatalytic behavior caused by the newly grown photocatalytic thin film, a comparison to a reference surface is required. One of the best established methods is the calibration with a standard reference thin film like Pilkington Activ^TM. The result of such a comparative study is shown in Fig. 7. The reference photocatalytic activity is estimated from the dye decomposition efficiency of various samples after 4 h of photocatalytic activity. As can be seen from this figure, the low pressure grown TiO₂ thin film on Si containing no LIPSS (LPTiO₂-NoLIPSS sample) shows much inferior photocatalytic activity than Pilkington Activ^TM however when it is grown on LIPSS (LPTiO₂-SiLIPSS) it can show 1.25 times higher photocatalytic activity than this reference material. The high pressure grown TiO₂ thin film on Si containing no LIPSS (HPTiO₂-NoLIPSS sample) on the other hand shows almost similar photocatalytic activity as Pilkington Activ^TM reference however when it is grown on LIPSS (HPTiO₂-SiLIPSS) it can reach about 1.5 times higher photocatalytic activity compared to the reference material.

It has to be noted that a number of methods like doping^[20–26], selective-fluorination, etching processes^[27], ion implantation^[28], laser annealing^[29], surface modifications^{[30, 31]} etc. have been used for enhancing the photocatalytic activity of TiO₂ thin films. All these methods involve the co-material growth-step or after-material-growth steps to enhance the photocatalytic activity. Here, co-material growth-step means that the processing step is done with thin film during its growth and after-material-growth step means that the processing step is done with the thin film after the growth of material. Unlike these methods, the enhancement method reported in this work involves the pre-material growth-step on the substrate i.e. though this method the surface area of the substrate is increased prior to the deposition of the thin film. So, all the aforementioned methods can also be used in conjunction with the method reported in this work. Thereby our method can open a possibility of enhancement of photocatalytic activity by exploiting multiple mechanisms. Lastly it has to be noted that recently some pre-material growth-step on substrate like in situ polymerization^[32], plasma surface modification^[33], multi-heterojunction^[34] have been demonstrated to enhance the photocatalytic activity of TiO₂ thin film. In terms of simplicity, reproducibility and controllability our method can be much better than the in situ polymerization and plasma surface modification whereas multi-heterojunction can be used in conjunction with our method thereby increasing the photocatalytic activity drastically.

To conclude, LIPSS on Si were generated by using a femtosecond laser. The typical period of the LIPSS was 500 nm and their orientation was found to appear perpendicular to the laser polarization. Photocatalytic substrates were prepared by depositing TiO₂ thin films on Si surface areas with and without LIPSS. For this purpose, sputtering from a Ti target into two types of oxygen atmospheres were applied. In the first case, the oxygen pressure in the sputtering chamber was chosen to be high (3 × 10^–3 mbar) whereas it was by one order of magnitude lower in the second case (2.1 × 10^–3 mbar). In the dye decomposition study with MB dye it was found that the presence of LIPSS leads to an enhancement in the photocatalytic reaction rate constant by 2.1 and 3.3 times for high pressure and low pressure grown TiO₂ thin films, respectively. The enhancement of surface area in presence of LIPSS is believed to be responsible for the enhanced photocatalytic activity in the present case. Moreover, in a comparative study it was found that the TiO₂ thin films grown on LIPSS under low and high oxygen pressure shows a 1.25 and 1.5 times higher photocatalytic activity than the standard reference sample Pilkington Activ^TM .

Deutsche Forschungsgemeinschaft (DFG), Germany (Grant number GR 1782/12) and Science and Engineering Research Board (SERB), India (Grant number EMR/2015/001175) are acknowledged by R.G. and S.K.D respectively for the financial support. Ms. M. Tischer is acknowledged for technical discussion and support.