Rutile TiO2 Flocculent Ripples with High Antireflectivity and

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Rutile TiO flocculent ripples with high anti-reflectivity and superhydrophobicity on the surface of titanium under 10-ns laser irradiation without focusing Aifei Pan, Wenjun Wang, Xuesong Mei, Kedian Wang, and Xianbin Yang Langmuir, Just Accepted Manuscript • Publication Date (Web): 28 Apr 2017 Downloaded from http://pubs.acs.org on May 6, 2017

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Rutile TiO2 flocculent ripples with high antireflectivity and superhydrophobicity on the surface of titanium under 10-ns laser irradiation without focusing Aifei Pan, Wenjun Wang*, Xuesong Mei, Kedian Wang, Xianbin Yang State Key Laboratory for Manufacturing System Engineering Xi’an Jiaotong University, Xi’an, China 710054 Keywords: Flocculent LIPSSs; Nanosecond laser; Anti-reflectivity; Super-hydrophobicity; Titanium; TiO2

Abstract: We report on the formation of rutile TiO2 flocculent laser-induced periodic surface structures (LIPSSs) with high anti-reflectivity and superhydrophobicity on the surface of titanium under 10-ns 1064-nm laser irradiation without focusing. The center part of Gaussion laser beam is used to deposit flocculent structure and the edge part used to produce LIPSSs. The melt and modification thresholds of titanium were determined first. And then, the melt and modification spot-overlap numbers, severally responsible for formation of flocculent structure and LIPSSs, were introduced. It is found that both the melt and modification spot-overlap numbers increase with an increase in laser fluence and spot-overlap number, contributing to the

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production of flocculent LIPSSs. LIPSSs are obtained with the modification spot-overlap number above 300 and the amount of flocculent structures increases with an increase in the peak laser fluence and spot-overlap number. Then, considering that the fine adjustment of the melt and modification spot-overlop numbers in one-time line scanning is quite difficult, the composite structure, of which both LIPSSs and flocculent structures are distinct, was optimized using laser line scanning for twice. On this basis, a characterization test shows the sample full of the flocculent LIPSSs represents best anti-reflectivity with the value around 10% in the waveband between 260 nm and 2600 nm (advance 5 times in infrared wavelengths compared to the initial titanium surface), and shows the no-stick hydrophobicity with the contact angle of 160° and rolloff angle of 25° because of the pure rutile phase of TiO2.

1. Introduction Titanium-based material has been widely applied in a wide range of areas, including aviation and space, health care, shipbuilding industry1. In recent years, surface modification of titanium, enhancing its performance in the specific application domains, has drawn much attention. In particular, titanium-based superhydrophobic surface can alleviate the bacterial contamination to reduce device-associated infection2. In addition, a low reflective surface of the titanium can be used in defense applications3. Laser-induced periodic surface structures (LIPSSs) have been one of the enabling technologies to modify the surface properties of titanium, for example, antireflective properties, hydrophilic-hydrophobic properties, and other properties can be improved1, 4, 5

. Some experimental studies have demonstrated that, upon ultra-fast laser irradiation, the

surface of titanium with pure LIPSSs had the hydrophobicity with the contact angle below 150°6 and its reflectivity in infrared wavelengths was above 20%3. It is well known that titanium oxide can enhance the laser absorption of titanium7, and pure rutile or anatase phase of TiO2 shows


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superhydrophobicity (the water contact angle greater than 150°)8. Therefore, the surface properties of LIPSSs obtained by ultra-fast laser aren’t excellent because of the low oxidization of titanium and lack of single-phase TiO26. In addition, the conventional methods to obtain the LIPSSs using ultra-fast laser are based on the focusing with a low laser fluence, which shows a low production efficiency3, 6, 9. In the conventional view, lasers with pulse duration in femtosecond (fs) and picosecond (ps) ranges are regarded to be suitable for the LIPSSs due to the extremely short laser-material interaction time10. It is well known that the laser beam obeys the Gaussion distribution both in space and in time11. In this regard, with the reasonable laser fluence, the effective pulse duration of nanosecond laser to ablate material can also be in the range of ps and even fs. It is therefore very likely to obtain LIPSSs under nanosecond laser irradiation with a low laser fluence. For titanium material, LIPSSs along laser polarization direction were obtained by monopulse with a pulse duration of 9 ns, and formation mechanism of this kind of LIPSSs can be explained by laser-induced oxidization12. Based on the reasonable laser parameters, nanosecond laser may also have the ability to produce the LIPSSs with an orientation perpendicular to laser polarization which have been obtained using ultra-short laser13-16. In addition, the surface of titanium can go through more oxidization and crystallization to form a TiO2 film with pure rutile or anatase phase due to the long pulse duration of nanosecond laser. Flocculent structures, based on the irregular permutation but not excessive agglomeration of nanoparticles, were characterized of the high porosity and effective surface. Previous research has demonstrated the antireflective effect can be enhanced by addition of flocculent structures due to the improved effective surface17. Nanoscale vaporized material can be deposited on the surface and forms flocculent structures around the ablation zone with a high laser energy18, 19. In


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this regard, it seems that the LIPSSs and flocculent structures can’t concurrence. In fact, titanium is such an active material that is oxidized when the temperature increases, and a thin titanium oxide layer will be easily formed on the surface of the titanium20, 21. The preformed titanium oxide layer would enhance the absorption of laser energy. In addition, the ablation threshold of titanium oxide is lower than that of titanium7. As a result, the laser fluence which is below the melt threshold of titanium will also be enough to ablate titanium oxide. On this basis, if the low laser fluence has a large distance away from the center to avoid the heat-conduction-induced melt and its effective pulse duration to ablate titanium oxide was in the range of ps and even fs, titanium oxide LIPSSs can be obtained. In addition, the high laser fluence in the center of the Gaussion beam is enough to ablate the titanium and produce nanoparticles which were diffused by heat expansion. In our previous studies, via the inversed pulsed laser deposition, flocculent structures formed by nanoparticles can be obtained in the zone with hundreds of micrometers away from the nanosecond laser irradiation zone22. Consequently, a number of nanoparticles were deposited on the surface of the preformed LIPSSs and shaped flocculent structures. The flocculent structures remained the 2D morphology of LIPSSs, which accompanied with LIPSSs were called as the flocculent LIPSSs here. The composite structures, equipped with the LIPSSsinduced sub-micron textures and large effective surface brought by flocculent structures, may have better properties than that of pure LIPSSs. Nevertheless, there is little research devoted to the production of flocculent LIPSSs using nanosecond laser. In this paper, experimental studies are devoted to preparation of titanium oxide flocculent LIPSSs on the surface of titanium under 10-ns 1064-nm-wavelength laser irradiation with the peak laser fluences (the two times as much as the average laser fluences23). The laser beam is not focused to avoid the heat-conduction-induced melt and improve the processing efficiency. The


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laser melt threshold and modification threshold of titanium were determined first. On this basis, the formation of LIPSSs was discussed, and then, the distinct flocculent LIPSSs were obtained via increase of peak laser fluence and optimization of other process parameters. Finally, the effect of the flocculent LIPSSs on the hydrophobicity and anti-reflectivity of the sample were detected. 2. Research method to produce flocculent LIPSSs The method to produce flocculent LIPSSs is shown in Figure 1. In pulsed laser line scanning, the chronological order of pulses is shown in Figure 1a. This way, in a certain zone, such as the black zone (Figure 1a), the laser fluence as the scanning time goes is shown in Figure 1b. Obviously, the laser fluence is also a Gaussion function of pulse serial number, which is the same to the energy density distribution across the laser beam. The previous pulses (the left of the yellow vertical line, corresponds to high laser fluences) can be used for melt, oxidization and deposition of ablated material, and the posterior pulses with low laser fluences are suitable to produce LIPSSs. In particular, the applied laser beam isn’t focused here to avoid the heatconduction-induced melt, making the preformed LIPSSs survive from in the laser irradiation with a high laser fluence. The details to produce flocculent LIPSSs are shown in Figure 1c. Formation of flocculent structures. A peak laser fluence, which is above the ablation threshold of titanium and enough to produce deposition of ablated material, is applied here. During laser line scanning, in the center of laser beam, the vaporized part leaves the body material, and meanwhile, the surface of titanium goes through melt, polishing and oxidization24. The vaporized material moved in all directions due to the heat-induced expansibility and then deposited with a large area distributions which may larger than the size of laser beam, forming the flocculent structures.


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Formation of LIPSSs. The ablation threshold of the zone after laser irradiation is much lower than the original material due to the heat accumulation. As a result, along with that titaniumoxide-induced absorption enhancement of laser energy, the needed laser fluence for nanoscale ablation of material can be far away from the center of the laser irradiation zone to avoid the heat-conduction-induced melt. It is well known that the laser beam also obeys Gaussion distribution in time. This way, the time duration of a pulse that laser fluence is enough to ablate material can be in dozens of ps, which can be regard as a ultra-short laser processing for production of LIPSSs. Furthermore, the surface of titanium after melt goes through polishing so that the role of the initial surface texture on the LIPSSs can be avoided25. Formation of flocculent LIPSSs. The vaporized material falls back after several nanoseconds as the pulse ends, and deposits on the surface of LIPSSs. When the posterior pulse with the laser fluence only for formation of LIPSSs continued irradiation, the flocculent structures in the convex part of the LIPSSs may be remained, agglomerated or partially melted, and in the concave part of the LIPSSs was ablated, forming the flocculent LIPSSs. In particular, as shown in the Figure 1c, When the posterior pulse with the laser fluence far less than the laser fluence for formation of LIPSSs continued irradiation, the preformed flocculent LIPSSs can also be deposited by the nanoparticles, where the flocculent structures were distinct. It is because that as the high laser fluence was applied, the deposition area of nanoparticle can be much larger than laser beam size via the laser-induced heat expansion, which is very common in the inversed pulsed laser deposition. This way, for a certain pulse, laser fluence in the center of the laser beam is used to produce the flocculent structure covered on the preformed LIPSSs produced by the previous pulses, and


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laser fluence the on the edge of the laser beam is used to produce the new LIPSSs on the surface of the material which has experienced melting.

Figure 1. Method for obtainment of flocculent LIPSSs. Inset “a” is the chronological order of pulses in laser line scanning. Inset “b” shows the irradiating laser fluence as the time goes (also mean the chronological order of pulses) for a certain zone. Inset “c” is the explanation of the formation mechanism of flocculent LIPSSs. From the results in the Ref. 26, the surface of the irradiated material would be smoothed if melt time is long. Therefore, the highest critical value for preparation of LIPSSs using nanosecond laser should be defined as the melt threshold of titanium. In addition, titanium is readily oxidized and the titanium oxide helps to low the ablation threshold of titanium, which should also be taken into consideration. As a result, the surface modification threshold, above which the laser fluence is large enough to make surface oxidize, should also be determined. In this regard, the met threshold of titanium reduces as the pulse number increases, and then more material goes through melting and ablation, which was deposited on the surface of titanium to form flocculent structures, and the laser fluence between melt and modification threshold of titanium can be used to produce LIPSSs.


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In this study, in order to investigate the role of spot-overlap number of the two types of laser fluences on the flocculent structure, the melt and modification effective lengths were introduced. For a certain zone, the irradiating laser fluence with the serial number of laser pulse also obeys Gaussion distribution, which is the same to the energy density distribution across the laser beam. In this regard, the melt effective length means the length that laser fluence above the melt threshold in Gaussion equation of laser beam (see Figure 2). The effect of the foregoing pulses with the laser fluence below melt threshold of titanium will be weakened by the posterior pulses with the laser fluence above melt threshold of titanium. As a result, the modification effective length is defined that the one-side length where the laser fluence below melt threshold but above the modifcation threshold of titanium. Here, we define that the melt spot-overlap number (Nmelt) and modification spot-overlap number (Nmodification) as followed: Nmelt =

Nmodification =

Lmelt × flaser , v Lmodification × flaser v

(1)

,

(2)

where Lmelt and Lmodification are the melt effective length and modfication effective length, respectively. flaser is the repetition rate of laser beam, and v is the scan speed.

Figure 2. Sketch map of the melt and modification effective lengths in laser line scanning 3. Experimental procedure


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Commercial pure titanium, with the titanium content above 99% and the oxygen content below 0.15%, was used here. Before laser irradiation, the titanium was polished by automatic polishing machine and surface roughness of the polished sample was about 100 nm. The polished samples were etched in solution of 100-ml 3% HF for 5 min to remove the texture and native oxide layer. Then, the samples were cleaned in an ultrasonic bath with acetone at room temperature (25°C) for 30 minutes and then rinsed by deionized water. A Nd:VAN nanosecond laser system (InnoLas, Germany), delivering pulses with a duration of 10 ns, was used for the irradiation. The maximum pulse energy of the laser system is 150 mJ. The laser system works at a tunable repetition rate between 1 and 100 Hz, and an optional wavelength of 1064 nm, 532 nm and 266 nm. The beam size reaching the motorized xyz stage is about 3.2 mm, and the energy density distribution across the laser beam is Gaussian (corresponding root-squares was about 95% detected by Laser Beam Profiler (Duma Optronics, BeamOn-VIS), see Figure 3a) with beam quality factor of (M2) ~1.3. As shown in the Figure 3b, in the course of the laser processing, a pyroelectric detector is used to monitor the pulse energy in real time with conjunction with a beam splitter in the primary laser path.


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Figure 3. The energy density distribution across the laser beam and the schematic diagrams of set-up for nanosecond laser irradiation In this study, the applied laser wavelength was 1064 nm and the repetition rate was 100 Hz. The laser beam is not focused and normally incident onto the surface of sample. A motorized xyz stage (Owis, Germany) controlled by computer is used for precise positioning of the samples. All laser ablation experiments were performed in air. The surface morphologies of titanium after laser irradiation were observed using a scanning electron microscope (Hitachi, Japan). XRD (Bruker, Germany) measurements with Cu radiation (λ=1.5406Å, operated at 40 KV, current 40 mA) were carried out at room temperature to determine the crystal phase composition of samples. The surface reflectivity of samples was examined by ultraviolet spectrophotometer (Shimadzu, Japan) to investigate the optical properties of the surface structures. Wettability of the surfaces was tested via an OCA20 Drop Shape Analysis System (Data physics, Germany). In detail, contact angles of deionized water droplet (5 µL of deionized water) at room temperature were obtained by applying the LaplaceYoung fitting algorithm to the images recorded with a CCD camera. 4. Results and discussion 4.1 The melt and surface modification thresholds of titanium In order to determine the melt and modification effective lengths in laser line scanning, the melt and modification thresholds of titanium were determined first. The melt and modification thresholds of titanium, with the peak laser fluences of 2.85 J/cm2 and 2.15 J/cm2 at the variable pulse numbers, are shown in Figure 4. For a single pulse, the melt threshold of titanium turns out to be 1.79 J/cm2 for the peak laser fluence of 2.85 J/cm2 and 1.94 J/cm2 for the peak laser fluence of 2.15 J/cm2. Considering the heat-conduction-induced redistribution of the laser energy, the


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melt and modification thresholds of titanium will increase when the peak laser fluence decreases. In addition, as is the same to the previous studies27, the melt threshold of titanium decreases with the increasing spot number due to the heat accumulation. The modification threshold, which is detected in SEM images, nearly keeps constant with the value about 0.28 and 0.37 J/cm2 for the peak laser fluences of 2.85 J/cm2 and 2.15 J/cm2, respectively. The melt threshold of titanium can be evaluated as given below φmelt ( N ) = φmelt (1) × N S −1 ,

(3)

ln ( N × φ melt ( N ) ) = S × ln ( N ) + ln φ melt (1)  ,

(4)

where N is the spot number, Φmelt(N) is the melt threshold of titanium with the spot number of N. S is accumulation factor, and is fitted to be 0.8928 and 0.8964 for the peak laser fluences of 2.85 J/cm2 and 2.15 J/cm2. All the corresponding root-squares are above 99.9%, which shows a satisfactory between the experimental data and the fitting values.

Figure 4. Melt and modification thresholds of titanium with the increasing spot number for the peak laser fluences of 2.85 J/cm2 and 2.15 J/cm2. The SEM images show the surface


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morphologies with different spot numbers for the peak laser fluence of 2.85 J/cm2.The modification threshold of titanium is detected by the surface white color in SEM images. The modification thresholds of titanium for the peak laser fluences of 2.85 J/cm2 and 2.15 J/cm2 are 0.28 J/cm2 and 0.37 J/cm2, respectively. S is accumulation factor, and is fitted to be 0.8928 and 0.8964 for the peak laser fluences of 2.85 J/cm2 and 2.15 J/cm2. The scale in insets is 1 mm. In order to obtain the melt and modification spot-overlap numbers for the other applied peak laser fluences, the deduced modification threshold and the single-pulse melt threshold of titanium with applied peak laser fluence can be fitted locally by Eq. (5) and Eq. (6) φmodification = a ×φpeak + b φm elt (1) = c × φ peak + d

,

(5)

,

(6)

where a, b, c and d are the fitting coefficients, which was -0.2143, 2.401, -0.1286 and 0.6464 here, respectively. Φpeak is the applied peak laser fluence. In addition, the accumulation factor, which is equal to 1 when the peak laser fluence is zero, can be fitted by Eq. (7) S = a × exp ( b × φ peak ) − a + 1

,

(7)

where a and b, the fitting coefficients, is 0.1095 and -1.362, respectively. On this basis, the melt and modification effective lengths of the other applied laser fluence and spot-overlap number can be deduced. 4.2 Formation of LIPSSs In order to avoid the deep groove was formed on the surface of titanium under the nanosecond laser irradiation, the applied peak laser fluence was below the single pulse ablation threshold of titanium (in the range of 3.0 and 3.3 J/cm2). However, because of the thermal accumulation effect, the ablation threshold of titanium may be below the applied peak laser fluence after several thousand pulses irradiation.


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First, a peak laser fluence of 2.15 J/cm2 was used to produce the LIPSSs. The surface morphologies for the spot-overlap number of 1600, 3200 and 6400 (with the corresponding scan speeds of 0.2, 0.1 and 0.05 mm/s) are shown in Figure 5. It can be noted that for the spot-overlap number of 1600, as shown in the Figure 5 a1 and a2, the surface of titanium is smoothed but no LIPSSs covers on the surface of titanium. When the spot-overlap number reaches 3200, as shown in the Figure 5 b1 and b2, LIPSSs with an orientation perpendicular to laser polarization are formed on the surface of titanium. The period of LIPSSs turns out to be about 971 nm (2/2.059 µm) via Fourier transform. In addition, when the spot-overlap number goes up to 6400, as shown in the Figure 5 c1 and c2, there are some flocculent LIPSSs covering on the surface of titanium. Consequently, the formation of both the LIPSSs and flocculent structures is affected by the spotoverlap number.

Figure 5. LIPSSs on the surface of titanium for the spot-overlap number of 1600, 3200 and 6400 with the peak laser fluence of 2.15 J/cm2. Insets “a2,” “b2” and “c2” are the enlarged views of the insets “a1,” “b1” and “c1,” respectively. The pictures on the top-right side of the insets “b1” and “c1” are the Fourier transform of surface LIPSSs. The laser polarization marked “E” is in the


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vertical direction. The numerals in the left bottom corner of the insets are the deduced melt (the upper one) and modification (the lower one) spot-overlap numbers. For the applied peak laser fluence of 2.15 J/cm2 and spot-overlap number of 1600, 3200, 6400, the melt spot-overlap numbers are 1055, 2190 and 4540 (the melt effective lengths are 2.11, 2.19 and 2.27 mm), and the modification spot-overlap numbers are 225, 400 and 720 (the modification effective lengths are 0.45, 0.40 and 0.36 mm), respectively. It can be seen that, for the modification spot-overlap number of 225, there is still no LIPSSs covering on the surface of titanium. When the modification spot-overlap number reaches 400 and 720, the LIPSSs can be obtained (see Figure 5b1 and c1). As a result, the influencing mechanism of spot-overlap number on the formation of LIPSSs is that modification spot-overlap number rises with increasing spotoverlap number. It is because that the laser fluence for surface modification should be low enough to avoid the melt-induced and heat-conduction-induced surface modification. Therefore, the ablation amount with single pulse for formation of LIPSSs is very little. 4.3 Formation of flocculent LIPSSs The ablated amount of material will increase as the melt spot-overlap number or laser fluence increases, and the deposited zone enlarges with the increasing laser fluence due to the heatinduced expansibility24, benefiting the deposition of ablated material. From the results above, it can be seen that the LIPSSs with many flocculent structures are not obtained because of the low peak laser fluence. Then, the peak laser fluence went up to 2.46 J/cm2 and 2.73 J/cm2 to investigate the evolution of surface morphologies with the increasing spot-overlap number. For the peak laser fluence of 2.46 J/cm2 and the spot-overlap number of 1600, 3200 and 6400, the melt spot-overlap number is 1160, 2400 and 5000, and the modification spot-overlap number is 220, 400 and 730,


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respectively; for the peak laser fluence of 2.73 J/cm2 and the spot-overlap number of 1600, 3200 and 6400, the melt spot-overlap number is 1236, 2547 and 5240, and the modification spotoverlap number is 223, 408 and 742, respectively. Compared to the peak laser fluence of 2.15 J/cm2, both the modification and melt spot-overlap numbers increase for peak laser fluence of 2.46 J/cm2 and 2.73 J/cm2, where the distinction of LIPSSs and the flocculent structures will be improved. The surface morphologies with the peak laser fluences of 2.46 and 2.73 J/cm2 for the spotoverlap number of 1600, 3200 and 6400 are shown in Figure 6. For the spot-overlap number of 1600, as shown in Figure 6a, d, there is no LIPSSs and the flocculent structures on the surface of titanium due to low modification spot-overlap number. When the spot-overlap number reaches 3200, for the peak laser fluence of 2.46 J/cm2, only the LIPSSs are obtained (see Figure 6b); for the peak laser fluence of 2.73 J/cm2, compared to the peak laser fluence of 2.46 J/cm2, the melt spot-overlap number increased (from 2400 to 2547), and there are LIPSSs with a small number of flocculent structures forming on the surface of titanium (see Figure 6c). In addition, the periods of these LIPSSs are about 940 nm for the peak laser fluence of 2.46 J/cm2 and 925 nm for the peak laser fluence of 2.73 J/cm2, respectively. According to the period of LIPSSs is about 970 nm for the peak laser fluence of 2.15 J/cm2, it seems that the periods of LIPSSs decreased with the applied laser fluence but not susceptive to the scanning speed. As the spot-overlap number goes up to 6400, the LIPSSs with flocculent structures cover on the surface of titanium for both the two laser fluences, as shown in Figure 6e, f. In particular, the deposited flocculent structures are too many so that the LIPSSs are less distinct. Therefore, the amount of flocculent structure increases with an increase in the peak laser fluence and spotoverlap number.


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Figure 6. Surface morphologies for the peak laser fluence of 2.46 and 2.73 J/cm2 with the spotoverlap number of 1600, 3200 and 6400. The laser polarization is in the vertical direction. The numerals in the left bottom corner of the insets are the deduced melt (the upper one) and modification (the lower one) spot-overlap numbers. 4.4 The fine adjustment using two-time laser line scanning to obtain distinct flocculent LIPSSs For laser line scanning, when the scan speed decreases, both the melt and modification spotoverlap numbers increase, and therefore, the fine adjustment of the two numbers in one-time line scanning is quite difficult. It is well known that the melt threshold of titanium is reduced with the increasing spot-overlap number because of the thermal incubation effect27. In this regard, the melt threshold of titanium can be adjusted using the two-time laser line scanning with the control of spot-overlap number in the first-time laser line scanning. In other words, the spot-overlap number of the first scanning is adjusted to modify the melt threshold so that the melt and modification spot-overlap number in the second scanning is slightly changed. From the results


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above, it is worth noting that, for the peak laser fluences of 2.15, 2.46 and 2.73 J/cm2, the best LIPSSs and flocculent LIPSSs can be obtained with the spot-overlap number of 3200. In this part, the surface flocculent LIPSSs were optimized using laser line scanning for two times with the fitted spot-overlap number of 3200 in the second laser line scanning. The schematic illustration of two-time laser line scanning and results are shown in Figure 7. It is worth noting that the both of the melt and modification spot-overlap numbers of the second laser line scanning increase with applied laser fluence for all the different spot-overlap numbers of the first-time laser line scanning. As a result, both the LIPSSs and flocculent structures become distinct as the peak laser fluence increases. In this regard, laser fluence is the key factor in preparation of the flocculent LIPSSs. In addition, as the spot-overlap number of the first-time laser line scanning increases, the melt spot-overlap number and modification spot-overlap number increases and decreases slightly, respectively. In details, for the laser fluence of 2.15 J/cm2 (see Figure 7 a-c), there is no flocculent structure formed on the surface of titanium. Furthermore, the flocculent structures can be obtained with the peak laser fluences of 2.46 J/cm2 and 2.73 J/cm2 (see Figure 7 d-i). In particular, when the applied peak laser fluence is 2.73 J/cm2, both of the LIPSSs and flocculent structures are distinct. Then, when the peak laser fluence increases to 2.85 J/cm2, as shown in Figure 6 h-k, the amount of flocculent structure is too much so that the LIPSSs are less distinct, but the LIPSSs do not disappear because of the large modification spot-overlap number. In this paper, the composite structures should retain the LIPSSs-induced sub-micron textures and large effective surface brought by flocculent structures. As a result, the desired flocculent LIPSSs at the peak laser fluence of 2.73 J/cm2 and spot-overlap number of the first-time laser line scanning of 6400.


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Figure 7. The schematic illustration of two-time laser line scanning and surface morphologies for various peak laser fluences after two-time line scanning with the fixed spot-overlap number of 3200 of the second-time laser line scanning. The laser polarization is in the vertical direction. The numerals in the left bottom corner of the insets are the deduced melt (the upper one) and modification (the lower one) spot-overlap numbers. The numerals in the left of all the insets are the applied peak laser fluences. The numerals in the upper of all the insets are the spot-overlap number of the first-time laser line scanning.


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4.5 The effect of flocculent LIPSSs on the Anti-reflection and hydrophobicity of titanium The melt effective length in laser line scanning is about 2.5 mm and the modification effective length is about 0.35 mm for the peak laser fluence of 2.73 J/cm2. In order to make the modification zone after laser irradiation can be overlapped to each other and obtain the flocculent LIPSSs via area scanning, the fixed interval of the two lines is 0.2 mm here. In addition, considering the overlap of modification zone, to make the melt spot-overlap number above 300, in this part, the fixed scan speed applied here is 0.5 mm/s. In addition, the laser scanning area of each sample was 288 mm2 (18mm×16 mm) and the process time of each sample was 48 min here. The experimental study was performed here to characterize surface properties of titanium obtained by various peak laser fluences (2.0, 2.15 and 2.73 J/cm2). The surface energy of titanium after laser processing is not stable and presents superhydrophobicity with the contact angles close to zero (45° for 2.0 J/cm2, 36° for 2.15 J/cm2 and 14° for 2.73 J/cm2). Then, the samples were stored in glass dish in air for this month, and the oxygen content (detected by EDS) increased for all the laser fluences. Therefore, the crystal structure, reflectivity and hydrophobicity of samples are detected after one month. The contact angle of the initial titanium surface is 70°. Figure 8 shows the typical structures for the various laser fluences and their corresponding contact angles (5µL of deionized water). It can be seen that as laser fluence increases, the amount of flocculent structure goes up, causing the increase of contact angle. In detail, for the laser fluences of 2 J/cm2 and 2.15 J/cm2, the flocculent structures incompletely cover on the surface of titanium so that the surfaces show adhesive hydrophobicity. When the laser fluence increases to 2.73 J/cm2, the titanium surface is full of flocculent structures, and the water contact angle is 160° and the water roll-off angle is about 25°. It is because air stored in


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the holes of the flocculent structures, and some air pockets are formed in the interface of the sample and deionized water28. In this regard, the water contact angle increases and roll-off angle decreases due to the reduction of the friction. The data of contact angle measurements shows that flocculent structures contribute to the super-hydrophobicity of titanium, which helps to improve the self-cleaning of titanium. The flocculent LIPSSs show much better self-cleaning property than the pure LIPSSs6.

Figure 8. Surface morphologies of titanium and the corresponding contact angles for the various peak laser fluences after one month In addition, the crystal structure and reflectivity of the sample are also detected, as shown in Figure 9. It can be seen from Figure 9a that, there is rutile TiO2 formed on the surface of titanium after nanosecond laser irradiation. As a result, both the flocculent structures and pure LIPSSs turn out to be rutile TiO2, which has the advantage of the long term chemical stability compared to the anatase phase17. The estimated grain size of TiO2 flocculent structure for the three laser fluences (measure the (1, -1, 0) peaks of XRD spectra) is about 43 nm according to the Scherrer


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equation29, which means the holes can also be very small. In other words, the effective surface of flocculent structure can be very large. In addition, it can be seen that the peaks of the XRD spectra are steep, showing high crystallinity. As a result, we can conclude that the flocculent structures and LIPSSs are the crystallized rutile TiO2, which is accord with the previous studies that pure rutile and anatase phases of TiO2 show superhydrophobicity8. Figure 9b shows the reflectivity of the surface of titanium in the waveband between 260 nm and 2600 nm. It can be seen that the surface exposed to nanosecond laser irradiation has a much low reflectivity in the waveband above visible light. In particular, the best anti-reflectivity of the sample can be obtained with the peak laser fluence of 2.73 J/cm2. The reflectivity is around 10% in the full spectrum and the reflectivity is about 7% at the light wavelength between 1000 nm and 1800 nm, which advances 5 times compared to the initial titanium surface. It is because coverage and the thickness of flocculent structures for the peak laser fluences of 2 and 2.15 J/cm2 is less than that for the peak laser fluences of 2.73 J/cm2. The flocculent LIPSSs have the better anti-reflectivity than the rutile TiO2 nanospikes which is also characterized of high effective area17. Therefore, the flocculent structure has can improve the anti-reflectivity of the sample.

Figure 9. XRD patterns and reflectivity of samples with the various peak laser fluences after one month


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The method for production of flocculent LIPSSs is based on nanosecond laser irradiation without focusing. In this regard, the size of laser beam can increase with the increasing laser energy. In addition, the applied repetition rate is 100 Hz in this paper, and the scanning speed can go up as the repetition rate increases. On this basis, the scanning speed and interval of the two lines can increase. Therefore, the process efficiency of laser processing can be improved. 5. Conclusion In this study, experimental studies were carried out to produce the rutile TiO2 flocculent LIPSSs with high anti-reflectivity and superhydrophobicity on the surface of titanium under 10ns 1064-nm laser irradiation without focusing. The melt and modification threshold of titanium using multi-pulse irradiation were determined first, and the melt as well as modification spotoverlap numbers was introduced. It is found that both the melt and modification spot-overlap numbers increase with an increase in peak laser fluence and spot-overlap number, contributing to the production of flocculent structure. A peak laser fluence of 2.15 J/cm2 was applied for laser line scanning. The pure LIPSSs are obtained at the spot-overlap number of 3200 and LIPSSs with few flocculent structures are obtained at the spot-overlap number of 6400. Then, peak laser fluence went up to 2.46 and 2.73 J/cm2 to obtain the flocculent LIPSSs. The results demonstrate that the amount of flocculent structure increases with an increase in the peak laser fluence and spot-overlap number. Then, the composite structure, of which both LIPSSs and flocculent structures are distinct, is optimized using two-time laser line scanning with the fixed spot-overlap number of 3200 in the second-time laser line scanning. The flocculent structure and LIPSSs turn out to be pure rutile TiO2. The particle size of flocculent structures is about 43 nm via XRD pattern, indicating the large effective surface of flocculent structures. The existence of flocculent LIPSSs can make the surface be super-hydrophobic with the water contact angle of 160° and


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roll-off angle of 25°. In addition, the surface reflectivity of titanium is about 10% in the waveband between 260 nm and 2600 nm. Therefore, the flocculent LIPSSs may be a new composite structure improving the surface property of material.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (grant nos.

51475361, 91323033 and 51421004) and the Program for Changjiang Scholars and Innovative Research Team in University (grant no. IRT_15R54).

REFERENCES

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Rutile TiO2 Flocculent Ripples with High Antireflectivity and

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