In this study, we create a multifunctional metal surface by producing a hierarchical nano/microstructure with femtosecond laser pulses. The multifunctional surface exhibits combined effects of dramatically enhanced broadband absorption, superhydrophobicity, and self-cleaning. The superhydrophobic effect is demonstrated by a falling water droplet repelled away from a structured surface with 30% of the droplet kinetic energy conserved, while the self-cleaning effect is shown by each water droplet taking away a significant amount of dust particles on the altered surface. The multifunctional surface is useful for light collection and water/dust repelling.

Nature provides many examples of multifunctional properties on a biological surface.1,2 One of the examples is the water-repelling lotus leaves.1,3,4 The lotus leaves have a number of functionalities, such as superhydrophobicity, self-cleaning, and defense against pathogens.1,3 Studies have shown that the lotus leaf surface has a hierarchical structure containing a larger micro-scale structure in the range of 10–50 μm and a finer structure in the range of 200 nm–2 μm.1,3 This hierarchical structure along with a hydrophobic epicuticular wax coating imparts the superhydrophobicity to lotus leaves. Furthermore, the hierarchical surface structure significantly reduces the adhesion of contaminants to the surface.3 Both enhanced hydrophobicity and reduced contaminant adhesion produce the lotus self-cleaning effect, often referred as the “lotus effect.”3 The lotus self-cleaning is achieved when water drops roll over the leaves, pick up the dust particles, and carry them away when rolling off the leaves. Another example of the multifunctional biological surface is the Morpho butterfly wing. The surface structures of the wings produce a blue color and also make the wing surface superhydrophobic and self-cleaning.5,6

Recently, studies have shown that femtosecond laser surface processing can produce surface structures that can significantly modify optical7–12 or wetting12–15 properties of metals. However, the metal blackening7–11 and wetting effect13–15 were each demonstrated individually. Here, we demonstrate a laser nano/microstructuring technique to create a combined black, superhydrophobic, and self-cleaning effect on a metal surface. The enhanced light absorption is useful whenever light collection is needed, for example, in sensors and solar energy absorbers. The superhydrophobicity and self-cleaning effects will repel water and dust, and improve the performance and reduce the maintenance of the devices that utilize these surfaces. Furthermore, the superhydrophobicity should also enable other highly desirable functionalities, such as anti-corrosion,16 anti-icing,17–19 anti-biofouling,19,20 anti-microbial,21 low flow resistance,21,22 and platelet anti-adhesion,23 which are intrinsically associated with the superhydrophobicity. Using anti-icing as an example, water drops do not have enough time to freeze on a superhydrophobic surface before they roll off the surface.24 Some potential applications for anti-icing surfaces include protection of aerofoils, power transmission lines, pipes of air conditioners and refrigerators, and radar or telecommunication antennas.24,25

In this study, we use an amplified Ti:sapphire laser system that generates 65-fs pulses with a central wavelength of 800 nm and at a maximum pulse repetition rate of 1 kHZ. The laser beam is focused onto the sample surface by a lens onto a sample mounted on a computerized XY-translation stage. The samples in our study are platinum, titanium, and brass. Each sample is textured with an array of parallel microgrooves covered by extensive nanostructures. The platinum sample is processed at laser fluence of 9.8 J/cm2. The titanium sample is processed at laser fluence of 7.6 J/cm2. Brass is processed at laser fluence of 3.9 J/cm2. The orientation of microgrooves is controlled by the scan direction. A scanning electron microscope (SEM) and a 3D laser scanning microscope are used to examine the surface structures. Superhydrophobic properties are studied by measuring both water contact angle and the surface tilt angle for water sliding. The self-cleaning properties are studied with real-life dust particles collected from a vacuum cleaner. For cleaning, we use rolling and falling water drops. The rolling drops with nearly zero kinetic energy are produced by pipetting water drops near the sample surface, while the falling drops are produced by pipetting drops at a height of 3–8 cm above the sample surface. The diameter of the pipetted water drops is in the range of 2–5 mm. The self-cleaning action is recorded with a video camera. To characterize the optical properties, we measure the total hemispherical optical reflection of the samples using a Perkin-Elmer Lambda 900 spectrophotometer and Bruker IFS 66/S FTIR spectrometer, each equipped with an integrating sphere. The two spectrometers allow us to measure the spectral reflectance in the wavelength range of 0.25–2.5 μm and 2.5–16 μm, respectively.

A laser-treated platinum surface is shown in Fig. 1(a). The treated surface appears velvet black at all viewing angles, indicating a significant increase of optical absorption. A hierarchical surface structure produced on platinum is shown in Figs. 1(b)–1(d). This structure is an array of parallel microgrooves covered by extensive nanostructures. The microgroove spacing is about 100 μm and the depth is about 75 μm. Our SEM study shows that the smallest nanoscale features are about 5–10 nm. Following the laser treatment, superhydrophobicity develops after the sample is exposed to air.14 To characterize the hydrophobicity of the treated platinum surface, we measure the water contact angle on the surface to be 158°, and a water drop will slide on the treated surface at a tilt angle of only 4°. More remarkably, when a drop of water is released and falls towards the treated surface, the water droplet is repelled by the treated surface to such a degree that it bounces off the surface, lands again due to gravity, and bounces again and off the treated surface area, as shown in Fig. 2. Here, the water drop is released 19 mm above the surface, reaches a height of 5.3 mm after the first bounce, and lands 13.75 mm away from the first bounce before bouncing off the surface. About 30% of the water droplet kinetic energy is conserved from the first bounce. The two bouncing motions last less than 0.5 s, and the laser-treated surface remains completely dry afterwards [Fig. 2(f)].

FIG. 1.

(a) Photograph of superhydrophobic black platinum; (b) laser microscopy image showing micro-structures on the platinum surface; (c) and (d) SEM images showing the detailed hierarchical structures on the platinum surface; (e) and (f) laser microscopy images showing surface structures on brass and titanium.

FIG. 1.

(a) Photograph of superhydrophobic black platinum; (b) laser microscopy image showing micro-structures on the platinum surface; (c) and (d) SEM images showing the detailed hierarchical structures on the platinum surface; (e) and (f) laser microscopy images showing surface structures on brass and titanium.

Close modal
FIG. 2.

(a)–(f) Video clips showing a water droplet bouncing off a superhydrophobic black platinum surface. The surface has a tilt angle of 8°.

FIG. 2.

(a)–(f) Video clips showing a water droplet bouncing off a superhydrophobic black platinum surface. The surface has a tilt angle of 8°.

Close modal

A decrease of surface tension on a solid surface can also enhance the hydrophobicity.26 Therefore, another approach to increase the hydrophobicity is to decrease the surface tension by coating a hydrophobic layer on the solid surface. The largest water contact angle ever achieved through coating on a smooth surface is only about 120°,21,27 which is far less than 150°, the minimum water contact angle required being qualified as superhydrophobicity. However, a combination of surface structuring and a hydrophobic chemical coating can produce strong superhydrophobicity.26 Metals are intrinsically hydrophilic; immediately after femtosecond laser surface structuring, they first become more hydrophilic, but the exposure to air turns the metals superhydrophobic. This transition is explained by chemical interaction between the surface and the ambient CO2, resulting in an accumulation of carbon and its compounds on the laser-treated surface.14,28 We believe that the laser-induced surface nanostructures also play an important role in enhancing this chemical interaction due to nanochemical effects.29 

In nature, self-cleaning occurs on a superhydrophobic surface with water from rain, dew, and fog. These water sources supply falling, rolling, and sliding drops. The rolling and falling drops are more efficient in removing dust particles than the sliding drops.30 Figure 3 shows self-cleaning of dust particles on the black platinum by applying a string of water drops. The dust particles are a collection of real-life dusts from a vacuum cleaner; the size of the particles is in the range of 0.1–2 mm. Video clips in Fig. 3 and a supplementary video in Fig. 4 (Multimedia view) show that the dust particles are taken away by the dropping water, and afterwards superhydrophobic surface becomes virtually clean and remains completely dry. In contrast, we can see that water sticks to the untreated area even upside down. We also apply water on an untreated platinum sample covered with dust particles. In contrast to our superhydrophobic surface, water remains on the untreated surface with all the dust particles floating inside [Fig. 3(f)]. After water vaporizes, all the dust particles will remain on the surface. In our study, we repeatedly perform 20 cleanings on the superhydrophobic surface and did not observe any degradation of the self-cleaning effect.

FIG. 3.

(a)–(e) Video clips showing a superhydrophobic platinum surface self-cleaned by water droplets. The surface has a tilt angle of 8°; (f) an untreated platinum surface accumulates a puddle of water with floating dusts.

FIG. 3.

(a)–(e) Video clips showing a superhydrophobic platinum surface self-cleaned by water droplets. The surface has a tilt angle of 8°; (f) an untreated platinum surface accumulates a puddle of water with floating dusts.

Close modal
FIG. 4.

Supplementary video demonstrating the self-cleaning effect of the platinum sample. (Multimedia view) [URL: http://dx.doi.org/10.1063/1.4905616.1]

FIG. 4.

Supplementary video demonstrating the self-cleaning effect of the platinum sample. (Multimedia view) [URL: http://dx.doi.org/10.1063/1.4905616.1]

Close modal

Self-cleaning surfaces should have the following properties: (i) large water contact angle exceeding 150°, (ii) small sliding angle (<10°) to cause water drops easily roll off the surface, and (iii) the adhesion between the surface and dust particles on the surface should be smaller than that between the dust particles and water.3,27,31–33 The surface structures we produced benefit self-cleaning in two ways. First, the surface structures turn a metal surface superhydrophobic; second, the surface structures also reduce the adhesion of dust particles to the solid surface.3 We also produce multifunctional black and superhydrophobic titanium and brass surfaces, and they show similar superhydrophobic and self-cleaning behaviors as Pt.

To characterize the optical property of the three multifunctional surfaces, we measure the wavelength dependent reflectance of their surfaces with the spectrophotometer and FTIR spectrometer, and results of these measurements are shown in Fig. 5. For comparison, we also measure the reflectance of mechanically polished surfaces of the three metals before laser treatment. We can see that the multifunctional surfaces have a very low reflectance over a broad range of wavelengths. The reflectance in the visible wavelengths is in a range of 1.3%–3.5%, 3.3%–4.1%, and 4.2%–4.5% for brass, Pt, and Ti, respectively. For comparison, the reflectance of mechanically polished surfaces of the three samples is much higher as shown in Fig. 5. Because of the extremely low reflectance, all three sample surfaces appear pitch black. Furthermore, these surfaces also have low reflectance in the near infrared, which increases with wavelength slightly for Pt and Ti but significantly for brass. At 16 μm, the absorption is 9% for Pt, 18% for Ti, and 73% for brass. The measured reflectance shows that the black Pt and Ti surfaces are excellent broadband absorbers of electromagnetic radiation from the ultraviolet to mid-infrared.

FIG. 5.

Spectral reflectance of the black brass, black platinum, and black titanium as a function of wavelength. Spectral reflectance of three mechanically polished metals without laser treatment is also shown for a comparison. Dashed line shows the spectral reflectance of an ideal solar absorber.

FIG. 5.

Spectral reflectance of the black brass, black platinum, and black titanium as a function of wavelength. Spectral reflectance of three mechanically polished metals without laser treatment is also shown for a comparison. Dashed line shows the spectral reflectance of an ideal solar absorber.

Close modal

It is known that absorptance, A, of a clean structured metallic surface is given by A(λ)=AINTR(λ)+ASS(λ), where AINTR is the intrinsic absorptance of a flat, clean, and ideally smooth surface and ASS is the contribution of surface structures.34 The dramatically enhanced absorption of our structured surface over a broad spectral range comes from several mechanisms. The surface structures smaller than light wavelength (nanostructures and fine microstructures) enhance absorptance through antireflection effect of the graded refractive index formed by subwavelength surface textures at the air/solid interface.35 Furthermore, these sub-wavelength surface structures significantly enhance absorptance through plasmonic absorption.36–39 On the other hand, the surface structures greater than the light wavelength enhance absorptance through light trapping in surface cavities and the Fresnel angular dependent reflection. All these absorption mechanisms contribute to the broadband high absorption, leading to the structural black color in the visible spectral range. Previously, it has been shown that the absorption of semiconductors can be enhanced through laser-assisted chemical reactions using SF6 and H2S that leave sulfur in the surface layer.40 In contrast, the blackening of metals in air is due to surface structuring and not associated with the change in elemental composition.8,10 The enhanced absorption can also be produced on metals with other femtosecond laser-induced surface structures.10,11 Finally, we note that the hierarchical surface structures we produced on Pt and Ti are more optimized for the broadband absorption in the wavelength range of 0.25–16 μm. In the past, we demonstrated that the surface structures on metals can also be optimized for efficient absorption in the THz range,41 where regular metals are perfect reflectors.

One application for the enhanced light absorption is building better solar absorbers for efficient conversion of solar energy to thermal42 or electrical energy through thermoelectric generators.43,44 Solar radiation is broadband and mainly composes of ultraviolet (λ < 0.4 μm), visible (0.4 < λ < 0.7 μm), and infrared radiation (0.7 < λ < 100 μm). At the sea level, the fraction of solar energy in the ultraviolet, visible, and infrared wavelengths are about 4%, 42%, and 54%, respectively. From practical point of view, almost all solar energy is contained in the wavelength range of 0.2 < λ < 3 μm.43 Fig. 5 shows that our samples have a very high absorptance in this wavelength range, especially for Pt and Ti. An ideal solar energy absorber should not only absorb solar energy efficiently in this wavelength range but also minimize radiative thermal loss to the environment at longer wavelengths. Therefore, the ideal wavelength dependent reflectance should be R(λ) = 0 at 0.3 < λ < 3 μm and R(λ) = 1 at 3 < λ < 50 μm.43 A dashed line in Fig. 5 shows this ideal reflectance. To provide high reflectance at λ > 3 μm, the surface structures we created on brass contain shallow microgrooves covered by nanostructures. Figure 1 shows a comparison of the microgrooves of brass versus Ti and Pt. The microgroove depth is about 10 μm for brass, but 50 and 75 μm for Ti and Pt. Because a shallower microgroove traps less infrared radiation at longer wavelengths, brass sample has a significantly higher reflectance in the infrared. We believe that a closer resemblance of the ideal reflection step function can be achieved by further optimizing the structural period and depth. In contrast to a femtosecond laser processing technique developed for enhancing light absorption in semiconductor photovoltaic cells,45,46 our work enables metals to absorb solar energy more efficiently for conversion to electricity in devices such as thermoelectric generators44 or conversion to heat in devices such as solar hot water tanks.42 

In summary, we create a multifunctional metal surface by producing a hieratical nano- and micro-structures with femtosecond laser pulses. The multifunctional surfaces exhibit combined effects of dramatically enhanced broadband light absorption, superhydrophobicity, and self-cleaning. This surface also has other highly desirable functionalities such as anti-corrosion, anti-icing, anti-biofouling, and self-sanitation, since these properties are directly related to superhydrophobicity.

This work was supported by Bill & Melinda Gates Foundation and U.S. Air Force Office of Scientific Research.

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