Fabrication of superhydrophilic surface on metallic nickel by sub-nanosecond laser-induced ablation

As an important property for solid surfaces, wettability is often used to evaluate the adhesion performance when a drop of liquid is in equilibrium on the surfaces.¹ The wettability can typically be characterized by the apparent contact angles (CAs). Generally, the surfaces with CAs higher than 90° can be treated as hydrophobic surfaces, and with CAs lower than 90° are defined as hydrophilic surfaces. Further, with the inspiration of natural instances (for example, lotus leaves^2,3 or mosses⁴), the concepts of superhydrophobic and superhydrophilic surfaces which yield CAs respectively larger than 150° or smaller than 10° are proposed.^5–7 Such unique features especially the superhydrophobic property have attracted many interests of researchers and have been designed onto materials and applied in various scientific areas and human’s daily life, such as self-cleaning,^8,9 water-repellency,¹⁰ anti-corrosion,¹¹ adhesion modification,¹² cooling enhancement,¹³ and biomaterials fabrication.¹⁴ 

Chemical modification and physical roughening are two ways that intrinsically change the wettability of solid surfaces. Based on these facts, various techniques/treatments such as sol-gel method,¹⁵ coating deposition,^16,17 plasma treatment,¹⁸ chemical etching,¹⁹ and surface mechanical attrition²⁰ have been used for controlling the wetting properties of the material surfaces. However, the key blemish and the localization presented in these methods, including poor flexibility, high-cost, time-consuming and even potential chemical hazard limit the further development of these techniques somewhat. Laser surface modification has also found increasing interests in this regard, since it is considered to be a practical method to cause a surface structuring and/or photochemical variation without any chemical assistant. Meanwhile, laser treatment shows more advantages in comparison with the conventional methods, where one of the most outstanding features is that laser could provide a controllable surface wettability by flexible combination of laser processing parameters.^21–25 

In this study, the emphasis is put on the superhydrophilic features. Actually, over the past decades, with a plethora of attention paid on the issues of superhydrophobicity, people have already made significant progress towards understanding the mechanisms involved in such phenomenon. However, relatively little investigation has been conducted on the topic of superhydrophilic surface which is supposedly the other extreme end of the wetting behavior spectrum. On the other hand, a universal laser approach with easier operations to fabricate superhydrophilic material is still a challenging issue which needs further studies before the high-throughput production of functional surfaces. In the present paper, an experimental attempt that sub-nanosecond laser irradiated on metallic nickel surfaces was made for discussing the effect of laser treatment on hydrophilic capacity promotion. Nickel, which is commonly used as a protective outer coating for softer metals was chosen here as the target material. In addition, despite the intensive studies and reports^23–25 that have been carried out to state the advantages of using ultrashort laser for surface structuring, here, the sub-nanosecond laser source was selected for the surface processing. More considerations were paid on the fabrication of the laser parameters-dependent micro/nanostructures and chemistry changes to give probable interpretations on wettability modification. We found that the hydrophilic behavior is dominated by the combined effect of laser induced surface structuralization and the variations in chemical natures, and can in principle be controlled by the accumulated fluence involving laser parameters and scanning strategy.

In our experiments, MOPA fiber laser source (PyroFlex, PyroPhotonics Lasers Inc.) with 1 ns pulse duration (FWHM) at wavelength of 1064 nm was used. The pulse repetition rate was kept constant at 10 kHz. The laser beam with Gaussian profile was focused over the sample surfaces through a x-y galvo scanning system, resulting in a focal spot with the diameter of about 30 μm. The laser fluence F₀ was varied from 0.6 J/cm² to 12.6 J/cm². Figure 1 is the schematic diagram of experimental setup for laser surface processing, where Fig. 1(b) shows the basic scanning strategy we employed. The hatch distance (L_y) was fixed at 30 μm in this research to omit the influence of hatch overlap, whereas the scanning overlap (L_x) was set to be varied from 25% to 75% with a step of 25% (i.e., scanning speed at 225 mm/s, 150 mm/s and 75 mm/s). Five repeat scanning passes were applied on each scanning area. The experiments were conducted under the laboratory environment with a relative humidity of 45% and room temperature at 21 °C. In addition, in order to reduce the probable arbitrary defects and scratches on as-received nickel samples, mechanical polishing and a 20 min acetone ultrasonic bath were carried out, with which the cleaned surfaces with an average roughness (Ra) of about 263 nm were obtained.

To evaluate the wettability of samples the contact angles (CAs) of DI water were measured based on sessile drop method. Droplets with volume of 5 μl were released on the processed surfaces and lateral images of stable droplets were recorded for post-analyzing using the software FTA32. Each of the contact angles is an average of five individual measurements taken at random locations on samples. The surface roughness of samples before and after laser treatment were determined by white light interferometry (WLI). The surface morphology and elemental compositions were inspected respectively using a scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS).

The plot of water CA variation versus laser fluences and scanning overlaps is depicted in Fig. 2. It is obviously that the sub-nanosecond laser treatment is effective in transforming nickel surface from relatively hydrophobic to superhydrophilic, where the CA has been reduced consistently from its intrinsic value (around 96°) to a minimal value of about 3.1°. It means, water is easier to be attracted and spread out over the surface rapidly, making the nickel or nickel-plating material a better heat dissipation medium in many cooling applications. The water CA decreases remarkably with increasing the laser fluence until a steady state with a CA below 4° is achieved. In addition, the scanning overlap also shows a significant impact on the wettability modifications, in which higher overlap helps to accelerate the decline in contact angle (CA), i.e., in the condition of same pulse fluence, higher overlap shows a more hydrophilic tendency. In this case, the CA is easier to be saturated, and therefore possessing the lower saturation fluence. For instance, in order to obtain the optimal hydrophilic performance, laser fluence of at least 8.0 J/cm² is required for laser scanning overlap at 50% while only about 4.0 J/cm² is necessary for the case of overlap at 75%. Apparently, overlap has the similar effect as the laser fluence in promotion of the wettability. The reason lies in that the higher overlap (i.e., lower scanning speeds) will result in excessive laser pulse input at one particular surface spot, which is equivalent to enlarge the initial incident fluence. Such increase in fluence will contribute to the enhancement in ablation processes, and consequently benefits the formation of surface topology and the modification of surface chemistry, which will be discussed in detail in Part B and C.

Further, as can be imagined, if the scanning overlap is further reduced the critical laser fluence for reaching the equilibrium CA then will also be enlarged accordingly. In reality, in consideration of processing efficiency, relatively low overlap is suggested to be employed to exchange for its equivalent higher scanning speed. Also, for a system which possess a broader range of laser fluence, lower overlap offers a more reasonable way to select the desirable wettability since it shows a more linear and stable falling in the full range of contact angle.

A further statistical analysis reveals that the CA has a roughly linear relation with the accumulated fluence, which is an integrated parameter that involves laser fluence and spatial overlap. The accumulated fluence can be defined as²⁶ 

where F₀ is the applied laser fluence, and N_eff represents the number of effective pulses where overlaps is generally involved, and given by

where N_s represents the scanning passes which is 5 in this work, ω₀ means the beam waist radius, f is the repetition rate, and v the laser scanning speed. The scanning overlap η = 1 − v/2ω₀f is embodied in Eq. (2). Since the hatch overlap is fixed to be zero the influence of pulse distance along Y-axis on accumulated fluence is omitted in this study.

The distribution of CA data over the accumulated fluence is shown in Fig. 3, where an approximate monotone decreasing region and a saturation region can be recognized within the fluence range. As is well known, the surface properties (including wettability) of materials are determined by surface states, where the processing parameters play a role of controlling the surface states directly. Therefore, in essence, this kind of trend reveals that a specific surface feature modification will be created at a given accumulated fluence to satisfy the corresponding wettability, and once this fluence used higher than a certain value, the surface states induced by laser irradiation will tend to be stable, thus resulting in the saturation in contact angle. For simplicity, the CA in decreasing interval is assumed to change linearly thus the CA can simply be determined by the accumulated fluence and is independent of single variables (i.e., pulse fluence and scanning overlap). That means, a theoretical one-to-one mapping correspondence can be established by means of curve fitting of the data located in the linear area. The non-strict linear fitting function is shown in Fig. 3. Moreover, the threshold for the formation of stable superhydrophilicity is obtained to be around 62.5 J/cm², which can be treated as a characteristic fluence of metallic nickel to reflect the relation between critical processing parameters and optimal hydrophilicity. The importance of establishing such mapping relationship is to allow us to make flexible combinations of laser parameters (laser fluence and processing speed) to achieve the controllable modification of superhydrophilicity in practical applications. Obviously, with it, a straightforward retrieval on achievable contact angle can be realized by direct using the corresponding accumulated fluence; the CA value can be confirmed by combining the equations 1 and 2 with the fitting function.

Despite the lack of a universal investigation on different metals in this work, one still can expect to achieve the similar mapping relationship between contact angle and accumulated fluence on other metals, as long as the scanning strategy, laser parameters, and processing conditions are similar to what we have suggested.

Modification of surface topography is commonly known as one of the key factors for controlling the materials’ wetting properties. It has been reported that the shape, distribution, density, and aspect ratio (height/width) of the surface structures have a great influence on surface properties.^27–29 Fig. 4 shows a sequence of SEM images of surface structures generated at various laser fluences and 75% overlap. The figures reveal that after sufficient laser ablation the original smooth surface has been completely changed; ablative debris, sub-microstructures, uneven nanoparticle groups, as well as the voids and cracks can be seen scattered throughout the ablated area. However, differing from the previous reports^22,25 where the hydrophilicity was generally fabricated via generating the microscale deep-grooves or deep-caves on surfaces under intense laser intensity, in this study, relative low laser fluences were used resulting in relatively intact and flat structured surfaces, such as the surface shown in Fig. 4(b). These grainy structures can be attributed to the thermodynamic processes during sub-nanosecond laser irradiation. For the case of 1 ns pulse duration, the energy transformation from electrons to lattices has already began and thermodynamic processes such as thermal diffusion, fusion and explosion will occur. Following these, fast resolidification of the molten material which is in hydrodynamic unstable state appears with the cutting-off of laser pulses, and thereafter the surface micro/nanostructures will be created. The particle structures also could be the ejected molten droplets that deposited back upon the surface under the effect of gravitation. Besides, the effect of transient thermal stress (or stain) is believed to be the possible reason for the present of cracks and voids observed in the experiments; the cracks also reflect the distribution of nickel grains.

The results show that the evolution of the surface morphology is basically consistent with the wettability behavior that shown in Fig. 2. As it can be clearly seen from Fig. 4(c)–(f), the size and distribution of the created surface structures are related tightly to the adopted laser fluence, where with the rise of the incident laser intensity the size of nanoparticles and cracks increase sharply and, the density, in contrast, decreases correspondingly. The average size of these self-organized nanoparticles as well as the depth of cracks are mainly distributed between tens to hundreds nanometer under the full range laser fluence we applied. At low and moderate laser intensities (below 6 J/cm²), the effect of structuralization is generally mild due to the nonviolent ablation processes, resulting in the formation of relatively flat surfaces which are covered with compact and relative uniform nanoparticles (as seen in Fig. 4(b)). In this situation, wettability will be improved but not toward extreme. Nevertheless, when it comes to the cases of using laser fluence above 6 J/cm², the size of the ablation structures will gain a sharp increase exhibited in the form of nanoparticle groups. It probably results from the enhancement of thermodynamic effects under higher repetitive laser intensities, making the adjacent particles fused together. Previous works³⁰ have declared that this kind of formation and aggregation of nanoparticles also could be blamed on the nucleation effect induced from the strong interaction between the plasma species and the air molecules during the laser ablation. Moreover, high laser dose also helps to form more cracks which have larger dimensions in horizontal and depth. These voids or cracks will promote the generation of micro-capillary force among the porous structures, and thus drive the liquid droplets spreading all over the surfaces easier. The above discussions imply that, proper enlargement of the “size” of laser-induced self-organized structures seems to be a useful way to enhance the wettability modification.

The Wenzel equation³¹ states that the roughness is a key factor which makes positive contribution to the decrease of CAs or the promotion in wetting ability. Therefore, in addition to the morphology, the influence of roughness on wettability were investigated simultaneously. Actually, owing to the effects of laser ablation that have successfully turned the smooth surfaces into rugged topographies, the local surface roughness should also increase correspondingly. Fig. 5 shows the dependence of CA on roughness, where the inset demonstrates the surface roughness that generated at different laser fluences. As increasing of laser fluence, surface roughness increases as well, where the more intensely rising trend can be observed under the gentle laser intensities and slows down when the laser fluence applied over about 8 J/cm². Within the laser fluence range we applied, maximum roughness (Ra) of approximately 624.6 nm is acquired, which affords almost three times higher than that of primordial surface. In regard to the intuitive relation between CA and roughness, an overall decrease trend in CAs over roughness can be observed till it gets saturated at a relative high roughness of about 500 nm. These results demonstrate that sub-nanosecond laser irradiation can effectively be used to improve the surface roughness and consequently enhance the wettability of nickel.

However, it should be noted that, in spite of the similar trends in both of roughness and wettability features, discrepancy is still presented, where the hydrophilia characteristics saturated faster than that of roughness. What’s more, the Wenzel equation cos θ_w = r cos θ₀ also points out that once the material has initial contact angle θ₀ greater than 90°, i.e., when the flat substrate is relatively hydrophobic, the apparent contact angle θ_w increases with increasing the value of roughness factor r and theoretically cannot cross over from hydrophobic to hydrophilic; then Cassie-Baxter equation is usually used for describing the hydrophobic feature. It is apparently contrary to the results we obtained in the experiments, which suggests that apart from the surface morphology and roughness, other potential factors such as chemistry should also be taken into account. Under the nanosecond laser irradiation, surface chemical changes can naturally be linked to the oxidizing processes. Therefore, a reasonable assumption can be made that the generated nanoparticles on the surface have already been transformed from pure nickel to nickel oxides (for instance, NiO, which is a kind of hydrophilic materials) or at least there has an oxide shell covering the nickel core (core-shell structure). In this case, θ₀ should be treated as the initial CA of nickel oxides, which is generally below 90°, to satisfy the condition of wetting transition from the Cassie-Baxter state to Wenzel state.

In addition to the topography, chemical property is considered to be another broad category of wetting domination, in which the attractive force is formed as a result of various bonding effects between the top atomic layer of a surface and liquids. In fact, a big advantage of using the sub-nanosecond laser in this study is that the laser caused thermal processes is comparatively strong in comparison to that of ultrashort laser. Besides, the increase in specific surface area that results from the surface roughening gives more opportunities for air diffusing to the metal material to reduce the limitations in chemical reactions between nickel and oxygen atoms; the oxidizing reaction continues even after the end of individual pulse irradiation. Both of these effects facilitate the formation of oxide layer.

Fig. 6 shows the results of XPS analysis of the sample surface treated at laser fluence of 10.4 J/cm² and scanning overlap of 75%. The major elements Ni, O, C, N can be distinguished from the survey spectrum shown in Fig. 6(a). After efficient laser ablation, the most prominent feature is that the oxygen content has increased substantially. Since the experiments were carried out in air condition the additional carbon and nitrogen observed in XPS spectrum can be regarded as the pollution species absorbed by the specimen, and the surface nanoparticles, therefore, is believed to be at least made up of nickel and nickel oxides. By further extracting of characteristic spectral peaks of 853.8 and 855.5 eV in Ni 2p_3/2 spectrum (as shown in Fig. 6(b)), the components of nickel dioxides, and nickel hydroxides were confirmed, which is in agreement with the previous reports.^32–34 The absence of characteristic peak of metal nickel, which is usually located at around 852-853 eV in Ni 2p_3/2 spectrum, indicates that the surface of nanoparticles or the self-assembly structures have been fully oxidized or hydroxylated after laser ablation. Among these components, Ni(OH)₂ is the chief constituent of the surface of nickel nanoparticles which has a proportion of 75.4%, and the rest is 24.6% NiO. Fig. 6(c) exhibits the spectrum of O 1s which contains the information about oxygen species. The peak at 529.5 eV is typically related to the Ni−O bonds of NiO;^35,36 other peaks like 531.5, 532.9 and 534.0 eV can be assigned to the hydroxyls of Ni(OH)₂,³⁶ O-CO bonds, or absorbed water/oxygen.³² Except for the unexpected foreign contaminations the oxygen content is primarily come from NiO and Ni(OH)₂. Despite of the uncertain of detail distribution of oxygen, these results support the assumption made in the above Section III B for the reason that NiO is a highly hydrophilic material³⁷ and Ni(OH)₂ nanoparticles also show great wettability.^33,38 In brief, the growth of polar components, especially the synthesis of NiO/Ni(OH)₂ on the top surface provides a great benefit to the formation of superhydrophilic nickel surfaces. As regards the formation of Ni(OH)₂, the further reactions of NiO with air components (such as moisture and oxygen) could be one of the reasons, or, it was directly created during the laser ablation in humid atmosphere.³⁹ 

We also found that, although the chemical nature also has a direct relation to the laser intensity, however, unlike the surface morphology or roughness, which shows obviously changes throughout the laser fluence range (see Fig. 4 and 5), the surface chemical properties exhibit a lower sensitivity to laser doses. The reason lies in the saturation of oxides formed on top surface, in which case, the changes in chemistry are more likely to be embodied in the form of oxide thickness rather than the species or concentrate. In the entire fluence range we adopted, the overall oxygen content and the ratio of NiO to Ni(OH)₂ were always remained at a high level in spite of their small decline. For example, for the sample treated with low laser fluence at 1.2 J/cm² and scanning overlap at 75%, the topography changed slightly (see Fig. 4(b)), with a limited increasement of only about 13.5% in roughness; nevertheless, as high as about 31.9% of oxygen still could be detected on the top surface, which comes from the mixture of nickel oxides/hydroxides. Compared to the small enhancement in roughness, such considerable increasement in oxygen concentration, especially the generation of hydroxyls is believed to provide a more substantial promotion in hydrophilicity. This indicates that the surface chemical nature plays the key role at low laser dose due to the fact of that the nickel trend towards hydrophilicity at the very beginning. Instead, with the increase in laser intensity, remarkable changes in surface morphology, as well as the significant increase in the corresponding surface roughness start to show up due to strengthening in ablation processes, while the change in elemental status is relatively small (oxygen content increased merely from 31.9% to 54.2% at the fluence range of 1.2 to 10.4 J/cm²). In this situation, the improvement of wettability, especially the formation of superhydrophilicity then counts mainly on the physical topology.

In this work, surface wettability behaviors of metallic nickel after sub-nanosecond fiber laser irradiation were investigated varying the different processing parameters, and the underlying mechanism was also discussed. Combinations of a wide range laser fluence from 0.6 to 12.6 J/cm² and three different scanning overlaps at 75%, 50%, and 25% were used to guarantee the extensive experimental results. The results show that both the increase in initial incident fluence and scanning overlap will contribute to the improvement in hydrophilicity of nickel surface, which is embodied in the falling of contact angle. By analyzing the distribution of contact angle over accumulated fluence, an approximate linear relationship between them was established, with which the flexible selection of contact angle is expected to be realized. Meanwhile, with the use of this linear dependence, a critical accumulated fluence of about 62.5 J/cm² was extrapolated, beyond which a stable superhydrophilicity with contact angle below 5° could be obtained.

The ablation-induced self-organized structures that consist of nanoparticle groups and cracks were found to have a great positive influence on the wettability improvement, since they would direct result in significant changes in surface morphology and the remarkable increase in surface roughness, which are believed to be one of the decisive reasons in the formation of superhydrophilicity. On the other hand, elemental analyses reveal that an oxide layer that made up of hydrophilic NiO/Ni(OH)₂ would be synthesized on the top surface of formed nanostructures after sufficient laser irradiation. These mixture polar components not only provide a “driving force” for transforming the nickel from initial hydrophobic to hydrophilic, but also further amplify the enhancement of surface topology on hydrophilic. All these cooperative physical changes and chemical properties together ultimately lead to the observed super-wetting behavior.

As next steps the universality of the linear dependence between hydrophilicity and accumulated fluence will be systematically analyzed through a statistical experiment that involves different metallic samples, and the attempt of extending this correlation over the whole wettability range (including the hydrophobicity) will be made.

This work was supported by National Natural Science Foundation of China (Grant No. 61605079).