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Abstract
We demonstrate the formation of laser-induced periodic surface structures (LIPSS) in boron-doped diamond (BDD) by irradiation with femtosecond near-IR laser pulses. The results show that the obtained LIPSS are perpendicular to the laser polarization, and the ripple periodicity is on the order of half of the irradiation wavelength. The surface structures and their electrochemical properties were characterized using Raman micro-spectroscopy, in combination with scanning electron and atomic force microscopies. The textured BDD surface showed a dense and large surface area with no change in its structural characteristics. The effective surface area of the textured BDD electrode was approximately 50% larger than that of a planar substrate, while wetting tests showed that the irradiated area becomes highly hydrophilic. Our results indicate that LIPSS texturing of BDD is a straightforward and simple technique for enhancing the surface area and wettability properties of the BDD electrodes, which could enable higher current efficiency and lower energy consumption in the electrochemical oxidation of toxic organics.
© 2017 Optical Society of America
1. Introduction
Diamond has attracted the interest of researchers for decades mainly because of its long list of outstanding properties: the highest atomic density of any bulk crystal, the highest Young modulus, largest transparency window of all solids and the highest thermal conductivity [1]. In addition, during the growth of diamond films using chemical vapor deposition (CVD) systems, dopants and impurities can be easily incorporated into the material, allowing for tuning its electric and optical properties. Substantial progress has been made in this area using boron for p-type doping and heavily boron-doped diamond (BDD) films (with B-doping levels > 1020 cm−3), which are now produced routinely for electrochemical investigations [2–5].
Boron-doped diamond (BDD) electrodes have emerged as ideal electrical interfaces for a variety of electrochemical applications as they are chemically stable and exhibit a large potential window with low background current [5, 6]. Furthermore, oxygen-terminated lightly boron-doped diamond thin films were synthesized as a semiconductor electron source to accelerate CO2 reduction in artificial photosynthetic systems [7]. With the aim of increasing the surface area of these electrodes while keeping the unique features of a smooth BDD interface, great efforts have been devoted to the formation of nanostructured BDD films [8–11]. Surface area of the electrode in electro-chemical applications is a critical factor because it determines its oxidation performance [12]. Among the different morphological forms of BDD reported in literature, 3D structures are common. For instance, it is possible to find reports on diamond powders coated with nanocrystalline BDD [13], BDD nanorod forests prepared on silicon nanowires [14], BDD coated ‘black silicon’ [15], and BDD ‘nano-grass’ arrays [16]. All of them have shown interesting electrochemical properties and, therefore, may be useful as electrochemical sensors. However, most reported methods present complex and very long synthesis steps [17]. Perhaps porous BDD has been shown to be one of the simplest alternatives to the fabrication of structured BDD electrodes, however still needs multiple fabrication steps [18].
Laser induced periodic surface structuring (LIPSS) is a simple ‘one-step’ surface functionalization technique that can be applied to nearly all kinds of material surfaces, including metals, semiconductors, and dielectrics [19–21]. In fact, LIPSS have been already associated with catalytic activity in electrochemical processes. Lange et al. used a ps-laser to process different surface morphologies on polished platinum electrodes, achieving enhancement of surface areas in the order of 1.75 compared to a polished surface [22]. In that work it was demonstrated that micro- and nano-structuring of plain platinum surfaces increased the electrochemically active surface area in a simple manner. Analogously, the formation of LIPSS yielded a considerable enhancement in the electrochemical activity of nickel surfaces in aqueous KOH [23]. Consequently, it is customary to assume that LIPSS on BDD should also enhance the electrochemical performance of BDD electrodes, although it remains to be experimentally demonstrated.
In this work, we explore the conditions for producing LIPSS on BDD surfaces using near-IR ultrafast pulses, and characterize their morphology and structural properties. Grating-like structures were fabricated using the LIPSS mechanism with aspect ratios close to 1:2, high reproducibility and accuracy, and enhanced surface area (1.5 × ) and wettability properties. To shed more light on the nano-structure characteristics, we performed atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman spectroscopy of the generated BDD nanostructures at low and moderate boron-doping (~800-1000 ppm) levels. Raman spectra obtained via micro-spectroscopy, indicated that no de-doping or substantial structural changes occur during the laser treatment. We demonstrate that LIPSS could be a simple alternative for the fabrication of surface enhanced BDD electrodes with improved surface and wettability properties.
2. Experiments
The experiments were carried out in a nitrogen atmosphere (in order to minimize non-linear effects occurring in air) with a Ti:Sapphire laser system (Coherent Inc. Legend) consisting of a mode-locked oscillator and a regenerative amplifier producing up to 5 mJ, 100 fs pulses at a 1 kHz repetition rate with a central wavelength of 800 nm. The output power was adjusted using a combination of a half-wave-plate and a polarizer as depicted in Fig. 1. The ~8 mm diameter laser beam was spatially filtered using an aperture and focused on the sample using a 500 mm focal length lens to a spot of approximately ~120 μm at FWHM. The sample was positioned at 45° incidence angle, and so the reported fluences take into account the projected elongated spot on the diamond surface. An X-Y translational stage was used to move the sample under the laser beam. A beam splitter with ~4% reflectivity was employed to monitor the spot in an equivalent image plane to the BDD sample as shown in Fig. 1. The repetition rate of the laser was selectable externally from single-shot to 1 kHz, allowing generating trains with an arbitrary number of pulses. The laser parameters (spot size, fluence, number of impinging pulses and repetition rate) were adjusted to produce LIPSS nanopatterns. Finally a CCD camera and a telephoto lens were used for real time monitoring of the machining process.
Fig. 1 Schematic layout of the femtosecond laser machining setup for processing boron doped diamond.
Ultra-polished (average roughness ~1 nm) polycrystalline boron doped diamond samples (Diamond Materials GmbH) with doping level ~800-1000 ppm were used in the study. The surfaces were cleaned using subsequent baths of boiling aqua regia and chromic acid to ensure clean surface before and after laser treatment.
3. Results
The spatial period Λ of LIPSS generated structures typically depends on the laser wavelength λ, the polarization of the laser electric field, the number of impinging laser pulses and the angle of incidence [24]. On strong absorbing materials, such as semiconductors and metals, the low spatial frequency LIPSS (LSFL) are usually oriented perpendicular to the laser beam polarization and exhibit periods close to the wavelength, i.e., deviating not more than a few tens of percent (ΛLSFL ∼λ). It is generally accepted that these structures are generated by interaction of the incident laser beam with an electromagnetic wave scattered at the surface and may involve the excitation of surface plasmon polaritons [24].
The dependence on the angle of incidence has already been studied in detail by [25–27]. Depending on the polarization direction (s- or p-polarized) and the angle of incidence, the LSFL period follows the relation ΛLSFL,p ∼λ/[ξ ± sin θ], and ΛLSFL,s ∼λ/[ξ2 − sin2 θ]1/2, with ξ2 = |Re(ε)|/[|Re(ε)| − 1] [24]. For strong absorbing and plasmonic materials [Re(ε) << −1] these equations simplify to the relations ΛLSFL,p ∼λ/[1 ± sin θ] and ΛLSFL,s ∼λ/cos θ [26]. For our particular case, assuming BDD is strongly absorbing 800 nm radiation, and taking into account the use of p-polarization at 45 degrees, the expected periodicity is ΛLSFL,p ∼468 nm.
3.1 Generation and morphology of BDD LIPSS
The laser fluence and number of pulses was scanned to produce a range of BDD structures with varied characteristics. The ablation threshold was measured to be around ~2 J/cm2 for areas irradiated with >1000 pulses, which is consistent with previous measurements employing similar irradiation conditions in un-doped diamond [28–32]. Figure 2(a) shows the measured threshold for LIPSS generation, which varies considerably between 2 – 23 J/cm2, depending mainly on the number of impinging pulses. We attribute this behavior to an incubation effect, where the threshold decreases with an increase in the number of pulses. The effect has been observed for several types of materials in experiments with laser pulse durations ranging from femtoseconds to nanoseconds [33]. In this way, the BDD surface is altered slightly by each pulse and the absorption of the laser energy increases. With a large number of pulses the difference in the threshold fluence becomes evident, as shown in Fig. 2(a).
Fig. 2 (a) LIPSS formation threshold as a function of laser fluence.and number of laser pulses (inset) Periodicity variation of LIPSS as a function of number of impinging pulses for 23 J/cm2. (b) SEM micrographs of LIPSS structures on boron doped diamond. (inset) Micrograph with 25,000x magnification shows typical ripple period about ~420 nm.
Scanning Electron Microscopy (SEM) images of the BDD samples, obtained by field-emission gun SEM (FEG-SEM Jeol JSM 7000F), are shown in Fig. 2(b) for a fluence of 12 J/cm2 and 100 pulses. The formation of surface periodic structures was evident for all processed samples, being perpendicularly oriented with respect to the laser beam polarization. Structures with similar regular periodicity were formed for a wide range of laser fluences ranging from 2 to 23 J/cm2. The SEM characterization yielded an average LIPSS periodicity of Λ = 420 ± 16 nm.
Figure 2(a) (inset) depicts the variation of the periodicity of the produced LIPSS as a function of number of pulses at 23 J/cm2. The results indicate that Λ remained basically constant under treatment with trains of 10 to 5000 pulses, being always in the range Λ = 400 – 450 nm, and insensitive to an increase in the photon dose. An analogous trend was observed for the other tested fluences, even at low fluence levels of just 2 J/cm2. This behavior could be explained by assuming a quasi-constant temperature of the BDD surface during the time the sample is being irradiated. The high thermal conductivity of the substrate diffuses the laser-generated heat fast enough to maintain the surface temperature constant and independent of an increase of laser power. It remains to be studied if performing the experiment at higher repetition rates leads to a different LIPSS periodicity.
The LIPSS depth was investigated by scanning atomic force microscopy (AFM model JPK NanoWizard). The nano-tip used in the AFM was a TAP300-G rotated monolithic silicon probe with a tip radius <10 nm, with half cone angle of 25°. A typical result of the measurements is depicted in Fig. 3, which shows the highly regular nature of the LIPSS patterns for a fluence of 12 J/cm2 and 100 pulses. The AFM analysis showed that the topology of the irradiated area contains nano-ripples with very similar height, and the average depth has been found to be around ~150 nm.
Fig. 3 AFM scan of the irradiated area showing an average depth of ~150 nm. The measured surface area enhancement was approximately ~50%.
3.2 Structural characterization
Raman spectroscopy was used to analyze if bulk physical properties were affected by the treatment. Spectra was acquired at room temperature with a Nd:YVO4 laser (532 nm), in back-scattering geometry, by using a Confocal Raman microscope Alpha 300R WiTec (~1 cm−1 resolution) equipped with a cryo-cooled charge coupled device (CCD) detector; the spot size was ~2 μm. Spectra of the investigated fs-laser treated BDD samples are shown in Fig. 4.
Fig. 4 Raman spectra of BDD before and after fs-laser treatment (23 J/cm2, 1000 pulses), red and blue lines, respectively. In the inset, it is noticeable the left shift of 1330 cm−1 diamond peak, pointing out a slight lattice stress.
The broad peaks centered at 500 and 1225 cm−1 for both recorded spectra are related to locally distorted lattice structure induced by the addition of boron dopant. The peak at 1332 cm−1 belongs to the triply degenerate zone-center phonon of the sp3 crystalline diamond lattice. The crystalline pure diamond peak intensity (1332 cm−1) decreases and its frequency exhibits a shift towards lower wavenumber (∼1330 cm−1) because of the B content. The recorded spectra showed no major changes in doping levels or structure between the laser treated and un-treated areas. The slight decrease in intensity of the diamond peak can be ascribed to Fano resonance, which quenches the diamond line but does not affect the quality of the diamond as B doping is increased. Furthermore, with higher concentrations of B, the 500 cm−1 and 1225 cm−1 broad bands usually become more intense too [34].
The inset of Fig. 4 shows a magnified plot of the diamond peak around 1330 cm−1, which position changes slightly after laser treatment. Since the FWHM of the Raman peaks remains fairly unchanged, it provides an indication that the disorder introduced into the crystal structure is negligible in terms of amount of plastic deformation. The downshift of diamond peak to lower wavenumbers (from 1330 cm−1 to 1325 cm−1) suggests the presence of lattice stress in the treated areas, although such a downshift could also be related to a size effect of the nano-structures.
3.3 Wettability properties
Among all the techniques that can be used to fabricate structures in the micro- or nano- scale, femtosecond laser machining has been demonstrated to be an effective technology to modify the wetting properties of a large list of material surfaces [35, 36]. In some works, the LIPSS generation process is accompanied by a chemical treatment in order to obtain extreme wetting properties such as superhydrophobicity [37]. But even with only the LIPSS nano-patterning is performed the wetting properties of the surface are changed [38].
Controlling the adhesion of water on the BDD electrode surface could play a significant role since the adhesive property ultimately determines the dynamic performance of BDD electrodes. The surface adhesion is ascribed to the surface chemical composition and the presence of different surface micro- and nano-structures, which result in different wetting states. For BDDs surfaces, the wettability control is useful in order to improve and maximize the contact with water and therefore its performance as electrode. In order to analyze the effect of the fabricated LIPSS nanopatterns, wettability tests have been performed.
In our experiments, we prepared a large LIPSS area of 4 × 4 mm2 by slightly overlapping numerous laser adjacent passes in a similar way to the one described in [30]. FEG-SEM images of the resulting structures are shown in Fig. 5(c) and 5(d). The spatial continuity of the LIPSS ripples extends indefinitely along multiple independent passes in a spatially coherent manner. The changes of wetting angle at specified experimental conditions were video recorded and displayed in Fig. 5(a) and (b). The volume of water droplet was controlled by microliter pipette (V ~5 μL). Figure 5(a) shows that the measured contact angle is approximately 75° for an untreated sample, while Fig. 5(b) shows that the treated sample has a highly hydrophilic behavior, with a decreased contact angle of 46° for both orientations.
Fig. 5 (a) Wettability test result for an untreated BDD surface yields a contact angle of 75°. (b) LIPSS area enhances the wettability properties by decreasing the contact angle to approximately 46°. (c) Laser treated 16 mm2 squared surface, scale bar = 100 μm (d) Detailed view of the LIPSS produced nano-ripples. Scale bar = 1 μm.
4. Conclusions
We studied the conditions for producing boron doped diamond LIPSS nano-structures over large areas using near-IR ultrafast laser pulses. The fabricated structures have a periodicity of 430 nm and average depth of 150 nm. Micro-Raman spectroscopic investigations showed no substantial structural change or de-doping in the BDD sample after laser treatment, while wettability tests show a decrease in contact angle from 75° to 46°. We found that this method could be a simple alternative for the fabrication of nano-structures for enhancing the surface area and wettability of electrodes in BDD, with applications in electrochemical oxidation of organics or water treatment for example.
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