Hydrogen production from water based on photoelectrochemical (PEC) reactions is feasible to solve the urgent energy crisis. Herein, hierarchical 3D self-supporting WO3 micro-nano architectures in situ grown on W plates are successfully fabricated via ultrafast laser processing hybrid with thermal oxidation. Owing to the large surface area and efficient interface charge transfer, the W plate with hierarchical porous WO3 nanoparticle aggregates has been directly employed as the photoanode for excellent PEC performance, which exhibits a high photocurrent density of 1.2 mA cm–2 at 1.0 V vs Ag/AgCl (1.23 V vs RHE) under AM 1.5 G illumination and reveals excellent structural stability during long-term PEC water splitting reactions. The nanoscale and microscale features can be facilely tuned by controlling the laser processing parameters and the thermal oxidation conditions to achieve improved PEC activity. The presented hybrid method is simple, cost-effective, and controllable for large-scale fabrication, which should provide a new and general route that how the properties of conventional metal oxides can be improved via hierarchical 3D micro-nano configurations.Keywords: hierarchical nanostructures; photoelectrochemical; ultrafast laser; water splitting; WO3;

Functional metal surfaces with minimum optical reflection over a broadband spectrum have essential importance for optical and optoelectronic devices. However, the intrinsically large optical impedance mismatch between metals and the free space causes a huge obstacle in achieving such a purpose. We propose and experimentally demonstrate a general pulse injection controlled ultrafast laser direct writing strategy for fabricating highly effective antireflection structures on metal surfaces. The presented strategy can implement separate and flexible modifications on both microscale frame structures and nanoscale particles, a benefit from which is that optimized geometrical light trapping and enhanced effective medium effect reducing the surface reflection can be simultaneously achieved within one hybrid structure. Thus, comprehensively improved antireflection performances can be realized. Hybrid structures with substantial nanoparticles hierarchically attached on regularly arrayed microcones are generally constructed on different metal surfaces, achieving highly efficient light absorption over ultraviolet to near-infrared broadband spectrum regions. Reflectance minimums of 1.4%, 0.29%, and 2.5% are reached on Cu, Ti, and W surfaces, respectively. The presented strategy is simple in process, adaptable for different kinds of metals, reproduceable in dual-scale structural features, and feasible for large-area production. All these advantages make the strategy as well as the prepared antireflection structures excellent candidates for practical applications.Keywords: antireflection; broadband; metal; micro−nano structure; ultrafast laser; ultralow reflectance;

In this paper, we report a unique approach to directly grow nano-scale cotton-like CuO in situ on a Cu current collector using a nano-second laser to ablate it, forming an excellent integrated electrode. This villous nano-cotton CuO is made of micro-scale cotton-like structures, which consist of a huge number of nano-scale particles around 5 nm in diameter. The CuO–Cu integrated anode prepared in 10 minutes by laser ablation exhibits excellent rate performance, coulombic efficiency and a long cycle life. After 800 cycles at 1.5 A g−1, the coulombic efficiency remains higher than 99% and the retention capacity reaches 393.6 mA h g−1. These, to our knowledge, are the highest achieved among currently available CuO integrated anodes. This unique performance is attributed to the villous cotton-like CuO structure, which effectively resists volume expansion and contraction, increases the adhesion of active materials to the current collector, and enhances the conduction of the anode. Laser ablation is a highly efficient and cost-effective approach to prepare integrated oxide-metal anodes for LIBs with a long cycle life. In addition, FexOy–Fe, NixOy–Ni integrated anodes were also prepared for LIBs to verify the wide applicability of this method.

W/Cu joining is key for the fabrication of plasma facing components of fusion reactors, which however is very challenging due to the low bonding strength and high W/Cu interface thermal stress as a result of the immiscible nature of W-Cu binary system and thermal expansion coefficient mismatch between W and Cu. In this paper, we proposed a method for comprehensive enhancement of the mechanical and thermo-mechanical properties of W/Cu joints based on femtosecond (fs) laser fabricated micro/nano interface structures. Four kinds of surface structures, namely pristine structure, nano ripples, micro-cubes and micro-pits were fabricated on W by fs laser ablation and introduced into W/Cu joining interface by hot pressing joining at 1000 °C, 80 MPa. The micro/nano interface structures contribute to significant enhancement of the tensile strength, shearing strength and thermal fatigue life of the W/Cu joints, which reach 101.58 MPa, 186.47 MPa and >800 thermal cycles, increased by about 150%, 320% and 400% compared with W/Cu joints with pristine interface structure. This research provides an approach for enhanced joining between dissimilar materials, including but not limited to W and Cu.

•The NiO hierarchical nanostructures grown on Ni plate as high performance bi-functional electrocatalysts is demonstrated.•Large-scale NiO/Ni electrode can be synthesized by laser ablation within short time and directly used for high efficient water splitting.•A series of hierarchical nanostructured metal oxides (MOx, M= Ti, Mn, Fe, Co, Ni, Cu, Mo, Ag, Sn, W and NiFe) have been successfully synthesized by the generalized and fast laser ablation process.There is a growing interest in oxide nanocrystal based electrocatalysts for overall water splitting. Despite tremendous efforts, large-scale fabrication of highly-active and durable oxide electrocatalytic electrodes remains as a great challenge. Herein, we report a fast and general strategy for manufacturing a series of hierarchical nanostructured metal oxides (MOx, M= Ti, Mn, Fe, Co, Ni, Cu, Mo, Ag, Sn, W and NiFe) as electrocatalysts by laser ablation on corresponding metal substrates. Particularly, the NiO nanocrystal electrocatalysts (~3 nm) grown on Ni plates have been directly employed as highly active and stable bifunctional electrodes for both hydrogen evolution and oxygen evolution reactions, by taking advantage of its large surface area, rich defects, high hydrophilicity and aerophobicity. The facile laser treatments to construct nanoporous electrodes should facilitate the development of low-cost and high-performance electrocatalytic overall water splitting with large-scale.The NiO/Ni plates with hierarchical nanostructures can be synthesized by a fast and general laser ablation process and directly employed as highly active and stable bifunctional electrodes for large-scale overall water splitting.Download high-res image (198KB)Download full-size image

•Diverse micro/nano surface structures fabricated by fs laser ablation on W.•Micro/nano structures introduced into W/Cu interface by hot pressing joining.•W/Cu bonding strength up to 120 MPa enhanced by micro/nano interface structures.•Enhanced thermal diffusivity and thermal stress resistance of W/Cu joining.W/Cu joining is key for the fabrication of plasma facing components for fusion reactors, which however is very challenging due to the immiscible nature of Cu-W binary system and the mismatch of thermal expansion coefficient between W and Cu. In this research, we proposed a method for comprehensive enhancement of the bonding strength and heat transfer capability of W/Cu joining based on femtosecond (fs) laser fabricated micro/nano interface structures. Four kinds of surface structures, namely pristine structure, nano ripples, micro-cubes array and micro-pits array were designed and fabricated on W by fs laser ablation, which were then introduced into W/Cu interfaces by hot pressing joining at 1000 °C, 80 MPa. The micro/nano interface structures lead to significant enhancement of W/Cu bonding strength, which reaches 101.58 MPa for W/Cu direct joining and 120.43 MPa for W/Cu joining with Cu interlayer, increased by about 150% and 190% compared with W/Cu joining with a flat interface. The thermal diffusivity and thermal stress resistance of W/Cu joining are also improved. The strengthening mechanism is recognized to be the combined effects of micro/nano interface structures related interdiffusion between W and Cu and increased resistance against W/Cu interfacial debonding. Our research provides a method for enhanced joining between W and Cu, as well as a vast range of dissimilar materials.Download high-res image (332KB)Download full-size image

Efficient solar energy harvesting and photothermal conversion have essential importance for many practical applications. Here, we present a laser-induced cauliflower-shaped hierarchical surface nanostructure on a copper surface, which exhibits extremely high omnidirectional absorption efficiency over a broad electromagnetic spectral range from the UV to the near-infrared region. The measured average hemispherical absorptance is as high as 98% within the wavelength range of 200–800 nm, and the angle dependent specular reflectance stays below 0.1% within the 0–60° incident angle. Such a structured copper surface can exhibit an apparent heating up effect under the sunlight illumination. In the experiment of evaporating water, the structured surface yields an overall photothermal conversion efficiency over 60% under an illuminating solar power density of ∼1 kW m−2. The presented technology provides a cost-effective, reliable, and simple way for realizing broadband omnidirectional light absorptive metal surfaces for efficient solar energy harvesting and utilization, which is highly demanded in various light harvesting, anti-reflection, and photothermal conversion applications. Since the structure is directly formed by femtosecond laser writing, it is quite suitable for mass production and can be easily extended to a large surface area.

Realizing superhydrophobicity, high transparency on polydimethylsiloxane (PDMS) surface enlarges its application fields. We applied a femtosecond laser to fabricate well-designed structures combining microgrooves with microholes array on mirror finished stainless steel to form a template. Then liquid PDMS was charged for the duplicating process to introduce a particular structure composed of a microwalls array with a certain distance between each other and a microprotrusion positioned at the center of a plate surrounded by microwalls. The parameters such as the side length of microwalls and the height of a microcone were optimized to achieve required superhydrophobicity at the same time as high-transparency properties. The PDMS surfaces show superhydrophobicity with a static contact angle of up to 154.5 ± 1.7° and sliding angle lower to 6 ± 0.5°, also with a transparency over 91%, a loss less than 1% compared with plat PDMS by the measured light wavelength in the visible light scale. The friction robust over 100 cycles by sandpaper, strong light stability by 8 times density treatment, and thermal stability up to 325 °C of superhydrophobic PDMS surface was investigated. We report here a convenient and efficient duplicating method, being capable to form a transparent PDMS surface with superhydrophobicity in mass production, which shows extensive application potentials.

Micro/nano surface structures show great significance in both fundamental research and practical applications. Imprinting technique is widely used to fabricate micro/nano surface structures on polymers at low temperature, but barely implemented on metals. In this paper, femtosecond (fs) laser was used to fabricate micro/nano surface structures on tungsten (W), which served as molds for subsequent solid state imprinting process to replicate the counter structures onto copper (Cu) surface at temperature up to 1000 °C. Nano-ripples superimposed on micro-bumps array are fabricated without surface oxidation, and the height of the micro-bumps is adjustable by varying the laser energy input for W molds preparation. The as-prepared Cu surface structures are hydrophobic and turn to superhydrophobic after chemical modification. To demonstrate the feasibility and capability of this technique, similar surface structures were fabricated on A356 aluminum alloy (A356 AA) by liquid state imprinting at 740 °C using M2 steel molds prepared by picosecond (ps) laser ablation. This approach combines the advantages of ultrafast laser ablation to fabricate molds with micro/nano surface structures and high-temperature imprinting process to replicate the micro/nano structures onto target metallic surfaces. It opens new possibility for the mass fabrication of functional micro/nano surface structures on metals.

The Cassie-state stability plays a vital role in the applications of metallic superhydrophobic surfaces. Although a large number of papers have reported the superhydrophobic performance of various surface micro/nanostructures, the knowledge of which kind of micro/nanostructure contributes significantly to the Cassie-state stability especially under low temperature and pressure is still very limited. In this article, we fabricated six kinds of typical micro/nanostructures with different topography features on metal surfaces by a femtosecond laser, and these surfaces were modified by fluoroalkylsilane to generate superhydrophobicity. We then systematically studied the Cassie-state stability of these surfaces by means of condensation and evaporation experiments. The results show that some superhydrophobic surfaces, even with high contact angles and low sliding angles under normal conditions, are unstable under low temperature or external pressure. The Cassie state readily transits to a metastable state or even a Wenzel state under these conditions, which deteriorates their superhydrophobicity. Among the six micro/nanostructures, the densely distributed nanoscale structure is important for a stable Cassie state, and the closely packed micrometer-scale structure can further improve the stability. The dependence of the Cassie-state stability on the fabricated micro/nanostructures and the laser-processing parameters is also discussed. This article clarifies optimized micro/nanostructures for stable and thus more practical metallic superhydrophobic surfaces.

Infrared antireflection is an essential issue in many fields such as thermal imaging, sensors, thermoelectrics, and stealth. However, a limited antireflection capability, narrow effective band, and complexity as well as high cost in implementation represent the main unconquered problems, especially on metal surfaces. By introducing precursor micro/nano structures via ultrafast laser beforehand, we present a novel approach for facile and uniform growth of high-quality oxide semiconductor nanowires on a Cu surface via thermal oxidation. Through the enhanced optical phonon dissipation of the nanowires, assisted by light trapping in the micro structures, ultralow total reflectance of 0.6% is achieved at the infrared wavelength around 17 μm and keeps steadily below 3% over a broad band of 14–18 μm. The precursor structures and the nanowires can be flexibly tuned by controlling the laser processing procedure to achieve desired antireflection performance. The presented approach possesses the advantages of material simplicity, structure reconfigurability, and cost-effectiveness for mass production. It opens a new path to realize unique functions by integrating semiconductor nanowires onto metal surface structures.

Superhydrophobic surfaces with tunable water adhesion have attracted much interest in fundamental research and practical applications. In this paper, we used a simple method to fabricate superhydrophobic surfaces with tunable water adhesion. Periodic microstructures with different topographies were fabricated on copper surface via femtosecond (fs) laser irradiation. The topography of these microstructures can be controlled by simply changing the scanning speed of the laser beam. After surface chemical modification, these as-prepared surfaces showed superhydrophobicity combined with different adhesion to water. Surfaces with deep microstructures showed self-cleaning properties with extremely low water adhesion, and the water adhesion increased when the surface microstructures became flat. The changes in surface water adhesion are attributed to the transition from Cassie state to Wenzel state. We also demonstrated that these superhydrophobic surfaces with different adhesion can be used for transferring small water droplets without any loss. We demonstrate that our approach provides a novel but simple way to tune the surface adhesion of superhydrophobic metallic surfaces for good potential applications in related areas.Keywords: copper; femtosecond laser; microdroplet manipulation; superhydrophobic surface; tunable water adhesion;

Corrosion of metals causes tremendous financial loss and disasters every year. Graphene is a promising candidate for anti-corrosion coating, due to its unique properties, e.g. chemical inertness, impermeability and high conductivity. Despite being a commercially important material, it is difficult to grow graphene on carbon steels and is therefore prominently grown on copper or nickel substrates. Here, we report a unique approach to grow graphene on carbon steel and explore its anti-corrosion application. By introducing Ni element into carbon steel through a laser alloying process to form a Ni/Fe alloy catalyst, we make it feasible to grow graphene on carbon steel. The corrosion rate of graphene covered carbon steel is only 0.05 mm/year, much lesser than that of the stainless steel (0.09 mm/year). The corrosion resistance is up to ∼1900 Ω cm2, which is almost 7 times that of original steel (270.7 Ω cm2). These results indicate that the in situ grown graphene coatings perform very well in resisting harsh environments, much better than stainless steel itself.

We report the anticorrosion capability of graphene directly and locally grown on a bulk nickel substrate by a high power laser beam irradiation at room temperature. The anticorrosive performance of the locally laser-grown graphene on Ni, together with Ni covered graphene transferred from the CVD method and bare Ni, was investigated by electrochemical measurements and immersion tests. The corrosion rate of laser-grown graphene evaluated from potentiodynamic polarizations was 4.5 times lower than that of bare Ni, and the corrosion potential was increased by 126 mV. Electrochemical impedance spectroscopy (EIS) measurements complemented the potentiodynamic results and indicated significant improvement in corrosion resistance in the presence of in situ grown graphene. The in situ grown graphene could protect the underlying substrate effectively, mainly due to its strong interaction with the substrate, and excellent barrier properties. This laser fabrication of graphene directly and locally on a bulk substrate provides a promising and workable method to protect metals from corrosion.

Graphene has many unique properties, most of them strongly depend on the number of layers. It is significant to develop a facile approach to realize the controllable growth of graphene with specific number of layers. We ever reported an efficient approach to grow graphene rapidly and locally by laser irradiation. In this work, we offers yet another important feature, to control the number of layers of graphene. Ni-Cu alloy has been reported to be used successfully as the catalyst for graphene growth with controllable number of layers. In that case, the Ni-Cu alloys with different compositions were normally formed by thermal evaporation. Here we provide an efficient way to fabricate the Ni-Cu alloy catalysts by laser cladding. Then the high power laser was employed to melt the Ni and Cu mixed powders. Different Ni-Cu alloy catalysts were formed in a high rate of 720 mm2/min with a thickness of 1.2 mm. Then the graphene with controllable layers was rapidly and locally grown on the Ni-Cu catalysts by laser irradiation at a high rate (18 cm2/min) at room temperature. We found that the Ni-Cu catalyst with 15 % Cu could be helpful to grow single layer graphene, which occupied 92.4 % of the entire film. Higher Cu content didn’t promote the growth due to the oxygen involved during the growth process. The controllable growth mechanism of graphene by laser processing was discussed. Combining the rapid catalyst fabrication and graphene synthesis make it a cost- and time-efficient method to produce the controllable graphene films.

Direct fabrication of graphene on solid carbon coated nickel surface was realized by laser irradiation at room temperature. High-quality graphene was obtained rapidly, e.g. 28.8 cm2/min. Arbitrary patterns designed by computer aided design (CAD) software were fabricated directly on Ni substrates without additional mask or setup. Raman mapping results showed that monolayer/bilayer graphene regions accounted for 64% of the film area. Graphene films exhibited excellent resistance to corrosion. The extremely low corrosion current density and high free corrosion potential in 3.5% NaCl aqueous (aq) solutions showed that the as-produced graphene had a superior anti-corrosion performance. The penetration and precipitation mechanism of carbon into Ni substrate during the fabrication process were also discussed. This approach may reach the scale large enough for practical applications.

•A method of fabricating nPRMMCs without using nano-powders as raw materials•Serial study on the effect of specific energy input on particle size and shape•Resultant high quality, uniform nano-TiCp/Inconel 625 composite coatings•Improved mechanical properties of Inconel 625 by introducing nano-TiC particlesNano-particulate reinforced metal matrix composites (nPRMMCs) exhibit excellent comprehensive properties unmatched by conventional micro-particulate reinforced metal matrix composites (μPRMMCs). However, current techniques for fabricating nPRMMCs usually use nano-powders as raw materials, which are not preferred due to their agglomeration trend and harmful size. In this paper, we developed a technique to fabricate nano-TiCp reinforced Inconel 625 composite coatings by laser cladding of an Inconel 625 + 5 wt.% TiC powder mixture, particle size of the raw powders both in micrometer range. By controlling the specific energy input, the micro-TiCp partially dissolved into nanometer scale. The influence of specific energy input on particle size, morphology and the microstructure, phase constitution and mechanical properties of the composite coatings were investigated by scanning electron microscopy, X-ray diffraction, transmission electron microscopy and nano-indentation test. Nano-TiCp reinforced Inconel 625 composite coatings were achieved at the specific energy input of 25.3 kJ/g. The hardness and modulus of the nPRMMCs are 3.36 GPa and 190.91 GPa, increased by 10.33% and 12.39% respectively compared to laser cladded Inconel 625 substrate. The nPRMMCs show potential in applications such as the fabrication of turbine blades and engine components with improved performance.

•Uniform few-layer graphene was fabricated at room temperature and ambient conditions.•Laser-assisted chemical vapor deposition was used to grow the layers in a few seconds.•The effect of process parameters on graphene growth was discussed.•This cost effective method could facilitate the integration of graphene in electronic devices.A fast, simple technique was developed to fabricate few-layer graphene films at ambient pressure and room temperature by laser-assisted chemical vapor deposition on polycrystalline Ni plates. Laser scanning speed was found as the most important factor in the production of few-layer graphene. The quality of graphene films was controlled by varying the laser power. Uniform graphene ribbons with a width of 1.5 mm and a length of 16 mm were obtained at a scanning speed of 1.3 mm/s and a laser power of 600 W. The developed technique provided a promising application of a high-power laser system to fabricate a graphene film.

A novel approach utilizing current-assisted CO2 laser irradiation was used to join two monolayer graphene flakes. Two partially overlapped graphene flakes were irradiated with a continuous wave CO2 laser, together with a current at a constant voltage of 30 V. Raman spectrometer and transmission electron microscope (TEM) analyses showed the joining signal at a laser power density of 8 W/cm2 with an irradiation time of 30 s and a current of 25 mA (30 V) for 5 min. The joining mechanism of graphene flakes was also investigated. We provide a novel route to realize large-area graphene joint for potential applications.

Micro-nano hierarchical structure on the substrate was fabricated by a hybrid approach including laser deposition, laser ablation and chemical dealloying. The structure consists of micro bumps with a width of 50 μm and a height of 100 μm, and nanoporous structures with a size of 70–150 nm on the micro bumps. XRD and XPS results confirm that these hierarchical structures were made of Cu2O. For use in comparison, three additional structures with feature size in milliscale, microscale, and nanoscale were also prepared respectively by the proposed methods. Under visible light, the micro-nano structure exhibited the best performance of photodegradation. It is the result of the large specific surface and the catalytic reaction driven by the cuprous oxides.Keywords: dealloying; laser ablation; micro-nano hierarchical structure; photo degradation;

Laser deposition of Inconel 738 on directionally solidified (DS) Ni-based superalloy demonstrates a strong susceptibility to cracking. Cracks originate from the liquation of low melting point eutectic on the DS grain boundary. Five boundary liquation and interface cracking styles were identified. Sound Inconel 738 deposition layers were achieved free of cracks.

The microstructural evolution in high power CO2 laser cladding of steel with Stellite 6+WC powder mixture was investigated, among which the WC vol.% was, respectively, 0, 9, 18, 27, 36, 45, 54, 72 and 100% by dual powder feeding Stellite 6 and WC powders with different feeding rates. Two significantly different solidification characteristics were found in the microstructural evolution in laser clads of Stellite 6+WC. The first one was characterized by the dendrites and interdendritic eutectics from 0 to 36% WC, in which the added WC was completely melted into the melt pool and the re-solidified structure contained α-Co,σ-CoCr and various carbides such as M7C3. The second one was characterized by various faceted dendrites in block, flower, butterfly, star shapes and the matrix from 45 to 100% WC, in which most of the WC was melted, the microstructure contained re-solidified WC, Co and various Co–W–C/Fe–W–C complex carbides. Maximum 26 and 64 wt.% W were, respectively, dissolved into the matrix and the faceted dendrites. When laser cladding WC powder with severe dilution from the substrate, a microstructure with different composition appeared within the faceted dendrites with interior approximately 90 wt.% of W and exterior approximately 70 wt.% of W. Such a structure was probably the frozen result of the peritectic reaction during laser cladding rapid solidification.