Peixun Fan

Superhydrophobic Surfaces Fabricated by Femtosecond Laser with Tunable Water Adhesion: From Lotus Leaf to Rose Petal

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.

Superhydrophilicity to superhydrophobicity transition of picosecond laser microstructured aluminum in ambient air

Studies regarding the wettability transition of micro- and nano-structured metal surfaces over time are frequently reported, but there seems to be no generally accepted theory that explains this phenomenon. In this paper, we aim to clarify the mechanism underlying the transition of picosecond laser microstructured aluminum surfaces from a superhydrophilic nature to a superhydrophobic one under ambient conditions. The aluminum surface studied exhibited superhydrophilicity immediately after being irradiated by a picosecond laser. However, the contact angles on the surface increased over time, eventually becoming large enough to classify the surface as superhydrophobic. The storage conditions significantly affected this process. When the samples were stored in CO2, O2 and N2 atmospheres, the wettability transition was restrained. However, the transition was accelerated in atmosphere that was rich with organic compounds. Moreover, the superhydrophobic surface could recover their original superhydrophilicity by low temperature annealing. A detailed XPS analysis indicated that this wettability transition process was mainly caused by the adsorption of organic compounds from the surrounding atmosphere onto the oxide surface.

Precise Control of the Number of Layers of Graphene by Picosecond Laser Thinning

The properties of graphene can vary as a function of the number of layers (NOL). Controlling the NOL in large area graphene is still challenging. In this work, we demonstrate a picosecond (ps) laser thinning removal of graphene layers from multi-layered graphene to obtain desired NOL when appropriate pulse threshold energy is adopted. The thinning process is conducted in atmosphere without any coating and it is applicable for graphene films on arbitrary substrates. This method provides many advantages such as one-step process, non-contact operation, substrate and environment-friendly, and patternable, which will enable its potential applications in the manufacturing of graphene-based electronic devices.

Cassie-State Stability of Metallic Superhydrophobic Surfaces with Various Micro/Nanostructures Produced by a Femtosecond Laser

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.

Robust and Stable Transparent Superhydrophobic Polydimethylsiloxane Films by Duplicating via a Femtosecond Laser-Ablated Template

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.

Broadband High-Performance Infrared Antireflection Nanowires Facilely Grown on Ultrafast Laser Structured Cu Surface.

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.

Wettability conversion of ultrafast laser structured copper surface

The wettability of metal oxides is vital to many applications including water erosion, filtration, and bioimplantation. In this work, the authors studied the wettability conversion behavior of picosecond laser structured copper surfaces in different atmospheres. The copper surfaces showed hydrophilicity initially after being irradiated by a picosecond laser. However, when they were stored in ambient air, their contact angles increased over time and became highly hydrophobic finally. The storage atmosphere influenced this process greatly, the atmosphere rich in CO2 or O2 would restrain the wettability transition, but the organic-rich and vacuum atmosphere would accelerate it. Detailed surface chemical analysis revealed that the adsorption of organic matters from the air played an important role in this wettability conversion process.

Rapid fabrication of surface micro/nano structures with enhanced broadband absorption on Cu by picosecond laser.

A surface micro/nano structuring technique was demonstrated by utilizing a picosecond laser beam to rapidly modify the optical property of copper surfaces with a scanning speed up to tens of millimeters per second. Three kinds of surface micro/nanostructures corresponding to three levels of reflectance were produced which are obviously different from those induced by a femtosecond or nanosecond laser. Specifically, a porous coral-like structure results in over 97% absorptivity in the visible spectral region and over 90% absorptivity in average in the UV, visible, and NIR regions (250 - 2500 nm). Potential applications may include solar energy absorbers, thermal radiation sources, and radiative heat transfer devices.

Angle-independent colorization of copper surfaces by simultaneous generation of picosecond-laser-induced nanostructures and redeposited nanoparticles

The simultaneous generation of nanostructures and redeposited nanoparticles on copper surfaces through their direct ablation in air by a high power and high repetition rate ps laser was demonstrated. Basic and detailed analysis on the formation and the size distribution of nanoparticles spreading over the nanostructured copper surfaces was performed. Lower scanning speed causes more laser pulse input onto the target surface, resulting in a more dense distribution of the nanoparticles with a nearly constant mean radius. The changes in the particle distribution render the copper surfaces to unique reflection spectra responses and surface colors, which are independent of the viewing angles. The present research can pave the way for the practical applications of ps laser in generating nanoparticles on metal surfaces and tailoring their optical properties.

Large-Scale Tunable 3D Self-Supporting WO3 Micro-Nano Architectures as Direct Photoanodes for Efficient Photoelectrochemical Water Splitting

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.