Haijun Tao

Icephobic/anti-icing potential of superhydrophobic Ti6Al4V surfaces with hierarchical textures

The main purpose of this paper was to investigate the icephobic potential of hierarchical superhydrophobic surfaces, which were prepared by modifying micro-nanostructures (constructed by the combination of sand blasting and hydrothermal treatment) on the surfaces of Ti6Al4V alloy with fluoroalkylsilane (FAS-17). We previously reported that this hierarchical superhydrophobic surface displayed excellent non-wettability with apparent contact angle of 161° and sliding angle of 3°. Thus, the present study focused on the systematic characterizations and analyses of the icephobic potential of the superhydrophobic surfaces around three parameters, including icing-delay time, ice adhesion strength, and contact time of an impacting droplet on cold superhydrophobic surfaces. The results indicated that the icing-delay time of a droplet on the superhydrophobic surface was many times longer than that of a droplet on the smooth Ti6Al4V substrate, and the ice adhesion strength on superhydrophobic surface was greatly reduced, which was attributed to the Cassie wetting state of a droplet on the surface. Additionally, the dynamic droplet impact and rebound assay demonstrated that water droplets always bounced off of the superhydrophobic surfaces before freezing under subzero conditions.

Anti-icing Potential of Superhydrophobic Ti6Al4V Surfaces: Ice Nucleation and Growth

On the basis of the icing-delay performance and ice adhesion strength, the anti-icing potential of the superhydrophobic surface has been well-investigated in the past few years. The present work mainly emphasized the investigations of ice nucleation and growth to fully explore the anti-icing potential of the superhydrophobic surface. We took the various surfaces ranging from hydrophilic to superhydrophobic as the research objects and, combining the classical nucleation theory, discussed the ice nucleation behaviors of the water droplets on these sample surfaces under the condition of supercooling. Meanwhile, the macroscopical growth processes of ice on these surfaces were analyzed on the basis of the growth mechanism of the ice nucleus. It was found that the superhydrophobic surface could greatly reduce the solid-liquid interface nucleation rate, owing to the extremely low actual solid-liquid contact area caused by the composite micro-nanoscale hierarchical structures trapping air pockets, leading to the bulk nucleation dominating the entire ice nucleation at the lower temperatures. Furthermore, ice on the superhydrophobic surface possessed a lower macroscopical growth velocity as a result of the less ice nucleation rate and the insulating action of the trapped air pockets.

Preparation and Characterization of TiO2 Nanotube Arrays via Anodization of Titanium Films Deposited on FTO Conducting Glass at Room Temperature

Abstract Self-organized TiO2 nanotube arrays with micro-scale length were prepared on fluorine-doped tin oxide (FTO) conducting glass in NH4F/glycerol electrolyte by electrochemical anodization of pure titanium films deposited by radio frequency magnetron sputtering (RFMS) at room temperature. The samples were characterized by means of field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and photoelectrochemistry methods. The results showed that Ti films prepared at the condition of Ar pressure 0.5 Pa, power 150 W, and 0.5 h at room temperature possessed the zone T model structure with good homogeneity and high denseness. When the anodization time was prolonged from 1 to 3 h at the voltage of 30 V, the pore diameter of TiO2 nanotubes increased from 50 to 75 nm, and the length increased from 750 to 1100 nm and then gradually decreased to 800 nm, while their wall morphology changed from smooth to rough. Also with increasing the anodization voltage, the pore diameter became larger, and the remaining oxide layer reduced, which could be easily removed by ultrasonic-chemical cleaning in 0.05% (w, mass fraction) diluted HF solution. Moreover, the photocurrent response curves and electrochemical impedance spectroscopy (EIS) results indicated that UV-illumination clearly enhanced the effective separation of the electron-hole pairs and the crystallized electrodes from the annealing treatment of as-anodized electrodes at 450 °C exhibited a better photoelectrochemical performance.

Preparation of TiO2 nanotube on glass by anodization of Ti films at room temperature

Abstract In order to fabricate titania nanotubes on glass substrate, Ti thin films (700–900 nm) were first deposited by radio-frequency(RF) magnetron sputtering and then anodized in an aqueous HF electrolyte solution at room temperature. The morphology and structure of the nanotubes were identified by means of field emission scanning electron microscopy(FE-SEM) and X-ray diffractometry(XRD). The effects of anodization parameters (concentration of electrolyte, applied voltage) on nanotube morphology were comprehensively investigated. The results show that the dense and crystalline Ti film can be obtained on the unheated glass substrate under the sputtering power of 150 W, and the anodization current and voltage play significant roles in the formation of titania nanotube with different tube sizes.

In situ synthesis of two-dimensional leaf-like Cu2ZnSnS4 plate arrays as a Pt-free counter electrode for efficient dye-sensitized solar cells

Kesterite-structure Cu2ZnSnS4 (CZTS) has been proved to be a high-performance Pt-free counter electrode (CE) material for dye-sensitized solar cells (DSSCs). Herein, a green but powerful two-step method based on solvothermal treatment was proposed to synthesize semi-transparent two-dimensional (2D) leaf-like CZTS plate arrays (PLAr) in situ on a FTO glass substrate, without any post-treatments, such as annealing, toxic sulfurization, or coating with other ancillary materials. The growth mechanism of the 2D leaf-like CZTS PLAr based on solvothermal treatment was illustrated. A power conversion efficiency (PCE) of 7.09% was obtained by utilizing leaf-like CZTS PLAr as the CE. Surprisingly, the PCE increased to 8.67% assisted by a mirror reflection. The excellent performance of DSSCs could be attributed to the high catalytic surface area, fast photo-generated electron transport at the counter electrode/redox electrolyte interface, remarkable electrocatalytic activity for I3− reduction, low charge transfer resistance toward the reduction of I3− ions, and high diffusion coefficient of the I3−. This work provides a green and feasible approach to construct high-quality metal sulfide nanoarrays on arbitrary conductive substrates under mild conditions (i.e. low temperature, no annealing, green, speediness) and promotes the development of Pt-free sulfide materials for sustainable photovoltaic applications.

Superhydrophobic Ti6Al4V surfaces with regular array patterns for anti-icing applications

In this article, we present a route to fabricate a robust anti-icing superhydrophobic surface containing the hierarchical structures of microscale array patterns (built by micromachining) and nanohairs (prepared via hydrothermal growth) on a Ti6Al4V substrate. In particular, the superhydrophobic surfaces not only exhibited high non-wettability and water repellency, but also generated a tremendous anti-icing potential. The results of the measurements indicated that the apparent contact angle reached 160°, the contact angle hysteresis reduced to 2°, and the spreading and recoiling process of an impact droplet can be completed within 12 ms. Furthermore, it also caused a longer icing-delay time (approximately 765 s) to hinder the ice formation and growth at −10 °C, and the ice adhesion strength was also only 70 kPa.

Approaching the theoretical contact time of a bouncing droplet on the rational macrostructured superhydrophobic surfaces

The aim of this study is to reveal theoretically and experimentally a limited contact time of a bouncing droplet on superhydrophobic surfaces with the rationally designed macrostructures. During impacting, the water droplet hydrodynamics is properly altered under the assistance of the macrotextures. As a consequence, the retracting process of the impact water droplet can be completely integrated into the process of spreading out to the maximal deformation, resulting in a limited overall contact time of approximately 5.5 ms, i.e., the time required for spreading out to the maximal deformation.

Relationship between Wetting Hysteresis and Contact Time of a Bouncing Droplet on Hydrophobic Surfaces

The contact time of impacting water droplets on superhydrophobic surfaces directly reflects the extent of thermal and energy conversions between the water droplet and the surface, which is also considered to be crucial to the practical applications. The purpose of this study was to reveal the relationship between the contact time and the wetting hysteresis. We designed and fabricated six classes of surfaces with different extent of hydrophobicity through modifying the microscale/nanoscale hierarchical textured titanium surfaces with 1H,1H,2H,2H-perfluorodecyltrimethoxysilane, and we filmed the contact process of the water droplet impacting on these surfaces using a high-speed camera. It can be concluded that wetting hysteresis played a significant role in determining how long the impacting water droplet can bounce off the surface, based on the interfacial wetting mechanism and the work done against the resistance force generated by contact angle hysteresis during the dynamic process.

Bouncing dynamics of impact droplets on the convex superhydrophobic surfaces

Bouncing dynamics of impact droplets on solid surfaces intensively appeal to researchers due to the importance in many industrial fields. Here, we found that droplets impacting onto dome convex superhydrophobic surfaces could rapidly bounce off with a 28.5% reduction in the contact time, compared with that on flat superhydrophobic surfaces. This is mainly determined by the retracting process of impact droplets. Under the action of dome convexity, the impact droplet gradually evolves into an annulus shape with a special hydrodynamic distribution. As a consequence, both the inner and external rims of the annulus shape droplet possess a higher retracting velocity under the actions of the inertia force and the surface energy change, respectively. Also, the numerical simulation provides a quantitative evidence to further verify the interpretation on the regimes behind the rapidly detached phenomenon of impact droplets.

Facile synthesis of single crystalline sub-micron Cu2ZnSnS4 (CZTS) powders using solvothermal treatment

Single crystalline sub-micron Cu2ZnSnS4 (CZTS) powders were successfully synthesized by a facile solvothermal method, using L-cysteine as sulfur precursor. It was confirmed that pure kesterite structured CZTS powders were synthesized at 400 °C after 5 h, with the irregular polygonal particle size being 500 nm to 1 μm. Interestingly, single crystalline CZTS particles were also obtained by simple solvothermal treatment without post annealing. The H2S released from L-cysteine had an impact on the growth of CZTS particles at high temperature. Further, the as-synthesized CZTS powders were used as counter electrode for dye-sensitized solar cells (DSSCs) and it is indicated that the CZTS counter electrode based DSSCs show a conversion efficiency of 4.243%.