Facile Fabrication of Superomniphobic Polymer Hierarchical Structures for Directional Droplet Movement

We report a facile method for fabricating polymer hierarchical structures, which are the engineered, ratchet-like microscale structures with nanoscale dimples, for the directional movement of droplets. The fabricated polymer hierarchical structures with no surface modifier show hydrophobic, superhydrophobic, or omniphobic characteristics depending on their intrinsic polymer properties. Further treatment with a surface modifier endows the polymer surfaces with superomniphobicity. The fabricated polymer substrates with no surface modifier enable the movement of the water droplet along the designed track at almost no inclination of the substrate

Electroassisted Transfer of Vertical Silicon Wire Arrays Using a Sacrificial Porous Silicon Layer

An electroassisted method is developed to transfer silicon (Si) wire arrays from the Si wafers on which they are grown to other substrates while maintaining their original properties and vertical alignment. First, electroassisted etching is used to form a sacrificial porous Si layer underneath the Si wires. Second, the porous Si layer is separated from the Si wafer by electropolishing, enabling the separation and transfer of the Si wires. The method is further expanded to develop a current-induced metal-assisted chemical etching technique for the facile and rapid synthesis of Si nanowires with axially modulated porosity

Three-Dimensional Hetero-Integration of Faceted GaN on Si Pillars for Efficient Light Energy Conversion Devices

An important pathway for cost-effective light energy conversion devices, such as solar cells and light emitting diodes, is to integrate III–V (<i>e</i>.<i>g</i>., GaN) materials on Si substrates. Such integration first necessitates growth of high crystalline III–V materials on Si, which has been the focus of many studies. However, the integration also requires that the final III–V/Si structure has a high light energy conversion efficiency. To accomplish these twin goals, we use single-crystalline microsized Si pillars as a seed layer to first grow faceted Si structures, which are then used for the heteroepitaxial growth of faceted GaN films. These faceted GaN films on Si have high crystallinity, and their threading dislocation density is similar to that of GaN grown on sapphire. In addition, the final faceted GaN/Si structure has great light absorption and extraction characteristics, leading to improved performance for GaN-on-Si light energy conversion devices

BiVO₄/WO₃/SnO₂ Double-Heterojunction Photoanode with Enhanced Charge Separation and Visible-Transparency for Bias-Free Solar Water-Splitting with a Perovskite Solar Cell

Coupling dissimilar oxides in heterostructures allows the engineering of interfacial, optical, charge separation/transport and transfer properties of photoanodes for photoelectrochemical (PEC) water splitting. Here, we demonstrate a double-heterojunction concept based on a BiVO₄/WO₃/SnO₂ triple-layer planar heterojunction (TPH) photoanode, which shows simultaneous improvements in the charge transport (∼93% at 1.23 V vs RHE) and transmittance at longer wavelengths (>500 nm). The TPH photoanode was prepared by a facile solution method: a porous SnO₂ film was first deposited on a fluorine-doped tin oxide (FTO)/glass substrate followed by WO₃ deposition, leading to the formation of a double layer of dense WO₃ and a WO₃/SnO₂ mixture at the bottom. Subsequently, a BiVO₄ nanoparticle film was deposited by spin coating. Importantly, the WO₃/(WO₃+SnO₂) composite bottom layer forms a disordered heterojunction, enabling intimate contact, lower interfacial resistance, and efficient charge transport/transfer. In addition, the top BiVO₄/WO₃ heterojunction layer improves light absorption and charge separation. The resultant TPH photoanode shows greatly improved internal quantum efficiency (∼80%) and PEC water oxidation performance (∼3.1 mA/cm² at 1.23 V vs RHE) compared to the previously reported BiVO₄/WO₃ photoanodes. The PEC performance was further improved by a reactive-ion etching treatment and CoO_<i>x</i> electrocatalyst deposition. Finally, we demonstrated a bias-free and stable solar water-splitting by constructing a tandem PEC device with a perovskite solar cell (STH ∼3.5%)

Branched TiO₂ Nanorods for Photoelectrochemical Hydrogen Production

We report a hierarchically branched TiO₂ nanorod structure that serves as a model architecture for efficient photoelectrochemical devices as it simultaneously offers a large contact area with the electrolyte, excellent light-trapping characteristics, and a highly conductive pathway for charge carrier collection. Under Xenon lamp illumination (UV spectrum matched to AM 1.5G, 88 mW/cm² total power density), the branched TiO₂ nanorod array produces a photocurrent density of 0.83 mA/cm² at 0.8 V versus reversible hydrogen electrode (RHE). The incident photon-to-current conversion efficiency reaches 67% at 380 nm with an applied bias of 0.6 V versus RHE, nearly two times higher than the bare nanorods without branches. The branches improve efficiency by means of (i) improved charge separation and transport within the branches due to their small diameters, and (ii) a 4-fold increase in surface area which facilitates the hole transfer at the TiO₂/electrolyte interface