Xianhua Tan

Quantum dot-sensitized hierarchical micro/nanowire architecture for photoelectrochemical water splitting.

We report the fabrication of quantum dot-sensitized hierarchical structure and the application of the structure as a photoanode for photoelectrochemical water splitting. The structure is synthesized by hydrothermally growing ZnO nanowires on silicon microwires grown with the vapor-liquid-solid method. Then the hierarchical structure is further sensitized with CdS and CdSe quantum dots and modified with IrOx quantum dots. As a result, the silicon microwires, ZnO nanowires, and the quantum dot/ZnO core/shell structure form a multiple-level hierarchical heterostructure, which is remarkably beneficial for light absorption and charge carrier separation. Our experimental results reveal that the photocurrent density of our multiple-level hierarchical structure achieves a surprising 171 times enhancement compared to that from simple ZnO nanowires on a planar substrate. In addition, the photoanode shows high stability during the water-splitting experiment. These results prove that the quantum dot-sensitized hierarchical structure is an ideal candidate for a photoanode in solar water splitting applications. Importantly, the modular design approach we take to produce the photoanode allows for the integration of future discoveries for further improvement of its performance.

Patterned gradient surface for spontaneous droplet transportation and water collection: simulation and experiment

We demonstrate spontaneous droplet transportation and water collection on wedge-shaped gradient surfaces consisting of alternating hydrophilic and hydrophobic regions. Droplets on the surfaces are modeled and simulated to analyze the Gibbs free energy and free energy gradient distributions. Big half-apex angle and great wettability difference result in considerable free energy gradient, corresponding to large driving force for spontaneous droplet transportation, thus causing the droplets to move towards the open end of the wedge-shaped hydrophilic regions, where the Gibbs free energy is low. Gradient surfaces are then fabricated and tested. Filmwise condensation begins on the hydrophilic regions, forming wedge-shaped tracks for water collection. Dropwise condensation occurs on the hydrophobic regions, where the droplet size distribution and departure diameters are controlled by the width of the regions. Condensate water from both the hydrophilic and hydrophobic regions are collected directionally to the open end of the wedge-shaped hydrophilic regions, agreeing with the simulations. Directional droplet transport and controllable departure diameters make the branched gradient surfaces more efficient than smooth surfaces for water collection, which proves that gradient surfaces are potential in water collection, microfluidic devices, anti-fogging and self-cleaning.

Investigation of Fog Collection on Cactus-inspired Structures

We demonstrate the application of cactus-inspired structures for fog collection. The drop-on-cone system is modeled to analyze Gibbs free-energy gradients, equilibrium positions and motion of drops. Normalized free energy and free energy gradient are presented to characterize barrel and clam-shell drops, revealing the relations of the driving force and wettability to the half-apex angle of the cones. Small half-apex angle results in long collecting length and weak driving force. Thus it is important for fog collection to balance the driving force and collecting length with a suitable half-apex angle. Fog collection experiments on cactus-inspired structures are conducted for verification. Inflection points around 1.1° are observed, where the fog collection ability is mainly limited by the weak driving force when below the inflection points, while increases with the collecting length when above the inflection points. These indicate that the half-apex angle at the inflection point is a compromise between the driving force and collecting length, agreeing with the normalized functions. The results also prove that the hydrophilic cones are more suitable for fog collection with regard to condensation and driving force. Our research offers design guidance for efficient fog collection structure.

Fabrication of micro/nanotubes by mask-based diffraction lithography

We demonstrate a scalable and repeatable method to fabricate micro/nanotubes. Mask-based diffraction lithography was modeled, and the effects of key parameters including the wavelength of the illuminant light, the thickness of the photoresist, the diameter of the photomask pattern and the exposure dose during the process were simulated. Analysis of the results indicates that the optical intensity distributions in the photoresist form separated regions, which can be used to fabricate micro/nanotubes. Experiments were then carried out, and we achieved the production of silicon microtubes and tube-in-tube arrays. It can be found that the outer diameter of the microtubes decreases while the inner diameter increases with the increment of exposure time, consistent with the simulation results. Thus, the optical model is suitable for explaining experimental phenomena, guiding experiments and optimizing the fabrication process. The obtained silicon microtubes exhibit fine hydrophobic properties and provide a good matrix for integrating active nanomaterials, which can provide great potential in their application to energy storage devices, solar cells, sensors and catalysts.