Nowadays, the serious resource shortage and environmental polluting issues have attracted tremendous attention worldwide. Renewable energy such as hydroenergy, wind energy and solar energy, has been in high demanded and emerged as promising substitutions of fossil fuel. Among them, solar irradiation is the most valuable energy source in the near future, which is abundant and naturally unlimited. Researchers have devoted to seek methods to efficiently utilize solar energy and practically applied in various areas. Solar evaporation is an attractive strategy to utilize solar energy for distillation without consuming fossil fuels. Recently, different solar absorbers based on diverse materials have been extensively studied to transfer solar energy into heat for vapor generation, such as gold nanoparticles and carbon-based bilayer structures.^{[1], [2]} However, most of the absorbers are fabricated complexly with extra cost, which limits their large-scale application.

In this work, the black polyurethane (PU) sponge with three dimensional porous structures was demonstrated as the solar light absorber for heat localization. The black PU sponge can be recycled from the used packaging materials, which are usually abandoned after utilization and difficult to decompose naturally. Every year, the production of black PU sponge as the packaging materials is numerous. Recycling and reusing the PU sponge contributes to sustainable development. Here, the PU sponge provides porous channels for fluent water supply, low thermal conductivity for heat localization and low intensity to create surface evaporation. A simple hydrophilic treatment of this PU sponge was applied to improve the wettability by being stirred in dopamine solution. An evaporation efficient of 52.2%, which is more than 3 times higher than natural evaporation process, was achieved by this modified sponge in a relatively simple and low cost method. Furthermore, it provides a new idea to reutilize the waste materials for solar energy conversion.

Acknowledgements

This work is financially supported by the Research Grants Council of Hong Kong, China (Project Number: GRF 152109/16E PolyU B-Q52T).

Reference

[1] Neumann, Oara, et al. "Solar vapor generation enabled by nanoparticles." Acs Nano 7.1 (2012): 42-49.

[2] Jiang, Qisheng, et al. "Bilayered Biofoam for Highly Efficient Solar Steam Generation." Advanced Materials 28.42 (2016): 9400-9407.

About 400 semiconductor solids are known to have photocatalytic activity for water splitting. Yet there is no single material that could satisfy all the requirements for desired photocatalysts: i) suitable band gap energy (1.7 eV< Eg < 2.3 eV) for high efficiency, ii) proper band position for reduction and/or oxidation of water, iii) long-term stability in aqueous solutions, iv) low cost, v) high crystallinity, and vi) high conductivity. Hence, in the selection of photocatalytic materials, we better start from intrinsically stable materials made of earth-abundant elements. Upon selection of the candidate materials, we can also modify the materials for full utilization their potentials. The main path of efficiency loss in photoelectrochemical water splitting process is recombination of photoelectrons and holes. We discuss the material designs to minimize the e^- - h⁺ recombination including; i) heterojunction photoanodes for effective charge separation, ii) band engineering to extend the range of light absorption, iii) metal or anion doping to improve conductivity of the semiconductor and, iv) one-dimensional nanomaterials to secure a short hole diffusion distance and vectoral electron transfer, and v) loading co-catalysts for facile charge separation. Finally, we need to construct a stand-alone solar fuel production system by combining with a solar cell in tandem, which provides bias voltage needed for the photolytic reactions making possible the fuel production only with solar energy without any external energy supply.

TiO₂ photocatalyst has attracted great interest owing to its wide applications in environmental cleaning and new energy production. Both the composition and the structure modifications affect its photocatalytic performances. Herein, we report the design of various TiO₂-hybrid nanomaterials like TiO₂ doped by CdS, CdSe, C₃N₄, Au, and Ag quantum dots with low band-gap. They exhibit high activity during photocatalytic/photoelectrocatalytic degradation of organic pollutants, H₂ evolution from water splitting, and CO₂ conversion by using sunlight. Meanwhile, the potential application of the photo-driven dual electrodes (Z-mechanism systems) in photocatalytic fuel cell (PEC) has been explored. Furthermore, better photocatalysts served as coating layer for electrodes through composition- and structure-engineering have been developed.

Silicon is a promising contender in the race for low-bandgap absorbers for use in a solar driven monolithic water splitting cell (PEC). However, given its role as the low-bandgap material the silicon must be situated behind the corresponding high-bandgap material and as such, it will be exposed to (red) light from the dry back-side – not from the wet front side, where the electrochemistry takes place.^1,2 Depending on the configuration of the selective contacts (junctions) this may lead to compromises between high absorption and low recombination.^2,3 We discuss the tradeoffs and compare modeling results to measurements. Regardless of configuration, the wet surface of the silicon is prone to passivation or corrosion and must therefore be carefully protected in service in order to remain active. TiO₂ has been found as an effective protection layer for both photoanodes and photocathodes in acid electrolyte ⁴ and NiCoO_x for photoanodes in alkaline electrolyte. ³ This is discussed in the context of general considerations for photoelectrode protection and state-of-the-art performance. ⁵

References:
[1]: B. Seger et al., Energ. Environ. Sci., 7 (8), 2397-2413 (2014), DOI:10.1039/c4ee01335b
[2]: D. Bae et al., Energ. Environ. Sci., 8 (2), 650-660 (2015), DOI: 10.1039/c4ee03723e
[3]: D. Bae et al., ChemElectroChem, 3 (10), 1546-1552  (2016), DOI: 10.1002/celc.201500554
[4]: B. Mei et al., J. Phys. Chem. C., 119 (27), 15019-15027 (2015), DOI: 10.1021/acs.jpcc.5b04407
[5]: D. Bae et al., Chem. Soc. Rev. (2017), DOI:10.1039/C6CS00918B

We report a highly sensitive ion detection method based on an optofluidic catalytic laser, which uses the fluorescent product generated by the enzyme-substrate reaction as the gain medium. Using ions as inhibitors, the catalytic reaction slows down, resulting in a delay in the lasing onset time, which can be used as the sensing signal. The sensing mechanism of the catalytic laser is theoretically analyzed and the performance is experimentally characterized. Sulfide anion is chosen as a model ion because of its broad adverse impacts on both environment and human health. Due to the optical feedback provided by the laser, the small difference in ion concentrations can be amplified. Consequently, a detection limit of 10 nM is achieved with a dynamic range as large as three orders of magnitude, representing significant improvement over the traditional fluorescence and colorimetric methods. This work will open a door to a new catalytic-laser-based chemical sensing platform for detecting a wide range of species that could inhibit the catalytic reaction.

It is well established that individual cells, even from the same origin, differ from each other in many aspects due to stochastic biological processes and differences in environmental perturbations. Cell heterogeneity has been found to play an important role in many biological processes, including cellular differentiation and immune response, as well as disease development. The heterogeneity of cells in culture and in organisms poses a challenge for many experimental measurements. Traditional ensemble analysis based on averaging a large population of cells, as a result, masks the behavior of minority subpopulations and effectively blinding researchers to possibly interesting differences between cells. Single-cell analysis is an important and emerging field that gives insights into heterogeneity between cells and advanced cellular processes at high resolution, which is important for cancer research, regenerative medicine, immune system research and diagnostics, as well as for the production of therapeutics. Microfluidics has proven to be a leading tool for single cell analysis since device dimensions are on the same scale as those of cells, allowing for precise fluid and cell manipulation at high throughput. In this talk, I will present our recent efforts on developing droplet microfluidic technology for high throughput single cell isolation, manipulation, and analysis at the DNA, RNA and protein level with single-molecule sensitivity.

Electric field effect on liquid drop rebounding is studied in interdigitated array (IDA) electrodes covered with superamiphiphobic coating under AC field. Coefficients of restitution are measured in different applied voltages. The results show that a medium voltage applied on IDA (~ 30V) can effectively change the drop rebounding on the IDA surface. We also directly measured the adhesive force curves and contact angles of drops on IDA under different voltages. The results suggest that only small fraction of the drop is tightly pinned on the surface while the other parts of the surface remain superhydrophobic. This new method of controlling liquid-solid interaction may enable new drop based microfluidic applications. A simple control of drop re-bouncing distance is also demonstrated.

Reversible adhesions of many animals are often mediated by micro- and nano-structures and liquids. Two distinctly different types of structures specialized for strong and reversible adhesion on a variety of substrates have been identified: smooth pads and hairy pads. Both types of structures can maximize the formation of effective contact on the surfaces with a wide range of roughness and chemical compositions under various environmental conditions. The liquids are transported to or drained from the contact interface in order to enhance or avoid adhesions of attachment systems, playing a rather complex role. Inspired by the wet adhesions in nature, polymeric structures have been fabricated based on “bottom-up” and “top-down” methods. The adhesion and friction performances of these bioinspired structures are then investigated with the presence of liquid at the contact interface.

Unlike rigid substrates, a sessile drop can deform a soft substrate by capillary forces, with the deformation being inversely proportional to the substrate modulus. The substrate deformation can affect the contact angle and contact angle hysteresis, and thus various capillary phenomena. The first part of this presentation will focus on the effects of substrate stiffness on spontaneous wetting. We show that the early stage of fast wetting of soft substrates is dominated by inertia, which is similar to that on rigid substrates, and the wetting dynamics follows a power law depending on wettability, but not on stiffness. However, the duration of the fast inertial wetting is controlled by substrate stiffness. This is an indication of a viscoelastic dissipation process occurring during wetting that starts after some characteristic time dependent on the surface tension of the liquid, on the viscosity of the surface, and on the speed of wetting. In the second part of the talk, drop impact dynamics on different soft substrates will be introduced. Several impact phenomena, which depend on the dynamic interaction between impinging droplets and soft substrates, were identified. We demonstrate that the effect of surface stiffness on drop impact is related to the dynamic response of viscoelastic substrates to impact. We also show that the dynamic wettability of soft substrates affects the post-impact droplet oscillations.

Water droplets impacting onto solid surfaces give rise to a broad diversity of fascinating physical phenomena including “crown formation”, jetting of secondary droplets and splashing. Due to the discovery of micro/nanostructured surfaces, the physics of droplet impact has been greatly enriched. An impacting droplet can exhibit wetting, pinning, partial rebound and complete rebound resulting from the generation of special wetting states at different impact conditions. Notably, the investigation of enhanced droplet mobility upon superwetting substrates is directly relevant to a wide range of applications, such as anti-icing, water-repellency or water-harvesting, anti-bacterial coatings and phase change heat transfer. In this talk, I will briefly discuss our recent efforts and exciting progress to this important problem. By designing novel surface made from an array of widely spaced tapered posts, the impinging droplet can bounce off with a pancake-like shape without retracting, leading to a fourfold reduction in contact time compared with conventional complete rebound. Then, I will discuss an asymmetric bouncing on cylindrical surfaces with a convex/concave architecture of size comparable to that of the drop, which leads to a 40% reduction in the total contact time. I will also discuss a new bouncing regime that combines the inherent advantage of lotus leaves and pitcher plant surfaces. We find that there exists a superhydrophobic-like bouncing on thin liquid films, characterized by the contact time, the spreading dynamics, and the restitution coefficient independent of the underlying liquid substrate. Finally, I will discuss the break of wetting symmetry of a droplet at high temperature by creating two concurrent thermal states (Leidenfrost and contact-boiling) on patterned surfaces, and thus engendering a preferential motion of a droplet towards the region with a higher heat transfer coefficient.