Efficient photoelectrochemical water splitting with ultrathin films of hematite on three-dimensional nanophotonic structures.

Photoelectrochemical (PEC) solar water splitting represents a clean and sustainable approach for hydrogen (H2) production and substantial research are being performed to improve the conversion efficiency. Hematite (α-Fe2O3) is considered as a promising candidate for PEC water splitting due to its chemical stability, appropriate band structure, and abundance. However, PEC performance based on hematite is hindered by the short hole diffusion length that put a constraint on the active layer thickness and its light absorption capability. In this work, we have designed and fabricated novel PEC device structure with ultrathin hematite film deposited on three-dimensional nanophotonic structure. In this fashion, the nanophotonic structures can largely improve the light absorption in the ultrathin active materials. In addition, they also provide large surface area to accommodate the slow surface water oxidation process. As the result, high current density of 3.05 mA cm(-2) at 1.23 V with respect to the reversible hydrogen electrode (RHE) has been achieved on such nanophotonic structure, which is about three times of that for a planar photoelectrode. More importantly, our systematic analysis with experiments and modeling revealed that the design of high performance PEC devices needs to consider not only total optical absorption, but also the absorption profile in the active material, in addition to electrode surface area and carrier collection.

A biomimetic approach to evaluate mineralization of bioactive glass-loaded resin composites.

PURPOSE: This study explores novel solutions other than standard SBF for biomimetic evaluations of mineralization particularly for resin composites containing bioactive glass (BAG). METHODS: Experimental UDMA/TEGDMA resin composites with 0.0, 1.9, 3.8 or 7.7 vol% of 45S5 BAG fillers were prepared. Besides simulated body fluid (SBF) as control, the specimens were immersed in three other solutions either with bicarbonate which are Hank's balanced salt solution (HBSS) and cell culture medium (MEM), or without bicarbonate which is a novel Simple HEPES-containing Artificial Remineralization Promotion (SHARP) solution, for 3, 7 and 14 days. These solutions were then analyzed by ICP-OES and pH meter, and the surfaces of the BAG composites were analyzed by SEM, XRD and FTIR. RESULTS: ICP-OES revealed Ca and P concentration continuously decrease, while Si concentration increases with time in the solutions other than SBF, which showed almost unchanged elemental concentration. Only SHARP solution is able to maintain a constant pH over the immersion time. SEM, together with XRD and FTIR, showed nano-sized octacalcium phosphate (OCP) nanospheres formation on 3.8 and 7.7 vol% BAG composites after 14 days immersion in HBSS (500-600 nm) and MEM (300-400 nm). SHARP solution enabled OCP formation after 3 days and then self-assembled into urchin-like carbonated hydroxyapatite (CHA) microspheres encompassed with nanorods of 100 nm width and 8 µm length after 14 days of immersion for 7.7 vol% BAG composites. CONCLUSION: This study suggests SHARP solution can evaluate mineralization biomimetically whereas CHA microspheres can be formed on BAG-containing resin composites.

Three-Dimensional Perovskite Nanophotonic Wire Array-Based Light-Emitting Diodes with Significantly Improved Efficiency and Stability.

Hybrid perovskites have emerged as promising candidates for highly efficient light-emitting diodes in the past few years due to their excellent crystallinity, high color purity, wide-range bandgap tunability, and solution processability. However, the reported device external quantum efficiency has not reached the level on par with that of conventional inorganic and organic light-emitting diodes. Moreover, device stability still needs substantial improvement. In this work, we demonstrate the fabrication of perovskite nanophotonic wire array-based light-emitting diodes with a capillary-effect-assisted template method. Compared with the planar control device, the nanostructured device demonstrates 45% improvement of external quantum efficiency from 11% to 16% owing to substantial enhancement on device light extraction efficiency verified by optical modeling. Intriguingly, it is also discovered that the nanostructured device possesses 3.89 times lifetime compared to the planar control device, due to effective template passivation. The results here have clearly shown that with a proper photonic device structure design, both the device performance and lifetime can be significantly improved.

A nanostructured anti-biofilm surface widens the efficacy against spindle-shaped and chain-forming rod-like bacteria.

Current control of pathogenic bacteria at all biomaterial interfaces is poorly attuned to a broad range of disease-causing pathogens. Leading antimicrobial surface functionalization strategies with antimicrobial peptides (AMPs), defensins, have not shown their promised efficacy. One of the main problems is the lack of stability and swift clearance from the surface. Surface nanotopography bearing sharp protrusions is a non-chemical solution that is intrinsically stable and long-lasting. Previously, the geometrically ordered arrays of nanotipped spines repelled or rapidly ruptured bacteria that come into contact. The killing properties so far work on cocci and rod-like bacteria, but there is no validation of the efficacy of protrusional surfaces on pathogenic bacteria with different sizes and morphologies, thus broadening the utility of such surfaces to cover increasingly more disease entities. Here, we report a synthetic analogue of nanotipped spines with a pyramidal shape that show great effectiveness on species of bacteria with strongly contrasting shapes and sizes. To highlight this phenomenon in the field of dental applications where selective bacterial control is vital to the clinical success of biomaterial functions, we modified the poly(methyl)-methacrylate (PMMA) texture and tested it against Streptococcus mutans, Enterococcus faecalis, Porphyromonas gingivalis, and Fusobacterium nucleatum. These nanopyramids performed effectively at levels well above those of normal and roughened PMMA biomaterials for dentistry and a model material for general use in medicine and disease transmission in hospital environments.

Low-cost, flexible, disinfectant-free and regular-array three-dimensional nanopyramid antibacterial films for clinical applications.

In this work, a low-cost, scalable and highly repeatable approach was developed to prepare polystyrene films with three-dimensional nanopyramids on the surface. The nanopyramids have a tunable aspect ratio and more importantly, their anti-bacterial performance has been systematically studied. The effectiveness of the nanopyramids on E. coli growth inhibition and the role of the nanostructure aspect ratio were carefully studied through scanning electron microscopy and confocal laser scanning microscopy. The results showed an excellent antibacterial performance with more than 90% reduction in the E. coli population in all nanopyramid samples after a 168 h prolonged incubation time. The nanopyramid film developed here can be used for clinical and commercial applications to prevent the growth of pathogenic bacteria on various surfaces.

3D Arrays of 1024-Pixel Image Sensors based on Lead Halide Perovskite Nanowires.

Large-scale and highly ordered 3D perov-skite nanowire (NW) arrays are achieved in nanoengineering templates by a unique vapor-solid-solid reaction process. The excellent material properties, in conjunction with the high integration density of the NW arrays, make them promising for 3D integrated nanoelectronics/optoelectronics. Image sensors with 1024 pixels are assembled and characterized to demonstrate the technological potency.

Highly Efficient Flexible Perovskite Solar Cells with Antireflection and Self-Cleaning Nanostructures.

Flexible thin film solar cells have attracted a great deal of attention as mobile power sources and key components for building-integrated photovoltaics, due to their light weight and flexible features in addition to compatibility with low-cost roll-to-roll fabrication processes. Among many thin film materials, organometallic perovskite materials are emerging as highly promising candidates for high efficiency thin film photovoltaics; however, the performance, scalability, and reliability of the flexible perovskite solar cells still have large room to improve. Herein, we report highly efficient, flexible perovskite solar cells fabricated on ultrathin flexible glasses. In such a device structure, the flexible glass substrate is highly transparent and robust, with low thermal expansion coefficient, and perovskite thin film was deposited with a thermal evaporation method that showed large-scale uniformity. In addition, a nanocone array antireflection film was attached to the front side of the glass substrate in order to improve the optical transmittance and to achieve a water-repelling effect at the same time. It was found that the fabricated solar cells have reasonable bendability, with 96% of the initial value remaining after 200 bending cycles, and the power conversion efficiency was improved from 12.06 to 13.14% by using the antireflection film, which also demonstrated excellent superhydrophobicity.

Low-cost, flexible, and self-cleaning 3D nanocone anti-reflection films for high-efficiency photovoltaics.

Low-cost engineered nanotemplates are used to mold flexible nanocone anti-reflection (AR) films. Both optical reflectance measurements and photovoltaics characterizations demonstrate that the flexible nanocone AR films can considerably suppress device front-side reflectance and thus improve the power conversion efficiency of high-efficiency thin-film CdTe solar cells. Additionally, these nanocone AR films are found to be superhydrophobic and thus possess self-cleaning capability.

Roll-to-roll fabrication of large scale and regular arrays of three-dimensional nanospikes for high efficiency and flexible photovoltaics.

Three-dimensional (3-D) nanostructures have demonstrated enticing potency to boost performance of photovoltaic devices primarily owning to the improved photon capturing capability. Nevertheless, cost-effective and scalable fabrication of regular 3-D nanostructures with decent robustness and flexibility still remains as a challenging task. Meanwhile, establishing rational design guidelines for 3-D nanostructured solar cells with the balanced electrical and optical performance are of paramount importance and in urgent need. Herein, regular arrays of 3-D nanospikes (NSPs) were fabricated on flexible aluminum foil with a roll-to-roll compatible process. The NSPs have precisely controlled geometry and periodicity which allow systematic investigation on geometry dependent optical and electrical performance of the devices with experiments and modeling. Intriguingly, it has been discovered that the efficiency of an amorphous-Si (a-Si) photovoltaic device fabricated on NSPs can be improved by 43%, as compared to its planar counterpart, in an optimal case. Furthermore, large scale flexible NSP solar cell devices have been fabricated and demonstrated. These results not only have shed light on the design rules of high performance nanostructured solar cells, but also demonstrated a highly practical process to fabricate efficient solar panels with 3-D nanostructures, thus may have immediate impact on thin film photovoltaic industry.

Programmable nanoengineering templates for fabrication of three-dimensional nanophotonic structures.

Porous anodic alumina membranes (AAMs) have attracted great amount of attention due to their potential application as templates for nanoengineering. Template-guided fabrication and assembly of nanomaterials based on AAMs are cost-effective and scalable methods to program and engineer the shape and morphology of nanostructures and nanomaterials. In this work, perfectly ordered AAMs with the record large pitch up to 3 µm have been fabricated by properly controlling the anodization conditions and utilization of nanoimprint technique. Due to the capability of programmable structural design and fabrication, a variety of nanostructures, including nanopillar arrays, nanotower arrays, and nanocone arrays, have been successfully fabricated using nanoengineered AAM templates. Particularly, amorphous Si nanocones have been fabricated as three-dimensional nanophotonic structures with the characterization of their intriguing optical anti-reflection property. These results directly indicate the potential application of the reported approach for photonics and optoelectronics.