3D hierarchical heterostructure NiFe LDH@NiCoP/NF electrodes are prepared successfully on nickel foam with special interface engineering and synergistic effects. This research finds that the as-prepared NiFe LDH@NiCoP/NF electrodes have a more sophisticated inner structure and intensive interface than a simple physical mixture. The NiFe LDH@NiCoP/NF electrodes require an overpotential as low as 120 and 220 mV to deliver 10 mA cm −2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 m KOH, respectively. Tafel and electrochemical impedance spectroscopy further reveal a favorable kinetic during electrolysis. Specifically, the NiFe LDH@NiCoP/NF electrodes are simultaneously used as cathode and anode for overall water splitting, which requires a cell voltage of 1.57 V at 10 mA cm −2 . Furthermore, the synergistic effect of the heterostructure improves the structural stability and promotes the generation of active phases during HER and OER, resulting in excellent stability over 100 h of continuous operation. Moreover, the strategy and interface engineering of the introduced heterostructure can also be used to prepare other bifunctional and cost-efficient electrocatalysts for various applications.

Reducing green hydrogen production costs is essential for developing a hydrogen economy. Developing cost-effective electrocatalysts for water electrolysis is thus of great research interest. Among various material candidates, transition metal phosphides (TMP) have emerged as robust bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) due to their various phases and tunable electronic structure. Recently, heterostructured catalysts have exhibited significantly enhanced activities toward HER/OER. The enhancement can be attributed to the increased amount of accessible active sites, accelerated mass/charge transfer, and optimized adsorption of intermediates, which arise from the synergistic effects of the heterostructure. Herein, a comprehensive overview of the recent progress of bifunctional TMP-based heterostructure is introduced to provide an insight into their preparation and corresponding reaction mechanisms. It starts with summarizing general fundamental aspects of HER/OER and the synergistic effect of heterostructures for enhanced catalytic activity. Next, the innovational strategies to design and construct bifunctional TMP-based heterostructures with enhanced overall water splitting activity, as well as the related mechanisms, are discussed in detail. Finally, a summary and perspective for further opportunities and challenges are highlighted for the further development of bifunctional TMP-based heterostructures from the points of practical application and mechanistic studies.

The authors present a technique that allows to modify the local characteristics of two-dimensional photonic crystals by controlled microinfiltration of liquids. They demonstrate experimentally that by addressing and infiltrating each pore with a simple liquid, e.g., water, it is possible to write pixel by pixel optical devices of any geometry and shape. Calculations confirm that the obtained structures indeed constitute the desired resonators and waveguide structures.

Through metal-assisted chemical etching (MaCE), superior purification of dirty Si is observed, from 99.74 to 99.9884% for metallurgical Si and from 99.999772 to 99.999899% for upgraded metallurgical Si. In addition, large area of silicon nanowires (SiNW) are fabricated. The purification effect induces a ∼35% increase in photocurrent for SiNW based photoelectrochemical cell.

deposit Si nanoparticles or Si nanowires (SiNWs) with dimension less than 100 nm. [ 7,8 ] However, high temperature growth, expensive precursors, and complex conducting layer coating are required. [8][9][10][11] On the other hand, top-down approaches are mostly based on expensive electronic grade silicon (EG-Si, purity > 99.999999%) and involve the use of templates or lithography and thus, lose their competence due to the increased process complexity. [ 12,13 ] Recently, metal-assisted chemical etching (MaCE) has emerged as a new, batch-processable top-down method to overcome these technical obstacles. [ 14,15 ] The technology is a room-temperature, wet chemical process that can produce nanostructured Si on a large scale and quantity. In addition, it spontaneously forms a unique internal mesoporosity, which has been recently recognized as the crucial aspect to maintain long-term cyclability. [ 12,16,17 ] The mesoporosity provides suffi cient inner space for the volume expansion of silicon. Moreover, the internal porosity provides good access to the electrolyte allowing fast charge transfer and full lithiation even when the silicon is completely amorphized. [ 18,19 ] Till date, Si particles and nanowires with mesoporosity have been synthesized via MaCE by the use of commercially available Si particles [ 18,19 ] and degenerated EG-Si wafers of high purity. [ 16,17,20 ] The porosity development strongly relies on heavy doping (dopant concentration > ≈10 19 cm −3 ). However, in both cases, the starting material costs are still high and at least in the order of $100 kg −1 . It is therefore highly desirable to explore inexpensive Si feedstock for meeting the needs of practical LIB applications. We have recently shown that MaCE can also be successfully applied to inexpensive metallurgical silicon. [ 21,22 ] In addition to the nanostructuring effect, MaCE also removes the impurities inside the metallurgical nanowires by complex redox reactions, thereby purifying the silicon by more than one order of magnitude. [ 21 ] In this communication, we present the large-area preparation of mesoporous SiNW from metallurgical silicon (MG-Si, purity ≈99.74%) with adjustable porosity by MaCE and their application as LIB anode. We demonstrate that this system has an encouraging cycling stability as the LIB anode. It exhibits a reversible capacity of about 2111 mAh g −1 at a current rate of 0.2 C, a promising stability of over 50 cycles, as well as a good rate capability, which is similar to a recent report on stainetched samples of ball-milled metallurgical-Si particles in ironcontaining HF solutions. [ 23 ] In comparison to recent reports on pillared silicon particle approaches (e.g., Nexeon's technology in the commercialization stage), [24][25][26] our technology requires no carbon coating and energy-intensive ball milling process, which can be easily upscaled in the industry and is compatible with roll-to-roll process. [ 27 ] Lithium-ion batteries (LIBs) are currently dominating the market of portable electronic devices as well as t...

We present an optical gas sensor based on the classical nondispersive infrared technique using ultracompact photonic crystal gas cells. The ultracompact device is conceptually based on low group velocities inside a photonic crystal gas cell and low-reflectivity antireflection layers coupling light into the device. Experimentally, an enhancement of the CO2 infrared absorption by a factor of 2.6 to 3.5 as compared to an empty cell, due to slow light inside a 2D silicon photonic crystal gas cell, was observed; this is in excellent agreement with numerical simulations. We show that, theoretically, for an optimal design enhancement factors of up to 60 are possible in the region of slow light. However, the overall transmission of bulk photonic crystals, and thus the performance of the device, i s limited by fluctuations of the pore diameter. Numerical estimates suggest that the positional variations and pore diameter fluctuations have to be well below 0.5% to allow for a reasonable transmission of a 1 mm device

Variations of the refractive index can be utilized in order to shift the stop band in periodic structures, such as photonic crystals. We report on investigations about three-dimensional macroporous silicon structures that are filled with a liquid crystal. Fourier transform infrared measurements indicate that a shift of the photonic band edge can be induced by changing the temperature. The director field in macropores within the silicon structure is investigated by H-NMR2 spectroscopy and compared to director field simulations. The latter method indicates a preferred parallel orientation of the director in the nematic state. Based on this finding, we analyze the optical properties.

Metal-assisted chemical etching (MaCE) has been shown to be a powerful and cost-effective method for surface nano-texturing and silicon micromachining. Since the motion of a metal catalyst during the etching process determines the etched morphology, understanding the mobility of the metal catalysts would enable precise control of the silicon structuring. Through the investigation of Pt nanoparticle (PtNP)-induced etching of silicon, we find that the Schottky barrier height of the metal-Si contacts strongly influences the charge transfer process during the etching. Consequently, the motion of the PtNPs is affected, which is different from previous understandings based on an electrokinetic model.