Methylammonium lead iodide perovskite, CH 3 NH 3 PbI 3 (MAPbI 3 ), has made great progress in its efficiency as used in solid-state solar cells during recent years. Meanwhile, the degradation of its performance in moisture has attracted great attentions, but the specific mechanismis not yet fully established. The water effects on the detailed structure and properties of the perovskite CH 3 NH 3 PbI 3 have been carefully explored based on first-principles calculations. The results reveals that the water adsorption energy on the CH 3 NH 3 PbI 3 (001) surface is about 0.30 eV, while the water can easily penetrate into the surface in the form of molecular state owing to the huge interspace of CH 3 NH 3 PbI 3 , which can further corrode down the whole structure gradually. More importantly, the deformation of the structure greatly affects the electronic structure, which decreases the optical absorption. Such work paves an important way to understand the initial degradation progress of the perovskite structure under the humidity condition, which should help to optimize the structure to prevent the penetration of water in the system. The conversion of solar energy into electricity has attracted great attentions because of the increasing energy demands of future generations without negatively impacting the global environment. 1-2 On the other hand, dye-sensitized solar cells (DSCs) based on nanocrystalline metal oxides like TiO 2 3-4 are a promising photovoltaic device for a renewable energy source. In recent years, new organic-inorganic hybrid perovskite compounds (MAPbX 3 , X=halogen; MA=CH 3 NH 3 ) 5-11 have been used as light harvesters for solid-state DSCs. These MAPbX 3 compounds stand out for their low cost, wide light absorption, ferroelectric properties and high efficiency. 12-18 In fact, since the first reported perovskite solar cell with power conversion efficiency (PCE) of 3.81% by Kojima and co-workers in 2009, 19 the amazing growth rate of PCE about these perovskite materials has been made in the following years. In 2011, Park et al. fabricated MAPbI 3 perovskite solar cells with PCE of 6.54%. 20 Then Kim et al. achieved a PCE of up to 9.7% based on spiro-MeOTAD as hole transport materials in 2012. 21 In 2013, Noh et al. demonstrated highly efficient solar cells of a PCE of 12.3% as a result of tunable composition for MAPb(I 1-x Br x ) 3 . 22 In 2014, Grätzel and co-workers reported an efficiency of 17.01% by controlling the size of MAPbI 3 cuboids during their growth. 23 Up to now, the PCE of perovskite-based solar cells reaches to nearly 20%. 7 Although the methylammonium lead iodide MAPbI 3 perovskite shows an outstanding performance and tantalizing prospect in solar cells, there are deficiencies needed to overcome at the same time. One vital problems is that MAPbI 3 perovskite films are extremely sensitive to moisture in air. 7-8, 24-27 Many experiments have demonstrated that the effect of moisture on MAPbI 3 plays a crucial role in the performance of perovskite solar cells. 22, 28-30 In spite of various...

We performed ab initio calculations on the basic set of MoS 2 single-layer materials, namely the 1H, 1T and 1T 0 polytypes, to lay a theoretical framework on the emerging breakthrough-discoveries of high activity towards the hydrogen evolution reaction (HER) in exfoliated MoS 2 and related materials. Our calculations show that for exfoliated MoS 2 , 1T 0 is the most HER active polytype, with active sites both on the basal plane and at the edges of the layered grains. In comparison, the basal planes of the 1H and 1T polytypes are HER inactive and their edge-sites are not as active as those of the 1T 0 polytype. We also found that 1T-MoS 2 is unstable and easily transforms into 1T 0 -MoS 2 , and the 1T 0 phase is metastable with a considerable barrier >0.7 eV to bar its transformation into the most stable 1H phase. Further, unlike the case of exfoliated WS 2 , the HER activity of exfoliated MoS 2 is not so critically affected by the lattice strain. In addition, in contrast to the 1T 0 -WS 2 , the 1T 0 -MoS 2 is not metallic but has a very small bandgap of 0.1-0.2 eV. Hence, 1T 0 -MoS 2 should have a high enough conductivity and other suitable properties for it to function as an electrochemical HER catalyst.

Bioimaging is a key technique for monitoring behavior and activity in vivo and plays an important role in the life science and medical fields. In the present work, for the first time, a new, safe cellulose based hybrid hydrogel was constructed from a cellulose solution containing rare-earth doped phosphor (PP) in an alkali/urea aqueous system using epichlorohydrin as a crosslinker. Its structure and properties were characterized by wide angle X-ray diffraction, FT-IR spectra, solid-state 13 C NMR, field emission scanning electron microscopy, UV-vis spectroscopy, fluorescence spectra, and compression tests. The results indicated that the PP particles were tightly embedded in the macroporous cellulose matrix, which not only supplied cavities for PP immobilization through relatively strong intermolecular hydrogen bonding interactions, but also supplied the pore wall as a shell to protect the structure and character of PP. Thus, the cellulose/PP hybrid (CPH) hydrogels emitted relatively strong green fluorescence under a UV lamp, as well as high brightness and long-lasting afterglow. This could avoid harmful radiation in the body and improve signal resolution with lower cell autofluorescence interference. Notably, CPH with strong afterglow could be detected both under the skin and in the stomach with and without excitation light, showing promising prospects as a candidate for bioimaging. Moreover, the hybrid hydrogels exhibited good compressive strength and processability.

Nonprecious metal catalysts for hydrogen evolution reaction (HER) have recently received growing attention. Herein, we designed a highly active MXene nanofiber catalyst with a high specific surface area (SSA) via the hydrolyzation of bulk MAX ceramics, and a subsequent HF etching process. Compared with traditional MXene flakes, the MXene nanofibers delivered a much higher SSA and exposed more active sites in the cross section. As a result, the MXene nanofiber delivered an enhanced HER activity with a low overpotential of 169 mV at a current density of 10 mA cm–2, a depressed Tafel slope of 97 mV dec–1, and low electrochemical resistance. The improved SSA and exposed active sites are responsible for the enhanced activity. This work provides a novel synthesis method for MXene nanofibers, and MXene nanofibers are also promising for applications in batteries, supercapacitors, and catalytic fields.

In this work, chitin microspheres (NCM) having a nanofibrous architecture were constructed using a "bottom-up" fabrication pathway. The chitin chains rapidly self-assembled into nanofibers in NaOH/urea aqueous solution by a thermally induced method and subsequently formed weaved microspheres. The diameter of the chitin nanofibers and the size of the NCM were tunable by controlling the temperature and the processing parameters to be in the range from 26 to 55 nm and 3 to 130 μm, respectively. As a result of the nanofibrous surface and the inherent biocompatibility of chitin, cells could adhere to the chitin microspheres and showed a high attachment efficiency, indicating the great potential of the NCM for 3D cell microcarriers.