Yanhang Ma

Cobalt phosphate-modified barium-doped tantalum nitride nanorod photoanode with 1.5% solar energy conversion efficiency

Spurred by the decreased availability of fossil fuels and global warming, the idea of converting solar energy into clean fuels has been widely recognized. Hydrogen produced by photoelectrochemical water splitting using sunlight could provide a carbon dioxide lean fuel as an alternative to fossil fuels. A major challenge in photoelectrochemical water splitting is to develop an efficient photoanode that can stably oxidize water into oxygen. Here we report an efficient and stable photoanode that couples an active barium-doped tantalum nitride nanostructure with a stable cobalt phosphate co-catalyst. The effect of barium doping on the photoelectrochemical activity of the photoanode is investigated. The photoanode yields a maximum solar energy conversion efficiency of 1.5%, which is more than three times higher than that of state-of-the-art single-photon photoanodes. Further, stoichiometric oxygen and hydrogen are stably produced on the photoanode and the counter electrode with Faraday efficiency of almost unity for 100 min.

Weaving of organic threads into a crystalline covalent organic framework

Weaving organic threads Woven fabrics are inherently flexible. Liu et al. created a molecular fabric analog using metal-organic frameworks (see the Perspective by Gutierrez-Puebla). Phenanthroline ligands on a copper metal complex directed the addition of organic linkers via imine bonds to create helical organic threads with woven texture. Removing the copper allowed the strands to slide against each other and increased the elasticity of the material 10-fold. Science, this issue p. 365; see also p. 336 A metal-organic framework templates the synthesis of a material made of woven organic polymers. [Also see Perspective by Gutierrez-Puebla] A three-dimensional covalent organic framework (COF-505) constructed from helical organic threads, designed to be mutually weaving at regular intervals, has been synthesized by imine condensation reactions of aldehyde functionalized copper(I)-bisphenanthroline tetrafluoroborate, Cu(PDB)2(BF4), and benzidine (BZ). The copper centers are topologically independent of the weaving within the COF structure and serve as templates for bringing the threads into a woven pattern rather than the more commonly observed parallel arrangement. The copper(I) ions can be reversibly removed and added without loss of the COF structure, for which a tenfold increase in elasticity accompanies its demetalation. The threads in COF-505 have many degrees of freedom for enormous deviations to take place between them, throughout the material, without undoing the weaving of the overall structure.

π–π interaction of aromatic groups in amphiphilic molecules directing for single-crystalline mesostructured zeolite nanosheets

One of the challenges in material science has been to prepare macro- or mesoporous zeolite. Although examples of their synthesis exist, there is a need for a facile yet versatile approach to such hierarchical structures. Here we report a concept for designing a single quaternary ammonium head amphiphilic template with strong ordered self-assembling ability through π-π stacking in hydrophobic side, which stabilizes the mesostructure to form single-crystalline mesostructured zeolite nanosheets. The concept is demonstrated for the formation of a new type of MFI (zeolite framework code by International Zeolite Association) nanosheets joined with a 90° rotational boundary, which results in a mesoporous zeolite with highly specific surface area even after calcination. Low binding energies for this self-assembling system are supported by a theoretical analysis. A geometrical matching between the arrangement of aromatic groups and the zeolitic framework is speculated for the formation of single-crystalline MFI nanosheets.

Enantiomerically enriched, polycrystalline molecular sieves

Significance Zeolites and zeolite-like molecular sieves are used as adsorbents and heterogeneous catalysts to prepare a wide variety of products ranging from gasoline to monomers for polymers such as polyethylene terephthalate, which is used in plastic bottles. Pharmaceuticals, pesticides, fragrances, and components in food can contain chiral centers. Here, we prepare enantioenriched polycrystalline samples of a molecular sieve and show that this type of porous material can function as an adsorbent to separate chiral molecules and as a heterogeneous catalyst to perform chiral reactions. This initial demonstration proves that bulk, enantioenriched chiral zeolites and zeolite-like molecular sieves can be synthesized and that this type of solid can be used to prepare chiral, small molecules. Zeolite and zeolite-like molecular sieves are being used in a large number of applications such as adsorption and catalysis. Achievement of the long-standing goal of creating a chiral, polycrystalline molecular sieve with bulk enantioenrichment would enable these materials to perform enantioselective functions. Here, we report the synthesis of enantiomerically enriched samples of a molecular sieve. Enantiopure organic structure directing agents are designed with the assistance of computational methods and used to synthesize enantioenriched, polycrystalline molecular sieve samples of either enantiomer. Computational results correctly predicted which enantiomer is obtained, and enantiomeric enrichment is proven by high-resolution transmission electron microscopy. The enantioenriched and racemic samples of the molecular sieves are tested as adsorbents and heterogeneous catalysts. The enantioenriched molecular sieves show enantioselectivity for the ring opening reaction of epoxides and enantioselective adsorption of 2-butanol (the R enantiomer of the molecular sieve shows opposite and approximately equal enantioselectivity compared with the S enantiomer of the molecular sieve, whereas the racemic sample of the molecular sieve shows no enantioselectivity).

A Synthetic Route for Crystals of Woven Structures, Uniform Nanocrystals, and Thin Films of Imine Covalent Organic Frameworks

Developing synthetic methodology to crystallize extended covalent structures has been an important pursuit of reticular chemistry. Here, we report a homogeneous synthetic route for imine covalent organic frameworks (COFs) where crystallites emerge from clear solutions without forming amorphous polyimine precipitates. The key feature of this route is the utilization of tert-butyloxycarbonyl group protected amine building blocks, which are deprotected in situ and gradually nucleate the crystalline framework. We demonstrate the utility of this approach by crystallizing a woven covalent organic framework (COF-112), in which covalent organic threads are interlaced to form a three-dimensional woven framework. The homogeneous imine COF synthesis also enabled the control of nucleation and crystal growth leading to uniform nanocrystals, through microwave-assisted reactions, and facile preparation of oriented thin films.

Phase Identification and Structure Solution by Three-Dimensional Electron Diffraction Tomography : Gd-Phosphate Nanorods

Hydrothermal synthesis of GdPO4 in the presence of poly(methacrylic acid) yields nanorods with a diameter of 15 nm and an aspect ratio of 20. Powder X-ray diffraction patterns showed that the GdPO4 nanorods display peaks characteristics for both monoclinic and hexagonal structures. Three-dimensional electron diffraction tomography (3D EDT) was used to determine the structures ab initio on the basis of reciprocal volume reconstruction of electron diffraction data sets collected from single nanorods. The crystal structure of the monoclinic form was shown to be P21/n, corroborating previous work. We were able to solve the 3D structure of the hexagonal P6222 form, which has not been reported previously. Our work shows that 3D EDT is a powerful method that can be used for solving structures of single nanocrystals.

A Hierarchical MFI Zeolite with a Two-Dimensional Square Mesostructure.

A conceptual design and synthesis of ordered mesoporous zeolites is a challenging research subject in material science. Several seminal articles report that one-dimensional (1D) mesostructured lamellar zeolites are possibly directed by sheet-assembly of surfactants, which collapse after removal of intercalated surfactants. However, except for one example of two-dimensional (2D) hexagonal mesoporous zeolite, no other zeolites with ordered 2D or three-dimensional (3D) mesostructures have been reported. An ordered 2D mesoporous zeolite can be templated by a cylindrical assembly unit with specific interactions in the hydrophobic part. A template molecule with azobenzene in the hydrophobic tail and diquaternary ammonium in the hydrophilic head group directs hierarchical MFI zeolite with a 2D square mesostructure. The material has an elongated octahedral morphology, and quaternary, ordered, straight, square channels framed by MFI thin sheets expanded along the a-c planes and joined with 90° rotations. The structural matching between the cylindrical assembly unit and zeolite framework is crucial for mesostructure construction.

Radical-Facilitated Green Synthesis of Highly Ordered Mesoporous Silica Materials

In the hydrothermal synthesis of highly ordered mesoporous silica material SBA-15, strong acid is typically required to catalyze the hydrolysis and condensation of silica species. Meanwhile, under strongly acidic conditions, the transition metal ions, e.g., iron ions, are difficult to incorporate into SBA-15 because of the facile dissociation of Fe-O-Si bonds. Here, we demonstrate an acid-free green synthetic strategy for the synthesis of highly ordered mesoporous SBA-15 and Fe-SBA-15 with the assistance of hydroxyl free radicals that are generated by physical or chemical methods. The prepared materials exhibit a large specific surface area compared to the counterparts prepared by conventional method under acidic conditions. Moreover, Fe-SBA-15 shows high metal loading efficiency as over 50%. Density functional theory calculations suggest that the hydroxyl free radicals exhibit higher catalytic activity than H+ ions for the hydrolysis of tetraethyl orthosilicate. This radical-facilitated synthesis approach overcomes the challenge to the direct synthesis of highly ordered SBA-15 and Fe-SBA-15 without adding any acid, providing a facile and environmentally friendly route for future large-scale production of ordered mesoporous materials.

Additive-free synthesis of mesoporous FAU-type zeolite with intergrown structure

Hierarchical porous zeolites attract great attention because of their porosity on different scales to improve molecular diffusion. Here, we report mesoporous Faujasite (FAU) zeolite nanosheets with intergrown structure synthesized in an additive-free system. The sample was composed of uniform nanosheets with a slice thickness of ∼50 nm, which held a honeycomb-like structure with abundant mesopores. This material exhibits both microporous and mesoporous structure: the intrinsic micropores with a diameter about 0.74 nm in the zeolite framework and the mesopores with a diameter about 10 nm existing within the zeolite nanosheets. The Si/Al ratios can be adjusted from 1.1 to 1.9 (zeolites X or Y). In addition, this simple and environmental method may provide inspiration to the synthesis of other hierarchical zeolites.摘要本文用无添加剂体系合成了具有共生结构的介孔FAU沸石. 该材料具有由约50纳米厚度的纳米片堆积形成的花状形貌, 而且具有两种孔道体系: 沸石骨架本身的微孔和存在于纳米片内部的晶间介孔. 通过对合成条件精确的控制以及对反应物比例的筛选, 分别合成了介孔X(硅铝比<1.5)型和Y(硅铝比>1.5)型沸石. 温和的温度以及高碱度使得在成核初期发生结晶-溶解-重结晶过程是形成介孔FAU沸石的关键条件. 该合成体系用简单和低成本的方法在FAU沸石中引入介孔, 有望突破沸石在大分子催化中的限制, 将有可能应用于工业化生产.

Structure Characterization of Mesoporous Materials by Electron Microscopy

The structural analysis of silica mesoporous crystals (SMCs) is a key step for understanding their properties, especially toward drug delivery. Although various techniques can be applied, electron crystallography has been proved the most powerful method in the structure solution of mesoporous materials. In this chapter, we will begin with introducing basic crystallography and extend to the 3D structure solution using a set of HRTEM images. Typical examples will also be given to show the structure features of 2D, 3D, cage-type, and hyperbolic surface-type mesostructures. A drug delivery system using SMCs as a reservoir will be given at the end.