C9H7NBrX (X = Cl, Br, NO3): Three Excellent Birefringent Crystals with Distinct Optical Anisotropy Regulated by Anions.

A large optical anisotropy is the most important parameter of birefringent crystals. Integrating π-conjugated groups with large polarizable anisotropy into target compounds is a common strategy for constructing brilliant birefringent crystals. However, the key problem is to enhance the density of the birefringence-active units and further arrange them parallelly. In this study, three novel birefringent crystals, C9H7NBrX (X = Cl, Br, NO3), are successfully synthesized by introducing a new birefringence-active [C9H7NBr]+ unit. Interestingly, these compounds feature similar layered structures but exhibit different optical anisotropies at 550 nm (0.277 for C9H7NBrCl, 0.328 for C9H7NBrBr, and 0.401 for C9H7NBrNO3) owing to the different anions in them. Particularly, the small trigonal planar NO3 anions perfectly fill the interstices of the π-conjugated [C9H7NBr]+ groups with large optical anisotropy, with the resulting compound C9H7NBrNO3 showing superior optical properties compared to the others. The above findings provide strategies for designing new optical materials with large birefringence by matching birefringence-active groups of different sizes. Additionally, a new theory for predicting and comparing the polarizability anisotropy of compounds is proposed, which would guide in exploring large birefringent crystals.

Noncentrosymmetric Crystal [C10H8NO2]2SiF6·H2O with Large Birefringence.

Crystals with noncentrosymmetric structures are applied in many fields, but reported compounds have a high probability of forming a centrosymmetric structure. Here, by hydrogen-bonding the π-conjugated [C10H8NO2]+ cation with the separated [SiF6]2- octahedron, a noncentrosymmetric isoquinoline hexafluorosilicate monohydrate optical crystal of [C10H8NO2]2SiF6·H2O was formed under the regulatory influence of hydrogen bonding. It not only possesses a moderate second harmonic generation response (1.0 × KDP) but also has a large birefringence (0.282 at 550 nm), which is greater than those of most commercial birefringent crystals. In addition, the UV-vis-NIR diffuse reflectance spectrum and thermal stability analysis are also reported. Our finding gives insight into how to design noncentrosymmetric structural compounds in the organic-inorganic system.

(C8H7N2O2)2[Bi2Br8]·2H2O and (C8H7N2O2)6[Bi2Cl10]Cl2·2H2O: Exploring Birefringent Crystals in Hybrid Halide Systems.

In this study, new hybrid birefringent crystals of (C8H7N2O2)2[Bi2Br8]·2H2O and (C8H7N2O2)6[Bi2Cl10]Cl2·2H2O were successfully synthesized by introducing a new birefringent group [C8H7N2O2]+ by a simple aqueous solution evaporation method. They crystallize in the P21/n space group, and their structure consists mainly of the π-conjugated group [C8H7N2O2]+ and the octahedron centered on Bi3+. By first-principles calculations, the birefringence response comes from the [C8H7N2O2]+ group with a planar π-conjugated structure. Meanwhile, the synthesis, structure, first-principles calculations, and optical properties are reported in this paper.

Naphthalene-Like Unit That Induces Strong Optical Anisotropy in C10H8NO2Br and C10H8NO2Br·H2O.

Two metal-free birefringent crystals, C10H8BrNO2 and C10H8BrNO2·H2O, which contain a new birefringence-active [C10H8NO2]+ gene, were synthesized via a mild solution evaporation method. In their crystal structures, the π-conjugated naphthalene-like [C10H8NO2]+ groups are basically aligned, which induces high optical anisotropy; i.e., the title compounds exhibit large birefringences of 0.36 and 0.41 at 550 nm according to first-principles calculations. Moreover, the UV-vis-near-IR diffuse-reflectance spectra suggest that they have similar optical band gaps. Structural analysis and theoretical calculations show that the [C10H8NO2]+ unit is responsible for the good optical anisotropy. These results make the naphthalene-like motif a good structural gene to search for new birefringent crystals.

(C3N6H7)2SbF5·H2O Exhibiting Strong Optical Anisotropy from the Optimal Arrangement of π-Conjugated (C3N6H7)+ Groups.

An antimony fluoride melamine birefringent crystal, (C3N6H7)2SbF5·H2O, was obtained by introducing the π-conjugated delocalized melamine and antimony trifluoride via a simple aqueous solution evaporation method. It features one-dimensional parallel [C3N6H7]∞ chains further connected by hydrogen bonds originated from [SbF5]2- groups with lone pairs. The experimental optical band gap (4.74 eV) allows it to be used in the ultraviolet region. The first-principles calculations suggest that (C3N6H7)2SbF5·H2O exhibits a large birefringence (∼0.38@550 nm), which is twice larger than that of the commercial CaCO3 crystal. Therefore, introducing the fluoride into π-conjugated melamine may be a good tactic to obtain birefringent crystals with large optical anisotropy.

CsY(SO4)2·4H2O: A Deep-Ultraviolet Birefringent Crystal Induced by an Edge-Sharing Connection.

SO4 tetrahedral groups have weak polarization anisotropy, which thus results in the small birefringence of sulfates. Here, we report new sulfate CsY(SO4)2·4H2O with unprecedented birefringence among deep-ultraviolet (deep-UV) sulfates. Its single crystal (10 mm × 3.5 mm × 1.5 mm) was simply grown by an aqueous solution evaporation technique, and it features a rare layered structure composed of YO9 polyhedra, SO4 tetrahedra, and H2O molecules. Interestingly, each SO4 group donates two oxygen atoms to edge-share with one adjacent YO9 polyhedron and thus causes severe distortion of these groups. The characteristic edge-sharing mode gives CsY(SO4)2·4H2O a large birefringence of ∼0.045@546 nm, which is the maximum among deep-UV sulfates and phosphates with similar non-π-conjugated anionic groups. The ultraviolet-visible-near-infrared diffuse reflection and transmission spectra, infrared spectrum, thermal stability, and theoretical calculations are also presented. The fascinating results will improve our understanding of sulfates and may provide useful insights into the exploration of deep-UV sulfates with large birefringence.

High electrochemical performance of Ni-foam supported Ti3C2TxMXene/rGO nanocomposite.

Two-dimensional (2D) materials have attracted extensive attention owing to their unique electronic/physiochemical properties, and wide application potential in energy storage and conversion. However, 2D materials are often tendency to aggregate due to the strong van der Waals interactions, leading to gradually decrease of an efficient mass transfer pathway and accessible surface area for the electrolyte. Here, we demonstrate an efficient approach for large-scale production of a hybrid nanostructures (Ti3C2Tx/rGO) based on ultrathin MXene nanosheets anchored on layered reduced graphene (rGO) supported by porous Ni-foam via a plain chemical dipping method followed by high temperature annealing process. Ti3C2Tx/rGO electrode exhibits a porous structure, excellent ionic and electrical conductivities, and remarkable specific capacitance. Furthermore, it shows ultra-high cycle stability, for example, 88.70% of its specific capacity can be maintained through 3000 cycles. This kind of porous nanostructure and integrated design idea is significant to design other energy storage modules.

Abrupt Structural Transformation in Asymmetric ABPO4F (A = K, Rb, Cs).

An asymmetric structure is the necessary requirement for second-order nonlinear-optical (NLO) materials, which have important applications in modern science and technology. Here we report two isostructural asymmetric compounds, RbBPO4F and CsBPO4F. Both compounds crystallize in cubic space group P213 (No. 198) with three-dimensional (3D) gismondine-like structures. Remarkably, in spite of the same basic structural units BO3F and PO4, both structures are distinct from the previously reported derivative KBPO4F, which crystallizes in a monoclinic space group Cc (No. 9) with a two-dimensional (2D)-layered structure. Careful structural analysis reveals that this structural transformation (from a monoclinic 2D structure to cubic 3D structures) should be aroused by the different alkaline ionic radii. To the best of our knowledge, such an abrupt structural transformation by alkaline elements is reported in all-inorganic asymmetric compounds for the first time. The structural transformation from 2D to 3D structures is favorable to eliminate the layered growth habit. This study will shed deep insight in the structural modulation of asymmetric compounds.

Mixing Halogens To Assemble an All-Inorganic Layered Perovskite with Warm White-Light Emission.

Most of single-component white-light-emitting materials focus on organic-inorganic hybrid perovskites, metal-organic frameworks, as well as all-inorganic semiconductors. In this work, we successfully assembled an all-inorganic layered perovskite by mixing two halogens of distinct ionic radii, namely, Rb2 CdCl2 I2 , which emits "warm" white light with a high color rendering index of 88. To date, Rb2 CdCl2 I2 is the first single-component white-light-emitting material with an all-inorganic layered perovskite structure. Furthermore, Rb2 CdCl2 I2 is thermally highly stable up to 575 K. A series of luminescence measurements show that the white-light emission arises from the lattice deformation, which are closely related to the [CdCl4 I2 ]2- octahedra with high distortion from the distinct ionic radii of Cl and I. The first-principles calculations reveal that both the Cl and I components make significant contributions to the electronic band structures of Rb2 CdCl2 I2 . These findings indicate that mixing halogens is an effective route to design and synthesize new single-component white-light-emitting materials.

A Langbeinite-Type Yttrium Phosphate LiCs2Y2(PO4)3.

Langbeinite-type inorganic compounds have a wide range of applications, but nonlinear-optical properties are rarely mentioned in this series. Here, we report a new orthophosphate, LiCs2Y2(PO4)3, with a langbeinite-type structure, which is the first example in the system of mixed alkali-metal yttrium phosphates. Notably, LiCs2Y2(PO4)3 exhibits a moderate second-harmonic-generation efficiency of 0.9KH2PO4 and is transparent down to 200 nm. In addition, the thermal stability and theory calculations, including the electronic band structure, second-order nonlinear-optical coefficients, and dipole moment analysis, are also reported. This work not only expands the langbeinite-type system but also inspires a study on their nonlinear-optical properties.

Broad-Band-Emissive Organic-Inorganic Hybrid Semiconducting Nanowires Based on an ABX3-Type Chain Compound.

Organic-inorganic hybrid lead halide (e.g., CH3NH3PbX3, where X = CI, Br, and I) nanowires (NWs) with remarkable electric and optical properties have recently garnered increasing attention, owing to their structural flexibility and tunability compared to inorganic semiconducting NWs. While most recently reported NWs are limited to methylammonium/formamidinium three-dimensional lead halide perovskites, it is urgent to develop new organic-inorganic hybrid semiconducting NWs. Here, broad-band-emissive single-crystal semiconductive NWs based on a new ABX3-type organic-inorganic chain hybrid, (2-methylpiperidine)lead tribromide, are reported. It is believed that this work will enrich the organic-inorganic hybrid semiconducting NWs and may provide potential applications for LED displaying.

Cooperation of Three Chromophores Generates the Water-Resistant Nitrate Nonlinear Optical Material Bi3 TeO6 OH(NO3 )2.

Nitrates have long been ignored for practical uses as nonlinear optical (NLO) materials because they are usually very easy to dissolve in water; despite this, the π-conjugated [NO3 ]- is among the most desirable NLO-active structural units. The cooperation of three structural chromophores, namely, Bi3 O6 OH short chains with 6s2 lone pair electrons, distorted TeO6 octahedra with d10 electrons, and π-conjugated [NO3 ]- triangles, generates a new nitrate NLO material, Bi3 TeO6 OH(NO3 )2 , which exhibits an enhanced phase-matchable NLO response of three times that of KH2 PO4 (3×KH2 PO4 ), exceeding those of most nitrate NLO materials. Remarkably, the new material did not show obvious weight loss and degeneration of NLO response after being dipped in de-ionized water for 24 h, indicating that it is highly resistant to water. Theoretical calculations reveal that foreign water molecules cannot stably stay in the crystal lattice of Bi3 TeO6 OH(NO3 )2 . These findings highlight the introduction of diverse chromophores into the nitrate systems as an effective approach for developing practical nitrate NLO materials that are of high water-resistance and good optical performance.

Designing a Beryllium-Free Deep-Ultraviolet Nonlinear Optical Material without a Structural Instability Problem.

A beryllium-free deep-ultraviolet (deep-UV) nonlinear optical (NLO) material K3Ba3Li2Al4B6O20F is developed mainly by the element substitution of Be for Al and Li from Sr2Be2B2O7 that was considered as one of the most promising deep-UV NLO materials. K3Ba3Li2Al4B6O20F preserves the structural merits of Sr2Be2B2O7 and thus exhibits no layering growth tendency and possesses the optical properties required for deep-UV NLO applications, including deep-UV transparency, phase-matchability, and sufficiently large second-harmonic generation (1.5 × KH2PO4). Furthermore, it overcomes the structural instability problem of Sr2Be2B2O7, which is confirmed by the obtainment of large single crystals and phonon dispersion calculations. These attributes make it very attractive for next-generation deep-UV NLO materials. The substitution of Be for Al and Li in beryllium borates provides a new opportunity to design beryllium-free deep-UV NLO materials with good performance.

Strong Nonlinear-Optical Response in the Pyrophosphate CsLiCdP2O7 with a Short Cutoff Edge.

A high efficiency of laser-light conversion and short cutoff edge is essential to an ultraviolet (UV) nonlinear-optical (NLO) material. Previous researches on phosphates were mainly centered on alkali-metal or alkali-earth-metal phosphate systems, resulting in relatively weak NLO responses. Through the introduction of transition-metal cadmium and alkali-metal cesium elements with large radii into the phosphate system, a new UV NLO pyrophosphate, CsLiCdP2O7, has been synthesized. It exhibits a high second-harmonic-generation (SHG) efficiency of 1.5KH2PO4 and is transparent down to 200 nm. This work provides a new path for the design of UV NLO materials with high SHG efficiencies and short cutoff edges by introducing a transition metal with a d10 electron configuration and an alkali metal with a large radius into phosphate systems.

C10H6NO2Cd(NO3)·2H2O: a birefringent crystal with an enlarged band-gap derived from covalent edge-sharing connection.

A new birefringent crystal, C10H6NO2Cd(NO3)·2H2O, was designed by combining a π-conjugated [C10H6NO2]- group with a CdO7N polyhedron. Notably, the experimental energy gap is 0.02 eV higher than that of the organic raw material, which resulted from a covalent edge-sharing connection, implying a new strategy to improve the band-gap of birefringent materials.

Ag-Modified ZnO Nanorod Array Fabricated on Polyester Fabric and Its Enhanced Visible-Light Photocatalytic Performance by a Built-in Electric Field and Plasmonic Effect.

ZnO nanorod arrays (NRAs) were fabricated on polyester fabrics (PFs) by a two-step method and modified with Ag by magnetron sputtering. The photogenerated charge transport properties of the Ag/ZnO nanorod heterojunctions were studied by a self-made Kelvin probe system and a surface photovoltage (SPV) test system. The measured work functions (WFs) of the deposited Ag and ZnO nanorod are 4.67 and 5.56 eV, respectively. The SPV spectra indicate that the direction of the inner electric field is from the Ag layer to the inner of the ZnO nanorod. The enhancement of light absorption by the local surface plasma resonance (LSPR) effect of Ag/ZnO NRA was observed by Raman microspectroscopy and UV-vis diffuse reflectance spectroscopy. The photocatalytic activity of the Ag/ZnO NRA-functionalized PFs was evaluated by the photocatalytic degradation of Rhodamine B (RB) solution under visible light. The full photo-oxidation of RB and the outperforming ZnO NRA-coated PFs demonstrate that the enhanced photocatalytic performance of Ag/ZnO NRA-coated PFs results from the cooperation of the inner electric field of the Ag/ZnO nanorod heterojunction and Ag LSPR.