Yangwu Guo

BaAu2S2: A Au-Based Intrinsic Photocatalyst for High-Performance Visible-Light Photocatalysis

A new Au-based sulfide BaAu2S2 was obtained through solid-state reaction. It crystallizes in the tetragonal space group I41/amd with unit cell parameters of a = 6.389 72(2) Å, b = 6.389 72(2) Å, c = 12.7872(1) Å, and Z = 4. Its structure features [AuS2/2]∞ zigzag chains composed of corner-sharing AuS2 linear units. With a direct band gap of 2.49 eV, BaAu2S2 is suitable for the visible-light harvesting. Moreover, it exhibits excellent visible-light photocatalytic activity, which is 1.3 times that of graphitic carbon nitride (g-C3N4) and also demonstrates excellent circulating stability. On the basis of the crystal and electronic structure analysis, the electrons are highly delocalized along the [AuS2/2] chains, and the electron effective mass of BaAu2S2 is only approximately one-fifth of that of g-C3N4, which may help the separation of the electron/hole pairs during the photocatalytic process. Additionally, the absorption coefficient of BaAu2S2 is extremely high, exceeding 1 × 104 cm-1 over the entire absorbable visible spectrum (hν > Eg), which is significantly higher than that of g-C3N4. Such factors may contribute to its outstanding photocatalytic performances. According to our best knowledge, BaAu2S2 is the first noble metal-based chalcogenide photocatalyst reported as intrinsic light-harvesting and electron/hole-generating centers. This study may provide valuable insights for further research on photocatalytic materials.

K2ZnSn3Se8: A Non-Centrosymmetric Zinc Selenidostannate(IV) Featuring Interesting Covalently Bonded [ZnSn3Se8]2− Layer and Exhibiting Intriguing Second Harmonic Generation Activity

Non-centrosymmetric zinc selenidostannate(IV) K2 ZnSn3 Se8 was synthesized. It features interesting covalently bonded [ZnSn3 Se8 ]2- layers with K+ cations filling in the interlayer voids. The phonon spectrum was calculated to clarify its structural stability. Based on the X-ray diffraction data along with the Raman spectrum, the major bonding features of the title compound were identified. According to the UV/vis-NIR spectroscopy, K2 ZnSn3 Se8 possesses a typical direct band gap of 2.10 eV, which is in good agreement with the band structure calculations. Moreover, our experimental measurements and detailed theoretical calculations reveal that K2 ZnSn3 Se8 is a new phase-matchable nonlinear optical material with a powder second harmonic generation (SHG) signal about 0.6 times of that of AgGaS2 .

K2ZnGe3S8: A Congruent-Melting Infrared Nonlinear-Optical Material with a Large Band Gap

K2ZnGe3S8 belonging to the noncentrosymmetric space group P21 of the monoclinic system was discovered via a solid-state method. It possesses two-dimensional [ZnGe3S8]2- layers, with alkali-metal cations K+ located between the layers. On the basis of UV-vis-near-IR diffuse-reflectance spectrometry, the band gap of K2ZnGe3S8 is 3.36(2) eV. According to powder second-harmonic-generation (SHG) measurements, the SHG response of K2ZnGe3S8 is about 0.9 times that of AgGaS2 at the particle size range of 20-41 μm. Experimental results demonstrate that K2ZnGe3S8 keeps a good balance between a large band gap (3.36 eV) and a moderate SHG response. Moreover, according to the differential scanning calorimetry measurements, K2ZnGe3S8 melts congruently at around 1023 K and recrystallizes at about 963 K. Therefore, it is possible to obtain bulk single crystals via the Bridgman-Stockbarger method. The first-principles calculations indicate that the optical properties of K2ZnGe3S8 are dominantly determined by the [GeS4] tetrahedra as well as a small contribution from the [ZnS4] tetrahedra.

Nonbonding Electrons Driven Strong SHG Effect in Hg2GeSe4: Experimental and Theoretical Investigations

A Hg-based ternary infrared nonlinear optical (NLO) material, Hg2GeSe4, with the defect diamond-like (DL) structure was systematically investigated for the first time. The experimental results show that Hg2GeSe4 exhibits an enhanced second harmonic generation (SHG) response about 2.1 times that of the normal DL selenide AgGaSe2 ( d36 = 33 pm/V) at the particle size of 150-200 μm, as well as good phase-matchable ability. Moreover, theoretical analysis reveals that the nonbonding electrons around Se atoms in the defect DL structure make a dominant contribution to the improvement of the NLO property: d36 = 78.83 pm/V and Δ n = 0.11. This study highlights the promise of electronic engineering strategies and opens new avenues toward the design of new infrared NLO crystals with high performance.

Pb0.65Mn2.85Ga3S8 and Pb0.72Mn2.84Ga2.95Se8: Two Quaternary Metal Chalcognides with Open-Tunnel-Framework Structures Displaying Intense Second Harmonic Generation Responses and Interesting Magnetic Properties

By combining different nonlinear optical-active structural chromophores with transition metal Mn into a crystal structure, two novel quaternary metal chalcogenides Pb0.65Mn2.85Ga3S8 (1) and Pb0.72Mn2.84Ga2.95Se8 (2) were successfully synthesized. Compounds 1 and 2 are isostructural, and they represent a new structure type that crystallizes in the space group P6̅ (No. 174) in the hexagonal system. Their structures feature an interesting three-dimensional open-tunnel framework composed of bridged infinite chains with Pb2+ cations filling in the biggest tunnels. Interestingly, both 1 and 2 demonstrate intense second harmonic generation responses at 2.09 μm that is about 1.5 and 4.4 times, respectively, of that of the benchmark material AgGaS2. However, 1 and 2 possess different optical diffuse reflectance spectra: 1 displays an evident multiband absorption characteristic with two distinguishing absorption edges of 738 and 551 nm, corresponding to two band gaps of 1.68 and 2.25 eV, respectively, while 2 exhibits only one sharp edge, and the corresponding band gap was estimated to be 1.65 eV. Moreover, apart from the considerable structural similarity between 1 and 2, the dc temperature dependent susceptibility measurements indicate that compound 1 is paramagnetic, while compound 2 exhibits spin-glass-like behavior.

Ba2M(C3N3O3)2 (M = Mg, Ca): Potential UV Birefringent Materials with Strengthened Optical Anisotropy Originated from (C3N3O3)3− Group

Demands for UV birefringent materials are growing dramatically owing to the rapid development of ultraviolet technology. Here, a new family of UV birefringent materials, Ba2Mg(C3N3O3)2 (BMCY) and Ba2Ca(C3N3O3)2 (BCCY), have been successfully discovered. It is the first time that the excellent birefringent properties of cyanurates have been studied. These materials exhibit an extremely large birefringence (Δn = 0.728–0.351 and 0.771–0.346 from 230 nm to 800 nm for BMCY and BCCY, respectively), much larger than that of the commercial UV birefringent crystal α-BaB2O4 (α-BBO) (Δn = 0.12@532 nm). Our study indicates that the impressive optical properties of BMCY and BCCY stem from the strengthened optical anisotropy of the planar (C3N3O3)3− group compared with the isoelectronic (B3O6)3− group. Besides, its congruent-melting properties make it feasible to grow a bulk crystal by the Bridgman–Stockbarger technique. The extraordinary properties of BMCY and BCCY may shed light on a new path to explore birefringent materials for practical application.

K2MnGe3S8: a new multifunctional semiconductor featuring [MnGe3S8]2− layers and demonstrating interesting nonlinear optical response and antiferromagnetic properties

A new multifunctional quaternary chalcogenide K2MnGe3S8 was discovered for the first time by using a conventional high-temperature solid-state reaction method. K2MnGe3S8 crystallizes in the space group P21 (no. 4) of the monoclinic system and features a 2∞[MnGe3S8]2− layer constructed by 1∞[MnGeS6]6− anionic chains and [Ge2S6]4− dimers. This air-stable compound exhibits a large band gap (about 2.95 eV) and a moderate nonlinear optical (NLO) response (about 0.6 × AgGaS2 with a particle size of 20–41 μm at a laser radiation of 2.09 mm). Moreover, despite the large Mn⋯Mn separation, namely, 7.1978 A for the shortest intralayer Mn⋯Mn and 6.6975 A for the neighboring interlayer Mn⋯Mn, K2MnGe3S8 exhibits an interesting antiferromagnetic transition at ∼8.1 K. The optical and magnetic properties are also investigated by first principles calculations. Our study demonstrates that the interplay between NLO-active structural units and transition metals may contribute to the discovery of new multifunctional materials.

Synthesis, Crystal Structure, and Optical Properties of Noncentrosymmetric Na2ZnSnS4

A new chalcogenide Na2ZnSnS4 has been successfully synthesized by using Na2S2 as reactive flux. Na2ZnSnS4 crystallizes in the tetragonal system with space group of I4̅. Its cell parameters are a = 6.4835(6) Å and c = 9.134(1) Å. The structure is a derivative of AgGaS2, in which the Ag+ ions are replaced by Na+ ions and the Ga3+ ions are replaced by Zn2+ and Sn4+ ions. All three cations are in seriously distorted tetrahedral geometry with a distortion factor (η = c/ a) of 1.4. Optical measurements show that the Na2ZnSnS4 powder sample has a large transparent range from 0.8 to 25 μm and a wide band gap of 3.1 eV. It exhibits large second-harmonic generation intensity of 0.9 × AgGaS2 in the grain size range from 41 to 74 μm. First-principles calculation results reveal that the valence band maximum and conduction band minimum are mainly composed of S 3p, Zn 3d orbitals and Sn 5s, S 3p orbitals, respectively.

Li7Cd4.5Ge4Se16 and Li6.4Cd4.8Sn4Se16: Strong Nonlinear Optical Response in Quaternary Diamond-Like Selenide Networks

Two new selenides with diamond-like structures, Li7 Cd4.5 Ge4 Se16 and Li6.4 Cd4.8 Sn4 Se16 , were synthesized by using a conventional high-temperature solid-state reaction method. They crystallize in the space group Pna21 (no. 33) of the orthorhombic system. Their three-dimensional frameworks consist of corner-sharing LiSe4 , CdSe4 , and MSe4 (M=Ge, Sn) tetrahedra. These two compounds exhibit strong powder second-harmonic generation responses that are about 1.2 and 2.5 times that of the benchmark AgGaS2 at a laser wavelength of λ=2.09 μm, and also demonstrate type I phase-matchable behavior. The optical bandgaps were determined to be 2.18 and 1.95 eV for Li7 Cd4.5 Ge4 Se16 and Li6.4 Cd4.8 Sn4 Se16 , respectively. Furthermore, these two materials exhibit congruent melting behavior at rather low temperatures of 985 and 1060 K, respectively, which makes bulk single crystal growth by using the Bridgman-Stockbarger method possible. Our study indicates that these two materials show advantages over the traditional IR NLO material CdSe and are promising for practical applications.