Preparation of Ag3PO4/CoWO4 S-scheme heterojunction and study on sonocatalytic degradation of tetracycline

In this study, 0.6Ag₃PO₄/CoWO₄ composites were synthesized by hydrothermal method. The prepared materials were systematically characterized by techniques of scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), N₂ adsorption/desorption, and UV–vis diffuse reflectance spectrum (DRS). Furthermore, the sonocatalytic degradation performance of 0.6Ag₃PO₄/CoWO₄ composites towards tetracycline (TC) was investigated under ultrasonic radiation. The results showed that, combined with potassium persulfate (K₂S₂O₈), the 0.6Ag₃PO₄/CoWO₄ composites achieved a high sonocatalytic degradation efficiency of 97.89 % within 10 min, which was much better than bare Ag₃PO₄ or CoWO₄. By measuring the electrochemical properties, it was proposed that the degradation mechanism of 0.6Ag₃PO₄/CoWO₄ is the formation of S-scheme heterojunction, which increases the separation efficiency of electron-hole pairs (e^--h⁺) and generates more electrons and holes, thereby enhancing the degradation activity. The scavenger experiments confirmed that hole (h⁺) was the primary active substance in degrading TC, and free radicals (OH) and superoxide anion radical (O₂^–) were auxiliary active substances. The results indicated that 0.6Ag₃PO₄/CoWO₄ nanocomposites could be used as an efficient and reliable sonocatalyst for wastewater treatment.     

Convergence properties of the supercell approach in the study of local defects in solids

The “supercell” scheme is applied to the study of local defects in MgO (Ca substitution, cation and anion vacancies) and bulk silicon (carbon substitution). The trend of the quantities of interest (defect formation energy, geometrical relaxation, charge distribution around the defect) as a function of the supercell size is explored; when neutral defects are considered, supercells containing 50 to 100 atoms are large enough to allow for most of the nuclear and electronic relaxation and to produce a negligible interaction between defects in different cells. These conclusions apply both to ionic and covalent host crystals. Present day ab initio quantum mechanical periodic computer programs can handle cells of such a size at a relatively low cost and high numerical accuracy.

When charged defects are considered (vacancies in MgO), the supercell scheme must be modified in order to avoid Coulomb divergencies, but the usually adopted correction, which consists in introducing a compensating uniform background of charge, generates spurious higher order electrostatic interactions, which are far from being negligible. The resulting defect formation energies show very slow, if any, convergence trends and “a posteriori” semiclassical corrections proposed in the literature do not represent a general solution to the problem. On the other hand, other properties, such as atom relaxation and charge distribution, show a much faster convergence than energy with respect to the cell size.     

夹持方式和加工误差对大口径薄型KDP晶体附加面形的影响

惯性约束聚变频率转换系统中,大口径薄型KDP晶体的面形质量是影响频率转换效率能否达到设计要求的关键因素之一。针对45放置状态下口径为400 mm400 mm的三倍频KDP晶体,采用ANSYS有限元分析软件,建立了不同夹持方式和具有不同加工误差的KDP晶体模型和夹具模型,分析了加工误差对不同夹持方式下KDP晶体附加面形的影响,给出了不同加工误差和不同夹持情况下,KDP晶体附加面形的P-V值和RMS值。研究结果表明,夹持方式和加工误差是引起KDP晶体附加面形变化的重要因素,正面压条夹持方式即使在晶体和夹具存在加工误差时也可以较好地控制晶体的附加面形。     

The superprism effect using large area 2D-periodic photonic crystal slabs

The “superprism effect” is an effect observed in photonic crystal structures whereby the direction of light propagation is extremely sensitive to the wavelength and angle of incidence. To realize the superprism effect, new structures are presented which rely on the sensitivity of the phase velocity in a two-dimensional (2D) photonic crystal slab to observe angular magnification outside the photonic crystal medium. Constant frequency contour calculations for a photonic crystal slab of finite thickness are used to predict the phase velocity superprism effect. Further analysis using 2D finite-difference time-domain simulations indicate that a large area photonic crystal and wide excitation beam are necessary for clear observation of the superprism effect. A fabrication technique is demonstrated to achieve the structure's required nanometer-sized features over centimeter-scale areas.     

Cavity modes in one-dimensional photonic crystal slabs

A theoretical study of cavity modes in one-dimensional photonic crystal slabs embedded in Silicon-on-Insulator structures is reported. Three different methods are employed, namely a guided-mode expansion in which the coupling to radiative modes is treated by perturbation theory, a grating or scattering-matrix method for calculating the surface reflectance, and a Fourier modal expansion for in-plane transmission calculations. It is shown that all methods lead to the same values for the quality factors of cavity modes for both first- and second-order Bragg mirrors. We conclude that the quality factor of a cavity mode can be determined with optical reflectance from the surface of the slab.     

Broadening compensation for ultrashort pulses in photonic crystals

Extremely large group velocity dispersion of both signs can be achieved at the band edges of guided modes in photonic crystal waveguides. The selection of a proper value and sign of this parameter allows the design of short-length waveguides to compensate for pulse broadening. This pulse broadening can be caused, for instance, when a photonic crystal delay line is introduced. We present a theoretical study of the possibilities of using photonic crystal waveguides as intra-circuital dispersion compensation elements for ultrashort pulses, so the width of a transmitted pulse can be reduced. However, we also demonstrate that recovering the original pulse shape is not possible for these large-bandwidth pulses due to higher-order dispersion terms.     

Subwavelength structures for high power laser antireflection application on fused silica by one-step reactive ion etching

In this paper we report a simple method to fabricate a novel subwavelength structure surface on fused silica substrate using one-step reactive ion etching with two-dimensional polystyrene colloidal crystals as masks. The etching process and the morphologies of the obtained structure are controlled. We show that the period of the obtained fused silica pillar-like arrays were determined by the initial polystyrene nanoparticle size. The height of pillar arrays can be adjusted by controlling the etching duration, which is proved to be of importance in tailoring the antireflection properties of subwavelength structures surface. The novel subwavelength structures surface exhibit excellent broadband antireflection properties, but the size of the pillar affects the antireflective properties in short wavelength region. We anticipate this method would offer a convenient and scalable way for inexpensive and high-efficiency high power laser field designs.     

Compton散射对磁化等离子体光子晶体缺陷模密温特性的影响

应用多光子非线性Compton散射模型和有限时域差分法,对Compton散射对磁化等离子体光子晶体缺陷模密温特性的影响进行了理论分析和数值模拟。结果表明,与Compton散射前的情况相比,Compton散射使低温低频处光子禁带中存在缺陷模的明显度降低,缺陷模频率增大,缺陷模和透射率峰值减小;使高温高频处缺陷模和透射率峰值、缺陷模频率显著增大,禁带宽减小,缺陷模位置向高频方向移动。随着电子密度的增大,散射减小了禁带增大效应和缺陷模减小效应,增强了缺陷模频率增大效应;随着电子密度的降低,散射增强了禁带变窄效应、缺陷模峰值增大效应和缺陷模频率减小效应。利用Compton散射,可实现对缺陷模密温特性的有效控制。     

Developing On-Site Trace Level Speciation of Lead,Cadmium and Zinc by Stripping Chronopotentiometry (SCP): Fast Screening and Quantification of Total Metal Concentrations

Electrochemical stripping techniques are interesting candidates for carrying out onsite speciation of environmentally relevant trace metals due to the existing low-cost portable instrumentation available and the low detection limits that can be achieved. In this work, we describe the initial analytical technique method development by quantifying the total metal concentrations using Stripping Chronopotentiometry (SCP). Carbon paste screen-printed electrodes were modified with thin films of mercury and used to quantify sub-nanomolar concentrations of lead and cadmium and sub-micromolar concentrations of zinc in river water. Low detection limits of 0.06 nM for Pb(II) and 0.04 nM for Cd(II) were obtained by the standard addition method using a SCP deposition time of 180 s. The SCP results obtained for Pb(II) and Cd(II) agreed with those of inductively coupled plasma mass spectrometry (ICP-MS). The coupling of SCP with screen-printed electrodes opens up excellent potential for the development of onsite speciation of trace metals. Due to the low analysis throughput obtained for the standard addition method, we also propose a new, more rapid screening Cd(II) internal standard methodology to significantly increase the number of samples that can be analyzed per day.     

Silver(I) ion induced formation of 2-D ternary lead(II)-silver(I)-4-tolythiolate [PbAg2(μ-Stol)2(μ4-Stol)2]n from 1-D binary lead(II)-4-tolythiolate [(μ-Cl)Pb2(Stol)(μ-Stol)2]n (tol = 4-tolyl)

Treatment of [Pb(Stol)₂]_n with an equiv of [Et₄N]Cl in DMF afforded a new 1-D polymeric complex, [(μ-Cl)Pb₂(Stol)(μ-Stol)₂]_n (1), with Pb–Cl–Pb links. Interaction of 1 with a suspension of [Ag(Stol)] in DMF resulted in formation of a 2-D polymeric complex, [PbAg₂(μ-Stol)₂(μ₄-Stol)₂]_n (2), with the planar four-membered Ag₂S₂ and PbAgS₂ rings bridged by μ-Stol and μ₄-Stol ligands. Both polymeric complexes were structurally characterized by single-crystal X-ray diffraction analysis.