S掺杂的还原态TiO_(2-x)的制备及其可见光催化性能（英文）

作为一种稳定、价廉的光催化剂,TiO_2被广泛应用于各种污染物的降解;但是,较大的宽禁带(~3.2 eV)和较低的电子迁移率不仅使TiO_2很难吸收可见光,而且光生电子和空穴的复合几率高,从而导致TiO_2的总体光电效率不高.因此,设计能够被可见光激发、并具有快速光生电子传输的TiO_2一直是研究热点.研究表明,Ti^(3+)自掺杂的TiO_2(还原态TiO_(2-x))不仅能够被可见光激发,而且使TiO_2具有良好的电子导电性,从而有利于提高TiO_2的光电转换效率.另外,非金属元素的掺杂能够减小TiO_2的禁带宽度,使TiO_2能够响应可见光并具有良好的可见光催化性能,其中S元素的掺杂被广泛研究.目前,S掺杂纳米TiO_2的制备通常采用TiS2,单质S,硫脲、二甲亚砜等为S源,但这类原料通常价格昂贵或者具有一定的毒性,因而实际应用受到限制.而制备Ti^(3+)自掺杂TiO_2的方法大都是基于'还原法',在真空或强还原性气氛如H_2,CO中加热TiO_2,或采用高能粒子(电子、氩离子)轰击.在实际应用中,这些方法存在步骤多、条件苛刻、反应时间长和设备昂贵等不足.而且,还原法反应通常发生在颗粒的表面,形成的Ti^(3+)很容易被空气和水中的溶解O2氧化,降低材料的稳定性.虽然在温和的液相中还原Ti4+可用于制备Ti^(3+)掺杂的TiO_2,但是由于反应过程中有副产物生成,需要进行后续处理才能得到纯的Ti^(3+)自掺杂TiO_2.因此,设计一种简单的制备S掺杂还原态TiO_(2-x)光催化剂仍具有十分重要的意义.前期我们采用H_2O_2氧化TiH_2得到不同状态的前驱体凝胶,然后进行不同方式的后处理得到Ti^(3+)自掺杂的纳米TiO_2.本文以TiH_2和H_2O_2反应得到的黄色前驱体凝胶为Ti源,以价格低廉、无毒、稳定的二氧化硫脲为S源和还原剂,采用不同的方法制备了S掺杂的还原态TiO_(2-x)光催化剂.本文初步研究了在凝胶中加入二氧化硫脲后进行水热处理,以及将干燥的凝胶粉末与二氧化硫脲混合热处理对所得产物的影响.并与纯的TiO_2、还原态TiO_(2-x)和S掺杂TiO_2的光吸收、电化学、光催化性能进行对比研究.采用X射线衍射、透射电子显微镜、高分辨透射电子显微镜、X-射线光电子能谱、紫外-可见漫反射光谱、比表面分析和电化学工作站等技术对产物的结构、形貌和光电性能进行了表征.以罗丹明B(RhB)溶液为模拟废水,考察样品的可见光催化性能.结果表明,不同的后续处理方式不仅影响S掺杂TiO_(2-x)的结晶性和形貌,而且影响产物的光吸收性能和电子传输性能,从而使不同条件下所得产物的可见光催化性能不同.其中,采用热处理方式得到的S掺杂TiO_(2-x)样品在可见光下降解RhB的速率分别是纯的TiO_2,TiO_(2-x)和S掺杂TiO_2的31,2.5和3.6倍,而且样品具有良好的循环稳定性.

Ti^(3+) self-doped TiO_2 photoelectrodes for photoelectrochemical water splitting and photoelectrocatalytic pollutant degradation

To improve the harvesting of visible light and reduce the recombination of photogenerated electrons and holes, Ti3+ self-doped TiO2 nanoparticles were synthesized and assembled into photoanodes with high visible light photoelectrochemical properties. X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectra, electron resonance spectroscopy and energy dispersive X-ray spectra were used to characterize the structure, crystallinity, morphology and other properties of the obtained nanoparticles. UV-visible diffuse reflectance spectra showed that the Ti3+ self-doped TiO2 nanoparticles had a strong absorption between 400 and 800 nm. Moreover, when hydrothermal treatment time was prolonged to 22 h, the heterogeneous junction was formed between the anatase and rutile TiO2, where the anatase particles exposed highly active {001} facets. Under visible light irradiation, the Ti3+ self-doped TiO2 electrode exhibited an excellent photoelectrocatalytic degradation of rhodamine B (RhB) and water splitting performance. Intriguingly, by selecting an appropriate hydrothermal time, the high photoconversion efficiency of 1.16% was achieved. (C) 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Prediction of large-gap quantum spin hall insulator and Rashba-Dresselhaus effect in two-dimensional g-TIA （A = N, P, As, and Sb） monolayer films

A new family of two-dimensional （2D） topological insulators （TIs） comprising g-TIA （A = N, P, As, and Sb） monolayers constructed by T1 and group-V elements is predicted by first-principles calculations and molecular-dynamics （MD） simulations. The geometric stability, band inversion, nontrivial edge states, and electric polarity are investigated to predict the large-gap quantum spin Hall insulator and Rashba-Dresselhaus effects. The MD results reveal that the g-T1A monolayers remain stable even at room temperature. The g-T1A （A = As, Sb） monolayers become TIs under the influence of strong spin-orbit couplings with large bulk bandgaps of 131 and 268 meV, respectively. A single band inversion is observed in each g-T1A （A = As, Sb） monolayer, indicating a nontrivial topological nature. Furthermore, the topological edge states are described by introducing a sufficiently wide zigzag-nanoribbon. A Dirac point in the middle of the bulk gap connects the valence- and conduction-band edges. The Fermi velocity near the Dirac point with a linear band dispersion is -0.51 × 106 m/s, which is comparable to that of many other 2D nanomaterials. More importantly, owing to the broken inversion symmetry normal to the plane of the g-T1A films, a promising Rashba-Dresselhaus effect with the parameter up to 0.85 eV-A is observed in the g-T1A （A = As, Sb） monolayers. Our findings regarding 2D topological g-T1A monolayers with room-temperature bandgaps, intriguing topological edge states, and a promising Rashba-Dresselhaus effect are of fundamental value and suggest potential applications in nanoelectronic devices.

Improving the photocatalytic hydrogen evolution of UiO-67 by incorporating Ce4+-coordinated bipyridinedicarboxylate ligands

UiO-67 is a Zr-based metal–organic framework(MOF) containing an organic linker namely, the dianion of biphenyl-4,40-dicarboxylic acid(bpdc). Ce4+metal ions(0.02 Ce to Zr atom ratio) were incorporated into UiO-67 via partially replacing bpdc with the dianion of 2,20-bipyridine-5,50-dicarboxylic acid(bpydc);thus, the latter forms a bpydc-Ce complex. The resulting product(i.e., UiO-67-Ce) demonstrated a photocatalytic hydrogen evolution rate that was over 10 times higher than that of UiO-67. Through this modification, a new energy transfer channel is opened up. The energy transfer between the bpdc and bpydc-Ce ligands(i.e., from excited bpdc to bpydc-Ce) weakened the recombination of the charge carriers, which was confirmed by photoluminescence, emission lifetime, and transient absorption measurements. This study presents a new way to construct highly efficient MOF photocatalysts.

Ultrasonic-assisted pyrolyzation fabrication of reduced SnO2-x/g-C3N4 heterojunctions： Enhance photoelectro- chemical and photocatalytic activity under visible LED light irradiation

Novel SnO2-x/g-C3N4 heterojunction nanocomposites composed of reduced SnO2 nanoparticles and exfoliated g-CBN4 nanosheets were prepared by a convenient one-step pyrolysis method. The structural, morphological, and optical properties of the as-prepared nanocomposites were characterized in detail, indicating that the aggregation of g-C3N4 nanosheets was prevented by small, well-dispersed SnO2_x nanoparticles. The ultraviolet-visible spectroscopy absorption bands of the nanocomposites were shifted to a longer wavelength region than those exhibited by pure SnO2 or g-CgN4. The charge transfer and recombination processes occurring in the nanocomposites were investigated using linear scan voltammetry and electrochemical impedance spectroscopy. Under 30-W visible-light-emitting diode irradiation, the heterojunction containing 27.4 wt.% SnO2-x exhibited the highest photocurrent density of 0.0468 mA.cm-2, which is 33.43 and 5.64 times larger than that of pure SnO2 and g-C3N4, respectively. The photocatalytic activity of the heterojunction material was investigated by degrading rhodamine B under irradiation from the same light source. Kinetic study revealed a promising degradation rate constant of 0.0226 min^-1 for the heterojunction containing 27.4 wt.% SnO2-x, which is 32.28 and 5.79 times higher than that of pure SnO2 and g-C3N4, respectively. The enhanced photoelectrochemical and photocatalytic performances of the nanocomposite may be due to its appropriate SnO2x content and the compact structure of the junction between the SnO2-x nanoparticles and the g-C3N4 nanosheets, which inhibits the recombination of photogenerated electrons and holes.

Two-dimensional square transition metal dichalcogenides with lateral heterostructures

Fabrication of lateral heterostructures （LHS） is promising for a wide range of next-generation devices and could sufficiently unlock the potential of two-dimensional materials.Herein,we demonstrate the design of lateral heterostructures based on new building materials,namely 1S-MX2 LHS,using first-principles calculations.1S-MX2 LHS exhibits excellent stability,demonstrating high feasibility in the experiment.The desired bandgap opening can endure application at room temperature and was confirmed in 1S-MX2 LHS with spin-orbit coupling （SOC）.A strain strategy further resulted in efficient bandgap engineering and an intriguing phase transition.We also found that black phosphorus can serve as a competent substrate to support 1S-MX2 LHS with a coveted type-Ⅱ band alignment,allowing versatile functionalized bidirectional heterostructures with built-in device functions.Furthermore,the robust electronic features could be maintained in the 1S-MX2 LHS with larger components.Our findings will not only renew interest in LHS studies by enriching their categories and properties,but also highlight the promise of these lateral heterostructures as appealing materials for future integrated devices.

Large valley-polarized state in single-layer NbX2 (X=S,Se):Theoretical prediction

Exploring two-dimensional valleytronic crystals with large valley-polarized state is of considerable importance due to the promising applications in next-generation information related devices.Here,we show first-principles evidence that single-layer NbX_(2)(X=S,Se)is potentially the long-sought two-dimensional valleytronic crystal.Specifically,the valley-polarized state is found to occur spontaneously in single-layer NbX_(2),without needing any external tuning,which arises from their intrinsic magnetic exchange interaction and inversion asymmetry.Moreover,the strong spin-orbit coupling strength within Nb-d orbitals renders their valley-polarized states being of remarkably large(NbS_(2)∼156 meV/NbSe_(2)∼219 meV),enabling practical utilization of their valley physics accessible.In additional,it is predicted that the valley physics(i.e.,anomalous valley Hall effect)in single-layer NbX_(2) is switchable via applying moderate strain.These findings make single-layer NbX_(2) tantalizing candidates for realizing high-performance and controllable valleytronic devices.

MoTe2 is a good match for Gel by preserving quantum spin Hall phase

Quantum spin Hall （QSH） insulator is a new class of materials that is quickly becoming mainstream in condensed-matter physics. The main obstacle for the development of QSH insulators is that their strong interactions with substrates make them difficult to study experimentally. In this study, using density functional theory, we discovered that MoTe2 is a good match for a GeI monolayer. The thermal stability of a van der Waals GeI/MoTe2 heterosheet was examined via molecular-dynamics simulations. Simulated scanning tunneling microscopy revealed that the GeI monolayer perfectly preserves the bulked honeycomb structure of MoTe2. The GeI on MoTe2 was confirmed to maintain its topological band structure with a sizable indirect bulk bandgap of 0.24 eV by directly calculating the spin Chern number to be -1. As expected, the electron mobility of the GeI is enhanced by MoTe2 substrate restriction. According to deformation- potential theory with the effective-mass approximation, the electron mobility of GeI/MoTe2 was estimated as 372.7 cm^2·s^-1·V^-1 at 300 K, which is 20 times higher than that of freestanding GeI. Our research shows that traditional substrates always destroy the topological states and hinder the electron transport in QSH insulators, and pave way for the further realization and utilization of QSH insulators at room temperature.

Third-order topological insulators with wallpaper fermions in Tl_(4)PbTe_(3)and Tl_(4)SnTe_(3)

Nonsymmorphic symmetries open up horizons of exotic topological boundary states and even generalize the bulk–boundary correspondence,which,however,the third-order topological insulator in electronic materials are still unknown.Here,by means of the symmetry analysis and k·p models,we uncover the emergence of long-awaited third-order topological insulators and the wallpaper fermions in space group I4/mcm(No.140).Based on this,we present the hourglass fermion,fourfold-degenerate Dirac fermion,and Möbius fermion in the(001)surface of Tl_(4)XTe_(3)(X=Pb/Sn)with a nonsymmorphic wallpaper group p4g.Remarkably,16 helical corner states reside on eight corners in Kramers pair,rendering the real electronic material of third-order topological insulators.More importantly,a time-reversal polarized octupole polarization is defined to uncover the nontrivial third-order topology,as is implemented by the 2nd and 3rd order Wilson loop calculations.Our results could considerably broaden the range of wallpaper fermions and lay the foundation for future experimental investigations of third-order topological insulators.

Intertwined ferroelectricity and topological state in twodimensional multilayer

The intertwined ferroelectricity and band topology will enable the non-volatile control of the topological states,which is of importance for nanoelectrics with low energy costing and high response speed.Nonetheless,the principle to design such system is unclear and the feasible approach to achieve the coexistence of two parameter orders is absent.Here,we propose a general paradigm to design 2D ferroelectric topological insulators by sliding topological multilayers on the basis of first-principles calculations.Taking trilayer Bi2Te3 as a model system,we show that in the van der Waals multilayer based 2D topological insulators,the in-plane and out-of-plane ferroelectricity can be induced through a specific interlayer sliding,to enable the coexistence of ferroelectric and topological orders.The strong coupling of the order parameters renders the topological states sensitive to polarization flip,realizing non-volatile ferroelectric control of topological properties.The revealed design-guideline and ferroelectric-topological coupling not only are useful for the fundamental research of the coupled ferroelectric and topological physics in 2D lattices,but also enable innovative applications in nanodevices.

2D spontaneous valley polarization from inversion symmetric single-layer lattices

2D spontaneous valley polarization attracts great interest both for its fundamental physics and for its potential applications in advanced information technology,but it can only be obtained from inversion asymmetric single-layer crystals,while the possibility to create 2D spontaneous valley polarization from inversion symmetric single-layer lattices remains unknown.Here,starting from inversion symmetric single-layer lattices,a general design principle for realizing 2D spontaneous valley polarization based on van der Waals interaction is mapped out.Using first-principles calculations,we further demonstrate the feasibility of this design principle in a real material of T-FeCl2.More remarkably,such design principle exhibits the additional exotic out-of-plane ferroelectricity,which could manifest many distinctive properties,for example,ferroelectricity-valley coupling and magnetoelectric coupling.The explored design-guideline and phenomena are applicable to a vast family of 2D materials.Our work not only opens up a platform for 2D valleytronic research but also promises the fundamental research of coupling physics in 2D lattices.

Intrinsic valley polarization and anomalous valley hall effect in single-layer 2H-FeCl_(2)

Valley,as a new degree of freedom for electrons,has drawn considerable attention due to its significant potential for encoding and storing information.Lifting the energy degeneracy to achieve valley polarization is necessary for realizing valleytronic devices.Here,on the basis of first-principles calculations,we show that single-layer FeCl_(2)exhibits a large spontaneous valley polarization(∼101 meV)arising from the broken time-reversal symmetry and spin-orbital coupling,which can be continuously tuned by varying the direction of magnetic crystalline.By employing the perturbation theory,the underlying physical mechanism is unveiled.Moreover,the coupling between valley degree of freedom and ferromagnetic order could generate a spin-and valley-polarized anomalous Hall current in the presence of the in-plane electric field,facilitating its experimental exploration and practical applications.