Computational study on the half-metallicity in transition metal—oxide-incorporated 2D g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> nanosheets

In this study, based on the first-principles calculations, we systematically investigated the electronic and magnetic properties of the transition metal–oxide-incorporated 2D g-C₃N₄ nanosheet (labeled C₃N₄–TM–O, TM = Sc–Mn). The results suggest that the TM–O binds to g-C₃N₄ nanosheets strongly for all systems. We found that the 2D C₃N₄–TM–O framework is ferromagnetic for TM = Sc, Ti, V, Cr, while it is antiferromagnetic for TM = Mn. All the ferromagnetic systems exhibit the half-metallic property. Furthermore, Monte Carlo simulations based on the Heisenberg model suggest that the Curie temperatures (T _c ) of the C₃N₄–TM–O (TM = Sc, Ti, V, Cr) framework are 169 K, 68 K, 203 K, and 190 K, respectively. Based on Bader charge analysis, we found that the origin of the half-metallicity at Fermi energy can be partially attributed to the transfer of electrons from TM atoms to the g-C₃N₄ nanosheet. In addition, we found that not only electrons but also holes can induce half-metallicity for 2D g-C₃N₄ nanosheets, which may help to understand the origin of half-metallicity for graphitic carbon nitride.     

第一性原理研究O和S掺杂的石墨相氮化碳(g-C_3N_4)_6量子点电子结构和光吸收性质

利用密度泛函和含时密度泛函理论研究了氧(O)和硫(S)原子掺杂的石墨相氮化碳(g-C_3N_4)_6量子点的几何、电子结构和紫外-可见光吸收性质.结果表明:掺杂后(g-C_3N_4)_6量子点杂质原子周围的C-N键长发生了一定的改变,最高电子占据分子轨道-最低电子未占据分子轨道(HOMO-LUMO)能隙显著减小.形成能的计算表明O原子取代掺杂的(g-C_3N_4)_6量子点体系更稳定,且O原子更易取代N3位点,而S原子更易取代N8位点.模拟的紫外-可见电子吸收光谱表明,O和S原子的掺杂改善了(g-C_3N_4)_6量子点的光吸收,使其吸收范围覆盖了整个可见光区域,甚至扩展到了红外区.而且适当的杂质浓度使(g-C_3N_4)_6量子点光吸收在强度和范围上都得到明显改善.通过O和S掺杂的比较,发现二者在可见光区对(g-C_3N_4)_6量子点的光吸收有相似的影响,然而在长波长区域二者的影响有明显差异.总体而言,O掺杂要优于S掺杂对(g-C_3N_4)_6量子点光吸收的影响.     

C_9N_4 as excellent dual electrocatalyst: A first principles study

We perform first principles calculations to investigate the catalytic behavior of C_9 N_4 nanosheet for water splitting.For the pristine C_9 N_4,we find that,at different hydrogen coverages,two H atoms adsorbed on the 12-membered ring and one H atom adsorbed on the 9-membered ring show excellent performance of hydrogen evolution reaction(HER).Tensile strain could improve the catalytic ability of C_9 N_4 and strain can be practically introduced by building C_9 N_4/BiN,and C_9 N_4/GaAs heterojunctions.We demonstrate that the HER performance of heterojunctions is indeed improved compared with that of C_9 N_4 nanosheet.Anchoring transition metal atoms on C_9 N_4 is another strategy to apply strain.It shows that Rh@C_9 N_4 exhibits superior HER property with very low Gibbs free energy change of-30 meV.Under tensile strain within ～2%,Rh@C_9 N_4 could catalyze HER readily.Moreover,the catalyst Rh_9 C_9 N_4 works well for oxygen evolution reaction(OER)with an overpotential of 0.58 V.Our results suggest that Rh@C_9 N_4 is favorable for both HER and OER because of its metallic conductivity,close-zero Gibbs free energy change,and low oneset overpotential.The outstanding performance of C_9 N_4 nanosheet could be attributed to the tunable porous structure and electronic structure compatibility.     

共聚改性g-C3N4-N可见光催化降解RhB的研究

染料废水的排放对人类健康和生态系统构成严重威胁,光催化技术因其操作简单、绿色环保等优点在解决环境污染问题上已显示出巨大的潜力。类石墨相氮化碳（g-C₃N₄）由于成本低且具有良好的化学稳定性,被认为是光催化领域最有前景的新型光催化剂之一,但由于单一g-C₃N₄的比表面积小、可见光吸收能力低、光生电子-空穴复合率高影响了其光催化性能。以邻氨基苯甲腈和尿素为原料,通过高温共聚改性制备高催化活性的g-C₃N₄-N光催化剂,研究g-C₃N₄-N在不同pH值、g-C₃N₄-N投加量和RhB溶液浓度条件下对RhB光催化降解的影响,并结合红外光谱、XRD、BET、UV-Vis对g-C₃N₄-N光催化降解RhB的机理和染料降解路径进行解析。结果表明,经共聚改性制备的碳化氮为类石墨型纯相g-C₃N₄-N,具有稳定的光催化活性、大的比表面积和多孔结构,在初始pH值为3时,加入50 mg的g-C₃N₄-N在可见光条件下光催化降解10 mg·L-1 的RhB可达到最好的光催化降解效果,RhB在暗反应30 min内的吸附去除率可达30%左右,120 min的去除率达到97.7%。在光催化作用下,g-C₃N₄-N将吸附在光催化剂表面的罗丹明B分子通过快速N-脱乙基过程形成DER、EER和AR等大分子中间体,它们在空穴与·OH和·O-₂作用下,共轭结构裂解、开环,生成丁二酸、间苯二酚、丙酸等小分子,脱除的乙基被逐步氧化为乙二醇,这些小分子可以被转化为CO₂和H₂O。     

Transition metal anchored on C9N4 as a single-atom catalyst for CO2 hydrogenation: A first-principles study

To alleviate the greenhouse effect and maintain the sustainable development, it is of great significance to find an efficient and low-cost catalyst to reduce carbon dioxide (CO₂) and generate formic acid (FA). In this work, based on the first-principles calculation, the catalytic performance of a single transition metal (TM) (TM = Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Ir, Pt, Au, or Hg) atom anchored on C₉N₄ monolayer (TM@C₉N₄) for the hydrogenation of CO₂ to FA is calculated. The results show that single TM atom doping in C₉N₄ can form a stable TM@C₉N₄ structure, and Cu@C₉N₄ and Co@C₉N₄ show better catalytic performance in the process of CO₂ hydrogenation to FA (the corresponding maximum energy barriers are 0.41 eV and 0.43 eV, respectively). The partial density of states (PDOS), projected crystal orbital Hamilton population (pCOHP), difference charge density analysis and Bader charge analysis demonstrate that the TM atom plays an important role in the reaction. The strong interaction between the 3d orbitals of the TM atom and the non-bonding orbitals (1π_g) of CO₂ allows the reaction to proceed under mild conditions. In general, our results show that Cu@C₉N₄ and Co@C₉N₄ are a promising single-atom catalyst and can be used as the non-precious metals electrocatalyst for CO₂ hydrogenation to formic acid.     

基于石墨相氮化碳量子点直接荧光猝灭法检测碘离子的研究

石墨相氮化碳(g-C₃N₄)荧光纳米材料具有原料便宜、制备容易、荧光量子产率高、光学稳定性好、毒性低等优点,并且避免有机荧光染料复杂的合成步骤或者金属半导体量子点对环境潜在的危害,这些优点使得g-C₃N₄纳米材料成为新兴的荧光探针用于检测金属离子。最近,已有文献报道重金属汞离子能够高灵敏高选择性地猝灭g-C₃N₄量子点的荧光,加入碘离子能够提取被键合的汞离子形成碘化汞(HgI₂)进而恢复g-C₃N₄量子点的荧光,从而建立一种高灵敏检测碘离子的荧光传感器。然而,该方法依然需要重金属汞离子的参与,限制了该方法的推广应用。通过硝酸氧化块体g-C₃N₄并结合水热法处理制备了一种水溶性好、荧光强度高的g-C₃N₄量子点。该量子点的荧光发射波长位于368 nm,且其荧光发射波长不随激发波长的改变而改变,表明该量子点的尺寸比较均一。笔者发现碘离子在220 nm处有一个较强的吸收峰,与该量子点的激发光谱(中心波长245 nm)具有较大的重叠,从而产生内滤效应引起该量子点的荧光发生猝灭。利用这一性质,构建了一种选择性检测碘离子的新型荧光传感器。在最优检测条件下,g-C₃N₄量子点的荧光猝灭强度(ΔF)与碘离子浓度(X,μmol·L-1)在10～400 μmol·L-1之间具有良好的线性关系,线性方程为ΔF=0.325 79X+6.039 05(R2=0.999 5),检出限为5.0 μmol·L-1。通过“混合即检测”并且不需要借助与重金属离子的配位作用就能够检测碘离子,因此该方法具有快速、环保以及操作简便等优点。     

First-principles Study on the Magnetic,Half-metal and Thermoelectric Transport Properties of Inorganic-Organic Hybrid Compounds [C4N2H12][Fe4Ⅲ(HPO3)2(C2O4)3]

The electronic structure, magnetic and half-metal properties of inorganic-organic hybrid compound [C₄N₂H₁₂][Fe₄^Ⅲ(HPO₃)₂(C₂O₄)₃] are investigated by using the full-potential linearized augmented plane wave (FPLAPW) method within density-functional theory (DFT) calculations. The density of states (DOS), the total energy of the cell and the spontaneous magnetic moment of [C₄N₂H₁₂][Fe₄^Ⅱ(HPO₃)₂(C₂O₄)₃] are calculated. The calculation results reveal that the low-temperature phase of [C₄N₂H₁₂][Fe₄^Ⅲ(HPO₃)₂(C₂O₄)₃] exhibits a stable ferromagnetic (FM) ground state, and we find that this organic compound is a half-metal in FM state. In addition, we have calculated antiferromagnetically coupled interactions, revealing the existence of antiferromagnetic (AFM), which is in agreement with the experiment. We have also found that [C₄N₂H₁₂][Fe₄^Ⅲ(HPO₃)₂(C₂O₄)₃] is a semiconductor in the AFM state with a band gap of about 0.40 eV. Subsequently, the transport properties for potential thermoelectric applications have been studied in detail based on the Boltzmann transport theory.     

The interaction of C2Cl4 with fe(110)

Quantitative adsorption studies, temperature programmed desorption (TPD) and Auger spectroscopy have been used to study the interaction of C₂Cl₄ with Fe(110) at 90 and 325 K. At 90 K, multilayer C₂Cl₄ adsorption occurs. The following desorption products are observed in the temperature range of 90–1050 K: C₂Cl₄ from the multilayer and the monolayer, FeCl₂, and a high mass iron chloride species with mass spectrometer cracking products FeCl⁺₂, FeCl⁺, and Fe⁺. Irreversible dissociative C₂Cl₄ adsorption occurs at 325 K and the only desorption product which is observed is the high mass iron chloride species. Auger spectroscopy shows that surface carbon from C₂Cl₄ starts to diffuse into the bulk of the crystal at ˜ 480 K while small coverages of chlorine remain on the surface of the crystal even after heating to 1050 K. Comparison of the behavior of C₂Cl₄ and CCl₄ on Fe(110) indicates that radical products (·CCl₃ and :CCl₂) observed to be produced from CCl₄ adsorption are not produced from C₂Cl₄ adsorption. This difference is probably due to the enhanced surface reactivity of the C=C bond in C₂Cl₄. A special reactivity of iron defect sites for C₂Cl₄ is observed through the production of associated FeCl₂ species which desorb via zero-order kinetics with an activation energy of 44.8 ± 8.5 kcal/mol, the sublimation enthalpy of FeCl₂.     

Graphitic carbon nitride based single-atom photocatalysts

Single-atom photocatalysts, due to their high catalysis activity, selectivity and stability, become a hotspot in the field of photocatalysis. Graphitic carbon nitride (g-C₃N₄) is known as both a good support for single atoms and a star photocatalyst. Developing g-C₃N₄-based single-atom photocatalysts exhibits great potential in improving the photocatalytic performance. In this review, we summarize the recent progress in g-C₃N₄-based single-atom photocatalysts, mainly including preparation strategies, characterizations, and their photocatalytic applications. The significant roles of single atoms and catalysis mechanism in g-C₃N₄-based single-atom photocatalysts are analyzed. At last, the challenges and perspectives for exploring high-efficient g-C₃N₄-based single-atom photocatalysts are presented.     

Rigorous reflectance performance analysis of Si3N4 self-suspended subwavelength gratings

In this paper, we make an investigation into the reflectance performance of the Si₃N₄ self-suspended subwavelength grating (SSG) via changing several parameters including the incident wavelength, the grating period, the grating thickness and the grating filling ratio. For TM polarization of incidence, numerical results by the rigorous coupled-wave method reveal that the reflectance spectrum of the Si₃N₄ SSG has a narrow linewidth (less than 1 nm), high peak reflectance (100%), and low side bands (less than 0.5%) simultaneously in the optical telecommunication wavelength region. By using a micro-electromechanical-systems actuator, it is believed that the Si₃N₄ SSG should serve as tunable optical filters in optical telecommunication systems.     

Deposition and characterization of carbon nitride films from hexamethylenetetramine/N2 by microwave plasma-enhanced chemical vapor deposition

Carbon nitride thin films were deposited on Si(1 0 0) substrate by microwave plasma-enhanced chemical vapor deposition (PECVD). Hexamethylenetetramine (HMTA) was used as carbon and nitrogen source while N₂ gas was used as both nitrogen source and carrier gas. The sp³-bonded C---N structure in HMTA was considered significantly in the precursor selection. X-ray diffraction analysis indicated that the film was a mixture of crystalline - and β-C₃N₄ as well as graphitic-C₃N₄ and β-Si₃N₄ which were not easily distinguished. Raman spectroscopy also suggested the existence of - and β-C₃N₄ in the films. X-ray photoelectron spectroscopy study indicated the presence of sp²- and sp³-bonded C---N structures in the films while sp³C---N bonding structure predominated to the sp² C---N bonding structure in the bulk composition of the films. N was also found to be bound to Si atoms in the films. The product was, therefore, described as CN_x:Si, where x depends on the film depth, with some evidences of crystalline C₃N₄ formation.     

Interaction of CO2 with Cs-promoted Fe(110) as compared to Fe(110)/K+CO2

The interaction of CO₂ with Cs-promoted Fe(110) at 85 K as well as temperature-dependent reactions between 100 and 700 K have been studied by means of ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Several surface species could be detected at 85 K, i.e. carbon monoxide (CO), carbonate (COⁿ⁻₃), physisorbed linear carbon dioxide (CO^lin₂) and very small amounts of oxidic oxygen (O_ox). An oxalate species (C₂O^m−₄) could not be identified definitively, but from comparison with the literature there is evidence that C₂O^m−₄ is present. Increasing the temperature after saturation with CO₂ leads to a complicated reaction behaviour. CO₂ either desorbs or dissociates into CO and COⁿ⁻₃ or forms C₂O^m−₄ at temperatures between 85 and 160 K. Above 160 K C₂O^m−₄, decomposes in parallel reactions into CO₂, COⁿ⁻₃ and CO. Above 320 K, adsorbed CO either desorbs into the gas phase or dissociates into C and O. In the temperature region between 500 and 700 K a recombination of C and O to CO and the desorption of Cs take place. As in the case of Fe(110)/K+CO₂, at high alkali coverages two carbonate species could be detected which dissociate upon heating at different temperatures. The system Fe(110)/Cs+CO₂ is proved to be very similar to the system Fe(110)/K+CO₂.     

Crystal structure and phase transitions of [(C2H5)4N]6Bi8Cl30

A new [(C₂H₅)₄N]₆Bi₈Cl₃₀ crystal of the family of alkylammonium halogenobismuthates was grown. X-ray diffraction studies showed that the crystals are monoclinic, space group C2/m with a = 20.117(5), b = 12.682(3), c = 20.396(5) Å, β = 93.03(3), Z =2. The lattice consists of (C₂H₅)₄N⁺ cations and a new type of Bi₈Cl⁶⁻₃₀ anion. Dielectric studies revealed two closely-lying structural phase transitions around 241 K (on cooling). They were interpreted as due to a freezing of the rotational motions of tetraethylammonium cations.     

On the controversial magnetic properties of the schiff-base metal-organic polymer {[Fe(C13H17N3)2]SO4·6H2O}n

Within the search for magnetic ordering in organic and metal-organic compounds, the polymeric material of formula {[Fe(C₁₃H₁₇N₃)₂]SO₄·6H₂O}_n has been extensively studied by magnetic measurements. Our results show a basic agreement with the previous ones. Nevertheless, our study, including more data, leads to an interpretation in terms of superparamagnetism rather than metamagnetism as was previously reported.     

Ca0.5Sr0.5TiO3弹性和热学性质的第一性原理研究

利用能量最小原理, 确定了Ca_0.5Sr_0.5TiO₃晶体中4c位置的Ca/Sr原子对称分布, 建立了Ca_0.5Sr_0.5TiO₃稳定的晶体结构, 在此基础上利用基于密度泛函理论第一性原理的平面波超软赝势方法, 采用局域密度近似和广义梯度近似函数, 计算了Ca_0.5Sr_0.5TiO₃的晶格参数、弹性常数、体弹模量、剪切模量、杨氏模量、泊松比, 并基于Christoffel方程的本征值研究了平面声波的特征, 基于Cahill和Cahill-Pohl模型研究了最小热导率的特征. 计算结果表明: Ca_0.5Sr_0.5TiO₃晶格参数和实验值很接近, 体弹模量大于剪切模量, [100], [010], [001]晶向的杨氏模量、泊松比、普适弹性常数(A^U)以及杨氏模量三维图均显示了弹性各向异性; 平面声波在(010), (001)平面呈现各向异性, 在(100)平面呈现各向同性, 平面声波大小与平均横波和平均纵波的数值很接近. Cahill模型最小热导率在各平面呈现各向同性, Cahill-Pohl模型最小热导率在高温时趋于恒定. 准谐德拜模型下Ca_0.5Sr_0.5TiO₃ 晶体的摩尔热容和热膨胀系数与CaTiO₃晶体的接近, 并且高温下具有稳定的热膨胀性能. 计算所得禁带宽度为2.19 eV, 导带底主要是Ti-3d与O-2p态电子贡献; 由电荷布居和电荷密度图理论证实Ca_0.5Sr_0.5TiO₃具有稳定的Ti-O八面体结构.     

过渡金属硼碳化物TM_3B_3C和TM_4B_3C_2稳定性和性能的理论计算

在过渡金属轻元素化合物中,寻找新的硬质或者超硬材料是当前的一个研究热点.目前寻找范围多集中在过渡金属硼化物、碳化物和氮化物等二元体系,三元相的研究则相对较少.本文以已知Nb_3B_3C和Nb_4B_3C_2结构为模板,采用元素替代法构建了29种TM_3B_3C (TM为过渡金属元素)结构和29种TM_4B_3C_2结构,采用基于密度泛函理论的第一性原理计算方法,成功找到了热力学、动力学以及力学都稳定的Ta_3B_3C和Ta4B_3C2两种新相.结构搜索计算确认了这两相为全局能量最优结构.能带结构和态密度的计算显示这两相均为导体,导电性主要源于Ta原子的d电子.这两种新相的硬度大约为26 GPa,说明Ta_3B_3C和Ta_4B_3C_2属于高硬度材料,但不是超硬材料.     

Raman scattering and optical absorption in S2N2 and partially polymerized S2N2 films

Raman scattering and optical absorption measurements between 0.5 and 5.0 eV on crystalline films of S₂N₂ and partly polymerized S₂N₂ have been made for the first time. The Raman active molecular and librational phonons have been observed and assigned. The Raman spectra of partly polymerized S₂N₂ show the appearance of weak scattering at 635 cm⁻¹ and appreciable broadening of the librational phonon peaks. Optical absorption maxima occur at 4.0 and 4.7 eV in pure S₂N₂ at 14 K, while absorptions at 0.86, 1.1 and 4 eV evolve with increasing polymerization.     

Two dimensional GeO2/MoSi2N4 van der Waals heterostructures with robust type-II band alignment

Constructing two-dimensional (2D) van der Waals heterostructures (vdWHs) can expand the electronic and optoelectronic applications of 2D semiconductors. However, the work on the 2D vdWHs with robust band alignment is still scarce. Here, we employ a global structure search approach to construct the vdWHs with monolayer MoSi₂N₄ and wide-bandgap GeO₂. The studies show that the GeO₂/MoSi₂N₄ vdWHs have the characteristics of direct structures with the band gap of 0.946 eV and type-II band alignment with GeO₂ and MoSi₂N₄ layers as the conduction band minimum (CBM) and valence band maximum (VBM), respectively. Also, the direct-to-indirect band gap transition can be achieved by applying biaxial strain. In particular, the 2D GeO₂/MoSi₂N₄ vdWHs show a robust type-II band alignment under the effects of biaxial strain, interlayer distance and external electric field. The results provide a route to realize the robust type-II band alignment vdWHs, which is helpful for the implementation of optoelectronic nanodevices with stable characteristics.     

高分辨电子显微镜研究α-Si3N4晶格缺陷

对于用MgO和LiF作为添加剂以热压法制备的α-Si₃N₄进行高分辨电子显微镜观察时,发现结构缺陷。观察到在三晶粒晶界处有分相现象,这表明晶界玻璃相化学成分不均匀。在晶粒中,由晶格变形和(100)晶面位移所引起应力区域时有发现。在有些区域晶格变形是如此严重,以致晶胞的六方对称性都失掉了。晶格的形变可借助于(100)晶面间距与正常值6.771?的偏离加以衡量。用高分辨电子显微镜,我们发现α-Si₃N₄存在辐射损伤,这可能 关键词：     

Linear and nonlinear optical response of g-C_3N_4-based quantum dots

Graphite carbon nitride(g-C_3N_4) attracts wide-ranging research interest due to its extraordinary physicochemical properties and promising applications ranging from heterogeneous catalysis to fuel cells. In this work, we design different g-C_3N_4-based quantum dots(g CNQDs), carry out a systematic study of optical properties, and elucidate the shape selectivity, composite nanostructure, and outfield effect. In particular, composites of g CNQDs and metal nanochains present excellent optical response, making it applicable to bioimaging, nano-plasma devices, and metalloenzyme in infrared light related fields. Besides, QDs which original bridging nitrogen atoms are replaced by amino(–NH_2), hydroxyl(–OH),and methyl(–CH_3) functional groups respectively, have excellent spectral selectivity in the deep ultraviolet region. More interestingly, in the study of the laser interaction with materials, the g CNQDs exhibit extremely high stability and light corrosion resistance. Phase transition from insulation to metal is observed under the critical condition of about 5 e V intensity or 337 nm wavelength. All provided theoretical support for designs and applications in g-C_3N_4 quantum devices.