H2O分子在Fe(100), Fe(110), Fe(111)表面吸附的第一性原理研究

采用第一性原理研究了H2O分子在Fe(100),Fe(110),Fe(111)三个高对称晶面上的表面吸附.结果表明,H2O分子在三个晶面上的最稳定结构皆为平行于基底表面的顶位吸附结构.H2O分子与三个晶面相互作用的吸附能及几何结构计算结果表明H2O分子与三个晶面的相互作用程度不同,H2O分子与Fe(111)晶面的相互作用最强,其次是Fe(100),相互作用最弱的是Fe(110)表面,而这与晶面原子的排列密度相关.吸附体系的电子结构计算结果也得出了相似的结论.同时电荷布居分析表明,H2O分子与Fe表面相互作用时,O原子与基底原子之间的电荷交换使基底Fe原子表面带负电,导致表面电位降低,也促使Fe表面更易于发生电化学腐蚀反应.     

Substitutional adsorption of K on Fe(100)

The adsorption of potassium on Fe(100) was studied by time-of-flight forward scattering and recoiling spectroscopy (TOF-SARS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After heating to 650 K of the potassium saturated surface the formation of a p(3 × 3) potassium superstructure was observed by LEED. TOF-SARS experiments ruled out the adsorption of potassium in the on-top, bridge and four-fold hollow site. The only site which is in agreement with all experimental results is the substitutional site where K replaces an Fe atom of the topmost layer of the crystal. This is the first time a substitutional adsorption site has been found on a bcc surface. On an fcc surface such an adsorption site has been found recently for adsorption of sodium and potassium on Al(111).     

First-principles study of Sr adsorption on InN (0001)

Structures of Sr adsorbed on InN (0001) surfaces are theoretically investigated by first-principles calculations. The adsorption energies of Sr on InN (0001) decrease with decreasing Sr coverage. An InN (0001)-(2 ×2) surface structure covered by a 1/4 monolayer of Sr at the T₄ sites may be the most energetically favourable. Sr atoms may substitute indium atoms, or accumulate at the voids inside InN films. The interstitial Sr defects may act as a potential source of compensation for the p-type behaviour of Sr-doped InN at the surface.     

First-principles study of hydrogen adsorption on titanium-decorated single-layer and bilayer graphenes

The adsorption of hydrogen molecules on titanium-decorated （Ti-decorated） single-layer and bilayer graphenes is studied using density functional theory （DFT） with the relativistic effect. Both the local density approximation （LDA） and the generalized gradient approximation （GGA） are used for obtaining the region of the adsorption energy of H2 molecules on Ti-decorated graphene. We find that a graphene layer with titanium （Ti） atoms adsorbed on both sides can store hydrogen up to 9.51 wt% with average adsorption energy in a range from -0.170 eV to 0.518 eV. Based on the adsorption energy criterion, we find that chemisorption is predominant for H2 molecules when the concentration of H2 molecules absorbed is low while physisorption is predominant when the concentration is high. The computation results for the bilayer graphene decorated with Ti atoms show that the lower carbon layer makes no contribution to hydrogen adsorption.     

The structure of atomic nitrogen adsorbed on Fe(100)

Nitrogen atoms adsorbed on a Fe(100) surface cause the formation of an ordered c(2 × 2) overlayer with coverage 0.5. A structure analysis was performed by comparing experimental LEED I–V spectra with the results of multiple scattering model calculations. The N atoms were found to occupy fourfold hollow sites, with their plane 0.27 Å above the plane of the surface Fe atoms. In addition, nitrogen adsorption causes an expansion of the two topmost Fe layers by 10% (= 0.14 Å). The minimum r-factor for this structure analysis is about 0.2 for a total of 16 beams. The resulting atomic arrangement is similar to that in the (002) plane of bulk Fe₄N, thus supporting the view of a “surface nitride” and providing a consistent picture of the structural and bonding properties of this surface phase.     

α-Al2O3(0001)表面原子缺陷对ZnO吸附影响

采用基于密度泛函理论的分子动力学方法，对α-A12O3(0001)表面A1，O原子空位缺陷及其对ZnO吸附进行了理论计算．电子局域函数显示了表面空位处的电子密度变化，表面Al原子空缺处有非常明显的缺电子区域，悬挂键临近O的电子密度增大，有利于对Zn的吸附；O原子空缺处的Al原子处存在孤立电子，其ELF值为0．05-0．3，将有利于同电负性较大的O或O^2-结合．通过吸附动力学模拟与体系能量的计算发现，表面缺陷显著增强了表面的化学吸附，空缺原子处都被吸附原子填补，吸附结合能远大于单晶表面的情况．在Al空缺的表面，由于ZnO的O与表面O形成双键，破坏了α-Al2O3(0001)表面O六角对称结构，减小了O的表面扩散，从而不利于规则的ZnO薄膜生长．相反，O的空缺表面，弥补了α-Al2O3(0001)表面O空位缺陷，不影响基片表面O六角对称结构．     

First-principles study of molecular NO dissociation on Ir(100) surface

The dissociation of NO on Ir(100) surface is investigated using density functional theory (DFT). The pathway and transition state (TS) of the dissociation of NO molecule are determined using climbing image nudge elastic band (CI-NEB). The prerequisite state of NO dissociation is determining the most stable sites of the reactant and products. We found that the most energetically stable sites are the hollow for N atom and the bridge for NO molecule as well as O atom. We found that the bending of NO is the first step of the dissociation reaction due to the increase of the back-donation from the d-band of Ir to 2π ^? orbital of NO, which causes the weakening of NO bond. The dissociation energy barrier of NO molecule on Ir(100) surface is 0.49 eV.     

Local atomic structure of ultra-thin Fe films grown on Cu(100)

Ultra-thin epitaxial Fe films grown by thermal deposition on Cu(100) are analyzed by scanning tunneling microscopy. Evidence is presented that the morphological characteristics and magnetic properties are a direct consequence of FCC-to-BCC transitions reminiscent of those occurring in bulk Fe. In contrast to the assumption of a ferromagnetic FCC phase in previous models of the Fe/Cu(100) system, we observe a tightly twinned and strained BCC-like phase termed nanomartensite in films below 5 ML thickness, which encompasses almost the entire film volume of 3 ML films. In addition, the surface of 7–8 ML films reconstructs by forming non-close-packed structures with BCC-like bond angles. The formation of these BCC-like phases is the reason for the expansion of the interlayer spacing observed in these films and correlates perfectly with their ferromagnetic ordering. PACS 68.55.-a; 64.70.-p; 81.30.-t     

Catalytic precursors: (Fe,Mn)3O4 and γ-(Fe,Mn)2O3

Iron-manganese oxide catalysts are known to influence the production of short chain hydrocarbons in the Fischer-Tropsch-synthesis process. XRD, XRF, gravity measurements and Mössbauer-effect spectroscopy has been used to study a catalytic precursor made by coprecipitation of iron and manganese oxides. The sample is characterized as a mixture of two compounds with the same iron to manganese ratio. The two compounds are cubic spinels with the same crystallographic parameters, however, one compound is a defect spinel while the other is not. In the Mössbauer studies the defect spinel shows superparamagnetic relaxation behavior—fluctuations of the magnetization vector along two opposite easy directions, while the non-defect spinel shows evidence of collective excitations—fluctuations of the magnetization vector in directions close to the easy direction. These properties are related to the particle sizes.     

The adsorption of hydrogen and the reaction of hydrogen with oxygen on Pt (100)

The adsorption of hydrogen on Pt (100) was investigated by utilizing LEED, Auger electron spectroscopy and flash desorption mass spectrometry. No new LEED structures were found during the adsorption of hydrogen. One desorption peak was detected by flash desorption with a desorption maximum at 160 °C. Quantitative evaluation of the flash desorption spectra yields a saturation coverage of 4.6 × 10¹⁴ atoms/cm² at room temperature with an initial sticking probability of 0.17. Second order desorption kinetics was observed and a desorption energy of 15–16 kcal/mole has been deduced. The shapes of the flash desorption spectra are discussed in terms of lateral interactions in the adsorbate and of the existence of two substates at the surface. The reaction between hydrogen and oxygen on Pt (100) has been investigated by monitoring the reaction product H₂O in a mass spectrometer. The temperature dependence of the reaction proved to be complex and different reaction mechanisms might be dominant at different temperatures. Oxygen excess in the gas phase inhibits the reaction by blocking reactive surface sites. At least two adsorption states of H₂O have to be considered on Pt (100). Desorption from the prevailing low energy state occurs below room temperature. Flash desorption spectra of strongly bound H₂O coadsorbed with hydrogen and oxygen have been obtained with desorption maxima at 190 °C and 340 °C.     

A metastable Fe(A) termination at the Fe3O4(001) surface

A metastable Fe(A) terminated Fe₃O₄(001) surface was prepared by tailoring the surface preparation conditions. STM, LEIS and LEED are utilized to demonstrate that annealing the Ar⁺ sputtered surface to 350 °C produces an Fe(A) terminated surface with a (√2 × √2)R45° superstructure. Within the superstructure both single Fe atoms and Fe dimer species are observed. The surface is reoxidized upon annealing to higher temperatures, eventually leading to the recovery of the energetically favorable Jahn–Teller distorted surface at 700 °C. The ability to reproducibly prepare the Fe(A) termination in this simple manner will allow investigations into the structure–function relationship for this important technological material.     

First-principles study on anatase TiO2（101） surface adsorption of NO

In this paper, the stable structure and the electronic and optical properties of nitric oxide （NO） adsorption on the anatase TiO2 （101） surface are studied using the plane-wave ultrasoft pseudopotential method, which is based on the density functional theory. NO adsorption on the surface is weak when the outermost layer terminates on twofold coordinated oxygen atoms, but it is remarkably enhanced on the surface containing O vacancy defects. The higher the concentration of oxygen vacancy defects, the stronger the adsorption is. The adsorption energies are 3.4528 eV （N end adsorption）, 2.6770 eV （O end adsorption）, and 4.1437 eV （horizontal adsorption）. The adsorption process is exothermic, resulting in a more stable adsorption structure. Furthermore, O vacancy defects on the TiO2 （101） surface significantly contribute to the absorption of visible light in a relatively low-energy region. A new absorption peak in the low-energy region, corresponding to an energy of 0.9 eV, is observed. However, the TiO2 （101） surface structure exhibits weak absorption in the low-energy region of visible light after NO adsorption.     

Investigation into the adsorption of atomic nitrogen on an Al2O3 (0001) surface

Computer simulation of sapphire nitridation used to obtain nitride-based heterostructures (GaN) on an Al₂O₃ substrate has been performed. The adhesion of atomic nitrogen to the sapphire (0001) surface is investigated ab initio. The possibility of replacing surface-layer oxygen atoms with nitrogen atoms has been examined. The calculated results indicate that adsorbed nitrogen atoms occupy the most stable positions above surface oxygen atoms at different nitrogen concentrations. The changes in the total system energy after replacement of surface oxygen atoms with nitrogen atoms have been calculated. It turns out that oxygen replacement is energetically unfavorable for a single nitrogen adatom. However, this process becomes energetically favorable if the concentration of nitrogen atoms increases. This outcome, obtained for the first time, enables better understanding of the atomic-scale mechanism of sapphire nitridation.     

The adsorption of hydrogen on the (100) surfaces of silicon and diamond

Energy and structural calculations of hydrogen interacting with 9-atom clusters modelling the (100) surfaces of silicon and diamond, using the MINDO/3 and MNDO semiempirical quantum chemistry procedures, are reported. The equilibrium structure of the monohydride phases are symmetric dimers with stretched dimer bond lengths (2.53 Å for Si and 1.67 Å for C). The dihydrides have bulk-like structures with the H layer raised about 20% relative to tetragonal layer separations. We show that the dimer bonds on the monohydride phases are unstable against corrosion by atomic hydrogen (in contrast to the back bonds) and that there is a large difference between the desorption energies of the two phases, in agreement with the observed difference in thermal desorption temperatures. Values are calculated for the work function changes on formation of both hydride phases.