Enhanced growth of crystalline-amorphous core-shell silicon nanowires by catalytic thermal CVD using in situ generated tin catalyst

In this work,we prepared silicon nanowires(Si NWs) on both fluorine-doped SnO 2(FTO) coated glass substrate and common glass substrate by catalytic thermal chemical vapor deposition(CVD) using indium film as the catalyst.It is confirmed that indium can catalyze the growth of Si NWs.More importantly,we found that tin generated in situ from the reduction of SnO 2 by indium can act as catalyst,which greatly enhances the growth of Si NWs on FTO substrate.The obtained Si NWs have a uniform crystalline-amorphous core-shell structure that is formed via vapor-liquid-solid and vapor-solid growth of silicon sequentially.This work provides a strategy to prepare Si NWs in high yield by catalytic thermal CVD using the low melting point metal catalysts.     

Surface electronic structure of Pt(1 1 0): comparison with Ni and Pd

The unoccupied electronic structure of Pt(1 1 0) was investigated by inverse photoemission. The results were compared with the data for Ni(1 1 0) and Pd(1 1 0) with particular emphasis on surface states. The surface states in the gap of Pt(1 1 0) are shifted upwards relative to Ni and Pd, as a consequence of the (1 × 2) missing-row reconstruction. In contrast, the surface state at is only weakly affected, which indicates that it is essentially a one-dimensional state, localized on the densely-packed atomic chains on the Pt(1 1 0) surface.     

Si衬底上生长的铁硅化合物结构和取向分析

采用分子束外延法分别在650-920℃的Si(110)和920℃的Si(111)衬底表面生长出铁的硅化物纳米结构,并主要分析了920℃高温下纳米结构的形貌、组成相及其与Si衬底的取向关系.扫描隧道显微镜(STM)研究表明,920℃高温下,Si(110)衬底上生长的铁硅化合物完全以纳米线的形式存在,且其尺寸远大于650℃低温下外延生长的纳米线尺寸;Si(111)衬底上生长出三维岛和薄膜两种形貌的铁硅化合物,其中三维岛具有金属特性且直径约300 nm、高约155 nm,薄膜厚度约2 nm.电子背散射衍射研究表明920℃高温下Si(110)衬底上生长的纳米线仅以β-FeSi2的形式存在,且β-FeSi2相与衬底之间存在唯一的取向关系:β-FeSi2(101)//Si(11 1);β-FeSi2[010]//Si[110];Si(111)衬底上生长的三维岛由六方晶系的Fe2Si相组成,Fe2Si属于164空间群,晶胞常数为a=0.405 nm,c=0.509 nm;与衬底之间的取向关系为Fe2Si(001)∥Si(111)和Fe2Si[1 20]//Si[112].     

Si衬底上生长的铁硅化合物结构和取向分析

采用分子束外延法分别在650-920 ℃的Si（110）和920 ℃的Si（111）衬底表面生长出铁的硅化物纳米结构,并主要分析了920 ℃高温下纳米结构的形貌、组成相及其与Si 衬底的取向关系. 扫描隧道显微镜（STM）研究表明,920 ℃高温下,Si（110）衬底上生长的铁硅化合物完全以纳米线的形式存在,且其尺寸远大于650 ℃低温下外延生长的纳米线尺寸;Si（111）衬底上生长出三维岛和薄膜两种形貌的铁硅化合物,其中三维岛具有金属特性且直径约300 nm、高约155 nm,薄膜厚度约2 nm. 电子背散射衍射研究表明920 ℃高温下Si（110）衬底上生长的纳米线仅以β-FeSi₂的形式存在,且β-FeSi₂相与衬底之间存在唯一的取向关系：β-FeSi₂（101）//Si（111）;β-FeSi₂ [010]//Si[110];Si（111）衬底上生长的三维岛由六方晶系的Fe₂Si 相组成,Fe₂Si 属于164 空间群,晶胞常数为a=0.405 nm,c=0.509 nm;与衬底之间的取向关系为Fe₂Si（001）∥Si（111）和Fe₂Si[120]//Si[112].     

Directly probing the hybrid bonding of styrene on Cu111

The interfacial electronic structure of chemisorbed styrene on Cu(111) was successfully investigated with two-photon photoemission spectroscopy. We observed unoccupied states 3.5 eV above the Fermi level and occupied states 2.0 eV below the Fermi level. Polarization results reveal that the occupied and unoccupied states arise from bonding and antibonding orbitals formed by hybridization of copper (surface state and d-band orbitals) and styrene (pi1* and pi2* orbitals).     

Phase-controlled growth of metastable Fe5Si3 nanowires by a vapor transport method

We report the synthesis of single-crystalline nanowires (NWs) of metastable Fe5Si3 phase via an iodide vapor transport method. Free-standing Fe5Si3 NWs are grown on a sapphire substrate placed on a Si wafer without the use of any catalyst. The typical size of the Fe5Si3 nanowires is 5-15 microm in length and 100-300 nm in diameter. Synthesis of the metastable phase is induced by composition-dependent nucleation from the gas-phase reaction. Depending on the concentration ratio of FeI2(g) to SiI4(g), different phases of iron silicides are formed. The growth of nanowires is facilitated by the initial nucleation of silicide particles on the substrate and further self-seeded growth of the NWs. The present work not only provides a method for the synthesis of metastable Fe5Si3 nanowires but also suggests that the phase controlled synthesis can be further optimized to produce other metal-rich silicide nanostructures for future spintronic devices.     

预制铂纳米颗粒对质子交换膜燃料电池用铂纳米线阴极的结构影响

铂纳米线（Pt NWs）由于其独特的结构特点,比商业Pt/C具有更高的氧还原反应（ORR）比活性。在本工作中,我们将预先制备好的铂纳米颗粒（Pt NPs）引入到碳基体中,用于诱导生长Pt NWs,获得了均匀分布Pt NWs的阴极。通过改变Pt NP载量（0~0.015 mg·cm^-2）和Pt NP来源（不同Pt含量的Pt/C）研究了所制备阴极的结构和性能。用扫描电镜对阴极表面进行了表征,并用透射电镜和X射线衍射分析了Pt NW的形貌和晶体结构。在单电池中分别进行了极化曲线和循环伏安曲线测试。当Pt NP来源为40% Pt/C且其载量为0.005 mg·cm^-2时,制备的Pt NW阴极具有最佳的单电池性能和最大的电化学表面积（ECSA）。最后,提出了预制Pt NP影响Pt NWs分布的可能机制。     

Early stages of oxide growth in H-terminated silicon nanowires: determination of kinetic behavior and activation energy

Silicon nanowires (Si NWs) terminated with hydrogen atoms exhibit higher activation energy under ambient conditions than equivalent planar Si(100). The kinetics of sub-oxide formation in hydrogen-terminated Si NWs derived from the complementary XPS surface analysis attribute this difference to the Si-Si backbond and Si-H bond propagation which controls the process at lower temperatures (T < 200 °C). At high temperatures (T≥ 200 °C), the activation energy was similar due to self-retarded oxidation. This finding offers the understanding of early-stage oxide growth that affects the conductance of the near-gap channels leading towards more efficient Si NW electronic devices.     

A new type of strong metal-support interaction and the production of H2 through the transformation of water on Pt/CeO2(111) and Pt/CeO(x)/TiO2(110) catalysts

The electronic properties of Pt nanoparticles deposited on CeO(2)(111) and CeO(x)/TiO(2)(110) model catalysts have been examined using valence photoemission experiments and density functional theory (DFT) calculations. The valence photoemission and DFT results point to a new type of "strong metal-support interaction" that produces large electronic perturbations for small Pt particles in contact with ceria and significantly enhances the ability of the admetal to dissociate the O-H bonds in water. When going from Pt(111) to Pt(8)/CeO(2)(111), the dissociation of water becomes a very exothermic process. The ceria-supported Pt(8) appears as a fluxional system that can change geometry and charge distribution to accommodate adsorbates better. In comparison with other water-gas shift (WGS) catalysts [Cu(111), Pt(111), Cu/CeO(2)(111), and Au/CeO(2)(111)], the Pt/CeO(2)(111) surface has the unique property that the admetal is able to dissociate water in an efficient way. Furthermore, for the codeposition of Pt and CeO(x) nanoparticles on TiO(2)(110), we have found a transfer of O from the ceria to Pt that opens new paths for the WGS process and makes the mixed-metal oxide an extremely active catalyst for the production of hydrogen.     

Pt3Te4 nanoparticles from tellurium nanowires

We report the synthesis of platinum telluride nanoparticles (Pt(3)Te(4) NPs) in the solution phase at room temperature using a template-assisted method. The dendrimeric aggregates formed are composed of several small units of Pt(3)Te(4) NPs of ～4 nm diameter. Tellurium nanowires (Te NWs) are used as the template and the reducing agent in the growth of NPs which occurs due to the galvanic replacement reaction between Te NWs and PtCl(6)(2-). Surface-enhanced Raman scattering (SERS) of the dispersed Pt(3)Te(4) NPs was studied using crystal violet (CV) as the analyte. SERS sensitivity up to 10(-8) M of CV was observed. The Raman enhancement factor (EF) of adsorbed CV on NP aggregates was calculated to be 1.74 × 10(5). The catalytic ability of the as-synthesized Pt(3)Te(4) NPs for the reduction of 4-nitrophenol (4-NP) was studied.     

High-resolution soft X-ray photoelectron spectroscopic studies and scanning auger microscopy studies of the air oxidation of alkylated silicon(111) surfaces

High-resolution soft X-ray photoelectron spectroscopy was used to investigate the oxidation of alkylated silicon(111) surfaces under ambient conditions. Silicon(111) surfaces were functionalized through a two-step route involving radical chlorination of the H-terminated surface followed by alkylation with alkylmagnesium halide reagents. After 24 h in air, surface species representing Si(+), Si(2+), Si(3+), and Si(4+) were detected on the Cl-terminated surface, with the highest oxidation state (Si(4+)) oxide signal appearing at +3.79 eV higher in energy than the bulk Si 2p(3/2) peak. The growth of silicon oxide was accompanied by a reduction in the surface-bound Cl signal. After 48 h of exposure to air, the Cl-terminated Si(111) surface exhibited 3.63 equivalent monoleyers (ML) of silicon oxides. In contrast, after exposure to air for 48 h, CH(3)-, C(2)H(5)-, or C(6)H(5)CH(2)-terminated Si surfaces displayed <0.4 ML of surface oxide, and in most cases only displayed approximately 0.20 ML of oxide. This oxide was principally composed of Si(+) and Si(3+) species with peaks centered at +0.8 and +3.2 eV above the bulk Si 2p(3/2) peak, respectively. The silicon 2p SXPS peaks that have previously been assigned to surface Si-C bonds did not change significantly, either in binding energy or in relative intensity, during such air exposure. Use of a high miscut-angle surface (7 degrees vs < or =0.5 degrees off of the (111) surface orientation) yielded no increase in the rate of oxidation nor change in binding energy of the resultant oxide that formed on the alkylated Si surfaces. Scanning Auger microscopy indicated that the alkylated surfaces formed oxide in isolated, inhomogeneous patches on the surface.     

Pt/Ti双层电极应力的微观分析研究

常温下用直流溅射法在Si（100）上淀积Pt/Ti电极薄膜.采用X射线衍射、原子力显微镜检测不同退火温度的薄膜,结果表明,退火初期Pt层内首先产生了压应力,从而使Pt表面形成了小丘凸起;退火过程产生的热应力在Pt层残余应力的产生中占主导地位,这种热应力使得Pt层最后的应力状态为张应力状态并且随着Pt层中Pt3Ti金属间化合物数量的增多而增大.     

Mesoscopic self-structuring on metal surfaces: Geometry and electronic structure

The relation between surface stress and surface reconstruction and the electronic origin of surface stress are briefly discussed. Three examples of stress induced mesoscopic self-structuring on metal surfaces are presented, namely O/Cu(110), Au(111) and Pt(110). While the principles underlying the domain formation on the first two systems seem to be understood quite well, the origin of the periodic hill-and-valley structure of Pt(110) is not yet clear. The confinement of electronic surface states in stress induced quasi-one-dimensional domains was studied recently by photoemission on O/Cu(110) and by scanning tunneling spectroscopy on Au(111). The results are briefly reviewed. The same basic effects were found in the two different studies.     

High resolution photoemission and x-ray absorption spectroscopy of a lepidocrocite-like TiO2 nanosheet on Pt(110) (1 × 2)

The electronic structure of TiO(2) nanosheets on the Pt(110)-(1 × 2) surface has been investigated by using high resolution photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The Ti 2p XAS spectra of the deposited TiO(2) films have been theoretically evaluated and, from the comparison with the experimental data, the assignment to a lepidocrocite-like structure is confirmed. Coexistence of TiO(2) islands with PtO(2) stripes for incomplete nanosheets is confirmed by high resolution photoemission data. The location of the valence and conduction band edges of the nanosheet has been experimentally determined allowing us to describe in details subtle electronic effects due to the interface with the substrate. The locations of the valence band maximum and the leading peak in the O 1s XAS spectrum indicate a band gap similar to anatase but with the Fermi level closer to mid-gap than found for bulk, n-type TiO(2).     

Vibrational analysis of H2 and D2 adsorption on Pt/SiO2

Vibrational properties of surface species formed upon H2 and D2 exposure of silica supported platinum particles have been investigated with in situ diffuse reflection infrared Fourier transform spectroscopy. Experiments have been performed at 50-250 degrees C, using different platinum loading of the samples in the absence and presence of oxygen. In addition, electronic structure calculations and vibrational analysis have been performed within the density functional theory for H adsorption on a silica cluster, (HO)3SiOSi(OH)3. The spectroscopy experiments showed reversible formation of isolated OH and OD groups on the silica surface when the samples were exposed to H2 and D2, respectively. In addition to the absorption peak corresponding to isolated OH and OD groups, an intense broad band was observed around 3270 cm(-1) (2500 cm(-1)) during H2 (D2) exposure. Supported by the calculations, this band was assigned to perturbed OH groups on the silica surface. The surface coverage of new OH groups was found to correlate to the platinum loading in the samples, indicating that the new silanol groups were formed in the vicinity of the Pt particles. In the investigated temperature interval, the formation rate of OH groups was not found to be temperature dependent.     

On the adsorption and formation of Pt dimers on the CeO2(111) surface

The direct adsorption of Pt(2) dimers on CeO(2)(111) and their formation from isolated adsorbed Pt atoms have been studied using periodic slab model calculations based on density functional theory and including the so-called on-site Hubbard parameter (GGA + U). In the most stable configuration Pt(2) is found to be almost parallel to the surface; the electronic ground state is closed shell and there is no evidence of charge transfer towards or from the surface. The formation of Pt(2) from two single adsorbed Pt atoms involves a rather small energy barrier of ~0.10 eV only. On the contrary, dissociation of adsorbed Pt(2) requires to overcome a considerable barrier of ~1.43 eV. This indicates that once Pt(2) is formed it will remain on the surface, thus likely triggering the growth of larger supported Pt particles.     

Electro-oxidation of carbon monoxide on well-ordered Pt(111)/Sn surface alloys

The electro-oxidation of CO on model platinum-tin alloy catalysts has been studied by ex-situ electrochemical measurements following the preparation of the Pt(111)/Sn(2x2) and Pt(111)/Sn(radical3 x radical3)R30 degrees surfaces. A surface redox couple, which is associated with the adsorption/desorption of hydroxide on the Sn sites, is observed at 0.28 V(RHE)/0.15 V(RHE) in H(2)SO(4) electrolyte on both surfaces. Evidence that it is associated with the adsorption of OH comes from ex-situ photoemission measurements, which indicate that the Sn atoms are in a metallic state at potentials below 0.15 V(RHE) and an oxidized state at potentials above 0.28 V(RHE). Specific adsorption of sulfate anions is not associated with the surface process since there is no evidence from photoemission of sulfate adsorption, and the same surface couple is observed in the HClO(4) electrolyte. CO is adsorbed from solution at 300 K, with saturation coverages of 0.37 +/- 0.05 and 0.2 +/- 0.05 ML, respectively. The adsorbed CO is oxidatively stripped at the potential coincident with the adsorption of hydroxide on the tin sites, viz., 0.28 V(RHE). This strong promotional effect is unambiguously associated with the bifunctional mechanism. The Sn-induced activation of water, and promotion of CO electro-oxidation, is sustained as long as the alloy structure remains intact, in the potential range below 0.5 V(RHE). The results are discussed in the light of the requirements for CO-tolerant platinum-based electrodes in hydrogen fuel cell anode catalysts and catalysts for direct methanol electro-oxidation.     

DFT study of interaction of O, O2, and OH with unreconstructed Pt(hkl) (h, k, l = 0, 1) surfaces—similarities, differences, and universalities

Adsorption of O, O₂, and OH on Pt(111), Pt(100), and Pt(110) surfaces was studied using periodic DFT calculations. It was found that generally adsorbate-surface interaction strengths increase with the decrease in surface packing density. On the Pt(111) surface the dissociation of O₂ molecule was not predicted, but it was predicted on Pt(100) and Pt(110) surfaces. While the strength of the adsorbate-substrate interaction decreases with the rise in surface coverage by O atoms, in the case of OH adsorption adsorbate layer gets stabilized at higher surface coverage through the hydrogen bonding. In spite of all the mentioned differences, single parameter of surface electronic structure was identified, being useful for the explanation of the adsorption trends at different adsorption sites for O and OH adsorption on Pt surfaces of various crystallographic orientations and also provided a deeper understanding of atomic oxygen adsorption as a function of surface coverage.     

基于密度泛函理论计算的CO2在SrTiO3(100)表面的吸附

通过密度泛函理论的第一性原理,模拟了CO2分子在SrTiO3(100)表面TiO2-和SrO-位点上的吸附行为,获得了CO2在几种不同吸附模型下的结构参数及表面吸附能,进而研究了吸附机理和结构稳定性.计算结果表明,当CO2的C原子吸附在SrTiO3(100)表面SrO-及TiO2-位点的氧原子上时,吸附结构较稳定,尤其是C、O原子共吸附在TiO2-位点时最稳定,而其余吸附模型则不稳定.对吸附稳定模型的Mulliken布局数及态密度分析显示:CO2分子在SrTiO3(100)表面吸附主要是由于SrTiO3(100)面的电子跃迁至CO2分子,CO2分子得到电子形成弯曲的CO2-阴离子结构,并伴随着C-O键的伸长,从而达到吸附活化CO2的目的.     

Initial oxidation and hydroxylation of the Ge(100)-2 x 1 surface by water and hydrogen peroxide

We use density functional theory to investigate the surface chemistry of initial oxidation and hydroxylation of the Ge(100)-2 x 1 surface by water and hydrogen peroxide. Comparison of the reaction of water on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces shows that the kinetics of oxidation of the Ge(100)-2 x 1 surface with water is slower. Our calculations also show that oxidation products on the Ge(100)-2 x 1 surface are less thermodynamically stable than on Si. We also investigate two competing dissociation reactions of H2O2 on the Ge(100)-2 x 1 surface. We find that dissociative adsorption via cleavage of the OH bond is less exothermic than OO dissociation. Furthermore, interdimer OO dissociation has a lower activation barrier than interdimer or intradimer OH dissociation, although interdimer dissociation products are found to be less stable compared than those formed from intradimer dissociation reactions. Finally, we find that the oxidation products formed from hydrogen peroxide are more stable than those formed from water.