Co-reporter:Song Ling Wang, Jing Li, Shijie Wang, Ji’en Wu, Ten It Wong, Maw Lin Foo, Wei Chen, Kai Wu, and Guo Qin Xu
ACS Catalysis October 6, 2017 Volume 7(Issue 10) pp:6892-6892
Publication Date(Web):August 28, 2017
DOI:10.1021/acscatal.7b02331
Developing catalysts to improve excitonic charge-carrier transfer and separation properties is critical for solar energy conversion through photochemical catalysis. Layer staking of two-dimensional (2-D) materials has opened up opportunities to engineer heteromaterials for strong interlayer excitonic transition. However, scalable fabrication of heteromaterials with seamless and clean interfaces remains challenging. Here, we report an in situ growth strategy for synthesizing a 2-D C/TiO2 heterogeneous hybrid. Layered structure of TiO2 and chemically bonded Ti–C between graphitic carbon and TiO2 generate synergetic effects, promoting interfacial charge transfer and separation, leading to more electrons participating in photoreduction for hydrogen evolution. The Ti–C bond as reactive sites, such as platinum behavior, makes it an interesting potential substitue for noble metals in hydrogen evolution. In the absence of noble metals, the C/TiO2 hybrid exhibits a significant enhancement of hydrogen evolution from water splitting using solar light, ∼3.046 mmol h–1 g–1. The facile and scalable fabrication of 2-D heterogeneous hybrid with enhanced interfacial charge transfer and separation provides perspectives for the creation of 2-D heteromaterials in optoelectronics and solar-light-harvesting applications.Keywords: graphitic carbon; heterogeneous hybrid; hydrogen evolution; photocatalysis; TiO2; water splitting;
Co-reporter:Song Ling Wang, Xin Luo, Xiong Zhou, Ye Zhu, Xiao Chi, Wei Chen, Kai Wu, Zheng Liu, Su Ying Quek, and Guo Qin Xu
Journal of the American Chemical Society November 1, 2017 Volume 139(Issue 43) pp:15414-15414
Publication Date(Web):October 11, 2017
DOI:10.1021/jacs.7b08229
The synthesis of free-standing two-dimensional titania (2-D TiO2) with a reduced band gap presents complex challenges to synthetic chemists. Here, we report a free-standing 2-D TiO2 sheet synthesized via a one-step solvothermal methodology, with a measured optical onset at ∼1.84 eV. Using first-principles calculations in combination with experiment, we propose that the as-formed 2-D TiO2 sheets are layers of the lepidocrocite TiO2 structure, but with large nonuniform strains consistent with its crumpled morphology. These strains cause a significant change in the quasiparticle band structure and optical absorption spectra, resulting in large absorption in the visible-light region. This narrow band gap 2-D TiO2 can catalyze the formation of singlet oxygen and the degradation of dye pollutants with low-energy photons of solar light. Our work demonstrates that lattice strains intrinsic to 2-D materials, especially its crumpled, free-standing forms, can result in new and useful properties.
Co-reporter:Fagen Wang;Leilei Xu;Weidong Shi;Jian Zhang;Kai Wu;Yu Zhao;Hui Li
Nano Research 2017 Volume 10( Issue 2) pp:364-380
Publication Date(Web):2017 February
DOI:10.1007/s12274-016-1296-2
In this study, the use of a thermally stable Ir/Ce0.9La0.1O2 catalyst was investigated for the dry reforming of methane. The doping of La2O3 into the CeO2 lattice enhanced the chemical and physical properties of the Ir/Ce0.9La0.1O2 catalyst, such as redox properties, Ir dispersion, oxygen storage capacity, and thermal stability, with respect to the Ir/CeO2 catalyst. Hence, the Ir/Ce0.9La0.1O2 catalyst exhibits higher activity and stabler performance for the dry reforming of methane than the Ir/CeO2 catalyst. This observation can be mainly attributed to the stronger interaction between the metal and support in the Ir/Ce0.9La0.1O2 catalyst stabilizing the catalyst structure and improving the oxygen storage capacity, leading to negligible aggregation of Ir nanoparticles and the Ce0.9La0.1O2 support at high temperatures, as well as the rapid removal of carbon deposits at the boundaries between the Ir metal and the Ce0.9La0.1O2 support.
Co-reporter:Song Ling Wang, Yan Lin Mak, Shijie Wang, Jianwei Chai, Feng Pan, Maw Lin Foo, Wei Chen, Kai Wu, and Guo Qin Xu
Langmuir 2016 Volume 32(Issue 49) pp:13046-13053
Publication Date(Web):November 22, 2016
DOI:10.1021/acs.langmuir.6b03594
Understanding and manipulating the one half-reaction of photoinduced hole-oxidation to oxygen are of fundamental importance to design and develop an efficient water-splitting process. To date, extensive studies on oxygen evolution from water splitting have focused on visible-light harvesting. However, capturing low-energy photons for oxygen evolution, such as near-infrared (NIR) light, is challenging and not well-understood. This report presents new insights into photocatalytic water oxidation using visible and NIR light. WO2–WO3 hybrid nanorods were in situ fabricated using a wet-chemistry route. The presence of metallic WO2 strengthens light absorption and promotes the charge-carrier separation of WO3. The efficiency of the oxygen evolution reaction over noble-metal-free WO2–WO3 hybrids was found to be significantly promoted. More importantly, NIR light (≥700 nm) can be effectively trapped to cause the photocatalytic water oxidation reaction. The oxygen evolution rates are even up to around 220 (λ = 700 nm) and 200 (λ = 800 nm) mmol g–1 h–1. These results demonstrate that the WO2–WO3 material is highly active for water oxidation with low-energy photons and opens new opportunities for multichannel solar energy conversion.
Co-reporter:Wei Mao, Jing Hui He, Yong Jie Xi, Wei Chen, Kai Wu, Chun Zhang, Eng Soon Tok, and Guo Qin Xu
The Journal of Physical Chemistry C 2016 Volume 120(Issue 43) pp:24780-24788
Publication Date(Web):October 17, 2016
DOI:10.1021/acs.jpcc.6b07477
Chemisorbed molecular nanocorrals on semiconductor surfaces are of both fundamental and technical importance. 1-Propanethiol molecules adsorbed on Si(111)-(7×7) were investigated using high-resolution energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM), and periodic density functional theory (DFT) calculations. HREELS spectra show that the 1-propanethiol molecules undergo S–H bond dissociative adsorption on Si(111)-(7×7). STM images reveal the temperature-dependent site selectivity for the binding of C3H7S- fragments. At room temperature, C3H7S- prefers to bind to faulted subunits compared to unfaulted subunits. At 110 K, C3H7S- binding on center adatoms over corner adatoms is dominant, resulting in an ordered array of molecular nanocorrals. DFT calculations were performed on a periodic slab including the entire 7×7 reconstruction rather than on a cluster model. The theoretical studies suggest that the temperature-dependent site selectivity originates from the thermal-plus-electron-induced diffusion of dissociative products and the site-preferential accommodation of the mobile physisorbed precursors. Our results provide a fundamental understanding on the origin of site selectivity of molecular binding on Si(111)-(7×7).
Co-reporter:Wei Mao, Jing Hui He, Jia Qiang Gu, Wei Chen, Kai Wu, Eng Soon Tok and Guo Qin Xu
Chemical Communications 2015 vol. 51(Issue 75) pp:14195-14198
Publication Date(Web):20 Jul 2015
DOI:10.1039/C5CC04713G
Adsorption of butadiene monoxide on Si(111)-(7×7) has been scrutinized by high-resolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The experimental results indicate that surface reaction occurs through a [2+2]-like cycloaddition, which is further supported by the DFT studies.
Co-reporter:Z. X. Chen, Y. J. Xi, L. Huang, W. C. Li, R. Li, G. Q. Xu and H. S. Cheng
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 40) pp:26740-26744
Publication Date(Web):14 Sep 2015
DOI:10.1039/C5CP05093F
Many applications of Sn-doped indium oxide (ITO) films in organic electronics require appropriate surface modifications of ITO nanocrystals with small organic molecules, such as silanes, phosophonic acids and carboxylic acids, to improve interfacial contacts and charge transfer. Here, we propose a new surface modification strategy via adsorption of acetylene molecules on an oxygen-terminated ITO(100) surface using a slab crystalline model to represent the nanocrystal surface. The adsorption was first studied using density functional theory. It was found that the chemisorption of C2H2 on two types of surface oxygen dimers is highly exothermic with the calculated adsorption energies of 3.80 eV and 5.19 eV, respectively. Electron population analysis reveals the origin of the strong interaction between the adsorbate and the ITO(100) surface. Experimental studies on the synthesized ITO nanocrystals using X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy confirm the predicted strong adsorption of C2H2 on ITO surfaces.
The Journal of Physical Chemistry C 2015 Volume 119(Issue 9) pp:4789-4795
Publication Date(Web):February 10, 2015
DOI:10.1021/jp5104164
The electrical and optical properties of transparent conducting oxides (TCOs) are of essential importance for optoelectronics. Electronic structures are keys to understanding these properties. In this work, the geometrical and electronic structures of body-centered-cubic In2O3 n-type-doped by group 14 and fifth-period main-group elements (Sb, Te, and I) were investigated systematically. The calculated electronic structures reveal a good hybridization between the O 2p states and the s states of Si, Ge, and Sn, resulting in superior electronic properties, such as a free-electron-like band feature, a large bandwidth (>2 eV), a low effective mass (m* = 0.2m0) and a high electron group velocity (≥8.35 × 105 m/s). The charge localization on the dopants leads to inferior electronic properties of In2O3 doped with other dopants. The calculated defect formation energies indicate that the formation of both neutral and 1+ charge-state Sn is spontaneous in indium oxide.
Nano Research 2015 Volume 8( Issue 10) pp:3177-3185
Publication Date(Web):2015 October
DOI:10.1007/s12274-015-0817-8
Two-dimensional (2D) materials are highly promising for flexible electronics, and graphene is the only well-studied transparent conductor. Herein, density functional theory has been used to explore a new transparent conducting material via adsorption of H on a 2D β-GaS sheet. This adsorption results in geometrical changes to the local structures around the H. The calculated electronic structures reveal metallic characteristics of the 2D β-GaS material upon H adsorption and a large optical band gap of 2.72 eV with a significant Burstein-Moss shift of 0.67 eV. The simulated electrical resistivity is as low as 10–4 O·cm, comparable to the benchmark for ITO thin films.
Co-reporter:Jing Hui He, Wei Mao, Wei Chen, Kai Wu, Han Song Cheng and Guo Qin Xu
Chemical Science 2014 vol. 5(Issue 11) pp:4447-4452
Publication Date(Web):2014/07/07
DOI:10.1039/C4SC01293C
Resolving orbitals using scanning tunneling microscopy (STM) provides an in-depth understanding of the chemical and electronic nature of molecule/substrate junctions. Most orbital resolving work was performed for molecules physisorbed on metal surfaces by inserting interfacial layers to decouple the interaction between the molecules and substrates. It remains challenging to image the orbitals of molecules directly chemisorbed on native surfaces because the linking chemical bonds likely induce severe coupling. Here we demonstrate that the π orbitals of the phenyl rings of nitrosobenzene chemisorbed on Ge(100) are electronically decoupled from the semiconductor surface and can be resolved by STM. Four types of dumbbell-like molecular features are imaged, corresponding to the intradimer and interdimer [2 + 2] nitrosoadducts. In these products, nitrosobenzene binds to Ge(100) through its NO group, which spatially separates and electronically decouples the phenyl ring (C6H5–) from the substrate. Theoretical calculations and STM simulation reveal that the dumbbell-like features resemble the occupied π orbitals of benzene. Our results show that electronic decoupling and orbital resolving can be achieved for molecules binding to highly reactive surfaces via the sacrifice of a double bond as the anchoring/spacing group.
Plasmon enhancement of optical properties is both fundamentally important and appealing for many biological and photonic applications. Although metal-enhanced two-photon excitation fluorescence has been demonstrated in the solid substrates, there is no report on metal enhanced overall two-photon excitation fluorescence in the colloid system. Here we systematically investigated gold nanorod enhanced one- and two-photon excitation fluorescence of a porphyrin molecule, T790. The separation distance between the metal core and T790 was varied by adjusting the silica shell thickness from 13 to 42 nm. One- and two-photon excitation fluorescence intensities of T790 were found to strongly depend on the thickness of silica shell that separates gold nanorod and T790. The optimum one- and two-photon excitation fluorescence enhancement was found to occur at shell thicknesses of 34 and 20 nm, with enhancement factors of 2.1 and 11.8, respectively. Fluorescence lifetime of T790 steadily decreased as the shell thickness decreased. The observed two-photon excitation fluorescence enhancement is ascribed to a combination effect of local electric field amplification and competition between increased radiative and non-radiative decay rates. Core-shell nanoparticles that displayed enhanced two-photon excitation fluorescence were also found to exhibit significantly improved singlet oxygen generation capability under two-photon excitation. The applications of these nanoparticles as effective agents for two-photon cell imaging and nano-photosensitizers for two-photon photodynamic therapy with improved efficiency have also been demonstrated in HepG2 cancer cells. The combined advantages of enhanced two-photon excitation fluorescence and two-photon induced singlet oxygen generation make these core-shell nanoparticles as attractive agents for two-photon imaging guided two-photon photodynamic therapy.Keywords: core−shell nanoparticles; gold nanorods; nano-photosensitizers; plasmon resonance enhancement; two-photon imaging; two-photon photodynamic therapy;
Oxygen vacancy is one type of the most important defects affecting the photocatalytic performance of WO3. In this paper, WO3 nanoplates with a high density of oxygen vacancies were synthesized from MWO4 (M = Zn, Cd, Co, Ni) precursors using a sacrificial template method. The structures and morphologies of WO3 nanoplates were investigated by field emission scanning electron microscopy (FE-SEM), high resolution Transmission Electron Microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) analysis, Photoluminescence (PL), Diffuse Reflectance UV-Vis (DRS UV-Vis) and Time-correlated single-photon counting (TCSPC). The metal tungstates were found to not only act as the precursors but also as structure-directing agents during the growth of WO3 nanoplates. XRD data revealed that two phases of WO3·xH2O (x = 1 or 2) were obtained after acid treatment of MWO4. WO3 nanoplates derived from NiWO4 were found to have the highest ratio of WO3·2H2O, highest concentration of oxygen vacancies, narrowest band gap, longest electron–hole recombination time, and in turn the highest rate of photodegradation of azo dye methylene blue. These results show that the structural, electronic and photocatalytic properties of synthesized WO3 nanoplates can be tuned by varying the transition metal tungstate precursors.
Chemistry - A European Journal 2014 Volume 20( Issue 46) pp:15095-15101
Publication Date(Web):
DOI:10.1002/chem.201403866
Abstract
Sword-like anatase TiO2 nanobelts exposed with 78 % clean {100} facets were synthesized and the facet-dependent photoreactivity of anatase TiO2 was investigated. By quantitative comparison with the reference {001} facets, the {100} facets possessed about ten-times higher active sites density than that on {001} facets, resulting in higher photoreaction efficiency. After the active sites density normalization, the {100} and {001} facets exhibited distinct wavelength-dependent photocatalytic performance, attributed to the anisotropic electronic structures in TiO2 crystals.
Co-reporter:Zhangxian Chen ; Qingfan Zhang ; Liang Huang ; Ran Li ; Wanchao Li ; Guoqin Xu ;Hansong Cheng
The Journal of Physical Chemistry C 2014 Volume 118(Issue 36) pp:21244-21249
Publication Date(Web):August 18, 2014
DOI:10.1021/jp5063447
Surface modification of ITO films is important for their applications in optoelectronics. Herein, trimesic acid was used to modify polycrystalline ITO nanoparticles. Spectroscopic results indicate the formation of carboxylate on the ITO nanoparticle surfaces upon modification. Density functional theory calculations reveal an upstanding adsorption structure of the acid molecule on the ITO (111) surface and the formation of a carboxylate surface species. The dissociative chemisorption of trimesic acid on the selected surface was found to be thermodynamically exothermic and kinetically facile. We show that the surface-modified ITO nanoparticles are capable of effectively enhancing the charge-transfer rate and substantially boosting electrocatalytic effect toward redox of ferrocene via a π–π interaction between ferrocene and the adsorbate.
Co-reporter:Wei Mao ; Jing Hui He ; Jia Qiang Gu ; Guo Qin Xu ;Eng Soon Tok
The Journal of Physical Chemistry C 2014 Volume 118(Issue 37) pp:21509-21516
Publication Date(Web):August 26, 2014
DOI:10.1021/jp506304d
The reaction mechanism and regioselectivity of methyl oxirane (C3H6O) adsorbed on Si(111)-(7 × 7) have been studied using high-resolution electron energy loss spectroscopy (HREELS), in situ scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The experimental results demonstrate that methyl oxirane chemically binds to Si(111)-(7 × 7) through both dative-bonded addition and ring-opening reaction via cleavage of C–C or one C–O bond within the epoxy group. The STM images also reveal that the adsorption is site-selective with a preference for center adatoms on the faulted half of the (7 × 7) unit cell. The DFT calculations further show that breaking the C(CH3)–O bond is most kinetically favorable with a relatively small barrier of ∼3 kcal/mol. The dative-bonded states are observable on the surface only after three adjacent adatom–rest atom pairs within one-half unit cell are fully reacted with the methyl oxirane molecules during the ring-opening reaction.
Co-reporter:Ji Hong Wu, Zhenping Guan, Su Ke Yang, Peiyan Yuan, Qing-Hua Xu and Guo Qin Xu
Nanoscale 2013 vol. 5(Issue 7) pp:2983-2989
Publication Date(Web):06 Feb 2013
DOI:10.1039/C3NR34000G
We report a new capping-agent-free strategy for the synthesis of substrate-supported porous icosahedral Au nanoparticles (NPs) with rough naked surfaces, based on the crystallization from substrate-supported thin solution layers followed by solid-phase thermolysis. The plasmonic properties of icosahedral Au NPs have been studied using single particle dark-field scattering microscopy and spectroscopy. The two distinct localized surface plasmon resonance (LSPR) bands observed in the single particle dark-field spectra can be ascribed to the quadrupole resonance at ca. 425 nm and the size-dependent dipole resonance in the red region (645–708 nm). The unique rough naked surface, the facile synthesis, together with the ability to control the nanoparticle size and to vary the LSPR frequency in the red region, would make the substrate-supported porous icosahedral Au NPs promising on multiple levels in the applications of catalysis, ultrasensitive biosensors, and in surface-enhanced Raman scattering (SERS).
Co-reporter:Zhangxian Chen, Wanchao Li, Ran Li, Yunfeng Zhang, Guoqin Xu, and Hansong Cheng
Langmuir 2013 Volume 29(Issue 45) pp:13836-13842
Publication Date(Web):October 11, 2013
DOI:10.1021/la4033282
Deposition technology of transparent conducting oxide (TCO) thin films is critical for high performance of optoelectronic devices. Solution-based fabrication methods can result in substantial cost reduction and enable broad applicability of the TCO thin films. Here we report a simple and highly effective solution process to fabricate indium–tin oxide (ITO) thin films with high uniformity, reproducibility, and scalability. The ITO films are highly transparent (90.2%) and conductive (ρ = 7.2 × 10–4 Ω·cm) with the highest figure of merit (1.19 × 10–2 Ω–1) among all the solution-processed ITO films reported to date. The high transparency and figure of merit, low sheet resistance (30 Ω/sq), and roughness (1.14 nm) are comparable with the benchmark properties of dc sputtering and can meet the requirements for most practical applications.
Co-reporter:Jing Hui He, Wei Mao, Jing Kun Gao, and Guo Qin Xu
The Journal of Physical Chemistry C 2013 Volume 117(Issue 33) pp:17111-17118
Publication Date(Web):July 23, 2013
DOI:10.1021/jp405602q
Oxynitridation of Ge surfaces by nitric oxide (NO) is an important method to synthesize the gate dielectric for Ge-based microelectronics. Understanding the atomic processes of NO oxynitridation on Ge(100) is highly desirable to improve the N incorporation efficiency. Adsorption and dissociation of NO on Ge(100) were investigated on periodic models using DFT calculations. The nondissociative precursors can transform into various dissociative products, resulting in lowering the system energy as well as increasing the coordination numbers of N and O atoms. The transition state search shows that both monomeric and dimeric dissociative pathways are possible. The interdimer route for monomeric dissociation is unfavorable at low temperatures due to the relatively large barriers. In contrast, the intradimer dissociation is preferable due to the existence of an intermediate state, in which the N–O bond is significantly weakened. When a high concentration of NO molecules is adsorbed on Ge(100), three dimeric adsorption structures with two O atoms attached on surfaces are thermodynamically and kinetically favorable to form but difficult to dissociate even at room temperature. Their further release of N2 at elevated temperatures would deteriorate the nitrogen incorporation ratio. Our results are useful for optimizing the oxynitridation of Ge(100) by nitric oxide.
Co-reporter:Jing Hui He, Wei Mao, Jia Qiang Gu, Guo Qin Xu, and Eng Soon Tok
The Journal of Physical Chemistry C 2013 Volume 117(Issue 37) pp:19115-19118
Publication Date(Web):August 24, 2013
DOI:10.1021/jp407996z
Adsorption of organic molecules on Ge(100) is important because of its potential applications in organic–semiconductor devices and semiconductor fabrication. On Ge(100), buckled Ge dimers composed of up-Ge and down-Ge atoms align to dimer rows separated by troughs. Adsorption of thiazole on Ge(100) surfaces was investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Two distinct features were observed when coverages were less than 0.25 monolayer (ML), including feature I on top of down-Ge atoms and feature II in dimer troughs. At 0.25 ML, feature II dominates and forms a highly ordered c(4 × 2) pattern. For coverage >0.25 ML, feature I self-assembles to one-dimensional molecular wires. DFT calculations reveal that feature I can be attributed to thiazole binding vertically to the down-Ge through N–Ge dative bonds, while feature II can be related to cross-dimer [4 + 2] cycloadducts through two C–Ge bonds. π–π interaction between T-shaped and parallel-displaced pairs of feature I facilitate the formation of the molecular wires. Our results indicate that thiazole may have application in both molecular electronics as well as Ge surface processing.
Gold nanorods with three different aspect ratios were prepared and their dual capabilities for two-photon imaging and two-photon photodynamic therapy have been demonstrated. These gold nanorods exhibit large two-photon absorption action cross-sections, about two orders of magnitude larger than small organic molecules, which makes them suitable for two-photon imaging. They can also effectively generate singlet oxygen under two-photon excitation, significantly higher than traditional photosensitizers such as Rose Bengal and Indocyanine Green. Such high singlet oxygen generation capability under two-photon excitation was ascribed to their large two-photon absorption cross-sections. Polyvinylpyrrolidone (PVP) coated gold nanorods displayed excellent biocompatibility and high cellular uptake efficiency. The two-photon photodynamic therapy effect and two-photon fluorescence imaging properties of PVP coated gold nanorods have been successfully demonstrated on HeLa cells in vitro using fluorescence microscopy and indirect XTT assay method. These gold nanorods thus hold great promise for imaging guided two-photon photodynamic therapy for the treatment of various malignant tumors.
Co-reporter:Wee Boon Tan, Jin Hongmei, Shuo-Wang Yang and Guo Qin Xu
Nanoscale 2012 vol. 4(Issue 23) pp:7557-7562
Publication Date(Web):02 Oct 2012
DOI:10.1039/C2NR31889J
Different from all reported sandwich molecular wires (SMWs), a novel class of SMW including vanadium boratabenzene (HBBz) clusters and their related one-dimensional (1D) SMWs are explored by using a density functional theory approach. The uniqueness of this class of SMWs lies in the boron heterocycles, where they possess a reactive functional atom. These features may overcome the limitation of existing SMWs which are inert and are difficult to be absorbed stably on surfaces. Theoretical calculations of the novel vanadium boratabenzene clusters indicated that they are energetically stable, with the rings having a restricted degree of rotation. In addition, its metallic 1D analog shows great promise in the applications of molecular electronics and spintronics.
Co-reporter:Jing Hui He, Yong Ping Zhang, Wei Mao, Guo Qin Xu, Eng Soon Tok
Surface Science 2012 Volume 606(9–10) pp:784-790
Publication Date(Web):May 2012
DOI:10.1016/j.susc.2012.01.007
CO adsorption on the Ge(100) surface has been investigated using a slab model with density functional theory implemented in SIESTA. CO was found to be exclusively adsorbed on the asymmetric dimer with C attaching on the lower Ge dimer atom. The adsorption process is barrierless. The calculated adsorption energy and vibration frequencies are comparable to previous experimental results. The crystal orbital Hamilton analysis showed that the bonding between Ge and CO is mainly attributable to the Ge 4pz orbital overlapping with C 2 s, or with CO molecular orbitals 3σ and 4σ. The repulsive energy between adsorbed CO molecules is less than 1 kcal/mol. The diffusion barrier of CO on the Ge(100) surface is about 14 kcal/mol.Highlights► DFT calculation found CO is exclusively adsorbed on lower Ge dimer atom on Ge(100). ► The bonding between Ge and CO is mainly attributable to the Ge 4pz orbital overlapping with C 2s. ► The adsorption and diffusion processes were studied. ► The adsorbate interaction is negligible.
The interaction of O2 and CO2 with the Si(111)-7 × 7 surface has been studied with X-ray photoelectron spectroscopy (XPS). It was found that both O2 and CO2 molecules can readily oxidize the Si(111)-7 × 7 surface to form thin oxide films. Two oxygen species were identified in the oxide film: oxygen atoms binding to on-top sites of adatom/rest atoms with an O 1s binding energy of ~ 533 eV as well as to bridge sites of adatom/rest atom backbonds at ~ 532 eV. These two oxygen species can be interconverted thermally during the annealing process. Due to the low oxidation capability, the silicon oxide film formed by CO2 has a lower O/Si ratio than that of O2.Both O2 and CO2 molecules can oxidize the Si(111)-7 × 7 surface and form thin oxide films. Two oxygen species in the oxide films were identified as oxygen atoms binding to on-top sites of adatom/rest atoms with the high binding energy, and oxygen on bridge sites of adatom/rest backbonds with the low binding energy.Highlights► O2 and CO2 both dissociatively adsorb on the Si(111)-7 x 7 surfaces. ► The oxide film contains two chemisorbed oxygen species. ► Oxygen atoms adsorb at the on-top site (SiOy) and the Si-Si bridge site (SiOx). ► Thermal-induced conversion between these two oxygen species was detected.
Co-reporter:Yong Ping Zhang, Jing Hui He, and Guo Qin Xu , Eng Soon Tok
The Journal of Physical Chemistry C 2012 Volume 116(Issue 16) pp:8943-8949
Publication Date(Web):March 21, 2012
DOI:10.1021/jp300173d
Controlled formation of covalently bonded N-benzylideneaniline-like molecular layers has been achieved by photoinduced reaction of benzonitrile molecules physisorbed on the 4-bromostyrene modified Si(111)-(7 × 7) surface. The photoinduced reaction results in the C–Br bond cleavage and produces a radical site which concurrently reacts with the C≡N cyano group of the physisorbed benzonitrile molecule above to form the −C–N═C– covalent linkage. X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spctroscopy (HREELS) experiments together with density functional theory (DFT) calculations confirm the formation of covalently bonded N-benzylideneaniline-like molecular layers on the silicon surface. The formation of this secondary molecular layer by photoinduced reaction may provide a direct pathway for further organic synthesis and fabrication of organic multilayers on semiconductor surfaces in a well-controlled enviornment.
Co-reporter:Ji Hong Wu, Tian Ze Xu, Siau Gek Ang, Qing-Hua Xu and Guo Qin Xu
Nanoscale 2011 vol. 3(Issue 4) pp:1855-1860
Publication Date(Web):07 Mar 2011
DOI:10.1039/C0NR01012J
Radially oriented anthracene nanowires and their self-assembled concentric ring arrays were prepared through a facial solvent-evaporation method. The successful growth of anthracene nanowires can be attributed to a combined mechanism of molecular self-assembly facilitated by strong π–π intermolecular interactions together with evaporation-induced capillary flow and fingering instability. Their radial orientation is determined by the capillary flow; their shape (either straight or curved nanowires) is governed by the competition between the capillary and Marangoni convectional flows. The self-assembly of nanowires into large-scale concentric ring arrays can be interpreted in terms of the repeated slipping-and-sticking motions of the contact line. The high-quality crystalline anthracene nanowire arrays exhibit size-dependent fluorescence emission with high-degree anisotropy.
The adsorption of S2 on the Si(1 1 1)-(7 × 7) surface and the interaction of copper and sulfur on this sulfur-terminated Si(1 1 1) surface have been studied using synchrotron irradiation photoemission spectroscopy and scanning tunneling microscopy. The adsorption of S2 at room temperature results in the passivation of silicon dangling bonds of Si(1 1 1)-(7 × 7) surface. Excessive sulfur forms Sn species on the surface. Copper atoms deposited at room temperature directly interact with S-adatoms through the formations of Cu–S bonds. Upon annealing the sample at 300 °C, CuSx nanocrystals were produced on the sulfur-terminated Si(1 1 1) surface.
Co-reporter:Ji Hong Wu, Zhenping Guan, Tian Ze Xu, Qing-Hua Xu, and Guo Qin Xu
Langmuir 2011 Volume 27(Issue 10) pp:6374-6380
Publication Date(Web):April 11, 2011
DOI:10.1021/la200569v
Large-scale tetracene-doped anthracene nanowire arrays were prepared, and the doping effects were studied. The high doping concentration up to 10% (molar ratio) has been achieved, attributed to both the unique long-nanowire geometry and the excellent structural compatibility of anthracene and tetracene. The incorporation of long tetracene molecules into the matrix of short anthracene molecules induced an enlarged interlayer thickness, a decreased nanowire thickness, and an expanded nanowire width. The tetracene molecules were homogeneously embedded into the anthracene matrix at low doping concentrations (<1%). The doping became inhomogeneous at high doping concentrations (≥1%). The energy transfer efficiency between anthracene and tetracene is nearly 100% at doping concentrations ≥1%.
The Journal of Physical Chemistry C 2011 Volume 115(Issue 5) pp:2140-2145
Publication Date(Web):December 20, 2010
DOI:10.1021/jp1098415
To prepare functional molecular templates for the development of hybrid molecular devices, multifunctional molecules may be employed and attached onto the silicon surfaces. Due to their possible multiple binding configurations, it is challenging to achieve the exclusive selectivity for the desired configuration. In this paper we describe a feasible way to tune the binding mechanism of pyridine through the coadsorption of pyrrole on the Si(111)-(7×7) surface at room temperature. Our X-ray photoelectron spectroscopy and high-resolution electron energy spectroscopy results clearly demonstrate that pyridine bonds to the clean Si(111)-(7×7) surface in the configuration of 1,4-cyclohexadiene-like adducts on the surface via Si−C(N) di-σ-linkages. However, on pyrrole-precovered surfaces, pyridine molecules are dative-bonded to the remaining adatoms on the surface. This fine-tuning of molecular configuration via surface premodification provides flexibility in controlling the molecular attachment on silicon surfaces.
The Journal of Physical Chemistry C 2011 Volume 115(Issue 31) pp:15496-15501
Publication Date(Web):July 2, 2011
DOI:10.1021/jp204113p
The covalent attachment of 4-bromostyrene on the Si(111)-(7 × 7) surface was investigated using X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory calculations. The HREELS spectra suggest that 4-bromostyrene covalently binds to the silicon surface through a [2 + 2]-like cycloaddition pathway between the external vinyl group and the adjacent adatom–rest atom pair of the Si(111)-(7 × 7) surface, forming 4-bromoethylbenzene-like binding configuration. The XPS results further confirm that only the vinyl double bond participates in the surface binding reaction, whereas the bromine atom remains unchanged during the adsorption process. The resulting 4-bromoethylbenzene-like structure on the Si(111)-(7 × 7) surface can be employed as a precursor for further chemical modification and functionalization.
Co-reporter:Ji Hong Wu, Binni Varghese, Xue Dong Zhou, Si Ying Teo, Chorng Haur Sow, Siau Gek Ang and Guo Qin Xu
Chemistry of Materials 2010 Volume 22(Issue 4) pp:1533
Publication Date(Web):January 11, 2010
DOI:10.1021/cm902490g
Highly oriented Zn(NO3)2·6H2O nanotubes were grown on mica substrates based on an epitaxy mechanism. The Zn(NO3)2·6H2O nanotubes with rectangular cross-section were self-assembled on mica surfaces into large-area, interconnected hexagonal networks. Fast evaporation of the solvent was found to be crucial for the growth of high-quality Zn(NO3)2·6H2O rectangular nanotubes. ZnO architectures with tailored porosity were achieved through controlled solid-phase thermal decomposition of the Zn(NO3)2·6H2O nanotubes. Defects in porous ZnO architectures and the photoluminescence (PL) properties could be well tuned by varying the annealing conditions. The porous ZnO interconnected networks were electrically interconnected and electrically functioned as a single integrated unit with symmetric, linear current−voltage (I−V) characteristic.
The Journal of Physical Chemistry C 2010 Volume 114(Issue 39) pp:16625-16629
Publication Date(Web):September 8, 2010
DOI:10.1021/jp105933f
This paper reports the template-induced formation of molecular corrals on the Si(111) surface by taking the advantage of the intrinsic property of the reconstructed (7 × 7) unit cell. Self-assembled molecular corrals have been formed on the Si(111)-(7 × 7) surface by binding pyrrole molecules chemically on the silicon center adatom through the breakage of the N−H bond. The dissociative adsorption of pyrrole on Si(111)-(7 × 7) leads to pyrroyl and H atom binding with an adatom and an adjacent rest atom, respectively. The molecular corral has dramatically modified the electronic property of the silicon surface, which leads to the formation of pyridine dative bonding to Si(111)-(7 × 7) surface at room temperature. The self-assembled molecular corral may provide a template for controlling the molecular binding configurations and quantum confinement effect of nanoclusters.
Co-reporter:Yan Xia Shao ; Dong Dong ; Ying Hui Cai ; Shuai Wang ; Siau Gek Ang
The Journal of Physical Chemistry C 2010 Volume 114(Issue 40) pp:17159-17165
Publication Date(Web):September 2, 2010
DOI:10.1021/jp103945m
3-Chloro-1-propanol (HO−CH2−CH2−CH2−Cl) covalently binds onto Si(100)-2×1 through the thermal dissociation of the OH group to form Si−O−CH2−CH2−CH2−Cl surface intermediates, evidenced by the appearance of the Si−H stretching mode (2110 cm−1) and the retention of C−Cl stretching mode (654 cm−1) in the high-resolution electron energy loss spectroscopy (HREELS) spectrum of chemisorbed 3-chloro-1-propanol molecules and the chemical downshift of O1s binding energy (BE) in the X-ray photoelectron spectroscopy (XPS) study. The C−Cl bonds in the chemisorbed 3-chloro-1-propanol can be cleaved upon 193 nm irradiation, resulting in Si−O−CH2CH2CH2−CH2CH2CH2−O−Si through lateral diradical coupling. Upon covering the chemisorbed 3-chloro-1-propanol with physisorbed molecules, photoinduced diradical coupling between physisorbed and chemisorbed molecules was also evidenced, achieving the secondary attachment of 3-chloro-1-propanol on the Si surface and forming Si−O−CH2CH2CH2−CH2CH2CH2−OH.
Co-reporter:Yue Sheng Ning, Yan Xia Shao and Guo Qin Xu
The Journal of Physical Chemistry C 2010 Volume 114(Issue 23) pp:10455-10462
Publication Date(Web):May 20, 2010
DOI:10.1021/jp9115707
The Covalent binding of p-benzoquinone (O═C6H4═O) and the formation of an aromatic ring (−O—C6H4—O−) on Si(111)-7×7 have been investigated by using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. For chemisorbed p-benzoquinone, the absence of v(C═O) at 1659 cm−1, the retention of sp2-v(C−H) at 3050 cm−1, the appearance of v(Si−O) at 824 cm−1, and aromatic v(C═C)/δip(C−H) at 1600/1505 cm−1 demonstrate that the molecule reacts with the surface in a [6+2]-like cycloaddition mode, which is further confirmed by XPS and density functional theory (DFT) vibrational calculations. DFT calculations indicate that the [6+2]-like cycloadduct (−O—C6H4—O−) bridging two nearest adatoms in neighboring half-unit cells is the most stable. This binding scheme may prove useful for chemical and electronic modification of the semiconductor surfaces.
Co-reporter:Yan Xia Shao, Dong Dong, Ying Hui Cai, Shuai Wang, Siau Gek Ang and Guo Qin Xu
The Journal of Physical Chemistry C 2010 Volume 114(Issue 6) pp:2701-2710
Publication Date(Web):January 22, 2010
DOI:10.1021/jp910314p
The adsorption of halogenated acetonitrile (X−CH2C≡N, X = F, Cl, Br) on Si(100)-2×1 has been studied using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculation. Fluoroacetonitrile chemisorbs on the Si(100)-2×1 surface via a C≡N [2 + 2] cycloaddition-like reaction, while chloroacetonitrile is chemically bound to the Si surface through both C≡N [2 + 2] cycloaddition-like and ene-like reactions. However, bromoacetonitrile was found to attach onto the surface exclusively through the ene-like reaction. The different reaction pathways of these halogenated compounds on the Si(100)-2×1 surface demonstrate that the halogen substitution groups play an important role in manipulating the reaction channels of bifunctional molecules, offering a great flexibility in surface reaction of silicon surfaces.
Novel multifunctional nanoparticles that combine two functionalities (a gold nanorod core and a porphyrin-doped mesoporous silica shell) into one entity were synthesized. Due to the encapsulation of mesoporous silica, the porphyrin can be well protected against the external bioenvironment. In addition, the generated singlet oxygen by porphyrin molecules can be easily released from the silica. We have demonstrated that these multifunctional nanoparticles can generate singlet oxygen with relatively higher yield compared to free porphyrin. These multifunctional nanocomposites are attractive candidates for simultaneous photosensitization and two-photon imaging as well as imaging guided therapy.
Chemical Physics Letters 2009 Volume 482(1–3) pp:77-80
Publication Date(Web):6 November 2009
DOI:10.1016/j.cplett.2009.09.080
Abstract
The vibrational and electronic features of propargyl chloride (HCCCH2Cl) attached on the Si(1 0 0) − 2 × 1 surface have been investigated using high resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The disappearance of the CCl stretching mode and the existence of CspH and CC stretching modes in the HREELS spectra of chemisorbed molecules strongly demonstrate the cleavage of C–Cl bonds and the retention of CCH linkage upon the chemisorption, further supported by the changes of the electronic features in chemisorbed XPS spectra.
Co-reporter:Ying Hui Cai, Yan Xia Shao, Dong Dong, Hai Hua Tang, Shuai Wang and Guo Qin Xu
The Journal of Physical Chemistry C 2009 Volume 113(Issue 10) pp:4155-4160
Publication Date(Web):2017-2-22
DOI:10.1021/jp8096549
The covalent attachment of 4-chloroaniline on the Si(111)-7×7 surface was investigated by using a combination of X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations. The HREELS spectra suggest that one of the N−H bonds dissociates to form Si−N and Si−H bonds with the phenyl ring and the C−Cl bond unperturbed upon chemisorption. The XPS results confirm that only the NH2 group participates in the surface binding. This binding mode and surface reaction pathway are further supported by the DFT calculation. The resulting chlorobenene-like structure on Si(111)-7×7 can be employed for further photochemical modification and functionalization.
The adsorption of naphthalene on Si(1 1 1)-7 × 7 at room temperature was studied using STM and DFT calculations. It is proposed that the major, if not exclusive, binding configuration of the adsorbed naphthalene involves the formation of covalent bonds between two opposite C atoms from one of the rings in naphthalene with an adjacent adatom–rest atom pair on the substrate. Combined data from STM and DFT studies shows that the chemisorption of naphthalene causes an increase in the charge density of the neighboring dangling bond sites that in turn enhance their reactivities with a naphthalene molecule. The faulted center and unfaulted corner adatoms are, respectively, the most and least reactive sites for naphthalene while the relative reactivities of the faulted corner and unfaulted center adatoms exhibit dependence on the naphthalene coverage.
High-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations were used to investigate the attachment of allyl and propargyl alcohols on Si(1 1 1)-7×7 under ultra-high vacuum conditions. The HREELS spectra of chemisorbed allyl alcohol (AA) show the concurrent appearance of characteristic stretching vibrations of Si−H (2104 cm−1) and Si−O (795 cm−1) coupled with the retention of vibrational features of CC stretching (1657 cm−1) and (sp2)C−H stretching (3012 and 3102 cm−1). These results clearly demonstrate the dissociative reaction nature via the hydroxyl group for the chemisorption of AA on Si(1 1 1)-7×7, which was further supported by XPS and DFT studies. A similar reaction pathway was found for propargyl alcohol (PA) adsorbed on the same Si(1 1 1)-7×7 surface. Our studies suggest that OH dissociation is highly favorable compared to [2+2]-like cycloadditions via CC/CC for organic reactions on silicon surfaces, which may be explained by the large spatial separation between the adjacent adatom-rest atom pair on Si(1 1 1)-7×7.
The Journal of Physical Chemistry C 2008 Volume 112(Issue 20) pp:7605-7610
Publication Date(Web):April 23, 2008
DOI:10.1021/jp800358s
Identically oriented NaCl nanocrystallites have been grown on mica surfaces based on a facile solution-evaporated method. With an increase in humidity, the morphology of the NaCl nanocrystallites evolves in succession from triangular pyramids to cubic islands and then to long lines with lengths of up to millimeters. These nanocrystallites can self-assemble into highly ordered arrays with large spatial extents of ∼10 mm2 under high humidity conditions when the value of relative humidity is higher than 40%. Their morphology transitions and self-assemblies can be understood in terms of controlled epitaxial crystallization together with water adsorption at the surface of the growing nanocrystallites.
The adsorption configurations of pyrimidine and triazine on Si(1 1 1)-7 × 7 were investigated using high-resolution electron energy loss spectroscopy (HREELS) X-ray photoelectron spectroscopy and density functional theory calculations. The HREELS spectra of chemisorbed monolayer show the coexistence of the C(sp2)–H and C (sp3)–H stretching modes together with the observation of the unconjugated CN(C) vibrational feature suggesting that the carbon atom and its para-nitrogen atom of pyrimidine and triazine directly participate in binding with the surface to form Si–C and Si–N σ-linkages. The core levels of the C-atom and its opposite nitrogen atom directly binding with Si-atoms experience a down-shifting by 1.8–1.9 and 1.4–1.6 eV, respectively. These experimental findings are consistent with the density functional theory calculations indicating that the carbon atom and its para-nitrogen atom favorably link with the adjacent adatom and rest atom pair to form C–Si and N–Si linkages.
The interaction of ethyl vinyl ketone (EVK) with Si(1 1 1)-7 × 7 has been investigated using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. The disappearance of both stretching vibrations of CH2 (3099 cm−1) and CO (1684 cm−1) coupled with the appearance of new CC stretching mode (1660 cm−1) in the HREELS spectra of chemisorbed EVK clearly demonstrates the direct involvement of conjugated CC and CO bonds to form a SiC1H2C2HC3(C4H2C5H3)OSi surface species via [4 + 2]-like cycloaddition in a highly selective manner. In addition, XPS studies show that the C1s binding energies of C1/C2 and C3 upon chemisorption display chemical downshifts of 0.8 eV and 2.2 eV, respectively, further confirming the proposed [4 + 2]-like cycloaddition reaction for the EVK/Si(1 1 1)-7 × 7 system. DFT theoretical calculations suggest that the proposed [4 + 2]-like cycloadduct is thermodynamically most favorable.
Co-reporter:Jing Yan Huang, Hai Gou Huang, Yue Sheng Ning, Qi Ping Liu, Solhe F. Alshahateet, Yue Ming Sun, Guo Qin Xu
Chemical Physics Letters 2005 Volume 411(1–3) pp:75-80
Publication Date(Web):5 August 2005
DOI:10.1016/j.cplett.2005.06.008
The covalent binding of acetyl cyanide on Si(1 0 0)-2 × 1 has been investigated using high-resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. The absence of the CO and CN stretching modes in the EELS spectra for chemisorbed molecules indicates the direct interactions between carbonyl, cyano groups and the dangling bonds on Si(1 0 0)-2 × 1. The characteristic CCN asymmetric stretching mode at 2031 cm−1 and CN stretching mode at 1667 cm−1 suggest the existence of both ketenimine species and tetra-σ adduct at the interface, further supported by XPS results and DFT calculations.
The electrocatalytic oxidation of glucose in alkaline medium directly at well-aligned multi-wall carbon nanotubes (MWNTs) electrodes has been investigated in the present study. Compared to glassy carbon electrode, a substantial (+400 mV) decrease in the overvoltage of the glucose oxidation reaction was observed at MWNTs electrodes with oxidation starting at ca. +0.10 V (vs. 3 M KCl–Ag|AgCl). The electrocatalytic effect is mainly attributed to carbon nanotubes with possible minor contributions from the Co catalysts present on the Ta substrate. At an applied potential of +0.20 V, MWNTs electrodes give a high and reproducible sensitivity of 4.36 μA cm−2 mM−1 in the presence of high concentration of chloride ion. The well-aligned MWNTs electrode thus allows highly sensitive, low-potential, stable, and fast amperometric sensing of glucose, promising for the development of nonenzymatic glucose sensors.
The reaction of formic acid on Si(1 1 1)-7 × 7 was investigated using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron energy loss spectroscopy (HREELS). The hydroxyl and carbonyl O 1s core levels of chemisorbed formic acid display chemical shifts of 2.4 and 0.2 eV respectively, compared with those of physisorbed molecules. The HREELS spectra of chemisorbed formic acid show the absence of stretching and bending modes of the O–H bond, the appearance of Si–H (2089 cm−1) and the Si–O (680 cm−1) stretching modes and the retained stretching mode of CO at 1703 cm−1. Our results clearly suggest that formic acid dissociates to form monodentate formate species and H-atom on the adatom–rest atom pair of Si(1 1 1)-7 × 7.
The adsorption and thermal reaction of pyrrole on Si(1 0 0)-2 × 1 have been studied using X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS) and high resolution electron energy loss spectroscopy (HREELS). At low exposures, Pyrrole chemisorbs molecularly at 120 K with its ring parallel to the surface via the π-interaction. The increase in coverage causes tilting of chemisorbed molecules towards the surface normal, attributable to the adsorbate–adsorbate interactions. At ∼350 K, the N–H bond scission of the π-bonded species occurs, resulting in Si–H and vertically N-bonded pyrrolyl on the surface. The pyrrolyl species is thermally stable to 700 K. Compared to furan or thiophene on Si(1 0 0), this higher thermal stability is ascribed to the passivation effect of the H-atoms from N–H bond dissociation and the less strain within the pyrrolyl–substrate complex. Further annealing to 900 K results in the formation of silicon carbide and silicon nitride on the substrate.
The binding and interaction of Cu on Si(1 1 1)-(7 × 7) at room temperature have been studied using scanning tunneling microscopy. Self-organized growth of ordered Cu nanostructures was observed. At low coverage, the two-dimensional Cu nanostructures are predominantly assembled at the faulted halves of the (7 × 7) unit cells. High coverage leads to Cu-binding on the unfauled halves. Our results also show that the two-dimensional nanostructure consists of six Cu-atoms with three on the center adatoms and the others on the rest atoms within one half of a unit cell. The Cu nanostructure formed may have potential applications in the fabrication of nanoscale devices and chemical modification of semiconductor surfaces.
The nitridation of ultra-thin Ti films on Si(1 0 0) have been studied using X-ray photoelectron spectroscopy (XPS) in a temperature range of 120–1000 K. Upon ammonia exposure to the multilayer Ti thin films at 120 K, three N 1s peaks at 397.8–398.1, 400.5–400.8 and 402.2–402.6 eV were observed, attributable to NHx (x=1 or 2), molecular NH3 and NH4δ+, respectively. Annealing of the NH3 saturated Ti/Si(1 0 0) surfaces results in the conversion of the NHx species. This species undergo two different pathways between 300 and 800 K, i.e. further dissociation to N(a) and H(a), and recombing with H(a) to form NH3(g). The atomic N reacts with Ti to yield a stable TiN film that retards significantly the interdiffusion at the Ti/Si interface.
Chemical Physics Letters 2001 Volume 338(Issue 1) pp:7-13
Publication Date(Web):13 April 2001
DOI:10.1016/S0009-2614(01)00179-8
Abstract
The molecular adsorption of benzene on the Si(1 1 1)-(7×7) surface has been studied using a combination of semi-empirical (PM3) and density functional theory (DFT) (pBP86/DN∗∗) methods with cluster models. Two di-σ bonded adsorption configurations corresponding to the 1,2 and 1,4 states are considered. Energy calculations by both theoretical methods have revealed that the 1,4 state is thermodynamically favoured over the 1,2 state. In particular, the adsorption energy and vibrational frequencies of the 1,4 state predicted by the DFT method are in excellent agreement with the experimental observations, which further confirms the 1,4-cyclohexadiene-like binding state of benzene on Si(1 1 1)-(7×7).
Activated carbon nanotubes were used as precursors for preparing copper- and nickel-decorated nanotubes. The nature of the activated and decorated nanotubes was studied using high-resolution transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). The inert surface of oxidized nanotubes was activated by the introduction of catalytic nuclei via a two-step sensitization—activation method and a single-step activation approach. The activated nanotubes catalyze metal deposition specifically onto their surfaces upon immersion in electroless plating baths. Both methods give tubes decorated with nickel. In addition, copper decoration was achieved for the first time. In the two-step sensitization–activation method, the extent of metal decoration was found to be dependent on the age of the sensitizing solution. Densely decorated nanotubes were obtained after three days of sensitizer aging. The single-step activation approach has previously not been applied to carbon nanotube substrates. Our results show that this method is an efficient and simple means of achieving carbon nanotube activation, with the activated tubes showing a high surface coverage of Pd–Sn catalytic nuclei. Electroless plating of such tubes readily results in nanotubes fully coated with nickel and copper.
Co-reporter:Meng Yin Li, Yao Quan Mao, Su Ke Yang, Ting Ting Dai, Hua Yang, Feng Feng, Tao Wu, Muzi Chen, Guo Qin Xu, and Ji Hong Wu
ACS Omega Volume 1(Issue 4) pp:696-705
Publication Date(Web):October 27, 2016
DOI:10.1021/acsomega.6b00149
Two types of out-of-substrate Ag–Ag2O nanoplates were grown on a ZnO substrate through a surfactantless photochemical method. First, the in situ photochemically synthesized Ag–Ag2O nanoparticles further crystallized into nanoplate-like superstructures with rough surfaces and ragged edges. The nanoparticle-mediated crystallization process was governed by a layer-by-layer crystallization mechanism. Our study should help fundamentally understand the formation mechanism of hierarchical nanoparticle superstructures. Under continuous UV illumination, the hundreds of nanometer-sized rough nanoplates (i.e., the nanoplate-like superstructures of nanoparticles) can be transformed into large smooth nanoplates with sizes of up to several micrometers. The out-of-substrate Ag–Ag2O nanoplates/ZnO heterostructures are potentially promising for photocatalytic applications.Topics: Diffraction; Microstructure; Nanostructures; Optical properties; Phase transition;
Co-reporter:Wei Mao, Jing Hui He, Jia Qiang Gu, Wei Chen, Kai Wu, Eng Soon Tok and Guo Qin Xu
Chemical Communications 2015 - vol. 51(Issue 75) pp:NaN14198-14198
Publication Date(Web):2015/07/20
DOI:10.1039/C5CC04713G
Adsorption of butadiene monoxide on Si(111)-(7×7) has been scrutinized by high-resolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The experimental results indicate that surface reaction occurs through a [2+2]-like cycloaddition, which is further supported by the DFT studies.
Co-reporter:Z. X. Chen, Y. J. Xi, L. Huang, W. C. Li, R. Li, G. Q. Xu and H. S. Cheng
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 40) pp:NaN26744-26744
Publication Date(Web):2015/09/14
DOI:10.1039/C5CP05093F
Many applications of Sn-doped indium oxide (ITO) films in organic electronics require appropriate surface modifications of ITO nanocrystals with small organic molecules, such as silanes, phosophonic acids and carboxylic acids, to improve interfacial contacts and charge transfer. Here, we propose a new surface modification strategy via adsorption of acetylene molecules on an oxygen-terminated ITO(100) surface using a slab crystalline model to represent the nanocrystal surface. The adsorption was first studied using density functional theory. It was found that the chemisorption of C2H2 on two types of surface oxygen dimers is highly exothermic with the calculated adsorption energies of 3.80 eV and 5.19 eV, respectively. Electron population analysis reveals the origin of the strong interaction between the adsorbate and the ITO(100) surface. Experimental studies on the synthesized ITO nanocrystals using X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy confirm the predicted strong adsorption of C2H2 on ITO surfaces.
Co-reporter:Jing Hui He, Wei Mao, Wei Chen, Kai Wu, Han Song Cheng and Guo Qin Xu
Chemical Science (2010-Present) 2014 - vol. 5(Issue 11) pp:NaN4452-4452
Publication Date(Web):2014/07/07
DOI:10.1039/C4SC01293C
Resolving orbitals using scanning tunneling microscopy (STM) provides an in-depth understanding of the chemical and electronic nature of molecule/substrate junctions. Most orbital resolving work was performed for molecules physisorbed on metal surfaces by inserting interfacial layers to decouple the interaction between the molecules and substrates. It remains challenging to image the orbitals of molecules directly chemisorbed on native surfaces because the linking chemical bonds likely induce severe coupling. Here we demonstrate that the π orbitals of the phenyl rings of nitrosobenzene chemisorbed on Ge(100) are electronically decoupled from the semiconductor surface and can be resolved by STM. Four types of dumbbell-like molecular features are imaged, corresponding to the intradimer and interdimer [2 + 2] nitrosoadducts. In these products, nitrosobenzene binds to Ge(100) through its NO group, which spatially separates and electronically decouples the phenyl ring (C6H5–) from the substrate. Theoretical calculations and STM simulation reveal that the dumbbell-like features resemble the occupied π orbitals of benzene. Our results show that electronic decoupling and orbital resolving can be achieved for molecules binding to highly reactive surfaces via the sacrifice of a double bond as the anchoring/spacing group.
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