Kinetics of dissociative O2 adsorption, OHad desorption, and oxygen reduction reaction （ORR） at Pt（111） electrode in 0.1 mol/L HClO4 has been investigated. Reversible OHad adsorption/desorption occurs at potential...Kinetics of dissociative O2 adsorption, OHad desorption, and oxygen reduction reaction （ORR） at Pt（111） electrode in 0.1 mol/L HClO4 has been investigated. Reversible OHad adsorption/desorption occurs at potentials from 0.6 V to 1.0 V （vs. RHE） with the exchange current density of ca. 50 mA/cm^2 at 0.8 V, the fast kinetics of OHad desorption indicates that it should not be the rate determining step for ORR. In the kineticor kinetic-mass transport mix controlled potential region, ORR current at constant potential displays slight decrease with reaction time. ORR current in the positive-going potential scan is slightly larger than that in the subsequent negative-going scan with electrode rotation speed （〉800 r/min） and slow potential scan rate （〈100 mV/s）. The open circuit potential of Pt/0.1 mol/L HClO4 interface increases promptly from 0.9 V to 1.0 V after switch from O2 free- to O2-saturated solution. The increase of open circuit potential as well as ORR current decays under potential control due to the accumulation of OHad from dissociative adsorption of O2. It indicates that at Pt（111） the net rate for O2 decomposition to OHad is slightly faster than that for OHad removal, one cannot simply use the assumption of rate determining step to discuss ORR kinetics. Instead, the ORR kinetics is determined by both the kinetics for O2 decomposition to OHad as well as the thermo-equilibrium of OHad＋H^＋＋e→←H2O.展开更多
Mo（CO）6 adsorption on the clean, oxygen-precovered and deeply oxidized Si（lll） surfaces was comparatively investigated by high-resolution electron energy loss spectroscopy. The downward vibrational frequency shift...Mo（CO）6 adsorption on the clean, oxygen-precovered and deeply oxidized Si（lll） surfaces was comparatively investigated by high-resolution electron energy loss spectroscopy. The downward vibrational frequency shift of the C-O stretching mode in adsorbed Mo（CO）6 illustrates that different interactions of adsorbed Mo（CO）6 occur on clean Si（111） and SiO2/Si（111） surfaces, weak on the former and strong on the latter. The strong interaction on SiO2/Si（111） might lead to the partial dissociation of Mo（CO）6, consequently the formation of molybdenum subcarbonyls. Therefore, employing Mo（CO）6 as the precursor, metallic molybdenum could be successfully deposited on the SiO2/Si（111） surface but not on the clean Si（111） surface. A portion of the deposited metallic molybdenum is transformed into the MoOa on the SiO2/Si（111） surface upon heating, and the evolved MoO3 finally desorbs from the substrate upon annealing at elevated temperatures.展开更多
In order to determine the structures of Si（111）-√7 √3-In surfaces and to understand their electronic properties, we construct six models of both hexagonal and rectangular types and perform first-principles calcula...In order to determine the structures of Si（111）-√7 √3-In surfaces and to understand their electronic properties, we construct six models of both hexagonal and rectangular types and perform first-principles calculations. Their scanning tunneling microscopic images and work functions are simulated and compared with experimental results. In this way, the hex-H3＇ and rect-T1 models are identified as the experimental configurations for the hexagonal and rectangular types, respectively. The structural evolution mechanism of the In/Si（lll） surface with indium coverage around 1.0 monolayer is discussed. The 4×1 and -√7× √3 phases are suggested to have two different types of evolution mechanisms, consistent with experimental results.展开更多
Fast scan voltammetry is an efficient tool to distinguish oxidative/reductive adsorp- tion/desorption from that for bulk reaction. In this work, we provide a methodology that the isotherm of oxidative/reductive adsorp...Fast scan voltammetry is an efficient tool to distinguish oxidative/reductive adsorp- tion/desorption from that for bulk reaction. In this work, we provide a methodology that the isotherm of oxidative/reductive adsorption desorption processes at electrode surface can be obtained using just one solution with relatively low reactant concentration, by taking the advantage of varying the potential scan rate （relative of the diffusion rate） to tune the adsorption rate and proper mathematic treatment. The methodology is demonstrated by taking acetate adsorption at Pt（lll） in acidic solution as an example. The possibility for extension of this method toward mechanistic studies of complicated electrocatalytic reactions is also given.展开更多
We report the formation and local electronic structure of Ge clusters on the Si（111）-7×7 surface studied by using variable temperature scanning tunnelling microscopy （VT-STM） and low-temperature scanning tunn...We report the formation and local electronic structure of Ge clusters on the Si（111）-7×7 surface studied by using variable temperature scanning tunnelling microscopy （VT-STM） and low-temperature scanning tunnelling spectroscopy （STS）. Atom-resolved STM images reveal that the Ce atoms are prone to forming clusters with 1.0 nm in diameter for coverage up to 0.12 ML. Such Ce clusters preferentially nucleate at the centre of the faulted-half unit cells, leading to the ＇dark sites＇ of Si centre adatoms from the surrounding three unfaulted-half unit cells in filled-state images. Biasdependent STM images show the charge transfer from the neighbouring Si adatoms to Ce clusters. Low-temperature STS of the Ce clusters reveals that there is a band gap on the Ce cluster and the large voltage threshold is about 0.9 V.展开更多
The adsorption and decomposition of trimethylgallium （Ga（CH3）3, TMG） on Pd（111） and the effect of pre-covered H and O were studied by temperature programmed desorption spectroscopy and X-ray photoelectron spectr...The adsorption and decomposition of trimethylgallium （Ga（CH3）3, TMG） on Pd（111） and the effect of pre-covered H and O were studied by temperature programmed desorption spectroscopy and X-ray photoelectron spectroscopy. TMG adsorbs dissociatively at 140 K and the surface is covered by a mixture of Ga（CH3）x （x=1, 2 or 3） and CHx（a） （x=1, 2 or 3） species. During the heating process, the decomposition of Ga（CH3）3 on clean Pd（111） follows a progressive Ga-C bond cleavage process with CH4 and H2 as the desorption products. The desorption of Ga-containing molecules （probably GaCH3） is also identi ed in the temperature range of 275-325 K. At higher annealing temperature, carbon deposits and metallic Ga are left on the surface and start to di use into the bulk of the substrate. The presence of precovered H（a） and O（a） has a signi cant effect on the adsorption and decomposition behavior of TMG. When the surface is pre-covered by saturated H2, CH4, and H2 desorptions are mainly observed at 315 K, which is ascribed to the dissociation of GaCH3 intermediate. In the case of O-precovered surface, the dissociation mostly occurs at 258 K, of which a Pd-O-Ga（CH3）2 structure is assumed to be the precusor. The presented results may provide some insights into the mechanism of surface reaction during the lm deposition by using trimethylgallium as precursor.展开更多
基金V. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China （No.20773116）, the National Instrumentation Program （No.2011YQ03012416）, and 973 Program from the Ministry of Science and Technology of China （No.2010CB923302）.
文摘Kinetics of dissociative O2 adsorption, OHad desorption, and oxygen reduction reaction （ORR） at Pt（111） electrode in 0.1 mol/L HClO4 has been investigated. Reversible OHad adsorption/desorption occurs at potentials from 0.6 V to 1.0 V （vs. RHE） with the exchange current density of ca. 50 mA/cm^2 at 0.8 V, the fast kinetics of OHad desorption indicates that it should not be the rate determining step for ORR. In the kineticor kinetic-mass transport mix controlled potential region, ORR current at constant potential displays slight decrease with reaction time. ORR current in the positive-going potential scan is slightly larger than that in the subsequent negative-going scan with electrode rotation speed （〉800 r/min） and slow potential scan rate （〈100 mV/s）. The open circuit potential of Pt/0.1 mol/L HClO4 interface increases promptly from 0.9 V to 1.0 V after switch from O2 free- to O2-saturated solution. The increase of open circuit potential as well as ORR current decays under potential control due to the accumulation of OHad from dissociative adsorption of O2. It indicates that at Pt（111） the net rate for O2 decomposition to OHad is slightly faster than that for OHad removal, one cannot simply use the assumption of rate determining step to discuss ORR kinetics. Instead, the ORR kinetics is determined by both the kinetics for O2 decomposition to OHad as well as the thermo-equilibrium of OHad＋H^＋＋e→←H2O.
文摘Mo（CO）6 adsorption on the clean, oxygen-precovered and deeply oxidized Si（lll） surfaces was comparatively investigated by high-resolution electron energy loss spectroscopy. The downward vibrational frequency shift of the C-O stretching mode in adsorbed Mo（CO）6 illustrates that different interactions of adsorbed Mo（CO）6 occur on clean Si（111） and SiO2/Si（111） surfaces, weak on the former and strong on the latter. The strong interaction on SiO2/Si（111） might lead to the partial dissociation of Mo（CO）6, consequently the formation of molybdenum subcarbonyls. Therefore, employing Mo（CO）6 as the precursor, metallic molybdenum could be successfully deposited on the SiO2/Si（111） surface but not on the clean Si（111） surface. A portion of the deposited metallic molybdenum is transformed into the MoOa on the SiO2/Si（111） surface upon heating, and the evolved MoO3 finally desorbs from the substrate upon annealing at elevated temperatures.
基金V. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China （No.20603032, No.20733004, No.21121003, No.91021004, No.20933006）, the National Key Basic Research Program （No.2011CB921400）, the Foundation of National Excellent Doctoral Dissertation of China （No.200736）, the Fundamental Research Funds for the Central Universities （No.WK2340000006 and No.WK2060140005）, and the Shanghai Supercompurer Center, the USTC-HP HPC Project, and the SCCAS.
文摘In order to determine the structures of Si（111）-√7 √3-In surfaces and to understand their electronic properties, we construct six models of both hexagonal and rectangular types and perform first-principles calculations. Their scanning tunneling microscopic images and work functions are simulated and compared with experimental results. In this way, the hex-H3＇ and rect-T1 models are identified as the experimental configurations for the hexagonal and rectangular types, respectively. The structural evolution mechanism of the In/Si（lll） surface with indium coverage around 1.0 monolayer is discussed. The 4×1 and -√7× √3 phases are suggested to have two different types of evolution mechanisms, consistent with experimental results.
基金This work was supported by one Hundred Talents＇ Program of the Chinese Academy of Science, the National Natural Science Foundation of China （No.20773116, No.21273215, and No.J1030412）, and 973 Program from theMinistry of Science and Technology of China （No.2010CB923302）. Many Thanks to Prof. Shen Ye from Hokkaido university for the help in establishing techniques for single crystalline electro- chemistry.
文摘Fast scan voltammetry is an efficient tool to distinguish oxidative/reductive adsorp- tion/desorption from that for bulk reaction. In this work, we provide a methodology that the isotherm of oxidative/reductive adsorption desorption processes at electrode surface can be obtained using just one solution with relatively low reactant concentration, by taking the advantage of varying the potential scan rate （relative of the diffusion rate） to tune the adsorption rate and proper mathematic treatment. The methodology is demonstrated by taking acetate adsorption at Pt（lll） in acidic solution as an example. The possibility for extension of this method toward mechanistic studies of complicated electrocatalytic reactions is also given.
基金Project supported by the National Natural Science Foundation of China （Grant Nos 90406022 and 10674159）.
文摘We report the formation and local electronic structure of Ge clusters on the Si（111）-7×7 surface studied by using variable temperature scanning tunnelling microscopy （VT-STM） and low-temperature scanning tunnelling spectroscopy （STS）. Atom-resolved STM images reveal that the Ce atoms are prone to forming clusters with 1.0 nm in diameter for coverage up to 0.12 ML. Such Ce clusters preferentially nucleate at the centre of the faulted-half unit cells, leading to the ＇dark sites＇ of Si centre adatoms from the surrounding three unfaulted-half unit cells in filled-state images. Biasdependent STM images show the charge transfer from the neighbouring Si adatoms to Ce clusters. Low-temperature STS of the Ce clusters reveals that there is a band gap on the Ce cluster and the large voltage threshold is about 0.9 V.
文摘The adsorption and decomposition of trimethylgallium （Ga（CH3）3, TMG） on Pd（111） and the effect of pre-covered H and O were studied by temperature programmed desorption spectroscopy and X-ray photoelectron spectroscopy. TMG adsorbs dissociatively at 140 K and the surface is covered by a mixture of Ga（CH3）x （x=1, 2 or 3） and CHx（a） （x=1, 2 or 3） species. During the heating process, the decomposition of Ga（CH3）3 on clean Pd（111） follows a progressive Ga-C bond cleavage process with CH4 and H2 as the desorption products. The desorption of Ga-containing molecules （probably GaCH3） is also identi ed in the temperature range of 275-325 K. At higher annealing temperature, carbon deposits and metallic Ga are left on the surface and start to di use into the bulk of the substrate. The presence of precovered H（a） and O（a） has a signi cant effect on the adsorption and decomposition behavior of TMG. When the surface is pre-covered by saturated H2, CH4, and H2 desorptions are mainly observed at 315 K, which is ascribed to the dissociation of GaCH3 intermediate. In the case of O-precovered surface, the dissociation mostly occurs at 258 K, of which a Pd-O-Ga（CH3）2 structure is assumed to be the precusor. The presented results may provide some insights into the mechanism of surface reaction during the lm deposition by using trimethylgallium as precursor.
