Login In
2018 Volume 34 Issue 12
2018, 34(12): 1299-1301
doi: 10.3866/PKU.WHXB201804192
Abstract:
2018, 34(12): 1302-1303
doi: 10.3866/PKU.WHXB201801192
Abstract:
2018, 34(12): 1304-1305
doi: 10.3866/PKU.WHXB201712061
Abstract:
2018, 34(12): 1306-1307
doi: 10.3866/PKU.WHXB201803021
Abstract:
2018, 34(12): 1308-1309
doi: 10.3866/PKU.WHXB201712082
Abstract:
2018, 34(12): 1310-1311
doi: 10.3866/PKU.WHXB201712142
Abstract:
2018, 34(12): 1312-1320
doi: 10.3866/PKU.WHXB201803011
Abstract:
The controlled coupling of spin centers is essential in the construction of molecular spin-based quantum information processing architectures. A major challenge is to induce the requisite coupling between two adjacent spins, while protecting them from neighboring spins and other environmental interactions. Owing to their native spin properties, endohedral fullerenes are attractive for use as elements in quantum information processing architectures. N@C60 is an endohedral fullerene molecule with a highly reactive nitrogen atom at the center of the carbon cage. The endohedral nitrogen is atomic and not covalently bound to the cage atoms; therefore, the nitrogen atom is chemically inert toward the outer environment. Owing to its remarkably long electron-spin lifetimes and sharp resonances, N@C60 has exceptional properties for quantum computing. The thermal stability and molecular structure of N@C60 make it a useful embodiment of a quantum bit — a fundamental element for a quantum computer. Several future quantum computer architectures based on N@C60 have been proposed, one of which is a two-dimensional, quantum-bit array on specific substrates. However, a challenging yet important task is to understand the effect of various substrates on the spin properties of the endohedral fullerene, since the interaction between the endohedral fullerene and the substrate may largely affect the spin characters of the endohedral N atom. The fabrication of an endohedral fullerene molecular array on substrates is also a challenge because high-temperature methods such as evaporation will cause decomposition of N@C60. Here we report our investigation on the electron spin resonance (ESR) of N@C60 molecules on various substrates such as Au(111), Si(111), and SiO2. In this study, N@C60 was prepared using the ion implantation method, and enrichment was performed using a multistep and recycling high-performance liquid chromatography (HPLC) system with a Cosmosil Buckyprep column. N@C60 molecular films on Au(111) substrates were prepared at room temperature. In addition, scanning tunneling microscope (STM) topography of the N@C60/C60 monolayer on Au(111) was obtained at a sample temperature of 5 K in ultra high vacuum (UHV). We found that the ESR signal of the N@C60 molecules decreases rapidly and disappeared approximately 360 min after the deposition of N@C60 on Au(111). In comparison, the ESR signal was maintained for a longer time on the Si(111) and SiO2 substrates. We propose that coupling between the Au(111) substrate and the endohedral N atoms quenches the ESR signal of the endohedral N atom, while the Si(111) and SiO2 substrates have a smaller effect on the ESR signal. This result offers useful information for the design of basic quantum computer architectures.
The controlled coupling of spin centers is essential in the construction of molecular spin-based quantum information processing architectures. A major challenge is to induce the requisite coupling between two adjacent spins, while protecting them from neighboring spins and other environmental interactions. Owing to their native spin properties, endohedral fullerenes are attractive for use as elements in quantum information processing architectures. N@C60 is an endohedral fullerene molecule with a highly reactive nitrogen atom at the center of the carbon cage. The endohedral nitrogen is atomic and not covalently bound to the cage atoms; therefore, the nitrogen atom is chemically inert toward the outer environment. Owing to its remarkably long electron-spin lifetimes and sharp resonances, N@C60 has exceptional properties for quantum computing. The thermal stability and molecular structure of N@C60 make it a useful embodiment of a quantum bit — a fundamental element for a quantum computer. Several future quantum computer architectures based on N@C60 have been proposed, one of which is a two-dimensional, quantum-bit array on specific substrates. However, a challenging yet important task is to understand the effect of various substrates on the spin properties of the endohedral fullerene, since the interaction between the endohedral fullerene and the substrate may largely affect the spin characters of the endohedral N atom. The fabrication of an endohedral fullerene molecular array on substrates is also a challenge because high-temperature methods such as evaporation will cause decomposition of N@C60. Here we report our investigation on the electron spin resonance (ESR) of N@C60 molecules on various substrates such as Au(111), Si(111), and SiO2. In this study, N@C60 was prepared using the ion implantation method, and enrichment was performed using a multistep and recycling high-performance liquid chromatography (HPLC) system with a Cosmosil Buckyprep column. N@C60 molecular films on Au(111) substrates were prepared at room temperature. In addition, scanning tunneling microscope (STM) topography of the N@C60/C60 monolayer on Au(111) was obtained at a sample temperature of 5 K in ultra high vacuum (UHV). We found that the ESR signal of the N@C60 molecules decreases rapidly and disappeared approximately 360 min after the deposition of N@C60 on Au(111). In comparison, the ESR signal was maintained for a longer time on the Si(111) and SiO2 substrates. We propose that coupling between the Au(111) substrate and the endohedral N atoms quenches the ESR signal of the endohedral N atom, while the Si(111) and SiO2 substrates have a smaller effect on the ESR signal. This result offers useful information for the design of basic quantum computer architectures.
2018, 34(12): 1321-1333
doi: 10.3866/PKU.WHXB201802081
Abstract:
Nucleobases (guanine (G), adenine (A), thymine (T), cytosine (C), and uracil (U)) are important constituents of nucleic acids, which carry genetic information in all living organisms, and play vital roles in structure formation, functionalization, and biological catalytic processes. The principle of complementary base pairing is significant in the high-fidelity replication of DNA and RNA. In addition to their specific recognition, the interaction between bases and other reactants, such as metals, salts, and certain small molecules, may cause distinct effects. Specifically, the interactions between bases and certain metal atoms or ions could damage nucleic acids, inducing gene mutation and even carcinogenesis. In the meantime, nanoscale devices based on metal-DNA interactions have become the focus of research in nanotechnology. Therefore, extensive researches on the interactions between metals and bases and the corresponding mechanism are of great importance and may make improvements in the fields of both biochemistry and nanotechnology. Scanning tunneling microscopy (STM) is a powerful tool for effectively resolving nanostructures in real space and on the atomic scale under ultrahigh vacuum (UHV) conditions. Moreover, density functional theory (DFT) calculations could help elucidate the reaction pathways and their mechanisms. In this review, we summarize the distinct interactions between bases (including their derivatives) and various metal species (comprising alkali, alkaline earth, and transition metals) derived from metal sources and the corresponding salts on the Au(111) substrate reported recently based on the results obtained by a combination of above two methods. In general, bases afford N and/or O binding sites to interact with metal atoms, resulting in various motifs via coordination or electrostatic interactions, and form intermolecular hydrogen bonds to stabilize the whole system. On the basis of high-resolution STM images and DFT calculations, structural models and the possible reaction pathways are proposed, and their underlying mechanisms, which reveal the nature of the interactions, are thus obtained. Among them, we summarize the construction of G-quartet structures with different kinds of central metals like Na, K, and Ca, which are directly introduced by salts, and their relative stabilities are compared. In addition, salts can provide not only metal cations but also halogen anions in modulating the structure formation with bases. The halogen species enable the regulation of metal-organic motifs and induce phase transition by locating at specific hydrogen-rich sites. Moreover, reversible structural transformations of metal-organic nanostructures are realized owing to the intrinsic dynamic characteristic of coordination bonds, together with the coordination priority and diversity. Furthermore, the controllable scission and seamless stitching of metal-organic clusters, which contain two types of hierarchical interactions, have been successfully achieved through STM manipulations. Finally, this review offers a thorough comprehension on the interaction between bases and metals on Au(111) and provide fundamental insights into controllable fabrication of nanostructures of DNA bases. We also admit the limitation involved in detecting biological processes by on-surface model system, and speculate on future studies that would use more complicated biomolecules together with other technologies.
Nucleobases (guanine (G), adenine (A), thymine (T), cytosine (C), and uracil (U)) are important constituents of nucleic acids, which carry genetic information in all living organisms, and play vital roles in structure formation, functionalization, and biological catalytic processes. The principle of complementary base pairing is significant in the high-fidelity replication of DNA and RNA. In addition to their specific recognition, the interaction between bases and other reactants, such as metals, salts, and certain small molecules, may cause distinct effects. Specifically, the interactions between bases and certain metal atoms or ions could damage nucleic acids, inducing gene mutation and even carcinogenesis. In the meantime, nanoscale devices based on metal-DNA interactions have become the focus of research in nanotechnology. Therefore, extensive researches on the interactions between metals and bases and the corresponding mechanism are of great importance and may make improvements in the fields of both biochemistry and nanotechnology. Scanning tunneling microscopy (STM) is a powerful tool for effectively resolving nanostructures in real space and on the atomic scale under ultrahigh vacuum (UHV) conditions. Moreover, density functional theory (DFT) calculations could help elucidate the reaction pathways and their mechanisms. In this review, we summarize the distinct interactions between bases (including their derivatives) and various metal species (comprising alkali, alkaline earth, and transition metals) derived from metal sources and the corresponding salts on the Au(111) substrate reported recently based on the results obtained by a combination of above two methods. In general, bases afford N and/or O binding sites to interact with metal atoms, resulting in various motifs via coordination or electrostatic interactions, and form intermolecular hydrogen bonds to stabilize the whole system. On the basis of high-resolution STM images and DFT calculations, structural models and the possible reaction pathways are proposed, and their underlying mechanisms, which reveal the nature of the interactions, are thus obtained. Among them, we summarize the construction of G-quartet structures with different kinds of central metals like Na, K, and Ca, which are directly introduced by salts, and their relative stabilities are compared. In addition, salts can provide not only metal cations but also halogen anions in modulating the structure formation with bases. The halogen species enable the regulation of metal-organic motifs and induce phase transition by locating at specific hydrogen-rich sites. Moreover, reversible structural transformations of metal-organic nanostructures are realized owing to the intrinsic dynamic characteristic of coordination bonds, together with the coordination priority and diversity. Furthermore, the controllable scission and seamless stitching of metal-organic clusters, which contain two types of hierarchical interactions, have been successfully achieved through STM manipulations. Finally, this review offers a thorough comprehension on the interaction between bases and metals on Au(111) and provide fundamental insights into controllable fabrication of nanostructures of DNA bases. We also admit the limitation involved in detecting biological processes by on-surface model system, and speculate on future studies that would use more complicated biomolecules together with other technologies.
2018, 34(12): 1334-1357
doi: 10.3866/PKU.WHXB201804201
Abstract:
Heterogeneous catalysts are usually synthesized by the conventional wet-chemistry methods, including wet-impregnation, ion exchange, and deposition-precipitation. With the development of catalyst synthesis, great progress has been made in many industrially important catalytic processes. However, these catalytic materials often have very complex structures along with poor uniformity of active sites. Such heterogeneity of active site structures significantly decreases catalytic performance, especially in terms of selectivity, and hinders atomic-level understanding of structure-activity relationships. Moreover, loss of exposed active components by sintering or leaching under harsh reaction conditions causes considerable catalyst deactivation. It is desirable to develop a facile method to tune catalyst active site structures, as well as their local chemical environments on the atomic level, thereby facilitating reaction mechanisms understanding and rational design of catalysts with high stability. Atomic layer deposition (ALD), a gas-phase technique for thin film growth, has emerged as an alternative method to synthesize heterogeneous catalysts. Like chemical vapor deposition (CVD), ALD relies on a sequence of molecular-level, self-limiting surface reactions between the vapors of precursor molecules and a substrate. This unique character makes it possible to deposit various catalytic materials uniformly on a high-surface-area support with nearly atomic precision. By tuning the number, sequence, and types of ALD cycles, bottom-up precise construction of catalytic architectures on a support can be achieved. In this review, we focus on the design and synthesis of supported metal catalysts using ALD. We first review strategies developed to precisely tailor the size, composition, and structures of metal nanoparticles (NPs) using ALD. Catalytic performances of these ALD metal catalysts are also discussed and compared to conventional catalysts. We highlight synthetic strategies for synthesis of metal single-atom catalysts and bottom-up precise synthesis of dimeric metal catalysts. Their impact on catalysis is discussed. We demonstrate that metal oxide ALD on metal NPs can enhance catalytic activity, selectivity, and especially stability. In particular, we show that site-selective blocking of metal NPs with an oxide overcoat improves selectivity and contributes to an understanding of the distinct functionalities of the low-and high-coordination sites in catalytic reactions on the atomic level. Next, we discuss an effective method to construct bifunctional catalysts via precisely-controlled addition of a secondary functionality using ALD. Finally, we summarize the advantages of ALD for the advanced design and synthesis of catalysts and discuss the challenges and opportunities of scaling up ALD catalyst synthesis for practical applications.
Heterogeneous catalysts are usually synthesized by the conventional wet-chemistry methods, including wet-impregnation, ion exchange, and deposition-precipitation. With the development of catalyst synthesis, great progress has been made in many industrially important catalytic processes. However, these catalytic materials often have very complex structures along with poor uniformity of active sites. Such heterogeneity of active site structures significantly decreases catalytic performance, especially in terms of selectivity, and hinders atomic-level understanding of structure-activity relationships. Moreover, loss of exposed active components by sintering or leaching under harsh reaction conditions causes considerable catalyst deactivation. It is desirable to develop a facile method to tune catalyst active site structures, as well as their local chemical environments on the atomic level, thereby facilitating reaction mechanisms understanding and rational design of catalysts with high stability. Atomic layer deposition (ALD), a gas-phase technique for thin film growth, has emerged as an alternative method to synthesize heterogeneous catalysts. Like chemical vapor deposition (CVD), ALD relies on a sequence of molecular-level, self-limiting surface reactions between the vapors of precursor molecules and a substrate. This unique character makes it possible to deposit various catalytic materials uniformly on a high-surface-area support with nearly atomic precision. By tuning the number, sequence, and types of ALD cycles, bottom-up precise construction of catalytic architectures on a support can be achieved. In this review, we focus on the design and synthesis of supported metal catalysts using ALD. We first review strategies developed to precisely tailor the size, composition, and structures of metal nanoparticles (NPs) using ALD. Catalytic performances of these ALD metal catalysts are also discussed and compared to conventional catalysts. We highlight synthetic strategies for synthesis of metal single-atom catalysts and bottom-up precise synthesis of dimeric metal catalysts. Their impact on catalysis is discussed. We demonstrate that metal oxide ALD on metal NPs can enhance catalytic activity, selectivity, and especially stability. In particular, we show that site-selective blocking of metal NPs with an oxide overcoat improves selectivity and contributes to an understanding of the distinct functionalities of the low-and high-coordination sites in catalytic reactions on the atomic level. Next, we discuss an effective method to construct bifunctional catalysts via precisely-controlled addition of a secondary functionality using ALD. Finally, we summarize the advantages of ALD for the advanced design and synthesis of catalysts and discuss the challenges and opportunities of scaling up ALD catalyst synthesis for practical applications.
2018, 34(12): 1358-1365
doi: 10.3866/PKU.WHXB201803071
Abstract:
Ni-based catalysts have been widely used in many important industrial heterogeneous processes such as hydrogenation and steam reforming owing to their sufficiently high activity yet significantly lower cost than that of alternative precious-metal-based catalysts. However, nickel catalysts are susceptible to deactivation. Understanding the adsorption and activation behavior of small molecules on the model catalyst surface is important to optimize the catalytic performance. Although many studies have been carried out in recent years, the initial oxidation process of nickel surface is still not fully understood, and the influence of the adsorption sequence of CO and O2 and their co-adsorption is controversial. In this study, the surface oxygen species on Ni(111) and the co-adsorption of CO and O2 were explored using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). HREELS can provide useful information about the surface structure, surface-adsorbed species, adsorption sites, and interactions between surface oxygen species and CO on the surface. The results showed that there were two kinds of oxygen species after the oxidation of Ni(111), and the energy loss peaks at 54–58 meV were ascribed to surface chemisorbed oxygen species, and the peak at 69 meV to surface nickel oxide. The chemisorbed oxygen at low coverage displayed a LEED pattern of (2×2), revealing the formation of an ordered surface structure. As the amount of oxygen increased, the energy loss peak at 54 meV shifted to 58 meV. At an O2 partial pressure of 1×10-8 Torr (1 Torr = 133.32 Pa), the AES ratio of O/Ni remained almost unchanged after dosing 48 L, which indicated that the surface nickel oxide was relatively stable. The surface chemisorbed oxygen species was less stable, which could change to surface nickel oxide after annealing in vacuum. CO adsorbed on Ni(111) at room temperature with tri-hollow and a-top sites. Upon annealing in vacuum, a-top CO weakened first and then disappeared completely at 520 K, whereas tri-hollow CO was much more stable. The pre-adsorption of CO could suppress O2 adsorption and oxidation of the Ni(111) surface. The presence of oxygen could then gradually remove and replace CO with O2. The surface oxygen species preferred the tri-hollow sites, resulting in more a-top adsorbed CO during the co-adsorption of CO and oxygen. The surface chemisorbed oxygen species were more active and could react with CO at room temperature; however, the surface nickel oxide was less active, and could only be reduced at a higher temperature and higher partial pressure of CO.
Ni-based catalysts have been widely used in many important industrial heterogeneous processes such as hydrogenation and steam reforming owing to their sufficiently high activity yet significantly lower cost than that of alternative precious-metal-based catalysts. However, nickel catalysts are susceptible to deactivation. Understanding the adsorption and activation behavior of small molecules on the model catalyst surface is important to optimize the catalytic performance. Although many studies have been carried out in recent years, the initial oxidation process of nickel surface is still not fully understood, and the influence of the adsorption sequence of CO and O2 and their co-adsorption is controversial. In this study, the surface oxygen species on Ni(111) and the co-adsorption of CO and O2 were explored using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). HREELS can provide useful information about the surface structure, surface-adsorbed species, adsorption sites, and interactions between surface oxygen species and CO on the surface. The results showed that there were two kinds of oxygen species after the oxidation of Ni(111), and the energy loss peaks at 54–58 meV were ascribed to surface chemisorbed oxygen species, and the peak at 69 meV to surface nickel oxide. The chemisorbed oxygen at low coverage displayed a LEED pattern of (2×2), revealing the formation of an ordered surface structure. As the amount of oxygen increased, the energy loss peak at 54 meV shifted to 58 meV. At an O2 partial pressure of 1×10-8 Torr (1 Torr = 133.32 Pa), the AES ratio of O/Ni remained almost unchanged after dosing 48 L, which indicated that the surface nickel oxide was relatively stable. The surface chemisorbed oxygen species was less stable, which could change to surface nickel oxide after annealing in vacuum. CO adsorbed on Ni(111) at room temperature with tri-hollow and a-top sites. Upon annealing in vacuum, a-top CO weakened first and then disappeared completely at 520 K, whereas tri-hollow CO was much more stable. The pre-adsorption of CO could suppress O2 adsorption and oxidation of the Ni(111) surface. The presence of oxygen could then gradually remove and replace CO with O2. The surface oxygen species preferred the tri-hollow sites, resulting in more a-top adsorbed CO during the co-adsorption of CO and oxygen. The surface chemisorbed oxygen species were more active and could react with CO at room temperature; however, the surface nickel oxide was less active, and could only be reduced at a higher temperature and higher partial pressure of CO.
2018, 34(12): 1366-1372
doi: 10.3866/PKU.WHXB201804161
Abstract:
Cu/ZnO/Al2O3 is one of the most widely used catalysts in industrial methanol synthesis. However, the reaction mechanism and the nature of the active sites on the catalyst for this reaction are still under debate. Thus, detailed information is needed to understand the catalytic processes occurring on the surface of this catalyst. H2 is one of the reaction gases in methanol synthesis. Studies of the activation and dissociation behaviors of H2 on ZnO surfaces are of great importance in understanding the catalytic mechanism of methanol synthesis. In this work, the activation and dissociation processes of H2 on a ZnO(1010) single crystal surface were investigated in-situ using ambient-pressure X-ray photoelectron spectroscopy (APXPS) and scanning tunneling microscopy (STM), two powerful surface characterization techniques. In the APXPS experiments, results indicated the formation of hydroxyl (OH) species on the ZnO single crystal surface at room temperature in 0.3 mbar (1 mbar = 100 Pa) H2 atmosphere. Meanwhile, STM measurements showed that the ZnO surface was reconstructed from a (1×1) to a (2×1) structure upon introduction of H2. These observations revealed adsorption behaviors of H2 the same as those of atomic H on a ZnO(1010) surface as seen in previous studies, which could be evidence of the dissociative adsorption of H2 on a ZnO surface. However, H2O adsorption on ZnO surfaces can also result in the formation of OH species, which can be observed using XPS. The STM results show that the exposure of H2O also leads to the reconstruction from a (1×1) to a (2×1) structure on the ZnO(1010) surface upon H2 introduction. Hence, it is necessary to exclude the influence of H2O in this work, because there may be trace amounts of H2O in the H2 gas. Therefore, we performed a comparative study of H2 and H2O on ZnO(1010) single crystal surface. A downward band bending of 0.3 eV was observed on the ZnO surface in 0.3 mbar H2 atmosphere using APXPS, while negligible band bending was shown in the case of the H2O atmosphere. Moreover, thermal stability studies revealed that the OH group formed in the H2 atmosphere desorbed at a higher temperature than the one resulting from H2O adsorption, meaning that the two OH groups formed on the ZnO surface were different. Results in this work provide evidence of the dissociative adsorption of H2 on the ZnO(1010) surface at room temperature and atmospheric pressure. This is in contrast to previous findings, in which no H2 dissociation on a ZnO(1010) surface under ultra-high vacuum conditions was observed, indicating that the activation of H2 on ZnO surfaces is a pressure dependent process.
Cu/ZnO/Al2O3 is one of the most widely used catalysts in industrial methanol synthesis. However, the reaction mechanism and the nature of the active sites on the catalyst for this reaction are still under debate. Thus, detailed information is needed to understand the catalytic processes occurring on the surface of this catalyst. H2 is one of the reaction gases in methanol synthesis. Studies of the activation and dissociation behaviors of H2 on ZnO surfaces are of great importance in understanding the catalytic mechanism of methanol synthesis. In this work, the activation and dissociation processes of H2 on a ZnO(1010) single crystal surface were investigated in-situ using ambient-pressure X-ray photoelectron spectroscopy (APXPS) and scanning tunneling microscopy (STM), two powerful surface characterization techniques. In the APXPS experiments, results indicated the formation of hydroxyl (OH) species on the ZnO single crystal surface at room temperature in 0.3 mbar (1 mbar = 100 Pa) H2 atmosphere. Meanwhile, STM measurements showed that the ZnO surface was reconstructed from a (1×1) to a (2×1) structure upon introduction of H2. These observations revealed adsorption behaviors of H2 the same as those of atomic H on a ZnO(1010) surface as seen in previous studies, which could be evidence of the dissociative adsorption of H2 on a ZnO surface. However, H2O adsorption on ZnO surfaces can also result in the formation of OH species, which can be observed using XPS. The STM results show that the exposure of H2O also leads to the reconstruction from a (1×1) to a (2×1) structure on the ZnO(1010) surface upon H2 introduction. Hence, it is necessary to exclude the influence of H2O in this work, because there may be trace amounts of H2O in the H2 gas. Therefore, we performed a comparative study of H2 and H2O on ZnO(1010) single crystal surface. A downward band bending of 0.3 eV was observed on the ZnO surface in 0.3 mbar H2 atmosphere using APXPS, while negligible band bending was shown in the case of the H2O atmosphere. Moreover, thermal stability studies revealed that the OH group formed in the H2 atmosphere desorbed at a higher temperature than the one resulting from H2O adsorption, meaning that the two OH groups formed on the ZnO surface were different. Results in this work provide evidence of the dissociative adsorption of H2 on the ZnO(1010) surface at room temperature and atmospheric pressure. This is in contrast to previous findings, in which no H2 dissociation on a ZnO(1010) surface under ultra-high vacuum conditions was observed, indicating that the activation of H2 on ZnO surfaces is a pressure dependent process.
2018, 34(12): 1373-1380
doi: 10.3866/PKU.WHXB201804131
Abstract:
The growth and structural properties of ZnO thin films on both Au(111) and Cu(111) surfaces were studied using either NO2 or O2 as oxidizing agent. The results indicate that NO2 promotes the formation of well-ordered ZnO thin films on both Au(111) and Cu(111). The stoichiometric ZnO thin films obtained on these two surfaces exhibit a flattened and non-polar ZnO(0001) structure. It is shown that on Au(111), the growth of bilayer ZnO nanostructures (NSs) is favored during the deposition of Zn in presence of NO2 at 300 K, whereas both monolayer and bilayer ZnO NSs could be observed when Zn is deposited at elevated temperatures under a NO2 atmosphere. The growth of bilayer ZnO NSs is caused by the stronger interaction between two ZnO layers than between ZnO and Au(111) surface. In contrast, the growth of monolayer ZnO NSs involves a kinetically controlled process. ZnO thin films covering the Au(111) surface exhibits a multilayer thickness, which is consistent with the growth kinetics of ZnO NSs. Besides, the use of O2 as oxidizing agent could lead to the formation of sub-stoichiometric ZnOx structures. The growth of full layers of ZnO on Cu(111) has been a difficult task, mainly because of the interdiffusion of Zn promoted by the strong interaction between Cu and Zn and the formation of Cu surface oxides by the oxidation of Cu(111). We overcome this problem by using NO2 as oxidizing agent to form well-ordered ZnO thin films covering the Cu(111) surface. The surface of the well-ordered ZnO thin films on Cu(111) displays mainly a moiré pattern, which suggests a (3 × 3) ZnO superlattice supported on a (4 × 4) supercell of Cu(111). The observation of this superstructure provides a direct experimental evidence for the recently proposed structural model of ZnO on Cu(111), which suggests that this superstructure exhibits the minimal strain. Our studies suggested that the surface structures of ZnO thin films could change depending on the oxidation level or the oxidant used. The oxidation of Cu(111) could also become a key factor for the growth of ZnO. When Cu(111) is pre-oxidized to form copper surface oxides, the growth mode of ZnOx is altered and single-site Zn could be confined into the lattice of copper surface oxides. Our studies show that the growth of ZnO is promoted by inhibiting the diffusion of Zn into metal substrates and preventing the formation of sub-stoichiometric ZnOx. In short, the use of an atomic oxygen source is advantageous to the growth of ZnO thin films on Au(111) and Cu(111) surfaces.
The growth and structural properties of ZnO thin films on both Au(111) and Cu(111) surfaces were studied using either NO2 or O2 as oxidizing agent. The results indicate that NO2 promotes the formation of well-ordered ZnO thin films on both Au(111) and Cu(111). The stoichiometric ZnO thin films obtained on these two surfaces exhibit a flattened and non-polar ZnO(0001) structure. It is shown that on Au(111), the growth of bilayer ZnO nanostructures (NSs) is favored during the deposition of Zn in presence of NO2 at 300 K, whereas both monolayer and bilayer ZnO NSs could be observed when Zn is deposited at elevated temperatures under a NO2 atmosphere. The growth of bilayer ZnO NSs is caused by the stronger interaction between two ZnO layers than between ZnO and Au(111) surface. In contrast, the growth of monolayer ZnO NSs involves a kinetically controlled process. ZnO thin films covering the Au(111) surface exhibits a multilayer thickness, which is consistent with the growth kinetics of ZnO NSs. Besides, the use of O2 as oxidizing agent could lead to the formation of sub-stoichiometric ZnOx structures. The growth of full layers of ZnO on Cu(111) has been a difficult task, mainly because of the interdiffusion of Zn promoted by the strong interaction between Cu and Zn and the formation of Cu surface oxides by the oxidation of Cu(111). We overcome this problem by using NO2 as oxidizing agent to form well-ordered ZnO thin films covering the Cu(111) surface. The surface of the well-ordered ZnO thin films on Cu(111) displays mainly a moiré pattern, which suggests a (3 × 3) ZnO superlattice supported on a (4 × 4) supercell of Cu(111). The observation of this superstructure provides a direct experimental evidence for the recently proposed structural model of ZnO on Cu(111), which suggests that this superstructure exhibits the minimal strain. Our studies suggested that the surface structures of ZnO thin films could change depending on the oxidation level or the oxidant used. The oxidation of Cu(111) could also become a key factor for the growth of ZnO. When Cu(111) is pre-oxidized to form copper surface oxides, the growth mode of ZnOx is altered and single-site Zn could be confined into the lattice of copper surface oxides. Our studies show that the growth of ZnO is promoted by inhibiting the diffusion of Zn into metal substrates and preventing the formation of sub-stoichiometric ZnOx. In short, the use of an atomic oxygen source is advantageous to the growth of ZnO thin films on Au(111) and Cu(111) surfaces.
2018, 34(12): 1381-1389
doi: 10.3866/PKU.WHXB201804092
Abstract:
CeO2-based catalysts are promising for use in various important chemical reactions involving CO2, such as the dry reforming of methane to produce synthesis gas and methanol. CeO2 has a superior ability to store and release oxygen, which can improve the catalytic performance by suppressing the formation of coke. Although the adsorption and activation behavior of CO2 on the CeO2 surface has been extensively investigated in recent years, the intermediate species formed from CO2 on ceria has not been clearly identified. The reactivity of the ceria surface to CO2 has been reported to be tuned by introducing CaO, which increases the number of basic sites for the ceria-based catalysts. However, the mechanism by which Ca2+ ions affect CO2 decomposition is still debated. In this study, the morphologies and electronic properties of stoichiometric CeO2(111), partially reduced CeO2-x(111) (0 < x < 0.5), and calcium-doped ceria model catalysts, as well as their interactions with CO2, were investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and synchrotron radiation photoemission spectroscopy. Stoichiometric CeO2(111) and partially reduced CeO2-x(111) films were epitaxially grown on a Cu(111) surface. STM images show that the stoichiometric CeO2 film exhibits large, flat terraces that completely cover the Cu(111) surface. The reduced CeO2-x film also has a flat surface and an ordered structure, but dark spaces are observed on the film. Different Ca-doped ceria films were prepared by physical vapor deposition of metallic Ca on CeO2(111) at room temperature and subsequent annealing to 600 or 800 K in ultrahigh vacuum. The different preparation procedures produce samples with various surface components, oxidation states, and structures. Our results indicate that the deposition of metallic Ca on CeO2 at room temperature leads to a partial reduction of Ce from the +4 to the +3 state, accompanied by the oxidation of Ca to Ca2+. Large CaO nanofilms are observed on CeO2 upon annealing to 600 K. However, small CaO nanoislands appear near the step edges and more Ca2+ ions migrate into the subsurface of CeO2 upon annealing to 800 K. In addition, different surface-adsorbed species are identified after CO2 adsorption on ceria (CeO2 and reduced CeO2-x) and Ca-doped ceria films. CO2 adsorption on the stoichiometric CeO2 and partially reduced CeO2-x surfaces leads to the formation of surface carboxylate. Moreover, the surface carboxylate species is more easily formed on reduced CeO2-x with enhanced thermal stability than on stoichiometric CeO2. On Ca-doped ceria films, the presence of Ca2+ ions is observed to be beneficial for CO2 adsorption; further, the carbonate species is identified.
CeO2-based catalysts are promising for use in various important chemical reactions involving CO2, such as the dry reforming of methane to produce synthesis gas and methanol. CeO2 has a superior ability to store and release oxygen, which can improve the catalytic performance by suppressing the formation of coke. Although the adsorption and activation behavior of CO2 on the CeO2 surface has been extensively investigated in recent years, the intermediate species formed from CO2 on ceria has not been clearly identified. The reactivity of the ceria surface to CO2 has been reported to be tuned by introducing CaO, which increases the number of basic sites for the ceria-based catalysts. However, the mechanism by which Ca2+ ions affect CO2 decomposition is still debated. In this study, the morphologies and electronic properties of stoichiometric CeO2(111), partially reduced CeO2-x(111) (0 < x < 0.5), and calcium-doped ceria model catalysts, as well as their interactions with CO2, were investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and synchrotron radiation photoemission spectroscopy. Stoichiometric CeO2(111) and partially reduced CeO2-x(111) films were epitaxially grown on a Cu(111) surface. STM images show that the stoichiometric CeO2 film exhibits large, flat terraces that completely cover the Cu(111) surface. The reduced CeO2-x film also has a flat surface and an ordered structure, but dark spaces are observed on the film. Different Ca-doped ceria films were prepared by physical vapor deposition of metallic Ca on CeO2(111) at room temperature and subsequent annealing to 600 or 800 K in ultrahigh vacuum. The different preparation procedures produce samples with various surface components, oxidation states, and structures. Our results indicate that the deposition of metallic Ca on CeO2 at room temperature leads to a partial reduction of Ce from the +4 to the +3 state, accompanied by the oxidation of Ca to Ca2+. Large CaO nanofilms are observed on CeO2 upon annealing to 600 K. However, small CaO nanoislands appear near the step edges and more Ca2+ ions migrate into the subsurface of CeO2 upon annealing to 800 K. In addition, different surface-adsorbed species are identified after CO2 adsorption on ceria (CeO2 and reduced CeO2-x) and Ca-doped ceria films. CO2 adsorption on the stoichiometric CeO2 and partially reduced CeO2-x surfaces leads to the formation of surface carboxylate. Moreover, the surface carboxylate species is more easily formed on reduced CeO2-x with enhanced thermal stability than on stoichiometric CeO2. On Ca-doped ceria films, the presence of Ca2+ ions is observed to be beneficial for CO2 adsorption; further, the carbonate species is identified.
2018, 34(12): 1390-1396
doi: 10.3866/PKU.WHXB201804191
Abstract:
The controllability of metal adatoms has been attracting ever-growing attention because the metal species in particular single-atom metals can play an important role in various surface processes, including heterogeneous catalytic reactions. On the other hand, organic self-assembly films have been regarded as an efficient and versatile bottom-up method to fabricate surface nanostructures, whose functionality and periodicity can be highly designable. In this work, we have developed a novel strategy to steer the generation and distribution of metal adatoms by combining the surface self-assemblies with exposure to small inorganic gaseous molecules. More specifically, we have prepared a honeycomb structure of melem (triamino-s-heptazine) on the Au(111) surface based on a well-structured hydrogen bonding network. The achieved melem self-assembly contains periodic hexagonal pores having diameters as large as around 1 nm. More importantly, the peripheries of the nanopores are decorated with heterocyclic N atoms that can probably form strong interactions with the metal species. Upon exposing the melem self-assembly to a CO atmosphere at room temperature, a fair number of Au adatoms were produced and trapped inside the nanopores encircled by the melem molecules. Single or clustered Au vacancies were concomitantly formed that were also trapped by the melem pores and stabilized by the surrounding molecules, as confirmed by high-resolution scanning tunneling microscopy (STM) images. Both types of added species showed positive correlations with the CO exposure and saturated at around 0.01 monolayer. In addition, owing to the large pore size, as well as the presence of multiple docking sites inside the nanopores, more than one Au adatom can reside in a melem nanopore; they can be distributed in a variety of configurations for bi-Au (two Au adatoms) and tri-Au (three Au adatoms) species, whose population can be manipulated with the CO exposure. Moreover, control experiments demonstrated that these CO-induced Au species, including the adatoms and vacancies, can survive annealing treatments up to the temperature at which the melem molecules start to desorb, indicating a substantial thermal stability. The formed Au species may hold great potential for serving as active sites for surface reactions. More interestingly, the bi-Au and tri-Au species have moderate Au-Au intervals, and can be potentially active for certain structurally sensitive bimolecular reactions. Considering all these aspects, we believe that this work presents a fresh approach to utilizing organic self-assembly films and has demonstrated a rather novel strategy for preparing various single-atom metal species on substrate surfaces.
The controllability of metal adatoms has been attracting ever-growing attention because the metal species in particular single-atom metals can play an important role in various surface processes, including heterogeneous catalytic reactions. On the other hand, organic self-assembly films have been regarded as an efficient and versatile bottom-up method to fabricate surface nanostructures, whose functionality and periodicity can be highly designable. In this work, we have developed a novel strategy to steer the generation and distribution of metal adatoms by combining the surface self-assemblies with exposure to small inorganic gaseous molecules. More specifically, we have prepared a honeycomb structure of melem (triamino-s-heptazine) on the Au(111) surface based on a well-structured hydrogen bonding network. The achieved melem self-assembly contains periodic hexagonal pores having diameters as large as around 1 nm. More importantly, the peripheries of the nanopores are decorated with heterocyclic N atoms that can probably form strong interactions with the metal species. Upon exposing the melem self-assembly to a CO atmosphere at room temperature, a fair number of Au adatoms were produced and trapped inside the nanopores encircled by the melem molecules. Single or clustered Au vacancies were concomitantly formed that were also trapped by the melem pores and stabilized by the surrounding molecules, as confirmed by high-resolution scanning tunneling microscopy (STM) images. Both types of added species showed positive correlations with the CO exposure and saturated at around 0.01 monolayer. In addition, owing to the large pore size, as well as the presence of multiple docking sites inside the nanopores, more than one Au adatom can reside in a melem nanopore; they can be distributed in a variety of configurations for bi-Au (two Au adatoms) and tri-Au (three Au adatoms) species, whose population can be manipulated with the CO exposure. Moreover, control experiments demonstrated that these CO-induced Au species, including the adatoms and vacancies, can survive annealing treatments up to the temperature at which the melem molecules start to desorb, indicating a substantial thermal stability. The formed Au species may hold great potential for serving as active sites for surface reactions. More interestingly, the bi-Au and tri-Au species have moderate Au-Au intervals, and can be potentially active for certain structurally sensitive bimolecular reactions. Considering all these aspects, we believe that this work presents a fresh approach to utilizing organic self-assembly films and has demonstrated a rather novel strategy for preparing various single-atom metal species on substrate surfaces.
2018, 34(12): 1397-1404
doi: 10.3866/PKU.WHXB201804022
Abstract:
In the past decade, fossil fuel resources have been exploited and utilized extensively, which could lead to increasing environmental crises, like greenhouse effect, water pollution, etc. Accordingly, many coping strategies have been put forward, such as water electrolysis, metal-air batteries, fuel cell, etc. Among the strategies mentioned above, water electrolysis is one of the most promising. Water splitting, which can achieve sustainable hydrogen production, is a favorable strategy due to the abundance of water resources. Splitting of water includes two half reactions integral to its operation: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, its practical application is mainly impeded by the sluggish anode reaction. Simultaneously, noble metal oxides (IrO2 and RuO2) and Pt-based catalysts have been recognized as typical OER catalysts; however, the scarcity of noble metals greatly limits their development. Hence, designing an alternative electrocatalyst plays a vital role in the development of OER. However, exploring a highly active electrocatalyst for OER is still difficult. Herein, a miraculous construction of a tree-like array of NiS/Ni3S2 heterostructure, which is directly grown on Ni foam substrate, is synthesized via one-step hydrothermal process. Since NiS and Ni3S2 have shown great OER performance in previous investigations, this novel NiS-Ni3S2/Nikel foam (NF) heterostructure array has tremendous potential as a practical OER catalyst. Upon application in OER, the NiS-Ni3S2/NF heterostructure array catalyst exhibits excellent activity and stability. More specifically, this novel tree-like NiS-Ni3S2 heterostructure array shows extremely low overpotential (269 mV to achieve a current density of 10 mA·cm-2) and small Tafel slope for OER. It also shows extraordinary stability in alkaline electrolytes. Compared with the Ni3S2 nanorods array, the NiS-Ni3S2 heterostructure array has a synergistic effect that can improve the OER performance. Due to the secondary structure (Ni3S2 nanosheets), the tree-like NiS-Ni3S2 array provides more active sites could have higher specific surface area. The greater activity of the NiS/Ni3S2 heterostructure may also stem from the tight conjunction between tree-like NiS/Ni3S2 and the Ni foam substrate, which is beneficial for electronic transmission. Hydroxy groups can accumulate in large amounts on the surface of the tree-like array, and it also generates some Ni-based oxides that are favorable to OER. Moreover, the synergistic effect of such heterostructure can intrinsically improve the OER activity. The unique tree-like NiS-Ni3S2 heterostructure array has great potential as an alternative OER electrocatalyst.
In the past decade, fossil fuel resources have been exploited and utilized extensively, which could lead to increasing environmental crises, like greenhouse effect, water pollution, etc. Accordingly, many coping strategies have been put forward, such as water electrolysis, metal-air batteries, fuel cell, etc. Among the strategies mentioned above, water electrolysis is one of the most promising. Water splitting, which can achieve sustainable hydrogen production, is a favorable strategy due to the abundance of water resources. Splitting of water includes two half reactions integral to its operation: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, its practical application is mainly impeded by the sluggish anode reaction. Simultaneously, noble metal oxides (IrO2 and RuO2) and Pt-based catalysts have been recognized as typical OER catalysts; however, the scarcity of noble metals greatly limits their development. Hence, designing an alternative electrocatalyst plays a vital role in the development of OER. However, exploring a highly active electrocatalyst for OER is still difficult. Herein, a miraculous construction of a tree-like array of NiS/Ni3S2 heterostructure, which is directly grown on Ni foam substrate, is synthesized via one-step hydrothermal process. Since NiS and Ni3S2 have shown great OER performance in previous investigations, this novel NiS-Ni3S2/Nikel foam (NF) heterostructure array has tremendous potential as a practical OER catalyst. Upon application in OER, the NiS-Ni3S2/NF heterostructure array catalyst exhibits excellent activity and stability. More specifically, this novel tree-like NiS-Ni3S2 heterostructure array shows extremely low overpotential (269 mV to achieve a current density of 10 mA·cm-2) and small Tafel slope for OER. It also shows extraordinary stability in alkaline electrolytes. Compared with the Ni3S2 nanorods array, the NiS-Ni3S2 heterostructure array has a synergistic effect that can improve the OER performance. Due to the secondary structure (Ni3S2 nanosheets), the tree-like NiS-Ni3S2 array provides more active sites could have higher specific surface area. The greater activity of the NiS/Ni3S2 heterostructure may also stem from the tight conjunction between tree-like NiS/Ni3S2 and the Ni foam substrate, which is beneficial for electronic transmission. Hydroxy groups can accumulate in large amounts on the surface of the tree-like array, and it also generates some Ni-based oxides that are favorable to OER. Moreover, the synergistic effect of such heterostructure can intrinsically improve the OER activity. The unique tree-like NiS-Ni3S2 heterostructure array has great potential as an alternative OER electrocatalyst.
