Chang Q. Sun

Oxidation electronics: bond–band–barrier correlation and its applications ☆

This report features the recent progress in understanding the behaviour of atoms and valence electrons involved in the process of oxidation, and some technological development driven by the new knowledge. It is initiated and verified that a chemical bond contracts spontaneously at a surface associated with magnitude rise of the bond energy due to the coordination imperfection and that an oxygen atom hybridizes its sp orbitals upon reacting with a solid surface. The former leads to the bond order–length–strength (BOLS) correlation for the physical aspect of a surface and a nano-solid and the latter to a bond–band–barrier (BBB) correlation for chemical reaction. In the process of oxidation, non-bonding lone pairs, anti-bonding dipoles and hydrogen-like bonds are involved, which add corresponding density-of-states (DOS) features to the valence band of the host. Bond forming also alters the sizes and valencies of the involved atoms and causes a collective dislocation of these atoms, which corrugate the morphology or the potential barrier of the surface. Based on the above premises, the oxidation of the low-index surfaces of transition metals Cu, Co, Ag and V, noble metals Rh, Ru, and Pd and non-metallic diamond has been consistently analyzed. Identities probed with various techniques, such as STM, LEED, XRD, STS, PES, TDS, EELS and Raman, have been systematically defined in terms of atomic valencies, bond geometry, valence DOS, bond strength and bond forming kinetics. It is understood that formation of the basic oxide tetrahedron, and consequently, the four discrete stages of bond forming kinetics and the oxygen-derived DOS features, are intrinsically common for all the analyzed systems though the patterns of observations may vary from situation to situation. What differs one oxide surface from another in observations are: (i) the site selectivity of the oxygen adsorbate, (ii) the order of the ionic bond formation and, (iii) the orientation of the tetrahedron at the host surfaces. The valencies of oxygen, the scale and geometrical orientation of the host lattice and the electronegativity of the host elements determine these specific differences extrinsically. Extending the premise of sp-orbital hybridization to the reactions of (C, N)–Ni(001) surfaces has led to a novel approach neutralizing the diamond–metal interfacial stress and hence strengthening the diamond–metal adhesion substantially. The BOLS correlation has provided consistent insight into the shape-and-size dependence of a number of properties for nano-solids. The BBB correlation has led to new findings in designing and fabricating materials for photoluminescence, electron emission and ultrahigh elasticity, etc.

Bond-order–bond-length–bond-strength (bond-OLS) correlation mechanism for the shape-and-size dependence of a nanosolid

A bond-order–bond-length–bond-strength (bond-OLS) correlation mechanism is presented for consistent insight into the origin of the shape-and-size dependence of a nanosolid, aiming to provide guidelines for designing nanomaterials with desired functions. It is proposed that the coordination number imperfection of an atom at a surface causes the remaining bonds of the lower-coordinated surface atom to relax spontaneously; as such, the bond energy rises (in absolute value). The bond energy rise contributes not only to the cohesive energy (ECoh) of the surface atom but also to the energy density in the relaxed region. ECoh relates to thermodynamic properties such as self-assembly, phase transition and thermal stability of a nanosolid. The binding energy density rise is responsible for the changes of the system Hamiltonian and related properties, such as the bandgap, core-level shift, phonon frequency and the dielectrics of a nanosolid of which the surface curvature and the portion of surface atoms vary with particle size. The bond-OLS premise, involving no assumptions or freely adjustable parameters, has led to consistency between predictions and experimental observations of a number of outstanding properties of nanosolids.

Thermally tuning of the photonic band gap of SiO2 colloid-crystal infilled with ferroelectric BaTiO3

SiO2 colloid crystal infilled with BaTiO3 was synthesized by a process of self-assembly in combination of a sol–gel technique. It is found that infilling of the fully crystallized BaTiO3 into the colloid assembly can enhance the photonic band gap significantly. In the vicinity of the ferroelectric phase transition point of BaTiO3 (100–150 °C), the photonic band gap of the assembly exhibits a strong temperature dependence. At the Curie point, the band gap has a 20 nm redshift, and the optical transmittance reaches its minimum. Such a temperature-tuning effect in the photonic band gap should be of high interest in device applications.

Nanophotocatalysts via microwave-assisted solution-phase synthesis for efficient photocatalysis

Nanostructured photocatalysts have attracted considerable interest due to their wide range of applications in processes such as organic pollutant degradation, heavy metal reduction, air and water purification, hydrogen production, etc. Pursuing high catalytic efficiency is the foremost goal in the field. One of the current key issues is to search for suitable photocatalysts to enhance light harvesting in the UV or visible light region. In this treatise, the microwave-assisted solution-phase synthesis of various nanomaterials including semiconductor oxides and sulfides, Bi-based oxides, as well as nanocomposites including carbon nanotube-based and graphene-based composites is systematically presented with demonstrations of the advantages of the microwave-assisted process over traditional synthesis methods including solid state or vapor reactions and hydrothermal or solvothermal processes. Application of these nanomaterials as photocatalysts for the degradation of pollutants in water or air, removal of Cr(VI) as well as hydrogen evolution is also demonstrated, showing the improved photocatalytic activities compared with the ones synthesized via traditional methods.

Density and Phonon-Stiffness Anomalies of Water and Ice in the Full Temperature Range.

Chang Q Sun, 2, ∗ Xi Zhang, Xiaojian Fu, 4 Weitao Zheng, Jer-lai Kuo, Yichun Zhou, Zexiang Shen, and Ji Zhou † Key Laboratory of Low-Dimensional Materials and Application Technologies, and Faculty of Materials and Optoelectronics and Physics, Xiangtan University, Hunan 411105, China School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 4 College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China School of Materials Science, Jilin University, Changchun 130012, China Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan School of Physics, Nanyang Technological University, Singapore 639798 (Dated: May 3, 2014)

Density, Elasticity, and Stability Anomalies of Water Molecules with Fewer than Four Neighbors

Goldschmidt-Pauling contraction of the H-O polar-covalent bond elongates and polarizes the other noncovalent part of the hydrogen bond (O:H-O), that is, the O:H van der Waals bond, significantly, through the Coulomb repulsion between the electron pairs of adjacent oxygen (O-O). This process enlarges and stiffens those H2O molecules having fewer than four neighbors such as molecular clusters, hydration shells, and the surface skins of water and ice. The shortening of the H-O bond raises the local density of bonding electrons, which in turn polarizes the lone pairs of electrons on oxygen. The stiffening of the shortened H-O bond increases the magnitude of the O1s binding energy shift, causes the blue shift of the H-O phonon frequencies, and elevates the melting point of molecular clusters and ultrathin films of water, which gives rise to their elastic, hydrophobic, highly-polarized, ice-like, and low-density behavior at room temperature.Chang Q Sun, 2, ∗ Xi Zhang, 3 Ji Zhou, Yongli Huang, Yichun Zhou, and Weitao Zheng † Key Laboratory of Low-dimensional Materials and Application Technologies, and Faculty of Materials and Optoelectronics and Physics, Xiangtan University, Hunan 411105, China School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 3 College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Key Laboratory of Low-Dimensional Materials and Application Technologies, and Faculty of Materials and Optoelectronics and Physics, Xiangtan University, Hunan 411105, China School of Materials Science, Jilin University, Changchun 130012, China (Dated: May 3, 2014)

Bond contraction and lone pair interaction at nitride surfaces

It is shown that bond contraction and nonbonding lone-pair interaction dominate at nitride surfaces. The maximum elastic recovery of a nitride surface was found to be 100% under a relatively lower nanoindentation load (<1.0 mN) and the hardness of the surface was found to be 100% higher than the bulk value. It is interpreted that the spontaneous bond contraction, estimated at 12%–14%, strengthens the binding energy and hence the hardness and Young’s modulus at the surface. The lone-pair weak interaction claims the responsibility for (i) the high elastic recovery, (ii) the lower Raman frequencies of vibration, and (iii) the existence of critical loads for slide friction or lone-pair broken.

An extended 'quantum confinement' theory: surface-coordination imperfection modifies the entire band structure of a nanosolid

With the miniaturization of a solid, quantum and interface effects become increasingly important. As a result, the band structure of a nanometric semiconductor changes: the band gap expands, the core level shifts, the bandwidth revises, and the sublevel separation within a band increases. Unfortunately, such a thorough change goes beyond the scope of currently available models such as the ‘quantum confinement’ theory. A consistent understanding of the factors dominating the band-structure change is highly desirable. Here we present a new approach for the size-induced unusual change by adding the effect of surface-coordination deficiency-induced bond contraction to the convention of an extended solid of which the Hamiltonian contains the intraatomic trapping interaction and the interatomic binding interaction. Agreement between modelling predictions and the observed size dependency in the photoluminescence of Si oxides and some nanometric III–V and II–VI semiconductors, and in the core-level shift of Cu–O nanosolids has been reached. Results indicate that the spontaneous contraction of chemical bonds at a surface and the rise in the surface-to-volume ratio with reducing particle size are responsible for the unusual change of the band structure of a nanosolid.

Hydrogen-bond relaxation dynamics: Resolving mysteries of water ice

Abstract We present recent progress in understanding the anomalous behavior of water ice under mechanical compression, thermal excitation, and molecular undercoordination (with fewer than four nearest neighbors in the bulk) from the perspective of hydrogen (O:H O) bond cooperative relaxation. We modestly claim the resolution of upwards of ten best known puzzles. Extending the Ice Rule suggests a tetrahedral block that contains two H2O molecules and four O:H O bonds. This block unifies the density-geometry-size-separation of molecules packing in water ice. This extension also clarifies the flexible and polarizable O:H O bond that performs like a pair of asymmetric, coupled, H-bridged oscillators with short-range interactions and memory as well as extreme recoverability. Coulomb repulsion between electron pairs on adjacent oxygen atoms and the disparity between the O:H and the H O segmental interactions relax the O:H O bond length and energy cooperatively under stimulation. A Lagrangian solution has enabled mapping of the potential paths for the O:H O bond at relaxation. The H O bond relaxation shifts the melting point, O 1s binding energy, and high-frequency phonon frequency whereas the O:H relaxation dominates polarization, viscoelasticity, and the O:H dissociation energy. The developed strategies have enabled clarification of origins of the following observations: (i) pressure-induced proton centralization, phase transition-temperature depression and ice regelation; (ii) thermally induced four-region oscillation of the mass density and the phonon frequency over the full temperature range; and (iii) molecular-undercoordination-induced supersolidity that is elastic, hydrophobic, thermally stable, with ultra-low density. The supersolid skin is responsible for the slipperiness of ice, the hydrophobicity and toughness of water skin, and the bi-phase structure of nanodroplets and nanobubbles. Molecular undercoordination mediates the O:H and H O bond Debye temperatures and disperses the quasi-solid phase boundary, resulting in freezing point depression and melting point elevation. O:H O bond memory and water-skin supersolidity ensures a solution to the Mpemba paradox — hot water freezes faster than its cold. These understandings will pave the way toward unveiling anomalous behavior of H2O interacting with other species such as salts, acids and proteins, and excitation of H2O by other stimuli such as electrical and magnetic fields.

Nanometric-layered CrN/TiN thin films: mechanical strength and thermal stability

Nanometric-layered CrNyTiN coatings were deposited using unbalanced magnetron sputtering.The layered coating structure was characterised by X-ray diffractometry, and the mechanical properties were measured by nano-indentation and scratch test. High temperature annealing at 400–750 8C was carried out to investigate the thermal stability of the coating structure and mechanical properties.For comparison, samples of TiN and CrN deposited under similar conditions were also annealed and tested.The results showed that nano-layered CrN yTiN has excellent mechanical and thermal properties.Nano-hardness of 40 GPa and scratch adhesion of 80 N were achieved at a wavelength of 7.5 nm and a substrate bias of y80 V.The coating demonstrates application prospects in the stampingycutting tools industry. 2002 Elsevier Science B.V. All rights reserved.