Xu-Ri Huang

Doping the Alkali Atom: An Effective Strategy to Improve the Electronic and Nonlinear Optical Properties of the Inorganic Al12N12 Nanocage

Under ab initio computations, several new inorganic electride compounds with high stability, M@x-Al12N12 (M = Li, Na, and K; x = b66, b64, and r6), were achieved for the first time by doping the alkali metal atom M on the fullerene-like Al12N12 nanocage, where the alkali atom is located over the Al-N bond (b66/b64 site) or six-membered ring (r6 site). It is revealed that independent of the doping position and atomic number, doping the alkali atom can significantly narrow the wide gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (EH-L = 6.12 eV) of the pure Al12N12 nanocage in the range of 0.49-0.71 eV, and these doped AlN nanocages can exhibit the intriguing n-type characteristic, where a high energy level containing the excess electron is introduced as the new HOMO orbital in the original gap of pure Al12N12. Further, the diffuse excess electron also brings these doped AlN nanostructures the considerable first hyperpolarizabilities (β0), which are 1.09 × 10(4) au for Li@b66-Al12N12, 1.10 × 10(4), 1.62 × 10(4), 7.58 × 10(4) au for M@b64-Al12N12 (M = Li, Na, and K), and 8.89 × 10(5), 1.36 × 10(5), 5.48 × 10(4) au for M@r6-Al12N12 (M = Li, Na, and K), respectively. Clearly, doping the heavier Na/K atom over the Al-N bond can get the larger β0 value, while the reverse trend can be observed for the series with the alkali atom over the six-membered ring, where doping the lighter Li atom can achieve the larger β0 value. These fascinating findings will be advantageous for promoting the potential applications of the inorganic AlN-based nanosystems in the new type of electronic nanodevices and high-performance nonlinear optical (NLO) materials.

Exceptionally Large Second-Order Nonlinear Optical Response in Donor–Graphene Nanoribbon–Acceptor Systems

Graphene nanoribbon (GNR) has been used, for the first time, as an excellent conjugated bridge in a donor-conjugated bridge-acceptor (D-B-A) framework to design high-performance second-order nonlinear optical materials. Owing to the unique diradical planar conjugated bridge of GNR, D(NH(2))-GNR-A(NO(2)) exhibits exceptionally large static first hyperpolarizability (β(0)) up to 2.5×10(6) a.u. (22000×10(-30) esu) for H(2)N-(7,3)ZGNR-NO(2) (ZGNR=zigzag-edged GNR), which is about 15 times larger than the recorded value of β(0) (1470×10(-30) esu) for the D-A polyene reported by Blanchard-Desce et al. [Chem. Eur. J. 1997, 3, 1091]. Interestingly, we have found that the size effect of GNR plays a key role in increasing β(0) for the H(2)N-GNR-NO(2) system, in which the width effect of GNR perpendicular to the D-A direction is superior to the length effect along the D-A direction.

Highly Active, Nonprecious Electrocatalyst Comprising Borophene Subunits for the Hydrogen Evolution Reaction

Developing nonprecious hydrogen evolution electrocatalysts that can work well at large current densities (e.g., at 1000 mA/cm2: a value that is relevant for practical, large-scale applications) is of great importance for realizing a viable water-splitting technology. Herein we present a combined theoretical and experimental study that leads to the identification of α-phase molybdenum diboride (α-MoB2) comprising borophene subunits as a noble metal-free, superefficient electrocatalyst for the hydrogen evolution reaction (HER). Our theoretical finding indicates, unlike the surfaces of Pt- and MoS2-based catalysts, those of α-MoB2 can maintain high catalytic activity for HER even at very high hydrogen coverage and attain a high density of efficient catalytic active sites. Experiments confirm α-MoB2 can deliver large current densities in the order of 1000 mA/cm2, and also has excellent catalytic stability during HER. The theoretical and experimental results show α-MoB2s catalytic activity, especially at large current densities, is due to its high conductivity, large density of efficient catalytic active sites and good mass transport property.

A Dependence on the Petal Number of the Static and Dynamic First Hyperpolarizability for Electride Molecules: Many-Petal-Shaped Li-Doped Cyclic Polyamines

Doping Li atom into higher flexible cyclic polyamines with many amine unit petals (ethyleneimine) forms the n-petal-shaped Li-doped cyclic polyamines (n = 3-5). Three structures, referred to as three-petal-shaped Li-[9]aneN(3), four-petal-shaped Li-[12]aneN(4), and five-petal-shaped Li-[15]aneN(5), with all-real frequencies are obtained at the MP2/6-31+G(d) level. Because the chemical doping with Li and the deformation of the complexant produce more diffuse excess electron, the three molecules with the excess electrons exhibit considerably large static first hyperpolarizabilities (beta(0)) at the MP2 level. Additionally, the beta(0) value increases with increasing the petal number (n) as follows: 52282 (n = 3) < 65505 (n = 4) < 127617 au (n = 5). This shows a new complexant effect on beta(0), that is, a dependence on the petal number (n) of beta(0) owing to the flexibility of the complexants increasing with the petal number. The MP2 frequency-dependent beta values are estimated by using the multiplicative approximation. The frequency dispersion is found to be strong. For the MP2 frequency-dependent beta values, the more pronounced dependence on the petal number (n) of beta (-2omega; omega, omega) and beta (-omega; omega, 0) are shown.

Understanding the Effects of Bidentate Directing Groups: A Unified Rationale for sp2 and sp3 C–H Bond Activations

Bidentate directing group (DG) strategy is a promising way to achieve sp(2) and more inert sp(3) C-H bond activations in transition metal (TM) catalysis. In this work, we systematically explored the assisting effects exerted by bidentate DGs in the C-H bond activations. Through DFT calculations and well-defined comparative analysis, we for the first time unified the rationale of the reactivity promoted by bidentate DG in sp(2) and sp(3) C-H activations, which are generally consistent with available experimental discoveries about the C-H activation reactivity up to date. In addition to the general rationale of the reactivity, the assisting effects of several typical bidentate DGs were also quantitatively evaluated and compared to reveal their relative promoting ability for C-H activation reactivity. Finally, the effect of the ligating group charge and the position of the ligating group charge in bidentate DGs were also investigated, based on which new types of DGs were designed and proposed to be potentially effective in C-H activation. The deeper understanding and new insight about the bidentate DG strategy gained in this work would help to enhance its further experimental development in sp(2) and sp(3) C-H bond activations.

Mechanism Insights of Ethane C-H Bond Activations by Bare [Fe-III=O](+): Explicit Electronic Structure Analysis

Alkane C-H bond activation by various catalysts and enzymes has attracted considerable attention recently, but many issues are still unanswered. The conversion of ethane to ethanol and ethene by bare [Fe(III)═O](+) has been explored using density functional theory and coupled-cluster method comprehensively. Two possible reaction mechanisms are available for the entire reaction, the direct H-abstraction mechanism and the concerted mechanism. First, in the direct H-abstraction mechanism, a direct H-abstraction is encountered in the initial step, going through a collinear transition state C···H···O-Fe and then leading to the generation of an intermediate Fe-OH bound to the alkyl radical weakly. The final product of the direct H-abstraction mechanism is ethanol, which is produced by the hydroxyl group back transfer to the carbon radical. Second, in the concerted reaction mechanism, the H-abstraction process is characterized via overcoming four/five-centered transition states (6/4)TSH_c5 or (4)TSH_c4. The second step of the concerted mechanism can lead to either product ethanol or ethene. Moreover, the major product ethene can be obtained through two different pathways, the one-step pathway and the stepwise pathway. It is the first report that the former pathway starting from (6/4)IM_c to the product can be better described as a proton-coupled electron transfer (PCET). It plays an important role in the product ethene generation according to the CCSD(T) results. The spin-orbital coupling (SOC) calculations demonstrate that the title reaction should proceed via a two-state reactivity (TSR) pattern and that the spin-forbidden transition could slightly lower the rate-determining energy barrier height. This thorough theoretical study, especially the explicit electronic structure analysis, may provide important clues for understanding and studying the C-H bond activation promoted by iron-based artificial catalysts.

The Effects of the Formation of Stone–Wales Defects on the Electronic and Magnetic Properties of Silicon Carbide Nanoribbons: A First‐Principles Investigation

Detailed first-principles density functional theory (DFT) computations were performed to investigate the geometries, the electronic, and the magnetic properties of both armchair-edged silicon carbide nanoribbons (aSiCNRs) and zigzag-edged silicon carbide nanoribbons (zSiCNRs) with Stone-Wales (SW) defects. SW defects in the center of aSiCNRs can remarkably reduce their band gaps, irrespective of the orientation of the defect, whereas zSiCNRs with SW defects in the center or at the edges exhibit degenerate energies of their ferromagnetic (FM) and antiferromagnetic (AFM) states, in which metallic and half-metallic behavior can be observed, respectively; half-metallic behavior can even be observed in both the FM and AFM states simultaneously. Further, it was shown that the formation energies of the SW defects in SiCNRs are orientation dependent, and the formation of edge defects is always favored over the formation of interior defects in zSiCNRs. The possible existence of SW defects in SiCNRs was further validated through exploring the kinetic process of their formation. These findings can be anticipated to provide valuable information in promoting the potential applications of SiC-based nanomaterials in multifunctional and spintronic nanodevices.

Successive hydrogenation starting from the edge(s): an effective approach to fine-tune the electronic and magnetic behaviors of SiC nanoribbons

Based on first-principles computations, the geometries, stabilities, electronic and magnetic properties of fully and partially hydrogenated silicon carbide nanoribbons (SiCNRs) were investigated. Independent of the ribbon width, the fully hydrogenated zigzag and armchair SiCNRs are all non-magnetic wide-band-gap semiconductors. By hydrogenating zigzag SiCNRs (zSiCNRs) from the edge(s) step by step, we have constructed partially hydrogenated zSiCNRs that can be viewed as the combination of hydrogenated and pristine zigzag SiC chain building blocks along the periodical direction. The computed results reveal that greatly enriched electronic and magnetic properties can be achieved in zSiCNRs: the transition of the antiferromagnetic spin gapless semiconductor (SGS)–ferromagnetic metal–antiferromagnetic half-metal–non-magnetic semiconductor can be achieved by controlling the hydrogenation pattern and ratio. Notably, this is the first time that the concept of successive hydrogenation starting from the edge(s) is proposed as an effective approach to fine-tune the electronic and magnetic behaviors of SiCNRs. These appealing features, especially the diverse electronic and magnetic transitions, in the unitary SiCNR-based nanostructures may provide tremendous potential applications for integrated multi-functional and spintronic nanodevices.

Electric field-driven acid-base chemistry: proton transfer from acid (HCl) to base (NH3/H2O).

It is well-known that single H3N-HCl and H2O-HCl acid-base pairs do not react to form the ion pairs, H4N(+)Cl(-) and H3O(+)Cl(-), in isolation. On the basis of ab initio method, we propose a physical method of external electric field (Eext) to drive the proton transfer from acid (HCl) to base (NH3/H2O). Our results show that when Eext along the proton-transfer direction achieves or exceeds the critical electric field (Ec), the proton transfer occurs, such as, the Ec values of proton transfer for H3N-HCl and H2O-HCl are 54 × 10(-4) and 210 × 10(-4) au, respectively. And the degree of the proton transfer can be controlled by modulating the strength of Eext. Furthermore, we estimate the inductive strength of an excess electron (Ee) equivalent to the Eext = 125 × 10(-4) au, which is greater than the Ec (54 × 10(-4) au) of NH3-HCl but less than the Ec (210 × 10(-4) au) of H2O-HCl. This explains well the anion photoelectron spectroscopy [Eustis et al. Science 2008, 319, 936] that an excess electron can trigger the proton transfer for H3N-HCl but not for H2O-HCl. On the basis of the above estimation, we also predict that two excess electrons can induce H2O-HCl to undergo the proton transfer and form the ion pair H3O(+)Cl(-).

Dihalogen edge-modification: an effective approach to realize the half-metallicity and metallicity in zigzag silicon carbon nanoribbons

By means of first-principles computations, we have systematically investigated the electronic and magnetic properties of the zSiCNR with not only homogeneous but also heterogeneous diatomic edge-modification by employing halogen atoms, where one edge is saturated by double halogen F/Cl atoms, and the other is terminated by single or double F/Cl/H atoms, respectively. The computed results reveal that this kind of edge-modification by dihalogen atoms can break the magnetic degeneracy of the pristine zSiCNR, and the intriguing electronic and magnetic behaviors invoking not only the antiferromagnetic (AFM) metallicity but also the AFM half-metallicity and ferromagnetic (FM) half-metallicity can be achieved. The decorated atoms at the C-edge of zSiCNR can play an important role in affecting the electronic and magnetic properties of the modified zSiCNR systems, and the heterogeneous asymmetric edge-modification by the halogen/hydrogen pair with great electronegative difference can more effectively strengthen the robustness of half-metallicity. Additionally, employing strong electron-withdrawing halogen atom to perform a diatomic edge-modification can also significantly lower the edge formation energy of the modified zSiCNR systems, endowing them with higher structural stability. Obviously, diatomic edge-modification with halogen atoms can be an effective strategy to modulate the electronic and magnetic behaviors of zSiCNRs, which can be of benefit in promoting excellent SiC-based nanomaterials in the application of spintronics and multifunctional nanodevices.