Nitrogen defects in wide band gap oxides: defect equilibria and electronic structure from first principles calculations

The nitrogen related defect chemistry and electronic structure of wide band gap oxides are investigated by density functional theory defect calculations of N, NH, and as well as and in MgO, CaO, SrO, Al(2)O(3), In(2)O(3), Sc(2)O(3), Y(2)O(3), La(2)O(3), TiO(2), SnO(2), ZrO(2), BaZrO(3), and SrZrO(3). The N acceptor level is found to be deep and the binding energy of NH with respect to and is found to be significantly negative, i.e. binding, in all of the investigated oxides. The defect structure of the oxides was found to be remarkably similar under reducing and nitriding conditions (1 bar N(2), 1 bar H(2) and 1 × 10(-7) bar H(2)O): NH predominates at low temperatures and predominates at higher temperatures (>900 K for most of the oxides). Furthermore, we evaluate how the defect structure is affected by non-equilibrium conditions such as doping and quenching. In terms of electronic structure, is found to introduce isolated N-2p states within the band gap, while the N-2p states of NH are shifted towards, or overlap with the VBM. Finally, we assess the effect of nitrogen incorporation on the proton conducting properties of oxides and comment on their corrosion resistance in nitriding atmospheres in light of the calculated defect structures.     

Ozonated graphene oxides as high efficient sorbents for Sr(II) and U(VI) removal from aqueous solutions

Ozone was used to oxidize graphene oxides (GO) to generate ozonated graphene oxides (OGO) with higher oxygen-containing functional groups. The as-prepared OGO was characterized by Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Based on the results of potentiometric acid-base titrations, the total carboxylic acid concentration on OGO surface was calculated to be 3.92 mmol/g, which was much higher than that on GO surface. The results of adsorption experiments indicated that the adsorption capacities of OGO for Sr(II) and U(VI) removal were improved significantly after ozonization.     

Removal of 4‐chlorophenol using graphene,graphene oxide,and a‐doped graphene (A = N,B): A computational study

Density functional theory (including van der Waals correction with the PBE‐D functional) is applied to the study of 4‐chlorophenol (4‐CP) adsorption on graphene oxide (GO), A‐doped graphene (A = N, B), and pristine graphene and test their possible application for 4‐CP removal. Results show that on GO adsorption is improved by the hydrogen bond interactions between the adsorbents and 4‐CP, suggesting that functionalized graphene is a preferable alternative than pristine graphene for 4‐CP removal. In addition, the stability of hydrogen bonds is confirmed by molecular dynamics calculations using the PM6 potential. Without hydrogen bonds, A‐doped graphene models show a comparable performance for 4‐CP removal than pristine graphene. Finally, even in a solvent medium, 4‐CP adsorption is strong. © 2013 Wiley Periodicals, Inc.     

Rules of Boron–Nitrogen Doping in Defect Graphene Sheets: A First‐Principles Investigation of Band‐Gap Tuning and Oxygen Reduction Reaction Catalysis Capabilities

Introduction of defects and nitrogen doping are two of the most pursued methods to tailor the properties of graphene for better suitability to applications such as catalysis and energy conversion. Doping nitrogen atoms at defect sites of graphene and codoping them along with boron atoms can further increase the efficiency of such systems due to better stability of nitrogen at defect sites and stabilization provided by B?N bonding. Systematic exploration of the possible doping/codoping configurations reflecting defect regions of graphene presents a prevalent doping site for nitrogen‐rich BN clusters and they are also highly suitable for modulating (0.2–0.9 eV) the band gap of defect graphene. Such codoped systems perform significantly better than the platinum surface, undoped defect graphene, and the single nitrogen or boron atom doped defect graphene system for dioxygen adsorption. Significant stretching of the O?O bond indicates a lowering of the bond breakage barrier, which is advantageous for applications in the oxygen reduction reaction.     

Effects of heteroatoms on the electronic,sensor, and adsorption properties of graphene

The effects of doping heteroatoms on the structure, electronic and adsorption properties of graphene are investigated using density functional theory calculations. Six different doped graphenes (with Al, B, Si, N, P, and S) are considered, and to obtain the interaction and adsorption properties, three sulfur-containing molecules (H₂S, SO₂, and thiophene) were interacted with selected graphenes. The adsorption energies (E _ad) in the gas phase and solvents show the exothermic interaction for all complexes. The maximum E _ad values are observed for aluminum doped graphene (AG) and silicon doped graphene (SiG), and adsorption energies in the solvent are not so different from those in the gas phase. NBO calculations show that the AG and SiG complexes have the highest E ⁽²⁾ interaction energies and simple graphene (G) and nitrogen doped graphene (NG) have the least E ⁽²⁾ energies. Population analyses show that doping heteroatoms change the energy gap. This gap changes more during the interaction and these changes make these structures useful in sensor devices. All calculated data confirm better adsorption of SO₂ by graphenes versus H₂S and thiophene. Among all graphenes, AG and then SiG are the best adsorbents for these structures.     

A Crystallographic Charge Density Study of the Partial Covalent Nature of Strong N⋅⋅⋅Br Halogen Bonds

The covalent nature of strong N?Br???N halogen bonds in a cocrystal ( 2 ) of N‐bromosuccinimide ( NBS ) with 3,5‐dimethylpyridine ( lut ) was determined from X‐ray charge density studies and compared to a weak N?Br???O halogen bond in pure crystalline NBS ( 1 ) and a covalent bond in bis(3‐methylpyridine)bromonium cation (in its perchlorate salt ( 3 ). In 2 , the donor N?Br bond is elongated by 0.0954 Å, while the Br???acceptor distance of 2.3194(4) is 1.08 Å shorter than the sum of the van der Waals radii. A maximum electron density of 0.38 e Å^?3 along the Br???N halogen bond indicates a considerable covalent contribution to the total interaction. This value is intermediate to 0.067 e Å^?3 for the Br???O contact in 1 , and approximately 0.7 e Å^?3 in both N?Br bonds of the bromonium cation in 3 . A calculation of the natural bond order charges of the contact atoms, and the σ*(N1?Br) population of NBS as a function of distance between NBS and lut , have shown that charge transfer becomes significant at a Br???N distance below about 3 Å.     

对苯二胺功能化还原氧化石墨烯的结构和官能团变化

采用一步水热回流法,选取对苯二胺(PPD)对氧化石墨烯(GO)进行还原与改性处理,制备了功能化还原氧化石墨烯(GOP-X).采用傅里叶变换红外光谱(FTIR)、X射线光电子能谱(XPS)及X射线衍射(XRD)等研究了PPD与GO的反应作用类型及结构变化.结果表明,随着PPD与GO质量比的增加,GOP-X层间距(d值)先增大后减小,GOP-X共轭结构逐渐恢复,与溶剂分子作用时,层间距增幅呈减小趋势,并最终趋于恒定.PPD单体与GO反应时存在3种键合类型:(1)GO含氧官能团和PPD分子之间的氢键作用(C—OH…H2N—X);(2)质子化PPD与弱酸性GO带负电位置之间的离子键作用(—COO-H3+N—X);(3)PPD中氨基(NH2)与GO含氧官能团之间形成的共价键作用.与GO中羧基(COOH)的酰胺化反应将先于与环氧基(C—O—C)的亲核取代反应.提出了相应的作用机理.     

Recent progress in applications of graphene oxide for gas sensing: A review

This paper is a review of the recent progress on gas sensors using graphene oxide (GO). GO is not a new material but its unique features have recently been of interest for gas sensing applications, and not just as an intermediate for reduced graphene oxide (RGO). Graphene and RGO have been well known gas-sensing materials, but GO is also an attractive sensing material that has been well studied these last few years. The functional groups on GO nanosheets play important roles in adsorbing gas molecules, and the electric or optical properties of GO materials change with exposure to certain gases. Addition of metal nanoparticles and metal oxide nanocomposites is an effective way to make GO materials selective and sensitive to analyte gases. In this paper, several applications of GO based sensors are summarized for detection of water vapor, NO₂, H₂, NH₃, H₂S, and organic vapors. Also binding energies of gas molecules onto graphene and the oxygenous functional groups are summarized, and problems and possible solutions are discussed for the GO-based gas sensors.     

Preparation and properties of polyimide/graphene oxide nanocomposite films with Mg ion crosslinker

Polyimide(PI)/graphene oxide(GO) nanocomposite films were prepared by chemical cross-linking using small amounts of divalent Mg ions. The PI/GO nanocomposites showed enhanced tensile properties compared to pristine PI due to the presence of exfoliated GO in the PI matrix as well as crosslinking between poly(amic acid) (PAA), which is a precursor of PI, and GO by Mg ions. The hydrogen bonds between PAA and GO suppressed the phase separation between PI and GO, and small amounts of Mg ions can bond between the oxygen functional groups and carboxylate groups of GO and PAA.     

A first principle study of adsorption of two proximate nitrogen atoms on graphene

Different possible configurations of two nitrogen‐adatoms on graphene are studied using density functional theory. Adsorption of single nitrogen atom on the bridge site of graphene is accompanied by distortion of the sheet. Electronically, this case amounts to p‐type doping. Two N atoms adsorbed on the graphene sheet can share a bond in two ways. They acquire positions either just above two adjacent carbon atoms or they form a bridge across opposite bonds of a hexagon in the sheet. Both these configurations also induce structural distortion of the sheet. Another stable configuration consists of two N atoms bonded as an N₂ molecule physisorbed on the graphene sheet. It is also possible to adsorb two N atoms on opposite sides of the graphene sheet, bonded to the same two C atoms. Moreover, two N atoms can be individually adsorbed on alternate bridge sites of neighboring hexagons experiencing a repulsion, the energy for which arises from the additional distortion of the graphene sheet. The densities of states near the Fermi level are found to be dependent on the adsorption configurations of two nitrogen atoms on graphene. Thus the electronic properties of graphene can be controlled by the selective adsorption of two nitrogen atoms. © 2014 Wiley Periodicals, Inc.     

New insight into selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites: a theoretical study

Density functional theory calculations were carried out to investigate the reaction mechanism of selective catalytic reduction of nitrogen oxides by ammonia in the presence of oxygen at the Br?nsted acid sites of H-form zeolites. The Br?nsted acid site of H-form zeolites was modeled by an aluminosilicate cluster containing five tetrahedral (Al, Si) atoms. A low-activation-energy pathway for the catalytic reduction of NO was proposed. It consists of two successive stages: first NH(2)NO is formed in gas phase, and then is decomposed into N(2) and H(2)O over H-form zeolites. In the first stage, the formation of NH(2)NO may occur via two routes: (1) NO is directly oxidized by O(2) to NO(2), and then NO(2) combines with NO to form N(2)O(3), which reacts with NH(3) to produce NH(2)NO; (2) when NO(2) exceeds NO in the content, NO(2) associates with itself to form N(2)O(4), and then N(2)O(4) reacts with NH(3) to produce NH(2)NO. The second stage was suggested to proceed with low activation energy via a series of synergic proton transfer steps catalyzed by H-form zeolites. The rate-determining step for the whole reduction of NO(x) is identified as the oxidation of NO to NO(2) with an activation barrier of 15.6 kcal mol(-1). This mechanism was found to account for many known experimental facts related to selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites.     

Strong N−X⋅⋅⋅O−N Halogen Bonds: A Comprehensive Study on N‐Halosaccharin Pyridine N‐Oxide Complexes

A study of the strong N?X???^?O?N⁺ (X=I, Br) halogen bonding interactions reports 2×27 donor×acceptor complexes of N‐halosaccharins and pyridine N‐oxides (PyNO). DFT calculations were used to investigate the X???O halogen bond (XB) interaction energies in 54 complexes. A simplified computationally fast electrostatic model was developed for predicting the X???O XBs. The XB interaction energies vary from ?47.5 to ?120.3 kJ mol^?1; the strongest N?I???^?O?N⁺ XBs approaching those of 3‐center‐4‐electron [N?I?N]⁺ halogen‐bonded systems (ca. 160 kJ mol^?1). ¹H NMR association constants (K_XB) determined in CDCl₃ and [D₆]acetone vary from 2.0×10⁰ to >10⁸ m ^?1 and correlate well with the calculated donor×acceptor complexation enthalpies found between ?38.4 and ?77.5 kJ mol^?1. In X‐ray crystal structures, the N‐iodosaccharin‐PyNO complexes manifest short interaction ratios (R_XB) between 0.65–0.67 for the N?I???^?O?N⁺ halogen bond.     

氮掺杂石墨烯的p型场效应及其精细调控

Functionalized graphene has attracted significant interest over the past decade due to its unique physical properties and potential applications. Graphene oxide (GO), a readily scaled-up product, is a basic material for further functionalization. Using reductive processes, highly conductive reduced graphene oxide (RGO) can be obtained, which exhibits electrical and optical properties analogous to those of graphene. Moreover, due to the presence of oxygen-containing functional groups, its chemical reactivity and electronic properties can be easily tailored by chemical doping with nitrogen. However, developing strategies for doping graphene is challenging and the fundamental roles of the doping atom configuration and its environment on the resulting properties of graphene remain poorly understood. These properties are important for electrical and catalytic applications of graphene. Thus, synthesizing specific configurations of nitrogen-doped graphene and consequently investigating the electrical and catalytic properties of the product is imperative. Herein, we demonstrate an approach that allows for successful production of nitrogen-functionalized RGO using Schiff base condensation between the amino groups in an o-aryl diamine compound and the carbonyl groups in GO. Three typical nitrogen-containing species including o-phenylenediamine (OPD), 2, 3-diaminopyridine (23DAP), and bis(trifluoromethyl)-1, 2-diaminobenzene (BTFMDAB) were used for functionalizing the GO samples, and the corresponding RGO derivatives (OPD-RGO, 23DAP-RGO, and BTF-RGO) were obtained by thermal annealing. Pyrazine nitrogen was successfully introduced into graphitic framework, as confirmed by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, thermal gravimetric analysis (TGA), Raman, and X-ray photoelectron spectroscopy (XPS). Field-effect transistors (FETs) based on the BTF-RGO exhibited hole-dominated ambipolar field-effect behavior with a Dirac point at a 9 V gate voltage and hole mobilities up to 2.5 times that of RGO. The weak p-type doping effect originated from the strongly electron-withdrawing trifluoromethyl groups. By studying the OPD-RGO and 23DAP-RGO-based FETs, containing pyrazine nitrogen and mixed pyrazine/pyridine nitrogen, respectively, we found that pyrazine nitrogen provided weak n-type doping effects, while pyridine nitrogen exhibited weak p-type doping effects due to its electron-withdrawing ability. Enhanced p-type doping effect was accompanied by the introduction of groups with stronger electron-withdrawing ability into the graphitic framework. Impressively, pyridine nitrogen in the pyrazine nitrogen-doped RGO yielded a weak p-type doped graphene due to the electron-withdrawing effect of the pyridine nitrogen. Nitrogen-doped graphene can be finely tuned from weak n-type to weak p-type doping by adjusting the electron-withdrawing ability of o-aryl diamine compounds. This study demonstrates the effect of nitrogen configuration and its surrounding environment on the electrical properties of RGOs, providing additional possible applications.     

Atomic layer deposition of metal oxides on pristine and functionalized graphene

We investigate atomic layer deposition (ALD) of metal oxide on pristine and functionalized graphene. On pristine graphene, ALD coating can only actively grow on edges and defect sites, where dangling bonds or surface groups react with ALD precursors. This affords a simple method to decorate and probe single defect sites in graphene planes. We used perylene tetracarboxylic acid (PTCA) to functionalize the graphene surface and selectively introduced densely packed surface groups on graphene. Uniform ultrathin ALD coating on PTCA graphene was achieved over a large area. The functionalization method could be used to integrate ultrathin high-kappa dielectrics in future graphene electronics.     

Titanium-decorated graphene oxide for carbon monoxide capture and separation

We propose titanium-decorated graphene oxide (Ti-GO) as an ideal sorbent for carbon monoxide (CO) capture and separation from gas mixtures. Based on first-principles calculations, Ti-GO exhibits a large binding energy of ~70 kJ mol(-1) for CO molecules, while the binding energies for other gases, such as N(2), CO(2), and CH(4), are significantly smaller. The gas adsorption properties of Ti-GO are independent of the local GO structures once Ti atoms are anchored by the oxygen-containing groups on the GO surface. The strong interaction between CO molecule and Ti is a result of dative bonding, i.e., hybridization between an empty d orbital of Ti and an occupied p orbital of CO. Adsorption isotherms from grand canonical Monte Carlo simulations clearly demonstrate the strong selectivity of Ti-GO for CO adsorption in a mixture with other gas.     

Synthesis and characterization nanocomposite of the graphene oxide/polyacrylic acid modified with 2-mercaptoethanol

In this research, synthesis and characterization of the nano-graphene oxide (GO) based on the modified polyacrylic acid (PAA) have been carried out. Formation of esteric bonds between the carboxyl functional groups of the GO surface and the hydroxyl groups of PAA was confirmed by FTIR spectroscopy. The result of this synthesis is covalent modification of graphene oxide during the polymerization process and this modification has caused improvement and change in some properties of graphene oxide including solubility of nanocomposite. Additionally, structure and stability of composite were studied by SEM, XRD and TGA.     

Does Fluorine Participate in Halogen Bonding?

When R is sufficiently electron withdrawing, the fluorine in the R?F molecules could interact with electron donors (e.g., ammonia) and form a noncovalent bond (F ??? N). Although these interactions are usually categorized as halogen bonding, our studies show that there are fundamental differences between these interactions and halogen bonds. Although the anisotropic distribution of electronic charge around a halogen is responsible for halogen bond formations, the electronic charge around the fluorine in these molecules is spherical. According to source function analysis, F is the sink of electron density at the F ??? N BCP, whereas other halogens are the source. In contrast to halogen bonds, the F ??? N interactions cannot be regarded as lump–hole interactions; there is no hole in the valence shell charge concentration (VSCC) of fluorine. Although the quadruple moment of Cl and Br is mainly responsible for the existence of σ‐holes, it is negligibly small in the fluorine. Here, the atomic dipole moment of F plays a stabilizing role in the formation of F ??? N bonds. Interacting quantum atoms (IQA) analysis indicates that the interaction between halogen and nitrogen in the halogen bonds is attractive, whereas it is repulsive in the F ??? N interactions. Virial‐based atomic energies show that the fluorine, in contrast to Cl and Br, stabilize upon complex formation. According to these differences, it seems that the F ??? N interactions should be referred to as “fluorine bond” instead of halogen bond.     

Low temperature growth of highly nitrogen-doped single crystal graphene arrays by chemical vapor deposition

The ability to dope graphene is highly important for modulating electrical properties of graphene. However, the current route for the synthesis of N-doped graphene by chemical vapor deposition (CVD) method mainly involves high growth temperature using ammonia gas or solid reagent melamine as nitrogen sources, leading to graphene with low doping level, polycrystalline nature, high defect density and low carrier mobility. Here, we demonstrate a self-assembly approach that allows the synthesis of single-layer, single crystal and highly nitrogen-doped graphene domain arrays by self-organization of pyridine molecules on Cu surface at temperature as low as 300 °C. These N-doped graphene domains have a dominated geometric structure of tetragonal-shape, reflecting the single crystal nature confirmed by electron-diffraction measurements. The electrical measurements of these graphene domains showed their high carrier mobility, high doping level, and reliable N-doped behavior in both air and vacuum.     

Role of Functional Groups in the Monomer Molecule on the Radical Polymerization in the Presence of Graphene Oxide. Polymerization of Hydroxyethyl Acrylate under Isothermal and Non-Isothermal Conditions

Functional groups in a monomer molecule usually play an important role during polymerization by enhancing or decreasing the reaction rate due to the possible formation of side bonds. The situation becomes more complicated when polymerization takes place in the presence of graphene oxide since it also includes functional groups in its surface. Aiming to explore the role of functional groups on polymerization rate, the in situ bulk radical polymerization of hydroxyethyl acrylate (HEA) in the presence or not of graphene oxide was investigated. Differential scanning calorimetry was used to continuously record the reaction rate under both isothermal and non-isothermal conditions. Simple kinetic models and isoconversional analysis were used to estimate the variation of the overall activation energy with the monomer conversion. It was found that during isothermal experiments, the formation of both inter- and intra-chain hydrogen bonds between the monomer and polymer molecules results in slower polymerization of neat HEA with higher overall activation energy compared to that estimated in the presence of GO. The presence of GO results in a dissociation of hydrogen bonds between monomer and polymer molecules and, thus, to higher reaction rates. Isoconversional methods employed during non-isothermal experiments revealed that the presence of GO results in higher overall activation energy due to the reaction of more functional groups on the surface of GO with the hydroxyl and carbonyl groups of the monomer and polymer molecules, together with the reaction of primary initiator radicals with the surface hydroxyl groups in GO.     

The nature of the N?O bond in amine oxides

Despite the ubiquitous presence of amine oxides in chemistry, there is no consensus about the nature of the N O bond in these compounds. In this work, we have used electron density analysis to investigate the nature of this bond in substituted amine oxides, R₃NO, and have compared it with the nature of the N O bond in hydroxylamines, R₂NOR, and model molecules that have well-established chemical bond character. The results showed that the N O bond length and relative stability are proportional to the inductive effect of the substituents. Quantum chemical topology, natural bond orbitals (NBO), and natural resonance theory (NRT) analyses indicated that the N O bond is polar covalent in all the studied amine oxides, but the ionic contribution is different. NBO and NRT analyses revealed that molecules with more electronegative substituents have strongly delocalized N O and N R bonds, whereas molecules with electropositive substituents have localized bonds.