Tuning the electronic and magnetic properties of germanene by surface adsorption of small nitrogen-based molecules

The structural, energetic and electronic properties of germanene adsorbed with small nitrogen-based molecules, including N₂, NH₃, NO₂ and NO, have been investigated by using first-principles calculations. The results show that all nitrogen-based molecules considered bind much stronger to germanene than to graphene due to the hybridized sp²-sp³ bonding of Ge atoms. The N₂, NO and NO₂ molecules all act as an acceptor, while the NH₃ molecule donates electrons to germanene. We also found sizable band gaps (2–158 meV) are opened at the Dirac point of germanene through N₂, NH₃, and NO₂ adsorptions, but with only slightly destroying its Dirac cone shape. The NO₂ molecule also shows a heavy p-type doping character and makes germanene to be metallic. Moreover, when adsorbed by NO molecule, the germanene can change to be a ferromagnetic half-metal with 100% spin-polarization at the Fermi level. Overall, the different adsorption behaviors of small nitrogen-based gas molecules on germanene provide a feasible way to exploit chemically modified germanene for a wide range of practical applications, such as field-effect transistors, gas sensors and spintronic devices.     

Adsorption of small molecules on transition metal doped rhodium clusters Rh3X (X = 3d, 4d atom): a first-principles investigation

The adsorption behaviours of seven molecules (CO, CO₂, N₂, NO, O₂, N₂O and NO₂) on Rh₃X (X?=Sc-Zn, Y-Cd) clusters are systematically investigated by density-functional calculations. Rh₃X clusters exhibit physical adsorption when interacting with CO₂, CO, N₂ and NO. The adsorption energies (E_ads) can be ranked as follows: NO?>?CO?>?CO_2?≥?N₂. Compared with pure Rh₄ cluster, the adsorption capacity changes with the doping element. Chemical adsorption can be obtained for Rh₃X when adsorbing O₂, N₂O and NO₂. E_ads shows an order of E_ads(O₂)?>?E_ads(NO₂)?>?E_ads(N₂O). When O₂ is adsorbed, energy barrier with doping Tc or Cr atom is substantially reduced, which indicates that chemical reactivity of O₂ on Rh₄ can be significantly enhanced. The doped rhodium clusters can be viewed as good candidates in the discrimination between different gas molecules.     

Capture of pure toxic gases through porous materials from molecular simulations

ABSTRACT

In the last three decades, the air pollution is the main problem to affect human health and the environment in China and its contaminants include SO_2, NH_3, H₂S, NO₂, NO and CO. In this work, we employed grand canonical Monte Carlo simulations to investigate the adsorption capability of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) for these toxic gases. Eighty-nine MOFs and COFs were studied, and top-10 adsorption materials were screened for each toxic gas at room temperature. Dependence of the adsorption performance on the geometry and constructed element of MOFs/COFs was determined and the adsorption conditions were optimised. The open metal sites have mainly influenced the adsorption of NH₃, H₂S, NO₂ and NO. Especially, the X-DOBDC and XMOF-74 (X = Mg, Co, Ni, Zn) series of materials containing open metal sites are all best performance for adsorption of NH₃ to illustrate the importance of electrostatic interaction. Our simulation results also showed that ZnBDC and IRMOF-13 are good candidates to capture the toxic gases NH_3, H₂S, NO₂, NO and CO. This work provides important insights in screening MOF and COF materials with satisfactory performance for toxic gas removal.     

Electronic transport calculation of adsorbate NO<Subscript>2</Subscript> and NO molecules on graphene using Maximally Localized Wannier functions

In this paper we have investigated the adsorption of the gas molecules (NO₂, NO) on graphene, using first-principles methods. For full geometric relaxation of the molecules in the vicinity of a graphene sheet, we obtain the adsorption geometry, adsorption energies, charge transfer and density of states (DOS). We can identify which of the adsorbate molecules is acting as donor or acceptor. We find that the conductance of graphene at the Fermi level decreases with adsorbing NO₂ molecules and increases with adsorbing NO molecules.     

Boron and nitrogen co-doped graphene nanosheets for NO and NO2 gas sensing

Using dispersion-corrected density functional theory calculations, the adsorption behavior of NO and NO₂ molecules is studied over B-doped and BN co-doped graphene sheets (BC_mN_n-Gr; $m,n=0,1,2,3$ and $m+n=3$). To examine practical gas sensing application and selectivity, the adsorption of H₂O, CO and CO₂ molecules is also studied on the BC_mN_n-Gr surfaces. It is found that the preferred adsorption site for the adsorption of these molecules is above the B atom due to accumulation of a local positive charge. Meanwhile, the incorporation of nitrogen atoms in BC_mN_n-Gr makes a substantial increase in the adsorption energies of NO and NO₂, mainly due to the shift in the Fermi energy and electron (donor) concentration states of these surfaces. According to our results, the electronic structure of BC₃-Gr, BC₂N-Gr and BCN₂-Gr is sensitive to NO and NO₂ as evidenced by relatively large variation of the electronic structure as well as charge-transfer values. To address the curvature effect of BC_mN_n-Gr nanosheets on the adsorption and sensing properties of NO and NO₂, the adsorption of these molecules is also investigated over B-doped and BN-codoped ($6,6$) carbon nanotubes. The calculations also indicate that BN co-doped graphene sheets can be used as an efficient and promising gas sensing material for detecting NO and NO₂ molecules in the presence of H₂O, CO and CO₂.     

Adsorption of NH3 and NO2 molecules on C48B6N6 heterofullerene: A DFT study on electronic properties

Adsorption of NH₃ and NO₂ molecules on the external surface of C₄₈B₆N₆ heterofullerene is investigated using DFT method. Attachment of NH₃ and NO₂ on C₄₈B₆N₆ heterofullerenes are compared with the bare C₄₈B₆N₆ model optimized at the B3LYP/6-31G^? level. The high surface binding energies indicates that ammonia undergoes chemical adsorption and could be compatible with the long recovery time but C₄₈B₆N₆ should be good NO₂ sensors with quick response as well as short recovery time. Total (TDOS) and partial (PDOS) density of state calculations is also considered to elucidate the difference in the NH₃ and NO₂ gas detection mechanism of C₄₈B₆N₆. The overlap population density of state (OPDOS) indicated that the chemical adsorption is due to the overlap of atomic orbitals below the Fermi level. The calculated results suggest that the C₄₈B₆N₆ heterofullerene is a suitable sensor material for NO₂ and is an ideal material for elimination and filtering of ammonia.     

The influence of NH3 on NO2 conversion in a dc corona discharge in N2:O2:CO2:NO2:NH3 mixture

The aim of this paper is to investigate the influence of NH₃ additive (540–1470 ppm) on the conversion of NO₂ and the creation of NO and N₂O in a mixture of N₂:O₂:CO₂: NO₂:NH₃ subjected to the so-called direct current (dc) corona discharge. The dc corona discharge was generated in a needle-to-plate reactor. Seven positively polarized needles were used as one electrode and a stainless steel plate as the other. The time-averaged discharge current was varied from 0 to 7 mA. It was found that the dc corona discharge decomposed NO₂ and produced NO and N₂O. The reduction of NO₂ was higher without NH₃ additive if the residence time of the operating gas was relatively short. However, in a longer corona discharge processing the NH₃ additive may be useful for reduction of NO₂.Supports from the Research and Development Commitee (KBN) under Programme KBN 0889/P4/93/04 and the Polish Academy of Sciences within IMP 3.1 project are gratefully acknowledged.     

Selectively strong molecular adsorption on boron nitride monolayer induced by transition metal substrate

We studied adsorption of several molecules (CO, CO₂, H₂O, N₂O, NO, NO₂, and O₂) on hexagonal boron nitride (h-BN) monolayers supported on transition metal (TM) surfaces, using density functional calculations. We observed that all the molecules bind very weakly on the pristine h-BN, with binding energies in the range of 0.02–0.03 eV. Interestingly, however, when h-BN is supported on the TM surface, NO₂ and O₂ become strongly chemisorbed on h-BN, with binding energies of >1 eV, whereas other molecules still physisorbed, with binding energies of ～0.1 eV at most. The electron transfer from TM to p_z states of h-BN played a substantial role in such strong bindings of NO₂ and O₂ on h-BN, as these molecules possess unpaired electrons that can interact with p_z states of h-BN. Such selective molecular binding on h-BN/TM originates from the peculiar distribution of the spin-polarized highest occupied and lowest unoccupied molecular orbitals of NO₂ and O₂. Strong molecular adsorption and high selectivity would make the h-BN/TM system possible for a variety of applications such as catalysts and gas sensors.     

An investigation of paddle wheel Cu2(μ2-O2CCH3)4 for gas molecule adsorptions

Metal organic frameworks (MOFs) have been well-known and extensively researched due to the high storage /good selectivity for gas molecules. Herein, the structures and electron paramagnetic resonance (EPR) spectra for dicopper paddle wheel MOF compound (Cu₂(µ₂-O₂CCH₃)₄ with various gas molecule are theoretically investigated by density functional theory (DFT) calculations. The adsorption energies and isotherms (including pure gas molecules and the mixed ones) are calculated for the gas molecules interacting with the unsaturated Cu₂(µ₂-O₂CCH₃)₄. Both quantities exhibit the roughly consistent orders (e.g. H₂S?>?NH₃?>?CO₂?>?CO?>?H₂O?>?N₂?>?NO?>?H₂ for isotherms and H₂S?>?NH₃?>?N₂?>?CO₂?>?NO?>?H₂O?>?H₂?>?CO for adsorption energies), possibly suggesting that this material may act as a potential adsorbent of these gas molecules. The catalytic property of Cu₂(µ₂-O₂CCH₃)₄ for oxidation of CO and NO into non-toxic molecules and splitting of H₂O into H₂ and O₂ in the solvent condition are uniformly discussed. Simulation of Grand Canonical Monte Carlo (GCMC) in MS 8.0 and calculations in Langmuir model reveal that Cu₂(µ₂-O₂CCH₃)₄ has good selectivity for CH₄ in natural gas (CH₄/CO₂/N₂) and SO₂ in fog (SO₂/NO/NO₂/H₂O/O₂), which would exhibit potential environmentally friendly applications.     

B-, N-doped and BN codoped C60 heterofullerenes for environmental monitoring of NO and NO2: a DFT study

ABSTRACT

Using density functional theory calculations, we investigate the gas sensing performance of B-, N-doped and BN-codoped C₆₀ fullerenes towards NO and NO₂ molecules. The calculated adsorption energies and net charge-transfer values indicate that NO and NO₂ molecules have a stronger interaction with the BN-codoped fullerenes compared to the B- or N-doped ones. It is also found that the electronic properties of the BN-codoped C₆₀ exhibit a larger sensitivity towards NO and NO₂ molecules. An increase in the concentration of doped/co-doped B and N atoms tends to weaken the gas sensing ability of these systems.     

Adsorption of pairs of NOx molecules on single-walled carbon nanotubes and formation of NO + NO3 from NO2

The adsorption of NO_x(x = 1, 2, 3) molecules on single-walled carbon nanotubes (SWCNTs) is investigated using first-principle calculations. Single NO, NO₂ and NO₃ molecules are found to physisorb on SWCNTs, but molecules can be chemisorbed in pairs on the top of carbon atoms at close sites of SWCNTs. The adsorption energy for pairs of NO or NO₃ molecules is larger than for pairs of NO₂ molecules. The local curvature is found to have a sizable effect on adsorption energies. The possibility of a surface reaction NO₂ + NO₂ → NO + NO₃ is examined and the relative pathway and barrier is calculated. The results are discussed with reference to available experimental results.     

Surface science studies of selective catalytic reduction of NO: Progress in the last ten years

Selective catalytic reduction (SCR) of NO_x is one of the important strategies in regulating NO_x emissions. In the past several decades, the reactions of NO_x (mainly NO) with H₂, CO, NH₃ and hydrocarbons have been extensively investigated under ambient conditions and have been summarized in numerous reviews. Nonetheless, many questions appear to be difficult to answer under ambient conditions, e.g., the pathways through which the reactions proceed. The introduction and development of modern surface science technology has played an indispensable role and is widely employed in the studies of the SCR of NO_x, greatly helping to elucidate the mechanisms of the reactions with CO, H₂, and NH₃. However, so far, there are few review papers systematically summarizing the progress of such studies.Recently, systematic surface science studies have been conducted on the mechanisms of SCR of NO with organic molecules including ethylene, benzene, and ethanol, which are much more complicated than those with H₂, CO and NH₃ and have drawn much less attention before. It is confirmed that these reactions can be reliably and most importantly, reproducibly probed by surface science technology, but a great deal of work remains to be done.Since Delmon et al. have provided a very thorough review of the researches on catalytic removal of NO (reactions with H₂, CO and NH₃) up to 1998, in which the reactions conducted both under ambient and UHV conditions were included, this review mainly concentrates on the progress made since 1998.     

Mott insulator-superfluid phase transition in p-band triangle optical lattices with on-site rotation

In order to exploit the potential applications of graphene as gas sensors, the adsorptions of a series of small gas molecules (such as CO, O₂, NO₂ and H₂O) on pristine graphene (PG) and Si-doped graphene (SiG) have been investigated by ab initio calculations. Our results indicate that the electronic properties of PG are sensitive to O₂ and NO₂ molecules, but not changed much by the adsorption of CO and H₂O molecules. Compared with PG, SiG is much more reactive in the adsorption of CO, O₂, NO₂ and H₂O. The strong interactions between SiG and the adsorbed molecules induce dramatic changes to the electronic properties of SiG. Therefore, we suggest that SiG could be a good gas sensor for CO, O₂, NO₂ and H₂O.     

Ab initio study of adsorption behaviors of molecular adsorbates on the surface and at the edge of MoS2

Two dimensional (2D) semiconducting materials such as MoS₂ have been actively investigated for their applications in nanodevices and gas sensors (or detectors). In this connection, we have investigated atomic and electronic structures of specific adsorbates on the surface of MoS₂ and the edge of MoS₂ armchair nanoribbons (ANRs) using density functional theory (DFT) calculations. Our calculations reveal that molecular adsorbates are well adsorbed at the edge of MoS₂ than on the surface of MoS₂. Despite the weak van der Waals (vdW) interaction between molecular adsorbates and MoS₂ surface, paramagnetic molecules such as NO and NO₂ induce the reduced band gap in MoS₂ by making the states within the bandgap. On the other hand, adsorbed CO, NO, NO₂, and O₂ at the edge of MoS₂ ANRs have much influence on the band structures of MoS₂ ANRs via dissociation into their constituent atoms, while adsorbed CO₂, NH₃, H₂, and N₂ at the edge of MoS₂ ANRs do not much change the band structure of MoS₂ ANRs due to no dissociation. Further, we identify that dissociated molecules rearrange the charge densities of MoS₂ ANRs by making the states within the bandgap.     

Atomic and molecular adsorption on Pd(111)

The adsorption properties of a variety of atomic species (H, O, N, S, and C), molecular species (N₂, HCN, CO, NO, and NH₃) and molecular fragments (CN, NH₂, NH, CH₃, CH₂, CH, HNO, NOH, and OH) are calculated on the (111) facet of palladium using periodic self-consistent density functional theory (DFT–GGA) calculations at ¼ ML coverage. For each species, we determine the optimal binding geometry and corresponding binding energy. The vibrational frequencies of these adsorbed species are calculated and are found to be in good agreement with experimental values that have been reported in literature. From the binding energies, we calculate potential energy surfaces for the decomposition of NO, CO, N₂, NH₃, and CH₄ on Pd(111), showing that only the decomposition of NO is thermochemically preferred to its molecular desorption.     

SERS and XPS observation of the adsorption behavior of NO and NO2 on a silver powder surface

The influence of H₂O on the adsorption behavior of NO or NO₂ on a silver powder surface was studied by SERS and XPS at room temperature. Water vapor was found to be responsible for the adsorption of NO on the silver powder surface. When surface species such as Ag₂O are present on the surface, some of the NO₂ molecules are adsorbed on the surface species to produce NO^-₃, whereas NO molecules are adsorbed on a different site to produce NO^-₂.     

A-Z-A型石墨烯场效应晶体管吸附 效应的第一性原理研究

利用第一性原理的计算方法, 研究了A-Z-A型GNR-FET的电子结构和输运性质及其分子吸附效应. 得到了以下结论: 纯净的A-Z-A型GNR-FET具有典型的双极型晶体管特性, 吸附分子的存在会使纳米带能隙变小. 对于吸附H, H₂, H₂O, N₂, NO, NO₂, O₂, CO₂和SO₂分子的情况, A-Z-A型GNR-FET仍然保持着场效应晶体管的基本特征, 但吸附不同类型的分子会使GNR-FET的输运特性发生不同程度的改变; 对于吸附OH分子的情况, 输运特性发生了本质的改变, 完全不具有场效应晶体管的特性. 这些研究结果将有助于石墨烯气体探测器的工程实现, 并对应用于不同环境中GNR-FET的设计具有重要指导意义.     

Conversion of nitrogen oxides in N2:O2:CO2 and N2:O2:CO2:NO2 mixtures subjected to a dc corona discharge

This paper concerns the influence of a direct current (dc) corona discharge on production and reduction of NO, NO₂ and N₂O in N₂:O₂:CO₂ and N₂:O₂:CO₂:NO₂ mixtures. The corona discharge was generated in a needle-to-plate reactor. The positively polarized electrode consisted of 7 needles. The grounded electrode was a stainless steel plate. The gas flow rate through the reactor was varied from 28 to 110 cm³/s. The time-averaged discharge current ranged from 0 to 6 mA. It was found that in the N₂:O₂:CO₂ mixture the corona discharge produced NO, NO₂ and N₂O. In the N₂:O₂:CO₂:NO₂ mixture the reduction of NO₂ was between 6–56%, depending on the concentration of O₂, gas flow rate and corona discharge current. The NO₂ reduction was accompanied by production of NO and N₂O. The results show that efficient reduction of nitrogen oxides by a corona discharge cannot be expected in the mixtures containing N₂ and O₂ if reducing additives are not employed.     

An experimental and modeling study on auto-ignition of ammonia in an RCM with N2O and H2 addition

Deep insights into the combustion kinetics of ammonia (NH₃) can facilitate its application as a promising carbon-free fuel. Due to the low reactivity of NH₃, experimental data of NH₃ combustion can only be obtained within a limited range. In this work, nitrous oxide (N₂O) and hydrogen (H₂) were used as additives to investigate NH₃ auto-ignition in a rapid compression machine (RCM). Ignition delay times for NH₃, NH₃/N₂O blends, and NH₃/H₂ blends were measured at 30 bar, temperatures from 950 to 1437 K. The addition of N₂O and H₂ ranged from 0 to 50% and 0 to 25% of NH₃ mole fraction, respectively. Time-resolved species profiles were recorded during the auto-ignition process using a fast sampling system combined with a gas chromatograph (GC). An NH₃ combustion model was developed, in which the rate constants of key reactions were constrained by current experimental data. The addition of N₂O affected the ignition of NH₃ primarily through the decomposition of N₂O (N₂O (+M) = N₂ + O (+M), R1) and direct reaction between N₂O and NH₂ (N₂H₂ + NO = NH₂ + N₂O, R2). The rate constant of R2 was constrained effectively by experimental data of NH₃/N₂O mixtures. Two-stage ignition behaviors were observed for NH₃/H₂ mixtures, and the corresponding first-stage ignition delay times were reported for the first time. Experimental species profiles suggested the first-stage ignition resulted from the consumption of H₂. The oxidation of H₂ provided extra HO₂ radicals, which promoted the production of OH radicals and initiated first-stage ignition. Reactions between HO₂ radicals and NH₃/NH₂ dominated the first-ignition delay times of NH₃/H₂ mixtures. Moreover, the first-stage ignition led to the fast production of NO₂, which acted as a key intermediate and affected the following total ignition. Consequently, the reaction NH₂ + NO₂ = H₂NO + NO (R3) was constrained by total ignition delay times.