NH3 on a BC3 nanotube: effect of doping and decoration of aluminum

The adsorption of the NH₃ molecule was investigated on pristine, Al-doped and Al-decorated BC₃ nanotubes (BC₃NT) using density functional theory calculations. It was found that NH₃ prefers to be adsorbed on a B atom of the tube wall, releasing energy of 1.02 eV. Al-doping increases the acidity of the tube surface and, therefore, its reactivity toward NH₃ so that the released energy in this case is about 1.62 eV, while decorating the BC₃NT with Al atom decreases the acidity and reactivity. Although Al-doping has no significant effect on the electronic properties of the BC₃NT, Al-decoration significantly reduces its HOMO/LUMO energy gap from 2.37 to 1.16 eV so that the tube becomes an n-type semiconductor. However, we believe that the acidity of the BC₃NTs may be controlled by doping or decoration of Al atoms.     

Density functional study of H<Subscript>2</Subscript>O molecule adsorption on α-U(001) surface

Periodic density functional theory (DFT) calculations were performed to investigate the adsorption of H₂O on U(001) surface. The metallic nature of uranium atom and different adsorption sites of U(001) surface play key roles in the H₂O molecular dissociate reaction. The long-bridge site is the most favorable site of H₂O-U(001) adsorption configuration. The triangle-center site of the H atom is the most favorable site of HOH-U(001) adsorption configuration. The interaction between H₂O and U surface is more evident on the first layer than that on any other two sub-layers. The dissociation energy of one hydrogen atom from H₂O is ?1.994 to ?2.215 eV on U(001) surface, while the dissociating energy decreases to ?3.351 to ?3.394 eV with two hydrogen atoms dissociating from H₂O. These phenomena also indicate that the O_ads can promote the dehydrogenation of H₂O. A significant charge transfer from the first layer of the uranium surface to the H and O atoms is also found to occur, making the bonding partly ionic.     

Nitrous oxide adsorption on pristine and Si-doped AlN nanotubes

Using density functional theory, we studied the adsorption of an N₂O molecule onto pristine and Si-doped AlN nanotubes in terms of energetic, geometric, and electronic properties. The N₂O is weakly adsorbed onto the pristine tube, releasing energies in the range of ?1.1 to ?5.7 kcal mol^-1. The electronic properties of the pristine tube are not influenced by the adsorption process. The N₂O molecule is predicted to strongly interact with the Si-doped tube in such a way that its oxygen atom diffuses into the tube wall, releasing an N₂ molecule. The energy of this reaction is calculated to be about ?103.6 kcal mol^-1, and the electronic properties of the Si-doped tube are slightly altered.     

Oxygen doped SiC nanocrystals: first principles study of the optical properties

We have studied a typical spherical SiC nanocrystal with a diameter of 1.2 nm (Si₄₃C₄₄H₇₆) using linear combination of atomic orbitals in combination with pesudopotential density functional calculation. The role of fluorine and oxygen impurities was investigated on the electronic and optical properties of the Si₄₃C₄₄H₇₆ nanocrystal. Total energy calculations show that the fluorine doped Si₄₃C₄₄H₇₆ nanocrystals are unstable. Oxygen doped Si₄₃C₄₄H₇₆ have different binding energies in various substitutional and interstitial defects. The maximum binding energy of the oxygen at carbon substitutional defect is about ?0.5 eV and at interstitial defect is ?0.18 eV. The HOMO-LUMO gap of the pure Si₄₃C₄₄H₇₆ is about 6.71 eV and after doping with oxygen changes on the order of 0.1 eV. Our studies show that the refractive index of the doped Si₄₃C₄₄H₇₆ nanocrystal significantly dispersed in comparison with pure SiC nanocrystal especially at the range of 6 to 8 eV.     

Fluorination of BC3 nanotubes: DFT studies

We have studied the adsorption of atomic and molecular fluorines on a BC₃ nanotube by using density functional calculations. It was found that the adsorption of atomic fluorine on a C atom of the tube surface is energetically more favorable than that on a B atom by about 0.97 eV. The adsorption of atomic fluorine on both C and B atoms significantly affects the electronic properties of the BC₃ tube. The HOMO-LUMO energy gap is considerably reduced from 2.37 to 1.50 and 1.14 eV upon atomic F adsorption on B and C atoms, respectively. Molecular fluorine energetically tends to be dissociated on B atoms of the tube surface. The associative and dissociative adsorption energies of F₂ were calculated to be about ?0.42 and ?4.79 eV, respectively. Electron emission density from BC₃ nanotube surface will be increased upon both atomic and molecular fluorine adsorptions due to work function decrement.     

Scytonin,a novel cyanobacterial photoprotective pigment: calculations of Raman spectroscopic biosignatures

Experimental studies have shown that chemical vapor decomposition method by using C₂H₂/C₂H₄ as carbon source could dramatically decrease the defects in prepared CNT. However, the inherent mechanism with regards to reduction of defects is quite unclear. In the present paper, density functional theory is used to study the healing process of CNT with divacancy defect by C₂H₂/C₂H₄ molecule. The healing processes undergo three evolution steps: (i) the chemisorption of the first C₂H₂/C₂H₄ molecule on defective CNT; (ii) the insertion of C atoms from C₂H₂/C₂H₄ molecule into defective CNT; (iii) the removal of the H atoms on CNT, forming perfect CNT. The estimated adsorption energy barrier of C₂H₂/C₂H₄ molecules on defective CNT is within the range from 1.10 to 1.63 eV, and the eventual formation of CNT is strongly exothermic (4.40/4.54 eV in (8, 0) CNT). In light of the unique conditions of CNT synthesis, i.e., high temperature in a closed container, such healing processes could most likely take place. Therefore, we propose that during CNT synthesis procedures, both C₂H₂ and C₂H₄ could act as a carbon source and the defect healer.

Carbon nanotube functionalization with carboxylic derivatives: a DFT study

Chemical functionalization of a single-walled carbon nanotube (CNT) with different carboxylic derivatives including –COOX (X?=?H, CH₃, CH₂NH₂, CH₃Ph, CH₂NO₂, and CH₂CN) has been theoretically investigated in terms of geometric, energetic, and electronic properties. Reaction energies have been calculated to be in the range of ?0.23 to ?7.07 eV. The results reveal that the reaction energy is increased by increasing the electron withdrawing character of the functional groups so that the relative magnitude order is ?CH₂NO₂?>?CH₂CN?>?H?>?CH₂Ph?>?CH₃?>?CH₂NH₂. The chemical functionalization leads to an increase in HOMO/LUMO energy gap of CNT by about 0.32 to 0.35 eV (except for ?H). LUMO, HOMO, and Fermi level of the CNT are shifted to lower energies especially in the case of ?CH₂NO₂ and _?CH₂CN functional groups. Therefore, it leads to an increment in work function of the tube, impeding the field electron emission.     

DFT studies of hydrocarbon combustion on metal surfaces

Catalytic combustion of hydrocarbons is an important technology to produce energy. Compared to conventional flame combustion, the catalyst enables this process to operate at lower temperatures; hence, reducing the energy required for efficient combustion. The reaction and activation energies of direct combustion of hydrocarbons (CH?→?C?+?H) on a series of metal surfaces were investigated using density functional theory (DFT). The data obtained for the Ag, Au, Al, Cu, Rh, Pt, and Pd surfaces were used to investigate the validity of the Brønsted-Evans-Polanyi (BEP) and transition state scaling (TSS) relations for this reaction on these surfaces. These relations were found to be valid (R²?=?0.94 for the BEP correlation and R²?=?1.0 for the TSS correlation) and were therefore used to estimate the energetics of the combustion reaction on Ni, Co, and Fe surfaces. It was found that the estimated transition state and activation energies (E_TS?=??69.70 eV and E_a?=?1.20 eV for Ni, E_TS?=??87.93 eV and E_a?=?1.08 eV for Co and E_TS?=??92.45 eV and E_a?=?0.83 eV for Fe) are in agreement with those obtained by DFT calculations (E_TS?=??69.98 eV and E_a?=?1.23 eV for Ni, E_TS?=??87.88 eV and E_a?=?1.08 eV for Co and E_TS?=??92.57 eV and E_a?=?0.79 eV for Fe). Therefore, these relations can be used to predict energetics of this reaction on these surfaces without doing the time consuming transition state calculations. Also, the calculations show that the activation barrier for CH dissociation decreases in the order Ag ? Au ? Al ? Cu ? Pt ? Pd ? Ni?>?Co?>?Rh?>?Fe.     

A Gaussian-3 theoretical study of the alkylthio radicals and their anions: structures, thermochemistry, and electron affinities

The optimized geometries, electron affinities, and dissociation energies of the alkylthio radicals have been determined with the higher level of the Gaussian-3(G3) theory. The geometries are fully optimized and discussed. The reliable adiabatic electron affinities with ZPVE correction have been predicted to be 1.860 eV for the methylthio radical, 1.960 eV for the ethylthio radical, 1.980 and 2.074 eV for the two isomers (n-C₃H₇S and i-C₃H₇S) of the propylthio radical, 1.991, 2.133 and 2.013 eV for the three isomers (n-C₄H₉S, t-C₄H₉S, and i-C₄H₉S) of the butylthio radical, and 1.999, 2.147, 2.164, and 2.059 eV for the four isomers (n-C₅H₁₁S, b-C₅H₁₁S, c-C₅H₁₁S, and d-C₅H₁₁S) of the pentylthio radical, respectively. These corrected EA_ad values for the alkylthio radicals are in good agreement with available experiments, and the average absolute error of the G3 method is 0.041 eV. The dissociation energies of S atom from neutral C_nH_2n+1S (n?=?1–5) and S^- from corresponding anions C_nH_2n+1S^- species have also been estimated respectively to examine their relative stabilities.     

Density functional studies of the stepwise substitution of pyridine,pyridazine, pyrimidine,pyrazine, and 1,3,5-triazine with BCO

Ozone (O₃) adsorption on pristine Stone–Wales (SW) defective BC₃ graphene-like sheets was investigated using density functional calculations. It was found that O₃ is weakly adsorbed on the pristine sheet. Two types of SW-defective sheets were studied, SW-CC and SW-BC, in which a defect is formed by rotating a C–C or B–N bond, respectively. O₃ molecules were found to be more reactive on SW-BC defective sheets. It was predicted that O₃ molecules are reduced to O₂ molecules on SW-BC sheets, overcoming an energy barrier of 34.2 kcal/mol^?1 at the B3LYP level of theory and 27.2 kcal/mol^?1 at the BP98 level of theory. Therefore, SW-BC sheets could potentially be employed as a metal-free catalyst for O₃ reduction. The HOMO–LUMO gap of a SW-BC sheet decreases from 2.16 to 1.21 eV after O₃ dissociation on its surface in the most stable state.     

The effect of C-vacancy on hydrogen storage and characterization of H2 modes on Ti functionalized C60 fullerene A first principles study

Density functional theory calculations were performed to examine the effect of a C vacancy on the physisorption of H₂ onto Ti-functionalized C₆₀ fullerene when H₂ is oriented along the x-, y-, and z-axes of the fullerene. The effect of the C vacancy on the physisorption modes of H₂ was investigated as a function of H₂ binding energy within the energy window (?0.2 to ?0.6 eV) targeted by the Department of Energy (DOE), and as functions of a variety of other physicochemical properties. The results indicate that the preferential orientations of H₂ in the defect-free (i.e., no C vacancy) C₆₀TiH₂ complex are along the x- and y-axes of C₆₀ (with adsorption energies of ?0.23 and ?0.21 eV, respectively), making these orientations the most suitable ones for hydrogen storage, in contrast to the results obtained for defect-containing fullerenes. The defect-containing (i.e., containing a C vacancy) C₅₉TiH₂ complex do not exhibit adsorption energies within the targeted energy range. Charge transfer occurs from Ti 3d to C 2p of the fullerene. The binding of H₂ is dominated by the pairwise support–metal interaction energy E(i)^Cn...Ti, and the role of the fullerene is not restricted to supporting the metal. The C vacancy enhances the adsorption energy of Ti, in contrast to that of H₂. A significant reduction in the energy gap of the pristine C₆₀ fullerene is observed when TiH₂ is adsorbed by it. While the C _n fullerene readily participates in nucleophilic processes, the adjacent TiH₂ fragment is available for electrophilic processes.

Effect of pristine and functionalized single- and multi-walled carbon nanotubes on CO<Subscript>2</Subscript> separation of mixed matrix membranes based on polymers of intrinsic microporosity (PIM-1): a molecular dynamics simulation study

Molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations were conducted to investigate the transport properties of carbon dioxide, methane, nitrogen, and oxygen through pure and mixed matrix membranes (MMMs) based on polymers of intrinsic microporosity (PIM-1). For this purpose, first, 0.5 to 3 wt% of pristine single-walled carbon nanotube (p-SWCNT) and multi-walled carbon nanotube (p-MWCNT) were embedded into the pure PIM-1, and then for better dispersion of CNT particles into the polymer matrix and to improve the performance of the resulting MMMs, polyethylene glycol (PEG) functionalized SWCNT and MWCNT (f-SWCNT and f-MWCNT, respectively) were loaded. The characterization of the obtained MMMs was carried out by using density, glass transition temperature, X-ray pattern, and fractional free volume calculations. Comparing the obtained results with the available reported experimental data, indicate the authenticity of the applied simulation approach. The simulation results exhibit that the pristine and PEG-functionalized CNT particles improve the transport properties such as diffusivity, solubility, and permeability of the PIM-1 membranes, without sacrificing their selectivity. Also, the MMMs incorporated with 2 wt% of the functionalized CNT particles indicate better performance for the CO₂ separation from other gases. According to the calculated results, the highest permeability and diffusivity for CO₂ are observed in the [PIM-1/f-SWCNT] MMM among the other membranes which represent that the loading of the f-SWCNTs can enhance the CO₂ separation performance of PIM-1 more than other CNTs studied in this work.     

A large gap opening of graphene induced by the adsorption of CO on the Al-doped site

We investigated CO adsorption on the pristine, Stone-Wales (SW) defected, Al- and Si- doped graphenes by using density functional calculations in terms of geometric, energetic and electronic properties. It was found that CO molecule is weakly adsorbed on the pristine and SW defected graphenes and their electronic properties were slightly changed. The Al- and Si- doped graphenes show high reactivity toward CO, so calculated adoption energies are about ?11.40 and ?13.75 kcal mol^?1 in the most favorable states. It was found that, among all the structures, the electronic properties of Al-doped graphene are strongly sensitive to the presence of CO molecule. We demonstrate the existence of a large E_g opening of 0.87 eV in graphene which is induced by Al-doping and CO adsorption.     

DFT studies of acrolein molecule adsorption on pristine and Al- doped graphenes

The ability of pristine graphene (PG) and Al-doped graphene (AlG) to detect toxic acrolein (C₃H₄O) was investigated by using density functional calculations. It was found that C₃H₄O molecule can be adsorbed on the PG and AlG with adsorption energies about ?50.43 and – v30.92 kcal mol^?1 corresponding to the most stable configurations, respectively. Despite the fact that interaction of C₃H₄O has no obvious effects on the of electronic properties of PG, the interaction between C₃H₄O and AlG can induce significant changes in the HOMO/LUMO energy gap of the sheet, altering its electrical conductivity which is beneficial to sensor designing. Thus, the AlG may be sensitive in the presence of C₃H₄O molecule and might be used in its sensor devices. Also, applying an external electric filed in an appropriate orientation (almost stronger than 0.01 a.u.) can energetically facilitate the adsorption of C₃H₄O molecule on the AlG.     

Hollow fiber membrane based H2 diffusion for efficient in situ biogas upgrading in an anaerobic reactor

Bubbleless gas transfer through a hollow fiber membrane (HFM) module was used to supply H₂ to an anaerobic reactor for in situ biogas upgrading, and it creates a novel system that could achieve a CH₄ content higher than 90 % in the biogas. The increase of CH₄ content and pH, and the decrease of bicarbonate concentration were related with the increase of the H₂ flow rate. The CH₄ content increased from 78.4 % to 90.2 % with the increase of the H₂ flow rate from 930 to 1,440 ml/(l?day), while the pH in the reactor remained below 8.0. An even higher CH₄ content (96.1 %) was achieved when the H₂ flow rate was increased to 1,760 ml/(l?day); however, the pH increased to around 8.3 due to bicarbonate consumption which hampered the anaerobic process. The biofilm formed on the HFM was found not to be beneficial for the process since it increased the resistance of H₂ diffusion to the liquid. The study also demonstrated that the biofilm formed on the membrane only contributed 22–36 % to the H₂ consumption, while most of the H₂ was consumed by the microorganisms in the liquid phase.     

A DFT study on the sensing behavior of a BC2N nanotube toward formaldehyde

We investigated the viability of using a BC₂N nanotube to detect formaldehyde (H₂CO) molecule by means of B3LYP and M06 density functionals. The results indicate that the molecule is weakly adsorbed on the intrinsic BC₂N nanotube releasing energy of 0.8 kcal mol^-1 (at B3LYP/6-31G(d)) without significant effect on the HOMO-LUMO energy gap and electrical conductivity of the tube. Thus, H₂CO cannot be detected using this intrinsic nanotube. To overcome this problem, a carbon atom of the tube wall was substituted by a Si atom. It was demonstrated that the Si-doped tube cannot only strongly adsorb the H₂CO molecule, but also may effectively detect its presence because of the increase in the electric conductivity of the tube.     

Interaction of a Ti-doped semi-fullerene (TiC<Subscript>30</Subscript>) with molecules of CO and CO<Subscript>2</Subscript>

Using density functional theory (DFT) and molecular dynamics (MD), we studied the interaction of a titanium atom with a half of a C₆₀ fullerene (i.e., C₃₀), formed from the corannulene structure with a pentagonal base. We considered atmospheric pressure and 300 K. We found that the most stable adsorption of the titanium atom on C₃₀ occurs in the concave surface of the molecule. Afterward, we investigated the interaction of the system C₃₀-titanium with carbon monoxide and carbon dioxide molecules, respectively. We found that each of these molecules is chemisorbed, with no dissociation. The value of the adsorption energy for the carbon monoxide molecule varies from ?0.897 to ?1.673 eV, and for the carbon dioxide molecule, it is between ?1.065 and ?1.274 eV. These values depend on the initial orientation of these molecules with respect to TiC₃₀.

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Graphical Abstract The TiC₃₀ system chemisorbs CO or CO₂?with no dissociation at atmospheric pressure and 300K

     

Theoretical study on the functionalization of BC2N nanotube with amino groups

Using density functional theory calculations, we investigated properties of a functionalized BC₂N nanotube with NH₃ and five other NH₂-X molecules in which one of the hydrogen atoms of NH₃ is substituted by X = ?CH₃, ?CH₂CH₃, ?COOH, ?CH₂COOH and ?CH₂CN functional groups. It was found that NH₃ can be preferentially adsorbed on top of the boron atom, with adsorption energy of ?12.0 kcal mol^?1. The trend of adsorption-energy change can be correlated with the trend of relative electron-withdrawing or -donating capability of the functional groups. The adsorption energies are calculated to be in the range of ?1.8 to ?14.2 kcal mol^?1, and their relative magnitude order is found as follows: H₂N(CH₂CH₃) > H₂N(CH₃) > NH₃ > H₂N(CH₂COOH) > H₂N(CH₂CN) > H₂N(COOH). Overall, the functionalization of BC₂N nanotube with the amino groups results in little change in its electronic properties. The preservation of electronic properties of BC₂N coupled with the enhancement of solubility renders their chemical modification with either NH₃ or amino functional groups to be a way for the purification of BC₂N nanotubes.     

Sensing behavior of Al-rich AlN nanotube toward hydrogen cyanide

In order to explore a sensor for detection of toxic hydrogen cyanide (HCN) molecules, interaction of pristine and defected Al-rich aluminum nitride nanotubes (AlNNT) with a HCN molecule has been investigated using density functional theory calculations in terms of energetic, geometric, and electronic properties. It has been found that unlike the pristine AlNNT, the Al-rich AlNNT can effectively interact with the HCN molecule so that its conductivity changes upon the exposure to this molecule. The adsorption energies of HCN on the pristine and defected AlNNTs have been calculated to be in the range of ?0.16 to ?0.62 eV and ?1.75 to ?2.21 eV, respectively. We believe that creating Al-rich defects may be a good strategy for improving the sensitivity of these tubes toward HCN molecules, which cannot be trapped and detected by the pristine AlNNT.     

Adsorption and decomposition mechanism of hexogen (RDX) on Al(111) surface by periodic DFT calculations

The adsorption of hexogen (RDX) molecule on the Al(111) surface was investigated by the generalized gradient approximation (GGA) of density functional theory (DFT). The calculations employ a supercell (4×4×3) slab model and three-dimensional periodic boundary conditions. The strong attractive forces between RDX molecule and aluminum atoms induce the N?O and N?N bond breaking of the RDX. Subsequently, the dissociated oxygen atoms, NO₂ group and radical fragment of RDX oxidize the Al surface. The largest adsorption energy is ?835.7 kJ mol^–1. We also investigated the adsorption and decomposition mechanism of RDX molecule on the Al(111) surface. The activation energy for the dissociation steps of V4 configuration is as large as 353.1 kJ mol^–1, while activation energies of other configurations are much smaller, in the range of 70.5–202.9 kJ mol^–1. The N?O is even easier than the N?NO₂ bond to decompose on the Al(111) surface.