A theoretical study of the ability of 2D monolayer Au (111) to activate gas molecules

The adsorption and activation of gas molecules are investigated substantially in solid-gas heterogeneous catalysis. Here we investigated the interaction between gas molecules and unique two-dimensional monolayer Au (111) structure using density functional theory. It is found that CO₂, H₂O, N₂ and CH₄ molecules are weakly adsorbed on the surface with the adsorption energies between ?0.150 and ?0.250 eV due to van der Waals interaction. While CO, NO, NO₂, and NH₃ molecules are adsorbed more stably with the adsorption energies between ?0.300 and ?0.470 eV. Especially, the bond length of CO is stretched by 0.038 Å and the bond angle of NO₂ is obviously enlarged by 10.460°. The activation originates from the rearrangement of molecule orbitals and the orbitals hybridization between the partial orbitals of gas molecules and Au-5d orbitals. The fundamental analyses of adsorption mechanism and electronic properties may provide guidance for the applications of two-dimensional monolayer metal catalysis.PACSnumbers 73.22.-f, 73.61.-r     

Hydrogen evolution and oxygen evolution reactions of pristine and alkali metal doped SnSe2 monolayer

Many transition metal di-selenides such as MoSe₂ and WSe₂ show good catalytic activity on their edges with limited active orientations. These metal di-selenides are actively being used as target material for increasing the number of electrocatalytic active sites and in turn to improve the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities by increasing the ratio of edges to the basal plane. In present work, we have studied the activity of pristine and alkali atoms (Na, K and Ca) doped-SnSe₂ for HER and OER catalyst. The state-of-art density functional theory (DFT) based computations are performed for estimating the catalytic activity of the pristine and doped SnSe₂ by means of evaluating the adsorption and Gibbs free energies subjected to hydrogen and oxygen adsorption. Further, to get better prediction of adsorption energy on the individual catalytic surface, we have included the dispersion correction term to exchange-correlation functional. Results show that the pristine SnSe₂ is not a good HER catalyst when hydrogen is adsorbed on its basal plane. However, edge-sites show the good hydrogen adsorption and indicates that the edges of SnSe₂ are the most preferential site for hydrogen adsorption. As far as the catalytic activity of SnSe₂ with dopants is concerned, the Na-doped SnSe₂ among all shows the best catalytic activity over its edge-site; whereas K and Ca doped SnSe₂ show basal plane as preferred catalytic site. It is interesting to note that the disadvantage of low catalytic activity on basal plane of SnSe₂ can be improved by selective doping of alkali metals.     

Origin of enhanced corrosion resistance of Ag and Au doped anatase TiO2

Anatase TiO₂ is a promising corrosion resistance material due to the excellent corrosion resistance, order array structure and good adsorption stability. However, the fundamental mechanism of interaction between TiO₂ and sulfur (S) is unclear. In particular, the nature of corrosion resistance of alloy-doped TiO₂ is not understood. By using the first-principles calculations, we study the sulfuretted mechanism of TiO₂ and explore the influence of Ag and Au on the corrosion resistance of TiO₂. The results show that sulfur is favorable to occupy the tetrahedral interstitial site because sulfur occupied this position can improve the localized hybridization between S-3p state and O-2p state, which forms the SO bond. The calculated bond length of SO bond is 1.967 Å. In particular, Ag dopant and Au dopant enhance the localized hybridization between sulfur and oxygen. The calculated bond length of SO bond for Ag dopant (1.483 Å) and Au dopant (1.506 Å) is shorter than that of S-doped TiO₂. As a result, those alloying elements improve the corrosion resistance of TiO₂. Compared to Au dopant, Ag dopant improves the corrosion resistance of TiO₂ because the bond strength of SO bond for Ag dopant is stronger than that of Au dopant.     

Ultra-low voltage adenine based gas sensor to detect H2 and NH3 at room temperature: First-principles paradigm

The molecular system-level detection of H₂ and NH₃ gas using an electrically doped Adenine bio-molecular gas sensor has been proposed and investigated using Density Functional Theory (DFT) combined with Non-Equilibrium Green's Function (NEGF) formalisms. First-principles calculations were applied and the structures and electronic properties of the Adenine gas sensor have calculated. This sensor reveals that the current-voltage response and conductivity of the bio-molecules increased evidently after the adsorption of these gas molecules. The Adenine sensor offers approximately 1800 times and 3300 times better current response during H₂ and NH₃ adsorption respectively. The significant gap between Highest Occupied Molecular Orbital (HOMO) and Lowest Un-occupied Molecular Orbital (LUMO) indicates the system's thermodynamic stability. Therefore, we hope that the Adenine monolayer could be a room temperature H₂ and NH₃ sensor with high selectivity and sensitivity and fast response and recovery time. Therefore, we hope that the Adenine monolayer will be a good candidate forH₂ and NH₃ work-function-type gas sensors.     

Hydrogen adsorption and diffusion on doped Zr(0001) surfaces: A first-principles study

The hydrogen adsorption and diffusion behaviors on the clean and a series of element doped Zr(0001) surfaces are studied through first-principles calculations. Among the studied doping elements, Cu, Co, Y, and Mg prefer to substitute Zr on the topmost surface layer, Al, Pd, Ir, and Si are favored from topmost layer to several surface layers down, while Mo is not favored. Independent of the substitution energies, Mo, Co, and Ir induce a symmetry-breaking local distortion surface structure. Based on the obtained geometries, it is found that most dopants promote the hydrogen adsorption on their next nearest neighbor sites but hinder it on the nearest neighbor sites. Most of the dopants also promote both the hydrogen diffusion on the surface plane and the hydrogen penetration into the subsurface layers. The results indicate that element doping may facilitate the hydride nucleation in Zr alloys.     

Strain effect on structural and dehydrogenation properties of MgH2 hydride from first-principles calculations

The structure, stability, dehydrogenation thermodynamic and kinetic properties of MgH₂ hydride under different biaxial strain conditions were investigated by using first-principles calculations based on the density functional theory (DFT). The results show that either biaxial tensile or compressive strain is likely to cause the structural deformation of MgH₂ crystal, and its lattice distortion becomes severe with increasing magnitude of strain. Due to the contribution of strain energy, the biaxial strain not only weakens the structural stability of MgH₂, but also lowers its hydrogen desorption enthalpy and dehydrogenation temperature. Furthermore, the diffusion activation energy of hydrogen atom in MgH₂ host is also decreased, which results in a remarkable improvement of dehydrogenation properties. Noticeably, the effect of tensile strain in improving dehydrogenation thermodynamics is relatively superior to that of compressive one, while the role of the latter in enhancing dehydrogenation kinetics is relatively stronger than that of the former. Further analysis of electronic structures suggests the strain-induced changes in structural and dehydrogenation properties of MgH₂ are closely associated with the value of total densities of states at the Fermi level as well as the bonding electrons number below Fermi level. These results provide an insight for developing better MgH₂-based nanocomposite hydrogen storage materials by introducing suitable interface misfit strain.     

The effect of functional groups (O,F, or OH) on reversible hydrogen storage properties of Ti2X (X=C or N) monolayer

The effect of functional groups (O, F, or OH) on the hydrogen storage properties of Ti₂X (X = C or N) monolayer was systematically investigated by first-principles calculations. The results show that the reversible hydrogen storage capacity of Ti₂X(OH)₂ monolayer is approximately 2.7 wt%, which is larger than that of Ti₂XO₂ and Ti₂XF₂ monolayers. The binding energy of the OH group at the F site is stronger than H atom. Thus, H₂ molecules will not be dissociated on Ti₂X(OH)₂ monolayer. At this time, the loss of 1.8 wt% hydrogen storage capacity is not produced in Ti₂X(OH)₂ monolayer. Furthermore, the PDOS, the population analysis, and the electron density difference explore that electron transfer appears between Ti and the second layer H₂ molecules on Ti₂X(OH)₂ monolayer, and a Dewar-Kubas interaction lies between second layer H₂ molecules and Ti₂X(OH)₂ monolayer. For Ti₂X(OH)₂ monolayer, the molecular dynamic simulation indicates that the H₂ molecules by Dewar-Kubas interaction sable adsorption at 300 K, and desorption at 400 K. Therefore, Ti₂X(OH)₂ is appropriate for reversible hydrogen sorbent storage materials under ambient conditions.     

Synthesis and characterization of TiO2 doped polyaniline composites for hydrogen gas sensing

The Polyaniline (PANI) and Titanium dioxide (TiO₂)/PANI composite thin film based chemiresistor type gas sensors for hydrogen (H₂) gas sensing application are presented in this paper. Pure PANI and TiO₂/PANI composites with different wt% of TiO₂ were synthesized by chemical oxidative polymerization of aniline using ammonium persulfate in acidic medium at 0-5 °C. Thin films of PANI and TiO₂/PANI composites were deposited on copper (Cu) interdigited electrodes (IDE) by spin coating method to prepare the chemiresistor sensor. Finally, the response of these chemiresistor sensors for H₂ gas was evaluated by monitoring the change in electrical resistance at room temperature. It was observed that the TiO₂/PANI composite thin film based chemiresistor sensors show a higher response as compared to pure PANI sensor. The structural and optical properties of these composite films have been characterized by X-ray diffraction (XRD) and UV-Visible (UV-Vis) spectroscopy respectively. Morphological and structural properties of these composites have also been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) respectively.     

Hydrogen adsorption on and diffusion through MoS2 monolayer: First-principles study

We investigate the hydrogen adsorption on and diffusion through the MoS₂ monolayer based on density-functional theory. We show that the hydrogen atom prefers to bond to the S atom at the monolayer, leading to enhanced conductivity. The hydrogen atom can also adsorb at the middle of the hexagon ring by overcoming an energy barrier of 0.57 eV at a strain of 8%. Also, we show that the MoS₂ monolayer is flexible and any mechanical deformation of the monolayer is reversible because the extension of the Mo–S bond is much smaller than the applied strain. The monolayer can block the diffusion of hydrogen molecule from one side to the other due to a high energy barrier (6.56 eV). However, the barrier can be reduced to 1.38 eV at a strain of 30% and even totally removed by creating S vacancies and applying a strain of 15%. The MoS₂ monolayer may find applications in sensors to detect hydrogen, and as mechanical valve to control the concentration of hydrogen gas.     

Surface and interface properties of monolayer graphene on hydrophobic and hydrophilic ultrananocrystalline diamond structures for hydrogen sensing applications

Here, for the first time, we synthesize hybrid hydrophilic and hydrophobic nanocarbon materials with reliable and stable gas sensing performance. The hybrid monolayer graphene (Gr)–nitrogen and argon (N₂ and Ar) gas incorporated ultra-nanocrystalline diamond (Gr/N₂@UNCD and Gr/Ar@UNCD) structures were synthesized using a microwave plasma enhanced chemical vapor deposition (MPECVD) method. The presented nanohybrid combinations have a unique morphology with diamond defects (sp³) covered by a graphene sheet (sp²). Sample sensors with metal electrodes were fabricated to study the H₂ gas sensing properties of the material. Thus, the as-fabricated Gr/N₂@UNCD exhibited higher sensor response (14.6%) than those of the as-fabricated Gr, N-UNCD and Gr/Ar@UNCD (3.6, 1.07 and 11.2%) based devices. The Gr/N₂@UNCD nanohybrid based sensor showed outstanding repeatability, selectivity and stability over ～56 days. The substantial improvement in the H₂ sensing performance of the as-fabricated Gr/N₂@UNCD nanohybrid based sensor was attributed to the modifications in surface morphology and resistance. The partial-hydrophobic surface of Gr/N₂@UNCD alters the beneficial resistivity and improved absorption, which assists in the efficient transport of electrons and H₂ gas molecules. The hybrid nanostructure of Gr-N₂@UNCD exhibits several unique properties that paves the way to future opportunities for advanced gas sensor fabrication.     

Structural,optical, and H2 gas sensing analyses of Cr doped CuO thin films grown by ultrasonic spray pyrolysis

Here, a specific metal oxide (CuO) and its impurity (Cr) added composites were grown onto glass substrates as nanostructured thin films by executing ultrasonic spray pyrolysis method. The effects of the varied Cr dopant concentration on the morphological, structural, optical and H₂ gas sensor properties of the synthesized CuO thin films were determined by conducting scanning electron microscopy, X-ray Diffraction, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, ultraviolet–visible spectroscopy, and gas detection analyses. The X-Ray Diffraction analysis revealed the presence of CuO crystals with predominant (111) plane and it changed to (002) orientation for the doped samples, where crystallite sizes varied between 32 and 46 nm. The structural studies disclosed that the crystalline structure modified due to the added impurities. The scanning electron microscopy observations unveiled polyhedron-like shape formations of the synthesized nanostructures which also showed clear indications of changed morphology due to the impacts of different Cr doping percentages. Besides, the presence of copper, oxygen, and chromium was confirmed by EDX elemental analysis as well as X-ray photoelectron spectroscopy. The optical examination concluded that absorbance values followed a random trend with respect to the increased impurity contents while bandgap decreased with the increase of doping concentration. And, it was also noted that the luminescent emission peaks decreased in the photoluminescence spectroscopy as a result of introduced impurity levels. Finally, H₂ responsivity was detected for the grown films and found out that the impurity doping notably increased the sensitivity of the gas sensor based on the prepared CuO nanostructures.     

High ionic conductivity in CeO2 SOFC solid electrolytes; effect of Dy doping on their electrical properties

Ionic conductors composed of lanthanide-doped ceria with general formula Dy_yCe_1-yO_2-δ (y = 0.05, 0.1 and 0.15) were synthesized by mechanochemistry (mechanical milling), and their electrical properties analyzed to be used as solid electrolytes in low-temperature SOFC. Starting oxide reagents were milled at different times in a planetary mill and the evolution of their structures and phases with milling time and temperature (up to 1500 °C) was followed by XRD. Just milled powders were also uniaxially pressed and sintered at different temperatures (1200, 1350 and 1500 °C), and analyzed by FE-SEM, to explore their morphologies as a function of temperature and Dy content. The electrical properties of these materials and undoped commercial CeO₂ were analyzed by impedance spectroscopy at different temperatures (200–650 °C) and frequencies (100 Hz - 1 MHz). Results showed that mechanochemistry is a suitable method to obtain the Dy_yCe_1-yO_2-δ systems after 20 h of milling, since XRD patterns of these milled powders reveal the formation of fluorite-type cubic solid solutions for all studied compositions. Increasing of temperature generates a higher crystallinity in these materials while the absence of phase transitions in them is corroborated at 1200 °C. Analysis of electrical properties of samples sintered a 1200 °C corroborates the viability of these systems to be used as solid electrolytes in the SOFC technology, being that high dc conductivities (σ_dc) were obtained for all doped samples, especially for the composition Dy_0.1Ce_0·9O_2-δ, which showed a σ_dc = 1 × 10^−1.91 S cm⁻¹ at 650 °C. This value represents an increase of almost three orders of magnitude for this composition with respect to the undoped CeO₂ sample (y = 0, σ_dc = 1 × 10 ^−4.83 Scm⁻¹).     

First-principles study on the dehydrogenation properties and mechanism of Al-doped Mg2NiH4

Mg₂NiH₄, with fast sorption kinetics, is considered to be a promising hydrogen storage material. However, its hydrogen desorption enthalpy is too high for practical applications. In this paper, first-principles calculations based on density functional theory (DFT) were performed to systematically study the effects of Al doping on dehydrogenation properties of Mg₂NiH₄, and the underlying dehydrogenation mechanism was investigated. The energetic calculations reveal that partial component substitution of Mg by Al results in a stabilization of the alloy Mg₂Ni and a destabilization of the hydride Mg₂NiH₄, which significantly alters the hydrogen desorption enthalpy ΔH_des for the reaction Mg₂NiH₄ → Mg₂Ni + 2H₂. A desirable enthalpy value of ∼0.4 eV/H₂ for application can be obtained for a doping level of x ≥ 0.35 in Mg_2−xAl_xNi alloy. The stability calculations by considering possible decompositions indicate that the Al-doped Mg₂Ni and Mg₂NiH₄ exhibit thermodynamically unstable with respect to phase segregation, which explains well the experimental results that these doped materials are multiphase systems. The dehydrogenation reaction of Al-doped Mg₂NiH₄ is energetically favorable to perform from a metastable hydrogenated state to a multiphase dehydrogenated state composed of Mg₂Ni and Mg₃AlNi₂ as well as NiAl intermetallics. Further analysis of density of states (DOS) suggests the improving of dehydrogenation properties of Al-doped Mg₂NiH₄ can be attributed to the weakened Mg-Ni and Ni-H interactions and the decreasing bonding electrons number below Fermi level. The mechanistic understanding gained from this study can be applied to the selection and optimization of dopants for designing better hydrogen storage materials.     

Room temperature hydrogen gas sensing properties of mono dispersed platinum nanoparticles on graphene-like carbon-wrapped carbon nanotubes

We present platinum nanoparticles dispersed wrinkled graphene-like carbon-wrapped carbon nanotubes (Pt/GCNTs) as a room temperature chemiresistive hydrogen gas sensor. Pt nanoparticles are decorated over GCNTs surface using poly (sodium 4-styrene sulfonate) (PSS) functionalization, followed by ethylene glycol reduction method. The highly defective wrinkled graphene-like surface of GCNTs provides large surface area and PSS functionalization provides stable immobilization of mono dispersed Pt nanoparticles on the carbon surface. A simple and inexpensive drop cast technique is used to fabricate the thick film sensor of the material. Hydrogen resistive gas sensing properties of Pt/GCNTs are studied at different gas concentrations, temperatures and Pt wt. % loadings. Pt/GCNTs sensor shows optimal sensitivity at room temperature with stable and reproducible response towards hydrogen. The sensor with 2 wt. % of Pt showed maximum sensitivity that is three fold higher than Pt decorated carbon nanotubes (Pt/CNTs) with the same Pt wt. % loading. The present study shows potential to explore novel H2 sensors^.     

Hydrogen gas sensing properties of microwave-assisted 2D Hybrid Pd/rGO: Effect of temperature,humidity and UV illumination

A novel microwave assisted two-dimensional (2D) hybrid material based on nanostructured reduced graphene oxide (rGO) doped with Pd nanoparticles (Pd/rGO) has been synthesised to investigate its hydrogen sensing performance at different operational conditions. The sensing performance has been evaluated at various operating temperatures (room temperature up to 120 °C), hydrogen concentrations (up to 1%), and relative humidity (up to ~44%). The material characterization of the hybrid Pd/rGO analysed by different techniques which confirms homogeneous distribution of Pd NPs (<35 nm) on the multi-layered porous structure of the rGO nanosheets (NSs) and forming the hybrid Pd/rGO NSs. Moreover, the fundamental hydrogen sensing mechanism as well as recovery enhancement by ultraviolet (UV) light are investigated. This work offers an environmentally friendly and energy-saving synthesis approach for hydrogen sensing with excellent control over experimental parameters which can lead to fabrication of a highly selective and sensitive hydrogen sensor.     

Electrical and hydrogen gas sensing properties of Co1-xZnxFe2O4 nanoparticles; effect of the sputtered palladium thin layer

Hydrogen gas sensing of Co_1-xZn_xFe₂O₄ (x = 0–0.45) nanoparticles synthesized by a simple hydrothermal process has been investigated. An n→p crossover in the electrical conductivity toward hydrogen gas was observed. However, no such charge carrier reversal is noticed at higher x values. In both cases, the related mechanisms are proposed. It has been found that reversal is temperature and doping ratio dependent. In this regard, the more compatible and realistic model is presented which explains the nature of our observations. By analyzing the adsorption kinetics of the surface, it is identified that at a higher percentage of Zn (x = 0.45) the sensor response deviates from the Freundlich isotherm and falls under the category of the Langmuir adsorption model toward H₂ gas exposure. These strong correlations between the results of gas sensing measurements and those calculated based on the DC electric resistivity would pave the way for further investigation of the gas sensors from a fundamental point of view. Deposition of Palladium nano-structures (possibly island-like) on the surface of the CoFe₂O₄ sensor appeared to be effective in speeding up the response time and increasing the sensitivity. The remarkable response time, as low as 3 s, is obtained after modifying the sensor surface with the palladium deposition.     

Effect of under nitrogen annealing on photo-electrochemical characteristics of films deposited from authentic Cu2SnSe3 sources by thermal vacuum under argon gas condensation

This communication describes how annealing under nitrogen affects photo-electrochemical characteristics of films deposited from authentic Cu₂SnSe₃ sources by vacuum evaporation under argon gas (low flow rate 5 cm³/min) using substrate 300 °C. Annealing lowered the photoresponse of the deposited film, by affecting crystallite structure, morphology, composition and pores in the films. Annealing at temperatures in the range 150–350 °C improved crystallinity of the film but lead to pore formation between adjacent, which lowered photoresponse by increased resistance across the electrode/redox interface. Higher temperature (450 °C) annealing lead to SnO₂ formation, as an additional phase, at the expense of Cu₂SnS₃ decomposition. Porosity and mixed phases with SnO₂ presumably increased film internal resistance and resulted in poor charge transfer across the solid/redox couple interface. By affecting film characteristics, annealing lowered photoresponse for the deposited films.     

Influence of titanium and nickel dopants on the dehydrogenation properties of Mg(AlH4)2: Electronic structure mechanisms

The structures and dehydrogenation properties of pure and Ti/Ni-doped Mg(AlH₄)₂ were investigated using the first-principles calculations. The dopants mainly affect the geometric and electronic structures of their vicinal AlH₄ units. Ti and Ni dopants improve the dehydrogenation of Mg(AlH₄)₂ in different mechanisms. In the Ti-doped case, Ti prefers to occupy the 13-hedral interstice (Ti_iA) and substitute for the Al atom (Ti_Al), to form a high-coordination structure TiH_n (n = 6, 7). The Ti 3d electrons hybridize markedly with the H 1s electrons in Ti_Al and with the Al 3p electrons in Ti_iA, which weakens the Al–H bond of adjacent AlH₄ units and facilitates the hydrogen dissociation. A TiAl₃H₁₃ intermediate in Ti_iA is inferred as the precursor of Mg(AlH₄)₂ dehydrogenation. In contrast, Ni tends to occupy the octahedral interstice to form the NiH₄ tetrahedron. The tight bind of the Ni with its surrounding H atoms inhibits their dissociation though the nearby Al–H bond also becomes weak. Therefore, Ti is the better dopant candidate than Ni for improving the dehydrogenation properties of Mg(AlH₄)₂ because of its abundant activated hydrogen atoms and low hydrogen removal energy.     

Ag monolayer doped by Pt atom on WC (0001) surface: A good catalyst for H2 dissociation with high sulfur tolerance

H₂ dissociation barriers on Ag (111), Ag monolayer on WC (0001) (Ag_ML/WC), Ag monolayer doped by Pt atom on WC (0001) (Ag_MLPt-d/WC) surface are decreasing from 1.07 to 0.03 eV. The ab initio atomistic thermodynamic data shows that at typical planar SOFC operating temperatures of 923–1073 K, under 100000-ppm pH₂S/pH₂, Ag_MLPt-d/WC can be sulfur free clean surface. Therefore, compared with traditional Nickel/Yttria-stabilized zirconia (Ni/YSZ) Solid Oxide Fuel Cells (SOFCs) anode, Ag_MLPt-d/WC shows high activity towards H₂ dissociation and high tolerance of sulfur poisoning.     

Dual gas sensing properties of graphene-Pd/SnO2 composites for H2 and ethanol: Role of nanoparticles-graphene interface

In the present work, role of palladium (Pd) and tin oxide (SnO₂) nanoparticles (NPs) deposited on graphene has been investigated in terms of dual gas sensing characteristics of ethanol and H₂ between two temperatures. The incorporation of nanoparticles into graphene has been observed which results a large change in the sensing response towards these gases. It is investigated that, incorporation of isolated Pd NPs on the graphene facilitates the room temperature sensing of H₂ gas with fast response and recovery time whereas, isolated SnO₂ NPs on graphene enables the detection of ethanol at 200 °C. However, combination of isolated Pd and SnO₂ NPs on graphene shows improved sensitivity and good selectivity towards H₂ and ethanol, usually not observed in chemiresistive gas sensors. Catalytic PdH interaction and corresponding change in work function of nanoparticles on hydrogenation resulting in modifications in electronic exchange between Pd, SnO₂ and graphene are responsible for the observed behavior. These results are important for developing a new class of chemiresistive type gas sensor based on change in the electronic properties of the graphene and NPs interfaces.