Mo Surface Research Articles

Minimum Thickness of Continuous Cu Layers on Mo from Fluoride Free Electrolytes

Mo as a refractory metal has physical properties which are of interest for applications in CIGS photovoltaic cells fabrication and for next generation of liners for Cu interconnects[1]. Therefore it is of fundamental and practical interest to develop a process for Cu metallization of Mo surfaces where contiguity and high coverage is achieved at minimum thickness. However, one of the obstacles that prevent readily use of Mo as substrate for Cu electrodeposition is tendency of its surface to form a high quality oxide[2] ,[3].The surface energy of Mo-oxide is much lower than Cu which results in sporadic 3D nucleation and low adhesion of Cu deposit[2]. The presented work addresses this fundamental problem by combining the knowledge/breakthroughs demonstrated in the field of electro-polishing of refractory metals such as Nb[4] with common pulse electrodeposition practice[1] ,[5]. A design of deposition pulse reverse potential/current function for Cu is presented which converts a Mo-oxide/Cu to a Mo/Cu interface. Elaborate measurements of the thin film impedance during the Cu growth complemented with Pb UPD decoration studies and AFM surface morphology analysis are presented. These data indicate that Cu layers reach full contiguity and coverage of the Mo substrate at the thickness level of 3-5 nm. Considering that common Cu electrodeposition bath chemistry for interconnects contains additives whose incorporation makes Cu deposit somewhat incompatible with photovoltaic layers[1], this work presents the Cu solution chemistry which is additive free and can serve as a base for further improvement of the specific Cu deposition processes depending on its application.[1] . J. Bi et al, Journal of Power Sources, 326, (2011) 211.[2] . D. Mercier et al, Journal of the Electrochemical Society, 160 (2013) 3103.[3] . M. Pourbaix, Atlas of electrochemical equilibria in aqueous solutions. 2d English ed. 1974, Houston, Tex.: National Association of Corrosion Engineers.[4] . Journal of The Electrochemical Society, 160 (9) E94-E98 (2013).[5] . J.C. Puippe and F. Leaman, Theory and Practice of Pulse Plating, AESF, (1986).

Efficient electrocatalytic conversion of CO2 to ethanol enabled by imidazolium-functionalized ionomer confined molybdenum phosphide

An effective electrochemical carbon dioxide reduction reaction (eCO2RR) requires the discovery of a catalytic system that is highly active and selective for multi-carbon products together with superior CO2 diffusion at a catalyst layer to minimize the reduction barriers. Here, we found a catalytic system that uses molybdenum phosphide (MoP) nanoparticles covered by imidazolium-functionalized ionomer (Im) that promotes CO2 diffusion at the catalyst layer toward the catalyst surface, where CO2 is reduced to ethanol (C2H5OH). The electrochemical results with the MoP-Im co-catalyst show a C2H5OH production Faradaic efficiency and a cathodic energy efficiency of 77.4% and 63.3%, respectively, at a potential as low as − 200 mV vs. RHE. The electrochemical experiments along with our physicochemical characterizations indicate that the Im improves CO2 diffusion and balances water content resulting in a higher CO2-to-water ratio at the catalyst layer and fine-tunes the electronic properties of Mo atoms at the MoP surface. In-situ Raman spectroscopy reveals that a high number of adsorbed *CO intermediates on the surface and a higher binding strength of *CO intermediates on the Mo surface sites in the presence of imidazolium molecules are the main reasons for a superior C-C coupling and thereby the improved C2H5OH formation.

Breaking the Linear Relation in the Dissociation of Nitrogen on Iron Surfaces.

Current industrial ammonia synthesis depends on the Haber-Bosch process, in which the activity of the catalyst is limited by the Brønsted-Evans-Polanyi (BEP) principle and Fe is used as a commercial catalyst. Herein, we found that the dissociation barriers of N2 on Fe(111), Fe(211), Fe(110), and Fe(100) surfaces do not follow the widely accepted BEP principle. N2 dissociation on Fe(111) surface has the smallest adsorption energy and the lowest energetic barrier. Such an abnormal phenomenon can be attributed to charge transfer from Fe surfaces to the anti-bonding orbital (π*) of the absorbed N2 . More charges transferred from the Fe surface to π* of N2 leads to a weaker N≡N triple bond and a lower adsorption energy of N atoms. However, the hydrogenation of N atoms and desorption of NH3 on the four Fe surfaces follow the BEP principle. Therefore, Fe(111) is found to be the most active surface to promote ammonia synthesis, and such a conclusion is also applicable to Ni and Mo surfaces.

Electrodeposition assisted sol-gel process to prepare CZTS thin films

The traditional sol-gel process usually takes a long time to form a stable Cu2ZnSnS4 (CZTS) sol-gel solution, which increases the whole process time. In this paper, an electrodeposition assisted sol-gel process was employed to reduce the solution preparation time. Cu was electrodeposited on Mo surface and ZnSnS sol-gel was rotated on Cu surface to prepare CZTS films. The solution preparation time was reduced from 12h to 30min. The mechanism of CZTS phase formation with sol-gel method assisted by electrodeposition was studied. Cu3SnS4 phase was formed during the electrodeposition assisted sol-gel process, while Cu2SnS3 phase was formed when the CZTS precursor during sol-gel process. Cu content was rich at the beginning of the reaction to form Cu3SnS4 during the electrodeposited assisted sol-gel process. CZTS films with Cu/(Zn + Sn) of 0.84 and Zn/Sn of 1.03 were prepared by sol-gel method assisted by electrodeposition. Compared with CZTS films prepared by sol-gel method, CZTS films prepared by this method have larger and more uniform grains. It is also found that the thickness of MoS2 layer was thinner than the one prepared by sol-gel method.

Long term antifouling performance of superhydrophobic surfaces in seawater environment: Effect of substrate material, hierarchical surface feature and surface chemistry

Biofouling is a serious concern for the materials that are in contact with seawater or brackish water. Superhydrophobic (SHP) surfaces have the potential to lower fouling. Although good progress is made on the design of SHP surfaces, the long-term antifouling performance of SHP surfaces is still a major concern. We report the effect of substrate material, hierarchical surface features, chemistry, and the nature of bacteria on the long-term antifouling properties of superhydrophobic for the first time. Three different superhydrophobic coatings on aluminum, Cr-Mo steel, and Ti substrates were studied in a seawater bacterial consortium. Silane based SHP Al with a water contact angle (WCA) ~170 ± 1.5°, cerium myristate based SHP Cr-Mo of WCA ~ 162 ± 2.4° and stearic acid based SHP Ti of WCA ~168 ± 3.3° were prepared by dip coating, electrodeposition, and rapid breakdown anodization, respectively. After 120 h of exposure in seawater bacterial consortium, the cell viability reductions were three, two and one order in SHP Al, SHP Cr-Mo, and SHP Ti, respectively. The SHP Al samples possessed both micron-sized structures (~2 µm) along with nano-sized spherical particles (~ 57 nm). The quat silane-like structures gave better antibacterial resistance to SHP Al. The predominant bacteria present on bare Al,Ti, Cr-Mo and SHP Al, and Ti surfaces after exposure were Bacillus and Pseudomonas sp., whereas cocci bacteria was seen on the SHP Cr-Mo. After 240 h of exposure, a thicker biofilm was seen on the SHP Cr Mo surface due to the densely populated nanostructures of sizes comparable to cocci diameter, which was favorable for the cocci bacterial attachment. Though the average WCA followed the trend SHP Ti> SHP Al>SHP Cr Mo, the bacterial adhesion trend was SHP Ti> SHP Cr Mo> SHP Al, after 120 h of exposure. This suggests that wettability is not the only parameter determining the antifouling performance of an SHP coating but is also influenced by the surface charge density, surface roughness, stiffness, topography, and environmental variables such as fluid dynamics, bacterial motility, and size and morphology of bacteria. Both the surface morphology and the chemical nature were responsible for this reduced cell viability of the SHP Al surface. These findings not only provide new insights into the understanding of long-term biofouling of SHP surfaces but also would provide new directions in the design of robust SHP surfaces.

Dual sites modulating MoO2 nanospheres for synergistically enhanced electrocatalysis of water oxidation

Oxygen evolution reaction (OER) is of critical importance in the area of electrocatalysis as it is frequently involved in a diversity of electrochemical devices. Molybdenum-based materials, thanks to their metallic character induced high conductivity, have been widely studied as catalysts for a variety of reactions but few reports for oxygen evolution reaction (OER). Herein, we report a dual-sites modifying strategy to modulate electrocatalytic properties of metallic MoO2 nanospheres toward OER, which is implemented by a one-pot synthesis process to prepare nickel and iron co-doped molybdenum oxide (Mo0.9Ni0.05Fe0.05O2). The introduction of dual sites (i.e., Ni and Fe) on MoO2 nanospheres surface can tune the surface electronic structure and local-chemical environment with generation of high-valence Mo and hydroxyl-rich surface, synergistically contributing to the improved electrocatalytic performance in terms of stability and activity. Density functional theory (DFT) calculation indicates Fe-doping site favors for enhancing the turnover efficiency (TOF) while the Ni-doping site facilitates the first proton coupled electron transfer (PCET) upon electrocatalysis of OER. The Mo0.9Ni0.05Fe0.05O2 requires only 249 mV of overpotential to reach a current density of 10 mA cm−2 with excellent stability for 80 h. This study expands the research on molybdenum-based OER catalysts and gains insight into the origin of both the activity and stability of Ni/Fe-doping reactive sites.

DFT study of Sb layers on the Mo(112) surface

Relativistic DFT calculations, performed for Sb layers adsorbed on the Mo (112) surface, have revealed a strong interaction of Sb adatoms with the surface. At low coverages, the layer is predicted to form a c (2 × 2) structure, while for a complete Sb monolayer the most favorable structure is the p (1 × 1), which forms despite a minor one-dimensional compressing of the layer in the direction along the Mo (112) surface furrows. This structure is favorable also for complete bilayer and 3-layer films, so that very thin Sb layers reproduce the furrowed structure of the Mo (112) surface. The adsorbed Sb layers cause a significant redistribution of the densities of states, which, however, does not increase enough at the Fermi level to produce any substantial metallization of the film. The spin-orbit coupling causes only minor changes in the band structure, while Sb layer on one face of the Mo (112) slab destroys the inversion symmetry of the adsorption system thus leading to the appearance of the Rashba-type splitting of the bands.

Exfoliation methods for compositional and electronic characterization of interfacial Mo(Sex,Sy) in Cu(In,Ga)(Se,S)2 solar cells by X‐ray and UV photoelectron spectroscopy

Mo(SexSy) is a transition metal dichalcogenide typically applied as a back contact interlayer in Cu(In,Ga)(Se,S)2 (CIGSSe) solar cells. Band alignment at the buried Mo/CIGSSe junction mediated by Mo(SexSy) is important for current transport and enables quasi‐ohmic behavior between the CIGSSe absorber and the Mo back electrode. Furthermore, the S/(Se + S) ratio is a crucial parameter that determines the height of the valence band offset at the CIGSSe/Mo(SexSy) interface. Because the interlayer is formed during rapid thermal processing, an MoSe2 or MoS2 thin film grown on free substrate surfaces will not be representative for a realistic solar cell device. Thus, for fundamental thin‐film material analysis, as well as functional characterization and modeling, appropriate preparation and analytical techniques are required in order to prevent artifacts. In principal, the weak van der Waals forces between two‐dimensional stacked Mo(SexSy) sheets allow the implementation of exfoliation procedures to generate free Mo(SexSy) surfaces out of CIGSSe solar cell layer stacks. In this article, two different exfoliation‐based Mo(Sex,Sy) preparation methods are investigated and evaluated with respect to subsequent surface analytical characterization by X‐ray and ultraviolet photoelectron spectroscopy. A special focus is laid on an artifact‐free characterization of chemical and electronical properties of the exposed layers for a number of samples. In a first instance, the compositional Se/S and (Se + S)/Mo ratios at the surface are quantitatively analyzed on the basis of dedicated peak‐fitting routines. Artifacts from carbonaceous contamination due to different exfoliation glues can be prevented through a detailed comparative analysis of carbon 1s and KLL Auger peaks. Furthermore, a significant surface band bending is observed that can be reduced by low‐energy Ar ion in situ sputtering. A simple model for the sputter removal of a charged surface layer is presented, which allows to approximately calculate the absolute valence band maximum (VBM) positions required for band alignment and numerical device simulations. The presented exfoliation surface analysis methodology is important for the whole CIGS(Se) solar cell community and may be of general interest for emerging applications of further 2D transition metal dichalcogenides as well.

Screening LiMn2O4 Surface Modification Schemes under Theoretical Guidance

Mn dissolution is one of the most important factors for the failure of LiMn2O4 batteries. Doping has been widely adopted in the modification of LiMn2O4 cathodes; however, there is still a lack of theoretical guidance on screening the dopants. Here, through first-principles calculations, we systematically investigated the effects of all 3, 4d transition metals as well as Mg, Ca, Sr, Al, Ga, and In on the surface oxygen stability of LiMn2O4 cathodes, which has been proved to be correlated with the stability of the surface Mn atoms. Six competitive dopants, namely Nb, Ru, Mo, V, Tc, and Ti, were screened out. Besides, for three dopants in low valence states (Mg, Cu, and Zn), their Li-site doping can more effectively stabilize the surface oxygen atoms compared with Mn-site doping. Finally, we synthesized LiMn2O4 samples with Mg, Mo, and Nb surface doping to validate the rationality of the computational results. We found that particle morphology should also be considered in addition to surface oxygen stability for controlling Mn dissolution. Moreover, the electrochemical performance of LiMn2O4 batteries is a more complex issue and cannot be solely regulated by Mn dissolution. During the experiments, we have explored novel efficient binary chromogenic reagents for ultraviolet-visible spectroscopy analysis that can be used for rapid and low-cost Mn dissolution detection. This work provides a paradigm for the systematic design of the surface modification of the LiMn2O4 cathode under theoretical guidance.

The Thermodesorption of CO from the Mo(110) Surface

The problem of the CO adsorption and dissociation on the Mo(110) surface has been studied by means of temperature-programmeddesorption (TPD) and density-functional (DFT) calculations. The TPD spectra show a first-order CO desorption, which indicates thedesorption from a virgin state, not a recombinative form of desorption. The height of the potential barrier for CO dissociation (2.75 eV), estimated from DFT calculations, substantially exceeds the energy of CO chemisorption on the Mo(110) surface (2.1 eV), which refutes a thermally induced CO dissociation. Monte Carlo simulations of TPD spectra, performed with the use of estimated chemisorption energies, are in good agreement with experiment and demonstrate that the two-peak shape of the spectra can be explained without involving the CO dissociation.

Surface relaxation and initial surface corrosion of strained Mo(100) surface

Metal surface corrosion is a common phenomenon under atmospheric conditions, however, the design of corrosion resistant materials at the atomistic level is hindered by the limited knowledge about the corrosion mechanism. Here, the initial surface corrosion of metallic molybdenum, an important in industrial materials, is investigated by using the density functional theory calculations. The adsorption energies of several key corrosion intermediates on Mo(100) surfaces are calculated under various strain (-5% to 5%). The concerted effect of strain and adsorbates can cause significant relaxation of surface atoms (the in-plane atom displacement is within 0.1 - 0.4 Å). The small repulsive lateral interactions among O* adsorbates allow high O coverages. The ensuing corrosion follows the uniform corrosion process rather than the pitting corrosion process. Tensile strain can accelerate the initial surface corrosion process. Further analyses indicate that higher coverage of adsorbates correlates to bigger variation of the interlayer distances, which is the consequence of the competition between the adsorption interactions and the interlayer interactions. The findings reveal the oxidation process over Mo(100) and provide a general understanding for metal corrosion.

The highly efficient cocatalyst of molybdenum powder enhancing low-cost and stable electro-Fenton system for organic contaminants degradation at wide pH range

To conquer the challenges in conventional electro-Fenton (EF) such as catalyst deactivation and expensive modified cathode material, commercial molybdenum powder (Mo) is firstly applied as co-catalyst in GACSS (the cathode fabricated by granular active carbon wrapped with stainless steel mesh) EF process for organic contaminants degradation. The intermittent energy saving system is proved to enhance the co-catalytic performance, without external O2 aeration, extra catalyst addition and pH adjusting. Optimal parameters are 50 mA applied current, 0.15 g/L Mo and 10 min on-10 min off power frequency in intermittent Mo-GACSS system, where 94.03% Rhodamine B (RhB) removal and 0.0467 min−1 degradation rate constant in 60 min is remarkable than other systems. Electron spin resonance (ESR) and quenching experiment confirm the major dominant radicals are superoxide radical (·O2−) and singlet oxygen (1O2), which originate from Fe2+ catalyzing and active site on Mo surface. Furthermore, the long-term consecutive experiments and characterization analysis are investigated to prove the stability of Mo-GACSS system. Various contaminants removal and electric energy consumption evaluation confirm the superiority of wide application and cost-effectiveness. The study combines co-catalyst Mo and low-cost GACSS to provide new perspective in practical organic wastewater treatment to reduce energy consumption.

Effects of annealing following back contact metal layer formation on CZTS solar cell properties

Cu2ZnSnS4 (CZTS) is interesting as a light absorption layer for thin-film solar cells because it consists of only earth-abundant materials such as copper, zinc, tin, and sulfur. The poor adhesion between CZTS and molybdenum (Mo), which is the back metal contact material, causes CZTS to peeling off during the chemical bath deposition (CBD) for the formation of CdS as a buffer layer. This induces severe degradation of the solar cell performance. In this study, we investigated the effect of annealing following back contact metal layer formation on CZTS stability and solar cell performance. By annealing Mo/soda lime glass at 400 °C in atmosphere, Mo was oxidized, and the surface roughness of Mo increased. On the Mo surface, the CZTS peeling off was suppressed during the CBD process, resulting in an improvement in the shunt resistance.

Smooth Cu electrodeposition for Cu(In, Ga)Se2 thin-film solar cells: Dendritic clusters elimination by Ag buffer layer

Smooth and compact Cu(In, Ga)Se2 (CIGSe) thin film is critical to prepare high-quality solar cells. However, the ”dendritic” morphology of Cu tends to form on Mo leading to a nonuniform Cu distribution during the electrodeposition process, which affects the uniformity of CIGSe and device performance. In this work, Ag was used as the underlying buffer layer and Cu was electrodeposited on Ag film instead of Mo film. By adjusting deposition parameters, smooth and compact Ag layer was obtained on the Mo surface. With the Ag buffer layer, the ”dendritic” Cu on Mo was successfully eliminated. The electrodeposited Ag/Cu/In/Ga metal precursors were annealed in Se-containing atmosphere at 450 °C∼550 °C for 15min to prepare (Ag, Cu)(In, Ga)Se2 (ACIGSe) absorbers. Compared with the CIGSe absorber, the crystallinity of the ACIGSe absorber was improved. The VOC, JSC, FF and efficiency of ACIGSe solar cells were improved compared with CIGSe solar cells. Ultimately, an 11.73% efficiency of ACIGSe thin-film solar cells was obtained.

A first-principles study of deposition of iridium coating on Mo (1 1 0) surface by atomic layer deposition

The reaction process of deposition of Iridium coating on Mo (1 1 0) surface by atomic layer deposition (ALD) was studied theoretically by using the density functional theory and periodical slab model. The results indicated that the whole ALD reaction cycle of iridium deposition can be divided into two half-reaction processes, that the half-reaction I was the reaction between iridium precursor (Ir(acac)3) and oxygen atom on Mo cluster, causing the chemical bond length of Ir–C to stretch and that of Mo–O to shorten, and forming a ring-like skeleton structure of transition state Mo–O–C–Ir, which was approximately a planar structure and presenting a fresh and active surface of (CH3)2*Ir–Mo–Ir(CH3)2*; In the half-reaction II, the oxygen atoms react with -(CH3)2* active group of (CH3)2*Ir–Mo–Ir(CH3)2* surface, causing the chemical bond length of Ir–C to stretch and that of O–O to shorten, forming a ring-like skeleton structure of transition state O–C–Ir–C, which was also approximately a planar structure and presenting a fresh and active O*-Ir-Mo-Ir-O* surface eventually. Furthermore, the by-products of every surface reaction were released in the form of CO2 and H2O. The experiment result shown that the surface of iridium coating deposited by ALD at 300 °C was flat and smooth, and the preferred growth crystal plane was (111). The theoretical conclusion was in good agreement with the experimental results.

First principles study of hydrogen adsorption and dissociation behavior on γ-U (100)/Mo surface

<sec>Uranium (U) is one of the most natural radioactive elements widely used in the nuclear industry. In the civilian field, uranium is the most important nuclear fuel in nuclear reactors; militarily, uranium is an important raw material for nuclear weapons. In addition, uranium is also used for radiation shielding and ship ballast due to its high-density properties. Depending on the temperature, U has three kinds of allotrope phases: the orthogonal α phase at temperature below 940 K, the body-centered tetragonal (BCT) β phase at temperature ranging from 940 K to 1050 K, and the body-centered cubic (BCC) γ phase at temperature above 1050 K. Compared with the other two structures, the crystal structure of γ phase has good symmetry and excellent mechanical properties. However, γ-U is extremely unstable at low temperature. No matter what heat treatment method is adopted, γ-U will undergo phase transformation and become α phase. It is shown that adding certain alloying elements, such as Mo, Nb, Zr, Ti and Hf, into uranium can stabilize γ-U to room temperature and improve the mechanical properties of uranium greatly. As an important uranium alloy, γ-U by doping Mo atom has excellent mechanical properties, structural stability and thermal conductivity, and is an important nuclear reactor fuel.</sec><sec>However, uranium has special physical and chemical properties due to its complex electronic structure and strong correlation of 5f electrons. Because of its special valence electron structure, it is highly susceptible to chemical and electrochemical reactions of environmental media. After the reaction between uranium and hydrogen, hydrogen embrittlement will occur, and even easily break into powder, which reduces the performance of uranium in service and brings hidden trouble to its storage. With the increase of service life, surface corrosion becomes more serious, and the safety and reliability of U alloys are seriously affected. The adsorption and dissociation of hydrogen on U alloy surface is the primary process of hydrogenation corrosion.</sec><sec>Based on density functional theory, first-principles study of hydrogen adsorption and dissociation on γ-U(100) surface by Mo atoms coatings is carried out in this work. The model of γ-U(100) and Mo atoms coatings on γ-U(100) surface are established, and the structural parameters, adsorption energy, Bader charge, surface work function, and electron state density of H<sub>2</sub> at highly symmetrical adsorption sites are calculated. The results show that H<sub>2</sub> molecule occurs when physical dissociation adsorption takes place on γ-U(100) and U(100)/Mo surface. The electron state density shows that H<sub>2</sub> does not bond to the surface atoms and no new hybridization peak appears. However, in the hollow parallel adsorption configuration, H<sub>2</sub> is completely dissociated into two H atoms and occurs chemical adsorption and dissociation on γ-U(100) and U(100)/Mo surface. The H/1s orbital electrons are hybridized with the U/6p, U/6d, Mo/5s, Mo/4p, Mo/4d orbital electrons, and the H atom forms stable chemical bonds with the Mo atoms. Bader charge distribution results show that the change of chemical adsorption net charge of H<sub>2</sub> on U(100)/Mo is more than that of physical adsorption. Because the adsorption energy of H<sub>2</sub> in the most stable configuration (H<sub>Mo</sub>-Hor) on U(100)/Mo is less than that of the most stable configuration (H<sub>U</sub>-Hor) on γ-U(100), the adsorption of H<sub>2</sub> on U(100)/Mo is more stable than that of γ-U(100) surface.</sec>

Dry reforming of methane on Ni(1 1 1) surface with different Mo doping ratio: DFT-assisted microkinetic study

Ni-based catalysts are one of the most promising Dry Reforming reaction (DRM) catalysts due to their cheapness and high catalytic activity, but the deactivation caused by severe carbon deposition is a common problem in Ni-based catalysts, and one solution is to dope with other elements. In this paper, density functional theory (DFT) combined with microkinetic simulation was used to study the effect of the surface doping concentration of Mo on the catalytic activity and carbon deposition resistance of Ni catalyst. In the surface doping model, the Mo surface doping concentration is set to 0, 1/9, 2/9, 3/9, 4/9 ML, namely Ni(111), NiMo1(111), NiMo2(111), NiMo3(111) and NiMo4(111). The electronic structure analysis shows that the doping of Mo improves the electron donating ability of the Ni catalyst and therefore generally increases the adsorption energy of the intermediate species in the DRM reaction. Compared with Ni(111), on NiMox(111)(x = 1–3), the activation energy of CH4 dissociation, the activation energy of CO2 dissociation, and the activation energy of C-O bond formation are generally reduced, while the activation energy of CH* dissociation and CHxO(x = 1–2) dehydrogenation increases. It was shown that the CH-O oxidation route is favorable for DRM and the Mo doped Ni catalyst showed lower free energy span. These DFT results clearly show the doping Mo not only improve the DRM catalytic activity but also enhance the carbon resistance ability. In the microkinetic model, the reaction sites on the catalyst surface were distinguished into sites affected by Mo (#) and sites not affected by Mo (*), and the influence of O coverage was considered on the activation energy of CO2 dissociation at the reaction sites affected by Mo. At 1073.15 K, the order of the catalytic activity of these catalysts for DRM reaction is NiMo2(111) > NiMo3(111) > NiMo1(111) > Ni(111), and NiMo2(111) shows the best catalytic activity for DRM. The microkinetic simulation results showed that without considering that adsorbed O may lead to the formation of NiMoO4 and thus affect the catalytic activity, the Mo-doped Ni catalyst, especially NiMo2(111), exhibits much higher catalytic activity than Ni(111) under high temperature conditions, and Ni(111) is mainly occupied by C* whereas NiMox(111)(x = 1–3) is basically not affected by C species when the simulation reaches a steady state. The strong electron donating ability of Mo is considered to be the basis for increasing the catalytic performance of Ni-based catalysts, which means that our conclusion can be more widely promoted, i.e. the catalytic activity and carbon deposition resistance of Ni-based catalysts can be improved by doping trace metals with strong electron donating ability.

First-Principles Study on Adsorption and Decomposition of NOx on Mo (110) Surface

Based on the density functional theory, the adsorption and decomposition of NOx (x = 1, 2) on Mo (110) surface are studied with first-principles calculations. Results show that the stable structures of NO2/Mo (110) are MoNO2 (T, μ1-N), MoNO2 (H, μ3-N, O, O′), MoNO2 (S, η2-O, O′), and MoNO2 (L, η2-O, O′). The corresponding adsorption energies for the structures are −3.83 eV, −3.40 eV, −2.81 eV, and −2.60 eV, respectively. Besides, the stable structures of NO/Mo (110) are MoNO (H, μ1-N), MoNO (H, μ2-N, O), and MoNO (H, η1-N) with the corresponding adsorption energies of −3.75 eV, −3.57 eV, and −3.01 eV, respectively. N and O atoms are easily adsorbed at the hollow sites on Mo (110) surfaces, and their adsorption energies reach −7.02 eV and −7.70 eV, respectively. The preferable decomposition process of MoNO2 (H, μ3-N, O, O′) shows that the first and second deoxidation processes need to overcome energy barriers of 0.11 eV and 0.64 eV, respectively. All these findings indicate that NO2 is relatively easy to dissociate on Mo (110) surface.

The nature of column boundaries in micro-structured silicon oxide nanolayers

Columnar microstructures are critical for obtaining good resistance switching properties in SiOx resistive random access memory (ReRAM) devices. In this work, the formation and structure of columnar boundaries are studied in sputtered SiOx layers. Using TEM measurements, we analyze SiOx layers in Me–SiOx–Mo heterostructures, where Me = Ti or Au/Ti. We show that the SiOx layers are templated by the Mo surface roughness, leading to the formation of columnar boundaries protruding from troughs at the SiOx/Mo interface. Electron energy-loss spectroscopy measurements show that these boundaries are best characterized as voids, which in turn facilitate Ti, Mo, and Au incorporation from the electrodes into SiOx. Density functional theory calculations of a simple model of the SiO2 grain boundary and column boundary show that O interstitials preferentially reside at the boundaries rather than in the SiO2 bulk. The results elucidate the nature of the SiOx microstructure and the complex interactions between the metal electrodes and the switching oxide, each of which is critically important for further materials engineering and the optimization of ReRAM devices.

Morphological and structural damage investigation of nanostructured molybdenum fuzzy surface after pulsed plasma bombardment

Steady high-flux helium (He) plasma with energy ranging from 50 eV to 90 eV is used to fabricate a fiber-form nanostructure called fuzz on a polycrystalline molybdenum (Mo) surface. Enhanced hydrogen (H) pulsed plasma in a wide power density range of 12 MW/m2–35 MW/m2 is subsequently used to bombard the fuzzy Mo, thereby simulating the damage of edge localized mode (ELM) to fuzz. The comparisons of surface morphologies, crystalline structures, and optical reflectivity between the original Mo and the Mo treated with various He+ energy and transient power densities are performed. With the increase of He ion energy, the Mo nano-fuzz evolved density is enlarged due to the decrease of filament diameter and optical reflectivity. The fuzz-enhanced He release should be the consequence of crystalline growth and the lattice shrinkage inside the Mo-irradiated layers (∼ 200 nm). The fuzz induced by lower energy experiences more severe melting damage and dust release under the condition of the identical transient H plasma-bombardment. The H and He are less likely to be trapped due to aggravated melting evidenced by the enhanced crystalline size and distinct lattice shrinkage. As the transient power density rises, the thermal effect is enhanced, thereby causing the fuzz melting loss to aggravate and finally to completely disappear when the power density exceeds 21 MW/m2. Irreversible grain expansion results in huge tensile stress, leading to the observable brittle cracking. The effects of transient thermal load and He ion energy play a crucial role in etching Mo fuzz during ELM transient events.