Mo Doping Research Articles

Elucidating the role of Mo doping in enhancing the meta-Xylene ammoxidation performance over the CeVO4 catalyst

Vanadium-based oxides-catalyzed conversion of meta-xylene (m-X) directly to isophthalonitrile (IPN) through vapor phase ammoxidation is a short-flow, sustainable and economic process. Herein, effects of molybdenum promoter loadings are systematically studied on the ammoxidation performance over the SiO2 supported V-Ce composite oxide catalyst prepared by spray drying method. The roles of Mo addition were elucidated by series of structure characterizations. It had been revealed that the addition of Mo can promote the formation of specific crystal face of CeVO4 species which works as the active phase of the catalyst. Meanwhile, the Mo addition can suppress the surface V2O5 species formation, and thus lower the oxidation state and the acid strength of the catalyst. The Ce3+/Ce4+ ratio is also increased with the rising of Mo amount, and the chemisorbed oxygen amount is lowered, leading to a moderate activation of the reactants. The modified physicochemical properties originated from the Mo promoter doped on the CeVO4 surface. At last, the performances of the Mo-modified and Mo-free catalysts are evaluated in the fluidized bed under 415 °C. The oxidation degree of the feeding m-X of the Mo containing catalyst is obviously lower than the Mo-free catalyst, and as a result the yield of IPN is improved. However, the Mo/V cannot be excessively increased, because at high Mo/V, the weaker adsorption of reactant would cause the incomplete m-X conversion, and the selectivity for monocyano product, i.e., metatolunitrile, rises, lowering the selectivity of the main product (IPN).

Effect of Mo doping in NiO nanoparticles for structural modification and its efficiency for antioxidant, antibacterial applications

Novel molybdenum (Mo)-doped nickel oxide (NiO) Nanoparticles (NPs) were synthesized by using a simple sonochemical methodology and the synthesized NPs were investigated for antioxidant, and antibacterial applications. The X-ray diffraction (XRD) analysis revealed that the crystal systems of rhombohedral (21.34 nm) and monoclinic (17.76 nm) were observed for pure NiO and Mo-doped NiO NPs respectively. The scanning electron microscopy (SEM) results show that the pure NiO NPs possess irregular spherical shape with an average particle size of 93.89 nm while the Mo-doped NiO NPs exhibit spherical morphology with an average particle size of 85.48 nm. The ultraviolet–visible (UV–Vis) spectrum further indicated that the pure and Mo-doped NiO NPs exhibited strong absorption band at the wavelengths of 365 and 349 nm, respectively. The free radical scavenging activity of NiO and Mo-doped NiO NPs was also investigated by utilizing several biochemical assays. The Mo-doped NiO NPs showed better antioxidant activity (84.2%) towards ABTS. + at 200 µg/mL in comparison to their pure counterpart which confirmed that not only antioxidant potency of the doped NPs was better than pure NPs but this efficacy was also concentration dependant as well. The NiO and Mo-doped NiO NPs were further evaluated for their antibacterial activity against gram-positive (Staphylococcus aureus and Bacillus subtilis) and gram-negative (Pseudomonas aeruginosa and Escherichia coli) bacterial strains. The Mo-doped NiO NPs displayed better antibacterial activity (25 mm) against E. coli in comparison to the pure NPs. The synthesized NPs exhibited excellent aptitude for multi-dimensional applications.

Remarkable formaldehyde photo-oxidation efficiency of Zn2SnO4 co-modified by Mo doping and oxygen vacancies

Formaldehyde (HCHO) is one of the major volatile organic compounds (VOCs) contributing to both urban and indoor air pollution. It is emitted from several industrial activities and construction materials. Photocatalytic oxidation is a green promising technology aiming at HCHO removal. However, it still has some scientific challenges not fully addressed, like insufficient reaction active sites and not enough active species. This results in low HCHO removal efficiency, several toxic intermediates and poor stability. Therefore, the design of photocatalysis with surface-active sites, improved mineralization activity and recycling stability is of crucial importance. Herein, we report a facile one-pot hydrothermal method to prepare Zn2SnO4 (ZSO) with transition metal Mo doping (using Na2MoO4·2H2O as molybdenum source). Our experiments show that Mo is successfully incorporated into the ZSO lattice and contributes to the generation of oxygen vacancies (OVs). Moreover, the introduction of Mo enhances the separation and migration of photogenerated carriers and hinders their recombination. Furthermore, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations reveal that switching the active sites from O terminals to Mo ones effectively improves adsorption and activation of the target molecules (O2, H2O, and HCHO), and promotes conversion of the reaction intermediates. Thus, ZSO co-modified by Mo doping and oxygen vacancies allows to achieve an efficient HCHO removal activity of 96%, which is much higher than pristine ZSO (21%) and other previously reported photocatalysts.

Novel cobalt-free perovskite PrBaFe1.9Mo0.1O5+δ as a cathode material for solid oxide fuel cells

Fe-based perovskite PrBaFe2O5+δ is inactive toward the oxygen reduction reaction (ORR) due to the sluggish oxygen-ion transport kinetics. Herein, Mo doping is developed as a simple and effective strategy to improve the ORR activity of PrBaFe2O5+δ. Mo-doped PrBaFe1.9Mo0.1O5+δ (PBFMO) is comparatively studied with undoped PrBaFe2O5+δ (PBFO) to reveal the effect of Mo doping on its crystal structure, thermal expansion behavior, electrical and electrochemical properties. PBFMO sample has a tetragonal structure with P4/mmm space group. Pr, Fe, and Mo in PBFMO sample exist as Pr3+/Pr4+, Fe2+/Fe3+, and Mo5+/Mo6+ mixed valence states. Mo doping reduces the average thermal expansion coefficient (TEC) from 18.8 × 10−6 K−1 down to 14.5 × 10−6 K−1, which is closer to the TEC of La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) electrolyte. Moreover, Mo doping results in the increment of oxygen vacancy content and thus the enhancement of electrochemical performance. The polarization resistance (Rp) is 0.092 Ω·cm2 at 800 °C for PBFMO and is reduced by 27.0% after Mo doping. The peak power density is 0.68 W·cm−2 at 800 °C for PBFMO-based single cell. These obtained results indicate Mo doped PBFMO can be a good candidate for cathode material of intermediate temperature SOFC.

Mo doping and Se vacancy engineering for boosting electrocatalytic water oxidation by regulating the electronic structure of self-supported Co9Se8@NiSe.

Oxygen evolution reactions (OERs) are regarded as the rate-determining step of electrocatalytic overall water splitting, which endow OER electrocatalysts with the advantages of high activity, low cost, good conductivity, and excellent stability. Herein, a facile H2O2-assisted etching method is proposed for the fabrication of Mo-doped ultrathin Co9Se8@NiSe/NF-X heterojunctions with rich Se vacancies to boost electrocatalytic water oxidation. After step-by-step electronic structure modulation by Mo doping and Se vacancy engineering, the self-standing Mo-Co9Se8@NiSe/NF-60 heterojunctions deliver a current density of 50 mA cm-2 with an overpotential of 343 mV and a cell voltage of only 1.87 V at 50 mA cm-2 for overall water splitting in 1.0 M KOH. Our study opens up the possibility of realizing step-by-step electronic structure modulation of nonprecious OER electrocatalysts via heteroatom doping and vacancy engineering.

Optimising the synthesis of LiNiO2: coprecipitation versus solid-state, and the effect of molybdenum doping

A detailed study on the optimisation of the synthesis of LiNiO2 is reported, with further preliminary results showing improved performance on Mo doping.

NaCl template-assisted construction of a CoP-MoP heterostructured electrocatalyst for electrocatalytic nitrogen reduction.

The electrocatalytic nitrogen reduction reaction (NRR) to ammonia is a promising technology to store renewable energy and mitigate greenhouse gas emissions. However, it usually suffers from low ammonia yield and selectivity because of the lack of efficient electrocatalysts. Herein, we report that the construction of metal phosphide heterojunctions is an efficient strategy for NRR activity enhancement. A CoP-MoP heterojunction electrocatalyst, which is fabricated by a facile NaCl template-assisted strategy, exhibits a favorable ammonia yield rate of 77.8 μg h-1 mgcat-1 (38.9 μg h-1 cm-2) and a high faradaic efficiency of 11.16% at -0.50 V versus the reversible hydrogen electrode. The high NRR electrocatalytic activity can be attributed to the electronic coupling effects and interfacial synergistic effects of CoP and MoP at the heterojunction interface, which accelerates the electron transfer rate. Moreover, Mo doping changes the d-band centers of metal sites on the CoP surface, which is conducive to N2 adsorption and promotes N2* adsorption in the competition of occupying active sites, thus inhibiting the HER. This work manifests the high potential of phosphide electrocatalysts and opens an alternative route toward NRR electrocatalysis.

From the single-atom limit to the mixed-metal phase: finding the optimum condition for activating the basal plane of a FePSe3 monolayer towards HER.

Layered ternary transition metal tri-chalcogenides are some of the most promising candidates for hydrogen evolution reaction (HER) because of their ease of synthesis and affordability. However, majority of the materials in this category have HER active sites only at their edges, rendering a large portion of the catalyst useless. In this work, ways for activating the basal planes of one of these materials, namely, FePSe3, are explored. The effects of substitutional transition metal doping and external biaxial tensile strain on the HER activity of the basal plane of a FePSe3 monolayer are studied via first principles electronic structure calculations based on density functional theory. This study reveals that although the basal plane of the pristine material is inactive towards HER (value of H adsorption free energy, ΔGH* = 1.41 eV), 25% Zr, Mo, and Tc doping makes it more active (ΔGH* = 0.25, 0.22 and 0.13 eV, respectively). The effect of reducing the doping concentration, moving to the single-atom limit, on the catalytic activity is studied for Sc, Y, Zr, Mo, Tc and Rh dopants. For Tc, the mixed-metal phase FeTcP2Se6 is also studied. Among the unstrained materials, 25% Tc-doped FePSe3 gives the best result. Significant tunability of HER catalytic activity in the 6.25% Sc doped FePSe3 monolayer via strain engineering is also discovered. An external tensile strain of 5% reduces ΔGH* to ∼0 eV from 1.08 eV in the unstrained material, making this an attractive candidate for HER catalysis. The Volmer-Heyrovsky and Volmer-Tafel pathways are examined for some of the systems. A fascinating correlation between the electronic density of states and HER activity is also observed in most materials.

Influence of Mo doping on interfacial charge carrier dynamics in photoelectrochemical water oxidation on BiVO4

Intensity of photocurrent during water oxidation in BiVO4 is predominantly limited by charge transfer resistance (Rct), rather than semiconductor bulk resistance (Rbulk). Mo doping of BiVO4 can slightly reduce Rbulk but obviously decreases Rct.

In situ grown high-valence Mo-doped NiCo Prussian blue analogue for enhanced urea electrooxidation.

Mo-doped NiCo Prussian blue analogue (PBA) electrocatalysts self-supported on Ni foam are elaborately designed, which exhibit a low potential of 1.358 V (vs. RHE) to reach 100 mA cm-2 for catalyzing the urea oxidation reaction (UOR). The incorporation of high-valence Mo (+6) modifies the electronic structure and improves the electron transfer ability. Using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) techniques, we confirm the effect of Mo doping on the NiCo PBA electronic structure.

Introducing high-valence molybdenum to stimulate lattice oxygen in a NiCo LDH cathode for chloride ion batteries.

Layered double hydroxides (LDHs) have been intensively investigated as promising cathodes for the new concept chloride ion battery (CIB) with multiple advantages of high theoretical energy density, abundant raw materials and unique dendrite-free characteristics. However, driven by the great compositional diversity, a complete understanding of interactions between metal cations, as well as a synergetic effect between metal cations and lattice oxygen on LDH host layers in terms of the reversible Cl-storage capability, is still a crucial but elusive issue. In this work, we synthesized a series of chloride-inserted trinary Mox-doped NiCo2-Cl LDH (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) with gradient oxygen vacancies as enhanced cathodes toward CIBs. The combination of advanced spectroscopic techniques and theoretical calculations reveals that the Mo dopant facilitates oxygen vacancy formation and varies the valence states of coordinated transition metals, which can not only tune the electronic structure effectively and promote Cl-ion diffusion, but improve the redox activity of LDHs. The optimized Mo0.3NiCo2-Cl LDH delivers a reversible discharge capacity of 159.7 mA h g-1 after 300 cycles at 150 mA g-1, which is almost a triple enhancement compared to that of NiCo2Cl LDH. The superior Cl-storage of trinary Mo0.3NiCo2Cl LDH is attributed to the reversible intercalation/deintercalation of chloride ions in the LDH gallery along with the oxidation state changes in Ni0/Ni2+/Ni3+, Co0/Co2+/Co3+ and Mo4+/Mo6+ couples. This simple vacancy engineering strategy provides critical insights into the significance of the chemical interaction of various components on LDH laminates and aims to effectively design more LDH-based cathodes for CIBs, which can even be extended to other halide-ion batteries like fluoride ion batteries and bromide ion batteries.

Mo-Pre-Intercalated MnO2 Cathode with Highly Stable Layered Structure and Expanded Interlayer Spacing for Aqueous Zn-Ion Batteries

Although manganese-based oxides possess high voltage and low cost, the sluggish reaction kinetics and poor structural stability hinder their applications in aqueous rechargeable Zn-ion batteries (ZIBs). Herein, a molybdenum (Mo) pre-intercalation strategy is proposed to solve the above issues of δ-MnO2. The pre-intercalated Mo dopants, acting as the interlayer pillars, can not only expand the interlayer spacing but also reinforce the layered structure of δ-MnO2, finally achieving enhanced reaction kinetics and superb cycling stability during carrier (de)intercalation. Moreover, oxygen defects, introduced due to Mo-pre-intercalation, play a critical role in the fast reaction kinetics and capacity improvement of the Mo-pre-intercalated δ-MnO2 (Mo-MnO2) cathode. Therefore, the Mo-MnO2 cathode displays a high energy density of 451 Wh kg-1 (based on cathode mass), excellent rate capability, and admirable long-term cycling performance with a high capacity of 159 mAhg-1 at 1.0 A g-1 after 1000 cycles. In addition, the energy storage mechanism of Zn2+/H+ stepwise reversible (de)intercalation is also revealed by ex situ experiments. This work provides an insightful guide for boosting the electrochemical performance of Mn-based oxide cathodes for ZIBs.

A first principles study on H-atom interaction with bcc metals

Solute hydrogen trapping has long been proposed as one of the mechanisms for hydrogen embrittlement in steel. It has been reported that the maximum hydrogen trapping energy of metallic solutes ranged from −0.7 eV to −0.9 eV. In this work, the mechanism of metal-H interaction in Cr-Mo steels was investigated with first principles calculations by modelling the binary alloy Fe-X (X = C, Si, Mn, Cr, Mo) system with reference to the chemical composition of Cr-Mo steels. The formation of hydrogen bonds in the case of H atoms located at different sites in Fe-X crystals was analyzed. Results indicated that various atomic doping had different roles in hydrogen effect in the steel, with C, Si and Mo doping making the solid solution of hydrogen in Fe crystals easier, while Mn and Cr doping was rather more difficult. In Fe-Mn and Fe-Cr crystals, the repulsion between Fe lattices was insignificant when H atoms were located in tetrahedral sites, which considerably reduced the binding energy in the crystal. When H atoms were dissolved into the crystal, the interatomic bonding interactions in Fe-X crystals were weakened, resulting in higher charge density fluctuations. The current work extends the understanding of H-atom diffusion and migration in steel from the microscopic scale to the atomic and electronic scales, which underpins the physics for tailoring chemical elements of bcc metals towards higher resistance to hydrogen embrittlement.

Novel Mo-doped WO3/ZnO nanocomposites loaded with polyvinyl alcohol towards efficient visible-light-driven photodegradation of methyl orange

The purpose of this work was to develop a molybdenum (Mo)-doped WO3/ZnO and polyvinyl alcohol (PVA) novel nanocomposite and then analyze and assess its photocatalyticperformance. The resulting composite was employed in a laboratory photoreactor to decompose methyl orange (OM) dye from synthetic wastewater. Fourier transform infrared (FT-IR) spectroscopy, UV–vis spectroscopy, and a transmission electron microscope (TEM)were utilized to analyze the preparednanocomposite. The impact of Mo dopant and PVA addition upon crystallinity, optical bandgap, and morphology was investigated. It was shown that 92 % of MO dye can be degraded by Mo-WO3/ZnO@PVA after 120 min of visible light irradiation.

Enhanced Electrocatalytic Activity of Mo-Doped NiFe Layered Double Hydroxide Nanosheet Arrays for the Hydrogen Evolution Reaction

NiFeMo layered double hydroxide (NiFeMo-LDH) nanosheet array structures in situ grown on Ni foam were rationally designed and fabricated by a convenient and green hydrothermal strategy. The systematic characterization proved that NixFe1Mo1-LDH/NF (x = 4, 6, 8, 10) composites grew on the surface of Ni foam as vertically interlaced LDH nanosheets. The electrochemical results showed that all composites possessed high electrocatalytic activity, and Ni6Fe1Mo1-LDH/NF exhibited the best hydrogen evolution reaction (HER) performance. Furthermore, the nanosheet array structure with the best active metal molar ratio was in situ grown on reduced graphene oxide (rGO) uniformly modified Ni foam (∼210 nm × 20 nm) with an overpotential of 90 mV at 10 cm–2 in an alkaline solution, which was superior to most non-noble metal-based catalysts. Experimental results and density functional theory with the Hubbard U (DFT + U) calculations confirmed that the active site for H adsorption changed from the Fe site to the Mo site after Mo doping in NiFe-LDH, which reduced the water dissociation energy barrier and the subsequent proton adsorption energy barrier and adjusted the electronic structure of Fe and Ni sites, thereby greatly promoting the whole Volmer–Heyrovsky process under alkaline conditions and improving the HER performance. The synergistic coupling of NiFeMo-LDH nanosheets with rGO enhanced conductivity and electrochemical performance. The growth of NiFeMo-LDH on rGO/NF facilitated the formation of bubbles and significantly improved electrochemical stability. The present electronic regulation strategies using metal doping can be extended to synthesize other types of transition metal composite catalysts and used for the development of renewable and friendly energy devices.

High contrast green luminescence from spin-coated Mo-doped β-Ga2O3 thin films

High-contrast green luminescence was obtained from molybdenum (Mo)-doped beta-phase gallium oxide (β-Ga2O3) by a sol-gel spin coating method. The X-ray diffraction, atomic force microscopy, and ultraviolet–visible–near infrared transmission results revealed that the Mo dopants did not significantly affect the fabricated films’ crystal structure, surface morphology, and energy bandgap (Eg). All spin-coated Mo-doped films formed a beta phase structure and exhibited smooth surface morphology with a root-mean-square surface roughness of less than 7 nm. The thickness of the coated film increased gradually with Mo doping concentration. In the photoluminescence measurements, a green luminescence band associated with the Mo ion dxz-dyz band transition slowly appeared, which hindered the original emission from the recombination of electron–hole pairs in β-Ga2O3 but increased overlapped green color luminescence. Compared with the undoped β-Ga2O3, the CIE chromaticity diagram showed that luminescence changed from blue to green, and the contrast of the green luminescence significantly improved from 0.33 to 0.73.

Microstructure and properties of Mo doped DLC nanocomposite films deposited by a hybrid sputtering system

A combination of low internal stress and hydrophobic performance for carbon film is crucial in some practical applications. In this work, Mo doped diamond-like carbon (DLC) films were deposited by a hybrid coating system. The designed DLC films with various Mo contents were obtained by adjusting the Mo target powers. The microstructure and comprehensive properties including wettability of Mo-DLC films were analyzed systematically. Results showed that with increasing the Mo target power, Mo content and deposition rate of films were continuously increased, and also the surface roughness slightly increased. At low Mo doping, films mainly exhibited amorphous feature, whereas, the MoC nanocrystallites were found at high Mo doping, implying the nanocomposite structure formed where MoC crystals were embedded in the carbon matrix. The sp3-C bond fraction was decreasing with increasing the Mo content due to the metal catalyst effect, which effectively promoted the sp3-C bonds transforming into sp2-C bonds. Meanwhile, the Mo addition had a significant positive effect on the reduction of residual stress. Moreover, the wettability of films was transferred from hydrophilicity state to hydrophobicity state, and the maximum water contact angle was 94.2° at 20.1 at. % Mo doping, then slightly diminished to 92.7° with further increasing Mo content. The present work would offer an available method to fabricate the DLC film with low internal stress and desirable wettability property.

The novel preparation method of thermochromic VO2 films with a low phase transition temperature by thermal oxidation of V–Mo cosputtered alloy films

Vanadium dioxide (VO2) has been studied extensively for its unique insulator-metal transition characteristics and potential applications in thermochromic smart windows, switching devices, and infrared detectors. However, how to balance the metal-insulator transition temperature, luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of VO2 thin films remains a challenge. In this work, high-quality thermochromic VO2 thin films were prepared by a two-step method of magnetron sputtering and thermal oxidation annealing. Metallic and alloyed V–Mo layers were first deposited by direct-current reactive magnetron sputtering, and then a thermal oxidation annealing process was used to obtain pure and Mo-doped VO2 thin films. The Mo content in the films was regulated by changing the sputtering power of the vanadium target, and the effect of Mo doping on the crystallinity, microstructure, phase transition temperature and optical properties of VO2 thin films was studied. The shift of the VO2(011) peak to a lower 2θ angle in the XRD patterns showed that Mo was successfully diffused into vanadium dioxide films. The phase transition temperatures were decreased continuously from 57.4 to 32.7 °C by decreasing the sputtering power of vanadium. The thinner Mo-doped VO2 thin films showed higher luminous transmittance and lower transition temperature. Our results were shown to be an innovative preparation method to fabricate thermochromic VO2 films with a low phase transition temperature, balanced luminous transmittance and solar modulation ability by thermal oxidation of V–Mo cosputtered alloy films.

Investigation of Mo Doping Effects on the Properties of AlN-Based Piezoelectric Films Using a Sputtering Technique

In this study, AlN-based films are deposited using a sputtering deposition method, and Mo dopants with different concentrations are added in the proposed system by controlling the sputtering power in order to improve the crystallinity and piezoelectric properties of AlN films. Through a detailed material analysis including energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), piezoresponse force microscopy (PFM), and nano-indentation, the piezoelectric property optimization mechanism of proposed films was explored and the best process parameters were determined. The piezoelectric coefficient d33 of AlN:Mo (3.46%) films reached 7.33 pm V−1, which is 82.79% higher than that of undoped AlN. As compared with the reported data about the dopants in AlN system, our proposed films have the better d33 values with those dopants in AlN-based films except Sc dopants. However, Sc is known as an expensive metal, our proposed films could be applied to low-cost piezoelectric MEMS applications.

Charge traps in Zn- and Mo-based oxide microstructures. The role of Mo

Peculiarities of the point defects creation under the X-ray irradiation and the influence of Mo doping in ZnO: Mo (10 and 30%) and MoO3 micro-powders fabricated by the hydrothermal growth method were studied in detail. It has been found that some new phases other than ZnO are created upon the heavy molybdenum doping. They were tentatively ascribed to complex zinc molybdates existing in the form of platelet flakes. The MoO3 sample contained both hydrated and anhydrous MoO3 phases in the form of microneedles. The molybdenum charge state was Mo6+ before X-ray irradiation in all the materials. X-ray irradiation resulted in creation of molybdenum- and oxygen-based charge trapping centres, i.e., Mo5+ and O− or . Part of them is expected to exist in the pure ZnO and MoO3 phases whereas the rest appears in the complex zinc molybdates.