Mo Doping Research Articles

High-valence metal doped Co2FeAl alloy as efficient noble-metal-free electrocatalyst for alkaline hydrogen evolution reaction

Most efficient electrochemical hydrogen evolution reaction (HER) catalysts in alkaline solutions are based on precious metals, whose rarity and high cost are severely limiting their large-scale applications. In this work, we synthesized Mo-doped and W-doped Co2FeAl alloys by a simple coprecipitation method. The HER in alkaline solution is promoted through the synergistic effect of thermal annealing and doping of high-valence metal atoms. Thermal annealing could optimize the surface morphology and improve the crystallinity. In the meantime, the doping of high-valence metal atoms could further regulate the crystal structure and electronic structure of alloys. Therefore, annealing temperatures and doping concentrations have a marked impact on the catalytic performance. In particular, when the Mo doping concentration is 27 at% and the annealing temperature is 750 ℃, the synthesized Co2FeAlMo1.5 (750 ℃) alloy electrocatalyst exhibits the superior HER performance in alkaline media. It displays a low overpotential of only 71 mV at 10 mA cm−2 and a Tafel slope of 110 mV dec−1. An excellent stability is also demonstrated that the electrocatalyst can work stably for 20 h and the overpotential is only shifted by 2 mV after 1000 cycles. A series of characterization tests indicate that the crystal structure and electronic structure of Co2FeAlMo1.5 (750 ℃) are optimized, which can improve the catalytic activity and promote alkaline HER. This work provides a new strategy for improving the catalytic performance of non-noble metal electrocatalysts for alkaline HER.

Mo-Doped Metal–Organic Frameworks for Efficient Nitrogen Reduction Reaction: A Density Functional Theory Study

The activation of nitrogen molecules is one of the main obstacles to electrocatalytic nitrogen reduction reaction (eNRR), which is closely related to the d–p degree of coupling between d-orbitals of metal active sites and p-orbitals of nitrogen. Herein, two types of MOFs─M2(DOBDC) and M2Cl2(BBTA) (M = V, Cr, Mn, Fe, Co, and Ni)─were systematically studied via density functional theory (DFT) calculations. In addition, modification strategies including Mo doping and coordination environment optimization were successfully implemented. These modified MOFs can not only effectively activate nitrogen but also maintain smooth ammonia desorption in the NRR process. Especially, Mo(II)-Fe2(DOAnBDC) and Mo(II)-Fe2F2(BBTA) were considered potential eNRR electrocatalysts, because of their high eNRR activity (UL = −0.60 V and −0.73 V, respectively) and selectivity. Furthermore, the detailed electronic structural analysis revealed that the increased eNRR activity is dependent on the unique d-orbitals arrangement of Mo itself. Moreover, the charge transfer between the −Fe–X–Mo– chains (X is a nonmetal anion) regulated the Mo-d orbitals to enhance the d-p coupling, contributing to the enhanced eNRR performance. These findings are of great significance to stimulate the enthusiasm for studying ligand effects to find high-performance eNRR catalysts.

In-situ solid solution of Mo or Cr in WB2: Densification, microstructure, and properties

Pure tungsten diboride (WB2) ceramic prepared by the boro/carbothermal reduction combined with spark plasma sintering (SPS) of WB2 can reach a relative density >90%. However, the final sintered density still needs further improvement to meet the application requirement. In this study, the WB2 ceramics doped with Mo or Cr (W0.95Mo0.05B2 and W0.95Cr0.05B2) were prepared and the effects of Mo or Cr doping on powder characters, sintering behavior, microstructure, mechanical and physical properties of WB2 were systematically investigated. The results showed that the average particle size of the WB2 powders was increased significantly after adding either dopant, and the relative density of WB2 ceramics was improved from ∼93% to ∼95% and 96% with Cr and Mo addition, respectively. However, the Vickers hardness of WB2 with either dopant was reduced. On the other hand, the W0.95Mo0.05B2 solid solution exhibited slight improvement on both of the thermal and electrical conductivity as compared with WB2. But, both of the thermal and electrical conductivity of W0.95Cr0.05B2 solid solution decreased significantly to about 50% of the values obtained for WB2. The decrease in thermal and electrical conductivity of W0.95Cr0.05B2 solid solution was mainly attributed to the formation of high concentration of atomic defects, i.e. dislocations.

Selective Oxidation of Biomass Molecules via ZnO Nanoparticles Modified Using Charge Mismatch of the Doped Co ions.

A charge mismatch between transition-metal-ion dopants and metal oxide nanoparticles (MO NPs) within an engineered complex engenders a significant number of oxygen vacancies (VO) on the surface of the MO NP construct. To elucidate in-depth the mechanism of this tendency, Co ions with different charge states (Co3+ and Co2+) were doped into ZnO NPs, and their atomic structural changes were correlated with their photocatalytic efficiency. We ascertained that the increase of the Zn-O bond distances was distinctly affected by Co3+-ion doping, and, subsequently, the number of VO was noticeably increased. We further investigated the mechanistic pathways of the photocatalytic oxidation of 2,5-hydroxymethylfurfural (HMF), which have been widely investigated as biomass derivatives because of their potential use as precursors for the synthesis of sustainable alternatives to petrochemical substances. To identify the reaction products in each oxidation step, selective oxidation products obtained from HMF in the presence of pristine ZnO NPs, Co3+- and Co2+-ion-doped ZnO NPs were evaluated. We confirmed that Co3+-ion-doped ZnO NPs can efficiently and selectively oxidize HMF with a good conversion rate (∼40%) by converting HMF to 2,5-furandicarboxylic acid (FDCA). The present study demonstrates the feasibility of improving the production efficiency of FDCA (an alternative energy material) by using enhanced photocatalytic MO NPs with the help of the charge mismatch between MO and metal-ion dopants.

Self-supported Mo-doped TiO2 electrode for ambient electrocatalytic nitrogen oxidation

Electrocatalytic nitrogen oxidation reaction (NOR) provides an eco-friendly pathway for ambient N2 fixation and nitrate/nitric acid synthesis but is plagued by sluggish kinetics due to the weak adsorption of N2 molecules and cleavage of NN triple bonds. Herein, a self-supported Mo-doped TiO2 electrode prepared by one-step hydrothermal method is reported for efficient nitrogen oxidation. Electrochemical measurements and spectroscopy investigations reveal that the inert TiO2 is effectively triggered by Mo substitution in terms of enhanced N2 adsorption and activation. As further confirmed by density function theory calculations, the incorporation of Mo reduces the barrier of the rate-determining step and provides an energetically favorable NOR pathway compared to pristine rutile TiO2. Electrochemical results show that Mo-doped TiO2 electrode achieves a nitrate yield of 5.44 μg h−1 mgcat−1 (or 8.77 ± 0.39 nmol h−1 cm−2) with a Faradaic efficiency of 2.88%. The formation of nitrate from N2 under positive bias is further demonstrated by in situ ultraviolet-visible spectrophotometry. This work provides experimental and theoretical evidences that Mo doping can stimulate the stable yet electrocatalytically inert Ti-based oxide toward NOR via promoting active sites formation and lowering activation energy barriers.

(Invited, Digital Presentation) Design of New Cathodes for Sodium-Ion Batteries Harnessing Anionic Redox Activity: Two Case Studies

Sodium-ion batteries (SIBs) are widely pursued as pragmatic alternative to the Li-ion battery technology having close operational similarity along with natural resource abundance. In this sector, apart from classical cationic redox-based cathodes, various new cathodes have been unveiled based on anionic (oxygen) redox activity. In this work, we will showcase two such case studies of novel cathode composition for SIBs exploiting anionic redox activity.(1) P2-type manganese-rich layered oxides offers high capacity along with fast rate kinetics. To improve their electrochemical properties, reduce voltage decay upon cycling and to mitigate irreversible phase transitions, cation doping or hybrid composite integrations can be explored. We have tested a unique Mn-rich layer-spinel composite, Na0.7(Li1/18Mn11/18Ni3/18Fe2/181 /18)O2–xNa2MoO4, leading to a synergistic effect of layered P2 and spinel phases. This layer/spinel biphasic composite was stabilised through Mo doping and its electrochemical activity was studied at different voltage windows. Upon cycling between 1.5–4.5 V, this Mo-based composite exhibited a high specific capacity of 183 mAh.g-1 involving both cationic and anionic (O2-/O2 n-) redox activity. The structural evolution during (dis)charge was studied by ex-situ X-ray diffraction and cyclic voltammetry. It is observed that mitigating P2-P2´´ phase transition at higher voltage is crucial to improve the electrochemical performance, cycling stability and reduce the voltage hysteresis. The structural and electrochemical properties of this new composite cathode will be described (Fig. a) (manuscript submitted).(2) The second part will dwell on anionic redox activity of Na-based mixed-metal oxide cathode [Na0.7Mn0.6Ni0.3Co0.1O2]. Combining cationic and anionic redox activity, it exhibited a reversible capacity of 120 mAh/g with anionic redox activity centered around 4.3 V (Fig. b). The underlying redox mechanism has been investigated using in-situ XRD and Raman spectroscopy showing peroxo bond formation (Fig. c). The structural and electrochemical properties of this new cathode composition will be explained (manuscript submitted). Figure 1

Coordination Effect-Promoted Durable Ni(OH)2 for Energy-Saving Hydrogen Evolution from Water/Methanol Co-Electrocatalysis

HighlightsA novel Ni(OH)2-based catalyst with ultralow Ni–Ni coordination is produced, exhibiting high activity (100 mA cm−2 at 1.39 V for methanol oxidation reactions) and outstanding stability in an industrial concentration electrolyte (over 500 mA cm−2 in 6 M KOH).Mechanistic studies show that the improved kinetics and durability are primarily due to ultralow Ni–Ni coordination, 3D-networking structures and the Mo dopant.

Mo doping of BiOBr nanoflowers for the degradation of tetracycline by heterogeneous activation of persulfate under visible light

Mo-doped BiOBr nanoflowers were prepared by hydrothermal method with various Bi:Mo molar ratios, and their degradation efficiency of tetracycline under visible light was investigated using K2S2O8 activation. Among the above series of catalysts, the sample containing 2% Mo dopant exhibited the optimum photocatalytic activity, which could efficiently remove tetracycline (TC) within 40 min. The mechanism of photocatalytic activity can be well explained by the generation of superoxide radicals (O2−), sulfate radicals (SO4−), hydroxyl radicals (OH) and holes (h+), which are responsible for the degradation of TC. In addition, the 2 %Mo-BiOBr catalyst has good recycling stability.

Role of molybdenum doping in reducing leakage current density of BiFeO3 based thin film devices

Leakage current properties of pure and Mo-doped BiFeO` (BFO) thin films have been studied in order to identify the cause of high leakage currents. Interestingly, it was observed that the leakage current density reduced by more than two orders of magnitude upon Mo doping at Fe site of BFO. According to J-V characteristics, the leakage current density decreased from 1.101 × 10-6 A/cm2 to 9.4 × 10-8 A/cm2 at an applied bias voltage of 2.75 V with Mo substitution. These findings suggest that oxygen vacancies, rather than Fe2+ ions, are the primary cause of BFO's high conductivity.

Evolution of superconductivity and charge density wave through Ta and Mo doping in CsV3Sb5

Here, we report the tunning study of superconductivity and charge density wave in $\mathrm{Cs}{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ through the V kagome net doping. Combining electrical transport and magnetic susceptibility measurements, we reveal the opposite effect of Ta and Mo doping. Ta doping promotes the superconductivity but suppresses charge density wave, which may be explained by the lift of van Hove singularity towards ${E}_{\mathrm{F}}$. On the contrary, Mo doping suppresses the superconductivity quickly and leads to a continuous increase of charge density wave temperature up to 100.7 K. Our result suggests that electron doping can also effectively tune charge density wave state in $\mathrm{Cs}{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$.

Improved solar water splitting performance of BiVO4 photoanode by the synergistic effect of Zr-Mo co-doping and FeOOH Co-catalyst layer

• Synthesis of Zr-Mo:BiVO 4 electrocatalyst via electrodeposition process. • Synergistic effect of Zr-Mo co-doping and FeOOH co-catalytic layer over BiVO 4 electrodes with enriched PEC performances. • A photocurrent of 3.34 mA cm −2 with 13-fold enhancement is achieved. In this work, Zr-Mo co-doping was successfully employed on electrochemically deposited BiVO 4 photoanodes, and tested for photoelectrochemical (PEC) water oxidation. Zr doping induced the formation of relatively smaller morphological features resulting in improved light absorption and PEC activity. Mo doping resulted in structural damage that enhanced the charge carrier separation and increased free carrier density. The synergetic effect of both dopants resulted in ∼5-fold enhancement of the PEC performance. When a FeOOH layer is hydrothermally grown on the Zr-Mo co-doped BiVO 4 electrode, the photocurrent density was enhanced by ∼13-fold achieving 3.34 mA/cm 2 at 1.23 V RHE . In addition, the onset potential was remarkably reduced from ∼0.45 V to <0.1 V.

Tuning the crystal structure of NiS/carbon by Mo doping for asymmetric supercapacitor application

A major challenge faced by NiS as positive electrode material for supercapacitors is the capacity attenuation caused by structural changes in the charging and discharging process. In this article, based on theoretical calculations and experimental studies, it is demonstrated that Mo doping can significantly improve the stability and specific capacitance of NiS for supercapacitor application. Experiments show that compared with original NiS/C, the cycle stability of the doped Mo-NiS/C increases by about 30% after 5,000 cycles, and the specific capacitance achieves 1,852 F/g at 1 A/g. Besides, Mo doping can bring better ion contact and increased reaction site to the original lamellar structure of NiS. The calculation results also show that the bond energy of the doped Mo-NiS/C is lower than that of the original NiS/C, which hinders the structural transformation or other side reactions and achieves a better cycle life. This article verifies the effect of Mo doping on the properties of NiS/C and provides a feasible scheme for the study of metal-doped sulfide.

Precursor Molarity Influence on Sprayed Mo-doped ZnO Films for solar cells

Objectives: Current research focuses on the role of precursor molarity effect on sprayed Mo-doped ZnO films and their suitability as window layers in solar cells. Methods: Molybdenum (Mo) doped zinc oxide (MZO) thin films were deposited by the technique of spray pyrolysis, varying the Zn molarity in the range, of 0.01 M to 0.20 M at a constant substrate temperature of 400 ◦C. The Mo doping concentration was constant at 2 at. %. Findings: The XPS studies witnessed the presence of Mo and Zn in +6 and +2 state respectively. The XRD pattern showed both (111) and (002) as strong peaks, confirming the hexagonal wurtzite crystal structure. The optical investigations showed that the MZO films with Zn molarity 0.10 M exhibited high optical transmittance having a wide energy band gap. Films with zinc molarity of 0.10 M showed low resistivity and high mobility. The prepared CTS/MZO hetero junction solar cells performance was studied by evaluating parameters such as open circuit voltage (Voc) of 0.14 V, short circuit current density (Jsc) of 6.46 mA cm-2, a fill factor (FF) of 0.27, and a conversion efficiency of 0.25 %. Novelty: Studies on the physical properties of Mo:ZnO layers such as Ritveld refinement analysis and Haze were reported first time. CTS/MZO hetero junction solar cells have not yet been published. The observed results are analogous to improve the efficiency of solar cells using environmentally benign materials. Keywords: Spray pyrolysis; Thin films; XRD; Optical properties; Haze; Hall measurements

Significant enhancement in photosensitivity, responsivity, detectivity and quantum efficiency of Co3O4 nanostructured thin film-based photodetectors through Mo doping developed by spray pyrolysis method

Herein, we have developed the high-performance photodetector diodes (p-Co3O4/n-Si and p-Mo@Co3O4/n-Si) by spray pyrolysis route for modern optoelectronic devices. The impact of different molybdenum (Mo) ions concentrations (0, 2, 4, and 6 wt.%) on the structural, morphological, optical and photo-electrical properties of Co3O4 films has been investigated. The X-ray diffraction study confirms the cubic phase of Mo-doped Co3O4 thin films with good crystallinity. From FESEM images, the unevenly sized sphere-like grains were clearly observed which are agglomerated at higher doping levels. UV-VIS spectra confirmed that the thin film owns good absorption and the energy gap varied with Mo doping. The p-Co3O4/n-Si and p-Mo@Co3O4/n-Si (Mo = 2, 4, and 6 wt.%) photodiodes were fabricated. The barrier height of the fabricated diodes was varied in the range of 0.65 to 0.98 eV. The maximum sensitivity of 10382% at 1.2 V has been achieved for 6 wt.% of Mo-doped diodes due to the creation of photo-created charge couples. Also, the quantum efficiency of the diode was enhanced from 39 to 164%. The 6 wt.% of p-Mo@Co3O4/n-Si junction diode exhibited the highest device performance among others and can be used as high performance photodetector.

The C-H Bond Activation Triggered by Subsurface Mo Dopant on MgO Catalyst in Oxidative Coupling of Methane

In this work, density functional theory calculations are performed to explore the unique role of Mo dopant on MgO in oxidative coupling of methane. It is revealed that subsurface Mo dopant significantly enhanced the adsorption and activation of oxygen molecules. The combination of adsorbed oxygen and surface Mg exhibited a balanced activity for C-H bond activation and release of methyl radical which paves the way to activate methane with a promising yield.

Doping Mo into NiFe LDH/NiSe Heterostructure to Enhance Oxygen Evolution Activity by Synergistically Facilitating Electronic Modulation and Surface Reconstruction

It is of great significance to design highly efficient electrocatalysts with abundant earth elements instead of precious metals for water splitting. Herein, Mo-doped NiFe-layered double hydroxides/NiSe heterostructure (Mo-NiFe LDH/NiSe) was fabricated by coupling Mo-doped NiFe LDH and NiSe on nickel foam (NF). The heterostructure electrocatalyst showed ultra-low overpotential (250 mV) and remarkable durability for oxygen evolution reaction (OER) at 150 mA cm-2 . Both theoretical and experimental results confirmed that Mo doping and interfacial synergism induced the interfacial charge redistribution and the lifted d-band center to weaken the energy barrier (EB) of the formation of OOH* . Mo doping also facilitated the surface reconstruction of NiFe LDH into Ni(Fe)OOH as the active sites under electro-oxidation process. This work provides a facile strategy for electronic modulation and surface reconstruction of OER electrocatalyst by transition metal doping and heterostructure generation.

Influence of Mo+2 ion concentration on crystallization, microstructure, crystal imperfection and morphology of WO3 sprayed thin films

Compositional dependence of microstructure and morphological properties of MoxW1−xO3 (0.2 ≥ x ≥ 0.0 at. %) thin films was studied. MoWO3 films were deposited by chemical spray pyrolysis technique on pre-heated glass substrates at 400 °C and the sedimentation rate was kept constant at 70 ml min−1 with the deposition time fixed at 10 min to keep the film thickness constant at about 650 ± 5 nm. The prepared thin films were studied using x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and energy dispersive x-ray analysis (EDAX) techniques. The nature of the as-deposited films displayed by XRD showed beyond a reasonable doubt that they are amorphous. After thermal annealing at 500 °C for 2 h, all samples will be transferred to a polycrystalline having a triclinic structure. The microstructural parameters of the pure and doped samples were measured, using different methods and a comparison between them to determine the accuracy percentage of the measurement of these methods. The results also showed that the average crystallite size of the films decreased from 24 to 12 nm, on the contrary the internal microstrain increased from 23 × 10−3 to 30 × 10−3 with the addition of Mo element in WO3. Field emission scanning electron microscopy (FE-SEM) analysis also revealed that Mo dopant caused significant changes in the surface morphology of the films as well as an increase in particle size with increasing molybdenum concentration. The EDX results exhibited that the percentages of the isotropic elements MoxW1−xO3 agree well with those determined by atomic weight. In general, the results obtained in this study confirm that MoxW1−xO3 with these properties, is suitable for photovoltaic and optoelectronic applications.

A molybdenum doped layer-spinel composite cathode material for sodium-ion battery

Sodium-ion batteries are pursued as pragmatic alternative to the Li-ion battery technology having operational similarity along with natural resource abundance. P2-type manganese-rich layered oxides are widely explored exhibiting high capacity along with fast rate kinetics. To improve their electrochemical performance and reduce voltage decay upon cycling and to mitigate irreversible phase transitions, cation doping or hybrid composite integrations have been proposed. Here, we report a unique Mn-rich layer-spinel composite, Na0.7(Li1/18Mn11/18Ni3/18Fe2/18χ1/18)O2–xNa2MoO4, leading to a synergistic effect of layered P2 and spinel phases. This stable layer/spinel biphasic composite was stabilized through Mo doping and its electrochemical activity was studied at different voltage windows. When cycled between 1.5–4.5 V, this composite delivered a high specific capacity of 183 mAh.g–1 involving both cationic and anionic (O2–/O2n–) redox. The structural evolution during (dis)charge was studied by ex-situ X-ray diffraction and cyclic voltammetry. It is observed that mitigating P2-P2′′ phase transition at higher voltage is crucial to improve the electrochemical performance, cycling stability and reduce the voltage hysteresis.

Crystallography and morphology evolution of MC carbides in Ni-12Mo-7Cr-2 Nb based superalloy

The crystallography and morphology evolution of MC carbides in Ni-12Mo-7Cr-2 Nb based superalloy have been systematically investigated in this study. After the thermal exposure at 750 °C, the secondary MC carbides can be observed in the matrix in the forms of stacking fault precipitates (SFP) and flake-shaped ones. At the first stage of exposure, the SFP preferentially nucleates around the primary MC carbides, and then spread throughout the matrix. The flake-shaped MC carbides were also formed at the dislocations and pile-ups. After the long-term exposure, the SFP starts to degenerate, while flake-shaped ones still remain and become coarsen. All secondary MC carbides possess the cube-on-cube orientation relationship with the matrix but the different interface structures. For the SFP, the semi-coherent interface is along the {111} planes. In the flake-shaped MC carbides, it is along {001} planes. All secondary MC carbides possess the same chemical constitution, that is approximately (Mo0.66Nb0.24Cr0.1) C. Based on the first-principle calculations, the Mo doping in NbC was prove to decrease the chemical interface energy and favor the nucleation process. The evolution mechanism of the secondary MC carbides was also briefly discussed by exploring the role of crystal defects (stacking faults and dislocations). Our results can help to deepen the understanding of the helium trapping mechanism of MC carbides.

Synergistic effect of trimetallic-based CuxMox/Co1−xO nanoparticles/reduced graphene oxide as high efficient electrocatalyst for oxygen evolution reaction

The development of an efficient electrocatalyst for the oxygen evolution reaction (OER) is urgently required for sustainable renewable energy production. Owing to the increase in chemical complexity, multimetallic catalysts provide the flexibility to alter their electronic structure to attain superior intrinsic catalytic activity via synergistic effects. In this study, trimetallic CuxMox/Co1−xO nanoparticles grown on reduced graphene oxide (RGO) as a catalyst for the OER were fabricated via a facile solution-hydrothermal synthesis assisted by a calcination process. In the fabricated CuxMox/Co1−xO NPs@RGO hybrid catalyst, the RGO sheet acted as a conductive network to promote electron transfer, thus increasing the electrocatalytic activity. Meanwhile, additional active sites as bridged sites from the Cu and Mo dopants improved the catalytic activity, resulting in outstanding electrochemical activity for the OER. The optimized Cu0.19Mo0.19/Co0.62O NPs@RGO catalyst exhibited a low overpotential of 250 mV and a small Tafel slope of 61 mV/dec at a current density of 10 mA cm−2 and an overpotential of 550 mV at a high current density of 150 mA cm−2. This study provides a novel strategy for the development of efficient OER electrocatalysts for large-scale applications.