Effect Of Transition Metal Dopants Research Articles

Effects of transition metal dopants and Se-vacancy defects on carbon monoxide sensitivity of WSe2/graphene heterostructures

In this study, we performed first principles calculations to explore the adsorption behavior, electronic structures, and magnetic properties of CO molecules on pristine transition metal (TM)-doped WSe2/graphene (WSe2/G) heterostructures with vacancy defects. Introducing the Se-vacancies and TM dopants (Ti, Cr, Ni, and Co) enhanced the adsorption stability of CO on WSe2/G. The adsorption mechanism of CO molecules on primordial WSe2/G was physisorption, with large interaction distances and small adsorption energies. However, when the CO molecules were adsorbed on the defected and TM-doped WSe2/G, the adsorption mechanism changed to chemisorption. The adsorption of TMs (Ti, Cr, Ni, and Co) on the WSe2/G heterostructures was systematically investigated to evaluate its potential for application in gas sensor devices. The recovery time of CO@Cr–WSe2/G after CO adsorption was suitable for CO detection gas sensors. The findings of our study provide new technical directions for developing CO gas sensor devices.

Effects of transition metal dopants (Mo and W) on electrical and optical properties of CdO thin films

Cadmium oxide (CdO) is known to have the highest electron mobility among common transparent conductive oxides (TCOs). Here, we carried out a systematic investigation on the properties of CdO films doped with transition metal donors, namely Mo (CMO) and W (CWO) which have been reported to enhance the mobility in In2O3. We find that the electron concentration N in as-grown CMO and CWO increases with dopant concentration to> 1021 cm−3. While the mobility μ increases with rapid thermal annealing (RTA) temperature, N decreases when the temperature is> 400 °C, suggesting that Mo and W are unstable donors. The stronger affinity of the Mo and W to diffuse out in an O2 environment results in an even stronger decrease in N when annealed in O2. Isothermal RTA at 300 °C further reveals that H interstitial donors incorporated during room temperature film growth are unstable at temperature< 300 °C. Overall, both CMO and CWO can achieve a low resistivity of ∼1.5 × 10−4 Ω-cm with μ > 100 cm2/V-s and N∼3.5 × 1020 and 4 × 1020 cm–3, respectively, as well as an overall transmittance> 80% up to λ > 1600 nm with x ≲ 0.01 after RTA at 600 °C in N2. Finally, changes in optical absorption edges Eopt of CMO and CWO are consistent with the modification of the conduction band (CB) due to anticrossing interactions of the dopant d–states with the CdO CB, which also flattens the CB dispersion which dramatically lowers the μ at high x.

Effects of transition metal dopants on the electronic structure of potassium niobate

Potassium niobate (KNbO3) is an important ferroelectric and photorefractive material that finds multiple nonlinear optical and photorefractive applications such as two-beam coupling and dynamic holography. We employ standard Density Functional Theory to examine two defect complexes in KNbO3 doped with 3-d transition metals. These defect complexes involve the substitution of a Nb with a 3-d transition metal element and (1) a coordinating oxygen vacancy induced in the nearest-neighbor oxygen shell or (2) a non-coordinating oxygen vacancy induced in the supercell as far away as possible from substitution impurity. With the exception of Ti and V, the 3-d transition metal dopants studied here may represent viable dopants for purposes of extending the photorefractive spectral response of KNbO3. Our results indicate that each of these complexes introduces defect states into the gap of KNbO3. Furthermore, we notice that the distribution of impurity states changes quite systematically across the 3-d transition metal row, and we note the effects of the disruption of the crystal field in the case of the coordinating O vacancy on the position and ordering of defect states within the gap. Substituting KNbO3 with transition metal ions can, therefore, significantly affect its optical and infrared properties, exemplifying how a detailed understanding of the electronic structure can be an important tool in tailoring the properties of this material for ferroelectric and photorefractive applications.

Hydrothermal synthesis and characterization of transition metal (Mn/Fe/Cu) co-doped cerium oxide-based nano-additives for potential use in the reduction of exhaust emission from spark ignition engines.

The goal of this work was to synthesize new cerium oxide-based nano-additives to minimise emissions from spark ignition (SI) engines fueled with gasoline blends, such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOx). To investigate the effect of transition metal dopants on their respective catalytic oxidation activity, nano-sized CeO2 catalysts co-doped with Mn, Fe, Cu and Ag ions were successfully produced by a simple hydrothermal technique. The synthesis of nano-catalysts with cubic fluorite geometry was confirmed by XRD data. The addition of transition metal ions to the CeO2 lattice increased the concentration of structural defects like oxygen vacancies and Ce3+ ions, which are advantageous for the catalytic oxidation reaction, as also supported by XAFS and RAMAN analysis. Further, nano-gasoline fuel emission parameters are measured and compared to straight gasoline fuel. The results demonstrated that harmful exhaust pollutants such as CO, HC and NOx were significantly reduced. The high surface area, better redox characteristics and presence of additional oxygen vacancy sites or Ce3+ ions have been linked to the improved catalytic performance of the synthesized catalyst.

Effects of Transition Metal Dopants (Mo and W) on Electrical and Optical Properties of Cdo Thin Films

Effects of Transition Metal Dopants (Mo and W) on Electrical and Optical Properties of Cdo Thin Films

Electrochemical performance of Li2MnSiO4/C lithium-ion cathode materials with low-level doping of Ru in the Mn site

Lithium manganese orthosilicate (Li2MnSiO4, LMS), a prospective high-voltage lithium-ion battery cathode material, exhibits poor cycling performance due to structural instabilities during charge and discharge. To alleviate cathode degradation and understand the effects of transition metal dopants on the material properties, Mn was partially substituted by Ru between 0.5 and 3 mol% in carbon-coated LMS (LMS/C) cathode. Undoped LMS/C had orthorhombic LMS structure in the Pmn21 space group. While Ru doping of 0.5 and 1 mol% yielded phase-pure orthorhombic Pmn21 phase, Ru doping of 3 mol% yielded an additional orthorhombic Pmnb phase. The galvanostatic charge-discharge profiles showed a slight improvement in first discharge capacity for 0.5 mol% Ru-doped LMS/C compared to undoped LMS/C, but the discharge capacity decreased with higher doping levels. The capacity retention, however, improved with increasing doping level. In situ XRD results showed that Ru-doped LMS/C was able to retain a higher level of crystallinity than undoped LMS/C during the first charge. By the end of the first charge, in situ XANES of LMS/C revealed that Mn2+ in [MnO4] was partially oxidized to Mn3+ in [MnO6] ligand upon first charging, which coincided with a reduction in crystallinity of LMS/C. As shown by in situ XANES, Ru doping decreased the extent of oxidation of Mn2+ to Mn3+ upon charging. The results revealed that Ru doping of LMS/C between 0.5 and 3 mol% in the Mn site could help maintain Li2MnSiO4 crystallinity during the initial charging step.

Tuning the electronic band gap of Cu2O via transition metal doping for improved photovoltaic applications

Cu 2 O is a widely known p -type semiconductor with a band-gap value suitable for photovoltaic applications. However, due to its parity-forbidden nature of the first interband transition and high carrier recombination currents, Cu 2 O has failed to reach commercial application. Hybrid density functional theory has been used to study the effect of transition metal dopants on the electronic and optical properties of Cu 2 O . Substitutional transition metal dopants, incorporated on the copper site, allow band-gap tunability by creating a range of defect states in the electronic structure, from shallow levels to deep intermediate bands. The preferred position of the dopants is in the vicinity of copper vacancies, which are naturally found in Cu 2 O and are responsible for its p -type conductivity. Impurity levels created via extrinsic transition metal dopants increase substantially the capacity of Cu 2 O to absorb light, reaching values close to 10%. First row transition metal dopants thus show potential for considerable improvement of the overall photovoltaic performance of Cu 2 O .

Transition-metal doped, magnesium oxide-supported terbium oxides as catalysts for the oxidative coupling of methane

The effects of transition metal dopants on the catalytic properties of magnesia-supported terbium oxide catalysts in the oxidative coupling of methane were investigated. A few transition metals were included based on their oxidation states relative to Tb in TbO1.81 (where Tb is between Tb3+ and Tb4+). Ni and Fe were selected as low valence dopants (LVDs) and Zr and V were considered high valence dopants (HVDs) in the study. The effect of dopant concentration was also investigated, between 0.2 and 1.0 % by weight (wt%), since high loadings could result in segregation of the transition metal (TM) dopants, while low loadings may not yield observable effects. At the higher loading (1.0 wt%) it appears that the oxidation state relative to that of Tb is of minor importance, and the redox properties of the dopant have a larger impact. At this loading, the best performing catalysts were the Ni- and Zr-doped TbOx/n-MgO, while the selectivities of the Fe- and V-doped catalysts toward OCM products were very low, particularly at the lower temperatures (<600 °C). The facile redox reactions that allow Fe and V to easily change oxidation state are likely detrimental to the OCM reaction, particularly at higher concentrations where segregation of the transition metal oxide may occur at the surface. In contrast, at the lower loading (0.2 wt%) all doped catalysts performed better than the undoped TbOx/n-MgO. At this loading, it appears that the LVDs resulted in better performing OCM catalysts, as the Ni- and Fe-doped TbOx/n-MgO both had higher activities and selectivities compared with the Zr- and V-doped catalysts.The characterization data revealed that even at the low concentration of 0.2 wt%, the transition metals markedly alters the properties of the doped TbOx/n-MgO catalysts. For example, the number of basic sites was significantly increased by adding 0.2 wt% of Fe or Zr to the TbOx/n-MgO catalyst and this led to an improvement in the performance of the catalysts in the OCM reaction. The dopants also affected the oxidation state of Tb in the TbOx lattice as well as the dispersion of TbOx on the surface of the MgO. Both XPS and XRD data indicated that the TbO1.81, which was present on the surface of the fresh catalysts, in most cases was reduced to Tb2O3 during reaction. However, there were no evident trends to indicate that LVDs promote, while HVDs suppress, surface reduction of the host oxide, i.e. TbOx. The lack of trends may in part be due to the presence of the MgO support, which results in a complex catalyst system. Nevertheless, the data revealed that even low loadings of a transition metal dopant can positively affect the activity and selectivity of a TbOx/n-MgO catalyst.

Effects of transition metal dopants on the calcination of CaCO3 under Ar, H2O and H2

This study compares thermal decomposition of pure and transition metal-doped (Fe, Co, Ni, Cu, and Zn) calcium carbonate samples under ˜0.1 mbar of Ar, H2O, and H2 in order to evaluate the effects of doping on CO2 release and conversion. All samples were synthesized via precipitation methods at room temperature from calcium chloride and sodium carbonate precursors, with additional doping of the relevant transition metal chloride. Structural and compositional analysis of the as-prepared and calcined materials is presented. TM-doping results in an earlier onset of CO2 release as compared to Pure CaCO3 irrespective of calcination gas. Cu-doping induced the largest temperature reduction. Calcination in H2O produces an additional lowering of the release temperature, as compared with calcination in Ar, with the Zn-doped sample exhibiting the largest enhancement. During calcination in H2, the Ni-, Co- and Fe-doped samples produce a significant enhancement of CO2 to CO conversion, whereas the overall conversion by the Cu- and Zn-doped samples remained comparable to that of Pure CaCO3. The Ni-doped samples, which produced the highest CO2 conversion, showed the largest comparative enhancement when the calcination gas was changed to H2.

Effect of transition metal dopants (M= Nb, La, Zr, and Y) on the M-TiO2 supported V2O5 catalysts in the selective oxidation of H2S to elemental sulfur

P-25 TiO2 was modified by a small amount addition of different transition metal dopants (Nb, La, Zr, and Y) and employed as the supports for preparation of the V2O5/TiO2 catalysts for the selective H2S oxidation at low temperature (130 °C). The H2S removal capability was ranged in the order: V2O5/Y-TiO2 > V2O5/Zr-TiO2 ≈ V2O5/TiO2 > V2O5/La-TiO2 > V2O5/Nb-TiO2, which corresponded well to the chemically adsorbed oxygen on oxygen vacancies at catalyst surface (or surface oxygen) as revealed by XPS results. The Y addition could promote the oxygen vacancies formation because of the Ti4+ substitution by less valence Y3+ cations, leading to the improvement of H2S conversion. Whereas the modification by dopants, which have the extra valence cations or larger ionic radius (i.e., Nb or La) suppressed the oxygen vacancies formation, thus leading to lower activity. Despite the stronger interaction between V and Zr-TiO2, the amount of surface oxygen and H2S conversion of the V2O5/Zr-TiO2 were less than the V2O5/Y-TiO2, suggesting that although the formation of oxygen vacancies depended on the interaction between V and M-TiO2, the effect of Ti4+ substitution by transition metal dopant was more pronounced than the degree of interaction between V and M-TiO2. The H2S removal capability corresponded well to the oxygen vacancies being formed more than the degree of interaction between V and M-TiO2. From this study, the V2O5/Y-TiO2 showed the highest H2S conversion at ∼77% with low SO2 formation (less than 10 ppm).

Effect of ultrathin Fe dusting layer on electrical transport properties of few-layer MoS2 field-effect transistors

Transition-metal-doped MoS2 has been predicted to be a potential candidate for a two dimensional dilute magnetic semiconductor, while the effect of transition metal dopants on the electrical properties of MoS2-based devices has received relatively less attention so far. Here, the authors report on a systematic electrical transport study of Fe-dusted few-layer molybdenum disulfide (MoS2) field-effect transistors via repeated in situ Fe deposition (total nominal thickness ≤ 2 nm) and electrical measurements in ultrahigh vacuum. It is found that an ultrathin Fe adsorption layer (≪0.5 nm) n-dopes MoS2 without noticeably affecting the electrical characteristics of the device. In contrast, a thicker Fe layer (0.5–2 nm) results in the loss of the carrier tunability and a nonlinear current-voltage characteristic with the differential conductance approximately linear to drain voltage. The authors show through result analysis and qualitative modeling that Fermi level pinning in MoS2 directly underneath Fe nanoclusters plays an important role in causing these degradations. The pinning effect can be partially removed by in situ oxidizing the Fe nanoclusters for a short duration of 16 min. The mechanism of Fermi level pinning is discussed.

Catalytic activity of transition metal doped Cu(111) surfaces for ethanol synthesis from acetic acid hydrogenation: a DFT study

The effects of transition metal dopants on Cu(111) surfaces for ethanol synthesis from acetic acid hydrogenation are investigated.

Effect of transition metal dopants on the optical and magnetic properties of semiconductor nanocrystals

This review discusses the recent developments in doped semiconductor nanocrystals with a special emphasis on the effect of dopant on the electronic structure of the host nanocrystals. The review focusses on 3d transition metal dopants with unique electronic structure making them receptive for dramatic changes in magnetism, absorption and photoluminescence properties by the successful introduction of a small percentage of dopants into the nanocrystals. Many of these properties are shown to be qualitatively different from that of the bulk properties, leading to challenges in understanding the nature and effects of the confinement of the host. The optical and magnetic changes induced by Mn doping is first reviewed, followed by the use of Cu as a probe to understand the bulk and surface electronic structure of the host. The review concludes with a short section on photomagnetism induced by Cu on the host nanocrystal and a summary of the work with other transition metal ions.

Model Studies on Heterogeneous Catalysts at the Atomic Scale

Thin single crystalline oxide films comprise perfect supports to grow nanoparticles of metals and other catalytically relevant materials. The model systems thus created can be thoroughly investigated with respect to structure and/or chemical activity applying both, the techniques of surface science under ultrahigh vacuum conditions as well as the traditional techniques applied in catalysis to study chemical kinetics under ambient conditions. We discuss here in particular the oxidation of methanol to formaldehyde on ceria supported vanadia nanoclusters as well as the effect of transition-metal dopants in single crystalline oxides, such as CaO, on the activation of molecular oxygen.

Transition Metal Doping of Manganese Cobalt Spinel Oxides for Coating SOFC Interconnects

Manganese cobalt spinel oxide coatings have been shown to be effective for preventing chromium volatilization from interconnect alloys and associated cathode poisoning in solid oxide fuel cells. A reaction layer consisting of chromium-containing spinel formed due to interaction of the coating with the chromia scale on the alloy surface and increases the overall area specific resistance (ASR) of the system. Reduction in ASR can be accomplished by reducing the thickness and/or increasing the electrical conductivity of the reaction layer, which includes both the high-chromium spinel phase and the chromia scale. This paper reports the effects of transition metal dopants in manganese-cobalt spinel on the reaction products and chromia scale formed on a ferritic stainless steel alloy. Dense single-phase doped spinel ceramics were prepared through process optimization. High temperature diffusion couple experiments with sintered spinel and ferritic stainless steel SS 441 were carried out to understand the spinel-alloy interactions. Doping of the spinel oxide led to a reduction in the thickness of the chromia scale formed on the alloy. The difference in the morphologies and compositions of interaction layers at 800°C and 900°C provides insight on the formation and growth mechanism of a high-chromium spinel.

Effect of Transition Metal Dopants on Mechanical Properties and Biocompatibility of Zirconia Ceramics

In this study, the effect of transition metal dopants, originally added as colouring agents, on the mechanical properties and biocompatibility of sintered zirconia was investigated. This study confirmed that transition metal dopants could have a slight detrimental effect on the mechanical properties of zirconia. The addition of metal dopants did not affect the adhesion and proliferation of gingival fibroblasts.

A theoretical study of the effects of transition metal dopants on the adsorption and dissociation of hydrogen on nickel clusters

The structure, stability, adsorption, and dissociation of H2 on nickel clusters doped with late transition metals were investigated using density functional theory with the BP86 functional. Molecular hydrogen physisorption occurred at a vertex atom with a low coordination number. Charge transfer between clusters and the H2 molecule stabilized the physisorption. The chemisorption of H2 occurred at the bridge sites, without any structural or spin change of the clusters. Among the pentamer clusters, Cd, Zn, and Au had the lowest chemisorption energies, while Ir and Pt had higher chemisorption energies for hydrogen. The computed reaction energies and activation barriers for the dissociation mechanism showed that dopants such as Rh, Pd, Pt, and Au have endothermic reaction energies and low activation barriers. This facilitates the reversible adsorption/dissociation of the H2 molecule on these metal‐doped clusters. The dopant atoms play a major role in modulating the physisorption, chemisorption, and dissociation mechanism of H2 on nickel clusters. © 2013 Wiley Periodicals, Inc.

Periodic trends of oxygen vacancy formation and C–H bond activation over transition metal-doped CeO2 (1 1 1) surfaces

Substitutional transition metal dopants in the cerium oxide surface can alter the surface reducibility and catalytic activity for hydrocarbon conversion. Density functional theory (DFT+U) methods are used to examine the electronic and structural effects of transition metal dopants (groups IV–XII) in the CeO2 (111) surface. Surface reducibility (oxygen vacancy formation) and dissociative adsorption of methane (forming H* and CH3*) are considered. Both the methane dissociative adsorption energy and activation barriers correlate linearly with the surface oxygen vacancy formation energy. Charge analysis is used to determine the role of dopant metal in serving as a reduction center or altering the Ce reducibility. Dopants in groups IV and V alter the reducibility of the surface and dopants in groups X–XII become the reduction center. The dopant plays the same role in both oxygen vacancy formation and methane adsorption. A Brønsted–Evans–Polanyi relationship is established between the methane activation barrier, through H-abstraction, and the dissociative adsorption energy. The sensitivity of quantitative and qualitative trends to the inclusion of U terms for the dopant transition metal d-states is considered. The optimal M/CeO2 dopant for methane conversion to CO or CO2 follows a volcano relationship with oxygen vacancy formation: Highly reducible surfaces will be limited by re-oxidation, whereas surfaces difficult to reduce will show high barriers for C–H bond activation. Transition metal dopants near the peak region of the volcano are Pd, Co, Ni, and Mn.

Ab Initio Computational Studies of Mg Vacancy Diffusion in Doped MgB2 Aimed at Hydriding Kinetics Enhancement of the LiBH4 + MgH2 System

We report an ab initio density functional theory (DFT) study that investigates the effect of transition metal dopants on diffusion of Mg vacancies in MgB2. This study has implications for the diffu...

Transition metal-doped titanium(IV) dioxide: Characterisation and influence on photodegradation of poly(vinyl chloride)

The effects of transition metal dopants (V(IV), V(V), Mn(II), Cr(III), Mo(V), and W(V)), introduced into TiO 2, upon the rate of photodegradation of poly(vinyl chloride) (PVC) films containing TiO 2 have been measured. The rates were determined mainly by monitoring carbonyl group formation. In another set of experiments, the rates of chloride ion release from irradiated PVC particles suspended in water undergoing agitation with air or O 2 in the presence of particles of doped TiO 2 were measured electrochemically. The doping of TiO 2 (rutile) with Cr(III), V(V) or Mn(II) reduces the photoactivity of the pigment, while doping by Mo(V) or W(V) enhances its photoactivity; the results obtained from carbonyl index measurements are paralleled closely by those from chloride ion release. Even the most aggressive doped pigments were less reactive than Degussa P25 pigment, while the greatest protection to PVC film was offered by TiO 2 particles coated with Al 2O 3 or SiO 2. Overall, the photoactivity of doped TiO 2 is a complex function of dopant concentration, the energy levels of the dopants in the TiO 2 lattice, their d electronic configuration and their local distribution. Photoactivity is also linked to other factors such as crystal type, particle size distribution and surface area. There is a clear relationship between the tendency of the dopant to induce the rutile-to-anatase transition and its effect in enhancing the photoactivity of the pigment. The characterisation of the doped pigments was achieved using X-ray powder diffraction, EPR spectroscopy, diffuse reflectance UV–vis spectrophotometry, scanning optical and electron microscopy and particle size analysis using LALLS.