Nb-doped Sample Research Articles

Catalytic activity of Nb-doped α-Fe2O3—Heterojunction structure, ferrimagnet-like character, and high activity

Photocatalytic activity of α-Fe2O3 under visible light had been studied intensively. Rapid electron-hole recombination is the main drawback of α-Fe2O3 and the photocatalytic activity can be enhanced by transition metal doping and heterojunction structure. In the present study, α-Fe2O3 doped with various amounts of Nb were synthesized using a simple sol–gel method, and further characterization and catalytic activity were investigated. PXRD, TEM-EDS and Mössbauer measurement showed the formation of weak-ferromagnetic Nb-doped α-Fe2O3 and FeNbO4, where heterojunction structure was suggested. UV–vis spectrum showed a broad absorption in visible-light regions with a narrow band gap between 2.27 and 2.97 eV. Photo-Fenton reaction using α-Fe2O3 to decompose methylene blue (MB) was conducted to evaluate the catalytic activity. Nb-doped sample showed a significant improvement compared with pure α-Fe2O3. PXRD and Mössbauer spectroscopy revealed the α-Fe2O3 and FeNbO4 phases, which were attached to a magnet due to the ferrimagentic character of Nb-doped α-Fe2O3. The high catalytic activity with k value 7.1 * 10−2/min was obtained for 7.4Nb600. Higher catalytic activity was also observed for 20Nb600 and 40Nb600, which were driven by higher surface area, Nb substitution and heterojunction structure. Mechanistic analysis revealed that the primary reaction of MB decomposition is the Photo-Fenton process.

Conduction mechanism and shallow donor defects in Nb-doped β-Ga2O3 single crystals

Electrical properties, electronic defects, and photoluminescence (PL) of Nb-doped β-Ga2O3 crystals grown by the floating zone method have been studied in the temperature range from 10 to 350 K. The activation energies of shallow and deep traps were obtained and compared with their counterparts. The 0.25-Nb-doped Ga2O3 crystal has a metallic behavior at room temperature and a semiconducting behavior at low temperatures. This behavior was interpreted by the quantum corrections to the electrical resistivity resulting from the weak localization and Coulomb interaction. In addition, the 0.25-Nb-doped Ga2O3 crystal has a shallow donor ionization energy of 0.03 eV, indicating that it requires less energy to enable n-type conductivity. Hence, Nb is a good choice for improving the electrical conductivity of the β-Ga2O3 crystal. Three defect trap states were found at EC-0.81 eV, EC-0.49 eV, and EC-0.12 eV below the conduction band edge. The PL spectra of the undoped sample show two characteristic bands in the UV and blue regions under the excitation wavelength of 256 nm. On the other hand, in the Nb-doped β-Ga2O3 sample, a novel UV emission band with a shorter wavelength was observed.

High Throughput Doping Study of Na-Mn-O Cathode Materials

With the large demand for energy consumption, sodium-ion batteries have been considered as an alternative to lithium-ion batteries for energy storage1. Among various cathode materials, the Na-Mn-O system is promising with its high specific capacities and excellent battery life. Herein, we applied our high throughput (HTP) system2,3 to explore Na0.66Mn0.9M0.1O2+δ (M: dopant) chemical compositions to perform fast screening of the doping effect in these materials. Using a systematic approach, we establish the relationships between the doping species, crystal structure, electrochemical performance, and air/moisture stability. In detail, 52 different dopants were utilized and prepared as precursor solutions to achieve the HTP sol-gel synthesis. HTP-XRD is performed after sintering to understand the structural changes occurring in doping. HTP-cyclic voltammetry (CV) is then performed to analyze the electrochemical properties. Additionally, to comprehend the relationship between dopants and air stability, an accelerated ageing process is carried out at various humidity levels for several weeks.In this study, we found a broad range of dopants with 20 distinct elements that can be integrated into the layered Na-Mn-O structures, including several previously unstudied dopants (Si, Sc, Ga, Rb, Rh, Cs, Re, and Tl), confirmed by XRD refinements. It's interesting to note that at 10% composition, the dopant species can significantly alter the crystal structure. For the undoped sample, a P2/P’2 biphasic material is obtained by slow cooling. However, with the presence of Li, Mg, Fe, Co, Ni, Zn, and Ga, pure P2 structures are obtained with smooth CV curves, suggesting fewer phase transitions via doping. Meanwhile, Si, B, and Mo-doped samples can introduce more distorted P’2 structures, giving more high-voltage plateaus to increase the voltage of the batteries. Nevertheless, the Rh-doped sample achieves a pure P3 structure, exhibiting smooth CV and higher voltage, but fair cycling. Tunnel structure is also found by Ti-doped samples, with an increased average discharged voltage of 2.74V (undoped:2.47V) and excellent rate performance. A series of novel cathodes yields excellent battery performance, including a high specific capacity of 200 mAh g-1 for the Rb-doped sample, and good capacity retention of 100% after 10 cycles for the Nb-doped sample (undoped showed 92% retention). Significantly, a number of doped samples exhibit high reversible anion redox which was shown to be strongly correlated with the bond valence mismatch of the M-O bonds, suggesting by tuning the polarity of M-O bonds triggered by dopants can stimulate the anionic redox behaviour. Moreover, air/moisture stability analysis is conducted for co-doped samples and found that as little as 5% Li in co-doped samples can stabilize the layered structure dramatically. Na0.66Mn0.9Li0.05Co0.05O2, for instance, substantially improves the air stability to over 99 % structure retention, compared to 0 % in the undoped P2 NMO after a one-month accelerated aging protocol. Therefore, these new insights into the impacts of doping via high throughput screening can serve as new guidelines for rational battery material design and development. Figure 1: Heat map of specific capacities in the periodic table with various dopants. The inset image demonstrates the workflow of the high throughput system.

Nb-doped NCM622 shows improved capacity under high-temperature cycling—An experimental and theoretical study

Ni-rich layered lithiated transition metal oxide LiNi0.6Co0.2Mn0.2O2 (NCM622) is a promising candidate for Li-ion batteries. However, the high nickel content in NCM622 leads to significant capacity degradation during battery cycling. In this study, we investigated the effectiveness of niobium (Nb) doping in enhancing the performance of NCM622. Our findings demonstrate that Nb-doping reduces cation mixing and preserves the solid solution state of NCM622, as confirmed by calculated formation energy. Furthermore, the introduction of Nb imparts remarkable electrochemical properties to the material. Even under high-temperature and high-voltage conditions, the Nb-doped sample exhibits significantly improved cycling performance compared to the undoped sample, starting from the fifth cycle onwards. Through a combination of atomic-level mechanisms and experimental techniques, this research establishes that Nb-doping can enhance the structural stability and electrochemical properties of NCM622, providing a viable pathway for the industrial advancement of Ni-rich materials.

Nb-doped double perovskite Sr2CoFeO6−δ as an efficient and prospective electrode for quasi-symmetrical solid oxide fuel cells

The utilization of identical anode and cathode materials in symmetrical solid oxide fuel cells (SSOFCs) can efficiently reduce their fabrication costs and prolong their operational life. Herein, Nb-doped double perovskite Sr2CoFeO6-δ of a traditional cathode was employed as the electrode material for SSOFCs. Notably, the electrochemical performance and reversibility were improved due to Nb doping. The Nb-doped sample Sr2CoFe0.7Nb0.3O6−δ.5 (SCFN3) was synthesized in air, which decomposed into Co–Fe alloy, SrCoO2.25, and Sr3FeNbO6−δ in H2. Following calcination in air, the sample reverted to the original perovskite structure. The SCFN3 sample exhibited good chemical compatibility with SOFC electrolytes such as La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM), Sm0.2Ce0.8O2−δ, and BaZr0.1Ce0.7Y0.1Yb0.1O3−δ. A single cell consisting of 300-μm LSGM electrolyte exhibited maximum power densities of 951, 510, 563, and 397 mW/cm2 at 850℃ when using H2, C3H8, liquid C2H5OH, and solid C as fuels, respectively. These findings adequately demonstrate the fuel-flexible characteristics of SOFCs.

Photoelectric properties of pristine and niobium-doped tungsten disulfide layered crystals

Tungsten disulfide (WS2) is a transition-metal dichalcogenide that is suited to next-generation electronic and optoelectronic devices. Doping is necessary for WS2 applications because it allows the inherent characteristics of WS2 to be tuned to increase its functionality. This study determines the effect of niobium (Nb) doping on the photoelectric properties of WS2 crystals. A chemical vapor transport process is used to grow pristine and Nb-doped WS2 crystals. Scanning electron microscopy images show that the WS2 specimens consist of layered crystals. The photoresponsivity of the samples increases as the incident laser power decreases. Nb-doped WS2 crystals are 92 times more photoresponsive than pristine WS2 crystals at a laser power of 2 × 10−8 W. The ratio of the normalized photocurrent for a Nb-doped sample to that for a pristine sample increases as the frequency of the illumination increases to a maximum value of 11.5 at 104 Hz. Time-dependent photocurrent measurements show that the rise and fall times for Nb-doped WS2 crystals are shorter than those for pristine WS2 crystals by at least an order of magnitude. The results of this study show that Nb doping significantly increases the optical response of WS2 crystals.

Luminescence in undoped and Nb-doped SrTiO3 crystals: Bulk and surface emission

The luminescenceof SrTiO3 depends on the sample type, either doped or stoichiometric, as grown or treated, the excitation conditions, and temperature. The origin of the luminescence emissions, blue, green, and infrared, remains controversial. In particular, the role played bydefects, mainly oxygen vacancies, impurities, and self-trapped holes and electrons on the different emissions are far to be elucidated. We present a cathodoluminescence (CL) and photoluminescence (PL) study of undoped and Nb-doped samples. The different excitation conditions of CL and PL permit to distinguish the luminescence emission from the bulk (CL) and from a surface skin region (PL). Significant differences between both techniques are seen for the undoped sample, while the Nb-doped sample presents less differences, highlighting the role played by the surface defects and the doped electrons. The study is complemented by the temperature dependence of the luminescence spectra and the emission due to defects generated by plastic deformation.

Band Bending and Ratcheting Explain Triboelectricity in a Flexoelectric Contact Diode.

Triboelectricity was recognized millennia ago, but the fundamental mechanism of charge transfer is still not understood. We have recently proposed a model where flexoelectric band bending due to local asperity contacts drives triboelectric charge transfer in non-metals. While this ab initio model is consistent with a wide range of observed phenomena, to date there have been no quantitative analyses of the proposed band bending. In this work we use a Pt0.8Ir0.2 conductive atomic force microscope probe to simultaneously deform a Nb-doped SrTiO3 sample and collect current-bias data. The current that one expects based upon an analysis including the relevant flexoelectric band bending for a deformed semiconductor quantitively agrees with the experiments. The analysis indicates a general ratcheting mechanism for triboelectric transfer and strong experimental evidence that flexoelectric band bending is of fundamental importance for triboelectric contacts.

Photoelectrochemical properties of butane flame-treated niobium-doped hematite thin films grown by the liquid-phase deposition method

In this study, thin films of undoped and Nb-doped hematite were grown on FTO-coated glass substrates via the liquid-phase deposition method as photoanodes for PEC water splitting applications. The samples were then annealed and flame-treated for 90 s with a butane flame. At all doping concentrations, the thinnest films (~200 nm) yielded the best properties. FESEM images showed that film morphology is not dependent on Nb-doping concentration. Compared with the undoped hematite film, the optimum Nb-doped sample showed an approximately five-fold increase in photocurrent density at 1.6 V vs. RHE and a ~0.3 eV decrease in energy band gap value. The charge carrier density value of the optimum Nb-doped sample was shown to increase from 1.38 × 1017 to 3.54 × 1018 cm-3 after flame treatment due to the introduction of oxygen vacancies. The number of times the reducing butane flame is applied was shown to affect the PEC performance, both positively and negatively, giving rise to an approximately sixteen-fold increase in photocurrent density at 1.6 V vs. RHE in comparison with the untreated sample after four times of application. Butane flame treatment was also shown to facilitate the process of charge transfer in Nb-doped hematite and its interface with the electrolyte.

Metal doping in topological insulators- a key for tunable generation of terahertz

The unique surface edge states make topological insulators a primary focus among different applications. In this article, we synthesized a large single crystal of Niobium (Nb)-doped Bi2Se3 topological insulator (TI) with a formula Nb0·25Bi2Se3. The single crystal has characterized by using various techniques such as Powder X-ray Diffractometer (PXRD), DC magnetization measurements, Raman, and Ultrafast transient absorption spectroscopy (TRUS). There are (00l) reflections in the PXRD, and Superconductivity ingrown crystal is evident from clearly visible diamagnetic transition at 2.5 K in both FC and ZFC measurements. The Raman spectroscopy is used to find the different vibrational modes in the sample. Further, the sample is excited by a pump of 1.90 eV, and a kinetic decay profile at 1.38 eV is considered for terahertz analysis. The differential decay profile has different vibrations, and these oscillations have analyzed in terms of terahertz. This article not only provides evidence of terahertz generation in Nb-doped sample along with undoped sample but also show that the dopant atom changes the dynamics of charge carriers and thereby the shift in the Terahertz frequency response. In conclusion, a suitable dopant can be used as a processor for the tunability of terahertz frequency in TI.

Preparation and Characterization of High Mobility Nb-Doped SnO<sub>2</sub> Transparent Conducting Films

Metal organic chemical vapor deposition (MOCVD) was employed for the preparation of niobium (Nb)-doped SnO2 films on SiO2 glass substrates. The structure, optical and electrical properties of the Nb-doped SnO2 films were systemically studied. The X-ray diffraction results indicated that the polycrystalline rutile SnO2 films were obtained with a preferred SnO2 [110] growth direction. Among which, the 5.4 at.% Nb-doped SnO2 film showed the lowest resistivity of 1.0×10-3 Ω∙cm and the highest Hall mobility of 74 cm2∙V-1∙s-1. The average visible light transmittance of the 5.4 at.% Nb-doped SnO2 sample was more than 79%. The obtained Nb-doped SnO2 films exhibited low resistivity, high Hall mobility and good transparency, which might have wide applications in electric and photoelectric devices.

Nb-doping in LiNi0.8Co0.1Mn0.1O2 cathode material: Effect on the cycling stability and voltage decay at high rates

Abstract Layered structure nickel-rich LiNi0.8Co0.1Mn0.1O2 possesses the superiority of high discharge capacity and voltage platform. However, the cycling instability and seriously voltage decay limited its cosmically commercial. Herein, Nb-doped Li(Ni0.8Co0.1Mn0.1)O2 cathode materials are synthesized by urea hydrothermal method assisted high temperature solid state process aimed at improving the cycle stability and inhibiting the voltage decay. The analysis results show that the Nb-doped samples reveal a silkworm shape with loosely aggregated surface, stable structure and enlarged diffusion coefficient of Li-ion. The electrochemical data indicates that Nb-doping not only enhance the rate and cycle performance at a high cut-off voltage of 4.6 V, but also inhibit the voltage decay at high rates. Between 2.8 V and 4.6 V, after 100 cycles, the capacity retention of 1% Nb-doped sample is 10.7% and 17% higher than that of the pristine sample at 1 C and 5 C, respectively.

Garnet-like Li7-xLa3Zr2-xNbxO12 (x = 0−0.7) solid state electrolytes enhanced by self-consolidation strategy

Garnet-like Li7La3Zr2O12 electrolytes with Nb doping are synthesized by self-consolidation method. Different from conventional methods such as cold or hot isostatic pressing, not any pressing assistance is employed throughout the preparation process. Although the preparation process is dramatically simplified, both density and ionic conductivity of the obtained samples are enhanced. Nb-doped content plays a key role in the sintering of the packed precursor powders and in the structure stabilization of the obtained bulk samples. The optimized 0.60 mol Nb-doped sample with relative density of 94%, fine particle boundaries, and solitary cubic structure possesses the maximum total ionic conductivity of 5.22 × 10−4 S cm−1 at 30 °C, which is comparable to the highest reported value of the samples prepared by conventional pressing methods. This work verifies that self-consolidation strategy is effective, reliable, and productive for the preparation of cubic Li7La3Zr2O12 electrolyte, which would significantly facilitate the development of ceramic electrolyte membrane technology.

Doping with Nb enhances the photoresponsivity of WSe2 thin sheets

In this study, we used chemical vapor transport to grow undoped and niobium (Nb)-doped tungsten diselenide (WSe2) thin sheets and then investigated the structural and photoelectronic characteristics of both samples. X-ray photoelectron spectroscopy confirmed the presence of Nb atoms in the Nb-doped WSe2 sample. Hall effect measurements of the transport behavior of the carriers in the undoped and Nb-doped WSe2 indicated that the Nb-doped WSe2 was of p-type and had a higher carrier concentration and lower mobility than the undoped WSe2. The current density–voltage characteristics indicated that doping with Nb led to a decrease in resistance. Photoconductivity measurements revealed that the responsivity of the Nb-doped WSe2 was an order of magnitude greater than that of the undoped WSe2. Thus, doping Nb atoms into WSe2 not only provides effective carriers but also enhances the photoresponsivity significantly. Accordingly, doping WSe2 with Nb atoms would appear to be useful for the fabrication of highly sensitive photodetectors.

Influence of Nb Doping Concentration on Bolometric Properties of RF Magnetron Sputtered Nb:TiO2−x Films

The present study directly addresses the improved bolometric properties by means of different Nb doping concentrations into TiO2−x films. The x-ray diffraction patterns do not display any obvious diffraction peaks, suggesting that all the films deposited at room temperature had an amorphous structure. A small binding energy shift was observed in x-ray photo electron spectroscopy due to the change of chemical composition with Nb doping concentration. All the device samples exhibit linear I–V characteristics, which attests to the formation of good ohmic contact with low contact resistance between the Nb:TiO2−x (TNO) film and the electrode (Ti) material. The performance of the bolometric material can be evaluated through the temperature coefficient of resistance (TCR), and the absolute value of TCR was found to be increased from 2.54% to 2.78% with increasing the Nb doping concentration. The voltage spectral density of 1/f noise was measured in the frequency range of 1–60 Hz and found to be decreased with increase of Nb doping concentration. As a result, for 1 at.% Nb-doped TNO sample exhibits improved bolometric properties towards the good infrared image sensor device.

Fabrication of Nb-doped ZnO nanowall structure by RF magnetron sputter for enhanced gas-sensing properties

An Nb-doped ZnO nanowall is fabricated by radio-frequency (RF) magnetron sputtering at 250 °C for application in a gas sensor. The fabricated Nb-doped ZnO nanowall is characterized by field emission scanning microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The Nb-doped ZnO nanowall is observed to show a predominant exposed polarized (002) plane. The specific surface area is increased relative to that of the parent ZnO nanowall from 9.2 ± 0.42 m2 g−1 to 9.8 ± 0.52 m2 g−1. The segregation of Nb does not occur in the Nb-doped sample observed by TEM; this indicates that the addition of Nb occurs within the solid-solution limit of the ZnO host lattice. The Nb-doped ZnO nanowall gas sensor presents excellent gas sensing properties for acetone and ethanol gases at a relatively low operating temperature. We confirm the effects of Nb-doping on the ZnO nanowall, such as the exposed (002) plane, specific surface area increase, and electronic structural changes. Nb doping of the ZnO host lattice is a promising method that enhances the acetone and ethanol gas-sensing capacity at a relatively low operating temperature.

Nb-doped Ga2O3 as charge-trapping layer for nonvolatile memory applications

The charge-trapping properties of Nb-doped Ga2O3 are investigated by using an Al/Al2O3/GaNbO/SiO2/Si structure. Compared with the memory capacitor with pure Ga2O3, the one with lightly Nb-doped Ga2O3 shows better charge-trapping characteristics, including larger memory window (5.5V at ±6V sweeping voltage), higher programming and erasing speeds due to its higher trapping efficiency resulted from increased trap density induced by the Nb doping. The sample with heavily Nb-doped Ga2O3 also shows improvements compared with the one with pure Ga2O3, but not as large as those for the lightly Nb-doped sample due to defects generated by excessive Nb doping. Erase saturation phenomenon is observed in all the samples, and can be suppressed by hole traps created by the Nb doping.

Effect of niobium addition on magnetization reversal behavior for SmCo-based magnets with TbCu7-type structure

AbstractThe effect of Nb addition on the microstructure and magnetic properties of nanocrystalline Sm(CobalNbxZr0.02)7 permanent magnet were investigated. The magnetization reversal behavior for ball milled Sm(CobalNbxZr0.02)7 samples with high coercivity was investigated by analyzing hysteresis curves and recoil loops of demagnetization curves. Nb addition proved to result in relevant improvement in the magnetic properties, especially in the coercivity Hc. It was shown that the magnetic properties of Sm(CobalNbx- Zr0.02)7 nanocrystalline magnets were improved by an additional 0.06 at.% Nb. In particular, Hc was improved from 602 to 786 kA/m at room temperature. The maximum value of the integrated recoil loops area for 0.06 at.% Nb-doped samples of 1.81 kJ/m3 was much lower than that of the Nb-free sample, which could be explained by a smaller recoverable portion of the magnetization remaining in the Nb-doped sample when the applied field was below the coercivity Hc. The nucleation field Hn for irreversible magnetization reversal of the magnetically hard phase were calculated by analyzed in terms of the ΔMirrev-H curve and the Kondorsky model.

Effect of Nb doping on electrochemical properties of LiNi1/3Co1/3Mn1/3O2 at high cutoff voltage for lithium-ion battery

Nb doped cathode materials with the formula LiNi1/3Co1/3Mn1/3−xNbxO2 (x=0, 0.01, 0.02, 0.03) have been prepared successfully by sol–gel method. The effect of Nb substitution on the crystal structure and electrochemical properties of LiNi1/3Co1/3Mn1/3O2 were studied systematically by X-ray diffraction (XRD) and various electrochemical measurements. The results showed Nb substitution played an important role in the good cycling performance of LiNi1/3Co1/3Mn1/3O2. Charge/discharge tests revealed that LiNi1/3Co1/3Mn1/3–0.02Nb0.02O2 showed a capacity retention of 94.1% at 1C after 50 cycles in a high cutoff voltage range (3.0–4.6V), while discharge capacity of LiNi1/3Co1/3Mn1/3O2 remains only 89.4% of that at 1C. Ex-situ XRD analysis and EIS analysis indicated that the improved electrochemical properties of Nb-doped sample result from the more stable structure and lower resistance during the electrochemical cycling.

Magnetic properties and microstructure of nanocomposite (Nd,Pr)–Fe–B ribbons by doping Nb element

Effects of Nb addition and annealing treatment on magnetic properties and microstructure of (Nd0.4Pr0.6)9Fe76–xNbxB15 (x = 0–4) ribbons were systematically investigated by means of vibrating sample magnetometer (VSM) and X-ray diffraction (XRD). The extra phases with nonmagnetic (Nd,Pr)1.1Fe4B4 phase and metastable compound (Nd,Pr)2Fe23B3 crystallized during quenching the Nb-free alloy. Moreover, the nonmagnetic (Nd,Pr)1.1Fe4B4 phase does not diminish during the following annealing treatment. The addition of Nb to (Nd,Pr)–Fe–B alloy suppresses metastable (Nd,Pr)2Fe23B3 and nonmagnetic (Nd,Pr)1.1Fe4B4 phases. The intrinsic coercivity increases from 397 kA·m−1 for the Nb-free sample to 1,091 kA·m−1 for the 4 at% Nb-doped sample optimally annealed. The Nb-free sample has the magnetic properties with Js = 1.04 T, Jr = 0.66 T, and (BH)max = 43.5 kJ·m−3. By comparison, the magnetic properties of the 4 at% Nb-doped sample were 0.97 T, 0.68 T, and 65.7 kJ·m−3, respectively. The significant improvement of magnetic properties mainly originates from the finer grains of the ribbons by introducing Nb.