Homogeneous Doping Research Articles

Performance modulation through selective, homogenous surface doping of lanthanum strontium ferrite electrodes revealed by in situ PLD impedance measurements.

Accelerating the oxygen reduction kinetics of solid oxide fuel cell (SOFC) cathodes is crucial to improve their efficiency and thus to provide the basis for an economically feasible application of intermediate temperature SOFCs. In this work, minor amounts of Pt were doped into lanthanum strontium ferrite (LSF) thin film electrodes to modulate the material's oxygen exchange performance. Surprisingly, Pt was found to be incorporated on the B-site of the perovskite electrode as non metallic Pt4+. The polarization resistance of LSF thin film electrodes with and without additional Pt surface doping was compared directly after film growth employing in situ electrochemical impedance spectroscopy inside a PLD chamber (i-PLD). This technique enables observation of the polarization resistance of pristine electrodes unaltered by degradation or any external contamination of the electrode surface. Moreover, growth of multi-layers of materials with different compositions on the very same single crystalline electrolyte substrate combined with in situ impedance measurements allow excellent comparability of different materials. Even a 5 nm layer of Pt doped LSF (1.5 at% Pt), i.e. a Pt loading of 80 ng cm−2, improved the polarization resistance by a factor of about 2.5. In addition, p(O2) and temperature dependent impedance measurements on both pure and Pt doped LSF were performed in situ and obtained similar activation energies and p(O2) dependence of the polarization resistance, which allow us to make far reaching mechanistic conclusions indicating that Pt4+ introduces additional active sites.

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Raman spectroscopy, laser conoscopy and photoinduced light scattering methods have been applied to comparatively study composition uniformity of strongly doped LiNbO3 crystals with a magnesium concentration close to a threshold value ≈5 mol% MgО, grown from a charge synthesized using precursor Nb2O5:Mg (homogeneous doping method) and at direct addition of magnesium to the melt (direct doping method). It has been shown that application of homogeneous doping method allows one to obtain compositionally more homogeneous heavily doped LiNbO3:Mg crystal than direct melt doping method.

Влияние способа сильного легирования на композиционную однородность и оптические свойства монокристаллов LiNbO-=SUB=-3-=/SUB=- : Mg

Raman spectroscopy, laser conoscopy and photoinduced light scattering methods have been applied to comparatively study composition uniformity of strongly doped LiNbO3 crystals with a magnesium concentration close to a threshold value ≈5 mol% MgО, grown from a charge synthesized using precursor Nb2O5:Mg (homogeneous doping method) and at direct addition of magnesium to the melt (direct doping method). It has been shown that application of homogeneous doping method allows one to obtain compositionally more homogeneous heavily doped LiNbO3:Mg crystal than direct melt doping method.

Фотолюминесценция в ближней ИК-области кристаллов ниобата лития различного состава и генезиса

The dependence of photoluminescent of lithium niobate crystals of different composition and genesis in the near IR- region on stoichiometry, Zn concentration, doping method was observed. It was found that increasing in stoichiometry leaded to decreasing in the photoluminescence. At Zn = 6.5 mol. % in the LiNbO3 crystal an intense emission was observed at 1.45 eV with simultaneous quenching of the NbLi emission. Homogeneous doping method of LiNbO3:Mg,Zn crystal allowed to enhance the luminescence in the region of 1.3–1.5 eV relative to the crystals obtained by direct doping method of the melt.

Polymorph controlled synthesis and photoluminescence properties of Eu3+ doped BiNbO4

Homogeneous doping of rare earth (RE) ions at atomic level is ideal for enhancing luminescence efficiency. Molten-salt synthesis facilitates crystal growth with improved phase-selectivity and dopant homogeneity. Herein, we present a facile and phase-selective synthesis of orthorhombic α-BiNbO4 and monoclinic β-BiNbO4 under Na2SO4-K2SO4 fluxes in the temperature range of 900 °C to 1050 °C. A series of Eu3+ doped BiNbO4 samples with different crystalline structures were obtained. The influences of Eu3+ doping on the crystal structure were systematicly investigated. α-BiNbO4 has a higher Eu3+-doping limit (>15 mol%) while β-BiNbO4 was unstable upon Eu3+ doping and gradually transformed into α-BiNbO4 with the increase of Eu3+ doping concentration. Moreover, all prepared Eu3+ doped BiNbO4 samples can be effectively excited by near-ultraviolet and blue light and exhibit intense red emissions due to the 5D0/7F2 transition of Eu3+ ions. In addition, the quenching concentration can reach above 10 mol% Eu3+ for both polymorphs. The temperature-dependent emission results revealed that α-BiNbO4 possesses a superior thermal stability with activation energy of 0.271 eV, larger than β-BiNbO4 with activation energy of 0.233 eV. The results reported here provide new insights into the understanding of polymorph-dependent property for photon down-conversion rare-earth luminescence of BiNbO4:Eu3+.

Slow oxidation of magnetite nanoparticles elucidates the limits of the Verwey transition

Magnetite (Fe3O4) is of fundamental importance for the Verwey transition near TV = 125 K, below which a complex lattice distortion and electron orders occur. The Verwey transition is suppressed by chemical doping effects giving rise to well-documented first and second-order regimes, but the origin of the order change is unclear. Here, we show that slow oxidation of monodisperse Fe3O4 nanoparticles leads to an intriguing variation of the Verwey transition: an initial drop of TV to a minimum at 70 K after 75 days and a followed recovery to 95 K after 160 days. A physical model based on both doping and doping-gradient effects accounts quantitatively for this evolution between inhomogeneous to homogeneous doping regimes. This work demonstrates that slow oxidation of nanoparticles can give exquisite control and separation of homogeneous and inhomogeneous doping effects on the Verwey transition and offers opportunities for similar insights into complex electronic and magnetic phase transitions in other materials.

Steering electron transfer using interface engineering on front-illuminated robust BiVO4 photoanodes

Front solar illumination on BiVO 4 is becoming important to design tandem structures in photoelectrochemical (PEC) water splitting. However, the interfacial recombination of BiVO 4 /substrate (e.g. FTO) is more serious than the bulk/surface recombination, which influences the electron transfer to the back side for hydrogen reaction. The BiVO 4 electrode therefore requires back illumination to obtain the best photoresponse. In this context, we proposed a universal strategy by applying a PdO layer functionalized SnO 2 interface as a useful electron transport layer to strikingly reduce the interface recombination. A trace amount of Pd was doped in the bulk BiVO 4 to modify the electronic structure by inducing a stronger dopant-host interaction than that of the surface-modified electrode and homogenous doping. The BiVO 4 /PdO electrodes exhibited similar PEC performance under both front and back illumination. The champion cocatalyst-free photoanode showed a record-high photocurrent of 4.90 mA cm −2 at 1.23 V RHE with a low onset potential of 0.15 V RHE in borate buffer electrolyte under AM 1.5 G front-illumination, accompanying with solar conversion efficiency of 1.72% at 0.6 V RHE . Equally importantly, a sustained long-term photostability over 80 h was achieved, which is the best performance and stability reported among the cocatalyst-free BiVO 4 photoelectrodes from front illumination. This work will open a new avenue for high-stability-performance photoelectrode design. The interfacial recombination of BiVO 4 /substrate is a serious issue for front-illuminated BiVO 4 on designing tandem structures with a wireless configuration. An effective PdO deposition strategy as gate interlayer is proposed to functionalize substrate and boost the electron transfer to the back side for hydrogen reaction. A trace amount of Pd doped into BiVO 4 increases the charge transfer rate of the bulk material. The electrode exhibits similar PEC performances under both front and back illumination, accomanying with long-term photostability over 80 h under solar front illumination. • Interface engineering by appplying PdO layer between the BiVO 4 /substrate is induced for steering electron transfer. • Cathodic photocorrosion is reduced for stabilizing front-illuminated BiVO 4 photoanode. • Effective carrier management is well controlled for reverse gradient Pd doping in BiVO 4 with increased oxygen vacancies. • A photocurrent of 4.9 mA cm −2 at 1.23 V RHE and a long-term photodurability over 80 h are obtained for cocatalyst-free BiVO 4 .

Valence control of charge and orbital frustrated system YbFe2O4 with electrochemical Li+ intercalation

We report an attempt valence control of the mixed valence iron triangular oxide YbFe2O4 to develop an effective technique controling the frustration of charges in strongly correlated electron systems. The electrochemical doping of Li + into YbFe2O4 was examined on a cell-type sample similar to the Li-ion secondary battery cell. Systematic changes in the lattice constant and Fe – Fe and Fe–Yb distance were observed with Li doping. Maximum value of the doping was over 300 mAh/g. An EXAFS experiment indicated that Li positioned between Yb octahedron layer (U-layer) and Fe-bipyramidal layer (W-layer). However, detailed change of iron valence state of YbFe2O4was not clearly observed because of the superimpose of the signal from iron metal nano particles in XANES observation. We discuss that the uncertainty might arise from the inhomogeneous distribution of the sample particle size, which might prevent the homogeneous doping of Li because the doping occurs on the surface of each nano-particles.

Bifunctional Catalytic Effect of CoSe2 for Lithium–Sulfur Batteries: Single Doping versus Dual Doping

AbstractDesigning catalysts to accelerate the conversion of soluble lithium polysulfides (LiPSs) is regarded as a promising strategy to inhibit the shuttle effect, improving cathode performance of lithium–sulfur batteries (LSBs). Herein, a bifunctional Ni/Zn dual‐doped CoSe2 is designed to enhance the catalytic effect of CoSe2. Specifically, Ni functions better in catalyzing the conversion of LiPSs into Li2S with shorter CoS bond and longer SS bonds than Zn, while Zn demonstrates a better catalytic effect for Li2S decomposition with reduced Li2S decomposition barrier and elongated LiS bonds than Ni. Those endows Ni/Zn dual‐doped CoSe2 with bifunction in catalyzing conversion reaction of LiPSs and Li2S precipitation as well as the decomposition of Li2S, originating from the modified molecular structure of LiPSs and Li2S. As a result, the sulfur cathode with Ni0.1Zn0.1Co0.8Se2 achieves a rate capability of 681.74 mAh g–1 at 2C, and a cycling stability for 400 cycles with a decay rate of 0.065%. This work provides an effective way to improve catalytic effect of transition metal compounds for advanced LSBs by homogenous dual doping.

Chromium Doped UO2-Based Ceramics: Synthesis and Characterization of Model Materials for Modern Nuclear Fuels.

Cr-doped UO2 as a modern nuclear fuel type has been demonstrated to increase the in-reactor fuel performance compared to conventional nuclear fuels. Little is known about the long-term stability of spent Cr-doped UO2 nuclear fuels in a deep geological disposal facility. The investigation of suitable model materials in a step wise bottom-up approach can provide insights into the corrosion behavior of spent Cr-doped nuclear fuels. Here, we present new wet chemical approaches providing the basis for such model systems, namely co-precipitation and wet coating. Both were successfully tested and optimized, based on detailed analyses of all synthesis steps and parameters: Cr-doping method, thermal treatment, reduction of U3O8 to UO2, green body production, and pellet sintering. Both methods enable the production of suitable model systems with a similar microstructure and density as a reference sample from AREVA. In comparison with results from the classical powder route, similar trends upon grain size and lattice parameter were determined. The results of this investigation highlight the significance of subtly different synthesis routes on the properties of Cr-doped UO2 ceramics. They enable a reproducible tailor-made well-defined microstructure, a homogeneous doping, for example, with lanthanides or alpha sources, the introduction of metallic particles, and a dust-free preparation.

Electrospinning of polyvinylidene fluoride membranes: effect of membrane composition and fabrication conditions

Membrane fabrication by the electrospinning can provide tunable structure membranes with interconnected pores, large surface area, and high porosity. This study focuses on studying the effect of different membrane compositions (i.e., polymer concentration and inorganic additives) and electrospinning conditions (i.e., internal needle diameter, tip to collector distance, and feed flow rate) on the quality of polyvinylidene fluoride (PVDF) flat sheet membrane. A homogeneous dope solution was prepared by dissolving the PVDF in a mixture of dimethylacetamide (DMAc) and acetone. By using a scanning electron microscope (SEM), the electrospinning conditions producing beadles fibers are as follows: 16 wt% polymer concentration, 0.004 wt% LiCl concentration, 0.51 mm internal needle diameter, 15 cm tip to collector distance, feed flow rate of 1 ml/hr. Further analysis was conducted by using Fourier Transform Infrared Spectrometer (FTIR) and X-Ray Diffraction (XRD) to assess the impact of electrospinning on fibers elemental composition. Additionally, static water contact angle, porosity, fiber diameter, and pore size measurements reveal the formation of submicron hydrophobic fibers with random alignment.

Accurate guided alternating atomic layer enhance internal electric field to steering photogenerated charge separation for enhance photocatalytic activity

Bi4O5BrxI2-xCy solid solutions with simultaneous bismuth-enriched and low temperature homogeneous C doping were obtained by the phase crystal reforming of BiOI with solvothermal alkali assisted molten salt method. The internal electric field between alternating atomic layers was significantly enhanced due to the synergism of formation of solid solutions, enrichment Bi and homogeneous C doping. Driven by the strong internal electric field, the photogenerated electrons and holes of Bi2.8C1.2O5Br0.5I1.5 were confined to the [Bi2.8C1.2O5]2+ layer and the double [Br0.5I1.5]− layer, respectively, then effective separation of charge carriers was achieved. The removal rates over Bi2.8C1.2O5Br0.5I1.5 of phenol, 4-chlorophenol, resorcinol and hydroquinone were 100 % in 1 h under visible light irradiation, which followed the parallel attack mechanism of O2- and h+. This study provided a reference for the multiple coordinated regulation of internal electric field by formation of solid solutions, enrichment Bi and homogeneous doping.

Ultrahigh activity of molybdenum/vanadium-doped Ni-Co phosphides nanoneedles based on ion-exchange for hydrogen evolution at large current density

Heteroatom doping is a promising strategy to optimize the electronic structure of transition metal phosphides so enhancing the hydrogen evolution reaction (HER). However, complex and harsh experimental design is often required to achieve homogeneous doping of corresponding elements while achieving the best regulating effect. Herein, a facile ion-exchange (IE) strategy is applied to dope Mo/V species evenly into Ni-Co phosphides under mild conditions while maintaining the nanoneedle morphology. The electrochemical characterization verifies Mo dopants have a better electronic regulation effect on NiCoP crystal than V dopants, corresponding to the better hydrogen evolution performance of Mo-NiCoP/NF. Notably, due to the highly dispersed nanoneedle morphology, the synergistic effect of Ni-Co phosphides, and the optimized electronic structure, Mo-NiCoP/NF demonstrates a higher activity than that of the noble metal Pt/C at the high current density (>99 mA cm−2). The present work is supposed to open new sights for the development of high-performance catalysts by ion-exchange strategy.

Laminar N-Doped Carbon Materials from a Biopolymer for Use as a Catalytic Support for Hydrodechlorination Catalysts.

Nitrogen-doped porous carbons were prepared using a chitosan biopolymer as both a carbon and nitrogen precursor and metallic salts (CaCl2 and ZnCl2-KCl) as a templating agent with the aim of evaluating their performance as catalyst supports. Mixtures of chitosan and templating salts were prepared by simple grinding subjected to pyrolysis and finally washed with water to remove the salts. The resulting materials were characterized, showing that homogeneous nitrogen doping of carbon was achieved (7–9% wt.) thanks to the presence of a nitrogen species in the chitosan structure. A lamellar morphology was developed with carbon sheets randomly distributed and folded on themselves, creating slit-shaped pores. Substantial porosity was observed in both the micropore and mesopore range with a higher surface area and microporosity in the case of the materials prepared by ZnCl2-KCl templating and a larger size of mesopores in the case of ZnCl2. Catalysts with well-dispersed Pd nanoparticles (around 10 nm in diameter size) were synthesized using the chitosan-based carbons obtained both by salt templating and direct chitosan pyrolysis and tested in the aqueous phase hydrodechlorination of 4-chlorophenol. The fast and total removal of 4-chlorophenol was observed in the case of catalysts based on carbons obtained by templating with CaCl2 and ZnCl2-KCl in spite of the low metal content of the catalysts (0.25% Pd, wt.).

Fabrication of Gd2O2S:Tb scintillation ceramics from the uniformly doped nanopowder

Abstract Single-phase 2Gd2O3·(Gd,Tb)2(SO4)3·nH2O precursor with strong crystallinity was synthesized by the water-bath method through optimizing the introducing method of Tb3+ doping, where Tb3+ was introduced from Tb(NO3)3. The formation process and phase evolution of the precursor were analyzed in detail. Compared with the conventional method of introducing Tb3+ via Tb4O7, the inhomogeneous doping caused by the appearance of 2Tb2O3·(Gd,Tb)2(SO4)3·nH2O in the precursor was avoided. Pure Gd2O2S:Tb nanopowders with layered structure were obtained after calcination in flowing H2-Ar. The Gd2O2S:Tb powder prepared with Tb(NO3)3 has higher XEL intensity than that with Tb4O7. Using the synthesized powders as starting material, Gd2O2S:Tb scintillation ceramics with high relative density (over 99% of the theoretical value) were fabricated by vacuum pre-sintering and HIP post-treatment in an argon atmosphere. The influences of the introducing method of Tb3+ doping on the microstructure, photoluminescence and X-ray excited luminescence of the Gd2O2S:Tb ceramics were studied. The ceramic sample prepared with Tb(NO3)3 has better luminescence properties due to the homogeneous doping in the ceramics. Temperature-dependent XEL measurements imply that the green emission of the ceramic sample is always dominant with the increase of temperature from 77 K to 340 K, attributed to the cross-relaxation between 5D3 and 5D4 levels of Tb3+.

Multifunctional red-emission graphene quantum dots with tunable light emissions for trace water sensing, WLEDs and information encryption

In this study, red-emission graphene quantum dots (R-GQDs), with tunable emission wavelength, are synthesized and explored for applications in trace water sensing, white light emitting diodes (WLEDs) and information encryption. Systematic experiments and characterizations demonstrate that the as-fabricated R-GQDs possess homogeneous size, good crystallinity and high doping concentration of nitrogen and oxygen. Moreover, tunable multicolor light emissions of R-GQDs are demonstrated by simply adjusting the polarity of surrounding solvents. As a result, solid-state fluorescence films are successfully fabricated to construct a high-quality WLED, with the CIE (Commission International d′Eclairage) coordinates (0.352, 0.380) and color temperature of 5121 K. Besides, the water quenchable fluorescence of R-GQDs enables the fabrication of a trace water sensing platform, with the low detection limit of 0.034%. Finally, the potential of our prepared R-GQDs for the application in color visual information encryption is developed. This study may pave the way for the development of GQDs, especially in optically sensing, optoelectronics and anti-counterfeiting technology.

Examination of structure and optical properties of Ce3+-doped strontium borate glass by regression analysis

Amorphous materials with non-periodic structures are commonly evaluated based on their chemical composition, which is not always the best parameter to evaluate physical properties, and an alternative parameter more suitable for performance evaluation must be considered. Herein, we quantified various structural and physical properties of Ce-doped strontium borate glasses and studied their correlations by principal component analysis. We found that the density-driven molar volume is suitable for the evaluation of structural data, while chemical composition is better for the evaluation of optical and luminescent data. Furthermore, the borate-rich glasses exhibited a stronger luminescence due to Ce3+, indicating a higher fraction of BO3/2 ring and larger cavity. Moreover, the internal quantum efficiency was found to originate from the local coordination states of the Ce3+ centres, independent of composition or molar volume. The comparison of numerical data of the matrix is useful not only for ensuring the homogenous doping of amorphous materials by activators, but also for determining the origin of physical properties.

Quantitative Assessment of Carrier Density by Cathodoluminescence. II. GaAs Nanowires

Precise control of doping in single nanowires (NWs) is essential for the development of NW-based devices. Here, we investigate a series of MBE-grown GaAs NWs with Be (p-type) and Si (n-type) doping using high-resolution cathodoluminescence (CL) mapping at low- and room-temperature. CL spectra are analyzed selectively in different regions of the NWs. Room-temperature luminescence is fitted with the generalized Planck law and an absorption model, and the bandgap and band tail width are extracted. For Be-doped GaAs NWs, the bandgap narrowing provides a quantitative determination of the hole concentration ranging from about $1\times 10^{18}$ to $2\times 10^{19}$~cm$^{-3}$, in good agreement with the targeted doping levels. High-resolution maps of the hole concentration demonstrate the homogeneous doping in the pure zinc-blende segment. For Si-doped GaAs NWs, the electron Fermi level and the full-width at half maximum of low-temperature CL spectra are used to assess the electron concentration to approximately $3\times 10^{17}$ to $6\times 10^{17}$~cm$^{-3}$. These findings confirm the difficulty to obtain highly-doped n-type GaAs NWs, maybe due to doping compensation. Notably, signatures of high concentration (5--9$\times 10^{18}$~cm$^{-3}$) at the very top of NWs are unveiled.

Microstructure manipulation in PVDF/styrene-maleic anhydride copolymer composite membranes: Effects of miscibility on the phase separation

Amphiphilic copolymers have increasingly attracted researchers’ attention to produce the hydrophilic and antifouling polymeric membranes. However, few studies has been reported on systematic manipulation of matrix miscibility to control the membrane microstructure during the nonsolvent induced phase separation (NIPS). Herein, a facile method is brought forward to investigate the effects of miscibility on the structure and performance of the membranes. Through the transformation of styrene-maleic anhydride copolymer (SMA) into sodium styrene-maleic anhydride copolymer (SMANa), a homogeneous and transparent dope solution was successfully prepared. Next, the crescent pores and spherical particles in the PVDF/SMA membrane were eliminated to achieve the uniformly porous PVDF/SMANa membranes by the exchange of nonsolvent and solvent components. Importantly, SMANa could segregate at the external surface and internal pore walls as SMA to enhance the hydrophilicity and antifouling properties of the membranes. Subsequently, by optimizing the dope solution composition and coagulation bath temperature, PVDF/SMANa ultrafiltration membranes exhibited the pure water flux of 1014 L/m2h and the rejection to BSA of 98.9%. These results outperformed previously reported ones for similar PVDF composite membranes. Significantly, this strategy can be applied to the large-scale manufacture of membranes.

Synthesis of Stoichiometric SrTiO3 and Its Carrier Doping from Air-Stable Bimetallic Complexes.

We demonstrate the synthesis of perovskite oxide SrTiO3 with ideal cation stoichiometry and homogeneous La doping using air-stable Sr-Ti and La-Ti bimetallic peroxo complexes with a 1:1 cation ratio. Phase-pure SrTiO3, La2Ti2O7, and LaTiO3 were successfully synthesized by thermal decomposition of those complexes. La-doped SrTiO3 was obtained by mixing the Sr-Ti and La-Ti complexes in an acid solution, followed by thermal decomposition. La-doped SrTiO3 showed systematic chemical trends in the lattice constant and electrical conduction. Not only those bulk polycrystals but also cation-stoichiometric SrTiO3 epitaxial thin films were grown with an atomically flat surface from the Sr-Ti complex.