Controllable Nitrogen Doping Research Articles

Synthesis of AlOxNy thin films using a two-step PE-ALD process

Homogeneous AlOxNy thin films with a controlled nitrogen composition, up to 7.1%, were grown on GaN substrate using plasma-enhanced atomic layer deposition. This was achieved through repeated cycles consisting of AlN thin layers deposition and subsequent in situ O2 plasma oxidation under optimized conditions. We demonstrate that the nitrogen concentration in AlOxNy films can be finely tuned from 1.5% to 7.1% by a precise control of the AlN thin film thickness from 0.25 to 0.96 nm before each oxidation step. It is shown also that higher AlN thickness, ≥1.96 nm, leads to an incomplete plasma oxidation of AlN films, resulting in the formation of AlOxNy/AlN-like structures. Several structural and physic-chemical characterizations, including focused ion beam-scanning transmission electron microscopy, Auger electron spectroscopy, XPS, and time-of-flight secondary ion mass spectrometry were performed to confirm these findings. These results highlight the importance of this approach for growing homogeneous AlOxNy thin films with controlled nitrogen doping, which can be used as dielectrics in gate stacks for high performance GaN-based transistors.

Controllable nitrogen doping of MOCVD Ga2O3 using NH3

We report on the controllable nitrogen doping of β-Ga2O3 as a deep acceptor dopant using ammonia diluted in nitrogen (NH3/N2) as a source of active nitrogen in the metal organic chemical vapor deposition epitaxy. The effects of the NH3/N2 flow rate and substrate temperature on the incorporation efficiency, reproducibility, and controllability of N doping into Ga2O3 were studied using secondary ion mass spectrometry measurements. With the increase in the NH3/N2 molar flow rate from ∼2 × 10−8 to ∼2 × 10−6 mol/min, the N impurities incorporated into the β-Ga2O3 increased linearly from ∼1 × 1018 to ∼2 × 1020 cm−3. At low substrate temperatures (<800 °C), hydrogen was incorporated into the film accompanying nitrogen with comparable concentrations. Despite this, the current–voltage measurements showed that the N and H co-doped films were resistive with a measured resistance of >70 MΩ for a film grown with [N] ≈ [H] of ∼8 × 1018 cm−3. X-ray on-axis (020) and off-axis (111) rocking curve ω-scans and atomic force microscopy measurements show no influence of NH3/N2 dopant on the structural and surface quality of the films. However, the presence of H promoted the growth of (110) and (1¯10) facets elongated along the [001] direction. At high growth temperatures (≥950 °C), the H concentration in the films was reduced by nearly ∼10×, but with a slight increase in the concentration of N. The results show that controllable and repeatable nitrogen doping into β-Ga2O3 can be achieved using ammonia to obtain deep acceptor doping or compensation needed for device engineering in β-Ga2O3-based power electronic devices.

Controlled Nitrogen Doping in Crumpled Graphene for Improved Alkali Metal‐Ion Storage under Low‐Temperature Conditions

AbstractThe significant performance decay in conventional graphite anodes under low‐temperature conditions is attributed to the slow diffusion of alkali metal ions, requiring new strategies to enhance the charge storage kinetics at low temperatures. Here, nitrogen (N)‐doped defective crumpled graphene (NCG) is employed as a promising anode to enable stable low‐temperature operation of alkali metal‐ion storage by exploiting the surface‐controlled charge storage mechanisms. At a low temperature of −40 °C, the NCG anodes maintain high capacities of ≈172 mAh g−1 for lithium (Li)‐ion, ≈107 mAh g−1 for sodium (Na)‐ion, and ≈118 mAh g−1 for potassium (K)‐ion at 0.01 A g−1 with outstanding rate‐capability and cycling stability. A combination of density functional theory (DFT) and electrochemical analysis further reveals the role of the N‐functional groups and defect sites in improving the utilization of the surface‐controlled charge storage mechanisms. In addition, the full cell with the NCG anode and a LiFePO4 cathode shows a high capacity of ≈73 mAh g−1 at 0.5 °C even at −40 °C. The results highlight the importance of utilizing the surface‐controlled charge storage mechanisms with controlled defect structures and functional groups on the carbon surface to improve the charge storage performance of alkali metal‐ion under low‐temperature conditions.

Efficient removal of H2S with zinc oxide/nitrogen-doped ordered mesoporous carbons at room temperature

Nitrogen-doped ordered mesoporous carbon (NMC) was prepared via an organic–inorganic co-assembly method to support ZnO for the efficient removal of H 2 S at room temperature. NMCs with large BET specific surface areas, copious ordered mesopores, and controllable nitrogen doping worked cooperatively with uniformly dispersed ZnO, to improve desulfurization capability at room temperature. Among these, NMC-2 possessed a BET surface area as high as 1172 m 2 /g, a large pore volume of 1.09 cm 3 /g, and a conducive nitrogen content. The appropriate residual ordered mesopores after ZnO loading provided a fast–H 2 S–diffusion channel in NMC-2-20%, an accessible interfacial area (925 m 2 /g), and a sufficient space for product storage (0.9 cm 3 /g). The introduction of nitrogen as Lewis basic sites not only promoted the dissociation of H 2 S but also accelerated the lattice diffusion rate during the desulfurization process. The synergy of the above advantages resulted in an excellent desulfurization performance of 151.2 mg/g. This study presents the effective application of NMC as a support for ZnO for a fine desulfurization at room temperature. • NMC was prepared via the organic-inorganic co-assembly method. • The ordered mesopores improve H 2 S diffusion and provide sufficient space for product. • N doping could promote the H 2 S dissociation and accelerate lattice diffusion rate. • NMC-2-20% has well desulfurization performance.

Boosting the OER Performance of Nitrogen-Doped Ni Nanoclusters Confined in an Amorphous Carbon Matrix.

Nanoclusters are ideal electrocatalysts due to their high surface activity. However, their high activities also lead to serious agglomeration and performance attenuation during the catalytic process. Here, highly dispersed Ni nanoclusters (∼3 nm) confined in an amorphous carbon matrix are successfully fabricated by pulsed laser deposition, followed by rapid temperature annealing treatment. Then, the Ni nanoclusters are further doped with nitrogen element through a clean N2 radio frequency plasma technology. It is found that the nitrogen-doped Ni nanoclusters obtained under optimized conditions showed superior OER performance with a very low overpotential of 240 mV at a current density of 10 mA/cm2, together with good stability. The excellent OER performance of the nanoclusters can be attributed to the unique confined structure and nitrogen doping, which not only provide more active sites but also improve the conductivity. Our work provides a controllable method for the construction of a novel confined structure with controllable nitrogen doping, which can be used as a high-efficiency OER electrocatalyst.

Nitrogen-doped Co3O4 nanowires enable high-efficiency electrochemical oxidation of 5-hydroxymethylfurfural

Developing highly efficient and cost-effective catalysts for electrochemically oxidizing biomass-derived 5-hydroxymethylfurfural (HMF) into value-added 2,5-furandicarboxylic acid (FDCA) is of great importance. Herein, we report a controllable nitrogen doping strategy to significantly improve the catalytic activity of Co3O4 nanowires for highly selective electro-oxidation of HMF into FDCA. The nitrogen doping leads to the generation of defects including nitrogen dopants and oxygen vacancies in Co3O4 nanowires, which is conducive to the formation of catalytically active sites. As a result, the electro-oxidation potential for HMF is only 1.38 V (vs. RHE) when the current density reaches 50 mA/cm2. More importantly, the conversion rate of HMF is as high as 99.5%, and the yield of FDCA is up to 96.4%.

Nitrogen-doped nanoporous graphene induced by a multiple confinement strategy for membrane separation of rare earth.

SummaryRare earth separation is still a major challenge in membrane science. Nitrogen-doped nanoporous graphene (NDNG) is a promising material for membrane separation, but it has not yet been tested for rare earth separation, and it is limited by multi-complex synthesis. Herein, we developed a one-step, facile, and scalable approach to synthesize NDNG with tunable pore size and controlled nitrogen content using confinement combustion. Nanoporous hydrotalcite from Zn(NO3)2 is formed between layers of graphene oxide (GO) absorbed with phenylalanine via confinement growth, thus preparing the sandwich hydrotalcite/phenylalanine/GO composites. Subsequently, area-confinement combustion of hydrotalcite nanopores is used to etch graphene nanopores, and the hydrotalcite interlayer as a closed flat nanoreactor induces two-dimensional space confinement doping of planar nitrogen into graphene. The membrane prepared by NDNG achieves separation of Sc3+ from the other rare earth ions with excellent selectivity (∼3.7) through selective electrostatic interactions of pyrrolic-N, and separation selectivity of ∼1.7 for Tm3+/Sm3+.

Solution to Electric Field Screening in Diamond Quantum Electrometers

There are diverse interdisciplinary applications for nanoscale-resolution electrometry of elementary charges under ambient conditions. These include characterization of two-dimensional electronics, charge transfer in biological systems, and measurement of fundamental physical phenomena. The nitrogen-vacancy center in diamond is uniquely capable of such measurements, however electrometry thus far has been limited to charges within the same diamond lattice. It has been hypothesized that the failure to detect charges external to diamond is due to quenching and surface screening, but no proof, model, or design to overcome this has yet been proposed. In this work we affirm this hypothesis through a comprehensive theoretical model of screening and quenching within a diamond electrometer and propose a solution using controlled nitrogen doping and a fluorine-terminated surface. We conclude that successful implementation requires further work to engineer diamond surfaces with lower surface-defect concentrations.

Hydrogen-dominated metal-free growth of graphitic-nitrogen doped graphene with n-type transport behaviors

Controllable nitrogen doping, an effective way to regulate the electronic property of graphene, has been achieved on metal catalyst which needs to be removed by additional processes when fabricating electronic devices, giving rise to structural damages and contaminations. Here, by using pyridine as the sole source of carbon and nitrogen, a metal-free thermal chemical vapor deposition method is demonstrated to synthesize nitrogen-doped graphene (NG) on dielectrics (SiO2/Si, quartz, rock crystal, and sapphire). It is discovered that increasing H2 flow could promote the quality of graphene and the proportion of graphitic-N. At an optimal growth temperature of 1000 °C, we realize the controllable synthesis of graphitic-N doped graphene. Impressively, field-effect transistors directly fabricated from the N-doped graphene (∼1.1%) films on SiO2/Si exhibit strong n-type semiconducting behaviors with electron mobilities of 110–283 cm2 V−1 s−1 in air, rivaling heavily doped NG grown on metals. As revealed by density functional theory calculations, compared with NG with multiple types of nitrogen atoms, single graphitic-N doping leads to more prominent n-type transport behaviors, owing to the absence of p-type pyrrolic-N and pyridinic-N.

A ball-milling synthesis of N-graphyne with controllable nitrogen doping sites for efficient electrocatalytic oxygen evolution and supercapacitors.

Low-cost and efficient multifunctional electrodes play an important part in promoting the practical application of energy conversion and storage. Herein, we report the facile synthesis of N-graphyne, with a novel structure, by one-step ball milling of CaC2 and pyrazine. The accurate doping of nitrogen atoms at the controllable sites of the molecular skeleton of γ-graphyne was achieved using the nitrogenous precursor (pyrazine) as a reactant. Various techniques were adopted for the investigation of the composition, structure, and morphology of the obtained samples. The electrochemical measurements demonstrated that N-graphyne can serve as an excellent electrode material for both electrocatalysis and supercapacitors. As an electrocatalyst, N-graphyne exhibited an overpotential of 280 mV at 100 mA cm-2 and a Tafel slope of 122 mV dec-1 for the oxygen evolution reaction with highly stable morphology and electrocatalytic performance. As a supercapacitor electrode, N-graphyne showed a maximum capacitance of 235 F g-1 at 1 A g-1, and capacitance retention of 87% after 3000 cycles. The superior electrochemical performance of N-graphyne is due to the nitrogen heteroatomic defects, large electrochemical active surface areas and fast electron migration. Our studies provide a facile synthesis of novel N-graphyne with controllable doping sites and promote its potential applications in electrocatalysis and supercapacitors.

Controlled Nitrogen Doping of Graphene Quantum Dots through Laser Ablation in Aqueous Solutions for Photoluminescence and Electrocatalytic Applications

Nitrogen-doped graphene quantum dots (N-GQDs) have promising applications in catalysis and photoluminescence, but many existing synthetic methods require uses of harsh chemicals, long reaction time...

Nitrogen plasma treatment of ZnO and TiO2 nanowire arrays for polymer photovoltaic applications

This work reports on a simple, yet unique approach to improving the opto-electronic properties of vertically-aligned arrays of rutile TiO2 and Wurzite ZnO nanowires by means of controlled nitrogen doping during exposure to highly kinetic radio-frequency generated N2 plasma radicals. Morphologically, the plasma treatment causes a distortion of the vertical alignment of the nanowires due to a dissociation of the weak Van der Waals force clustering the nanowires. Optical spectroscopy show that plasma treatment increases the light transmission of TiO2 arrays from 48% to 90%, with the ZnO arrays exhibiting an increase from 70% to 90% in the visible to UV range. The as-synthesized TiO2 array has an indirect band gap of 3.13 eV, which reduces to 3.03 eV after N2 treatment, with the ZnO equivalent decreasing from 3.20 to 3.17 eV post plasma exposure. A study of the 3d transition metal near edge fine structure of both Ti and Zn show that the N2 plasma treatment of the nanowires results in nitrogen doping of both TiO2 and ZnO lattices; this is confirmed by scanning transmission electron microscopy coupled with energy dispersive spectroscopy x-ray maps collected of single nanowires, which show a clear distribution of nitrogen throughout the metal-oxide. Application of these structures in P3HT:PCBM polymer blends shows progressive improvement in the photoluminescence quenching of the photoactive layer when incorporating both undoped and nitrogen-doped nanowires.

Fabrication of Porous Carbon with Controllable Nitrogen Doping as Anode for High‐Performance Potassium‐Ion Batteries

AbstractPotassium‐ion batteries (PIBs) are a potential alternative for lithium‐ion batteries (LIBs) due to the abundant reserves of potassium and the low redox potential of K+/K. Nonetheless, the practical application of PIBs is hampered by the fact that K+ has a large ion radium and cannot smoothly and reversibly insert/extract into the existing electrode materials. Herein, a series of nitrogen‐doped porous carbon materials with varied N content are synthesized and the effect of nitrogen content on the performance of PIBs is investigated. It is demonstrated that the N content is not in direct proportion to the K‐storage capacity of porous carbon considering the irregular change of specific surface area. Porous carbon with 12.44 at% N content (NPC‐2) exhibits the best performance among the synthesized samples on account of its large specific surface area associated with the superior porous structure as well as abundant defects. It delivers a high reversible capacity of 385 mAh g−1 at 0.05 A g−1, remarkable rate performance of 218 mAh g−1at 2 A g−1, and long‐term cycling stability maintaining 242 mAh g−1 after 800 cycles at 0.5 A g−1. The quantitative kinetics analysis verify that the K‐storage mechanism is mainly dominated by surface‐driven capacitive controlled process, which explains the superior rate capability. The theoretical simulation of different types of nitrogen adsorption and density of states (DOS) prove that pyridinic‐N and pyrrolic‐N exhibit higher affinity to K atoms and thus N‐doping is beneficial to the adsorption of K+ on the surface of electrode. This work provides a basic reference for the subsequent development of carbon anode materials for potassium‐ion batteries.

Identification of active sites for acidic oxygen reduction on carbon catalysts with and without nitrogen doping

Owing to the difficulty in controlling the dopant or defect types and their homogeneity in carbon materials, it is still a controversial issue to identify the active sites of carbon-based metal-free catalysts. Here we report a proof-of-concept study on the active-site evaluation for a highly oriented pyrolytic graphite catalyst with specific pentagon carbon defective patterns (D-HOPG). It is demonstrated that specific carbon defect types (an edged pentagon in this work) could be selectively created via controllable nitrogen doping. Work-function analyses coupled with macro and micro-electrochemical performance measurements suggest that the pentagon defects in D-HOPG served as major active sites for the acidic oxygen reduction reaction, even much superior to the pyridinic nitrogen sites in nitrogen-doped highly oriented pyrolytic graphite. This work enables us to elucidate the relative importance of the specific carbon defects versus nitrogen-dopant species and their respective contributions to the observed overall acidic oxygen reduction reaction activity.

Achieving High Volumetric Lithium Storage Capacity in Compact Carbon Materials with Controllable Nitrogen Doping

AbstractAlthough nanostructured/nanoporous carbon and silicon‐based materials are a potential replacement for graphite as cost‐effective anodes for lithium ion batteries (LIBs), their extremely low packing density leads to considerably reduced volumetric capacities. Herein, a highly compact carbon anode material constructed from sub‐2 nm nanosized graphitic domains is reported that exhibits excellent capacity density. By introducing a coordination agent in the synthesis precursors, an unusually high concentration of N‐doping (≈26.56 wt%) is achieved, which is mainly confined at the graphitic edges with the pyrrolic‐N and pyridinic‐N configurations. As further supported experimentally and theoretically, the edge‐N dopants, particularly the pyrrolic‐N, favor both ion diffusion kinetics and lithium storage via adsorption. Based on the lithiation‐state electrode volume, the compact anode shows a capacity density of 951 mAh cmtotal−3that is comparable with Si anodes and surpasses all reported carbon‐based anodes, revealing its potential in promoting the performance of future LIBs.

Controllable nitrogen doping and specific surface from freestanding TiO2@carbon nanofibers as anodes for lithium ion battery

Further modification of carbon and transition metal composites has become a hot spot in the preparation of anode materials for lithium ion battery, including various morphologies, nitrogen doping and porous introduction. However, the synergistic effect of specific surface area and nitrogen doping content of composite materials on the electrochemical performance as anode materials for lithium ion batteries has not been revealed. In this paper, the carbon nanofibers loaded with titanium dioxide are fabricated via electrospinning method followed by calcination process with simple addition admixture of diisopropyl azodiformate in precursor solution. The pores are introduced into the composite with controllable nitrogen doping and surface area simultaneously. The specific capacity of titanium dioxide @carbon nanofibers has been increased from 192.2 mAh g−1 to 336 mAh g−1 due to the increased nitrogen content of the composite from 7.18% to 10.21%, and elevated specific surface area from 67.23 to 111.15 m2 g−1, which can endow the composite superior conductivity and more active sites. The capacity contribution of the total specific capacity has decreased from 60.8% to 44.7% compared with original sample, proving that increasing diffusion controlled Faradaic Li-ion insertion origins from nitrogen doping.

Controllable Nitrogen Doping in Multicrystalline Silicon by Casting Under Low Cost Ambient Nitrogen

We have utilized cheaper ambient nitrogen for casting multicrystalline silicon ingots. Ambient nitrogen serves as the N doping source, it causes Si3N4 inclusions and related cracks when the doping amount is too high, while N doping can be effectively controlled by tuning the pressure of ambient nitrogen. The maximum value of the N concentration is measured up to 9.4 × 1015/cm3 by SIMS. The increase of nitrogen concentrations can reduce wafer breakage rate by significantly improving the fracture strength of wafers. The influence of N-O complexes in N-doped MC-Si on electric properties is also discussed. Solar cell efficiencies of N-doped wafers are almost comparable to those from ingots grown under ambient argon.

Computational study of precision nitrogen doping on graphene nanoribbon edges

Nitrogen doping in graphene is important for applications spanning from electronics to metal-free electrocatalysts. Despite much experimental study, limited theoretic work has been done in understanding the mechanism of the doping process, especially from a precision perspective. Herein, we present a computational study on precision nitrogen doping on edges of graphene nanoribbons (GNRs) by combining molecular dynamics (MD) simulation at a time scale of 40 ns and density function theory (DFT) calculation. In the MD study both ammonia and acetonitrile were used as nitrogen sources. MD results revealed that the ammonia produces almost all amine-type dopants, while the acetonitrile produces a considerable amount of pyrrolic and pyridinic nitrogen dopants which are beneficial to electronics and electrocatalysts. Results also show that the concentration of pyrrolic and pyridinic dopants can be precisely controlled by the edge geometries of the GNRs. Furthermore, DFT calculation illustrated the reaction mechanism in these different types of the GNRs when using acetonitrile as the nitrogen source. The calculated energies in different reaction stages indicate the stability of dopants on various GNRs, agreeing well with the MD results. The disclosed mechanism of controllable nitrogen doping on the edges of the GNRs would provide guidance to experimental realization, paving new routes to widespread applications.

Reduced-graphene-oxide supported tantalum-based electrocatalysts: Controlled nitrogen doping and oxygen reduction reaction

Controlled N-doping is feasible to engineer the surface stoichiometry and the electronic configuration of metal-oxide electrocatalysts toward efficient oxygen reduction reactions (ORR). Taking reduced graphene oxide supported tantalum-oxides (TaOx/RGO) for example, this work illustrated the controlled N-doping in both metal-oxides and carbon supports, and the contribution to the improved ORR activity. The active N-doped TaOx/RGO electrocatalysts were fabricated via SiO2-assisted pyrolysis, in which the amount and kind of N-doping were tailored toward efficient electrocatalysis. The optimal nanocomposites showed a quite positive half-wave potential (0.80V vs. RHE), the excellent long-term stability, and the outstanding tolerance to methanol crossing. The improvement in ORR was reasonably attributed to the synergy between N-doped TaOx and N-doped RGO. Elucidating the importance of controlled N-doping for electrocatalysis, this work will open up new opportunities to explore noble-metal-free materials for renewable energy applications.

Effect of Nitrogen Doping Level on the Performance of N-Doped Carbon Quantum Dot/TiO2 Composites for Photocatalytic Hydrogen Evolution.

Carbon quantum dots (CQDs) have attracted widespread interest for photocatalytic applications, owing to their low cost and excellent electron donor/acceptor properties. However, their advancement as visible-light photosensitizers in CQDs/semiconductor nanocomposites is currently impaired by their poor quantum yields (QYs). Herein, we describe the successful fabrication of a series of nitrogen-doped CQDs (NCDs) with N/C atomic ratios ranging from 0.14-0.30. NCDs with the highest N-doping level afforded a remarkable external QY of 66.8 % at 360 nm, and outstanding electron transfer properties and photosensitization efficiencies when physically adsorbed on P25 TiO2 . A NCDs/P25-TiO2 hybrid demonstrated excellent performance for hydrogen evolution in aqueous methanol under both UV and visible-light illumination relative to pristine P25 TiO2 . Controlled nitrogen doping of CQDs therefore represents a very effective strategy for optimizing the performance of CQDs/semiconductor hybrid photocatalysts.