N-doped Surface Research Articles

Real-time first-principles simulations of thermionic emission from N-doped diamond surfaces

We investigate thermionic emission from N-doped C(100) surfaces terminated with H or Li atoms using finite-temperature real-time density functional theory simulations. The current–temperature characteristics are found to follow the Richardson–Dushman (RD) equation, which was derived from a semiclassical theory. However, the Richardson constants are two orders of magnitude smaller than the ideal values from the RD theory. This considerable reduction is attributed primarily to the extremely low transmission probability of electrons from the surfaces toward the vacuum. The present method enables straightforward evaluation of the ideal efficiency of a thermionic energy converter.

Hierarchically Porous N-Doped Carbon Nanotubes/Reduced Graphene Oxide Composite for Promoting Flavin-Based Interfacial Electron Transfer in Microbial Fuel Cells.

Interfacial electron transfer between an electroactive biofilm and an electrode is a crucial step for microbial fuel cells (MFCs) and other bio-electrochemical systems. Here, a hierarchically porous nitrogen-doped carbon nanotubes (CNTs)/reduced graphene oxide (rGO) composite with polyaniline as the nitrogen source has been developed for the MFC anode. This composite possesses a nitrogen atom-doped surface for improved flavin redox reaction and a three-dimensional hierarchically porous structure for rich bacterial biofilm growth. The maximum power density achieved with the N-CNTs/rGO anode in S. putrefaciens CN32 MFCs is 1137 mW m-2, which is 8.9 times compared with that of the carbon cloth anode and also higher than those of N-CNTs (731.17 mW m-2), N-rGO (442.26 mW m-2), and the CNTs/rGO (779.9 mW m-2) composite without nitrogen doping. The greatly improved bio-electrocatalysis could be attributed to the enhanced adsorption of flavins on the N-doped surface and the high density of biofilm adhesion for fast interfacial electron transfer. This work reveals a synergistic effect from pore structure tailoring and surface chemistry designing to boost both the bio- and electrocatalysis in MFCs, which also provide insights for the bioelectrode design in other bio-electrochemical systems.

Structural and electronic properties of nitrogen-doped TiO2 nanocrystals and their effects on the adsorption of CH2O and SO2 molecules investigated by DFT

The interaction of CH2O and SO2 molecules with TiO2 anatase nanocrystals were studied using the density functional theory calculations. Several adsorption positions of CH2O and SO2 molecules on the TiO2 surface were examined in detail. The results include the calculations of the adsorption energies, electronic structures and structural parameters of the adsorption systems. We found that both oxygen and carbon atoms of the CH2O molecule interact with TiO2 surface, while the hydrogen atoms does not contribute to the adsorption process. Besides, the oxygen atom of SO2 molecule strongly interacts with the TiO2. The adsorption of CH2O and SO2 on the N-doped surface is more favorable in energy than the adsorption on the pristine surface, suggesting that the N-doped nanocrystal acts as an appropriate sensing material. The substantial changes in the electronic structure near the fermi level reveal that the nitrogen modified TiO2 would be a promising sensing material for CH2O and SO2 detection. The charge density difference calculations reveal that the electronic density increases at the middle of the newly formed bonds, as evidenced by the significant overlaps of the projected density of states between the interacting atoms. Besides, the distribution of spin densities reveals that the magnetization was mainly located on the adsorbed CH2O molecule, being useful for the design and characterization of highly efficient gas sensors.

Electronic, magnetic and optical properties of C- and N-doped CeO2 bulk and (111) surface from first-principles

The electronic, magnetic and optical properties of C- and N-doped CeO2 bulk and (111) surface have been systematically investigated by first-principles calculations. The results show that the spin splitting occurs when doping atoms replace the anion sites in the CeO2 matrix, causing a local magnetic moment of 2.00, and 1.00 μB for C- and N-doping, respectively. The strong hybridization between dopants 2p and O 2p triplet-states around the Fermi level gives rise to the half-metallic character for doped bulk systems, while substitution onto the surface eliminates the degeneration of dopants 2p orbitals, which results in anisotropic spin atmosphere. Especially, owing to the low formation energy and available RTFM, N-doped CeO2 would be easily realized in the experiment and should also be wonderful candidate materials for oxide spintronics. In addition, compared with N-doped CeO2, the calculated optical properties reveal that C-doped CeO2 (111) surface is able to enhance the absorption of the visible light.

Charge-modulated CO2 capture

Recently, the concept of using charge-modulation to manipulate the interaction of adsorbate molecules such as CO2 on certain sorbent materials has been advanced through a series of first principle computational predictions. The interactions switched on through charge-modulation shifting the Fermi level are in part, but by no means exclusively, electrostatic. In addition to electrostatics, the Fermi level shifting can be viewed as a way to modulate not only the position but also the identity and character of the frontier orbitals on the sorbent materials that engage with the adsorbate molecules. Thus, a rich new space for electrochemical modulation of surface-molecular interactions—guided by first principle computational modeling—suggests itself. Here we summarize the growing computational literature on switchable CO2 capture strategies. We also provide some new insights into contrasting electrostatic versus chemical responses to charge-modulation as exemplified by water versus CO2 on N-doped graphene surfaces, which suggest that this CO2 capture strategy could be water tolerant.

Large-size graphene-like porous carbon nanosheets with controllable N-doped surface derived from sugarcane bagasse pith/chitosan for high performance supercapacitors

The large-size graphene-like porous carbon nanosheets (LGPCN) with controllable N-doped surface are successfully prepared by using a simple and effective integrated “self-template” and “designing” strategy through KOH activation. Biomass-sugarcane bagasse pith and chitosan are used as self-template carbon source and controlled nitrogen source, respectively. The relationship between structure of the LGPCN-X (X represents the activation temperature) and its electrochemical performance at different activation temperatures is explored in detail. The large-size multiscale wrinkled nanosheets architecture, high surface area, and appropriate controllable surface nitrogen doping state of the LGPCN-800 shows the best electrochemical performance in 6 M KOH. The largest specific electrode capacitance is about 339 F g−1 (at 0.25 A g−1), which still as high as 280 F g−1 at 100 A g−1. The maximum energy density is about 11.77 Wh kg−1 at a power density of 34.11 W kg−1. The LGPCN-800 also shows an excellent cycling stability with 97.9% capacitance retention after 10,000 cycles.

Dual–phase Spinel MnCo2O4 Nanocrystals with Nitrogen-doped Reduced Graphene Oxide as Potential Catalyst for Hybrid Na–Air Batteries

The electrochemical performance of Na–air batteries have a significant dependence on the type of electrocatalyst used for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, a dual-phase spinel MnCo2O4 with nitrogen-doped reduced graphene oxide hybrid (dp-MnCo2O4/N-rGO) employed in a hybrid Na-air battery system was demonstrated simultaneous electrocatalytic characteristic toward both ORR and OER, which showed comparable ORR and superior OER catalytic performance over commercial Pt/C. The fabricated dual-phase cobalt manganese spinel nanoparticles, the introduced dopants (nitrogen atoms), and the electrical and chemical coupling between the oxide nanoparticles and nanocarbon backbones significantly contribute to the high catalytic performance of dp-MnCo2O4/N-rGO. The proposed catalyst exhibited a slightly higher discharge voltage of 2.75V and lower charge voltage of 3.14V compared to that of a commercial Pt/C (2.73 and 3.35V, respectively). The hybrid battery showed a low overpotential of 0.39V and improved round-trip energy efficiency. The battery exhibited excellent discharge stability and no significant degradation was observed during the 25 cycles due to the moderate particle size of dp-MnCo2O4 and large active N-doped surface area. Therefore, dp-MnCo2O4/N-rGO could be a promising bifunctional catalyst for enhancing the electrochemical performance of hybrid Na-air batteries.

Influence of a local electric field on the light harvesting efficiency of a cyclopentadithiophene-bridged D-A-π-A indoline dye on pure and N-doped TiO2 surfaces

The primary objective of this paper is to study the influence of the local electric field at the dye/semiconductor (pure and nitrogen doped TiO2) interface on the performance of dye sensitized solar cells (DSSCs). In this regard, as a first step, we explored the influence of electric field on the performance of the WS-51 dye by computing its properties in an electric field up to 15 × 10−4 a.u. The electronic and optical properties of an established, efficient, donor-acceptor-π-acceptor dye in different electric field strengths were probed with density functional theory (DFT) and time-dependent DFT. The calculated results indicate that, under an electric field, the dye shows significant changes in absorption spectrum due to considerable changes in molecular structure. The TD-DFT results indicate that the absorption spectrum of the dye in acetonitrile solution have shown a blue shift with decreasing molecular extinction coefficient by rising electric field strength. Secondly, we investigated the dye adsorbed on Ti48O96 and Ti48O95N clusters. In the applied field off state, the absorption spectrum of the dye/Ti48O95N system is red shifted with an improved molecular extinction coefficient as compared to the dye/Ti48O96 system, indicating that the nitrogen doped TiO2 surface is more favourable for enhancing the efficiency of the DSSCs. Finally, the calculated results suggested that the light harvesting efficiency (LHE) of the dye/Ti48O95N system is higher than that of the dye/Ti48O96 system. But, under an electric field the LHE of the dye on Ti48O96 and Ti48O95N clusters is likely to decrease with increasing electric field strength. The photon to current response of the DSSCs is limited by the local electric field generated at the dye – semiconductor interface. Finally, results indicate that the doping of a nitrogen atom in TiO2 moderates the electric field intensity at the dye – semiconductor interface. Therefore, the results obtained in this study will provide a valuable reference for understanding the role of local electric field for the further optimization of DSSCs.

Surface analysis of N-doped TiO2 nanorods and their enhanced photocatalytic oxidation activity

We investigated the presence of Ti3+ defect sites on the surfaces of N-doped TiO2 nanorods by using scanning transmission X-ray microscopy (STXM) and high-resolution photoemission spectroscopy (HRPES). The photo-oxidation activities of different types of N-doped TiO2 nanorods were compared with each other and with their undoped nanorods. These nanorods were used to photocatalyze the oxidation of thiol molecules (i.e. 2-mercaptoethanol, benzenethiol, and 2-aminothiophenol) to disulfide and sulfonic (SO3H) species, and the conversion of CO to CO2, and their photocatalytic activities towards these reactions were assessed using HRPES and a residual gas analyzer, respectively. Conversion to further oxidized sulfonic species was only achieved on the N-doped TiO2 surface compared to the non-doped TiO2 nanorods. In addition, we found that longer N-doped TiO2 nanorods (NTR-150) showed higher photo-oxidation activity than NTR-60, which resulted from their increased number of defect sites and narrowed band-gap.

All-flexible lithium ion battery based on thermally-etched porous carbon cloth anode and cathode

Flexible electrode material with high mechanical strength and excellent electrical stability is still a great challenge for the fabrication of highly flexible energy storage devices. Commercial carbon cloth has been long reported as flexible substrate for many electrode materials due to their high mechanical strength and flexibility. However, their application directly as electrode material for flexible lithium ion batteries is yet to be reported. In this paper, commercial carbon cloth was thermally etched and used directly as electrode material in the half-cell and all-flexible full lithium ion batteries. Upon the mass weight and the large diameter of the carbon fiber, the as-prepared free-standing N-doped Porous carbon cloth delivered an initial capacity of 1.75mAh/cm2 (190mAh/g) and capacity up to 1.65mAh/cm2 (168mAh/g) after long electrochemical cycles in the half-cell. The all-flexible device exhibits a high working potential of 3.4V, remarkable lithium storage performance and excellent flexibility. It also achieves a maximum volumetric energy density of 43Wh/cm3 at 0.125mA/cm2 and power density 800W/cm3 at 5.0mA/cm2. The excellent performance can be attributed to N-doped porous surfaces, which provide large surface area for more lithium storage.

Spillover of hydrogen on SiC-ML surface: Doping effect and bond exchange mechanism

The dispersion corrected density functional theory has been employed in this work to explore the binding and splitting of H2 molecule on pure and N doped silicon carbide monolayer (SiC-ML) and spillover of H atoms. Small charge transfer from the donor level of N-doped SiC-ML surface to the σ∗ orbital helps to anchor the hydrogen molecule. The N-doped SiC-ML surface showed excellent catalytic behavior through lowering the H–H bond breaking energy to 0.87 eV, and thus helping in the facile spillover process. It is shown that the migration of H atoms on the surface to form a partially hydrogenated SiC-ML surface requires very high activation barrier. Here we have explained the need and role of borane (BH3) molecule as secondary catalysts for lowering of the migration energy barrier via bond exchange mechanism to facilitate easy and fast migration of H atoms. The nature of interaction of BH3 molecules with the various surfaces has been studied in detail. This work ensures the possibility of the spillover mechanism, and takes us a step ahead towards the implementation of the light-weight material as Hydrogen storage medium.

Highlighting the role of nitrogen doping in enhancing CO2 uptake onto carbon surfaces: a combined experimental and computational analysis

Enhanced interactions between CO2 and N-doped carbon surfaces are elucidated by a combined experimental and computational analysis.

What Makes the Photocatalytic CO2 Reduction on N-Doped Ta2O5 Efficient: Insights from Nonadiabatic Molecular Dynamics.

Recent experimental studies demonstrated that photocatalytic CO2 reduction by Ru catalysts assembled on N-doped Ta2O5 surface is strongly dependent on the nature of the anchor group with which the Ru complexes are attached to the substrate. We report a comprehensive atomistic analysis of electron transfer dynamics in electroneutral Ru(di-X-bpy) (CO)2Cl2 complexes with X = COOH and PO3H2 attached to the N-Ta2O5 substrate. Nonadiabatic molecular dynamics simulations indicate that the electron transfer is faster in complexes with COOH anchors than in complexes with PO3H2 groups, due to larger nonadiabatic coupling. Quantum coherence counteracts this effect, however, to a small extent. The COOH anchor promotes the transfer with significantly higher frequency modes than PO3H2, due to both lighter atoms (C vs P) and stronger bonds (double vs single). The acceptor state delocalizes onto COOH, but not PO3H2, further favoring electron transfer in the COOH system. At the same time, the COOH anchor is prone to decomposition, in contrast to PO3H2, making the former show smaller turnover numbers in some cases. These theoretical predictions are consistent with recent experimental results, legitimating the proposed mechanism of the electron transfer. We emphasize the role of anchor stability, nonadiabatic coupling, and quantum coherence in determining the overall efficiency of artificial photocatalytic systems.

A Theoretical Study of the Effect of Dopants on Diamond (100) Surface Stabilization for Different Termination Scenarios

The effect of dopants (N or B) on differently terminated diamond (100)-2 × 1 surfaces has in the present study been studied theoretically by using DFT (density functional theory) under periodic boundary conditions. The terminating species, X, include H, OH, Oontop, and Obridge. As a result of geometry optimization, the C–N and C–B bond lengths were calculated to be longer than for the situation with saturated binding conditions (i.e., the situation where N (or B) are binding to three other atoms, instead of four). Moreover, the X–Csurface-dopant angles were observed to decrease for the N-doped and increase for the B-doped senarios. In addition, the atomic charges and bond populations for the region surrounding the dopants were also carefully analyzed in order to compare the surface stabilization situations for non-, N- and B-doped diamond surfaces. For the H-terminated diamond surfaces, the C–H bonds became weakened when substituationally doped with either N or B. For the O-terminated diamond surfaces (i.e., both Oontop, and Obridge), the results showed opposite trends by strengthening (or weakening) the C–O bonds for the N- (or B-) doped system, respectivly. The adsorption energies for the various terminating species were observed to decrease when going from a nondoped to an N-doped situation and finally over to a B-doped situation. This is a result that strongly correlates with the calculated Csurface–X (X = H, OH, Oontop, Obridge) bond lengths. In addition, the effect of surface termination on the diamond surface stabilization energy, was observed to be in the following order: Obridge > Otop > H > OH. This result was valid for both non-, N- and B-doped diamond surfaces. The calculated spin density calculations indicated a local distribution of the unpaired electron in the N- and B-doped systems, respectively. This is a result that showed a strong correlation to the bond lengths surrounding the dopants and to the calculated adsorption energies for the terminating species, X. Moreover, the surface electronic structures (i.e., surface states) for the N- and B-doped systems were calculated and visualized by performing pDOS calculations. The results showed a shift of the Fermi levels for the N- and B-doped situations. As expected, the Fermi level was shifted toward the conduction band for the N-doped surfaces and toward the valence band for the B-doped systems. In addition, the pDOS spectra for the Oontop-termination showed extra states around the Fermi level, which were the result induced by the radical nature of this type of termination species.

N-doped structures and surface functional groups of reduced graphene oxide and their effect on the electrochemical performance of supercapacitor with organic electrolyte

Nitrogen-doped reduced graphene oxide (N-rGO) has been synthesized using a simple, efficient method combining instant thermal exfoliation and covalent bond transformation from a melamine-graphene oxide mixture. The capacitive performance of N-rGO has been tested in both aqueous (0.5 M H2SO4) and organic (1 M tetraethyl-ammonium tetrafluoroborate (TEABF4) in propylene carbonate (PC)) electrolytes, which are compared with those obtained from thermal-reduced graphene oxide (T-rGO) and chemical-reduced graphene oxide (C-rGO). The contributions of scan-rate-independent (double-layer-like) and scan-rate-dependent (pseudo-capacitance-like) capacitance of all reduced graphene oxides in both aqueous and organic electrolytes were evaluated and compared. The results show that relatively rich oxygen-containing functional groups on C-rGO form significant ion-diffusion barrier, resulting in worse electrochemical responses in organic electrolyte. By contrast, the N-doped structures, large surface area, and lower density of oxygen-containing groups make N-rGO become a promising electrode material for organic electric double-layer capacitors (EDLCs). The capacitance rate-retention of N-rGO reaches 71.1% in 1 M TEABF4/PC electrolyte when the scan rate is elevated to 200 mVs−1, demonstrating that N-rGO improves the relatively low-power drawback of EDLCs in organic electrolytes. The specific energy and power of a symmetric N-rGO cell in the organic electrolyte reach 25 Wh kg−1 and 10 kW kg−1, respectively.

Nitrogen doped carbon catalyzing acetylene conversion to vinyl chloride

Commercial production of vinyl chloride from acetylene relies on the use of HgCl2 as the catalyst, which has caused severe environmental problem and threats to human health because of its toxicity. Therefore, it is vital to explore alternative catalysts without mercury. We report here that N-doped carbon can catalyze directly transformation of acetylene to vinyl chloride. Particularly, N-doped high surface area mesoporous carbon exhibits a rather high activity with the acetylene conversion reaching 77% and vinyl chloride selectivity above 98% at a space velocity of 1.0 mL·min−1·g −1 and 200 °C. It delivers a stable performance within a test period of 100 h and no obvious deactivation is observed, demonstrating potentials to substitute the notoriously toxic mercuric chloride catalyst.

Molybdenum Sulfide/N-Doped CNT Forest Hybrid Catalysts for High-Performance Hydrogen Evolution Reaction

Cost effective hydrogen evolution reaction (HER) catalyst without using precious metallic elements is a crucial demand for environment-benign energy production. Molybdenum sulfide is one of the promising candidates for such purpose, particularly in acidic condition, but its catalytic performance is inherently limited by the sparse catalytic edge sites and poor electrical conductivity. We report synthesis and HER catalysis of hybrid catalysts composed of amorphous molybdenum sulfide (MoSx) layer directly bound at vertical N-doped carbon nanotube (NCNT) forest surface. Owing to the high wettability of N-doped graphitic surface and electrostatic attraction between thiomolybdate precursor anion and N-doped sites, ∼2 nm scale thick amorphous MoSx layers are specifically deposited at NCNT surface under low-temperature wet chemical process. The synergistic effect from the dense catalytic sites at amorphous MoSx surface and fluent charge transport along NCNT forest attains the excellent HER catalysis with onset overpotential as low as ∼75 mV and small potential of 110 mV for 10 mA/cm(2) current density, which is the highest HER activity of molybdenum sulfide-based catalyst ever reported thus far.

Cost-effective and eco-friendly synthesis of novel and stable N-doped ZnO/g-C3N4 core–shell nanoplates with excellent visible-light responsive photocatalysis

N-doped ZnO/g-C3N4 hybrid core-shell nanoplates have been successfully prepared via a facile, cost-effective and eco-friendly ultrasonic dispersion method for the first time. HRTEM studies confirm the formation of the N-doped ZnO/g-C3N4 hybrid core-shell nanoplates with an average diameter of 50 nm and the g-C3N4 shell thickness can be tuned by varying the content of loaded g-C3N4. The direct contact of the N-doped ZnO surface and g-C3N4 shell without any adhesive interlayer introduced a new carbon energy level in the N-doped ZnO band gap and thereby effectively lowered the band gap energy. Consequently, the as-prepared hybrid core-shell nanoplates showed a greatly enhanced visible-light photocatalysis for the degradation of Rhodamine B compare to that of pure N-doped ZnO surface and g-C3N4. Based on the experimental results, a proposed mechanism for the N-doped ZnO/g-C3N4 photocatalyst was discussed. Interestingly, the hybrid core-shell nanoplates possess high photostability. The improved photocatalytic performance is due to a synergistic effect at the interface of the N-doped ZnO and g-C3N4 including large surface-exposure area, energy band structure and enhanced charge-separation properties. Significantly, the enhanced performance also demonstrates the importance of evaluating new core-shell composite photocatalysts with g-C3N4 as shell material.

Highly n-doped Surfaces on n-type Silicon Wafers by Laser-chemical Processes

For a novel cell concept based on the combination of silicon heterojunction (SHJ) with metal wrap through (MWT) metallization [1] a highly doped area underneath the metallization grid is required. In the past years, alternative processes for realizing defined doping profiles in n- and p-type silicon solar cells were published [2,3]. Laser doping offers the advantage of a locally confined heat impact causing diffusion of phosphor atoms into the molten silicon.Aim of this work is the development of a low cost laser-chemical doping process resulting in a shallow phosphor profile with high concentrations of electrically active phosphor on the surface. Application of the phosphor source and the diffusion of phosphor into the silicon is done in a one-step process with a single laser type. Sheet resistances of the laser doped areas are below 15 Ohm/square when using a pulsed wave (picosecond) laser source in the infrared region.

Combined embedding of N-doping and CaCO3 surface modification in the TiO2 photoelectrodes for dye-sensitized solar cells

A successive embedding of N-doping and CaCO3 surface modification was carried out in the TiO2 photoelectrodes for dye-sensitized solar cells (DSSCs). The combined effect was revealed with the great increase of the open-circuit voltage (Voc), short-circuit current (Jsc), and photoelectric conversion efficiency (η) of the prepared cells; the efficiency (η) was improved from 5.42% of a commercial TiO2 photoelectrode to 7.47% of an unmodified N-doped electrode, and to 9.03% of a N-doped and CaCO3 surface modified electrode. An enhanced photoresponse in N-doped TiO2 nanoparticles generate more photo-excited electrons in adsorbed dye, as supported by measured UV–vis diffuse reflectance spectra and incident photon to current conversion efficiency (IPCE). A successive CaCO3 surface modification then form a barrier on the surface of N-doped TiO2 particles, suppressing charge recombination of photo-generated electrons from N-doped TiO2 to dye or electrolyte, and thus extending their life time in the electrode, as supported by electron impedance spectroscopy (EIS). Furthermore, the higher basicity of the CaCO3 modified TiO2 facilitates the dye adsorption, as supported by the direct measurement of the amount of adsorbed dye.