N-doped Graphene Films Research Articles

Synthesis of N-doped graphene film with tunable graphitic and pyridinic doping content

Nitrogen-doped (N-doped) graphene films with up to a 17.39 at.% doping level were synthesized with the chemical vapor deposition growth method. An extensive range of tunable nitrogen doping levels was achieved by selecting organic molecules as precursors grown at a wide temperature range of 300 to 1000 °C. The benzene rings in the precursor significantly reduced the energy requirements of N-doped graphene. The incorporation of bridgehead nitrogen and amino groups ensured a high nitrogen doping level and offered the tunable modulation of nitrogen dopant concentrations. The influence of temperature on the nitrogen content was investigated, distinguishing the graphitic and pyridinic dopant types, each demonstrating unique trends considering the dopant stability and growth mechanism. Specifically, the proportion of graphitic doping could be modulated from 40 % to 100 %. Notably, the pyrrolic type was absent at lower temperatures and was nearly negligible at elevated temperatures. Therefore, we developed a synthesis method for N-doped graphene growth that allowed for the selective and prioritized tuning of nitrogen dopant types. This method is particularly favorable when specific nitrogen dopants at certain doping levels are required for electrical and chemical catalytic applications.

Nitrogen doped single layer graphene for CZTS-based thin film solar cells

CdS thin films are commonly utilized as a buffer layer in chalcopyrite thin-film solar cells. However, due to the toxic nature of Cadmium (Cd), ongoing efforts are being directed towards exploring alternative options. In this contribution, doped graphene film with remarkable optical and electrical properties has been introduced for the first time as an alternative buffer layer, replacing CdS in CZTS thin-film solar cell application. In this study, nitrogen-doped graphene (N-doped graphene) film was utilized as a substitute buffer layer in the CZTS thin-film solar cell structure, replacing the conventional CdS thin film. For comparative analysis, CZTS/N-doped graphene and CZTS/CdS traditional solar cell structures were fabricated and separately characterized. The CZTS thin films produced were examined through EDX, XRD, SEM, Raman, optical transmission and PL spectroscopy measurements. According to performed analyses, the Cu-poor and Zn-rich kesterite CZTS thin films exhibited a uniform and dense polycrystalline microstructure as observed in surface and cross-sectional SEM images. XRD spectra of the kesterite CZTS thin film displayed predominant peaks corresponding to the (112), (220/204), and (312/116) diffraction planes of the kesterite CZTS phase. Raman spectra showed a dominant peak at ∼336 cm−1 associated with the kesterite CZTS phase. PL emission spectra indicated transitions from the conduction band to defect levels. The CVD-grown doped graphene film exhibited a 3.43 I2D/IG ratio and a 25 cm−1 FWHM of the 2D peak, indicating a single-layer graphene according to Raman analysis. Permanent nitrogen doping with a 2% atomic concentration was confirmed by XPS measurement. The optical transmission measurement of the single layer doped graphene film showed a 95% transmittance value. Nitrogen doping was contributed to decrease the sheet resistance of the graphene film. The Glass/Mo/CZTS/N-doped graphene/i-ZnO/ITO/Al solar cell displayed a VOC of 0.267 V, JSC of 24.6 mA/cm2, FF of 36.46, and η of 2.37%, showing higher FF and Jsc values but lower conversion efficiency compared to the Glass/Mo/CZTS/CdS/i-ZnO/ITO/Al traditional solar cell structure. Hence, the superior working function and transparency properties position the N-doped graphene film as a competitive buffer layer for use in CZTS-based thin-film solar cells.

Synthesis of N-Doped Graphene Photo-Catalyst for Photo-Assisted Charging of Li-Ion Oxygen Battery.

In this work, nitrogen (N)-doped graphene film is synthesized, as a photo-catalyst, on one side of the copper foam by chemical vapor deposition and the copper foam is directly used as an electrode after porous Pd@rGO cathode loading to the other side of the foam for the photo-assisted charging of the Li-ion oxygen battery. The amount of urea (CO(NH2)2), which is used as N atom source, is optimized to get maximum photo-anodic currents from the n-type graphene films. The optical band gap and the valance band edge potential of the optimized N-doped graphene film are determined as 2.00eV and 3.71VLi+/Li, respectively. X-ray photoelectron spectra provided that the atomic percent of N atoms in the graphene film is 1.34% and the graphitic, pyrrolic and pyridinic N atom percentages are 54.01%, 42.20% and 3.79%, respectively. The photo-assisted charging tests indicated that the N-doped graphene film photo-catalyst reduced the charging potential significantly even at 1000mAg-1 (0.1mAcm-2) current density and improved the cyclic discharge-charge performance of the Li-ion oxygen battery considerably.

N-doped graphene film prepared by rapid thermal shock for ultra-sensitive temperature reading

Graphene is restricted by its zero-band gap when applying in functional components. However, its band structure can be adjusted appropriately through nitrogen doping. At present, many nitrogen doping processes have been developed such as chemical vapor deposition, thermal annealing and plasma sputtering, but in general these methods are time-consuming and difficult to control the type of nitrogen atom doped. In this work, an in situ high-temperature thermal shock process, with an extremely fast rate of temperature change of more than 104 K/s, is developed and used to prepare the nitrogen doped graphene (NG) films. This novel N-doped process enables the preparation of NG films within 10 s, which is the fastest to the best of our knowledge. Moreover, the type and content of doped nitrogen atoms can be effectively regulated by controlling the composition of precursor films. Optimally, the nitrogen content of NG film reaches 2.26 %, and the proportion of graphite nitrogen is more than 60 %. The electrical conductivity and the average Seebeck coefficient of the NG film are 13,870 S·m−1 and −46.5 μV/K, respectively. Its thermoelectric power factor reaches 30 μW·m−1·K−1, indicating that the NG film has a good prospect of functional application. Furtherly, A NG film-based temperature reading device is developed, which shows ultra-high accuracy and sensitivity in temperature reading within a rather wide temperature range.

Dual role of N-doped graphene film as a cathode material for anodic organic oxidation and persulfate production and as a planar carbocatalyst for non-electrochemical persulfate activation

In this study, a large-area N-doped graphene film (NG/NF) was fabricated on a nickel foam (NF) substrate via thermal chemical vapor deposition along with the flow of gaseous C2H4/NH3 mixtures.

A Novel Freestanding Porous N-Doped Carbon-Coating Silicon/Carbon Nanotube in N-Doped Graphene Film Anode for High Performance Li-Ion Batteries

A Novel Freestanding Porous N-Doped Carbon-Coating Silicon/Carbon Nanotube in N-Doped Graphene Film Anode for High Performance Li-Ion Batteries

N-Doped Graphene Film Prepared by Rapid Thermal Shock for Ultra-Sensitive Temperature Reading

N-Doped Graphene Film Prepared by Rapid Thermal Shock for Ultra-Sensitive Temperature Reading

Low-Temperature Nitrogen Doping of Nanocrystalline Graphene Films with Tunable Pyridinic-N and Pyrrolic-N by Cold-Wall Plasma-Assisted Chemical Vapor Deposition.

We report a viable method to produce nanocrystalline graphene films on polycrystalline nickel (Ni) with enhanced N doping at low temperatures by a cold-wall plasma-assisted chemical vapor deposition (CVD) method. The growth of nanocrystalline graphene films was carried out in a benzene/ammonia/argon (C6H6/NH3/Ar) system, in which the temperature of the substrate heated by Joule heating can be further lowered to 100 °C to achieve a low sheet resistance of 3.3 kΩ sq–1 at a high optical transmittance of 97.2%. The morphological, structural, and electrical properties and the chemical compositions of the obtained N-doped nanocrystalline graphene films can be tailored by controlling the growth parameters. An increase in the concentration of atomic N from 1.42 to 11.28 atomic percent (at.%) is expected due to the synergetic effects of a high NH3/Ar ratio and plasma power. The possible growth mechanism of nanocrystalline graphene films is also discussed to understand the basic chemical reactions that occur at such low temperatures with the presence of plasma as well as the formation of pyridinic-N- and pyrrolic-N-dominated nanocrystalline graphene. The realization of nanocrystalline graphene films with enhanced N doping at 100 °C may open great potential in developing future transparent nanodevices.

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.

High-quality nitrogen-doped graphene films synthesized from pyridine via two-step chemical vapor deposition

Modulation of the electrical properties of graphene is of significant importance in advancing graphene electronics: it can be achieved by a Fermi level shift induced by electron acceptor/donor doping. Suitable doping methods involving low-temperature processes and offering long-term stability are imperative to practical applications for such materials. Here, we demonstrate a two-step chemical vapor deposition (CVD) technique for direct synthesis of N-doped graphene film from a pyridine feed-stock at 300 °C under ambient pressure. We extended the synthesis—classified into nucleation and lateral growth steps—by controlling the carbon partial pressure in the processing gases. This led to large-area, continuous N-doped graphene films of excellent quality with full surface coverage: for example, a film size of 2 in2, optical transmittance of 97.6%, and electron mobility of 1400 cm2 V−1 s−1. Our modified CVD method is expected to facilitate the direct synthesis of N-doped graphene in device manufacturing processes toward practical applications while keeping the underlying devices intact.

Dependence of plasma power for direct synthesis of nitrogen-doped graphene films on glass by plasma-assisted hot filament chemical vapor deposition

Metal-free synthesis of nitrogen-doped (N-doped) graphene films on glass is important for modulating the properties of graphene glass but has so far met with limited success. In this study, direct synthesis of N-doped graphene films on glass with eco-friendly N2 dopant through a novel plasma-assisted hot filament chemical vapor deposition (HFCVD) approach was reported. Influence of plasma power on the structural and electrical properties of N-doped graphene films was investigated. The filament and plasma source were found to be both crucial for depositing high-quality N-doped graphene films with N2 dopant. With a small N2 flow of 5 sccm, the N content of graphene films synthesized by plasma-assisted HFCVD could be modulated from 0.6 to 3.0 at.% through adjusting the plasma power from 0 to 130 W. A lowest resistivity of 4.68 × 10−3 Ω cm was obtained at 130 W. Temperature-dependence of resistance measurement revealed that the carrier mobility of N-doped graphene films decreased by raising the plasma power, which was attributed to the increase of conductive activation energy. This work provides an alternative method for direct, controllable and green preparation of N-doped graphene films on glass.

Novel sponge-like N-doped graphene film as high-efficiency electrode for Li-ion battery

The novel sponge-like N-doped reduced graphene oxide films (N-rGO) were synthesized successfully via a facile one-step route at 800 °C. The as-prepared films are few-layer and have loose sponge-like structure between layers and the N-doping content can reach to 7.11% and C/O is 17:1. As an anode material of Li-ion battery (LiBs), the as-prepared N-rGO flim exhibit excellent electrochemical properties. It delivers an initial discharge capacity of 1673 mAh g−1 and the reversible specific discharge capacity of 906 mAh g−1 at the current density of 100 mA g−1. Even at the current density of 1.6 A g−1, the reversible specific capacity still could remain at 346 mAh g−1 after 500 cycles. The appearance of N-doping microstructure of rGO with a high content and loose sponge-like framework might be applied to explain thus enhanced electrochemical properties.

Electroanalytical Performance of Nitrogen-Doped Graphene Films Processed in One Step by Pulsed Laser Deposition Directly Coupled with Thermal Annealing.

Graphene-based materials are widely studied to enable significant improvements in electroanalytical devices requiring new generations of robust, sensitive and low-cost electrodes. In this paper, we present a direct one-step route to synthetize a functional nitrogen-doped graphene film onto a Ni-covered silicon electrode substrate heated at high temperature, by pulsed laser deposition of carbon in the presence of a surrounding nitrogen atmosphere, with no post-deposition transfer of the film. With the ferrocene methanol system, the functionalized electrode exhibits excellent reversibility, close to the theoretical value of 59 mV, and very high sensitivity to hydrogen peroxide oxidation. Our electroanalytical results were correlated with the composition and nanoarchitecture of the N-doped graphene film containing 1.75 at % of nitrogen and identified as a few-layer defected and textured graphene film containing a balanced mixture of graphitic-N and pyrrolic-N chemical functions. The absence of nitrogen dopant in the graphene film considerably degraded some electroanalytical performances. Heat treatment extended beyond the high temperature graphene synthesis did not significantly improve any of the performances. This work contributes to a better understanding of the electrochemical mechanisms of doped graphene-based electrodes obtained by a direct and controlled synthesis process.

Modification of N-doped graphene films and their applications in heterojunction solar cells

A two-step growth process was developed to synthesis N-doped graphene films by using pentachlorop-yridine as nitrogen source and methane as carbon source. The configuration and compositions of modified graphene films were characterized by X-ray photoelectron spectroscopy (XPS), and its bonding properties were obtained with Raman spectroscopy. The films were found to be N-doped graphene films with the N/C ratio of 2.5%–4%. The electronic properties of the films have been tested by using Hall Effect Measurement System. The nitrogen doped graphene film exhibits lower resistance (7.26 × 10−6 Ω·cm) but smaller mobility (521.8 cm2/V·S). Current density-voltage feature of solar cells with different structures were measured to research the photovoltaic characteristic of Graphene/n-Si solar cells. It was found that solar cells with N-doped graphene layer shown better performance of photovoltaic in illumination. The maximum energy conversion efficiency achieved was 6.24%. The inserted nitrogen atoms can effectively reduce the resistivity of graphene films, thus solve the problem of carrier recombination which caused by slow transmission speed. As a result, N-doped graphene film can highly facilitate the transmission and exportation of current, along with the improvement of short-circuit current density, which further enhance the overall efficiency of the solar cells. This work can promote the development of graphene film in application of transparent conductive electrode fields.

Engineering of highly conductive and ultra-thin nitrogen-doped graphene films by combined methods of microwave irradiation, ultrasonic spraying and thermal annealing

We report a new method for the fabrication of highly conductive and transparent ultrathin nitrogen (N) doped graphene films from graphene inks by combining a microwave treatment, ultrasonic nebulizer coating and thermal annealing. The starting graphene oxide (GO) solution was mixed with poly(ionic liquids) (PIL) and treated with microwave (Mw) irradiation to prepare Mw-rGO@PIL inks, which is a gentle reduction of PIL-attached reduced graphene oxide (rGO). In this non-contacting heating method, the PIL was used to not only mediate microwave irradiation and prevent disorder of the graphitic structure, but also repair the lattice defects and introduce nitrogen into the graphitic structure. The ultra-thin graphene films were prepared using the nebulizer for controlling the aerosol droplet distribution of the Mw-rGO@PIL inks coated onto quartz or glass substrates. The prepared films displayed a surface resistance of ∼1.45 × 107 Ω/sq at a transparency of ∼87%. A further thermal treatment was conducted to improve the conductivity of the prepared films by annealing at a high temperature (900 °C), which allowed complete reduction of oxygen containing groups, enhanced graphitization, and reordering of the basal graphene plane and N-doping of the carbon lattice (pyrolytic PIL). The resulting thin films significantly reduced the surface resistance in the range of 1.5 × 103 to 6.2 × 103 Ω/sq at a transparency ranging from 68 to 82%, respectively. The presented method involving in situ N-doping offers a promising environmentally-friendly, low-cost and scalable manufacture of high-quality conductive N-doped graphene films.

Hierarchically porous hexagonal microsheets constructed by well-interwoven MCo2S4 (M = Ni, Fe, Zn) nanotube networks via two-step anion-exchange for high-performance asymmetric supercapacitors

We report a significant advance in the design and fabrication of MCo2S4 (M = Ni, Fe, Zn) complex hierarchical structures with well-defined morphologies by achieving novel hierarchically porous hexagonal microsheets constructed by well-interwoven nanotube networks using a controllable two-step anion-exchange technique. Uniform and smooth hexagonal sheets are initially achieved for the first anion-exchange leading to nanowire-woven hexagons, followed by transformation of each nanowire to rough MCo2S4 nanotube via the second anion-exchange. The involved mechanism of this general top-down method allowing fine nanostructure control is clarified based on our proposed new insights into ion-induced anisotropic growth and time-dependent anion-exchange reaction kinetics. The merits of both maximized porosity and low resistance facilitate fast electron transfer/ion diffusion, thus NiCo2S4 electrode material exhibits a higher specific capacitance of 1780Fg−1 and superior rate capability than most reported NiCo2S4 nanostructures with different morphologies as well as excellent stability (92.4% capacity retention after 10,000 cycles at 10Ag−1). Furthermore, an asymmetric solid-state supercapacitor using such NiCo2S4 as positive and N-doped graphene film as negative electrodes achieves outstanding cycle ability (92.1% retention over 5000 cycles at 20Ag−1) and higher energy density of 67.2Whkg−1 (at 900Wkg−1) than that of similar devices. Such MCo2S4 electrode materials are promising for the future generation of high performance supercapacitors.

Plasmons in N-doped graphene nanostructures tuned by Au/Ag films: a time-dependent density functional theory study.

The energy resonance point of the prominent peak of the absorption spectrum of nitrogen-doped graphene is in the ultraviolet region. This limits its application as a co-catalyst in renewable hydrogen evolution through photocatalytic water splitting in the visible light region. It is well known that noble metal films show active absorption in the visible region due to the existence of the unique feature known as surface plasmon resonance. Here we report tunable plasmons in nitrogen-doped graphene nanostructures using noble metal (Au/Ag) films. The energy resonance point of the prominent peak of the composite nanostructure is altered by changing the separation space of two-layered nanostructures. We found the strength of the absorption spectrum of the composite nanostructure is much stronger than the isolated N-doped graphene monolayer. When the separation space is decreased, the prominent peak of the absorption spectrum is red-shifted to the visible light region. Moreover, currents of several microamperes exist above the surface of the N-doped graphene and Au film composite nanostructure. In addition, the field enhancement exceeds 1000 when an impulse excitation polarized in the armchair-edge direction (X-axis) when the separation space is decreased to 3 Å and is close to 100 when an impulse excitation polarized in the zigzag-edge direction (Y-axis). The N-doped graphene and noble metal film composite nanostructure is a good candidate material as a co-catalyst in renewable hydrogen production by photocatalytic water splitting in the visible light region.

Achieving High Capacitance of Paper-Like Graphene Films by Adsorbing Molecules from Hydrolyzed Polyimide.

To date, graphene-based electric double layer supercapacitors have not shown the remarkable specific capacitance as theoretically predicted. An efficient strategy toward boosting the overall capacitance is to endow graphene with pseudocapacitance. Herein, molecules of hydrolyzed polyimide (HPI) are used to functionalize N-doped graphene (NG) via π-π interaction and the resulting enhanced electrochemical energy storage is reported. These aromatic molecules in monolayer form on graphene contribute strong pseudocapacitance. Paper-like NG films with different areal mass loadings ranging from 0.5 to 4.8 mg cm-2 are prepared for supercapacitor electrodes. It is shown that the gravimetric capacitance can be increased by 50-60% after the surface functionalization by HPI molecules. A high specific capacitance of 553 F g-1 at 5 mV s-1 is achieved by the HPI-NG film with a graphene mass loading of 0.5 mg cm-2 in H2 SO4 aqueous electrolyte. For the HPI-NG film with highest mass loading, the gravimetric specific capacitance drops to 340 F g-1 while the areal specific capacitance reaches a high value of 1.7 F cm-2 . HPI-NG films are also tested in Li2 SO4 aqueous electrolyte, over an extended voltage window of 1.6 V. High specific energy densities up to 40 Wh kg-1 are achieved with the Li2 SO4 electrolyte.

Design and fabrication of N-doped graphene decorated LiFePO4@C composite as a potential cathode for electrochemical energy storage

Olivine-structured LiFePO4@C nanoparticles grown on N-doped graphene (NG) sheets have been fabricated by using a simple sol-gel method with post calcinations. During the synthesis process, both the carbon film and NG sheets can inhibit the growth of LiFePO4 particles. Meanwhile, the constructed conductive network between the NG and carbon film can greatly enhance the electronic conductivity of LiFePO4 material. These unique properties lead to markedly improved lithium storage performance. The NG-decorated LiFePO4@C (NG-LiFePO4@C) composite presents high specific capacity (163.1mAhg−1, 0.1C) and excellent rate capability (118.6mAhg−1, 10C). Therefore, this NG-LiFePO4@C composite can be regarded as a potential electrode for electrochemical energy storage.

Dual-wavelength, passively Q-switched thulium-doped fiber laser with N-doped graphene saturable absorber

Demonstrations of dual-wavelength Q-switched fiber lasers in the wavelength regions of 1.0μm and 1.55μm have been widely reported but not in the longer wavelength of 2.0μm region. In this paper, a dual-wavelength Q-switched thulium-doped fiber laser is proposed and demonstrated using an N-doped graphene film as the saturable absorber, while the dual-wavelength mechanism is achieved with the use of a tunable Mach-Zehnder filter. The dual-wavelength Q-switched laser produced consistent outputs at 1922.0nm and 1961.0nm with a wavelength spacing of 39.0nm. The repetition rate of the pulses obtained covers a range from 17.68kHz to 35.21kHz corresponding to an increasing pump power of 112.55mW until 147.78mW. The narrowest pulse width obtained is 4.4μs and a pulse energy as high as 25.56nJ is generated. To the best of our knowledge, this is the first demonstration of a dual‐wavelength Q-switched thulium-doped fiber laser operating in the 2.0μm wavelength region using a fiber ring cavity design.