Heterogeneous single-cluster catalysts(Mn_(3),Fe_(3),Co_(3),and Mo_(3))supported on nitrogen-doped graphene for robust electrochemical nitrogen reduction

Electrochemical nitrogen reduction reaction(NRR)is one of the most promising alternatives to the traditional Haber-Bosch process.Designing efficient electrocatalysts is still challenging.Inspired by the recent experimental and theoretical advances on single-cluster catalysts(SCCs),we systematically investigated the catalytic performance of various triple-transition-metal-atom clusters anchored on nitrogen-doped graphene for NRR through density functional theory(DFT)calculation.Among them,Mn_(3)-N4,Fe_(3)-N4,Co_(3)-N4,and Mo_(3)-N4 were screened out as electrocatalysis systems composed of non-noble metal with high activity,selectivity,stability,and feasibility.Particularly,the Co_(3)-N4 possesses the highest activity with a limiting potential of-0.41 V through the enzymatic mechanism.The outstanding performance of Co_(3)-N4 can be attributed to the unique electronic structure leading to strong π backdonation,which is crucial in effective N_(2) activation.This work not only predicts four efficient non-noble metal electrocatalysts for NRR,but also suggest the SCCs can serve as potential candidates for other important electrochemical reactions.

Selective glycerol oxidation using platinum nanoparticles supported on multi-walled carbon nanotubes and nitrogen-doped graphene hybrid

Selective oxidation of glycerol is a hot topic.Increased biodiesel production has led to glycerol oxidation over Au- and Pt-based catalysts being widely studied.However,Pt catalysts suffer from deactivation because of weak metal-support interactions.In this study,multi-walled carbon nanotube（MWCNTs）-pillared nitrogen-doped graphene（NG） was prepared by direct pyrolysis of melamine on MWCNTs,and the synthesized NG-MWCNT composite was used as the support for Pt.Characterization results showed that the surface area（173 m^2/g） and pore volume of the NG-MWCNT composite were greater than those of bare MWCNTs and the separated melamine pyrolysis product（CH_x）.Pt（1.4±0.4 nm） dispersion on the NG-MWCNTs was favorable and the Pt/NG-MWCNT catalyst was highly active and selective in the oxidation of glycerol to glyceric acid（GLYA） in base-free aqueous solution.For example,the conversion of glycerol reached 64.4% with a GLYA selectivity of 81.0%,whereas the conversions of glycerol over Pt/MWCNTs and Pt/CN_x were 29.0% and 31.6%,respectively.The unique catalytic activity of the Pt/NG-MWCNTs is attributed to well-dispersed Pt clusters on the NG-MWCNTs and the electron-donating effect of the nitrogen dopant in the NG-MWCNTs.

Zeolitic imidazolate framework-67 derived Al-Co-S hierarchical sheets bridged by nitrogen-doped graphene:Incorporation of PANI derived carbon nanorods for solid-state asymmetric supercapacitors

Metal sulfides have been widely enticed as battery-type electrodes in supercapacitor devices because of their maximal theoretical capacitance.Nevertheless,their lower conductivity and ion transport kinetics can largely restrict their rate performance,hence the practical usage in fields of demanding high power devices.Therefore,the design of new electrodes with higher energy and power densities remains a highly challenging task.To the best of our knowledge,a novel hierarchical composite of Al-CoS_(2) on nitrogendoped graphene(NG)is prepared based on a zeolite imidazole framework using a simple and scalable hydrothermal process.In this hybrid,ultrathin Al-CoS_(2) nanosheet arrays are vertically orientated on the NG framework to limit self-aggregation,hence increasing the electrical property and cycle stability of composite.It is investigated that the Al/Co feeding ratio plays a crucial role in controlling the obtained hierarchical structure of Al-Co-S sheets and their electrode performance.Also,Al^(3+) can influence remarkably the morphology and electrochemical property of the resultant graphene composite.An effective synergism is noticed between the redox Al-CoS_(2) and NG resulting in fast electron transfer and chargingdischarging processes.Surprisingly,when the as-developed composite is utilized as a positive electrode at an applied current density of 1 A/g,a specific capacitance of 1915.8 F/g is attained with ultra-long cycle stability(96%,10,000 cycles)and an excellent retention rate(~89%).As a consequence,when a solid-state asymmetric supercapacitor(ASC)device is made by combining an Al-CoS_(2) @NG hybrid with a negative electrode made of polyaniline(PANI)derived carbon nanorods(PCNRs),it demonstrates remarkable specific capacitance(188 F/g),energy density(66.9 Wh/kg),and cyclic stability of 92%after 10,000 cycles.This may open the pathway for the application of the next-generation supercapacitors in the future.

Novel Formaldehyde Sensor based on Hydrogen Peroxide/Melamine Modulated Photoluminescence of Nitrogen-doped Graphene Quantum Dots

A modulated photoluminescence nanosensor was developed for the quantitative detection of formaldehyde with nitrogen-doped graphene quantum dots and melamine. The sensing system was based on the different activated effects of melamine and hydrogen peroxide on the photoluminescence intensity of nitrogendoped graphene quantum dots. Under the optimal conditions, the modulated photoluminescence sensing system can be used to detect formaldehyde with a good linear relationship between the nitrogen-doped graphene quantum dots photoluminescence difference and the concentration of formaldehyde. The novel sensing system provided new directions for the detection of formaldehyde with high selectivity and quick response.

Modified Nitrogen-Doped Graphene Electrocatalyst for Oxygen Reduction Reaction in Alkaline Fuel Cells

We report modified nitrogen-doped graphene （CN） as electrocatalyst for ORR （oxygen reduction reaction） in alkaline medium. CN was synthesized by a novel procedure based on graphite oxide thermally treated with cyanamide suitable for facile N-doping and large-scale production, whereas cyanamide was used as N-precursor. The structure of the material was characterized by TEM （transmission electron microscopy）, SEM （scanning electron microscopy）, Raman spectroscopy and XPS （X-ray photoelectron spectroscopy）. Structural and electrochemical properties of CN were compared with those of non-modified graphene （TRGO （thermally reduced graphite oxide））. The electrochemical characterization of TRGO and CN in alkaline solution demonstrates enhanced electrocatalytic ORR activity and improved long-term stability for N-doped CN. Voltammetric studies confirmed that, oxygen reduction on CN rather follows four-electron pathway. Compared with commercial 20% PtC catalyst, CN is characterized by exceptional methanol crossover resistance and superb long-term operation stability. Owing to these factors, nitrogen-doped graphene has a great potential to be used as metal-free electrocatalyst in cathodes of alkaline fuel cells.

Nitrogen-doped graphene:Synthesis,characterizations and energy applications

Nitrogen-doped（N-doped） graphene has attracted increasing attentions because of the significantly enhanced properties in physic, chemistry, biology and material science, as compared with those of pristine graphene. By date, N-doped graphene has opened up an exciting new field in the science and technology of two-dimensional materials. From the viewpoints of chemistry and materials, this article presents an overview on the recent progress of N-doped graphene, including the typical synthesis methods, characterization techniques, and various applications in energy fields. The challenges and perspective of Ndoped graphene are also discussed. We expect that this review will provide new insights into the further development and practical applications of N-doped graphene.

Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphene with Urea

Chemical doping is an effective method to intrinsically modify the chemical and electronic property of graphene. We propose a novel approach to synthesize the nitrogen-doped graphene via thermal annealing graphene with urea, in which the nitrogen source can be controllably released from the urea by varying the annealed temperature and time. The doped N content and the configuration N as well as the thermal stabilities are also evaluated with X-ray photoelectron spectroscopy and Raman spectra. Electrical measurements indi- cate that the conductivity of doped graphene can be well regulated with the N content. The method is expected to produce large scale and controllable N-doped graphene sheets for a variety of potential applications.

High efficient catalytic oxidation of 5-hydroxymethylfurfural into 2,5-furandicarboxylic acid under benign conditions with nitrogen-doped graphene encapsulated Cu nanoparticles

Selective oxidation of 5-hydroxymethylfurfual(HMF) to 2,5-furandicarboxylic acid(FDCA) as a bioplastics monomer is efficiently promoted by a simple system without noble-metal and base additives. In this work, graphene oxide(GO) was first synthesised by an electrochemical method with flexible graphite paper(FGP) as start carbon material, then, nitrogen-doped graphene(NG) layers encapsulated Cu nanoparticles(NPs) was prepared by one-step thermal treatment of GO supported Cu2+ in flowing NH3 atmosphere. Compared with NG supported Cu NPs prepared by the traditional impregnation method, enhanced catalytic activity was achieved over Cu/NG and an FDCA yield of 95.2% was achieved under mild reaction conditions with tert-butylhydroperoxide(t-BuOOH) as the oxidant. Control experiments with different catalysts and different addition procedure of t-BuOOH showed the yield of HMF and various intermediates during reaction. From the changing of intermediates concentrations and reaction rates, a reaction pathway through HMF-DFF-FFCA-FDCA was proposed. This work gives a more convenient, more green,more economical and effective method in encapsulated metal NPs preparation and high selectivity in HMF oxidation to FDCA under mild conditions.

Isolated Co single atoms in nitrogen-doped graphene for aluminum-sulfur batteries with enhanced kinetic response

Al-S batteries are promising next generation energy storage devices due to their high theoretical energy density(1340 Wh kg^(-1)),low cost,and safe operation.However,the electrochemical performance of Al-S batteries suffers poor reversibility owing to slow kinetic processes determined by the difficulty of reversible conversion between Al and S.Here,we proposed a single-atom catalysts comprising Co atoms embedded in a nitrogen-doped graphene(Co NG)as an electrochemical catalyst in the sulfur cathode that renders a reduced discharge-charge voltage hysteresis and improved sulfur utilization in the cathode.The structural and electrochemical analyses suggest that the Co NG facilitated both the formation and oxidation of Al S;during the electrochemical reactions of the sulfur species.Consequently,the Co NG-S composite can deliver a considerably reduced voltage hysteresis of 0.76 V and a reversible specific capacity of 1631 m Ah g^(-1) at 0.2 A g^(-1) with a sulfur utilization of more than 97%.

Luminescence and magnetic properties of bifunctional nanoparticles composited by nitrogen-doped graphene quantum dots and gadolinium

In this paper,nitrogen-doped graphene quantum dots(N-GQDs)were combined with gadolinium ions(Gd^(3+))by a surface modification to obtain magneto-optical dual-functional N-GQDs/Gd^(3+)nanoparticles.The morphology of obtained composite was characterized by field emission scanning electron microscopy and transmission electron microscopy.Luminescence and magnetic properties were measured by a fluorescence spectrophotometer and a vibrating sample magnetometer,respectively.Results indicate that well-dispersed spherical N-GQDs/Gd^(3+)nanoparticles have an average diameter of 7 nm.N-doping significantly increases the luminesce nce of particles with an optimal luminescence intensity at 20℃and pH=9.X-ray photoelectron spectroscopy results indicate that the N-doping introduces pyrrolic N as an electron donor,enhancing fluorescence by increasing the surface electron cloud density of N-GQDs.In addition,density functional theory calculation results reveal that N-doping reduces the band gap of NGQDs/Gd^(3+),enabling electronic transitions to higher energy levels and generating more activation sites,thereby enhancing luminescence.Compared to N-GQDs/Gd^(3+)prepared at 20℃,the saturated magnetization of particles prepared at 40℃is 0.85 emu/g,indicating a better magnetic response.The above results suggest that bifunctional nanomaterials N-GQDs/Gd^(3+)with excellent optical properties and magnetism can be better used for fluorescence and magnetic resonance imaging.

Synthesis of nitrogen-doped reduced graphene oxide/magnesium ferrite/polyaniline composite aerogel as a lightweight,broadband and efficient microwave absorber

The fabrication of graphene-based microwave absorbing materials with low density,small filling ratio,broad bandwidth and strong absorption remains a huge challenge.In this work,nitrogen-doped reduced graphene oxide/magnesium ferrite/polyaniline(NRGO/MgFe_(2)O_(4)/PANI)composite aerogel was synthesized by a three-step method of solvothermal reaction,in situ chemical oxidation polymerization and hydrothermal self-assembly.The results showed that the obtained aerogels had a unique three-dimensional(3D)porous network structure and low bulk density(11.1-13.0 mg cm^(−3)).It was worth noting that in the NRGO/MgFe_(2)O_(4)/PANI ternary composite aerogel,MgFe_(2)O_(4)coated with a thin PANI layer was anchored on the surface of NRGO sheets.Furthermore,the NRGO/MgFe_(2)O_(4)/PANI ternary composite aerogel showed much better microwave absorbing capacity compared with pure NRGO aerogel and NRGO/MgFe_(2)O_(4)binary composite aerogel.When the filling ratio was as low as 11.5 wt.%,the obtained ternary composite aerogel exhibited the maximum effective absorption bandwidth of 7.0 GHz at a matching thickness of 2.1 mm,and the minimum reflection loss of-42.9 dB at a thickness of 3.57 mm.Additionally,the prob-able microwave dissipation mechanism was also elucidated.It was believed that this study would pave the way for the construction of 3D graphene-based composites as lightweight,broadband and efficient microwave absorbents.

Functionalization of nitrogen-doped graphene quantum dot:A sustainable carbon-based catalyst for the production of cyclic carbonate from epoxide and CO_(2)

A series of organic compounds were successfully immobilized on an N-doped graphene quantum dot (N-GQD) to prepare a multifunctional organocatalyst for coupling reaction between CO_(2)and propylene oxide (PO).The simultaneous presence of halide ions in conjunction with acidic-and basic-functional groups on the surface of the nanoparticles makes them highly active for the production of propylene carbonate (PC).The effects of variables such as catalyst loading,reaction temperature,and structure of substituents are discussed.The proposed catalysts were characterized by different techniques,including Fourier transform infrared spectroscopy (FTIR),field emission scanning electron microscopy/energy dispersive X-ray microanalysis (FESEM/EDX),thermogravimetric analysis (TGA),elemental analysis,atomic force microscopy (AFM),and ultraviolet–visible (UV-Vis) spectroscopy.Under optimal reaction conditions,3-bromopropionic acid (BPA) immobilized on N-GQD showed a remarkable activity,affording the highest yield of 98%at 140℃ and 106Pa without any co-catalyst or solvent.These new metal-free catalysts have the advantage of easy separation and reuse several times.Based on the experimental data,a plausible reaction mechanism is suggested,where the hydrogen bonding donors and halogen ion can activate the epoxide,and amine functional groups play a vital role in CO_(2)adsorption.

Nitrogen-doped graphene hydrogel-supported NiPt-CeOx nanocomposites and their superior catalysis for hydrogen generation from hydrazine at room temperature

The safe and efficient storage and release of hydrogen is one of the key technological challenges for the fuel cell-based hydrogen economy. Hydrazine monohydrate has attracted considerable attention as a safe and convent chemical hydrogen-storage material. Herein, we report the facile synthesis of NiPt-CeOx nanocomposites supported by three-dimensional nitrogen-doped graphene hydrogels （NGHs） via a simple one-step co-reduction synthesis method. These catalysts were composition-dependent for hydrogen generation from an alkaline solution of hydrazine. （NisPt5）I-（CeOx）0.B/NGH exhibited the highest catalytic activity, with 100% hydrogen selectivity and turnover frequencies of 408 h^-1 at 298 K and 3,064 h^-1 at 323 K. These superior catalytic performances are attributed to the electronic structure of the NiPt centers, which was modified by the electron interaction between NiPt and CeOx and the strong metal-support interaction between NiPt-CeOx and the NGH.

Nitrogen-doped graphene nanosheets as reactive water purification membranes

Oxidation of organic pollutants by sulfate radicals produced via activation of persulfate has emerged as a promising advanced oxidation technology to address various challenging environmental issues. The development of an effective, environmentally-friendly, metal-free catalyst is the key to this technology. Additionally, a supported catalyst design is more advantageous than conventional suspended powder catalysts from the point of view of mass transfer and practical engineering applications （e.g. post-use separation）. In this study, a metal-free N-doped reduced graphene oxide （N-rGO） catalyst was prepared via a facile hydrothermal method. N-rGO filters were then synthesized by facile vacuum filtration, such that water can flow through nanochannels within the filters. Various advanced characterization techniques were employed to obtain structural and compositional information of the as-synthesized N-rGO filters. An optimized phenol oxidative flux of 0.036 ＋_ 0.002 mmol.h ~ was obtained by metal-flee catalytic activation of persulfate at an influent persulfate concentration of 1.0 mmol-L 1 and filter weight of 15 rag, while a N-free rGO filter demonstrated negligible phenol oxidation capability under similar conditions. Compared to a conventional batch system, the flow-through design demonstrates obviously enhanced oxidation kinetics （0.036 vs. 0.010 retool-h-I）, mainly due to the liquid flow through the filter leading to convection-enhanced transfer of the target molecule to the filter active sites. Overall, the results exemplified the advantages of organic compound removal by catalytic activation of persulfate using a metal-free catalyst in flow- through mode, and demonstrated the potential of N-rGO filters for practical environmental applications.

Enhanced formaldehyde sensitivity of two-dimensional mesoporous SnO_(2) by nitrogen-doped graphene quantum dots

Formaldehyde(HCHO) is widely known as an indoor air pollutant,and the monitoring of the gas has significant importance.However,most HCHO sensing materials do not have low detection limits and operate at high temperatures.Herein,two-dimensional(2D) mesoporous ultrathin SnO_(2) modified with nitrogen-doped graphene quantum dots(N-GQDs) was synthesized.The N-GQDs/SnO_(2) nanocomposite demonstrated high efficiency for HCHO detection.With the addition of 1.00 wt%N-GQDs,the response(Ra/Rg) of SnO_(2) gas sensor increased from 120 to 361 at 60℃ for the detection of 10×10^(-6) HCHO.In addition,the corresponding detection limit was as low as 10×10^(-9).Moreover,the sensor exhibited excellent selectivity and stability for the detection of HCHO.The enhanced sensing performance was attributed to both the large specific surface area of SnO_(2) and electron regulation of N-GQDs.Therefore,this study presents a novel HCHO sensor,and it expands the research and application potential of GQDs nanocomposites.

Three-dimensional nitrogen-doped graphene hydrogel supported Co-CeOx nanoclusters as efficient catalysts for hydrogen generation from hydrolysis of ammonia borane

The development of highly active noble-metal-flee catalysts for catalytic hydrolysis of ammonia borane is mandatory for its application in hydrogen storage. Herein, Co-CeOx nanoclusters have been successfully anchored on a three-dimensional nitrogen-doped graphene hydrogel （NGH） by a simple coreduction method and further used as efficient catalysts to catalytic hydrolysis of ammonia borane at room temperature. Thanks to the strong synergistic electronic effect between Co and CeOx, as well as the strong metal-support interaction between Co-CeOx and 3D NGH, the as-synthesized Co-（CeOx）0.91/NGH catalyst exhibits superior catalytic activity toward hydrolysis of ammonia borane, with the turnover frequency （TOF） value of 79.5 min 1, which is almost 13 times higher than that of Co]NGH, and higher than most of the reported noble-metal-free catalysts.

Design ofL-Cysteine Functionalized Au@SiO2@Fe3O4/Nitrogen-doped Graphene Nanocomposite and Its Application in Electrochemical Detection of Pb2＋

A novel magnetic electrochemical sensor was designed for determination of lead ions based on gold na- noparticles（AuNPs）@SiO2@Fe3O4/nitrogen-doped graphene（NG） composites functionalized with L-cysteine. The Au@SiO2@Fe3O4/NG was synthesized by the electrostatic adsorption between AuNPs and SiO2-coated Fe304 NPs（SiO2@Fe304） and the amide bond between Au@SiO2@Fe3O4 and NG. L-Cysteine was successfully functionalized on the surface of Au@SiO2@Fe3O4/NG nanocomposites via the S--Au bond between L-cysteine and AuNPs. Owing to numerous active sites in L-cysteines and high conductivity of Au@SiO2@Fe3O4/NG composites, the pro- posed electrochemical sensor exhibited a well-distributed nanostructure and high responsivity toward Pb（II）. The sensor linearly responded to Pb2＋ concentration in the range of 5-80 μg/L with a detection limit of 0.6 μg/L, indicating that this L-cysteine functionalized Au@SiO2@Fe3O4/NG composite could be a promising candidate material for the detection of Pb2＋.

Cuprous Oxide/Nitrogen-doped Graphene Nanocomposites as Electrochemical Sensors for Ofloxacin Determination

Cu2O/nitrogen-doped grapheme（NG） nanocomposite material was prepared via a facile one step chemical reduction and characterized by means of X-ray diffraction（XRD） and scanning electron microscopy（SEM）. A new electrochemical sensor was then fabricated by coating Cu2O/nitrogen-doped graphene nanocomposite with Nation on glassy carbon electrode（Cu2O/NG/Nation/GCE）. The electrochemical response of this modified electrode toward of- loxacin was examined by cyclic voltammetry. The results indicate that Cu2O/NG/Nafion composite-modified elec- trode exhibits higher catalytic activity in the electrochemical oxidation of ofloxacin compared with glassy carbon electrode（GCE）, Cu2O/Nafion modified electrode（Cu2O/Nafion/GCE）, and N-doped graphene/Nation modified electrode（NG/Nafion/GCE）. Under optimal conditions, the peak current was found to be linearly proportional to the concentration of ofloxacin in the 0.5-27.5 μmol/L and 27.5-280 μmol/L ranges with a lower detection limit of 0.34 μmol/L, higher sensitivity of 39.32 μA-L-mmoV1 and a shorter reaction time of less than 2 s. In addition, Nation can enhance the stability of the modified electrode and prevent some negative species. Thus the modified electrode exhibits good selectivity and a long working life. The Cu2O/NG/Nafion composite modified electrode shows promising application in electrochemical sensors, biosensors, and other related fields because of its excellent properties.

Facile Synthesis of Nitrogen-Doped Graphene Aerogels for Electrode Materials in Supercapacitors

Three-dimensional porous nitrogen-doped graphene aerogels （NGAs） were synthesized by using graphene oxide （GO） and chitosan （CS） via a self-assembly process by one-pot hydrothermal method. The morphology and struc- ture of the as-prepared materials were characterized by means of scanning electron microscopy, transmission elec- tron microscopy, X-ray diffraction, XPS spectroscopy, Raman spectroscopy, nitrogen adsorption/desorption meas- urement and Fourier transform infrared spectroscopy. The electrochemical performance of NGAs was studied by cyclic voltammetry, galvanostatic charge/discharge and impedance spectroscopy measurements. The microstructure, surface area and capacitance of NGAs could be facilely controlled by adding different amounts of chitosan. The prepared NGA-4 showed a specific capacitance of 148.0 F/g at the discharge current density of 0.5 A/g and also re- tained 95.3% of the initial capacitance after 5000 cycles at the scan rate of 10 mV/s. It provided a possible way to obtain graphene based materials with high surface area and capacitance.

CVD synthesis of nitrogen-doped graphene using urea

This work provides an effective low-cost synthesis and in-depth mechanistic study of high quality large-area nitrogen-doped graphene(NG) films. These films were synthesized using urea as nitrogen source and methane as carbon source, and were characterized by scanning electron microscopy(SEM), Raman spectroscopy and X-ray photoelectron spectroscopy(XPS). The N doping level was determined to be 3.72 at.%, and N atoms were suggested to mainly incorporated in a pyrrolic N configuration. All distinct Raman peaks display a shift due to the nitrogen-doping and compressive strain. The increase in urea concentration broadens the D and 2D peak's Full Width at Half Maximum(FWHM), due to the decrease of mean free path of phonons. The N-doped graphene exhibited an n-type doping behavior with a considerably high carrier mobility of about 74.1 cm2/(V s), confirmed by electrical transport measurements.