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Journal ArticleDOI

Facile synthesis of nitrogen-doped graphene via low-temperature pyrolysis: The effects of precursors and annealing ambience on metal-free catalytic oxidation

01 May 2017-Carbon (Pergamon)-Vol. 115, pp 649-658
TL;DR: In this article, a green and facile protocol of thermal treatment of graphene oxide (GO) with urea was adopted to synthesize nitrogen-doped graphene (NG-Urea-air) at a low temperature (350°C) in the static air.
About: This article is published in Carbon.The article was published on 2017-05-01 and is currently open access. It has received 299 citations till now. The article focuses on the topics: Graphene & Carbocatalysis.

Summary (3 min read)

1. Introduction

  • The exfoliation of graphite, the stacked layers of graphene, has attracted extensive interests because of the cheap and abundant carbon source and mild synthesis conditions.
  • Therefore, N-doping has been demonstrated to be able to endow pristine graphene with impressively enhanced activity toward chemical reactions and catalysis [15-17].
  • The derived carbocatalysts were utilized to activate peroxymonosulfate (PMS) for catalytic oxidation.
  • Moreover, the N-doping procedure was investigated by exploring the effects of carbon matrix (oxygen-rich/deficient carbon precursors), N-doping precursors (organic/inorganic nitrogen-containing substances), and annealing conditions (oxidative or non-oxidative ambience).

2.1. Materials preparation

  • The chemicals in this study were purchased from Sigma-Aldrich, Australia with a reagent grade and used as received without further purification.
  • The nitrogen doped graphene (NG) was synthesized by a facile pyrolysis of GO with nitrogen precursors.
  • The obtained furry black powder was then washed with ethanol and deionized water several times and then dried in an oven at 60 °C overnight for further use.
  • The products were ground and denoted as NG-Urea-air.
  • Besides, different N-precursors were also investigated with other organic precursors such as melamine, cyanamide, and inorganic salts such as ammonium nitrate (NH4NO3) and ammonium chloride (NH4Cl).

2.2. Characterization of nanocarbons

  • The structure and morphology of the nanocarbons was revealed by a Zeiss Neon FIBSEM.
  • The elemental distribution of nitrogen-doped graphene from different precursors was monitored by the EDS elemental mapping in Figs. S1 – S5.
  • FTIR spectra were acquired from a Bruker spectrometer.
  • The BET surface area, nitrogen sorption isotherms, and pore diameter distributions were measured at -196 °C on a TriStar II apparatus.
  • The surface elemental information was probed with a Kratos X-ray photoelectron spectroscopy (XPS) instrument equipped with a monochromated Al Kα X-ray gun.

2.3. Catalytic oxidation procedure

  • The catalytic performance of the nanocarbon catalysts was evaluated by PMS activation and oxidation of toxic organic contaminants.
  • The aqueous reactions were carried out in a batch reactor with the organics, PMS, and carbocatalysts at the usage of 20 ppm organic chemicals, 6.5 mM PMS, and 0.4 g/L carbocatalysts at 25 ºC.
  • The solution was withdrawn at set intervals and filtered to remove the solid catalyst, quenched by methanol, and analyzed on an ultra-high performance liquid chromatography (UHPLC, ThermoFisher Scientific) with an UltiMate™ 3000 RSLCnano system.
  • The generated singlet oxygen was captured by 2,2,6,6-tetramethyl-4piperidone (TMP) and tested on a Bruker electron paramagnetic resonance (EPR) instrument with the settings at following parameters: center field 3515 G, sweep width 100.0 G, power 20.0 mW, sweeping time 30 s, and scan number 2.

3.1. Characterization of samples

  • The SEM images in Fig. 1 and Fig. 2 display the morphologies of the nanocarbon catalysts.
  • Different from the smooth lamellar and severe stacked layers of GO (Figs. 1a and 2a), the obviously expanded and corrugated thin layers were spotted for rGO-air (Figs. 1b and 2b).
  • Besides, urea may work as the reducing agent during the thermal treatment to result in a better reducibility of rGO.
  • The moderate pyrolysis can achieve a very high doping level compared with graphene annealed at high temperatures (less than 10 at.%), as the nitrogen dopants would decompose with the raising temperature and lead to a greater proportion of graphitic N [24, 25].
  • The N-doping dramatically interrupted the carbon lattice and induced high defective degrees for NG-Urea-air (1.41) and N-rGO-air (1.32).

3.2. Evaluation of catalytic oxidation over the carbocatalysts.

  • The catalytic performances of the as-made nanocarbons were evaluated in activation of peroxymonosulfate for phenol oxidation.
  • It is also supposed that the formation of a dipole moment in Ndoped graphene may act as a crucial step for the enhanced catalytic performances for PMS activation.
  • Nevertheless many studies utilized various nitrogen-containing compounds as nitrogen sources to synthesize N-doped graphene via thermal annealing or hydrothermal approaches, the influences of the precursors on N-doping and catalytic performances of the products have not been systematically investigated yet.
  • The FTIR spectra in Fig. S10 suggests that there existed condensed CxNy substances due to the polymerization of the N-rich organic precursors.

3.3. Mechanism of PMS activation

  • In a preliminary study, the authors discovered that N-doping into the highly graphitic single-walled carbon nanotubes (N-SWCNT) would mediate a nonradical oxidative pathway, in which the organics was oxidized directly on the carbon surface without generation of free radicals [43].
  • A high ratio of the radical scavengers (MMeOH : MPMS =1000) did not completely terminate the oxidative reactions, and an excellent phenol oxidation efficiency was still achieved with complete organics removal in 120 min, which is intrinsically different from the classical radical-based Co3O4/PMS system in which the reaction was dramatically shut down by the alcohols (Fig. S16).
  • The ketonic groups (C=O) at the boundaries of sp2-conjugated carbon lattice could be the active sites for the evolution of singlet oxygen as elucinated in Fig.
  • As a result, N-doped graphene/PMS is a complicated system involving multi-oxidative pathways, which is intrinsically different from the radical-dominated metal-based AOPs systems.

3.4. Effects of solution pH and halogen ions

  • The impact of chloride ions was studied in the carbon-catalyzed oxidative system.
  • The chloride ions not only scavenge the reactive radicals which partially contribute to the oxidation, but also compete with the organics to donate an electron to form Cl via the nonradical process [47, 48].
  • When excessive chloride ions (5 and 10 mM) were introduced into the system, the produced Cl would then quickly react with superabundant Cl− to form chlorine radicals (Cl2 −, 2.1 V) with a higher oxidative potential, which tend to attack the electron-rich organics and induce the enhanced efficiency of phenol removal [49].
  • The influence of initial solution pH was also investigated in PMS activation with NG-Urea-air.
  • The pH has been reported to dramatically affect the catalytic efficiency of heterogeneously metal catalysts for PMS activation [50, 51].

4. Conclusions

  • In summary, nitrogen-doped graphene was synthesized via a facile thermal combustion approach and applied for heterogeneous activation of peroxymonosulfate for organic oxidation.
  • The N-doped graphene derived from urea demonstrated a significant enhancement toward catalytic decomposition of toxic contaminants.
  • The influences of precursors and doping conditions were investigated.
  • The high phenol removal efficiency with carbocatalysis was benefited from the nonradical oxidation meanwhile involving singlet oxygen, and a few other free radicals.
  • This study provides a fundamental understanding of the nitrogen doping progress in carbon framework as well as the mechanistic insights into peroxymonosulfate activation by nitrogendoped graphene.

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Citations
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Journal ArticleDOI
TL;DR: This Critical Review comparatively examines the activation mechanisms of peroxymonosulfate and peroxydisulfates and the formation pathways of oxidizing species and the impacts of water parameters and constituents such as pH, background organic matter, halide, phosphate, and carbonate on persulfate-driven chemistry.
Abstract: Reports that promote persulfate-based advanced oxidation process (AOP) as a viable alternative to hydrogen peroxide-based processes have been rapidly accumulating in recent water treatment literature. Various strategies to activate peroxide bonds in persulfate precursors have been proposed and the capacity to degrade a wide range of organic pollutants has been demonstrated. Compared to traditional AOPs in which hydroxyl radical serves as the main oxidant, persulfate-based AOPs have been claimed to involve different in situ generated oxidants such as sulfate radical and singlet oxygen as well as nonradical oxidation pathways. However, there exist controversial observations and interpretations around some of these claims, challenging robust scientific progress of this technology toward practical use. This Critical Review comparatively examines the activation mechanisms of peroxymonosulfate and peroxydisulfate and the formation pathways of oxidizing species. Properties of the main oxidizing species are scrutinized and the role of singlet oxygen is debated. In addition, the impacts of water parameters and constituents such as pH, background organic matter, halide, phosphate, and carbonate on persulfate-driven chemistry are discussed. The opportunity for niche applications is also presented, emphasizing the need for parallel efforts to remove currently prevalent knowledge roadblocks.

1,412 citations

Journal ArticleDOI
TL;DR: This Account showcases the recent contributions to metal-free catalysis in advanced oxidation, including design of nanocarbon catalysts, exploration of intrinsic active sites, and identification of reactive species and reaction pathways, and offers perspectives on carbocatalysis for future environmental applications.
Abstract: ConspectusCatalytic processes have remarkably boosted the rapid industrializations in chemical production, energy conversion, and environmental remediation. As one of the emerging applications of carbocatalysis, metal-free nanocarbons have demonstrated promise as catalysts for green remediation technologies to overcome the poor stability and undesirable metal leaching in metal-based advanced oxidation processes (AOPs). Since our reports of heterogeneous activation of persulfates with low-dimensional nanocarbons, the novel oxidative system has raised tremendous interest for degradation of organic contaminants in wastewater without secondary contamination. In this Account, we showcase our recent contributions to metal-free catalysis in advanced oxidation, including design of nanocarbon catalysts, exploration of intrinsic active sites, and identification of reactive species and reaction pathways, and we offer perspectives on carbocatalysis for future environmental applications.The journey starts with the dis...

872 citations

Journal ArticleDOI
TL;DR: This study not only provides robust and cheap carbonaceous materials for environmental remediation but also enables the first insight into the graphitic biochar-based nonradical catalysis.
Abstract: Environmentally friendly and low-cost catalysts are important for the rapid mineralization of organic contaminants in powerful advanced oxidation processes (AOPs). In this study, we reported N-doped graphitic biochars (N-BCs) as low-cost and efficient catalysts for peroxydisulfate (PDS) activation and the degradation of diverse organic pollutants in water treatment, including Orange G, phenol, sulfamethoxazole, and bisphenol A. The biochars at high annealing temperatures (>700 °C) presented highly graphitic nanosheets, large specific surface areas (SSAs), and rich doped nitrogen. In particular, N-BC derived at 900 °C (N-BC900) exhibited the highest degradation rate, which was 39-fold and 6.5-fold of that on N-BC400 and pristine biochar, respectively, and the N-BC900 surpassed most popular metal or nanocarbon catalysts. Different from the radical-based oxidation in N-BC400/PDS via the persistent free radicals (PFRs), singlet oxygen and nonradical pathways (surface-confined activated persulfate–carbon compl...

752 citations

Journal ArticleDOI
Eun Tae Yun1, Jeong Hoon Lee1, Jaesung Kim1, Hee Deung Park1, Jaesang Lee1 
TL;DR: This study explored singlet oxygenation and mediated electron transfer as plausible nonradical mechanisms for organic degradation by carbon nanotube (CNT)-activated peroxymonosulfate (PMS) and suggested that CNT-mediated electron transfer from organics to persulfate was primarily responsible for the nonradical degradative route.
Abstract: Select persulfate activation processes were demonstrated to initiate oxidation not reliant on sulfate radicals, although the underlying mechanism has yet to be identified. This study explored singlet oxygenation and mediated electron transfer as plausible nonradical mechanisms for organic degradation by carbon nanotube (CNT)-activated peroxymonosulfate (PMS). The degradation of furfuryl alcohol (FFA) as a singlet oxygen (1O2) indicator and the kinetic retardation of FFA oxidation in the presence of l-histidine and azide as 1O2 quenchers apparently supported a role of 1O2 in the CNT/PMS system. However, the 1O2 scavenging effect was ascribed to a rapid PMS depletion by l-histidine and azide. A comparison of CNT/PMS and photoexcited Rose Bengal (RB) excluded the possibility of singlet oxygenation during heterogeneous persulfate activation. In contrast to the case of excited RB, solvent exchange (H2O to D2O) did not enhance FFA degradation by CNT/PMS and the pH- and substrate-dependent reactivity of CNT/PMS ...

682 citations

Journal ArticleDOI
TL;DR: In this article, the authors present state-of-the-art research on nonradical pathways in persulfate-based AOPs, with emphases on the controversial methodologies for identifying the oxygen reactive species (ROS), ambiguous reaction mechanisms, intrinsic impacts of metal/carbon catalysts and organic substrates in the nonradical-based catalytic oxidation reactions.
Abstract: Recent discoveries of nonradical oxidation in aqueous-phase advanced oxidation processes (AOPs) have induced tremendous interest in environmental remediation of wastewater, whereas different findings from a variety of investigations have also raised severe controversies in the occurrence and mechanism of the nonradical reaction. Hence, critical understandings of the nonradical reaction will significantly advance the knowledge and its application for catalytic oxidation and wastewater treatment. In this review, we would like to present state-of-the-art research on nonradical pathways in persulfate-based AOPs, with emphases on the controversial methodologies for identifying the oxygen reactive species (ROS), ambiguous reaction mechanisms, intrinsic impacts of metal/carbon catalysts and organic substrates in the nonradical-based catalytic oxidation reactions. Moreover, further research directions on mechanistic investigation of the nonradical pathway with rational experimental design and advanced strategies, as well as the potential applications of the nonradical system are proposed.

619 citations

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TL;DR: Monocrystalline graphitic films are found to be a two-dimensional semimetal with a tiny overlap between valence and conductance bands and they exhibit a strong ambipolar electric field effect.
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Additional excerpts

  • ...Several protocols of exfoliation have been developed such as micromechanical exfoliation [6], electrochemical exfoliation [7], and chemical exfoliation [8, 9]....

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TL;DR: Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena can now be mimicked and tested in table-top experiments.
Abstract: Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.

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"Facile synthesis of nitrogen-doped ..." refers background in this paper

  • ...Geim uncovered the existence of single-layered graphite, namely graphene, with carbon atoms perfectly packed into the honeycomb lattice in a uniform sp(2)-hybridized configuration, graphene-based materials have opened up a new avenue to the world of materials science, nanotechnology, and chemical/energy conversions [1, 2]....

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TL;DR: This work shows that graphene's electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers, and allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area.
Abstract: Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality We show that its electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers The D peak second order changes in shape, width, and position for an increasing number of layers, reflecting the change in the electron bands via a double resonant Raman process The G peak slightly down-shifts This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area

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TL;DR: In this paper, a colloidal suspension of exfoliated graphene oxide sheets in water with hydrazine hydrate results in their aggregation and subsequent formation of a high surface area carbon material which consists of thin graphene-based sheets.

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  • ...Among these methods, the Hummers’ approach was most widely adopted to synthesize single or few-layered chemically exfoliated graphene oxide (GO), followed by post-processing with thermal-treatment or chemical reduction to achieve a better reductive degree of graphene, also known as reduced graphene oxide (rGO) [10-12]....

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Frequently Asked Questions (19)
Q1. What are the contributions in "Facile synthesis of nitrogen-doped graphene via low-temperature pyrolysis: the effects of precursors and annealing ambience on metal-free catalytic oxidation" ?

In this paper, the authors applied a facile strategy by direct treatment of GO and urea at a moderate temperature ( 350 °C ). 

The annealing process with a slowheating rate (5 °C/min) also effectively inhibited the polycondensation of urea as the process only occurs during the rapidly thermal polymerization in the presence of large amounts of urea. 

the acidic/basic condition of the reaction solutions can impact the catalytic performance of metal catalysts as the strong acidic condition can destroy the metal crystalline structure and lead to severe metal leaching. 

Introducing heteroatoms (B, N, O, P, S, and so forth) into the carbon lattice can effectively disorientate the homogeneously conjugated electron network and modulate the surface properties by tweaking the charge distribution and spinning culture of the doped domains [13, 14]. 

Their pioneering studies integrating with material design and theoretical calculations revealed that, due to a greater electronegativity (χN = 3.04) compared with the carbon atom (χC = 2.55), N-doping can effectivtly interrupt the highly-conjugated carbon network of graphene and induce the electron transport from the neighbouring carbon to nitrogen atoms, giving rise to positively charged carbon atoms [37, 38]. 

Many studies have reported that N-doping can effectively break the inertness of sp2 hybridized carbon lattice and dramatically tailor the electron density and spin culture of the adjacent carbon, giving rise to the superb catalytic performances in electrochemistry, hydrocarbon conversion, and superoxide activation [34-36]. 

Their previous study also indicated raising the annealing temperature in inert atmosphere would result in a lower N-doping level due to the breakup of C-N bond while affording a higher proportion of quaternary N benefiting from the better thermal stability of graphitic N, which is simultaneously bonded with three carbon atoms with substitutional doping in the carbon framework [26]. 

The mild annealing and doping processes favored the oxidative atmosphere due to the induced structure defects such as edging sites and vacancies which enabled carbon reconstruction for N-doping. 

the rGO can be used as a better carbon precursor to prepare nitrogen-doped graphene with a desirable SSA, N-doping level, defective degree, and stunning catalytic activity. 

With respect to rGO, the re-constructing of graphene boundaries (edging sites) and lattice during annealing may also help attain certain amounts of N-doping as shown in this study. 

The green and efficient carbocatalysts have demonstrated extraordinary potentials for activating various superoxides (e.g. peroxymonosulfate, persulfate, hydrogen peroxide, and ozone) for the oxidative removal of toxic pollutants in wastewater without any secondary contamination [18-21]. 

Both organic substances and inorganic salts were applied as the nitrogen precursors for nitrogen doping and urea was discovered to be the best precursor with a high doping level and better reducibility without any polycondensation. 

The experimental results indicated that the nitrogen-doped graphene with a higher proportion of graphitic N at a similar doping level exhibited a better catalytic activity for phenol oxidation, and the densities functional theory (DFT) calculations further evidenced that the adsorption of PMS molecules at the adjacent carbon of graphitic N exhibited the lowest adsorption energy and greatest tendency for electron transfer from carbon lattice to PMS for the activation of superoxide O-O bond [26]. 

The proportion of graphitic N of NG-Urea-air and N-rGO-air in total nitrogen was low due to the fact that the mild annealing temperature cannot produce high contents of substitutional N-doping into the carbon lattice. 

The outstanding efficiency of PMS activation on nitrogen-doped graphene is also contributed by the non-radical process, in which the PMS molecules are activated at N-doped domains and followed by readily oxidation of target organics via electron transfer as illustrated in Fig. 

due to the complicated structure and surface chemistry of nanocarbons, the mechanism of carbocatalysis in metal-free oxidation remains ambiguous, leaving more blanks for mechanisticstudy. 

Fig. S8 manifested that the phenol oxidation rate was speeded up with the increased nitrogen amount in graphene, suggesting that the high doping level leads to a promoted catalytic performance for PMS activation. 

The ammonium salts would decompose during the thermal annealing and release NH3 (the doping agent) and other gasses (N2, N2O, or HCl), which facilitate the formation of porous N-doped graphene with larger SSAs, pore size, and pore volume (Fig. S9 and Table S2). 

The XPS surveys in Fig. 3a illustrate that thermal annealing effectively eliminated the oxygen groups of the graphene oxide (31.4 at.%, Table 1) to form a more reductive surface of rGO-air (14.3 at.%).