Zhimin Ao

Nitrogen-doped graphene for generation and evolution of reactive radicals by metal-free catalysis.

N-Doped graphene (NG) nanomaterials were synthesized by directly annealing graphene oxide (GO) with a novel nitrogen precursor of melamine. A high N-doping level, 8-11 at. %, was achieved at a moderate temperature. The sample of NG-700, obtained at a calcination temperature of 700 °C, showed the highest efficiency in degradation of phenol solutions by metal-free catalytic activation of peroxymonosulfate (PMS). The catalytic activity of the N-doped rGO (NG-700) was about 80 times higher than that of undoped rGO in phenol degradation. Moreover, the activity of NG-700 was 18.5 times higher than that of the most popular metal-based catalyst of nanocrystalline Co3O4 in PMS activation. Theoretical calculations using spin-unrestricted density functional theory (DFT) were carried out to probe the active sites for PMS activation on N-doped graphene. In addition, experimental detection of generated radicals using electron paramagnetic resonance (EPR) and competitive radical reactions was performed to reveal the PMS activation processes and pathways of phenol degradation on nanocarbons. It was observed that both (•)OH and SO4(•-) existed in the oxidation processes and played critical roles for phenol oxidation.

Al doped graphene: A promising material for hydrogen storage at room temperature

A promising material for hydrogen storage at room temperature–Al doped graphene is proposed theoretically by using density functional theory calculation. Hydrogen storage capacity of 5.13 wt % is predicted at T=300 K and P=0.1 GPa with an adsorption energy Eb=−0.260 eV/H2. This is close to the target specified by U.S. Department of Energy with a storage capacity of 6 wt % and a binding energy of −0.2 to −0.4 eV/H2 at ambient temperature and modest pressure for commercial applications. It is believed that the doped Al alters the electronic structures of both C and H2. The bands of H2 overlapping with those of Al and C simultaneously are the underlying mechanism of hydrogen storage capacity enhancement.

Microwave-assisted Synthesis of Mesoporous Co3O4 Nanoflakes for Applications in Lithium Ion Batteries and Oxygen Evolution Reactions

Mesoporous Co3O4 nanoflakes with an interconnected architecture were successfully synthesized using a microwave-assisted hydrothermal and low-temperature conversion method, which exhibited excellent electrochemical performances as anode materials in lithium ion batteries and as catalysts in the oxygen evolution reaction (OER). Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) observations showed the unique interconnected and mesoporous structure. When employed as anode materials for lithium ion batteries, mesoporous Co3O4 nanoflakes delivered a high specific capacity of 883 mAh/g at 0.1C current rate and stable cycling performances even at higher current rates. Post-mortem analysis of ex situ FESEM images revealed that the mesoporous and interconnected structure had been well maintained after long-term cycling. The mesoporous Co3O4 nanoflakes also showed both OER active properties and good catalytic stability. This could be attributed to both the stability of unique mesoporous structure and highly reactive facets.

Density functional theory calculations on the CO catalytic oxidation on Al-embedded graphene

The oxidation of CO molecules on Al-embedded graphene has been investigated by using the first principles calculations. Both Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) oxidation mechanisms are considered. In the ER mechanism, an O2 molecule is first adsorbed and activated on Al-embedded graphene before a CO molecule approaches, the energy barrier for the primary step (CO + O2 → OOCO) is 0.79 eV. In the LH mechanism, O2 and CO molecules are firstly co-adsorbed on Al-embedded graphene, the energy barrier for the rate limiting step (CO + O2 → OOCO) is only 0.32 eV, much lower than that of ER mechanism, which indicates that LH mechanism is more favourable for CO oxidation on Al-embedded graphene. Hirshfeld charge analysis shows that the embedded Al atom would modify the charge distributions of co-adsorbed O2 and CO molecules. The charge transfer from O2 to CO molecule through the embedded Al atom plays an important role for the CO oxidation along the LH mechanism. Our result shows that the low cost Al-embedded graphene is an efficient catalyst for CO oxidation at room temperature.

Micelle-template synthesis of nitrogen-doped mesoporous graphene as an efficient metal-free electrocatalyst for hydrogen production.

Synthesis of mesoporous graphene materials by soft-template methods remains a great challenge, owing to the poor self-assembly capability of precursors and the severe agglomeration of graphene nanosheets. Herein, a micelle-template strategy to prepare porous graphene materials with controllable mesopores, high specific surface areas and large pore volumes is reported. By fine-tuning the synthesis parameters, the pore sizes of mesoporous graphene can be rationally controlled. Nitrogen heteroatom doping is found to remarkably render electrocatalytic properties towards hydrogen evolution reactions as a highly efficient metal-free catalyst. The synthesis strategy and the demonstration of highly efficient catalytic effect provide benchmarks for preparing well-defined mesoporous graphene materials for energy production applications.

Insights into N-doping in single-walled carbon nanotubes for enhanced activation of superoxides: a mechanistic study.

Emerging characteristics upon nitrogen-doping were differentiated in the activation of superoxides over single-walled carbon nanotubes. Both experimental and theoretical studies revealed that enhanced peroxymonosulfate (PMS) activation is ascribed to a nonradical process while persulfate (PS) activation is accelerated via directly oxidizing water, yet hydrogen peroxide (H2O2) activation is inert to N-doping. This study details the first insights into versatile N-doping in carbocatalysis for organic oxidation in sustainable remediation.

Reversible Hydrophobic to Hydrophilic Transition in Graphene via Water Splitting Induced by UV Irradiation

Although the reversible wettability transition between hydrophobic and hydrophilic graphene under ultraviolet (UV) irradiation has been observed, the mechanism for this phenomenon remains unclear. In this work, experimental and theoretical investigations demonstrate that the H2O molecules are split into hydrogen and hydroxyl radicals, which are then captured by the graphene surface through chemical binding in an ambient environment under UV irradiation. The dissociative adsorption of H2O molecules induces the wettability transition in graphene from hydrophobic to hydrophilic. Our discovery may hold promise for the potential application of graphene in water splitting.

Enhanced stability of hydrogen atoms at the graphene/graphane interface of nanoribbons

The thermal stability of graphene/graphane nanoribbons (GGNRs) is investigated using density functional theory. It is found that the energy barriers for the diffusion of hydrogen atoms on the zigzag and armchair interfaces of GGNRs are 2.86 and 3.17 eV, respectively, while the diffusion barrier of an isolated H atom on pristine graphene was only ∼0.3 eV. These results unambiguously demonstrate that the thermal stability of GGNRs can be enhanced significantly by increasing the hydrogen diffusion barriers through graphene/graphane interface engineering. This may provide new insights for viable applications of GGNRs.

Topotactic Transformation of Metal–Organic Frameworks to Graphene-Encapsulated Transition-Metal Nitrides as Efficient Fenton-like Catalysts

Innovation in transition-metal nitride (TMN) preparation is highly desired for realization of various functionalities. Herein, series of graphene-encapsulated TMNs (FexMn6-xCo4-N@C) with well-controlled morphology have been synthesized through topotactic transformation of metal-organic frameworks in an N2 atmosphere. The as-synthesized FexMn6-xCo4-N@C nanodices were systematically characterized and functionalized as Fenton-like catalysts for catalytic bisphenol A (BPA) oxidation by activation of peroxymonosulfate (PMS). The catalytic performance of FexMn6-xCo4-N@C was found to be largely enhanced with increasing Mn content. Theoretical calculations illustrated that the dramatically reduced adsorption energy and facilitated electron transfer for PMS activation catalyzed by Mn4N are the main factors for the excellent activity. Both sulfate and hydroxyl radicals were identified during the PMS activation, and the BPA degradation pathway mainly through hydroxylation, oxidation, and decarboxylation was investigated. Based on the systematic characterization of the catalyst before and after the reaction, the overall PMS activation mechanism over FexMn6-xCo4-N@C was proposed. This study details the insights into versatile TMNs for sustainable remediation by activation of PMS.

Electrospun cobalt embedded porous nitrogen doped carbon nanofibers as an efficient catalyst for water splitting

The major challenge in water splitting is to develop low cost electrocatalysts as alternatives for simultaneously generating oxygen and hydrogen. Herein, we report the successful synthesis of cobalt nanoparticle embedded porous nitrogen doped carbon nanofibers (Co-PNCNFs) by a facile and scalable electrospinning technology. The electrospun Co-PNCNF composite exhibits a low onset potential of 1.45 V (vs. RHE) along with high current density (overpotential of 285 mV for 10 mA cm−2) towards the oxygen evolution reaction (OER). The exceptional performance could be ascribed to the bi-functionalized CNFs with nitrogen doping and cobalt encapsulation. Moreover, the porous structure and synergistic effect further provide a highly effective surface area and facilitate a fast electron transfer pathway for the OER process. Interestingly, the Co-PNCNF composite also displays the capability for the hydrogen evolution reaction (HER) in alkaline solution. A water electrolyzer cell fabricated by applying Co-PNCNFs as both anode and cathode electrocatalysts in alkaline solution can achieve a high current density of 10 mA cm−2 at a voltage of 1.66 V.