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Two-dimensional crystals for energy conversion and storage
( Javeed Mahmood ),백종범 한국공업화학회 2016 한국공업화학회 연구논문 초록집 Vol.2016 No.0
Despite recent explorations in 2D materials science and engineering, easy and scalable methods to produce uniformly doped 2D materials are limited. To overcome these problems, a new layered 2D network structure with uniformly distributed holes and nitrogen atoms was synthesized and its stoichiometry of basal plane is C2N1. The structure of the C2N was confirmed by scanning tunneling microscopy (STM). Its calculated and experimental band-gaps are 1.7 and 2.0 eV, respectively, in the semiconductor region, suggesting a clear advantage over conducting graphene and insulating h-BN. The C2N structure was used to encapsulate iron (Fe) and cobalt (Co) particles by in situ reduction to give Fe@C2N and Co@C2N, which were used as electrocatalysts for energy conversion and storage.
Nitrogenated holey two-dimensional structures
( Javeed Mahmood ),이은광,정민복,심동빈,전인엽,배서윤,손소담,정선민,최현정,서정민,박노정,오준학,신현준,백종범 한국공업화학회 2015 한국공업화학회 연구논문 초록집 Vol.2015 No.0
Recent graphene research has triggered enormous interest in new twodimensional ordered crystals constructed by the inclusion of elements other than carbon for band-gap opening. The design of new multifunctional two-dimensional materials with proper bandgap has become an important challenge. Here we report a layered two-dimensional network structure that possesses evenly distributed holes and nitrogen atoms and a C2N stoichiometry in its basal plane. The two-dimensional structure can be efficiently synthesized via a simple wet-chemical reaction and confirmed with various characterization techniques, including scanning tunneling microscopy. Furthermore, a field-effect transistor device fabricated using the material exhibits an on/off ratio of 107, with calculated and experimental bandgaps of approximately 1.70 and 1.96 eV, respectively. In view of the simplicity of the production method and the advantages of the solution processability, the C2N-h2D crystal has potential for use in practical applications.
Fe@C<sub>2</sub>N: A highly-efficient indirect-contact oxygen reduction catalyst
Mahmood, Javeed,Li, Feng,Kim, Changmin,Choi, Hyun-Jung,Gwon, Ohhun,Jung, Sun-Min,Seo, Jeong-Min,Cho, Sung-June,Ju, Young-Wan,Jeong, Hu Young,Kim, Guntae,Baek, Jong-Beom Elsevier 2018 Nano energy Vol.44 No.-
<P><B>Abstract</B></P> <P>Converting unstable earth-abundant group VIIIB transition metals into stable catalysts with high oxygen reduction reaction (ORR) performances remains a critical challenge for electrochemical technologies. Iron (Fe)-nitrogen (N)-carbon (C)-based electrocatalysts have recently demonstrated ORR performances comparable to platinum (Pt)-based catalysts. However, as their poor stability remains a critical issue, which needs to be resolved to satisfy commercial requirements. Here, we describe a methodology for preparing a high-performance and stable Fe-based ORR catalyst. The catalyst was obtained by the <I>in-situ</I> sandwiching of a Fe<SUP>3+</SUP> precursor in a nitrogenated holey two-dimensional network (denoted as C<SUB>2</SUB>N). Reduction of the sandwiched Fe<SUP>3+</SUP> results in the formation of Fe oxide (Fe<SUB>x</SUB>O<SUB>y</SUB>) nanoparticles, which are simultaneously transformed into highly crystalline Fe<SUP>0</SUP> nanoparticle cores, while the C<SUB>2</SUB>N is catalysed into well-defined, encapsulating, nitrogenated graphitic shells (Fe@C<SUB>2</SUB>N nanoparticles) during heat-treatment. The resultant Fe<SUP>0</SUP>@C<SUB>2</SUB>N nanoparticles are uniformly distributed on the C<SUB>2</SUB>N substrate, becoming the Fe@C<SUB>2</SUB>N catalyst, which displayed ORR activities superior to commercial Pt/C in both acidic and alkaline media. Furthermore, the Fe@C<SUB>2</SUB>N catalyst remained rust-free during harsh electrochemical testing even after 650h, suggesting that its unusual durability originates from indirect-contact electrocatalysis.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Iron (Fe) nanoparticle cores encapsulated in electrochemically transparent and stable shells (Fe@C<SUB>2</SUB>N) are prepared. </LI> <LI> The Fe@C<SUB>2</SUB>N catalyst displays superb oxygen reduction (ORR) performance in both alkaline and acidic media. </LI> <LI> The advantages of Fe@C<SUB>2</SUB>N catalyst over commercial Pt/C are low-cost and comparable ORR activity and superior durability. </LI> <LI> The unusual ORR performance is the result of the electrochemically stable and transparent encapsulating shells. </LI> <LI> Encapsulating shells allow not only an efficient electron tunneling but also protecting unstable active Fe cores. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Oxygen reduction catalyst from iron encapsulated in C<SUB>2</SUB>N framework (Fe@C<SUB>2</SUB>N) exhibits outstanding catalytic activities in both alkaline and acidic media. The Fe@C<SUB>2</SUB>N catalyst holds great potential for commercialization.</P> <P>[DISPLAY OMISSION]</P>
Nitrogenated two-dimensional structure, a semiconducting covalent-organic framework
Javeed Mahmood,Eun Kwang Lee,Minbok Jung,Dongbin Shin,In-Yup Jeon,Sun-Min Jung,Hyun-Jung Choi,Jeong-Min Seo,Seo-Yoon Bae,So-Dam Sohn,Noejung Park,Joon Hak Oh,Hyung-Joon Shin,Jong-Beom Baek 한국고분자학회 2016 한국고분자학회 학술대회 연구논문 초록집 Vol.41 No.2
An efficient and pH-universal ruthenium-based catalyst for the hydrogen evolution reaction
Mahmood, Javeed,Li, Feng,Jung, Sun-Min,Okyay, Mahmut Sait,Ahmad, Ishfaq,Kim, Seok-Jin,Park, Noejung,Jeong, Hu Young,Baek, Jong-Beom Nature Publishing Group, a division of Macmillan P 2017 Nature nanotechnology Vol.12 No.5
<P>The hydrogen evolution reaction (HER) is a crucial step in electro-chemical water splitting and demands an efficient, durable and cheap catalyst if it is to succeed in real applications(1-3). For an energy-efficient HER, a catalyst must be able to trigger proton reduction with minimal overpotential(4) and have fast kinetics(5-9). The most efficient catalysts in acidic media are platinum-based, as the strength of the Pt-H bond(10) is associated with the fastest reaction rate for the HER11,12. The use of platinum, however, raises issues linked to cost and stability in non-acidic media. Recently, non-precious-metal-based catalysts have been reported, but these are susceptible to acid corrosion and are typically much inferior to Pt-based catalysts, exhibiting higher overpotentials and lower stability(13-15). As a cheaper alternative to platinum, ruthenium possesses a similar bond strength with hydrogen (similar to 65 kcal mol(-1))(16), but has never been studied as a viable alternative for a HER catalyst. Here, we report a Ru-based catalyst for the HER that can operate both in acidic and alkaline media. Our catalyst is made of Ru nanoparticles dispersed within a nitrogenated holey two-dimensional carbon structure (Ru@C2N). The Ru@C2N electrocatalyst exhibits high turnover frequencies at 25 mV (0.67 H-2 s(-1) in 0.5 M H2SO4 solution; 0.75 H-2 s(-1) in 1.0 M KOH solution) and small overpotentials at 10 mA cm(-2) (13.5 mV in 0.5 M H2SO4 solution; 17.0 mV in 1.0 M KOH solution) as well as superior stability in both acidic and alkaline media. These performances are comparable to, or even better than, the Pt/C catalyst for the HER.</P>
Encapsulated cobalt oxide in 2D conjugated network polymer as a hydrogen evolution catalyst
( Javeed Mahmood ),정선민,김석진,박정민,유정우,백종범 한국공업화학회 2015 한국공업화학회 연구논문 초록집 Vol.2015 No.0
In the hydrogen generation field, cobalt oxides have been regarded as promising catalysts, 2D network polymer-supported cobalt-oxide catalysts with good crystallinity are highly anticipated to enhance catalytic performance. Here we report the fabrication of a 2D nitrogenated network polymerencapsulated cobalt-oxide (Co@C2N) catalyst via an in situ solvothermal synthesis. Co@C2N exhibits outstanding catalytic activities for hydrogen (H2) generation from the hydrolysis of alkaline sodium borohydride (NaBH4) solutions. The rate of maximum hydrogen generation is comparable to the best reported values for catalysts containing other noble metals in alkaline solutions. Moreover, Co@C2N can also catalyze the in situ reduction of a nitro group into an amino group (4-nitrophenol to 4-aminophenol) in the presence of NaBH4. The origin of high catalytic activity with enhanced stability could be due to the strong interaction between the cobalt-oxide nanoparticle and the C2N framework, which contains a large portion of nitrogen.
Two-dimensional polyaniline (C<sub>3</sub>N) from carbonized organic single crystals in solid state
Mahmood, Javeed,Lee, Eun Kwang,Jung, Minbok,Shin, Dongbin,Choi, Hyun-Jung,Seo, Jeong-Min,Jung, Sun-Min,Kim, Dongwook,Li, Feng,Lah, Myoung Soo,Park, Noejung,Shin, Hyung-Joon,Oh, Joon Hak,Baek, Jong-Beo National Academy of Sciences 2016 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF Vol.113 No.27
<P>The formation of 2D polyaniline (PANI) has attracted considerable interest due to its expected electronic and optoelectronic properties. Although PANI was discovered over 150 y ago, obtaining an atomically well-defined 2D PANI framework has been a longstanding challenge. Here, we describe the synthesis of 2D PANI via the direct pyrolysis of hexaaminobenzene trihydrochloride single crystals in solid state. The 2D PANI consists of three phenyl rings sharing six nitrogen atoms, and its structural unit has the empirical formula of C3N. The topological and electronic structures of the 2D PANI were revealed by scanning tunneling microscopy and scanning tunneling spectroscopy combined with a first-principle density functional theory calculation. The electronic properties of pristine 2D PANI films (undoped) showed ambipolar behaviors with a Dirac point of -37 V and an average conductivity of 0.72 S/cm. After doping with hydrochloric acid, the conductivity jumped to 1.41 x 10(3) S/cm, which is the highest value for doped PANI reported to date. Although the structure of 2D PANI is analogous to graphene, it contains uniformly distributed nitrogen atoms for multifunctionality; hence, we anticipate that 2D PANI has strong potential, from wet chemistry to device applications, beyond linear PANI and other 2D materials.</P>
Spatiotemporal Low-Rank Modeling for Complex Scene Background Initialization
Javed, Sajid,Mahmood, Arif,Bouwmans, Thierry,Jung, Soon Ki IEEE 2018 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDE Vol.28 No.6
<P>Background modeling constitutes the building block of many computer-vision tasks. Traditional schemes model the background as a low rank matrix with corrupted entries. These schemes operate in batch mode and do not scale well with the data size. Moreover, without enforcing spatiotemporal information in the low-rank component, and because of occlusions by foreground objects and redundancy in video data, the design of a background initialization method robust against outliers is very challenging. To overcome these limitations, this paper presents a spatiotemporal low-rank modeling method on dynamic video clips for estimating the robust background model. The proposed method encodes spatiotemporal constraints by regularizing spectral graphs. Initially, a motion-compensated binary matrix is generated using optical flow information to remove redundant data and to create a set of dynamic frames from the input video sequence. Then two graphs are constructed, one between frames for temporal consistency and the other between features for spatial consistency, to encode the local structure for continuously promoting the intrinsic behavior of the low-rank model against outliers. These two terms are then incorporated in the iterative <I>Matrix Completion</I> framework for improved segmentation of background. Rigorous evaluation on severely occluded and dynamic background sequences demonstrates the superior performance of the proposed method over state-of-the-art approaches.</P>