Browsing by Author "Gu, Siyong"

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Amino-functionalization on graphene oxide sheets using an atomic layer amidation technique
(2020-01-14)
Gu, Siyong
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Hsieh, Chien-Te
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Mallick, Bikash Chandra
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Gandomi, Yasser Ashraf
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Chang, Jeng-Kuei
Nitrogen functionalization on graphene sheets has solely been demonstrated with a stepwise amidation implementation within the literature. The present work proposes an atomic layer amidation (ALN) technique to linearly adjust the surface amidation level on the carbon nanostructures. The ALN cycling protocol proposed in this work consists of an ammonia pulse, nitrogen pulse, and pumping at 100 and 150 degrees C. Both surface amidation and oxidation levels were linearly controlled by the ALN cycle number. The ALN technique was demonstrated as a self-limiting process capable of step-by-step N-functionalizing graphene oxide (GO) sheets on the atomic scale. The amidation reaction rates were ca. 0.023 (100 degrees C) and 0.028 atomic percent (at%) cycle(-1) (150 degrees C), and the apparent activation energy was determined to be 14.3 kJ mol(-1) (= 0.148 eV atom(-1)) from the Arrhenius equation. The amidation mechanism for forming the "chemical N" and "lattice N" at an atomic level was also proposed. Linearly controlling the optical band-gap with the ALN cycle number was suggested as a robust method for tuning the oxidation as well as the amidation level on the GO sheets. In this sense, nitrogen functionalization can be utilized for engineering any desirable properties within the graphene sheets.
Atomic layer oxidation on graphene sheets for tuning their oxidation levels, electrical conductivities, and band gaps
(2018-09-07)
Gu, Siyong
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Hsieh, Chien-Te
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Lin, Tzu-Wei
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Yuan, Chun-Yao
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Gandomi, Yasser Ashraf
Graphene sheets that can exhibit electrical conducting and semiconducting properties are highly desirable and have potential applications in fiber communications, photodetectors, solar cells, semiconductors, and broadband modulators. However, there is currently no efficient method that is able to tune the band gap of graphene sheets. This work adopts an efficient atomic layer oxidation (ALO) technique to cyclically increase the oxidation level of graphene sheets, thus, tuning their electrical conductance, band-gap structure, and photoluminescence (PL) response. The O/C atomic ratio as an increasing function of the ALO cycle number reflects two linear regions: 0.23% per cm(2) per cycle (0-15 cycles) and 0.054% per cm(2) per cycle (15-100 cycles). The excellent correlation coefficients reveal that the ALO process follows a self-limiting route to step-by-step oxidize graphene layers. The interlayer distance of ALO-graphene sheets shows an obvious increase after the ALO treatment, proved by X-ray diffraction. As analyzed by X-ray photon spectroscopy, the hydroxyl or epoxy group acts as a major contributor to the interlayer spacing distance and oxidation extent in the initial ALO stage, as compared to carbonyl and carboxyl groups. The ALO mechanism, based on Langmuir-Hinshelwood and Eley-Rideal models, is proposed to clarify the formation of oxygen functionalities and structural transformation from pristine graphene sheets to oxidized ones during the ALO cycle. With a tunable oxidation level, the electrical resistivity, semiconductor character, and PL response of ALO-graphene samples can be systematically controlled for desired applications. The ALO approach is capable of offering a straightforward route to tune the oxidation level of graphene sheets or other carbons.
Fluorescence of functionalized graphene quantum dots prepared from infrared-assisted pyrolysis of citric acid and urea
(2020-01-01)
Gu, Siyong
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Hsieh, Chien-Te
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Yuan, Chun-Yao
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Gandomi, Yasser Ashraf
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Chang, Jeng-Kuei
This paper reports an efficient fabrication of N-doped graphene quantum dots (GQDs) showing controllable chemical and fluorescence (FL) properties through infrared carbonization (IRC) of citric acid and urea. The GQDs prefer to form an equilibrium shapes of circle with an average particle size ranged from 5 to 10 nm. The N/C atomic ratio in GQDs can be precisely tailored in a range from 21.6 to 49.6 at.% by simply controlling the weight ratio of citric acid to urea. With increasing the urea content, the GQDs not only contain N-doped graphene but also incorporate with crystalline cyanuric acid, forming a binary crystallinity. The quantum yield of 22.2% is achieved by N-doped GQDs, prepared from the IRC synthesis of chemical precursor at the citric acid/urea at 3:1. Excessive N and cyanuric acid can lead to FL quenching, red shift and wide spectral distribution. The design of GQDs possesses a multiple chromophoric band-gap structure, originated from the presence of cyanuric acid, defect-related emissive traps, and functional group distributions. This work offers an effective and inspiring approach to engineering both chemical compositions and unique crystalline structures of GQDs, and will therefore facilitate their fundamental research and applications to optical, sensing, energy and biological fields.
Improved lithium storage capacity and high rate capability of nitrogen-doped graphite-like electrode materials prepared from thermal pyrolysis of graphene quantum dots
(2020-09-10)
Gu, Siyong
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Christensen, Tommiejean
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Hsieh, Chien-Te
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Mallick, Bikash Chandra
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Gandomi, Yasser Ashraf
Adopting a solid-phase microwave-assisted technique followed by thermal pyrolysis of N-functionalized graphene quantum dots, novel nitrogen-doped graphite-like (NGL) electrode materials were synthesized and served as the anode for Li-ion batteries. The NGL anode demonstrated reversible capacity of 530 mAh g(-1) at 0.1C, superior rate capability at high C rate operation (420 mAh g(-1) at 5C), remarkable initial coulombic efficiency (>95.7%), and excellent cyclic stability along with high efficiency (>99.1%) during entire cycling. The NGL anode nanostructure enables improved lithium ion mobility and reversible Li+ storage during cycling. The analysis of the Ragone plots revealed that the specific energy of NGL anode reaches to ca. 840 Wh kg(-1) at the power density of 4200 W kg(-1). The diffusion coefficient of Li ions was measured as 1.69 x 10(-9) cm(2) s(-1) for the NGL anode material, substantially improving over commonly used graphite electrodes (15-26 times higher Li+ diffusivity). The high-rate cyclability as well as the cyclic stability of the NGL anodes were also confirmed via long-term cycling of full pouch cells assembled with ternary cathode and NGL anode. The robust design of the NGL anode materials introduced in this work, paves the way for designing next-generation lithium-ion batteries operating at ultrahigh C rates. (C) 2020 Elsevier Ltd. All rights reserved.
Microwave growth and tunable photoluminescence of nitrogen-doped graphene and carbon nitride quantum dots
(2019-05-14)
Gu, Siyong
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Hsieh, Chien-Te
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Gandomi, Yasser Ashraf
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Chang, Jeng-Kuei
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Li, Ju
Tunable photoluminescent nitrogen-doped graphene and graphitic carbon nitride (g-C3N4) quantum dots are synthesized via a facile solid-phase microwave-assisted (SPMA) technique utilizing the pyrolysis of citric acid and urea precursors. The atomic ratio, surface functionalization, and atomic structure of as-prepared quantum dots strongly depend on the ratio of citric acid to urea. The quantum dots have a homogeneous particle size and tend to form a circle and/or ellipse shape to minimize the edge free energy. The atomic ratio of surface nitrogen to carbon (N/C) in the quantum dots can reach as high as 1.74, among the highest values reported in the literature. The SPMA technique is capable of producing high-quality quantum dots with photoluminescence (PL) emission at various wavelengths on a pilot scale. The atomic structures of the N-doped graphene and g-C3N4 quantum dots are explored using molecular dynamics simulations. Increasing the urea concentration increases the tendency of in-plane N (i.e., quaternary N) substitution over that of other amino functionalizations, such as pyrrolic and pyridinic N. The PL emission can be precisely tuned via a one-step SPMA method by adjusting the precursor composition. A high quantum yield of 38.7% is achieved with N-doped graphene quantum dots, indicating the substantial influence of the N- and O-rich edge groups on the enhancement of PL efficiency. A bandgap structure is proposed to describe the interstate (*-) transition of quantum dots. This work introduces a novel approach for engineering the chemical composition and atomic structure of graphene and g-C3N4 quantum dots, facilitating their research and applications in optical, electronic, and biomedical devices.
Tailoring fluorescence emissions, quantum yields, and white light emitting from nitrogen-doped graphene and carbon nitride quantum dots
(2019-09-21)
Gu, Siyong
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Hsieh, Chien-Te
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Gandomi, Yasser Ashraf
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Li, Jianlin
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Yue, Xing Xing
Highly fluorescent N-doped graphene quantum dots (NGQDs) and graphitic carbon nitride quantum dots (CNQDs, g-C3N4) were synthesized using a solid-phase microwave-assisted (SPMA) technique. The SPMA method, based on the pyrolysis of citric acid and urea with different recipes, is capable of producing quantum dots with coexisting NGQDs and CNQDs at 280 degrees C within only five minutes. The photoluminescence (PL) emissions from NGQD and CNQDs are strongly dependent on the excitation wavelength and the solvent type, i.e., water, ethanol, and N-methyl pyrrolidinone. The unique attribute of the quantum dots, possessing a multiple chromophoric band-gap structure, originates from the presence of g-C3N4, defect-related emissive traps, and grain boundaries. Thus, an appropriate excitation wavelength induces a conjugated pi electron system to fulfill the most probable absorption band, resulting in wavelength-dependent emissions including ultraviolet, visible and infrared light. The quantum yield of the NGQD and CNQD samples can reach as high as 68.1%. Accordingly, a light-emitting device using the combination of the NGQD and CNQD powder embedded polymeric film can emit white-like light with ultra-high power-conversion efficiency.