Hierarchical material of carbon nanotubes grown on carbon nanofibers for high performance electrochemical capacitor

Kshetri, Tolendra,Thanh, Tran Duy,Singh, Soram Bobby,Kim, Nam Hoon,Lee, Joong Hee Elsevier 2018 CHEMICAL ENGINEERING JOURNAL -LAUSANNE- Vol.345 No.-

<P><B>Abstract</B></P> <P>The development of new advanced nanostructures based on the hybridization of different carbon nanomaterials to obtain enhanced performance of energy storage devices has attracted considerable attention. Herein, a hierarchical nanostructure of carbon nanotubes supported electrospun carbon nanofiber networks (CNTs@CNFs) was successfully fabricated by using two facile techniques: – electrospinning and chemical vapor deposition (CVD). Such CNTs@CNFs hybrid showed the uniform and high density of CNTs directly grown on the surface of carbon nanofiber networks, leading to the formation of a hierarchical nanostructure with a large surface area and highly porous characteristics. The enhanced interactions between the CNTs and the CNFs networks were found to improve the electrical conductivity and electrochemical stability of the material. Owing to its unique nanoarchitectures and physicochemical properties, the CNTs@CNFs hybrid was demonstrated to be a potential electrode material for an electrochemical capacitor, in which a high specific capacitance of 464.2 F g<SUP>−1</SUP> at 0.5 A g<SUP>−1</SUP> and long-term stability with 97% retention after 10,000 repeated charge–discharge cycles were achieved. The obtained results suggest that the present CNTs@CNFs hybrid is a promising candidate for an electrochemical capacitor in energy storage technologies.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A hierarchical nanostructure of CNTs@CNFs has been successfully fabricated. </LI> <LI> The CNTs@CNFs has large surface area and strong interfaces between hybrids. </LI> <LI> It shows a high specific capacitance of 464.2 F g<SUP>−1</SUP> at 0.5 A g<SUP>−1</SUP> and stability. </LI> </UL> </P>

Blue, green, red and near-infrared emissions via upconversion in α-sialon

Kshetri, Yuwaraj K.,Joshi, Bhupendra,Kim, Tae-Ho,Lee, Soo Wohn Elsevier 2017 Materials letters Vol.199 No.-

<P>Near-infrared to visible and near-infrared upconversion emission has been reported in Yb3+/Ln(3+) (Ln(3+) = Er3+/Tm3+/Ho3+)-alpha-sialon ceramic prepared by hot press sintering technique. Under 980 nm excitation, blue, green, red and near-infrared emissions are observed simultaneously as a result of the efficient energy transfer from Yb3+ to Er3+, Tm3+, and Ho3+ ions leading to a near-white light emission with CIE color coordinate of (0.38, 0.45). The two-and three-photon absorption processes are responsible for the observed upconversion emissions. The ceramic has moderately low phonon energy of 835 cm(-1). These novel properties make the material a promising candidate for upconversion applications. (C) 2017 Elsevier B.V. All rights reserved.</P>

Visible and near-infrared upconversion in α-sialon ceramics

Kshetri, Yuwaraj K.,Joshi, Bhupendra,Kim, Tae-Ho,Lee, Soo W. Royal Society of Chemistry 2017 Journal of Materials Chemistry C Vol.5 No.14

<▼1><▼1><P>Intense near-infrared to visible and near-infrared upconversion emissions are realized in Er<SUP>3+</SUP>/Ho<SUP>3+</SUP>/Tm<SUP>3+</SUP> doped α-sialon ceramics.</P></▼1><▼2><P>α-Sialon ceramics are well known for highly stressed structural engineering applications owing to their inherently outstanding mechanical and thermo-chemical stability. However, the possibilities of the functional applications such as frequency upconversion in α-sialon ceramics have been scarcely investigated. Here, very intense visible and near-infrared frequency upconversion as well as frequency downconversion emissions are reported for the first time in Er<SUP>3+</SUP>, Ho<SUP>3+</SUP>, and Tm<SUP>3+</SUP> triply doped α-sialon ceramics under 980 nm excitation. α-Sialon ceramics are prepared using a hot press sintering technique. Efficient energy transfer processes between the dopant ions are found to be responsible for the observed emissions. The upconversion process is governed by a two-photon absorption process. The energy transfer efficiency in the triply doped system is 95.8% while that in Er<SUP>3+</SUP>/Ho<SUP>3+</SUP> and Er<SUP>3+</SUP>/Tm<SUP>3+</SUP> systems is 71.6% and 66.3%, respectively. Triple doping results in the combination of the emission properties found in the co-doped Er<SUP>3+</SUP>/Ho<SUP>3+</SUP> and Er<SUP>3+</SUP>/Tm<SUP>3+</SUP> systems and it also reveals a cooperative emission behavior which is not possible with only two dopants. These findings open up the possibility of potential application of the Er<SUP>3+</SUP>/Ho<SUP>3+</SUP>/Tm<SUP>3+</SUP> doped α-sialon ceramic as an optical material in which efficient luminescent properties can co-exist along with the excellent mechanical and thermo-chemical stability.</P></▼2></▼1>

Efficient near infrared to visible and near‐infrared upconversion emissions in transparent (tm3+, er3+)‐α‐sialon ceramics

Kshetri, Yuwaraj K.,Joshi, Bhupendra,Diaz‐,Torres, Luis Armando,Lee, Soo Wohn American Ceramic Society 2017 Journal of the American Ceramic Society Vol. No.

Microwave hydrothermal synthesis and upconversion properties of Yb³⁺/Er³⁺ doped YVO₄ nanoparticles

Kshetri, Yuwaraj K,Regmi, Chhabilal,Kim, Hak-Soo,Lee, Soo Wohn,Kim, Tae-Ho IOP 2018 Nanotechnology Vol.29 No.20

<P>Yb<SUP>3+</SUP> and Er<SUP>3+</SUP> doped YVO<SUB>4</SUB> (Yb<SUP>3+</SUP>/Er<SUP>3+</SUP>:YVO<SUB>4</SUB>) nanoparticles with highly efficient near-infrared to visible upconversion properties have been synthesized by microwave hydrothermal process. Uniform-sized Yb<SUP>3+</SUP>/Er<SUP>3+</SUP>:YVO<SUB>4</SUB> nanoparticles were synthesized within 1 h at 140 °C which is relatively faster than the conventional hydrothermal process. Under 980 nm laser excitation, strong green and less strong red emissions are observed which are attributed to <SUP>2</SUP>H<SUB>11/2</SUB>, <SUP>4</SUP>S<SUB>3/2</SUB> to <SUP>4</SUP>I<SUB>15/2</SUB> and <SUP>4</SUP>F<SUB>9/2</SUB> to <SUP>4</SUP>I<SUB>15/2</SUB> transitions of Er<SUP>3+</SUP> respectively. The emission intensity is found to depend strongly on the concentration of Yb<SUP>3+</SUP>. The quadratic dependence of upconversion intensity on the excitation power indicates that the upconversion process is governed by two-photon absorption process.</P>

Green Luminescence in a Translucent (Mg, Yb)-<i>α</i>-Sialon Phosphor Under Blue Light Irradiation

Joshi, Bhupendra,Kshetri, Yuwaraj K.,Gyawali, Gobinda,Tripathi, Khagendra,Lee, Soo Wohn American Scientific Publishers 2018 Science of advanced materials Vol.10 No.1

<P>The translucent green alpha-Sialon ceramics was fabricated by the hot press sintering method. The fabricated alpha-Sialon ceramics co-doped with Yb2O3 shows green emission under blue light irradiation. The alpha-Sialon phase was the main phase obtained as observed by XRD. The effect of thicknesses on the optical properties was also analyzed. The green luminescence was observed at a 555 nm wavelength under 460 nm excitation. The thinner samples transmit higher blue light and hence show high luminous efficacy as coupled with the blue LED. Whereas, the 500 mu m thick sample shows green luminescence near the white region of CIE co-ordinates with a low luminous efficacy of 20.62l m/W.</P>

Near infrared and visible luminescence in transparent (Er, Yb)-α-sialon ceramics under 980 nm laser irradiation

Bhupendra Joshi,KSHETRI YUWARAJ KHATRI,Gobinda Gyawali,이수원 한양대학교 세라믹연구소 2016 Journal of Ceramic Processing Research Vol.17 No.3

The novel Er3+ and Yb3+ doped transparent Sialon ceramics were fabricated by hot pressing sintering method as a nearinfrared (NIR) as well as visible luminescent material. The optical transmittance, down-conversion luminescence and NIRluminescence were investigated. The samples show around 30% transmittance in visible region whereas 70% in IR regionaround 3000 nm. The samples were excited within the range of 350-400 nm wavelength, the highest emission was in greenregion (~ 550 nm). Also, the upconversion luminescence depending upon the thicknesses of the samples in the green and redregion have been investigated. The NIR luminescence was observed around 1524 nm wavelength under 980 nm wavelengthirradiation. The decay time profiles have been investigated for higher Er3+ concentration and higher Yb3+ co-doped with Er3+α-Sialon samples. In this study, the addition of Yb3+ has poor sensitizing effect on Er3+ ion to enhance the luminescence.

Visible-light-induced Fe-doped BiVO 4 photocatalyst for contaminated water treatment

Regmi, Chhabilal,Kshetri, Yuwaraj K.,Kim, Tae-Ho,Pandey, Ramesh Prasad,Lee, Soo Wohn Elsevier 2017 Molecular catalysis Vol.432 No.-

<P>A monoclinictetragonal heterostructured BiVO4 photocatalyst doped with varying quantities of Fe was synthesized by a microwavehydrothermal method and employed in the photodegradation of ibuprofen and inactivation of Escherichia coli under irradiation with visible light. An improvement in the photocatalytic activity for the Fe-doped BiVO4 was confirmed relative to that determined for the undoped BiVO4. The in-gap state formed between the valence band and conduction band of BiVO4 as a result of doping, as established by first principles DFT calculations, facilitates an easy transition of electrons from the valence band to the conduction band. This transition enhances the tendency of electronhole separation and improves the visible light absorption capacity, thereby improving the photocatalytic activity. Among the various concentrations of Fe dopant examined, the highest visible light photoactivity is demonstrated for the 1 wt% Fe-doped BiVO4 photocatalyst, which degrades 80% of ibuprofen within 180 min and inactivates 90% of Escherichia coli within 5 h. No decrease in the efficiency of the 1 wt% Fe-doped BiVO4 photocatalyst was observed during the degradation of ibuprofen over three consecutive cycles, thereby demonstrating the stability of the semiconductor towards photocorrosion. Possible mechanisms for both the enhanced photocatalytic activity and the degradation of ibuprofen are proposed on the basis of the experimental observations. (C) 2017 Elsevier B.V. All rights reserved.</P>

Understanding the multifunctionality in Cu-doped BiVO4 semiconductor photocatalyst

Regmi, Chhabilal,Kshetri, Yuwaraj K.,Pandey, Ramesh Prasad,Kim, Tae-Ho,Gyawali, Gobinda,Lee, Soo Wohn Elsevier 2019 Journal of environmental sciences Vol.75 No.-

Visible-light-driven S and W co-doped dendritic BiVO 4 for efficient photocatalytic degradation of naproxen and its mechanistic analysis

Regmi, Chhabilal,Kshetri, Yuwaraj K.,Pandey, Ramesh Prasad,Lee, Soo Wohn Elsevier 2018 Molecular catalysis Vol.453 No.-

<P>An inherently low photocatalytic efficiency due to electron-hole recombination greatly limits the practical application of BiVO4. It is crucial to improve this photocatalytic efficiency; we therefore, designed a novel sulphur (S) and tungsten (W) co-doped BiVO4 photocatalyst through a facile microwave hydrothermal process. The synthesized samples were characterized using a wide variety of techniques. Compared to undoped and S and W single-doped BiVO4, the co-doped system exhibits a significantly enhanced photocatalytic activity towards difficult-to-degrade naproxen drug (76.5%) upon visible light irradiation. The results indicate that co-doping with S and W increases the specific surface area while improving absorbance in the visible light region. Thus, the co doping of S and W gives a synergistic effect, leading to an enhanced photocatalytic efficiency, as confirmed by photoluminescence measurements. This study suggests a new strategy for the design of novel materials with enhanced photocatalytic performance for use in practical applications.</P>