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Azam, Ashraful,Kim, Jungmo,Park, Junyong,Novak, Travis G.,Tiwari, Anand P.,Song, Sung Ho,Kim, Bumsoo,Jeon, Seokwoo American Chemical Society 2018 NANO LETTERS Vol.18 No.9
<P>Two-dimensional (2D) transitional metal oxides (TMOs) are an attractive class of materials due to the combined advantages of high active surface area, enhanced electrochemical properties, and stability. Among the 2D TMOs, 2D tungsten oxide (WO<SUB>3</SUB>) nanosheets possess great potential in electrochemical applications, particularly in electrochromic (EC) devices. However, feasible production of 2D WO<SUB>3</SUB> nanosheets is challenging due to the innate 3D crystallographic structure of WO<SUB>3</SUB>. Here we report a novel solution-phase synthesis of 2D WO<SUB>3</SUB> nanosheets through simple oxidation from 2D tungsten disulfide (WS<SUB>2</SUB>) nanosheets exfoliated from bulk WS<SUB>2</SUB> powder. The complete conversion from WS<SUB>2</SUB> into WO<SUB>3</SUB> was confirmed through crystallographic and elemental analyses, followed by validation of the 2D WO<SUB>3</SUB> nanosheets applied in the EC device. The EC device showed color modulation of 62.57% at 700 nm wavelength, which is 3.43 times higher than the value of the conventional device using bulk WO<SUB>3</SUB> powder, while also showing enhancement of ∼46.62% and ∼62.71% in switching response-time (coloration and bleaching). The mechanism of enhancement was rationalized through comparative analysis based on the thickness of the WO<SUB>3</SUB> components. In the future, 2D WO<SUB>3</SUB> nanosheets could also be used for other promising applications such as sensors, catalysis, thermoelectric, and energy conversion.</P> [FIG OMISSION]</BR>
Tiwari, Anand P.,Azam, Ashraful,Novak, Travis G.,Prakash, Om,Jeon, Seokwoo The Royal Society of Chemistry 2018 Journal of Materials Chemistry A Vol.6 No.17
<P>Researchers have revealed that the electrocatalytic activity can be improved by creation of defects in the crystal lattice of 2D layered transition metal dichalcogenides (TMDCs) or ternary metal chalcogenides (TMCs) such as MoS2 or Cu2MoS4, respectively. However, the role of anion substitution in the enhancement of overall electrocatalytic activity in TMCs remains unproven. Here, we show the substitution of anion atom sulfur (S) with selenium (Se) in a new electrocatalyst Cu2WS4 for efficient hydrogen evolution reaction (HER) activity. The higher electrocatalytic activity of Cu2WS4 after anion atom substitution can be attributed to the creation of chemical strain in the lattice, which causes an increase of active sites for hydrogen adsorption and desorption. Experimentally, the anion substituted Cu2W(SySe1 − y)4 samples show superior electrocatalytic activities with a low onset potential of −0.320 V at 10 mA cm<SUP>−2</SUP> for the HER, which is two-fold lower than that of the pristine Cu2WS4 (−0.650 V at 10 mA cm<SUP>−2</SUP>) sample. In addition, after 1000 cycles with continuous electrolysis in an acid electrolyte for 12 h, the anion substituted samples Cu2W(SySe1 − y)4 preserve their structure and robust catalytic activity perfectly. As a result, our work demonstrates a new approach for developments of real applications of TMCs in energy conversion.</P>
Novak, Travis G.,Shin, Hosun,Kim, Jungmo,Kim, Kisun,Azam, Ashraful,Nguyen, Chien Viet,Park, Sun Hwa,Song, Jae Yong,Jeon, Seokwoo American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.21
<P>In recent years, two-dimensional black phosphorus (BP) has seen a surge of research because of its unique optical, electronic, and chemical properties. BP has also received interest as a potential thermoelectric material because of its high Seebeck coefficient and excellent charge mobility, but further development is limited by the high cost and poor scalability of traditional BP synthesis techniques. In this work, high-quality BP is synthesized using a low-cost method and utilized in a PEDOT:PSS film to create the first ever BP composite thermoelectric material. The thermoelectric properties are found to be greatly enhanced after the BP addition, with the power factor of the film, with 2 wt % BP (36.2 μW m<SUP>-1</SUP> K<SUP>-2</SUP>) representing a 109% improvement over the pure PEDOT:PSS film (17.3 μW m<SUP>-1</SUP> K<SUP>-2</SUP>). A simultaneous increase of mobility and decrease of the carrier concentration is found to occur with the increasing BP wt %, which allows for both Seebeck coefficient and electrical conductivity to be increased. These results show the potential of this low-cost BP for use in energy devices.</P> [FIG OMISSION]</BR>
층간삽입 반응을 이용한 그래핀/탄소나노튜브 동시 개별 분산 및 전도성 복합 필름으로의 응용
김정모 ( Jungmo Kim ),김진 ( Jin Kim ),윤혜원 ( Hyewon Yoon ),박민수 ( Minsu Park ),( Travis Novak ),( Azam Ashraful ),이진호 ( Jinho Lee ),전석우 ( Seokwoo Jeon ) 한국복합재료학회 2016 Composites research Vol.29 No.3
본 논문은 층간 삽입 반응을 이용하여 그래핀을 박리하는 동시에 탄소나노튜브를 개별 분산시키고 이를 복합 필름으로 제조한 결과를 보고한다. 일반적으로 그래핀과 탄소나노튜브의 경우 흑연과 탄소나노튜브 번들로부터의 개별적 박리를 통해 제조될 수 있으나 장시간의 공정 시간을 요구하게 된다. 본 연구에서는 그래핀과 탄소나노튜브의 동시 박리 및 분산을 위해 흑연 및 탄소나노튜브 번들 내로 포타슘 소듐 타르트레이트를 삽입했으며 XRD 분석을 통해 흑연 층간 거리 및 탄소나노노튜브 번들 내 거리의 증가를 확인했다. 제조된 층간삽입화합물로부터 박리된 그래핀 및 탄소나노튜브의 경우 매우 낮은 산화도(< 8.3 at%)를 나타냈으며 제조된 물질을 여과 장치 및 스프레이 전사를 통해 복합 필름으로 제조한 결과 그래핀 및 탄소나노튜브의 단일 필름에 대비하여 복합 필름의 경우 전도성의 향상을 보였다. This paper reports a novel method for simultaneous exfoliation of graphene and dispersion of carbon nanotube by using intercalation method. In common, graphene flake and carbon nanotubes can be produced through individual exfoliation or debundling process, but the process require significant amount of time. Here, potassium sodium tartrate was thermally intercalated into graphite and carbon nanotube bundle for simultaneous exfoliation and dispersion of graphene and carbon nanotubes. We confirmed expansion of interlayer distance via XRD, and also found that oxidation level of the exfoliated materials were significantly low (below 8.3 at%). The produced materials are fabricated in to conductive composite film via vacuum filtration and spray deposition to show enhancement of conductive properties.