Low-Cost Black Phosphorus Nanofillers for Improved Thermoelectric Performance in PEDOT:PSS Composite Films

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>

Analysis of contact resistance in single-walled carbon nanotube channel and graphene electrodes in a thin film transistor

Baek Jinwook,Novak Travis G.,vKim Houngkyung,Lee Jinsup,Jang Byoungwook,Lee Junseok,전석우 나노기술연구협의회 2017 Nano Convergence Vol.4 No.35

In this work, we present the experimental investigation on the contact resistance of graphene/single-walled carbon nanotube (SWCNT) junction using transfer length method with the simple equivalent circuit model. We find that p–n like junctions are formed in graphene/SWCNT transistors, and the contact resistance in the junction is observed to be ~ 494 and ~ 617 kΩ in case of metallic SWCNT (m-SWCNT) and semiconducting SWCNT (s-SWCNT), respectively. In addition, the contact resistance increases from 617 to 2316 kΩ as Vg increases from − 30 to − 10 V. Through our study, high carrier density induced from doping in both graphene and SWCNT leads to low contact resistance. This development of contact engineering, namely modulation of carrier density in the junction and contact length (Lcon) scaling shows the potential for all-carbon based electronics.

3D ordered nanoelectrodes for energy conversion applications: thermoelectric, piezoelectric, and electrocatalytic applications

김기선,Tiwari Anand P.,Novak Travis G.,전석우 한국세라믹학회 2021 한국세라믹학회지 Vol.58 No.4

To date, many methods have been suggested to improve the performance of materials in various applications by applying new physical and chemical properties at the nanometer scale in the form of nanodots, nanowires, and nanofi lms. However, most of the proposed methods are diffi cult to apply to industrial settings due to their size limitations. In that sense, the realization of 3D nanostructured materials is signifi cant for practical use of nanotechnology. The continuous 3D nanostructuring insures the maximum utilization of materials effi ciency and improves the stability through well-ordered structures. In this respect, 3D nanostructures of materials can be useful for energy conversion applications such as thermoelectric, piezoelectric, and electrocatalytic applications. Herein, we briefl y overview 3D nanofabrication methods to convert the materials in the 3D nanostructures, followed by a review on the advantages of 3D ordered nanoelectrodes for high-performance energy conver- sion applications.

Improving electrochemical active area of MoS₂ via attached on 3D-ordered structures for hydrogen evolution reaction

Kim, Kisun,Tiwari, Anand P.,Hyun, Gayea,Novak, Travis G.,Jeon, Seokwoo Elsevier 2019 International journal of hydrogen energy Vol.44 No.52

<P><B>Abstract</B></P> <P>To date, researchers have revealed that the electrocatalytic activity of 2-dimensional (2D) layered transition metal dichalcogenides (TMDCs) such as MoS<SUB>2</SUB> can be improved by making free standing vertical structures to expose edge sites for efficient water splitting. However, poor electrical conductivity and structural instability restrict the practical application of vertical structures for efficient electrocatalytic activities. Here, a homogeneously attached MoS<SUB>2</SUB> structure on well-ordered 3-dimensional nickel (3D-Ni) is reported for efficient hydrogen evolution reaction (HER). This homogeneously attached structure of MoS<SUB>2</SUB> leads to abundant active sites and well-ordered 3D-Ni structures, solving the conductivity issue of MoS<SUB>2</SUB> and ensuring the structural stability during electrocatalytic processes. By controlling the amount of MoS<SUB>2</SUB> on the 3D-Ni, it is found that the electrochemical active area (ECSA) is increased by 5 times (50 cm<SUP>2</SUP> of active sites) compared to normal MoS<SUB>2</SUB> grown on 2D-Ni (9 cm<SUP>2</SUP> of active sites). It is also found that the charge transfer resistance (R<SUB>ct</SUB>) of attached MoS<SUB>2</SUB> structures on 3D-Ni (1 Ω) is 16 times lower than MoS<SUB>2</SUB> grown on 2D-Ni (16 Ω). In addition, the proposed attached structure of MoS<SUB>2</SUB> is stable in acidic electrolytes for continuous electrocatalytic activity and can be mass producible for practical applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A well-ordered 3-dimensional metal (3D-Ni) nanostructures is developed. </LI> <LI> 2D-MoS<SUB>2</SUB> sheets are homogeneously grown on the 3D-Ni by solvothermal method. </LI> <LI> Homogeneously attached 2D-MoS<SUB>2</SUB> on 3D-Ni abundant active sites for hydrogen evolution. </LI> <LI> Well-ordered 3D-Ni ensures stability and high electrochemical conductivity. </LI> <LI> The 2D/3D structure combo can be used in other electrochemical TMDC applications. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Two-Dimensional WO₃ Nanosheets Chemically Converted from Layered WS₂ for High-Performance Electrochromic Devices

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>

Chemical strain formation through anion substitution in Cu₂WS₄ for efficient electrocatalysis of water dissociation

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>