Growth of 2D materials and application in electrochemical energy conversion

The discovery of graphene in 2004 has generated numerous interests among scientists for graphene’s versatile potentials. The enthusiasm for graphene has recently been extended to other members of two-dimensional(2D) materials for applications in electronics, optoelectronics, and catalysis. Different from graphene, atomically-thin transition metal dichalcogenides (TMDs) have varied band gaps and would benefit for applications in the semiconductor industry. One of the promising applications of 2D TMDs is for 2D integrated circuits to replace current Si based electronics. In addition to electronic applications, 2D materials are also good candidates for electrochemical energy storage and conversion due to their large surface area and atomic thickness. This thesis mainly focuses on the synthesis of 2D materials and their application in energy conversion. Firstly, we focus on the synthesis of two-dimensional Tin Disulfide(SnS2). SnS2 is considered to be a novel material in 2D family. 2D SnS2 has a large band gap (~ 2.8 eV) and high carrier mobility, which makes it a potential applicant for electronics. Monolayer SnS2 with large scale and high crystal quality was successfully synthesized by chemical vapor deposition (CVD), and its performance as a photodetector was examined. The next chapter demonstrated a generic method for growing millimeter-scale single crystals as well as wafer-scale thin films of TMDs. This generic method was obtained by studying the precursors’ behavior and the flow dynamics during the CVD process of growing MoSe2, and was extended to other TMD layers such as millimeter-scale WSe2 single crystals. Understanding the growth processes of high quality large area monolayers of TMDs is crucial for further fundamental research as well as future development for scalable complex electronics. Besides the synthesis of 2D materials with high qualities, we further explored the relationship between defects and electrochemical properties. By directly observing and correlating the microscale structural changes of TMD monolayers such as MoS2 to the catalytic properties, we were able to provide insight on the fundamental catalytic mechanism for hydrogen evolution reaction. Finally, we used the 2D materials to build up 3D architectures, showing excellent performance in energy storage and conversion. For example, we used graphene as a conductive scaffold to support vanadium oxide (V2O5) on nanoscale, and achieved high performances for supercapacitors. Also, we applied the Pt anchored N-doped graphene nanoribbons as the catalyst for methanol electro oxidation, and reported the best performance among Pt/Carbon-based catalysts

3D reduced graphene oxide coated V2O5 nanoribbon scaffolds for high-capacity supercapacitor electrodes

Sem informação3D architecture V2O5 nanoribbon/reduced graphene oxide is successfully fabricated as an electrode material for supercapacitors. In combination with the advantages from the good rate performance of carbon‐based materials and the high specific capacitance of metal oxides, as well as its 3D architecture, this material shows high specific capacitance, superior rate performance, and stability.328817821Sem informaçãoSem informaçãoSem informaçãoG.Y. and Y.G. contributed equally to this work. This work was financially supported by US Army Research Office through a MURI Grant No. (W911NF‐11‐1‐0362) on Novel Free‐Standing 2D Crystalline Materials, titled “Atomic layers of Nitrides, Oxides, and Sulfides” and U.S Air Force Office of Scientific Research for the MURI Grant No. (FA 9550‐12‐1‐0035), titled “Synthesis and Characterization of 3D Carbon Nanotube Solid Networks.

Boron- and Nitrogen-Doped Graphene Quantum Dots/Graphene Hybrid Nanoplatelets as Efficient Electrocatalysts for Oxygen Reduction

The scarcity and high cost of platinum-based electrocatalysts for the oxygen reduction reaction (ORR) has limited the commercial and scalable use of fuel cells. Heteroatom-doped nanocarbon materials have been demonstrated to be efficient alternative catalysts for ORR. Here, graphene quantum dots, synthesized from inexpensive and earth-abundant anthracite coal, were self-assembled on graphene by hydrothermal treatment to form hybrid nanoplatelets that were then codoped with nitrogen and boron by high-temperature annealing. This hybrid material combined the advantages of both components, such as abundant edges and doping sites, high electrical conductivity, and high surface area, which makes the resulting materials excellent oxygen reduction electrocatalysts with activity even higher than that of commercial Pt/C in alkaline media

Origami-inspired 3D interconnected molybdenum carbide nanoflakes

High-temperature stable transition metal carbides are one of the promising classes of materials for next-generation energy applications such as water splitting catalysis and electrodes for energy storage devices. Herein, origami-like molybdenum carbide flakes with interfacially connected structures in various orientations using an easily scalable chemical vapor deposition method are synthesized. Interestingly, each individual flake of similar orientation is interconnected across different planes. The interconnected architectures are found to be highly elastic and behave in a sponge-like manner. In addition, the surface energy of each plane is calculated using the first-principle density functional theory. The molybdenum carbide shows excellent activity for the hydrogen evolution reaction, with the onset over potential occurring around ?16 to ?25 mV with high stability. The material is used as an electrode for supercapacitors as a second demonstration. The supercapacitor constructed with polypyrrole reaches the specific capacitance of ?279 F g?1 at a current density of 0.5 A g?1.by Ryota Koizumi, Sehmus Ozden, Atanu Samanta, Ana Paula P. Alves, Avanish Mishra, Gonglan Ye, Glaura G. Silva, Robert Vajtai, Abhishek K. Singh, Chandra S. Tiwary and Pulickel M. Ajaya

CoMoO₄ Nanoparticles Anchored on Reduced Graphene Oxide Nanocomposites as Anodes for Long-Life Lithium-Ion Batteries

A self-assembled CoMoO₄ nanoparticles/reduced graphene oxide (CoMoO₄NP/rGO), was prepared by a hydrothermal method to grow 3–5 nm sized CoMoO₄ particles on reduced graphene oxide sheets and used as an anode material for lithium-ion batteries. The specific capacity of CoMoO₄NP/rGO anode can reach up to 920 mAh g^–1 at a current rate of 74 mA g^–1 in the voltage range between 3.0 and 0.001 V, which is close to the theoretical capacity of CoMoO₄ (980 mAh g^–1). The fabricated half cells also show good rate capability and impressive cycling stability with 8.7% capacity loss after 600 cycles under a high current density of 740 mA g^–1. The superior electrochemical performance of the synthesized CoMoO₄NP/rGO is attributed to the synergetic chemical coupling effects between the conductive graphene networks and the high lithium-ion storage capability of CoMoO₄ nanoparticles

Defects Engineered Monolayer MoS₂ for Improved Hydrogen Evolution Reaction

MoS₂ is a promising and low-cost material for electrochemical hydrogen production due to its high activity and stability during the reaction. However, the efficiency of hydrogen production is limited by the amount of active sites, for example, edges, in MoS₂. Here, we demonstrate that oxygen plasma exposure and hydrogen treatment on pristine monolayer MoS₂ could introduce more active sites via the formation of defects within the monolayer, leading to a high density of exposed edges and a significant improvement of the hydrogen evolution activity. These as-fabricated defects are characterized at the scale from macroscopic continuum to discrete atoms. Our work represents a facile method to increase the hydrogen production in electrochemical reaction of MoS₂ via defect engineering, and helps to understand the catalytic properties of MoS₂

Carbon Nitrogen Nanotubes as Efficient Bifunctional Electrocatalysts for Oxygen Reduction and Evolution Reactions

Oxygen reduction and evolution reactions are essential for broad range of renewable energy technologies such as fuel cells, metal-air batteries and hydrogen production through water splitting, therefore, tremendous effort has been taken to develop excellent catalysts for these reactions. However, the development of cost-effective and efficient bifunctional catalysts for both reactions still remained a grand challenge. Herein, we report the electrocatalytic investigations of bamboo-shaped carbon nitrogen nanotubes (CNNTs) having different diameter distribution synthesized by liquid chemical vapor deposition technique using different nitrogen containing precursors. These CNNTs are found to be efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions. The electrocatalytic activity strongly depends on the nanotube diameter as well as nitrogen functionality type. The higher diameter CNNTs are more favorable for these reactions. The increase in nanotube diameter itself enhances the catalytic activity by lowering the oxygen adsorption energy, better conductivity, and further facilitates the reaction by increasing the percentage of catalytically active nitrogen moieties in CNNTs

Chemical Vapor Deposition Of Monolayer Rhenium Disulfide (res2).

The direct synthesis of monolayer and multilayer ReS2 by chemical vapor deposition at a low temperature of 450 °C is reported. Detailed characterization of this material is performed using various spectroscopy and microscopy methods. Furthermore initial field-effect transistor characteristics are evaluated, which highlight the potential in being used as an n-type semiconductor.274640-464

Vertical and in-plane heterostructures from WS2/MoS2 monolayers

Layer-by-layer stacking or lateral interfacing of atomic monolayers has opened up unprecedented opportunities to engineer two-dimensional heteromaterials. Fabrication of such artificial heterostructures with atomically clean and sharp interfaces, however, is challenging. Here, we report a one-step growth strategy for the creation of high-quality vertically stacked as well as in-plane interconnected heterostructures of WS2/MoS2 via control of the growth temperature. Vertically stacked bilayers with WS2 epitaxially grown on top of the MoS2 monolayer are formed with preferred stacking order at high temperature. A strong interlayer excitonic transition is observed due to the type II band alignment and to the clean interface of these bilayers. Vapour growth at low temperature, on the other hand, leads to lateral epitaxy of WS2 on MoS2 edges, creating seamless and atomically sharp in-plane heterostructures that generate strong localized photoluminescence enhancement and intrinsic p–n junctions. The fabrication of heterostructures from monolayers, using simple and scalable growth, paves the way for the creation of unprecedented two-dimensional materials with exciting properties