2D heterostructures have attracted considerable attention owing to their unique electrical and optical properties such as tunable bandgaps, excitonic spectra and work functions. Therefore, they can be used for a wide range of applications including p-n diodes, photodetectors, transistors, sensors, light-emitting diodes and catalysis.
We have synthesized 2D vertical or lateral heterostructure by using chemical vapor deposition method and studied their unique properties or growth behaviors.
Vacancy-Engineering & HER Activity
Activation of the TMDs basal plane is crucial factor for the development of high-performance HER catalysts. Introduction of vacancy in TMDs lattice is one of the methods to activate the basal plane. Various post-treatment such as hydrogen annealing, chemical doping, and phase transition have been proposed to introduce vacancy. Introduction of vacancy through post-treatment complicates the process which limits in commercialization.
We demonstrate in-situ modulation of chalcogen vacancy sites during the chemical vapor deposition synthesis of molybdenum diselenides (MoSe2) for application in the HER. We demonstrate that the Tafel reaction can be activated via in-situ vacancy-engineered MoSe2, resulting in improved onset potential and an exceptionally low Tafel slope, which exhibits one of the lowest values reported for TMDs to date. Density functional theory calculations revealed that vacancy coalescence in the MoSe2 lattice reduced the hydrogen adsorption free energy and diffusion barrier to activate the Tafel reaction.
Although intensive research for graphene oxide (GO) has been conducted, the broad size distribution of as-synthesized GO flakes still remains a challenging issue since flake size has a direct impact on the material properties and related applications.
We developed a facile and scalable approach for the size fractionation of GO flakes by manipulating the surface charge of GO with an organic solvent-mediated aqueous solution. Both the experimental and simulation results confirm that the dispersion of GO flakes is notably influenced by the bond strength with solvent, which is attributed by the charge amount of oxygen in solvent molecule. From the understanding of the dispersion mechanism, an optimal solvent for the size fractionation of GO flakes was determined and GO flakes were successfully sorted into various size groups with a narrow distribution. The size effects of GO flakes as template and hole transfer layer in perovskite solar cell was confirmed through various analyses. We believe that our results could bring significant advances in producing two-dimensional materials with reliable and reproducible material properties toward practical applications.
Synthesis of 2D materials based composite via liquid based processes
– Liquid exfoliation and size selection
– Chemical exfoliation
– Electrochemical exfoliation
Enhanced dispersion in environmentally solvents
Phase transition of TMDs
2D materials based composite for energy application
– Electrocatalysts based on Pure 2D TMDs nanosheets
– Bifunctional electrocatalysts for overall water splitting
Hydrogen energy has many advantages in terms of energy supply and demand, economic effect, environment, etc., and is anticipating the advent of hydrogen economy at the national level and undergoing a preparatory process in earnest. Therefore, various studies for hydrogen generation have been carried out. Among them, hydrogen generation through water electrolysis is a relatively simple method, and catalyst selection is important because overvoltage is determined depending on the catalyst. At present, noble metal catalysts such as Pt and IrO2 are vulnerable to stability and cost. To solve this problem, our laboratory is studying the water splitting catalysts which can solve economical, stability and performance problems. It is mainly composed of transition metal-based catalysts (various perovskite oxides based on transition metals, Co, Ni and Fe-based compounds, transition metal-based alloys, layered double hydroxide, transition metal dichalcogenides, transition metal carbides, transition metal nitrides, and transition metal oxides). We focus on high performance, high stability, and economical preparation of electrocatalyst via control the ratio of elements according to synthesis conditions, control of the structural properties of catalyst (size, shape, crystallinity), electrical structure control (d-band center position, work function, Fermi level) and optimization of hydrogen absorption/desorption energy and active site.
Organic Solar Cells
Recently, graphene has attracted significant attention as a transparent conductive electrode (TCE) owing to its outstanding optical, electrical, and mechanical properties. To date, several methods to prepare OSCs using graphene-based TCE that is grown by chemical vapor deposition (CVD) have been investigated.
However, the major issue with graphene electrodes in OSCs is commonly related to the use of the organic material-based solutions that are not well-coated on the hydrophobic surface of graphene. For instance, a nonuniform coverage of charge transporting layers on graphene TCEs can cause the degradation of devices; this results in decreased efficiency by providing the leakage current pathways. Different strategies have been investigated to overcome this issue, such as the use of solvent- or surfactant-modified charge transporting materials, chemical doping of graphene, and insertion of additional metal oxide layer on graphene
Perovskite Solar Cells
TMDs has attracted significant attention due to its superior electronic and optical properties
We introduced the exfoliated TMDs into the PEDOT:PSS to apply TMDs as new PEDOT:PSS modifier and efficiency enhancer for perovskite device
Schottky Solar Cells
With the rise of graphene, its applications as the active component in various types of solar cells, such as transparent conductors, additives, or interfacial charge transport layers, have been intensively investigated. Among them, graphene-based Schottky junction solar cells have been rapidly developed due to their relatively simple device structures compared to conventional p–n junction type solar cells. Through various modifications such as chemical doping, antireflection coating, and interfacial oxide layer control, a power conversion efficiency of over 15% was successfully reported. However, graphene-based Schottky junction type solar cells often suffer from s-shaped current density–voltage characteristics, which leads to the inevitable performance degradation, particularly for the fill factor. In this work, we investigate the origin of such aforementioned behaviors and propose a facile approach to suppress the s-shape character in the operation of graphene-based Schottky junction solar cells. Through the careful modulation of the graphene integration process, the interfacial charge recombination seemed to be significantly suppressed leading to a notably improved device performance (from 0.8% to 12.5%). Our findings shall provide valuable insights into the operating principle of graphene-based Schottky junction solar cells, which can play an important role as one of the primary suppliers of next-generation renewable clean energy.
2D Material-Based Hybrid TCE
Indium tin oxide (ITO) is not suitable for future application in flexible and stretchable devices due to its fragile property.
Various features should be considered for TCE toward high-performance OPV; electrical conductivity, optical transparency, surface roughness, permeability, chemical and thermal durability, mechanical stability.