Multidimensional Transition Metal Dichalcogenides for Hydrogen Evolution and White Light Emitting Devices Applications
Date Issued
2016
Date
2016
Author(s)
NIKAM, REVANNATH DNYANDEO
Abstract
The aim of this Ph.D. thesis is to synthesize multidimensional TMDs and their heterostructures by chemical vapor deposition (CVD) for light emitting diode (LED) and electrocatalytic hydrogen evolution (HER) applications. In the first part of this thesis, we briefly introduce TMDs. The formation of artificial lateral or vertical heterostructure between two types of TMDs is an effective strategy to tune the electronic and optical property of individual TMDs. The vertical heterostructures formed in TMDs have tremendous future applications in the fields of energy and optoelectronics such as in HER and LED applications. In this connection, we briefly review the underlying mechanism for the formation of vertical heterostructures using TMDs by CVD. In the second part of this thesis, we demonstrate that molybdenum sulfide (MoSx) tetracyanoquinodimethane (TCNQ) hybrid material shows excellent catalytic effects in HER applications. We have found a significant enhancement in the electrocatalytic activity of MoSx when it is grown on carbon cloth treated with TCNQ, where the MoSx is synthesized by thermolysis from the ammonium tetrathiomolybdate ((NH4)2MoS4) precursor at 170 °C. The pyridinic-N and graphitic N-like species on the surface of carbon cloth arising from the TCNQ treatment facilitate the formations of Mo5+ and S22– species in the MoSx, especially with S22– serving as an active site for HER. Also, the smaller particle size of the MoSx grown on the TCNQ-treated carbon cloth reveals a high ratio of the edge sites to the basal plane sites, indicating more adequate reaction sites are created with superior electrocatalytic characteristics. This study provides the fundamental concepts useful in the design and preparation of TMDs, beneficial in the development of clean energy. In the third part of this thesis, we synthesized MoS2 nanosheets on 3D conductive MoO2 via a two-step chemical vapor deposition (CVD) reaction. The 3D MoO2 structure can create structural disorders in MoS2 nanosheets, which lead to superior HER activity by exposing tremendous active sites in the form of terminal disulfur of and utilizing the backbone conductive oxide layer to facilitate the interfacial charge transport for proton reduction. Also, the MoS2 nanosheets could protect the inner MoO2 core from the acidic electrolyte in HER reaction. The high activity of the as-synthesized 3D MoS2/MoO2 hybrid material in HER is attributed to the small onset overpotential of 142 mV, a largest cathodic current density of 85 mA cm-2, a little Tafel slope of 35.6 mV dec-1, and the robust electrochemical durability. In the fourth part of this thesis, we demonstrate the novel vertically stacked heterostructures of p-GaN/p-n MoS2 by chemical vapor deposition (CVD). The as-grown novel hybrid heterostructures were applied to fabricate white light emitting diode (WLED) based on a three-emitter system. We report the electroluminescence (EL) from vertically stacked p-GaN/p-n MoS2 heterojunction LEDs under forward bias. The EL spectra were composed of three emissions centered at 480 nm (from p-GaN), 530 nm (p-MoS2) and 650 nm (n-MoS2) under forward biases, and were dominated by the broad emission at 650 nm from n-MoS2. Our WLED device with the p-GaN/p-n MoS2 structure showed the luminance of 100 cd m−2 at a current density of 2.5 mA cm−2.
Subjects
Transition Metal Dichalcogenides
Chemical Vapor Deposition
Hydrogen Evolution Reaction
Light Emitting Diode
Type
thesis
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