Development and characterization of catalysts based on two-dimensional materials for photoelectrochemical water splitting

다국어 초록 (Multilingual Abstract)

The development of hydrogen energy as eco-friendly and sustainable energy has become increasingly important toward realizing a low-carbon society. Among various hydrogen production methods, the solar-energy-driven photoelectrochemical (PEC) water splitting technology using photoelectrodes is one of the promising ways to produce clean and carbon-free hydrogen. A semiconductor photocathode, p-type silicon (p-Si), is widely used as a photoactive material of the PEC water splitting cell for hydrogen evolution reaction (HER). However, the semiconductor photocathodes are typically very unstable in acid solutions and have the high kinetic energy barriers required for electrochemical hydrogen reduction occurring on the photocathode/electrolyte interface. To efficiently reduce kinetic energy barriers, an appropriate catalyst is required on the photocathode surface while being inexpensive and stable. Additionally, it is highly demanded to develop a reliable characterization technique that clearly analyzes and correlates the catalytic activity with the properties of the catalyst.
Transition metal dichalcogenides (TMDs) with two-dimensional (2D) layered structures, such as molybdenum disulfides (MoS2) and Tungsten disulfides (WS2), have recently received the spotlight as a promising candidate for the HER catalyst because they are cheap and earth-abundant. Although the catalyst activity at the atomic edge sites of TMDs is known to be high, studies have recently been actively conducted on the technology to activate the catalytic activity of the relatively inert basal plane to take advantage of the large surface-to-edge ratio of these 2D layered materials. The studies tried so far include the uses of naturally-existing or intentionally-strained atomic vacancies and grain boundaries, catalytic dopants incorporated on the basal surface, etc.
This dissertation mainly focused on the development and characterization of atomically thin heterojunction catalysts using TMDs with different energy band structure without additional surface engineering as an efficient HER catalyst. The heterojunction catalyst using energy-band-engineered semiconductors has been widely used in conventional bulk material systems. The formation of type-II heterojunction can efficiently reduce the kinetic energy barriers that exist at the photocathode/electrolyte interfaces. In this respect, a heterojunction consisting of atomically thin TMD layers is expected to form a physically limited deletions region, enabling the ultrafast transfer of photo-excited charge carriers by the strong built-in electric field, resulting in optimizing the interfacial kinetics of photoelectrolysis.
However, in commonly used PEC measurements using large-area photoelectrode under global illumination, it is experimentally very difficult to correlate the PEC performances with specific properties of the catalyst because of the ensemble averaging effects caused by various inhomogeneous factors, such as step edges, vacancies, and grain boundaries, as well as other extrinsic factors in the electrode fabrication processes such as contact resistances, the number of layers, and local strains. To resolve this critical issue, we have developed a new PEC characterization technique, named scanning photoelectrochemical microscopy (SPECM), using a microscale device. This SPECM analysis enables the spatial resolved characterization and imaging of PEC performance of the designed catalytic surfaces, thus visualizing the HER catalytic activity on various catalytic surfaces. The technique uses a focused laser with a diameter less than 1 µm, and its incident power is very low at around 100 nW, excluding the photothermal effects caused by local heating.
Using the methodology developed above, the difference in catalytic activity between the TMD heterojunction and the single TMD layer was clearly revealed, while excluding other influential effects. In particular, when the MoS2/WS2 heterojunction is introduced as a catalyst on the photocathode, a significant reduction in both overpotential and charge-transfer resistance is achieved even without intentional generation of catalytic sites. The beneficial effect of atomically thin vertical heterojunction is explained by the built-in potential resulted from the efficient charge transfer of excited carriers in type-II heterojunction with the theoretical support of the first-principles calculation. Based on this, the large-scale atomically thin TMD heterojunction catalyst was demonstrated, and their PEC performances were characterized. The large-area MoS2/WS2 heterojunction catalyst was fabricated on a p-Si photocathode using 2-inch TMD films grown by metal-organic chemical vapor deposition. The fabricated photocathode resulted in a very low onset potential and a high photocurrent density at 0 V versus a reversible hydrogen electrode.
Furthermore, the spatially resolved PEC characterization using SPECM clearly revealed the PEC performance of more complex catalytic structures, including the atomic edge sites, different thickness of TMD layer, vertically aligned TMD with nanoparticles. Our demonstration not only provides an unprecedented approach to fundamentally investigating the PEC performances about the tailored properties of catalysts but also proposing a new catalytic architecture, thereby enabling the design of highly efficient energy converging systems, including PEC water splitting cell.

목차 (Table of Contents)

• Table of contents
• Abstract........................................................................i
• Table of contents........................................................vi
• List of figures............................................................xii
• Chapter 1. Introduction.............................................1
• 1.1 Motivation: The potential and issues of photoelectrochemical (PEC) water splitting for hydrogen production..............................1
• 1.2 Objective and approach............................................................3
• Chapter 2. Literature review......................................7
• 2.1 PEC water splitting for sustainable hydrogen production...........7
• 2.1.1 Principles of PEC water splitting................................................10
• 2.1.2 Semiconductor-electrolyte junction...........................................12
• 2.1.3 Photocathodes for hydrogen evolution reaction (HER)..............14
• 2.1.4 HER mechanism.........................................................................18
• 2.2 Transition metal dichacogenides (TMDs) as HER catalysts.....20
• 2.2.1 Properties of TMDs....................................................................20
• 2.2.2 HER catalytic activities of TMDs...............................................24
• 2.2.3 Identification & activation of HER catalytic sites on TMDs..................................................................................................27
• 2.2.4 TMD heterojunction...................................................................31
• 2.3 Spatially resolved PEC characterization...................................37
• 2.3.1 Conventional characterization....................................................37
• 2.3.2 Scanning electrochemical microscopy........................................38
• 2.3.3 Other spatially resolved PEC characterization............................42
• Chapter 3. Experimental methods...........................44
• 3.1 Synthetic methods of two dimensional (2D) materials.............44
• 3.1.1 Mechanical exfoliation...............................................................44
• 2.1.2 Chemical vapor deposition.........................................................47
• 3.1.3 Metal organic chemical vapor deposition...................................49
• 3.2 Fabrication of 2D heterostructure.............................................50
• 3.2.1 Polymer-assisted mechanical transfer.........................................50
• 3.2.2 van der Waals layer by layer assembly........................................51
• 3.3 Materials characterization........................................................53
• 3.3.1 Raman and photoluminescence (PL) spectroscopies..................53
• 3.3.2 Scanning Kelvin probe microscopy (SKPM)..............................57
• 3.3.3 Reflection measurements of 2D materials..................................58
• 3.4 PEC characterization................................................................59
• 3.4.1 Conventional PEC characterization setup...................................59
• 3.4.2 Scanning photoelectrochemical microscopy setup.....................61
• 3.4.3 Spatially defined PEC characterization setup.............................63
• Chapter 4. An atomically thin 2D heterojunction catalyst.......................................................................64
• 4.1 Introduction..............................................................................64
• 4.2 Model system and fabrication of an atomically thin heterojunction catalyst for HER on a p-type semiconducting photocathode..................................................................................66
• 4.2.1 Model system of the monolayer MoS2/WS2 heterojunction catalyst on a WSe2 photocathode.........................................................66
• 4.2.2 Fabrication procedure of the monolayer MoS2/WS2 heterojunction catalyst on a WSe2 photocathode.................................68
• 4.3 Material characteristics of the 2D heterostructure....................71
• 4.3.1 Atomic force microscopy (AFM) and Raman spectroscopy measurements.....................................................................................71
• 4.3.2 PL spectroscopy measurements..................................................73
• 4.3.3 Surface potential measurement using SKPM..............................75
• 4.3.4 Absorption measurements..........................................................77
• 4.4 PEC characteristics..................................................................79
• 4.4.1 Spatially resolved PEC characterization.....................................79
• 4.4.2 Spatially defined PEC characterization......................................85
• 4.4.3 Electrochemical performance of each catalyst............................88
• 4.4.4 Electrochemical impedance spectroscopy (EIS) measurements.....................................................................................91
• 4.4.5 Stability test using Raman spectra before and after PEC measurements.....................................................................................93
• 4.5 DFT calculation........................................................................94
• 4.6 Summary..................................................................................97
• Chapter 5. Large-scale atomically thin 2D heterojunction catalyst.............................................98
• 5.1 Introduction..............................................................................98
• 5.2 Band diagram and fabrication procedure of the 2D heterojunction catalyst on p-Si photocathode...............................100
• 5.2.1 Band diagram of the 2D heterojunction catalyst on p-Si photocathode.....................................................................................100
• 5.2.2 Fabrication procedure of the 2D heterojunction catalyst on p-Si photocathode.....................................................................................102
• 5.3 Material characterization of the 2D heterostructure...............104
• 5.3.1 AFM measurements.................................................................104
• 5.3.2 Optical characteristics..............................................................106
• 5.3.3 X-ray photoelectron spectroscopy measurements....................108
• 5.4 PEC characteristics................................................................110
• 5.4.1 Conventional PEC characterization..........................................110
• 5.4.2 EIS and Incident-photon-to-current conversion efficiency measurements....................................................................................113
• 5.4.3 Stability test and Faradaic efficiency measurements.................115
• 5.5 Summary................................................................................117
• Chapter 6. Visualizing the PEC performances on different catalytic structures..................................118
• 6.1 Introduction............................................................................118
• 6.2 Heterojunctions with different thicknesses of TMD...............120
• 6.3 Heterojunction versus edge site..............................................122
• 6.4 3D/2D heterostructures..........................................................124
• 6.5 Summary................................................................................129
• Chapter 7. Conclusion............................................130
• Referneces...............................................................133
• Summary in Korean...............................................146
• Curriculum Vitae....................................................150