Materials Synthesis Methods

There are many material synthesis methods for fabricating the metal and oxide components of oxide encapsulated electrocatalysts. Common techniques for metal deposition include: electrodeposition, physical vapor deposition (sputtering and evaporation), and chemical vapor deposition. Conventional methods for oxide synthesis include: ultraviolet ozone photochemistry, sol-gel synthesis, and chemical vapor deposition. Each synthesis technique has specific advantages (e.g. purity, scalability, or controllability over thickness and geometry) and disadvantages (e.g. high temperature, low pressure vacuum, low throughput). Therefore, different techniques may be appropriate for one application but not another depending on the design specifications. The following provides brief descriptions of the material synthesis methods used in this dissertation.

2.2.1 Electrodeposition

Electrodeposition is a room temperature and scalable method for depositing metals. From an economic standpoint, this technique also optimizes the use of raw materials, equating to major cost savings. In order to fabricate the metal components of MIS electrodes at lower cost and improved scalability, room temperature electrodeposition processes are preferred to high temperature and vacuum processes (e.g. physical vapor deposition). Electrodeposition involves an electrode substrate that is immersed in metal-ion containing electrolyte and subjected to reducing conditions, at potentials more negative than the standard reduction potential of the metal ion species of interest. Illumination is also required for electrodeposition onto a photocathode (p-type

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semiconductor) to generate photoelectron minority carriers. Electrons having electric potential energy more negative than the thermodynamically required potential to reduce the metal ion are able to transfer to metal ions in the electrolyte and create a metallic deposit. Tuning the applied potential/current can allow for control over particle morphology, size, and coverage.

For reproducible results, the electrodeposition process must be finely tuned to control parameters such as particle density and size.25,26 This can be accomplished by determining the nucleation and growth regimes for different metal deposits. The deposition reaction of interest, in this dissertation, is the reduction of platinum species from tetrachloroplatinate (II) ions:

[PtCl4]-2 + 2 e- ↔ Pt + 4 Cl- E° = + 0.755 V vs. NHE (2.1) Pt electrodeposition is unique in that hydrogen underpotential deposition (section 2.4.3) can also occur on Pt surfaces. This property was leveraged by Liu et al. to controllably deposit Pt monolayers through an atomic layer-type deposition mechanism.27 After the potential is quickly pulsed to deposit Pt, the potential is adjusted to favor hydrogen termination on the Pt surface, effectively quenching further Pt deposition. This process can be repeated to obtain particles of relatively uniform size or films of controlled thickness. A similar procedure was implemented to deposit Pt onto Si photocathodes in Chapter 3.

Pt nanoparticles were electrodeposited onto p-type Si(100) with native silicon oxide, in 3 mM potassium tetrachloroplatinate (K2PtCl4) solution containing 0.5 M sodium chloride (NaCl) supporting electrolyte under 100 mW cm-2 illumination intensity.

2.2.2 Electron Beam Physical Vapor Deposition

Physical vapor deposition (PVD) is a vacuum deposition method used to deposit thin films from a material in vapor phase. Electron beam physical vapor deposition (EBPVD) is a type of PVD that is a line-of-sight evaporative deposition method with nanometer control over thickness.

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The material source, commonly in the form of metal pellets, is heated by a power-adjustable electron beam source (charged tungsten filament) to evaporate the metal into the gaseous phase. The target substrate is located above the material source allowing for metal vapors to precipitate into a solid film on its surface. This technique coats everything within the vacuum chamber, such that a portion of the source material is wasted.

A titanium (99.99%) adhesion layer and Pt (99.99%) layers were sequentially deposited at 0.2 A s−1 by EBPVD onto the silicon substrates without breaking vacuum and without substrate heating in a Angstrom EvoVac evaporator system with a base pressure of 1.0 × 10−7 Torr. Film thicknesses were monitored with quartz crystal thickness monitors. The amount of metal deposited was then controlled by adjusting the amount of time that the substrate was exposed to the source.

2.2.3 Ultraviolet Ozone Photochemistry

Ultraviolet (UV) ozone photochemistry converts PDMS to SiOx films.28,29 _{This process} occurs at room temperature and atmospheric pressure, which allows for the formation of SiOx films on various material surfaces that PDMS can wet. The UV ozone photochemical conversion of PDMS to SiOx has been proposed to occur via the generation of radical species from the polymer backbone due to atomic oxygen attack. During the UV ozone conversion process, ozone first forms under ultraviolet (< 240 nm) photodissociation of molecular oxygen (O2). Next, gaseous ozone decomposes to oxygen atoms and molecules by absorbing photons in the UV and visible spectrum. Photons having 254 nm wavelength cause a chain decomposition to form highly energetic and reactive atomic oxygen species, which then initiates the decomposition of silicones. Previous literature proposed atomic oxygen cleaves C-H bonds, in the methyl groups, or Si-C bonds to create intermediate radical species that crosslink. The products of the crosslinked structures are the silicon oxide film, and carbon dioxide and water volatiles.

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To synthesize SiOx films, a spincoater (Laurell Technologies Corp.) was used to coat PDMS on the substrate from an organic toluene solution.29,30 _{Due to the low surface energy,} (section 2.1.3) the PDMS uniformly coats the substrate. The samples were then dried in a vacuum oven at 70 °C for 60 minutes to evaporate the solvent. Afterwards, the PDMS was converted to SiOx in a UV ozone cleansing chamber in air for 2 hours (UVOCS, T10X10/OES). The mercury light source generates UV light in the 254 nm and 185 nm range relevant for generating ozone and exciting organic molecules.28,29 _{The thickness of the resulting film was controlled by adjusting the} concentration of PDMS in organic solution and the spin coating speed rate. This relation between thickness, concentration and spin rate produces reproducible films with known thickness. Specific details regarding the concentration of PDMS/toluene solution (mg ml-1) and the spincoating speed rate (rpm) are provided in the individual chapters, as these parameters varied with each study.