Designing and understanding hematite based nanomaterials for solar energy applications

2019-02-18T15:02:44Z (GMT) by Melissa P. Cardona
Solar water splitting is a sustainable method of hydrogen fuel production. Hematite (α-Fe2O3) is a promising photocatalytic material due to its favorable band gap (2.1 eV), stability and low cost. However, overcoming hematite’s poor surface and electrical properties is crucial for improving its solar to hydrogen efficiency. This dissertation designs and investigates nanomaterial-based techniques to promote electron/hole pair generation and separation to enhance hematite’s photocurrent density. <br> To this end, I first demonstrate that single layer graphene (SLG) can be transferred to the surface of nanostructured, titanium doped hematite. The transfer is confirmed and the SLG improves the photocurrent density by 1.6 times compared to uncoated hematite. Electrochemical techniques are employed to reveal the role of the single layer graphene overlayer and the reason for improved water oxidation. First, Electrical Impedance Spectroscopy under AM 1.5G solar illumination shows that the SLG affects the resistance and capacitance at the surface-electrolyte interface. Direct kinetic studies were carried out using Intensity Modulated Photocurrent Spectroscopy. This revealed that the SLG results in a decreased charge transfer rate and a decreased charge recombination rate. Therefore, though SLG inhibits charge transfer, its role as an overlayer on hematite is reduced recombination at the electrode surface, allowing for higher yield of charge carriers available for the oxygen evolution reaction. This demonstrates that SLG acts as a passivation layer on hematite. SLG overlayers also improve the photocurrent density of TiO2 photoanodes, demonstrating that SLG overlayers have the potential to act as a general surface-modification technique to improve the efficiency of semiconductor photocatalysts. <br> Second, I conduct theoretical studies to calculate the ideal photocurrent density of metal-semiconductor core-shell (CS) and semiconductor-metal-semiconductor core-multishell (CMS) nanowires (NWs). Using emerging plasmonic metals TiN and ZrN as the metal core or metal interlayer, the photocurrent density of sub-50 nm hematite features is enhanced. One key to fabricating these CS and CMS NWs is the synthesis of ultra-thin plasmonic metal nitride materials. To achieve this, a TiN synthesis is developed via plasma enhanced atomic layer deposition (PE-ALD) using TDMATi and NH3 plasma precursors. Using Raman spectroscopy, it is observed that this ALD method produces nitrogen vacant TiNx. Ellipsometry results confirm that the TiN is plasmonic with a broad visible light plasmon resonance. On silicon substrates, the plasmon wavelength is at 646 nm and 651 nm for TiN film thicknesses of 50 and 10 nm, respectively. Therefore, showing that plasmonic TiN thin film fabrication is possible using a PE-ALD method and that the plasmonic properties are retained for ultra-thin films, making them suitable for plasmon enhancement of hematite nanomaterials. <br><br>