GRADUATE SCHOOL OF SCIENCES & ENGINEERING
CHEMICAL AND BIOLOGICAL ENGINEERING
MS THESIS DEFENSE BY GÖZDE TEKELİ
Title: Photocatalytic Water Splitting on Tungsten (VI) Oxide Films Prepared by Supercritical Deposition in a Cold-wall Reactor and by a Hydrothermal Technique
Speaker: Gözde Tekeli
Time: August 09, 2018, 13.00
Rumeli Feneri Yolu
Thesis Committee Members:
Prof. Dr. Can Erkey (Advisor, Koç University)
Asst. Prof. Sarp Kaya (Co-advisor, Koç University)
Assoc. Prof. Dr. Uğur Ünal (Koç University)
Asst. Prof. Zeynep Ülker Demir (Altınbaş University)
Asst. Prof. Erdal Uzunlar (İzmir Institute of Technology)
Energy demand is growing and there is a vital need to find new energy sources that are sustainable, clean, and renewable. Hydrogen that is produced from water has been considered as one the most promising energy sources for decades. One of the technologies for production of hydrogen from water is photoelectrochemical water splitting where water and sunlight react on photocatalytic surfaces. In this manner, photoelectrochemical cell (PEC) configurations have been developed for the photo-electrolysis of water within a photoelectrode which is used both to absorb the light and also to catalyze the splitting of water into hydrogen and oxygen. Many studies have concentrated on development of photocatalytic materials for this purpose. Tungsten (VI) trioxide (WO3) is a promising photoelectrode material since it has an appropriate band gap and high stability in acidic conditions.
In this study, supercritical fluid deposition method (SCD) was developed as an alternative technique to produce WO3 films on a conductive glass, fluorine doped tin oxide (FTO) for photoelectrochemical water splitting. For this purpose, a cold-wall reactor was designed for deposition of the films onto heated substrates from supercritical solutions at substantially lower temperatures than the substrate temperature. WO3 films were deposited using 0.4 wt% tungsten carbonyl (W(CO)6) solutions in supercritical CO2 . Four types of deposition procedures were investigated with no additional oxygen (O2) gas and with addition of O2 in different steps of the deposition. Depositions were carried out at 8.27 MPa CO2 and a heating stage temperature of 300 °C for each method. Coating of the FTO surface were confirmed via scanning electron microscopy (SEM) and energy dispersive X-ray analysis. SEM results indicated that the produced films were very thin (most probably less than 100 nm). For some cases, SEM analysis from cross-sectional view showed the incomplete film growth on some parts of the sample. Structure of the WO3 films were determined as mixtures of monoclinic WO3 and an unknown phase of tungsten oxide species with a different state of tungsten from the analysis results of the grazing incidence X-ray diffraction spectra. Photoelectrochemical characterization was performed with cyclic voltammetry method under illumination at 1.0 sun AM 1.5 G in 1 M sulfuric acid solution (H2SO4) with and without using 10 vol.% methanol as a hole scavenger. The best photocurrent was obtained as 0.0225 mA/cm2 at 1.23 V from the film produced with the addition of oxygen before starting the deposition in 1 M H2SO4. Increase in the photocurrent activity was observed with the addition of hole scavenger to the electrolyte. To make comparison between the films deposited by conventional method and new developed supercritical carbon dioxide technique, hydrothermal synthesis of WO3 films were performed. Films were deposited by direct growth on FTO substrate and by drop casting of a colloidal solution of WO3 powder. The effect of the post calcination and hydrothermal reaction period on the film structure and morphology were investigated. The best method was determined as the 6 h reaction time and annealing at 500 °C for 1 h. The highest photocurrent was obtained as 0.76 mA/cm2 in 1 M H2SO4 from the film which was produced by drop casting.