Nanostructured electrodes for photoelectrochemical water splitting

Burch, Henry Arthur (2016). Nanostructured electrodes for photoelectrochemical water splitting. University of Birmingham. Ph.D.

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Abstract

Nanostructured MoS $_2$ and ZnFe $_2$ O $_4$ were synthesised and tested as catalytic water splitting photoelectrodes. MoS $_2$ was nanopatterned from a bulk crystal using a combination of nanosphere lithography and plasma etching. Three morphologies were produced: nanospheres deposited with interstices between them produced nanopillars, nanospheres squashed into hexagons imprinted a nanowell pattern, and linked nanopillars resulted from parts of each. The MoS $_2$ was tested as a photocathode and morphologies with linkages between features had improved catalysis than those without. This was attributed to the layered structure of MoS $_2$ . These samples degraded in air to MoSxO( $_2$ $_-$ $_x$ ), and an electrochemical technique utilising Na $_2$ S $_2$ O $_3$ was used to re-sulfidate the MoSxO( $_2$ $_-$ $_x$ ). The technique decreased the onset potential from -0.27 V SHE to -0.17 V SHE, and the Tafel slope from 282 mV dec $^{-1}$ to 87 mV dec $^{-1}$ .
ZnFe $_2$ O $_4$ electrodes were deposited by AACVD from a precursor molecule. The deposition solvent composition was systematically altered between methanol and ethanol to examine its effect on the nanostructure. ZnFe $_2$ O $_4$ electrodes deposited from predominantely methanol solvent had compact morphologies due to heterogenous nucleation, while the electrodes deposited from predominantly ethanol solvent had high surface area structures due to homogeneous nucleation. The more exothermic enthalpy of combustion of ethanol was deemed responsible.