Here, we present a new microwave-solvothermal method for the preparation of iron oxide nanostructures using deep eutectic solvents as a more sustainable reaction medium. By varying the synthesis temperature and solvent water fraction, the methodology offers control over iron oxide phase, size, and morphology, using efficient, rapid (10 minute) microwave heating. Synthesis with pure DES gives small (<5 nm) superparamagnetic samples of γ-Fe2O3 or α-Fe2O3, whereas hydrated DES yielded either nanoshards or large rhombohedral nanoparticles without the superparamagnetic response. Nanostructures were solution-cast onto F[thin space (1/6-em)]:[thin space (1/6-em)]SnO2 films. The photoelectrochemical response of the prepared photoanodes was assessed, with a maximum measured photocurrent response of 0.7 mA cm−2 at 1.23 V vs. RHE. We measured the solvent structure using synchrotron WAXS, demonstrating the differences between the dry and hydrated solvent before and after heat-treatment, and showing that the hydrated solvent is remarkably resilient to extensive degradation.