High-Throughput Experimental Study of Wurtzite Mn_1- Zn O Alloys for Water Splitting Applications

We used high-throughput experimental screening methods to unveil the physical and chemical properties of Mn_1- Zn O wurtzite alloys and identify their appropriate composition for effective water splitting application. The Mn_1- Zn O thin films were synthesized using combinatorial pulsed laser deposition, permitting for characterization of a wide range of compositions with x varying from 0 to 1. The solubility limit of ZnO in MnO was determined using the disappearing phase method from X-ray diffraction and X-ray fluorescence data and found to increase with decreasing substrate temperature due to kinetic limitations of the thin-film growth at relatively low temperature. Optical measurements indicate the strong reduction of the optical band gap down to 2.1 eV at x = 0.5 associated with the rock salt-to-wurtzite structural transition in Mn_1- Zn O alloys. Transmission electron microscopy results show evidence of a homogeneous wurtzite alloy system for a broad range of Mn_1- Zn O compositions above x = 0.4. The wurtzite Mn_1- Zn_xO samples with the 0.4 < x < 0.6 range were studied as anodes for photoelectrochemical water splitting, with a maximum current density of 340 μA cm^-2 for 673 nm-thick films. These Mn_1- Zn O films were stable in pH = 10, showing no evidence of photocorrosion or degradation after 24 h under water oxidation conditions. Doping Mn_1- Zn O materials with Ga dramatically increases the electrical conductivity of Mn_1- Zn O up to ∼1.9 S/cm for x = 0.48, but these doped samples are not active in water splitting. Mott-Schottky and UPS/XPS measurements show that the presence of dopant atoms reduces the space charge region and increases the number of mid-gap surface states. Overall, this study demonstrates that Mn_1- Zn O alloys hold promise for photoelectrochemical water splitting, which could be enhanced with further tailoring of their electronic properties.