High-Throughput Experimental Study of Wurtzite Mn(1–x)Zn(x)O Alloys for Water Splitting Applications

[Image: see text] We used high-throughput experimental screening methods to unveil the physical and chemical properties of Mn(1–x)Zn(x)O wurtzite alloys and identify their appropriate composition for effective water splitting application. The Mn(1–x)Zn(x)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–x)Zn(x)O alloys. Transmission electron microscopy results show evidence of a homogeneous wurtzite alloy system for a broad range of Mn(1–x)Zn(x)O compositions above x = 0.4. The wurtzite Mn(1–x)Zn(x)O 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–x)Zn(x)O films were stable in pH = 10, showing no evidence of photocorrosion or degradation after 24 h under water oxidation conditions. Doping Mn(1–x)Zn(x)O materials with Ga dramatically increases the electrical conductivity of Mn(1–x)Zn(x)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–x)Zn(x)O alloys hold promise for photoelectrochemical water splitting, which could be enhanced with further tailoring of their electronic properties.