High-Throughput Experimental Study of Wurtzite Mn1-xZnxO Alloys for Water Splitting Applications

Paul Ndione, Emily Warren, Stephan Lany, Mowafak Al-Jassim, Todd Deutsch, Aaron Macomber, David Ginley, Brian Gorman, Erin Ratcliff, Suhash Dey, Aaron Holder

Research output: Contribution to journalArticlepeer-review

5 Scopus Citations

Abstract

We used high-throughput experimental screening methods to unveil the physical and chemical properties of Mn1-xZnxO wurtzite alloys and identify their appropriate composition for effective water splitting application. The Mn1-xZnxO 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 Mn1-xZnxO alloys. Transmission electron microscopy results show evidence of a homogeneous wurtzite alloy system for a broad range of Mn1-xZnxO compositions above x = 0.4. The wurtzite Mn1-xZnxO samples with the 0.4 < x < 0.6 range were studied as anodes for photoelectrochemical water splitting, with a maximum current density of 340 uA cm-2 for 673 nm-thick films. These Mn1-xZnxO films were stable in pH = 10, showing no evidence of photocorrosion or degradation after 24 h under water oxidation conditions. Doping Mn1-xZnxO materials with Ga dramatically increases the electrical conductivity of Mn1-xZnxO 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 Mn1-xZnxO alloys hold promise for photoelectrochemical water splitting, which could be enhanced with further tailoring of their electronic properties.
Original languageAmerican English
Pages (from-to)7436-7447
Number of pages12
JournalACS Omega
Volume4
Issue number4
DOIs
StatePublished - 2019

NREL Publication Number

  • NREL/JA-5900-73457

Keywords

  • analytical chemistry
  • catalysts
  • combinatorial chemistry
  • crystal structure
  • electric properties
  • energy level
  • phase
  • phase transition
  • semiconductors
  • solid state electrochemistry
  • solubility
  • spectra

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