Optical excitations in MnO and MnO:ZnO via embedded CASPT2 theory and their implications for solar energy conversion

MnO:ZnO was recently proposed as a novel, potentially visible-light absorbing catalyst with many advantages for water splitting. However, little is known about its optical absorption properties and whether they are suited for solar applications. We address this issue by examining excitations in MnO:ZnO and pure MnO using electrostatically embedded cluster models treated with complete active space second order perturbation (CASPT2) theory. The lowest-lying excitations in MnO are single Mn d → d ligand field excitations (∼2.6 eV, ∼10⁸ s lifetime), followed by double d → d excitations (∼5.2 eV, ∼10⁶ s lifetime), Mn 3d → 4s excitations (∼6.3 eV, ∼10^-3 s lifetime), and higher-lying O 2p → Mn 3d ligand-to-metal charge-transfer (LMCT) excitations (∼10.1 eV, ∼10^-4 s lifetime). The longer-lived transitions should exhibit better electron-hole pair separation and enhance photoconductivity depending on ease of carrier transport. Alloying MnO with ZnO notably lowers the LMCT transition to ∼8.3 eV with the metal orbital comprised mainly of Zn 4s states, leaving most other absorption properties relatively unchanged. We find near degeneracies among spin-allowed and spin-forbidden LMCT states that could facilitate intersystem crossing resulting in longer lifetimes. We suggest seeking other materials that exhibit similar LMCT excitations but that are visible-light activated as a design strategy for further enhancing photon conversion efficiencies.