Fabrication of pristine Mn₂O₃ and Ag-Mn₂O₃ composite thin films by AACVD for photoelectrochemical water splitting

Pristine Mn₂O₃ and Ag-Mn₂O₃ composite thin films have been developed on fluorine doped tin oxide (FTO) coated glass substrates at 450 °C by aerosol assisted chemical vapor deposition (AACVD) using a methanol solution of a 1:2 mixture of acetatoargentate(i), Ag(CH₃COO), and a newly synthesized manganese complex, [Mn(dmae)₂(TFA)₄] (1) (dmae = N,N-dimethylaminoethanolate, TFA = trifluoroacetate). The phase purity and stoichiometric composition of the films were investigated by X-ray diffraction (XRD) and Raman spectroscopy techniques. Energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) analyses revealed a Ag to Mn ratio of 1:2 and further confirmed the uniform dispersion of Ag nanoparticles into the Mn₂O₃ structure. Optical studies showed a direct band gap of 2.0 eV for the pristine Mn₂O₃ film that was lowered to 1.8 eV for Ag-Mn₂O₃ due to the plasmonic interaction of Ag with Mn₂O₃. The Ag-Mn₂O₃ composite film displayed enhanced photocatalytic activity in photoelectrochemical (PEC) water splitting and yielded a photocurrent of 3 mA cm^-2 at 0.7 V versus Ag/AgCl which was 1.6 times higher than a pristine Mn₂O₃ film alone, under AM 1.5 G illumination (100 mW cm^-2). The high PEC efficiency is mainly due to the plasmonic effect of Ag nanoparticles, which enhances the visible light absorption, efficient electron-hole separation and high carrier mobility of the Ag-Mn₂O₃ photoelectrode. The charge carrier density of Ag-Mn₂O₃ is two times higher than the pristine Mn₂O₃ as calculated by the Mott-Schottky plot. Based on the PEC studies a mechanism is proposed to elucidate the high activity of Ag-Mn₂O₃ in PEC water splitting.