Band Gap Engineering of MnO via ZnO Alloying: A Potential New Visible-Light Photocatalyst

Abstract

Practical implementation of solar-powered water splitting and CO 2 reduction to fuels requires the discovery of efficient and inexpensive photocatalytic (PC) materials. One possible materials design strategy aims to tune properties of relatively inexpensive transition metal oxide catalysts to increase sunlight absorption while preserving potential redox reactivity.Here we consider MnO for PC use by alloying it with ZnO in solid solutions. A combined density functional theory and GW scheme is used to study the band gap and band edge placements as a function of composition. We predict that alloying MnO with ZnO in varying amounts reduces MnO's band gap for more efficient light absorption while maintaining advantageous band edge placements. The 1:1 alloy of MnO and ZnO is identified as a new (2.6 eV band gap) visible-light-absorbing material with band edges suitably placed with respect to both water-oxidation and CO 2 -reduction reactions, making it a compelling candidate for solar PC chemistry.

Design of Semiconducting TetrahedralMn1−xZnxOAlloys and Their Application to Solar Water Splitting

“…This finding suggests that the Mn 1−x Zn x O alloys have the potential for optimizing the tradeoff between the magnitude of the band gap and the alignment of the band edges with the redox potentials. A recent theoretical study [12] also arrived at this conclusion, albeit for the octahedral rocksalt structure of Mn 1−x Zn x O, which was shown here to be unstable in the composition range of interest and which would not benefit from the improved optical, transport, and doping properties present in the tetrahedral wurtzite structure.…”

“…There is a body of literature on thin-film growth of Mn-doped ZnO by pulsed laser deposition, and Mn 1−x Zn x O alloys in the Zn-rich composition range have attracted much interest as diluted magnetic oxides, with focus on the magnetic properties of Mn impurities in ZnO [10]. However, there are no experimental reports of alloys in the Mn-rich composition range, where the structural transition between the octahedral rocksalt structure (MnO ground state) and the tetrahedral wurtzite structure (ZnO ground state) is expected [11], and previous theoretical investigations for such alloys were restricted to the RS structure [12,13]. The alloy composition range of interest lies inside the miscibility gap [14], and the properties of such alloys are unknown.…”

Modelling materials for solar fuel synthesis by artificial photosynthesis; predicting the optical, electronic and redox properties of photocatalysts

“…Vertical potentials, based on approach I or II, are routinely reported in the literature [33,[89][90][91][92][93][94][95][96][97][98][99]. See the work of Stevanovic et al [89] for a nice illustration for the case of GW vertical potentials for the surfaces of typical crystalline photocatalysts, which also clearly illustrates that potentials are surface rather than bulk properties (see figure 3).…”

Phase stability and properties of manganese oxide polymorphs: Assessment and insights from diffusion Monte Carlo

“…The presence of electron correlations also render transition metal oxides very challenging to model from first principles. One classic example of a correlated metal oxide is manganese oxide (MnO), which is of interest for several potential applications including solar energy conversion 7 , photoelectrochemical water splitting 8,9 , and as a magneto-piezoelectric semiconductor 10 . While the ground state of MnO is rock salt (RS), a few years ago metastable wurtzite (WZ) was grown by thermal decomposition on a carbon template 11 .…”