Electronic structures of silver oxides

The oxides of silver have a number of important technological applications, including use in battery technology, catalysis, and in the treatment of dermatological conditions. However, only the Ag${}_2$O phase has been well characterized in previous work. To this end, this paper characterizes the electronic structures of the major oxide forms, namely, Ag${}_2$O, Ag${}_2$O${}_3$, and AgO, using standard density functional theory (DFT), Hartree-Fock, and hybrid-DFT approaches. The optical properties are also assessed for these materials, enabling comparisons to be drawn to experiment and the origin of optical band gaps to be explained. The calculated optical gaps also suggest that electrodeposited Ag${}_2$O films may not consist of pure material. Hartree-Fock calculations are seen to fail to model Ag(I) species correctly, due to the neglect of correlation. DFT and hybrid-DFT methods are seen to perform better, but problems due to the lack of van der Waals interactions are identified. Preliminary calculations using empirical dispersion corrections are discussed.

Figure 1

Atomic structure of (a) the unit cell of Ag${}_2$O and (b) the coordination environment of Ag(I) ions, with experimental bond lengths (Ref. 5). Silver (I) and oxygen atoms are colored green (light gray) and red (dark gray), respectively.

Figure 2

Calculated band structure for Ag${}_2$O using the (a) GGA and (b) HSE06 functional. The top of the valence band in set to $0\mathrm{eV}$, or, in the case of the GGA functional, the Fermi energy.

Figure 3

HSE06-calculated electronic density of states for Ag${}_2$O. (a) Total EDOS, (b) Ag 5s and 5p PEDOS, (c) Ag 4d PEDOS, (d) Ag 4d PEDOS (multiplied to show the CB features), and (e) O PEDOS. The s, p, and d states are colored blue (dashed-dotted), red (dashed), and green (solid), respectively. The Fermi energy is represented by the vertical solid gray line, which has been set to 0 eV.

Figure 4

Atomic structure of (a) the unit cell of Ag${}_2$O${}_3$ and (b) the coordination environment around a silver atom, with bond distances from the experimental structure (Ref. 7). Silver (III) atoms are colored light blue (light gray) and oxygen atoms are colored red (dark gray).

Figure 5

Calculated band structure for Ag${}_2$O${}_3$ using the (a) GGA and (b) HSE06 functional. The top of the valence band in set to $0\mathrm{eV}$.

Figure 6

HSE06-calculated electronic density of states for Ag${}_2$O${}_3$. (a) Total EDOS, (b) Ag 5s and 5p PEDOS, (c) Ag 4d PEDOS, (d) O${}_1$ PEDOS, and (e) O${}_2$ PEDOS, where O${}_1$ is a combination O${}_{1a}$, O${}_{1b}$, and O${}_{1c}$ states. The s, p, and d states are colored blue (dashed-dotted), red (dashed), and green (solid), respectively. The Fermi energy is represented by the vertical solid gray line, which has been set to 0 eV.

Figure 7

Atomic structure of (a) the unit cell of AgO, grown $2\times 2\times 2$ for clarity, and (b) the coordination environment around an Ag(III) ion, with corresponding bond lengths. Green (medium gray), light blue (light gray), and red (dark gray) spheres represent the Ag(I), Ag(III), and oxygen atoms, respectively. See text for details of bond angles around the Ag(III) ion.

Figure 8

Schematic showing (a) the minimized unit cell structure of AgO as predicted using a GGA method, grown $1\times 2\times 2$ for clarity. Ag ions are colored purple (medium gray) and O ions are red (dark gray). The bonding environment of the silver ions, with corresponding bond lengths, is detailed in (b), while (c) shows the corresponding bonding environment in the experimental structure of Cu(II)O (Ref. 99), where copper ions are colored yellow (light gray).

Figure 9

Energy-volume curves for AgO. The system was minimized as two different structures, namely, the experimental (Ref. 9) Ag(I,III)O and GGA-predicted Ag(II)O structures. These are indicated by the black (solid circles) and red (triangles) lines, respectively. Dashed lines (open circles) represent calculations where the unit-cell dimensions and shapes were held fixed as full minimization resulted in a structural change. The simulations were trialled with GGA, GGA + U (U varied from 1 to $7\mathrm{eV}$), and HSE06 methodologies.

Figure 10

Calculated band structure for AgO using the (a) GGA and (b) HSE06 functional. The top of the valence band in set to $0\mathrm{eV}$, or, for GGA, the Fermi energy.

Figure 11

HSE06-calculated electronic density of states for AgO. (a) Total EDOS, (b) Ag(I) 5s and 5p PEDOS, (c) Ag(I) 4d PEDOS, (d) Ag(III) 5s and 5p PEDOS, (e) Ag(III) 4d PEDOS, and (f) O PEDOS. The s, p, and d states are colored blue (dashed-dotted), red (dashed), and green (solid), respectively. The Fermi energy is represented by the vertical solid gray line, which has been set to 0 eV.

Figure 12

HSE06-calculated absorption spectra ($\alpha$h$\nu$)${}^2$ for AgO (black/dashed-dotted), Ag${}_2$O${}_3$ (blue/dashed), and Ag${}_2$O (red/solid).