Electron Energy Bands of Liquid Water

SoUd ice forms a crystal of diamond structure, in which one water molecule is hydrogen-bonded with four adjacent water molecules. Most (85%) of the hydrogen bonds remain even after solid ice melts into liquid water. The structure of electron energy bands of liquid water (hydrogen oxide) is basically similar to that of metal oxides, 6dthough the band edges are indefinite due to its amorphous structure. 

The conduction band electron of liquid water, which cotild be produced by photoelectron emission fi m metal electrodes, is very unstable with its lifetime being 10 seconds it is readily captured by water molecules to form a hydrated electron. The hydrated electron is also very unstable being rapidly absorbed in electron scavenger particles such as H3O, NO 2 and O2. The level of the hydrated electron has been estimated at 0.3 to 0.5 eV below the conduction band edge [Battisi-Trasatti, 1977 Watanabe-Gerischer, 1981]. 

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Figure 2-34 shows the diagram of electron energy bands of liquid water. As in the case of metal oxides, the oxygen 2p orbital constitutes the valence band. 

Fig. 2-34. Electron energy bands of liquid water formed from atomic orbitals of hydrogen and oxygen atoms.

FIGURE 22.1 Electron energy bands of liquid water made of atomic orbitals of hydrogen and oxygen [5] CB = conduction band, VB = valence band, sc > —1.2 eV, gv < —9.3 eV, gg > 8 eV. 

FIGURE 13.3 Energy levels of the OHVOH" couple aligned with the band edges of liquid water. The data for the vertical IP of OH", electron affinity EA of OH, VBM, and CBM of water are from Table 13.3. The OH reduction potential (AA) is taken from Table 13.2. All levels, except the CBM, of water are computed from the total energy differences. 

Ethylene glycol is a very viscous liquid and the molecule presents two close OH groups. It has to be noticed that, among all the different solvents studied by pulse radiolysis, the transition energy of the solvated electron absorption band is maximum in liquid ethylene glycol. For these reasons, the electron in EG seems to have a special behaviour and it is of great interest to study the dynamics of the formation of equilibrated solvated electron. Within this context, the present communication deals with the dynamics of solvation in EG of electrons produced by photoionisation of the solvent at 263 nm. The formation of solvated electrons is followed by pump-probe transient absorption spectroscopy in the visible spectral range from 425 to 725 nm and also in near IR. For the first time, the absorption spectrum of the precursor of the equilibrated electron is observed in EG. Our results are shortly compared by those obtained in water and methanol. 

A water-splitting device has been invented [4], where photo-semiconductor and platinum are used as the cathode and the anode, respectively, instead of setting both the solar cell and the electrolyzer, separately. This method is called photoelectrochemical (PEC) water-splitting or photo semiconductor electrode method . The key phenomenon of PEC watersplitting is the steep rise (fall) of the potential at the interface between the n-(p-) semiconductor and the liquid electrolyte (e.g., KOH). If photons irradiate onto the interface, both the electrons (e ) and positive holes (IT) are excited to their conductive energy bands where they can move freely, so that e and h+ are separated by the interface potential difference. The h+ react with water by the equation ... 

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