Bulk water-semiconductor interface

Pant and Levinger have measured the solvation dynamics of water at the surface of semiconductor nanoparticles [48,49]. In this work, nanoparticulate Zr02 was used as a model for the Ti02 used in dye-sensitized solar photochemical cells. Here, the solvation dynamics for H2O and D2O at the nanoparticle surface are as fast or faster than bulk water motion. This is interpreted as evidence for reduced hydrogen bonding at the particle interface. 

In all the studies mentioned, the optical effect caused by the accumulated products of any electrode reactions (formally, by the appearance of an ultrathin film at the interface) was ignored. This effect produces the baseline distortions in the regions of the absorption bands of the solvent (Fig. 3.27). Using the air-water interface as an example, Buffeteau et al. [247] demonstrated that to reproduce these distortions correctly, the optical constants of interfacial rather than bulk water should be used in the spectrum simulations. As underlined by Chazalviel et al. [146, 147] for ATR spectra of a film at the semiconductor-water interface, the uncompensated absorption of the solvent due to increasing the film thickness can be either negative or positive. If the film is compact and has a low refractive index (this is... 

It is generally accepted that three major processes limit the photoelectrochemical current in semiconductors after a bandgap excitation [76]. These processes are schematically illustrated in the band diagram shown in Fig. 3.2. The bold arrows show the desired processes for efficient water splitting PEC cell after a bandgap excitation the transport of electrons to the back contact, the transfer of the hole to the semiconductor surface and the oxidation of water at the semiconductor/electrolyte interface. The three major limiting processes are a) bulk recombination via bandgap states, or b) directly electron loss to holes in the... 

It is important to determine the conductivity and flat-band potential ( ft) of a photoelectrode before carrying out any photoelectrochemical experiments. These properties help to elucidate the band structure of a semiconductor which ultimately determines its ability to drive efficient water splitting. Photoanodes (n-type conductivity) drive the oxygen evolution reaction (OER) at the electrode-electrolyte interface, while photocathodes (p-type conductivity) drive the hydrogen evolution reaction (HER). The conductivity type is determined from the direction of the shift in the open circuit potential upon illumination. Illuminating the electrode surface will shift the Fermi level of the bulk (measured potential) towards more anodic potentials for a p-type material and towards more cathodic potentials for a n-type material. The conductivity type is also used to determine the potential ranges for three-electrode j-V measurements (see section Three-Electrode J-V and Photocurrent Onset ) and type of suitable electrolyte solutions (see section Cell Setup and Connections for Three- and Two-Electrode Configurations ) used for the electrochemical analyses. 

The development of photocathode materials for either single- or dual-absorber cells has also received considerable attention [80, 101, 102]. Thermodynamic equilibrium dictates that p-type semiconductors will exhibit upward band bending when in contact with a liquid electrolyte. This behaviour is the opposite to that of n-type semiconductors described previously, and will result in the movement of photogenerated electrons towards the semiconductor-electrolyte interface while the holes are driven into the bulk of the electrode, towards the electrical back contact. At the surface, provided that the energy carried by the electrons is sufficient, H2 is evolved. As discussed previously, one of the electronic properties of metal oxides that makes them suitable for water photo-oxidation purposes is the O 2p character of the valence electrons, which places the VB edge at potentials... 

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