Polycrystalline Hematite

Fig. 3.4. pH dependence of on-set potential, Uon and flat-band potential, Up, vs. NHE for various hematite materials as collected from Table 2. The solid line, with a Nemstian slope of -59 mV/pH-unit, shows the potential for hydrogen generation as a function of pH. Ebias stands for the bias, which at least is needed at pH 10 to photogenerate hydrogen at the polycrystalline hematite electrode described by McGregor et al [7]. 

It can be concluded that doped polycrystalline hematite indeed can be interesting for photoelectrolysis of water. The studied area has been intensively researched and some of the findings represent promising photoelectrochemical properties. In combination with the ease of preparation and good stability towards photocorrosion this make doped polycrystalline electrodes of hematite an interesting material for direct conversion of solar energy into dihydrogen. 

The earliest work by Morin on pure sintered polycrystalline hematite describes very low electrical conductivities (ca. 10 " cm ) [36], conduction electron... 

As a result, nanometer-sized metallic Au particles were selectively deposited onto monodispersed polycrystalline ellipsoidal hematite particles without addition of any specific reducing agent, as shown in Figure 5. 

Figure 6 shows transmission electron micrographs of Au particles supported by (a) monocrystalline ellipsoidal (B), (b) monocrystalline pseudocubic, and (c) monocrystalline platelet-type hematite particles (see also Figure 5 for Au particles on polycrystalline ellipsoidal (A) particles). Figure 7 shows Au particles deposited on (a) a-FeOOH, (b) P-FeOOH, (c) ZrOj (A), (d) ZrOj (B), and (e) Ti02 (anatase). 

Figure 5. Transmission electron micrographs of Au nanoclusters deposited on the surfaces of the polycrystalline ellipsoidal hematite particles. The left photograph is a close-up view of the right one.

Finally, based on the lifetime and charge carrier mobility found in hematite, the diffusion length of excited carriers is expected to be small. Kennedy and Frese calculated the diffusion length for holes in n-type (Ti doped) polycrystalline samples using the experimental photocurrent efficiency and a band-bending... 

It is clear, by looking at Table 4.1, that while many methods of stmcturing hematite have been reported, the IPCE values at ca. 400 nm rarely come close that of the single crystal or sintered polycrystalline electrodes prepared during the first... 

Photolysis in ice has a role also for inorganic compounds. As an example, it was found that the photoreductive dissolution of iron oxide particles to form bioavailable iron (Fe(II)aq) was slow in aqueous solution (pH 3.5) but was significantly accelerated in polycrystalline ice. This occurred independently on the irradiation wavelength and on the type of oxides [hematite. 

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