Polycrystalline Doped Hematite

The most frequently used substances for n-doping of hematite are oxides of silicon and titanium. Photo-oxidation of water at Si- and Ti-doped hematite has accordingly been studied extensively [1, 6, 7, 12, 18, 20, 21, 61]. In general, the conductivity is improved by these dopants. However, there seems to be a limit for the improvement by doping. For example, concentrations over 0.5 atom percent of titanium in hematite have been stated to have no effect on the resistivity [7]. 

Iodine-doped hematite has also been studied. The iodine-doped thin films of iron oxide were obtained by spray pyrolysis. The condition for the preparation was not described in detail, apart that a mixture of an 80% ethanolic solution of 0.01 M iodine and 0.1 M FeCl3 was used. Undoped films of 50 nm film thickness showed a maximum photocurrent density of 1.1 mA/cm2, while a 100 nm thick iodine-doped films had a maximum photocurrent density of 5 mA/cm2 at 0.82 V vs. NHE at pH 13 [37]. These measurements were performed with a xenon lamp with a light irradiation of only 150 mW/cm2. At the same condition, it was suggested by model calculation that an optimized stack of five iodine-doped hematite electrodes was expected to yield a photocurrent density of 15 mA/cm2. 

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. 

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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... 

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