Hematite Single Crystal

It can be concluded that single crystals of hematite generally exhibit low current response and high resistivity, which make them a less attractive candidate for photo- 

---

Aust. J. Soil Res. 23 543-567 Flanders, P.J. Remeika, J.P. (1964) Magnetic properties of hematite single crystals. Phil. 

Fig. 4.3 Challenges presented by hematite at left is an energy diagram showing a typical value of the Hatband potential, Vp, of n-type hematite, and the PEC water-splitting operation of a hematite photoanode under illumination and with an applied external bias, Vf,. The right graph (from [53], with permission) shows the quantum efficiency vs. wavelength for Nb-doped and Ge-doped hematite single crystals at 0 V vs. SCE in 1 M NaOH (1.06 V vs. RHE). The efficiency is expressed in units of percent...

The structures of iron oxides have been determined principally by single crystal X-ray diffraction or neutron diffraction with supplementary information coming from infrared spectroscopy, electron diffraction and high resolution electron microscopy. A few years after the first successful application of X-ray diffraction to crystal structure determination, this technique was used to establish the major features of the structures of magnetite (Bragg, 1915 Nishikawa, 1915) and hematite (Bragg Bragg, 1918). 

Low levels of structural Ge" have also been observed in natural hematite from the Apex mine, Utah (Bernstein Waychunas, 1987) and to achieve charge balance, incorporation of two Fe for one Ge", i.e. similar to the two Fe" for one in ilme-nite, has been suggested. Synthetic, single crystals of Ge substituted hematite have also been grown by a chemical vapour transport method (Sieber et al. 1985). A range of elements including Zr, Ge, Hf, V, Nb, Ta, W and Pb has been used as low level dopants (2 10 - 0.2 g kg ) to improve the semiconductor behaviour of hematite anodes (Anderman Kermedy, 1988). The increase in unit cell c from 1.3760 to 1.3791 nm and in a from 0.50378 to 0.50433 nm indicated that Nd (as an inactive model for trivalent actinides of similar ionic size (Am r = 0.0983 nm Nd " r = 0.098 nm)) was incorporated in the structure (Nagano et al. 1999). 

Single crystals from 50-800 pm across can be synthesised by chemical vapour transport. Hematite powder is held for on week in a silica tube with a temperature gradient of 1000-850 °C in an atmosphere of HCl(5.3kPa)/02 (133 Pa) (Moukassi et al., 1984). 

Figure 2.1. Various forms exhibited by crystals, (a) Polyhedral crystals (b) hopper crystal (c) dendritic crystal (snow crystal, photographed by the late T. Kobayashi) (d) step pattern observed on a hematite crystal (0001) face (e) internal texture of a single crystal (diamond-cut stone, X-ray topograph taken by T.Yasuda) (f) synthetic single crystal boule. Si grown by the Czochralski method (g) synthetic corundum grown by the Verneuil method.

Figure 28.5 High-magnification TEM image of a-Fe203 single crystal. Insets show lattice fringes of the (012) plane of hematite and the corresponding single crystal diffraction pattern.

The phase transformations in the catalyst play an important role in determining the activity, attrition resistance, and deactivation of this catalyst. Activation of this precipitated catalyst transforms single crystals of hematite to smaller crystallites of carbide. While the transformation from hematite to magnetite is extremely rapid, the magnetite to carbide transition is much slower under the conditions of temperature and pressure employed in this study. As carbon deposits on the carbide particles, it serves to further prise the carbide particles apart. In a commercial slurry phase reactor the carbide particles break away leading to catalyst attrition. The implication of this work for the attrition resistance of iron FT catalysts is explored in detail elsewhere.18... 

In general, it is accepted that recombination of electrons and holes, trapping of electrons by oxygen deficiency sites and a low mobility of the holes, cause a low conductivity and accordingly a low photoresponse for hematite. Electron mobility in the range 0.01 [60] to 0.1 cm2/V-s [17] has been reported. In the latter case, it was found that the electron mobility was independent of donor concentration. More recently, an electron mobility of about 0.1 cm2/V-s has been measured with doped single crystals and the mobility was also here independent of donor concentration [5]. A diffusion length of holes has been determined to be only of 2-4 nm [6], which is about 100 times lower than many other (III-V) oxides. 

---