Hematite preparation

It is suggested that P occupies an empty tetrahedral position. Yubero et al. (2000) claim, on the basis of Rutherford backscattering spectroscopy, that up to 0.04 mol mol of Ar could be incorporated in the structure of hematite prepared from Fe(CO)s by ion beam assisted deposition in the presence of 0 and Ar", followed by annealing at 500 °C. 

J. Colloid Interface Sci. 109 219-228 Van San, E. De Grave, E. Vandenberghe, R.E. Desseyn, H.O. Datas, L. Barron,V. Rousset, A. (2001) Study of Al-substituted hematites, prepared from thermal treatment... 

Table A3.1 Hematite preparation procedures and colloid characteristics, ferric chloride floe characteristics for comparison. ...

Sivula K, Zboril R, Le Formal F et al (2010) Photoelectrochemical water splitting with mesoporous hematite prepared by a solution-based colloidal approach. J Am Chem Soc 132 7436-7444... 

Sivula K., Zboril R., Le Formal F., Robert R., Weidenkaff A., Tucek J., Frydrych J. and Gral tzel M. (2010). Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach. Journal of the American Chemical Society, 132(21), 7436-7444. 

Fig. 4.5 Examples of hematite nanowire morphologies SEM images of hematite prepared by the thermal oxidation method left) from [78] and aqueous method (right) from [77] used with permission...

In general, it is clear that major advances have been made recently in the nanostructuring of hematite by electrochemical methods. However, the potential to oxidize water with hematite prepared in the methods described above has been limited by the crystallinity and ultimate feature size after the requisite annealing. 

The first surface challenge can be addressed by strong oxidation conditions [46] and careful hematite preparation, but the slow water oxidation kinetics are probably intrinsic to hematite. Nevertheless, methods have recently been found to increase the oxidation rate and thus reduce the overpotential. For example, the water oxidation by cobalt has been extensively studied and is known to be particularly rapid [114]. Indeed the treatment of Fe203 photoanodes (prepared by APCVD) with a monolayer of Co " resulted in a ca. 0.1 V reduction of the photocurrent onset potential [105]. Since this treatment also increased the plateau photocurrent it is good evidence that the reaction rate was increased, and the Co " did not just fill surface traps. Following the report of a remarkably effective cobalt-phosphate (Co-Pi)- based water oxidation catalyst [115], the overpotential was reduced even further on hematite photoanodes by Gamelin and coworkers [116]. Their results are shown in Fig. 4.11. 

This was observed to occur at similar temperatures of 800°C. In addition, nanoparticles of hematite prepared via an aqueous precipitation have shown a size and temperature-dependent concentration of spinel defects at the surface. The formation of these defects on hematite was shown to be governed by the thermodynamics of the ly-y phase transition with decreasing particle size [121], Raman spectroscopy has shown that similar defects on the surface of highly efficient anodes prepared by APCVD are important to the performance [13]. 

Similarly as for goethite, a linear relationship of R as a function of A1 content and particle size has been proposed [ 102] which is valid at RT and for concentrations less than 10 %A1. However, all these results are derived from synthetic samples, mostly obtained from goethite. Therefore, particularly at RT, the magnetic hyperfine field will still be largely influenced by morphological effects. Moreover, most preparation methods, based on the decomposition of oxyhydroxides, result in inhomogeneous A1 substitution [96]. A more clear-cut picture for the dependence of the hyperfine field on A1 substitution is obtained for hematites prepared from oxinates [103] where a reduction of 0.061 at RT and 0.032 at 80 K per at % A1 is observed. 

G.M. da Costa, E. Van San, E. De Grave, R.E. Vandenberghe, V. Barron, L. Datas, A1 hematites prepared by homogeneous precipitation of oxinates material characterization and determination of the Morin transition. Phys. Chem. Miner. 29, 122-131 (2002)... 

Figure 2. Mossbauer spectra of lanthanum-doped hematite prepared witli ammonium hydroxide (AH), sodium hydroxide (NH) and sodium carbonate (NC)...

The influence of the preparation method on the properties of lanthanum-doped hematite prepared with potassium carbonate was studied aiming to get catalysts for styrene production from ethylbenzene dehydrogenation. The most active catalyst was obtained by adding the reactants on water. This solid has the highest intrinsic activity, the highest resistance against reduction and the lowest conversion drop. These properties were related to the presence of potassium eompounds on the surface and to the presence of Fe species (active phase) stabilized in lanthanum oxide lattice. 

---