Hematite electrode

Rapid oxidation of polished samples of iron in an air/acetylene flame produces a hematite coating of 50-100 um in thickness for use as a hematite electrode (Curran... 

Doped hematite electrodes are produced by mixing hematite (99.998 purity) with T1O2 of Sn02 of the same purity, in acetone, evaporating the mixture at room temperature and compressing 0.5 g of the hematite powder in a die at 20-34 MPa the resulting pellet is sintered at 1350 °C for 5-6 hr (Balko and Clarkson, 2001). 

Balko, B.A. Clarkson, K.M. (2001) The effect of doping with Ti(lV) and Sn(IV) on oxygen reduction at hematite electrodes. J. Electro-chem. Soc. 148 E85-E91 Balkwill, D. Maratea, D. Blakemore, R.P. (1980) Ultrastructure of a magnetotactic spirillum. J. Bacteriol. 141 1399-1408 Ballko, B.A. Tratnyek, P.G. (1998) Photoeffects on the reduction of carbon tetrachloride by zero-valent iron. J. Phys. Chem. B 102 1459-1465... 

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]. 

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. 

In a porous film consisting of interconnected nanometer sized semiconductor particles the effective surface area can be enhanced 1000-fold [121]. Therefore, nanostructured electrodes can be good for unravel the surface phenomena. By scrutinizing the effect of iodine on the performance of the electrode it was concluded that the effect of surface states was small in the nanostructured hematite electrode. It was stated that the bulk and grain boundary recombination remained dominant. This is in consistency with the report from Cherepy et al [43]. 

The surface charging of materials that show a certain degree of electric conductivity can be measured directly by electrical methods. Such measurements are not used to determine the PZC. The potential of a hematite electrode prepared as a coating on Pt [243] was Nemstian (59 mV/pH unit) at 20°C in 0.005 M KCl. Also, a monocrystalline hematite electrode [644] had a nearly Nemstian potential in the acidic range in 0.0005 M NaNO, but the slope was lower in the basic range and... 

Fig. 4.4 Scanning electron micrograph (left) of the edge of a porous electrode made from sintered Fc203 sols on a conducting cassiterite (Sn02) support (magnification 5,800 times). Absorption spectrum of the hematite electrode and quantum efficiency as a function of wavelength (right) obtained in 0.1 M NaOH at 1.4 V vs. RHE when illuminating the electrode through the substrate (SE) and directly onto the interface with the electrolyte (EE). Note that the scale of the IPCE values differs with a factor of 100 in the two cases. From [67] used with permission...

Lindgren, T., Vayssieres, L., Wang, H., Lindquist, S.E. Photo-oxidation of water at hematite electrodes. In Kokorin, A.I., Bahnemann, D.W. (eds.) Chemical Physics of Nanostructured Semiconductors, pp. 83-103. VSP International Science Publishers, The Netherlands (2003)... 

Values of kf and obtained using low-intensity small-amplitude intensity modulation (IMPS) are illustrated in Figure 18.6 for a hematite electrode that had been treated with a cobalt(II) solution to enhance performance [26]. It can be seen that kf is indeed of the order of 10 s and it is almost independent of potential k, by contrast, changes by over three orders of magnitude as the potential is made more positive. The crossover point of the two plots is found to correspond to the half-way point on the rising photocurrent-voltage curve, as predicted by Equation (18.17). Interestingly, however, the potential dependence... 

Brillet, J., Gratzel, M. and Sivula, K. (2010) Decoupling feature size and functionality in solution-processes porous hematite electrodes for water splitting. Nano Letters, 10, 4155 160. 

Meitl LA, Eggleston CM, Colberg PJS, Khare N, Reardon CL, Shi L. Electrochemical interaction of Shewanella oneidensis MR-1 and its outer membrane cytochromes QmcA and MtrC with hematite electrodes. Geochim Cosmochim Acta 2009 73 5292-5307. 

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