Photoelectrolysis of Water with Natural Mineral TiO2 Rutile Electrodes

Abstract

Natural mineral rutile (TiO2) was studied as anode for photoelectrolysis of water. Both its photoelectrochemical behavior and semiconducting properties are examined in comparison with those of Fe-doped synthetic single crystal TiO2, the latter independently studied to simulate the high impurity content existent in the mineral. The natural and synthetic Fe-doped TiO2 electrodes showed a performance comparable to that of arc-plasma sprayed raw rutile, including a low breakdown potential. Flatband potentials and donor densities were determined from Mott-Schottky plots and evidence for deep and shallow donors is discussed. Photoresponse measurements allowed us to verify the influence of the iron impurities on the performance of such electrodes.The photoelectrolysis of water has been investigated by several authors (1-7) in the last few years. Electrodes of polyerystalline TiO2 made by hot-pressing powdered rutile (2), by oxidation of metal foils (2-4), and by chemical vapor deposition (CVD) (5, 6) show a behavior similar to that of single crystal TiO., ( 7) and have the advantage of being easier to prepare. Until now, little attention has been paid to the semiconductor properties of natural TiO2 rutile crystals, perhaps because these minerals are usually impure (8). However, it has been recently shown that impure polycrystalline TiO2 electrodes, prepared by arcplasma sprayed raw rutile powder (9), behave similarly to other polycrystalline ones with respect to photoeleetrolysis. In the present paper we report the photoelectrochemical behavior of electrodes made either of single crystal or polycrystalline natural TiO2, obtained from mines located in different regions of Brazil. These electrodes showed a performance comparable to that of synthetic single crystal or other polycrystalline TiO2 electrodes. The results demonstrate the possibility of using low cost raw materials in primitive form for photoelectrolysis of water.Since large amounts of iron impurities were detected by x-ray analysis in the mineral samples, their effect on the performance of the photoelectrolytic cell is discussed by comparing the behavior of the natural futile electrodes with that of two intentionally Fe-doped TiO2 synthetic single crystal electrodes. Experimental ProcedureSample characterization and heat-treatments.--Sam-* Electrochemical Society Student Member. ** Electrochemical Society Active Member.

Photoelectrochemical and Corrosion Properties of Fe2 O 3 ‐ TiO2 Artificial Passivation films

“…The quantum yield of the films, , was obtained from the measured photocurrent spectra using the Eq. 1 19,20 ϭ N e /N p [1] where N e is the number of electrons for photocurrent and N p is the number of incident photons. Then the relationship between the quantum yield and the composition of the films was examined.…”

Photoelectrochemical Properties of Polycrystalline TiO2Thin Film Electrodes on Quartz Substrates

“…Table II summarizes some calculated data for Preparation 3c (least squares fitted). In the calculation of ND a value e ----120 (3,19) has been used. It can be concluded (see Fig.…”

“…10 are plotted in accordance with Eq. [3], Lp + w can be determined from the slope of the curve (Fig. 12).…”

“…During the last decade much interest has been devoted to TiO2 as a semiconducting photoanode in photoelectrochemical cells with the aim of finding a material suitable for the production of hydrogen from water using light as the energy source. Studies have been performed on single crystals of rutile structure (1)(2)(3)(4)(5)(6)(7) and on polycrystalline material either as a sintered powder (7)(8)(9) or as thin layers of TiO2 films (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22). Experiments to extend the photoresponse into the visible region by doping have also been reported (6)(7)(8)(9)(10)(11)(12)(13).…”

Epitaxial TiO2 Thin Film Photoanodes: Influence of Crystallographic Structure and Substrate Nature

“…For this purpose, we modeled our system with a R S -RCPE circuit of equivalent impedance Z given by where R S and R are the series and space charge layer resistances respectively and i ω n Q the admittance of CPE, where n is between 0.9 and 1 ( n = 1 for perfect capacitance). Then, we estimated the capacitance value of the CPE using the formula The Mott–Schottky plots A 2 / C 2 = f ( V ) were fitted using eq to determine the flat-band potential ( V fb ) and the carrier concentration ( N D )­ where A is the illuminated surface area, ε 0 the dielectric constant of vacuum (8.85 × 10 –12 F/m), ε r is the dielectric constant of titanium dioxide taken equal to 31 for anatase and to 173 for rutile, V is the potential applied to the electrochemical cell, k B is the Boltzmann constant (1.38 × 10 –23 J/K), and e is the elementary charge (1.6 × 10 –19 C). The flat-band potential corresponds to the difference between the electrochemical potentials of the semiconductor (Fermi level) and of the electrolyte (redox level).…”