The Characterization of Doped Iron Oxide Electrodes for the Photodissociation of Water: Stability, Optical, and Electronic Properties

Abstract

A characterization of optical and electronic properties is presented for p‐type (Mg‐doped) and n‐type (Si‐doped) iron oxides used in the photoelectrolysis of water. Photocurrent vs. wavelength spectra for these electrodes indicate that α‐Fe2O3 is the active optical component for both p‐type and n‐type materials. Band‐edge locations for p‐type and n‐type iron oxides in sodium hydroxide aqueous solution are determined from differential capacitance measurements. The thermodynamic feasibility of the catalytic photodissociation of water without external potential is demonstrated for a short‐circuited p/n diode assembly on an energy level diagram of the electrode/electrolyte interfaces. The open‐circuit voltage false(VOCfalse) and short‐circuit current false(ISCfalse) generated by the p/n assembly as a function of the intensity of laser irradiation indicate that these doped iron oxides are low mobility, high carrier density semiconductors. Photo‐oxidation of water at the n‐type anode is verified through oxygen detection. Gas evolution is monitored from an operating diode assembly using mass spectrometry and isotopically labeled water false(H218Ofalse) . Photocurrents from these p/n assemblies show excellent long‐term stability in aqueous solution and Auger analysis of the semiconductor surfaces indicates no evidence of electrode dissolution.

Solar Water Splitting: Progress Using Hematite (α‐Fe2O3) Photoelectrodes

“…Typically, the capacitance is determined by fitting the frequency responses with a simple resistor-capacitor (RC) circuit model (see Figure 8 a). A single frequency, [30,50,56] multiple frequencies, [105] extrapolating to an infinite frequency, [54,57,126] or an entire range of high frequencies [65,[127][128] (for heavily doped samples) have been reported for both planar and structured electrodes. [24,[127][128] However since the classic MS relation relies on a simple parallel-plate capacitor model the application of EIS data from real systems has often been problematic.…”

Electrodeposition of Nanometer-Sized Ferric Oxide Materials in Colloidal Templates for Conversion of Light to Chemical Energy

“…Hematite, α-Fe 2 O 3 , has a much more favorable 2.2 eV band gap for solar harvesting and therefore absorbs about 40% of the air mass 1.5 solar spectrum [6][7][8]. Previous researchers found that the energetic position of the valence band edge in hematite is appropriate for water oxidation [9][10][11][12][13][14][15][16][17][18][19][20][21]. In fact, iron oxides are amongst the smallest band gap semiconductors that are stable toward oxygen evolution and are certainly the least expensive of them.…”

Nanostructuredα-Fe2O3 in PEC Generation of Hydrogen

“…They found the acceptor density to be 4.41 Â 10 17 and 5.57 Â 10 17 cm À3 at frequencies of 791 and 250 Hz, respectively. The acceptor densities obtained for Cu- [37], Zn- [36] and Mg-doped [6] a-Fe 2 O 3 are 2-3 orders of magnitude lower than for undoped n-Fe 2 O 3 [35]. These lower acceptor densities may be responsible for the lower photocurrent density obtained for p-type a-Fe 2 O 3 samples.…”

“…Doping of Mg 2 þ and Zn 2 þ in a-Fe 2 O 3 has been found to exhibit some peculiar characteristics related to p-type behavior [6,28]. These electrodes exhibited the phenomenon of transformation from negative to positive photocurrent, when the external potential was varied from negative to positive.…”

“…The addition of palladium (75-100 A ) on the hetrojunction electrode was done by vacuum evaporation, while RuO 2 loading was done by covering the electrode with RuCl 3 butanol solution. PEC measurements in this study were carried out photodynamically at 0.1 V s À1 in 0.2 M KOH solution containing 0.2 M K 4 (Fe (CN) 6 ) and 0.01 M K 3 (Fe(CN) 6 ) in a purified Ar gas atmosphere, with an Hg/HgO electrode as the reference electrode. The open-circuit voltage V oc for the photoanodes Fe 2 O 3 /n-Si, Pd/ Fe 2 O 3 /n-Si and RuO 2 /Fe 2 O 3 /n-Si exhibited a constant value of 0.330 V, while the short-circuit photocurrent density I sc , increased with Pd to 7.95 mA cm À2 and with RuO 2 to 10.5 mA cm À2 .…”