Layered Structures
In studies on nanostructured thin films, layered structures have also attracted the attention of researchers and are known to offer considerably higher phototoelectrochemical and photo-catalytic activity. A combination of small- and large-bandgap material deposited one over the other, as shown in Figure 13.11, may absorb the full solar spectrum more efficiently. If energy-band edges match at the junction, better and efficient separation of photogenerated electrons and holes is possible.
Liou et al. prepared an a-Fe2O3/TiO2 heterojunction electrode for photoelectrolysis of water [86] using iron foils, as early as in 1982. The heterojunction electrodes were made by the
- Figure 13.11 Schematic representation of layered structure of low and high bandgap material for PEC application.
chemical vapor deposition (CVD) of TiO2 films on a-Fe2O3 substrates. The photocurrent of the a-Fe2O3 electrode in the positive-bias region was about half of that of a CVD TiO2 electrode. They observed that the heterojunction electrode had the same flatband potential as that of CVD TiO2 on Pt, though the shapes of the I V curves were different. With an increase in bias potential, the photoresponse starts at a longer wavelength, and the peak of the photocurrent also shifts toward a longer wavelength, which strongly indicates a hole contribution from the a-Fe2O3 substrate. It appears that the holes generated in the a-Fe2O3 substrate do contribute to the photocurrent of the a-Fe2O3/TiO2 electrode. The heterojunction thus exhibited the relatively large flatband potential of TiO2 and the spectral threshold of a-Fe2O3.
An iron-oxide/n-Si heterojunction photoanode (Fe2O3/n-Si,) was studied by Osaka et al., in 1985, for a regenerative PEC cell [87]. The electrode was prepared by vacuum evaporation of a 400 500 A iron film onto a (100) single crystal of 5 7 O cm n-Si substrate, followed by vacuum-furnace heating at 400 ° C for 1 hr. The addition of palladium (75 100 A) on the hetrojunction electrode was done by vacuum evaporation, while RuO2 loading was done by covering the electrode with RuCl3 butanol solution. PEC measurements in this study were carried out photodynamically at 0.1 V s 1 in 0.2 M KOH solution containing 0.2 M K4(Fe (CN)6) and 0.01 M K3(Fe(CN)6) in a purified Ar gas atmosphere, with an Hg/HgO electrode as the reference electrode. The open-circuit voltage Voc for the photoanodes Fe2O3/n-Si, Pd/ Fe2O3/n-Si and RuO2/Fe2O3/n-Si exhibited a constant value of 0.330 V, while the short-circuit photocurrent density Isc, increased with Pd to 7.95 mA cm 2 and with RuO2 to 10.5 mA cm 2. The energy-conversion efficiency of the PEC cell with RuO2/Fe2O3/n-Si anode was 1.6% for illumination by a 55 mW cm 2 Xe lamp. The heterojunction electrode showed high stability against photocorrosion. RuO2 layering on Rh/Co/Fe2O3 was also found to increase the photoresponse of spray-pyrolytically deposited a-Fe2O3 thin films [34], whereas layering of
PbO2 and MnO2 on similar films was observed to decrease the photoresponse, and no such effect was observed in the case of Co2O3.
Miller et al. designed and developed a multijunction photoelectrode for hydrogen production using an a-Fe2O3/electrolyte as the top junction with two underlying amorphous silicon/ germanium (a-Si:Ge) solid-state junctions, fabricated onto stainless-steel foil coated with a thin film of nickel molybdenum hydrogen catalyst on the back surface [55]. The construction of the hybrid cell involved sputter deposition of an a-Fe2O3 layer over single-junction a-Si:Ge n-i-p device. The focus of the experiment was on the demonstration of stable operation and bias saving. The hybrid photoelectrode, on testing, showed a 0.6 0.65 V bias saving under AM1.5 illumination in potassium hydroxide.
Recently, Luo et al. examined the composite structure of WO3/Fe2O3 by admixing both WO3 and a-Fe2O3 [82]. WO3 films were coated on an as-grown film of a-Fe2O3. XRD analysis confirmed the presence of a-Fe2O3 and WO3 on the films, with no traces of formation of the ternary compound Fe(WO4). The bandgaps of a-Fe2O3 and WO3, as calculated by transmission spectra analysis, were found to be 1.97 and 2.53 eV, respectively. The photocurrent obtained for the WO3/Fe2O3 heterojunction was found to be 6 ||A cm 2 under illumination at 440 nm, which was higher than for both WO3 and a-Fe2O3 films.
The energy-band diagram of a-Fe2O3/TiO2, with respect to the redox level of water at pH 13 is shown in Figure 13.12, which indicates that the structure allows absorption of almost the whole range of the solar spectrum lying in the visible and UV much more efficiently, compared to a single-material electrode. A bilayered photoelectrode a-Fe2O3/TiO2 system incorporating TiO2 thin films prepared by the sol-gel technique over a spray-pyrolytically deposited thin film of a-Fe2O3 has been studied by the authors' group [88]. The photocurrent density exhibited by the various samples prepared was observed to vary with the thickness of the TiO2 layers, as shown in Figure 13.13. The maximum photocurrent density, ^945 |A cm 2 at 0.6 V versus SCE, was obtained for a bilayered structure of TiO2 (thickness 0.65 |m, particle size 24 nm) and a-Fe2O3 (thickness 0.96 |m, particle size 60 nm). This photoresponse was much larger than those of the single-material electrodes, which were ^12 |Acm 2 and 148 |Acm 2 for a-Fe2O3 and TiO2 thin films, respectively. An increase in the photovoltage was also observed from 0.08 for a-Fe2O3 to 0.26 V versus SCE in a sample of the bilayered structure of total
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