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42860-GB5
P-Type Cu2O Nanocrystalline Films for Photoelectrochemical Energy Conversion
Porous p-Cu2O films prepared from an ethanol dispersion of Cu2O powder showed a photocurrent density (650 ?A/cm2 at -0.4 V vs. SCE) appreciably greater than electrodeposited Cu2O films (100 ?A/cm2 at -0.4 V vs. SCE) in 0.5 M Na2SO4 aqueous solution under the same illumination conditions. The three-dimensional nature of the porous film increased the interfacial area between the film and the solution, resulting in an enhanced electron transfer from the Cu2O to the dissolved O2. The surface area of a porous Cu2O film was measured to be 2.8 x 10-3 m2/(cm2 film), by using the BET measurement value, 0.33 m2/g for Cu2O powder heated at 350 °C for 1 hr and the quantity of Cu2O powder spread to 1 cm2 of film (0.0083 g). The surface area of an electrodeposited film was estimated to be 1.7 x 10-4 m2/(cm2 film), by assuming that each crystallite has four equilateral triangle sides (1 ?m at the base) exposed and that there are 104 x 104 crystallites per 1 cm2 of the film. Although this estimate is a rough value, it is not unreasonable to believe that the porous film has an order of magnitude greater surface area than that of the electrodeposited film. Therefore, it is reasonable to attribute the enhanced photocurrent density to the increased surface area of the porous Cu2O.
No chemical change was observed and detected by XRD by illuminating the porous Cu2O film for 2 hr. Unlike single crystal Cu2O photocathodes that are unstable, electrodeposited Cu2O photocathodes have been reported to be stable. However, the reason for this stability has not been investigated. The instability of a single crystal Cu2O is due to the reduction of Cu2O by the photoexcited electrons:
Cu2O + 2H+ + 2e- ? 2Cu + H2O
This reaction is assisted by H+ which is attracted to O2-, hence, this reaction occurs at an O2-terminated surface, but not at a Cu+-terminated surface. On the contrary, the reduction of dissolved O2 that stabilizes the photocathode, occurs at a Cu+-terminated surface, but not at an O2--terminated surface because the initial step of this reaction is the adsorption of O2 to two Cu+ sites.
O2 + 2H+ + 2e- ? H2O2
This proposed mechanism resembles the well-accepted mechanism of electroreduction of O2 at Pt and Au surfaces.
Instability was reported for single crystals with the predominately (211) and (311) surfaces. The decomposition of Cu2O can occur at the (211) surface terminated by Cu+ and O2- plane, because of the exposure of O2- to the solution. The same reaction is also possible at the (211) surface terminated by Cu+ only plane, because of the exposure of O2- from the plane directly beneath. All atomic layers perpendicular to the [311] direction contain only Cu+ or O2-, and the O2-- Cu+- O2- planes repeat. Hence, the stoichiometric termination of the Cu2O (311) is O2- plane, where the decomposition of Cu2O occurs. This is a plausible explanation for the instability of Cu2O photocathodes.
On the other hand, electrodeposited Cu2O films have been reported to be photostable. The crystallites are four-sided pyramidal with a four-fold symmetry axis along the [100] direction. From the average angle between the sides, the four sides of the pyramids are (111) surfaces. The termination of the (111) surface of a stoichiometric four-sided pyramid depends on the first deposition plane. If the deposition is initiated with an O2- plane, the Cu2O is fairly well protected from the corrosion reaction. Therefore, it is possible for the electrodeposited Cu2O film to be a stable photocathode. We are currently further testing our hypothesis, and results will be reported in a future paper.
