Stabilization of n ‐ GaAs in Acidic Concentrated Iodide Electrolytes

Abstract

Although n-GaAs is known to be unstable in presence of iodine, it is shown that this semiconductor can be efficiently stabilized by using highly concentrated acidified iodide aqueous solutions. The effects of proton activity and iodine and iodide concentrations on the electrode stability are investigated by means of rotating ring-disk experiments, RHEED observations, and chemical analysis of the electrolyte. Impedance measurements show a positive shift of the bandedges of GaAs with iodine concentration and illumination level. At a nominal pH of 0, the stabilization ratio S is measured close to 100% (80%) for n-GaAs in contact with a 7M (2M) NaI solution, respectively. Under illumination, the I-/I3-redox level is found to be favorably located near midgap. Kinetic analysis of the competing hole reactions yields a satisfactory simulation of S vs. I concentration. The hole capture by reduced species is found to be proportional to the concentration of iodide ions, not to their activity, in contrast to expectation. Furthermore, it is shown that chemisorbed Ru 3+ cations act as a catalyst for hole transfer and thus improve the stability of n-GaAs in 7M NaI solution.) unless CC License in place (see abstract).ABSTRACT A general mathematical model for studying the kinetics of electrochemical reactions at a rotating disk electrode under steady-state potentiostatic conditions is presented. The model, apart from predicting the net and partial current densities at given values of the applied potential, the ohmic potential drop, and the concentration and potential profiles in the solution, also accounts for homogeneous reactions of any order in the solution and noncharge transfer reactions at the electrode surface. The versatility of the model is demonstrated by the application of the model to a variety of complex reaction schemes.

Charge Transfer Process at Illuminated Semiconductor/Electrolyte Junctions Modified by Electrodeposition of Microscopic Metal Grain

“…In the 7M NaI (pH = 0) solution [the pH is adjusted with H2SO4 as in Ref. (18)], the sensitivity of the RRDE technique is not able to detect any improvement of S because it is initially >0.99 (18). Figure 3 also shows that AV decreases in 1M NaI while S increases.…”

“…Before concluding, we want to stress that this work deals with the long-term stability of GaAs. In 7M NaI (pH = 0) the remaining corrosion is initially 10 -3 of the total photocurrent for a naked surface (18). According to Fig.…”

Reaction Mechanisms at the n ‐ FeS2 / I Interface: An Electrolyte Electroreflectance Study

“…The main source of photovoltage limitations is the high electronic affinity of FeS2 (i.e., the low energetic position of Fe:eg empty levels), which theoretically allows a maximum photovoltage [V~ -E~ of about 300 mV. With GaAs, for instance, a semiconductor far less stable than FeS2 in concentrated I-/I2 solutions, higher photovoltages can be reached as it has a flatband potential ~700 mV more negative than FeS~ (22). An additional source of photovoltage limitations is the high density of bandgap energy states, both in the bulk and at the surface.…”

Photoelectrochemical Behavior of n-Type GaAs(100) Electrodes Coated by a Single Layer of Graphene

“…Thin noble metals such as Pt, Au, and Rh have been applied on GaAs, and although these metals improve the anodic stability of GaAs, they also form a Schottky barrier that pins the Fermi level, leading to nonoptimal photovoltages. − Polymer films such as polypyrrole, , polystyrene, and polythiophene , have been coated on GaAs, but the stability of these systems is limited by peeling of the film, and the unusually high photovoltages exhibited by such electrodes are indicative of active photocorrosion. Another method that has been used to stabilize the n-GaAs/H 2 O interface in regenerative photoelectrochemical cells involves enhancing kinetic competition for photogenerated holes by using electrolytes containing redox couples, such as Se – /Se 2– (aq) − or I – /I 3 – (aq), , at high concentrations, and by adding metal ions that chemisorb to the electrode to increase the rate of transport of photogenerated holes away from the GaAs surface. Furthermore, thick (∼60–120 nm) amorphous TiO 2 films grown by atomic-layer deposition have demonstrated the ability to protect illuminated np + -GaAs and GaAs/InGaP tandem devices for tens of hours of operation as oxygen-evolving photoanodes in water-splitting devices that use 1 M KOH­(aq) as an electrolyte.…”