Competitive Photo-Oxidation of Water and Hole Scavengers on Hematite Photoanodes: Photoelectrochemical and Operando Raman Spectroelectrochemistry Study

Abstract

Hematite (α-Fe 2 O 3 ) is one of the most studied photoanode materials due to its stability in alkaline electrolytes and visible light absorption. However, its reported performance lags significantly behind its theoretical limit. Toward determining routes for efficient photo-oxidation on hematite, we investigated the role of hole-trapping surface states as reaction intermediates using sacrificial reductant reagents as hole scavengers, H 2 O 2 and FeCN. Photoelectrochemical characterization at low scavenger concentrations and intermediate potentials, as opposed to previous studies at high scavenger concentrations, has shown the reaction mechanism to include competitive photo-oxidation between water molecules and the hole scavengers, similar for both H 2 O 2 and FeCN. Using operando Raman spectroelectrochemistry, we show similar transient features for both scavengers, interpreted as scavenger adsorption to a two-site reaction intermediate participating in the photogenerated hole transfer of water photo-oxidation, hence the competition. These findings strengthen the significance of hole-trapping surface states for water photo-oxidation on hematite and the previously suggested two-site reaction pathway for efficient hole transfer in this reaction. A better understanding of the mechanisms of photoelectrochemical water splitting can assist in improving the efficiency of solar hydrogen production.

Roles of Surface States in Cocatalyst-Loaded Hematite Photoanodes for Water Oxidation

“…14 The positive sides of S2 can be highlighted with sulfite oxidation, which is more kinetically favorable than water oxidation. 34 Following the results shown in Figures 2c and 3b, S1 was passivated in the presence of Na 2 SO 3 because of alteration on surface radical species, 35 and S2 became the main pathway for charge transfer for sulfite oxidation (illustrated in Figure 4b). Since the holes trapped in S2 had a lifetime much longer than those in S1, α-Fe 2 O 3 showed excellent PEC performance with an E on close to its flat band potential (Figure 1b).…”

Interfacial Se–O Bonds Modulating Spatial Charge Distribution in FeSe2/Nb:Fe2O3 with Rapid Hole Extraction for Efficient Photoelectrochemical Water Oxidation

“…As it shows, the peaks are indexed to hematite completely. 45 The Raman shift of Nb:Fe 2 O 3 locates almost the same as those of Fe 2 O 3 , but the peaks of FeSe 2 /Nb:Fe 2 O 3 exhibit a positive shift compared to that of Nb:Fe 2 O 3 . It is attributable to the lessened oxygen vacancy defects after surface treatment.…”

“…Raman spectra in Figure5dfurther proves that. As it shows, the peaks are indexed to hematite completely 45. The Raman shift of Nb:Fe 2 O 3 locates almost the same as those of Fe 2 O 3 , but the peaks of FeSe 2 /Nb:Fe 2 O 3 exhibit a positive shift compared to that of Nb:Fe 2 O 3 .…”

In situ and operando Raman spectroscopy of semiconducting photoelectrodes and devices for photoelectrochemistry

“…Note that the notch/LWP filters installed in any Raman setup dedicated to the detection of the Stokes lines and used to suppress the Rayleigh line can also serve this purpose, depending on the specifications of the particular optical analysis system in use. Adopting this approach, Ramakrishnan and coworkers recently investigated the role of hole-trapping surface states in hematite photoanodes as reaction intermediates using sacrificial reducing compounds such as H 2 O 2 and FeCN as hole scavengers [54]. Using operando RS coupled with a PEC investigation of the system above, the authors observed a competitive photo-oxidation between H 2 O 2 , FeCN, and water at low scavenger concentrations and intermediate potentials.…”

“…To the best of our knowledge, only one report of semiconducting photoelectrodes and devices for photoelectrochemistry using in situ and operando RS exists. Recently, Ramakrishnan et al reported a competitive photo-oxidation of water and hole scavengers on hematite photoelectrodes using operando RS [54]. A similar approach was conducted in studying a PEC dye-sensitized solar cell, directly detecting charge transfer processes [55].…”