Water oxidation reaction is as follows:
2H2O = O2 + 4e- + 4H+
and standard reduction potential for the O2/H2O couple is 1.23 V (vs NHE). Therefore what exact potential (-1.23V or +1.23 V) we need for water oxidation? As we know that the water oxidation happens in a photocatalytic reaction using a semiconductor's holes from its valence band, what should be the position(-1.23V or +1.23 V vs NHE) for valence band maxima (VBM) of that semiconductor?
The potential for water oxidation is pH dependent and also catalyst modified electrode dependent.
You might find some information from the following paper.
Water Oxidation and Oxygen Monitoring by Cobalt-Modified Fluorine-Doped Tin Oxide Electrodes
J. Am. Chem. Soc., 2013, 135 (23), pp 8432–8435
On the basis of cyclic voltammetry measurements, surface-bound Co(II) undergoes a pH-dependent 1e–/1H+ oxidation to Co(III), which is followed by pH-dependent catalytic water oxidation. O2 reduction at FTO occurs at −0.33 V vs NHE,
Dear Sir
My question was to know for a ideal situation as I depicted in my question! Let me be more specific about my question. You definitely have noticed in literatures (I have attached 2 pages from 2 different journals) that the representations of the band positions of different semiconductors relative to the redox potentials of water reduction (also can be said as proton reduction) (around -0.41 V at pH 7 vs RHE) and water oxidation (around 0.81 V vs RHE at pH 7). Now my very basic question was why should the potential for water oxidation (2H2O = O2 + 4e- + 4H+) be 0.81 V RHE at pH 7 and hence the VBM position of any semiconductor be more positive than 0.81 V RHE at pH 7, whereas the standard reduction potential (O2 + 4e- + 4H+ =2H2O) of the O2/H2O couple is also conventionally taken as 0.81 V RHE at pH 7.
The standard reduction potential for the O2/H2O couple is 1.23 V (vs NHE)
The standard Oxidation potential for the H2O/O2 couple is 1.23 V (vs NHE)
The standard oxidation potential and the standard reduction potential are opposite in sign to each other for the same chemical species.
Relation Between Standard Reduction Potential (SRP) and the Standard Oxidation Potential (SOP)
Eo0(SRP)=−Eo0(SOP)
@Prof. Yurii V Geletii Thank you so much Sir for your kind answer. I'm glad to see that you mentioned the sign convention. According to my understanding, this sign convention has huge impact on the spontaneity of a reaction. As you mentioned that the standard Oxidation potential for the H2O/O2 couple is -1.23 V (vs NHE) and actually water oxidation happens(instead of oxygen reduction) by utilizing the holes from the Valence Band of a semiconductor, therefore the potential of Valence Band Maximum of that semiconductor should also be around -1.23 V (vs NHE). However in every literature it shows as 1.23 V (vs NHE), not -1.23 V (vs NHE). Somehow I don't get the logic behind this convention! If you can illuminate some light on that part, that would be very nice of you Sir! Thanks again.
just move your thoughts into the four quadrants of the electrochemical cycle and go to the bottom left. Remember to draw electrochemical quadrants in the classical format and not the current format. The current format makes life easier for people to understand a basic operating principle of electrochemistry but makes it harder for a person to become an expert at electrochemistry. My book is coming out from Singapore soon and you will understand what I mean. My book is Neuroimaging-Sensing Biochemistry in the Brain. My best regards, Dr. Broderick
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