Hi, everyone.
I have a question about the photocatalyst related to the Z scheme heterojunction.
First of all, sorry for my poor English.
The research that I read is as follows:
[Hierarchically Porous WO3/CdWO4 Fiber-in-Tube Nanostructures Featuring Readily Accessible Active Sites and Enhanced Photocatalytic Effectiveness for Antibiotic Degradation in Water] (ACS Appl. Mater. Interfaces 2021, 13, 21138-21148.)
(Article Hierarchically Porous WO3/CdWO4 Fiber-in-Tube Nanostructures...
)Figure 4. (a) and (b) shows the light absorption properties and calculated band gap of heterojunction.
In Figure 4a, the absorbance edges of CdWo4 and WO3 occur at about 330 nm and 500 nm, respectively.
When a heterojunction is formed with WO3/CdWO4, the absorption edge is almost at 440 nm, demonstrating that the absorption property is improved as heterojunction is formed.
In Figure 4b, the band gap of CdWo4 and WO3 were 2.80 eV and 3.70 eV, respectively, and when heterojunction was formed, the band gap of WO3/CdWO4 was adjusted to a value of 3.10 eV.
I have a question at this point.
I wonder why the bandgap decreases (or changes to an intermediate value) as the heterojunction is formed.
As far as I know, the Z-scheme is explained by the following mechanism.
- The two materials generate photogenerated electron-hole by photon energy (hv).
- Then, O2 radicals are generated by the electrons at lower potential CB, and OH radicals are generated by the holes at higher potential VB.
- At the same time, the recombination between the electrons at higher potential CB and the holes at lower potential VB, resulting in increased photocatalytic efficiency.
Here, for proper operation of the Z-scheme, each material must be able to generate photogenerated electron-holes.
So, I think that photon energy corresponding to a material with a larger band gap is required.
However, as shown in Figure 4b, when two materials form a heterojunction, the band gap changes to the value between each material.
I think this energy is insufficient to excite the material with a larger band gap energy.
How is this mechanism possible?
If you have any ideas or related research about this, please introduce them to me.
Thanks for reading.
First of all, Z-Scheme is just a type II staggered band aligmenent which is differentiated by only the charge transfer route in comparison to the type-II assembly. While the reduction or alteration of the bandgap can be ascribed to band bending and the formation of intermediate bandgap states between the conduction and valence bands due to doping or other structural modifications, resulting in narrowing of bandgap.
Hassan Ali
Thank you for your kind reply!
According to the answer, it can be understood that band bending occurs due to doping or other modification, resulting in the change of the band energy.
However, I think when heterojunction was constituted with different band energy, band bending takes place according to the fermi energy level.
However, as can be seen in the figure, the researchers suggested the mechanism of photocatalytic degradation as follows: In both WO3 and CdWO4, energies of 2.80 eV and 3.70 eV are required to generate electrons and holes, respectively.
Although the band gap is adjusted to 3.10 eV by the junction between two materials, I wonder if it is plausible to represent the photocatalytic mechanism as in this figure.
The overall bandgap of the composite material is 3.10 eV corresponding to 400 nm implying that a minimum wavelength of 400 nm or less would be required for bulk excitation of the material. However, this value is usually higher due to the overpotential. For optimized functioning, it is imperative that individual components of the samples should be irradiated with the wavelength corresponding to bandgap. The construction of heterojunction is attractive mainly due to its ability to join two materials with high conduction band (more reductive) and another one with low valance band (more oxidative).
Hassan Ali
Thanks for your comments!
Ambiguous points were completely cleared up by your explanation.
From now on, I'll try to clarify these findings through experiments about heterojunction.
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