Abstract
Recently, Dai et al. published a paper in Research Materials 2019; 22(5): e20190311. They have investigated the visible-light photocatalytic activity of the synthesized Bi2WO6 powders1. In the context of determining of band gap energy based on diffuse reflectance measurements, the cited authors make some mistakes, which are clarified in the present letter.
Keywords:
Diffuse Reflectance Spectroscopy; Kubelka-Munk function; Absorption coefficient
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1
was designed by Kubelka-Munk (K-M) equation11 Dai B, Xuan M, Lv Y, Jin C, Ran S. Molten Salt Synthesis of Bi2WO6 Powders and its Visible-Light Photocatalytic Activity. Mater Res. 2019;22(5):e20190311. http://dx.doi.org/10.1590/1980-5373-mr-2019-0311.
https://doi.org/10.1590/1980-5373-mr-201... , which is incorrect. In fact, is a classical equation from the optical transitions theory in semiconductor/insulator materials22 Landi S. Comment on “Photocatalytic degradation of RhB from an aqueous solution using Ag3PO4/N-TiO2 heterostructure” and “Evaluation of the effect of dose change of Fe3O4 nanoparticles on electrochemical biosensor compatibility using hydrogels as an experimental living organism model”. J Mol Liq. 2021;338:116635. http://dx.doi.org/10.1016/j.molliq.2021.116635.
https://doi.org/10.1016/j.molliq.2021.11... . On the other hand, the dimensionless K-M function/equation () is defined as the ratio between the K-M absorption and scattering coefficients33 Milosevic M, Berets SL. A review of FT-IR of diffuse reflection sampling considerations. Appl Spectrosc Rev. 2002;37(4):347-64. http://dx.doi.org/10.1081/ASR-120016081.
https://doi.org/10.1081/ASR-120016081... . In the vicinity of , the scattering phenomenon can be neglected, that is, the scattering coefficient is treated as a constant44 Liu S, Wang Z, Bao Q, Li X, Chen Y, Wang Z, et al. Abnormal thermal quenching and blue-shift of Zn3(BO3)(PO4): Inducing host T defect by doping Mn2+ and Tb3+. Dyes Pigments. 2021;165:44-52. http://dx.doi.org/10.1016/j.dyepig.2019.01.048.
https://doi.org/10.1016/j.dyepig.2019.01... , therefore: . -
2
According to the referred authors11 Dai B, Xuan M, Lv Y, Jin C, Ran S. Molten Salt Synthesis of Bi2WO6 Powders and its Visible-Light Photocatalytic Activity. Mater Res. 2019;22(5):e20190311. http://dx.doi.org/10.1590/1980-5373-mr-2019-0311.
https://doi.org/10.1590/1980-5373-mr-201... , the parameter is usually 1. In fact, the value of does not interfere in the determination of the band gap energy () because it is an independent parameter of photon energy ()55 Malainho E, Vasilevskiy MI, Alpuim P, Filonovich SA. Dielectric function of hydrogenated amorphous silicon near the optical absorption edge. J Appl Phys. 2009;106:073110. http://dx.doi.org/10.1063/1.3240203.
https://doi.org/10.1063/1.3240203... . However, its value depends on properties such as the number of unit cell or even the type of electronic transition between the valence and conduction bands66 Elliott RJ. Intensity of Optical Absorption by Excitons. Phys Rev. 1957;108:1384-9. http://dx.doi.org/10.1103/PhysRev.108.1384.
https://doi.org/10.1103/PhysRev.108.1384... . In this sense, it is unlikely that its value is usually 1. -
3
According to inset of Figure 5, is plotted against the , where is the absorption coefficient of material and represents the probability of light being absorbed per unit path length77 Yang L, Kruse B. Revised Kubelka-Munk theory. I. Theory and application. J Opt Soc Am A Opt Image Sci Vis. 2004;21:1933-41. http://dx.doi.org/10.1364/josaa.21.001933.
https://doi.org/10.1364/josaa.21.001933... . Consequently, the unit of measure for is unit of energy divided by unit of length and not eV as declared by the authors11 Dai B, Xuan M, Lv Y, Jin C, Ran S. Molten Salt Synthesis of Bi2WO6 Powders and its Visible-Light Photocatalytic Activity. Mater Res. 2019;22(5):e20190311. http://dx.doi.org/10.1590/1980-5373-mr-2019-0311.
https://doi.org/10.1590/1980-5373-mr-201... . However, as the values presented on the ordinate axis in the inset shown in Figure 5 were obtained from the absorbance measurements, the most appropriated unit for is “arbitrary unit”.
References
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1Dai B, Xuan M, Lv Y, Jin C, Ran S. Molten Salt Synthesis of Bi2WO6 Powders and its Visible-Light Photocatalytic Activity. Mater Res. 2019;22(5):e20190311. http://dx.doi.org/10.1590/1980-5373-mr-2019-0311.
» https://doi.org/10.1590/1980-5373-mr-2019-0311 -
2Landi S. Comment on “Photocatalytic degradation of RhB from an aqueous solution using Ag3PO4/N-TiO2 heterostructure” and “Evaluation of the effect of dose change of Fe3O4 nanoparticles on electrochemical biosensor compatibility using hydrogels as an experimental living organism model”. J Mol Liq. 2021;338:116635. http://dx.doi.org/10.1016/j.molliq.2021.116635.
» https://doi.org/10.1016/j.molliq.2021.116635 -
3Milosevic M, Berets SL. A review of FT-IR of diffuse reflection sampling considerations. Appl Spectrosc Rev. 2002;37(4):347-64. http://dx.doi.org/10.1081/ASR-120016081.
» https://doi.org/10.1081/ASR-120016081 -
4Liu S, Wang Z, Bao Q, Li X, Chen Y, Wang Z, et al. Abnormal thermal quenching and blue-shift of Zn3(BO3)(PO4): Inducing host T defect by doping Mn2+ and Tb3+ Dyes Pigments. 2021;165:44-52. http://dx.doi.org/10.1016/j.dyepig.2019.01.048.
» https://doi.org/10.1016/j.dyepig.2019.01.048 -
5Malainho E, Vasilevskiy MI, Alpuim P, Filonovich SA. Dielectric function of hydrogenated amorphous silicon near the optical absorption edge. J Appl Phys. 2009;106:073110. http://dx.doi.org/10.1063/1.3240203.
» https://doi.org/10.1063/1.3240203 -
6Elliott RJ. Intensity of Optical Absorption by Excitons. Phys Rev. 1957;108:1384-9. http://dx.doi.org/10.1103/PhysRev.108.1384.
» https://doi.org/10.1103/PhysRev.108.1384 -
7Yang L, Kruse B. Revised Kubelka-Munk theory. I. Theory and application. J Opt Soc Am A Opt Image Sci Vis. 2004;21:1933-41. http://dx.doi.org/10.1364/josaa.21.001933.
» https://doi.org/10.1364/josaa.21.001933
Publication Dates
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Publication in this collection
23 July 2021 -
Date of issue
2021
History
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Received
22 Apr 2021 -
Accepted
24 June 2021