Comment on “Molten Salt Synthesis of Bi₂WO₆ Powders and its Visible-Light Photocatalytic Activity” in Materials Research. 2019; 22(5): e20190311

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

1. 1

   $\alpha h\nu =A{\left(h\nu -E_g\right)}^2$ was designed by Kubelka-Munk (K-M) equation¹, which is incorrect. In fact, $\alpha h\nu =A{\left(h\nu -E_g\right)}^2$ is a classical equation from the optical transitions theory in semiconductor/insulator materials². On the other hand, the dimensionless K-M function/equation ($F\left(R\right)$) is defined as the ratio between the K-M absorption and scattering coefficients³. In the vicinity of $E_g$, the scattering phenomenon can be neglected, that is, the scattering coefficient is treated as a constant⁴, therefore: $F\left(R\right)\propto \alpha$.

2. 2

   According to the referred authors¹, the parameter $A$ is usually 1. In fact, the value of $A$ does not interfere in the determination of the band gap energy ($E_g$) because it is an independent parameter of photon energy ($h\nu$)⁵. 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 bands⁶. In this sense, it is unlikely that its value is usually 1.

3. 3

   According to inset of Figure 5, ${\left(\alpha h\nu \right)}^{1/2}$ is plotted against the $h\nu$, where $\alpha$ is the absorption coefficient of material and represents the probability of light being absorbed per unit path length⁷. Consequently, the unit of measure for $\alpha h\nu$ is unit of energy divided by unit of length and not eV as declared by the authors¹. 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 $\alpha h\nu$ is “arbitrary unit”.

References

• ¹
  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-2019-0311
• ²
  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.2021.116635
• ³
  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
• ⁴
  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.2019.01.048
• ⁵
  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
• ⁶
  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
• ⁷
  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

Publication Dates

• Publication in this collection
  23 July 2021
• Date of issue
  2021

History

• Received
  22 Apr 2021
• Accepted
  24 June 2021