UV-Vis of zinc oxide nanoparticles

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Discussion Overview

The discussion centers on the UV-Vis spectroscopy results of zinc oxide nanoparticles, focusing on the observed absorbance characteristics and potential explanations for the unusual spectral shape. Participants explore the implications of particle size, scattering effects, and the expected absorption peaks of ZnO in the context of their experimental findings.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes their preparation of zinc oxide nanoparticles and presents UV-Vis results showing a "v" shaped trough in absorbance, questioning the reasons for absorbance in the visible spectrum and the shape of the spectrum.
  • Another participant suggests that the observed trough might be due to scattering rather than absorption, prompting a consideration of particle size effects.
  • A participant confirms their confidence in measuring absorbance rather than transmittance, despite the unusual results, and mentions a different sample that produced a peak.
  • One participant emphasizes the importance of having the analyte dissolved in solution for accurate UV-Vis results, noting that slurries may lead to poor data.
  • A later reply speculates that the expected sharp peak in ZnO nanoparticles could be related to an excitonic band, although the participant expresses uncertainty due to their lack of expertise in semiconductor materials.

Areas of Agreement / Disagreement

Participants generally agree on the potential influence of scattering on the UV-Vis results, but there is no consensus on the reasons for the observed spectral features or the interpretation of the data. Multiple competing views remain regarding the nature of the absorbance and the expected behavior of ZnO nanoparticles.

Contextual Notes

Participants note that the results may be influenced by factors such as particle size and the physical state of the sample (e.g., slurry vs. solution), which could affect the accuracy of the UV-Vis measurements.

goggles31
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I prepared zinc oxide nanoparticles by reacting zinc sulfate with sodium hydroxide and urea. Then, I centrifuged the reaction mixture to obtain the white powder and washed it with deionized water several times. I then mixed the powder with deionized water for UV-Vis. My results are a bit weird and I need help understanding it. I performed UV-Vis for wavelengths between 200-800nm. Starting at 800nm, the absorbance is 0.045 and increases slowly to 0.0483 at 600nm. Then, it slowly decreases to 0.033 at 382nm. Here, it increases rapidly forming a "v" shape until 375nm and the spectrum continues until 200nm. I have a few questions:

  1. Why does ZnO show absorbance in the visible spectrum from 400-700nm? I made sure that my prepared solution was transparent before testing.
  2. ZnO should have an absorption peak around 350-390nm. Apparently mine is a "v" shaped trough. How did this occur?
 
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Are you sure you're looking at absorbance and not transmittance? That would cause peaks to look like troughs.
 
TeethWhitener said:
Are you sure you're looking at absorbance and not transmittance? That would cause peaks to look like troughs.
Yes, I am sure. I have another sample in which I varied the concentration of precursor. It produces a peak.
 
I expect your particles are scattering the wavelengths, you're not looking at absorption or transmission in the visible. Have you done PSD analysis on your sample? A particle size of 2λ will scatter a wavelength of λ.
 
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PS: For good UV-Vis spectroscopy, the analyte must be dissolved in solution. Slurries do not give good results.
 
goggles31 said:
I prepared zinc oxide nanoparticles by reacting zinc sulfate with sodium hydroxide and urea. Then, I centrifuged the reaction mixture to obtain the white powder and washed it with deionized water several times. I then mixed the powder with deionized water for UV-Vis. My results are a bit weird and I need help understanding it. I performed UV-Vis for wavelengths between 200-800nm. Starting at 800nm, the absorbance is 0.045 and increases slowly to 0.0483 at 600nm. Then, it slowly decreases to 0.033 at 382nm. Here, it increases rapidly forming a "v" shape until 375nm and the spectrum continues until 200nm. I have a few questions:

  1. Why does ZnO show absorbance in the visible spectrum from 400-700nm? I made sure that my prepared solution was transparent before testing.
  2. ZnO should have an absorption peak around 350-390nm. Apparently mine is a "v" shaped trough. How did this occur?
1. has already been mentioned by Kebin McHugh as scattering, and I agree to this.

I am really not sure about 2. since my field of research is not semiconductor materials, but quick research gave me information that ZnO nanoparticles are supposed to have a sharp peak around that range. I speculate that it is an excitonic band.