UV-Vis of zinc oxide nanoparticles

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In summary: Excitons are energy levels that are split between two valence electrons.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.PS: For good UV-Vis spectroscopy, the analyte must be dissolved in solution. Slurries do not give good results.
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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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  • #2
Are you sure you're looking at absorbance and not transmittance? That would cause peaks to look like troughs.
 
  • #3
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.
 
  • #4
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.
 
  • #6
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.
 

1. What is the UV-Vis spectrum of zinc oxide nanoparticles?

The UV-Vis spectrum of zinc oxide nanoparticles typically shows a strong absorption peak in the UV region with a maximum at around 370-380 nm. This peak is due to the bandgap absorption of the nanoparticles.

2. How does the size of zinc oxide nanoparticles affect the UV-Vis spectrum?

The size of zinc oxide nanoparticles has a significant impact on the UV-Vis spectrum. As the size decreases, the bandgap energy increases, leading to a blue shift in the absorption peak. This means that smaller nanoparticles will have a higher energy absorption peak in the UV region.

3. Can the UV-Vis spectrum of zinc oxide nanoparticles be used to determine their size?

Yes, the UV-Vis spectrum can be used to estimate the size of zinc oxide nanoparticles. As mentioned before, smaller nanoparticles will have a blue shift in the absorption peak compared to larger nanoparticles. By analyzing the peak position and shape, the size of the nanoparticles can be estimated.

4. How does the concentration of zinc oxide nanoparticles affect the UV-Vis spectrum?

The concentration of zinc oxide nanoparticles can also affect the UV-Vis spectrum. As the concentration increases, the intensity of the absorption peak also increases. This is due to the increased number of nanoparticles present, leading to a higher chance of light being absorbed.

5. What factors can cause changes in the UV-Vis spectrum of zinc oxide nanoparticles?

There are several factors that can cause changes in the UV-Vis spectrum of zinc oxide nanoparticles. These include changes in the size, shape, concentration, and surface chemistry of the nanoparticles. Other external factors such as temperature, pH, and light exposure can also affect the spectrum.

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