UV-Vis of zinc oxide nanoparticles

[goggles31]

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?
   

 

[TeethWhitener]

Are you sure you're looking at absorbance and not transmittance? That would cause peaks to look like troughs.

 

[goggles31]

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.

 

[Kevin McHugh]

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.

 

[Kevin McHugh]

PS: For good UV-Vis spectroscopy, the analyte must be dissolved in solution. Slurries do not give good results.

 

[HAYAO]

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.