Band gap energy of powder photocatalysts

U3
Band gap energy of powder photocatalysts
Objective: Determine approximate value of Eg for selected powder photocatalyst
Principle:
Certain semiconductor like TiO 2 , ZnO, ZnS, CdS are widely used for photocatalysis of
chemical reactions and are used in nanotechnology. The main advantage of TiO 2 was
discussed in U13. To evoke the photocatalytical properties of photocatalysts, they have to be
irradiated with light having the same (or higher) energy as band gap energy of given
semiconductor. If the transition of electron from valence to conductive band occurs, the
electron – hole pair is generated.
For the evaluation of band gap energy of photocatalyst UV-VIS spectrophotometry equipped
with diffuse reflectance accessory is being used very often. On the surface of finely
powderized sample the incident beam is diffusely scattered. The diffusely scattered beam is
concentrated in the reflectance sphere.
Questions:
1. Find E g values for TiO 2 (anatase and rutile), CdS, ZnO, ZnS.
2. For each semiconductor (listed in question 1) calculate the wavelength of the light
which has to be used for transition of electron from valence band to conductive band.
3. List the examples of practical utilization of photocatalysis.
Materials and equipment:
Procedure:
samples of semiconductor, 2x Spectralon standards (barite targets), spectrophotometer
CINTRA303, diffuse reflectance accessories, 5mm UV-VIS absorption cells (cuvetes),
manual CINTRA303, flash disc.
1. Ask the assistant for installation of diffuse reflectance accessories.
2. Set up the parameters for registration of diffuse reflectance spectra; refer to
CINTRAL-Manual for students.
3. Using the Spectralon standards measure the reference line. During this measurement
behind both entrance slits the Spectralon standards are placed (see positions x1 and x2
in Fig. 1).

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1
2
Fig. 1 a) installation of diffuse reflectance accessory, b) positioning of sample to diffuse
reflectance accessory
4. Pour carefully the sample to measuring cuvete. Clean the outer side of cuvete.
5. Replace the Spectralon standard in position 1 with sample in cuvete.
6. Measure diffuse reflectance spectra of samples, for measurement refer to manual.
7. Import the measured data (*.csv format), the data are imported in two columns – 1 st -
wavelengths, 2 nd - relative reflectance.
8. Using EXCEL (or using other software) perform the evaluation of the measured data:
multiple measured reflectance with the value 100 (the reflectance is now expressed in
the form of %), next construct the relationship Reflectance(%)=f(wavelength).
Translate the wavelength values to energy in eV, construct the relationship
Reflectance(%)=f(E).
9. Perform the Kubelka-Munk transformation of measured reflectance (according to eq.
1).
2
(1 R)
K (1)
2R
where K is reflectance transformed according to Kubelka Munk, R is reflectancy (%).
2
10. Construct the relationship ( K * h
) f ( h * ) [1] - (see Fig. 2).
11. Find the E g value.
1
[1] J. Tauc, R. Grigorovici and A. Vancu, Phys. Status Solidi 15 (1966), p. 627.
Conclusion:
Summarize the measurement, list the found E g values and comment them.
Acknowledgement:
Author tanks to RNDr. J. Jirkovský, CSc. from J. Heyrovsky Institute of Physical Chemistry for consultation
with interpretation of diffuse reflectance spectra.

U3
5,5
5,0
4,5
4,0
(K*h) 1/2 [eV 1/2 ]
3,5
3,0
2,5
2,0
1,5
1,0
E g
0,5
0,0
5,0 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0
h* [eV]
2
Fig. 2 Relationship ( K * h
) f ( h * )
1