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ELECTRON MICROSCOPY AND STRUCTUR

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					                                 Acta Microscopica, Vol. 18, Supp. C, 2009                        253




ELECTRON MICROSCOPY AND STRUCTURAL STUDIES OF UNDOPED ZINC OXIDE
THIN FILMS DEPOSITED BY CHEMICAL SPRAY FOR PHOTOCATALYTIC
DEGRADATION OF METHYLENE BLUE: EFFECT OF THE WATER AND SUBSTRATE
TEMPERATURE.

A.Guillén-Santiago a, S. A. Mayén a, G. Torres-Delgado a, R. Castanedo-Pérez a,
A. Maldonado b, M. de la L. Olvera-Amador b
a
  Lab. de Investigación en Materiales, CINVESTAV-IPN, U. Querétaro, Apdo. Postal 1-798,
Querétaro, Qro. 76001, México
b
  Depto. de Ing. Eléctrica, CINVESTAV-IPN, Apdo. Postal 14-740, México D. F., 07000, México


Undoped zinc oxide thin films were deposited on glass substrates by the spray pyrolysis technique.
Zinc acetate hydrated was used as the starting reagent. The effect of the water content in the starting
solution as well as the substrate temperature of deposition on the photo-catalytic degradation of
methylene blue (MB), were studied. The X-ray diffraction measurements show that the samples fit
well with hexagonal zinc oxide, although the intensity of the peaks is affected by the water content.
The degradation of MB dissolved in water, as a result of the reaction produced on the surface of the
films, and promoted by ultraviolet irradiation, was measured by the variation of the corresponding
absorption spectra. The results show that as the water content increases, the degradation increases,
reaching an optimum value in the water content, where an order of 18% of MB remains after 5
hours of irradiation. Further increase in water content decreases the photo-catalytic activity and
hence the degradation of MB. On the other hand, the ZnO films deposited at 450 ºC show more
activity than those deposited at 500 ºC. The results are consistent with the model of surface
modification for the increase in photo-catalytic activity. Morphological studies show that changes in
the surface occur, due to the water content in the starting solution, affecting the photo-catalytic
performance.

Key words: zinc oxide, photocatalysis, chemical spray, thin films



Photocatalytic processes in water through the use of UV-irradiated organic oxides degrade the
concentration of contaminants. ZnO is very competitive on the photo-catalytic degradation of
methylene blue (MB) [1]. Besides ZnO thin films have been found to decompose aqueous solutions
of reactive dyes [2,3]. ZnO thin films have been prepared by several techniques, such as sol-gel
[4,5], chemical vapor deposition [6], among others. In this work, ZnO thin films were prepared by
the spray pyrolysis technique [7].

Solutions were prepared using zinc acetate hydrated as precursor. The water content was varied in
order to observe its effect on the degradation process. X-ray diffraction patterns were obtained using
a Rigaku D/max-2100 diffractometer (CoKα1 radiation, 1.78899 Å). The surface morphology of
ZnO thin films was observed by Scanning Electron Microscopy using a Philips XL30 microscope.
The photo-catalytic activity was quantified by means of the bleaching of MB solution dropped into
a quartz cell where a ZnO thin film is located. The arrangement is exposed to UV light, as described
by G. Torres at al. [1]. The MB residual concentration was determined using the UV-Vis
absorbance spectra registered on a Perkin-Elmer Lambda-2 spectrophotometer.
                                                                                                          Acta Microscopica, Vol. 18, Supp. C, 2009                   254




                                                      1000                   ZnO               H2O



                                                                                           300 ml/L
                       Intensidad de rayos X (u.a.)




                                                        0
                                                      1000




                                                                                          100 ml/L

                                                        0
                                                      1000




                                                                                               0%
                                                                                                                                                                   2um
                                                        0
                                                                 30   40         50       60         70
                                                                          2 θ (grados)                                         Fig. 4 SEM micrograph for a TiO2 thin film
                                                                                                                               with 100 mL/L wáter content in solution
           ray
Fig. 1 X-ray diffraction measurements for
different water content.
                                                                                                                               References
                      1.2
                                                                                                                                                      ,
                                                                                                                               [1] G. Torres Delgado, C. I. Zuñiga Romero,
                      1.0
                                                                                                           0mL
                                                                                                          100 mL
                                                                                                                               S. A. Mayén Hernández, R. Castanedo Pérez
                                                                                                          300 mL               and O. Zelaya Angel. Solar Energy Materials
                      0.8                                                                                                      and Solar Cells 93 (2009) 55.
    [MB] Normalized




                      0.6
                                                                                                                               [2] C. A. K. Gouvêa, F. Wypych, S. G.
                      0.4
                                                                                                                                                                  Peralta-
                                                                                                                               Moraes, N. Durán, N. Nagata and P. Peralta
                                                                                                                               Zamora. Chemosphere 40 (2000) 433.
                      0.2


                                                                                                                               [3] C. Lizama, J. Ferrer, J. Baeza and H. D.
                      0.0
                                                             0        1               2         3          4       5           Mansilla. Catal. Today 76 (2002) 235.
                                                                                   Time (Hours)


                                                                                                                               [4] B. Pal and M. Sharon. Mater. Chem.
Fig. 2 MB normalized concentration against                                                                                     Phys. 76 (2002) 82.
the time of irradiation.
                                                                                                                               [5] F. Peng, S.H. Chen, L. Zhang, H.J.
                                                                                                                               Huang and Z. Y. Xie. Acta Phys. Chem. Sin.
                                                                                                                               21 (2005) 944

                                                                                                                               [6] H. Y. Ha, S. W. Nam, T. H. Lim, I. H.
                                                                                                                                                   .
                                                                                                                               Oh and S. A. Hong. J. Membr. Sci. 111
                                                                                                                               (1996) 81.

                                                                                                                                                                  McManus-
                                                                                                                               [7] M. Wei, D. Zhi and J. L. McManus
                                                                                                                               Driscoll. Scr. Mater. 54 (2006) 817.

                                                                                                               2um


Fig. 3 SEM micrograph for a TiO2 thin film
with 0% wáter content in solution

				
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