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Y.J. Jang et al. / Materials Research Bulletin 41 (2006) 67–77
1. Introduction
Nanoparticles, smaller than several tens of nanometer in primary particle diameter, frequently show
behavior which is intermediate between that of a macroscopic solid and that of an atomic or molecular
system. Nanoparticles display fascinating electronic and optical properties as a consequence of their
dimensions and they may be synthesized from a wide range of materials. The dimensions of these
particles make them ideal candidates for the nanoengineering of surfaces and the fabrication of functional
nanostructures that are useful in optoelectronics, chemical and biosensors, drug delivery, catalysis, etc.
Nanoparticles of semiconductors such as titanium dioxide (TiO2), zinc oxide (ZnO), iron oxide (Fe2O3),
and cadmium sulfide (CdS) have attracted extensive attention as a photocatalyst for the degradation of
organic pollutants in water and air [1,2].
Photocatalyst is also called photochemical catalyst and the function is similar as the chlorophyll in the
photosynthesis. In a photocatalytic system, photo-induced molecular transformation or reaction takes
place at the surface of catalyst. A basic mechanism of photocatalytic reaction on the generation of
electron–hole pair and its destination is as follows; when a photocatalyst is illuminated by the light
stronger than its band gap energy, electron–hole pairs diffuses out to the surface of photocatalyst and
participates in the chemical reaction with electron donor and acceptor. Those free electrons and holes
transform the surrounding oxygen or water molecules into OH free radicals with super strong
oxidization. It can oxygenolyse various kinds of organic compounds and part of minerals. It may also
deoxidize harmful substances like benzene, formaldehyde, and ammonia into CO2 and water free of
poison, harm, and odor. Therefore, photocatalyst may kill germs, viruses, epiphytes, pollen, and the like
and may decompose formaldehyde, benzenes, ammonia, and other harmful gases, and it will not bring
secondary environmental pollution [3,4].
TiO2 nanoparticles have been known as a representative of the photocatalyst among semiconductors
due to their high photocatalytic activity, stable chemical and physical properties [5–10].
Although TiO2 nanoparticles are used for many environmental applications, ZnO are a suitable
alternative to TiO2 so far as band gap energy is concerned. It has also suggested that ZnO is a low cost
alternative photocatalyst to TiO2 for degradation of organics in aqueous solutions [11].
Dinda and Icli [12] found ZnO was as reactive as TiO2 for the photocatalytic degradation of phenol
under concentrated sunlight. Chakrabarti and Dutta [13] explored ZnO particles of 146 nm in average
diameter as an effective catalyst for the photodegradation of model textile dyes under the illumination of
ultraviolet light. Wang et al. [14] reported enhanced photocatalytic activity for methyl orange degrada-
tion using ZnO nano-crystalline particles that are larger than 100 nm in average particle diameter but
smaller than 100 nm in mean grain size.
From the analysis of the previous results, we found that nanoparticles and nano-crystalline particles of
ZnO are both effective photocatalyst on the photocatalytic degradation of organic compounds. But there
was no report on the comparison of photocatalytic activity of those particles having different morphol-
ogy.
In this study, photocatalytic activity of ZnO nanoparticles was compared with that of ZnO nano-
crystalline particles on the degradation of methylene blue in water under the illumination of ultraviolet
rays for the first time. Flame spray pyrolysis and spray pyrolysis assisted with an electrical furnace were
employed to synthesize the nanoparticles and the nano-crystalline particles, respectively. Then, effect of
particle morphology, ZnO catalyst loading, and concentration of the methylene blue on the photocatalytic
degradation of methylene blue in water under the illumination of ultraviolet light was investigated.