In this research, Aluminum (Al) and Calcium
(Ca) co-doped zinc oxide (ZnO) nanoparticles synthesized by the sol-gel method
successfully. The synthesized nanoparticles were characterized using X-Ray
diffraction (XRD), transmission electron microscopy (TEM) and spectrophotometer
UV-Vis. The results revealed hexagonal and rod-like dominant morphology for undoped
and Al-Ca co-doped ZnO nanoparticles, respectively. UV–vis absorption spectra showed that Al-Ca co-doped ZnO exhibited a red-shift of the
band-edge compared to the pure ZnO. Co-doping of Al and Ca as an intermediate
metal inside of ZnO structure caused decreasing of band gap quantity and
increasing of absorption rate in visible area. The photocatalytic activity of
the prepared catalysts were measured by degradation of methyl orange (MO) under
ultraviolet light irradiation. Al-Ca co-doped ZnO nanoparticles showed proper
photocatalytic activity compared to pure ZnO. The results revealed
that the best photocatalytic performance was obtained at 5% Al and 3% Ca (Zn0.92
Al0.05 Ca0.03 O).

Keywords: ZnO nanoparticles, Co-doped, Sol-gel, Photocatalysis, Methyl orange.









1. Introduction

In the past decade, reclamation of environment and pollution clean-up has
been one of the global preferences 1. Daily, a lot of harmful industrial
pollutants release into the environment 2. With the lapse of time and
industrialize, chromatic materials divided into two categories of natural and
synthetic which widely used in the industries of weaving, papermaking,
impression, leather,  cosmetics,  and sanitary materials, plastic, and
alimentary 3.Industrial wastewater containing toxic dye effluents has raised
severe environmental threat 4.  Dyes are
frequently poisonous, carcinogen, biological irresolvable and stable, that by
releasing to the environment, create undesirable effects 5. Methyl orange uses
as an organic dye in the weaving, cosmetics and pharmaceuticals industries and
has medical dangers for humans and animals 6.  Use of nanocatalysts is one of the pollution elimination
methods, which can achieve modern catalysts that have higher specific surface
area compared to current catalysts 7-9. Recently, nanosized metal-oxide
semiconductor materials have played an important role in the photo degradation
of organic contaminants 10. Zinc oxide also has been considered as a suitable
alternative for TiO2 due to its similar band (3.3eV), lower cost and
better stability 11-12. However, the
application of ZnO semiconductor is limited because of its higher charge
carrier recombination rate 13. The presence of corrector ingredients in the ZnO
crystals changes the coupling velocity of load carriers and relocation width of
the band into visible area remarkably 14. Doping is a powerful way to enhance
the separation of charge carries in semiconductor photocatalyst 15. Doping with metallic cations or non-metallic anions has been widely used
for the modification of ZnO to improve its photocatalytic activity or to extend
its light absorption into visible region 16. Undoped ZnO nanoparticles have
low activity in the visible region, while show the most photocatalytic activity
in the ultraviolet region. since the radiance of sunlight includes mostly
visible light, doping by metal to increase the activity of nanoparticles
absorption is very important 17.

Recently, Ahmad et al. 18 synthesized doped nanoparticles with Al by combustion method and
investigated photocatalytic degradation of methyl orange under radiation of
visible light at different concentrations. Their results showed that Al
concentration of 4% is optimum concentration to improvement photocatalytic
activity. Slama et al. 19 reported photo-degradation of Methyl blue by
doped ZnO nanoparticles with Calcium. In their study the maximum photocatalytic
activity occurs at Ca concentration of 4 %. Furthermore, Zhang et al. 20 synthesized Er-Al co-doped ZnO nanoparticles
by a solvothermal method and reported photocatalytic activity of particles by
degradation of methyl orange under radiation of visible light. Results of their
research showed high photocatalytic activity of co-doped particles compared to undoped
and doped by a single metallic. Senthilraja 21 and Huoj 22 demonstrated use
of two metals synthetically in doping zinc oxide nanoparticles that increased photocatalytic
activity remarkably. These observations are attributed to superficial
interactions between ZnO and doped metals and also reduction of coupling
velocity of electrons and holes. The metals of calcium and aluminum are very
suitable impurity for improvement of photocatalytic activity due to low cost, abundance,
and exclusive physical properties 23. Although the use of co-doped ZnO
nanoparticles caused enhancement of photocatalytic activity, selection of
suitable doping metals and their effectiveness on the photocatalytic activity
needs more research and experiments.

In this research, Al-Ca co-doped ZnO nanoparticles were synthesized with
polyhedral structure by the sol-gel method, without any template or
surfactants. Furthermore, the structure, morphology, absorption spectra and
photocatalytic activity of synthesized pure ZnO nanoparticles were considered. Photocatalytic
properties of synthesized nanoparticles were investigated by degradation of
methyl orange as a pollutant under radiation of ultraviolet light.



2. 1. Materials and method synthesis of

Al-Ca co-doped ZnO nanoparticles were prepared at different
concentration of Al and Ca by the sol-gel method, respectively. Firstly 100 ml
of 0.15 M zinc acetate (Zn (CH3COO)2. 2H2O)
solution made by use of methanol. After that Aluminum nitrate (Al (NO3)3)
and of calcium nitrate (Ca (NO3)2) were added to the
solution gradually, and the mixture stirred for 2 hours. Then one molar sodium
hydroxide was added slowly to the solution to reach pH of 10 and stirred 60
minutes at the room temperature. Afterward obtained sol centrifuged 30 minutes
at 6000 rpm rounds until complete the sol-gel process. Sediment with white
color washed several times by using water and ethanol. Products were dried at
the temperature of 120 °C for 2 hours. Finally,
the white powder was produced. It should be noted tha