Solution combustion synthesis of ZnO powders using CTAB as fuel
Introduction
Zinc oxide (ZnO) is an interesting material for many applications such as electro-optics, spintronics and gas sensors, due to its wide band gap (3.27 eV), large exciton binding energy (60 meV), cost effectiveness, nontoxicity, and structural stability [1], [2], [3]. The physical and chemical properties of ZnO strongly depend on structure, particle size and shape and composition which are tunable via synthesis method [4]. Zinc oxide powders have been prepared by various routes such as solid state, hydrothermal, precipitation, solvothermal, solution combustion synthesis, etc. [5], [6], [7], [8], [9]. Among the methods, solution combustion synthesis (SCS) as a simple, versatile, time- and energy-efficient and environmentally friendly route has received a great attention for rapid and mass production of nanomaterials [10]. SCS involves a self-sustained exothermic reaction between oxidizers (e. g. metal nitrates) and organic fuels (e. g. urea, glycine, citric acid) [11]. The molecular mixing of cations together with released heat by combustion reaction guarantee the formation of complicated structures at low temperatures [12]. Furthermore, the combustion of organic fuels generates a large amount of gaseous products such as CO2, H2O, H2, N2, CO, etc., leading to porous structures. The outgoing gases rapidly cool the as-combusted products, preventing particle growth and sintering [13], [14], [15].
Powder characteristics of the as-combusted ZnO powders such as structure, particle size and shape, specific surface area and optical properties can be mainly tuned by fuel nature and fuel content, metal precursor, pH, gel rheology, etc. [16], [17]. Urea and glycine are popular fuels in SCS for their low decomposition temperature, high solubility, readily available and low cost [18], [19]. However, other organic fuels such as oxalic acid, dextrose, oxalyl dihydrazide, L-Glutamine, Leucine, L-Valine, etc. were also employed for solution combustion synthesis to improve the specific surface area and crystallinity of the as-combusted ZnO powders [20], [21]. Cetyltrimethylammonium bromide (CTAB) as a cationic surfactant has been widely employed in material synthesis to reduce particle size by capping or protecting [22]. CTAB has been used in solution combustion synthesis of oxide nanoparticles such as CuO [23], Co3O4 [24], SrFe12O19 [25], BiVO4 [22]. Bedi and Singh compared the effects of CTAB on the morphology and particle size of the as-combusted CuO powders [26]. The surfactant molecules separate the sol particle due to the electrostatic stabilization, leading to spherical CuO nanoparticles (∼20 nm) with a minimum degree of agglomeration.
In the present work, the effects of fuel to oxidant ratio on the combustion behavior, phase, microstructure, optical properties and photocatalytic activity of the solution combusted ZnO powders using CTAB as fuel were reported. The slow combustion reaction rate of CTAB fuel led to the formation of small platelet ZnO particles at high fuel content.
Section snippets
Experimental procedures
The required amounts of zinc nitrate (Zn(NO3)2) and cetyltrimethylammonium bromide ([(C16H33)N(CH3)3]Br) as fuel were dissolved in distilled water in which the different fuel to oxidant ratios (ϕ = 0.5, 0.75, 1 and 1.5) were used. After homogenization, the precursor solution was poured into a dish and heated till to transform into a gel while by further heating up to 250 °C on a hot plate, ignition reaction started from a point and propagated spontaneously. The resulted powders were
Results and discussion
On the base of propellant chemistry, the redox reaction during combustion process by considering CO2, N2, Br2 and H2O as byproducts can be written as follows [27]:
Here, the stoichiometric mixture (ϕ = 1) means no need to atmospheric oxygen for completing fuel oxidation, while the ϕ> 1 (or ϕ < 1) corresponds to the fuel-rich (fuel-lean) condition.
FTIR spectra of the dried gels at 80 °C and the as-combusted ZnO powders
Conclusions
Single phase ZnO powders were directly synthesized by solution combustion method using CTAB as fuel at various fuel contents. The formation of small platelet ZnO particles can be attributed to the slow combustion reaction rate of CTAB fuel. The specific surface area and pore volume of the as-combusted ZnO powders increased with the increase of fuel content due to the exhausting of a large amount of gaseous products. The photodegradation of MB dye under UV light irradiation increased from 35% to
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