**1. Introduction**

Nanoscale ZnO powder has attracted great attention due to its excellent physical and chemical properties they are widely used in nanoscale devices such as nanogenerators [1], ultraviolet photodetectors [2], gas sensors [3], solar cells [4], field emission displays [5], electrical and optical devices [6, 7], photocatalysis [8, 9], medical [10] and environmental applications [11]. These nanomaterials have novel electronic, structural and thermal properties which have potential interest in basic and applied research. ZnO is a wide bandgap (Eg = 3.37 eV) semiconductor some basic properties listed in **Table 1** [12].

Semiconductor nanocrystals or nanoparticles may have superior optical and antibacterial properties than bulk crystals due to quantum confinement effects and the large surface to volume ratio. The synthesis and properties of ZnO nanostructure such as nanowires [13], nanotubes [14], nanorods [15] and nanoparticles [16] have been reported. The nanoparticles have great significance


### **Table 1.**

*Basic properties of ZnO [12].*

as three dimensional confined systems bridging the gap between bulk materials and molecular compounds. A variety of techniques have been employed for the synthesis of ZnO nanoparticles such as sol-gel synthesis [17], the hydrothermal method [18], the solution combustion method [19] and solid state reactions [20]. Among these, the combustion technique is noteworthy as a fast method to synthesize nanocrystalline materials in as-synthesized form with large surface area without the further need of heat treatment. Nanocrystalline oxides are produced through the redox reaction between an oxidizer containing the metal precursor and anorganic fuel at a moderately low initiation temperature of around 350–600°C within a few minutes [21]. The main advantage of this method is that the high temperature of the exothermic reaction assures high purity and well crystallized powder. In combustion synthesis, the type of fuel and the fuel to oxidizer ratio (F/O) play critical roles in influencing the nature of combustion reaction and the flame temperature. Selection of a suitable fuel and the F/O ratio influences the combustion process and the properties of the product. The F/O ratio of unity is known to produce highest exothermicity with complete combustion. An arbitrary ratio of fuel to oxidizer (F/O—1) sometimes leads to formation of intermediate phases raw materials in the final product [22]. In this regard, various fuels have been tested to synthesize nanocrystalline ZnO. Sousa et al. [23] used metallic nitrate and urea to synthesis ZnO nanopowder with a size about 400–500 nm for various applications. Hwang et al*.* [24] worked on ZnO nanopowder synthesized by a combustion method with glycine as a fuel and metal nitrate mixed in a stoichiometric ratio.

In the present work, we report the synthesis of nanocrystalline ZnO powders by combustion technique using new, eco-friendly and cost-effective organic fuels as urea, glycine and citric acid. The effect of fuel in different ratio of two fuels combinations on the properties of the final product has been studied. The structure and luminescence properties of ZnO nanoparticles are also being studied in this work.

**17**

**Table 4.**

**Table 2.**

**Table 3.**

*Characteristics of raw material.*

*Structural and Luminescence Properties of ZnO Nanoparticles Synthesized by Mixture of Fuel…*

Synthesis of ZnO NPs the different materials were used such as zinc nitrate, urea, glycine, and citric acid. **Table 2** shows the characteristic of the raw materials. The chemical reaction used in synthesis of ZnO powder are given in **Table 3**.

The zinc nitrate hexahydrate and fuel were dissolved in 5 ml of double distilled water and stirred thoroughly to obtain a transparent solution, which was placed inside a preheated muffle furnace at 600°C to initiate the combustion process. Within a short time the mixture ignited with a flame and the rapid evolution of enormous amounts of gases produced a voluminous foamy product (ash). This was ground using an agate pestle and mortar to produce the final powder, without any additional heat treatment. The fuels used in this synthesis have different combina-

**Raw materials Formulation Molecular weight (g/mol) Manufacturer** Zinc nitrate Zn(NO3)2.6H2O 297.49 Sigma Aldrich Urea NH2CONH2 60.06 Sigma Aldrich Glycine NH2.CH2.COOH 75.06 Sigma Aldrich Citric acid C6H8O7.H2O 210.14 Sigma Aldrich

The synthesis process of ZnO NPs is illustrated in **Figure 1**.

**Fuel Combustion reaction**

*Chemical reaction in combustion synthesis of ZnO using different fuels [25].*

*Sample name with respect to used fuels in different combination of fuels.*

Urea 3Zn(NO3)2 + 5CO(NH2)2 → 3ZnO + 5CO2 + 10H2O + 8N2 Glycine Zn(NO3)2 + 2CH2(NH2)(COOH) + 2O2 → ZnO + 4CO2 + 5H2O + 2N2 Citric acid Zn(NO3)2 + C3H5O(COOH)3 + 2O2 → ZnO + 6CO2 + 4H2O + N2

**Sample name Fuels contents (%)**

UC1 75 — 25 UC2 50 — 50 UC3 25 — 75 UG1 75 25 — UG2 50 50 — UG3 25 75 —

**Urea Glycine Citric acid**

*DOI: http://dx.doi.org/10.5772/intechopen.82467*

**2.1 Preparation of ZnO nanoparticles**

tion of fuels and shown in **Table 4**.

**2. Experimental**
