Dependence of optical properties on doping metal, crystallite size and defect concentration of M-doped ZnO nanopowders (M = Al, Mg, Ti)

Abstract

ZnO, Al-, Mg- and Ti-doped ZnO nanopowders were synthesized from CTAB-assisted oxalate intermediate by thermal decomposition method at 600 °C in air. All samples presented a hexagonal wurtzite structure. The spherical nanoparticles assembled in a porous octahedron-like shape for all samples. The size of Al-doped ZnO nanopowders increased as a function of Al ion concentration whereas the size of Mg- and Ti-doped ZnO nanopowders decreased when Mg and Ti ion concentrations were increased. The increment and reduction of their sizes can be explained by the Zener pinning effect. The E_g value of Al-doped ZnO nanopowders slightly decreased when Al ions were increased due to the crystallite size and defect concentration increased. In contrast, the E_g value of Mg- and Ti-doped ZnO nanopowders increased as a function of Mg and Ti ion concentration which can be explained by the Moss–Burstein effect.

Introduction

For almost a decade, nanomaterials have been attracting interest in the world of science. Great efforts have been recently devoted to the preparation of nanostructured metal oxides with a variety of shapes and sizes, because of their strikingly uncommon properties [1], [2], [3], [4], [5]. Among the nanostructured metal oxides, ZnO is considered to be one of the best metal oxides that can be used at a nanoscale level. ZnO itself has normally a hexagonal or wurtzite structure and it is well-known as an n-type II–VI semiconductor with a wide direct band-gap of about 3.37 eV and a large exciton binding energy of 60 meV [6]. From this point of view, nanostructured ZnO powders display a great power in many applications such as gas sensors [7], solar cells [8], varistors [9] and photocatalyst with high chemical activity [10]. A number of chemical methods of synthesis have been applied to synthesize nanostructured ZnO powders, for example by using hydrothermal [11], precipitation [6], sol–gel [12] and thermal decomposition [13] methods. Among these chemical routes, precipitation and thermal decomposition are a common method that can be used for large scale production. Moreover, the particle shape can be controlled easily by modifying the precursor solution with an appropriate surfactant such as diethylene glycol (DEG) [14], cetyltrimethylammonium bromide (CTAB) [15], sodium dodecyl sulfate (SDS) [16], monoethanolamine (MEA), diethanolamine (DEA) or triethanolamine (TEA) [17]. To date, a number of research workers have reported the fabrication of nanostructured ZnO powders with spherical [14], rod-like [18], flower-like [10] and sheet-like [19] structures. Only a few reports [11], [20], [21] have involved the synthesis of porous ZnO through an oxalate intermediate by a chemical route. Likewise only a few researchers have investigated the construction of porous metal-doped ZnO powders, for example, Peiteado et al. [22] studied the phase formation of a Zn_1−xMn_xO system. Kanade et al. [23] reported the formation of self-assembled aligned Cu-doped ZnO nanoparticles for photocatalytic hydrogen production under visible light irradiation. Astonishingly enough, the publication concerning porous Al-, Mg- and Ti-doped ZnO nanopowders was still incomplete. Therefore, a synthesis of porous Al-, Mg- and Ti-doped ZnO nanopowders via the oxalate precursor by thermal decomposition method in the presence of CTAB could be worthwhile investigating.

In this paper, we report the influence of a small amount of Al, Mg and Ti ion concentrations in the precursor solution modified with CTAB on the self-assembled aligned Al-, Mg- and Ti-doped ZnO nanopowders and we also report the effects of Al, Mg and Ti ion concentration on the crystallite size, defect concentration and optical properties.