**1. Introduction**

In recent years, several dyes have been frequently used in textiles, printing, paper and pharmaceutical industries. The untreated hazardous dyes are discharged into the water, leading to enormous environmental problems, like perturbation of aquatic life and human health. Therefore, the removal of these dyes from water is of the utmost priority for the scientific community. Several approaches have been made to remove the toxic dye molecules from wastewater, such as adsorption, coagulation, membrane separation and ion exchange process. However, these methods fail on a larger scale due to their expensive equipments, slow processes and toxic byproducts [1]. Effective and successful methods to remove dye include photocatalytic activity in which metal oxide semiconductors are used as catalysts due to their large specific surface area, chemical stability and high photocatalytic response [2,3]. It is vitally important to establish the stability and activity of the photocatalyst to propose a photocatalytic system. From the existing transition metal oxide semiconductors, iron oxide has drawn scientific interest due to its outstanding physical and chemical properties. A variety of crystalline phases are exhibited by iron oxides, such as hematite (α-Fe2O3), akaganeite (β-Fe2O3), maghemite (γ-Fe2O3) and magnetite (Fe3O4) [4]. Among them, α-Fe2O<sup>3</sup> exhibits thermodynamical, as well as chemical stability, over a broad pH scale. This compound has drawn significant interest for their

potential applications such as photocatalysts, magnetic data storage, gas sensors, lithium-ion batteries, spintronics and ferrofluids [1,5–8]. The atomic arrangement possessed by hematite is similar to that of corundum α-Al2O<sup>3</sup> structure, in which anions (O2<sup>−</sup> ions) are stacked in hexagonal close-packed arrangement (framework by the regular alternating layers, in each layer the atoms lie at the vertices of a series of equilateral triangles and the atoms overlie one another in one layer), with cations (Fe3+) occupying 2/3 octahedral coordination geometry [9].

α-Fe2O<sup>3</sup> is a promising photocatalyst with optical band gap of ~2.6 eV. Also, hematite is one of the few semiconductors having valence band edge position suitable for oxygen evolution and the conduction band edge is more negative than the redox potential of H+/H2, thus, requiring an electrical bias to generate hydrogen [10]. However, the catalytic activity of α-Fe2O<sup>3</sup> nanoparticles remains much lower due to rapid recombination of charge carriers, which reduces the degradation performance [1]. Thus, several methodologies have been made in order to sort out this problem. An effective process is doping of α-Fe2O<sup>3</sup> with other metal ions, which can overcome their limitations. Doping of various metal ions such as Cr, Ti, Mn, Al, Zn, Ni, Ga, Rh, Zr and Co at Fe site in hematite influence the physical and photocatalytic properties. It is observed that Zr dopant limits the recombination of electron hole pairs in Fe2O<sup>3</sup> nanorods array that act as a better catalyst for dye degradation [11]. Similarly, Ti-doped Fe2O<sup>3</sup> enhances the donor density and lowers the electron-hole pair recombination rate that improves the photocatalytic activity [12].

The influence of divalent Zn cation on structural, electrical and optical behavior of hematite has become a field of scientific research. The substitution of Zn2<sup>+</sup> at Fe3<sup>+</sup> site causes the charge imbalance in the host lattice [13]. In order to maintain charge neutrality, one or more of the following mechanisms can occur: Transformation of Fe3<sup>+</sup> to Fe2<sup>+</sup> state, creation of cation vacancies and filling of oxygen vacancies. The physical properties of hematite are effected by the degree of crystallinity, particle size, doping and pressure [14–17]. A report by Velev et al. [18] showed that Zn2<sup>+</sup> affects electronic properties of hematite that causes the creation of a hole in the oxygen valence band. The extra hole from Zn2<sup>+</sup> is situated on the neighboring O sites inducing an acceptor level just below the fermi energy. This hole is relatively delocalized, and hence, provides good hope for high conductivity. The purpose of incorporation of Zn2<sup>+</sup> ions is to promote the hopping mechanism of electrons by Fe3+-Fe2<sup>+</sup> pairs and also modifying the optical properties. Based on these factors, we have synthesized Zn-doped Fe2O<sup>3</sup> nanoparticles, with dilute concentrations, to study their structural, optical, dielectric and photocatalytic properties.
