INTRODUCTION
A promising and rapidly expanding field of study, nanotechnology has achieved great success in the age of contemporary technology. Materials with distinctive size (often falling between 1 and 100 nm), structure, electric, magnetic, mechanical physio-chemical, thermal, catalytic, optical scattering and form characteristics are called nanoparticles [1]. The ultra-small size and large surface area is largely determining the properties and reactivity of the nanoparticles [2]. The smaller sizes magnetic nanoparticles which are less than 100 nm and narrow particle size distribution with high magnetization values are essentially required for applications Reduction in size results in notable effects on the magnetic ordering within the particles because the surface layer’s magnetic structure is different from the particle core’s [3]. Hematite, maghemite, and magnetite are the three major types of iron oxides, which are extremely significant minerals. Hematite (Fe2O3) is the more stable of them under ambient conditions and also the more ecologically friendly and semiconductor (e.g., 2.1 eV) [4]. Hematite (Fe2O3) is more stable with anti-corrosive abilities, tunable optical and magnetic properties, high chemical stability and biological compatibility with inexpensiveness that suitable for a wide range of technological applications. These benefits of hematite allowed for development of creative nano technologies for applications in catalysts, anticorrosive agents high-density magnetic storage media, pigments, water splitting, water purification, solar energy conversion and gas sensors [5]. Iron oxide nanoparticles have been synthesized by using the hydrothermal process [6], co-precipitation [7], micro emulsion, thermal decomposition [8], sol-gel processing, and other methods. Therefore, the preparations and applications of iron oxide nanoparticles have attracted more and more researchers [9].
Hydrothermal synthesis of nanoparticles is more preferable because it can be appropriately expanded for large-scale nanoparticle synthesis [10]. Hydrothermal synthesis would be more commercially viable if the nanoparticles could be synthesized more faster in the reaction containers.  One of the most used techniques for producing nanoparticles under high pressure and temperature is hydrothermal synthesis [11]. The ease of large-scale manufacture, high-crystallized powders with narrow particle size distribution, excellent purity, and adjustable process parameters like solute concentration are some advantages of the hydrothermal synthesis approach [12]. There are two primary components to a hydrothermal system: i) The unit that controls temperature. ii)The reactor; which consists of Teflon lined stainless steel autoclaves.
In this work, we aimed to investigate the effect of temperature change for synthesis of iron oxide at the shape, optical band gap, size and morphology of the particles. We used hydrothermal technique to synthesize pure Fe2O3 nanoparticles. Their optical, morphological and structural properties were ascertained using characterization methods such as the UV-Vis (Ultraviolet-visible analysis), SEM (Scanning Electron Microscopy) and XRD (X-ray diffraction).

MATERIALS AND METHODS
Materials
Ferric chloride hexahydrate (FeCl3.6H2O), Ammonium hydroxide (NH4OH), distilled water and ethanol. 

Methods
Hydrothermal method was used to prepare iron oxide nanoparticle (Fe2O3 NPs). Firstly, 50ml of distilled water was used to dissolve 2g of FeCl3.6H2O before heating at 80 °C for 30 min with magnetic stirring. To maintain the pH value to 9.2, 25ml of 2M ammonium hydroxide was added. Then, the solution was hydrothermally treated after transferred it into the reactor (Teflon-lined stainless-steel autoclave) for several temperatures of (60,100,160) °C for 8 hours. Centrifugation was used for 5 minutes at 3500 rpm in order to separate the yield and after that washed for several times with distilled water and ethanol, and finally, the yield was dried in air for 1 hour at 80 °C and calcined for 4 hours at 700 °C.

RESULTS AND DISCUSSION
UV-Visible Analysis
The UV-Vis absorption spectra of the three samples hydrothermally produced of iron (III) oxide nanoparticles at varying temperatures showed that all absorption UV-Vis curves indicate a highly absorption between 550and 650 nm wavelength. UV-Vis spectra showed absorption peaks at 573, 610 and 568 nm for Fe2O3 NPs synthesized at three different temperatures of 60,100, and 160 oC, respectively (Fig. 3).  The absorption results are in compatible with data from other studies [13]. The band gap energy Eg values can be calculated.by applying the following Eq. 1 [14]:

Where h is the Plank constant = (4.135667x10-15eV.s), C is the speed of light in vacuum = (3.00x108 m/s), and λ is the maximum wavelength of the absorption peaks in nm (λ max).
Maximum wavelength of the absorption peak is 573nm at a temperature of 60 oC, the value of Eg was found to be 2.16. At 100 oC, λmax of the absorption peak was 610nm and the value of Eg was 2.03. While for 160 oC, λmax of the absorption peak is 568nm with 2.18 Eg value. Generally. It was showed that when the wavelength of the absorption UV-Vis peaks reduced, the Eg values increased.  The higher band gap energy tends to shift the absorption to higher energies (short wavelength), which means that near states from conduction band are full of electrons and for this reason the electrons will need more energy to transfer, this appears as high Eg value [15].

XRD crystallography
Nature of crystals, crystal size, shape, purity and crystallinity of Fe2O3 NPs are given by XRD crystallography. Wurtzite crystallites with hexagonal shape match the structural pattern for pure iron (III) oxide as reported by JCPDS (Joint Committee on Powder Diffraction Standards) card no 33-0664. At various temperatures of 60, 100 and 160 oC, XRD crystallography revealed strong diffraction angles for the indices of 012, 104, 110, 113, 024, 116, 018, 214 and 300 across a range of diffraction angles of 10 to 80 degree (Fig. 4). 
The mean crystallite size was calculated by applying Debye–Scherrer Eq. 2:

Where D is the crystallite size (nm), k is the Scherrer’s constant of value 0.89, λ is the X-ray wavelength (λ=1.54056 Å), β is the full width of the assessment peak at half maximum (in radian) and ϴ is Bragg’s angle [16]. Thus, at three varying temperatures of 60, 100 and 160 oC the mean crystallite size of Fe2O3 NPs was decreased from 27 to 12 to 11.7nm, respectively, which means that Fe2O3 NPs that prepared by hydrothermal method is in nanoscale. Additionally, it was discovered that the peak intensities for Fe2O3 NPs had decreased, which clearly suggested a decreased in crystallite size.

FE-SEM microscopy
Fig. 5 shows FE-SEM images for Fe2O3 NPs synthesized at three different temperatures of 60 oC (a,b), 100 oC (c,d) and 160 oC (e,f) and at two different scales bars of 500nm and 1 μm.  Fe2O3 NPs synthesized under lower temperature were agglomerated into dots-like shape. While the nanorods’ shape is formed at temperature rising (160 oC). In Fig. 5(a, b), it was revealed that Fe2O3 NPs had dots-like forms and ranged in size from 100 to 200 nm on average. In Fig. 5(c, d) it was revealed that Fe2O3 NPs had dots-like forms and ranged in size from 90 to 200 nm on average. In Fig. 5(e, f), it was revealed that Fe2O3 NPs had variation from dots to rod like forms and ranged in size from 60 to 200 nm on an average. The nanocrystals agglomerated as well, which is mainly consist of nanoparticles that agglomerate with each others. These nanoparticles tend to agglomerate because of its high surface energy and high surface stress [17].

CONCLUSION 
Fe2O3 NPs can be synthesized by hydrothermal method using Ferric chloride hexahydrate (FeCl3.6H2O) at range of temperatures of 60, 100 and 160 oC for 8 hours.
In this study all techniques that was used refer to successful synthesis for Fe2O3 crystallites with a hexagonal short elongation; which agglomerated into nanoparticles measuring, on average, (23, 12, 11.7) nm in size crystallites with wurtzite-like shapes at 60, 10 and, 160 oC, respectively.
 XRD Crystallography showed that Fe2O3 NPs prepared at 160 oC were smaller than that prepared at 60 and 100 oC.

CONFLICTS OF INTEREST
The authors have no conflicts of interest to declare that are relevant to the content of this article.