Synthesis of Pyramid-Shaped NiO Nanostructures using Low-Temperature Composite- Hydroxide- Mediated Approach

Composite-hydroxide-mediated (CHM) approach was used to synthesize NiO nanocrystals. The proposed method makes use of molten composite hydroxides; providing reaction media and lower the process temperature. Processing temperature and reaction time are the two potential parameters to control the growth of a nanomaterial. The method was used at temperatures in the range of 180-250°C and formation of the nanomaterial was monitored using XRD, SEM, EDX, FTIR, and UV-visible spectroscopy. The produced nanomaterial was purely polycrystalline with an average crystallite size in the range of 23.71-36.92 nm. Method suggested formation of pyramid shaped NiO nanocrystals in the temperature range 220-250°C. Evidence on the elemental composition, purity, and chemical bonding were obtained from EDX and FTIR analysis respectively. Estimation on direct bandgap was made from the optical analysis and found to be in the range 4.0-4.8 eV. The method is attractive and seems a cost effective route for the growth of transition metal oxides for research purpose. For further efficacy, the approach can be examined for other technologically significant nanostructures. *Corresponding author: Khan TM, National Institute of Lasers and Optronics (NILOP), Nilore 45650, Islamabad, Pakistan, Tel: 92-51-9248801; Fax: 92512208051; E-mail: tajakashne@gmail.com Received August 29, 2016; Accepted October 04, 2016; Published October 14, 2016 Citation: Shahid T, Khan TM, Zakria M, Shakoor RI, Arfan M, et al. (2016) Synthesis of Pyramid-Shaped NiO Nanostructures using Low-Temperature CompositeHydroxideMediated Approach. J Material Sci Eng 5: 287. doi:10.4172/21690022.1000287 Copyright: © 2016 Shahid T, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
Nanotechnology has emerged potentially a versatile and interdisciplinary field involving many subjects of science and engineering; leading to a wide-range of new developments, innovation, and advanced significant research. Past few years are witnessed major mechanical advancements which are based on remarkable nanostructures level progress. Also, metal oxide nanomaterials including nanoparticles (NPs), nanowires, nanosheets, and nanofibers, has got an expanding attention of scientific researchers due to their potential applications in biomedical, optical, and electronic fields [1].
In the transition metal oxides, nickel oxide (NiO) is potentially important with a cubic lattice structure. It is a p-type semiconductor with a wide bandgap ranging from 3.6 eV to 4.0 eV. Recently, efforts have been placed to develop, characterize and describe the physio-chemical properties of NiO nanostructures. NiO has a number of intensive applications generally a pure material for specialty applications and metallurgical grade; mainly used for the manufacturing of alloys, frits, ferrites, porcelain glazes [2,3], magnetic materials [4], alkaline batteries cathode [5], anode material for Li-ion batteries [6,7], gas sensors [8], anti-ferromagnetic layers [9], solid-oxide fuel cells [10,11], drug delivery and magnetic resonance imaging (MRI) [12]. NiO use in nanoscale optoelectronics devices such as electrochromic display optical fibers, photovoltaic applications [13,14], p-n heterojunctions [15], catalysis [16,17], the electrode material for electrochemical capacitors, [18] and smart windows [19]. The requirements of these applications include many factors regarding small size and particle distribution. The factors include volume effect, the quantum size effect and the surface effect with improved properties for these various attractive applications [20]. In the literature, various methods have been reported for the synthesis of nanocrystalline NiO from different starting materials for some valuable applications. Most of these methods are; thermal decomposition [21,22], carbonyl method, sol-gel technique [23], microwave pyrolysis [24], solvothermal [25], anodic arc plasma [26], Sonochemical [27], precipitation-calcination [28] and microemulsion [29]. However, mostly these are high-temperature and high-pressure methods; rely on surface-capping agents or demand for organic-metallic precursors and seems to be expensive for an industrial scale application. So it is always desired to seek for a simple approach to be; cost effective, workable at lower temperature, and has potential for a large-scale and controlled growth of oxide nanostructures at atmospheric pressure.
The method we have proposed here for the preparation of NiO nanomaterial is the composite-hydroxide-mediated (CHM) approach. The method of CHM is basically a technically sound, environmentally friendly methodology for creating a wide range of significant nanostructures [30]. The method is quite cheap and single step technique provides a fast nano synthesis route for nanomaterials. The synthesis is usually carried out at a lower temperature ∼200°C in the ambient atmosphere [30,31]. This method has the advantages to synthesize functional nanowires, nanorods, nanobelts and several other oxide nanostructures. Recently, Khan et al. applied this method for the synthesis of CdO and ZnO nanostructures and a temperature dependent study was established [32,33]. Previously in our group, a comprehensive study was done on the feasibility of CHM approach for the synthesis of doped Cu 1-x Zn x O nanostructures [34]. Interesting nanostructures with flower-like features and morphological peculiarities were obtained. These structures seem to strongly depend on the content of the incorporated Zn +2 . Recently, C An et al. applied this method to prepare NiO nanocrystals using NiCl 2 .6H 2 O as the source material. However, in their limited study, they showed the dependence of synthesis process on the amount of solvents used with no further understanding of optical and morphological structures [35]. The method of CHM is extensively under investigation in direction of basic understanding for research purposes. The method seems has a potential to be a viable synthesis route for the high-tech industrial scale application if it is established.
In this report we present synthesis of purely NiO nanocrystals using a cost effective CHM approach and formation of the nanostructures is established using; X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), UV-Visible spectroscopy, scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDXS). We established that well defined interesting pyramid type nanocrystals are obtained in a certain temperature range and showed a temperaturedependent mechanism of the nanostructures. This study will help to understand and control morphology of the nanomaterial with the processing temperature. The proposed approach seems to provide a fast and cheap nano-synthesis route for a verity of novel nanomaterials of technological importance for research purposes.

Synthesis of NiO nanomaterial
The synthesis of NiO was performed using CHM approach. All the chemicals were the analytical grade, used without further purification and purchased from Merk Co. The main source materials used for this experiment are; Ni (NO 3 ) 2 .6H 2 O, NaOH, and KOH. The synthesis procedure has been given in our previous reports [32][33][34]. Briefly, for the synthesis of NiO, an appropriate amount of mixed hydroxide (NaOH: KOH=51.5:48.5) were added in a Teflon beaker, and covered beaker was then placed in an electric furnace at 180 o C followed by further experiments at temperatures; 200°C, 220°C, and 250°C. Until the hydroxides were totally in the molten state, the beaker was taken out from the furnace. The molten hydroxide mixture was stirred well by a Teflon bar and Ni (NO 3 ) 2 .6H 2 O was added. Then Teflon beaker was put in the furnace for 24 h (processing time) at the above-described temperatures for various repeated experiments. After a reaction time of 24 h, the beaker was taken out and cooled to room temperature naturally. The product was washed with distilled water to remove the hydroxide. The black powder was obtained and collected for further characterization. A general sketch showing the procedure of the process is given in Figure 1.
The reaction mechanism to form NiO nanocrystals can be formulated as follow: 2 is chemically unstable at high temperature and splits to form NiO and water molecule. The precipitated NiO nanocrystals so formed are:

Characterization methods
X-ray diffraction (XRD) analysis of the prepared nanomaterial was carried out by PANalytical diffractometer equipped with a CuKα monochromatic radiation (λ=1.5406 Å). SEI detector in the scanning electron microscopy (SEM) and an energy-dispersive X-ray spectroscopy (EDX) was used to analyze the morphology and chemical composition of the as-prepared NiO.
FTIR spectrum was studied on a Perkin Elmer FTIR spectrometer in the spectral range of 400-4000 cm -1 . The UV-visible absorption spectrum analysis was carried out in the spectral range 250 -800 nm. The optical bandgap was estimated by using UV-visible absorption spectroscopy.  200) and (220) reflections are much stronger compared to other reflections in the XRD pattern. This gives strongly oriented crystallites in these planes directions. However, the growth along (200) is more prominent. The sharpness and strong intensity of these very certain peaks indicate a good crystalline nature of the product. The peak broadening in XRD pattern is a characteristic of the produced nanostructures. The average crystallite size of NiO is estimated by the well-known Debye-Sherrer formula using peak broadening [36],

XRD analysis
Here β denotes "full width at half maximum" of the diffraction peak in radian units, K is the correction factor and its value is assumed to be 0.94 for FWHM of the crystals, λ is the wavelength in unit of nm of Cu-Kα radiations and θ is the angular position in radian units (the Bragg's angle). The calculated values of crystallite sizes, corresponding to each plane along with FWHM are given in Table 1  is due to the viscosity of the hydroxide melts. As the viscosity of the hydroxide melts is greater at temperatures higher than 180 o C, this causes slow nucleation and crystallization which result in formation of bigger nanosized crystallites. The variation in d-spacing and diffraction density with processing temperature are displayed in Figure 2b. The value of d-spacing shows an increasing trend in the temperature range 180-220 o C followed by a reverse dropping trend at 250 o C. Reflection density shows an opposite behavior to that observed for d-spacing in the same temperature range. Figures 3a and 3b shows SEM images and EDX of NiO nanomaterial prepared at different temperatures. These images give information on the morphology and composition of nanosized NiO. It seems that at temperatures 180 o C and 200 o C the prepared samples have no well-defined morphological structure. This predicts that a minimum temperature is required in the CHM process for different materials to starts to grow and nucleate in well-defined morphological peculiarities and structure. This also shows a temperature-dependent mechanism of nanomaterial synthesis by the CHM method. This temperature dependent nucleation can also be associated with the viscosity of the melts which is strongly influenced by the temperature factor and suggests viscosity is playing a significant role in the formation and growth of a nanomaterial in the CHM approach. As the temperature is raised to 220 and 250 o C, a significant change is observed in the nanostructure of as-prepared samples. Interesting nanopyramid type structures are obtained with a high density. The color line circle in the image for 250 o C shows two enclosed nanopyramids with its magnified marked image alongside EDS to manifests a clear visual picture of the

SEM characterization
Then the cell volume of prepared cubic NiO nanostructures was estimated by the formula; The X-ray density was calculated using molecular weight and volume of unit cell of the samples by the following formula [37]; Where ''Z'' is the number of molecules per formula unit, ''M'' is the molar mass, ''N A '' have its usual meanings and the V cell is volume of a unit cell. All the calculated structural parameters are listed in the Table 1. It can be clearly seen that the lattice parameters have good agreement with the standard values. A small decrease is seen for the samples from 180 o C to 220 o C, where at 250 o C the lattice parameter is very close to standard value. The cell volume also has the same tendency but values of the X-ray density has increased with processing temperature. The average size of crystallites obtained are; 23.71 nm (at  prepared nanopyramids. The length of sides of these nanopyramids is in the range of 0.3-0.5 µm with random orientation. These NiO crystallites self-organized nanopyramids are high-density assemblies with a clear appearance. For the chemical composition, energydispersive X-ray (EDX) was performed on each sample and it was confirmed each sample showed the existence of Ni and O elements. This gives a strong signature on the chemically pure nature of the prepared product by the CHM method.

FTIR studies
The FTIR spectra were studied in the spectral range of 400-4000 cm -1 shown in Figure 4. FTIR is carried out in order to understand the chemical and structural roots of the prepared samples of NiO and the effects of reaction materials used. The absorption band in the region of 600-630 cm -1 is the characteristic stretching vibration band of NiO and is an obvious signature on the formation of NiO. The broadness of this band indicates the presence of NiO nanocrystals in the prepared samples. The observed vibration band for all the samples is wellconsisted with the literature reported vibration band of NiO [38]. The small double peak at 2362 and 2346 cm -1 is caused by the presence of CO 2 molecules. The intensity of these bands decreases with increasing temperature [39,40]. FTIR study strongly supports the XRD results in respect of purity of the samples showing the formation of a single phase NiO.

UV-visible spectroscopy
This technique is based on the fact that an electron is moved from valence band to conduction band due to the absorbed photon having energy larger than the band gap energy of semiconductor. The electronic transition happened due to absorbed photon provides rich information about the type of transition. When the electron momentum is conserved the transition is direct otherwise indirect and energy of the transitions is measured using Tauc's relation [41,42].
UV-visible spectroscopy was performed to investigate the optical behavior of the nanostructures and to make an estimation of the bandgap. The optical absorption spectra of NiO samples at 180°C, 200°C, 220°C and 250°C were taken at room temperature by dispersing them in absolute ethanol which is a low absorption medium [43]. UVvis absorption spectra of the samples are shown below in Figure 5a. A slight shift towards higher wavelength for the absorption edges was observed as the process temperature increased. This shift indicates a decrease in bandgap value, which can be attributed to an increase in the particle size. The value of absorption edges of NiO prepared at 180 o C, 200 o C, 220 o C, 250 o C were 295, 317, 321, 322 nanometer respectively. Also, it can be seen from the Figure 5 that there is an exponential decrease in the intensity of absorption with the increase in wavelength. This decreasing behavior in intensity is attributed for many semiconductors and may be due to many reasons similarly to internal electric fields within the crystal, deformation of lattice due to strain caused by imperfection and inelastic scattering of charge carriers by phonons [44]. Absorption coefficient (α) associated with the strong absorption region of the sample was calculated from the following relation [45]: Here t' stands for the path length in cm and is taken to the cuvette length of 1 cm. According to the data of the absorption spectra, the optical bandgap (Eg) of NiO can be estimated by using the following Tauc equation [46]: Where hν is photo energy, α is absorption coefficient, B is a constant relative to the material and n is either 2 for direct band gap material or 1/2 for an indirect band gap material. According to the equation, the optical bandgap for the absorption peak can be obtained by extrapolating the linear portion of the (αhν) n -hν curve to zero of the energy axis shown in Figure 5b. The corresponding bandgap    energies of produced nano-NiO at different temperature are given in Table 1. No linear relation was found for n = 1/2, suggesting that the as-synthesized NiO is semiconductor material with a direct transition at this energy [47]. The bandgap energies calculated are; 4.8 eV (at 180 o C), 4.5 eV (at 200 o C), 4.4 eV (220 o C), and 4.0 eV (at 250 o C). These values well consist reported by Khashan et al. [48] and Rifaya et al. [49]. This bandgap is the p-d character of NiO has been investigated by a range of spectroscopic techniques including optical absorption. The increasing trends of the bandgap energy upon the decreasing particles size can be associated nanosized structures. This effect is likely due to the chemical defects or vacancies present in the intergranular regions generating new energy level to reduce the band gap energy [50]. The NiO bandgap showed an increase with processing temperature with an overall maximum increased value up to 4.5 eV. The variation in E g with processing temperature along with the crystallite size is depicted in Figure 5c.

Conclusion
In conclusion, pyramid-shaped NiO nanostructures were synthesized by using a low temperature and feasible CHM approach. A minimum limit on the processing temperature was established to nucleate NiO with a well-defined morphological structure. At temperatures below 220 o C, NiO was presenting no definite morphology; while at temperatures in the range 220-250 o C well-defined pyramid shaped morphological structures were obtained. The nanostructures were polycrystalline with an estimated average crystallite size in the range of 23.71 -36.92 nm. The increase in average crystallite size assumed was caused by viscosity effect of the melts; playing a significant role in the nucleation process. Morphology showed a temperature dependent nature and was influenced by the processing temperature. The product was a pure crystalline material; composed of Ni and O elements with Ni-O stretching vibration bonding. The process temperature showed influence on the optical properties. The direct bandgap estimated was tuned in the range of 4.8-4.0 eV with the rise in process temperature. The present study would be beneficial as a new cost-effective route for the development of novel nanomaterials and temperature dependent morphological structures for prospective application in optoelectronics.