Synthesis of ZnO/Al:ZnO nanomaterial: structural and band gap variation in ZnO nanomaterial by Al doping

Pure ZnO and Al-doped ZnO nanomaterial have been successfully fabricated using zinc acetate dihydrate in a basic aqueous solution of KOH through solution precipitation method then treated at 600°C in air. The XRD analysis confirms the Wurtzite hexagonal crystal structure of the product with crystallite size in 32–53 nm range. The morphology of the product has been studied under scanning electron microscopy (SEM). The simultaneous differential scanning calorimetry and thermogravimetric analyses were used to investigate thermal decomposition temperature and different phase transitions up to 800°C. The optical properties and variation in band gap of ZnO by Al doping were investigated by ultraviolet–visible spectroscopy.


Introduction
ZnO nanomaterials are extensively used in many applications, which have attracted much attention in the present years. Bulk ZnO have a direct band gap of 3.37 eV (at bulk state) and a larger exciton binding energy (60 meV). The electronic band gap of ZnO has been predicted theoretically and calculated by many people (Oshikiri and Aryasetiawan 2000;Muscat et al. 2001;Usuda and Hamada 2002;Uddin and Scuseria 2006;Shishkin and Kresse 2007;Christoph Friedrich et al. 2011;Dixit et al. 2011;Yan et al. 2011), and lot of experimental work have been done to find out the band gap of ZnO (varying 2.9-3.7 eV) (Alhamed and Abdullah 2010;Ma et al. 2011;Sakthivelu et al. 2011;Zandi et al. 2011;Tan et al. 2005;Bandyopadhyay et al. 2002;Inamdar et al. 2007;Ananthakumar et al. 2010). ZnO is very useful in several opto-electronic field such as optical sensors and light emitters (RF Service 1997; Makino et al. 2000), etc. In addition, ZnO is also very useful in gas detecting devices and piezoelectric application (Fortunato et al. 2005;Gong et al. 2006;Song et al. 2006;Jeong et al. 2003;Zhang et al. 2006). In fact, many application and devices having bulk ZnO, and ZnO nanomaterial have been established (Ma et al. 2011;R.F. Service 1997;Makino et al. 2000;Fortunato et al. 2005;Gong et al. 2006;Song et al. 2006;Jeong et al. 2003;Zhang et al. 2006;Yu et al. 2006).
There are many methods to synthesize ZnO nanomaterial such as, preparation by sputtering (Yan et al. 2011), chemical vapor deposition (Park et al. 2006), molecular beam epitaxy (MBE) (Fons et al. 2006), spry pyrolysis (Joseph et al. 1999), laser deposition , and the soft chemical method (Ristic et al. 2005;Kuo et al. 2006). Alhamed and Abdullah (2010) has discussed structural and optical properties of ZnO:Al films prepared by the sol-gel method. The solution precipitation method is predominantly gorgeous because of its simplicity, low costs, and obtained product of good crystalline quality, which makes it superior to the other methods. Here, we report a very simple solution precipitation method to synthesize ZnO nanomaterial and Al-doped ZnO with different doping concentration (3, 5, and 10%) to study the effect of doping concentration on structural and optical properties.

Materials
All chemicals used in this experiment having analytical grade purity were obtained from the commercial market, used without further any purification. For all the reactions, zinc acetate dihydrate Zn(CH 3 CO 2 ) 2 Á2H 2 O, aluminum hydroxide acetate AlOH(CH 3 CO 2 ) 2 , and potassium hydroxide KOH were used for synthesis.
Calculated stoichiometric ratio of zinc acetate dihydrate Zn(CH 3 CO 2 ) 2 Á2H 2 O was taken in a beaker containing distilled water and stirred for 30 min. Potassium hydroxide KOH mixed in distilled water was added to the acetate solution. In case of Al-doped (3, 5, and 10%) ZnO nanomaterial different concentrations of aluminum hydroxide acetate AlOH(CH 3 CO 2 ) 2 were added into the zinc solution before potassium hydroxide KOH solution. The reaction was stirred for 30 min at room temperature. When reaction was completed, we filtered the resultant white suspension using the centrifuge machine. Each centrifuge step had 5 min of rotation with the speed of 2,500 rpm. We obtained the white precipitates and then washed with distilled water and ethyl alcohol many times. The obtained white precipitates of zinc hydroxide/aluminum hydroxide were dried at  60°C. Finally the thermal decomposition of zinc hydroxide/ aluminum hydroxide was done to obtain the nanocrystals of ZnO and ZnAlO. We placed these zinc hydroxide and aluminum hydroxide in furnace chamber at 600°C for 1 h. The involved reactions are shown in Table 1.

Characterization of the product
Powder X-ray diffraction (XRD) data were recorded and collected on the XRD model MPD X'PERT PRO of PANalytical Company Ltd., Holland using Cu-Ka as characteristic radiation (k = 0.15418 nm) with h-h configuration. The measurements were made in 2h ranging from 20 to 70°. Study was mainly done by the software X'Pert HighScore of the same company. Scanning electron microscopy (SEM) images were taken on a scanning electron microscope (JEOL JSM-6480). A differential scanning calorimetery (DSC) and thermal thermogravimetric analyses (TGA) were performed by SDT Q600 of TA Instrument; the optical properties were investigated by ultraviolet-visible spectroscopy using UV/Vis spectrophotometer CECIL2700.

TGA and DSC analysis
To determine the thermal decomposition temperature of zinc hydroxide/aluminum hydroxide, differential scanning calorimetery (DSC) and thermal thermogravimetric analyses (TGA) were carried out by SDT Q600 of TA Instrument; the thermal decomposition curves (DSC/TGA) of zinc hydroxide/aluminum hydroxide are depicted in Fig. 1. The specimen was heated from room temperature to 800°C with an increment of 20°C/min in air. The TGA data plots the weight variation of the specimen, DSC designates whether reaction is endothermic or exothermic and weight derivative (temp) gives information about the rate of change of weight with respect to temperature. The specimen suffered the weight losses at 100 and 250°C, respectively, which is due to evaporation of ethyl alcohol, water, and organic by products. The evaporation is endothermic reaction; corresponding peaks emerge at DSC curve, weight derivative peaks also confirm the said conversion, we can observe another weight loss in TGA curve at 400°C, analogous large exothermic peak and weight derivative peaks are also exhibited at same temperature; due to the formation and crystallization of ZnO. Therefore, the crystallization of ZnO nanomaterial occurred at temperatures over 400°C. For this reason, we used 600°C for the thermal decomposition of prepared precursors. Figure 2a shows the XRD patterns of ZnO/Al:ZnO nanomaterial annealed at 600°C for 1 h. All ZnO/Al:ZnO samples are polycrystalline and correspond to hexagonal structure that can be indexed by comparison with data from JCPDS file no. 03-065-3411 with lattice constants a = 3.2495 Å , b = 3.2495 Å , and c = 5.2069 Å . No diffraction peaks of Al 2 O 3 or other impurities are observed, which show that the Al ions successfully reside in the lattice site rather than interstitial ones.

X-ray diffraction
By comparing with undoped ZnO, the doped samples show lowering of intensity and decrease in full width at half maxima (FWHM). Average crystallite size calculated from XRD peaks is about 32 nm for undoped sample which goes up to 53 nm for the Al-doped samples.
We find the major diffraction peaks decreasing as the Al concentration increases, which indicates Al-doping effects and decreases the crystalline quality. Comparing the crystallization of ZnO with Al:ZnO, a large amount of Al dopants produce lattice disorder, which is linked with the reduction in lattice strain in ZnO. Moreover, the stain reduction, the grains grew much easier when Al dopants were included with ZnO.
To understand the crystalline mechanism of ZnO/ Al:ZnO, the crystallite size of the ZnO/Al:ZnO nanomaterial has been estimated from the FWHM of diffraction peak using the Scherrer formula (Klug and Alexander 1974).
where D is crystallite size, k, h, and b are the wavelengths of X-ray, the Bragg's diffraction angle and full width at half maximum (FWHM) of the diffraction peak, respectively. The graph between average crystallite size and doping concentration is shown in Fig. 2b.
The lattice strain (e) has been determined by using the tangent formula (Klug and Alexander 1974). The graph Fig. 3 a SEM image for ZnO nanomaterial. b SEM image for 3% Al:ZnO nanomaterial. c SEM image for 5% Al:ZnO nanomaterial between average lattice strain (e av ) and doping concentration is shown in Fig. 2c. e ¼ b=ð4 tan hÞ: The position (2h), FWHM (b), crystallite size, and lattice strain of ZnO/Al:ZnO thus obtained are listed in Tables 2 and 3.

SEM results
The structural morphologies of the synthesized product were observed by the scanning electron microscopy model JSM 6480LV JEOL Japan. SEM images are collected at high magnifications to investigate the morphology of  samples. Figure 3a shows the SEM image for pure ZnO nanomaterial. The magnification of this image is 25,000 times and reference bar of 1 lm. SEM micrograph shows that particles are random in shape with average particle size 125 nm, estimated by pixel analysis using ImageJ and Micro-Manager 1.4 softwares. Figure 3b, c show SEM images for 3 and 5% Al-doped ZnO nanomaterial, also having same magnification and reference bar, average particle sizes estimated are 150 and 176 nm, respectively. These SEM images show a narrow particle size distribution and particle size is growing as Al doping increases.
Optical analysis Figure 4 shows the absorbance spectra of the undoped and Al-doped ZnO films for wavelength 200-800 nm The optical band gap (E g ), was estimated from the extrapolation of the linear portion in a plot of (Aht) 2 against ht, where A is the absorbance and ht is the photon energy Table 4. It is observed that E g in the undoped ZnO nanomaterial is *3.01 eV which is lower than value of bulk ZnO (3.37 eV). This deviation may be due to the structural defects takes place at the time of synthesis and thermal treatment of precursor, On doping at 3% of Al, E g is found to decrease to the 2.99 eV because of big crystallite size and small lattice strain as observed in the X-ray study. Further E g decreases and becomes 2.97 and 2.94 eV, in the case of 5 and 10% Al doping, respectively. Similar type of E g behavior have been reported by different researchers for sol-gel spin-coating to develop ZnO thin film (Natsume and Sakata 2000) and pulsed laser ablation (3.1 eV) (Narasimhan et al. 1999). Figure 4a, b, c, d show the graph for the calculation of the band gap of undoped ZnO and doped ZnO with 3, 5, and 10% Al concentration. The graph between band gap and doping concentration is shown in Fig. 4e.

Conclusion
We have fabricated zinc oxide and aluminum-doped zinc oxide nanomaterial of different sizes. XRD data confirms the hexagonal phase of the synthesized materials. The crystallite size increases with increase in the doping concentration. The surface morphologies of the synthesized product were observed by the scanning electron microscopy (SEM). DSC/TGA analysis was done to study the phase changes during fabrication of materials. In DSC/TGA analysis it was found that Zn(OH) 2 /Al(OH) 3 decompose to form the required product. A spectrophotometer was used to attain the absorption spectrum in ultraviolet-visible region.
It was found that absorption is maximum for highest doping and band gap of the materials was also calculated.