One step sonochemical synthesis of single phase Sr-doped ZnO2 nanorods

Single phase SrZnO2 nanorods with average size of around 100 nm are synthesized by probe sonicator using ultrasonic waves at 60 W, 6 kHz for 3 h to observe the changes in optoelectronic properties of ZnO. The as grown SrZnO2 has a single phase rod shaped structure. These nanorods were broadly characterized by different instrumental techniques like TEM, FESEM, EDAX, XRD, UV–Visible and FTIR for analysis of size, crystal structure, morphology and composition which acknowledge the formation of single phase crystalline ZnO and slightly amorphous SrZnO2 nanorods. All these results suggest that single phase SrZnO2 nano-composites can easily be synthesized by one step sono-chemical method.


Introduction
Recently, most of the research work on photo anodes has been focused on the nanostructures of TiO 2 [1] . However, the ZnO semiconductor could be a substitute for TiO 2 since, it has optoelectronics and luminescent pre-eminent properties with electronic affinity similar to that of TiO 2 [2]. ZnO has a comparable bandwidth gap of 3.37 eV, a high electronic mobility of 115-155 cm 2 V −1 s −1 , high exciton binding energy of 60 eV and stability against photo corrosion [3]. Photovoltaic applications of ZnO and its nanonomposites are well studied but the applications of ZnO incorporated with differrent metal oxides are less explored in the field of photo catalysis and photoluminescence [4]. So far ZnO intrgrated with SrO are synthesized by hydrothermal, high energy ball milling, carbonate decomposition, aerosol-assisted chemical vapour deposition (AACVD) [4] and combustion synthesis [7].
In recent time, ZnO coupled with SrO gained much attention as the photocatalyst and phosphore mainly due to its cost effectiveness. Moreover, the high band gap of SrO (Eg = 5.5-5.9 eV) with low conduction band in comparison of ZnO (Eg = 3.37 eV) also makes it a good choice. The position of valence band and conduction band of ZnO in ZnO/SrO heterojunction is between and above the valance band and conduction band of SrO respectively. This could help in improving the photo catalytic performance of these nanocomposites by hinder charge recombination to overcome with the problems like high energy and long reaction period as required by the methods [5] mentioned above Herein, in this work we proposed a novel sonochemical synthesis of rod shaped SrZnO 2 nanocomposites in a 180 min. The as synthesized SrZnO 2 nanocomposites were characterized with different instrumental techniques to establish its crystal structure and surface properties. This is possibly the first report of sonochemical synthesis of single phase nanorods of SrZnO 2 as per best of the author's knowledge.

Synthesis of rod shaped SrZnO 2
SrZnO 2 nanorods were synthesized using the ultrasonic irradiation via probe-sonicator taking Zn and Sr in 1:1 molar concentration. To obtain SrZnO 2 , 0.1 M strontium chloride hexahydrate solution (SrCl 2 .6H 2 O) was added to the 0.1 M [Zn(CH 3 OO) 2 .2H 2 O] in 100 ml of distilled water. The resulting aqueous solution was stirred for 30 min before adding 2 ml of ethylene-di-amine as the capping agent. The solution was then sonicated for 3 h at 60 W, 6 kHz with the simultaneous addition of 50 ml 0.2 M potassium hydroxide (KOH) as a reducing agent. The Yellowish-white precipitate obtained was than washed 5-6 times with water-ethanol (3: 1) solution and dried for 8 h at 85 °C.
SrZnO 2 nanorods were also synthesized through wet chemical synthesis without using ultrasonic waves keeping all the concentrations of reagents same as used in sonochemical synthesis at 80 °C on hot plate magnetic stirrer. The as prepared sample is than dried at 80 °C in oven, washed with 25% ethanol and stored for characterization.
The ZnO without doping Sr is also synthesized using ultrasonic waves as mentioned above using 0.01 M ZnSO 4, 7.5% NH 3 and distilled water at 80 °C are used as precursors. The 0.01 M ZnSO 4 was mixed with 7.5% of NH 3 and taken in 100 ml burette . It was than drop wise added to a distilled water preheated at 80 °C and simultaneously sonicated for 3 h at 150watt and 6kH. The white ppt thus formed is washed with distilled water 6-7times and dried in oven at 80 °C.

Mechanism of rod shaped SrZnO 2
Acoustic cavitation [6], formation of phase isolated microreactor [7] and the microsized droplets that confine chemical reactions within their interior are responsible for the formation of nanocomposites. These transient, localized hot spot with extremely high temperature and pressures are primarily responsible for chemical effects of ultrasound.
When 2 ml of ethylene-di-amine as the capping agent was added to a well stirred mixture of strontium chloride hexahydrate solution (SrCl 2 .6H 2 O) and zinc acetate [Zn(CH 3 OO) 2 .2H 2 O]. The solution is then sonicated with dropwise addition of KOH as reducing agents, then zinc (Zn(OH) 2 ) and strontium hydroxide (Sr(OH) 2 ) are formed [8,9]. When the sonochemical process continued, the Zn(OH) 2 and Sr(OH) 2 is separated into Zn 2+ and Sr 2+ ions and OH − radical forming ZnO and SrO nuclei according to the last reaction as given in the Fig. 1. The dissolution-nucleation cycle, produces nanocomosites of SrZnO 2 .

Characterization
To establish the Physiochemical structure of the synthesized material, different analytical techniques are used. X-ray diffraction (XRD) analysis is done using a fifth generation Rigaku (Modal no Mini Flex 600 XRD). Morphology and Fig. 1 Schematic representation of mechanism of synthesis particle size of the composites are characterized by scanning electron microscopy (JEOLJSM6390LV). The chemical composition of synthesized material is also evaluated using Bruker Nano GmbH, Germany-EDAX. FTIR &UV-Vis spectroscopic techniques are also used to establish the synthesis of SrZnO 2 and change in energy bandgap.

FT-IR analysis
FTIR analysis (Fig. 2) in a region between 4000 cm -1 and 450 cm -1 has been done to identify the synthesis of SrZnO 2 . FTIR spectra give a wide and intense band between 550 cm −1 and 450 cm −1 [10] respectively for ZnO and SrO. Stretching mode of vibrations of the OH group of water molecules 1 can be attributed to the absorption peak at 3261 cm −1 . The presence of EDA in the SrZnO 2 nanostructures is confirmed by to the C-H elongation and flexion modalities attributed to the 3951 cm −1 and 2914 cm −1 respectively for SrZnO 2 and ZnO peaks. The band observed at 1578 cm −1 and 1642 cm −1 corresponds to the vibration C=O, due to the acetate group and zinc acetate used as one of the precursor. The peaks 1070-1012 cm −1 are due to stretching vibration SrO. The band observed at 854 cm −1 is attributed to the vibration of anti symmetric stretching of SrO. It is important to take into account that the SrO species affects the spectra and it is observed that the displacement in the ZnO band at 514 and 461 cm −1 attached to the ZnO peaks which is assigned as a mode of elongation of Sr-O which is due to the Sr +2 present in the composite  (JCPDS no. 0060520) [14]. The difference in atomic size causes changes in defect density, induces stress, network distortion and leads to a reduction in oxygen supply. The reticular constants calculated from the most important peaks and the relation of the relative intensities I (191)/I (109) in the diffraction model of the ZnO and SrZnO 2 at room temperature, respectively.

TEM analysis
TEM images (Fig. 4)   in the SAED pattern of ZnO correspond to its hexagonal shape [13]

SEM and EDAX analysis
Typical SEM micrograph (Fig. 5) of the SrZnO2 nanoparticles shows that the shapes of nanocomposites are close to rod shaped. The flower like shapes of hexagonal rods of ZnO is also confirmed form the previous work of the author [8]. It is expected that the initially form flower like structures can split into its hexagonal rods during sonochemical synthesis. EDAX (Energy-dispersive X-ray spectroscopy) spectrum (Fig. 6) shows four peaks which were identified as atom % of oxygen (66.1%), zinc (16.7%) and strontium (17.1%) (Fig. 6). This percentage of oxygen, zinc and strontium is in agreement of the formation of SrZnO 2 nanocomposites. Figure 7 shows the UV optical studies of SrZnO 2 composites. Optical studies confirm that the bandgap of SrZnO 2 nano composites clearly shifts to higher value as compare to pure ZnO from 3.31 [15] to 3.66. This increase in bandgap is due to the decrease in lattice spacing because of assimilation of Sr into ZnO lattice. Bandgap value can also be affected by factors like impurity, lattice distortion, and strain [16].

Conclusion
It is clearly established from this work that Sr integrated ZnO nanorods were synthesized by a simple, low-cost, ultrasonic method. Structural characterization revealed the incorporation of Sr +2 into the ZnO network. This is the first report of SrZnO 2 nanorods by sonochemical method as per the best knowledge of authors. The inclusion of strontium in ZnO and formation of SrZnO 2 is increased to 3.66 from 3.31 of pure ZnO.

Conflict of interest The authors declare that they have no conflict of interest
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