Facile and Clean Solution Synthesis of Large-Scale ZnO Nanorods Assisted with Aliquat 336

Zinc oxide (ZnO), a wide band-gap (3.37 eV) II–VI compound semiconductor, has a stable wurtzite structure and large exciton binding energy (60meV) at room temperature [1]. ZnO nanostructures, especially one-dimensional nanoZnO such as rods, wires, and tubes, have attracted much attention due to their many unique properties and the possibility that they may be used as building blocks for future electronics and photonics [2–4]. Various chemical, physical, and electrochemical methods, for instance, solidstate routes, hydrothermal reactions, sol-gel, microwaveor ultrasound-assisted synthesis, electrodeposition, and vapor transport process, have been applied for fabrication of 1D ZnO nanostructures [5–12]. Among these methods, solution chemical route has become a promising option for large-scale production of nanoscale materials because of its simple, fast, less expensive virtues and flexibility of controlling shape and size of the structures by tuning different growth conditions. In solution synthesis of nano-ZnO, an organic additive such as surfactants, soluble polymers, and recent ionic liquids for the control of sizes andmorphologies is usually necessary [13, 14]. On the other hand, Aliquat 336, a 2 : 1 mixture of methyl trioctyl ammoniumand methyl tridecyl ammonium chloride (Figure 1) with a proposed mean mole weight of 432 g/mol, is conventionally employed as a phase-transfer catalyst [15, 16],metal extraction reagent [17], andwaste water treatment chemical [18]. It presents liquid state at ambient temperatures and is in fact a room temperature ionic liquid (RTIL) or acts as a versatile and affordable cation source for quaternary ammonium ionic liquids [19, 20]. Moreover, Aliquat 336 is provided with special salts-induced phase behavior [21]. It can be almost completely separated out from aqueous phase and thus be recovered easily. Recently, various RTILs have been used as the solvents, morphology controlled templates, reactants, or so-called all-in-one for fabrication of nanostructured metal oxides because of their negligible vapor pressure, low interfacial energy and tension, ordering of aggregation, high ionic conductivity, and thermostability [22–26]. Although, Aliquat 336 is a conventional and cheap RTIL, there are few reports on using Aliquat 336 for synthesis of inorganic nanomaterials. In present work, we report a facile synthesis of ZnO nanorods with Aliquat 336 as a soft template, and the NaSO 4 induced recovery of Aliquat 336 is demonstrated.


Introduction
Zinc oxide (ZnO), a wide band-gap (3.37 eV) II-VI compound semiconductor, has a stable wurtzite structure and large exciton binding energy (60 meV) at room temperature [1].ZnO nanostructures, especially one-dimensional nano-ZnO such as rods, wires, and tubes, have attracted much attention due to their many unique properties and the possibility that they may be used as building blocks for future electronics and photonics [2][3][4].Various chemical, physical, and electrochemical methods, for instance, solidstate routes, hydrothermal reactions, sol-gel, microwave-or ultrasound-assisted synthesis, electrodeposition, and vapor transport process, have been applied for fabrication of 1D ZnO nanostructures [5][6][7][8][9][10][11][12].Among these methods, solution chemical route has become a promising option for large-scale production of nanoscale materials because of its simple, fast, less expensive virtues and flexibility of controlling shape and size of the structures by tuning different growth conditions.In solution synthesis of nano-ZnO, an organic additive such as surfactants, soluble polymers, and recent ionic liquids for the control of sizes and morphologies is usually necessary [13,14].
On the other hand, Aliquat 336, a 2 : 1 mixture of methyl trioctyl ammonium-and methyl tridecyl ammonium chloride (Figure 1) with a proposed mean mole weight of 432 g/mol, is conventionally employed as a phase-transfer catalyst [15,16], metal extraction reagent [17], and waste water treatment chemical [18].It presents liquid state at ambient temperatures and is in fact a room temperature ionic liquid (RTIL) or acts as a versatile and affordable cation source for quaternary ammonium ionic liquids [19,20].Moreover, Aliquat 336 is provided with special salts-induced phase behavior [21].It can be almost completely separated out from aqueous phase and thus be recovered easily.Recently, various RTILs have been used as the solvents, morphology controlled templates, reactants, or so-called all-in-one for fabrication of nanostructured metal oxides because of their negligible vapor pressure, low interfacial energy and tension, ordering of aggregation, high ionic conductivity, and thermostability [22][23][24][25][26]. Although, Aliquat 336 is a conventional and cheap RTIL, there are few reports on using Aliquat 336 for synthesis of inorganic nanomaterials.
In present work, we report a facile synthesis of ZnO nanorods with Aliquat 336 as a soft template, and the NaSO 4induced recovery of Aliquat 336 is demonstrated.

Experimental
All reagents employed were commercially available and were directly used without further purification.in 50 mL deionized water and 2.0 g (2.3 mmol) Aliquat 336 (Aladdin Chemistry Co., Ltd., 98%) was added to the solution.The mixture was irradiated with ultrasound (SK500LH, Shanghai Kudos, 59 KHz, 100 W) at room temperature for 30 min.Then NaOH solution (2 M, 50 mL) was poured, and the formative white slurry was irradiated with ultrasound for another 15 min.The mixture was stood at 80 ∘ C for 24 h.The product was separated by centrifugation at 2500 rpm/min for 20 min.The solid was washed with water and ethanol several times and dried at 60 ∘ C for 6 h under vacuum to get the ZnO nanorod samples.The liquid phase was neutralized with saturated NaHSO 4 solution to the pH = 7.0.The liquid was irradiated with ultrasound for 30 min and then was centrifuged for collecting the upper oils.
The X-ray diffraction pattern (XRD) of the product was collected on an RIGAKU D/max-TTR III diffractometer with Cu-K radiation ( = 0.15406 nm).Scanning electron microscopy (SEM) images were obtained on an FEI Quanta 200 equipped with energy dispersive X-ray spectroscopy (EDS).For the measurement, the ZnO powders were laid on carbon films, and the samples were dried by critical point dryer.Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were taken on a JEM-2100 apparatus with an acceleration voltage of 200 KV.The ZnO sample was redispersed in ethanol, and a drop of this slurry was deposited on a carbon film supported on a copper grid.Photoluminescence (PL) spectra were measured on a Hitachi F-4500 fluorospectrophotometer using a Xe lamp with an excitation wavelength of 325 nm at room temperature.Raman spectra were recorded by Renishaw inVia microscopic confocal Raman spectrometer system using 514.5 nm line of an Ar + laser at room temperature in the air operating at 10 mW laser power.

Results and Discussion
ZnO nanorods have grown in an aqueous solution including Zn(CH 3 COO) 2 , Aliquat 336, and NaOH.After the formative slurry was pretreated under an ultrasonic water bath, the reaction system was stood under 80 ∘ C and kept for 24 h.The process is simple and the mass production is easy to achieve.The overall reaction for ZnO nanostructures in an alkaline solution can be proposed as follows: The growth behavior of crystals is primarily determined by the internal structure of a given crystal and affected by external conditions [27,28].With the presence of Aliquat 336 at high concentration of OH − , the growth habit of ZnO is restricted to one direction and leads to the formation of 1D nanorods.The formative mechanism of ZnO nanorods in current process may be consistent with previous literatures using other cationic surfactants for the synthesis of ZnO 1D nanostructures in alkaline solution, such as CTAB [29][30][31].However, it is a significant advantage that Aliquat 336 can be recovered easily after the filtrate was neutralized with saturated NaHSO 4 solution and the oil-water phase separation occurred when the filtrate was centrifuged [21].The average recovery of Aliquat 336 exceeded 90%.No anionic exchange such as SO 4 2− and CH 3 COO − was detected in the regenerated Aliquat 336.
The powder XRD pattern of as-prepared sample is shown in Figure 2. The diffraction peaks of ZnO powder coincide with the peaks of ZnO wurtzite structure with high crystallinity (JCPDF card number 36-1451, space group P6 3 mc).The calculated lattice constants ( = 0.3252 nm,  = 0.5206 nm) are in good agreement with the values in the standard card.No characteristic peaks of other impurities such as Zn(OH) 2 are detected in the diffractogram.XRD studies also may suggest strong preferred orientation along the c-axis because the (002) reflection is greatly enhanced relative to the standard spectrum of ZnO powders.The diameters of the samples calculated using Scherrer formula range between about 20 nm and 80 nm.
The morphology of the as-obtained products was investigated using SEM and TEM.Typical SEM and TEM images are shown in Figure 3.It reveals that the products with rod-like shapes are successfully fabricated under simple processing.The SEM and TEM images show as-obtained nanorods with diameters ranging from 20 to 80 nm and length from 400 nm to 1.2 m which also corresponds to their diameters calculated by Scherrer formula according to XRD analysis.In addition, the obtained rod-like crystals are widespread over the whole sample stand, shown in the large-scale SEM at low magnifications in Figure 3(a).These images imply that  Aliquat 336-assisted chemical solution method has wide application in the fabrication of large-scale ZnO nanocrystals with rod-like shapes.The Raman scattering spectrum of the samples is shown in Figure 4(a), and the frequencies of the Raman active phonon modes are presented in Table 1.This is consistent with previously reported results about optical phonon modes of ZnO nanorods, and no impurity relevant Raman peaks appear in the Raman spectrum [32][33][34].The Raman active zone-center optical phonons predicted by the group theory are  1 +  1 + 2 2 .The phonons of  1 and  1 symmetry are polar phonons and hence exhibiting different frequencies for the transverse-optical (TO) and longitudinal-optical (LO) phonons.Nonpolar phonon modes with symmetry  2 have two frequencies. 2H is associated with oxygen atoms and  2L is associated with Zn sublattice.The presence of sharp  2H (438 cm −1 ) mode with high intensity and the second-order Raman mode at 333 cm −1 in the Raman scattering indicates that the obtained sample possesses the wurtzite hexagonal phase of ZnO with good crystal quality.The  1 (LO) (582 cm −1 ) is associated with lattice defects, such as oxygen vacancy and zinc interstitial [35], which is in accordance with the result provided by EDS.According to the previous research, the acoustic combination of  1 and  2 was observed around 1101 cm −1 [36].Our result shows a broad band between 1050 and 1200 cm −1 , which confirms before the mentioned report.The broad and weak peaks at 661, 982 cm −1 should belong to the multiphonons process [37].
Figure 4(b) shows the room temperature PL spectrum of the ZnO nanorod samples.The UV emission at ∼399 nm is attributed to the band edge exciton emission [38], and the blue-green emission with PL peaks at 452, 469, 482, and 493 nm mainly originates from structural defects such as oxygen vacancies which often were detected in the ZnO nanocrystals prepared by low temperature grown methods [39][40][41][42].In addition, the broad green emission around 520 nm can be observed.The PL spectral characteristics differ somewhat from those of typical spectra of ZnO crystals, which usually exhibit a narrow UV absorption peak at 370-390 nm and a broad green emission at 510-550 nm [38].The exact mechanisms for the ZnO visible emission are controversial and not clear so far.However, the visible emission should be related to localized states in the band gap since the photon energy of visible light is far less than the band gap of ZnO.Therefore, it is a widely accepted view that the ZnO visible emission is due to radial recombination of the photogenerated hole with the electrons that belong to the oxygen vacancies [43][44][45].Combined with the previous analysis, the presence of blue-green emission should further confirm that the structural defects such as oxygen vacancies exist in the lattice of the obtained ZnO nanorods.

Conclusion
In summary, large-scale ZnO nanorods with diameters ranging from 20 to 80 nm and length from 400 nm to 1.2 m were successfully fabricated using solution chemical route assisted with Aliquat 336 as a soft template.Aliquat 336 can be simply recovered by salts-induced phase separation.A facile and clean synthetic process of ZnO nanorods was demonstrated.The XRD pattern and SEM and TEM analysis reveal that the ZnO nanorods have the wurtzite single crystalline structure with uniform morphologies and smooth surfaces, and a preferential growth direction along the c-axis.In the Raman spectra, the intense  2 modes, which include low-and highfrequency vibrations, approve the wurtzite structure of ZnO nanorods.The  1 (LO) process shows the presence of lattice defects in the ZnO nanorod crystals.The added appearances of blue-green emission in the PL spectrum further confirm the existence of oxygen vacancies in the ZnO nanorod lattices.

Figure 2 :
Figure 2: A representative XRD pattern of as-prepared ZnO nanorods.

Figure 3 :
Figure 3: (a) Low magnification SEM image of the as-prepared ZnO nanorods with the corresponding EDS (inset).(b) A representative SEM image of the ZnO nanorods.(c) A representative TEM image of the ZnO nanorods with the corresponding SAED pattern (inset).(d) HRTEM image showing the difference between two lattice fringes, which is about 0.26 nm.

Figure 3 (
c) shows the TEM image of ZnO nanorods with sharp tips and smoother surfaces.No amorphous layer was found at the surface of nanorods.The corresponding SAED pattern obtained from the shown nanorods confirmed that the synthesized products have single crystalline structures and grew along the [0001] direction (shown as inset in Figure 3(c)).

Figure 3 (
d) shows the HRTEM image of the marked nanorod.The image clearly reveals that only the fringes of (002) planes with a lattice spacing of about 0.26 nm can be observed, indicating that the individual ZnO nanorod is a single crystal.Furthermore, the spacing of 0.26 nm between two adjacent lattice planes corresponds to the distance between (002) planes indicating that [0001] is the growth direction of the ZnO nanorods.This is consistent with the XRD inferences.To further confirm the composition of the prepared product, EDS coupled to SEM was recorded.The inset of Figure3(a) is the EDS from the corresponding SEM, showing that zinc and oxygen are the only detected elements, without any other element contamination.The calculated atomic ratio of Zn and O is about 1.15 : 1, likely indicating the presence of oxygen deficiency.

Figure 4 :
Figure 4: (a) Raman spectrum of the as-prepared ZnO nanorods with excitation wavelength 514.5 nm.(b) Room temperature PL spectrum of the ZnO nanorods with excitation wavelength 325 nm.

Table 1 :
The wave numbers (in cm −1 ) of the first-and second-order Raman spectra observed in ZnO nanorods.