Synthesis of High Crystalline Al-Doped ZnO Nanopowders from Al 2 O 3 and ZnO by Radio-Frequency Thermal Plasma

High crystalline Al-doped ZnO (AZO) nanopowders were prepared by in-flight treatment of ZnO and Al 2 O 3 in Radio-Frequency (RF) thermal plasma. Micron-sized (∼1 μm) ZnO and Al 2 O 3 powders were mixed at Al/Zn ratios of 3.3 and 6.7 at.% and then injected into the RF thermal plasma torch along the centerline at a feeding rate of 6.6 g/min. The RF thermal plasma torch system was operated at the plate power level of ∼140 kVA to evaporate the mixture oxides and the resultant vapor species were condensed into solid particles by the high flow rate of quenching gas (∼7000 slpm).The FE-SEM images of the as-treated powders showed that the multipod shaped and the whisker type nanoparticles were mainly synthesized. In addition, these nanocrystalline structures were confirmed as the single phase AZO nanopowders with the hexagonal wurtzite ZnO structure by the XRD patterns and FETEM results with the SAED image. However, the composition changes of 0.3 and 1.0 at.% were checked for the as-synthesized AZO nanopowders at Al/Zn ratios of 3.3 and 6.7 at.%, respectively, by the XRF data, which can require the adjustment of Al/Zn in the mixture precursors for the applications of high Al doping concentrations.


Introduction
Zinc oxide is a typical n-type semiconductor with transparency to visible light [1][2][3].Because of these electrical and optical properties, ZnO nanopowders have attracted much attention as the building block for transparent electrodes in RFID tags, light emitting diodes (LEDs), and solar cells [4][5][6].Furthermore, as an earth-abundant material, ZnO can be used as an alternative to indium tin oxide (ITO), whose supply may be depleted in the near future.Compared with the electrical conductivity of indium tin oxide (ITO), however, that of ZnO needs to be improved for practical application as a TCO (transparent conductive oxide) material.For this purpose, ZnO is often doped with group III and IV metals, such as Al, Ga, and In [2,[7][8][9].By acting as an impurity in the ZnO structure, these metals can enhance the high n-type conductivity of the doped ZnO as well as the optical transparency [7][8][9].Accordingly, many efforts have been made to offer the doped ZnO nanopowders as the building block for TCO electrodes.Among them, the Al-doped ZnO (AZO) nanostructures are promising as a low-cost TCO material to substitute the ITOs because Al metal is a typical earthabundant material.In addition, Al metal can greatly improve the electrical conductivity of ZnO [10].For example, AZO nanowires have been reported to show a steep increase of the electrical conductivity up to 9.73 × 10 −5 S/cm compared with the value of 2.55 × 10 −5 S/cm for the electrical conductivity of the pure ZnO nanowires [10].The physical properties and the morphology of AZO nanopowders strongly depend on their preparation methods [7,11,12].Conventional wet chemical processes including precipitation, sol-gel, and hydrothermal routes can produce AZO nanopowders with the specific compositions and various morphologies from flower-like ones to nanowhisker types [7,11,12].However, these conventional processes may produce nanopowders of poor crystalline quality because of the relatively low synthesis temperatures, in which case an additional calcination process would be needed to produce nanopowders of high crystalline quality.Recently, RF (Radio-Frequency) thermal plasma has been gradually adopted as a unique heat source in making nanosized powders [13][14][15][16][17]. Since no electrode is required for sustaining plasma state, RF plasma torches can produce high enthalpy flow fields without electrode erosion and arc jet fluctuation, which are inevitable in the applications based on DC (Direct Current) arc plasma torches.These well-known advantages of RF plasma torch can be beneficial to the synthesis of nanocomposite powders by the in-flight heat treatment of mixture precursors.As for the doped ZnO, for example, Cheng et al. reported that AZO nanopowders can be prepared by RF thermal plasma treatment of the aqueous suspension with an oxide mixture of 10 wt.% [17].Although Cheng et al. 's work was also intended to utilize those advantages of RF plasma torch for the synthesis of AZO nanopowders, the in-flight treated aqueous solution may bring the poor crystallinity to the as-prepared nanopowders due to the latent heat of water vaporization and relatively low RF power level (∼21 kW) [17,18].Considering the crystallinity of the asprepared powders and the simplification of the preparation process, we employed a solid mixture consisting of Al 2 O 3 and ZnO powders as a precursor for RF thermal plasma synthesis of AZO nanopowders in this work.By removing the vaporization process of water and using a high-powered (∼140 kVA) RF thermal plasma system, the plasma enthalpy can be used efficiently in heating not only the injected oxide powders but also the as-synthesized particles.

RF Thermal Plasma Synthesis of Al-Doped ZnO Nanopowders.
A schematic of the commercial RF thermal plasma system used in this work is illustrated in Figure 1.In this system, electromagnetic energy with oscillation frequencies of 1-4 MHz was generated from a self-excited type RF oscillator with the maximum plate power level of 350 kW and transferred to the RF thermal plasma in an inductively coupled way through the induction coil of the RF plasma torch (Tekna, PS-100).For the confinement of the generated RF thermal plasma, a water-cooled Si 3 N 4 tube with an inner diameter of 100 mm was employed inside the RF plasma torch (Tekna, PS-100), which was mounted on a 700 mm diameter × 1000 mm long synthesis reactor.The upper part of synthesis reactor was equipped with a carbon liner for thermal insulation, as shown in Figure 1.This carbon liner helped to expand the high enthalpy plasma flame in the synthesis reactor and to heat the injected oxides up to their evaporation points.Then, nanopowders were synthesized by quenching the vapor species of the injected materials.In this quenching process, a gas mixture of Ar and O 2 was poured into the synthesis reactor at a flow rate of ∼7000 slpm.Finally, the synthesized nanopowders were collected at the sintered stainless-steel filters with a total surface area of 2.9 m 2 .
For the preparation of the solid precursors, micron-sized powders of ZnO (∼1 m) and Al 2 O 3 (∼1 m) were weighed and mixed at Al/Zn ratios of 3.3 and 6.7 at.%.The mixture precursors were fed along the centerline of the RF thermal plasma torch through a water cooled injector, which was connected to the powder feeder, as shown in Figure 1.The injected precursors were treated by the RF thermal plasma generated at the plate power level of ∼140 kVA and the plasma forming gas flow rate of 260 slpm, where the plasma forming gas consisted of a central gas (Ar only, 60 slpm) and a sheath gas (Ar 100 slpm and O 2 100 slpm), as depicted in Figure 1.In the RF thermal plasma system, all gases including quenching gas were designed to move continuously at the reactor pressure of 89.3 kPa by mechanical vacuum pump and gas recycling system.

Characterization of Al-Doped ZnO Nanopowders.
Field emission-scanning electron microscopy (FE-SEM, S4800, Hitachi Ltd., Japan) and field emission-transmission electron microscopy (FE-TEM, JEM-2200FS, Jeol, Japan) were used to observe the particle sizes and morphologies of the mixture precursors and the RF plasma treated powders.The electron microscopes were operated at acceleration voltages of 10 kV and 200 kV, respectively.The transmission electron microscopy/energy dispersive X-ray spectroscopy (TEM-EDX, INCA X-sight, Oxford, UK) was used to observe the result of Al doping.For the preparation of TEM sample, the as-synthesized powders were dispersed in ethanol by an ultrasonic vibrator for 5 min and then loaded on a copper grid.The phase and structural analyses were carried out by X-ray diffraction (XRD, D8 Advance, Bruker, Germany) with Cu K radiation ( = 1.5405Å) at 2-theta angles ranging from 20 ∘ to 80 ∘ and scan speed of 10 min −1 .Finally, difference in the composition between precursors and the as-synthesized powders was checked by X-ray fluorescence analysis (XRF, S4 Pioneer, Bruker, Germany).

Results and Discussion
Figures 2(a) and 2(b) show the FE-SEM images of the mixture precursors and the RF plasma treated powders for Al/Zn doping concentration = 3.3 at.%, respectively.From these images, one can see that the irregular mixture precursors were mainly reformed into multipod shaped or whisker type nanocrystals after plasma processing.Figures 3(a) and 3(b) show the FE-SEM images of a similar change in the morphology and particle sizes of the mixture precursors after processing for Al/Zn doping concentration = 6.7 at.%.Since the micronsized (∼1 m) solid precursors will be evaporated due to the huge enthalpy of the high-powered RF thermal plasma (∼140 kVA), these changes in the morphology and sizes can take a place during the condensation and nucleation process of the vapor species.In these processes, accordingly, Al atoms can be expected to be doped into ZnO nanocrystals with the whisker-like morphology or the multipods.In order to confirm Al doping in the as-synthesized nanocrystals, TEM-EDX mapping images were obtained for the as-synthesized powders at Al/Zn ratio = 6.7 at.%, which are presented in Figure 4.As expected, this figure shows that the elements Al and Zn were relatively uniformly dispersed in the assynthesized nanocrystals consisting of the multipods and the whiskers.Additionally, the SAED (Selected Area Electron Diffraction) patterns of the TEM images for the multipod and the whisker, as shown in Figures 5(a) and 5(b), reveal that the as-synthesized nanocrystals had a wurtzite structure of ZnO.Since Al elements were detected in the as-prepared powders by TEM-EDX (see Figure 4), these SAED results indicate that AZO nanopowders can be synthesized as a single phase of ZnO.The formation of the AZO nanocrystals with a single phase of ZnO can also be confirmed by comparing the XRD patterns of the mixture precursors and the assynthesized powders, as shown in Figures 6 and 7.In Figure 6, the crystalline peaks for Al 2 O 3 were not detected in the XRD graph of the as-synthesized powders, although they were present in the mixture precursors.In addition, the diffraction peaks corresponding to the wurtzite structure of ZnO shifted to higher Bragg angles (2-theta) for the assynthesized powders as observed in Figure 7, indicating the replacement of Zn 2+ by Al 3+ in the ZnO lattice.Together with the TEM-EDX mapping images (see Figure 4) and SAED data (see Figures 5(a Finally, the change in the composition between the mixture precursors and the nanopowders was checked by XRF analyses, as listed in Table 1.In this table, one can see that the Al contents of the as-synthesized powders were reduced with the decrement of 0.3 and 1.0 at.% for the Al/Zn ratio = 3.3 at.% and 6.7 at.%, respectively.These changes of Al content  may be attributed to not only the relatively complicated vapor species generated from Al 2 O 3 at high temperatures but also their cocondensation processes for the synthesis of AZO nanopowders.For example, ZnO is simply decomposed into Zn and O at its decomposition point of 2,248 K [19].During the quenching process, accordingly, Zn element has a single route to the formation of AZO nanocomposite.Since Al 2 O 3 can be vaporized into Al and aluminium suboxides such as AlO, Al 2 O, and AlO 2 , around the boiling point of Al 2 O 3 (∼3,000 K) [19,20], however, Al elements participating in the synthesis of AZO nanocomposite come from not only Al but also various aluminium suboxides such as AlO, Al 2 O, and AlO 2 , which can exist in the gas phase at different temperature ranges.As a result, some of the suboxides can be oxidized into Al 2 O 3 without participating in the cocondensation of ZnO vapor species, which may lead to the Zn rich AZO nanoparticles collected at filtration as listed in Table 1 although the exact mechanism has not been clarified.

Conclusion
In summary, high crystalline Al-doped ZnO nanopowders were synthesized by RF thermal plasma treatment of Al 2 O 3 and ZnO solid mixture precursors.From the FE-SEM images of the as-synthesized powders, the mixture oxide precursors were mainly reformed into nanocrystals with a multipod or a whisker morphology, which is attributed to the high crystallinity of the as-synthesized powders, as shown in XRD data.In addition, the TEM-EDX images, SAED data, and the XRD pattern of the as-synthesized powders confirmed that Al 3+ was incorporated into the ZnO lattices of wurtzite structure, and consequently the as-synthesized AZO nanopowders had a single phase of ZnO.However, the XRF data showed a small reduction of Al content in the composition of the assynthesized AZO nanopowder.For the practical applications requiring a high doping concentration of Al, accordingly, the experimental results indicate that the ratio of Al/Zn in the mixture precursors needs to be set higher than the targeted Al content for the adjustment of the composition of the assynthesized AZO powder.

sLpm + O 2 Figure 1 :
Figure 1: A schematic of the RF thermal plasma system for the synthesis of nanosized AZO powders.
) and 5(b)) of the nanocrystals, these XRD results show that high crystalline AZO nanopowders with a single phase of ZnO were synthesized by RF thermal plasma treatment of Al 2 O 3 and ZnO powders.

Figure 5 :
Figure 5: HR-TEM images of the as-synthesized nanocrystals with morphologies of (a) a multipod and (b) a whisker, at Al/Zn doping concentration = 6.7 at.%.The insets represent the selected area diffraction patterns corresponding to each particle in HR-TEM images.

Figure 6 :
Figure 6: XRD patterns of (a) the as-mixed precursor powders and (b) the as-synthesized powders, at Al/Zn doping concentrations = 6.7 at.%.

Figure 7 :
Figure 7: Comparison of XRD patterns between the pure ZnO powders and the as-synthesized powders.The main peak corresponding to ZnO wurtzite structure shifted to the higher Bragg angle (2-theta) due to the incorporation of the Al cation into the ZnO lattice.

Table 1 :
Relative cation composition changes between the asmixed precursors and the as-synthesized powders based on X-ray fluorescence data.