Ultrasonic-Assisted Synthesis , Characterization , and Optical Properties of Sb Doped ZnO and Their Photocatalytic Activities

1 Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand 2Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 3 Electron Microscopy Research and Service Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 4Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 5Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand

In this research, a facile and environment-friendly lowtemperature route was used to synthesize Sb doped ZnO by ultrasonic-assisted solution method.Phase, morphologies, optical properties, and photocatalytic properties of Sb doped ZnO were also studied and discussed in this report.

Experimental Procedures
Sb doped ZnO nanostructures were synthesized by the ultrasonic-assisted solution method using zinc nitrate hexahydrate (Zn(NO 3 ) 2 ⋅6H 2 O), antimony chloride (SbCl 3 ), and ammonium hydroxide (NH 4 OH) as starting materials.All the chemicals for this synthesis were purchased from Aldrich Chemical Corporation and used without further purification.
For the typical experimental procedure, 0.01 mol of Zn(NO 3 ) 2 ⋅6H 2 O and 1-5% by mole of SbCl 3 were dissolved in 100 mL of deionized water.Aqueous solution of 28% ammonium hydroxide was dropped in precursor solutions until reaching the pH of 8.5 with continuous stirring to precipitate Zn 2+ and Sb 3+ ions into metal hydroxide compound.Subsequently, the resultant solutions were transferred into sonication bath (35 kHz) and sonicated at 80 ∘ C for 3 h.In the end, the precipitates were filtered and washed with methanol several times to remove ionic impurities and finally dried at room temperature.
Crystalline phases of the as-synthesized nanostructured materials were analyzed by an X-ray diffractometer (XRD, Philips X'Pert MPD) with Cu-K  radiation in the 2 = 15 ∘ -75 ∘ range.The morphology investigation was carried out by field emission scanning electron microscopy (FE-SEM, JEOL JSM-6335F) and transmission electron microscopy (TEM, JEOL JEM-2010) operating at 35 kV and 200 kV, respectively.The optical properties were studied by a Perkin Elmer, Lambda 25 UV-visible spectrometer.
Photocatalytic activity was tested by decolorization of methylene blue (MB) in aqueous solution under UV light.The 150 mg pure ZnO and Sb doped ZnO as photocatalysts were suspended in 150 mL 10 −5 M MB solutions and were magnetically stirred for 30 min in the dark environment to establish an adsorption/desorption equilibrium of MB on surfaces of the photocatalysts.After UV irradiating, the concentrations of MB were determined by a UV-visible spectrophotometer (Lambda 25, Perkin Elmer) using a wavelength of 664 nm.The decolorization efficiency (%) was calculated as follows: where   and  were the initial concentration of MB and the concentration of MB after UV irradiation, respectively.

Results and Discussion
The purity and crystalline properties of the as-synthesized ZnO and Sb doped ZnO samples were determined by X-ray diffraction (XRD) as shown in Figure 1  the major diffraction peaks shifted slightly towards smaller diffraction angle compared to the pure ZnO phase due to the ionic radius of Sb 3+ of 0.76 Å [15,16] >ionic radius of Zn 2+ of 0.74 Å [16,17].Upon increasing the Sb concentration doped in ZnO structure of more than 3%, mixed phases of Zn(OH) 2 and ZnO (JCPDS no.38-0385 [14] for Zn(OH) 2 and no.36-1451 [14] for ZnO) were detected.The XRD results show that the limited Sb concentration doped in ZnO is 3 wt% in this research.
Figures 2 and 3 show the FE-SEM images of the assynthesized 0-3% Sb doped ZnO products with low and high magnifications.A morphology of pure ZnO as shown in Figure 2(a) was well-defined flower-like three-dimensional ZnO nanostructures in a large-scale area with diameters in the range of 0.5-1 m.It should be noted that the flowerlike three-dimensional ZnO nanostructures were composed of assemblies of nanorods as petals.At high magnification image of the nanorod-built flower-like ZnO nanostructures in Figure 3(a), they revealed that each petal was about 300 nm long and 100 nm in diameter.For the SEM images of Sb doped ZnO, the morphologies of rice kernel-like ZnO nanostructures formed instead of flower-like structures.They show the rice kernel-like ZnO nanorods in the range of 300-400 nm long.However, no flower-like structures were detected in the Sb doped ZnO samples.At high magnification, the products were composed of assembled nanorods to build rice kernel-like ZnO nanorods.These different morphologies of ZnO and 1-3% Sb doped ZnO can  be explained in terms of a thermodynamic barrier arising from the Sb 3+ dopant that slowed down the nucleation and inhibited the further growth of Sb doped ZnO crystals [18].Chemical composition of the as-synthesized products was observed using EDX analysis.Figures 4 and 5 show the typical EDX spectra of 0-3% Sb doped ZnO and EDX mapping of 3% Sb doped ZnO.EDX spectra show that the products consisted of zinc and oxygen for pure ZnO and zinc, oxygen, and antimony atoms for 1-3% Sb doped ZnO.Intense peaks of Cu and Au were also detected in the spectra due to the Cu stubs and sputtered Au.There was no detection of other impurities in the products, indicating that they had very high purity.Figure 5 shows selected area elemental mapping of 3% Sb doped ZnO.The mapping was Figure 7 shows the typical TEM images of 1% and 3% Sb doped ZnO nanostructures.It is apparent that 1% Sb doped ZnO exhibits well-defined rice kernel-like colonies with an average size of 300-400 nm.The nanorods serving as building blocks were tightly packed as colonies of rice kernel-like shaped particles.It can be concluded that the rice kernel-like ZnO colonies formed from the attachment of ZnO nanorods.The magnified TEM image in Figure 7(b) shows the detailed colonies of rice kernel-like ZnO.The colonies of rice kernels were composed of densely arrayed nanorods with diameter of about 10 nm.While the colonies of the 3% Sb doped ZnO nanocrystallites as shown in Figure 7(c) present the rice kernel-like colonies of many closely packed nanorods of about 90 nm in diameter and 1.2 m in length similar to 1% Sb doped ZnO sample.It also shows that the ends of the nanorods have relatively smaller diameters compared to those of the middle parts.The enlarged TEM image of 3% Sb doped ZnO sample as shown in Figure 7(d) shows the colonies of rice kernel-like ZnO particles with very rough surface.It is noteworthy that the rice kernel-like structure was sufficiently stable, which cannot be destroyed even after ultrasonication for a long time.The insets of Figures 7(a For the present research, the products were polycrystalline in nature.They were the (100), (002), ( 101), ( 102), ( 110), ( 103), (112), and (201) planes which were in accordance with those of the JCPDS database for hexagonal ZnO phase.
The optical properties of as-synthesized 0-3% Sb doped ZnO samples were studied by UV-visible absorption as shown in Figure 8.The spectrum of pure phase ZnO sample exhibits a broad absorption band at around 373 nm, blue shift relative to 380 nm of bulk ZnO [23].However, the spectra of 1%, 2%, and 3% Sb doped ZnO samples exhibit sharp bands at 356 nm, 350 nm, and 343 nm, respectively.It should be noted that the absorption peaks became sharper.They were blue-shift from 373 nm of pure ZnO sample to 343 nm of 3% Sb doped ZnO sample.The band gaps were calculated by the equation of   = 1240/ [24,25].They are 3.32 eV, 3.48 eV, 3.54 eV, and 3.61 eV for ZnO, 1% Sb doped ZnO, 2% Sb doped ZnO, and 3% Sb doped ZnO, respectively.These can be explained by the decreasing in size of the particles and consequently the increasing band gap between the valence and conduction bands.A blue shift of the absorption peak in the UV-visible spectra of these samples was successfully and clearly observed.
Upon the illumination of UV light, ZnO can transform the photonic energy into chemical energy, in a similar way for the synthesis or the decomposition of organic materials.Its remarkable oxidation reduction capability, high chemical stability, and harmless characteristics are most commonly applied in pollutant removal and disinfectants.When the ZnO samples are illuminated by ultraviolet of wavelength less than 400 nm, electrons of the valence band were excited by the photonic energy of the ultraviolet to the conduction band.At the same time, the valence band created electronic holes carrying positive electricity.These holes reacted with the absorbed O 2 or H 2 O to create OH • free radicals, which further generated the reaction such as disinfection or deodorization [26].Figure 9(a) shows the UV-visible absorption spectral change of MB during the photocatalytic degradation in the presence of ZnO under UV light over the wavelength range of 400-800 nm.The intensity of main absorption peaks of the MB solutions at approximately 664 nm decreases continuously with the length of UV irradiation time.It indicates that MB molecules could be degraded in the presence of ZnO.The photocatalytic mechanism of ZnO is as follows: where h VB + and e CB − are the electron vacancies in the valence band and the photogenerated electrons in the conduction band, respectively.The conduction-band electrons and valence-band holes are generated on the surfaces of ZnO nanostructures when they are illuminated by UV light with energy greater than the band gap.Holes react with water molecules adhering to the surfaces of ZnO nanostructures to form highly reactive hydroxyl radicals (OH • ) which have a powerful oxidation ability to degrade organic dye [7].Figures 9(b) and 9(c) show the UV-visible absorption spectra of the aqueous solutions of MB with 1% Sb doped ZnO and 3% Sb doped ZnO samples as photocatalysts and illuminated to UV light for different time intervals.The characteristic absorption of MB at 664 nm decreases rapidly with the prolonging time and almost disappears after about 300 min.Further exposure leads to no absorption peak in the whole spectrum, indicating that almost none of the MB remain.These photocatalysis results clearly demonstrate that Sb doped ZnO exhibited higher photocatalytic activity as compared with ZnO sample.
Figure 10 shows MB degradation efficiency of the assynthesized ZnO and Sb doped ZnO samples.The Sb doped ZnO samples exhibit much higher photocatalytic activities than that of the pure ZnO one.It took only 102 min for 3% Sb doped ZnO and 134 min for 1% Sb doped ZnO to decolorize 50% of MB while pure ZnO took more than 275 min to decolorize the same amount of MB.This faster degradation rate of MB under UV irradiation using Sb doped ZnO is attributed to the increase in defect sites caused by Sb 3+ doping, leading to an enhanced optical absorption in the UV region.After 300 min of irradiation, the values of degradation efficiency are 56, 90, and 95% for pure ZnO, 1% Sb doped ZnO, and 3% Sb doped ZnO, respectively.This clearly demonstrates that ZnO doped with Sb 3+ degrades MB more efficiently than undoped ZnO.In this research, the 3% Sb doped ZnO shows the highest photocatalytic activity.Under illumination with UV light, Sb doped ZnO generates electron-hole pairs at the tail states of conduction and valence bands.The generated  electrons diffused to the adsorbed MB molecules on the surface of Sb doped ZnO.The excited electrons from the photocatalyst conduction band migrated into the molecular structure of MB and by forming the conjugated system which then led to the complete decomposition of MB.Holes at the valence band generated OH • via reaction with water or OH − might be used for oxidation of other organic compounds.
The photocatalytic properties of as-synthesized photocatalysts were evaluated by measuring the absorption intensity of MB at 664 nm after UV irradiation at different lengths of time.Both of these photodegradation reactions were determined by pseudo-first-order reactions [27][28][29][30].The reaction rate constants of MB degradation calculated for ZnO, 1% Sb doped ZnO, and 3% Sb doped ZnO are 1.47×10 −3 , 6.30×10 −3 , and 8.65 × 10 −3 min −1 , respectively.This clearly demonstrates that ZnO doped with antimony can be used as a potential photocatalyst illuminated with UV light.antimony added shows a profound effect on morphology which changed from flower-like structure of nanorods for ZnO to rice kernel-like structure for Sb doped ZnO.Assynthesized doped and undoped ZnO crystals were tested and compared for their photocatalytic activities by decolorization of MB under UV light.It was clear that 3% Sb doped ZnO showed the highest photocatalytic activity toward the MB solution.

Figure 1 :
Figure 1: XRD patterns of the products synthesized by ultrasonicassisted solution method.

Figure 6 :Figure 7 :
Figure 6: TEM images and SAED pattern of flower-like ZnO structure.
) and 7(c) show the SAED pattern taken from their corresponding rice kernel-like Sb-doped ZnO samples.The diffraction patterns were composed of a number of bright spots arranged in concentric rings, with the calculated lattice planes obtained from the diameters of the diffraction rings.
Ultrasonic-assisted synthesis of Sb doped ZnO at room temperature has been introduced.XRD results showed the formation of wurtzite ZnO and the upper bound of 3 wt% doped Sb.No other phases were detected.The amount of

Figure 10 :
Figure 10: Decolorization efficiencies of ZnO with and without Sb doping.
[3]b 2 O 3 , and Sb were detected in these samples, suggesting that Sb 3+ ions could uniformly substitute into the Zn 2+ sites or interstitial sites of ZnO lattice by forming 2Sb Zn and V Zn[3].Moreover,