Sonochemical Synthesis of Er 3 +-Doped ZnONanospheres with Enhanced Upconversion Photoluminescence

ZnO semiconductors, with a direct wide bandgap of 3.37 eV and aesthetic nanoscale morphologies, have been intensively studied due to their multifunctional characteristics for a diverse range of applications in optical, electronic, optoelectronic, piezoelectric, photocatalytic, and power devices [1]. In the past decade, ZnO has been explored for new device applications when extra functionalities are intentionally introduced through proper doping or alloying with impurity ions despite the considerable challenges. It is worth noting that much effort has also been made through doping rare earth (RE) ions (e.g., Er and Eu) into the ZnO host, which undergoes upconversion (UC) luminescence and/or energy transfer, in realizing new optoelectronic and photonic device applications such as solid-state full-colour displays, infrared detectors, solar cells, biological fluorescent labels, and all-solid compact lasers [2]. The Er-doped semiconductors are the potential optoelectronic materials [3] due to the Er intra-4f shell transition with a photoemission at a wavelength of 1.54 μm, which lies in the minimum loss region of silica-based optical fibers [4]. Up to now, physical doping methods such as ion implantation [5], laser ablation [6], magnetron sputtering [7], and high temperature calcinations [8] have mainly been used to introduce Er into ZnO substrate, while most researches focused on the formation of films. Some chemical doping methods, such as sol-gel [9], colloidal [10], and hydrothermal processes [11], have also been reported to prepare Er-doped ZnO nanocrystals. However, most products were nanoparticles. Only a few groups reported the morphology-controlled synthesis, such as Er-doped ZnO nanowires fabricated using Er-ion implantation [5], flower-like structures prepared by a wet chemical reaction [12], and prickly sphere-like, column-like, prism-like, and grain-like structures prepared via hydrothermal process [13]. Therefore, it is still a challenge to prepare Er-doped nanocrystals with special morphology through a fast and convenient method. Here, a facile sonochemical route has been used to prepare Er-doped ZnO nanospheres. Under the irradiation of ultrasound, it only took 30 min of reaction time to effectively dope Er into ZnO host. The photoluminescence and upconversion photoluminescence properties of the asprepared Er-doped ZnO nanospheres were investigated, and the results showed enhanced emission due to effective doping.


Introduction
ZnO semiconductors, with a direct wide bandgap of 3.37 eV and aesthetic nanoscale morphologies, have been intensively studied due to their multifunctional characteristics for a diverse range of applications in optical, electronic, optoelectronic, piezoelectric, photocatalytic, and power devices [1].In the past decade, ZnO has been explored for new device applications when extra functionalities are intentionally introduced through proper doping or alloying with impurity ions despite the considerable challenges.It is worth noting that much effort has also been made through doping rare earth (RE) ions (e.g., Er 3+ and Eu 3+ ) into the ZnO host, which undergoes upconversion (UC) luminescence and/or energy transfer, in realizing new optoelectronic and photonic device applications such as solid-state full-colour displays, infrared detectors, solar cells, biological fluorescent labels, and all-solid compact lasers [2].The Er 3+ -doped semiconductors are the potential optoelectronic materials [3] due to the Er intra-4f shell transition with a photoemission at a wavelength of 1.54 μm, which lies in the minimum loss region of silica-based optical fibers [4].
Up to now, physical doping methods such as ion implantation [5], laser ablation [6], magnetron sputtering [7], and high temperature calcinations [8] have mainly been used to introduce Er into ZnO substrate, while most researches focused on the formation of films.Some chemical doping methods, such as sol-gel [9], colloidal [10], and hydrothermal processes [11], have also been reported to prepare Er 3+ -doped ZnO nanocrystals.However, most products were nanoparticles.Only a few groups reported the morphology-controlled synthesis, such as Er 3+ -doped ZnO nanowires fabricated using Er-ion implantation [5], flower-like structures prepared by a wet chemical reaction [12], and prickly sphere-like, column-like, prism-like, and grain-like structures prepared via hydrothermal process [13].Therefore, it is still a challenge to prepare Er 3+ -doped nanocrystals with special morphology through a fast and convenient method.
Here, a facile sonochemical route has been used to prepare Er 3+ -doped ZnO nanospheres.Under the irradiation of ultrasound, it only took 30 min of reaction time to effectively dope Er 3+ into ZnO host.The photoluminescence and upconversion photoluminescence properties of the asprepared Er 3+ -doped ZnO nanospheres were investigated, and the results showed enhanced emission due to effective doping.

Synthesis.
Typically, Zn(NO 3 ) 2 •6H 2 O (0.005 mol) was dissolved in H 2 O (100 mL), and then specific amount of Er(NO 3 ) 3 •5H 2 O (2, 5, 10% doping, resp.),PEG-20000 (1 g) and TEA (2 mL) were introduced sequentially with stirring to form a clear solution.The transparent mixture solution was exposed to high-intensity ultrasound irradiation under ambient air for 30 min.Ultrasound irradiation was generated with a high-intensity ultrasonic probe (Xinzhi Co., China, JY92-2D, 0.6 cm diameter; Ti-horn, 20 kHz, 60 W cm −2 ) immersed directly in the reaction solution.A white precipitate was centrifuged, washed with distilled water and absolute ethanol in sequence, and finally dried in air.The final products were collected for characterizations and further preparations.

Characterization. The X-ray powder diffraction (XRD)
analysis was performed on a Philips X'-pert X-ray diffractometer at a scanning rate of 4 • min −1 in the 2θ range from 10 • to 80 • , with graphite-monochromatized Cu-Kα radiation (λ = 0.15418 nm).The scanning electron micrographs (SEM) were taken on a LEO-1530VP fieldemission scanning electron microscope.Transmission electron microscopy (TEM) was carried out on a JEOLJEM 200CX transmission electron microscope, using an accelerating voltage of 200 kV.High-resolution transmission electron micrographs (HRTEM) were obtained by employing a JEOL-2010 high-resolution transmission electron microscope with an accelerating voltage of 200 kV.Photoluminescence spectra (PL) were measured on a Shimadzu RF-5301PC fluorescence spectrometer under the excitation of 325 nm at room temperature.The upconversion (UC) photoluminescence spectra were recorded on a spectrometer (Zolix, China) equipped with a 980 nm laser diode as the excitation source.

Characterization of the Final Products.
Figure 1 shows the XRD pattern of the as-prepared products.Evidently, all of the diffraction peaks in the XRD pattern are well assigned to hexagonal wurtzite ZnO as reported in JCPDS card no.36-1451, and no impurity phase related to Er 3+ could be found.The diffraction peaks shift to lower angel slightly with the Er-doping concentration increasing from 2-10%, indicating that the lattice parameter gets larger when Er 3+ are incorporated into ZnO lattice, which is consistent with the fact that the radii of Er 3+ (R = 0.89 Å) are bigger than that of Zn 2+ (R = 0.74 Å).
The as-prepared Er 3+ -doped ZnO samples appeared as uniform solid nanospheres with diameter of ca.400 nm, as shown in Figures 2(a

Possible Sonochemical Formation Mechanism.
In recent years, ultrasonic irradiation has been extensively used in the synthesis of nanomaterials.The effects of high-intensity ultrasound result primarily from acoustic cavitation are [14]: the formation, growth, and implosive collapse of bubbles in liquids.During the acoustic cavitation process, very high temperatures (>5000 K), pressures (>20 MPa), and cooling rates (>10 10 K s −1 ) can be achieved upon the collapse of the bubble, which provides a unique platform for the growth of nanostructures including 0D nanoparticles [15], 1D nanorods [16], to 2D nanoplates [17], and even mesoporous [18] or hollow structures [19].Nanocomposites [20] and doped nanomaterials [21] have also been prepared through sonochemical methods.
In the present case, Zn(NO 3 ) 2 •6H 2 O was used as precursor, TEA as complexing and precipitating agent and PEG as capping agent.TEA has the chemical formula N(CH 2 CH 2 OH) 3 , which acts generally as a weak base due to the single lone pair of electrons on nitrogen atom and accordingly could coordinate to zinc ions forming [Zn(TEA) 4 ] 2+ complex [22].Under ultrasonic irradiation, the complex would dissociate and a part of TEA molecules would hydrolyze to build up an alkaline environment, which leads to the controlled release of free zinc ions and hydroxide ions ((1) and ( 2)).The released Zn 2+ ion and OH − ion would combine and transform to ZnO under sonochemical conditions ((3) and ( 4)).As a popular shape modifier, PEG was used here to control the morphology of formed ZnO nanoparticles and also showed effective role to make uniform ZnO nanospheres.Therefore, the mechanism of the formation of ZnO nanospheres is probably related to the coordination of Zn 2+ and TEA to form Zn-TEA complex, the dissociation of the complex under sonication and PEGmodified formation of ZnO nanospheres.The probable reaction process in aqueous solution can be summarized as follows: High-intensity ultrasonic irradiation also played an important role.The transient high temperature and high pressure field produced during ultrasonic irradiation provides a favorable environment for the growth of nanocrystals.Cavitations and shock waves created by ultrasound can accelerate solid particles to high velocities leading to interparticle collisions and inducing effective fusion at the point of collision [23].We consider that the high temperature, high pressure, and shock waves caused by ultrasound induced the effective doping of Er 3+ into ZnO host.The energy generated during collision can induce the crystallization of the amorphous particles, responsible for the further crystallization process.It only took 30 min of reaction time to conduct effective doping under ultrasonic irradiation, while rather long reaction time (e.g., 8-12 hours) is needed in hydrothermal and other synthetic routes.

Photoluminescence Properties.
Figure 3 shows the roomtemperature photoluminescence spectra of pure and Er 3+doped ZnO with different Er 3+ doping concentrations using the same excitation line at 325 nm.All samples show a strong and broad green emission around 540 nm which can be attributed to deep-level emission (DLE) caused by defects and impurities [24].The emission intensity of ZnO : Er is increased enormously as compared with that of undoped ZnO.The integral emission intensity of ZnO : Er with 10% Er 3+ doping concentration is 3 times higher than that of the undoped ZnO, which indicates an unusual improvement on the light yield by Er doping.
The PL spectra of the ZnO : Er crystal with Er 3+ doping concentration varying from 2% to 10% (Figures 3(b)-3(d)) show that the emission intensity enhanced with the increasing doping concentration.The heavily doped product appeared to have an extremely high PL intensity in the present case, which might be related to the interaction between ZnO and Er.Previous studies reported that the intentionally doped impurities could provide a significant contribution to DLE, for example, by providing donoracceptor pairs in ZnO [24].The majority of Er in ZnO is likely to be associated with rare earths occupying substitutional Zn sites [25], and the local lattice is modified because of the different oxidation states and ionic sizes of Er and Zn ions [26].The incorporation of Er ions in the ZnO lattice is therefore expected to increase the deep-level states and resulted in an increase in DLE.Energy transfer might also occur between excitons and Er 3+ ions [27]; however, we did not observe Er 3+ emission peaks in the visible region.It might be possible that the weak emission peaks of Er 3+ ions were embedded in the trace of a strong DLE.
Obviously, the upconversion luminescence intensity of Er 3+ -doped ZnO is much higher than that of pure ZnO, which indicate the effective doping could enhance the upconversion emission.It is found that with the Er doping concentration increasing from 2% to 5%, the integral upconversion luminescence intensity enhanced.However, with the Er-doping concentration varying from 5% to 10%, the green emission decreased slightly while the red emission enhanced, which is similar with the literature [28].Therefore, the upconversion light output could be tuned by adjusting the dopant concentration.Further detailed investigation is still going on.

Conclusion
Er 3+ -doped ZnO nanospheres have been synthesized via a facile sonochemical route.Ultrasonic irradiation played an important role in the formation of ZnO nanospheres and resulted in the effective doping.The photoluminescence intensity of Er 3+ -doped nanocrystals enhanced with the increasing Er doping concentrations.Higher upconversion photoluminescence emissions of the Er 3+ -doped ZnO nanocrystals were observed, which suggests that the Er 3+ ions have been incorporated inside the crystalline ZnO grains.
) and 2(b).The HRTEM image recorded on the surface of ZnO : Er nanospheres (Figure2(c)) shows lattice fringes with interplanar spacing of 0.25 nm for the (101) faces of hexagonal ZnO.