Kirkendall void formation and selective directional growth of urchin-like ZnO/Zn microspheres through thermal oxidation in air

Abstract

Microparticles with nanostructures on the surface have the characteristics of nanomaterials, yet they avoid aggregation and dispersion problems due to the nature of the nanomaterials. We report a simple, cost-effective, catalyst-free, and scalable production of urchin-like ZnO/Zn microspheres through thermal oxidation from Zn microparticles at 350–550 °C in air for a period of 6–36 hours. The novel, urchin-like microspheres are composed of (from inner to outer): a hollow core, a thin-shell of Zn, a thick-shell of ZnO and ZnO nanowires grown radially on the surface with a tip diameter of 20 nm, with a high aspect ratio (length/diameter), and a high population density. In our study, the Kirkendall effect was proposed for the formation of the hollow core/void of the urchin-like microspheres. Selective directional growth of ZnO nanowires with (100) orientation, followed by interdiffusion and propagation of oxygen and zinc for the lengthening the ZnO nanowires, were proposed for the formation mechanism of the ZnO nanowires. Studies on the field emission properties of the microspheres show that the microspheric emitter has a low turn-on field of 2.2 V μm⁻¹, a high field enhancement factor of 6804.8, a high luminance of 24 000 cd m⁻², and excellent luminance uniformity.

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1. Introduction

Zinc oxide is a wide-band-gap (3.37 eV) n-type semiconductor with a high-excitation binding energy (60 meV). Hexagonal wurtzite and cubic zinc-blends are the two main crystal structures of zinc oxide. In recent decades, zinc oxide nanostructures have been extensively studied including zero dimensional (0D) nanoparticles.^1,2 One dimensional (1D) nanorods and nanowires,^3–6 two dimensional (2D) nanofilms,^7,8 three dimensional (3D) nanoflowers,^9,10 and urchin-like nano structures^11–14 are evaluated. Due to the unique characteristics of nanostructures and the excellent properties of ZnO, many applications have been explored such as piezoelectric devices,^15–17 optoelectronic devices,^5,18,19 dye sensitized solar cells,^20,21 light emitting diodes,^22–25 electromagnetic absorption,^14,26 photosensors,^27,28 gas sensors,^8,29 solar water splitting,^30,31 and field emission applications.^32,33

Various methods were reported for the synthesis of ZnO nanostructures. These methods include: chemical vapor deposition,^34,35 thermal evaporation,^36,37 wet oxidation,^38,39 solution deposition,^40,41 thermal decomposition,^42,43 hydrothermal methods,^9,44 sol–gel methods^45,46 and thermal oxidation.^32,47 Among the methods mentioned above, thermal oxidation is a simple, cost-effective, catalyst-free, and one-step process for the formation of 1D ZnO nanostructures. The in situ process only requires Zn bulks/films and air as the sources for forming 1D ZnO nanostructures and an oven for thermal oxidation of the Zn source at a desired temperature. In addition, the fabrication method is also easy to scale-up for the mass production. 1D ZnO nanostructures can be formed with good crystallinity and high purity.^48–50

An urchin-like structure is one of the novel 3D nanostructures for ZnO material. The nanowires/nanorods were grown radially on the surface of a spherical substance forming an urchin-like structure. Thermal evaporation^51–55 and the hydrothermal method⁵⁶ are used for the synthesis of the urchin-like structure. According to our literature review, no study was found on the formation of urchin-like spheres with hollow cores and Zn/ZnO shell structures by thermal oxidation and its characteristics with field emission applications.

In this study, we report the formation of urchin-like spheres with a hollow-core and a Zn/ZnO shell structure by a simple, catalyst-free, cost-effective, one-step thermal oxidation process. The unique structure is examined with a variety of methods and the field emission properties of the material are studied in terms of turn-on field, field enhancement factor, luminance, and luminance uniformity.

2. Experimental

Zn microspheres (Alfa Aesar, purity 97.5 percent) with a solid core and diameters ranging from 6 to 9 μm were used for the formation of the urchin-like microspheres with hollow cores and Zn/ZnO shells. The formation process was carried out with Zn microspheres randomly sprayed on silicon substrates and heated at a desired temperature in air for a dwell time of up to 72 hours. The characterization of the produced materials was conducted by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800), high-resolution transmission electron microscopy (HR-TEM, Philips Tecnai F20), X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD), focused ion beam microscopy (FEI, Quanta 3D FEG), and X-ray diffraction (XRD, Rigaku Miniflex) using Cu Kα radiation.

The field emission characteristics of the urchin-like microspheres were studied. The device is composed of two parallel plates namely, an anode and a cathode, with a distance of 700 μm between them. For the preparation of the cathode, the Zn microspheres were first screen-printed on a 1.0 × 1.0 cm² indium tin oxide (ITO) glass followed by thermal oxidation at 500 °C to form the urchin-like microspheres on the ITO glass in situ. As-grown ZnO nanowires act as field emitters. For the preparation of an anode, green phosphors were used and screen-printed on an ITO glass followed by thermal treatment at 400 °C for a better adhesion on the glass. The device was loaded in a vacuum chamber with a pressure of 10⁻⁶ torr. The voltage–current relationship to the device was monitored using a Keithley 2410 source meter while luminance and luminance uniformity were measured using a luminance colorimeter (Topcon, BM-7A).

3. Results and discussion

3.1 Formation of the urchin-like structure

Fig. 1(a)–(d) show the SEM images of the Zn microspheres after thermal oxidation at temperatures of 400 °C (no dwell time), 550 °C (no dwell time), and 550 °C with a 36 hour dwell time, respectively, The Zn microsphere was of a relatively smooth surface and the diameter was about 8 μm. Thermal oxidation was conducted with a ramping rate of 5 °C per minute to 550 °C. As can be seen, as the temperature increased, the roughness of the surface of the Zn sphere increased. An island-like surface was created, as shown in Fig. 1(b). We believe that a layer of oxide was formed on the Zn sphere. As the temperature increased to 550 °C, the roughness of the surface increased more than the surfaces observed at lower temperature. A severe oxidation condition led to the change of the surface morphology from an island-like surface to a tumor-like surface. In addition, a few nanowires were found on the sphere, as shown in Fig. 1(c). Fig. 1(d) shows that an urchin-like microsphere formed at 550 °C with a dwell time of 36 hours. The mean length and population density of the nanowires were approximately 3.5 μm and 12.5 nanowires per μm², respectively. The mean average of the tip diameters was about 18 nm. A large amount of urchin-like microspheres can be synthesized via the method showing in Fig. S1 in the ESI.† Fig. 2 shows the relationship between the length and population density of the nanowires at various dwell times at 550 °C. It reveals that as the dwell time increased, the mean length and population density increased. The growth of the nanowires was terminated (or very slow) after a dwell time of 36 hours. It was expected that the weight of the microsphere increased during the thermal oxidation. In the case of the microsphere treated at 550 °C with a dwell time of 36 hours, the weight gained was about 23.5 percent measured by a thermal gravimetric analyzer.

Fig. 1 FE-SEM images of (a) Zn microsphere and Zn microsphere after thermal oxidation at (b) 400 °C (dwell time: 0 hours), (c) 550 °C (dwell time: 0 hours), and (d) 550 °C (dwell time: 36 hours).

Fig. 2 Relationship between length and population density of nanowires with various dwell times at 550 °C.

The urchin-like microspheres were characterized in terms of the nanowires, the surface of the sphere, and the inner part of the spheres. The nanowires were removed from the urchin-like spheres using an ultrasonic method. Fig. 3(a)–(c) show the TEM image, selected area electron diffraction (SAED) pattern, and the HR-TEM image of a nanowire, respectively. Fig. 3(a) illustrates that the nanowire had a wide root tapering to a sharp tip and the diameter of the tip is about 20 nm. The HR-TEM image [Fig. 3(c)] and corresponding SAED pattern of the nanowire reveal that the nanowire is a single-crystalline ZnO with a hexagonal close packed structure. The growth direction of the ZnO crystal is along [110] direction, which is parallel to the long axis of the nanowires. The d-spacing of 0.28 nm is consistent with the (110) interplanar spacing of ZnO. The energy-dispersive X-ray spectrum (EDS) in the inset of Fig. 3(c) shows that the nanowire is composed of Zn and O elements.

Fig. 3 (a) TEM image of the ZnO nanowires, (b) selected area electron diffraction pattern of the nanowires, and (c) HR-TEM image of the ZnO nanowire [inset: d-spacing analysis (top) and energy dispersive X-ray spectrum (bottom)].

The body of the urchin-like microsphere was characterized by various methods. Fig. 4(a) shows the XRD pattern of the microsphere, which was thermally treated at 550 °C for a dwell time of 36 hours. The diffraction peaks of the microspheres correspond to ZnO and Zn crystals. However, the intensity of the peaks, which represent ZnO, is higher than the intensity that represents Zn. The result implies that the oxidation process was completed with thermal oxidation for 36 hours at 550 °C in air. In addition, the peak of (110) orientation can be regarded as the contribution of ZnO nanowires, which was confirmed by HR-TEM analysis. Fig. 4(b) illustrates the Raman spectrum of the urchin-like microspheres. The inset is the optical image where Raman analysis was conducted. The peaks at 331.4, 382, 439.3 and 571.4 cm⁻¹ correspond to the standard 2E₂ (337 cm⁻¹), A₁ (387 cm⁻¹), E₂ (442 cm⁻¹), and E₁ (588 cm⁻¹) modes of ZnO, respectively. The result is in close agreement with a previously reported data.⁵⁷ Fig. 4(c) and (d) show the XPS spectra of Zn 2p and O 1s, respectively. In Fig. 4(c), the peaks at 1045.3 and 1022.3 eV correspond to the binding energy of the oxygen with Zn 2p_1/2 and Zn 2p_3/2, respectively. In Fig. 4(d), the peak can be decomposed by a curve-fitting method into two subpeaks centered at 531.5 and 530.2 eV. Both subpeaks represent the oxygen–zinc bondings. As a result, the spectra are in good agreement with the ZnO spectra reported in the literature.^58,59 The surface analysis of the urchin-like microspheres by Raman spectroscopy and XPS indicates that no Zn was found on the surface of the microspheres. However, XRD analysis for the bulk microsphere shows that two materials, ZnO and Zn, were in the microsphere. We believe that Zn existed in the inner part of the microspheres. The outer shell of the microspheres was analyzed via HR-TEM, a UV-vis spectrometer, and a photoluminescence spectroscope. The outer shell was confirmed as ZnO material. The results of the aforementioned analyses are shown in the ESI in Fig. S2 and S3.†

Fig. 4 (a) XRD pattern of urchin-like microparticles, (b) Raman spectrum of urchin-like microsphere (inset: the optical image of the sample in which the Raman analysis was conducted), and XPS spectra of (c) Zn 2p region and (d) O 1s region.

3.2 Effects of oxidation temperature and dwell time on the formation of the urchin-like microspheres

A systematic study was conducted with various oxidation temperatures (350 °C, 450 °C, and 550 °C) and dwell times (0 to 72 hours) to find the optimal operation condition for the formation of an urchin-like microsphere with the desired characteristics (e.g., length, tip diameter, and population density) of the ZnO nanowires. Furthermore, this study was conducted to understand the growth mechanism of the urchin-like microspheres. Fig. 5 shows the XRD analysis of the urchin-like microsphere after thermal oxidation at 450 °C for dwell times of 0 to 72 hours. As expected, due to the thermal oxidation, the intensity of the peaks corresponding to Zn decreased with increased dwell time, while the intensity of the peaks corresponding to ZnO increased with increased dwell time. In addition, the peaks corresponding to Zn was found with a dwell time of 72 hours, which indicates that an incomplete oxidation process was found. The result is consistent with the result obtained from the study at 550 °C as previously described.

Fig. 5 XRD analysis of the urchin-like microsphere after thermal oxidation at 450 °C with dwell times of 0 to 72 hours.

A focus ion beam (FIB) microscope equipped with EDS was employed to understand the morphology and composition of the outer and inner structures of the urchin-like microsphere. Fig. 6(a) and (b) show SEM images of a Zn sphere and a Zn hemisphere sectioned by FIB-milling, respectively. These figures illustrate that the Zn sphere has a solid core and a diameter of a few micrometers. Fig. 6(c) and (d) depict the surface and inner structure of the urchin-like microsphere, respectively, with thermal oxidation at 450 °C for a dwell time of 6 hours. Some ZnO nanowires were grown on the surface, and the core of the sphere was solid. Fig. 6(e) illustrates that by increasing the dwell time to 24 hours, the length and population density of the ZnO nanowires increased on the sphere. Furthermore, a hollow core was created, as depicted in Fig. 6(f). Fig. 6(g) and (h) show the surface and inner structure of the urchin-like microsphere, respectively, with a dwell time of 72 hours. We found that the length and population density of the ZnO nanowires increased slightly, but the size of the hollow core significantly increased. EDS was employed to understand the composition of the urchin-like microsphere. Fig. 7 shows EDS analysis of zinc and oxygen elements using mapping and line scanning techniques. The result revealed that no oxygen was found near the edge of the hollow core while zinc was present in the whole sphere. We therefore believe that a zinc layer existed around the hollow core as an inner shell and a zinc oxide layer formed an outer shell. As a consequence, the urchin-like microsphere is composed of, from inner to outer, a hollow core, a Zn shell, a ZnO shell, and ZnO nanowires.

Fig. 6 SEM images of the outer and inner structures of (a and b) Zn spheres and the sphere after thermal oxidation at 450 °C for dwell times of (c and d) 6 hours (e and f) 24 hours, and (g and h) 72 hours.

Fig. 7 EDS analysis of the urchin-like microspheres (mapping and line scanning of zinc and oxygen elements).

The study was also conducted with thermal oxidation of Zn microspheres at 350 °C for a dwell time from 0 to 72 hours. The observation of growth of the urchin-like microspheres by SEM and temperature profiles was summarized together with those synthesized at 450 °C and 550 °C in the ESI seen in Fig. S4 and S5.† As expected, the formation mechanisms of the urchin-like Zn microsphere were similar in cases with the thermal process at 350 °C, 450 °C, and 550 °C. The process started with the surface oxidation to form a layer of ZnO, followed by the formation of ZnO nanowires on the surface of the spheres. As the oxidation time increased, the length and population density of the ZnO nanowires increased. The growth of ZnO nanowires finally reached a limit and the formation of the urchin-like microsphere was terminated. According to our observations, the formation of the urchin-like microsphere was terminated at 48 hours, 36 hours, and 30 hours at the oxidation temperatures of 350 °C, 450 °C, and 550 °C, respectively. Fig. 8 shows the tip diameter and aspect ratio of the ZnO nanowires synthesized at 350 °C, 400 °C, 450 °C, 500 °C and 550 °C with the terminated dwell time. We found that as the oxidation temperature increased, the mean tip diameter decreased from 27 nm (350 °C), 21 nm (450 °C) to 180 nm (550 °C), while the aspect ratio (length/diameter) was increased from 30 nm (350 °C), 127 nm (450 °C) to 184 nm (550 °C). In addition, the population density (number of nanowire per μm²) increased from 9.4 nm (350 °C), to 10.1 nm (450 °C), and to 13.0 nm (550 °C) with an increase in oxidation temperature. We think that a higher oxidation temperature led to a higher oxidation rate, which is preferable for ZnO growth with [100] direction resulting in the formation of ZnO nanowires. The TEM and XRD analyses indicate that the growth direction of the ZnO nanowire was [100]. In addition, a high growth rate also refers to a rapid lengthening rate of the ZnO nanowires and the growth of large numbers of ZnO nanowires. As a result, the aspect ratio and population density increased with an increase of oxidation temperature. The weight gained for the urchin-like microspheres with various oxidation temperatures and dwell times is shown in the ESI shown in Fig. S6(a).† The weight of the urchin-like microsphere increased dramatically in the initial stage of the thermal process due to a rapid oxidation on the surface of the microspheres. With the increase of oxidation time, the speed of oxidation gradually slowed down and, finally, the weight of the urchin-like microsphere reached a limit. In addition, the weight gained for the microspheres increased with the increase of oxidation temperature. For example, the weight gained by the urchin-like microspheres was 2.42 percent, 5.76 percent, 12.36 percent, 18.78 percent, and 22.34 percent for the oxidation temperature at 350 °C, 400 °C, 450 °C, 500 °C, and 550 °C, respectively, for a dwell time of 24 hours.

Fig. 8 Tip diameter and aspect ratio of the urchin-like ZnO/Zn microspheres with various oxidation temperatures.

The growth mechanism of the urchin-like microsphere with a Zn/ZnO shell structure through thermal oxidation has not been reported in the literature although a vapor-solid (VS) model,⁶⁰ a self-catalyzed vapor-liquid-solid (VLS) model,⁶¹ a solid-state based-up diffusion model,⁵⁰ and a stress-induced mass transport mechanism³² were proposed for the growth of ZnO nanowires on a thin film/foil by thermal oxidation. However, we found that none of the studies reported the generation of the void (the hollow core in our case) during the oxidation process. We think that the formation mechanism of the urchin-like microspheres might involve several stages for the formation of the ZnO nanowires, an outer shell of the ZnO layer, an inner shell of the Zn layer, and a hollow core. Fig. S6(b)† shows the thermogravimetric analysis (TGA) and thermogravimetric analysis (DTA) as the information of the first stage of the oxidation process. At this stage, the weight of the microsphere increased slowly, which implies a slow oxidation rate due to the oxidation being at low temperature. The percentages of the weight gain were about 0.12 percent, 0.34 percent, 1.14 percent, 2.52 percent, and 4.83 percent at the temperatures of 350 °C, 400 °C, 450 °C, 500 °C, and 550 °C, respectively. A sharp endothermic peak is shown at 420 °C in the DTA curve, which represents the melting point of Zn. However, no significant change in terms of the weight was seen in this region. The exothermic process obviously took place after the melting point of ZnO. SEM images shown in the inset of Fig. S6(b)† reveal that a layer of zinc oxide with a rough surface was generated. This finding might be the result of a mild oxidation rate at a related low temperature (say, 20–300 °C) and a uneven oxidation rate on the surface caused by a native oxide on Zn microspheres.

At the second stage, with an increase to an elevated temperature (say, 400 °C to 550 °C), the oxidation rate was increased, resulting in the creation of oxide “islands” and a server rough surface. In addition, a few ZnO nanowires were found with a low aspect ratio on the surface of the microspheres. This was regarded as the beginning phase of the growth of ZnO nanowires. At the third stage, with increased dwell time at the elevated temperature, the length and population density of ZnO nanowires increased rapidly. A hollow core in the microsphere was formed, and then the size of the hollow core increased with increased dwell time. At the final stage, the oxidation rate decreased as the rate of weight gain decreased, as shown in the ESI S6(a).† The length and population density of the ZnO nanowires reached a limit in this stage. The structure of the urchin-like microsphere was analyzed as, from outer to inner, having radial ZnO nanowires on the surface of the microsphere, a thick shell of ZnO, a thin shell of ZnO, and a hollow core. We believe that thermal oxidation was involved in the chemical reaction of oxygen molecules and zinc atoms on the surface and then in the interdiffusion of oxygen and zinc atoms in the solid microsphere. As the reaction continued, with the diffusion of the oxygen atoms towards the core and the diffusion of the zinc atoms towards the surface, the thickness of the ZnO layer increased, resulting in the volumetric expansion of the microsphere. During the oxidation, a hollow core was formed. We think that the hollow core/void was caused as a consequence of the difference in diffusion rates of the oxygen and zinc atoms known as the Kirkendall effect.^62–66 The Kirkendall effect describes that the movement of the boundary layer between a diffusion couple is attributed to the different diffusion rate of these two species and atomic diffusion through a vacancy exchange. The void/hollow core was created because of the outward transport of the zinc ion with a fast-moving rate through the oxide layer and a balancing inward flow of vacancies to the vicinity of the metal–oxide interface.⁶⁴ In recent decades, hollow nanostructures based on the Kirkendall effect such as hollow nanocrystals^64,67 and nanotubes^68–70 have been developed.

In our previous studies, the urchin-like Fe₂O₃ microspheres^71,72 and urchin-like CuO microspheres⁷³ were synthesized, and the mechanism for the hollow core formation was discussed. However, we found that the core formation might be differently revealed by the SEM observation and FIB analysis.^71–73 This might be because Fe₃O₄ and Cu₂O are regarded as transient states of the Fe₂O₃ and CuO, respectively, and should be considered as the factors for the formation mechanism. ZnO nanowires formed by thermal oxidation were studied in the literature.^32,50,61 In addition to ZnO nanowires, formation of Fe₂O₃ nanowires,^74–77 and CuO nanowires^78–80 were studied and the formation mechanism was discussed. We think that the formation of the ZnO nanowires was due to the selective oxidation/growth of ZnO in a preferred [100] orientation during the thermal process. The interdiffusion of zinc and oxygen species led to the migration and propagation of Zn species toward the tip of the nanowires. The migrated Zn in the tip of the nanowire reacted with oxygen in the gas phase to form ZnO crystals for the lengthening of the nanowires. The ZnO nanowires with high aspect ratios were therefore formed.

3.3 Field emission properties of the urchin-like microsphere

The advantage of the urchin-like microsphere for field emission applications was the radial feature of the ZnO nanowires on the sphere, resulting in the tips of some nanowires pointing towards the anode. As a consequence, the nanowires as field emitters do not require an activation process such as a tapping process⁸¹ and/or electrical aging.⁸² Tapping entails lifting the tips of the emitters embedded in the electrically conductive layer after the screen-printing process. In addition, in situ growth of the urchin-like microspheres on the ITO glass led to the embedding of the microspheres in the ITO glass. A good contact between urchin-like microspherical emitters and the conductive layer ensures a low contact resistance so that the performance of the field emission will be enhanced. Fig. 9(a) shows the current density–electric field (J–E) curve for the urchin-like emitters formed at various temperatures. As can be seen, no current was observed between the cathode and anode in the low electrical field. As the electrical field increased, the current started to rise. A steep slope represented a good field emission efficiency. The turn-on field is defined as the applied field required to obtain an emission current density of 10 μA cm⁻². The turn-on fields were 2.22 V μm⁻¹, 2.44 V μm⁻¹, 3.12 V μm⁻¹, and 6.5 V μm⁻¹ for the urchin-like emitters synthesized at 500 °C, 450 °C, 400 °C, and 350 °C, respectively. The result shows that the urchin-like emitter formed at 500 °C had a better turn-on field and J–E curve. We believe that the small-tip diameter of the emitters, a high aspect ratio of length/diameter, and moderate population density enhanced the field emission performance of the emitters. The turn-on field of 2.22 V μm⁻¹ is relatively low for ZnO nanowires.^{32,33,83–85} The feature of the urchin-like emitters provided a higher local electrical field for nanowires on the spherical support, which enhanced the rate of electron tunneling and reduced the turn-on field.⁷¹ Fig. 9(b) shows a Fowler–Nordheim (F–N) plot of 1/E versus ln(J/E²) for the urchin-like emitters. The linear relationship between 1/E and ln(J/E²) indicates that the field emission behavior of the urchin-like emitter fits the F–N mechanism. The field enhancement factors, β, obtained from F–N equation were 6804.8, 5168.5, 3425.9, and 2354.5 for the emitters formed at 500 °C, 450 °C, 400 °C, and 350 °C, respectively. As expected, the emitters synthesized at 500 °C had the highest β value.

Fig. 9 (a) Current density (J) as a function of electric field (E) for urchin-like microspheres formed at 350 °C, 400 °C, 450 °C, and 500 °C, (b) F–N plot of urchin-like microsphere formed at 350 °C, 400 °C, 450 °C, and 500 °C.

Fig. 10(a) shows the luminance, current density, and fluorescence images varying with applied electric fields using the urchin-like emitters formed at 500 °C for 48 h. Both luminance and current density curves show the same trend. As the electric field increased, the luminance and current density increased. The bright spots become visible on the anode screen at 1.2 kV (electrical field of 2.0 V μm⁻¹). With an increase in the electric field, the emission spot density increased. At the applied voltage of 2.0 kV (electric field of 2.86 V μm⁻¹), the bright spots were uniformly distributed on the entire screen. A good uniformity was therefore achieved with a current density of 5 μA cm⁻² and a luminance of 303 cd m⁻². When the applied electric field was beyond 2.86 V μm⁻¹, the luminance and current density increased rapidly. A maximum luminance of 24 010 was obtained at an electric field of 8.00 V μm⁻¹ and a current density of 118 μA cm⁻². Fig. 10(b) shows an example of the luminance uniformity of the anode screen. A nine-point measurement procedure and an equation for the determination of the luminance uniformity were conducted.^86,87 The results show that a luminance uniformity greater than 90 percent was achieved for most of the cases in our study. As a consequence, the urchin-like emitters exhibited a low turn-on field, high β, excellent luminance, and luminance uniformity.

Fig. 10 (a) Luminance, current density, and fluorescence images of the urchin-like microsphere varying with applied electric field, (b) photograph of the luminance image operating at 4.8 kV, and (c) corresponding luminance uniformity distribution.

4. Conclusion

We have developed a simple, cost-effective, template-free and catalyst-free approach for the production of the urchin-like ZnO/Zn microsphere with ZnO nanowires on the surface. The formation of the urchin-like microsphere was studied through thermal oxidation from Zn microspheres at 350–550 °C in air for a few days. The tip of the ZnO nanowires on the sphere was about a few tens of nanometers while the length of the ZnO nanowires was about a few micrometers. The characterization of the microspheres with a variety of methods led to the understanding of both the inner and outer structures of the microspheres and a possible growth mechanism of the microsphere. We think that the Kirkendall effect played an important role in the formation of the hollow-core of the microsphere and the growth of the ZnO nanowires was due to the selective directional growth of ZnO with [100] orientation followed by interdiffusion and propagation of oxygen and zinc for nanowire lengthening. Field emission studies of the urchin-like microspheres reveal that the urchin-like microspheres are excellent emitters for the field emission lighting applications. The urchin-like structure formed at 500 °C for 48 h was suggested to be the optimal material for the field emission applications.

Acknowledgements

The authors would like to thank the Ministry of Science and Technology, Taiwan, and Lightlab Co., Sweden, for supporting the research under grant NSC 102-2221-E-194-008-MY3 and 103-10723-01, respectively.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19168h

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