Due to the special characteristic and excellent performance in optics, catalyst, optoelectronic properties differing from the bulk materials, micro/nano-materials have attracted great interests [1,2]. And now, micro/nano-materials play an increasingly significant role in basic scientific research and high-end potential application [3,4]. As one of them, the ultrafine ZnO powder is an attractive material used in a wide variety of application fields, including pigments, catalysts, photocatalysis, ceramics and phosphors [[1], [2], [3]]. With the rapid development of industrialization and urbanization, environmental pollution has become a global issue. Among them, dye wastewater pollution is particularly serious because of its color depth, high concentration, high toxicity and difficult degradation [5,6]. The photocatalytic technology is an effective way to mitigate this kind of pollution. Therefore, the low-cost ultrafine ZnO powder with good photoactivity and high photocatalytic efficiency has been paid further attention, and it is expected to substitute the catalytic powder TiO₂ [[7], [8], [9]]. Multiple methods are adopted to synthesize the ultrafine ZnO powder, and the wet chemical processes are the most significant for simplicity and low temperature [10,11]. Hydrothermal method, one of the wet chemical processes, is widely used in the field of ZnO crystal preparation with different morphologies due to its controllability [12]. Furthermore, its advantages attracting people’s attention also include simplicity, low temperature, high yield, better crystal quality and catalyst free [13,14]. So the hydrothermal method is usually employed to prepare ultrafine particles with different morphologies and better crystal quality [15]. Generally, zinc acetate and zinc nitrate are often chosen as raw materials in preparing the ultrafine ZnO powder [16], a few papers were reported using zinc sulfate as raw material [17]. The preparation of ultrafine ZnO with flower-like structure from zinc oxide ore has not been reported. An improved process for clean and effective utilization of zinc oxide ore has been proposed, including mixing, roasting, water leaching, purification [18,19]. All of the valuable elements in zinc oxide ore, such as zinc, silica, iron, lead and strontium are made into the corresponding products. As the key step of producing high value-added ultrafine ZnO powder, the synthesis of ultrafine ZnO powders with different morphologies was investigated in detail. The influencing factors, including dosage of dispersant, reaction temperature, molar ratio of OH^- to Zn²⁺, reaction time and Zn²⁺ concentration were systematically discussed. The crystal structures and morphologies of ZnO powders were detected and abserved by using X-ray diffraction (XRD) and scanning electric microscopy (SEM). The growth mechanism of ZnO powders with different morphologies was described. Finally, the photocatalytic degradation of rhodamin B (RhB) over ZnO with flower-like structure was studied.

The zinc oxide ore from Lanping, Yunnan Province, China was dried and ground to about 72-80 μm before mixing. The main components in zinc oxide ore are ZnO in a grade of 36.29% (exists as smithsonite and willemite), SiO₂ with the content of 17.85% (exists as quartz and Zn₂SiO₄), Fe₂O₃ with the content of 9.02% (exists as hematite), PbO in a grade of 3.66% (exists as cerusite and anglesite) and CaO with the content of 2.50% (exists as limestone). Aluminum and strontium are in a low grade. The major mineral phases are smithsonite, quartz and hematite, while the minor mineral phases are Zn₂SiO₄, cerusite and limestone and so on. Sodium hydroxide and polyethylene glycol (PEG20000) with analytic grade were used as precipitant and dispersant.

The obtained zinc sulfate solution was used as raw material, which was obtained through a series of processes, including mixing, roasting, water leaching and purification. And the Zn²⁺ concentration in solution was adjusted to a suitable range. NaOH solutions with variable concentrations were pre-made. The required molar ratio of ZnSO₄ and NaOH solutions were added into a 200 mL Teflon lined reactor, and then PEG20000 solution with 10% mass fraction as dispersant was added. The homogeneous solution was transfered to the autoclave and placed into an oven that was pre-heated to a fixed temperature for a certain time after being stirred magnetically. The autoclave was cooled down to room temperature after reaction. Then the slurry was filtered and the product was washed by distilled water repeatedly and by ethanol twice, respectively. The ZnO powder was obtained after being dried.

Appropriate weight of ZnO powders ranging from 0 to 500 mg with different morphologies were added into 20 mL RhB solution with concentration in the range of 10-25 mg L^-1 and ultrasonically dispersed. Then the suspensions were magnetically stirred and exposed to a lamp (JT8-Y20W) with a wavelength of 365 nm in a homemade case. The distance from lamp to specimen was 10 cm. Every 1 h, the suspension was centrifugated and 2 mL supernatant was taken and diluted to 50 mL in each test. The absorbency of the specimens was measured.

The structures of the ZnO powders were identified using a Japan Rigaku Ultima IV X-ray diffractometer, employing CuKα radiation with a voltage 40 kV, at a scanning rate of 6° min^-1 with 2θ ranging from 5° to 90°. The morphologies of the ZnO powders were observed by a Japan Hitachi 8010 scanning electron microscope. The absorbance of specimens was measured using a TU-1900 ultraviolet spectrophotometer.

3.1.1. Influence of dosage of PEG20000

The influence of dosage of PEG20000 on the structures and morphologies of ZnO powders was investigated under the reaction conditions: reaction temperature at 150 °C, molar ratio of OH^-/Zn²⁺ of 5, reaction time for 8 h and Zn²⁺ concentration of 1 mol L^-1. The results shown in Fig. 1 indicated that the dosage of PEG20000 had seldom effect on ZnO crystal structure. All diffraction data were in good agreement with JCPDS files No. 361451 (a = 0.324982 nm and c = 0.520661 nm). No other phases were detected, indicating that ZnO powders were hexagonal wurtzite structure with regular crystal form and high purity. But the influence on the morphologies of ZnO powders was obvious. In Fig. 1(b), the ZnO powder presented irregular flower-like structure. In Fig. 1(c), regular flower-like structure ZnO was observed with uniform micro-rods, each of them having a tip. Increasing the dosage of PEG20000 to 8 mL, more fascicular particles were yielded. Based on those evidence, the reason is PEG20000 [HO-(C₂H₄O)_nH] releases -OH groups in solution. In the initial growth stage of ZnO crystal, the main reaction is dehydration polymetization of Zn(OH)₄^2-. The nucleation activation energy was reduced by the combination of Zn²⁺ with O in C-O-C groups in PEG20000, promoting the steady growth of ZnO crystals. The dosage of PEG20000 with 10% mass fraction of 5 mL was chosen in following.

3.1.2. Influence of molar ratio of OH^- to Zn²⁺

The effect of molar ratio of OH^- to Zn²⁺ on the structures and morphologies of ZnO particles was studied under dosage of PEG20000 solution of 5 mL, temperature of 150 °C, reaction time of 8 h and Zn²⁺ concentration of 1 mol L^-1. The results displayed in Fig. 2 indicated that all the obtained specimens were ZnO with hexagonal wurtzite structure and the effect of molar ratio on the morphologies of ZnO powders were significant. In Fig. 2(b), the ZnO particle obtained at OH^-/Zn²⁺ = 2 were cluster structure composed of sheet particles. Increasing OH^-/Zn²⁺ to 5, regular flower-like structure ZnO particles with uniform micro-rods was obtained, as observed in Fig. 2(c). The micro-rods grew uniformly in a radiative way from a shared center. However, when OH^-/Zn²⁺ was too high, the micro-rods grew bigger and longer, resulting in that the flower-like strictures were broken, as shown in Fig. 2(d), some of the micro-rods had been stripped from the flower-like strictures. This is because the ZnO crystal has the ability of self-assembled growth. Moreover, NaOH solution was alkaline with strong polarity. Increasing the molar ratio of OH^- to Zn²⁺, the solution polarity was enhanced, which led to the excessive growth of ZnO micro-rods.

3.1.3. Influence of reaction temperature

The effect of the reaction temperature on the structures and morphologies of ZnO powders was also studied under conditions of dosage of PEG20000 solution 5 mL, molar ratio of OH^-/Zn²⁺ 5, reaction time 8 h and Zn²⁺ concentration 1 mol L^-1, as shown in Fig. 3. The reaction temperature had no obvious effect on the structures of ZnO powders, as the XRD patterns (Fig. 3(a)) showed that the obtained ZnO powders were hexagonal wurtzite structure with high purity. However, the reaction temperature had significant effect on the morphologies of ZnO powders. The ZnO powder obtained at 120 °C displayed a cluster structure composing of flaky particles with smooth surface. The polarity growth was unconspicuous. A high polarity solution may be help to obtain the ZnO powder with flower-like structure. Increasing the temperature to 150 ℃, ZnO powder with flower-like structure was observed, as shown in Fig. 3(c). However, when the temperature was excess high, for example 180 °C, the flower-like structure of ZnO was destroyed due to the excessive growth of ZnO micro-rods, which were stripped from the shared center, and incomplete and irregular flower-like structure were left. It was confirmed by the Fig. 3(d).

3.1.4. Influence of reaction time

The influence of reaction time on the structures and morphologies of the ZnO particles was also examined under the condition of dosage of PEG20000 solution of 5 mL, molar ratio of OH^-/Zn²⁺ of 5, reaction temperature of 150 °C and Zn²⁺ concentration of 1 mol L^-1, the results were showed in Fig. 4. The as-obtained ZnO powders were hexagonal wurtzite structure with high purity. The ZnO powder synthesized at 150 °C for 4 h (Fig. 4(b)) was cluster structure with flaky and short rod-like particles. Obviously, the ZnO powder was still in growth stage. Prolonging the reaction time to 8 h (Fig. 4(c)), the uniform and regular ZnO powder with flower-like structures was successfully synthesized, and all the micro-rods aligned in a radiative type from the shared center. When the reaction time reached up to 12 h, the branches of flower-like structure ZnO grew longer and thicker, so that they destroyed the flower-like structure, as shown in Fig. 4(d). It can be said that 8 h is enough for obtaining ZnO powder with flower-like structure.

3.1.5. Influence of Zn²⁺ concentration

The influence of Zn²⁺ concentration was also discussed under the conditions: PEG20000 solution of 5 mL, OH^-/Zn²⁺ = 5 at 150 °C for 8 h. As shown in Fig. 5, the location and intensity of all the diffraction peaks were in good agreement with JCPDS files No. 361452, indicating Zn²⁺ concentration had no obvious effect on the phase of ZnO powder. The ZnO powder obtained at Zn²⁺ concentration 0.5 mol·L^-1 was irregular and nonuniform flower-like structure (Fig. 5(b)). The micro-rods were discrepancy in size. Increasing Zn²⁺ concentration to 1 mol L^-1, the desired ZnO powder with flower-like structure was obtained (Fig. 5(c)). When Zn²⁺ concentration was 2 mol L^-1, although the prepared ZnO presented flower-like structure, the aggregation and adhesion was obvious (Fig. 5(d)). This is because increasing the Zn²⁺ concentration resulted in yielding a large number of crystal nuclei in the initial growth stage so that the micro-rods had no sufficient space to grow reasonably.

The SEM image zoomed on the tip of ZnO rod and EDS pattern were displayed in Fig. 6. It was obvious that the polarized tip-growth was observed, revealing a multistep figure. The main elements from EDS pattern are Zn, O (from ZnO rod) and Au (from the gold spraying before testing), confirming that the as-prepared ZnO powder with flower-like structure was pure.

It is known that the crystal formation is composed of two stages: nucleation and growth, and the crystal figure depends on nucleation speed and growth rate [3,20]. Based on the experimental results and previous work [3,18,19], the growth mechanism of ZnO powders with different morphologies was deduced and presented in Fig. 7. As we known, the nucleation speed and growth rate are fast. The large amounts ZnO nuclei formed in the initial stage converged due to the surface energy and electronstatic effect to form tiny crystals. And those tiny crystals in supersaturated solution kinetically favored to grow large crystals along with close-over plane [21]. Therefore, the ZnO crystals grew directionally in solution through Zn(OH)₄^2- polymerization dehydration, nucleation, crystal growth, then became the micro-rods of flower-like structure ZnO. Based on the above, the growth process of ZnO particles could be formulated by the following chemical reactions, and the schematic growth diagram of ZnO particles is shown in Fig. 7 [3,20].

3.3.1. Influence of ZnO powders with different morphologies

The influence of ZnO powders with different morphologies on the degradation ratio of RhB at different time was investigated under conditions of 20 mL RhB solution with 10 mg L^-1 and ZnO 300 mg. The results plotted in Fig. 8(a) indicated that increasing the lighting time, the degradation ratio of RhB increased. The degradation ratios under photocatalytic degradation with flower-like structure, sheet and rod-like ZnO were 95%, 85% and 73% at 4 h, respectively. This is because the wide gap and effective surface area in flower-like structure ZnO provide more opportunities for the diffusion and motion of RhB molecules, increasing the contact opportunities between active points of ZnO powder and RhB molecules, then promotes the photocatalytic reaction [22,23].

3.3.2. Influence of dosage of ZnO powder

The investigation of the effect of dosage of ZnO powder with flower-like structure on the degradation of RhB was performed under conditions of 20 mL RhB solution with 10 mg L^-1. The results displayed in Fig. 8(b) indicated that the concentration of RhB was almost unchanged by lighting for 4 h without adding ZnO. It demonstrated that the self-degradation of RhB is negligible. The degradation ratio of RhB was enhanced by rising lighting time when ZnO powder was added into the solutions. ZnO powder with flower-like structure showed a good photolytic activity. Increasing the dosage of ZnO powder, the degradation ratio of RhB increased. However, when the dosage of ZnO powder was higher than 300 mg, the degradation ratio decreased. This is because the photocatalytic reaction is inadequate when the ZnO powder in the degradation solution is insufficient. With the increase of ZnO powder adding, the photocatalytic activity increases and the degradation rate of RhB increases. But when the dosage of ZnO powder is too high, the solid particles hinder the light transmittance in solution so that reduces the photocatalytic efficiency.

3.3.3. Influence of initial concentration of RhB

The absorption equilibrium of RhB over ZnO powder with flower-like structure reached 3.2% within 1 h under dark environment. Hence, the concentration change of RhB was mainly attributed to the photocalytic degradation. The investigation of the relationship between the initial concentration of RhB and the degradation of RhB was carried out under conditions of dosage of ZnO 300 mg and 20 mL RhB solution with variable concentration. The results presented in Fig. 8(c) indicated that the degradation ratio of RhB was enhanced by rising lighting time. Increasing the concentration of RhB, the degradation ratio increased first and then decreased. The RhB solution with concentration of 15 mg L^-1 exhibited the best photocatalytic efficiency and was almost completely degraded over ZnO powder 300 mg and lighting for 3 h. This is because the higher the concentration of RhB is, the weaker the light transmittance is. And the being covered active sites also results in lower catalytic activity. The suitable concentration of RhB is 15 mg L^-1.

As mentioned in the reports, there are several factors influencing the photocatalytic property, including the morphology, specific surface area and the oxygen defects [4,7]. During the photocatalysis process, the reaction primarily occurs at the interface because of the close contact between the RhB and the active sites [4,7,24,25]. When ZnO powder is irradiated by UV light, it absorb photons with energy equal to or greater than its band gap, resulting in the electrons (e^-) in the valence band are excited to the conduction band and leave the holes (h⁺) in the valence band. The holes will react with the water molecules and hydroxide anions absorbed on the surface of ZnO powder to form reactive ·OH radicals [4,7,24,25]. ·OH radicals are the major active species, which has strong oxidative ability and are able to oxidize RhB into the final products of CO₂ and H₂O [25].

The ZnO powder with flower-like structure has a large specific surface area, which is favorable for photocatalytic activity [4]. And the flower-like structure would effectively prevent the aggregation and maintain the high specific surface area. The ZnO powder with flower-like structure is polar crystal with Zn²⁺-rich positive polar planes. More polar faces derived from the multistep tips can provide more oxygen defects (Fig. 6(a)). Furthermore, the oxygen vacancy was detected on the surface of flower-like structure ZnO and rod-like structure ZnO, respectively. But the content of oxygen vacancy in flower-like structure is larger than that in rod-like structure [26]. Hence more OH radicals are generated to participate the photocatalytic reaction [3,27,28]. The ZnO powder with flower-like structure can provide more opportunity for the surface reaction so that the flower-like structure ZnO powder showed higher photocatalytic activity than the rod-like ZnO powder. Thus, the photocatalytic process can be displayed as below:

The stability of the ZnO powder with flower-like structure was also measured after three cycles of operation, and it was relatively stable and its photocatalytic efficiency was a little decrease of 2.4%. Moreover, the morphology of the specimen has no obvious change after three cycles.

The dosage of PEG20000 solution, molar ratio of OH^-/Zn²⁺, reaction temperature, reaction time and Zn²⁺ concentration had significant influences on the morphologies of the ZnO powders. The micro-rods of the flower-like structures grew larger when the reaction temperature, molar ratio of OH^-/Zn²⁺ and reaction time increased. When the reaction temperature, molar ratio and time increased to a certain degree, the flower-like structures would be destroyed and formed micro-rods. The appropriate conditions for synthesizing ZnO powder with flower-like structure were the dosage of PEG20000 with 10% mass fraction 5 mL, molar ratio of OH^- to Zn²⁺ 5, reaction temperature 150 °C, reaction time 8 h at Zn²⁺ concentration 1 mol L^-1. The obtained ZnO powders with different morphologies were hexagon wurtzite structure with high purity. The growth process of ZnO powder was complex, including: formation of Zn(OH)₄^2-, polymerization dehydration of Zn(OH)₄^2-, formation of ZnO nuclei and ZnO crystals growth. The ZnO powder with flower-like structure has good photocatalytic degradation ability to degrade RhB. 20 mL RhB solution with 15 mg L^-1 was almost completely degraded over ZnO powder 300 mg within 3 h.

This work was supported financially by the National Natural Science Foundation of China (Nos. 51774070 and 51574084) and the National Key R&D Program of China (No. 2017YFB0305401).