View PDF VersionPrevious ArticleNext Article
 
DOI: 10.1039/C5RA24492G (Paper) RSC Adv., 2016, 6, 7403-7408

Microstructure, growth process and enhanced photocatalytic activity of flower-like ZnO particles

Yumin Sua, Jibiao Lia, Zhanzhou Luob, Bin Lua and Ping Li*a
aKey Laboratory of Inorganic Nanomaterials of Hebei Province, School of Chemistry and Materials Science, Hebei Normal University, Shijiazhuang 050024, China. E-mail: pingl@126.com; Tel: +86 311 80787436
bSchool of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China

Received 19th November 2015 , Accepted 8th January 2016

First published on 13th January 2016

Abstract

Flower-like ZnO microstructures are prepared by a simple one-step hydrothermal process at 85 °C for 30 min. ZnO hierarchical structures about 3.5 μm in size are assembled by rod-like crystals with sharp ends. The growth of flower-like ZnO is rapid. OH−-driven oriented aggregation and multistep nucleation result in the formation of flower-like ZnO structures. The products exhibit excellent photocatalytic performance in the degradation of methyl orange under UV-light irradiation, which can be attributed to their assembled open structures.

1. Introduction

ZnO is one of the most promising semiconductor materials with a wide band gap of 3.37 eV and a high exciton binding energy of 60 meV.1,2 The unique optical and electrical properties of ZnO enable its use in solar cells, gas sensors, varistors, light-emitting devices, nanolasers, piezoelectric devices, and some biomedical applications.3–9 Physical and chemical properties of ZnO are known to be closely related to its morphology, size, and dimension.10–13 In recent years, the self-assembly of nanoscale building blocks into complex structures, especially three-dimensional (3D) hierarchical structures, has been a focus in the fabrication of ZnO nanocrystals.14–19 Flower-like ZnO with 3D branched structures is very popular and attracting considerable attention because of its outstanding photoelectric and photocatalytic properties.20–24 Many techniques can be used to synthesize ZnO nano- or microstructures such as chemical vapor deposition (CVD),25 thermal evaporation processes,26 sputter deposition techniques,27 electrochemical deposition techniques,28 hydrothermal methods20 and so on. Among the reported methods, the hydrothermal process is one of the most important and widely used methods for the preparation of flower-like ZnO. The advantages of this process include low cost, simple operation, and easy regulation of morphology and particle size.

The morphology and size of crystals grown in aqueous solutions are reportedly due to their intrinsic structures and the nature of solution. Our previous work has revealed that the alkalinity of a solution is a decisive factor affecting particle crystallization.29,30 When NaOH is used as precipitant, the components of a solution may include Zn(OH)2, Zn(OH)3−, and Zn(OH)42− precursors, depending on alkali concentration. ZnO precipitate from precursors with elevated temperature, and reaction rate largely depends on precipitant concentration.

In this work, we develop a simple one-step hydrothermal process using only Zn(NO3)2 (0.2 M) and high-concentration (1.2 M) NaOH as reactants. Flower-like ZnO with branched structures were harvested from the hydrolysis of Zn(OH)42− precursors. This approach is rapid, pollution free, and does not require any surfactant and additive. The growth of flower-like ZnO is concluded as a mechanism of OH−-driven oriented aggregation and multistep nucleation, which is a new viewpoint first presented in this paper.

2. Experimental

2.1 Sample preparation and characterization

Analytical-grade zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and sodium hydroxide (NaOH) were purchased from Tianjin Chemical Reagent Company and used without further purification. Zn(NO3)2·6H2O and NaOH were dissolved at room temperature in deionized water to obtain 1.0 and 4.0 M standard solutions, respectively. About 30 mL of NaOH solution was added dropwise to 20 mL of Zn(NO3)2 solution with an additional 50 mL of deionized water under vigorous stirring at 3 °C, resulting in a 100 mL clear solution containing Zn(OH)42− precursors ([Zn2+] = 0.2 M, [OH−] = 1.2 M). The mixture was heated to 85 °C and aged at constant temperature for 30 min. The white product flower-like ZnO was then harvested after the reaction. The morphology and microstructure of the sample were characterized with a Hitachi S-4800 field-emission scanning electron microscopy (FESEM) system, a Bruker-AXS D8 ADVANCE X-ray diffractometer (XRD), and a JEOL JEM-2010 high-resolution transmission election microscopy (HRTEM) system equipped with selected area electron diffraction (SAED).

2.2 Growth process

The growth of flower-like ZnO was studied by a time-dependent experiment. During the reaction, samples were collected from the mixture at regular time intervals. After filtration and washing, the solid phase was characterized by XRD and FESEM. The residual soluble zinc concentration was analyzed by EDTA chelate titration method.

2.3 Photocatalytic experiments

The photocatalytic performance of ZnO was evaluated using methyl orange (MO) as a model dye under UV-light irradiation. A 250 W high-pressure Hg lamp (λmax = 365 nm) was used for photodegradation test. An aqueous MO solution (10 mg L−1, 100 mL) and a photocatalyst (50 mg) were placed in a 200 mL beaker. The suspension was pretreated by ultrasonication to disperse the catalyst and then magnetically stirred in the dark for 30 min to adsorption–desorption equilibrium. The suspension was then placed under UV-light irradiation. A quantity of suspension was collected every 30 min to measure MO degradation using a 752 N UV-vis spectrophotometer.

3. Results and discussion

3.1 Characterization of as-prepared sample

Reaction for the growth of flower-like ZnO was performed at 85 °C. Fig. 1a shows the FESEM images of particles formed at 30 min. The general shape was monodispersed branched structure with a size of around 3.5 μm and radiated aggregation consisting of many rod-like ZnO with a mean diameter of 100 nm. The assembled structures looked like beautiful flowers each with hexagonal prismatic “flower petals”. Most of the “petals” had a sharp, pointed end extending outward, with a few hexagonal plane ends growing perpendicularly to the “pistil” (white box in Fig. 1a). Fig. 1b shows the HRTEM image taken from an individual flower petal. The image clearly reveals only the fringes of (002) planes can be observed, indicating that the aggregated ZnO nanorod is a single crystal. In addition, the spacing of 0.26 nm between two adjacent lattice planes corresponds well to the distance between (002) planes', indicating that [0001] is the growth direction of the ZnO nanorods. Fig. 1c shows the SAED pattern of the middle part of an individual petal. The regularly arrangement of spots reveals the single crystalline nature and hexagonal phase of the individual petal. The SAED pattern taken from the pistil part (Fig. 1d) is identified as symmetrical stripes rather than polycrystalline circles or arrayed spots, which confirmed the presence of some ordered arrangement of crystallites in this terrain. According to the polar growth nature of ZnO,13 the fastest growth rate of the crystal was along +c-axial direction. As a result, the crystal face of (0001) eventually disappeared into a point, and the sharp end of the “petal” should be (0001) face with an abundance of Zn2+ ions, whereas the hexagonal plane end should be (000[1 with combining macron]) face with an abundance of O2− ions. The petal junction showed strong negative polarity because it was the gathering site of many hexagonal plane ends rich in O2− ions.
image file: c5ra24492g-f1.tif
Fig. 1 Flower-like ZnO microstructures grown at 85 °C for 30 min. (a) General-view FESEM image; (b) HRTEM image of an individual flower petal; (c) SAED pattern of the middle part of an individual petal and (d) SAED pattern of the pistil part.

3.2 Growth mechanism of flower-like ZnO particles

A time-dependent experiment was conducted to better understand the formation mechanism of flower-like ZnO microstructures. We obviously observed that at the beginning of this process, a Zn(OH)2 precipitate was obtained. As more of the NaOH solution was added, the Zn(OH)2 precipitate dissolved to yield a homogenous aqueous solution containing Zn(OH)42− ions. Therefore, Zn(OH)42− is proposed to be the growth unit that is directly incorporated into ZnO crystallites under given conditions. The growth process of ZnO can be formulated as follows:
 
Zn2+ + 2OH− → Zn(OH)2↓ (1)
 
Zn(OH)2 + 2OH− → Zn(OH)42− (2)
 
Zn(OH)42− → ZnO + 2H2O + 2OH− (3)

Fig. 2 displays the XRD patterns of samples taken from different growing times. The composition of samples grown in the first 2 min (curve a) was determined as Zn(OH)2. At 5 min (curve b), weak characteristic peaks of ZnO (indicated with asterisks) appeared in the pattern. At 10 min (curve c), samples were determined to be pure ZnO with a hexagonal wurtzite structure (JCPDS 36-1451). At 30 min (curve d), the intensity of the ZnO diffraction peaks significantly increased, indicating very good crystallization of sample.


image file: c5ra24492g-f2.tif
Fig. 2 XRD patterns of samples at different growth times: (a) 2 min; (b) 5 min; (c) 10 min and (d) 30 min.

Fig. 3 shows the morphologies of the particles at different growth stages. At 2 min (Fig. 3a), the particles exhibited plank-shaped structures that can be identified as Zn(OH)2 based on XRD data. At 5 min (Fig. 3b), some flower-like structures with an average size of 2.9 μm were present in the sample, along with plank-shaped Zn(OH)2. At 10 min (Fig. 3c), flower-like ZnO predominated in the samples with an increased size of 3.3 μm. At 30 min (Fig. 1a), only a slight increase in particle size and no obvious change in morphology were detected.


image file: c5ra24492g-f3.tif
Fig. 3 FESEM images of samples collected after different growth times: (a) 2 min; (b) 5 min and (c) 10 min.

The concentration of soluble Zn(II) that remained in the mother solution as a function of time was also determined (Fig. 4). In the first 2 min, Zn(II) concentration linearly decreased, indicating the precipitation of plank-shaped Zn(OH)2. Then, the curve had a small rise between 2 and 5 min. Combined with XRD and FESEM data in this stage, the plank-shaped Zn(OH)2 can be inferred to have begun dissolving, accompanied by ZnO nucleation. Zn(OH)2 dissolution was also slightly faster than ZnO formation. The curve had a slow decline between 5 and 30 min and eventually plateaued after 30 min. In this stage, flower-like ZnO grew with the consumption of Zn2+ ions until dissolution–crystallization equilibrium of ZnO was established.


image file: c5ra24492g-f4.tif
Fig. 4 Soluble Zn(II) concentration versus time in the mother liquid during reaction.

The above results showed that flower-like ZnO very rapidly grew because the particularly high alkalinity of the solution provided abundant precursors of Zn(OH)42−, which accelerated reaction rate. Subsequently, the “explosive” nucleation of ZnO occurred when the solution reached its critical temperature. A large number of primary bullet-shaped nuclei with structural polarity precipitated out of the solution in a short time. The tiny particles tended to aggregate because of their high surface energy and frequent collisions among them. Given that the high alkalinity of solution provided a negative environment, most pointed ends (electropositive) of the bullets were aligned toward the solution, and most of the plane ends (electronegative) gathered around a few pointed ends. Thus, secondary nuclei formed in a radiated mode. Subsequently, every single nucleus of the secondary nuclei grew preferentially along its c-axial direction, which led to the formation of the ultimate flower-like ZnO structures. Fig. 5 shows a schematic illustration of the multistep nucleation of flower-like ZnO.


image file: c5ra24492g-f5.tif
Fig. 5 Schematic illustration of the formation process of flower-like ZnO microstructures.

3.3 Effect of NaOH on the formation of flower-like ZnO

NaOH concentration is also a decisive factor affecting the formation of flower-like ZnO. The same experiments with diversed molar ratio of (Zn2+[thin space (1/6-em)]:[thin space (1/6-em)]OH−) were performed when the Zn2+ concentration remained unchanged. Results showed that flower-like ZnO can be obtained only under moderate alkaline conditions (Zn2+[thin space (1/6-em)]:[thin space (1/6-em)]OH− = 1[thin space (1/6-em)]:[thin space (1/6-em)]5–1[thin space (1/6-em)]:[thin space (1/6-em)]8), as shown in Fig. 6. Precipitation from aqueous solution required a threshold dose rate to produce critical nuclei, so when NaOH concentration was too high, a great deal of OH− ions inhibited the hydrolysis of Zn(OH)42− and greatly reduced reaction rate. By contrast, when NaOH concentration was too low, the decrease in Zn(OH)42− precursors also reduced reaction rate. Therefore, flower-like ZnO structures can grow only in certain alkaline environments. In addition, the numbers of “petals” in each “flower” increased with the increase of NaOH concentration. When NaOH concentration was relatively high (Zn2+[thin space (1/6-em)]:[thin space (1/6-em)]OH− = 1[thin space (1/6-em)]:[thin space (1/6-em)]8), the shape of the “petals” turned into hexagonal prisms (Fig. 6e).
image file: c5ra24492g-f6.tif
Fig. 6 FESEM images of ZnO microstructures prepared with different molar ratio of (Zn2+[thin space (1/6-em)]:[thin space (1/6-em)]OH−): (a) 1[thin space (1/6-em)]:[thin space (1/6-em)]4; (b) 1[thin space (1/6-em)]:[thin space (1/6-em)]5; (c) 1[thin space (1/6-em)]:[thin space (1/6-em)]6; (d) 1[thin space (1/6-em)]:[thin space (1/6-em)]7; (e) 1[thin space (1/6-em)]:[thin space (1/6-em)]8 and (f) 1[thin space (1/6-em)]:[thin space (1/6-em)]9.

3.4 Photocatalytic activities

The photocatalytic performance of ZnO can reportedly be affected by particle morphology.11,12 Considering that flower-like ZnO exhibits assembled open structures and maintains a large active surface area, they can be efficient photocatalysts for the degradation of organic pollutants. In the current study, the photocatalytic performance of ZnO was evaluated using MO as a model dye under UV-light irradiation. As shown in Fig. 7a, almost no MO degradation occurred in blank solution (no photocatalyst), whereas samples of ZnO and P25 TiO2 showed obvious MO degradation, with ZnO sample exhibiting better activity than P25 TiO2. MO-degradation efficiency was about 96% for ZnO after 180 min of UV-light irradiation, which was slightly higher than that for P25 TiO2 (93%). To evaluate the stability of the flower-like ZnO catalyst, the cyclic experiments of the sample were carried out (Fig. 7b). After three cycles, there was no apparent decrease in the photodegradation efficiency, suggesting that the flower-like ZnO maintained relatively high photocatalytic activity.
image file: c5ra24492g-f7.tif
Fig. 7 (a) Photodegradation curves of MO over ZnO and P25 TiO2; (b) photocatalytic recycling experiments using flower-like ZnO as the photocatalyst.

As we know, the photocatalytic degradation mechanism of MO in the presence of ZnO can be summarized as follows. First, when ZnO photocatalysts are irradiated by UV light with energy higher than or equal to their band gap, electrons (e−) in the valence band (VB) can be excited to the conduction band (CB) with the simultaneous generation of holes (h+) in the VB. Then, the holes (hVB+) will react with water or hydroxyl groups to generate ˙OH. Finally, hydroxyl radical ˙OH can react with MO molecules to exert the degradation of MO. The corresponding photocatalytic reaction routes are expressed by eqn (4)–(7).31

 
ZnO + hν → ZnO (eCB− + hVB+) (4)
 
hVB+ + H2O → H+ + ˙OH (5)
 
hVB+ + OH− → ˙OH (6)
 
˙OH + MO → H2O + CO2 (7)

Based on the proposed mechanism, hierarchical flower-like ZnO may contribute to their superior photocatalytic performance. The open structures of ZnO samples enable the better light harvesting and scattering, hence the utilization of the UV light which is the “direct stimulation” of the photocatalytic reactions will be heightened.32 Besides, ZnO intrinsic defects such as oxygen vacancies can capture photogenerated electrons temporarily to restrain the recombination of e− and h+, which is beneficial to the photocatalytic performance.33 The room-temperature photoluminescence (PL) spectrum of flower-like ZnO is shown in Fig. 8. The strong UV emission is attributed to the near-band-edge (NBE) transition. The broad band in the visible region is related to the surface defects in ZnO, which is in favor of the photodegradation of MO.


image file: c5ra24492g-f8.tif
Fig. 8 Room-temperature PL spectrum of flower-like ZnO microstructures.

4. Conclusions

Flower-like ZnO with assembled structures were prepared via a simple one-step hydrothermal process. This approach is rapid, environment friendly, and does not require any surfactant or additive. The particles had assembled structures and showed high photocatalytic activity toward MO under UV-light irradiation. The intrinsic polar structure of ZnO and the high alkalinity of the solution enabled OH−-driven oriented aggregation and multistep nucleation, which ultimately resulted in the formation of flower-like ZnO. These results provided new insight into the growth of complex hierarchical structures in aqueous solutions.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (No. 21203052), the Science and Technology Project of Hebei Province (No. 14211105D), and the Science Foundation of Hebei Normal University (No. L2012K03, L2012B07, and L2015Z03). We also thank EnPapers for its linguistic assistance during manuscript preparation.

References

  1. A. Escobedo-Morales, D. Téllez-Flores, L. R. Peralta, J. Garcia-Serrano, A. M. Herrera-González, E. Rubio-Rosas, E. Sánchez-Mora and O. O. Xometl, Mater. Chem. Phys., 2015, 151, 282–287 CrossRef CAS.
  2. N. Roy and A. Roy, Ceram. Int., 2015, 41, 4154–4287 CrossRef CAS.
  3. W. S. Chiu, A. Yaghoubi, M. Y. Chia, N. H. Khanis, S. A. Rahman, P. S. Khiew and Y.-L. Chueh, CrystEngComm, 2014, 16, 6003–6009 RSC.
  4. Y.-H. Tseng, M.-H. Liu, Y.-W. Kuo, P. Chen, C.-T. Chen, Y.-F. Chen and C.-Y. Mou, Chem. Commun., 2012, 48, 3215–3217 RSC.
  5. M.-K. Liang, M. J. Limo, A. Sola-Rabada, M. J. Roe and C. C. Perry, Chem. Mater., 2014, 26, 4119–4129 CrossRef CAS.
  6. S. H. Xu, L. Fu, T. S. H. Pham, A. M. Yu, F. G. Han and L. Chen, Ceram. Int., 2015, 41, 4007–4013 CrossRef CAS.
  7. A. Pimentel, D. Nunes, P. Duarte, J. Rodrigues, F. M. Costa, T. Monteiro, R. Martins and E. Fortunato, J. Phys. Chem. C, 2014, 118, 14629–14639 CAS.
  8. P. Srinivasan, B. Subramanian, Y. Djaoued, J. Robichaud, T. Sharma and R. Bruning, Mater. Chem. Phys., 2015, 155, 162–170 CrossRef CAS.
  9. S. Suwanboon, S. Klubnuan, N. Jantha, P. Amornpitoksuk and P. Bangrak, Mater. Lett., 2014, 115, 275–278 CrossRef CAS.
  10. A. Anžlovar, K. Kogej, Z. C. Orel and M. Žigon, Cryst. Growth Des., 2014, 14, 4262–4269 Search PubMed.
  11. R. Khan, M. S. Hassan, H.-S. Cho, A. Y. Polyakov, M.-S. Khil and I.-H. Lee, Mater. Lett., 2014, 133, 224–227 CrossRef CAS.
  12. X. Y. Li, J. Wang, J. H. Yang, J. H. Lang, J. Cao, F. Z. Liu, H. G. Fan, M. Gao and Y. H. Jiang, Mater. Chem. Phys., 2013, 141, 929–935 CrossRef CAS.
  13. F. Wang, X. F. Qin, D. D. Zhu, Y. F. Meng, L. X. Yang and Y. F. Ming, Mater. Lett., 2014, 117, 131–133 CrossRef CAS.
  14. M. Yang, K. Sun and N. A. Kotov, J. Am. Chem. Soc., 2010, 132, 1860–1872 CrossRef CAS.
  15. Y. Hong, C. G. Tian, B. J. Jiang, A. P. Wu, Q. Zhang, G. H. Tian and H. G. Fu, J. Mater. Chem. A, 2013, 1, 5700–5708 CAS.
  16. H. R. Liu, M. Li, Y. J. Wei, Z. Liu, Y. C. Hu and H. Ma, Mater. Lett., 2014, 137, 300–303 CrossRef CAS.
  17. G. F. Du, Y. J. Liu, Y. H. Li, K. Zhao and H. Y. Zhao, Mater. Lett., 2014, 137, 68–71 CrossRef CAS.
  18. S. Cho, J.-W. Jang, J. S. Lee and K.-H. Lee, Langmuir, 2010, 26, 14255–14262 CrossRef CAS.
  19. M. K. Reyes, U. Dashdorj, A. Amarjargal, C. H. Park and C. S. Kim, Ceram. Int., 2015, 41, 2022–2027 CrossRef CAS.
  20. D. Bhuyan, B. Malakar, S. S. Arbuj and L. Saikia, RSC Adv., 2014, 4, 8256–8259 RSC.
  21. J. X. Zhan, H. X. Dong, Y. Liu, Y. L. Wang, Z. H. Chen and L. Zhang, CrystEngComm, 2013, 15, 10272–10277 RSC.
  22. G. H. Chen, Y. Wang, Q. H. Shen, Y. J. Song, G. L. Chen and H. Yang, Mater. Lett., 2014, 123, 145–148 CrossRef CAS.
  23. Y. Q. Wang, J. Yang, Y. C. Li, T. T. Jiang, J. Y. Chen and J. X. Wang, Mater. Chem. Phys., 2015, 153, 266–273 CrossRef CAS.
  24. S. Chakraborty, C. S. Tiwary and P. Kumbhakar, J. Phys. Chem. Solids, 2015, 78, 84–89 CrossRef CAS.
  25. S. K. Mohanta, D. C. Kim, H. K. Cho, S. J. Chua and S. Tripathy, J. Cryst. Growth, 2008, 310, 3208–3213 CrossRef CAS.
  26. A. Umar, B. Karunagaran, S. H. Kim, E.-K. Suh and Y. B. Hahn, Inorg. Chem., 2008, 47, 4088–4094 CrossRef CAS.
  27. S. Eisermann, J. Sann, A. Polity and B. K. Meyer, Thin Solid Films, 2009, 517, 5805–5807 CrossRef CAS.
  28. Q. Y. Li, C. L. Wang, M. L. Ju, W. L. Chen and E. B. Wang, Microporous Mesoporous Mater., 2011, 138, 132–139 CrossRef CAS.
  29. P. Li, H. Liu, B. Lu and Y. Wei, J. Phys. Chem. C, 2010, 114, 21132–21137 CAS.
  30. P. Li, H. Liu, F. X. Xu and Y. Wei, Mater. Chem. Phys., 2008, 112, 393–397 CrossRef CAS.
  31. W. W. Xia, C. Mei, X. H. Zeng, G. K. Fan, J. F. Lu, X. D. Meng and X. S. Shen, ACS Appl. Mater. Interfaces, 2015, 7, 11824–11832 CAS.
  32. Y. S. Liu, N. Q. Zhao and W. Gao, RSC Adv., 2013, 3, 21666–21674 RSC.
  33. J. X. Zhan, H. X. Dong, Y. Liu, Y. L. Wang, Z. H. Chen and L. Zhang, CrystEngComm, 2013, 15, 10272–10277 RSC.
This journal is © The Royal Society of Chemistry 2016
Click here to see how this site uses Cookies. View our privacy policy here.