Journal of the Korean Ceramic Society Vol. 55, No. 3, pp. 261~266, 2018.

− 261 −

https://doi.org/10.4191/kcers.2018.55.3.03

†Corresponding author : Young-In Lee E-mail : youngin@seoultech.ac.kr Tel : +82-2-970-6646 Fax : +82-2-973-6657

Facile and Selective Synthesis of ZnO Hollow or Crumpled Spheres and Their Photocatalytic Degradation Activities

Yomin Choi* and Young-In Lee**,†

*Material Technology Center, Korea Testing Laboratory, Ansan 15588, Korea**Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Korea

(Received January 31, 2018; Revised March 19, 2018; Accepted March 19, 2018)

ABSTRACT

Hollow or bumpy ZnO structures with micrometer-size features have been investigated as photocatalysts for water purification

due to their high surface area available for reaction with harmful organic molecules and relatively large size for easy separation

after finishing the photocatalytic reaction. In this study, selective synthesis of ZnO hollow or crumpled microspheres was per-

formed using a simple and versatile ultrasonic spray pyrolysis process with various zinc precursors. The morphologies, phases,

specific surface areas, and optical properties of the microspheres were characterized using X-ray diffraction, scanning electronic

microscopy, nitrogen adsorption-desorption isotherms, and UV-vis spectroscopy. In addition, the mechanism underlying the for-

mation of different morphologies and their photocatalytic activities were systematically investigated.

Key words : ZnO, Optical properties, Microsphere, Ultrasonic spray pyrolysis, Photocatalytic degradation

1. Introduction

inc oxide (ZnO) is a versatile material due to its uniquematerial properties and is used in diverse applications

because it provides electronic, photonic, and spin-basedfunctionality through its wide band gap (3.37 eV) and largeexciton binding energy of ~ 60 meV.1,2) These properties areparticularly useful in applications such as piezoelectrictransducers, optical coatings, gas sensors, photocatalysts,and photovoltaics. In addition, photocatalytic degradation oforganic pollutants is an efficient remediation method com-pared with traditional advanced oxidation processes (AOPs)such as ozone, hydrogen peroxide, and UV light treat-ments.3,4) To meet the wider application and higher activitydemands for environmental remediation, developing advancedphotocatalysts with high specific surface areas has becomeincreasingly important.

ZnO nanostructures with high specific surface areaincluding nanoparticles,5) nanowires,6) nanofibers,7) nano-sheets,8) and hierarchical forms9) have been developed toachieve improved photocatalytic activity. However, nano-scale photocatalysts tend to agglomerate, minimizing itssurface energy and leading to decreased photocatalytic effi-ciency. Moreover, since dispersed nanoscale photocatalystsin waste media are hard to collect after decomposition oforganic pollutants, it remains in the mixture as a pollutantitself. Hollow or bumpy ZnO structures with sub-microme-ter-size features are particularly attractive as photocata-

lysts for a water purification due to their high surface areaavailable to react with harmful organic molecules and largesize for easy separation after finishing the photocatalyticreaction compared to microspheres and nanoparticles.

Ultrasonic spray pyrolysis (USP) is an aerosol processingmethod that can be used to produce relatively monodis-persed sub-micron particles in a simple and continuousprocess using inexpensive precursors.10-12) This allows forthe industrially-scalable production of various metal andoxide powders. Conventional USP processes generally syn-thesize a sub-micron or micrometer sized particle becausethe reduction of initial droplet size using ultrasound is lim-ited. However, the size and morphology of the powder syn-thesized by USP can be readily modified by changing thetype of precursor salt, which has distinct impacts on thedecomposition and crystallization rate during pyrolysis.

In this study, controllable synthesis of sub-micron hollowand bumpy ZnO powders was achieved by changing theprecursor for USP. The influence of precursor solutiontypes (aqueous zinc nitrate and zinc acetate) on the mor-phological, structural, and optical characteristics of theZnO samples was also systematically investigated. In addi-tion, the mechanism by which the morphological differ-ences occur are discussed. Finally, the photocatalyticactivity of the synthesized ZnO powders were evaluated bymonitoring the degradation of rhodamine-B under irradia-tion with a xenon lamp.

2. Experimental Procedure

All the reagents and solvents were purchased fromSigma-Aldrich and used directly without further purifica-

Z

Communication

262 Journal of the Korean Ceramic Society - Yomin Choi and Young-In Lee Vol. 55, No. 3

tion. The starting solutions for USP were prepared by dis-solving zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99.9%)and zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 99.9%) toconcentrations of 100 mM in distilled water. The preparedprecursor solutions were ultrasonically nebulized at a fre-quency of 1.7 MHz into microdroplets, which were then car-ried by a gas flow into a tube furnace where solventevaporation and precursor decomposition occurred produc-ing ZnO powders. The furnace with a quartz tube (30 mminner diameter and 500 mm long) was preheated to 500,700, and 900°C, and compressed pure oxygen was used as acarrier gas at a flow rate of 2 L/min. The ZnO powders werecollected by the incorporation of filter paper (QualitativeFilter Paper, 5 - 8 mm, Hyundai Micro Co., Ltd., Korea)into the carrier gas stream. For the sake of simplicity, thesamples prepared from Zn(NO3)2 and Zn(CH3COO)2 at vari-ous temperatures are denoted as ZnO-N(temp.) and ZnO-A(temp.), respectively.

The crystal structures of the powders were characterizedby X-ray diffractometry (XRD, X’Pert3 Powder, PANalyti-cal, Netherlands) with a radiation wavelength of λ =1.54056 Å. XRD patterns were obtained from 2θ = 20 to 80°at a scan rate of 4° min−1. Microstructural characterizationof the ZnO powders was performed by field emission scan-ning electron microscopy (FE-SEM, S-4800, Hitachi, Japan).The decomposition and crystallization behavior of the pre-cursors was monitored using thermogravimetric analysisand differential scanning calorimetry (TG/DSC, STA409PC,Netzsch). The specific surface area of the powders wasdetermined using the Brunauer-Emmett-Teller (BET)method from the N2 adsorption isotherm. Absorption spec-tra were measured from 200 - 900 nm with an ultraviolet/visible spectrophotometer (UV-vis DRS, UV-2600, SHI-MADZU, Japan).

The photocatalytic activity of the ZnO powders was stud-ied by measuring the photodegradation rate of rhodamineB (Rh B). Their activity was confirmed with the decoloriza-tion of Rh B through photocatalytic degradation based onthe Beer-Lambert law13) which is the linear relationshipbetween absorbance and concentration of an absorbing spe-cies. The photocatalytic experiments were performed by

adding 0.10 g of the ZnO powder to 100 mL of a 0.5 × 10−5 MRh B aqueous solution. The mixture was kept in the darkfor 30 min to reach the adsorption-desorption equilibriumbetween the photocatalyst, Rh B, and water before irradia-tion. The photocatalytic experiment was performed using asolar simulator equipped with a 300 W xenon lamp and anAir Mass 1.5 Global Filter simulating solar radiation reach-ing the surface of the earth at a zenith angle of 48.2°. Themixture was irradiated using the xenon lamp with mag-netic stirring in a water bath at 25°C. A series of aliquotswere withdrawn at selected times for analysis and werecentrifuged before measurement. The residual concentra-tion of Rh B in each sample was determined by UV-visspectrometry (UV-vis DRS, UV-2600, SHIMADZU, Japan).

3. Results and Discussion

Figure 1 shows the XRD patterns of the powders synthe-sized by USP of a 100 mM aqueous solution of Zn(NO3)2and Zn(CH3COO)2 at various temperatures. The diffractionpeaks can be indexed according to the wurtzite ZnO struc-ture (JCPDS card No. 36-1451) except for some peaks inthe ZnO-N(500) spectrum which match zinc hydroxyni-trates (Zn3(OH)4(NO3)2, JCPDS card No. 52-0627) as shownin Fig. 1(A). The diffraction peaks at 31.7, 34.4, and 36.3°correspond to the (100), (002), and (101) planes, respec-tively. The intensities of the diffraction peaks increasedwith pyrolysis temperature suggesting that the samplessynthesized at higher temperatures are more crystalline.Except for ZnO-N(500), no impurity signals were detected,indicating the high purity of the products.

The morphologies of the ZnO-N and ZnO-A samples wereexamined by SEM, as shown in Figs. 2 and 3. All samplesconsisted of large amounts of polydispersed microspheresranging from several hundred nanometers to 2 µm in diam-eter. The micrographs in Fig. 2 show the porous sphericalshape of the ZnO-N powder, which consisted of multiplenanosized primary particles with a three-dimensional net-work. In addition, abundant mesopores were presentbetween adjacent particles. As the synthesis temperatureincreased, particle growth due to sintering between the pri-

Fig. 1. XRD patterns of the powders synthesized by the ultrasonic pyrolysis process with different zinc precursors; (a) zincnitrate and (b) zinc acetate, at 500oC, 700oC and 900oC.

May 2018 Facile and Selective Synthesis of ZnO Hollow or Crumpled Spheres and Their Photocatalytic Degradation Activities

mary particles was observed and resulted in a smoothersurface and shrinkage. On the other hand, the ZnO-A pow-ders exhibited dimples and wrinkles, as shown in Fig. 3.The difference in morphology can be attributed to the dif-ferent thermal and crystallization behaviors of the zinc pre-cursors.

The mechanism underlying the difference in morphologywas identified by TG analysis of the zinc precursors. Fig.4(a) shows the TG analysis results obtained during thethermal decomposition of zinc nitrate and acetate used forpreparing the ZnO powders by USP. During initial stagebelow 200°C, the mass loss corresponds to water evapora-tion from both precursors without the loss of nitrate or ace-tate groups. Above 200°C, the total weight loss of the

precursors differed according to the chemical formula butthe overall weight loss trends were similar. However, asshown in Fig. 4(b), the DSC curves of the ZnO powdersshowed different behaviors, especially the final peaks of thezinc nitrate and acetate indicate an exothermic and endo-thermic reaction, respectively. This can be explained by thedifference in thermal decomposition and crystallizationbehavior of each precursor. For zinc nitrate, the crystalliza-tion of ZnO occurs on the surface of dried precursor drop-lets as it slowly thermally decomposes. As a result, a core(precursor)/shell (ZnO) structure is formed during pyroly-sis, which leads to the formation of hollow structures withan empty interior. This interpretation is consistent withthe XRD analysis shown in Fig. 1(a). On the other hand, for

Fig. 2. FE-SEM images of the powders synthesized by USP using a zinc nitrate salt at (a, d) 500°C, (b, e) 700°C, and (c, f)900°C.

Fig. 3. FE-SEM images of the powders synthesized by USP using a zinc acetate salt at (a, d) 500°C, (b, e) 700°C, and (c, f)900°C.

264 Journal of the Korean Ceramic Society - Yomin Choi and Young-In Lee Vol. 55, No. 3

zinc acetate the final reaction peak indicated an endother-mic reaction, suggesting that crystallization occurs afterquick thermal decomposition of the precursor. As a result,dried precursor droplet contraction caused by the rapidthermal decomposition of the precursor occurs, and ZnOsubsequently crystallizes, creating a wrinkled morphology.

Figure 5 shows the nitrogen adsorption/desorption iso-therms and pore-size distribution curves of the ZnO-N(500), ZnO-N(700), and ZnO-A(500) samples. All three

samples can be classified as type IV in the IUPAC classifi-cation with a distinct hysteresis loop in the range of 0.5 -1.0 P/P0, which is characteristic of porous materials. TheBrunauer-Emmett-Teller (BET) specific surface area of theZnO-N(500), ZnO-N(700), and ZnO-A(500) samples are13.18, 12.14, and 21.19 m2 g−1, respectively. Fig. 5(b) showsthe Barrett-Joyner-Halenda (BJH) pore-size distributionplot for the N2 sorption isothermals of the prepared sam-ples. It is clear that the ZnO-A(500) powder has a large

Fig. 4. (a) TG and (b) DSC curves of the Zn precursors (black) zinc nitrate and (blue) zinc acetate.

Fig. 5. (a) N2 adsorption-desorption isotherms and (b) pore size distribution curves of the ZnO powders synthesized using differ-ent Zn precursors at 500 or 700°C.

Fig. 6. (Inset) UV-vis diffuse absorption spectra and plots of the transformed Kubelka-Munk function as a function of absorbedlight energy of the ZnO powders synthesized using different Zn precursors; (a) zinc nitrate and (b) zinc acetate at 500,700, and 900°C.

May 2018 Facile and Selective Synthesis of ZnO Hollow or Crumpled Spheres and Their Photocatalytic Degradation Activities

number of pores in the macropore range between 10 and100 nm which originated from its bumpy structure. For theZnO-N(500) sample, the plot indicates that the powder hasa relatively small number of pores that are large comparedto those of the ZnO-A(500) powder. This resulted in the hol-low structure of the ZnO-N(500) powder. The main poresize of the ZnO-N(700) sample was slightly smaller com-pared to the ZnO-N(500) powder and mesopores of approxi-mately 5 nm was observed because of shrinkage bysintering of the primary particles. These analyses indicatethat the ZnO-A(500) powder exhibited the largest surfacearea of all samples, and it is predicted to provide the bestphotocatalytic activity of all prepared samples.

Figure 6(a) shows UV-vis diffuse reflectance spectra forthe synthesized powders. The absorption spectra exhibitedan absorption shoulder at approximately 385 nm, which issimilar to the absorption band of bulk ZnO. The band gapof the sample can be estimated from the spectrum in Fig.6(A) using Kubelka-Munk theory.14) The converted Kubelka-Munk function graph is shown in Fig. 6(b) and extrapolat-ing the linear sections of the curves gives the band gap ofapproximately 3.2 eV. This is consistent with the previ-ously reported optical bandgap of 3.1 - 3.3 eV. The smallband gap differences between each sample are thought tooriginate from the transitions involving extrinsic states,such as surface defects and oxygen vacancies.15)

The samples synthesized from different precursors andtemperatures exhibited different morphologies, and showcorrespondingly different photocatalytic performance. Fig.7 shows the changes in the UV-vis spectra during theremoval of aqueous Rh B by the ZnO powders. As shown inFig. 7, the dark absorptions of the different samples aresimilar. The degradation percentage of the Rh B dye wasapproximately 75%, 73%, and 92% for the ZnO-N(500),ZnO-N(700), and ZnO-A(500) samples, respectively, afterphoto-irradiation for 30 min. This photocatalytic perfor-mance difference can be explained as follows. Photocata-lytic activity is generally related to the morphology andsurface area of the particles. High BET surface areas arefavorable for increased photocatalytic activity. In thisstudy, the specific surface area of the samples increased inthe order of ZnO-A(500) < ZnO-N(500) < ZnO-N(700), asshown in Fig. 5. Thus, the ZnO-A(500) samples was expected

to should show the highest photocatalytic activity.

4. Conclusions

In summary, we have demonstrated that USP using dif-ferent zinc precursors provides an effective route for theselective synthesis of ZnO powder with hollow or bumpyshapes. The different morphologies have a significant influ-ence on the specific surface area and photocatalytic activi-ties of the prepared ZnO powders. TG-DTA analysisindicated that the morphology difference originated fromthe decomposition and crystallization behavior of the zincprecursors during pyrolysis. The ZnO powder with abumpy structure synthesized from zinc acetate exhibited ahigher specific surface area and pore volume than the pow-ders with a hollow interior prepared from zinc nitrate,resulting in superior photocatalytic activity. This studydemonstrates that various powder morphologies can beselectively synthesized with USP.

Acknowledgments

This research was supported by the Basic ScienceResearch Program through the National Research Founda-tion of Korea (NRF) funded by the Ministry of Science, ICT,and Future Planning (NRF-2015R1C1A1A02037745).

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