Feng Zhang ,Zhilong Xu ,Kun Wang ,Rizhi Chen ,Zhaoxiang Zhong ,*,Weihong Xing ,*

1 State Key Laboratory of Materials-Oriented Chemical Engineering,National Engineering Research Center for Special Separation Membrane,Nanjing Tech University,Nanjing 210009,China

2 Changzhou Lvxin Advanced Material Technology Co.,Ltd.,Changzhou 213200,China

Keywords:Three dimensional Porous ZnO Flower-like Membrane dispersion Growth mechanism Photocatalytic

ABSTRACT Three dimensional(3D) flower-like basic zinc carbonate constructed by multilayered nanoplates were rapidly prepared atroomtemperature through the directprecipitation method coupled with membrane dispersion technology,and porous ZnO with similar structures could be obtained after calcining the precursor.The structural properties of the products before and after the calcining process were characterized by SEM,TEM and XRD.The supersaturation of the reaction system due to the membrane dispersion played an important role in the formation of uniform Zn5(CO3)2(OH)6 precursors.A plausible mechanism was proposed for the formation of the flower-like ZnO assembled by nanoplates composed of nanoparticles.The obtained ZnO microspheres showed excellent photocatalytic properties,which could be attributed to the open structure and remarkable amount of porous nanoplates.

1.Introduction

ZnO is one of the most important semiconducting oxides with wide band gap(3.37 eV)and large exciting binding energy(60 meV).With the developmentofnanotechnology,nano-structured ZnOhas attracted extensive research attentions over the past decades[1,2].Due to the advantages of high specific surface area and stable nanostructure,ZnO has been widely used in the fields of photo-catalysis,gas sensors and semiconductors[3,4].

The particle size distribution,morphology and specific surface area of the nano-particles play an important role in their catalytic properties[5,6].Xing found that improved photo degradation property was achieved with three dimensional(3D)structured ZnO[7].Compared with the common ZnO,the photo-catalytic property was 15%higher for the 3D ZnO micro-tubes[8].

More attentions have turned to 3D ZnOwith hierarchical nanostructures for their larger specific surface area and catalytic activity.At the same time,the stable three-dimensional structure can effectively avoid the agglomeration ofparticles[9].Many methods forsynthesizing basic zinc carbonate precursor with nanostructures have been proposed,including the chemical precipitation method and hydrothermal method[10-12].At present,the main method to prepare 3D hierarchicalnano-structured ZnOis the hydrothermal method.Zhou synthesized multilayered ZnO nano-sheets with hierarchically porous structures in the presence of cetyltrimethylammonium bromide with the hydrothermal method under 150°C for 24 h[13].Guo prepared hierarchical ZnO porous microspheres with monoethanolamine under 120°C for 12 h[14].In general,pore-directing reagents or templates were used which may suffer from contamination during the synthesis process in these methods mentioned above.Furthermore,the shortcomings of the hydrothermal method were time consuming,large energy consumption and difficult to industrialization.The direct chemical precipitation method is one of the most promising ways to fabricate nano-structured ZnO for its low cost,simple process and high purity[15].Mixing plays an important role in the solution phase synthesis of nanoparticles[16].However,the mass transfer process is seriously limited in the typical precipitation process in which one feedstock is slowly added to another one dropwise.High super-saturation ratio is notavailable in whole, finally leading to the wide distribution ofparticle diameter.3D ZnO with hierarchical microstructures could be prepared rapidly with the manufacturing of direct precipitation,but the size and the morphology of the particles appeared to be out of order[17].

Membrane dispersion method has been extensively used in the preparation of the nanoparticles[18,19].Barium sulfate nanoparticles with an average particle size of 70 nm were prepared with hollow fiber membrane,and results showed uniform morphology and narrow diameter distribution[20].However,the study ofthe membrane dispersion method is mainly on the size of the particles,rarely on the morphology of the precursor.

The morphology still remained is another process for the preparation of the particles with hierarchical nanostructures,but the precursor is easily destroyed during the calcining process.The morphology of nanoparticles prepared by direct chemical precipitation is greatly affected by the calcining conditions.Porous ZnO was obtained after the precursor annealed at 300°C[21].Li found that the precursor changed into ZnO at 400°C without collapse of the morphology[22].However,the effects of the calcining processes,such as the cooling manner and the calcining temperature,on the morphology of the nanoparticles are rarely systematically studied.

3D flower-like ZnO precursor with uniformparticle sizes and hierarchicalnanostructures had been successfully fabricated through a simple and fastchemicalprecipitation method coupled with membrane dispersion technology at room temperature and the morphology of ZnO still remained during the calcining process in this paper.The synthesis of the precursor involves the controlling of super-saturation with membrane dispersion,oriented attachment and self-assembly process.Also,the effect of the cooling manner and the calcining temperature was also important in the calcining process.As expected,the porous ZnO exhibited excellent photo-catalytic properties due to its large specific surface area.

2.Experimental

2.1.Materials

All the chemical reagents used in our experiments were obtained from commercial sources and used without any other purification.Zinc acetate dehydrate(Zn(CH3COO)2·2H2O)and commercial ZnO were purchased from Xi Long Chemical Co.,Ltd.and NH4HCO3was purchased from Shanghai Si Shi He Wei Chemical Co.,Ltd.Microporous tubular ceramic membrane(Φ10 mm ×40 mm)with a pore size of 50 nm was supplied by Nanjing Jiusi High-Tech Co.Ltd.,China.

2.2.Synthesis of ZnO

NH4HCO3and Zn(AC)2·2H2O of analytical grade were used in the reaction as precipitant and zinc salt respectively.

Fig.1.Schematic diagram of membrane dispersion reactor.

Fig.1 showed the membrane reactor experimental setup and the principle of membrane dispersion.A tubular ceramic membrane with a pore size of 50 nm took on the role of dispersion medium in the reactor.The hierarchical Zn5(CO3)2(OH)6precursor was prepared as follows:Under the pressure of 0.1 MPa,the addition of the 1.0 mol·L-1Zn(AC)2·2H2O solution into the 1.0 mol·L-1NH4HCO3solution through the ceramic membrane was adopted in the precipitation process with the feed flow rates of 10 ml·min-1.The zinc salt solution reacted with NH4HCO3solution,and white precursor could be generated.Following the process,the white precursor was washed by distilled water three times to remove foreign ions,and then dried at 70°C for several hours.The 3D hierarchically porous flower-like ZnO architectures were obtained by subsequent calcination of the precursor in a muf fle furnace.Related reaction equations were shown as formulas(1)and(2).

2.3.Characterization

The morphology of the samples was studied by scanning electron microscopy(Hitachi,S-4800,Japan).The crystal structures of the prepared catalysts were observed with the help of an X-ray diffraction(XRD)instrument(Bruker,Mini Flex 600,Germany)using a Cu target Kαray(λ =0.15405 nm).The accelerating voltage and the applied current were 40 kV and 30 mA,respectively.The diffraction patterns were collected at room temperature in the range of 10°-80°.The average crystallite sizes were calculated with the Scherrer equation with the full width at half maxima(FWHM)data.The Brunauer-Emmett-Teller(BET)surface areas were obtained from the nitrogen adsorption apparatus(Micromeritics,ASAP2010,America).FT-IR studies were carried out with the help of infrared spectroscopy(Thermo Fisher Scientific,NICOLET 8700,China).

2.4.Photocatalytic activity measurements

Photocatalytic activities of the as-synthesized ZnO catalysts were evaluated by measuring the degradation efficiencies of methyl orange(10 mg·L-1)under UV light.The experiments under UV light irradiation were carried out with an affixed catalyst concentration of 1 g·L-1in a 250 ml reactor.A recycle water jacket was used to keep the reactor temperature constant at(25 ± 1)°C.A 40 W UV lamp(UVB,λmax=365 nm)was used as UV light source.The light irradiated outside(at a distance of 25 cm from the solution surface)upon the reactor.The light intensity near the solution surface was about 18 mW·cm-2,measured by a Multi-Sense UV-B UV radiometer(Beijing Normal university photo electricity Instruments Plant,Beijing,China).

The UVirradiation time varied from 10 to 90 min.Ateach time interval,hierarchical ZnO nanostructures were recovered by suction filtration,and the light absorption of the clear solution was measured by the spectrophotometer instrument(LAMBDA 35,Perkin Elmer,USA).The remaining concentration of the methyl orange in the solution could be calculated by the ratio between the light absorptions of photocatalyst-treated and untreated methyl orange solutions.For the comparison purpose,the concentration changes of methyl orange solution were also investigated with the same experimental setup in the absence of hierarchical ZnO nanostructures and under light illumination.

3.Results and Discussion

3.1.Preparation of basic zinc carbonate precursor

3.1.1.Effect of the feeding method on precursor

Fig.2(A)and(B)shows the SEMimages ofthe morphology ofprecursor at different feeding methods.Compared with the directprecipitation method,the Zn5(CO3)2(OH)6microspheres had uniform morphology and particle size with membrane dispersion technology.The droplet of the dispersed phase fed with membrane reached the scale of membrane pore,which intensified the process of mass transfer and small scale microspheres could be guaranteed.Fig.2(C)is the magnification of Fig.2(B)which shows that the morphology is sphere like and the radius is about4μm.Also,an interesting flower-like morphology atmicrometer size was prepared and the Zn5(CO3)2(OH)6microspheres were assembled by the 2D nanoplates.It was found that the thickness of the nanoplates was about 40 to 60 nm,and the surface of the nanoplates was smooth.Also,an open structure was formed by the nanoplates,which facilitated the diffusion and mass transportation in the photocatalytic process.

The preparation process of ZnO is shown in Fig.3.The nucleation was formed with the Zn(AC)2·2H2O solution added drop by drop when the super-saturation exceeded the critical value.After then,the precipitant of the precursor was also formed.The size of the precipitant increased for the process of reaction,and the nucleation changed into a new precipitant at the same time.So the sizes of the precipitants were different with the direct precipitation method.Zn(AC)2·2H2O solution was dispersed into tiny drops with membrane,which detached rapidly fromthe membrane surface with a shearforce produced by stirring.The size of the drops was greatly decreased due to the porous structure of the membrane,and the mass transfer was enhanced.Thus,the degree of super-saturation of the reaction system tended to be uniform with the membrane dispersion technology and the crystal growth environment was bene ficial to explosive nucleation,which was conducive to formthe particle with uniformsize and morphologicalstructure.Finally,the tiny drops reacted with NH4HCO3,and white precursor Zn5(CO3)2(OH)6could be generated.The particle size distribution in Fig.3 showed that precursor prepared with the membrane dispersion technology had uniform size about 3 μm,but the precursor prepared with dropwise presented a multi-peak distribution.

3.1.2.TG–DSC of the precursor

Fig.4.TG-DSC curve chart of basic zinc carbonate.

Fig.3.ZnO precursor preparation process with membrane and dropwise.

Fig.4 shows a two-step weight loss of the precursor.The first mass loss of the precursor started at 175 °C,till 215 °C,and the mass loss was attributed to the released of crystalline water and carbon dioxide from the annealing process.The mass loss of the precursor between 215 °C and 390 °C was assigned to the desorption of the acetate anion.Through the annealing process,the total mass loss was about 25.5%,which was close to the theoretical value(25.87%).From the DSC analysis,it was obvious that the exothermic process was taken at about 200°C,which corresponds to the loss of the O--H and CO32-.

3.2.Preparation of ZnO

3.2.1.Effect of the cooling manner

Fig.5(A)and(B)shows the SEM images of ZnO with natural and rapid cooling after the calcination.As shown in Fig.5,there was a distinct change in the morphology of ZnO with different cooling manner.The flower-like nanostructure of the powders was maintained in the rapid cooling manner.However,the sphericalstructure ofthe precursor and the structure of the nanoplates were destroyed with natural cooling.

The size ofthe ZnOgrain continued to grow up in the naturalcooling process in a long time ata relatively high temperature.The binding force of the ZnO grain decreased and the flower-like morphological structure of the particles was destroyed.The size of the ZnO grain remained unchanged after the rapid cooling process.So the morphology of ZnO could be maintained.

3.2.2.Effect of the calcination temperature

Fig.6 shows the SEM images of ZnOobtained at differentcalcination temperatures through rapid cooling.It was obvious that the original nano-plate structure of the precursor was destroyed at 200°C and 300°C,as the binding force between the basic zinc carbonate and the ZnO was small,and the structure of the particles was destroyed.The morphology was maintained at 350°C.

Fig.5.In fluence of cooling means on morphology of ZnO.(A)Natural cooling and(B)rapid cooling.

Fig.6.SEM images of ZnO obtained at different calcination temperatures.(A)200,(B)300 and(C)350°C.

Fig.7.Magnified SEM images of ZnO which calcinated at 350°C.(A)2D nanoplates with an open porous structure and(B)porous structure in these nanoplates.

The 3D flower-like ZnO obtained at 350°C had two pore structures,in which the nanoplates formed an open porous structure(as shown in Fig.7(A))and the pores between these nanoparticles in these nanoplates(as shown in Fig.7(B)).The high magnification photograph(Fig.7(A))showed that the thickness of the 2D nanoplates was about 40-60 nm,which were composed of primary nanoparticles.The nanoplates became coarse and porous,which could be attributed to the distortion of the nanoplates and the removal of H2O and CO2during the calcining process.

The structure of the ZnOparticles was further characterized by TEM.As shown in Fig.8,the sizes ofthese particles were about 20-30 nm and the mesopores with an estimated size of about 25 nm.

Fig.8.TEM image of ZnO which calcinated at 350°C.

3.2.3.Growth mechanism of 3D flower-like porous ZnO

As a result,a mechanism could be put forward for the formation of these uniform flower-like ZnO particles assembled by 2D nanoplates composed of nanoparticles based on the above experiment results in Fig.9.Lots of uniform primary precursor nuclei were formed with the help of supersaturation controlling by membrane dispersion.would not participate in any chemical reaction but adsorb onto the surface of crystal just precipitated in the reaction between ammonium salt and zinc salt to prepare basic zinc carbonate,which could form a monolayer onto the surface of these nanoparticles.And then,could induce the crystal growth along the direction of flake due to the hydrogen-bond interaction,and the basic zinc carbonate crystal grows into two dimensional nanoplates due to oriented attachment.And these nanoplates linked to form different 3D structures by self-assembly to minimize the surface energy.Wang and Yang also found that the formation of the nano flakes was greatly affected by thein the reaction liquid[23,24].A 3D flower-like porous ZnOconstructed with 2D nanoplates was evolved by the decomposition of the precursor.

3.3.Characterization of the precursor ZnO

The structure of the precursor and calcined samples was further characterized by XRD,and the XRD spectra were shown in Fig.10.The white precursor obtained by the precipitation reaction was crystallized Zn5(CO3)2(OH)6(JCPDS 19-1458).The XRD pattern demonstrated that the precipitation obtained through rapid cooling was well indexed into hexagonal wurtzite ZnO(JCPDS 36-1451).No other obvious Znbased compounds could be detected,which indicated thatthe precursor was transformed into a high crystallization degree of ZnO after the calcining process.According to the half peak width of the diffraction peak,the sizes of ZnO on different crystal faces were calculated with the Scherrer formula.The peaks of the three crystal planes(100,002 and 101)were the main characteristic peaks,and the sizes of the three crystal plane were 23.4,27.9 and 23.0 nm.Thus,the ZnO with hierarchical structure was composed of nanoparticles about 24.7 nm,which is consistent with the TEM result.

Fig.10.XRD pattern of the precursor and ZnO.

Fig.11 showed the FT-IR of the precursor and ZnO.The peak at 3367 cm-1in the IR spectrum of the precursor was due to the stretching vibrations of the OH group,and the peaks at 706,834,1392 and 1503 cm-1were assigned to the stretching vibrations of CO3[2-25,26].Only part of the basic zinc carbonate changed into ZnO at 200 °C and 300 °C,as shown in Fig.11.However,all the basic zinc carbonate changed into ZnO at 350°C.The characteristic peaks oftended to disappear at 350°C,which showed that the precursor was decomposed into ZnO completely.Furthermore,the absorption peak of the ZnO product was severely weakened at 3367 cm-1indicating the dehydration of the precursor.So,the flower-like morphology structure could be maintained.Based on the analysis of TG,XRD and FT-IR,the precursor had changed into ZnO completely by the calcining process in air at 350°C.

Fig.9.Schematic of the formation process of hierarchically porous flower-like ZnO microstructures.

Fig.11.IR spectra of samples calcined at different temperatures.

Further information of the pore in the 3D flower-like porous ZnO was carried out using N2BET adsorption-desorption analysis.Fig.12 is the adsorption-desorption curves and pore distribution curves(insets).The nitrogen adsorption of the ZnO obtained through rapid cooling was greater than the ZnO obtained through natural cooling,and far greater than the ordinary ZnO.The pore sizes of the ordinary ZnO and the ZnO obtained through rapid cooling were 4 and 25 nm respectively.According to the BET formula,the specific surface areas were 5.36 and 35.88 m2·g-1.The catalytic performance was associated with the porous structure of the nanostructured ZnO powders,which had larger specific surface area and higher light use efficiency[27].

3.4.Photocatalytic performance of the ZnO

Fig.13 is the UV-vis absorption curves of methyl orange absorbance over different time in the photocatalytic experiment.The absorbance of methyl orange decreased with duration time of ultraviolet light extended and tended to disappear after 90 min.

In order to examine the catalytic performance of ZnO prepared in our study,titanium dioxide P25 was also used as comparison.Fig.14 is the photodegradation curve of methyl orange as a function of time.According to the photodegradation curve,there was almost no degradation under the irradiation of ultraviolet light,which illustrated that methyl orange could not be effectively degraded without a photocatalyst.The flower-like nanostructure had higher specific surface area,which could increase the surface reaction active sites.Kansalfound that the catalytic performance of the flower-like ZnO was higher than the ordinary ZnO due to their higher specific surface area[8].

Fig.12.N2 isothermal adsorption stripping and pore size distribution curve.(A)Common ZnO and(B)ZnO obtained in the rapid cooling manner.

Fig.13.UV-vis absorption curves of the methyl orange solution after different irradiation time.

Fig.14.Photocatalytic effect on the methyl orange solution of different catalysts.

In the experiment with a photocatalyst of as prepared ZnO,the degradation rate reached 80%after 60 min,and methyl orange could be degraded completely after 90 min with the ZnO obtained at 350°C.From Fig.15,it was obvious that the color of the methyl orange solution gradually changed from orange to colorless.However,when the titanium dioxide P25 was used as a catalyst under the same conditions,the degradation rate of methyl orange was only about 75%.So,the photocatalytic performance of ZnO prepared in our study was superior to titanium dioxide P25 owing to the special three-dimensional hierarchical structure and the rich internal channel structure.

Fig.15.Color of methyl orange solution after different irradiation time.

4.Conclusions

In summary,3D hierarchical ZnO constructed by 2D porous nanostructures had been successfully prepared though a novel membrane dispersion reactor with the direct precipitation method and optimizing crucial calcined parameters.According to the morphological study on the evolution of porous ZnO,a possible formation mechanism was proposed from the viewpoint of nucleation and self-assembly of building blocks.The fabrication of 3D hierarchical nanostructures involved an oriented attachment and self-assembly process.The obtained ZnO products with hierarchical nanostructures exhibited excellent photocatalytic performance due to the high surface area and 3D morphology.Considering their unique structures,the porous ZnO is also expected to serve as an ideal candidate for more potential applications such as photocatalysis,lithium-ion batteries and photoluminescence.Moreover,this work hints thatthe facile self-assembly technique has opened a new pathway for synthesizing other porous metal oxide materials with unique morphologies.