research article facile synthesis of rambutan-like zno...

Research ArticleFacile Synthesis of Rambutan-Like ZnO Hierarchical HollowMicrospheres with Highly Photocatalytic Activity

Ke-Jian Ju1 Ming Zhang2 Qian-Li Zhang1 Jie Wei1 and Ai-Jun Wang23

1School of Chemistry and Biological Engineering Suzhou University of Science and Technology Suzhou 215009 China2School of Chemistry and Chemical Engineering Henan Normal University Xinxiang 453007 China3College of Geography and Environmental Science Zhejiang Normal University Jinhua 321004 China

Correspondence should be addressed to Qian-Li Zhang zqlmhb163com and Ai-Jun Wang ajwangzjnucn

Received 11 February 2015 Accepted 17 March 2015

Academic Editor Aiping Chen

Copyright copy 2015 Ke-Jian Ju et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Rambutan-like ZnO hierarchical hollow microspheres (ZnO HHMs) were constructed under hydrothermal conditions usingcarboxyl methyl starch (CMS) as a soft template The resulting products were characterized by using X-ray diffraction(XRD) scanning electron microscopy (SEM) and transmission electron microscopy (TEM) The experimental parameters andgrowth mechanism of rambutan-like ZnO HHMs were discussed in some detail The as-prepared samples displayed improvedphotocatalytic activity for the degradation of rhodamine B under ultraviolet (UV) irradiation

1 Introduction

ZnO has attracted global interest as a promising alternativesemiconductor to TiO

2in dye-sensitized solar cells owing to

its direct wide band gap (337 eV) and high electron mobility(17 cm2 sdot Vminus1 sdot sminus1) for single-crystal ZnO nanostructures[1 2] Furthermore single-crystalline ZnO displays efficientelectron transport collection and a faster charge transfer forits 23-fold electron mobility than TiO

2 As a result single-

crystalline ZnO with a variety of sizes and shapes have beenprepared including nanowires [3 4] nanofiber [5] nanodiscs[6] nanorods [7 8] nanotubes [9 10] nanonails [11] core-shell [12] and hierarchical structures [13 14]

Now one-dimensional (1D) ZnO nanostructures suchas nanowires and nanotubes have been prepared to reducerecombination phenomena upon the electron transport pro-cess and improve the electron collection efficiency [15]However these 1D structures suffer from a rather low specificsurface area To solve this issue ZnO hollow structures haveattracted great attention because of their huge active surfacearea stability high porosity and permeability (mesoporousnature) [16 17]

To date hollow ZnO spheres have mainly been synthe-sized by hard template [18] For instance ZnO hollow spheres

have been prepared using spherobacterium Streptococcusthermophilus as a biotemplate [19] However the incubationof bacteria is time-consuming and complicated and requiresspecial agents Furthermore expensive raw materials com-plex process control and sophisticated equipment are oftenneeded which is unfavorable for potential large-scale synthe-sis of single-crystal ZnO structures

Alternatively soft templates are expected to be moreflexible in preparing ZnO hollow structures For examplehierarchically nanoporous ZnO hollow spheres were facilelyobtained using glucose as a template [20] where calciningthe products is essential Lately flower-like and double-cagedpeanut-like ZnO hollow structures were prepared in ourgroup [21 22] which showed enhanced surface area andimproved catalytic activity Moreover hollow microsphereassembled by the units (eg nanoparticles and nanorods)have some additional advantages such as enlarged surfacearea easy separation rich interface and good stability [20ndash22]

Hydrothermal method is usually used for the synthesis ofhigh-crystallized powders owing to its simplicity facility lowcost and scalability [23 24] Besides the products will ownhigh purity and narrow particle size distribution Herein asimple hydrothermal method was developed for large-scale

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2015 Article ID 242798 10 pageshttpdxdoiorg1011552015242798

2 Journal of Nanomaterials

synthesis of rambutan-like ZnO hierarchical hollow micro-spheres (HHMs) under low temperature (120∘C) without thecalcination procedure using carboxylmethyl starch (CMS) asa soft template The catalytic activity of the resulting productwas examined by the photodegradation of rhodamine B as amodel system

2 Experimental Section

21 Chemical and Reagents All the reagents were of analyti-cal grade and were purchased from Aladdin Industrial Cor-poration and used without further purification All aqueoussolutions were prepared with twice-distilled water

22 Preparation of Rambutan-Like ZnO HHMs The rambu-tan-like ZnO HHMs were prepared with the assistance ofCMS under hydrothermal conditions In a typical procedure065 g of CMSwas dissolved in 25mL of water under stirringfollowed by the addition of 625mL of zinc nitrate solu-tion (50mM) into the above solution under stirring Next0625mL of ammonia solution (25 wV) was dropwiseput into the mixed solution After stirring for 15min themixture was transferred to a 50mL Teflon-lined autoclavesealed and kept at 120∘C for 12 h and finally cooled toroom temperature naturally The final white precipitate wascollected by centrifugation thoroughly washed with waterand ethanol and dried at 60∘C in vacuum

Control experiments with different amounts of ammoniaand CMS and different temperature were performed respec-tively And the time-dependent experiments were conductedto investigate the formation mechanism of rambutan-likeZnO HHMs

23 Characterization Field emission scanning electronmicroscopy (FESEM) was performed with a JSM-6390LVmicroscope Transmission electron microscope (TEM) andhigh-resolution TEM (HRTEM) images were recorded ona JEOL JEM-2100F using an accelerating voltage of 200 kVX-ray diffraction (XRD) analysis was carried out on a RigakuDmax-2000 diffractometer with CuK120572 radiation Thesurface areas were calculated by the Brunauer-Emmett-Teller(BET) method and the pore size distribution was calculatedfrom the adsorption branch using the Barett-Joyner-Halenda(BJH) theoryTheUV-vis spectra were recorded on a Lambda950 UVvis spectrometer (PerkinElmer USA)

24 Photocatalytic Experiments The photocatalytic activityof the samples was evaluated by the photocatalytic decol-orization of rhodamine B (RhB Amresco Inc) in aqueoussolution at ambient temperature To improve the degree ofcrystallinity the productwas calcined at 400∘C for 2 h Photo-catalytic experiments were performed as follows the reactionsystem containing 50mL of RhBwith an initial concentrationof 25 times 10minus5M and 15mg of the ZnO samples was stirred inthe dark for 1 h to reach adsorption-desorption equilibriumbefore ultraviolet (UV) light irradiation A 300W high-pressure Hg lamp (Yaming Company Shanghai 8 cm awayfrom the suspension) was used as a light source to trigger

the photocatalytic reaction The solutions were collected andcentrifuged every 10min to measure the degradation of RhBin solution by UV-vis spectroscopy

3 Results and Discussion

31 Characterization of Rambutan-Like ZnO HHMsFigure 1(a) shows the typical SEM image of rambutan-likeZnO HHMs with the average diameter of sim1 120583m where ZnOnanorods perpendicularly grow outwards on the surface ofthe shell regardless of the curvature of ZnO microspheres(Figures 1(a)ndash1(c)) This observation is different from thatof commercial ZnO samples with irregular nanoparticles(Figures S1 and S2A in Supplementary Material availableonline at httpdxdoiorg1011552015242798 supportinginformation) Higher magnification SEM image of thecracked one demonstrates their hollow spheres again(Figure 1(c))

As illustrated by the XRD pattern of typical ZnO HHMs(Figure 1(d)) all the diffraction peaks are well indexed tothe wurtzite hexagonal ZnO with lattice constants 119886 =3249 A and 119888 = 5207 A (JCPDS card 36-1451) [25 26]respectively No any other peaks from impurity are observedrevealing complete removal of the soft template in our systemas further proved by FTIR analysis (Figure S3 supportinginformation) in which all the characteristic peaks match wellwith pure ZnO nanoparticles [27]

TEM measurements were conducted to provide moredetailed information about ZnO HHMs (Figure 2) Notablythe slight pale center together with the deep dark edgeoutside evidences the sphere with a large void spaceinside (Figure 2(a)) Similar spheres were observed usingpolystyrene microspheres as a hard template [28] while solidsphereswere obtainedwith larger sizes (5120583m) in their systemThe selected area electron diffraction (SAED) patterns takenfrom the selected area of the TEM image (Figure 2(b)) showtheir single-crystalline nature Additionally Figures 2(c)-2(d)show that all parts of the nanorods on the ZnO spheressurface only have the fringes of planes with 119889 value of025 nm [29] suggesting that the nanorods exhibit the 1011orientation The commercial ZnO samples are also highlycrystalline with a plane spacing of 026 nm correspondingto the distance between two (0002) crystal planes whichindicates that they preferentially grow along the [0001]directions [29] (Figure S2B supporting information)

Figure 3 provides the nitrogen adsorption-desorptionisotherm of the typical sample calcined at 300∘C Theisotherm displays a typical IV curve while the hysteresisloop is associated with narrow slit-like pores in the sampleas confirmed by the pore size distribution curve (Inset inFigure 3) The BET specific surface area of the product isabout 415m2 sdot gminus1 which is much larger than that ofcommercial ZnO powders with a value of 364m2 sdot gminus1 [30]This value is also higher than those of ZnO hollow sphereswith the value of 977m2 sdot gminus1 [30] and ZnO flowers withthe value of 2516m2 sdot gminus1 [31] The corresponding pore sizedistribution curve exhibits that most of the pores have thesize of ca 20 nm The enlarged surface area of ZnO HHMsis expected to have excellent photocatalytic performance

Journal of Nanomaterials 3

1120583m

(a)

500nm

(b)

500nm

(c)

202004201

200

112103110102

Inte

nsity

cou

nts

100002

101

2120579 (deg)20 30 40 50 60 70 80

(d)

Figure 1 Morphological and structural characterizations of rambutan-like ZnO HHMs the corresponding low (a) and high ((b)-(c))magnification of the SEM images and the XRD patterns (d) The inset shows the high magnification of the SEM images of (b)

500nm

(a)

025nm

(d)

2nm2nm

025nm

(c)

(b)

Figure 2 (a) TEM image of rambutan-like ZnOHHMs (b)The SAEDpatterns corresponding to the selected area of the TEM imageHRTEMimages of the tip (c) and the main body (d) of the nanorods on the hollow sphere


0

20

40

60

80

003

006

009

012

015dV

(d) (

ccn

mg

)

Diameter (nm)Volu

me (

ccg

)

00 02 04 06 08 10

0 20 40 60 80 100

Relative pressure PP0

Figure 3 Typical N2

gas adsorption-desorption isotherm oframbutan-like ZnOHHMs Inset shows the corresponding pore sizedistribution

32 Effects of Ammonia The amount of ammonia is crucialfor the formation of unique rambutan-like ZnO HHMs(Figure 4) When 03mL ammonia is put into the reactionsystem ZnO spheres are basically formed with rough surface(Figure 4(a)) while some ZnO spheres were obtained witha few nanorods on the surface (Figure 4(b)) by using 05mLammonia However the density of the nanorods decreases onthe spheres in the case of 07mL ammonia (Figure 4(c)) andbaldZnOparticles emerge by the addition of 12mL ammoniato the reaction system (Figure 4(d))

33 Effects of CMS CMS is served as prior nucleation sitesfor ZnO at the initial crystallization process When theCMS concentration is low (2mgsdotmLminus1) there are a lot ofbare ZnO spheres with rough surfaces (Figure 5(a)) Andvertical ZnO nanorods grow on the ZnO spheres in thepresence of 26mgsdotmLminus1 CMS resulting in rambutan-likemorphology (Figure 1(a)) However sufficient CMS such as4mgsdotmLminus1 would prevent the growth of nanorods on thesurface of ZnO hollow spheres causing the lower density ofthe nanorods (Figure 5(b)) Besides 6mgsdotmLminus1 CMS inducesthe emergence of many deformed and even broken ZnOflowers (Figure 5(c)) because the nucleation takes placealong the prior nucleation sites When the amount of CMSfurther increased to 12mgsdotmLminus1 (Figure 5(d)) disorderedaggregates are formed

34 Effects of the Reaction Temperature Different morpholo-gies are obtained by adjusting the reaction temperature from80 to 160∘C The aggregation of ZnO spheres (Figure 6(a))transforms to immature rambutan-like crystals below 100∘C(Figure 6(b)) and well-defined rambutan-like ZnO HHMswere obtained at 120∘C (Figure 2(b)) However the densityof the nanorods on the surfaces of ZnO HHMs drops downat 140∘C (Figure 6(c)) and broken bald spheres show up asthe temperature is up to 160∘C (Figure 6(d)) These resultsdemonstrate that the rates of the nucleation and crystal

growth are sensitive to the reaction temperature [22] Atlower temperature the crystal growth rate is higher thanthat of nucleation The increase of the temperature greatlyfacilitates the nucleation rate and the newly generated crystalnuclei are easy to aggregate together realizing crystal growthfor adequate space present leading to the formation oframbutan-like ZnO HHMs On the contrary at much highertemperature the nucleation rate is much higher than that ofcrystal grain growth and thereby crystal nuclei are produced

35 Effects of the Reaction Time and Formation MechanismTime-dependent experiments were carried out to deeplyunderstand the formation mechanism of ZnO HHMs (Fig-ures 7(a)ndash7(d)) Firstly the products are just composed ofsolid spheres with the reaction time of 2 h (Figure 7(a))When the reaction time is 5 h small nanorods emerge onZnO spheres (Figure 7(b)) and rambutan-like ZnOHHMs asthe reaction time is 12 h (Figure 2(b)) However the density ofthe nanorods is decreased by extending the reaction time to24 (Figure 7(c)) and 48 h (Figure 7(d)) owing to the Ostwaldripening effects (Figure 8)

In general ZnO has a strong tendency to self-orientedgrowth At the initial stage newly generated ZnO is quicklycongregated to spherical aggregates to decrease their surfaceenergies and hence amorphous solid spheres are formedthrough the following reactions [21 32]

NH3+H2OlArrrArr NH

3sdotH2OlArrrArr NH

4

++OHminus

Zn(OH)4

2minus 4OHminus

larr997888997888997888997888 Zn2+4NH3

997888997888997888997888rarr Zn(NH3)

4

2+

Zn(OH)4

2minus997888rarr ZnO +H

2O + 2OHminus

Zn(NH3)

4

2+

+ 2OHminus 997888rarr ZnO + 4NH3+H2O

(1)

It should be mentioned that the original solid phasemight not be well crystallized owing to rapid spontaneousnucleation Thus Ostwald ripening dictates growth andrecrystallization after hydrolysis With time extending thesurface layer of the spheres firstly crystallizes due to thedirect contact with the surrounding solution As a result thematerials inside the solid spheres have a strong tendency todissolve which provides the driving force for the spontaneousOstwald ripening [33]This assumption is strongly supportedby the above experimental data while it is different from thatdescribed by Shenrsquos group [34] In their work original solidmicrospheres were composed by Zn(OH)

2 rather than solid

ZnO spheres in our case (Figure S4 supporting information)The formation process of rambutan-like ZnO HHMs can

be further addressed by TEM measurements (Figure 9) Atthe very early stage the precipitated crystallites assembletogether to form solid ZnO spheres (Figures 9(a) and 9(b))With the increase of the reaction time the inner crystalliteswith higher surface energy would dissolve and transferoutside to produce channels connecting inner and outerspaces in the oxide shells (Figure 9(c)) [35] Finally thehollow interior of the ZnO spheres emerged (Figure 9(d))which is also supported by the broken one from the SEMimage (Figure 1(c))


500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)

Figure 4 SEM images of the products prepared with different volumes of ammonia 03mL (a) 05mL (b) 07mL (c) and 12mL (d)

500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)

Figure 5 SEM images of the products preparedwith different amounts of CMS 2mgsdotmLminus1 (a) 4mgsdotmLminus1 (b) 6mgsdotmLminus1 (c) and 12mgsdotmLminus1(d)


1120583m

(a)

1120583m

(b)

1120583m

(c)

1120583m

(d)

Figure 6 SEM images of the products obtained at different reaction temperature 80∘C (a) 100∘C (b) 140∘C (c) and 160∘C (d)

2120583m

(a)

2120583m

(b)

2120583m

(c)

2120583m

(d)

Figure 7 SEM images of the products obtained at different reaction time 2 h (a) 5 h (b) 24 h (c) and 48 h (d) Inset shows highmagnificationof the images (b) and (c)


Ostwald ripening

Figure 8 The formation mechanism of rambutan-like ZnO HHMs based on time-dependent evolution of the growth process

500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)

Figure 9 TEM images of the products obtained at different reaction time 2 h (a) 5 h (b) 8 h (c) and 12 h (d)

In our study CMS as a soft template not only facilitatesthe formation of well-dispersed rambutan-like ZnO HHMsbut also serves as a diffusion boundary to restrain rapidcrystal growth and prevent direct fusion among the ZnOspheres during the ripening process Based on the analysisthe formation of rambutan-like ZnO HHMs is ascribed toCMS-assisted Ostwald ripening mechanism

36 Photocatalytic Activity In the last decade photocatalyticoxidation process provides an effective route for the destruc-tion of hazardous and toxic pollutants [36] RhB were chosen

as a model pollutant to evaluate the photocatalytic activity oframbutan-like ZnO HHMs

Figure 10(a) shows the photocatalytic degradation of RhBin the aqueous solution as a function of exposure time inthe presence of the ZnO catalysts under UV light irradiationWith the increase of the reaction time the absorption peakintensity of RhB greatly drops down and even disappearswithin 90min in the presence of rambutan-like ZnO HHMsThe time for photodegradation of RhB is much shorter thanthat of hollow ZnO nanospheres (150min) [37]The apparentreaction rate constant (119896app) is found to be 415 times 10minus2minminus1for rambutan-like ZnO HHMs (Figure 10(b)) while 119896app is


00

05

10

15

20

25Ab

sorb

ance

Wavelength (nm)450 500 550 600 650 700

0min10min20min30min40min


(a)

00

03

06

09

12

c

b

Irradiation time (min)

a

0 20 40 60 80 100

CC

0(b)

Figure 10 (a) UV-vis absorption spectra from the 25 times 10minus5MRhB solution with 03mgsdotmLminus1 hollow rambutan-like ZnOHHMs at differenttime intervals and (b) RhB photodegradation curves of the119862119862

0versus time in the absence (curve a) and presence of commercial ZnO (curve

b) or rambutan-like ZnO HHMs (curve c)

212 times 10minus2minminus1 for the blank system (without ZnO) The119896app of rambutan-like ZnO HHMs is much higher thanthe photocatalysis of Congo red (415 times 10minus2minminus1) usinghollow flower-like ZnO structures as a catalyst [21] althoughthe amount of the catalysts used is at the same level After90min of UV irradiation almost 100 of RhB moleculesare decomposed for rambutan-like ZnO HHMs unlike thatof commercial ZnO with 23 of RhB molecules remainingIt indicates that the photocatalytic activity of rambutan-likeZnO HHMs is much higher than that of commercial ZnOMoreover the rambutan-like ZnOHHMs also display highlyphotocatalytic stability after five times recycling (Figure S5supporting information)

Figure S6 (supporting information) shows the photo-catalytic performance when using the samples obtained atdifferent reaction time It is found that the photocatalyticactivity increased when the morphology is more close to thestructure of ZnO HHMs owing to the enlarged surface areaof the unique structures RhB molecules are excited by theabsorption of UV light and electrons are injected into theconduction band of ZnO particles facilitating the oxidationof RhB molecules [38] These results clearly demonstratethat ZnO HHMs have enhanced photocatalytic activity Inaddition the exposed facets of the ZnO structures also playan important role The nanorods of ZnO HHMs are growingalong the 1011 planes The weaker coordinated O atomson the surface are more likely to be saturated by H atomsin aqueous solution thereby releasing more free OH radicalsunder irradiation as supported by the previous work [39]

4 Conclusions

In summary a simple and green pathway has been exploredfor the construction of rambutan-like ZnO HHMs underhydrothermal conditions CMS as a soft template plays animportant role in the synthesis procedure The formation oframbutan-like ZnO HHMs was attributed to CMS-assistedOstwald ripening process The as-prepared ZnO structureswere demonstrated as a good catalyst for photocatalyticdegradation of RhB This work not only provides a simplemethod to prepare ZnO hollow structures but also shedssome light on the improvement of the photocatalytic perfor-mance by designing efficient catalysts

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgment

This work was financially supported by the National NaturalScience foundation of China (21275130 and 51178283)

References

[1] P Baviskar A Ennaoui and B Sankapal ldquoInfluence of process-ing parameters on chemically grown ZnO films with low costEosin-Y dye towards efficient dye sensitized solar cellrdquo SolarEnergy vol 105 pp 445ndash454 2014


[2] A B Gurav S S Latthe R S Vhatkar et al ldquoSuperhydrophobicsurface decorated with vertical ZnO nanorods modified bystearic acidrdquo Ceramics International vol 40 no 5 pp 7151ndash7160 2014

[3] P Labouchere A K Chandiran T Moehl et al ldquoPassivation ofZnO nanowire guests and 3D inverse opal host photoanodes fordye-sensitized solar cellsrdquoAdvanced EnergyMaterials vol 4 no12 2014

[4] L Liu K Hong X Ge D Liu and M Xu ldquoControllable andrapid synthesis of long ZnO nanowire arrays for dye-sensitizedsolar cellsrdquoThe Journal of Physical Chemistry C vol 118 no 29pp 15551ndash15555 2014

[5] A Katoch S-W Choi H W Kim and S S Kim ldquoHighlysensitive and selective H

2sensing by ZnO nanofibers and

the underlying sensing mechanismrdquo Journal of HazardousMaterials vol 286 no 9 pp 229ndash235 2015

[6] S Hussain T Liu M Kashif et al ldquoSurfactant dependentgrowth of twinned ZnO nanodisksrdquo Materials Letters vol 118pp 165ndash168 2014

[7] T Bai Y Xie C Zhang Y Zhang J Hu and J Wang ldquoFacilefabrication of ZnO nanorodsZnO nanosheet-spheres hybridphotoanode for dye-sensitized solar cellsrdquo Functional MaterialsLetters vol 8 no 1 Article ID 1550012 2015

[8] P P Das S A Agarkar SMukhopadhyay UManju S B Ogaleand P S Devi ldquoDefects in chemically synthesized and thermallyprocessed ZnO nanorods implications for active layer proper-ties in dye-sensitized solar cellsrdquo Inorganic Chemistry vol 53no 8 pp 3961ndash3972 2014

[9] J Yang Y Lin Y Meng and Y Lin ldquoOriented ZnO nanotubesarrays decorated with TiO2 nanoparticles for dye-sensitizedsolar cell applicationsrdquo Applied Physics A vol 114 no 4 pp1195ndash1199 2014

[10] J-S Jeong B-H Choe J-H Lee J-J Lee and W-Y ChoildquoZnO-coated TiO

2nanotube arrays for a photoelectrode in dye-

sensitized solar cellsrdquo Journal of ElectronicMaterials vol 43 no2 pp 375ndash380 2014

[11] T A Safeera N Johns P V Athma P V Sreenivasan and E IAnila ldquoSynthesis and characterization of ZnO nanonailsrdquo AIPConference Proceedings vol 1620 no 1 pp 572ndash577 2014

[12] L Song Q Jiang P Du Y Yang J Xiong and C Cui ldquoNovelstructure of TiO

2-ZnO core shell rice grain for photoanode of

dye-sensitized solar cellsrdquo Journal of Power Sources vol 261 no1 pp 1ndash6 2014

[13] S Zhu L Shan X Chen et al ldquoHierarchical ZnO architecturesconsisting of nanorods and nanosheets prepared via a solutionroute for photovoltaic enhancement in dye-sensitized solarcellsrdquo RSC Advances vol 3 no 9 pp 2910ndash2916 2013

[14] T-T Miao Y-R Guo and Q-J Pan ldquoThe SL-assisted synthesisof hierarchical ZnO nanostructures and their enhanced photo-catalytic activityrdquo Journal of Nanoparticle Research vol 15 no6 pp 1ndash12 2013

[15] K Wijeratne and J Bandara ldquoAspect-ratio dependent electrontransport and recombination in dye-sensitized solar cells fabri-cated with one-dimensional ZnO nanostructuresrdquo Electrochim-ica Acta vol 148 no 1 pp 302ndash309 2014

[16] X Zhou W Feng C Wang et al ldquoPorous ZnOZnCo2O4

hollow spheres synthesis characterization and applications ingas sensingrdquo Journal of Materials Chemistry A vol 2 no 41 pp17683ndash17690 2014

[17] J Shi Y Liu X Zhou Q Su J Zhang and G Du ldquoHierarchicalZnO hollow microspheres with strong violet emission and

enhanced photoelectrochemical responserdquo Materials Lettersvol 132 no 1 pp 421ndash424 2014

[18] C Ottone K Bejtka A Chiodoni et al ldquoComprehensive studyof the templating effect on the ZnO nanostructure formationwithin porous hardmembranesrdquoNew Journal of Chemistry vol38 no 5 pp 2058ndash2065 2014

[19] R Selvakumar N Seethalakshmi P Thavamani R Naidu andM Megharaj ldquoRecent advances in the synthesis of inorganicnanomicrostructures using microbial biotemplates and theirapplicationsrdquo RSC Advances vol 4 no 94 pp 52156ndash521692014

[20] J Yu and X Yu ldquoHydrothermal synthesis and photocatalyticactivity of zinc oxide hollow spheresrdquo Environmental Science ampTechnology vol 42 no 13 pp 4902ndash4907 2008

[21] J-J Feng Q-C Liao A-J Wang and J-R Chen ldquoMannitesupported hydrothermal synthesis of hollow flower-like ZnOstructures for photocatalytic applicationsrdquo CrystEngComm vol13 no 12 pp 4202ndash4210 2011

[22] A-J Wang Q-C Liao J-J Feng P-P Zhang A-Q Li andJ-J Wang ldquoApple pectin-mediated green synthesis of hollowdouble-caged peanut-likeZnOhierarchical superstructures andphotocatalytic applicationsrdquo CrystEngComm vol 14 no 1 pp256ndash263 2012

[23] A B Djurisic X Y Chen and Y H Leung ldquoRecent progressin hydrothermal synthesis of zinc oxide nanomaterialsrdquo RecentPatents on Nanotechnology vol 6 no 2 pp 124ndash134 2012

[24] Y-W Cheng H-L Su W-H Lin and C-F Lin ldquoFormingextremely smooth ZnO thin film on silicon substrates forgrowth of large and well-aligned ZnO rods with the hydrother-mal methodrdquo Journal of Sol-Gel Science and Technology vol 70no 1 pp 81ndash89 2014

[25] W Guo M Fu C Zhai and Z Wang ldquoHydrothermal syn-thesis and gas-sensing properties of ultrathin hexagonal ZnOnanosheetsrdquo Ceramics International vol 40 no 1 pp 2295ndash2298 2014

[26] C Wang S Ma A Sun et al ldquoCharacterization of electrospunPr-doped ZnO nanostructure for acetic acid sensorrdquo Sensorsand Actuators B Chemical vol 193 no 31 pp 326ndash333 2014

[27] A C Janaki E Sailatha and S Gunasekaran ldquoSynthesischaracteristics and antimicrobial activity of ZnO nanoparti-clesrdquo Spectrochimica Acta Part A Molecular and BiomolecularSpectroscopy vol 144 no 5 pp 17ndash22 2015

[28] VM Guerin J Elias T T Nguyen L Philippe and T PauporteldquoOrdered networks of ZnO-nanowire hierarchical urchin-likestructures for improved dye-sensitized solar cellsrdquo PhysicalChemistry Chemical Physics vol 14 no 37 pp 12948ndash129552012

[29] J-J Feng Z-Z Wang Y-F Li J-R Chen and A-J WangldquoControl growth of single crystalline ZnO nanorod arrays andnanoflowers with enhanced photocatalytic activityrdquo Journal ofNanoparticle Research vol 15 no 4 pp 1ndash12 2013

[30] Z Deng M Chen G Gu and L Wu ldquoA facile method to fabri-cate ZnO hollow spheres and their photocatalytic propertyrdquoTheJournal of Physical Chemistry B vol 112 no 1 pp 16ndash22 2008

[31] B Li and Y Wang ldquoFacile synthesis and enhanced photocat-alytic performance of flower-like ZnO hierarchical microstruc-turesrdquo The Journal of Physical Chemistry C vol 114 no 2 pp890ndash896 2010

[32] J Duan X Huang and E Wang ldquoPEG-assisted synthesis ofZnO nanotubesrdquoMaterials Letters vol 60 no 15 pp 1918ndash19212006


[33] X W Lou L A Archer and Z Yang ldquoHollow micro-nanos-tructures synthesis and applicationsrdquo Advanced Materials vol20 no 21 pp 3987ndash4019 2008

[34] Y F Zhu D H Fan and W Z Shen ldquoTemplate-free synthesisof zinc oxide hollow microspheres in aqueous solution at lowtemperaturerdquo The Journal of Physical Chemistry C vol 111 no50 pp 18629ndash18635 2007

[35] J Li and C Z Hua ldquoHollowing Sn-doped TiO2nanospheres

viaOstwald ripeningrdquo Journal of theAmericanChemical Societyvol 129 no 51 pp 15839ndash15847 2007

[36] Z Zhang M Zhang J Deng et al ldquoPotocatalytic oxida-tive degradation of organic pollutant with molecular oxygenactivated by a novel biomimetic catalyst ZnPz(dtn-COOH)

4rdquo

Applied Catalysis B Environmental vol 132-133 no 27 pp 90ndash97 2013

[37] C Zhu B Lu Q Su E Xie and W Lan ldquoA simple method forthe preparation of hollow ZnO nanospheres for use as a highperformance photocatalystrdquoNanoscale vol 4 no 10 pp 3060ndash3064 2012

[38] W Teng X Li Q Zhao J Zhao and D Zhang ldquoIn situ captureof active species and oxidation mechanism of RhB and MBdyes over sunlight-driven AgAg

3PO4plasmonic nanocatalystrdquo

Applied Catalysis B Environmental vol 125 pp 538ndash545 2012[39] J Chang and E R Waclawik ldquoFacet-controlled self-assembly

of ZnO nanocrystals by non-hydrolytic aminolysis and theirphotodegradation activitiesrdquo CrystEngComm vol 14 no 11 pp4041ndash4048 2012

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Nano

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Journal ofNanomaterials


synthesis of rambutan-like ZnO hierarchical hollow micro-spheres (HHMs) under low temperature (120∘C) without thecalcination procedure using carboxylmethyl starch (CMS) asa soft template The catalytic activity of the resulting productwas examined by the photodegradation of rhodamine B as amodel system

2 Experimental Section

21 Chemical and Reagents All the reagents were of analyti-cal grade and were purchased from Aladdin Industrial Cor-poration and used without further purification All aqueoussolutions were prepared with twice-distilled water

22 Preparation of Rambutan-Like ZnO HHMs The rambu-tan-like ZnO HHMs were prepared with the assistance ofCMS under hydrothermal conditions In a typical procedure065 g of CMSwas dissolved in 25mL of water under stirringfollowed by the addition of 625mL of zinc nitrate solu-tion (50mM) into the above solution under stirring Next0625mL of ammonia solution (25 wV) was dropwiseput into the mixed solution After stirring for 15min themixture was transferred to a 50mL Teflon-lined autoclavesealed and kept at 120∘C for 12 h and finally cooled toroom temperature naturally The final white precipitate wascollected by centrifugation thoroughly washed with waterand ethanol and dried at 60∘C in vacuum

Control experiments with different amounts of ammoniaand CMS and different temperature were performed respec-tively And the time-dependent experiments were conductedto investigate the formation mechanism of rambutan-likeZnO HHMs

23 Characterization Field emission scanning electronmicroscopy (FESEM) was performed with a JSM-6390LVmicroscope Transmission electron microscope (TEM) andhigh-resolution TEM (HRTEM) images were recorded ona JEOL JEM-2100F using an accelerating voltage of 200 kVX-ray diffraction (XRD) analysis was carried out on a RigakuDmax-2000 diffractometer with CuK120572 radiation Thesurface areas were calculated by the Brunauer-Emmett-Teller(BET) method and the pore size distribution was calculatedfrom the adsorption branch using the Barett-Joyner-Halenda(BJH) theoryTheUV-vis spectra were recorded on a Lambda950 UVvis spectrometer (PerkinElmer USA)

24 Photocatalytic Experiments The photocatalytic activityof the samples was evaluated by the photocatalytic decol-orization of rhodamine B (RhB Amresco Inc) in aqueoussolution at ambient temperature To improve the degree ofcrystallinity the productwas calcined at 400∘C for 2 h Photo-catalytic experiments were performed as follows the reactionsystem containing 50mL of RhBwith an initial concentrationof 25 times 10minus5M and 15mg of the ZnO samples was stirred inthe dark for 1 h to reach adsorption-desorption equilibriumbefore ultraviolet (UV) light irradiation A 300W high-pressure Hg lamp (Yaming Company Shanghai 8 cm awayfrom the suspension) was used as a light source to trigger

the photocatalytic reaction The solutions were collected andcentrifuged every 10min to measure the degradation of RhBin solution by UV-vis spectroscopy

3 Results and Discussion

31 Characterization of Rambutan-Like ZnO HHMsFigure 1(a) shows the typical SEM image of rambutan-likeZnO HHMs with the average diameter of sim1 120583m where ZnOnanorods perpendicularly grow outwards on the surface ofthe shell regardless of the curvature of ZnO microspheres(Figures 1(a)ndash1(c)) This observation is different from thatof commercial ZnO samples with irregular nanoparticles(Figures S1 and S2A in Supplementary Material availableonline at httpdxdoiorg1011552015242798 supportinginformation) Higher magnification SEM image of thecracked one demonstrates their hollow spheres again(Figure 1(c))

As illustrated by the XRD pattern of typical ZnO HHMs(Figure 1(d)) all the diffraction peaks are well indexed tothe wurtzite hexagonal ZnO with lattice constants 119886 =3249 A and 119888 = 5207 A (JCPDS card 36-1451) [25 26]respectively No any other peaks from impurity are observedrevealing complete removal of the soft template in our systemas further proved by FTIR analysis (Figure S3 supportinginformation) in which all the characteristic peaks match wellwith pure ZnO nanoparticles [27]

TEM measurements were conducted to provide moredetailed information about ZnO HHMs (Figure 2) Notablythe slight pale center together with the deep dark edgeoutside evidences the sphere with a large void spaceinside (Figure 2(a)) Similar spheres were observed usingpolystyrene microspheres as a hard template [28] while solidsphereswere obtainedwith larger sizes (5120583m) in their systemThe selected area electron diffraction (SAED) patterns takenfrom the selected area of the TEM image (Figure 2(b)) showtheir single-crystalline nature Additionally Figures 2(c)-2(d)show that all parts of the nanorods on the ZnO spheressurface only have the fringes of planes with 119889 value of025 nm [29] suggesting that the nanorods exhibit the 1011orientation The commercial ZnO samples are also highlycrystalline with a plane spacing of 026 nm correspondingto the distance between two (0002) crystal planes whichindicates that they preferentially grow along the [0001]directions [29] (Figure S2B supporting information)

Figure 3 provides the nitrogen adsorption-desorptionisotherm of the typical sample calcined at 300∘C Theisotherm displays a typical IV curve while the hysteresisloop is associated with narrow slit-like pores in the sampleas confirmed by the pore size distribution curve (Inset inFigure 3) The BET specific surface area of the product isabout 415m2 sdot gminus1 which is much larger than that ofcommercial ZnO powders with a value of 364m2 sdot gminus1 [30]This value is also higher than those of ZnO hollow sphereswith the value of 977m2 sdot gminus1 [30] and ZnO flowers withthe value of 2516m2 sdot gminus1 [31] The corresponding pore sizedistribution curve exhibits that most of the pores have thesize of ca 20 nm The enlarged surface area of ZnO HHMsis expected to have excellent photocatalytic performance


1120583m

(a)

500nm

(b)

500nm

(c)

202004201

200

112103110102

Inte

nsity

cou

nts

100002

101

2120579 (deg)20 30 40 50 60 70 80

(d)


500nm

(a)

025nm

(d)

2nm2nm

025nm

(c)

(b)



0

20

40

60

80

003

006

009

012

015dV

(d) (

ccn

mg

)

Diameter (nm)Volu

me (

ccg

)

00 02 04 06 08 10

0 20 40 60 80 100


Figure 3 Typical N2








NH3+H2OlArrrArr NH

3sdotH2OlArrrArr NH

4

++OHminus

Zn(OH)4

2minus 4OHminus

larr997888997888997888997888 Zn2+4NH3

997888997888997888997888rarr Zn(NH3)

4

2+

Zn(OH)4


2O + 2OHminus

Zn(NH3)

4

2+


(1)


2 rather than solid




500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)


500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)



1120583m

(a)

1120583m

(b)

1120583m

(c)

1120583m

(d)


2120583m

(a)

2120583m

(b)

2120583m

(c)

2120583m

(d)



Ostwald ripening


500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)







00

05

10

15

20

25Ab

sorb

ance

Wavelength (nm)450 500 550 600 650 700



(a)

00

03

06

09

12

c

b


a

0 20 40 60 80 100

CC

0(b)






4 Conclusions




Acknowledgment


References
















































4rdquo














CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




1120583m

(a)

500nm

(b)

500nm

(c)

202004201

200

112103110102

Inte

nsity

cou

nts

100002

101

2120579 (deg)20 30 40 50 60 70 80

(d)


500nm

(a)

025nm

(d)

2nm2nm

025nm

(c)

(b)



0

20

40

60

80

003

006

009

012

015dV

(d) (

ccn

mg

)

Diameter (nm)Volu

me (

ccg

)

00 02 04 06 08 10

0 20 40 60 80 100


Figure 3 Typical N2








NH3+H2OlArrrArr NH

3sdotH2OlArrrArr NH

4

++OHminus

Zn(OH)4

2minus 4OHminus

larr997888997888997888997888 Zn2+4NH3

997888997888997888997888rarr Zn(NH3)

4

2+

Zn(OH)4


2O + 2OHminus

Zn(NH3)

4

2+


(1)


2 rather than solid




500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)


500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)



1120583m

(a)

1120583m

(b)

1120583m

(c)

1120583m

(d)


2120583m

(a)

2120583m

(b)

2120583m

(c)

2120583m

(d)



Ostwald ripening


500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)







00

05

10

15

20

25Ab

sorb

ance

Wavelength (nm)450 500 550 600 650 700



(a)

00

03

06

09

12

c

b


a

0 20 40 60 80 100

CC

0(b)






4 Conclusions




Acknowledgment


References
















































4rdquo














CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0

20

40

60

80

003

006

009

012

015dV

(d) (

ccn

mg

)

Diameter (nm)Volu

me (

ccg

)

00 02 04 06 08 10

0 20 40 60 80 100


Figure 3 Typical N2








NH3+H2OlArrrArr NH

3sdotH2OlArrrArr NH

4

++OHminus

Zn(OH)4

2minus 4OHminus

larr997888997888997888997888 Zn2+4NH3

997888997888997888997888rarr Zn(NH3)

4

2+

Zn(OH)4


2O + 2OHminus

Zn(NH3)

4

2+


(1)


2 rather than solid




500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)


500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)



1120583m

(a)

1120583m

(b)

1120583m

(c)

1120583m

(d)


2120583m

(a)

2120583m

(b)

2120583m

(c)

2120583m

(d)



Ostwald ripening


500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)







00

05

10

15

20

25Ab

sorb

ance

Wavelength (nm)450 500 550 600 650 700



(a)

00

03

06

09

12

c

b


a

0 20 40 60 80 100

CC

0(b)






4 Conclusions




Acknowledgment


References
















































4rdquo














CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)


500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)



1120583m

(a)

1120583m

(b)

1120583m

(c)

1120583m

(d)


2120583m

(a)

2120583m

(b)

2120583m

(c)

2120583m

(d)



Ostwald ripening


500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)







00

05

10

15

20

25Ab

sorb

ance

Wavelength (nm)450 500 550 600 650 700



(a)

00

03

06

09

12

c

b


a

0 20 40 60 80 100

CC

0(b)






4 Conclusions




Acknowledgment


References
















































4rdquo














CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




1120583m

(a)

1120583m

(b)

1120583m

(c)

1120583m

(d)


2120583m

(a)

2120583m

(b)

2120583m

(c)

2120583m

(d)



Ostwald ripening


500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)







00

05

10

15

20

25Ab

sorb

ance

Wavelength (nm)450 500 550 600 650 700



(a)

00

03

06

09

12

c

b


a

0 20 40 60 80 100

CC

0(b)






4 Conclusions




Acknowledgment


References
















































4rdquo














CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Ostwald ripening


500nm

(a)

500nm

(b)

500nm

(c)

500nm

(d)







00

05

10

15

20

25Ab

sorb

ance

Wavelength (nm)450 500 550 600 650 700



(a)

00

03

06

09

12

c

b


a

0 20 40 60 80 100

CC

0(b)






4 Conclusions




Acknowledgment


References
















































4rdquo














CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




00

05

10

15

20

25Ab

sorb

ance

Wavelength (nm)450 500 550 600 650 700



(a)

00

03

06

09

12

c

b


a

0 20 40 60 80 100

CC

0(b)






4 Conclusions




Acknowledgment


References
















































4rdquo














CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials

















































4rdquo














CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



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