Ultrasonics Sonochemistry 21 (2014) 1335–1342

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Ultrasonics Sonochemistry

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Synthesis of morphology-controlled ZnO microstructures viaa microwave-assisted hydrothermal method and theirgas-sensing property

http://dx.doi.org/10.1016/j.ultsonch.2014.02.0071350-4177/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +86 755 26558134.E-mail address: pxzhang@szu.edu.cn (P. Zhang).

1 These authors contributed equally to this work.

Sa Liang a,1, Lianfeng Zhu b,1, Guosheng Gai b, Youwei Yao c, Jue Huang c, Xuewen Ji d, Xiaoming Zhou a,Dongyun Zhang a, Peixin Zhang a,⇑a School of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, PR Chinab Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR Chinac Graduated School at Shenzhen, Tsinghua University, Shenzhen 518055, PR Chinad Shenzhen Dovelet Sensors Technology Co., Ltd., Shenzhen 518057, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 June 2013Received in revised form 20 November 2013Accepted 7 February 2014Available online 18 February 2014

Keywords:ZnOMicrowaveHydrothermalEthanolGas sensors

Controllable ZnO architectures with flower-like and rod-like morphologies were synthesized via a micro-wave-assisted hydrothermal method. By adjusting the concentration of Zn2+ in the aqueous precursors,different morphologies of ZnO microstructures were obtained. The size of ZnO was uniform after ultra-sonic treatment. The growth process of ZnO in solution was studied by monitoring the intermediate prod-ucts, which were extracted at different stages of the reactions: (i) precursor preparation, (ii) microwaveirradiation heating, (iii) natural cooling. Studies of the SEM images and XRD data revealed that the for-mation of ZnO occurred via in situ assembly or dissolution–reprecipitation of zinc hydroxide complexes.The morphology-dependent ethanol sensing performance was observed; the seven-spine ZnO structuresexhibit the highest activity.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction now broadly understood [11–15]. Briefly, the ZnðOHÞ2�4 species,

ZnO is an important crystalline functional semiconductor with alarge binding energy (Eg = 60 meV) and a wide band gap (3.37 eV).It has attracted research attention because it is non-toxic, inexpen-sive and suitable for applications in solar cells [1], luminescentmaterials [2], optical devices [3], gas sensors [4,5] and others.Controlling the size, shape and orientation of ZnO crystallites is aprerequisite for high device performances. To date, considerableefforts have been devoted to develop syntheses of ZnO with tun-able size and morphology; synthetic methods have included zincoxidation [6], vapor phase deposition [7], metal-organic chemicalvapor deposition (MOCVD) [8], sol–gel [9] and hydrothermalsyntheses [10], as well as others. Of these methods, hydrothermalsyntheses is particular interesting because ZnO microstructurescan be fabricated under mild conditions (aqueous solution,<100 �C) and are highly reproducible.

A number of previous studies have investigated hydrothermalsyntheses of zinc oxide microstructures and the principles are

which make up the majority of the solution species in alkalinesolution (i.e. pH 11.5) [16], decompose into Zn(OH)2 at moderatetemperature; this decompositions is followed by further condensa-tion and dehydration to form ZnO. The growing units, which arethe ZnðOHÞ2�4 species, are stored in the solution [17,18] or gener-ated from the dissolution of Zn(OH)2, which may precipitate fromthe aqueous precursor [15]. The shapes and sizes of the zinc oxidecrystals are variable and depend upon the decomposition kineticsof the ZnðOHÞ2�4 species [18], as well as the morphologies ofnucleation sites [19]. Slow decomposition of the ZnðOHÞ2�4 speciesfacilitates the self-assembly of Zn(OH)2 conforming to the crystalnature of ZnO on the nucleation sites. Due to the different face-polarities of hexagonal ZnO [18], small molecules (or unreactivecomplexes) can selectively adsorb to different crystal faces duringgrowth; these ‘‘capping agents’’ prevent the self-assembly ofZnðOHÞ2�4 ions and lead to slower growth of ZnO normal to thecapped crystalline face. Therefore, organic additives or auxiliariesare usually introduced to tune the shapes of the products effec-tively: metal sulfate hydrates for the synthesis of ZnO nanoplatesor nanowires [18], citrate generates oriented ZnO columns andplates [20], ascorbate triggers the formation flower-like ZnOmicrostructures [21], and ethanolamine facilitates the growth ofnano-rods [22].

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Previous works concerning hydrothermal synthesis methodsprovide an array of phenomenology and understanding of themechanisms for the formation of ZnO. However, the batchprocessing techniques utilized in hydrothermal synthesis arerather inefficient; some of them involve tens of hours of thermaltreatment at a constant temperature. It is a challenge to fabricateZnO microstructures with superior control of shape and size effi-ciently when considering the cost of manufacture. Alternatively,the microwave-assisted hydrothermal method has a number ofadvantages, such as higher reaction rate, rapid volumetric heating,higher yields of products and energy savings [23–25]. In this paper,we rapidly synthesize ZnO with varied morphologies through amicrowave-assisted hydrothermal method. Flower-like or rod-likemicrostructures can be fabricated by simply changing the concen-tration of the zinc ions in the aqueous precursors instead of usingany auxiliaries. This process is rapid and reproducible compared tothe conventional heating and hydrothermal routes. Additionally,the gas sensing properties of morphologically different ZnOmaterials are compared.

Fig. 1. Flow-charts representing the experiment procedure for the fabrication ofZnO particles with flower- and rod-like morphologies. [Zn2+] represents theconcentration of zinc ion in solution.

2. Experimental

2.1. Preparation of ZnO microstructures

All chemicals used were analytical-grade and used without fur-ther purification. Different morphologies of ZnO were preparedwith a microwave-assisted hydrothermal method. Flow chartscontaining the experiment procedure are illustrated in Fig. 1. Typ-ically, 6 ml of ammonia (95%) were dissolved in deionized water toform a 100 ml solution. The diluted ammonia solution was addeddropwise to 100 ml of aqueous zinc nitrate hexahydrate,[Zn2+] = 0.02 or 0.06 M, with magnetic stirring. [Zn2+] representsthe concentration of zinc ion in solution. Subsequently, the solu-tions were irradiated with a power-controlled microwave synthe-sis system (GALANZ WD800 (B123)) for 8 min at 800 W ultrasonictreated for 30 min, then cooled naturally to room temperature. Fi-nally, the white products were collected by filtration before beingwashed with deionized water three times and anhydrous ethanolone time; the resultant solids were dried under vacuum at 130 �Cfor 5 h. Hereafter, the mixture of aqueous precursor of NH3�H2Omixed and the initial solution of Zn(NO3)2 with [Zn2+] = 0.02 M willbe called precursor A, while the mixture with [Zn2+] = 0.06 M willbe called precursor B.

2.2. Gas sensing measurement

The ZnO microstructures were coated directly onto the surfaceof an alumina substrate (2 � 2 mm2) with a pair of printed goldelectrodes already installed; the system was dried at 60 �C forapproximately 2 h. A ceramic heater was printed onto the backsideof the alumina substrate to provide the working temperature of thegas sensor. The working temperature of the sensor was adjusted bychanging the heating voltage and the substrate temperature wasmeasured with a thermocouple. To improve the long-term stabil-ity, the sensors were maintained at the working temperature for2 days. A stationary-state gas distribution method was used whiletesting gas responses in dry air. The gases for detection, which in-cluded C2H5OH, were injected into a test chamber and mixed withair. The volume of the chamber was 16 L and vaporizing 1 M ofC2H5OH generates 22.4 L of pure ethanol gas. The gas concentra-tion was determined via the volume ratio of ethanol and the cham-ber dimensions. The electrical current of the sensors wasmonitored with a picoammeter (Keithley 6487) and an appliedvoltage of 5.0 V. The current values were recorded twice per sec-ond by computer. The gas response of the sensor in this paper

was defined as S = Ra/Rg, where Ra and Rg were the resistance ofthe gas sensor in air and the test gas [4], respectively.

2.3. Characterization

Phase identification for the powders obtained was performedwith a D8ADVANCE X-ray diffraction (XRD) instrument. The micro-structures and morphologies of the particles were determined viascanning electron microscopy (SEM) on a Hitachi S-3400N (II)instrument. The change in the zinc ion concentrations over timewas recorded by inductively coupled plasma emission spectrome-try (ICP – OPTIMA2100DV).

3. Results and discussion

3.1. Formation process for the flower-like and rod-like ZnO

The zinc oxide was produced by microwave irradiating theaqueous precursors. The flow chart was shown in Fig. 1, the useof different aqueous precursors resulted in products with two dis-tinct morphologies. Using precursor A yielded flower-like materials,whereas using precursor B yielded rod-like microcrystalline. Tostudy the growth process of these ZnO products, we carried outthe following experiments. The growing process was divided intothree steps for study: (i) preparing the aqueous precursor; (ii)heating through microwave irradiation; (iii) cooling naturally.

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The intermediate products would be extracted at each step foranalysis.

In the first step, ammonium hydroxide was added into theZn(NO3)2 solutions dropwise to prepare aqueous precursor. Drop-ping ammonium hydroxide in aqueous Zn(NO3)2 with [Zn2+] of0.02 M would generated lots of white precipitates at the start butdissolved finally. After addition of all ammonium hydroxide, a col-orless transparent aqueous precursor was formed (as shown inFig. 2a left, precursor A). In contrast, as shown in Fig. 2a right, pre-cursor B, parts of precipitates still appeared in the aqueous precur-sor when all ammonium hydroxide was dropped. The whiteprecipitates in such aqueous sol was filtrated and analyzed bySEM and XRD (Fig. 2b). The SEM image shown that the precipitateswere irregular shapes of poorly defined. And the X-ray diffractionpatterns indicated that the precipitates were mixture of ZnO andZn(OH)2 with bad crystallinity because of the intensity and halfwidth of the XRD pattern were low.

Subsequently, the aqueous precursors were heated by micro-wave irradiation followed by natural cooling. Samples were with-drawn from the reaction solutions at regular intervals. Duringthe treatment of precursor A, flower-like microstructure was ap-peared by microwave irradiating for 4 min (Fig. 3a). The XRD pat-tern shown in Fig. 3e (tm = 4 min) was consistent with zincite,JCPDS 36-1451, which indicated that mostly ZnO precipitated.When the irradiation time was extended, the microstucture be-came large and more spines grew epitaxially from the core of theflower-like microstructure (Fig. 3b). The corresponding XRD pat-tern indicated that there were Zn(OH)2 in the precipitates in addi-tion to the ZnO (Fig. 3e tm = 8 min). Subsequently, the aqueous

Fig. 2. (a) Photograph of precursor A (left) and precursor B (right). The precursorswere mixed solutions of NH3�H2O and Zn(NO3)2. The [Zn2+] in the initiated Zn(NO3)2

solutions were 0.02 M (left) and 0.06 M (right), respectively. (b) SEM image of theprecipitates filtered from the cloudy solution (right solution in (a)), the XRD patternof the precipitation is depicted in the inset.

mixtures of the irradiated precursor A were cooled naturally inair. During this process, no obvious change was observed in themorphology and size of the flower-like ZnO microstructure (asshown in Fig. 3c and d). However, it was observed (Fig. 3f) thatthe crystallinity of ZnO increased and the Zn(OH)2 disappeared.

During the similar treatment of precursor B, there were amor-phous precipitates after 4 min heat treatment (as shown inFig. 4a). When the irradiating time was prolonged to 8 min, ZnOrods began to emerge (Fig. 4b). The XRD pattern indicated thatthe precipitates in precursor B during the entire microwave irradi-ation process were mixtures of Zn(OH)2 and ZnO (Fig. 4e). Duringthe natural cooling process of the microwave-irradiated precursorB, large amounts of rods emerged after 10 min (Fig. 4c); after30 min almost all of the precipitates transformed into rods(Fig. 4d). XRD confirmed that a small amount Zn(OH)2 was left inthe precipitates after the cooling process (Fig. 4f).

ZnO microstructures with two distinct morphologies can befabricated through the same process by simply changing the con-centrations of Zn2+ in the aqueous precursors. A detailed discussionis provided.

The different appearances of precursor A and precursor B wererelated to the solubility of the zinc hydroxide complexes in solu-tion. Zinc hydroxide complexes formed when the ammoniumhydroxide was added to the Zn(NO3)2 solutions. These complexesincluded Zn(OH)+, Zn(OH)2, ZnðOHÞ�3 and ZnðOHÞ2�4 depending onthe amount of ammonium hydroxide added. When the molar ratioof OH�:Zn2+ was 2:1, the reaction product was Zn(OH)2 [26,27].Zn(OH)2 had a low solubility in water (Ksp = 3.5 � 10�17 at 25 �C)[28,29]. If more hydroxide was added, more soluble, higher-orderhydroxide complexes of zinc (ZnðOHÞ�3 or ZnðOHÞ2�4 ) formed [14].Therefore, the appearances of precursors (transparent or turbid)were attributed to the molar ratio of OH�:Zn2+ in the solutions.The white precipitates in precursor B should be Zn(OH)2, but dehy-dration of the unstable wet Zn(OH)2 might occur during the oven-drying process in our experiment and result in formation of ZnO.The occurrence of ZnO in the initial precipitate has been reportedin previous reports [13,30].

While heating the precursors to elevated temperatures withmicrowave irradiation, the growing units, which were ZnðOHÞ2�4

species, decomposed into the less soluble Zn(OH)2. To study thegrowth process of ZnO with both precursors, various [Zn2+] in solu-tions with different reaction times were surveyed (Fig. 5). Duringthe microwave irradiation process (0–8 min), a sharp decrease in[Zn2+] was observed in both precursors. These results indicatedthat nucleation was taking place to result in the formation of flow-er-like microstructures or amorphous precipitates, which are de-picted in Figs. 3a, b and 4a, b. During the natural cooling process,the concentrations of Zn2+ in solution hardly changed in eitherprecursor.

The formation routes for the flower- and rod-like microstruc-tures of ZnO were different. As shown in Fig. 3a and b, the flow-er-like microstructure formed after 4 min of microwaveirradiation for precursor A. Additionally, when the irradiating timewas prolonged, more spines grew. The ZnðOHÞ2�4 in solution di-rectly assembled onto the nucleation sites. The formation of theflower-like configuration could be understood as follows: hexago-nal nuclei were formed initially because ZnO has an innately hex-agonal structure. Twinned ZnO crystals grew when defectsdeveloped in the nuclei formed earlier in the process. The ZnOwould twin along the (1122) planes to produce fourling structures.Each spine of the fourling elongated along the c-axis. The anglesbetween the pairs of spines were 97.6� or 115.3� [12,13,31,32].During the continued grow of ZnO, more spines emerged and devi-ated from this ideal structure.

When irradiating the precursor B, the Zn(OH)2 that decomposedfrom the ZnðOHÞ2�4 would assemble onto the surface of the

Fig. 3. (a–d) SEM images of the precipitates that were collected from the precursor A during the successive rounds of microwave irradiation and natural cooling. The durationsof microwave irradiation and natural cooling are represented by tm and tc, respectively. (a) tm = 4 min, (b) tm = 8 min, (c) tc = 10 min, (d) tc = 30 min. (e) and (f) are thecorresponding XRD patterns of precipitates formed during microwave irradiation and natural cooling.

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amorphous precipitates that were previously suspended in thesolution. Meanwhile, the driving force for the nucleation of precur-sor B was greater than the one observed for precursor A becausegreater super-saturation of Zn(OH)2 was generated. The rapid con-densation of the zinc hydroxide in precursor B was not suitable forassembling ZnO in its preferred crystal-growth motif. Therefore,larger amorphous aggregates formed instead of flower-like micro-structures (Fig. 4a and b). Rod-like ZnO formed during the coolingof irradiated precursor B. Based on the above observations, thegrowth of the rod-like ZnO involved the dissolution-reprecipitationmechanism [15]. Growth units of ZnðOHÞ2�4 were provided by redis-solution of the Zn(OH)2 in high temperature alkaline solutions.Additionally, the surface dehydration of Zn(OH)2 led to the forma-tion of ZnO nuclei. Subsequently, the growth units (ZnðOHÞ2�4 ) couldreach the nuclei and facilitate the further growth of the ZnO rods.During the growth process of the ZnO rods, the Zn(OH)2 held theZnO rods until it was completely dissolved. The growth of rod-likeZnO might be a process of restructuring the Zn and O atoms in thesolid via surface diffusion and dehydration [33].

In order to investigate the influence of [Zn2+] in aqueous precur-sors, ZnO samples were also synthesized with varying [Zn2+] underthe same microwave irradiation conditions. The results weredisplayed in Fig. 6. When the [Zn2+] increased, the morphologies

occurred in the order as follows: seven-spine, flower-like, urchin-shaped and rod-like architectures. The results clearly suggestedthat [Zn2+] produced significantly affected the size and shape ofthe ZnO samples.

At the same time, it was found out that the ultrasonic treatmentwas very effective to the uniformity. Comparative experiments ofZnO nanostructure growing with and without ultrasonic were car-ried out. Nanorods or nanoflowers generated under treatment ofultrasonic own better uniformity which showed in Fig. 7.

Systematically studies shown that morphology of ZnO was con-trolled by [Zn2+] in water. While the high efficiency and good uni-formity of synthesis of ZnO nanostructure was leaded by thesynergetic effect of microwave and ultrasonic treatment. Theshapes and sizes of the zinc oxide crystals were variable and de-pend upon the decomposition kinetics of the ZnðOHÞ2�4 species.So that heating manners would be the key factor.

Microwaves act as high frequency electric fields and will heatmolecules of polar solvent. Molecules of the solvent are forced torotate with the field and lose energy in collisions. The frequencyof microwave radiation emitted from the microwave oven is about2.45 GHz, causing dielectric heating primarily by absorption of theenergy in water. Conventional heating usually involves the use of afurnace or oil bath, which heats the reactor by heat conduction.

Fig. 4. SEM images (a–d) and XRD patterns (e and f) of the precipitates collected from the precursor B during the successive rounds of microwave irradiation and naturalcooling. The durations of microwave irradiation and natural cooling were represented by tm and tc, respectively.

Fig. 5. Varied [Zn2+] in solutions with time during the reaction process.

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The inside of the sample takes much longer to achieve target tem-perature. Microwave heating is able to heat the whole compoundswithout heating the wall of reactor, which saves time and energy.It is also able to heat objects throughout their volume, in theory

producing more uniform heating. On the other hand, it’s possiblethat specific molecules or functional groups in water could be ex-cited by microwave, and decomposed quickly.

When the solution is irradiated by high intensity of ultrasound,acoustic cavitation in solvent occurs. Bubbles collapse in liquid andproduce enormous amount of heat energy. The compression of thebubbles during cavitation is more rapid than thermal transport,which generates a short-lived localized hot-spot. These hot-spotcould decomposed the ZnðOHÞ2�4 species more evenly. Meanwhile,ultrasound irradiation will generate strong tiny jets in water andfacilitate the nanoparticles to distribute uniformly. Once cavitationoccurs near an extended solid surface, cavity collapse drives high-speed jets of liquid to the surface. These jets and associated shockwaves can damage the new highly heated surface. Liquid-powdersuspensions produce high velocity interparticle collisions.

3.2. Gas-sensing properties

ZnO has been used for gas sensors. To demonstrate the potentialof the ZnO microstructures for applications in gas sensing, ethanolwas chosen as the testing gas. Fig. 7a depicts the real-time re-sponse curves of the ZnO microstructures when exposed to differ-ent concentrations of ethanol at a working temperature of 300 �C.The concentrations of ethanol were 150, 100, 50, 30, 25, 10 and

Fig. 6. Effect of [Zn2+] on the morphology of ZnO particles. The [Zn2+] in the initiated Zn(NO3)2 solutions were (a) 0.01, (b) 0.02, (c) 0.04, (d) 0.06 M, which correspond toseven-spine, flower-like, sea urchin and rod-like architectures, respectively.

Fig. 7. Comparative experiments of ZnO nanostructure growing with and without ultrasonic. (a) and (c) were the ZnO nanostructures grown under ultrasonic treatment,while (b) and (d) were grown without ultrasonic treatment.

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5 ppm, respectively. All of the ZnO microstructures generated agood response. The magnitude of the current variations increasedwhen the concentration of ethanol gas increased. Importantly,morphology-induced enhancements to the gas sensing perfor-mance of the ZnO microstructures were observed. As illustratedin Fig. 8a, the gas sensor constructed from the flower-like ZnO pos-sessed higher resistivity than the rod-like samples. Additionally,the seven-spine ZnO displayed the best gas sensing performancesin terms of sensitivity (Ra/Rg), as depicted in Fig. 8b.

It’s well known that the sensing mechanism of semiconductormaterials involves surface-potential-barrier-controlled processes.According to Barsan and Weimar [34], when the temperaturewas between 100 and 500 �C, the surface-adsorbed of oxygenwould be ionized to form molecular (O�2 ) and atomic (O�) species.

The adsorbed oxygen species trapped the electrons of the grainsand led to the formation of an electron depleted layer at the sur-face. Therefore, there were double-potential barriers at each con-tact junction of the grains [34]. The electrons transportedbetween grains had to pass through this double-potential barrier.The electron transport in the junction could be described as a ther-moelectronic emission mechanism [35]. The high resistivity of ZnOin air was attributed to the above reasons. When the ethanol gaswas introduced, the ethanol reduced the surface oxygen speciesand liberated the electrons that were trapped at the surface. Sub-sequently, the thickness of the depleted surface layer decreasedand resulted in lower double-potential barriers. Therefore, theZnO exhibited a decrease in resistance when exposed to ethanol.According to this mechanism, the gas sensing performance of the

Fig. 8. Real-time response curve (a) and sensor responses, (b) of the ZnOmicrostructures when exposed to different concentrations of ethanol at a workingtemperature of 300 �C.

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sensor might be greatly affected by the morphologies of the sens-ing materials. Compared to the rod-like ZnO, the superior gas-sens-ing performances of the seven-spine ZnO most likely originatefrom the large number of point-contacts and low stacking density.These advantages facilitated the surface-potential-barrier-con-trolled processes and percolation of gas molecules.

4. Conclusions

In conclusion, a straightforward, one-step, microwave-assistedhydrothermal method was demonstrated to produce ZnO micro-structures with variable morphologies. Seven-spine, flower-like,urchin-shaped and rod-like ZnO could be fabricated by modulatingthe [Zn2+] in aqueous precursors without using any auxiliaries. Thegrowth process of the ZnO in solution was studied by monitoringthe intermediate products, which were extracted during differentstages of the reactions. SEM imaging and XRD analysis allowedus to conclude that the formation of ZnO might be attributed toin situ assembly or dissolution-reprecipitation of ZnðOHÞ2�4 . Themorphology-dependent ethanol sensing performances were ob-served; the seven-spine ZnO structures exhibited the highest activ-ity. These findings provide a better understanding of the growthhabits of ZnO crystals formed in alkaline solutions in hydrothermalenvironments and improve the efficiency of ZnO microstructurefabrication by more effectively controlling the shape and size ofthe crystals.

Acknowledgment

This work was financially supported by the National NaturalScience Foundation of China (grant #51174138 and ShenzhenGovernment’s Plan of Science and Technology (grant#JCYJ20130329102720840)

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