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Current Applied Physics 14 (2014) 359e365

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Current Applied Physics

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High-temperature-mixing hydrothermal synthesis of ZnOnanocrystals with wide growth window

Jun Wen a, Yonghong Hu b, Kongjun Zhu a, Yufang Li a,*, Jizhong Song a,*

aCollege of Material Science and Technology, State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronauticsand Astronautics, Nanjing 210016, ChinabHubei University of Science and Technology, School of Nuclear Technology and Chemistry & Biology, Xianning 437100, China

a r t i c l e i n f o

Article history:Received 29 September 2013Received in revised form23 November 2013Accepted 25 November 2013Available online 4 December 2013

Keywords:ZnOHigh-temperature-mixing hydrothermalColloid nanocrystalsSolution-process

* Corresponding authors. Tel./fax: þ86 025 843032E-mail addresses: jizhongsong@gmail.com, 396017

1567-1739/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.cap.2013.11.033

a b s t r a c t

High-quality and controllable growth of nanocrystals (NCs) have been attracting great attention. Here, ahigh-temperature-mixing hydrothermal (HTMH) method was designed to synthesize ZnO NCs with highcrystallinity and narrow size distribution in a wide growth window. Compared with conventional hy-drothermal (CH) growth, zinc source and alkali precursors were intentionally separated in temperature-rising stage and permitted to mix at the starting of heat preservation stage of HTMH growth. Highlycrystalline ZnO NCs with uniform spherical morphology can be formed at alkali concentration andtemperature windows as wide as 0.1e0.5 M and 160e200 �C, respectively. However, the products via CHmethod have much larger changes in not only morphology but also size. These results demonstrated thatthe high-temperature-mixing reaction greatly facilitates nucleation but depresses grain growth.Considering the simplicity and reproducibility, such HTMH method could have wide potentials for thefabrication of various functional nanocrystals.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

As materials with a tremendously wide variety of applications,zinc oxide (ZnO) nanostructures have attracted intense attention inrecent years [1,2]. Among them, pure and doped ZnO sphericalnanocrystals (NCs) have been considered as important buildingblocks in advanced functional optoelectronic device applications,including photodetectors [3], light-emitting diodes (LEDs) [4], solarcells [5] and transparent conductive electrodes [1,6,7], efficient andtransferable field emitters [6]. Compared with conventional ZnOfilms grown by high temperature gas-phase methods, the filmsbased on low temperature formed colloidal ZnO NCs have manysignificant virtues in above applications [7]. Firstly, the optoelec-tronic performances have been demonstrated to be improved bythe characters of ZnO NCs, such as large specifical surface area andquantum effect [8,9]. Secondly, the colloidal growth of NCs iscompatible with large scale and low cost assembly processes, suchas roll-to-roll [10] and ink printing techniques [11]. But, suchassembled NCs devices make serious requests on the high-qualityof NCs, especially on morphology uniformity and narrow size

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distribution. Although many methods, such as sol-gel [12,13],chemical bath deposition [13,14], pyrolysis [15] and laser ablation inliquid (LAL) [16], have been reported for the growth of colloidal ZnONCs, high-quality and large scale fabricationwith very wide growthwindow is still a serious challenge.

Among various liquid-phase growth methods, hydrothermalprocess [17,18] is a low cost and large scale method. However, themorphology and size of products are strongly influenced by reac-tion time [18], temperature [19,20] and pH value [21] during hy-drothermal process. As for conventional hydrothermal (CH)process, the raw materials are pre-mixed at room temperature andthen heated-up. Due to the pre-mixing followed by temperature-rising process, the nucleation and growth behaviors greatly over-lap each other in a large temperature range, thus it is difficult tocontrol the nucleation and growth of ZnO NCs.

In this article, a high-temperature-mixing hydrothermal(HTMH) method was designed to decrease the heating-up processinduced overlap of nucleation and growth, and reported thefabrication of ZnO NCs in a wide-growth-window. During HTMHprocess, the raw material solutions are separated and heated-up toa preset temperature in a double chambered autoclave, and mixedat high temperature to start the rapid nucleation and growth. ZnONCs with high crystallinity and narrow size distribution were suc-cessfully prepared by this method. The dependence of ZnO NCs

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morphology on alkalinity and temperature were investigated.Interestingly, the results implied that high-quality ZnO NCs can beobtained by HTMH method at 0.1e0.5 M alkali concentration and160e200 �C, whereas the products via CH method evolved fromnanoprisms to resemble thin flake-like nanosheets and thennanosheets. Considering the simplicity and reproducibility, suchHTMH method can be extended and used in the preparation ofvarious nanocrystals.

2. Experimental section

To synthesize ZnO NCs via HTMH method, A teflon lined auto-clave (as shown in Scheme 1) with two separated chambers wasdesigned. A certain pre-determined amounts of sodium hydroxide(NaOH, 96%) were dissolved in deionized water to form solutionswith different concentrations. With appropriate amounts of zincnitrate (Zn(NO3)2$6H2O, 98%) and deionized water charged intoone chamber, and an alkaline solution into the other. Both cham-bers were heated to the preset temperature before the solutionsweremixed together, and the subsequent hydrothermal treatmentswere carried out at preset temperature for a period of time, andthen the autoclave was cooled to room temperature gradually. Theformed products were washed with deionized water and ethanolseveral times. The parallel products were performed underdifferent conditions, including the alkalinity (0.1, 0.2 and 0.5 M) andthemixing temperature (160, 180, 200 �C). The concentration of theZn(NO3)2$6H2O and reaction time was fixed 1 � 10�2 M and 6 hrespectively for all experiments. In order to compare the influenceof NaOH concentrations and temperature on the morphology ofZnO nanoparticle synthesized by the HTMH method, the sameexperimental parameters are performed by CH method.

The X-ray diffraction (XRD) patterns of samples were recordedon a multipurpose X-ray diffraction system (D8 ADVANCE, Bruker)with a Cu-Ka radiation at l ¼ 1.5406 �A, The morphology of the as-prepared samples was observed on a Hitachi S-4800 field-emissionscanning electron microscopy (FE-SEM). For transmission electronmicroscopy measurements, one drop of the aqueous redispersedsuspension of sample was placed on a carbon-coated copper gridand allowed to dry in air. Then samples were observed by a TecnaiG2 F30 S-TWIN transmission electron microscope. Photo-luminescence (PL) measurements were performed under the

Scheme 1. Schematic illustration of a double chambered autoclave during HTMH.

325 nm UV fluorescent light excitation using a fluorescence spec-trophotometer (Hitachi F-4500).

3. Results and discussion

Generally speaking, the formation of ZnO NCs in hydrothermalgrowth can be described as following. Zn(OH)2 is formed in situ viathe reaction of Zn2þ and OH�. At the same time, part of the Zn(OH)2colloids dissolves into Zn2þ and OH�. When the concentration ofZn2þ and OH� reaches the super saturation degree, ZnO nuclei areformed. The growth units of [Zn(OH)4]2� is of importance for thegrowth of ZnO. The preparation process can be easily affected byslight fluctuation of temperature and solution concentration.Therefore, after a default reaction time, all sorts of ZnO nano-structures were obtained [22,23]. For hydrothermal growth, theformation process of ZnO nanostructures includes temperature-rising, temperature-keeping and temperature-down stages, whichcorresponds to Zn-O cluster nucleation, ZnO growth, and growthtermination, respectively. These three stages need some time tofinish every single process and are so important for conventionalhydrothermal process. The nucleation is not at the push of a button,which may lead to the diversity of ZnO morphology. The HTMHmethod reported here is an instantaneous nucleation process andcan be used to prepare uniform ZnO nanocrystals. As shown inFig. 1(a) SEM image, the size of ZnO nanocrystals prepared with0.1 M Zn(NO3)2 and NaOH aqueous solution at 200 �C for 6 h is veryuniform and large-scale. The diameter is about w25 nm. The ZnONC size distribution is rather narrow, with an average size of25 � 7 nm (Fig. 1(b)). Transmission electron microscopy (TEM)analyses (Fig. 1(c)) show the presence of spherical-like nano-particles for ZnO. The ZnO NC structure is analyzed by high-resolution TEM (HRTEM) (Fig. 1(d)). We observe that the ZnO NCis a single crystal with a lattice fringe distance of 0.281 nm (Fig. 1(d)inset), illustrative of the crystal lattice spacing of the (100) planes,which is quite different from nanowires and nanorods with the(002) preferred orientation.

Fig. 2 presents the XRD spectra of the ZnO powders synthesizedat 200 �C for 6 h with a stationary NaOH concentration (0.1 M) byHTMH and CH, respectively. The formation of well crystallinehexagonal ZnO for all the samples can be observed. The XRD pat-terns of our samples were indexed, and they were agreed well withthe standard ZnO of hexagonal structure (JCPDS card No. 36-1451).No characteristic peaks of other impurities were detected in thepatterns. The sharp diffraction peaks indicate the good crystallinityof the prepared nanocrystals.

The ZnO nanocrystals can be prepared in a wide window, suchas large temperature range and different precursor concentrationvia HTMH method. We investigated the effect of temperature andconcentration on the ZnO morphology. It is well known that tem-perature is an important influence factor of the morphology evo-lution under hydrothermal reaction. We elaborate the effect of thetemperature on themorphology by comparing HTMH and CH. Fig. 3presents typical FE-SEM images of ZnO nanoparticles prepared atdifferent temperatures with fixed alkalinity concentrations (0.1 M)by HTMH and CH method, respectively. As shown in the SEM im-ages, the spherical nanoparticle morphology in Fig. 3(aec) has noobvious change mixed under the temperature of 160 �C, 180 �C and200 �C, which is quite different from the literature [21] undersimilar experimental conditions. With the same experimental pa-rameters for the CH method, regular hexagonal nanoprism wasobtained as shown in Fig. 3(def). For CH, we believe that there is apreset temperature range having an influence on the overlappednucleation with the same alkalinity concentration. As for HTMH,the rapid uniformed nucleation is key to form sphere-like nano-particle without the effect of overlapped nucleation, and such

Fig. 1. (a) SEM image, (b) the size distribution, (c) TEM (d) high resolution TEM image of ZnO nanocrystals prepared with 0.1 M Zn(NO3)2 and NaOH aqueous solution at 200 �C for6 h, respectively.

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method is not sensitive to the mixing temperature. We have manyattempts to broaden the reaction temperature. For example, whenthe reaction temperature reaches to 220 �C, we cannot obtainuniform ZnO nanocrystals but ZnO nanorod as shown in Fig. 4. Thisis due to when the temperature is high to 220 �C, the Zn(NO3)2 inaqueous solution may be degraded to form Zn-O cluster [24].Therefore, Zn-O cluster as nucleus can quickly grow in case NaOH isadded, and ZnO nanorods were obtained after reaction over.

Fig. 2. XRD patterns of ZnO NCs synthesized at NaOH concentration (0.1 M) with200 �C by HTMH and CH, respectively.

In order to clarify the effect of NaOH concentration on themorphology variation, the experiments were performed withaqueous solutions containing Zn(NO3)2 (0.1 M) and different NaOHconcentrations of 0.1, 0.2 and 0.5 M by HTMH and CH, respectively.According to the SEM results in Fig. 5(aec), surprisingly, it can beseen that as-obtained spherical ZnO NCs can be hardly influencedby NaOH concentration of the starting solution during HTMH.However, by CH method, regular hexagonal prisms, resemble thinflake-like nanosheets and nanosheets of ZnO powders were ob-tained with NaOH concentration of 0.1 M, 0.2 M and 0.5 M,respectively, shown in Fig. 5(def). It is obvious that the morphologyof the products fabricated by CH is sensitive to NaOH concentration,which is quite different from HTMH.

By comparing with the CH method above, the HTMH methodhas distinct advantage to obtain the uniformed size of sphere-likeZnO NCs. In order to analyze the determined size, we counted thesize distribution of spherical ZnO NCs with different test parame-ters. Fig. 6(a) illustrates the nanoparticle diameter on dependenceof NaOH concentration and mixing temperature prepared byHTMH, respectively. From the results, when the NaOH concentra-tion changes from 0.1 M to 0.5 M, the ZnO nanoparticle diameterfluctuates between 20 and 30 nm, estimated by using full widthhalf maximal (FWHM) from the line broadening of the XRD peaks,which is well consistent with the SEM images (Fig. 5(aec)). At thesame time, the results imply the as-obtained ZnO nanoparticlewithnarrow spherical particle size distributions. The mixing tempera-ture dependence of the diameter of nanoparticle by HTMH inFig. 6(b) shows the nanoparticle size increase slightly with the in-crease of mixing temperature from 160 to 200 �C. In conclusion,

Fig. 3. FE-SEM images of ZnO nanoparticles synthesized at different temperature with fixed NaOH concentration (0.1 M) of the starting solution: (a, d) 160 �C, (b, e) 180 �C, (c, f)200 �C by HTMH and CH, respectively.

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HTMH provides a narrow distribution with wide alkalinity andtemperature windows for prepared ZnO NCs.

In order to systemically analyze the mechanism of such simplemethod of preparing uniform ZnO nanocrystals, we gave a detailedsummary about ZnO prepared in different conditions. For tradition

Fig. 4. The SEM image of uniform ZnO nanorods was obtained when the reactiontemperature is at 220 �C by HTMH method.

route, as shown in Fig. 7(a), the reaction and nucleation start at (T1,t1) during the temperature-rising stage. On the one hand, theprecursors are mixed at room temperature. However, as pointed byWilliam A. Ducker et al. [25] the temperature of Zn(OH)2 convertingto ZnO can be as low as 50 �C. Therefore, it is obvious that the re-action and nucleationwill occur as soon as the pre-mixed solutionsreaching the critical reaction temperature T1 (about 50 �C) at timet1. This temperature is much lower than the usually preset keeping-temperature T2 (about 100 �C), so the nucleationwill be overlappedby the growth during the whole temperature-rising andtemperature-keeping stages. This means that new nucleus will becontinually formed and the growth is maintained at the same time.Due to the c-axis growth preference habit of ZnO, the early formednucleus will become larger and form rod-like shape, while thesubsequently formed nucleus form spherical particles with smallsize. As a result, NCs with different size and morphology will beformed as shown in Fig. 7(b).

To preclude the effect of overlapped nucleation by the growthduring the whole temperature-rising and temperature-keepingstages for CH method, a new route so-called HTMH method wasdesigned as shown in Fig. 7(c). During HTMH, the starting solutionsand raw materials are directly heated to a preset keeping-temperature T2 separately in a double chambered autoclavewithout mixing during the whole temperature-rising, the startingsolutions were mixed quickly to start the hydrothermal reactions,

Fig. 5. FE-SEM images of ZnO NCs synthesized at different NaOH concentrations, (a, d) 0.1 M, (b, e) 0.2 M, and (c, f) 0.5 M by HTMM and CH method at 200 �C, respectively.

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which could efficiently avoid the new continually formed nucleusand the maintained growth during CH method. Thus, for the wholeHTMH process, the nucleation and growth will occur simulta-neously and instantaneously at the same starting time and tem-perature (T2, t2) as soon as mixing the raw materials. Comparingwith the CH method, ZnO nuclei can be more easily formed by

Fig. 6. (a) NaOH concentrations and (b) mixing temperature dependen

HTMH method at such high reaction temperature. Considering thesimultaneous uniformly rapid nucleation and depress growth atpreset keeping-temperature T2 (about 100 �C) instead of the criticalreaction temperature T1 (about 50 �C) at time t1, the uniformedspherical particles with small size was easily obtained by HTMHmethod.

ce of ZnO nanoparticles diameter prepared by HTMH, respectively.

Fig. 7. The schematic illustration of the nucleation and growth mechanisms duringhydrothermal process. (a) shows the temperature as a function of reaction time duringCH and HTMH process. The mechanisms of nucleation and growth during CH andHTMH were presented by (b) and (c), respectively.

Fig. 8. PL spectrum of ZnO fabricated by HTMH with NaOH concentrations (0.1 M) at200 �C excited at 325 nm. The inset shows the good dispersed ZnO NCs in ethanol andisopropanol, respectively.

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As mentioned above, the explanation can be based oncombining the effect of overlapped nucleation with the electro-static interaction [26] between [Zn(OH)4]2� and the polar sur-faces, which can be affected by NaOH concentration during thewhole temperature-rising and temperature-keeping stages in CH.Thus, when NaOH concentration is 0.1 M, the quantity of thecorresponding Zn(OH)2 is larger while there is no enough growthunit ([Zn(OH)4]2�) to grow ZnO, so the electrostatic interaction isweak. Considering new overlapped nucleus and the preferredorientation of ZnO, nanoprisms formed at last. While for theincreasing NaOH concentration (0.5 M), a larger quantity ofgrowth unit ([Zn(OH)4]2�) and a smaller quantity of Zn(OH)2 areobtained, contributing to the strong electrostatic interaction.Allowing for the rich positive polar plane Zn2þ-terminated (001)ZnO surface with minimum lattice mismatch [27] and the moststable morphology with crystal elongated along c-axis direction inpure solution [28], the formation of ZnO nanosheet with highproportion of polar {001} plane is certainly due to strongly sup-pressed crystal growth along high-energy [001] axis and relativeenhancement along of growth non-polar f210g and {010} di-rections [29]. Thus, the growth along the low surface energy and(010) planes results in the format of nanosheets shown in Fig. 5(f).So the resemble thin flake-like nanosheets were formed usingNaOH (0.2 M) as the mineralizer. While for HTMH, the spherenanoparticles were easily obtained by directly mixing the startingsolutions and raw materials at preset keeping-temperature, as-obtained sphere nanoparticle was based on the simultaneousrapid nucleation and depress growth at preset keeping-temperature (about 100 �C) with efficiently avoiding the newcontinually formed nucleus and the maintained growth duringCH. Based on above facts, it is reasonable to hold that the

overlapped nucleation does affect the product morphology duringthe whole temperature-rising and temperature-keeping stages inCH method. The temperature effect in CH method is evidentshown in Fig. 3(def). The short rod about 200 nm can be obtainedby CH method in the temperature range of 160e200 �C, which isdifferent from the uniform ZnO NCs prepared by HTMHmethod. Ina word, uniform ZnO NCs can be easily obtained by HTMHmethodin a wide window in the temperature range 160e200 �C and thebase concentration range 0.1e0.5 M.

The room temperature PL spectrum of ZnO NCs is one of themost interesting and important properties that have beenintensively studied recently, indirectly presenting the crystallinityof materials. Fig. 8 shows the room-temperature PL spectrum ofZnO fabricated by HTMH mixed at 200 �C with 0.1 M NaOHcontent. This spectrum was recorded at wavelength from 360 to600 nm corresponding to photon energy from 3.6 to 1.55 eV witha fluorescent spectrophotometer equipped with a Xenon lamp(excitation wavelength: 325 nm). The obtained ZnO nanoparticlesonly emit ultra-violet (UV) luminescence at photon energy ofw3.23 eV (384 nm). The energy of the observed UV emissionoriginating from the near-band-edge (NBE) is in the range ofvalues reported for as-deposited ZnO nanostructures prepared byelectrodeposition (3.12e3.30 eV) [30e33] and hydrothermaldeposition (3.12e3.29 eV) techniques [34,35]. There is no visibleemission related to the defect states as compared with the UVemission. It indirectly indicates that the crystalline quality is quitewell, which agrees well with the above analysis, and has prom-ising applications in optics and optoelectronics. ZnO colloid so-lution products dispersed in ethanol and isopropyl alcohol areshown in Fig. 8 inset 1#, 2#. These samples are so stable storedabout for 3 months. Such sample can be potentially applied insolution-processed optoelectronics by roll-to-roll, ink-jet printingtechnique.

4. Conclusion

In summary, we have successfully prepared hexagonal ZnOspherical nanoparticles with high crystalline quality by HTMHmethod. The alkalinity concentration hardly has effect on the size ofspherical nanoparticles of the products prepared by HTMH, whichis quite different from CH method evolved from nanoprisms toresemble thin flake-like nanosheets and then nanosheets. It isattributed to highly uniform ZnO nuclei rate and slowly growthduring HTMH process. High-quality ZnO NCs can be formed by

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HTMHmethod at alkali concentration and temperaturewindows aswide as 0.1e0.5 M and 160e200 �C, respectively. Considering thesimplicity and reproducibility, through such HTMH method, wideapplications for the formation of various nanocrystals can be found.

Acknowledgment

This work was financially supported by the National BasicResearch Program of China (2014CB931702), NSFC (61222403), theDoctoral Program Foundation of China (20123218110030), theFundamental Research Funds for the Central Universities (No.30920130111017 and NE2012004), the Opened Fund of the StateKey Laboratory on Integrated Optoelectronics (IOSKL2012KF06)and the Natural Science Foundation of Hubei Province under grantno. 2013CFC013.

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