Author's Accepted Manuscript SolÀ gel-based hydrothermal method for the synth- esis of 3D flower-like ZnO microstructures com- posed of nanosheets for photocatalytic applications Xiaohua Zhao, Feijian Lou, Meng Li, Xiangdong Lou , Zhenzhen Li, Jianguo Zhou PII: S0272-8842(13)01412-0 DOI: http://dx.doi.org/10.1016/j.ceramint.2013.10.140 Reference: CERI7567 To appear in: Ceramics International Received date: 13 August 2013 Revised date: 29 October 2013 Accepted date: 29 October 2013 Cite this article as: Xiaohua Zhao, Feijian Lou, Meng Li, Xiangdong Lou, Zhenzhen Li, Jianguo Zhou, SolÀ gel-based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic applications, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2013.10.140 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. P lease note that during the production process errors may be d iscovered which could affect the content, and all legal disclaimers that apply to the journal pertain. www.elsevier.com/locate/ceramint 1 Sol  gel-based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic applications Xiaohua Zhao a, b , Feijian Lou c , Meng Li b , Xiangdong Lou b, * , Zhenzhen Li b , Jianguo Zhou a, * a School of Environment, Henan Normal University, Xinxiang 453007, PR China b School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang 453007, PR China c School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210097, PR China  Corresponding authors. Tel.: +86 13 623731736; fax: +86 37 33326336. chemenglxd@126.com xhzhao79@163.com Abstract: Self-assembled 3D flower-like ZnO microstructures composed of nanosheets have been prepared on a large scale through a solgel-assisted hydrothermal method using Zn(NO 3 ) 2 ·6H 2 O, citric acid, and NaOH as raw materials. The product has been characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The optical properties of the product have been examined by room temperature photoluminescence (PL) measurements. A possible growth mechanism of the 3D flower-like ZnO is proposed based on the results of experiments carried out for different hydrothermal treatment times. Experiments at different hydrothermal treatment temperatures have also been carried out to investigate their effect on the final morphology of the ZnO. The photocatalytic activities of the as-prepared ZnO have been evaluated by photodegradation of Reactive Blue 14 (KGL) under ultraviolet (UV) irradiation. The experimental results demonstrated that self-assembled 3D * Corresponding author. Tel.: +86 13623731736; Fax: +86 3733326336 E-mail address: chemenglxd@126.com (Xiangdong Lou) E-mail address: xhzhao79@163.com (Jianguo Zhou) 2 flower-like ZnO composed of nanosheets could be obtained over a relatively broad temperature range (90150 ºC) after 17 h of hydrothermal treatment. All of the products showed good photocatalytic performance, with the degree of degradation of KGL exceeding 82% after 120 min. In particular, the sample prepared at 120 ºC for 17 h exhibited superior photocatalytic activity to other ZnO samples and commercial ZnO, and it almost completely degraded a KGL solution within 40 min. The relationship between photocatalytic activity and the structure, surface defects, and surface areas of the samples is also discussed. Key words: nanosheets; self-assembled 3D flower-like structures; ZnO; solgel-based hydrothermal method; photocatalysis 1. Introduction As one of the most important metal oxides and semiconductors, zinc oxide (ZnO) has found applications in a wide range of fields, including light-emitting diodes [1], nanolasers [2], field-effect transistors [3], solar cells [4], gas sensors [5,6], among others [7]. Besides the above applications, ZnO is also regarded as a promising photocatalytic material in the UV spectral range due to its wide direct band gap (3.37 eV), high exciton binding energy (60 meV), excellent chemical/thermal stability, high transparency, and non-toxicity [810]. It has been proven that the activity of photocatalysts is strongly influenced by the microstructures of the photocatalytic materials, such as crystal size, orientation and morphology, aspect ratio, and even crystalline density [11]. Therefore, study of the microstructure of ZnO is highly relevant to research and applications in photocatalysis. 3 The self-assembly of nanoscaled building blocks into complex structures has been a recent hot topic in research. Much attention has been paid to the organization of complex micro-/nanoarchitectures, especially three-dimensional (3D) hierarchical architectures [12]. Compared with low-dimensional structures, 3D ZnO hierarchical architectures provide an effective means of maintaining high specific surface area and preventing aggregation during photocatalytic reaction processes, leading to enhanced photocatalytic performance [13]. One of the 3D hierarchical architectures adopted by ZnO has a flower-like appearance, and there have been many reports on the synthesis of flower-like ZnO during the past few years [5,1236]. However, most of these reports have been concerned with flower-like ZnO composed of nanorods [1424], including hexagonal nanorods [1518], sword-like nanorods [1722], needle-like nanorods [23,24], or other forms of flower-like ZnO [5,2531]. Reports about flower-like ZnO composed of nanosheets have been rare [12,13,3235]. Moreover, some methods have been based on tedious operations and rigorous experimental conditions, and have required expensive substrates, complex template agents, or high temperatures [3335]. Compared with other flower-like ZnO, self-assembled flower-like ZnO composed of nanosheets usually shows more of the (0001) plane and a greater surface area, which should improve its photocatalytic activity [13]. There is still a need to develop a method for preparing 3D flower-like ZnO microstructures composed of nanosheets that avoids the use of toxic reagents or expensive substrates. In our previous work [37], we synthesized ZnO with different microstuctures, including 3D flower-like ZnO composed of nanosheets. However, the detailed formation mechanism and photocatalytic activity of this flower-like ZnO were not investigated. Herein, we report the further use of this facile, low-cost, green solgel-based hydrothermal method and an investigation of the dependence of the morphology evolution of the self-assembled flower-like ZnO composed of nanosheets on the hydrothermal treatment time (017 h) and temperature (90150 ºC). 4 Moreover, the corresponding photocatalytic activities have also been studied. The experimental results have indicated that the self-assembled flower-like ZnO composed of nanosheets could be obtained from 90 to 150 ºC after 17 h of hydrothermal reaction, or at 120 ºC after 4 h. The ZnO synthesized at 120 ºC for 17 h exhibited superior photocatalytic activity to other ZnO samples, such that the degree of degradation of KGL reached almost 100% after 40 min of UV irradiation. This could be attributed to the particular morphology, surface defects, and surface area of the photocatalyst. 2. Experimental section 2.1. Materials All chemicals were purchased from Shanghai Chemical Industrial Co. Ltd. (Shanghai, China), and were used without further purification. Distilled water was used in the reaction system as the solvent medium. 2.2. Synthesis of 3D flower-like ZnO microstructures composed of nanosheets <1> In this work, 3D flower-like ZnO samples were synthesized by the following procedure. A sol was first prepared by adding Zn(NO 3 ) 2 ·6H 2 O (3.756 g) and citric acid (C 6 H 8 O 7 ; 5.250 g) to distilled water (100 mL) and stirring at 70 ºC. The sol was then placed in an oven at 100 ºC to form the gel. Secondly, 1 m NaOH was directly dropped into the dry gel under constant stirring, until a suspension of pH 14 was obtained. The resulting suspension was then transferred to a 100 mL Teflon-lined stainless steel autoclave and heated at 120 ºC for 17 h. Finally, the white precipitate was collected, washed thoroughly with distilled water, and then dried at 100 °C to obtain the final sample. <2> In order to reveal the growth mechanism of the 3D flower-like ZnO, experiments were conducted for different hydrothermal treatment times (0, 4, 8, and 12 h) while the other conditions were 5 kept unchanged. <3> In order to investigate the influence of hydrothermal treatment temperature on the final morphology of the ZnO, experiments at different hydrothermal treatment temperatures (90, 150 ºC) were also carried out while the other conditions were kept unchanged. 2.3. Characterization The as-synthesized samples were characterized by X-ray diffraction (XRD) (Bruker Advance-D8 XRD with Cu-KĮ radiation, Ȝ=0.154178 nm, the accelerating voltage was set at 40 kV with a 100 mA flux). Microstructures and morphologies were investigated by field-emission scanning electron microscopy (FESEM; JSM-6701F, JEOL), transmission electron microscopy (TEM; JEM-2100, JEOL), and scanning electron microscopy (SEM; JSM-6390LV, JEOL). Photoluminescence (PL) spectra were measured on a Shimadzu RF-5301PC fluorescence spectrophotometer. The surface areas of the samples were determined from nitrogen adsorptiondesorption isotherms using an ASAP 2000 instrument and the BrunauerEmmettTeller (BET) method was used for surface area calculation. 2.4. Photocatalytic experiments The photocatalytic activities of the as-synthesized samples were evaluated by the degradation of aqueous KGL solution. Photocatalyst (100 mg) was added to 250 mL of 20 mg/L KGL solution and the mixture was stirred for 20 min to reach absorption equilibrium and then exposed to UV light (300 W high-pressure Hg lamp; maximum emission at 365 nm). In order to minimize temperature fluctuations, water at room temperature was employed to absorb the heat generated from the UV light and the test tube containing the KGL solution was rotated at a distance of 10 cm from the center of the lamp. Samples 6 were collected at intervals of 20 min, centrifuged, and the supernatants were characterized by UV/Vis spectrophotometry (UV-5100, Shanghai Metash Instruments Co. Ltd., China) to monitor the degradation of the KGL. The characteristic absorption peak of KGL at Ȝ=608 nm was chosen to monitor the photocatalytic degradation process. For comparison, commercial ZnO powder purchased from Tianli Chemical Reagent Co. Ltd. (99.0%; product number XK 13-201-00578; BET: 2.93) was also used for photocatalytic experiments. The photocatalytic degradation efficiency was calculated from the following expression (1): Degradation (%) = 0t 0 CC C  100%u = 0t 0 A A A  100%u (1) where C 0 and A 0 are the initial concentration and absorbance of KGL, and C t and A t are the concentration and absorbance of KGL at a certain reaction time t. 3. Results and discussion 3.1. Structure and morphology Figs. 1 ac show FESEM images of the ZnO microstructures synthesized at 120 ºC for 17 h at low, medium, and high magnifications, respectively. From the FESEM images, it can be seen that the ZnO product consisted of numerous 3D flower-like aggregates, with single flowers having diameters in the range 23 ȝm. In addition, each flower was made up of many thin nanosheets as “petals”, and these nanosheets were about 30 nm in thickness. Further information about the ZnO product was obtained from TEM and HRTEM images and the associated SAED patterns. Fig. 1d shows a typical TEM image of a flower-like ZnO microstructure, confirming the 3D structure with a diameter of about 2 ȝm and its construction from numerous nanosheets. The SAED pattern shown in the inset of Fig. 1d indicates the single-crystalline nature of the nanosheet. The HRTEM image shown in Fig. 1e exhibits well-resolved 7 lattice fringes with a spacing of 0.26 nm, which is in good agreement with the interplanar spacing of the (0001) plane. The XRD pattern of the flower-like ZnO microstructure is displayed in Fig. 1f. All of the diffraction peaks could be well indexed to hexagonal wurtzite ZnO (JCPDS Card No. 36-1451). No characteristic peaks from any impurities were detected. In addition, the strong and sharp peaks indicated that the prepared ZnO was highly crystalline. 3.2. Effect of hydrothermal treatment time In order to reveal the formation mechanism of the 3D flower-like ZnO, SEM images and XRD patterns were acquired at appropriate intervals during the time-dependent evolution process. Fig. 2a shows relatively uniform microspheres with an average diameter of 2 ȝm, which were collected before being transferred to the Teflon-sealed autoclave. From the magnified image shown as an inset in Fig. 2a, one can see that the microspheres were composed of tiny nanosheets. When the reaction time was extended to 4 h (Fig. 2b), these tiny nanosheets were gradually extended. When the hydrothermal treatment time was increased to 8 h and 12 h, more and larger nanosheets grew and the shapes of the flower-like ZnO microstructures were further developed. Finally, well-defined 3D flower-like microstructures were obtained after extending the reaction time to 17 h (Fig. 2e). Therefore, it can be concluded that flower-like ZnO is produced after a hydrothermal treatment time of 4 h, and further increasing the hydrothermal treatment time makes the diameters of the nanosheets more uniform and the flower-like ZnO more defined in accordance with the Ostwald ripening mechanism [25]. The surface areas of the samples increased accordingly with extending the hydrothermal time; Table 1 lists the surface areas of different samples. It can be seen that the surface areas of the samples increased from 2.25 to 11.05 m 2 /g when the hydrothermal time was increased from 0 h to 17 h. XRD patterns of the samples 8 obtained after different reaction times are shown in Fig. 2f. It is worth noting that all of the diffraction peaks could be well indexed to hexagonal wurtzite ZnO (JCPDS card No. 36-1451). 3.3. Possible growth mechanism of the 3D flower-like ZnO A schematic illustration of the formation process is presented in Fig. 3, and a possible formation mechanism for the 3D flower-like ZnO is proposed as follows. Firstly, in the solgel process, it is supposed that Zn(II)citric acid chelate complexes are formed during the gelation of the sol [38], and then the gel is dissolved by adding a certain amount of NaOH. Some of the OH  ions in the solution might neutralize H + ions derived from the citric acid, while other OH  ions might react with the Zn(II)citric acid chelate complexes to form [Zn(OH) 4 ] 2 complexes, which will decompose into ZnO nuclei (Eqs. (2)(4)) [39,40]. Zn(OH) 2 + 2OH - o Zn(OH) 4 2- (2) Zn 2+ + 4OH - o Zn(OH) 4 2- (3) Zn(OH) 4 2- o ZnO + H 2 O + 2OH - (4) At the same time, further OH  left in the solution will affect the morphology of the ZnO. It is known that ZnO forms polar crystals, with a positive polar (0001) plane rich in Zn 2+ cations and a negative polar (000 1 ) plane rich in O 2 anions [41]. Usually, hexagonal rod-like ZnO elongated along the c-axis direction would be obtained due to the intrinsic anisotropy in its growth rate v with Ȟ[0001] >> Ȟ[01 1 0] >> Ȟ[000 1 ] [42]. However, in the present case, because the molar ratio of Zn 2+ to OH  is about 1:12 (in solution at pH 14), a very high concentration of OH  is present in the aqueous medium. Following the decrease in the concentration of Zn(OH) 4 2 due to the initial fast nucleation of ZnO, the absorption of OH  ions on the positively charged Zn-(0001) plane would dominate in the competition 9 with Zn(OH) 4 2 . Therefore, the excess OH  ions stabilize the surface charge and the structure of the Zn-(0001) surfaces to some extent, allowing fast growth along the [01 1 0] direction, which leads to the formation of ZnO nanosheets with a {2 1 1 0}-plane surface [12]. In order to minimize the total surface energy, numerous spherical ZnO aggregates composed of tiny nanosheets are then formed in the reaction system (Fig. 2a) [40, 43]. Secondly, in the hydrothermal process, the ZnO aggregates would tend to further decrease their energy through surface reconstruction, which would provide more active sites for further heterogeneous nucleation and growth. Thus, the nanosheets would grow out continuously from the surface of the primary structures (Fig. 2b) [40]. Subsequently, with increasing hydrothermal treatment time, more and more nanosheets with a {2 1 1 0}-planar surface become interlaced and overlapped with each other to form a multilayer network structure, and thereby the flower-like ZnO nanostructures are shaped (Figs. 2c2e). 3.4. Photocatalytic activities of ZnO samples synthesized for different hydrothermal treatment times To demonstrate their potential environmental application in the removal of contaminants from wastewater, the photocatalytic activities of the as-synthesized ZnO samples were investigated by the degradation of KGL. From Fig. 4(a), it is clear that the ZnO samples synthesized for different hydrothermal treatment times exhibited different photocatalytic activities. Their photocatalytic activities increased with increasing hydrothermal treatment time. The ZnO synthesized for a hydrothermal time of 17 h showed superior photocatalytic activity, degrading KGL by 96.7 % after irradiation for 60 min. It is known that when semiconductor materials are irradiated with light of energy higher than or equal to the band gap, an electron (e cb ) in the valence band (VB) can be excited to the conduction band (CB) with the simultaneous generation of a hole (h vb+ ) in the VB. Excited-state e cb and h vb+ can recombine and [...]... SEM images of the time-dependent evolution in the formation of 3D flower-like ZnO synthesized 18 at 120 ƕC for 0 h (a), 4 h (b), 8 h (c), 12 h (d), 17 h (e) and XRD patterns (f), respectively Insets in (a–e) are the high-magnification images of the corresponding ZnO samples Fig 3 Schematic illustration of the formation process of the 3D flower-like ZnO Fig 4 (a) Bar graph illustration of the photocatalytic... degradation of KGL using ZnO samples synthesized at 120 ƕC for different hydrothermal treatment times and (b) PL spectra of the as-synthesized ZnO samples Fig 5 SEM images of the ZnO samples prepared at 90 ƕC (a), 150 ƕC (b) for 17 h hydrothermal treatment, and XRD patterns (c) Insets in (a,b) are the high-magnification images of the corresponding ZnO samples Fig 6 (a) Photocatalytic degradation of KGL with ZnO. .. necessary for effective photodegradation of the KGL dye [51] After irradiation for 120 min, the degrees of degradation of KGL were about 82.17% for the ZnO (150 ºC) sample, and almost 100% for ZnO (120 ºC) and ZnO (90 ºC) samples and a commercial ZnO sample It should be noted that after irradiation for 40 min, only for the ZnO (120 ºC) sample did the degree of degradation approach 100%, and accordingly the. .. degradation of KGL with ZnO samples synthesized at different hydrothermal temperatures, (b) the change photographs of KGL solution with the photocatalytic degradation time in the presence of ZnO (120 ƕC, 17 h) sample and (c) PL spectra of the ZnO samples synthesized at different hydrothermal temperatures Table 1 The surface area of ZnO samples synthesized at different hydrothermal time sample 0h 4h 8h 12... photocatalytic activity 10 3.5 Effect of hydrothermal treatment temperature In order to further study the effects of the hydrothermal treatment temperature on the ZnO product, samples were also synthesized at 90 ºC and 150 ºC for 17 h The morphologies of the ZnO samples prepared at different hydrothermal treatment temperatures are shown in Figs 5a and 5b It can be observed that the sample prepared at 90 ºC... formation of 3D flower-like ZnO synthesized at 120 ◦C for 0 h (a), 4 h (b), 8 h (c), 12 h (d), 17 h (e) and XRD patterns (f), respectively Insets in (a–e) are the high-magnification images of the corresponding ZnO samples Figure Fig 3 Schematic illustration of the formation process of the 3D flower-like ZnO Figure Fig 4 (a) Bar graph illustration of the photocatalytic degradation of KGL using ZnO samples... samples synthesized at 120 ◦C for different hydrothermal treatment times and (b) PL spectra of the as-synthesized ZnO samples Figure a b c Fig.5 SEM images of the ZnO samples prepared at 90 ◦C (a), 150 ◦C (b) for 17 h hydrothermal treatment, and XRD patterns (c) Insets in (a,b) are the high-magnification images of the corresponding ZnO samples Figure Fig 6 (a) Photocatalytic degradation of KGL with ZnO. .. amount of oxygen vacancies can entrap electrons from the semiconductor, the holes can diffuse to the surface of the semiconductor and cause oxidation of the organic dye Therefore, a high density of surface oxygen defects is beneficial for efficient separation of electron-hole pairs, minimizes the radiative recombination of electrons and holes, and increases the lifetime of the charge carriers, thereby... indexed to hexagonal wurtzite ZnO (JCPDS Card No 36-1451) 3.6 Photocatalytic activity of ZnO samples synthesized at different hydrothermal temperatures Fig 6a shows the degradation rates of KGL as a function of irradiation time in the presence of ZnO samples synthesized at different hydrothermal treatment temperatures In the absence of light or catalyst, the concentration of KGL showed no obvious change... and the ZnO (150 ºC) sample, respectively 4 Conclusions In summary, 3D flower-like ZnO composed of nanosheets has been successfully synthesized by a solgel -based hydrothermal method over a relatively broad temperature range (90150 ºC) after 17 h of hydrothermal reaction or at 120 ºC after 4 h A possible formation mechanism has been proposed based on the experimental results Compared with the other . apply to the journal pertain. www.elsevier.com/locate/ceramint 1 Sol  gel -based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic. Xiangdong Lou, Zhenzhen Li, Jianguo Zhou, SolÀ gel -based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic applications, Ceramics International,. report the further use of this facile, low-cost, green solgel -based hydrothermal method and an investigation of the dependence of the morphology evolution of the self-assembled flower-like ZnO