Original Research Paper Sol–gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity Xiaohua Zhao, Meng Li, Xiangdong Lou ⇑ School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang 453007, Henan, China article info Article history: Received 15 November 2012 Received in revised form 4 June 2013 Accepted 11 June 2013 Available online xxxx Keywords: Sol–gel assisted hydrothermal method pH Morphology Photocatalysis abstract ZnO microstructures of different morphologies were synthesized by the sol–gel assisted hydrothermal method using Zn(NO 3 ) 2 , citric acid and NaOH as raw materials. Twining-hexagonal prism, twining-hex- agonal disk, sphere and flower-like ZnO microstructures could be synthesized only through controlling the pH of the hydrothermal reaction mixture at 11, 12, 13 and 14, respectively. The as-synthesized sam- ples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM). Optical proper- ties were examined by UV–Vis absorption/diffuse reflectance spectroscopy and room-temperature photoluminescence measurements (PL). Photocatalytic activities of the samples were evaluated by deg- radation of Reactive Blue 14 (KGL). The results indicated that the flower-like ZnO composed of nano- sheets possessed superior photocatalytic activity to other ZnO microstructures and commercial ZnO, which could be attributed to the morphology, surface defects, band gap and surface area. The formation mechanisms of different ZnO morphologies were also investigated based on the experimental results. Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. 1. Introduction Zinc oxide (ZnO), an II–VI compound semiconductor with a wide and direct band gap of 3.37 eV at room temperature, large exciton binding energy of 60 meV [1], excellent chemical and ther- mal stability, outstanding optical and electrical properties [2], has been extensively studied and applied in transistors [3], solar cells [4], piezoelectric transducers [5], gas sensors [6] and photocata- lysts [7]. As one of the most important semiconductor photocata- lysts, ZnO has attracted considerable attention due to its high photosensitivity, nontoxic nature and low cost [8]. It is reported that the photocatalytic activity of ZnO is strongly influenced by the methods and conditions of preparation which have great ef- fects on the microstructures of the materials, such as crystal size, orientation and morphology, aspect ratio and even crystalline den- sity [9]. Therefore, it is essential to develop facile method to pre- pare high quality ZnO with uniform morphologies. Among different methods [1–11], hydrothermal technique which has several advantages over other growth processes such as the simplicity of operation, low energy consumption and poten- tial large-scale industrialization, has been successfully used in preparation of ZnO with different morphologies [12]. In the typical hydrothermal process, the three most popular means to control the morphology of ZnO are (i) Adding additives (such as polyelectro- lytes, polymers, anions, surfactants or amino acids). For example, Wang et al. synthesized ZnO particles with controllable shape through a cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal method [13]. Yogamalar et al. prepared various shapes of ZnO by a poly-ethylene glycol (PEG 4000)-assisted hydrothermal method [14]. (ii) Changing the alkaline environ- ments (such as monoethanolamine, diethanolamine). For example, Lu et al. used monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA) and NH 4 OH as the alkaline sources to obtain ZnO particles via the hydrothermal process [15]. Sun et al. selected TEA as alkaline sources to synthesize flower-like ZnO through the hydrothermal method [16]. (iii) Changing the pH of the reactants. For example, Jang et al. studied on the morphology change of ZnO nanostructures with different pH in the hydrothermal process [17]. Alver et al. investigated the influence of pH on the optical and morphological properties of ZnO fabricated by hydrothermal method [18]. The surfactants or alkaline used in the reaction is inconsistent with the concept of ‘‘green’’ chemistry that is utilization of non- toxic, environmentally benign, easily available, and relatively inex- pensive chemicals [12]. Among various additives, citric acid which could function as capping agent to manipulate the morphology of ZnO is a comparatively inexpensive and environmentally friendly reagent [2,19,20]. Herein, the sol–gel assisted hydrothermal meth- od was adopted to synthesis controllable morphologies of ZnO microstructures with citric acid as additive. The major differences 0921-8831/$ - see front matter Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2013.06.004 ⇑ Corresponding author. Tel.: +86 13782507167; fax: +86 3733326336. E-mail addresses: xhzhao79@yahoo.com.cn (X. Zhao), chemenglxd@126.com (X. Lou). Advanced Powder Technology xxx (2013) xxx–xxx Contents lists available at SciVerse ScienceDirect Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt Please cite this article in press as: X. Zhao et al., Sol–gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity, Advanced Powder Technology (2013), http://dx.doi.org/10.1016/j.apt.2013.06.004 from the available literatures are: (1) Citric acid was used as a kind of gelata in this work other than capping agent. (2) Twining-hexag- onal prism, twining-hexagonal disk, sphere and flower-like ZnO microstructures were obtained by only adjusting the pH of the solution. (3) To our best knowledge, flower-like ZnO composed of nanosheets has not been achieved by only using the raw materials of Zn(NO 3 ) 2 , citric acid and NaOH. In addition, photocatalytic deg- radation of dye pollutant (KGL, the chemical structure is presented in Fig. 1) was performed by using these different morphologies of ZnO as photocatalysts. The experiment results revealed that the flower-like ZnO exhibited enhanced photocatalytic activity which may be affected by the morphology, surface defects and surface area of the ZnO materials. 2. Experimental 2.1. Materials All the chemicals were used as received without any further purification. Distilled water was used in the reaction system as the solvent medium. 2.2. Preparation of different ZnO microstructures ZnO samples were synthesized by two main procedures: (1) sol–gel process, 3.756 g Zn(NO 3 ) 2 Á6H 2 O and 5.250 g citric acid (C 6 H 8 O 7 ) were dissolved in 100 mL distilled water and stirred at the temperature of 70 °C to form the sol, then the sol was put into the oven at 100 °C to form the gel. (2) Hydrothermal process, 1 M NaOH was directly dropped into the gel under constant stirring. The pH of the suspension was adjusted to 11, 12, 13 and 14, respec- tively. Then the resulting suspension (75 mL) was transferred into 100 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 17 h. The product was then collected and washed by distilled water thoroughly, then dried at 100 °C. The ZnO samples prepared at the pH of 11, 12, 13 and 14 were denoted as ZnO-11, ZnO-12, ZnO-13 and ZnO-14, respectively. In order to investigate the influence of the sol–gel process on the final morphology of the ZnO microstructure, a comparative experiment was carried out. 1 M NaOH was directly dropped into the mixed solution of 3.756 g Zn(NO 3 ) 2 Á6H 2 O and 5.250 g citric acid (C 6 H 8 O 7 ) to the pH of 14. With the same hydrothermal process, the product thus obtained was denoted as ZnO-s. 2.3. Characterization The as-synthesized samples were characterized by X-ray dif- fraction (XRD) (Bruker advance-D8 XRD with Cu K a radiation, k = 0.154178 nm, the accelerating voltage was set at 40 kV with a 100 mA flux). Microstructures and morphologies were investigated using JEOL JSM-6390LV scanning electronic microscopy (SEM). The photoluminescence (PL) spectra were measured by SHIMADZU RF- 5301PC fluorescence spectrophotometer. The UV–Vis absorption spectra were obtained on a UV-1700 PharmaSpec UV–Vis spectro- photometer. The UV–Vis diffuse reflection spectra were obtained on the Lambda 950 UV–Vis spectrophotometer. Surface areas of the samples were determined from nitrogen adsoption–desorption isotherms using the ASAP 2000 instrument and the Brunauer–Em- mett–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. 100 mg photocatalyst was added into 250 mL, 20 mg/L KGL solu- tion and stirred for 20 min to reach the absorption equilibrium, then exposed to UV light (300 W high pressure Hg lamp, the stron- gest emission at 365 nm). In order to expel the temperature influ- ence, water at room temperature was applied to absorb the heat generated from the UV light and the test tube containing KGL solu- tion was circled 10 cm away from lamp center. Every 20 min, a sample was collected by centrifugation and characterized with UV–Vis spectrophotometer (UV-5100, Shanghai Metash Instru- ments Co., Ltd., China.) to monitor the degradation of KGL mole- cules. The characteristic absorption peak of KGL at k = 608 nm was chosen to monitor the photocatalytic degradation process. For comparison, ZnO powder purchased from Tianli Chemical re- agent Co., Ltd. (99.0%, product number XK 13-201-00578) was used for the photocatalytic experiments. The photocatalytic degradation efficiency was calculated from the following expression (1): Degradationð%Þ¼ C 0 À C t C 0 Â 100% ¼ A 0 À At A 0 Â 100% ð1Þ where C 0 is the initial concentration of KGL, C t is the KGL concentra- tion at certain reaction time t. A 0 is the initial absorbance of KGL, A t is the KGL absorbance at certain reaction time t. 3. Results and discussion 3.1. XRD analysis XRD patterns of the as-syntheized samples are shown in Fig. 2. All the diffraction peaks of the samples can be well indexed as the Fig. 1. Chemical structure of Reactive Blue 14 (KGL). Fig. 2. XRD patterns of the as-synthesized ZnO samples. 2 X. Zhao et al. / Advanced Powder Technology xxx (2013) xxx–xxx Please cite this article in press as: X. Zhao et al., Sol–gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity, Advanced Powder Technology (2013), http://dx.doi.org/10.1016/j.apt.2013.06.004 wurtzite-structrued hexagonal ZnO (JCPDS card No. 36-1451). No significant characteristic peaks of impurities could be detected, which indicates that all the samples were pure ZnO. The high intensities of the XRD peaks suggest that the ZnO phase synthe- sized in this work is highly crystalline [21]. Calculated via the Sher- rer formula [10], the average crystalline size for ZnO-11, ZnO-12, ZnO-13 and ZnO-14 samples is about 37 nm, 45 nm, 33 nm and 36 nm, respectively. Compared with the standard card, the ratios of relative XRD intensities of (100)/(002) are remarkably different, which may attribute to different degrees of growth in preferred orientation [22]. 3.2. SEM analysis The morphology and the size of the samples were examined by SEM. Fig. 3 presents the SEM images of the as-synthesized ZnO samples. It could be seen that adjustment of the solution pH led to the formation of different morphologies. Hexagonal prism-like ZnO with twinning microstructures was obtained at the pH of 11 (Fig. 3a). Each prism is about 2 l m in length and 1 l m in diameter. Fig. 3b shows the morphology of the ZnO obtained at the pH of 12, disk-like ZnO with twinning microstructures was formed under this condition. Each disk is about 2 l m in length and 5 l m in diam- eter. Meanwhile, there were some amorphous structures scattered among these disks. Comparing Fig. 3a with b, the length of the microstructures hardly changes but the diameter increases with the addition of NaOH under the same condition. With the further increase of the solution pH to 13, sphere-like ZnO with 3–5 l m in diameter were obtained (Fig. 3c). It should be noted that there exists sphere-like ZnO with twinning microstructures (arrow pointed in Fig. 3c). Fig. 3d is the image of ZnO obtained at the pH of 14. It reveals that the morphology of ZnO product is well-defined flower-like three-dimension (3D) microstructures with the diame- ter of 3–5 l m, assembled by many nanosheets as ‘‘petals’’. These nanosheets are about 30 nm in thickness, and they alternately con- nect with each other to form the flower. 3.3. Possible growth mechanisms of different ZnO microstructures The schematic illustration of the formation process is presented in Fig. 4 and the possible formation mechanisms of different mor- phologies of ZnO were proposed as follows. Firstly, the sol–gel pro- cess surely has impact on the final morphology of the ZnO, which could be confirmed from the SEM result (Fig. 3e). In the absence of the sol–gel process, the ZnO product (ZnO-s) had no specific mor- phology. It is supposed that Zn(II) citric acid chelate complexes are formed during the gelation of sol [20], and then the gel was dis- solved by adding certain amount of NaOH. In this process, part of OH À ions in the solution might neutralize H + ions which come from citric acid, part of OH À ions might react with Zn(II) citric acid che- late complexes to form [Zn(OH) 4 ] 2À complexes which will decom- pose into ZnO nuclei in the hydrothermal process (Eqs. (2)–(4)) [23]. The rest part of OH À will be left in the solution, which will af- fect the morphology of ZnO in the hydrothermal process. ZnðOHÞ 2 þ 2H 2 O ! ZnðOHÞ 2À 4 þ 2H þ ð2Þ Fig. 3. SEM images of the as-synthesized ZnO samples: (a) ZnO-11; (b) ZnO-12; (c) ZnO-13; (d) ZnO-14. (e) ZnO-s. X. Zhao et al. / Advanced Powder Technology xxx (2013) xxx–xxx 3 Please cite this article in press as: X. Zhao et al., Sol–gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity, Advanced Powder Technology (2013), http://dx.doi.org/10.1016/j.apt.2013.06.004 Zn 2þ þ 4OH À ! ZnðOHÞ 2À 4 ð3Þ ZnðOHÞ 2À 4 ! ZnO þ H 2 O þ 2OH À ð4Þ Secondly, the OH À left in the solution must have played a key role in the ZnO morphology formation process considering that the pH of the solution was the only variable in this work. It is known that ZnO is a polar crystal with a positive polar (0 001) plane rich in Zn 2+ cations and a negative polar ð000  1Þ plane rich in O 2À anions [24]. Under pH 11 hydrothermal circumstances (the molar ratio of Zn 2+ /OH À is about 1:6), [Zn(OH) 4 ] 2À complexes preferably adsorb on the positively charged Zn-(0001) plane of ZnO nuclei, which leads to form ZnO twining-hexagonal prism elongated along the c-axis direction due to the intrinsic anisotropy in its growth rate v with m [000 1] >> m ½01  10 >> m ½000  1 [25]. With the pH increasing to 12 (the molar ratio of Zn 2+ /OH À is about 1:6.5), the concentration of OH À presented in the aqueous solution increased, hence the absorption of OH À on the positively charged Zn-(00 0 1) plane would compete with that of the [Zn(OH) 4 ] 2À complexes [26]. The OH À ions could stabilize the surface charge and the structure of Zn-(0001) surfaces, leading to the growth rate along the c-axis direction being suppressed to some extent, thus twining-hexagonal disks were formed (Fig. 2b). When pH = 13 (the molar ratio of Zn 2+ / OH À is about 1:7.5), based on surface energy minimization, the nanoparticles would rearrange themselves on the surface of ZnO nuclei for lowering the surface energy, so the sphere-like (Fig. 2c) ZnO nanostructure was formed [27]. But for pH 14 (the molar ratio of Zn 2+ /OH À is about 1:12), the concentration of OH À presented in the aqueous solution had a significant increase, following the de- crease in the concentration of [Zn(OH) 4 ] 2À due to the initial fast nucleation of ZnO. Hence, the absorption of OH À on the positively charged Zn-(0001) plane would dominate in the competition with [Zn(OH) 4 ] 2À complexes. Based on the above discussion, the growth rate along the c-axis direction would be descended, leading to the formation of nanosheets that are preferentially grown along the [000 1] and ½01  10 directions within the f2  1  10g plane. Subse- quently, more and more nanosheets with a f2  1  10g-planar surface interlaced and overlapped with each other into a multilayer and network structure, and the flower-like ZnO nanostructures were shaped (Fig. 3d) [26]. 3.4. UV–Vis absorption analysis The UV–Vis absorption spectra of the as-synthesized samples were carried out for further analysis of the optical absorption prop- erties of the materials. Fig. 5 illustrates the UV–Vis absorption spectra of the samples at room temperature. The absorption spec- tra of these samples have a narrow absorption peak located at about 370 nm, which is the characteristic of wide-band gap ZnO [28]. It is significant that except characteristic wide-band gap ZnO absorption band, no other band is observed in the obtained UV–Vis spectra, which confirms that the synthesized samples are pure ZnO [29] and also reveals the good optical properties of the as-synthesized ZnO [30]. Fig. 4. Schematic illustration of the formation process of the as-synthesized ZnO samples. Fig. 5. UV–Vis absorption spectra of the as-synthesized ZnO samples. 4 X. Zhao et al. / Advanced Powder Technology xxx (2013) xxx–xxx Please cite this article in press as: X. Zhao et al., Sol–gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity, Advanced Powder Technology (2013), http://dx.doi.org/10.1016/j.apt.2013.06.004 3.5. Photocatalytic activity 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 degradation of KGL. Because the yield of ZnO-11 was very small, no other studies were taken on it. The photocatalytic activity of the commercial ZnO powders was also tested for comparison. Fig. 6a shows the degradation rate of KGL as a function of irradia- tion time in presence of different samples. In the absence of light or catalyst, the concentration of KGL had no obvious change for 120 min, indicating that both light and catalyst were necessary for the effective photodegradation of KGL dye [31]. From Fig. 6a, it is clear that the morphology of the ZnO sample has great influ- ence on its photocatalytic activity. After irradiation for 120 min, the degradation ratios of KGL were about 27.8% for ZnO-13, while nearly 100% for ZnO-12, ZnO-14 and commercial ZnO. It should be noted that after irradiation for 40 min, only the degradation ratio of ZnO-14 was nearly 100%. Obviously, ZnO-14 showed the best photocatalytic activity. Fig. 6b shows the UV–Vis absorption spec- tra of KGL for ZnO-14 sample. It is obvious that the absorbance for the maximum peak at 608 nm decreases, suggesting the occur- rence of the destruction of KGL and the formation of some interme- diates. When the solution was irradiated for 40 min, the absorption peaks in the curve almost disappeared (the KGL solution decolor- ized completely). The reason may be explained as follows, when semiconductor materials are irradiated by light with energy higher or equal to the band gap (E g ), an electron e À cb ÀÁ in the valence band (VB) can be excited to the conduction band (CB) with the simultaneous gen- eration of a hole h þ v b ÀÁ in the VB. Excited state e À cb and h þ vb can recombine and get trapped in metastable surface states, or react with electron donors and electron acceptors adsorbed on the semi- conductor surface. In other words, the photoelectron is easily trapped by electron acceptors like adsorbed O 2 , whereas the photo- induced holes can be easily trapped by electronic donors, such as OH À or organic pollutants, to further oxidize organic dye [7]. There are many factors that could influence the photocatalytic activity of the ZnO, such as morphology, surface defects, surface areas, and so on [32]. Firstly, as reported in the literature, photocatalysts with higher surface energy show better photocatalytic performance [33]. Among different crystal planes of ZnO, the (0 0 0 1) plane exhibits the highest surface energy [34]. Therefore, ZnO samples with larger proportion of (00 0 1) plane would improve the photocatalytic activities. From morphology analysis in 3.3 part, it is known that ZnO-14 in the shape of nanosheets composed flower-like and ZnO-12 in the shape of twining-hexagonal disk both have larger area of (0001) plane than ZnO-13 in the shape of sphere, so they exhibit enhanced photocatalytic activities than ZnO-13. Secondly, oxygen defects may be considered to be the active sites of the ZnO photocatalyst [35]. Since proper amount of oxygen vacancies can entrap electrons from semiconductor, the holes can diffuse to the surface of the semiconductor and cause oxidation of the organic dye. Therefore, surface oxygen detects of high den- sity benefit the efficient separation of electron–hole pairs, mini- mize the radiative recombination of electron and hole and increase the lifetime of the charge carriers, hence improve the pho- tocatalytic activity [36]. It has been reported that stronger exci- tonic PL intensity suggests there exists more surface defects [37]. Fig. 7 is the photoluminescence spectrum of the as-synthesized ZnO samples measured at room temperature with the excitation wavelength of 350 nm. It can be seen that there are two main emission peaks for all three samples, one is the strong peaks at $388 nm which correspond to the near band-edge emission (NBE) [38], the other is the wide band emission extending from 400 to 650 nm which covers the blue–yellow regions. The emission in the visible-light region is attributed to ZnO surface detects, in Fig. 6. (a) Photocatalytic degradation of KGL with different ZnO samples under different conditions. (b) Time-dependent absorption spectra of KGL solution in the presence of ZnO-14 sample after UV irradiation. Fig. 7. PL spectra of the as-synthesized ZnO samples. X. Zhao et al. / Advanced Powder Technology xxx (2013) xxx–xxx 5 Please cite this article in press as: X. Zhao et al., Sol–gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity, Advanced Powder Technology (2013), http://dx.doi.org/10.1016/j.apt.2013.06.004 which oxygen vacancies are the most suggested defects [39].Asis shown in Fig. 7, the PL intensity varies in the following order: ZnO- 14 > ZnO-12 > ZnO-13, suggesting that ZnO-14 possesses surface defects of the highest density, which is responsible for the best photocatalytic activity, followed by ZnO-12, and then ZnO-13. This result is in accordance with the photocatalytic activities. Thirdly, one of the other key factors controlling the photocata- lytic activity of the ZnO samples is the optical absorption and the migration of the light-induced electrons and holes [40]. UV–Vis dif- fuse reflectance spectroscopy was used to characterize the optical absorption properties of the ZnO samples, and the spectrum is dis- played in Fig. 8. It demonstrates that all the defined excitonic absorption peaks could correspond to the wurtzite hexagonal ZnO. It has been recognized that the E g value occurred from an electronic transition between the filled valence bands to the empty conduction bands [11]. According to the principle of the photocat- alytic reaction, the smaller the band gap of the photocatalyst, the better photogenerated of the electron–hole pairs, and the stronger of the photocatalytic activity [41]. The band gap energies (E g ) of the as-synthesized ZnO samples could be obtained from the plots of (ahv) 2 versus photon energy (hv), and the values estimated from the intercept of the tangents to the plots were shown in Table 1. The samller E g value of ZnO-14 indicates that it could be better photogenerated than that of ZnO-12 or ZnO-13. In addition, surface area would also affect the photocatalytic activity of the catalyst [34]. The surface areas of the as-synthesized ZnO samples are presented in Table 1. From Table 1, one can see that ZnO-14 possesses much bigger surface area than ZnO-12 or ZnO-13. Furthermore, the nanosheet-composed micro-flowers shape of ZnO-14 could effectively prevent aggregation, thus main- tain larger active surface area, offer more opportunity to the diffu- sion and mass transportation of KGL molecules and hydroxyl radicals (OH Å ) in the photocatalytic degradation process than those of ZnO-12 and ZnO-13 [25]. Therefore, attributing to the largest area of (0001) plane, nar- rowest band gap, highest density of surface defects and the largest surface area of ZnO-14, it exhibits the highest photocatalytic activ- ity among all the three samples. As for ZnO-12 and ZnO-13, although the surface area of ZnO-12 is smaller than that of ZnO- 13, it exhibits more surface defects, larger areas of (0001) plane and narrower band gap. It can be concluded that in the photocata- lytic reactions, the influence of the integrated factors ((000 1) plane, E g , surface defects) may exceed the impact factor of surface area, leading to the better photocatalytic activity of ZnO-12 than that of ZnO-13. 4. Conclusion In summary, we have synthesized twining-hexagonal prism, twining-hexagonal disk, sphere, flower-like ZnO microstructures using sol–gel assisted hydrothermal method by just controlling the pH of the hydrothermal reaction mixture. Possible growth mechanisms have been proposed. The nanosheet composed flow- er-like ZnO exhibits the best photocatalytic activity, which could be attributed to the morphology, surface defects, band gap and sur- face areas. The enhanced performance of the flower-like ZnO indi- cates that it can be used as a promising photocatalyst for the practical application in photocatalytic degradation of organic dyes. 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Zhao et al., Sol–gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity, Advanced Powder Technology (2013), http://dx.doi.org/10.1016/j.apt.2013.06.004 . xxxx Keywords: Sol–gel assisted hydrothermal method pH Morphology Photocatalysis abstract ZnO microstructures of different morphologies were synthesized by the sol–gel assisted hydrothermal method. Original Research Paper Sol–gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity Xiaohua Zhao, Meng Li, Xiangdong Lou ⇑ School of Chemistry and. at 100 °C. The ZnO samples prepared at the pH of 11, 12, 13 and 14 were denoted as ZnO- 11, ZnO- 12, ZnO- 13 and ZnO- 14, respectively. In order to investigate the influence of the sol–gel process