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Sol−gel-based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic applications
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 5507–5514 www.elsevier.com/locate/ceramint
Sol gel-based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic applications Xiaohua Zhaoa,b, Feijian Louc, Meng Lib, Xiangdong Loub,n, Zhenzhen Lib, Jianguo Zhoua,n b
a School of Environment, Henan Normal University, Xinxiang 453007, PR PR China 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
Received 13 August 2013; received in revised form 29 October 2013; accepted 29 October 2013 Available online 6 November 2013
Abstract Self-assembled 3D flower-like ZnO microstructures composed of nanosheets have been prepared on a large scale through a solgel-assisted hydrothermal method using Zn(NO3)2 6H2O, 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 flower-like ZnO composed of nanosheets could be obtained over a relatively broad temperature range (90150 1C) 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 1C 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. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: D. ZnO; Nanosheets; Self-assembled 3D flower-like structures; 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 n
Corresponding authors. Tel.: þ86 13 623731736; fax: þ86 37 33326336. E-mail addresses: [email protected] (X. Lou), [email protected] (J. Zhou).
morphology, aspect ratio, and even crystalline density [11]. Therefore, study of the microstructure of ZnO is highly relevant to research and applications in photocatalysis. 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
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.10.140
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[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, selfassembled 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 microstructures, 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 1C). Moreover, the corresponding photocatalytic activities have also been studied. The experimental results have indicated that the selfassembled flower-like ZnO composed of nanosheets could be obtained from 90 to 150 1C after 17 h of hydrothermal reaction, or at 120 1C after 4 h. The ZnO synthesized at 120 1C 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(NO3)2 6H2O (3.756 g) and citric acid (C6H8O7; 5.250 g) to distilled water (100 mL) and stirring at 70 1C. The sol was then placed in an oven at 100 1C 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 1C for 17 h. Finally, the white precipitate was collected, washed thoroughly with distilled water, and then dried at 100 1C to obtain the final sample.
(2) In order to reveal the growth mechanism of the 3D flowerlike ZnO, experiments were conducted for different hydrothermal treatment times (0, 4, 8, and 12 h) while the other conditions were 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 1C) 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-Kα radiation, λ ¼ 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-6701 F, 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 were collected at intervals of 20 min, centrifuged, and the supernatants were characterized by UV/Vis spectrophotometry (UV5100, Shanghai Metash Instruments Co. Ltd., China) to monitor the degradation of the KGL. The characteristic absorption peak of KGL at λ¼ 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): C0 Ct A0 At Degradation ð%Þ ¼ 100% 100% ð1Þ C0 A0 where C0 and A0 are the initial concentration and absorbance of KGL, and Ct and At are the concentration and absorbance of KGL at a certain reaction time t.
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3. Results and discussion 3.1. Structure and morphology Fig. 1a c shows FESEM images of the ZnO microstructures synthesized at 120 1C 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 μm. 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 μm 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 wellresolved 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
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with an average diameter of 2 μm, 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 m2/g when the hydrothermal time was increased from 0 h to 17 h. XRD patterns of the samples 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 þ
Fig. 1. FESEM (a–c), TEM and SAED pattern (inset) (d), HRTEM (e) and XRD (f) patterns of the ZnO sample synthesized at 120 1C for 17 h hydrothermal treatment.
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Fig. 2. SEM images of the time-dependent evolution in the formation of 3D flower-like ZnO synthesized at 120 1C 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.
Table 1 The surface area of ZnO samples synthesized at different hydrothermal time. Sample
0h
4h
8h
12 h
17 h
BET(m2/g)
2.25
8.74
9.05
10.5
11.05
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 þ 2OH -Zn(OH)24
(2)
Zn2 þ þ 4OH -Zn(OH)24
(3)
Zn(OH)24 -ZnO þ H2O þ 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 Zn2 þ cations and a negative polar (0001) plane rich in O2 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 ν[0001]⪢ν[0110]⪢ν [0001] [42]. However, in the present case, because the molar ratio of Zn2 þ 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)24 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 with Zn(OH)24 . 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 [0110] direction, which leads to the formation of ZnO nanosheets with a {21 10}-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 {21 10}-planar surface become interlaced and overlapped with each other to form a multilayer network structure, and thereby the flower-like ZnO nanostructures are shaped (Fig. 2c e). 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. 4a, 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 (ecb ) in the valence band (VB) can be excited to the conduction band (CB) with the simultaneous generation of a hole (hvb þ ) in the VB. Excited-state ecb and hvb þ can recombine and become trapped in metastable surface states, or react with electron donors and electron acceptors adsorbed on
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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 ◦C for different hydrothermal treatment times and (b) PL spectra of the as-synthesized ZnO samples.
the semiconductor surface. In other words, the photoelectron is easily trapped by electron acceptors such as adsorbed O2, whereas the photoinduced holes can be easily trapped by electron donors such as OH or organic pollutants, to further oxidize organic dyes [44]. There are many factors that may influence the photocatalytic activity of ZnO, such as morphology, surface area, surface defects, and so on [45]. On the one hand, the larger surface area (Table 1) in the ZnO (120 1C, 17 h) sample can provide more active sites for the adsorption of KGL, and then facilitate the diffusion and mass transportation of KGL molecules and hydroxyl radicals during the photochemical reaction [14]. On the other hand, oxygen defects may be considered to be the active sites of the ZnO photocatalyst [46]. Since an appropriate 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 improving the photocatalytic activity [47]. To study the surface defects of the as-synthesized ZnO samples, room temperature photoluminescence (PL) spectra were measured using Xe light (350 nm) as the excitation source, and representative spectra are shown in Fig. 4b. Two main emission peaks are seen for all three samples, a strong peak at around 388 nm, corresponding to the near band-edge emission (NBE) [48], and a broad band emission extending from 400 to 650 nm, covering the blue to yellow region. The emission in the visible-light region is attributed to ZnO surface defects, among which oxygen vacancies are likely to be the most prominent [49]. The PL intensity varied in the following order: 17 h 412 h 4 8 h4 4 h4 0 h, suggesting that ZnO (17 h) possessed the highest density of surface defects, and this may have been responsible for its superior photocatalytic activity.
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Fig. 5. SEM images of the ZnO samples prepared at 90 ◦C (a), 150 1C (b) for 17 h hydrothermal treatment, and XRD patterns (c). Insets in (a,b) are the highmagnification images of the corresponding ZnO samples.
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 1C and 150 1C for 17 h. The morphologies of the ZnO samples prepared at different hydrothermal treatment temperatures are shown in Fig. 5a and b. It can be observed that the sample prepared at 90 1C (Fig. 5a) showed a flowerlike morphology with a diameter of 1 2 μm, but the edges of the nanosheets were not obvious. When the temperature was increased to 150 1C (Fig. 5b), the diameter of the sample reached 3 4 μm, and the nanosheets accumulated more densely and became thicker. Comparing Fig. 5a,b with Fig. 2e, it can be concluded that flower-like ZnO can be obtained over a relatively wide hydrothermal temperature range (90 150 1C), but that the interspace of nanosheets in the ZnO sample synthesized at 120 1C is larger than that in the samples synthesized at either lower (90 1C) or higher hydrothermal temperature (150 1C). This may be because the growth rate of nanosheets is slower at low reaction temperature (90 1C) [50], and faster at high reaction temperature (150 1C); consequently, in the same hydrothermal time, the nanosheets in the flower-like microstructures are either not fully formed or accumulate more densely. A larger interspace between nanosheets means greater surface area, which will influence the photoactivity of the sample to some extent. Comparison of the surface areas listed in Table 2 shows that the ZnO sample synthesized at 120 1C indeed had the largest surface area. The XRD pattern of this sample (Fig. 5c) could also be well indexed to hexagonal wurtzite ZnO (JCPDS Card No. 36-1451).
Table 2 The surface area of ZnO samples synthesized at different hydrothermal temperatures. Sample
90 1C
120 1C
150 1C
BET (m2/g)
10.98
11.05
7.16
sample, and almost 100% for ZnO (120 1C) and ZnO (90 1C) samples and a commercial ZnO sample. It should be noted that after irradiation for 40 min, only for the ZnO (120 1C) sample did the degree of degradation approach 100%, and accordingly the color of the KGL had disappeared (Fig. 6b). The ZnO (120 1C) sample clearly showed the best photocatalytic activity. The reasons can be explained as follows. Firstly, as mentioned above, the ZnO (120 1C) sample had a larger surface area and a greater interspace between the nanosheets than the other samples. A greater interspace between the nanosheets can effectively prevent aggregation during the photodegradation, and a larger surface area can provide more active sites for the adsorption of KGL. Hence, the photocatalytic activity of the ZnO (120 1C) sample was enhanced. Secondly, as shown in Fig. 6c, the PL intensity varied in the following order: 120 1C4 90 1C 4 150 1C, meaning that the ZnO (120 1C) sample separated electrons and holes more efficiently than the other samples. Hence, the ZnO (120 1C) sample showed the best photocatalytic activity, followed by the ZnO (90 1C) sample and the ZnO (150 1C) sample, respectively.
4. Conclusions 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 over 120 min, indicating that both light and catalyst were 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 1C)
In summary, 3D flower-like ZnO composed of nanosheets has been successfully synthesized by a sol gel-based hydrothermal method over a relatively broad temperature range (90 150 1C) after 17 h of hydrothermal reaction or at 120 1C after 4 h. A possible formation mechanism has been proposed based on the experimental results. Compared with the other samples, the ZnO prepared at 120 1C for 17 h exhibited enhanced photocatalytic activity, indicating that it may potentially be used as a promising photocatalyst for practical application in the photocatalytic degradation of organic dyes.
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Fig. 6. (a) Photocatalytic 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 1C, 17 h) sample and (c) PL spectra of the ZnO samples synthesized at different hydrothermal temperatures.
Acknowledgment This project is supported by the National Natural Science Foundation of China (Grant No. 21073055).
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 5507–5514 www.elsevier.com/locate/ceramint
Sol gel-based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic applications Xiaohua Zhaoa,b, Feijian Louc, Meng Lib, Xiangdong Loub,n, Zhenzhen Lib, Jianguo Zhoua,n b
a School of Environment, Henan Normal University, Xinxiang 453007, PR PR China 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
Received 13 August 2013; received in revised form 29 October 2013; accepted 29 October 2013 Available online 6 November 2013
Abstract Self-assembled 3D flower-like ZnO microstructures composed of nanosheets have been prepared on a large scale through a solgel-assisted hydrothermal method using Zn(NO3)2 6H2O, 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 flower-like ZnO composed of nanosheets could be obtained over a relatively broad temperature range (90150 1C) 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 1C 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. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: D. ZnO; Nanosheets; Self-assembled 3D flower-like structures; 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 n
Corresponding authors. Tel.: þ86 13 623731736; fax: þ86 37 33326336. E-mail addresses: [email protected] (X. Lou), [email protected] (J. Zhou).
morphology, aspect ratio, and even crystalline density [11]. Therefore, study of the microstructure of ZnO is highly relevant to research and applications in photocatalysis. 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
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.10.140
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[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, selfassembled 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 microstructures, 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 1C). Moreover, the corresponding photocatalytic activities have also been studied. The experimental results have indicated that the selfassembled flower-like ZnO composed of nanosheets could be obtained from 90 to 150 1C after 17 h of hydrothermal reaction, or at 120 1C after 4 h. The ZnO synthesized at 120 1C 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(NO3)2 6H2O (3.756 g) and citric acid (C6H8O7; 5.250 g) to distilled water (100 mL) and stirring at 70 1C. The sol was then placed in an oven at 100 1C 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 1C for 17 h. Finally, the white precipitate was collected, washed thoroughly with distilled water, and then dried at 100 1C to obtain the final sample.
(2) In order to reveal the growth mechanism of the 3D flowerlike ZnO, experiments were conducted for different hydrothermal treatment times (0, 4, 8, and 12 h) while the other conditions were 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 1C) 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-Kα radiation, λ ¼ 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-6701 F, 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 were collected at intervals of 20 min, centrifuged, and the supernatants were characterized by UV/Vis spectrophotometry (UV5100, Shanghai Metash Instruments Co. Ltd., China) to monitor the degradation of the KGL. The characteristic absorption peak of KGL at λ¼ 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): C0 Ct A0 At Degradation ð%Þ ¼ 100% 100% ð1Þ C0 A0 where C0 and A0 are the initial concentration and absorbance of KGL, and Ct and At are the concentration and absorbance of KGL at a certain reaction time t.
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3. Results and discussion 3.1. Structure and morphology Fig. 1a c shows FESEM images of the ZnO microstructures synthesized at 120 1C 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 μm. 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 μm 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 wellresolved 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
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with an average diameter of 2 μm, 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 m2/g when the hydrothermal time was increased from 0 h to 17 h. XRD patterns of the samples 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 þ
Fig. 1. FESEM (a–c), TEM and SAED pattern (inset) (d), HRTEM (e) and XRD (f) patterns of the ZnO sample synthesized at 120 1C for 17 h hydrothermal treatment.
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Fig. 2. SEM images of the time-dependent evolution in the formation of 3D flower-like ZnO synthesized at 120 1C 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.
Table 1 The surface area of ZnO samples synthesized at different hydrothermal time. Sample
0h
4h
8h
12 h
17 h
BET(m2/g)
2.25
8.74
9.05
10.5
11.05
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 þ 2OH -Zn(OH)24
(2)
Zn2 þ þ 4OH -Zn(OH)24
(3)
Zn(OH)24 -ZnO þ H2O þ 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 Zn2 þ cations and a negative polar (0001) plane rich in O2 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 ν[0001]⪢ν[0110]⪢ν [0001] [42]. However, in the present case, because the molar ratio of Zn2 þ 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)24 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 with Zn(OH)24 . 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 [0110] direction, which leads to the formation of ZnO nanosheets with a {21 10}-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 {21 10}-planar surface become interlaced and overlapped with each other to form a multilayer network structure, and thereby the flower-like ZnO nanostructures are shaped (Fig. 2c e). 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. 4a, 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 (ecb ) in the valence band (VB) can be excited to the conduction band (CB) with the simultaneous generation of a hole (hvb þ ) in the VB. Excited-state ecb and hvb þ can recombine and become trapped in metastable surface states, or react with electron donors and electron acceptors adsorbed on
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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 ◦C for different hydrothermal treatment times and (b) PL spectra of the as-synthesized ZnO samples.
the semiconductor surface. In other words, the photoelectron is easily trapped by electron acceptors such as adsorbed O2, whereas the photoinduced holes can be easily trapped by electron donors such as OH or organic pollutants, to further oxidize organic dyes [44]. There are many factors that may influence the photocatalytic activity of ZnO, such as morphology, surface area, surface defects, and so on [45]. On the one hand, the larger surface area (Table 1) in the ZnO (120 1C, 17 h) sample can provide more active sites for the adsorption of KGL, and then facilitate the diffusion and mass transportation of KGL molecules and hydroxyl radicals during the photochemical reaction [14]. On the other hand, oxygen defects may be considered to be the active sites of the ZnO photocatalyst [46]. Since an appropriate 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 improving the photocatalytic activity [47]. To study the surface defects of the as-synthesized ZnO samples, room temperature photoluminescence (PL) spectra were measured using Xe light (350 nm) as the excitation source, and representative spectra are shown in Fig. 4b. Two main emission peaks are seen for all three samples, a strong peak at around 388 nm, corresponding to the near band-edge emission (NBE) [48], and a broad band emission extending from 400 to 650 nm, covering the blue to yellow region. The emission in the visible-light region is attributed to ZnO surface defects, among which oxygen vacancies are likely to be the most prominent [49]. The PL intensity varied in the following order: 17 h 412 h 4 8 h4 4 h4 0 h, suggesting that ZnO (17 h) possessed the highest density of surface defects, and this may have been responsible for its superior photocatalytic activity.
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Fig. 5. SEM images of the ZnO samples prepared at 90 ◦C (a), 150 1C (b) for 17 h hydrothermal treatment, and XRD patterns (c). Insets in (a,b) are the highmagnification images of the corresponding ZnO samples.
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 1C and 150 1C for 17 h. The morphologies of the ZnO samples prepared at different hydrothermal treatment temperatures are shown in Fig. 5a and b. It can be observed that the sample prepared at 90 1C (Fig. 5a) showed a flowerlike morphology with a diameter of 1 2 μm, but the edges of the nanosheets were not obvious. When the temperature was increased to 150 1C (Fig. 5b), the diameter of the sample reached 3 4 μm, and the nanosheets accumulated more densely and became thicker. Comparing Fig. 5a,b with Fig. 2e, it can be concluded that flower-like ZnO can be obtained over a relatively wide hydrothermal temperature range (90 150 1C), but that the interspace of nanosheets in the ZnO sample synthesized at 120 1C is larger than that in the samples synthesized at either lower (90 1C) or higher hydrothermal temperature (150 1C). This may be because the growth rate of nanosheets is slower at low reaction temperature (90 1C) [50], and faster at high reaction temperature (150 1C); consequently, in the same hydrothermal time, the nanosheets in the flower-like microstructures are either not fully formed or accumulate more densely. A larger interspace between nanosheets means greater surface area, which will influence the photoactivity of the sample to some extent. Comparison of the surface areas listed in Table 2 shows that the ZnO sample synthesized at 120 1C indeed had the largest surface area. The XRD pattern of this sample (Fig. 5c) could also be well indexed to hexagonal wurtzite ZnO (JCPDS Card No. 36-1451).
Table 2 The surface area of ZnO samples synthesized at different hydrothermal temperatures. Sample
90 1C
120 1C
150 1C
BET (m2/g)
10.98
11.05
7.16
sample, and almost 100% for ZnO (120 1C) and ZnO (90 1C) samples and a commercial ZnO sample. It should be noted that after irradiation for 40 min, only for the ZnO (120 1C) sample did the degree of degradation approach 100%, and accordingly the color of the KGL had disappeared (Fig. 6b). The ZnO (120 1C) sample clearly showed the best photocatalytic activity. The reasons can be explained as follows. Firstly, as mentioned above, the ZnO (120 1C) sample had a larger surface area and a greater interspace between the nanosheets than the other samples. A greater interspace between the nanosheets can effectively prevent aggregation during the photodegradation, and a larger surface area can provide more active sites for the adsorption of KGL. Hence, the photocatalytic activity of the ZnO (120 1C) sample was enhanced. Secondly, as shown in Fig. 6c, the PL intensity varied in the following order: 120 1C4 90 1C 4 150 1C, meaning that the ZnO (120 1C) sample separated electrons and holes more efficiently than the other samples. Hence, the ZnO (120 1C) sample showed the best photocatalytic activity, followed by the ZnO (90 1C) sample and the ZnO (150 1C) sample, respectively.
4. Conclusions 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 over 120 min, indicating that both light and catalyst were 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 1C)
In summary, 3D flower-like ZnO composed of nanosheets has been successfully synthesized by a sol gel-based hydrothermal method over a relatively broad temperature range (90 150 1C) after 17 h of hydrothermal reaction or at 120 1C after 4 h. A possible formation mechanism has been proposed based on the experimental results. Compared with the other samples, the ZnO prepared at 120 1C for 17 h exhibited enhanced photocatalytic activity, indicating that it may potentially be used as a promising photocatalyst for practical application in the photocatalytic degradation of organic dyes.
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Fig. 6. (a) Photocatalytic 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 1C, 17 h) sample and (c) PL spectra of the ZnO samples synthesized at different hydrothermal temperatures.
Acknowledgment This project is supported by the National Natural Science Foundation of China (Grant No. 21073055).
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