photocatalytic properties of hierarchical zno flowers synthesized by a

Applied Surface Science 259 (2012) 557– 561 Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Photocatalytic properties of hierarchical ZnO flowers synthesized by a sucrose-assisted hydrothermal method Wei Lv a , Bo Wei b , Lingling Xu a,b,∗ , Yan Zhao c,∗∗ , Hong Gao a , Jia Liu a a Key Laboratory of Photonic and Electric Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, PR China b Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150080, PR China c Department of Physics, Northeast Forestry University, Harbin 150040, PR China a r t i c l e i n f o Article history: Received 6 November 2011 Received in revised form 4 April 2012 Accepted 5 April 2012 Available online 24 July 2012 Keywords: ZnO flowers Photocatalytic properties Hydrothermal method Sucrose a b s t r a c t In this work, hierarchical ZnO flowers were synthesized via a sucrose-assisted urea hydrother- mal method. The thermogravimetric analysis/differential thermal analysis (TGA–DTA) and Fourier transform infrared spectra (FTIR) showed that sucrose acted as a complexing agent in the synthe- sis process and assisted combustion during annealing. Photocatalytic activity was evaluated using the degradation of organic dye methyl orange. The sucrose added ZnO flowers showed improved activity, which was mainly attributed to the better crystallinity as confirmed by X-ray photoelec- tron spectroscopy (XPS) analysis. The effect of sucrose amount on photocatalytic activity was also studied. © 2012 Elsevier B.V. All rights reserved. 1. Introduction In the last decade, zinc oxide (ZnO) nanostructures have aroused tremendous attention due to its distinguished performance in piezoelectric systems, optoelectronics, photovoltaic energy con- version, photocatalytic decomposition of organic pollutants and as chemical sensing elements. Also, it has been found that those prop- erties can be improved with special morphologies, shapes, sizes and crystallinity of ZnO nanostructures [1–6]. Thus, the designed and controllable fabrications of ZnO with specific morphologies and structures have been explored to gain superior properties in recent years [7–10]. Three-dimensional hierarchical ZnO exhibited excellent optical and catalytic properties. Primary routes for three-dimensional hier- archical ZnO synthesis include vapor–liquid–solid (VLS) growth at relatively high temperature, electrochemical and solution-based methods for self-assembly of hierarchical ZnO [11,12]. Among these synthesis methods, the hydrothermal method is a simple, facile and ∗ Corresponding author at: Key Laboratory of Semiconducter Nanocomposite Materials, Ministry of Education Department of Physics, School of Physics and Elec- tronic Engineering, Harbin Normal University, Harbin 150025, PR China. Tel.: +86 451 88060526; fax: +86 451 88060629. ∗∗ Corresponding author. Tel.: +86 451 88060526; fax: +86 451 88060629. E-mail addresses: xulingling hit@163.com (L. Xu), zhaoyan516@126.com (Y. Zhao). controllable way to obtain large yields with unique morphology. ZnO can be used as a kind of photocatalyst, which decomposes organic pollutants with ultra-violet light excitation [2,4,13]. The hierarchical structures increased the efficiency of optical absorp- tion and enhanced the photocatalytic activity. To synthesize the hierarchical mesoporous ZnO, the multi-layered basic zinc car- bonate (LBZC) was reported to be used as a precursor in the urea precipitation or hydrothermal method [14,19]. Several reports about the fabrication of LBZC have concerned about the effects of surfactants. In the past decade, kinds of morphologies of ZnO can be synthesized with different surfactant, like cetyltricetyl- trimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), polyethylene glycol (PEG) and so on [21,4,22,23]. Usu- ally, the environmentally-friendly, low-cost and easily-obtainable sucrose is used as fuel in the combustion synthesis procedure for ceramic material fabrication [5,6,15–17]. Also, it is reported that sucrose can play the role of chealting agent after the hydroly- sation in acid solution. In this work, sucrose was introduced in the urea hydrothermal procedure to fabricate hierarchical ZnO flowers as a chelating agent and fuel. The annealing process of sucrose added precursor was performed and more heat and gases were released, resulting in the good crystallization and large reaction areas in ZnO flowers. The photocatalytic proper- ties of ZnO flowers dependent on the sucrose content were also discussed. 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.182 558 W. Lv et al. / Applied Surface Science 259 (2012) 557– 561 2. Experimental 2.1. Preparation of ZnO flowers All the chemicals were analytical grade reagents and were used without further purification. Firstly, 0.002 M zinc nitrate solution was prepared by dissolving proper Zn(NO 3 ) 2 in deionized water. In a typical procedure, 0.006 mol urea powder was added into 20 mL 0.002 M Zn(NO 3 ) 2 solution with variable quantity of sucrose. After a continuous stirring for 15 min, the mixed solution was transferred into a 50 mL Teflon bottle held in a stainless steel autoclave, which was kept at 90 ◦ C for 2 h. The white precursor was washed for sev- eral times with deionized water followed by drying in air at 75 ◦ C for 12 h. Further heat-treated was carried out to obtain the final ZnO at 300 ◦ C for 2 h. The samples with 0.08 g and 0 g sucrose added were labeled as P 0 and S 0 . In order to assess the relationship between the amount of sucrose and the photocatalytic activity of ZnO, vari- able amount of sucrose added samples were prepared through the similar process, labeled as P −2 , P −1 , P 1 and P 2 for 0.04 g, 0.06 g, 0.1 g and 0.25 g, respectively. 2.2. Characterization The thermal decomposition process of the precursors was inves- tigated by thermogravimetric analysis/differential thermal analysis (TGA–DTA) using a TA SDT 2960 instrument. It was performed in air from 40 to 1000 ◦ C with a heating rate and flow rate of 10 ◦ C min −1 and 100 mL min −1 , respectively. Powder X-ray diffraction (XRD) analysis was carried out by a Rigaku D/Max-2550/pc diffractome- ter using Cu-K␣ radiation. The IR spectra of sucrose and samples before/after heat treatment were determined by Fourier trans- form infrared spectroscopy (FTIR, Bruker IFS 66 v/s) using KBr disc method. The ratio of KBr to samples was about 300:1 in weight. The morphologies of ZnO flowers obtained with various sucrose amounts were revealed by a scanning electron microscope (SEM, Hitachi S-4800). X-ray photoelectron spectroscopy (XPS) experi- ments were measured with a K-Alpha (Thermofisher Scienticfic Company) X-ray photoelectron spectrometer using Al K␣ radiation (12 kV, 6 mA). The binding energies of elements were calibrated by taking carbon C1s (285.06 eV) as reference. 2.3. Photocatalytic activities tests In this work, the photocatalytic activities of hierarchical struc- tures ZnO were tested by using methyl orange (MO) as the model pollutant. 0.02 g sample was added into 50 mL, 1.2 × 10 −5 M MO solution and mechanically stirred in dark for 20 min to achieve the adsorption equilibrium of MO with ZnO before the UV irradiation. In a cool water bath, the mixture was irradiated by two UV lamps (Philips, 8 W) with continuous stirring. The samples were taken out from the mixed suspension at every 20 min to check the changes of MO concentration. To remove the catalysts of ZnO, centrifugation was carried out at 10,000 rpm for 10 min. The UV–vis absorption spectra of the centrifuged solutions were measured on the HITACHI UV/vis spectrometer (U-3010). 3. Results and discussion To investigate the appropriate calcinations temperature for the transformation of the precursor to ZnO, the thermal analysis in air atmosphere was conducted. Typical TGA/DTA plots for the pre- cursor of sample P 0 is shown in Fig. 1. At the beginning, a small endothermic peak with 5.4% weight loss can be observed, which is mainly attributed to the evaporation of water in the precursors. In the temperature range of 100–400 ◦ C, an obvious endothermic Fig. 1. TGA–DTA curves of the precursor of P 0 . Fig. 2. XRD patterns of the samples: (a) the precursor of P 0 , (b) P 0 . peaks centered at 259.3 ◦ C can be found in DTA curve. Simultane- ously, a faster weight loss stage, claimed as 25.8% can be observed in TGA curve. The thermal decomposition processes can be ascribed to the decomposition and oxidation of the precursor by the releasing of water and carbon dioxide. Therefore, the annealing temperature was chosen at 300 ◦ C to obtain the final products. The purity and crystalline phase of P 0 and the precursor of P 0 were determined by XRD. Fig. 2(a) showed the XRD patterns of the precursor. As a comparison, the XRD pattern of ZnO product (P 0 ) after calcination was also presented (Fig. 2(b)). The diffraction peaks in Fig. 2(a) can be identified as the Zn 4 (CO 3 )(OH) 6 H 2 O, which was consistent with JCPDS Card No.11-0287. While, the diffraction peaks of P 0 can be identified as pure hexagonal ZnO (JCPDS Card No. 36-1451). The XRD patterns of P 0 and the precursor are con- sistent with our previous results with no sucrose added synthesis procedure [19]. It shows that the sucrose as complexing agent will not influence the formation of the precursor (Zn 4 (CO 3 )(OH) 6 H 2 O) and the final product ZnO. In the synthesis process, sucrose was introduced into the urea hydrothermal procedure. To clarify the role of sucrose acting in the crystal growth, FTIR spectra were measured to verify the possi- ble intermediate by-products and the results were shown in Fig. 3. We found that sucrose played the roles of complexing agent and fuel in the synthesis process. In acidic solution, the sucrose firstly hydrolyzes into glucose and fructose, which can be further oxidized into saccharic acid, glycolic acid and trihydroxy-butyric acid with a large number of–COOH and–OH groups. Furthermore, the COOH groups can easily combine with metal ions in the solution, which is quite similar to the citric acid complexing mechanisms. W. Lv et al. / Applied Surface Science 259 (2012) 557– 561 559 Fig. 3. FTIR spectra of samples (a) sucrose (b) the precursor of P 0 (c) calcined at 300 ◦ C (P 0 ). Fig. 3(a) shows the FTIR spectrum of sucrose and its typical absorptions are in agreement with the spectrum in database [18]. It is worth noticing that no obvious absorption is present between 1500 cm −1 and 2500 cm −1 . While, the spectrum for the precursor of P 0 shown in Fig. 3(b) clearly shows the coordinated COO − sym- metric stretching with broad absorption around 1618 cm −1 , which comes from the products of the sucrose hydrolyzation [20]. Consid- ering the complexing ability mentioned above, it can be identified that the metal ions are well complexed by the COOH groups, form- ing stable COOZn 2+ . And in fact, no precipitation was observed during the stirring. Moreover, the broad absorption band centered at 3400 cm −1 can be observed due to the OH stretching vibration, which can be attributed to the existence of crystallization water in the precursor. The absorption band around 1385 cm −1 is typ- ical asymmetric stretching vibration of NO 3 − , which comes from the raw material Zn(NO 3 ) 2 . After calcination at 300 ◦ C (Fig. 3c), the chelating complexes decomposed and a mass of gases are gener- ated, which are favored for the formation of porous product. As curve (b) showed, in infrared absorption spectra of the precursor, the absorption peak at 1048 cm −1 , 830 cm −1 , 711 cm −1 are ascribed to CO 3 2− lattice vibration induced infrared absorption. Therefore, the FTIR shows the precursor is the Zn 4 (CO 3 )(OH) 6 H 2 O, which is consistent with the XRD results. After annealing at 300 ◦ C, the infrared absorption spectra (Fig. 3(c)) shows that a new absorption peak centered at 474 cm −1 appears, indicating the formation of ZnO and the complete decomposition of the precursors. Fig. 4(a) shows the typical SEM images of the products after annealing at 300 ◦ C. Obviously, the hierarchical structure was con- structed by large quantities of fluffy nanosheetes with a uniform size distribution of micro-flowers. The enlarge view of the P 0 in Fig. 4(b) shows that the diameter of ZnO flowers is about 10 ␮m. The nanosheets petals are narrow in width and ended with a sharp tip. The abundance of petals will greatly increase the con- tact area between the catalysts and organic dyes. Moreover, the gap formed by the adjacent nanosheets would enhance the absorption of exciting light and promote the photocatalytic activities of ZnO. The optical absorption efficiency increased by the diffuse reflec- tion happens among the petals, as shown in the inserted figure of Fig. 4(b). On the other hand, the microstructure of the nanosheets petals also shows differences between sucrose adding sample P 0 and no sucrose adding one S 0 . The high magnification SEM images of petals from S 0 and P 0 were shown in Fig. 4(c, d). Apparently, the pores on the nanosheets are quite distinguished from each other. The microstructure of S 0 presents that the pores are embedded in the petals, like large number of holes on a flat surface. While, for the sucrose added sample P 0 , the pores were formed by the Fig. 4. SEM images. (a) Flower-like ZnO of P 0 . (b) An enlarge view of P 0 . The inserted shows the abridged general view of the possible light absorption in the sample P 0 . (c) The microstructure of S 0 (d) the microstructure of P 0 . 560 W. Lv et al. / Applied Surface Science 259 (2012) 557– 561 Fig. 5. Photodegradation of MO in the solution with S 0 and P 0 ZnO flowers. connection of a great quantities of ZnO nanoparticles presenting larger surface areas compared with S 0 . In fact, there is no obvi- ous difference in the flowerlike status between the precursors of P 0 and S 0 , indicating that the sucrose effects on the morphology of LBZC (Zn 4 (CO 3 )(OH) 6 H 2 O) is not obvious. However, to gain the final ZnO hierarchical structures, annealing process was carried out and the role of sucrose was activated during the decomposition of LBZC. In the process of synthesis, the sucrose hydrolyzes into two kinds of monosaccharides, glucose and fructose that is homodis- perse in the Zn 4 (CO 3 )(OH) 6 H 2 O and assist combustion during the annealing. Considering the sucrose can be used as fuel in the fab- rication of oxides, the high temperature decomposition process of LBZC with sucrose adding can be treated as a more intensive and rapid combustion, leading to the precursor burning much more sufficiently and the crystallinity of ZnO particles improved. Good crystalline quality can be reflected from the micro structure of sam- ples. Spherical nanoparticles constituting the resultant nanosheets were formed by the additional heating from the added sucrose, which would be beneficial to the photocatalytic activity.To evaluate the sucrose effects on the photocatalytic activity, the performances of S 0 and P 0 were investigated by the degradation of MO dye under UV irradiation. Fig. 5 compares the photodegradation of MO as a function of irradiation time for the P 0 and S 0 samples. As clearly shown, after irradiation for 100 min, the photocatalytic degradation of MO on S 0 is 80%. In fact, we have discussed the superior photocat- alytic properties of the multi-layered mesoporous ZnO structures (S 0 ) decomposing the MO, which showed the superior photocat- alytic activity to the commercial ZnO 19 . Surprisingly, in comparison with the S 0 , a small amount of sucrose adding sample P 0 displayed much higher decomposition efficiency with a degradation rate of nealy 100% after irradiation for 80 min. Considering the differences in the synthesized procedure, sucrose adding plays an important role in improving the photocatalytic properties. The surface sensitive diagnostic test XPS was conducted to elu- cidate the oxidation states of S 0 and P 0 . Fig. 6 demonstrates the high-resolution XPS spectra of O1s states of sample S 0 and P 0 . Obvi- ously, the XPS spectra of O1s peaks is asymmetric and broadening, which can be resolved into two peaks by a Gaussian distribution fitting centered at 530.1 ± 0.2 eV and 531.7 ± 0.2 eV, respectively. The fitting indicates that at least two oxygen species are present in the near-surface region. O A signal peaks are centered at 530.1 ± 0.2 eV is due to oxygen in the wurtzite structure of ZnO (lattice oxygen), and the intensity of this peak is a measure of fully oxidized oxygen atoms [24]. O B signal 526 528 530 53 2 534 (a) S 0 Intensity (a.u.) Bindin g Energ y (e V) O1s Scan B O1s Scan A 526 52 8 530 532 534 (b) P 0 Intensity (a.u.) Binding En ergy (eV) O1s Scan A O1s Sc an B Fig. 6. The high-resolution XPS spectra of O1s states of sample S 0 (a) and P 0 (b). peaks at 531.7 ± 0.2 eV corresponds to the adsorbed oxygen, which is ascribed to the presence of adsorbed oxygen, including hydroxyl and carbonate groups adsorbed on the material surface.[25–28] The integrated intensity of peak O A can be compared with that of peak O B using the O A to O B integrated intensity ratio “X,” which was approximately 2.0 and 1.7 for P 0 and S 0 , respectively. Apparently, the lattice oxygen in the sucrose added sample P 0 is higher than that of sample S 0 . This result also indicates that the crystallinity of P 0 is superior to S 0 due to the added sucrose providing with more energy during annealing. Under the UV excitation, electron-hole pairs carried out redox reaction and more surface defects will be companied with higher combination probability of surface states and hole. However, the high crystallinity would decrease surface defects and the combination probability of surface states and holes that can enhance photocatalytic activity. [5,6] Considering the pho- tocatalytic activity of P 0 and S 0 , the sucrose induced crystallinity improvement is an effective treatment to increase the photoactiv- ity of ZnO photocatalysts.In order to find the relationship between the amount of sucrose and the photocatalytic activity of ZnO, vari- able amount of sucrose added samples were prepared through the similar process. Fig. 7 shows the plot of the decolorization efficien- cies of MO by the ZnO with variable sucrose after 40 min reaction time. It can be seen that no sucrose added ZnO S 0 show nearly 60% decolorization efficiency. With the sucrose added, ZnO samples showed much better photocatalytic activity and the decolorization efficiencies were greatly increased. As shown in Fig. 7, P 0 shows the superior photocatalytic activity and decolorization efficiency was achieved 95%. While, other ZnO sample with fewer or more sucrose W. Lv et al. / Applied Surface Science 259 (2012) 557– 561 561 Fig. 7. Photocatativity comparison of ZnO flowers after MO degradation for 40 min. The sucrose contents of S 0 , P −2 , P −1 , P 0 , P 1 , P 2 were 0 g, 0.04 g, 0.06 g 0.08 g 0.1 g and 0.25 g, respectively. added show lower decolorization efficiencies during the same reac- tion time. Since the small amount of sucrose added can result in negligible effects on the morphology, the crystallinity and agglom- eration of photocatalysts should be considered. In some cases, it was found that the heat generated during the reaction could be more prominent to cause sintering or agglomeration of particles, resulting in grain growth and low photocatalytic reaction sites. Therefore, the optimization of reaction condition was established for 0.08 g sucrose added ZnO flowers. 4. Conclusion In this study, hierarchical structures ZnO was successfully syn- thesized via a sucrose added urea hydrothermal method. The prepared ZnO flowers were characterized by TG-DTA, FTIR, XRD and SEM. The photocatalytic activities of ZnO flowers were evalu- ated by the degradation of MO and results show that the sucrose added sample presents superior decolorization efficiency. The XPS analysis reflected that the adding of sucrose can improve the crystallization of ZnO. The ZnO flowers synthesized via variable sucrose amount were also estimated by the decolorization effi- ciency of MO after 40 min reaction time. It was found that higher sucrose added would induce a slightly reduction effect on the photocatalytic activities and the optimized reaction condition was estimated. 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