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Growth mechanism, optical and photocatalytic properties of ZnO nanorods@nanoflowers (quantum dots) hybrid nanostructures
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Growth mechanism, optical and photocatalytic properties of ZnO nanorods@nanoflowers (quantum dots) Hybrid nanostructures Xiaoyan Liu, Jian Cao, Lili Yang, Maobin Wei, Xiuyan Li, Jihui Lang, Xuefei Li, Yang Liu, Jinghai Yang, Yaohui Liu
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S0272-8842(15)01165-7 http://dx.doi.org/10.1016/j.ceramint.2015.06.049 CERI10788
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Cite this article as: Xiaoyan Liu, Jian Cao, Lili Yang, Maobin Wei, Xiuyan Li, Jihui Lang, Xuefei Li, Yang Liu, Jinghai Yang, Yaohui Liu, Growth mechanism, optical and photocatalytic properties of ZnO nanorods@nanoflowers (quantum dots) Hybrid nanostructures, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.06.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Growth mechanism, optical and photocatalytic properties of ZnO nanorods@nanoflowers (quantum dots) hybrid nanostructures Xiaoyan Liu,ab Jian Cao,b Lili Yang,b Maobin Wei,b Xiuyan Li,b Jihui Lang,b Xuefei Li,b Yang Liu,b Jinghai Yang*b and Yaohui Liu*a a. Key Laboratory of Automobile Materials of Ministry of Education & School of Materials Science and Engineering, Nanling Campus, Jilin University, No. 5988 Renmin Street, Changchun 130025, P.R. China b. Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, P.R. China
Abstract ZnO nanorods@nanoflowers (quantum dots) hybrid nanostructures have been successfully fabricated by the spin coating method. The transmission electron micrograph (TEM) and field-emission scanning electron microscopy (FE-SEM) images showed that ZnO quantum dots (QDs) and ZnO nanoflowers (NFs) had been assembled on the surfaces of ZnO nanorods (NRs) or in the upper parts of the gaps between the rods. The morphology of ZnO nanoflowers was formed through a general route of zero-dimensional (0D) → 2D → 3D, controlling by sodium dodecyl sulfate (SDS) during the experiment. The blue shifts of Raman and PL spectra were observed for the hybrid nanostructures at room temperature, which were explained by the quantum confinement effects. Compared with the photocatalytic activities of the samples, ZnO NRs@NFs showed enhanced photocatalytic performance for the degradation of RhB under UV radiation, which could be attributed to the special structural feature with an open and porous nanostructured surface layer and the surface defects. ZnO NRs@NFs would show a wider application for its particular porous morphology. * Corresponding author E-mail: [email protected] (J. Yang), [email protected] (Y. Liu). Tel./Fax: +86 434 3294566. 1
Keywords: Hybrid nanostructures; Self-assembly; Optical property; Photocatalytic property
1. Introduction The photocatalytic degradation is one of the important methods for purifying water pollutants due to the requirements of sustained developments. It is well-known that zinc oxide (ZnO) as an important II–VI group semiconductor with a wide band gap semiconductor (~ 3.37 eV) and large exciton binding energy (~ 60 meV), is currently attracting worldwide intense interests because of its numerous applications especially as solar cells [1], gas sensors [2], antibacterial agent [3], photocatalysts [4-7], etc. The dimensions and morphologies of ZnO nanostructures can strongly influence their electrical, optical, and catalytic properties [2, 8-12]. Various dimensional and morphological ZnO nanostructures, such as spheres [8], rods [9], nanoplates [10], screw caps [11], and pyramids [12], have been synthesized and characterized. Especially, the porous 3D ZnO nanostructures assembled from 0D, 1D and 2D nanostructure components have attracted great research interest [2, 7, 13-15], which exhibit potentially useful properties that combine the features of the micrometer and nanometer scale building blocks. These porous structures have an increased number of surface-active sites compared to other forms of ZnO that may improve the photoelectric and catalytic properties. Furthermore, hybrid nanostructures, combining more than two types of nanostructures with prominent advantages, are important building blocks for advanced materials, and will improve the original properties of the pure components [16-19]. For example, Dong et al. [16] reported nanosphere@nanorod hybrid arrays (Ns@NrHys) on substrates, which were applied on the front surface of silicon solar cells, the average total reflectance was reduced from 15.05% to 4.14%, while the short-circuit current density (Jsc) was improved from 33.59 to 36.54 mA cm–2. Among the promising components used in the fabrication of hybrid nanostructures, nanoflowers and one-dimensional nanostructures are still quite rare. To date, some complex preparation processes and multiple steps have been employed for the
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synthesis of nanorod-based hybrid arrays on nanorod hosts, such as sequential plasma deposition [17], atomic layer deposition (ALD) [18] and metal-organic vapour-phase epitaxy (MOVPE) [19]. So, the development of simplified and low-cost approaches for large-scale production is a priority in the fabrication of hybrid nanostructures. Herein, we presented a facile and effective way to prepare ZnO nanorods@quantum dots (NRs@QDs) and nanorods@nanoflowers (NRs@NFs) hybrid nanostructures. It was found that the porous ZnO NFs were self-assembled by irregular nanosheets consisting of numerous ZnO QDs with hexagonal wurtzite structure. On the basis of the preparation process and morphology, we proposed the possible formation mechanism of ZnO NFs and explored the influence of SDS. The optical and photocatalytic properties of the as-prepared hybrid nanostructures were investigated in detail. Because of its particular and complicated structure, the ZnO NRs@NFs would be promising for various applications such as photocatalysis, optoelectronics, biomedicine, etc.
2. Experimental Materials Zinc acetate dihydrate (Zn (OOCCH3)2 · 2H2O, 99.9% purity), zinc nitrate hexahydrate (Zn (NO3)2·6H2O, 99.9% purity), methenamine (C6H12N4, 99.9% purity), zinc chloride (ZnCl2), ammonium bicarbonate (NH4HCO3), sodium dodecyl sulfate (SDS) and pure ethanol were all analytical grade (Sinopharm Chemical Reagent Co., Ltd) and used without further purification. Distilled water was made in analytical laboratory. Preparation of ZnO nanorods (NRs) ZnO NRs were grown on indium tin oxides (ITO) substrates by a two step CBD method, i.e., a substrate treatment prior to the CBD growth, details of the sample preparation can be found elsewhere in our previous work [20, 21]. Firstly, the pretreatment of the substrates, by coating the substrate for several times with a 10 mM solution of Zn (OOCCH3)2 · 2H2O dissolved in pure ethanol. This solution was coated four times onto ITO substrate by a spin coater (Laurell WS-400-8TFW-Full) at the
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rate of 2000 rpm for 30 s, and then the ITO substrate with a seed layer was annealed at 300 °C for 1 h. Subsequently, the CBD growth, the 0.1 M aqueous solutions of Zn (NO3)2·6H2O and 0.1 M aqueous solutions of C6H12N4 were first prepared and mixed together. The pretreated ITO substrates were immersed into the aqueous solution and kept at 95 °C for 5 h with sealing the beaker. After growth, the samples were washed by distilled water for several times and dried at room temperature. Preparation of ZnO quantum dots (QDs) ZnO QDs were synthesized by the following procedure [22, 23]. 40 ml of aqueous solutions of NH4HCO3 (1 M) and 12.5 µl of aqueous solutions of SDS (1 M) were first mixed together in one beaker. Under constant magnetic stirring, 25 ml of aqueous solutions of ZnCl2 (1 M) was added slowly (dropwise for 10 min) into the resulting solution. During the dropping process, the white precipitations were gradually formed in the solution. After 2 h reaction, the precursors were filtered and washed with pure ethanol several times to remove the impurities, followed by drying in an oven for 4 h at 50 °C. Finally, ZnO QDs with a light yellow color were obtained after annealing the precursors in air under 200 °C for 1 h in the chamber furnace. Preparation of hybrid nanostructures Two different hybrid nanostructures were synthesized in our experiments by the following procedures. Sample 1: The obtained ZnO QDs were dispersed in pure ethanol solution and stirred on a hotplate at 50 °C for 1 h. ZnO QDs were spin coated onto ZnO NRs as compact layer for several times. Each spin coating process was followed by annealing the sample at 60 °C in air for 5 min. The substrate with ZnO NRs@QDs was annealed at 200 °C for 1 h, and labeled as S-1. Sample 2: It was different from S-1, the precursors of ZnO QDs were spin coated onto ZnO NRs for several times at first. Then the obtained substrate was also annealed at 200 °C for 1 h, and labeled as S-2. Characterization of products X-ray diffraction (XRD) pattern was collected on D/max2500PC copper rotating-anode X-ray diffractometer with Cu Kα radiation. Transmission electron 4
microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were taken on JEOL JEM-2100HR electron microscope. The specimen was prepared by depositing a drop of the dilute solution of the sample in pure ethanol on a carbon coated copper grid and drying at room temperature. Field-emission scanning electron microscopy (FE-SEM) images were taken on JEOL JSM-7800F electron microscope. Additionally, a Renishaw-inVia confocal Raman spectrometer equipped with a research-grade Leica microscope was used to collect Raman and photoluminescence (PL) spectra. For Raman spectra, a 514.5nm laser line from Argon laser was utilized as excitation. For PL, a 325nm laser line from He-Cd laser was utilized as excitation. Photocatalytic activity measurements The photocatalytic activities of the samples were evaluated by measuring the photodegradation efficiency of RhB aqueous solutions under the irradiation of UV. The photocatalytic reactions in the solution were carried out at room temperature in a closed system using a high-pressure Hg lamp (λ = 365 nm). Prior to UV light exposure, the samples immersed in the same solution were aged in the dark for 1h to reach the adsorption equilibrium. After different irradiation intervals, the solution concentration of RhB was analyzed by the UV-Vis spectrophotometer (UV-5800PC, Shanghai Metash Instruments Co., Ltd) at room temperature. The degradation efficiency of RhB was calculated from the following expression: Degradation (%) = (1
－C/C )×100%, where C 0
0
is the initial concentration of RhB and C is the residual
concentration of RhB at different irradiation intervals.
3. Results and discussion Fig. 1 shows the structure and morphology of ZnO NRs and ZnO QDs. All of the XRD patterns can be indexed as the hexagonal wurtzite phase of ZnO, which are consistent with the standard card (JCPDS No. 80-0074). As seen from the XRD pattern of ZnO NRs (Fig.1 (a)), the (002) diffraction peak is stronger and narrower than the other peaks, which illustrates the highly preferential orientation of ZnO NRs along c-axis. The TEM image (Fig.1 (b)) shows that the surfaces of ZnO NRs are
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smooth, and all the nanorods are preferential orientation grown perpendicular to ITO substrate. The HRTEM image (see inset of Fig. 1 (b)) further proves that the NRs grow along the (002) direction, which exhibits a well oriented and good crystallization. According to the XRD pattern of ZnO QDs (Fig.1 (c)), the average grain size of ZnO QDs estimated through the Debye-Scherer formula, is approximately 4.3 nm. From the TEM and HRTEM images (Fig.1 (d)), it can be seen that the average grain size of the sample is about 4.7 nm, in agreement with the XRD result. Comparing the two HRTEM images (the inset of Fig. 1 (b) and (d)), the different lattice fringes between ZnO NRs and ZnO QDs can be clearly identified. The TEM images of the prepared hybrid nanostructure S-1 are shown in Fig. 2. It can be seen that the surfaces of ZnO NRs are not smooth, the dark contrast demonstrates that ZnO QDs exist on the surfaces of ZnO NRs, indicating that ZnO NRs@QDs (S-1) has been successfully prepared. The inset of Fig. 2 (b) is the HRTEM image of ZnO NRs@QDs, which shows that the quantum dots are about 5.3 nm in diameter and attached to the surface of the nanorod. The morphology of the hybrid nanostructure S-2 was examined by FE-SEM. As can be seen from the Fig. 3, the obtained hybrid nanostructure is composed of ZnO nanorods and nanoflowers. The nanoflowers (NFs) mainly assemble on the top surfaces of ZnO NRs and in the upper parts of the gaps between the rods. The typical FE-SEM images (Fig. 3 (b-d)) show clearly that the nanoflowers are self-assembled with irregular nanosheets. The detailed microstructure of the prepared hybrid nanostructure S-2 was further characterized by TEM. Fig. 4 (a) shows the nanoflowers are randomly attached on the top surfaces of the nanorods or in the upper parts of the gaps between the rods. Fig. 4 (b) is the enlarged image of nanoflower in (a), which shows the nanoflower is made up of irregular porous nanosheets. From the HRTEM image of the oval region in Fig. 4 (b), it can be seen clearly that the nanosheet is self-assembled by numerous quantum dots with the size of about 3.6–5.4 nm. The EDS spectrum of the whole nanoflower in Fig. 4 (b) exhibits the presence of the Zn and O element, which unambiguously confirms that the nanoflowers are only composed of ZnO QDs. These 6
results further demonstrate the results of SEM, indicating that ZnO NRs@NFs has been also successfully prepared. Based on the FE-SEM and TEM results, the possible formation mechanism of ZnO NFs is schematically illustrated in Fig. 5. According to the preparation process, the precursor nuclei are formed by the chemical precipitation method as the SDS exists. And, the negative charged SDS anions are preferably adsorbed onto the positive lattice planes of the precursors while the white precipitations gradually formed in the solution [14]. The precursor nuclei have the tendency to aggregate into layered nanosheet structures with the assistance of SDS during the spin coating process. To minimize system’s surface energy, the crystalline nuclei self-assemble into 3D structures (flower-like architecture) with the increase of spin coating times. Because of the introduction of anionic surfactant SDS, the precursors are surrounded by them during the annealing process. As a result, the samples keep the original flower-like architecture, and ZnO NFs are obtained after annealing the precursors. The whole process can be summarized as two steps: (i) Aggregation of 0D quantum dots to 2D nanosheets via SDS. (ii) Formation of 3D structures (porous nanoflowers) via self-assembly of 2D units. For the hybrid nanostructure S-1, SDS has decomposed before the spin coating, and ZnO QDs would not self-assemble into nanosheets. It indicates that SDS plays an important role in the formation of the flower-like architecture. Raman scattering is always used to study the vibrational properties of materials and to identify the crystallization, structural disorder, and defects. Fig. 6 shows the Raman-scattering spectra of the samples performed at room temperature. According to group theory analysis, Raman-active modes for wurtzite ZnO are A1 + 2E2 + E1 [24]. For all spectra, the sharp, strong and dominant peak located at about 438 cm-1 is observed, which is the characteristic scattering peak of the Raman-active dominant E2 (high) mode of wurtzite hexagonal ZnO [25]. Two weaker and broader peaks at 333 and 380 cm-1 are assigned to E2H – E2L (multi-phonon) and A1 (TO) modes, respectively. In addition, the low band centered at 583 cm-1 is a superposition of the A1 (LO) mode at 574 cm-1 and the E1 (LO) mode at 591 cm-1 [5, 26], which is usually 7
ascribed to the presence of structural defects (oxygen vacancies and zinc interstitials, etc.) and impurities in the grown products [27]. Therefore, the higher the ratio of A1&E1 (LO) to E2 (high) is, the more the oxygen vacancies or Zn interstitials present. The Raman results indicate that the hybrid nanostructures keep the overall crystal structure of the bulk ZnO, but have more structural defects. The phonon modes in the specta of the hybrid nanostructures are slightly blue-shifted in the comparison with the spectrum of bare ZnO NRs. Those only few cm−1 blue-shifts derive from quantum confinement effects [22], because of ZnO QDs exist in the hybrid nanostructures, which further confirms that hybrid nanostructures have been successfully fabricated. PL study is also an effective way to investigate the structural properties and defects of the semiconductors [9, 13]. Fig. 7 shows the PL spectra measured at room temperature consisting of a UV peak located at about 380 nm in wavelength and a deep level emission (visible emission) band centered at 530−550 nm with a broad feature. The UV emission band edge is a characteristic band edge of ZnO materials originated from the recombination of free excitons related to a near band edge (NBE) transition [9]. Compared with the spectra, the intensity of UV emission peak of the hybrid nanostructures is much lower than the bare ZnO NRs, which demonstrates that the hybrid nanostructures can effectively suppress photogenerated electron–hole recombination and probably could result a higher photocatalytic performance. In addition, the position of UV emission peak of the hybrid nanostructures shows a blue-shift in the comparison with the bare ZnO NRs. ZnO QDs in our samples is smaller than 7 nm in dimension, and so this blue-shift behavior of PL peak position is naturally explained by the quantum confinement effect [28], which is in agreement with the results from Raman. The deep level emission (DLE) band in the range of 430−670 nm is known as defect luminescence band, which has been reported to originate from various intrinsic and extrinsic defects in ZnO crystals and also influenced by defects, impurities and absorbed molecules on the sample surface [29, 30]. It’s generally agreed that DLE emission is largely related to oxygen vacancies (VO) and surface defects. The inset of Fig. 7 displays a column graph of the DLE to 8
NBE ratio from the three samples, the higher value indicates higher defect density [30]. It can be seen that the hybrid nanostructures show a higher concentration of surface defects as a result of ZnO QDs or ZnO NFs assembled onto the nanorods surface. ZnO NRs@NFs exhibits the highest defect emission, which is attributed to high concentration of inherent surface defects (i.e., oxygen vacancies) contained in the complicated structure of ZnO NFs. The photocatalytic activities of the as-prepared samples were evaluated by measuring the degradation of RhB as a representative pollutant under UV light illumination. Curves of degradation efficiency versus reaction time are shown in Fig. 8. A blank experiment without any catalyst is usually performed at the same experimental conditions to monitor the auto-photocatalytic property of the dye. After irradiation for 8 h, the degradation efficiency of RhB is about 77% (with bare ZnO NRs), 79% (with ZnO NRs@QDs) and 87% (with ZnO NRs@NFs), respectively. It is noted that ZnO NRs@NFs exhibits enhanced photocatalytic performance compared to the other two samples. It is well known that the photocatalytic reaction occurs at the interface between catalyst surfaces and organic pollutants [31]. Thus, the morphology of the catalyst plays a vital role in the photocatalysis, a particular surface and complicated structure will adsorb more dye molecules. The reason for the higher photocatalytic activity of ZnO NRs@NFs is due to the special structural feature of ZnO NFs with an open and porous nanostructured surface layer that significantly facilitates the diffusion and mass transportation of RhB molecules and oxygen species in photochemical reaction of RhB degradation [32]. Furthermore, it is generally accepted that the electron–hole separation efficiency also plays an important role in promoting the photocatalytic activity of semiconductor materials. If a suitable scavenger or surface defect state is available to trap the electron or hole, recombination is prevented and subsequent redox reactions may occur [33]. From Fig. 7, it can be clearly seen that the defect-related emission from ZnO NRs@NFs is relatively stronger than the other samples, indicating more surface defects exist in ZnO NRs@NFs, in which oxygen vacancies are the most 9
suggested defects [34]. Since the oxygen vacancies can serve as the electrons (e–) capturing center to restrain the recombination of photogenerated electrons and holes, the holes (h+) can react with either adsorbed H2O or with the surface OH– groups on the semiconductor materials to form hydroxyl radicals (·OH), and cause oxidation of the organic dye. In addition, the oxygen vacancies are helpful to the generation of active species (·O2ˉ, ·HO2, or ·OH) on the surfaces of semiconductor materials, therefore, are beneficial to the photodegradation of organic dye [35, 36].
4. Conclusion In conclusion, we have successfully synthesized ZnO NRs@QDs and ZnO NRs@NFs hybrid nanostructures by the spin coating method. SDS had an important effect on the morphologies of the as-synthesized samples, and then influencing on their optical and photocatalytic performances. The whole formation process of ZnO NFs could be summarized as two steps: (i) 0D quantum dots with size of 3.6–5.4 nm were assembled to 2D nanosheets via SDS. (ii) 2D units were organized toward 3D porous ZnO NFs. ZnO NRs@NFs showed a dramatically enhanced deep-level emission, indicating the presence of higher defect concentration as compared to the other samples. In addition, ZnO NRs@NFs exhibited the best photocatalytic activity for degradation of RhB under UV light irradiation, which could be attributed to the particular hybrid nanostructure, morphology and surface defects. The particular hybrid nanostructure, the simple and low-cost preparation procedure may promote the assembly for various applications such as photocatalysis, optoelectronics, biomedicine, etc.
Acknowledgments This work is supported by National Programs for High Technology Research and Development of China (863) (Item No. 2013AA032202), the National Natural Science Foundation of China (Grant Nos. 61178074, 61378085, 51479220 and 61475063), the National Youth Program Foundation of China (Grant Nos. 11204104 and 61308095) and Program for the Development of Science and Technology of Jilin province (Item No. 20150101180JC and 20140101205JC). 10
References [1] L.L. Yang, G. Chen, Y.F. Sun, D.L. Han, S. Yang, M. Gao, P. Zou, H.M. Luan, X.W. Kong, J.H. Yang, Effects of interface modification with self-assembled monolayers on the photovoltaic performance of CdS quantum dots sensitized solar cells, Electrochim. Acta 164 (2015) 38-47. [2] J. Chang, Muhammad Z. Ahmad, Wojtek Wlodarski and Eric R. Waclawik, Self-Assembled 3D ZnO Porous Structures with Exposed Reactive {0001} Facets and Their Enhanced Gas Sensitivity, Sensors 13 (2013) 8445-8460. [3] J.Z. Ma, J.L. Liu, Y. Bao, Z.F. Zhu, X.F. Wang, J. Zhang, Synthesis of large-scale uniform mulberry-like ZnO particles with microwave hydrothermal method and its antibacterial property, Ceram. Int. 39 (2013) 2803-2810. [4] L.Y. Zhang, L.W. Yin, C.X. Wang, N. Lun, and Y.X. Qi, Sol-Gel Growth of Hexagonal Faceted ZnO Prism Quantum Dots with Polar Surfaces for Enhanced Photocatalytic Activity, ACS Appl. Mater. Interfaces 2 (2010) 1769-1773. [5] Soumita Mukhopadhyay, Partha Pratim Das, Suvendu Maity, Prasanta Ghosh, P. Sujatha Devi, Solution grown ZnO rods: Synthesis, characterization and defect mediated photocatalytic activity, Appl. Catal. B: Environ. 165 (2015) 128-138. [6] Mansoor Farboda, Esmat Jafarpoora, Hydrothermal synthesis of different colors and morphologies of ZnO nanostructures and comparison of their photocatalytic properties, Ceram. Int. 40 (2014) 6605-6610. [7] X.H. Zhao, F.J. Lou, M. Li, X.D. Lou, Z.Z. Li, J.G. Zhou, Sol-gel-based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic applications, Ceram. Int. 40 (2014) 5507-5514. [8] Z.W. Deng, M. Chen, G.X. Gu, L.M. Wu, A facile method to fabricate ZnO hollow spheres and their photocatalytic property, J. Phys. Chem. B 112 (2008) 16-22. [9] Hye Won Jeong, Seung-Yo Choi, Seong Hui Hong, Sang Kyoo Lim, Dong Suk Han, Ahmed Abdel-Wahab, and Hyunwoong Park, Shape-Dependent Charge Transfers in Crystalline ZnO Photocatalysts: Rods versus Plates, J. Phys. Chem. C 118 (2014) 21331-21338. [10] H. Yang, S.Q. Ni, X. Jiang, W. Jiang, J. Zhan, In situ fabrication of single-crystalline porous ZnO nanoplates on zinc foil to support silver nanoparticles as a stable SERS substrate,
11
CrystEngComm 14 (2012) 6023-6028. [11] G.R. Li, T. Hu, G.L. Pan, T.Y. Yan, X.P. Gao, H.Y. Zhu, Morphology-Function Relationship of ZnO: Polar Planes, Oxygen Vacancies, and Activity, J. Phys. Chem. C 112 (2008) 11859-11864. [12] F. Lu, W.P. Cai, Y.G. Zhang, ZnO Hierarchical Micro/Nanoarchitectures: Solvothermal Synthesis and Structurally Enhanced Photocatalytic Performance, Adv. Funct. Mater. 18 (2008) 1047-1056. [13] X.H. Wang, Y.K. Zhang, C. Hao, F. Feng, H.B. Yin, and N.C. Si, Solid-Phase Synthesis of Mesoporous ZnO Using Lignin-Amine Template and Its Photocatalytic Properties, Ind. Eng. Chem. Res. 53 (2014) 6585-6592. [14] X.S. Chen, J.Y. Liu, X.Y. Jing, J. Wang, D.L. Song, L.H. Liu, Self-assembly of ZnO nanosheets into flower-like architectures and their gas sensing properties, Mater. Lett. 112 (2013) 23-25. [15] X. Wang, W. Liu, J. Liu, F. Wang, J. Kong, S. Qiu, C. He, L. Luan, Synthesis of Nestlike ZnO Hierarchically Porous Structures and Analysis of Their Gas Sensing Properties, ACS Appl. Mater. Interfaces 4 (2012) 817-825. [16] W.J. Dong, C.Y. Huang, T.X. Wei, Y. Zhang, K.N. Zhang, Y. Sun, X. Chen and N. Dai, Nanosphere@nanorod hybrid arrays generated on substrates by a one-pot process as low-reflecting surfaces, RSC Adv. 3 (2013) 21039-21045. [17] M.L. Wang, C.G. Huang, Y.G. Cao, Q.J. Yu, Z.H. Deng, Y. Liu, Z. Huang, J.Q. Huang, Q.F. Huang, W. Guo and J.K. Liang, Dye-sensitized solar cells based on nanoparticle-decorated ZnO/TiO2 core/shell nanorod arrays, J. Phys. D: Appl. Phys. 42 (2009) 155104. [18] S. Bang, S. Lee, T. Park, Y. Ko, S. Shin, S.-Y. Yim, H. Seo and H. Jeon, Dual optical functionality of local surface plasmon resonance for RuO2 nanoparticle-ZnO nanorod hybrids grown by atomic layer deposition, J. Mater. Chem. 22 (2012) 14141-14148. [19] E.S. Jang, J.Y. Bae, J. Yoo, W.I. Park, D.W. Kim, G.C. Yi, T. Yatsui and M. Ohtsu, Quantum confinement effect in ZnO/Mg0.2Zn0.8O multishell nanorod heterostructures, Appl. Phys. Lett. 88 (2006) 023102. [20] Q.X. Zhao, L.L. Yang, M. Willander, B.E. Sernelius, P.O. Holtz, Surface recombination in ZnO nanorods grown by chemical bath deposition, J. Appl. Phys. 104 (2008) 073526. 12
[21] X.Y. Liu, J. Cao, B. Feng, L.L. Yang, M.B. Wei, H.J. Zhai, H.L. Liu, Y.R. Sui, J.H. Yang, Y.H. Liu, Facile fabrication and photocatalytic properties of ZnO nanorods/ZnSe nanosheets heterostructure, Superlattices Microstruct. 83 (2015) 447-458. [22] J.H. Yang, X.Y. Liu, L.L. Yang, Y.X. Wang, Y.J. Zhang, J.H. Lang, M. Gao, B. Feng, Effect of annealing temperature on the structure and optical properties of ZnO nanoparticles, J. Alloys Comp. 477 (2009) 632-635. [23] L.L. Yang, J.H. Yang, X.Y. Liu, Y.J. Zhang, Y.X. Wang, H.G. Fan, D.D. Wang, J.H. Lang, Low temperature synthesis and characterization of ZnO quantum dots, J. Alloys Comp. 463 (2008) 92-98. [24] C.A. Arguello, D.L. Rousseau and S.P.S. Porto, First-Order Raman Effect in Wurtzite-Type Crystals, Phys. Rev. 181 (1969) 1351-1355. [25] A. Umar, Y.B. Hahn, Aligned hexagonal coaxial-shaped ZnO nanocolumns on steel alloy by thermal evaporation, Appl. Phys. Lett. 88 (2006) 173120. [26] N. Ashkenov, B.N. Mbenkum, C. Bundesmann, V. Riede, M. Lorenz, D. Spemann, E.M. Kaidashev, A. Kasic, M. Schubert, M. Grundmann, G. Wagner, H. Neumann, V. Darakchieva, H. Arwin and B. Monemar, Infrared dielectric functions and phonon modes of high-quality ZnO films, J. Appl. Phys. 93 (2003) 126-133. [27] J.J. Wu, S.C. Liu, Catalyst-Free Growth and Characterization of ZnO Nanorods, J. Phys. Chem. B 106 (2002) 9546-9551. [28] J.G. Lu, Z.Z. Ye, Y.Z. Zhang, Q.L. Liang, Sz. Fujita, Z.L. Wang, Self-assembled ZnO quantum dots with tunable optical properties, Appl. Phys. Lett. 89 (2006) 023122. [29] A. B. Djurišić and Y. H. Leung, Optical Properties of ZnO Nanostructures, Small 2 (2006) 944-961. [30] Firat Güder, Yang Yang, Andreas Menzel, Chunyu Wang, Julia Danhof, Kittitat Subannajui, Andreas Hartel, Daniel Hiller, Rajeevan Kozhummal, Niranjan S. Ramgir, Volker Cimalla, Ulrich T. Schwarz and Margit Zacharias, Superior Functionality by Design: Selective Ozone Sensing Realized by Rationally Constructed High-Index ZnO Surfaces, Small 8 (2012) 3307-3314. [31] S. Girish Kumar and K. S. R. Koteswara Rao, Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental 13
applications, RSC Adv. 5 (2015) 3306-3351. [32] B.X. Li and Y.F. Wang, Facile Synthesis and Enhanced Photocatalytic Performance of Flower-like ZnO Hierarchical Microstructures, J. Phys. Chem. C 114 (2010) 890-896. [33] J.C. Wang, P. Liu, X.Z. Fu, Z.H. Li, W. Han, X.X. Wang, Relationship between Oxygen Defects and the Photocatalytic Property of ZnO Nanocrystals in Nafion Membranes, Langmuir 25 (2009) 1218-1223. [34] X.H. Zhao, M. Li, X.D. Lou, Sol-gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity, Adv. Powder Technol. 25 (2014) 372-378. [35] Y. Liu, Z.H. Kang, Z.H. Chen, I. Shafiq, J.A. Zapien, I. Bello, W.J. Zhang, S.T. Lee, Synthesis, Characterization, and Photocatalytic Application of Different ZnO Nanostructures in Array Configurations, Cryst. Growth Des. 9 (2009) 3222-3227. [36] Z.Z. Han, L. Liao, Y.T. Wu, H.B. Pan, S.F. Shen, J.Z. Chen, Synthesis and photocatalytic application of oriented hierarchical ZnO flower-rod architectures, J. Hazard. Mater. 217-218 (2012) 100-106.
Figure Captions
Fig. 1 XRD patterns of (a) ZnO NRs and (c) ZnO QDs. TEM images of (b) ZnO NRs and (d) ZnO QDs. The inset of (b) and (d) is the HRTEM image of ZnO NRs and ZnO QDs, respectively.
Fig. 2 (a, b) TEM images of ZnO NRs@QDs (S-1), the inset figure of (b) is the corresponding HRTEM image.
Fig. 3 FE-SEM images of ZnO NRs@NFs (S-2).
Fig. 4 (a) TEM image of ZnO NRs@NFs (S-2). (b) The enlarged image of nanoflower in (a). (c) HRTEM image of the oval region in (b). (d) EDS image of the whole nanoflower in (b). 14
Fig. 5 Schematic illustration of the formation mechanism of ZnO NFs.
Fig.6 Raman spectra of (a) bare ZnO NRs, (b) ZnO NRs@QDs, (c) ZnO NRs@NFs.
Fig. 7 Room-temperature PL (excited at 325nm) of (a) bare ZnO NRs, (b) ZnO NRs@QDs, (c) ZnO NRs@NFs. Inset is the DLE to NBE ratio of the three samples.
Fig. 8 Degradation efficiency versus reaction time for (a) bare ZnO NRs, (b) ZnO NRs@QDs, (c) ZnO NRs@NFs.
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Growth mechanism, optical and photocatalytic properties of ZnO nanorods@nanoflowers (quantum dots) Hybrid nanostructures Xiaoyan Liu, Jian Cao, Lili Yang, Maobin Wei, Xiuyan Li, Jihui Lang, Xuefei Li, Yang Liu, Jinghai Yang, Yaohui Liu
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PII: DOI: Reference:
S0272-8842(15)01165-7 http://dx.doi.org/10.1016/j.ceramint.2015.06.049 CERI10788
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Ceramics International
Received date: Revised date: Accepted date:
27 May 2015 10 June 2015 10 June 2015
Cite this article as: Xiaoyan Liu, Jian Cao, Lili Yang, Maobin Wei, Xiuyan Li, Jihui Lang, Xuefei Li, Yang Liu, Jinghai Yang, Yaohui Liu, Growth mechanism, optical and photocatalytic properties of ZnO nanorods@nanoflowers (quantum dots) Hybrid nanostructures, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.06.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Growth mechanism, optical and photocatalytic properties of ZnO nanorods@nanoflowers (quantum dots) hybrid nanostructures Xiaoyan Liu,ab Jian Cao,b Lili Yang,b Maobin Wei,b Xiuyan Li,b Jihui Lang,b Xuefei Li,b Yang Liu,b Jinghai Yang*b and Yaohui Liu*a a. Key Laboratory of Automobile Materials of Ministry of Education & School of Materials Science and Engineering, Nanling Campus, Jilin University, No. 5988 Renmin Street, Changchun 130025, P.R. China b. Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, P.R. China
Abstract ZnO nanorods@nanoflowers (quantum dots) hybrid nanostructures have been successfully fabricated by the spin coating method. The transmission electron micrograph (TEM) and field-emission scanning electron microscopy (FE-SEM) images showed that ZnO quantum dots (QDs) and ZnO nanoflowers (NFs) had been assembled on the surfaces of ZnO nanorods (NRs) or in the upper parts of the gaps between the rods. The morphology of ZnO nanoflowers was formed through a general route of zero-dimensional (0D) → 2D → 3D, controlling by sodium dodecyl sulfate (SDS) during the experiment. The blue shifts of Raman and PL spectra were observed for the hybrid nanostructures at room temperature, which were explained by the quantum confinement effects. Compared with the photocatalytic activities of the samples, ZnO NRs@NFs showed enhanced photocatalytic performance for the degradation of RhB under UV radiation, which could be attributed to the special structural feature with an open and porous nanostructured surface layer and the surface defects. ZnO NRs@NFs would show a wider application for its particular porous morphology. * Corresponding author E-mail: [email protected] (J. Yang), [email protected] (Y. Liu). Tel./Fax: +86 434 3294566. 1
Keywords: Hybrid nanostructures; Self-assembly; Optical property; Photocatalytic property
1. Introduction The photocatalytic degradation is one of the important methods for purifying water pollutants due to the requirements of sustained developments. It is well-known that zinc oxide (ZnO) as an important II–VI group semiconductor with a wide band gap semiconductor (~ 3.37 eV) and large exciton binding energy (~ 60 meV), is currently attracting worldwide intense interests because of its numerous applications especially as solar cells [1], gas sensors [2], antibacterial agent [3], photocatalysts [4-7], etc. The dimensions and morphologies of ZnO nanostructures can strongly influence their electrical, optical, and catalytic properties [2, 8-12]. Various dimensional and morphological ZnO nanostructures, such as spheres [8], rods [9], nanoplates [10], screw caps [11], and pyramids [12], have been synthesized and characterized. Especially, the porous 3D ZnO nanostructures assembled from 0D, 1D and 2D nanostructure components have attracted great research interest [2, 7, 13-15], which exhibit potentially useful properties that combine the features of the micrometer and nanometer scale building blocks. These porous structures have an increased number of surface-active sites compared to other forms of ZnO that may improve the photoelectric and catalytic properties. Furthermore, hybrid nanostructures, combining more than two types of nanostructures with prominent advantages, are important building blocks for advanced materials, and will improve the original properties of the pure components [16-19]. For example, Dong et al. [16] reported nanosphere@nanorod hybrid arrays (Ns@NrHys) on substrates, which were applied on the front surface of silicon solar cells, the average total reflectance was reduced from 15.05% to 4.14%, while the short-circuit current density (Jsc) was improved from 33.59 to 36.54 mA cm–2. Among the promising components used in the fabrication of hybrid nanostructures, nanoflowers and one-dimensional nanostructures are still quite rare. To date, some complex preparation processes and multiple steps have been employed for the
2
synthesis of nanorod-based hybrid arrays on nanorod hosts, such as sequential plasma deposition [17], atomic layer deposition (ALD) [18] and metal-organic vapour-phase epitaxy (MOVPE) [19]. So, the development of simplified and low-cost approaches for large-scale production is a priority in the fabrication of hybrid nanostructures. Herein, we presented a facile and effective way to prepare ZnO nanorods@quantum dots (NRs@QDs) and nanorods@nanoflowers (NRs@NFs) hybrid nanostructures. It was found that the porous ZnO NFs were self-assembled by irregular nanosheets consisting of numerous ZnO QDs with hexagonal wurtzite structure. On the basis of the preparation process and morphology, we proposed the possible formation mechanism of ZnO NFs and explored the influence of SDS. The optical and photocatalytic properties of the as-prepared hybrid nanostructures were investigated in detail. Because of its particular and complicated structure, the ZnO NRs@NFs would be promising for various applications such as photocatalysis, optoelectronics, biomedicine, etc.
2. Experimental Materials Zinc acetate dihydrate (Zn (OOCCH3)2 · 2H2O, 99.9% purity), zinc nitrate hexahydrate (Zn (NO3)2·6H2O, 99.9% purity), methenamine (C6H12N4, 99.9% purity), zinc chloride (ZnCl2), ammonium bicarbonate (NH4HCO3), sodium dodecyl sulfate (SDS) and pure ethanol were all analytical grade (Sinopharm Chemical Reagent Co., Ltd) and used without further purification. Distilled water was made in analytical laboratory. Preparation of ZnO nanorods (NRs) ZnO NRs were grown on indium tin oxides (ITO) substrates by a two step CBD method, i.e., a substrate treatment prior to the CBD growth, details of the sample preparation can be found elsewhere in our previous work [20, 21]. Firstly, the pretreatment of the substrates, by coating the substrate for several times with a 10 mM solution of Zn (OOCCH3)2 · 2H2O dissolved in pure ethanol. This solution was coated four times onto ITO substrate by a spin coater (Laurell WS-400-8TFW-Full) at the
3
rate of 2000 rpm for 30 s, and then the ITO substrate with a seed layer was annealed at 300 °C for 1 h. Subsequently, the CBD growth, the 0.1 M aqueous solutions of Zn (NO3)2·6H2O and 0.1 M aqueous solutions of C6H12N4 were first prepared and mixed together. The pretreated ITO substrates were immersed into the aqueous solution and kept at 95 °C for 5 h with sealing the beaker. After growth, the samples were washed by distilled water for several times and dried at room temperature. Preparation of ZnO quantum dots (QDs) ZnO QDs were synthesized by the following procedure [22, 23]. 40 ml of aqueous solutions of NH4HCO3 (1 M) and 12.5 µl of aqueous solutions of SDS (1 M) were first mixed together in one beaker. Under constant magnetic stirring, 25 ml of aqueous solutions of ZnCl2 (1 M) was added slowly (dropwise for 10 min) into the resulting solution. During the dropping process, the white precipitations were gradually formed in the solution. After 2 h reaction, the precursors were filtered and washed with pure ethanol several times to remove the impurities, followed by drying in an oven for 4 h at 50 °C. Finally, ZnO QDs with a light yellow color were obtained after annealing the precursors in air under 200 °C for 1 h in the chamber furnace. Preparation of hybrid nanostructures Two different hybrid nanostructures were synthesized in our experiments by the following procedures. Sample 1: The obtained ZnO QDs were dispersed in pure ethanol solution and stirred on a hotplate at 50 °C for 1 h. ZnO QDs were spin coated onto ZnO NRs as compact layer for several times. Each spin coating process was followed by annealing the sample at 60 °C in air for 5 min. The substrate with ZnO NRs@QDs was annealed at 200 °C for 1 h, and labeled as S-1. Sample 2: It was different from S-1, the precursors of ZnO QDs were spin coated onto ZnO NRs for several times at first. Then the obtained substrate was also annealed at 200 °C for 1 h, and labeled as S-2. Characterization of products X-ray diffraction (XRD) pattern was collected on D/max2500PC copper rotating-anode X-ray diffractometer with Cu Kα radiation. Transmission electron 4
microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were taken on JEOL JEM-2100HR electron microscope. The specimen was prepared by depositing a drop of the dilute solution of the sample in pure ethanol on a carbon coated copper grid and drying at room temperature. Field-emission scanning electron microscopy (FE-SEM) images were taken on JEOL JSM-7800F electron microscope. Additionally, a Renishaw-inVia confocal Raman spectrometer equipped with a research-grade Leica microscope was used to collect Raman and photoluminescence (PL) spectra. For Raman spectra, a 514.5nm laser line from Argon laser was utilized as excitation. For PL, a 325nm laser line from He-Cd laser was utilized as excitation. Photocatalytic activity measurements The photocatalytic activities of the samples were evaluated by measuring the photodegradation efficiency of RhB aqueous solutions under the irradiation of UV. The photocatalytic reactions in the solution were carried out at room temperature in a closed system using a high-pressure Hg lamp (λ = 365 nm). Prior to UV light exposure, the samples immersed in the same solution were aged in the dark for 1h to reach the adsorption equilibrium. After different irradiation intervals, the solution concentration of RhB was analyzed by the UV-Vis spectrophotometer (UV-5800PC, Shanghai Metash Instruments Co., Ltd) at room temperature. The degradation efficiency of RhB was calculated from the following expression: Degradation (%) = (1
－C/C )×100%, where C 0
0
is the initial concentration of RhB and C is the residual
concentration of RhB at different irradiation intervals.
3. Results and discussion Fig. 1 shows the structure and morphology of ZnO NRs and ZnO QDs. All of the XRD patterns can be indexed as the hexagonal wurtzite phase of ZnO, which are consistent with the standard card (JCPDS No. 80-0074). As seen from the XRD pattern of ZnO NRs (Fig.1 (a)), the (002) diffraction peak is stronger and narrower than the other peaks, which illustrates the highly preferential orientation of ZnO NRs along c-axis. The TEM image (Fig.1 (b)) shows that the surfaces of ZnO NRs are
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smooth, and all the nanorods are preferential orientation grown perpendicular to ITO substrate. The HRTEM image (see inset of Fig. 1 (b)) further proves that the NRs grow along the (002) direction, which exhibits a well oriented and good crystallization. According to the XRD pattern of ZnO QDs (Fig.1 (c)), the average grain size of ZnO QDs estimated through the Debye-Scherer formula, is approximately 4.3 nm. From the TEM and HRTEM images (Fig.1 (d)), it can be seen that the average grain size of the sample is about 4.7 nm, in agreement with the XRD result. Comparing the two HRTEM images (the inset of Fig. 1 (b) and (d)), the different lattice fringes between ZnO NRs and ZnO QDs can be clearly identified. The TEM images of the prepared hybrid nanostructure S-1 are shown in Fig. 2. It can be seen that the surfaces of ZnO NRs are not smooth, the dark contrast demonstrates that ZnO QDs exist on the surfaces of ZnO NRs, indicating that ZnO NRs@QDs (S-1) has been successfully prepared. The inset of Fig. 2 (b) is the HRTEM image of ZnO NRs@QDs, which shows that the quantum dots are about 5.3 nm in diameter and attached to the surface of the nanorod. The morphology of the hybrid nanostructure S-2 was examined by FE-SEM. As can be seen from the Fig. 3, the obtained hybrid nanostructure is composed of ZnO nanorods and nanoflowers. The nanoflowers (NFs) mainly assemble on the top surfaces of ZnO NRs and in the upper parts of the gaps between the rods. The typical FE-SEM images (Fig. 3 (b-d)) show clearly that the nanoflowers are self-assembled with irregular nanosheets. The detailed microstructure of the prepared hybrid nanostructure S-2 was further characterized by TEM. Fig. 4 (a) shows the nanoflowers are randomly attached on the top surfaces of the nanorods or in the upper parts of the gaps between the rods. Fig. 4 (b) is the enlarged image of nanoflower in (a), which shows the nanoflower is made up of irregular porous nanosheets. From the HRTEM image of the oval region in Fig. 4 (b), it can be seen clearly that the nanosheet is self-assembled by numerous quantum dots with the size of about 3.6–5.4 nm. The EDS spectrum of the whole nanoflower in Fig. 4 (b) exhibits the presence of the Zn and O element, which unambiguously confirms that the nanoflowers are only composed of ZnO QDs. These 6
results further demonstrate the results of SEM, indicating that ZnO NRs@NFs has been also successfully prepared. Based on the FE-SEM and TEM results, the possible formation mechanism of ZnO NFs is schematically illustrated in Fig. 5. According to the preparation process, the precursor nuclei are formed by the chemical precipitation method as the SDS exists. And, the negative charged SDS anions are preferably adsorbed onto the positive lattice planes of the precursors while the white precipitations gradually formed in the solution [14]. The precursor nuclei have the tendency to aggregate into layered nanosheet structures with the assistance of SDS during the spin coating process. To minimize system’s surface energy, the crystalline nuclei self-assemble into 3D structures (flower-like architecture) with the increase of spin coating times. Because of the introduction of anionic surfactant SDS, the precursors are surrounded by them during the annealing process. As a result, the samples keep the original flower-like architecture, and ZnO NFs are obtained after annealing the precursors. The whole process can be summarized as two steps: (i) Aggregation of 0D quantum dots to 2D nanosheets via SDS. (ii) Formation of 3D structures (porous nanoflowers) via self-assembly of 2D units. For the hybrid nanostructure S-1, SDS has decomposed before the spin coating, and ZnO QDs would not self-assemble into nanosheets. It indicates that SDS plays an important role in the formation of the flower-like architecture. Raman scattering is always used to study the vibrational properties of materials and to identify the crystallization, structural disorder, and defects. Fig. 6 shows the Raman-scattering spectra of the samples performed at room temperature. According to group theory analysis, Raman-active modes for wurtzite ZnO are A1 + 2E2 + E1 [24]. For all spectra, the sharp, strong and dominant peak located at about 438 cm-1 is observed, which is the characteristic scattering peak of the Raman-active dominant E2 (high) mode of wurtzite hexagonal ZnO [25]. Two weaker and broader peaks at 333 and 380 cm-1 are assigned to E2H – E2L (multi-phonon) and A1 (TO) modes, respectively. In addition, the low band centered at 583 cm-1 is a superposition of the A1 (LO) mode at 574 cm-1 and the E1 (LO) mode at 591 cm-1 [5, 26], which is usually 7
ascribed to the presence of structural defects (oxygen vacancies and zinc interstitials, etc.) and impurities in the grown products [27]. Therefore, the higher the ratio of A1&E1 (LO) to E2 (high) is, the more the oxygen vacancies or Zn interstitials present. The Raman results indicate that the hybrid nanostructures keep the overall crystal structure of the bulk ZnO, but have more structural defects. The phonon modes in the specta of the hybrid nanostructures are slightly blue-shifted in the comparison with the spectrum of bare ZnO NRs. Those only few cm−1 blue-shifts derive from quantum confinement effects [22], because of ZnO QDs exist in the hybrid nanostructures, which further confirms that hybrid nanostructures have been successfully fabricated. PL study is also an effective way to investigate the structural properties and defects of the semiconductors [9, 13]. Fig. 7 shows the PL spectra measured at room temperature consisting of a UV peak located at about 380 nm in wavelength and a deep level emission (visible emission) band centered at 530−550 nm with a broad feature. The UV emission band edge is a characteristic band edge of ZnO materials originated from the recombination of free excitons related to a near band edge (NBE) transition [9]. Compared with the spectra, the intensity of UV emission peak of the hybrid nanostructures is much lower than the bare ZnO NRs, which demonstrates that the hybrid nanostructures can effectively suppress photogenerated electron–hole recombination and probably could result a higher photocatalytic performance. In addition, the position of UV emission peak of the hybrid nanostructures shows a blue-shift in the comparison with the bare ZnO NRs. ZnO QDs in our samples is smaller than 7 nm in dimension, and so this blue-shift behavior of PL peak position is naturally explained by the quantum confinement effect [28], which is in agreement with the results from Raman. The deep level emission (DLE) band in the range of 430−670 nm is known as defect luminescence band, which has been reported to originate from various intrinsic and extrinsic defects in ZnO crystals and also influenced by defects, impurities and absorbed molecules on the sample surface [29, 30]. It’s generally agreed that DLE emission is largely related to oxygen vacancies (VO) and surface defects. The inset of Fig. 7 displays a column graph of the DLE to 8
NBE ratio from the three samples, the higher value indicates higher defect density [30]. It can be seen that the hybrid nanostructures show a higher concentration of surface defects as a result of ZnO QDs or ZnO NFs assembled onto the nanorods surface. ZnO NRs@NFs exhibits the highest defect emission, which is attributed to high concentration of inherent surface defects (i.e., oxygen vacancies) contained in the complicated structure of ZnO NFs. The photocatalytic activities of the as-prepared samples were evaluated by measuring the degradation of RhB as a representative pollutant under UV light illumination. Curves of degradation efficiency versus reaction time are shown in Fig. 8. A blank experiment without any catalyst is usually performed at the same experimental conditions to monitor the auto-photocatalytic property of the dye. After irradiation for 8 h, the degradation efficiency of RhB is about 77% (with bare ZnO NRs), 79% (with ZnO NRs@QDs) and 87% (with ZnO NRs@NFs), respectively. It is noted that ZnO NRs@NFs exhibits enhanced photocatalytic performance compared to the other two samples. It is well known that the photocatalytic reaction occurs at the interface between catalyst surfaces and organic pollutants [31]. Thus, the morphology of the catalyst plays a vital role in the photocatalysis, a particular surface and complicated structure will adsorb more dye molecules. The reason for the higher photocatalytic activity of ZnO NRs@NFs is due to the special structural feature of ZnO NFs with an open and porous nanostructured surface layer that significantly facilitates the diffusion and mass transportation of RhB molecules and oxygen species in photochemical reaction of RhB degradation [32]. Furthermore, it is generally accepted that the electron–hole separation efficiency also plays an important role in promoting the photocatalytic activity of semiconductor materials. If a suitable scavenger or surface defect state is available to trap the electron or hole, recombination is prevented and subsequent redox reactions may occur [33]. From Fig. 7, it can be clearly seen that the defect-related emission from ZnO NRs@NFs is relatively stronger than the other samples, indicating more surface defects exist in ZnO NRs@NFs, in which oxygen vacancies are the most 9
suggested defects [34]. Since the oxygen vacancies can serve as the electrons (e–) capturing center to restrain the recombination of photogenerated electrons and holes, the holes (h+) can react with either adsorbed H2O or with the surface OH– groups on the semiconductor materials to form hydroxyl radicals (·OH), and cause oxidation of the organic dye. In addition, the oxygen vacancies are helpful to the generation of active species (·O2ˉ, ·HO2, or ·OH) on the surfaces of semiconductor materials, therefore, are beneficial to the photodegradation of organic dye [35, 36].
4. Conclusion In conclusion, we have successfully synthesized ZnO NRs@QDs and ZnO NRs@NFs hybrid nanostructures by the spin coating method. SDS had an important effect on the morphologies of the as-synthesized samples, and then influencing on their optical and photocatalytic performances. The whole formation process of ZnO NFs could be summarized as two steps: (i) 0D quantum dots with size of 3.6–5.4 nm were assembled to 2D nanosheets via SDS. (ii) 2D units were organized toward 3D porous ZnO NFs. ZnO NRs@NFs showed a dramatically enhanced deep-level emission, indicating the presence of higher defect concentration as compared to the other samples. In addition, ZnO NRs@NFs exhibited the best photocatalytic activity for degradation of RhB under UV light irradiation, which could be attributed to the particular hybrid nanostructure, morphology and surface defects. The particular hybrid nanostructure, the simple and low-cost preparation procedure may promote the assembly for various applications such as photocatalysis, optoelectronics, biomedicine, etc.
Acknowledgments This work is supported by National Programs for High Technology Research and Development of China (863) (Item No. 2013AA032202), the National Natural Science Foundation of China (Grant Nos. 61178074, 61378085, 51479220 and 61475063), the National Youth Program Foundation of China (Grant Nos. 11204104 and 61308095) and Program for the Development of Science and Technology of Jilin province (Item No. 20150101180JC and 20140101205JC). 10
References [1] L.L. Yang, G. Chen, Y.F. Sun, D.L. Han, S. Yang, M. Gao, P. Zou, H.M. Luan, X.W. Kong, J.H. Yang, Effects of interface modification with self-assembled monolayers on the photovoltaic performance of CdS quantum dots sensitized solar cells, Electrochim. Acta 164 (2015) 38-47. [2] J. Chang, Muhammad Z. Ahmad, Wojtek Wlodarski and Eric R. Waclawik, Self-Assembled 3D ZnO Porous Structures with Exposed Reactive {0001} Facets and Their Enhanced Gas Sensitivity, Sensors 13 (2013) 8445-8460. [3] J.Z. Ma, J.L. Liu, Y. Bao, Z.F. Zhu, X.F. Wang, J. Zhang, Synthesis of large-scale uniform mulberry-like ZnO particles with microwave hydrothermal method and its antibacterial property, Ceram. Int. 39 (2013) 2803-2810. [4] L.Y. Zhang, L.W. Yin, C.X. Wang, N. Lun, and Y.X. Qi, Sol-Gel Growth of Hexagonal Faceted ZnO Prism Quantum Dots with Polar Surfaces for Enhanced Photocatalytic Activity, ACS Appl. Mater. Interfaces 2 (2010) 1769-1773. [5] Soumita Mukhopadhyay, Partha Pratim Das, Suvendu Maity, Prasanta Ghosh, P. Sujatha Devi, Solution grown ZnO rods: Synthesis, characterization and defect mediated photocatalytic activity, Appl. Catal. B: Environ. 165 (2015) 128-138. [6] Mansoor Farboda, Esmat Jafarpoora, Hydrothermal synthesis of different colors and morphologies of ZnO nanostructures and comparison of their photocatalytic properties, Ceram. Int. 40 (2014) 6605-6610. [7] X.H. Zhao, F.J. Lou, M. Li, X.D. Lou, Z.Z. Li, J.G. Zhou, Sol-gel-based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic applications, Ceram. Int. 40 (2014) 5507-5514. [8] Z.W. Deng, M. Chen, G.X. Gu, L.M. Wu, A facile method to fabricate ZnO hollow spheres and their photocatalytic property, J. Phys. Chem. B 112 (2008) 16-22. [9] Hye Won Jeong, Seung-Yo Choi, Seong Hui Hong, Sang Kyoo Lim, Dong Suk Han, Ahmed Abdel-Wahab, and Hyunwoong Park, Shape-Dependent Charge Transfers in Crystalline ZnO Photocatalysts: Rods versus Plates, J. Phys. Chem. C 118 (2014) 21331-21338. [10] H. Yang, S.Q. Ni, X. Jiang, W. Jiang, J. Zhan, In situ fabrication of single-crystalline porous ZnO nanoplates on zinc foil to support silver nanoparticles as a stable SERS substrate,
11
CrystEngComm 14 (2012) 6023-6028. [11] G.R. Li, T. Hu, G.L. Pan, T.Y. Yan, X.P. Gao, H.Y. Zhu, Morphology-Function Relationship of ZnO: Polar Planes, Oxygen Vacancies, and Activity, J. Phys. Chem. C 112 (2008) 11859-11864. [12] F. Lu, W.P. Cai, Y.G. Zhang, ZnO Hierarchical Micro/Nanoarchitectures: Solvothermal Synthesis and Structurally Enhanced Photocatalytic Performance, Adv. Funct. Mater. 18 (2008) 1047-1056. [13] X.H. Wang, Y.K. Zhang, C. Hao, F. Feng, H.B. Yin, and N.C. Si, Solid-Phase Synthesis of Mesoporous ZnO Using Lignin-Amine Template and Its Photocatalytic Properties, Ind. Eng. Chem. Res. 53 (2014) 6585-6592. [14] X.S. Chen, J.Y. Liu, X.Y. Jing, J. Wang, D.L. Song, L.H. Liu, Self-assembly of ZnO nanosheets into flower-like architectures and their gas sensing properties, Mater. Lett. 112 (2013) 23-25. [15] X. Wang, W. Liu, J. Liu, F. Wang, J. Kong, S. Qiu, C. He, L. Luan, Synthesis of Nestlike ZnO Hierarchically Porous Structures and Analysis of Their Gas Sensing Properties, ACS Appl. Mater. Interfaces 4 (2012) 817-825. [16] W.J. Dong, C.Y. Huang, T.X. Wei, Y. Zhang, K.N. Zhang, Y. Sun, X. Chen and N. Dai, Nanosphere@nanorod hybrid arrays generated on substrates by a one-pot process as low-reflecting surfaces, RSC Adv. 3 (2013) 21039-21045. [17] M.L. Wang, C.G. Huang, Y.G. Cao, Q.J. Yu, Z.H. Deng, Y. Liu, Z. Huang, J.Q. Huang, Q.F. Huang, W. Guo and J.K. Liang, Dye-sensitized solar cells based on nanoparticle-decorated ZnO/TiO2 core/shell nanorod arrays, J. Phys. D: Appl. Phys. 42 (2009) 155104. [18] S. Bang, S. Lee, T. Park, Y. Ko, S. Shin, S.-Y. Yim, H. Seo and H. Jeon, Dual optical functionality of local surface plasmon resonance for RuO2 nanoparticle-ZnO nanorod hybrids grown by atomic layer deposition, J. Mater. Chem. 22 (2012) 14141-14148. [19] E.S. Jang, J.Y. Bae, J. Yoo, W.I. Park, D.W. Kim, G.C. Yi, T. Yatsui and M. Ohtsu, Quantum confinement effect in ZnO/Mg0.2Zn0.8O multishell nanorod heterostructures, Appl. Phys. Lett. 88 (2006) 023102. [20] Q.X. Zhao, L.L. Yang, M. Willander, B.E. Sernelius, P.O. Holtz, Surface recombination in ZnO nanorods grown by chemical bath deposition, J. Appl. Phys. 104 (2008) 073526. 12
[21] X.Y. Liu, J. Cao, B. Feng, L.L. Yang, M.B. Wei, H.J. Zhai, H.L. Liu, Y.R. Sui, J.H. Yang, Y.H. Liu, Facile fabrication and photocatalytic properties of ZnO nanorods/ZnSe nanosheets heterostructure, Superlattices Microstruct. 83 (2015) 447-458. [22] J.H. Yang, X.Y. Liu, L.L. Yang, Y.X. Wang, Y.J. Zhang, J.H. Lang, M. Gao, B. Feng, Effect of annealing temperature on the structure and optical properties of ZnO nanoparticles, J. Alloys Comp. 477 (2009) 632-635. [23] L.L. Yang, J.H. Yang, X.Y. Liu, Y.J. Zhang, Y.X. Wang, H.G. Fan, D.D. Wang, J.H. Lang, Low temperature synthesis and characterization of ZnO quantum dots, J. Alloys Comp. 463 (2008) 92-98. [24] C.A. Arguello, D.L. Rousseau and S.P.S. Porto, First-Order Raman Effect in Wurtzite-Type Crystals, Phys. Rev. 181 (1969) 1351-1355. [25] A. Umar, Y.B. Hahn, Aligned hexagonal coaxial-shaped ZnO nanocolumns on steel alloy by thermal evaporation, Appl. Phys. Lett. 88 (2006) 173120. [26] N. Ashkenov, B.N. Mbenkum, C. Bundesmann, V. Riede, M. Lorenz, D. Spemann, E.M. Kaidashev, A. Kasic, M. Schubert, M. Grundmann, G. Wagner, H. Neumann, V. Darakchieva, H. Arwin and B. Monemar, Infrared dielectric functions and phonon modes of high-quality ZnO films, J. Appl. Phys. 93 (2003) 126-133. [27] J.J. Wu, S.C. Liu, Catalyst-Free Growth and Characterization of ZnO Nanorods, J. Phys. Chem. B 106 (2002) 9546-9551. [28] J.G. Lu, Z.Z. Ye, Y.Z. Zhang, Q.L. Liang, Sz. Fujita, Z.L. Wang, Self-assembled ZnO quantum dots with tunable optical properties, Appl. Phys. Lett. 89 (2006) 023122. [29] A. B. Djurišić and Y. H. Leung, Optical Properties of ZnO Nanostructures, Small 2 (2006) 944-961. [30] Firat Güder, Yang Yang, Andreas Menzel, Chunyu Wang, Julia Danhof, Kittitat Subannajui, Andreas Hartel, Daniel Hiller, Rajeevan Kozhummal, Niranjan S. Ramgir, Volker Cimalla, Ulrich T. Schwarz and Margit Zacharias, Superior Functionality by Design: Selective Ozone Sensing Realized by Rationally Constructed High-Index ZnO Surfaces, Small 8 (2012) 3307-3314. [31] S. Girish Kumar and K. S. R. Koteswara Rao, Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental 13
applications, RSC Adv. 5 (2015) 3306-3351. [32] B.X. Li and Y.F. Wang, Facile Synthesis and Enhanced Photocatalytic Performance of Flower-like ZnO Hierarchical Microstructures, J. Phys. Chem. C 114 (2010) 890-896. [33] J.C. Wang, P. Liu, X.Z. Fu, Z.H. Li, W. Han, X.X. Wang, Relationship between Oxygen Defects and the Photocatalytic Property of ZnO Nanocrystals in Nafion Membranes, Langmuir 25 (2009) 1218-1223. [34] X.H. Zhao, M. Li, X.D. Lou, Sol-gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity, Adv. Powder Technol. 25 (2014) 372-378. [35] Y. Liu, Z.H. Kang, Z.H. Chen, I. Shafiq, J.A. Zapien, I. Bello, W.J. Zhang, S.T. Lee, Synthesis, Characterization, and Photocatalytic Application of Different ZnO Nanostructures in Array Configurations, Cryst. Growth Des. 9 (2009) 3222-3227. [36] Z.Z. Han, L. Liao, Y.T. Wu, H.B. Pan, S.F. Shen, J.Z. Chen, Synthesis and photocatalytic application of oriented hierarchical ZnO flower-rod architectures, J. Hazard. Mater. 217-218 (2012) 100-106.
Figure Captions
Fig. 1 XRD patterns of (a) ZnO NRs and (c) ZnO QDs. TEM images of (b) ZnO NRs and (d) ZnO QDs. The inset of (b) and (d) is the HRTEM image of ZnO NRs and ZnO QDs, respectively.
Fig. 2 (a, b) TEM images of ZnO NRs@QDs (S-1), the inset figure of (b) is the corresponding HRTEM image.
Fig. 3 FE-SEM images of ZnO NRs@NFs (S-2).
Fig. 4 (a) TEM image of ZnO NRs@NFs (S-2). (b) The enlarged image of nanoflower in (a). (c) HRTEM image of the oval region in (b). (d) EDS image of the whole nanoflower in (b). 14
Fig. 5 Schematic illustration of the formation mechanism of ZnO NFs.
Fig.6 Raman spectra of (a) bare ZnO NRs, (b) ZnO NRs@QDs, (c) ZnO NRs@NFs.
Fig. 7 Room-temperature PL (excited at 325nm) of (a) bare ZnO NRs, (b) ZnO NRs@QDs, (c) ZnO NRs@NFs. Inset is the DLE to NBE ratio of the three samples.
Fig. 8 Degradation efficiency versus reaction time for (a) bare ZnO NRs, (b) ZnO NRs@QDs, (c) ZnO NRs@NFs.
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