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ZnO heterostructure composites with enhanced photocatalytic performance
Superlattices and Microstructures 65 (2014) 134–145
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Synthesis of spindle-like Ag/ZnO heterostructure composites with enhanced photocatalytic performance Zhenfei Zhang a,b, Hairui Liu d, Hua Zhang a,b, Hailiang Dong a,b, Xuguang Liu a,c, Husheng Jia a,b,⇑, Bingshe Xu a,b a Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Ministry of Education, Taiyuan, Shanxi 030024, PR China b College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, PR China c College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, PR China d College of Physics and Electronic Enginee Henan Normal University, Xinxiang 453007, PR China
a r t i c l e
i n f o
Article history: Received 18 July 2013 Received in revised form 16 October 2013 Accepted 31 October 2013 Available online 7 November 2013 Keywords: Ag/ZnO Spindle Heterostructure Electron transfer Photoluminescence Photocatalysis
a b s t r a c t Spindle-like Ag/ZnO heterostructure composites were synthesized through a solution-based surface modification method, during which Ag nanoparticles were deposited on the surfaces of ZnO spindles prepared in advance. The obtained samples were characterized by XRD, SEM, TEM, XPS, PL and UV–vis absorption spectroscopy. The photocatalytic activity of the as-prepared spindle-like Ag/ZnO samples with different Ag contents was tested with the photocatalytic degradation of methylene blue. Results showed that the photocatalytic activity of Ag/ZnO was obviously improved compared with the pure ZnO, and the Ag/ZnO-2 sample prepared with an AgNO3 concentration of 0.05 M had the highest photocatalytic activity. The Ag/ZnO heterostructure composites promoted the separation of photo-induced electrons and holes, which was proved by surface photovoltage spectroscopy. Ó 2013 Elsevier Ltd. All rights reserved.
⇑ Corresponding author at: College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, PR China. Tel./fax: +86 0351 6014138. E-mail address: [email protected] (H. Jia). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.10.045
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1. Introduction Semiconductors, especially metal-oxide semiconductors, have become more and more attractive due to unusual optical, electrical and catalytic properties [1–4]. ZnO, as an important wide-direct band gap (3.37 eV) metal-oxide semiconductor, is promising photocatalyst owing to its high catalytic efficiency, broader ultraviolet (UV) light absorption, lower cost than TiO2, and environmental sustainability [5,6]. Photocatalytic efficiency of ZnO nanoparticles mainly depends on the ability of electron–hole (e/h) pair formation under light illumination. That is to say, the rapid recombination of photoexcited electrons and holes is the main factor to hinder the improvement of its photocatalytic efficiency. The most efficient way to accelerate the charge carrier separation is to decorate ZnO with noble metals or fabricate composite semiconductors [7–11]. For example, Zhang et al. [10] fabricated hierarchical ZnO/ NiO nanowires using a facile electron spinning technique. Wang et al. [11] synthesized ZnO/Au composite nanoparticles by a two-step way. Among multitudinous metals, Ag has been widely used in the fields of biomedicine, electronics and optics. The combination of ZnO with Ag can produce peculiar properties in optical and electrical characteristics and especially is an effective way to modulate optical phenomena including emission, harvesting, and concentration of electromagnetic radiation, which are related to optimizing photocatalytic and photovoltaic systems and to the development of sensing and optoelectronic devices [12]. In the past few years, there have been many reports about the preparation of Ag/ZnO composites. For example, Hong et al. [13] reported the preparation of metallic Ag nanoparticles on ZnO films. Han et al. [14] fabricated Ag/ZnO flower heterostructures via a photoreduction method; Yin et al. [15] prepared Ag nanoparticle/ZnO nanorod nanocomposites by a seedmediated method; Gu et al. [16] synthesized dendrite-like ZnO/Ag heterostructures with Ag nanowires as cores and ZnO nanorods as shells through a two-step hydrothermal method. Xie and Wu [17] synthesized Ag/ZnO nanosphere composites through a template solvothermal reaction method. However, there is no report about spindle-like Ag/ZnO heterostructural composites. Moreover, the above-cited methods require complex and expensive equipments, severe synthesis conditions or high synthesis temperature. In this work, spindle-like Ag/ZnO heterostructural composites were prepared through a solutionbased surface modification method, which is used to modify the surfaces of materials through surface adsorption, coating, deposition and other physical or chemical means [18,19]. A systematic study is presented for detailed characterizations of the phase, microstructure, composition, optical and photocatalytic property of the samples. Finally, the optical properties and photocatalytic activity of as-prepared composites with different content of Ag are studied.
2. Experimental 2.1. Chemicals and materials All the reagents were used as received without further purification. Silver nitrate and sodium peroxide were obtained from Aldrich. Zinc acetate was obtained from Beijng Chem. Co. (China). Ethylene glycol (EG) was purchased from Huadong Chem. Co. (China). Ethanol was purchased from Beijng Chem. Co. (China). The water used was purified through a distillation system.
2.2. Synthesis of ZnO spindles In a typical procedure, 100 mL of aqueous solution containing 0.01 M zinc acetate (Zn(Ac)22H2O) (99.9%) and 0.015 M sodium peroxide (Na2O2) was prepared and stirred for 30 min to obtain a white slurry. And then the solution was put into a Teflon-lined stainless autoclave of 100 mL capacity. Subsequently, the sealed autoclave was put into an oven and heated at 90 °C for 5 h. After the reaction was completed, the resulting solid products were centrifuged, washed with distilled water and ethanol to remove the ions possibly remaining in the final products, and finally dried at 60 °C for 12 h in air.
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2.3. Synthesis of spindle-like Ag/ZnO composites Spindle-like Ag/ZnO composites were synthesized through the surface modification method of depositing Ag nanoparticles on the surfaces of ZnO spindles in solution. In a typical synthesis process, 0.03 g of spindle-like ZnO was dispersed into 20 mL of ethylene glycol solution containing desired amount of AgNO3 with vigorous stirring for about 30 min at room temperature. Then 20 mL of ethylene glycol solution with 0.7 g of PVP was dropped into above solution under quick stirring. After that, the mixed solution was transferred to a microwave oven, heated with a power of 500 W for 10 min, and then cooled down. Then, the precipitates were centrifuged and washed with distilled water and ethanol several times to remove the impurities. The products were dried in vacuum at 60 °C for 12 h, and spindle-like Ag/ZnO composites were obtained. Different Ag/ZnO composites were obtained by using 0.01, 0.05 and 0.075 M AgNO3 glycol solutions in the above way, and they were labeled as Ag/ ZnO-1, Ag/ZnO-2 and Ag/ZnO-3, respectively. For comparison, the pure Ag nanoparticles were obtained in the absence of ZnO during the process of preparation of the Ag/ZnO composites for further test.
2.4. Photocatalytic activity of spindle-like Ag/ZnO composites The photocatalytic performance of the samples was evaluated by the degradation of methylene blue (MB) used as a representative dye pollutant. In detail, 7.5 mg of Ag/ZnO photocatalysts was dispersed into 100 mL of an aqueous methylene blue solution (30 ppm) under magnetic stirring. After stirring under dark for enough time to attain the adsorption equilibrium, the mixture was irradiated by a 15-W UV light-tube (365 nm). After a given irradiation time, 2 mL of the mixture was withdrawn and centrifuged to separate the residual catalyst, and then the absorbance intensity at 664 nm was monitored by a UV–vis spectrophotometer (Tuopu 756PC, China). The degradation rate (R) was calculated according to the sample absorbance variation:
R ¼ ðA0 AÞ=A0 ¼ ðC 0 CÞ=C 0 where C0 represents the initial concentration of the mixture and C the concentration after degradation for a given time, while A0 and A represent the initial absorbance and absorbance after degradation for a given time, respectively.
Fig. 1. XRD patterns of the pure ZnO particles and Ag/ZnO composites.
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3. Results and discussion 3.1. XRD analysis The power XRD patterns of ZnO particles, and Ag/ZnO composites are illustrated in Fig. 1. From Fig. 1(a), it can be found that pure ZnO sample had sharp peaks at 31.8°, 34.4°, 36.3°, 47.6°, 56.7°, 62.9°, 68.0° and 69.2°. The positions of all the peaks can be indexed to the wurtzite ZnO structure (JCPDS File No. 36-1451) and correspond to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2) and (2 0 1), respectively. Compared with the result obtained from ZnO particles, Ag/ZnO composite samples had four more peaks at 38.1°, 44.3°, 64.5° and 77.5°, which were readily assigned to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of face center cubic (fcc) structure of silver (JCPDS File No. 04-783), respectively. Additionally, the peak intensity of Ag phase gradually increased with the increase of Ag amount in the Ag/ZnO composites, and no other peaks and notable shifts were observed in the powder diffraction peaks. These results further confirm that Ag/ZnO composites were made up with ZnO and Ag phases and no other phases. 3.2. Morphology and composition In order to characterize the morphologies of as-prepared ZnO particles, Ag/ZnO composites coated on the surface by Ag particles, SEM and TEM observation were carried out. Also the average size of the products was statistically analyzed by JEOL SmileView software. The SEM image in the Fig. 2(a) clearly show that as-prepared ZnO particles were uniform and have the representative spindle-like shape with average length 250 nm and diameter of 20–100 nm. In addition, the surfaces of ZnO spindles were very rough, which suggests that ZnO spindles were made up with many even smaller nanoparticles. Fig. 2(b) shows the SEM image of Ag/ZnO particles prepared with an AgNO3 concentration of 0.01 M. It can be found that the Ag/ZnO particles remained spindle-like structure, and spherical Ag particles with an average diameter of 20 nm were observed on the surfaces of ZnO spindles as labelled
Fig. 2. FESEM images of as-synthesized ZnO (a); Ag/ZnO composites prepared with different concentrations of AgNO3: (b) 0.01 M, (c) 0.05 M, and (d) 0.075 M.
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by the circles, which suggests that the structure of ZnO was almost maintained after the formation of Ag nanoparticles. Fig. 2(c) shows the SEM image of Ag/ZnO particles prepared with an AgNO3 concentration of 0.05 M. A great amount of spherical Ag particles with an average diameter of 30 nm were observed on the surfaces of ZnO spindles. The SEM image of Ag/ZnO particles prepared with an AgNO3 concentration of 0.075 M is shown in Fig. 2(d). Much aggregation of Ag particles was observed on the surfaces of ZnO spindles, and the ZnO spindles were wrapped by Ag nanoparticles. Fig. 3(a) and (b) shows the TEM images of the Ag/ZnO heterostructures prepared with an AgNO3 concentration of 0.01 M. It is obvious that the spindle-like Ag/ZnO heterostructures were consisting of metallic Ag nanoparticles with an average diameter of 20 nm (highlighted by circles) and ZnO spindles. Fig. 3(c) shows an HRTEM image of the selected area in Fig. 3(b). Two kinds of lattice spacing and a distinguished interface were observed. The plane fringe with a crystalline plane spacing of 0.236 nm was assigned to the (1 1 1) plane of Ag with fcc structure; the other one 0.256 nm was corresponding to the spacing of (0 0 2) planes of wurtzite ZnO. These results demonstrate that Ag particles in the Ag/ ZnO composites were fcc structures and ZnO crystals wurtzite structures, which are in good agreement with the XRD results. Fig. 3(d) shows the EDS spectrum of Ag/ZnO composites prepared with an AgNO3 concentration of 0.01 M. Oxygen, zinc and silver element were observed, which provides powerful evidence for the formation of Ag on ZnO spindles. 3.3. Possible growth mechanism for the spindle-like ZnO nanoparticles The processes of the formation of ZnO are shown as follows:
Fig. 3. TEM (a–b) and HRTEM (c) images of Ag/ZnO-3 composites prepared with an AgNO3 concentration of 0.075 M; and the elemental spectrum of Ag/ZnO-3 composites revealed by EDS (d).
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2Na2 O2 þ 2H2 O ! 4NaOH þ O2
ð1Þ
Zn2þ þ 2OH ! ZnðOHÞ2
ð2Þ
ZnðOHÞ2 þ 2OH ! ½ZnðOHÞ4 2
ð3Þ
½ZnðOHÞ4 2 ! ZnO þ 2OH þ H2 O
ð4Þ
According to the above chemical reactions which take place in the aqueous solution, the final morphologies of ZnO crystals are determined by the intrinsic crystal structures and the characteristic of [Zn(OH)4]2 precursors which might affect the competition between thermodynamics and kinetics during the process of nucleation and ZnO crystals growth [20]. As it is known that ZnO is a kind of hexagonal polar crystal with a close packing of Zn2+ and O2, and the main crystal planes include a facet, and six symmetric nonpolar planes. The positive polar zinc (0 0 0 1) facet, a polar oxygen (0 0 0 1) polar surfaces have high surface enerZn2+ terminated (0 0 0 1) and negative O2 terminated (0 0 0 1) gies, and these facets tend to grow along their normal direction and eventually disappear from the final appearance according to the GibbsWulff theorem. Moreover, the negative [Zn(OH)4]2 complexes can be preferably adsorbed on the surface of ZnO nuclei, which is helpful to the growth of ZnO nuclei along the [0 0 0 1] direction [21]. According to the above discussion, a possible growth mechanism of the ZnO spindles can be proposed. The formation of ZnO spindles may follow the growth mechanism of ‘‘nucleation–aggregation– self-assembly’’ process [22]. Initially, when Na2O2 is added into the zinc salt solution, OH ions would be first produced. And then Zn(OH)2 is precipitated from the solution (Eqs. (1) and (2)). When the concentration of OH ion is excessive enough, [Zn(OH)4]2 complexes would be formed (Eq. (3)). A large quantity of small primary ZnO nuclei are intensively generated by the decomposition of [Zn(OH)4]2 species when the concentrations of ZnO reached the supersaturation degree of ZnO (Eq. (4)). The freshly formed ZnO nuclei are unstable because of their high surface energy and they tended to aggregate rapidly. In addition, the negative ions [Zn(OH)4]2 could be adsorbed onto the polar (0 0 0 1) surface terminated by Zn2+ as a result of charge compensation, which results in the redistribution of the surface energy, and the change in the growth rate of different facets. After the nuclei aggregation through assembly, most probably by electrostatic gravitation and intermolecular force, well-defined ZnO spindles are formed, which is similar to the formation process of YF3 nanospindles [23]. Scheme 1 shows the schematic illustration for the possible formation process of the ZnO spindle-like structure. 3.4. XPS analysis In order to clarify the elements and chemical states of Ag/ZnO composites, the surface structure of the Ag/ZnO synthesized with an AgNO3 concentration of 0.01 M was investigated by XPS analysis. The high-resolution spectra of O, Zn and Ag are shown in Fig. 4(a)–(c). In Fig. 4(a), the asymmetric spectra of the O1s were fitted into two symmetrical peaks (a and b locating at 530.0 eV and 531.7 eV, respecively), which indicates two different kinds of O in the sample. They should be associated with the lattice oxygen (OL) of ZnO and chemisorbed oxygen caused by the surface hydroxyl (OH) [24], respectively. Fig. 4(b) displays a strong and symmetric peak profile centered at 1021.1 eV correspond-
Scheme 1. The schematic illustration for the possible formation process of the ZnO spindle-like structure.
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Fig. 4. High-resolution XPS spectra of Ag/ZnO-1 composites prepared with an AgNO3 concentration of 0.01 M for the elements of: (a) O, (b) Zn, and (c) Ag.
ing to the Zn2p3/2 of ZnO, corroborating the existence of Zn atom in the form of zinc ion on the sample surface [25]. Fig. 4(c) shows the Ag3d XPS spectra for the sample. The two peaks locating at 367.0 and 372.9 eV shifted notably to lower binding energy compared with the pure metallic Ag (the standard binding energies of Ag3d5/2 and Ag3d3/2 are about 368.2 eV and 374.2 eV, respectively), indicating that the electron density of Ag decreased [26]. For Ag/ZnO, the negative shifts of Ag3d were due to the interaction between Ag nanoparticle and ZnO. Because the work function of ZnO is larger than that of Ag, once the ZnO (work function = 5.3 eV) particles come into contact with Ag (work function = 4.26 eV), electron transfer occurs from Ag to the conduction band (CB) of ZnO to obtain the equilibrium of Fermi energy level. Finally, a new Fermi energy level in Ag/ZnO heterostructures is formed, resulting in the tendency of Ag to higher valence. Because the binding energy of oxidation state Ag+ is lower than that of substance Ag (Ag is a kind of transition metal), the peaks of Ag3d in Ag/ZnO heterostructure shift obviously. Moreover, the shifts of Ag3d5/2 and Ag3d3/2 to lower binding energies also demonstrate the strong interaction between ZnO and Ag, which further confirms the formation of Ag/ZnO heterostructural composites. 3.5. UV–vis spectra The UV–vis absorption spectra of pure ZnO, pure Ag and Ag/ZnO-1, Ag/ZnO-2 and Ag/ZnO-3 composites are presented in Fig. 5. The pure ZnO spindles (curve b) showed a steep absorption at 368 nm, owing to the excitonic absorption, while pure Ag particles (curve a) displayed a surface plasmon resonance peaks at 422 nm, which corresponds to transverse plasmon resonance [27]. For Ag/ZnO composites, their absorption was not a simple superposition of ZnO and Ag (labeled as curve c, d and e), and the surface plasmon band was notably broadened and red-shifted, probably resulting from a strong interfacial electronic coupling between neighboring ZnO particles and Ag, which might
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Fig. 5. UV–vis absorption spectra of Ag particles (a), ZnO (b), Ag/ZnO-1 (c), Ag/ZnO-2 (d), and Ag/ZnO-3 (e).
enhance the photoactivities of materials under visible light [28]. As is known, the electron density of metal determines the position of plasmon absorption of metal nanocrystals [29], which is given as follows: 1=2
k ¼ ½4p2 c2 meffe =Ne2
where k is the plasmon absorption wavelength, Ne is the electron density of metal, meffe the effective mass of the free electron of metal. It can be known that the plasmon absorption (k) of metal depends on the electron density of metal. In our experiment, the red shift of the plasmon peak of Ag/ZnO suggests that the electron density of Ag was decreased. The decrease of the electron density of Ag nanocrystals may be caused by the transfer of the electron from the Ag nanocrystals to ZnO particles. Because the work function of ZnO (5.2 eV) is larger than that of Ag (4.26 eV), the Fermi energy level of Ag is higher than that of ZnO (first electron affinity is around 4.3 eV). Thus, as illustrated in Fig. 6, the transfer of electrons from Ag to ZnO will occur to make the two achieve equilibration when they contact. The electron transfer from Ag to ZnO results in the reduction of electrons in the Ag nanocrystals. Therefore the surface plasmon absorption of Ag/ZnO composites gets red-shifted. In addition, a blue shift in ZnO/Ag-3 sample was observed in comparison to Ag/ZnO-2. The blue shift may be attributed to the size decrease of Ag nanoparticles [30], which could be seen from Fig. 2(c) and (d). Similar results for ZnO/Au composites have been reported [30].
Fig. 6. The mechanism of electron transfer between ZnO and Ag without UV irradiation. E : Fermi level; CB: conduction band; VB: valance band; : electron.
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3.6. Photoluminescence spectra The photoluminescence (PL) spectra of as-prepared ZnO particles are shown in Fig. 7(a). A narrow UV peak centered at about 370 nm and a broad peak at 600 nm were observed. The UV peak was assigned to the recombination of electron–hole pairs in the semiconductor through an exciton–exciton collision process and the broad peak was caused by surface states or oxygen vacancies [31]. In PL spectra of Ag/ZnO composites with different Ag content (curve b–d), two emission bands were observed in the UV/vis range, which came from the ZnO. One the other hand, the PL spectra of Ag/ZnO showed a red-shift in the wavelength and a decrease in the intensity of UV emission peak, which resulted from oxygen vacancies in Ag/ZnO composites and electron transfer from ZnO to Ag on the interface [32], respectively. Moreover, it can be seen clearly that the sample Ag/ZnO-2 showed the minimum PL intensity. For Ag/ZnO heterostructure systems, Ag as shell and ZnO as core, Ag crystals grew up on the surfaces of ZnO spindles as seeds, and formed a thin positively charged layer. Because the bottom energy level of the conduction band of ZnO is higher than the new equilibrium Fermi energy level (Ef) of Ag/ZnO, the photoexcited electrons on the conduction band could transfer from ZnO to Ag nanoparticles, causing the reduction in combination between electrons and holes for ZnO, which results in the decrease of the PL intensity in Ag/ZnO system. With the increase of Ag content, as sample Ag/ZnO-2, more metal sites were formed and available to accept electrons, which led to a corresponding increase in separation effects for the photoinduced electrons and holes, and a declined intensity of PL emission [3]. However, when the Ag content exceeded certain value (as sample Ag/ZnO-3), the PL intensity increased again. This was attributed to the absorption or reflection of emission at the interface between ZnO and Ag, which was mainly induced by the strong surface plasmon absorption of Ag particles [33]. Moreover, as can be seen from Fig. 1(c) and (d) with the increase of Ag content, Ag particles aggregated seriously, and even formed cluster, which would reduce the possibility of forming homogeneous composites, and thus, the interfacial interaction between ZnO and Ag nanoparticles got weakened, which, in a certain degree, improved the possibility of recombination between holes with electrons. 3.7. Photocatalytic performance Methylene blue(MB) was used as a representative organic pollutant to evaluate the photocatalytic performance of ZnO spindles, Ag nanoparticles and Ag/ZnO composites with different Ag contents. Fig. 8 shows the photodegradation curves of methylene blue (MB) as a function of time. It can be found that the degradation of MB over pure Ag particles was negligible, and Ag/ZnO composites with different Ag contents obviously had higher photocatalytic activities than pure ZnO spindles. Moreover, the Ag/ZnO-2 sample exhibited the highest catalytic activity. After 60 min, it gave the degradation rate of 83.5%, while the degradation rate on ZnO particles reached only 43.2%.
Fig. 7. PL spectra of Ag/ZnO composites prepared with various Ag contents.
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Fig. 8. Photocatalytic degradation kinetics of the synthesized samples for the methylene blue (MB) solution.
The photocatalytic process of Ag/ZnO composites can be analyzed through their energy band structure. Because the Fermi energy level of ZnO is lower than that of Ag, the transfer of electrons from Ag to the conduction band (CB) of ZnO will occur to make the two achieve equilibration when the heterostructure is formed. The process shown in Fig. 6 can be expressed as:
Ag ! Agþ þ e
ð5Þ
When Ag/ZnO composites are irradiated by UV light, some electrons (e) of ZnO in the valence band (VB) will be excited to the conduction band (CB) with the same amount of holes (h+) coming out in the VB. As it is shown in Fig. 9, the bottom energy level of the conduction band (CB) of ZnO is higher than the equilibrium Fermi energy level (Ef) of Ag/ZnO heterostructure, so photoexcited electrons will transfer from ZnO particles to Ag particles driven by the potential energy. Furthermore, Ag particles or clusters on the surface of ZnO particles work as a sink, and they not only reduce the recombination of photoinduced electrons and holes but also prolong the lifetime of photogenerated pairs. And then the adsorbed O2 as electronic acceptors can easily trap photoelectrons and produce a superoxide anion radical (O2), while superoxide anion radical and photoinduced holes can also readily react with H+ or OH to generate hydroxyl radical species (OH), and the hydroxyl radical is an extremely strong oxidant for the degeneration of organic chemicals [34]. The photocatalytic reaction process can be proposed as follows: [34,35]: þ
ZnO þ hmðUVÞ ! ZnOðecb þ hvb Þ
ð6Þ
ecb þ O2 ! O2
ð7Þ
Agþ þ ecb ! Ag
ð8Þ
þ
hvb þ OH ! OH
ð9Þ
O2 þ 2Hþ ! 2 OH
ð10Þ
OH þ methylene blue ! Degradation Products
ð11Þ
Therefore the photocatalytic activity of Ag/ZnO photocatalysts was enhanced, and higher than that of ZnO. And photocatalysis results demonstrate that with the increase of Ag content, the photocatalytic activity of Ag/ZnO composites increased firstly and then decreased, and the Ag/ZnO-2 heterostructural composites had the highest photocatalytic activity because the photoinduced electrons
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Fig. 9. Charge separation process and the photocatalytic mechanism of as-prepared Ag/ZnO heterostructure under UV irradiation.
and holes were effectively separated. However, when the silver content of the composites was too large for Ag/ZnO-3, ZnO particles were tightly wrapped by the layer of silver nanoparticles and the Ag/ZnO heterostructural composites aggregated seriously, as can been seen from Fig. 2(d), which may reduce the possibility of semiconductor surface for light absorption and pollutant surface contact and decrease the catalytic efficiency of the ZnO particles. Therefore the Ag/ZnO-3 composites showed lower photocatalytic activity. What is more, it is found that the photocatalytic activities and PL intensities of the samples showed an opposite variation tendency with the change of Ag content. The Ag/ ZnO-2 composite sample had the highest photocatalytic activity, while its PL intensity is the weakest. This finding proved the existence of separation effects between photoinduced electrons and holes when the samples were irritated by UV light. 4. Conclusions In summary, spindle-like Ag/ZnO heterostructural composites were prepared through a solutionbased surface modification method. Detailed structural characterization and elemental analysis demonstrate that the surfaces of wurtzite ZnO spindles were coated by face-center-cubic Ag nanoparticles. The absorption band of Ag/ZnO composites exhibit some red-shifts and the PL was weakened, revealing the strong interfacial interaction between ZnO and Ag nanoparticles. This would improve the utilization efficiency of the light in the UV/visible light range when the composites are used as photocatalysts. The effect of Ag amount on the optical properties of Ag/ZnO composites was also studied, and results show that the intensity of PL for Ag/ZnO composites decreased firstly and then increased. The photocatalytic activity of Ag/ZnO composites was higher than that of ZnO, and Ag/ ZnO-2 heterostructural composites prepared with an AgNO3 concentration of 0.05 M showed the highest photocatalytic activity because the photoinduced electrons and holes were effectively separated. Acknowledgments This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0972), the National Natural Science Foundation of China (No. 51152001), the International Science &Technology Cooperation Program of China (2011DFA52290, 2012DFR50460), and the Research Project Supported by Shanxi Scholarship Council of China (2012-038). References [1] M. Afzaal, M.A. Malik, P. O’Brien, New J. Chem. 31 (2007) 2029–2040.
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Synthesis of spindle-like Ag/ZnO heterostructure composites with enhanced photocatalytic performance Zhenfei Zhang a,b, Hairui Liu d, Hua Zhang a,b, Hailiang Dong a,b, Xuguang Liu a,c, Husheng Jia a,b,⇑, Bingshe Xu a,b a Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Ministry of Education, Taiyuan, Shanxi 030024, PR China b College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, PR China c College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, PR China d College of Physics and Electronic Enginee Henan Normal University, Xinxiang 453007, PR China
a r t i c l e
i n f o
Article history: Received 18 July 2013 Received in revised form 16 October 2013 Accepted 31 October 2013 Available online 7 November 2013 Keywords: Ag/ZnO Spindle Heterostructure Electron transfer Photoluminescence Photocatalysis
a b s t r a c t Spindle-like Ag/ZnO heterostructure composites were synthesized through a solution-based surface modification method, during which Ag nanoparticles were deposited on the surfaces of ZnO spindles prepared in advance. The obtained samples were characterized by XRD, SEM, TEM, XPS, PL and UV–vis absorption spectroscopy. The photocatalytic activity of the as-prepared spindle-like Ag/ZnO samples with different Ag contents was tested with the photocatalytic degradation of methylene blue. Results showed that the photocatalytic activity of Ag/ZnO was obviously improved compared with the pure ZnO, and the Ag/ZnO-2 sample prepared with an AgNO3 concentration of 0.05 M had the highest photocatalytic activity. The Ag/ZnO heterostructure composites promoted the separation of photo-induced electrons and holes, which was proved by surface photovoltage spectroscopy. Ó 2013 Elsevier Ltd. All rights reserved.
⇑ Corresponding author at: College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, PR China. Tel./fax: +86 0351 6014138. E-mail address: [email protected] (H. Jia). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.10.045
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1. Introduction Semiconductors, especially metal-oxide semiconductors, have become more and more attractive due to unusual optical, electrical and catalytic properties [1–4]. ZnO, as an important wide-direct band gap (3.37 eV) metal-oxide semiconductor, is promising photocatalyst owing to its high catalytic efficiency, broader ultraviolet (UV) light absorption, lower cost than TiO2, and environmental sustainability [5,6]. Photocatalytic efficiency of ZnO nanoparticles mainly depends on the ability of electron–hole (e/h) pair formation under light illumination. That is to say, the rapid recombination of photoexcited electrons and holes is the main factor to hinder the improvement of its photocatalytic efficiency. The most efficient way to accelerate the charge carrier separation is to decorate ZnO with noble metals or fabricate composite semiconductors [7–11]. For example, Zhang et al. [10] fabricated hierarchical ZnO/ NiO nanowires using a facile electron spinning technique. Wang et al. [11] synthesized ZnO/Au composite nanoparticles by a two-step way. Among multitudinous metals, Ag has been widely used in the fields of biomedicine, electronics and optics. The combination of ZnO with Ag can produce peculiar properties in optical and electrical characteristics and especially is an effective way to modulate optical phenomena including emission, harvesting, and concentration of electromagnetic radiation, which are related to optimizing photocatalytic and photovoltaic systems and to the development of sensing and optoelectronic devices [12]. In the past few years, there have been many reports about the preparation of Ag/ZnO composites. For example, Hong et al. [13] reported the preparation of metallic Ag nanoparticles on ZnO films. Han et al. [14] fabricated Ag/ZnO flower heterostructures via a photoreduction method; Yin et al. [15] prepared Ag nanoparticle/ZnO nanorod nanocomposites by a seedmediated method; Gu et al. [16] synthesized dendrite-like ZnO/Ag heterostructures with Ag nanowires as cores and ZnO nanorods as shells through a two-step hydrothermal method. Xie and Wu [17] synthesized Ag/ZnO nanosphere composites through a template solvothermal reaction method. However, there is no report about spindle-like Ag/ZnO heterostructural composites. Moreover, the above-cited methods require complex and expensive equipments, severe synthesis conditions or high synthesis temperature. In this work, spindle-like Ag/ZnO heterostructural composites were prepared through a solutionbased surface modification method, which is used to modify the surfaces of materials through surface adsorption, coating, deposition and other physical or chemical means [18,19]. A systematic study is presented for detailed characterizations of the phase, microstructure, composition, optical and photocatalytic property of the samples. Finally, the optical properties and photocatalytic activity of as-prepared composites with different content of Ag are studied.
2. Experimental 2.1. Chemicals and materials All the reagents were used as received without further purification. Silver nitrate and sodium peroxide were obtained from Aldrich. Zinc acetate was obtained from Beijng Chem. Co. (China). Ethylene glycol (EG) was purchased from Huadong Chem. Co. (China). Ethanol was purchased from Beijng Chem. Co. (China). The water used was purified through a distillation system.
2.2. Synthesis of ZnO spindles In a typical procedure, 100 mL of aqueous solution containing 0.01 M zinc acetate (Zn(Ac)22H2O) (99.9%) and 0.015 M sodium peroxide (Na2O2) was prepared and stirred for 30 min to obtain a white slurry. And then the solution was put into a Teflon-lined stainless autoclave of 100 mL capacity. Subsequently, the sealed autoclave was put into an oven and heated at 90 °C for 5 h. After the reaction was completed, the resulting solid products were centrifuged, washed with distilled water and ethanol to remove the ions possibly remaining in the final products, and finally dried at 60 °C for 12 h in air.
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2.3. Synthesis of spindle-like Ag/ZnO composites Spindle-like Ag/ZnO composites were synthesized through the surface modification method of depositing Ag nanoparticles on the surfaces of ZnO spindles in solution. In a typical synthesis process, 0.03 g of spindle-like ZnO was dispersed into 20 mL of ethylene glycol solution containing desired amount of AgNO3 with vigorous stirring for about 30 min at room temperature. Then 20 mL of ethylene glycol solution with 0.7 g of PVP was dropped into above solution under quick stirring. After that, the mixed solution was transferred to a microwave oven, heated with a power of 500 W for 10 min, and then cooled down. Then, the precipitates were centrifuged and washed with distilled water and ethanol several times to remove the impurities. The products were dried in vacuum at 60 °C for 12 h, and spindle-like Ag/ZnO composites were obtained. Different Ag/ZnO composites were obtained by using 0.01, 0.05 and 0.075 M AgNO3 glycol solutions in the above way, and they were labeled as Ag/ ZnO-1, Ag/ZnO-2 and Ag/ZnO-3, respectively. For comparison, the pure Ag nanoparticles were obtained in the absence of ZnO during the process of preparation of the Ag/ZnO composites for further test.
2.4. Photocatalytic activity of spindle-like Ag/ZnO composites The photocatalytic performance of the samples was evaluated by the degradation of methylene blue (MB) used as a representative dye pollutant. In detail, 7.5 mg of Ag/ZnO photocatalysts was dispersed into 100 mL of an aqueous methylene blue solution (30 ppm) under magnetic stirring. After stirring under dark for enough time to attain the adsorption equilibrium, the mixture was irradiated by a 15-W UV light-tube (365 nm). After a given irradiation time, 2 mL of the mixture was withdrawn and centrifuged to separate the residual catalyst, and then the absorbance intensity at 664 nm was monitored by a UV–vis spectrophotometer (Tuopu 756PC, China). The degradation rate (R) was calculated according to the sample absorbance variation:
R ¼ ðA0 AÞ=A0 ¼ ðC 0 CÞ=C 0 where C0 represents the initial concentration of the mixture and C the concentration after degradation for a given time, while A0 and A represent the initial absorbance and absorbance after degradation for a given time, respectively.
Fig. 1. XRD patterns of the pure ZnO particles and Ag/ZnO composites.
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3. Results and discussion 3.1. XRD analysis The power XRD patterns of ZnO particles, and Ag/ZnO composites are illustrated in Fig. 1. From Fig. 1(a), it can be found that pure ZnO sample had sharp peaks at 31.8°, 34.4°, 36.3°, 47.6°, 56.7°, 62.9°, 68.0° and 69.2°. The positions of all the peaks can be indexed to the wurtzite ZnO structure (JCPDS File No. 36-1451) and correspond to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2) and (2 0 1), respectively. Compared with the result obtained from ZnO particles, Ag/ZnO composite samples had four more peaks at 38.1°, 44.3°, 64.5° and 77.5°, which were readily assigned to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of face center cubic (fcc) structure of silver (JCPDS File No. 04-783), respectively. Additionally, the peak intensity of Ag phase gradually increased with the increase of Ag amount in the Ag/ZnO composites, and no other peaks and notable shifts were observed in the powder diffraction peaks. These results further confirm that Ag/ZnO composites were made up with ZnO and Ag phases and no other phases. 3.2. Morphology and composition In order to characterize the morphologies of as-prepared ZnO particles, Ag/ZnO composites coated on the surface by Ag particles, SEM and TEM observation were carried out. Also the average size of the products was statistically analyzed by JEOL SmileView software. The SEM image in the Fig. 2(a) clearly show that as-prepared ZnO particles were uniform and have the representative spindle-like shape with average length 250 nm and diameter of 20–100 nm. In addition, the surfaces of ZnO spindles were very rough, which suggests that ZnO spindles were made up with many even smaller nanoparticles. Fig. 2(b) shows the SEM image of Ag/ZnO particles prepared with an AgNO3 concentration of 0.01 M. It can be found that the Ag/ZnO particles remained spindle-like structure, and spherical Ag particles with an average diameter of 20 nm were observed on the surfaces of ZnO spindles as labelled
Fig. 2. FESEM images of as-synthesized ZnO (a); Ag/ZnO composites prepared with different concentrations of AgNO3: (b) 0.01 M, (c) 0.05 M, and (d) 0.075 M.
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by the circles, which suggests that the structure of ZnO was almost maintained after the formation of Ag nanoparticles. Fig. 2(c) shows the SEM image of Ag/ZnO particles prepared with an AgNO3 concentration of 0.05 M. A great amount of spherical Ag particles with an average diameter of 30 nm were observed on the surfaces of ZnO spindles. The SEM image of Ag/ZnO particles prepared with an AgNO3 concentration of 0.075 M is shown in Fig. 2(d). Much aggregation of Ag particles was observed on the surfaces of ZnO spindles, and the ZnO spindles were wrapped by Ag nanoparticles. Fig. 3(a) and (b) shows the TEM images of the Ag/ZnO heterostructures prepared with an AgNO3 concentration of 0.01 M. It is obvious that the spindle-like Ag/ZnO heterostructures were consisting of metallic Ag nanoparticles with an average diameter of 20 nm (highlighted by circles) and ZnO spindles. Fig. 3(c) shows an HRTEM image of the selected area in Fig. 3(b). Two kinds of lattice spacing and a distinguished interface were observed. The plane fringe with a crystalline plane spacing of 0.236 nm was assigned to the (1 1 1) plane of Ag with fcc structure; the other one 0.256 nm was corresponding to the spacing of (0 0 2) planes of wurtzite ZnO. These results demonstrate that Ag particles in the Ag/ ZnO composites were fcc structures and ZnO crystals wurtzite structures, which are in good agreement with the XRD results. Fig. 3(d) shows the EDS spectrum of Ag/ZnO composites prepared with an AgNO3 concentration of 0.01 M. Oxygen, zinc and silver element were observed, which provides powerful evidence for the formation of Ag on ZnO spindles. 3.3. Possible growth mechanism for the spindle-like ZnO nanoparticles The processes of the formation of ZnO are shown as follows:
Fig. 3. TEM (a–b) and HRTEM (c) images of Ag/ZnO-3 composites prepared with an AgNO3 concentration of 0.075 M; and the elemental spectrum of Ag/ZnO-3 composites revealed by EDS (d).
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2Na2 O2 þ 2H2 O ! 4NaOH þ O2
ð1Þ
Zn2þ þ 2OH ! ZnðOHÞ2
ð2Þ
ZnðOHÞ2 þ 2OH ! ½ZnðOHÞ4 2
ð3Þ
½ZnðOHÞ4 2 ! ZnO þ 2OH þ H2 O
ð4Þ
According to the above chemical reactions which take place in the aqueous solution, the final morphologies of ZnO crystals are determined by the intrinsic crystal structures and the characteristic of [Zn(OH)4]2 precursors which might affect the competition between thermodynamics and kinetics during the process of nucleation and ZnO crystals growth [20]. As it is known that ZnO is a kind of hexagonal polar crystal with a close packing of Zn2+ and O2, and the main crystal planes include a facet, and six symmetric nonpolar planes. The positive polar zinc (0 0 0 1) facet, a polar oxygen (0 0 0 1) polar surfaces have high surface enerZn2+ terminated (0 0 0 1) and negative O2 terminated (0 0 0 1) gies, and these facets tend to grow along their normal direction and eventually disappear from the final appearance according to the GibbsWulff theorem. Moreover, the negative [Zn(OH)4]2 complexes can be preferably adsorbed on the surface of ZnO nuclei, which is helpful to the growth of ZnO nuclei along the [0 0 0 1] direction [21]. According to the above discussion, a possible growth mechanism of the ZnO spindles can be proposed. The formation of ZnO spindles may follow the growth mechanism of ‘‘nucleation–aggregation– self-assembly’’ process [22]. Initially, when Na2O2 is added into the zinc salt solution, OH ions would be first produced. And then Zn(OH)2 is precipitated from the solution (Eqs. (1) and (2)). When the concentration of OH ion is excessive enough, [Zn(OH)4]2 complexes would be formed (Eq. (3)). A large quantity of small primary ZnO nuclei are intensively generated by the decomposition of [Zn(OH)4]2 species when the concentrations of ZnO reached the supersaturation degree of ZnO (Eq. (4)). The freshly formed ZnO nuclei are unstable because of their high surface energy and they tended to aggregate rapidly. In addition, the negative ions [Zn(OH)4]2 could be adsorbed onto the polar (0 0 0 1) surface terminated by Zn2+ as a result of charge compensation, which results in the redistribution of the surface energy, and the change in the growth rate of different facets. After the nuclei aggregation through assembly, most probably by electrostatic gravitation and intermolecular force, well-defined ZnO spindles are formed, which is similar to the formation process of YF3 nanospindles [23]. Scheme 1 shows the schematic illustration for the possible formation process of the ZnO spindle-like structure. 3.4. XPS analysis In order to clarify the elements and chemical states of Ag/ZnO composites, the surface structure of the Ag/ZnO synthesized with an AgNO3 concentration of 0.01 M was investigated by XPS analysis. The high-resolution spectra of O, Zn and Ag are shown in Fig. 4(a)–(c). In Fig. 4(a), the asymmetric spectra of the O1s were fitted into two symmetrical peaks (a and b locating at 530.0 eV and 531.7 eV, respecively), which indicates two different kinds of O in the sample. They should be associated with the lattice oxygen (OL) of ZnO and chemisorbed oxygen caused by the surface hydroxyl (OH) [24], respectively. Fig. 4(b) displays a strong and symmetric peak profile centered at 1021.1 eV correspond-
Scheme 1. The schematic illustration for the possible formation process of the ZnO spindle-like structure.
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Fig. 4. High-resolution XPS spectra of Ag/ZnO-1 composites prepared with an AgNO3 concentration of 0.01 M for the elements of: (a) O, (b) Zn, and (c) Ag.
ing to the Zn2p3/2 of ZnO, corroborating the existence of Zn atom in the form of zinc ion on the sample surface [25]. Fig. 4(c) shows the Ag3d XPS spectra for the sample. The two peaks locating at 367.0 and 372.9 eV shifted notably to lower binding energy compared with the pure metallic Ag (the standard binding energies of Ag3d5/2 and Ag3d3/2 are about 368.2 eV and 374.2 eV, respectively), indicating that the electron density of Ag decreased [26]. For Ag/ZnO, the negative shifts of Ag3d were due to the interaction between Ag nanoparticle and ZnO. Because the work function of ZnO is larger than that of Ag, once the ZnO (work function = 5.3 eV) particles come into contact with Ag (work function = 4.26 eV), electron transfer occurs from Ag to the conduction band (CB) of ZnO to obtain the equilibrium of Fermi energy level. Finally, a new Fermi energy level in Ag/ZnO heterostructures is formed, resulting in the tendency of Ag to higher valence. Because the binding energy of oxidation state Ag+ is lower than that of substance Ag (Ag is a kind of transition metal), the peaks of Ag3d in Ag/ZnO heterostructure shift obviously. Moreover, the shifts of Ag3d5/2 and Ag3d3/2 to lower binding energies also demonstrate the strong interaction between ZnO and Ag, which further confirms the formation of Ag/ZnO heterostructural composites. 3.5. UV–vis spectra The UV–vis absorption spectra of pure ZnO, pure Ag and Ag/ZnO-1, Ag/ZnO-2 and Ag/ZnO-3 composites are presented in Fig. 5. The pure ZnO spindles (curve b) showed a steep absorption at 368 nm, owing to the excitonic absorption, while pure Ag particles (curve a) displayed a surface plasmon resonance peaks at 422 nm, which corresponds to transverse plasmon resonance [27]. For Ag/ZnO composites, their absorption was not a simple superposition of ZnO and Ag (labeled as curve c, d and e), and the surface plasmon band was notably broadened and red-shifted, probably resulting from a strong interfacial electronic coupling between neighboring ZnO particles and Ag, which might
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Fig. 5. UV–vis absorption spectra of Ag particles (a), ZnO (b), Ag/ZnO-1 (c), Ag/ZnO-2 (d), and Ag/ZnO-3 (e).
enhance the photoactivities of materials under visible light [28]. As is known, the electron density of metal determines the position of plasmon absorption of metal nanocrystals [29], which is given as follows: 1=2
k ¼ ½4p2 c2 meffe =Ne2
where k is the plasmon absorption wavelength, Ne is the electron density of metal, meffe the effective mass of the free electron of metal. It can be known that the plasmon absorption (k) of metal depends on the electron density of metal. In our experiment, the red shift of the plasmon peak of Ag/ZnO suggests that the electron density of Ag was decreased. The decrease of the electron density of Ag nanocrystals may be caused by the transfer of the electron from the Ag nanocrystals to ZnO particles. Because the work function of ZnO (5.2 eV) is larger than that of Ag (4.26 eV), the Fermi energy level of Ag is higher than that of ZnO (first electron affinity is around 4.3 eV). Thus, as illustrated in Fig. 6, the transfer of electrons from Ag to ZnO will occur to make the two achieve equilibration when they contact. The electron transfer from Ag to ZnO results in the reduction of electrons in the Ag nanocrystals. Therefore the surface plasmon absorption of Ag/ZnO composites gets red-shifted. In addition, a blue shift in ZnO/Ag-3 sample was observed in comparison to Ag/ZnO-2. The blue shift may be attributed to the size decrease of Ag nanoparticles [30], which could be seen from Fig. 2(c) and (d). Similar results for ZnO/Au composites have been reported [30].
Fig. 6. The mechanism of electron transfer between ZnO and Ag without UV irradiation. E : Fermi level; CB: conduction band; VB: valance band; : electron.
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3.6. Photoluminescence spectra The photoluminescence (PL) spectra of as-prepared ZnO particles are shown in Fig. 7(a). A narrow UV peak centered at about 370 nm and a broad peak at 600 nm were observed. The UV peak was assigned to the recombination of electron–hole pairs in the semiconductor through an exciton–exciton collision process and the broad peak was caused by surface states or oxygen vacancies [31]. In PL spectra of Ag/ZnO composites with different Ag content (curve b–d), two emission bands were observed in the UV/vis range, which came from the ZnO. One the other hand, the PL spectra of Ag/ZnO showed a red-shift in the wavelength and a decrease in the intensity of UV emission peak, which resulted from oxygen vacancies in Ag/ZnO composites and electron transfer from ZnO to Ag on the interface [32], respectively. Moreover, it can be seen clearly that the sample Ag/ZnO-2 showed the minimum PL intensity. For Ag/ZnO heterostructure systems, Ag as shell and ZnO as core, Ag crystals grew up on the surfaces of ZnO spindles as seeds, and formed a thin positively charged layer. Because the bottom energy level of the conduction band of ZnO is higher than the new equilibrium Fermi energy level (Ef) of Ag/ZnO, the photoexcited electrons on the conduction band could transfer from ZnO to Ag nanoparticles, causing the reduction in combination between electrons and holes for ZnO, which results in the decrease of the PL intensity in Ag/ZnO system. With the increase of Ag content, as sample Ag/ZnO-2, more metal sites were formed and available to accept electrons, which led to a corresponding increase in separation effects for the photoinduced electrons and holes, and a declined intensity of PL emission [3]. However, when the Ag content exceeded certain value (as sample Ag/ZnO-3), the PL intensity increased again. This was attributed to the absorption or reflection of emission at the interface between ZnO and Ag, which was mainly induced by the strong surface plasmon absorption of Ag particles [33]. Moreover, as can be seen from Fig. 1(c) and (d) with the increase of Ag content, Ag particles aggregated seriously, and even formed cluster, which would reduce the possibility of forming homogeneous composites, and thus, the interfacial interaction between ZnO and Ag nanoparticles got weakened, which, in a certain degree, improved the possibility of recombination between holes with electrons. 3.7. Photocatalytic performance Methylene blue(MB) was used as a representative organic pollutant to evaluate the photocatalytic performance of ZnO spindles, Ag nanoparticles and Ag/ZnO composites with different Ag contents. Fig. 8 shows the photodegradation curves of methylene blue (MB) as a function of time. It can be found that the degradation of MB over pure Ag particles was negligible, and Ag/ZnO composites with different Ag contents obviously had higher photocatalytic activities than pure ZnO spindles. Moreover, the Ag/ZnO-2 sample exhibited the highest catalytic activity. After 60 min, it gave the degradation rate of 83.5%, while the degradation rate on ZnO particles reached only 43.2%.
Fig. 7. PL spectra of Ag/ZnO composites prepared with various Ag contents.
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Fig. 8. Photocatalytic degradation kinetics of the synthesized samples for the methylene blue (MB) solution.
The photocatalytic process of Ag/ZnO composites can be analyzed through their energy band structure. Because the Fermi energy level of ZnO is lower than that of Ag, the transfer of electrons from Ag to the conduction band (CB) of ZnO will occur to make the two achieve equilibration when the heterostructure is formed. The process shown in Fig. 6 can be expressed as:
Ag ! Agþ þ e
ð5Þ
When Ag/ZnO composites are irradiated by UV light, some electrons (e) of ZnO in the valence band (VB) will be excited to the conduction band (CB) with the same amount of holes (h+) coming out in the VB. As it is shown in Fig. 9, the bottom energy level of the conduction band (CB) of ZnO is higher than the equilibrium Fermi energy level (Ef) of Ag/ZnO heterostructure, so photoexcited electrons will transfer from ZnO particles to Ag particles driven by the potential energy. Furthermore, Ag particles or clusters on the surface of ZnO particles work as a sink, and they not only reduce the recombination of photoinduced electrons and holes but also prolong the lifetime of photogenerated pairs. And then the adsorbed O2 as electronic acceptors can easily trap photoelectrons and produce a superoxide anion radical (O2), while superoxide anion radical and photoinduced holes can also readily react with H+ or OH to generate hydroxyl radical species (OH), and the hydroxyl radical is an extremely strong oxidant for the degeneration of organic chemicals [34]. The photocatalytic reaction process can be proposed as follows: [34,35]: þ
ZnO þ hmðUVÞ ! ZnOðecb þ hvb Þ
ð6Þ
ecb þ O2 ! O2
ð7Þ
Agþ þ ecb ! Ag
ð8Þ
þ
hvb þ OH ! OH
ð9Þ
O2 þ 2Hþ ! 2 OH
ð10Þ
OH þ methylene blue ! Degradation Products
ð11Þ
Therefore the photocatalytic activity of Ag/ZnO photocatalysts was enhanced, and higher than that of ZnO. And photocatalysis results demonstrate that with the increase of Ag content, the photocatalytic activity of Ag/ZnO composites increased firstly and then decreased, and the Ag/ZnO-2 heterostructural composites had the highest photocatalytic activity because the photoinduced electrons
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Fig. 9. Charge separation process and the photocatalytic mechanism of as-prepared Ag/ZnO heterostructure under UV irradiation.
and holes were effectively separated. However, when the silver content of the composites was too large for Ag/ZnO-3, ZnO particles were tightly wrapped by the layer of silver nanoparticles and the Ag/ZnO heterostructural composites aggregated seriously, as can been seen from Fig. 2(d), which may reduce the possibility of semiconductor surface for light absorption and pollutant surface contact and decrease the catalytic efficiency of the ZnO particles. Therefore the Ag/ZnO-3 composites showed lower photocatalytic activity. What is more, it is found that the photocatalytic activities and PL intensities of the samples showed an opposite variation tendency with the change of Ag content. The Ag/ ZnO-2 composite sample had the highest photocatalytic activity, while its PL intensity is the weakest. This finding proved the existence of separation effects between photoinduced electrons and holes when the samples were irritated by UV light. 4. Conclusions In summary, spindle-like Ag/ZnO heterostructural composites were prepared through a solutionbased surface modification method. Detailed structural characterization and elemental analysis demonstrate that the surfaces of wurtzite ZnO spindles were coated by face-center-cubic Ag nanoparticles. The absorption band of Ag/ZnO composites exhibit some red-shifts and the PL was weakened, revealing the strong interfacial interaction between ZnO and Ag nanoparticles. This would improve the utilization efficiency of the light in the UV/visible light range when the composites are used as photocatalysts. The effect of Ag amount on the optical properties of Ag/ZnO composites was also studied, and results show that the intensity of PL for Ag/ZnO composites decreased firstly and then increased. The photocatalytic activity of Ag/ZnO composites was higher than that of ZnO, and Ag/ ZnO-2 heterostructural composites prepared with an AgNO3 concentration of 0.05 M showed the highest photocatalytic activity because the photoinduced electrons and holes were effectively separated. Acknowledgments This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0972), the National Natural Science Foundation of China (No. 51152001), the International Science &Technology Cooperation Program of China (2011DFA52290, 2012DFR50460), and the Research Project Supported by Shanxi Scholarship Council of China (2012-038). References [1] M. Afzaal, M.A. Malik, P. O’Brien, New J. Chem. 31 (2007) 2029–2040.
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