ZnO Core–Shell Heterostructural Composites

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Worm-Like Ag/ZnO Core−Shell Heterostructural Composites: Fabrication, Characterization, and Photocatalysis H. R. Liu,†,‡ G. X. Shao,†,‡ J. F. Zhao,†,§ Z. X. Zhang,†,‡ Y. Zhang,†,‡ J. Liang,†,‡ X. G. Liu,†,§ H. S. Jia,*,†,‡ and B. S. Xu†,‡ †

Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Ministry of Education, Taiyuan, Shanxi 030024, P. R. China ‡ College of Materials Science and Engineering, and §College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, P. R. China S Supporting Information *

ABSTRACT: Novel worm-like Ag/ZnO core−shell heterostructural composites were fabricated using a two-step chemical method. As-prepared silver nanowires were soaked in a solution of zinc acetate and triethanolamine to form worm-like Ag/ZnO core−shell composites under ultrasonic irradiation. Samples were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy, and UV−vis spectrophotometer. The results show that the core−shell composites are composed of single-crystal Ag nanowires serving as the core, on which dense ZnO particles grow as the shell. The surface plasmon absorption band of Ag/ZnO composites is distinctly broadened and red shifted to monometallic Ag nanowires. The PL intensity of Ag/ZnO heterostructural composites varies and has the minimum intensity for the sample prepared with Ag of 2.8 atom %. Moreover, photocatalytic tests show that the Ag/ZnO composites exhibit higher photocatalytic activity compared to pure ZnO particles.

1. INTRODUCTION Synthesis of materials with well-defined morphologies based on construction of nanounits has gained considerable attention over the past few years.1−5 The properties of many materials can be particularly modified by addition of other materials with some complementary properties to form core−shell, hetero-, and/or doped structures.6−8 This method not only finely adjusts materials to possess desirable properties but also makes them multifunctional. ZnO, as an important wide and direct band-gap semiconductor, is a promising photocatalyst because of its high catalytic efficiency, low cost, and environmental sustainability.9−11 However, rapid recombination of photoexcited electrons and holes weakens its photocatalytic efficiency. To solve this problem, one of the most efficient ways is to modify ZnO using noble metals.12−14 For example, when Ag is attached to ZnO, Ag will act as an electron sink to allow electron transfer from ZnO to the metal Ag through the interface between them. This inevitably results in an increased rate of electron transfer in the metal Ag and keep holes remaining on the ZnO surface. Thus, with these unique properties, as well as optical and electrical characteristics, the Ag/ZnO composites with a heterostructure can be not only used in photocatalysis but also in special applications requiring antibacterial effects,15,16 surface-enhanced Raman scattering,17,18 gas sensors,19,20 and so on. On the basis of the © 2012 American Chemical Society

above reasons, the study of Ag/ZnO heterostructural composites, especially prolonging the lifetime of the electron−hole pair will be beneficial for photocatalysis, has become an exciting area for developing photophysical and photochemical applications. In recent years, the Ag/ZnO heterostructural composites with different morphologies have been successfully synthesized using different methods. For example, Ren et al.21 reported that uniformly aligned ZnO nanorods coated with Ag nanoparticles could be prepared using a wet chemical route and a photodeposition method. Radical-shaped Ag/ZnO microstructures were fabricated by depositing metal Ag onto the surface of ZnO microprisms.22 Dendrite-like ZnO@Ag heterostructural nanocrystals were synthesized with Ag nanowires as trunks and highly dense ZnO nanorods as branches by a two-step hydrothermal method.23 Some researchers also reported fabrication of Ag/ZnO composites through a facile one-step hydrothermal method.24,25 From the studies reported above, it can be found that Ag nanoparticles are present mainly in the form of aggregates in Ag/ZnO heterostructure. The disadvantage of Ag aggregation will directly lead to a decrease of contacting chance between Ag and ZnO and further Received: November 30, 2011 Revised: July 3, 2012 Published: July 6, 2012 16182

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Figure 1. (a, b) SEM images scanned at different magnifications, and (c) TEM image of Ag nanowire. (d) HRTEM image of the interface of Ag and corresponding FFT pattern (inset).

were separated from the excess EG and PVP by addition of acetone and ethanol, followed by sonication and centrifugation. Finally, purified nanostructural Ag products were dried in a vacuum oven and then mixed into 1 mM ethanol for fabrication of Ag/ZnO core−shell composites. Synthesis of Worm-Like Ag/ZnO Core−Shell Composites. To fabricate Ag/ZnO core−shell heterostructural composites with different Ag content, first, 10 mL of triethanolamine (TEA) was added into 60 mL of 1 mM aqueous zinc acetate (Zn(Ac)2) solution. Meanwhile, 1 mM Ag nanowire in ethanol was ultrasonically shaken for about 30 min to obtain a well-dispersed Ag suspension. Second, a specific volume of the Ag suspension was introduced into the above mixed solution of triethanolamine and aqueous zinc acetate with a designed atom percent of Ag to zinc (about 1.0, 3.0, and 5.0 atom %). Finally, the mixture was ultrasonically stirred at 200 W and 20 MHz for about 2 h. After completion of the reaction, the precipitates were separated via centrifugation, washed thoroughly with distilled water and ethanol several times, and then dried in a vacuum oven. The corresponding final products were detected by EDS denoted as Ag/ZnO core−shell composites with Ag concentrations of 0.8, 2.8, and 4.9 atom %, respectively. Synthesis of spherical ZnO particles was conducted through a reduction reaction between Zn(Ac)2 and TEA in aqueous solution.28,29 In general, a surfactant is used in the synthesis of Ag/ZnO systems to form the heterointerface.30,31 However, in the present work, no surfactants are used in the fabrication of Ag/ZnO core−shell heterostructure, since intermolecular dehydrolysis has been

significantly affects the efficiency of photocatalysis and photoelectric properties of the prepared material. Therefore, to prepare Ag/ZnO heterostructure with good properties as required we have to find a way to avoid aggregation during the preparation process. Compared with other methods, ultrasonic irradiation is considered as a useful method to generate cavitations in solution and eliminate agglomeration of the products.26 This method has been widely used to design and synthesize heterostructural materials with controllable morphology and is an efficient route for optimizing some properties of heterostructural semiconductors. In this work, a hydrothermal approach combined with an ultrasonic-assisted chemical method is reported for fabrication of novel worm-like Ag/ZnO core−shell heterostructural composites. A systematic study is presented for detailed characterization of microstructure, phase, composition, and optical property of the samples. Finally, the photocatalytic activity of as-prepared composites is compared, and a possible mechanism of photocatalysis is also discussed and proposed.

2. EXPERIMENTAL SECTION Synthesis of Ag Nanowires. Ag nanowires were prepared using a classical polyol process.27 First, 0.075 mM polyvinylpyrrolidone (PVP, MW ≈ 40 000) was added into 10 mL of 2 mM Na2S in ethylene glycol (EG), and the mixed solution was vigorously stirred. This mixture was then added dropwise using a syringe pump (40 r/min) into a 10 mL solution of AgNO3 (0.05 mM) in EG with magnetic stirring. Second, the solution was transferred to a microwave oven, heated with a power of 450 W, and then cooled down. The silver products 16183

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Figure 2. (a) SEM image of spherical ZnO particles; (inset) high-magnification SEM image of broken ZnO particles. (b) Particle size distribution of ZnO particles. (c) TEM image of ZnO particle. (d) HRTEM image of the interface of ZnO and corresponding FFT pattern (inset).

centrifugation. Photocatalytic degradation process was monitored using a UV−vis spectrophotometer (Shimadzu UV-2450, Japan) to measure the absorption of RhB at 554 nm.

suggested to play a significant role in formation of Ag/ZnO heterostructures in the absence of surfactants.32 Characterization. The morphologies and microstructures of the as-synthesized Ag nanowires, ZnO particles, and Ag/ ZnO core−shell composites were investigated by field emission scanning electron microscopy (FESEM; JSM-6700F, Japan) and high-resolution transmission electron microscopy (HRTEM; JEM-2010, Japan). Chemical compositions were analyzed by X-ray energy-dispersive spectroscopy (EDS) equipped to the SEM. The crystal structure was determined by powder X-ray diffraction (XRD) with a 0.154178 nm Cu Kα rotating anode point source operating at 40 kV and 40 mA. Xray photoelectron spectroscopy (XPS; PHI-5300, ESCA, USA) was used to analyze elemental and chemical states in the core− shell composites. Photoluminescence (PL; Renishaw1000, UK) spectra were measured at room temperature using a He−Cd laser as the excitation light source at 325 nm. The photocatalytic activities of the Ag/ZnO products were evaluated by examining the decomposition of rhodamine B (RhB) used as a standard system. First, 10 mg of as-prepared Ag/ZnO core−shell photocatalysts was ultrasonically dispersed into 200 mL of an aqueous RhB solution (30 ppm); the mixture was magnetically stirred overnight in the dark to attain equilibrium adsorption on the catalyst surface. Then, ultraviolet (UV) irradiation was carried out with a 500 W fluorescent Hg lamp. After a given irradiation time, about 5 mL of the mixture was withdrawn and the catalysts were separated by

3. RESULTS AND DISCUSSION Morphological Observations. Figure 1 shows SEM and TEM images of as-synthesized Ag nanowires. It can be clearly seen from Figure 1a that the Ag products mainly consist of monodispersed Ag nanowires with some Ag nanoparticles, and Ag nanowires have a feature length ranging from 5 to 20 μm and a diameter ranging from 80 to 120 nm. The aspect ratio of the Ag nanowires is within a range of 40−200, which has similar orders of magnitude with the Ag nanowires fabricated by the hydrothermal method.33 Figure 1b reveals a high-magnification SEM image of Ag nanowires. In general, the evolution of Ag nanowires by PVPassisted polyol reduction is the interactive result of multiply twinned particles (MTP) of silver (MTP with 5-fold symmetry) and PVP (as a capping reagent). As Ag nanowires possess a five-twinned structure with a pentagonal cross-section, bounded by (100) planes and capped by (111) planes, they can readily grow into longer nanowires along the ⟨110⟩ direction.34,35 Figure 1c shows a typical TEM image of a single Ag nanowire with a diameter of about 90 nm. The fringe image of Ag nanowire is shown in Figure 1d, where the spacing between adjacent lattice fringes is 0.236 nm, close to the d-spacing value of the (111) plane. This finding indicates that the ⟨110⟩ 16184

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Figure 3. SEM images of Ag/ZnO core−shell composites with Ag contents of (a) 0.8, (b) 2.8, and (c) 4.9 atom %. The inset in b is a highmagnification SEM image of the composite with an Ag content of 2.8 atom %. (d) Elemental spectrum of Ag/ZnO core−shell composites revealed by EDS. (e) TEM image of worm-like Ag/ZnO core−shell composites with an Ag content of 2.8 atom % on a conductive carbon tape; (inset) SAED patterns corresponding to Ag nanowire and ZnO particle. (f) HRTEM image of the interface of ZnO and corresponding FFT pattern (inset).

presented in Figure 2c, indicates an integrated sphere with a diameter of about 520 nm. The spheres have some features of mesocrystals, which are easily assembled by rather small crystallites. From the HRTEM image presented in Figure 2d, no dislocations or stacking faults can be observed from the ZnO sample, which reveals that the crystallites are of highly crystalline nature with a lattice spacing of 0.263 nm, corresponding to the interlayer spacing of (0002) planes in wurtzite ZnO. Figure 3a, 3b, and 3c shows SEM images of Ag/ZnO core− shell composites synthesized with Ag concentrations of 0.8, 2.8, and 4.9 atom %, respectively. From Figure 3a, it is clearly seen that worm-like Ag/ZnO composites have been formed with

direction (c axis) is the preferred growth direction of Ag nanowires. The inset in Figure 1d is the FFT pattern of Ag nanowire. Figure 2 shows SEM and TEM images as well as the size distribution of the as-prepared ZnO samples produced after 2 h of ultrasonic irradiation. From Figure 2a, it can be clearly observed that the ZnO products grow into spherical structures. The inset in Figure 2a presents a magnified SEM image of a broken ZnO sphere with mesopores and a rough surface (see the Supporting Information, SI-1). The particle size distribution of ZnO is given in Figure 2b. The diameters of ZnO particles range from 400 to 700 nm, and the average diameter of ZnO particles is about 549 ± 10 nm. A typical TEM image, as 16185

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Table 1. Weights and Atomic Percentages of Elements in Ag/ZnO Composites sample

1

2

3

element

O

Zn

Ag

O

Zn

Ag

O

Zn

Ag

wt % atom % error

21.0 52.4 ±5.0

76.8 46.8 ±4.8

2.2 0.8 ±0.1

20.1 51.6 ±5.2

72.6 45.6 ±5.1

7.3 2.8 ±0.1

18.9 50.3 ±4.9

68.7 44.8 ±4.6

12.4 4.9 ±0.1

XRD Analysis. The XRD patterns of pure Ag nanowires, ZnO particles, and Ag/ZnO core−shell composites in Figure 5

some aggregation of ZnO particles (indicated by the red circles). With increasing Ag content, aggregation of ZnO particles reduces and the worm-like Ag/ZnO composites increase. The inset in Figure 3b shows a high-magnification SEM image of the Ag/ZnO products in which the worm-like composites are fully covered by ZnO spherical particles without seeing Ag nanowires. However, when the Ag content is continuously increased to 4.9 atom %, as shown in Figure 3c, the worm-like Ag/ZnO core−shell composites increase but some Ag nanowires are not entirely coated by ZnO particles (shown by red arrows). Figure 3d shows the EDS spectrum of Ag/ZnO core−shell composites dispersed onto a conductive carbon tape. Elemental zinc, oxygen, and silver are detected, and the corresponding weight and atomic percentages for the three samples are listed in Table 1. Some deviations of the Zn and O contents are observed because of the contamination of the samples due to exposure to air. TEM images, as shown in Figure 3e and 3f, further confirm the results obtained from SEM observations. From Figure 3e, it can be seen that a dark Ag nanowire with a diameter of about 100 nm in the center has been fully covered by ZnO particles. The shell is composed of spherical ZnO particles with a shell thickness ranging from 280 to 320 nm. The inset in Figure 3e is a selected area electron diffraction (SAED) pattern of the interface, which consists of two sets of zone diffraction patterns, corresponding to the ZnO [1210] zone diffraction and the Ag [110] zone diffraction.32,36 These mixed diffraction patterns further indicate the presence of the ZnO crystalline nucleus on the interface of Ag nanowire. Figure 3f shows a HRTEM image of the exterior of ZnO particles. The plane fringe with a crystalline plane spacing of 0.263 nm is assigned to the (0002) plane of ZnO with a hexagonal wurtzite structure. These results demonstrate that ZnO particles in the composites are single crystals with wurtzite structures. On the basis of the above observations, a possible growth mechanism of the Ag/ZnO heterostructure is proposed in Figure 4. First, single-crystal Ag nanowires are dispersed into

Figure 5. XRD patterns of Ag/ZnO core−shell composites, pure ZnO particles, and pure Ag nanowires.

evidently reveal the crystal structure and phase purity of Ag/ ZnO core−shell heterostructural composites. In Figure 5, compared with the results obtained from ZnO particles and Ag nanowires, two sets of diffraction peaks mixed together can be observed from the spectrum of Ag/ZnO composite sample, which can be indexed to hexagonal wurtzite ZnO (JCPDS File No. 36-1451) and face-centered-cubic (fcc) metallic Ag (JCPDS File No. 04-783). No remarkable shifts in diffraction peaks are detected; this further confirms that the as-synthesized samples are composed of ZnO and Ag phases. Using the Debye−Scherrer formula, the average crystal sizes of Ag and ZnO in Ag/ZnO heterostructural composites are calculated to be about 24.8 ± 2.0 and 31.4 ± 2.5 nm, respectively. Compared to the average crystal sizes of 22.2 ± 2.0 nm for the pure Ag nanowires and 29.8 ± 2.5 nm for ZnO particles, it seems that the average crystal size of as-prepared ZnO has only a little change with addition of Ag nanowires; this is similar to those results obtained from samples fabricated by the hydrothermal method.36,37 XPS Analysis. To clarify the elemental and chemical states of Ag/ZnO core−shell composites, XPS measurement is conducted on Ag/ZnO composites prepared with an Ag content of 2.8 atom %. The corresponding results for samples with and without Ar ion etching are shown in Figure 6. All peaks on XPS curve for the composites etched for 90 s are ascribed to Ag, Zn, O, and C. No any other peaks can be observed. This result confirms that Ag/ZnO core−shell composites are composed of three elements of Zn, Ag, and O, which is consistent with the XRD and EDS results discussed above. However, XPS measurement for as-prepared Ag/ZnO composites with Ag 2.8 atom % reveal that there are only peaks of elemental Zn, O, and C; no Ag peak can be found. Comparison for XPS results again firmly verifies that Ag nanowires have been fully covered by ZnO spherical particles.

Figure 4. Schematic illustration of the growth of Ag/ZnO core−shell composites.

the mixed Zn(Ac)2 and TEA aqueous solution. Some ZnO particle seeds are then partially deposited on the surface of Ag nanowires to form nucleation sites for subsequent growth of ZnO particles. During early stages of the reaction, ZnO crystal nuclei are formed on the Ag lateral surface by the reduction reaction of Zn(Ac)2. With increasing reaction time, initial particle seeds continuously aggregate and crystallize, leading to increasing diameters of the ZnO shell. Finally, a dense and complex worm-like heterostructure is formed. 16186

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Figure 6. (a) Complete XPS spectra of Ag/ZnO core−shell composites prepared with 2.8 atom % Ag with and without etching. High-resolution spectra of etched sample for the elements of (b) Ag, (c) Zn, and (d) O.

associated with the lattice oxygen of ZnO and chemisorbed oxygen of the surface hydroxyls, respectively.31,41 It is worth mentioning herein that the surface hydroxyl can produce primary active hydroxyl radicals, which are capable of trapping photoinduced electrons and holes. Thus, the surface hydroxyl is very important for photocatalysis,25 as will be discussed later. UV−Vis Absorption and Photoluminescence. UV−vis absorption spectra of pure ZnO particles, pure Ag nanowires, and Ag/ZnO core−shell composites prepared with 2.8 atom % Ag are shown in Figure 7. ZnO particles exhibit a UV absorption band at 362 nm (labeled as line a), which is where exciton absorption of the particles occurs.42 Ag nanowires display two surface plasmon resonance (SPR) peaks at 350 and

The presence of C likely originates from chamber contamination in the XPS equipment. High-resolution spectra of Zn, Ag, and O species obtained from the etched sample are shown in Figure 6b, 6c, and 6d, respectively. From Figure 6b, it can be seen that two peaks centered at 367 and 372.9 eV can be attributed to Ag 3d5/2 and Ag 3d3/2, respectively. Peak positions of Ag 3d shift remarkably to lower binding energies compared with those of bulk Ag (Ag 3d5/2, 368.2 eV; Ag 3d3/2, 374.2 eV38); this phenomenon is similar to the results obtained from dendrite-like ZnO@Ag heterostructures23 and Ag-ZnO heterostructural nanofibers.39 The binding energy shift of Ag is mainly attributed to electron transfer from metallic Ag to ZnO crystals (i.e., formation of monovalent Ag). The Fermi levels of two components equilibrate when the metal particles come into contact with a semiconductor, such as Au/ZnO and Ag/ZnO heterostructures. Accordingly, when Ag (work function = 4.26 eV) is attached to ZnO (work function = 5.3 eV), some of the electrons are transferred from Ag to ZnO at the interfaces of Ag/ZnO core−shell heterostructures, resulting in the higher valence of Ag. The binding energy of monovalent Ag is lower than that of zero-valent Ag; therefore, the shift to lower binding energies of Ag 3d5/2 and Ag 3d3/2 further verifies formation of Ag/ZnO heterostructural composites. The Zn 2p3/2 peak for the Ag/ZnO sample, as shown in Figure 6c, has a value of about 1021.1 eV, which is similar to that of pure ZnO nanorods; this finding confirms that Zn exists mainly in the Zn2+ chemical state on the sample surface.40 In Figure 6d, the O 1s profile is asymmetric and can be fitted into two symmetrical peaks (α and β located at 530.0 and 531.7 eV, respectively). This implies the presence of two different kinds of O species in the sample. The peaks of α and β are

Figure 7. UV−vis absorption spectra of ZnO particles (a), Ag nanowires (b), and Ag/ZnO core−shell composites with 2.8 atom % Ag (c). 16187

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380 nm (labeled as line b). The maximal peak at ∼380 nm corresponds to transverse plasmon resonance, while the weaker peak at ∼350 nm is attributed to quadrupole resonance excitation.35 However, it is noticed that the absorption of Ag/ ZnO core−shell composites is not a simple superposition of individual single-component materials (labeled as line c). The surface plasmon band of Ag/ZnO core−shell composites is distinctly broadened and shows a red shift compared to that of pure Ag nanowires, probably due to a strong interfacial electronic coupling between neighboring ZnO particles and Ag nanowires.43,44 In detail, the electron transfer from Ag to ZnO in Ag/ZnO core−shell composites is because the Fermi energy level of Ag is higher than that of ZnO. This transfer will result in a deficit of electrons on the surface of Ag nanowires, leading to the red shift in the surface plasmon absorption.45 Similar results for ZnO−Au composites have been reported elsewhere.46 Figure 8 demonstrates the room-temperature photoluminescence (PL) spectra (excitation at 325 nm) of Ag/ZnO core−

overaccumulation of electrons on the metal deposits could attract photogenerated holes to the metal sites and encourage a recombination of charge carriers. In such a case, the metal deposits conversely behave as recombinant centers.23,42 Under the given conditions in this study, the composites prepared with less and more Ag content have higher PL intensity, which is not good for photocatalysis as will be addressed below; therefore, only 2.8 atom % Ag content is assumed to be the optimum concentration for preparing Ag/ZnO composites with the homogeneous distribution of ZnO particles on Ag nanowires. Photocatalytic Performance. Rhodamine B (RhB) is selected as a representative organic pollutant to evaluate the photocatalytic performance of Ag/ZnO core−shell composites prepared with various Ag contents. Commercial TiO2 (Degussa P-25) is used as a photocatalytic reference to qualitatively understand the photocatalytic activity of Ag/ZnO composites used as catalysts. If the photodegradation of RhB is considered as a pseudo-first-order reaction;39,48 its photocatalytic reaction kinetics can be expressed as follows C = C0e−kt

where k is the degradation rate constant and C0 and C are the initial concentration of RhB and the concentration of the pollution at a reaction time of t, respectively. Figure 9 shows

Figure 8. PL spectra of Ag/ZnO core−shell composites prepared with various Ag contents.

shell composites prepared with different Ag contents. All individual curves similarly show a UV emission band centered at around 370 nm, corresponding to the near band edge emission of ZnO, namely, recombination of free excitons through an exciton−exciton collision process.47 Moreover, the PL intensity of Ag/ZnO heterostructural composites varies and appears at minimum intensity for the sample prepared with a molar basis (Ag) of 2.8 atom %. This phenomenon can be interpreted as that with increasing Ag content from 0.8 to 2.8 atom %, more metal sites are formed and available to accept electrons; this leads to a corresponding increase in separation effects for the photoinduced electrons and holes and a declined intensity of PL emission.12 Furthermore, when the Ag content exceeds 2.8 atom %, the PL intensity increases again; this can be attributed to absorption or reflection of emission at the Ag/ ZnO interface, which is mainly induced by the strong surface plasmon absorption of Ag nanowires.39 Moreover, when using a higher Ag concentration, even if using ultrasonic irradiation assists the synthesis, Ag nanowires are still easy to aggregate together to form clusters locally or partly coated by ZnO particles; this will reduce the possibility of forming the homogeneous composites and increase the amount of metal sites on the nanoparticles surface and, thus, further enhance recombination between charge carriers with holes. With regard to this, several groups have suggested that when the metal content is higher than a considerable optimum value,

Figure 9. Rhodamine B degradation curves of ln(C0/C) versus time for Ag nanowires, ZnO particles, P25, and Ag/ZnO heterostructural composites used as catalysts, and reaction rate constant, k, obtained from linear fitting.

the photodegradation curves of RhB in the form of ln(C0/C) as a function of time and the determined values of the reaction rate constant, k, from linear fitting for different samples. It can be found that the degradation of RhB over pure Ag nanowires is negligible (k = 6.68 × 10−5 min−1), as confirmed by Gu et al. and Zheng et al.23,32 However, Ag/ZnO core−shell composites apparently exhibit higher photocatalytic activities compared with pure ZnO particles. The Ag/ZnO core−shell samples prepared with different Ag concentrations all reveal increased photocatalytic activities. In particular, the sample with 2.8 atom % Ag shows the highest catalytic activity with a k of 0.0793 min−1, about double compared to those of the Ag/ZnO composites with 0.8 (0.032 min−1) and 4.9 atom % Ag (0.041 min−1). Zheng et al.32 also found that the photocatalytic activity of the Ag/ZnO heterostucture increases with increasing Ag content up to 5.0 atom % and attributed this phenomenon to oxygen-related defects. For the dendrite-like ZnO@Ag heterostructure, it is 16188

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latter is an extremely strong oxidant for degeneration of organic chemicals. The photocatalytic reaction process can be proposed as follows32,39,42,51

reported that an Ag content of about 8 atom % yields maximum photocatalytic activity.23 For Ag/ZnO composites with different morphologies used as catalysts, the difference in photocatalytic activities is probably due to different sizes of the nanoparticles and crystallinity of the Ag nanoparticles in contact with the ZnO particles. For any particular system, small particle size with large specific surface could shorten the route for an inner photoinduced electron migrating to the surface and provide more surface active sites, where photogenerated charge carriers are able to react with surface-adsorbed molecules to form active radicals. Besides, it is well known that the defects may serve as the recombination centers for photoexcited electron−hole pairs during photocatalysis, which would decrease the photocatalytic activity.49,50 Thus, the high crystallinity with few defects in the composites will be beneficial to photocatalytic reaction. It can be concluded that small crystal sizes with larger specific surface areas and high crystallinity of the samples may play important roles in the enhancement of photocatalytic activities. Compared with commercial TiO2 (Degussa P-25), Ag/ZnO heterostructures have higher performance in terms of photocatalytic activity; similar results have been reported.25,32 In particular, the sample with 2.8 atom % Ag shows the highest catalytic activity, with a rate constant (k) of 0.0793 min−1, almost double that of commercial P-25 (0.0497 min−1). Since the photocatalytic process of Ag/ZnO heterostructural composites is complex, it is necessary to discuss the band structure of Ag/ZnO heterostructures in detail. When the metallic Ag nanowires are attached to ZnO particles, as the Fermi energy level of Ag is higher than that of ZnO, it will lead to migration of electrons from Ag to the conduction band (CB) of ZnO in order to achieve Fermi energy level equilibration (Ef). This process can be expressed as Ag → Ag + + e−

ZnO + hν(UV) → ZnO(e−cb + h+vb)

(2)

e−cb + O2 → •O2−

(3)

Ag + + ecb− → Ag

(4)

h vb+OH− → •OH

(5)

•

OH + RhB → degradation products

(6)

Photocatalysis test results demonstrate that an appropriate Ag content for preparing Ag/ZnO core−shell heterostructural composites with a higher photocatalytic activity is about 2.8 atom %, which can effectively separate photoinduced electrons and holes to obtain the highest photocatalytic performance. By comparison, the photocatalytic activities and PL intensities of the samples show an opposite variation tendency. This finding confirms the occurrence of separation effects between photoinduced electrons and holes.

4. CONCLUSION In summary, worm-like Ag/ZnO core−shell heterostructural composites have been fabricated by a two-step chemical method. Single-crystal Ag nanowires were first synthesized by the polyol process. Highly dense ZnO particles were then grown on the lateral surfaces of Ag nanowires by an ultrasonicassisted aqueous solution method. SEM and TEM observations confirm that the surfaces of Ag core nanowires have been coated by a shell of ZnO particles. PL results reveal that the intensity of Ag/ZnO heterostructural composites decreases to a minimum with increasing Ag content to 2.8 atom % and thereafter enhances with further increase of Ag content. Photocatalysis test results show that Ag/ZnO core−shell heterostructural composites exhibit higher photocatalytic activities in comparison with pure ZnO particles; in particular, samples prepared with 2.8 atom % Ag have the highest photocatalytic activity. These metal core−semiconductor shell composites are photocatalytically active and useful to promote light-induced electron-transfer reactions. Although the fabricated Ag/ZnO heterostructures involve the expensive noble metal Ag, as a result of its higher photocatalytic efficiency, the preparation method should be important. The method presented here can be extended to the synthesis of other binary complex semiconductor heterostructures or ZnO/ ternary complex heterostructures for various applications; also, it is possible to find other cheaper substitutes for designing such a composite material in a further study. Thus, exploration of the catalytic activity of such heterostructural composites could pave the way for designing novel lightharvesting systems. Future research work will address such feasibilities.

(1)

When the catalysts are irradiated by UV light with photon energy higher than or equal to the band gap of ZnO crystals, electrons (e−) in the valence band (VB) will be excited to the CB with simultaneous generation of the same amount of holes (h+) in the VB. As presented in Figure 10, the bottom energy

Figure 10. Light-induced charge separation mechanism of the Ag/ ZnO core−shell composites and the proposed band structure and photocatalytic mechanism.

level of CB of ZnO is higher than the Ef of Ag/ZnO heterostructure, so that photoexcited electrons can transfer from ZnO particles to Ag nanowires driven by the potential energy. Ag nanowires, acting as electron sinks, not only reduce the recombination of photoinduced electrons and holes but also prolong the lifetime of photogenerated pairs. Subsequently, electronic acceptors, such as adsorbed O2, can easily trap photoelectrons to produce a superoxide anion radical (•O2−), while photoinduced holes can also readily react with surfacebound OH− to generate hydroxyl radical species (•OH); the

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ASSOCIATED CONTENT

* Supporting Information S

Nitrogen adsorption-desorption isotherm and pore size distribution curves for porous ZnO particles and related explanation. This material is available free of charge via the Internet at http://pubs.acs.org. 16189

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0972), the National Natural Science Foundation of China (No. 51002102 and No. 51152001), the Natural Science Foundation of Shanxi Province (No. 2009021026), and the Top Young Academic Leaders of Higher Learning Institutions of Shanxi 2011.

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dx.doi.org/10.1021/jp2115143 | J. Phys. Chem. C 2012, 116, 16182−16190