Journal of Hazardous Materials 287 (2015) 59–68

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Ag/ZnO heterostructures and their photocatalytic activity under visible light: Effect of reducing medium Yangsi Liu ∗ , Shanghai Wei, Wei Gao Department of Chemicals and Materials Engineering, the University of Auckland, PB 92019, Auckland 1142, New Zealand

h i g h l i g h t s • • • • •

Ag/ZnO nanocomposites were synthesized by hydrothermal method and photoreduction. Ag nanostructures were reduced in air, water and water with ethanol. The influence of reducing mediums on Ag nanostructures was investigated. The microstructure and formation mechanism of Ag components were studied. The photocatalysis of Ag/ZnO heterostructures under visible light was discussed.

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Article history: Received 16 October 2014 Received in revised form 19 December 2014 Accepted 23 December 2014 Keywords: Ag/ZnO heterostructure Reducing medium Microstructure Visible light Photocatalysis

a b s t r a c t Decoration of ZnO by Ag is a promising method to improve its photocatalytic activity and extend the photoreactivity to the visible light. In this paper, Ag/ZnO heterostructures have been synthesised by photoreduction in various reducing mediums. When the Ag/ZnO nanocomposite arrays were obtained in the air, only a small amount of Ag was reduced. Ag nanosheets and nanoparticles were formed in the water and attached on the top and side surfaces of ZnO nanorods, forming Ag/ZnO heterostructures with a nano(sheet-rod-particle) multi-level structure. In the mixture of water and ethanol, a large amount of Ag nanoclusters was produced and embedded in the ZnO nanorod arrays. The influence of reducing mediums on the microstructure, morphology, quantity and dispersion of Ag nanostructures was investigated; and the effect of Ag component on the optical properties and visible light driven photocatalytic behaviour of the Ag/ZnO heterostructures was discussed. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Environmental problems relating to organic and toxic waste in water have urged increasing attention on account of the potential hazard to human health and inhibition for economic development, which also provides impetus for sustained fundamental research in environmental remediation [1–4]. Metal oxide semiconductors, such as TiO2 and ZnO, have been considered as one kind of the most promising materials for wastewater treatment by virtue of their photocatalytic activity [5,6]. ZnO is a direct wide band gap semiconductor (3.37 eV) with a high exciton binding energy at room temperature (60 meV) [7]. It has been intensively studied in photodegradation of various pollutants with respect to the merits of its

∗ Corresponding author. Tel.: +64 9 3737599x89840. E-mail addresses: [email protected], [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.jhazmat.2014.12.045 0304-3894/© 2014 Elsevier B.V. All rights reserved.

high photoreactivity, high photosensitivity, controlled morphologies, easy synthesis, low cost and low environmental toxicity [8,9]. However, the photocatalytic activity of ZnO is restricted by two major intrinsic drawbacks. Owing to its wide band gap, ZnO has a narrow spectral response range since the photocatalysis can be only activated by UV light irradiation, which is only 3–5% of the total solar irradiance. This limits the practical applications in normal light condition [4]. Another issue is the high recombination rate of photo-generated electrons and holes, which brings about low quantum efficiency and deteriorates its photocatalytic performance [4,10]. To extend the photo response of ZnO toward the visible light region and allow efficient charge carrier separation and transport, ZnO has been combined with noble metals, such as Pt [11,12], Au [13], Ag [4,10,14–16], Pd [17] to form plasmonic metal/semiconductor heterostructures. Nobel metals are well known for the extended light absorption and intense interactions with visible-light, facilitating the creation of charge carriers

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induced by the surface plasmon resonance (SPR) effect [18–20]. Moreover, they have been found to efficiently hinder the recombination of electron–hole pairs and improve the charge separation by establishing the Schottky barrier at the metal-semiconductor interface [21]. Among noble metals, Ag is stable, non-toxic, and comparatively cost effective, and it has the highest electrical and thermal conductivity among all metals, giving it a remarkable catalytic potential [22]. Thus, modification of ZnO using Ag has been extensively investigated and become one of the most popular approaches for enhancing the photocatalytic efficiency in UV and visible light conditions. The photoreduction method has been frequently used to reduce Ag+ ions to zerovalent Ag on the surface of ZnO to obtain Ag/ZnO heterostructures because of its simplicity and convenience [4,10,23–25]. Ag/ZnO flower heterostructures as visible light driven photocatalysts were prepared via this method without surfactants by Han et al. [4]. Ren et al. [10] modified the ZnO nanorod arrays with Ag nanoparticles using a photodeposition method and obtained the enhanced UV photocatalytic activity. Deng et al. [26] discovered that the rate of degradation of Ag nanoparticle decorated nanoporous ZnO microrods was much faster than that of bare ZnO microrods. Wang et al. [23] demonstrated that uniform Ag nanoparticles could be directionally grafted on the tip of ZnO nanowire arrays by photoreduction, and proposed that the Ag/ZnO heterostructures could promote the effective separation and directional transfer of photo-excited electron-hole pairs, and thus enhance the photoconversion properties. Although there are studies of photoreduced Ag/ZnO heterostructures and their photocatalytic applications, the influence of reducing mediums on the microstructure of Ag/ZnO heterostructures and their photocatalytic efficiency has rarely been explored. Herein, we report the synthesis of Ag/ZnO heterostructures based on ZnO nanorod arrays by photoreduction of AgNO3 in the air, water and water with ethanol. Microstructure and optical properties of the Ag/ZnO heterostructures were characterized and the formation mechanism of various Ag nanostructures was suggested. It was found out that the reducing mediums can significantly tailor the amount, position, dispersion and morphology of Ag nanostructures in ZnO nanorod arrays, which will further affect the photocatalytic activity of the Ag/ZnO heterostructures under visible light. 2. Experimental 2.1. Sample preparation 2.1.1. Growth of ZnO nanorods All chemicals of analytical grade and de-ionized (DI) water were used throughout this study. ZnO nanorods were grown on glass substrates (25 mm × 10 mm) into immobilized arrays via a facile hydrothermal method as we reported before [5]. Briefly, clean glass slides were pre-coated with ZnO seed layers by magnetron sputtering. Aqueous solution containing 25 mM zinc nitrate (Zn(NO3 )2 ·6H2 O, 98%) and hexamethylenetetramine (HMT) (C6 H12 N4 , 99%) with an equal molar ratio was prepared as the reactant resource. The substrates were immersed into a sealable glass jar with the reactant solution and kept face- down at 95 ◦ C for 4 h, followed by rinsing and drying. 2.1.2. Synthesis of Ag/ZnO heterostructures Ag/ZnO heterostructures were produced by a photoreduction method. Two kinds of 0.05 M silver nitrate (AgNO3 ) solutions (A and B) were prepared by diluting 0.1 M AgNO3 aqueous solution with DI water (A) and pure ethanol (B). Three ZnO nanorod samples (#1, #2 and #3) were selected for Ag decoration. Sample #1 and #2

were soaked respectively into 3 ml solution A and sample #3 was soaked into 3 ml solution B. After 1 h, Sample #1 was taken out and exposed in the air, but sample #2 and #3 were still kept in the AgNO3 solutions. The three samples were then irradiated by two 9 W UV lamps ( = 365 nm, Philip) for 10 min to reduce Ag+ to zerovalent Ag. The resultant Ag/ZnO heterostructures were rinsed with stirring water to remove the residual Ag+ ions and dried. There were three kinds of reducing mediums that were used in the photoreduction process as shown in Fig. 1. The Ag/ZnO heterostructures were named after their reducing mediums, e.g. AgZNRa for Ag decorated ZnO nanorod (ZNR) samples by photoreduction in the air; AgZNRw for Ag decorated ZNR samples by photoreduction in the water; AgZNRe for Ag decorated ZNR samples by photoreduction in the water with ethanol. 2.2. Sample characterization The crystal structure of the Ag/ZnO heterostructures was identified by X-ray diffraction (XRD, Bruker D2 phaser) using Cu K␣ radiation. The morphology and microstructure of the Ag/ZnO samples were characterized by a field-emission gun scanning electron microscope (FEG-SEM, Philips XL-30S) and a high resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F20, 200 kV). The composition of the Ag/ZnO heterostructures was conducted with energy dispersive spectroscopy equipped (EDS) on SEM. Room temperature photoluminescence (PL) properties were measured by a PerkinElmer LS55 luminescence spectrophotometer with a Xe lamp as the excitation source at a wavelength of 325 nm. The UV–vis absorption spectra were recorded on an Agilent 8453 UV–vis spectrophotometer. 2.3. Photocatalytic activity Degradation of rhodamine B (RhB) dye in form of aqueous solution was used to evaluate the photocatalytic activity of the Ag/ZnO heterostructures under visible light. The Ag/ZnO samples were immersed into RhB solutions (3 ml, 2 mg/L). Prior to the degradation test, they were kept in dark for 30 min to establish an adsorption/desorption equilibrium of RhB with photocatalysts. This guarantees that the measured concentration changes during UV irradiation would be solely caused by photocatalysis. Then the RhB solutions were placed on the plate of a home-made stainless steel photoreaction enclosure with the light source, two 9 W blue light lamps ( = 450 nm, Osram Dulux), 9 cm above. The variation of RhB concentration with irradiation time was measured using a UV–vis spectrophotometer (Perkin Elmer Lambda 35). The intensity of the absorption band peak (553 nm) was recorded at a certain time interval. The degradation rate was estimated as C/C0 , where C0 is the equilibrium concentration before UV irradiation and C is the concentration at the sampling time. According to the Beer–Lambert law, the concentration of RhB is linearly proportional to the absorbance value (A) at 553 nm, thus C/C0 = A/A0 . 3. Results and discussion 3.1. Morphology The morphology of Ag/ZnO heterostructures was studied by SEM as shown in Fig. 2. The Ag/ZnO heterostructures were developed on the basis of ZnO nanorod arrays, which are composed of vertically arranged ZnO nanorods with the diameter of 60–80 nm and the height of ∼1 ␮m. Many pieces of nanosheets disperse on the ZnO nanorod array of AgZNRw. These nanosheets are so thin that their thickness is

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Fig. 1. Schematic illustration of synthesis of Ag/ZnO heterostructures by photoreduction in various reducing mediums.

about 20 nm, but their areas are quite large as most of them are in microscale (Fig. 2a). Apart from the nanosheets originated from the top surfaces of ZnO nanorods, much smaller nanoparticles can be observed. The sizes of these nanoparticles are 10–20 nm and they distribute separately on the lateral surfaces of ZnO nanorods (Fig. 2b and c). With these nanostructures, AgZNRw exhibits a nano(sheetrod-particle) multi-level structure with various dimensions (2D1D-0D). The top section of AgZNRe is densely covered by many nanoclusters, which are in irregular polygon shape with the size of 500–800 nm (Fig. 2d). The nanoclusters are composed of numerous aggregated nanocrystals (Supporting Information, Fig. S1). They are deposited on the upper part of the ZnO nanorod array and each of them is supported by several ZnO nanorods. The side surfaces of the ZnO nanorods are smooth and without any decoration (Fig. 2e and f). The morphology of Ag/ZnO heterostructures is significantly different between AgZNRw and AgZNRe. As for AgZNRa, no major structural development can be seen. Several curly nanoribbons are placed sparely on the ZnO nanorod arrays, which can hardly be captured in cross section view due to the small amount and size (Fig. 2g and h). 3.2. Elemental composition The elemental composition of the Ag/ZnO heterostructures was analysed by EDS. Fig. 3a gives the elemental spectra of AgZNRa, AgZNRw and AgZNRe detected from the top view of their SEM images. The main elements detected for all samples include Si and Ga from glass substrates, Pt from pre-SEM coating and O, Zn and Ag from the heterostructures. The intensity of Ag peak enhances dramatically from AgZNRa to AgZNRw, and to AgZNRe. The atom percentages (at %) of Ag increase from 1.35% to 7.68% and to 26.1% accordingly, implying that only a small amount of Ag was produced in the air, whereas a large quantity of Ag was reduced with the existence of ethanol in the water. According to the SEM results, it can be

also noticed that the coverage of Ag on the ZnO nanorods increases from AgZNRa to AgZNRw, and to AgZNRe. The elemental distribution of AgZNRw and AgZNRe was detected along their cross sections in different selected areas (500 nm × 500 nm). The Ag peak magnifies from area #2 to area #1 of AgZNRw, indicating the total amount of Ag element from the nanoparticles on the lateral surfaces of ZnO nanorods is less than the Ag from the top part, where the top surfaces of ZnO nanorods are connecting with many nanosheets (Fig. 3b and c). Similarly, the Ag peak in area #1 is much bigger than that in area #2 of AgZNRe, signifying that more Ag element exists in the upper part than the lower part (Fig. 3d and e). Although there are no other materials attached on the side surfaces of ZnO nanorods, some smaller nanoclusters fell into the gaps among the nanorods and embedded into the array (Supporting Information, Fig. S1). The total amount of Ag in AgZNRa is very small and the Ag nanostructures are not obvious along the cross section, therefore the selected EDS was not conducted on AgZNRa. 3.3. Crystal structure The XRD patterns reveal the crystal structure of the Ag/ZnO heterostructures as represented in Fig. 4. Two sets of diffraction peaks can be found in their patterns. The sharp peaks at 34.4◦ are indexed to the (0 0 2) phase of hexagonal wurtzite ZnO crystal. The strongest intensity of this peak in each curve confirms that the Ag/ZnO heterostructures were based on the ZnO nanorods arrays with the preferential growth along c-axis in [0 0 0 1] direction. Other three main diffraction peaks of AgZNRe with 2 value of 38.1◦ , 44.3◦ and 64.4◦ are corresponded to (1 1 1), (2 0 0) and (2 2 0) crystal planes of face-centred-cubic (fcc) metallic Ag, respectively (JCPDS No. 04–0783), indicating the as-synthesized sample is composed of ZnO and Ag. Only the (1 1 1) Ag peak is obvious for AgZNRw, which probably because of the reduced percentage of Ag element in the ZnO nanorod array. No Ag peaks can be seen for AgZNRa, implying the total amount of Ag is too small to be detected.

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Fig. 2. SEM images from (a) top view of AgZNRw, (b,c) cross section of AgZNRw, (d) top view of AgZNRe, (e,f) cross section of AgZNRe, (g) top view of AgZNRa and (h) cross section of AgZNRa.

There is no notable shift of any diffraction peak, suggesting that Ag did not change the bulk intrinsic property of ZnO nanocrystals or incorporate into the lattice of ZnO to form Ag related solid solution. The lattice expansion or shrinkage of the Ag/ZnO heterostructures can be negligible. Additionally, no trace of impurities and other phases such as Zn(OH)2 and Ag2 O are observed, confirming that the additional nanostructures including nanosheets, nanoparticles and nanoclusters, are metallic Ag.

3.4. TEM analysis The typical TEM/HRTEM images in Fig. 5 illustrate the crystallographic details and microstructure of AgZNRw (Fig. 5a–d) and AgZNRe (Fig. 5e–i). The Ag nanosheet of AgZNRw has a flat and smooth surface with a large area. Its selected area electronic diffraction (SAED) displays a set of clear spots aligned with hexagonal symmetry (Fig. 5a), evidence of the metallic silver with a fcc structure and single crystals with (1 1 1) plane. This is a

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Fig. 3. (a) EDS spectra of the Ag/ZnO heterostructures; (b,c) EDS spectra from selected areas of AgZNRw; (d,e) EDS spectra from selected areas of AgZNRe.

common structural configuration in plate-like Ag crystals, such as Ag nanoplates [27], nanoprisms [28] and nanodisks [29,30]. Many Ag nanoparticles are randomly dispersed along the side surfaces of ZnO nanorods and most of them are attached on the surfaces, while several isolated ones would be detached during the ultrasonic treatment before TEM test. The size of sphere shaped Ag nanoparticles is 10–20 nm and the diameter of ZnO nanorods is 40–60 nm, in good agreement with the SEM results (Fig. 5b and

c). A closer observation at the junction part of a nanoparticle and a nanorod shows a distinguished interfaces and the continuity of lattice fringes between ZnO and metallic Ag, indicating a good interface connection and the lattice relationship (Fig. 5d). Ag nanoclusters of AgZNRe were separated into smaller and irregular shaped pieces by the ultrasonication, which can be found on the top surfaces of ZnO nanorods (Fig. 5e and g). Orderly arranged 2D lattice fringes can be seen in the HRTEM image of the interstitial part of an Ag nanocluster. The distance between the lattice planes is 0.234 nm, which is close to the lattice space of Ag (1 1 1) planes. The fast Fourier transform (FFT) pattern and the uniform lattice structure indicate the high quality of Ag crystallinity (Fig. 5f). The Ag (2 0 0) phases with a 0.20 nm crystalline plane also exist as shown in the HRTEM image of the corner part of an Ag nanocluster (Fig. 5i). The clear lattice fringes and spots pattern of the corresponding FFT in the top part of a ZnO nanorod evidence the good crystalline nature of ZnO. The spacing between adjacent planes is 0.26 nm, which can be assigned to the (0 0 2) plane of wurtzite ZnO, indicating the [0001] direction (c-axis) being the preferential growth direction of ZnO nanorods (Fig. 5h). The TEM/HRTEM results verify the crystal structure and phase information of Ag nanostructures and ZnO nanorods from the XRD pattern. 3.5. Formation mechanism of Ag nanostructures

Fig. 4. XRD patterns of the Ag/ZnO heterostructures.

As a semiconductor, when ZnO is irradiated by UV light, electrons (e− ) will be excited from the valance band (VB) to the conduction band (CB) with the simultaneous occurrence of the same amount of holes (h+ ), resulting in electron-hole pairs. The electrons can be captured by Ag+ ions surrounding ZnO nanorods,

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Fig. 5. (a–d) TEM/HRTEM images of AgZNRw: (a) TEM image of a Ag nanosheet (The inset is the corresponding SAED pattern); (b and c) TEM images of Ag nanoparticles and ZnO nanorods; (d) HRTEM image of a Ag nanoparticle on ZnO surface. (e–i) TEM/HRTEM images of AgZNRe: (e and g) TEM images of Ag nanoclusters and ZnO nanorods; (f and i) HRTEM images of a Ag nanocluster (The inset in Fig. 5f is the corresponding FFT pattern); (h) HRTEM image of a ZnO nanorods (The inset is the corresponding FFT pattern).

which will be reduced into metallic Ag and heterogeneously nucleate on the surface of ZnO nanorods. The top surfaces of ZnO nanorods are highly polarised and energetic, which are more photoelectrically active than other non-polar lateral surfaces [31–33]. Therefore, more electrons can be accumulated on the top surfaces and more metallic Ag will form on the top part of ZnO nanorod arrays, leading to the uneven elemental distribution as shown in the EDS results. The Ag nuclei would prioritize the growth within the (1 1 1) plane as it possesses the lowest surface energy [29,34], making the (1 1 1) diffraction the highest one for Ag crystals in the XRD patterns. Lattice mismatch is known to have a significant impact on the epitaxial growth of heterogeneous structures. A high degree of lattice mismatch will prevent the nucleation and growth on a substrate due to the high structural strain and energy barrier [35,36]. The d-spacing values of Ag (1 1 1) plane, ZnO (0 0 2) plane, and ZnO (1 0 0) plane are 0.236, 0.26 and 0.28 nm, respectively. The Ag nuclei attached on the lateral surfaces of ZnO nanorods have a large lattice mismatch with their substrates, therefore they would more likely to develop into 0D sphere shaped nanoparticles to minimize the interfacial energy. The formation of large 2D Ag nanosheets on the top surfaces of ZnO nanorods would benefit from the less lattice mismatch and more electrons as reducing agents.

The photo-induced electrons would be easily recombined with the holes, which is adverse for the photoreduction of Ag+ ions. When the AgNO3 solution was mixed with ethanol, ethanol can act as the hole scavenger to consume the photo-induced holes, leaving the unpaired electrons on ZnO surfaces [37–39]. Therefore, the supply of reducing agents is larger than those without the hole scavenger, which will accelerate the reduction rate and form more metallic Ag. In this case, massive Ag crystals would tend to aggregate into large nanoclusters; and the big volume of these nanoclusters can provide the opportunity for more Ag crystals with other than the (1 1 1) crystal orientations such as (2 0 0) and (2 2 0), as shown in the TEM and XRD results. The lack of Ag nanostructures on the lateral surfaces of ZnO nanorods can be attributed to the rapid consumption of Ag+ ions on the top part of AgZNRe. When the ZnO nanorods were taken out from the AgNO3 solution and exposed in the air, the available Ag resource, Ag+ ions absorbed on the ZnO surfaces, was very limited, so the resultant metallic Ag was too little to be detected by XRD.

3.6. UV–vis absorption Fig. 6 is the UV–vis absorbance spectra of the Ag/ZnO heterostructures. The wavelength distribution of the absorbed light

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Fig. 8. Schematic diagram of photocatalytic mechanism of Ag/ZnO heterostructures under visible light.

Fig. 6. UV–vis absorption spectra of the Ag/ZnO heterostructures.

is an important property of photocatalysts, corresponding to the region of light that can be utilized. The absorption of AgZNRa and AgZNRw shows similar edges at ∼373 nm, which is mainly related to the band edge absorption of ZnO nanorods. Compared to AgZNRa, the absorption of AgZNRw strengthens greatly from 400 to 950 nm, showing enhanced absorption in the visible range. The promotion of the photon absorption would result from the optical vibration by surface plasmon resonance (SPR) of Ag nanostructures in AgZNRw [40,41], making the Ag/ZnO heterostructures practical for photoreaction under visible light. The absorption edge of AgZNRe, however, experiences an obvious red-shift to ∼400 nm and a broad and intense absorption covers nearly the whole visible region. This unique absorption character would mainly because the large quantity of Ag nanoclusters played the major role to overlap the optical function of ZnO nanorods, which absorbed a large proportion of incident light as AgZNRe showed the strongest deep black colour. 3.7. Photocatalysis The photocatalytic activity of the Ag/ZnO heterostructures was evaluated by the photodegradation of RhB under visible light irradiation. Fig. 7 shows the degradation rate of RhB by different samples

Fig. 7. Photodegradation rate of RhB by Ag/ZnO heterostructures.

over the same periods of time. The blank experiment in absence of any photocatalysts shows a very low degradation (∼5%) of RhB by visible light irradiation, so the photolysis of RhB can be negligible. All Ag/ZnO heterostructures exhibit the visible light driven photocatalysis. About 82% of RhB is degraded by AgZNRw by visible light after 4 h reaction, which is the most effective photocatalysts among the Ag/ZnO heterostructure samples. This is followed by AgZNRa, which removes 39.4% of RhB. AgZNRe shows the lowest photocatalytic activity since 72.7% of RhB still remained. The UV–vis absorption spectra of RhB corresponding to photodegradation rate can be seen in Supporting Information, Fig. S2. The photocatalytic process of Ag/ZnO heterostructures under visible light irradiation is based on the mechanism related to the photo-induced charge generation, separation and migration at Ag–ZnO interfaces [42–45]. Normally, when two materials with different work functions are combined with each other, a Schottky barrier will be established and electrons will transfer from the materials with a low work function to the materials with a high work function. In our case, the work function of ZnO and Ag is 5.2 eV and 4.26 eV respectively, and the first electron affinity of ZnO is 4.3 eV. The Fermi energy of Ag is higher than that of ZnO due to the larger work function of ZnO. This leads to the transfer of electrons from the Fermi level of Ag to the Fermi level of ZnO, until the two levels reach equilibrium and form a new Fermi energy level [45]. The Fermi levels of Ag and ZnO were adjusted to the same value when the heterostructures were formed and created a new Fermi energy level with more free electrons [42–44]. Since the energy of visible light is not great enough to trigger the photo-excitation state of semiconductors attributed to their wide band gap, the visible light driven photocatalysis of Ag/ZnO heterostructures is initiated from the Ag component as illustrates in Fig. 8. When they are illuminated by visible light, the equilibrated Fermi level electrons are excited into ZnO conduction band via surface plasmon resonance (SPR) oscillations of Ag nanostructures through the electric force driven by the incident photons [46]. The injected electrons can be trapped by surface-absorbed O2 molecules or the dissolved oxygen to produce superoxide rad• ical anions (O2 − ), which will further react with H2 O to produce hydroxyl radicals (• OH) [4,42]. These • OH radicals are reactive oxidative species, which will attack the RhB molecules and subsequently decompose them into CO2 and H2 O. The major reaction steps in this plasmonic photocatalytic mechanism under visible light irradiation (h␯) can be formulated by the following equations [4,42]: (1) (2) (3) (4)

Ag/ZnO + h → Ag/ZnO* (e− ) Ag/ZnO* (e− ) → Ag*+ZnO (e− ) e− +O2 → O2 − O2 − + H+ → HO2 •

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(5) (6) (7) (8)

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HO2 • + H2 O → H2 O2 + • OH H2 O2 + O2 •− → • OH + O2 + OH− H2 O2 + e− → • OH + OH− • OH + RhB → CO + H O 2 2

Moreover, Ag nanostructures can transfer the plasmonic energy to ZnO nanorods via resonant energy transfer, inducing electron–hole pairs in ZnO nanorods [19], which will facilitate the formation of hydroxyl radicals and subsequently enhance the visible light photocatalysis. Hence, the photocatalytic activity of Ag/ZnO heterostructures can be stimulated by visible light. To further confirm the role of • OH, photocatalysis of AgZNRw under visible light was investigated by adding dimethyl sulfoxide (DMSO). The RhB solution with the same volume and concentration (3 ml, 2 mg/L) was prepared by the solvent of the mixture of DI water and DMSO (90:10, vol:vol). According to the previous reports [47,48], DMSO is a typical • OH scavenger in photocatalytic systems, which reacts with • OH to form methyl radicals [49]. Compared with the photodegradation rate of RhB in the absence of DMSO (Supporting Information, Fig. S3), the photocatalytic performance of AgZNRw decreases drastically (more than 4 times) when DMSO is added, because most of the generated • OH radicals are consumed by DMSO. Above results provide strong evidence that • OH radicals generated by SPR of Ag/ZnO composites play a vital role in their photocatalytic activity. According to the discussion above, the Ag component of Ag/ZnO heterostructures plays a crucial role in the visible light driven photocatalysis, therefore the quantity and dispersion of Ag nanostructures have a considerable effect on the photocatalytic performance of Ag/ZnO heterostructures. The highest photocatalytic efficiency of AgZNRw is largely related to the favourable Ag distribution on the ZnO nanorods. The Ag nanosheets and nanoparticles allow the SPR to happen on both the top and side surfaces of ZnO nanorods, providing a large number of active sites for photocatalysis process. The 2D Ag nanosheets with extended surface area also supply more direct conduction pathways for electron quick transfer, which will accelerate the generation of hydroxyl radicals, resulting in the fast RhB photodegradation. Furthermore, when the visible light passes through the Ag/ZnO nanocomposite array, multiple scattering and reflection could happen among the nano(sheet-rod-particle) multi-level structure, increasing the energy utilization and light harvesting as shown in the UV–vis spectra. Additionally, since Ag nanoparticles have a solid interface connection with ZnO nanorods (TEM results), the integrated Ag–ZnO heterojunctions facilitate the localized electromagnetic field formed by SPR of Ag nanostructures. It has already proved to be able to increase the formation rate of electron-hole pairs of the semiconductor by a few orders of magnitude [50,51]. The small amount of Ag in AgZNRa cannot bring about enough SPR effect, thus its light absorption is much weaker in the visible range compared to AgZNRw, which is not beneficial for the visible light driven photocatalysis. As for AgZNRe, the Ag content mainly aggregates on the top surfaces of ZnO nanorods into large nanoclusters, rather than dispersing on different parts as those of AgZNRw. The specific surface area of the Ag/ZnO nanocomposite array will be severely diminished, reducing the contact area for RhB molecules and reaction sites for photocatalysis. On the other hand, the high coverage of Ag nanoclusters on ZnO surfaces may inhibit the access of visible light irradiation to the interface of Ag and ZnO, which will deteriorate the photon utilization and retard the SPR occurrence, reducing the amount of photo-excited charge carriers and decreasing photocatalytic performance. Additionally, the excess Ag loading may act as charge carrier recombination centres, also depressing the photocatalytic efficiency. All of these drawbacks may account for the lowest photocatalytic activity of AgZNRe. Thus, the suitable pro-

Fig. 9. PL spectra of the Ag/ZnO heterostructures.

portion and distribution of Ag content are important for the visible light driven photocatalysis of Ag/ZnO heterostructures. From AgZNRa to AgZNRw and to AgZNRe, the weight percentages (wt %) of Ag increase from 5.06% to 22.90% and to 52.73% accordingly (EDS results). The Ag components change from few thin nanoribbons to moderate nanosheets and nanoparticles and to overloaded nanoclusters, the best photocatalytic efficiency lies in the Ag/ZnO heterostructures with appropriate amount of Ag percentage and favorable Ag shape for inspiring SPR. Varying the synthesis parameters of AgZNRw could result in the optimized shape and percentage of Ag for the visible-light driven photocatalysis, which is under further study. 3.8. PL study In photocatalytic reactions, the separation and recombination of photo-induced charge carriers are two competitive processes, and photocatalytic activity is closely related to the lifetime of photogenerated electrons and holes [38]. The PL technique is an effective way to study the electronic, optical and photochemical properties of photocatalysts, from which the efficiency of charge carrier trapping, immigration and transfer can be evaluated [52]. The PL analysis was carried out to compare the extent of the charge recombination and migration efficiency in Ag/ZnO heterostructures, as shown in Fig. 9. A strong UV emission at 395 nm and several relatively weak visible emissions in the range of 400–500 nm are notable for AgZNRa. The UV emission is corresponding to free excitonic emission at the near band edge (NBE); and the visible emissions are attributed to the transition in various kinds of defect states [53,54]. A significant quenching of PL emissions can be seen for AgZNRw and AgZNRe. It is generally believed that a lower intensity of PL emission means a lower electron-hole recombination rate [52]. The decreased PL intensity of AgZNRw and AgZNRe can be ascribed to the larger amount of Ag than those of AgZNRa. Electronhole pairs will be generated in ZnO under the PL excitation source (325 nm). Under this similar circumstance as UV light irradiation, Ag will show the electron trapping ability and act as an electron sink, which will enhance the separation and transfer of charge carriers [38,43]. These photo-generated carriers with prolonged life time will improve the photocatalytic efficiency. Based on this consideration, it seems that both AgZNRw and AgZNRe should have better photocatalytic activity than AgZNRa, and AgZNRe should be the best among the Ag/ZnO heterostructures as it has the lowest PL

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emission. However, the photocatalytic result contradicts the expectation since AgZNRe showed the lowest photocatalytic efficiency. This phenomenon can be explained by the lowest separation efficiency of photo-generated electrons and holes as proposed by Xie et al. [38]. The possibility of hole capture increases by the large number of negatively charged Ag on ZnO because of the over accumulation of electrons and hence Ag begins to conversely behave as a recombinant centre and reduces the efficient charge separation [55]. This process of hole capture probably proceeds in a non-radiative pathway, which could not be detected by conventional PL measurement [38]. Other disadvantages associated with the Ag quantity and dispersion in AgZNRe also contribute to its poor photocatalytic activity. 4. Conclusions Ag/ZnO heterostructures were synthesized by photoreduction of Ag+ ions on the basis of ZnO nanorod arrays. The Ag nanostructures have a major (1 1 1) phase and mainly distribute on the top part of ZnO nanorods. The reducing mediums have a considerable effect on the quantity and dispersion of Ag component and further influence their visible light driven photocatalysis. The Ag/ZnO heterostructures reduced in the water have a nano(sheet-rod-particle) multi-level structure, which exhibited the best photocatalytic activity. The Ag/ZnO heterostructures reduced in the air showed the inferior photocatalytic degradation and the worst one is the Ag/ZnO heterostructures reduced in water with ethanol. The reducing mediums have a big influence on the Ag nanostructure parts of the nanocomposites, which further affect the surface phonon resonance, resulting different photocatalytic performance under visible light. Acknowledgement The authors gratefully appreciate the assistance from staff members in the Department of Chemical and Materials Engineering and the Research Centre for Surface and Materials Science at the University of Auckland, especially Ms Catherine Hobbis. The authors would also like to thank the scholarship from China Scholarship Council (CSC). Appendix A. Supplementary data Supplementary data associated with can be found, in the online cle http://dx.doi.org/10.1016/j.jhazmat.2014.12.045.

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