Synthesis and characterization of polymer-coated AgZnO nanoparticles with enhanced photocatalytic activity

Abstract

The polymer-coated AgZnO nanoparticles were synthesized via a one-pot process with the assistance of the triblock copolymer, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEO-PPO-PEO). The nanoparticles were characterized by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) including the mode of high resolution (HRTEM) and by energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible light absorbance spectrometry (UV-vis) and photoluminescence spectrophotometry (PL). It is found that the nanoparticles reveal high crystallinity, good dispersibility, and excellent optical performance both in organic and aqueous solvents. The photocatalytic behavior of the AgZnO nanoparticles was scrutinized using rhodamine B (RhB) as probe molecule. The results show that the degradation efficiency of the AgZnO nanoparticles is higher than that of the ZnO nanoparticles under both UV and sunlight irradiation, 89.5% and 66.9% for the former and 52.7% and 37.5% for the latter after 90 minute exposure, respectively. Moreover, the AgZnO nanoparticles were found to have the photocatalytic efficiency unaltered after 5 cycles of use as tested. Our results demonstrate that the nanostructured AgZnO nanoparticles exhibit enhanced photocatalytic performance and high stability. Consequently, the AgZnO nanoparticles are favorable candidates for potential application as a promising photocatalyst.

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1 Introduction

ZnO, a well-known II–VI semiconductor, has attracted extensive investigation because it possesses merits such as high photosensitivity, low cost, non-toxic nature, and environmentally friendly features for photocatalytic applications.¹ Additionally, the wurtzite ZnO structure shows a wide band gap and a large exciton binding energy which could lead to lasing action based on exciton recombination even at room temperature.^2,3 Due to the high rate of electron–hole recombination and visible blindness, however, its applications are somewhat restricted.^4,5 In order to enhance the photocatalytic performance with improved light harvesting efficiency, the separation efficiency of photo-induced electron–holes should be boosted. Numerous studies are available with respect to the addition of noble metals, including Au,^6–9 Ag,^10,11 Pd,¹² and Pt.^13–15 Such an element provides trapping sites for photoexcited electrons from the valence band of semiconductor oxides, leading to lower the band gap that the photocatalytic activity of the material is substantially increased by visible light irradiation. In particular, silver is a well studied material, possessing localized surface plasmon resonance, surface-enhanced Raman scattering and metal-enhanced fluorescence, and displaying some unique features in chemical and biological sensing.¹⁶ It shows great potential to decorate ZnO for improving the photocatalytic activity. After nano-engineering both materials into a single entity, the ensuing nanostructure exhibits multiple functionalities and novel properties, such as optical, electrical, magnetic, and chemical properties. The properties would not only arise from the unique properties of silver and the semiconductor, but also be generated from original collective phenomena based on the interaction between Ag and ZnO. Thus, the synthesis of AgZnO could be favored for photocatalysis, photodegradation, and optoelectronic devices.^{10,11,16–21}

Furthermore, the natural covering of nanoparticle surfaces with biofriendly and hydrophilic molecules is exceedingly desired for imminent biomedical applications, especially if completed in situ during the synthesis procedure of the nanoparticles.^22,23 In our research, excellent nanoparticles could be harvested by nanoemulsion approaches with the support of a triblock copolymer such as poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEO-PPO-PEO). The triblock copolymer demonstrates distinct advantages of non-charging trait, biocompatibility, non-toxicity, and aqueous solubility and has been employed in a variety of fields.^24–28 In the nanoemulsion synthesis, the PEO-PPO-PEO molecules primarily participate in the reactions as a surfactant, besides playing a role in stabilizing the nanoparticles formed, and acting as a reducing agent. In our previous research, experiments have produced highly crystalline, long-term stable, monosized FeAu, Fe₃O₄/Ca₃(PO₄)₂, ZnO–Au, and Fe₃O₄/ZnO nanoparticles.^23,29–31 In this paper, we report a simple and efficient method for the synthesis of polymer-coated AgZnO nanoparticles using biocompatible and non-toxicity triblock copolymer PEO-PPO-PEO as the surfactant. The resulting nanoparticles possess high crystallinity, excellent dispersibility and optical performance. The photocatalytic behavior of the nanoparticles is evaluated using RhB as a probe molecule under UV and sunlight irradiation, and the reusability of AgZnO catalyst is studied as well. The results demonstrate that the nanostructured AgZnO moieties unveil enhanced photocatalytic performance and high stability. Based on the experimental observations, the as-synthesized AgZnO nanoparticles could be treated as a promising photocatalyst candidate in the decomposition of organic pollutants.

2 Experimental

2.1 Materials

Ag acetylacetonate (Ag(acac), 99.9%), and zinc(II) acetylacetonate (Zn(acac)₂, 99.9%) were used as the precursors. The surfactant was the block copolymer PEO-PPO-PEO (M_w = 5800), while the solvent for the nanoemulsion reaction was octyl ether (C₈H₁₇OC₈H₁₇, 99%) and 1,2-hexadecanediol (C₁₄H₂₉CH(OH)CH₂(OH), 90%) acted as the reductant. Other chemicals included hexane and ethanol. All materials were used as received without further processing.

2.2 Synthesis of AgZnO nanoparticles coated by PEO-PPO-PEO

A typical synthesis was carried out in a 100 mL flask. The AgZnO sample was obtained by mixing Ag(acac) (0.125 mmol or 0.0259 g) and Zn(acac)₂ (0.375 mmol or 0.0989 g) in 10 mL octyl ether with 1,2-hexadecanediol (0.6468 g) and PEO-PPO-PEO (0.7878 g) under vigorous stirring. The reaction mixture was first heated to 125 °C in 2 h and maintained for 1 h at 125 °C, then rapidly raised to 280 °C in 15 min and refluxed at the temperature for 1 h. After cooling down to room temperature, a pale gray product was separated from the supernatant by centrifugation, which was washed with ethanol–hexane in a volume ratio of 2 : 1 several times, and re-dispersed in hexane for further use.

2.3 Structural characterization and measurements of the nanoparticles

The crystal structures of the PEO-PPO-PEO-covered AgZnO nanoparticles were analyzed by X-ray powder diffraction (XRD, X'Pert Pro) and transmission electron microscopy (TEM, JEOL 2010F) including the mode of high resolution (HRTEM) and energy dispersive X-ray analysis (EDX) were used to characterize the morphologies and elemental compositions. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo ESCALAB 250XI photoelectron spectrometer with Al K_α X-ray as the excitation source. The UV-vis spectra were measured by a UV-vis spectrometer (UV-vis near IR spectrophotometer, Hitachi U4100), revealing the band edge and surface plasmon resonance features of the nanostructured ZnO and Ag materials in the composite nanoparticles. The fluorescence spectra were obtained with a spectrofluorophotometer (Hitachi F7000). In the Fourier transform infrared spectroscopy (FTIR) studies, the washed PEO-PPO-PEO coated AgZnO nanoparticles and the pure PEO-PPO-PEO polymer were separately crushed with a pestle in an agate mortar. The individually crushed material was mixed with KBr in about 1 : 100 proportion. The mixture was then compressed into a 2 mm semi-transparent disk by applying a force of 10 T for 2 min. The FTIR spectra were recorded in the wavenumber range of 4000–400 cm⁻¹ using an Avatar 360 FTIR spectrometer (Nicolet Company, USA).

2.4 Characterization of photocatalytic activity

The photocatalytic activities of the nanostructured catalysts were performed under a variety of conditions by measuring the degradation rate of a RhB dye solution at room temperature. RhB was prepared with a concentration of 2.5 mg L⁻¹, by dissolving the dye powder in distilled water. The photocatalytic reaction was conducted at room temperature under a UV light 36 W UV-A tube predominantly emitting at 365 nm (Philips) with the distance of 10 cm from the lamp to the reaction beaker. The reaction was carried out with 5 mg of a catalyst dispersed in 25 mL of 2.5 ppm RhB aqueous solution. Prior to irradiation, the mixtures were stirred in the dark for 30 min for the establishment of adsorption–desorption equilibrium between RhB molecules and the surface of the photocatalyst. After illumination, the samples (volume of each is about 3.5 mL) were withdrawn from the reaction beaker every 15 min, centrifuged at 4500 rpm for 10 min and filtered to remove the particles. The filtrate was then analyzed using a UV-vis spectrophotometer (Beijingpuxitongyong TU-1900) to measure the absorption of RhB in the range of 200 nm to 800 nm.

3 Results and discussion

3.1 Nanostructures and TEM morphology of PEO-PPO-PEO-capped AgZnO nanoparticles

As shown in Fig. 1, the crystal structure of the AgZnO nanoparticles was recorded by XRD and analyzed together with the results of the ZnO nanoparticles with similar sizes as both nanoparticles were prepared by analogous synthesis methods. Fig. 1a represents the diffraction pattern obtained from the AgZnO nanoparticles, dominated by the Ag diffractions as labelled (JCPDS no. 87-0718) with ZnO peaks (not labelled), whereas the diffraction pattern of the ZnO nanoparticles is shown in Fig. 1b, in match to the standard diffraction peaks of the corresponding bulk material (JCPDS no. 65-3411). As shown in Fig. 1b, the diffraction peaks positioning at 32.6°, 34.7°, 36.9°, 48.1°, 57.3°, 63.4°, 66.8°, and 68.5° are indexed to the (100), (002), (101), (102), (110), (103), (200), and (112) planes of the ZnO hexagonal wurtzite, and the peaks in Fig. 1a at 38.8°, 44.9°, 65.2°, 77.9° and 82.1° are indexed to the Ag positions of (111), (200), (220), (311) and (222) planes. The relatively stronger reflection intensities from Ag compared with ZnO in Fig. 1a are mainly attributed to the much stronger scattering power of Ag than that of ZnO, as Ag has a higher atomic number and is much mass heavier than ZnO, offseting the effect of the concentration difference in Ag and ZnO. In addition, negligible changes of all diffraction peak positions of ZnO in the AgZnO samples compared to that of pure ZnO suggests that Ag does not incorporate into the lattice of ZnO, and supports the nanostructuring of ZnO and Ag in a single motif.¹⁸ Subsequently, the average particle size of the AgZnO nanoparticles is estimated to be approximately 24.6 nm by the Scherrer equation based on the full width at half maximum (FWHM), comparable to that from the statistical size counting of the TEM analysis as analyzed below, supposing that the broadening of the peaks in the XRD pattern is largely due to the finite-size effect of the nanoparticles.³² The well-defined, strong, distinct peaks show that the AgZnO nanoparticles are in a highly crystalline state.

Fig. 1 XRD patterns for the respective PEO-PPO-PEO–AgZnO and ZnO nanoparticles. (a) AgZnO nanoparticles (indexed for Ag), (b) ZnO nanoparticles (indexed for ZnO). Bar diagram for the JCPDS of ZnO (in square).

The morphology and particle size of the prepared AgZnO nanoparticles were recorded by TEM. As shown in Fig. 2a, the nanoparticles apparently are highly crystalline, virtually uniform and spherical in shape. The size distribution of the nanoparticles acquired from Fig. 2a is presented in Fig. 2b, in which the histogram has a tight size distribution and gives an average particle size of ∼25 nm in diameter. The size distribution is described quite satisfactorily by the Gaussian function. Fig. 2c represents the high-resolution TEM image of a single polymer-coated AgZnO nanoparticle with highly regular lattices running over the nanocrystal. As labeled, the spacing of 2.35 Å is assigned to the projection of the (111) Ag plane, whereas the spacing of 2.81 Å is originated from the (100) ZnO plane, clearly indicating the integration of both Ag and ZnO compositions in the same nano-entity. Fig. 2d shows a typical TEM-EDX point-detection instance for the composition, clearly exposing the simultaneous presence of both zinc and silver elements.

Fig. 2 TEM analyses of the PEO-PPO-PEO-capped AgZnO nanoparticles. (a) Bright-field image, (b) particle size histogram with Gaussian fit, (c) HRTEM of an individual AgZnO nanoparticle, (d) point-detection EDX analysis of the composition.

The XPS analysis was carried out to investigate the chemical composition of the AgZnO nanoparticle, and the corresponding experiment results are shown in Fig. 3. The binding energies in the XPS spectra were calibrated using C 1s (284.8 eV). There are no peaks for other elements except for Zn, O, Ag, and C observed from the full XPS spectra of Fig. 3a. The presence of carbon comes largely from the surfactant (PEO-PPO-PEO) molecules on the surface of the resulting nanoparticles. Therefore, it is concluded that the nanoparticle is mostly composed of three elements, Zn, Ag, and O. The peaks observed at 367.2 eV and 373.2 eV (Fig. 3b) correspond to the Ag 3d_5/2 and Ag 3d_3/2 states of the metallic silver. The binding energy of the Ag 3d states is shifted obviously to lower values as observed for other AgZnO composites when compared to bulk Ag (about 368.2 and 374.2 eV, respectively³³). This effect is often ascribed to the fact that the interaction between Ag and ZnO, namely, the electron transfer takes place from Ag to ZnO at the interfaces when Ag and ZnO come into contact.¹⁸ Thus, the analysis illustrates that the AgZnO nanoparticle dominantly comprises Ag and ZnO, which is in agreement with the XRD and TEM results as previously addressed.

Fig. 3 XPS spectra of the PEO-PPO-PEO-capped AgZnO nanoparticles. (a) Full XPS spectrum, (b) details of Ag 3d_5/2 and Ag 3d_3/2 bonding.

The presence of the PEO-PPO-PEO macromolecules on the surface of the AgZnO nanoparticles in this work was justified by FTIR on the purified nanoparticles in comparison to the pure polymer.^23,29 Fig. 4 evaluates the FTIR spectrum of the PEO-PPO-PEO–AgZnO nanoparticles after purification against that of the PEO-PPO-PEO molecules used in the synthesis as the surfactant. In Fig. 4b, the pure PEO-PPO-PEO polymer molecules show one strong characteristic band at the position of ∼1110.3 cm⁻¹ due to C–O–C stretching vibration of the ether bonding which usually ranges between 1250 cm⁻¹ and 1000 cm⁻¹ and a sharp band for the C–H bending vibration at the position of ∼1463.6 cm⁻¹.^23,29 As given in Fig. 4a, these characteristic vibration and bending modes reappear in the FTIR spectrum of the PEO-PPO-PEO-capped AgZnO nanoparticles, but instead shifting to the positions of ∼1120.4 cm⁻¹ for the C–O–C stretching vibration and ∼1576.9 cm⁻¹ for the C–H bending vibration, respectively. Likewise, the band shapes and absorption intensities differ noticeably from the pure PEO-PPO-PEO molecules to the composite AgZnO nanoparticles. The blue-shift and shape change of the C–O–C stretching and C–H bending may be credited to the coordination of the oxygen atoms in the PEO-PPO-PEO main chains to the Ag and Zn atoms.^23,29,34 As the extra PEO-PPO-PEO molecules were removed by the purification process, the outcome overtly draws to the conclusion of covering of the PEO-PPO-PEO molecules onto the surface of the AgZnO nanoparticles, which can be further corroborated by the other relevant absorption bands in the spectra.

Fig. 4 FTIR spectra of (a) the PEO-PPO-PEO-capped AgZnO nanoparticles and (b) the pure PEO-PPO-PEO polymer.

3.2 Optical properties of PEO-PPO-PEO-capped AgZnO nanoparticles

The nanoparticles are both hydrophobic and hydrophilic which of the attribute enables a bi-phase dispersible function for an easy transport of the nanoparticles between non-polar and polar solvents without further surface modification, as a result of the PEO-PPO-PEO lacing on the surfaces of AgZnO nanoparticles. Fig. 5 shows the UV-vis spectra of the AgZnO nanoparticles dispersed in hexane (a), ethanol (b) and water (c), together with those of Ag (d) and ZnO (e) nanoparticles in similar sizes dispersed in hexane. Evidently, there are two kinds of absorption bands, one from the band edge of ZnO and the other from the surface plasmon resonance of the nanosized Ag. In Fig. 5a, the AgZnO nanoparticles dispersed in hexane exhibit one well-defined absorption band around 359 nm, representing the most distinctive absorption of the ZnO semiconductor, which corresponds to blue-shifting from the absorption peak of the ZnO nanoparticles in hexane at the position of ∼365 nm, as shown in Fig. 5e. The effects of solvents on the characteristic absorption band are detected in the UV-vis spectra of the polymer-laced AgZnO nanoparticles dispersed in ethanol and water, which are slightly red-shifted to 362 nm for the former and 363 nm for the latter when comparative to that in hexane but still blue-shifted against the ZnO nanoparticles. The blue-shifting of the ZnO absorption may be construed in the quantum confinement due to the reduced particle dimension and the solvent effects. Furthermore, the second well-defined absorption between ∼400 nm and ∼500 nm features the optical property of surface plasmon resonance (SPR) owing to Ag nanostructuring. It is well established that the SPR band of metal nanoparticles strongly depends on the size, shape, composition, and dielectric property of the nanoparticles and the local environment.^15,35,36 In this work, dependent on the dielectric property of a solvent, the position of the surface plasmon band in the solution of the AgZnO nanoparticles varies from ∼441 nm in hexane, ∼425 nm in ethanol, to ∼431 nm in water, in comparison to the Ag nanoparticles in hexane which has an absorption peaking at ∼430 nm, as shown in Fig. 5d. The distinctly band shape change and shift of the surface plasmon spectra of the AgZnO composite nanoparticles could be attributable to the fact that the strong interfacial coupling between Ag and ZnO results in electrons transfer from Ag to ZnO resulting from the formation of AgZnO nanocomposites.^37,38

Fig. 5 UV-visible absorbance spectra of the PEO-PPO-PEO-capped AgZnO nanoparticles dispersed in different solvents. Hexane (a), ethanol (b) and water (c), in comparison to Ag (d) and ZnO (e) nanoparticles (both in hexane).

The PL emission spectra of the PEO-PPO-PEO-capped AgZnO nanoparticles respectively dispersed in hexane, ethanol and water were examined under the excitation wavelength of 360 nm. In Fig. 6a, the AgZnO nanoparticles in hexane show a strong emission at approximately 402 nm, a distinct plateau at around 475 nm and a relatively strong emission at approximately 580 nm. In Fig. 6b, the nanoparticles in ethanol manifest a strong emission at approximately 403 nm, an analogous plateau at around 473 nm and a second emission at approximately 579 nm. In the case of water (Fig. 6c), the nanoparticles likewise demonstrate a strong emission at approximately 411 nm, with a weak but firm plateau at around 473 nm and a second emission at approximately 579 nm. Overall, the blue bands around 400 nm most likely occurs from the donor level of interstitial Zn to the acceptor energy level of Zn vacancy, and the emission at approximately 580 nm is commonly endorsed to the singly ionized oxygen vacancy in ZnO which is caused by the recombination between the electrons in a deep defect level or a shallow surface defect level and the holes in a valence band.³⁹ The concentration increase of the excitons is considered responsible for the enhancement of visible emission after nanosized Ag combined with ZnO.⁴⁰ Nevertheless, the contributions of the Ag nanocrystallites to the PL emissions may be further divided into three main factors: first, on the premise that the PL emission is related to structural defects, the decrease of the PL emission and the appearance of the visible emission indicates that adding silver acetylacetonate to the reaction system disrupts the crystallinity of ZnO resulting in more defects. Second, the formation of the interface between silver and ZnO lead to an increased concentration of the excitons, which amplifies the visible emission.⁴⁰ Finally, when the silver nano-islands are formed on the surface of ZnO, a semiconductor–metal junction is built up and the silver acts as an electron acceptor.

Fig. 6 Photoluminescence emission spectra of the PEO-PPO-PEO-capped AgZnO nanoparticles dispersed in different solvents. Hexane (a), ethanol (b), and water (c), with the excitation of λ_ex = 360 nm.

3.3 Photocatalytic degradation of RhB

Rhodamine B (RhB), a basic dye of the xanthene class with highly water soluble, is widely used as a probe molecule to appraise the photocatalytic activity of a catalyst. Besides, it also frequently used as a colorant in textiles and food stuffs. The dye is harmful if swallowed by human beings and animals, causing irritation to the skin, eye and respiratory tract.⁴¹ Thus, the reduction or elimination of the dye pollutant is imperative and its degradation, however, can be conveniently monitored by optical absorption spectroscopy. Fig. 7a shows the absorption spectra of a RhB solution containing the AgZnO nanoparticle, taken at different intervals after exposing the solution under UV irradiation. The intensity of the absorption spectra monotonically decreases as the exposure time increases from 0 to 90 min. The reduced intensity in the main absorption peaks is assigned to the degradation of RhB. Fig. 7b reports a comparative study for the degradation of RhB under different conditions. The coordinate is referred as C/C₀ in which C is the concentration of RhB at each irradiated time interval determined at λ_max and C₀ is the starting concentration when the adsorption–desorption equilibrium was achieved. The effects of photodegradation were comparatively investigated under the conditions of (1) AgZnO under UV, (2) ZnO under UV, (3) AgZnO under sunlight, (4) ZnO under sunlight, (5) no photocatalyst under UV irradiation, and (6) ZnO in dark. It is apparent that RhB decomposes 6.4% in the presence of ZnO without irradiation (curve 6), compared to only 7.2% photolysis when barely exposed to UV irradiation (curve 5). However, substantial degradation of RhB with AgZnO as catalyst is obviously witnessed, 89.5% decomposed after 90 min of UV irradiation (curve 1), which is much higher than that under sunlight, 66.9% decomposed (curve 3). The observation points to the fact that the catalyzing activity of AgZnO is stronger under UV than sunlight irradiation,²¹ but in turn, AgZnO under both conditions are more effective than ZnO respectively under UV and under sunlight, 52.7% and 37.5% decomposed (curves 2 and 4), clearly showing the greater efficiency of AgZnO than ZnO. The higher efficiency of AgZnO can be explained by two reasons: one is that Ag acts as an electron acceptor, by which chemisorbed molecular oxygen reacts with photogenerated electrons to form active oxygen species, facilitates the trapping of photogenerated electrons; the other one is that Ag can increase the separation efficiency of photogenerated electrons and holes in ZnO.¹⁹

Fig. 7 (a) Absorption spectral changes of a RhB solution degraded by AgZnO over irradiation time of 0–90 min, (b) degradation of RhB under different conditions. Curves: (1) AgZnO under UV, (2) ZnO under UV, (3) AgZnO under sunlight, (4) ZnO under sunlight, (5) no photocatalyst under UV, and (6) ZnO in dark.

In order to explore the recyclability potential of the AgZnO catalyst, it was recovered from the degradation mixture through filtration and was then washed with distilled water. The recovered catalyst was reused for the photochemistry of RhB under the identical reaction conditions. The catalytic activity of AgZnO was tested for 5 times, as showed in Fig. 8a, even after being reused for 5 times, the AgZnO catalyst preserved its photocatalytic behavior and the dye degradation efficiency is practically the same. This recyclability of AgZnO is attributed to the stability and resistance to photocorrosion.¹⁹ Such properties of the AgZnO nanoparticles may be understood in the context of possible morphological alterations happening in the original and reused AgZnO samples. Fig. 8b is a representative TEM image of the AgZnO sample after five cycling test under UV, indicative of no distinct change from the as-synthesized one as shown in Fig. 2a. Consequently, the AgZnO nanoparticles as prepared in this work demonstrate robustness in the photocatalytic degradation process. As it is well-known, for a greener and environment-friendly approach, the reusability is desired as this makes the process free of waste and also reduces the operational cost. So the reusability of the AgZnO catalyst plays a vital role for the degradation of dyes.

Fig. 8 (a) Reusability of AgZnO catalyst for the degradation of RhB for 5 cycles, (b) TEM image of the reused AgZnO sample after five cycles under UV.

4 Conclusions

In summary, we have successfully synthesized the polymer-coated AgZnO nanoparticles by the one-pot process adopting the biocompatible and non-toxicity triblock copolymer PEO-PPO-PEO as the surfactant. The FTIR assessment confirms that the PEO-PPO-PEO macromolecules are capping on the surface of the nanoparticles. The morphological and structural analyses show the narrow particle size distribution with average diameter ∼25 nm and the high crystallinity of the nanoparticles. Both EDX and XPS characterization endorses the co-existence of Ag and Zn in the AgZnO composite nanoparticles. The UV-vis and fluorescence measurements show the well-defined optical absorption and emission properties with the nanoparticles dispersed in both hydrophilic and hydrophobic solvents, whereas the PL emission spectra further manifest the excellent optical properties of the nanoparticles. The photocatalytic behavior of the AgZnO nanoparticles has been scrutinized using rhodamine B (RhB) as the probe molecule. AgZnO was found to be a better photocatalyst than ZnO under both UV and sunlight irradiation, and its photocatalytic efficiency was unaltered ever after 5 cycles of use. Thus, AgZnO could be employed as an effective catalyst for the degradation of dyes.

Acknowledgements

This work was supported in part by the Scientific and Technological Development Projects, Science and Technology Department of Henan Province, China, the National Natural Science Foundation of China (no. 51172064), the Research Initiative Fund from South University of Science and Technology of China and Shenzhen Government, the National Research Foundation of Korea (no. 2012-0005657, 2012-0001067), the Industrial Core Technology Development Program funded by the Ministry of Trade, Industry and Energy (no. 10033183), and the Seoul R&BD Program (no. 10920).

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