Elsevier

Materials Research Bulletin

Volume 46, Issue 10, October 2011, Pages 1625-1631
Materials Research Bulletin

Facile synthesis of Ag/ZnO heterostructures assisted by UV irradiation: Highly photocatalytic property and enhanced photostability

https://doi.org/10.1016/j.materresbull.2011.06.016Get rights and content

Abstract

We report a new method to synthesize Ag/ZnO heterostructures assisted by UV irradiation. The formation of Ag/ZnO heterostructures depends on photogenerated electrons produced by ZnO under UV light to reduce high valence silver. Functional property of the Ag/ZnO heterostructures is evaluated by photodegradation of methylene blue (MB) under UV illumination. Results of photodegradation tests reveal that the optimal photocatalytic activity of as-syntheszied samples is about 1.5 times higher than the pure ZnO synthesized in the same condition or commercial TiO2 (P-25), showing the advantage of the unique structure in the Ag/ZnO heterostructure. Besides, due to the reduced activation of surface oxygen atom, photocatalytic activity of the photocatalysts has no evident decrease even after three recycles.

Highlights

► Fabrication of Ag/ZnO heterostructure between the two incompatible phases is realized under UV irradiation in the absence of surfactant. ► The synthetic method is facile, low cost, and low carbon, which depends on the photogenerated electrons produced by ZnO under UV light. ► Photocatalytic property of the as-synthesized samples is 3.0 times as good as the pure ZnO synthesized under the same condition or the commercial TiO2 (Degussa, P-25). ► The heterostructures exhibit good durability without significant change in the activity even after the third cycle compared to the pure ZnO.

Introduction

In the past few decades, heterostructures have been extensively studied because of its potential applications in nanodevices [1], biomedicine [2], and photocatalysis [3], [4], [5]. Among various heterostructure materials, metal/semiconductor is one of the most popular heterostructures due to its unique optical, electrical, and catalytic properties [6]. With increasing environmental problems, it is very significative to develop metal/semiconductor heterostructures for high performance photocatalysts. Recently, Ag/ZnO heterostructures with designed structures have been successfully synthesized due to its high photocatalytic activity, for example, Ag/ZnO 3D hollow microspheres [7], Ag/ZnO heterostructured nanofibers [8], dendrite-like Ag/ZnO nanocrystals [9] and so on. But silver and zinc oxide have a high degree of lattice mismatch, which prevents the nucleation and growth of an overlayer on a substrate, so Ag/ZnO heterostructure may not be obtained [10]. So far, many methods have been made to synthesize Ag/ZnO heterostructures such as hydrothermal or solvothermal method [7], [11], electrospinning method [8], sol–gel [12], flame spray pyrolysis (FSP) [13], RF magnetron sputtering [14], and so on. However, most of these methods are either inefficient, or need expensive instruments. Therefore, it is necessary to find a shortcut to synthesize objective photocatalysts with high performance and low cost. In this paper, we report a modified photodeposition method, which utilizes photogenerated electrons produced by ZnO under UV irradiation to reduce high valence silver, and simultaneously form Ag/ZnO heterostructures. The as-synthesized Ag/ZnO samples show a high crystallinity and a strong contact between metallic silver and zinc oxide by chemical bonds.

It is well-known that when a wide-band-gap ZnO was irradiated by UV light with photon energy higher than or equal to the band gap, electrons in the valence band (VB) can be excited to the conduction band (CB) with simultaneous generation of the same amount of holes in the VB. The formation of Ag/ZnO heterostructure can be formulated as follows:Zn2+ + 4OH  Zn(OH)42−Ag+ + 2OH  Ag(OH)2Zn(OH)42− + 2Ag(OH)2  Ag2O/ZnO + 2H2O + 4OHAg2O/ZnOhvAg/ZnO+O2We expect to utilize the photogenerated electrons to reduce the high valence silver (Eq. (4)), and form heterojunctions at the interfaces. In this system, reduction by photogenerated electrons at the interfaces plays an important role in the formation of heterostructure between the two incompatible phases. The process can be roughly depicted as Scheme 1.

In summary, we develop a UV-assisted method to synthesize Ag/ZnO heterostructures, and explore the influence of UV irradiation on formation of Ag/ZnO heterostructures. The catalytic efficiency was evaluated by degradation of methylene blue (MB) under UV irradiation, and the degradation extent can be easily detected by the absorbance. We believe that our method can open a new avenue for formation of metal/semiconductor heterostructures for desired properties.

Section snippets

Materials

Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), sodium hydroxide (NaOH), silver nitrate (AgNO3), ethanol (C2H5OH), commercial TiO2 (Degussa, P-25), and methylene blue (MB) are all analytical grade and used without further purification.

Preparation of Ag/ZnO heterostructures

All experiments were carried out at room temperature. In a typical procedure, a certain Zn(NO3)2·6H2O and AgNO3 was dissolved in 100 mL absolute ethanol under vigorously stirring (the content of the Ag element is 3.0, 5.0, 7.0, 9.0, and 15.0 at.%, respectively) to form a

Morphology and structure of Ag/ZnO heterostructure

The XRD patterns of the samples with different Ag contents are shown in Fig. 1. The two sets of strong diffraction peaks show that the as-synthesized samples are dimer-type nanocrystals with high crystallinity. All the peaks in Fig. 1 can be indexed to hexagonal wurtzite ZnO and face-centered-cubic (fcc) metallic Ag, which are consistent with the data of JCPDS 36-1451 and JCPDS 4-0783 respectively. No impurity peaks are detected from the patterns. In addition, there is no remarkable shift of

Conclusions

In conclusion, photocatalytically active Ag/ZnO heterostructures have been successfully prepared via a UV-assisted method, where photogenerated electrons play an important role in the formation of the heterostructure between the two incompatible phases. The synthetic method is very simple, economical, low-carbon and free from organic contaminations. The resultant heterostructures show much better photocatalytic activity in photodegradation of MB under UV light compared to the pure ZnO and

Acknowledgements

This work has been financially supported by the National Natural Science Foundation of China (No. 51002128), Open Project Program of Chemical Process Simulation and Optimization Research Center of Ministry of Education in Xiangtan University (No. 2009HGZX03).

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