The characteristics and photocatalytic activities of silver doped ZnO nanocrystallites

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Abstract

Silver ions doped ZnO nanocrystallites were synthesized from the precursor of Ag-doped Zn5(CO3)2(OH)6 applying hybrid induction and laser heating (HILH) techniques. The physical properties of prepared nanoparticles were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and UV-Vis methods. The results of the photocatalytic degradation of methylene blue (MB) and phenol in aqueous suspensions showed that silver ions doping greatly improved the photocatalytic efficiency of ZnO nanocrystallites. This was attributed to the change of surface properties of metal ions doped semiconductor, such as O vacancies, crystal deficiencies, and increased specific surface area (SSA). Increasing the amount of silver ions resulted in increased lattice deficiency and hence enhanced the photocatalytic activity. The quantum confinement of silver ions doped ZnO nanocrystallites was also investigated in this study.

Introduction

In the past decades, the scientific and engineering interest in the application of semiconductor photocatalysis has grown exponentially [1], [2], [3], [4], [5], [6], [7], [8]. ZnO can act as a sensitizer for light-induced redox due to its electronic structure, which is characterized by a filled valence band and an empty conduction band. When a photon with an energy of matches or exceeds the bandgap energy, Eg, of the semiconductor, an electron, ecb, from the valence band (VB), is excited and enters into the conduction band, CB, leaving a hole, hvb+ behind. Excited state conduction band electrons and valence band holes can recombine and release energy as heat, get trapped in metastable surface states, or react with electron donors and electron acceptors adsorbed on the semiconductor surface or within the surrounding electrical double layer of the charged particles. If a suitable scavenger or surface defect is available to trap the electron or hole, recombination is prevented and subsequent redox reactions may occur. Metal-doped ZnO nanocrystallites have attracted much attention, because metal ions introducing could enhance the photocatalytic activity of ZnO in some cases. Although the photophysical mechanisms of doped ZnO are not well understood, generally speaking, the formation of shallow charge trapping on the surface of the ZnO nanocrystallites due to the replacement of ZnII by metal ions is accepted. Surface properties, such as area, oxygen deficiency and hydroxyl attachments play an important role when ZnO nanocrystallites are used to remove non-biodegradable recalcitrant chemicals in wastewater or are used in heterogeneous applications.

In the recent years, silver ions become the interests of several research works [7], [8], [9], both becaused of their novel effects on the improvement of photoactivity of semiconductor photocatalysis nanocrystallites [8], [9], and their effects on antibacterial activity [7]. These properties can be applied to a tremendous range of applications, for instance, environment, textiles, engineering materials and so on. However, the studies on silver-doped photocatalyst nanocrystallites are still limited in the literature. Those of published mainly using dipping photolysis [8], [9] and co-precipitate thermal hydrolysis method [7]. In this study, HILH method a thermal physical nanocrystallites synthesis technique, has been applied to synthesis silver ions doped ZnO nanocrystallites, which is totally different from the approaches mentioned above. Series of metal, oxide, carbon have been synthesized successfully by our group previously [10], [11], [12], [13]. This study focused on the morphological, photocatalytical and optical characteristics of silver ions doped ZnO systems synthesized by HILH method.

Section snippets

The preparation of silver doped ZnO

The synthesis procedure can be divided into two parts, one is the preparation of precursor by co-precipitate methods and the other is the formation of silver doped ZnO nanocrystallites by HILH. The precursor was prepared according to the method documented in reference [7]. The detailed process can be described as follows. Firstly, under vigorous stirring, the aqueous solution of complex silver ions coordinated by ammonia was sprayed into Zn(NO3)2 aqueous solution, subsequently NaOH aqueous

The size, morphology and phase of silver ions doped ZnO nanocrystallites

The TEM images showed that undoped ZnO nanocrystallites were tetrapod nanowhiskers with needles of 8 nm in diameter and 260 nm in length, similar to a previous report by our group [11]. However, the needle shaped nanocrystallites were much less in silver ions doped systems. It is found that the more the dosage of the silver ions, the less the ratio of the structure of nanowhiskers. The average grain size D of the silver ions doped ZnO nanocrystallites can be calculated from the width of the lines

Conclusion

The silver ions doped ZnO nanocrystallites synthesized by HILH techniques have a wurtzite structure with the uniform sizes and morphologies. It is found that the doping of silver ions could greatly improve the photocatalytic degradation activity of ZnO nanocrystallites, which was considered to be attributed to the surface properties of metal ions doped semiconductor, such as O vacancies, crystal deficiencies and increased specific surface area. By applying AFM, the silver particles

Acknowledgements

The work was partially supported by the R&D center of Shenzhen Junye Nano Materials Co. Ltd. The authors also would like to acknowledge the funding from the Hong Kong Polytechnic University.

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