Ag3PO4/ZnO: An efficient visible-light-sensitized composite with its application in photocatalytic degradation of Rhodamine B

doi:10.1016/j.materresbull.2012.10.015

Materials Research Bulletin

Volume 48, Issue 1, January 2013, Pages 106-113

https://doi.org/10.1016/j.materresbull.2012.10.015 Get rights and content

Abstract

The efficient visible-light-sensitized Ag₃PO₄/ZnO composites with various weight percents of Ag₃PO₄ were prepared by a facile ball milling method. The photocatalysts were characterized by XRD, DRS, SEM, EDS, XPS, and BET specific area. The radical dot OH radicals produced during the photocatalytic reaction was detected by the TA–PL technique. The photocatalytic property of Ag₃PO₄/ZnO was evaluated by photocatalytic degradation of Rhodamine B under visible light irradiation. Significantly, the results revealed that the photocatalytic activity of the composites was much higher than that of pure Ag₃PO₄ and ZnO. The rate constant of RhB degradation over Ag₃PO₄(3.0 wt.%)/ZnO is 3 times that of single-phase Ag₃PO₄. The optimal percentage of Ag₃PO₄ in the composite is 3.0 wt.%. It is proposed that the radical dot OH radicals produced in the valence band of ZnO play the leading role in the photocatalytic degradation of Rhodamine B by Ag₃PO₄/ZnO systems under visible light irradiation.

Graphical abstract

The free OH radicals generated in the VB of ZnO play the primary role in the visible-light photocatalytic degradation of RhB in Ag₃PO₄/ZnO system. The accumulated electrons in the CB of Ag₃PO₄ can be transferred to O₂ adsorbed on the surface of the composite semiconductors and H₂O₂ yields. H₂O₂ reacts with electrons in succession to produce active radical dot OH to some extent.

Highlights

► Efficient visible-light-sensitized Ag₃PO₄/ZnO composites were successfully prepared. ► Effect of Ag₃PO₄ content on the catalytic activity of Ag₃PO₄/ZnO is studied in detail. ► Rate constant of RhB degradation over Ag₃PO₄(3.0 wt.%)/ZnO is 3 times that of Ag₃PO₄. ► The active species in RhB degradation are examined by adding a series of scavengers. ► Visible light degradation mechanism of RhB over Ag₃PO₄/ZnO is systematically studied.

Introduction

ZnO has been widely used as a photocatalyst, owing to its high activity, low cost, and environmentally friendly feature [1]. However, the photocatalytic activity of ZnO is limited to irradiation wavelengths below 387 nm because ZnO semiconductor has a wide band-gap of about 3.2 eV and only 3–5% of the whole solar energy is effectively utilized, while 43% of the visible light is open to exploit. Therefore, various methods have been adopted to improve the utilization of the visible light [2], [3], [4], [5]. Among them, a good strategy is coupling of two semiconductors to form composite photocatalysts, because it has been reported that if two semiconductors are properly integrated into one system, this system can be expected to achieve high photocatalytic activity [6]. For example, the SnO₂–ZnO [7], TiO₂–ZnO [8], WO₃–ZnO [9] composite heterostructures appear to be very efficient for the photodecomposition of organic dyes.

So far, a great number of semiconductors with narrow band-gaps, such as BiOI, CdS, CuO, V₂O₅, C₃N₄, loaded on the surface of ZnO have been investigated to design visible-light-driven composite photocatalysts [10], [11], [12], [13], [14]. The narrow band-gap semiconductors usually act as visible-light sensitizes, and some of the photogenerated electrons or holes excited by visible light irradiation will then transfer to ZnO. As for numerous sensitizers, the conduction band (CB) edge lies above that of ZnO, and so does the valence band (VB) position. Under visible light irradiation, the photogenerated electrons transfer to the CB of ZnO, while the holes remain in the VB of sensitizers. Because the radical dot OH radicals are unavailable on the surface of ZnO photocatalyst, it is difficult for this type composite photocatalysts to induce complete mineralization of organics [15]. On the contrary, if the VB level of a sensitizer is lower than that of ZnO in the coupled structure, some of the electrons in the VB of ZnO can be transferred to that of the sensitizer, and consequently the holes generated in the VB of ZnO can initiate photocatalytic oxidation reactions. The system is considered to induce complete decomposition of pollutants because of the powerful oxidative ability of the holes located in the VB of ZnO [16]. However, it is difficult to find out appropriate sensitizers whose VB is lower than that of ZnO, owing to the scarcity of suitable sensitizers. Therefore, research about the composite photocatalyst with a lower VB level sensitizer compared to ZnO is of significant importance and should be paid more attention. And it is a possible way to develop new visible-light-responsive photocatalysts with high activity.

Recently, Ag₃PO₄ has attracted considerable attention as a potential visible light photocatalyst, and it is a pale yellow semiconductor with a band gap of ca. 2.45 eV [17], [18], [19], [20]. It has been reported that the CB and VB edge potential of Ag₃PO₄ is 0.45 eV and 2.9 eV (vs. NHE), respectively [17], [18]. The VB potential of Ag₃PO₄ is lower than that of ZnO with 2.6 eV [21], and so Ag₃PO₄ is considered to be an appropriate sensitizer to improve photocatalytic activity in the Ag₃PO₄/ZnO system, in which ZnO works as a substrate, while the role of Ag₃PO₄ is a sensitizer absorbing visible light. To the best of our knowledge, however, the study of the composite photocatalyst Ag₃PO₄/ZnO has not been reported.

In this study, Ag₃PO₄ was prepared by the chemical precipitation method and the efficient visible-light-sensitized Ag₃PO₄/ZnO composites with different weight percents of Ag₃PO₄ were prepared by the facile ball milling method. The composites were characterized by X-ray powder diffraction (XRD), UV–vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) specific surface area and porosity. The radical dot OH radicals produced on the photocatalyst surface were detected by the TA–PL technique. The photocatalytic activity of the photocatalyst was evaluated by photocatalytic degradation of Rhodamine B (RhB) under visible light irradiation. The results showed that photocatalytic activity of Ag₃PO₄/ZnO was much higher than that of single Ag₃PO₄ or ZnO. The rate constant of RhB degradation over Ag₃PO₄(3.0 wt.%)/ZnO is 3 times that of pure Ag₃PO₄. The optimal percentage of Ag₃PO₄ in the composite is 3.0 wt.%. The stability of the prepared photocatalysts was also investigated. The possible mechanisms of photocatalytic degradation of RhB over Ag₃PO₄/ZnO were discussed in detail. This work will play a significant role in guiding the treatment of dye pollutants.

Section snippets

Materials

Silver nitrate (AgNO₃, purity ≥99.8%), disodium hydrogen phosphate (Na₂HPO₄, purity >99.0%) and zinc oxide (ZnO, purity ≥99.8%) with crystallite size of about 50 nm used in the experiments were supplied by Sinopharm Chemical Reagent Co. Ltd., China. Rhodamine B (RhB), Degussa P25, ethyl alcohol and other chemicals used in the experiments were of analytically pure grade. They were purchased from Shanghai and other China Chemical Reagent Ltd. and used without further purification. Deionized water

XRD analysis

In order to determine the crystal phase composition and the crystallite size of the photocatalyst, a powder XRD study was carried out. Fig. 1 shows the XRD patterns of different the Ag₃PO₄/ZnO composites with different Ag₃PO₄ amount. For comparison, the XRD patterns for the pure Ag₃PO₄ and ZnO are also given. The patterns showed that ZnO substrate was hexagonal wurtzite crystal and Ag₃PO₄ was of body-centered cubic structure type. As shown in Fig. 1, the diffraction peaks of ZnO are clearly

Conclusions

We successfully prepared the efficient visible-light-sensitized Ag₃PO₄/ZnO composites with different Ag₃PO₄ contents by the facile ball milling method. The absorption intensity in the visible light region of the Ag₃PO₄/ZnO increases compared with pure ZnO. Remarkably, the application in photocatlytic degradation of RhB demonstrated that the visible-light photocatalytic activity of the composites was much higher than that of pure Ag₃PO₄ and ZnO. The rate constant of RhB degradation over Ag₃PO₄

Acknowledgements

This work was supported by the National Basic Research Program of China (No. 2011CB302004), the Natural Science Foundation of China (Nos. 20973071, 51172086, 51272081 and 21103060) and the Foundation for Young Talents in College of Anhui Province, China (No. 2011SQRL072ZD).

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Ag3PO4/ZnO: An efficient visible-light-sensitized composite with its application in photocatalytic degradation of Rhodamine B

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Materials

XRD analysis

Conclusions

Acknowledgements

Appl. Catal. B: Environ.

Mater. Chem. Phys.

Mater. Res. Bull.

Mater. Lett.

Chin. J. Catal.

Int. J. Hydrogen Energy

J. Catal.

J. Hazard. Mater.

Mater. Res. Bull.

Dyes Pigments

J. Hazard. Mater.

Catal. Commun.

Chem. Eng. J.

Appl. Catal. B: Environ.

J. Phys. Chem. C

Nano Lett.

Chem. Rev.

Inorg. Chem.

J. Phys. Chem. C

Langmuir

Energy Environ. Sci.

Ag₃PO₄/ZnO: An efficient visible-light-sensitized composite with its application in photocatalytic degradation of Rhodamine B