Enhanced photoactivity on Ag/Ag3PO4 composites by plasmonic effect
Graphical abstract
Highlights
► Ag/Ag3PO4 composites with different molar ratios have been prepared. ► The photoactivity is strongly dependent on the wavelength of the incident light. ► The photoactivity is dependent on the molar ratio of Ag/Ag3PO4 as well. ► Simulation results suggest a good agreement with our experimental observations.
Introduction
Heterogeneous photocatalysis technology has become a promising approach to solve energy and environmental problems since 1972 [1]. The concept of “low-carbon & green life” as well as the pioneer achievements by scholars such as Asahi et al. [2], Zou et al. [3] are inspiring and great enthusiasm to develop highly efficient photocatalysts to improve the quantum efficiency under visible light (380–780 nm), which represents around 42 % of the total light spectrum of the solar radiations [4], [5].
In early 2010, silver orthophosphate (Ag3PO4) was reported as an active semiconductor, which showed an extremely high photocatalytic performance with quantum efficiency over 90% at wavelengths longer than 420 nm [6]. Soon after, synthesis of single-crystalline Ag3PO4 sub-microcrystals with selective facets [7], colloidal Ag3PO4 nanocrystals [8], Ag3PO4 sub-microcrystals [9], [10], Ag3PO4 nanowires [11], and AgX/Ag3PO4 (X = Cl, Br, I) [12] composites were reported. It has been widely documented and experimentally confirmed that the Ag3PO4 could be partially reduced to metallic Ag under visible light illumination, which undesirably leads to the catalyst leaching. However, a recent work by Huang et al. demonstrated that a combination of metallic silver with a series of silver salts based photocatalysts can improve their photocatalytic activity to different extents, depending on the types of negatively charged ions (e.g., Cl−, Br−, I−, , , and ) in the corresponding semiconductor crystals [13]. The photocatalytic activity enhancement was attributed to localized surface plasmon resonance (LSPR) induced by metallic silver nanoparticles deposited on the surfaces of the silver salt photocatalysts.
Generally, surface plasmon resonance (SPR) for planar surfaces or localized surface plasmon resonance (LSPR) for nanoscaled metal particles can be induced by the excitation of surface plasmons (also known as surface plasmon polaritons, SPP) by using proper wavelength photons. The unique properties of surface plasmon have found a number of promising applications, such as heterogeneous photocatalysis [14], [15], sensors [16], and photovoltaic cell [17], [18].
Although the plasmonic effect of metallic nanoparticles on photocatalysis has been reported recently using wide spectrum of visible light for various photocatalysts, the report on monochromatic responses of these materials in terms of photocatalytic enhancement is very rare. As it is known, the extent of SPR and LSPR is highly dependent on the wavelengths of incident photons, so investigation into monochromatic response of these materials deserves much more attentions than it currently receives.
In this investigation, a series of Ag/Ag3PO4 composites were prepared by a facile photochemical method, and their monochromatic response (i.e., 350, 420, 450, 500 nm) was investigated by photodegradation of a model compound (i.e., Rhodamine B (Rh. B)). Rh. B photodegradation in water was found to be strongly dependent on the wavelength of incident light and the amount of surface silver nanoparticles on Ag3PO4, which is substantiated well by physics simulation results.
Section snippets
Preparation of Ag/Ag3PO4 composites
All the chemicals used in this study were of analytical grade and used as received without further purification. Absolute ethanol (C2H5OH), silver nitrate (AgNO3), disodium hydrogen phosphate (Na2HPO4), and Rhodamine B (Rh. B) were purchased from Sigma-Aldrich. Ultrapure Milli-Q water obtained by Millipore was used for the preparation of solutions.
Semiconductor compound of Ag3PO4 was prepared as per procedure described in the literature [6] by using AgNO3 and Na2HPO4 as the starting materials.
Results and discussion
Fig. 1a depicts XRD patterns of the Ag/Ag3PO4 composites synthesized under different irradiation time. Clearly, after certain time duration of visible light irradiation, the characteristic XRD patterns of the Ag3PO4 remained more or less unaffected. However, some new peaks, such as at 38.1°, 44.2°, 64.5°, and 77.4°, emerged, which can be ascribed to the silver crystal indexes of (1 1 1), (2 0 0), (2 2 0) and (3 1 1), respectively. This confirms the formation of Ag on the surfaces of Ag3PO4. Fig. 1b
Conclusions
In summary, this paper reports the photodegradation enhancement on Ag/Ag3PO4 composites by plasmonic effect. It was found that both factors such as combined metallic silver amount and the wavelength of incident photons could dramatically influence the photoactivity improvement. The highest photoactivity was obtained under an optimum coverage amount of silver nanoparticles (2.7%) and incident lights having a wavelength of 350 nm. Simulation results suggest that SPR have a good spectral agreement
Acknowledgments
The support by King Fahd University of Petroleum and Minerals under approved Laser Research Group projects # Rg1011-1 and Rg1011-2 is gratefully acknowledged. This study is partially supported by National Natural Science Foundation of China (No. 41103076), Special Foundation of President of the Chinese Academy of Science (No. 312B11YBLWYZJ2011001) and Youth Innovation Promotion Association, CAS.
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