Short CommunicationHighly efficient and stable Ag/Ag3PO4 plasmonic photocatalyst in visible light
Graphical abstract
Highlights
► Ag/Ag3PO4 photocatalyst was synthesized by light-induced formation method. ► Content of Ag0 reaches to 5.25% (mol %) after 2 hour irradiation. ► MB decomposition rate over Ag/Ag3PO4 dozens of times quicker than over N-TiO2. ► It maintains a high level activity after recycling five times.
Introduction
Environmental pollution is affecting human survival and development. Photocatalytic technology is efficient, stable, and environmentally friendly, much to be accomplished in the field of environmental pollution control [1], [2]. TiO2 shows relatively high reactivity and is stable to photo or chemical corrosion [3], [4]. However, TiO2 has a large band gap (3.18 eV for anatase and 3.02 eV for rutile) which cannot make use of visible light effectively. Moreover, the low separation rate of the photoexcited electron–hole in TiO2 leads to its limited quantum efficiency. Therefore, the discovery of a new active and efficient visible-light-driven photocatalyst attracts much attention [5], [6].
More recently, Ye et al. [7] reported the new use of Ag3PO4 semiconductor in photocatalytic applications, where it exhibits extremely high photooxidative capabilities for the O2 evolution from water and the decomposition of organic dyes under visible-light irradiation. More specifically, this novel photocatalyst can achieve a quantum efficiency of up to 90% at wavelengths greater than 420 nm, which is significantly higher than the previous reported values. Zhu et al. investigated the origin of photocatalytic activation of Ag3PO4 using first-principle density functional theory incorporating the LDA + U formalism. They found that Ag3PO4 had a large dispersion of conduction band and the inductive effect of PO43 −, which helped the separation of electron–hole pairs. It was demonstrated theoretically that the Ag vacancies with a high concentration in Ag3PO4 have a significant effect on the separation of electron–hole pairs and optical absorbance in the visible-light region [8]. But Ag3PO4 is photosensitive. Silver orthophosphate grain absorbs a photon to generate an electron and a hole, and then the electron combines with an interstitial silver ion to give a silver atom. Therefore, it is a highly crucial task to improve the photocatalytic stability of Ag3PO4 while maintaining its high photocatalytic activity.
Since the conception of plasmonic photocatalyst was proposed [9], [10], the localized surface plasmon resonance (LSPR) effect of noble-metal nanoparticles on photocatalyst has been the focus of intense study [11]. For instance, ZnO photocatalyst decorated with Ag nanoparticles [12], [13] and cosputtered M:TiO2 (M = Au, Ag, Cu) nanocomposite system [14], [15] have been synthesized for efficient degradation of dyes under visible illumination. Recently, Ag/silver halide structure has been developed as a visible-light photocatalyst to enhance the photocatalytic activity of semiconductor, such as Ag/AgCl [10], [16], [17], Ag/AgBr [18], [19], Ag/AgI [20]. These catalysts displayed high photocatalytic activity and good stability under visible light due to the LSPR of silver nanoparticles produced on the surface of silver halide.
In this paper, we prepare Ag/Ag3PO4 plasmonic photocatalyst with the purpose of improving the stability of Ag3PO4. Since the LSPR effect of silver nanoparticles and a large negative charge of PO43 − ions, the reducibility of Ag+ ions in the Ag3PO4 lattice decreased significantly when the surface of Ag3PO4 is covered by Ag0 nanoparticles. Thus, the highly efficient and stable photocatalyst is obtained. Additionally, the mechanism of photocatalytic degradation MB of the Ag/Ag3PO4 plasmonic photocatalyst was also proposed.
Section snippets
Sample preparation
All reagents were analytical grade and obtained from Shanghai Reagents Company (Shanghai, China). Ag3PO4 powder samples were prepared by the simple ion-exchange method: 3.581 g of Na2HPO4 ·12H2O and 1.6987 g of AgNO3 were thoroughly mixed in an agate mortar and ground until the initially white color changed to yellow. The mixture was then washed with distilled water to dissolve any unreacted raw material. Last, the vivid yellow powders obtained were dried at 70 °C for 8 h in the dark.
Ag/Ag3PO4
Characterization of photocatalyst
Ag3PO4 is a body-centred cubic structure type with a lattice parameter of 6.004 Å [22]. The structure consists of isolated, regular PO4 tetrahedra (P–O distance of ~ 1.539 Å) forming a body-centred cubic lattice. Six Ag+ ions are distributed among twelve sites of two-fold symmetry. XRD patterns of our sample (Fig. 1) prepared by the simple ion-exchange method confirmed this crystal structure. All patterns matched very well with the JPCDS (74-1876) standard data of Ag3PO4. As for Ag/Ag3PO4 samples
Conclusions
In summary, the highly efficient Ag/Ag3PO4 photocatalyst was prepared by a simple ion-exchange method followed by light-induced reduction route. The content of Ag0 on Ag3PO4 surface increased with irradiation time. It reached to 5.25% (mol %) after 2 hour irradiation and then remained stable. The rate of MB dye decomposition over AAP-2 is far more quickly than that over N-TiO2. The photocatalyst still maintained high level activity after recycling five times. High efficiency and stability of
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