AgBr/Ag3PO4 photocatalyst was synthesized using a facile coprecipitation method. The photocatalyst was characterized by X-ray powder diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface areas, and photoluminescence (PL) technique. The activity of the photocatalyst was evaluated by the degradation of methyl orange (MO) and rhodamine B (RhB). The results showed that the prepared AgBr/Ag3PO4 exhibited excellent performance and much higher photocatalytic activity than the single one under visible-light irradiation. The optimum mole ratio of Br/P in AgBr/Ag3PO4 samples is 0.3. The prepared AgBr/Ag3PO4 photocatalyst was transformed to Ag/AgBr/Ag3PO4 system with excellent property and good stability in the photocatalytic process. The possible mechanisms of the enhanced photocatalytic activity for the AgBr/Ag3PO4 were also discussed in detail.
Photocatalysis is a promising technology for the treatment of contaminants, especially for the removal of organic pollutants with solar energy [
Silver halides (AgX, X = Cl, Br, I) are photosensitive materials extensively used as source materials in photographic films. When absorbing a photon, silver halide particle may generate an electron and a hole, so AgX can be used as a potential photocatalyst. But the photoinduced electrons will combine with interstitial Ag+ ions to form a cluster of Ag0 atoms within the silver halide particle, which results in the instability of AgX under light irradiation [
Recently, Ag3PO4 has attracted considerable attention as a new visible-light photocatalyst, and it is a pale yellow semiconductor with a bandgap of 2.45 eV [
In the study, a novel AgBr/Ag3PO4 was constructed and synthesized by a facile coprecipitation method. Methyl orange (MO) and rhodamine B (RhB) were used as model pollutants to evaluate the photocatalytic activity of the AgBr/Ag3PO4 composites under visible-light irradiation (
All reagents are of analytical purity and were used without further purification. Silver nitrate (AgNO3), potassium bromide (KBr), disodium hydrogen phosphate (Na2HPO4), methyl orange (MO), rhodamine B (RhB), absolute ethanol, terephthalic acid (TA), benzoquinone (BQ), isopropanol (IPA), potassium iodide (KI), and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used throughout this study.
AgBr/Ag3PO4 was prepared by the coprecipitation method in a dark room to facilitate the experimental manipulation and prevent the decomposition of AgBr. In a typical procedure, 1.42 g of Na2HPO4 dispersed in 500 mL of deionized water was placed in a 1000 mL pyrex glass beaker. Then, 0.119 g of KBr in 100 mL of deionized water was added to the above suspension and stirred magnetically for 2 h. Subsequently, 5.3 g of AgNO3 in 100 mL of deionized water was quickly added to the mixture. The resulted suspension was vigorously stirred for 12 h. The product was filtered and washed with absolute ethanol and deionized water for several times and dried at 60°C for 24 h. Finally, the obtained 0.10-AgBr/Ag3PO4 with theoretical Br/P molar ratio of 0.10 : 1 was collected. Varying the amount of AgBr, different AgBr/Ag3PO4 photocatalysts were prepared, respectively, and defined as BP-0.1, BP-0.3, BP-0.5, and BP-0.7. Pure Ag3PO4 and AgBr samples were prepared using the same method but with only one kind of anion in the solution (
X-ray diffraction (XRD) measurements were carried out at room temperature using a BRUKER D8 ADVANCE X-ray powder diffractometer with Cu K
The photocatalytic degradations of MO and RhB were adopted to evaluate the photocatalytic activity of the samples in a photoreaction apparatus [
In order to determine the crystal phase composition and the crystallite size of the photocatalyst, XRD study of the samples was carried out. Figure
XRD patterns of (a) Ag3PO4, (b) BP-0.1, (c) BP0.3, (d) BP-0.5, (e) BP-0.7, and (f) AgBr.
XRD patterns of (a) fresh BP0.1, (b) used one time BP0.1 and (c) used five times BP-0.1.
To further confirm the existence of Ag in the used AgBr/Ag3PO4 sample, the used BP-0.1 sample was examined by XPS. The results are shown in Figure
XPS survey spectrum (a) and Ag 3d XPS spectrum (b) of the used AgBr/Ag3PO4.
SEM was used to investigate the morphology of the photocatalysts. Figure
SEM images of (a) Ag3PO4 and (b) AgBr/Ag3PO4 (BP-0.3).
UV-Vis diffuse reflectance spectroscopy was carried out to investigate the optical properties of the samples. Figure
(a) DRS of AgBr, Ag3PO4, and BP-0.3 samples. (b) DRS of the fresh BP-0.3, the used one time BP-0.3, and the used five times BP-0.3.
From Figure
In this equation,
The band edge positions of CB and VB of semiconductor can be determined with a simple approach. The valance band edge (
The photocatalytic activities of as-prepared samples were evaluated by the degradation of MO and RhB under visible-light irradiation. The blank test shows that photoinduced self-sensitized photodegradation has little influence on the results of the experiment. At the same time, the dark absorption test in the absence of irradiation but with the catalysts shows that no significant change in the substrate concentration is found.
Figure
(a) The degradation process of MO with different photocatalysts. (b) The degradation process of RhB with different photocatalysts.
Figures
(a) The first-order kinetics of MO degradation with different photocatalysts. (b) The first-order kinetics of RhB degradation with different photocatalysts.
The rate constants of MO and RhB degradation with different samples: Ag3PO4 (a), BP-0.1 (b), BP-0.3 (c), BP-0.5 (d) and BP-0.7 (e).
The BET surface areas of Ag3PO4, BP-0.1, BP-0.3, BP-0.5, and BP-0.7 are 2.3, 2.6, 3.1, 3.7, and 3.7 m2/g, respectively. It is clear that the BET surface areas of the samples are increased gradually with the increase in the amount of AgBr. It is obvious that the increased photocatalytic activity is not in obvious correspondence with the BET surface area of the samples. Therefore, the enhanced photocatalytic activity of the samples can only be ascribed to the presence of AgBr.
The catalyst’s lifetime is an important parameter of the photocatalytic reaction process, so it is essential to evaluate the stability of the catalyst for practical application. As shown in Figure
(a) Cyclic experiments of the BP-0.3 photocatalyst for MO degradation. (b) Cyclic experiments of the BP-0.3 photocatalyst for RhB degradation.
The high photocatalytic activity and good stability are closely related to the efficient separation of photoexcited electron-hole pairs derived from the matching band potentials between AgBr and Ag3PO4, as well as the surface plasmon resonance of Ag nanoparticles formed on the surface of the photocatalyst during the photocatalytic reaction process. The presence of metal Ag can restrain the further decomposition of AgBr under visible-light irradiation conditions [
The photocatalytic mechanism was investigated for the excellent photocatalytic property of the prepared AgBr/Ag3PO4. It is generally accepted that the dyes and organic pollutants can be photodegraded via photocatalytic oxidation process. A large number of main reactive species including
The effects of a series of scavengers on the degradation efficiency of MO and RhB (the dosage of scavengers = 0.1 mmol/L, illumination times are 50 min and 10 min for MO and RhB, resp.).
To further research whether
The PL spectra of AgBr/Ag3PO4 in TA solution under visible-light irradiation.
Based on bandgap structure of the prepared AgBr/Ag3PO4 and the effects of scavengers, a possible mechanism of AgBr/Ag3PO4 photocatalyst for degradation dyes was proposed. The AgBr/Ag3PO4 photocatalyst was transformed into a plasmonic Z-scheme mechanism of Ag3PO4/Ag/AgBr system during the photocatalytic oxidation process. In other words, the AgBr/Ag3PO4 photocatalyst was firstly transformed into the Ag3PO4/Ag/AgBr photocatalyst under visible-light irradiation. And then, AgBr, Ag, and Ag3PO4 can be simultaneously excited and produce photogenerated electrons and holes. The plasmon-induced electrons of Ag nanoparticles are injected into the CB of AgBr, while the holes remain on the Ag nanoparticles. As for Ag3PO4, the photogenerated electrons move to the Ag nanoparticles to recombine with the plasmon-induced holes produced by plasmonic absorption of Ag nanoparticles, while the holes in the VB of Ag3PO4 may oxidize MO and RhB directly. Besides, it was reported that electrons in the CB of AgBr could probably be excited up to a higher potential edge (−0.39 eV) under visible-light illumination with energy less than 2.95 eV [
Schematic diagram of photoexcited electron-hole separation process.
AgBr/Ag3PO4 photocatalyst was synthesized using a facile coprecipitation method. The prepared AgBr/Ag3PO4 exhibited excellent performance for the degradation of MO and RhB and displayed a much higher photocatalytic activity than the single one under visible-light irradiation. The optimum mole ratio of Br/P in AgBr/Ag3PO4 samples is 0.3. The AgBr/Ag3PO4 photocatalyst was transformed to Ag/AgBr/Ag3PO4 photocatalyst quickly, and the formed Ag/AgBr/Ag3PO4 photocatalyst remained with high photocatalytic property and good stability in the photocatalytic process. The reason is attributed to the efficient separation of photoexcited electron-hole pairs of the photocatalyst. The degradation of MO and RhB for the AgBr/Ag3PO4 photocatalyst is mainly via
This study was supported by the Natural Science Foundation of China (NSFC, Grant nos. 20973071, 51172086, 21103060, and 51272081).