AgCl/Ag/g-C3N4 Hybrid Composites: Preparation, Visible Light-Driven Photocatalytic Activity and Mechanism
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The ternary plasmonic AgCl/Ag/g-C3N4 photocatalysts were successfully fabricated by a modified deposition–precipitation method, through which Ag/AgCl nanoparticles (5–15 nm in size) were evenly dispersed on the surface of g-C3N4. The AgCl/Ag/g-C3N4 composites exhibited higher photocatalytic activity than Ag/AgCl and g-C3N4. The enhanced photocatalytic performance could be attributed to an efficient separation of electron–hole pairs through a Z-scheme mechanism, in which Ag nanoparticles acted as charge separation centers.
KeywordsAgCl/Ag/g-C3N4 Hybrid Photocatalytic activity
As a new metal-free semiconductor, polymeric graphitic carbon nitride (g-C3N4) has been developed to cope with environmental pollutants due to its outstanding mechanical, optical, electronic, and catalytic properties as well as its high thermal and chemical stability [1, 2, 3, 4]. However, its practical application is quite limited owing to its appreciable drawbacks, including low specific surface area, high photogenerated electron–hole recombination rate, and the limited range of visible light photo-responses [5, 6]. To tackle these issues, many methods have been proposed, such as doping extraneous elements [7, 8, 9, 10], designing porous structures [11, 12, 13, 14, 15], depositing noble metals [16, 17, 18, 19], and coupling with other semiconductors [20, 21, 22, 23]. Although some progresses have been achieved, the light harvesting ability and quantum efficiency of these modified g-C3N4 systems are still poor.
Noble metal nanoparticles have attracted considerable attention due to their application as active components for the preparation of various efficient visible light photocatalysts. Logar et al. reported Ag/TiO2 plasmonic photocatalyst that showed high efficiency for degradation of methyl orange (MO) . Parida et al. developed Au/g-C3N4 plasmonic photocatalyst with enhanced photocatalytic activity under irradiation of visible light . Besides, Ag/AgCl [25, 26, 27], Ag/AgBr , and Ag/AgI [29, 30] have also been used as co-catalysts to enhance the photocatalytic activity of semiconductors under visible light irradiation. It is believed that noble metal nanoparticles can act as active sites and play vital roles in effective visible light absorption and subsequent photocatalytic reactions. The possible reason is that noble metal nanoparticles can strongly absorb visible light because of their localized surface plasmon resonance (LSPR), which can be tuned by varying their size, shape, and surroundings .
Recently, two types of Ag/AgCl/g-C3N4 composites were fabricated by Yao et al.  and Zhang et al. . The as-prepared products showed efficient photocatalytic degradation activity. However, their precipitation methods not only lack precise control over the morphology and size of final products, but also induce reunion reaction of AgCl .
In this work, AgCl/Ag/g-C3N4 composites were fabricated by a modified deposition–precipitation method to overcome the above shortcomings. The photocatalytic activities of the AgCl/Ag/g-C3N4 composites were evaluated by the photocatalytic degradation of Rh B and MO aqueous solution under irradiation of visible light. A plasmonic Z-scheme photocatalytic mechanism was then proposed to explain the enhancement of the photocatalytic activity of the AgCl/Ag/g-C3N4 photocatalysts.
All reagents supplied by Sinopharm Chemical Reagent co., Ltd. are of analytical grade and used as received without further purification.
2.1 Preparation of Photocatalysts
2.1.1 Preparation of g-C3N4 Powders
The metal-free g-C3N4 powders were fabricated by heating melamine in a muffle furnace. Typically, 5 g of melamine was placed in a semi-closed alumina crucible with a cover. The crucible was heated to 550 °C at a heating rate of 10 °C min−1 and held for 4 h. After the reaction, the alumina crucible was cooled to room temperature. The products were collected and ground into powders.
2.1.2 Preparation of AgCl/Ag/g-C3N4 Hybrid Composites
In a typical preparation process, 0.4 g of g-C3N4 powders and 0.64 g of hexadecyl trimethyl ammonium chloride (CTAC) were added into 200 mL of deionized water, and the suspension was stirred for 30 min and sonicated for 30 min. Then, 4.4 mL of 0.1 M AgNO3 was quickly added to the above mixture. During this process, the excessive surfactant CTAC not only adsorbed onto the surface of g-C3N4 to limit the number of nucleation sites for AgCl to grow, resulting in homogenously dispersed AgCl, but also induced Cl− to precipitate Ag+ in the suspension. The resulting suspension was stirred for 1 h and then placed under irradiation of 300 W Xe lamp for 30 min. The suspension was filtered, washed using deionized water, and dried at 80 °C for 8 h. And then, the gray powder was calcined at 300 °C for 3 h. Different molar ratios of AgCl/Ag/g-C3N4 photocatalysts (3, 5, 10, 15, 20, and 40 at%) were fabricated with the similar procedure.
2.2 Characterization of Photocatalysts
X-ray diffraction (XRD) data were collected on a D-MAX 2500/PC diffractometer (Japan). The surface morphologies of the as-prepared samples were characterized with field emission scanning electron microscopy (FESEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEOL JEM-2100F). Photoluminescence (PL) spectra were measured at room temperature on F-4600 fluorescence spectrometer (Hitachi, Japan) with an excitation wavelength of 365 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB 250 X-ray photoelectron spectrometer. The Fourier transform infrared spectra (FTIR) of the samples were recorded using IRAffinity-1 spectrometer (Shimadzu, Japan). Ultraviolet visible (UV–Vis) diffuse reflectance spectra (DRS) of the samples were obtained on an UV–vis spectrophotometer (Shimadzu UV-2600, Japan) in the range of 200 to 800 nm and BaSO4 as a standard reference. The Brunauer–Emmett–Teller (BET) surface area (SBET) of the samples were measured by nitrogen adsorption–desorption isotherm measurements on a micromeritics ASAP2020 system. The electron spin resonance (ESR) signals of ·OH and ·O2 − radicals spin-trapped with the spin-trap reagent DMPO (5, 5-dimethyl-1-pyrroline-N-oxide) in water and methanol were examined on an ESR spectrometer (ER200-SCR, Germany), respectively.
2.3 Adsorption Experiment
Adsorption experiments were carried out in the dark. In a typical adsorption procedure, 100 mg of 5 at% AgCl/Ag/g-C3N4 powers mixed with 100 mL 10 mg L−1 Rh B aqueous solution in a glass conical beaker was shaken at ambient temperature. At given time intervals, about 3 mL solution suspension was sampled and immediately centrifuged. The concentration of Rh B solution was analyzed with a UV–Vis spectrophotometer at the maximal absorption wavelength of Rh B, whose characteristic absorption peak was chosen to be 554 nm.
2.4 Test of Photocatalytic Activity
The photocatalytic activities of samples were evaluated by the degradation of Rh B and MO under 300 W Xe lamp with a 420 nm cutoff filter. In brief, 100 mg of photocatalyst was dispersed in 100 mL of a 10 mg L−1 aqueous solution of Rh B or MO in a reactor with a double layer cooled by running water to keep the temperature unchanged. Prior to irradiation, the suspensions were magnetically stirred in the dark for 1 h to ensure the establishment of an adsorption/desorption equilibrium between the photocatalyst and dye molecules. Then, the suspension was illuminated by the Xe lamp combined with magnetic stirring. At given time intervals, about 3 mL solution suspension was sampled and centrifuged. The concentrations of Rh B and MO were measured by UV–Vis spectrophotometer. Additionally, the recycle experiments were performed for five consecutive cycles to test the durability. After each cycle, the catalyst was centrifuged and washed thoroughly with distilled water several times to remove residual dye impurities and then dried at 80 °C for the next test.
3 Results and Discussion
3.1 Characterization of Photocatalysts
3.2 Adsorption Kinetics
3.3 Photocatalytic Activities of the AgCl/Ag/g-C3N4 Hybrid Composites
3.4 Photocatalytic Mechanism
The g-C3N4 with a band gap of 2.7 eV is a novel metal-free visible light photocatalyst . Under visible light irradiation, the g-C3N4 absorbs visible light photons to produce photogenerated electrons and holes. The photogenerated electrons react with O2 that existed in the photodegradation system, reducing it to superoxide radical anion ·O2 −. The dye molecules are degradated by photogenerated holes and ·O2 −. Ag/AgCl has been demonstrated to be an efficient visible light photocatalyst . As AgCl cannot absorb visible light due to its wide band gap of 3.25 eV, the visible light absorption in Ag/AgCl is attributed to the plasmonic absorption of Ag nanoparticles which can absorb visible light and convert the plasmonic energy into LSPR oscillation. Then, the plasmon-induced electrons from the photoexcited Ag nanoparticles transfer to the CB of AgCl and the electrons on the surface of AgCl are trapped by the adsorbed O2 to form ·O2 − active species, and the plasmon-induced holes stay on the surface of Ag nanoparticles and oxidize the dye molecules [41, 42].
The hybrid AgCl/Ag/g-C3N4 photocatalysts were successfully fabricated by a modified deposition–precipitation method, which was effective for the control of photocatalyst morphology and size. The Ag/AgCl particles with the size of approximately 5–15 nm were evenly dispersed on the surface of g-C3N4. The AgCl/Ag/g-C3N4 composites exhibited the higher photocatalytic performance than Ag/AgCl and g-C3N4 over the degradation of Rh B or MO dyes, which was attributed to Ag nanoparticles act as the charge separation center to form the visible light-driven AgCl/Ag/g-C3N4 Z-scheme system. This study provides new insight into the design of highly efficient and stable g-C3N4-based plasmonic Z-Scheme photocatalysts and facilitates their practical application.
The authors are grateful to the financial aid from the National Natural Science Foundation of China (NSFC No. 51472133).
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