Heterogeneous photocatalysis is a promising approach to development of technology for environmental remediation, solar energy conversion, and hydrogen production. The pioneering work of Fujishima et al. [1] spurred interest in the development of visible light driven photocatalysis for application in environmental technology. Subsequent achievements [2, 3, 4, 5] have contributed to a drive towards the development of efficient photocatalysts with high quantum efficiency in the visible portion of the solar spectrum (380-780 nm) [6].
Silver orthophosphate (Ag3PO4) is an active semiconductor that can take part in photooxidation processes. Ag3PO4 also has a quantum efficiency of 90% at wavelengths longer than 420 nm [7] and shows high photocatalytic activity owing to its absorption in the visible portion of the solar spectrum and high charge carrier mobility. This is because of the delocalized charge distribution of the conduction-band minimum, which results in a small electron effective mass, which is beneficial for the surface carrier mobility [8].
Graphitic carbon nitride (g-C3N4) is based on the stacked two-dimensional structure analogous of graphite with N replacing non-adjacent carbon atoms. This material has drawn attention for its potential in water splitting application as a metal-free photocatalyst operating under visible light irradiation, as reported by Wang et al. [9]. However, the potential of this material is limited by inherent constraints such as inefficient use of the visible portion of the solar spectrum and a high electron/hole recombination rate. The photocatalytic efficiency of g-C3N4 needs further enhancement prior to any practical application. Various g-C3N4 based hybrid photocatalysts including g-C3N4/BiPO4 [10], graphene/g-C3N4 [11], g-C3N4/ Bi2WO6 [12], Fe-g-C3N4-LUS-1 [13], g-C3N4/SiO2-HNb3O8 [14], and g-C3N4/TaON [15] have been developed to further extend the visible light absorption range and the photogenerated carriers separation efficiency.
Described herein is a simple photochemical precipitation based preparation of a series of novel ternary Ag/Ag3PO4/ g-C3N4 hybrid photocatalysts and a detailed investigation of the catalytic activity using rhodamine B (RhB) as a model contaminant. Our photocatalytic experiments indicate that the Ag/Ag3PO4/g-C3N4 hybrid system exhibits superior photocatalytic performance for RhB degradation compared with a binary Ag3PO4/g-C3N4 photocatalyst, or Ag3PO4 and g-C3N4 as individual components. It was found that RhB photodegradation strongly depends on the proportion of silver nanoparticles on the Ag3PO4 surface and the ratio of the components in the Ag3PO4/g-C3N4 hybrid. The enhanced photoactivity can be attributed to the surface plasmon resonance (SPR) originating from silver nanoparticles and the heterojunction-like interface between Ag3PO4 and g-C3N4. This work is a continuation of our commitment to developing environmentally friendly technologies for clean fuel by using the photocatalytic technique [16, 17, 18, 19 20 21].
All the chemicals used in this study were of analytical grade and used as received without further purification. Absolute ethanol (C2H5OH), sodium bismuthate (NaBiO3), disodium hydrogen phosphate (Na2HPO4), anatase-TiO2, and rhodamine B (RhB) were purchased from Sinopharm Chemical Reagent Co. Melamine and silver nitrate (AgNO3) were obtained from Shanghai Lingfeng Chemical Reagent Co. Ltd and Jiangsu Qiangsheng Chemical Co. Ltd, respectively. Distilled water was produced using a Direct-Q Millipore filtration system to a resistivity of 18.2 MΩcm (Millipore Limited, Watford, UK).
g-C3N4 was prepared through a pyrolysis process using melamine as the starting material [22], and then binary Ag3PO4/g-C3N4 (x) (where x denotes the mass fraction of Ag3PO4 in the sample) hybrid photocatalysts were prepared by a chemical deposition-precipitation method. In a typical preparation for the case of Ag3PO4/g-C3N4 (0.8), 2.3 g of g-C3N4 was dispersed into an aqueous solution of AgNO3 (1.26 g/50 mL). Then a solution of Na2HPO4 (0.9 g/50 mL) was added drop-wise over 30 min into the above suspension with stirring until the precipitation was completed. The mixture was then centrifuged, washed with water, and dried overnight at 50 °C. The obtained Ag3PO4/g-C3N4 (0.8) powder (2.0 g) was dispersed in 200 mL of water and irradiated by a 300 W Xenon lamp (Beijing Trusttech Co. Ltd., PLS-SXE-300) equipped with a visible light band pass filter (400-800 nm) for 0.5-3 h. The final product was collected by centrifugation at 4000 r/min, washed with absolute ethanol/water several times, and then dried at 50 °C. The series of Ag/Ag3PO4/g-C3N4 hybrid photocatalysts were denoted Ag/Ag3PO4/g-C3N4 (x; t), where t is the irradiation time (0.5-3 h) for the preparation.
The crystal structure of the ternary Ag/Ag3PO4/g-C3N4 hybrid photocatalysts was analyzed using a wide-angle X-ray diffractometer (Rigaku SmartLab) employing Cu Kα radiation (λ = 0.15418 nm). Scanning electron microscope (SEM) imaging of samples was performed using a FEI F50 SEM. Diffuse reflectance of samples was measured with a JASCO V-670 UV-Vis-NIR spectrophotometer. The photoluminescence (PL) spectra were recorded on a PL spectrofluorometer (Horiba Jobin) at an excitation wavelength of 365 nm.
The degradation of RhB was used as a model reaction to evaluate photodegradation behavior of the ternary Ag/Ag3PO4/ g-C3N4 hybrid photocatalyst. For the photodegradation experiments, a 150 mL solution of RhB (7 mg/L) was prepared, into which 150 mg of photocatalyst was added. The suspension of the RhB solution and photocatalyst was magnetically stirred in the dark for 60 min to ensure adsorption-desorption equilibrium before the irradiation. The suspension was then irradiated with a 300 W Xenon lamp equipped with visible light band pass filter (400-800 nm). Portions of the reaction mixture slurry were taken from the mixture and filtered to remove Ag/Ag3PO4/g-C3N4 hybrid photocatalyst at regular intervals. The concentration of RhB in the aqueous solution was determined using a UV-VIS spectrophotometer (Hitachi U-3900).
Figure 1 depicts XRD patterns of the ternary Ag/Ag3PO4/ g-C3N4 hybrid photocatalyst prepared at different exposure time. The diffraction peaks of both g-C3N4 and silver nanoparticles cannot be observed clearly in XRD patterns because of strong interference with the Ag3PO4 signals. However, a new peak appeared at 38.1° in the Ag3PO4/g-C3N4 sample irradiated for 3 h that could be indexed as the silver (111).
The enlarged XRD patterns (Fig. 2) from 37° to 40° and 26.5° to 29° show the Ag(111) and g-C3N4 (002) peaks, respectively. These results confirmed the formation of silver particles on the surface of the Ag3PO4 particles and the existence of a g-C3N4 phase. The diffraction intensity of the Ag(111) peak increased and saturated with prolonged irradiation time, which may suggest that the silver nanoparticles formed in situ on surface of Ag3PO4 and inhibited further photochemical reduction of the Ag3PO4 to Ag0. The size of the silver nanoparticles from different light exposed samples was calculated and summarized in Table 1 using the Scherrer equation based on full width at half maximum (FWHM) value for Ag(111) peak at 38.1°. By a semi quantitative analysis, 0.6%, 1.0%, 2.1%, and 2.8% metallic silver was estimated in samples irradiated for 0.5, 1, 2, and 3 h, respectively. Figure 3 shows a typical SEM image of Ag/Ag3PO4/g-C3N4 (0.8; 1 h) photocatalyst. The silver nanoparticles can be clearly seen in the SEM images, and the particle size is estimated at around 40-50 nm, which is in agreement with the XRD analysis.
Figure 4 illustrates the optical absorption spectra of g-C3N4, Ag3PO4, Ag3PO4/g-C3N4 (0.8), and Ag/Ag3PO4/g-C3N4 (0.8; 1 h). The peaks at around 393 and 454 nm in the ternary Ag/Ag3PO4/ g-C3N4 hybrid photocatalyst can be attributed to the absorptions of g-C3N4 and Ag3PO4, respectively. Ag/Ag3PO4/g-C3N4 exhibits much higher absorption, which may be attributed to the surface plasmon resonance (SPR) of the silver nanoparticles [23].
PL spectra of g-C3N4 and Ag/Ag3PO4/g-C3N4 (0.8; t) samples are shown in Fig. 5. A strong emission band centered at 459 nm in g-C3N4 can be assigned to the emission of g-C3N4 with an energy corresponding to its band gap. This indicates more efficient radiative recombination of the photogenerated carriers inside the g-C3N4 semiconductor in samples prepared with longer irradiation time. The overall PL emission intensity of the Ag/Ag3PO4/g-C3N4 (0.8; t) hybrid photocatalysts decreased with respect to the overall PL intensity of g-C3N4with increasing irradiation time. This may be attributed to charge transfer between Ag3PO4 and g-C3N4 or trapping of photoexcited electrons by the silver nanoparticles.
The effect of the Ag3PO4/C3N4 mass ratio from Ag3PO4 to g-C3N4 on the photodegradation performance was investigated before studying the enhancement of photodegradation by the silver nanoparticles in the ternary systems. Figure 6 illustrates the photocatalytic activity of a series of Ag3PO4/C3N4 hybrid photocatalysts investigated. The Ag3PO4/g-C3N4 hybrid photocatalysts showed considerably enhanced photocatalytic performance compared with either Ag3PO4 or g-C3N4 alone, suggesting a synergic effect as reported by Zhang et al. [24] and Kumar et al. [25]. The optimum composition ratio was found to be 4:1 for the Ag3PO4/g-C3N4 hybrid, and this photocatalyst completely decomposed the model contaminant over the course of the experiment. Only 63% and 12% degradation of RhB were observed within 30 min using pure Ag3PO4 and g-C3N4, respectively.
The curves in Fig. 7 depict the removal of RhB as a function of irradiation time using the ternary Ag/Ag3PO4/g-C3N4 hybrid photocatalysts. Further enhancement in photodegradation performance was observed from the as-prepared Ag/Ag3PO4/ g-C3N4 samples. Almost 50% of the RhB in the aqueous suspension was decomposed using Ag/Ag3PO4/g-C3N4 (0.8; 1 h) under broadband visible light irradiation for 5 min. This ratio may be considered as the optimal compositional ratio of the ternary photocatalyst. Around 40% degradation was obtained from Ag3PO4/g-C3N4 (0.8) under identical experimental conditions.
The corresponding UV-Vis spectral changes (inset in Fig. 7) show the intensity of the absorption peak at 554 nm decreasing dramatically as the photodegradation reaction progresses. It is notable that no blue-shift of the RhB absorption peak centered at 554 nm was observed, suggesting a decomposition mechanism involving cleavage of the whole conjugated chromophore structure in RhB during the photochemical reaction, rather than an N-deethylation process [26].
Figure 8 shows a comparison of the apparent rate constants (k) of the Ag/Ag3PO4/g-C3N4 (0.8; 1 h) hybrid photocatalysts and Ag3PO4/g-C3N4 calculated by the Langmuir-Hinshelwood model [27]. Using the binary Ag3PO4/g-C3N4 photocatalyst as a baseline, an apparent rate of 0.105 min−1 was obtained for the reaction in the first 5 min. The Ag/Ag3PO4/g-C3N4 hybrid photocatalyst prepared by light irradiation for 1 h showed a 25% enhancement (0.129 min−1). Figure 8 (b) shows the effect of irradiation time during the preparation of Ag/Ag3PO4/g-C3N4 (i.e. which correlates with the loading of silver nanoparticles) on the kinetic rate constant. It is clear that the kinetic rate constant increases initially with the silver nanoparticle loading and subsequently decreases. The highest performance was obtained from Ag/Ag3PO4/g-C3N4 (0.8; 1 h).
Compared with other popular photocatalytic systems, the as-prepared ternary Ag/Ag3PO4/g-C3N4 (0.8; 1 h) hybrid photocatalyst exhibits superior performance. A comparison shown in Fig. 9 suggests that the kinetic rate constant of Ag/Ag3PO4/ g-C3N4 (0.8; 1 h) at 0.129 min−1 within 6 min is higher than the kinetic rate constants observed on anatase TiO2 (0.009 min−1), P25 (0.016 min−1), NaBiO3 (0.054 min−1), Ag3PO4 (0.073 min−1), and Ag3PO4/g-C3N4 (0.8) (0.105 min−1). The low photodegradation rate of RhB on TiO2 (or P25) may be attributed to the wide band gap of the materials, which restricts photosensitization by visible light [28]. Compared with the commercial photocatalyst P25, a 6-fold improvement of photodegradation activity was achieved for the Ag/Ag3PO4/g-C3N4 (0.8; 1 h) hybrid photocatalyst. In addition, as shown in Fig. 9, the high density of the hybrid photocatalyst (ρ = 1.2 g/cm3) indicates that it would be more convenient to separate it from the aqueous phase for regeneration and reuse, compared with P25 (ρ = 0.56 g/cm3).
The proposed ternary Ag/Ag3PO4/g-C3N4 hybrid photocatalyst can be easily recycled using a simple filtration technique and then washed with water to remove adsorbed RhB. As depicted in Fig. 10, after the first three cycles, the kinetic rate constants remained above 0.12 min−1. The kinetic constant rate decreased to 0.04 min−1 after the 6th cycle, likely because of catalyst fouling [29].
The adsorption of RhB molecules to the Ag3PO4 or g-C3N4 photocatalyst surfaces was investigated, revealing that less than 4% of the RhB adsorbed to Ag3PO4 or g-C3N4. The weak interaction of the RhB with this material may be considered one of the reasons for the enhanced photoactivity following introduction of silver nanoparticles. It is also notable that the enhancement in activity arises from the bifunctional heterojunction structure at the interface of Ag3PO4/g-C3N4,as indicated by PL data and the SPR from the silver nanoparticles of the ternary Ag/Ag3PO4/g-C3N4 hybrid photocatalyst. The proposed mechanism of charge carrier separation in system is illustrated in Fig. 11.
It is reported [24, 25] that the conduction band edges of g-C3N4 and Ag3PO4 are at −1.12 eV and 0.45 eV (vs NHE), respectively. Thus photoexcited electrons from the conduction band (CB) of g-C3N4 are injected into the CB of Ag3PO4 because of favorable offset between the CB edges of g-C3N4 and Ag3PO4 as illustrated in Fig. 11. The photogenerated holes move from the valance band (VB) of the Ag3PO4 towards the VB of g-C3N4 because of the more positive VB edge of Ag3PO4 (2.9 eV, vs NHE) than that of g-C3N4 (1.57 eV, vs NHE).
Harnessing the plasmonic effects of silver nanoparticles, to generate highly-active composite photocatalysts through combining the nanoparticles with semiconducting materials has been reported including Ag-TiO2 nanocomposite hollow spheres [30] and Ag/AgCl/TiO2 nanotube arrays [31], etc. Our previous simulation results [32] have clearly shown that the highest surface plasmon polariton momentum of silver nanoparticles is achieved by irradiation of photons with a wavelength of 359 nm in RhB aqueous solution, by considering the Brendel-Bormann model. The surface plasmon polariton momentum is decreased by 25% when changing the wavelength of the excitation source from 359 to 400 nm. This may also explain why the apparent photocatalytic enhancement of the Ag/Ag3PO4/g-C3N4 hybrid photocatalyst can be observed under visible light irradiation between 400 and 800 nm, even though the most efficient excitation wavelength for the SPR effect is λ = 359 nm, which has a naturally low intensity in the solar spectrum. Decreased catalytic activity was observed for samples that were highly decorated with silver nanoparticles. Further reduction of silver on the surface of Ag3PO4 not only destroyed the SPR because of agglomeration of silver nanoparticles but also reduced the surface area available for the reaction on Ag3PO4.
The preparation of a ternary Ag/Ag3PO4/g-C3N4 hybrid photocatalyst and its improved photodegradation performance are reported. The highest photoactivity was achieved by optimization of the silver nanoparticle (w = 1.0%) surface coverage on the Ag3PO4 and the composition fraction (x = 0.8) of Ag3PO4 and g-C3N4. The as prepared hybrid photocatalyst can be regenerated and separated from the aqueous phase easily. In addition, the catalyst also exhibits high photostability under visible light exposure and shows enhanced photodegradation of RhB dye in solution compared with other typical photocatalysts.
2014, Vol. 35 Issue (1): 78-84
PDF (985KB)
2014, Vol. 35 Issue (1): 78-84
PDF (985KB)