Abstract

1 Introduction

As a result of an imminent energy crisis and growing pollution issues, many researchers are aiming at the utilization of renewable energy sources such as wind or solar light¹1 Sang Y, Liu H, Umar A. Photocatalysis from UV/Vis to near-infrared light: towards full solar-light spectrum activity. ChemCatChem. 2015;7:559-573. http://dx.doi.org/ 10.1002/cctc.201402812
https://doi.org/10.1002/cctc.201402812... . Among all the approaches for renewable energy utilization, one of the most remarkable one is the use of solar energy for photocatalytic degradation of organic pollutants and for hydrogen generation via water splitting²2 Sang Y, Zhao Z, Zhao M, Hao P, Leng Y, Liu H. From UV to near-infrared, WS2 nanosheet: a novel photocatalyst for full solar light spectrum photodegradation. Advanced Materials. 2015;27:363-369. http://dx.doi.org/10.1002/adma.201403264
https://doi.org/10.1002/adma.201403264... . However, some traditional photocatalysts such as TiO₂, ZnO, NiO can absorb only ultraviolet light due to their wide band gaps. Thus, the development of visible-light-driven (VLD) photocatalysts has attracted increasing attention in order to efficiency utilize solar light in the visible region (700 nm > λ >400 nm). Recently, Ye and co-workers reported that Ag₃PO₄ semiconductor exhibited a high-efficient usage of visible light (up to 90%) for O₂ evolution from water as well as effective photodecomposition of organic compounds³3 Yi Z, Ye J, Kikugawa N, Kako T, Ouyang S, Stuart-Williams H, et al. An orthophosphate semiconductor with photooxidation properties under visible-light irradiation. Nature Materials. 2010;9:559-564. http://dx.doi.org/10.1038/nmat2780
https://doi.org/10.1038/nmat2780...

4 Bi Y, Ouyang S, Umezawa N, Cao J, Ye J. Facet effect of single-crystalline Ag3PO4 sub-microcrystals on photocatalytic properties. Journal of the American Chemical Society. 2011;133:6490-6492. http://dx.doi.org/10.1021/ja2002132
https://doi.org/10.1021/ja2002132... ^-55 Bi Y, Hu H, Ouyang S, Lu G, Cao J, Ye J. Photocatalytic and photoelectric properties of cubic Ag3PO4 sub-microcrystals with sharp corners and edges. Chemical Communication. 2012;48:3748-3750. http://dx.doi.org/10.1039/c2cc30363a
https://doi.org/10.1039/c2cc30363a... . Unfortunately, one major limitation of this novel photocatalyst is unstable upon photo-illumination, and it is easily corroded by the photogenerated electrons($4A{g}_{3}P{O}_4+6H_{2}O+12h^{+}+12e^{-}\to 12Ag+4H_{3}P{O}_4+3O_2$)³3 Yi Z, Ye J, Kikugawa N, Kako T, Ouyang S, Stuart-Williams H, et al. An orthophosphate semiconductor with photooxidation properties under visible-light irradiation. Nature Materials. 2010;9:559-564. http://dx.doi.org/10.1038/nmat2780
https://doi.org/10.1038/nmat2780... . Therefore, a key issue is how to enhance the stability of Ag₃PO₄ photocatalyst by improving the separation efficiency of the photogenerated electrons and holes.

It has been reported that coupling two or more semiconductors with appropriate band positions is an efficient strategy to effectively enhance the photocatalytic activities of the semiconductor photocatalysts, because it can improve the separation efficiency of photogenerated electron-hole pairs and solar light absorption ability⁶6 Wang H, Zhang L, Chen Z, Hu J, Li S, Wang Z, et al. Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances. Chemical Society Reviews. 2014;43:5234-5244. http://dx.doi.org/10.1039/c4cs00126e
https://doi.org/10.1039/c4cs00126e... ^,77 Lv Y, Huang K, Zhang W, Yang B, Chi F, Ran S et al. One step synthesis of Ag/Ag3PO4/BiPO4 double-heterostructured nanocomposites with enhanced visible-light photocatalytic activity and stability. Ceramics International. 2014;40:8087-8092. http://dx.doi.org/10.1016/j.ceramint.2013.12.162
https://doi.org/10.1016/j.ceramint.2013.... . In particular, the fabrication of a p-n junction photocatalyst is believed to be very effective strategy to significantly enhance the photocatalytic activity of photocatalysts because of the existence of an internal electric field⁸8 Wang W, Wang J, Wang Z, Wei X, Liu L, Ren Q, et al. p-n junction CuO/BiVO4 heterogeneous nanostructures: synthesis and highly efficient visible-light photocatalytic performance. Dalton Transactions. 2014;43:6735-6743. http://dx.doi.org/10.1039/c3dt53613k
https://doi.org/10.1039/c3dt53613k... . For instance, it has been reported that BiOI/SnS₂ heterojunction flowerlike structure demonstrate an enhanced visible-light photocatalytic activity due to the formation of the p-n junction between p-type BiOI and n-type SnS₂⁹9 Wang T, Meng H, Yu X, Liu Y, Chen H, Zhu Y, et al. p-BiOI/n-SnS2 heterojunction flowerlike structure with enhanced visible-light photocatalytic performance. RSC Advances. 2015;5:15469-15478. http://dx.doi.org/10.1039/c4ra15770b
https://doi.org/10.1039/c4ra15770b... .

Cerium dioxide (CeO₂) is an n-type semiconductor with a wide energy band gap of 3.2 eV, indicating that CeO₂ can only respond to ultraviolet light. In addition, the photocatalytic properties of CeO₂ are predominantly restricted by the rapid recombination of photo-induced electrons and holes. Although a single CeO₂ material exhibits poor photocatalytic performance owing to its low charge-transfer rate, previous investigations indicated that CeO₂ is an excellent cocatalyst with other semiconductors such as Fe₂O₃, Bi₂O₃, CdS, g-C₃N₄, and so on¹⁰10 Arul NS, Mangalaraj D, Ramachandran R, Grace AN, Han JI. Fabrication of CeO2/Fe2O3 composite nanospindles for enhanced visible light driven photocatalysts and supercapacitor electrodes. Journal of Materials Chemistry A. 2015;3:15248-15258. http://dx.doi.org/10.1039/c5ta02630j
https://doi.org/10.1039/c5ta02630j...

11 Wang Q, Yu S, Tan Z, Zhang R, Li Z, Gao X, et al. Synthesis of monodisperse Bi2O3-modified CeO2 nanospheres with excellent photocatalytic activity under visible light. CrystEngComm. 2015;17:671-677. http://dx.doi.org/10.1039/c4ce02053g
https://doi.org/10.1039/c4ce02053g...

12 Lu X-H, Xie S-L, Zhai T, Zhao Y-F, Zhang P, Zhang Y-L et al. Monodisperse CeO2/CdS heterostructured spheres: one-pot synthesis and enhanced photocatalytic hydrogen activity. RSC Advances. 2011;1:1207-1210. http://dx.doi.org/10.1039/c1ra00252j
https://doi.org/10.1039/c1ra00252j... ^-1313 Huang L, Li Y, Xu H, Xu Y, Xia J, Wang K, et al. Synthesis and characterization of CeO2/g-C3N4 composites with enhanced visible-light photocatatalytic activity. RSC Advances. 2013;3:22269-22279. http://dx.doi.org/10.1039/c3ra42712a
https://doi.org/10.1039/c3ra42712a... , because of the improved charge separation and oxygen reduction at the interfaces between the two coupled catalysts. Ag₃PO₄ is a p-type semiconductor³3 Yi Z, Ye J, Kikugawa N, Kako T, Ouyang S, Stuart-Williams H, et al. An orthophosphate semiconductor with photooxidation properties under visible-light irradiation. Nature Materials. 2010;9:559-564. http://dx.doi.org/10.1038/nmat2780
https://doi.org/10.1038/nmat2780... with indirect and direct band gaps are 2.36 eV and 2.43 eV, respectively. On account of that the combination of CeO₂ and Ag₃PO₄ possess well matched overlapping band structure¹⁴14 Yang ZM, Huang GF, Huang WQ, Wei JM, Yan XG, Liu YY, et al. Novel Ag3PO4/CeO2 composite with high efficiency and stability for photocatalytic applications. Journal of Materials Chemistry A. 2014;2:1750-1756. http://dx.doi.org/10.1039/c3ta14286h
https://doi.org/10.1039/c3ta14286h... , p-n hetero-junctions could be fabricated by coupling CeO₂ with Ag₃PO₄, which will bring more effective interface transfer of photo-generated electrons and holes to restrain the recombination. Besides, owe to its narrower band gap relative to CeO₂, Ag₃PO₄ is able to act as efficient photosensitizer to enlarge the light response range under solar light irradiation.

In this paper, we applied a facile process to successfully fabricate Ag₃PO₄/CeO₂ hetero-junctions by an in situ precipitation method. The as-prepared Ag₃PO₄/CeO₂ hetero-junctions demonstrated much higher activity than that of single Ag₃PO₄ or CeO₂ under the irradiation of visible light as well as UV light. Furthermore, the stability of the Ag₃PO₄/CeO₂ photocatalyst was investigated and a photocatalytic mechanism under visible-light irradiation was proposed.

2 Experimental section

2.1 Preparation of CeO2 nano-particles

All reagents are of analytic grade and used without further purification. In a typical procedure, 1.0 g of cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) was fully dissolved in 40 mL formalin solution under vigorous magnetic stirring for 10 min. Afterwards, 1.5 g of sodium hydroxide (NaOH) was added into the above solution and was transferred in to a 50 mL capacity Teflon-lined stainless steel autoclave. Subsequently, the autoclave was laid in an oven at 140 ^oC for 40 h under autogenous pressure and static conditions. After reaction, the suspension was cooled down to room temperature. The as-obtained powder samples were centrifuged and washed with distilled water, and then dried completely in an oven at 80 ^oC for 12 h. The dried powders were heated to 450 ^oC in air at a rate of 5 ^oC min^-1 and calcined for 5 h. The light yellow powders were obtained.

2.2 Preparation of Ag3PO4/CeO2 heterostructure composite

The preparation of the Ag₃PO₄/CeO₂ heterostructure composite was carried out by an in situ precipitation method. In a typical synthesis process, 0.1 g of as-prepared CeO₂ nano-particles were dispersed in 150 mL deionized water and sonicated for 30 min. Immediately after sonication, AgNO₃ aqueous solution (100 mL, 0.012 mol L^-1) was added to the white CeO₂ dispersed solution, followed by magnetic stirring. Na₂HPO₄ aqueous solution (200 mL, 0.003 mol L^-1) was then added dropwise, accompanied with thorough stirring until the color of the solution changed from white to yellow. The precipitate was centrifuged and washed several times with deionized water and absolute ethanol, and dried at 80 ^oC for 10 h. For comparison, Ag₃PO₄ particles were also prepared under the same conditions without the presence of CeO₂ nanoparticles.

2.3 Characterization

X-ray powder diffraction (XRD) patterns of the samples were recorded on a Bruke D8 Advance powder X-ray diffractometer with Cu Kα (λ=0.15406 nm) over a range of 10-70^o (2θ) with 0.02^o per step. A HITACHI S-4800 field emission scanning electron microscope (FE-SEM) was used to characterize the morphologies of the synthesized samples. High resolution transmission electron microscopic (HRTEM) images were acquired with a JOEL JEM 2100 microscope. UV-Vis diffuse reflectance spectra (DRS) of the samples were recorded on a Schimadzu UV 2550 spectrophotometer with an integrating sphere attachment within the range of 200 to 800 nm and with BaSO₄ as the reflectance standard.

2.4 Photocatalytic activity measurements

The photocatalytic activity of the Ag₃PO₄/CeO₂ heterostructures was investigated by the photodegradation of Rhodamine B (RhB, 10 mg/L). Prior to the irradiation, the suspension containing 250 mL of RhB solution and 0.10 g catalysts was magnetically stirred in the dark for 30 min to establish adsorption-desorption equilibrium between the surface of the catalysts and the dye under ambient conditions. A 300 W mercury lamp with a maximum emission at 356 nm was used as the UV source, a 300 W Xe arc lamp as the visible light source where the UV components were filtered out during visible light photocatalysis. The distance between it and the photocatalyst was 50 cm. The light intensity reaching the samples was measured using a radiometer and was found to be approximately 35 W m^-2 in the visible-light range. At varied irradiation time intervals, an aliquot of the mixed solution was collected and centrifuged, and the residual RhB concentration in the supernatant was analyzed by UV-vis spectroscopic measurements (Hitachi UV-3100)

3 Results and discussion

3.1 Morphology and structure characterization of catalyst

Figure 1 illustrates the XRD patterns of the obtained pure CeO₂ nanoparticles, Ag₃PO₄ and Ag₃PO₄/CeO₂ p-n heterojunctions. It is observed that all of the diffraction peaks shown in Figure 1a and b can be indexed to the cubic fluorite-type CeO₂ structure (JCPDS no. 43-1002) and body-centered cubic structure of Ag₃PO₄ (JCPDS no. 06-0505), respectively. The sharp diffraction peaks of both Ag₃PO₄ and CeO₂ indicate their good crystallinity. No traces of other phases are detected, confirming the high purity of the samples. The XRD pattern of the Ag₃PO₄/CeO₂ p-n heterojunctions (Figure 1c) clearly matches with the polycrystalline structures of Ag₃PO₄ (the peak without the triangle mark) and CeO₂ (the peak with the triangle mark), indicating that the Ag₃PO₄/CeO₂ p-n heterojunctions has been successfully prepared.

Figure 1
XRD patterns of (a) CeO₂, (b) Ag₃PO₄ and (c) Ag₃PO₄/ CeO₂.

The morphology of the as-synthesized pure CeO₂, Ag₃PO₄ and the Ag₃PO₄/CeO₂ p-n heterojunctions was examined by FE-SEM, as shown in Figure 2. It can be seen that pure CeO₂ (Figure 2a) exhibits spherical-shaped nanoparticls with diameters of about 30 nm, whereas pure Ag₃PO₄ is about 200-300 nm with a smooth surface (Figure 2b). When CeO₂ was deposited on the surface of Ag₃PO₄via a facile precipitation-deposition process (Figure 2c-d), the resulting Ag₃PO₄/CeO₂ composite sample exhibits a similar morphology and size as compared to that of pure Ag₃PO₄. Obviously, the coexistence of CeO₂ and Ag₃PO₄ did not significantly affect their morphology. It is noticeable that the CeO₂ nanoparticls are uniformly distributed in the Ag₃PO₄ crystallites and form a composite structure. To further confirm the crystallographic structure of the Ag₃PO₄/CeO₂ p-n heterojunctions, high-resolution TEM (HRTEM) measurement was carried out (Figure 3). As shown in Figure 3b, the synthesized Ag₃PO₄/CeO₂ composite are of sphere morphology and is composed with inner Ag₃PO₄ core (Region I) and outer layer (Region II) of CeO₂ nanoparticle. Clear lattice fringes of the small CeO₂ nanocrystals can also be observed in Figure 3c, and inter-planar spacings are measured to be 0.313 nm, which agrees with the d values of the (111) plane of the cubic fluorite-type CeO₂.

Figure 2
FE-SEM images of (a) CeO₂, (b) Ag₃PO₄ and (c-d) Ag₃PO₄/ CeO₂.

Figure 3
HRTEM images (a-c) of as prepared Ag₃PO₄/ CeO₂ heterojunction.

3.2 UV-Vis absorption spectra

The optical absorption plays an important role in the photocatalysis, especially in the visible-light photodegradation of contaminants. The optical absorption properties of pure CeO₂, Ag₃PO₄ and the Ag₃PO₄/CeO₂ p-n heterojunctions were measured by UV-vis DRS and demonstrated in Figure 4. For pure CeO₂, an adsorption edge can be observed at 420 nm and can absorb solar energy with a wavelength shorter than 420 nm (Figure 4a), which is in agreement with the results previously reported ¹⁰10 Arul NS, Mangalaraj D, Ramachandran R, Grace AN, Han JI. Fabrication of CeO2/Fe2O3 composite nanospindles for enhanced visible light driven photocatalysts and supercapacitor electrodes. Journal of Materials Chemistry A. 2015;3:15248-15258. http://dx.doi.org/10.1039/c5ta02630j
https://doi.org/10.1039/c5ta02630j... ^,1111 Wang Q, Yu S, Tan Z, Zhang R, Li Z, Gao X, et al. Synthesis of monodisperse Bi2O3-modified CeO2 nanospheres with excellent photocatalytic activity under visible light. CrystEngComm. 2015;17:671-677. http://dx.doi.org/10.1039/c4ce02053g
https://doi.org/10.1039/c4ce02053g... . It can be clearly seen that pure yellow Ag₃PO₄ can absorb energy with wavelengths shorter than 550 nm (Figure 4b), in agreement with the results previously reported¹⁵15 Lin H, Ye H, Xu B, Cao J, Chen S. Ag3PO4 quantum dot sensitized BiPO4: a novel p-n junction Ag3PO4/BiPO4 with enhanced visible-light photocatalytic activity. Catalysis Communications. 2013;37:55-59. http://dx.doi.org/10.1016/j.catcom.2013.03.026
https://doi.org/10.1016/j.catcom.2013.03... . Upon the loading of the CeO₂ nanoparticles on the surface of Ag₃PO₄, the absorption edge of heterocrystals was drastically extended to around 560 nm and the absorption intensity in the region of 200-800 nm has been evidently increased. The red-shift of the absorption wavelength indicated that the photocatalyst could absorb more photons. Therefore, the red-shift in the absorption band could be favorable for photocatalytic reaction. These results clearly reveal that the in situ deposited of CeO₂ nanoparticles on the crystal surfaces of Ag₃PO₄ can serve as an effective strategy for enhancing their visible-light absorption.

Figure 4
UV-vis absorption spectra of (a) CeO₂, (b) Ag₃PO₄ and (c-d) Ag₃PO₄/ CeO₂

3.3 Photocatalytic activity

3.3.1 Visible light photocatalytic activities of Ag3PO4/CeO2

In this study, Rhodamine B (RhB), with a major absorption band at 554 nm, was chosen as a model pollutant for testing photocatalytic activity of the as-prepared products. The absorption spectra of RhB (Figure 5a), with 0.1 g of the Ag₃PO₄/CeO₂ p-n heterojunction photocatalyst under visible light irradiation, clearly show that the characteristic absorption peaks corresponding to RhB decrease rapidly as the exposure time increases, indicating the decomposition of RhB and the significant reduction in the RhB concentration. In this experiment, the photodegradation process is studied by monitoring the change in RhB concentration. The degradation efficiency of RhB over pure CeO₂, Ag₃PO₄ and the Ag₃PO₄/CeO₂ p-n heterojunctions under visible irradiation is presented in Figure 5b. It can be seen that 88.0% of the RhB is photocatalytically degraded after 60 min irradiation for the Ag₃PO₄/CeO₂ composite. For comparison, the activity of the Ag₃PO₄ photocatalyst was carried out under the same conditions. As shown in Figure 5b, the photocatalytic activity of the Ag₃PO₄ and CeO₂ sample for RhB degradation is much lower than that of the Ag₃PO₄/CeO₂ p-n heterojunctions. For the pure Ag₃PO₄ and CeO₂ samples, the RhB is degraded by only 47.0% and 10%, respectively.

Figure 5
UV-vis absorbance spectra of RhB solution after photocatalytic degradation with Ag₃PO₄/ CeO₂ heterojunction (a) and RhB concentration C_t/C₀ and ln(C₀/C_t) (inset)vs. time for the photocatalytic degradation of RhB with CeO₂, Ag₃PO₄ and Ag₃PO₄/ CeO₂ (b) under visible light irradiation.

To quantitatively investigate the reaction kinetics of the RhB degradation, the experimental data were fitted by a first-order model as expressed by the formula¹³13 Huang L, Li Y, Xu H, Xu Y, Xia J, Wang K, et al. Synthesis and characterization of CeO2/g-C3N4 composites with enhanced visible-light photocatatalytic activity. RSC Advances. 2013;3:22269-22279. http://dx.doi.org/10.1039/c3ra42712a
https://doi.org/10.1039/c3ra42712a... :

where C₀ and C are the dye concentrations in solution at times 0 and t, respectively, and k is the apparent first-order rate constant.

The kinetic data curves for RhB photocatalytic degradation with photocatalysts in the inset of Figure 5b show that the relationship between ln(C/C₀) and irradiation time is almost linear, suggesting that the photocatalytic reaction follows pseudo-first-order kinetics. As can be seen in Figure 5b, the pseudo-first-order rate constants (k) for RhB degradation with CeO₂, Ag₃PO₄ and Ag₃PO₄/CeO₂ composite were estimated to be 0.0004 min^-1, 0.0082 min^-1, 0.0331 min^-1, respectively. The rate constant of the Ag₃PO₄/CeO₂ composite is 82.8 times as high as that of CeO₂ and 4.0 times as that of Ag₃PO₄. The results demonstrate that the degradation efficiency of the Ag₃PO₄/CeO₂ composite to RhB in much higher than those of pure Ag₃PO₄ and CeO₂ under visible light irradiation. To elucidate the photocatalytic reaction mechanism, the main species including h⁺, •O²⁻ and •OH involved in the photocatalytic process was examined. The method was applied according to the Xiang's report without any modifications¹⁶16 Xiang QJ, Lang D, Shen TT, Liu F. Graphene-modified nanosized Ag3PO4 photocatalysts for enhanced visible-light photocatalytic activity and stability. Applied Catalysis B: Environmental. 2015;162:196-203. http://dx.doi.org/10.1016/j.apcatb.2014.06.051
https://doi.org/10.1016/j.apcatb.2014.06... . The results indicated that h⁺ and •O²⁻ were the main reactive oxidizing species in photocatalytic reaction process of Ag₃PO₄/Ag composites.

3.3.2 UV light photocatalytic activities of Ag3PO4/CeO2

Although CeO₂ has poor capacity for RhB degradation under visible light irradiation due to its higher band gap energy, it shows high photocatalytic capability in the UV region¹⁷17 Song S, Xu L, He Z, Ying H, Chen J, Xiao X et al. Photocatalytic degradation of C.I. Direct Red 23 in aqueous solutions under UV irradiation using SrTiO3/CeO2 composite as the catalyst. Journal of Hazardous materials. 2008;152:1301-1308. http://dx.doi.org/10.1016/j.jhazmat.2007.08.004
https://doi.org/10.1016/j.jhazmat.2007.0... ^,1818 Lu X, Zhai T, Cui H, Shi J, Xie S, Huang Y, et al. Redox cycles promoting photocatalytic hydrogen evolution of CeO2 nanorods. Journal of Materials Chemistry. 2011;21:5569-5572. http://dx.doi.org/10.1039/c0jm04466k
https://doi.org/10.1039/c0jm04466k... . To investigate the effect of the coupling Ag₃PO₄ sensitizer on the photocatalytic capability of CeO₂, the photocatalytic degradation of RhB over CeO₂, Ag₃PO₄ and the Ag₃PO₄/CeO₂ p-n heterojunction under 300 W UV irradiation has also been performed. As shown in Figure 6, CeO₂ and Ag₃PO₄ show only 35% and 86% RhB degradation, whereas the Ag₃PO₄/CeO₂ composite renders 98.3% RhB degradation after 10 min of photocatalytic reaction. The Ag₃PO₄/CeO₂ composite exhibits the highest photocatalytic degradation efficiency, followed by the pure Ag₃PO₄ and CeO₂ photocatalysts. Additionally, pseudo-first-order rate constants (k) for RhB decomposition by the Ag₃PO₄/CeO₂ composite under UV light irradiation is about 0.2262 min^-1, faster than that with Ag₃PO₄ (0.0909 min^-1) and pure CeO₂ (0.0205 min^-1) by a factor of 2.5 and 11.0, respectively. The results demonstrate that the degradation efficiency of the Ag₃PO₄/CeO₂ composite to RhB in much higher than those of pure Ag₃PO₄ and CeO₂ under UV light irradiation.

Figure 6
RhB concentration C_t/C₀ and ln(C₀/C_t) (inset)vs. time for the photocatalytic degradation of RhB with CeO₂, Ag₃PO₄ and Ag₃PO₄/ CeO₂ under UV light irradiation.

3.3.3 photocatalytic stability

In addition to photocatalytic efficiency, the stability of photocatalyst is also very important for practical application. To evaluate the stability of the photocatalytic performance of Ag₃PO₄/CeO₂ composite, the circulating run in the photocatalytic degradation of RhB was carried out under visible light irradiation. As shown in Figure 7, the photocatalytic degradation efficiency of RhB still reached 82% after 3 cycles in 3 h. The decrease of degradation efficiency under the third run may be due to the loss of sample during the cycling reaction. Furthermore, the XRD analysis of Ag₃PO₄/CeO₂ photocatalyst before and after photocatalytic reaction indicated no difference between the two lines. The result demonstrates that the Ag₃PO₄/CeO₂ composite, formed by the coupling of CeO₂ and Ag₃PO₄, shows excellent photocatalytic performance, as well as good stability.

Figure 7
Cycling runs in the photocatalytic degradation of RhB with Ag₃PO₄/ CeO₂ heterojunction under visible light irradiation.

3.4 Photocatalytic mechanism of Ag3PO4/CeO2 heterojunctions

Based on the above results, it is evident that the enhanced activity of the hybrid photocatalyst involving CeO₂ nanoparticles and Ag₃PO₄ can be attributed to the synergistic effects of visible-light sensitization and p-n junction structure¹⁹19 Dai G, Yu J, Liu G. Synthesis and enhanced visible-Light photoelectrocatalytic activity of p−n junction BiOI/TiO2 nanotube Arrays. The Journal of Physical Chemistry C. 2011;115:7339-7346. http://dx.doi.org/10.1021/jp200788n
https://doi.org/10.1021/jp200788n... . Ag₃PO₄ is a typical p-type semiconductor with a narrow band gap¹⁵15 Lin H, Ye H, Xu B, Cao J, Chen S. Ag3PO4 quantum dot sensitized BiPO4: a novel p-n junction Ag3PO4/BiPO4 with enhanced visible-light photocatalytic activity. Catalysis Communications. 2013;37:55-59. http://dx.doi.org/10.1016/j.catcom.2013.03.026
https://doi.org/10.1016/j.catcom.2013.03... while CeO₂ is an n-type semiconductor with a large band gap²⁰20 Yang ZM, Hou SC, Huang GF, Duan HG, Huang WQ. Electrospinning preparation of p-type NiO/n-type CeO2 heterojunctions with enhanced photocatalytic activity. Materials Letters. 2014;133:109-112. http://dx.doi.org/10.1016/j.matlet.2014.06.169
https://doi.org/10.1016/j.matlet.2014.06... . In order to fully understand the energy band structure of the p-Ag₃PO₄/n- CeO₂ heterojunction, the original energy band structures of Ag₃PO₄ and CeO₂ were provided. The valence band edge positions of Ag₃PO₄ and CeO₂ were estimated in this study according to the concept of electronegativity¹⁸18 Lu X, Zhai T, Cui H, Shi J, Xie S, Huang Y, et al. Redox cycles promoting photocatalytic hydrogen evolution of CeO2 nanorods. Journal of Materials Chemistry. 2011;21:5569-5572. http://dx.doi.org/10.1039/c0jm04466k
https://doi.org/10.1039/c0jm04466k... . The conduction band (CB) and valence band (VB) positions of the two semiconductors at the point of zero charge are predicted theoretically by the following empirical equations²¹21 Jiang D, Chen L, Zhu J, Chen M, Shi W, Xie J. Novel p-n heterojunction photocatalyst constructed by porous graphite-like C3N4 and nanostructured BiOI: facile synthesis and enhanced photocatalytic activity. Dalton Transactions. 2013;42:15726-15734. http://dx.doi.org/10.1039/c3dt52008k
https://doi.org/10.1039/c3dt52008k... :

where E_CB is the valence band (CB) potential; E_VB is the conduction band (VB) potential; X is the absolute electronegativity of the semiconductor, which is defined as the geometric mean of the absolute electronegativity of the constituent atoms; E^e is the energy of free electrons on the hydrogen scale (ca. 4.5 eV); and E_g is the band gap of the semiconductor. The conduction band (CB) position can be deduced by eqn (2). The X values for Ag₃PO₄ and CeO₂ are ca. 5.959 and 5.696 eV, respectively. The band-gap energies of Ag₃PO₄ and CeO₂, in our experiment, are estimated to be 2.32 and 2.86 eV, respectively. Herein, based on eqn (1) and (2), the top of the VB and the bottom of the CB for Ag₃PO₄ are calculated to be 2.619 and 0.299 eV respectively. Accordingly, the VB and CB of CeO₂ are estimated to be 2.626 and -0.234 eV, respectively. The energy band structure diagram of p-type Ag₃PO₄ and n-type CeO₂ is thus schematically illustrated in Figure 8a.

Figure 8
Schematic diagrams for (a) energy bands of p-Ag₃PO₄ and n-CeO₂before contact and (b) the formation a p-n junction and its energy banddiagram at equilibrium and transfer of photoinduced electrons form p-Ag₃PO₄ and n-CeO₂ under visible-light irradiation.

When p-type Ag₃PO₄ and n-type CeO₂ are in contact, The Fermi level of Ag₃PO₄ is raised up, while the Fermi level of CeO₂ is lowered until an equilibrium state is formed as shown in Figure 8b. Meanwhile, with the raising up and/or lowering of the Fermi level, the energy bands of Ag₃PO₄ shift upward along the Fermi level (E_FP) and those of the CeO₂ shift downward along its Fermi level (E_Fn), and as a result, the conduction band edge of p-type Ag₃PO₄ is higher than that of n-type CeO₂, leading to the formation of a p-n junction at the interface between Ag₃PO₄ and CeO₂ crystals. Thus, at the thermodynamic equilibrium, an inner electric field orienting from CeO₂ to Ag₃PO₄ was established in the interface between CeO₂ and Ag₃PO₄.

Therefore, we speculated a photocatalytic mechanism of the p-Ag₃PO₄/n-CeO₂ heterojunction as follows: under visible light illumination, Ag₃PO₄ acting as a photosensitizer could be easily activated by visible light and generated electrons and holes. From Figure 8b, it can be found that under the function of an internal electric field, the electrons from the excited Ag₃PO₄ transfer to the conduction band of CeO₂, and simultaneous holes remain in the p-Ag₃PO₄ valence band. In such a way, the photogenerated electron-hole pairs will be separated effectively by the p-n junction formed in the heterostructured p-Ag₃PO₄/n-CeO₂ interface and the photocatalytic activity is much enhanced. That is to say, to some extent, the recombination of photogenerated electrons and holes can be restrained. Furthermore, the migration rates of the photogenerated electrons and holes are promoted by the internal electric field in the p-n heterojunctions. The efficient charge separation could increase the lifetime of the charge carriers and has enough time to react with the reactants adsorbed onto the photocatalyst surfaces so as to improve the photocatalytic activity. According to the above results and discussion, it is apparent that the synergetic effects of the p-n junction formed between Ag₃PO₄ and CeO₂ were responsible for the enhanced visible-light photocatalytic activity.

4 Conclusions

In summary, we have successfully fabricated Ag₃PO₄/CeO₂ p-n heterojunctions by depositing small CeO₂ nanoparticles sized of 10-15 nm on the surface of Ag₃PO₄ via a facile precipitation route. The obtained Ag₃PO₄/CeO₂ p-n heterojunctions exhibited enhanced photocalytic activity toward RhB degradation under ultra-violet and light irradiation than that of pure Ag₃PO₄ and CeO₂ nanoparticles. Such enhanced photocatalytic activity of the Ag₃PO₄/CeO₂ p-n heterojunctions could be attributed to high dispersibility of small CeO₂ nanoparticles, broadened of the optical absorption range as well as the generation of a p-n junction between p-type Ag₃PO₄ and n-type CeO₂. This work not only supports the possibility of using cost-effective n-type CeO₂ for photocatalytic degradation organic dye but also shows that a suitable p-n junction structure is crucial for high photocatalytic activity in a hybrid photocatalyst.

Acknowledgements

This research was financially supported by the Natural Science Foundation of AnHui Provincial Education Department (KJ2015A085) and National Undergraduate Training Programs for Innovation and Entrepreneurship

References

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