Microwave heating preparation of phosphorus doped g-C3N4 and its enhanced performance for photocatalytic H2 evolution in the help of Ag3PO4 nanoparticles

https://doi.org/10.1016/j.ijhydene.2020.03.169Get rights and content

Highlights

  • Ag3PO4/P-g-C3N4 was prepared via a combination of microwave-assisted heating and ion-exchange processes.

  • Ag3PO4/P-g-C3N4 presented much better performance in photocatalytic H2 evolution than g-C3N4.

  • The high performance was mainly ascribed to the improved charge separation in the bulk and surface of g-C3N4.

  • Z-scheme mechanism worked in the Ag3PO4/P-g-C3N4 composite.

Abstract

This paper synthesized a novel Ag3PO4/P-g-C3N4 via a two-step chemical route i.e. microwave-assisted heating and ion-exchange procedures. The as-synthesized hybrid presented high efficiency for photocatalytic hydrogen production. Systematic investigation indicated that phosphorus was successfully doped into the g-C3N4 framework through microwave heating the mixture of melamine and NH4H2PO4 for 40 min, which increases the BET surface area, broadens the visible light response region, and elevates the separation efficiency of electron-hole pairs. The Ag3PO4 nanoparticles were decorated on the optimal P-g-C3N4 sample via an ion-exchange process. Due to the instability of Ag3PO4, the formed composite is actually Ag/Ag3PO4/P-g-C3N4 photocatalyst. The introduced Ag3PO4 nanoparticles further improves the charge separation efficiency of P-g-C3N4, but slightly affects the surface area and optical property, which highlights the key role of the separation efficiency of electron-hole pairs in photocatalytic reaction. The best Ag3PO4/P-g-C3N4 hybrid shows a photocatalytic H2 production rate of 1221 and 90.2 μmol g−1 h−1 under simulated sunlight and visible light, respectively. This value is 2.1 and 1.4 times greater than that of g-C3N4 and P-g-C3N4, respectively. Meanwhile, the Ag3PO4/P-g-C3N4 displayed high photocatalytic stability. A probable photocatalytic mechanism of the hybrid was also suggested.

Introduction

Photocatalytic water splitting to hydrogen is one of the most important potential application of photocatalysis technique. Especially in the current state, the global energy shortage and environmental pollution have significantly highlighted the importance of photocatalytic H2 evolution (PHE). Since the work of Fujishima was firstly informed in 1972 [1], scientists have made great effort to develop highly effective photocatalysts and understand the photocatalytic mechanism. Many efficient semiconductor materials including TiO2 [2], ZnO [3], g-C3N4 [4], and CdS [5] have been developed and employed in H2 generation via photocatalysis. However, the photocatalytic activity of these single semiconductors cannot meet the requirements of practical applications, which triggers the modification of these photocatalytic materials to improve the PHE performance. Meanwhile, due to the advantages of carbon nitride materials in cost, visible light response, good photoactivity, and sustainability, g–C3N4–based materials have become a hot spot in current photocatalysis research.

The PHE process can be briefly described as the following three procedures: electrons and holes are generated in photocatalysts under light illumination, the photoinduced electrons move from the interior of the catalyst to the surface, and the surface electrons reduce the hydrogen ions into H2. Correspondingly, three characteristics of photocatalysts, i.e. optical property, bulk charge separation, and surface charge separation can greatly influence the photoactivity. Therefore, the modification of g-C3N4 is mainly focused on improving the above three characteristics. For instance, Xu et al. modified g-C3N4 using upconversion agents (Er3+ and Tm3+), which endowed the doped g-C3N4 ability in RhB photodegradation under red light irradiation [6]. Zhao et al. synthesized C-doped g-C3N4 and extend the light absorption region from visible light to near-infrared, which is an important cause for the enhanced performance in NO removal [7]. The element doping is also an effective approach to modify the bulk charge separation. Chen et al. reported that the B doping slightly enhanced the visible light response [8]. Nevertheless, the PHE rate was greatly improved, which was mainly ascribed to the increased bulk charge separation efficiency. Similar results were also reported in other B doped g-C3N4 photocatalysts [[9], [10], [11]]. For the surface charge separation, the surface medication (noble metal loading or composite fabrication) is usually applied. Liu et al. prepared ZnO/g-C3N4 composite and applied it in photocatalytic degradation of RhB under visible light [12]. The introduced ZnO acted as electrons accepter to improve the surface charge separation and hastened the photocatalytic reaction. Yu et al. coupled g-C3N4 with KNbO3 and obtained enhanced PHE performance driven by visible light [13]. The role of KNbO3 is the same as that of ZnO. There are many similar examples, such as SnO2/g-C3N4 [14], KTa0.75Nb0.25O3/g-C3N4 [15], YVO4/g-C3N4 [16], BiOCl/g-C3N4 [17]. Of course, the above purposeful modulation to g-C3N4 can cause changes in other properties which may also contribute to the photocatalytic reaction. For instance, while the S doping improved the photoabsorption performance of g-C3N4, the surface area of the catalyst was also increased to some extent, which enhanced the number of surface reactive sites and promoted photocatalytic reaction [18]. For g-C3N4 based hybrid catalyst, if the coupled semiconductor has good visible light responsiveness (such as CdS [19], Bi4O5I2 [20], AgNbO3 [21], BiOI [22], Ag3VO4 [23], V2O5 [24], and SnSe [25]), the visible light absorption capacity and carrier separation efficiency of the catalyst can be simultaneously improved, which can enhance the photocatalytic activity more effectively. Notably, a single modification is difficult to achieve a satisfactory improvement of all the properties of g-C3N4. The combined application of multiple approaches is essential for obtaining g-C3N4 based photocatalyst with high performance. Some novel effective photocatalysts (such as S-g-C3N4/Au/CdS [26], S-g-C3N4/BiVO4 [27], Ag/S-g-C3N4 [28], and P-g-C3N4/Zn0.8Cd0.2S [29]) have been developed using this strategy. Nevertheless, currently, more work is still needed to get satisfactory efficiency in photocatalytic H2 evolution.

Herein, we display a successful application of the multiple modification strategy in the preparation of Ag3PO4/P-g-C3N4 composite. The phosphorus doping and Ag3PO4 coupling were simultaneously adopted to increase the visible-light response, bulk charge separation and surface charge separation of g-C3N4. Although some nonmetal ions doped g-C3N4 have been reported [[8], [9], [10], [11], [12],[30], [31], [32], [33]], Ma et al. reported that P doping shows a better potential for changing the bulk charge separation than other elements [34]. For the surface charge separation, our previous work has confirmed that Ag3PO4 nanoparticles showed exceptional promotion effect [35]. Additionally, the P doping and Ag3PO4 coupling can also promote the visible light absorption [[35], [36], [37]]. Thus, Ag3PO4/P-g-C3N4 composite was prepared for the first time in the paper. Meanwhile, a new and facile route was designed to prepare the composite. P-g-C3N4 was firstly synthesized via microwave-assisted heating method with melamine and NH4H2PO4 as precursors. The high temperature generated by microwave allows the fast preparation of g-C3N4 and the facile doping of P into the matrix of g-C3N4. The residual free PO43− ions were used to form Ag3PO4 nanoparticles on P-g-C3N4 surface and construct a Z-scheme Ag3PO4/P-g-C3N4 hybrid. As far as we know, both the Ag3PO4/P-g-C3N4 photocatalyst and the preparation route have not been reported before. The photocatalytic test has demonstrated that the as-synthesized composite presents a high photocatalytic activity in H2 production under simulate sunlight and visible light. A thorough characterization of the Ag3PO4/P-g-C3N4 is also executed to reveal the nature behind the improved photoactivity.

Section snippets

Experimental part

All of the chemical reagents employed to prepare the photocatalysts are purchased commercially and used without further purification. P doped g-C3N4 was prepared via a microwave-assisted heating process. Take 5 %P-g-C3N4 (denoted as PCN-5) as an example, 8.0 g melamine and 0.4 g NH4H2PO4 were mixed in 40 ml deionized water and stirred for 60 min. Then, the water in the mixture was evaporated at 60 °C, and the obtained powders were further dried at the same temperature for 24 h. Finally, the

Characterizations of the Ag3PO4/P-g-C3N4

Due to the decomposition and thermal polymerization of melamine during the synthesis of g-C3N4 based hybrid, the weight loss of the catalyst precursor is unavoidable. TG analysis is thus executed to determine the real concentration of g-C3N4 in P doped g-C3N4 and Ag3PO4/PCN-5 photocatalysts. The result in Fig. 2a suggests that neat g-C3N4 is completely decomposed in the range of 550–750 °C, which agrees well with the reported literature [38]. Compared to g-C3N4, phosphate ion is stable and is

Conclusion

Novel Ag3PO4/P-g-C3N4 composite was successfully prepared through a combination of microwave heating and ion-exchange process. The as-synthesized composite displayed much better performance than pure g-C3N4 in photocatalytic H2 evolution under simulated solar light and visible light, which was mostly attributed to the elevated separation efficiency of charge carriers. The doping of P into the g-C3N4 matrix increases the charge migration in the g-C3N4, which induces the improvement of bulk

Acknowledgements

The work was financially supported by Nature Science Foundation of Zhejiang Province (Grant No. LY20B030004) and Science and Technology Innovation Project for College Students of Zhejiang Province (Grant No.2019R404007).

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