Elsevier

Materials Chemistry and Physics

Volume 177, 1 July 2016, Pages 529-537
Materials Chemistry and Physics

A facile fabrication of plasmonic g-C3N4/Ag2WO4/Ag ternary heterojunction visible-light photocatalyst

https://doi.org/10.1016/j.matchemphys.2016.04.065Get rights and content

Highlights

  • g-C3N4/Ag2WO4/Ag ternary nanocomposite photocatalyst was prepared.

  • g-C3N4/Ag2WO4/Ag showed high photocatalytic activity.

  • g-C3N4/Ag2WO4/Ag showed long reusable life.

Abstract

It's important to reduce recombination of electrons and holes and enhance charge transfer through fine controlled interfacial structure. In this work, novel graphitic-C3N4 (g-C3N4)/Ag2WO4/Ag ternary photocatalyst has been synthesized by deposition of Ag2WO4 onto g-C3N4 template and followed by sun light reduction of Ag2WO4 into Ag2WO4/Ag. As-prepared g-C3N4/Ag2WO4/Ag presented significantly enhanced photocatalytic performance in degrading methylene blue (MB) under 410 nm LED light irradiation. Metallic Ag0 is used as plasmonic hot spots to generate high energy charge carriers. Optimal g-C3N4 content has been confirmed to be 40 wt%, corresponding to apparent pseudo-first-order rate constant kapp of 0.0298 min−1, which is 3.3 times and 37.3 times more than that of pure g-C3N4 and Ag2WO4, respectively. This novel ternary g-C3N4/Ag2WO4/Ag structure material is an ideal candidate in environmental treatment and purifying applications.

Graphical abstract

A high efficient plasmonic graphitic-C3N4/Ag2WO4/Ag ternary nanocomposite photocatalyst was synthesized.

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Introduction

The continuous increasing world's population, together with the substantial development of industry has brought about imperious attention for the environmental protection [1], [2], [3], [4], [5]. Many technologies, such as reverse osmosis [6], [7], ultrafiltration [8], [9], electrodeionization [10], [11], ion exchange [12], [13], vacuum evaporation [14], [15], absorption [16], [17], and biological water treatment [18], [19], have been put forward to degrade toxic pollutants. But how to treat organic waste water with low price, unsurpassable efficiency and stability is still a challenge. Recently, semiconductor-based visible sunlight energy conversion materials have attracted sustained attention owing to their great potential in renewable energy systems and environmental applications [20], [21], [22], [23], [24]. However, the major challenge in conventional processes is the limitation in absorption of solar energy in the visible range or insufficient carrier separation ability [25], [26], [27], [28]. To address this issue, substantial effort has been devoted to the exploration and fabrication of novel photocatalytic materials for expanding the energy utilization to the visible range of the solar spectrum [29], [30], [31], including dye-sensitization [32], [33], [34], development of new narrow-band gap semiconductors and band engineering of semiconductor composites [35], [36], [37], [38].

In recent years, graphitic carbon nitride (g-C3N4), a novel “sustainable” photocatalysis with superior electronic structure (2.7 eV), have aroused great attention due to their potential applications in photocatalytic field [39], [40], [41], [42], [43], [44]. It possesses many excellent properties such as inexpensiveness, high photooxidative capabilities, highly thermal and chemical stability, abundancy, innocuity and easy preparation. Wang et al. reported that g-C3N4 has the photocatalytic activities for H2 or O2 production from water splitting under visible light excitation [45]. However, the photocatalytic performance of pure g-C3N4 is limited due to poor light absorption and fast recombination of electron–hole pairs. Up to now, continuous attempts have been carried out to promote the photocatalytic efficiency of g-C3N4, such as doping metal, elements and construct the heterojunction between g-C3N4 and another semiconductor with suitable band potential such as Zn2GeO4 [46], TiO2 [47], Co3O4 [48], MoS2 [49], Bi2WO6 [50], Ag3VO4 [51], In2O3 [52], SnS2 [53], or even g-C3N4 itself [54]. The results showed that the photoactivity of nanocomposites was dramatically improved and g-C3N4 could be used as efficient cocatalyst to increase the photocatalytic activity of the semiconductor. At the same time, the layered structure of g-C3N4 is beneficial to the transfer of electrons. However, since g-C3N4 was poorly dispersed in the solution, the obtained insufficient contact interface would limit the transfer of photogenerated charges. The obtained insufficient contact interface would limit the transfer of photogenerated charges, which need to be further explored and improved for practical application.

Recently, the incorporation of Ag+ ions into metal/non-metal oxides constructing Ag-based composite oxide photocatalysts, such as Ag3PO4 [55], Ag2Ta4O11 [56], Ag2CO3 [57], Ag2Mo2O7 [58], Ag2Nb4O11 [59], Ag3VO4 [60], have been proved to be an effective strategy for adjusting band structure and position of the semiconductor to improve photocatalytic activity. It mainly benefits from the uniquely filled d10 electronic configurations of Ag+ ions taking part in composition and hybridization of the energy band in the majority of Ag-based compounds [61], [62]. Consequently, designing Ag-based composite oxides based on this strategy can adjust electronic structure and light absorption property of photocatalysts. Currently, there are many studies for Ag-based composite photocatalysts, which display high-efficiency decomposing organic pollutants and O2 evolution ability under light irradiation and are believed to be a kind of promising photocatalytic materials. Furthermore, Ag-based photocatalysts are not stable, Ag0 metal always generates from Ag-based composite, and Ag metal-semiconductor materials have gained much interest because the presence of surface plasmon resonance (SPR) of Ag nanoparticles can also improve light harvesting [63], [64].

However, the durable operation of Ag-based photocatalysts is difficult to realize owing to their high photocorrosion feature. In fact, for all kinds of photocatalysts, it is the stability and durability that are very significant for their actual application. Nowadays, avoiding and mitigating deactivation of Ag-based photocatalysts is still one major challenge. Although many methods, such as doping, loading and constructing heterojunction, have been developed to improve photocatalytic activity and stability of Ag-based photocatalysts, they also increased the difficulty and complexity in the synthesis and can not solve deactivation problems completely. Therefore, if we can develop a method which can maintain the long-term stability of Ag-based photocatalyst, we will solve the problem fundamentally.

Beside the importance of high heterojunction area for effective reducing recombination of free electron and holes, heterostructure photocatalysts design requires two semiconducting materials which has matching energy band, and it favours the charge separation. To the best of our knowledge, there are no reports which study coupling mode of g-C3N4/Ag2WO4/Ag with metallic Ag0 as plasmonic material. And the photocatalytic mechanism of those inorganic-organic hybrid photocatalyst remains far from clear.

Inspired by the role of SPR to improve charge transfer in the structure design of noble metal–semiconductor photocatalysts [65], [66], we synthesized a ternary composite composed of g-C3N4, Ag2WO4 and Ag in this work. The reduced photoluminescence (PL) peak intensity and enhanced photocatalytic performance have shown the effectively improvement of the electron and hole transfer between Ag, g-C3N4 and Ag2WO4. This novel mechanism has open up new opportunities for the development of visible light driven photocatalysts with high efficiency and stability.

Section snippets

Materials

Commercial P25 TiO2 (P25, 20% rutile and 80% anatase) was purchased from Degussa, Germany. Methylene blue (MB), sodium tungstate dihydrate (Na2WO4), melamine were provided by Shanghai Chemical Reagent Co. Ltd., China. Silver nitrate (AgNO3) was purchased from Sinopharm Chemical Reagent Co. Ltd., China. 18 MΩ deionized water was used for solution preparation.

Preparation of g-C3N4

Pure g-C3N4 was synthesized by simply heating melamine. 5.0 g melamine powder was put into a muffle furnace and heated to 500 °C for 4 h

Results and discussion

As indicated in Fig. 1, the formation of as-prepared g-C3N4/Ag2WO4/Ag composited samples can be divided into three stages. At stage (I), when AgNO3 mixed with Na2WO4 and g-C3N4, initial Ag2WO4 nuclei were formed on g-C3N4 by the reaction of AgNO3 and Na2WO4. As illustrated in stage (II), Ag2WO4 nanorods grown directly on Ag2WO4 nuclei via a hydrothermal reaction may be simplified as follows:2AgNO3+Na2WO4Ag2WO4+2NaNO32Ag2WO4+2H2O+4h++4e4Ag+2H2WO4+O2

At stage (III), the Ag+ ions on the outer

Conclusions

In summary, a novel g-C3N4/Ag2WO4/Ag ternary composite with well-designed structure has been synthesized and showed higher visible-light photocatalytic activity for degradation of MB than those of pure g-C3N4 and Ag2WO4. The experiment results showed that the 40%g-C3N4/Ag2WO4/Ag had the highest photoactivity. The special plasmonic ternary composite favor the enhancement of light absorption and effective electron-hole separation, which exhibit wide spectral response and provide a new strategy

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51572103, 51302101 and 21303129), the Natural Science Foundation of Anhui Province (1408085QE78), the Foundation for Young Talents in College of Anhui Province (12600941) and Collaborative Innovation Center of Advanced Functional Materials (XTZX103732015008).

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