MO degradation by Ag–Ag2O/g-C3N4 composites under visible-light irradation
© Wang et al. 2016
Received: 12 December 2015
Accepted: 12 February 2016
Published: 24 March 2016
The paper demonstrated the synthesis of Ag–Ag2O/g-C3N4 nanoparticles via a simple liquid phase synthesis path and a facile calcination method. The synthesized Ag–Ag2O/g-C3N4 composites were well characterized by various analytical techniques, such as X-ray diffraction, Fourier transform infrared (FT-IR), X-ray photoemission spectroscopy, transmission electron microscopy, scanning electron microscopy, high resolution transmission electron microscopy, the UV–Vis diffuse-reflectance spectra and transient photocurrent. From the structure and surface characterization, it indicated that Ag–Ag2O/g-C3N4 composites were formed by an effective covering of g-C3N4 with Ag–Ag2O. The results revealed that the 50 wt% nanoparticle had a great effection on the degradation of the methyl orange (MO), which was almost 7.5 times as high as that of g-C3N4. Based on the experimental results, the possible photocatalytic mechanism with photogenerated holes as the main active species was presented.
With the development of the society, the environmental pollution has become one of the important problems which aroused more and more focus. It is well known that the TiO2 has been proved to be the most distinguished and widely used in the photocatalytic degradation of dyes (Liu et al. 2008; Chang et al. 2014) and H2 production (Cho et al. 2011; Park et al. 2006; Yang et al. 2009). However, with the increasing demands of the photocatalytic materials searching for more semiconductor photocatalysts is becoming more urgent. Thus, the mental and non-mental composites with g-C3N4 have attracted more attention (Peng et al. 2013; Zong et al. 2013).
As a good photocatalyst, Graphitic carbon nitride (g-C3N4) has been widely investigated since the discovery of its excellent properties by Liu and Cohen (1989). To date, it exhibits catalytic activity for extensive reactions, such as water splitting, oxidation reaction, dye photodegradation, nitric oxide (NO) decomposition and so on (Huang et al. 2013; Vignesh and Kang 2015; Chen et al. 2015; Yu et al. 2015; Dong et al. 2015; Chang et al. 2013; Su et al. 2010). This new material also possesses the good capabilities such as environmental friendly, stable, low cost and efficient. The reason why the g-C3N4 has a good photocatalytic activity is that the g-C3N4 possesses special optical characteristics and outstanding chemical stability. But, even with those merits, the g-C3N4 still has some disadvantages which show the limited photocatalytic property, such as the poor dispersion, easy agglomeration, recycling difficulties and so on. Yet combined with other materials such as the g-C3N4/MoO3 (Huang et al. 2013), g-C3N4/Ni(dmgH)2 (Cao et al. 2014), g-C3N4/Bi2O2CO3 (Tian et al. 2014), g-C3N4/Ag3PO4 (Xiu et al. 2014) and so on could enhance the catalytic activity of g-C3N4.
For example, in recent years, a g-C3N4 was modified with a composite semiconductor could possess the performance of water splitting and remove organic pollutants, which were reported by Wang et al. (2009) and Zhao et al. (2012). Wang and Zhang (2012) reported a g-C3N4–TiO2 pohotocatalyst fabricated by a simple impregnation method which has good activities for the H2 production. In fact, the approach indicates a synergetic effect of the impregnation preparation which provides a better junction between g-C3N4 and TiO2. It can be seen that the composites may have better photoactivities. However, not only can TiO2 doped possess the properties of degrading the pollutants, but also other mental and non-mental materials doped could have good activities. As we all know, the Ag-based materials have good photocatalytic activity. Thus, enormous efforts have been made to study more photocatalysts which needed Ag-based materials modification, such as Ag/C3N4 (Li et al. 2015), Ag/AgVO3/g-C3N4 (Zhao et al. 2015), Ag/AgCl/g-C3N4 (Yao et al. 2014), Ag–AgBr/g-C3N4 (Li et al. 2014) and so on.
Simultaneously, Ag2O nanoparticles were partially reduced to Ag0 as it was calcined at 220 °C for 90 min to prepare the desired Ag–Ag2O photocatalysts. This method is also used the same as the preparation of Ag–Ag2O/g-C3N4 nanocomposites. Then the intimate contacted interfaces between the Ag–Ag2O and g-C3N4 were also developed. In addition, prepared g-C3N4 via Ag–Ag2O doping has been proved to control the migration photon-generated carriers, so that the electrons and holes could be separated selectively at the edges, respectively. The mechanism of this report can explain phenomenon for it which indicates Ag–Ag2O has a great potential to be used as a stable and highly efficient photocatalyst to degrade the pollutants under the visible-light irradiation. MO, a representative of dyestuffs resistant to biodegradation, was selected as a model for the study. From our study, we find that the proportion of Ag–Ag2O loading on g-C3N4 surface has the most enhanced adsorption capacity and the best photocatalytic activity is 50 wt%. Therefore, both Ag and Ag2O maybe act as traps to capture photogenerated electrons which contribute to the separation of electron–hole pairs (Yu et al. 2005, 2012; Zhou et al. 2010; Subramanian et al. 2001; Xie et al. 2011). Based on the experimental results, a possible photocatalytic mechanism for the degradation of MO over Ag–Ag2O doped g-C3N4 nanosheets under visible-light irradiation was proposed.
All reagents in this work were AR grade and used without further purification.
Preparation of g-C3N4
The g-C3N4 was synthesized by calcination method. In a typical process, 6 g dicyandiamide was put into three crucibles with three covers, sealed in a quartz tube partially backfilled with pure nitrogen, annealed at 350 °C for 2 h and annealed at 600 °C for 2 h again. Then the crucibles were cooled to room temperature.
Preparation of Ag–Ag2O/g-C3N4 nanoparticles
The Ag–Ag2O/g-C3N4 was also synthesized via a simple liquid phase synthesis path and a facile calcination method. The method was as follow: 0.2 g of g-C3N4 was added into 20 ml of deionized water. Then they were magetic stirred for 5 min and sonicated for 15 min. Further, 0.2932 g of silver nitrate (AgNO3) was added into the solution and sonicated for 15 min. Next, 0.5 ml hydrated ammonia (NH3·H2O) was also added into the solution, which was still magetic stirred for 15 min. In addition, 0.1829 g of sodium carbonate (Na2CO3) was added drop by drop under stirring in 15 min. Moreover, the pH of the solution was adjusted to 7 and heated in water bath at 25 °C for 1 h. Next, the product was obtained by centrifugation, washed with ethanol and deionized water for several times and dried at 60 °C for 8 h. At last, the sample was annealed at 220 °C for 90 min. The 50 wt% Ag–Ag2O/g-C3N4 could be obtained. All the experiments were carried out at room temperature. The Ag–Ag2O/g-C3N4 composites with different mass ratios were synthesized using the same method through changing the amount of g-C3N4, AgNO3 and Na2CO3, such as 5, 10, 30 and 40 wt%, respectively.
The crystal phase of Ag–Ag2O, g-C3N4 and Ag–Ag2O/g-C3N4 powders were analyzed by X-ray diffraction (XRD) analysis using a Bruker D8 diffractometer with Cu-Kα radiation (λ = 1.5418 Å) in the 2θ range of 20°–80°. Scanning electron microscopy (SEM) image and transmission electron microscopy (TEM) micrographs were taken with a JEOL-JEM-2010 (JEOL, Japan) operating at 200 kV. High resolution transmission electron microscopy (HR-TEM) micrographs were taken with a FEI F20. Energy Dispersive spectrum (EDS) measurements were performed by a JEM-2100F electron microscope. The UV–Vis diffuse-reflectance spectra (DRS) of the samples were obtained on a UV–Vis spectrophotometer (UV-2450, Shimadzu Corporation, Japan). They were measured in solid state, and BaSO4 powder was used as the substrate. Fourier transform infrared (FT-IR) spectra of all the catalysts (KBr pellets) were recorded on Nicolet Model Nexus 470 IR equipment. X-ray photoemission spectroscopy (XPS) was measured on a PHI5300 with a monochromatic Mg Kα source to explore the elements on the surface. The photocurrents were measured with an electrochemical analyzer (CHI660B, CHI Shanghai, Inc.).
Results and discussion
In addition, the metallic Ag can further complete efficient electron migration process to efficiently inhibit the recombination of the photoexcited pairs (Xu et al. 2013). So it can be seen that even the VB and CB of g-C3N4 are higher than that of Ag2O, the Ag can be worked as the charge transmission bridge, which transfers the photogenerated electrons from the CB of Ag2O to Ag0 and then the photogenerated electrons were trapped by O2 to produce ·O2 −. At last, the ·O2 − transformed into ·OH. As a result, with the assistance of Ag–Ag2O, the Ag–Ag2O/g-C3N4 photocatalysts could effectively enhance the separation of photoexcited electron-hole pairs and reduced the recombination of electrons and holes. Thus, the Ag–Ag2O nanoparticles loaded on the surface of the g-C3N4 could form the heterojunction structure, which contributed to the promotion of the photocatalytic activity.
In summary, we have demonstrated that Ag–Ag2O nanophases were active catalysts for degrading MO. The results revealed that the optimal activity of Ag–Ag2O/g-C3N4 is 7.5 times as high as that of g-C3N4 and even better than that of Ag–Ag2O. In this investigation the as-synthesized samples were characterized by a collection of techniques, such as XRD, SEM, TEM, HR-TEM, DRS, EDS, XPS and FT-IR. Based on structural analysis, we concluded that the Ag–Ag2O nanoparticles are dispersed on the surface of the g-C3N4. The modified g-C3N4 samples were robust and able to show better photocatalytic activities than Ag–Ag2O and g-C3N4. In addition, the photocatalysis mechanism was also investigated by entrapping active species. These results indicated that the holes played important roles in the degradation of MO over sample Ag–Ag2O/g-C3N4.
XW prepared the sample, had done the experiment of the XRD, FT-IR, XPS, TEM, SEM, HR-TEM, DRS, transient photocurrent and MO dye degradation and drafted the manuscript. JY provided with design ideas and teaching methods to improve the article. HYJ, ZGC, YGX, LYH, QZ, YHS checked and improved the manuscript. HX conceived of the study, and participated in its design and coordination and helped to draft the manuscript. HML conceived of the study, and participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.
The authors genuinely appreciate the financial support of this work from the National Nature Science Foundation of China. The authors genuinely appreciate the financial support of this work by the National Nature Science Foundation of China (21476097, 21476098,21407065 and 21406094), the Natural Science Foundation of Jiangsu Province (BK20131207 and BK2012717, BK20140533).
All authors declare that they have no competing interests.
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