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Article

Ultrasonic-Assisted Synthesis of 2D α-Fe2O3@g-C3N4 Composite with Excellent Visible Light Photocatalytic Activity

by
Huoli Zhang
1,
Changxin Zhu
1,
Jianliang Cao
1,*,
Qingjie Tang
1,
Man Li
1,
Peng Kang
2,
Changliang Shi
1,* and
Mingjie Ma
1
1
School of Chemistry and Chemical Engineering, Henan Key Laboratory of Coal Green Conversion, The Collaboration Innovation Center of Coal Safety Production of Henan Province, Henan Polytechnic University, Jiaozuo 454003, China
2
College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
*
Authors to whom correspondence should be addressed.
Catalysts 2018, 8(10), 457; https://doi.org/10.3390/catal8100457
Submission received: 28 September 2018 / Revised: 13 October 2018 / Accepted: 16 October 2018 / Published: 16 October 2018
(This article belongs to the Special Issue Semiconductor Catalysis)
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Versions Notes

Abstract

:
In this study, α-Fe2O3@g-C3N4 photocatalyst was synthesized using an ultrasonic assisted self-assembly preparation method. The α-Fe2O3@g-C3N4 photocatalyst had a stronger optical absorption in the visible light region than pure graphitic carbon nitride (g-C3N4). The Z-Scheme heterojunction between α-Fe2O3 and g-C3N4 significantly inhibited the recombination of electrons and holes. The photocatalytic performances of α-Fe2O3@g-C3N4 photocatalyst were excellent in degradation of Rhodamine B (RhB) under visible light irradiation. The results indicated that 5 wt.% α-Fe2O3/g-C3N4 had the optimal photocatalytic activity because two-dimension (2D) α-Fe2O3 nanosheets can be well-dispersed on the surface of g-C3N4 layers by ultrasonic assisted treatment. A possible photocatalytic mechanism is also discussed.

1. Introduction

Developing novel and facile preparations of composite photocatalysts is an effective attempt to satisfy high efficiency application in wastewater treatment through sunlight irradiation [1,2,3]. Heterojunction can promote fast separation of photoinduced electron–hole pairs, especially for composite photocatalysts. To obtain a visible light responsive photocatalytic material, graphitic carbon nitride (g-C3N4) is an appropriate choice as it has a suitable bandgap (2.7 eV); its heterojunction composites have become a focus for research aiming to decrease the recombination of electrons and holes. The advantage of α-Fe2O3/g-C3N4 photocatalyst with the Z-scheme heterojunction is that it can realize high quantum efficiency and significantly restrict the recombination of electrons in the g-C3N4 conduction band (CB) and holes in the α-Fe2O3 valence band (VB), while retaining strong reducibility and oxidizability; this differs from results for other photocatalysts reported in the literature, such as g-C3N4/BiVO4 [4], C3N4/Cu2O [5], AgFeO2/g-C3N4 [6], C3N4/graphene oxide heterostructure [7] and Pt/g-C3N4 heterostructure [8]. The preparation of Z-scheme heterojunction has attracted great interest due to its adjusting ability of oxidation and reduction [9,10,11,12,13]. More recently, numerous Z-scheme heterojunction composites have been synthesized successfully, including g-C3N4/Au/C-TiO2 [14], g-C3N4/SnS2 [15], CdS/Co9S8 [16], g-C3N4/MnO2 [17], BiVO4/CdS [18], g-C3N4/ZnO [19], TiO2/CdS [20], WO3/g-C3N4 [21], ZnIn2S4/TiO2 [22] and Bi12GeO20/g-C3N4 [23].
Hematite (α-Fe2O3) is an n-type semiconductor with a suitable bandgap (2.0–2.2 eV) that is employed as a visible light responsive photocatalytic material. α-Fe2O3 can absorb visible light (absorbance edge ∼600 nm) [24,25]. Therefore, the Z-scheme α-Fe2O3/g-C3N4 heterojunction exhibits an excellent visible light absorbing ability, which can promote photocatalytic activity of water-splitting and CO2 reduction. It has been demonstrated that the Z-scheme heterostructure between α-Fe2O3 and g-C3N4 effectively separates electrons in the CB of g-C3N4 and holes in the VB of α-Fe2O3 [26,27,28]. However, existing synthesis methods for α-Fe2O3/g-C3N4 heterojunction are time-consuming and involve a complicated process. For example, 2D α-Fe2O3/g-C3N4 used for water-splitting was synthesized by high-temperature calcination at 550 °C twice [26]. 2D/2D Fe2O3/g-C3N4 used for H2 generation was synthesized by rotary evaporation, washing and drying at 80 °C overnight [27]. α-Fe2O3/g-C3N4 used for CO2 reduction was synthesized by stirring for 24 h to obtain a homogeneous suspension and using the hydrothermal method at 150 °C for 4 h [28].
Herein, α-Fe2O3@g-C3N4 photocatalyst was synthesized using a simple ultrasonic assisted self-assembly preparation method. Due to the unique hexagonal 2D α-Fe2O3 nanostructure, 2D α-Fe2O3 nanosheets can be dispersed on the surface of g-C3N4 layers to fabricate the Z-scheme heterojunction. Therefore, the representative 5 wt.% α-Fe2O3@g-C3N4 composite exhibited excellent catalytic activity for Rhodamine B (RhB) photodegradation, which also had good recycling stability.

2. Results and Discussion

2.1. Catalyst Characterization

As seen in Figure 1, it can be noticed that all the peaks of pure 2D α-Fe2O3 nanosheets showed the typical characteristic diffraction peaks of α-Fe2O3 phase, which are at 24.1°, 33.1°, 35.6°, 40.9°, 49.5°, 54.1°, 62.5° and 63.9° (JCPDS No.33-0664) [29,30]. For the pure 2D g-C3N4, two diffraction peaks appearing at 13.2° and 27.8° were observed, which can be attributed to the (100) and (002) diffraction planes respectively (JCPDS No. 87-1526). For 5 wt.% α-Fe2O3@g-C3N4 photocatalyst, the characteristic peaks belonging to g-C3N4 (at 13.2° and 27.8°) were detected. Meanwhile, the remaining obvious peaks belonged to α-Fe2O3 phase characteristic diffraction peaks, which were also detected. This indicated that α-Fe2O3 and g-C3N4 were combined together, implying the formation of α-Fe2O3@g-C3N4 composite photocatalyst.
The microstructures and morphologies of the samples were observed by scanning electron microscopy (SEM). As seen in Figure 2a,b, the as-obtained α-Fe2O3 and g-C3N4 materials were both 2D nanosheet structure; the 2D α-Fe2O3 nanosheets were uniform in size. The 2D g-C3N4 were wrinkled due to accumulation and aggregation of g-C3N4 layers. As seen in Figure 2c, 2D α-Fe2O3 nanosheets can be well-dispersed on the surface of g-C3N4 layers without obvious agglomeration. The size of the hexagonal 2D α-Fe2O3 nanosheets is about 100–150 nm. This shows that the heterojunction can be effectively formed for the representative 5 wt.% α-Fe2O3@g-C3N4 photocatalyst. The obtained heterostructure interfaces are beneficial to accelerate the separation of photogenerated electrons and holes during the photocatalytic reaction process. From energy dispersive spectrometry (EDS) of 5 wt.% α-Fe2O3@g-C3N4 photocatalyst, the elements of C, N, O and Fe were all detected, as seen in Figure 2d. The coexistence of C, N, O and Fe also indicated the α-Fe2O3@g-C3N4 composite formation. Furthermore, the numerous Z-scheme heterojunction interfaces across α-Fe2O3 and g-C3N4 can effectively improve charge separation efficiency of α-Fe2O3@g-C3N4 photocatalyst [26,27,28].
In order to investigate optical harvest property, the light absorption of the samples was analyzed in the range of 400–750 nm. As seen in Figure 3a, the absorption edge of pure g-C3N4 is at about 460 nm. By comparison, the 5 wt.% α-Fe2O3@g-C3N4 composite exhibited obvious redshift, which might be the strong interface interaction of α-Fe2O3 and g-C3N4 leading to redshift. Therefore, the representative 5 wt.% α-Fe2O3/g-C3N4 photocatalyst shows a stronger visible light harvest ability than 2D g-C3N4. The bandgaps of 2D g-C3N4 and 5 wt.% α-Fe2O3@g-C3N4 photocatalyst were analyzed from the intercept of the tangents to the plots of the (ahν)1/2 versus photon energy. The bandgap was also analyzed through the intercept of the tangents to the plots of the (ahν)2 versus photon energy for 2D α-Fe2O3. The results show that the bandgaps of 2D g-C3N4, 5 wt.% α-Fe2O3@g-C3N4 and 2D α-Fe2O3 are 2.71 eV, 2.32 eV and 2.10 eV respectively, as seen in Figure 3b. This revealed that 2D α-Fe2O3 nanosheets can effectively broaden the visible light response of the α-Fe2O3@g-C3N4 photocatalyst. The enhanced photoabsorption ability of the α-Fe2O3@g-C3N4 photocatalyst will moderately improve the photocatalytic performance.

2.2. Catalytic Performance

The photocatalytic performances were investigated through photodegradation RhB reaction under visible light irradiation. The results indicate that the photodegradation percentages of α-Fe2O3, g-C3N4, 5 wt.% α-Fe2O3@g-C3N4 photocatalyst, 10 wt.% α-Fe2O3@g-C3N4 photocatalyst and 20 wt.% α-Fe2O3@g-C3N4 photocatalyst are 20%, 26%, 90%, 75% and 60% after 120 min, respectively. In addition, in order to demonstrate the visible light photocatalytic system, Evonik P-25 was used in the photocatalytic system under the same conditions. The result showed that P-25 had no photocatalytic activity, which indicates that the photodegradation RhB system is the visible light photocatalytic system, as seen in Figure 4A. As seen in Figure 4B, there was a linear relationship of photodegradation RhB between ln(C0/C) and reaction time, which is well followed the first-order kinetics according to the regression factors (R2). As seen in Figure 4C, the values of rate constant k are 0.0024 min−1, 0.0179 min−1, 0.0111 min−1 and 0.0080 min−1 for g-C3N4, 5 wt.% α-Fe2O3@g-C3N4 photocatalyst, 10 wt.% α-Fe2O3@g-C3N4 photocatalyst and 20 wt.% α-Fe2O3@g-C3N4 photocatalyst respectively. The rate constant k value of 5 wt.% α-Fe2O3@g-C3N4 photocatalyst is nearly 7.4 times larger than the pure 2D g-C3N4. As seen in Figure 4D, 5 wt.% α-Fe2O3@g-C3N4 photocatalyst showed the optimal photocatalytic activity, which was used for the recycling test. The results showed that 5 wt.% α-Fe2O3@g-C3N4 photocatalyst still had a good stability after four cycles. In addition, as the amount of 2D α-Fe2O3 nanosheets increased in the α-Fe2O3@g-C3N4 photocatalyst, the rate constant k values of the α-Fe2O3@g-C3N4 photocatalyst were gradually decreased. This can be attributed to the agglomeration of 2D α-Fe2O3 nanosheets on the surface of g-C3N4. As seen in Figure 5, for 10 wt.% α-Fe2O3@g-C3N4 photocatalyst and 20 wt.% α-Fe2O3@g-C3N4 photocatalyst, excessive agglomeration of 2D α-Fe2O3 nanosheets was observed.

2.3. Photocatalytic Mechanism

For the enhanced photocatalytic activity of α-Fe2O3@g-C3N4 photocatalyst, the charge carrier separation and transfer are discussed, as seen in Figure 6. A reasonable photocatalystic mechanism can be described as follows:
α-Fe2O3@g-C3N4 + visible light → e− + h+
e− + O2 → O2−
h+ + H2O → OH + H+
O2− + OH + RhB → degradation products → CO2 + H2O
Both α-Fe2O3 and g-C3N4 can be excited under visible light irradiation. Photoinduced holes and electrons can be obtained in their VB and CB, respectively. The CB location of g-C3N4 (−1.1 eV vs. normal hydrogen electrode (NHE)) is more negative than that of α-Fe2O3 (0.3 eV vs. NHE), and the VB location of α-Fe2O3 (2.4 eV vs. NHE) is more positive than that of g-C3N4 (1.6 eV vs. NHE) [31,32]. Through Z-scheme heterojunction, the photoinduced electrons in the CB location of α-Fe2O3 can migrate to the VB location of g-C3N4 and further combine with photogenerated holes in the VB location of g-C3N4. The charge migration follows the Z-scheme route, which can effectively inhibit the recombination of the photogenerated electrons in the CB location of g-C3N4. The photogenerated holes and electrons respectively accumulate in the VB location of α-Fe2O3 and the CB location of g-C3N4, keeping the strong oxidizability and reducibility [27,28]. Significantly, the Z-scheme heterojunction not only enhances visible light absorption ability, but also improves charge separation efficiency, leading to a notable enhancement for photocatalytic activity of α-Fe2O3@g-C3N4 photocatalyst.

3. Materials and Methods

3.1. Synthesis

The chemical reagents in this study were of analytical grade. Deionized water was used in this work. The nanosheets of 2D g-C3N4 and hexagonal 2D α-Fe2O3 were synthesized according to previously reported methods [33,34,35]. In a typical synthesis, the 2D α-Fe2O3@g-C3N4 photocatalyst was prepared using an ultrasonic assisted self-assembly method. First, 95 mg of 2D g-C3N4 was added into 10 mL ethanol by stirring in an ultrasonic bath. Subsequently, 5 mg of 2D α-Fe2O3 nanosheets and 50 μL of Nafion solution (5 wt.% Nafion, Sigmα-Aldrich, DuPont, Wilmington, DE, USA), as stabilizing agents, were added to the mixture in that order. The mixture was irradiated under the 40 kHz ultrasonic wave with power output of 240 W for 30 min. The as-obtained suspension was dried at 80 °C for 12 h. The dried product was collected. In order to remove stabilizing agent, the product was washed several times with ethanol and deionized water. Then the product was dried at 60 °C for 12 h. Finally, the as-obtained 5 wt.% α-Fe2O3@g-C3N4 composite was obtained used as visible light photocatalyst. Different amounts of α-Fe2O3 were used for preparing 10 wt.% and 20 wt.% α-Fe2O3@g-C3N4 photocatalyst under the same preparation conditions. The schematic illustration is shown in Figure 7. The 2D α-Fe2O3@g-C3N4 photocatalyst can be synthesized though the ultrasonic assisted self-assembly route.

3.2. Characterizations

Scanning electron microscopy (SEM) was used to observe morphology of different samples using FEI (Quanta 250 FEG, Eindhoven, The Netherlands) microscopy. The energy dispersive spectrometry (EDS) was analyzed by X-Max (Oxford Instruments, Oxford, UK). The X-ray powder diffraction (XRD) patterns were recorded by AXSD8 (Bruker, Madison, WI, USA) with CuKα as a radiation source. The operating voltage and current were at 40 kV and 40 mA, respectively. The UV-vis diffuse reflectance spectra (UV-vis DRS) was investigated by using UV-240 (Shimadzu, Kyoto, Japan).

3.3. Photocatalytic Tests

2D α-Fe2O3@g-C3N4 photocatalyst was used for photodegradation RhB to evaluate the photocatalytic activity. The photocatalytic tests were performed at visible light irradiation of a halogen lamp (Osram, Munich, Germany, 220 V and 500 W) with a UV-cutoff filter (Shanghai Seagull Colored Optical Glass Co., Ltd., Shanghai, China) (λ > 420 nm). First, 100 mg as-prepared sample was added in 100 mL RhB (10 mg/L). Subsequently, the as-obtained solution was kept in the dark and stirred for 30 min to ensure an adsorption and desorption equilibrium. After this, the mixture solution at 10 cm from the halogen lamp was irradiated under visible light. Additionally, a certain amount of the mixture solution was pipetted and centrifuged every 30 min until 120 min. The RhB concentration of the centrifuged aqueous solution was measured by UV–vis spectrophotometer using TU1900 (Beijing purkinje general instrument Co., Ltd., Beijing, China).

4. Conclusions

In summary, 2D α-Fe2O3@g-C3N4 photocatalyst was successfully synthesized using a simple ultrasonic assisted self-assembly preparation method. The results indicate that 2D α-Fe2O3 were dispersed on the surface of g-C3N4. The 2D α-Fe2O3@g-C3N4 photocatalyst is very suitable to obtain Z-scheme heterojunction as charge transfer route. The 5 wt.% α-Fe2O3@g-C3N4 photocatalyst showed excellent catalytic activity and a good recycling stability for photodegradation RhB under visible light irradiation. The α-Fe2O3@g-C3N4 photocatalyst, synthesized using a simple ultrasonic assisted self-assembly method, obtained an improvement of visible light photocatalytic activity, providing a route to synthesize g-C3N4-based photocatalysts.

Author Contributions

H.Z., C.Z. and Q.T. performed the experiments; P.K. provided the characterization of the samples; M.L. and M.M. analyzed the data; J.C. and C.S. provided the idea of this work and managed the experimental section as the corresponding authors.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (U1704146), Program for Science & Technology Innovation Talents in Universities of Henan Province (17HASTIT029), the Fundamental Research Funds for the Universities of Henan Province (NSFRF1606, NSFRF170201), Colleges and Universities in Henan Province Key Scientific Research Project Plan (17A430019), Starting Funds for Post-Doctoral Research Projects of Henan Province and Program for Innovative Research Team of Henan Polytechnic University (T2018-2).

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 08 00457 g001 550
Figure 1. XRD patterns of different samples.
Figure 1. XRD patterns of different samples.
Catalysts 08 00457 g001
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Figure 2. SEM images of different samples: (a) 2D α-Fe2O3, (b) 2D g-C3N4, (c) 5 wt.% α-Fe2O3@g-C3N4 photocatalyst and (d) EDS of 5 wt.% α-Fe2O3@g-C3N4 photocatalyst.
Figure 2. SEM images of different samples: (a) 2D α-Fe2O3, (b) 2D g-C3N4, (c) 5 wt.% α-Fe2O3@g-C3N4 photocatalyst and (d) EDS of 5 wt.% α-Fe2O3@g-C3N4 photocatalyst.
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Figure 3. (a) UV-vis diffuse reflectance spectra (UV–vis DRS) of different samples, (b) plots of the (ahν)1/2 versus photon energy (hv) for g-C3N4 and 5 wt.% α-Fe2O3@g-C3N4 photocatalyst; plots of the (ahν)2 versus photon energy (hv) for α-Fe2O3.
Figure 3. (a) UV-vis diffuse reflectance spectra (UV–vis DRS) of different samples, (b) plots of the (ahν)1/2 versus photon energy (hv) for g-C3N4 and 5 wt.% α-Fe2O3@g-C3N4 photocatalyst; plots of the (ahν)2 versus photon energy (hv) for α-Fe2O3.
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Figure 4. (A) Photocatalytic performances of different samples, (B) kinetic curves of the as-prepared samples, (C) rate constant k of the samples: (a) P-25, (b) α-Fe2O3, (c) g-C3N4, (d) 5 wt.% α-Fe2O3@g-C3N4 photocatalyst, (e) 10 wt.% α-Fe2O3@g-C3N4 photocatalyst, (f) 20 wt.% α-Fe2O3@g-C3N4 photocatalyst and (D) recycling runs of 5 wt.% α-Fe2O3@g-C3N4 photocatalyst.
Figure 4. (A) Photocatalytic performances of different samples, (B) kinetic curves of the as-prepared samples, (C) rate constant k of the samples: (a) P-25, (b) α-Fe2O3, (c) g-C3N4, (d) 5 wt.% α-Fe2O3@g-C3N4 photocatalyst, (e) 10 wt.% α-Fe2O3@g-C3N4 photocatalyst, (f) 20 wt.% α-Fe2O3@g-C3N4 photocatalyst and (D) recycling runs of 5 wt.% α-Fe2O3@g-C3N4 photocatalyst.
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Figure 5. SEM images of (a) 10 wt.% α-Fe2O3@g-C3N4 photocatalyst and (b) 20 wt.% α-Fe2O3@g-C3N4 photocatalyst.
Figure 5. SEM images of (a) 10 wt.% α-Fe2O3@g-C3N4 photocatalyst and (b) 20 wt.% α-Fe2O3@g-C3N4 photocatalyst.
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Figure 6. Schematic illustration of the charge carrier separation and transfer on α-Fe2O3@g-C3N4 photocatalyst.
Figure 6. Schematic illustration of the charge carrier separation and transfer on α-Fe2O3@g-C3N4 photocatalyst.
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Figure 7. Schematic illustration for the 2D α-Fe2O3@g-C3N4 photocatalyst synthesis.
Figure 7. Schematic illustration for the 2D α-Fe2O3@g-C3N4 photocatalyst synthesis.
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MDPI and ACS Style

Zhang, H.; Zhu, C.; Cao, J.; Tang, Q.; Li, M.; Kang, P.; Shi, C.; Ma, M. Ultrasonic-Assisted Synthesis of 2D α-Fe2O3@g-C3N4 Composite with Excellent Visible Light Photocatalytic Activity. Catalysts 2018, 8, 457. https://doi.org/10.3390/catal8100457

AMA Style

Zhang H, Zhu C, Cao J, Tang Q, Li M, Kang P, Shi C, Ma M. Ultrasonic-Assisted Synthesis of 2D α-Fe2O3@g-C3N4 Composite with Excellent Visible Light Photocatalytic Activity. Catalysts. 2018; 8(10):457. https://doi.org/10.3390/catal8100457

Chicago/Turabian Style

Zhang, Huoli, Changxin Zhu, Jianliang Cao, Qingjie Tang, Man Li, Peng Kang, Changliang Shi, and Mingjie Ma. 2018. "Ultrasonic-Assisted Synthesis of 2D α-Fe2O3@g-C3N4 Composite with Excellent Visible Light Photocatalytic Activity" Catalysts 8, no. 10: 457. https://doi.org/10.3390/catal8100457

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