g-C3N4 for enhanced visible-light-driven photocatalysis

Accepted Manuscript Title: 0D/2D Fe2 O3 Quantum Dots/2D-C3 N4 for Enhanced Visible-Light-Driven Photocatalysis Authors: Quanguo Hao, Zhao Mo, Zhigang Chen, Xiaojie She, Yuanguo Xu, Yanhua Song, Haiyan Ji, Xiangyang Wu, Shouqi Yuan, Hui Xu, Huaming Li PII: DOI: Reference:

S0927-7757(18)30029-3 https://doi.org/10.1016/j.colsurfa.2018.01.023 COLSUA 22218

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

17-11-2017 9-1-2018 10-1-2018

Please cite this article as: Hao Q, Mo Z, Chen Z, She X, Xu Y, Song Y, Ji H, Wu X, Yuan S, Xu H, Li H, 0D/2D Fe2 O3 Quantum Dots/2D-C3 N4 for Enhanced Visible-Light-Driven Photocatalysis, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2010), https://doi.org/10.1016/j.colsurfa.2018.01.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

0D/2D Fe2O3 Quantum Dots/2D-C3N4 for Enhanced Visible-Light-Driven Photocatalysis Quanguo Haoa, Zhao Moa, Zhigang Chena, Xiaojie Shea, Yuanguo Xua, Yanhua Songb, Haiyan Jia, Xiangyang Wua, Shouqi Yuana, Hui Xua,* Huaming Lia,* a

School of the Environment and Safety Engineering, Institute for Energy Research,

b

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Jiangsu University, Zhenjiang 212013, P. R. China.

School of Environmental and Chemical, Engineering, Jiangsu University of Science

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and Technology, Zhenjiang 212003, P. R. China

*Corresponding author: Tel.:+86-0511-88799500; Fax: +86-0511-88799500; E-mail address: [email protected], [email protected]

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Graphical abstract

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The Fe2O3 QDs/2D-C3N4 composites are synthesized by combining ultrasonic

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dispersion with low temperature calcination. The composites feature several nanometers sized Fe2O3 QDs well dispersed on 2D-C3N4, and the composites show

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high photocurrent response and photocatalytic activity. In addition, the Fe2O3

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QDs/2D-C3N4 sample still maintains its satisfying stability with negligible activity

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reduction after four photoreactions.

Abstract 0D/2D photocatalysts have aroused concern due to the high charge mobility. Herein, a 0D/2D structure is formed by employing a facile method to disperse Fe2O3 Quantum-Dots (QDs) on 2D-C3N4 without aggregation at a relatively low temperature of 200◦C. The ultrathin monolayer 2D structure, highly dispersed Fe2O3 QDs, the strong coupling as well as quantum-sized confinement effect between them lead to

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superior visible-light-driven photocurrent performance and photocatalytic activity. Particularly, the 0.5% Fe2O3 QDs/2D-C3N4 composite exhibits the highest

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photocatalytic activity under visible light. More importantly, the 0D/2D

photocatalysts show superior durability with little photocatalytic activity decline in the circular reaction for the photocatalytic destruction of dyes. Therefore, this work

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supplies a new opportunity to construct multifunctional 0D/2D photocatalysts for

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degradation of pollutants in water.

Keywords

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1. Introduction

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Fe2O3 QDs; 2D-C3N4; photocatalytic degradation; high stability

The the organic pollution in water environment is becoming more and more

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serious in recent years [1], so we urgently need a way to solve this problem. The photocatalytic degradation of organic pollutant is one of the most effective ways [2-7].

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The key to this technology is the choice of photocatalyst. Hence, many kinds of semiconductor photocatalysts (TiO2 [8], transition metal dichalcogenides [9], two-dimensional materials [10, 11], various quantum dots load on 2D ultrathin

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materials [12-15]) have been prepared for further improving photocatalytic performance. Among them, TiO2 is the most widespread researched material because of its good stability, strong oxidation capacity, low cost and toxicity [8]. However, the realistic applications of TiO2 are quite limited due to the lack of visible absorption [16]. Hence, the development of steady, green and effective photocatalysts is still an enormous challenge for practical applications.

Since Wang reported that the carbon nitride possesses the performance of hydrogen from water under visible light [17], the study towards the photocatalyst has drawn extensive concern. Graphitic carbon nitride (g-C3N4), as a semiconductor material with some unique properties: such as suitable energy band structure, cost-effective metal free composition, and thermal stability, has been considered to be a candidate for photocatalytic water splitting [18-21], artificial photosynthesis,

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decomposition of contaminants [20, 22] and CO2 reduction [23, 24]. However, the

bulk g-C3N4 prepared by traditional calcination thermal condensation of precursors

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(melamine, urea, thiourea, dicyanodiamine ) possesses the deficiencies of the fast

electron hole recombination and insufficient solar-light absorption, which block its wide applications [25, 26]. Therefore, many methods are proposed to boost the

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photocatalytic efficiency, such as nanostructure design [27, 28], element doping [29,

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30], molecule incorporation [31, 32], reaction environment modulation [33] and in

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situ defect modification of g-C3N4 [34]. In addition, theoretical investigations reveal that two-dimensional g-C3N4 exhibits unique electronic and optical properties: 1)

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large surface area with abundant active sites to construct strong electronic coupling

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with photocatalyst; 2) 2D structure to prompt interfacial photoexcited charge transfer and effective utilization of visible light; 3) tunable band gap offering great opportunities to form favorable band structure [18, 25]. Unfortunately, the 2D-C3N4 is

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still subjected to quick recombination of the photogenerated electron-hole pairs, insufficient photoabsorption and limited active sites, contributing to low

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photocatalytic performance [34, 35]. Hence it is quite eager to seek for a way to optimize the band structure or obtain more active sites so as to improve the

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photocatalytic activity of 2D-C3N4. Fe2O3 attracts extensive attention on account of its large overpotential for

hydrogen evolution, natural storage, and price moderate [12]. Therefore, there are many papers reported Fe2O3 can work as the anode of high-voltage asymmetric supercapacitors [36-38], cocatalyst for hydrogen evolution [39], and the degradation of pollutants [40]. Nevertheless, the morphology and poor electrical conductivity still restrain its utilization completely. Among various structural, Fe2O3 quantum dots

(QDs) (size of 2~10 nm [41]) and their composites have been widely applied in electro-optic examination, photocatalysis, and sensor fields [12, 42-44]. So we utilize an appropriate novel strategy to get the smaller Fe2O3 dispersed on the surface of 2D-C3N4 to solve the problem mentioned above. These reasons to design the 0D/2D structure are as follows： 1) 2D-C3N4 possesses huge surface areas to provide numerous active sites and more attachment sites [45]. 2) Fe2O3 QDs with the

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quantum-sized confinement effect are anchored on the surface of 2D-C3N4 to defer the recombination of the charge carriers.

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From what has been discussed above, it is worth combining 2D-C3N4 with Fe2O3 QDs to obtain a 0D/2D photocatalyst with high photocatalytic performance. 0D/2D composite photocatalysts have many unique properties: 1) 2D semiconductor can fix

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the 0D semiconductor QDs on its surface to inhibit QDs self-aggregation and

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instability [46]. 2) 0D semiconductor QDs tend to increase the absorption of visible

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light effectively. 3) 0D semiconductor QDs can improve the transfer rate of carrier quickly due to the quantum confinement effect. Although numerous reports have

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reported about g-C3N4 and Fe2O3 composites, there are very few reports about Fe2O3

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QDs composited with g-C3N4. Here, we report a simple and facile method to synthesis the Fe2O3 QDs/2D-C3N4 composite. The effect of Fe2O3 QDs modification to the morphology structure and performance of 2D-C3N4 is comprehensively researched.

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The result is testified against the degradation of MB. The composites exhibit much enhanced photocatalytic activities than the 2D-C3N4 material. The mechanism of

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photocatalysis is tentatively proposed on the grounds of many experimental results. 2. Experimental section

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2.1. Materials

Melamine (>99.0%), Ethanol (99.7%), Fe(NO3)3·9H2O (95.0–98.0%) are

purchased from Sinopharm Chemical Reagent Co., Ltd., (China). 2.2. Synthesis of 2D-C3N4 2 g melamine is calcined at 550◦C for 4 h at 2.0◦C min−1 in the furnace. Then, the material ground into powder, and then heated at 550◦C again. The obtained material is denoted as 2D-C3N4.

2.3. Synthesis of Fe2O3 QD/2D-C3N4 composite A certain amount of Fe(NO3)3·9H2O is first dissolved in 35 mL ethanol, and then 0.1 g 2D-C3N4 is added followed for 15 min ultrasonic processing to form a suspending liquid. The liquid is magnetically stirred with ethanol evaporating slowly in a fume cupboard. Next, the obtained sample is dried at 40℃ for all day in a drying closet. The obtained material is denoted as Fe(NO3)3·9H2O/2D-C3N4 composite. And

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then the sample is calcinated at 200℃ for 10 h at 2.0℃min−1 in a muffle furnace to gain the Fe2O3 QDs/2D-C3N4 composite. Different contents of Fe(NO3)3·9H2O are

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added to form final Fe2O3 QDs/2D-C3N4 composites with theoretical mass ratios of

1%, 0.5%, and 0.2% (Fe2O3 QDs to 2D-C3N4) are prepared. Meanwhile, pure Fe2O3 QDs are obtained 4 in the same way without the addition of 2D-C3N4.

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2.4. Synthesis of the photocatalyst by mixing Fe2O3 and 2D-C3N4 physically

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0.1 g 2D-C3N4 is first put in the mortar, and then a certain amount of Fe2O3 QDs

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is added. The mixture is grinded to form the photocatalyst by mixing Fe2O3 QDs and

2.5. Photocatalytic activity

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2D-C3N4 physically.

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The visible photocatalytic properties of the materials are assessed by the degradation rate of the MB, MO, RhB under visible light. In detail, the obtained photocatalyst (15 mg) is put into 50 mL MB, MO or RhB (10 mg/L) solution in a

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reactivator under a constant temperature (30◦C). After connecting the device, the air is pumped into the system at 2 L/min which is irradiated by a 300 W Xe lamp. Before

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irradiation, the solution is stirred for 30 min in the dark to guarantee MB adsorbed/desorpted equilibrium completely. Extract 3 ml of solution and then filter at specific time separation. The filtrates are tested in the spectrophotometer (UV-2450

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Shimadzu) to confirm concentration of the solvent after photocatalytic degradation through recording the diversification of the absorbency value at maximum wavelength (664, 463, 553 nm). 3. Results and discussion In the experiments, 2D-C3N4 is prepared by our previous method [46]. The Fe2O3 QDs/2D-C3N4 composite is prepared by the step above. During the reaction process,

with the ethanol evaporating, Fe(NO3)3·9H2O is induced to attach on the 2D-C3N4 surface. The Fe(NO3)3·9H2O/2D-C3N4 composite is immediately disposed in muffle furnace to obtain Fe2O3 QDs/2D-C3N4 composite, as shown in scheme 1. The 2D-C3N4 with huge surface areas and numerous active sites serves as the growth sites for the controlled growth of Fe2O3 QDs [12, 48]. For the composite system, the 2D-C3N4 with ultrathin structure (Fig. 1b) exposing more oxygen-containing

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functional groups have a strong influence on the growth of Fe2O3 QDs, contributing to the Fe2O3 QDs dispersed on the ultrathin 2D-C3N4 uniformly. The existence of

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covalent bonding developed through oxygen defect sites on the 2D-C3N4 surface affords a possibility to tightly anchor Fe2O3 QDs [47]. Finally, a 0D/2D

nanocomposite is constructed with better long-term stability which can overcome

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some inherent shortcomings of QDs [39]. By taking advantage of 0D and 2D structure,

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the composite exhibits tremendously enhance optoelectronic and photocatalytic

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performance. And then with a proper mass ratio control for the Fe2O3/2D-C3N4, Fe2O3

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QDs can be well dispersed on the 2D-C3N4 evenly without aggregation (Fig. 1a).

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Scheme 1. Schematic illustration of preparing Fe2O3 QDs/2D-C3N4 samples.

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Fig. 1. The transmission electron microscopy (TEM) patterns of Fe2O3 QDs/2D-C3N4 (a) and 2D-C3N4 (b) XRD patterns (c) and Fourier transform infrared (FT-IR) spectra

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(d) of all kinds of Fe2O3 QDs/2D-C3N4 samples. From the XRD patterns (Fig. 1c), all samples exhibit the stronger characteristic

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peak at 27.3◦ of the (002) peak, which is due to the layered structure of graphite-like interlayer stacking [18]. The characteristic peak of 2D-C3N4 does not have any change,

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which shows that the structure of 2D-C3N4 is maintained in the process of the whole experiment. Meanwhile, there is a characteristic peak at 57.5◦ (of Fe2O3) [40] appearing with Fe2O3 the content increasing little by little, which is in accord with

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XRD patterns of monomer Fe2O3 QDs (Supplementary Fig. S1). However, no other diffraction peaks of Fe2O3 in the 0.2% Fe2O3 QDs/g-C3N4 are observed, which is most likely due to the low amount of Fe2O3. The characteristic peak of monomer Fe2O3 QDs may be hidden by the characteristic peak of 2D-C3N4. As shown in Fig.1d. FT-IR spectra show that all kinds of Fe2O3 QDs/g-C3N4 samples reveal analogical FT-IR vibration modes. The peaks ranging from 1200 to

1600 cm-1 are ascribed to the representative extending vibrations of CN heterocycles, and the shrill peaks at 810 cm-1 originates from the breathing mode of the triazine units [18]. The wide regions at 2900~3300 cm-1 are characteristic signals of N-H or O-H oscillations, which are attributed to the terminal incoagulable amino radicals and water absorbability [27]. Above all, the characteristic units synchronously appear in all kinds of Fe2O3 QDs/2D-C3N4 samples. It is also suggested that the 2D-C3N4 structure

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characteristics are preserved in the process of the whole experiment. No obvious peak of Fe2O3 QDs presents because of less content. But it can be observed by increasing

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Fe2O3 QDs loading amount on the surface of 2D-C3N4.

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Fig. 2 (a) XPS survey spectra of the Fe2O3 QDs/2D-C3N4 (0.5%) sample (b) Fe 2p (c) C 1s (d) N 1s (e) O 1s. Furthermore, XPS analysis is carried out to understand the electronic structures of the composites, which exhibits the existence of C 1s, O 1s, N 1s, and Fe 2p in the

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sample. The spectrum confirms that the Fe2O3 QDs/2D-C3N4 is made up of carbon,

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nitrogen, oxygen and iron (Fig. 2a). Simultaneously, the Fe2O3 QDs/2D-C3N4 has very

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slightly increased compared with the oxygen content of 2D-C3N4. Obviously, in the

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spectrum of Fe 2p (Fig. 2b), two sharp peaks is located at 724.95 eV for Fe 2p1/2 and 711.34 eV for Fe 2p3/2, respectively. Meanwhile, two small peaks at 719.4 eV and

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733.2 eV can be observed, which agrees with literature reports for Fe2O3 [30, 39, 49]. The high resolution C 1s shows two distinct peaks at the cohesive energies of 288.7eV

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and 288.1 eV (Fig. 2c), which stand for a standard carbon specialized for the sample which can be attributed to the C-O or C=O bond [25, 50, 51]. Simultaneously, the

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binding energies of 284.9 eV is assigned to the C-C (additional carbon) in carbon environment, which may be due to the agraphitic carbon adhered to the surface [52]. Fig. 2d is the spectrum of N 1s. The peak is divided into four obviously peaks at

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around 398.7 eV, 399.8 eV, 401.1 eV and 404.4 eV, which are connected with sp2 nitrogen atoms (C–N=C), the two middle peaks of nitrogen in the form of N-(C)3 groups and amino groups (C-N-H), respectively [51, 52]. Another tiny one at 404.4 eV is determined as the charging impacts or positive charge existence in the heterocycles [49, 51, 53, 54]. In the spectrum of O 1s (Fig. 2e), the O 1s peak can be splited into two peaks. One peak is located at 531.8 eV corresponding to the water

molecule adsorbed on the surface or the C-O or C=O bond. The other at 530.2 eV is caused by lattice oxygen atoms in the Fe2O3 QDs, which gives eloquent proof of the existence of covalent chemical bonding between 2D-C3N4 and Fe2O3 QDs. The above

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analysis confirms that Fe2O3 QDs are inserted into 2D-C3N4 successfully.

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Fig. 3. UV-vis absorption spectra of all kinds of Fe2O3 QDs/2D-C3N4 samples.

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The different Fe2O3 QDs loading amount on the surface of 2D-C3N4 samples brings about different photocatalytic performances. To reveal the phenomenon, the

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light absorption properties of the materials are studied by UV-vis absorption spectra

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(DRS), and the spectra of the different photocatalysts are compared in the Fig. 3. The absorption edge of the pure 2D-C3N4 is about 440 nm and the pure Fe2O3 QDs is extended to about 600 nm (Supplementary Fig. S2a). And the absorption bands of

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Fe2O3 QDs/2D-C3N4 samples are extended towards larger wavelength compared with 2D-C3N4. The results obvious indicate the composites generate the red shift of the

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absorption band due to the implantation of Fe2O3 QDs. The band gap energy (Eg) of Fe2O3 QDs is calculated to 1.5 eV (Supplementary Fig S2b), which is lower than that of the 2D-C3N4. Therefore, the hybrid nanocomposites give rise to an appreciable

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decrease of Eg, increasing the absorption of visible light. At the same time, it is beneficial to improving the photocatalytic activities under visible light irradiation [55].

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Fig. 4. Photoluminescence emission spectra (a) and time-resolved fluorescence

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excited by the incident illumination of 337 nm (b) of the 2D-C3N4 and 0.5% Fe2O3 QDs/2D-C3N4.

Fig. 4a shows the photoluminescence emission spectra (PL) of 2D-C3N4 and

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0.5% Fe2O3 QDs/2D-C3N4 composite. It is apparent that a high emission peak appears

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at approximately 450 nm. However, compared with the 2D-C3N4, the 0.5% Fe2O3

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QDs/2D-C3N4 shows the PL emission peak with lower intensity, which can indirectly

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illustrate that the modifications will significantly suppress the rapid charge carrier recombination [14, 27]. The results can contribute to improve the quantum efficiency.

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Simultaneously, the PL maximum emission peak spectrum exhibits a slightly blue shifted, which is on account of Fe2O3 QDs quantum-sized confinement effect. What’s

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more, the experiments of the fluorescence life-time (FL) are implemented to study the charge transfer situation of the materials (Fig. 4b). Based on the attenuation curves,

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the fluorescence strengths of the materials show departure from the exponential attenuation. And FL is prolonged, which indicates that there is an important electronic interaction between the Fe2O3 QDs/2D-C3N4 and electrophilic groups. After fitting,

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the FL of the 2D-C3N4 and the 0.5% Fe2O3 QDs/2D-C3N4 are ∼3.99 ns and ∼4.98 ns. The result shows that FL of the charge carriers of 0.5% Fe2O3 QDs/2D-C3N4 is enhanced. Hence, it has lower recombination rates than the 2D-C3N4. It is known that FL of electrons is related to enhanced electron transport and electronic band structure shifts [56]. The FL of electrons plays a vital role in increasing the possibility of charge carriers to participate in photocatalytic reactions before recombination [40]. Thus, it is

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most likely to improve the photocatalytic activities under visible light irradiation.

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Figure 5. Electrochemical impedance spectroscopy in a 0.1 M potassium chloride

solution with 5 mM ferricyanide/ferrocyanide (a) and transient photocurrent curves (b) of 2D-C3N4 and all kinds of Fe2O3 QDs/2D-C3N4 samples.

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To guarantee that the Fe2O3 QDs/2D-C3N4 composites are superb photocatalysts, electronic structure, electron shift property and charge carrier separation are

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investigated by electrochemical impedance spectroscopy (EIS) and photocurrent. EIS

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is used to investigate the electrical conductivity. The arc radius is equivalent to the

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electron-transfer course and the semicircle diameter is in accordance with the electron-transfer resistance [27]. As shown in Fig. 5a, the decreased arc radius of

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Nyquist plot for the Fe2O3 QDs/2D-C3N4 composites suggests a smaller charge transfer resistance than that of 2D-C3N4. And the 0.5% Fe2O3 QDs/2D-C3N4 has a

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lower arc radius of Nyquist plots, which suggests that it has a superior electron-transfer velocity. Hence, it is beneficial to improve photocatalytic activity. A

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similar result can also be found in photoelectric response tests, which are recorded for several intermittent lights, as shown in Fig. 5b. The photocurrent tests investigate the photo-induced charge transfer and separation behaviors [57]. Compared with the

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2D-C3N4, every composite has a larger photocurrent response. Meanwhile, the 0.5% Fe2O3 QDs/2D-C3N4 has the maximum photoelectric response， which is in accordance with the EIS tests. The reason of the above results is as follows： the Fe2O3 QDs promotes the electron shifting from CB of 2D-C3N4 to O2， which delays the photoinduced electron-holes recombination and improves photogenerated charges mobility [41].

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Fig. 6. The photocatalytic degradation of MB (a) and photocatalytic degradation

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kinetics of MB by Fe2O3 QDs/2D-C3N4 samples under visible light irradiation (b);

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The electron spin resonance (ESR) spectra of DMPO-O2•- (c) and DMPO-•OH (d) adducts over the 0.5% Fe2O3 QDs/2D-C3N4 in water solution before and after visible light irradiation.

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The different photocatalysts of photocatalytic activities are evaluated by photocatalytic destruction of MB under visible radiation (Fig. 6a). The result displays

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2D-C3N4 with low photocatalytic activity (about 55% MB degraded in 180 min). However, the photocatalytic activities of the composites are significantly enhanced, owing to the reduction in charge combination. Among them, the 0.5% Fe2O3

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QDs/2D-C3N4 exhibits much highest photocatalytic activity (about 75% MB degraded in 180 min)，which shows that 0.5% Fe2O3 QDs is the optimum composition, which is consistent with the aforementioned results. The over increasing content of Fe2O3 QDs leads to a remarkable decline of photocatalytic activity, but which is still superior to the pure 2D-C3N4. The reason why excess Fe2O3 QDs in the Fe2O3 QDs/2D-C3N4 samples decreases the photocatalytic degradation performance can be illustrated as

follows: 1) A great quantity of Fe2O3 QDs will cover the partial active sites, which hinders electron transfer from 2D-C3N4. 2) Too many Fe2O3 QDs will restrict the light absorption of 2D-C3N4. 3) A great number of Fe2O3 QDs can be agglomerated increasing particle size and influencing quantum effect of Fe2O3 QDs. Although the excess Fe2O3 QDs can promote electron shift, the photo-induced electron from 2D-C3N4 will be suppressed. In addition, the Fe2O3 QDs/2D-C3N4 sample shows

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superior photocatalytic performance towards MO and RhB degradation (Fig. S4).

Above all, the Fe2O3 QDs/2D-C3N4 sample still maintains its satisfying stability with

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negligible activity reduction and well-preserved crystalline structure after four cycling photoreactions (Fig. S3).

To further confirming the introduction of Fe2O3 QDs will effectively facilitate

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the photocatalytic activity. The kinetics of MB photocatalytic degradation by the pure

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2D-C3N4 and the Fe2O3 QDs/2D-C3N4 composites are researched, as shown in Fig. 6b.

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The data is fitted in a first-order kinetic model. The reaction rate constants (k) is gained by appropriately processing the data with the formula: ln (Ct/C0) = -k t, where

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C0 is the adsorption equilibrium concentration of MB, t is the reaction time, and Ct is

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the concentration of MB at arbitrary time t. From the Fig. 6b, it is obviously observed that the photodegradation rate of all the samples is higher than that of the pure 2D-C3N4 under visible light irradiation (Table S1). The data shows that Fe2O3 QDs on

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the surface of g-C3N4 do good to improve photocatalytic performance. However, with the increasing content of Fe2O3 QDs, the photodegradation rate first increases and

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then reduces. Particularly, The photodegradation reaction rate constants (k) of the 0.5% Fe2O3 QDs/2D-C3N4 composite is 0.4724 min-1, which is obviously superior to the pure 2D-C3N4. Simultaneously, the photocatalyst prepared by mixing Fe2O3 QDs

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and 2D-C3N4 physically is prepared for comparaion. And it can be seen that only about 50% of MB is degraded after 3 h irradiation by Fe2O3 QDs and 2D-C3N4 by mechanical mixing, which is lower degradation performance than that of Fe2O3 QDs/2D-C3N4 (Figure S3). The consequences also indicate that the introduction of Fe2O3 QDs on the surface of 2D-C3N4 enhances the photoinduced electron-holes separation and thus improves the photocatalytic activities.

Generally, when radiated, a catalyst can be excited to generate the photoelectrons on the CB and holes on the VB, respectively. Undoubtedly, the holes have oxidation capacity to oxidate H2O/OH-1 (1.99 eV vs. NHE) into •OH. Meanwhile, the photoelectrons with the reduction capacity can produce reduction reactions, and also can convert O2 into O2•- in the solution. To further explore the photocatalytic mechanism of the composites, and to ensure the primary reactive species display

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function during photocatalytic degradation process, the ESR signals of radicals trapping by 5,5-dimethyl-1-pyrrolin-N-oxide (DMPO), which acts as a radical

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scavenger of DMPO/O2•- or DMPO/•OH, are recorded during photocatalytic

degradation process. Apparently, the distinctive signals of the DMPO-O2•- are

detected (Fig. 6c), but no obvious signals of the DMPO-•OH are observed under the

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same condition (Fig. 6d). This consequence shows that O2•- radical is the dominating

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oxidation groups.

Scheme 2. The photocatalytic mechanism of Fe2O3 QDs/2D-C3N4 composites for photocatalytic degradation of MB.

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In our work, according to the aforementioned experimental results, a mechanism

is proposed for the improved photocatalytic performance among Fe2O3 QDs/2D-C3N4 composites, as illustrated in Scheme 2 and Fig. S4. When the photocatalysts are radiated by visible light, the electron on the VB of 2D-C3N4 will be excited to transfer to its CB, and thus electrons and holes can be generated on the CB and VB of 2D-C3N4, respectively. The photoinduced electron transfer to the surface of Fe2O3

QDs rapidly [12]. The intimate contact between Fe2O3 QDs and 2D-C3N4 offers a fast carriers transmission path which is of great importance for photocatalytic activities [28]. The Fe2O3 QDs act as the main catalytic centers for the photocatalytic degradation of MB. The electrons can be scavenged by O2 absorbed on the surface of Fe2O3 QDs to produce O2•-, while the holes of 2D-C3N4 can not oxidize OH- or H2O molecules to form •OH due to the lower oxidation capacity of 2D-C3N4, but the holes

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on the valence band of 2D-C3N4 can directly oxidize MB to form the oxidation

product [3, 58]. Synthetically considering above all factors, the Fe2O3 QDs/2D-C3N4

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composites show the favourable photocatalytic performance under visible light irradiation. 4. Conclusion

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In short, we have presented a facile method to synthesize Fe2O3 QDs/2D-C3N4

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composites featuring several nanometers sized Fe2O3 QDs well dispersed on 2D-C3N4.

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With the increasing of Fe2O3 QDs amount, the absorption of visible light for the composites is enhanced, and first the photocatalytic performance first increases and

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then declines. PL spectra, FL spectra, EIS and transient photocurrent curves results

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show that the Fe2O3 QDs can facilitate effectually shift of photoelectrons from CB of 2D-C3N4 to O2, inhibiting the recombination of photogenerated electron-hole pairs. And then the as-prepared hybrid exhibit enhanced photocatalytic performance. Above

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all, the Fe2O3 QDs/2D-C3N4 composites still maintain its satisfying stability and well-preserved crystalline structure after four cycles. The work presents a new

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possibility to develop a 0D/2D visible-light-driven photocatalysts for degradation of pollutants in water.

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Acknowledgment The authors genuinely appreciate the financial support of this work by the

National Nature Science Foundation of China (21476097, 21776118, 21507046). Six talent peaks project in Jiangsu Province (2014-JNHB-014), the Natural Science Foundation of Jiangsu Province (BK20161363) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

References [1] L.A. Schaider, K.M. Rodgers, R.A. Rudel, Environmental Science & Technology 51 (2017) 7304-7317. [2] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269-271. [3] L. Ge, C. Han, J. Liu, Journal of Materials Chemistry 22 (2012) 11843. [4] F. Chen, Q. Yang, S. Wang, F. Yao, J. Sun, Y. Wang, C. Zhang, X. Li, C. Niu, D. Wang, G. Zeng,

IP T

Applied Catalysis B: Environmental 209 (2017) 493-505.

[5] Y. Cui, Z. Ding, P. Liu, M. Antonietti, X. Fu, X. Wang, Physical Chemistry Chemical Physics :

SC R

PCCP 14 (2012) 1455-1462.

[6] L.L. Yun Zheng, Bo Wang, and X. Wang, Angewandte Chemie Int. Ed. 2015, 54 (44) 12868-84. [7] X.B. Chen, S.H. Shen, L.J. Guo, S.S. Mao, Chem Rev 110 (2010) 6503-6570.

U

[8] J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Chem Rev 114 (2014) 9919-9986.

N

[9] L. Ge, C. Han, X. Xiao, L. Guo, International Journal of Hydrogen Energy 38 (2013) 6960-6969.

A

[10] J. Di, J. Xia, H. Li, Z. Liu, Nano Energy 35 (2017) 79-91.

M

[11] T. Low, A. Chaves, J.D. Caldwell, A. Kumar, N.X. Fang, P. Avouris, T.F. Heinz, F. Guinea, L. Martin-Moreno, F. Koppens, Nature Materials 16 (2017) 182-194.

ED

[12] H. Xia, C. Hong, B. Li, B. Zhao, Z. Lin, M. Zheng, S.V. Savilov, S.M. Aldoshin, Advanced Functional Materials 25 (2015) 627-635.

PT

[13] L. Yao, D. Wei, Y. Ni, D. Yan, C. Hu, Nano Energy 26 (2016) 248-256. [14] S. Fang, Y. Xia, K. Lv, Q. Li, J. Sun, M. Li, Applied Catalysis B: Environmental 185 (2016)

CC E

225-232.

[15] S. Manzeli, D. Ovchinnikov, D. Pasquier, O.V. Yazyev, A. Kis, Nature Reviews Materials 2 (2017) 17033.

A

[16] H. Hirakawa, M. Hashimoto, Y. Shiraishi, T. Hirai, Journal of the American Chemical Society 139 (2017) 10929-10936. [17] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nature Materials 8 (2009) 76-80. [18] S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P.M. Ajayan, Advanced Materials 25 (2013) 2452-2456.

[19] Y. Cui, G. Zhang, Z. Lin, X. Wang, Applied Catalysis B: Environmental 181 (2016) 413-419. [20] C. Han, Y. Gao, S. Liu, L. Ge, N. Xiao, D. Dai, B. Xu, C. Chen, International Journal of Hydrogen Energy 42 (2017) 22765-22775. [21] C. Han, L. Wu, L. Ge, Y. Li, Z. Zhao, Carbon 92 (2015) 31-40. [22] H. Xu, J. Yan, Y. Xu, Y. Song, H. Li, J. Xia, C. Huang, H. Wan, Applied Catalysis B:

IP T

Environmental 129 (2013) 182-193. [23] C. Han, Y. Lu, J. Zhang, L. Ge, Y. Li, C. Chen, Y. Xin, L. Wu, S. Fang, Journal of Materials Chemistry A 3 (2015) 23274-23282.

SC R

[24] G. Zhang, Z.-A. Lan, X. Wang, ChemCatChem 7 (2015) 1422-1423.

[25] X. She, L. Liu, H. Ji, Z. Mo, Y. Li, L. Huang, D. Du, H. Xu, H. Li, Applied Catalysis B: Environmental 187 (2016) 144-153.

U

[26] G. Zhang, Z.A. Lan, X. Wang, Chemical Science 8 (2017) 5261-5274.

N

[27] Z.-A. Lan, G. Zhang, X. Wang, Applied Catalysis B: Environmental 192 (2016) 116-125.

A

[28] G. Dong, L. Zhang, J. Mater. Chem. 22 (2012) 1160-1166.

[29] Y. Zheng, Z. Yu, F. Lin, F. Guo, K.A. Alamry, L.A. Taib, A.M. Asiri, X. Wang, Molecules 22

M

(2017).

ED

[30] L. Ge, C. Han, Applied Catalysis B: Environmental 117-118 (2012) 268-274. [31] L. Ge, C. Han, X. Xiao, L. Guo, Applied Catalysis B: Environmental 142-143 (2013) 414-422. [32] J. Fu, J. Yu, C. Jiang, B. Cheng, Advanced Energy Materials (2017) 1701503.

PT

[33] G. Liu, T. Wang, H. Zhang, X. Meng, D. Hao, K. Chang, P. Li, T. Kako, J. Ye, Angewandte

CC E

Chemie Int. Ed. 54 (2015) 13561-13565. [34] G.G. Liu, G.X. Zhao, W. Zhou, Y.Y. Liu, H. Pang, H.B. Zhang, D. Hao, X.G. Meng, P. Li, T. Kako, J.H. Ye, Advanced Functional Materials 26 (2016) 6822-6829. [35] J. Jin, Q. Liang, C. Ding, Z. Li, S. Xu, Journal of Alloys and Compounds 691 (2017) 763-771.

A

[36] Y. Li, J. Xu, T. Feng, Q. Yao, J. Xie, H. Xia, Advanced Functional Materials 27 (2017) 1606728. [37] X. Lu, Y. Zeng, M. Yu, T. Zhai, C. Liang, S. Xie, M.S. Balogun, Y. Tong, Advanced Materials 26 (2014) 3148-3155. [38] S. Shivakumara, T.R. Penki, N. Munichandraiah, ECS Electrochemistry Letters 2 (2013) A60-A62. [39] X. She, J. Wu, H. Xu, J. Zhong, Y. Wang, Y. Song, K. Nie, Y. Liu, Y. Yang, M.-T.F. Rodrigues,

R. Vajtai, J. Lou, D. Du, H. Li, P.M. Ajayan, Advanced Energy Materials (2017) 1700025. [40] K.C. Christoforidis, T. Montini, E. Bontempi, S. Zafeiratos, J.J.D. Jaén, P. Fornasiero, Applied Catalysis B: Environmental 187 (2016) 171-180. [41] Q. Liu, T. Chen, Y. Guo, Z. Zhang, X. Fang, Applied Catalysis B: Environmental 193 (2016) 248-258.

IP T

[42] Y. Dong, J. Choi, H.K. Jeong, D.H. Son, Journal of the American Chemical Society 137 (2015) 5549-5554.

[43] Z. Song, T. Lin, L. Lin, S. Lin, F. Fu, X. Wang, L. Guo, Angewandte Chemie Int. Ed. 55 (2016)

SC R

2773-2777.

[44] Q. Wang, Y. Wen, P. He, L. Yin, Z. Wang, F. Wang, K. Xu, Y. Huang, F. Wang, C. Jiang, J. He, Advanced Materials 28 (2016) 6497-6503.

U

[45] Q. Han, Z. Cheng, J. Gao, Y. Zhao, Z. Zhang, L. Dai, L. Qu, Advanced Functional Materials 27

N

(2017) 1606352.

A

[46] M.Y. Ye, Z.H. Zhao, Z.F. Hu, L.Q. Liu, H.M. Ji, Z.R. Shen, T.Y. Ma, Angewandte Chemie Int. Ed. 56 (2017) 8407-8411.

ED

Nano Energy 27 (2016) 138-146.

M

[47] X. She, J. Wu, J. Zhong, H. Xu, Y. Yang, R. Vajtai, J. Lou, Y. Liu, D. Du, H. Li, P.M. Ajayan,

[48] M. Zheng, D. Qiu, B. Zhao, L. Ma, X. Wang, Z. Lin, L. Pan, Y. Zheng, Y. Shi, RSC Adv. 3 (2013) 699-703.

PT

[49] C. Jaramillo-Páez, J.A. Navío, M.C. Hidalgo, A. Bouziani, M.E. Azzouzi, Journal of

CC E

Photochemistry and Photobiology A: Chemistry 332 (2017) 521-533. [50] J. Li, B. Shen, Z. Hong, B. Lin, B. Gao, Y. Chen, Chemical Communications 48 (2012) 12017-12019.

[51] H.J. Li, B.W. Sun, L. Sui, D.J. Qian, M. Chen, Physical Chemistry Chemical Physics : PCCP 17

A

(2015) 3309-3315.

[52] W. Shan, Y. Hu, Z. Bai, M. Zheng, C. Wei, Applied Catalysis B: Environmental 188 (2016) 1-12. [53] B. Long, Y. Zheng, L. Lin, K.A. Alamry, A.M. Asiri, X. Wang, J. Mater. Chem. A 5 (2017) 16179-16188. [54] Z. Mo, H. Xu, Z. Chen, X. She, Y. Song, P. Yan, L. Xu, Y. Lei, S. Yuan, H. Li, Applied Catalysis B: Environmental 225 (2018) 154-161.

[55] F. Wang, P. Chen, Y. Feng, Z. Xie, Y. Liu, Y. Su, Q. Zhang, Y. Wang, K. Yao, W. Lv, G. Liu, Applied Catalysis B: Environmental 207 (2017) 103-113. [56] P. Niu, L. Zhang, G. Liu, H.-M. Cheng, Advanced Functional Materials 22 (2012) 4763-4770. [57] Z. Jiang, C. Zhu, W. Wan, K. Qian, J. Xie, J. Mater. Chem. A 4 (2016) 1806-1818. [58] H. Xu, Y. Song, Y. Song, J. Zhu, T. Zhu, C. Liu, D. Zhao, Q. Zhang, H. Li, RSC Advances 4

A

CC E

PT

ED

M

A

N

U

SC R

IP T

(2014) 34539.