MnO2 Nanocomposite as a Direct Z-Scheme

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2D/2D g-C3N4/MnO2 nanocomposite as a direct Zscheme photocatalyst for enhanced photocatalytic activity Pengfei Xia, Bicheng Zhu, Bei Cheng, Jiaguo Yu, and Jingsan Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03289 • Publication Date (Web): 04 Nov 2017 Downloaded from http://pubs.acs.org on November 6, 2017

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ACS Sustainable Chemistry & Engineering

2D/2D g-C3N4/MnO2 Nanocomposite as a Direct Z-scheme Photocatalyst for Enhanced Photocatalytic Activity Pengfei Xia,† Bicheng Zhu,† Bei Cheng,† Jiaguo Yu,*,†,§ and Jingsan Xu*,‡ †

State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, 430070, P. R. China. E-mail: [email protected] §

Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah

21589, Saudi Arabia. ‡

School of Chemistry, Physics and Mechanical Engineering, Queensland University

of Technology. 2 George Street, Brisbane, QLD 4001, Australia. E-mail: [email protected]

1

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ABSTRACT: Constructing two-dimensional (2D) composites using layered materials is considered to be an effective approach to achieve high-efficiency photocatalysts. Herein, 2D/2D g-C3N4/MnO2 heterostructured photocatalyst was synthesized via in-situ growth of MnO2 nanosheets on the surface of g-C3N4 nanolayers using a wet-chemical method. The hybrid nanomaterial was characterized by a range of techniques to study its micromorphology, structure, chemical composition/states and so on. The g-C3N4/MnO2 nanocomposite exhibited greatly improved photocatalytic activities for dye degradation and phenol removal, in comparison to the single g-C3N4 or MnO2 component. Based on the electron paramagnetic resonance spectra, X-ray photoelectron spectra and the Mott-Schottky measurements, we consider that a Z-scheme heterojunction was generated between the g-C3N4 nanosheets and the MnO2 nanosheets, wherein the photo-induced electrons in MnO2 combined with the holes in g-C3N4, leading to enhanced charge carrier extraction and utilization upon photoexcitation. This work provides an effective approach to construct the 2D/2D heterojunctions for the application in solar-to-fuel conversion and photocatalytic water treatment. Keywords: graphitic carbon nitride, two-dimensional nanocomposite, photocatalytic degradation, Z-scheme heterojunction

INTRODUCTION 2


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One green and promising approach to decomposing organic pollutants into nontoxic compounds is the use of the artificial photocatalytic system in modern society.1 As a typical metal-free semiconductor photocatalyst, graphitic carbon nitride (g-C3N4) has attracted growing interest in the fields of solar-to-fuel conversion, optoelectronics and catalytic field,2-4 since Wang and coworkers first revealed the photocatalytic activity of g-C3N4 for water splitting. g-C3N4 has a layered structure similar to graphite and can be exfoliated into nanosheets via bath sonication or acid/base etching.5-7 g-C3N4 nanosheets can have increased photocatalytic performance owing to the high specific surface area and shortened path length of charge carriers.8,9 However, the photoresponsive range of this type of nanomaterials becomes narrow due to quantum size effect, leading to a blue-shifted optical absorption and thus lowered utilization of solar irradiation. Constructing

heterojunctions

is

an

effective

strategy

to

broaden

the

photoresponsive range of photocatalysts and meanwhile to facilitate charge separation and charge transfer.10,11 To date, various semiconductor photocatalysts have been coupled with g-C3N4 to form heterojunctions, such as metal oxides,12-16 organic semiconductors17,18 and metal chalcogenides.19 These heterostructures showed high efficiency in photocatalytic water treatment, H2 evolution and CO2 reduction. Especially, 2D/2D heterojunctions possess large interfacial area between the two types of nanosheet due to face-to-face contact,20,21 leading to effective separation and low recombination rate of photogenerated electron-hole pairs.22 As one type of special heterojunction, Z-scheme configurations have been developed by combining two 3



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semiconductor materials that have appropriate band structures to improve the charge separation efficiency and the redox ability upon photo-excitation. To date, a large number of g-C3N4 based Z-scheme photocatalysts have been designed and prepared, including

g-C3N4/TiO2,25,26

g-C3N4/AgCrO3,27

g-C3N4/MoS2,28

g-C3N4/ZnO,29

g-C3N4/BiWO3.30 However, to search and construct new types of 2D/2D Z-scheme heterojunctions to further improve photocatalytic performance is still an ongoing and challenging task. Herein, we report a 2D/2D nanocomposite built by in-situ growing MnO2 nanosheets on the surface of exfoliated g-C3N4 nanosheets using a simple solution deposition method. A range of techniques including AFM, XRD, SEM/TEM, XPS, nitrogen sorption, EPR and Mott-Schottky were employed to fully characterize the obtained materials. We found that the g-C3N4/MnO2 nanocomposite showed significantly enhanced photocatalytic activities for dye degradation and phenol removal, in comparison to the individual g-C3N4 or MnO2 component. Based on the analysis of EPR spectra, XPS spectra and the Mott-Schottky plotting, we proposed a direct Z-scheme heterojunction generated between the g-C3N4 nanosheets and the MnO2 nanosheets, wherein the photo-induced electrons in MnO2 combined with the holes in g-C3N4, leading to improved charge carrier extraction and utilization upon photoexcitation and hence increased efficiency for photocatalytic reactions.

EXPERIMENTAL SECTION 4


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Preparation of g-C3N4 nanosheets. All the reagents used in this work were purchased without any further purification. The bulk g-C3N4 was fabricated according to the previously reported method.9,31 Specifically, 10 g of urea was placed in an crucible with cover, then the crucible was put in a muffle furnace and heated to 550 ℃ for 3 h. After naturally cooling to room temperature, the light-yellow g-C3N4 powder was carefully collected. Afterwards, 1g of the as-obtained g-C3N4 was added into 50 ml of the mixed solution of concentrated nitric acid and hydrogen peroxide (1:1 volume ratio), then keeping the mixture stirring for 24 h. After that, the above mixture was neutralized by 1 M KOH. Then it was centrifuged under 3000 r/min, with washing for many times by distilled water and ethanol. The mother liquor was retained and then centrifuged again under 5000 r/min for 30 min. The precipitates were collected and dried under the vacuum freeze drying, thus obtaining g-C3N4 nanosheets. Preparation of g-C3N4/MnO2 nanocomposite. 0.2 g of the g-C3N4 nanosheets were dispersed into 30 ml of deionized water under sonication for 20 min. Then 0.25 mmol of MnCl2·4H2O was slowly added and dissolved into the suspension under stirring. The pH of the suspension was adjusted to 8.5 by tetramethylammonium hydroxide (TMA•OH) after 1 h. Subsequently, 0.55 ml of H2O2 (30 v%) was dropwise added into the mixed suspension under rapid stirring and keeping it for 30 min. After that, the suspension was centrifuged and washed for several cycles by distilled water and ethanol. Finally, the obtained precipitates were dried under vacuum freeze drying. Preparation of MnO2 nanosheets. MnO2 nanosheets were synthesized using the 5



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preparing method of g-C3N4/MnO2 nanocomposite but without addition of g-C3N4 nanosheets. Specifically, 0.25 mmol of MnCl2·4H2O was dissolved into 30 ml of deionized water. The pH of the solution was adjusted to 8.5 by tetramethylammonium hydroxide (TMA•OH) after 1 h. Subsequently, 0.55 ml of H2O2 (30 v%) was dropwise added into the mixed solution under rapid stirring. After 30 min, the suspension was centrifuged and washed for several cycles by distilled water and ethanol and finally dried by vacuum freeze-drying.

RESULTS AND DISCUSSION Figure 1a illustrates the preparation process of the 2D/2D nanocomposite of g-C3N4/MnO2. Briefly, g-C3N4 framework composed of nanosheets were obtained after exfoliation, having a large number of dangling bonds, hydroxyl groups and exposed lone pair electrons from N atoms.9,32 These defects and surface groups could provide the Mn2+ ions with many adsorption sites due to electrostatic attraction and coupling actions.20,33 The adsorbed Mn2+ cations on the surface of g-C3N4 nanosheets can be gradually oxidized to Mn4+ and finally generated MnO2 nanosheets upon the addition of TMA•OH and H2O2. Thus, 2D/2D g-C3N4/MnO2 nanocomposite was achieved with closely coupled interface. This result can be verified by the atomic force microscope (AFM) images (Figure 1b-d). The thickness of g-C3N4 nanosheets, MnO2 nanosheets and the g-C3N4/MnO2 hybrid was around 3.4, 2.2 and 5.7 nm, respectively. That is, the thickness of g-C3N4/MnO2 nanocomposite was substantially equivalent to the stacking distance of the g-C3N4 nanosheets and MnO2 nanosheets, 6


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suggesting that the MnO2 nanosheets had a tight connection to the surface of g-C3N4 nanosheets.

Figure 1. (a) The schematic illustration of g-C3N4/MnO2 nanocomposite synthesis by in-situ growth of MnO2 on the g-C3N4 nanosheets; The atomic force microscope (AFM) images of (b) g-C3N4 nanosheets, (c) MnO2 nanosheets and (d) g-C3N4/MnO2 nanocomposite.

Figure 2a shows the X-ray diffraction (XRD) patterns of MnO2 nanosheets, g-C3N4 nanosheets and the g-C3N4/MnO2 nanocomposite. The as-fabricated MnO2 nanosheets were confirmed to be the δ phase MnO2 (JCPDS NO.18-0802), revealing a typical layered material.34 The diffraction peaks centered at 12.1, 25.1, 36.7 and 65.1° were assigned to the (001), (002), (100) and (110) facets of δ-MnO2, respectively.34-36 The XRD pattern of the g-C3N4 nanosheets shows a strong peak at 27.1°, which was indexed as the (002) facet, corresponding to the stacking of the tri-s-triazine connected layers. 9,37,38

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Figure 2. (a) The X-ray diffraction (XRD) patterns of MnO2, g-C3N4 and g-C3N4/MnO2; (b) Scanning electron microscope (SEM) image, (c) Transmission electron microscope (TEM) image and (d) Energy disperse spectroscopy (EDS) spectrum of g-C3N4/MnO2 nanocomposite.

The morphology and microstructure of the g-C3N4/MnO2 nanocomposite were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The SEM image (Figure 2b) still exhibits layered morphology after loading MnO2 onto the g-C3N4 nanosheets, which can not only provide abundant adsorption sites for the dye molecules, but also greatly shorten the transport distance of photo-generated charge carriers, as will be shown below. The TEM images (Figure 2c) show a close integration between the MnO2 nanosheets and the g-C3N4 layers, 8


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implying the formation of heterojunction via the in-situ growth of MnO2 on the g-C3N4 nanosheets. Further, the lattice fringes reveal a lattice spacing of 0.731 nm, corresponding to (001) facet of MnO2 with δ phase35 Furthermore, the energy disperse spectroscopy (EDS) spectra (Figure 2d) and elemental mapping images (Figure S1) confirm that the nanocomposite was composed of Mn, O, N and C elements with homogeneous distribution (aluminum signal from the substrate). The surface groups of the as-prepared samples were investigated by Fourier transform infrared spectroscopy (FTIR) as shown in Figure 3a. As for the single g-C3N4 sample, the absorption peaks at 3100-3400 cm-1 were ascribed to the amino and adsorbed hydroxyl groups.39,9 The peaks at 1200-1600 cm-1 can be attributed to the various vibrations of the C-N bonds.40 The peak at 810 cm-1 was assigned to the characteristic breathing-vibration mode of the (tri-s-)triazine units in the g-C3N4 framework.41 The FTIR spectrum of MnO2 shows an absorption peak at 493 cm-1, corresponding to the stretching vibration of Mn-O bond.35 The g-C3N4/MnO2 hybrid shows a combined spectra of g-C3N4 and MnO2 with lowered transmittance, resulting from the generation of the nanocomposite.

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Figure 3. (a) The fourier transform infrared spectroscopy (FTIR) spectra, (b) The N2 adsorption-desorption isotherms and (c) pore size distribution, (d) The ultraviolet visible spectra (UV-vis) of g-C3N4, MnO2 and g-C3N4/MnO2 samples.

N2 adsorption-desorption isotherms were examined to study the surface area (SBET) and pore structure of the samples (Figure 3b and c). The hysteresis loops of the three samples (g-C3N4, MnO2 and g-C3N4/MnO2 nanocomposite) were all regarded as the H3 type, indicating the presence of slit-shaped pores aggregated from flake-like materials.42-44 The SBET of the sample g-C3N4, MnO2 and the g-C3N4/MnO2 hybrid was calculated to be 113, 58 and 91 m2/g, respectively (Table 1). Interestingly, the g-C3N4/MnO2 composite exhibited higher total pore volume (Vpore) than either g-C3N4 or MnO2. Besides, in comparison with individual g-C3N4 or MnO2, more large pores (20-100 nm) were generated for the nanocomposite. 10


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Table 1: Vpore represents Total Pore volume (cm3/g), Dpore represents Average Pore Width (nm), SBET is the specific surface area (m2/g), K is the degradation rate constant (min-1) of photocatalytic RhB degradation. Sample

SBET

K

K/SBET (*10-5)

Vpore

Dpore

g-C3N4

0.276

29.8

113

0.004

3.54

g-C3N4/MnO2

0.398

27.2

91

0.034

37.35

MnO2

0.147

15.3

58

0.007

12.07

The ultraviolet visible (UV–vis) spectra (Figure 3d) and the photoluminescence (PL) spectra were measured to investigate the optical properties. The absorption edge of g-C3N4 and MnO2 nanosheets was determined to be 370 nm and 580 nm, respectively. After loading MnO2 onto the g-C3N4 nanosheets, the obtained nanocomposite showed remarkably enhanced optical absorption extending to the visible-light region. This UV-vis absorption feature of the nanocomposite revealed that the MnO2 nanosheets were well coupled with the g-C3N4 nanosheets. Moreover, the PL spectra of the materials were recorded in Figure S2 (excitation wavelength 300 nm). The significantly decreased PL intensity of the g-C3N4/MnO2 nanocomposite with respect to the g-C3N4 implied the charge transfer between the two components upon photo-excitation, as will be discussed below.

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Figure 4. X-ray photoelectron spectroscopy (XPS) spectra of (a) Mn 2p, (b) O 1s, (c) N 1s, (d) C 1s for the g-C3N4, MnO2 and g-C3N4/MnO2 nanocomposite.

The surface chemical states of the materials were examined by the X-ray photoelectron spectroscopy (XPS) spectra (Figure 4 and Figure S3). All of the binding energies in the XPS spectra have been calibrated by the C 1s at 284.8 eV.45 The Mn 2p spectrum of the MnO2 nanosheets (Figure 4a, top) shows two main peaks located at 641.9 and 654.0 eV, corresponding to Mn 2P3/2 and Mn 2P1/2,35,36 while as for the spectra of the g-C3N4/MnO2 nanocomposite (Figure 4a, bottom), the binding energies shifted to 641.6 and 653.7 eV, respectively. The O1s spectrum of the MnO2 sample shows two fitted peaks at 529.5 eV and 531.4 eV, which can be attributed to the lattice

12


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oxygen (Mn–O) and surface adsorbed oxygen species (e.g., O–H), respectively.35,38 For the g-C3N4 sample, only the adsorbed oxygen species and other oxygen impurities were observed (Figure 4b, middle).46,47 With respect to the g-C3N4/MnO2 nanocomposite, a combined O1s spectrum was obtained (Figure 4b, bottom), with the binding energy of the lattice oxygen shifted to 529.3 eV. The N1s spectrum of the g-C3N4 sample shown in Figure 4c can be deconvoluted into three main peaks at 398.2, 399.7 and 401.2 eV, which can be assigned to the sp2-hybridized nitrogen (C=N–C), bridging nitrogen ((C)3–N) and N–H bonding in the carbon nitride framework, respectively.48,49 After combining with the MnO2 nanosheets, the N1s binding energy of the C=N–C bond showed a minor shift to higher value. These results might indicate the formation of heterojunction by the growth of MnO2 onto g-C3N4 nanosheets, with electron flowing from g-C3N4 to MnO2 and reaching thermodynamic equilibrium. The C1s spectra of the g-C3N4 sample and the g-C3N4/MnO2 hybrid were shown in Figure 4d. To further study the heterojunction formed between the g-C3N4 and MnO2 nanosheets, their work functions were calculated relative to the vacuum energy level. Figure 5a presents the optimized structure of g-C3N4/MnO2 composite. Figure 5b shows the calculated electrostatic potentials of MnO2 (001) and g-C3N4 (001). The work function of MnO2 (001) and g-C3N4 (001) was calculated to be 6.8 eV and 4.5 eV, respectively, indicating that the electrons in g-C3N4 will flow to the MnO2 through the heterojunction.16,50 Therefore, the g-C3N4 is more positively charged and accordingly the MnO2 is more negatively charged near the interface of the 13



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heterojunction. These results are illustrated by the charge density difference as shown in Figure 5c. The cyan and yellow region represent electron depletion and accumulation, respectively, displaying that the electrons migrate from g-C3N4 to the MnO2 when the heterojunction between the g-C3N4 and MnO2 forms. The analysis is in agreement with the XPS results as described above.

Figure 5. (a) Optimized structure model of the g-C3N4/MnO2 nanocomposite. The blue, gray, red and purple spheres are N, C, O and Mn atoms, respectively; (b) Calculated electrostatic potentials for g-C3N4 and MnO2 nanosheets, respectively; (c) Charge density difference model of the g-C3N4/MnO2 nanocomposite. The isosurface is 0.0004 eV Å–3.

In the next set of experiments, the photocatalytic activities were evaluated by photocatalytic degradation of rhodamine B (RhB) and phenol in aqueous solution under the illumination of Xenon lamp. An adsorption-desorption equilibrium between the photocatalysts and the molecules were reached in dark prior to irradiation. As

14


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shown in Figure 6a, the degradation rate of RhB was up to 91.3% after 60 min irradiation in the presence of the g-C3N4/MnO2 nanocomposite, while the degradation rate was only 19.6% and 22.3% for the g-C3N4 and MnO2 sample, respectively. Moreover, their decomposition kinetic behaviors obeyed the pseudo-first-order kinetics based on Langmuir-Hinshelwood model (Figure 6b).10,51 The apparent reaction rate constant (K) of the g-C3N4/MnO2 nanocomposite was 0.033 min-1, which was ~9 and ~5 times higher than that of g-C3N4 and MnO2, respectively. Importantly, the 2D/2D hybrid also showed significantly enhanced photocatalytic efficiency over phenol removal. The removal rate of phenol reached around 73.6% after 180 min illumination over the nanocomposite, while the removal rate was only 12.3% and 35.4% when using the g-C3N4 and MnO2 sample as the photocatalyst (Figure 6c). The corresponding apparent reaction rates were determined and displayed in Figure 6d, further demonstrating the high efficiency of the 2D/2D hybrid photocatalyst for potential wastewater treatment.

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Figure 6. The photocatalytic degradation rate of (a) RhB and (c) phenol over g-C3N4, MnO2 and g-C3N4/MnO2 nanocomposite. The pseudo-first-order kinetics fitted curves and corresponding apparent reaction rates of (b) RhB and (d) phenol over g-C3N4, MnO2 and g-C3N4/MnO2 nanocomposite under light irradiation.

To evaluate the durability of the g-C3N4/MnO2 heterostructured photocatalyst, the recycling photodegradation of RhB and phenol were carried out. The removal efficiency of RhB and phenol upon the g-C3N4/MnO2 nanocomposite did not show any decrease after fourth cycles of photodegradation, indicating the good durability of the as-prepared g-C3N4/MnO2 photocatalysts. Additionally, the XRD pattern of the g-C3N4/MnO2 heterostructured material was measured before and after the photocatalytic reaction. As shown in Figure S4, the diffraction peaks remained almost 16


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the same, suggesting the stability of the as-prepared materials during the photocatalytic reactions.

Figure 7. Recycling photodegradation of (a) RhB and (b) phenol upon the g-C3N4/MnO2 heterostructured photocatalyst.

In order to investigate the photogenerated charge transfer dynamics, transient photocurrents and electrochemical impedance spectra (EIS) were measured. The transient photocurrent response was recorded by repeatedly switching on and off the light illumination. As can be seen in Figure 8a, the photocurrent density of the g-C3N4/MnO2 nanocomposite was much higher than that of individual g-C3N4 and MnO2 component, affirming the enhanced photoactivity by combining g-C3N4 and MnO2 materials.52 Figure 8b shows the EIS spectra of the samples and the corresponding analog equivalent circuit (inset). Rctr, Rsr and CPE represent the charge-transfer resistance of the working electrode, electrolyte solution resistance (0.5M Na2SO4 in the present case) and the constant phase element, respectively. A smaller Rctr value reflects lower charge-transfer resistance and faster transfer rate.53 17



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After fitting the Nyquist plots, the Rctr value of g-C3N4, MnO2 and g-C3N4/MnO2 was calculated to be 1275, 936 and 651 Ω, respectively, suggesting that the recombination of photogenerated carriers can be significantly inhibited due to g-C3N4/MnO2 heterogeneous structure, and thus, leading to improved efficiencies for photocatalytic RhB degradation and phenol removal.

Figure 8. (a) The transient photocurrent response and (b) electrochemical impedance spectra (EIS) spectra of g-C3N4, MnO2 and g-C3N4/MnO2 nanocomposite.

Electronic paramagnetic resonance (EPR) analysis was carried out to determine the reactive species in the photocatalytic system. 5,5-dimethyl-l-pyrroline N-oxide (DMPO) was used to trap the hydroxyl radicals (•OH) and superoxide anion radicals (•O2−), producing the adducts of DMPO-•OH and DMPO-•O2− in aqueous solution and methanol, respectively.54,55 Figure 9a shows the EPR signals of DMPO-•OH adduct in the presence of the samples after irradiation for 120s. The strongest DMPO-•OH signal was observed in the presence of the g-C3N4/MnO2 nanocomposite, while for the MnO2 the signal was much weaker and for g-C3N4 only negligible EPR 18


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signals can be seen. These results clearly demonstrate the enhanced activity of the g-C3N4/MnO2 nanocomposite to produce •OH radicals than the individual component. Similar results were obtained in the •O2− trapping experiments (Figure 9b), except in this case that the g-C3N4 gave rise to relatively strong EPR signal while much weaker signals were recorded for MnO2, suggesting that •O2− radicals were preferably generated for g-C3N4 upon photoexcitation.

Figure 9. EPR spectra of (a) DMPO-•OH with irradiation for 120 s in aqueous dispersion and (b) DMPO-•O2− with irradiation for 120 s in methanol dispersion in the presence of g-C3N4, MnO2 and g-C3N4/MnO2 nanocomposite.

Mott-Schottky analysis and XPS valence band spectroscopy were carried out to investigate the band structures of the g-C3N4/MnO2 hybrid. The positive slopes of the Mott-Schottky plots indicate the n-type semiconducting feature of both g-C3N4 and MnO2, as shown in Figure 9a and b. Besides, the flat-band potential for n-type semiconductors can be approximately regarded as the conduction band.56-58 Therefore, the conduction band of g-C3N4 and MnO2 was determined to be -1.61 V and 1.22 V, 19



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respectively (vs. NHE, pH 7), according to the intercepts with the X-axis (Figure 10a,b). The valence band positions of the materials can be determined by XPS valence band spectroscopy (Figure 10c), which indicates that the valence band of g-C3N4 and MnO2 was 1.81 and 3.26 V, respectively. Based on these results as well as the EPR analysis (shown above), a Z-scheme charge transfer process has been proposed to understand the improved photocatalytic activity of the 2D/2D g-C3N4/MnO2 hybrid. With regard to the single g-C3N4 or MnO2 component, the photogenerated holes in g-C3N4 are not able to oxidize OH- to generate •OH radicals and the photogenerated electrons in MnO2 cannot effectively produce •O2− radicals, respectively, due to thermodynamic restriction (Figure 10d). Therefore, either single g-C3N4 or MnO2 material did not show optimized photocatalytic activities. After the formation of the 2D/2D heterojunction between g-C3N4 nanosheets and MnO2 nanosheets, nevertheless, the photo-induced electrons in the CB of MnO2 could transfer to the VB of g-C3N4 and combine with the holes there. Owing to this Z-scheme configuration, the extraction and utilization of the charge carriers (electrons from g-C3N4 and holes from MnO2) were significantly improved, leading to superior photocatalytic efficiency of the g-C3N4/MnO2 nanocomposite for dye/phenol degradation.

20


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Figure 10. Mott–Schottky plots: (a) g-C3N4 nanosheets, (b) MnO2 nanosheets. (c) XPS valence band spectra of MnO2 and g-C3N4 nanosheets. (d) Band structure and photocatalytic mechanism of the g-C3N4/MnO2 nanocomposite.

CONCLUSIONS In summary, a 2D/2D g-C3N4/MnO2 heterostructured photocatalyst was prepared via a simple wet-chemical method to grow MnO2 nanosheets onto the surface of g-C3N4 nanolayers. The nanocomposite was characterized by a range of techniques to study its micromorphology, structure, and chemical composition/states. The g-C3N4/MnO2 nanocomposite illustrated significantly improved photocatalytic activities for dye degradation and phenol removal, in comparison to the single g-C3N4 or MnO2 component. Based on the EPR and XPS spectra as well as the Mott-Schottky 21



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measurements, we suppose that a Z-scheme heterojunction was formed between the g-C3N4 nanosheets and the MnO2 nanosheets, thereby the photo-induced electrons in MnO2 combining with the holes in g-C3N4, resulting in enhanced charge carrier extraction and utilization upon photoexcitation. This work provides an effective approach to construct 2D/2D heterostructured nanomaterials for high-efficiency light harvesting.

ASSOCIATED CONTENT Supporting

Information

Characterization

details,

DFT

calculations,

(photo)electrochemical measurements, photocatalytic tests, EPR measurement and analysis, elemental mapping images, PL spectra, survey XPS spectra, XRD patterns. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The financial supports on this work are provided by 973 program (2013CB632402), 22


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NSFC (51372190, 51472191, 51320105001 and 21433007). Additional support is provided by the Innovative Research Funds of SKLWUT (2017-ZD-4), and the Natural Science Foundation of Hubei Province of China (2015CFA001). J.X. thanks the financial support from Australian Research Council for financial support (DE160101488).

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Table of Content

Constructing a direct Z-scheme photocatalyst of 2D/2D g-C3N4/MnO2 nanocomposite provides a sustainable pathway for water treatment upon solar irradiation.

33


MnO2 Nanocomposite as a Direct Z-Scheme

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