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ZnO nanorods arrays for enhanced photocatalytic hydrogen performance
Accepted Manuscript Title: Light-assisted preparation of heterostructured g-C3 N4 /ZnO nanorods arrays for enhanced photocatalytic hydrogen performance Authors: Jing Liu, Xiao-Ting Yan, Xu-Sen Qin, Si-Jia Wu, Heng Zhao, Wen-Bei Yu, Li-Hua Chen, Yu Li, Bao-Lian Su PII: DOI: Reference:
S0920-5861(19)30068-9 https://doi.org/10.1016/j.cattod.2019.02.028 CATTOD 11980
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
31 October 2018 7 February 2019 13 February 2019
Please cite this article as: Liu J, Yan X-Ting, Qin X-Sen, Si-Jia W, Zhao H, WenBei Y, Chen L-Hua, Li Y, Su B-Lian, Light-assisted preparation of heterostructured g-C3 N4 /ZnO nanorods arrays for enhanced photocatalytic hydrogen performance, Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.02.028 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.
Light-assisted nanorods
preparation
arrays
for
of
heterostructured
enhanced
photocatalytic
g-C3N4/ZnO hydrogen
performance
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Jing Liu1, †, Xiao-Ting Yan1,†, Xu-Sen Qin1, Si-Jia Wu1, Heng Zhao1, Wen-Bei Yu1, Li-Hua
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Chen1,2, Yu Li1,2* and Bao-Lian Su1,3,4
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University
of Technology, 122 Luoshi Road, 430070 Wuhan, Hubei, China.
Nanostructure Research Centre (NRC), Wuhan University of Technology, 122 Luoshi Road, 430070 Wuhan,
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2
Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, 61 rue de Bruxelles, B-5000
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3
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Hubei, China.
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Namur, Belgium.
Clare Hall, University of Cambridge, Herschel Road, Cambridge CB3 9AL, United Kingdom.
*
Corresponding author. Email: [email protected].
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† These authors contributed equally to this work.
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Graphical abstract
The type-II heterostructured g-C3N4/ZnO NRAs films are synthesized via a light-assisted
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Highlights
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method
A dense interface connection between g-C3N4 and ZnO is achieved
The heterostructured g-C3N4/ZnO NRAs films exhibit enhanced photocatalytic H2
The g-C3N4 layer modification can inhibit ZnO photocorrosion
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production
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Abstract:
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The type II heterostructured g-C3N4/ZnO nanorods arrays (NRAs) with intimate interface
connection were successfully synthesized via a light-assisted method for photocatalytic H2 production. Such type-II heterojunction between g-C3N4 and ZnO not only promotes the charge separation and transport but also extends light absorption to the visible light region. In addition, the high physicochemical stability of g-C3N4 layer keeps ZnO from photocorrosion during the 2
reaction. Consequently, the heterostructured g-C3N4/ZnO NRAs demonstrate enhanced photocatalytic H2 performance. In particular, the highest photocatalytic activity enhancement of g-C3N4/ZnO heterostructure is 3.3 times that of ZnO NRAs with a good stability of 85% retention rate after 5 cycles. Our work here presents an effective strategy to construct
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heterojunction between g-C3N4 and metal oxide semiconductors for significantly enhanced
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photocatalytic H2 production.
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Keywords: heterostructured g-C3N4/ZnO; light-assisted; type II heterojunction; H2 production
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1. Introduction Photocatalytic strategy has been considered as one of the most promising techniques to solve global energy crisis and environment problems due to its unique advantages, such as energy-saving and environmentally friendliness [1, 2]. Among the various semiconductors for
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photocatalysis, zinc oxide (ZnO) has been attracted tremendous attentions in environmental and energy-related applications owing to its various nanostructures, large exciton, binding energy
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(60 meV), high mobility of conduction electrons (200 cm2/(Vs)) and good chemical and thermal
stability [3-5]. Particularly, ZnO nanorod arrays (NRAs) owing to its high aspect ratio, short
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diffusion length have been widely used in photocatalytic or photoelectrochemical water
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splitting [6-10]. In addition, the highly ordered architecture allows light absorption in the axial
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direction and carrier separation in the radial direction, providing the reduction of carrier
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recombination before charge separation and direct pathways for efficient charge carrier
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transport [1, 4, 8]. However, two major limitations hinder the application of ZnO NRAs: (1) large bandgap (3.2 eV) restricting its utilization in the visible light region of the solar spectrum
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and (2) high recombination rate of photogenerated electron-hole pairs and inevitable
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photocorrosion [11-13]. Accordingly, much efforts have been conducted to explore the effective strategies to address the problems mentioned above. Constructing heterojunction structures using the semiconductors with suitable energy band
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gaps are quite promising due to their further advantages and wider range of potential applications [1, 14-16] This strategy endows the heterostructures exhibiting broadband light response and high hydrogen evolution rates, through prolonging the lifetime of photogenerated electrons from one semiconductor to the other. So far, various ZnO heterojuncted 4
nanostructures have been designed and investigated such as ZnO/CdS, ZnO/TiO2, ZnO/ BiVO4, Cu2O/ZnO, ZnO/g-C3N4, CdS/Au/ZnO [6, 7, 10, 17-22] to improve the photocatalytic performances in a certain extent. Among these semiconductors, graphitic carbon nitride (gC3N4), a typical metal-free conjugated polymeric semiconductor, catches much attention by its
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appealing electronic band structure, high physicochemical stability and earth-abundant nature [23-26]. The sp2-hybridized carbon and nitrogen in g-C3N4 establish the 2D morphology and
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π-conjugated electronic structures, making it a promising candidate for photocatalysis-based applications [25, 27]. Indeed, just simply mixing the exfoliated g-C3N4 dispersion with ZnO
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nanostructures, the formed g-C3N4/ZnO heterostructures have already demonstrated enhanced
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photocatalysis to a certain degree [28-33]. For example, Wu et.al reported the hierarchically
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porous ZnO/graphitic g-C3N4 microspheres with type-II heterojunction and demonstrated the
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highest the highest photocatalytic activity with good stability and higher photocatalytic
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degradation rate comparing to pure g-C3N4 and pure ZnO [33]. As we all know, the good interconnection between two semiconductors with well-developed structures is the key
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influence for photocatalytic activity enhancement because it can guarantee the photogenerated
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charges transfer smoothly across the interface to achieve the desired performance [29]. Recently, several heterostructured g-C3N4/ZnO have been constructed and exhibited enhanced photoelectrocatalytic activity for methylene blue (MB) decolorization under visible light
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illumination by a thermal condensation of melamine process [7, 8]. Previous works have demonstrated that the wettability of the ZnO NRAs films can be tunable via solar light irradiation which can highly affect the behavior of liquids on the surface including the contact angle, liquid mobility and effective area of the solid-liquid interface [5]. Although many works 5
have been done on the investigation of the g-C3N4/ZnO heterostructures, most researches neglect the wettability of the ZnO NRAs films even though it would affect the interface connection between ZnO based heterojunctions with other semiconductors [5, 34, 35]. Thus, it is still a big challenge to synthesize the heterostructured g-C3N4/ZnO NRAs with good interface
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connection for enhanced photocatalysis. We herein for the first time, synthesize the heterojuncted g-C3N4/ZnO NRAs films via a
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light-assisted method by tuning the wettability of the ZnO nanorods arrays under the light irradiation [5]. Our results show that the obtained heterojuncted g-C3N4/ZnO NRAs films
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exhibit the broad light response range to visible region and benefit to the effective separation
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of photogenerated electron-holes pairs and fast interfacial charge transfer because of the type-
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II heterojunction system. As a result, the obtained heterojunction g-C3N4/ZnO NRAs films
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show the high photocatalytic hydrogen production and stability comparing with the pure ZnO
2.1 Materials
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2. Experimental section
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NRAs film.
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Zinc acetate dihydrate (Zn(CH3COO)22H2O), Ethanolamine (C2H7NO),
Melamine,
Ethylene glycol, Sodium sulfite(Na2SO3), Sodium sulfidenonahydrate (Na2S∙9H2O), Zinc
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nitrate hexahydrate (Zn(NO3)2·6H2O) and Sodium hydroxide (NaOH) were purchased from Aldrich. All the chemical reagents were analytically grade and used directly without further purification. 2.2 Preparation of heterojunction g-C3N4/ZnO NRAs films The heterojunction g-C3N4/ZnO NRAs films were prepared via light-assisted method. 6
Typically, ZnO NRAs were grown vertically on the clean fluorine tin oxide (FTO) by seedmediated growth in a solutions containing NaOH (0.4mol/L) and Zn(NO3)2 (0.01mol/L) at the reaction temperature of 90 C for 180min [9, 36]. After washed with deionized water and ethanol for several times, the obtained ZnO NRAs were irradiated under ultraviolet visible light
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for 30 min and then immersed in the melamine solutions with different concentrations (0.1mg/L, 0.2mg/L, 0.3mg/L) using ethylene glycol as solvent which were also ultrasonic dispersed at the
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ambient temperature for 20min. After the penetration of melamine, further thermal
condensation treatment was performed at 500 C for 2 h in Ar atmosphere with a ramp rate of
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5 C/min and then cooled down to room temperature to form g-C3N4. The heterojunction g-
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C3N4/ZnO NRAs films obtained with different melamine solutions concentrations (0.1mg/L,
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0.2mg/L, 0.3mg/L) are designated as g-C3N4/ZnO-1, g-C3N4/ZnO-2 and g-C3N4/ZnO-3,
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respectively.
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2.3 Characterization
The crystalline phases of the samples were examined by X-ray diffraction (XRD) with a
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Bruker D8 Advanced diffractometer using Cu Kα radiation (λ=1.54Å) in the 2θ range of 5-80º.
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Field emission scanning electron microscopy (FESEM) was performed on a Hitachi S-4800 electron microscope to character the sample morphology. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on a
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JEOL JEM-2100F microscope with an acceleration voltage of 200 kV. The electronic absorbance band of g-C3N4/ZnO NRAs was recorded with a SHIMADZU UV2550 UV-vis spectrometer. Static contact angle (CA) measurements were performed using an OCA35 automatic contact angle instrument with droplets in 3 mL volume ethylene glycol. The surface 7
chemical composition and valence states of the products were studied using X-ray photoelectron spectroscope (XPS, Thermofisher) with monochromatic Al Kα radiation. The photocurrent measurements were performed on an electrochemical workstation (CHI 660D, Chenhua Instrument Co., China) in a standard three-electrode electrochemical cell with a Pt
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plate counter electrode and an Ag/AgCl reference electrode in our experiments. The electrolyte solution was performed using 0.1M Na2SO4 and the working electrodes with the exposed area
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of 0.5 × 1 cm2 were illuminated from the front side with solar light sources. All the heterostructured g-C3N4/ZnO NRAs photoanodes were measured at 0.5 V external potential vs.
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hydrogen electrode (RHE).
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2.4. Photocatalytic hydrogen production
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The photocatalytic reactions were carried out in a closed circulation system (Beijing
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Perfectlight Science & Technology Co., LTD) using a PLS-SXE-300C lamp with an UV light
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intensity of 34 mW/cm2 and visible-light intensity of 158 mW/cm2. Typically, one piece of the obtained g-C3N4/ZnO NRAs film coated on FTO substrate (geometrical area of each piece is
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1.5cm×2cm) was placed in the reaction cell with 100mL of an aqueous solution containing
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0.1M Na2S and 0.1 M Na2SO3 as the sacrificial agents. The gas products were analyzed periodically by an Agilent 7890 A gas chromatograph (GC) with a thermal conductivity detector (TCD). While proceeding the recycle process, the g-C3N4/ZnO NRAs film was washed by the
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deionized water and placed again in the reaction cell with 100mL of new sacrificial agents and repeat the same testing process.
3. Results and discussion Fig. 1a illustrates the synthesis process for the hetreostructured g-C3N4/ZnO NRAs films. 8
As the only difference for these heterojuncted g-C3N4/ZnO NRAs films is the decoration amount of g-C3N4, the SEM, TEM and XRD characterizations of g-C3N4/ZnO-2 are selected to demonstrate the quality of the heterojuncted g-C3N4/ZnO NRAs. As the wettability of the ZnO NRAs films would affect the interface connection between ZnO and g-C3N4, the morphology
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and microstructure of the obtained hetreostructured g-C3N4/ZnO NRAs before and after UVvisible light irradiation were characterized by SEM. Fig. 1b-c shows the morphology of
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obtained hetreostructured g-C3N4/ZnO NRAs before UV-visible light irradiation. The SEM observation shows that the surface of the ZnO NRAs films is inhomogeneously covered by the
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g-C3N4 nanoparticles without UV-visible light irradiation. The contact angle (CA) of the ZnO
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NRAs film (inset in Fig.1b) shows a contact angle of ~65, indicating the hydrophobicity of the
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ZnO NRAs surface. After UV-visible light irradiation for 0.5 h, the spherical ethylene glycol
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droplet spreads out with a contact angle of at ~8 (inset in Fig.1d). This means that the ZnO
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NRAs film can be tuned from hydrophobicity to hydrophilicity by UV-visible light irradiation. This is because the UV-visible light irradiation can generate electron-hole pairs in the ZnO
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surface in air, and some of the holes react with lattice oxygen to form surface oxygen vacancies
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[5, 34, 35]. As the wettability of the ZnO NRAs film has potential effect on the interface contact between g-C3N4 and ZnO, we also conducted the SEM observation on the obtained hetreostructured g-C3N4/ZnO NRAs after UV-visible light irradiation (Fig1d-e). The SEM
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images show that the g-C3N4 are uniformly and densely deposited on ZnO NRAs after the thermal condensation of melamine. In particular, the g-C3N4/ZnO NRAs remain the original shape. Fig S1 displays the morphology of ZnO NRAs with smooth surface and average diameter around ~50 nm. From the morphology comparison between heterostructured g-C3N4/ZnO 9
NRAs films before and after the UV-visible light irradiation, one can see that the hydrophilic ZnO NRAs is preferable for the formation of the homogeneously densely contacted core-shell
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heterostructured g-C3N4/ZnO NRAs films.
Fig. 1. (a) Schematic illustration of the preparation of g-C3N4/ZnO NRAs films. FESEM images of the asprepared heterostructured g-C3N4/ZnO-2 NRAs films (b-c) before and (d-e) after UV-visible irradiation (inset
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Photographs of ethylene glycol droplet shape on ZnO NRAs films before and after UV-visible irradiation for 0.5 h, respectively), and (f-g) TEM images of the obtained heterostructured g-C3N4/ZnO-2 films after UV-
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visible irradiation.
Fig.1f-g show the TEM images of the heterostructured g-C3N4/ZnO-2. The average
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diameter of the nanorods is around ~70nm with rough surface of g-C3N4 layer depositing on the surface of ZnO NRAs and the thickness of the g-C3N4 layer is ~7nm. Figure S2 presents the location of g-C3N4 by EDX Mapping, which shows that the C and Zn elements are distributed uniformly. Both the SEM and TEM results demonstrate that the heterostructured g-C3N4/ZnO NRAs are successfully obtained via the light-assisted method. 10
Fig. 2 presents the XRD patterns collected from ZnO, g-C3N4 and heterostructured gC3N4/ZnO NRAs films. The result shows that the heterostructured g-C3N4/ZnO NRAs film grown on FTO is a hexagonal wurtzite structure (JCPDS No. 079-0206) with representative three strong diffraction peaks, corresponding to (100), (002) and (101) respectively. The
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diffraction peaks observed at 37.76º and 51.75º belong to FTO substrate. The characteristic diffraction peak at 27.5º is ascribed to the characteristic inter planar staking peak (002) of
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aromatic system [25]. The g-C3N4 sample has two characteristic peaks at 13.0 º and 27.5 º, associating with the interlayer stacking of poly-aromatic rings for graphitic materials and the
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interlayer distance on the g-C3N4 [7, 27]. The XRD patterns of g-C3N4/ZnO NRAs show that
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the g-C3N4 decoration cannot influence the phase of ZnO. This indicates the successful
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synthesis of the g-C3N4/ZnO NRAs heterostructure.
Fig. 2. XRD patterns of the as-prepared pure ZnO, g-C3N4 and heterostructured g-C3N4/ZnO NRAs films.
UV-Vis absorption measurement was performed to reveal the electronic structures and the optical absorption properties of the pure ZnO NRAs and g-C3N4/ZnO NRAs samples (Fig. 3a). 11
For the pure ZnO NRAs, the strong absorption at wavelength below 400 nm matches the intrinsic inter-band transition absorption of hexagonal ZnO, showing its UV absorption character. After the deposition of g-C3N4 nanoparticles, the absorption edges of the heterostructured g-C3N4/ZnO NRAs red-shift and the absorption edge drastically extends to
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~405, ~480 and ~440 nm for g-C3N4/ZnO-1, g-C3N4/ZnO-2 and g-C3N4/ZnO-3, respectively. These results imply that the heterostructured g-C3N4/ZnO NRAs films can lead to the
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significant photocatalytic enhancement due to the broadened light utilization region and the
effectively improved photogenerated electron-hole separation [37]. In particular, the g-
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C3N4/ZnO-2 displays the broadest light absorption and highest light utilization, indicating the
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highest photocatalytic activity.
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To investigate the mechanism of the heterostructured g-C3N4/ZnO NRAs, the
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photoelectrochemical measurements were conducted as shown in Fig. 3b-c. The Fermi level of
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g-C3N4/ZnO is then investigated by Mott-Schottky (MS) plots measured at 500 Hz and 1000 Hz to determine the conduction band (CB) and valence band (VB) positions of the obtained
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composites. The calculated flat band potentials (Vfb) for ZnO and g-C3N4 are -0.9 V and -1.5 V
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respectively (the insets in Fig. 3b-c), which means that the Fermi level of g-C3N4 is lower than that of ZnO. The valence band spectra of XPS were carried out to determine the distance from VB maximum to Fermi level and the VB maximum of ZnO and g-C3N4 are 2.6 eV and 2.4 eV,
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respectively (Fig.3b and c). According to the VB-XPS and Mott-Schottky plots shown in Fig. 3, the CB and VB positions of ZnO (-0.8eV and 2.4eV versus the normal hydrogen electrode) and g-C3N4 (-1.2eV and 1.5eV versus the normal hydrogen electrode) are accurately determined as illustrated in Fig. 3d. Accordingly, in the heterostructured g-C3N4/ZnO NRAs system, the 12
photogenerated electrons can easily transfer from CB of g-C3N4 into CB of ZnO owing to the more negative CB position of CdS and the photogenerated holes can easily transfer from VB of ZnO into VB of g-C3N4 owing to the more positive VB position of ZnO. This typical typeII heterojunction system is beneficial for effective separation of photogenerated electron-holes
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pairs and faster interfacial charge transfer as illustrated in Fig 1a.
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Fig. 3. (a) UV-Vis absorption spectra of the heterostructured g-C3N4/ZnO NRAs films with different amount of g-C3N4, valence band XPS (VB-XPS) spectra of (b) ZnO and (c) g-C3N4 (The inset shows the corresponding Mott-Schottky plots in 0.5 m Na2SO4 aqueous solution) and (d) schematic energy-level diagrams of ZnO and g-C3N4 in comparison with the potentials for water reduction and oxidation.
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To further demonstrate the photogenerated charge transfer property of the heterostructured
g-C3N4/ZnO NRAs films and their photocatalytic activities, representative curves of chopped light-current density versus time (I-t) characterizations under UV-visible light were carried out as presented in Fig. 4. It shows that the current densities in the dark can sharply increase from 13
several µA to mA scale in the presence of light irradiation, confirming that all the g-C3N4/ZnO NRAs photoanodes exhibit excellent sensitivity to light irradiation. In addition, the photocurrent densities of the heterostructured g-C3N4/ZnO NRAs films are higher than the pure ZnO-NRAs due to the effective separation of photogenerated electron-holes pairs in such a
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typical type-II heterojunction system. In particular, the photocurrent density of g-C3N4/ZnO-2 reaches 0.76 mA, much higher than those of g-C3N4/ZnO-1 at 0.50 mA and g-C3N4/ZnO-3 at
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0.28 mA, indicating that the g-C3N4/ZnO-2 will exhbit the best performance for photocatalytic
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H2 production.
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Fig.4. Photocurrent response of the ZnO NRAs and heterostructured g-C3N4/ZnO NRAs photoanodes at an external potential of 0.5V vs. RHE with repeated on/off circles of UV-visible light illumination.
The photocatalytic H2 production on the heterostructured g-C3N4/ZnO NRAs under UV-
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visible light irradiation was performed using 0.1M Na2S and Na2SO3 as hole-scavenger. As shown in Fig. 5a, all the heterostructured g-C3N4/ZnO NRAs samples show significant enhanced photocatalytic H2 production. The hydrogen amount is 16, 33 and 23 μmol for gC3N4/ZnO-1, g-C3N4/ZnO-2 and g-C3N4/ZnO-3, respectively. However, the hydrogen amount of pure ZnO NRAs is only at ~10 μmol. The photocatalytic H2 production for g-C3N4/ZnO-2 14
shows the highest photocatalytic activity, which is 3.3 times comparing with the pure ZnONRAs. On the one hand, the heterostructured g-C3N4/ZnO NRAs can extend the light absorption from UV region to the visible light region, resulting in a significantly enhanced photocatalytic H2 production. On the other hand, the type II heterojunction formed between g-
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C3N4 and ZnO can effectively separate the photogenerated electron-holes pairs thus leading to the higher photocatalytic hydrogen production and stability. However, the excessive decoration
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amount of g-C3N4 nanoparticles may cause the aggregation of g-C3N4. This not only reduces
the utilization of solar light (Fig.3a) but also decreases the photogenerated electron transport
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efficiency between g-C3N4 and ZnO because the charge carriers have to diffuse longer to reach
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the g-C3N4/ZnO interface, and finally leads to the decreased photocatalytic hydrogen
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production performance [8]. Therefore, g-C3N4/ZnO-2 exhibits the best photocatalytic
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performance with the suitable g-C3N4 content.
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Fig.5. Photocatalytic H2 production for heterostructured g-C3N4/ZnO NRAs with different amount of g-C3N4 layer and ZnO NRAs under UV-visible light irradiation (a); 5 successive photocatalytic activity cycles of gC3N4/ZnO-2 films for photocatalytic H2 production (b).
The stability and recyclability of the obtained heterostructured g-C3N4/ZnO NRAs films was also investigated. Fig. 5b shows the result of photocatalytic activity of g-C3N4/ZnO-2 for 15
5 cycles. The result demonstrates that the photocatalytic activity of g-C3N4/ZnO-2 only decreases from 33μmol to 28μmol with 85% retention rate after 5 cycles. The very good stability of g-C3N4/ZnO-2 indicates that high physicochemical stability of g-C3N4 layer can
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keep ZnO from the photocorrosion.
4. Conclusion
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In summary, heterostructured g-C3N4/ZnO NRAs films were successfully synthesized via
a light-assisted method to form a dense interface connection between g-C3N4 and ZnO. The XRD results reveal that the obtained heterostructured g-C3N4/ZnO NRAs is pure hexagonal
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wurtzite structure and typical g-C3N4. All the heterostructured g-C3N4/ZnO NRAs films exhibit
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higher photocatalytic hydrogen production than the pure ZnO NRAs under UV-visible light
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irradiation. In particular, the heterostructured g-C3N4/ZnO-2 demonstrates the highest photocatalytic activity with 3.3 times comparing to the pure ZnO-NRAs. The enhanced
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photocatalytic activity can be attributed to the effectively extended light absorption to the
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visible light region and the formation of the type-II heterojunction which can effectively separate the photogenerated electron-holes pairs and increase the interfacial charge transfer.
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Moreover, the g-C3N4/ZnO-2 shows very good stability with 85% retention rate after 5 cycles, indicating that high physicochemical stability of g-C3N4 layer can inhibit ZnO photocorrosion.
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Our work here provides an efficient pathway to construct g-C3N4 modified heterostructured photocatalysts with improved photocatalytic performance.
Acknowledgements Y. Li acknowledges Hubei Provincial Department of Education for the “Chutian Scholar” 16
program. B. L. Su acknowledges the Chinese Central Government for an “Expert of the State” position in the Program of the “Thousand Talents”. This work is supported by National Key R&D Program of China (2016YFA0202602), National Natural Science Foundation of China (U1663225, 21671155, 21805220), Hubei
Provincial Natural
Science Foundation
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(2018CFB242, 2018CFA054), Major programs of technical innovation in Hubei (2018AAA012), Program for Changjiang Scholars Innovative Research Team in University
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(IRT_15R52).
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S0920-5861(19)30068-9 https://doi.org/10.1016/j.cattod.2019.02.028 CATTOD 11980
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
31 October 2018 7 February 2019 13 February 2019
Please cite this article as: Liu J, Yan X-Ting, Qin X-Sen, Si-Jia W, Zhao H, WenBei Y, Chen L-Hua, Li Y, Su B-Lian, Light-assisted preparation of heterostructured g-C3 N4 /ZnO nanorods arrays for enhanced photocatalytic hydrogen performance, Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.02.028 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.
Light-assisted nanorods
preparation
arrays
for
of
heterostructured
enhanced
photocatalytic
g-C3N4/ZnO hydrogen
performance
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Jing Liu1, †, Xiao-Ting Yan1,†, Xu-Sen Qin1, Si-Jia Wu1, Heng Zhao1, Wen-Bei Yu1, Li-Hua
1
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Chen1,2, Yu Li1,2* and Bao-Lian Su1,3,4
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University
of Technology, 122 Luoshi Road, 430070 Wuhan, Hubei, China.
Nanostructure Research Centre (NRC), Wuhan University of Technology, 122 Luoshi Road, 430070 Wuhan,
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2
Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, 61 rue de Bruxelles, B-5000
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3
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Hubei, China.
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Namur, Belgium.
Clare Hall, University of Cambridge, Herschel Road, Cambridge CB3 9AL, United Kingdom.
*
Corresponding author. Email: [email protected].
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† These authors contributed equally to this work.
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Graphical abstract
The type-II heterostructured g-C3N4/ZnO NRAs films are synthesized via a light-assisted
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Highlights
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method
A dense interface connection between g-C3N4 and ZnO is achieved
The heterostructured g-C3N4/ZnO NRAs films exhibit enhanced photocatalytic H2
The g-C3N4 layer modification can inhibit ZnO photocorrosion
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production
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Abstract:
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The type II heterostructured g-C3N4/ZnO nanorods arrays (NRAs) with intimate interface
connection were successfully synthesized via a light-assisted method for photocatalytic H2 production. Such type-II heterojunction between g-C3N4 and ZnO not only promotes the charge separation and transport but also extends light absorption to the visible light region. In addition, the high physicochemical stability of g-C3N4 layer keeps ZnO from photocorrosion during the 2
reaction. Consequently, the heterostructured g-C3N4/ZnO NRAs demonstrate enhanced photocatalytic H2 performance. In particular, the highest photocatalytic activity enhancement of g-C3N4/ZnO heterostructure is 3.3 times that of ZnO NRAs with a good stability of 85% retention rate after 5 cycles. Our work here presents an effective strategy to construct
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heterojunction between g-C3N4 and metal oxide semiconductors for significantly enhanced
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photocatalytic H2 production.
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Keywords: heterostructured g-C3N4/ZnO; light-assisted; type II heterojunction; H2 production
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1. Introduction Photocatalytic strategy has been considered as one of the most promising techniques to solve global energy crisis and environment problems due to its unique advantages, such as energy-saving and environmentally friendliness [1, 2]. Among the various semiconductors for
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photocatalysis, zinc oxide (ZnO) has been attracted tremendous attentions in environmental and energy-related applications owing to its various nanostructures, large exciton, binding energy
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(60 meV), high mobility of conduction electrons (200 cm2/(Vs)) and good chemical and thermal
stability [3-5]. Particularly, ZnO nanorod arrays (NRAs) owing to its high aspect ratio, short
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diffusion length have been widely used in photocatalytic or photoelectrochemical water
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splitting [6-10]. In addition, the highly ordered architecture allows light absorption in the axial
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direction and carrier separation in the radial direction, providing the reduction of carrier
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recombination before charge separation and direct pathways for efficient charge carrier
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transport [1, 4, 8]. However, two major limitations hinder the application of ZnO NRAs: (1) large bandgap (3.2 eV) restricting its utilization in the visible light region of the solar spectrum
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and (2) high recombination rate of photogenerated electron-hole pairs and inevitable
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photocorrosion [11-13]. Accordingly, much efforts have been conducted to explore the effective strategies to address the problems mentioned above. Constructing heterojunction structures using the semiconductors with suitable energy band
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gaps are quite promising due to their further advantages and wider range of potential applications [1, 14-16] This strategy endows the heterostructures exhibiting broadband light response and high hydrogen evolution rates, through prolonging the lifetime of photogenerated electrons from one semiconductor to the other. So far, various ZnO heterojuncted 4
nanostructures have been designed and investigated such as ZnO/CdS, ZnO/TiO2, ZnO/ BiVO4, Cu2O/ZnO, ZnO/g-C3N4, CdS/Au/ZnO [6, 7, 10, 17-22] to improve the photocatalytic performances in a certain extent. Among these semiconductors, graphitic carbon nitride (gC3N4), a typical metal-free conjugated polymeric semiconductor, catches much attention by its
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appealing electronic band structure, high physicochemical stability and earth-abundant nature [23-26]. The sp2-hybridized carbon and nitrogen in g-C3N4 establish the 2D morphology and
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π-conjugated electronic structures, making it a promising candidate for photocatalysis-based applications [25, 27]. Indeed, just simply mixing the exfoliated g-C3N4 dispersion with ZnO
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nanostructures, the formed g-C3N4/ZnO heterostructures have already demonstrated enhanced
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photocatalysis to a certain degree [28-33]. For example, Wu et.al reported the hierarchically
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porous ZnO/graphitic g-C3N4 microspheres with type-II heterojunction and demonstrated the
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highest the highest photocatalytic activity with good stability and higher photocatalytic
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degradation rate comparing to pure g-C3N4 and pure ZnO [33]. As we all know, the good interconnection between two semiconductors with well-developed structures is the key
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influence for photocatalytic activity enhancement because it can guarantee the photogenerated
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charges transfer smoothly across the interface to achieve the desired performance [29]. Recently, several heterostructured g-C3N4/ZnO have been constructed and exhibited enhanced photoelectrocatalytic activity for methylene blue (MB) decolorization under visible light
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illumination by a thermal condensation of melamine process [7, 8]. Previous works have demonstrated that the wettability of the ZnO NRAs films can be tunable via solar light irradiation which can highly affect the behavior of liquids on the surface including the contact angle, liquid mobility and effective area of the solid-liquid interface [5]. Although many works 5
have been done on the investigation of the g-C3N4/ZnO heterostructures, most researches neglect the wettability of the ZnO NRAs films even though it would affect the interface connection between ZnO based heterojunctions with other semiconductors [5, 34, 35]. Thus, it is still a big challenge to synthesize the heterostructured g-C3N4/ZnO NRAs with good interface
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connection for enhanced photocatalysis. We herein for the first time, synthesize the heterojuncted g-C3N4/ZnO NRAs films via a
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light-assisted method by tuning the wettability of the ZnO nanorods arrays under the light irradiation [5]. Our results show that the obtained heterojuncted g-C3N4/ZnO NRAs films
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exhibit the broad light response range to visible region and benefit to the effective separation
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of photogenerated electron-holes pairs and fast interfacial charge transfer because of the type-
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II heterojunction system. As a result, the obtained heterojunction g-C3N4/ZnO NRAs films
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show the high photocatalytic hydrogen production and stability comparing with the pure ZnO
2.1 Materials
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2. Experimental section
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NRAs film.
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Zinc acetate dihydrate (Zn(CH3COO)22H2O), Ethanolamine (C2H7NO),
Melamine,
Ethylene glycol, Sodium sulfite(Na2SO3), Sodium sulfidenonahydrate (Na2S∙9H2O), Zinc
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nitrate hexahydrate (Zn(NO3)2·6H2O) and Sodium hydroxide (NaOH) were purchased from Aldrich. All the chemical reagents were analytically grade and used directly without further purification. 2.2 Preparation of heterojunction g-C3N4/ZnO NRAs films The heterojunction g-C3N4/ZnO NRAs films were prepared via light-assisted method. 6
Typically, ZnO NRAs were grown vertically on the clean fluorine tin oxide (FTO) by seedmediated growth in a solutions containing NaOH (0.4mol/L) and Zn(NO3)2 (0.01mol/L) at the reaction temperature of 90 C for 180min [9, 36]. After washed with deionized water and ethanol for several times, the obtained ZnO NRAs were irradiated under ultraviolet visible light
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for 30 min and then immersed in the melamine solutions with different concentrations (0.1mg/L, 0.2mg/L, 0.3mg/L) using ethylene glycol as solvent which were also ultrasonic dispersed at the
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ambient temperature for 20min. After the penetration of melamine, further thermal
condensation treatment was performed at 500 C for 2 h in Ar atmosphere with a ramp rate of
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5 C/min and then cooled down to room temperature to form g-C3N4. The heterojunction g-
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C3N4/ZnO NRAs films obtained with different melamine solutions concentrations (0.1mg/L,
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0.2mg/L, 0.3mg/L) are designated as g-C3N4/ZnO-1, g-C3N4/ZnO-2 and g-C3N4/ZnO-3,
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respectively.
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2.3 Characterization
The crystalline phases of the samples were examined by X-ray diffraction (XRD) with a
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Bruker D8 Advanced diffractometer using Cu Kα radiation (λ=1.54Å) in the 2θ range of 5-80º.
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Field emission scanning electron microscopy (FESEM) was performed on a Hitachi S-4800 electron microscope to character the sample morphology. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on a
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JEOL JEM-2100F microscope with an acceleration voltage of 200 kV. The electronic absorbance band of g-C3N4/ZnO NRAs was recorded with a SHIMADZU UV2550 UV-vis spectrometer. Static contact angle (CA) measurements were performed using an OCA35 automatic contact angle instrument with droplets in 3 mL volume ethylene glycol. The surface 7
chemical composition and valence states of the products were studied using X-ray photoelectron spectroscope (XPS, Thermofisher) with monochromatic Al Kα radiation. The photocurrent measurements were performed on an electrochemical workstation (CHI 660D, Chenhua Instrument Co., China) in a standard three-electrode electrochemical cell with a Pt
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plate counter electrode and an Ag/AgCl reference electrode in our experiments. The electrolyte solution was performed using 0.1M Na2SO4 and the working electrodes with the exposed area
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of 0.5 × 1 cm2 were illuminated from the front side with solar light sources. All the heterostructured g-C3N4/ZnO NRAs photoanodes were measured at 0.5 V external potential vs.
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hydrogen electrode (RHE).
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2.4. Photocatalytic hydrogen production
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The photocatalytic reactions were carried out in a closed circulation system (Beijing
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Perfectlight Science & Technology Co., LTD) using a PLS-SXE-300C lamp with an UV light
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intensity of 34 mW/cm2 and visible-light intensity of 158 mW/cm2. Typically, one piece of the obtained g-C3N4/ZnO NRAs film coated on FTO substrate (geometrical area of each piece is
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1.5cm×2cm) was placed in the reaction cell with 100mL of an aqueous solution containing
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0.1M Na2S and 0.1 M Na2SO3 as the sacrificial agents. The gas products were analyzed periodically by an Agilent 7890 A gas chromatograph (GC) with a thermal conductivity detector (TCD). While proceeding the recycle process, the g-C3N4/ZnO NRAs film was washed by the
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deionized water and placed again in the reaction cell with 100mL of new sacrificial agents and repeat the same testing process.
3. Results and discussion Fig. 1a illustrates the synthesis process for the hetreostructured g-C3N4/ZnO NRAs films. 8
As the only difference for these heterojuncted g-C3N4/ZnO NRAs films is the decoration amount of g-C3N4, the SEM, TEM and XRD characterizations of g-C3N4/ZnO-2 are selected to demonstrate the quality of the heterojuncted g-C3N4/ZnO NRAs. As the wettability of the ZnO NRAs films would affect the interface connection between ZnO and g-C3N4, the morphology
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and microstructure of the obtained hetreostructured g-C3N4/ZnO NRAs before and after UVvisible light irradiation were characterized by SEM. Fig. 1b-c shows the morphology of
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obtained hetreostructured g-C3N4/ZnO NRAs before UV-visible light irradiation. The SEM observation shows that the surface of the ZnO NRAs films is inhomogeneously covered by the
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g-C3N4 nanoparticles without UV-visible light irradiation. The contact angle (CA) of the ZnO
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NRAs film (inset in Fig.1b) shows a contact angle of ~65, indicating the hydrophobicity of the
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ZnO NRAs surface. After UV-visible light irradiation for 0.5 h, the spherical ethylene glycol
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droplet spreads out with a contact angle of at ~8 (inset in Fig.1d). This means that the ZnO
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NRAs film can be tuned from hydrophobicity to hydrophilicity by UV-visible light irradiation. This is because the UV-visible light irradiation can generate electron-hole pairs in the ZnO
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surface in air, and some of the holes react with lattice oxygen to form surface oxygen vacancies
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[5, 34, 35]. As the wettability of the ZnO NRAs film has potential effect on the interface contact between g-C3N4 and ZnO, we also conducted the SEM observation on the obtained hetreostructured g-C3N4/ZnO NRAs after UV-visible light irradiation (Fig1d-e). The SEM
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images show that the g-C3N4 are uniformly and densely deposited on ZnO NRAs after the thermal condensation of melamine. In particular, the g-C3N4/ZnO NRAs remain the original shape. Fig S1 displays the morphology of ZnO NRAs with smooth surface and average diameter around ~50 nm. From the morphology comparison between heterostructured g-C3N4/ZnO 9
NRAs films before and after the UV-visible light irradiation, one can see that the hydrophilic ZnO NRAs is preferable for the formation of the homogeneously densely contacted core-shell
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heterostructured g-C3N4/ZnO NRAs films.
Fig. 1. (a) Schematic illustration of the preparation of g-C3N4/ZnO NRAs films. FESEM images of the asprepared heterostructured g-C3N4/ZnO-2 NRAs films (b-c) before and (d-e) after UV-visible irradiation (inset
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Photographs of ethylene glycol droplet shape on ZnO NRAs films before and after UV-visible irradiation for 0.5 h, respectively), and (f-g) TEM images of the obtained heterostructured g-C3N4/ZnO-2 films after UV-
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visible irradiation.
Fig.1f-g show the TEM images of the heterostructured g-C3N4/ZnO-2. The average
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diameter of the nanorods is around ~70nm with rough surface of g-C3N4 layer depositing on the surface of ZnO NRAs and the thickness of the g-C3N4 layer is ~7nm. Figure S2 presents the location of g-C3N4 by EDX Mapping, which shows that the C and Zn elements are distributed uniformly. Both the SEM and TEM results demonstrate that the heterostructured g-C3N4/ZnO NRAs are successfully obtained via the light-assisted method. 10
Fig. 2 presents the XRD patterns collected from ZnO, g-C3N4 and heterostructured gC3N4/ZnO NRAs films. The result shows that the heterostructured g-C3N4/ZnO NRAs film grown on FTO is a hexagonal wurtzite structure (JCPDS No. 079-0206) with representative three strong diffraction peaks, corresponding to (100), (002) and (101) respectively. The
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diffraction peaks observed at 37.76º and 51.75º belong to FTO substrate. The characteristic diffraction peak at 27.5º is ascribed to the characteristic inter planar staking peak (002) of
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aromatic system [25]. The g-C3N4 sample has two characteristic peaks at 13.0 º and 27.5 º, associating with the interlayer stacking of poly-aromatic rings for graphitic materials and the
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interlayer distance on the g-C3N4 [7, 27]. The XRD patterns of g-C3N4/ZnO NRAs show that
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the g-C3N4 decoration cannot influence the phase of ZnO. This indicates the successful
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synthesis of the g-C3N4/ZnO NRAs heterostructure.
Fig. 2. XRD patterns of the as-prepared pure ZnO, g-C3N4 and heterostructured g-C3N4/ZnO NRAs films.
UV-Vis absorption measurement was performed to reveal the electronic structures and the optical absorption properties of the pure ZnO NRAs and g-C3N4/ZnO NRAs samples (Fig. 3a). 11
For the pure ZnO NRAs, the strong absorption at wavelength below 400 nm matches the intrinsic inter-band transition absorption of hexagonal ZnO, showing its UV absorption character. After the deposition of g-C3N4 nanoparticles, the absorption edges of the heterostructured g-C3N4/ZnO NRAs red-shift and the absorption edge drastically extends to
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~405, ~480 and ~440 nm for g-C3N4/ZnO-1, g-C3N4/ZnO-2 and g-C3N4/ZnO-3, respectively. These results imply that the heterostructured g-C3N4/ZnO NRAs films can lead to the
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significant photocatalytic enhancement due to the broadened light utilization region and the
effectively improved photogenerated electron-hole separation [37]. In particular, the g-
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C3N4/ZnO-2 displays the broadest light absorption and highest light utilization, indicating the
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highest photocatalytic activity.
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To investigate the mechanism of the heterostructured g-C3N4/ZnO NRAs, the
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photoelectrochemical measurements were conducted as shown in Fig. 3b-c. The Fermi level of
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g-C3N4/ZnO is then investigated by Mott-Schottky (MS) plots measured at 500 Hz and 1000 Hz to determine the conduction band (CB) and valence band (VB) positions of the obtained
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composites. The calculated flat band potentials (Vfb) for ZnO and g-C3N4 are -0.9 V and -1.5 V
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respectively (the insets in Fig. 3b-c), which means that the Fermi level of g-C3N4 is lower than that of ZnO. The valence band spectra of XPS were carried out to determine the distance from VB maximum to Fermi level and the VB maximum of ZnO and g-C3N4 are 2.6 eV and 2.4 eV,
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respectively (Fig.3b and c). According to the VB-XPS and Mott-Schottky plots shown in Fig. 3, the CB and VB positions of ZnO (-0.8eV and 2.4eV versus the normal hydrogen electrode) and g-C3N4 (-1.2eV and 1.5eV versus the normal hydrogen electrode) are accurately determined as illustrated in Fig. 3d. Accordingly, in the heterostructured g-C3N4/ZnO NRAs system, the 12
photogenerated electrons can easily transfer from CB of g-C3N4 into CB of ZnO owing to the more negative CB position of CdS and the photogenerated holes can easily transfer from VB of ZnO into VB of g-C3N4 owing to the more positive VB position of ZnO. This typical typeII heterojunction system is beneficial for effective separation of photogenerated electron-holes
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pairs and faster interfacial charge transfer as illustrated in Fig 1a.
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Fig. 3. (a) UV-Vis absorption spectra of the heterostructured g-C3N4/ZnO NRAs films with different amount of g-C3N4, valence band XPS (VB-XPS) spectra of (b) ZnO and (c) g-C3N4 (The inset shows the corresponding Mott-Schottky plots in 0.5 m Na2SO4 aqueous solution) and (d) schematic energy-level diagrams of ZnO and g-C3N4 in comparison with the potentials for water reduction and oxidation.
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To further demonstrate the photogenerated charge transfer property of the heterostructured
g-C3N4/ZnO NRAs films and their photocatalytic activities, representative curves of chopped light-current density versus time (I-t) characterizations under UV-visible light were carried out as presented in Fig. 4. It shows that the current densities in the dark can sharply increase from 13
several µA to mA scale in the presence of light irradiation, confirming that all the g-C3N4/ZnO NRAs photoanodes exhibit excellent sensitivity to light irradiation. In addition, the photocurrent densities of the heterostructured g-C3N4/ZnO NRAs films are higher than the pure ZnO-NRAs due to the effective separation of photogenerated electron-holes pairs in such a
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typical type-II heterojunction system. In particular, the photocurrent density of g-C3N4/ZnO-2 reaches 0.76 mA, much higher than those of g-C3N4/ZnO-1 at 0.50 mA and g-C3N4/ZnO-3 at
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0.28 mA, indicating that the g-C3N4/ZnO-2 will exhbit the best performance for photocatalytic
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H2 production.
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Fig.4. Photocurrent response of the ZnO NRAs and heterostructured g-C3N4/ZnO NRAs photoanodes at an external potential of 0.5V vs. RHE with repeated on/off circles of UV-visible light illumination.
The photocatalytic H2 production on the heterostructured g-C3N4/ZnO NRAs under UV-
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visible light irradiation was performed using 0.1M Na2S and Na2SO3 as hole-scavenger. As shown in Fig. 5a, all the heterostructured g-C3N4/ZnO NRAs samples show significant enhanced photocatalytic H2 production. The hydrogen amount is 16, 33 and 23 μmol for gC3N4/ZnO-1, g-C3N4/ZnO-2 and g-C3N4/ZnO-3, respectively. However, the hydrogen amount of pure ZnO NRAs is only at ~10 μmol. The photocatalytic H2 production for g-C3N4/ZnO-2 14
shows the highest photocatalytic activity, which is 3.3 times comparing with the pure ZnONRAs. On the one hand, the heterostructured g-C3N4/ZnO NRAs can extend the light absorption from UV region to the visible light region, resulting in a significantly enhanced photocatalytic H2 production. On the other hand, the type II heterojunction formed between g-
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C3N4 and ZnO can effectively separate the photogenerated electron-holes pairs thus leading to the higher photocatalytic hydrogen production and stability. However, the excessive decoration
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amount of g-C3N4 nanoparticles may cause the aggregation of g-C3N4. This not only reduces
the utilization of solar light (Fig.3a) but also decreases the photogenerated electron transport
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efficiency between g-C3N4 and ZnO because the charge carriers have to diffuse longer to reach
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the g-C3N4/ZnO interface, and finally leads to the decreased photocatalytic hydrogen
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production performance [8]. Therefore, g-C3N4/ZnO-2 exhibits the best photocatalytic
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performance with the suitable g-C3N4 content.
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Fig.5. Photocatalytic H2 production for heterostructured g-C3N4/ZnO NRAs with different amount of g-C3N4 layer and ZnO NRAs under UV-visible light irradiation (a); 5 successive photocatalytic activity cycles of gC3N4/ZnO-2 films for photocatalytic H2 production (b).
The stability and recyclability of the obtained heterostructured g-C3N4/ZnO NRAs films was also investigated. Fig. 5b shows the result of photocatalytic activity of g-C3N4/ZnO-2 for 15
5 cycles. The result demonstrates that the photocatalytic activity of g-C3N4/ZnO-2 only decreases from 33μmol to 28μmol with 85% retention rate after 5 cycles. The very good stability of g-C3N4/ZnO-2 indicates that high physicochemical stability of g-C3N4 layer can
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keep ZnO from the photocorrosion.
4. Conclusion
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In summary, heterostructured g-C3N4/ZnO NRAs films were successfully synthesized via
a light-assisted method to form a dense interface connection between g-C3N4 and ZnO. The XRD results reveal that the obtained heterostructured g-C3N4/ZnO NRAs is pure hexagonal
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wurtzite structure and typical g-C3N4. All the heterostructured g-C3N4/ZnO NRAs films exhibit
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higher photocatalytic hydrogen production than the pure ZnO NRAs under UV-visible light
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irradiation. In particular, the heterostructured g-C3N4/ZnO-2 demonstrates the highest photocatalytic activity with 3.3 times comparing to the pure ZnO-NRAs. The enhanced
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photocatalytic activity can be attributed to the effectively extended light absorption to the
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visible light region and the formation of the type-II heterojunction which can effectively separate the photogenerated electron-holes pairs and increase the interfacial charge transfer.
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Moreover, the g-C3N4/ZnO-2 shows very good stability with 85% retention rate after 5 cycles, indicating that high physicochemical stability of g-C3N4 layer can inhibit ZnO photocorrosion.
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Our work here provides an efficient pathway to construct g-C3N4 modified heterostructured photocatalysts with improved photocatalytic performance.
Acknowledgements Y. Li acknowledges Hubei Provincial Department of Education for the “Chutian Scholar” 16
program. B. L. Su acknowledges the Chinese Central Government for an “Expert of the State” position in the Program of the “Thousand Talents”. This work is supported by National Key R&D Program of China (2016YFA0202602), National Natural Science Foundation of China (U1663225, 21671155, 21805220), Hubei
Provincial Natural
Science Foundation
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(2018CFB242, 2018CFA054), Major programs of technical innovation in Hubei (2018AAA012), Program for Changjiang Scholars Innovative Research Team in University
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(IRT_15R52).
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