g-C3N4 nanosheet heterostructures with improved photocatalytic activity

Journal Pre-proof Construction of 2D/2D TiO2 /g-C3 N4 Nanosheet Heterostructures with Improved Photocatalytic Activity Huihui Zhu, Xiaoju Yang, Min Zhang, Qiuye Li, Jianjun Yang

PII:

S0025-5408(19)32259-7

DOI:

https://doi.org/10.1016/j.materresbull.2019.110765

Reference:

MRB 110765

To appear in:

Materials Research Bulletin

Received Date:

30 August 2019

Revised Date:

27 December 2019

Accepted Date:

31 December 2019

Please cite this article as: Zhu H, Yang X, Zhang M, Li Q, Yang J, Construction of 2D/2D TiO2 /g-C3 N4 Nanosheet Heterostructures with Improved Photocatalytic Activity, Materials Research Bulletin (2020), doi: https://doi.org/10.1016/j.materresbull.2019.110765

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Construction of 2D/2D TiO2/g-C3N4 Nanosheet Heterostructures with Improved Photocatalytic Activity

Huihui Zhu, Xiaoju Yang, Min Zhang*, Qiuye Li*, Jianjun Yang

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National & Local Joint Engineering Research Center for Applied Technology of Hybrid Nanomaterials, Engineering Research Center for Nanomaterials, Henan University, Kaifeng, 475004, China.

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*Corresponding author, e-mail address: [email protected], [email protected]

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Graphical abstract

Highlights  2D/2D TiO2/g-C3N4 heterostructure was constructed by a facile hydrothermal method.  Tight ultrathin 2D heterostructure can shorten the migration 1

distance of photogenerated carriers.  The exposed oxygen vacancy can expand light absorption and improve charge separation.

Abstract: Constructing two-dimensional (2D) heterostructure is an effective method to obtain highly efficient photocatalysts. In this work, 2D/2D TiO2/g-C3N4

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heterostructures were constructed via in-situ growth of TiO2 nanosheets on the surface of g-C3N4 nanosheets by a facile hydrothermal co-assembly technique. The increased dispersion is favorable to expose more vacancies, which is beneficial to the

enhancement of visible light absorption. This unique 2D/2D heterostructure exhibits

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ultrathin thickness and larger interfacial contact area, which results in shorter

migration distance of photo-generated carriers to the surface and faster participation

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in photocatalytic water splitting. As expected, the optimized TiO2/g-C3N4 nanocomposites showed a higher photocatalytic hydrogen evolution performance

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under visible light irradiation than that of pristine g-C3N4 and TiO2 nanosheets. The improved performance can be attributed to the ultrathin nanosheet structure, the

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intimate interfacial contact and the presence of surface oxygen vacancies. More appealingly, this environment-friendly 2D/2D TiO2/g-C3N4 nanohybrid would attract more attention in water splitting.

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KEYWORDS: A. TiO2/g-C3N4; A. 2D/2D heterostructure; D. Oxygen vacancy; C. Transmission electron microscopy

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* To whom correspondence should be addressed.

E-mail: [email protected] (M. Zhang*); [email protected] (Q. Li*)

1. Introduction To alleviate the energy shortage crisis and environmental pollution issues, solar-driven photocatalytic hydrogen production from water has been widely studied 2

by providing a green and efficient way [1-4]. As a typical metal-free semiconductor photocatalyst, graphitic carbon nitride (g-C3N4) has attracted much attention in the field of photocatalytic water splitting due to its high reduction ability, visible-light driven bandgap and facile synthesis [5-8]. However, the photocatalytic activity of pure g-C3N4 photocatalyst is greatly limited by its fast recombination of photogenerated charge carriers and low specific surface area [9-11]. Therefore, it is highly necessary to develop a reasonable approach to accelerate the separation efficiency of photocarriers and increase specific surface area of g-C3N4 to meet the

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needs of practical applications. Constructing g-C3N4-based heterojunctions with other semiconductors is an efficient way to expand visible light absorption range as well as promote charge carrier separation and transfer [10-11]. One candidate of photocatalyst for

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hybridization of g-C3N4 is TiO2, which has proper band edge [12-14]. To date, a large number different morphology of g-C3N4/TiO2 photocatalysts has been successfully

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prepared, such as 0D g-C3N4 QDs/1D TiO2 nanowire or nanotube arrays [15-16], TiO2 nanoparticles/2D g-C3N4 nanosheets [17-18], core-shell structural TiO2/g-C3N4

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[19-20], 1D TiO2 nanobelts or nanowires/2D g-C3N4 [21] and 2D/2D g-C3N4/TiO2 [22-23]. Among them, 2D/2D g-C3N4/TiO2 heterostructure is likely to a new tendency

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in the design of highly active photocatalysts owing to its larger lateral size, abundant contact areas and the short migration distance. For example, Gu et al. reported that ultrathin 2D g-C3N4/TiO2 nanosheets exhibits a high photocatalytic evolution rate of

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hydrogen under UV-vis light [22]. They concluded ultrathin face-to-face contact is favorable for higher charge mobility across the heterointerfaces, shorting charge

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transport distance and time, and therefore reducing the recombination of charge carriers. Zhong et al. reported the photocatalytic activity of hydrogen evolution was 6.1 times higher than that of the simple physical mixture under visible light irradiation for 2D/2D TiO2 and g-C3N4 nanocomposites [23]. They proposed that the interfacial Ti-O-N covalent bonding promotes the charge carrier transfer and separation effectively, thus accelerating the photocatalytic H2 production. In a word, relatively 3

scarce studies have been reported concerning construction of ultrathin 2D/2D TiO2/g-C3N4 heterostructure, moreover, the underlying structural factors responsible for the interfacial charge transfer have not been clearly identified. Therefore, to study the relation between structural and properties of 2D/2D TiO2/g-C3N4 heterostructure is still ongoing and challenging task. In this work, we designed and constructed 2D/2D TiO2/g-C3N4 heterostructure by a simple hydrothermal co-assembly method. The as-prepared samples were thoroughly characterized by X-ray diffraction (XRD), Transmission electron

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microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectroscopy (FT-IR) and Electron spin resonance spectra (ESR), etc. The photoelectrochemical performance

and photocatalytic activity of hydrogen evolution were evaluated. The optimized

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TiO2/g-C3N4 nanocomposites showed a higher photocatalytic hydrogen evolution

performance under visible light irradiation than pristine g-C3N4 and TiO2 nanosheets.

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The photocatalytic mechanism for 2D/2D TiO2/g-C3N4 heterostructure was discussed

2. Experimental Section 2.1 Material Synthesis

Synthesis of g-C3N4 Nanosheets

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2.1.1

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in detail.

g-C3N4 nanosheets were prepared by thermally method under environmental atmosphere. The precursor (urea) was placed in an alumina crucible with a cover and

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kept at 550 ℃ for 4 h with a heating rate of 5℃·min-1. The obtained yellowish resultants were collected and ground into fine powder as the final products. Synthesis of TiO2 Nanosheets

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2.1.2

Ultrathin TiO2 nanosheets were prepared by a solvothermal method by Wang et

al [24]. In a typical synthetic procedure, under stirring conditions, 2ml TiCl3 aqueous solution and 2ml deionized water were mixed together until no HCl gas yield. Then 60mL ethylene glycol was introduced into the mixture. The resulting homogeneous solution was sealed in a 100ml Teflon-lined autoclave, and then kept at 170℃ for 4h 4

in an oven. The obtained white products were washed with water and ethanol for four times, dried at 60℃ in a vacuum drying oven. 2.1.3

Synthesis of 2D/2D TiO2/g-C3N4 Heterostructures The synthesis method of 2D/2D TiO2/g-C3N4 heterostructures is similar to pure

TiO2 nanosheets, only add different mass of g-C3N4 in the solution before hydrothermal reaction. The 2D/2D TiO2/g-C3N4 composites with different amount of g-C3N4 (0.3g, 0.9g, 2.1g, 2.7g) were denoted as TCN-1:1, TCN-1:3, TCN-1:7, TCN-1:9, respectively.

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2.2 Characterization The crystal structures of the samples were confirmed by X-ray diffractometer

(XRD, Bruker D8-AVANCE, Germany). The surface groups of prepared samples were investigated on Fourier transform infrared spectrometer (FT-IR, Bruker

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VERTEX 70, Germany). The morphologies of the samples were characterized by high

resolution transmission electron microscope (HRTEM, JEOL JEM-2100, Japan) and

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Scanning electron microscope (SEM, Carl Zeiss, GeminiSEM 500, England), respectively. The surface chemical states were recorded using X-ray photoelectron

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spectroscopy (XPS, Thermo ESCALAB 250Xi, USA). The binding energy of contaminant carbon (284.8 eV) was used as the reference. Electron spin resonance

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spectra (ESR) were determined on a Bruker E500 spectrometer at 70 K. The Brunauer-Emmett-Teller (BET) special surface areas (SBET) of samples were determined by nitrogen adsorption-desorption isothermals (Quadrasorb SI instrument,

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USA). The UV–vis diffuse reflection spectra (DRS) were recorded on a UV-vis spectrophotometer (SHIMADZU, UV-2600, Japan). BaSO4 was as the reference

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sample. The steady-state PL spectra were measured on the HORIBA Fluoromax+. The time-resolved fluorescence decay spectra were determined by an FLS980 fluorescence spectrophotometer. 2.3 Photoelectrochemical Measurements Photoelectrochemical measurements were carried out on an electrochemical workstation

(Chenhua,

CHI660E,

China)

using

a

conventional

standard 5

three-electrode cell under visible light irradiation (λ > 420 nm). A saturated calomel electrode was used as the reference electrode and Pt filament as the counter electrode. The working electrodes were prepared as follows: 2 mg of the prepared samples and 5 μL of Nafion117 solution (Aladdin, 5% in a mixture of lower aliphatic alcohols and water) were ultrasonically dispersed in 1 mL absolute ethanol for 30 min. Then, the above suspension was spin-coated onto the ITO conductive glass. After that, the prepared ITO was calcined at 200 ℃ for 2h to obtain an uniform and compact working electrode. 0.1 mol·L-1 Na2SO4 solution was used as electrolyte.

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2.4 Photocatalytic Activity Measurement

Photocatalytic H2 production reaction was conducted in a Pyrex cell with a top

flat window. A 300W Xenon lamp (Beijing China Education Au-light Co., Ltd) coupled with a UV cut-off filter (λ ≥ 420 nm) was used as the light source and the

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light intensity of the reactor was 200 mW·cm-2. Typically, 50 mg photocatalyst

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powder was added into 100 mL H2O containing 20 vol% (v/v) triethanolamine (TEOA, pH = 7) as sacrificial reagent under stirring. Subsequently, H2PtCl6·6H2O

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aqueous solution (about 1 wt % Pt) was then added into the system to load Pt onto the surface of the photocatalyst via a photochemical reduction deposition method. Before illumination, the system was evacuated for 20 minutes to remove any residual O2. The

Ar carrier).

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amounts of hydrogen were measured through a gas chromatography (GC-7920, TCD,

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3. Results and Discussion

In order to study the morphology of TiO2/g-C3N4 nanocomposites, transmission

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electron microscopy (TEM) images and scanning electron microscope (SEM) images were measured. As shown in Fig.1a, TiO2 is ultrathin nanosheet structures, and the size is in the range of 40-65 nm. The g-C3N4 obtained by calcining urea is 2D sheet structure (Fig.1b), which has a large lateral dimension about 200-300nm. In the Fig.1c of TCN-1:7 sample, it can be seen the bottom layer is g-C3N4, and the pleated TiO2 nanosheets are grown on g-C3N4. This can prove that they form a better 2D/2D hybrid. In addition, from the high-resolution TEM image (Fig.1d), it can be seen that the clear 6

0.31nm and 0.35nm correspond to the lattice fringes of g-C3N4 and TiO2, respectively [18]. Such intimate interface indicates the successful construction of 2D/2D TiO2/g-C3N4 nanocomposites via the one-step hydrothermal process. Furthermore, the energy-disperse

spectroscopy

(EDS)

spectrum

(Fig.1f)

confirm

that

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nanocomposites were composed of Ti, O, N, and C elements with homogeneous distribution (aluminum signal from the substrate). This can also be used to verify the

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successful synthesis of TCN samples by a simple hydrothermal method.

Figure.1 TEM images of (a) TiO2 nanosheets, (b) g-C3N4 nanosheets, (c) TCN-1:7, (d) HRTEM

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images, (e) SEM image, (f) energy- disperse spectroscopy (EDS) spectrum of TCN-1:7.

Figure.2 (a) Power X-ray diffraction (XRD) pattern and (b) Fourier transform infrared spectra (FT-IR) of g-C3N4, TiO2 and TCN sample.

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Fig.2a shows that the X-ray diffraction (XRD) patterns of the synthesized TiO2 nanosheets, g-C3N4 nanosheets, and TiO2/g-C3N4 nanocomposite. Insert of Fig.2a is the XRD patterns of TiO2, diffraction peaks centered at 14.8°, 24.6°, 27.8° and 48.3° corresponds to (001), (110), (002) and (020) are well matched to ultrathin nanosheets of metastable phase TiO2 (JCPDS 74-1940) [24-25]. XRD patterns of g-C3N4 nanosheets exhibits characteristic diffraction peaks at 13.6° and 27.5°, which are assigned to the diffraction peaks of (100) and (002) plane, corresponding to the repeated stacking structure of the g-C3N4 triazine heterocycle and the like-graphite

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structure of the superimposed peaks between the characteristic layers, respectively [26-27]. For TCN nanocomposites, the XRD patterns are observed as the combination

of TiO2 nanosheets and g-C3N4 nanosheets. Moreover, the diffraction peak intensity of

g-C3N4 nanosheets gradually increase, the diffraction peak intensity of TiO2

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nanosheets gradually decreases with increasing contents of g-C3N4 in the composites.

These results indicate successful combination of TiO2 and g-C3N4 in TCN

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nanocomposites.

Fig. 2b depicts the Fourier transform infrared (FT-IR) spectra to investigate the

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surface groups of the prepared g-C3N4 nanosheets, TiO2 nanosheets and TCN composites. For g-C3N4, the peak at 811 cm − 1 is assigned to the characteristic

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breathing-vibration mode of the tri-s-triazine units in the g-C3N4 framework [28]. The absorption peaks at 1200−1650 cm−1 can be attributed to the typical stretching modes of C−N bonds [29]. The broad absorption peaks of 3100−3500 cm−1 are ascribed to

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residual uncondensed amino (N-H) and absorbed hydroxyl groups [30]. The FTIR spectrum of TiO2 shows a peak at 496 cm-1 belongs to the stretching vibration of the

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Ti-O-Ti bonds [31]. According to the above analysis, the introduction of g-C3N4 does not change the chemical structure both of 2D g-C3N4 and TiO2. In order to test the specific surface area, we made N2 adsorption-desorption isotherm. The BET surface areas of g-C3N4, TiO2 and TCN-1:7 are calculated to be 47.4 m2·g-1, 401.2 m2·g-1 and 111.7 m2·g-1, respectively. Comparing with pure g-C3N4, the specific surface area of TCN-1:7 composites increased nearly 2.4 times. The increasing specific surface area 8

can effectively improve the contact area between the two nanosheets. In order to investigate the surface chemical states, XPS spectra of g-C3N4, TiO2 and TCN-1:7 samples were measured. All binding energies were calibrated by C1s binding energy at 284.8eV. In the full survey spectra, the signals of C, N, Ti and O elements are found in the TCN-1:7 composites, which could confirm the presence of g-C3N4 and TiO2 in the composites. The Ti 2P spectrum of TiO2 nanosheets indicates two major peaks locate at 457.6 and 463.3 eV, which can be assigned to Ti4+ species. (Fig.3a). However, for the spectra of TCN-1:7 nanocomposite (Fig.3b), the binding

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energies of Ti4+ shifted to 457.9 and 463.6 eV, respectively. Moreover, new peaks at 457.3 and 463.1eV exist for TCN-1:7 nanocomposite, which are ascribed to Ti3+ [32, 33]. This may be due to the fact that ultrathin TiO2 lamellar structure after g-C3N4

intercalation can expose more basal planes, making coordination unsaturated [31].

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Just like we know, Ti3+ species always co-exists with the oxygen vacancy. These results show that obvious oxygen defects in the TCN-1:7 nanocomposites. Namely,

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the increased dispersion of two kinds of nanosheets is favorable for TiO2 to expose more vacancies, when TiO2 nanosheets were deposited on the surface of g-C3N4 to

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form 2D/2D heterostructure.

Figure. 3 High resolution Ti 2p of (a) TiO2 and (b) TCN-1:7 nanocomposites.

In attempt to obtain more evidence relative to Ti3+ and oxygen vacancies, the low temperature ESR was conducted to detect the spin state of unpaired electron. Pure TiO2 nanosheets exhibits a very weak peak compared to the other samples. But as we 9

can see from the enlarged image (Fig. 4a), they have a distinct g value peak at 2.002, which can be ascribed to the appearance of oxygen vacancies [34-35]. Probably because the ultrathin structure makes Ti-O bond coordination unsaturated in the laminate, and lattice O is more likely to be missing, leaving surface oxygen vacancy [31]. For bare g-C3N4 nanosheets, they have one single Lorentzian line with an obvious peak appeared at g=2.002, due to the unpaired electron of the π-bonded aromatic rings [36-39]. The ESR peaks at g =2.002 for 2D/2D TCN-1:7 nanocomposites are greatly enhanced, which may be due to the interaction of oxygen

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vacancies in TiO2 nanosheets with unpaired electrons in g-C3N4 nanosheets. Moreover, a shoulder peak appeared at g=2.02, which can be ascribed to surface Ti3+ species,

indirectly confirm the existence of oxygen vacancies [40]. When TiO2 nanosheets

were deposited on the surface of g-C3N4 to form 2D/2D heterostructure, the

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dispersion of two kinds of nanosheets increased with respect to a bare material, which is more favorable for TiO2 to expose more vacancies. The above analysis confirms the

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successful synthesis of TCN samples with oxygen vacancies by a simple

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hydrothermal method.

Figure. 4 (a) ESR spectra (b) UV-vis diffuse reflectance spectra (UV-vis DRS) (c) The transient

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photocurrent response curves and (d) Electrochemical impedance spectra (EIS) of investigated photocatalysts.

To further exploring the optical absorption ability of TCN samples, UV-vis diffuse reflectance spectra (UV-vis DRS) was studied as demonstrated in Fig.4b. Pure TiO2 nanosheets showed a strong light absorption in the ultraviolet region below 380nm, corresponding to a band gap 3.2 eV. Pure g-C3N4 nanosheets exhibit the obvious absorption in the ultraviolet and visible light region. When TiO2 nanosheets were combined with g-C3N4 nanosheets, the obtained TCN-1:7 sample exhibits the

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similar light absorption region compared to that of pure g-C3N4 nanosheets. However, the light absorption of TCN-1:7 at wavelength between 470nm and 800nm is stronger than that of pure g-C3N4 nanosheets, which may be due to the existence of oxygen

vacancy. Namely, when TiO2 nanosheets were uniformly deposited on the surface of

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g-C3N4 to form 2D/2D heterostructure, it is more favorable for TiO2 to expose more

oxygen vacancies, which has been confirmed by XPS and ESR results. The above

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results display TiO2 nanosheets were well-combined with g-C3N4 nanosheets that makes more efficient use of visible light.

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To further verify the charge transfer process of the photo-induced electrons and holes, the transient photocurrent responses and electrochemical impedance spectra

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(EIS) were measured. As shown in Fig.4c, TCN-1:7 nanocomposites exhibit higher photocurrent response than pure g-C3N4 or TiO2 nanosheets under visible-light irradiation, which indicates the higher charge separation efficiency. Fig. 4d shows the

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results of EIS experiments. Pure TiO2 nanosheets have largest impedance radius, followed by g-C3N4 nanosheets. When TiO2 nanosheets were combined with g-C3N4

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nanosheets, the impedance radius is significantly reduced. Small electrochemical impedance radius indicates that TCN-1:7 nanocomposites possess significantly enhanced separation efficiency of photoexcited charge carriers by forming 2D/2D tight heterostructure [28]. This is consistent with the I-t results. Based on the above discussion, it can be concluded that the charge separation efficiency was improved due to the formation of tight contact interfaces, which may be bring about the 11

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enhanced photocatalytic activity of H2 production.

Figure. 5. (a) The photocatalytic hydrogen evolution rate (HER) over different samples under

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visible light irradiation. (b) Stability test of TCN-1:7 nanocomposites for photocatalytic H2 production under visible light irradiation. (c) Steady-state PL spectra of pure g-C3N4, TiO2 and

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TCN-1:7 nanocomposites. (d) Suggested mechanism for photocatalytic H2 production by optimized 2D/2D TiO2/g-C3N4 nanocomposites under visible light irradiation.

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Decay Time (ns)

Relative Amplitude (%)

Samples

Average Life Time (τ, ns)

2

χ

τ2

B1

B2

TiO2

1.05

53.37

296.28

42.84

47.104

1.237

g-C3N4

2.70

17.92

697.34

142.98

11.473

1.171

TCN-1:7

1.98

9.77

739.45

169.82

6.118

1.001

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τ1

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Table1. Time-resolved fluorescence decay spectra of g-C3N4, TiO2 and TCN-1:7 samples.

The photocatalytic activity of H2 evolution was evaluated for the as-prepared samples under visible light irradiation. As can be seen from Fig. 5a, pure TiO2 nanosheets exhibit almost no photocatalytic activity of H2 evolution due to the weaker visible light absorption. However, pure g-C3N4 nanosheets indicate the obvious activity in photocatalytic H2 evolution (107 μmol·g − 1·h − 1). After coupling TiO2 nanosheets with g-C3N4 nanosheets, the photocatalytic activity of H2 generation is obviously enhanced. The photocatalytic activity of hydrogen production increased

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initially and then decreased with the increase of g-C3N4 amount. The coupled g-C3N4

amount has an optimum value, TCN-1:7 exhibits the highest hydrogen production activity of 189 μmol·g−1·h−1. In addition, another mixing-1:7 sample was prepared by

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the simple physical mixture of TiO2 nanosheets and g-C3N4 nanosheets with the ratio

of TiO2/g-C3N4 = 1:7. It was found that the photocatalytic activity of mixing-1:7 is

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only the simple additive sum of the photocatalytic activity of TiO2 nanosheets and g-C3N4 nanosheets, which is lower than that of TCN-1:7, indicating that the tight

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interfacial coupling plays an important role in boosting the photocatalytic activity[41]. This phenomenon can be attributed to the following reasons: one reason is the formation of ultrathin tight 2D/2D heterojunctions to shorten the transfer distance of

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charge carriers; another reason is the increasing the dispersion of samples to expose more oxygen vacancies to absorb visible light. Under the co-effect of the above two

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aspects, the hydrogen production activity can be effectively improved, which cannot be achieved by simple physical mixing.

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Furthermore, to evaluate the long-term stability, five runs of recycling hydrogen evolution reaction for the optimized TCN-1:7 nanocomposites were tested under visible light irradiation for 5h as shown in Fig.5b. The results reveal that the H2 evolution rate of TCN-1:7 nanocomposites decline very slowly, but it still maintains a high value after 25h of visible light irradiation, indicating that the TCN-1:7 composite photocatalyst has enough high stability in practical photocatalytic hydrogen production. 13

The ability of charge separation is of critical importance to the photocatalytic activity of TCN nanocomposites. Charge separation efficiency can be characterized by the fluorescence emission spectra and time-resolved fluorescence decay spectra. Steady-state PL spectra of g-C3N4 nanosheets, TiO2 nanosheets and TCN-1:7 nanocomposites are shown in Fig. 5c. In general, the stronger intensity of the fluorescent peak, the easier recombination of electrons and holes, which more detrimental to photocatalytic hydrogen production [18]. It can be seen that pure g-C3N4 nanosheets have a strong fluorescent peak. After coupling g-C3N4 nanosheets

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with TiO2 nanosheets, the fluorescence intensity of TCN-1:7 is significantly reduced. While the fluorescence intensity of mixed g-C3N4 and TiO2 sample (named mixing) is less than that of g-C3N4 nanosheets, but is still stronger than that of TCN-1:7 sample. It indicates the recombination rate of photogenerated electrons and holes reduced for

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TCN-1:7 samples due to the formation of 2D/2D heterojunction, which are beneficial to the increase of photocatalytic hydrogen production activity. This is also consistent

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with the hydrogen production (Fig.5a). This result is further evidenced by time-resolved transient PL measurements in Table 1. The average PL lifetimes of TiO2

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nanosheets, g-C3N4 nanosheets and TCN-1:7 nanocomposites are 47.1 ns, 11.5 ns and 6.1 ns, respectively. Taken together with the reduced PL intensity of TCN-1:7

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nanocomposites, they show the shortest average lifetimes, which means higher separation efficiency of electrons and holes for TCN-1:7 nanocomposites due to the better contact between TiO2 nanosheets and g-C3N4 nanosheets [6,42-44].

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Furthermore, the formation 2D heterostructure can promote the separation and transfer of electrons and holes between layers, which can also explain the

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improvement of H2 production activity. Fig. 5d exhibits the schematic diagram of the proposed mechanism for H2

evolution of TCN nanocomposites under visible light irradiation. By the light illumination, the electrons in the valence band (VB) of g-C3N4 could be excited to the conduction band (CB), generating photoinduced electrons and holes. Simultaneously, TiO2 nanosheets would also be excited. Due to the large interfacial contact area and 14

the short migration distance of TiO2/g-C3N4, the transfer rate of photo-induced carriers can be significantly improved. Therefore, the photogenerated electrons in the CB of g-C3N4 would quickly jump to the CB of TiO2 to reduce H2O (or H+) to H2, the photogenerated holes in the VB of TiO2 to the VB of g-C3N4 and consumed by the sacrificial TEOA solution. Thus, the photoinduced electrons and holes in the 2D/2D TiO2/g-C3N4 hybrids would be effectively separated and transferred, which leads to remarkable enhancement the performance of photocatalytic H2 evolution. Conclusions

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To conclude, the 2D/2D TiO2/g-C3N4 heterostructure were successfully constructed by a facile feasible hydrothermal co-assembly technique. Via in-situ growth of TiO2 nanosheets with oxygen vacancies on the surface of g-C3N4

nanosheets, the optimized 2D/2D TiO2/g-C3N4 provided stronger visible light

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absorption, abundant intimate interfaces and the short migration distance charge carriers for accelerating the transfer and separation of photoinduced electrons, further

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improved hydrogen production activity. We believe that this successful construction of 2D/2D TiO2/g-C3N4 may pave the way for designing other environment-friendly

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2D/2D g-C3N4-based nanosheet heterostructure photocatalysts.

Conflict of Interest

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No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my

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co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.

Author Statement Min Zhang: Ideas; formulation of overarching research goals and aims; Review & Editing 15

Huihui Zhu: Experimental preparation; Writing- Original draft preparation Xiaoju Yang: Organization of data Qiuye Li & Jianjun Yang: Resources; Management and coordination responsibility for the research activity planning and execution

Acknowledgements

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Science Foundation of China (No 21673066, 21703054).

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The authors gratefully acknowledge to the financial support of National Natural

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