Na2Fe2Ti6O16 as a hybrid co-catalyst on g-C3N4 to enhance the photocatalytic hydrogen evolution under visible light illumination

Journal Pre-proofs Full Length Article Na2Fe2Ti6O16 as a hybrid co-catalyst on g-C3N4 to enhance the photocatalytic hydrogen evolution under visible light illumination Ze-Qing Guo, Qi-Wen Chen, Jian-Ping Zhou PII: DOI: Reference:

S0169-4332(20)30113-6 https://doi.org/10.1016/j.apsusc.2020.145357 APSUSC 145357

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

19 November 2019 4 January 2020 9 January 2020

Please cite this article as: Z-Q. Guo, Q-W. Chen, J-P. Zhou, Na2Fe2Ti6O16 as a hybrid co-catalyst on g-C3N4 to enhance the photocatalytic hydrogen evolution under visible light illumination, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145357

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© 2020 Published by Elsevier B.V.

Na2Fe2Ti6O16 as a hybrid co-catalyst on g-C3N4 to enhance the photocatalytic hydrogen evolution under visible light illumination Ze-Qing Guo,a,b Qi-Wen Chen,a Jian-Ping Zhoua,* School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710119, People’s Republic of China a

Key Laboratory of Functional Materials and Devices for Informatics of Anhui Educational Institutions, Fuyang Normal University, Fuyang, 236037, People’s Republic of China b

Abstract: A few Na2Fe2Ti6O16 (NFTO) nanosheets were loaded on graphitic carbon nitride with a salt melt method to form heterojunctions. NFTO as a co-catalyst can extend the light harvesting, increase conductivity and promote photogenerated electronhole separation, as a result, improve the hydrogen generation of NFTO/g-C3N4 nanocomposite, which is 1.8 times than that of pure g-C3N4. The hydrogen production rate steadily reaches 389 μmol·g–1·h–1 with a little NFTO content of 2% in the NFTO/gC3N4 composites under visible light illumination. These results demonstrate that NFTO nenosheets hold great promise as a co-catalyst for solar energy harvesting of g-C3N4based composite.

Keywords: Hydrogen generation, Co-catalyst, Na2Fe2Ti6O16/g-C3N4 nanocomposite

*

Corresponding author. E-mail address: [email protected], ORCID: 0000-0003-0807-1404 1

1

Introduction Currently, the rapid development of industrialization brings many global problems,

such as environmental pollutant, greenhouse effect, and energy crisis. It is desirable to construct clean renewable energy to solve these issues. Photocatalytic hydrogen evolution from water without sacrificial agents is considered as a promising approach to relieve the pressure of increasing energy and environmental demands by transforming the solar energy to clean hydrogen energy [1-3]. Many efforts have been devoted to pursue new photocatalysts of pure semiconductors and composites for hydrogen evolution with considerable efficiencies. Among the numerous photocatalysts, graphitic carbon nitride (g-C3N4) shows a superior photocatalytic hydrogen production by splitting water in visible-light range due to its appropriate band structure, thermal and chemical stability against corrosive chemical environments, facile fabrication, and abundant in nature [4-9]. g-C3N4 has a graphite-like sp2-bonded C-N structure and suitable bandgap of 2.7 eV. Its positions of conduction band and valence band edges (–0.8 V and 1.9 V) are theoretically suitable for the photoreduction and oxidation of absorbed water [4]. However, g-C3N4 obtained by the thermal polymerization generally suffers from fast recombination of photogenerated carriers, low surface area and low density of active sites, resulting in moderate photocatalytic efficiency [7,10,11]. Then, it is necessary to design new structures to improve the photocatalytic activity of g-C3N4 for industrial application. The photocatalytic activity of g-C3N4 can be enhanced by controlling its size, thickness, porous structure, which provide large specific surface area and accelerate the photogenerated carrier transfer to surface [6,7,9,11-13]. Element doping can also promote its photocatalytic activity by improving its electronic structure [5,14,15]. Especially, g-C3N4 based hybrid photocatalysts, such as Au/g-C3N4 [16], α-Fe2O3/gC3N4 [10], MoS2/g-C3N4 [17-20], PTCDA/g-C3N4 [21], CeO2/g-C3N4 [22,23], carbon nanotubes/g-C3N4 [24], Pt/t-ZrO2/g-C3N4 [25], TiO2/g-C3N4 [26,27], and TiO2(B)/gC3N4 [28] are greatly significant to enhance the light harvesting ability and hider the recombination of photoinduced electron-hole pairs. 2

Na2Fe2Ti6O16 (NFTO) is a TiO2(B) based compound with monoclinic crystal structure, sharing same space group (C2/m) with TiO2(B). NFTO nanosheets enjoy multifunctions. Its double absorption extends the light harvesting to visible light range, its magnetic behavior allows a convenient collection from aqueous solution for recycling, and its high adsorption capacity is helpful for reaction [29]. Herein, we load a few NFTO nanosheets on g-C3N4 to efficiently improve the photocatalytic hydrogen generation rate. This work provides a new insight to apply the multifunctional material as a high-efficiency and low-cost co-catalyst.

2

Experimental section

2.1 Synthesis of NFTO-C3N4 The fabrication process of NFTO/g-C3N4 composites includes two steps. Firstly, the NFTO nanosheets used in this study were prepared via a hydrothermal method [29]. Typically, Fe(NO3)3·9H2O and TiO2 with Na2Fe2Ti6O16 stoichiometry were added into 1.4 M NaOH aqueous solution. After continuously magnetic stirring for 30 min, the slurry mixture was transferred into a 140 mL stainless steel autoclave filled to 80% of its volume, sealed and heated up to 270 °C for hydrothermal treatment 80 min under mechanical stirring. The khaki precipitate was collected and washed with deionized water and absolute ethanol before drying at 70 °C for 12 h. g-C3N4 was synthesized by preheated melamine as a starting material in combination with the salt melt method [8]. Certain mass melamine was added and pressed into a crucible and then heated to 550 °C for 4 h at a rate of 12 °C/min in a muffle furnace under air atmosphere, the yellow block product g-C3N4 was collected and grinded into powder. Subsequently, NFTO/g-C3N4 composites with different proportions were obtained as follows: a given mass of powder g-C3N4 was well dispersed into 30 mL methanol solution by ultrasonication for 30 min, and then NFTO was added into above suspension and stirred in a fume hood for 24 h until the methanol volatilization. Finally, the resultant samples were collected and calcined at 250 °C for 2 h to provide enough thermal energy to form a tight chemical binding between g-C3N4 and NFTO, which could afford a spatial condition for charge transport from one 3

material to another through the interfaces. The NFTO/g-C3N4 composites with final NFTO mass percents of 1%, 2% and 3% were signed as samples 1%-NFTO/CN, 2%NFTO/CN and 3%-NFTO/CN, respectively. 2.2 Material characterization X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku D/Max2550 diffractometer under a scan rate of 8 °/min. The morphology was characterized by a field emission scanning electron microscope (Nova Nano SEM-450, FEI, USA) at an accelerating voltage of 20 kV. An FEI Tecnai G20 electron microscope was employed to conduct transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analysis. The Fourier transform infrared (FTIR) measurement was carried out on a Tensor27 infrared spectrophotometer (Bruker, Germany). X-ray photoelectron spectroscopy (XPS) data were obtained on an ESCALAB MKII instrument (VG Scientic, UK) with Al Kα radiation for the chemical analysis. All binding energies were referenced to the C1s peak at 284.6 eV arising from adventitious carbon. The optical absorption spectra were recorded in a Lambda 950 UV-vis-NIR spectrophotometer with BaSO4 as a reference. Nitrogen adsorption-desorption isotherms were collected at 77 K using a Micromeritics ASAP 2010M system. The photoluminescence spectra were recorded on a Varian Cary-Eclipse 500 fluorescence spectrometer at room temperature with excitation at 460 nm. Time-resolved fluorescence decay spectra were acquired on a luminescence spectrometer (FLS 1000, Edinburgh Instruments Ltd., UK) with an excitation wavelength at 370 nm. 2.3 Electrochemical methods The preparation of electrode was on the base of a previous method [30]. The indium doped tin oxide (ITO) was cut into square (15 mm×15 mm) as substrate and cleaned by ultrasonication in deionized water, absolute ethanol, and isopropanol for 15 min in sequence, and then dried in vacuum. 5 mg of sample and 10 μL of nafion solution (5 wt%) were dispersed in 1 mL water/isopropanol mixed solvent (3:1 V/V) by at least 30 min sonication to obtain a homogeneous sample colloid. Then, 150 μL of the sample colloid was deposited on the ITO substrate and dried in air to form a working electrode. 4

The photoelectrochemical and electrochemical impedance measurements were performed on a CHI660E electrochemical analyzer (CHI Shanghai, Inc.) with a threeelectrode cell containing 0.2 M Na2SO4 aqueous solution as electrolyte. The aforesaid as-prepared electrode, Pt plate and Ag/AgCl electrode were used as the working electrode, counter-electrode and reference electrode, respectively. The visible light was generated by a 300 W xenon lamp with a 420 nm cutoff filter. The photocurrent with ON/OFF cycles was measured under an applied potential of 1.2 V versus Ag/AgCl. Mott-Schottky plots were obtained under direct current potential polarization at 600 Hz with the potential range from –1.0 to 1.0 V (vs. Ag/AgCl). The measured potential versus Ag/AgCl is converted to the NHE potential according to the Nernst equation [17,31,32]

ENHE  E Ag / AgCl  0.0591pH  EAg / AgCl

(1)

 where ENHE is the converted potential versus NHE, EAg / AgCl = 0.1976 V at 25 °C, and

EAg/AgCl is the experimental potential measured against the Ag/AgCl reference electrode. 2.4 Hydrogen Evolution Experiments The photocatalytic hydrogen evolution was carried out in a 150 mL three-necks pyrex flask containing 100 mL 10 vol% of triethanolamine (TEOA) aqueous solution, which was used as a sacrificial electron donor [9,24,33]. 50 mg of catalyst photodeposited in situ by 3 wt% Pt was used in the reaction. Before the irradiation [33], the reaction cell was connected to a gas-closed system with a gas-circulated pump and evacuated several times to remove the air completely. The visible light source was generated by a 300 W xenon lamp with a 420 nm cutoff filter. The H2 evolution was analyzed by an online gas chromatograph (Clarus 480, Perkin-Elmer) equipped with a thermal conductivity detector.

3

Results and discussion

3.1 Structure and morphology XRD patterns were used to identify the crystal structure of the samples as shown 5

in Fig. 1. g-C3N4 powders give two peaks near 13° and 27°, which is indexed to (0 0 1) and (0 0 2) reflections (JCPDS no. 87-1526), corresponding to the in-plane structure packing motif and interlayer stacking of carbon nitride, respectively [5-7,17,31]. XRD pattern of NFTO powders is assigned to pure Na2Fe2Ti6O16 (JCPDS no. 70-0637) [29]. NFTO diffraction peaks are not obvious in the 1%-NFTO/CN and 2%-NFTO/CN composites, attributing to the well-dispersed NFTO and its low amount in the composites. However, several distinguishable peaks appear in the 3%-NFTO/CN composite, suggesting that monoclinic NFTO nanosheets have been composited onto

(1 0 0)

(0 0 2)

the graphite g-C3N4 nanopowders. Pure g-C3N4

Intensity (a.u.)

g-C3N4: JCPDS no.87-1526 1%NFTO/CN 2%NFTO/CN

*

*

*

3%NFTO/CN Pure NFTO NFTO: JCPDS no.70-0637

10

20

30

40

50

60

70

80

2- (deg.)

Fig. 1. XRD patterns of NFTO/g-C3N4 composites with different NFTO contents, as well as pure g-C3N4 and Na2Fe2Ti6O16. Their standard diffraction lines are also displayed. Fig. 2 shows typical SEM images of the samples. The pure g-C3N4 powder [Fig. 2(a)] exhibits typical irregular topography similar to the recent results [8], whereas pure NFTO powder [Fig. 2(b)] shows regular long hexagonal plates with about 1.0 μm in length, 400 nm in width and 50 nm in thickness [29]. The SEM images of NFTO/gC3N4 composites [Figs. 2(c)~2(e)] directly show NFTO nanosheets are deposited and well dispersed on g-C3N4 nanopowders. TEM images [Figs. 3(a)~3(c)] also confirm these characteristics.

6

Fig. 2. SEM images of (a) g-C3N4, (b) Na2Fe2Ti6O16, (c) 1%-NFTO/CN, (d) 2%NFTO/CN and (e) 3%-NFTO/CN.

Fig. 3. Typical TEM images of the as-prepared samples: (a) pure g-C3N4, (b) pure Na2Fe2Ti6O16, (c) composite 2%-NFTO/CN sample, and (d, e) HRTEM images of the 2%-NFTO/CN photocatalyst. HRTEM images in Figs. 3(d) and 3(e) reveal a close interface between NFTO and g-C3N4 in the composite. g-C3N4 has a layered structure with several stacking layers and NFTO displays high monodispersity with long hexagonal plates. The lattice 7

spacings of 4.12 Å and 3.21 Å correspond to the (1 0 0) and (0 0 2) lattice planes in gC3N4 while the spacings of 3.60 Å and 6.20 Å are relative to the (1 1 0) and (0 0 1) lattice planes in NFTO, respectively. Fig. 3(e) shows a well matched heterostructure with crystal orientation relationships of (1 0 0)CN plane on (0 0 1)NFTO plane to form heterojunctions, which is hence favorable for the carrier transfer from one semiconductor to another through the matched interfaces. This is helpful for the

Transmittance (a.u.)

photocatalytic activity.

Pure-C3N4 3167

1321 1246 1632 1408

808

1%-NFTO/CN

2%-NFTO/CN

3%-NFTO/CN Pure-NFTO

4000 3500 3000 2500 2000 1500 1000 500 850 800 750

Wavenubers (cm-1)

Fig. 4. FTIR spectra of pure g-C3N4, Na2Fe2Ti6O16 and composite NFTO/g-C3N4 samples. The crystal structure of the samples was further investigated by FTIR spectroscopy as shown in Fig. 4. Pure g-C3N4 displays several distinct bands in three main absorption regions, indicating the typical molecular structure of g-C3N4. The broad peak between 3000 and 3600 cm–1 is attributed to the stretching vibration of N-H and O-H of the physically adsorbed water [32]. Besides, the peak at 1632 cm–1 is attributed to C=N stretching vibration modes, while those at 1408, 1321, and 1246 cm–1 belong to the characteristic stretching modes of C-N heterocycles [26]. The peak near 808 cm–1 originates from the characteristic breathing mode of tri-s-triazine units [7,14]. Pure NFTO displays a distinct absorption band at 500 ~ 1000 cm–1, which is related to Ti-O and Fe-O stretching vibration [34,35]. FTIR spectra of NFTO/g-C3N4 heterojunctions exhibits similar characteristic peaks with pure g-C3N4 in the three main absorption

8

regions. However, it is important to notice that the vibration band at 808 cm–1 in NFTO/g-C3N4 composite reveals a slight distinguishable shift to higher wavenumber, supporting the formation of chemical bonding between NFTO and g-C3N4 instead of a simple physical attachment [22,36]. This strong interfacial interaction can largely improve the carrier transfer.

Intensity (a.u.)

2%-NFTO/CN

296 294 292 290 288 286 284 282 280

Raw data Fitted curve

2%-NFTO/CN

Raw data Fe 2p1/2 Fe 2p3/2

3%-NFTO/CN 1071

1068

Binding energy (eV)

Binding energy (eV)

1065

Raw data Ti 2p1/2 Ti 2p3/2

(e) Fe 2p

(d) Na 1S

3%-NFTO/CN

1074

540 538 536 534 532 530 528 526 524

Binding energy (eV)

Intensity (a.u.)

Intensity (a.u.)

2%-NFTO/CN

1077

3%-NFTO/CN

408 406 404 402 400 398 396 394 392

Binding energy (eV)

735

730

725

(c) O1S

2%-NFTO/CN

3%-NFTO/CN

3%-NFTO/CN

1080

Raw data Fitted curve Fitted curve

(b) N 1S

Intensity (a.u.)

Intensity (a.u.)

2%-NFTO/CN

Raw data Fitted curve Fitted curve Fitted curve

Intensity (a.u.)

(a) C 1S

Raw data Fitted curve Fitted curve Fitted curve

720

715

710

705

(f) Ti 2p

2%-NFTO/CN

3%-NFTO/CN 472 470 468 466 464 462 460 458 456 454

Binding energy (eV)

Binding energy (eV)

Fig. 5. XPS high-resolution spectra of (a) C 1s, (b) N 1s, (c) O 1s, (d) Na 1s, (e) Fe 2p, and (f) Ti 2p for 2%-NFTO/CN and 3%-NFTO/CN composites. The element chemical states of NFTO/g-C3N4 composites were analyzed via XPS as shown in Fig. 5. The C 1s spectra in Fig. 5(a) is deconvoluted into two peaks centered at 284.6 and 288.2 eV, which are ascribed to surface adventitious carbon C–C and sp2 bonded carbon N–C=N in triazine rings in the polymeric g-C3N4 framework, respectively [5,14,17]. The N 1s XPS spectra of NFTO/g-C3N4 in Fig. 5(b) are also divided into two peaks at 397.8 and 400.2 eV. The main peak at 397.8 eV corresponds to sp2-hybridized nitrogen in the tri-s-triazine rings C–N=C, while the peak at 400.2 eV is related to the tertiary nitrogen of N–(C)3 groups [5,6,14,32]. The binding energy at 529.4 eV in the O 1s XPS spectra of Fig. 5(d) is ascribed to the O2– chemical state and the peak at 532.5 eV is attributed to the adsorbed CO2 and H2O on the surface of the 9

sample [29,37]. The binding peaks of Na 1s at 1071.5 eV, Fe 2p at 724.1 eV and 710.5 eV, Ti 2p at 464.6 eV and 458.8 eV correspond to the Na+, Fe3+ and Ti4+ chemical states in Na2Fe2Ti6O16 [29], respectively, indicating the existence of NFTO in the composites. 3.2

Enhanced photocatalysis and photoelectrochemical property Photocatalytic H2 evolution of the catalysts was evaluated using TEOA as

sacrificial electron donor to quench holes generated from band-gap excitation. NFTO loaded onto g-C3N4 improves the photocatalytic performance because its two absorption edges can extend the light harvest in visible range although NFTO has poor photocatalytic property [29]. NFTO, as a co-catalyst for the first time, significantly influences the photocatalytic H2 production of NFTO/g-C3N4 composites in water under visible light illumination (λ > 420 nm) as described in Fig. 6(a). The amount of H2 evolved over pure g-C3N4 for 4 h reaction reaches 42.4 μmol, but all the NFTO/gC3N4 composites exhibit an enhanced photocatalytic H2 production activity in comparison with bare g-C3N4, achieving maximum for 2%-NFTO/CN, which is about 1.8 times higher as that of pure g-C3N4.

45

90 243

212

g-C

30

N4

3

FT -N 1%

N O/C

FT -N 2%

N O/C

FT -N 3%

N O/C

Pure g-C3N4 1%-NFTO/CN 2%-NFTO/CN 3%-NFTO/CN

15 0

(a)

271

0

30

60

90 120 150 180 210 240

Irradiation time (min)

2nd run

1st run

389

3rd run

4th run

75

Bare g-C3N4 3% NFTO/g-C3N4 without Pt g-C3N4 loaded 3% Pt

50

60

3%-NFTO/CN

40 30

45

20

30

10

15

0

0

(b)

60

H2 evolution (mol)

60

450 400 350 300 250 200 150 100 50 0

H2 evolution (mol)

H2 evolution (mol)

75

H2 evolution (mol h-1g-1)

90

0

2

4

6

8

10

12

Irradiation Time (h)

14

16

(c)

0

30

60

90 120 150 180 210 240

Irradiation time (min)

Fig. 6. (a) Hydrogen evolution from water under visible light (λ > 420 nm) by 3.0 wt% Pt-deposited NFTO/g-C3N4 and pure g-C3N4 photocatalysts. The inset exhibits H2 production rate of the samples. (b) Recycle runs of H2 evolution over 2%-NFTO/CN composite. (c) Time course of hydrogen evolution over bare g-C3N4, 3.0 wt% Ptdeposited g-C3N4 and 3% NFTO/CN without Pt under visible light irradiation. The generation efficiency was calculated as shown in the inset in Fig. 6(a). The highest H2 production rate steadily reaches 389 μmol·g–1·h–1 with a little NFTO content of 2% in the NFTO/g-C3N4 composite, over 1.8 times as that of bare g-C3N4, which is an important improvement in comparison with the recent results as listed in Table 1[18,23,24], especial with the complex oxides. 10

Table 1 Comparison between the BET specific surface area and H2 production under visible light irradiation (300 W xenon lamp, λ > 420 nm) over the typical g-C3N4 based catalysts. H2 evolution

BET

(μmol·g–1·h–1)

(m2·g–1)

1% Pt

230

10

10% TEOA

2% acetylene black

348

not provided [39]

g-C3N4/NiS

17% TEOA

0.5% acetylene black 366.4

63.4

[40]

g-C3N4/NiS

15% TEOA

0.5% Pt

590

63.4

[41]

g-C3N4/MoS2

10% TEOA

not provided

257.9

not provided [42]

MoS2/g-C3N4

10% TEOA

not provided

500

16.4

g-C3N4/Ni2P

25% TEOA

not provided

160

not provided [43]

CeO2/g-C3N4

1.5% TEOA

1.5 wt% Pt

830

not provided [23]

10% TEOA

3 wt% Pt

175.5

16.2

[24]

15% TEOA

1 wt% Pt

212.8

51

[44]

t-ZrO2/g-C3N4

10% TEOA

3 wt% Pt

722.5

not provided [25]

g-C3N4/TiO2

10% TEOA

not provided

210

not provided [45]

g-C3N4/TiO2

10 vol% TEOA

3 wt% Pt

513

67.1

MoP/g-C3N4

10 vol% TEOA

1 wt% Pt

327.5

not provided [47]

Cr2O3/g-C3N4

10 vol% TEOA

1 wt% Pt

207

34.2

0.6 wt% Pt

212

not provided [49]

10 vol% TEOA

1 wt% Pt

158

not provided [50]

10 vol% TEOA

3 wt% Pt

389

16.02

Catalyst

Sacrificial agent Co-catalys

g-C3N4/Cu3P

TEOA

g-C3N4/CuS

carbon nanotubes/gC3N4 amorphous carbon/gC3N4

g-C3N4/InVO4 LaFeO3/g-C3N4 Na2Fe2Ti6O16/gC3N4

20%

methanol

aqueous

Ref.

[38]

[17]

[46]

[48]

This work

However, further increasing the NFTO content will decrease the H2 evolution rate to 271 μmol·g–1·h–1 for 3%-NFTO/CN. This is probably attributed to the shielding of the active sites on the g-C3N4 and decreasing efficient utilization of visible light by excess NFTO in the photocatalytic system [24]. Furthermore, the H2 evolution rate of NFTO/g-C3N4 displays no obvious decrease after four runs within 16 h as shown in Fig. 6(b), indicating that NFTO/g-C3N4 has outstanding stability. 11

We have not observed clear hydrogen evolution over pure NFTO because it has poor photocatalytic property, but it can be used as a co-catalyst because its two absorption edges can extend the light harvest in visible range [29]. We measured the hydrogen production over 3% NFTO/g-C3N4 composite without Pt nanoparticles as an example to further illustrate the NFTO effect. As shown in Fig. 6(c), the 3% NFTO/CN without Pt can generate 35.95 μmol of hydrogen within 4 h under visible light irradiation, which is much higher than bare g-C3N4 (5.08 μmol) and even close to the g-C3N4 loaded the same mass ratio of noble metal Pt (42.47 μmol). Therefore, the semiconductor NFTO with excellent visible light harvesting could be used as an effective co-catalyst for g-C3N4. The specific surface area of the samples was measured to understand the photocatalytic activity deeply. Fig. 7 exhibits nitrogen adsorption-desorption isotherms at 77 K and corresponding pore size distributions of the samples, which show type IV hysteresis loops at P/P0 = 0.8~1.0, indicating a mesoporous structure. Their specific surface areas are obtained on the base of the Brunauer-Emmet-Teller (BET) method and displayed in the figures. SBET is 10.02 cm2·g–1 for pure g-C3N4, and increases a little after loading NFTO, attributing to the higher SBET of NFTO. The enhanced catalyst surface will help to improve the photocatalytic activity. The enhanced H2 production of the NFTO/g-C3N4 composites is revealed by photoelectrochemical responses. Photoluminescence intensity can effectively reveal the intrinsic recombination of photoexcited carriers during the photocatalytic process. As depicted in Fig. 8(a), NFTO/g-C3N4 composites, especial 2%-NFTO/CN composite, exhibit distinct weak peak at approximately 456 nm in contrast to the bare g-C3N4, implying that the photogenerated electron-hole recombination of g-C3N4 is efficiently suppressed after loading NFTO. Obviously, the outstanding charge separation over the NFTO/g-C3N4 photocatalysis system is benefited from the formed heterojunction [18,22,23,27]. The increasing charge separation revealed by the photoluminescence analysis is also reflected in the photocurrent.

12

Pure C3N4 Pure NFTO

200

1%-NFTO/CN SBET= 14.68 m /g

160

80

SNFTO/BET= 75.37 m2/g

40

SCN/BET= 10.02 m2/g

20

40 0 80 60

2%-NFTO/CN

60

3%-NFTO/CN 2

2

SBET= 16.02 m /g

SBET= 17.24 m /g

40

40

20

20

0

(a)

0 0.0

0.2

0.4

0.6

1.0

0.8

0.0

0.2

0.6

0.4

0.8

Na2Fe2Ti6O16

200

160

Pore diameter (nm)

0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01

1.0

g-C3N4 1%-NFTO-CN 2%-NFTO-CN 3%-NFTO-CN

0

Relative pressure (P/P0)

120

80

40

0

0 80

Pore volume (cm3/g)

Volume adsorbed (cm3/g)

120

60

2

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

Pore volume (cm3/g)

80

240

20

(b)

40

60

80

100

Pore diameter (nm)

Fig. 7. (a) Nitrogen adsorption-desorption isotherms of pure g-C3N4, NFTO and photocatalysts NFTO/g-C3N4 composites, (b) corresponding pore size distribution. The open dots represent adsorption process while the solid dots desorption process. Fig. 8(b) shows the photocurrent responses of pure g-C3N4 and NFTO/g-C3N4 composites under visible light irradiation. The distinct enhanced photocurrent of NFTO/g-C3N4 over the pristine g-C3N4 are observed, indicating that the construction of NFTO/g-C3N4 heterojunction efficiently improves transport and separation of charge carrier between g-C3N4 and NFTO, and thus a better photocatalytic performance can be

(a)

400 440 480 520 560 600 640

Wavelength (nm)

g-C3N4 2%-NFTO/CN

0.8

1%-NFTO/CN 3%-NFTO/CN

50 40

0.6

30

0.4

Z" (k)

g-C3N4 1%-NFTO/CN 2%-NFTO/CN 3%-NFTO/CN

Current density (Acm-2)

Intensity (a.u.)

highly anticipated [23,27,28].

0.2

20

Pure g-C3N4 1%-NFTO/CN 2%-NFTO/CN 3%-NFTO/CN

10

0.0

(b)

5

10

15

20

25

Irradiation time (s)

30

35

0

(c)

0

1

2

3

4

5

Z' (k)

6

7

8

9

10

Fig. 8. (a) Photoluminescence spectra of pure g-C3N4 and NFTO/g-C3N4 composites with an excitation wavelength of 360 nm in air, (b) transient photocurrent density for g-C3N4 and NFTO/g-C3N4 composites. (c) Nyquist plots of the impedance spectra for g-C3N4 and NFTO/g-C3N4 photocatalysts. The separation and transport of the photogenerated electrons are further investigated by an electrochemical impedance spectroscopy. As displayed in Fig. 8(c), 13

the smaller arc diameter in Nyquist plots indicates an easier carrier transfer in the NFTO/g-C3N4 composite. Efficient charge separation is achieved by decreasing the recombination of electron-hole pairs in NFTO/g-C3N4 heterojunctions. 2%-NFTO/CN has the lowest impedance, corroborating that the higher conductivity and stronger electron-hole separation contributes to the better photocatalytic H2 evolution [9,18]. Hence, all the above results reveal that NFTO is a promising co-catalyst in photocatalytic H2-production. The time-resolved photoluminescence decay is employed to accurately determine the lifetime of the photoexcited electron-hole pairs because the transport behaviors of charge carriers in photocatalyst are closely related to the fluorescence emission. Fig. 9 shows the time-resolved photoluminescence spectra to quantitatively investigate the photo-induced charge carrier lifetimes of the photocatalysts. For in-depth understanding the charge separation in photocatalysts, the spectra are fitted with a double-exponential photoluminescence function I (t )  A1 exp(t /  1 )  A2 exp(t /  2 )

 ave 

A1 12  A2 22 A1 1  A2 2

(2) (3)

where τ1 and τ2 are the decay times for the faster and the slower components, A1 and A2 are the contrition of each component; τ1 is the fluorescence lifetime, corresponding to the non-radiative binding process on the catalyst surface, and τ2 is the lifetime caused by the excited state electrons generated by catalyst, τave is the average decay time [9]. All of the fitting decay data are listed in the inset of Fig. 9. Obviously, the contribution of the fast time decay component to the overall photoluminescence lifetime increases from 92.6% to over 98.4% with the increase in NFTO amount, suggesting the strong interactive nature between g-C3N4 and NFTO. The average photoluminescence lifetime increases from 2.01 ns to 3.04 ns for 2%NFTO/CN. The results indicate a high separation efficiency and long lifetime of photogenerated charge carriers in the NFTO/g-C3N4 heterojunctions, which can provide 14

a higher possibility of charge carrier in the photocatalytic reaction, thus playing a pivotal role in surface redox reactions [12,14,18]. Sample g-C3N4 1%-NFTO 2%-NFTO 3%-NFTO

1(ns) 1.73

Intensity (a.u.)

800

A1(%) 2(ns) A2(%)  (ns)

600

92.6 5.48 7.38 2.01

2.02 94.9 6.85 5.10 2.76

400

2.17 96.8 8.63 3.21 2.92

g-C3N4 1%-NFTO/CN 2%-NFTO/CN 3%-NFTO/CN

200 0

2.17 98.4 12.6 1.56 3.04

0

10

20

30

Time (ns)

40

50

60

Fig. 9. Time-resolved transient photoluminescence decay of g-C3N4 and NFTO/g-C3N4 photocatalysts. 3.3 Optical absorption and band structure

ln(hv-2.8) -2.7 -2.4 -2.1 -1.8 y = 0.507x-1.53 Pure C3N4

6

2%-NFTO/CN 3%-NFTO/CN Pure NFTO

(hv)2 (eV2)

0.1-3.0

8

Pure C3N4 1%-NFTO/CN

0.6 0.4 0.2

0.01 200

(a)

300

400

500

4

2

ln(hv)

Absorptance (a.u.)

1

600

Wavelength (nm)

700

800

0 1.8

(b)

y = 21.01x-58.87 y = 15.85x-44.42 y = 13.08x-36.40 y = 10.92x-30.37 y = 16.71x-42.22 y = 8.27x-18.06 Pure C3N4 1%-NFTO/CN 2%-NFTO/CN 3%-NFTO/CN pure NFTO

2.0

2.2

2.4

2.6

2.8

3.0

3.2

Photon energy (eV)

Fig. 10. (a) UV-vis diffuse reflection spectra of the pure g-C3N4, NFTO and NFTO/gC3N4 composite samples with different NFTO amounts, (b) plot of (αhν)2 versus photon energy (hν) to determine the bandgap. The inset shows ln(αhν) versus ln(hν – 2.8), revealing a direct-gap semiconductor behavior of pure g-C3N4. The green ellipses indicate the two absorption platform and the red rectangular box shows the enhanced visible-light absorption of NFTO/g-C3N4 composites. Fig. 10(a) displays the optical properties of the pure g-C3N4, NFTO and NFTO/gC3N4 composites evaluated by UV-vis diffuse reflection spectra. Pure g-C3N4 product is a typical direct-bandgap semiconductor because n = 1 when fitting the experimental n2 data near the absorption edge with  hv  A(hv  Eg ) as shown in the inset in Fig.

15

10(a). Its band gap is estimated as 2.8 eV by linearly fitting the data near the absorption edge as shown in Fig. 10(b), which is consistent with the recently reported values [6,13,15,27]. As a light harvester, the NFTO exhibits double absorptions with directbandgaps of 2.18 eV and 2.53 eV [29], enhancing the light absorption of NFTO/g-C3N4 heterostructure in visible region as shown by the red square box in Fig. 10(a). The bandgap investigated from the (ahv)2 versus photon-energy plots [Fig. 10(b)] further reveals that the band characteristic for NFTO/g-C3N4 composites can be adjusted by loading NFTO onto g-C3N4. The g-C3N4 is a disclose n-type semiconductor with the valence band maximum at 1.58 eV vs. NHE and conduction band minimum at –1.12 eV vs. NHE [28,51]. But NFTO, as a novel semiconductor, need investigated to build energy band structure to understand the NFTO/g-C3N4 heterostructure. Herein, the electrochemical MottSchottky measurement was carried out in darkness at the following conditions: an AC frequency of 600 Hz with amplitude of 10 mV, scanning from –1.0 V to 1.0 V vs. Ag/AgCl and in 0.2 M Na2SO4 solution. A positive slope in Mott-Schottky plot described in Fig. 11 indicates an n-type semiconductor characteristic of NFTO [13]. Meanwhile, the position of conduction band minimum was estimated by Mott-Schottky equation, expressed as [13,52]

kT 1 2  ( Eappl  EFB  ) 2 CSC  0 qN D q

(4)

where CSC is the capacitance (F·cm–2), ND is the donor density (cm–3), ε and ε0 are the relative dielectric constant and the permittivity of vacuum, respectively, Eappl is the applied potential, EFB is the flat band potential, k is the Boltzmann’s constant, T is the absolute temperature, and q is the electronic charge. EFB, at a given T and pH of the electrolyte solution, can be obtained by extrapolating the linear part to zero in the Csc-2 vs. Eappl plot in Fig. 11, where EFB = Eappl – kT/q. The obtained flat band potential for the NFTO semiconductor is about –0.014 V vs. NHE at pH 7.0, which is further verified by the empirical method proposed by Butler and Ginley. The band positions of the semiconductor NFTO were estimated from the absolute electronegativity of the atoms 16

with the following formulas [53,54]

EVB    Ee  0.5Eg

(5)

Eg  EVB  ECB

(6)

where EVB and ECB are the valence band and conduction band edge potentials, respectively; χ is the absolute Mulliken electronegativity of the semiconductor, which is defined as the geometric mean of the absolute electronegativity of the constituent atoms; Ee (= 4.5 eV) is the energy of free electrons on the hydrogen scale; and Eg is the band gap energy of the semiconductor (Herein, Eg = 2.20 eV). The absolute electronegativity of Na, Fe, Ti, O is 2.85, 4.03, 3.45 and 7.54 eV, respectively [55,56]. Thus, the EVB and ECB of NFTO are estimated at 2.166 and – 0.033 eV, respectively. The ECB is very close to the above result of Mott-Schottky method, and we consider ECB more negative than EVB. 18

1/C2 (×109 F-2cm4)

16 14 12 10 8 6 4 2 0 -0.9

0.013 V -0.6

-0.3

0.0

0.3

0.6

0.9

1.2

Potential (V vs NHE)

Fig. 11. Mott-Schottky plot measured in 1.0 M Na2SO4 electrolyte (pH =7, at 298 K) to determine the semiconductor type and the Fermi level of novel material NFTO. The intercept of the straight line with the x-axis corresponds to EFB + kT/q, the inset is NFTO band structure. 3.4

Mechanism discussion On the basis of the above band energy level analysis, a related mechanism for the

enhanced photocatalytic activity over NFTO/g-C3N4 heterojunction was proposed. The EVB and ECB of n-type semiconductor NFTO were estimated at 2.167 and – 0.033 eV, respectively, and it is well known that the g-C3N4 is also a typical n-type semiconductor 17

with the EVB at 1.58 eV and ECB at –1.12 eV vs. NHE [57]. When NFTO and g-C3N4 composite together, Fermi level equilibration achieves to form an n-n type heterojunction as shown in Fig. 12. Meanwhile, the alignment of Fermi level results in more balanced negative redox potential of both NFTO and g-C3N4 compared to H+/H2 (0 eV vs. NHE) [58], which is favorable and catalytically active in water splitting reaction [59].

Fig. 12. Photoinduced electron-hole pair separation under visible light irradiation (λ > 420 nm) for hydrogen evolution from water containing 10 vol% triethanolamine (TEOA) as the sacrificial agent by 3.0 wt% Pt-deposited NFTO/g-C3N4 heterojunction. Under visible light irradiation, electrons from the valence bands of NFTO and gC3N4 are excited to their corresponding conduction bands, leaving holes in the conduction bands, respectively. The photogenerated electrons transfer from g-C3N4 to NFTO owing to the existence of a potential gradient at the interface between g-C3N4 and NFTO. At last, the photogenerated electrons tend to be trapped by the Pt nanoparticles on the surface of photocatalysts, and used to reduce H2O to H2. Meanwhile, the photogenerated holes transfer from the conduction band of NFTO to that of g-C3N4, and are consumed by the sacrificial reagent TEOA, which is helpful to decrease the concentration of holes in the photocatalytic reaction system. As a consequence, the NFTO/g-C3N4 heterojunction significantly improve the separation of photogenerated electron/hole pairs, resulting in an enhancement of photocatalytic performance for H2 evolution. 18

4

Conclusions In summary, NFTO nanosheets as a co-catalyst were loaded on g-C3N4

nanoparticles to form heterojunctions for the first time. The NFTO/g-C3N4 heterojunctions promote the light harvesting, increase conductivity, hinder the recombination of photogenerated electron-hole pairs, and improve the carrier transfer efficiency. Therefore, the NFTO/g-C3N4 heterojunctions exhibit approximately 1.8 times higher hydrogen generation as pure g-C3N4 nanoparticles. This work provides new semiconductor co-catalyst as low-price, highly efficiency for promoting the photocatalytic hydrogen production.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51672168) and the Fundamental Research Funds for the Central Universities (No. GK201901005).

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Title: Na2Fe2Ti6O16 as a hybrid co-catalyst on g-C3N4 to enhance the photocatalytic hydrogen evolution under visible light illumination Authors: Ze-Qing Guo, Qi-Wen Chen, Jian-Ping Zhou

The authors claim that none of the material in the paper has been published or is under consideration for publication elsewhere.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Title: Na2Fe2Ti6O16 as a hybrid co-catalyst on g-C3N4 to enhance the photocatalytic hydrogen evolution under visible light illumination Authors: Ze-Qing Guo, Qi-Wen Chen, Jian-Ping Zhou 4.1 Highlights A few NFTO nanosheets were loaded on g-C3N4 to form heterojunctions. NFTO extends the light harvesting and promotes electron-hole separation. NFTO improves the hydrogen generation of NFTO/g-C3N4 nanocomposite. The H2 production rate reaches 389 μmol·g–1·h–1 under visible light illumination.

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