Sulfur- and Carbon-Codoped Carbon Nitride for Photocatalytic

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Sulfur and Carbon Co-doped Carbon Nitride for Photocatalyltic Hydrogen Evolution Performance Improving Huiwen Tian, Xiaoying Zhang, and Yuyu Bu ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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ACS Sustainable Chemistry & Engineering

Sulfur and Carbon Co-doped Carbon Nitride for Photocatalyltic Hydrogen Evolution Performance Improving Huiwen Tiana, Xiaoying Zhanga and Yuyu Bub* a

Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology,

Chinese Academy of Sciences, Qingdao, 266071, China b

Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices, School of

Microelectronics, Xidian University, Xi’an, 710071, China.

Corresponding author E-mail: [email protected] and [email protected]

ABSTRACT: In this work, trithiocyanuric acid and graphene oxide quantum dots (GOQDs) served as sulfur (S) and carbon (C) sources to prepared S, C co-doped carbon nitride (SCCN). The morphology of SCCN changes from lamellar to airbag-like nano-structure with GOQDs addition. The photocatalytic hydrogen evolution performance of the optimized SCCN photocatalyst shows 5.6 times higher than that of the CNS photocatalyst. The mechanism of the improved performance is further investigated. It is found that the concentration of hydroxide radical increases during the photocatalytic process, based on the valence band potential of the

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SCCN photocatalyst moves positively and the surface oxidation energy barrier decreasing. Furthermore, the charge transfer capacity of the SCCN photocatalyst is improved also, so that decrease the recombination rate of the photogenerated free charges.

Keywords: conduction band positive moving; carbon self-doping; co-doped C3N4; Graphene oxide quantum dots; charge transfer

INTRODUCTION

Graphite carbon nitride (g-C3N4) is widely considered as a potential organic polymer semiconductor for solar energy conversion.[1-5] Up to now, it has been used in the fields of photocatalytic water splitting,[6-8] organic pollutions degradation,[9-11] antibacterial,[12-14] CO2 reduction,[15-17] organic waste gas degradation,[18-21] Electrocatalytic water splitting,[22] and bifunctional oxygen electrocatalyst for rechargeable Zn−Air batteries.[23-25] A urgent issue of g-C3N4 is that its valance band (VB) potential is merely near the water oxidation potential, resulting in weak oxidation activity of the photogenerated holes. Thus, it is very urgency to move positively the VB potential of g-C3N4 semiconductor photocatalyst.[26-28] To address this issue mentioned above, two strategies have been carried out. The one is to deposit transition metal oxides[29, 30] or phosphide[31, 32] on the surface of g-C3N4 to decrease the energy barrier of the oxidation reaction. The other one is to regulate the state of electrons on the VB by doping other elements. It is a direct and essential method to improve the oxidation capacity of g-C3N4.[33]


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Recently, carbon doping has been investigated to regulate the VB band potential of g-C3N4 and therefore improve the oxidation capacity of the corresponding photogenerated holes. Liu etc.[34,35] found that the VB potential of g-C3N4 could move positively to near 1.6 V (vs RHE) by carbon doping, and therefore increasing the oxidation capacity of photogenerated holes. Cheet. al.[26] copolymerized C3N4 matrix with the carbon rings to form an in-plane g-C3N4-based heterostructure. Through x-ray absorption near-edge spectroscopy, electrochemical impedance spectroscopy (EIS) and first-principles calculations, it is indicated that the VB potential of the C3N4 moves positively and the charge transfer capacity is also improved obviously. On the other hand, the electronic structure of g-C3N4 has also been modified by sulfur doping, which can substitute the lattice N atoms on the C3N4 matrix. After S doping, the conduction band potential of g-C3N4 would move positively, thus narrowing its width of band gap and then enlarging the absorption range of photons.[37-40] EXPERIMENTALSECTION

Preparation of SCCN

S doped g-C3N4, marked as CNS, was prepared by thermal decomposition 5 g trithiocyanuric acid under 650 °C for 2 h with a heating rate of 10 °C min−1 in Ar atmosphere. For SCCN, 5 g trithiocyanuric acid dissolved in 80 mL deionized water, and then different dosages (0.02, 0.05, 0.1 and 0.15 mL) of GOQDs dispersion solution (Purchase from Nanjing JCNANO Technology Co., LTD, Nanjing, China, and the HRTEM of GOQDs shows in Fig. S1.) With a concentration


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1 mg/mL were added into the trithiocyanuric acid solution above, respectively. So, the concentration of the GOQDs in the stock solution can be defined as 0.25 µg/mL, 0.63 µg/mL, 1.26 µg/mL and 1.89 µg/mL. The mixtures were stirred under 60 °C to complete dry. Subsequently, these mixture powders were under 650 °C for 2 h with a heating rate of 10 °C min−1 in Ar atmosphere. The end-product powders were marked as SCCN 0.02, SCCN 0.05, SCCN 0.1 and SCCN 0.15, respectively. Except GOQDs, all other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received without further purification.

Characterization

Scanning electron microscope (SEM, EVO18; ZEISS, Germany) was employed to analyze the morphology of these samples. The crystalline structures of these samples were tested by X-ray diffraction (XRD, D/MAX-2500/PC; Rigaku Co., Tokyo, Japan). The nano-morphology of CNS, SCCN 0.1 and SCCN 0.15 was observed by field emission transmission electron microscope (FE-HRTEM, Tecnai G2 F20, FEI Company, USA). The elements information and bonding information of the samples were analyzed by energy dispersive spectrometer (EDS, EVO18; ZEISS, Germany) and X-ray photo electron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd., England). Optical absorption properties of these materials were tested using UV-Visible diffuse

reflectance

spectrophotometer

(U-41000;

HITACHI,

Tokyo,

Japan).

The

photoluminescence intensities of the prepared photocatalysts were characterized by fluorescence


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spectrometer (PL, Fluoro Max-4, HORIBA Jobin Yvon, France). DMPO-•O2− (adding 0.01 g photocatalyst into 50 mL methanol) and DMPO-•OH (adding 0.01g photocatalyst into 50 mL DMPO) of CNS and SCCN0.1were tested by electron spin resonance spectrometer (ESR; JESFA series; JEOL; Japan) under visible light illumination. The Brunauer−Emmett−Teller (BET) surface area was determined by a multipoint BET method (BET, BK132F, JWGB Sci. & Tech., China). The Fourier transform spectrophotometer (FT-IR, VERTEX 70, Bruker) with KBr was employed to test the chemical bonds changing on the materials.

Assessment of the photocatalytic hydrogen evolution performance

In this test, 0.05 g photocatalyst was added into a mixture including 10 mL triethanolamine and 90 mL H2O. Subsequently, 1wt% HPtCl4 added into the mixture. A 150 W Xe lamp (PLSSXE300, Beijing Changtuo Co. Ltd., China) was used as light source with a 420nm cut-off filter to remove ultraviolet light, and the light density at the liquid surface was adjusted to 200 mW/cm2 by a spectroradiometer (Thorlabs, PM100D). The testing temperature was controlled at 5 °C by a condensate water system.

The AQE performances of CNS, SCCN0.02, SCCN0.05, SCCN0.1 and SCCN0.15 photocatalysts were tested under various wavelengths (385 nm, 447 nm, 470 nm, 530 nm, 590 nm, 617 nm and 680 nm, Zaher, Germany) LED light illumination, and the incident light intensity was measured by Spectroradiometer (Thorlabs, PM100D). The AQE values under different LED light stimulation were calculated according to the following Equ.


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AQE = 2 × number of evolved H2 molecules / number of incident photons

Photoelectrode preparation and electrochemical measurements

A FTO glass (13 × 10 mm) was cleaned in acetone and water mixture solution, and then dried by dry airflow. Subsequently, 0.01 g photocatalyst powder was mixed with 4 µL naphthol and 0.1 mL of deionized water in an agate mortar, and ground for 10 min to form a homogeneous suspension. Then, 0.025 ml of the as-prepared suspension was dropped on FTO glass with a active area 1 × 1 cm2, then heated at 120 °C for 2 h under vacuum condition. A copper wire was connected to the conductive side of the FTO glass using conductive silver tape. Uncoated part of the FTO glass and cooper wire were isolated by parafilm.

Electrochemical measurements were tested by CHI660D Electrochemical Workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). The prepared photoelectrodes, Ag/AgCl and Pt electrode served as the working electrode, reference electrode, and counter electrodes, respectively. The photoinduced current density with time (i-t curve) was measured at a bias potential 0.5 V under visible light illumination. Electrochemical impedance spectroscopy (EIS) tests were performed at an open circuit potential over the frequency range between 104 and 10−1 Hz, with an AC voltage magnitude of 5 mV, using 12 points/decade. Mott-Schottky plots was measured in the potential range of -1.0 V ~ 0.5 V and the frequency used as 10 Hz with an AC voltage magnitude of 10 mV. Above electrochemical tests all carried out in 0.1 M Na2SO4 electrolyte. The linear scan voltammograms (LSV) tests were performed in 1 M KOH with scan


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rates of 10 mV s-1 in the potential range of 0 to 1.0 V (vs Ag/AgCl). The LSV potentials vs Ag/AgCl were converted to a reversible hydrogen electrode (RHE) scale (ERHE = EAg/AgCl + 0.059 pH + 0.197).

RESULTS AND DISCUSSION

The SEM images of as-prepared CNS and SCCN 0.1 are shown in Fig. 1. From the lowresolution SEM image of CNS (Fig. 1A), typical layer structure of C3N4 which fabricate by some large-scale nanosheets, meanwhile some smaller nanosheets with diameter less than 1µm accumulate around the layer structure. Under the high-resolution, as shown in Fig. 1B, CNS shows a smooth surface and distinct layer structure. Fig. 1C presents the SEM image of SCCN 0.1 at low resolution. Interestingly, the layer structure of CNS has become airbag structure. Since the GOQD dispersion liquid mixture with the trithiocyanuric acid, because of high density oxidation chemical groups on the surface of GOQDs, the trithiocyanuric acid would be captured by them to form mixed coacervates which provide a template for airbag structure formation after CNS polymerization under high temperature. Fig. 1D shows the high-resolution SEM morphology of SCCN 0.1. Many loopholes can be observed on the surface of the airbag like structure. This phenomenon indicates that airbag like structure of SCCN 0.1 can be created with the addition of GOQDs, and the loopholes on the surface of the airbag structure demonstrate that many gases such as NH3, CO2 and H2S have produced during the precursor decomposition,


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increasing the internal pressure of the airbag like structure of SCCN 0.1 and then inducing loopholes formation.

Fig. 1 The low (A) and high (B) resolution SEM images of CNS, respectively; (C) and (D) are the low and high-resolution SEM images of SCCN 0.1.

Fig. 2 shows the TEM images of CNS, SCCN 0.1 and SCCN 0.15 samples. Fig. 2A is the lowresolution TEM image of CNS, a large scaled nanosheet can be observed, which is exfoliated from the layered structure of CNS during the preparation process of TEM sample. From Fig. 2B, the nanosheet structure can be observed more clearly. Fig. 2C shows the HRTEM images of CNS, which presents a typical HRTEM morphology of carbon nitride-based materials. Fig. 2D shows the low resolution TEM image of SCCN 0.1, comparing with Fig. 2A, many independent airbag structures can be observed obviously, which corresponds to the SEM image of SCCN 0.1 sample (Fig. 1 C). Further increasing the magnification factor, as shown in Fig. 2E, a will-


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defined airbag structure can be found. Fig. 2F presents the HRTEM image of SCCN 0.1. No GOQDs particles can be distinguished on the surface of SCN nanosheet, and this phenomenon need to be proved by other technologies further.. Fig. 2 G to 2 I show the low and high resolution TEM images of SCCN 0.15. These images present similar structures with the sample of SCCN 0.1, indicating that the airbag morphology of SCCN 0.15 has not changed with the added amount of GOQDs increasing. In addition, from the SAED images showed in Fig. 2F and 2I, we can get the information that both SCCN samples are amorphous, thus the lattice spacing can’t be found on the HRTEM of these samples. Furthermore, these results also indicate that there are no crystal GOQDs on the surface of SCCN. The possible formation mechanism of SCCN airbag structure presented in Fig. 2J. In the precursor solution which contain GOQDs and trithiocyanuric acid, the trithiocyanuric acid molecules would capture by GOQDs, because there is a van der Waals interaction between the sulfydryl groups on trithiocyanuric acid and oxidation groups on GOQDs, respectively. Thus, the trithiocyanuric acid molecules would anchor around the GOQDs. After polymerization under high temperature, the anchoring structure would shrink inward, meanwhile the NH3 and H2S would production during this processing to facilitate the airbag like structure of SCCN.


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J

Fig. 2. TEM images of CNS (A) ~ (C) from low to high resolution; SCCN0.1 (D) ~ (F) from low to high-resolution; SCCN0.15 (G) ~ (I) from low to high-resolution. Inset images are the SEAD of SCCN0.1 and SCCN0.15, respectively; (J) Possible mechanism of airbag SCCN formation.


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The small angle XRD results of CNS and SCCN samples are shown in Fig. S2. Strongly peak at 2θ = 27.4° is observed on CNS. This peak corresponds to the interlayer stacking of aromatic series.[1] However, with GOQDs added amount increasing, these peak shift toward a lower 2θ angle direction gradually. This crystal lattice distortion phenomenon similar the previous result reporting by Dong et al., and this peak shifting attributed to the carbon self-doping in heptazine heterocyclic ring of C3N4.[41]

The FTIR results of CNS, SCCN0.05, SCCN0.1 and SCCN0.15 showed in Fig. 3A. As reported by the references[42,43], for a typical C3N4, the IR peaks at 1636, 1571, 1454, 1325 and 1250 cm-1 corresponded to the stretching vibration modes of heptazine heterocyclic ring units. After S doping, as showed in Fig. 3A, the peak at 1250 cm-1 of C3N4 blue shift to 1233 cm-1, meaning that the S element doped into the heptazine heterocyclic ring of C3N4. Comparing the four samples at1558 cm-1, it can be observed that the peaks appear at this site on the IR curves of SCCN0.1 and SCCN0.15. This IR peaks attributed to the stretching vibration of C=C bond, indicating that for the samples of SCCN0.1 and SCCN0.15, the carbon atoms doped into the heptazine heterocyclic ring of C3N4. In addition, the IR spectral of GOQDs provided in Fig. S1, comparing it with these four samples, the difference between them are highly significant, this result indicting that the GOQDs do not exist on SCCN, but doped into it.

The XPS was employed to analyze the doping situation of the CNS, SCCN 0.1 and SCCN0.15. The XPS survey spectrum of these three samples were showed in Fig. S3A. C, N and O elements


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can be observed on these three curves. The signal of oxygen peaks much lower than C and N elements, indicating trace amount of oxygen in the SCN and SCCN samples. The O 1s XPS curves of these three samples present in Fig. S3B, the intensity of O 1s XPS peaks enhances lightly with the GOQDs adding amount increasing, which induce more oxygen elements in the SCCN.

Fig. 3B shows the C 1s of CNS,SCCN 0.1 and SCCN0.15 samples, peaks at 288.3 eV and 284.8 eV are corresponding to C-N in triazine, defect C=C respectively.[35] In addition, the intensity of C=C peaks in SCCN are higher than that of CNS, indicating that the C atoms doped into the heptazine heterocyclic ring units of CNS, thus increasing the density of C=C bonds in the SCCN samples.[40] The peaks locating at 289.3 eV attribute to the O=C-O bonds, the peak intensity of SCCN 0.1 and SCCN0.15 samples are higher than that of SCN, meaning that the density of surface O=C-O bonds on SCCN samples are increased after GOQDs doping, which result identify with the O 1s results showed in Fig. S3B Fig. 3C presents the N 1s spectrum of these three samples. The peaks at 399.6 eV and 400.7 eV attributed to O=C-N and tertiary N (N-(C)3) groups, respectively.[35] Comparing these peaks, no obvious peak shift can be observed. However, the C-N=C peaks of these three samples are shifted to higher energy direction with the GOQDs adding amount increasing. This phenomenon indicating that C atoms doped in the heptazine heterocyclic ring units of CNS by substitution the N atoms, thus inducing a peak shift of the CN=C peaks.


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Fig. 3D shows the S2p on the samples of CNS, SCCN0.1 and SCCN0.15. Comparing these three curves, we can find that the peak intensities of S2p on SCCN are much higher than that of CNS. The possible reason is that the GOQDs shows higher capture capacity of the sulfur group in trithiocyanuric acid, thus inducing higher sulfur content in the SCCN samples.

For further confirmation this result, the EDS and ICP-MS were employed to analyze the contents of C and S elements in CNS and SCCN 0.1, respectively. And the results are presented in Fig. S4 and Tab. S1. As shown in Fig. S4, the sign of S element on SCCN 0.1 is much stronger than that of CNS. Tab. S1 presents their ICP-MS results. For CNS, the atomic percentages of C, N, S and O elements are 39.48%, 59.16%, 0.25% and 1.11%, respectively. While the corresponding atomic percentages of C, N, S and O elements in SCCN0.1 are 41.25%, 56.39%, 0.34% and 2.02%, respectively. It is found that after S-C co-doping, the C content not increases obviously, because just litter GOQDs was added in the reaction precursor. Whereas, after GOQDs doping, the N content dose reduce from 59.16% (CNS) to 56.39% (SCCN0.1), meanwhile, the S content dose increase from 0.25% (CNS) to 0.34 % (SCCN0.1). The O atomic percentage dose increase from 1.11% to 2.02%. These changes indicate that carbon atoms replace the N atoms in the heptazine heterocyclic ring units by GOQDs adding to induce the atomic percentage of N, meanwhile, increase the atomic percentage of S and O.


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Intensity (a.u.)

0.4

289.3 eV O=C-O

284.8 eV C=C 1571 1636

0.6

(B) C1s 1325

CNS SCCN0.05 SCCN0.1 SCCN0.15 1233

Intensity (a.u.)

0.8

1405

(A)1.0

0.2

0.0

SCCN 0.15

288.3 eV C-N

SCCN 0.1

CNS

1558

1000 1100 1200 1300 1400 1500 1600 1700 1800 -1

284

285

286

Wavelength (cm )

(C) N1s

C-N=C

399.6 eV 400.7 eV O=N-C N-(C3)

287

288

289

290

291

Binding energy (eV) (D) S 2p SCCN 0.15

SCCN 0.15

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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SCCN 0.1

SCCN 0.1

CNS CNS 396

397

398

399

400

401

402

Binding energy (eV)

158

159

160

161

162

163

164

165

166

167

Binding energy (eV)

Fig. 3. (A) FTIR curves of CNS and SCCN samples; (B) C 1s, (C) N 1s and (D) S 2p XPS results of CNS and SCCN 0.1 and SCCN0.15.

The optical characters of SCCN samples were tested by UV-Vis DRS, and the results were shown in Fig. 4. It can be seen that all samples present direct absorption edges near 460 nm, which is a typical absorption of carbon nitride semiconductor.[1] At the area of wavelength less than 460 nm, CNS shows highest absorption capacity, and with the doping amount of GOQDs increasing, absorption capacity decreases gradually. This phenomenon maybe attribute to the electron structure change with the C doped in. When the wavelength higher than 460 nm, the CNS presents a wide indirect absorption extending to near 700 nm, which contribute to the S doping.[37-39]


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1.2

a b c d e

1.0

0.8

Abs. (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

CNS SCCN 0.02 SCCN 0.05 SCCN 0.1 SCCN 0.15

a b c d e

0.2

0.0 200

300

400

500

600

700

800

Wavelength (nm)

Fig. 4. UV-Vis DRS results of CNS and different SCCN samples.

Fig. 5A presents the photocatalytic hydrogen evolution performance of SCCN samples. The hydrogen evolution amount of CNS is about 900 µmol/g in 4 h. Compared with CNS, all codoped samples present higher photocatalytic hydrogen evolution properties. In addition, the photocatalytic hydrogen evolution properties of SCCN photocatatysts improves gradually with the GOQDs doping amount increasing to 0.1 mL, and it does not enhance with GOQDs further increasing. The highest performance was achieved by SCCN 0.1, and the corresponding total amount of photocatalytic hydrogen evolution reaches 5050 µmol/g in 4 h, which is 5.6 times higher than that of CNS. However, the corresponding value reduces to 1950 µmol in 4 h with the GOQDs adding amount reaching 0.15 mL, indicating that excessive levels of GOQDs amount will inhibit the photocatalytic hydrogen evolution performance. Therefore, it evidences that the photocatalytic hydrogen evolution performance is strongly dependent on the GOQDs added amount.


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Fig. 5B shows the AQE value of these photocatalysts. From this result, the SCCN 0.1 possesses highest AQE value, which is in accord with the situation in Fig. 5A. Furthermore, all samples show high AQE value in the wavelength range of 385 nm to 470 nm, and the photocatalytic performance of the SCCN samples can be exceeded to near 620 nm, indicating that these samples possess excellent photocatalytic performance in visible light region. Fig. 5C presents the performance stability of the SCCN 0.1 sample. There is not an obvious decay of the photocatalytic water reduction performance of this sample, indicating that SCCN0.1 is a stable photocatalyst with S and C co-doping. After photocatalytic water reduction for 4 cycles, the SEM image of the SCCN0.1 was tested, and the result showed in Fig. S5. Comparing it with Fig. 1C, the airbag structure has been changed slightly, however, the photocatalytic water reduction performance could be maintained.

The Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves of CNS, SCCN0.05, SCCN0.1 and SCCN0.15 are showed in Fig. S6, the specific surface area of these samples are 69.513, 73.814, 80.364 and 84.227 m2/g respectively. Previous results showed that the airbag structure of SCCN formed when the GOQDs doping in. Thus, with the airbag structure formation, the specific surface area shows upward tendency. For all samples, the pore diameters distribute near 2 to 20 nm, no obvious macropore can be observed.


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5500 5000 4500 4000

(B)24


(C)

22

a b c d e

20 18 16

3500

d

3000 2500

c

2000 1500

b

1000

e a

500

14

Amount of H2 evolved (µmol*g-1)

a b c d e

6000

AQE (%)

(A)6500 Amount of H2 evolved (µmol*g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



12 10 8 6 4 2 0

0 0

1

2

Irradiation Time (h)

3

4

SCCN 0.1

5000

1 st

3 th

2 nd

4 th

4000

3000

2000

1000

0 400

450

500

550

600

650

700

Wavelength (nm)

0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16

Time (h)

Fig. 5. (A) photocatalytic water reduction for hydrogen evolution performance of CNS and different CNS-GOQD samples in 10 v% triethanolamine solution, illuminated under visible light (> 420 nm, 200 mWcm-2); (B) Wavelength-dependent apparent quantum efficiency (AQE) of hydrogen evolution (385 nm, 447 nm, 470 nm, 530 nm, 590 nm, 617 nm and 680 nm); (C) stability on photocatalytic water reduction performance of SCCN0.1.

Fig. 6 shows the photoelectrochemical and electrochemical performances of CNS and series of SCCNs photoelectrodes. From Fig. 6A, we can find that CNS shows the lowest photocurrent density. The photocurrent densities of SCCNs photoelectrodes enhance gradually with the GOQDs doped content up to SCCN0.1. While further increasing the GOQDs doped amount, the photocurrent density of SCCN 0.15 decreases dramatically. The SCCN 0.1 achieves the highest photocurrent density, meaning higher photon quantum efficiency than other photocatalysts. Fig. 6B shows the EIS results of these samples. The radius of impedance arc on EIS curve is mainly in direct proportion to the charge transfer resistance.

[44]

As shown in Fig. 6B, the radiuses of

impedance arc are decreased with the GOQDs doped amount, indicating the conductivity of the samples enhance gradually with the C doping. This is a positive factor for the photocatalytic


17


performance improving. In addition, these EIS date were fitted by the equivalent circuit presenting in Fig. S7, and the fitting date showed in Tab S2. Rfilm is the resistance of the thin film. With

the

carbon

doping

amount

in

CNS

increasing,

their

Rfilm

reducing

near

two orders of magnitude. Rss is defined as surface charge transfer resistance, comparing these date, it can be found that the Rss values of SCCN0.1 and SCCN0.15 are very close, but much smaller than CNS and SCCN0.05. Normally, the sample which possess smaller Rfilm and Rss should present higher photocatalysis performance, however, as shown in Fig. 5A and Fig. 6A, SCCN0.15 presents the lowest photocatalytic property and photocurrent density, meaning that new charge recombination centers would be formed since excess carbon doping in the CNS semiconductor, thus inducing photocatalytic water reduction performance decreasing.

Fig. S8 shows the PL spectrum of CNS and CNS-GOQD photocatalysts. The higher PL peak intensity indicates higher free charges recombination rate. From this result we can find that CNS and SCCN 0.1 present the highest and lowest PL peak intensity, indicating highest and lowest free charges recombination rate of the photocatalysts, respectively. (A)3.5

(B)200000 a

3.0

b 150000

2.5

c

1.5

1.0

c

SCCN 0.05

b SCCN 0.02 d SCCN 0.15

0.5

5000

100000

50000

CNS SCCN 0.02 SCCN 0.05

d e

SCCN 0.1 CNS-GOQD-0.1 SCCN 0.15 CNS-GOQD-0.15

4000

d

3000 2000

e

1000 0 0

a CNS

0.0

a b c 7000 6000

SCCN 0.1

Z' (Ohm)

e 2.0

Z' (Ohm)

J (µA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

1000 2000 3000 4000 5000 6000 7000

Z (Ohm)

0 0

50

100

150

200

250

300

350

400

450

0

50000

Time (s)

100000

150000

200000

Z (Ohm)


18

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1.60E+010

a b c d e

(D)


0.0008

a 0.0006

b

1.20E+010

J (A/cm2)

(C)2.00E+010

1/C2 (cm4 F-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c 8.00E+009

d


a b c d e

0.0004

dec b a

0.0002

4.00E+009

e 0.00E+000 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.0000 0.60

0.65

Potential (V)

0.70

0.75

0.80

0.85

0.90

0.95

Potential (V)

Fig. 6. Photoelectrochemical and electrochemical performances of CNS and series of SCCNs photoelectrodes, (A) photoinduced i-t curves; (B) EIS; (C) Mott-Schottky plots; (D) LSV test for oxygen evolution in 1 M KOH solution, vs Ag/AgCl.

Mott-Schottky method was employed to test the flat band of these samples and the results are showed in Fig. 6C. From this result, we can find that all samples show n-type characteristic, because the slopes of the linear region are positive. The flat band potentials of CNS and SCCN 0.02 are both keep at -0.84 V (vs Ag/AgCl), while they move positively to -0.52 V (vs Ag/AgCl), when the GOQDs added amount exceed 0.05 mL. This result indicates that the conduction band potential of the carbon nitride can be moved positively by S and C co-doping. Furthermore, as shown in Fig. 6C, the slopes of the tangent line, are change to smaller with the GOQDs added amount, indicating that the free charge density can be enhanced with the S-C co-doping.[3]

With the energy band positive movement, the surface oxidation energy barrier of the samples may be decreased. Herein, the LSV for oxygen evolution potential of these samples were measured, and the results show in Fig. 6D. From this result, we can get the information that after S and C co-doping, the oxygen evolution potential will move negatively, and the SCCN 0.1


19


sample displays lowest oxygen evolution potential, indicating that the surface electron structure of SCCN 0.1 sample is the most favorable one to push the oxidation reaction occurring. Thus, we can infer that more oxidation catalysis centers can be formed with S and C co-doped into the carbon nitride, to provide more active centers with lower energy barrier for the oxidation reaction of photogenerated holes. (A)

(B)

Hydroxyl radical

Superoxide radical SCCN 0.1

SCCN 0.1 Visible light 10 min SCCN 0.1 dark

CNS Visible light 10 min

Intensity (a.u.)

Visible light 10 min

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

SCCN0.1 dark

CNS Visible light 10 min

CNS dark

CNS dark 317.8

317.9

318.0

318.1

318.2

318.3

317.8

317.9

318.0

B (mT)

318.1

318.2

318.3

318.4

B (mT)

Fig. 7. DMPO-ESR spin-trapping spectra of CNS and SCCN 0.1, (A) hydroxyl radical sign and (B) superoxide radical sign, under dark or by visible light stimulate for 10 min.

The DMPO-ESR spin trapping was employed to detect the concentration of •O2 and •OH radicals production by the CNS and SCCN 0.1 under visible light illumination.[45] It is well known the •OH radicals is mainly produced by photogenerated holes oxidation, and the sign intensity of •O2 radicals is related to the yields of photogenerated electrons. As shown in Fig. 7, under dark，there is no any sign of •OH radicals or •O2 radicals, indicating that both •OH radicals and •O2 radicals can’t be produced spontaneously. After visible light illumination for 10 min, the •OH radicals sign of SCCN 0.1 is much stronger than that of CNS, while the intensities


20

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of •O2 radicals sign on SCCN 0.1 is a litter stronger than CNS also. The increase of •OH radicals sign intensity means that the oxidation energy barrier decrease or more photogenerated holes production during photocatalytic process. In addition, the unobvious change of intensity indicates similar yields of photogenerated electrons in both photocatalysts. Thus, C-S co-doping provides more active centers with lower energy barrier for the oxidation reaction of photogenerated holes, which is in accord with the LSV tests shown in Fig. 6D.

Fig. 8 Mechanism of S and GOQDs co-doped carbon nitride for photocatalytic performance improving.

The possible mechanism of enhanced photocatalytic performance on carbon nitride by S and C co-doping is showed in Fig. 8. Combination of the Mott-schottky plots and UV-vis DRS results, the band energy information of CNS and SCCN was provided in Fig. 8. The CB potential of CNS locates at -0.84 V (vs. Ag/AgCl). After S-C co-doping, the CB potential of SCCN can shift to -0.52 V positively. In addition, from the UV-vis DRS curves, we find that all bandgap width of the samples keep at 1.77 eV, thus the VB band potentials of CNS and SCCN positive shift to


21


Page 22 of 32

0.93 V and 1.25 V.[40,3] With the VB potential positive shift, it can increase the oxidation energy of the photogenerated holes and reduce the surface oxidation reaction energy barrier, and then enhance the concentration of •OH radicals in the photocatalytic system. Furthermore, with the Rfilm decreasing, the recombination ratio of photogenerated electrons and holes will be inhabited, thus more photogenerated holes can transform into •OH radicals to oxide the TEA. Meanwhile, more photogenerated electrons will reduce the water to hydrogen.

CONCLUSIONS

In this study, series of SCCN photocatalysts were been prepared by mixture the trithiocyanuric acid with different contents of GOQDs. SCCN photocatalysts show an airbag like nano-structure, which is very different from the layered structure of CNS. SCCN 0.1 photocatalyts could produce 5050 µmol/g hydrogen in 4 h under visible light illumination, which is 5.6 times higher than that of CNS. Further electrochemical, photoelectrochemical and SER measurements indicated that the VB band potential of C3N4 moved positively by S and C co-doping, decreasing surface oxidation energy barrier, thus increasing the concentration of the hydroxide radicals obviously. At the same time, the charge transfer resistance of the SCCN 0.1 was charged to smooth with the C doped in, to prolong the life-time of the photogenerated free charges. Thus, SC co-doping is an efficiency strategy to improve the photocatalytic hydrogen evolution property of the C3N4.

ASSOCIATED CONTENT


22

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Supporting Information

TEM, EDS, ICP-MS and Photoluminescence spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] and [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (grant number 51609233), the Shandong Provincial Natural Science Foundation (grant number ZR2016EEQ16), Shandong Provincial Key Research and Development Plan (grant number 2017GHY15118) and the Nantong science and technology plan (grant number GY12016046).

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Positive moving the valance band potential and decreasing the surface oxidation energy barrier of C3N4 by sulfur and carbon co-doping.


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Sulfur- and Carbon-Codoped Carbon Nitride for Photocatalytic

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