Noble-metal-free photocatalysts consisting of graphitic carbon nitride

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Functional Nanostructured Materials (including low-D carbon)

Noble-metal-free photocatalysts consisting of graphitic carbon nitride, nickel complex and nickel oxide nanoparticles for efficient hydrogen generation Yun-Xiao Zhang, Shuang Tang, Wei-De Zhang, and Yu-Xiang Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01704 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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ACS Applied Materials & Interfaces

Noble-Metal-Free Photocatalysts Consisting of Graphitic Carbon Nitride, Nickel Complex and Nickel Oxide Nanoparticles for Efficient Hydrogen Generation Yun-Xiao Zhang, Shuang Tang, Wei-De Zhang*, Yu-Xiang Yu Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou, 510640, People’s Republic of China

KEYWORDS: Photocatalysts; Carbon nitride; Nickel complex; NiOx nanoparticles; Hydrogen evolution.

ABSTRACT: A facile and simple synthetic route is developed to prepare earth-abundant and noble-metal-free hybrid photocatalysts, which are composed of graphitic carbon nitride, nickel complex, and NiOx nanoparticles. Bimolecular nucleophilic substitution reaction was employed to attach nickel complex onto graphitic carbon nitride framework through covalent bond to support its high loading and dispersion. NiOx nanoparticles were further incorporated into the catalysts to serve as hole transporting medium to improve the separation of photogenerated carriers for higher photocatalytic activity. Both yNiL/CN and yNiL/NiOx/CN exhibit superb H2 evolution activity. The optimum H2 evolution rate of the binary 1

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photocatalysts yNiL/CN reaches 303.3 μmol·h-1·g-1, while that of the ternary photocatalysts yNiL/NiOx/CN reaches 524.1 μmol·h-1·g-1 and the apparent quantum efficiency reaches 1.46 % at 450 nm. This finding reveals that coordination of nickel complex is significant in promoting photocatalytic performance and the incorporation of NiOx nanoparticles as hole transporting medium is beneficial for separation of the photogenerated charge carriers. The novel hybrid system offers a new horizon for designing transition metal complexes-modified graphitic carbon nitride as noble-metal-free and highly active photocatalysts for efficient visible light-driven hydrogen generation.

INTRODUCTION Successful development of graphitic carbon nitride (CN) spell a brand-new chapter for the study on a new generation of photocatalysts for storing solar energy in the form of renewable and sustainable clean energy aiming to settle the increasing energy crisis and serious environmental pollution.1 Graphitic carbon nitride, a metal-free polymeric photocatalyst, is formed by repeating triazine units with π-conjugated system.2-4 In view of the unique electronic2 and optical properties5 (Eg = 2.7 eV, active under ultraviolet and a portion of visible light), high thermal, chemical and optical stability,1 graphitic carbon nitride has been demonstrated to be an outstanding visible-light-driven photocatalyst to generate hydrogen from water. During the photocatalytic process, co-catalysts based on noble metals like Pt and Pd, are usually necessary to promote the separation of photogenerated carriers and reduce 2


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the hydrogen evolution overpotential.6,

7

Consequently, various strategies, such as

controlling morphologies, combining with molecular dopants or constructing heterojunction structures,8-10 have been approved to enhance the photocatalytic activity of CN. Nevertheless, the utilization of noble metals still suppresses the application of CN for hydrogen generation. Therefore, it is extremely urgent to find efficient, earth-abundant and inexpensive photocatalysts for photocatalytic hydrogen evolution. In addition, self-assembly of non-noble metal complexes onto semiconductors holds great potential for substitution of noble metals in photocatalytic reactions.11-18 Chen and his colleagues demonstrated that CdSe/ZnS quantum dots (QDs) could serve as photosensitizers and electron-transfer medium containing surface-bound molecular cobaloxime via a phosphonate for efficient H2 production, in which cobaloxime was bonded onto CdSe/ZnS QDs hybrids.11 Furthermore, high recombination of photogenerated carriers caused by the high-electronegativity nitrogen in the repeating triazine units must be restrained for the purpose of achieving higher photocatalytic activity of CN.19 Intriguingly, nickel oxide (NiOx) nanoparticles exhibit the superb conductivity and electron blocking ability.20 Guo et al. demonstrated a facile photoelectrochemical water-splitting system for excellent water oxidation, which is composed of polycrystalline n+p-Si and NiFe-layered double hydroxide nanosheet that is arrayed by a partially activated Ni (Ni/NiOx) bridging layer. In such a system, NiOx exhibits a superior capacity of hole accumulation and facilitates migration of accumulated holes to NiFe-layered double hydroxide 3



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nanosheet.21 In addition, for organic solar cells, NiOx nanoparticles were also used as hole transporting medium.22-24 Therefore, the incorporation of NiOx nanoparticles may facilitate the separation of photogenerated electrons and holes for efficient photocatalytic performance. Herein, ternary hybrid catalysts composed of CN, NiOx nanoparticles and nickel complex (co-catalyst) were developed, in which nickel complex (NiL) was successfully anchored onto CN frameworks by making use of the intrinsic nitrogen atoms

that

exist

in

both

the

backbone

and

edges

of

CN

with

5-chloromethyl-2-hydroxybenzaldehyde via a simple bimolecular nucleophilic substitution reaction (SN2). The robust binding between CN and NiL promotes the dispersion of NiL on CN framework. Consequently, the incorporation of NiOx promotes the separation of photogenerated charge carriers. NiOx functioned as hole transporting medium to facilitate the transfer of the holes. In addition, the essential influences of NiL and NiOx on morphology, optical absorption and separation efficiency of charge carriers are discussed in detail. Under irradiation of visible light, the hybrid system exhibits superior photocatalytic performance towards H2 generation. The optimized hydrogen evolution rate over yNiL/CN reaches 303.3 μmol·h-1·g-1, while that over the optimized yNiL/NiOx/CN reaches 524.1 μmol·h-1·g-1.

RESULTS AND DISCUSSIONS As shown in Figure 1, the morphologies of CN, 4NiL/CN, and 4NiL/NiOx/CN catalysts were detected by SEM and TEM. In Figure 1A, CN exhibits typical 2D 4


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morphology in the shape of randomly crumpled and aggregated sheets. Interestingly, the folds of the 2D nanosheets effectively keep them from stacking together, which could offer more active sites for photocatalytic reactions.25 In Figure 1B and 1C, 4NiL/CN and 4NiL/NiOx/CN catalysts show similar 2D outlook with CN. Notably, there are no obvious nanoparticles on CN framework, offering a proof that nickel complex is homogeneously scattering on the nanosheets. It is also hard to identify NiOx nanoparticles in 4NiL/NiOx/CN by SEM observation. As shown in Figure 1D-1F, TEM images of CN, 4NiL/CN, and 4NiL/NiOx/CN catalysts are consistent with SEM observation. In Figure 1F, the inset indicates the existence of NiOx nanoparticles. Corresponding elemental mapping and HAADF-STEM images of 4NiL/CN and 4NiL/NiOx/CN catalysts provide direct evidence of homogeneous distribution of Ni, C, O, and N atoms in the stacked sheets (Figure 1G-1P). Furthermore, the insets in Figure 1L-1P offer a direct evidence of successful incorporation of NiOx nanoparticles in 4NiL/NiOx/CN catalyst. The high distribution of all atoms over the whole samples proves that nickel complex is homogeneously distributed in CN framework.

5



Figure 1. SEM images of (A) CN, (B) 4NiL/CN, and (C) 4NiL/NiOx/CN. TEM images of (D) CN, (E) 4NiL/CN, and (F) 4NiL/NiOx/CN. Elemental mappings of (G-K) 4NiL/CN and (L-P) 4NiL/NiOx/CN.

XRD patterns of CN, 4NiL/CN, and 4NiL/NiOx/CN catalysts are depicted in Figure 2. All of the three catalysts exhibit two obvious diffraction peaks located at 12.9° (100) and 27.2° (002) which are corresponding to in-plane packing of repeating heptazine framework and graphitic stacking feature of conjugated aromatic systems, respectively.26 Obviously, three diffraction peaks located at 37.1°, 43.2°, and 62.8° appeared in the XRD pattern of 4NiL/NiOx/CN catalyst, corresponding to (111), (200), and (220) diffraction planes of NiOx,23, 27 respectively. Due to the high dispersion and low contents of NiL, it is hard to identify them by XRD patterns. The result is consistent with SEM observation. The phase structure of CN, yNiL/CN and yNiL/NiOx/CN catalysts are shown in Figure S2. With the increased contents of NiL, 6


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yNiL/CN catalysts still display two typical diffraction peaks of CN framework (Figure S2A), while yNiL/NiOx/CN catalysts present the characteristic peaks of CN and NiOx (Figure S2B). According to the XRD patterns of yNiL/CN and yNiL/NiOx/CN catalysts, the coordination of NiL or introduction of NiOx does not affect the CN framework.

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


*

* *

4NiL/NiOx/CN

4NiL/CN

CN

20

40 60 2 Theta / degree

80

Figure 2. XRD patterns of CN, 4NiL/CN, and 4NiL/NiOx/CN catalysts.

The surface structural composition of CN and 4NiL/NiOx/CN are elucidated by XPS analyses (Figure 3). XPS survey spectra disclose the existence of O, C, and N elements in CN, while O, C, N, and small amounts of Ni in 4NiL/NiOx/CN (Figure 3A). Table S1 lists the elemental composition of CN and 4NiL/NiOx/CN according to XPS analyses. The C/N atom ratio of CN is 0.76, which is almost the theoretical value of ideal CN (0.75).1 Notably, the C/N atom ratio of 4NiL/NiOx/CN catalyst is 1.24, which is higher than that in CN. Obviously, the introduction of NiL causes the 7



increased contents of carbon species. In Figure 3B, high-resolution XPS spectra of C 1s for CN exhibits the intrinsic NC=N and sp2 C=C species at binding energy of 288.3 and 284.8 eV, respectively.28 For 4NiL/NiOx/CN catalyst, the binding energy at 288.2 eV is attributed to NC=N species, which is lower than that of CN. This was caused by the incorporation of NiL.29 Notably, compared to CN, a binding energy at 286.3 eV for CN species30,

31

of 4NiL/NiOx/CN appears, which is ascribed to the

binding group of CN and NiL. Table S2 lists the relative ratios of carbon species of CN and 4NiL/NiOx/CN. Compared with CN, the ratios of C=C and CN for 4NiL/NiOx/CN increase. The increased ratios are ascribed to the successful incorporation of NiL. High resolution XPS spectra of N 1s for CN and 4NiL/NiOx/CN exhibit four characteristic peaks (Figure 3C). For CN, the peaks located at 404.7, 400.9, 399.4, and 398,7 eV, correspond to π excitations, amino functional groups (quaternary N, N1C), tertiary nitrogen (N(C)3, N3C), and sp2-hybrized nitrogen (CN=C, N2C).32 Compared to CN, with the incorporation of NiL, the four characteristic peaks for 4NiL/NiOx/CN shifted to higher binding energy, indicating the decreased electron density around the N atoms in CN framework.33 The decreased electron density of CN framework in 4NiL/NiOx/CN catalyst means that NiL and NiOx attract electrons from CN after their combination. This is beneficial for the migration of photogenerated electrons from CN to Ni-species, hence improving the photocatalytic activity of 4Ni/NiOx/CN. Table S3 lists the relative ratios of nitrogen species of CN and 4NiL/NiOx/CN. Compared to CN, the area ratio of N1C decreased from 14.9 to 12.8 in 4NiL/NiOx/CN. The result further verifies that NiL was 8


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successfully anchored onto CN framework via SN2 reaction. High-resolution XPS spectra of O1s for 4NiL/NiOx/CN is depicted in Figure 3D, which exhibits two characteristic peaks located at 532.6 eV corresponding to NiOOH and at 531.5 eV corresponding to Ni2O3 or OH absorbed on the surface, revealing the existence of NiOx.23, 34 The binding energy of Ni 2p in 4NiL/NiOx/CN is located at 852.8, 855.5, 858.6, 861.9, 867.3 and 873.8 eV (Figure 3E). The peak at 855.5 eV comes from Ni2O3 species and the one at 861.9 eV is related to NiO species.23 The peak centered at 852.8 eV is assigned to the characteristic Ni 2p3/2 peak of Ni0, whereas the peaks centered at 873.8 and 856.8 eV represent the characteristic Ni 2p1/2 and Ni 2p3/2 peaks of Ni2+, respectively.35 The peak at 867.3 eV also corresponds to Ni 2p1/2 of Ni2+.36 The results further verify the successful incorporation of NiL and NiOx with CN.

C 1s

4NiL/NiOx/CN

288.2 eV

C 1s

O 1s

Intensity / a.u.

B

4NiL/NiOx/CN

N 1s

284.8 eV

Intensity / a.u.

A

Ni 2p N 1s

CN

286.3 eV

288.3 eV

CN

C 1s

O 1s 284.8 eV

1200

C

1000

800

600 400 Binding energy / eV

200

0

292

290

288

286

284

282

Binding energy / eV

D

4NiL/NiOx/CN

N 1s

O 1s

4NiL/NiOx/CN

398.9 eV

532.6 eV 399.5eV 401.2 eV

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


404.8 eV 398.7 eV

400.9 eV

CN

399.4 eV

531.5 eV

404.7 eV

408

406

404

402

400

398

396

536

534

532

530

Binding energy / eV

Binding energy / eV

9


528


E

Ni 2p

4NiL/NiOx/CN 855.5 eV

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

873.8 eV

861.9 eV

858.6 eV

867.3 eV

880

870

852.8 eV

860

850

Binding energy / eV

Figure 3. (A) XPS survey spectra, high-resolution XPS spectra of (B) C 1s and (C) N 1s of CN and 4NiL/NiOx/CN. High-resolution XPS spectra of (D) O 1s and (E) Ni 2p of 4NiL/NiOx/CN.

FTIR spectra were recorded to get further insights into the influence of incorporation of NiL and NiOx on the structure of CN. As depicted in Figure 4A, the as-prepared CN, 4NiL/CN, and 4NiL/NiOx/CN catalysts exhibit similar FTIR spectra. The weak and broad peaks in the range of 3000-3400 cm-1 are ascribed to NH stretching vibration modes owing to partial condensation.37 The intense peaks in the range of 1200-1700 cm-1 are assigned to the stretching vibration modes of CN heterocycles.38 The peak centered at 810 cm-1 is attributed to the breathing vibration modes of heptazine rings.39 Close inspection of the FTIR spectra of CN, 4NiL/CN, and 4NiL/NiOx/CN catalysts at 2800-3400 cm-1 and 1000-1750 cm-1 is shown in Figure 4B and 4C, respectively. As shown in Figure 4B, compared to CN, the characteristic peak located at 2974 cm-1 appears in the spectra of 4NiL/CN and 4NiL/NiOx/CN, which corresponds to the stretching vibration of CH in =CH.40 The mode of =CH is ascribed to the carbon species in NiL. As depicted in Figure 4C, 10


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compared to CN, three weak peaks at 1164, 1545, and 1632 cm-1 appear, corresponding to CH breathing vibration modes, aromatic CC stretching vibration and C=N stretching vibration, respectively.40,

41

The result further confirms the

existence of NiL in 4NiL/CN and 4NiL/NiOx/CN catalysts. The FTIR spectra of CN, yNiL/CN and yNiL/NiOx/CN catalysts are also displayed in Figure S3. All of the as-prepared samples exhibit similar peaks, which confirm that the incorporation of NiL and NiOx does not change the basic structure of CN. A

B

4NiL/NiOx/CN

4NiL/NiOx/CN

4NiL/CN

T%

T%

4NiL/CN CN

CN N-H

C-N-C N-(C)3 N

N N

4000

3000

2000

3400

1000

3200

3000

2800

-1

Wavenumber / cm

C

4NiL/NiOx/CN 4NiL/CN CN

T%

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


1600

1400

1200

1000

-1

Wavenumber / cm

Figure 4. FTIR spectra of CN, 4NiL/CN, and 4NiL/NiOx/CN catalysts, (A) Full scale, (B, C) high resolution spectra of the selected areas in (A).

Photocatalytic hydrogen evolution activity was assayed to evaluate the photocatalytic performance over the prepared catalysts taking TEOA as a hole 11



sacrificial agent under visible light irradiation (λ > 420 nm). No H2 was detected by just using NiL, NiOx, or NiL/NiOx as a photocatalyst without CN. As shown in Figure 5A, CN exhibits negligible activity. Upon increasing the content of NiL, the hydrogen evolution rates of yNiL/CN increased markedly. After reaching the highest value (303.3 μmol·h-1·g-1) on 4NiL/CN, further increasing NiL resulted in the decline of hydrogen generation rate. This is because that higher loading amount of NiL could serve as recombination sites for photogenerated electrons and holes.42 The effect of different contents of NiOx on photocatalytic performance is displayed in Figure 5B. Taking 4NiL/CN as an example, introduction of NiOx plays a positive role on its hydrogen production and the optimized content of NiOx is 1 mg. Figure 5C and 5D display the photocatalytic hydrogen evolution performance of yNiL/CN and 4NiL/yNiOx/CN in 4 h. On the basis of the controlled experiments, the influences of NiL and NiOx on photocatalytic performance are shown in Figure 6. After introduction of 1 mg NiOx, the hydrogen evolution rates of yNiL/NiOx/CN increased obviously. The maximum rate reaches 524.1 μmol·h-1·g-1 for 4NiL/NiOx/CN (Figure 6A), which is 1.73 times of that of 4NiL/CN. Figure 6B shows the photocatalytic hydrogen evolution activity of yNiL/NiOx/CN, while Figure 6C illustrates the photocatalytic hydrogen production rate of 4NiL/NiOx/CN in 4-cycle test for 16 h. The slight reduction in hydrogen generation rate indicates the high stability of 4NiL/NiOx/CN catalyst. In addition, after the photocatalytic reaction, XRD patterns and FT-IR spectra of 4NiL/NiOx/CN were investigated and shown in Figure S4 and S5. Remarkably, the active components exhibit no obvious changes after 12


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photocatalytic reaction. The result also indicates the superior stability of 4NiL/NiOx/CN catalyst. UV-vis diffuse reflection spectra and corresponding hydrogen production rates under monochromatic light (400, 450, 500, 550, and 600 nm) of CN and 4NiL/NiOx/CN catalyst are shown in Figure 6D. It is noted that there is no appreciable H2 using CN as a photocatalyst. However, 4NiL/NiOx/CN still exhibits response to monochromatic light with wave length of 600 nm to generate H2, which is in line with the UV-vis diffuse reflection spectra. According to Figure 6D, the hydrogen evolution rate of 4NiL/NiOx/CN at 450 nm reaches 154.0 μmol·h-1·g-1 and the corresponding AQE is 1.46 %. The solar-to-hydrogen (STH) efficiency over 4NiL/NiOx/CN under AM 1.5G solar simulator irradiation was examined. The incident power over the irradiation area of 38.47 cm2 was 3.85 W. The total input solar energy for 6 h was 83095 J. During the photocatalytic reaction for 6 h, 4138 μmol H2 was obtained, corresponding to 986 J free energy. Thus, the STH conversion efficiency of 4NiL/NiOx/CN was 1.18 %. Figure 6E exhibits the comparison of hydrogen rates on yNiL/CN and yNiL/NiOx/CN catalysts, indicating that the introduction of NiOx promotes their photocatalytic performance effectively. To better clarify the photocatalytic performance of the photocatalysts, Table S4 lists some reported H2 evolution rates over transitional metal complexes-modified CN catalysts. Remarkably, 4NiL/NiOx/CN reported in this study displays excellent photocatalytic performance.

13



Figure 5. Photocatalytic hydrogen evolution rate and performance of (A and C) yNiL/CN, (B and D) 4NiL/yNiOx/CN under visible light irradiation (λ > 420 nm).

14


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Figure 6. (A) Photocatalytic hydrogen evolution rates of yNiL/NiOx/CN catalysts under visible light irradiation (λ > 420 nm). (B) Photocatalytic hydrogen evolution performance of yNiL/NiOx/CN catalysts. (C) Stability tests of 4NiL/NiOx/CN catalyst for 4 cycles in 16 h. (D) Wavelength dependence of the hydrogen evolution rates of CN and 4NiL/NiOx/CN. (E) Photocatalytic hydrogen evolution rate of yNiL/CN and yNiL/NiOx/CN catalysts.

UV-vis diffuse reflection spectra were recorded to disclose the optical properties of CN, yNiL/CN, and yNiL/NiOx/CN catalysts. As depicted in Figure 7A and 7B, the samples exhibit strong absorption peaks in the region of 200-400 nm, corresponding to π-π* electronic transitions in repeating heptazine of CN and n-π* electronic transitions relating to lone pairs of nitrogen atoms.43, 44 Compared to CN, an obvious visible light absorption was observed after being modified by NiL and NiOx. It is clearly that yNiL/CN and yNiL/NiOx/CN catalysts display increased tail absorption, which is accompanied with the increased contents of NiL or NiL/NiOx. The enhanced absorption in visible region is beneficial for the utilization of solar energy, which is consistent with the photocatalytic performance under monochromatic light (550 and 15



600 nm) over 4NiL/NiOx/CN catalyst (Figure 6D). UV-vis diffuse reflection spectra of CN, 4NiL/CN, 4NiL/NiOx/CN catalysts, HL, OPDA and NiCl2·6H2O are shown in Figure S6. The different absorption of the samples reveals the successful assembly of 4NiL/CN and 4NiL/NiOx/CN catalysts rather than just a physical mixture of the precursors. PL spectra were recorded to explore the charge transfer properties of the as-prepared catalysts (Figure 8). As shown in Figure 8A and 8B, pure CN shows intense PL peak, indicating high recombination rate of photogenerated charge carriers. However, the peak intensity of yNiL/CN and yNiL/NiOx/CN catalysts is weaker than that of CN upon the increased contents of NiL or NiL/NiOx. The weaker PL peak is originated from rapid migration of electrons and holes and then, generating more electrons to participate in photocatalytic reaction for hydrogen production.45 A

200

B

CN 1NiL/CN 2NiL/CN 3NiL/CN 4NiL/CN 5NiL/CN 6NiL/CN

400

600

CN 1NiL/NiOx/CN 2NiL/NiOx/CN 3NiL/NiOx/CN 4NiL/NiOx/CN

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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800

5NiL/NiOx/CN 6NiL/NiOx/CN

200

Wavelength / nm

400

600

800

Wavelength / nm

Figure 7. UV-vis diffuse reflection spectra of (A) CN and yNiL/CN, and (B) CN and yNiL/NiOx/CN.

16


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A

350

B

CN 1NiL/CN 2NiL/CN 3NiL/CN 4NiL/CN 5NiL/CN 6NiL/CN

400

450

500

550

CN 1NiL/NiOx/CN 2NiL/NiOx/CN 3NiL/NiOx/CN

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


600

350

4NiL/NiOx/CN 5NiL/NiOx/CN 6NiL/NiOx/CN

400

Wavelength / nm

450

500

550

600

Wavelength / nm

Figure 8. Photoluminescence spectra of (A) CN and yNiL/CN, and (B) CN and yNiL/NiOx/CN.

Electrochemical impedance spectra (EIS) and photocurrent density-potential profiles were measured to demonstrate the charge transportation in the catalysts. As described in Figure 9A, CN, 4NiL/CN, and 4NiL/NiOx/CN catalysts display photocurrent response under visible light irradiation. 4NiL/NiOx/CN catalyst reaches the optimum photocurrent intensity, which is 3.5 times of that of CN. Higher photocurrent density implies higher utilization of visible light and more efficient photocarrier transportation.46 EIS was assayed to estimate the rate of photogenerated carrier transfer (Figure 9B). With modification of NiL or NiL/NiOx, the semicircular diameter of Nyquist curves for 4NiL/CN and 4NiL/NiOx/CN decreases than that of CN, indicating a smaller internal charge-transfer resistance.47 4NiL/NiOx/CN exhibits the smallest diameter among the samples, indicating the highest efficiency in photogenerated carrier transfer, which is consistent with its highest hydrogen evolution rate.

17



B 100

CN 4NiL/CN 4NiL/NiOx/CN

-2

A

80 -2

-Z" / k

Photocurrent density / A·cm

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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-4

-6

0

On

50

100

40 20

CN

Off

60

4NiL/CN 4NiL/NiOx/CN

150

200

0

250

0

50

100

150

200

Z' / k

Time / sec

Figure 9. (A) Transient photocurrent response under visible light irradiation (λ > 420 nm) and (B) electrochemical impedance spectroscopy in the dark.

Linear sweep voltammograms (LSV) of the CN, NiL/CN, and NiOx/CN catalysts were measured to study the role of NiL and NiOx in the photocatalytic reaction. CN and NiOx/CN exhibit similar onset potential and photocurrent density at -1.2 to 0 V (Figure 10). Compared to CN and NiOx/CN, an obvious positive shift of onset potential and increased photocurrent density appear at the NiL/CN electrode, indicating that NiL serves as a co-catalyst during photocatalytic hydrogen evolving reaction.48 In the range of 0 to 1.7 V, CN and NiL/CN display similar onset potential and photocurrent density. After modification with NiOx, NiOx/CN demonstrates a noticeable negative shift of onset potential and enhanced photocurrent density than that of CN and NiL/CN. Obviously, NiOx works as a hole transporting material to promote the separation of photogenerated carriers.22, 48, 49 To further confirm the role of NiL on the photocatalytic reaction, DFT calculations were carried out to calculate the electron band structure and optimized HOMO and LUMO energy levels of CN and NiL/CN (Figure 11). For CN, its HOMO energy level mainly derives from lone 18


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pair of the nitrogen atoms and LUMO energy level mostly roots in π-conjugated sp2 C=N.50,

51

After modification with NiL, the HOMO and LUMO energy levels of

NiL/CN change a lot. The HOMO energy level mostly derives from the NiL while LUMO energy level tends to better delocalize in CN framework. On the basis of the DFT calculations, the bandgaps of CN and NiL/CN are resolved to be 3.6 and 3.0 eV, respectively. The narrow bandgap of NiL/CN is ascribed to positive shift of LUMO energy level from -2.61 (CN) to -2.26 eV (NiL/CN) and positive shift of HOMO energy level from -6.21 (CN) to -5.26 eV (NiL/CN). The introduction of NiL accelerates the delocalization of π-conjugated electrons in the CN framework, resulting in higher photocatalytic hydrogen production.37 According to the above discussion, possible photocatalytic mechanism of NiL/CN catalysts is shown in Scheme 1. Firstly, under visible light irradiation, electrons and holes are generated. On one hand, photogenerated electrons transmit from CB of CN to NiL through the binding bond between CN and NiL to generate the reduced Nickel(I) species. With the existence of H+, the highly reactive intermediate [Nickel(I)] yields Ni(III)H species. Extra addition of H+ to Ni(III)H species harvests hydrogen, and recovers to NiL.52-54 Meanwhile, photogenerated holes transfer to NiOx and then be captured by TEOA.

19



-2

1

Current density / mA·cm

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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0

CN NiL/CN NiOx/CN

-1

-1

0

1

Potential / V vs. RHE

Figure 10. Linear sweep voltammograms (LSV) of the CN, NiL/CN, and NiOx/CN catalysts.

Figure 11. Electronic structure of polymeric trimmer models and optimized HOMO and LUMO energy levels of CN and NiL/CN trimmer.

20


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Scheme 1. Schematic diagram of photocatalytic mechanism of NiL/NiOx/CN catalysts under visible light irradiation.

CONCLUSIONS In this work, we successfully prepared noble-metal-free and earth-abundant hybrid photocatalysts, which are composed of NiOx nanoparticles, nickel complex (NiL), and graphitic carbon nitride (CN) via a bimolecular nucleophilic substitution (SN2) reaction by making use of the intrinsic nitrogen atoms that exist in both the backbone and edges of CN with 5-chloromethyl-2-hydroxybenzaldehyde. The utilization of SN2 reaction provides higher loading and dispersion for the nickel complex. The coordination of NiL working as a co-catalyst is significant in enhancing photocatalytic performance. Meanwhile, the incorporation of NiOx nanoparticles working as a hole transporting medium is beneficial for separation of photogenerated charge carriers. The optimized hydrogen evolution rate over yNiL/CN achieves 303.3 21



μmol·h-1·g-1, while that of yNiL/NiOx/CN reaches 524.1 μmol·h-1·g-1 with apparent quantum efficiency of 1.46 % at 450 nm. The ternary hybrid system offers a novel proposal to boost hole transporting for effective separation of photogenerated charge carriers and design transition metals complexes-modified CN as noble-metal-free and highly active photocatalysts towards solar-to-hydrogen conversion.

EXPERIMENTAL SECTION Preparation of photocatalysts Representation of the synthetic process of NiL/CN and NiL/NiOx/CN catalysts is illustrated in Scheme S1 and Scheme 2, respectively.

Scheme 2. Representation of the synthetic process of NiL/NiOx/CN catalysts.

Preparation of graphitic carbon nitride (CN): CN nanosheets were synthesized as follows. 5 g urea in a porcelain boat was annealed at 550 °C in a tube 22


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furnace for 4 h under Ar atmosphere. After cooling down to ambient temperature, the as-prepared samples were marked as CN. Synthesis of 5-chloromethyl-2-hydroxybenzaldehyde (1): 1 was prepared by following a previous report.55 In a 250 mL two-necked flask, paraformaldehyde (9.0 g, 0.30 mol), salicylaldehyde (12.2 g, 0.10 mol) and 100 mL concentrated hydrochloric acid were mixed under rapid stirring. The mixture was then heated to 60 °C and refluxed for 3 h. After cooling down to room temperature, the obtained mixture was extracted with 100 mL DCM for three times. The obtained extracts were dehydrated over anhydrous Na2SO4 and the crude product was collected after filtration and rotary evaporation. After recrystallization with n-hexane, the purified solid was afforded to compound 1 (5.16 g, 30%). 1H NMR of 1 was shown in Figure S1. 1H NMR (400 MHz, CDCl3) δ 11.06 (s, 1H), 9.89 (s, 1H), 7.58 (d, J = 2.2 Hz, 1H), 7.55 (dd, J = 8.5, 2.3 Hz, 1H), 6.99 (d, J = 8.5 Hz, 1H), 4.58 (s, 2H). Synthesis of NiOx nanoparticles: Based on the previous literature,23 0.13 g Ni(acac)2 and 14 mL tert-butanol were mixed under stirring for 30 min to obtain a light green suspension. The suspension was transferred into a Teflon lined autoclave and then heated in an oven at 220 °C for 20 h. After cooling down to ambient temperature, the yellow NiOx powder was obtained by being centrifuged and then dried under vacuum. Synthesis of yNiL/CN: 100 mg CN was dispersed in 20 mL DMF and sonicated. NaOH and 1 (0, 0.5, 1, 5, 10, 15, 20 mg) were put into the above suspension. After churning for 2 h, corresponding amounts of O-phenylenediamine (0, 0.16, 0.32, 1.6, 23



3.2, 4.8, 6.4 mg) were dropped into the mixture and stirred for 2 h. After filtering and drying, the as-prepared samples were labeled as yL/CN (y = 0, 1, 2, 3, 4, 5, 6). In order to prepare yNiL/CN, the obtained yL/CN was dispersed into 20 mL DMF and then sonicated for 30 min. NiCl2·6H2O (0, 0.35, 0.70, 3.5, 7.0, 10.5, 14 mg) was added into the above suspension and stirred overnight. The yNiL/CN (y = 0, 1, 2, 3, 4, 5, 6) samples were obtained by filtering and drying. Synthesis of 4NiL/yNiOx/CN: 100 mg CN and certain amounts of NiOx (0, 0.1, 0.5, 1.0, 2.0, 3.0 mg) were dispersed in 20 mL DMF and sonicated for 30 min. 10 mg 1, NaOH, 3.2 mg O-phenylenediamine and 7.0 mg NiCl2·6H2O were added into the suspension while the other steps followed those of synthesis of 4NiL/CN. According to the contents of NiOx, the as-prepared samples were labeled as 4NiL/yNiOx/CN (y = 0, 1, 2, 3, 4, 5). Synthesis of yNiL/NiOx/CN: 100 mg CN and 1 mg NiOx were dispersed in 20 mL DMF and sonicated for 30 min. NaOH, 1 (0, 0.5, 1, 5, 10, 15, 20 mg), O-phenylenediamine (0, 0.16, 0.32, 1.6, 3.2, 4.8, 6.4 mg) and NiCl2·6H2O (0, 0.35, 0.70, 3.5, 7.0, 10.5, 14 mg) were added into the suspension whereas the other steps followed those of synthesis of yNiL/CN. The obtained samples were labeled as yNiL/NiOx/CN (y = 0, 1, 2, 3, 4, 5, 6). The detailed synthetic conditions for yNiL/CN and yNiL/NiOx/CN catalysts were listed in Table S5. Photoelectrochemical test Photoelectrochemical measurements including liner sweep voltammetry (LSV), photocurrent response, and electrochemical impedance spectra (EIS) were carried out 24


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on an electrochemical workstation (CHI 660C, Chenhua, Shanghai) with a traditional three-electrode cell composed of a KCl-saturated Ag/AgCl electrode, a Pt plate electrode, and the sample deposited fluoride-tin oxide (FTO) electrode, corresponding to reference electrode, counter electrode, and working electrode, respectively. The electrolyte is 0.5 M Na2SO4 aqueous solution (pH = 6.5). The working electrode was prepared in this way: 1 mg ethyl cellulose and 10 mg photocatalyst were dispersed in 3 mL ethanol to obtain a suspension by sonication. The suspension was transferred onto a 1×1 cm2 FTO conductive glass. The working electrode was gained after drying at 100 °C for 1 h. For the on/off photocurrent response, a 300 W Xenon lamp (PLS-SXE 300/300UV) was served as the visible light source (λ > 420 nm, 100 mW·cm-2). LSV was scanned in the range of -0.58 to -1.38 V and a scan speed was 5 mV·s-1. EIS was texted in 0.01 M K3Fe(CN)6/K4Fe(CN)6 (1:1) solution at open circuit potential over the frequency of 0.01 Hz to 10 KHz in the dark. Photocatalytic hydrogen evolution assessment Photocatalytic performance was studied in a closed gas circulation and evacuation system (Labsolar III AG, Beijing Perfectlight Technology Co. Ltd., China) consisted of a quartz reaction vessel. The light intensity was adjusted to 100 mW·cm-2. 50 mg as-prepared photocatalyst was dispersed in 100 mL 10 vol% triethanolamine aqueous solution and sonicated for 30 min. The suspension was vacuumed for 30 min to eliminate the air before photocatalytic reaction. A 300 W Xenon lamp mounted with a 420 nm cut-off filter was put on the top of reactor. After visible light irradiation for several hours, the evolved gas was determined by a gas 25



Page 26 of 36

chromatography (GC 7806, Beijing Shiweipuxin Analytical Instruments Co. Ltd, China) consisting of a thermal conductive detector and a 5A molecular sieve column, with nitrogen as a carrier gas. The apparent quantum efficiency (AQE) was tested as follows. Under the similar experimental conditions with photocatalytic reaction, the suspension of photocatalysts was irradiated under monochromatic light (400, 450, 500, 550, and 600 nm) with a band-pass filter. The light intensity was tested to be 2.04 mW·cm-2 of the monochromatic light at 450 nm. The number of incident photons (N) was computed using equation (1). The value of AQE was computed according to equation (2) as follows. The solar to hydrogen (STH) conversion was evaluated by using AM 1.5G solar simulator (100 mW·cm-2) as the light source with 4NiL/NiOx/CN as the catalyst. STH was determined with equation (3).56 (1) (2)

(3)

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details about reagents, equipments, DFT calculations, spectra of 1H NMR, XRD, FT-IR and UV-vis, elemental composition from XPS analyses are included in Electronic Supplementary Information (ESI). 26


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AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected]. Tel and Fax: 86-20-8711 4099.

NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21773074, 21273080).

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Nerve

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acetylcholinesterase. Bioorg. Med. Chem. 2017, 25, 4497-4505. (56)Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z.; Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-Electron Pathway. Science 2015, 347, 970-974.

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Noble-metal-free photocatalysts consisting of graphitic carbon nitride

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