Nickel Boride Cocatalyst Boosting Efficient Photocatalytic Hydrogen

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Cite This: Ind. Eng. Chem. Res. 2018, 57, 8125−8130

Nickel Boride Cocatalyst Boosting Efficient Photocatalytic Hydrogen Evolution Reaction Qiaohong Zhu, Bocheng Qiu, Mengmeng Du, Mingyang Xing,* and Jinlong Zhang* Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P.R. China

Ind. Eng. Chem. Res. 2018.57:8125-8130. Downloaded from pubs.acs.org by OPEN UNIV OF HONG KONG on 01/23/19. For personal use only.

S Supporting Information *

ABSTRACT: Noble metals have been extensively used as catalyst promoters to driven highly efficient photocatalytic hydrogen evolution reaction (HER). However, noble-metal-based promoters are limited by their expensive costs and scarcities. Recently, massive efforts have been focused on metal phosphides, metal sulfides, and metal carbides as cocatalysts to achieve high HER performance. Currently, metal borides were reported as promoters for hydrogen evolution. Here, we demonstrate amorphous nickel boride (NiB) with a suitable band gap prepared by the chemical reduction of nickel nitrate hexahydrate using sodium borohydride as a cocatalyst over graphite carbon nitride (C3N4), thus achieving higher hydrogen evolution performance than C3N4. With the B(δ−)−Ni(δ+)−N(δ−) bonds between NiB and C3N4, the as-prepared C3N4/NiB7.5 shows a dramatically enhanced photocatalytic hydrogen generation rate (464.4 μmol h−1 g−1), and reaches a quantum efficiency of 10.92% with 365 nm light irradiation.

1. INTRODUCTION Photocatalytic conversion of water to hydrogen by making use of semiconductor photocatalysts offers a low-cost and sustainable solution to alleviate current energy-related issues.1−5 Various semiconductor photocatalysts, such as TiO2,6,7 CdS,8−10 and C3N4,11,12 have been extensively studied. Particularly, C3N4, as a nonmetal material, has boosted much interest of researchers in recent years,13−15 attributed to its excellent electronic, structural, and unique optical features.16 However, the photocatalytic efficiency of C3N4 is confined by the low separation rate of photogenerated charges. To solve these problems, plenty of strategies have been employed, including coupling with other semiconductors,17 heteroatoms doping, and supramolecular preorganization.18 However, with these strategies, only bulk separation can be efficiently enhanced, and surface recombination is still hard suppressed with use of the above-mentioned modifications. Differently, introduction of cocatalysts onto the surface of photocatalysts is a relatively effective way to accelerate the surface charge separation. Some noble metals, like platinum,19 palladium,20 aurum (gold),21 and argentum (silver),22 have been used as promoters © 2018 American Chemical Society

to enhance the photocatalytic activity of C3N4. However, the future development of these precious metals in catalysis applications is confined by their scarcity and high cost. Therefore, it is extremely desired to develop some highly efficient and cheap promoters. Recently, massive efforts have been focused on metal phosphides,23,24 metal sulfides,25 and metal carbides,26 and C3N4 coupling with these cocatalysts have shown HER performance superior to that of pristine C3N4. Besides, the metal boride prepared through one-step synthesis also processes suitable energy band structure and stable chemical properties,27,28 which has a large application potential in photocatalytic HER. However, compared to other widely used cocatalysts, metal boride has not gained much attention in photocatalysis. As far as we know, we are not aware of any literature about metal borides as cocatalysts enhancing the HER performance of C3N4. Received: Revised: Accepted: Published: 8125

March 29, 2018 May 28, 2018 May 29, 2018 May 29, 2018 DOI: 10.1021/acs.iecr.8b01376 Ind. Eng. Chem. Res. 2018, 57, 8125−8130

Article

Industrial & Engineering Chemistry Research

electrode that was prepared through a clean fluoride-tin oxide (FTO) deposited with a sample film, a Pt wire as the counter electrode, and a saturated calomel electrode as the reference electrode. For Mott−Schottky plots, 0.5 M Na2SO4 solution purged with high-purity nitrogen was used as the bath solution. The EIS mseasurements were performed in a 25 mmol/L K3Fe(CN)6 and K4Fe(CN)3 and 0.1 mol/L KCl mixture aqueous solution. 2.4. Photocatalytic Measurements. To assess the photocatalytic performance of C3N4/NiB materials, hydrogen evolution under light irradiation was measured with the solution in a closed Pyrex reactor with a closed gas circulation. For H2 evolution, 100 mg of catalyst was added into 100 mL of 10 vol % triethanolamine (TEOA) aqueous solution. The system was vacuumed for 30 min to remove air in the asprepared solution before light irradiation. Hydrogen evolution was then launched by irradiating using a 300 W Xe lamp outfitted with the optical filter AM1.5. Gas chromatography outfitted with a thermal conductive detector (TCD) and a 5 Å molecular sieve column was applied to analyze the gas product, in the whole process. Argon was used as the carrier gas.

Here, we prepared nickel boride (NiB) nanoparticles (NPs) as the cocatalyst combined with C3N4 photocatalyst. Especially, by coupling 7.5 wt % NiB with C3N4 (C3N4/NiB7.5), the yielded C3N4/NiB7.5 demonstrates enhanced HER activity of 464.4 μmol g−1 h−1 and a remarkably high apparent quantum yield of 10.92% with 365 nm light irradiation. Such dramatically improved HER activity resulted from the fast electron transmission from C3N4 to NiB through the N−Ni−B bond.

2. EXPERIMENTAL SECTION 2.1. Materials. Every chemical is shown in Table S1: melamine at 99.0% purity, ammonium chloride at 99.5% purity, nickel chloride hexahydrate (AR), sodium borohydride at 96.0% purity, and sodium hydroxide at 96.0% purity were used as received without any further purification. All the reagents were supplied by General-reagent. Deionized water with the indicated number of 18.25 MΩ·cm was applied in our experiment. 2.2. Preparation of C3N4/NiB Hybrids. The material of C3N4 was fabricated through heating the mixture of melamine and ammonium chloride in the air atmosphere for 4 h at 550 °C with a heating rate of 2 °C min−1. The amorphous NiB cocatalyst was grown on C3N4 through a simple chemical reduction. Samples with different theoretical addition contents of NiB (2.5, 5, 7.5, and 10 wt %) were labeled C3N4/NiB2.5, C3N4/NiB5, C3N4/NiB7.5, and C3N4/NiB10, respectively. A typical preparation of 7.5 wt % NiB-loaded C3N4 is as follows: C3N4 (500 mg), nickel nitrate hexahydrate (143 mg), and sodium borohydride solution (67.6 mg in 3 mL of H2O) were used as the precursors. Then the NaBH4 solution was added into the mixture of C3N4 and nickel nitrate hexahydrate under stirring with an ice bath.9 And then it was continuously stirred for 30 min and then the stirring was turned off. The resulting precipitate was separated and washed with distilled water and ethanol three times and finally dried in vacuum at 60 °C overnight. Pure amorphous NiB nanoparticles were prepared with the same process without C3N4. 2.3. Characterization Methods. The X-ray diffraction (XRD) using a RigakuD/MAX 2550 diffractometer (Cu K radiation, λ = 1.5406 Å, 40 kV, 100 mA) was applied to characterize the composition of samples. The morphologies of our prepared samples were characterized by field-emission scanning electron microscope (FESEM; TESCAN MAIA3; Hitachi S4800; JEOL 6700), transmission electron microscope (TEM; JEOL, JEM-1400), and high-resolution transmission electron microscope (HRTEM; JEOL 2100F). The energydispersive X-ray spectroscopy (EDX) attached to the HRTEM was applied to investigate the composition of the as-prepared samples. For the X-ray photoelectron spectroscopy (XPS) analysis, a PerkinElmer PHI 5000C ESCA system with Al Kα radiation (250 W) was used. The shift of the binding energy was corrected using the C 1s peak at 284.6 eV as standard. The ultraviolet−visible diffuse reflectance spectra resulted from a Scan UV−vis spectrophotometer (Varian, Cary 500) equipped with an integrating sphere assembly (with BaSO4 as reference sample).The photoluminescence (PL) was carried out through an Hitachi F-4600 fluorescence spectrophotometer with an incident ray at 370 nm. BET surface area measurement was carried out with nitrogen adsorption at 77 K using an ASAP2020 instrument. Mott−Schottky plots and the electrochemical impedance spectroscopy (EIS) were all recorded on an electrochemical workstation (Zahner, Zennium). The standard three-electrodes system consists of a working

3. RESULTS AND DISCUSSION 3.1. Composition and Microstructure of C3N4/NiB. In Figure 1a, the X-ray diffraction (XRD) patterns of as-made

Figure 1. (a) XRD patterns of C3N4, C3N4/NiB7.5, and NiB. (b) FESEM image of C3N4/NiB7.5. (c, d) TEM images of C3N4/NiB7.5, and the inset in (d) is the corresponding SAED patterns. (e) HAADFSTEM image of C3N4/NiB7.5 and its corresponding elements mapping of Ni, B, C, and N. The scale bar is 100 nm.

C3N4, NiB, and C3N4/NiB7.5 can be observed. The two main characteristic diffraction peaks appeared at 2θ = 13.1° and 27.4° are assigned to (100) and (002) planes of hexagonal C3N4 (JCPDS card no. 87-1526); the former is related to the in-plane repeating tri-s-triazine units of C3N4 and the latter is connected with the conjugated aromatic stacking in C3N4.15 After coupling with NiB, no obvious changes for characteristic peaks of C3N4 appeared. Moreover, accompanied by the loading increment of NiB (Figure 2), no diffraction peaks for NiB are observed, which suggests the relatively low loading amount and high 8126

DOI: 10.1021/acs.iecr.8b01376 Ind. Eng. Chem. Res. 2018, 57, 8125−8130

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Industrial & Engineering Chemistry Research

Figure 2. XRD patterns of C3N4, C3N4/NiB2.5, C3N4/NiB5, C3N4/ NiB7.5, C3N4/NiB10, and NiB.

dispersion of NiB.8 The field-emission scanning electron microscope (FESEM) image shows the surface of C3N4/ NiB7.5 with a well-defined sheetlike nanostructure (Figure 1b). In the transmission electron microscopy (TEM) images of C3N4/NiB7.5 (Figure 1c, d), the highly dispersed NiB NPs, whose sizes are around 10 nm, can be observed on the surface of C3N4 nanosheets. The inset of Figure 1d is a selected area electron diffraction (SAED) pattern, which shows the amorphous structure of NiB NPs. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of C3N4/NiB7.5 hybrids in Figure 1e evidently indicates the uniform hybridization of NiB and C3N4. The corresponding energy dispersive X-ray (EDX) spectrum (Figure 3) reveals there exists Ni and B elements,

Figure 5. XPS spectra of C 1s (a), N 1s (b), Ni 2p (c), and B 1s (d) of C3N4 and C3N4/NiB7.5.

400.9, 404.0 eV, corresponding to sp2-hybridized nitrogen (C− NC), tertiary nitrogen (N−(C)3), and positive charge localization in heterocycles (C−N−H), respectively. The banding energy of N 1s peak of tertiary nitrogen groups in C3N4/NiB7.5 shifts to 399.3 eV as compared to that in C3N4 (399.9 eV), which indicates that NiB nanoparticles chemically interact with C3N4 by forming Ni−N bond. However, there is no shift for the N 1s peak of triazine rings in C3N4/NiB7.5 located at 398.3 eV. Hence, Ni atoms may connect only with N atoms in tertiary nitrogen groups instead of triazine rings. Such chemical connections may lead to the increase of electron density in N atoms while the electron density in Ni atoms decreases. The Ni 2p3/2 peaks at 855.5 and 861.3 eV can be assigned to Ni atoms in NiB and Ni2+ formed by the surface oxidation, respectively (Figure 5c). Compared to the typical binding energies of Ni 2p3/2 in NiB (855.4 and 861.1 eV),29 the binding energies of Ni 2p3/2 in C3N4 /NiB7.5 shift to 855.5 and 861.3 eV, indicating the formation of Ni−N bonding state. Similarly, Ni 2p1/2 edge also shows the contribution of both states. The peaks at 865.9 and 883.3 eV are indexed to the satellite shake-up peaks. There is no obvious peak at 852.3 eV, suggesting that metallic nickel does not exist and NiB is stable. The XPS spectrum of B 1s (Figure 5d) shows two peaks at 188.0 and 191.8 eV, corresponding to an alloying state and a boron oxide state, respectively, suggesting the formation of NiB. Moreover, the peak of B 1s at 188.0 eV exhibits a negative shift relative to that of elemental B in NiB (around 188.3 eV).30 These results suggest that there also exists Ni(δ+)−B(δ−) bonding states in C3N4/NiB7.5 because of the charge redistribution between Ni and B atoms. Overall, the coupling between the Ni(δ+) atoms and N atoms in tertiary nitrogen can probably be stabilized in the form of B(δ−)−Ni(δ+)− N(δ−) bonding states on the surface of C3N4/NiB7.5. 3.3. Hydrogen Generation Activity of C3N4/NiB. The photocatalytic HER activity of the as-made samples are examined in 10 vol % triethanolamine (TEOA) under the imitated solar light irradiation (Xenon light outfitted with AM1.5 filter). After coupling with NiB, an obvious enhancement of HER performance can be observed. As demonstrated in Figure 6a, pristine C3N4 shows negligible activity toward H2

Figure 3. EDX spectrum of C3N4/NiB7.5.

evidencing the formation of NiB on C3N4. Besides, the surface area of C3N4/NiB7.5 increased as compared to C3N4 (Figure 4), demonstrating the successful introduction of NiB into C3N4. 3.2. XPS Analysis of C3N4/NiB. The surface chemical states of C3N4/NiB7.5 were investigated by X-ray photoelectron spectroscopy (XPS). In Figure 5a, there exist two peaks at 284.6 and 287.9 eV, corresponding to the C−C bonds and N−CN in C3N4. The core level of N 1s spectrum (Figure 5b) can be analyzed into four peaks at 398.3, 399.9, and

Figure 4. Nitrogen adsorption−desorption spectrum of C3N4 and C3N4/NiB7.5. 8127

DOI: 10.1021/acs.iecr.8b01376 Ind. Eng. Chem. Res. 2018, 57, 8125−8130

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C3N4, we measured their photoelectric response under irradiation. As shown in Figure 6d, a more intense photocurrent response for C3N4/NiB7.5 can be observed, which indicates the higher separation efficiency of the charge carriers. Besides, the smaller semicircle in electrochemical impedance spectroscopy Nyquist plot can be observed (Figure 7), indicating faster electron transfer and lower charge-transfer resistance in C3N4/ NiB7.5.

Figure 7. EIS Nyquist plots of as-prepared samples.

Photoluminescence (PL) with the incident ray at 370 nm was used to investigate the behaviors of electrons and holes. As indicated in Figure 6e, pristine C3N4 shows a strong and wide peak near 470 nm, corresponding to the intrinsic recombination of electrons and holes of C3N4. After coupling with NiB, an obviously decreased PL emission intensity can be observed, indicating the recombination of electrons and holes in C3N4/ NiB7.5 is restrained. Additionally, the enhanced charge separation efficiency is further confirmed by the fluorescent lifetime spectra (Figure 6f). It is obviously seen that the fluorescent intensity of the C3N4/NiB7.5 shows slow decay kinetics as compared to pristine C3N4 counterpart. The three radiative lifetime constants τ1, τ2, and τ3 of C3N4/NiB7.5 are 0.67, 3.07, and 15.20 ns (Table 1), respectively, which are all smaller than the corresponding values for pristine C3N4.

Figure 6. (a) Photocatalytic hydrogen production rates on C3N4/NiB photocatalysts loaded with different amounts of NiB under the irradiation of a 300 W Xe lamp (AM1.5 air mass filter). (b) Recyclability of hydrogen evolution tests (300 W xenon lamp with AM1.5). (c) Wavelength-dependent quantum efficiency of hydrogen evolution over C3N4/NiB7.5. (d) Transient photocurrent of C3N4 and C3N4/NiB7.5. (e) Photoluminescence spectra of C3N4 and C3N4/ NiB7.5. (f) Time-resolved fluorescence decay spectra of C3N4 and C3N4/NiB7.5.

evolution under solar light irradiation. However, after deposition of a small amount of NiB (2.5 wt %), an improved HER rate (99.9 μmol h−1 g−1) can be observed. With the increment of the loading amount of NiB, the hydrogen amount of C3N4/NiB is increased correspondingly. A highest HER rate of 464.4 μmol h−1 g−1 can be achieved on C3N4/NiB7.5. Further increment of loading amount of NiB leads to a sharp decline of HER activity, which is caused by a large amount of NiB covering the surface-active sites on C3N4 and impeding the light absorption of C3N4. Besides, it should be noted that pristine NiB has no HER activity, suggesting its role as a cocatalyst rather than a photocatalyst. And our material is relatively cheap and abundant among many industrial photocatalysts (Table S2). The stability of the optimized C3N4/ NiB7.5 is further evaluated by conducting the hydrogen evolution under the same conditions for four cycles. As shown in Figure 6b, the hydrogen evolution rate shows no significant decrease, indicating the present HER evolution system is quite stable and robust. Furthermore, our synthesized photocatalyst still shows hydrogen evolution activity after irradiating for 40 h (Figure S1). Figure 6c shows the quantum efficiency values of C3N4/NiB7.5 related to the corresponding wavelength for hydrogen evolution, thus suggesting the correspondence of its variation trend of quantum efficiency curves with its diffuse reflectance spectra. The quantum efficiency at 365 nm can reach 10.92%, which is extensively enhanced as compared to pristine bulk C3N4.31,32 To further evaluate the charge separation efficiency in C3N4/NiB7.5 and

Table 1. Kinetic Parameters of Time-Resolved Fluorescence Decays of C3N4 and C3N4/NiB7.5 under 470 nm Excitation sample

τ1/ns

τ2/ns

τ3/ns

τ/ns

C3N4 C3N4/ NiB7.5

0.80 (40.72%) 0.67 (40.72%)

3.43 (40.24%) 3.07 (40.24%)

17.09 (19.04%) 15.20 (19.04%)

4.959 3.919

3.4. Charge Separation and Transfer Mechanism of C3N4/NiB. To investigate the origin of the remarkably high HER performance of C3N4/NiB7.5, UV−vis spectrum was indicated in Figure 8a. The light absorption of C3N4/NiB7.5 obviously increased in the region of 200−800 nm as compared to that of pristine C3N4 on account of the black color of deposited NiB nanoparticles (Figure 8a, inset). Similarly, this phenomenon can also be found for C3N4/NiB2.5, C3N4/NiB5, and C3N4/NiB10 (Figure 9). Moreover, it should be noted that no apparent shift in the absorption edge of C3N4/NiB7.5 is observed, ruling out that Ni and B atoms are undoped into the crystal lattice of C3N4. Besides, according to the Mott− Schottky tests (Figure 8b, c), the conduction band (CB) potential of C3N4/NiB7.5 is reduced to −1.35 V as compared to that of C3N4, suggesting that the photogenerated electrons can transfer from C3N4 to NiB. As shown in Figure 8d, the photoinduced electrons of C3N4 can quickly transfer to NiB nanoparticles through the B(δ−)−Ni(δ+)−N(δ−) bonds. 8128

DOI: 10.1021/acs.iecr.8b01376 Ind. Eng. Chem. Res. 2018, 57, 8125−8130

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irradiation of a 300 W Xe lamp (AM1.5 air mass filter) for 40 h (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mingyang Xing: 0000-0002-0518-2849 Jinlong Zhang: 0000-0002-1334-6436 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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Figure 8. (a) UV−vis spectra of as-prepared samples. Inset: Plots of (ahυ)2 versus the energy of exciting light of C3N4 and C3N4/NiB7.5. (b, c) Mott−Schottky plots of as-prepared samples. (d) Charge separation and transfer in the C3N4/NiB7.5 system under irradiation. Red and blue spheres denote photoinduced electrons and holes, respectively.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21773062, 21577036, 21377038, 5171101651), the State Key Research Development Program of China (2016YFA0204200), the Science and Technology Commission of Shanghai Municipality (16JC1401400, 17520711500), the Shanghai Pujiang Program (17PJD011), and the Fundamental Research Funds for the Central Universities (22A201514021).

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Figure 9. UV−vis spectra of C3N4, C3N4/NiB, and NiB. (a, C3N4; b, C3N4/NiB2.5; c, C3N4/NiB5; d, C3N4/NiB7.5; e, C3N4/NiB10; f, NiB). The inset shows the colors of the above samples.

Subsequently, NiB as a cocatalyst exposes these photoinduced electrons to the active sites on the surface to participate in HER. Meanwhile, photoinduced holes of C3N4 are consumed by TEOA molecules.

4. CONCLUSION In summary, an efficient and robust C3N4/NiB7.5 photocatalytic system has been constructed. As favored by the B(δ−)−Ni(δ+)−N(δ−) bonds between NiB and C3N4, the achieved C3N4/NiB7.5 shows a dramatically enhanced photocatalytic H2 evolution rate (464.4 μmol h−1 g−1) as opposed to pristine C3N4. This work not only demonstrates enormous potential in coupling NiB with other semiconductor photocatalysts but also represents a vital step for developing highly efficient cocatalysts using inexpensive and abundant elements in nature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b01376. Materials and cost, comparison of photocatalysts loaded with cocatalysts, and hydrogen evolution tests under the 8129

DOI: 10.1021/acs.iecr.8b01376 Ind. Eng. Chem. Res. 2018, 57, 8125−8130

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DOI: 10.1021/acs.iecr.8b01376 Ind. Eng. Chem. Res. 2018, 57, 8125−8130