English

• Graphitic carbon nitride (g-C₃N₄), the most popular metal-free semiconductor, has been widely used for sunlight-driven water splitting^[1-3], carbon dioxide photoreduction^[4,5] and organic pollutant photodegradation^[6,7]. However, the somewhat wide bandgap of g-C₃N₄ means that it can absorb only the ultraviolet and blue fraction of the solar spectrum (λ < 450 nm), which has seriously limited its photocatalytic performance^[8,9]. It has been found that the bandgap can be reduced significantly by increasing the N/C ratio^[10]. Thereby, it is quite necessary to develop N-rich carbon nitride materials.

  g-C₃N₅, as a new carbon nitride photocatalyst with high nitrogen content and narrower bandgap, showed extraordinary properties in the field of photocatalysis^[11]. The C₃N₅ framework contains heptazine moieties bridged together by azo linkage (−N=N−). The presence of azo linkage extends the π conjugated network due to overlap between the p orbitals on N atoms constituting the azo bond and π system of heptazine motif, which resulted in the reduction of the electronic bandgap^[12]. Like most of semiconductors, g-C₃N₅ also suffers the innate drawback of carrier recombination and low specific surface area of bulk g-C₃N₅^[13]. In the previous reports, various hybrid composites have been developed to improve the catalytic activity of C₃N₅. For example, the CeTi₂O₆/g-C₃N₅ heterojunction exhibited outstanding photocatalytic response under the visible light towards the degradation of endocrine rupture material 2,4-dichlorophenol (2,4-DCP) than its single component^[14]. Nitrogen vacancies g-C₃N₅/BiOBr composites also exhibited excellent PEC NRR performance without the addition of noble metals^[15].

  Nickel (II) oxide (NiO), as a non-noble metal p-type semiconductor monoxide, plays an important role in the photocatalytic hydrogen production process due to it can offer more active sites for hydrogen-releasing^[16-21]. Moreover, NiO can promote the separation of photogenerated electron-hole pairs by formation of p-n junctions with other n-type semiconductors. Recently, NiO modified g-C₃N₄ non-noble metal heterojunction photocatalyst exhibited enhanced phototcatalytic performance for hydrogen production^[22-24]. Therefore, it is possible to design NiO/C₃N₅ p-n junctions photocatalysts with desirable performance. To the best of our knowledge, there is no report available for NiO/g-C₃N₅ composite towards the photocatalytic hydrogen evolution.

  In the present work, we aimed to construct a NiO modified C₃N₅ non-noble metal photocatalyst with enhanced hydrogen evolution performance. Varying amounts of Ni modified C₃N₅ samples were prepared and characterized. Based on the experimental results, it can be found that the migration and separation of photogenerated electron-hole pairs by formation of p-n junctions is beneficial to the photocatalytic hydrogen evolution performance. Furthermore, the possible photocatalytic hydrogen evolution mechanism was discussed.

  1.   Experimental

  1.1   Preparation of bulk C₃N₅

  All the reagents in this work were of analytical grade and used as received without any purification. Bulk C₃N₅ had been prepared expediently from the thermal polymerization method^[25]. In brief, 8 g 3-amino-1,2,4-triazole (3-AT) and 8 g NH₄Cl was grounded to form a homogeneous solid mixture. The mixture was heated at 550 °C for 3 h with a ramping rate of 15 °C/min, and cooled naturally to room temperature. Finally, the obtained chocolate brown color product was dried at 70 °C in a vacuum oven for 5 h.

  1.2   Preparation of C₃N₅ nanosheets

  The C₃N₅ nanosheets were obtained according to the literature with a few modifications^[26]. Simply, 1.0 g bulk C₃N₅ was added into round-bottom flask containing 20 mL 3.0 mol/L nitric acid solution. The solution was stirred continuously for 24 h till it turned yellow. Then product was diluted with 1.0 L deionized water, collected by suction filtration with membrane and dried at 70 °C for 4 h. The obtained powder was named as C₃N₅ nanosheets.

  1.3   Preparation of 9-Ni/C₃N₅ composite

  The 9-Ni/C₃N₅ sample was prepared by a hydrothermal method. Firstly, 50 mg C₃N₅ nanosheets powder was dispersed in 80 mL of distilled water with vigorous ultrasound for 1.0 h. Thereafter, a desired amount of NiCl₂·6H₂O was added with continuous stirring, and then adjust the pH value 12.0 using 28% ammonia. After 0.5 h, the suspension was transferred to a 100 mL Teflon-lined stainless-steel autoclave and maintained at 150 °C for 12 h. Subsequently, the powder product was centrifugated, washed and dried at 70 °C in the vacuum drying oven. The final product was obtained after heat treatment at 300 °C for 4 h in air. The samples were named as x-Ni/C₃N₅, where x is the weight ratio of NiCl₂·6H₂O (0, 3%, 5%, 9% and 18%) to the composite.

  1.4   Photocatalytic H₂ generation

  Photocatalytic H₂ generation experiments were measured in a lab solar H₂-evolution system. Xe lamp with an AM 1.5 G filter (CEL-HXF300, 300 W, λ ≥ 420 nm) was used as a simulated solar light and the light density was 160 mW/cm². In a typical measurement, 50 mg NiO/C₃N₅ composite was dispersed by ultrasound into a 100 mL triethanolamine (TEOA) solution (15%) and the system was kept at −0.1 MPa and 5 °C. GC-7900 gas chromatograph was employed to detect the H₂ on line after every 1 h.

  2.   Results and discussion

  2.1   Characterizations of NiO/C₃N₅ composite

  The structure and morphology were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as shown in Figure 1.

  Figure 1

  

  Figure 1.  (a) SEM image of C₃N₅ nanosheets; (b) SEM image of 9-Ni/C₃N₅; (c) and (d) TEM images of 9-Ni/C₃N₅, (e )− (g) the corresponding elemental mapping of C, N and Ni (scale bar: 200 nm)

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  From Figure 1(a), it can be seen that C₃N₅ shows an obvious sheet-like structure by acidifying. After modified with NiO, the dense multilayer structure was formed (Figure 1(b)), this structure was also verified by high magnification TEM image (Figure 1(c)). From Figure 1(d), we can observe that some dark nanoparticles dispersed uniformly on the multilayer of 9-Ni/C₃N₅ and the average diameter of the black spots is about 5−6 nm, which demonstrate that NiO particles were successfully decorated on the surface of C₃N₅. The elemental mapping on the selected area shown in Figure 1(e)−(f) demonstrates a uniformly distribution of C, N and Ni atoms on the surface of the NiO/C₃N₅ composite.

  XRD patterns of the as-prepared samples are presented in Figure 2(a). All samples show strong diffraction peak at 28.1°, which is assigned to the inter-layer stacking of graphitic-like structure, indicating that the lattice structure of C₃N₅ remains unchanged after NiO modification, which is beneficial for photocatalytic properties of NiO/C₃N₅. Moreover, all of the NiO/C₃N₅ samples exhibit a little blue-shift to about 27.1°−27.6°, suggesting that parts of Ni may implant into the lattice of C₃N₅. No obvious peak was detected after loading NiO species onto pure C₃N₅, probably because of low crystallinity and amorphous structure.

  Figure 2

  

  Figure 2.  XRD patterns (a) and FT-IR spectra (b) for C₃N₅ and x-Ni/C₃N₅ composites

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  To gain an insight into the surface functional group of as prepared samples, the FT-IR analysis was carried out. As exhibited in Figure 2(b), there is an obvious peak at about 808 cm⁻¹, which can be assigned to the bending vibration of triazine units^[27]. In addition, the band ranged from 1200 to 1600 cm⁻¹ belongs to the stretching in aromatic C−N. The broad peak at 3000−3600 cm⁻¹ is attributed to the stretching vibration of N−H or O−H groups. The bare C₃N₅ and Ni/C₃N₅ nanocomposites show similar absorption bands, indicating that the structure of C₃N₅ remains unchanged after NiO modification.

  To analysis the composition and chemical state of constituent elements in 9-Ni/C₃N₅ sample, XPS measurements were conducted. Referring to Figure 3(a), the XPS survey spectra of 9-Ni/C₃N₅ sample not only exhibits the peaks of C 1s peak and N 1s peak, but also exhibits a relatively weak O 1s and Ni 2p peak, indicating the well combination of C₃N₅ and NiO. In the C 1s spectrum (Figure 3(b)), the binding energies at 285 and 288.25 eV can be attributed to C−C and N−C=N peaks, respectively^[28]. There are three peaks for the N 1s XPS peak in Figure 3(c), which are ascribed to the amino groups (400.7 eV), N(C)₃ (399.7 eV) and C−N=C (398.6 eV), respectively^[3]. The peak of Ni spectra (Figure 3(d)) centered at 872.7 and 855.4 eV is related to Ni 2p_1/2 and Ni 2p_3/2, respectively^[29-31]. The two relatively weak satellite peaks located at 879.9 and 862.2 eV belong to the shake-up types of Ni 2p_1/2 and Ni 2p_3/2. This result was consistent to previous literature.

  2.2   Photocatalytic H₂-production

  The photocatalytic H₂-production activity of the x-Ni/C₃N₅ nanosheets was evaluated using triethanolamine as an electron donor. As shown in Figure 4(a), as a controlled experiment, negligible H₂ was detected without either photocatalyst or irradiation. Trace H₂ evolution was observed for bare C₃N₅, while Ni-modified C₃N₅ nanosheets displayed better H₂ evolution rate than bare C₃N₅. After loading different Ni proportions (3%, 5%, 9% or 18%) on C₃N₅ nanosheets, the photocatalytic performance of NiO/C₃N₅ has been remarkably improved, and the highest amount of H₂ evolution of 9-Ni/C₃N₅ can reach 357 μmol/(g·h) in the first 4 h reaction. The excellent photocatalytic H₂-production activity is probably due to the separation of photogenerated electron-hole pairs by formation of p-n junctions, which can be confirmed by PL (Figure 5(a)). The PL emission intensity of 9-Ni/C₃N₅ was the lowest among all the samples, indicating 9-Ni/C₃N₅ has the highest separation efficiency of electrons and holes. Moreover, the BET surface area of 9-Ni/C₃N₅ was the largest among all the samples (the BET surface area of 9-Ni/C₃N₅ was found to be 24.6425 m²/g, which was larger than bulk C₃N₅ (20.7077 m²/g), 3-Ni/C₃N₅ (22.1024 m²/g), 5-Ni/C₃N₅ (24.4835 m²/g) and 18-Ni/C₃N₅ (9.3544 m²/g)), which was benefit to catalytic hydrogen evolution reaction.

  Figure 3

  

  Figure 3.  XPS spectra of (a) survey scan, (b) C 1s, (c) N 1s and (d) Ni 2p spectra of 9-Ni/C₃N₅

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  In order to demonstrate the stability and durability of 9-Ni/C₃N₅, the recycling experiments were performed under similar experimental conditions, and each test cycle time is 4 h. As shown in Figure 4(b), the results show that the amount of H₂ produced was retained by about 82% after four cycles, indicating the high stability properties of 9-Ni/C₃N₅ composite.

  Figure 4

  

  Figure 4.  (a) Histogram of the photocatalytic H₂ production rate of C₃N₅ with varied Ni content, (b) recycle experiments of 9-Ni/C₃N₅ photocatalyst

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  2.3   Mechanism analysis

  According to the results of mentioned above, the enhanced H₂-production performance of 9-Ni/C₃N₅ photocatalyst mainly attributes to three reasons: (1) NiO nanoparticles offer more active sites for hydrogen-releasing, (2) broaden the photo light absorption region and (3) the effective separation of photogenerated electron-hole pairs by formation of p-n junctions, which were verified further by the following experiments.

  Photoluminescence (PL) spectroscopy is usually employed to investigate the optical properties and charge-separation efficiency. As shown in Figure 5(a), PL intensity of NiO/C₃N₅ gradually increases first, and then decreases with the increase of Ni content. These results indicate that the NiO nanoparticles on C₃N₅ are able to effectively promote the transfer of charge and thus inhibit photogenerated electron-hole pairs recombination. The PL emission intensity of 9-Ni/C₃N₅ was the lowest among all the samples, indicating 9-Ni/C₃N₅ has the highest separation efficiency of electrons and holes. The time-resolved PL (TRPL) decay profiles of C₃N₅ and 9-Ni/C₃N₅ in Figure 5(b) showed that the charge carrier lifetime in 9-Ni/C₃N₅ (2.386 ns) was longer than that of C₃N₅ (2.006 ns). The result further indicated that 9-Ni/C₃N₅ has excellent separation efficiency of electrons and holes.

  Figure 5

  

  Figure 5.  PL spectra (a) and TRPL spectra (b) of different samples

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  The optical absorption properties of the samples were tested by UV-vis absorption spectroscopy. As shown in Figure 6(a), both C₃N₅ and 9-Ni/C₃N₅ exhibit clear absorption in the region of visible light. Their band-gap energies are 1.95 and 1.78 eV, respectively (Figure 6(b)), which agree with previous reports well. Furthermore, 9-Ni/C₃N₅ can harvest visible light more efficiently for above catalytic reaction, which may be due to a broader tail (bathochromic/red shift) in the absorption spectrum. The band positions were confirmed by Mott-Schottky plots, as depicted in Figure 6(c). The flat band or CB potential of C₃N₅ was −0.67 V.

  Figure 6

  

  Figure 6.  (a) UV-vis absorption spectra, (b) bandgap energy, (c) Mott-Schottky plots and (d) Electrochemical impedance spectra of different samples

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  From the obtained CB and band gap potential, it is easy to estimate the VB position of C₃N₅, which was calculated to be 1.28 V. The positive slopes could prove the n-type semiconductors properties of C₃N₅, which was favorable to form p-n junction with p-type NiO nanoparticles. The p-n junction will promote the separation of electron-hole pairs. This result was further clarified by the electrochemical impedance spectroscopy (EIS) plot (Figure 6(d)). 9-Ni/C₃N₅ has the smaller semicircle diameter compared to pure C₃N₅ in higher frequency region, which indicated enhanced charge-carrier transfer ability of 9-Ni/C₃N₅^[32].

  Based on the results and analysis above, a possible photocatalytic H₂-production mechanism over 9-Ni/C₃N₅ photocatalyst was summarized in Figure 7.

  Figure 7

  

  Figure 7.  Proposed H₂ evolution mechanism by 9-Ni/C₃N₅ photocatalyst

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  Under visible light irradiation, only C₃N₅ can easily absorb visible light and generate photo-induced electron-hole pairs. Photogenerated electrons in the CB of C₃N₅ migrate to the more negative potential CB of NiO (E_CB = −0.5 V vs NHE) for proton reduction, and photogenerated holes in the VB of C₃N₄ were consumed by the sacrificial electron donor TEOA. In a word, the photo-induced electron-hole pairs can be efficient separation by constructing an inner electric field of NiO/C₃N₅ p-n heterojunction.

  3.   Conclusions

  In summary, we successfully synthesized 9-Ni/C₃N₅ p-n junctions photocatalyst through a facile hydrothermal method. This photocatalyst showed excellent hydrogen production efficiency under visible light. The hydrogen production rate reached 357 μmol/(g·h), which was 107-fold higher than that of pristine C₃N₅. This mainly attributed to the NiO modification being able to promote photoinduced electron-hole pair separation, thus promote the hydrogen evolution reaction. This work has provided a feasible strategy to design non-noble metal modified carbon nitride for high-efficient solar conversion.

  
  
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• 

  Figure FIG. 1270. 

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  Figure 1  (a) SEM image of C₃N₅ nanosheets; (b) SEM image of 9-Ni/C₃N₅; (c) and (d) TEM images of 9-Ni/C₃N₅, (e )− (g) the corresponding elemental mapping of C, N and Ni (scale bar: 200 nm)

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  Figure 2  XRD patterns (a) and FT-IR spectra (b) for C₃N₅ and x-Ni/C₃N₅ composites

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  Figure 3  XPS spectra of (a) survey scan, (b) C 1s, (c) N 1s and (d) Ni 2p spectra of 9-Ni/C₃N₅

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  Figure 4  (a) Histogram of the photocatalytic H₂ production rate of C₃N₅ with varied Ni content, (b) recycle experiments of 9-Ni/C₃N₅ photocatalyst

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  Figure 5  PL spectra (a) and TRPL spectra (b) of different samples

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  Figure 6  (a) UV-vis absorption spectra, (b) bandgap energy, (c) Mott-Schottky plots and (d) Electrochemical impedance spectra of different samples

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  Figure 7  Proposed H₂ evolution mechanism by 9-Ni/C₃N₅ photocatalyst

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