Article (Special Issue on Photocatalytic H2 Production and CO2 Reduction)
The embedded CuInS2 into hollow-concave carbon nitride for photocatalytic H2O splitting into H2 with S-scheme principle

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Abstract

It is still a great challenge to effectively optimize the electronic structure of photocatalysts for the sustainable and efficient conversion of solar energy to H2 energy. To resolve this issue, we report on the optimization of the electronic structure of hollow-concave carbon nitride (C3N4) by deviating the sp2-hybridized structure of its tri-s-triazine component from the two-dimensional plane. The embedded CuInS2 into C3N4 (CuInS2@C3N4) demonstrates an increased light-capturing capability and the promoted directional transfer of the charge carrier. Research results reveal that the hollow structure with an apparent potential difference between the concave and convex C3N4 drives the directional transfer of the photoinduced electrons from the Cu 2p orbital of CuInS2 to the N 1s orbital of C3N4 with the S-scheme principle. The H2 evolution efficiency over CuInS2@C3N4 is up to 373 µmol·h−1 g−1 under visible irradiation, which is 1.57 and 1.35 times higher than those over the bulk g-C3N4 with 1 wt% Pt (238 µmol·h−1 g−1) and g-C3N4 with 3 wt% Pd (276 µmol·h−1 g−1), respectively. This suggests that the apparent potential difference of the hollow C3N4 results in an efficient reaction between the photogenerated electrons and H2O. This work supplies a new strategy for enhancing the sustainable solar conversion performance of carbon nitride, which can also be suitable for other semiconductors.

Graphical Abstract

S-scheme principle for water splitting into H2 over CuInS2@C3N4 with directional charge-transfer under the effect of an apparent potential difference of the hollow-concave C3N4 caused by deviating the sp2-hybridized structure of its tri-s-triazine component from the two-dimensional plane.

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Introduction

Photocatalytic H2 production from H2O splitting is suggested to be a promising strategy for solar–photon energy conversion owing to its green and renewable characteristics [1, 2, 3]. In a general artificial photocatalytic procedure, the photocatalytic process involves four procedures, i.e., photon absorption, generation of e/h+, charge transfer from provider to active sites, and reduction of absorbed H2O or H3O+ to H2 [3, 4]. Achieving enhanced photon capture and efficient utilization of solar energy is still challenging [5]. As a result, photon energy needs to exceed the transition energy of photogenerated e in the bandgap; the directional charge-transfer and its designated interaction with the absorbed H2O are required. The former requires significant photon energy to excite photogenerated charges; however, this requirement cannot be met. The second factor dynamically reflects a competition in the utilization of the photogenerated charge, and e transfers to H2O slowly (~μs); however, e recombines with h+ rapidly (~ps) [6, 7]. Although there are many strategies to be attempted [8, 9], such as the heteroatom doping, heterojunction construction, and plasmonic noble metal loading, it is still difficult to integrate the electricity and optics of semiconductors to simultaneously satisfy the thermodynamic and dynamic requirements of photogenerated carriers.

Taking 2D π-conjugated carbon nitride as a representative, g-C3N4 is a typical visible-excitation semiconductor (~2.7 eV), and its CB potential is −1.23 V (vs. NHE at pH = 7) [10, 11]. Generally, g-C3N4 has been used as a photocatalyst for reductive photocatalytic reactions, e.g., splitting H2O into H2 [12, 13, 14, 15, 16, 17, 18], CO2 [19, 20, 21, 22, 23], O2 [24, 25, 26], and/or heavy metal reduction [27, 28, 29, 30]. It is known that g-C3N4 is usually in the form of the layered bulk agglomeration in thermal polymerization, which results in a small surface area, low-photon absorption, and serious charge–carrier recombination [31]. Thus, noble metal (Pt) is often used as a support for g-C3N4 to enhance its photo-induced electron migration and utilization in H2O splitting into H2. However, its H2 evolution efficiency does not match the photocatalyst cost requirements [32, 33]. Essentially, it is necessary to understand that the migration and action mechanism of photogenerated electrons from the atomic orbital structures of g-C3N4 and its electronic structure can be consequently adjusted to optimize the utilization of photo-induced electrons and H2 production performance over g-C3N4. It is revealed that g-C3N4 is composed of sp2-hybridized C-N bonds with lone electrons on the pz orbitals, which form π-conjugated structures [34, 35, 36]. When the sp2-hybridized structure of g-C3N4 deviates from the two-dimensional plane, the π-electron density shifts from the concave to the convex sp2-hybridized structure along with the hybridization intermediate between sp2 and sp3 in the g-C3N4 layers [36, 37, 38]. This causes the electron to directly trend from concave to convex. Moreover, the curved-hollow g-C3N4 can increase its surface density, and the undulating surface increases its absorbance and improves the photosensitivity of its intrinsic or composite materials [39]. In view of this, we synthesized hollow-concave carbon nitride (C3N4), and CuInS2, as the typical narrow-bandgap sample, was embedded and grown in the cavity of C3N4 by hydrothermal method. Research results confirm that the constructed CuInS2@C3N4 photocatalysts demonstrated increased light-absorption intensity from normal 455 to 480 nm due to the light scattering effect caused by the hollow structure. The good directional migration capability of the charge carriers from CuInS2 to C3N4 has been achieved under the action of the apparent potential difference between the concave to the convex of the sp2-hybridized structure of C3N4, which thus promotes the effective spatial separation, and enhances the utilization of photogenerated carriers with the direct S-scheme mechanism. As a result, CuInS2@C3N4 photocatalysts exhibit higher efficiency toward H2 evolution compared with the bulked g-C3N4 supported 1 wt% Pt and g-C3N4 loaded 3 wt% Pd.

Section snippets

Synthesis of hollow-concave C3N4

SiO2 nanospheres with an average size in the range of 300–350 nm were prepared according to a previous report [40]. C3N4 was synthesized by direct heat treatment using SiO2 nanospheres as a template. Specifically, 2.0 g of melamine powder was evenly mixed with 1.0 g of SiO2 nanospheres. Finally, the mixture was placed in a crucible (20 mL). The crucible, including precursors and templates, was firstly heated in a muffle furnace at 320 °C for 2 h at a linear ramp rate of 10 °C min−1, and then

Crystal composite and morphology

The size of the pristine CuInS2 is about 100–120 nm (Fig. 1(A)), and C3N4 demonstrates the uniform hollow-concave structure with a diameter of 300–350 nm (Fig. 1(B)). Using these hollow concave as nucleation sites, the grown CuInS2 has a larger size of 250–300 nm than that of the pristine CuInS2, and finally covers the surface of C3N4 (Fig. 1(C)). Element mappings confirm the uniform distributions of C, N, Cu, In, and S for the CuInS2@C3N4 sample in Fig. 1(D). The XRD spectra of the as-prepared

Conclusions

In summary, utilizing the advantage of the apparent potential difference of hollow-concave carbon nitride, CuInS2 nanoparticles have been impregnated within C3N4 for the photocatalytic H2O splitting into H2. The constructed CuInS2@C3N4 exhibits higher photocatalytic performance than the bulk g-C3N4 with 1% Pt and g-C3N4 with 3% Pd in a visible-light hydrolysis system. Experimental analyses confirm that the high efficiency of the H2 evolution over CuInS2@C3N4 mainly arises from (1) the increased

Acknowledgments

Study was supported by the National Natural Science Foundation of China (21871155), the K. C. Wong Magna Fund in Ningbo University, Fan 3315 Plan, and Yongjiang Scholar Plan.

References (60)

  • R.C. Shen et al.

    Chin. J. Catal.

    (2019)
  • J.J. Liu et al.

    Appl. Catal. B: Environ.

    (2019)
  • Y.J. Ren et al.

    Chin. J. Catal.

    (2019)
  • Y.J. Wang et al.

    Chin. J. Catal.

    (2019)
  • Z. Li et al.

    Chin. J. Catal.

    (2019)
  • Y.B. Li et al.

    Chin. J. Catal.

    (2019)
  • N. Xiao et al.

    Chin. J. Catal.

    (2019)
  • X.H. Wu et al.

    Appl. Catal. B: Environ.

    (2019)
  • Y. Li et al.

    J. Colloid Interf. Sci.

    (2018)
  • Y.Q. Zhu et al.

    Appl. Surf. Sci.

    (2019)
  • Y. Jiang et al.

    Appl. Surf. Sci.

    (2019)
  • X.B. Li et al.

    J. Alloys Compd.

    (2019)
  • X.X. Zhang et al.

    Appl. Surf. Sci.

    (2019)
  • N. Tian et al.

    Appl. Catal. B: Environ.

    (2019)
  • Q.L. Wang et al.

    Nano Energy

    (2019)
  • S.Q. Song et al.

    Appl. Catal. B: Environ.

    (2018)
  • L.P. Yang et al.

    J. Catal.

    (2017)
  • C.H. Lu et al.

    Appl. Surf. Sci.

    (2016)
  • C.H. Lu et al.

    Appl. Catal. B: Environ.

    (2017)
  • J. Zhang et al.

    Chem. Eng. J.

    (2019)
  • D.D. Chen et al.

    J. Hazard. Mater.

    (2019)
  • B.C. Zhu et al.

    Appl. Catal. B: Environ.

    (2017)
  • Y.J. Yan et al.

    Ceram. Int.

    (2019)
  • S.Q. Song et al.

    Appl. Surf. Sci.

    (2018)
  • J.J. Wang et al.

    Int. J. Hydrogen Energy

    (2019)
  • H.G. Yu et al.

    Chem. Eng. J.

    (2019)
  • S.Q. Song et al.

    Appl. Catal. B: Environ.

    (2016)
  • J.W. Fu et al.

    Appl. Catal. B: Environ.

    (2019)
  • Q.J. Xiang et al.

    J. Phys. Chem. Lett.

    (2013)
  • S.W. Cao et al.

    J. Phys. Chem. Lett.

    (2014)
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    Published 5 January 2020

    Study was supported by the National Natural Science Foundation of China (21871155), the K. C. Wong Magna Fund in Ningbo University, Fan 3315 Plan, and Yongjiang Scholar Plan.

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