Article (Special Issue on Photocatalytic H2 Production and CO2 Reduction)
Photocatalytic H2 generation via CoP quantum-dot-modified g-C3N4 synthesized by electroless plating

https://doi.org/10.1016/S1872-2067(19)63459-5Get rights and content

Abstract

Photocatalytic water splitting is a promising method for hydrogen production. Numerous efficient photocatalysts have been synthesized and utilized. However, photocatalysts without a noble metal as the co-catalyst have been rarely reported. Herein, a CoP co-catalyst-modified graphitic-C3N4 (g-C3N4/CoP) is investigated for photocatalytic water splitting to produce H2. The g-C3N4/CoP composite is synthesized in two steps. The first step is related to thermal decomposition, and the second step involves an electroless plating technique. The photocatalytic activity for hydrogen evolution reactions of g-C3N4 is distinctly increased by loading the appropriate amount of CoP quantum dots (QDs). Among the as-synthesized samples, the optimized one (g-C3N4/CoP-4%) shows exceptional photocatalytic activity as compared with pristine g-C3N4, generating H2 at a rate of 936 μ mol g−1 h−1, even higher than that of g-C3N4 with 4 wt% Pt (665 μmol g−1 h−1). The UV-visible and optical absorption behavior confirms that g-C3N4 has an absorption edge at 451 nm, but after being composited with CoP, g-C3N4/CoP-4% has an absorption edge at 497 nm. Furthermore, photoluminescence and photocurrent measurements confirm that loading CoP QDs to pristine g-C3N4 not only enhances the charge separation, but also improves the transfer of photogenerated e-h+ pairs, thus improving the photocatalytic performance of the catalyst to generate H2. This work demonstrates a feasible strategy for the synthesis of highly efficient metal phosphide-loaded g-C3N4 for hydrogen generation.

Graphical Abstract

The photocatalytic activity of H2 generation over g-C3N4/CoP composites prepared by an electroless plating method is much higher than that over pure g-C3N4. The composites boosted the separation and migration of photogenerated carriers, resulting in a higher photocatalytic activity.

  1. Download : Download high-res image (73KB)
  2. Download : Download full-size image

Introduction

In recent decades, global energy consumption has increased rapidly. Developing renewable energy technologies has thus become a critical task. Keeping this in mind, researchers in various communities are searching for alternate energy resources. Recently, great effort has been expended to search for an inexhaustible, renewable, and clean energy resource to satisfy future energy demands and cope with environmental hazards [1, 2, 3, 4, 5]. Hydrogen is an ideal source that can produce clean and green energy without forming secondary pollution, and it also has a high combustion efficiency.

In the modern era, hydrogen production through water splitting utilizing semiconductor photocatalysis is a promising method that has received much attention. The ideal source for hydrogen generation is water because of its high abundance, availability, and renewability. Many photocatalysts and photoelectrodes have been employed for hydrogen generation. For example, in the recent past, various types of photocatalysts, including TiO2, LaFeO3, CdS, SrTiO3, and CoP, have been utilized for hydrogen generation [6, 7, 8, 9, 10, 11, 12, 13]. However, because of various limitations, the hydrogen generation performance via such materials is inconsistent. Accordingly, photocatalytic water splitting to generate H2 needs to be further pursued.

More recently, among many investigated photocatalysts, non-metal materials like g-C3N4 have attracted a great amount of attention because of their suitable band gap (2.7 eV) and stability. g-C3N4 is considered to be a highly efficient photocatalyst in many reactions, such as CO2 reduction [14, 15], water splitting [16, 17], and organic pollutant degradation [18, 19]. Most importantly, g-C3N4 fulfills the standard thermodynamic requirements of water splitting by light, i.e., g-C3N4 has a suitable band gap and appropriate position for the conduction band (CB) and valence band (VB) [20, 21, 22]. However, the photocatalytic activity of g-C3N4 is not satisfactory because of various limitations, including the low efficiency of visible light adsorption and high recombination rate of photogenerated carriers. Various approaches have been adopted to improve the performance of pristine g-C3N4. Among these strategies, forming a heterojunction between two semiconductors with wide and narrow band gaps [23, 24, 25, 26, 27], surface modification [28, 29, 30, 31], metal and nonmetal doping [32], and utilization together with a co-catalyst are all well known. Many investigators have reported that some noble metals, including Au, Pt, Pd, and Ag are efficient [33, 34, 35, 36, 37, 38], their high cost restricts large-scale usage, making it urgent to develop new strategies.

Transition metal-based phosphides, including FeP [39], CoP [40], NiP [41, 42, 43, 44], and CuP [45], have been extensively used as co-catalysts in photocatalysis reactions. Moreover, surveying the relevant literature reveals that the photocatalytic reaction pathway and mechanism of Co-based phosphides are identical to what the hydrogenases do naturally. It is expected that Co-based phosphides can also be utilized as efficient catalysts for the hydrogen evaluation reaction because of their unique physical and chemical properties and easy synthesis [46]. To the best of our knowledge, limited work has been performed to utilize CoP in the field of photocatalysis as a co-catalyst, i.e., forming g-C3N4/CoP for efficient photocatalytic water splitting to produce H2. Thus, it is worth investigating how CoP can be loaded on g-C3N4 and applied as a co-catalyst.

In this work, g-C3N4 loaded with various amounts of CoP quantum dots (QDs) is fabricated by two steps, including pyrolysis and electroless plating. The bare g-C3N4 is prepared by a previously reported method, i.e., thermal polymerization of urea. After this, in the second step, an electroless plating process is adopted for CoP growth in situ on the surface of g-C3N4. The optimized structure is g-C3N4/CoP-4%, which shows higher photocatalytic activity for H2 production than that of pristine g-C3N4 under simulated solar light radiation, even higher than that of Pt-modified g-C3N4. Finally, this work demonstrates that g-C3N4/CoP nanocomposites demonstrate promising applications in the fields of photocatalysis and H2 production.

Section snippets

Preparation

Typically, g-C3N4 is synthesized via heating of urea. A total of 15 g of dry and solid urea was transferred to a crucible, after which the crucible was heated in a muffle furnace at 500 °C for 2 h, with a heating rate of 10 °C min−1. After the prepared material cooled down naturally, the product was ground into powder and stored. For coating CoP on the surface of g-C3N4, an electroless plating technique was employed. A total of 1 g of g-C3N4 was placed in 40 mL of H2O and treated ultrasonically

XRD

The crystallinity and structure of the as-prepared samples were investigated by XRD. Fig. 1 shows the XRD patterns of pristine g-C3N4 and g-C3N4/CoP composites loaded with various amounts of CoP. The two predominant peaks are at 13.1° and 27.5°, respectively, representing the g-C3N4 itself (JCPDS Card No 87-1526). The diffraction peaks at 27.5° correspond to the (002) typical plane with a planar distance of 0.33 nm, analogous to the interlayer stacking of the related aromatic segments. The

Conclusions

A simple method is reported for the preparation of CoP QDs by an in situ growth strategy on the surface of g-C3N4. CoP QDs as a co-catalyst were loaded on the surface of g-C3N4 by a simple pyrolysis-based method and electroless plating. The as-prepared g-C3N4/CoP composite shows much higher photocatalytic activity for H2 production than that of pristine g-C3N4. Among the as-prepared samples, the optimized one, i.e., g-C3N4/CoP-4%, shows remarkable improvement with respect to hydrogen generation

References (59)

  • K. Qi et al.

    Chin. J. Catal.

    (2018)
  • K. Qi et al.

    J. Alloys Compd.

    (2017)
  • K. Qi et al.

    Sol. Energy Mater. Sol. Cells

    (2018)
  • K. Qi et al.

    Chin. J. Catal.

    (2017)
  • S. Wei et al.

    Chin. J. Catal.

    (2018)
  • X. Zhu et al.

    Chin. J. Catal.

    (2018)
  • J. Fu et al.

    Appl. Catal. B Environ.

    (2018)
  • Q.-F. Liu et al.

    Chin. J. Catal.

    (2018)
  • J. Fu et al.

    Appl. Catal. B Environ.

    (2019)
  • C. Bie et al.

    Appl. Surf. Sci.

    (2018)
  • D. Xu et al.

    Appl. Catal. B Environ.

    (2018)
  • K. He et al.

    Appl. Surf. Sci.

    (2018)
  • Z. Dong et al.

    Appl. Surf. Sci.

    (2018)
  • D. Ma et al.

    Chem. Eng. J.

    (2017)
  • J. Wen et al.

    Appl. Surf. Sci.

    (2017)
  • Z. Liu et al.

    Chin. J. Catal.

    (2018)
  • Q. Xu et al.

    Mater. Today

    (2018)
  • K. Pandiselvi et al.

    J. Hazard. Mater.

    (2016)
  • G. Wang et al.

    Appl. Surf. Sci.

    (2019)
  • R. He et al.

    Appl. Surf. Sci.

    (2018)
  • H. Yang et al.

    Appl. Surf. Sci.

    (2018)
  • T. Tong et al.

    Appl. Surf. Sci.

    (2018)
  • Q. Xu et al.

    Carbon

    (2017)
  • X. Li et al.

    Appl. Surf. Sci.

    (2018)
  • H. Li et al.

    Appl. Surf. Sci.

    (2018)
  • T. Hu et al.

    Appl. Surf. Sci.

    (2019)
  • K. Qi et al.

    Appl. Surf. Sci.

    (2019)
  • Q. Lin et al.

    Appl. Catal. B Environ.

    (2015)
  • S. Huang et al.

    Colloids Surf. A

    (2015)
  • Cited by (168)

    View all citing articles on Scopus

    Published 5 January 2020

    This work was supported by the National Natural Science Foundation of China (51602207), the Doctoral Scientific Research Foundation of Liaoning Province (20170520011), the Program for Liaoning Excellent Talents in Universities (LR2017074), the Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment (SKLPEE-201810), Fuzhou University, the Scientific Research Project of the Educational Department of Liaoning Province (LQN201712), and Shenyang Excellent Talents in Universities (RC180211).

    View full text