Ni, Co-Based Selenide Anchored g-C3N4 for Boosting Photocatalytic Hydrogen Evolution

Developing novel and efficient catalysts is a significant way to break the bottleneck of low separation and transfer efficiency of charge carriers in pristine photocatalysts. Here, two fresh photocatalysts, g-C3N4@Ni3Se4 and g-C3N4@CoSe2 hybrids, are first synthesized by anchoring Ni3Se4 and CoSe2 nanoparticles on the surface of well-dispersed g-C3N4 nanosheets. The resulting materials show excellent performance for photocatalytic in situ hydrogen generation. Pristine g-C3N4 has poor photocatalytic hydrogen evolution activity (about 1.9 mu mol.h(-1)) because of the rapid recombination of electron-hole pairs. However, the hydrogen generation activity is well improved after growing Ni3Se4 and CoSe2 on the surface of g-C3N4, owing to the unique effect of these selenides in accelerating the separation and migration of charge carriers. The hydrogen production activities of G-C3N4@Ni3Se4 and g-C3N4@CoSe2 are about 16.4 mu mol.h(-1) and 25.6 mu mol.h(-1), which are 8-fold and 13-fold that of pristine g-C3N4, respectively. In detail, coupling Ni3Se4 and CoSe2 with g-C3N4 greatly improves the light absorbance density and extends the light response region. The photoluminescence intensity of the photoexcited Eosin Y dye in the presence of g-C3N4@Ni3Se4 and g-C3N4@CoSe2 is weaker than that in the presence of pure g-C3N4. On the other hand, the upper limit of the electrontransfer rate constants in the presence of g-C3N4@Ni3Se4 and g-C3N4@CoSe2 is greater than that in the presence of pure g-C3N4. Among the g-C3N4@Ni3Se4@FTO, g-C3N4@CoSe2@FTO, and g-C3N4@FTO electrodes, the g-C3N4@FTO electrode has the lowest photocurrent density and the highest electrochemical impedance, implying that the introduction of CoSe2 and Ni3Se4 onto the surface of g-C3N4 enhances the separation and transfer efficiency of photogenerated charge carriers. In other words, the formation of two star metals selenide based on g-C3N4 can efficiently inhibit the recombination of photogenerated charge carriers and accelerate photocatalytic water splitting to generate H-2. Meanwhile, the right shift of the absorption band edge effectively reduces the transition threshold of the photoexcited electrons from the valence band to the conduction band. In addition, the more negative zeta potential for the g-C3N4@Ni3Se4 and g-C3N4@CoSe2 catalysts as compared with that for pure g-C3N4 leads to a notable enhancement in the adsorption of protons by the sample surface. Moreover, the results of density functional theory calculations indicate that the hydrogen adsorption energy of the N sites in g-C3N4 is -0.22 eV; further, the hydrogen atoms are preferentially adsorbed at the bridge site of two selenium atoms to form a Se-H -Se bond, and the adsorption energy is 1.53 eV. In-depth characterization has been carried out by transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, transient photocurrent measurements, and Fourier transform infrared spectroscopy; the results of these experiments are in good agreement with one another.