3.6 Photocatalytic H2 evolution activity measurements
Fig. 7a depicts the photocatalytic hydrogen evolution performance of different samples under visible light irradiation (λ ≥ 400 nm) with lactic acid (LA) as the hole scavenger. The original ZnCdS nanoparticles are visibly active to catalyze the splitting of water under 5 W LED light irradiation, giving a H2-evolving rate of 8.2 mmol h-1 g-1. However, after coupling 0.5 wt.% CoP polyhedron or 0.5 wt.% Co3O4 QDs with ZnCdS nanoparticles, the as-obtained ZCS/CoP and ZCS/Co3O4 QDs hybrids show an improved hydrogen evolution activity, which manifests that the introduction of both CoP polyhedron and Co3O4 QDs can accelerate the separation and migration of photo-excited charges from ZCS. More importantly, while 0.5 wt.% CoP polyhedron and 0.5 wt.% Co3O4 QDs were introduced concurrently, the constructed hierarchical CoP/ZnCdS/Co3O4QDs (800>40>4.5 nm) bi-heterostructure cages exhibits more high-performance photocatalytic hydrogen production, which strongly indicates that the parallel pathways of electron migration was successfully established for more rapider charge transport. Fig. 7b shows the comparison of hydrogen evolution rate of ZCS/CoP with different wt.% of CoP polyhedron. It can be clearly observed that ZCS/CoP with 1.5 wt.% CoP exhibits higher hydrogen generation rate (12.4 mmol h-1 g-1) which is 1.5 times that of pristine ZCS, again demonstrating the significant role of CoP for promoting the hydrogen evolution activity of ZCS. Fig. 7c displays the hydrogen production activities after assembling different content of Co3O4QDs on ZCS/CoP-3. As observed, the O/ZCS/P-3 with 1.5 wt.% Co3O4 QDs exhibits the highest photocatalytic hydrogen evolution property (24.2 mmol h-1 g-1) under 5 W LED light irradiation, which is about 2.9 times that of original ZCS. Further, O/ZCS/P-3 possesses more prominent hydrogen generation performance under 300 W Xe lamp irradiation close to sunlight (Fig. 7d), and a large number of bubbles can be clearly observed in a closed reaction vessel as shown in Fig. 7g, which seriously reveal the ultrahigh photocatalytic performance of the hierarchical O/ZCS/P-3 bi-heterostructure cages for hydrogen evolution. In order to more intuitively evaluate the photocatalytic hydrogen evolution rate of the bi-heterostructured O/ZCS/P-3 cages, the reaction vessel was equipped with H2-collection-device (HCD). Dynamic bubbles in hydrogen evolution reaction is provided in a separate document (H2-evolution video). While the hydrogen evolution reaction proceed for 0 h, the HCD of easy expansion is downcast. After 5 h, it was obviously bulging. However, it’s completely propped up after 10 h. Both lots of bubbles and the rapid bulging of elastic HCD indicate the ultrahigh photocatalytic hydrogen evolution performance of the as-constructed O/ZCS/P-3 bi-heterostructure cages. Additionally, the purity of evolved H2 also was tested via ignition experiment, bright flame means the high purity of H2from O/ZCS/P-3 driven water splitting under visible light irradiation. Undoubtedly, the hierarchical O/ZCS/P-3 bi-heterostructure cages should be one of the world-level photocatalysts for efficient H2 evolution up to now. The hydrogen generation performance of O/ZCS/P-3 under different wavelength was carried out (1 h), the results are shown in Fig. 7e. Obviously, the decrease of photocatalytic hydrogen evolution rate is drastic at the wavelengths greater than 500 nm, which indicates that the water splitting reaction for H2 evolution is indeed titillated by the light excitation of the O/ZCS/P-3 photocatalyst. And the H2 evolution rate of O/ZCS/P-3 is the fastest at the wavelength of 450 nm. Furthermore, it also can be observed from Fig. 7f that the hierarchical O/ZCS/P-3 bi-heterostructure photocatalyst not only exhibits ultrahigh H2 production properties but also very good stability, which are mainly contributed to the construction of the two efficient parallel approaches applied to the fast migration of photo-induced charges in the architecture of hierarchical O/ZCS/P-3 bi-heterostructure cages.