3.4 Brunauer-Emmett-Teller analysis
The Brunauer-Emmett-Teller (BET) specific surface area (S BET) and the pore diameter distribution of ZnCdS nanoparticles, CoP polyhedrons, Co3O4 QDs and bi-heterostructured O/ZCS/P-3 cages were analyzed by N2adsorption-desorption measurements as displayed in Fig. 5. The type IV isotherms with type H3 hysteresis loops in Fig. 5a and the pore diameter distribution curves in Fig. 5b indicate the presence of mesoporous structure in the four as-synthesized photocatalysts, which will facilitate the mass transfer for heterogeneous catalysis [41]. And the average pore size of ZnCdS nanoparticles, CoP polyhedrons, Co3O4QDs and O/ZCS/P-3 is 20, 18, 4 and 22 nm, respectively. In comparison with CoP polyhedrons and Co3O4 QDs, ZnCdS nanoparticles possess a relatively small surface area of 30 m2·g-1 owing to its relatively larger density. However, an enlarged BET surface area of 37 m2·g-1 for hierarchical O/ZCS/P-3 bi-heterostructure cages was gained because of the integration of both CoP (79 m2·g-1) polyhedrons and Co3O4 QDs (178 m2·g-1) with ZnCdS nanoparticles, which ensures more active sites for photocatalytic hydrogen evolution reaction.