We have realized in-situ growth of ultrathin ZnIn₂S₄ nanosheets on the sheet-like g-C₃N₄ surfaces to construct a “sheet-on-sheet” hierarchical heterostructure. The as-synthesized ZnIn₂S₄/g-C₃N₄ heterojunction nanosheets exhibit remarkably enhancement on the photocatalytic activity for H₂ production. This enhanced photoactivity is mainly attributed to the efficient interfacial transfer of photoinduced electrons and holes from g-C₃N₄ to ZnIn₂S₄ nanosheets, resulting in the decreased charge recombination on g-C₃N₄ nanosheets and the increased amount of photoinduced charge carriers in ZnIn₂S₄ nanosheets. Meanwhile, the increased surface-active-sites and extended light absorption of g-C₃N₄ nanosheets after the decoration of ZnIn₂S₄ nanosheets may also play a certain role for the enhancement of photocatalytic activity. Further investigations by the surface photovoltage spectroscopy and transient photoluminescence spectroscopy demonstrate that ZnIn₂S₄/g-C₃N₄ heterojunction nanosheets considerable boost the charge transfer efficiency, therefore improve the probability of photoinduced charge carriers to reach the photocatalysts surfaces for highly efficient H₂ production.

The size-dependent temperature sensitivity is observed on the upconversion luminescence of NaYF₄:Er,Yb microspheres with sizes between 0.7 and 2 μm that are prepared by a poly(acrylic acid)-assisted hydrothermal process. It is found that the fluorescence intensity ratio (FIR) of their green upconversion emissions (with peaks at 521 and 539 nm) is strongly size-dependent at temperatures between 223 and 403 K. As the size of the spheres increases from 0.7 to 1.6 μm, the maximum sensitivity decreases from 36.8 × 10⁻⁴ to 24.7 × 10⁻⁴ K⁻¹. This effect is mainly attributed to the larger specific surface area of the smaller spheres where a relatively large number of Er(III) ions are located at the surface. This results in an increase in the efficiency of the ⁴S_3/2 → ²H_11/2 population process of the Er(III) ions due to stronger electron–phonon interactions with increasing T. Heating of the spheres by NIR light is also supposed to cause enhanced electron–phonon interactions in such particles.