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Fabricating heterojunction photocatalysts is an important strategy for speeding up the separation rate of photogenerated charge carriers, which is attracting greater interest. However, the choice of three factors, individual materials, band offsets, and effective interfaces, is still important for fabricating efficient heterojunction photocatalysts. Herein, efficient g-C3N4/Zn2GeO4 photocatalysts with effective interfaces were designed by controlling the surface charges of the two individual materials inside the same aqueous dispersion medium, making use of the electrostatic attraction between oppositely charged particles. The g-C3N4/Zn2GeO4 heterojunction with opposite surface charge (OSC) showed higher visible-light photocatalytic activity for degradation of methylene blue than those of pure g-C3N4, pure Zn2GeO4, and the g-C3N4/Zn2GeO4 with identical surface charge (ISC). The investigation of the light absorption spectrum, adsorption ability, and photocurrent responses revealed that the improved separation of photogenerated carriers was the main reason for the enhancement of the OSC g-C3N4/Zn2GeO4 sample's photocatalytic activity. By combining with theoretical calculations, we investigated the microscopic mechanisms of interface interaction and charge transfer between g-C3N4 and Zn2GeO4. The photogenerated electrons in the g-C3N4 N 2p states directly excited into the Zn 4s and Ge 4s hybrid states of Zn2GeO4. The strategy of designing and preparing a g-C3N4/Zn2GeO4 composite catalyst in this work is very useful for fabricating other efficient heterojunction photocatalysts.

Two comparable models of BiOI/BiOCl heterojuctions with different interface structures (crystal surface orientation and crystal surface combination), denoted as BiOI(001)/BiOCl(001) and BiOI(001)/BiOCl(010), have been prepared via integrating heterojuncton nanostructure construction with crystal facet engineering. BiOI(001)/BiOCl(010) had a greater degree of lattice mismatch and displayed higher visible-light photocatalytic activity than BiOI(001)/BiOCl(001). In general, the activity of a photocatalyst (ηPC) has a positive correlation with light harvesting (ηLH), charge separation (ηCS), and charge injection (ηCI). On the basis of the experimental results, we considered that the higher ηCI value of BiOI(001)/BiOCl(010) was the main reason for its better visible-light photocatalytic performance. In combination with theoretical calculations, we found that the higher ηCI value of BiOI(001)/BiOCl(010) was the result of a shorter photogenerated electron diffusion distance, assisted by the self-induced internal electric fields of the BiOCl slabs. This indicated that the crystal facet combination is the key to enhancing the photocatalytic activity of BiOI/BiOCl. Our work offers an archetype for the further design of heterojunction photocatalysts with a fine tuning of the interface structures in order to reach optimized charge injection and enhanced photocatalytic activity.