Very recently, some impressive breakthroughs in efficiency improvement have been reported from other groups. Liu et al. proposed a reproducible and economical pre-encapsulation technique for stabilizing highly dispersed and highly loaded (1.5 wt%) Cu single atoms (CuSA-TiO
2) on the surface of TiO
2 [
24]. During the photocatalytic HER process, the reversible change of Cu
2+ and Cu
+ greatly facilitated the separation and transfer of photogenerated electrons and holes, enabling CuSA-TiO
2 to achieve higher photocatalytic activity than conventional Pt/TiO
2. The resulting CuSA-TiO
2 showed a high hydrogen evolution rate of 101.7 mmol g
−1 h
−1 and an apparent quantum efficiency of 56% at 365 nm, which exceeded all previously reported TiO
2-based photocatalysts. It is worth mentioning that the sample still has good performance equivalent to that of the freshly prepared sample after storage for 380 days. This work provides an efficient, low-cost, high-stability, and easy-to-prepare TiO
2-based single atom catalyst for solar hydrogen production. Yang et al. found that the lifetime of charge carriers could be extended by introducing a suitable donor-acceptor structure (β-ketene-cyano) into covalent organic framework nanosheets [
25]. By combining this organic nanosheet with a Pt cocatalyst, a record-breaking apparent quantum efficiency of 82.6% at 450 nm was achieved, surpassing all previously reported polymeric semiconductors for photocatalytic HER. This work provides an effective solution to enhance the photocatalytic activity of polymeric semiconductors. Li et al. reported a CdTe/V-In
2S
3 heterojunction photocatalyst, in which CdTe quantum dots were anchored on surface of V-In
2S
3 via an electrostatic self-assembly method and Pt and CoO
x dual-cocatalysts were loaded as the H
2- and O
2-evolving sites (
Fig. 2c) [
23]. Under the synergistic effect of robust interfacial built-in electric field and cascade energy band structure, the charge separation kinetics and multi-exciton generation effect of CdTe-4.2/V-In
2S
3-3 hybrid were effectively promoted and utilized, resulting in an internal quantum efficiency of up to 114% and an apparent quantum yield of 73.25% at 350 nm (
Fig. 2d). Nevertheless, the STH efficiency of this work was only 1.31% under the simulated solar light, which was at the same low level as most reported STH efficiencies of PC water splitting. Jiang et al. constructed reductive high index facet (002) and oxidative low index facet (110) co-exposed CdS by a one-step hydrothermal method [
26]. They found that optimizing the ratio of high and low index facets could tune the
d-band center, and subsequently affect chemisorption and conversion of intermediates (*-OH and *-O) on reduction and oxidation sites. Finally, an improved STH efficiency of 2.20% was achieved. Mi et al. recently reported a new record-setting STH efficiency from PC water splitting [
27], using a highly integrated InGaN/GaN nanowire arrays on commercial silicon wafers through molecular beam epitaxy growth technology. The InGaN/GaN nanowire was decorated by Rh/Cr
2O
3/Co
3O
4 cocatalyst by in situ photodeposition. It was found that the infrared thermal effect generated by high-intensity concentrated solar light could not only promote the forward water splitting reaction but also inhibit the reverse hydrogen-oxygen recombination during the PC overall water splitting (
Fig. 3a-c). This strategy enabled the as-prepared photocatalysts to exhibit an STH efficiency of up to 9.2% under concentrated simulated solar light (
Fig. 3d), which is much higher than that of previously reported unassisted PC water splitting systems and close to the requirement of industrial applications (10% for STH efficiency [
28]).