*3.1. Modification of g-C3N4*

Improving the photocatalytic activity of g-C3N4 by introducing various nanostructures such as nanoparticles, nanosheets, nanorods, and nanowires has recently been studied [52–56]. Surface modification of the catalytic structure and morphology has the potential to promote charge separation and narrow the band gap due to increased surface area and efficient charge-carrier separation [57,58].

In 2016, Han et al. [59] reported an atomically thin mesoporous nanomesh of g-C3N4 for hydrogen evolution by highly efficient photocatalysts (Figure 4a) fabricated via the solvothermal exfoliation of mesoporous g-C3N4 prepared by the thermal polymerization of freeze-dried nanostructured precursors. The delamination of the layer material to provide the two-dimensional single-atom sheet has led to unique physical properties such as a large surface area, a very high unique carrier mobility, and a significant change in the energy band structure [60]. The mesoporous g-C3N4 nanomesh shows inherent structural advantages, electron transfer capability, and efficient light harvesting. Figure 4b shows the electronic band structure of the monolayer mesoporous g-C3N4 nanomesh and bulk counterparts. The band gap is 2.75 eV for the monolayer mesoporous g-C3N4 nanomesh and 2.59 eV for the bulk counterpart, as determined from optical absorption spectra. The VB of the monolayer mesoporous g-C3N4 nanomesh (2.41 eV) identified via X-ray photoelectron spectroscopy is also 0.35 eV higher than the bulk counterparts (2.06 eV). The CB is upshifted by 0.51 eV when considering the 0.16 eV increase in the VB and a negative shift of 0.35 eV. The monolayer mesoporous g-C3N4 nanomesh exhibits significantly improved the light-harvesting ability mainly due to the multiple scattering effect and the presence of defect sites associated with the mesoporous surface. A 30 h reaction was performed with intermittent evacuation every 5 h to confirm the hydrogen production ability of mesoporous g-C3N4

nanomesh under visible-light irradiation (Figure 4c). As a result, the 2.6 mmol H2 gas (59 mL) produced by the atomically thin mesoporous g-C3N4 nanomesh was not visibly deactivated and the H2 gas was generated continuously. Wavelength-dependent H2 evolution shows the optical absorption spectrum of monolayer g-C3N4 nanomesh, indicating that the H2 generation is driven by photoinduced electrons in g-C3N4 (Figure 4d). In conclusion, the mesoporous g-C3N4 nanomesh produces an atomically thin mesoporous layer during the freeze-dried assembly and solvothermal exfoliation. Its good application benefits from structural advantages for light harvesting, electron transport, and accessible reaction sites [61]. This new type of mesoporous g-C3N4 nanomesh could be applied to photocatalytic and various engineering fields.

**Figure 4.** (**a**) Schematic illustration of atomically thin mesoporous g-C3N4 nanomesh photocatalyst and (**b**) a band gap schematic of the monolayer mesoporous g-C3N4 nanomesh and bulk counterparts. (**c**) Hydrogen production rate of the monolayer mesoporous g-C3N4 nanomesh, the bulk counterpart, and the traditional g-C3N4 bulk under visible-light irradiation. (**d**) H2 evolution rate on the monolayer mesoporous g-C3N4 nanomesh with wavelength dependence. Reproduced with permission from [59]; copyright (2016), American Chemical Society.

In 2018, Zhao et al. [62] reported the fabrication of a mesoporous g-C3N4 consisting of hollow nanospheres (MCNHN) via a simple vapor-deposition method that improved hydrogen production under visible-light irradiation. Figure 5a shows the photocatalytic hydrogen evolution by MCNHN under visible-light irradiation. Both MCNHN and bulk g-C3N4 achieved a stable average rate of hydrogen production within 4 h, but the hydrogen evolution of MCNHN was 659.8 μmol g−<sup>1</sup> h−1, which is 22.3 times greater than bulk g-C3N4 (29.6 μmol g−<sup>1</sup> h<sup>−</sup>1). The excellent hydrogen production activity of MCNHN is due to its well-defined structure. The increased surface area provides more active sites in the photocatalytic reaction, thereby allowing more light to be harvested. Moreover, the planarized unit layer and the decreased interlayer space of g-C3N4 crystals facilitate the transfer and separation of photoinduced charge carriers in MCNHN. As a result, photocatalytic hydrogen generation is significantly improved due to the large surface area and decreased interlayer space of g-C3N4. Figure 5b shows the proposed photocatalytic mechanism of H2 evolution for MCNHN based on the aforementioned results and the literature. The active site of MCNHN absorbs visible

light. Electrons in the VB are excited to the CB by absorption of photons, and are then transferred to the Pt nanoparticles loaded on the surface of MCNHN; the corresponding photoexcited holes remain in the VB. The electron-rich Pt nanoparticles become active sites where water can be split into hydrogen. In addition, multiple reflections of visible light in the MCNHN with Pt nanoparticles improves light absorption.

**Figure 5.** (**a**) Time course of H2 evolution and (**b**) a schematic mechanism for photocatalytic H2 evolution on MCNHN. Reproduced with permission from [62]; copyright (2018), Elsevier.
