The surface composition of Cu@f-gC3N4 was confirmed by X-ray photoelectron spectroscopy (XPS). Figure (see in ESI) exhibits the XPS peaks attributed to the g-C3N4 phase: (i) at 288.1 eV (sp 2 -bonded C atoms in N-C=N) in the C 1s region, as well as (ii) at 398.6 eV (sp 2 -hybridized N atoms in C-N=C), 400.1 eV (bridging N atoms in N-(C)3) and 401.2 eV (N atoms in amino groups) in the N 1s region . Additional confirmation of the presence of a properly developed g-C3N4 phase is the determined atomic ratio N/C = 1.4, which is very close to the expected one. The XPS Cu 2p spectrum shows the lack of distinct peaks, indicating a low content and high dispersion of Cu atoms on the surface. The presence of isolated Cu single-atom active sites was further confirmed by analyzing the Cu@f-gC3N4 through XANES and EXAFS experiments, which show a clear fingerprint of the valence state and neighboring environment of the Cu atoms in the support matrix, respectively. Figure compares the Cu K-edge XANES spectra of the photocatalyst with the reference spectra of Cu foil, CuO, CuO2, and Cu(II)Pc, the near-edge feature of Cu@f-gC3N4 sample was in between of those of Cu2O and CuO, indicating that the Cu species were partially positively charged (Cu δ+ , 1< δ< 2) . Fourier-transformed k-weighted extended X-ray absorption fine structure (EXAFS) in R 20 space was performed to elucidate the coordination environments of Cu atoms anchored on modified g-C3N4 (Figure ). For Cu@f-gC3N4, the major distinct scattering was observed at around 1.5 Å which corresponds to the first coordination shell of Cu-N/O and the minor peak at around 2.52 Å (not phase corrected), which seems to be at the same distance for C32H16CuN8, which corresponds to the second coordination shell of Cu-C. The main peak at 1.50 Å and the higher shell peak at 2.52 Å coincide well with the Cu-N and Cu-C peaks for Cu(II)Pc (C32H16CuN8), respectively . Additionally, the peak at around 2.2 Å that is ascribed to Cu-Cu scattering cannot be observed in Cu@f-gC3N4. This evidence Cu exists as an isolated atom that is atomically dispersed on a modified g-C3N4 matrix through Cu-N-C bonding . These findings are in perfect agreement with the HAADF-STEM results described above. Figures show the comparison between the FT of the experimental spectra (empty circles) and the bestfit simulations (colored lines) for the synthesized material. The coordination numbers, bond lengths, Debye-Waller factors (σ 2 ), and the energy shift parameter (ΔE0) were refined. The bestfit parameters are summarized in Table in ESI. The first shell best-fit results show that each Cu atom is coordinated with 2 N/O atoms on average. Further evidence about the existence of Cu single atom sites as well as their coordination to N atoms was obtained from EPR spectroscopy. The EPR spectrum of the Cu@f-gC3N4 catalyst (Figure , ESI) exhibited an axial signal at g^= 2.06 and gǁ = 2.173 with well-resolved hyperfine structure (HFS) Aǁ = 198 G due to the coupling of the single electron spin of Cu II (d 9 , S = 1/2) with the nuclear spin of Cu (I = 3/2), indicating the presence of isolated Cu II ions. Moreover, the perpendicular component of the g tensor showed a 9-line super-hyperfine structure (SHFS) as evident from the corresponding 1 st derivative of the EPR spectrum (Figure ). This splitting is on the order of 2nI+1 (n = 4; I = 1) and with a coupling constant of 16 G, indicating a coordination of four nitrogen atoms to Cu sites ( 14 N, I = 1) .