Plasmonic ternary hybrid photocatalyst based on polymeric g-C3N4 towards visible light hydrogen generation

Surface plasmon resonance (SPR) effect of noble metal nanoparticles (NPs) for photocatalysis has a significant enhancement. In this system, a plasmonic ternary hybrid photocatalyst of Ag/AgBr/g-C3N4 was synthetized and used in water splitting to generation H2 under visible light irradiation. 18%Ag/AgBr/g-C3N4 showed the highest photoactivity, with the efficiency of hydrogen generation as high as 27-fold to that of pristine g-C3N4. Compared to simple mixture of Ag/AgBr and g-C3N4, hetero-composite Ag/AgBr/g-C3N4 showed a higher photoactivity, even though they contained same content of Ag/AgBr. We find that significant factors for enhancing properties were the synergistic effect between Ag/AgBr and g-C3N4, and the light absorption enhancing by SPR effect of Ag NPs. Ag/AgBr NPs firmly anchored on the surface of g-C3N4 and their high dispersion were also responsible for the improved activity and long-term recycling ability. The structure of Ag/AgBr/g-C3N4 hybrid materials and their enhancement to photocatalytic activity were discussed. Meanwhile, the possible reaction mechanism of this system was proposed.


3.
Fourier-transform infrared (FT-IR) spectra Figure S3 depicted the FT-IR spectra of the bare g-C3N4 and Ag/AgBr/g-C3N4 composites with different contents. In the case of pure g-C3N4, the strong bands which corresponding to the feature-distinctive stretch modes of aromatic CN heterocycles in the 1200-1650 cm −1 region were found in the spectrum. The absorption bands near at 1562 and 1639 cm −1 were attributed to C=N stretching, while the other four bands at 1240, 1319, 1410 and 1456 cm −1 were attributed to aromatic C-N stretching. Additionally, the characteristic breathing mode of triazine units at 808 cm −1 was observed 8,9 . A broad band near 3100 cm −1 corresponding to the stretching modes of terminal NH2 or NH groups at the defect sites of the aromatic ring were also been observed. It could also be clearly seen that the main characteristic peaks of g-C3N4 appeared in all Ag/AgBr/g-C3N4 photocatalysts 8,10 .
In the case of the Ag/AgBr/g-C3N4 composites, the characteristic peaks of g-C3N4 did not move after the introduction of Ag/AgBr nanoparticles. It was clear that the modification with Ag/AgBr did not alter the FT-IR spectrum of the g-C3N4, which indicated that there were no covalent bondz formed between Ag/AgBr and g-C3N4. The Ag/AgBr nanoparticles were deposited and well dispersed on the g-C3N4 surface, and the EDS, SEM and TEM characterization would be analyzed in detail. Figure S3 FT-IR spectra of (a) g-C3N4, (b) 5%Ag/AgBr/g-C3N4, (c) 10%Ag/AgBr/g-C3N4, (d) 15%Ag/AgBr/g-C3N4, (e) 18%Ag/AgBr/g-C3N4, and (f) 21%Ag/AgBr/g-C3N4

X-ray photoelectron spectroscopy (XPS) Analysis
In order to further ascertain in-depth, the information about the functional group and surface electronic state of pure g-C3N4 and 18%Ag/AgBr/g-C3N4 composite, the X-ray photoelectron spectroscopy (XPS) spectra were detected and shown in Figure S4. The survey spectrum in Figure   S3Aa indicated that the main elements on the surface of the g-C3N4 sample were C, N and O. And it clearly showed that 18%Ag/AgBr/g-C3N4 composite ( Figure S4Ab) consisted of C, N, Ag, Br and O elements. The appearance of O was due to the adsorption of O2 onto the surface of the samples. The corresponding high-resolution XPS spectra were shown in Figure S4B-E. The XPS peaks of the C 1s were at 284.8 and 288.2 eV, as shown in Figure S3B, mainly derived from the g-C3N4 and the adventitious hydrocarbon of the XPS instrument. The peak at 398.6 eV for the pure g-C3N4 was identified as nitrogen atoms in C-N-C groups 1 . And the N 1s in the 18%Ag/AgBr/g-C3N4 had a little shift from 398.6 to 398.7 eV ( Figure S4C), which indicated that the N chemical environment in the Ag/AgBr/g-C3N4 had changed. It also suggested the existence of the interaction between Ag/AgBr and g-C3N4, and the Ag/AgBr was combined with the N site in the g-C3N4. Figure S3D showed the high-resolution XPS spectra of Ag 3d spectrum, the two peaks located at 367.6 and 373.6 eV were attributed to Ag 3d5/2 and Ag 3d3/2, respectively, suggesting the presence of Ag + species for AgBr.
The Ag 3d5/2 peak was further divided into two different peaks at 367.6 and 368.6 eV, and the Ag 3d3/2 peak was divided into two different peaks at 373.6 and 374.0 eV [2][3][4] . The peaks at 368.6 and 374.0 eV were attributed to metal Ag and the peaks at 367.6 and 373.6 eV were attributed to Ag + of AgBr. The spectrum of Br 3d in Figure S3E showed that the binding energies of Br 3d5/2 and Br 3d3/2 appeared at 68.3 and 69.1 eV, respectively [5][6][7] . The XPS results of Ag 3d and N 1s confirmed the existence of metal Ag (agree with the XRD result, as shown in Figure 4) and the interaction between Ag/AgBr and

Analysis of Hydrogen Generation with Gas Chromatographic
The amount of hydrogen generation was determined by gas chromatography. Gas chromatographic conditions: the column type was 5A (15m*3 mm*3 mm, capillary column); the inlet temperature was 120 ℃; the column temperature was 60 ℃; the TCD temperature was 120 ℃；the carrier gas was Ar with a flow rate of 1 mL min -1 ; the injection mode was splitless injection; the amount of injection was 1 mL; the current was 60 mA. We injected the same amount of standard gas containing different H2 to get the following standard curve: y=165546x, R 2 =0.9998 (as shown in Figure S6). We used the standard curve to calculate the hydrogen generation based on the measured peak area.