## 3 Results and Discussion
### Characterization of H\({}_{2}\) Production from Water Splitting  
The photocatalytic efficiency is measured by the rate of H\({}_{2}\) generation from water splitting under visible-light illumination (Figures 5a and S4a,b). When the water content was controlled, the highest photocatalytic activity was exhibited by 10H\({}_{2}\)O-CN, for which the total H\({}_{2}\) production was 7.3 \(\mu\)mol following 6 h of reaction. When the ratio of melamine and 2,5-hiophenedicarboxylic acid was altered, the photocatalytic activity of these five samples varied slightly, which implies that the water content in the precursor is more important. This result is consistent with the XRD analysis. It is evident that H\({}_{2}\) generation dramatically increases following further heating to become 14 times higher for FCN than for CN. Figure 4: (**a**,**b**) SEM images of FCN. (**c**,**d**) TEM images of FCN. Figure 3: (**a**–**d**) SEM images for xH\({}_{2}\)O–CN. The AQYs of FCN are 2.45%, 2.16%, 0.95%, and 0.17% at 420 nm, 450 nm, 470 nm, and 550 nm, respectively (Figure 5b). The change of the AQY is consistent with the absorption spectrum, indicating that the photoexcitation promotes the H\({}_{2}\) formation. It has higher light excitation effect between 420 nm and 450 nm. It is evident from Figure 5c that FCN remained stable under visible-light illumination is quite stable following 10 cycles. The yield of H\({}_{2}\) steadily increased in the initial three cycles, following which, it oscillated about a mean point. This phenomenon may be due to TEOA being consumed as the reaction proceeds, because excess TEOA molecules participate in the reflection and absorption of incident photons [22]. The effect of excessive TEOA content on H\({}_{2}\) production gradually decreases, and the H\({}_{2}\) evolution rate is maximized in the third cycle. As the TEOA content continued decreasing till the 10 th cycle, the shortage of the sacrificial agent caused partial electron-hole recombination, leading to reduced H\({}_{2}\) production. Similarly, we can see from the CN cycle (Figure S5) that the sample showed a small increase in the second cycle, and then the hydrogen production gradually stabilized. Photoluminescence spectra were used to verify the recombination rate of photoexcited electrons and holes [26; 27]. As shown in Figure S4c, the PL spectra of the 0H\({}_{2}\)O-CN and 50H\({}_{2}\)O-CN exhibited weaker signals, although all the samples exhibited emissions centered approximately at 520 nm. The PL intensity of 10H\({}_{2}\)O-CN was smaller than 20H\({}_{2}\)O-CN, indicating a lower rate of recombination and a longer lifetime of charge carriers, which improves the photocatalytic activity. The order of PL intensity was (Figures 5d and S4d): FCN < 1:4 CN < 1:3 CN < 1:1 CN; therefore, FCN exhibits the lowest rate of recombination of photoexcited electrons and holes. The mechanism of transfer of the photogenerated charge carriers was studied by electrochemical testing. The photocurrent responses of CN and FCN are shown in Figure 5e. The composite efficiency of photogenerated electrons can be obtained by analyzing the transient photocurrent response [23]. Evidently, further heating enhances the photocurrent generation. FCN delivered a six times higher photocurrent response than that of CN. This result reveals that the separation efficiency of photoelectrons and holes for FCN is higher than that for the CN sample, which explains the higher photocatalytic activity of FCN. Figures 5f and S6 show the UV-vis DRS of the obtained samples. Except two samples (0H\({}_{2}\)O-CN and 50H\({}_{2}\)O-CN) with nearly zero absorbance, the spectrum of 10H\({}_{2}\)O-CN was redshifted compared to that of 20H\({}_{2}\)O-CN, and the absorbance peaks of 10H\({}_{2}\)O-CN and 20H\({}_{2}\)O-CN all lay between 400 and 800 nm. These results indicate that appropriately controlling the water content in precursor increases the optical absorptivity in the visible region.