Sugars induced exfoliation of porous graphitic carbon nitride for efficient hydrogen evolution in photocatalytic water-splitting reaction

Photocatalytic hydrogen evolution holds great promise for addressing critical energy and environmental challenges, making it an important area in scientific research. One of the most popular photocatalysts is graphitic carbon nitride (gCN), which has emerged as a noteworthy candidate for hydrogen generation through water splitting. However, ongoing research aims to enhance its properties for practical applications. Herein, we introduce a green approach for the fabrication of porous few-layered gCN with surface modifications (such as oxygen doping, carbon deposition, nitrogen defects) with promoted performance in the hydrogen evolution reaction. The fabrication process involves a one-step solvothermal treatment of bulk graphitic carbon nitride (bulk-gCN) in the presence of different sugars (glucose, sucrose, and fructose). Interestingly, the conducted time-dependent process revealed that porous gCN exfoliated in the presence of fructose at 180 °C for 6 h (fructose_6h) exhibits a remarkable 13-fold promotion of photocatalytic hydrogen evolution compared to bulk-gCN. The studied materials were extensively characterized by microscopic and spectroscopic techniques, allowing us to propose a reaction mechanism for hydrogen evolution during water-splitting over fructose_6h. Furthermore, the study highlights the potential of employing a facile and environmentally friendly fructose-assisted solvothermal process to improve the efficiency and stability of catalysts based on graphitic carbon nitride.


Duration optimization of fructose-assisted solvothermal modification of bulk graphitic carbon nitride
In the subsequent step, 40 mg of fructose (Sigma-Aldrich) was dissolved in a solution of 60 mL of distilled water and ethanol, mixed in a 1:1 volume ratio.Next, 400 mg of bulk-gCN was added to the prepared solution.The mixture was subjected to vigorous stirring for 0.5 h, followed by 0.5 h of sonication.Afterward, the resulting suspension was transferred into a 100 mL Teflon-lined autoclave, where it was maintained at a temperature of 180 ℃ for different durations (3, 6, 12, 18, or 24 h).After cooling down the suspension was centrifuged and washed three times with distilled water and ethanol, followed by drying at 60 ℃ overnight.The naming convention of the samples involved using "fructose_xh", where "xh" indicates the duration of the reaction in the autoclave.For instance, "fructose_12h" signifies that bulk graphitic carbon nitride (bulk-gCN) was modified with fructose, and the reaction duration was 12 h.The XRD, and TGA results of the sugar-assisted solvothermal modification of graphitic carbon nitride Fig. S2.In detail, after sugar-assisted modification of graphitic carbon nitride, the peak at around 27° shifts towards higher angles, which indicates a concentration of the interlayer distance that has been correlated to the increased interaction induced by the more electronegative O-atoms replacing the C-atoms in the layer (oxygen doping) [S1, S2].The diffractograms of pure sugars (glucose, sucrose, and fructose) are presented in Fig. S3a.

Results and discussion
Glucose exhibits peaks at 10. 49 (110), 11.97 (020), 14.77 (120), 17.21 (200), 18.88 (011)  in the range of 3000-3600 cm -1 are attributed to the O-H stretching [S5].Notably, the spectra of sucrose reveal overlapping absorption peaks primarily in the 3000-3600 cm -1 range, originating from both glucose and fructose.This is due to the fact that sucrose, known as disaccharide, is composed of these two monosaccharides.The XRD, FTIR-ATR, UV-vis with corresponding Tauc plot, PL, CA, and EIS results of the fructose-assisted solvothermal modification of graphitic carbon nitride for optimization of duration are depicted in Fig. S4.The description of graphitic carbon nitride is fully described in the main manuscript, thus in Supplementary Material, the authors described the differences of fructose-assisted modification of graphitic carbon nitride -duration of reaction dependence.
In detail, the shift in peak position at around 27° (Fig. S4a) is influenced by the duration of the solvothermal reaction of fructose-assisted modification.This peak gradually shifts towards higher angles until 12 hours of reaction.Further extension of the solvothermal duration of the reaction (fructose_18h and fructose_24h) results in a shift in the opposite direction, indicating an improved interlayer stacking order [S2].The absorption spectra of graphitic carbon nitride modified with fructose (Fig. S4b) reveal similar absorption peaks as observed in pristine bulk-gCN, indicating that the primary chemical structure of the graphitic carbon nitride structure remains intact, which aligns with XRD results.The energy band gaps (Fig. S4cd) were determined as 2.75, 2.80, 2.89, 2.84, 2.83, and 2.80 eV for bulk-gCN, fructose_3h, fructose_6h, fructose_12h, fructose_18h, and fructose_24h, respectively.Interestingly, the PL spectra (Fig. S4e) show that the duration of solvothermal reaction between 6-24 h has no significant impact on the recombination process.Both CA and EIS (Fig. 4fg) confirm that fructose_6h and fructose_12h have the highest mobility of charge carriers, thus both samples have similarly high photoactivity toward hydrogen production.

Fig.
Fig.S3bdisplays the TGA/DTA results of pure sugars (glucose, sucrose, and fructose).The weight loss for fructose commenced at 130 ℃, while glucose exhibited weight loss at 161 ℃, and sucrose started losing weight at 194 ℃[S3].Furthermore, the melting points of fructose, glucose, and sucrose are 103, 148, and 179 ℃, respectively[S3, S4].To effectively modify graphitic carbon nitride using various sugars, it is advisable to maintain the reaction temperature above the respective melting points of utilized sugars.The FTIR-ATR spectra of pure sugars (glucose, sucrose, and fructose) are depicted in Fig.S3c.All samples display similar absorption patterns, indicating a resemblance in the chemical structures of the green reducing agents used for the exfoliation of graphitic carbon nitride purpose.To elaborate, in the region of 1000 to 1200, the spectra exhibit vibration modes associated with C-C and C-O bonds, typical for carbohydrates[S5].The region from 1350 to 1500 cm -1 shows the combination bands of C-O-C and C-O-H deformations, while absorption bands around 2900 cm -1 correspond to the aliphatic C-H stretching, and the absorption bands

Fig. S5 .
Fig. S5.(a) SEM image, (b, c) TEM images, (d) AFM image, (e) XRD diffractogram of bulk-gCN after solvothermal reaction in the absence of sugars, and (f) hydrogen evolution from water splitting of studied materials.

Table S1 .
AFM data of graphitic carbon nitride modified with different sugars.