Heptazine Ring Research Articles

Defective Potassium Poly(Heptazine Imide) Preventing Spin Delocalization and Hole Transfer Deactivation for Efficient Solar Energy Conversion and Storage.

Anti-site defective potassium poly(heptazine imide) (KPHI) with the central nitrogen atoms partially replaced by graphitic carbon atoms in the flawed heptazine rings is prepared by direct ionothermal treatment of the rationally designed supramolecular complex in KSCN salt molten. Compared to the KPHIs without the anti-site defect, the anti-site defective KPHI demonstrates significantly improved photocatalytic and dark photocatalytic performances for H2 evolution reaction (HER). In the presence of the hole scavenger, the anti-site defective KPHI exhibits superior photocatalytic stability for HER lasting 20h, whereas the deactivation is observed from the ordinary KHPIs after 3h HER. Moreover, the H2 yield in the dark by the stored photoelectrons in the anti-site defective KPHI increases by more than an order of magnitude. Density functional theory calculations reveal that the anti-site defective unit in KPHI not only prevents spin delocalization but also inhibits the deactivation of hole transfer, which are beneficial to photoelectron storage and photocatalytic activity. The findings in this study provide insight into the photophysical and catalytic properties of KPHI, which conclude a strategy to improve the performances for solar energy conversion and storage by incorporating intrinsic anti-site defects in KPHI.

Realigning the melon chains in carbon nitride by rubidium ions to promote photo-reductive activities for hydrogen evolution and environmental remediation

The photocatalytic efficiency of polymeric carbon nitride (PCN) suffers from unsatisfactory charge separation because of its amorphous structure. Herein, we report a simple bottom-up method to synthesize a novel structure of rubidium ion inserted PCN (Rb-PCN), which involves the regular alignment of melon chains to endow a crystalline feature in PCN. The insertion of Rb+ decreased not only the N p electrons in the heptazine ring but also the plane angle of the heptazine motifs in the melon chain, which promoted the long-range periodicity and crystallinity of carbon nitride. This structurally rearranged crystalline Rb-PCN demonstrated considerably enhanced separation of charge carriers, resulting in six-fold higher photocatalytic hydrogen evolution activity than its amorphous counterpart. Furthermore, the photoexcited electrons can be efficiently trapped by O2 to generate H2O2, which facilitates the production of reactive oxygen species to inactivate bacteria and degrade organic pollutants, showing great potential for use in both energy and environmental applications.

Enhanced electron density of the π-conjugated structure and in-plane charge transport to boost photocatalytic H2 evolution of g-C3N4

Graphite carbon nitride (g-C3N4) has become an emerging metal-free photocatalyst and attracted the attention of researchers. However, pure g-C3N4 (PCN) prepared by traditional thermal condensation polymerization has a low polymerization of heptazine ring chains and a large number of charge recombination centers, which hinder in-plane charge transport, leading to undesirable photocatalytic performance. Herein, PCN fabrication by thermal treatment with ammonium oxalate monohydrate was performed to produce highly dispersed porous g-C3N4 nanosheets, and the integrity of their in-plane heptazine ring chains was successfully improved without introducing heteroatoms. Electron paramagnetic resonance and 13C nuclear magnetic resonance analyses revealed significantly increased electron density and delocalization of the π-conjugated structure. The obtained samples exhibited acceptable stability and photocatalytic activity, and the hydrogen reduction rates without and with the Pt co-catalyst were 11.2 and 5.3 times that of PCN.

Carbonyl heterocycle modified mesoporous carbon nitride in photocatalytic peroxydisulfate activation for enhanced ciprofloxacin removal: Performance and mechanism

Polymer carbon nitride is considered to be a promising photocatalyst with broad application prospects in water treatment. However, the defects of pristine polymer carbon nitride (PCN), such as small specific surface area, fast photogenerated electron-hole recombination, and low mass transfer efficiency, limit its photocatalytic activity. In this work, by introducing 2-thiouracil into the precursor, a carbonyl heterocycle-containing mesoporous carbon nitride photocatalyst (TCN) was successfully obtained with significantly enhanced peroxydisulfate (PDS) photocatalytic activity. In this study, the modulation mechanism of carbonyl heterocycle introduction on surface electronic structure and the band structure were fully discussed by means of a combination of experiments and theoretical calculations. The carbonyl and vicinal carbon-modified heterocycles dominated the electrons, while the adjacent heptazine ring dominated the holes. The photogenerated electron-hole pair recombination efficiency and the electron transition energy barrier were greatly reduced. According to the findings of density functional theory (DFT) calculations, the introduction of carbonyl and vicinal C modulated the electronic structure of catalyst, enhanced the adsorption of PDS at the carbonyl ortho N site, which promoted the electronic interaction between TCN and PDS molecules. Experiments showed that the free radical pathway and non-radical pathway coexisted in TCN/PDS/Vis system. The reactive oxygen species were mainly derived from PDS molecules. DFT calculations provided a more comprehensive theoretical basis for the experimental results. This study provided a fresh perspective on the rational design of carbon nitride-based catalysts and the reaction mechanism of persulfate advanced oxidation systems.

The synergistic effect of potassium ions and nitrogen defects on carbon nitride for enhanced photocatalytic hydrogen evolution

Ion doping is an effective method to improve the photocatalytic activity of graphitic carbon nitride (g-C3N4) by providing a photocarriers transfer channel. But limited by the bonds in heptazine rings, photoelectrons are still trapped in the structure. Therefore, both potassium ions and nitrogen defects were successfully introduced into g-C3N4 by high temperature calcination to accelerate the charges transfer between both interlayers and intralayer of g-C3N4. The results showed that the hydrogen production rate of g-C3N4 modified simultaneously by nitrogen defects and potassium ions reaches 1722.4 μmol·g−1·h−1, which is 8 times that of pristine g-C3N4. Based on various characterization techniques and DFT calculations, we attributed the enhanced photocatalytic hydrogen evolution to the improved light adsorption, more delocalized HOMO-LUMO, and stronger interlayer interactions. This work will provide a promising way to enhance photocatalytic hydrogen evolution of g-C3N4 and a possible mechanism was proposed.

Donor-acceptor engineered g-C3N4 enabling peroxymonosulfate photocatalytic conversion to 1O2 with nearly 100% selectivity

Singlet oxygen (1O2) is a thrilling active species for selectively oxidating organic substances. However, the efficient and selective generation of 1O2 maintains a great challenge. Here, we develop a donor-acceptor structured g-C3N4 by covalently engineering benzenetricarboxaldehyde (BTA) onto the fringe of g-C3N4. The g-C3N4-BTA exerts high-efficiency 1O2 generation with nearly 100% selectivity via peroxymonosulfate (PMS) photocatalytic activation upon visible light illumination, exhibiting obviously boosted efficiency for selective elimination of atrazine (ATZ). The consequences of experiments and theoretical calculations demonstrate that BTA units serve as electron-withdrawing sites to trap photogenerated electrons and facilitate the adsorption of PMS on the electron-deficient heptazine rings of g-C3N4. As such, PMS can be in-situ oxidated by the photogenerated holes to selectively produce 1O2. Besides, the g-C3N4-BTA/PMS system delivers high stability and strong resistance to the coexisting organic ions and natural organic matter, demonstrating great potential for selectively removing targeted organic contaminants with high efficiency.

Fabrication of dual Z-scheme g-C3N4/Fe2TiO5/Fe2O3 ternary nanocomposite using natural ilmenite for efficient photocatalysis and photosterilization under visible light

The advanced oxidation process is a prominent method available to remove dyes released to normal water reservoirs to alleviate water scarcity. We report the fabrication of novel g-C3N4/Fe2TiO5/Fe2O3 using natural ilmenite sand as the precursor of the metallic semi-conductors exploration of a heterostructure for photodegradation of methylene blue under sunlight. Ternary composites were synthesized by varying g-C3N4 with respect to Fe2TiO5/Fe2O3 and varying Fe2TiO5/Fe2O3 with respect to g-C3N4 where the varying component was varied as 8, 24 and 40%, respect to the constant material. The hybridization of the three semi-conductors has been confirmed by the microscopic, chemical, and structural analyses. X-ray diffraction patterns show the presence of all three g-C3N4, Fe2TiO5 and α-Fe2O3 while the transmission electronic microscopic and scanning electronic microscopic images show the heterogeneous distribution of the metal oxide nanoparticles on g-C3N4 matrix forming the composite. HRTEM images further reveal the junction of Fe2TiO5 and α-Fe2O3. X-ray photoelectron spectra show the existence of s-triazine and heptazine rings in the composites with Fe3+ and Ti4+ as the only oxidation states of Fe and Ti. Fe2TiO5/Fe2O3/40% g-C3N4 with bandgap of 2.63 eV calculated by diffuse reflectance UV-Visible spectroscopy showed the highest photocatalytic activity (0.009 min−1) being 1.3 times greater than the Fe2TiO5/Fe2O3 nanoparticles. Enhanced photocatalytic activity over the fabricated composites was observed due to the increased visible light absorption, efficient charge separation and improved charge transportation. g-C3N4 coupled with 40% Fe2TiO5/Fe2O3 showed the highest antibacterial activity against gram-negative E.Coli. The synthesis of dual Z-scheme g-C3N4/Fe2TiO5/Fe2O3 ternary composite provides new sights in developing novel photocatalysts using natural ilmenite sand for environmental applications.

Carbon defective g-C3N4 thin-wall tubes for drastic improvement of photocatalytic H2 production

Both morphology and defects play a vital role for boosting photocatalytic efficiency. Herein, a vapor-assisted surface reconstruction was developed to synthesize g-C3N4 thin-wall tubes with carbon defects. The significantly reduced wall thickness delivers rich disclosed active sites, ensures efficient the mass transport as well as suppresses interior recombination of photoexcited carriers. The intentionally introduced carbon defects in the heptazine rings owing to the altered surface charge state, optimizes the band structure and suppresses exterior recombination of photoexcited carriers. The resultant g-C3N4 with vigoroso reduction potential possess a tremendous improvement (25-fold) for H2 production under visible light, which outperforms the majority of the previously reported g-C3N4 tubes. With pressing research demands, the functionality of vapor-assisted surface reconstruction was deeply revealed by theoretical and experimental evidences. This clean strategy enriches the rational design of high-performance g-C3N4 photocatalysts.

Insight into the effect of B-doping on electronic structure and photocatalytic H2 evolution performance of C3N5 nanosheets

Graphitized carbon nitride (g-C3N4) is a star polymer for photochemical H2 production, but its broad bandgap and severe charge recombination usually limit its practical application. Herein, a novel B-doped N-rich carbon nitride (C3N5) material (BCN) is prepared through a one-pot thermal polymerization method, whereby C3N5 contains heptazine rings connected by two N-rich triazole terminal units with different structures, and the incorporated B-atoms replace the C-sites of heptazine rings. These structural features make C3N5 and BCN have larger conjugated structure, smaller electronic bandgap and more effective charge separation than g-C3N4. Moreover, B-doping can further significantly promote the photogenerated charge carrier separation and modulate the electronic energy bands of C3N5, and 1.3 wt% BCN delivers an apparent quantum yield of 12.5 % at 420 nm. The present results provide a promising strategy to improve the solar H2 production performance of carbon nitride by altering component and incorporating heteroatoms.

Engineering doping and defect in graphitic carbon nitride by one-pot method for enhanced photocatalytic hydrogen evolution

Hydrogen energy, as one clean energy, is one of the important ways to decarbonize in the future. The advancement of doping and constructing defect engineering in graphitic carbon nitride (g-C3N4) show promise for artificial photosynthesis for H2 evolution using solar energy. Unfortunately, it is still difficult to generate g-C3N4 that has been co-modified by doping and defect and investigate their synergistic effects. In this study, potassium thioacetate was used first-time as a potassium source and dehydrogenation agent to prepare g-C3N4 decorated by K+ ions and cyano groups with enhanced H2 evolution by a one-step method in an air atmosphere. The presence of K+ ions can increase the density of electron clouds in the delocalized π bond of the heptazine ring. The cyano group may successfully delocalize the solitary valence electrons in conjugated heterocycles since it is a potent electron-withdrawing group. These outcomes allow the modified catalysts to achieve improved H2 evolution activity, reduced interfacial transfer resistance, and increased light absorption and photocurrent. Finally, K(0.05)-CN's photocatalytic hydrogen evolution rate (HER) rose to 1319 μmol h−1 g−1, which is five-fold better than that of CN, and the normalized HER reached 118 μmol h−1 m−2, which was much higher than other reports. This research not only offers a fresh approach to creating photocatalysts with good H2 evolution efficiency but also advances a deep understanding of how structural defects and alkali metal doping affect photocatalytic activity.

H2+CO2 Synergistic Plasma Positioning Carboxyl Defects in g-C3N4 with Engineered Electronic Structure and Active Sites for Efficient Photocatalytic H2 Evolution.

Defective functional-group-endowed polymer semiconductors, which have unique photoelectric properties and rapid carrier separation properties, are an emerging type of high-performance photocatalyst for various energy and environmental applications. However, traditional oxidation etching chemical methods struggle to introduce defects or produce special functional group structures gently and controllably, which limits the implementation and application of the defective functional group modification strategy. Here, with the surface carboxyl modification of graphitic carbon nitride (g-C3N4) photocatalyst as an example, we show for the first time the feasibility and precise modification potential of the non-thermal plasma method. In this method, the microwave plasma technique is employed to generate highly active plasma in a combined H2+CO2 gas environment. The plasma treatment allows for scalable production of high-quality defective carboxyl group-endowed g-C3N4 nanosheets with mesopores. The rapid H2+CO2 plasma immersion treatment can precisely tune the electronic and band structures of g-C3N4 nanosheets within 10 min. This conjoint approach also promotes charge-carrier separation and accelerates the photocatalyst-catalyzed H2 evolution rate from 1.68 mmol h−1g−1 (raw g-C3N4) to 8.53 mmol h−1g−1 (H2+CO2-pCN) under Xenon lamp irradiation. The apparent quantum yield (AQY) of the H2+CO2-pCN with the presence of 5 wt.% Pt cocatalyst is 4.14% at 450 nm. Combined with density functional theory calculations, we illustrate that the synergistic N vacancy generation and carboxyl species grafting modifies raw g-C3N4 materials by introducing ideal defective carboxyl groups into the framework of heptazine ring g-C3N4, leading to significantly optimized electronic structure and active sites for efficient photocatalytic H2 evolution. The 5.08-times enhancement in the photocatalytic H2 evolution over the as-developed catalysts reveal the potential and maneuverability of the non-thermal plasma method in positioning carboxyl defects and mesoporous morphology. This work presents new understanding about the defect engineering mechanism in g-C3N4 semiconductors, and thus paves the way for rational design of effective polymeric photocatalysts through advanced defective functional group engineering techniques evolving CO2 as the industrial carrier gas.

Bimetal single atom on defect-tailoring carbon nitride that boosts photocatalytic hydrogen evolution and superfast contaminant degradation

Bimetal single atom functionalized porous carbon nitride (K1Na1/pCN) is designed by one-step method of forming defective carbon nitride and anchoring single atom, which constructs an efficient transfer channel for charge carriers via selective tailoring and linking heptazine rings in frameworks. The existence of single atom over pCN substrate is proved by special aberration corrected transmission electron microscope, X-ray absorption near-edge structures and X-ray photoelectron spectroscopy. The as-prepared K1Na1/pCN presents superior activity towards hydrogen evolution (4,561 μmol/h/gPt, AQY is 21.8% at 420 nm) and ciprofloxacin degradation (apparent kinetic constant of 0.74 min−1), which are almost one-order and two-order of magnitude larger than the traditional carbon nitride, respectively. Results of experimental data and density functional theory reveal that the K1Na1/pCN can narrow bandgap, form midgap state and provide more effective active sites, which have a synergistic effect on inhibiting charge carrier recombination and improving their directional transfer. This paper proposes an alternative strategy to synthesize carbon nitride-supported single atom by a one-step method and extends to support other metal single atoms via facile alternatives of molten salts.

Highly efficient hydrogen evolution from water splitting on heptazine polymer with three types of defects

Three types of defects were successfully introduced into heptazine polymer by calcinating the mixture of melamine and high proportion of KOH. The formation mechanism and effects of defects were studied in detail. In particular, cyano groups and sp2-nitrogen defects were introduced into samples when KOH content was low. With increasing usage of KOH, the heptazine rings tend to open at the position of sp2-nitrogen defects to form sp3-nitrogen defects. Different defects would introduce different defect level within the bandgap, thus adjusting the band structure of heptazine polymer and enhancing its response to visible light.The sp2-nitrogen defects acted as electron donor and introduced donor level at bottom of conduction band, while -C≡N groups and sp3-nitrogen defects acted as electron acceptor and introduced accept level above the valence band. The molecular dipole was increased due to three types of defects coexistence in heptazine polymer, which contributed to enlarge the built-in electric field and promote the separation of photogenerated carriers. As a result, the optimized photocatalyst 3MK-400 exhibits the highest hydrogen evolution rate of 4.37 mmol g-1h−1 under solar driven, which was 13.7 times higher than melem-400.

Band structure tuning of g-C3N4 via sulfur doping for broadband near-infrared ultrafast photonic applications.

Graphitic carbon nitride (g-C3N4) featuring a stable heptazine ring structure and high polymerization degree, was indexed as a high thermochemical stability material, attracting rising research enthusiasm for diverse applications. However, the poor near-infrared (NIR) optical absorption and resulting limited NIR applications were pronounced for g-C3N4 due to its large bandgap of 2.7eV. In the present work, sulfur-doping was manifested by first-principles calculations to introduce impurity level and result in anisotropic spin splitting in g-C3N4 for enhancing broadband nonlinear optical characteristics in NIR regime. The modified sulfur-doped g-C3N4 (S-C3N4) exhibited the maximum effective nonlinear absorption coefficient to be-0.82cm/GW. Pulse duration within hundred nanoseconds was realized with high modulation stability employing S-C3N4 as saturable absorber in Q-switching operations. Moreover, broadband ultrafast photonics properties were successfully demonstrated in constructed ytterbium-doped and erbium-doped fiber lasers, generating highly stable dissipative soliton and traditional soliton mode-locking pulses. The presented S-C3N4 nanomaterial with remarkable nonlinear optical performances might explicitly boost the development and application of g-C3N4 materials in advanced optoelectronic and ultrafast photonic devices.

Platinum-silicon doped graphitic carbon nitride: A first principle calculation

Modifications of electronic and optical properties due to platinum, silicon, and platinum-silicon co-doping in the graphitic carbon nitride (g-C3N4) monolayer have been explored through density functional theory (DFT). The bandgap value of 1.2 eV has been obtained for the pristine g-C3N4 using generalized gradient approximation (GGA) employing Perdew-Burke-Ernzerhof (PBE) functional. The doping process has resulted in a fall in bandgap value. The platinum-silicon co-doping into g-C3N4 provides two new paths, C–N–Pt–N–C, and C–N–Si–N–C for the relocation of electron-hole couples across the adjacent heptazine rings. There is approximately a 25% increment (at 1 eV) in the optical conductivity in the infrared region after co-doping. The co-doped g-C3N4 also shows enriched light absorption. The work function is found to be decreased from 4.29 eV for the pristine g-C3N4 to 3.58 eV for the Pt/Si-co-doped g-C3N4. Hence, platinum-silicon co-doping should be considered as a suitable means to advance the photocatalytic performance of g-C3N4.

Freeze-dried dicyandiamide-derived g-C3N4 as an effective photocatalyst for H2 generation

BackgroundHydrogen (H2) is viewed as a clean, green, and sustainable energy source. The photocatalytic H2 evolution using light-activated photocatalyst under light irradiation is attractive to convert light into chemical energy in a feasible way. In the present work, photocatalysis of H2 generation was explored, using graphitic carbon nitride (g-C3N4) synthesized from freeze-dried dicyandiamide (DICY). MethodsThe DICY underwent rapid dissolution-recrystallization during the freeze-drying process, enabling more complete polymerization of the heptazine units of g-C3N4 than occurred in the same compound synthesized from untreated DICY. After loading with Pt as a cocatalyst, and under light irradiation (metal halide lamp) in the presence of triethanolamine, g-C3N4 synthesized from freeze-dried DICY (DCN) showed an increased H2 evolution rate (∼20 μmol h‒1) compared with that using g-C3N4 derived from untreated DICY (CN) (12 μmol h‒1). In addition, DCN had a higher H2 generation rate than CN under light of different wavelengths (400, 450, and 550 nm). Significant FindingsThe improved activity of DCN could be attributed to inhibition of charge recombination (evidenced by photoluminescence), fast charge transfer (evidenced by electrochemical impedance spectra and photocurrent measurements), and a suitable energy bandgap (evidenced by Mott-Schottky and UV–vis measurements) resulting from better-polymerized heptazine rings (evidenced by nuclear magnetic resonance spectroscopy). In summary, DCN prepared in this study could be used as a visible-light activated and effective metal-free material for photocatalytic H2 generation.

Modulating Local Charge Distribution of Carbon Nitride for Promoting Exciton Dissociation and Charge-Induced Reactions.

The sustainable light can generate reduction and oxidation centers in situ through the generation of photoexcited electrons and holes in the presence of photocatalyst. However, the photoexcited electrons and holes have huge Coulombic attraction and high exciton binding energy due to the weak screening effect and dielectric properties in many low-dimensional conjugated polymers, such as carbon nitride. Reducing the exciton binding energy of carbon nitride and promoting the conversion of excitons into free charge carriers are necessary for improving the activity of photocatalytic reactions but still very challenging. Here, by introducing amino-cyano functional groups into carbon nitride, it is demonstrated that excitons can be effectively dissociated into electrons and holes by finely controlling the charge distribution of heptazine ring. It is found that carbon nitride with heptazine rings of positive charge distribution can greatly reduce the exciton binding energy to 24 from 71 meV. Compared with heptazine ring having negative charge distribution, heptazine ring with positive charge distribution can increase photocatalytic hydrogen production of carbon nitride by up to ten times. This work provides an easy way to promote the dissociation of excitons in carbon nitride by regulating the charge distribution.

Insights into the photocatalytic peroxymonosulfate activation over defective boron-doped carbon nitride for efficient pollutants degradation

The metal-free graphitic carbon nitride is a promising photocatalyst for peroxymonosulfate (PMS) activation towards water decontamination, but bearing low efficiency due to its electronic structure and surface chemistry. Herein, the non-metallic element boron was adopted for catalyst development. The boron dopants and defects were simultaneously introduced by potassium borohydride, resulting in an excellent activity towards PMS activation. The dominant reactive oxygen species was singlet oxygen, which was determined to originate from PMS activation over photo-induced holes initiated by an electron transfer process. Calculations based on density functional theory revealed that at excited states, due to the dopants and defects, the electron-hole distribution was altered from an even population to a significant separation, which was beneficial for photocatalytic performance. Besides, the engineered electronic structure weakened the catalyst resistance to charge transfer, enabling easier electron transfer between the catalyst and the PMS. Moreover, the strengthened and enlarged positive electrostatic potential areas on heptazine rings oriented the electron transfer process from the negatively charged PMS to the catalyst, facilitating the generation of singlet oxygen. These findings provide underlying mechanism insights into the contribution of dopants and defects to catalytic performance on persulfate-based photocatalytic water treatment.

Metal-ion-assisted construction of cyano group defects in g-C3N4 to simultaneously degrade wastewater and produce hydrogen

Converting inexhaustible solar energy into chemical energy for environmental contaminant control and fossil fuel substitution has been considered a sustainable and promising strategy. In this paper, an easy-to-operate salt-assisted method is used to synthesize a bifunctional graphitic carbon nitride (BaCN-C3N4) co-decorated with Ba2+ doping and cyano defects. This novel BaCN-C3N4 is applied to produce hydrogen from organic wastewater efficiently and stably. Experimental characterization and DFT calculation suggest that Ba2+ is trapped in the carbon nitride intra-layer ring cavity through ion–dipole interaction, meanwhile the presence of Ba2+ promotes the production of cyano defects by partial heptazine ring opening. The mechanism of the excellent photocatalytic activity of BaCN-C3N4 is that the synergistic action of Ba2+ and cyano group promotes stronger visible light absorption, narrower band gap and higher photogenerated electron-hole separation. As a consequence, BaCN-C3N4 achieves an outstanding hydrogen production rate (10316 μmol/g/h) from wastewater, which is nearly 5.5 times that of the pristine g-C3N4. This investigation cleverly used the salt-assisted method to construct cation and cyano defects co-modified carbon nitride, achieving the goal of coinstantaneous pollutant degradation and energy development, and providing a new strategy for semiconductor photocatalysts to convert solar energy.

First-principles study of S-doped point defects with different charge states in monolayer g-C3N4

Non-metal-doped graphitic carbonitride (g-C3N4) can improve the redox capacity and photocatalytic activity, as a photocatalyst for the degradation of organic pollutants. In the present study, the theoretical defect models of S-doping with different doping sites and valence states were built in g-C3N4. The stability of charged defects was systematically investigated by the first-principles calculations based on the density functional theory. The effects of S-doping on the electronic structures and optical properties of g-C3N4 were revealed. According to the results, the g-C3N4 doped with positive divalent S is the most favorable defect form in energy. A new electron transfer pathway between adjacent heptazine rings on S-doped g-C3N4 was proposed. It is found that the special electronic states and the enhanced local charge density of S-doped g-C3N4 could effectively optimize the electron transport dynamic, leading to the increased reduction ability of photo-generated holes. As a result, the S-doped g-C3N4 shows better ability to separate photo-generated electron/hole pairs and improve photocatalytic efficiency with respect to the pristine g-C3N4.