關(guān)鍵詞：光催化制氫；Cu2O@NC；共價三嗪框架（CTF）；S型異質(zhì)結(jié)；氮摻雜碳

1 Introduction

The global economic expansion has intensified the demandfor energy， exacerbating the depletion of finite naturalresources and heightening environmental concerns due to thereliance on fossil fuels 1–6. Photocatalytic hydrogen productionhas garnered significant attention as a viable method to harnesssolar energy for water splitting. However， currentphotocatalytic systems are hindered by several criticalchallenges， including rapid recombination of photogeneratedelectron-hole pairs， limited light absorption capabilities， andpoor stability under operational conditions 7–10.

One promising approach involves the use of Cu2O， asemiconductor known for its suitable bandgap for theabsorption of visible light. Despite these advantages，Cu2O-based photocatalysts suffer from stability issues andrapid charge recombination. To enhance the photocatalyticperformance of Cu2O， innovative modifications andcombinations with other materials are required. Constructingheterojunctions has been proposed as an effective strategy forachieving the desired outcome. The formation ofheterojunctions at the interface between two distinctsemiconductor materials has the potential to markedly enhancecharge separation and extend the absorption range ofphotocatalysts. Notably， S-scheme heterojunctions havedemonstrated considerable potential for enhancingphotocatalytic efficiencies 11–13. In 2019， Yu et al. first proposedthe innovative mechanism of an S-scheme heterojunction， anovel heterojunction system capable of maintaining a highredox potential 14，15. The transfer of photogenerated electronsfrom oxidised photocatalysts （OP） to reduced OP is facilitatedby S-scheme heterojunctions， which efficiently separateelectron-hole pairs with strong redox capabilities. The energyband alignment and built-in electric field enable the migrationof photogenerated electrons from the conduction band of OP tothe valence band of RP， resulting in the formation ofheterojunctions and the retention of the strong photogeneratedholes and electrons while eutralizing the unwanted electronsand holes 16，17. A substantial body of prior research hassubstantiated the superiority of S-scheme heterojunctions 18–22.

Covalent Triazine Frameworks （CTF） have emerged as anoteworthy new kind of materials for photocatalyticapplications owing to their tunable porosity， large surface area，and customisable electronic properties 23. Nevertheless，standalone CTF frequently encounter constraints， includingrestricted electron mobility and inefficient charge carrierdynamics. The integration of CTF with other materials canserve to mitigate these issues and thereby enhance overallphotocatalytic performance 24，25. Moreover， the utilization oforganic semiconductors， such as CTF， as the oxidation end inS-scheme heterojunctions remains relatively unexplored. Theadvantages of utilizing CTF as the oxidation end include itsexceptional chemical stability， adaptable electronic properties，and potential for visible-light-driven photocatalysis. Theseproperties render CTF an appealing option for enhancing thefunctionality of S-scheme heterojunctions 26，27.

To further enhance the photocatalytic performance of Cu2O，nitrogen-doped carbon materials have been incorporated.Nitrogen doping can introduce additional active sites andenhance the conductivity of carbon materials， facilitatingefficient charge transfer 28–30. The formation of nitrogen-dopedcarbon-coated Cu2O （Cu2O@NC） was achieved throughhigh-temperature annealing of Cu-based MOF. When combinedwith Cu2O， a semiconductor with a suitable bandgap for visiblelight absorption， NC can significantly boost the photocatalyticactivity and stability of the overall system 31. This combinationleverages the broad visible light response of Cu2O and theconductive properties of nitrogen-doped carbon， providing asynergistic effect that enhances overall photocatalyticperformance 32. The integration of CTF with nitrogen-dopedcarbon-coated Cu2O systems， forming the S-schemeheterojunction named CTF-Cu2O@NC， represents a novelapproach to address the limitations of individual componentsand achieve superior photocatalytic performance.

Herein， we probe the enhancement of Covalent TriazineFrameworks （CTF） through modification with NC-coated Cu2Osystems， forming the S-scheme heterojunction CTF-Cu2O@NC（Scheme 1）. By integrating nitrogen-doped carbon coatingswith CTF and Cu2O， we aim to improve photocatalytichydrogen production efficiency and stability. The formation ofthis S-scheme heterojunction enhances charge separation andtransfer， significantly reducing recombination rates.Nitrogen-doped carbon further boosts conductivity andstability， ensuring efficient and sustained photocatalyticactivity. The composite CTF-7% Cu2O@NC system achieves ahydrogen-evolution rate of 15645 μmol·g?1·h?1， which is a5.85-fold and 651.87-fold increase compared to those of theCTF （2673 μmol·g?1·h?1） and Cu2O@NC （24 μmol·g?1·h?1），respectively. These findings underscore the potential of tailoredmodifications in CTF to advance sustainable photocatalytictechnologies for practical applications.

2 Experimental

All chemicals and reagents were of analytical gradematerials and utilized as received without deep purification.The 1，3，5-benzenetricarboxylic acid， 98%）， benzimidazole（98.0%） and cupric nitrate （99.0%） were purchased fromMacklin Chemicals. 1.4-diamidinobenzene （97%） and4，4′-bis（hydroxymethyl）-2，2′-bipyridine （97%） were suppliedby Shanghai Haohong scientific Co.， Ltd. The dimethylsulfoxide （DMSO， 99.9%）， N，N-dimethylformamide （DMF）and tetrahydrofuran （THF， 99.5%） were supplied byGuangzhou Chemical Reagent Factory.

2.1 Chemicals and materials

All chemicals utilized were of analytical grade and utilizedwithout deep purification. The materials included bipyridyl，various oxidizing agents for the stepwise synthesis of CTF， andprecursors for the MOF construction. The sources and purity ofeach chemical are detailed in the Supporting Information.

2.2 Synthesis of CTF

The synthesis of CTF was based on previous studies andimproved by adding 1.0 mmol [2，2′′-bipyridine]-5，5′′-diyldimethanol， 4.4 mmol of cesium carbonate， and 2.0 mmolterephthalamidine dihydrochloride into the DMSO in a roundbottom flask. The mixture was stirred at 100， 120， 140， and180 °C for 6， 6， 12， and 24 h， respectively， according to theprogrammed heating route. After natural cooling to roomtemperature， the mixture was purified with water， anhydrousethanol， and tetrahydrofuran， respectively， and the yellowpowders were freeze-dried for 12 h to obtain the CTF 23.

2.3 Synthesis of Cu2O@NC

To synthesize Cu2O@NC， initially， a precise mixture ofbenzimidazole （0.201 g， 1.7 mmol） and Cu（NO3）2·3H2O （0.024g， 0.1 mmol） was skillfully blended in a solvent composed ofN，N-dimethylformamide （DMF， 9 mL） and ethanol （9 mL），resulting in a uniform solution. Additionally，1，3，5-benzenetricarboxylic acid （0.210 g， 0.1 mmol） wasdissolved in a similar ethanol/DMF mixture to create solution（b）. This solution was then carefully combined with solution（a）， and the mixed solution was continuously agitated at 90 °Cfor 4 h. The obtained product was then centrifuged and dried，undergoing five exhaustive purification procedures withanhydrous ethanol and vacuum drying at 60 °C to achievecomplete desiccation. The dry powder was finally annealed at300 °C for 30 min under an argon gas flow， with a heating rateof 2 °C?min?1， to produce the Cu2O@NC compound 32.

2.4 Synthesis of Cu2O

Dissolve 26.2 mg CuSO4·5H2O in 50 mL water. Add 10 mLof 698 mg NaOH solution with stirring at 60 °C. After 5 min，add 10 mL of 76.5 mg ascorbic acid solution. Stir for 30 min，then for 4 more hours. Isolate the solid by centrifugation， washwith ethanol， and freeze-dry at 35 °C for 12 h to obtain Cu2O.

2.5 Synthesis of CTF-Cu2O@NC

The CTF-Cu2O@NC S-scheme heterojunction wassynthesized by combining CTF and Cu2O@NC in a solventmixture. The components were dispersed using ultrasonicationto ensure a homogeneous mixture， followed by continuousstirring to promote the interaction between CTF andCu2O@NC. The resulting material was collected， washed， anddried for further characterization and photocatalytic testing.

3 Results and discussion

3.1 Compositions and structures of photocatalysts

The chemical structures of CTF， Cu2O@NC， and CTF-7%Cu2O@NC were analyzed using X-ray photoelectronspectroscopy （XPS）. In the C 1s XPS spectrum of CTF-7%Cu2O@NC， core-level peaks at 288.63， 286.02， and 284.74 eVcorresponded to N―C ＝ N， C―O， and C―C bonds，respectively （Fig. 1a）. For N 1s， core level peaks at 399.71eV（graphitic-N）， 398.87 eV （pyrrolic-N） and 398.39 eV（pyrridine-N） are characteristic of nitrogen doped carbon layer（Fig. 1b）. In the O 1s XPS spectra of CTF-7% Cu2O@NC， thebinding energy peak at 533.13 eV is assigned to O―H groups，while the peak at 531.74 eV is associated with metal-boundoxygen in the Cu―O bonds of Cu2O@NC materials （Fig. 1c）.The principal peaks at 953.15 and 933.18 eV were attributed toCu 2p1/2 and Cu 2p3/2 of Cu+ （Fig. 1d）， respectively. Satellitepeaks suggest the existence of Cu2+， potentially resulting fromthe partial oxidation of surface Cu2O to CuO during treatmentsand air exposure 33. The XPS spectrum of Cu2O@NC exhibitsthe corresponding elemental fitting peaks （Fig. S1）， confirmingthe synthesis of Cu2O@NC. Additionally， Fourier transforminfrared （FT-IR） analyses detected characteristic vibrations at1353 and 1517 cm?1 attributed to ―C―N＝ and ―C＝N―，indicating the successful formation of triazine frameworks inboth CTF-7% Cu2O@NC and CTF （Fig. 1e） 34–36. In thesolid-state 13C CP/MAS spectra （Fig. 1f）， the signals at 127.8ppm and 137.4 ppm are attributed to pyridine carbons locatednear the nitrogen atom. Notably， the two carbons adjacent tonitrogen exhibit chemical shifts at 150 ppm and 158 ppm，respectively 37. Furthermore， the chemical shift at 168.9 ppm isassigned to the carbon in the triazine ring， thereby confirmingthe perfect synthesis of the CTF triazine ring. PXRD analysisconfirmed the crystallinity of CTF. As shown in Fig. S2a， thePXRD pattern of CTF is consistent with the presence of aneclipsed AA stacking model for X-ray diffraction （XRD）analysis， further confirming the successful synthesis ofCTF-Cu2O@NC， CTF and Cu2O@NC （Fig. S2b） 38. TheBrunauer-Emmett-Teller （BET） surface area analysis isessential for evaluating the porosity and surface area ofphotocatalysts， which directly influences their photocatalyticefficiency. The permanent porosity of CTF andCTF-Cu2O@NC was determined by investigating N2adsorption/desorption isotherms at 77 K （Fig. 1g）. The N2adsorption-desorption isotherms show a typical type IVhysteresis loop for CTF and CTF-7% Cu2O@NC， which isfavorable for efficient mass transfer 39. The specific surfaceareas were determined to be 352 m2?g?1 for CTF and 213 m2?g?1for CTF-Cu2O@NC. The distribution of pore sizes， measuredusing the Barrett-Joyner-Halenda （BJH） method， shows a valueof 3.9 nm （Fig. 1h） 40. The analysis revealed a high surfacearea， indicating the presence of a porous structure， which isadvantageous for photocatalytic reactions. The thermalstabilities of CTF and CTF-7% Cu2O@NC were confirmedthrough thermogravimetric analysis （TGA） （Fig. 1i）， whichindicated the ability of both CTF and CTF-Cu2O@NC tomaintain stability at temperatures below 300 °C.

The microstructure and morphology of the synthesized CTF，Cu2O@NC and CTF-7% Cu2O@NC were analyzed usingtransmission electron microscopy （TEM）. As shown in Fig. 2a，the CTF consists of nanomaterials without specificmorphological features， which， when combined with scanningelectron microscopy （SEM） tests （Fig. S3）， show a prominentlamellar morphology. TEM analysis provides a nanoscale view（Fig. 2b）， revealing Cu2O nanoparticles enclosed by anitrogen-doped carbon matrix， creating a core-shell structurethat boosts effective charge transfer andseparation. In the caseof CTF compounded with Cu2O@NC， the CTF and Cu2O@NCare observed to be in close contact （Fig. 2e） with a visibleinterface， indicating a critical and strong interaction necessaryfor the heterojunction. Under the HRTEM test， clear latticefringes are observed for both CTF （Fig. 2d） and Cu2O@NC（Fig. 2f）， and a measurement of 0.212 nm fringes correspondsto the （200） crystalline surface of Cu2O （Fig. 2c）. Theseobservations confirm the successful synthesis of CTF-7%Cu2O@NC. The EDX elemental mapping pictures demonstratethe uniform dispersion of C， N， O， elements in the CTF-7%Cu2O@NC （Fig. 2g–j）. Together， SEM and TEManalyses provide a comprehensive understanding of themorphological features of the CTF-7% Cu2O@NCheterojunction， characterized by a lamellar structure andwell-dispersed nanoparticles. This nanostructured design iscrucial for enhancing photocatalytic performance byminimizing charge transfer distance， reducing recombination，and improving hydrogen production efficiency.

3.2 Photocatalytic hydrogen evolution performance

The UV-Vis diffuse reflectance spectroscopy （DRS）spectrum of the as-fabricated samples is presented in Fig. 3a.The UV-Vis spectrum of CTF reveals an absorption band withan onset at around 500 nm. Cu2O@NC， covered by anitrogen-doped carbon layer， exhibits absorption throughout thespectrum. Separately prepared Cu2O demonstrates strongabsorption at 640 nm in the visible light range， reflecting theenergy band information of the semiconductor Cu2O. CTF-7%Cu2O@NC has a broader absorption band than CTF， and itsenhanced light absorption is attributed to the successfulcombination of CTF and Cu2O@NC.

The photocatalytic efficiency of H2 evolution under visiblelight （≥ 420 nm） was evaluated as illustrated in Fig. 3b， usingthe material irradiated with the addition of 3% Pt in aqueoustriethanolamine-methanol solutions （TEOA： 26.25% v/v， +methanol： 12.5% v/v）. Due to the rapid complexation ofphotogenerated carriers， hydrogen production was limited to3673 μmol·g?1·h?1 for pure CTF and was negligible for pureCu2O@NC. The photocatalyst Cu2O@NC was optimised，along with the type of sacrificial agent and other factors， inorder to obtain the most suitable reaction conditions. It isnoteworthy that， in comparison to the weak hydrogenproduction activity of triethylamine （TEA） and methanol（MeOH）， which were used as sacrificial agents， thecombination of triethanolamine （TEOA） and methanol （MeOH）not only maintained the high activity of TEOA but also resultedin an increase in the overall rate of the reaction. This could beattributed to two factors： firstly， the provision of protonsrequired for the reaction; and secondly， the improvement ofcatalyst solubility or dispersion in water （Fig. 3d） 41. Thesynergistic effect of enhanced charge separation and transfer inthe optimized CTF-7% Cu2O@NC resulted in a remarkablehydrogen production rate of 15645 μmol·g?1·h?1， representing a5.85-fold increase compared to that of pure CTF. The superiorphotocatalytic activity of the CTF-7% Cu2O@NCheterojunction， compared to that of pure CTF， can be attributedto the efficient electron acceptance and transport properties ofCu2O@NC. The stability of the photocatalyst was confirmedby multiple hydrogen evolution cycles， showing stablephotocatalytic performance for more than 18 h （Figs. 3e andS4）. Notably， the AQY for CTF reached up to 1.67% at 420 nmand even 0.13% at the longer wavelength of 650 nm （Fig.3f） 42. These results highlight the excellent stability andpotential for long-term applications （Fig. S5）. Mechanistically，the enhanced performance is ascribed to the favorable transferand separation of photogenerated electron-hole pairs to thehydrogen evolution sites. This process ensures that the carriershave the necessary redox potential to drive the hydrogenevolution reaction. The photocatalytic performance of CTF-7%Cu2O@NC surpasses most of the reported CTF-basedphotocatalysts for hydrogen evolution （please refer to Fig. 3gand Table S1 in the Supporting Information for acomprehensive comparison） 34，43–63.

3.3 S-scheme heterojunction photocatalyticmechanism analysis

The electronic energy band structures of semiconductors arecrucial in determining hydrogen production activity. Therelative positions of the valence band （VB） and conductionband （CB） for CTF and Cu2O@NC were estimated bycombining Mott-Schottky analysis， VB-XPS measurements andtheir optical band gaps. As shown in Fig. 4a，b， the CB potentialclose to the flat band potential was determined using theequation： ECB = E（Ag/AgCl） + 0.197 + 0.0591 × pH， （similarlyfor EVB）. Cu2O was used to accurately measure the band gap ofCu2O@NC， as the NC layer obstruction affects the correctevaluation of the optical band gap. Fig. 4c shows that the bandgap of CTF was experimentally measured to be 2.21 eV， whilethat of Cu2O@NC was determined to be 2.15 eV 64，65. The VBlevel for the VB-XPS test was calculated using the equation：EVB （representing the potential of VB） = φ （instrument workfunction， 4.40 eV） + EVB-XPS ? 4.44 + 0.0591 × pH （pH = 7）.The VB-XPS test （Fig. 4d，e） resulted in an estimated EVB of2.09 V vs. RHE for CTF and 1.88 V vs. RHE for Cu2O@NC.The band levels clearly show that both CTF and Cu2O@NCmeet the required thermodynamic conditions for photocatalytichydrogen evolution reaction （Fig. 4f）.

The band structures of CTF and Cu2O were analyzed usingdensity functional theory （DFT） simulations 66. The calculatedband gaps for CTF （1.302 eV） and Cu2O （1.226 eV） aredepicted in Fig. 5a，b， respectively， with the correspondingdensity of states shown in Fig. S6. These results are inexcellent agreement with the experimental band data shownin Fig. 4f 20，67. The formation mechanism of S-schemeheterojunction has fundamentally elucidated the interactionsand processes that enhance the photocatalytic activity of theCTF-Cu2O@NC heterojunction. Advanced characterizationtechniques have provided insights into the mechanism ofphotocatalytic hydrogen evolution. The work function （Φ） ispivotal in unveiling the charge transfer at the interface. It canbe calculated as Φ = EVAC ? EF， where EVAC represents theenergy of the vacuum potential， and EF denotes the Fermienergy. The work function at the CTF and Cu2O@NC surfaceswas calculated using DFT simulations （Figs. 5c and S7），yielding values of 5.69 eV for CTF （001） and 5.12 eV for Cu2O（111）， respectively 68，69. To better understand the charge transfermechanism in CTF-7% Cu2O@NC， the Kelvin probe （KP） wasutilized to evaluate the work function. The average contactpotential difference obtained by KP measurements was 360meV for CTF and ?10 meV for Cu2O@NC （Figs. S8 and S9）.The corresponding work functions were converted to 5.23 eVfor CTF and 4.86 eV for Cu2O@NC （Fig. 5d）. The calculatedvalues align well with the experimental data， indicating that thework function is crucial for the formation of S-schemeheterojunctions. Since the work function of CTF is higher thanthat of Cu2O@NC， it implies that CTF has a lower Fermienergy level. In the absence of light， the contact between CTFand Cu2O@NC leads to band bending due to the difference inFermi energy levels， forming an internal electric field pointingfrom Cu2O@NC to CTF.

The charge-transfer mechanisms at the heterojunctioninterface under photoirradiation can be evaluated using in situXPS. Under the illumination of 365 nm light， the chargetransfer between CTF and Cu2O@NC is monitored by in situXPS. The binding energy of Cu 2p and O 1s in CTF-7%Cu2O@NC shows a negative shift in comparison with the XPSdata collected in darkness （Fig. 5e，f） 65，70–72， implying anenhancement in the electron density of Cu2O@NC. The abovetests confirm that CTF-Cu2O@NC is a typical S-schemeheterojunction.

The electrochemical and photophysical properties of theCTF-7% Cu2O@NC S-scheme heterojunction play a crucialrole in its photocatalytic behavior. Insights from transientsurface photocurrent measurements indicate a higherphotocurrent intensity for the heterojunction compared to thoseof pure CTF and pure Cu2O@NC （Figs. 5g and S10） 73–76，suggesting more effective charge separation and longer lifetimeof charge carriers. This is attributed to the S-scheme structurethat promotes electron flow and minimizes recombination.Electrochemical Impedance Spectroscopy （EIS） reveals asmaller semicircle radius in the Nyquist plots for the CTF-7%Cu2O@NC， indicating a lower charge transfer resistance andfaster interfacial charge transfer， which are key for improvedphotocatalytic activity.

To reveal the underlying reasons for the enhancedperformance observed in the CTF-7% Cu2O@NC composite，we carried out the time-resolved photoluminescence andphotoluminescence spectra of representative samples. Asdepicted in Fig. 5h， the PL peak intensity of CTF-7%Cu2O@NC is significantly lower than those of CTF andCu2O@NC， indicating that the heterojunction formed via theS-scheme connection effectively promotes the transfer andseparation of photogenerated electron-hole pairs 77. Notably，the CTF-7% Cu2O@NC sample displays the shortest averagefluorescence lifetime of 1.54 ns， confirming its best ability forphotogenerated charge separation （Fig. 5i）. This observationis in agreement with its remarkable H2 evolution activity.

According to previous results， we propose a possiblephotocatalytic mechanism for CTF-7% Cu2O@NC. As shownin Fig. 6， in the absence of light， when CTF and Cu2O@NC arein contact， electrons from Cu2O@NC with a higher Fermilevel， flow through the NC layer to CTF， which has a lowerFermi level. The respective Fermi energy levels are aligned，thereby forming an internal electric field directed fromCu2O@NC to CTF. Under light illumination， both CTF andCu2O@NC undergo electron generation， where electrons areexcited from the VB to the CB， resulting in the formation ofholes in the VB. The photogenerated electrons then migratefrom the CB of CTF to the NC layer， and are subsequentlytransferred to the VB of Cu2O， where they recombine with theholes 78. The remaining electrons in the Cu2O are injected tothe surface of the Pt cocatalysts， thereby driving the surfacehydrogen evolution reaction. Meanwhile， the remainingphotogenerated holes in the CTF are utilzied by TEOA andMeOH. In a word， the achievement of the S-schemeheterojunction mechanism effectively improves the migrationand separation of electron-hole pairs， thereby inhibitingphoto-corrosion of Cu2O.

4 Conclusions

Extensive research on the CTF-Cu2O@NC S-schemeheterojunction has yielded significant insights into its potentialfor photocatalytic hydrogen production. The CTF-7%Cu2O@NC composite achieves remarkable photocatalytichydrogen evolution rate of 15645 μmol·g?1·h?1， which issubstantially significantly higher than that of pure CTF. Thisenhanced performance is attributed to the S-scheme chargetransfer mechanism， which efficiently separates photogeneratedholes and electrons， while preserving their redox potentials.Mechanistic insights were gained through advancedcharacterization techniques such as in situ XPS， Kelvin probemeasurements， and DFT calculations. These analyses confirmeffective charge carrier separation and favorable energy levelalignment， contributing to the superior photocatalytic activityof the CTF-7% Cu2O@NC heterojunction. Future researchcould focus on further optimizing the composition and structureof such heterojunction composites to enhance theirphotocatalytic performance. Additionally， exploring broaderapplications of these materials could contribute toadvancements in sustainable energy technologies， particularlyin areas like solar-driven hydrogen production andenvironmental remediation.