Isotype Heterojunction Research Articles

Photocatalytic removal of organic dyes mediated by p-NiO/p-CuO isotype heterojunctions under sunlight illumination

Building heterojunctions between different semiconductors is one of the most typical approaches to separating photo-induced electrons and holes in order to enhance photocatalytic efficiency. In this study, NiO, CuO, and NiO–CuO nanocomposites thin films are obtained under the same deposition conditions by a simple spray pyrolysis technique. The structural, morphological, and optical properties of the deposited thin films were investigated using different techniques such as XRD, Raman, SEM, EDX, PL, and UV–visible spectroscopy. XRD and Raman analysis revealed that NiO, CuO, and NiO–CuO have crystalline cubic, monoclinic, and mixed phases, respectively. EDX analysis confirmed the chemical composition, and SEM images reveal different morphologies depending on the Ni:Cu molar ratio. The photodegradation of MB dye in the presence of the thin films was examined under sunlight. Compared with individual metal oxides, NiO–CuO nanocomposites are better as photocatalysts. Optical measurements were able to connect these enhanced photocatalytic activities to the synergistic effects of combining NiO and CuO, which promoted the creation of defects and highly reactive species on surfaces and an effective separation of photogenerated charges. The benefits of p-p isotype heterojunctions for dye photodegradation that this study has illustrated provide a novel perspective on metal oxydes-based nanocomposites for the development of efficient and novel photocatalysts.

Role of oxygen vacancy enriched m-BiVO4/t-BiVO4 isotype heterojunction for enhanced photocatalytic degradation of rhodamine B dye and in ferroelastic to paraelastic phase transition

Mixed-phase isotype heterojunction oxide materials with superior photocatalytic activities have recently revealed a significant coupling between individual phases with controlled optical properties. Therefore, we focus on synthesizing a mixed-phase monoclinic-tetragonal (m-t) BiVO4 photocatalyst using hydrothermal method. The increased recombination rate of electron-hole (e−-h+) pairs limits the photocatalytic performance of monoclinic (m) BiVO4 photocatalysts. Therefore, effective spatial charge carrier separation can be improved by creating an isotype heterojunction between the phases m-BiVO4 and t-BiVO4. The precursor pH, mechanical grinding, and calcination temperature regulate the formation of m-t BiVO4. The phase fractions of the two phases in m-t BiVO4 sample are 85% monoclinic and 15% tetragonal. Herein, we demonstrate the optical properties and photocatalytic activity based on the crystal chemistry of m-BiVO4 and m-t BiVO4. Temperature-dependent Raman measurements suggest a second order ferroelastic monoclinic to paraelastic tetragonal phase transition above 530 K owing to optical phonon anharmonic coupling interactions. The small increase (5 °C) in Tc in m-t BiVO4 compared to m-BiVO4 is due to the coexistence of tetragonal and monoclinic phases. The optical band gap of m-t BiVO4 is larger than that of m-BiVO4. Additionally, the evaluation of BiVO4 photocatalytic activity over rhodamine-B (Rh-B) dye suggests that the m-t BiVO4 with 97.5% degradation efficiency is superior compared to m-BiVO4 with 89% degradation efficiency. This is because, the two phases (m and t) of BiVO4 form an isotype heterojunction interface, which results in a high charge separation efficiency, as demonstrated by photocurrent studies and electrochemical impedance spectroscopy. Moreover, oxygen vacancies tune the charge carrier dynamics through the modification of band edges. These findings regarding the isotype phase-junction mechanism provide possibilities for innovative ideas for highly efficient mixed-phase isotype heterojunction photocatalysts for wastewater remediation.

Porous two-dimensional CuSe@BiOI isotype heterojunction with highly exposed (1 0 2) facets for efficient photoelectrocatalytic CO2 reduction and photodetection

Using photoelectrochemical technology to construct efficient photoelectric materials has aroused extensive research interest. In this study, we propose a two-dimensional porous CuSe@BiOI isotype heterojunction photocathode for CO2 reduction to formic acid. The heterojunction was constructed by the epitaxial growth of high-quality oriented BiOI nanosheets onto the porous CuSe framework. The CuSe framework as a template to ensure the formation of the (102) facets of BiOI and the isotropic charge transport characteristics of the (102) surface greatly improve the efficiency of charge separation and transfer. This CuSe@BiOI isotype heterojunction electrode can efficiently reduce CO2 to formic acid with a selectivity of up to 80.5 % in 0.5 M potassium bicarbonate aqueous solution. Meanwhile,the performance of the photodetector based on CuSe@BiOI heterostructure is 6.97 × 1011 Jones and a response rate of 87.13 mA W−1. This finding provides a new approach to designing stable heterojunction photoelectrocatalysts with isotropy.

Supercritical CH3OH-Triggered Isotype Heterojunction and Groups in g-C3N4 for Enhanced Photocatalytic H2 Evolution.

The structure tuning of bulk graphitic carbon nitride (g-C3N4) is a critical way to promote the charge carriers dynamics for enhancing photocatalytic H2-evolution activity. Exploring feasible post-treatment strategies can lead to effective structure tuning, but it still remains a great challenge. Herein, a supercritical CH3OH (ScMeOH) post-treatment strategy (250-300 °C, 8.1-11.8 MPa) is developed for the structure tuning of bulk g-C3N4. This strategy presented advantages of time-saving (less than 10 min), high yield (over 80%), and scalability due to the enhanced mass transfer and high reactivity of ScMeOH. During the ScMeOH post-treatment process, CH3OH molecules diffused into the interlayers of g-C3N4 and subsequently participated in N-methylation and hydroxylation reactions with the intralayers, resulting in a partial phase transformation from g-C3N4 into carbon nitride with a poly(heptazine imide)-like structure (Q-PHI) as well as abundant methyl and hydroxyl groups. The modified g-C3N4 showed enhanced photocatalytic activity with an H2-evolution rate 7.2 times that of pristine g-C3N4, which was attributed to the synergistic effects of the g-C3N4/Q-PHI isotype heterojunction construction, group modulation, and surface area increase. This work presents a post-treatment strategy for structure tuning of bulk g-C3N4 and serves as a case for the application of supercritical fluid technology in photocatalyst synthesis.

Narrow-band n-GaN/n-Si isotype heterojunction photodiode: A simplified approach for photodiode development

An innovative approach has been explored to fabricate a high-performance photodiode in semiconductor device production, utilizing the thermal evaporation technique to grow GaN thin films. Structural analyzes were conducted on both as-deposited and annealed samples, and their properties were determined using various characterization techniques, including FESEM, XRD, XPS, Hall effect, photoluminescence, and UV-Vis spectrophotometer measurements. Valuable information has been gained about the potential of this alternative approach for fabricating GaN-based electronic devices. In this study, Al/n+-GaN/n-Si/Ag (as-deposited GaN) and Al/n-GaN/n-Si/Ag (annealed GaN) structures have been fabricated. It was found that the device produced from the as-deposited GaN thin film exhibited better performance than the annealed one. The isotype heterojunction n+-GaN/n-Si photodiode exhibited exceptional performance in detecting NIR light at a wavelength of 1050 nm under a 0 V bias. It has a full-width at half-maximum (FWHM) value of 120 nm and achieved a photoresponsivity value of 355 mA/W at a light output of 10 mW/cm². It is further supported by its external quantum efficiency of 42% and detectivity value of 6.93×1011 Jones at the peak wavelength and very fast response speed with ∼33 µs rise/fall times. These findings highlight the potential of photodiode as an efficient narrow-band in the NIR region.

Visible-light-assisted peroxymonosulfate activation by metal-free g-C3N4 isotype heterojunction for water purification: Surface-activated complex and 1O2 dominated non-radical mechanisms

Visible-light-assisted peroxymonosulfate (PMS) activation is flourishing in wastewater decontamination, but exploiting safe and efficient activators remains challenging due to a deficient cognition of the activated mechanisms. Hereby, a metal-free g-C3N4 isotype heterojunction (HCN) was successfully synthesized and activated PMS under visible light. The Z-scheme isotype heterojunction significantly meliorated light absorption and carrier migration, thereby enhancing PMS activation. The optimal HCN + PMS system almost completely removed ranitidine (RAN) in 30 min with 7.46–8.48 times higher activity than the monomers. Through systematic experiments and characterizations, we proposed a non-radical mechanism of PMS activation, which generated predominated surface-activated complex and singlet oxygen (1O2) for pollutant removal. The surface-activated complex degraded RAN by drawing its electrons. This non-radical pathway exhibited tolerance towards background substances and pH fluctuations. Additionally, the broad-spectrum decontamination and recyclability of the system further enhanced its practical applicability. Notably, intermediate analyses confirmed the RAN degradation pathways predicted by theoretical calculations, with the resulting intermediates found to be non-toxic. This study significantly contributes to our understanding of the non-radical mechanisms of PMS activation on g-C3N4-based activators for micropollutant degradation.

Synthesis of isotype heterojunction by series calcination of multicomponent for high efficiency photocatalytic reduction of uranium

Photocatalytic reduction of soluble U(VI) to insoluble U(IV) is considered to be one of the most effective strategies for uranium contaminant removal. Graphite carbon nitride (g-C3N4) was widely used in the photocatalytic reduction of uranium due to its adjustable band structure and excellent chemical stability. Herein, through supramolecular self-assembly method, using thiourea as precursors, a carbon nitride photocatalyst (sp-ICN) with an isotype heterojunction structure was prepared by multicomponent series calcination for photocatalytic reduction of U(VI). The effects of pH, photocatalyst dosage and initial uranium concentration were examined on the effectiveness of the sp–ICN–1.0 photocatalytic reduction of uranium. The optimal reaction rate of sp–ICN–1.0 was 0.088 min−1, being 2.8 and 2.6 times that for pristine TCN (0.031 min−1) and sp-MCN (0.034 min−1), respectively. After 5 recycling experiments, more than 90% of U(VI) can still be reduced, indicating that sp–ICN–1.0 has good stability and repeatability in photocatalytic reduction of uranium. The electrochemical characterization of sp–ICN–1.0 showed that the formation of carbon nitride isotype heterojunction could improve the efficiency of photogenerated carrier separation and transfer, also reduce the resistivity of electron migration. A potential mechanism for photocatalytic reduction in U(VI) has been proposed. The result provides an idea for the further development of a novel semiconductor carbon nitride photocatalyst for the photocatalytic reduction of uranium.

Construction of isotype heterojunctions in polymeric carbon nitride with thermal modulation and improved photocatalytic hydrogen production activity

Subtle structural changes in polymeric carbon nitrides (PCN) influence their photocatalytic activities. Herein, we present a strategy to construct isotype heterojunctions by thermal condensation of dicyandiamide as a single precursor. This avoids the complications that differences in the electronegativity of the coupled materials incur. Heterojunctions were created by depositing a carbon nitride layer processed at 500 °C over the carbon nitride processed at 600 °C utilizing structural defects and uncondensed moieties to form an interconnected structure. This enables the directional stacking of the PCN layers leading to changes in physicochemical properties and electronic band structure. The detailed characterization and testing revealed a type-II heterojunction configuration, whereby photoluminescence quenching measurements confirm longer exciton lifetimes and differences in Mott-Schottky flat band potential energies and low interfacial resistance facilitate mobility of charge carriers. Heterojunction materials exhibit high photocurrent generation and H2 production with visible light, and AQY surpasses 5%. Overall, efficient charge separation owing to the intrinsic electric field of closely related band offsets at the interface of the heterojunction materials enhances the photocatalytic response. The work highlights the importance of fine structural changes in improving the activity of polymeric photocatalytic systems.

A graphitic carbon nitride photocatalyst with a benzene-ring-modified isotype heterojunction for visible-light-driven hydrogen production

Benzene rings modified g-C3N4 with an iso-type heterojunction structure (UTPh) promotes significantly higher visible-light driven photocatalytic H2 production due to the efficient photo-charge separation and transfer, and donor-acceptor mechanism.

Isotype heterojunction: tuning the heptazine/triazine phase of crystalline nitrogen-rich C3N5 towards multifunctional photocatalytic applications.

Photocatalytic technology has been well studied as a means to achieve sustainable energy generation through water splitting or chemical synthesis. Recently, a low C/N atomic ratio carbon nitride allotrope, C3N5, has been found to be highly prospective due to its excellent electronic properties and ample N-active sites compared to g-C3N4. Tangentially, crystalline g-C3N4 has also been a prospective candidate due to its improved electron transport and extended π-conjugated system. For the first time, our group successfully employed a one-step molten salt calcination method to prepare novel N-rich crystalline C3N5 and elucidate the effect of calcination temperature on the heptazine/triazine phase. Calcination temperatures of 500 °C (CC3N5-500) and 550 °C (CC3N5-550) lead to crystalline carbon nitride with both heptazine and triazine phases, forming an intimate isotype heterojunction for robust interfacial charge separation. An excellent photocatalytic hydrogen evolution rate (359.97 μmol h-1; apparent quantum efficiency (AQE): 12.86% at 420 nm) was achieved using CC3N5-500, which was 15-fold higher than that of pristine C3N5. Furthermore, CC3N5-500 exhibited improved activity for simultaneous benzyl alcohol oxidation and hydrogen production, as well as H2O2 production (AQE: 9.49% at 420 nm), signifying its multitudinous photoredox capabilities. Moreover, the recyclability tests of the optimal CC3N5-500 on a 3D-printed substrate also showed a 92% performance retention after 4 cycles (16 h). This highlights that crystalline C3N5 significantly augmented the reaction performance for diverse multifunctional solar-driven applications. As such, these results serve as a guide toward the structural tuning of 2D metal-free carbon nanomaterials with tunable crystallinity toward achieving boosted photocatalysis.

Reusable isotype heterojunction g-C3N4/alginate hydrogel spheres for photocatalytic wastewater treatment.

Various visible-light-driven photocatalysts have been studied for practical applications in photocatalytic wastewater treatment via solar irradiation. Among them, g-C3N4 has attractive features, including its metal-free and environmentally friendly nature; however, it is prone to charge recombination and has low photocatalytic activity. To solve these problems, isotype heterojunction g-C3N4 was recently developed; however, the methods employed for synthesis suffered from limited reproducibility and efficiency. In this study, isotype heterojunction g-C3N4 was synthesized from various combinations of precursor materials using a planetary ball mill. The isotype heterojunction g-C3N4 synthesized from urea and thiourea showed the highest photocatalytic activity and completely decolorized Rhodamine B (RhB; 10 ppm) in 15 min under visible-light irradiation. Furthermore, to improve recyclability, isotype heterojunction g-C3N4 was immobilized in alginate hydrogel spheres. The isotype heterojunction g-C3N4/alginate hydrogel beads were used in 10 repeated RhB degradation experiments and were able to maintain their initial photocatalytic activity and mechanical strength. These achievements represent an advance towards practical, sustainable photocatalytic wastewater treatment.

Isotype heterojunction graphitic carbon nitride photocathode for photo-accelerated zinc-ion capacitors

A g-C3N4 isotype heterojunction photo-electrode is reported for high-performance photo-accelerated zinc-ion capacitors.

3D black g-C3N4 isotype heterojunction hydrogels as a sustainable photocatalyst for tetracycline degradation and H2O2 production

The sustainable visible light active g-C3N4 hydrogel is synthesized and employed for water reclamation and H2O2 production. In this study, citric acid-assisted chemically functionalized black g-C3N4 isotype heterojunction (BCN) hydrogel was prepared using the blend-crosslinking method. The comprehensive characterization of the BCN hydrogels affirmed its tuned optical bandgap with effective visible light utilization, greater separation efficiency, and restricted recombination ratio of electrons and holes. The BCN hydrogels exhibited a phenomenal photocatalytic efficiency towards tetracycline degradation (86% in 40 min) and H2O2 production (987 µmol in 60 min). The superoxide, holes, and singlet oxygen demonstrated a synergistic role in photocatalytic activity through the effective migration of electrons. As an immobilized system, the predominant advantages of BCN hydrogel beads include ease of recovery and remarkable reusability up to 7 cycles. This study provides insights into systematically modifying g-C3N4 toward sustainable energy and environmental applications.

Construction of novel an in-situ m-BiVO4/t-BiVO4 isotype heterojunction photocatalyst for improved visible light-dependent degradation of dye

In this article, we have studied the photocatalytic characteristics of isotype heterostructure Bismuth vanadate BiVO4 (m-BiVO4/t-BiVO4). The isotype Bismuth vanadate m-BiVO4/t-BiVO4 (MTBV) was formed by a facile hydrothermal method. It was observed that the crystalline phase of isotype MTBV could be controlled by modifying the pH values of the precursors in the hydrothermal method. The isotype MTBV heterojunction was investigated for its physicochemical features by different techniques including X-ray powder diffraction (XRPD), Field emission scanning electron microscopy (FESEM), Energy dispersive microscopy (EDS), Fourier transfer infrared (FTIR) spectroscopy, UV–visible diffuse reflectance spectroscopy (UV–Vis DRS), Photoluminance (PL), and Electrochemical impedance spectroscopy (EIS). Photocatalytic degradation of Congo red (CR) dye under visible light irradiation was used to determine the photocatalytic activity of a well-developed MTBV heterostructure. The isotype MTBV phase resulted in an increased response due to a faster electron-hole pair formation rate by reducing charge recombination processes, a reduction in optical energy band gap (2.52 eV), and good compatibility between Preston t-BiVO4 and m-BiVO4 photocatalyst. In 1 hr, the optimized isotype MTBV heterojunction showed a CR degradation of 98%. According to the results of the PL, EIS, and UV–Vis studies, isotype MTBV might be an excellent option for a blue/green visible light-dependent optical photocatalyst. A feasible photocatalytic mechanism was suggested and discussed in particular to relate the physiochemical properties and photocatalytic properties; furthermore, kinetics decomposition process study, scavenger effect, dye concentration, catalyst dose effect, reusability and stability were checked, respectively.

Dual-channel charge transfer over g–C3N4/g–C3N4/bismuth-based halide perovskite composite for improving photocatalytic degradation of tetracycline hydrochloride

Photocatalytic technology represents a cutting-edge approach for addressing water pollution. The performance of photocatalysts can be improved by constructing of heterojunctions that facilitate the charges transfer between the two components. Herein, we produced a three-component g–C3N4/g–C3N4 isotype heterojunction/bismuth-based halide perovskites (g–C3N4–ISO/BHP) composite using a solvothermal method to grow bismuth-based halide perovskite (BHP) onto the surface of g–C3N4/g–C3N4 isotype heterojunction (g–C3N4–ISO) layers. Several methods were used to investigate crystalline structure, morphology, chemical composition/states, optical characteristics, and charge-carrier dynamics of the three-component composite. The g–C3N4–ISO/BHP composite exhibited superior photocatalytic activity under white LED light with a degradation percentage of 73.2%, which is higher than the mixed-phase g–C3N4–ISO (61.4%) and the bare BHP (62.5%). The control of component content improved the transmission of charges between the composite components. The S-scheme charge transfer mechanism was verified to function on the g–C3N4–ISO/BHP composite as a result of its high compatibility and adequate band structure, resulting in robust redox ability, effective charge carrier separation and extended lifetime. Our study presents a practical approach for constructing three-component composites for efficient photocatalytic water treatment.

One-step fabrication of unique 3D/2D S, O-doped g-C3N4 S-scheme isotype heterojunction for boosting CO2 photoreduction

Solar irradiation-based CO2 conversion to energy-rich products is an efficient way to reduce excessive CO2 emissions and solve resource depletion. Unfortunately, achieving a cost-effective and dependable method for converting CO2 is still a significant challenge. Here, we introduce a hydrothermal approach followed by calcination to design a new-fashioned 3D S-doped g-C3N4/2D O-doped g-C3N4 (3D/2D SOCN) based step-scheme (S-scheme) heterojunction as an efficient photocatalysts for CO2 reduction. The adjusted sample demonstrated significantly greater CO2 photoreduction conversion rates compared to the blank control, including 3D S-doped g-C3N4 (3D SCN), 2D O-doped g-C3N4 (2D OCN), and bulk g-C3N4 (CN). The exceptional photocatalytic performance can be ascribed to multiple factors, including the S-Scheme isotype heterojunction that suppresses the recombination of photogenerated charge carriers, the material's high specific surface areas, which highly enhance the abundance of active sites, and the synergistic effect of the heteroatom as a dopant. The validity of the S-Scheme photogenerated charge transfer process is supported by measuring the work function and electron paramagnetic resonance (EPR). This research presents a practical method for fabricating multi non-metal-doped S-scheme isotype heterojunction photocatalysts that display remarkable efficiency for solar fuel conversion.

Two-dimensional interface engineering of g-C3N4/g-C3N4 nanohybrid: Synergy between isotype and p-n heterojunctions for highly efficient photocatalytic CO2 reduction

The creation of an isotype phase junction has been recognized as an effective means of improving the spatial charge separation and migration in g-C3N4 materials. Nevertheless, due to a lack of electrostatic control in the internal electric field, isotype heterojunction alone is incapable of providing a sufficient driving force to maximize charge carrier transfer. Herein, we present an interface engineering strategy for co-integrating isotype and p-n heterojunctions to fabricate g-C3N4/g-C3N4 nanohybrids using a facile ultrasonic-assisted self-assembly method. The coherent boundary between the two distinct phases of g-C3N4 demonstrates that an intimate 2D interfacial contact can be easily established by π-π stacking interactions because of their high structural similarities and low lattice strain. Furthermore, their compatible and well-matched electronic band structures led to a staggered type II alignment in the lateral heterojunction, which confers the resulting composite with strong redox ability, efficient exciton dissociation, and desirable optoelectronic characteristics beyond those of the two constituents. Essentially, the isotype and p-n heterojunctions work in tandem to create a robust built-in electric field, allowing for effective interfacial charge steering for directional electron migration from BCN to CNx. Benefiting from these merits, the BCN/CNx nanohybrid manifested remarkable CH4 generation from photocatalytic CO2 reduction, outperforming its CNx and BCN counterparts by 1.91 and 6.88-fold, respectively. The midgap states induced by nitrogen defects in CNx also pose a positive effect on the improved photoactivity of the binary composite by acting as an electron reservoir to mediate electron transfer and further impede charge carrier recombination. This proof-of-concept study highlights the significance of interface engineering in charge flow manipulation and utilization for photocatalytic enhancement.

A high-cyano groups-content amorphous-crystalline carbon nitride isotype heterojunction photocatalyst for high-quantum-yield H2 production and enhanced CO2 reduction

Polymeric carbon nitride suffers terrible recombination of photo-generated carriers and thus usually results in limited photocatalytic activity. Improving crystallinity and creating novel structure can effectively alleviate the aforementioned problem. Herein, a high-cyano groups-content and amorphous-crystalline carbon nitride isotype heterojunction (BCN-NaK) was obtain via a post treatment strategy in eutectic salt. The cyano groups acted as electron acceptors, and the intimate isotype heterojunction constructed by amorphous carbon nitride and crystalline poly-heptazine-imide built interfacial electric field. Both synergistically enabled BCN-NaK more efficient charge transfer than other amorphous and crystalline carbon nitrides. Comprehensive and systematic analysis demonstrated that the promoted charge transfer made the predominant contribution to the ultrahigh photocatalytic H2-production activity of BCN-NaK with an excellent apparent quantum yield of 68.9 % at 405 nm. In addition, BCN-NaK exhibited a good photocatalytic CO2-to-CO conversion rate of 22.8 μmol h−1 gcat−1 without adding sacrificial agent or loading metal under visible light.

High‐performance perovskite/silicon heterojunction solar cells enabled by industrially compatible postannealing

AbstractIn recent years, developing dopant‐free carrier‐selective contacts, instead of heavily doped Si layer (either externally or internally), for crystalline silicon (c‐Si) solar cells have attracted considerable interests with the aim to reduce parasitic light absorption and fabrication cost. However, the stability still remains a big challenge for dopant‐free contacts, especially when thermal treatment is involved, which limits their industrial adoption. In this study, a perovskite material ZnTiO3 combining with an ultrathin (~1 nm) SiO2 film and Al layer is used as an electron‐selective contact, forming an isotype heterojunction with n‐type c‐Si. The perovskite/c‐Si heterojunction solar cells exhibit a performance‐enhanced effect by postmetallization annealing when the annealing temperature is 200–350°C. Thanks to the postannealing treatment, an impressive efficiency of 22.0% has been demonstrated, which is 3.5% in absolute value higher than that of the as‐fabricated solar cell. A detailed material and device characterization reveal that postannealing leads to the diffusion of Al into ZnTiO3 film, thus doping the film and reducing its work function. Besides, the coverage of SiO2 is also improved. Both these two factors contribute to the enhanced passivation effect and electron selectivity of the ZnTiO3‐based contact and hence improve the cell performance.

Single-atom cobalt-incorporating carbon nitride for photocatalytic solar hydrogen conversion: An X-ray spectromicroscopy study

The use of carbon nitride-based materials and light to drive catalytic water splitting has enormous potential for the production of hydrogen. Revealing the processes of molecular conjugation, nucleation, and crystallization in crystalline carbon nitride is expected to enhance the photocatalytic activity through the creation of isotype heterojunctions and active sites. In this work, the addition of cobalt salts in ionothermal synthesis was found to promote the phase transition of heptazine-based crystalline carbon nitride (CCN) to triazine-based poly (triazine imide) (PTI), resulting in the formation of a single-atom cobalt-doped coordinated isotype CCN/PTI heterojunction. The new hybrid orbital modulates the atomic/electronic structure and the band gap of the CCN/PTI heterojunction, and synergistically increases the absorption of visible light, accelerating the separation and transfer of photoexcited electrons and holes. Synchrotron-based X-ray spectroscopy and microscopy are used to identify the origin of the improved performance of the single-atom cobalt-doped CCN/PTI heterojunction in the photocatalytic hydrogen evolution reaction. This work demonstrates that synchrotron X-ray spectroscopy is a promising tool for designing materials aimed at enhancing photocatalytic activity in solar energy conversion applications.