Brief introduction of the results
Understanding charge transfer kinetics and carrier separation pathways is challenging due to the lack of suitable characterization techniques. Based on this,Professor Xiang Quanjun, Professor Qiao Liang of University of Electronic Science and Technology of China, and Professor Lv Kangle of South-Central University for Nationalities (co-corresponding author) and othersThe crystalline triazine heptazine carbon nitride homojunction was selected as a model system to demonstrate the electron transport mechanism at the interface. During in-situ photoemission, surface bimetallic cocatalysts are used as sensitive probes to trace the S-type transfer path of interfacial photoelectrons from the triazine phase to the heptazine phase.
The change in the surface potential of the sample under light confirms the dynamic transfer of the S-type charge. Further theoretical calculations show that there is an interesting reversal of the electron transfer path at the interface under light and dark conditions, which also supports the experimental results of the S-type transport mechanism. Due to the unique advantages of S-type electron transfer, the CO2 photoreduction activity of homojunction is significantly enhanced. This work provides a strategy to explore dynamic electron transport mechanisms and design fine material structures for efficient CO2 photoreduction.
Background:
Solar-powered semiconductor photocatalysis is an environmentally friendly energy conversion method that does not require an external energy supply and has become a promising solution to alleviate the energy and environmental crises. It is necessary and challenging to design reliable protocols to accurately track the migration of photogenerated electrons and holes, to study charge transfer dynamics, and even to regulate the separation of charges.
In this study, a high-quality crystalline TCN HCN homojunction (Th1:4) was synthesized by a combination of asynchronous crystallization and electrostatic self-assembly strategies, and the S-type electron migration direction was directly measured under illumination using state-of-the-art in-situ photoemission and scanning probe techniques. Mnox and PTO nanoparticles are deposited on the surface of the catalyst as redox co-catalysts, and they can be used as probes under co-excitation of X-ray and visible light to determine the true band arrangement under photoexcitation.
The in-situ Kelvin probe force microscopy intuitively reveals the potential difference between TCN and HCN and the change of surface potential, and elucidates the S-type transfer mechanism of photogenerated electrons at the TCN HCN interface. Further theoretical calculations confirm that the interfacial electron transfer path in TCN HCN homojunction follows the S-type transport mechanism. In conclusion, due to the unique advantages of S-type electron transfer, TCN HCN homojunction has significantly enhanced charge separation efficiency and photoreduced CO2 activity.
**Reading guide
Figure 1Characterization of crystal structure and morphology
TEM (Figures 1c and F)** shows that layered TCN and tubular HCN were successfully prepared. Figure 1G shows the staggered growth of TCN and HCN, with lattice fringes of two crystalline phases appearing at the interface, indicating the successful construction of the homojunction. The XRD (Fig. 1H) spectra showed the presence of characteristic peaks of both TCN and HCN on the Th1:4 sample, further confirming the successful preparation of TCN HCN homojunctions.
Figure 2Formation of an interfacial electric field.
Through theoretical calculations, the work functions of TCN and HCN are 6., respectively03 and 469 EV (Figures 2a and b). The band gaps for TCN, HCN, and homojunction were determined by DRS measurements and Kubelka-Munk analysis (Figure 2E). 80 and 292 ev。The XPS and UPS test results (Figures 2C and D) determined the valence band positions for TCN, HCN, and homojunction, respectively. 41 and 164 ev。According to the obtained band-edge positions and work functions of TCN and HCN, a schematic diagram of the built-in electric field at the TCN HCN interface in a dark environment was obtained (Fig. 2F). Due to the small work function of Hcn in homojunction, electrons migrate from the Fermi level of Hcn to the Fermi level of Tcn, thus forming an internal built-in electric field pointing from Hcn to Tcn.
Figure 3Dynamic tracking of S-type electron transfers.
MNOX and PTO were loaded on the homojunction and monitored using in-situ XPS irradiated with a xenon lamp as electron transfer indicators (Figures 3A and B). Among them, PTO is mainly concentrated on the TCN surface, while MNOX is mainly concentrated on the HCN surface (Fig. 3c and D). After 15 min of illumination, the binding energy of MN2P shifted to the left by 06 EV and return to the energy position in the "dark state" after turning off the light (Figure 3G), indicating that MNOX gains electrons under the action of light. At the same time, we noticed that the binding energy of PT 4F under light appeared 11 EV shifts to the left and does not fully return to the "dark" energy position after the lights are turned off (Figure 3F), which involves two electron transfer processes for TCN-loaded PTO. While the electron transfer pathways involved in PTO are relatively complex, electron transfer on MNOX provides evidence of the transfer of photogenerated electrons from TCN to HCN.
Figure 4Verification and mechanism of S-type electron migration.
The figure above shows the change in homogeneous outcome potential before and after illumination using light-assisted KPFM (Figure 4A). Atomic force microscopy (AFM) images and corresponding height distribution curves indicate that the selected region is the interface between TCN and HCN in the homojunction (Figures 4B and C). As shown in Figures 4d and e, the distribution of the surface potentials of the two phases in the "dark state" indicates that the surface potential of TCN in the homojunction is greater than that of HCN, and 42The difference of 1mV proves that the work function of TCN in the homojunction is greater than HCN, which is consistent with the theoretical calculation results.
After 15 minutes of illumination, the surface potential at the junction of TCN and HCN in the same area changed significantly. The surface potential of TCN is determined by 3692 to 3717 MV while HCN from 3271 reduced to 3195 mV (Figures 4F and G). The change in the surface potential of the KPFM induced by light irradiation directly shows the dynamic transfer of photoelectrons from TCN to HCN at the real-time spatial and real-time scales (Fig. 4H), thus confirming the S-type charge transfer mechanism.
Figure 5Theoretical perspectives on electron changes caused by s-type electron transfer
The work function of the TCN HCN homojunction is 586 EV with an inter-layer distance of 33 Figure 5a). Mulliken charge analysis revealed the opposite electron transfer pathway of the TCN HCN homojunction in dark and illuminated environments (Fig. 5B). Under dark conditions, the surface electrons of HCN are depleted and the surface of TCN is enriched with electrons, indicating that electrons are shifting from HCN to TCN. However, under light conditions, the electron transfer paths are completely opposite (Figure 5c). These results further support that the photogenerated electrons at the TCN HCN interface follow the S-type transfer mechanism.
Figure 6Effect of S-type electron transfer on the photoreduction performance of CO2
In the absence of sacrificial agents and cocatalysts, the optimized ratio (TH1:4) TCN HCN homojunction had the best CO2 photoreduction activity, and the yields of CO and CH4 were 2 higher than those of bulk carbon nitride (BCN).09 and 945x (Figures 6a and b). In addition, the Th1:4 sample had the highest rate of electron consumption during CO2 photoreduction (Figure 6C). With the increase of the concentration of heptazine phase in the homojunction, the CO2 photoreduction activity first increased, and then began to decay gradually after reaching the threshold. When the ratio of triazine to heptazine reaches 1:4, the interfacial electric field strength is the strongest and the photocatalytic activity is the highest (Fig. 6d and e). The 13C isotope tracking experiment showed that the CH4 and CO produced by the photoreduction experiment were from the 13CO2 reaction gas (Figure 6F).
Bibliographic information
understanding the unique s-scheme charge migration in triazine/heptazine crystalline carbon nitride homojunction. nat commun 14, 3901 (2023).