Hot Electrons Induced by Localized Surface Plasmon Resonance in Ag/g-C3N4 Schottky Junction for Photothermal Catalytic CO2 Reduction

Converting carbon dioxide (CO2) into high-value-added chemicals using solar energy is a promising approach to reducing carbon dioxide emissions; however, single photocatalysts suffer from quick the recombination of photogenerated electron–hole pairs and poor photoredox ability. Herein, silver (Ag) nanoparticles featuring with localized surface plasmon resonance (LSPR) are combined with g-C3N4 to form a Schottky junction for photothermal catalytic CO2 reduction. The Ag/g-C3N4 exhibits higher photocatalytic CO2 reduction activity under UV-vis light; the CH4 and CO evolution rates are 10.44 and 88.79 µmol·h−1·g−1, respectively. Enhanced photocatalytic CO2 reduction performances are attributed to efficient hot electron transfer in the Ag/g-C3N4 Schottky junction. LSPR-induced hot electrons from Ag nanoparticles improve the local reaction temperature and promote the separation and transfer of photogenerated electron–hole pairs. The charge carrier transfer route was investigated by in situ irradiated X-ray photoelectron spectroscopy (XPS). The three-dimensional finite-difference time-domain (3D-FDTD) method verified the strong electromagnetic field at the interface between Ag and g-C3N4. The photothermal catalytic CO2 reduction pathway of Ag/g-C3N4 was investigated using in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS). This study examines hot electron transfer in the Ag/g-C3N4 Schottky junction and provides a feasible way to design a plasmonic metal/polymer semiconductor Schottky junction for photothermal catalytic CO2 reduction.


Introduction
Gradually increasing carbon dioxide (CO 2 ) content in the ambient atmosphere has resulted in the greenhouse effect, which has caused serious ecological damage and climate change [1][2][3][4].The conversion of carbon dioxide into high-value chemicals and energetic fuels to achieve carbon neutrality could contribute to a new balance of the carbon cycle.This could not only reduce carbon dioxide emissions, but also alleviate the energy crisis.It is a promising way to convert carbon dioxide into high-value-added chemicals and energycontaining fuels through solar energy [5][6][7][8].Photocatalysis combined with the additional driving force provided by the thermal effect is beneficial to increasing CO 2 reduction and value-added product generation.Photothermal catalysis integrating photochemical and thermochemical processes is considered a promising technology to convert solar energy into chemical energy [9][10][11][12].
Compared with the thermocatalytic process, which consumes considerable energy, photothermal catalysis induced by the photothermal effect is energy-saving and environmentally friendly [13][14][15][16].Photothermal catalytic CO 2 reduction could not only improve the utilization of solar energy but also promote the improvement of catalytic efficiency.
Polymers 2024, 16, 2317 2 of 13 Heat generated by the photothermal effect promotes the photocatalytic CO 2 reaction process through thermochemical pathways, and photon energy also significantly contributes to reaction activity, resulting in a synergistic effect of thermochemical and photochemical pathways that is different to simply adding these two pathways together [17,18].Photothermal catalysts dissipate absorbed photon energy into thermal energy under light irradiation, which is conducive to the transfer of charge carriers and improves catalytic CO 2 reaction activity [19,20].Hot electrons are generated at photothermal catalysts upon irradiation and then participate in the photocatalytic CO 2 reaction.Photothermal catalysis enhances the reaction activity because of the synergy between the photochemical process and the thermochemical process, thereby exhibiting excellent photothermal catalytic CO 2 reduction performance [21,22].
Single photocatalysts show unsatisfactory photocatalytic CO 2 reaction performances due to the rapid recombination of photogenerated electrons and holes.It is necessary to address the issue of rapid recombination of photogenerated electron-hole pairs to improve the photocatalytic CO 2 reaction performance.Designing plasmonic metal/semiconductor Schottky junction materials can improve the efficiency of spatial charge separation, thereby suppressing photogenerated electron and hole pair recombination and enhancing photocatalytic CO 2 reaction performance [23][24][25][26].Additionally, the plasmonic metal/semiconductor Schottky junction can be used for photothermal catalytic CO 2 reduction due to the photothermal effect induced by the localized surface plasmon resonance (LSPR) of plasmonic metal [27,28].Therefore, designing plasmonic metal/semiconductor Schottky junction materials has become a promising solution to realize photothermal catalytic CO 2 reduction.
As a promising polymer semiconductor, graphitic carbon nitride (g-C 3 N 4 ) has attracted increasing attention in photocatalytic CO 2 reduction due to its low cost, high thermal and chemical stability, easy preparation process, semiconductivity, and appropriate band gap [29][30][31][32].However, g-C 3 N 4 exhibits low photocatalytic CO 2 reduction activity owing to its rapid recombination of photogenerated electrons and holes and poor photoreduction ability.Noble metals can enhance the optical absorption of their adjacent semiconductors.Gold, silver, palladium, and other noble metals featuring LSPR can decay through the radiative pathway to create local thermal heating and electromagnetic field enhancements near the particle and generate hot electrons in a nonradiative way [33][34][35][36][37]. Hot electrons with high energy overstep the Schottky barrier at the interfaces of the plasmonic metal/polymer semiconductor Schottky junction.They can directly inject into the conduction band of polymer semiconductors to drive the surface photocatalytic CO 2 reduction [38,39].Thus, the coupling of g-C 3 N 4 and noble metals is expected to form a plasmonic metal/polymer semiconductor Schottky junction which can promote the separation and transfer of photogenerated electrons and holes, thereby exhibiting excellent photothermal catalytic CO 2 reduction performance.
Herein, the Ag/g-C 3 N 4 Schottky junction featuring LSPR is designed for photothermal catalytic CO 2 reduction.Compared with g-C 3 N 4 , the Ag/g-C 3 N 4 Schottky junction photocatalyst exhibits higher CH 4 (10.44 µmol•h −1 •g −1 ) and CO (88.79 µmol•h −1 •g −1 ) evolution rates with enhanced local reaction temperatures.The Ag/g-C 3 N 4 Schottky junction is designed for photothermal catalytic CO 2 reductions.Still, the underlying photothermal catalytic CO 2 reduction mechanism, including hot electron transfer routes for photothermal catalytic CO 2 reduction and the reduction of CO 2 to CH 4 and CO pathways, is unknown.Therefore, in situ irradiated X-ray photoelectron spectroscopy (XPS) was employed to investigate electron transfer in the Ag/g-C 3 N 4 Schottky junction, and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were applied to detect the photothermal catalytic CO 2 reduction pathway over the Ag/g-C 3 N 4 photocatalyst.Three-dimensional finite-difference time-domain (3D-FDTD) verified the strong electromagnetic field at the interface between Ag and g-C 3 N 4 .Efficient hot electron transfer in the Ag/g-C 3 N 4 Schottky junction improves the local reaction temperature and promotes the separation and transfer of photogenerated charge carriers, thereby enhancing photocatalytic CO 2 reduction performance.This study reveals the efficient electron transfer Polymers 2024, 16, 2317 3 of 13 and photothermal CO 2 reduction mechanism of the Ag/g-C 3 N 4 Schottky junction and provides a feasible way to design a plasmonic metal/polymer semiconductor Schottky junction for photothermal catalytic CO 2 reduction.

Materials
Melamine (C 3 H 6 N 6 , 99%) was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China.Silver nitrate (AgNO 3 , 99.99%) was bought from China National Pharmaceutical Group Chemical Reagent Co., Ltd., Beijing, China.Deionized water was purchased from Sinopharm Chemical Reagent Co., Ltd, Beijing, China.All chemicals were used without further purification.

Synthesis of Ag/g-C 3 N 4
An amount of 5 g of melamine was put into the crucible, and then calcinated at 500 • C for 6 h with a ramp rate of 2 • C min −1 in a muffle furnace.After cooling to room temperature, bulk g-C 3 N 4 was obtained.The bulk g-C 3 N 4 was ground with a mortar to obtain the g-C 3 N 4 (CN) powder.Next, 1 g of g-C 3 N 4 powder and 0.05 g of AgNO 3 were dispersed into 30 mL of deionized water under magnetic stirring for 5 h.Then, the above-obtained suspension was irradiated by a Xenon lamp under magnetic stirring for 6 h.Finally, the mixture was dried in a baking oven at 60 • C for 24 h to obtain Ag/CN-1.Meanwhile, 1 g of g-C 3 N 4 powder and 0.1g of AgNO 3 were dispersed into 30 mL of deionized water under magnetic stirring for 5 h.Then, the above-obtained suspension was irradiated by a Xenon lamp under magnetic stirring for 6 h.Finally, the mixture was dried in a baking oven at 60 • C for 24 h to obtain Ag/CN-2.

Photocatalytic CO 2 Reduction
We added 50 mg of catalyst (CN, Ag/CN-1, and Ag/CN-2) into a 200 mL stainless steel reactor with an optical quartz window at the top.The reactor was first vacuumed, and then H 2 and CO 2 were introduced into the stainless steel reactor at a volume ratio of 4:1 for an hour to blow out the air in the reactor.A 300 W Xenon lamp (PLS-SXE300, Beijing PerfectLight, Beijing, China) with a filter (AM 1.5 G, Ceaulight Technology Co., Ltd., Beijing, China) was employed to simulate solar illumination for photocatalytic CO 2 reduction with about 100 mW•cm −2 .A Xenon lamp irradiated the reactor for 3 h.During the photocatalytic CO 2 reduction process, the gaseous mixture was periodically sampled from the stainless steel reactor every 0.5 h and analyzed using gas chromatography (GC 9790 II, Fuli Instruments, Wenling, China).

Microstructure and Physical Properties Analysis
The preparation process of Ag/g-C 3 N 4 is illustrated in Figure 1a.Firstly, melamine is calcined to become g-C 3 N 4 at high temperatures.Afterward, the Ag nanoparticles are coated on the surface of the g-C 3 N 4 to form the Ag/g-C 3 N 4 Schottky junction.The fieldemission scanning electron microscopy (FSEM) was employed to study the microstructure of the photocatalysts.As shown in Figure 1b, the g-C 3 N 4 shows a bulk structure with a size of 5 µm.As Figure 1c,d shows, the Ag nanoparticles are highly dispersed on the surface of the g-C 3 N 4 , indicating the loading of Ag nanoparticles on the g-C 3 N 4 .The high-resolution TEM (HRTEM) image shows that the lattice-fringe spacing of 0.23 nm is indexed into the (111) plane of Ag (Figure 1e), implying the existence of Ag nanoparticles in the Ag/g-C 3 N 4 Schottky junction.
the (111) plane of Ag (Figure 1e), implying the existence of Ag nanoparticles in the Ag/g-C3N4 Schottky junction.X-ray powder diffraction (XRD) patterns were applied to investigate the phase structures of the photocatalysts.As shown in Figure 1f, all the CN, Ag/CN-1, and Ag/CN-2 peaks correspond to the typical diffraction pattern of g-C3N4 (PDF#87-1526) [29,40,41].The crystallinity percentages of different g-C3N4-based photocatalysts are the same [42].The diffraction peaks of Ag are not observed in the CN, Ag/CN-1, and Ag/CN-2 samples because of the low content of Ag.The above results suggest that the Ag/g-C3N4 Schottky junction photocatalyst was successfully prepared.The nitrogen adsorption-desorption isotherms of CN, Ag/CN-1, and Ag/CN-2 display typical type IV isotherms with an H3 hysteresis loop (Figure S1).Brunauer-Emmett-Teller surface areas (SBET) of CN, Ag/CN-1, and Ag/CN-2 are 13.5, 12.2, and 11.4 m 2 •g −1 , respectively (Table S1).The light absorption abilities of CN, Ag/CN-1, and Ag/CN-2 were measured using UV-vis diffuse reflectance spectra (UV-vis DRS spectra).In Figure 1g, the light adsorption intensities of Ag/CN-1 and Ag/CN-2 are higher than those of CN, especially in the visible light region.Notably, compared with the CN, the absorption edge of Ag/CN-1 and Ag/CN-2 shows no shift.The result of UV-vis DRS spectra suggests that loading Ag nanoparticles on the g-C3N4 improves visible light adsorption ability, which is advantageous for photocatalytic CO2 reduction.

Photothermal Catalytic CO2 Reduction Performance
The photothermal catalytic CO2 reduction performances of CN, Ag/CN-1, and Ag/CN-2 photocatalysts were measured under UV-vis light.As shown in Figure 2a, CH4 and CO are the major photoreduction products in the photocatalytic CO2 reduction processes of CN, Ag/CN-1, and Ag/CN-2.The CH4 and CO evolution rates of Ag/CN-1 and Ag/CN-2 are higher than those of CN, indicating that the loading of Ag nanoparticles enhanced the photocatalytic CO2 reduction performance.The CH4 and CO evolution rates of Ag/CN-2 are up to 10.44 and 88.79 µmol•h −1 •g −1 , respectively, which are higher than those of CN and Ag/CN-1.The CH4 and CO evolution rates of CN are 4.17 and 35.51  S1).The light absorption abilities of CN, Ag/CN-1, and Ag/CN-2 were measured using UV-vis diffuse reflectance spectra (UV-vis DRS spectra).In Figure 1g, the light adsorption intensities of Ag/CN-1 and Ag/CN-2 are higher than those of CN, especially in the visible light region.Notably, compared with the CN, the absorption edge of Ag/CN-1 and Ag/CN-2 shows no shift.The result of UV-vis DRS spectra suggests that loading Ag nanoparticles on the g-C 3 N 4 improves visible light adsorption ability, which is advantageous for photocatalytic CO 2 reduction.

Photothermal Catalytic CO 2 Reduction Performance
The photothermal catalytic CO 2 reduction performances of CN, Ag/CN-1, and Ag/CN-2 photocatalysts were measured under UV-vis light.As shown in Figure 2a, CH 4 and CO are the major photoreduction products in the photocatalytic CO 2 reduction processes of CN, Ag/CN-1, and Ag/CN-2.The CH 4 and CO evolution rates of Ag/CN-1 and Ag/CN-2 are higher than those of CN, indicating that the loading of Ag nanoparticles enhanced the photocatalytic CO 2 reduction performance.The CH 4 and CO evolution rates of Ag/CN-2 are up to 10.44 and 88.79 µmol•h −1 •g −1 , respectively, which are higher than those of CN and Ag/CN-1.The CH 4 and CO evolution rates of CN are 4.17 and 35.51 µmol•h −1 •g −1 , respectively.The CH 4 and CO evolution rates of g Ag/CN-1 are 6.26 and 53.27 µmol•h −1 •g −1 , respectively.The CH 4 evolution rates of Ag/CN-2 are 2.5 and 1.7 times higher than those of CN and Ag/CN-1, respectively.The CO evolution rates of Ag/CN-2 are 2.5 and 1.7 times higher than those of CN and Ag/CN-1, respectively.The CH 4 and CO evolution rates of Ag/CN-2 grow nearly linearly with time (Figure 2b,c) and are much higher than those of CN and Ag/CN-1 during the photothermal catalytic CO 2 reduction process.Compared with g-CN and Ag/CN-1, Ag/CN-2 exhibits excellent photothermal catalytic CO 2 reduction performance.The photothermal catalytic CO 2 reduction performance of Ag/CN-2 is higher than that of many other g-C 3 N 4 -based photocatalytic materials under similar conditions (Table S2, Supporting Information).For instance, the CO evolution rates of Na 3 PO 4 /g-C 3 N 4 and coral tubular g-C 3 N 4 are 7.33 and 5.38 µmol•h −1 •g −1 , respectively.The CH 4 and CO yield of Ag/CN-2 during the cycled stable test (Figure 2d) and the XRD patterns hardly change before and after the stability tests involving Ag/CN-2 (Figure S2), indicating that the structure of Ag/CN-2 remains stable during the photothermal catalytic CO 2 reduction.2024, 16, x FOR PEER REVIEW 5 of 13 µmol•h −1 •g −1 , respectively.The CH4 and CO evolution rates of g Ag/CN-1 are 6.26 and 53.27 µmol•h −1 •g −1 , respectively.The CH4 evolution rates of Ag/CN-2 are 2.5 and 1.7 times higher than those of CN and Ag/CN-1, respectively.The CO evolution rates of Ag/CN-2 are 2.5 and 1.7 times higher than those of CN and Ag/CN-1, respectively.The CH4 and CO evolution rates of Ag/CN-2 grow nearly linearly with time (Figure 2b,c) and are much higher than those of CN and Ag/CN-1 during the photothermal catalytic CO2 reduction process.Compared with g-CN and Ag/CN-1, Ag/CN-2 exhibits excellent photothermal catalytic CO2 reduction performance.The photothermal catalytic CO2 reduction performance of Ag/CN-2 is higher than that of many other g-C3N4-based photocatalytic materials under similar conditions (Table S2, Supporting Information).For instance, the CO evolution rates of Na3PO4/g-C3N4 and coral tubular g-C3N4 are 7.33 and 5.38 µmol•h −1 •g −1 , respectively.The CH4 and CO yield of Ag/CN-2 during the cycled stable test (Figure 2d) and the XRD patterns hardly change before and after the stability tests involving Ag/CN-2 (Figure S2), indicating that the structure of Ag/CN-2 remains stable during the photothermal catalytic CO2 reduction.From Figure 2e-g, the local temperatures of Ag/CN-1 (53.1 • C) and Ag/CN-2 (70.6 • C) reaction systems are higher than that of CN (46.3 • C).The higher reaction temperatures of Ag/CN-1 and Ag/CN-2 in comparison to that of CN imply that Ag nanoparticles increase visible light absorption and generate hot electrons by LSPR, thereby increasing the local reaction temperature.Thus, the enhanced CH 4 and CO evolution rates of Ag/CN-2 could be related to the hot electrons induced by the LSPR of Ag nanoparticles.The CH 4 and CO evolution rates of Ag/CN-2 indicate that the efficient hot electron transfer in the Ag/g-C 3 N 4 Schottky junction improves photocatalytic CO 2 reduction activity.

Hot Electron Transfer Route
In order to investigate the mechanism of hot electrons induced by localized surface plasmon resonance in the Ag/g-C 3 N 4 Schottky junction for photothermal catalytic CO 2 reduction, X-ray photoelectron spectroscopy (XPS) was applied to identify the electronic chemical states and electron transfer of the photocatalyst.The XPS spectra show that C, N, and Ag elements can be detected in the Ag/g-C 3 N 4 , indicating the successful construction of the Ag/g-C 3 N 4 Schottky junction.As presented in Figure 3a, the peaks for the g-C 3 N 4, located at 284.8 and 288.1 eV, can be attributed to C-C and N-C=C of C, respectively.Furthermore, the peaks centered at 398.7, 399.8, and 401.0 eV are assigned to C-N=C, N-(C) 3, , and C-N-H, respectively (Figure 3b) [29,43].In general, the change in electron binding energy reflects the change in electron density.Therefore, the change in electron binding energy can verify the direction of electron transfer in the Schottky junction photocatalyst [44][45][46].As Figure 3a,b show, the binding energies of C-C and N-C=C in Ag/g-C 3 N 4 shift toward higher energy levels, indicating that g-C 3 N 4 loses electrons.Meanwhile, the binding energy of C-N=C, N-(C) 3, , and C-N-H in Ag/g-C 3 N 4 shifts to a higher energy level, implying that g-C 3 N 4 loses electrons.The peaks centered at 368.4 and 374.4 eV are assigned to Ag 3d5/2 and Ag 3d3/2 in Ag/g-C 3 N 4 , respectively (Figure 3c) [47,48].The electrons transfer from g-C 3 N 4 to Ag in the Ag/g-C 3 N 4 Schottky junction.In contrast, the binding energies of C 1s and N 1s in Ag/g-C 3 N 4 shift toward lower energy levels upon light irradiation, while the binding energies of Ag 3d shift to a higher energy level.The in situ high-resolution XPS spectra indicate that the hot electrons transfer from Ag to g-C 3 N 4 .Therefore, the electrons firstly transfer from g-C 3 N 4 to Ag, and the hot electrons migrate from Ag to g-C 3 N 4 in the Ag/g-C 3 N 4 Schottky junction upon irradiation.
Based on the results of in situ irradiated XPS, the electron and hot electron transfer routes between g-C 3 N 4 and Ag in the Ag/g-C 3 N 4 Schottky junction are summarized.The work functions (Φ) of Ag and g-C 3 N 4 were calculated using density functional theory (DFT).Figure 3d,e show that the work function of g-C 3 N 4 (0.174 Ha) is smaller than that of Ag (0.201 Ha), which indicates that the Fermi level of Ag is lower than that of g-C 3 N 4 .As shown in Figure 3f, before the contact of Ag and g-C 3 N 4 , g-C 3 N 4 has a smaller W 1 and higher E f1 , while Ag has a larger W 2 and lower E f2 .The difference in work function will lead to band bending at the interface between Ag and g-C 3 N 4 .After Ag makes contact with g-C 3 N 4 , driven by band bending, the electrons transfer from the g-C 3 N 4 to the Ag.When the Ag/g-C 3 N 4 Schottky junction is formed upon irradiation, the electrons are excited from the VB to the CB of g-C 3 N 4 , and hot electrons are generated at the surface of Ag.Then, the hot electrons can overstep the Schottky barrier and transfer from Ag into the CB of g-C 3 N 4 at the interface because hot electrons possess higher energies than those of normally excited electrons, leading to the efficient separation and transfer of photogenerated electron-hole pairs in g-C 3 N 4 .The efficient separation and transfer of photogenerated electron-hole pairs in the Ag/g-C 3 N 4 Schottky junction improved the photocatalytic CO 2 reduction performance.Electrochemical impedance is the electrical resistance that occurs during charge carrier transport in photocatalysts [42].The photocurrent in semiconductors is caused by light irradiation.The photons excite the electrons in the valence band of the semiconductor to the conduction band and generate an electric current upon irradiation.Electrochemical impedance spectra (EIS) and transient photocurrent (TPC) spectra can be employed to investigate the charge carriers separation and transfer behavior in photocatalysts.As shown in Figure 4a, the photocurrent densities of Ag/CN-1 or Ag/CN-2 are higher than that of CN.Ag/CN-1 and Ag/CN-2 show a smaller electrochemical impedance spectroscopy radius than that of CN (Figure 4b).The TPC and EIS results illustrate that the hot electrons of Ag migrate to g-C3N4, improving the efficient photogenerated charge carrier separation and transfer in the Ag/g-C3N4 Schottky junction.
To better determine the electromagnetic field and electric-field vector of Ag/g-C3N4, three-dimensional finite-difference time-domain (3D-FDTD) simulations were performed to calculate the spatial electric field distributions as a function of the incident wavelength of light.The FDTD simulation model of Ag/g-C3N4 is illustrated in Figure 5a.The electromagnetic field of Ag nanoparticle loading at the g-C3N4 is shown in Figure 5c.The Electrochemical impedance is the electrical resistance that occurs during charge carrier transport in photocatalysts [42].The photocurrent in semiconductors is caused by light irradiation.The photons excite the electrons in the valence band of the semiconductor to the conduction band and generate an electric current upon irradiation.Electrochemical impedance spectra (EIS) and transient photocurrent (TPC) spectra usually can be employed to investigate the charge carriers separation and transfer behavior in photocatalysts.As shown in Figure 4a, the photocurrent densities of Ag/CN-1 or Ag/CN-2 are higher than that of CN.Ag/CN-1 and Ag/CN-2 show a smaller electrochemical impedance spectroscopy radius than that of CN (Figure 4b).The TPC and EIS results illustrate that the hot electrons of Ag migrate to g-C 3 N 4, improving the efficient photogenerated charge carrier separation and transfer in the Ag/g-C 3 N 4 Schottky junction.
To better determine the electromagnetic field and electric-field vector of Ag/g-C 3 N 4 , three-dimensional finite-difference time-domain (3D-FDTD) simulations were performed to calculate the spatial electric field distributions as a function of the incident wavelength of light.The FDTD simulation model of Ag/g-C 3 N 4 is illustrated in Figure 5a.The electromagnetic field of Ag nanoparticle loading at the g-C 3 N 4 is shown in Figure 5c.
The electromagnetic field at the interface between the Ag nanoparticle and the g-C 3 N 4 is stronger than that of the g-C 3 N 4 (Figure 5b) under the excitation of visible light, which indicates that more hot electrons have emerged at the interface between the Ag nanoparticle and the g-C 3 N 4 .Figure 5d shows the electric-field vector for Ag nanoparticle loading at g-C 3 N 4 , indicating that electrons have transferred from the TiO 2 to the Pt nanoparticle; hot electrons could then be generated on the surface of the Ag nanoparticle and transferred from the Ag nanoparticle to the g-C 3 N 4 .The FDTD simulation results are very consistent with the experimental and characterization results.
Polymers 2024, 16, x FOR PEER REVIEW 8 of 13 electromagnetic field at the interface between the Ag nanoparticle and the g-C3N4 is stronger than that of the g-C3N4 (Figure 5b) under the excitation of visible light, which indicates that more hot electrons have emerged at the interface between the Ag nanoparticle and the g-C3N4.Figure 5d shows the electric-field vector for Ag nanoparticle loading at g-C3N4, indicating that electrons have transferred from the TiO2 to the Pt nanoparticle; hot electrons could then be generated on the surface of the Ag nanoparticle and transferred from the Ag nanoparticle to the g-C3N4.The FDTD simulation results are very consistent with the experimental and characterization results.Polymers 2024, 16, x FOR PEER REVIEW 8 of 13 electromagnetic field at the interface between the Ag nanoparticle and the g-C3N4 is stronger than that of the g-C3N4 (Figure 5b) under the excitation of visible light, which indicates that more hot electrons have emerged at the interface between the Ag nanoparticle and the g-C3N4.Figure 5d shows the electric-field vector for Ag nanoparticle loading at g-C3N4, indicating that electrons have transferred from the TiO2 to the Pt nanoparticle; hot electrons could then be generated on the surface of the Ag nanoparticle and transferred from the Ag nanoparticle to the g-C3N4.The FDTD simulation results are very consistent with the experimental and characterization results.

Photothermal Catalytic CO2 Reduction Mechanism
In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were applied to investigate the photothermal catalytic CO2 reduction reaction pathway and possible reaction carbon intermediates on the surface of the Ag/g-C3N4 photocatalyst.As shown in Figure 6, DRIFTS spectra of Ag/g-C3N4 in dark and light were collected in the photothermal catalytic CO2 reduction process.Initially, no absorption peaks can be detected in the spectrum of the Ag/g-C3N4 photocatalyst in the dark (0 min).With the introduction of CO2 and H2 (10-20 min), monodentate carbonate species (m-CO3 2− , 1303, 1419, 1484, and 1558 cm −1 ) and bidentate carbonate species (b-CO3 2− , 1311,1523, and 1650 cm −1 ) appear in the spectra [49][50][51][52]  Thus, the probable mechanism of the Ag/g-C3N4 Schottky junction for photothermal catalytic CO2 reduction can be proposed (Figure 7).Under light irradiation, photogenerated electrons are excited from the valence band to the conduction band of g-C3N4, while hot electrons are generated at the surface of Ag due to LSPR.Then, hot electrons at the surface of Ag can overstep the Schottky barrier and transfer from Ag to the CB of g-C3N4, promoting the separation and transfer of photogenerated electron-hole pairs in the Ag/g-C3N4 Schottky junction.Meanwhile, H2 is oxidized into H + on the VB, and CO2 is reduced into CH4 and CO on the CB.Additionally, the transfer of hot electrons from Ag to g-C3N4 improves the local reaction temperature, thereby enhancing the photocatalytic CO2 reduction performance.The efficient hot electron transfer in the Ag/g-C3N4 Schottky junction enhances the photocatalytic CO2 reduction performance.Thus, the probable mechanism of the Ag/g-C 3 N 4 Schottky junction for photothermal catalytic CO 2 reduction can be proposed (Figure 7).Under light irradiation, photogenerated electrons are excited from the valence band to the conduction band of g-C 3 N 4 , while hot electrons are generated at the surface of Ag due to LSPR.Then, hot electrons at the surface of Ag can overstep the Schottky barrier and transfer from Ag to the CB of g-C 3 N 4 , promoting the separation and transfer of photogenerated electron-hole pairs in the Ag/g-C 3 N 4 Schottky junction.Meanwhile, H 2 is oxidized into H + on the VB, and CO 2 is reduced into CH 4 and CO on the CB.Additionally, the transfer of hot electrons from Ag to g-C 3 N 4 improves the local reaction temperature, thereby enhancing the photocatalytic CO 2 reduction performance.The efficient hot electron transfer in the Ag/g-C 3 N 4 Schottky junction enhances the photocatalytic CO 2 reduction performance.

Figure 1 .
Figure 1.(a) Scheme of the preparation of Ag/g-C 3 N 4 ; FSEM of (b) g-C 3 N 4 and (c,d) Ag/g-C 3 N 4 ; (e) high-resolution TEM (HRTEM) images of Ag/g-C 3 N 4 ; (f) XRD patterns of g-C 3 N 4 , TiO 2 and g-C 3 N 4 /TiO 2 ; (g) UV-vis DRS of CN, Ag/CN-1 and Ag/CN-2.X-ray powder diffraction (XRD) patterns were applied to investigate the phase structures of the photocatalysts.As shown in Figure1f, all the CN, Ag/CN-1, and Ag/CN-2 peaks correspond to the typical diffraction pattern of g-C 3 N 4 (PDF#87-1526)[29,40,41].The crystallinity percentages of different g-C 3 N 4 -based photocatalysts are the same[42].The diffraction peaks of Ag are not observed in the CN, Ag/CN-1, and Ag/CN-2 samples because of the low content of Ag.The above results suggest that the Ag/g-C 3 N 4 Schottky junction photocatalyst was successfully prepared.The nitrogen adsorption-desorption isotherms of CN, Ag/CN-1, and Ag/CN-2 display typical type IV isotherms with an H 3 hysteresis loop (FigureS1).Brunauer-Emmett-Teller surface areas (S BET ) of CN, Ag/CN-1, and Ag/CN-2 are 13.5, 12.2, and 11.4 m 2 •g −1 , respectively (TableS1).The light absorption abilities of CN, Ag/CN-1, and Ag/CN-2 were measured using UV-vis diffuse reflectance spectra (UV-vis DRS spectra).In Figure1g, the light adsorption intensities of Ag/CN-1 and Ag/CN-2 are higher than those of CN, especially in the visible light region.Notably, compared with the CN, the absorption edge of Ag/CN-1 and Ag/CN-2 shows no shift.The result of UV-vis DRS spectra suggests that loading Ag nanoparticles on the g-C 3 N 4 improves visible light adsorption ability, which is advantageous for photocatalytic CO 2 reduction.

Figure 3 .
Figure 3.In situ high-resolution XPS spectra of (a) C 1s, (b) N 1s, and (c) Ag 3d of g-C3N4 and Ag/g-C3N4; work functions of (d) Ag and (e) g-C3N4; (f) schematic illustrations of electron transfer mechanism between g-C3N4 and Ag before contact, after contact, and after contact under light irradiation.

Figure 3 .
Figure 3.In situ high-resolution XPS spectra of (a) C 1s, (b) N 1s, and (c) Ag 3d of g-C 3 N 4 and Ag/g-C 3 N 4 ; work functions of (d) Ag and (e) g-C 3 N 4 ; (f) schematic illustrations of electron transfer mechanism between g-C 3 N 4 and Ag before contact, after contact, and after contact under light irradiation.

Figure
Figure In situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) of Ag/g-C 3 N 4 .