Plasma Ag-Modified α-Fe2O3/g-C3N4 Self-Assembled S-Scheme Heterojunctions with Enhanced Photothermal-Photocatalytic-Fenton Performances

Low spectral utilization and charge carrier compounding limit the application of photocatalysis in energy conversion and environmental purification, and the rational construction of heterojunction is a promising strategy to break this bottleneck. Herein, we prepared surface-engineered plasma Ag-modified α-Fe2O3/g-C3N4 S-Scheme heterojunction photothermal catalysts by electrostatic self-assembly and light deposition strategy. The local surface plasmon resonance effect induced by Ag nanoparticles broadens the spectral response region and produces significant photothermal effects. The temperature of Ag/α-Fe2O3/g-C3N4 powder is increased to 173 °C with irradiation for 90 s, ~3.2 times higher than that of the original g-C3N4. The formation of 2D/2D structured S-Scheme heterojunction promotes rapid electron-hole transfer and spatial separation. Ternary heterojunction construction leads to significant enhancement of photocatalytic performance of Ag/α-Fe2O3/g-C3N4, the H2 photocatalytic generation rate up to 3125.62 µmol g−1 h−1, which is eight times higher than original g-C3N4, and the photocatalytic degradation rate of tetracycline to reach 93.6%. This thermally assisted photocatalysis strategy improves the spectral utilization of conventional photocatalytic processes and provides new ideas for the practical application of photocatalysis in energy conversion and environmental purification.


Introduction
Energy crisis and environmental pollution have become urgent problems to be solved, and photocatalytic technology is a green way to tackle them [1][2][3][4]. The core issue of photocatalysis is to increase the activity of the photocatalysts, and many semiconductor materials have been used in photocatalysis so far, such as TiO 2 , CdS, MoSe 2 , g-C 3 N 4 , and perovskite-type composite oxide. Among them, g-C 3 N 4 is a novel nonmetallic polymer semiconductor, which has been widely used in photocatalysis because of its suitable forbidden bandwidth and tunable energy band structure [5][6][7][8]. The architecture of g-C 3 N 4 is similar to that of graphene, and it has excellent thermal and chemical stability, which is attributed to the aromatic conjugated system formed by C-N heterocycles and van der Waals forces of interlayer interaction [9,10]. However, for a single g-C 3 N 4 photocatalyst, the high exciton binding energy makes the photo-generated electrons and holes generated by photoexcitation easily complexed, and the oxidation capacity of the holes on its valence band is not sufficient to generate strongly oxidizing hydroxyl radicals, which is extremely unfavorable for photocatalytic reactions [11,12]. A new S-Scheme heterojunction was Moreover, as a narrow bandgap semiconductor, α-Fe 2 O 3 can effectively broaden the spectral absorption interval of g-C 3 N 4 , improve the spectral utilization of sunlight, and enhance the photothermal conversion effect of the catalyst [28,29]. Therefore, the photocatalytic-Fenton coupling system composed of α-Fe 2 O 3 /g-C 3 N 4 composites can further improve its photocatalytic performance in theory [30,31].
As a promising photocatalytic technique, surface plasmon resonance has received great attention in recent years [32][33][34][35]. Plasma photocatalysis can extend the absorption range of light through localized surface plasmon resonance (LSPR) [36][37][38]. LSPR of noble metal nanoparticles creates an enhanced surrounding local electric field that can linearly increase the concentration of photogenerated carriers. The effect of the enhanced electric field can also extend to the space charge layer of the adjacent semiconductors, increasing the concentration of photogenerated carriers near the semiconductor surface [39,40]. In addition, the Schottky barrier between the metal nanoparticles and the semiconductor can promote the transfer of the charge carriers to opposite directions, further injecting energetic (hot) electrons into the semiconductor, causing a much higher concentration of energetic electrons on the surface of the photocatalyst [41][42][43]. The energy of the oscillating electrons excited by plasma resonance can be decayed by non-radiative pathways, and the high-energy electrons are coupled to the phonon mode heating the metal lattice through electron-phonon scattering. This heat is eventually diffused into the environment by phonon-phonon relaxation, creating a local thermal effect at the catalyst interface, which speeds up the reaction rate on the catalyst surface [44,45]. In addition, the local electric field generated by LSPR of noble metals facilitates the polarized adsorption of reactant molecules on the surface of the photocatalytic material, which promotes the catalytic activity of the catalyst. Ag has been widely used in photocatalysis because of its cost advantage, abundant reserves, stable nature, and unique antibacterial properties. For instance, Xing et al. introduced silver nanoparticles into the catalytic systems of titanium dioxide and molybdenum sulfide, which showed enhanced light absorption and photocatalytic ac-tivity [46,47]. Therefore, the introduction of silver nanoparticles is the preferred way to construct efficient photocatalytic materials.
In this work, Ag/α-Fe 2 O 3 /g-C 3 N 4 ternary composite photocatalysts were prepared by electrostatic self-assembly and light deposition strategy. The construction of S-Scheme heterojunction in Ag/α-Fe 2 O 3 /g-C 3 N 4 can effectively drive photogenerated carrier separation and transfer. Ag nanoparticles can expand the light absorption range, provide additional active sites, and also produce excellent photothermal effects through LSPR. The prepared Ag/α-Fe 2 O 3 /g-C 3 N 4 shows much higher performance than pristine g-C 3 N 4 for hydrogen production and degradation of tetracycline (TC). The special 2D/2D combination and charge carriers transfer pathway of Ag/α-Fe 2 O 3 /g-C 3 N 4 were verified by detailed characterization, and the enhanced photothermal effect of Ag/α-Fe 2 O 3 /g-C 3 N 4 ternary photocatalysts in both gas-phase and liquid-phase was also demonstrated by photothermal testing, then we proposed a synergistic photothermal-photocatalytic-Fenton catalytic mechanism. This work provides a new option for the design of cost-effective, highly active catalysts, as well as a potential way to solve the growing energy crisis and environmental pollution problems.

Experimental Section
The preparation process of Ag/α-Fe 2 O 3 /g-C 3 N 4 ternary heterojunction photocatalyst is shown in Figure 1. First, the supramolecular precursors composed of melamine and cyanuric acid were calcined to obtain g-C 3 N 4 ultrathin nanosheets, and hexagonal α-Fe 2 O 3 nanosheets were prepared by one step hydrothermally. The zeta potentials of g-C 3 N 4 and α-Fe 2 O 3 were measured to be −23.9 mV and +28.3 mV, respectively ( Figure S1). g-C 3 N 4 and α-Fe 2 O 3 were assembled into heterojunctions driven by electrostatic forces between positive and negative charges, and then Ag/α-Fe 2 O 3 /g-C 3 N 4 was obtained by modifying Ag nanoparticles on the α-Fe 2 O 3 /g-C 3 N 4 surface via light deposition process. The specific experimental parameters and test procedures are detailed in the supplemental material. dioxide and molybdenum sulfide, which showed enhanced light absorption and photocatalytic activity [46,47]. Therefore, the introduction of silver nanoparticles is the preferred way to construct efficient photocatalytic materials.
In this work, Ag/α-Fe2O3/g-C3N4 ternary composite photocatalysts were prepared by electrostatic self-assembly and light deposition strategy. The construction of S-Scheme heterojunction in Ag/α-Fe2O3/g-C3N4 can effectively drive photogenerated carrier separation and transfer. Ag nanoparticles can expand the light absorption range, provide additional active sites, and also produce excellent photothermal effects through LSPR. The prepared Ag/α-Fe2O3/g-C3N4 shows much higher performance than pristine g-C3N4 for hydrogen production and degradation of tetracycline (TC). The special 2D/2D combination and charge carriers transfer pathway of Ag/α-Fe2O3/g-C3N4 were verified by detailed characterization, and the enhanced photothermal effect of Ag/α-Fe2O3/g-C3N4 ternary photocatalysts in both gas-phase and liquid-phase was also demonstrated by photothermal testing, then we proposed a synergistic photothermal-photocatalytic-Fenton catalytic mechanism. This work provides a new option for the design of cost-effective, highly active catalysts, as well as a potential way to solve the growing energy crisis and environmental pollution problems.

Experimental Section
The preparation process of Ag/α-Fe2O3/g-C3N4 ternary heterojunction photocatalyst is shown in Figure 1. First, the supramolecular precursors composed of melamine and cyanuric acid were calcined to obtain g-C3N4 ultrathin nanosheets, and hexagonal α-Fe2O3 nanosheets were prepared by one step hydrothermally. The zeta potentials of g-C3N4 and α-Fe2O3 were measured to be −23.9 mV and +28.3 mV, respectively ( Figure S1). g-C3N4 and α-Fe2O3 were assembled into heterojunctions driven by electrostatic forces between positive and negative charges, and then Ag/α-Fe2O3/g-C3N4 was obtained by modifying Ag nanoparticles on the α-Fe2O3/g-C3N4 surface via light deposition process. The specific experimental parameters and test procedures are detailed in the supplemental material.

Preparation of g-C 3 N 4 ultra-Thin Nanosheets
The bottom-up supramolecular self-assembly method was used to prepare g-C 3 N 4 ultrathin nanosheets. In a typical synthesis process, 0.01 mol melamine and 0.01 mol cyanuric acid were dissolved in 50 mL of deionized water and stirred at room temperature for 12 h after being mixed. Subsequently, the white melamine-melamine supramolecules were centrifuged and freeze-dried for 24 h. Then, they were warmed up to 550 • C in a muffle furnace at a rate of 5 • C min −1 and kept for 4 h. The light-yellow 3D g-C 3 N 4 was gained after cooling.

Preparation of Hexagonal α-Fe 2 O 3 Nanosheets
Hexagonal α-Fe 2 O 3 nanosheets were prepared by one-step solvothermal synthesis. In a typical synthesis, 2.0 mmol of FeCl 3 ·6H 2 O was dissolved into a mixed solution of 20 mL ethanol and 1.4 mL water under vigorous magnetic stirring, and 1.6 g of sodium acetate was added after complete dissolution of FeCl 3 ·6H 2 O and stirring was continued for 0.5 h. Subsequently, the homogeneous solution was transferred to a 50 mL Teflon-lined reactor sealed with a stainless-steel sheath and kept at 180 • C for 12 h. After cooling to room temperature, the resulting precipitate was rinsed several times with alternating deionized water and ethanol and dried overnight at 60 • C to obtain α-Fe 2 O 3 hexagonal nanosheets.

Preparation of α-Fe 2 O 3 /g-C 3 N 4 Heterojunction Photocatalyst
α-Fe 2 O 3 /g-C 3 N 4 heterojunction photocatalysts were prepared by electrostatic selfassembly method. A total of 50 mg of g-C 3 N 4 nanosheets were dispersed in 80 mL of deionized water. Then different amounts of α-Fe 2 O 3 powder were taken and ultrasonically dispersed in 50 mL of deionized water, and the obtained α-Fe 2 O 3 solution was added to the g-C 3 N 4 nanosheet suspension under vigorous stirring, and a uniform suspension was obtained after slow stirring for 6 h. After the suspension is filtered, it is washed several times with deionized water and methanol. The obtained mixed powder was dried overnight in an oven at 60 • C to obtain α-Fe 2 O 3 /g-C 3 N 4 heterojunction photocatalyst.

Preparation of Ag/α-Fe 2 O 3 /g-C 3 N 4 Ternary Composite Photocatalyst
Ag quantum dots were deposited on the surface of α-Fe 2 O 3 /g-C 3 N 4 using light deposition process. About 0.2 g of α-Fe 2 O 3 /g-C 3 N 4 was added to 100 mL of deionized water and stirred for 30 min, then different volumes of AgNO 3 (0.1 M) solution were added and the mixed solution was irradiated with a 300 W xenon lamp for 30 min under stirring. Finally, the mixed solution was filtered, washed several times with deionized water, and dried in an oven at 50 • C to obtain the Ag/α-Fe 2 O 3 /g-C 3 N 4 ternary product.

Experiment Characterizations
The X-ray diffractometer (XRD, Rigaku, Japan, λ = 1.5418 Å) was used to determine the phase structure of the prepared photocatalysts. The transmission electron microscope (TEM, JEM-2100, Japan) and scanning electron microscope (SEM, Nova nano SEM 450) were used to examine the microstructure of the photocatalysts, and the distribution of the elements was detected by TEM equipped with the energy dispersive spectrometer (EDS). X-ray photoelectron spectroscopy (XPS) was performed by a photoelectron spectrometer (Thermo ESCALAB 250Xi, USA). The ultraviolet-visible (UV-Vis) diffuse reflectance spectra of the catalysts were measured by a spectrophotometer (UV-2600i, Shimadzu, Japan). Photoluminescence (PL) spectra of the photocatalysts were measured with an FLS 1000 fluorescence spectrophotometer (Edinburgh Instruments). The work function of samples was tested by Scanning Kelvin Probe (SKP) (SKP5050 system, Scotland).

Photothermal Test
The photothermal test of as-prepared samples was carried out as follows. About 0.1 g of sample was laid out flat on white weighing paper ( Figure S2) and the initial temperature was controlled at room temperature. The temperature of the sample was Nanomaterials 2022, 12, 4212 5 of 17 measured using the Testo 865 infrared thermography. In the water temperature evolution test, 0.05 g photocatalyst was dispersed in 30 mL of water and kept magnetically stirred, and the corresponding temperature was recorded every 10 min by infrared thermal imager Testo 865. A 300 W xenon lamp (CEL-HXF300, Beijing China Education Au-light Co., Ltd., Beijing, China) was used as the light source for all the photothermal experiments, with a distance of approximately 20 cm between the light source and the sample.

Photocatalytic Performance Assessment
The photocatalytic pollutant degradation reactions were carried out on a multipurpose photochemical reaction system (CEL-LAB500E4, Beijing China Education Au-light Co., Ltd.). About 50 mg of the photocatalyst was added to 100 mL of a solution containing TC (10 mg/L). Before the photocatalytic experiments, the solution containing the pollutant and the photocatalyst was placed in a dark room for 30 min to get the adsorption-desorption equilibrium. Then, the solution was irradiated under a 300 W xenon lamp with circulating water jacket (CEL-HXF300, Beijing China Education Au-light Co., Ltd.). Every 20 min, 3 mL of each liquid sample was removed from the beaker and filtered with 0.22 µm Millipore filter heads. Concentrations were subsequently tested using a high-performance liquid chromatograph (LC-3100).
The photocatalytic hydrogen evolution reaction was performed on the all-glass automatic online trace gas analysis system (Labsolar-6A, Beijing Perfect light Technology Co., Ltd., Beijing, China). With Labsolar-6A, a 300 W Xenon lamp (Microsolar300, Beijing Perfect light Technology Co., Ltd.) was used as the simulated sunlight spectral source. The as-prepared catalyst (10 mg) was uniformly dispersed by using a magnetic stirrer in 120 mL of methanol solution (containing H 2 O/methanol, v/v = 90:30). The temperature of the reaction was kept at 298 K by cool flowing water. During the irradiation process, a hydrogen sample (0.5 mL) was extracted from the reactor at a given interval and the amount of hydrogen produced was analyzed by an online gas chromatograph (GC-7900).

Physical Phase Composition and Energy Band Structure
The crystal structures of g-C 3 N 4 nanosheets, hexagonal α-Fe 2 O 3 nanosheets, α-Fe 2 O 3 /g-C 3 N 4 2D/2D heterojunction and Ag/α-Fe 2 O 3 /g-C 3 N 4 ternary heterojunction were analyzed by XRD pattern, as shown in Figure 2a. The g-C 3 N 4 has two distinct peaks at 12.8 • and 27.7 • , which were derived from the repeating unit and interlayer stacking within the carbon nitride plane, corresponding to two crystal planes (100) and (002), respectively [48]. The characteristic peaks of α-Fe 2 O 3 at 24.1, 33. The characteristic peaks of g-C 3 N 4 and α-Fe 2 O 3 are observed in the α-Fe 2 O 3 /g-C 3 N 4 composite sample, indicating that the α-Fe 2 O 3 /g-C 3 N 4 heterojunction photocatalyst was successfully prepared. The diffraction peaks of Ag were not observed in the XRD patterns of the Ag/α-Fe 2 O 3 /g-C 3 N 4 ternary composites due to the low loading of Ag nanoparticles [40,43,47]. However, in the XPS with higher sensitivity, all the characteristic peaks of the α-Fe 2 O 3 and g-C 3 N 4 appeared in the XPS survey spectrum of Ag/α-Fe 2 O 3 /g-C 3 N 4 composites, and the characteristic peaks of Ag were observed near 367 and 373 eV, indicating that the Ag/α-Fe 2 O 3 /g-C 3 N 4 ternary composites were successfully prepared ( Figure 2b) [47,49]. The light absorption properties of each prepared catalytic material were characterized by UV-vis diffuse reflectance spectroscopy (UV-vis DRS), Figure 2c shows that the optical response interval of the α-Fe 2 O 3 /g-C 3 N 4 expands from 483 nm to 647 nm. The optical response of the Ag/α-Fe 2 O 3 /g-C 3 N 4 ternary composite interval was further extended to the NIR region due to the LSPR of Ag nanoparticles. The intrinsic band gaps of g-C 3 N 4 and α-Fe 2 O 3 were estimated from the Tauc plots in Figure 2d, and the forbidden bandwidths of 2.04 and 2.58 eV for g-C 3 N 4 and α-Fe 2 O 3 were calculated from the intercepts of extrapolated lines in the X-axis, respectively. The energy band struc-Nanomaterials 2022, 12, 4212 6 of 17 ture of Ag/α-Fe 2 O 3 /g-C 3 N 4 was determined by VB-XPS. The test results of the VB-XPS in Figure 2e show that the VB positions of α-Fe 2 O 3 and g-C 3 N 4 are 2.36 and 1.35 eV relative to the normal hydrogen electrode, respectively. The relative energy band positions of α-Fe 2 O 3 and g-C 3 N 4 can be calculated based on the relationship between the semiconductor VB and CB, as shown in Figure 2f, the energy band arrangement of α-Fe 2 O 3 and g-C 3 N 4 is similar to Type II heterojunction [50]. The g-C 3 N 4 possesses a higher conduction band position and is more electronically reductive, and the α-Fe 2 O 3 has a lower valence band position and strong hole oxidation, which means that α-Fe 2 O 3 and g-C 3 N 4 will form S-Scheme heterojunctions with both strong proton reduction and strong hole oxidation ability. was further extended to the NIR region due to the LSPR of Ag nanoparticles. The intrinsic band gaps of g-C3N4 and α-Fe2O3 were estimated from the Tauc plots in Figure 2d, and the forbidden bandwidths of 2.04 and 2.58 eV for g-C3N4 and α-Fe2O3 were calculated from the intercepts of extrapolated lines in the X-axis, respectively. The energy band structure of Ag/α-Fe2O3/g-C3N4 was determined by VB-XPS. The test results of the VB-XPS in Figure  2e show that the VB positions of α-Fe2O3 and g-C3N4 are 2.36 and 1.35 eV relative to the normal hydrogen electrode, respectively. The relative energy band positions of α-Fe2O3 and g-C3N4 can be calculated based on the relationship between the semiconductor VB and CB, as shown in Figure 2f, the energy band arrangement of α-Fe2O3 and g-C3N4 is similar to Type II heterojunction [50]. The g-C3N4 possesses a higher conduction band position and is more electronically reductive, and the α-Fe2O3 has a lower valence band position and strong hole oxidation, which means that α-Fe2O3 and g-C3N4 will form S-Scheme heterojunctions with both strong proton reduction and strong hole oxidation ability.

Morphological and Structural Analysis
The morphological and structural characteristics of different photocatalysts were analyzed by TEM and SEM. In Figure 3a, g-C3N4 exhibits a self-supporting nanosheet morphology with a thickness of about 10 nm, which is further observed by the nearly transparent TEM image ( Figure 3d). As shown in Figure 3b,g, the SEM and TEM images of α-Fe2O3 indicate that it is a hexagonal nanosheet with a size of about 220 nm and a thickness of about 20 nm. The SEM and TEM of α-Fe2O3/g-C3N4 composites are shown in Figure  3c,e, respectively, where two differently charged sheets of α-Fe2O3, g-C3N4 are closely adhered by electrostatic force. The hexagonal α-Fe2O3 nanosheets tightly adhere to the substrate composed of g-C3N4 with large size. This 2D/2D heterojunction shortens the distance that charge carriers have to migrate to the surface and facilitates the rapid involvement of carriers in chemical reactions [7]. Figure 3f clearly shows that Ag nanoparticles are dispersed on the α-Fe2O3/g-C3N4 surface. Figure 3h,i is high-resolution TEM images of the circled region, respectively, which show that the fully exposed active crystal plane of α-Fe2O3 is the (110) plane, while the 0.24 nm lattice spacing corresponds to the (111) crystal plane of Ag [51,52]. Figure 3j-n shows the energy dispersive X-ray spectroscopy (EDX) of

Morphological and Structural Analysis
The morphological and structural characteristics of different photocatalysts were analyzed by TEM and SEM. In Figure 3a, g-C 3 N 4 exhibits a self-supporting nanosheet morphology with a thickness of about 10 nm, which is further observed by the nearly transparent TEM image ( Figure 3d). As shown in Figure 3b,g, the SEM and TEM images of α-Fe 2 O 3 indicate that it is a hexagonal nanosheet with a size of about 220 nm and a thickness of about 20 nm. The SEM and TEM of α-Fe 2 O 3 /g-C 3 N 4 composites are shown in Figure 3c,e, respectively, where two differently charged sheets of α-Fe 2 O 3 , g-C 3 N 4 are closely adhered by electrostatic force. The hexagonal α-Fe 2 O 3 nanosheets tightly adhere to the substrate composed of g-C 3 N 4 with large size. This 2D/2D heterojunction shortens the distance that charge carriers have to migrate to the surface and facilitates the rapid involvement of carriers in chemical reactions [7]. Figure 3f clearly shows that Ag nanoparticles are dispersed on the α-Fe 2 O 3 /g-C 3 N 4 surface. Figure 3h,i is high-resolution TEM images of the circled region, respectively, which show that the fully exposed active crystal plane of α-Fe 2 O 3 is the (110) plane, while the 0.24 nm lattice spacing corresponds to the (111) crystal plane of Ag [51,52]. Figure 3j-n shows the energy dispersive X-ray spectroscopy (EDX) of Ag/α-Fe 2 O 3 /g-C 3 N 4 , which shows the presence of N (green), C (yellow), O (red), Fe (violet), and Ag (indigo) elements, further demonstrating the successful synthesis of Ag/α-Fe 2 O 3 /g-C 3 N 4 ternary composite photocatalyst. are dispersed on the α-Fe2O3/g-C3N4 surface. Figure 3h,i is high-resolution TEM images of the circled region, respectively, which show that the fully exposed active crystal plane of α-Fe2O3 is the (110) plane, while the 0.24 nm lattice spacing corresponds to the (111) crystal plane of Ag [51,52]. Figure 3j-n shows the energy dispersive X-ray spectroscopy (EDX) of Ag/α-Fe2O3/g-C3N4, which shows the presence of N (green), C (yellow), O (red), Fe (violet), and Ag (indigo) elements, further demonstrating the successful synthesis of Ag/α-Fe2O3/g-C3N4 ternary composite photocatalyst.

Analysis of Charge Transfer Mechanism
The charge transfer upon contact in compound semiconductors is generally related to the work function of the materials. To study the charge movement between g-C3N4 and α-Fe2O3, the work functions of them were analyzed by SKP (Figure 4a). The test results are presented as relative values of Au, and the work function of the standard Au sample

Analysis of Charge Transfer Mechanism
The charge transfer upon contact in compound semiconductors is generally related to the work function of the materials. To study the charge movement between g-C 3 N 4 and α-Fe 2 O 3 , the work functions of them were analyzed by SKP (Figure 4a). The test results are presented as relative values of Au, and the work function of the standard Au sample was 5.1 eV. The work functions of α-Fe 2 O 3 and g-C 3 N 4 were derived to be 5.61 and 4.54 eV from the difference between the test results and the work function of the standard Au sample. Apparently, g-C 3 N 4 has a higher Fermi energy position relative to α-Fe 2 O 3 , and when they are in close contact to form a heterojunction, the difference in work function leads to the spontaneous diffusion of electrons from g-C 3 N 4 to α-Fe 2 O 3 and form electron depletion and electron aggregation layers at the contact interface, which is the reason for the formation of the internal electric field. In addition, changes in element valence and electron density lead to chemical shifts in XPS, so the charge transfer mechanism of α-Fe 2 O 3 /g-C 3 N 4 heterojunction can be analyzed by in situ irradiated XPS. Peaks in the Ag 3d XPS spectrum appear at 373.3 and 367.3 eV, originating from Ag 3d 5/2 and Ag 3d 3/2 with a difference of 6.0 eV between the two peaks, indicating that Ag exists as a monomeric form (Figure 4b) [49]. Figure 4c shows that the peaks of C1s located at 286.5 and 288.3 eV are attributed to C-N and C-N=C in g-C 3 N 4 , and the carbon peak at 284.8 eV is used to calibrate the other peaks. The peaks of N1s at 398.5, 399.8, and 400.8 eV in Figure 4d are assigned to C-N=C, C-(N) 3 and C-N-H in g-C 3 N 4 [53]. Upon contact between α-Fe 2 O 3 and g-C 3 N 4 , electrons transfer from g-C 3 N 4 to α-Fe 2 O 3 due to the difference in work functions. Both the peaks of C and N move toward higher binding energies after losing electrons. The electrons in the CB of α-Fe 2 O 3 move to g-C 3 N 4 driven by the internal electric field under light, and the peaks of N and C move toward lower binding energy after g-C 3 N 4 gaining electrons. The peaks of Fe 2p appeared at 711.2, 719.0, and 724.3 eV in Figure 4e, corresponding to Fe 2p 3/2 , Fe 3+ , and Fe 2p 1/2 , respectively. Figure 4f shows that the characteristic peaks of O1s are located at 529.7 and 531.4 eV, attributing to lattice oxygen and adsorbed oxygen in α-Fe 2 O 3 , respectively, and the peak at 532.8 eV is from water adsorbed by the composite photocatalyst [54,55]. The shifting trend of O and Fe in α-Fe 2 O 3 /g-C 3 N 4 before and after light illuminating is opposite to that of g-C 3 N 4 . α-Fe 2 O 3 gets electrons when a heterojunction is formed, and its binding energy decreased. However, after illumination, α-Fe 2 O 3 loses electrons and its binding energy rises [13,14,19]. Kelvin probe and in situ irradiated XPS results demonstrate the charge transfer mode of α-Fe 2 O 3 /g-C 3 N 4 S-Scheme heterojunction, as shown in Figure 4g. Electrons move from g-C 3 N 4 to α-Fe 2 O 3 due to the difference of work function between α-Fe 2 O 3 and g-C 3 N 4 , and an internal electric field is formed at the interface of the heterojunction, and the energy band bending of α-Fe 2 O 3 and g-C 3 N 4 caused by the electrostatic repulsion. When the α-Fe 2 O 3 /g-C 3 N 4 heterojunction photocatalyst is excited by light, the electrons in the CB of α-Fe 2 O 3 move to the VB of g-C 3 N 4 and compound with the holes under the effect of internal electric field and energy band bending, so that the α-Fe 2 O 3 /g-C 3 N 4 heterojunction maintains strong redox properties.

Photothermal Effect Evaluation
The photothermal effect of the catalyst was characterized by infrared thermography. Figure 5a,b shows the change of surface temperature of the pure g-C 3 N 4 and ternary heterojunction photocatalysts with the increased light time in the air, respectively. The initial temperature of the g-C 3 N 4 powder surface was 26.6 • C, and then increased quickly to 54.9 • C after 90 s of light illumination, while the temperature of Ag/α-Fe 2 O 3 /g-C 3 N 4 powder increased from 26.5 • C to 173 • C at the same condition, which was 5.57 times higher than that of the original g-C 3 N 4 , indicating that the construction of heterojunction and the modification of Ag nanoparticles significantly improved the photothermal conversion ability of the ternary composite. The photothermal performance of the powder catalysts in real reactions was investigated by photothermal experiments in water. Figure 5c,d shows the temperature changes of pristine g-C 3 N 4 and the ternary composite sample with the increase of light time in the water, respectively. The temperature of g-C 3 N 4 solution system under light increased from the initial 19.4 • C to 32.3 • C with a temperature increase of 12.9 • C, while the temperature of the ternary heterojunction increased from the initial 19.7 • C to 59.1 • C at the same condition, which was three times higher than that of pure g-C 3 N 4 , indicating that Ag/α-Fe 2 O 3 /g-C 3 N 4 offers a more significant photothermal effect. Ag/α-Fe 2 O 3 /g-C 3 N 4 ternary photocatalysts can convert part of the absorbed photon energy into thermal energy through the photothermal effect to form a local thermal environment near the catalyst surface in a photocatalytic process, thus increasing the near-field chemical reaction rate of the catalyst and enhancing the photocatalytic performance of the catalyst [13,56,57].

Photothermal Effect Evaluation
The photothermal effect of the catalyst was characterized by infrared thermography. Figure 5a,b shows the change of surface temperature of the pure g-C3N4 and ternary heterojunction photocatalysts with the increased light time in the air, respectively. The initial temperature of the g-C3N4 powder surface was 26.6 °C, and then increased quickly to 54.9 °C after 90 s of light illumination, while the temperature of Ag/α-Fe2O3/g-C3N4 powder increased from 26.5 °C to 173 °C at the same condition, which was 5.57 times higher than that of the original g-C3N4, indicating that the construction of heterojunction and the modification of Ag nanoparticles significantly improved the photothermal conversion ability of the ternary composite. The photothermal performance of the powder catalysts in real reactions was investigated by photothermal experiments in water. Figure 5c,d shows the temperature changes of pristine g-C3N4 and the ternary composite sample with the increase of light time in the water, respectively. The temperature of g-C3N4 solution system under light increased from the initial 19.4 °C to 32.3 °C with a temperature increase of 12.9 °C, while the temperature of the ternary heterojunction increased from the initial 19.7 °C to 59.1 °C at the same condition, which was three times higher than that of pure g-C3N4, indicating that Ag/α-Fe2O3/g-C3N4 offers a more significant photothermal effect. Ag/α-Fe2O3/g-C3N4 ternary photocatalysts can convert part of the absorbed photon energy into thermal energy through the photothermal effect to form a local thermal environment near the catalyst surface in a photocatalytic process, thus increasing the near-field chemical reaction rate of the catalyst and enhancing the photocatalytic performance of the catalyst [13,56,57].

Photocatalytic Hydrogen Production and Pollutant Degradation Testing
Photocatalytic hydrogen production and degradation of TC experiments were performed to evaluate the activity of prepared catalysts. The synthesis process was optimized by degrading TC experiment. The photocatalytic performance of α-Fe2O3/g-C3N4 heterojunction was affected by the ratio of α-Fe2O3. The best degradation effect of TC was obtained when the weight ratio of α-Fe2O3 was 30wt%. The α-Fe2O3/g-C3N4 will produce

Photocatalytic Hydrogen Production and Pollutant Degradation Testing
Photocatalytic hydrogen production and degradation of TC experiments were performed to evaluate the activity of prepared catalysts. The synthesis process was optimized by degrading TC experiment. The photocatalytic performance of α-Fe 2 O 3 /g-C 3 N 4 heterojunction was affected by the ratio of α-Fe 2 O 3 . The best degradation effect of TC was obtained when the weight ratio of α-Fe 2 O 3 was 30wt%. The α-Fe 2 O 3 /g-C 3 N 4 will produce serious agglomeration if α-Fe 2 O 3 is too much (Figure S3), and the performance improvement is not obvious if α-Fe 2 O 3 is too low ( Figure S4). When the loading amount of Ag nanoparticles is 0.5wt%, Ag/α-Fe 2 O 3 /g-C 3 N 4 has the best photocatalytic performance ( Figure S5). The performance of the catalyst is not improved as the silver nitrate increased, indicating that the content of Ag particles loaded on α-Fe 2 O 3 /g-C 3 N 4 by photoreduction process was relatively low, which is beneficial to improve the dispersion of Ag nanoparticles. Therefore, samples with optimal composite ratios were used to assess the contribution of constructing heterojunctions and plasma Ag modifications. Figure 6a shows that the original g-C 3 N 4 photocatalytic hydrogen production rate was 387.45 µmol g −1 h −1 , and the hydrogen production rate of ternary heterojunction reached 3125.62 µmol g −1 h −1 . After constructing the S-Scheme heterojunction and completing the surface-engineered plasma Ag modification, the hydrogen production rate reached more than eight times of the original one, and there was no significant decay during the 1200 min cycle (Figure 6b). Ag/α-Fe 2 O 3 /g-C 3 N 4 has a high hydrogen production efficiency compared to other recently reported photocatalysts of the same type (Table S1). Ag nanoparticles not only improve carrier separation through LSPR but also provide additional active sites for hydrogen production [33,34]. In addition, Ag/α-Fe 2 O 3 /g-C 3 N 4 also showed the best performance in the degradation experiments of TC with a 93.6% degradation rate in 150 min (Figure 6c), and approximately maintained this degradation rate for all five cycles (Figure 6d). The first-order reaction kinetic curves show that apparent reaction rate of Ag/α-Fe 2 O 3 /g-C 3 N 4 was 2.6 times that of pure g-C 3 N 4 (Figure 6e), and Ag/α-Fe 2 O 3 /g-C 3 N 4 has relatively excellent TC degradation compared to other recently reported photocatalysts of the same type (Table S2), which was the combined effect of S-Scheme heterojunction and photothermal effect to enhance the chemical reaction rate [19,56]. The radical quenching experiments showed that ·OH played a dominant role in the degradation of TC, followed by vacancies, and the superoxide radical (·O -2 -2 ) at last (Figure 6f) [58]. The mineralization rate of ternary heterojunction for TC reached 49.26%, which was 44.3 times that of pristine g-C 3 N 4 , indicating that the redox ability of the composite sample was substantially enhanced (Figure 6g). To investigate the promotion of photocatalytic degradation reaction by photothermal effect, we conducted a circulating cooling experiment. The effect of temperature on the degradation of TC by Ag/α-Fe 2 O 3 /g-C 3 N 4 was relatively obvious from Figure 6h, and the degradation efficiency decreased significantly when the circulating cooling water was turned on. The reaction rate in the Ag/α-Fe 2 O 3 /g-C 3 N 4 degradation of TC without cooling water was 1.5 times higher than that with cooling water (Figure 6i), which indicated that the heat generated by the photothermal effect could effectively promote the photocatalytic catalytic reaction rate. effect, we conducted a circulating cooling experiment. The effect of temperature on the degradation of TC by Ag/α-Fe2O3/g-C3N4 was relatively obvious from Figure 6h, and the degradation efficiency decreased significantly when the circulating cooling water was turned on. The reaction rate in the Ag/α-Fe2O3/g-C3N4 degradation of TC without cooling water was 1.5 times higher than that with cooling water (Figure 6i), which indicated that the heat generated by the photothermal effect could effectively promote the photocatalytic catalytic reaction rate.

Photoelectrochemical Performance Analysis
To investigate the charge transport kinetics of the photocatalyst, a series of photoelectrochemical tests was performed. Figure 7a shows that Ag/α-Fe 2 O 3 /g-C 3 N 4 has the best transient photocurrent response, indicating its high concentration of photogenerated carriers. The low impedance implies a low carrier migration resistance in Ag/α-Fe 2 O 3 /g-C 3 N 4 , which facilitates charge separation (Figure 7b). The steady-state PL shows the lowest luminescence intensity in Ag/α-Fe 2 O 3 /g-C 3 N 4 , demonstrating that the S-Scheme heterojunction effectively suppresses the radiative relaxation in Ag/α-Fe 2 O 3 /g-C 3 N 4 (Figure 7c). Time-resolved photoluminescence spectroscopy (TRPL) can accurately analyze the carrier lifetime in the catalysts, and the fitted results in Figure 7d show that the fluorescence lifetime of Ag/α-Fe 2 O 3 /g-C 3 N 4 is 4.57 ns, which is 3.7 times that of the pristine g-C 3 N 4 , indicating that the S-Scheme heterojunction and surface plasmon resonance effects enhance the separation and transport of carriers. The electron paramagnetic resonance (EPR) experiments reveal the mechanism of the interaction between carriers and surface-adsorbed molecules in the catalysts. The ·O 2 and ·OH production under light for each catalyst is shown in Figure 7e,f, respectively. The results of DMPO-·O showed that α-Fe 2 O 3 could not generate ·O -2 under light, which is due to its conduction band electron reduction abil-ity is not sufficient to reduce O 2 to generate ·O -2 . The signal of ·O -2 was detected after α-Fe 2 O 3 was compounded with g-C 3 N 4 , and the Ag nanoparticle modification further enhanced the ·O -2 signal. The DMPO-·OH results showed that no ·OH signal is detected in pure g-C 3 N 4 due to insufficient oxidation capacity of the hole. α-Fe 2 O 3 /g-C 3 N 4 and Ag/α-Fe 2 O 3 /g-C 3 N 4 showed obvious ·OH characteristic signals, which indicated that the composite photocatalytic system can generate a large number of strongly oxidizing hydroxyl radicals under light to participate in the degradation reaction. This is consistent with the results of the free radical quenching experiments. The ERP test results further reveal that charge transfer similar to that of Type II heterojunctions does not occur in the composite photocatalytic system and that electrons and holes with strong redox capabilities are retained in the system. The mode of charge transfer in ternary heterojunctions is consistent with the S-Scheme mechanism [13,14].
the separation and transport of carriers. The electron paramagnetic resonance (EPR) experiments reveal the mechanism of the interaction between carriers and surface-adsorbed molecules in the catalysts. The ·O2 -and ·OH production under light for each catalyst is shown in Figure 7e,f, respectively. The results of DMPO-·O -2 showed that α-Fe2O3 could not generate ·O -2 under light, which is due to its conduction band electron reduction ability is not sufficient to reduce O2 to generate ·O -2 . The signal of ·O -2 was detected after α-Fe2O3 was compounded with g-C3N4, and the Ag nanoparticle modification further enhanced the ·O -2 signal. The DMPO-·OH results showed that no ·OH signal is detected in pure g-C3N4 due to insufficient oxidation capacity of the hole. α-Fe2O3/g-C3N4 and Ag/α-Fe2O3/g-C3N4 showed obvious ·OH characteristic signals, which indicated that the composite photocatalytic system can generate a large number of strongly oxidizing hydroxyl radicals under light to participate in the degradation reaction. This is consistent with the results of the free radical quenching experiments. The ERP test results further reveal that charge transfer similar to that of Type II heterojunctions does not occur in the composite photocatalytic system and that electrons and holes with strong redox capabilities are retained in the system. The mode of charge transfer in ternary heterojunctions is consistent with the S-Scheme mechanism [13,14].

Mechanistic Analysis
Based on the above experimental phenomena and characterization results, we propose a photocatalytic mechanism for Ag/α-Fe 2 O 3 /g-C 3 N 4 heterojunction. S-Scheme heterojunction, photothermal and photocatalytic-Fenton synergy for Ag/α-Fe 2 O 3 /g-C 3 N 4 performance enhancement (Figure 8). The carrier generated by photoexcitation migrates directionally under the action of internal electric field and interfacial energy band bending, and its migration path follows the S-Scheme mechanism that electrons and holes stored in the VB of α-Fe 2 O 3 and in the CB of g-C 3 N 4 , respectively. The electron with strong reducibility in g-C 3 N 4 can capture free hydrogen and dissolved oxygen and generate H 2 O 2 rapidly. The free electrons in the photocatalytic system can reduce part of Fe 3+ to Fe 2+ in α-Fe 2 O 3 , Fe 2+ then react with H 2 O 2 to produce a large amount of ·OH through the chain Fenton reaction. Moreover, a large number of high-energy electrons generated by the LSPR effect of Ag nanoparticles are injected into the S-Scheme heterojunction photocatalytic system, which facilitates the generation of strong oxidizing ·OH and ·O -2 and increases the concentration of free radicals in the system, and the heat generated will promote the rate of chemical reactions on the surface. This photocatalytic reaction process consists of the following steps: Nanomaterials 2022, 12, x FOR PEER REVIEW 14 of 18 Figure 8. Schematic diagram of photocatalytic mechanism of Ag/α-Fe2O3/g-C3N4 S-Scheme heterojunction.

Conclusions
In conclusion, we have successfully prepared plasma Ag-modified α-Fe2O3/g-C3N4 S-Scheme heterojunctions by electrostatic self-assembly and light deposition strategy. The photocatalytic hydrogen production rate of Ag/α-Fe2O3/g-C3N4 reached 3125 µ mol g −1 h −1 , eight times that of pristine g-C3N4, and the photocatalytic TC degradation rate was as high as 93.6% within 120 min. The reason for this enhanced performance may arise from the following: (1)The S-Scheme heterojunction composed of 2D/2D nanosheets facilitates the spatial separation of charge carriers and the full exposure of active sites; (2) the photocatalytic-Fenton reaction helps to improve the carrier conversion and to enrich the number of active radicals in the reaction system; (3) the LSPR effect of Ag nanoparticles provides high-energy electrons the system to promote the separation of carriers, and generates the photothermal effect to facilitate surface reaction kinetic. This synergistic catalytic strategy provides valuable insights for the construction of highly active photocatalytic systems.

Supplementary Materials:
The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1: Zeta potential of the g-C3N4 and α-Fe2O3 dispersed in deionized water at pH 7.; Figure S2: The sample patterns of g-C3N4 and Ag/α-Fe2O3/g-C3N4. Figure S3: SEM images of α-Fe2O3/g-C3N4 for α-Fe2O3 weight ratios 20% (a), 30% (b), 40% (c). Figure S4: Degradation of TC by different weight ratios of α-Fe2O3/g-C3N4. Figure S5: Degradation of TC by catalysts with different Ag loadings. Figure S6: IR images of the pure water under illumination with a 300 w xenon lamp as a function of time. Figure S7: Photocatalytic degradation of other pollutants by Ag/α-Fe2O3/g-C3N4. Table S1: Comparison of the performance of Ag/α-Fe2O3/g-C3N4 and other similar photocatalysts for H2 evolution.; Table S2: Comparison of the photocatalytic performance of Ag/α- The photocatalytic-Fenton coupling system increases the number of active radicals effectively, and the internal energy generated by the photothermal effect can be activated to promote the dissociation of water molecules and accelerate the chemical reaction kinetics. The synergistic effect of S-Scheme heterojunctions, photothermal and photocatalytic-Fenton enabled the Ag/α-Fe 2 O 3 /g-C 3 N 4 ternary heterojunction to exhibit satisfactory photocatalytic performance [23][24][25].

Conclusions
In conclusion, we have successfully prepared plasma Ag-modified α-Fe 2 O 3 /g-C 3 N 4 S-Scheme heterojunctions by electrostatic self-assembly and light deposition strategy. The photocatalytic hydrogen production rate of Ag/α-Fe 2 O 3 /g-C 3 N 4 reached 3125 µmol g −1 h −1 , eight times that of pristine g-C 3 N 4 , and the photocatalytic TC degradation rate was as high as 93.6% within 120 min. The reason for this enhanced performance may arise from the following: (1)The S-Scheme heterojunction composed of 2D/2D nanosheets facilitates the spatial separation of charge carriers and the full exposure of active sites; (2) the photocatalytic-Fenton reaction helps to improve the carrier conversion and to enrich the number of active radicals in the reaction system; (3) the LSPR effect of Ag nanoparticles provides high-energy electrons the system to promote the separation of carriers, and generates the photothermal effect to facilitate surface reaction kinetic. This synergistic catalytic strategy provides valuable insights for the construction of highly active photocatalytic systems.