3.1. XRD analysis and Raman spectra
Figure 1 (a) and (b) show the crystal structure of the binary and ternary nanocomposite. In the XRD pattern of AFNS, the peaks generally revealed a decent crystal structure and phase. The diffraction peaks were located at 2θ = 15.63, 31.32, 37.96, and 49.85°, which are respectively assigned to the (111), (113), (004) and (115) crystal planes of FeNi2S4 (JCPDS. 96-900-0979). After combining with graphene, the diffraction peak becomes sharp; the diffraction peak of graphene was observed at 2θ = 21.68°, corresponding to the (100) crystal plane. The peak intensity was low because the quantity was low. In the AFNSGT nanocomposite, the XRD peaks of TiO2 were observed at 2θ = 25.28, 38.05, and 55.02°, which were assigned to the (011), (004) and (121) crystal planes of anatase TiO2 (JCPDS №.21-1272). The reference XRD peak of anatase TiO2 and ternary nanocomposite data is shown in Fig. 1 (a). The catalytic activity of the anatase phase is more active than that of the rutile phase, and the electron-hole pair life of anatase is favorable for the surface reaction. The crystallite sizes of each nanocomposite were calculated using the Debye-Scherrer equation; these were found to be 3.37, 6.31, and 3.68 nm. The crystallite sizes of a particle can affect the catalytic activity of the photocatalyst. Several previous research papers (Xueqin et al. 2014; Mathias et al. 2014) have analyzed the effect of crystallite size on photocatalytic performance. Figure 1 (b) shows the XRD patterns of the Ag2NiS2, FeNiS2 and AgFeNi2S4 semiconductors, which helps to demonstrate the difference between ternary and quaternary semiconductors. The peaks of NiS, Ag2S, and FeS were observed in this XRD pattern, as shown in Fig. 1 (b). The XRD peaks of AFNS were clear and distinct.
The state of the carbon material (D- and G-band) and molecular interactions between AgFeNi2S4, graphene, and TiO2 were confirmed by the results of Raman spectroscopy. The full Raman data of the binary and ternary nanocomposites are shown in detail in Fig. 1 (c). In AFNSG, graphene had two sharp peaks in the 1348.6 and 1588.2 cm− 1 shift region, which indicates A1g and E2g symmetry (D- and G-mode). The two types of feature peaks were observed at 1390 and 1583 cm− 1 (Malard et al. 2009). The peak position shifted from the lower to the higher region or vice versa. The quaternary semiconductor had one wide and sharp peak in the 551.5 cm− 1 region. According to previous studies, chalcogenide-based semiconductors have characteristic peaks in the 100–600 cm− 1 region (Pandiaraman and Soundararajan 2012). Some main peaks were not observed in the current study due to modification of the crystal lattice vibrations of the nanomaterial. AFNSGT included five main peaks, which were assigned to TiO2 and graphene. The Raman band of anatase-structured TiO2 was obtained in the 100–700 cm− 1 shift region, which included the Eg, B1g, and A1g modes (Pierre et al. 2018). The Eg mode peak appeared at 613.5 cm− 1, the B1g mode peak was observed at 420.4 cm− 1, and the last peak, SEO, appeared at 292.6 cm− 1, which indicates the motions of atoms. The characteristic peaks of the quaternary semiconductors were not present in AFNSGT because the symmetric motion of quaternary semiconductors was distorted and the peak intensity of TiO2 was sharp and strong. The peak position of the D- and G-band was shifted from the higher to the lower shift regions. Shifting of peaks towards a lower or higher wave number is related to chemical bond length. If the chemical bond length of the molecules changes for any internal or external reasons, the wave number may shift. Based on the above theory, the chemical bond length of graphene changed due to the bonding interaction between graphene and TiO2. The ID/IG ratio was 0.85 in AFNSG and 0.84 in AFNSGT. The change in the value indicated the presence of lattice defects, which arise from the combination of a metal chalcogenide and graphene.
3.2. Morphological analysis
The surface of the photocatalyst was analyzed via SEM. Figure 2 (a) and (b) show the SEM analysis of the quaternary semiconductors, which were wrinkled and elliptical. The preparation of the quaternary semiconductor based on the Pechini method helped to establish a chemical bond between the transition metal and the organic molecules in order to adjust the metal chain. This could demonstrate successful fabrication of the metal chalcogenide semiconductor. After combining with graphene, all particles agglomerated on the graphene exterior owing to the functional group on the graphene surface. The variously structured particles were irregularly agglomerated, as shown in Fig. 2 (c)-(d). In the SEM image of AFNSGT, the TiO2 particle showed a white, wrinkled and oval structure. Figure 2 (e) and (f) demonstrate the successful fabrication of AgFeNi2S4-Graphene-TiO2. Furthermore, the successful interconnection of AgFeNi2S4, graphene and TiO2 can be helpful for formation of the hetero-junction structure with good charge carrier properties. The role of graphene is as a bridge and electron acceptor that can support the photogenerated charge carrier and increase the photocatalytic capacity. Figure 2 shows the 3D surface plot, which reveals the roughness of each sample. The surface of AFNS was uneven and sharp, while that of AFNSG was convex. The surface of AFNSGT was smooth and convex.
The internal structure and crystallographic facet of each nanocomposite were analyzed by TEM and HRTEM. Figure 3 (a) shows the presence of the quaternary semiconductor as an elliptical structure on the graphene, which indicates the successful interaction between AFNS and graphene. The morphology of graphene was clear and provided the large surface needed for growth of the metal compound, as shown in Figure S1 (a)-(b). As shown in Fig. 3 (d), the pentagonal TiO2 appeared as a dark black entity that was uniformly spread on the graphene exterior in the AFNSGT ternary nanocomposite. The TiO2 and quaternary semiconductors were evenly distributed on the surface of graphene. The oxygenated functional group on the graphene exterior prevents the formation of metal or metal oxide agglomerates. Figure 3 (b) and (e) show the HRTEM findings of the crystallographic facets of each nanocomposite. The lattice fringes show the d-spacing values of the quaternary semiconductor and TiO2, 0.235 nm and 0.351 nm, respectively, corresponding to the AFNS (201) and TiO2 (101) crystal planes, as shown in Fig. 3 (b) and (e). The average particle size histograms of the binary, ternary and quaternary semiconductors are shown in Fig. 3 (c), (f) and (g). The average particle sizes were 3.08 (AFNS in AFNSG), 2.75 (AFNS in AFNSGT), and 3.08 nm (TiO2 in AFNSGT).
Additionally, quantitative element analysis was done using an EDX instrument. Figure 4 shows the microanalyses of all as-synthesized samples, which reveal the presence of the main elements. Ag, Fe, Ni, S, Ti, and O were obtained from the metal and metal-oxide component, and C was derived from the main adsorbent material, graphene, in the ternary photocatalyst. Each element had its own Kα and Kβ values in the appropriate region. These elemental analyses are summarized in the 3D pie graph.
3.3. FTIR and XPS analysis
FTIR was used to identify the presence of the functional groups of the nanocomposites. Figure 5 shows the FTIR results of the nanocomposites. The adsorption band of the carbonyl group (C-O) appeared at 1582.9 cm− 1, the carboxylic acid group (-COOH) was observed at 1412.1 cm− 1, and asymmetric and symmetric stretching of C-O-C appeared at the wave number regions of 1089.7 and 617.3 cm− 1. All of those peaks corresponded to graphene (Mohammad et al. 2020). The blue line revels the adsorption peaks for AFNS at 2917.8, 1433.0, 1117.9, 998.0, 665.2 and 622.9 cm− 1, which were ascribed to the (R-C(O)-OH), (C(O)-OH), S = O, CH2 rocking, Fe-O and S-O vibration mode (Nguyen et al. 2012). Most of the peaks were ascribed to the citric acid and the adsorption bands, confirming the successful chemical bonding state of the metal chalcogenide semiconductor. After combination with graphene, the peak intensity and vibration mode changed due to the interaction between graphene and AFNS. In the FTIR spectra of AFNSGT, the wide adsorption band of the Ti-O-Ti of TiO2 appeared at 1079.49 and 584.5 cm− 1 (Lei et al. 2019). The FTIR analysis of the samples confirmed the successful formation of the AgFeNi2S4-Graphene-TiO2 ternary nanocomposite.
XPS was used to analyze the chemical states and interactions among AgFeNi2S4, graphene, and TiO2. Figure 6 shows the XPS spectrum of each element. The binding energy depends on the chemical interactions and form of the samples. Figure 6 (a) shows the XPS survey spectra, which indicated the existence of each element and confirmed the successful formation of AFNSGT. Figure 6 (b) shows two peaks at the 368.35 and 374.37 eV binding regions, which are related to the Ag3d5/2 and Ag3d3/2 spectra of Ag (I) in AFNSGT (Gondal et al. 2016). The XPS spectrum of Fe2p (Fig. 6 (c)) displayed four deconvoluted peaks at 711.14, 714.89, 724.76, and 731.75 eV. The photoelectron peak at 711.14 eV corresponds to the binding energy of Fe3+ 2p3/2, while the peak at 724.76 eV can be assigned to Fe2+ 2p3/2. The peak at 731.75 eV corresponds to the 2p1/2 of Fe (II) and Fe (III) ions (Hongliang et al. 2013). The last peak, at 714.89 eV, is a satellite peak that indicates the coexistence of Fe (II) and Fe (III) in the quaternary semiconductor. Figure 6 (d) shows that the Ni2p XPS spectrum was deconvoluted into two spin-orbit doublets. The photoelectron peaks at 856.52 and 873.72 eV indicate the 2p3/2 and 2p1/2 spin orbitals of Ni3+. The other two peaks are assigned to the high-spin divalent state (satellite peaks) of Ni2+ (Yang et al. 2015). The S2p XPS spectrum consisted of four peaks, located at 163.86, 165.04, 169.29, and 170.35 eV, as shown in Fig. 6 (e). These four peaks can be assigned to monosulfide (S2−) and disulfide (S22−) (Diptiman et al. 2016). The XPS peaks of sulfide were observed at 158–161 eV, and the disulfide peaks were located in the 162–168 eV region.
The electron configuration of C1s consists of three peaks, which correspond to the C-C (aliphatic), C-O-C, and C = O groups, as shown in Fig. 6 (f) (Demri and Muster 1995). The high-resolution Ti2p spectrum of AFNSGT displayed two peaks at 459.42 and 465.18 eV, which are related to Ti4+2p 3/2 and Ti4+2p 1/2. In addition, the 2p3/2 spin-orbitals clearly indicate the presence of Ti (IV); the ratio of these two peaks is 2:1, as shown in Fig. 6 (g) (Pyrgiotakis and Wolfgang 2010). The O1s spectrum is composed of four peaks, located at 530.7, 532.65, 532.69 and 535.34 eV. The first peak is related to the C-O group, whereas the second peak is assigned to the metal-carbonate forms (O-Me), as shown in Fig. 6 (h) (Hussein et al. 2012). The existence of the metal carbonate forms confirms the interaction between metal and oxygen. The next two peaks correspond to the carbonyl and carboxyl groups.
3.4. PL, photocurrent response, Nyquist plot and DRS analysis
The charge transfer and interactions among AFNS, TiO2, and graphene were studied using the DRS, photocurrent response, PL, and EIS methods. The charge separation capacity of AgFeNi2S4-Graphene-TiO2 was analyzed by the PL method. The intensity of PL depends on the recombination rate of the electron-hole pairs; a decrease in PL intensity indicates that the charge carrier has a long lifetime. All photocatalysts showed a light response under the 514 nm laser, as shown in Fig. 7 (a). The emission peaks of the samples appeared in the 540–660 nm range, in the visible light region. In the PL spectra, two sharp emission peaks appeared, at wavelengths of 546.51 and 551.95 nm. Additionally, pure AFNS and AFNSG had wide emission peaks near 604.66 nm. The wide-emission peak area and shape changed after the addition of TiO2 to yield AFNSG. The emission peak of the metal-based composite was classified in two sectors: (1) UV emission (NBE) and (2) visible emission (DF defect) (Zhou et al. 2017). The rapid charge recombination rate of AFNS is confirmed by the high-intensity PL peak. The PL peak intensity was reduced in AFNSG due to the high conductivity and charge transfer capacity of the 2D-structured graphene. In AFNSGT, the intensity of the peak was somewhat reduced due to the efficient interfacial contact between each part, which bolstered the charge carrier separation. The results of the above analysis show that the proposed ternary nanocomposite, consisting of a metal chalcogenide, graphene and TiO2, has a high charge carrying capacity. The low PL intensity reflected the recombination rate of electron and holes, which is related to rapid conversion from CO2 into hydrocarbon fuel.
The photocurrent response of all samples showed repeatable signals in fewer than five on-off cycles, as shown in Fig. 7 (b). All nanocomposites had a prompt and stable photocurrent response for each cycle. The photocurrent response decayed in the dark but recuperated within 60 s once the light was turned on. The above procedure can be described in terms of the presence of charge transmission from VB to CB under light irradiation. The 2D-structured graphene had a low current value (2.156 × 10− 6 mA/cm2) in each cycle. The pure quaternary nanocomposite had a lower current value (2.818 × 10− 6 mA/cm2) in every cycle, which indicated low separation efficiency of the photo-generated charge. AFNSG has efficient photo-generated charges because of its good photocurrent response (1.091 × 10− 5 mA/cm2); the graphene supports charge separation and promotes electron flow in the light-on state. AFNSGT showed a higher photocurrent value (1.272 × 10− 5 mA/cm2) than did AFNS during six on-off cycles of light irradiation. The final ternary photocatalyst may show better catalytic activity due to the efficient separation of photogenerated charges and interfacial contact among AgFeNi2S4, graphene, and TiO2, and each nanocomposite had high conductivity.
Figure 7 (c) shows the electrochemical impedance spectroscopy (EIS) findings of all samples. The EIS profile is identical to the charge transfer resistance across the electrode/electrolyte. A wide semicircle profile is indicative of a poor charge carrier with low conductivity. Conversely, a small semicircle can indicate good conductivity with an effective charge carrier, and thus confirm the high catalytic performance of a photocatalyst. Figure 9 (c) shows the Nyquist plot of all samples at room temperature for the frequency range from 5 to 1 mHz with a half-cell. The AFNSGT sample had a smaller semicircle than did the pure AFNS and AFNSG samples. The size of the semicircle depends on the chemical composition and degree of interconnection of AFNS, graphene, and TiO2. Graphene had a small semicircle profile because its high electrical conductivity is capable of sustaining the conductivity of the quaternary chalcogenide nanocomposite. The semicircular profile of the AFNSG binary nanocomposite is located between that of AFNS and AFNSGT, depending on the electrical conductivity and interfacial connection of AFNS and graphene. Figure 7 (d) shows the DRS spectra of the samples. All samples had light responses in the visible and UV light regions. Figure 7 (e) expresses the bandgap energy value of the unary, binary and ternary nanocomposites.
The band gap energy value of each nanocomposite was computed by UV-vis DRS, using Equation  (Kumar et al. 2015):
[hvF (R∞)]1/n =A(hv Eg ) 
where Eg is the energy of the band gap, F (R) (1 R)2/2R is the Kubelka-Munk function, A is the constant of proportionality, and F (R) is the absorption coefficient. hv = hc/λ is the energy of the incident photon. The calculated band gaps of the three nanocomposites were 2.67, 2.59, and 1.95 eV; the band gap value of AFNSGT was lower than that of AFNS and AFNSG. The band gap energy analysis confirmed that AFNSGT had high catalytic activity and efficiently reduced CO2 into methanol. Nickel sulfide-based or quantum dot united carbonaceous materials or semiconductor photocatalysts are generally used for H2 evolution (Mathias et al. 2014; Malard et al. 2009). The one-spot and seed-mediated hydrothermal methods and a sol-gel method are commonly used to prepare NiS photocatalysts (Habisreutinger et al. 2013; Windle and Perutz 2012). The structure of the photocatalyst is the main factor that determines catalytic activity. Recently, ternary structured nanocomposites have become popular in the catalytic field because of their advantages over binary materials, which include strong light absorption rates and enhanced photocatalytic potency. In the last decade, several studies have reported on use of NiS-based photocatalysts for CO2 reduction under light illumination (Pandiaraman and Soundararajan 2012; Pierre et al. 2018; Mohammad et al. 2020).
According to our analysis, the ternary nanocomposite formed successfully and showed a high degree of interconnectivity, highly efficient charge transfer, and suppression of the photogenerated e− and h+ recombination rate. Also, the combination of 2D graphene and TiO2 with a metal semiconductor showed more intimate contact, superior electronic coupling, and a more sensitive photosensitizer, which may lead to better CO2 reduction under light irradiation. Based on the results of both previous studies and the current study, we assume that the ternary nanocomposite has high photocatalytic activity for CO2 reduction.
Currently, ternary chalcogenide photocatalyst materials are commonly used in CO2 evolution, including ZnO/ZnSe (Shuangfang et al. 2018), WO3-TiO2/Cu2ZnSnS4 (Adil et al. 2020), and NiS/CQDs/ZnIn2S4 (Bingqing et al. 2019; Zambaga and Oh 2019) nanomaterials. AgFeNi2S4-Graphene-TiO2 exhibits higher photocatalytic CO2 conversion activities than did the other ternary chalcogenide photocatalysts. Therefore, a noble metal joined quaternary chalcogenide catalyst can still achieve high-efficiency CO2 reduction. The utility of TiO2 in the CO2 photoreduction test is due to the band gap and the location of the energy band, which is comparable to the CO2 reduction capacity and H2O oxidation capacity. Theoretically, redox reactions could be occurring on TiO2 due to its conduction band energy and valence band energy being available to reduce CO2 to formic acid and methanol, and to oxidize H2O to form H+.
In addition, an NiS-based photocatalyst has p-type semiconductor properties, which are favorable to water oxidation and do not strongly affect CO2 reduction. The combination of TiO2 and AgFeNi2S4 was evenly spread across the graphene exterior and the successful interaction of those nanocomposites can determine the band structure and photocatalytic potency of the newly modeled ternary nanocomposite.
3.5. Evolution of CO2 into methanol
The conversion of CO2 to methanol was carried out by two different experimental methods, photocatalytic and electrochemical. According to the test results, the newly modeled ternary nanocomposite had high catalytic activity and successfully reduced CO2 into methanol. Figure 8 ((a), (c) and (e)) shows the quantification of the final methanol yield under different light conditions. The unary, binary, and ternary nanocomposites were used for the photocatalytic reduction of CO2 to methanol. The entire test process was carried out under two different conditions, with-scavenger and without-scavenger. The final concentration of methanol was analyzed by the "Quantitative Analysis of Alcohol" method using CrO3 as a strong oxidizing agent. During the experiment, the final methanol was oxidized, and the oxidation state of CrO3 decreased. The color of the base solution changed – as the methanol concentration increased, the oxidation state of CrO3 decreased, and the color of the base solution changed accordingly. Figure 8 (a) shows the final methanol yield over the AFNS unary photocatalyst under different types of light irradiation for 48 hours. Under visible light, the highest methanol yield was 2.339%, in the with-scavenger condition. During UV radiation, the methanol yield increased to 4.136%; this increase in yield was directly related to the effect of ultraviolet photon energy. The band gap value of the AFNS was 2.67 eV, which confirms that the newly modelled quaternary semiconductor showed a light response for both visible and UV light. Figure 8 (b) shows the cyclic voltammogram of the AFNS working electrode in the CO2-dissolved solution and the final product at a scan rate of 100 mV s− 1. The CV graph occurred in a negative potential range (-0.9 to -0.05 V) on the AFNS working electrode system. The results of the post-electrolysis test show that the highest oxidation peak appeared around 1.20 V (vs. Ag/AgCl). The cyclic voltammogram was performed under pure methanol conditions using the as-prepared working electrodes, and the highest oxidation peaks were found in the potential range of 0.39 to 1.20 V (V vs. Ag/AgCl), as shown in Figure S2. The abovementioned results suggest that unary nanomaterials play an important role in reducing carbon dioxide to methanol. Figures 8 (c) and (d) show the results of methanol production with the AFNSG nanocomposites. The experimental results confirm that the CO2 conversion efficiency of this catalyst is better than that of the AFNS quaternary nanocomposite. The methanol yields were 3.592% and 4.845% in the without-scavenger condition. The methanol yields were increased to 4.738% and 6.349% in the system with the electron-donor scavenger. In the cyclic voltammogram, the highest oxidation peak appeared at a potential of approximately 1.2 V (vs. Ag/AgCl) and the current density increased.
Compared to the unary and binary composites, the ternary composite showed high conversion efficiency for the CO2 evolution reaction. Continuous production of CH3OH was obtained using all types of samples, but there was an increasing trend in the evolution of CH3OH that was directly related to the properties of the photocatalyst. The methanol yields were increased when AFNSGT was used due to the properties of 2D-structured graphene. The graphene increases the light absorption rate and conductivity, which strongly enhances the photocatalytic potency of the AFNSGT ternary nanocomposite. The highest methanol yields were obtained when using AFNSGT nanocomposite; the yields were 5.504 and 5.971% under the without-scavenger condition. The methanol yields increased by approximately 1.5-fold and the highest yields were obtained when using the AFNSGT nanocomposite under light irradiation. The highest methanol evolution of 6.291 and 8.679% was achieved over AFNSGT in the with-scavenger condition. The CV graph appeared in a negative potential range (-0.9 to -0.76 V) in a CO2-dissolved solution. In addition, AFNSGT showed a high CO2 conversion efficiency in the electrochemical reduction process. In the cyclic voltammogram data, the highest oxidation peaks appeared at 1.18 V (V vs. Ag/AgCl), as shown in Fig. 8 (f). The abovementioned CV graph location is the same as that shown in Figure S2. This result suggests that ternary nanomaterials play an important role in reducing carbon dioxide to methanol.
The results of the abovementioned photocatalytic experiments imply that the best CO2 reduction was obtained in the UV light region, because UV light can supply efficient photon energy which can activate the photocatalyst. To increase the CO2 reduction and catalytic activity of the nanocomposite, the electron-donor scavenger (Na2SO3) was used because it allows CO2 to combine with water. There are two reasons for this phenomenon: (i) an increase in the amount of dissolved CO2, and (ii) a reduction in recombination of hole-electron pairs, leading to a longer decay time of surface electrons and facilitation of CO2 reduction.
Prior to the electrochemical procedure, a 0.04 M NaHCO3 solution was placed in a sealed three-electrode glass cell and a CO2-saturated electrolyte was prepared by blowing CO2 at a pressure of 0.2 kg/cm2 for 30 min. Three different types of electrodes were used during the experiment, and the CO2-dissolved solution was electrolyzed. After electrolysis, a voltammogram was obtained for the final product, taking into account the highest oxidation peak current. The cyclic voltammogram test was performed under two different conditions: (i) CO2-dissolved solution before electrolysis, and (ii) methanol solution after electrolysis.
The electrochemical mechanism of CO2 reduction was determined by analyzing the Faraday efficiency of the final product. According to Eq. (1), thermodynamically, 6 electrons are required for the conversion of CO2 to methanol:
CO2 + 6H+ + 6e− = CH3OH + H2O (1)
The Faraday efficiency of the quaternary nanocomposite was 25.46%. The value of the Faraday efficiency increased in the AFNSG binary nanocomposite due to the junction of 2D-structured graphene and metal chalcogenide quaternary photocatalyst, and the final calculated efficiency was around 39.97%. The value of the Faraday efficiency for the AFNSGT ternary nanocomposite increased to 44.25%, proving that the successful design of the newly modeled nanocomposite increases the activity of the working electrode and has a positive effect on the electrochemical reduction of CO2. The peak separation and peak current state depend on the electron transfer properties of the working electrode.
For the electrochemical approach, a solution of 0.04 M NaHCO3 saturated with CO2 was electrolyzed on working electrode (vs. Ag/AgCl) and the methanol concentration in the electrolyte solution was determined during a half-time test afterward using a calibration curve via the cyclic voltammetry technique, as shown in in Fig. 9.
Stability and reusability are the main factors that define the sturdiness of a photocatalyst in practical applications. In the recycling test, the photocatalyst (AFNSGT) was used four times (192 h) under light irradiation with (0.3 g) and without scavengers in the CO2 evolution reaction. The ternary photocatalyst was very sturdy during four cycles, with no significant deactivation towards methanol production, as shown in Fig. 10. According to the recycling test, the ternary photocatalyst can be considered for generation of solar fuels in practical applications. The recombination rate of the hole-electron pairs is the main parameter that defines the catalytic activity of the photocatalyst. The electron-donor scavenger (sodium sulfite) can reduce the recombination rate of the pairs and increase the number of electrons, while also acting as a sufficient cyclic electron donor. Together, these factors produce a photocatalyst with excellent activity.
FTIR was used to identify the presence of the final products and semi-product. The final methanol product (after 1 and 4 cycles) and commercial methanol solution were used in this FTIR analysis. The FTIR spectrum of methanol, which is shown in Figure S3, consisted of four characteristic peaks located at the theoretical peak region. All key data are summarized in Table S1. The FTIR spectrum of all samples was exhibited in a range of 600–3900 cm− 1. The peak location and intensity of the reference and methanol products were the same, which confirms that the ternary photocatalyst reduced the CO2 into methanol. During the test, the semi-product was withdrawn from the reactor and analyzed by FTIR. The results suggest that the chemical structure of CO2 had changed and confirmed that carbon dioxide can be reduced to methanol.
The CO2 reduction pathway can be described by a chemical reaction, given below. The reduction of CO2 to methanol involves unique reaction pathways. In general, the reduction of CO2 requires a more negative potential level in the conduction band (CB), while water oxidation needs a more positive level in the valence band (VB), thereby increasing the number of protons and enhancing the reduction of carbon dioxide. Under Vis/UV light irradiation, both AgFeNi2S4 and TiO2 absorb energy from photons, and electrons are excited from VB to CB. Graphene can accelerate the charge carrier between AFNS and TiO2 and suppress charge recombination.
TiO2 + hν → e- (TiO2) + h+ (TiO2) (2)
AgFeNi2S4 + hv → e- (AgFeNi2S4) + h+(AgFeNi2S4) (3)
e- (TiO2) + Graphene → TiO2 + e- (Graphene) (4)
e- (Graphene) + AgFeNi2S4 → Graphene + e- (AgFeNi2S4) (5)
H2O + h+ → H+ + OH- (6)
(Formic acid) (8)
CO2 + 2H+ + 2e- → CO + H2O (Carbon monoxide) (9)
CO2 + 4H+ + 4e- → HCHO + H2O (Formaldehyde) (10)
CO2 + 6H+ + 6e- → CH3OH + H2O (Methanol) (11)
Eqs. (2)–(5) describe the activation states of AgFeNi2S4, TiO2, and graphene that enable them to produce photoexcited electron-hole pairs. Eqs. (7)–(11) describe the redox reaction, in which the holes are used for oxidation and the electrons are used for reduction. The positively charged holes reacted with water to form hydrogen, and this hydrogen source favored the formation of methanol. Meanwhile, the purged CO2 was adsorbed on the surface of the photocatalyst, so the photo-excited electrons on the surface can reduce/convert the CO2 into methanol. The conversion of CO2 into CH3OH required 6e− /6H+.