New Modeling of AgFeNi2S4-Graphene-TiO2 Ternary Nanocomposite Synthesized Via the Pechini Method for the Enhanced Electrophotocatalytic Reduction of CO2


 For reduction of CO2 into hydrocarbon fuels, an AgFeNi2S4-Graphene-TiO2 ternary nanocomposite material was synthesized via the Pechini method. The Pechini method is based on a chelating agent which, together with ethylene glycol (C2H6O2) and citric acid (C6H8O7) as a chelating cation, can affect the structure and stability of the nanocomposite. The catalytic activity of the photocatalyst for photocatalytic and electrochemical CO2 evolution into hydrocarbon fuels was tested. The methanol yield under UV light was 8.679 %, 6.349 %, and 4.136 %, and the methanol yields under visible light was 6.291 %, 4.738 %, and 2.339 %, respectively. The stability and reusability of the photocatalyst remained high after a 4-cycle recycling test without a decrease in yield of the final photocatalytic CO2 reduction product. The enhanced photoreduction of CO2 through the as-prepared ternary photocatalyst can be ascribed to the catalyst's conformation and low recombination rate. In electrochemical CO2 reduction, the Faraday efficiency is the main parameter that defines the performance of the working electrode and the evolution of methanol. The Faraday efficiency of AFNSGT ternary nanocomposite was 44.25 %; this is an increase in the value of the Faraday efficiency, which proves that the design of the new nanocomposite successfully increases the activity of the working electrode and has a positive effect on the electrochemical reduction of CO2. The photocatalytic and electrochemical CO2 reduction data show that the preparation method, morphological state, and charge carrier properties of the photocatalyst are important for the catalytic activity and efficiency of the methanol evolution pathway. This study provides a strategy for fabrication of a new ternary nanocomposite based on 2D-structured graphene, TiO2, and a quaternary nanocomposite.


Introduction
Industrial development is increasing both the use of fossil fuels and the amount of carbon dioxide and air pollution [1][2][3][4][5]. The simplest and most commonly used solution to these problems is a photocatalyst-based process. Crystal structure, nanoparticle size, band structure, and charge transfer are the main parameters that determine the catalytic activity of a photocatalyst, and these values vary depending on the synthetic method [6][7][8]. Many different methods are used to synthesize photocatalysts, such as the sol-gel, hydrothermal, combustion and ultrasonic-assisted methods [9][10][11][12][13][14][15]. Among them, the Pechini method, which is a type of modified sol-gel method, allows control of the particle size and stoichiometry. The basic process is chelation based on alpha-hydroxide carboxylic acid and organic solvents to form a metal chain by creating chemical interactions between metals and organic molecules [16]. The transition metal chalcogenide is mainly used in the catalytic research field because it has unique properties. Nickel sulfide serves as a good co-catalyst, as it has low Fermi-level energy which can capture the photo-generated electron and push the reduction reaction on the catalyst [17][18][19][20]. Quaternary nanocomposites are attracting the attention of researchers, and it is possible to synthesize quaternary nanocomposites with semi-insulating properties and high catalytic activity by combining nickel sulfide with transition metals using appropriate synthetic methods.
In addition, 2D graphene and titanium oxide (TiO2) photocatalysts are widely studied in photocatalytic CO2 reduction due to their physical and chemical properties. Graphene acts as a better catalyst than 0D and 1D nanomaterials and has a high surface area and high charge transfer capacity [21][22][23]. TiO2 offers a strong oxidizing ability and good physical stability, but this semiconductor material can become activated in the UV region (< 387 nm) and is subject to rapid convergence of electrons and holes due to its band gap energy (3.20 eV) [24]. Previous studies have shown that the graphene-TiO2 binary photocatalyst material showed high conversion efficiency for CO2 under light irradiation. Graphene can strengthen the physical and chemical stability of the metal-junctioned binary material [25]. Recently, novel structured ternary photocatalysts have shown advantages over binary catalysts because of their strong light absorption rate and enhanced photocatalytic potency. The combination of two or more semiconductors has many advantages, such as band structure, energy adjustment, efficient charge transport, and high catalyst activity; these properties have allowed nanomaterials to find a range of new applications [26,27].
Based on the above mentioned superiority, our research proposes a new method of modeling a ternary nanocomposite based on graphene, TiO2, and a quaternary semiconductor. 2D-structured graphene increases not only the surface area but also the light absorption capacity and can act as a bridge between semiconductors to enhance the degree of photogenerated charge transfer. The catalytic activity of the newly modeled ternary nanocomposite was evaluated based on the conversion of CO2 to hydrocarbon fuel under light radiation using an electron donor scavenger.
Various methods have been used for CO2 reduction, such as photocatalytic, electrochemical, and photo-electrochemical techniques. Among them, the electrochemical and photocatalytic reduction of CO2 to value-added products has been shown to be effective due to its simplicity and ability to operate under ambient circumstances [28]. The carbon dioxide conversion rate depends on several fundamental factors, such as the surface and morphology of the nanocomposite, composition and experimental conditions. The electron-donor purifier can provide a sufficient number of cycle electrons and allow CO2 to combine with water, thus improving the CO2 conversion process and supporting the catalytic activity of the photocatalyst [29].
Herein, we prepared an AgFeNi2S4-Graphene-TiO2 ternary nanocomposite material synthesized via the Pechini method. The final ternary photocatalyst demonstrated the ability to efficiently separate and transfer charges due to the synergistic effects of the new design. Also, the good electrical conductivity of graphene provides semiconductor conduction and the 2D structure provides charge transfer fields. The CO2 conversion efficiency of the ternary photocatalyst was greater than that of unary and binary catalysts in the photocatalytic and electrochemical reduction process. Sufficient electron-donor reagents were used to improve the performance of the photocatalyst, and the experimental results showed that the final methanol content was relatively high. Our work demonstrates a new technique to assemble a graphene-based catalyst with systematized CO2 reduction activity.

Preparation of quaternary nanocomposite
The quaternary nanocomposite was synthesized by the Pechini method. The precursor solution was prepared using a 3:2 ratio of ethylene glycol (C2H6O2) and chelate-cationic citric acid (C6H8O7). The precursor agents, in a mmol ratio of 0.02:0.03:0.04:0.08, were dispersed into the citric acid solvent and stirred at 60℃ for 6 h until it became viscous. The process of preparing the mortar is called chelating, and its main function is to establish a chemical bond between the transition metal and the organic molecules in order to adjust the metal chain. The mixture was transferred into 100 ml Teflon-lined clave and heated at 150°C for 15 h, then allowed to cool to room temperature (25°C). The final product was washed with DI water and dried at 100℃, then the powder was calcined at 700℃ for 2 h. The final nanomaterial was denoted AFNS.

Preparation of binary and ternary nanocomposite
The binary and ternary composites were prepared by the Pechini method. The graphene was prepared by Hummer and Offeman's method [30], the details of which were described previously [31]. First, the organic solvent was prepared using absolute ethanol and DI water (volume ratio = 5:4) with continuous stirring at room temperature. Then, 0.7 g of AFNS was added to the solvent and stirred for 1 h. Graphene (0.7 g) was added to the mixture, which was stirred at 120°C for 6 h, then transferred to a 100 ml Teflon-lined stainless-steel autoclave and heat-treated at 150°C for 15 h before being cooled to 25°C. The final product was rinsed with DI water, dried completely at 90°C, and then calcined at 700℃ for 2 h. The prepared sample was denoted AFNSG. In the next step, TiO2 was combined with AFNSG. The reactants were combined in a molar ratio of ethanol to H2O to TNB of 30 : 15 : 4 and stirred continuously for 1 h. One-half gram of AFNSG nanocomposite was added to the prepared solution, which was stirred at room temperature for 6 h, then transferred to an autoclave and kept in an electric oven at 150°C for 15 h. The obtained product was rinsed with DI water and dried until it became a powder and calcined under the same conditions. The final sample was renamed AFNSGT. The morphology state and electrochemical analysis method was detail explained in the supplementary material. Photocatalytic CO2 reduction The photoreduction of CO2 was conducted in a three-part closed reactor under light irradiation (Scheme S1). Two different electron-donor scavengers were used in this experiment, which provided a supply of cyclic electrons to increase the catalytic activity of the photocatalyst. First, a 0.04 M NaHCO3 solution was prepared. Then, 0.1 g of photocatalyst and scavenger were dispersed in solvent and stirred for 1 h. A highly pure input gas (CO2, 99.99%) was purged into the mixture to obtain a gas/solvent mixture; the gas was controlled by the mass flow controller and the total purging time was 30 min. Next, pure nitrogen was purged into the mixture to remove the gas from the glass reactor. The lamp was then switched on and the light source (500 W halide lamp) was placed at the top of the reactor at a distance of about 10 cm. In detail, 100 mg of photocatalyst and 0.3 g scavenger were dispersed in 0.04 M NaHCO3 containing 50 ml solvent and stirred for 1 h. The pure input gas was controlled by the mass flow controller and the total gaspurging time was 30 min. The total reaction time was 48 hours; every 12 hours, a certain volume of solution was withdrawn from the reactor using a syringe, and the solution was filtered through a membrane filter with a pore size of 0.45 μm and a diameter of 47 mm. The amount of alcohol in the solution was analyzed using a "Quantitative analysis of alcohol" method. In this method, 10 ml of 0.1 M CrO3 was added to a 1 ml sample and agitated for 15 min, then the suspension was centrifuged (10000 ppm/15 min). The concentration of the acquired solution was examined by a UV spectrophotometer (Optizen POP, Korea) using a quartz cell (1 × 4.5 cm). The functional group of the final methanol was analyzed by a FTIR spectrometer (FTIR iS5, Thermoscience).

Electrochemical CO2 reduction
The electrochemical CO2 reduction process was conducted on a PGP201 Potentiastat (A41A009) using a three-electrode system. Ag/AgCl and platinum were used as the reference and counter electrode, respectively. The as-prepared binary and ternary nanocomposites were used for the working electrode (WE) preparation. The WE were prepared by following the "Doctor blade" method [32]. Ethyl cellulose was used as a binding material and mixed with the as-prepared nanocomposite in a 1:3 ratio. Then, a few drops of pure ethanol were added, and the resulting mixture was ground and used to veneer the top of the Ni foil. The electrode preparation process is presented in Scheme S2. The current density determines the amount of electric current per unit cross-section of the material. The input CO2 gas-purging speed and the amount of gas were controlled by the mass-flow controller and 50 ml of 0.04 M NaHCO3 solvent was used as the electrolyte solution. The cyclic voltammetry (CV) measurement was conducted in a potential range of (-1.2 to 1.5 V vs. Ag/AgCl) at a scan rate of 100 mV s -1 . After electrolysis, a voltammogram was obtained for the final product cycle considering the highest oxidation peak current. In a reaction that reduces the electrochemical composition of CO2, the catalyst can direct a specific reaction to produce a certain amount of product. In CO2RR, the following equation considers the Faraday efficiency (FE) to confirm the selected product during the reaction [33].
where, nmethanol is the number of moles of formate and n represents the number of electrons transferred from CO2 to produce one molecule of methanol. In this case, n = 6, F is Faraday's constant (96,485 C mol -1 of electrons), and I is the circuit current (measured by the Potentiastat).
Reaction time (t) is measured in s.

Result and Discussion
3.1. XRD analysis and Raman spectra 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 Figure 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 [34,35] 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 Figure 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 Figure 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 [36]. 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 [37]. 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 [38].
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.

Morphological analysis
The surface of the photocatalyst was analyzed via SEM. 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   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.

FTIR and XPS analysis
FTIR was used to identify the presence of the functional groups of the nanocomposites. Figure   5 shows 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 [41]. 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.  and Fe (III) ions [43]. 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. 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 Figure 6 (f) [46]. The high-resolution Ti2p spectrum of AFNSGT displayed two peaks at 459.42 and 465.18 eV, which are related to Ti 4+ 2p 3/2 and Ti 4+ 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 Figure 6 (g) [47]. 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 Figure 6 (h) [48]. 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.  (2) visible emission (DF defect) [49]. 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 Figure 7   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 eand 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 [51], WO3-TiO2/Cu2ZnSnS4 [52], and NiS/CQDs/ZnIn2S4 [53][54] 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.

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 changedas the methanol concentration increased, the oxidation state of CrO3 decreased, and the color of the base solution changed accordingly. Figure 8  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 Figure 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 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 Figure 9. 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 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 + .

Conclusion
In conclusion, we successfully fabricated a new ternary nanocomposite by the Pechini method.
The advantage of the Pechini method is that it allows us to create a chemical interaction between the transition metal and organic molecules to form a metal chain. The morphology, functional groups and electrochemical properties of the unary, binary and ternary nanocomposites were analyzed by spectroscopic techniques. The ternary structured nanocomposite had more stable properties and charge carrier separation due to the presence of 2D-structured graphene, TiO2 and the quaternary semiconductor. 2D-structured graphene can act as a bridge between the semiconductor (TiO2) and a quaternary semiconductor that increases light absorption and enhances photo-generated charge transfer. The conduction band structure of TiO2 makes it available to reduce carbon dioxide to methanol under light irradiation, thus enhancing the catalytic activity of the ternary nanocomposite. The CO2 reduction process was conducted through photocatalytic and electrochemical methods. The highest methanol evolutions of 6.291 and 8.679% were achieved over AFNSGT, while methanol yields of 4.738 and 6.349% were achieved over the AFNSG nanocomposite in the photocatalytic process. The ternary catalyst was reusable after four cycles, which confirmed that the newly modeled nanocomposite will be useful for practical applications. In electrochemical CO2 reduction, the Faraday efficiency is the main parameter that defines the performance of the working electrode and the evolution of methanol. The Faraday efficiency of AFNSGT increased to 44.25%, which proves that 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. In this work, we synthesized the AgFeNi2S4-Graphene-TiO2 ternary nanocomposite by the Pechini method. The catalytic activity tests confirmed that the ternary nanocomposite had high catalytic activity for photocatalytic and electrochemical reduction of CO2. This work confirms that the new structured nanomaterial can be applied to reduce environmental pollution and fabricate hydrocarbon fuels.

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