Cubic AgBiS2 Powder Prepared Using a Facile Reflux Method for Photocatalytic Degradation of Dyes

The ternary chalcogenide AgBiS2 has attracted widespread attention in the field of photovoltaic and photoelectric devices due to its excellent properties. In this study, AgBiS2 powders with an average diameter of 200 nm were prepared via a simple and convenient reflux method from silver acetate, bismuth nitrate pentahydrate, and n-dodecyl mercaptan. The adjustment of the ratios of Ag:Bi:S raw materials and of the reaction temperatures were carried out to investigate the significance of the synthesis conditions toward the composition of the as-synthesized AgBiS2. The results of XRD indicated that the powders synthesized at a ratio of 1.05:1:2.1 and a synthesis temperature of 225 °C have the lowest bismuth content and the highest purity. The synthesized AgBiS2 crystallizes in a rock salt type structure with the cubic Fm3¯m space group. Scanning and transmission electron microscopy, thermogravimetric analysis, ultraviolet–visible–near-infrared spectra, and photocatalytic degradation performance were employed to characterize the as-synthesized samples. The results demonstrated that AgBiS2 powders display thermal stability; strong absorption in the ultraviolet, visible, and partial infrared regions; and an optical bandgap of 0.98 eV. The obtained AgBiS2 powders also have a good degradation effect on the methylene blue solution with a degradation efficiency of 58.61% and a rate constant of 0.0034 min−1, indicating that it is an efficient strategy for sewage degradation to reduce water pollution.


Introduction
With the rapid development of the industry, chemical plants continue to discharge organic wastewater, leading to increasingly severe water pollution.This pollution has a serious impact on the water used in human daily life and the environment and poses a significant threat to human health.Therefore, it is crucial to find a simple, economical, and efficient method for sewage degradation to reduce pollution and achieve sustainable development.The traditional sewage treatment methods include precipitation, ion exchange, membrane separation, and adsorption, etc., but these methods have the disadvantages of high cost, low selectivity, and serious secondary pollution, which limits their practical application.In recent years, the photocatalytic degradation of organic matter by semiconductor materials has attracted much attention due to its simplicity, high efficiency, low energy consumption, and environmental friendliness.
The ternary chalcogenide semiconductor material AgBiS 2 exhibits unique properties, such as a high absorption coefficients and environmental stability, making it an attractive material for various applications [1].It belongs to the I-V-VI 2 family of compounds (where I = Cu/Ag/Au; V = As/Sb/Bi; and VI = S/Se/Te) [2], which are composed of earth-abundant, non-toxic elements.It has been proven that AgBiS 2 has potential in fields like the photovoltaic, photoelectric, thermoelectric, and so on, but there are few reports focusing on its ability for photocatalytic degradation.Ganguly et al. [3] synthesized the AgBiS 2 -TiO 2 composite to photodegrade doxycycline, and the degradation effect exceeded 95% within 180 min, which demonstrated the photocatalytic degradation ability of AgBiS 2 .
The history of the synthesis and the crystal growth of the ternary chalcogenide semiconductor material AgBiS 2 can be traced to the first report by Wernick [4,5] in 1959.He reported a complete series of AgBiSe 2 /AgBiS 2 of β (low temperature, hexagonal) and α (high temperature, cubic) solid solutions with transformation temperatures ranging from 200 to 300 • C [4].Since then, various methods have been reported for preparing AgBiS 2 , such as solvothermal [6][7][8], hot-injection solution [9][10][11], hydrothermal synthesis [12], sonochemical [13], microwave [14], colloidal [15], solution-phase ligand-exchange [16], and a solid phase sintering method [17], among which the solvothermal and hot-injection solution methods are the most commonly used.Manimozhi et al. [6] synthesized AgBiS 2 nano-/microspheres through the solvothermal method using ethylene glycol as the solvent and polyethylene glycol as a soft template at 180 • C for 20 h.Thongtem et al. [7] produced and purified AgBiS 2 nanostructured flowers from CH 3 COOAg, Bi(NO 3 ) 3 •5H 2 O, and thiosemicarbazide using different solvents in Teflon-lined stainless steel autoclaves via the solvothermal method at the reaction temperature of 200 • C for 24 h.Bernechea et al. [9] developed a hot-injection synthetic route for the synthesis of colloidal AgBiS 2 nanocrystals in an Ar atmosphere.The significance of the composition and sensitivity toward synthesis conditions of the hot-injection synthesis method was investigated by adjusting the concentrations and ratios of Ag and Bi precursors, as reported by Öberg et al. [10].Kaowphong [12] successfully prepared AgBiS 2 nanostructures via biomolecule-assisted hydrothermal synthesis at 200 • C for 12-72 h.Pejova et al. [13] synthesized 3D arrays of close-packed AgBiS 2 quantum dots (QDs) in thin film form using the sonochemical approach with an annealing treatment in an air atmosphere at 250 • C. Cubic AgBiS 2 nanoparticles and flower-like clusters were successfully synthesized via 100 W and 800 W microwave refluxing in ethylene glycol for 60 min [14].Bellal et al. [17] investigated the physical and photoelectrochemical properties of α-AgBiS 2 , synthesized from Bi 2 S 3 and Ag 2 S in an evacuated Pyrex ampoule at 550 • C.
Although all of the aforementioned methods can synthesize high-quality AgBiS 2 , they also require complex equipment to achieve high temperatures or high pressures, and the reaction times are usually over 12 h.In this study, high-quality AgBiS 2 powders were synthesized using a simple and convenient reflux method, and their physical performance and the photocatalytic degradation of the methylene blue solution under the simulated sunlight were studied.

Preparation of AgBiS 2 by the Reflux Method
Silver acetate and bismuth nitrate pentahydrate were dissolved in 30 mL of n-dodecyl mercaptan solution in a three-port flask.The mixture was stirred with a magnetic stirrer and aided by ultrasonic shaking to promote dissolution.The molar ratios of the silver source, bismuth source, and sulfur source were set at 1:1:2, 1.05:1:2.1,1.1:1:2.2,and 1.15:1:2.3,respectively.Thiourea was then added to this orange solution.Subsequently, a condenser tube and a vacuum pump were connected to the completely dissolved solution, and the solution was vacuumed at 130 • C for 5 min.Vacuuming was then stopped, and the temperature was raised to 175 • C and held for 30 min, followed by heating to the desired reaction temperature and maintaining it for an hour.The reaction temperatures were set at 175, 200, 225, 250, and 275 • C, respectively.After the completion of the reaction, the solution was naturally cooled to about 60 • C, and the solution was poured into the mixed solution consisting of methanol and anhydrous ethanol with a volume ratio of 1:1 in a beaker.The solids on the wall of the three-port flask were cleaned with n-hexane and then poured into the beaker.A significant number of black solids were obtained after centrifuging this mixed solution.Finally, the black solids were washed several times with anhydrous ethanol and ultrapure water.After cleaning, the AgBiS 2 black solids were then dried in a blast drying oven at 60 • C for 2 h to obtain AgBiS 2 powders.

Photocatalytic Degradation Experiment
The 40 mg catalyst was added to the prepared methylene blue solution at a concentration of 5 mg/L.The mixture was stirred in a dark environment for one hour to ensure the adsorption-analytical equilibrium between the catalyst and the methylene blue molecules.The solution was then divided into two parts, one of which was filtered to remove the catalyst, leaving only the methylene blue solution to be tested, named Solution A, and the other solution containing the catalyst, named Solution B. After that, Solution A and B were placed under a 400 W high-pressure sodium lamp to simulate sunlight for the photocatalytic degradation experiment to ensure that the distance between each sample and the light source was the same and to eliminate the influence of the light source intensity on the experiment.After 1, 2, 3, and 4 h of simulated light irradiation, a 4 mL solution was extracted from Solution B, the methylene blue solution was left after removing the catalyst via filtration, and the 4 mL solution was also extracted from Solution A as the blank control sample, respectively.The light absorption intensity of the methylene blue solution was determined using a UV-visible near-infrared spectrophotometer (U-4100).

Characterization
The structure and phase purity of the synthesized powders were characterized using X-ray diffraction (XRD) performed on a DX-2700BH instrument (Haoyuan Instrument, Dandong, China).The XRD analysis utilized Cu Kα radiation (λ = 0.154 nm) in the 2θ range of 5-90 • and an increment of 0.026 • and operated at 40 kV and 30 mA.The morphology of the samples was recorded via ultra-high resolution field emission scanning electron microscopy (FE-SEM) (SU8020, Hitachi SU8000 series, Hitachi City, Japan) operated at 3 kV.The light absorption intensity of samples were characterized using the ultraviolet-visiblenear-infrared spectrophotometer (U-4100, Hitachi, Hitachi City, Japan) with a scan speed of 100 nm min −1 and a slit width of 2 nm within the wavelength range of 200-1400 nm at room temperature.A highly refined barium sulfate (BaSO 4 ) plate was used as the reference.The chemical elements in the samples were detected using a 300 kV field emission transmission electron microscope (FE-TEM) (Tecnai G2 F30 S-TWIN, FEI, Hillsboro, OR, USA) in a multi-functional, multi-user environment.The thermal stability and composition of the AgBiS 2 powders were measured via thermogravimetric (TG) analysis (TG-DTA8122, Rigaku, Akishima, Japan).

Structural Characteristics
As shown in Figure 1, the XRD patterns were employed to investigate the influence of different reaction temperatures on AgBiS 2 powders prepared using the reflux method with a raw material ratio of Ag:Bi:S of 1:1:2.The cubic AgBiS 2 (PDF# 89-2045) was synthesized at different reaction temperatures, which resulted a face-centered statistically NaCl-type [5] structure with the cubic Fm3m space group and a unit-cell parameter of 5.648 Å (Figure 2a).In the cubic AgBiS 2 , Ag and Bi atoms are distributed indistinguishably and on the average in one set of positions (0, 0, 0; 0, 1 2 , 1 2 ;

Structural Characteristics
As shown in Figure 1, the XRD patterns were employed to investigate the influence of different reaction temperatures on AgBiS2 powders prepared using the reflux method with a raw material ratio of Ag:Bi:S of 1:1:2.The cubic AgBiS2 (PDF# 89-2045) was synthesized at different reaction temperatures, which resulted a face-centered statistically NaCltype [5] structure with the cubic 3  space group and a unit-cell parameter of 5.648 Å (Figure 2a).In the cubic AgBiS2, Ag and Bi atoms are distributed indistinguishably and on the average in one set of positions (0, 0, 0; 0, ½, ½; ↻ ), and the S atoms are distributed on the average in the other set of positions (½, ½, ½; 0, 0, ½; ↻) [5].The diffraction angles of the XRD patterns 2θ = 27.327°,31.657°,45.380°, 53.786°, 56.385°, 66.121°, 72.950°, 75.167°, and 83.846° correspond to the AgBiS2 diffraction crystal planes (111), ( 200), ( 220), (311), ( 222), (400), ( 331), (420), and (422), respectively.The remainder of the diffraction peaks are consistent with the characteristic peaks of bismuth, indicating the presence of Bi as the only impurity in the product.The XRD pattern at 175 °C shows the characteristic peak of AgBiS2, indicating that the synthesis temperature of AgBiS2 is below this temperature, which is consistent with previous reports.Using silver sulfide and bismuth sulfide as raw materials, Bellal et al. [17] concluded that AgBiS2 was endothermic when formed at 173 °C.As shown in Figure 2b, the binary phase diagram of Ag2S-Bi2S3 also supports the formation of AgBiS2 after exceeding 175 °C [18][19][20].It is worth noting that, except for the XRD pattern at the reaction temperature of 225 °C, the peak intensities at 2θ = 27.327° are higher than that at 2θ = 31.657°,which is due to the mechanical superposition of the characteristic peaks of AgBiS2 and Bi.This result indicates that the reaction yield is the highest and the proportion of raw material wastage is the smallest at 225 °C.Therefore, based on these results, a synthesis temperature of 225 °C was used for further experiments.), and the S atoms are distributed on the average in the other set of positions ( 12 , 1 2 , 1 2 ; 0, 0, 1 2 ; Micromachines 2023, 14, x FOR PEER REVIEW 4 of 12

Structural Characteristics
As shown in Figure 1, the XRD patterns were employed to investigate the influence of different reaction temperatures on AgBiS2 powders prepared using the reflux method with a raw material ratio of Ag:Bi:S of 1:1:2.The cubic AgBiS2 (PDF# 89-2045) was synthesized at different reaction temperatures, which resulted a face-centered statistically NaCltype [5] structure with the cubic 3  space group and a unit-cell parameter of 5.648 Å (Figure 2a).In the cubic AgBiS2, Ag and Bi atoms are distributed indistinguishably and on the average in one set of positions (0, 0, 0; 0, ½, ½; ↻ ), and the S atoms are distributed on the average in the other set of positions (½, ½, ½; 0, 0, ½; ↻) [5].The diffraction angles of the XRD patterns 2θ = 27.327°,31.657°,45.380°, 53.786°, 56.385°, 66.121°, 72.950°, 75.167°, and 83.846° correspond to the AgBiS2 diffraction crystal planes (111), ( 200), ( 220), ( 311), ( 222), (400), ( 331), (420), and (422), respectively.The remainder of the diffraction peaks are consistent with the characteristic peaks of bismuth, indicating the presence of Bi as the only impurity in the product.The XRD pattern at 175 °C shows the characteristic peak of AgBiS2, indicating that the synthesis temperature of AgBiS2 is below this temperature, which is consistent with previous reports.Using silver sulfide and bismuth sulfide as raw materials, Bellal et al. [17] concluded that AgBiS2 was endothermic when formed at 173 °C.As shown in Figure 2b, the binary phase diagram of Ag2S-Bi2S3 also supports the formation of AgBiS2 after exceeding 175 °C [18][19][20].It is worth noting that, except for the XRD pattern at the reaction temperature of 225 °C, the peak intensities at 2θ = 27.327° are higher than that at 2θ = 31.657°,which is due to the mechanical superposition of the characteristic peaks of AgBiS2 and Bi.This result indicates that the reaction yield is the highest and the proportion of raw material wastage is the smallest at 225 °C.Therefore, based on these results, a synthesis temperature of 225 °C was used for further experiments.) [5].The diffraction angles of the XRD patterns 2θ = 27.327200), ( 220), (311), ( 222), (400), (331), (420), and (422), respectively.The remainder of the diffraction peaks are consistent with the characteristic peaks of bismuth, indicating the presence of Bi as the only impurity in the product.The XRD pattern at 175 • C shows the characteristic peak of AgBiS 2 , indicating that the synthesis temperature of AgBiS 2 is below this temperature, which is consistent with previous reports.Using silver sulfide and bismuth sulfide as raw materials, Bellal et al. [17] concluded that AgBiS 2 was endothermic when formed at 173 • C. As shown in Figure 2b, the binary phase diagram of Ag 2 S-Bi 2 S 3 also supports the formation of AgBiS 2 after exceeding 175 • C [18][19][20].It is worth noting that, except for the XRD pattern at the reaction temperature of 225 • C, the peak intensities at 2θ = 27.327• are higher than that at 2θ = 31.657• , which is due to the mechanical superposition of the characteristic peaks of AgBiS 2 and Bi.This result indicates that the reaction yield is the highest and the proportion of raw material wastage is the smallest at 225 • C. Therefore, based on these results, a synthesis temperature of 225 • C was used for further experiments.

Structural Characteristics
As shown in Figure 1, the XRD patterns were employed to investigate the influence of different reaction temperatures on AgBiS2 powders prepared using the reflux method with a raw material ratio of Ag:Bi:S of 1:1:2.The cubic AgBiS2 (PDF# 89-2045) was synthesized at different reaction temperatures, which resulted a face-centered statistically NaCltype [5] structure with the cubic 3  space group and a unit-cell parameter of 5.648 Å (Figure 2a).In the cubic AgBiS2, Ag and Bi atoms are distributed indistinguishably and on the average in one set of positions (0, 0, 0; 0, ½, ½; ↻), and the S atoms are distributed on the average in the other set of positions (½, ½, ½; 0, 0, ½; ↻) [5].The diffraction angles of the XRD patterns 2θ = 27.327°,31.657°,45.380°, 53.786°, 56.385°, 66.121°, 72.950°, 75.167°, and 83.846° correspond to the AgBiS2 diffraction crystal planes (111), ( 200), ( 220), (311), ( 222), (400), (331), (420), and (422), respectively.The remainder of the diffraction peaks are consistent with the characteristic peaks of bismuth, indicating the presence of Bi as the only impurity in the product.The XRD pattern at 175 °C shows the characteristic peak of AgBiS2, indicating that the synthesis temperature of AgBiS2 is below this temperature, which is consistent with previous reports.Using silver sulfide and bismuth sulfide as raw materials, Bellal et al. [17] concluded that AgBiS2 was endothermic when formed at 173 °C.As shown in Figure 2b, the binary phase diagram of Ag2S-Bi2S3 also supports the formation of AgBiS2 after exceeding 175 °C [18][19][20].It is worth noting that, except for the XRD pattern at the reaction temperature of 225 °C, the peak intensities at 2θ = 27.327° are higher than that at 2θ = 31.657°,which is due to the mechanical superposition of the characteristic peaks of AgBiS2 and Bi.This result indicates that the reaction yield is the highest and the proportion of raw material wastage is the smallest at 225 °C.Therefore, based on these results, a synthesis temperature of 225 °C was used for further experiments.To purify the AgBiS 2 product and minimize the formation of the secondary phase, subsequent experiments were performed by increasing the molar amounts of silver and sulfur sources.The initial molar ratios of raw materials of Ag:Bi:S were set at 1:1:2, 1.05:1:2.1,1.1:1:2.2,and 1.15:1:2.3,respectively.Figure 3 shows the XRD patterns of the synthesized products with different molar ratios at 225 • C. With the increase in silver and sulfur content, the characteristic peaks of bismuth are weakened, but they do not disappear when the contents of silver and sulfur continue to increase.This is because the generation temperature of bismuth is lower than that of AgBiS 2 , resulting in bismuth nucleation occurring prior to AgBiS 2 nucleation.Therefore, the weak characteristic peak of bismuth still exists in the spectrum.It is worth noting that the ratio of the peak intensity of the characteristic peak of Bi at 2θ = 37.954 • to that of AgBiS 2 at 2θ = 31.657• is the smallest in the XRD spectrum of AgBiS 2 synthesized with a ratio of raw materials of 1.05:1:2.1.This proves that the powder synthesized with this ratio has the lowest bismuth content and the highest purity.Therefore, the raw material ratio of 1.05:1:2.1 was selected for the preparation of AgBiS 2 in the subsequent experiments.To purify the AgBiS2 product and minimize the formation of the secondary phase, subsequent experiments were performed by increasing the molar amounts of silver and sulfur sources.The initial molar ratios of raw materials of Ag:Bi:S were set at 1:1:2, 1.05:1:2.1,1.1:1:2.2,and 1.15:1:2.3,respectively.Figure 3 shows the XRD patterns of the synthesized products with different molar ratios at 225 °C.With the increase in silver and sulfur content, the characteristic peaks of bismuth are weakened, but they do not disappear when the contents of silver and sulfur continue to increase.This is because the generation temperature of bismuth is lower than that of AgBiS2, resulting in bismuth nucleation occurring prior to AgBiS2 nucleation.Therefore, the weak characteristic peak of bismuth still exists in the spectrum.It is worth noting that the ratio of the peak intensity of the characteristic peak of Bi at 2θ = 37.954° to that of AgBiS2 at 2θ = 31.657° is the smallest in the XRD spectrum of AgBiS2 synthesized with a ratio of raw materials of 1.05:1:2.1.This proves that the powder synthesized with this ratio has the lowest bismuth content and the highest purity.Therefore, the raw material ratio of 1.05:1:2.1 was selected for the preparation of AgBiS2 in the subsequent experiments.To purify the AgBiS2 product and minimize the formation of the secondary phase, subsequent experiments were performed by increasing the molar amounts of silver and sulfur sources.The initial molar ratios of raw materials of Ag:Bi:S were set at 1:1:2, 1.05:1:2.1,1.1:1:2.2,and 1.15:1:2.3,respectively.Figure 3 shows the XRD patterns of the synthesized products with different molar ratios at 225 °C.With the increase in silver and sulfur content, the characteristic peaks of bismuth are weakened, but they do not disappear when the contents of silver and sulfur continue to increase.This is because the generation temperature of bismuth is lower than that of AgBiS2, resulting in bismuth nucleation occurring prior to AgBiS2 nucleation.Therefore, the weak characteristic peak of bismuth still exists in the spectrum.It is worth noting that the ratio of the peak intensity of the characteristic peak of Bi at 2θ = 37.954° to that of AgBiS2 at 2θ = 31.657° is the smallest in the XRD spectrum of AgBiS2 synthesized with a ratio of raw materials of 1.05:1:2.1.This proves that the powder synthesized with this ratio has the lowest bismuth content and the highest purity.Therefore, the raw material ratio of 1.05:1:2.1 was selected for the preparation of AgBiS2 in the subsequent experiments.In the process of powder preparation via the reflux method, a yellow emulsion was formed when the silver source and bismuth source were added to the solvent.This occurrence arises from the ultrasonic dissolution of silver acetate and bismuth nitrate pentahydrate in n-dodecyl mercaptan (NDM), resulting in the formation of light yellow [Ag(C 12 H 25 SH) m ] + (Ag-NDM) turbid liquid and yellow [Bi(C 12 H 25 SH) n ] 3+ (Bi-NDM).Subsequently, upon the addition of thiourea (TU), an orange substance formed, indicating a reaction of the solution system within thiourea.The color of the solution system turns black after gradually heating up, and finally, black AgBiS 2 particles are generated.It is worth noting that, thiourea in the reaction mixture not only plays a role in forming a complex but also as the source of sulfur.All the reactions are expressed via the following equations: At the beginning of the reaction, NDM was utilized as the solvent, resulting in coordination with Ag + and Bi 3+ ions to form a covalent complex as given above (where m and n are positive integers).With the addition of thiourea, these complexes then react with thiourea to form complexes as {Ag[SC(NH 2 ) 2 ] m } + and {Bi[SC(NH 2 ) 2 ] n } 3+ .As the temperature increased, the stability of the complexes decreased to produce AgBiS 2 , which is consistent with the reaction reported by Ganguly et al. [3] Field emission scanning electron microscopy (FE-SEM) and field emission transmission electron microscopy (TEM) tests were conducted on the as-prepared AgBiS 2 powders, as depicted in Figures 4 and 5, respectively.The AgBiS 2 prepared via the reflux method under an ambient atmosphere was granular with an average particle size of about 200 nm, and the particle size and morphology obtained by TEM and SEM were consistent.The lattice fringe labeled in Figure 5d was shown to be 0.325 nm, corresponding to the (111) crystal face of AgBiS 2 .Mak et al. [21] prepared nanocrystals with an average grain size of about 10 nm under room temperature and ambient atmosphere, and Akgul et al. [22] also synthesized nanocrystals with an average grain size of about 5 nm under the same conditions.At room temperature, AgBiS 2 nanoparticles nucleated but did not grow.The synthesis of the particles shown in Figure 4 was carried out under an ambient atmosphere at elevated temperatures; thus, the AgBiS 2 nanoparticles under these conditions underwent a grain growth process as compared to the nanoparticles in the above literature.The thermal stability of the material has a great influence on its performance.Thermogravimetric (TG) analysis was adopted to investigate the thermal stability of the as-prepared AgBiS 2 powders under an ambient atmosphere.As shown in Figure 6, the black thermogravimetric (TG) curve represents the weight of the powders, the derivative thermogravimetric (DTG) curve represents the rate of weight change of the powders, and the blue differential thermal analysis (DTA) curve corresponds to the heat absorption and exothermic change of the powder sample as the temperature rises.It can be seen from the figure that when the temperature rises from room temperature to 700 • C, the weight loss of the powder sample does not exceed 5%; thus, it can be considered that AgBiS 2 has thermal stability below 700 • C.These results are comparable to previously reported values [23].The observed weight gain or loss, as well as the absorptive or exothermic processes of the system observed in the curves, occur due to the small number of heterogeneous phases in the powder beyond the accuracy of XRD testing, including the desorption of adsorbed water, the heat absorption process of impurity phase oxidation, and the heat release process of decomposition.All in all, the thermal stability of the AgBiS 2 powder was determined via the thermogravimetric analysis curve, and its test atmosphere was an air atmosphere, thus confirming that the synthesis of AgBiS 2 powder could take place in an air atmosphere.

UV-Vis Spectroscopy and Band Gap Characterizations
UV-Vis-NIR absorption spectrum was adopted to describe the optical characteristic of the as-synthesized AgBiS 2 powders, as shown in Figure 7a.The absorption curve of AgBiS 2 displays high absorption values in the ultraviolet, visible, and partial infrared regions, and the absorption coefficient decreases gradually when the optical wavelength reaches 1100 nm.As for cubic AgBiS 2 , most reports claim that it is a direct bandgap material [6,17], but some argue that it is an indirect bandgap material [8,24].Here, the estimated value of bandgap is around 0.98 eV by assuming direct bandgap via plotting (αhν) 2 vs. (hν), as shown in Figure 7b.This result is in a good agreement with the reported values of AgBiS 2 grown via other methods [6,23].While the AgBiS 2 powders provide a bandgap of 0.92 eV via the indirect bandgap Tauc plot, their linear fitting relationship is worse than that of the direct bandgap.Therefore, we conclude that the obtained AgBiS 2 material is a direct semiconductor and features two optical transition mechanisms, with a direct bandgap of 0.98 eV and an indirect band gap of 0.92 eV.These values of optical band gaps are comparable to the previously reported values [23,25].Adeyemi et al. [23] synthesized AgBiS 2 and Cu 3 BiS 3 with a rod-like morphology via a microwave-assisted solution route using a deep eutectic solvent and measured the optical band gap.The black powder of AgBiS 2 showed an indirect band gap of ∼0.90 eV and a direct band gap of 0.98 eV, which is consistent with our results.Ju et al. [26] investigated the electronic properties of various AgBiS 2 nanocrystals using first-principles computation and demonstrated that the optoelectronic properties of bulk AgBiS 2 are highly dependent on the M-S-M-S-(M: Ag or Bi) orderings, and Ag-S-Ag-S-and Bi-S-Bi-S-in AgBiS 2 bulk crystals contributed, respectively, to the valence band maximum and conduction band minimum.They found that AgBiS 2 nanocrystals can exhibit markedly different optoelectronic properties depending on their stoichiometry.This may be the main reason for the different band gap values reported.
Micromachines 2023, 14, x FOR PEER REVIEW 8 of 12 The thermal stability of the material has a great influence on its performance.Thermogravimetric (TG) analysis was adopted to investigate the thermal stability of the asprepared AgBiS2 powders under an ambient atmosphere.As shown in Figure 6, the black thermogravimetric (TG) curve represents the weight change of the powders, the red derivative thermogravimetric (DTG) curve represents the rate of weight change of the powders, and the blue differential thermal analysis (DTA) curve corresponds to the heat absorption and exothermic change of the powder sample as the temperature rises.It can be seen from the figure that when the temperature rises from room temperature to 700 °C, the weight loss of the powder sample does not exceed 5%; thus, it can be considered that AgBiS2 has thermal stability below 700 °C.These results are comparable to previously reported values [23].The observed weight gain or loss, as well as the absorptive or exothermic processes of the system observed in the curves, occur due to the small number of heterogeneous phases in the powder beyond the accuracy of XRD testing, including the desorption of adsorbed water, the heat absorption process of impurity phase oxidation, and the heat release process of decomposition.All in all, the thermal stability of the AgBiS2 powder was determined via the thermogravimetric analysis curve, and its test atmosphere was an air atmosphere, thus confirming that the synthesis of AgBiS2 powder could take place in an air atmosphere.

UV-Vis Spectroscopy and Band Gap Characterizations
UV-Vis-NIR absorption spectrum was adopted to describe the optical characteristic of the as-synthesized AgBiS2 powders, as shown in Figure 7a.The absorption curve of AgBiS2 displays high absorption values in the ultraviolet, visible, and partial infrared regions, and the absorption coefficient decreases gradually when the optical wavelength reaches 1100 nm.As for cubic AgBiS2, most reports claim that it is a direct bandgap material [6,17], but some argue that it is an indirect bandgap material [8,24].Here, the estimated value of bandgap is around 0.98 eV by assuming direct bandgap via plotting (αhν) 2 vs. (hν), as shown in Figure 7b.This result is in a good agreement with the reported values of AgBiS2 grown via other methods [6,23].While the AgBiS2 powders provide a bandgap of 0.92 eV via the indirect bandgap Tauc plot, their linear fitting relationship is worse than that of the direct bandgap.Therefore, we conclude that the obtained AgBiS2 material is a direct semiconductor and features two optical transition mechanisms, with a direct bandgap of 0.98 eV and an indirect band gap of 0.92 eV.These values of optical band gaps are comparable to the previously reported values [23,25].Adeyemi et al. [23] synthesized

Photocatalytic Activity
Figure 8 shows the light absorption intensity of the methylene blue solution of the as-prepared AgBiS2 samples and the blank control sample after simulated illumination at different times.In the figure, 0 h represents the light absorption intensity of the methylene

Photocatalytic Activity
Figure 8 shows the light absorption intensity of the methylene blue solution of the as-prepared AgBiS 2 samples and the blank control sample after simulated illumination at different times.In the figure, 0 h represents the light absorption intensity of the methylene blue solution at the time when the adsorption-desorption equilibrium is reached under dark conditions and before simulated sunlight irradiation.The peak position of the highest intensity in the absorption peak of the methylene blue solution in the figure is consistent with its characteristic absorption wavelength of 662 nm.Therefore, the change value of its light absorption intensity can be used to represent the change in methylene blue solution concentration to characterize the efficiency of photocatalytic degradation of the methylene blue solution by the catalyst.It is clear that the AgBiS 2 powder prepared by the reflux method has a certain degradation efficiency within 4 h.However, as to the photocatalytic degradation of the blank control samples, there was almost no spontaneous decomposition of methylene blue under this degradation test condition, so it can be determined that methylene blue underwent photocatalytic degradation under the actions of light and catalysts.The degradation was possible due to the hole, hydroxyl, and superoxide radical, which are the dominant active species generated by the photocatalytic process.
degradation efficiency of the blank control sample reached 11.10% with a rate constant of 5 × 10 −4 min −1 .This indicates that AgBiS2 powders prepared via the reflux method have a good degradation effect on methylene blue solution.These degradation results are within the range of previously reported values [3,29].Ajiboye et al. [29] synthesized graphitic carbon nitride functionalized with ternary silver bismuth sulfide (AgBiS2/gC3N4) for the photocatalytic removal of Pb(II) from water in the presence of methylene blue, crystal violet, and methyl orange.It was observed that the dyes were degraded simultaneously as the divalent lead was reduced to metallic lead, indicating that AgBiS2 was a good photocatalyst for removing both dyes and heavy metal ions from water.Ajiboye et al. investigated the degradation rate of dyes in the presence of Pb(II), and the percentage degradation of methyl orange, crystal violet, and methylene blue are 94.85, 71.81, and 39.64%, respectively (corresponding to the pseudo-first-order rate constant of 0.0443, 0.0233, and 0.0104 min −1 , respectively).The value of 58.61% for the degradation efficiency of the asprepared AgBiS2 powders via the reflux method is better than that of the methylene blue solution reported by Ajiboye et al., while the value of 0.0034 min −1 for the degradation rate constant of the methylene blue solution is lower than the reported 0.0104 min −1 of the Ag-BiS2/gC3N4 compounds.

Conclusions
In summary, a simple and convenient reflux method was adopted to synthesize the completely environmentally friendly AgBiS2 powders with a particle size of about 200 nm.The thermal stability of the as-synthesized AgBiS2 powders was investigated, revealing The above data were plotted with time as the abscissa and degradation rate as the ordinate to obtain the photocatalytic degradation efficiency diagram and time as the abscissa and ln(c 0 /c) as the ordinate to obtain the photocatalytic degradation kinetics diagram, as shown in Figure 9.To quantitatively compare the photocatalytic [27] efficiency of all the samples, a pseudo-first-order reaction kinetic model (ln(c 0 /c) = kt + b, where t is time and b is constant) was employed to calculate the rate constants (k) of the photodegradation experiment over AgBiS 2 [28].The degradation of the samples basically occurred via a pseudo-first-order reaction, and the degradation efficiency of the as-prepared AgBiS 2 sample reached 58.61% with a rate constant of 0.0034 min −1 , whereas the degradation efficiency of the blank control sample reached 11.10% with a rate constant of 5 × 10 −4 min −1 .This indicates that AgBiS 2 powders prepared via the reflux method have a good degradation effect on methylene blue solution.These degradation results are within the range of previously reported values [3,29].Ajiboye et al. [29] synthesized graphitic carbon nitride functionalized with ternary silver bismuth sulfide (AgBiS 2 /gC 3 N 4 ) for the photocatalytic removal of Pb(II) from water in the presence of methylene blue, crystal violet, and methyl orange.It was observed that the dyes were degraded simultaneously as the divalent lead was reduced to metallic lead, indicating that AgBiS 2 was a good photocatalyst for removing both dyes and heavy metal ions from water.Ajiboye et al. investigated the degradation rate of dyes in the presence of Pb(II), and the percentage degradation of methyl orange, crystal violet, and methylene blue are 94.85, 71.81, and 39.64%, respectively (corresponding to the pseudofirst-order rate constant of 0.0443, 0.0233, and 0.0104 min −1 , respectively).The value of 58.61% for the degradation efficiency of the as-prepared AgBiS 2 powders via the reflux method is better than that of the methylene blue solution reported by Ajiboye et al., while the value of 0.0034 min −1 for the degradation rate constant of the methylene blue solution is lower than the reported 0.0104 min −1 of the AgBiS 2 /gC 3 N 4 compounds.

Conclusions
In summary, a simple and convenient reflux method was adopted to synthesize the completely environmentally friendly AgBiS2 powders with a particle size of about 200 nm.The thermal stability of the as-synthesized AgBiS2 powders was investigated, revealing

Figure 1 .
Figure 1.XRD patterns of AgBiS2 synthesized at different temperatures with a raw material ratio of 1:1:2.

Figure 1 .
Figure 1.XRD patterns of AgBiS2 synthesized at different temperatures with a raw material ratio of 1:1:2.

Figure 1 .
Figure 1.XRD patterns of AgBiS2 synthesized at different temperatures with a raw material ratio of 1:1:2.

Figure 3 .
Figure 3. XRD patterns of AgBiS2 synthesized with different raw material ratios at 225 °C.

Figure 3 .
Figure 3. XRD patterns of AgBiS2 synthesized with different raw material ratios at 225 °C.

Figure 3 .
Figure 3. XRD patterns of AgBiS 2 synthesized with different raw material ratios at 225 • C.

Figure 7 .
Figure 7.The optical properties of the as-synthesized AgBiS2 powder.(a) UV-Vis-NIR absorption spectra; (b) the corresponding Tauc plots displaying the extrapolated optical band gap.

Figure 7 .
Figure 7.The optical properties of the as-synthesized AgBiS 2 powder.(a) UV-Vis-NIR absorption spectra; (b) the corresponding Tauc plots displaying the extrapolated optical band gap.

Figure 8 .
Figure 8.The light absorption intensity of methylene blue solution after photocatalytic degradation of different samples for different times.(a) The blank control group; (b) AgBiS2.

Figure 8 .
Figure 8.The light absorption intensity of methylene blue solution after photocatalytic degradation of different samples for different times.(a) The blank control group; (b) AgBiS 2 .

Figure 8 .
Figure 8.The light absorption intensity of methylene blue solution after photocatalytic degradation of different samples for different times.(a) The blank control group; (b) AgBiS2.