Synthesis and Photocatalytic Applications of Silver Sulfide Nanostructures: Recent Advancement

Abstract

Silver sulfide nanoparticles belong to the family of important metal chalcogenides. Silver sulfide has been extensively studied in recent years due to its applications in various fields ranging from biosensors, infrared detectors, and optoelectronics to catalysis. Silver sulfide is considered as a potential photocatalyst due to narrow band gap energy, mechanical and thermal stability, cyclability, and ease of synthesis. Different methods have been investigated to produce various forms of silver sulfide nanoparticles. The present chapter focuses on the recent progress in synthesis of silver sulfide nanoparticles using hydrothermal method, chemical bath deposition, microwave-based approach, sonochemical method, single molecular precursor’s decomposition, and green synthesis. Moreover, the application of silver sulfide nanoparticles in photocatalytic degradation of organic dyes is discussed in details.

1. Introduction

Energy crisis and water contamination are among the serious problems in the present world of industrialization and globalization. To address these issues, research on development of advanced processes and materials is the need of the hour. In recent years, photocatalysis has emerged as an effective technique to address various issues related to energy and environment [1]. It is an advanced oxidative process that involves the use of solar energy and semiconductor materials for various applications including solar energy conversion and water treatment. Metal sulfide nanoparticles are well-studied chalcogenides for multiple applications in energy conservation, environmental issues [2], and medicines and biological sciences due to their unique optoelectronic and catalytic properties [3]. Slightly tunable band gap with different shapes makes them ideal candidates for photocatalytic applications. Among various metal sulfides, silver sulfide (Ag₂S) is a distinctive binary metal chalcogenide semiconductor material with a narrow and direct band gap of around 1ev and high absorption coefficient of approximately 10⁶ cm [4, 5, 6]. Studies suggest that silver sulfide has a layered structure that exists in three polymeric forms [7]. Phase diagrams [8] predict that below 450 K α-Ag₂S, the semiconducting phase exists. It has a monoclinic structure. As the temperature rises in the range of 452–859 K, it changes into body-centered cubic form (bcc) β-Ag₂S, a phase with superionic conductivity (Figure 1). At a higher temperature of about 860 K, face-centered cubic (fcc) γ-phase Ag₂S is obtained with the isolation of metallic silver. α-Ag₂S is semiconductor, while β-Ag₂S shows metallic properties. It demonstrates that α-β conversion has the property of semiconducting metal conversion.

Figure 1.

Change of crystal structure of silver sulfide during reversible phase transformations. [Reproduced from Ref. [7], copyright© 2022 by the authors. Licensee MDPI, Basel, Switzerland].

The physicochemical properties and hence applications of nanoparticles are highly dependent on the shape and size of nanoparticles. During last few decades, various rapid and simple synthetic methods (viz hydrothermal methods, solvothermal methods, microwave-assisted methods, sonochemical methods, template synthesis, sol gel methods, decomposition of molecular precursors, green synthesis using plant extracts and microorganisms, etc.) have been developed to synthesize Ag₂S nanostructures of different morphology and size [9, 10] including nanoparticles, quantum dots [11, 12], nanoplatelets [13], nanowires [6], nanorod [14], nanoleaf [15], nanochains [16], nanospheres [17], nanorice [18], nanocubes [19], and many more [20]. Few representative morphologies of silver sulfide nanostructures are given in Figure 2 and Table 1.

Figure 2.

Different morphology of silver sulfide nanoparticles: a) nanocubes [Reproduced from Ref. [19], copyright © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim], b) nanopowder [Reproduced from Ref. [21] copyright © 2020 American Chemical Society], c) nanorice [Reproduced from Ref. [18], copyright © 2014 Elsevier B.V.], d) nanochains [Reproduced from Ref. [22], copyright © 2020 American Chemical Society], e) CNT-Ag₂S nanotubes [Reproduced from Ref. [23], copyright© 2023 by the authors. Licensee MDPI, Basel, Switzerland], f) tetrahedral nanocrystals. [Reproduced from Ref. [24], copyright © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].

Precursors	Type of Ag2S-based nanostructure	Methodology	Application	References
Silver nitrate, sodium sulfide with EDTA/sodium citrate complexing agent	Ag2s/Ag hetero-nanostructures	Chemical bath deposition	Photocatalytic activity for hydrogen evolution from aqueous solution	[25]
Silver nitrate in 1-dodecanthiol	Ag2S quantum dots/Li-doped Ag2S	Ultrasonic irradiation	Photodetector application	[26]
Silver nitrate, sodium sulfide squalene	Self-assembled Ag2S nanorods	Microwave	—	[27]
Melamine, silver nitrate, sodium sulfide, ammonium oxalate	graphitic carbon nitride/Ag2S irregular plates	in-situ deposition of Ag2S on g-C3N4	Photocatalytic disinfection	[28]
Silver nitrate, sodium sulfide, thioglycolic acid	Ag2S quantum dots	One pot chemical synthesis	Antimicrobial application	[29]
Silver nitrate, ammonia, carbon disulfide, alcohol, water	Leaf-like Ag2S nanosheets	Hydrothermal synthesis	—	[15]
Silver nitrate, sodium sulfide, 3-Mercaptopropyl-; trimethoxysilane	Ag2S colloidal nanoparticles	Self-assembly	—	[22]
Silver nitrate, sodium sulfide	Ag2S crystal grains aggregate	Ion exchange	Photodegradation of dyes	[30]
Silver nitrate, silicon wafer, sulfur powder	Ag-Ag2S hetrodendrites	Electrodeposition followed by in-situ sulfuration	Photocatalytic activity	[31]
Silver nitrate, thiourea, (NH2)2CS, and polyvinyl alcohol	Ag2S photocatalyst Film	Solution casting method	Photodegradation of dyes	[32]

Table 1.

Synthesis of silver sulfide nanostructures with different morphology.

Owing to narrow band gap, high electrical conductivity, excellent catalytic properties, high stability, and ease of synthesis and low toxicity, silver sulfide nanoparticles find attractive applications (Figure 3) in sensors [33], infrared detectors [34], photocatalytic cells [35], optoelectronic devices [26], photocatalytic semiconducting materials [36], and bio-imaging [37]. Among various applications, Ag₂S nanostructures are becoming popular material as ultraviolet-visible/near infrared (UV-visible/NIR) light-responsive photocatalysts for degradation and removal of water pollutants due to their exceptional absorption ability in the UV-visible/NIR region [38].

Figure 3.

Applications of silver sulfide nanoparticles.

Group of Jing Xue used a facile reverse microemulsion method for the synthesis of silver sulfide quantum dots with a tunable size of 3–8 nm [39]. A detailed analysis on the structural properties and various modifications of silver based materials including silver sulfide is reported for the various photocatalytic applications [40]. For photodetector application, silver sulfide nanoparticles with Lithium (Li) doping are synthesized using simple and rapid sonochemical methods. Li doping is observed to significantly enhance the photoluminescence properties [26]. The synthesized Ag₂S nanoparticles (NPs) were employed to fabricate photodetectors with excellent photo response based on graphene nanosheets. Pioneering work on synthesis of silver sulfide is reported by Sadovnikov et al. for photocatalytic applications [41]. Vijayen et al. demonstrated the temperature dependence change in optical and morphological properties of silver sulfide NPs synthesized by coprecipitation method at various temperatures [42]. They observed a crystallite size of prepared silver sulfide nanoparticles as 11.59 nm with needle-like to porous spherical nanostructure on increasing the temperature. The band gap of prepared Ag2S ranges from 0.86–0.96 eV suggested their application in photocatalysis. A simple hydrothermal method was used to obtain rice-shaped monoclinic Ag₂S NPs using silver ammonia complex and sodium sulfide with the presence of polyvinyl pyrrolidone. Rice-shaped nanoparticles are found to be 70–9o nm long and 30–35 nm wide [18]. Hollow nanosphere of Ag₂S is synthesized using simple wet chemistry by sacrificial core sulfur atom; when compared to solid silver sulfide, it was observed to have 67% greater light emission capacity [43]. Rod-like Ag₂S nanocrystals of the size between 200 and 400 nm were synthesized using precursor sodium thiosulfate [44]. More recently, the coprecipitation method has been used to prepare Ag₂S-chitosan composite, and investigation is made on the effect of chitosan on the morphological and optical properties of Ag₂S. The potential of as prepared composite was demonstrated for photocatalytic applications [45]. Singh et al. developed Ag₂S inside titanium dioxide with the average particle size of 19 nm for photocatalytic hydrogen-generation application [46].

Keeping in view the rapid development of silver sulfide-based nanostructure for a variety of applications, this chapter is focused on the advancement and comparison of commonly used synthetic methodologies to obtain silver sulfide of different morphologies and sizes. Moreover, recent development of photocatalytic applications of Ag₂S nanostructures for catalytic degradation of organic dyes is discussed in detail. Finally, we conclude with current trends, limitations, and challenges that might be useful for further research.

2. Synthesis of silver sulfide nanostructures

Easy synthetic routes are considered among one of the reasons for versatile applications of silver sulfide nanomaterials. The nanomaterial should be carefully designed to control the properties of nanomaterial required for different applications. Synthetic methods play a pivotal role in this regard to obtain the material of desired characteristics including surface properties, optoelectronic properties, size, and morphology, which in turn influence the potential of the materials for different applications [47]. Various synthetic approaches have been explored to synthesize silver sulfide of varying shape and size required for various applications [48]. Widely used methods include hydrothermal/solvothermal methods, hydrochemical bath deposition, microwave-assisted methods, ultrasonic methods, and green methodology. Each method has its own advantages and limitations. Few methods are eco-friendly, and others can be used universally to synthesize nanostructures of different properties.

2.1 Hydrothermal approach

Hydrothermal approach is based on the simple wet chemical reaction in solution. Temperature of the reaction varies from room temperature to very high. It is one of the most widely used methods for the synthesis of nanomaterials of wide range of shapes and sizes. Many forms of silver sulfide nanostructures have been synthesized using hydrothermal approaches. Zhou et al. synthesized single crystalline hollow nano hexagons of Ag₂S using a simple chemical route [49]. Aqueous solution of silver nitrate (AgNO₃), sodium sulfite, and cetyltrimethylammonium bromide (CTAB) was used as precursors. X-ray diffraction (XRD) results confirm the monoclinic Ag₂S, and scanning electron microscope (SEM) and transmission electron microscope (TEM) results indicate the formation of hollow hexagons of silver sulfide nanomaterials. The nano hexagons have been found to be self-assembled in an ordered array, which makes them suitable for designing of nanodevices. NIR emitting colloidal quantum dots of Ag₂S coated with 2-mercaptopropionic acid (MPA) was synthesized using a simple hydrothermal method. The effect of sodium sulfide (Na₂S) and MPA to control the size of Ag₂S quantum dots is studied. The size was observed in the range of approximately 2–3 nm [50]. Chen et al. used a hydrothermal approach to synthesize unique leaf-like Ag₂S nanosheets using ammonia and carbon disulfide. The medium of the reaction was alcohol-water, which determines this unique morphology. XRD results indicate the formation of pure monoclinic β-Ag₂S form [15]. Wang and coauthors used organometallic compounds Ag[S2P(OR)2] (R = CnH2n + 1) to synthesize monodisperse Ag₂S nanocrystals [51]. The methodology involves the tedious route of synthesis of air-sensitive organometallic compounds. Dong et al. modified the hydrothermal method substituting the organometallic compounds with thiourea and surfactant (CTAB) to obtain spherical to hexagons to cubic Ag₂S nanoparticles by controlling the ratio of silver salt, sulfur source, and surfactant. Aqueous medium was used for synthesis, which makes the method environmentally friendly. Monoclinic α-Ag₂S in the range of 40–80 nm is confirmed by XRD and SEM analysis [52]. A simple chemical route at 65°C is employed to synthesize highly crystalline and single-phase Ag₂S nanoparticles using variable concentration of silver nitrate and sodium dodecyl sulfate surfactant. Crystallite size of as-prepared nanoparticles was 39–44 nm, and particle size was 40–60 nm [53]. Thio-Schiff base (2-(benzylidene amino) benzenethiol) has been used as a novel sulfur source and capping agent to synthesize Ag₂S nanoparticles in the presence of various solvents. It has been observed that morphology and size of nanoparticles can be controlled by varying reaction conditions, surfactant, and solvent systems [54]. Use of hydrothermal methods for the formation of various forms of nano Ag₂S using Schiff base by varying the reaction conditions is given in Figure 4. More recently, Munaro et al. have compared the microemulsion and hydrothermal approaches to prepare silver sulfide nanoparticles. Authors concluded that hydrothermal approach give nanoparticles with greater purity and long-term stability against oxidation [47].

Figure 4.

Synthesis of silver sulfide nanoparticles by varying reaction condition, surfactant, and solvent system [54].

Though hydrothermal processes produce a variety of products, there are some limitations associated with viz use of toxic reagents, complicated systems, and nonuniform particle size [9].

2.2 Hydrochemical deposition

Hydrochemical bath deposition or chemical bath deposition is considered as among the most popular methods for the synthesis of metal sulfide nanocrystals. The comparison of various synthetic methods for silver sulfide concluded that the hydrochemical method is a universal method to synthesize silver sulfide colloidal solution, quantum dots, crystalline powder, and Ag₂S/Ag heterostructures [55]. Hydrochemical deposition methods involve the use of aqueous solution of silver nitrate, sodium sulfide, and complexing agent to prepare silver sulfide nanostructures. Silver sulfide has a very low solubility product (6.3 × 10−50 at 298 K); therefore, in the presence of sufficient amount of sodium sulfide, deposition of silver sulfide starts readily. The important point is that various forms of nanoparticles can be produced with a single precursor with slight variation in reaction conditions and concentration. Literature suggests the wide use of sodium citrate as a complexing agent to synthesize various forms of Ag₂S. Sodium citrate is an environmentally benign reagent and a harmless food additive E331, so the method is considered as eco-friendly. Use of chemical bath deposition to synthesize various forms of silver sulfide using sodium citrate is reviewed comprehensively [41]. Silver nitrate, sodium sulfide, and sodium citrate have been used to develop a very simple and facile chemical bath deposition method for the synthesis of monoclinic core silver sulfide having citrate shell. The size of core and shell can easily be tuned by controlling the reaction conditions. It is suggested that the synthesized silver sulfide core @ citrate shell might find application in biology by being nontoxic [56]. In another study, stable colloidal solution and nanocrystalline powder of Ag₂S have been prepared using sodium citrate or sodium-ethylenediaminetetraacetic acid (EDTA) complexing agent. By varying the concentration of silver source, sulfur source, and complexing agent, the size of nanoparticles varied between 500 nm and 40 nm. On decreasing the average size of nanoparticles, the band gap of silver sulfide increased from 0.88 to 1.21 ev. As-synthesized nanoparticles have shown potential antibacterial action [57]. Silver sulfide nanopowder in the range of 1000–50 nm and colloidal solution in the range 15–20 nm have been prepared using chemical deposition from supersaturated solution of AgNO₃ and Na₂S with the help of sodium citrate as the complexing and stabilizing agent [58, 59]. The effect of temperature on the stability of synthesized Ag₂S nanoparticles by chemical deposition from aqueous solution has been studied. Authors observed that by increasing the temperature to around 450 K, nanoparticles grow insignificantly, which indicates the thermal stability of silver sulfide up to this temperature [60]. The authors of the work reported the phase transformation of silver sulfide synthesized via hydrochemical deposition method over a wide range of temperatures [7]. Raid A Ismail et al. investigated the effect of deposition time on the synthesis of Ag₂S nanofilm. They concluded that deposition time affects the size, morphology, optical behavior, and energy gap of synthesized silver sulfide nanofilm [61]. In a very recent work, Stanislav I. Sadovnikov has synthesized Ag₂S quantum dots of varying size ranging from 1 to 30 nm dispersed in polyvinyl alcohol. The author determined the long-term stability of synthesized quantum dots. It was observed to be stable for up to thousand days. The long-term stability might be owing to the fixing of colloidal particles on polymer film. The optical properties study of prepared silver sulfide quantum dots indicated that the decrease in size to 2–3 nm is supported by an increase in their band gap from 1.0 to 1.85 eV [62]. In another study, Raman spectra of silver sulfide powder synthesized via chemical bath deposition is investigated. Raman spectra indicates the photodecomposition of Ag₂S to give metallic silver [63].

2.3 Microwave-assisted methods

In recent years, microwave-assisted techniques have gained considerable attention for the synthesis of nanoparticles. Rapid reaction time with uniform heating makes microwave-based methods advantageous over traditional methods. Due to uniform heating and interaction of electromagnetic radiation with source material, nanocrystals with uniform and narrow size distribution with higher phase purity become possible. Microwave-assisted synthesis has been used successfully over the years to synthesize metal sulfide nanostructures of controlled shape and size. [64]. Synthesis of worm-shaped Ag₂S nanoparticles using microwave irradiation is reported by Xing et al. [65]. Silver nitrate was used as a source of silver and thioacetamide was used as source of sulfur in the presence of aqueous solution of bovine serum albumin (BSA). BSA acts as a stabilizing agent in synthesis. Formation of nano-worm morphology is evident from SEM/TEM results, and monoclinic acanthite α-Ag₂S phase is confirmed by XRD analysis (Figure 5). Nano sulfide was observed as 50 nm wide and 100 nm long. The synthesized nano-worm exhibited potential cytotoxicity over HeLa cells. Sousa et al. investigated the effect of light irradiation and time on the microwave synthesis of silver sulfide and silver nanoparticles. Three different precursors silver nitrate, silver oxide, and silver fluoride were used for the synthesis along with sulfur source 1-dodecanethiol. It was observed that silver nanoparticles were obtained with silver fluoride, whereas silver sulfide nanoparticles were obtained with silver nitrate and silver oxide. Authors of this work concluded that in the presence of light (smaller reaction time), spherical nanoparticles of smaller size of around 6 nm were formed, and in the absence of light (larger reaction time), nanoparticles of larger size of around 100 nm were obtained [21]. Long Ag₂S nanowires were synthesized via microwave irradiation at different power using thioglycolic acid as sulfur source. The study indicated the effect of microwave power on morphology of synthesized nanowires [66]. Yaghmour and coauthor developed a squalene-assisted microwave irradiation method to synthesize monoclinic rod-shaped Ag₂S with a diameter of around 10 nm and length of around 32 nm [27]. In another study, Ren et al. developed a facile and rapid microwave method to synthesize water-soluble silver sulfide quantum dots with silver nitrate and d-penicillamine at a low temperature. As-prepared quantum dots possess bright red luminescence with excellent stability and superb biocompatibility, which makes them suitable for bio-imaging of HeLa cells [67]. Al-Shehri et al. employed a 700 W microwave irradiation for rapid synthesis of irregular shaped silver sulfide nanoparticles in the presence of varying concentrations of surfactant, sodium dodecyl sulfate. The crystallite size of crystalline nanoparticles was observed in the range of 44–49 nm. Results indicated that concentration of sodium dodecyl sulfate regulates the shape and size of nanoparticles. With increasing concentration of surfactant, the size was found to decrease and energy gap was found to decrease [68]. Mirahmadi et al. explored microwave-activated synthesis of silver sulfide nanoparticles [69]. Furthermore, the microwave-assisted method is used successfully for synthesizing carambola-shaped silver sulfide microspheres [70].

Figure 5.

XRD patterns (A) and transmission electron microscopy images (TEM) (B) of the synthesized nanoparticles, starting from AgNO3 (1), Ag2O (2), and AgF2 (3) [Reproduced from [21], copyright © 2020 American Chemical Society].

2.4 Sonochemical synthesis

Since the first synthesis of metal chalcogenides nanomaterials using ultrasonic approach [71], it has widely been used for the synthesis of different nanomaterials [72]. Owing to the interaction of ultrasonic energy and matter, the unique reaction conditions of high temperature and high pressure followed by rapid cooling increase the possibility of the formation of nanomaterials of a uniform shape and size. Acoustic cavitation, that is, formation, growth and impulsive collapse of gas bubbles in ultrasonically irradiated liquid, is considered to be responsible for the synthesis of unique nanomaterials with high purity and large surface area [73, 74, 75]. In recent years, silver chalcogenides nanostructures have been successfully synthesized using simple and rapid sonochemical approaches [76]. The very first synthesis of silver sulfide polymer composite using a sonochemical approach was reported by Kumar et al. [77]. Du et al. synthesized long silver sulfide nanowire via ultrasonic irradiation of the solution of silver nitrate and thioglycolic acid. The authors investigated the effect of time of irradiation and observed long width and smooth surfaced nanowire of Ag₂S after 60 min of irradiation [78]. A simple ultrasonic induced methodology at room temperature was used to prepare silver sulfide and Li-doped silver sulfide using silver nitrate and 1-dodecanethiol precursors [26]. As-synthesized Ag₂S nanoparticles exhibited excellent absorption and emission behavior in the NIR region. Authors used first principle calculation to study the effect of Li doping, and it was found to enhance NIR photoluminescence properties. Moshafi et al. employed micellization-assisted ultrasonic method to synthesize Chitosan crosslinked Ag₂S nanocomposites for antibacterial action [79]. Sulfuration mechanism of sulfuration of silver nanowire using sodium polysulfide under ultrasonic treatment is investigated by Ren and coworkers [80].

2.5 Synthesis using decomposition of single molecular precursors

Single molecular precursor decomposition approach has attracted considerable attention in recent years to synthesize nanoparticles with higher purity and higher yield. In the procedure, by injecting organometallic compounds into a hot solvent, pure nanomaterials with desired morphology can be obtained by decomposition of the precursor. This method is advantageous over other methods by being rapid, safe, and compatible. Moreover, using this approach, highly monodisperse nanoparticles of similar morphology and size can be obtained [51]). In a very first report, silver thiobenzoate was used as a precursor in the presence of amine to synthesize nanocrystals of Ag₂S with different morphology including nanocubes, nanorods, and faceted nanocrystals by varying reaction conditions (Figure 6) [19]. Later, the method was modified by Tang and coworkers to synthesize Ag₂S without any alkyl amine activator. At the temperature of 160°C silver thiobenzoate was injected into a preheated system to give spherical Ag₂S nanocrystals. At a temperature lower than 160°C, a mixture of silver and silver sulfide was obtained [81]. Wang et al. [82] reported a simple pyrolysis route to synthesize crystallite Ag₂S using silver diethyldithiocarbamate (Ag DDTC) single molecular precursor at 200°C. XRD results indicated the formation of pue α-Ag₂S form. Crystallite particles of size 20 nm–1.1 μm were confirmed by SEM micrograph. In another study, Zhang and coauthors used silver diethyldithiocarbamate precursor to produce surface ligand coated silver sulfide quantum dots. The uniform size nanoparticles was obtained in the range of 2.4–7 nm [83]. Synthesis of NIR photoluminescent Ag₂S was reported by Wang and his group via thermal decomposition of Ag-DDTC in the presence of oleic acid, octadecylamine, and 1-octadecene solvents. NIR emission was observed at 1058 nm [84]. Khirid et al. fabricated silver sulfide nanowire using [(PPh3)2AgS2P(OiPr)2] single precursor in the presence of a mixture of amines for catalytic application in hydrogen evolution reaction [85]. In a more recent study, Ag(I) complex of N-methyl-N-phenyl dithiocarbamate has been used as a single source precursor to obtain oleylamine capped monoclinic Ag₂S nanoparticles at 180°C [86]. Khan and coworkers described single molecular precursor method for the synthesis of silver-based metallic, intermetallic, and metal sulfide nanoparticles using xanthate precursors. Potential of the synthesized nanoparticles as a catalyst for oxygen reduction reaction was investigated [87].

Figure 6.

Synthesis of silver sulfide nanocrystals of varying shape by decomposition of silver thiobenzoate at different reaction conditions [Reproduced from Ref. [19], copyright © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].

Though precursor decomposition methods produce pure compounds with controlled shape and size, there are certain limitations that should be addressed, such as complex procedures to form air-sensitive organometallic compounds, involvement of high temperature, use of solvents, and production of relatively larger-sized nanomaterials. It is therefore crucial to develop more precursors that can be easily synthesized and can decompose under mild conditions of temperature and pressure. Moreover, eco-friendly solvent-free synthetic methods should be investigated for the decomposition of precursors. To overcome the limitation of pre-synthesis of organometallic precursors, interest is arousing to develop synthetic routes without prior precursor injection. Li and coworkers developed one pot synthesis to self-assemble Ag₂S nanocrystals into tetrahedral aggregates with colloidal properties. Dodecanethiol (DDT) acted as both sulfur source and structure-directing agent [24]. Group of Tang prepared Ag and Ag₂S nanocrystals by selective cleavage of Ag-S and C-S bonds. In the method, no precursor was injected; silver salt was heated in the presence of DDT. Reaction temperature, amount of DDT, and concentration of silver salt controlled the phase purity and structural features of prepared nanocrystals [88].

2.6 Biosynthesis/green synthesis

By considering disadvantages of conventional synthetic routes using environmentally hazardous chemicals, research is going on to develop eco-friendly methods to synthesize nanoparticles. There is significant progress in developing green chemistry based methods for the synthesis of silver sulfide nanoparticles [89, 90]. Several bio-organisms including plant extract and microorganisms have been used successfully for the synthesis of silver sulfide nanostructures for a variety of applications. Group of Awwad used rosemary leaves extract to prepare silver sulfide nanoparticles for antibacterial properties [91]. Recently, Upadhyay and Kothari have developed a simple and rapid method to synthesize spherical silver sulfide nanoparticles of 50 nm size using Cinnamomum tamala leaves extract as a reducing and capping agent for pharmacological applications [92]. Another study described the novel green synthesis of silver sulfide nanoparticles using Pleurotus ostreatus mycelium [93]. SEM results confirm spherical-shaped particles in the range of 10–15 nm. The synthesized nanoparticles had shown to have antibacterial action against Escherichia coli and Bacillus thuringiensis. Ayodhya and Veerabhadram reported a green synthesis of spherical silver sulfide nanoparticles of 25 nm size using Cochlospermum gossypium [94]. Moreover, the potential of Diospyros kaki fruits was demonstrated for eco-friendly synthesis of silver sulfide for photocatalytic activity in hydrogen evolution [95]. Moloto and coauthor analyzed the effect of pH on the biosynthesis of silver sulfide nanoparticles using green tea Combretum molle, black wattle extract, and chitosan [96]. Zahedifar et al. developed a simple green method for the synthesis of silver sulfide nanoparticles on the cellulose/Fe3O4 nanocomposite. They used leaf and seed of Pistacia atlantica plants as a reducing and capping agent. As-synthesized nanoparticles were efficiently used for the degradation of organic dyes [97].

Further, biosynthesis of nanoparticles using microbacteria is also gaining popularity by being eco-friendly without the use and generation of toxic waste material [98, 99]. Gram-negative bacteria Shewanella oneidensis MR-1 is widely used for the green synthesis of functional nanomaterials [99]. Representative work based on biosynthesis of silver sulfide using microbes is given in Table 2.

Precursor/Reaction conditions	Microorganism	Shape of the nanoparticles	Size of the nanoparticles	References
AgNO3 and Sodium thiosulfate (Na2S2O3); Room Temp	Shewanella oneidensis MR-1	Spheres	2–16 nm	[100]
AgNO3 and Na2S2O3, liquid nutrient medium containing the grown cells	Shewanella oneidensis MR-1	Spherical	4–10 nm	[101]
AgNO3 and Na2S2O3, living as well as ultrasonically disrupted cells	Shewanella oneidensis MR-1 cells	Spherical	7.8 ± 1.5 nm (in presence of living cells); 6.5 ± 2 nm (in presence of ultrasonically disrupted cells)	[102]
AgNO3 and Na2S2O3	Shewanella oneidensis MR-1 cells	Spherical	9 ± 3.5 nm	[103]
AgNO3 and Na2S2O3, aerobic condition	Shewanella oneidensis, Escherichia coli K12, Bacillus subtilis 168	Spherical	8 ± 2 nm; (using S. oneidensis MR-1 and Escherichia coli K12); 10 ± 3 nm (using B. subtilis 168)	[104]

Table 2.

Biosynthesis of Ag₂S nanoparticles using microbes.

3. Photocatalytic applications of silver sulfide nanoparticles

The catalyst absorbs light and is involved in the change in the rate of a chemical reaction called photocatalytic reaction. There are different nanostructures used as photocatalyst for various photocatalytic reactions. Photocatalysis have become highly useful techniques to resolve environmental and energy issues. In photocatalytic reactions, the energy is obtained directly from sunlight, which degrade harmful water pollutants, split water to produce oxygen and hydrogen, and induce nitrogen fixation and CO₂ reduction [40]. The sunlight coming to the Earth’s surface has a composition of 49% IR, 46% visible, and 5% UV light [105]. The efficacy of semiconductors depends upon the value of the band gap; the band gap higher than 3 eV is excited by UV light called wide-band-gap photocatalyst, and the band gap lower than 3 eV is excited in visible light called narrow-band-gap photocatalyst [106]. Generally, the silver sulfide nanostructures possess narrow band gap (around 1 eV) and high absorption coefficient (∼10⁶ cm). Silver sulfide shows strong absorption in the range of 200–600 nm. These properties are the reason for the high stability and low toxicity of Ag₂S nanoparticles and thus facilitating their photocatalytic properties. In this chapter, we are discussing the photocatalytic behavior of silver sulfide nanostructures for dye degradation.

3.1 Dye degradation

Dye is generally used for coloring purposes, and hence, its requirement is always high due to the textile industry. Hence, dyes as industrial pollutants have a negative impact on the environment, especially water pollution. Due to this water pollution, not only are our natural food chains affected; it also affects the aquatic habitat. Therefore, the degradation of these dyes is highly appreciable. There are different types of dye used for industrial purpose; degradation of some of the frequently used dyes is discussed below:

3.1.1 Methylene blue (MB)

Methylene blue (MB) is one of the widely used dyes. It absorbs at around lambda max (λ max) 665 nm. The discharging of MB into the water bodies is measured as a major threat for human and aquatic animals due to aesthetical and toxicological reasons [32]. At a definite concentration, it affects the human health such as respiratory distress, blindness, abdominal disorders, digestive irregularities, and mental illness [107].

Z. R. Mubarokah et al. successfully synthesized silver sulfide nanoparticles incorporated into cellulose and compared their photocatalytic properties with commercially available materials for the degradation of MB dye. The results indicated that the commercial samples showed an increase in their activity by up to 100% degradation in 2 hrs of sunlight exposure with 0.1 g Ag₂S catalyst loading onto the cellulose film, whereas the synthesized samples showed 98.6% degradation [32]. A. Pourahmad et al. employed ion exchange method to synthesize Ag₂S/MCM-41 photocatalyst for the degradation of methylene blue [108]. They have evaluated the effect of parameters such as catalyst loading, pH, and initial concentration of dye on degradation using Ag₂S/MCM-41 photocatalyst. It was observed that the degradation reaction follows pseudo-first order kinetics, and 0.6 g/L of photocatalyst is an optimum value for the catalytic degradation. Interestingly, it was found that the degradation efficiency decreases in dye concentrations above 3.2 ppm. OH Won-Chun and coworker have been reported the synthesis of Ag₂S/TiO₂ nanocomposite by hydrothermal process for the photocatalytic degradation of MB dye. It was observed that the photocatalytic activity of the Ag₂S/TiO₂ photocatalyst under visible light was much higher than that of TiO₂ due to the excellent effects of the Ag₂S semiconductor [109].

3.1.2 Methyl orange (MO)

X. Wang et al. synthesized Ag₂S NCs and deposited them on the surface of Ag₈W₄O₁₆ by anion exchange technique for the photocatalytic degradation of methyl orange dye. The photocatalytic results revealed that the as-synthesized Ag₂S/Ag₈W₄O₁₆ photocatalyst with 2.16 wt% Ag₂S NCs showed the highest activity with a k value of 20.8 × 10⁻³ min⁻¹ for the degradation of MO under visible light. This result is 20 times higher than that of pure Ag₈W₄O₁₆ and by 10 times of N-TiO₂. In the structural analysis of the band gap of Ag₂S and Ag₈W₄O₁₆ semiconductors, it was observed that the photocatalytic activity is significantly dependent on the size of Ag₂S NCs [110]. Yi Xie et al. reported the synthesis of Ag₂S-coupled TiO₂ nanocomposite by following a wet chemical method for visible-light-induced photocatalytic degradation of methyl orange dye. This nanocomposite has been found to have a higher visible light absorption capability than that of pure TiO₂. The photocatalytic efficiency of synthesized Ag₂S/TiO₂ nanocomposites is found higher than that of commercial TiO₂ [111]_.

3.1.3 Methyl green (MG)

M. S. Hamdy and coworker have prepared Ag₂S NPs using different amount of sodium dodecyl sulfate surfactant by the microwave-assisted method and named the samples as S1, S2, S3, S4, and S5. The prepared nanoparticles have 3D irregular shape, close to fused-spheres or worm-like structure with size in range from 44 to 49 nm. The synthesized samples S1, S2, S3, S4, and S5 were examined for the photocatalytic decolourization of methyl green dye under UV/visible light. The photocatalytic results under UV light after 70 min analysis with 12–15% adsorption of the dye over the surface of Ag2S indicate that the 83 and 99% of the dye was decolorized using S1 and S5, respectively. However, in visible light, 62 and 85% of the dye was decolorized over S1 and S5 after 75 min, respectively. Hence, the excellent photocatalytic behavior of Ag₂S samples is highly suitable for water treatment applications [68].

3.1.4 Alizarin yellow R (AYR)

Alizarin yellow R (AYR) is a highly hazardous water-soluble anionic dye that was prepared in 1887 by Rudolf Nietzki by the reaction of m-nitroaniline and salicylic acid. AYR is an important dye widely used in various industries such as textile, paints, leather, and plastic industries. In addition, it is also used for laboratory purposes like as an acid-base indicator, stains, and nutrient media preparations. Gururaj M. Neelgund et al. prepared CNTs-Ag₂S nanocomposite by the facile hydrothermal process. The Ag₂S nanoparticles were deposited on functionalized carbon nanotubes (CNTs). The CNTs-Ag₂S completely degrades the hazardous AYR present in water under the illumination of natural sunlight. The analyzed Langmuir-Hinshelwood (L-H) model indicated the degradation of AYR proceeded through pseudo-first-order kinetics. The analyzed Langmuir-Hinshelwood (L-H) model indicated the degradation of AYR proceeded through pseudo-first-order kinetics. The apparent rate constant values of Ag₂S (0.0035 min⁻¹), CNTs (0.0108 min⁻¹) and CNTs-Ag₂S (0.0378 min⁻¹) determined that the CNTs-Ag₂S was 11 times greater than the Ag₂S and about four-fold higher than CNTs (Figure 7) [23].

Figure 7.

Langmuir-Hinshelwood plot for the degradation of AYR in the presence of CNTs-Ag₂S under sunlight [Reproduced from Ref. [23], copyright© 2023 by the authors. Licensee MDPI, Basel, Switzerland].

Gururaj M. Neelgund et al. [23] also proposed the possible mechanism for the degradation of AYR dye (Figure 8). They suggested that the AYR degradation could happen in the presence of the catalyst by breaking its conjugated system under sunlight irradiation. They have given the plausible mechanism for the rapid degradation of AYR by CNTs-Ag₂S, which is explained by the following equations:

Figure 8.

The proposed mechanism for the degradation of AYR using CNTs-Ag₂S nanocomposite under sunlight [Reproduced from Ref. [23], copyright© 2023 by the authors. Licensee MDPI, Basel, Switzerland].

$$\text{CNTs}‐{\mathrm{Ag}}_2\mathrm{S}+\mathrm{h\nu }\to {\mathrm{e}}^{-}+{\mathrm{h}}^{+}$$E1

$$\mathrm{h}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{OH}}^{-}+{\mathrm{H}}^{+}$$E2

$${\mathrm{e}}^{-}+{\mathrm{O}}_2\to {{\mathrm{O}}_2}^{·-}$$E3

$${{\mathrm{O}}_2}^{·-}+{\mathrm{H}}^{+}\to {\mathrm{HO}}_2^{·}$$E4

$${\mathrm{HO}}_2^{·}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{HO}}^{·}$$E5

$$\mathrm{AYR}+{\mathrm{HO}}^{·}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2+\text{Nontoxic\hspace{0.17em}product}$$E6

4. Conclusion

During last few decades, silver sulfide synthesized via various routs has found multifaceted applications ranging from electronics to photocatalysis. Through a comprehensive literature survey, the present chapter summarized the recent progress in the synthesis of various forms of nanostructured silver sulfide using different synthetic methods. Hydrothermal process, chemical bath deposition, microwave-assisted methods, sonochemical method, single molecular precursors decomposition, and biosynthesis are described and analyzed in details. Each method offers its own advantages and limitations in terms of controlled shape and size enabling the altering of silver sulfide nanoparticles for distinct applications. Moreover, photocatalytic application of silver sulfide nanoparticles for the degradation of organic dyes is systematically presented for environmental problems. Though various synthetic routes with improved features are devised for the synthesis of silver sulfide nanoparticles, still there are some challenges and future perspectives that warrant attention. To mention a few, low concentration of colloidal silver sulfide, wide size distribution, heterogeneous size and shape of prepared nanoparticles, and the scalability and cost-effectiveness of synthesis methods should be addressed to empower the practical application of silver sulfide nanostructures. These issues can be solved in coming years by considering the development of more focused and environment-friendly methods. Further research is needed to understand the basic mechanisms of the photocatalytic activity of silver sulfide nanostructures, allowing for the rational design of improved silver sulfide based photocatalytic materials. Continual efforts in the area will surely contribute to advancement in photocatalytic process and pave the way for cleaner future.

Acknowledgments

The authors are thankful to University of Delhi for funding under Innovation Project scheme 2015-2016 (DBC-303 and DBC-308).

Conflict of interest

The authors declare no conflict of interest.