Synthesis and Functions of Ag2S Nanostructures

The paper presents a review about synthesis and applications of Ag2S nanostructures. As the modern photoelectric and biological materials, Ag2S nanomaterials are potentially useful for both structure and function purposes. Ag2S is a direction narrow band gap semiconductor with special properties. Ag2S nanostructures have been widely researched in chemistry and biochemistry fields because of their unusual optical, electrical, and mechanical properties. It can also be used in many fields, such as photovoltaic cells and infrared detector. In the past few years, Ag2S nanostructures have been synthesized by various methods. The article mainly discusses the four types of preparation methods. Moreover, this article shows a detailed review on the new properties, fabrication, and applications of Ag2S nanocrystals.


Introduction
Nanoparticles are different from molecular and block materials and have many special physical and chemical properties. In the past decades, the synthetic methods and tuning morphologies of single-component nanocrystalline have made great progress. The metal sulfide nanomaterials have attracted widespread attention due to their suitable band gap, easy manufacturing, low cost, and high performance [1][2][3][4].
As an important metal sulfide, Ag 2 S is a direction narrow band gap semiconductor (~1.5 eV), and highabsorption coefficient is approximately 10 4 m −1 [5,6]. It also has good chemical stability and optical properties [7,8], so it is widely used in various fields, such as semiconductor, photovoltaic cells, infrared detector, and superionic conductor [9][10][11][12]. Recently, it also has been used in the photoelectric switch and oxygen sensor at room temperature [13]. So far, Ag 2 S nanoparticles were successfully prepared by microemulsion method, sol-gel method, particles embedded technology template method, etc. [14][15][16][17]. But, it often requires strict preparation conditions, time-consuming, energy consumption, and large size distribution in the traditional production methods. Therefore, simple and economic routes of uniform size distribution of Ag 2 S nanomaterials still have challenges.
In this review, we summarized the preparation and properties of Ag 2 S materials as follows.

Synthetic Methods
Ag 2 S nanostructures have been synthesized by various methods, such as sol-gel method, particles embedded technology template method, etc. Every method has both advantages and limitations. So the preparation methods have still challenges. The new performance of the Ag 2 S nanostructures needs to be further exploded. So, the article summarized four types of preparation methods to expect to provide a little help for the workers who are engaging in this field.

Formation of Different Forms of Ag 2 S Nanoparticles
Recently, the great efforts have been focused on the morphology control of the semiconductor nanocrystals due to their morphology-dependent properties [18]. Thus, the preparation of different forms of Ag 2 S nanoparticles (such as urchin- [19], snow- [20], dendrite-like [21,22] nanocrystals, and so on) has been caused many scientific research in order to expend their current applications. Various methods in the preparation of Ag 2 S nanostructures and their mechanism have been explored. For example, Zhao et al. [23] prepared rod-like Ag 2 S nanocrystallines by using Na 2 S 2 O 3 as a chalcogen source via gamma ray irradiation at room temperature. In this experiment, polyvinylpyrrolidone (PVP) as a guide reagent of crystal growth plays an important role in the formation of rod-like Ag 2 S nanocrystallines. It is well known that high-energy gamma ray irradiation can make H 2 O producing strong reductive e aq − that can initiate many redox reactions to generate ions, which cannot happen in a common atmosphere. And, the homogeneously dispersed S 2 O 3 2− ions reacted with the generated reductive particles to form S 2− [24]. And then, Ag + could react with S 2− to form Ag 2 S nanoparticles. Ag 2 S nanorods were formed in the solution of the PVP [25]. Figure 1 shows that the diameters of the nanorods rang from 200 to 500 nm, and the length is up to several ten micrometers. The band gap of Ag 2 S nanorods calculated from the UV-vis spectrum is 2.34 eV, which shows an obvious blue shift in UV-vis absorption compared with the bulk Ag 2 S (Fig. 2).
Chen et al. [26] reported that the leaf-like Ag 2 S nanosheets were prepared successfully by a facile hydrothermal method in alcohol-water homogenous medium. In the experiment, CS 2 used as sulfur source was dissolved in alcohol and AgNO 3 and water at the beginning, respectively. Then the two solutions were mixed together. The reaction is as follows: The picture of TEM (Fig. 3) shows all of the samples are leaf-like.
However, Xu et al. [27] reported that the Ag 2 S was prepared with an alcohol solution method using CS 2 as sulfur source also. But, all the products were irregular nanoparticles. The reaction medium was changed from water and alcohol-water to alcohol. So, the morphology changed from big irregular nanosheets and microspheres, leaf-like nanosheets nanoparticles to big microspheres, respectively. It is found that alcohol-water homogenous condition and sulfur source of NH 3 -CS 2 played the key roles in constructing this unique morphology. That is because the formation of nucleation and growth is expected to be strongly dependent on the properties of the solvent during processes such as coarsening and aggregation [28,29]. The different morphologies of   [23] Ag 2 S nano-and micromaterials, including spokewise micrometer bars, nanowires, and nanopolyhedrons have been gained by a facile one-step method at room temperature [30]. In the route, organic template materials were not added into the reaction container. It only changed the ratios of Ag + , S 2− , and ammonia, which may produce the different size and morphologies of the products. In this route, the spokewise microbars of Ag 2 S were successfully prepared. Firstly, 10 mL of 0.2 M Tu ((NH 2 ) 2 CS) was added in 10 mL of 0.7 % ammonia. Then, 5 mL of 0.3 M Ag 2 NO 3 was quickly added in the solution with stirring for 20 min. Last, the product was purified and redispersed in the water several times by centrifugation and sonication (Fig. 4). The methods with other morphologies of Ag 2 S were similar to the above process, except for the concentration of ammonia, AgNO 3 , or Tu. By controlling the concentration of ammonia, AgNO 3 , or Tu, the morphology of Ag 2 S could be easily tuned (Table 1 and Figs. 5, 6,  and 7).
Recently, polyhedral nanocrystals including nanocubes have been attracted intensive attention and a variety of face-centered cubicin organic nanocrystals [31][32][33] have been successfully fabricated. The Ag 2 S nanocrystals were also prepared by Lim et al. [34] and Wang et al. [35] by decomposing exothermically organometallic precursor silver thiobenzoate (Ag(SCOPh)) and Ag[S 2 P(OR) 2 ] (R = C n H 2n + 1 ), respectively. However, the method often suffers from elaborate  preparation of air sensitive, expensive organometallic complexes that unstable in air. However, the temperature was high during the experimental with the inert gases protecting. And, the solvent was needed to coordinate. The above method has been improved by Dong et al. [36]. A simple hydrothermal route was reported by modulating the ratio of Tu and AgNO 3 with assistance of cetyltrimethyl ammonium bromide (CTAB), respectively. It was also found that the cooperation effect of CTAB and Tu should be responsible for the formation of the asobtained Ag 2 S nanocrystals. Face-centered cubic Ag 2 S nanocrystals were synthesized successfully in aqueous medium, which makes the synthesis environmentally benign, user-friendly, economical, and practicable to industry production. The images of SEM showed that the size of the particles ranged from 40 to 80 nm, and the particles with a size of over 100 nm were found occasionally. The typical TEM image showed that most of the Ag 2 S nanocrystals particles appeared hexagonal in shape confirming the faceted nature of the nanocrystal. The UV-vis absorption spectrum of the products showed obvious blue shift owe to the small size.
Ag 2 S nanomaterials were synthesized by a large number of methods. For example, single-crystalline Ag 2 S hollow nanohexagons with better quality and narrower size distribution were successfully reported in aqueous solution at low temperature [37]. The formation mechanism of Ag2S hollow nanohexagons is shown in Fig. 8. Figure 9 displays the SEM images of a typical of Ag 2 S nanohexagons. It indicates that the products are good uniformity. In addition, they are hexagonal in shape with a narrower plane size distribution. And, an edge-to-edge distance of 48.9 ± 1.83 nm was achieved by using this approach. From highmagnification TEM image and high-magnification SEM image show that some of these nanohexagons broke. It is shown that they are hollow inside and single crystalline. Because of the high uniformity and Vander Waals interactions, the hollow nanohexagons spontaneously assemble into high-quality ordered arrays [38,39]. Due to the character, the hollow nanohexagons may have potential applications in fabricating new useful nanodevices in the future. Meanwhile, Rajib Ghosh Chaudhuri et al. [40] reported an easy and novel route for the synthesis of hollow Ag 2 S particles by a sacrificial core method in surfactant containing aqueous media. High-aspect-ratio of worm-like Ag 2 S nanocrystal with length up to several micrometers and the diameter of 25~50 nm has been successfully prepared by a Triton X-100/cyclohexane/ hexanol/water W/O reverse microemulsion in the presence of TAA (thioacetamide) as a sulfur source Table 1 Starting chemicals used in the syntheses of Ag2S and the morphologies of the products [30] No.
Concentration of reactants  and EDTA (ethylenediaminetetraacetic acid) as a chelating ligand [41]. The as-synthesized Ag 2 S nanocrystals exhibit strong absorption in UV region, and the absorption edge at about 290 nm ( Fig. 10) corresponding to the band gap of 4.3 eV. Compared to the absorption band of bulk Ag 2 S (1240 nm, Eg = 1.0 eV) [42], the observed absorption edge is a significant blue-shift. The result is due to the positiondependent quantum-size effect and shape effect. Figures 11,12, and 13 show the TEM images of Ag 2 S nanocrystal synthesized with the typical experimental procedure, in which change one condition.
The results indicate that the morphology and size of Ag 2 S nanocrystal can be readily controlled by modulating the mole ratio of Ag + to EDTA, the molar ratio of water to surfactant (ω 0 ), and the aging time.
The diameter distribution of Ag 2 S nanocrystal becomes wider with the increasing ω 0 . It can be explanted that at low ω 0 water inside the reverse micelles is considered to be "bound". Therefore, insufficiently available to dissolve the surfactant head group and counterion, the microemulsion becomes more fluid with increasing ω 0 , which accelerated the growth of nanocrystalline Ag 2 S. The effect of EDTA concentration on the formation of worm-like Ag 2 S nanocrystals is important. It can coordinate Ag + to form relative stable Ag-EDTA complex, which lowers the effective concentration of Ag + ; TAA could release S 2− very slowly in the solution. The two aspects make the Ag + and S 2− react slowly, which could result in the separation of nucleation and growth step and is favorable for the directional growth of the crystal nuclei [43]. Ag 2 S nanorice was synthesized by reaction between [Ag(NH 3 ) 2 ] + and Na 2 S·9H 2 O in the presence of PVP through hydrothermal method [44]. And, the feature of the Ag 2 S nanostructure depends mainly on the type of sliver source, influence of the pyrrolidone rings of PVP, reaction time, and temperature. TEM technique was employed to inspect the morphological variation of the Ag 2 S nanoparticles obtained by using different sliver sources (Fig. 14 a and b) at 160°C for 10 h. It was found that the well-dispersed Ag 2 S nanoparticles presented an approximately uniformed rice-shaped morphology. But when AgNO 3 was changed into Ag[(NH 3 ) 2 ] + as the sliver source without a control  [30] over the Ag + release rate provided by ammonia complexation during the reaction, the rice-shaped morphological feature of Ag 2 S nanoparticles would not been seen, and anamorphous appearance would be presented instead. So, Ag + concentration had a key impact over the formation of the Ag 2 S nanorice. Similar condition also happened while reaction time influences on the experimental results (Fig. 14 c and d). It can be explained by the famous Ostwald ripening mechanism. The FI-IR spectra of pure PVP, Ag 2 S nanorice-associated PVP are shown in Fig. 15. It is easy to find that the pyrrolidone ring plays an important role in the formation of the Ag 2 S nanorice. Normal and flattened rhombic dodecahedral submicrometer Ag 2 S particles were prepared by adjusting the ratio of Tu to AgNO 3 and volume of concentrated HCl aqueous solution [45].

Preparation by Using Bionic Technology
In recent years, the synthesis of quantum dots (QDs) with biological macromolecules route has attached great attention [46]. In previous works, CdTe QDs with good biocompatibility by using RNase A as the template was synthesized successfully [48]. But, it is not popular The inset is the corresponding electronic diffraction pattern from one nanohexagon. Scale bars, a 100 nm, b 5 nm, and c 100 nm [37] enough because of the toxic nature of Cd and Te. Ag 2 S QDs is treated as an ideal optical probe because it has lower toxicity compared with previous prepared nearinfrared (NIR) QDs which was synthesized in organic phase. But, the process may cause extra environment pollution [49,50]. The synthesis of QDs with fluorescence emission from UV to NIR regions has made great progresses as optical probes for in vitro and in vivo molecular imaging [47]. So, the highly monodisperse and water soluble RNase-Acopped-Ag 2 S QDs clusters were synthesized via biomimetic route in aqueous phase [51]. The QDs have low cytotoxicity and good biocompatibility. Meanwhile, the produce process is environmental friendly. From the images (Fig. 16), it indicates that RNase A-Ag 2 S QDs clusters with irregular morphologies were dispersed, and the Ag 2 S nanocrystals have clear lattice fringes. Furthermore, X-ray diffraction (XRD) image shows that the prepared Ag 2 S QDs assumed the crystalline structure of monoclinic α-Ag 2 S. Tetrazolium-based colorimetric assay (M77 test) shows that RNase A dose not only serves as a stabilizer agent in the formation of Ag 2 S QDs to avoid aggregation but also is a biomolecule to modify the surface of Ag 2 S QDs to decrease toxicity. So, the products have great potential application in molecular imaging in living cells and tissues. Biomolecules assisted the formation of inorganic nanostructures, facilitate the electrostatic stabilization, and thus improving optical properties. Siva C et al. [52] reported that Ag 2 S nanostructure was obtained using Ag 0 nuclei as a core. Biomolecule (L-cysteine) can act as a sulfur source and stabilizing agent which prevents the agglomeration [53]. Meanwhile, they also have obtained the novel Ag 3 AuS 2 nanocrystals by adopting the gold ions in the L-cysteine-assisted Ag 2 S formation. The FT-IR images (Fig. 17) show that one L-cysteine molecule is interconnected to another via hydrogen bonding. From Fig. 18, it is clear that the Ag 2 S nanocrystals are almost uniform in their size and are interconnected among themselves, and their average particle size is 5.2 nm. Meanwhile, the Ag 3 AuS 2 nanocrystals are also connected with themselves. The Ag 3 AuS 2 nanocrystals are non-uniform in their sizes, and the average size is 9.1 nm. Self-organized nanocrystal architectures with subnanometric spatial resolution also obtained by mimicking the biological crystal growth [54].

Preparation by a Single Molecular Precursor of Decomposition
Among the many methods for synthesizing metal chalcogenide materials, the single molecular precursor route has some appealing features [55,56]. On the one hand, it offers the distinct advantages of mildness, simplicity, safety, and particular compatibility with the metalorganic chemical vapor deposition [57]. On the other hand, the molecular precursor may be related to the unusual crystal growth selectivity or metastable phase formation of the resultant products, which are sometimes unattainable via conventional synthesis techniques [55,56]. For example, Ag 2 S nanocrystals were achieved via a modified hotinjection process from a single-source precursor molecule Ag(SCOph). When the precursor molecule is injected into a preheated reaction system at 160°C, spherical Ag 2 S nanocrystals are directly obtained [58]. Wang et al. [59] obtained the Ag 2 S crystallites by heating the Ag-DDTA in air at 200°C for 3 h and used an air-stable single-source molecular precursor (Ag-DDTA) as the react source. The method is both economic and non-toxic. Monodispersed and size-controlled Ag 2 S nanoparticles were synthesized successfully via a green and simple surfactant-free solventless thermolysis of silver xanthates as singlesource precursors [60]. In the experiment, the diameter of Ag 2 S nanoparticles is from 8.9 ± 1.2 nm to 48.3 ± 3.6 nm (Fig. 19). And, the "Size control" was  achieved by simply changing the alkyl chain length in the precursor. The grain size of Ag 2 S nanoparticles decreases with the increase of the alkyl chain length of the precursors. At the same time, with the temperature increasing, the xanthate ligand will be absorbed onto the surface of Ag 2 S nanoparticles to control particle growth. Figure 20 shows the possible growth mechanism.

Other Methods of Preparation
In addition, there are many other methods. For example, Ag 2 S nanocrystals were prepared via a facile solution growth method, in which Ag 2 S and sulfur powder are used as precursors. Oleylamine is used and act as both reducing agent and stabilizer during the synthetic process [61]. The obtained Ag 2 S nanocrystals can be used as substrates for surface-enhanced Raman scattering (SERS) detection in this method. SERS spectra of rhodamine 6G can be detected, the synthesis strategy is simple, and the obtained samples have great potential for high sensitive optical detection. This character may attract much interest in fundamental physics as well as device application points of view. Maryam Shakouri-Arani et al. [62] have produced the Ag 2 S nanoparticles by a solvothermal process; a new sulfuring agent from class of thio Schiff-base benzenethiol was used in the presence of various solvents. In this paper, we also found that the shape and size of the Ag 2 S can be controlled by means of setting certain reaction parameters such as the reaction temperature, presence of surfactant, and type of solvent (Fig. 21).

Application in Biotechnology
In recent years, Ag 2 S nanometer materials in the application of biotechnology have gradually aroused people's concern and attention. Many efforts have been devoted to identifying NIR-II emitting agents for in vivo imaging applications. QDs such as PbSe, [63] PbS, [64] and CdHgTe [65], with NIR emission, were successfully   [44] obtained. But, the highly toxic nature of Pb, Cd, and Hg is of concern for in vivo applications [66]. And, well-designed carbon nanotubes also have been regarded as biological imaging agent in the NIR-II region. However, the disadvantage is the lower fluorescence quantum yield of carbon nanotubes [67,68]. So, Zhang et al. [69] made a study for Ag 2 S QDs, which combined with other biomacromolecules to become the imaging agent. Because Ag 2 S QDs should be more biocompatible owning to the absence of any toxic metals such as Cd, Pb, and Hg. And, Ag 2 S also exhibits an ultra-low solubility product constant (Ksp = 6.3 × 10 −50 ) which ensures the minimum amount of Ag + released into the biological surroundings. Ag 2 S QDs have high-emission efficiency in the unique NIR-II imaging window. So, there are a lot of characters such as deep tissue penetration, high sensitivity, and elevated spatial and temporal resolution; the watersoluble Ag 2 S QDs terminated with carboxylic acid group were synthesized by one-step method reported [70]. The Ag 2 S QDs exhibited bright photoluminescence and excellent photo stabilities. Therefore, the photoluminescence emission could be turned from visible region to near-infrared (NIR) region (from 510 to 1221 nm). So, it has the opportunity to study nanodiagnostics and imaging. In vivo imaging experiment, the Ag 2 S QDs were injected into the nude mice subcutaneous tissue or abdominal cavity. As c Powder X-ray diffraction (XRD) pattern of RNase A-CdS QDs. d EDS spectrum of RNase A-CdS QDs [51] shown in Fig. 22a-c, bright spot of Ag 2 S QDs fluorescence was observed in the mice with subcutaneous and celiac injection compared with the ordinary mice. From the PL spectra (Fig. 22d), it can be seen that the fluorescence emitted from the injection region differ with the auto fluorescence from the other region of the mice body. It indicated that the fluorescence of the as-prepared Ag 2 S QDs can penetrate the body of nude mice. And, the fluorescence emitted from the celiac region was clear and bright. It suggested that the Ag 2 S QDs fluorescence was less affected by the body auto fluorescence. At the same time, the Ag 2 S QDs do not contain toxic elements to body. Thus, it has great potential in vivo imaging. And then, Ag 2 S nanocrystals were applied into DNA hybridization analysis [71]. A DNA probe labeled with Ag 2 S nanoparticles, which detection limit can be attained up to picomoles per liter. It indicated that the product have high sensibility and selectivity. Furthermore, this surfactant-capped Ag 2 S product is likely to be of potential application value in electrochemical detection and biosensors.

Application in the Catalytic and Decomposition
Nowadays, a shortage of clean water can lead to serious problems and diseases. So, water purification problems become more and more important. But, many textile dyes are difficult to degrade with the common methods due to their synthetic origin and the presence of a complex aromatic structure [72,73] TiO 2 is often used as Fig. 17 a FT-IR spectrum of L-cysteine, Ag 2 S, and Ag 3 AuS 2 nanocrystals. b and c are the magnified graphs to show the-SH stretching, and carboxylic group interaction [52] catalytic to remove dyes and phenolics for their higher photocatalytic activity, good photo stability, non-toxicity, and low price. However, the large band gap of TiO 2 (3.2 eV) limits its photocatalytic applications in the UV range and reduces its catalytic efficiency. Because of the unique structure of Ag 2 S, it is expected to be the new type catalyst. And because of the unique structure of Ag 2 S, it can expect to be the new type catalyst. For example, Ag 2 S nanoparticles were prepared by using a hydrothermal method and Ni was doped via a photo-assisted deposition method [74]. The XRD images of the parent Ag 2 S and Ni/ Ag 2 S nanoparticles are contrasted in Fig. 23. They found that the structural characteristics of Ag 2 S and Ni/Ag 2 S are mainly composed of Ag 2 S. It indicated that the Ag 2 S structure remained conserved after the application of the photo-assisted deposition methodology. From the UV-vis diffuse reflectance spectra of the Ag 2 S and Ni/Ag 2 S nanoparticles, they calculated that the energy gap decreased with the increasing Ni ions. Ni used as a trapping site captures photogenerated electrons from the conduction band and separates the photo-generated electron-hole pairs. This change would force Ag 2 S to be activated more easily in the visible region, so it can enhance the light absorption ability of the catalysts. And, the catalyst could be reused without any loss in activity for the first 5 cycles. Compared with pure semiconductors, Ag 2 S loaded mesoporous materials in general possess greater photocatalytic activity.
The advantages using zeolite or mesoporous support for semiconductor photocatalysis include formation of ultrafine semiconductor particles during sol-gel deposition, increased adsorption in the pores, surface acidity which enhances electron-abstraction, and decreased UV-light scattering as the main component of Insets are the histogram of particle size distribution [52] Fig. 19 TEM images and corresponding size distribution of Ag 2 S nanoparticles synthesized by solventless thermolysis of silver octyl xanthate (a-c), silver hexadecyl xanthate (d-f), and silver carnaubyl xanthate (g-i) [60] Fig. 20 Schematic diagram of the possible growth mode [60] zeolite is silica [72,73]. A. Pourahmad prepared the Ag 2 S/MCM-41 photocatalysts by ion exchange method and is used for the photocatalytic degradation of methylene blue [75]. Figure 24 is the time-dependent electronic absorption spectra of dye during photo irradiation. After 20 min of irradiation under UV light in a Ag 2 S/MCM-41 suspension, 94 % of dye was decomposed and decolorized. And, no new bands appear in the UV-vis region due to the reaction intermediates formed during the degradation process. Under UV irradiation, Ag 2 S, MCM-41, and Ag 2 S/ MCM-41 materials on photodegradation of methylene blue are shown in Fig. 25. It is observed that Ag 2 S supported system has a higher rate of degradation than Ag 2 S or MCM-41 alone. And, there are many factors that can influence the efficiency of nanocomposite catalyst, such as the amount of Ag 2 S loading, PH, and initial concentration of dye. Several methods have been reported concerning the photosensitization of TiO 2 by M x S y or M x O y nanoparticles for heterogeneous photocatalysis [76] including CdS [77] or WO 3 [78]. In fact, nanocrystalline Ag 2 S is a good candidate for the photosensitization of TiO 2 catalysts, for Ag 2 S has a direct band gap of 0.9-1.05 eV, and its conduction band (−0.3 eV) is less anodic than the corresponding TiO 2 band (−0.1 eV), and the valence band (+0.7 eV) is more cathodic than the TiO 2 valence band (+3.1 eV). So, distinct TiO 2 /Ag 2 S nanocomposites were prepared by a single-source decomposition method [79]. After, the sensitized TiO 2 materials were evaluated as photocatalysts on the degradation of aqueous phenol solutions, and the photocatalytic activity of nanocomposites was enhanced with the existence of Ag 2 S over the TiO 2 surface. And, the efficiency of this photocatalysts is considerably improved comparing with pure TiO 2 . The best phenol photocatalyst was obtained when atomic ratio of Ti/Ag is 2.40. Nanostructured Ag 2 S/CdS was synthesized by two-step precipitation method [80]. And, the composite materials have certain photocatalytic performance. When the concentration of Ag 2 S was 5 % by weight, Ag 2 S/Cds showed the highest photocatalytic activity for hydrogen evolution, with the solar-hydrogen energy conversion efficiency approximately 0.7 %. So, after doped Ag 2 S, the photocatalytic activity of CdS have enhanced obviously.

Application in Optoelectronic Devices
Compared with bulk counterparts, the sheet-like photocatalysts are much better for continuous flow system because of ease separation and recovery from the reaction system [86]. The sheet-like photocatalysts can also help to harvest light more efficiently [87]. So, a novel graphene sheet/Ag 2 S composite was synthesized through a facile solvothermal method, and its electrochemical performance was carried on a modified glassy carbon electrode in a three-electrode Fig. 21 Schematic diagram illustrating the formation of Ag2S samples at various conditions [62] electrochemical cell [81]. In Fig. 26, it can be clearly seen that the pure graphene oxide sheets naturally aggregate and stack to multilayers with numerous edges, and the surface of graphene oxide was very smooth compared with graphene sheets doped with Ag 2 S NPs. So, the morphology of G-Ag 2 S composites has a substantial difference from that of the Go sheet. Figure 27 shows that the composite modified GCE shows redox peaks, but graphene modified exhibits a perfect rectangle curve, and the redox peaks of the composites, often a characteristic of pseudocapacitance mainly result from the redox transition of Ag 2 S between a semiconducting state and a conducting state. And, it is calculated that the redox peaks  [70] with the specific capacitance is 1063 Fg −1 , but the specific capacitance of graphene modified is 316 Fg −1 . So, it is believed that the nanocomposites would be a promising candidate as supercapacitor materials for practical applications in future electronic devices. At the same time, graphene-like Co 3 S 4 nanosheet/Ag 2 S nanocomposite was prepared using a simple method. The nanocomposite photocatalyst displays excellent stability and photocatalytic activity compared with pure Co 3 S 4 nanosheet or Ag 2 S nanoparticles [85]. Ag 2 S is an important material for optoelectronic, because it has an energy gap of Eg~1.1 eV, which is similar to the ideal band gap of 1.13 eV for a photovoltaic device [82] indicating that Ag 2 S could be an optimal solar absorber performance which was measured to the battery. The Ag 2 S QDs were synthesized by the successive ionic layer adsorption and reaction deposition method [83]. And, the assembled Ag 2 S-QD solar cell in λ = 530 nm has the biggest external quantum efficiency (EQE) which was 59 %, and when the spectral range in 400-1000 nm, the average of EQE was 42 %. The effective scope of photovoltaic is full of visible light and near-infrared spectral regions. Therefore, the results indicate that Ag 2 S QDs can be used as a highly efficient and broad band sensitizer for solar cells. R. Karimzadeh et al. [84] found that about 3 nm Ag 2 S semiconductor nanocrystals in concentrations of dimethyl sulfoxide solution has different non-linear refractive properties; it can be used as a low power opticallimiting device. In addition, Ag 2 S also has important application in other areas, for example nanometerscale non-volatile memory devices. [88].

Other Applications
Ag 2 S nanomaterials also have many other properties in various fields, such as electronic, magnetic, and so on. Ag 2 S belongs to I-VI semiconductor materials with monoclinic crystal structure. Thin films of Ag2S have applications in photoconducting cells [89], IR detectors [90], and solar selective coating. Thus, Ag 2 S is a promising material for the conversion of solar energy into electricity as its band gap is between 1 and 2 eV. Usually, the material design for these technological applications is based on thin film preparation techniques, in which the film thickness ranges from micrometer to submicrometer. It is well known that the surface contribution to the electric transport process could be evaluated when thin films with different thicknesses, i.e., with various surfaceto-volume ratios, are investigated [91]. So, D. Karashanova et al. [92] evaluated the surface contribution to the electronic or ionic transport in the epitaxial silver sulfide films using electron-conducting and electronblocking contacts, respectively. At the same time, Ag/Ag 2 S also can become the electrode materials, but disadvantages of the solid-state Ag/Ag 2 S electrode such as non-ideal response, signal drifting, and a long response time at low sulfide levels have   [75] limited its application [93]. Thus, increasing the precision of the Ag/Ag 2 S electrode by surface micromotion can help the research work under extreme circumstances, such as hydrothermal vents [94,95]. So, Ding et al. [96] have enhanced the sensitivity of the Ag/Ag 2 S electrode by using direct current carrier power to electroplate silver nanoparticles on a silver wire. Three types of the Ag/Ag 2 S electrode each had different physical structures under SEM (Fig. 28), which indicated that the different surfaces of these electrodes demonstrated that the preparation procedures affected the physical structures of the electrodes. Among all these electrodes, the direct current carrier electroplating electrode has the highest detection limit while the typical electrode has the lowest limit. From Table 2, we can see that the response time of the electrode prepared by direct   current carrier electroplating for detecting a concentration of 10 −7 mol L −1 S 2− was less than 60 s. And, the detection limits of the Ag/Ag 2 S electrodes prepared by direct current electroplating and direct current carrier electroplating were improved to 1 × 10 −5 and 1 × 10 −7 mol L −1 , respectively. The RMSE (root mean square deviation) of the linear regression for the electrode using the direct current carrier electroplating method verified the accuracy and precision of this type of electrode (Fig. 29). In addition to the above mention, Ag 2 S also has many other   29 Response curves of the three types of electrode. The linear correlation curve is generated using the direct current carrier electroplating method [96] properties in applications, but it still needs people to explore it gradually.

Conclusions
Ag 2 S, playing important functions in a number of optical, electrochemical, and biochemical process, has been regarded as a promising sensor and biological imaging agent in living creature. The preparation process and product of Ag 2 S have many disadvantages in traditional preparation methods. For example, it usually needs high temperature, complicated processes, easy gather, and particle size bed control. Recently, considerable efforts have been made to optimize the productive process of Ag 2 S and enhance properties and values of products. This work reviewed the progress in the development of Ag 2 S nanomaterials in the field of synthesis and application. Different forms of Ag 2 S nanostructures have been synthesized such as rod-shaped, leaf-shaped, and cubic. Ag 2 S nanostructure obtained by bionic technology and precursor of decomposition were prepared successfully. Meanwhile, it has been applied to many fields successfully, such as optical, electrical, and biology, and it is expected to use in other fields. In fact, there are still limitations for their practical use in photoelectric and medical fields because it often requires complex preparation process, and the yield is very low. In most cases, Ag 2 S nanoparticles are very prone to gather, which will greatly reduce its optical properties. Therefore, it is often necessary to composite with other materials to achieve a good effect. Although, there are so many challenges, the advances in nanoscience and nanotechnology of Ag 2 S still promise a better future for kinds of industries.