**2. Indium sulfide series**

Research on indium chalcogenide nanomaterials predominantly focuses on the indium sulfide series, attributed to readily available, abundant, cheap and stable precursors. This series finds applications in various applications, typically in optoelectronics. Among recent advancements is the selective NO2 gas sensing abilities of β-In2S3 thin films prepared by spray pyrolysis; this preliminary study introduces β-In2S3 thin films as less toxic and cheaper alternatives to highly selective and sensitive cadmium sulfide-based NO2 gas sensors [15]. In other work, In2S3

## *Indium Chalcogenide Nanomaterials in the Forefront of Recent Technological Advancements DOI: http://dx.doi.org/10.5772/intechopen.94558*

thin films prepared by ultrasonic spray pyrolysis was evaluated, for the first time, as photoelectrodes (**Figure 1**) for all-vanadium photoelectrochemical batteries [16]. The efficiency was linked to the degree of optical and photoelectrochemical behavior associated with the thickness of the In2S3 thin films. In both works, it becomes apparent that the physical alterations of the films are necessary to improve selectivity, sensitivity and efficiency. There are two notable recent reports which have provided preliminary solutions as per above: (i) band gap (1.9–2.3 eV) and electrical resistivity (5.5 x 100 –6.0 x 103 Ωm) control through thermal treatment of as-prepared In2S3 thin films at different temperatures in the presence/absence of sulfur powder [17], and (ii) tunable morphological (root mean square roughness) and optical properties (transmittance and photoluminescence) of the In2S3 thin films by varying the S/In molar ratio in spray pyrolysis deposition experiments [18].

Similar to the research objectives in Ref [15], indium sulfide is yet again demonstrated as a promising alternative to the cadmium sulfide, in this case as a buffer layer in the Cu(In,Ga)Se2 solar cell [19]. It was found that the indium sulfide-based solar cell achieved 15.3% efficiency compared to 17.1% recorded for the cadmium sulfide counterpart. The authors report that the observed efficiency is attributed to substrate temperature optimization during the sputtering method-based experiments. According to the study, the increase in substrate temperature tempers with the InxSy and Cu(In,Ga)Se2 compositions; an increase in temperature resulted to a sulfur-rich InxSy buffer layer, as well as copper depletion observed in the Cu(In,Ga) Se2 absorber layer, as seen in **Figure 2**. Furthermore, sodium doping was observed in both the InxSy layer and in the InxSy and Cu(In,Ga)Se2 interface. Thus, it is these features which were identified to play a major role in the increase of the solar efficiency.

Other efforts to improve attractive properties of In2S3 thin films have been reported, such as silver doping as means of improving electrical transport [20], as well as plasma treatment which consequently results to the self-formation of metallic indium arrays at the surface thus presenting opportunities in fabricating

#### **Figure 1.**

*Schematic representation of the photoelectrochemical VR-flow cell based on the In2S3-type photoelectrode. Reprinted with permission from Ref. [16]. Copyright 2020 American Chemical Society.*

**Figure 2.**

*Elemental composition of InxSy and Cu(In,Ga)Se2 layers deposited at different substrate temperatures. Reprinted with permission from Ref. [19]. Copyright 2020 MDPI.*

heterostructures for potential use optoelectronics [21]. Bilayer and trilayer InS triangular nanoflakes have also been prepared by chemical vapor deposition [22], potential applications envisaged as heterojunctions in nanoelectronic devices.

The films outlined above are predominantly obtained from existing technologies such as spray pyrolysis, thermal evaporation and chemical vapor deposition, where the films are directly prepared on a substrate. A new, solution-based synthesis of suspended 2D ultrathin sheets was developed [23]. This novel strategy, optimized through the synthesis of In2S3 sheets, exploits a self-assembling anisotropic growth mechanism templated by a combination of amine ligand with a geometricallymatched alkane. The obtained In2S3 sheets exhibited high photoelectric activities best suited for photoelectrochemical applications. Preliminary experiments displayed versatility of the method, attributed to the successful preparation of other 2D nanostructures such as Co9S8, MnS, SnS2, Al2S3 and MoS2. Thus, this presents an alternative route to easily prepare functional thin films which could ultimately be transferred to desired substrate post preparation and manipulation processes.

It has been observed that the recent advances in indium sulfide nanomaterials outlined above predominantly use the multiple precursor route. Although progress has been made in the past few years, the search for novel metalorganic single-source precursors for indium sulfide continues. New indium complexes with aminothiolate ligands have been synthesized and characterized fully [24], their structures are provided in **Figure 3**. Preliminary evaluations as potential single-source precursors showed that complex **1** is able produce β-In2S3 nanoparticles, complexes **2** and **3**

**Figure 3.** *Chemical structures of novel indium (III) aminothiolate complexes prepared by Ref. [24].*

*Indium Chalcogenide Nanomaterials in the Forefront of Recent Technological Advancements DOI: http://dx.doi.org/10.5772/intechopen.94558*

however need extensive work, as the diffraction studies for phase identification were inconclusive. However, microelemental analyses suggest that the nanomaterial exhibit general formulae In2S3 and In2Se2S from complexes **2** and **3**, respectively.

#### **2.1 Indium sulfide-based ternary and quaternary nanomaterials**

The main interest on indium sulfide-based ternary and quaternary nanomaterials is their optical properties, predominantly exploited in emission/ photoluminescence applications. Recent studies in this field has focused on the chalcopyrite-type materials, copper indium sulfide (CuInS2) and silver indium sulfide (AgInS2) in particular. A recent, concise review on the synthesis and applications of CuInS2 is available in Ref [25]. However, there are interesting literature reports which emerged subsequent to the publication of the review. For example, there is a study which has evaluated the influence of halide ions on the optical properties of CuInS2 quantum dots [26]. Similar to our work where we evaluated the influence of halide ligands in the single-source precursors on the morphological and optical properties of cadmium sulfide [27, 28] and lead sulfide [29] nanoparticles, the authors in this case follow a multiple-source precursor route (through the solvothermal synthetic protocols) using CuX (where X = I, Cl and Br) salts. The optical properties show unique behavior with respect to the metal salt used, attributed to the physicochemical properties resulting from the growth processes which consequently promote accumulation of the halide ions in the crystal lattices of the quantum dots. In another report, the importance of controlling the Cu:In ratio

#### **Figure 4.**

*(a) Resonant photoluminescence (PL) measurements of CuxInS2 quantum dots where x = 0.47 (dashed lines) and x = 0.85 (solid lines), at different excitation energies. (b) the PL peak energies extracted from (a). (c) Simulated absorption (lines) and corresponding PL (shaded peaks) spectra of CuxInS2 quantum dots with respect to Cu1+ and Cu2+ defects. Reprinted with permission from Ref. [30]. Copyright 2020 American Chemical Society.*

in CuInS2 quantum dots to harness different properties for various applications is discussed [30]. The report suggests that the change in the Cu:In ratio induces defects attributed to what the authors refer to as Cu1+ *vs* Cu2+ concentration defects, resulting in different optical emission behaviors as observed in **Figure 4**.

Although CuInS2 is a reputably known non-toxic material displaying attractive properties which are already exploited intensively in biomedical-based applications, there are however recent reports which have shown compelling experimental evidence contradicting this non-toxic behavior. A recent research study has observed the instability of zinc sulfide (ZnS) shell-free CuInS2 quantum dots relative to the shelled counterparts in the *in vitro* studies [31], degradation was demonstrated by rapid dissolution in simulated biological fluid (SBF) and artificial lysosomal fluid (ALS) through absorption spectroscopy measurements shown in **Figure 5**. Furthermore, it was demonstrated that shell-free CuInS2 induces severe toxicity in the *in vivo* studies compared to the infamous, toxic cadmium selenide. In another report, CuInS2 nanocrystals were exposed in environment-like conditions (including alkaline and acidic settings) thereby promoting weathering [32]. It was observed that when the environmental pathogenic bacteria *Staphylococcus aureus* CMCC 26003 strain is exposed to weathered CuInS2 nanocrystals, it develops increased tolerance to certain antibiotics such as penicillin G, tetracycline and ciprofloxacin. Thus, these two studies are a constant reminder with regards to creating awareness that alternative "green" approaches require concise evaluations and any possible adverse effects towards disruption of natural and crucial processes.

#### **Figure 5.**

*(A) Comparative study on the dissolution of CIS (CuInS2), CISZ (zinc-alloyed CuInS2) and CIS/ZnS' (CuInS2/ZnS core/shell) quantum dots by absorption spectroscopy measurements. (B) Visual evidence of dissolution in SBF. (C-E) dissolution studies in various media. Reprinted with permission from Ref. [31]. Copyright 2020 American Chemical Society.*

*Indium Chalcogenide Nanomaterials in the Forefront of Recent Technological Advancements DOI: http://dx.doi.org/10.5772/intechopen.94558*

This should however not deter attempts in developing similar technologies such as AgInS2 quantum dots which have recently shown ultralong PL decay time attributed to the coordinating ligands which bear electron rich groups capable of passivating surface trap centers and achieving strong emissions [33].

Apart from the chalcopyrite series, other indium sulfide-incorporated multinary nanomaterials have made significant technological progress. Recently, CdIn2S4 and ZnIn2S4 nanostructures have been prepared by solvent-free green reaction protocols at moderately low temperatures [34]; the nanostructures displayed good photocatalytic activities in hydrogen evolution reactions through the splitting of hydrogen sulfide and water under visible light conditions. The activities were however lower than those of other CdIn2S4 and ZnIn2S4 nanostructures reported elsewhere [35, 36], attributed to synthetic the method limitations particularly on poor control of physical features such as particle size and shape. The quaternary system which has been recently reported is the Zn2xCu1 − xIn1 − xS2 [37] and Zn2xAg1 − xIn1 − xS2 [38] nanomaterials which display unique optical properties with varying composition and an active component potential in the light harvesting inorganic–organic hybrid nanomaterial, respectively.
