Synthesis and Applications of Dimensional SnS2 and SnS2/Carbon Nanomaterials

Dimensional nanomaterials can offer enhanced application properties benefiting from their sizes and morphological orientations. Tin disulfide (SnS2) and carbon are typical sources of dimensional nanomaterials. SnS2 is a semiconductor with visible light adsorption properties and has shown high energy density and long cycle life in energy storage processes. The integration of SnS2 and carbon materials has shown enhanced visible light absorption and electron transmission efficiency. This helps to alleviate the volume expansion of SnS2 which is a limitation during energy storage processes and provides a favorable bandgap in photocatalytic degradation. Several innovative approaches have been geared toward controlling the size, shape, and hybridization of SnS2/Carbon composite nanostructures. However, dimensional nanomaterials of SnS2 and SnS2/Carbon have rarely been discussed. This review summarizes the synthesis methods of zero-, one-, two-, and three-dimensional SnS2 and SnS2/Carbon composite nanomaterials through wet and solid-state synthesis strategies. Moreover, the unique properties that promote their advances in photocatalysis and energy conversion and storage are discussed. Finally, some remarks and perspectives on the challenges and opportunities for exploring advanced SnS2/Carbon nanomaterials are presented.


Introduction
The fast depletion of fossil fuel and its environmental implications have led to the development of technologies for green-energy production and storage as well as environmental remediation. These technologies are of great interest to the research community to minimize carbon footprints. Therefore, breakthroughs in nanotechnology research could enable the creation of unique materials at the molecular level, opening up a slew of green industrial possibilities.
SnS 2 nanomaterials have made impactful strides in the synthesis of dimensional nanomaterials, due to their unique hexagonal nanostructures and the ability to have sulfur chains with variable lengths. In addition, SnS 2 has a favorable energy bandgap, low cost, low toxicity, excellent stability, and abundant reserves in nature. However, its wide application in batteries is hindered by low intrinsic conductivity and poor cycling stability [9,10]. One of the most effective techniques used to tackle these problems is the synthesis of SnS 2 in composite nanomaterials. Carbon materials are economically abundant and have  [53]. Copyright 2014, Elsevier. (c) Self-assembly synthesis process (d) XRD, of 3D SnS2/Graphene aerogels. Adapted with permission from [54]. Copyright 2013, Elsevier.

Wet Chemical Synthesis of SnS2/Carbon Nanomaterials
Just like SnS2 nanomaterials, the structure of SnS2/carbon composite nanomaterials depends on the precursors and the synthesis conditions. Normally, SnS2/carbon composites result in different dimensional architectures, where the SnS2 nanostructures anchor themselves onto interpenetrated carbon materials with varied architectures. Figure 1c is a clear representation of self-assembled SnS2/carbon composite nanostructures. It shows 3D SnS2/graphene aerogel nanostructures fabricated through in situ macroscopy self-assembly using a hydrothermal process, followed by freeze-drying to preserve its 3D architectures. Figure 1d shows the crystalline structure of SnS2 in composite materials; however, it could not detect the carbon peaks due to its amorphous form. For that matter, Raman spectroscopy was proposed for the detection of carbon in the composite.
Controlling the growth orientations of SnS2 nanostructures on nanocarbon surfaces has been reported as a challenging concept as seen in the fabrication of parallel and vertically aligned SnS2 nanostructures on graphene nanosheets [55]. Hence, an understanding of the mechanism of SnS2/carbon hybrid synthesis, with desired properties and varied nanostructures, is important in nanotechnological applications. Liu et al. synthesized SnS2/bacterial-cellulose-derived carbon nanofiber (BC-CNF) first by the hydrolysis of thioacetamide, followed by in situ metathesis reactions, and finally by self-assembly and oriented crystallization processes [56]. The resultant BC-CNFs had a highly porous 3D network with an average diameter of 30-50 nm. Among the methods adopted for synthesizing SnS2/Carbon nanomaterials, the most prevalent is the hydrothermal process. This technique can enhance the characteristics and stability of nanomaterials while concurrently  [53]. Copyright 2014, Elsevier. (c) Self-assembly synthesis process (d) XRD, of 3D SnS 2 /Graphene aerogels. Adapted with permission from [54]. Copyright 2013, Elsevier.

Wet Chemical Synthesis of SnS 2 /Carbon Nanomaterials
Just like SnS 2 nanomaterials, the structure of SnS 2 /carbon composite nanomaterials depends on the precursors and the synthesis conditions. Normally, SnS 2 /carbon composites result in different dimensional architectures, where the SnS 2 nanostructures anchor themselves onto interpenetrated carbon materials with varied architectures. Figure 1c is a clear representation of self-assembled SnS 2 /carbon composite nanostructures. It shows 3D SnS 2 /graphene aerogel nanostructures fabricated through in situ macroscopy self-assembly using a hydrothermal process, followed by freeze-drying to preserve its 3D architectures. Figure 1d shows the crystalline structure of SnS 2 in composite materials; however, it could not detect the carbon peaks due to its amorphous form. For that matter, Raman spectroscopy was proposed for the detection of carbon in the composite.
Controlling the growth orientations of SnS 2 nanostructures on nanocarbon surfaces has been reported as a challenging concept as seen in the fabrication of parallel and vertically aligned SnS 2 nanostructures on graphene nanosheets [55]. Hence, an understanding of the mechanism of SnS 2 /carbon hybrid synthesis, with desired properties and varied nanostructures, is important in nanotechnological applications. Liu et al. synthesized SnS 2 /bacterial-cellulose-derived carbon nanofiber (BC-CNF) first by the hydrolysis of thioacetamide, followed by in situ metathesis reactions, and finally by self-assembly and oriented crystallization processes [56]. The resultant BC-CNFs had a highly porous 3D network with an average diameter of 30-50 nm. Among the methods adopted for synthesizing SnS 2 /Carbon nanomaterials, the most prevalent is the hydrothermal process. This technique can enhance the characteristics and stability of nanomaterials while concurrently controlling the structures of the hybrid composites. These allow for interconnected networks with a high surface area which enhances the synergetic interactions between the layered SnS 2 and the carbon by increasing their contact areas [57]. Furthermore, the interconnected network helps SnS 2 in alleviating the mechanical stress, preventing its aggregation, and accommodating its volume change during cycling [58,59]. For instance, SnS 2 /graphene oxide nanocomposites were synthesized by reflux condensation together with a hydrothermal strategy using an anionic surfactant, sodium dodecyl sulfate (SDS) [60]. Zhang et al. also proposed a means for fabricating hierarchical polyaniline/SnS 2 @carbon nanotubes onto the carbon fiber (CF) surface [61]. However, synthesis limitations, such as low pressure and temperature can seriously affect the rate performance of composite materials [62,63].
Solvothermal synthesis has been used to investigate the synthesis of SnS 2 /carbon composite nanomaterials, but only a handful of reports exist on the use of this method. For example, Zhang et al. synthesized one-pot flexible SnS 2 /CNT (2D nanosheet/3D self-assembled flower) composites which were controlled by a time-dependent process [64]. Moreover, functionalized graphene sheets (FGS) were used to synthesize graphene-SnS 2 nanocomposites via a solvothermal method [65]. In most cases, annealing is used for further treatment to improve the phase purity and crystallinity of nanomaterials before use in various applications [10,66].

Solid-Phase Synthesis of SnS 2 and SnS 2 /Carbon Nanomaterials
Solid-phase synthesis is a top-down approach to synthesizing inorganic nanomaterials. The procedure involves milling and may include many annealing steps with several intermediate milling procedures to heighten the uniformity of the mixture and reduce the sizes of the fabricated materials [67][68][69][70][71]. Additional milling tends to make the particles more sinter active in the heat treatment procedures that follow [72,73]. Furthermore, huge quantities of materials can be synthesized in a reasonably straightforward method, but the resulting nanomaterials have a comparatively high agglomeration compared to the wet synthesis processes discussed above [74,75]. As a result, solid-phase synthesis produces relatively large particle sizes and poor homogeneity which are somewhat unavoidable.

Solid-Phase Synthesis of SnS 2 Nanomaterials
Solid-phase synthesis of SnS 2 nanomaterials is an alternative fabrication method that helps the growth of SnS 2 nanostructures by supplying an adequate amount of precursors [76]. Usually, this is carried out without the aid of a template, inert gas protection, or a vacuum environment but by heating the solid precursor mixtures in air at certain temperatures and time followed by washing treatment [77]. In some reports, SnS 2 nanoflakes were fabricated using a suitable amount of SnCl 4 ·5H 2 O and thiourea mixed and grounded thoroughly until a homogeneous mixture was acquired and subsequently heated in a crucible [78,79]. Owing to the intrinsic anisotropic nature of SnS 2 crystals, solid-phase synthesis tries to enhance its surface area to achieve desired nanostructures through the milling process. Xiao et al. and Wang et al. prepared SnS 2 nanomaterials by heating the precursors at their liquid-solid phase, i.e., at the melting points and boiling points of tin (Sn), sulfur (S), and ammonium chloride (NH 4 Cl) in air [80,81]. In addition, the presence of NH 4 Cl aided and promoted the synthesis of pure SnS 2 under mild conditions. It is worth noting that the annealing process of nanomaterials can also bring about self-purification due to the impurities and intrinsic material defects that prefer moving toward the surface during the annealing process.

Solid-Phase Synthesis of SnS 2 /Carbon Nanomaterials
To the best of our knowledge, limited literature exists on the solid-state synthesis of SnS 2 /Carbon nanomaterials. The solid synthesis of SnS 2 /Carbon nanomaterials may involve microwave heating, calcination, milling, or a combination of these processes to achieve a homogeneous crystalline product. The mechanical energy used creates phase transformations and chemical reactions at very low temperatures. For instance, ball milling enables the reduction in particle sizes and characteristic lengths in addition to the effective mutual dispersion of the processed phases. Wang et al. synthesized a SnS 2 /Carbon compos-ite by annealing metallic Sn, S powder, and polyacrylonitrile (PAN) mixed in a sealed glass tube under vacuum at 600 • C for 3 h [82]. This resulted in SnS 2 nanostructures embedded in the carbon matrix that was generated by the carbonization of PAN. The morphologies are shown in Figure 2a and the synthesis process is schematically shown in Figure 2b. Furthermore, the synthesized SnS 2 /carbon composite was directly milled in NaCl which reduced the crystal structure of the SnS 2 /Carbon nanocomposite, and this improved the overall battery performance of the synthesized SnS 2 /Carbon nanomaterial [83]. Figure 2c shows the SEM of un-milled and directly milled SnS 2 /Carbon structures, respectively and Figure 2d shows the schematic fomation of SnS 2 /Carbon composite.
To the best of our knowledge, limited literature exists on the solid-state synthesis of SnS2/Carbon nanomaterials. The solid synthesis of SnS2/Carbon nanomaterials may involve microwave heating, calcination, milling, or a combination of these processes to achieve a homogeneous crystalline product. The mechanical energy used creates phase transformations and chemical reactions at very low temperatures. For instance, ball milling enables the reduction in particle sizes and characteristic lengths in addition to the effective mutual dispersion of the processed phases. Wang et al. synthesized a SnS2/Carbon composite by annealing metallic Sn, S powder, and polyacrylonitrile (PAN) mixed in a sealed glass tube under vacuum at 600 °C for 3 h [82]. This resulted in SnS2 nanostructures embedded in the carbon matrix that was generated by the carbonization of PAN. The morphologies are shown in Figure 2a and the synthesis process is schematically shown in Figure 2b. Furthermore, the synthesized SnS2/carbon composite was directly milled in NaCl which reduced the crystal structure of the SnS2/Carbon nanocomposite, and this improved the overall battery performance of the synthesized SnS2/Carbon nanomaterial [83]. Figure 2c shows the SEM of un-milled and directly milled SnS2/Carbon structures, respectively and Figure 2d shows the schematic fomation of SnS2/Carbon composite. In some other processes, solid-state synthesis has been indicated to require low temperatures and help to improve the purity of the resultant substances. [84,85] The solidstate syntheses have also been applied in the synthesis of other SnS2/composites including Phosphorus-SnS2 composites, which is not the focus of this review. [86] Figure 3a demonstrates the simple synthesis routes of SnS2/Carbon nanomaterials by wet chemical and Figure 3b by solid-state synthesis. Table 1 shows the various SnS2/Carbon nanomaterials achieved through wet and solid-state synthesis approaches. In other instances, a hybrid synthesis method was employed to achieve SnS2/carbon composite nanomaterials. [87,88] In one instance, the carbon precursor was synthesized at elevated temperatures before it was further combined with the Sn 2+ and S 2− precursors to form the composite SnS2/carbon nanomaterials [89,90]. In some other processes, solid-state synthesis has been indicated to require low temperatures and help to improve the purity of the resultant substances [84,85]. The solid-state syntheses have also been applied in the synthesis of other SnS 2 /composites including Phosphorus-SnS 2 composites, which is not the focus of this review [86]. Figure 3a demonstrates the simple synthesis routes of SnS 2 /Carbon nanomaterials by wet chemical and Figure 3b by solid-state synthesis. Table 1 shows the various SnS 2 /Carbon nanomaterials achieved through wet and solid-state synthesis approaches. In other instances, a hybrid synthesis method was employed to achieve SnS 2 /carbon composite nanomaterials [87,88]. In one instance, the carbon precursor was synthesized at elevated temperatures before it was further combined with the Sn 2+ and S 2− precursors to form the composite SnS 2 /carbon nanomaterials [89,90].

Dimensional Characteristics of SnS 2 and SnS 2 /Carbon Nanomaterials
Nanomaterials possess a variety of shapes and sizes. In some cases, their names are generated and characterized by their shapes or orientations. For example, nanospheres are spherical, nanotubes are tube-shaped, etc. Nanostructure classifications are also based on their dimensions, compositions, uniformity, and agglomeration. Classification based on dimensionality is a generalization of the concept based on the aspect ratio of 0D, 1D, 2D, or 3D. These dimensions or morphologies result from a variety of precursors, temperature, pH, templates, the mode of reagent dosage during synthesis, etc. The ability to control the morphology of nanomaterials is crucial in exploiting their properties for applications. As a measure of the dimensional characteristics of SnS 2 and SnS 2 /Carbon nanomaterials in this review article, TEM and SEM analyses were mainly used.
3.1. Dimensional Characteristics of SnS 2 Nanomaterials 3.1.1. Zero-Dimensional (0D Nanodots) SnS 2 Nanomaterials SnS 2 quantum dots (QDs) possess strong luminescence, good aqueous stability, and biocompatibility. Therefore they are often used in the field of sensing and biology [115]. Excitation and emission properties exhibited by SnS 2 QDs were credited to the polydispersity of SnS 2 -QDs and its characteristic feature of quantum confinement and edge effects [116]. Negatively charged SnS 2 QDs were made by inserting electrons into vacant molecular orbitals, whereas positively charged SnS 2 QDs were made by injecting holes into the highest occupied molecular orbitals, and these collided with the stable SnS 2 QDs to produce excited SnS 2 QDs that could emit light [117]. Figure 4a-d show representative transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) details of SnS 2 QDs. Figure 4a,b are nearly monodispersed SnS 2 QDs with a mean size of 6.5 nm. A single particle lattice spacing of 0.32 nm is seen in Figure 4d corresponding to the (200) plane of hexagonal SnS 2 [118]. Nanomaterials 2022, 12, x FOR PEER REVIEW 8 of 41 1 NSDC, Nitrogen, Sulfur-doped carbon nanofibers; 2 SIBs, Sodium ion batteries; 3 LIBs, Lithium ion batteries; 4 CNT, Carbon nanotube; 5 GF, Graphene fiber; 6 rGO, Reduced graphene oxide; 7 g-C3N4, graphitic carbon nitride.

Dimensional Characteristics of SnS2 and SnS2/Carbon Nanomaterials
Nanomaterials possess a variety of shapes and sizes. In some cases, their names are generated and characterized by their shapes or orientations. For example, nanospheres are spherical, nanotubes are tube-shaped, etc. Nanostructure classifications are also based on their dimensions, compositions, uniformity, and agglomeration. Classification based on dimensionality is a generalization of the concept based on the aspect ratio of 0D, 1D, 2D, or 3D. These dimensions or morphologies result from a variety of precursors, temperature, pH, templates, the mode of reagent dosage during synthesis, etc. The ability to control the morphology of nanomaterials is crucial in exploiting their properties for applications. As a measure of the dimensional characteristics of SnS2 and SnS2/Carbon nanomaterials in this review article, TEM and SEM analyses were mainly used.

Dimensional Characteristics of SnS2 Nanomaterials
3.1.1. Zero-Dimensional (0D Nanodots) SnS2 Nanomaterials SnS2 quantum dots (QDs) possess strong luminescence, good aqueous stability, and biocompatibility. Therefore they are often used in the field of sensing and biology. [115] Excitation and emission properties exhibited by SnS2 QDs were credited to the polydispersity of SnS2-QDs and its characteristic feature of quantum confinement and edge effects. [116] Negatively charged SnS2 QDs were made by inserting electrons into vacant molecular orbitals, whereas positively charged SnS2 QDs were made by injecting holes into the highest occupied molecular orbitals, and these collided with the stable SnS2 QDs to produce excited SnS2 QDs that could emit light. [117] Figure 4a-d show representative transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) details of SnS2 QDs. Figure 4a,b are nearly monodispersed SnS2 QDs with a mean size of 6.5 nm. A single particle lattice spacing of 0.32 nm is seen in Figure 4d corresponding to the (200) plane of hexagonal SnS2. [118]   The surface energy and crystal structures of the SnS 2 QDs are dependent on synthesis conditions. Optimized synthesis conditions could result in a significant increase in the surface-to-volume ratio and influence the surface energy and phase stability greatly. Hy-Nanomaterials 2022, 12, 4497 9 of 38 drothermally synthesized SnS 2 QDs are in situ functionalized and pH sensitive [115,119]. In application, these nanomaterials can connect and partly fuse to adjacent ones leading to much flatter structures after annealing. In the end, this is valuable to form good contact between the active layer and the electrode material [120].
3.1.2. One-Dimensional (1D) SnS 2 Nanomaterials One-dimensional nanostructures are of interest due to their potential to serve as the basis for determining the size and dimensionality dependence of a material's physical properties. Many solid structures of chalcogenide grow from 1D nanostructures. Onedimensional nanomaterials have been exploited as a novel model while investigating the size and dimensional dependence of functional properties. They also play an important role as interconnected nanostructures and as the key units in fabricating electronic, optoelectronic, and electrochemical energy devices with nanoscale dimensions. SnS 2 nanowires were synthesized by sulfurizing the Sn nanowires, which were embedded in the nanochannels of anodic aluminum oxide (AAO) templates. The characterization of these nanowires is shown in Figure 5 [121]. After detaching from the AAO templates, SnS 2 nanowires achieved a diameter of about 40 nm. It is worth noting that reports on the synthesis of 1D nanomaterial are rare, owing to the fact that most synthesized SnS 2 nanomaterials are the building blocks for achieving other dimensionally structured nanomaterials. The surface energy and crystal structures of the SnS2 QDs are dependent on synthesis conditions. Optimized synthesis conditions could result in a significant increase in the surface-to-volume ratio and influence the surface energy and phase stability greatly. Hydrothermally synthesized SnS2 QDs are in situ functionalized and pH sensitive [115,119]. In application, these nanomaterials can connect and partly fuse to adjacent ones leading to much flatter structures after annealing. In the end, this is valuable to form good contact between the active layer and the electrode material [120].
3.1.2. One-Dimensional (1D) SnS2 Nanomaterials One-dimensional nanostructures are of interest due to their potential to serve as the basis for determining the size and dimensionality dependence of a material's physical properties. Many solid structures of chalcogenide grow from 1D nanostructures. One-dimensional nanomaterials have been exploited as a novel model while investigating the size and dimensional dependence of functional properties. They also play an important role as interconnected nanostructures and as the key units in fabricating electronic, optoelectronic, and electrochemical energy devices with nanoscale dimensions. SnS2 nanowires were synthesized by sulfurizing the Sn nanowires, which were embedded in the nanochannels of anodic aluminum oxide (AAO) templates. The characterization of these nanowires is shown in Figure 5 [121]. After detaching from the AAO templates, SnS2 nanowires achieved a diameter of about 40 nm. It is worth noting that reports on the synthesis of 1D nanomaterial are rare, owing to the fact that most synthesized SnS2 nanomaterials are the building blocks for achieving other dimensionally structured nanomaterials.  Researchers have made significant advances in the preparation, characterization, adjustment, and theoretical investigation of 2D materials. The abundance of 2D materials has elevated them with a range of material frameworks in methodological studies for the development of nano-and atomic-level applications. Two-dimensional SnS 2 nanocrystals exhibit semiconductor characteristics, [122] owing to their high carrier movement and large bandgap [123]. SnS 2 has a sandwich-like structure with an S plane held in between two Sn planes, all in hexagonal order. The adjacent sulfur atoms in the sulfur layers are bonded, allowing for easy layer separation via chemical or mechanical exfoliation [41]. However, vacancy defects in 2D SnS 2 nanomaterials are known to have a major influence on material characteristics and are unavoidable during exfoliation [124]. Sun et al. confirmed the formation of micrometer-sized SnS 2 nanosheets with exposed (011) facets as the primary surfaces [125]. These 2D nanosheets could be reconstructed by lateral confinement with longitudinal extension, and a typical 2D SnS 2 structure is displayed in Figure 6. The as-grown SnS 2 nanosheets were quasi-vertically oriented and standing free on the fluorine-doped tin oxide (FTO) substrate (Figure 6a-c). The SnS 2 nanostructures displayed a well-defined semi-hexagonal shape. Similarly, Li et al. achieved 2D SnS 2 nanoflakes grown perpendicular to the substrate in a low-temperature zone of a SiO 2 /Si substrate [123]. Strong light absorption, short minority-carrier transport distances, and a wide exposed surface area for catalytic reactions have all helped 2D SnS 2 nanomaterials to effectively harvest photocurrent [50]. Furthermore, 2D SnS 2 with characteristics such as mono-dispersity, high compactness, open morphology, well-defined structures, and maximally exposed surfaces/edges are favorable [51].
Nanomaterials 2022, 12, x FOR PEER REVIEW 10 of 41 development of nano-and atomic-level applications. Two-dimensional SnS2 nanocrystals exhibit semiconductor characteristics, [122] owing to their high carrier movement and large bandgap. [123] SnS2 has a sandwich-like structure with an S plane held in between two Sn planes, all in hexagonal order. The adjacent sulfur atoms in the sulfur layers are bonded, allowing for easy layer separation via chemical or mechanical exfoliation. [41] However, vacancy defects in 2D SnS2 nanomaterials are known to have a major influence on material characteristics and are unavoidable during exfoliation. [124] Sun et al. confirmed the formation of micrometer-sized SnS2 nanosheets with exposed (011) facets as the primary surfaces [125]. These 2D nanosheets could be reconstructed by lateral confinement with longitudinal extension, and a typical 2D SnS2 structure is displayed in Figure  6. The as-grown SnS2 nanosheets were quasi-vertically oriented and standing free on the fluorine-doped tin oxide (FTO) substrate (Figure 6a-c). The SnS2 nanostructures displayed a well-defined semi-hexagonal shape. Similarly, Li et al. achieved 2D SnS2 nanoflakes grown perpendicular to the substrate in a low-temperature zone of a SiO2/Si substrate [123]. Strong light absorption, short minority-carrier transport distances, and a wide exposed surface area for catalytic reactions have all helped 2D SnS2 nanomaterials to effectively harvest photocurrent [50]. Furthermore, 2D SnS2 with characteristics such as monodispersity, high compactness, open morphology, well-defined structures, and maximally exposed surfaces/edges are favorable [51].

Three-Dimensional (3D) Self-Supporting SnS2 Nanoflowers
Three-dimensional SnS2 nanomaterials have hierarchical flower-like architectures with nanosized building blocks and a complex assembled architecture. Their large surface area can reduce the concentrated polarization and offer more sites for accommodation [127]. Figure 7a,b are flower-shaped nanostructures analyzed by TEM and SEM analysis. Figure 7c,d represents the TEM images of SnS2 nanoflowers and the fringe interval with the d-spacing of hexagonal SnS2, respectively. The schematic approach leading to the for-

Three-Dimensional (3D) Self-Supporting SnS 2 Nanoflowers
Three-dimensional SnS 2 nanomaterials have hierarchical flower-like architectures with nanosized building blocks and a complex assembled architecture. Their large surface area can reduce the concentrated polarization and offer more sites for accommodation [127]. Figure 7a,b are flower-shaped nanostructures analyzed by TEM and SEM analysis. Figure 7c,d represents the TEM images of SnS 2 nanoflowers and the fringe interval with the d-spacing of hexagonal SnS 2 , respectively. The schematic approach leading to the formation of SnS 2 nanoflowers is demonstrated in Figure 7e [128,129]. Xiong et al. also described well-defined SnS 2 nanoflowers for NH 3 detection by a facile solvothermal method [130]. On the other hand, 3D hierarchical SnS 2 microspheres consisting of thinlayered nanosheets were synthesized via a one-pot hydrothermal method [131]. By altering the ratio of SnCl 4 to L-cysteine, they were able to keep their morphologies under control. In addition, mild hydrothermal treatment in the presence of octyl-phenol-ethoxylate (Triton X-100) at 160 • C led to the achievements of 3D nanoflowers with a spot-like appearance along the [010] axis of the SnS 2 crystal [132]. mation of SnS2 nanoflowers is demonstrated in Figure 7e [128,129]. Xiong et al. also described well-defined SnS2 nanoflowers for NH3 detection by a facile solvothermal method [130]. On the other hand, 3D hierarchical SnS2 microspheres consisting of thin-layered nanosheets were synthesized via a one-pot hydrothermal method [131]. By altering the ratio of SnCl4 to L-cysteine, they were able to keep their morphologies under control. In addition, mild hydrothermal treatment in the presence of octyl-phenol-ethoxylate (Triton X-100) at 160 °C led to the achievements of 3D nanoflowers with a spot-like appearance along the [010] axis of the SnS2 crystal [132]. Reproduced with permission from [128]. Copyright 2018, Elsevier. (e) Schematic illustration of 3D SnS2 nanoflowers. Reproduced with permission from [129]. Copyright 2013, Elsevier.

Dimensional Characteristics of SnS2/Carbon Composite Nanomaterials
The hybrid structures of SnS2/Carbon materials at the nano level have permitted the control of desired properties and features. Table 2 suggests the possible hierarchical formation of SnS2/Carbon composite nanomaterials. It tries to depict the synergies between SnS2 nanostructures and carbon nanomaterials in the hybridized structures. The concept of composites further tries to enhance the nanomaterials' internal and external capabilities as well as the physical/chemical compatibility. The unique properties of these composite nanomaterials are of great interest for their environmental and energy storage applications.

Zero-Dimensional (0D) SnS2/Carbon Composite Nanodots
Various questions come up about what constitutes 0D hybrid materials. The literature captures this concept of zero-dimensional composites giving preference to one component. The individual nanostructures of SnS2 and carbon have achieved great heights in many applications, for example, in the fields of photoelectric detectors, solar photocatalysts, and photovoltaic solar cell applications. To the best of our knowledge, there are no reports on zero-dimensional composite SnS2/Carbon nanomaterials. SnS2/Carbon nanostructures could result from a combination of SnS2 QDs and Carbon QDs in a hybridized synthesis approach. The achievement of this structural material at a low dimension Reproduced with permission from [128]. Copyright 2018, Elsevier. (e) Schematic illustration of 3D SnS 2 nanoflowers. Reproduced with permission from [129]. Copyright 2013, Elsevier.

Dimensional Characteristics of SnS 2 /Carbon Composite Nanomaterials
The hybrid structures of SnS 2 /Carbon materials at the nano level have permitted the control of desired properties and features. Table 2 suggests the possible hierarchical formation of SnS 2 /Carbon composite nanomaterials. It tries to depict the synergies between SnS 2 nanostructures and carbon nanomaterials in the hybridized structures. The concept of composites further tries to enhance the nanomaterials' internal and external capabilities as well as the physical/chemical compatibility. The unique properties of these composite nanomaterials are of great interest for their environmental and energy storage applications.

Zero-Dimensional (0D) SnS 2 /Carbon Composite Nanodots
Various questions come up about what constitutes 0D hybrid materials. The literature captures this concept of zero-dimensional composites giving preference to one component. The individual nanostructures of SnS 2 and carbon have achieved great heights in many applications, for example, in the fields of photoelectric detectors, solar photocatalysts, and photovoltaic solar cell applications. To the best of our knowledge, there are no reports on zero-dimensional composite SnS 2 /Carbon nanomaterials. SnS 2 /Carbon nanostructures could result from a combination of SnS 2 QDs and Carbon QDs in a hybridized synthesis approach. The achievement of this structural material at a low dimension has proven beneficial in its application in catalysis and electrochemistry [133]. Chen et al. found that the tiny size of SnS 2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS 2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS 2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS 2 QDs and carbon QDs could be a powerhouse for future applications. has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. [91] 1D has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. [58,92,95,[135][136][137][138] 2D Nanomaterials 2022, 12, x FOR PEER REVIEW 12 of 41 has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. [89,[139][140][141][142][143] 3D Nanomaterials 2022, 12, x FOR PEER REVIEW 12 of 41 has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS2/Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. [111,136,[144][145][146][147][148][149][150] Potential structures of SnS 2 /Carbon composite nanomaterials Nanomaterials 2022, 12, x FOR PEER REVIEW 12 of 41 has proven beneficial in its application in catalysis and electrochemistry. [133] Chen et al. found that the tiny size of SnS2 QDs makes them easy to insert into graphene nanosheets which prevents the restacking of graphene nanosheets [134]. Meanwhile, the inserted SnS2 QDs showed an enhanced photocatalytic effect. The carbon dots-SnS2 nanomaterials could show excellent photocatalytic adsorption capacity by acting as a good electron acceptor. Therefore, a synergetic combination of SnS2 QDs and carbon QDs could be a powerhouse for future applications.

One-Dimensional (1D) SnS 2 /Carbon Composite Nanomaterials
Controlling the orientation and polymer chain alignment of 1D nanostructures can increase their multifunctional features such as thermal and electrical conductivity [151]. The characteristic properties of hierarchical 1D composite nanomaterials are usually realized by using one of the components as a backbone or template material, and the other component is deposited on the surface or within it [152][153][154]. An understanding of 1D nanostructures has been intensively covered by Wei et al. [155], where the fabrications and applications of 1D mono and hybrid nanomaterials are touched on. In most 1D SnS 2 /Carbon hybrid formations, the SnS 2 part, usually nanosheets or nanoflakes is embedded in the 1D carbon template parallelly or inclined at an angle. This enhances the properties of SnS 2 /Carbon nanomaterials and correspondingly influences their applications [138]. SnS 2 /CNT composite nanomaterials are gaining attention because of their large surface area and the improved conductivity compared to SnS 2 . The sheet size of SnS 2 is greatly reduced when it is clustered in SnS 2 /CNT hybrid nanocomposites, indicating that the introduction of CNTs refined the sheet size of SnS 2 [156]. This leads to the CNTs evenly wrapped on the surface of or interspersed in the SnS 2 sheets which is beneficial for improving the conductivity of the SnS 2 . In addition, SnS 2 /CNTs can be attached to the surface of the separators without any peeling and blanking, thus showing good flexibility and mechanical stability [94,157]. In an extraordinary case, the CNTs could act as templates for SnS 2 materials. Jin et al. demonstrated this by filling hard CNT templates with Sn materials and sulfurizing beyond 300 • C to achieve SnS 2 nanostructures as a dominant phase within the CNTs [97]. Again, the diffraction peak corresponding to the (001) plane of SnS 2 /CNT hybrid nanostructures exhibits preferentially oriented growth along this plane. Figure 8 demonstrates a typical formation of 1D SnS 2 /Carbon composite nanostructure through a facile templating synthesis using MnO x nanorods as templates [136]. By adjusting the sulfurization temperature, it aided in the structural control during the formation of the nanocomposite, such that the SnS 2 nanosheets were encapsulated in amorphous carbon nanotubes. and applications of 1D mono and hybrid nanomaterials are touched on. In most 1D SnS2/Carbon hybrid formations, the SnS2 part, usually nanosheets or nanoflakes is embedded in the 1D carbon template parallelly or inclined at an angle. This enhances the properties of SnS2/Carbon nanomaterials and correspondingly influences their applications [138]. SnS2/CNT composite nanomaterials are gaining attention because of their large surface area and the improved conductivity compared to SnS2. The sheet size of SnS2 is greatly reduced when it is clustered in SnS2/CNT hybrid nanocomposites, indicating that the introduction of CNTs refined the sheet size of SnS2 [156]. This leads to the CNTs evenly wrapped on the surface of or interspersed in the SnS2 sheets which is beneficial for improving the conductivity of the SnS2. In addition, SnS2/CNTs can be attached to the surface of the separators without any peeling and blanking, thus showing good flexibility and mechanical stability [94,157]. In an extraordinary case, the CNTs could act as templates for SnS2 materials. Jin et al. demonstrated this by filling hard CNT templates with Sn materials and sulfurizing beyond 300 °C to achieve SnS2 nanostructures as a dominant phase within the CNTs [97]. Again, the diffraction peak corresponding to the (001) plane of SnS2/CNT hybrid nanostructures exhibits preferentially oriented growth along this plane. Figure 8 demonstrates a typical formation of 1D SnS2/Carbon composite nanostructure through a facile templating synthesis using MnOx nanorods as templates [136]. By adjusting the sulfurization temperature, it aided in the structural control during the formation of the nanocomposite, such that the SnS2 nanosheets were encapsulated in amorphous carbon nanotubes.  The carbon nanofiber network has also made feasible contributions in terms of SnS 2 /Carbon nanomaterial formation. For instance, Xia et al. prepared SnS 2 embedded in nitrogen and sulfur dual-doped carbon (SnS 2 /NSDC) nanofibers by a facile electrospinning technique as indicated in Figure 9 [92]. Figure 9d-f illustrates the various morphological features of the SnS 2 /Carbon composite nanofibers. The carbon nanofiber framework provides a conductive host and is tolerant to the volume variation of SnS 2 during the charging/discharging processes, thereby maintaining the structural stability of the SnS 2 /Carbon electrode [158]. Furthermore, the microstructures of SnS 2 nanosheets can provide rich migration paths of sodium ions and electrons; therefore, the hybridized synergy realizes a rapid and efficient electron transport, which leads to an enhanced performance of the SnS 2 /Carbon system [159][160][161][162]. The carbon nanofiber network has also made feasible contributions in terms of SnS2/Carbon nanomaterial formation. For instance, Xia et al. prepared SnS2 embedded in nitrogen and sulfur dual-doped carbon (SnS2/NSDC) nanofibers by a facile electrospinning technique as indicated in Figure 9 [92]. Figure 9d-f illustrates the various morphological features of the SnS2/Carbon composite nanofibers. The carbon nanofiber framework provides a conductive host and is tolerant to the volume variation of SnS2 during the charging/discharging processes, thereby maintaining the structural stability of the SnS2/Carbon electrode [158]. Furthermore, the microstructures of SnS2 nanosheets can provide rich migration paths of sodium ions and electrons; therefore, the hybridized synergy realizes a rapid and efficient electron transport, which leads to an enhanced performance of the SnS2/Carbon system [159][160][161][162].

Two-Dimensional (2D) SnS2/Carbon Composite Nanomaterials
The sheet-like nature of 2D nanomaterials makes them attractive for resolving diverse application demands. Coupled with SnS2, 2D SnS2/carbon nanomaterials display a wide range of extraordinary properties especially in alleviating volume expansion [163]. Especially, minimal stacking of 2D layered materials can be achieved for better application performance due to the introduction of the conductive graphene layers, which conveniently protects the SnS2 nanosheets from breakdown and weakens their agglomerating and re-stacking trends. Through an all-solid-state synthesis approach, Lonkar et al. achieved the minimal stacking of SnS2 nanosheets and realized a scalable 2D SnS2 and graphene layered nanosheets (SnS2/G) via ball milling using robust mixed precursors and sufficient metal-sulfur intercalation within the GO substrate [141]. Furthermore, it showed great inherent conductivity, high specific surface area, and high catalytically active planes, which is a plus in battery applications.
Two-dimensional nanostructures are considered as architectural building blocks to hasten reaction kinetics and shorten the transport paths of electrons and ions. Therefore,

Two-Dimensional (2D) SnS 2 /Carbon Composite Nanomaterials
The sheet-like nature of 2D nanomaterials makes them attractive for resolving diverse application demands. Coupled with SnS 2 , 2D SnS 2 /carbon nanomaterials display a wide range of extraordinary properties especially in alleviating volume expansion [163]. Especially, minimal stacking of 2D layered materials can be achieved for better application performance due to the introduction of the conductive graphene layers, which conveniently protects the SnS 2 nanosheets from breakdown and weakens their agglomerating and restacking trends. Through an all-solid-state synthesis approach, Lonkar et al. achieved the minimal stacking of SnS 2 nanosheets and realized a scalable 2D SnS 2 and graphene layered nanosheets (SnS 2 /G) via ball milling using robust mixed precursors and sufficient metal-sulfur intercalation within the GO substrate [141]. Furthermore, it showed great inherent conductivity, high specific surface area, and high catalytically active planes, which is a plus in battery applications.
Two-dimensional nanostructures are considered as architectural building blocks to hasten reaction kinetics and shorten the transport paths of electrons and ions. Therefore, the 2D synergetic combination of SnS 2 with 2D carbon materials would be vital in enhancing its application. For example, SnS 2 itself experiences low catalytic and electrical activity, but the existence of strong interfaces between SnS 2 and graphene might facilitate and ease charge transportation [164]. Furthermore, the carbon component serves as a bridge for SnS 2 nanomaterials which serves as a transfer highway to improve the efficiency of charge transportation. This has proven to be beneficial in improving the overall charge transportation of the resulting nanocomposites. Figure 10 shows fabricated 2D SnS 2 nanoplates anchored on rGO nanosheets by a one-step controllable hydrothermal synthesis approach followed by a slight reduction reaction [165]. The face-to-face (FTF) nanostructure allowed for a large contact area, which improved the composite's conductivity and reduced the migration distance of Na + and electrons between rGO and SnS 2 . the 2D synergetic combination of SnS2 with 2D carbon materials would be vital in enhancing its application. For example, SnS2 itself experiences low catalytic and electrical activity, but the existence of strong interfaces between SnS2 and graphene might facilitate and ease charge transportation [164]. Furthermore, the carbon component serves as a bridge for SnS2 nanomaterials which serves as a transfer highway to improve the efficiency of charge transportation. This has proven to be beneficial in improving the overall charge transportation of the resulting nanocomposites. Figure 10 shows fabricated 2D SnS2 nanoplates anchored on rGO nanosheets by a one-step controllable hydrothermal synthesis approach followed by a slight reduction reaction [165]. The face-to-face (FTF) nanostructure allowed for a large contact area, which improved the composite's conductivity and reduced the migration distance of Na + and electrons between rGO and SnS2. In addition, the charge transfer resistance tests of 2D nanocomposites demonstrated superior transportation kinetics as shown in Figure 11, which may originate from the fast electron transport of FTF SnS2/Carbon composites [104]. These features have also been seen in photocatalytic SnS2/Graphene hybrid nanosheets with identically 2D structural configurations where SnS2 nanoplates were evenly distributed across the graphene framework [166]. Wang et al. used mixed processes of hydrothermal and vapor-phase polymerization to successfully produce triaxial nanocables of conducting polypyrrole@SnS2@carbon nanofiber (PPy@SnS2@CNF) [93]. The nanostructures showed a porous and interconnected nanofiber network with outstanding battery application. In addition, the charge transfer resistance tests of 2D nanocomposites demonstrated superior transportation kinetics as shown in Figure 11, which may originate from the fast electron transport of FTF SnS 2 /Carbon composites [104]. These features have also been seen in photocatalytic SnS 2 /Graphene hybrid nanosheets with identically 2D structural configurations where SnS 2 nanoplates were evenly distributed across the graphene framework [166]. Wang et al. used mixed processes of hydrothermal and vapor-phase polymerization to successfully produce triaxial nanocables of conducting polypyrrole@SnS 2 @carbon nanofiber (PPy@SnS 2 @CNF) [93]. The nanostructures showed a porous and interconnected nanofiber network with outstanding battery application.

Three-Dimensional (3D) Self-Supporting SnS2/Carbon Nanomaterials
Three-dimensional nanostructures of SnS2/Carbon nanomaterials are usually not confined to the nanoscale in any dimension. Three-dimensional nanostructures offer appreciable expanded levels of functionality compared to 2D counterparts because the strains of the 3D shape can induce bending and twisting below the maximum endurance limit for each layer in the construct [167]. Three-dimensional SnS2/Carbon composite nanostructures could result from different synthesis approaches with different combinations of SnS2 and carbon precursors. In general, 0D, 1D, and 2D nanomaterials are the building blocks to achieving desired structural nanocomposites. The dispersions of the nanomaterials could include, for example, nanodots, nanotubes, or nanosheets as well as multi-nano layers. These structural elements are usually in close contact with each other, thereby resulting in 3D interfaces. Many 3D nanocomposite combinations have been reported in the literature [168][169][170]. For example, through the hydrothermal synthesis method, carbon nanotubes formed a cross-winding network on the surface of SnS2 nanoplates. This resulted in flower-like SnS2/Carbon composite nanostructures via electrostatic interactions as shown in Figure 12a-d [111]. The diameter of the CNTs was 25 nm with a length of 1-3 µm (Figure 12d). The hybridized 3D SnS2/Carbon structures could alleviate the internal stress induced by the volumetric expansion/contraction during Li + insertion/extraction processes [148]. Liu et al. obtained uniform 3D interpenetrating porous

Three-Dimensional (3D) Self-Supporting SnS 2 /Carbon Nanomaterials
Three-dimensional nanostructures of SnS 2 /Carbon nanomaterials are usually not confined to the nanoscale in any dimension. Three-dimensional nanostructures offer appreciable expanded levels of functionality compared to 2D counterparts because the strains of the 3D shape can induce bending and twisting below the maximum endurance limit for each layer in the construct [167]. Three-dimensional SnS 2 /Carbon composite nanostructures could result from different synthesis approaches with different combinations of SnS 2 and carbon precursors. In general, 0D, 1D, and 2D nanomaterials are the building blocks to achieving desired structural nanocomposites. The dispersions of the nanomaterials could include, for example, nanodots, nanotubes, or nanosheets as well as multi-nano layers. These structural elements are usually in close contact with each other, thereby resulting in 3D interfaces. Many 3D nanocomposite combinations have been reported in the literature [168][169][170]. For example, through the hydrothermal synthesis method, carbon nanotubes formed a cross-winding network on the surface of SnS 2 nanoplates. This resulted in flower-like SnS 2 /Carbon composite nanostructures via electrostatic interactions as shown in Figure 12a-d [111]. The diameter of the CNTs was 25 nm with a length of 1-3 µm (Figure 12d). The hybridized 3D SnS 2 /Carbon structures could alleviate the internal stress induced by the volumetric expansion/contraction during Li + insertion/extraction processes [148]. Liu et al. obtained uniform 3D interpenetrating porous membrane nanostructures of SnS 2 /Carbon fabricated via non-solvent-induced phase separation (NIPS) membrane technology, and this technology offered an abundant membrane pore space for uniform SnS 2 nanosheet development via C-S covalent bonding [171].  Aside from 3D network composite nanomaterials, hollow 3D composite nanostructures are quite common and have shown unique properties in energy storage fields. Li et al. reported hollow 3D SnS2/Carbon nanospheres that were designed through a facile solvothermal route followed by an annealing treatment (Figure 13a). The SnS2/Carbon nanocomposite resulted from using SnO2@C hollow nanospheres as a template and thioacetamide as a sulfur source as shown in Figure 13c. Moreover, the hollow structure and morphology were maintained during the synthesis process. The 3D SnS2/Carbon nanospheres showed substantial structural integrity reinforcement during electrochemical reactions with improved sodium storage properties. Furthermore, there was high reversible capacity due to a large number of active sites, ideal void space and porosity for volume expansion, high surface permeability, and favorable kinetics due to the high face-to-volume ratio of the hollow structure. Aside from 3D network composite nanomaterials, hollow 3D composite nanostructures are quite common and have shown unique properties in energy storage fields. Li et al. reported hollow 3D SnS 2 /Carbon nanospheres that were designed through a facile solvothermal route followed by an annealing treatment (Figure 13a). The SnS 2 /Carbon nanocomposite resulted from using SnO 2 @C hollow nanospheres as a template and thioacetamide as a sulfur source as shown in Figure 13c. Moreover, the hollow structure and morphology were maintained during the synthesis process. The 3D SnS 2 /Carbon nanospheres showed substantial structural integrity reinforcement during electrochemical reactions with improved sodium storage properties. Furthermore, there was high reversible capacity due to a large number of active sites, ideal void space and porosity for volume expansion, high surface permeability, and favorable kinetics due to the high face-to-volume ratio of the hollow structure.
Nowadays, 3D composites of SnS 2 /Carbon architectures are becoming an academic hotspot with optimal rate capability and cycling stability owing to the synergism of active SnS 2 particles and an extremely conductive carbon framework. Three-dimensional carbon fiber and graphene foam have served as a conductive and robust skeleton for SnS 2 , and their TEM imaging demonstrated that the SnS 2 nanoflakes were strongly attached to these materials [172,173]. The graphene-assembled architectures can adapt hierarchical morphology with high surface-area-to-volume ratios and construct macroscopic and largesize monolithic materials, indicating that they have considerable technological promise for a variety of sustainable applications [174]. Nowadays, 3D composites of SnS2/Carbon architectures are becoming an academic hotspot with optimal rate capability and cycling stability owing to the synergism of active SnS2 particles and an extremely conductive carbon framework. Three-dimensional carbon fiber and graphene foam have served as a conductive and robust skeleton for SnS2, and their TEM imaging demonstrated that the SnS2 nanoflakes were strongly attached to these materials [172,173]. The graphene-assembled architectures can adapt hierarchical morphology with high surface-area-to-volume ratios and construct macroscopic and largesize monolithic materials, indicating that they have considerable technological promise for a variety of sustainable applications [174].

Applications of Synthesized SnS2 and SnS2/Carbon Nanomaterials in Environmental Remediation, Electrochemical Energy Conversion, and Storage
Due to its extensive availability, biocompatibility, cheap cost, low toxicity, and high chemical stability, SnS2 is one of the most economically viable materials exploited in a wide range of applications. In addition, SnS2 possesses good qualities such as a high surface area with increased active sites, good ion exchange capability, and loading capacity. The hybridization of SnS2 with carbon materials has been explored in catalysis, biomedicine, supercapacitors, electrochemical sensors, batteries, photocatalysis, and so on. In particular, their capacity to build dimensionally variable structures gives SnS2 and SnS2/Carbon nanomaterials significant structural advantages in environmental remediation and electrochemical energy conversion and storage. The applications of SnS2 and SnS2/Carbon nanomaterials have been briefly summarized in Figure 14.

Applications of Synthesized SnS 2 and SnS 2 /Carbon Nanomaterials in Environmental Remediation, Electrochemical Energy Conversion, and Storage
Due to its extensive availability, biocompatibility, cheap cost, low toxicity, and high chemical stability, SnS 2 is one of the most economically viable materials exploited in a wide range of applications. In addition, SnS 2 possesses good qualities such as a high surface area with increased active sites, good ion exchange capability, and loading capacity. The hybridization of SnS 2 with carbon materials has been explored in catalysis, biomedicine, supercapacitors, electrochemical sensors, batteries, photocatalysis, and so on. In particular, their capacity to build dimensionally variable structures gives SnS 2 and SnS 2 /Carbon nanomaterials significant structural advantages in environmental remediation and electrochemical energy conversion and storage. The applications of SnS 2 and SnS 2 /Carbon nanomaterials have been briefly summarized in Figure 14.

Photocatalyst in Pollutant Degradation
Photocatalysis has shown great potential in hydrogen production, antibacterial activity, pollutant degradation, air purification, etc. [175][176][177][178][179]. Amongst them, photocatalytic pollutant degradation is a particularly appealing technology since organic pollutants can be entirely degraded into CO 2 , H 2 O, and inorganic compounds leaving minimum detrimental leftovers [180,181]. For decades, semiconductor-based photocatalysts such as SnO 2 , ZnO, TiO 2 , etc., have gained prominence as breakthrough material for organic pollutant degradation [182]. This is due to their ability to use solar energy to carry out the catalytic reaction [183]. Amongst them, TiO 2 has gained wider recognition due to its abundance and low cost. However, drawbacks of TiO 2 such as a wide bandgap (3.2 eV), limited active sites, low absorption of UV light, and low quantum efficiency impede its versatility in the efficient degradation of pollutants [184]. Therefore, it is imperative to design a unique photocatalyst with high absorption capacity and a narrow bandgap for photocatalysis. SnS 2 and its hybrid nanocomposites are gaining massive recognition in the scientific community as alternative photocatalytic materials to TiO 2 as a result of their narrow bandgap and high quantum yield [185][186][187][188]. SnS 2 composite nanomaterials have also shown higher catalytic performance than SnS 2 nanomaterials themselves in pollutant removal.

Photocatalyst in Pollutant Degradation
Photocatalysis has shown great potential in hydrogen production, antibacterial activity, pollutant degradation, air purification, etc. [175][176][177][178][179]. Amongst them, photocatalytic pollutant degradation is a particularly appealing technology since organic pollutants can be entirely degraded into CO2, H2O, and inorganic compounds leaving minimum detrimental leftovers. [180,181] For decades, semiconductor-based photocatalysts such as SnO2, ZnO, TiO2, etc., have gained prominence as breakthrough material for organic pollutant degradation [182]. This is due to their ability to use solar energy to carry out the catalytic reaction [183]. Amongst them, TiO2 has gained wider recognition due to its abundance and low cost. However, drawbacks of TiO2 such as a wide bandgap (3.2 eV), limited active sites, low absorption of UV light, and low quantum efficiency impede its versatility in the efficient degradation of pollutants [184]. Therefore, it is imperative to design a unique photocatalyst with high absorption capacity and a narrow bandgap for photocatalysis. SnS2 and its hybrid nanocomposites are gaining massive recognition in the scientific community as alternative photocatalytic materials to TiO2 as a result of their narrow bandgap and high quantum yield [185][186][187][188]. SnS2 composite nanomaterials have also shown higher catalytic performance than SnS2 nanomaterials themselves in pollutant removal.

SnS2 Nanomaterials in Photocatalysis
SnS2 nanoparticles are known to exhibit photocatalytic properties under visible light [189][190][191]. As a semiconductor metal sulfide, SnS2 can act as capable sensitizers and harvest visible light for narrow bandgap semiconductors in some photocatalytic applications. Srinivas et al. found the bandgap of SnS2 nanostructures is around 2.50 eV as the photocatalyst of the irradiation of visible light [192]. SnS2 QDs have shown a bandgap that matches the absorption spectra of sunlight, a huge extinction coefficient due to quantum confinement, and large intrinsic dipole moments. However, the reduction in particle size has shown an increase in the bandgap of the semiconductor nanomaterials [193]. Nonetheless, various dimensions of SnS2 nanomaterials have reported successes in photocata-  [189][190][191]. As a semiconductor metal sulfide, SnS 2 can act as capable sensitizers and harvest visible light for narrow bandgap semiconductors in some photocatalytic applications. Srinivas et al. found the bandgap of SnS 2 nanostructures is around 2.50 eV as the photocatalyst of the irradiation of visible light [192]. SnS 2 QDs have shown a bandgap that matches the absorption spectra of sunlight, a huge extinction coefficient due to quantum confinement, and large intrinsic dipole moments. However, the reduction in particle size has shown an increase in the bandgap of the semiconductor nanomaterials [193]. Nonetheless, various dimensions of SnS 2 nanomaterials have reported successes in photocatalytic activities. For example, 1D SnS 2 nanotubes have demonstrated big potential in photocatalysis with more active sites for adsorption and catalysis [194]. These properties have also been exhibited by 2D SnS 2 nanomaterials [77,195]. For instance, atomically ultrathin 2D SnS 2 conducting channels helped to achieve rapid carrier transport in photoelectrodes which greatly reduced the recombination rate with a bandgap of 2.29 eV [196]. Moreover, the lower thickness of 2D SnS 2 structures provided an easy pathway for photogenerated electrons and holes to move toward the surface reaction sites [195]. Hence, the possibility of recombination is reduced, and photocatalytic effectiveness is improved. Ullah et al. in a comparative study observed that SnS 2 and conventional cadmium sulfide (CdS) films have direct bandgap values of 2.20 eV and 2.45 eV, respectively [197].
Moreover, it was discovered that SnS 2 film has a higher photocurrent of 140 µA than CdS films with 80 µA. Thus, compared with CdS, SnS 2 nanostructures offer a better bandgap, superior cycling stability, and bigger reversible capacities that are desirable for photocatalysis and electrocatalytic applications. Three-dimensional SnS 2 nanoflowers prepared at 120 • C in solvent ethylene glycol have been proven to have high adsorption capability and visible light photocatalytic activity for dyes (Methyl Blue and Methyl Orange) and heavy metal ions (Pb 2+ and Cd 2+ ) [198]. Microwave-assisted synthesis of hexagonal SnS 2 allowed for the simultaneous adjustment of morphologies and nanostructures under atmospheric pressure and low temperature [199]. Moreover, it showed advantages in the photoreduction of stable azo-dye. In addition, SnS 2 nanostructures showed excellent photocatalytic activity in the reduction of hazardous Cr(VI) to harmless Cr(III) in environmental conditions, as well as effectively decomposing mutagenic dyes (Methyl Orange and Rhodamine Blue) to benign compounds in a brief duration [192].
It is apparent that all dimensional SnS 2 nanomaterials can be harnessed for photocatalytic applications due to their semiconductor nature. Researchers are drawn to this exceptional catalytic feat because it allows them to include and synthesize composite nanostructures with improved performance. Moreover, the bandgap is an important parameter in photocatalytic activities. Besides adsorption capacities, semiconductor catalysts with a narrow bandgap can absorb more photons, resulting in better catalytic activity when exposed to visible light [200]. New modifications are being used during synthesis to further optimize the bandgap for visible light application. One such modification is the introduction of carbon precursors to achieve SnS 2 /Carbon composite nanomaterials with desired morphological orientations and photocatalytic properties.

SnS 2 /Carbon Nanomaterials in Photocatalysis
Carbon materials can form unique chemical bonding thus providing strong interactions with SnS 2 , which leads to a bandgap narrowing effect [201]. SnS 2 /Carbon composite nanomaterials show more active sites, electron acceptors, and transport channels with improved structural stability and adsorption ability [202]. SnS 2 /Carbon nanomaterials have been reported to have the ability to degrade organic pollutants and carcinogens more effectively as compared to SnS 2 (i.e., CO 2 ) [103,203]. Xue et al. in their research used heterojunction bio-carbon/SnS 2 nanocomposites with a narrow bandgap to efficiently photocatalyze the conversion of Arsenic(III) and calcium arsenate removal [204]. The -C=Sn-S bonds efficiently prevented SnS 2 agglomeration, extended the photoresponse range, and enhanced the hydrophilicity of the bio-carbon/SnS 2 nanocomposites while reducing their transfer resistance. For example, Figure 15 shows the sheet-like SnS 2 nanoparticles uniformly incorporated on rGO sheets. Because of the increase in interfacial charge carriers, the addition of rGO to the composite nanomaterials improved the photocatalytic activity of Cr (VI) reduction. The SnS 2 /rGO composite photocatalysts also outperformed pure SnS 2 QDs in terms of photocatalysis. So, the synergy between SnS 2 and carbon materials at the nanoscale can provide a sufficient bandgap to catalyze photocatalytic reactions. A substantial bandgap is necessary to significantly promote the photocatalytic abilities of SnS 2 /Carbon composite nanomaterials [205].
The recombination inhibition of charge carriers between SnS 2 and the carbon materials has been observed to bring about the optimization of charge carriers at the SnS 2 /carbon interfaces to photodegrade Cr(VI) [207]. This was achieved through the coupling effect and the strong electrostatic attraction of carbon materials, which served as the electron acceptor to trap the photoinduced electrons from SnS 2 and thus enhances the separation efficiency of electrons and holes [208]. However, the degradation of toxic substances is further impacted by the concentration of the pollutants and the dosage of the catalysts. Figure 16 further shows a schematic illustration of the mechanism in the photocatalytic breakdown of organic and inorganic pollutants by SnS 2 /Carbon composite nanomaterials. As shown in the diagram, once illuminated by light, electrons get excited and then migrate from the valence band (VB) to the conductor band (CB) of SnS 2 QDs. Subsequently, the SnS 2 electrons transfer to the associated carbon nanostructures that act as electron acceptors. This suppresses the recombination of photogenerated electron-hole pairs leading to˙OH and˙O 2− radical species, which can lead to the removal of pollutants by their superior activities. The recombination inhibition of charge carriers between SnS2 and the carbon materials has been observed to bring about the optimization of charge carriers at the SnS2/carbon interfaces to photodegrade Cr(VI) [207]. This was achieved through the coupling effect and the strong electrostatic attraction of carbon materials, which served as the electron acceptor to trap the photoinduced electrons from SnS2 and thus enhances the separation efficiency of electrons and holes [208]. However, the degradation of toxic substances is further impacted by the concentration of the pollutants and the dosage of the catalysts. Figure 16 further shows a schematic illustration of the mechanism in the photocatalytic breakdown of organic and inorganic pollutants by SnS2/Carbon composite nanomaterials. As shown in the diagram, once illuminated by light, electrons get excited and then migrate from the valence band (VB) to the conductor band (CB) of SnS2 QDs. Subsequently, the SnS2 electrons transfer to the associated carbon nanostructures that act as electron acceptors. This suppresses the recombination of photogenerated electron-hole pairs leading to ˙OH and ˙O 2− radical species, which can lead to the removal of pollutants by their superior activities.    Table 3 shows the comparative photocatalytic performances of SnS 2 and SnS 2 /Carbon composite nanomaterials under visible light from various literature. It can be observed that the synergistic combination of SnS 2 nanomaterials and the carbon allotropes significantly enhanced the photocatalytic efficiency of the SnS 2 /Carbon composite nanomaterials compared with SnS 2 . Notably, dimensional SnS 2 /Carbon nanomaterials exhibited remarkable degrading effects especially on chromium (VI). Overall, SnS 2 /Carbon nanomaterials hold great degradation potential toward wastewater treatments. Photocatalysts are usually made of costly precious metals that are not in abundance. With the availability and low cost of SnS 2 and carbon materials, researchers can venture more into creating SnS 2 /Carbon photocatalytic nanomaterials to harness its potential in photocatalysis at a large scale.

Electrochemical Conversion and Energy Storage Applications of SnS 2 and SnS 2 /Carbon Nanomaterials
The ever-growing demands for energy resources and environmental concerns have paved the way for the exploration and development of clean and sustainable energy alternatives. Electrochemical energy conversions and storage devices including supercapacitors, fuel cells, solar cells, and metal ions or air batteries have gained attention due to their environmentally benign nature and hold great potential as a fossil fuel replacement. Since its discovery in 2004, graphene has become one of the most promising materials in energy storage due to its remarkable electrochemical properties [212,213]. Furthermore, graphene has the tendency to form composite nanomaterials of different dimensions which helps to boost the overall catalytic and electrochemical performance. SnS 2 possesses a theoretical capacity of~1136 mAhg −1 [214] which is higher than that of graphene (744 mAh g −1 ), [215,216] making it valuable for battery application, solely or in a composite material. SnS 2 /Carbon (including various carbon allotropes) composite nanomaterials have also proven to be more efficient for energy storage systems because of their high conductivity, mechanical and thermal stability, and long cycle ability [217]. The relationship between SnS 2 and SnS 2 /Carbon nanoarchitectures and their electrochemical performances are discussed below.

SnS 2 Nanomaterials in Electrochemical Conversion and Energy Storage
SnS 2 nanomaterials exhibit enhanced electrochemical performance due to their compact and consistent crystal structure with a reasonable thickness and crystallinity [218], which is also favorable for structural stability and quick ion transport during lithiation/delithiation processes. Various synthesis approaches are geared at improving the performance of SnS 2 nanomaterials as alternative electrode materials. However, the capacity fading of SnS 2 electrode materials persists due to significant volume changes during charging/discharging processes [219,220]. SnS 2 nanostructures with different morphologies have been fabricated to resolve these challenges.
Studies on the use of low-dimensional SnS 2 nanomaterials in electrochemical energy conversion and storage applications are scant, to the best of our knowledge. This is because SnS 2 materials can be hindered by sluggish diffusion kinetics and an unavoidable volume change during discharging and charging processes. Nonetheless, a SnS 2 nanowall electrode realized a high reversible capacity of 576 mAh g −1 at 500 mA g −1 and an excellent rate capability of~370 mAh g −1 at 5 A g −1 in sodium ion batteries [221]. The sulfide matrix acts as a buffer to decrease the large strain caused by the volume expansion of tin nanostructures [222]. Unfortunately, in some cases, the large volume expansion induces aggregation of the Sn particles. As such, it can bring about the cracking, pulverization, and degradation of the electrode material which leads to capacity loss [223][224][225]. Nevertheless, flowerlike-SnS 2 nanostructures with large specific surface areas and better average pore sizes have exhibited remarkable battery performance with excellent long-term cycling stability [49,226,227]. In addition, binders with superior dispersion and cohesiveness in electrodes have shown to improve the electrochemical performance of SnS 2 as the anode for LIBs [228]. SnS 2 monolayers boost Lithium mobility, although their adsorption strength is moderate compared to other nanostructures. Rolling the monolayer into a one-dimensional nanotube increases Lithium ions' adsorption strength and diffusion rates [229].
In terms of air batteries, Khan et al. used 3D SnS 2 nanopetals as an air electrode material for hybrid Na-air batteries. It displayed a low overpotential gap of 0.52 V, high round trip efficiency of 83%, high power density of 300 mW g −1, and good rechargeability of up to 40 cycles [230]. Moreover, their electrocatalytic performance was linked to oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The hybrid cell charge potential (OER) is 3.57 V at the high current density of 20 mA g −1 , which is comparable to the charge potential (3.47 V) of a hybrid cell with Platinum on Carbon (Pt/C), known to be the best catalyst for ORR at low current density (5 mA g −1 ) [226]. Chia et al. explored the prospects of SnS 2 materials as alternative electrocatalysts in ORR, OER, and HER [227]. It was proven that SnS 2 has high inherent electrocatalytic activity and a fast heterogeneous electron transfer (HET) rate. Moreover, Xia et al. recently used first-principle methods based on the density functional theory to study the electrocatalytic performance of transition metal atoms supported on a SnS 2 monolayer [231]. The catalytic performance of SnS 2 for OER and ORR was shown to be significantly enhanced by the surface of the SnS 2 monolayer. There is limited literature on the use of SnS 2 solely or as a composite electrocatalyst, but gaps created in this field could be harnessed to create high-performance bifunctional ORR, OER, and HER electrocatalysts in the future.

SnS 2 /Carbon Nanomaterials in Electrochemical Conversion and Energy Storage
Carbon nanostructures have been demonstrated to have the ability to confine active materials in composite nanostructures. The addition of heteroatoms to carbon could increase its affinity for active materials, form a strong architecture, and speed up the electron and ion transfer process [173,232,233]. When associated with SnS 2 nanostructures, SnS 2 /Carbon composite nanomaterials can tolerate the volume change and enhance the ion diffusion rate through porous structure construction; thus, it is valuable in resolving the rapid battery capacity fading [234][235][236]. Furthermore, it can enhance the weak interaction between non-polar carbon and polar polysulfides which reduces polysulfide leakage from carbon materials in lithium-sulfur batteries (LSBs) [156,157]. SnS 2 /Carbon nanomaterials as electrode materials in LSBs are fairly recent in research but have shown to have the ability to reduce the "shuttle effect". Zhou et al. tried to resolve the limitations in LIBs by embedding SnS 2 nanoparticles into 2D porous carbon nanosheet (PCN) interlayers to form a multi-functional (PCN-SnS 2 ) nanocomposite as illustrated in Figure 17 [237]. The synergy between PCN and SnS 2 nanoparticles resulted in a fast conversion of long-chain polysulfides to Li 2 S. The constant conversion of polysulfides on PCN-SnS 2 to the final Li 2 S product assisted in reducing polysulfide shuttle during the cycling process. The best performance was demonstrated by PCN-SnS 2 with dual physicalchemical confinement. It also improved the chemical reaction kinetics thereby diminishing the transfer of polysulfides to the lithium anode. This, in turn, reduced the "shuttle effect" during the entire charging/discharging process. Figure 17d shows a schematic illustration of the conversion process of sulfur on SnS 2 embedded in PCNs. Wei et al. in Figure 18 also created a flexible electrocatalytic membrane that could reduce polysulfide shuttling and capacity fading in LSBs with different SnS 2 /HCNF (hollow carbon nanofiber) interlayers that are 2D nanostructured. The SnS 2 /HCNF in the LSBs displayed a high-rate discharge capacity (694 mAh g −1 at 3C) and low-capacity fading rate (0.056% per cycle during 500 cycles at 1C). Additionally, it showed that the nanocomposite efficiently alleviated the "shuttle effect" as a result of the composite nanostructure synergy [159,238].  Three-dimensional nanomaterials are the most popular dimensional SnS2/Carbon composite nanostructures, and these nanostructures have also made an impact in energy storage applications. They have been beneficial for resolving the structure pulverization and poor electrical conductivity of metal dichalcogenides that could lead to adverse capacity decay both in LIBs and SIBs. Figure 19a,b shows the SEM image of 3D honeycomblike rGO anchored with SnS2 quantum dots (3D SnS2 QDs/rGO) through spray-drying and sulfidation processes. The 3D features allowed for the volume change of SnS2 QDs during the lithiation/delithiation and sodiation/desodiation processes. It also made provision for electrolyte reservoirs to promote the conductivity of the SnS2 QDs. In addition, the 3D SnS2 QDs/rGO nanocomposite electrode delivered a high capacity and long cycling stability of 862 mAh g −1 for LIB at 0.1 A/g after 200 cycles (Figure 19c) and 233 mAh g −1 for SIB at 0.5 A g −1 after 200 cycles (Figure 19d). The improved battery performance, according to Chang et al., can be due to the composite structure's robustness and the synergistic effects among a few layers of SnS2 and graphene [240]. Moreover, in situ-grown SnS2 nanoparticles have been homogeneously confined in rGO and CNT porous carbon nanostructures, which resulted in 3D architectures that demonstrated outstanding performance [55].
At present, the hybridization synthesis of SnS2/Carbon nanomaterials focuses on improving the capability and cycling stability of the electrodes [89,241]. To achieve a stable SIB/LIB electrode, Cui et al. developed self-standing electrodes with rational SnS2 nanosheets restricted into bubble-like carbon nanoreactors anchored on N, S doped carbon nanofibers [242]. The electrodes demonstrated a very steady capacity of 964.8 and 767.6 mAh g −1 at 0.2 A g −1 , as well as strong capacity holding of 87.4% and 82.4% after 1000 cycles at high current density, respectively. It was stated further that the addition of N, S components improved the wettability of the carbon nanofiber matrix to the electrolyte and Li ions and the electrode's overall electrical conductivity. The performances of SnS2 and SnS2/Carbon composite nanomaterials in battery applications are summarized in Table 4 and compared with graphene as a reference material. Numerous synthesis approaches are Three-dimensional nanomaterials are the most popular dimensional SnS 2 /Carbon composite nanostructures, and these nanostructures have also made an impact in energy storage applications. They have been beneficial for resolving the structure pulverization and poor electrical conductivity of metal dichalcogenides that could lead to adverse capacity decay both in LIBs and SIBs. Figure 19a,b shows the SEM image of 3D honeycomb-like rGO anchored with SnS 2 quantum dots (3D SnS 2 QDs/rGO) through spray-drying and sulfidation processes. The 3D features allowed for the volume change of SnS 2 QDs during the lithiation/delithiation and sodiation/desodiation processes. It also made provision for electrolyte reservoirs to promote the conductivity of the SnS 2 QDs. In addition, the 3D SnS 2 QDs/rGO nanocomposite electrode delivered a high capacity and long cycling stability of 862 mAh g −1 for LIB at 0.1 A/g after 200 cycles (Figure 19c) and 233 mAh g −1 for SIB at 0.5 A g −1 after 200 cycles (Figure 19d). The improved battery performance, according to Chang et al., can be due to the composite structure's robustness and the synergistic effects among a few layers of SnS 2 and graphene [240]. Moreover, in situ-grown SnS 2 nanoparticles have been homogeneously confined in rGO and CNT porous carbon nanostructures, which resulted in 3D architectures that demonstrated outstanding performance [55].
At present, the hybridization synthesis of SnS 2 /Carbon nanomaterials focuses on improving the capability and cycling stability of the electrodes [89,241]. To achieve a stable SIB/LIB electrode, Cui et al. developed self-standing electrodes with rational SnS 2 nanosheets restricted into bubble-like carbon nanoreactors anchored on N, S doped carbon nanofibers [242]. The electrodes demonstrated a very steady capacity of 964.8 and 767.6 mAh g −1 at 0.2 A g −1 , as well as strong capacity holding of 87.4% and 82.4% after 1000 cycles at high current density, respectively. It was stated further that the addition of N, S components improved the wettability of the carbon nanofiber matrix to the electrolyte and Li ions and the electrode's overall electrical conductivity. The performances of SnS 2 and SnS 2 /Carbon composite nanomaterials in battery applications are summarized in Table 4 and compared with graphene as a reference material. Numerous synthesis approaches are being harnessed to tackle these issues and formulate hybrid nanostructures with effective outcomes. This can perhaps shorten the pathway and improve the transportation speed of electrolyte ions at electrode surfaces [29]. being harnessed to tackle these issues and formulate hybrid nanostructures with effective outcomes. This can perhaps shorten the pathway and improve the transportation speed of electrolyte ions at electrode surfaces [29]. Adapted from [243]. Copyright 2019, Springer, Open Access.
Electrochemical reactions, such as ORR, OER, and hydrogen evolution reaction (HER) in fuel-cell and metal-air battery applications, have also shown promising successes in electrochemical energy conversion technologies [244][245][246][247]. However, research on SnS2/Carbon composite nanostructures as electrocatalysts is rarely reported. For instance, Cheng et al. fabricated stable SnS2 nanosheets incorporated with carbon dots, which exhibited an OER rate of up to 1.1 mmol g −1 h −1 under simulated sunlight irradiation [248]. Moreover, through a simple solid-state synthesis, a 2D SnS2/Graphene nanocomposite was achieved, and it showed an electrocatalytic (HER) overpotential of 0.36 V and a specific capacitance of 565 F g −1 [141]. In addition, a 3D hollow C@SnS2/SnS nanosphere was discovered to have outstanding OER performance through structural phase transitions [145]. The Sn 4+ in the composite readily received electrons in water which is vital for improving the OER activity. More so, it measured a low overpotential of 380 mV at 10 mA cm −2 current density. Additionally, Chen et al. recently engineered SnS2 nanosheet arrays on carbon paper with surface oxygen adjustment under the directions of density function theory (DFT) calculations to efficiently electroreduce CO2 into formate and syngas (CO Electrochemical reactions, such as ORR, OER, and hydrogen evolution reaction (HER) in fuel-cell and metal-air battery applications, have also shown promising successes in electrochemical energy conversion technologies [244][245][246][247]. However, research on SnS 2 /Carbon composite nanostructures as electrocatalysts is rarely reported. For instance, Cheng et al. fabricated stable SnS 2 nanosheets incorporated with carbon dots, which exhibited an OER rate of up to 1.1 mmol g −1 h −1 under simulated sunlight irradiation [248]. Moreover, through a simple solid-state synthesis, a 2D SnS 2 /Graphene nanocomposite was achieved, and it showed an electrocatalytic (HER) overpotential of 0.36 V and a specific capacitance of 565 F g −1 [141]. In addition, a 3D hollow C@SnS 2 /SnS nanosphere was discovered to have outstanding OER performance through structural phase transitions [145]. The Sn 4+ in the composite readily received electrons in water which is vital for improving the OER activity. More so, it measured a low overpotential of 380 mV at 10 mA cm −2 current density. Additionally, Chen et al. recently engineered SnS 2 nanosheet arrays on carbon paper with surface oxygen adjustment under the directions of density function theory (DFT) calculations to efficiently electroreduce CO 2 into formate and syngas (CO and H 2 ) [249]. The SnS 2 nanosheets that were modified with surface oxygen exhibited a notable Faradaic efficiency of 91.6% for carbonaceous products at −0.9 V vs. reversible hydrogen electrode (RHE), including 83.2% for formate creation and 16.5% for syngas. These dimensional SnS 2 and SnS 2 /Carbon composite nanostructures can shorten electron transfer channels in electrochemical application because of their high surface-to-volume ratio, which probably have promoted their electrochemical performance.

Conclusions and Perspectives
SnS 2 nanomaterials of different dimensional morphological orientations have made ample progress in photocatalysis and energy storage batteries. Meanwhile, they have presented some limitations which need further modifications to enhance their practical application potential. The broad bandgap and volume expansion during the charging/discharging processes of SnS 2 are well-known drawbacks that limit its applicability. Hybridization of SnS 2 with appropriate carbon materials, synthesizing composite nanomaterials, and developing innovative structures or morphologies dimensionally have been developed in order to overcome the aforementioned difficulties. Many novel and cost-effective synthetic methodologies have offered ways to achieve better performance in photocatalysis and energy storage batteries. SnS 2 /Carbon architectural nanomaterials have become an academic hotspot with outstanding reports on rate capability and cycling stability due to the synergism of active SnS 2 particles and a very conductive carbon framework.
Here, we have summed up some recent research on SnS 2 and SnS 2 /Carbon composite nanomaterials and reviewed the progress made on the wet and solid-phase fabrication methods to achieve various morphological structures of tin disulfide (SnS 2 ) and SnS 2 /Carbon nanomaterials such as nanodots, nanofibers, nanowires, nanotubes, nanorods, nanosheets, nanoflowers, and nanospheres in (0D-3D) dimensional states and their applications in photocatalysis, electrochemical conversion, and energy storage. We tried to bridge the knowledge gap presented in SnS 2 , SnS 2 /Carbon nanostructures, and their application performances in photocatalytic degradation and energy storage batteries.
Although the understanding of dimensional hybrid nanomaterials has made some achievements, there is still room to harness their dimensional capabilities, as this field of study has a lot of promise for the development of high-performance nanomaterials. In the meantime, more research into the compatibility of carbon nanomaterials with SnS 2 functional nanomaterials is needed to enhance the utilization of these hybrid nanocomposites in photocatalytic and energy storage applications. Furthermore, a deeper knowledge of the mechanisms involved in the formation of SnS 2 /Carbon nanohybrids can be used to develop novel methods for producing optimal, cost-effective, and environmentally benign composite nanomaterials. Realizing these possibilities may necessitate the efforts of researchers as well as a fresh look at hierarchical nanocomposites.