Transition Metal Dichalcogenides Nanoscrolls: Preparation and Applications

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) nanosheets have shown extensive applications due to their excellent physical and chemical properties. However, the low light absorption efficiency limits their application in optoelectronics. By rolling up 2D TMDCs nanosheets, the one-dimensional (1D) TMDCs nanoscrolls are formed with spiral tubular structure, tunable interlayer spacing, and opening ends. Due to the increased thickness of the scroll structure, the light absorption is enhanced. Meanwhile, the rapid electron transportation is confined along the 1D structure. Therefore, the TMDCs nanoscrolls show improved optoelectronic performance compared to 2D nanosheets. In addition, the high specific surface area and active edge site from the bending strain of the basal plane make them promising materials for catalytic reaction. Thus, the TMDCs nanoscrolls have attracted intensive attention in recent years. In this review, the structure of TMDCs nanoscrolls is first demonstrated and followed by various preparation methods of the TMDCs nanoscrolls. Afterwards, the applications of TMDCs nanoscrolls in the fields of photodetection, hydrogen evolution reaction, and gas sensing are discussed.

A great deal of effort has been developed to improve the light absorption of TMDCs nanosheets, such as plasma treatment, formation of van der Waals heterojunction, utilization of plasmonic effect, integration of quantum dots [18][19][20][21][22], etc. Recently, rolling up the monolayer TMDCs nanosheet to form the TMDCs nanoscroll (TMDCs-NS) has been reported as a promising method to improve their optoelectronic performance [19]. With the aid of volatile organic solvents or alkaline solution [19,[23][24][25][26][27], the monolayer TMDCs nanosheets can be transformed into one-dimensional (1D) nanoscrolls with tubular and spiral structures [23,28,29]. The as-obtained TMDCs nanoscroll (TMDCs-NS) showed enhanced light absorption due to the increased cross-section. In addition, the TMDCs-NS also inherits the excellent properties from the monolayer TMDCs nanosheet [21,[30][31][32][33][34][35]. Furthermore, the curved structure of the nanoscroll exhibits strain, which can modulate the band gap of TMDCs nanosheet [33]. Moreover, its scrolled structure has tunable interlayer space with open ends, in which other functional nanomaterials can be encapsulated [35,36]. Therefore, the TMDCs-NS have attracted great attention in optoelectronics in recent years.  [56]. (e) The MoS2 sheets of the same size roll through the armchair (left) and zigzag (right) orientation molecular dynamics (MD) simulations before (upper) and after (lower) structural relaxation [53]. (f) The relationship between unit atomic energy and simulation time of MoS2 nanoscrolls with armchair, zigzag, and chiral orientation [53]. (g) The atomic energy is a function of MoS2 nanoscroll spacing. The red area indicates the energy-favorable interlayer distance [53].  [56]. (e) The MoS 2 sheets of the same size roll through the armchair (left) and zigzag (right) orientation molecular dynamics (MD) simulations before (upper) and after (lower) structural relaxation [53]. (f) The relationship between unit atomic energy and simulation time of MoS 2 nanoscrolls with armchair, zigzag, and chiral orientation [53]. (g) The atomic energy is a function of MoS 2 nanoscroll spacing. The red area indicates the energy-favorable interlayer distance [53].
Theoretical investigation indicates that there is an energy barrier between the nanosheet and nanoscroll [71][72][73]. A driving activation energy is required to initialize the scrolling automatically [74]. By scrolling or folding the nanosheet into a nanoscroll, the lowest energy form is presented [74]. The barrier can be overcome with the help of external force in liquid or in air, where the TMDCs-NS is formed.

Solvent Evaporation to Make Nanoscrolls
Many organic solvents are liquid at ambient conditions with large volatility, which can be used to assist or induce the scrolling of TMDCs nanosheets [23,33,35,[75][76][77], such as acetone, ethanol, and isopropanol [24,[78][79][80]. In 2018, we proposed the preparation of MoS 2 nanoscroll by dropping an ethanol or acetone droplet on monolayer MoS 2 nanosheet [23], as shown in Figure 2. Monolayer MoS 2 nanosheets were first obtained by chemical vapor deposition (CVD) on SiO 2 /Si (Figure 2a,b), and a drop of ethanol was deposited on the MoS 2 nanosheets (Figure 2c). Due to its low surface tension, the ethanol can wet MoS 2 and SiO 2 /Si substrates easily. During the evaporation process, a thin ethanol layer is formed near the contact line ( Figure 2d). With the rapid evaporation of ethanol, a temperature gradient is generated near the contact line, creating a surface tension gradient to induce fluid flow. Such kind of fluid flow could roll up the edge of MoS 2 . As the contact line moves, the MoS 2 nanosheet continues to be rolled up until a complete MoS 2 nanoscroll is formed (Figure 2e) [23].
By dropping the mixture of ethanol and water on CVD-grown monolayer MoS 2 nanosheet, Jian Zheng et al. also successfully prepared MoS 2 nanoscrolls, as shown in Figure 3 [24]. Large-area monolayer MoS 2 nanosheets were obtained firstly by CVD (Figure 3a). MoS 2 nanoscrolls were then fabricated in a short time in a mixture solution of ethanol and water with a volume ratio of 2:1 (ethanol:water = 2:1).
Due to the high temperature-causing mismatch between the MoS 2 nanosheet and substrate during CVD growth (Figure 3a), a strain equilibrium is balanced between them when the temperature decreases to room temperature (Figure 3b). When the mixture solution is dropped onto the MoS 2 nanosheet, the ethanol intercalates between the MoS 2 and substrate, and the upper part of MoS 2 is detached from the substrate. In this case, the strain balance is broken (Figure 3c), and the released portion curls into a roll (Figure 3d). Due to the adhesion from the substrate, the left portion remains intact [24]. With the evaporation of ethanol, the surface tension at the air-solvent-MoS 2 interface is greater than that between MoS 2 and the substrate. As a result, the strain-adhesion balance is broken continuously, generating MoS 2 nanoscroll finally ( Figure 3e).  [24]. Large-area monolayer MoS2 nanosheets were obtained firstly by CVD ( Figure  3a). MoS2 nanoscrolls were then fabricated in a short time in a mixture solution of ethanol and water with a volume ratio of 2:1 (ethanol:water = 2:1).
Due to the high temperature-causing mismatch between the MoS2 nanosheet and substrate during CVD growth (Figure 3a), a strain equilibrium is balanced between them when the temperature decreases to room temperature (Figure 3b). When the mixture solution is dropped onto the MoS2 nanosheet, the ethanol intercalates between the MoS2 and substrate, and the upper part of MoS2 is detached from the substrate. In this case, the strain balance is broken (Figure 3c), and the released portion curls into a roll ( Figure 3d). Due to the adhesion from the substrate, the left portion remains intact [24]. With the evaporation of ethanol, the surface tension at the air-solvent-MoS2 interface is greater than that between MoS2 and the substrate. As a result, the strain-adhesion balance is broken continuously, generating MoS2 nanoscroll finally (Figure 3e). For bilayer and multilayer nanosheets, it is difficult to roll them up effectively due to the strong adhesion force from the substrate. Since TMDCs nanosheets are usually grown   [24]. Large-area monolayer MoS2 nanosheets were obtained firstly by CVD ( Figure  3a). MoS2 nanoscrolls were then fabricated in a short time in a mixture solution of ethanol and water with a volume ratio of 2:1 (ethanol:water = 2:1).
Due to the high temperature-causing mismatch between the MoS2 nanosheet and substrate during CVD growth (Figure 3a), a strain equilibrium is balanced between them when the temperature decreases to room temperature ( Figure 3b). When the mixture solution is dropped onto the MoS2 nanosheet, the ethanol intercalates between the MoS2 and substrate, and the upper part of MoS2 is detached from the substrate. In this case, the strain balance is broken (Figure 3c), and the released portion curls into a roll (Figure 3d). Due to the adhesion from the substrate, the left portion remains intact [24]. With the evaporation of ethanol, the surface tension at the air-solvent-MoS2 interface is greater than that between MoS2 and the substrate. As a result, the strain-adhesion balance is broken continuously, generating MoS2 nanoscroll finally (Figure 3e).

Alkaline Droplet-Assisted Fabrication of Nanoscrolls
For bilayer and multilayer nanosheets, it is difficult to roll them up effectively due to the strong adhesion force from the substrate. Since TMDCs nanosheets are usually grown

Alkaline Droplet-Assisted Fabrication of Nanoscrolls
For bilayer and multilayer nanosheets, it is difficult to roll them up effectively due to the strong adhesion force from the substrate. Since TMDCs nanosheets are usually grown on SiO 2 /Si substrate, etching the SiO 2 layer beneath them could eliminate the strong adhesion from the substrate. Therefore, alkaline solution has been used to etch the SiO 2 layer, and thus break the adhesion energy between the nanosheet and substrate ( Figure 4). As a result, the strain equilibrium is broken, rolling up the TMDCs nanosheet from edges to form spiral nanoscrolls [19,51].
The materials for fabricating nanoscrolls are derived from 2D nanosheets produced by mechanical stripping [78] or CVD. The experiments were dominated by CVD-prepared nanosheets, but the nanosheets prepared by CVD in most cases are accompanied by the production of defects, homojunctions, and heterojunctions [81]. Therefore, we successfully fabricated nanoscrolls using CVD-prepared heterojunction nanosheets. In 2020, we proposed a method to scroll the MoS 2 /WS 2 heterostructures nanosheet by using an alkaline solution. Firstly, large-area MoS 2 /WS 2 heterostructure nanosheets were grown on SiO 2 /Si substrate by CVD (Figure 4a) [19,81]. Afterwards, 50 µL of alkaline aqueous solution (0.1 M KOH or NaHCO 3 ) was dropped on the nanosheets. The SiO 2 layer was etched by the alkaline solution, allowing penetration of the alkaline solution into the interface of the nanosheet and SiO 2 /Si substrate (Figure 4b), which could further etch the SiO 2 layer to release the edge of the nanosheet. To decrease the energy form, the released edge of the nanosheet tends to be scrolled (Figure 4c). With the further etching of the SiO 2 layer, more portions of the nanosheet were released and continuously scrolled till forming nanoscroll (Figure 4c,d) [19]. The as-obtained nanoscroll was then rinsed with deionized (DI) water and dried with N 2 for characterization and device fabrication (Figure 4d). Furthermore MoS 2 /WS 2 heterostructures nanosheet, we found that the silver nanoparticles decorated monolayer MoS 2 and WS 2 nanosheets could also be rolled up effectively by using an alkaline solution [35].
Nanomaterials 2023, 13, x FOR PEER REVIEW 5 of on SiO2/Si substrate, etching the SiO2 layer beneath them could eliminate the strong adh sion from the substrate. Therefore, alkaline solution has been used to etch the SiO2 lay and thus break the adhesion energy between the nanosheet and substrate (Figure 4). A result, the strain equilibrium is broken, rolling up the TMDCs nanosheet from edges form spiral nanoscrolls [19,51]. The materials for fabricating nanoscrolls are derived from 2D nanosheets produc by mechanical stripping [78] or CVD. The experiments were dominated by CVD-prepar nanosheets, but the nanosheets prepared by CVD in most cases are accompanied by t production of defects, homojunctions, and heterojunctions [81]. Therefore, we succes fully fabricated nanoscrolls using CVD-prepared heterojunction nanosheets. In 2020, w proposed a method to scroll the MoS2/WS2 heterostructures nanosheet by using an alk line solution. Firstly, large-area MoS2/WS2 heterostructure nanosheets were grown SiO2/Si substrate by CVD ( Figure 4a) [19,81]. Afterwards, 50 µL of alkaline aqueous sol tion (0.1 M KOH or NaHCO3) was dropped on the nanosheets. The SiO2 layer was etch by the alkaline solution, allowing penetration of the alkaline solution into the interface the nanosheet and SiO2/Si substrate (Figure 4b), which could further etch the SiO2 layer release the edge of the nanosheet. To decrease the energy form, the released edge of t nanosheet tends to be scrolled (Figure 4c). With the further etching of the SiO2 layer, mo portions of the nanosheet were released and continuously scrolled till forming nanoscr (Figure 4c,d) [19]. The as-obtained nanoscroll was then rinsed with deionized (DI) wat and dried with N2 for characterization and device fabrication (Figure 4d). Furthermo MoS2/WS2 heterostructures nanosheet, we found that the silver nanoparticles decorat Duan et al. found that 80% of the bilayer and trilayer TMDCs heterostructure nanosheets could be transformed into 1D nanoscrolls by using a mixture of ethanol and water. However, many scrolls are incompletely rolled up with tightly pinned edges, indicating the mixed solvent does not work well to completely delaminate the edges of a thick 2D nanosheet. The yield of the nanoscroll increased to 90% by adding 5% ammonia into the mixed solvent, with 60% of them showing a closely stacked scroll structure. The result indicated that the etching of the SiO 2 layer by alkaline solution played an important role in peeling off the 2D nanosheet from the substrate even the existence of strong edge-substrate interaction [21,24].

Fabrication of TMDCs Nanoscrolls by Dragging Water Droplets
Due to the low surface tension of organic solvent, it is easily adsorbed on TMDCs nanosheets. Thus, it is inevitable to trap organic solvent in the TMDCs-NS during the scrolling process. It is known that the adsorbed organic solvent could greatly influence Nanomaterials 2023, 13, 2433 6 of 20 the properties and device performance of TMDCs nanosheets. Therefore, it is desirable to fabricate TMDCs-NS without using organic solvent. Recently, we reported an organic solvent-free method to fabricate tightly-packed TMDCs-NS [52]. Firstly, CVD-grown monolayer MoS 2 nanosheets were heated at 100 • C, as shown in Figure 5a,b. After 3 µL of deionized (DI) water droplet was dropped on the MoS 2 nanosheets (Figure 5c), The coverslips were dragged from one end to the other at a speed of 3 mm/s ( Figure 5d). Afterwards, large scale closely packed MoS 2 nanoscrolls were fabricated (Figure 5e).

Fabrication of TMDCs Nanoscrolls by Dragging Water Droplets
Due to the low surface tension of organic solvent, it is easily adsorbed on TMDCs nanosheets. Thus, it is inevitable to trap organic solvent in the TMDCs-NS during the scrolling process. It is known that the adsorbed organic solvent could greatly influence the properties and device performance of TMDCs nanosheets. Therefore, it is desirable to fabricate TMDCs-NS without using organic solvent. Recently, we reported an organic solvent-free method to fabricate tightly-packed TMDCs-NS [52]. Firstly, CVD-grown monolayer MoS2 nanosheets were heated at 100 °C, as shown in Figure 5a,b. After 3 µL of deionized (DI) water droplet was dropped on the MoS2 nanosheets (Figure 5c), The coverslips were dragged from one end to the other at a speed of 3 mm/s (Figure 5d). Afterwards, large scale closely packed MoS2 nanoscrolls were fabricated (Figure 5e). The detailed mechanism for the formation of nanoscrolls could be explained as follows, as shown in Figure 5f. When the monolayer MoS2 nanosheets on SiO2/Si substrate were heated at 100 °C, the adhesion force between the nanosheets and substrate was weakened. Therefore, with the movement of water droplets, the scrolling occurs first at the edges of MoS2 nanosheets. Due to the hydrophobicity of MoS2 nanosheets, the low friction between water and MoS2 and the high surface tension of water contribute to the following scrolling of MoS2 nanosheets. More importantly, due to the hydrophobicity of MoS2, the water molecules were difficult to trap in the nanoscrolls, obtaining solvent-free and closely packed nanoscrolls [52]. The detailed mechanism for the formation of nanoscrolls could be explained as follows, as shown in Figure 5f. When the monolayer MoS 2 nanosheets on SiO 2 /Si substrate were heated at 100 • C, the adhesion force between the nanosheets and substrate was weakened. Therefore, with the movement of water droplets, the scrolling occurs first at the edges of MoS 2 nanosheets. Due to the hydrophobicity of MoS 2 nanosheets, the low friction between water and MoS 2 and the high surface tension of water contribute to the following scrolling of MoS 2 nanosheets. More importantly, due to the hydrophobicity of MoS 2 , the water molecules were difficult to trap in the nanoscrolls, obtaining solvent-free and closely packed nanoscrolls [52].

Amine-Functional Amphiphilic Molecule Assisted Fabrication of TMDCs Nanoscrolls
The involvement of amphiphilic molecules in the preparation of MoS 2 , MoSe 2 , and MoTe 2 nanoscrolls has also been investigated [48,49,82,83]. By mixing N-(2-aminoethyl)-3αhydroxy-5β-cholan-24-amide (LCA) and exfoliated TMDCs nanosheets in orthodichlorobenzene (ODCB) for 24 h at room temperature, TMDCs-NS was obtained in large scale (Figure 6a). The formation of MoS 2 nanoscrolls with the help of LCA could be explained as follows. Firstly, the LCA molecules were self-assembled into fibers in ODCB. The amine group of LCA fiber has a stronger interaction with the edges of TMDCs nanosheets than the entire plane. Thus, the edges start scrolling around the LCA fibers. With the gradual self-assembly of LCA fiber, the interaction of fiber with the edges of TMDCs nanosheets is enhanced. In this case, the original equilibrium state of TMDCs nanosheets is broken and the nanosheet scrolls spontaneously. Therefore, TMDCs-NS was formed. By using this method, MoS 2 nanoscrolls, MoS 2 -Ag nanoparticles nanoscrolls, and MoS 2 -Au nanoparticles nanoscrolls have been successfully prepared [49,83].
3α-hydroxy-5β-cholan-24-amide (LCA) and exfoliated TMDCs nanosheets in orthodichlo robenzene (ODCB) for 24 h at room temperature, TMDCs-NS was obtained in large scal (Figure 6a). The formation of MoS2 nanoscrolls with the help of LCA could be explaine as follows. Firstly, the LCA molecules were self-assembled into fibers in ODCB. The amin group of LCA fiber has a stronger interaction with the edges of TMDCs nanosheets tha the entire plane. Thus, the edges start scrolling around the LCA fibers. With the gradua self-assembly of LCA fiber, the interaction of fiber with the edges of TMDCs nanosheet is enhanced. In this case, the original equilibrium state of TMDCs nanosheets is broke and the nanosheet scrolls spontaneously. Therefore, TMDCs-NS was formed. By usin this method, MoS2 nanoscrolls, MoS2-Ag nanoparticles nanoscrolls, and MoS2-Au nano particles nanoscrolls have been successfully prepared [49,83].

Supercritical Fluid-Assisted Fabrication of Nanoscrolls
Supercritical fluids (SCFs) are fluids with much more space and are highly compress ible than ordinary fluids above their critical temperatures and pressure [84,85]. By con trolling the temperature or pressure, the density and solvent strength of SCFs can be tune from gas-like to solid-like [49,83,86,87]. SCF has unique properties such as gas diffusivity liquid solubility, low interfacial tension, excellent surface wettability, and high diffusio coefficient [88][89][90]. Thus, SCFs processing has been used as a promising and effective rout to exfoliate layered materials into 2D nanosheets, such as graphene, BN, and MoS2 due t their simplicity, rapidity, and short reaction time [68,69,88,91]. It has been reported tha the as-exfoliated 2D nanosheets can be rolled up into nanoscrolls in order to minimiz their surface energy. Therefore, MoS2 and WS2 nanoscrolls can be formed by using SC processing of MoS2 nanosheets in 30 min, as shown in Figure 7a-g [68,88]. The X-ra

Supercritical Fluid-Assisted Fabrication of Nanoscrolls
Supercritical fluids (SCFs) are fluids with much more space and are highly compressible than ordinary fluids above their critical temperatures and pressure [84,85]. By controlling the temperature or pressure, the density and solvent strength of SCFs can be tuned from gas-like to solid-like [49,83,86,87]. SCF has unique properties such as gas diffusivity, liquid solubility, low interfacial tension, excellent surface wettability, and high diffusion coefficient [88][89][90]. Thus, SCFs processing has been used as a promising and effective route to exfoliate layered materials into 2D nanosheets, such as graphene, BN, and MoS 2 due to their simplicity, rapidity, and short reaction time [68,69,88,91]. It has been reported that the as-exfoliated 2D nanosheets can be rolled up into nanoscrolls in order to minimize their surface energy. Therefore, MoS 2 and WS 2 nanoscrolls can be formed by using SCF processing of MoS 2 nanosheets in 30 min, as shown in Figure 7a-g [68,88]. The X-ray diffraction (XRD) patterns of the MoS 2 nanoscrolls clearly show that the surfaces of the MoS 2 nanosheets are not oxidized and are free of impurities [68]. Meanwhile, the lattice structure of the SCF-prepared MoS 2 is essentially unchanged, making this method a convenient and efficient way to prepare nanoscrolls. diffraction (XRD) patterns of the MoS2 nanoscrolls clearly show that the surfaces of the MoS2 nanosheets are not oxidized and are free of impurities [68]. Meanwhile, the lattice structure of the SCF-prepared MoS2 is essentially unchanged, making this method a convenient and efficient way to prepare nanoscrolls.

Shear Force Assisted Fabrication of Nanoscrolls
By using a vortex flow device (VFD), MoS2 nanosheets have been successfully transformed into nanoscrolls under continuous flow [50]. In a tilted quartz tube with rapid rotation, a dynamic thin film was generated in VFD, providing mechano-energy as high shear stress during intense micro-mixing. Therefore, the MoS2 nanosheets were firstly exfoliated from bulk material due to the strong shear stress. Simultaneously, the as-exfoliated MoS2 nanosheets were rolled up in-situ to form scrolls with high yield (Figure 8a,b). At low-speed rotation (4000 rpm), the shear stress is mainly governed by the Typhoonlike toroidal flow, generating centrifugal forces on the tube wall. As a result, the MoS2 nanosheets were exfoliated first and then scrolled as the toroidal flow moved upwards (Figure 8c,d) [50,92,93]. As the rotation speed increased to 8000 rpm, the dominated double-helical twisted Faraday wave vortex flow cannot curl and roll up MoS2 nanosheets effectively. The morphology of MoS2 nanostructure can be changed from lamellae to scroll, by controlling the solvent selection, concentration of bulk material, and the processing parameters of VFD, including rotation speed and rotation angle. The VFD has been widely used to synthesize nanoscrolls from 2D nanosheets, such as graphite, graphene oxide, and hexagonal boron nitride [92][93][94].

Shear Force Assisted Fabrication of Nanoscrolls
By using a vortex flow device (VFD), MoS 2 nanosheets have been successfully transformed into nanoscrolls under continuous flow [50]. In a tilted quartz tube with rapid rotation, a dynamic thin film was generated in VFD, providing mechano-energy as high shear stress during intense micro-mixing. Therefore, the MoS 2 nanosheets were firstly exfoliated from bulk material due to the strong shear stress. Simultaneously, the as-exfoliated MoS 2 nanosheets were rolled up in-situ to form scrolls with high yield (Figure 8a,b). At lowspeed rotation (4000 rpm), the shear stress is mainly governed by the Typhoon-like toroidal flow, generating centrifugal forces on the tube wall. As a result, the MoS 2 nanosheets were exfoliated first and then scrolled as the toroidal flow moved upwards (Figure 8c,d) [50,92,93]. As the rotation speed increased to 8000 rpm, the dominated double-helical twisted Faraday wave vortex flow cannot curl and roll up MoS 2 nanosheets effectively. The morphology of MoS 2 nanostructure can be changed from lamellae to scroll, by controlling the solvent selection, concentration of bulk material, and the processing parameters of VFD, including rotation speed and rotation angle. The VFD has been widely used to synthesize nanoscrolls from 2D nanosheets, such as graphite, graphene oxide, and hexagonal boron nitride [92][93][94].

Plasma-Assisted Fabrication of MoS 2 Nanoscrolls
In 2016, Zhang et al. proposed the preparation of MoS 2 nanoscrolls by treating CVD-grown monolayer MoS 2 nanosheets in a weak Ar plasma environment, as shown in Figure 9 [18]. Upon plasma bombardment, the top layer sulfur atoms of the MoS 2 nanosheets are partially removed as the kinetic energy of Ar + is larger than the binding energy of the Mo-S bond. As a result, the MoS 2 lattice is disrupted and unsaturated dangling bonds are left, leading to out-of-plane strain. Such kind of strain induces out-ofplane distortion, which rolls up the edge of the MoS 2 nanosheet to form nanoscrolls [18,58]. The optimum power for fabricating nanoscrolls was 25 W. If the power was too strong, short nanoscrolls were obtained. While a longer time was needed to trigger the scrolling when the power was too weak. When the adjacent edges of MoS 2 nanosheets are not parallel, a kink will be formed, preventing the formation of a long scroll [18]. In addition, Nanomaterials 2023, 13, 2433 9 of 20 WS 2 and WSe 2 nanoscrolls were also prepared by treating the monolayer WS 2 and WSe 2 nanosheets in Ar plasma.

Plasma-Assisted Fabrication of MoS2 Nanoscrolls
In 2016, Zhang et al. proposed the preparation of MoS2 nanoscrolls by treating CVDgrown monolayer MoS2 nanosheets in a weak Ar plasma environment, as shown in Figure  9 [18]. Upon plasma bombardment, the top layer sulfur atoms of the MoS2 nanosheets are partially removed as the kinetic energy of Ar + is larger than the binding energy of the Mo-S bond. As a result, the MoS2 lattice is disrupted and unsaturated dangling bonds are left, leading to out-of-plane strain. Such kind of strain induces out-of-plane distortion, which rolls up the edge of the MoS2 nanosheet to form nanoscrolls [18,58]. The optimum power for fabricating nanoscrolls was 25 W. If the power was too strong, short nanoscrolls were obtained. While a longer time was needed to trigger the scrolling when the power was too weak. When the adjacent edges of MoS2 nanosheets are not parallel, a kink will be formed, preventing the formation of a long scroll [18]. In addition, WS2 and WSe2 nanoscrolls were also prepared by treating the monolayer WS2 and WSe2 nanosheets in Ar plasma.

Plasma-Assisted Fabrication of MoS2 Nanoscrolls
In 2016, Zhang et al. proposed the preparation of MoS2 nanoscrolls by treating CVDgrown monolayer MoS2 nanosheets in a weak Ar plasma environment, as shown in Figure  9 [18]. Upon plasma bombardment, the top layer sulfur atoms of the MoS2 nanosheets are partially removed as the kinetic energy of Ar + is larger than the binding energy of the Mo-S bond. As a result, the MoS2 lattice is disrupted and unsaturated dangling bonds are left, leading to out-of-plane strain. Such kind of strain induces out-of-plane distortion, which rolls up the edge of the MoS2 nanosheet to form nanoscrolls [18,58]. The optimum power for fabricating nanoscrolls was 25 W. If the power was too strong, short nanoscrolls were obtained. While a longer time was needed to trigger the scrolling when the power was too weak. When the adjacent edges of MoS2 nanosheets are not parallel, a kink will be formed, preventing the formation of a long scroll [18]. In addition, WS2 and WSe2 nanoscrolls were also prepared by treating the monolayer WS2 and WSe2 nanosheets in Ar plasma.

Strain-Assisted Fabrication of TMDCs Nanoscrolls
Because of the different thermal expansion coefficients of the MoS 2 and SiO 2 , there are thermal strain gradients between the interface of CVD-grown MoS 2 nanosheets and SiO 2 /Si substrate [58,75,95]. Upon quenching, the orientation-specific fractures are formed on CVD-grown MoS 2 nanosheets due to the existing S vacancies (Figure 10a,b). Since the cooling rate of the top MoS 2 layer is greater than the bottom SiO 2 layer, strong lattice contraction of the MoS 2 layer is observed due to the temperature difference, which induces self-curling at the fractures of the MoS 2 layer (Figure 10c). Afterwards, the curled edge continues to form a scroll in order to minimize the surface energy (Figure 10d). are thermal strain gradients between the interface of CVD-grown MoS2 nanosheets and SiO2/Si substrate [58,75,95]. Upon quenching, the orientation-specific fractures are formed on CVD-grown MoS2 nanosheets due to the existing S vacancies (Figure 10a,b). Since the cooling rate of the top MoS2 layer is greater than the bottom SiO2 layer, strong lattice contraction of the MoS2 layer is observed due to the temperature difference, which induces self-curling at the fractures of the MoS2 layer (Figure 10c). Afterwards, the curled edge continues to form a scroll in order to minimize the surface energy (Figure 10d).  Table 1 summarizes the preparation methods of TMDCs nanoscrolls to show the advantages and disadvantages in detail. To date, the TMDCs nanoscrolls have been prepared mainly from CVD-grown monolayer nanosheets or film in large-area. It is known that large-area TMDCs nanosheets can also be obtained by using molecular beam epitaxial (MBE) method and mechanical exfoliation (ME). Therefore, TMDCs nanosheets prepared by the MBE or ME method provide an alternative way for fabricating TMDCs nanoscrolls. Currently, it is difficult to control the geometry of nanoscrolls by using organic or alkaline droplets to directly treat CVD-grown large-area monolayer TMDCs films. In this case, TMDCs nanoscrolls with lengths of several tens to hundreds of micrometers were obtained in random orientation. By using a focused ion beam (FIB), the large-area monolayer TMDCs film can be shaped as long parallel ribbons with controlled width and direction. After dropping the mixture of ethanol and water on these ribbons, long straight TMDCs nanoscrolls can be obtained in array form [24]. The diameter of TMDCs nanoscroll could be tuned by controlling the width of the original ribbon. By using FIB to cut the long TMDCs nanoscrolls, the TMDCs nanoscrolls were patterned into arrays with controlled lengths and locations. By etching the large-area monolayer TMDCs films along the crystalline orientation, TMDCs nanoscrolls arrays with controlled chirality could also be prepared.  Table 1 summarizes the preparation methods of TMDCs nanoscrolls to show the advantages and disadvantages in detail. To date, the TMDCs nanoscrolls have been prepared mainly from CVD-grown monolayer nanosheets or film in large-area. It is known that large-area TMDCs nanosheets can also be obtained by using molecular beam epitaxial (MBE) method and mechanical exfoliation (ME). Therefore, TMDCs nanosheets prepared by the MBE or ME method provide an alternative way for fabricating TMDCs nanoscrolls. Currently, it is difficult to control the geometry of nanoscrolls by using organic or alkaline droplets to directly treat CVD-grown large-area monolayer TMDCs films. In this case, TMDCs nanoscrolls with lengths of several tens to hundreds of micrometers were obtained in random orientation. By using a focused ion beam (FIB), the large-area monolayer TMDCs film can be shaped as long parallel ribbons with controlled width and direction. After dropping the mixture of ethanol and water on these ribbons, long straight TMDCs nanoscrolls can be obtained in array form [24]. The diameter of TMDCs nanoscroll could be tuned by controlling the width of the original ribbon. By using FIB to cut the long TMDCs nanoscrolls, the TMDCs nanoscrolls were patterned into arrays with controlled lengths and locations. By etching the large-area monolayer TMDCs films along the crystalline orientation, TMDCs nanoscrolls arrays with controlled chirality could also be prepared.

Advantages Challenges References
In air Plasma-assisted fabrication of TMDCs nanoscroll Simple process, high yield Small dimension, structural damage [18,20] Strain-assisted fabrication of TMDCs nanoscroll Simple and repeatable Complex process, low productivity, incomplete nanoscroll [17]

Photodetector Based on TMDCs-NS
Compared to monolayer TMDCs nanosheet, the TMDCs-NS shows much better light absorption efficiency due to its increased thickness of spirally scrolled structure. Thus, the TMDCs-NS should exhibit excellent optoelectronic performance. Recently, the photodetection performance of TMDCs-NS has been investigated [51,98]. Photosensitivity, described by the ratio of photocurrent to dark current (PDR), is one of the most important parameters to evaluate the performance of a photodetector [19,23,35]. Figure 11a shows the PDRs of MoS 2 nanosheet and nanoscroll-based photodetectors under blue light irradiation with a bias voltage of 0.1 V. The PDR of a nanoscroll-based photodetector is about 400, which is about 100 times higher than that of a nanosheet-based one [23]. In addition, the response and recovery time of a nanoscroll-based photodetector is less than the nanosheet-based photodetector. Similar enhanced PDR was also observed in the MoSe 2 nanoscroll-based device [17]. These results indicate that the TMDCs-NS shows much better photodetection performance than the TMDCs nanosheet, which should be attributed to the enhanced light absorption and rapid electron transportation along the 1D structure [24,52]. The photodetection enhancement of TMDCs nanoscrolls could be explained in detail as follows. Firstly, the increased thickness of TMDCs nanoscroll increases the light absorption as the light permeates each layer of the nanoscroll. Although each layer shows low light absorption, the total light absorption of the nanoscroll has increased. Therefore, increased photocurrent is obtained in nanoscroll. In addition, the one-dimensional structure of the nanoscroll confines the electron transportation along the axis direction, and thus rapid electron movement is realized in the nanoscroll compared to that in a 2D nanosheet. Moreover, due to the large surface-to-volume ratio of the MoS 2 nanosheet, the adsorbates, such as O 2 and H 2 O molecules, greatly reduce the photoresponse of the MoS 2 nanosheet-based device. The MoS 2 nanoscroll has a much smaller surface-to-volume ratio than the nanosheet, which can decrease the influence of adsorbates on the photoresponse. nanosheet. Moreover, due to the large surface-to-volume ratio of the MoS2 nanosheet, the adsorbates, such as O2 and H2O molecules, greatly reduce the photoresponse of the MoS2 nanosheet-based device. The MoS2 nanoscroll has a much smaller surface-to-volume ratio than the nanosheet, which can decrease the influence of adsorbates on the photoresponse. Compared to the MoS2 nanoscroll prepared by ethanol droplet (MoS2 NS-ethanol), the MoS2 nanoscroll prepared by water droplet (MoS2 NS-water) shows higher PDR (Figure 11b) [52]. The ethanol molecules trapped in MoS2 NS-ethanol reduce the light absorption and hinder the interlayer transport of photogenerated carriers, and thus decrease the Compared to the MoS 2 nanoscroll prepared by ethanol droplet (MoS 2 NS-ethanol), the MoS 2 nanoscroll prepared by water droplet (MoS 2 NS-water) shows higher PDR (Figure 11b) [52]. The ethanol molecules trapped in MoS 2 NS-ethanol reduce the light absorption and hinder the interlayer transport of photogenerated carriers, and thus decrease the photoresponse. In addition, the ethanol can donate an electron to MoS 2 and thus increase the dark current of MoS 2 NS-ethanol, which in turn decreases the PDR. Moreover, the MoS 2 NS-water shows a slightly higher photocurrent than MoS 2 NS-ethanol.

Photodetector Based on TMDCs-NS Composite
In bilayer WS 2 /MoS 2 heterostructure, the carriers can be transferred from MoS 2 to WS 2 within 50 fs under illumination, indicating the important role of interface. However, there is only one interface in the bilayer WS 2 /MoS 2 heterostructure. The photoresponse performance of WS 2 /MoS 2 heterostructure could be further improved if multiple interfaces can be established. By scrolling the bilayer WS 2 /MoS 2 heterostructure into WS 2 /MoS 2 heterostructure nanoscroll, multiple hetero-interfaces are formed, which could show better photoresponse than the bilayer WS 2 /MoS 2 heterostructure with one hetero-interface. After the bilayer WS 2 /MoS 2 heterostructure was grown by CVD, the alkaline droplet was dropped on it to roll up the bilayer heterostructure into a nanoscroll [19]. As shown in Figure 12a, the PDR of bilayer WS 2 /MoS 2 heterostructure-based photodetector is~180 under a blue laser, which is much higher than that of monolayer MoS 2 or WS 2 nanosheets. The PDR of WS 2 /MoS 2 heterostructure nanoscroll-based photodetector is 2700, about an order of magnitude higher than that of bilayer WS 2 /MoS 2 heterostructure-based photodetector.
Nanomaterials 2023, 13, x FOR PEER REVIEW 13 of 21 excellent photodetection performance [24,70], indicating it could be a promising candidate for high-performance optoelectronics.

Hydrogen Evolution Reaction
The conductivity and effective active site of the catalyst are two key factors in improving the hydrogen precipitation reaction (HER) [99][100][101]. TMDCs are considered promising candidates after noble metals for catalytic hydrogen precipitation due to their good electrical conductivity and abundant active edges [66,102,103]. The introduction of a small amount of MoSx greatly enhanced the HER activity of NbS2 nanoflakes [100]. The theoretical calculation indicates that Mo edge sites are identified as the catalytically active site for HER [104,105].
A number of efforts have been employed to increase the active sites and conductivity By encapsulating photoactive PbI 2 nanocrystals into MoS 2 nanoscroll, the PDR of MoS 2 nanoscroll can be enhanced by two orders of magnitude (Figure 12b) [51]. Similarly, the PDRs of MoS 2 and WS 2 nanoscrolls also increased by two orders of magnitude after Ag nanoparticles were trapped in a nanoscroll (Figure 12c) [35]. Moreover, the photoresponsivity of the MoS 2 nanoscroll was also enhanced by about two orders of mag-nitude when BaTiO 3 nanoparticles were encapsulated into it [27]. By doping the WS 2 nanoscroll with CdSe-ZnS quantum dots, the photosensitivity can be enhanced 3000-fold (Figure 12d) [79]. Compared to single TMDCs nanoscrolls, the nanoscroll composite shows excellent photodetection performance [24,70], indicating it could be a promising candidate for high-performance optoelectronics.

Hydrogen Evolution Reaction
The conductivity and effective active site of the catalyst are two key factors in improving the hydrogen precipitation reaction (HER) [99][100][101]. TMDCs are considered promising candidates after noble metals for catalytic hydrogen precipitation due to their good electrical conductivity and abundant active edges [66,102,103]. The introduction of a small amount of MoSx greatly enhanced the HER activity of NbS 2 nanoflakes [100]. The theoretical calculation indicates that Mo edge sites are identified as the catalytically active site for HER [104,105].
A number of efforts have been employed to increase the active sites and conductivity of TMDCs materials [106][107][108]. Among them, transforming the TMDCs nanosheets into nanoscrolls with active edges is of great interest in electrocatalytic HER [50]. By curling the TMDCs nanosheets to form nanoscrolls, the curled edges provide highly active edge sites for efficient catalysis. Meanwhile, the bending strain of the basal plane also provides more active sites due to the scrolled structure [50,[109][110][111]. In addition, the specific surface area of the nanoscroll increases, converting the solution contact from single-sided contact of the nanosheet to multi-layer contact [112]. As a result, the TMDCs-NS provides higher catalytic activity and better conductivity. Thus, the HER activity of TMDCs-NS is greatly enhanced. Figure 13a shows the polarization curves of metallic WSe 2 (M-WSe 2 ) and 2H-WSe 2 nanoscrolls compared to the commercial Pt/C catalyst. The M-WSe 2 nanoscroll exhibits higher HER activity, smaller overpotential, and larger current densities than the 2H-WSe 2 nanoscroll, attributed to its good conductivity and enhanced catalytic activity from the scrolled structure [103]. Figure 13b shows the linear scanning voltammogram curves of current density and potential for Pt electrode, MoS 2 sheet, MoS 2 nanoscroll, MoS 2 nanoscrolls@Pt, MoS 2 @Pt sheet, and MoS 2 @Pt nanoscroll [48]. The MoS 2 nanosheet shows overpotential over 400 mV, while that of the MoS 2 nanoscroll decreases. The overpotential of MoS 2 nanoscroll decorated by Pt nanoparticles (NPs) greatly decreases, indicating the important role of Pt NPs in enhancing the HER activity. By decorating Pt NPs on MoS 2 nanosheet (MoS 2 @Pt sheet), the overpotential further decreases to around 300 mV. After the MoS 2 @Pt sheet was rolled up to form a MoS 2 @Pt scroll, the overpotential was reduced to 200 mV, implying the basal plane bending of the nanoscroll can further improve the HER activity.

Gas Sensor
In recent years, TMDCs nanosheets have attracted great attention for gas sensing because of their large surface-to-volume ratio and good electrical properties [113][114][115][116]. The TMDCs nanosheet-based gas sensors show high sensitivity for trace gas molecules and good selectivity at room temperature [60,[117][118][119]. However, they suffer from incomplete

Gas Sensor
In recent years, TMDCs nanosheets have attracted great attention for gas sensing because of their large surface-to-volume ratio and good electrical properties [113][114][115][116]. The TMDCs nanosheet-based gas sensors show high sensitivity for trace gas molecules and good selectivity at room temperature [60,[117][118][119]. However, they suffer from incomplete recovery and poor stability [120], which limit their practical applications. The TMDCs-NS has good conductivity due to its 1D structure and specific surface area from the tubular structure. Meanwhile, the nanoscroll structure also provides tunable space to encapsulate functional nanomaterials for further enhanced performance. Therefore, the TMDCs-NS has been considered to present promising performance in the field of gas sensing [121].
By electrochemically exfoliating InSe crystal in electrolytes containing cetyltrimethylammonium bromide (CTAB), the CTAB-functionalized InSe nanosheets (C-InSe) were obtained [120]. With the aid of solvent evaporation, the C-InSe nanosheets rolled up to form C-InSe nanoscroll. After the as-obtained C-InSe nanoscrolls were deposited on interdigitated electrodes (Figure 14b), they were placed into a chamber with a fixed concentration of NO 2 gas. An LED lamp illuminates light into the chamber through a glass cover (Figure 14a). The response of the C-InSe nanoscrolls-based sensor increases under light illumination (Figure 14c). The enhanced response has arisen from the increased adsorption of NO 2 molecules favored by photogenerated electrons. While desorption of NO 2 is observed as the light intensity increases further, which decreases the response of the sensor. As the concentration of NO 2 increases from 100 ppb to 25 ppm, the response of C-InSe nanosheets and nanoscrolls-based sensors largely increases (Figure 14d). However, the C-InSe nanoscrolls-based sensor exhibits a much better response than the C-InSe nanosheets-based sensor, indicating the superiority of nanoscrolls in gas sensing.

Conclusions
In this review, we summarize a series of fabrication methods of TMDCs nanoscrolls, and briefly demonstrate their applications in photodetection, HER, and gas sensing (Figure 15). Compared to the 2D TMDCs nanosheet, the 1D TMDCs nanoscroll presents higher light absorption efficiency and faster electron transport because of the scrolled structure. Due to their higher specific surface area and active edges, the TMDCs nanoscrolls have shown excellent performance in catalytic reactions. In conclusion, the TMDCs nanoscrolls are emerging materials with many novel physical and chemical properties that are promising for optoelectronics, catalysis, energy storage, and sensing.

Conclusions
In this review, we summarize a series of fabrication methods of TMDCs nanoscrolls, and briefly demonstrate their applications in photodetection, HER, and gas sensing ( Figure 15). Compared to the 2D TMDCs nanosheet, the 1D TMDCs nanoscroll presents higher light absorption efficiency and faster electron transport because of the scrolled structure. Due to their higher specific surface area and active edges, the TMDCs nanoscrolls have shown excellent performance in catalytic reactions. In conclusion, the TMDCs nanoscrolls are emerging materials with many novel physical and chemical properties that are promising for optoelectronics, catalysis, energy storage, and sensing.

Conclusions
In this review, we summarize a series of fabrication methods of TMDCs nanoscrolls, and briefly demonstrate their applications in photodetection, HER, and gas sensing (Figure 15). Compared to the 2D TMDCs nanosheet, the 1D TMDCs nanoscroll presents higher light absorption efficiency and faster electron transport because of the scrolled structure. Due to their higher specific surface area and active edges, the TMDCs nanoscrolls have shown excellent performance in catalytic reactions. In conclusion, the TMDCs nanoscrolls are emerging materials with many novel physical and chemical properties that are promising for optoelectronics, catalysis, energy storage, and sensing.

Conflicts of Interest:
The authors declare no conflict of interest.