## 8 Applications of Two-Dimensional Nanoscale SnS
have prepared SnS hexagonal nanosheets with diameters 50-150 nm via the formation and decomposition of a tin diethylditiocarbamate. The photovoltaic parameters obtained were: J\({}_{\text{SC}}\): 17.7 mA cm\({}^{-2}\), V\({}_{\text{OC}}\): 740 mV, fill factor: 50%, R\({}_{\text{S}}\). 634 \(\Omega\), R\({}_{\text{K}}\): 0.94 \(\Omega\); the efficiency was 6.56%, which compared favourably to platinum (7.57%). The cyclic voltammetry profile showed a similar catalytic activity to platinum with a smaller peak to peak splitting (225 mV vs. 260 mV), indicating that SnS may be superior to Pt with regards to electrocatalytic activity and reversibility of the I\({}_{3}\)-/I\({}^{-}\) redox reaction [9]. Fewer dimensional SnS quantum dot sensitised mesoporous TiO\({}_{2}\)-based cells [156] and solution-processed SnS quantum dot solar cells have also been produced [157], as well as 1D SnS nanorod-based cells [158]. The nonlinear optical absorption coefficient of SnS (50.5 \(\times\) 10\({}^{-3}\) cm GW\({}^{-1}\), an order of magnitude higher than black phosphorous) makes SnS a potentially effective material for ultrafast laser generation. Optical absorption peaks at 1180 nm and 1480 nm suggest that SnS nanosheets may be a promising optical material for the telecommunications band. Liquid-phase exfoliated nanosheets of SnS produced by [PERSON] et al. have been found to have rapid response times of \(\tau_{1}\) (short decay time) = 873 fs and \(\tau_{2}\) (long decay time) = 98.9 fs. These times correspond to the electron thermalisation in the band and the recombination of the carriers, respectively. A pulse width of 656 fs was obtained. The rapid response times allow SnS to be used as a modulator in the GHz to THz range [107]. For comparison, black phosphorous lasers have been found to have a pulse width of 946 fs [159]. Due to the ability to successfully intercalate ions between the atomic layers, SnS has attracted significant interest in energy storage, with the large interlayer spacing allowing the intercalation of larger Na\({}^{+}\) ions as well as smaller Li\({}^{+}\) ions. SnS also has a lower volume expansion than SnS\({}_{2}\) (242% vs. 324%), making it more suitable for repeated cycling [160]. Two-dimensional SnS has also been used successfully in the creation of a variety of energy storage devices, such as lithium-ion batteries, with nanosheets showing improved properties compared to bulk [161]. This is due partly to the increase in effective dielectric constants from around 15 for bulk to around 25-30 for monolayer SnS, as calculated from the DFT ab initio package by [PERSON] et al. [92]. Batteries utilising  
Figure 14: SEM of (**a**) free-coated, (**b**) Ag-coated, and (**c**) Au-coated buffer layer SnS nanosheet films; (**d**) photoresponses of free-coated, Ag-, and Au-coated films. Scale bars: 500 nm photoresponse of SnS films. Adapted with permission from _Sol. Energy Mater. Sol. Cells_, **2016**, 49–56. SnS nanomaterials typically exhibit a reduction in discharge capacity due to the reaction: SnS + 2 Li \(\rightarrow\) Sn + Li\({}_{2}\)S [161, 162, 22]. Composites utilising 2D SnS show stable capacities of 595 mAg\({}^{-1}\), 580 mAg\({}^{-1}\)[22], 660 mAg\({}^{-1}\)[163], and have been obtained by a variety of groups with the respective capacities maintained over tens [22, 164] and even thousands of cycles [163]. Hybrid SnS/rGO nanosheets have been used by [PERSON] et al. for the creation of a lithium-ion battery hybrid anode. The composite showed improved performance due to the high capacity of SnS combined with the conductivity of graphene and the large surface area of nanomaterials; the SnS was found to be around 10-20 nm in thickness. The reversible capacity was determined to be 791 mAh g\({}^{-1}\) with 595 mAh g\({}^{-1}\) retained after 50 cycles at 100 mA g\({}^{-1}\) (Figure 15) [164]. Hierarchical structures utilising nanoscale SnS allow for the potential improvement of lithium-ion storage devices [81]. SnS nanosphere composites with an inner 2D SnS layer have been prepared by [PERSON] et al.