## 8 Applications of Two-Dimensional Nanoscale SnS
A reduction in resistance is present under exposure to these solvents due to adsorption of the alcohol molecules reducing the electron density of the SnS [24]. An ethanol sensor based on a ZnO\({}_{2}\)-based hybrid thin film showed limits of detection of 600 ppm [182]; however, novel nanosheet-based ethanol sensors have limits of detection down to 0.1 ppm [183]. Two-dimensional SnS has also been used successfully to create NO\({}_{2}\) sensing devices by [PERSON] et al., with the potential application of testing emissions which can lead to the formation of acid rain; the device was capable of sensing down to 3.75 ppm [23], a higher limit of detection than black phosphorus (20 ppb) [184], however SnS lacks the stability issues typically associated with black phosphorous. Two-dimensional SnS prepared by liquid-phase exfoliation by [PERSON] et al. gave nanosheets with an average sheet thickness of 4.5 nm and a modal lateral size between 500 and 600 nm. The physisorption of NO\({}_{2}\) to the SnS surface led to the formation of dipoles, redistributing photoexcited charges, and modifying the SnS photoluminescence. The limit of detection obtained was 17 ppb. The response time and magnitude were increased up to 2000 ppb, where a saturation point is reached. The devices showed good selectivity with the response magnitude of CO, H\({}_{2}\)S and SO\({}_{2}\) being \(\approx\)1% compared to NO\({}_{2}\) (68%). The photoluminescence was improved by an order of magnitude upon thinning, as determined via time-resolved photoelectron spectroscopy (Figure 16) [185]. SnS nanosheets with Schottky contacts on flexible and solid substrates were prepared by mechanical exfoliation and have been used for humidity sensors for potential use in biomedical applications. The SnS produced was 30 nm thick with a lateral size around 10 \(\upmu\)m. The sensors had a response of 67,600% at 10% relative humidity (RH) with a detection range between 3% to 99% RH; rise and fall times of 6 s and 4 s were obtained. DFT showed that H\({}_{2}\)O adsorbs onto monolayer SnS with a large binding energy of \(-\)0.388 eV with a charge transfer of \(-\)0.055 eV, indicating a good specificity and selectivity [186]. Detectors made from MoSe\({}_{2}\) gave an RH range between 0% and 90% with a resolution of 0.04% RH [187]. Other examples of SnS-based sensors include SnS/SnO\({}_{2}\) composites for the detection of oxygen with detection limits lower than graphitic carbon nitride on rGO (3 ppm [188] vs. 20 ppm [189]). Zero-dimensional SnS nanoparticles have been used for NO\({}_{2}\) sensors [190], ethanol sensors [191], and LPC (liquid petroleum gas) sensors [192]. SnS nanoflowers have been used for ethanol sensing [193], and a SnS/TiO\({}_{2}\) and graphene oxide composite for the  
Figure 16: (**a**) SnS sensor at NO\({}_{2}\) concentrations of 150–3750 ppb; (**b**) example height profile by AFM of the nanosheets produced. Adapted with permission from _Appl. Mater. Interfaces_**2019**, 45, 42462–424680. detection of paracetamol, tryptophan, and caffeine in pharmaceuticals [194]. This indicates that SnS has a high versatility and range of sensing applications. SnS also has applications in catalysis. SnS is considered an effective hydrogen evolution catalyst due to its favourable band alignment and good photoresponse. SnS layers produced by [PERSON] et al. were formed via sputtering of SnS\({}_{2}\) followed by phase transformation into layers up to 100 nm thick and formed of 2D nanodiscs. From this device, hydrogen was capable of being generated up to a rate of 140 umol h\({}^{-1}\) cm\({}^{-2}\) within an acidic electrolyte. The hydrogen evolution rate under photoelectrocatalytic reaction conditions was 100 umol h\({}^{-1}\) cm\({}^{-2}\). The hydrogen evolution rate correlated with the photocurrent [195]. Hydrothermally synthesised materials such as N-doped graphene/SnS nanosheet composites prepared by [PERSON] et al.