Effect of Temperature and Capping Agents on Structural and Optical Properties of Tin Sulphide Nanocrystals

SnS nanocrystals were synthesized using bis(phenylpiperazine dithiocarbamate)tin(II) in oleic acid (OA) and octadecylamine (ODA) at three different temperatures (150, 190, and 230°C). XRD diffraction pattern confirms that OASnS and ODASnS nanoparticles are in the orthorhombic phase and the type of capping agent used affects the crystallinity. Transmission electron microscopy (TEM) images shows spherically shaped nanocrystals for oleic acid capped SnS (OASnS) while octadecylamine (ODASnS) are cubic. Monodispersed SnS of size range 10.67–17.74 nm was obtained at 150°C for OASnS while the biggest-sized nanocrystals were obtained at 230°C for ODASnS. Temperature and capping agents tuned the crystallite sizes and shapes of the asprepared nanocrystals. Electron dispersive X-ray spectroscopy indicates the formation of tin sulphide with the presence of Sn and S peaks in the nanocrystals. Flowery and agglomerated spherical-like morphology were observed for ODASnS and OASnS nanocrystals, respectively, using a SEM (scanning electron microscope). Direct electronic band gaps of the synthesized SnS nanocrystals are 1.71–1.95 eV and 1.93–2.81 eV for OASnS and ODASnS nanocrystals, respectively.


Materials and Physical
Measurements. N-phenyl piperazine, carbon disulphide, sodium hydroxide, tin(II) chloride dihydrate, trioctylphosphine, oleic acid, and octadecylamine of analytical grade were purchased from Sigma-Aldrich and used without further purifications. e FTIR of the ligand, complex and nanocrystals spectra were recorded using an Agilent technologies Cary 630 FTIR spectrometer in the frequency range of 4000-650 cm −1 . 1 H and 13 C-NMR of ligand and complex were recorded on an 400 MHz Bruker Avance III NMR spectrometer with tetramethylsilane serving as internal standard. Absorption spectra were obtained using an Agilent technologies Cary 100 UV-Vis spectrophotometer in the wavelength range of 200-800 nm. Photoluminescence emission was obtained using a PerkinElmer LS 45 fluorescence spectrometer. Melting point calculated using stuart SMP3 version 5.0. Transmission electron microscope (TEM) images of the nanocrystals were determined using a JEOL JEM-1400 transmission electron microscope with an accelerating voltage of 120 kV. e powder X-ray diffraction analysis was done using a Bruker D8 advanced diffractometer (Billerica, MA, USA). e morphology and compositions of tin sulphide nanocrystals were analysed using a Zeiss Evols 15 scanning electron microscope equipped with energy dispersive X-ray spectroscopy.

Synthesis of N-Phenyl Piperazine Dithiocarbamate Sn(II)
Complex. Bis(phenylpiperazine dithiocarbamato)tin(II) was synthesized using modified literature procedures [33]. Briefly, 1.2 g (0.03 mol) of sodium hydroxide was dissolved in distilled water and allowed to cool in ice to which N-phenyl piperazine (4.58 mL, 0.03 mol) was added followed by cold carbon disulphide and allowed to stir in ice bath for 4 h. e product obtained (PhPipdtc) was filtered and washed using diethyl ether and allowed to dry. e dried sodium N-phenyl piperazine dithiocarbamate (PhPipdtc) was then dissolved in 50 mL of methanol, and 0.015 mol of tin(II) chloride dihydrate dissolved in 25 mL of methanol was added dropwise to the dithiocarbamate methanolic solution and stirred at room temperature for 3 hours (Scheme 1). e resulting precipitate was washed with water then followed by diethyl ether. e complex [Sn(PhPipdtc) 2   e resulting solution was injected into hot 4 g of octadecylamine in a three-necked round bottom flask at 150, 190, and 230°C. ere was a reduction in temperature of about 12-25°C, after which it was allowed to stabilize at the desired temperature and then stirred for 1 hour. e reaction was left to cool to 70°C followed by the addition of cold methanol. e flocculate was then centrifuged at 3500 rpm for 30 minutes, and the supernatant was decanted and washed severally. e nanocrystals were dispersed in hexane for further analysis. e same procedure was repeated using 6 mL of oleic acid. e resulting nanocrystals prepared from octadecylamine were labelled ODASn1 (150°C), ODASn2 (190°C), and ODASn3 (230°C).

Spectroscopic Studies of PhPipdtc and [Sn(PhPipdtc) 2 ].
e phenyl-piperazine dithiocarbamate (Phpipdtc) electronic spectrum exhibited three absorption bands attributed to π⟶π * of υ|N−C�S| at 252 nm, π⟶π * of υ|C−S�S| at 277 nm, and n⟶π * of the sulphur atoms at 337 nm [50]. On complexation, the absorption band of υ|C�N| chromophore was observed at 265 nm due to the thioureide group intramolecular transition in the Sn(II) dithiocarbamate complex [51]. e υ|N−CS 2 | band observed at 1462 cm −1 in the ligand spectrum shifted to 1489 cm −1 in the complex due to the delocalization of the electron from the nitrogen attached to the thioureide moiety confirming the coordination of tin(II) ion to the ligand [52]. e ν|C−S| symmetrical vibrational band peak observed in the ligand at 994 cm −1 as two peaks shifted to 915 cm −1 as a single peak on coordination to tin(II) ion. It has been established that a single band around 1000 ± 70 cm −1 in metal dithiocarbamate complexes is attributed to bidentate coordination of the dithiocarbamate moiety while the splitting of the band in this region indicates the monodentate coordination mode [53]. e 1 H-NMR spectrum of PhPipdtc showed the methylene proton of piperazine signals at 3.23 ppm and 4.50 ppm as triplet. e downfield resonance at 4.50 ppm is because of the thioureide nitrogen while the phenyl proton resonated at a higher frequency of 7.07-7.41 ppm. On complexation, phenyl protons shifted to 6.88-7.27 ppm, while the methylene protons resonated at 3.18 and 3.31 ppm. e desheilding effect is due to delocalization of thioureide nitrogen towards the sulphur atoms which increases electronegativity of the surrounding protons [54].

X-Ray Diffraction of SnS Nanocrystals.
e XRD patterns of oleic acid capped tin sulphide nanoparticles (Figure 1 e relatively smaller broad diffraction peaks suggest smaller particle sizes. Sharp intense peaks in the XRD patterns of ODASnS (Figure 1 [56] observation using HDA (hexadecylamine) as a capping agent. e sharp and narrow peaks of (120) and (101) with high intensity show that the nanoparticles are purely SnS which is also of orthorhombic phase. e difference in the XRD pattern is due to the different capping agents used.

Transmission Electron Microscopic Analysis of SnS
Nanocrystals. TEM images of SnS nanocrystals showed spherical and cubic shapes as presented in Figure 2. e assynthesized SnS nanocrystals show diverse size and shape by varying the temperature and capping agents. OASnS nanocrystals has monodispersed spherical shape with different sizes at various temperatures such as 10.67-28.74 nm, 12.82-17.10 nm, and 35.25-75.10 nm, respectively, for 150, 190, and 230°C showing that temperature influences the sizes of the nanocrystals obtained. ODASnS nanocrystals are spherical shaped of size range 22.00-28.32 nm at 150°C, which changed to agglomerated nanocrystals cubes at 190°C with particle size in the range of 28.15-33.36 nm. At 230°C, there was increase in size to 67.04-80.15 nm while maintaining the cubic shape. Agglomeration was observed to increase as the temperature increases this could be due to surface attraction of the nanocrystals. OA and ODA capped SnS nanocrystals followed the trend of increase in size with increase in temperature. e capping agents used showed various degrees of sizes. Oleic acid gave smaller size SnS nanocrystals of monodispersed spherical shape, while octadecylamine has bigger agglomerated nanocrystals with cubelike shape.

SEM Analysis and EDX Spectra of SnS Nanocrystals.
e influence of capping agents and reaction temperature on the SnS nanoparticle morphology was investigated using SEM as shown in Figure 3. OASnS nanocrystals had spherical aggregates. e morphologies of ODASnS were different from those of OASnS as they showed flaky-like morphology [57]. e SEM images show diversity in morphology due to different capping agents used in the preparation of tin sulphide. Temperature has no effect on the morphology of OASnS and ODASnS. Carbon and oxygen signals were observed in the EDX (electron dispersive X-ray) spectra (Figure 4) of SnS nanocrystals; this is because of the capping agents used and the carbon tape. e presence of Sn and S peaks is an indication that SnS nanocrystals were formed. OA-capped SnS showed elemental composition of 53% to 47% of Sn and S at all temperatures, and ODASnS nanocrystals composition was 52% to 48%. e presence of excess sulphur is an indication of dangling sulphur bonds. In all the SnS nanocrystals of various capping agents, the composition of Sn is slightly more than S, while a change in temperature has no effect on the composition of SnS nanocrystals showing its stability [58].

Optical Studies of SnS
Nanocrystals. Quantum connement makes semiconductors' nanoscale band gap deviate from their bulk band gap, and this is caused by photogenerated electron-hole pairs [40,59]. To obtain the band gap of the synthesized SnS nanocrystals, the absorption spectra data were measured in the wavelength range of 300-800 nm at room temperature. Tauc's analysis was used for conversion by utilizing the near-edge absorption equation (αhv) n A(hv − Eg), where α is the wavelengthdependent absorption coe cient, A is a constant, hv is  the photon energy, and Eg is the band gap. e transition process was denoted as "n" which can be 1/2 for indirect allowed transitions, 1/3 for indirect forbidden transition, 2/3 for direct forbidden transition, and 2 for direct allowed transition. In this study, the direct allowed transition was used as a good t of extrapolation was obtained by plotting (αhv) 2 against hv and extrapolating the x-axis value [33,60]. e band gap is in the range of 1.71-1.95 eV for OASnS and 1.93-2.81 eV for ODASnS nanocrystals ( Figure 5) which is higher than 1.4 eV of bulk SnS [61] exhibiting blue shift of the band gap. SnS band gap for direct allowed transitions of 1.0-1.7 eV has been reported [62][63][64]. Increase in the band gap of the SnS nanoparticle could be attributed to combination of strain, particle size con nement, and defects and disorder of grain boundaries [33,65]. e capping agents and temperature in uence the band gap; the highest band gap energy (2.81 eV) was obtained at 230°C for ODASnS nanoparticle. e UV-Vis absorption spectra presented in Figure 5 show the nanocrystals having absorption at 337 nm for OASnS1 and 345 nm for ODASnS1 while those of the remaining nanocrystals occurred at 348 nm. Wavelength band shows quantum con nement e ect as lower wavelength implies smaller diameter and vice versa [59,66]. e photoluminescence spectra of SnS nanocrystals were recorded at room temperature at an excitation wavelength of 350 nm (Figures 6-7); strong and broad emission peaks maximum was obtained at 408-431 nm for ODASnS while OASnS has a sharp emission band [51,67].

FTIR Studies of SnS Nanocrytals.
e FTIR spectra of OASnS and ODASnS nanocrystals were compared with their respective capping agents for the con rmation of their coating (Figure 8). NH 2 bending modes were observed at 927-966 and 1058-1070 cm −1 in ODASnS nanocrystals which were present at 964 cm −1 in the capping agents alongside the N-H wagging mode at 793 cm −1 which is absent in the nanocrystal spectra. is can be attributed to the inhibition of the wagging mode due to its attachment to the surface of SnS as reported in literature [68][69][70]. C-N bending vibrations were observed at 1468 cm −1 , and the combination of NH 2 scissor and bending vibration at 1668 cm −1 in the capping agents and their nanocrystals. e di erence observed is that the nanocrystals has less intensity compared to the capping agents, and a shift of 20-60 cm −1 was observed for the NH 2 scissors and bending vibrations.  Journal of Nanotechnology e N-H asymmetric peak observed at 3329 cm −1 is weak in the nanocrystals con rming interaction between the amine and the SnS surface [71]. C-H vibrations were found at 2851-2919 cm −1 which is common to (CH 2 ) n chains more than 3. Carbonyl vibration, deformed vibration, and rocking vibration of CH 2 were observed, respectively, at 1710, 1465, and 722 cm − in OASnS nanocrystals apart from the carbonyl bond which disappeared and a new band at 1599-1606 cm −1 appeared which is the asymmetric mode of carboxylate metal salt [68,72,73].

Conclusion
In conclusion, we report the synthesis and use of bis(phenylpiperazine dithiocarbamate)tin(II) complex as a single molecular precursor to prepare oleic acid and octadecylamine capped at 150, 190, and 230°C. e crystallinity of the synthesized tin sulphide nanoparticles was a ected by the capping agents used though the same orthorhombic phase was observed with the XRD pattern. Monodispersed spherical-shaped SnS nanoparticles were obtained in OASnS, while ODASnS nanocrystals gave aggregated cubic nanocrystals. At 150°C, small-sized nanocrystals in the range of 10-28 nm were favoured in all the capping agents, while at 230°C, bigger size in the range of 35-80 nm was obtained. e morphology was in uenced by the capping agents used as ower-like, and spherical-like morphologies were obtained for ODASnS and OASnS nanocrystals. Band gap measurements were in the range of 1.71-1.95 and 1.93-2.81 eV for OASnS and ODASnS, respectively; the highest band gap is a ected by particle size con nement. Temperature and capping agents were found to in uence the size and shape of the nanocrystals. SnS FTIR showed that they were coated with the capping agents.
Data Availability e data used to support the ndings of this study are available from the authors upon request.

Conflicts of Interest
e authors declare that there are no con icts of interest.    Journal of Nanotechnology