A SnS 2 Molecular Precursor for Conformal Nanostructured Coatings

: We present a simple, versatile and scalable procedure to produce SnS 2 nanostructured layers based on an amine/thiol-based molecular ink. The ratios amine/thiol and Sn/S, and the reaction conditions are systematically investigated to produce phase-pure SnS 2 planar and conformal layers with a tremella-like SnS 2 morphology. Such nanostructured layers are characterized by excellent photocurrent densities. The same strategy can be used to produce SnS 2 -graphene composites by simply introducing graphene oxide (GO) into the initial solution. Conveniently, the solvent mixture is able to simultaneously dissolve the Sn and Se powders and reduce the GO. Besides, SnS 2-x Se x ternary coatings and phase pure SnSe 2 can be easily produced by simply incorporating proper amounts of Se into the initial ink formulation. Finally, the potential of this precursor ink to produce gram scale amounts of unsupported SnS 2 is investigated.


INTRODUCTION
2][3] Toward this goal, solution-based strategies based on the decomposition, assembly and/or deposition of salts, molecular precursors or colloidal nanoparticles offer obvious advantages over physical and chemical vacuum-based technologies. 4However, the use of salts generally leads to the incorporation of large amounts of impurities in the final material and may result in inhomogeneous layer compositions.][7][8][9][10][11] On the other hand, molecular precursors generally provide a homogeneous mixing of the precursor elements that ensures a uniform layer composition without impurities.Additionally, the crystallization of the material post deposition can considerably improve charge transfer/transport properties and layer adherence. 2 2004, Mitzi and coworkers reported the use of hydrazine to produce a molecular ink from the dissolution of bulk SnS2 and SnSe2 in the presence of excess elemental chalcogen. 12The process was facilitated by the in-situ reduction of the elemental chalcogen to produce chalcogenide anions that subsequently started a series of nucleophilic reactions with the bulk chalcogenide, breaking it down to eventually yield soluble molecular metal chalcogenide species. 12The obtained molecular inks could then be spin-coated and thermally annealed at 350 °C to yield phase-pure SnS2-xSex thin films with high charge carrier mobility. 12After this pioneer report, the hydrazine route was demonstrated effective in producing a plethora of chalcogenides, including GeSe2, 13 Cu1.9S, 14In2Se3, 15 ZnTe, 16 In2Te3, 17 GeSbSe 18 , CuInSe2 19 , and CuInTe2 17 .The use of hydrazine finds advantages on its high reduction power, small size, volatility and on leaving no carbon residue.However, hydrazine is highly toxic and explosive, thus it is not convenient for scale-up and commercial applications. 20 avoid the disadvantages of hydrazine, the Brutchey group demonstrated a binary solvent mixture based on ethanedithiol (Edt) and ethylenediamine (En), named the ¨alkahest solvent¨, to dissolve and recover several V2VI3 chalcogenides, including As2S3, As2Se3, As2Te3, Sb2S3, Sb2Se3, Sb2Te3, Bi2S3, Bi2Se3 and Bi2Te3, 21 at room temperature and ambient pressure, under air atmosphere and in just few minutes. 21,22This amine-thiol solution system dissolves the bulk chalcogen through an amine-catalyzed reaction resulting in alkyl di-, tri-, and tetrachalcogens while -SR species bind to the metal ions. 20,21,23Various combinations of mono-or dithiols with primary mono-and/or diamines, as well as different amine/thiol ratios have been used to dissolve a plethora of metal and chalcogen precursor materials 20,24,25 .As an example, Cu, Zn, Sn, and In are soluble in butylamine and Edt; 26 MaXn (M = Cu + , Cu 2+ , Zn 2+ , Sn 2+ , In 3+ , Ga 3+ ; X = Cl -, acac -, OAc -, O 2-) can be easily dissolved in a mixture of hexylamine and propanethiol; and MbYn (M = Cu + , Cu 2+ , Sn 2+ , In 3+ ; Y = S 2-, Se 2-) are highly soluble in hexylamine and Edt. 27In the amine-thiol system, the main difficulty to prepare a phase pure material is to find the proper amine and thiol mixture, their ratio and reaction conditions to crystallize the targeted product in the desired phase, preventing the formation of unwanted secondary phases.
SnS2 is an indirect n-type semiconductor that is attracting significant interest as absorber material for solar energy conversion due to the abundance of its constituting elements and its excellent optoelectronic properties in part related to its layered structure.0][41] Sn has been also dissolved in 10/1 of En/Edt to recover SnS, 42 but the production of pure SnS2 by this method has been so far elusive.
Herein, we present a simple route based on a molecular ink to produce phase pure SnS2 from a Edt and En mixture.The ratio of Sn/S, En/Edt and the annealing process are investigated to define the optimal conditions to recover pure SnS2 planar and conformal layers.We additionally study the production of ternary SnS2-xSex and phase pure SnSe2, the one step production of composite of SnS2/reduced graphene oxide (SnS2/rGO) composites and the photocatalytic properties of the produced layers.

RESULTS AND DISCUSSION
In our first attempts to produce SnS2, stoichiometric molar amounts of sulfur and tin powders (Sn/S=1/2) were dissolved in a volume ratio En/Edt=10/1 at 70 ºC for 1h to obtain an optically clear pale yellow solution.The precursor ink was coated onto a glass substrate and the obtained film was annealed at 320 °C for 30 min.Upon annealing, the layer turned grey yellow.Scanning electron microscope (SEM) characterization showed the layer to contain a large density of quasi-spherical flower-like structures (Figure 1a), but powder X-ray diffraction (XRD) analysis identified the main crystal phase of the obtained material as SnS (Figure 1b), with only a minor amount of SnS2 (Figure 1b).
The precursor solution was modified by incorporating an additional amount of sulfur (Sn/S=1/3) to correct for the low quantity of this element in the final layer.This precursor solution displayed a much more intense yellow color (Figure 1c).XRD analyses of the layers produced from the coating and posterior annealing of this precursor ink showed a notable increase of the amount of SnS2, but SnS impurity peaks were still evident.Besides, the structure of the obtained film changed from the quasi-spherical flowers to tremella-like particles (Figure 1d).Similar results were obtained when instead of increasing the amount of sulfur, we decreased the concentration of Edt in the initial solution (Sn/S=1/2; En/Edt=20/1).This precursor solution decomposed into tremella-like structures with a main SnS2 phase and a residual SnS component (Figure 1b,e).
To further adjust the stoichiometry and crystal phase of the final layer, we simultaneously increased the amount of sulfur (Sn/S=1/3) and reduced the amount of Edt (En/Edt=20/1).Using this new precursor solution (Sn/S=1/3, En/Edt=20/1), which displayed a green color (Figure 1c), phase-pure tremella-like SnS2 could be finally recovered (Figure 1b,f).Energy dispersive X-ray spectroscopy (EDX) analysis further confirmed the composition of the obtained layer to be consistent with stoichiometric SnS2 (Table S1).
Generally, lower amounts of Edt favored the formation of the SnS2 phase over SnS.Pure SnS2 could be obtained with Edt contents of En/Edt=20/1 or lower when using an excess of S, Sn/S≤1/3 (Figures 2a and S1).However, without Edt, the Sn and S powder mixture could not be fully dissolved (Figure 2b, S1) and no SnS2 could be recovered.On the amine side, replacing En by hexylamine or butylamine did not allow the dissolution of proper amounts of Sn and S (Figure S2).The higher the Edt concentration, the lower the time required to obtain an optically clear ink (Table S2).When using dissolution times in the range between 20 min and 1 h, no major effect was observed on the phase and structure of the produced material.However, when increasing the dissolution time, the SnS phase appeared even when using the previously defined as proper precursor compositions: Sn/S=1/3, En/Edt=20/1 (Figure 3a).EDX analysis further confirmed the Sn/S ratio of the annealed material to increase with the time of preparation of the precursor solution (Table S3).Additionally, the color of the ink also faded with the dissolution time (Figure 3b).Besides, the morphology of the final material turned toward sunflower-like structures when dissolution time increased to 3 h and above (Figure S3).We hypothesize that all these observations were associated with a sulfur loss during ink formulation.This loss was in the form of H2S that was produced by the reaction of elemental sulfur with amine, 43,44 To prove this hypothesis, we added an additional amount (0.5 mmol) of sulfur powder into a SnS2 molecular ink after 6 h at 70 ºC.This ink was mixed for additional 30 min to dissolve the added sulfur.After this time, the intense green color was recovered, and pure SnS2 phase with a tremella morphology was obtained after annealing (Figure S4).
When applying no heat, a suitable ink could be prepared by dissolving proper amounts of Sn and S (Sn/S=1/3) in En/Edt=20/1 at ambient temperature for 6 h.In this condition, no significant loss of sulfur was detected and pure SnS2 could be recovered after annealing (Figure S5).Overall, to optimize time and ensure a proper product phase with tremella like morphology (Figure 3a,c), we fixed the ink preparation time at 30 min, dissolution temperature at 70 ºC, the element ratio at Sn/S=1/3 and the solvent ratio at En/Edt=20/1.Thermogravimetric (TG) analysis of the yellowish gel-like paste collected after drying the conventional ink at 200 ºC (Figure 4a) displayed a two-step mass loss in the temperature range 200-300 ºC.These steps were associated with the ink decomposition and overall accounted for ca.65 wt% of the initial mass.At temperatures above 500 ºC an additional mass loss, associated with the sulfur evaporation, was observed.Fourier transform infrared spectroscopy (FTIR, Figure 4b) corroborated the loss of organics after annealing the material at 320 ºC for 30 min, as evidenced by the complete loss of strong (N-H)/(O-H) and (C-H) stretching bands originating from the solvent mixture.The absence of bands corresponding to the ν(S-H) thiol group at 2560 cm -1 implies that Edt is fully deprotonated by En. 42 EDX elemental mapping of the films obtained from the deposition and annealing of the conventional ink showed a uniform distribution of S and Sn (Figure S6).High-resolution transmission electron microscopy (HRTEM) micrographs confirmed the SnS2 hexagonal phase (space group = P63mc) with a=b=3.6450Å, c=11.8020Å (Figure 4a, S7).HRTEM analysis reported in Figure 4d shows the overlapping of two 2 different [0001]-oriented nanosheets.The power spectrum (Fast Fourier Transform) applied to the HRTEM image with the applied reciprocal space frequency (structural) filtering showed the presence of a 21 o rotation between the overlapping nanosheets (Figure 4d).These results confirmed the pileup of different nanosheets, consistently with SEM results.
UV-vis analysis of recovered SnS2 films (Figure 4c) showed their bandgap to be around 2.5 eV, which is consistent with the relatively wide range of optical bandgaps reported for SnS2. 41,45The small deviation on the experimental band gap values measured here with respect to theoretical SnS2 values can be ascribed to the reduced thickness of the produced SnS2 layer (Figure 1f, 3c and 4d), consistently with previous report. 46) FTIR spectra of a SnS2 molecular ink dried at room temperature and of the material recovered after annealing at 320 ºC for 30 min.c) UV-Vis spectrum of the material recovered after annealing at 320 ºC for 30 min.d) HRTEM micrograph of the material recovered after annealing at 320 ºC for 30 min, detail of the orange squared region and its corresponding power spectrum.From the analyzed crystalline domain, the SnS2 lattice fringe distances were measured to be 0.316 nm, 0.314 nm, 0.310 and 0.181 nm, at 62.65, 123.67 and 94.00 o , which could be interpreted as the hexagonal SnS2 phase, visualized along its [0001] zone axis.Green and yellow squared images correspond to the reciprocal space frequency filtered reconstructed images corresponding to each individual overlapping nanosheet, as extracted from the orange squared region.The spots used in the frequency reconstruction have been circled in red and green, respectively, in the power spectra provided below.
To study the photocatalytic properties of SnS2 tremella layers obtained from the molecular ink, the ink was deposited on fluorine doped tin oxide (FTO)-coated glass substrates by drop casting.The annealed layer was used as anode in a photoelectrochemical (PEC) cell for water splitting.XRD and SEM analysis confirmed the phase and structure of the SnS2 to be maintained when depositing the ink over FTO instead of glass (Figure S8).Using these SnS2 layers, current densities reaching up to ~160 A/cm 2 (1.23 V vs. RHE) were measured (Figure 5).9][50][51][52] We ascribed this excellent photocurrents to: 1) the tremella morphology that provided a larger interface area with the electrolyte and effective paths for the solution diffusion; 2) the intimate contact of the SnS2 absorber with the FTO current collector, thus decreasing the interphase electrical resistance and facilitating the transfer of photogenerated charges from the SnS2 photocatalyst films to the FTO.We further investigated the conformal coating of SnS2 on different substrates.As an example, stainless steel (SS) meshes were coated through a quick immersion of the preheated support on the molecular ink and the posterior drying and annealing of the obtained film (see experimental section for details).Using this simple procedure, homogeneous layers of nanostructured SnS2 could be coated on the surface of SS meshes as observed by SEM, EDX and SEM mapping (Figures S9 and S10).
Notice that since the mixture of En/Edt has a high metal dissolution power, the coating of Ni and Cu foams/meshes using this ink inevitably resulted in Ni or Cu contamination in the final SnS2 layer.As an alternative, instead of SS, corrosion-free carbon cloth (CC) was used as current collector.Figure S11 displays SEM micrographs of bare and SnS2-coated CCs.
A different strategy was used to coat graphene flakes.GO produced by the Hummers method was introduced in the precursor solution.During the ink formulation, simultaneously to the dissolution of Sn and S powders, GO was reduced by the amine, 53,54 yielding an ink containing a mixture of Sn-S molecular precursor and rGO.This composite inkcould be used to coat virtually any substrate with SnS2/rGO composite layers using the above coating and annealing steps (Figure S12).We used the same strategy to produce SnS2-xSex coatings with controlled S/Se ratios, by simply adding the proper amount of Se in the precursor solution (see experimental section for details).As can be seen in Figure 6a, an obvious XRD peak shift towards lower angle occurred when Se ratio increased from 0 to 100 %, indicating the Se ion entered within the crystal structure.No impurity peaks were detected, even when pure SnSe2 was recovered.The phase purity obtained in the present work was in contrast with results obtained by McCarthy et al., who systematically obtained sulfide impurities when attempting to produce pure CoSe2. 55Differences are associated with the very little amount of Edt used in the present work.Figure S13 shows how the color of the precursor solution changed from green to dark brown with the selenium dosage increase.Figure 6b displays how the color of spin-coated thin-film changed after annealing.SEM-EDX data (Table S4) demonstrated the sum of Se and S to be always around 67 %, while the tin composition was around 33 %, confirming that Se ions occupied the position of S within SnS2-xSex ternary chalcogenides.SEM analysis further showed the tremella-like morphology to change into sheet-like with the addition of Se (Figure 6c).
Among the samples containing Se, the highest photocurrents were measured from SnSe2/FTO photoelectrodes, reaching up to 10 A/cm 2 at -0.1 V vs. Ag/AgCl (Figure S14).We additionally studied the potential of this precursor ink to produce gram scale amounts of unsupported particles, which can be potentially used in catalysis 36 , batteries, 8 photodetectors, 56 sensors, 57 field-effect transistor devices, 58 etc.With this aim, large amounts of SnS2 molecular ink (5 mL) were decomposed inside a heated flask (see experimental section and Figure S15a for details).
After annealing, a gray powder was collected, which turned gray-yellow after washing.XRD analysis showed the powder to be phase pure SnS2 (Figure S15b).X-ray photoelectron spectroscopy (XPS) analysis showed the Sn 3d region to display a unique doublet at 495.9 eV (Sn 3d3/2) and 487.5 eV (Sn 3d5/2), associated with Sn 4+ within the SnS2 lattice, [59][60][61] while the S 2p region showed a doublet at 163.5 eV (S 2p1/2) and 162.3 eV (S 2p3/2), ascribed to S 2-in the SnS2 lattice 60,61 (Figure 7a).Besides, SEM-EDX data in Figure 7b further confirmed that the composition of Sn/S was ca.1/2, consistent with stoichiometric SnS2.Interestingly, the powder displayed a flake-like morphology in contrast to the tremella-like particles recovered on substrates, which was ascribed to the additional growth of the crystals recovered in powder form associated with the larger amount of available precursor.

CONCLUSIONS
In summary, a Sn-S molecular ink was produced by dissolving Sn and S elemental powders in an amine/thiol co-solvent.The decomposition and annealing of this ink at 320 ºC for 30 min resulted in the formation of nanostructured tin sulfide.Pure SnS2 layers with a tremella morphology were obtained using an ink containing an elemental ratio Sn/S=1/3 and a solvent ratio En/Edt=20/1.A loss of sulfur in the form of H2S gas during dissolution was identified as the reason behind the presence of SnS impurities in inks produced using too long dissolution times.This molecular ink could be applied to produce planar and conformal SnS2 nanostructured layer on conductive substrates, such as FTO, SS and CC.SnS2/FTO photoanodes provided outstanding photocurrent densities under illumination with simulated solar light.Besides, SnS2/rGO composites and SnS2-xSex were easily
Ink Preparation.Tin and sulfur elemental powders, other than their salts, were selected as solutes to avoid any potential counterion contamination.To prepare a Sn-S molecular ink, elemental Sn (0.5 mmol) and S (1.5 mmol) powders were weighted and transferred into a glass vial (4 mL).En (2 mL) and Edt (0.1 mL) were sequentially added into the vial.The mixture was then stirred at 750 rpm and heated to 70 °C and kept at this temperature for 30 min to get an optical transparent green ink solution.
To study the influence of ratio between En and Edt, different amount of Edt was used, while En amount kept unchanged.To study the influence of sulfur amount, we used different amount of sulfur.
Recovery of Semiconductor Film on Planar Glass Slides.Typically, 0.1 mL of molecular ink was drop casted on a microscope glass slide (~1×1 cm), and then the deposited substrate was heated on a hot plate (Corning, PC400D) at 200 °C to dry the ink.The dried precursor film was afterward annealed at 320 °C on the hotplate for 30 min.These operations were conducted in the argon-filled glovebox.The obtained film was scraped off of the substrate for posterior characterization by XRD, SEM and HRTEM.
Recovery of Semiconductor Film on FTO-coated Glass Slides.20 L of molecular ink was drop casted on the surface of FTO with an active area of 1cm×1cm (an additional area of 1cm×1cm was covered by the high-temperature bearable tape to leave a clean area for electrical connection).The tip of the pipette-pipe was used to pull the solution in order to produce a uniform coverage (in this operation, the tip shouldn't touch the FTO surface), after then dried at 200 °C and finally annealed on the hot plate at 320 °C for 30 min.To coat CC with a SnS2 layer, 40 μL of molecular ink was evenly drop casted on two sides of a 1x1 cm 2 carbon cloth using the tip of the pipette to spread the precursor solution and then put close to the surface of the hot plate to dry, subsequently annealed in Ar filled tubular furnace at 320 °C for 30 min with a temperature ramp of 3 °C/min.

Recovery of Semiconductor
Recovery of SnS2/rGO Composites.GO was produced by the Hummers method as originally presented by Kovtyukhova. 62In the next step, 10 mg of GO was added together with Sn powder and S powder to formulate the Sn-S molecular ink.The same ratios of Sn/S and En/Edt and dissolution and annealing times and temperatures as used in conventional inks were used to recover SnS2/rGO composite layers.

Recovery of SnS2-xSex.
A proper amount of elemental Se powder was introduced in the initial mixture in replacement of the same molar amount of S powder.The dissolution time was adjusted to make sure that all the precursor powders were dissolved and an optical transparent molecular solution was obtained.Dissolution time was typically 30 min for SnS2 and increased up to 80 min with Se amount increasing.Dissolution temperature was set at 70 ºC in all cases.The same drying and annealing steps used to recover pure-phase SnS2 were used to recover SnS2-xSex, except that the annealing time was increase to 60 min.
To coat SnS2-xSex on FTO for posterior PEC measurements, we noticed that the recovered SnS2-xSex, especially SnSe2, using the same spin coating method as we did on SnS2/FTO showed almost no i-t response.We attributed this result to the SnSe2 layer being too thick.Notice SnSe2 has a much lower band gap and thus absorbs light much more strongly, thus thinner layers are required.Too thick layers prevent light from arriving to the semiconductor-electrode interphase and decrease performance.We thus used the spin coating method to produce thinner layers, and kept all the deposition process the same to make the results comparable.
Recovery of Large-scaled Powder.5 mL of Sn-S molecular ink was injected into a 50 mL threeneck flask preheated 300 °C and containing an Ar atmosphere.A safety flask containing water/ethanol was used to collect the gas/vapor emanated from the ink solution.Upon ink injection, temperature decreased to ~200 °C and then increased slowly to 300 °C.The flask was kept at this temperature for additional 10 min to obtain a dry powder.The powder was then grinded and annealed in tubular furnace under Ar atmosphere at 350 °C for 2 hours with a ramp rate of 3 °C/min or lower.The annealed powder was washed using CS2 and ethanol by sonication to remove the potential sulfur residues.The sample was finally dried for posterior characterizations and use.It should be noted that we injected the precursor solution very fast in order to make the heating process more uniform and synthesis more repeatable, and thus minimizing the difference among different batches.

Figure 3 .
Figure 3. a) XRD patterns of samples recovered after annealing the layers obtained from molecular inks prepared after different dissolution times from 0.5 h to 9 h, as noted; b) Optical photos of respective molecular inks before and after heating at 70 C for varied range of times.Ink composition and dissolution temperature: Sn/S=1/3, En/Edt=20/1, 70 ºC.c) Large area SEM micrograph and higher magnification image of sample recovered from the annealing of a layer produced from a standard ink: Sn/S=1/3, En/Edt=20/1, 70 ºC, 0.5 h dissolution time.Inset shows optical photograph of the tremella fungus.

Figure 4 .
Figure 4. SnS2 recovered from a conventional molecular ink: a) TG profile of a SnS2 molecular ink after drying at 200 ºC.b) FTIR spectra of a SnS2 molecular ink dried at room temperature and of the material recovered after annealing at 320 ºC for 30 min.c) UV-Vis spectrum of the material recovered after annealing at 320 ºC for 30 min.d) HRTEM micrograph of the material recovered after annealing at 320 ºC for 30 min, detail of the orange squared region and its corresponding power spectrum.From the analyzed crystalline domain, the SnS2 lattice fringe distances were measured to be 0.316 nm, 0.314 nm, 0.310 and 0.181 nm, at 62.65, 123.67 and 94.00 o , which could be interpreted as the hexagonal SnS2 phase, visualized along its [0001] zone axis.Green and yellow squared images correspond to the reciprocal space frequency filtered reconstructed images corresponding to each individual overlapping nanosheet, as extracted from the orange squared region.The spots used in the frequency reconstruction have been circled in red and green, respectively, in the power spectra provided below.

Figure 5 .
Figure 5. a) Linear sweep voltammogram and b) amperometric i-t curve at 1.23 V vs RHE of a SnS2/FTO sample under chopped illumination.RHE: reversible hydrogen electrode.

Figure 6 .
Figure 6.SnS2-xSex ternary chalcogenides obtained from the annealing for layers produced from inks containing different S/Se ratios: a) XRD patterns; b) optical photograph of SnS2-xSex/FTO layers before and after annealing; c) SEM micrographs; d) UV-Vis spectra.

Figure 7 .
Figure 7. SnS2 powder recovered from the annealing of large amounts of molecular ink: a) XPS spectra of Sn 3d region and S 2p regions.b) SEM micrographs and EDX data.
same strategy by simply introducing GO and Se powder in the initial precursor solution.
Film on Stainless Steel and Carbon Cloth.Typically, a piece of precleaned SS was firstly heated on the hot plate at 200 °C for several minutes, then quickly immerse into molecular ink and hold for 30 s.The wet SS was then held by a tweezer and put close to the surface of the hot plate to dry, after then annealed in Ar filled tubular furnace at 320 °C for 30 min with a temperature ramp of 3 °C/min.SS was preheated to decrease the local viscosity of the molecular ink close to the SS surface, and thus facilitate it diffusion through the porous electrode structure.The heated support also promoted the film drying and facilitated the formation of more uniform coatings.
Photoelectrochemical Measurements.PEC characterization was performed in a three-electrode system using an electrochemical workstation (Metrohm Autolab).A Pt mesh (2 cm 2 surface area) and Ag/AgCl (3.3 M KCl) were used as the counter and reference electrodes, respectively.SnS2-xSex films deposited on FTO were used as working electrode.Before film deposition, the FTO glass was washed with acetone-isopropanol (1-1 volume), ethanol, and then deionized water.An aqueous solution of Na2SO4 (0.5 M, pH = 7) was used as the electrolyte.Relatively thick SnS2 films deposited by drop casting were used to test the SnS2 PEC performance.On the other hand, thinner films produced by spin coating were used to the SnSe2 performance owing to the higher light absorption capability of