PROCESS FOR PREPARING CUBIC PI-PHASE MONOCHALCOGENIDES

Abstract

The invention provides process for preparing tin or germanium monochalcogenides of cubic crystalline structure, the process comprises combining a source of tin or germanium and a source of chalcogenide in a reaction vessel in the presence of uncharged liquid primary amine R—NH.sub.2 and a charged form R—NH.sub.3+ associated with a counter anion, wherein R is saturated or unsaturated hydrocarbyl, which may be the same or different in the uncharged and charged forms, and recovering from the reaction mixture an essentially pure cubic phase of the monochalcogenides.

Claims

1. A process for preparing tin or germanium monochalcogenides of cubic crystalline structure, the process comprises combining a source of tin or germanium and a source of chalcogenide in a reaction vessel in the presence of uncharged liquid primary amine R—NH.sub.2 and a charged form R—NH.sub.3.sup.+ associated with a counter anion, wherein R is saturated or unsaturated hydrocarbyl, which may be the same or different in the uncharged and charged forms, and recovering from the reaction mixture an essentially pure cubic phase of the monochalcogenides.

2. A process according to claim 1, wherein R is the same for the uncharged and charged forms.

3. A process according to claim 1, wherein the monochalcogenide is selected from the group consisting of tin sulfide and tin selenide.

4. A process according to claim 1, comprising dissolving a source of tin in the uncharged liquid primary amine R—NH.sub.2, to form a first solution, dissolving a source of chalcogenide in the uncharged liquid primary amine R—NH.sub.2, to form a second solution, and combining said first and second solutions, wherein the charged form R—NH.sub.3.sup.+ is supplied to the first solution and/or the second solution.

5. A process according to claim 4, wherein the charged form R—NH.sub.3.sup.+ is supplied to the reaction in the form of ex-situ prepared salt of the formula R—NH.sub.3.sup.+ Hal−, wherein Hal is halide, added to the first solution and/or the second solution.

6. A process according to claim 4, wherein the charged form R—NH.sub.3.sup.+ is supplied to the reaction in the form of ex-situ prepared pair of charged species of the formula [R—NH.sub.3.sup.++R—NHCO.sub.2.sup.−], added to the first solution and/or the second solution.

7. A process according to claim 4, wherein the charged form R—NH.sub.3.sup.+ is supplied to the reaction in-situ, by treating the uncharged liquid primary amine R—NH.sub.2 in the first solution and/or in the second solution with an acidic gas under anhydrous conditions.

8. A process according to claim 7, comprising introducing hydrogen halide to the first solution and/or the second solution, thereby converting the primary amine to the corresponding ammonium halide R—NH.sub.3.sup.+ Hal.sup.− in-situ.

9. A process according to claim 7, comprising introducing carbon dioxide to the first solution and/or the second solution, thereby converting the primary amine to the corresponding pair of charged species of the formula [R—NH.sub.3.sup.++R—NHCO.sub.2.sup.−] in-situ.

10. A process according to claim 3, wherein the source of tin is a stannous halide salt and the sources of sulfur and selenium are NH.sub.2C(S)NH.sub.2 and NH.sub.2C(Se)NH.sub.2, respectively.

11. A process according to claim 1, wherein the group R of the uncharged liquid primary amine R—NH.sub.2 is alkyl or alkenyl that contains not less than 8 carbon atoms.

12. A process according to claim 11, wherein the R group is straight C12-C18 alkenyl group bearing one or more non-terminal carbon-carbon double bonds.

13. A process according to claim 1, wherein the uncharged liquid primary amine is oleyl amine and the charged form is oleyl ammonium associated with a counter anion selected from the group consisting of halides and oleyl carbamate.

14. A process according to claim 1, wherein the amounts of the uncharged primary amine and the corresponding charged form thereof are proportioned to maximize the polymorphic purity of the product in favor of the cubic phase.

15. A process for preparing tin or germanium monochalcogenides of cubic crystalline structure, the process comprises combining a source of tin or germanium and a source of chalcogenide in a reaction vessel in the presence of uncharged liquid primary amine R—NH.sub.2 and an additive which is R—COOH, wherein R is saturated or unsaturated hydrocarbyl, which may be the same or different in the uncharged and charged forms, and recovering from the reaction mixture an essentially pure cubic phase of the monochalcogenides.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 shows the XRPD pattern of π-SnS.

[0037] FIG. 2 shows the XRPD pattern of π-SnSe.

[0038] FIG. 3 presents 1D 1H NMR spectra of oleylamine, oleylammonium chloride and oleylammonium carbamate.

[0039] FIG. 4 provides TEM images (4a-4f) and corresponding XRPD patterns (4h-4j) of tin sulfide produced by Examples 1 to 3 and reference XRPD patterns (4g-4k) of pure phase tin sulfide (α-SnS and π-SnS), illustrating the effect of addition of ex-situ prepared oleylammonium chloride to the reaction mixture.

[0040] FIG. 5 provides TEM images (5a-5d) and corresponding XRPD patterns (5e-5h) of tin selenide produced by Examples 4 to 7, illustrating the effect of addition of ex-situ prepared oleylammonium chloride to the reaction mixture.

[0041] FIG. 6 is a TGA curve of oleylamine exposed to CO.sub.2 at 110° C. The mass gain is expressed as percentage of mass gained with respect to the initial mass. The molar concentrations were calculated from this mass gain curve.

[0042] FIG. 7 provides TEM images (7a-7d) and corresponding XRPD patterns (7e-7h) of tin selenide produced by Examples 8 to 11, illustrating the effect of in-situ formation of oleylammonium-oleyl carbamate in the reaction mixture.

[0043] FIG. 8 provides TEM and SEM micrographs (8a-8g) and X-ray diffractograms (8h-8l) of tin sulfide produced by Examples 12 to 16, testing the effect of addition of oleic acid to the reaction mixture.

EXAMPLES

[0044] Methods

[0045] Transmission electron microscopy (TEM) was carried out using a Tecnai G2 instrument operating at 120 kV. TEM samples were prepared by solvent evaporation from chloroform suspensions. Powder X-ray Diffraction (XRD) was performed on a Panalytical Empyrean powder diffractometer equipped with a position sensitive X'Celerator detector using Cu Kα radiation (λ=1.5418 Å).

[0046] Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker AVANCE III 500 MHz spectrometer equipped with broad-band 1H decoupling probe (BBO) at room temperature. NMR samples were prepared by adding 0.25 g (for solid samples) or 0.3 mL (for liquid samples) to 10 mL of CDCl3 in a glass vial. The samples were completely dissolved in the solvent, and 0.5 mL of that solution was transferred to an NMR tube for analysis. Chemical shifts are given in parts per million relative to the residual solvent peak. Processing was carried out using Topspin 2.1 software.

[0047] High resolution scanning electron microscopy (SEM) was carried out using a Thermo-Fisher Verios 460L.

[0048] Materials

[0049] Tin(II) chloride (SnCl.sub.2, reagent grade, 98%), oleylamine (OLA, >98%), oleic acid (Tech grade, 90%), thiourea (reagent grade, 98%), selenourea (reagent grade, 98%) were purchased from Sigma-Aldrich and used without further purification with the exception of oleylamine, which was heated to 150° C. and degassed under vacuum for at least 3 h, after which it was stored in a glovebox. This was done in order to release CO.sub.2 captured by the amine headgroup and to eliminate low boiling point impurities such as water. Hydrochloric acid (32%), sulfuric acid (95%), methanol (99.8%) and chloroform (99.9%) were purchased from Bio-Lab and used without further purification.

Preparation 1

Synthesis of Oleylammonium Chloride (OACl)

[0050] OACl was prepared by titrating hydrochloric acid over sulfuric acid while the evolved HCl gas was dried and bubbled into oleylamine. This process was stopped when bubbles started evolving from the oleylamine, indicating complete reaction with HCl. Standard Schlenk line reaction set-up was used for the reaction.

Preparation 2

Synthesis of Oleylammonium Oleylcarbamate (OAOC)

[0051] OAOC was prepared by bubbling CO.sub.2 gas into purified oleylamine inside a closed vessel. This process was terminated when CO.sub.2 bubbles evolved from the oleylamine, indicating complete conversion to OAOC. Standard Schlenk line reaction set-up was used for the reaction.

[0052] FIG. 3 presents 1D 1H NMR spectra of oleylamine, oleylammonium chloride and oleylammonium carbamate (the as-supplied oleylamine was purified according to the procedure described above).

[0053] The resultant NMR spectra obtained for oleylamine coincides with the chemical moieties expected from this molecule. The chemical shifts at 0.75, 1.95, and 5.23 ppm correspond to the methyl, allyl, and alkene groups, respectively. The range of chemical shifts from 1.1 to 1.5 ppm arises from protons that reside on the alkyl chain. The protons on the amine headgroup appear at 1 ppm and are marked with an arrow in FIG. 3.

[0054] Comparing the 1H NMR spectra of the purified oleylamine with that of oleylamine exposed to an excess of HCl gas shows complete transformation of oleylamine to oleylammonium chloride. The amine moiety is shifted downfield to 6.75 ppm and substantially broadened, indicating deshielding and chemical exchange with other equivalent protons. This is in accordance with the transformation of the amine group into ammonium. The α-amine protons are also shifted downfield as expected from reaction with the amine headgroup. Reaction of oleylamine with HCl can occur at the amine headgroup but is also possible at the C═C bond, resulting in possible formation of alkyl-chlorides. The ratio of the integrated signal from the methyl group and alkene group shows it remains unaffected after the reaction, ruling out reaction with the double bond and verifying reaction with the amine headgroup.

[0055] The 1H NMR spectrum of purified oleylamine reacted with an excess of CO.sub.2 gas confirms complete conversion of the amine headgroups to oleylammonium-oleylcarbamate molecular pairs. The amine moiety is shifted to a broad pair of peaks around 4.2 ppm. These peaks correspond to protons that reside directly on the nitrogen in carbamate and ammonium head groups, as shown in the inset in the second row in FIG. 3.

Examples 1 to 3

Synthesis of SnS in Oleyl Amine and in a Medium Consisting of Oleyl Amine and Ex-Situ Prepared Oleyl Ammonium Chloride

[0056] General Procedure

[0057] 56.8 mg of SnCl.sub.2 and 5.5 ml of oleyl amine were placed in a 3-necked flask in a glove-box and transferred to the Schleck-line. 22.83 mg of thiourea were dissolved in 3 ml of oleyl amine and placed in a 1-necked flask in a glove-box and transferred to the Schleck-line. Glove-box was used in order to prevent moisture to react with the precursors. The Sn-precursor was heated to 180° C. for 1 hr, until the SnCl.sub.2 completely dissolved. Meanwhile the S precursor was heated to 170° C. for 1 hr and injected to the Sn precursor. Instantaneous color change to deep-brown indicated the occurrence of the reaction.

[0058] The reaction was terminated by removing the reaction flask from the heating mantle and immediately quenching it to RT by pouring the content of the flask into a 50 mL test tube which was filled with methanol. The test tube was centrifuged at 2800 rpm for 5 min which after the solution was decanted. The test tube was filled with a mixture of 1:10 of chloroform and methanol and the process of washing and centrifuging was repeated two more times. The washed nanoparticles were then dried in a ventilated area and kept in powder form for storage.

[0059] Oleyl Amine/Oleyl Ammonium Chloride Reaction Medium

[0060] To test the effect of a combination consisting of a free primary amine and its chloride salt, the general procedure was repeated but oleyl ammonium chloride of Preparation 1 was added to the synthesis by weighing specific amounts of and subtracting the same number of moles of oleyl amine. Oleyl ammonium chloride was added to the Sn-precursor flask while handled in a glovebox.

[0061] The compositions of the reaction medium tested, consisting of free amine or mixtures of free amine/chloride salt are tabulated in Table 1. The compositions are expressed in terms of molar fractions of the free amine/chloride salt.

TABLE-US-00001 TABLE 1 Oleyl amine Oleyl ammonium chloride Example (mole fraction) (mole fraction) 1 100 0 2 91 9 3 75 25

[0062] Results

[0063] The results are shown by TEM micrographs (FIGS. 4a-4f) and corresponding XRD diffractograms (FIGS. 4g-4k).

[0064] Starting with the TEM images, it is seen that in the absence of the chloride salt, i.e., using neat oleyl amine (Example 1), the reaction product consists of nanoparticles exhibiting irregular shapes and sizes (295±216 nm; FIGS. 4a and 4b). Addition of low amount of the chloride salt, namely, carrying out the reaction in the presence of a mixture consisting of the free amine:chloride salt mixture at 10:1 molar ratio (Example 2), results in tetrahedron-shaped nanoparticles that are more uniform in size (183±23 nm; FIGS. 4c and 4d). Further increase the amount of chloride salt at the expense of the free amine, that is, when the reaction medium consists of 3:1 proportioned mixture (by moles, in favor of the free amine) induces shape transformation to rounded particles which are smaller in size and similar distribution (138±25 nm; FIGS. 4e and 4f).

[0065] Turning now to the XRD diffractograms, FIGS. 4g and 4k are simulated powder diffraction patterns of the pure π-SnS and α-SnS phases, whereas FIGS. 4h, 4i and 4j are the powder diffraction patterns of the experimentally obtained samples of Example 1, 2 and 3, respectively. The results indicate that particles synthesized using only oleyl amine consist of both cubic (n) and orthorhombic (a) phases (FIG. 4h). In contrast, particles synthesized using mixtures of oleyl amine and its chloride salt gave diffractograms consistent with that of the π-SnS crystal structure, as shown in FIGS. 4g-k. From this comparison we can conclude that the presence of oleyl ammonium chloride alongside the free amine stabilizes the π-phase of the nanoparticles and can result in powders consisting of pure π-SnS.

[0066] We have also tested for the extreme condition where neat oleyl ammonium chloride is used as the reaction medium. In this case injection of the precursor solutions did not result in the characteristic color change; the reaction solution remained transparent and no solid material could be centrifuged out from this solution. An aliquot was taken from the reaction mixture and transferred to a TEM grid for further analysis. TEM showed that the sample is composed mostly of large aggregates of amorphous material and some small crystalline particles, as confirmed by electron diffraction analysis (not shown).

Examples 4 to 7

Synthesis of SnSe in Oleyl Amine and in a Medium Consisting of Oleyl Amine and Ex-Situ Prepared Oleyl Ammonium Chloride

[0067] General Procedure

[0068] 56.8 mg portion of SnCl.sub.2, 5 mL of oleyl amine, and 0.5 mL of oleic acid were placed in a three necked flask in a glovebox and transferred to a Schlenk-line. An 18 mg portion of selenourea and 1.5 mL of oleyl amine were placed in an amber vial in a glovebox and transferred to a Schlenk-line. The flask and vial connected to the Schlenk line were cycled with inert gas and vacuum three times. After the solutions were subjected to vacuum, an inert atmosphere was flushed again to the flasks. The Sn precursor flask was heated to 180° C. using a heating mantle until the SnCl.sub.2 completely dissolved and was kept at that temperature for 30 min. The temperature of the Sn precursor was lowered to 110° C. and kept for 30 min while the Se precursor was transferred to a benchtop sonicator and sonicated at room temperature for 30 min. The Se precursor was injected into the Sn precursor and the reaction was initiated, as indicated by instantaneous color change from transparent yellowish to opaque deep-brown solution.

[0069] The reaction was terminated by removing the reaction flask from the heating mantle and immediately quenching it to RT by pouring the content of the flask into a 50 mL test tube which was filled with methanol. The test tube was centrifuged at 2800 rpm for 5 min which after the solution was decanted. The test tube was filled with a mixture of 1:10 of chloroform and methanol and the process of washing and centrifuging was repeated two more times. The washed nanoparticles were then dried in a ventilated area and kept in powder form for storage.

[0070] Oleyl Amine/Oleyl Ammonium Chloride Reaction Medium

[0071] To test the effect of a combination consisting of a free primary amine and its chloride salt, the general procedure was repeated but oleyl ammonium chloride of Preparation 1 was added to the synthesis by weighing specific amounts thereof and subtracting the same number of moles of oleyl amine (from the solution of the Sn precursor). The compositions of the reaction medium tested, consisting of free amine or mixtures of free amine/chloride salt at various proportions, are tabulated in Table 1. The compositions are expressed in terms of molar concentration of the chloride salt in the free amine solvent (in the Sn solution).

TABLE-US-00002 TABLE 2 Example Oleyl ammonium chloride (M) 4 0 5 0.4 6 0.75 7 1M

[0072] Results

[0073] The results are shown by TEM micrographs (FIGS. 5a-5d), and corresponding XRD diffractograms (FIGS. 5e-5h), referring to Examples 4 to 7, respectively.

[0074] The TEM micrograph series shown in FIGS. 5a-d show that initially, the favored shape of the resultant nanoparticles is cubic, but rod-like nanoparticles dominate at the highest OACL concentration. For instance, the product of Example 5 (produced in the presence of 0.4M oleyl ammonium chloride) consists of nanoparticles the majority of which (75%) is cube shaped, with a minority of 11% platelets with additional 14% of other irregular morphologies. But on increasing the concentration of OACL to 1M (Example 7), rod-like particles with short edge of 13.5±1.8 nm and long edge of 40±4.2 become predominant: the 1M oleyl ammonium chloride concentration yielded majority of rods that consist about 85% of the synthesis product.

[0075] Turning now the corresponding XRPD patterns, it is seen that particles recovered from a reaction mixture devoid of oleyl ammonium chloride (Example 4) exhibit a broad peak around 2θ=30.27 degrees as seen in FIG. 5e. Increasing OACL content to 0.4M (Example 5) induces the emergence of new peaks at 20=angles of 29.8°, 30.7° and 31.6° as shown in FIG. 5f. These peaks that were obtained from a sample synthesized in the presence of 0.4M OACL match to (400), (401) and (330) Miller indices, three peaks which appear together as a signature mark for π-SnSe. Peak intensities agree well with the assignment to π-SnSe, as can be seen in the reference diffractogram in FIG. 5g, and the cube morphology of the nanoparticles correlates well with the cubic crystal system. As we further increase the OACL concentration to 0.75M (Example 6), we observe minor changes in the X-ray diffractogram presented in FIG. 5g. The intensity ratio of the (400), (401) and (330) reflection is changed due to increased intensity of the (401) reflection which is shifted to 2θ=30.5°, indicating gradual replacement of π-SnSe with α-SnSe since the strongest reflection of the former is centered around 2θ=30.2 degrees. The appearance of platelets in the TEM micrograph in FIG. 5c confirms this interpretation, as it is well established that α-SnSe nanoparticles usually appear in platelet morphology. Once the OACL concentration is further increased to 1M (Example 7), the dominant XRD peak reverts to the orthorhombic α-SnSe phase, as evident by the total disappearance of the π-SnSe peaks and the appearance of a new peak centered around 2θ=30.2° that corresponds to the (111) reflection of α-SnSe (FIG. 5h). Therefore, the rod-like nanoparticle morphology presented in FIG. 5d is clearly correlated with the orthorhombic α-SnSe phase.

[0076] The results indicated that with suitably proportioned mixture of R—NH.sub.2/R—NH.sub.3.sup.+Hal.sup.− as the reaction medium, it is possible to achieve phase selective synthesis, which allows for stabilization of the cubic π-SnSe phase. The study reported in these Examples shows that a concentration of 0.2 to 0.6 M oleyl ammonium chloride in the primary amine reaction solvent affords nanoparticles population consisting essentially of pure crystalline phase cubic π-SnSe phase.

Examples 8 to 11

Synthesis of SnSe in Oleyl Amine and in a Medium Consisting of Oleyl Amine and In-Situ Formed Oleyl Ammonium-Oleyl Carbamate

[0077] General Procedure

[0078] The same general procedure of the previous set of Examples was used. However, in place of added oleyl ammonium chloride, the effect of in-situ formed oleyl ammonium-oleyl carbamate was tested. This pair of charged species was formed by exposing the solution of stannous chloride in oleyl amine to CO.sub.2 for predetermined time periods. This exposure resulted in the following reaction:

CO.sub.2+2CH.sub.3(CH.sub.2).sub.7CH═CH(CH.sub.2).sub.7CH.sub.2—NH.sub.2.fwdarw.CH.sub.3(CH.sub.2).sub.7CH═CH(CH.sub.2).sub.7CH.sub.2—NH.sub.3.sup.++CH.sub.3(CH.sub.2).sub.7CH═CH(CH.sub.2).sub.7CH.sub.2—NHCO.sub.2.sup.−

[0079] Subsequently, the Se solution was injected to the Sn solution.

[0080] Oleyl Amine/Oleyl Ammonium-Oleyl Carbamate Reaction Medium

[0081] The concentration of the oleyl ammonium-oleyl carbamate pair in the primary amine reaction solvent was varied by increasing the exposure time of the tin solution to CO.sub.2, as tabulated in Table 3 (CO.sub.2 was introduced into the Schlenk line]

TABLE-US-00003 TABLE 3 CO.sub.2 Exposure Oleyl ammonium - Example time (minutes) oleyl carbamate (M) 8 0 0 9  8 min 0.15 10 12 min 0.25 11 30 min 0.63

[0082] The molar concentrations of the oleyl ammonium-oleyl carbamate pair in the reaction mixture (in the Sn solution) set out in Table 3 were calculated from a calibration curve, shown in FIG. 6. The calibration curve was generated with the aid of thermogravimetric analysis (TGA). The TGA was performed with a TA Instruments Q500 analyzer, using an alumina crucible at a scan rate of 5° C./min. The oleylamine sample was placed in the alumina crucible that was constantly purged with argon gas. The sample was heated to 150° C. so that residual evolved CO.sub.2 was effectively removed. The sample was then cooled to 110° C. and exposed to CO.sub.2 at 110° C., and the mass gain was recorded over time. This temperature was chosen in order to simulate the absorption of CO.sub.2 at the reaction temperature of colloidal synthesis of SnSe, which takes place at 110° C.—see general procedure (it is worthy of note that oleylamine can capture CO.sub.2 until equilibrium is reached, depending on the equilibrium constant, which is temperature dependent).

[0083] The results were corrected to compensate for mass loss due to evaporation. The % mass gain of oleylamine owing to CO.sub.2 intake was determined from the calibration curve of FIG. 6, and subsequently converted to molar concentration of OAOC tabulated in Table 3.

[0084] Results

[0085] The results are shown by TEM micrographs of the SnSe nanoparticles (FIGS. 7a-7d), and corresponding XRD diffractograms (FIGS. 7e-7h), referring to the products of Examples 8 to 11, respectively. The magnified part of each diffractogram in the 29 range of 28-33° is shown respectively on the right. * denotes a SnSe2 impurity peak (JCPDS no. 23-0602). The red and blue lines indicate reference α-SnSe and π-SnSe peak positions, respectively.

[0086] Without exposure of the reaction mixture to CO.sub.2 (Example 8), the nanoparticles formed appear to possess irregular shapes, large size distribution, and poor crystallinity. The corresponding TEM and XRD results are presented in FIGS. 7a and 7e, respectively.

[0087] Exposing the reaction mixture to CO.sub.2 for 8 min (reaching concentration of 0.15 M OAOC—see Example 9) results in a cube-like nanoparticle morphology, as shown in the TEM micrograph presented in FIG. 7b. The size of the nanoparticles was 17±1.6 nm. The reaction product consists of 70% cube-like nanoparticles in admixture with other irregular morphologies. The corresponding X-ray diffractogram from this sample (FIG. 7f) matches to π-SnSe, as indicated by the emergence of the π-signature peaks at 29 angles of 29.8°, 30.7°, and 31.6° that correspond to π-SnSe (400), (401), and (330), respectively.

[0088] After exposure of the reaction mixture to CO.sub.2 for 12 min, to reach 0.25 M OAOC—see Example 9), a subtle change is noticed in the XRD (FIG. 7g). The peak at 2θ=30.7° which corresponds to π-SnSe (401) shifts to 2θ=30.5°. This may be explained by appearance of α-SnSe as a minority phase in this sample. The strongest reflection of α-SnSe, (111), is at position 2θ=30.2°, and the presence of this phase would affect the XRD peak position by shifting the π-SnSe (401) peak to lower 20 angles.

[0089] Exposure of the reaction mixture to CO.sub.2 for 37 minutes (resulting in buildup of 0.63 M OAOC), leads to significant changes. FIG. 7d is the TEM micrograph of the resultant SnSe nanoparticles, shows rod-like morphology of the nanoparticles, with a short edge size of 10±1.3 nm and long edge size of 20.7±2.7 nm. The rod-like nanoparticles constitute about 83% of the reaction product, while the rest show irregular morphologies. The corresponding XRD (FIG. 7h) shows that the intensity of the peak positioned around 30.2 increases at the expense of the π-SnSe peaks. This peak is attributed to the (111) reflection of α-SnS. A similar trend is noted by the peak positions at 20 of 37.4° and 38.3° that correspond to the (131) α-SnS and (413) π-SnSe Bragg reflections, respectively.

[0090] The results indicate that if the reaction takes place in a suitably proportioned mixture of R—NH.sub.2/[R—NH.sub.3.sup.++R—NHCO.sub.2.sup.−], stabilization of the cubic π-SnSe phase is achieved. The study reported in these Examples shows that a concentration of 0.1 to 0.2 M of oleyl ammonium-oleyl carbamate in the primary amine reaction solvent affords nanoparticles population consisting essentially of pure cubic π-SnSe phase.

Examples 12 to 16

Synthesis of SnS in Oleyl Amine and in a Medium Consisting of Oleyl Amine and Oleic Acid

[0091] General Procedure

[0092] 56.8 mg of SnCl.sub.2 and 5.5 ml of oleylamine were placed in a 3-necked flask in a glove-box and transferred to a Schlenk-line. 22 mg of thiourea and 3 ml of oleylamine were placed in 3-necked flask in a glove-box and transferred to a Schlenk-line. Connected flasks were cycled with inert gas and vacuum three times. The Sn and S precursors flasks was heated to 180° C. and 170° C. respectively using heating mantle. The precursors were kept at that temperature for 1 hr while SnCl.sub.2 and thiourea were completely dissolved. The S precursor was injected into the Sn precursor and the reaction initiated, as indicated by instantaneous color change from transparent yellowish to opaque deep-brown solution.

[0093] The reaction was terminated by removing the reaction flask from the heating mantle and immediately quenching it to RT by pouring the content of the flask into a 50 ml test tube which was filled with methanol. The test tube was centrifuged at 2800 rpm for 5 minutes which after the solution was decanted. The test tube was filled with a mixture of 1:10 of chloroform and methanol and the process of washing and centrifuging was repeated two more times. The washed nanoparticles were then dried in a ventilated area and kept in powder form for storage.

[0094] Oleyl Amine/Oleic Acid Reaction Medium

[0095] To test the effect of a combination consisting of a free primary amine R—NH.sub.2 and a corresponding carboxylic acid R—COOH, the general procedure was repeated but oleic acid (OA) was introduced to the reaction by systematically subtracting a well-defined amount of oleylamine volume and replacing it with the same amount of OA volume (the acid was added to the Sn-precursor flask while handled in a glovebox).

[0096] The compositions of the reaction medium tested, consisting of oleylamine or mixtures of the oleylamine and oleic acid are tabulated in Table 1. The compositions are expressed in terms of molar concentration of the chloride salt in the free amine solvent.

TABLE-US-00004 TABLE 4 Example Oleic acid (M) 12 0 13 0.2 14 0.5 15 0.75 16 0.9

[0097] Results

[0098] The results are shown by TEM micrographs of the SnS nanoparticles (FIGS. 8a-8e), SEM micrographs (FIGS. 8f-8g) and corresponding XRD diffractograms (FIGS. 8h-8l), referring to the products of Examples 8 to 11, respectively.

[0099] Without the addition of oleic acid (Example 12) the nanoparticles exhibit polydispersity in morphology and size (295±216 nm) as shown in FIG. 8a. The corresponding XRPD pattern (FIG. 8i) indicates that cubic and orthorhombic polymorphs coexist in the sample, suggested by the overlapping reflection from π-SnS 401 and α-SnS 111. The reflection overlap results in higher intensity at 2θ=31.6° compared to reference π-SnS XRD pattern. Additional support for the coexistence of both phases could be found by examining the α-SnS 120 and 131 reflections which appear at 2θ=26.06 and 2θ=38.9 respectively.

[0100] With the addition of oleic acid to the reaction mixture, the following trend was noted. Synthesis of SnS nanoparticles in the presence of 0.2 M OA (Example 13) induces the appearance of tetrahedral nanoparticles with better polydispersity. The morphology of the nanoparticles was determined by examining the TEM and SEM micrographs presented in FIGS. 8b and 8f, respectively. Increasing further the OA concentration to 0.5M (Example 14).fwdarw.0.75M (Example 15).fwdarw.0.9M (Example 16), resulted in the formation of SnS nano-cubes (FIGS. 8c, 8d, 8e, respectively; FIG. 8g). XRD of the samples that were synthesized in the presence of OA indicates the polymorphic purity of the product, which consist of π-SnS polymorph: the presence of the orthorhombic polymorphs was ruled out, as all of the peaks could be indexed as π-SnS.

[0101] The results indicated that with suitably proportioned mixture of R—NH.sub.2/R—COOH as the reaction medium, it is possible to achieve phase selective synthesis, which allows for stabilization of the cubic π-SnS phase. The study reported in this Example shows that a concentration of 0.2 to 1.0 M of oleic acid in the primary amine reaction solvent (namely, the oleylamine) affords nanoparticles population consisting essentially of the π-SnS cubic polymorph.