Tin chalcogenides – a group of multi utility inorganic materials

3.1.1 Tin(II) precursors for the synthesis of tin chalcogenide

Divalent tin chalcogenolates (e.g., _∞ ¹[Sn(EPh)₂] (E = S, Se, Te), ¹⁰² [M(Sepy)₂], ⁹⁷ [Sn{ESi(SiMe₃)₃}₂] (E = S, Se, Te), ⁹⁸ undergo a single step decomposition to yield ME with concomitant elimination of chalcogeno ethers. The complexes _∞ ¹[Sn(EPh)₂] start decomposing under a flow of helium gas at 265, 200 and 126 °C for E = S, Se, Te, respectively with the formation of phase pure SnE. ¹⁰¹ The complex [Sn{TeSi(SiMe₃)₃}₂] either under flowing nitrogen at 250 °C or in refluxing toluene affords phase pure SnTe, whereas other complexes, [Sn{ESi(SiMe₃)₃}₂] (E = S or Se) under similar conditions yield SnE contaminated with metallic impurities. ⁹⁸ Nanocrystals of cubic phase of SnTe have been isolated by rapidly injecting a mixture of [Sn{N{SiMe₃)₂}₂]₂ and trioctylphosphine telluride in ODE containing OLA at 150 °C. ¹⁰² These SnTe NCs in tetrachloroethylene when evaporated slowly under low pressure resulted in the formation of self-assembled superlattice (Figure 6). The size of NCs varied in the range 4.5–15 nm and can be managed by fine tuning the rate of injection, thermolysis temperature and concentration of OLA in the reaction. The optical band gap values for 7.2 and 14 nm NCs are 0.54 and 0.39 eV, respectively. ¹⁰²

Figure 6:

TEM images of as-synthesized SnTe NCs (10.2 nm) capped with oleic acid, (a) bar indicates 50 nm, (b) bar represents 10 nm. (Reproduced with permission from American Chemical Society ¹³⁷ ).

Bis(3-mercapto-1-propanethiolato)tin(II), [Sn(SCH₂CH₂CH₂SH)₂] has been employed to grow thin films of orthorhombic SnS in the temperature range 300–400 °C on glass substrates. ¹⁰³ Films could be deposited in the absence of H₂S gas, unlike tin(IV) precursors, Sn(SR)₄ which required H₂S as an additional sulfur source (see later). The SnS films showed an optical band gap of ∼1.31 eV. Tin(II) aminothiolate complexes, [Sn{(SCH₂CH₂)₂NR}] (R = Me or Et) (1) (Scheme 4) ⁸⁷ and [Sn{SC(Me₂)CH₂NMe₂}₂] (2) (Scheme 4) ¹⁰⁴ undergo a single step decomposition to give SnS. Microwave-assisted solvothermal decomposition of [Sn{(SCH₂CH₂)₂NMe}] in N-methyl-2-pyrrolidone gave flake-like nanocrystals of SnS while the thermolysis of [Sn{SC(Me₂)CH₂NMe₂}₂] in OLA at 270 °C afforded orthorhombic SnS nanocrystals. The complex [Sn{SC(Me₂)CH₂NMe₂}₂] has also been used for growing thick and well-faceted orthorhombic SnS thin films on SiO₂ substrate at 300 °C. ¹⁰⁴ Similarly, a toluene solution of tin(II) thioamidate, [Sn{SCPrⁱ)NR}₂] (R = Prⁱ or Bu^t) (3) (Scheme 4) has been used for growing SnS thin films on SiO₂-coated glass substrate in the temperature range 200–400 °C. ¹⁰⁵ Another precursor, tin(II)thio-ureido (4) (Scheme 4) has been used for selective deposition of orthorhombic (α-SnS) and cubic zinc blende SnS (π-SnS) films of ∼800 nm thickness on glass substrates by AACVD. ¹⁰⁶ The TG analysis revealed a single step decomposition with an onset temperature of ∼270 °C. The cubic phase SnS films were formed at 300 °C whereas orthorhombic SnS films were produced at higher temperatures (350–450 °C). These films showed optical band gaps of 1.78 and 1.34 eV_(direct), respectively. ¹⁰⁶ The photoelectrochemical analysis revealed that α-SnS films exhibited cathodic photocurrent (p-type behaviour) while π-SnS films showed both p- and n-type photocurrent behaviour. Mishra and coworkers synthesized a mononuclear tin(II) thiolate, [Sn(SBu^t)(tfb-dmeda)] by the reaction of [Sn(SBu^t)₂] with Htfb-dmeda which undergoes decomposition in a single step in the temperature range 250–275 °C. ¹⁰⁷ Using this precursor, films of various thickness (80 nm – 2 μm) of orthorhombic SnS have been deposited on different supports, like FTO, Mo-coated soda lime glass, in the temperature range of 330–450 °C by CVD method. ¹⁰⁷

Scheme 4:

Structures of tin(II)precursors used for the synthesis of tin chalcogenides.

Tin(II) 1,1′-dithiolate/-diselenolate complexes have been used successfully as single source molecular precursors for the synthesis of tin chalcogenides. ^{108 , 109 , 110 , 111} The dithiocarbamate complexes, [Sn(S₂CNRR’)₂] (R/R’ = Et/Et, Me/Buⁿ, Et/Buⁿ) undergo thermal decomposition in the temperature range 210–360 °C yielding SnS. ^{109 , 110} However, in coordinating solvents decomposition of [Sn(S₂CNEt₂)₂] takes place at much lower temperatures. In hexadecyl amine or oleyl amine the complex decomposes at temperature as low as 85 °C yielding nanocrystals of orthorhombic SnS. The shape and size of NCs can be modified by the solvent, duration and temperature of thermolysis. ¹¹⁰ Interestingly, 1,10-phenanthroline adduct, [Sn(S₂CNEt₂)₂(phen)], on the other hand can be employed for the preparation of two different tin sulfides, viz. SnS and SnS₂ by subtle variation in reaction parameters. ¹¹¹ Thermolysis in a mixture of OLA and ODE (1:1 ratio) at 300 °C yields nanosheets (7 μm × 3 μm × 20 nm) of orthorhombic SnS which exhibits electrochemical properties with capacity of 350 mAhg⁻¹ around 1.2 V. However, thermolysis at 280 °C in a mixture of OLA–OA- ODE (1:1:2 ratio) affords nanoplates (6 × 150 nm) of hexagonal SnS₂. ¹¹¹ A 1,1′-diselenolato complex, [Sn(Se₂PPh₂)₂] has been successfully employed for orthorhombic SnSe films deposition on borosilicate glass plates at 400 °C under a flowing argon atmosphere by AACVD method. ¹⁰⁸

3.1.2 Tin(IV) precursors for the synthesis of tin chalcogenides

Different classes of both classical and organotin(IV) chalcogenolates have been used for the synthesis of tin chalcogenides. Homoleptic tin chalcogenolates, viz., [Sn(SCH₂CF₃)₄], ¹¹² [Sn(SPh)₄], ¹¹³ [Sn(SCH₂CH₂S)₂], ¹¹⁴ [Sn(SePh)₄] ¹¹⁵ and [Sn(Sepy)₄], ⁹⁷ on pyrolysis yield SnE or SnE₂ depending on the nature of the precursor. The thiolate complexes, [Sn(SR)₄] (R = CH₂CF₃, Ph) and [Sn(SCH₂CH₂S)₂] require H₂S as an additional source and yield different tin sulfides depending on the decomposition temperatures. ^{112 , 113 , 114} The complexes [Sn(SR)₄] give SnS₂ at 300–450 °C and SnS at 500–600 °C ^{112 , 113} whereas [Sn(SCH₂CH₂S)₂] at 350, 400 and 500 °C produces yellow SnS₂ (spherical particles of diameter 300 nm), brown Sn₂S₃ (needle like growth) and silver-grey SnS (overlapping plates of 2 × 2 µm), respectively thin films on glass substrates under AACVD conditions. ¹¹⁴ Penta- and hexa-coordinated tin(IV) selenolate complexes, [SnCl₂{(SeCH₂CH₂)₂NMe}] and [Sn{(SeCH₂CH₂)₂NMe}₂], respectively decompose in four steps to result in the formation of orthorhombic SnSe via SnSe₂ intermediate generated at lower temperatures. The bis complex either on thermolysis at 350 °C or on microwave-assisted thermolysis in N-methyl-2-pyrrolidone at 300 °C yields hexagonal plates of diameter ranging from 7.5 to 20 μm of SnSe₂. ¹¹⁶

Pyrolysis of [Sn(Sepy)₄] in a sealed tube at 300 °C gives SnSe₂ and py₂Se, ⁹⁷ whereas [Sn(SePh)₄] on thermolysis at 300 °C produced NCs of SnSe. ¹¹⁵ Similarly bis(chalcogenolates), [Sn(EPh)₂{N(SiMe₃)₂}₂], obtained by oxidative addition of Ph₂E₂ on stannylene, [Sn{N(SiMe₃)₂}₂], undergo single step decomposition (TGA) and on thermolysis in OLA at 210 °C yields nanocrystals of SnE (E = S, Se, Te). The size of NCs is influenced by duration of thermolysis. ¹¹⁷ For instance, the size of SnSe nanocrystals can vary from 60–90 nm, 0.5–1.0 µm and 250–500 nm, respectively for 20, 40 and 120 min. Reid and coworkers examined a series of butyltin chalcogenolates, [Buⁿ ₃Sn(EBuⁿ)] (E = S, Se, Te) and [Buⁿ ₂Sn(SBuⁿ)₂] as molecular precursors for growing thin films of SnE on silica plates by LPCVD techniques. ^{118 , 119} The complexes, [Buⁿ ₃Sn(EBuⁿ)] (E = Se, Te) and [Buⁿ ₂Sn(SBuⁿ)₂] produced orthorhombic SnE (E = S or Se) and cubic SnTe films, but [Buⁿ ₃Sn(SBuⁿ)] gave sulfur deficient SnS films. Cubic SnTe films were found to be p-type semiconductor. Their temperature dependent thermoelectric performance revealed peak Seebeck coefficient of 78 µVK⁻¹ and power factor of 8.3 µWK⁻² cm⁻¹ at 342 °C. ¹¹⁹

Authors’ group ^{120 , 121 , 122 , 123 , 124 , 125 , 126 , 127 , 128 , 129} has developed diorganotin(IV) complexes derived from N-heterocyclic chalcogen ligands (Scheme 5) as molecular precursors. The complexes have been examined by NMR spectroscopy and single crystal X-ray diffraction analyses. These complexes have been successfully used for the synthesis of phase pure tin chalcogenides (SnE and SnE₂) either in NC form or for growing thin films without any requirement of additional chalcogen source.

Scheme 5:

N-Heterocyclic chalcogenolate ligands (E = S or Se).

A number of diorganotin 2-pyridyl/2-pyrimidylthiolates, [R₂Sn(Spy)₂] (R = Me, Et, Bu^t), [R₂Sn(Cl)(Spy)] (R = Et, Bu^t) and [R₂Sn(Cl)(SpymMe₂)] (R = Me, Et, Bu^t) on thermolysis through heat-up approach in OLA at 240 °C yields hexagonal nanosheets (thickness of 30–80 nm) (e.g. from Et₂Sn(Spy)₂]) and rectangular sheets (301 × 390 × 58 nm) (e.g. from [Et₂Sn(Cl)(SpymMe₂)]) of orthorhombic SnS with direct band gap of 1.61–1.90 eV. ^{124 , 129} Morphology and crystallite size of nanosheets of SnS produced from [Bu^t ₂Sn(SpymMe₂)₂] in high boiling coordinating solvents (oleyl amine, octadecene, dodecane thiol), can be controlled by the reaction solvent. ¹²² For instance, SEM and TEM images of SnS prepared in OAm (Figure 7a and b) depicts rectangular nanosheets while those produced in DDT reveal a mixture of rectangular and hexagonal sheets (Figure 7d and e). On contrary, SnS obtained in ODE exhibit irregular nanosheets (Figure 7g and h). The interplanar spacing depicted in the respective HRTEM images point to orthorhombic SnS (Figure 7c, f and i). The thickness of these SnS nanosheets vary depending on the precursor employed in the thermolysis. For instance, the thickness of SnS nanosheets was around 50 nm in the case of [^tBu₂Sn(Spy)₂] while dimensions of nanoplatelets obtained by thermolysis of [^tBu₂SnCl(Spy)] is larger due to relatively faster growth kinetics of nanoplatelet formation. ¹²⁹ These precursors have also been utilized for the deposition of orthorhombic SnS thin films on glass and silicon substrates at 400 °C through AACVD. Microscopic images unveiled randomly oriented rectangular blocks and hierarchical flowers on the glass and silicon substrates. The morphological difference of SnS with respect to nature of the substrate has been ascribed to the orientation and interaction of the substrate atoms with the droplets of the precursor approaching the substrate surface. ¹²⁴ Similarly, thermolysis of [^tBu₂Sn(Spyz)₂] (HSpyz = 2-pyrazinyl thiol) in OLA at 300 °C gave rectangular sheets (size ∼30 nm) of orthorhombic SnS. ¹²⁸

Figure 7:

SEM, TEM and HRTEM images of orthorhombic SnS nanostructures synthesized by the thermolysis of [^tBu₂Sn(SpymMe₂)₂] at 250 °C for 10 min in (a–c) OAm, (d–f) DDT and (g–i) ODE, respectively (Reproduced with permission from Elsevier ¹²² ).

Diorganotin 2-pyridyl, 3-methyl-2-pyridyl and 5-methyl-2-pyridyl, 4,6-dimethyl-2-pyridyl and 4,6-dimethyl-2-pyrimidyl selenolates undergo clean thermolysis in OLA to give different morphologies of orthorhombic SnSe (e.g. from [Me₂Sn(Sepy)₂], [Me₂Sn(SepyMe₂)₂]) or hexagonal SnSe₂ (e.g. from [R₂Sn(SepymMe₂)₂]) nanoparticles depending on the nature of precursor. ^{121 , 123 , 125 , 126} For example, different nanostructures of tin selenide can be prepared by [^tBu₂Sn{SeC₅H₃(Me-5)N}₂] on injecting in different solvent systems (ODE, OLA, OA or in 1:1 (v/v) ODE:OLA and OA:OLA). ¹²⁶ Thermolysis of [^tBu₂Sn{SeC₅H₃(Me-5)N}₂] in ODE and OLA yield hexagonal sheets of hexagonal phase SnSe₂ and irregular sheets of orthorhombic SnSe (average thickness 18 and 12 nm), respectively. While in OA and in solvent mixtures (ODE:OLA and OA:OLA), a mixture of sheets of different shapes could be obtained. The nanoflakes of thickness as tiny as 10 nm are formed in OA:OLA mixture which has been attributed due to stronger binding of the oleate group. The complex, [Bu₂ ^tSn(Sepy)₂] has been employed to deposit films of orthorhombic SnSe on glass and p-type silicon(100) substrates at substrate temperature 490 °C. Similarly photo-responsive nanosheets of SnSe₂ can be deposited from [Bu₂ ^tSn(SepyMe-5)₂]. ¹²⁶ The complex [Bu₂ ^tSn(SepymMe₂)₂] has been engaged to coat SnSe₂ thin films on silicon wafers at 375 °C by AACVD. The direct band gaps of as deposited and annealed thin films are 2.12 and 2.06 eV, respectively. ¹²⁵ The optical band gaps (direct band gaps for SnSe 1.68–1.78 eV ¹²¹ and for SnSe₂ 1.76–2.30 eV ¹²⁵ ) of these nanostructures are blue shifted with reference to bulk material indicative of quantum confinement.

1,1′-Dithiolate complexes of tin(IV) are yet another family of precursors employed for the synthesis of tin sulfide NCs and for thin films deposition. ^{130 , 131 , 132 , 133 , 134} Heteroleptic dithiocarbamates, [Sn(SR)₂(S₂CNEt₂)₂] (R = CH₂CF₃, c-Hex) on heating undergo reductive elimination of disulfide (R₂S₂) with concomitant formation of tin(II) complex, [Sn(S₂CNEt₂)₂] which after subsequent loss of (Et₂NCS)₂S yields SnS. ¹³¹ In contrast, tetrakis derivative, [Sn(S₂CNEt₂)₄] takes a different decomposition path. ¹³¹ The complex starts decomposing either on leaving in CHCl₃/CH₂Cl₂ at RT or on heating at 200 °C to a dimeric sulfido-bridged complex, [Sn(µ-S)(S₂CNEt₂)₂]₂, which on further heating to 375 °C leads to the formation of hexagonal SnS₂. ¹³¹ Organotin dithiocarbamates, [Me₃Sn(S₂CNMeBuⁿ)] and [BuⁿSn(S₂CNMeBuⁿ)₃], on the other hand, requires H₂S as additional sulfur source when used for growing thin films of SnS. ¹³² In the absence of H₂S gas no films could be grown. ¹³² In contrast, dibutyltin dithiocarbamates, [Bu₂Sn(S₂CNRR’)₂] (R/R’ = Et/Et; Me/Buⁿ; Buⁿ/Buⁿ; Me/n-Hex), in the absence of H₂S gas, could produce orthorhombic SnS films on glass substrate in the temperature range 400–530 °C by AACVD method. ¹³⁴ The films consisted of sheet-like crystallites and the presence of a small amount of SnO₂. The nature of precursor and deposition temperature greatly influenced the band gap of the films which varied in the range 1.21–1.71 eV and the photo-sensitivity varied from 0.4 to 2.1 % for films deposited at 500 °C. ¹³⁴ Thermolysis of dibutyltin bis(piperidine dithiocarbamate), [Bu₂Sn{S₂CN(CH₂)₅}₂] in OLA at 230 °C yields nanosheets (998 × 465 nm) of phase pure orthorhombic SnS. ¹³³ Like dibutyltin dithiocabamates, diphenyltin xanthates, [Ph₂Sn(S₂COR)₂] (R = CH₂CH₂OMe, Buⁱ) behave similarly. ¹³⁰ SnS films deposited using these precursors on glass substrates in 400–575 °C range by AACVD were contaminated with some SnO₂. ¹³⁰ Thio-/seleno-benzoate complexes, [Bu₂Sn(ECOPh)₂] (E = S or Se) have been used for the preparation of orthorhombic SnE. ^{135 , 136} OLA capped nanosheets of SnSe have been prepared by hot injection method by injecting TOP dispersion of selenobenzoate complex to preheated OLA at 200 °C. Thin films of SnSe could also be grown on a glass substrate using a THF solution of the complex by AACVD. Uniform deposition of crystalline films takes place in the temperature range 375–475 °C. ¹³⁵

Organotin chalcogenides have also been investigated as SSMPs for tin chalcogenides. Boudjouk and coworkers employed diorganotin chalcogenides, [(R₂SnE)₃] (R = Ph or Bz; E = S or Se) ^{94 , 137} and bis(triorganotin) chalcogenides, (R₃Sn)₂E (R = Ph or Bz; E = S, Se, Te) ^{94 , 138} as molecular precursors for the synthesis of SnE. Pyrolysis of [(R₂SnE)₃] under an inert atmosphere yields NCs of phase pure orthorhombic SnE (E = S or Se). ^{94 , 137} Pyrolysis of (Ph₃Sn)₂E under flowing nitrogen in a tube furnace at above 330 °C yields a dark grey powder of micro-crystalline orthorhombic phase of SnE (E = S or Se) and cubic SnTe. ¹³⁸ In contrast, the corresponding benzyltin derivatives, (Bz₃Sn)₂E (E = S or Se) yield SnE contaminated with some elemental tin. ⁹⁴ Flow pyrolysis of structurally characterized dibenzyltin telluride, [(Bz₂SnTe)₃] at 200–275 °C affords phase pure cubic SnTe nanocrystals of size ∼1 µm. ¹³⁹ Chalcogenido-bridged diorganotin complexes, [Sn(µ-E){CH(SiMe₃)₂}₂]₂ (E = S, Se, Te) (5) (Scheme 6), obtained by oxidative addition of chalcogen on tin(II) compound [Sn{CH(SiMe₃)₂}₂], are thermally quite stable. ^{140 , 141} Only tellurido-bridged complexes undergo decomposition and could be used for growing SnTe films on gold seeding layers at 400 °C. ¹⁴⁰ Similarly other tin(IV) chalcogenide complexes (6–8) (Scheme 6) were obtained by oxidative addition of chalcogen on tin(II) complexes. ^{86 , 104 , 142} The complexes 6 can also be prepared by salt metathesis reaction between [Sn{(N-c-Hex)₂CNMe₂}₂Br₂] and Li₂E ⁸⁶ which on thermolysis in OLA at 210 °C yields nanocrystals of SnE. ⁸⁶

Scheme 6:

Some representative tin(IV) precursors used for the synthesis of tin chalcogenides.

Reid and coworkers employed Lewis acid-base adducts of the type trans-[SnCl₄(ER₂)₂] (E = S or Se; R = Et or Buⁿ) and cis-[SnCl₄(RE^∩ER)] (E = S or Se; RE^∩ER = BuⁿE(CH₂)_nEBuⁿ (n = 2 or 3); o-(MeECH₂)₂C₆H₄) for deposition of tin chalcogenide thin films of various thickness on different substrates (Si; SiO₂; TiN) using LPCVD methods. ^{143 , 144 , 145} Depending on precursor and decomposition temperatures, films of different tin chalcogenides could be grown. Complexes with chelating ligands decompose at higher temperatures with respect to those containing monodentate ligands. ^{144 , 145} In general, at lower deposition temperatures SnE₂ is preferentially formed whereas at higher temperatures films of SnE are formed. For instance, cis-[SnCl₄(BuⁿE(CH₂)₃EBuⁿ)] at 286 and 480–500 °C gave SnS₂ and SnSe₂, respectively, but at 558 and 588 °C produced SnS and SnSe. ¹⁴⁴ Boscher et al. instead of Lewis acid-base adducts, employed dual source – SnCl₄ and Et₂Se for growing thin films of SnSe and SnSe₂ on glass substrate under LPCVD conditions. ¹⁴⁶ SnSe films (100 nm thick) could be deposited at 650 °C whereas SnSe₂ films of 10–80 μm size adherent crystals were formed at 600-650 °C. ¹⁴⁶ Similarly, the dual source approach using SnCl₄ and H₂S, has been used to deposit different tin sulfides depending on the deposition temperatures. Thus, films of hexagonal SnS₂, orthorhombic Sn₂S₃ mixed with some SnS₂ and orthorhombic SnS could be deposited on glass substrate by APCVD method at 500, 525 and 545 °C, respectively. ¹⁴⁷ The use of dual source precursors for growing thin films can be traced back to 1970s when Manasevit and Simpson deposited SnE (E = S, Se, Te) films on various substrates using tetramethyl tin and H₂S/H₂Se/Me₂Te under a flow of hydrogen at 550, 500 and 650 °C, respectively. ¹⁴⁸ Besides chalcogeno ether adducts, adducts of other chalcogen ligands, like thiosemicarbazone (e.g., [Bz₃Sn(Cl){S=C(NH₂)NH–N(=CHAr)}], where Ar = 2-HOC₆H₄; 4-ClC₆H₄), ¹⁴⁹ N,N-dimethyl selenourea (e.g., [SnCl₄{Se=C(NMe₂)NH₂}₂]), ⁸⁷ have also been explored as templates for tin chalcogenides. The selenourea complex, [SnCl₄{Se=C(NMe₂)NH₂}₂] undergoes a two-step decomposition. It yields SnSe₂ at 400 °C (from TGA), but thermolysis either in OA or in an OA-OLA mixture at 350 °C produces nano-flakes of a mixture of SnSe and SnSe₂. ⁸⁷

3.2.1 SnE_xE’_y materials

The optical band gap and thermal conductivity of binary tin chalcogenides can be further fine-tuned in anion-alloyed ternary systems for their thin-film based solar-cell and thermoelectric applications. These materials can be generated by direct heating of stoichiometric quantities of constituent elements, ¹⁵⁰ by heat-up method ¹⁵¹ or by molecular precursor route. ^{94 , 152 , 153}

Organotin chalcogen compounds have been effectively probed as precursors for the synthesis of anion-alloyed materials. Pyrolysis of a mixture of [Bz₂SnS]₃ and [Bz₂SnSe]₃ in different molar ratios at 450 °C afforded ternary tin chalcogenides, SnS_0·75Se_0.25, SnS_0·5Se_0.5 and SnS_0·25Se_0.75, respectively as plate-shaped (∼4 μm) NCs. ⁹⁴ The entire range of SnS_1−xSe_x has been prepared by thermolysis of bis(thiobenzoato)dibutyltin(IV) and bis(selenobenzoato)dibutyltin(IV) in an appropriate molar ratios in OLA at 230 °C. A steady variation in the band gap on going from SnS (1.48 eV) to SnSe (1.1 eV) has been observed as revealed by UV-Vis-NIR analysis. ¹³⁶

Oxidative addition of chalcogen on organotin(II) chalcogenides, (RSn)₂X yields (RSnE)₂X (R = 2,6-(Me₂NCH₂)₂C₆H₃ ⁻; X = S, Se and E = S, Se, Te) (Scheme 7) ^{152 , 153} which serve as a single source precursors for ternary tin selenides. These compounds contain two terminal Sn = E bonds ¹⁵³ and decompose in multiple steps to give tin chalcogenides. Thermolysis of (RSnSe)₂S yields SnS_0·5Se_0.5 whereas (RSnTe)₂X (X = S or Se) gives only SnTe as the main product. However, SnSe_xTe_1-x (x = 0–0.15) has been prepared by heating pure elemental Sn, Se and Te in stoichiometric amounts in a sealed quartz tube at 900 °C. ¹⁵⁰

Scheme 7:

Oxidative addition of chalcogen on organotin(II) complexes.

The band gap of SnS_xSe_1-x nanocrystals (7–15 nm), obtained by hot injection method employing an ODE–OA solution of SnO treated with the solutions of TOP/Se and OLA/S at 270 °C, can be tuned from 0.92 to 1.24 eV with a linear change of S/(S + Se) and fall within the range of band gaps of SnS (1.6 eV) and SnSe (1.1 eV). ¹⁵⁴ The band gap energy can effectively be modulated in a wide range of 0.96–2.26 eV in SnS_xSe_1-x, Sn(S_xSe_1-x)₂, and Sn₂S₃ NCs (Figure 8). ¹⁵⁵ Thin films of semiconducting SnSSe material have been deposited by spin-coating methods at varying spinning speeds. For instance, amorphous films of Sn₄₂S₄₁Se₁₇ with an optical band gap of 1.79 eV are deposited at 1,500 rpm. ¹⁵³ Thin films deposited using a hydrazine solution of SnS₂, SnSe₂ and sulfur followed by annealing at 270 °C had the composition SnS_2-xSe_X. ¹⁵¹ These films exhibited n-type transport properties with large current densities greater than 10⁵ A cm⁻² and mobilities higher than 10 cm² V⁻¹ s⁻¹.

Figure 8:

Schematic representation of band gap energy of SnS_xSe_1-x, Sn(S_xSe_1-x)₂, and Sn₂S₃ NCs (reproduced with permission from Royal Society of Chemistry ¹⁵⁵ ).

3.2.2 Sn_1-x-IV_x-E materials

A number of cation-alloyed ternary IV–VI group semiconductors have been synthesized by several methods. Earlier methods relied on direct heating of stoichiometric quantities of constituent elements in sealed ampules (such as Sn_xGe_1-xSe (0 ≤ x ≤ 1)) at high temperatures, ¹⁵⁶ but the current methods employ solution techniques. Nanocrystals of Sn_xGe_1-xSe (0 ≤ x ≤ 1) have been prepared by injecting a hexamethyldisilazane solution of Bu^t ₂Se₂ into hot (95 °C) dodecyl amine solution of GeI₄ and SnI₄ mixture in a desired ratio and heating further at 225 °C. ¹⁵⁷ Powder X-ray diffraction patterns of Sn_xGe_1-xSe (0 ≤ x ≤ 1) point out orthorhombic Pmna crystal structure for these nanocrystals (Figure 9). The indirect band gap energy of these NCs varied in the range 0.87–1.13 eV as a function of x = 1.0–0.2. Nanosheets of SnGeS₃ have been isolated by thermolysis of xanthate complexes, [M(S₂COPrⁱ)₄] (M = Ge and Sn). These photo-responsive nanosheets exhibit a band gap of 1.59 eV as measured from diffuse reflectance spectroscopy (DRS). ¹⁵⁸

Figure 9:

XRD patterns of Sn_xGe_1−xSe nanocrystals with different compositions. (Reproduced with permission from American Chemical Society ¹⁵⁷ ).

Ternary Pb_1-xSn_xE compounds are narrow band gap semiconductor materials. They exist in a cubic rock salt crystal structure. Nanocrystals of Pb_1-xSn_xS (av. size 8 nm) have been isolated by solvothermal route. In this method an OLA solution of sulfur is swiftly added to a hot solution of a mixture of stannous oxide and lead oxide in ODE and OA and heated further at 270 °C. The band gap (1.57 (when x = 0.54) to 1.96 eV (when x = 0.85)) varied nearly linearly with the gradual rise of Pb/(Pb + Sn) ratio. ¹⁵⁹ A series of Pb_1-xSn_xTe nanocrystals (∼7.5 nm in size) have been synthesized by the reaction of lead oleate and [(Me₃Si)₂N]₂Sn mixture with a TOP solution of tellurium at 150 °C. These NCs unveil band gaps in the mid-IR region. TEM images of the pristine Pb_1-xSn_xTe NCs are nearly spherical with the average particle size varying between 6.0 and 8.7 nm irrespective of all compositions (Figure 10). Here, Pb_0·86Sn_0·14Te and Pb_0·14Sn_0·86Te NCs are monodispersed and spherical in shape while the morphology of Pb_0·5Sn_0·5Te NCs varies from spherical to oval and are polydispersed in nature. The change in morphology of NCs has been attributed due to the different binding affinity of Pb and Sn precursors with oleic acid and amine moieties. ¹⁶⁰

Figure 10:

TEM images of the (a) Pb_0·86Sn_0·14Te, (b) Pb_0·8Sn_0·2Te, (c) Pb_0·5Sn_0·5Te, (d) Pb_0·33Sn_0·67Te, (e) Pb_0·2Sn_0·8Te and (f) Pb_0·14Sn_0·86Te NCs. (Reproduced with permission from American Chemical Society ¹⁶⁰ ).

3.2.3 M_xSn_yE_z materials

Various synthetic methods for the NCs of Cu₂SnE₃ (E = S, ^{56 , 161 , 162 , 163} Se ^{67 , 164 , 165 , 166 , 167 , 168 , 169 , 170} ) have been reported and include heat-up, hot-injection, solvothermal/hydrothermal routes. Reactions in these synthetic routes are carried out in different solvents, like OLA, ODE, OA, HDA, TOPO, using a range of copper (CuCl, CuI, CuCl₂, Cu(acac)₂) and tin (SnCl₂, SnBr₂, SnCl₄, Sn(acac)₂, Sn(OAc)₂) precursors and a chalcogen source (S, Se, SeO₂, DDT, thio-/seleno-urea, Bu^t ₂Se₂, Ph₂Se₂). ¹⁷¹ Several factors like metal precursor reactivity, nature of chalcogen source, reaction medium, and the reaction temperature, have a strong impact on the phase of the NCs. The use of different sulfur sources can give different phases. NCs of Cu₂SnS₃ synthesized by hot-injection route using an ODE solution of CuCl and SnCl₂ on treatment with OLA solution of sulfur at 240 °C affords the zinc blende phase, whereas treatment with dibutyldithiocarbamic acid (DBDCA) in ODE with a suitable capping agent (DDT, TOPO, OA) results in the formation of wurtzite phase. ¹⁷² Formation of different phases of Cu₂SnS₃ NCs in the reaction of CuI, Sn(OAc)₂ with DDT could be managed by proper selection of solvent. The use of ODE at 220 °C results in the formation of zinc blende phase whereas in OLA at 190 °C wurtzite phase of NCs is formed. ¹⁷³

Hot injection method has been frequently used for the preparation of Cu₂SnSe₃ NCs. Wang et al. from the reaction of CuCl, SnCl₂ in oleyl amine with Ph₂Se₂ at 310 °C isolated NCs of Cu₂SnSe₃ as tetrapods consisting of cubic core with four wurtzite arms. ^{67 , 170} Brutchey and co-workers ¹⁶⁹ synthesized NCs (15.1 ± 2.9 nm) of wurtzite phase by treatment of a dodecyl amine-DDT solution of CuCl and SnI₄ with di-tert-butyl-diselenide (Bu^t ₂Se₂). In these reactions, different phases could be conveniently controlled by suitable choice of capping agent and reaction temperatures. ^{169 , 170} Different crystal phases, cubic, zinc blende, hexagonal, wurtzite and monoclinic with varied morphologies of CTSe could be isolated by slight variation in the reaction parameters. ¹⁷⁴ The wurtzite/monoclinic phase is typically formed at lower temperatures but on increasing the temperature the cubic phase is obtained. ^{174 , 175}

Authors’ group employed organotin precursors for the synthesis of Cu₂SnE₃ (E = S or Se) (Figure 11). ^{127 , 128} Thermolysis of [Bu^t ₂Sn(Spyz)₂] and [Cu(Spyz)(PPh₃)₂] (HSpyz = 2-mercapto pyrazine) in 1:2 ratio in OLA at 300 °C yields nano-crystals (∼22 nm) of phase pure monoclinic Cu₂SnS₃. However, thermolysis either at lower temperatures or in a mixture of OLA-OA gave monoclinic CTS slightly contaminated with binary sulfides. ¹²⁸ The band gaps of Cu₂SnS₃ nanostructures (1.4 eV) are blue shifted relative to bulk counterparts. Similarly, co-thermolysis of [Cu(SepyMe-3)]₄ and [Me₂Sn(SepyMe-3)₂] in 1:2 M ratio in OLA or TOPO at 300 °C produced cubic Cu₂SnSe₃ nano-crystals (11 ± 1 nm). These NCs could be isolated in flower-shaped, dish-like, nano-sheets, polyhedra morphologies by subtle variation in the reaction conditions. For instance, SEM of Cu₂SnSe₃ isolated by hot injection of [Me₂Sn(SepyMe-3)₂] and [Cu(SepyMe-3)]₄ in OLA at 300 °C revealed polyhedra shape of NCs (Figure 12). ¹²⁷

Figure 11:

Synthesis of Cu₂SnE₃ (E = S or Se) NCs via organometallic precursor route (structures are drawn using Mercury Program (Mercury: Visualization and analysis of crystal structures). ^{127 , 128 , 176}

Figure 12:

SEM image of Cu₂SnSe₃ polyhedra obtained by hot injection of [Me₂Sn(SepyMe-3)₂] and [Cu(SepyMe-3)]₄ in OLA at 300 °C. (Reproduced with permission from Springer Nature Publishers ¹²⁷ ).

O’Brien and coworkers deposited thin films of cubic Cu₂SnSe₃ using a THF solution of Cu(acac)₂ and [Sn(Se₂PPh₂)₂] in 2:1 stoichiometric ratio by AACVD at 400 and 450 °C. Films grown at 400 °C consisted of nanocrystalline flakes of CTSe while those deposited at 450 °C consisted of semi-spherical crystallites. ¹⁰⁸

Nanocrystals of Cu₃SnS₄ have been isolated by hot injection and solvothermal methods. Orthorhombic Cu₃SnS₄ NCs (31.8 ± 4.9 nm) and nanosheets (15 nm thick with a lateral dimension of 209 ± 33 nm) have been prepared by hot-injection route employing Cu(acac)₂, SnCl₄, followed by either seeding with monoclinic Cu₃₁S₁₆ NCs or by adding DDT. ¹⁷⁷

ZnSnS₃ is another interesting material. It exists in three closely related phases, viz. non-polar monoclinic and ilmenite and a polar lithium niobate (LN) form. The latter with a band gap of 1.28 eV exhibits ferroelectric behaviour. ¹⁷⁸ Being a material with a narrow band gap, it is highly effective in countering the major limitation of traditional ferroelectric PV devices which employ oxide materials that have large band gaps (>3 eV). The films of LN-ZnSnS₃ and LN-ZnSnS₃@rGO have been prepared by hydrothermal method using ZnSn(OH)₆ and thiourea. ^{179 , 180}

The compound In₄SnSe₄ (band gap = 1.6 eV) is another narrow band gap ternary system. Nanowires (length 5–20 µm and width 100–400 nm) of In₄SnSe₄ have been prepared by heat-up method using SnCl₂, InCl₃, and Ph₂Se₂ in a mixture of ODE and OLA containing excess hexamethyldisilazane at 300 °C. ¹⁸¹

3.3.1 Synthesis of Cu₂ZnSnE₄ (E = S or Se)

Although there are several synthetic approaches to prepare Cu₂ZnSnS₄ (CZTS), to isolate high-quality stoichiometric materials over a large area remains a challenging task. Synthetic strategies employed for the preparation of CZTS include hydrothermal/solvothermal, ^{183 , 184} hot-injection method, ^{60 , 66 , 185 , 186} heating-up method, ultrasound-assisted microwave, ¹⁸⁷ etc., hot-injection method being the most common technique. In these synthetic methods stoichiometric quantities of metal salts of copper (CuCl₂·2H₂O, Cu(OAc)₂.H₂O, Cu(acac)₂), zinc (ZnCl₂, Zn(OAc)₂.2H₂O, Zn(acac)₂) and tin (SnCl₂·2H₂O, SnCl₄·5H₂O, Sn(OAC)₄) are dissolved in a suitable solvent (ethylene glycol, OLA, ODE, TOPO, monoethanolamine, etc.) and then treated with a sulfur source like elemental sulfur in OLA, thiourea, or thiols (DDT, tert-dodecane thiol). Alternatively, metal and the sulfur source can be substituted by metal complexes derived from sulfur ligands like diethyldithiocarbamate ^{1 , 188 , 189 , 190 , 191} and xanthates. ¹⁹² Diethyldithiocarbamate complexes, [Cu(S₂CNEt₂)₂], [Zn(S₂CNEt₂)₂] and [Sn(S₂CNEt₂)₄] on thermolysis in oleyl amine/trioctyl amine have been successfully used to prepare NCs of CZTS. ^{1 , 190 , 191} Capping agents like OA, ^{1 , 193} hexadecane thiol, ¹⁹⁰ are quite often employed to stabilize the NCs. For instance, Lu et al. generated wurtzite CZTS nano-prisms of size of 20 × 28 nm and nanoplates of thickness 14 nm in DDT/OLA and DDT/OA, respectively (Figure 13). Nanostructured thin films of CZTS have been grown by air spraying a THF solution of Cu(S₂CNEt₂)₂, [Zn(S₂CNEt₂)₂] and [Sn(^tBu)₂(S₂CNEt₂)₂] in 2: 1: 1 M ratio on a heated (450 °C) glass substrate under an argon atmosphere. ¹⁸⁸ Similarly, CZTS thin films have been deposited by AACVD on a glass substrate using a toluene solution of Cu(S₂CNEt₂)₂, [Zn(S₂CNEt₂)₂] and [Bu₂Sn(S₂CNEt₂)₂] in 2: 1: 1 M ratio. ¹⁸⁹ Thin films of CZTS were grown on a glass substrate by spin coating technique using a THF solution of [(Ph₃P)₂Cu(S₂COEt)], [Zn(S₂COⁿBu)₂] and [Sn(S₂COEt)₂] in 2: 1: 1 M ratio and finally heating in a furnace at an appropriate temperature. The films annealed at temperatures <375 °C produced hexagonal phase whereas annealing between 375 and 475 °C produced tetragonal material. ¹⁹² Copper zinc tin selenide (Cu₂ZnSnSe₄ (CZTSe) has also been prepared by different methods. Hot-injection route using metal salts in an amine (e.g., HDA) and trioctylphosphine selenide (TOPSe) was first reported. ¹⁹⁴ Polycrystalline NCs of size 25–30 nm of CZTSe have been prepared by hot-injection method at 280 °C employing CuCl₂, ZnCl₂, SnCl₄ and (Et₃Si)₂Se in ODE and OA mixture. ¹⁹⁵ The samples prepared by different routes generally show compositional inhomogeneities. ¹⁷¹ Fuhrmann and coworkers synthesized Cu₂ZnSnE₄ by co-thermolysis of [(tmeda)Zn(µ-E)₂{(SnR₂)₂E}] (R = Me, Bu^t or Ph; E = S or Se) with [{(Prⁱ ₃P)Cu}₂(ECH₂CH₂E)]. ¹⁹⁶

Figure 13:

TEM images of wurtzite CZTS a) nano-prisms and b) nanoplates prepared in DDT/OLA and DDT/OA, respectively (reproduced with permission from Royal Society of Chemistry ¹⁹³ ).

Depending on reaction temperature and the nature of sulfur source, different crystallographic forms can be isolated ^{197 , 198} whereas solvent can contribute to the final shape of CZTS nanoparticles. ¹⁹⁹ The reaction between metal salts and sulfur powder in an appropriate solvent at high temperatures generally yields nanocrystals of kesterite CZTS. ¹⁹⁵ Owing to strong coordinating ability, thiols as a sulfur source, on the other hand, allow the isolation of metastable phases. Thus, the reaction between metal salts and alkanethiols, such as DDT ^{186 , 193} or even the thermolysis of metal dithiocarbamate complexes in the presence of thiols (hexadecane thiol) ¹⁹⁰ results in the creation of wurtzite CZTS. Thiols also serve as an effective passivating agent for the wurtzite facets and help in decreasing the overall size of the NCs. Another metastable phase–orthorhombic CZTS, could be prepared via hydrothermal method employing stoichiometric quantities of metal chlorides and thiocarbamide dissolved in a mixture of water-ethylenediamine, followed by heating at 200 °C. Orthorhombic CZTS undergoes phase transition on annealing at 500 °C to the tetragonal kesterite structure. ⁶⁵ The films deposited by thermal degradation of spin-coated CZTS nanoparticles ink gave different CZTS phases depending on the annealing temperature. Annealing at 200 °C produced a mixture of cubic and hexagonal phases, at 350 °C only cubic CZTS phase was formed, while at 500 °C the tetragonal kesterite structure existed. ⁶⁶

3.3.2 Synthesis of Cu₂MSnS₄ (M = Mn, Fe, Co, Ni)

There are several protocols to prepare nanocrystals of first-row transition metal ions incorporated compounds, Cu₂MSnS₄ (M = Mn, Fe, Co, Ni). These include hydrothermal/solvothermal, ^{200 , 201 , 202 , 203} hot injection, ²⁰⁴ spray pyrolysis, ²⁰⁵ direct spin-coating or spin-coating of nanoparticles/sol–gel, ^{206 , 207 , 208 , 209} electrodeposition, ²¹⁰ etc. In these preparative methods stoichiometric quantities of metal salts of copper (CuCl, CuCl₂·2H₂O, Cu(NO₃)₂·6H₂O, Cu(OAc)₂.H₂O, Cu(acac)₂), tin (SnCl₂·2H₂O, SnCl₄·5H₂O) and transition metal salts (MnCl₂·4H₂O, Mn(OAc)₂.4H₂O/FeCl₃·6H₂O, FeSO₄·7H₂O, Fe(acac)₂/CoCl₂·6H₂O/NiCl₂·2H₂O, Ni(NO₃)₂·6H₂O) either as an aqueous solution or in an organic solvent (2-methoxy ethanol, OLA, or organic acids like acetic/citric acid) is treated with a sulfur source like thiourea, Na₂S, Na₂S₂O₃·5H₂O, elemental sulfur in OLA. Quite often traces of secondary phases (binary and ternary) are also formed. Cui et al. ²¹¹ described a general solvothermal method in which a hexylamine solution of CuCl, SnCl₂ and MCl₂/(NO₃)₂ is treated with a mixture of carbon disulfide and 3-mercaptopropionic acid, followed by heating in an autoclave. The method is employed for the preparation of nanocrystals of these compounds in both zinc blende and wurtzite phases. ²¹¹

Tuneable crystal phases can be prepared by subtle variation in the reaction conditions. ^{212 , 213} NCs with the stannite structure of Cu₂CoSnS₄ (band gap 1.58 eV) have been prepared by solvothermal methods, ^{214 , 215} while the wurtzite structure could be synthesised by a hot-injection route. ²¹⁶ On annealing the NCs of wurtzite phase at 400 °C led to the conversion in pure stannite structure. The structure of Cu₂MSnS₄ can also be modified by employing a suitable sulfur source in the reaction. For instance, the use of thiourea as a sulfur source in hydrothermal synthesis of Cu₂MnSnS₄ results in the formation of microspheres of a mixture of hexagonal and tetragonal forms, whereas use of Na₂S in place of thiourea in the reaction afforded fine crystallites of tetragonal phase. ²¹⁷

Transition metal incorporated ternary materials show magnetic behaviour. Nanocrystals of Cu₂MnSnS₄ and Cu₂NiSnS₄ display superparamagnetic behaviour at low temperature, whereas NCs of Cu₂FeSnS₄ and Cu₂CoSnS₄ exhibit ferromagnetic behaviour. ²¹¹ Cu₂MnSnS₄ (band gap 1.1 eV) shows feeble ferromagnetic behaviour at 2 K. ²¹⁸

4.3.1 Photo-electrocatalysts for water splitting

Photo-electrochemical hydrogen evolution reaction (HER) for generation of hydrogen by splitting of water is another important reaction where layered metal chalcogenides have been explored as catalysts. HER involving post transition metal chalcogenides has been reviewed recently. ⁷ HER is a two-step process. It involves the reduction of proton and desorption of H₂ molecules from the electrode surface. Both mono and dichalcogenides of tin, SnE and SnE₂ (E = S or Se) have been examined as catalysts for HER. ⁷ Density functional theory (DFT) calculations have revealed that SnS₂ indicated stronger HER electrocatalytic behaviour than SnS. ²⁵²

Water splitting by single-layers of SnS₂ deposited on indium-tin-oxide (ITO) coated glass has been investigated by photoelectrochemical (PEC) measurements in a 0.5 M Na₂SO₄ electrolyte using a three-electrode setup. ²⁵³ The single-layers of SnS₂ could produce photocurrent density up to 2.75 mA cm⁻² at 1.0 V versus Ag/AgCl, which is 70 times higher than the bulk material. They showed conversion efficiency of 38.7 % under visible light. Lattice mismatched layered SnS₂/SnS superstructures show photocatalytic activity for H₂ generation under blue light (450 nm) irradiation with H₂ evolution rates of ∼ 100 μmol h⁻¹, but there was negligible evolution of H₂ under green light (500 nm) irradiation. ²⁵⁴ Orthorhombic SnSe/FTO films, grown from [Bu₂Sn(SeCOPh)₂], show photoelectrochemical (PEC) behaviour. The films show bifunctional behaviour, i.e., can be utilized for both HER and oxygen evolution reaction (OER) by swapping the applied potential. ¹³⁵ At 0 V, hydrogen evolution was noted but at 0.1 V the electrode switched to OER. ¹³⁵

4.3.2 Photocatalytic activity

Binary, ternary and quaternary tin chalcogenides show photocatalytic activity under solar irradiation. A typical photocatalysis mechanism for binary metal chalcogenides is shown in Figure 19. SnS quantum dots (2.5–3.0 nm size), produced by co-precipitation method using SnCl₂·2H₂O and thiourea in ethylene glycol, act as a catalyst for photodegradation of eosin yellow and brilliant green dyes under sunlight. ²²² Superoxide ions formed during solar illumination are responsible for photodegradation of these dyes.

Figure 19:

Photocatalysis mechanism for binary metal chalcogenides.

Visible-light-driven photocatalytic activity of Cu₂SnE₃ compounds has been investigated in photo-degradation of organic dyes. ^{163 , 255 , 256} Nanosheets of Cu₂SnS₃ (band gap ∼ 2.42 eV) ¹⁶³ and nanoflakes of Cu₂SnSe₃ (band gap ∼1.14 eV) ²⁵⁶ with rough surfaces show photo-catalytic activity in photo-degradation reaction of methylene blue (MB). The latter could degrade 73 % of the MB dye in 3 h of light irradiation. The Cu₂SnS₃@rGO (3 %) nanocomposite unveils superior photocatalytic activity than that of pure Cu₂SnS₃ in photo-degradation of eosin under visible-light. ²⁵⁵ The improved photocatalytic activity of Cu₂SnS₃@rGO has been ascribed to the creation of distinct interface between constituents of the composite facilitating charge carrier separation in the Cu₂SnS₃.

The photocatalytic activity of CZTS has also been investigated through dye degradation and hydrogen evolution reaction. ²⁵⁷ CZTS exhibited photocatalytic activity through photodegradation of Congo red azo dye ¹⁸⁴ and RhB ²⁵⁸ under simulated solar irradiation. The catalyst could degrade 90.2 % Congo red azo dye in just 60 min and could also be reused repeatedly for dye degradation. ²⁰¹

Nano-crystals of Cu₂MSnS₄ (M = Fe or Co), synthesized by hydrothermal route, exhibited photocatalytic activity, NCs with rough surfaces being more active. ^{201 , 256} The Cu₂FeSnS₄ nanoflakes could degrade almost 74 % of methyl red dye within 3 h of light illumination, ²⁵⁶ whereas the nanocrystals of Cu₂CoSnS₄ (size 26–40 nm) photo-catalytically degraded methylene blue dye (78 %) and crystal violet dye (84 %). ²⁰¹