The in situ synthesis of PbS nanocrystals from lead(II) n-octylxanthate within a 1,3-diisopropenylbenzene–bisphenol A dimethacrylate sulfur copolymer

The synthesis of lead sulfide nanocrystals within a solution processable sulfur ‘inverse vulcanization’ polymer thin film matrix was achieved from the in situ thermal decomposition of lead(II) n-octylxanthate, [Pb(S2COOct)2]. The growth of nanocrystals within polymer thin films from single-source precursors offers a faster route to networks of nanocrystals within polymers when compared with ex situ routes. The ‘inverse vulcanization’ sulfur polymer described herein contains a hybrid linker system which demonstrates high solubility in organic solvents, allowing solution processing of the sulfur-based polymer, ideal for the formation of thin films. The process of nanocrystal synthesis within sulfur films was optimized by observing nanocrystal formation by X-ray photoelectron spectroscopy and X-ray diffraction. Examination of the film morphology by scanning electron microscopy showed that beyond a certain precursor concentration the nanocrystals formed were not only within the film but also on the surface suggesting a loading limit within the polymer. We envisage this material could be used as the basis of a new generation of materials where solution processed sulfur polymers act as an alternative to traditional polymers.

The synthesis of lead sulfide nanocrystals within a solution processable sulfur 'inverse vulcanization' polymer thin film matrix was achieved from the in situ thermal decomposition of lead(II) n-octylxanthate, [Pb(S 2 COOct) 2 ]. The growth of nanocrystals within polymer thin films from single-source precursors offers a faster route to networks of nanocrystals within polymers when compared with ex situ routes. The 'inverse vulcanization' sulfur polymer described herein contains a hybrid linker system which demonstrates high solubility in organic solvents, allowing solution processing of the sulfur-based polymer, ideal for the formation of thin films. The process of nanocrystal synthesis within sulfur films was optimized by observing nanocrystal formation by X-ray photoelectron spectroscopy and X-ray diffraction. Examination of the film morphology by scanning electron microscopy showed that beyond a certain precursor concentration the nanocrystals formed were not only within the film but also on the surface suggesting a loading limit within the polymer. We envisage this material could be used as the basis of a new generation of materials where solution processed sulfur polymers act as an alternative to traditional polymers.

Introduction
The in situ growth of nanomaterials from single-source precursors in polymers offers a potentially shorter route to thin films of bi-continuous phases of inorganic nanocrystals and polymers, by growing the nanocrystals in the polymer rather than synthesizing them ex situ followed by incorporation into the polymer. The technique involves the dissolution of a precursor for the inorganic nanocrystals and a polymer in a mutual solvent, before coating a desired substrate [1]. The substrate is then heated causing the decomposition of the precursor to given nanocrystals in the polymer film (scheme 1). This has the advantage over ex situ routes as surface ligands, a necessity for colloidal stability, can be avoided thus improving contact between the particles and polymer in the film, a particular advantage when producing bulk heterojunction photovoltaics [1]. The wide range of single-source precursors and polymers available make the growth of nanomaterials directly within polymers an attractive route for the synthesis of composite thin films and has been used for PbS [2], CdS [3][4][5][6][7], Bi 2 S 3 [8], CuInS 2 [9,10], Sb 2 S 3 [11] in photovoltaic and thermoelectric devices, e.g. PbS in semiconducting polymers for photovoltaics. Potential applications for nanocrystal-containing thin films include: solar cells [9,10,[12][13][14][15], catalysis [16][17][18], absorptive optical filters [19,20], sensors [21][22][23][24][25] and antimicrobial surfaces [26][27][28][29].
The use of sulfur as a feedstock for polymeric materials has arisen due to the abundance of elemental sulfur, with approximately 60 million tons produced annually [49][50][51]. Sulfur polymers are known as 'inverse vulcanization' materials due to the reaction of elemental sulfur at elevated temperatures with divinylic species [52][53][54], forming sulfur chains cross-linked with organic linkers, in contrast with traditional vulcanization used to strengthen rubber where poly(isoprene) chains are cross-linked with di-sulfide bridges. These polymeric materials have potential as battery materials [55,56], infrared lenses [57,58] and as effective filters for heavy metal ions [59][60][61] due to their ease of manufacture, chemical inertness and optical translucence. These materials are worth investigating due to the large quantity of sulfur waste created from petroleum refining, thus alleviating pressure on traditional polymer feedstocks, namely crude oil.
We present the synthesis of PbS nanocrystals in a solution processable sulfur polymer from the in situ decomposition of lead(II) n-octylxanthate, [Pb(S 2 COOct) 2 ]. This new material has the potential to replace conventional polymer-nanocrystal materials by taking advantage of elemental sulfur as an alternative feedstock. The nanocrystal-polymer thin films have potential as absorptive optical filters, or colour gels, where the traditionally used polycarbonate or polyester has been replaced with a solution processable sulfur polymer. By variation of the ratio of polymer to precursor prior to spin coating, decomposition temperature and polymer composition, the influence on film morphology has been explored. The suitability of the sulfur polymer for thin film formation was optimized through the addition of bisphenol A dimethacrylate to the 1,3-diisopropenylbenzene linker, which raised the melting and decomposition temperatures as well as the hardness of the polymer, a key advantage for the formation of robust polymer-nanocrystal thin films [62].
Solvents were purchased from Sigma-Aldrich Ltd and used as received. Deionized (DI) water with a resistivity of not less than 18.2 MΩ cm −1 (Millipore, UK) was used for aqueous solutions.  2.2.2. Synthesis of poly(sulfur-co-1,3-diisopropenylbenzene), 50 wt% ratio of linker (1) Poly(sulfur-co-1,3-diisopropenylbenzene) was prepared according to Chung et al. [62]. Briefly, elemental sulfur (S 8 , 2 g, 7.81 mmol) was added to a glass vial and heated to 185°C in a graphite bath under vigorous stirring. Once 185°C was reached, 1,3-diisopropenylbenzene (2.16 ml, 12.6 mmol) was injected, the whole mixture agitated with a glass rod and stirred for 4-5 min. The temperature was then raised to 200°C and the polymer was allowed to cure at that temperature for 30 min.

Synthesis of poly(sulfur-co-bisphenol A dimethacrylate) (2)
Poly(sulfur-co-bisphenol A dimethacrylate) was synthesized using the procedure for (1), except that bisphenol A dimethacrylate (4.59 g, 12.6 mmol) was used instead of 1,3-diisopropenylbenzene, and vigorous mechanical stirring was required to ensure full mixing of the monomer and the molten sulfur.

Synthesis of poly(sulfur-co-linker)-lead sulfide thin films
Poly(sulfur-co-linker)-lead sulfide thin films were synthesized using a protocol outlined by Lewis et al. [2]. Briefly, poly(sulfur-co-bisphenol A dimethacrylate-co-1,3-diisopropenylbenzene) (85 mg) (4) was dissolved in chloroform (2.5 ml) by heating and swirling at 50°C for an hour. [Pb(S 2 COOct) 2 ] (60 mg, 0.1 mmol) was then added, resulting in a dark brown solution. Glass slides (20 × 15 × 1 mm) were cleaned by sonication in 2-propanol, and dried in air. A total of 100 µl of the lead(II) n-octylxanthate-polymer solution was coated onto the glass slide by spin coating at 1250 r.p.m. for 20 s followed by 4000 r.p.m. for 10 s. The films were loaded into a quartz tube for heating and placed under N 2 using standard Schlenk techniques. The tube was inserted into the furnace preheated at 150°C and held at the chosen temperature for the specified time. Once finished, the tube was removed from the furnace and allowed to cool in air taking approximately 3 min.

Instrumentation
High-resolution transmission electron microscopy (HRTEM) measurements were conducted using an FEI Tecnai G20 TEM with a LaB 6 source operating at an acceleration voltage of 200 kV. EDS spectra were taken with an Oxford XMax 80 TLE. TEM samples were prepared dissolving the films in 1,2dichlorobenzene followed by drop casting. UV/visible absorption spectra were recorded using a Shimadzu UV-1800 instrument over a range of 350-1000 nm. X-ray photoelectron spectroscopy (XPS) was undertaken using a Thermo Scientific K-alpha spectrometer with monochromated Al Kα radiation, a dual beam charge compensation system and constant pass energy of 50 eV (spot size 400 µm). Survey scans were collected in the range of 0-1200 eV. High-resolution peaks were used for the principal peaks of S (2p), O (1 s) and C (1 s) and fitted with CASA XPS software. Elemental analysis, thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) and temperature cycling DSC were performed by the University of Manchester Microanalytical Laboratory on a Mettler Toledo TGA/DSC 1 Stare System (between 30°C and 1000°C) and a Perkin Elmer-Jade DSC (between 0°C and 200°C), respectively. Grazing incidence X-ray diffraction (XRD) patterns were acquired using a Bruker D8 Advance diffractometer, using a Cu Kα (λ = 1.5418 Å) source with a Göbel mirror optic and Soller slits on the detector side. Scans were acquired at a grazing incidence of 3°over a 2θ range of 10−80°w ith 0.02°steps and 3 s per step. Scanning electron microscopy (SEM) images were acquired using an FEI Quanta 650 SEM using an accelerating voltage of 20 kV.

Solution processable sulfur polymer synthesis and characterization
Bisphenol A dimethacrylate was reacted with sulfur in conjunction with 1,3-diisopropenylbenzene in an 'inverse vulcanization' process to create a sulfur-based polymer that was more robust with a higher melting point and decomposition temperature than pure poly(sulfur-co-1,3-diisopropenylbenzene), shown in scheme 2. This combination of cross-linkers results in the increased solubility in organic solvents due to the formation of a suspected hyperbranched structure allowing solution processing [63]. Derivatives of bisphenol A are a common component of epoxy resins, acting as the cross-linking polymer backbone to hardeners such as aliphatic amines in the epoxy blend [64]. It was found that molten sulfur and bisphenol A dimethacrylate exhibit poor miscibility, with constant, strong mechanical agitation required to form the polymer. The miscibility improved upon addition of 1,3-diisopropenylbenzene to the sulfur/bisphenol A dimethacylate mixture.
On heating, yellow sulfur S 8 rings melt between 120°C and 124°C [65,66] before the critical temperature of approximately 159°C, the λ-transition, at which the specific heat capacity, viscosity, density and thermal expansion all change dramatically. The changes are due to the ring opening of S 8 , forming long-chain oligomeric short and long [-S-S-] x chains with radical end groups [62]. This is accompanied by a darkening in colour to red potentially due to organic impurities within the sulfur [62,65]. The radical end-groups react with divinylic species of the bisphenol A dimethacrylate     (2), (c) poly(sulfur-co-bisphenol A dimethacrylate-co-1,3-diisopropenylbenzene), higher ratio of 1,3-diisopropenylbenzene (3) and (d) poly(sulfur-co-bisphenol A dimethacrylate-co-1,3-diisopropenylbenzene), higher ratio of bisphenol A dimethacrylate (4).
TGA and DSC were used to examine the thermal properties of the copolymers and showed that the addition of the bisphenol A increases the onset of decomposition for the polymer (figure 1). For example, the melting point range (from capillary melting point apparatus) of pure poly(sulfur-co-1,3diisopropenylbenzene) (1) at 50 wt% organic content is 15°C-35°C compared to that of 190°C-245°C (starts to decompose) for pure poly(sulfur-co-bisphenol A dimethacrylate) (2). Composite polymers (3) and (4) were shown to have intermediate melting temperatures (180°C-200°C for (3) and 205°C-225°C for (4)), commensurate with the molar ratios of 1,3-diisopropenylbenzene to bisphenol A dimethacrylate.
Transition temperatures were determined from the DSC traces in electronic supplementary material, figure S4. There is a large decomposition step present in all of the polymer traces: onset 228.5°C for polymer (1), 250.7°C for polymer (2), 263.5°C for polymer (3) and 247.4°C for polymer (4) commensurate with elemental sulfur leaching out of the structure and carbonization [61] as well as direct evidence that the presence of bisphenol A dimethacrylate contributes to the thermal stability of the polymer. Owing to the presence of this decomposition at temperatures less than 200°C, DSC thermal cycles were undertaken between 0°C and 200°C at a heating rate of 10°C min −1 before cooling to 0°C at 10°C min −1 before undertaking a second heating cycle. Polymer (1) showed a small feature at 32.2°C which was assigned as the T g of the polymer, and is comparable with the T g observed for the 50 wt% 1,3-diisopropenylbenzene sulfur polymer by Pyun et al. [62] of 28.4°C (see electronic supplementary material, figures S5 and S6). The DSC curves for polymers (2), (3) and (4) showed no T g peaks on the heating or cooling curves, and only small melting transitions observed on the heating curves. There were no corresponding peaks on the cooling curves for polymers (3) and (4), indicating the properties of the material do not change with temperature until the onset of decomposition (see electronic supplementary material, figures S7, S8 and S9).
Powder XRD of polymers (1)- (4) showed no reflections indicating that the polymers are amorphous and that the polymerization has gone to completion as no residual S 6 or S 8 is present (see electronic supplementary material, figure S10).
The solubility of a polymer is of vital importance for applications that require solution processing. Bear et al. [50] reported that higher sulfur content poly(sulfur-co-1,3-diisopropenylbenzene) was insoluble in all organic solvents except for very limited solubility in 1,2-dichlorobenzene. When linker content is near 50 wt%, solubility in chlorinated solvents increases, making chloroform an ideal solvent for dissolution of the polymer and allows solution processing with [Pb(S 2 COOct) 2 ]. Work by Pyun et al. indicated that the reactor in which the 'inverse vulcanization' polymer was synthesized played an important role in the solubility of the 1,3-diisopropenylbenzne-sulfur polymers [57]. Other groups have published examples of 'inverse vulcanization' polymers with solubility in a wider range of organic solvents using different linkers including: divinylbenzene [67], allyl functionalized benzoxazine [68], S-farnesol [69] and S-limonene [60]. In this work, polymers (1) and (3) exhibited solubility in a wide range of organic solvents including: n-hexane, 1,2-dichlorobenzene, chloroform, toluene and dichloromethane. Polymers (2) and (4) dissolved partially, leaving a transparent, insoluble loose residue in an otherwise clear orange solution. These polymers had a high weight fraction of bisphenol A dimethacrylate, and potentially difficulties in obtaining a homogeneous reaction mixture may have contributed to poor incorporation of bisphenol A dimethacrylate causing the partial solubility.
Polymer (1), however, has a T g close to room temperature [62] and is very tacky, which limits the use for formation of robust thin films. Upon heating (1) did not withstand the elevated temperatures required for the rapid decomposition of [Pb(S 2 COOct) 2 ]. The only PbS nanocrystal-polymer film created using (1) was annealed at 70°C for 72 h, yielding a film with irregular substrate coverage (see electronic supplementary material, figure S11). Therefore, polymer (3) was used for temperature and concentration studies in PbS nanocrystal-polymer film formation.

Polymer thin film synthesis and characterization
The influence of breakdown temperature was investigated by fixing the S-polymer:[Pb(S 2 COOct) 2 ] mass ratio at 6:1 (90 mg or 0.15 mmol of xanthate). Four identical films and a polymer control of (3) were heated at 50°C, 100°C and 150°C for 15 min. The effect of varying the [Pb(S 2 COOct) 2 ] concentration was also investigated, with 0.05, 0.1, 0.15 and 0.2 mmol of precursor heated at 150°C for 15 min.

Variable temperature study
A series of decomposition temperatures (namely 25°C (room temperature), 50°C, 100°C and 150°C) were investigated to ensure the complete breakdown of precursor within a fixed time, i.e. 15 min, and the progress of the reaction was followed using XPS and XRD. XPS for all heated films demonstrated characteristic peaks for lead(II) sulfide, with added sulfur environments attributed to the sulfur polymer, vide infra. The valence spectra of the variable temperature samples showed a change in the Fermi level from the precursor-in-polymer control to the 150°C sample, which is characteristic of lead sulfide, figure 2d [70].
The Pb 4f XPS spectrum, figure 2c, was typical of the composite films heated at 150°C, regardless of the ratio of [Pb(S 2 COOct) 2 ] to polymer. The envelope exhibited two environments, with the main region at 137.3 eV (Pb4f 7/2 ) assigned as PbS (galena) [71][72][73][74][75] and the second region (138.6 eV) as a lead sulfate, PbSO x [76]. With increasing temperature, the binding energy of the Pb4f 7/2 region decreases: 138.0 eV (50°C), 137.7 eV (100°C) and 137.3 eV (150°C). This is due to the decomposition of a greater proportion of [Pb(S 2 COOct) 2 ] at higher temperatures and the resulting change in the overall Pb chemical environment.
The S2p region of the XPS spectra with variable temperature allows monitoring of the decomposition of [Pb(S 2 COOct) 2 ] at the different heating temperatures (see electronic supplementary material, figure S12). Electronic supplementary material, figure S12 shows S2p scans from sulfur in the native polymer (S2p 3/2 , 167.6 eV) and scans from the different temperatures which show the emergence of another feature from the formation of PbS (160.2 eV). A fitted example of an S2p region, electronic supplementary material, figure S13, after heating at 150°C, contains fitted regions for PbS (160.8 eV), elemental sulfur (S 8       decomposition. In all cases, XRD patterns yielded reflections for PbS (see electronic supplementary material, figure S14), and identical XPS spectra to those obtained for the variable temperature study at 150°C. EDS analysis was also consistent, showing the presence of PbS (see electronic supplementary material, figure S15). Backscatter electron (BSE) SEM was used to examine the morphology of the networks of PbS nanocrystals formed within the sulfur polymer films ( figure 4). Use of the BSE allows for greater Z contrast and a deeper sampling region, ideal for the examination of Pb-containing nanocrystals within a polymer film.  figure S11). The SEM images show that the nanocrystals are incorporated within the S-polymer in the same manner as when grown in polystyrene according to our previous report [2].

Variable concentration study
TEM of PbS nanocrystals extracted from the sulfur polymer films followed by dissolution in chloroform showed well-defined cubes with occasional slight elongation forming irregular

Optical characterization
UV/visible spectra of the films allowed observation of the concentration of nanocrystals within the films upon variation of [Pb(S 2 COOct) 2 ]:S-polymer (polymer (4)) concentration ratios (figure 7). All films exhibited broad absorption over the entire visible range, which is typical of PbS (see electronic supplementary material, figure S17). The broad absorption increased between samples of polymer (3), an unreacted film of polymer (3)

Conclusion
Films of a solution processable sulfur polymer containing networks of PbS nanocrystals have been produced via spin coating of sulfur polymers with lead(II) n-octylxanthate, [Pb(S 2 COOct) 2 ], and the subsequent heating of the films. The solubility of the sulfur polymer was optimized for solution processing by the addition of bisphenol A dimethacrylate and 1,3-diisopropenylbenzene to molten elemental sulfur in an 'inverse vulcanization' process to form poly(sulfur-co-bisphenol A dimethacrylateco-1,3-diisopropenylbenzene). Upon heating films of sulfur polymer and lead(II) n-octylxanthate, PbS nanocrystals were found to form within the sulfur polymer film. Monitoring the formation of PbS with XPS and XRD showed the optimum temperature for the breakdown of [Pb(S 2 COOct) 2 ] was 150°C. The influence of the ratio of sulfur polymer to [Pb(S 2 COOct) 2 ] on the morphology of the films showed that a limit of PbS nanocrystals within the polymer was passed when 0.05 mmol of [Pb(S 2 COOct) 2 ] was added resulting in a 79.8 wt% film of PbS in polymer. Above this value ejection of PbS nanocrystals from the film occurred causing formation of nanocrystals on the surface. The edge length and monodispersity of the PbS nanocrystals showed a minimum when 0.1 mmol of [Pb(S 2 COOct) 2 ] was used. The thin films produced allow the growth of PbS nanocrystals within the S-polymer distributed in the film similar to our previous report when using lead(II) alkylxanthates and polystyrene. The development of a solution processable process to thin films of sulfur-based polymers containing highly optically absorbent inorganic nanocrystals demonstrates the potential for the use of sulfur-based polymers as an alternative to traditional polymers for absorbent optical filters, or colour gels.