Direct synthesis of nanostructured silver antimony sulfide powders from metal xanthate precursors

Silver(I) ethylxanthate [AgS2COEt] (1) and antimony(III) ethylxanthate [Sb(S2COEt)3] (2) have been synthesised, characterised and used as precursors for the preparation of AgSbS2 powders and thin films using a solvent-free melt method and spin coating technique, respectively. The as-synthesized AgSbS2 powders were characterized by powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The crystalline AgSbS2 powder was investigated using XRD, which shows that AgSbS2 has cuboargyrite as the dominant phase, which was also confirmed by Raman spectroscopy. SEM was also used to study the morphology of the resulting material which is potentially nanostructured. EDX spectra gives a clear indication of the presence of silver (Ag), antimony (Sb) and sulfur (S) in material, suggesting that decomposition is clean and produces high quality AgSbS2 crystalline powder, which is consistent with the XRD and Raman data. Electronic properties of AgSbS2 thin films deposited by spin coating show a p-type conductivity with measured carrier mobility of 81 cm2 V−1 s−1 and carrier concentration of 1.9 × 1015 cm−3. The findings of this study reveal a new bottom-up route to these compounds, which have potential application as absorber layers in solar cells.

A variety of techniques have been established for the formation of AgSbS 2 thin films including thermal evaporation 39 , pulsed-laser deposition 40 , RF-magnetron sputtering 41 and laser ablation 42 . For each of the above techniques the starting material is prepared by direct fusion of stoichiometric quantities of the elements, which can be problematic due to the formation of sub phases caused by inequivalent ion migration rates in the solid state. The use of metal xanthate precursors, however, may circumvent this problem as the mixing prior to thermal decomposition occurs at the nanoscale and hence final products should be homogeneous and of a single crystalline phase, with the bottom up nature of the process allowing for exquisite control of elemental constitution. Due to the pre-formed bonds between metal and chalcogenide atoms, metal xanthates can act as efficient precursors for the formation of solid state metal sulfides. This has led to the extensive application of, for instance, xanthate complexes for the production of thin films 43,44 . Advantages conferred by this sort of synthetic route include the ability to carry out low temperature decomposition, the ease of synthesis and stability of the resulting compound in air, along with the fact that by-products for these materials are generally gaseous. O'Brien & Lewis have reported a number of such syntheses for a range of main group and transition metal sulfides [45][46][47][48][49][50] .
In this paper, we describe a metal xanthate precursor route to produce ternary silver antimony sulfide (AgSbS 2 ) as a single well-defined phase via thermal decomposition of metal xanthate precursors in stoichiometric ratios. AgSbS 2 is rarely found in nature but possesses potentially excellent properties for solar cell applications 51,52 .

Thermogravimetric analysis of [AgS 2 COEt] (1) and [Sb(S 2 COEt) 3 ] (2) complexes.
Thermogravimetric analysis of (1) and (2) was performed in the temperature range of 0 °C to 550 °C under a nitrogen atmosphere. Both complexes exhibited a large mass loss between 80 and 250 °C (Fig. 2). The decomposition of (1) started at 96 °C and ended at 177 °C with the remaining weight determined to be 54%, which is matching the calculated value of 54%. Both experimental and theoretical values confirmed the phase of Ag 2 S. In a similar manner, the TGA profile of (2) exhibits the main decomposition step between 100 and 161 °C. The final residue www.nature.com/scientificreports/ of 35% is in good agreement with the calculated value of 35% which confirms the formation of Sb 2 S 3 . The minor decomposition step with < 3% mass loss is attributed to loss of sulfur which was also observed by Alqhatani et al 54 . TGA of mixtures of the two complexes ( Fig. 2) shows a single step decomposition at ca. 200 °C with a remaining weight of 41% which corresponds to the formation of AgSbS 2 . This low temperature decomposition of the complexes to produce AgSbS 2 means that it could potentially be produced within polymer matrices and can be used as an absorber layers in polymer nanocrystal based hybrid solar cells [55][56][57] . O'Brien et al. has reported the preparation of PbS nanocrystals in polymer matrix via decomposition of lead(II) xanthates in polystyrene matrices as a potential absorber material for flexible hybrid photovoltaic devices 58 . The mixtures of the solid precursors form a homogenous molten intermediate reactive melt, when the temperature increased. Before undergoing decomposition to form the final solid products. The volatile organic components are evacuated through the constant nitrogen flow 53,59,60 . The mechanism of xanthate decomposition follows a Chugaev elimination reaction which involves the production of a cyclic transition state to produce carbonyl sulfide molecules (OCS) and alkenic side products (Scheme 1) 61,62 . Alanazi et al. has previously reported the synthesis of stannite Cu 2 FeSnS 4 (CFTS) quaternary chalcogenides from mixtures of metal (O-ethylxanthato) (M = Cu, Fe and Sn) complexes, which shows that combining xanthate precursors in tandem in reactive melts is a promising approach to these materials 63 .
Raman spectroscopy was conducted on the AgSbS 2 powder produced at 500 °C (Fig. 5). Raman resonances are observed at 80.2, 115.4, 185.9, 249.2, 368.9 and 448.1 cm −1 respectively, and the spectral positions of these peaks agree with those reported previously for AgSbS 2 68 . The electrical properties of AgSbS 2 films were measured using the Van Der Pauw method. Silver paste was used to form the four contacts on 7 × 7mm 2 sample area. The     63 . Hall effect measurements revealed that the films exhibit p-type conductivity. Secondary electron scanning electron microscopy (SEM) was used to interrogate the surface morphologies of the powders produced at different temperatures. Cubic structures are revealed for powders produced at 400 °C which changed to a porous appearance when the temperature of the synthesis was increased to 450 °C. When the temperature was increased to 500 °C, the morphology changed to flakes as shows in Fig. 6 and ESI (Fig. S2.6). Influence of the increasing temperature on the crystal structure has been reported by Habe et al. 69 . The AgSbS 2 powders prepared at 400, 450 and 500 °C were analysed using energy-dispersive X-ray (EDX). EDX mapping gives information on the spatial distribution of elements at the micro to nanoscale and to ensure that the distribution of elements is homogeneous. Representative elemental mapping (Fig. 6) of these components showed a uniform distribution of the silver, antimony and sulfur. EDX spectra show that the samples consist only of the elements silver, antimony and sulfur, suggesting that decomposition is clean and produces high quality crystalline materials which is consistent with the XRD and Raman data from the same materials (see supporting information for EDX sum spectra Figs. S2.7 to S2.11).

Conclusions
A novel, efficient and low temperature method for the synthesis of AgSbS 2 powders has been demonstrated. Silver(I) ethylxanthate [AgS 2 COEt] (1) and antimony(III) ethylxanthate [Sb(S 2 COEt) 3 ] (2) precursors have been used to produce crystalline powders of AgSbS 2 with a high degree of atom efficiency. Ternary cubic AgSbS 2 (cuboargyrite) was successfully produced which was evidenced by XRD and Raman spectroscopy. XRD data shows that crystallite size increase with increasing synthesis temperature. SEM images show a change in the surface morphology of these powders from cubic crystallites to flakes upon increasing the synthesis temperature. EDX mapping gives a clear indication of the presence of spatially co-localised Ag, Sb and S with no other elemental impurities. Use of solvents can be avoided altogether through the melt method which has great potential for the mass production of nanocrystalline powders of ternary materials.

Synthesis of AgSbS 2 powders.
A homogenised mixture of [AgS 2 COEt] (1) and [Sb(S 2 COEt) 3 ] (2) complexes (1:1 mol ratio) was placed in a ceramic boat that was subsequently placed in the centre of a glass tube which was then inserted into a Carbolite tube furnace. One end of the glass tube was directly connected to nitrogen gas through a Schlenk line in the fume hood, and the other end of the tube was carefully sealed with a rubber septum. A vacuum was used to remove any oxygen from the glass tube, and the glass tube was then refilled with nitrogen gas. After that the mixture was heated in the Carbolite furnace at 400 °C, 450 °C and 500 °C, respectively and kept it at each temperature for 1 h under nitrogen atmosphere to produce AgSbS 2 powders. The final product was collected for further analysis after the system was slowly cooled to room temperature. In addition, AgSbS 2 thin films were also deposited by spin coating technique using the same complexes, as per the synthesis of AgSbS 2 powders. Full details of thin film deposition and characterisation are presented in Sect. 3 of ESI.

Materials characterisation.
A Specac single reflectance ATR instrument (4000-400 cm −1 ) with resolution 4 cm −1 was used to record the infrared spectra (IR). Melting points of the complexes were obtained using a Barloworld SMP10. 13 C NMR spectra were obtained using a Bruker AC400 FT-NMR spectrometer. Elemental analysis was performed with a Carlo Erba EA 1108 instrument. Thermogravimetric analysis (TGA), was performed using a Seiko SSC/S200 at a heating rate of 10 °C min −1 under nitrogen. Powder X-ray diffraction (XRD) measurements were carried out by a Bruker Xpert diffractometer, utilising Cu-Ka radiation (1.5406 Å). Raman spectra were recorded using a Renishaw 1000 microscope system equipped with laser excitation of 514 nm. Scanning electron microscopy (SEM) images were obtained using a Tescan SC Oxford SEM. Electrical properties of the thin films were measured using the Van Der Pauw method by means of a custom-build Hall effect measurement system. www.nature.com/scientificreports/