(176b) Solvothermal Route to the Synthesis of Iron Sulfide Nanomaterials

Recently, nanomaterials have employed as the new building blocks to construct solar energy harvesting devices. Of particular interest are the size dependent properties such as size quantization effects in semiconductor nanomaterials. Many semiconductor nanomaterials have been developed for solar cells application [1]. However, the use of toxic elements such as cadmium or lead remains a limitation as the environmental issues need to be addressed and these elements are not abundant [2]. Herein, we proposed a cheap, non-toxic and abundant semiconductor nanomaterials-pyrite nanowires, which has an indirect energy transition at 0.95 eV, a direct transition at 1.03 eV and an integrated absorption coefficient of 3.3×10⁵ cm^-1 for the energy spectrum of wavelength values (λ) between 300 and 750 nm [3]. With a high absorption coefficient from visible to near-infrared region, material consumption is significantly reduced in semiconductors for thin layer solar cells compared to conventional silicon-based solar cells. Pyrite has previously been prepared using several high temperature approaches including sulfurization of iron films, sulfurization of iron oxide films, reactive sputtering, and spray pyrolysis, yet at elevated temperatures, segregation of iron and sulfur species is unavoidable, which could change the stoichiometry and material phase of the deposited film. Thus, solvothermal process is a prefered method used for producing pyrite nanocrystals [4]. Despite the recent advancement of solvothermal routes to the synthesis of pyrite FeS₂ nanocrystals, to obtain pure pyrite phase and controllable shape and size of pyrite nanocrystals still remain challenges. In this work, we have synthesized iron sulfur nanowires, microflowers and microplates by using FeCl₃∙6H₂O, FeCl₂ ∙ 4H₂O, Fe(NO₃)₃ ∙ 9H₂O and FeSO₄ ∙ 7H₂O as the iron source and thiourea, sulfur and Na₂S₂O₃ as the sulfur source. The solvent used is limited to ethylenediamine and ethylenediamine+ethyleneglycol mixture. Scanning electron microscope (SEM) was used to determine the morphology of the materials and energy dispersive x-ray analysis (EDAX) was used to determine the chemical composition of these materials. By tuning the concentration, we can obtain the length of wires ranging from 100-500 nanometers to 1-10 micrometers with 100-200 nm in diameter. X-ray photoelectron spectroscopy (XPS) results indicate that these materials are iron monosulfide along with some oxidized products. These wire-like materials are not well-crystallized which can be seen from high-resolution transmission electron microscopy (HRTEM) images. We found temperature is also an important factor in controlling the morphology of the iron sulfur materials. Microplates can be obtained at high temperature (210 ͦ C). Only wires can be obtained at 180 ͦ C . We also found that the counter-ion and valence state of the iron source play an important role in determining the morphology and phase of the final products. By changing the solvent from ethylenediamine to water, significant change in the phase of the product was observed (Pyrite along with macarsite), which is verified by X-Ray Diffraction (XRD). Further characterization of the magnetic and electronic property of these iron sulfur materials is carrying out.

References

[1] Kamat, P.V. J. Phys. Chem. C 2008, 112, 18737–18753.

[2] Alivisatos, A.P. et al. Chem. Mater. 2009, 21, 2568–2570.

[3] Chen, C.W. et al. Nanotechnology, 2009, 20, 405207-405212.

[4] Chaudhuri, S. et al. Chem. Phys. Lett. 2004, 398, 22–26.