发表文章毕业论文201502-Yuan binxia-New J Chem-One-step synthesis of pure pyrite FeS2 with different morphaologies in water

This journal isThe Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015New J. Chem., 2015, 39, 3571-3577 |3571 Cite this: NewJ.Chem., 2015, 39, 3571 One-step synthesis of pure pyrite FeS2with diff erent morphologies in water Binxia Yuan,aWeiling Luan,*bShan-tung Tuband Jiang Wua In this study, pyrite FeS2with novel nanostructures had been obtained via an ethanolamine (ETA)water binary system. This method provided a uniform and homogenous environment for the nucleation and growth of FeS2. When only pure water was used as the reaction solvent, marcasite FeS2with a hollow sphere structure was achieved, and then transformed into the pyrite phase with a rod-like structure piled up by nanoparticles with the increase of the reaction time. Moreover, cubic, flake-like, and two types of tetrakaidecahedron struc- tures were obtained via the adjustment of the volume ratio of ETA and H2O. The transformation mechanism from flower-like amorphous to different types of morphologies of pyrite FeS2crystals was analyzed through the time-dependent controlled experiments. The Raman spectra of the samples with different morphologies were investigated, which were consistent with the XRD analysis. The studies of optical properties indicate that the morphologies had a great influence on the absorption properties. This study provided a very simple and low cost method to control the morphologies of FeS2crystals, which would be of great potential for the synthesis of other metal chalcogenides and lay the foundation for the development of solar cells. Introduction Over the past years, there has been considerable interest in the synthesis and characterization of transition-metal chalcogenides owing to their excellent optical properties and wide applications in optical cells and devices.14In particular, iron pyrite (FeS2) is significantly attractive in both cost and availability compared with other compounds.5Pyrite FeS2is applied as a photovoltaic cell,6lithium batteries as a cathode material,7and hydrogen productionasa depolarizer anode.8Relatedreports haveproved that pyrite FeS2has a high optical absorption coefficient (4105cm?1) and asuitableband gapenergy(Eg= 0.95eV),9,10indicatingpotential applications in solar cells. The optical properties of pyrite FeS2are highly related with their morphologies. For instance, the team of Alivistatos employed a novel single source molecular precursor to obtain pure pyrite FeS2nanocrystals, showing quasi-cubic morphologies with a size over 100 nm.11The Law group successfully prepared oblate and spheroid single crystals with diameters in the range of 520 nm by a solvothermal process in diphenyl ether media, which estimates the band gap of the NCs to be 0.880.91 eV.12 The Wang group first reported the synthesis of pyrite FeS2with cubic and octahedral shapes via a simple surfactant-assisted ethyleneglycol-mediatedsolvothermalmethod.13Huangs colleagues reported pyrite NCs with a cubic shape using a surfac- tant assisted hot-injection method that yielded pure phase, highly crystalline,and surface stableNCs,indicating thatthedirectoptical band gap was 1.38 eV.14The Kotovhad group obtained FeS2 nanoparticles, nanowires, and nanosheets in polar solvent and aqueous dispersions.15Our group have previously reported the synthesis of pyrite FeS2with cubic shape (about 100 nm) through an oleylamine-mediated solvothermal process, corresponding to a band gap of 1.05 eV.16However, most of the synthetic route mainly depends on the complex molecular precursor or process route. Herein, we adopted the hydrothermal synthesis method in an autoclave, which is comparably simple and low cost. In this paper, we had reported a hydrothermal process in an ethanolamine (ETA)water binary solvent for the synthesis of FeS2crystals without expensive single-source precursors and complex craft. The reaction time and the volume ratio of ETA and H2O played a critical role in the formation of pure pyrite FeS2crystals. Under the conditions of pure water, the pyrite FeS2products were not obtained. However, interestingly, the hollow sphere shape of marcasite FeS2was observed, which was then converted into the rod-like FeS2crystals piled up with nanoparticles with the elongation of the reaction time. Moreover, pyrite crystals with four types of morphologies, such as cubic, flake-like, and two types of tetrakaidecahedron, were also obtained. The formation mechanism of these morphologies had also been studied. Accordingly, the Raman and optical properties were measured in order to study their relationship with their morphologies. aShanghai Engineering Research Center of Power Generation Environment Protection, Shanghai University of Electric Power, Shanghai 200090, P. R. China bThe State Key Laboratory of Safety Science of Pressurized System, School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China. E-mail: luan; Fax: +86 21 6425 3513 Received (in Montpellier, France) 6th December 2014, Accepted 16th February 2015 DOI: 10.1039/c4nj02243b /njc NJC PAPER Published on 16 February 2015. Downloaded by East China University of Science more- over, each Fe atom is coordinated by six sulfurs in a slightly distorted octahedron and each S atom is bonded to three Fe atoms and its dimer pair. To obtain the morphology and size information of the as- prepared FeS2samples, the technique of SEM was employed. The SEM images (Fig. 2) depicted various crystals in diff erent volume ratios of the solvents at 200 1C for 48 h. When the volume ratio of ETA and H2O (sample A in Table 1 and Fig. 2a) was adjusted to 8:1, as shown in Fig. 2a, cubic-like FeS2 particles with a length of 400 nm were clearly observed. By simply decreasing the volume ratio of ETA and H2O to 1:1 (sample B in Table 1 and Fig. 2b), desultorily flake-like struc- tures were obtained, as presented in Fig. 2b. Interestingly, a polyhedron shape was formed when the volume ratio of ETA and H2O was reduced to 1:8 (sample C in Table 1 and Fig. 2c). To further understand the morphology diff erence, it was worth- while to investigate the change in the relative intensity in each crystal lattice. The crystal strength of the diff erent morpholo- gies of FeS2samples obtained under various reaction condi- tions is listed in Table 2. From the table, it can be found that the intensity values of sample A and the theoretical sample in each lattice plane were essentially identical, which means that sample A did not have an orientated growth along the specific crystal. With the increase of H2O content (VETA:VH2O= 1:1), the strongest peak became a (311) crystal plane. This proved that the (311) crystal face was dominant and had more chances to be diff racted, which indicated that sample B grew up along the (311) plane. This result coincided with the SEM images with a flake-like structure. When the volume ratio of ETA and H2O was reduced to 1:8, the position of three strong peaks had no Table 1Summary of the various morphologies of the FeS2samples SamplePhaseVETA:VH2OTemperature (1C)Time (h)Morphology APyrite8:120048Cubic BPyrite1:120048Flake-like CPyrite1:820048Tetrakaidecahedron DMarcasitePure water20024Hollow sphere EPyrite and marcasitePure water20048Rod-like PaperNJC Published on 16 February 2015. Downloaded by East China University of Science (b) sample B, VETA:VH2O= 1:1; (c) sample C, VETA:VH2O= 1:8, with a reaction for 48 h at 200 1C, (B) the crystal structure of pyrite FeS2. Fig. 2SEM images of FeS2 crystals obtained under diff erent conditions: (a) VETA:VH2O= 8:1; (b) VETA:VH2O= 1:1; (c) VETA:VH2O= 1:8, with a reaction for 48 h at 200 1C. Table 2The relative intensities of crystal faces in measurement and theory Crystal face Sample Theoretical modelABC (111)35.041.338.131.0 (200)100.098.2100.0100.0 (210)57.760.666.053.0 (211)53.553.250.440.0 (220)45.760.948.836.0 (311)72.5100.071.269.0 (222)14.816.613.911.0 (023)15.815.518.213.0 (321)8.224.019.316.0 Fig. 3 Evolution of the XRD patterns with diff erent reaction time: (a) 24 h, (b) 48 h, and a reaction temperature of 200 1C. NJCPaper Published on 16 February 2015. Downloaded by East China University of Science (b) 36 h; (c) and (d) 48 h, with synthesis in pure water. Fig. 5Transformation mechanism from the hollow sphere to rod-like morphology: the formation process of marcasite nuclei (1), aggregation of nanocrystallites into a single-hollow sphere (2 and 3), allotropic transfor- mation (4 and 5), and creation of a rod-like via Ostwald ripening (6). PaperNJC Published on 16 February 2015. Downloaded by East China University of Science (c) 12 h; (d) 24 h; (e) 36 h; (f) 48 h. Fig. 7SEM images for the shape-evolution process of FeS2crystals under diff erent reaction time in VETA/VH2O= 1:1 (a) 24 h, (b) 36 h, (c) 48 h. Fig. 8SEM images for the shape-evolution process of FeS2crystals under diff erent reaction time in VETA/VH2O= 1:8 (a) and (b) 24 h, (c) 30 h, (d) 36 h, (e) 48 h. NJCPaper Published on 16 February 2015. Downloaded by East China University of Science & Technology on 08/02/2018 12:59:45. View Article Online 3576| New J. Chem., 2015, 39, 3571-3577This journal isThe Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015 of diff erent sizes. When the reaction time was further increased to 36 h (Fig. 8d), the products consisted of lots of tetrakaide- cahedron structures. When the reaction time was extended to 48 h (Fig. 8e), the morphology of tetrakaidecahedron structure had some changed. On the basis of the experimental results and analyses, the entire formation process of nucleation growth, oriental assembly, and subsequent cubic-deformation was proposed to illustrate the growthmechanism.Attheinitialstageofthehydrothermal reaction, free Fe3+could coordinate withL-cysteine to form a Fecysteine precursor, which then decomposed to FeS2nuclei. Then, the flocculentmorphologieswereimmediatelyobtainedduetotheeffect of water and further cubic-shape FeS2crystals were obtained owing to the effect of ethanolamine. Table 3 showed the relative intensities of FeS2crystals in different crystal planes. When the reaction time was shorter than 30 h, the relative intensities of FeS2crystals in a different crystal panel were close to the theoretical value. Although the shapes of inorganic materials often convey their intrinsic crystal structure nature, crystals can display diversiform morphologies undertheinfluenceofextrinsicenvironmentalfactors.Thisbehavior leads to distinct surface energies, directly resulting in the different growth rates of the crystal face. Herein, the morphology of the FeS2 sample was subtly predominated by the facet-selective adsorption characteristic of ETA, which served as a structure-directing coordi- nate template. Moreover, ETA acted as a phase catalyst in this system.AccordingtotheSEMimages(Fig.8ce),thesizesofsamples remainedunchangedatabout3mmwiththeincreaseofthereaction time. The results indicated that the particle did not grow large but underwent morphology transformation upon prolonging the reac- tion time. When the reaction time was 36 h, the relative intensity of the (111) crystal plane was greater than that of the sample prepared at24hand30h.Itwasindicatedthatthe(111)crystalfacehadmore chance to be diffracted, which meant the phase structure or morphology had changed a little. At 48 h, the intensity of the same plane was decreased but that of (210) and (023) showed a certain degree of increase, indicating a dominant role with the increase of the reaction time. In order to further understand the formation process, the possible schematic is shown in Fig. 9. Raman and optical properties of FeS2. The Raman spectra of the FeS2 samples obtained under diff erent reaction conditions are showed in Fig. 10. In general, high-purity, crystalline FeS2 with a pyrite structure usually exhibits three characteristic Raman peaks at 340353 cm?1(Eg, Tg), 377383 cm?1(Ag, Tg), and 427466 cm?1(Tg).2223The peak at 340 cm?1was Egmode, which corresponded to the sulfur atoms that were displaced perpendicularly to the dimmer axes, the peak at 377 cm?1was of the Agmode, which corresponded to the in-phase stretching vibration of the SS dimmer of pyrite FeS2: moreover, no other peaks were observed. Thus, the Raman results further indicated that samples A, B and C were of the pure pyrite structure without marcasite FeS2or other impurities. On the contrary, the Raman peaks of sample D were located at 323 cm?1and 385 cm?1, indicating a marcasite phase instead of the pyrite phase. While the Raman spectrum of sample E showed three peaks at 323 cm?1, 340 cm?1and 377 cm?1, revealing a mixture of pyrite and marcasite FeS2. Based on the above mentioned analysis, the phase structures of the as-prepared samples were further proved to be in agreement with the XRD analysis. The optical properties of FeS2dispersed in ethylene tetra- chloride (C2Cl4) by sonication were studied by ambient tem- perature absorption spectroscopy from 300 nm to 2500 nm, as shown in Fig. 11. It was found that absorption properties were quite diff erent among the samples because of their diverse morphologies. Although samples A, B, and C were all pyrite FeS2 crystals, the absorption curves were diff erent due to diff erent morphologies. Sample A exhibited a broad absorption peak at 1200 nm (1.03 eV) and a sharp optical absorption located at 1724 nm (0.72 eV). In contrast to the direct band Table 3 The relative intensities of the diff erent crystal faces (VETA/VH2O= 1:8) Crystal indices Sample Theoretical model24 h30 h36 h48 h (111)37.236.441.038.131.0 (200)100100100.0100.0100.0 (210)54.755.356.366.053.0 (211)48.940.0 (220)46.246.747.248.836.0 (311)72.371.670.871.269.0 (222)12.812.112.713.911.0 (023)15.814.314.818.213.0 (321)18.019.120.519.316.0 Fig. 9Schematic illustration of the proposed formation mechanism of FeS2crystals. Fig. 10 Raman spectra of products obtained in the diff erent samples A, B, C, D and E. PaperNJC Published on 16 February 2015. Downloaded by East China University of Science & Technology on 08/02/2018 12:59:45. View Article Online This journal isThe Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015New J. Chem., 2015, 39, 3571-3577 |3577 gap (0.96 eV) of bulk FeS2, the results showed that the absorp- tion peaks had a blue shift. Other reports showed that the band gap of bulk FeS2 was much larger due to its size eff ect. Sample B did not show evident absorption peaks and the absorption curve decreased from 300 nm to 2000 nm. Compared with sample B, the absorption curve of sample C and sample A showed more similarity due to their similar morphologies. Moreover, the absorption curves of the samples D and E were also diff erent due to diff erent phases (pyrite and marcasite) and morphologies (hollow sphere and rod-like). The hollow sphere marcasite FeS2(sample D) had a broad and weak absorption peak at about 1700 nm (0.73 eV) and 1000 nm (1.24 eV), because of a small amount of pyrite FeS2. In contrast to sample D, sample E possessed a well-defined, broad optical absorption spanning from 1200 nm to 1600 nm with a distinct peak at 1380 nm (0.90 eV). Optical performance was mainly influenced by its crystal structure and morphologies. Conclusion In summary, by adjusting the reaction time and volume ratio of ethanolamine and H2O, pure pyrite FeS2crystals with several types of morphologies were obtained via a hydrothermal pro- cess. The hollow sphere of marcasite FeS2was synthesized with pure water, and the morphology was converted into rod-like along with the increase of pyrite FeS2phase as the reaction time was increased. When the ratio of VETA:VH2Owas kept at 8:1, the initial flower-like amorphous crystal transformed into flake- like pyrite FeS2and then into cubic shape. While at 1:8, the flocculent morphologies firstly transformed into a cubic shape, and further transformed into a tetrakaidecahedron shape. At the initial stage of the hydrothermal reaction, free Fe3+could coordinate withL-cysteine to form a Fecysteine precursor, which then decomposed to FeS2nuclei. The cubic-shape FeS2 was obtained and then changed owing to the effect of ethanol- amine. The absorption properties showed a corresponding change with diverse morphologies, implying that the optical performance was influenced by the structure and morphologies. Acknowledgements This study was supported by the financial support from the Fundamental Research Funds for National Nature Science Foundation of China (51172072, 21237003), the Innovation ProgramofShanghaiMunicipalEducationCommission (Z2014-070), and the Talent Fund of Shanghai University of Electric Power (K2014-004). Notes and references 1 W. Y. Kim, Y. C. Choi, S. K. Min and K. S. Kim, Chem. Soc. Rev., 2009, 38, 2319. 2 S. Kumar and T. Nann, Small, 2006, 2, 316. 3 G. D. Scholes, Adv. Funct. Mater., 2008, 18, 1157. 4 P. Altermatt, T. Kiesewetter, M. Kunst and H. Tributsch, Sol. Energy Mater. Sol. Cells, 2002, 71, 181. 5 C. Wadia, A. P. Alivisatos and D. M. Kamme