| Pyrite(cubic β-FeS2), commonly known as an earth-abundant and non-toxic compound, has been regarded as one of the most promising semiconductor materials for solar cells and photodetectors, due to its ideal band gap of 0.95 eV, which perfectly matches the solar spectrum, and excellent properties with high absorption coefficient(~105 cm-1, hν>1.3 eV) and carrier mobility(~360 cm2V-1s-1). In this work, we demonstrate a cost-effective, environment friendly and controllable way called hydrothermal method to synthesize phase-pure single pyrite nanocrystal(NC), using FeCl2·4H2O and S as precursors, gelatin as surfactant and NaOH as mineralizer. XRD、SEM、HRTEM、EDS and UV-vis are employed to characterize the pyrite NCs. Subsequently, different reaction time and S/Fe molar ratio are adopted to explore the influence on pyrite’s components, structure, morphology and defects. Finally, simple photodetector is fabricated to analyze the photoelectric property of pyrite NCs. The achieved results are stated as follows:1) Method to synthesize pyrite NCsThe experiment is carried out at the conition of S/Fe=3.75/1,t=48 h,pH=12.3,T=200℃ and finally obtain phase-pure single pyrite NC without any impurities such as marcasite and FexO1-x. The lattice constant and size distribution are calculated as 0.5417 nm and 20~40 nm, respectively. The morphology is mainly spherical, cubic and polyhedral. The decomposition of surfactant leads to serious agglomeration of NCs.2) Process Optimization with different reaction time and S/Fe molar ratioCertain S/Fe molar ratio(S/Fe=2.5/1) with different reaction time(12, 24, 48 h, respectively) is investigated. The experiment result indicates that shorter time will give rise to more impurities of FexO1-x with dispersive, uniform, smaller and spherical morphology; while the pyrite NCs are found to be dominant with the increase of reaction time. Moreover, the size distribution displays a wider range. In addition, other morphology such as nanocubes and agglomeration of NCs are observed.Meantime, certain reaction time(24 h) with different S/Fe molar ratio(1/1.5, 1/0.75, 2.5/1, respectively) is conducted. Interestingly, smaller S/Fe molar ratio is found to facilitate the growth of spherical pyrite NCs. With the increase of molar ratio, the yield of pyrite NCs will change from low to high and NCs will gradually grow larger accompanied by slight agglomeration and better crystallinity.At last, longer reaction time(48 h) with larger S/Fe molar ratio(2.5/1, 3/1, 3.75/1, respectively) is further demonstrated. The result reveals that higher molar ratio can obtain more pure pyrite NCs until single phase pyrite NCs are achieved at S/Fe=3.75/1. At the same time, the morphology varies from sphere, nanocube to polyhedron with serious agglomeration and greater size.3) Photoelectric property of pyrite NCsSimple prototype photodetector is prepared by lithography, electron beam evaporation and focus ion beam etching. Meanwhile, uniform pyrite NCs thin film is deposited on the device, using drop coating and annealing disposition. The current density under the illumination of incandescent lamp is measured as 1~102 mA●cm-2, while the band gap Eg is calculated as 1.53 eV via UV-vis characterization, which is much higher than theoretical value due to the quantum size effect and other possible factors. |
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