发表文章毕业论文2012-Fu-DT-A simple route for synthesis of PbSe nanocrystals_ shape control by ligand and reaction time

Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2012, 41, 12254 /daltonPAPER A simple route for synthesis of PbSe nanocrystals: shape control by ligand and reaction time Honghong Fu, Weiling Luan* and Shan-Tung Tu Received 3rd May 2012, Accepted 5th August 2012 DOI: 10.1039/c2dt30962a The shape controlled synthesis of high quality colloidal lead selenide (PbSe) nanocrystals (NCs) was achieved through a simple solvothermal process. By using oleic acid (OA) as a ligand and activating agent for the Pb precursor, the evolution of the NCs from nanospheres to nanofl owers and fi nally to nanocubes was achieved by increasing the reaction time. Further, the shape variation from nanospheres to polyhedrons was readily realized through the increase of OA concentration in the stock solution. More interestingly, the change of the anion ligand was proven to be a facile method to control the structure and size of the nanofl owers. The X-ray diffraction and TEM analysis demonstrated the cubic rock salt structure of the synthesized PbSe NCs. Accompanied by comprehensive analytics, the discussion on the possible mechanisms for the shape evolution was provided. Introduction As one of the most important IVVI semiconductor materials, lead selenide (PbSe) has become an attractive photovoltaic can- didate in recent decades,14due to its excellent properties, especially the high absorption coeffi cient. The organic and most of the inorganic semiconductor NCs in these photovoltaic devices generally absorb visible light. However, half of the solar energy reaching the Earth lies in the infrared spectrum. PbSe is a narrow band gap semiconductor with the value of 0.28 eV in bulk, which can harvest the energy in the infrared area. Besides, PbSe exhibits a large Bohr exciton radius of 46 nm, offering important access for the extreme quantum confi nement regime. Moreover, PbSe NCs received signifi cant attention because of their multi-exciton generation (MEG) property.5 Should effi cient MEGbeachievable,thetheoreticalphotovoltaicdevice effi ciency could be increased to 43%.6 The electric-optical properties of NCs, which are dependent on the sizes and morphologies of the NCs, have a great infl uence on the performance of their photovoltaic devices. To date, several methods have been reported for the synthesis of PbSe NCs. PbSe NCs were prepared using several high temperature approaches such as decomposition of a single molecular precur- sor,7electrodeposition,8chemical vapor deposition (CVD)9and so on.1012However, the obtained products at high temperatures were extremely big sizes, much larger than the Bohr radius. Syn- thetic routes at low temperatures, including hydrothermal and solvothermal synthesis had also been tried.1318PbSe NCs with nanowire, star, spindle, nanocube and hexapod structures were achieved. In general, for PbSe nanocrystals, they were syn- thesized through the reaction between Se stock solution and a Pb organic precursor.19Among these reports, due to its low price, oleic acid (OA) was considered as one of the suitable ligands topreparecationprecursorsforthesynthesisofmany inorganic compounds such as CdSexTe1x,20CuInS2,21PbS22 and PbSexS1x 23 with tuneable size and shape. In addition, most of the Se stock solutions were prepared by dissolving Se powders in an anion ligand such as trioctylphosphine (TOP) or tributylphosphine (TBP). As anion ligands, TOP and TBP usually played important roles during the growth of the NCs. The application of TBP as an anion ligand generally favored the synthesisofmonodispersedsmallerNCscomparedwith those NCs synthesized using TOP. However, the reports on the morphology control by using different anion ligands were scarce.24 Besides, the shape evolution of PbSe NCs was generally monitored through dynamic injection technology.25The dynamic injection process requires complex procedures and a long reac- tion time, which makes it diffi cult to achieve the shape control. Herein, a simple route has been developed to control the morphology of PbSe NCs, and high quality cubic PbSe NCs were achieved. Systematic investigation of the reaction para- meters has been conducted to study the shape evolution of PbSe NCs with reaction time, studying ligand as well as the pre- cursor concentration. Furthermore, co-ligands of TOP and TBP were used for the Se precursor to control the size and mor- phology of the PbSe NCs. The as prepared PbSe NCs were characterized by X-Ray diffraction (XRD) and transmission elec- tron microscopy (TEM). The discussions on the possible mech- anism of the growth as well as shape evolutions were also provided. The Key Laboratory of Safety Science of Pressurized System, Ministry of Education, East China University of Science and Technology, Shanghai 200237, China. E-mail: luan; Fax: +86-21-64253513; Tel: +86-21-64253513 12254| Dalton Trans., 2012, 41, 1225412258This journal isThe Royal Society of Chemistry 2012 Published on 28 August 2012. Downloaded by East China University of Science & Technology on 18/12/2014 11:52:10. View Article Online / Journal Homepage / Table of Contents for this issue Experimental Chemicals Lead(II) oxide (PbO, powder, analytical grade 99%), OA (techni- cal grade 90%), octadecene (ODE, technical grade, 90%), tri- octylphosphine (TOP, technical grade, 90%), tri-n-butylphosphine (TBP, technical grade, 90%) were purchased from Sigma Aldrich. Selenium (Se, 99.99%), anhydrous ethanol, chloroform, acetone and tetrachloroethylene (TCE) were purchased from Sinopharm Chemical Reagent Co. Ltd. Purifi cation process TCE was used as the particular solvent for PbSe NCs, because it does not have any CH absorption peaks in the near infrared region of the optical spectrum. The detailed procedures about purifi cation are as follows. The aliquot was swiftly taken out and quenched by room-temperature chloroform. The purifi cation of NCs was performed by repeated extraction of the reaction mix- tures dissolved in chloroform. An equal volume mixture of chloroform and methanol (CHCl3: CH3OH = 1:1) was used as the extraction solvent to separate the un-reacted materials such as OA. After exaction, the chloroform/ODE phase containing the nanocrystals was precipitated with excess acetone to remove the ODE and chloroform. The precipitate was isolated by centrifu- gation and decantation. In order for complete purifi cation, the obtained NCs should be extracted several times by repeating the steps stated above. The purifi ed PbSe NCs were dried under nitrogen and fi nally dispersed in TCE for characterization. The rotational speed and time of the centrifuge were set as 8000 r min1and 10 min, respectively. Sample characterization Powder XRD characterization was performed using a Rigaku D/ max 2250 V diffractometer operating with Cu K radiation ( = 1.5405 ), and the operation voltage and current were 40 kVand 100 mA, respectively. The samples for XRD patterns were pre- pared by depositing PbSe NCs onto Si substrate. TEM and energy dispersive X-ray spectroscopy (EDS) were carried out by using a JEOL JEM-2100F operated at an acceleration voltage of 200 kV, and samples for TEM images were prepared by dropping a PbSe solution onto a carbon coated copper grid and allowing the solvent to evaporate at room temperature. Selected area elec- tron diffraction (SAED) patterns were obtained on the TEM. The UV-NIR absorption spectrum was recorded on a Varian Cary 500 spectrometer by dispersing PbSe NCs in TCE. Results and discussion Shape evolution with reaction time For a typical reaction, a mixture of 1 mmol PbO powder, 1 mL of OA and 5 mL of ODE were loaded in a 50 mL three neck fl ask. The mixture was then stirred and heated to 150 C for 60 min to get a clear yellow solution. Then, Se stock solution was prepared by dissolving 1 mmol Se powder in 1.5 mL of TOP and 1.5 mL of TBP at room temperature. The as prepared Se stock solution was then rapidly injected into the Pb precursor solution at 170 C. Samples were taken at different time intervals for characterization. All the procedures were carried on in a nitrogen atmosphere. The phase and the purity of the as obtained PbSe NCs were studied by XRD analysis, see Fig. 1. All the diffraction peaks observed in the XRD spectra proved the cubic phase of the obtained PbSe NCs (JCPDS card no. 06-0354), and no impurity peaks were detected. The sharp and narrow peaks illustrate the high crystallization of the samples. Fig. 2 shows the crystal struc- ture of cubic phase PbSe with the space group Fm3m (225) (cell parameters a = b = c = 6.124 ). To elucidate the morphology evolution of the PbSe NCs, reac- tion time-dependent experiments were conducted. The sizes and morphologies of the as-synthesized PbSe products were charac- terized by HR-TEM images (shown in Fig. 3). Upon injection, small PbSe clusters immediately nucleated, and well crystallized NCs with a quasi-spherical shape were obtained. The average diameter of the NCs is around 5 nm (Fig. 3a). The particles tend to aggregate because of high surface energy, resulting in unclear boundaries. With the increase of reaction time to 4 min, the average diameter of the PbSe NCs increased to 6 nm, and each 56 NCs were aggregated into fl ower-like clusters with an average size of 30 nm, as shown in Fig. 3b and c. The SAED pattern (inset image in Fig. 3c) is taken from the edge of the fl ower-like clusters and it reveals poly-crystallinity. The diffraction planes can be indexed to the (111), (200), (220) and (311) planes for the PbSe NC, exhibiting cubic rock salt structure. The HRTEM image (see Fig. 3c) further implied that the small crystallites have been coupled with each other, showing that the lattice planes were mostly aligned. Upon magnifi cation, the lattice fringes image reveals that the obtained PbSe NCs are well crystallized. The lattice d-spacing of Fig. 1XRD pattern of the obtained PbSe NCs. Fig. 2Molecular structure of the PbSe NC (a) (100) faces, (b) (110) faces, (c) (111) faces. This journal isThe Royal Society of Chemistry 2012Dalton Trans., 2012, 41, 1225412258 |12255 Published on 28 August 2012. Downloaded by East China University of Science & Technology on 18/12/2014 11:52:10. View Article Online PbSe NCs is calculated as 3.516 , which is consistent with the (111) lattice planes for bulk PbSe (JCPDS card no. 06-0354, 3.536 ). Furthermore, EDX analysis was performed to confi rm the composition of the PbSe NCs. Pb and Se peaks are observed in EDX spectra (as shown in Fig. 4), suggesting that the PbSe NCs are composed of Pb and Se atoms. The EDX spectrum displayed a ratio of 1: 0.9 for Pb to Se atoms (here, the Cu peak in the EDX spectrum is corresponding to the copper grid from TEM). According to the general growth mechanism proposed in the La Mer model,26many systems exhibit an Ostwald ripening process, small particles tend to grow into larger ones to lower their surface energy (see Fig. 3df). The morphologies of the PbSe NCs have been developed into cubic as the reaction time elapses to 40 min, as shown in Fig. 3d and e. It is observed that the average size of the crystals was around 15 nm. The lattice d spacing in HR-TEM image (inset image in Fig. 3e) was calcu- lated as 3.056 , which is consistent with the (200) plane. The mechanism for the morphology evolution of PbSe In order to correlate the morphology of the nanocrystals and the effect of the growth kinetics of the (200) plane, a series of XRD studies were conducted as in Fig. 5. All the diffraction peaks in the XRD patterns were indexed and no impurities were found. This indicated that all the obtained PbSe products were highly crystallized. With the increase of reaction time, the diffraction peaks became sharper. It could be evidenced that the particle grew large with reaction time. The diffraction peaks at 2 values of 24, 29 and 42 correspond to the (111), (200), and (220) planes of the PbSe structure. As the time elapses, the ratio of intensity of (111) to the (200) plane decreased. This proved that the (200) peak became more dominant with increasing reaction time. This observation indicated that the nanocubes were present, as the 100 surfaces of the nanocubes were preferen- tially parallel to the Si substrate, and the (200) facets had more chances to be diffracted, which coincides with the TEM images. The formation process of the PbSe NCs with different mor- phologies was shown in Fig. 6. After injection, the monomers are immediately nucleated, and many nuclei started to rapidly evolve into spherical nanocrystals with the stabilization provided by the OA ligand. When the size of the spherical NCs are small, the surface energy is high enough to lead agglomeration. Moreover, by the HRTEM, it was found that the fl ower-like clusters were formed by the aggregation of smaller quasi spheri- cal PbSe NCs at 4 min (see Fig. 1b). The main driving force for these fl ower-like clusters is the van der Waals forces between the capping ligands around the NCs. Van der Waals forces are the attractive forces between molecules. With the elapse of reaction time, the concentration of the monomers remaining in the bulk solution greatly decrease, then the PbSe NCs exhibit an Ostwald ripening procedure, which results in the small sized NCs fusing to form larger ones. For a cubic face centred structure, the surface energy is considered to be in the order 111 100 110,27and the growth preferentially occurred along the 111 facets, forming the cubic shaped NCs. Optical properties of PbSe NCs The optical properties of the PbSe NCs were characterized and shown in Fig. 7. Two discrete absorption peaks appear at about Fig. 3TEM and HRTEM images of the synthesized PbSe NCs at differ- ent reaction times, (a) 1 min, (b) and (c) 4 min (inset: SAED pattern), (d) 10 min, (e) and (f) 40 min (inset: HRTEM image of the NC). Fig. 4EDX pattern of PbSe NCs obtained at 4 min. Fig. 5X-Ray diffraction patterns of PbSe NCs with different reaction time. Fig. 6Schematic diagram of the formation process of PbSe NCs. 12256| Dalton Trans., 2012, 41, 1225412258This journal isThe Royal Society of Chemistry 2012 Published on 28 August 2012. Downloaded by East China University of Science & Technology on 18/12/2014 11:52:10. View Article Online 1400 nm (0.9 eV) and 2080 nm (0.6 eV), respectively. Compared with bulk PbSe, the fi rst absorption peak (2080 nm) of the PbSe NCs exhibits a dramatic blue shift. The fi rst absorption feature corresponds to an interband transition. The appearance of two well-separated absorption peaks and the blue shift of the fi rst absorption are the results of quantum confi nement effects. As a narrow optical band gap semiconductor material, the broader absorption spectrum and the larger absorption peak wavelength indicated that more solar photons could be harvested, exhibiting great potential application in the optoelectronic device. Infl uence of cation ligand concentration It is worthwhile to mention that the cation ligand concentration has strong effects on the sizes and morphologies of PbSe samples. The obtained products prepared at different cation con- centrations are shown in Fig. 8. The OA :Pb precursor ratios were changed to investigate their infl uence on the size and shape of the obtained PbSe NCs. The experiment was operated by tuning the molar ratio of OA, while keeping the amount of Pb precursor constant. PbSe nanocrystals shown in Fig. 8 were prepared with OA: Pb precursor ratios of 1.5 : 1, 4: 1 and 6:1, respectively. The average size of the PbSe products increases from 5 nm to 11 nm with the increment of OA concentration. A high ligand concentration suppresses the activities of the monomers. Consequently, the number of nuclei formed is low, which results in relatively large size. Another important effect of OA concentration is to tailor the shape of the PbSe NCs. As shown in Fig. 8, PbSe NCs with different shapes were obtained by tuning the OA :Pb precursor ratio. When the OA : Pb precursor ratio is 1.5 : 1, spherical shaped and not well dispersed PbSe NCs were detected (Fig. 8a). This tendency of agglomeration of NCs arises due to the high surface energy of small particles. PbSe NCs exhibit monodispersed and polyhedral features when the OA : Pb precur- sor ratio is increased to 4: 1 and 6:1, see Fig. 8b and c. It is generally believed that28the surface must be a polyhedron with high-index crystallography planes for small sized spherical crys- tals, which possibly results in a higher surface energy. The TEM image reveals that these PbSe NCs are highly crystalline with rock salt structures of ordered arrays. PbSe has an fcc structure and the morphology of an fcc NC is mainly determined by the ratio (R) between the growth rates along the and directions.29,30Perfect nanocubes will be formed when R = 0.58, while an R = 0.70.87 favors the formation of truncated NCs (polyhedrons). For the structure of PbSe nanocrystals, the 100 faces consist of both Pb and Se atoms, while the 111 faces consist of Pb or Se atoms only (see Fig. 2). OA are expected to preferentially absorb at the 111 facets. This would leave the 100 facets poorly capped or uncapped with surfactant molecules. In the present study, when the concentration of OA is 1.5 mmol, the ligands on the surface of the NC were few, leading to the small sized quasi spherical shaped NCs. Meanwhile with the increase of OA concentration in the reaction solution, the preferential growth at the 100 facets would become prominent, which resulted in a larger R, and the polyhedrals were obtained. Shape variation with anion ligand In addition to adjusting the concentration of the cation ligand (OA), the possibility of morphology control by the modulation of the anion ligand was also studied. The experiment was conducted by changing the volume ratio of TBP and TOP while keeping the total volume as 3 mL. Differ- ent mixtures of TBP and TOP were prepared with volume r