发表文章毕业论文2011-Wan-JoCaIS-Continuous synthesis of CdSexTe1x nanocrystals_ Chemical composition gradient and single-step capping

Continuous synthesis of CdSexTe1?xnanocrystals: Chemical composition gradient and single-step capping Zhen Wan, Weiling Luan, Shan-tung Tu Key Laboratory of Pressure Systems and Safety (MOE), School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, China a r t i c l ei n f o Article history: Received 24 October 2010 Accepted 11 January 2011 Available online 15 January 2011 Keywords: Microreactor CdSeTe Composition gradient Core/shell Luminescence a b s t r a c t A capillary microreactor was fi rstly utilized to continuously synthesize near-infrared emitting CdSexTe1?x nanocrystals (NCs). By using trioctylphosphine oxide and trioctylphosphine as the solvents for anion pre- cursor as well as oleic acid and oleylamine as the solvents for cation precursor, high quantum yield zinc- blend CdSexTe1?xNCs with a chemical composition gradient internal structure and tunable emission from 634 to 783 nm were synthesized. Thus, the nonlinear relationship between the composition and the emission energies were verifi ed. Moreover, a facile single-step capping approach was developed by using the dissolution of cadmium oxide and free element sulfur in oleic acid, and a very thin CdS shell was suc- cessfully epitaxial grown around the as-prepared CdSexTe1?xNCs to enhance the photostability. After the capping process, the core/shell structured CdSexTe1?x/CdS NCs remained 1540% of their initial PL inten- sity after 3 h of illumination. ? 2011 Elsevier Inc. All rights reserved. 1. Introduction Fluorophors with emission in the near-infrared (NIR) spectral range (650900 nm) is attracting improving attention for vivo fl uo- rescence image. In particular, biological autofl uorescence and absorbance can be reduced to their minimum in this window 1 3. However, conventional organic-based NIR emitting fl uorophors present several drawbacks, including low photoluminescence (PL) quantum yields (QYs) and susceptibility to photobleaching as well as broad emission full width at half-maximum (FWHM), which limit the application of organic fl uorophors for biological imaging and detection 4. Colloidal semiconductor nanocrystals (NCs), also known as quantum dots (QDs), have the potential to overcome these drawbacks, due to their excellent photoelectric properties such as tunable emission wavelength, high PL QYs, narrow PL peak width and well photostability 5,6. These nanoscale materials, especially the binary Cd-based QDs, have generated a great deal of interest in the past decade. Their applications in biological imag- ing, including single molecule tracking 7, multiplexed bimolecu- lar labeling 8, vivo imaging 9,10, and molecular diagnostics 11 and so on, have been proved as a great success. However, the CdSe- based QDs can only reach typically 650 nm in emission, which is below the NIR range from 650 nm to 900 nm 12,13. Although CdTe QDs with a diameter as 7 nm can reach 720 nm in emission, the low PL QYs and rapid photobleaching restricted their applica- tion in vivo imaging 14. Ternary alloyed QDs of CdSexTe1?xwith nonlinear composition effect opens up additional perspectives in the band gap engineering and in developing NIR fl uorescent probes for vivo fl uorescence image. To date, synthetic approaches of oil soluble and water soluble CdSexTe1?xNCs have been studied. Generally, conventional batch process relies on the injection of the anion Se and Te precursors into a preheated mixture of solvents, ligands and Cd precursors were utilized to prepare alloyed CdSexTe1?xNCs. However, phos- phonic acid and trioctylphosphine (TOP) were almost used as coor- dinating solvents, and high reaction temperature above 300 ?C are required to promote the activity of precursors for the synthesis of oil soluble CdSexTe1?xNCs. Due to the fast kinetics at such a high temperature, the reaction process was fi nished in several minutes or tens of minutes. In this case, the sluggish response of tempera- ture and heat control in batch processes made it diffi cult to achieve NCs with the desired optical properties 1518. Furthermore, long reaction time as several hours to tens of hours was needed in the aqueous synthesis, giving rise to a poor reaction yield and a high operation cost 1922. Therefore, the development of low-cost and easy-to-manipulate processes for the synthesis of high-quality CdSexTe1?xNCs is favorable both in academia and industry. Capillary microreaction provides a controllable way to synthe- size alloyed CdSexTe1?xNCs in an accelerated manner due to high heat and mass transfer effi ciency as well as stable reaction envi- ronment in the microchannels. In particular, the residence time (also called reaction time) could be precisely regulated from sev- eral seconds to several minutes through the adjustment of the length of reaction channel and the fl ow rate of stock solutions 2325. All these features make microreactor be an effi cient tool to realize the preparation of CdSexTe1?xNCs with desired optical properties. Furthermore, considering that the free elemental Te 0021-9797/$ - see front matter ? 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.01.039 Corresponding author. Fax: +86 21 6425 3513. E-mail address: luan (W. Luan). Journal of Colloid and Interface Science 356 (2011) 7885 Contents lists available at ScienceDirect Journal of Colloid and Interface Science has a high melting point of 437 ?C and the Te-based NCs is prone to oxidation 26, the addition of a small amount of trioctylphosphine oxide (TOPO) could signifi cantly improve the solubility of Te and enhance the PL effi ciency of the products. However, the obtained CdSexTe1?xNCs still showed poor photostability. The surface cap- ping CdS was conducted to resolve this problem. In the core/shell structured CdSexTe1?x/CdS NCs, the shell provides a physical bar- rier between the optical active core and the surrounding medium to make the NCs less sensitive to environmental changes and pho- tooxidation. Furthermore, the shell provides an effi cient passiv- ation of surface traps, resulting in enhanced PL QYs 27,28. According to the previous reports, most of the CdS capped NCs were synthesized based on the approach of successive ion layer adsorption and reaction (SILAR) in batch process 29,30. However, the procedures such as repetitious injections and temperature vari- ations lead to low preparation effi ciency. A novel and facile single- step capping approach was developed to realize the shell growth. In this study, a capillary microreaction was utilized for the con- trollable synthesis of high-quality NIR emitting CdS capped CdSexTe1?xNCs. A complex ligands system (oleic acid (OA)oleyl- amine(OLA)TOPTOPO)wasappliedtopreparealloyed CdSexTe1?xNCs. Compared the growth kinetics with binary CdSe and CdTe NCs, basic optical and structural characterization of CdSexTe1?xNCs were systematically investigated, and the nonlin- ear dependence of the band gap of alloyed NCs on various Se/Te ra- tios was demonstrated. Moreover, a single-step capping approach was developed to realize the CdS growth on the surface of CdSexTe1?xNCs, and a single ligand system (OA) was utilized as capping precursors. Different capping temperatures, residence times as well as concentration of precursors were consistently in- spected, and the PL stability of capped and uncapped NCs was con- ducted.ThesuccessfulformationofCdSshellwasfurther confi rmed by X-ray powder diffraction (XRD) pattern, energy-dis- persive X-ray spectroscope (EDS). 2. Experimental 2.1. Chemicals Cadmium oxide (CdO, 99.9%), selenium powder (Se, 99.5%), tel- lurium powder (Te, 99.5%), sulfur powder (S, 99.5%), OA (90%), ana- lytic grade Rhodamine 6G, methanol, chloroform, and acetone were purchased from Sinopharm Chemical Reagent Co. Ltd. TOP (90%), 1-octadecene (ODE, 90%), and OLA (70%) were purchased from Fluka. TOPO (98%) was purchased from Alfa Aesar. All chem- icals and solvents were used directly without further purifi cation. 2.2. Set-up of the capillary microreactor The capillary microreactor consisted of a precursor delivering system, a micromixer system, a reaction system and a sample col- lecting part, as shown in Fig. 1. The convective micromixer was uti- lized to realize the effi cient mixing for the anion and cation precursors under low fl ow rates. A polytetrafl uoroethylene (PTFE) capillary (462lm I.D.) with a length of 80 cm and a thermal-stable oil bath compose the reaction system. Thereinto, the nucleation and growth of NCs were conducted in the PTFE capillary channel, while stable heating source was provided by a thermal-stable oil bath. 2.3. Synthesis of CdSexTe1?x, CdSe and CdTe NCs Typically, CdO (64.20 mg, 0.5 mmol) was added into a fl ask con- taining OA (0.8 ml), OLA (1.5 ml) and ODE (2.7 ml), and the mixture was heated at 150 ?C for 1 h with vigorous stirring to form a clear Cd stock solution. The hybrid Se and Te stock solution was pre- pared by dissolving Se powder and Te powder in the mixture of TOP (2 ml), ODE (2.5 ml) and TOPO (0.5 g) at 250 ?C under the mag- netic stirring. After for 1 h, an optically clear solution was formed. The ratio of total Cd to Se and Te was kept constant as 1:1. Five dif- ferent mixtures of Se and Te mixtures were prepared with molar ratios of 0:1, 1:3, 1:1, 3:1 and 1:0. During the operation, equal-vol- ume solutions of cation and anion stock solutions were delivered by a syringe pump (Harvard 22, USA) under the same fl ow rate, then combined and mixed in a convective micromixer. After that, the mixed solution entered the heating zone to initiate the nucle- ation. After dwelling in the heating section for different periods, the solutions fl owed out of the outlet and were collected and di- luted with chloroform for analysis without any size sorting. In the following discussion, the alloyed NC samples were denoted by providing in subscript the relative molar amounts of Se and Te precursors used in the synthesis, for example CdSe0.5Te0.5, the sum of the relative molar amounts for Se and Te gives 1. It is noticeable that the actual ratio of Se and Te in the CdSexTe1?x NCs strongly deviates from the initial ratio of Se and Te precursors used in the synthesis, as was demonstrated below. 2.4. CdS shell synthesis Single-step capping approach was developed to realize the preparation of core/shell structure NCs. Typically, for a CdS shell synthesis, CdSexTe1?xcore solution (3 ml) were directly utilized without any further treatment. A suspension of CdO (38.52 mg, 0.3 mmol), OA (0.48 ml) and ODE (1.02 ml) was heated at 150 ?C with stirring to prepare a clear yellow Cd precursor solution. Meanwhile, a S stock solution was obtained by dissolving S powder (9.62 mg, 0.3 mmol) in ODE (1.5 ml) under 150 ?C and stirring for 1 h. After cooling to room temperature, Cd and S precursor solu- tions suffi ciently mixed to form hybrid precursor for the growth of shell. The same set-up was also applied for the capping process, as shown in Fig. 1. Equal-volume of CdSexTe1?xcore and the mix- ture of Cd and S stock precursors were drew into syringes, respec- Fig. 1. Schematic for the set-up of capillary microreaction. Z. Wan et al./Journal of Colloid and Interface Science 356 (2011) 788579 tively. And the following steps were similar to the above-men- tioned. Thus, the CdSexTe1?x/CdS NCs were obtained from the out- let of capillary. 2.5. Purifi cation of the as-prepared NCs The obtained NCs solution was mixed with chloroform to form a optical clear solution, and the chloroform solution was extracted twice with an equal-volume of methanol by centrifugation. Then the ODE phase was mixed with excess acetone to totally precipi- tate the obtained NCs. The purifi ed NCs were dried under nitrogen and fi nally redispersed in chloroform for characterization. There- into, the rotational speed and time of centrifuge set as 8000 r/ min and 10 min, respectively. 2.6. Characterization UV/Vis and PL spectra were recorded on a Varian Cary 100 UV/ Vis spectrometer and Cary Eclipse fl uorescence spectrophotometer at room temperature, respectively. The PL QYs of the as-prepared CdSexTe1?xNCs was determined by comparing the integrated emission of the QDs samples in chloroform with that of a fl uores- cent dye Rhodamine 6G with identical optical density (0.1) in the excitation wavelength as 510 nm. High resolution transmission electron microscope (HRTEM) images were acquired using a JEOL JEM-2100F operated at an acceleration voltage of 200 kV, and the sample was prepared by dipping an amorphous carboncopper grid in a dilute solution of NCs dispersed in chloroform. XRD pat- tern was recorded with a Rigaku D/max 2550 V diffractometer. Ele- ment analysis was performed by EDS on a JEOL JSM-6360LV scanning electron microscope. The composition of the as-prepared CdSexTe1?xNCs was measured by means of Varian 710 inductively coupled plasma spectrometry (ICP-MS), while the NCs were dis- solved in the mixture of HNO3/HCl. 3. Results and discussion 3.1. Optical properties The synthesis of ternary alloyed CdSexTe1?xNCs introduced here is based on the utilization of CdOAOLA and TOPTOPOSe and -Te as precursors. The nucleation and growth of the CdSexTe1?x NCs was achieved by the simultaneous reaction of Se and Te pre- cursors with Cd precursor when the temperature of premixed pre- cursor solution rapidly enhanced to the desired value. During the reaction, the change of the solution appearance from clear to dark red was observed. Thereinto, the OA played as the potential metal- site-coordinating ligand (via oxygen) and TOP acted as the poten- tial chalcogen-site-coordinating ligand (via the P atom). Due to the existence of phosphines in the synthesis recipe, a high reaction temperature (?300 ?C) is conventionally needed under the batch reaction, which results in a fast reaction rate. Thus, the reaction al- ways fi nishes in several minutes, making it diffi cult to precisely control the growth of the CdSexTe1?xNCs with ideal optical prop- erties. Microreaction provides a controllable manner to synthesize CdSexTe1?xNCs. The residence time can be accurately tuned via the variation of the fl ow rates of precursor solutions. As a result of the enhancement of heat and mass transfer intrin- sic in a microchannel, the reaction temperature would be de- creased and the residence time would be shortened. Here, a reaction temperature as 280 ?C was applied for the synthesis. In or- der to investigate the growth kinetics of alloyed CdSexTe1?xNCs, binary CdSe and CdTe QDs were also synthesized under the similar procedure. Fig. 2 compared the temporal evolution of absorption and PL spectra of CdSe, CdSexTe1?xand CdTe NCs prepared with various Se and Te feeding molar ratios. Thereinto, the evolution of residence time from 9 s to 270 s was realized by the varying of the fl ow rate from 35.76 ml h?1to 0.89 ml h?1. The band-edge absorbance was clearly observed from the absorption spectra, indi- cating the well size distribution of NCs. The small non-resonant Stokes shift and the symmetrical shape of PL spectra indicated that the PL of alloyed CdSexTe1?xNCs was mainly determined by the exciton recombination rather than by the surface traps. Further- more, by varying the residence time and the molar ratio of Se and Te precursors used in the synthesis, the PL peak wavelength of alloyed CdSexTe1?xNCs can be tuned over the wavelength region Fig. 2. Temporal evolution of absorption and PL spectra of the CdSexTe1?xNCs synthesized at 280 ?C with various initial Se and Te molar ratios (a) 1:0 (b) 3:1 (c) 1:1 (d) 1:3 (e) 0:1. The molar ratio of Cd/(Se + Te) in the precursors was fi xed at 1:1. 80Z. Wan et al./Journal of Colloid and Interface Science 356 (2011) 7885 from 634 to 735 nm, compared with the range from 530 to 579 nm of CdSe QDs and 648712 nm for CdTe QDs, which were caused by the nonlinear change of their optical bandgap on composition. ThePL emission wavelengthandFWHM ofthe alloyed CdSexTe1?xNCs in comparison with the bare CdSe and CdTe are presented in Fig. 3. The alloyed CdSexTe1?xNCs show the similar growth process with CdSe and CdTe QDs, which can be divided into three periods. During the initial 54 s, a high concentration of monomers resulted in the instantaneous nucleation and fast growth of nucleus, accordingly led to rapid red-shift of the PL emis- sion wavelength, accompanying with the decrease of the value of FWHM and the focusing of size distribution. Due to the depletion of the monomers in the second stage from 54 s to 162 s, the growth rate began to decrease steadily, resulted from the slowly growth of NCs. Moreover, the continuous growth of large NCs on the sacrifi ce of dissolved small NCs induced a broader size distribution (also called Ostwald ripening). Finally, the as-prepared NCs nearly reached a constant particle size under prolonging growth time to 270 s, which revealed the terminal of the reaction. Thegrowthandopticalpropertiesoftheas-prepared CdSexTe1?xNCs were found to have a strong correlation with the Se and Te feeding molar ratio in the precursors, as shown in Fig. 3a. When the Se:Te molar ratio set as 1:1, the PL peak wave- length of the alloyed CdSexTe1?xNCs was between those of CdSe and CdTe QDs. However, due to the different atomic sizes and elec- tronegativity existed in different ions, the relaxation of the anion cation bonds to their equilibrium positions results in local struc- tural ordering and a particularly large band gap reduction of al- loyed NCs. When molar ratios of Se and Te as 1:3 and 3:1 were applied, the alloyed NCs demonstrated a longer PL peak wave- length than those of binary QDs due to the nonlinear relationship between the composition and the band gap of the CdSexTe2?x QDs resulting from the optical bowing effect 15. Furthermore, CdSexTe1?xNCs were also prepared under the various Cd precursor concentrations, while keeping the Se:Te molar ratio as 1:1.