Effects of Pb Treatment on Optical Properties of Aqueous CdSe

Dec 18, 2015 - By treating CdSe AQDs with Pb precursor in solution, the photoluminescence (PL) intensity and quantum yield of the CdSe AQDs doubled...
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Effects of Pb Treatment on Optical Properties of Aqueous CdSe Quantum Dots Cheng-Hsin Lu,† Wan Y. Shih,‡ and Wei-Heng Shih*,† †

Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, Pennsylvania 19104, United States

‡

S Supporting Information *

ABSTRACT: Cadmium selenide (CdSe) aqueous quantum dots (AQDs) with broadband trap-state emissions were synthesized using an environmentally friendly, aqueous method under room temperature. By treating CdSe AQDs with Pb precursor in solution, the photoluminescence (PL) intensity and quantum yield of the CdSe AQDs doubled. Furthermore, the absorption edge and PL peak wavelength of CdSe blue-shifted. It was found that the presence of excess Cd in the CdSe is essential to the observed behavior. A possible cation exchange between Pb and the excess Cd is proposed to explain the observation. Our study provides a unique way to increase the photoluminescence intensity of trap-state emission quantum dots.

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INTRODUCTION Quantum dots (QDs) are semiconductor nanocrystals (NCs) that offer unique optoelectronic properties due to the quantum confinement effect.1 The band gap between the valence band and the conduction band of QDs is tunable by their size. The unique optoelectronic properties of QDs make them useful for a variety of applications including biological imaging,2,3 photodetection,4 laser,5 and light-emitting diode (LED).6 The potential of utilizing the multiple exciton generation (MEG) effect7 and hot electron transfer8 makes QDs an emerging field in photovoltaic (PV) applications.9 Compared to traditional organic dye, QDs have better photochemical stability10 allowing longer time for observation in biomedical imaging applications. Though hot injection synthesis has been widely used in QD synthesis,11 it requires organic solvent and high temperature treatment, which is energy consuming and not friendly to the environment. Also, the applications of organic QDs (OQDs) are limited because bioimaging usually requires water-soluble fluorescent labels12 and the long-chain alkyl capping molecules may hinder the electron transfer in solar cell applications.13 Typically further ligand exchange process is required in order to make them water-soluble or having capping molecules of a shorter chain length. Direct synthesis of QDs via aqueous route is a good alternative to the hot injection synthesis to solve these two problems. Various approaches to prepare water-soluble QDs have been studied such as hydrothermal method14 or microwave irradiation method.15 Shih et al. have developed an environmentally friendly, aqueous method for synthesizing aqueous QDs (AQDs) under room temperature.16 These water-soluble QDs are capped with short-chain capping molecule 3-mercaptopropionic acid (MPA) and are suitable for bioapplications.17 In contrast to the OQDs synthesized by hot injection approach generally having edge-state emissions with narrow peaks, the AQDs tend to have trap-state emissions with broad peaks. Broadband trap-state emissions are bright and have applications in imaging18 and white light LEDs.19 Enhancing the PL intensity and quantum yield (QY) of AQDs is critical for their successful applications. Previously it © 2015 American Chemical Society

was shown that by coating a wide band gap material such as ZnS on the surface of QDs, the PL intensity and quantum yield of OQDs with edge-state emission can be enhanced.20 However, there has been little information about the enhancement on the PL and QY of trap-state emission. Furthermore, most of the PL enhancement involves the redshift of the emission wavelength by adding a shell creating core−shell quantum dots.21 Here we report the study of enhancing trap-state emission with reduced emission wavelength by using the CdSe AQD as a model system since it has good photoluminescence stability in air and is well suspended over a long period of time.18 It was found that addition of lead precursor enhanced the PL of CdSe AQDs by doubling their quantum yield without red shift of wavelength. Our result is interesting because, in contrast to high stability aqueous CdSe QDs, aqueous PbSe QDs have poor stability and would aggregate and precipitate easily in a short period of time due to the fact that Pb has strong reaction with Se and can grow very quickly.22 In fact, CdSe shell has been coated on PbSe QDs using ion exchange method23 or successive ionic layer adsorption and reaction (SILAR) method24 forming PbSe/ CdSe core−shell QDs to enhance the stability and quantum yield of PbSe QDs in organic system. However, in this study, it was found that aqueous CdSe QDs with excess Cd ion on the surface can prevent the added Pb ion from interacting with the Se ion in CdSe QDs forming unstable PbSe and thus create a stable Pb-treated CdSe with enhanced trap-state emission at reduced wavelength.

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EXPERIMENTAL SECTION Aqueous MPA-capped CdSe QDs used in this study were synthesized by a procedure adapted from our previous paper18 which is briefly described as follows: Cd and Se precursor Received: Revised: Accepted: Published: 99

May 20, 2015 December 2, 2015 December 18, 2015 December 18, 2015 DOI: 10.1021/acs.iecr.5b01871 Ind. Eng. Chem. Res. 2016, 55, 99−106

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Industrial & Engineering Chemistry Research solutions were prepared separately in advance(1) 0.08 M cadmium nitrate tetrahydrate (Fisher Scientific) solution and (2) 0.08 M sodium selenosulfate (Na2SeSO3, prepared by mixing selenium powder (200 mesh, Sigma-Aldrich) with sodium sulfite anhydrous (Sigma-Aldrich) with continuous stirring and heating) solution. First, 28 μL of 3-mercaptopropionic acid (MPA) (as purchased, Alfa Aesar) was added to 40 mL of deionized (DI) water followed by continuous stirring for 5 min. MPA acts as the capping molecule of CdSe QDs. The pH of MPA solution was then adjusted to 11 using tetramethylammonium hydroxide (TMAH) (as purchased, Alfa Aesar) followed by continuous stirring for 10 min. After that, 1 mL of 0.08 M Cd precursor was added to the MPA solution and the pH was maintained at 11 by adding more TMAH. After another 5 min stirring, 1 mL of 0.08 M Se precursor was added to the solution followed by continuous stirring for another 10 min. After that, another 2 mL of 0.08 M Cd precursor (as excess cation) was added to the suspension and the pH was adjusted to 12 by adding TMAH with continuous stirring for 10 min. Finally, the final volume of the solution was made to 50 mL by adding additional DI water. The synthesized MPA-capped CdSe QD suspension had a final [Se2−] concentration of 1.6 mM (the particle concentration is roughly 1.35 μM) with a molar ratio of capping molecule (MPA):cation (Cd + excess Cd):anion (Se) = 320 μmol:(80 + 160) μmol:80 μmol = 4:3:1. It was found that this specific molar ratio with MPA:excess Cd equals 2:1 gives the optimal PL intensity.18 Similar to 4:3:1 CdSe, 4:2:1 and 4:1:1 CdSe QDs with different amount of excess Cd were synthesized using the same approach but changing the additional amount of excess Cd precursor from 2 to 1 and 0 mL, respectively. The appearances of the CdSe QDs had a yellow color under ambient light and a yellow fluorescent color under ultraviolet (UV) lamp. For Pb treatment process, various amounts of Pb precursor of 0.08 M lead (II) nitrate (Sigma-Aldrich) solution were added to the postsynthesized MPA-capped CdSe QDs (as synthesized, without any purification) and stirred for 5 min to modify the surface of CdSe QDs forming Pb-treated CdSe QDs (noted as CdSe/xPb QDs hereafter, where x is the molar ratio of added Pb compared to the stoichiometric Se). For example CdSe (4:3:1)/1Pb QDs were obtained by adding 1 mL of 0.08 M Pb precursor into the 50 mL postsynthesized CdSe (4:3:1) QDs (which also contain 1 mL of 0.08 M Se precursor) and stirring for 5 min. For comparison purpose, aqueous MPAcapped PbSe QDs were synthesized using a similar method as aqueous CdSe QDs but with a molar ratio of MPA:Pb:Se = 8:2:1 instead of 4:3:1. The reason for using more capping molecules but less excess cations for synthesizing PbSe QDs was because Pb reacts with Se strongly and can grow very quickly leading to QD aggregation. The quantum yield for the QDs can be determined by measuring the integrated fluorescence intensity and absorbance of the QD suspensions at different concentrations under certain excitation wavelength and comparing the results with that of a standard. Rhodamine 101 ethanol solution was chosen as the standard here since it has a known quantum yield of 100%.25 Quantum yield can be obtained by a comparative method using equation: ϕX = ϕST (GradX/GradST) (ηX2/ηST2), where X in the equation denotes the test (CdSe with or without Pb treatment) sample and ST denotes the standard (rhodamine 101) sample. The ϕ, Grad, and η in the equation denote the quantum yield, slope of the integrated fluorescence intensity versus absorbance, and the refractive index of the solvent (ηwater = 1.33 and ηethanol

= 1.36), respectively. The photoluminescence (PL) emission intensities of the untreated and Pb-treated QDs were measured by Photon Technology International (PTI) QuantaMaster spectrofluorometer and the ultraviolet−visible (UV−vis) absorbance spectra were measured by Ocean Optics USBISS-UV/VIS combined with a USB 4000 spectrometer. The excitation source of PTI QuantaMaster is a Xenon arc lamp combined with a monochromator, and the excitation wavelengths in this study for untreated CdSe and Pb-treated CdSe QDs were fixed at 460 and 455 nm, respectively, unless otherwise specified. Note that all QD samples were diluted with pH controlled DI water from 1.6 to 0.16 mM before measuring their optical absorption and emission spectra to prevent any potential scattering or reabsorption effect due to high concentration. The elemental analyses were done by FEI XL30 environmental scanning electron microscope (ESEM) using its energy-dispersive X-ray microanalysis (EDAX) detector. The crystal structures of the untreated and Pb-treated QDs were studied using Rigaku SmartLab X-ray diffractometer. The transmission electron microscopy (TEM) images were taken by JEOL JEM2100 transmission electron microscope to determine the size of QDs. Note that the QD suspensions for PL and absorption optical measurements were tested as synthesized without any further purification. However, the QD powders for structural and elemental studies were washed before observation by adding ethanol, precipitated by centrifugation, and repeat the washing steps by resuspending the collected powders in ethanol and centrifuge again. This is to ensure that the elemental analysis only examines the QD itself without any unwanted excess ions in the water attached to the QD surface.

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RESULTS Figure 1 shows the optical characteristics of the CdSe (4:3:1)/ 1Pb Pb-treated QDs compared to that of the untreated CdSe

Figure 1. Absorbance spectra (dashed lines, left Y axis) and photoluminescence emission spectra (solid lines, right Y axis) of untreated CdSe quantum dot suspension (black lines) versus Pbtreated CdSe quantum dot suspension (red lines) with molar ratios of MPA:Cd:Se = (4:3:1) and MPA:Cd:Se/Pb = (4:3:1/1), respectively. The excitation wavelengths in this study for untreated CdSe and Pbtreated CdSe were fixed at 460 and 455 nm, respectively, unless otherwise specified.

(4:3:1) QDs. It was found that the emission peak intensity almost doubled, increased from ∼45 000 to ∼82 000 after Pb treatment while the PL peak wavelength blue-shifted from ∼600 to ∼570 nm. Meanwhile the absorption edge blue-shifted from ∼500 to ∼480 nm and a slightly higher absorption was 100

DOI: 10.1021/acs.iecr.5b01871 Ind. Eng. Chem. Res. 2016, 55, 99−106

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Industrial & Engineering Chemistry Research

observed that an optimal amount of Pb (where x is between 0.5 and 1.5) exists for the highest PL intensity. Meanwhile, the absorption edges and the emission peak wavelengths all blueshifted to shorter wavelength after Pb treatment. Another important aspect of the effect of Pb addition is that it requires the presence of excess Cd. By adding a fixed molar ratio of Pb precursor into the suspensions of postsynthesized CdSe QD with different molar ratios of excess Cd (molar ratio of MPA:Cd:Se = 4:3:1, 4:2:1, or 4:1:1), the effect of excess Cd on the QDs with added Pb was examined. Using CdSe/1Pb as a model, the optical behaviors were examined with varied amount of excess Cd. The optical absorption and emission spectra of CdSe (4:3:1), CdSe (4:2:1), CdSe (4:1:1) (solid lines) versus CdSe (4:3:1)/1Pb, CdSe (4:2:1)/1Pb, and CdSe (4:1:1)/1Pb (dashed lines) were summarized in Figure 4a and 4b; and their appearances under ambient light and ultraviolet lamp were shown in Figure 4c and 4d, respectively. From Figure 4a it was found that, when there was excess Cd present on the surface, the absorption edge of MPA-CdSe (4:3:1)/1Pb and CdSe (4:2:1)/1Pb QDs (which means with 2 molar ratio of excess Cd and 1 molar ratio of excess Cd, respectively) both blueshifted to a shorter wavelength compared to the untreated ones. And their PL intensities both increased accompanied by blueshifted peak wavelengths. However, when there was no excess Cd on the surface, in the case of MPA-CdSe (4:1:1)/1Pb, the absorption edge and emission peak both red-shifted. The instrument has a detection limit of around 825 nm, any peak larger than 825 nm was cut off and therefore the exact PL peak position for CdSe (4:1:1)/1Pb could not be determined. For comparison purposes, PbSe (8:2:1) QDs were used to gain more information about the reactions involved in the Pb treatment. The X-ray diffraction (XRD) patterns of untreated and Pb-treated CdSe QDs versus PbSe QDs are shown in Figure 5a−c. From Figure 5a and b it can be seen that the crystal structures of CdSe QDs did not change much before and after Pb treatment. The XRD patterns of the untreated CdSe QDs were cubic zinc-blende structure and remain the same structure after Pb treatment. On the other hand, the XRD patterns of PbSe QDs in Figure 5c have a cubic rock-salt structure. This result confirmed that there was no PbSe formed after Pb treatment, or the amount of PbSe formed was so little that no significant crystal structure change can be found in XRD patterns. The appearances of (1) CdSe (4:3:1) QDs, (2) CdSe (4:3:1)/1Pb QDs, and (3) PbSe (8:2:1) QDs under ambient light and ultraviolet lamp are shown in Figure 5d and e, respectively. It can be seen that the color of CdSe QDs under ambient light and UV lamp still maintain yellow after the Pb treatment, while the color of PbSe QD is dark brown in ambient light and no PL emission in the visible range under UV lamp. The absorbance and emission spectra of PbSe QDs compared to CdSe and CdSe/1Pb QDs are shown in Figure 5f. It can be seen that the absorption edge of PbSe QDs was >800 nm, while no emission peak was found below 800 nm. The reaction involved with Pb treatment is further examined by adding excess Se precursor to the suspension after CdSe QDs have been treated with Pb in order to see whether PbSe will form on the CdSe surface. One molar ratio of Pb precursor was first added to the postsynthesized CdSe QD suspension and stirred for 5 min. After that, different molar ratios of excess Se precursor were added to the CdSe (4:3:1)/1Pb QD suspension which was denoted CdSe/1Pb/ySe QD (where y is the molar ratio of additional Se compared to core stoichiometric Se). The optical absorption and emission spectra

found near the exciton peak after Pb treatment. Figure 2 shows the integrated fluorescence intensity versus absorbance of CdSe

Figure 2. Integrated fluorescence intensity versus absorbance of untreated CdSe and Pb-treated CdSe quantum dot suspension with molar ratios of MPA:Cd:Se = (4:3:1) and MPA:Cd:Se/Pb = (4:3:1/1) respectively, versus rhodamine 101 ethanol solution (reference). The excitation wavelength of CdSe and CdSe/1Pb was fixed at 455 nm while the excitation wavelength of rhodamine 101 was fixed at 567 nm.

AQDs with and without Pb treatment compared to rhodamine 101. The quantum yields of aqueous CdSe QD before and after Pb treatment are 19.4% and 40.3% respectively calculated from the slopes in Figure 2. It can be seen that the Pb treatment doubled the quantum yield, consistent with the increased PL intensity by the Pb treatment shown in Figure 1. The amount of added Pb relative to the stoichiometric Se was varied to study the systematic effect of the Pb treatment on CdSe (4:3:1) QDs. The optical properties of CdSe, CdSe/ 0.5Pb, CdSe/1Pb, CdSe/1.5Pb, and CdSe/2Pb QDs were summarized in Figure 3 with the PL intensity, emission peak wavelength, and absorption edge plotted as a function of the amount of Pb added. It is shown that the Pb treatment can increase emission peak intensities. Even with very little amount of Pb added (x = 0.5), the PL peak intensity almost doubled. As Figure 3 shows, PL intensity increased to maximum at x = 1 and decreased at x = 1.5, while x = 2 decreased even more. It is

Figure 3. Photoluminescence intensities (black squares, left Y axis), emission peak wavelengths (blue triangles, right Y axis), and absorption edge wavelengths (red triangles, right Y axis) of different Pb treatment contents in CdSe/xPb where x = 0, 0.5, 1, 1.5, and 2 corresponded to untreated and Pb-treated CdSe quantum dot suspension with molar ratio of MPA:Cd:Se/Pb = (4:3:1/0), (4:3:1/ 0.5), (4:3:1/1), (4:3:1/1.5), and (4:3:1/2), respectively. See Figure S2 in the Supporting Information for more details. 101

DOI: 10.1021/acs.iecr.5b01871 Ind. Eng. Chem. Res. 2016, 55, 99−106

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Industrial & Engineering Chemistry Research

Figure 4. (a) Absorbance and (b) photoluminescence emission spectra of untreated CdSe quantum dot suspensions with molar ratios of MPA:Cd:Se = (4:3:1), (4:2:1), and (4:1:1) (solid lines) versus Pb-treated CdSe quantum dot suspensions with molar ratios of MPA:Cd:Se/Pb = (4:3:1/1), (4:2:1/1), and (4:1:1/1) (dashed lines) respectively. (b inset) Emission peak of CdSe (4:1:1) between 550 and 750 nm. The appearances of untreated CdSe and Pb-treated CdSe quantum dot suspensions with different amount of excess Cd under (c) ambient light and (d) ultraviolet lamp.

The inset of Figure 4b shows the emission peak of CdSe (4:1:1) between 550 and 750 nm with an intensity of ∼2000, which is almost ten times lower than that of the CdSe (4:2:1). Meanwhile the excess Cd can affect the absorption edge as well because the optical properties of QDs are not only related to their core but also related to how their electrons (or holes) delocalize to their shells. Therefore, QDs with a thicker shell will have a slightly red-shifted optical properties compared to the thinner one (absorption edge: 4:3:1 > 4:2:1 as shown in Figure 4a). However, we found that the one without shell (4:1:1) has a larger absorption edge compared to the ones with shells (4:3:1 and 4:2:1). This is likely due to the fact that without the excess Cd layer, the core has the potential to grow with time and becomes bigger because short-chain capping molecule MPA does not prohibit growth effectively. Indeed we found that the absorption edge and emission peak of (4:1:1) continue to red-shift with time. Literature has shown that larger QDs will have both red-shifted absorption edge and trap-state emission wavelength compared to smaller QDs.27 From Figure 4a and b, we can see that the optical properties (both absorption and emission spectra) of QDs with excess Cd all blue-shifted while QDs without excess Cd all red-shifted after Pb treatment. If there is contact between Pb and Se, they can react and form PbSe which is a strong absorber with smaller band gap than CdSe. In the case of QDs without excess Cd, Pb

of CdSe, CdSe/1Pb, CdSe/1Pb/0.5Se, CdSe/1Pb/1Se, CdSe/ 1Pb/1.5Se, and CdSe/1Pb/2Se were summarized in Figure 6a and b; their appearances under ambient light and ultraviolet lamp were shown in Figure 6c and d, respectively. It was found that with increasing excess Se, the CdSe/1Pb/ySe QD absorb more light and the absorption edges red-shifted to longer wavelength compared to the CdSe/1Pb (Figure 6a) while the PL emission intensities were quenched and red-shifted to longer wavelength as well (Figure 6b).

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DISCUSSION In order to better understand the results presented above, here a possible mechanism for the Pb treatment was proposed. Further studies will be performed in the future to verify the possible mechanism. We first try to understand the role of excess cations in trap state emission QDs. The trap states in CdSe are generally believed to be Se vacancies.26 From Figure 4b we found that the PL intensities of untreated CdSe QDs increase with the increasing amount of excess Cd (PL intensities: 4:3:1 > 4:2:1 > 4:1:1). This is due to more excess Cd will introduce more Se vacancies to the QD surface (or more defects at the interface of CdSe core and excess Cd shell) and therefore increase the trap state emission intensities. If there is no excess Cd on the surface (i.e., CdSe (4:1:1)), there will be very little trap state emission. 102

DOI: 10.1021/acs.iecr.5b01871 Ind. Eng. Chem. Res. 2016, 55, 99−106

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Industrial & Engineering Chemistry Research

Figure 5. X-ray diffraction (XRD) patterns of (a) untreated CdSe QDs (with molar ratio of MPA:Cd:Se = 4:3:1), (b) Pb-treated CdSe QDs (with molar ratio of MPA:Cd:Se/Pb = 4:3:1/1), and (c) pure PbSe QDs (with molar ratio of MPA:Pb:Se = 8:2:1). The appearances of CdSe, Pb-treated CdSe, and PbSe QDs under (d) ambient light and (e) ultraviolet lamp. The absorbance (dashed lines, left Y axis) and emission (solid lines, right Y axis) spectra of CdSe QDs (blue lines) and CdSe/1Pb QDs (red lines) compared to PbSe QDs (black lines) are shown in part f.

can directly react with Se forming dark brown PbSe and lead to strong red-shift of absorption edge and emission peak in the CdSe (4:1:1)/1Pb case. However, we found that there was no absorption above 500 nm after Pb treatment with the presence of excess Cd indicating that the added Pb did not react with the Se in the CdSe (4:3:1)/1Pb and CdSe (4:2:1)/1Pb cases. One possible way to explain the blue-shifted absorption edge and emission peak wavelength is the cation exchange between the added Pb ion and the excess Cd ion on the QD surface leading to shrinking the size of overall QDs. Previously it has been shown that the addition of Pb is able to displace the Cd in aqueous CdS QDs forming CdS/PbS QDs due to the lower solubility of Pb compared to Cd in water.28 The energydispersive X-ray spectroscopy (EDS) elemental analyses of the untreated and Pb-treated CdSe QDs of Figure 4 were summarized in Table 1 (the EDS elemental analyses and the optical properties of Figure 3 can be found in Table S1, Figure S1, and Figure S2 in the Supporting Information). It can be seen that the atomic percentage of Cd and Se both decreased while that of Pb increased with the Pb treatment. This indicates that the added Pb was indeed with the QDs, however, we did not know exactly where the added Pb was within the QDs nor did we know if there was some kind of compound or alloy formed. From the fact that when there was excess Cd presence, the QD did not turn brown after adding 1Pb (Figure 4) or after adding 1Se after 1Pb treatment (Figure 6) indicating that there was no PbSe formed (neither in the core nor on the surface), it was speculated that the added Pb is contained inside the excess Cd layer. When the amount of added Se is more than the amount of excess Cd (as in the case of CdSe/1Pb/2Se) then the reaction of Pb with Se occurred and the QD turned brown again. From the EDS results, we speculate that the added Pb ions replaced some excess Cd on the surface, and at the same time some ions (both Cd and Se) in the core might lose during

the ion exchange process due to dissolution. As a result, we ended up a slightly smaller Pb-treated particle than the untreated particle. A smaller particle leads to a wider band gap due to the quantum confinement effect thus causing the absorption edge to blue-shift. The proposed mechanism of Pb treatment is schematically illustrated in Scheme 1. Scheme 1a− c shows the synthesis of untreated CdSe QDs with excess Cd ion on the surface. Scheme 1d−f shows the partial cation exchange process of Pb-treated CdSe QDs where the excess Cd shell turned into a Pb-containing Cd shell. The size of the final Pb-treated QD is slightly smaller than the untreated QD due to ion dissolution. The defects generated at the interface of CdSe core and the Pb-containing Cd shell can increase the trap state emission intensities. In Figure 3, the PL intensity first increased then decreased again along with the increasing amount of additional Pb was probably due to the thickness of the Pb-containing Cd shell. With a thin Pb-containing Cd shell (when the amount of added Pb is small), defects can accumulate and lead to more trap state emission. However, when the added Pb is too much, surface strain due to the lattice mismatch between the Pbcontaining Cd shell and the core can result in nonradiative defects leading to lower PL intensity. In addition, when adding too much Pb (when x > 2) the CdSe QD suspension is no longer stable and will lead to QD aggregation and precipitation. The reason is that after Pb replacing Cd, the number of MPAs on the QD surface decrease (because MPAs were bonded to Cd ions) leading to instability of QDs. The more Pb is added, the less stable the QDs are. Meanwhile, MPAs can be photooxidized by the presence of light and oxygen causing MPAs to detach from QD surface forming disulfide bond between two MPAs.29 Excess MPAs in the suspension could replace the oxidized MPAs and maintain the stability of the suspension. This is the reason why Pb-treatment was 103

DOI: 10.1021/acs.iecr.5b01871 Ind. Eng. Chem. Res. 2016, 55, 99−106

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Industrial & Engineering Chemistry Research

Figure 6. (a) Absorbance and (b) photoluminescence emission spectra of untreated CdSe quantum dot suspension with molar ratio of MPA:Cd:Se = (4:3:1) versus CdSe/Pb/Se quantum dot suspension with molar ratio of MPA:Cd:Se/Pb/Se = (4:3:1/1/0), (4:3:1/1/0.5), (4:3:1/1/1), (4:3:1/1/ 1.5), and (4:3:1/1/2). The appearance of CdSe and CdSe/Pb/Se quantum dot suspensions under (c) ambient light and (d) ultraviolet lamp, respectively.

Table 1. Summary of EDS of Untreated CdSe and Pb-Treated CdSe Quantum Dot Powders with Molar Ratios of MPA:Cd:Se = (4:3:1), (4:2:1), (4:1:1) and MPA:Cd:Se/Pb = (4:3:1/1), (4:2:1/1), (4:1:1/1), Respectively CdSe (4:3:1)

CdSe/Pb (4:3:1/1)

CdSe (4:2:1)

CdSe/Pb (4:2:1/1)

CdSe (4:1:1)

CdSe/Pb (4:1:1/1)

element

at %

at %

at %

at %

at %

at %

CdL SeK PbL Others total

38.41 14.10 0.15 47.35 100.00

30.61 10.83 10.79 47.77 100.00

37.54 15.72 0.00 46.75 100.00

33.82 14.26 19.48 32.45 100.00

40.63 37.41 0.00 21.96 100.00

23.74 19.62 27.76 28.88 100.00

size before Pb treatment is around 4.3 nm, roughly in the same scale of the sizes obtained by TEM. When carefully examining Figure 7f (note that the scale bar in Figure 7f is 10 nm, while the scale bars in others are 5 nm), it can be seen that it is composed of two sizes of quantum dots: small size quantum dots of around 5 nm and large size quantum dots (or aggregates) of around 10−30 nm. We speculate that CdSe (4:1:1)/1Pb is a mixture of CdSe and PbSe QDs. XRD patterns of CdSe (4:1:1) versus CdSe (4:1:1)/1Pb are shown in Figure S5 in the Supporting Information. From Figure S5 we can see that the XRD patterns of CdSe (4:1:1)/1Pb is a combination of CdSe (Figure 5a) and PbSe (Figure 5c).

performed on the unpurified CdSe QD suspension instead of purified suspension. The unpurified CdSe suspensions had excess free MPAs in the suspension, which can replace the detached MPA molecules to keep the QDs remain capped. Pbtreated unpurified CdSe suspension can be stable and well suspended up to several months if stored properly under dark while Pb-treated purified CdSe suspension would aggregate and precipitate at the bottom in less than 1 h (see Figure S3 and Figure S4 in the Supporting Information for more details). The sizes of quantum dots before and after Pb treatment were examined by TEM. Figure 7 shows that the quantum dot sizes before and after Pb treatment are both roughly around 5 nm without any significant change except the CdSe (4:1:1)/ 1Pb (Figure 7f) case. The quantum dot size was calculated using the Brus equation by the energy band gap obtained from absorption edge in Tauc plot and the obtained quantum dot

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CONCLUSIONS MPA-capped CdSe AQDs have been synthesized using an environmentally friendly, aqueous method under low temper104

DOI: 10.1021/acs.iecr.5b01871 Ind. Eng. Chem. Res. 2016, 55, 99−106

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Industrial & Engineering Chemistry Research

highest PL intensity. Further addition of Pb led to QD aggregation and the PL intensity decreased. It was found that accompanied by the PL intensity increase, the absorption edge and emission peak wavelength both blue-shifted after the Pb treatment due to core size shrinkage by partial cation exchange process. The excess Cd ion on the CdSe surface plays an important role in the Pb-treated CdSe. With enough excess Cd ions on the CdSe surface, it can prevent direct reaction of the added Pb with the Se in the CdSe core and therefore prevent the formation of PbSe without significant red-shift in the optical response nor significant changes in the crystal structure. On the other hand, when there is no excess Cd ions on the CdSe surface, the Pb ion will react with the Se forming strong absorber PbSe and therefore lead to emission quenching.

Scheme 1. (a) Addition of Cd and Se Precursors for the Formation of CdSe Core; (b) Addition of Excess Cd (c) Forming Excess Cd Shell on CdSe Core;a (d) Addition of Pb Precursor, (e) Causing Removal of Some Excess Cd through Dissolution and Cation Exchange with the Added Pb and (f) Resulting in a Smaller Particle with a Pb-Containing Cd Shellb

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01871. Elemental analyses and optical responses of CdSe (4:3:1) QDs with different amount of Pb treatments, optical responses of purified and unpurified CdSe (4:3:1) QDs after Pb treatment, and the XRD patterns of CdSe (4:1:1) versus CdSe (4:1:1)/1Pb (PDF)

a

Together a−c show the synthesis process of untreated CdSe QDs. Together d−f show the cation exchange process of Pb-treated CdSe QDs.

b

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Mr. Shan Mei for his help in taking TEM images.

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REFERENCES

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Figure 7. TEM images of untreated CdSe quantum dots with molar ratios of MPA:Cd:Se = (4:3:1) (a), (4:2:1) (c), and (4:1:1) (e) versus Pb-treated CdSe quantum dots with molar ratios of MPA:Cd:Se/Pb = (4:3:1/1) (b), (4:2:1/1) (d), and (4:1:1/1) (f). The scale bars in all images are 5 nm, except the one in part f is 10 nm.

ature. Pb-treated CdSe QDs were obtained by adding Pb precursor to postsynthesized CdSe QDs. The trap state emission PL intensity and the quantum yield of CdSe QDs were roughly doubled by the Pb treatment due to the introduction of more defects. An optimal amount of additional Pb (where x in CdSe/xPb is between 0.5 and 1.5) exists for the 105

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