Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Chiroptical Activity of Type II Core/Shell Cu2S/CdSe Nanocrystals Xiao Shao,† Tianyong Zhang,*,†,‡,§ Bin Li,*,†,§ Minghao Zhou,† Xiaoyuan Ma,† Jingchao Wang,† and Shuang Jiang*,† †
Tianjin Key Laboratory of Applied Catalysis Science and Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300354, People’s Republic of China ‡ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People’s Republic of China § Tianjin Engineering Research Center of Functional Fine Chemicals, Tianjin 300354, People’s Republic of China
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S Supporting Information *
ABSTRACT: Ligand-induced chirality in core/shell nanocrystals (NCs) has attracted extensive attention because of many valuable potential applications. However, the cause of chirality especially in semiconductor nanomaterials is still under debate despite the creation of chiral type I core/shell structures. Herein, we synthesized a kind of new Cu2S/CdSe core/shell nanostructure to study the underlying reason. Four samples of Cu2S/CdSe were synthesized utilizing successive ion layer adsorption and reaction to vary the thickness of the CdSe shell upon a Cu2S core with 5 nm diameter. The chirality of type II Cu2S/CdSe NCs is imparted by L-/D-cysteine and penicillamine, which could be modulated with an increasing thickness of the CdSe shell. To the best of our knowledge, this is the first report of chiral type II core/shell semiconductor NCs. The hybridization theory can explain the variation trend of g factors with every increase in shell thickness from four monolayers (4 ML) to 7 ML. The results indicate that the chiroptical activity of semiconductor NCs is mainly due to hybridization between the holes in the valence band of NCs and the highest occupied molecular orbitals of the chiral ligands. In addition, Cu2S/CdSe NCs show a better chiroptical intensity in comparison with the type I structure according to previous work. The first design of chiral type II Cu2S/CdSe core/shell NCs and a detailed investigation of chiral variation trend help to give a better understanding of the chiral interaction between ligands and core/shell semiconductor nanostructures.
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INTRODUCTION Chiral inorganic nanostructure, a fascinating field of nanomaterials, shows an extensive range of potential applications in chiral sensing, biochemistry, asymmetric catalysis, medicine, enantiomeric separation, spintronics, and molecular recognition.1−8 Chiral noble-metal nanomaterials with plasmons have been researched deeply in theoretical and experimental aspects, unveiling attractive properties and applications.9,10 Also, significant advancements have been made in the preparation of chiral semiconductor nanomaterials and development of their applications. For example, Kotov et al. found that the transparency of Co3O4 nanoparticle gels to circularly polarized light beams could be reversibly modulated by magnetic fields.7 Chiral CdTe nanoparticles have been used in gene editing to conduct site-selective cleavage and profiling of DNA.8 In a word, chiral inorganic nanomaterials have a fascinating charm. The enantiotopic chiral nanomaterials show symmetrical circular dichroism (CD) signals, on account of the different absorption of left and right circularly polarized light.9,11 Specifically, in chiral semiconductor quantum dots (QDs), the quantum confinement effects affect CD, resulting in sizetunable chiroptical properties. There have been abundant reports on chiroptical nanocrystals (NCs).12−17 There are several different theories explaining the cause of chirality in chiral semiconductor NCs.13,18−24 For example, the chiral ligands induce the surface atoms to a chiral distortion, or there © XXXX American Chemical Society
is a chiral defect to form an enantiomeric sample with enantiomeric ligands, or there is a dipolar interaction between NC and ligands, and so on.13,18−22 Despite a mass of studies, the origin of chirality in inorganic semiconductor NCs is still under debate and hence needs further investigation.25−31 In a recent article, the chirality derives from the hybridization between the QD hole level in the valence band and the highest occupied molecular orbitals (HOMO) of chiral ligands.32 However, an accurate interpretation of the origin of the induced CD in QDs or core/shell NCs has not been obtained and still lacks experimental data to support it. Over the last 10 years, chiral (reverse) type I core/shell NCs (CdSe/CdS, CdS/CdSe, and CdSe/ZnS) have been studied.3,9,10,16,33−36 Gun’ko et al. demonstrated that chiroptical properties of type I CdSe/CdS NCs depend on the shell thickness, and the intensity of chirality was decided by varying the hybridization of the hole level of NCs and the HOMO level of the ligand.15 In the type I or reverse type I core/shell structures, both electrons and holes are confined in the cores or shells, which could provide the lowest energy states for holes and electrons. However, almost no one has studied type II core/shell NCs in terms of chirality. In contrast, holes and electrons with the lowest energy states remain in different Received: March 18, 2019
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DOI: 10.1021/acs.inorgchem.9b00769 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
electrons and holes into the core side or shell side, leading to a distinct interaction with chiral ligands. A successive ion layer adsorption and reaction (SILAR) synthetic approach was employed to prepare and grow CdSe shells layer by layer to control the shell thickness, with a subsequent ligand exchange process to obtain chiral properties. The variation trend of chiral intensity has been investigated in detail during the increase in shell thickness. 54,55 From 4 ML (ML = monolayers) to 7 ML, the spatial separation degree changes and energy barrier increases, leading to a varying CD signal, indicating a hybridization interaction between chiral ligand and semiconductor. We believe that these investigations will bring a deeper understanding of chirality inside semiconductor structures and shed light on the design of chiral nanostructures.
semiconductors for type II NCs. The energy gradient existing at the interfaces tends to spatially separate electrons and holes to different sides of the heterojunction. These materials with this characteristic may provide evidence for mechanisms to explain the induction of chirality in semiconductor NCs. In any event, chiral core/shell NCs offer an attractive platform for understanding the fundamental chirality mechanism in chiral semiconductor NCs, which is very important for the advancement of chiral nanotechnology and nanoscience in general. Type II NCs possess novel properties of great optical anisotropy,37 long radiative lifetime38−40 or controllable singleand multiple-exciton lifetimes,41,42 and band-bending effects43,44 that are fundamentally different from those of the type I NCs due to the spatial separations of carriers, which causes these materials to be more suitable in applications of photoconduction and NC lasing technologies.45 Therefore, studying chiral type II NCs is meaningful and will bring new findings. There is a great deal of room for constructing type II core/shell NCs. Copper sulfides (Cu2−xS) have attracted great interest and have been considered as potential materials for application in photocatalysis,46 photothermal therapy,47−49 photovoltaics,5 sensing,50 and nanoplasmonics. They have been found to show intriguing and stable localized surface plasmon resonance (LSPR) behavior, which had only been reported previously in noble metals such as Au and Ag nanomaterials, due to the energy levels of d−d transitions.51,52 The conduction bands (CB) and valence bands (VB) of Cu2S are both higher than those of CdSe by a comparison of their energy band gaps, and there is only about 4−8% lattice mismatch between these two materials (Scheme 1 and Table
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EXPERIMENTAL SECTION
Chemicals. All chemical reagents were of analytical grade and used as purchased without further purification. Copper(I) chloride (CuCl, 99.95%), cadmium oxide (CdO, 99.995%), selenium (Se, 99.999%), L-cysteine and D-cysteine (L-Cys and D-Cys, respectively), L-penicillamine and D -penicillamine (L-Pen and D -Pen, respectively), tetramethylammonium hydroxide (TMAH), oleylamine (OM, 95%), oleic acid (OA, AR), trioctylphosphine (TOP, 97%) and 1octadecene (ODE, 90%) were purchased from the Aladdin Reagent Company. Sulfur powder (S, 99.5%) was purchased from SigmaAldrich. The solvents used in the experiments or tests, including methanol, ethanol, trichloroethylene (TCE), and chloroform, were obtained from Tianjin Chemical Reagent, China. Water throughout all experiments was obtained via a Milli-Q water system. Synthesis of Cu2S Core NCs. For a typical synthetic reaction, airfree conditions accomplished by the standard Schlenk-line technique are necessary for all procedures carried out in this work. The S-OA precursor solution was prepared by dissolving 0.128 g (4 mmol) of S powder in 24 mL of oleic acid in a 50 mL three-necked flask, and then the mixture was heated to 190 °C. At the same time, the Cu-OM precursor solution was prepared by adding 0.396 g (4 mmol) of CuCl to 40 mL of OM in a 100 mL four-necked flask. Afterward, the mixture was heated to 190 °C to obtain a yellow clear solution, followed by the quick injection of 11 mL of the S-OA solution into the Cu-OM solution. Cu2S QDs were formed when the color changed to black from yellow. Subsequently, the reaction temperature was kept at 190 °C for 5 min to meet the growth condition of Cu2S NCs. After the reaction was complete, the mixture was cooled to room temperature by placing the four-necked flask in cool water. On addition of excess ethanol to the solution and centrifugation at 6000 rpm for 10 min, a sediment of NCs was obtained. A certain amount of chloroform was used to redisperse the precipitate, followed by adding excess ethanol and centrifuging at 6000 rpm for another 5 min. The above purification procedure was repeated three times to give pure core QDs. Preparation of CdSe Precursor and Calculation for the Injection. There were two stock solutions. One was a cadmium stock solution (0.2 M) prepared by adding 0.176 g of CdO to a mixture of 3.5 mL of OA and 3.5 mL of ODE, which was degassed for 30 min and then heated to 240 °C under argon. After the mixture turned colorless, the solution was cooled to 100 °C for shell synthesis. The other was a selenium stock solution (TOPSe, 0.2 M) prepared by dissolving selenium in TOP, which was then sonicated to get a clarified solution. All of the above stock solutions were prepared under a nitrogen flow. The SILAR technique is based on the alternating injections of the Cd and Se precursors into a solution containing Cu2S NCs for the growth of Cu2S/CdSe core/shell NCs. The amount of cadmium or selenium precursors required for each layer was determined by the number of surface atoms of a given size of a core/shell NC. Only about a 4−8% lattice mismatch existed between Cu2S and CdSe crystals, leading to an insignificant loss of Cd and Se precursors.
Scheme 1. Type II Band-Edge Alignments at the Heterointerface between Two Semiconductors
S1).50,53−57 The CB and VB values of Cu2S, CdS, and CdSe reported in their respective papers are not exactly the same due to the difference in size, morphology, ligand, test conditions, and other factors.50,54−57 Nevertheless, the common law is that the VB and CB of Cu2S are higher than those of CdS54,57 and CdSe is embraced inside of CdS.53,55 Therefore, the VB and CB are both higher than those of CdSe according to the intermediation of CdS. Thus, this provides the possibility for us to design new type II core/shell NCs based on Cu2S and CdSe. Herein, we synthesized a new core/shell nanomaterial of Cu2S/CdSe, and the chirality was imparted by L-/D-cysteine and penicillamine. This is the first study of a chiral type II core/shell structure, with a unique advantage over a chiral type I material. Type II materials have a good spatial separation of B
DOI: 10.1021/acs.inorgchem.9b00769 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Therefore, the calculations of injection volume were based on the wurtzite structure of CdSe NCs. The average thickness of one monolayer of CdSe was taken as 0.35 nm; as a result, one additional shell layer growth would increase the diameter of a NC by 0.7 nm. Synthesis of Cu2S/CdSe Core/Shell NCs. Cu2S/CdSe core/ shell NCs were synthesized according to a previously reported method with certain modifications.54,58−60 The injection volumes used in the preparation procedure were calculated using the SILAR method to precisely control the thickness of CdSe deposited.54,58−60 First, 30 mg of crude Cu2S NCs (5 nm in diameter, 1.35 × 10−4 mmol of nanoparticles), 4 mL of ODE, and 2 mL of OM were mixed in a 25 mL three-neck flask. The flask was heated to 80 °C and degassed for 30 min to remove any residual air from the system. Then, 390 μL of the Se stock solution was added to the reaction system that was then kept at 80 °C for 10 min. Subsequently, the system was switched to an N2 flow and the reaction mixture was further heated to 200 °C, where the shell growth was performed. The injection volumes of Cd and Se precursors were calculated according to the respective volumes of concentric spherical shells with 0.35 nm thickness for one monolayer (1 ML) of CdSe. The respective volumes of the injection solution for each ML were as follows: Se/Cd (390/390 μL)−Se/Cd (490/490 μL)−Se/Cd (618/618 μL)−Se/Cd (753/753 μL)−Se/Cd (901/901 μL)−Se/Cd (1.06/1.06 mL)−Se/Cd (1.24/1.24 mL). After that, different numbers of layers of CdSe shells from one to seven on the Cu2S core NCs were obtained. A 10 min interval is necessary for a sufficient reaction because the UV−vis spectra demonstrated no further fluctuations after this time period. The injection rate of the precursors (injection speed about 150 μL/min) plays a vital role in the epitaxial growth of CdSe shells onto the Cu2S cores. Improper injection will cause independent homogeneous nucleation of CdSe; thus, slow injection is necessary to guarantee the heterogeneous growth of Cd/Se precursors onto the Cu2S core. With the addition of the Se/Cd precursors, the original dark brown solution turned dark yellow, then became reddish brown, and eventually changed to deep red. The reaction was terminated by lowering the reaction temperature to room temperature. For purification, methanol was added to precipitate the core/shell NCs with subsequent centrifugation to remove excess ligands (OA). Chiral Phase Transfer (Ligand Exchange of the Cu2S/CdSe Core/Shell NCs with Chiral Cysteine and Penicillamine Molecules). Ligand exchange was carried out by the methods in relevant articles with some modifications.61,62 The water solubility of the as-prepared oil-soluble Cu2S NCs was obtained by replacing the initial hydrophobic ligands (OA) with hydrophilic cysteine molecules. D- or L-Cys (70 mg) was dissolved in methanol (5 mL), and the pH was adjusted to 8.0 with TMAH. The cysteine−methanol solution was then dropped into 3 mL of a chloroform solution containing Cu2S/CdSe core/shell NCs (A(λexc) = 1.50) followed by stirring for 1 min. A 3 mL portion of water was then added to the mixture, and stirring was continued for another 1 min. The solution was separated into two phases finally. Nearly all of the Cu2S/CdSe core/shell NCs transferred into the supernatant aqueous phase to be collected. The free cysteine molecules in the aqueous solution were removed by adding acetone to precipitate the Cu2S/CdSe core/shell NCs. D- or LPen capped Cu1.94S NCs were obtained by using similar procedures except with a change from cysteine to penicillamine. Characterization. Transmission Electron Microscopy (TEM). Samples for TEM were prepared by ultrasonic dispersion of the Cu2S and Cu2S/CdSe core/shell NCs in chloroform. The suspension was then drop-cast onto carbon-coated copper grids and dried in air. TEM tests were carried out with a JEM-2100F transmission electron microscope with an acceleration voltage of 100 kV. High-resolution transmission electron microscopy (HRTEM) images were obtained on a JEM-2100F instrument at an acceleration voltage of 200 kV. X-ray Diffraction (XRD). The XRD patterns of the prepared Cu2S and Cu2S/CdSe core/shell NCs were obtained with a Rigaku D/max 2500 V/PC X-ray diffractometer (Cu Kα, λ 0.154 nm, 40 kV, 200 mA). UV−Vis Absorption. A UV−vis spectrophotometer (U-3010, Hitachi) was used to characterize the UV−vis absorption spectra of
Cu2S and Cu2S/CdSe core/shell NCs (400−700 nm). A quartz cuvette with a 0.5 cm path length was used for all UV−vis experiments. Photoluminescence Spectra (PL) and Photoluminescence Lifetime Measurements. PL spectra (λexc = 380 nm) and PL lifetime decays were recorded for Cu2S/CdSe core/shell NCs using a FluoroMax-4 instrument. Fourier Transform Infrared Spectroscopy (FTIR). Fourier-transform infrared (FTIR) spectra were obtained on a NICOLET380 instrument using the pressed KBr pellet technique. Circular Dichroism Spectroscopy (CD). CD spectroscopy was carried out with a Jasco J-810 CD spectrometer operating under a N2 flow of 5−8 L/min at 25 °C. The spectra of the chloroform and aqueous phases were measured separately at a 500 nm/min scan rate. The data pitch was set to 1 nm, the digital integration time to 1 s, and the bandwidth to 4 nm. Quartz cuvettes with a 5.0 mm path length were used for all experiments. All g factors were calculated from absorbance data measured simultaneously with a Jasco J-810 circular dichroism spectrometer.
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RESULTS AND DISCUSSION Preparation and Characterization of Type II Cu2S/ CdSe Core/Shell NCs in Organic Phase. Cu2S/CdSe core/ shell NCs were synthesized using a two-step procedure. First, Cu2S core NCs were produced according to previous work by using a simple hot-injection method with slight modification.63 After that, Cu2S core NCs were cleaned and the shell of CdSe was grown onto the Cu2S cores in a separate reaction. A CdSe shell with a variable thickness grows around the Cu2S core by using the method of successive ion layer absorption and reaction (SILAR). The volume of the CdSe precursor was calculated by the SILAR method, and the thickness of CdSe deposition was precisely controlled. The detailed procedure of the preparation of Cu2S and Cu2S/CdSe core/shell NCs is given in the Experimental Section. Several techniques were used to monitor the growth of NCs and the structure and optical properties of prepared particles, including TEM, XRD, PL, and UV−vis spectroscopy.55 The increase in NC diameter revealed by TEM or highresolution TEM (HRTEM) is regarded as the most direct evidence of shell growth. It is extremely difficult to accurately determine the increment of NC size by TEM after coating one additional monolayer shell of CdSe. Therefore, the size growth of the core/shell structure is described with every two additional monolayers of NCs. Figure 1 shows TEM images of plain Cu2S cores and three representative core/shell NCs with 3, 5, and 7 ML of CdSe shell (for simplicity, hereafter denoted as Cu2S/CdSe 3 ML, Cu2S/CdSe 5 ML, and Cu2S/ CdSe 7 ML, respectively) derived from the same batch of cores. Figure 1a demonstrates the HRTEM image of Cu2S NCs with an average size of 5 ± 0.5 nm. The insert shows the lattice-resolved image, revealing that d = 1.9 Å in the (110) planes of Cu2S, which is consistent with that of hexagonal Cu2S (JCPDS No. 26-1116; a = b = 3.961 Å, c = 6.722 Å). TEM images of Cu2S/CdSe 3 ML, Cu2S/CdSe 5 ML, and Cu2S/ CdSe 7 ML, respectively, are displayed in Figure 1b−d. The average diameters of Cu2S/CdSe 3 ML, Cu2S/CdSe 5 ML and Cu2S/CdSe 7 ML NCs were found to be 6.8 ± 1.5, 8.3 ± 1.8, and 9.8 ± 2.1 nm by statistically evaluating 500 particles, respectively, without any size sorting or postpreparation fractionation. TEM analysis of these samples and the corresponding size distribution histograms were carried out as shown in Figure S1 in the Supporting Information. These sizes of core/shell NCs approximately match the theoretical thicknesses calculated from the amount of injected stock C
DOI: 10.1021/acs.inorgchem.9b00769 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
their indirect band gap. Cu2S/CdSe NCs with the first and second CdSe layer shelld present no sharp distinctive feature in the absorption profile and are not shown here. The absence of features is due to the inconspicuous optical properties of the Cu2S/CdSe system with a thin CdSe layer (
