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Preparation of anisotropic CdSe-P3HT core-shell nanorods using directly synthesized Br-functionalized CdSe nanorods
Surface & Coatings Technology 362 (2019) 84–89
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Preparation of anisotropic CdSe-P3HT core-shell nanorods using directly synthesized Br-functionalized CdSe nanorods
T
Jaehan Jung Department of Materials Science and Engineering, Hongik University, Sejong 30016, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Organic-inorganic Hybrid CdSe P3HT Nanorods Surface engineering
The simple yet robust synthetic strategy toward organic-inorganic nanocomposites was developed by capitalizing on robust click coupling between functionalized nanocrystals (NCs) and conjugated polymers (CPs). The functionalized NCs were directly synthesized dispensing with surface engineering of NCs such as ligand exchange by employing bifunctional ligand (i.e., 4-bromomethyl benzoic acid) at the NC synthesis stage. The direct synthetic conditions toward bromine-functionalized CdSe nanorods (NRs) were scrutinized by precisely tailoring the ratio of 4-bromomehtyl benzoic acid over aliphatic ligands. Subsequent substitution of bromine moiety at the NRs into azide yielded azide-terminated NRs. Finally, ethynyl-terminated poly(3-hexylthiophene) (P3HT) were grafted onto azide-functionalized NR surface via click chemistry, forming intimately contact P3HT-CdSe NR nanocomposites. Transmission electron microscope measurement revealed that CPs render the effective dispersion of NRs in the CP matrices. The success of grafting between CPs and NCs was substantiated by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. The optical properties of P3HTCdSe NR nanocomposites were explored with absorption and photoluminescence studies.
1. Introduction Conjugated polymers (CPs) have drawn considerable attention as promising materials in the field of solar cells, light-emitting diodes, and sensors [1–3]. Their advantages such as low-weight, flexibility, and large area production open up new opportunities for easy and low-cost manufacturing of optoelectronic devices [4–7]. Among various CPs, poly(3-hexylthiophene) (P3HT) is one of the most extensively studied semiconductor CPs owing to its solution-processability, tailorable electrochemical properties, and excellent optoelectronic properties [8]. Semiconducting nanocrystals (NCs) exhibit size- and composition-dependent tunable optoelectronic properties, including band gap, emission, and absorption range due to the quantum-confinement effect [9,10]. In this regard, hybrid nanocomposites composed of semiconductor nanocrystals (NCs) and conjugated polymers (CPs) have garnered considerable attention due to their complementary merits. Especially, a one dimensional NCs/CPs core/shell architecture is a promising building block for the use in a wide range of optoelectronic devices owing to its peculiar properties [11]. However, hybrid nanocomposites prepared via a simple physical blending of CPs and NCs have suffered from several problems, including microscopic phase separation and the existence of insulating interfacial layer, thus limiting the performance
of the resulting devices [12]. To address these problems, the chemical tethering of CPs onto the NC surface was introduced to achieve an excellent dispersion of NCs in the CP matrices as well as to promote the electronic interaction between these two semiconductors [13,14] It mostly involves the ligand exchange procedure (e.g., refluxing with pyridine or inorganic ligands) to engineer the NC surface with bifunctional ligands for chemical coupling with CPs [15,16]. However, such typical routes to CP-grafted NCs greatly relies on the ligand exchange process, which is usually multi-steps thus time-consuming. Moreover, it is often not effective resulting in residue surfactants on NC surface [16]. Therefore, a simple yet effective alternative synthetic route that eliminates the need for the ligand exchange is demanded. Herein, we report on a simple yet robust synthetic strategy toward crafting anisotropic CdSe-P3HT core-shell nanocomposites by capitalizing on the direct synthesis of functionalized NCs and subsequent click coupling with end-functionalized CPs. 4-bromomethyl benzoic acids (BrCH2-BA) were employed as bifunctional ligands in this study as carboxylic acids are most widely used capping agent for CdSe NCs and their bromine moiety can be used in a wide range of chemical coupling [17]. It is noteworthy that their aromatic ring can facilitate the electronic interaction and charge carrier transport [16]. Specifically, the synthetic condition toward Br-functionalized CdSe nanorods was discovered by tailoring the ratio of 4-bromomehtyl benzoic acid over
E-mail address: [email protected]. https://doi.org/10.1016/j.surfcoat.2019.01.093 Received 29 August 2018; Received in revised form 6 January 2019; Accepted 25 January 2019 Available online 26 January 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 362 (2019) 84–89
J. Jung
Scheme 1. Grafting ethynyl-terminated P3HT (i.e., P3HT-≡) onto N3CH2-BA-functionalized CdSe NRs by catalyst-free click chemistry, yielding P3HT-CdSe NR nanocomposites.
solution became clear and transparent, 0.7 ml of 1 M Se/TOP solution injected swiftly at 300 °C to initiate the nucleation and growth. The reaction was allowed for 10 min at 300 °C. Then heating mantle was removed to stop the reaction. When the solution cooled down to 60 °C, 2 ml of THF were added. The BrCH2-BA-capped CdSe NRs were purified three times by precipitation with the excess amount of methanol.
aliphatic ligands. Bromine moieties of BrCH2-BA-capped CdSe NRs were then converted into azide groups, yielding N3CH2-BA-functionalized CdSe NRs. Subsequently, synthesized ethynyl-terminated P3HT were grafted onto CdSe NR surface via click chemistry, producing anisotropic P3HT-CdSe NR nanocomposites. The success of grafting was confirmed by Fourier transform infrared spectroscopy and nuclear magnetic resonance. The optical properties of P3HT-CdSe NR nanocomposites were explored with absorption and photoluminescence studies. To this end, the simple synthetic procedure toward producing anisotropic NC-CP core-shell hybrid nanocomposites possibly can be employed in various applications including laser, solar cells, and LEDs where the proper alignment is required.
2.3. Grafting ethynyl-terminated P3HT onto CdSe NRs As synthesized Br-CH2-BA-capped CdSe NRs were mixed with Sodium azide (NaN3) in THF solution and then sealed and stirred at room temperature for three days. Excess amount of NaN3 was removed by centrifugation for three times with methanol/water. The resulting azidomethyl benzoic acid (N3CH2-BA-CdSe NRs) was then precipitated with the excess amount of methanol. Subsequently, 50 mg of P3HT-≡ and 50 mg of N3CH2 -BA-CdSe NRs were mixed in 20 ml THF and stirred at 65 °C under Ar for two days. The final product (i.e., P3HT-CdSe NR nanocomposites) was cooled to room temperature and diluted 10 times with THF. The resulting solution was precipitated with methanol twice to remove uncoupled P3HT.
2. Experimental All chemicals, including 4-bromomethyl benzoic acid, cadmium oxide, sodium azide, selenium powder, 2,5-dibromo-3-hexylthiophene, Ni(dppp)Cl2, tert-butylmagnesium chloride (2 mol/l in diethyl ether), and ethynylmagnesium bromide (0.5 mol/l in THF) from Sigma Aldrich, tetradecyl phosphonic acid (TDPA,97%), and tri-n-octylphosphine (TOP, 90%), from Alfa Aesar, and tri-n-octylphosphine oxide (TOPO, 90%) from Strem chemicals were used as received. THF (VWR, 99%) was refluxed over sodium wire and distilled from sodium naphthalenide solution.
2.4. Characterizations The morphology of BrCH2-BA-capped CdSe NCs and P3HT-CdSe NR nanocomposites were characterized by transmission electron microscopy (TEM) (JEOL 100CX and Tecnai F30). 1H NMR spectra (Varian VXR-400 spectrometer) and Fourier transform infrared spectroscopy (FTIR) were examined to confirm the occurrence of grafting between P3HT-≡ and N3CH2-BA-capped CdSe NRs. The absorption and emission spectra were recorded with a UV–vis (UV-2600, Shimadzu) and photoluminescence spectrometers.
2.1. Synthesis of ethynyl-terminated poly(3-hexylthiophene) (P3HT) Ethynyl-terminated P3HT was synthesized through a quasi-living Grignard metathesis (GRIM) method [18]. 2,5-Dibromo-3-hexylthiophene (0.815 g, 2.5 mmol) was dissolved in anhydrous THF (5 ml) in a round bottom flask (250 ml) and stirred under Ar. Subsequently, 1.25 ml of 2.0 M tert-butylmagnesium chloride was added and then the mixture was stirred for 2 h at room temperature. After it was diluted with 25 ml of anhydrous THF, Ni(dppp)Cl2 (56 mg, 0.1 mmol) was added to initiate polymerization. The resulting solution was stirred for 30 min at room temperature, producing intermediate P3HT; it was then reacted with ethynylmagnesium bromide (2 ml, 1 mmol) in THF for 30 min, yielding ethynyl-terminated P3HT (P3HT-≡). The product was obtained by precipitating the reaction mixture in methanol, filtering in an extraction thimble, and washing by Soxhlet extraction with methanol, hexane, and chloroform sequentially. It was recovered after chloroform evaporated.
3. Results and discussion The synthesis of inorganic-organic CdSe-P3HT core-shell nanorod nanocomposites is depicted in Scheme 1. It involves a direct synthesis of bromine-functionalized CdSe nanorods (NRs) and robust catalyst-free click chemistry between ethynyl-terminated poly(3-hexylthiophene) (P3HT-≡) and functionalized CdSe NRs. It is worth noting that the direct synthesis of functionalized CdSe NRs by utilizing bifunctional ligands at NC synthesis stage dispenses with the need for the tedious ligand exchange procedure. In this study, 4-bromomethyl benzoic acid (Br-CH2-BA) possessing carboxylic acid at one end and bromine moiety at the other end were utilized as bifunctional ligands with alkylphosphonic acids (i.e., tetradecylphosphonic acid (TDPA)) as carboxylic acids are broadly used anchoring agents for CdSe NCs as well as bromine moiety can easily enable coupling reaction [19]. The addition of aliphatic phosphonic acid ligands (e.g., TDPA) prevents the aggregation by providing enough hindrance among NCs and induces elongated growth of CdSe NCs [20]. Specifically, BrCH2-BA-capped CdSe NRs
2.2. Preparation of BrCH2-BA-capped CdSe NRs 1 mmol of CdO, 2 mmol of surfactants (4-bromomethyl benzoic acid (BrCH2-BA) and tetradecyl phosphonic acid (TDPA)), and 1.2 g of TOPO were placed in a three-neck flask. The molar fraction of BrCH2-BA/ TDPA was 1:0, 3:1, 1:3, and 0:1 respectively. The mixture then degassed at 120 °C for 60 min and was then heated to 300 °C under Ar. After the 85
Surface & Coatings Technology 362 (2019) 84–89
J. Jung
Fig. 1. TEM images of CdSe NCs at different molar ratio of BrCH2-BA to TDPA. (a) BrCH2-BA:TDPA = 0:1, (b) BrCH2-BA:TDPA = 1:3, (c) BrCH2-BA:TDPA = 3:1, and (d) BrCH2-BA:TDPA = 1:0.
surface. Anisotropic semiconductor nanorods exhibit intriguing opto-electronic properties such as peculiar electronic structures and polarizations with respect to spherical nanocrystals since their carriers experience strong confinement only along two dimensions, whereas they can delocalize along the long axis of the rods [20,23]. Thereby nanorods offer great potentials in a wide range of applications especially when proper alignment can be achieved [24]. In this context, multifunctional inorganic-organic core-shell nanorods architecture can be served as promising building blocks for the use in opto-electronic applications including lasers, LEDs, and solar cells. In this regard, semiconductor CP grafted CdSe NRs core-shell nanocomposites were synthesized via click coupling between azide functionalized CdSe NRs and ethynyl-terminated P3HT. The high quality BrCH2-BA-capped CdSe NRs were prepared with the molar ratio of TDPA: BrCH2-BA of 3:1 to trigger anisotropic growth but to prevent the formation of branched structure. The high-resolution TEM images revealed that prepared BrCH2-BA-capped CdSe NRs were 40.2 nm in length with a diameter of 5.1 nm and exhibited a good crystallinity (Fig. 2a). The lattice spacing of 0.351 nm in an inset of Fig. 2a corresponds to d-spacing of (0002) plane of wurtzite CdSe crystals, substantiating the induced growth along c-axis. X-ray diffraction (XRD) characterization of resulting CdSe nanorods in Fig. 2c also clearly supported the formation of wurtzite structure. The bromine functional groups of as-synthesized BrCH2-BA-capped CdSe NRs were then converted into azide moieties with sodium azide in THF and followed by coupling with ethynyl-terminated P3HT via catalyst-free click chemistry. Notably, the resulting P3HT-CdSe NR nanocomposites can be readily dispersed in the P3HT homo polymer matrices (Fig. 2b). The success of grafting P3HT chains onto N3CH2-BA-functionalized
were prepared employing bifunctional ligands followed by azidation of bromine group into azide group. Subsequently, P3HT-≡ were grafted onto azide-capped CdSe NR surface via catalyst-free click chemistry between these two constituents. The morphology of Br-terminated CdSe nanocrystals (BrCH2-BAcapped CdSe NCs) were first precisely controlled by tuning the ratio between bifunctional ligands (i.e., BrCH2-BA) and aliphatic ligands (i.e., TDPA). To investigate the influence of the molar ratio of BrCH2-BA over TDPA on the shape of CdSe NCs, their faction was systematically controlled to be 1:0, 3:1, 1:3, and 0:1, respectively. TEM measurement shown in Fig. 1 revealed that CdSe QDs were synthesized when TDPA was solely used while the addition of short mobile ligands (i.e., BrCH2BA) caused anisotropic or branched structures, depending on the molar ratio of TDPA and BrCH2-BA [21]. It is worth noting that the lower monomer (i.e., cadmium complexes) concentration resulted in QD shape despite the use of phosphonic acids (i.e., TDPA) that often induces elongated growth due to their peculiar binding nature. Phosphonic acid is known to bind stronger to the (0110) or (112 0) surfaces than to the (0001) and (0001) facets (c-axis direction), thereby effectively facilitating anisotropic growth [20]. Intriguingly the addition of BrCH2-BA leads to CdSe nanorods as their mobile and reactive nature boosts reaction kinetic and facilitates the anisotropic growth [22]. However, uncontrolled shape was observed in the absence of TDPA (Fig. 1d) possibly because the mobile and reactive nature of Cd-BACH2Br monomers resulted in faster growth and thus became more susceptible to defects such as stacking faults or twinning, showing extensive branching. [21] It is noteworthy that BrCH2-BA clearly played a significant role in determining the shape of NCs, which can serve as indirect evidence of the existence of BrCH2-BA ligands on CdSe NR 86
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Fig. 2. TEM images of (a) Br-functionalized CdSe NRs (inset: close-up of individual Br-functionalized CdSe NRs, a scale bar is 5 nm) and (b) P3HT-CdSe NR nanocomposites in P3HT matrix. (c) X-ray diffraction (XRD) pattern of CdSe NRs prepared with TDPA:BrCH2-BA =3:1. The standard XRD pattern of wurtzite CdSe is shown.
CdSe NRs was confirmed by the Fourier transform infrared spectroscopy (FT-IR) and 1H NMR measurements. Fig. 3 compares the FTIR spectra of Br-CH2-BA-CdSe NRs (black), N3-CH2-BA-CdSe NRs (red), and P3HT-CdSe NR nanocomposites (blue), respectively. Strong absorption at 2954 cm−1, 2915 cm−1, and 2850 cm−1 were assigned to the asymmetric CeH stretching vibrations in –CH3, –CH2, and the symmetric CeH stretching vibration in –CH2, respectively, from the alkyl side chains in P3HT and aliphatic ligands (i.e., TDPA) [16]. Obviously, the characteristic eN3 vibration was appeared at 2040 cm−1 after the bromine groups of BrCH2-BA-functionalized CdSe NRs were converted into azide groups, supporting the successful conversion of bromine into azide moiety [25]. This peak was then weakened after the N3CH2-BA-functionalized CdSe NRs coupled with P3HT-≡ via a 1,3dipolar cycloaddition between ethynyl groups and azide groups, suggesting some fraction of N3CH2-BA ligands on CdSe NRs still remain. It should be noted that the residue sodium azide were washed several times with water/methanol mixture. In addition, 1H NMR characterization also clearly substantiates the occurrence of coupling between P3HT-≡ and N3CH2-BA-capped CdSe NR. A proton signal at 3.5 ppm from the ethynyl functional moieties on the P3HT chains (Fig. 4a) was completely disappeared after click chemistry (Fig. 4b). It is worth noting that the 1H NMR sample was prepared without separating detached P3HT. On the basis of the disappearance of ethynyl peak at 3.5 ppm from 1H NMR and the existence of weakened azide peak at 2040 cm−1 from FTIR, it is clear that the amount of azide functional
Fig. 3. FTIR spectra of BrCH2-BA-functionalized CdSe NRs (black), N3-CH2-BAfunctionalized CdSe NRs (red), and P3HT-CdSe NR nanocomposites (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. 1H NMR spectra of (a) ethynyl-terminated P3HT, and (b) P3HT-CdSe NR nanocomposites.
Fig. 5. (a) The absorption spectra of ethynyl-terminated P3HT (black), BrCH2BA-capped CdSe NRs (red), and P3HT-CdSe NR nanocomposites (blue) prepared by click chemistry. (b) Emission spectra of ethynyl-terminated P3HT (black) and P3HT-CdSe NR nanocomposites (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
groups on the CdSe NR surface was enough to render the complete coupling of ethynyl groups on P3HT-≡. Photophysical properties of N3-CH2-BA capped CdSe NR, ethynylterminated P3HT, and P3HT-CdSe NR nanocomposites in THF were explored by UV–vis absorbance and photoluminescence (PL) spectroscopies as shown in Fig. 5, respectively. Obviously, the absorption spectrum of P3HT-CdSe NR nanocomposites showed the characteristic peaks of both constituents, with a 445-nm peak from P3HT and the absorption edge around 600 nm originated from CdSe NRs; this served as another evidence of the successful formation of nanocomposites [26,27]. Notably, the strong quenching of the emission at 574 nm that is designated to P3HT from P3HT-CdSe NRs nanocomposites relative to the pristine P3HT homopolymer was observed (Fig. 5b), indicating the charge transfer between P3HT and CdSe nanorods.
NMR. The complete disappearance of ethynyl peak and the weakened absorption of azide characterization peak clearly demonstrated the occurrence of coupling. Moreover, the weakened azide peak obviously suggested that the amount of N3CH2-BA molecules on the CdSe NR surface was enough to enable complete grafting of ethynyl-terminated P3HT. To this end, this novel yet robust strategy toward producing anisotropic CP-NC nanocomposites may offer opportunities in various optoelectronic applications including LEDs, lasers, and bio-imaging. Acknowledgement This work was supported by the Hongik University new faculty research support fund.
4. Conclusion
References
In this study, we have successfully crafted anisotropic CdSe-P3HT core-shell nanocomposites via catalyst-free click chemistry between ethynyl group in P3HT and functionalized CdSe NRs which were directly synthesized by utilizing bifunctional ligands, thereby dispensing with the need for ligand exchange process. The influence of the ratio between fatty aliphatic ligands and mobile aromatic ligands on the shape of CdSe NCs was scrutinized to optimize the shape of functionalized CdSe NRs for a wide range of potential opto-electronic applications. These parameters had a significant impact on the shape evolution of CdSe NCs. The grafting of ethynyl terminated P3HT with azide functionalized CdSe NRs were successful as confirmed by FTIR and 1H
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Preparation of anisotropic CdSe-P3HT core-shell nanorods using directly synthesized Br-functionalized CdSe nanorods
T
Jaehan Jung Department of Materials Science and Engineering, Hongik University, Sejong 30016, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Organic-inorganic Hybrid CdSe P3HT Nanorods Surface engineering
The simple yet robust synthetic strategy toward organic-inorganic nanocomposites was developed by capitalizing on robust click coupling between functionalized nanocrystals (NCs) and conjugated polymers (CPs). The functionalized NCs were directly synthesized dispensing with surface engineering of NCs such as ligand exchange by employing bifunctional ligand (i.e., 4-bromomethyl benzoic acid) at the NC synthesis stage. The direct synthetic conditions toward bromine-functionalized CdSe nanorods (NRs) were scrutinized by precisely tailoring the ratio of 4-bromomehtyl benzoic acid over aliphatic ligands. Subsequent substitution of bromine moiety at the NRs into azide yielded azide-terminated NRs. Finally, ethynyl-terminated poly(3-hexylthiophene) (P3HT) were grafted onto azide-functionalized NR surface via click chemistry, forming intimately contact P3HT-CdSe NR nanocomposites. Transmission electron microscope measurement revealed that CPs render the effective dispersion of NRs in the CP matrices. The success of grafting between CPs and NCs was substantiated by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. The optical properties of P3HTCdSe NR nanocomposites were explored with absorption and photoluminescence studies.
1. Introduction Conjugated polymers (CPs) have drawn considerable attention as promising materials in the field of solar cells, light-emitting diodes, and sensors [1–3]. Their advantages such as low-weight, flexibility, and large area production open up new opportunities for easy and low-cost manufacturing of optoelectronic devices [4–7]. Among various CPs, poly(3-hexylthiophene) (P3HT) is one of the most extensively studied semiconductor CPs owing to its solution-processability, tailorable electrochemical properties, and excellent optoelectronic properties [8]. Semiconducting nanocrystals (NCs) exhibit size- and composition-dependent tunable optoelectronic properties, including band gap, emission, and absorption range due to the quantum-confinement effect [9,10]. In this regard, hybrid nanocomposites composed of semiconductor nanocrystals (NCs) and conjugated polymers (CPs) have garnered considerable attention due to their complementary merits. Especially, a one dimensional NCs/CPs core/shell architecture is a promising building block for the use in a wide range of optoelectronic devices owing to its peculiar properties [11]. However, hybrid nanocomposites prepared via a simple physical blending of CPs and NCs have suffered from several problems, including microscopic phase separation and the existence of insulating interfacial layer, thus limiting the performance
of the resulting devices [12]. To address these problems, the chemical tethering of CPs onto the NC surface was introduced to achieve an excellent dispersion of NCs in the CP matrices as well as to promote the electronic interaction between these two semiconductors [13,14] It mostly involves the ligand exchange procedure (e.g., refluxing with pyridine or inorganic ligands) to engineer the NC surface with bifunctional ligands for chemical coupling with CPs [15,16]. However, such typical routes to CP-grafted NCs greatly relies on the ligand exchange process, which is usually multi-steps thus time-consuming. Moreover, it is often not effective resulting in residue surfactants on NC surface [16]. Therefore, a simple yet effective alternative synthetic route that eliminates the need for the ligand exchange is demanded. Herein, we report on a simple yet robust synthetic strategy toward crafting anisotropic CdSe-P3HT core-shell nanocomposites by capitalizing on the direct synthesis of functionalized NCs and subsequent click coupling with end-functionalized CPs. 4-bromomethyl benzoic acids (BrCH2-BA) were employed as bifunctional ligands in this study as carboxylic acids are most widely used capping agent for CdSe NCs and their bromine moiety can be used in a wide range of chemical coupling [17]. It is noteworthy that their aromatic ring can facilitate the electronic interaction and charge carrier transport [16]. Specifically, the synthetic condition toward Br-functionalized CdSe nanorods was discovered by tailoring the ratio of 4-bromomehtyl benzoic acid over
E-mail address: [email protected]. https://doi.org/10.1016/j.surfcoat.2019.01.093 Received 29 August 2018; Received in revised form 6 January 2019; Accepted 25 January 2019 Available online 26 January 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 362 (2019) 84–89
J. Jung
Scheme 1. Grafting ethynyl-terminated P3HT (i.e., P3HT-≡) onto N3CH2-BA-functionalized CdSe NRs by catalyst-free click chemistry, yielding P3HT-CdSe NR nanocomposites.
solution became clear and transparent, 0.7 ml of 1 M Se/TOP solution injected swiftly at 300 °C to initiate the nucleation and growth. The reaction was allowed for 10 min at 300 °C. Then heating mantle was removed to stop the reaction. When the solution cooled down to 60 °C, 2 ml of THF were added. The BrCH2-BA-capped CdSe NRs were purified three times by precipitation with the excess amount of methanol.
aliphatic ligands. Bromine moieties of BrCH2-BA-capped CdSe NRs were then converted into azide groups, yielding N3CH2-BA-functionalized CdSe NRs. Subsequently, synthesized ethynyl-terminated P3HT were grafted onto CdSe NR surface via click chemistry, producing anisotropic P3HT-CdSe NR nanocomposites. The success of grafting was confirmed by Fourier transform infrared spectroscopy and nuclear magnetic resonance. The optical properties of P3HT-CdSe NR nanocomposites were explored with absorption and photoluminescence studies. To this end, the simple synthetic procedure toward producing anisotropic NC-CP core-shell hybrid nanocomposites possibly can be employed in various applications including laser, solar cells, and LEDs where the proper alignment is required.
2.3. Grafting ethynyl-terminated P3HT onto CdSe NRs As synthesized Br-CH2-BA-capped CdSe NRs were mixed with Sodium azide (NaN3) in THF solution and then sealed and stirred at room temperature for three days. Excess amount of NaN3 was removed by centrifugation for three times with methanol/water. The resulting azidomethyl benzoic acid (N3CH2-BA-CdSe NRs) was then precipitated with the excess amount of methanol. Subsequently, 50 mg of P3HT-≡ and 50 mg of N3CH2 -BA-CdSe NRs were mixed in 20 ml THF and stirred at 65 °C under Ar for two days. The final product (i.e., P3HT-CdSe NR nanocomposites) was cooled to room temperature and diluted 10 times with THF. The resulting solution was precipitated with methanol twice to remove uncoupled P3HT.
2. Experimental All chemicals, including 4-bromomethyl benzoic acid, cadmium oxide, sodium azide, selenium powder, 2,5-dibromo-3-hexylthiophene, Ni(dppp)Cl2, tert-butylmagnesium chloride (2 mol/l in diethyl ether), and ethynylmagnesium bromide (0.5 mol/l in THF) from Sigma Aldrich, tetradecyl phosphonic acid (TDPA,97%), and tri-n-octylphosphine (TOP, 90%), from Alfa Aesar, and tri-n-octylphosphine oxide (TOPO, 90%) from Strem chemicals were used as received. THF (VWR, 99%) was refluxed over sodium wire and distilled from sodium naphthalenide solution.
2.4. Characterizations The morphology of BrCH2-BA-capped CdSe NCs and P3HT-CdSe NR nanocomposites were characterized by transmission electron microscopy (TEM) (JEOL 100CX and Tecnai F30). 1H NMR spectra (Varian VXR-400 spectrometer) and Fourier transform infrared spectroscopy (FTIR) were examined to confirm the occurrence of grafting between P3HT-≡ and N3CH2-BA-capped CdSe NRs. The absorption and emission spectra were recorded with a UV–vis (UV-2600, Shimadzu) and photoluminescence spectrometers.
2.1. Synthesis of ethynyl-terminated poly(3-hexylthiophene) (P3HT) Ethynyl-terminated P3HT was synthesized through a quasi-living Grignard metathesis (GRIM) method [18]. 2,5-Dibromo-3-hexylthiophene (0.815 g, 2.5 mmol) was dissolved in anhydrous THF (5 ml) in a round bottom flask (250 ml) and stirred under Ar. Subsequently, 1.25 ml of 2.0 M tert-butylmagnesium chloride was added and then the mixture was stirred for 2 h at room temperature. After it was diluted with 25 ml of anhydrous THF, Ni(dppp)Cl2 (56 mg, 0.1 mmol) was added to initiate polymerization. The resulting solution was stirred for 30 min at room temperature, producing intermediate P3HT; it was then reacted with ethynylmagnesium bromide (2 ml, 1 mmol) in THF for 30 min, yielding ethynyl-terminated P3HT (P3HT-≡). The product was obtained by precipitating the reaction mixture in methanol, filtering in an extraction thimble, and washing by Soxhlet extraction with methanol, hexane, and chloroform sequentially. It was recovered after chloroform evaporated.
3. Results and discussion The synthesis of inorganic-organic CdSe-P3HT core-shell nanorod nanocomposites is depicted in Scheme 1. It involves a direct synthesis of bromine-functionalized CdSe nanorods (NRs) and robust catalyst-free click chemistry between ethynyl-terminated poly(3-hexylthiophene) (P3HT-≡) and functionalized CdSe NRs. It is worth noting that the direct synthesis of functionalized CdSe NRs by utilizing bifunctional ligands at NC synthesis stage dispenses with the need for the tedious ligand exchange procedure. In this study, 4-bromomethyl benzoic acid (Br-CH2-BA) possessing carboxylic acid at one end and bromine moiety at the other end were utilized as bifunctional ligands with alkylphosphonic acids (i.e., tetradecylphosphonic acid (TDPA)) as carboxylic acids are broadly used anchoring agents for CdSe NCs as well as bromine moiety can easily enable coupling reaction [19]. The addition of aliphatic phosphonic acid ligands (e.g., TDPA) prevents the aggregation by providing enough hindrance among NCs and induces elongated growth of CdSe NCs [20]. Specifically, BrCH2-BA-capped CdSe NRs
2.2. Preparation of BrCH2-BA-capped CdSe NRs 1 mmol of CdO, 2 mmol of surfactants (4-bromomethyl benzoic acid (BrCH2-BA) and tetradecyl phosphonic acid (TDPA)), and 1.2 g of TOPO were placed in a three-neck flask. The molar fraction of BrCH2-BA/ TDPA was 1:0, 3:1, 1:3, and 0:1 respectively. The mixture then degassed at 120 °C for 60 min and was then heated to 300 °C under Ar. After the 85
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Fig. 1. TEM images of CdSe NCs at different molar ratio of BrCH2-BA to TDPA. (a) BrCH2-BA:TDPA = 0:1, (b) BrCH2-BA:TDPA = 1:3, (c) BrCH2-BA:TDPA = 3:1, and (d) BrCH2-BA:TDPA = 1:0.
surface. Anisotropic semiconductor nanorods exhibit intriguing opto-electronic properties such as peculiar electronic structures and polarizations with respect to spherical nanocrystals since their carriers experience strong confinement only along two dimensions, whereas they can delocalize along the long axis of the rods [20,23]. Thereby nanorods offer great potentials in a wide range of applications especially when proper alignment can be achieved [24]. In this context, multifunctional inorganic-organic core-shell nanorods architecture can be served as promising building blocks for the use in opto-electronic applications including lasers, LEDs, and solar cells. In this regard, semiconductor CP grafted CdSe NRs core-shell nanocomposites were synthesized via click coupling between azide functionalized CdSe NRs and ethynyl-terminated P3HT. The high quality BrCH2-BA-capped CdSe NRs were prepared with the molar ratio of TDPA: BrCH2-BA of 3:1 to trigger anisotropic growth but to prevent the formation of branched structure. The high-resolution TEM images revealed that prepared BrCH2-BA-capped CdSe NRs were 40.2 nm in length with a diameter of 5.1 nm and exhibited a good crystallinity (Fig. 2a). The lattice spacing of 0.351 nm in an inset of Fig. 2a corresponds to d-spacing of (0002) plane of wurtzite CdSe crystals, substantiating the induced growth along c-axis. X-ray diffraction (XRD) characterization of resulting CdSe nanorods in Fig. 2c also clearly supported the formation of wurtzite structure. The bromine functional groups of as-synthesized BrCH2-BA-capped CdSe NRs were then converted into azide moieties with sodium azide in THF and followed by coupling with ethynyl-terminated P3HT via catalyst-free click chemistry. Notably, the resulting P3HT-CdSe NR nanocomposites can be readily dispersed in the P3HT homo polymer matrices (Fig. 2b). The success of grafting P3HT chains onto N3CH2-BA-functionalized
were prepared employing bifunctional ligands followed by azidation of bromine group into azide group. Subsequently, P3HT-≡ were grafted onto azide-capped CdSe NR surface via catalyst-free click chemistry between these two constituents. The morphology of Br-terminated CdSe nanocrystals (BrCH2-BAcapped CdSe NCs) were first precisely controlled by tuning the ratio between bifunctional ligands (i.e., BrCH2-BA) and aliphatic ligands (i.e., TDPA). To investigate the influence of the molar ratio of BrCH2-BA over TDPA on the shape of CdSe NCs, their faction was systematically controlled to be 1:0, 3:1, 1:3, and 0:1, respectively. TEM measurement shown in Fig. 1 revealed that CdSe QDs were synthesized when TDPA was solely used while the addition of short mobile ligands (i.e., BrCH2BA) caused anisotropic or branched structures, depending on the molar ratio of TDPA and BrCH2-BA [21]. It is worth noting that the lower monomer (i.e., cadmium complexes) concentration resulted in QD shape despite the use of phosphonic acids (i.e., TDPA) that often induces elongated growth due to their peculiar binding nature. Phosphonic acid is known to bind stronger to the (0110) or (112 0) surfaces than to the (0001) and (0001) facets (c-axis direction), thereby effectively facilitating anisotropic growth [20]. Intriguingly the addition of BrCH2-BA leads to CdSe nanorods as their mobile and reactive nature boosts reaction kinetic and facilitates the anisotropic growth [22]. However, uncontrolled shape was observed in the absence of TDPA (Fig. 1d) possibly because the mobile and reactive nature of Cd-BACH2Br monomers resulted in faster growth and thus became more susceptible to defects such as stacking faults or twinning, showing extensive branching. [21] It is noteworthy that BrCH2-BA clearly played a significant role in determining the shape of NCs, which can serve as indirect evidence of the existence of BrCH2-BA ligands on CdSe NR 86
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Fig. 2. TEM images of (a) Br-functionalized CdSe NRs (inset: close-up of individual Br-functionalized CdSe NRs, a scale bar is 5 nm) and (b) P3HT-CdSe NR nanocomposites in P3HT matrix. (c) X-ray diffraction (XRD) pattern of CdSe NRs prepared with TDPA:BrCH2-BA =3:1. The standard XRD pattern of wurtzite CdSe is shown.
CdSe NRs was confirmed by the Fourier transform infrared spectroscopy (FT-IR) and 1H NMR measurements. Fig. 3 compares the FTIR spectra of Br-CH2-BA-CdSe NRs (black), N3-CH2-BA-CdSe NRs (red), and P3HT-CdSe NR nanocomposites (blue), respectively. Strong absorption at 2954 cm−1, 2915 cm−1, and 2850 cm−1 were assigned to the asymmetric CeH stretching vibrations in –CH3, –CH2, and the symmetric CeH stretching vibration in –CH2, respectively, from the alkyl side chains in P3HT and aliphatic ligands (i.e., TDPA) [16]. Obviously, the characteristic eN3 vibration was appeared at 2040 cm−1 after the bromine groups of BrCH2-BA-functionalized CdSe NRs were converted into azide groups, supporting the successful conversion of bromine into azide moiety [25]. This peak was then weakened after the N3CH2-BA-functionalized CdSe NRs coupled with P3HT-≡ via a 1,3dipolar cycloaddition between ethynyl groups and azide groups, suggesting some fraction of N3CH2-BA ligands on CdSe NRs still remain. It should be noted that the residue sodium azide were washed several times with water/methanol mixture. In addition, 1H NMR characterization also clearly substantiates the occurrence of coupling between P3HT-≡ and N3CH2-BA-capped CdSe NR. A proton signal at 3.5 ppm from the ethynyl functional moieties on the P3HT chains (Fig. 4a) was completely disappeared after click chemistry (Fig. 4b). It is worth noting that the 1H NMR sample was prepared without separating detached P3HT. On the basis of the disappearance of ethynyl peak at 3.5 ppm from 1H NMR and the existence of weakened azide peak at 2040 cm−1 from FTIR, it is clear that the amount of azide functional
Fig. 3. FTIR spectra of BrCH2-BA-functionalized CdSe NRs (black), N3-CH2-BAfunctionalized CdSe NRs (red), and P3HT-CdSe NR nanocomposites (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. 1H NMR spectra of (a) ethynyl-terminated P3HT, and (b) P3HT-CdSe NR nanocomposites.
Fig. 5. (a) The absorption spectra of ethynyl-terminated P3HT (black), BrCH2BA-capped CdSe NRs (red), and P3HT-CdSe NR nanocomposites (blue) prepared by click chemistry. (b) Emission spectra of ethynyl-terminated P3HT (black) and P3HT-CdSe NR nanocomposites (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
groups on the CdSe NR surface was enough to render the complete coupling of ethynyl groups on P3HT-≡. Photophysical properties of N3-CH2-BA capped CdSe NR, ethynylterminated P3HT, and P3HT-CdSe NR nanocomposites in THF were explored by UV–vis absorbance and photoluminescence (PL) spectroscopies as shown in Fig. 5, respectively. Obviously, the absorption spectrum of P3HT-CdSe NR nanocomposites showed the characteristic peaks of both constituents, with a 445-nm peak from P3HT and the absorption edge around 600 nm originated from CdSe NRs; this served as another evidence of the successful formation of nanocomposites [26,27]. Notably, the strong quenching of the emission at 574 nm that is designated to P3HT from P3HT-CdSe NRs nanocomposites relative to the pristine P3HT homopolymer was observed (Fig. 5b), indicating the charge transfer between P3HT and CdSe nanorods.
NMR. The complete disappearance of ethynyl peak and the weakened absorption of azide characterization peak clearly demonstrated the occurrence of coupling. Moreover, the weakened azide peak obviously suggested that the amount of N3CH2-BA molecules on the CdSe NR surface was enough to enable complete grafting of ethynyl-terminated P3HT. To this end, this novel yet robust strategy toward producing anisotropic CP-NC nanocomposites may offer opportunities in various optoelectronic applications including LEDs, lasers, and bio-imaging. Acknowledgement This work was supported by the Hongik University new faculty research support fund.
4. Conclusion
References
In this study, we have successfully crafted anisotropic CdSe-P3HT core-shell nanocomposites via catalyst-free click chemistry between ethynyl group in P3HT and functionalized CdSe NRs which were directly synthesized by utilizing bifunctional ligands, thereby dispensing with the need for ligand exchange process. The influence of the ratio between fatty aliphatic ligands and mobile aromatic ligands on the shape of CdSe NCs was scrutinized to optimize the shape of functionalized CdSe NRs for a wide range of potential opto-electronic applications. These parameters had a significant impact on the shape evolution of CdSe NCs. The grafting of ethynyl terminated P3HT with azide functionalized CdSe NRs were successful as confirmed by FTIR and 1H
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