Polyhedral Oligomeric Silsesquioxane as a Ligand for CdSe Quantum

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Polyhedral Oligomeric Silsesquioxane as a Ligand for CdSe Quantum Dots Yu Wang,† Aleksandar Vaneski,† Haihua Yang,† Shuchi Gupta,† Frederik Hetsch,† Stephen V. Kershaw,† Wey Yang Teoh,‡ Huanrong Li,*,§ and Andrey L. Rogach*,† †

Department of Physics and Materials Science and Centre for Functional Photonics (CFP), City University of Hong Kong, Hong Kong SAR ‡ Clean Energy and Nanotechnology (CLEAN) Laboratory, School of Energy and Environment, City University of Hong Kong, Hong Kong SAR § School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin, 300130, People’s of Republic of China ABSTRACT: We report the synthesis of CdSe quantum dots (QDs) using a mercapto-substituted polyhedral oligomeric silsesquioxane (SH-POSS). The bulky siloxane cage-like core of the ligand makes this an ideal steric stabilizer, and comparison with conventional branched alkyl phosphonic acid capped CdSe QDs shows SH-POSS capped QDs to have superior optical properties including photoluminescence quantum efficiencies and fluorescence lifetimes. The POSS shell allows for the access of small electrolyte ions and electron transport from the surface of the QDs, evidenced by better performance as a photosensitizer in conjunction with a titania nanotube array electron acceptor in comparison to the conventionally stabilized QDs.

1. INTRODUCTION Polyhedral oligomeric silsesquioxane (POSS) has attracted a great deal of interest owing to its unique cage-like molecular structure containing an inorganic siloxane core surrounded by eight organic corner groups (Figure 1a)1−3 These organic groups can serve as reaction sites for further functionalization, while the rigid inorganic silica-like structure of the core makes POSS an ideal building block for constructing novel functional materials with enhanced thermomechanical properties, thermal stability, as well as oxygen and corrosion resistance.4−6 POSS derivatives have been tailored for various applications in nanocomposites,7 porous materials,8 medicine carriers, 9 catalysis,10 self-assembled structures,11 bioimaging,12 as well as in photonics and electroluminescence devices.13 Recently, POSS has been employed as an excellent covalent ligand to stabilize graphene nanosheets,14 gold15 and palladium nanoparticles.16 Chang and co-workers15 utilized a thiol-monofunctionalized mercaptopropylisobutyl-POSS (denoted as SHPOSS further below, Figure 1a) as a ligand for fabrication of POSS-Au hybrid nanoparticles. The search for novel ligands for semiconductor nanocrystals (colloidal quantum dots, QDs) providing them with useful functionalities constitutes an active area of research nowadays.17 Rizvi et al. recently showed that the simultaneous use of SH-POSS alongside two other ligands mercaptosuccinic acid and D-cysteine favorably reduced the cytotoxicity of aqueous-based CdTe QDs.18 Herein, we report a synthesis of SH-POSS capped CdSe QDs (denoted as POSSCdSe QDs, sketched in Figure 1b), highlight their favorable © XXXX American Chemical Society

light-emission characteristics, and demonstrate their use as efficient photosensitizers for TiO2 nanotube (TNT) arrays.

2. EXPERIMENTAL SECTION 2.1. Synthesis of POSS-CdSe QDs. The synthesis was carried out in a 50 mL three necked round-bottomed flask equipped with a cooling condenser, temperature controller, septum, and a stirring rod. A precursor solution of 0.21 g (0.79 mmol) of cadmium acetate (Cd(CH3COO)2), 2.78 g (11.515 mmol) of n-hexadecylamine (HDA), and 2.41 g (2.705 mmol) of mercaptopropylisobutyl-POSS (SH-POSS, Hybrid Plastics) was heated to 80 °C, degassed by applying vacuum on a Schlenk line, and heated while stirred under nitrogen atmosphere to 235 °C. At this temperature, a solution of degassed 0.2 g of selenium in 4 g trioctylphosphine (TOP) was quickly injected using a syringe. After desired reaction time, ranging from 2 to 30 min, the reaction was quenched by removing the heating mantle and the mixture was allowed to cool down to 80 °C, followed by addition of 20 mL of chloroform. The as-prepared POSS-CdSe QDs were subsequently precipitated by excess of ethanol, and redispersed in chloroform. 2.2. Synthesis of Ref-CdSe QDs.19 A precursor solution of 0.21 g (0.79 mmol) of cadmium acetate (Cd(CH3COO)2), 2.78 g (11.515 mmol) of n-hexadecylamine (HDA), and 0.75 g Received: November 14, 2012 Revised: January 4, 2013

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dx.doi.org/10.1021/jp3112843 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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images were taken on a JEOL JEM-2100F instrument operated at 200 kV. UV−vis absorption spectra were recorded using a Varian Cary 50 UV−visible spectrophotometer. Steady-state and time-resolved photoluminescence (PL) spectra were measured on an Edinburgh Instrument FLS920P spectrometer, with a 450 W xenon lamp as the steady-state excitation source, and a picosecond pulsed diode laser (EPL-405 nm, pulse width: 48.9 ps) as the single wavelength excitation light source for time-correlated single-photon counting (TCSPC) measurements. All spectra were obtained at room temperature. The PL quantum yield (QY) of QDs was evaluated according to the procedure described in details recently21 using Rhodamin 6G (PL QY = 95%) as reference standards. The thermogravimetric analysis (TGA) was performed on a SDT Q-600 analyzer at a heating rate of 10 °C/min from 30 to 700 °C under slow argon flow. Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer 2000 FTIR spectrometer. Elemental concentration of QDs deposited on TNT was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Perkin-Elmer, Optima 2100DV). Field-effect transistor measurements were done with a Keithley 2612 sourcemeter. 2.5. Photoelectrochemical (PEC) Measurements. The PEC set up followed a classic three-electrode system, comprising of POSS-CdSe QDs or TDPA-CdSe QDs sensitized TNT photoanode, platinum counter electrode and Ag/AgCl reference electrode. Unless otherwise stated, the electrolyte consisted of 0.1 M Na2S and 0.01 M sulfur aqueous solution deaerated with N2 for 30 min prior to use. A 300 W xenon lamp (Newport) was used as the light source in the presence of 420 nm cutoff filter (GG420, Newport). A Solartron Modulab potentiostat was employed to record the photocurrent and open-circuit voltage characteristics. Excitation under solar irradiance was simulated by replacing the cutoff filter with an A.M. 1.5 global filter (Newport) and incident filtered light intensity adjusted to 100 mW cm−2. Monochromatic photoresponse was measured in a Teflon cell with quartz window in the presence of 0.1 M Na2SO4 aqueous electrolyte. Focused irradiation beam was provided by a 300 W Xenon lamp (Newport) coupled to a monochromator unit (Cornerstone 130).

Figure 1. (a) Chemical structure of mercaptopropylisobutyl-POSS, (b) schematic drawing of a CdSe QD capped with SH-POSS ligands, (c) FT-IR spectra of SH-POSS ligand, ref-CdSe QDs, and POSS-CdSe QDs, (d) TGA curves of SH-POSS ligand, ref-CdSe QDs (4.3 nm), and POSS-CdSe QDs (4.3 nm), all in Ar, (e) TEM overview image, and (f) HRTEM image of POSS-CdSe QDs. The inset in (e) shows the SAED pattern of the sample.

3. RESULTS AND DISCUSSION 3.1. Synthesis, Characterization and Luminescence Properties of POSS-CdSe QDs. The synthesis of POSS-CdSe QDs has been conducted in a similar fashion as for the CdSe nanocrystals grown in a commonly used three-component coordinating solvent mixture of trioctylphosphine (TOP), hexadecylamine (HDA), and tetradecylphosphonic acid (TDPA). This latter material is denoted as ref-CdSe QDs further below. TDPA was substituted by the equimolar amount of SH-POSS, and the reaction temperature was lowered from 270 to 235 °C in order to slow down the otherwise very fast growth kinetics. The resulting QDs could be conveniently purified by precipitating with ethanol, and were easily soluble in a variety of organic solvents, including chloroform, tetrahydrofuran, chlorobenzene, toluene, and hexane. Colloidal suspensions of POSS-CdSe QDs in all of these solvents were stable in air without aggregation or change of the optical properties for at least half a year, when stored in darkness or under day light. Comparison of FT−IR spectra of the SH-POSS ligand, refCdSe QDs, and POSS-CdSe QDs (Figure 1c) clearly showed

(2.705 mmol) of tetradecylphosphonic acid (TDPA) was heated to 80 °C, degassed by applying vacuum using Schlenk line technique, and heated while stirred under nitrogen atmosphere to 270 °C. At this temperature, a solution of degassed 0.2 g of selenium in 4 g trioctylphosphine (TOP) was quickly injected using a syringe. Other steps were the same as for POSS-CdSe QD synthesis. 2.3. Deposition of CdSe QDs onto TiO2 Nanotube Arrays. The deposition procedures were carried out as described in ref 20 by immersion of titania nanotube arrays into the chloroform solutions of POSS-CdSe or TDPA-CdSe QDs for 10 h. The obtained composite samples with the same particle size of POSS-CdSe QDs and TDPA-CdSe QDs were sintered at 150 °C for 1 h under N2 atmosphere after evaporation of solvent. 2.4. Characterization. X-ray diffraction (XRD) patterns were collected with a Philips X’pert X-ray diffractometer using Cu−Kα radiation. Transmission electron microscopy (TEM) B



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Figure 2. (a) Absorption and (b) PL (λex = 405 nm) spectra of POSS-CdSe QDs, (c) PL decay curves for POSS-CdSe QDs samples excited at λex = 405 nm, and (d) a photograph showing the photoluminescence color change of the POSS-CdSe QDs under a UV lamp during the growth. The color coding for all spectra in frames (a)−(c) corresponds to different sizes of CdSe QDs, as summarized under frame (a).

the presence of a characteristic peak at 1100 cm−1 attributed to the asymmetric stretching of the Si−O−Si unit of the POSS cage15,22 for POSS-CdSe QDs. The ref-CdSe QDs possess a distinctly different FT−IR spectrum most notably indicating the existence of the surface-bound amino group of HDA23 at 2360 cm−1, along with features at 1080 cm−1, 2345 cm−1 and 1035 cm−1 assigned to PO (which may originate both from TDPA and from the TOP complex with Se), N−H and C−N group vibrations, respectively. The thermogravimetric analysis (TGA) shows that the thermal decomposition occurs very similarly for SH-POSS and POSS-CdSe QDs, in stark contrast to the two-step curve of the ref-CdSe QD sample (Figure 1d). The TGA data gives compelling evidence that the SH-POSS ligand dominates the surface relative to the two other coligands. An 86% weight percentage of SH-POSS could be estimated from TGA curves for 4.3 nm sized POSS-CdSe QDs, in line with near complete surface site occupancy, which indicates that SH-POSS serves as the major capping agent for CdSe nanoparticles. A simple geometrical analysis based on the QD diameter, and primitive unit cell dimensions for the bulk wurtzite CdSe lattice leads to an estimated average of 140 available POSS coordination sites per dot, which would give a theoretical weight loss of

approximately 74%, in reasonable agreement with the experimental value. Transmission electron microscopy (TEM) images of a representative sample of the as-synthesized (no size-selective fractionation procedure has been applied) sample showed that the POSS-CdSe QDs were roughly spherical with a size dispersion of 15% (Figure 1e). The corresponding highresolution TEM image of a single POSS-CdSe QD (Figure 1f) indicates a high crystallinity with a lattice fringe distance of 0.37 nm, which is in a good agreement with the (100) plane of the hexagonal CdSe structure. Both the selected area electron diffraction (SAED) pattern (the inset in Figure 1e) and the powder X-ray diffraction pattern (not shown) further confirm the formation of wurtzite (hexagonal phase) CdSe nanocrystals. The growth kinetics of the POSS-CdSe QDs can be conveniently monitored by their absorption and photoluminescence (PL) spectra, which are presented in Figure 2a,b for the 2 to 30 min particle growth interval. Particles formed after 2 min of growth, with a characteristic absorption maximum at ∼430 nm and an estimated size of 1.8 nm24 are clusters,25 possessing a broad PL spectrum (Figure 2b) corresponding to their broad, near-white light emission as shown on the digital photograph of Figure 2d. The absorption peak shifted toward ∼527 nm after 3 min of growth (estimated C



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Table 1. Photoluminescence Characteristics, Including Full Width at Half Maximum (FWHM) and PL Room Temperature Quantum Yields (QY) of the Band-Edge PL of As-Prepared POSS-CdSe QDs of Different Sizes, And the Fitting Parameters of the Corresponding Photoluminescence Decay Curvesa samples 1.8 2.8 3.3 3.7 3.8 4.0 4.2 4.3 a

nm nm nm nm nm nm nm nm

band-edge PL peak [nm] 470 545 575 589 599 603 605 606

± ± ± ± ± ± ± ±

2 2 2 2 2 2 2 2

fwhm [nm] 31 29 32 34 38 41 45

± ± ± ± ± ± ±

2 2 2 2 2 2 2

PL QY [%]

B1 [%]

± ± ± ± ± ± ±

35 21 30 38 40 41 43 45

8.1 6.3 4.5 3.5 3.2 2.3 1.5

0.7 0.5 0.4 0.3 0.3 0.2 0.2

τ1 [ns] 117.0 29.5 27.5 23.5 23.0 22.0 21.5 20.0

± ± ± ± ± ± ± ±

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

B2 [%] 65 79 70 62 60 59 57 55

τ2 [ns] 296.0 132.5 108.0 81.5 80.5 80.0 68.0 62.5

± ± ± ± ± ± ± ±

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

B1 and B2 represent the amplitudes of the fast and slow components and τ1 and τ2 represent the time constants, respectively.

Figure 3. (a) I−V curves (in chopping illumination mode) of bare titania nanotubes, TiO2 nanotubes sensitized with POSS-CdSe and TiO2 nanotubes sensitized with ref-CdSe QDs under λ > 420 nm irradiation, light source: 300 W xenon lamp with 420 nm cutoff filter, Ag/AgCl reference electrode, Pt counter electrode. (b) Schematic drawing of the electron injection from CdSe QDs into TiO2 nanotubes. (c) Monochromatic photoresponse of TiO2 nanotubes and the QDs-sensitized photoanodes, and (d) comparison of photocurrent stability of the latter under A.M. 1.5 global filter (one sun), the inset showing initial photocurrent response when irradiation begins at 10 s.

ligand have on average 20% longer PL lifetimes (the fast components in the biexponential decay curves, ranging from 117 to 20 ns, summarized in Table 1) as compared with refCdSe QDs grown with conventional ligands. 3.2. Photoelectrochemical Performance of POSS-CdSe QDs. We further evaluated the photosensitization efficiency of POSS-CdSe QDs and compared their performance with refCdSe QDs in terms of collection of photogenerated electrons by vertically aligned TiO2 nanotube arrays, a well-known electron acceptor.28 Ordered TiO2 nanotube arrays (with nanotubes 6 μm in length and 80 nm inner pore diameter) were fabricated by the electrochemical etching of titanium foil using sodium fluoride as the complexation etching source.20 The inner surface area of the nanotubes was decorated with QDs by immersion deposition in respective chloroform solutions of POSS-CdSe and ref-CdSe QDs with the same particle size (4.3 nm) and equal concentration (1.7 × 10−6 M). This resulted in nearly the same deposition density of both

QD size: 2.8 nm), and the narrow band-edge PL peak in the green spectral range became dominant, even so the trap-related red-shifted emission was still present. The latter practically disappeared after 10 min of growth (particle size: 3.7 nm) and the emission color of the growing samples gradually changed from green to red (Figure 2d) due to the weakening of the size related quantum confinement effect. The band-edge PL peaks remained remarkably narrow during the growth (fwhm in the range of 3045 nm), indicating good monodispersity of QDs. Figure 2c shows the PL decay curves for POSS-CdSe QDs samples with excitation at 405 nm, with a corresponding trend of reducing PL lifetimes (and PL quantum yields, ranging from 8% for 2.8 nm QDs to 1.5% for 4.3 nm QDs, as summarized in Table 1) with increasing the particle size.26 The observed PL QYs are reasonably high for the core-only CdSe QDs, in particular taking into account that thiols have been reported to be efficient quenchers of CdSe QD emission in the ligand exchange experiments.27 CdSe QDs with a SH-POSS capping D



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nanocrystals could also find use as building blocks for semiconductor chalcogenide aerogels.31

types of QDs (4 × 1013 QDs/cm2), as confirmed by the inductively coupled plasma-atomic emission (ICP-AE) spectroscopy. Photoelectrochemical properties of the QD-sensitized TiO2 nanotube samples were evaluated by employing a standard three-electrode photoelectrochemical cell-containing aqueous Na2S (0.1 M solution with 0.01 M sulfur) as a redox electrolyte. Figure 3a shows a remarkable increase in photocurrent relative to bare TiO2 nanotubes resulting solely from injection of electrons from the CdSe QDs (Figure 3b), as we used excitation at λ > 420 nm where TiO2 is not absorbing. Extended photocurrent response up to 620 nm was measured (Figure 3c), consistent with the absorption edge of the CdSe QDs. Here, the high photocurrent shows that not only are POSS-CdSe QDs efficient photosensitizers of TiO2 nanotubes but also that the capping POSS molecules provide channels to transport electrolytes for reactions on the surface of QDs, similar to findings for POSS-stabilized metal nanoparticles by Chang and co-workers.15 Importantly, as we show in Figure 3d, POSS capping layer maintained a much higher inherent stability as compared to traditionally used stabilizers on refCdSe QDs under A.M. 1.5 global filter (one sun). The photocurrent of POSS-CdSe QDs was consistently higher than that of the ref-CdSe QDs, which is also reflected in Figure 3a. Absolute quantum efficiencies for both systems at the conditions of our experiments were relatively low (∼1%), which is also related to the particular electrolyte used. We further evaluated the conductivity of close-packed thin films of POSS-CdSe QDs in a field effect transistor (FET) configuration. A silicon wafer with either 100 or 300 nm thermally grown SiO2 as a dielectric layer was used as a back gate, with an interdigit electrode of 5 nm Cr followed by 50 nm of Au deposited on its top by thermal evaporation. CdSe QDs were transferred into hexane and deposited on the interdigit electrodes by spin coating, and the resulting field-effect devices were tested under inert atmosphere in a glovebox. Over a voltage range between −40 V and +40 V for both source-drain and backgate voltage, we could not observe any field effect. The current magnitudes were in the nA range and comparable with data for ref-CdSe QDs, which is not surprising given the bulky nature of the POSS ligand leading to the large interparticle separation in the film. The available literature data on CdSe QD FETs generally report very low conductivity for as-synthesized, untreated QDs films.29,30 Closed-packed films of such QDs may be suitable for applications where rather isolating behavior is needed, like in a memory device.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. hk. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Research Grant Council of Hong Kong S.A.R. (Projects No. T23-713/11 and 103311), and by the National Nature Science Foundation of China (Project No. 21171046). We thank T. F. Hung and H. H. Chan (CityU) for HRTEM and ICP-AE analysis.

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REFERENCES

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4. CONCLUSIONS We have successfully employed SH-POSS as a ligand to produce wurtzite phase CdSe QDs with variable sizes and improved light-emission characteristics. This is attributed to the benefit of the bulkiness of the siloxane core as a steric stabilizer. Despite the nominal bulkiness of the cage-like ligand, compared to the TDPA and HDA, carrier and electrolyte species still have reasonable access to the CdSe QD surface and thus POSSCdSe QDs deposited on TiO2 nanotube arrays served as an efficient photosensitizer able to inject electrons into TiO2, leading to the better and more stable performance as compared to ref-CdSe QDs. The POSS-CdSe QDs introduced here may therefore be promising for a wide range of applications ranging from QD-based solar cells to memory devices. Owing to the availability of siloxane cages on the QD surface, these E



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Polyhedral Oligomeric Silsesquioxane as a Ligand for CdSe Quantum

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