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Aminoalkanoic Acids as Alternatives to Mercaptoalkanoic Acids for the Linker-Assisted Attachment of Quantum Dots to TiO 2

Natalia Rivera-Gonzalez, Saurabh Chauhan, and David F. Watson Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02704 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 29, 2016

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Aminoalkanoic Acids as Alternatives to Mercaptoalkanoic Acids for the Linker-Assisted Attachment of Quantum Dots to TiO2 Natalia Rivera-González, Saurabh Chauhan, and David F. Watson* Department of Chemistry, University at Buffalo, The State University of New York Buffalo, New York 14260-3000

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Abstract Linear aminoalkanoic acids (AAAs) and mercaptoalkanoic acids (MAAs) were characterized as bifunctional ligands to tether CdSe QDs to nanocrystalline TiO2 thin films and to mediate excited-state electron transfer (ET) from the QDs to TiO2 nanoparticles. The adsorption of 12-aminododecanoic acid (ADA) and 12-mercaptododecanoic acid (ADA) to TiO2 followed the Langmuir adsorption isotherm. Surface adduct formation constants (Kad) were ~104 M-1; saturation amounts of the ligands per projected surface area of TiO2 (Γ0) were ~10-7 mol cm-2. Both Kad and Γ0 differed by 20% or less for the two linkers. CdSe QDs adhered to ADA- and MDA-functionalized TiO2 films; data were well-modeled by the Langmuir adsorption isotherm and Langmuir kinetics. For ADA- and MDA-mediated assembly, respectively, values of Kad were (1.8 ± 0.4) × 106 M-1 and (2.4 ± 0.4) × 106 M-1, values of Γ0 were (1.6 ± 0.3) × 10-9 mol cm-2 and (1.2 ± 0.1) × 10-9 mol cm-2, and rate constants were (14 ± 5) M-1 s-1 and (60 ± 20) M-1 s-1. Thus, the thermodynamics and kinetics of linker-assisted assembly were slightly more favorable for MDA than ADA. Steady-state and time-resolved emission spectroscopy revealed that electrons were transferred from both band-edge and surface states of CdSe QDs to TiO2 with rate constants (ket) of ~107 s-1. ET was approximately twice as fast through the thiol-bearing linker 4mercaptobutyric acid (MBA) as through the amine-bearing linker 4-aminobutyric acid (ABA). Photoexcited QDs transferred holes to adsorbed MBA. In contrast, ABA did not scavenge photogenerated holes from CdSe QDs, which maximized the separation of charges following ET. Additionally, ABA shifted electron-trapping surface states to higher energies, minimizing the loss of potential energy of electrons prior to ET. These tradeoffs involving the kinetics and thermodynamics of linker-assisted assembly; the driving force, rate constant, and efficiency of ET; and the extent of photoinduced charge separation can inform the selection bifunctional ligands to tether QDs to surfaces.


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Introduction Quantum dots (QDs) are intriguing harvesters of light and donors of excited charge carriers for solar energy conversion.1-4 Size-dependent energetics enable control over band gaps and band-edge potentials, and multielectron and hot-carrier extraction from QDs have been demonstrated.5-14 Linkerassisted assembly, in which bifunctional ligands tether colloidal QDs to substrates, is an attractive strategy to prepare QD-semiconductor interfaces for charge transfer.15,16 Tethering pre-synthesized colloidal QDs to substrates facilitates the tuning of electronic and photophysical properties. Moreover, the distance and electronic coupling between QDs and the substrate are programmable with the properties of molecular linkers.17-22 Linear mercaptoalkanoic acids (MAAs), particularly 3-mercaptopropanoic acid (MPA), have received the most attention thus far as linkers to tether cadmium chalcogenide QDs to TiO2 and other metal oxides.15-18,23-27 The carboxylic acid group of MAAs coordinates to TiO2, usually as the deprotonated carboxylate.15,16,28 The thiol group of MAAs interacts with QDs, presumably by coordination of the deprotonated thiolate to surface Cd2+ centers.28,29 Spectroscopic studies have revealed that electrons can be transferred efficiently from photoexcited QDs, through MPA, to TiO2.1719,30-32

Notably, the most efficient QD-sensitized solar cells, reported recently by Zhong and coworkers,

incorporate CdSe or CdSe/ZnS QDs tethered via MPA to TiO2.33,34 Despite the established use of MAAs as linkers, they are far from ideal. It is well-established that photogenerated holes can be transferred from CdSe QDs to adsorbed thiolates.35-37 This hole-scavenging mechanism, which is active at QD-MAA-TiO2 interfaces,18,38 reduces the oxidizing potential of photogenerated holes and the distance between electrons (in TiO2) and holes (on thiolate radicals) in the charge-separated state. Additionally, the transfer of holes to adsorbed thiolates may oxidatively degrade MAAs. For these reasons, hole-transfer from QDs to adsorbed bifunctional ligands is undesirable. Amine-bearing bifunctional ligands may be attractive alternatives to MAAs. Primary alkylamines have been reported to scavenge holes from QDs less efficiently than thiol-bearing ligands, if at all, due to the more positive oxidation potential of the alkylamines.37 Additionally, alkylamines can 3 ACS Paragon Plus Environment

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enhance and blue-shift band-edge and surface emission from QDs.37,39-44 This effect has been attributed to surface passivation through the coordination of amines to Cd2+-derived surface electron-trap states, shifting these states to higher energy and eliminating them from radiative and non-radiative decay pathways.39-41,43-46 We hypothesized that linear aminoalkanoic acids (AAAs) may have two distinct advantages over MAAs as bifunctional ligands to tether QDs to TiO2. First, AAAs should not accept photogenerated holes from QDs, maximizing the spatial separation of photogenerated charges following ET to TiO2, minimizing the dissipation of the oxidizing potential of the hole, and preventing the oxidative degradation of linkers and other ligands on QDs. Second, adsorption of AAAs may shift surface states of QDs to higher energies, eliminating non-radiative electron-hole recombination pathways and minimizing the undesired decrease of the potential energy of electrons prior to their extraction from QDs. We are aware of one prior report on AAAs as bifunctional linkers between QDs and TiO2: Hines and Kamat reported that the dynamics of excited-state ET were similar within heterostructures of CdSe QDs and TiO2 tethered by β-alanine and MPA.47 Their work provides evidence that AAAs are intriguing alternatives to MAAs and warrant further investigation. In this article, we report on (1) the kinetics and thermodynamics of the linker-assisted attachment of CdSe QDs to nanocrystalline TiO2 films using 12-aminododecanoic acid (ADA) and 12mercaptododecanoic acid (MDA) and (2) the spectroscopic characterization of excited-state ET through 4-aminobutyric acid (ABA) and 4-mercaptobutyric acid (MBA). These bifunctional linkers are depicted in Chart 1. The surface-attachment experiments revealed that ADA and MDA performed similarly as molecular linkers in terms of both thermodynamics and kinetics, with MDA being slightly preferable. Time-resolved spectroscopic measurements revealed that ABA did not accept photogenerated holes from QDs, whereas MBA did, but that the rate constants and efficiencies of QD-to-TiO2 electron transfer (ET) were slightly greater with MBA as a linker than with ABA. Taken together, our results indicate that AAAs are intriguing alternatives to MAAs as bifunctional linkers for the assembly of QDcontaining heterostructures for photoinduced charge transfer. AAAs are particularly attractive from the 4 ACS Paragon Plus Environment

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standpoint of retaining photogenerated holes in light-harvesting QDs and maximizing electron-hole separation. Chart 1. Names, abbreviations, and structures of bifunctional ligands. Name 12-aminododecanoic acid 12-mercaptododecanoic acid 4-aminobutyric acid 4-mercaptobutyric acid

Abbrev. ADA MDA ABA MBA

Structure HO2C-(CH2)11-NH2 HO2C-(CH2)11-SH HO2C-(CH2)3-NH2 HO2C-(CH2)3-SH

Experimental section Materials. Reagents and their commercial sources are as follows: selenium powder (Alfa Aesar); zirconium(IV) n-propoxide (TCI America); cadmium acetate dihydrate, tri-n-octylphosphine oxide (TOPO) (90%), titanium(IV) isopropoxide, poly(ethylene glycol) (PEG), MDA, ADA, MBA, and ABA (Sigma-Aldrich). Acetone, toluene, dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), ethanol (EtOH), methanol (MeOH), and concentrated nitric acid were obtained from various sources. Reagents and solvents were used as received. Synthesis of TiO2 films. Nanocrystalline TiO2 films on glass microscope slides were prepared by the hydrolysis of titanium(IV) isopropoxide as described previously.28,29 Synthesis of CdSe QDs. Nominally TOPO-capped CdSe QDs were synthesized following the method of Peng and co-workers.48,49 Details are provided in Appendix S1 in Supporting Information. Step 1 of Linker-Assisted Assembly: Adsorption of Linkers to TiO2 films. In equilibrium binding experiments, freshly annealed TiO2 films (3-4 per each concentration of linker) were immersed in toluene solutions of MDA (0.05-60 mM) for 16-24 h or in DMSO solutions of ADA (0.05-60 mM) for 72 h. In experiments to probe the kinetics of adsorption, TiO2 films (4 per each immersion time) were immersed in solutions of the ligands for varying durations. Adsorption reactions were performed at room temperature. Films were removed and rinsed by immersing for 2-5 s (while being moved back and forth) in toluene (for MDA) or acetone (for ADA). Films were dried in air and stored in the dark until characterization by FTIR spectroscopy.


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FTIR Spectral Characterization of Adsorption of Linkers. Linker-functionalized TiO2 films were characterized by the acquisition of FTIR spectra in transmission mode (3200-2600 cm-1). Spectra were acquired by positioning glass slides coated with linker-functionalized TiO2 films perpendicular to the IR beam. The glass slides and TiO2 absorbed below 2300 cm-1 and did not interfere with the C-H stretching region of MDA and ADA. Spectra were baseline corrected by subtracting a Gaussian and a constant to minimize the absorbance from 3200 to 3000 cm-1 and 2800 to 2600 cm-1 using a leastsquares method. Amounts of adsorbed MDA and ADA per projected surface area of TiO2, hereafter referred to as “surface coverages,” were calculated by dividing the integrated absorbance within the C-H stretching region (3000-2800 cm-1) by the integrated absorption coefficient (1.96 × 107 cm mol-1) determined from Beer-Lambert plots for MDA dissolved in CCl4.26,29,50 We assumed that the integrated absorption coefficients were equal for MDA and ADA (because ADA is insoluble in CCl4) and were unchanged upon adsorption of the linkers to TiO2. MDA and ADA were chosen for fundamental studies of adsorption and linker-assisted assembly because their eleven CH2 groups give rise to intense C-H stretching bands.51 Absorbances at the asymmetric CH2 stretching (νa(CH2)) maximum of TiO2 films functionalized with MDA or ADA at saturation surface coverage were 0.06-0.08. Step 2 of Linker-Assisted Assembly: Attachment of CdSe QDs to Linker-Functionalized TiO2 films. MDA- or ADA-functionalized TiO2 films were prepared and characterized by attenuated total reflectance (ATR) FTIR (4000-400 cm-1) to ensure consistency in the surface coverages of linkers. MDA- or ADA-functionalized TiO2 films were immersed in THF dispersions of CdSe QDs (0.25-30 µM) for 0.5-22 h. Concentrations of dispersed QDs were determined from measured absorbances at the first excitonic maximum and reported molar absorption coefficients.52 Four films were used for each concentration of QD. UV/vis absorption spectra were acquired to monitor the attachment of QDs to TiO2. TiO2 films were removed from dispersions of CdSe QDs, rinsed with THF, and dried in air. Absorption spectra were obtained in transmission mode by holding the TiO2-coated glass slide perpendicular to the UV/vis beam. Amounts of QDs per projected surface area of TiO2, hereafter


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referred to as “surface coverages,” were calculated by dividing the absorbance of the QD-coated TiO2 film at the first excitonic absorption maximum of the CdSe QDs by the reported molar absorption coefficient (in cm2 mol-1).52 Mixed Dispersions for Emission Spectroscopy. Steady-state and time-resolved emission measurements were performed on dispersions containing the following components: (1) CdSe QDs, (2) CdSe QDs, dissolved MBA or ABA, and TiO2 nanoparticles (NPs) (hereafter referred to as QD-ABATiO2 or QD-MBA-TiO2 dispersions), and (3) CdSe QDs, MBA or ABA, and ZrO2 NPs (hereafter referred to as QD-ABA-ZrO2 or QD-MBA-ZrO2 dispersions.) The solvent for MBA-containing dispersions was 1:1 toluene/EtOH by volume; the solvent for ABA-containing dispersions was 5:3:2 THF/DMSO/ETOH by volume. The choice of solvents was dictated by solubility of the linkers. The preparation of these dispersions is described in Appendix S2 in the Supporting Information. Spectroscopic Instrumentation. FTIR spectra were acquired with a Nicolet Magna-IR 550 spectrometer or a Perkin Elmer Spectrum Two ATR-FTIR spectrometer. UV/vis absorption spectra were obtained with an Agilent 8453 diode array spectrophotometer. Energy-dispersive X-ray spectroscopy (EDS) was performed with a Hitachi SU70 field emission instrument with an Oxford Inca SDD EDS detector. Steady-state emission spectra were obtained with a Varian Cary Eclipse fluorimeter with excitation at 430 nm. Time-resolved emission data were obtained by time-correlated single photon counting (TCSPC) using the system described previously and in Appendix S3 in Supporting Information.53,54

Results and Discussion Adsorption of Linkers to TiO2. Immersion of TiO2 films in solutions of ADA or MDA resulted in the growth of νa(CH2) and symmetric CH2 stretching (νs(CH2)) bands centered at approximately 2929 cm-1 and 2857 cm-1, respectively, in IR spectra (Fig. 1), indicating that the ligands adsorbed to TiO2. Equilibrium surface coverages of ADA and MDA increased with concentration (insets to Fig. 1). Data


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followed the Langmuir adsorption isotherm,55 as evidenced by the linearity of plots of 1/Γ vs 1/C and C/Γ vs C (Fig. S1 in Supporting Information), where Γ is surface coverage and C is concentration.46,56 Surface adduct formation constants (Kad), determined from the slopes of linear fits to plots of 1/Γ vs 1/C (Fig. S1a),46 for ADA and MDA were (1.0 ± 0.2) × 104 M-1 and (1.2 ± 0.2) × 104 M-1, respectively. Thus, the values of Kad were, within the precision of our measurements, identical for the two linkers. Saturation surface coverages (Γ0), determined from the slopes of linear fits to plots of C/Γ vs C (Fig. S1b),56 for ADA and MDA were (1.7 ± 0.2) × 10-7 mol cm-2 and (2.1 ± 0.1) × 10-7 mol cm-2, respectively. The measured values of Kad and Γ0 are consistent with reported parameters for the adsorption of alkanoic acids and other carboxylic acid-bearing ligands to nanocrystalline TiO2 films,29,50,51,56-58 suggesting that ADA and MDA adsorbed through their carboxyl groups.

0.07 0.06

[ADA] (mM) 1.00 0.45 0.15 0.05

(b)

-7

2.0x10

0.09

1.5x10-7 -7

1.0x10

0.08

-8

5.0x10

0.0 0.0

0.05

0.2

0.4

0.6

0.8

1.0

[ADA] (mM)

0.04 0.03

0.07 0.06

[MDA] (mM) 1.00 0.60 0.15 0.05

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0.05 0.04 0.03

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0.01 0.00 3000

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Γ (mol cm-2)

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Absorbance

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Γ (mol cm-2)

(a)

Absorbance

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0.01 2950

2900

2850

2800

0.00 3000

2950

-1

Wavenumber (cm )

2900

2850

2800

-1

Wavenumber (cm )

Fig. 1. FTIR spectra of TiO2 films that were functionalized with ADA (a) and MDA (b) by equilibrating with solutions of the linkers at varying concentrations. Insets: Surface coverage (Γ) as a function of concentration and fits to Langmuir adsorption isotherm. Error bars are plus-or-minus one standard deviation relative to the average absorbance or Γ of 3-4 films. Attachment of QDs to Linker-Functionalized TiO2. CdSe QDs adhered to TiO2 films functionalized with ADA or MDA at saturation surface coverages, as evidenced by (1) the growth of excitonic absorption bands of CdSe in UV/vis absorption spectra of the films (Fig. 2) and (2) the peaks at 3.1 and 3.3 keV (Cd) and 1.4 keV (Se) in EDS spectra (Fig. S2 in Supporting Information). To evaluate surface-attachment equilibria, linker-functionalized films were immersed in THF dispersions of ACS Paragon Plus Environment

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QDs with varying concentrations for 22 h. The equilibrium surface coverage of QDs increased with concentration (Fig. 3a). Data were well-modeled by the Langmuir adsorption isotherm; Kad and Γ0 were calculated from linear fits to plots of 1/Γ vs 1/C (Fig. 3b) and C/Γ vs C (Fig. 3c), as described above. Values of Kad for the attachment of QDs to TiO2 films functionalized with ADA and MDA were (1.8 ± 0.4) × 106 M-1 and (2.4 ± 0.4) × 106 M-1, respectively. These values are higher than previously-reported values of Kad of 103-105 M-1 for the adsorption of thiols and amines to planar cadmium chalcogenides and CdSe QDs.37,59,60 Our experiments differ from prior reports in that ADA and MDA were tethered to a porous TiO2 substrate. The higher Kad values that we measured may have arisen from the coordination of multiple TiO2-adsorbed linkers per QD. However, direct comparison of our values of Kad with prior reports is additionally complicated by differences in solvation, exposed crystal facets of CdSe, size of QDs, and/or the identity and fractional coverage of native capping groups. Importantly, our internal comparison reveals that Kad for attachment of CdSe QDs to MDA-functionalized films was (1.4 ± 0.4)fold greater than for attachment to ADA-functionalized films. Alternatively, the change of free energy (∆G) was (13 ± 1)-fold more negative upon attachment of the QDs to MDA-functionalized TiO2. These relative values are consistent with a report by Bullen and Mulvaney that Kad for the adsorption of octanethiol to CdSe QDs was approximately double that for the adsorption of n-decylamine.37 Values of Γ0 for the attachment of QDs to ADA- and MDA-functionalized films were (1.6 ± 0.3) × 10-9 mol cm-2 and (1.2 ± 0.1) × 10-9 mol cm-2, respectively. These values are approximately two orders of magnitude lower than for the adsorption of ADA and MDA to TiO2, consistent with the greater footprint of the QDs and, probably, their more limited access to the pore structure of the nanocrystalline films.


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0.35

Absorbance

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Adsorption time

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425

450

475

500

525

550

575

600

Wavelength (nm) Fig. 2. Representative UV/vis absorption spectra of CdSe-coated TiO2 films prepared by adsorbing ADA at saturation surface coverage then reacting with dispersed CdSe QDs for the indicated times. The absorption band centered at 485 nm corresponds to the first excitonic transition of the CdSe QDs.

(a)

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ADA MDA 1 / Γ (cm mol )

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0

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Fig. 3. Equilibrium binding data for the linker-assisted attachment of CdSe QDs to ADA- and MDAfunctionalized TiO2 films: plots of surface coverage (Γ) as a function of concentration of dispersed CdSe QDs ([CdSe QDs]) (a) and corresponding plots of 1/Γ vs. 1/[CdSe QDs] (b) and [CdSe QDs]/Γ vs. [CdSe QDs] (c). Superimposed on the data are fits to the Langmuir adsorption isotherm. Error bars are plus-or-minus one standard deviation relative to the average value for 4 films. Having evaluated the thermodynamics of linker-assisted assembly, we also compared the kinetics of the attachment of CdSe QDs to ADA- and MDA-functionalized TiO2 films. Linker-modified films were immersed in THF dispersions of CdSe QDs with concentrations of 0.25-30 µM. The attachment of QDs was monitored by the growth of the excitonic absorption bands of CdSe in UV/vis absorption spectra of films (Fig. 2). The surface coverage of QDs increased with time until equilibrating ACS Paragon Plus Environment

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after 2 to 24 h; attachment reactions reached equilibrium more rapidly at lower concentrations. Adsorption kinetics were well-modeled by the following integrated rate law:

ln(1 − θ ) = −k ad Ct

(1)

where kad is the rate constant of adsorption, t is time, and θ is the fractional surface coverage of QDs calculated by dividing the measured surface coverage by Γ0. For both ADA- and MDA-functionalized films, plots of [ln(1-θ)]/C vs t were linear (Fig. S3), indicating that the data followed Langmuir kinetics and that the linker-assisted attachment of QDs to TiO2 was pseudo-first order with respect to concentration of QDs.50,55,61 The value of kad for each linker was calculated from the average of the slopes of linear fits to plots of [ln(1-θ)]/C vs t from adsorption kinetics data acquired at seven different concentrations of QDs. The resulting values of kad for the attachment of QDs to ADA- and MDAfunctionalized TiO2 films were (14 ± 5) M-1 s-1 and (60 ± 20) M-1 s-1, respectively. These values are of the same order of magnitude as reported values of kad for the adsorption of linear alkanoic acids to nanocrystalline TiO2 films.50 Summary of Results from Adsorption and Linker-Assisted Assembly. Equilibrium-binding and adsorption-kinetics data are summarized in Table 1. The experiments revealed (1) that all data were well-modeled by Langmuir kinetics and isotherms, (2) that ADA and MDA adsorbed to TiO2 with essentially identical values of Kad and Γ0, (3) that values of Γ0 were similar (within (25 ± 4)%) for the attachment of CdSe QDs to ADA- and MDA-functionalized TiO2 films, (4) that CdSe QDs exhibited a slightly greater affinity for the terminal thiol of MDA than for the terminal amine of ADA, as evidenced by the (1.4 ± 0.4)-fold higher value of Kad, and (5) that the kinetics of attachment of the QDs to TiO2 films functionalized with MDA and ADA were similar, as evidenced by the similarity of kad values (within a factor of 5 ± 2). Overall, ADA and MDA performed similarly as molecular linkers, but thiolbearing MDA was slightly more favorable from the standpoints of both thermodynamics and kinetics.


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Table 1. Parameters from equilibrium binding and adsorption kinetics experiments.

1.2 CdSe QDs QD-ABA-ZrO2

1.0

1000

QD-ABA-TiO2 800

0.8 600

0.6

400

0.4

200

0.2 0.0 400

0 450

500

550

600

650

700

750

Γ0 (mol cm-2) (1.7 ± 0.2) × 10-7 (2.1 ± 0.1) × 10-7 (1.6 ± 0.3) × 10-9 (1.2 ± 0.1) × 10-9

(b)

1.2

kad (M-1 s-1)

14 ± 5 60 ± 20

800

CdSe QDs QD-MBA-ZrO2

1.0

QD-MBA-TiO2

600

0.8 0.6

400

0.4 200 0.2 0.0 400

0 450

500

550

600

650

700

750

Photoluminescence intensity (AU)

(a)

Kad (M-1) (1.0 ± 0.2) × 104 (1.2 ± 0.2) × 104 (1.8 ± 0.4) × 106 (2.4 ± 0.4) × 106

Absorbance

Attachment of QDs to linkerfunctionalized TiO2

Linker ADA MDA ADA MDA

Photoluminescence intensity (AU)

Adsorption of ligands to TiO2

Absorbance

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Wavelength (nm)

Wavelength (nm)

Fig. 4. Absorbance spectra (dashed lines) and steady-state emission spectra (solid lines) of CdSe QDs, QD-linker-ZrO2 dispersions, and QD-linker-TiO2 dispersions with ABA (a) and MBA (b) as linkers. Solvents were 5:3:2 THF/DMSO/ETOH (a) and 1:1 toluene/EtOH (b). Spectra of mixed dispersions were acquired 40 min after mixing.

Our Strategy to Characterize ET from QDs to TiO2 Using Emission Spectroscopy. Excitedstate ET from QDs competes with electron-hole recombination and results in dynamic quenching of emission.17,18,38,62 Thus, steady-state and time-resolved emission measurements can be used to quantify ET at QD-molecule-TiO2 interfaces. Kamat and coworkers and our group have established that in situ linker-assisted assembly yields molecularly-tethered QD-NP heterostructures within mixed dispersions of QDs, molecular linkers, and NPs.17-19,30,38 Such dispersions can be preferable to thin films for emission spectroscopy. We chose MBA and ABA, each of which contains a three-carbon alkyl chain, for these experiments because the rate constant and efficiency of QD-to-TiO2 ET decrease with the alkyl chain length of MAA linkers.17,18,22,63 Dispersions containing ZrO2 NPs served as controls. The conduction band edge potential of ZrO2 is nearly 1 V more negative than that of bulk CdSe;64-67 therefore, ET from photoexcited CdSe QDs to ZrO2 NPs was thermodynamically unfavorable. ACS Paragon Plus Environment

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Steady-State Emission Spectroscopy. Dispersions of CdSe QDs exhibited first excitonic absorption bands centered at 488-492 nm (Fig. 4). The band was red-shifted by ~4 nm for QDs dispersed in 1:1 toluene/EtOH relative to QDs dispersed in 5:3:2 THF/DMSO/EtOH. The QDs exhibited band-edge emission bands centered at 505 nm, and broad surface (trap-state) emission bands extending from approximately 550 nm into the near-IR (Fig. 4). Intensities at the maxima of band-edge and surface emission were nearly equal for QDs dispersed in 1:1 toluene/EtOH (Fig. 4b), whereas band-edge emission was much less intense than surface emission for QDs dispersed in 5:3:2 THF/DMSO/EtOH (Fig. 4a). (Differences in the solubility of ABA and MBA necessitated the use of different solvents for dispersions containing the two linkers.) Emission spectra of dispersions containing CdSe QDs, ABA or MBA, and TiO2 or ZrO2 NPs were obtained as a function of time after mixing. Any spectral changes ceased after approximately 1.5 to 2 h (Fig. S4 in Supporting Information), indicating that the mixtures had equilibrated. Band-edge emission from QD-MBA-ZrO2 dispersions was quenched significantly (by ~85%) relative to dispersions of QDs alone; surface emission was quenched to a much lesser extent and red-shifted slightly (Fig. 4b). These changes are consistent with the formation of QD-MBA-ZrO2 heterostructures. The transfer of photogenerated holes from CdSe to the adsorbed thiolate of MBA, which is well-established,18,35-38 was probably the predominant contribution to quenching. Additionally, the incorporation of QDs into QDMBA-ZrO2 heterostructures may have altered interfacial polarity and increased the local concentration of QDs, both of which may have contributed to the quenching of emission. We previously reported that tethering CdSe QDs to ZrO2 NPs can quench emission even in the absence of ET.38,68 Combining CdSe QDs, ABA, and ZrO2 NPs gave rise to strikingly different effects (Fig. 4a). Band-edge emission was enhanced significantly (by approximately 1.5-fold), the surface-emission maximum was blue-shifted, and the high-energy region of surface emission was enhanced in QD-ABA-ZrO2 dispersions relative to dispersions of QDs alone. These changes suggest that QD-ABA-ZrO2 heterostructures formed via coordination of the carboxyl group of ABA to ZrO2 and the amine of ABA to CdSe. Both the enhancement of band-edge emission and the blue-shift of surface emission are consistent with the 13 ACS Paragon Plus Environment

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reported shifting of electron-trapping surface states to higher energy upon coordination of amines to CdSe.37,39-46 The enhancement of emission from QD-ABA-ZrO2 further indicates that, as expected and desired, the amine of ABA did not scavenge photogenerated holes efficiently from CdSe QDs. (This enhancement of emission apparently outweighed any assembly-induced quenching upon tethering the QDs to ZrO2 NPs.) Thus, ABA is advantageous relative to MBA from the standpoints of (1) minimizing the decrease of potential energy of electrons upon relaxation into surface states and (2) maximizing the distance between photogenerated electrons and holes in the charge-separated state. Band-edge and surface emission from QD-ABA-TiO2 and QD-MBA-TiO2 dispersions were quenched by 40-80% relative to the emission from QD-ABA-ZrO2 and QD-MBA-ZrO2 dispersions (Fig. 4). The conduction band edge potential of TiO2 is more positive than that of bulk or quantumconfined CdSe;66,67 therefore, ET from QDs to TiO2 is thermodynamically favorable. We attribute the significant quenching of emission from QD-ABA-TiO2 and QD-MBA-TiO2 dispersions, relative to the corresponding QD-ABA-ZrO2 and QD-MBA-ZrO2 dispersions, to ET. This interpretation is consistent with previous reports on CdSe QD-linker-TiO2 dispersions with thiol-bearing linkers.17,30,38,68 Time-Resolved Emission Spectroscopy. Time-resolved emission measurements using TCSPC were performed to evaluate the dynamics and efficiency of excited-state ET. Emission decay traces were obtained at wavelengths throughout the band-edge and surface emission bands of dispersions of QDs alone and dispersions containing QDs, ABA or MBA, and ZrO2 or TiO2 NPs. Decay traces were acquired in two different windows: over the first 50 ns and over the first 1 µs after pulsed excitation of samples. Emission typically decayed to baseline within approximately 200-600 ns; thus, the longer-time window enabled the measurement of full decay traces. Measurements with the shorter time window had superior temporal resolution and were more useful for characterizing ET-induced differences in initial decay dynamics. Decay traces obtained with the shorter (50 ns) time window were fitted to the following equation using multi-exponential reconvolution:


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 t − t'  t n dt ' I (t ) = ∫−∞ IRF (t ')∑i =1 Ai exp −  τ i  

(2)

where t is time, I(t) is intensity, Ai is amplitude of the ith component, τi is lifetime of the ith component, and IRF(t′) is the instrument response function. The quality of multiexponential fits was evaluated by the distribution of residuals around zero and by chi-square with values of 0.95-1.40 considered acceptable. For a given decay trace, an additional exponential component was added if it improved chisquare and the distribution of residuals significantly. Representative emission decay traces for dispersions of CdSe QDs alone and corresponding mono-, bi-, and triexponential reconvolution fits, residuals, and values of chi-square are in Figure S5 in Supporting Information. Triexponential reconvolution yielded the highest-quality fits for both band-edge and surface emission. The lone exception was that biexponential reconvolution yielded the best fits for band-edge emission from QDMBA-TiO2 dispersions. Multiexponential decay kinetics, which formally implies multiple populations of fluorophores that decay single-exponentially, have been observed often for colloidal dispersions of QDs with inherent heterogeneities in size and surface chemistry.31,38,68-71 Our data were modeled more precisely by multiexponential kinetics than distributed kinetics. For ease of comparison of our data from different QD-containing dispersions, and to enable the estimation of rate constants of ET, we calculated intensity-weighted average lifetimes ():38,68,72-74

τ

Aiτ i2 ∑ = ∑ Aiτ i

(3)

Representative decay traces and fits for band-edge and surface emission from CdSe QD-containing dispersions are shown in Figure 5 (50 ns window) and in Figure S6 in Supporting Information (1 µs window). Values of Ai, τi, and from fits to emission decay traces (50 ns window) at wavelengths near the maximum of the band-edge emission band (500 nm) and within the surface emission band (672 nm) are compiled in Table S1 in Supporting Information.


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We first consider dispersions containing CdSe QDs, ABA, and metal oxide NPs. Values of for free QDs dispersed in 5:3:2 THF/DMSO/ETOH for band-edge and surface emission were (19.3 ± 0.5) ns and (37 ± 2) ns, respectively. The corresponding values of for band-edge and surface emission from QD-ABA-ZrO2 dispersions were (22.3 ± 0.6) ns and (39 ± 2) ns, respectively. The increase of for band-edge emission is consistent with the enhancement of steady-state band-edge emission upon exposure of the QDs to ABA and ZrO2 NPs (Fig. 4a). Both band-edge and surface emission decayed more rapidly for QD-ABA-TiO2 dispersions than for QD-ABA-ZrO2 dispersions (Figs. 5a,b and S6a,b). Values of for band-edge and surface emission from QD-ABA-TiO2 dispersions were (9.7 ± 0.5) ns and (18 ± 2) ns, respectively. We attribute the dynamic quenching of band-edge and surface emission from QD-ABA-TiO2 dispersions, relative to QD-ABA-ZrO2 dispersions, to ET from both band-edge and surface states of QDs to TiO2 NPs.


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0

(a)

Normalized Intensity


0

(b)

10

-1

10

2 1 -2

10

3

Band-edge emission CdSe QDs (1) QD-ABA-ZrO2 (2)

-3

10

10

1, 2

-1

10

3 -2

10

Surface emission CdSe QDs (1) QD-ABA-ZrO2 (2)

-3

10

QD-ABA-TiO2 (3)

QD-ABA-TiO2 (3)

-4

-4

10

10 0

5

10

15

20

25

30

35

40

0

5

10

Time (ns) 0

10

QD-MBA-TiO2 (3)

-1

20

25

30

35

40

1 -2

10

2

-3

10

0

10

(d)

Band-edge emission CdSe QDs (1) QD-MBA-ZrO2 (2)

10

15

Time (ns)


(c) Normalized Intensity

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3

1

-1

10

2

3

-2

10

Surface emission CdSe QDs (1) QD-MBA-ZrO2 (2)

-3

10

QD-MBA-TiO2 (3) -4

-4

10

10

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

Time (ns)

Time (ns)

Fig. 5. Emission decay traces and corresponding fits for band-edge emission and surface emission from dispersions of free CdSe QDs, QD-linker-ZrO2 dispersions, and QD-linker-TiO2 dispersions with ABA (a, b) and MBA (c, d) as linkers. Solvents were 5:3:2 THF/DMSO/ETOH (a, b) and 1:1 toluene/EtOH (c, d). Table 2. Values of for band-edge and surface emission from QD-containing dispersions and corresponding values of ket and ηet. Sample

λemc (nm) d

CdSe QDsa QDs-ABA-ZrO2a QDs-ABA-TiO2a CdSe QDsb QDs-MBA-ZrO2b QDs-MBA-TiO2b

500 (BE ) 672 (surfe) 500 (BEd) 672 (surfe) 500 (BEd) 672 (surfe) 500 (BEd) 672 (surfe) 500 (BEd) 672 (surfe) 500 (BEd) 672 (surfe)

(ns) 19.3 ± 0.5 37 ± 2 22.3 ± 0.6 39 ± 2 9.7 ± 0.5 18 ± 2 23.0 ± 0.6 36 ± 2 13.7 ± 0.9 30 ± 1 4.6 ± 0.3 10.5 ± 0.6

ket /107 (s-1)

ηet (%)

5.9 ± 0.5 3.1 ± 0.4

57 ± 2 54 ± 4

14 ± 2 6.2 ± 0.6

66 ± 3 65 ± 3


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Solvent was 5:3:2 THF/DMSO/ETOH by volume; b solvent was 1:1 toluene/ETOH by volume; c λem is the central wavelength of the channel; d BE = band-edge; e surf = surface. a

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Rate constants of ET (ket) were estimated as follows:18,19,31,62,68

k et =

1

τ

QD-ABA-TiO 2

−

1 (4)

τ

QD-ABA-ZrO2

This analysis is predicated on two assumptions: (1) that ET was the only additional deactivation pathway in QD-ABA-TiO2 dispersions relative to QD-ABA-ZrO2 dispersions and (2) that rate constants for all other excited-state deactivation processes of QDs were identical in QD-ABA-TiO2 and QDABA-ZrO2 dispersions. Variation of rate constants for competing decay pathways would introduce error into our estimation of ket. Additionally, equation 4 is rigorously correct only for monoexponential emission-decay kinetics, which were not observed for our QD-containing dispersions. Thus, the values of ket calculated from intensity-weighted average lifetimes are estimates, which we find useful for comparing QD-to-TiO2 ET mediated by ABA and MBA. Importantly, because surface emission reached the maximum intensity within the instrument response, photogenerated electrons had equilibrated into a distribution of band-edge and surface states on time scales faster than the instrument response. Therefore, ET from band-edge states did not affect surface-emission decay kinetics, enabling the estimation of ket from surface states using equation 4. Estimated values of ket from band-edge and surface states of QDs, via ABA, to TiO2 NPs were (5.9 ± 0.5) × 107 s-1 and (3.1 ± 0.4) × 107 s-1, respectively (Table 2). Slower ET from surface states is consistent with the decreased driving force.68 Alternatively, the localization of electrons in surface states that are not in close proximity to TiO2 may have contributed to the slower ET. Finally, we estimated efficiencies of ET (ηet), which are more important than ket from the standpoint of energy conversion, as follows:68,75

 τ η et = 1 − τ  

QD-ABA-TiO 2 QD-ABA-ZrO2

  × 100   ACS Paragon Plus Environment

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Values of ηet from band-edge and surface states of QDs for QD-ABA-TiO2 dispersions were (57 ± 2)% and (54 ± 4)%, respectively (Table 2). The increase of ket from band-edge states, relative to surface states, was offset by the faster excited-state decay (lower ) from band-edge states, such that ET was equally efficient from band-edge and surface states. Time-resolved emission data were also acquired for dispersions containing CdSe QDs, MBA, and metal oxide NPs. Decay traces are shown in Figs. 5 and S6, parameters from fitting are in Table S1, and extracted values of ket and ηet are in Table 2. Band-edge emission from free CdSe QDs dispersed in 1:1 toluene/EtOH decayed slightly more slowly than for the QDs dispersed in 5:3:2 THF/DMSO/ETOH, whereas surface emission-decay kinetics were independent of solvent. Band-edge emission decayed more rapidly for QD-MBA-ZrO2 dispersions than for dispersions of free QDs, as evidenced by the (40 ± 4)% decrease of . Surface emission was quenched dynamically but to a lesser extent, as decreased by just (18 ± 2)%. This dynamic quenching of emission upon exposure of the QDs to MBA and ZrO2 stands in contrast to the deceleration of emission decay for QD-ABA-ZrO2 dispersions and is consistent with hole-transfer from CdSe to MBA. Both band-edge and surface emission decayed much more rapidly for QD-MBA-TiO2 dispersions than for QD-MBA-ZrO2 dispersions, providing evidence for ET to TiO2. Estimated values of ket from band-edge and surface states were (1.4 ± 0.2) × 108 s-1 and (6.2 ± 0.6) × 107 s-1, respectively (Table 2), which are approximately twice the corresponding values of ket for QD-ABA-TiO2 dispersions. The difference probably reflects differences in interfacial electronic coupling through MBA and ABA, as the QD-to-TiO2 distances should be, to a first approximation, nearly identical. Values of ηet from band-edge and surface states of QDs within QD-MBA-TiO2 dispersions were (66 ± 3)% and (65 ± 3)%, respectively (Table 2). Thus, ET from both band-edge and surface states was approximately 1.2-fold more efficient within QD-MBA-TiO2 dispersions than QDMBA-ZrO2 dispersions.

Conclusions ACS Paragon Plus Environment

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We compared amine-bearing AAAs with the more commonly-used thiol-bearing MAAs as bifunctional ligands to tether CdSe QDs to TiO2 and to mediate excited-state ET. The adsorption of ADA and MDA to TiO2 followed the Langmuir adsorption isotherm. Values of Kad for the two ligands were essentially identical, and values of Γ0 were similar (within approximately 20%). The attachment of QDs to ADA- and MDA-functionalized TiO2 films was well-modeled by the Langmuir adsorption isotherm and by Langmuir kinetics. The two linkers exhibited similar overall performances: Kad was (1.4 ± 0.4)-fold greater for the attachment of QDs to MDA-modified films, whereas Γ0 was (1.3 ± 0.3)fold greater for the attachment of QDs to ADA-modified films. The attachment of QDs to linkerfunctionalized films followed first-order kinetics with respect to QDs with values of kad on the order of 10-102 M-1 s-1 for both linkers. Steady-state and time-resolved emission measurements revealed that electrons were transferred from both band-edge and surface states of QDs to TiO2, with estimated values of ket on the order of 107 s-1. ET was approximately twice as fast from band-edge states as from surface states, probably due to the greater driving force and/or decreased charge-transfer distance. Irrespective of the state from which electrons were extracted, ET via MBA was approximately twice as fast as ET via ABA, probably due to differences in electronic coupling. However, AAAs did not accept photogenerated holes from QDs, which increased the distance between electrons and holes in the charge-separated-state. Additionally, AAAs shifted electron-trapping surface states to higher energies, minimizing the trapping-induced loss of potential energy of electrons and maximizing the driving force for ET to TiO2. Our results reveal that thiol-bearing MAAs slightly outperform structurally analogous aminebearing AAAs in terms of the kinetics and thermodynamics of linker-assisted assembly and the kinetics and efficiencies of photoinduced QD-to-TiO2 ET. However, amine-bearing AAAs do not scavenge holes from QDs and shift surface states to higher energies, preserving more effectively the potential energy of photogenerated electrons. Understanding these tradeoffs should aid in the selection of bifunctional ligands for linker-assisted assembly.


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Acknowledgements. This material is based upon work supported by the National Science Foundation under Grant No. CHE-1306784.

Supporting information. Equilibrium binding data, energy-dispersive X-ray spectra, QDattachment kinetics data, and steady-state and time-resolved emission data.

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