Synergic Influence of Reverse Micelle ... - ACS Publications

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Synergic Influence of Reverse Micelle Confinement on the Enhancement in Photoinduced Electron Transfer to and from Carbon Nanoparticles Somen Mondal, Tarasankar Das, Arnab Maity, Sourav Kanti Seth, and Pradipta Purkayastha* Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, West Bengal 741246, India S Supporting Information *

ABSTRACT: The photoexcited behavior of carbon nanoparticles (CNPs) and the effect of confinement on photoinduced electron transfer (PET) to and from the CNPs have been examined by confining the nanoparticles and electron donor−acceptor systems in aerosol OT (AOT)/hexane/water reverse micelles (RMs). The CNPs and the electron donor dimethylaniline (DMA) are captured in the nonpolar environment of the RMs, while methyl viologen (MV2+), the electron acceptor, readily goes into the water pool. This confined medium facilitates experimentation on the electron transfer dynamics between two different phases. PET from DMA to MV2+ via CNPs is the expected phenomenon. The ultrafast photogenerated MV•+ cation radical acts as an electron sink scavenging electrons from DMA. PET has been confirmed from steady-state and time-resolved fluorescence along with ultrafast transient absorption measurements. The kinetic details of PET in the DMA−CNP− MV2+ assembly in a confined RM medium provide prospects toward development of light energy conversion devices.

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INTRODUCTION Interfacial electron transfer (IET) plays a key role in many applications, such as water purification,1 solar energy conversion,2 molecular electronics,3 etc., and hence has attracted the attention of scientists. In ET phenomena, the injection and recombination rates depend on the strength of electronic coupling between light-harvesting molecules serving as the electron donor and a charge transport semiconductor acting as the electron acceptor. A popular example can be cited for application of photoinduced ET (PET) in semiconductor quantum dot (QD) solar cells where the QDs act as lightharvesting materials.4−6 ET rates in donor−bridge−acceptor systems can be controlled by tuning the electronic coupling strength through the use of polymeric bridges between QDs and mesoporous oxides.4 Boehme et al. studied PET between CdTe and CdSe QDs in a QD film.5 They reported that efficient electron trapping in CdTe QDs obstructs electron transfer to CdSe QDs under most conditions. These examples show that QD−metal oxide junctions are integral parts of nextgeneration solar cells, light-emitting diodes, and nanostructured electronic arrays. Comprehensive examination of ET at metal oxide junctions by Tvrdy et al. using a series of CdSe QD donors and metal oxide nanoparticle acceptors shows that the ET rate constants depend strongly on the change in the system free energy.6 With a similar purpose, Zhao et al. investigated the rate of PET from PbS@CdS core@shell QDs to wide band gap semiconducting mesoporous films.7 They could fine-tune the electron injection rate by determining the width and height of the energy barrier for tunneling from the core to the oxide films using different electron affinities of the metal oxides, core sizes, © 2015 American Chemical Society

and shell thicknesses. Although there have been a wide range of studies on this aspect, controversies often emerge regarding the quantum efficiency and the rate of charge transfer.8 Harris et al. attempted to provide a better understanding of the events that influence the photoconversion yields and mapped the charge transfer kinetics at different time scales.9 They probed the PET events between 3 nm CdSe semiconductor nanocrystals and methyl viologen (MV2+) by confining them in an aerosol OT (AOT)/heptane reverse micelle (RM). Their work shows that the ultrafast ET to MV2+ is completed with an average rate constant on the order of 1011 s−1. Works on the enhancement in photocatalytic performance of nanocrystals have recently been reported by Ma et al. and Wang et al.10,11 Reverse micelles are thermodynamically stable nanostructures composed of a polar core encapsulated by surfactant molecules in nonpolar media. Dioctyl sulfosuccinate, or aerosol OT, is a commonly used surfactant for attaining reverse micelles in nonpolar media that accommodate spherical water pools of controlled size.12 The size of the water pool is dependent on the molar water-to-surfactant ratio, w0 = [H2O]/ [AOT].13 Biphasic ultrafast vibrational energy transfer in a suspension of reverse micelles, in which AOT separates a water nanodroplet from a bulk nonpolar CCl4 phase, has recently been reported.14 It has also been shown that, unlike in bulk water, no intermolecular transfer of OH-stretching quanta occurs among the interfacial water molecules or from the Received: March 2, 2015 Revised: May 28, 2015 Published: May 28, 2015 13887

DOI: 10.1021/acs.jpcc.5b02026 J. Phys. Chem. C 2015, 119, 13887−13892

Article

The Journal of Physical Chemistry C

Figure 1. Characterization of CNPs: (A) absorption and emission spectra, (B) DLS histogram, and (C) FTIR spectrum of CNPs in hexane showing the amide stretch at 1640 cm−1.

detection unit. A 402 nm laser head was used as the excitation source. The FTIR spectrum was recorded using a Perkin− Elmer Spectrum RX1 spectrophotometer. The dynamic light scattering (DLS) measurements were taken using a Malvern Zetasizer Nano equipped with a 4.0 mW HeNe laser operating at λ = 633 nm. All samples were measured in a nonaqueous system at room temperature with a scattering angle of 173°. The size distribution is calculated by Nano software using a non-negative least-squares analysis. A very dilute solution was prepared for the DLS experiment. A 5 μL volume of the stock solution of CNPs was added to 1 mL of hexane, and dust particles were removed by filtration. Atomic force microscopy (AFM) was performed using an NT-MDT NTEGRA instrument procured from NTMDT, California. The transient absorption spectroscopy experiments were done using a FemtoFrame-II UV−vis femtosecond transient absorption pump−probe spectrometer with an intrinsic temporal resolution of 7 fs. The pump laser was used at 400 nm. Preparation of Carbon Nanoparticales. Hydrophobic CNPs were synthesized by mixing glucose (160 mg) and dodecylamine (1.6 g) and heating the solution at 80 °C for 30 min.16 A brown-colored solid was obtained on cooling and was dissolved in hexane. The soluble CNPs were purified by solvent extraction using hexane and methanol. CNPs are soluble in hexane since they are hydrophobic in nature as are coated with dodecylamine. Glucose and free dodecylamine are soluble in methanol. The hexane part was separated, and the solvent was evaporated to obtain the brown solid CNPs, which were solubilized in common organic solvents such as acetone, hexane, and DMSO. This method is supposed to provide >99% pure CNPs.

hydration shell to the bulklike core, indicating that the hydrogen bond network near the H2O/AOT interface is strongly disrupted.15 In a track similar to that of Harris et al., we have used the RM environment to trap a potential electron donor, dimethylaniline (DMA), and MV2+, an electron acceptor.9 Along with this duo, we have used hydrophobic carbon nanoparticles (CNPs) to relay electrons from DMA to MV2+. DMA and the synthesized CNPs are hydrophobic and hence can be tactically put into the hydrophobic zone of the RM, whereas MV2+ is hydrophilic and readily moves into the aqueous core. This device has been used to transfer electrons from a nonpolar to a polar zone controlled by CNPs. The dual electron donor−acceptor property of CNPs helps to relay the electrons from one phase to the other. Steady-state and time-resolved fluorescence coupled with ultrafast transient absorption spectroscopy were used to show that highly efficient PET is actually taking place in this system.

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EXPERIMENTAL SECTION Materials. Glucose, dodecylamine, sodium bis(2ethylhexyl)sulfosuccinate (AOT), and hexane were purchased from Sigma-Aldrich and used as received. Triple-distilled water was used to prepare the experimental solutions. Methods. The absorption spectra were recorded in a Varian Cary 300 Bio UV−vis spectrophotometer. Fluorescence measurements were done on a QM-40 spectrofluorimeter procured from PTI. The fluorescence lifetimes were measured by the method of time-correlated single-photon counting (TCSPC) using a picosecond spectrofluorimeter from Horiba Jobin Yvon IBH equipped with a FluoroHub single-photon counting controller and an FC-MCP-50SC MCP-PMT 13888


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Figure 2. (A) Change in the absorption spectrum, (B) emission spectrum, (C) steady-state fluorescence anisotropy, and (D) fluorescence lifetime of CNPs in AOT RMs and RMs with w0 up to 15.

Table 1. Fluorescence Decay Parameters of CNPs in Hexane and 0.1 M AOT and in a w0 = 15 Micelle (λex = 402 nm, λem = 460 nm)a

a

sample

τ1(a1) (ns)

τ2(a2) (ns)

τ3(a3) (ps)

⟨τ⟩ (ns)

χ2

CNPs CNPs + 0.1 M AOT CNPs in w0 = 15 micelle

1.61 (30.9) 2.28 (32.3) 2.31 (32.4)

5.25 (54.4) 6.80 (57.7) 7.31 (54.8)

252 (14.7) 275 (10.0) 289 (13.8)

3.39 4.68 4.79

1.09 1.15 1.08

The values in parentheses indicate respective contributions to the decay components. χ2 values denote the goodness of the fit.

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to enhance the ET rate constant by an order of magnitude.9 To evoke a similar synergic influence on PET across the hydrophobic and hydrophilic phase boundary, we put the hydrophobic electron donor DMA and the CNPs into the hydrophobic kernel of the reverse micelle and the hydrophilic MV2+ inside the aqueous core. CNPs have a dual character of electron donation and acceptance.19 A small enhancement in the absorbance of CNPs at 345 nm on dissolving them in AOT/hexane indicates entrapment of CNPs inside the hydrophobic cage of the surfactant in the ground electronic state (Figure 2A). Controlled addition of water (w0 = 5−15) results in formation of the AOT RMs. It is known that water molecules can penetrate to some extent inside the RM, slightly altering the polarity.20 This is reflected in Figure 2B, where there is an enhancement in the fluorescence intensity of CNPs in hexane on addition of AOT due to the hydrophobic entrapment, followed by a slight decrease with a red shift on addition of water (w0 = 15). Motional restriction of the CNPs on gradual formation of the aqueous core in RMs is suggested by the enhancement in the steady-state fluorescence anisotropy (Figure 2C) since the CNPs penetrate further toward the aqueous core. The process may also be consolidated from the fluorescence study of the

RESULTS AND DISCUSSION The synthesized CNPs were characterized from their absorption and fluorescence spectra as shown in Figure 1A. The absorption spectrum exhibits a peak at 300 nm and a weak shoulder at 400 nm which may be attributed to the π−π* transition in the aromatic CC bonds and the n−π* transition, respectively. Blue fluorescence was observed from the CNPs dissolved in n-hexane with a maximum at 460 nm upon excitation at 400 nm. Variation in the emission peak position with varying excitation wavelengths, as shown in Figure 1A, arises from the “surface states” formed by the particle size effect.17 Xiao et al. proposed that the shift in fluorescence is due to the functional groups, such as hydroxyl, ketonic carbonyl, and ester carbonyl, residing on the CNPs, which are responsible for the excitation-dependent fluorescence change.18 DLS from the CNPs shows that the average particle size is around 5 nm (Figure 1B). Hydrophobicity has been imposed on the CNPs deliberately by coating them with dodecylamine, where the amine groups react with the carboxyl functional groups to form amide bonds (Figure 1C). Harris et al. used the dual role of TiO2 as an electron shuttle and a rectifier by putting it in the reverse micelle core along with MV2+, which created a synergistic effect 13889


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Figure 3. Absorption (A) and emission (B) spectra of CNPs in reverse micelles with the gradual addition of MV2+ (0−6.32 mM) after initial addition of DMA (0−80 mM). (C) Relative quenching of CNP fluorescence by DMA in the presence (black) and absence (red) of MV2+.

Table 2. Fluorescence Decay Parameters of CNPs in the Presence of DMA and MV2+ in Reverse Micelles (λex = 402 nm, λem = 460 nm)a compd added to CNPs in RMs with w0 = 15 0 mM DMA 20 mM DMA 40 mM DMA 80 mM DMA 80 mM DMA + 1.5 mM MV2+ 80 mM DMA + 6.0 mM MV2+ a

τ1(a1) (ns)

τ2(a2) (ns)

τ3(a3) (ps)

2.31 2.06 2.00 1.93 1.62 1.54

7.31 6.07 5.97 5.81 4.86 4.48

289 287 300 303 301 302

(32.4) (33.1) (35.6) (35.2) (33.1) (34.4)

(54.8) (50.7) (43.6) (43.8) (42.3) (35.9)

(13.8) (16.2) (18.8) (21.0) (24.6) (29.7)

⟨τ⟩ (ns)

χ2

4.79 3.80 3.36 3.28 2.66 2.22

1.08 1.15 1.11 1.06 1.11 1.05

The values in parentheses indicate the respective contributions to the decay components. χ2 values denote the goodness of the fit.

acceptance of an electron from DMA.21 The absorbance spectrum of the CNPs remains unchanged on addition of DMA, showing that the electron transfer is photoinduced. Our intention is to facilitate interphase PET using CNPs. Hence, we added hydrophilic MV2+ to the RMs containing CNPs and DMA in the hydrophobic region. MV2+ enters the aqueous core of the RM.9 A new band at 550 nm can be seen in the absorption spectrum of the CNPs after irradiation in the presence of MV2+ (Figure 3A), which indicates formation of a stable MV•+ radical.22 With an increase in the concentration of MV2+ in the DMA- and CNP-impregnated RMs, we observed further quenching of the DMA-quenched CNP fluorescence (Figure 3B). The extent of fluorescence quenching is determined by plotting the relative change in the fluorescence intensity (F0/F, where F0 and F denote the fluorescence intensities of CNP in the absence and presence of a quencher,

CNPs in RMs (Figure 2D). The results obtained from the fluorescence decay rate for CNPs are provided in Table 1. The multicomponent lifetime decays of CNPs suggest that multiple radiative species are present in the samples. Hence, it is instructive to consider the average lifetimes of the species. In RMs formed by AOT/hexane and with w0 = 15, the fluorescence decay of CNPs slows, suggesting a decrease in the nonradiative pathways. This is apparent since the CNPs exist in a more rigid environment with formation of a water pool as described earlier. It has been mentioned previously that DMA is a well-known electron donor and is hydrophobic in nature. Hence, DMA can readily be incorporated into the hydrophobic region of the RM. We used the characteristics of DMA to allow it to coexist with the hydrophobic CNPs in the hydrocarbon tails of the RMs. This results in quenching of CNP fluorescence due to 13890


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The prompt appearance of the excited-state absorption (ESA) band peaking at 470 nm confirms the formation of the DMA radical cation23 and hence provides decisive evidence for ET from DMA to CNPs in RMs. A second prominent band appears at 600 nm simultaneously with the DMA radical cation (Figure. 4), indicating formation of the MV•+ radical.22 Thus, DMA acts as an electron donor to the CNPs, which quickly scavenge the photogenerated electrons and transfer them to MV2+ in confinement. The PET in these systems is relatively fast and occurs within ∼10 ps. Figure S2 (Supporting Information) shows the data of some control experiments to support our proposition. Parts A and B of Figure 5 show the changes in absorbance versus delay times monitored at the absorption peak of DMA (480 nm) and the MV•+ radical (605 nm). The change in excited-state absorption, as shown in Figure 5, can be analyzed by fitting the data with biexponential eq 1, where the parameters a1 and a2 are the relative amplitudes of each lifetime component and τ1 and τ2 are the fast and slow components of the excited-state lifetimes, respectively, as provided in Table 3.

respectively) against the quencher concentrations (Figure 3C). The plot indicates that MV2+ acts as a catalyst in the PET process between CNPs and DMA. MV2+ is a well-known electron acceptor.9 CNPs have excellent electron-donating and -accepting properties. Hence, in the present case, an environment is created where DMA donates an electron to MV2+ in the excited state via CNPs executing electron-accepting and -donating properties. DMA is a low quantum yield fluorophore and absorbs at ∼295 nm. PET may take place between the DMA and MV2+ if DMA is excited at 295 nm. However, in the present case, all excitations have been performed at 400 nm, which practically does not excite DMA. Hence, this possibility of interference is eliminated. Possible control experiments needed to consolidate our proposal were performed, and the results are shown in Figure S1 (Supporting Information). PET could be established by measuring the fluorescence decay of the CNPs in the presence of DMA. Enhancement of the DMA concentration in an RM containing CNPs induces a faster decay of the CNP fluorescence that gets reinforced on further addition of MV2+ as shown in Table 2. A reduction in the average lifetime ⟨τ⟩ of the CNPs confirms the occurrence of PET. The excited-state lifetime of CNPs is usually very low and requires detection with subnanosecond resolution. For this purpose, time-resolved transient absorption spectroscopy is a suitable method to establish the excited-state behavior of the CNPs and consolidate PET. Transient absorption spectra recorded at different delay times following 400 nm laser pulse excitation of DMA−CNP−MV2+ in RMs are shown in Figure 4.

ΔA(t ) = a1e(t / τ1) + a 2e(t / τ2)

(1)

Table 3. Kinetic Parameters of CNPs in Excited-State Absorption DMA−CNP−MV2+

τ1(a1) (ps)

τ2(a2) (ps)

⟨τ⟩ (ps)

χ2

with AOT@480nm with AOT@600nm

0.08 (0.86) 0.13 (0.91)

5.75 (0.14) 3.42 (0.09)

5.30 2.51

0.98 0.99

The average lifetime, ⟨τ⟩, is determined as ⟨τ ⟩ =

∑ (Aiτi 2)/ ∑ (Aiτi)

(2)

From the estimated average lifetime, it can be said that the electron transfer process is faster in the AOT/hexane medium because the MV2+ is confined within the nanosized RM core. The rate constant of PET is defined as the reciprocal of the average lifetime. We can estimate the rate of ET using the following equation: ket = 1/τ(CNP + DMA + MV2+)AOT − 1/τ(CNP + DMA + MV2+)

(3)

The calculated rate constants from the traces are 1.12 × 1011and 3.9 × 1011 s−1 at 480 and 600 nm, respectively. The presence of MV2+ in the RM core enhances the rate of ET by

Figure 4. Transient absorption spectra recorded of DMA−CNP− MV2+in AOT/hexane RMs following 400 nm laser pulse excitation.

Figure 5. Absorption−time profiles monitored at (A) 480 nm and (B) 600 nm following 400 nm laser pulse excitation of DMA−CNP−MV2+ in an AOT/hexane RM. The biexponential fits are shown as solid red lines. 13891


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an order of magnitude. In this case, AOT/hexane RMs with w0 = 15 induce a synergistic effect on the electron transfer rate when CNP is coupled with both DMA and MV2+.

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CONCLUSION In summary, we report the effect of confinement in RMs on PET to and from the CNPs. CNPs and the electron donor DMA are captured in the nonpolar environment of the RMs while MV2+, the electron acceptor, readily goes into the water pool. PET from DMA to MV2+ via CNPs is expected. It is found that the ultrafast photogenerated MV•+ cation radical acts as an electron sink scavenging electrons from DMA. We believe that our work on PET in the DMA−CNP−MV2+ assembly in RM confinement would provide prospects toward light energy conversion.

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ASSOCIATED CONTENT

S Supporting Information *

Absorption and emission spectra of CNPs in reverse micelles with gradual addition of DMA and addition of MV2+ and transient absorption spectra of CNP−DMA and CNP−MV2+ in AOT/hexane RMs following 400 nm laser pulse excitation. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02026.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Department of Science and Technology (DST) (Grant SR/S1/PC-35/2011) and Council of Scientific and Industrial Research (CSIR) (Grant 01(2690)/ 12/EMR-II). S.M., T.D., A.M., and S.K.S. acknowledge their fellowships from the CSIR and University Grants Commission (UGC). The experimental support in capturing the transient absorption studies of Dr. Pratik Sen, Indian Institute of Technology (IIT) Kanpur, India, is gratefully acknowledged.

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REFERENCES

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Synergic Influence of Reverse Micelle ... - ACS Publications

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