Diffusion of Solvent-Separated Ion Pairs Controls Back Electron

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C: Energy Conversion and Storage; Energy and Charge Transport

Diffusion of Solvent-Separated Ion-Pairs Controls Back Electron Transfer Rate in Graphene Quantum Dots Ayan Bhattacharyya, Anju Chadha, Soumalya Mukherjee, and Edamana Prasad J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02526 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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The Journal of Physical Chemistry

Diffusion of Solvent-Separated Ion-Pairs Controls Back Electron Transfer Rate in Graphene Quantum Dots Ayan Bhattacharyya a, Soumalya Mukherjee b, Anju Chadha* b,c and Edamana Prasad*a a. Department of Chemistry, Indian Institute of Technology Madras, Chennai-600036, Tamil Nadu, India b. Department of Biotechnology, Indian Institute of Technology Madras, Chennai-600036, Tamil Nadu, India c. National Center for Catalysis Research, Indian Institute of Technology Madras, Chennai600036, Tamil Nadu, India AUTHOR INFORMATION Corresponding Author Edamana Prasad Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: In the present study, the stability of the photogenerated, solvent-separated charged states of graphene quantum dots (GQDs) in presence of N,N-diethylaniline (DEA) has been evaluated in a series of organic solvents. The results indicate that rate constant for back electron transfer (kBET) from GQD radical anion to DEA radical cation is diffusion controlled. As a result of the diffusion-controlled back-electron transfer (BET), kBET exhibits an inverse exponential relation with (a) the viscosity coefficient (η) of the solvent and (b) average radius of the graphene quantum dots. An analytical expression for the diffusion-controlled back electron transfer rate constant has been formulated. The kBET dependency on the diffusion of solventseparated ion-pairs (SSIP’s) has been evaluated for the first time for quantum dot systems and the results provide an efficient method for enhancing the lifetime of the photogenerated charge separated states from graphene quantum dots. The present findings can potentially improve the performance of GQD based photovoltaic and optoelectronic devices.


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INTRODUCTION

Graphene based materials have been extensively used for photovoltaic and optoelectronic applications in the past couple of decades.1-3 In recent years, graphene quantum dots (GQDs) have emerged as a new class of graphene based nano-material with unique photophysical properties.4-7 The band gap tunability of GQDs based on its size, functionalization and doping have made them extremely important candidates for photovoltaic and optoelectronic applications.8-12 Several theoretical studies have also been performed over the years to understand the nature and origin of the luminescence in GQDs.13, 14

Generation, separation and stabilization of charge separated states in GQDs are important for various applications such as photovoltaic, OLED and solar cell.15-20 A slow back electron transfer rate is necessary for the stabilization of the photo-generated charge-separated states. Several strategies have been developed over the years for generating long-lived charge separated states by reducing the back electron transfer rate constant.21-25 However, most of the studies have been confined to organic donor-acceptor systems and investigations to control the back electron transfer process involving quantum dot based donor-acceptor systems have not been reported.

Since GQD’s have significantly larger size as compared to common organic molecules, we hypothesized that the rate of back electron transfer (BET) from the solvent-separated ion-pairs (SSIP’s) would be limited by the rate at which the ion-pair diffuses towards each other leading to a collision event. As a result of the diffusion-controlled BET, lower magnitude of kBET is expected in solvents with a larger viscosity coefficient (η).The opposing viscous drag (vopp) experienced by a molecule is directly proportional to the radius of the molecule(r), according to


3


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Einstein-Smoluchowsky equation. Hence, the size of the quantum dot and the magnitude of kBET should also follow an inverse relation. In the present study, we present a systematic investigation on the effects of (i) the solvent medium and (ii) the size of the GQDs on the stability of the charge separated states formed via the photoinduced electron transfer (PET) processes of the GQDs. Nitrogen-functionalized graphene quantum dot (NGQD), pristine graphene quantum dot (GQD) and boron-doped graphene quantum dot (BGQD) were the electron-acceptor systems for the PET studies against a common ground state electron donor molecule, N,N-diethylaniline (DEA). Steady state and time resolved luminescence quenching studies have been carried out to determine the bimolecular quenching constants (kq). Nanosecond laser flash photolysis experiments have been carried out to verify the formation of the transient radical ions and to determine the rate constant of back electron transfer (kBET). The results have been analyzed in the light of the driving force dependence for the forward and back electron transfer processes. The impact of the solvent viscosity and size of the graphene quantum dots on the back electron transfer dynamics have been discussed in detail.

EXPERIMENTAL SECTION 1.1 a. Synthesis of nitrogen-functionalized graphene quantum dots (NGQDs) The water-soluble nitrogen-functionalized graphene quantum dots were synthesized via a microwave mediated hydrothermal process.

26, 27

Citric acid and urea were used as the carbon

and the nitrogen source respectively. Equimolar mixture of citric acid and urea was prepared in de-ionized water. The mixture was then transferred to a borosilicate bowl and subjected to 700W microwave radiation for 5 minutes. The microwave treatment resulted in the formation of a


4

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brown aggregated cluster. The clustered mass was then crushed and dried in a vacuum oven at 80 degree centigrade for 1 hour. This process removes moisture and other volatile gases like ammonia that might have formed during the microwave treatment. The brown powder was then cooled down to room temperature and then dispersed in water, forming a black dispersion. The dispersion was then centrifuged (1500G) and filtered (through a 10kDa filter) to produce a pale yellow solution of nitrogen functionalized graphene quantum dots (NGQDs). Tyndall effect was observed using a 5mW laser pointer elucidating the formation of colloidal solution of NGQDs. 1.1b: Dispersion of the NGQDs in other solvents: The aqueous solution of NGQDs was lyophilized to get a pale yellow solid mass. The as produced powder was then taken and dispersed in different solvents. 1.2: Characterization of NGQD The transmission electron microscopy images of NGQDs were obtained using Technai G2 20 TEM, FEI, Netherlands, operating at 200kV. The X-Ray diffraction characterizations were carried out by a powder X-Ray diffractometer using Cu-Kα radiation with 2θ spanning from 0 to 80 degree. Fourier transform infrared spectroscopy [FT-IR, Perkin Elmer] (scanning over a wide range of wavenumber viz. 500cm-1 to 4000 cm-1) was used for the chemical characterization of NGQDs. The lyophilized NGQDs were thoroughly mixed with KBr pellets before recording the FT-IR data. 1.3: Synthesis and TEM characterizations of GQDs from Citric acid: The graphene quantum dots (GQDs) were synthesized by a microwave mediated hydrothermal process according to a


5


reported procedure

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exactly similar to that of in the case of NGQD with the exception that in

this particular synthesis, NaOH has been used instead of urea. In short, 10 ml of 1 M citric acid was prepared in de-ionized water (D.I. water) and mixed with equimolar mixture of NaOH to form a transparent solution. The solution was then subjected to a microwave radiation of power 700 W for about 5 minutes. The dispersion of the GQDs in various solvents has been carried out using the same process as that of NGQD. The characterization of GQD has been carried out using the same instruments as that of NGQD. 1.4. Synthesis and characterization of B-GQD: Boron doped graphene quantum dots were synthesized by the hydrothermal treatment of aqueous solution of 0.5 M boric acid and 0.5 M glucose, as mentioned in a recent report. 28 In short, the transparent mixture was transferred to a commercial microwave and heated at 700 W for about 5 minutes. Microwave treatment of the above two aquoeus solution results in the formation of a brown aggregated mass, that was heated under vaccuum to remove volatile components. Subsequent dispersion in D.I. water and cycles of centrifugation (1500G) and filtration against a 10kDa filter results in the formation of a brown dispersion. The characterizations for BGQD were done using the same instruments as described for NGQD and GQD. All the characterization data for NGQD, GQD and BGQD are provided in Supporting Information( Figure S1 to S6). 1.5: Experimental details of the PET studies UV-Visible absorption studies: The UV-visible absorption studies were carried out using a JASCO V-660 spectrophotometer. The samples were taken in a two faced, transparent, square shaped quartz cuvette with a path length of 1 cm. All the experiments were carried out at room


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temperature (298 K). Concentration dependent absorption studies were carried out keeping the concentration of N-GQD at 0.02 mg/ml while the concentration of DEA was varied from 0 M to 0.12 M. Steady state photo-luminescence experiments: Steady state photo-luminescence experiments were carried out using a Horiba Jobin Yvon Fluoromax- 4 instrument. A four faced transparent quartz cuvette was used for all the experiments. The excitation/emission slit widths were kept at 3/3 for every experiment. Luminescence quenching studies were carried out with a series of DEA solutions with varying concentrations (0 M to 0.12 M). Time

resolved

luminescence

decay

measurements:

Time

resolved

luminescence

measurements were carried out in a Horiba Jobin Yvon Fluorocube instrument in a time correlated single photon counting (TCSPC) arrangement. A 440 nm LED with a pulse repetition rate of 1 MHz was used as the light source. The instrument response function (IRF) was collected using a scatterer (Ludox AS40, colloidal silica, Sigma Aldrich).For the 440 nm LED light source, the full width at half maximum including detector response is approximately 1 ns. The excited state decay of the samples was collected fixing the wavelength at near their emission maximum. The decay was fitted using the IBH software DAS6 according to the following equation: I (t) = ΣAi exp (- t/τi) where τi is the luminescence lifetime and Ai represents the amplitude of the corresponding decay. Laser flash photolysis studies: Transient Absorption Spectra (TAS) was recorded using a nanosecond laser flash photolysis instrument (Applied Photophysics, U.K.). The third harmonic of a Q-switched Nd: YAG laser (Quanta Ray, Lab 150, Spectra Physics, USA) was used to photo


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excite the samples. The pulse width of the laser light was 8 ns. The signals from the transients were probed using a 150 W pulsed xenon lamp, a Czerny – Turner monochromator, and HamaMatsu R-928 photomultiplier tube as a detector. The transient signals were captured using an Agilent Infiniium digital storage oscilloscope. The data were transferred to the computer for further analysis. All the samples were purged with argon for 30 minutes before recording the TAS. All the experiments pertaining to the PET studies of GQD and BGQD have been carried out in a similar fashion as that of NGQD with the exception that the concentration of GQD for the experiments were kept at 0.04 mg/mL [for BGQD it was same as NGQD viz. 0.02mg/mL]. The luminescence decay measurements for BGQD were carried out using a 370 nm LED. Materials: Citric acid, urea, boric acid and glucose was bought from Sigma Aldrich (USA) and used without any further purification. Spectroscopic grade solvents viz acetone, acetonitrile, tetrahydrofuran, dimethylformamide and dimethylsulphoxide was obtained from Spectrochem. India Pvt. Ltd. and used without any further purification. N, N-diethylaniline was obtained from Merck and used without any further purification. RESULTS AND DISCUSSIONS: The as-prepared NGQDs and GQDs showed a green luminescence under the UV-illumination (λex= 365 nm) while BGQD has a blue luminescence (Figure S1). The transmission electron microscopy (TEM) image of NGQD is shown below revealing a distribution of spherical particles (Figure 1). The average radius of NGQD was obtained from the TEM data as 7.2 nm (figures 1 and figure S3). Similarly, average radius of GQD and BGQD has been found to be 3 nm and 1 nm, respectively (figures S4 and S6, respectively).


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Figure 1: The TEM image of NGQD. The initial PET studies were carried out for the NGQD-DEA system. To verify our hypothesis, we have carried out the PET studies in solvents of varied viscosity viz. acetone [ACE] (η = 0.30 cP at 25 °C), acetonitrile [ACN] (η = 0.4 cP at 25 °C), tetrahydrofuran [THF] (η = 0.56 cP at 25 °C), dimethylformamide [DMF] (η = 0.92 cP at 25 °C) and dimethylsulphoxide [DMSO] (η = 1.99 cP at 25 °C).29 Steady state luminescence quenching studies revealed a significant decrease in the emission intensity of NGQD with increasing concentration of the quencher DEA (in all the stated solvents) with a little or no change in the absorption spectra (Figure 2, Figure S8-S11).These observations suggested that the quenching is solely dynamic in nature and there is no ground state interaction between the donor and the acceptor. Time-resolved luminescence quenching studies also revealed a significant lifetime quenching in all the cases asserting the dynamic nature of the quenching mechanism (Figure 3a and 3b, Figure S12-S13, Table S1-S5).


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2a

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2b

Figure 2: (a) the UV-visible absorption spectra of NGQD with increasing concentration of the quencher (DEA) in DMF (b) the steady state luminescence quenching studies of NGQD with increasing concentration of the quencher (DEA) in DMF (λex = 440 nm).

Figure 3: (a) the time-resolved luminescence quenching studies of NGQD with increasing concentration of DEA in DMF (λex= 440 nm, λem = 500 nm) (b) the time-resolved luminescence quenching studies of NGQD with increasing concentration of DEA in DMSO (λex= 440 nm, λem = 500 nm).


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The quenching constant (kq) values were obtained from Stern-Volmer plots (Figure S14-S16). The observations have been summarized in Table S6.The possibility of an excited state energy transfer is ruled out based on the fact that there is no spectral overlap between the emission spectra of NGQD with the absorption spectra of DEA (Figure S17). The possibility of an excited state intermolecular proton transfer is also ruled out since DEA is a tertiary amine. The results point towards the fact that photoinduced electron transfer (PET) is the sole reason behind the observed luminescence quenching of the NGQDs. Calculation of Free Energy Change of PET: To realize the thermodynamic feasibility of the electron transfer processes between NGQD with the aromatic amine DEA, the free energy of PET (∆GPET) has been calculated using the oxidation potential of DEA and reduction potential of NGQD. Since, GQDs possess an electrical double-layer capacitor behavior,

30, 31

cyclic

voltametry experiments didn’t show any obvious redox peaks for NGQD and hence, a value of the reduction potential for NGQD has been calculated using a method reported in the literature.32-35 The oxidation potential value of DEA was obtained from literature reports.36,37 These values have been utilized to evaluate the free energy of PET in all the solvents according to the modified Rehm-Weller equation 38:

∆ = ( ⁄) − (⁄ ) − − + ! "# − !% − ∗

$

&''

(1)

In the equation (1) above, E0(D+/D) = 0.72 eV in acetonitrile [vs SCE]. E0(A/A-) = 0.80 eV, is the reduction potential of NGQD [vs Ag/AgCl] as determined from calculations.

32-35

The other

parameters are (i) e = charge of an electron (1.602 x 10-19 C),(ii) ε0 = permittivity of free space,(iii) εs = dielectric constant of the solvent where we want to measure the feasibility of electron transfer,(iv) rD = 0.4 nm is the radius of DEA and (v) and rA has been considered ~ 8 nm


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(approx.) for the calculations and (vi) dcc = the centre to centre distance of the donor and the acceptor in the encounter complex (dcc ~ rD + rA). The E00 value for all the cases were calculated from the point of intersection of the absorption and emission spectrum of the GQDs. A negative ∆GPET in all the solvents, for the NGQD-DEA pair confirmed PET as the reason behind the observed luminescence quenching. Additionally, we calculated the driving force for the back electron transfer process (∆GBET) utilizing the value of E00 and ∆GPET according to the following formula: ∆GBET = -E00-∆GPET

39, 40

(2)

The obtained values have been tabulated below in Table 1. Table 1: The driving force dependence of forward and back electron transfer processes for the DEA-NGQD electron donor-acceptor system in different solvents [E0(D+/D) = 0.76 eV vs Ag/AgCl (satd.KCl)]. Solvent

E00 [in eV]

∆GPET [in eV]

∆GBET [in eV]

ACE

2.69

-1.26

-1.43

ACN

2.71

-1.13

-1.58

THF

2.70

-1.13

-1.57

DMF

2.64

-1.25

-1.39

DMSO

2.72

-1.34

-1.38

Laser Flash Photolysis Studies: In order to assert the formation of the charge separated states, nanosecond laser flash photolysis experiments were carried out. The peak for the DEA radical cation at ~ 450 nm confirmed the PET process.35, 37, 41, 42 In accordance with some of the reported literature,

43

no distinct peak has been observed for the NGQD radical anion in the laser flash


12

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experiments. The probable explanations for this particular observation can be two-fold: (a) the NGQD radical anion absorbs in the infrared-region which is beyond our instrument detection limit and, (b) the NGQD radical absorption coefficient under the experimental conditions is extremely less compared to the DEA radical cation, making it indistinguishable in presence of the broad absorption of the DEA radical cation.The rate constant of back electron transfer (kBET) was calculated using the equation kBET = 1/τradical.35, 37, 41 It was observed that the longevity of the charged states increased when the solvent medium changed from ACN (τradical = 300 ns) to DMSO (τradical = 10 µs) (Figure 4). Figure 4a and 4b shows the decay of DEA radical cation at 450 nm in DMSO and ACN, respectively. Figure 4c shows a plot of the two decays together, clearly demonstrating the significant enhancement in τradical when the solvent medium is changed from ACN to DMSO. The obtained τradical and kBET values for NGQD are summarized in Table 2 along with the solvent viscosity coefficient η. The transient absorption spectra (TAS) of NGQD+DEA in dimethylsulphoxide showing the formation of the DEA radical cation are illustrated in Figure 5. The TAS of NGQD + DEA in DMF, THF, ACN and ACE are provided in the supporting information (Figure S18 and Figure S19). It is clear from Table 1 and Table 2 that the free energy change of back electron transfer (∆GBET) does not exhibit any correlation with the back electron transfer rate constant (kBET) indicating that the back electron transfer process is not activation controlled. It has been known that, in polar solvents, the solvation and separation of ion-pairs takes place [formation of SSIP] and hence, the rates of recombination/back electron transfer decrease. Based on the solvent polarity effect on the rate of BET, the expected trend for the NGQD-DEA pair should be: kBET (THF, ϵ = 8.0) > kBET (ACE, ϵ = 21.0) > kBET (ACN, ϵ = 37.0) ≈ kBET (DMF, ϵ = 38.0) > kBET (DMSO, ϵ = 47.0). However, from table 2, the observed trend is different viz. kBET (ACE, ϵ =21.0) > kBET


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(ACN, ϵ =37.0) > kBET (THF, ϵ =8.0) > kBET (DMF, ϵ = 38.0) > kBET (DMSO, ϵ =47.0). Thus, we conclude that solvation of the generated radical ion-pairs is not the major factor controlling the rate of BET. However, a continuous decrease in the magnitude of kBET with increase in the medium viscosity coefficient (η) is clearly observed from Table 2. This clearly indicated that the major factor governing the BET dynamics is the viscosity of the medium.

Figure 4: The 450 nm decay for the DEA radical cation (a) in DMSO (b) in ACN (c) the two decays plotted together clearly reveals that the back electron transfer rate constant (kBET) is much slower in DMSO compared to ACN.


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Table 2: The variation of the radical lifetime [τradical] and kBET with the solvent viscosity coefficient [η] Solvent

η[in cP at 25oC]

τ radical [in µs]

kBET[s-1]

ACE

0.30

Not observed

> 1.25 x 108

ACN

0.38

0.30 ± 0.01

(3.3± 0.1) x 106

THF

0.55

0.85 ± 0.05

(1.18 ± 0.07) x 106

DMF

0.92

3.01 ± 0.15

(3.3± 0.2) x 105

DMSO

1.99

10.0 ± 0.20

(0.99 ± 0.1) x 105

Figure 5: The TAS of NGQD [0.02 mg/mL] + DEA [0.120 M] in DMSO revealing a distinct peak at 450 nm for the DEA radical cation confirming the formation of the charge-separated states.


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The inverse relation between the magnitude of kBET and η prompted us to examine the tunability of the BET rate constant with the size of the graphene quantum dots. The viscous drag (vopp) experienced by a molecule depends on its radius and hence, tunability of the back electron transfer rate constant can be expected depending upon the size of the graphene quantum dots. The size dependence of the back electron transfer rate constant (kBET) was evaluated by comparing the kBET values of NGQD with two other sets of graphene quantum dots of different sizes viz. pristine graphene quantum dot (GQD) and boron-doped graphene quantum dot (BGQD) respectively (Figure S20-S23). The solvent medium was kept constant in all the three cases in order to minimize the effect of the outer sphere solvent reorganization energies on the rate of BET. The rate constant for the back electron transfer process increased in the order NGQD (ravg= 7.2 nm)-DEA < GQD (ravg= 3nm)-DEA < BGQD (ravg= 1nm)-DEA in DMSO. The observations have been summarized in Table S9. The magnitude of kBET was found to be inversely related to the size corroborating our hypothesis that diffusion of the ion-pair is the major factor controlling the rate of back electron transfer. Further, for the GQD-DEA system, increase in medium viscosity led to a decrease in the rate constant for the back-electron transfer process, similar to that observed for the NGQD-DEA system (Table S11). For the BGQD-DEA system, such comparative studies were not possible since the rate of BET was too fast and beyond our instrument resolution (~ 8 ns). Vauthey et.al had demonstrated that the BET dynamics within an exciplex (CIP) depends upon the rate of the conformational changes in viscous non-polar solvents viz. decalin.40 However, in case the back electron transfer is from the solvent-separated ion-pairs (SSIPs), the equations in the exciplex model can be modified, keeping in mind the fact that the conformational changes can occur provided the SSIPs have already diffused towards each other. Considering this


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particular fact and using the relation kBET = [V(t)]2,

44

[V(t) being the electronic coupling

constant], a mathematical model can be built, demonstrating the variation of kBET (SSIP) with η and R (radius of the dots) [details of the derivation is provided in the supporting information SI 2]. We find that, kBET has a c.a. inverse exponential relation with both η and R. Since we have a distribution of particle size in case of GQDs, in place of R, ravg can be used. The mathematical model provides a justification for the substantial changes observed in the magnitude of kBET with slight changes in the viscosity coefficient (η) of the medium and size of the quantum dots. A plot of kBET vs η for the NGQD-DEA system showed a fit via an exponential decay function (Figure 6).

Figure 6: Plot of kBET vs viscosity for the NGQD-DEA donor-acceptor pair revealing an exponential relationship Kinetic Model: A kinetic model has been formulated to obtain an analytical expression for the diffusion controlled rate constant for the back electron transfer process in the system. The overall back electron transfer process has been represented in Scheme 1 below:


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Scheme 1: BET in solvent-separated radical ion pair

The first step is a reversible diffusion process with rate constants kdiff and k-diff for the forward and backward diffusion processes respectively. This is followed by the conformational changes of the radical ions with rate constant denoted as kcc. The third step is the electron transfer step from the acceptor radical anion to the donor radical cation regenerating the ground state donor and acceptor with rate constant k’BET . The rate of formation of D is given by: &()* &+

= ,′- ( − *c

Applying steady-state approximation for [D+ [D+

A-] c = (

.''

.′/01

(3)

A-] c we obtain:

*[D+

A-]

(4)

Similarly, applying steady state approximation, we obtain an expression for the other intermediate [D+

A-] in terms of [D+] [A-]. [D+

A-] = (

.2344

.5234467''

* [D+] [A-]

(5)

Substituting and rearranging equation (3) in terms of (4) and (5) we obtain:


18

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&()* &+

.'' ∗ .2344

= 8.

52344 .''

9 ( *( *

(6)

The detailed steps are provided in SI 3. Assuming, k-diff

Diffusion of Solvent-Separated Ion Pairs Controls Back Electron

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