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Ultraslow recombination in AOT-capped TiO2 nanoparticles sensitized by protoporphyrin IX Sudipta Biswas,a Swati De*a and Arunkumar Kathiravanb Aerosol OT (AOT) capped TiO2 nanoparticles have been prepared by a phase transfer mechanism. The TiO2 nanoparticles have a diameter of 5–10 nm, are highly crystalline (anatase) and show high photoluminescence. They are effectively sensitized by protoporphyrin IX (PPIX) and show high electron injection
Received 26th June 2014, Accepted 30th July 2014 DOI: 10.1039/c4dt01926a www.rsc.org/dalton
1.
rates while the rate of back recombination is much slower than those reported previously. Thus the AOT capped TiO2 nanoparticles synthesized in this work are highly effective not only in promoting ultrafast electron injection from PPIX to TiO2 but more importantly they lead to extremely slow back recombination rates. The significance of this work is in the synthesis of highly photoluminescent TiO2 nanoparticles which can be easily sensitized by a porphyrin dye, whereby ultraslow recombination is observed.
Introduction
Over the past few years sensitization of large band-gap semiconductor materials with organic dyes has been a field of extensive research because semiconductor materials such as TiO2 have been widely used as photocatalysts for solar energy conversion.1–3 In addition, one of the third-generation solar cells, i.e. the dye-sensitized solar cell (DSC) has received wide attention due to its low production cost and flexible production methods. Up to now, DSCs’ performance has reached power conversion efficiencies exceeding 12% by using ruthenium and porphyrin sensitizers.4–7 In DSC, the photoanodes play a vital role in the conversion of light into electrical energy. Dye sensitized TiO2 material has served as the frequently used material for DSC photoanodes. Effective electron transfer from the excited dye to the TiO2 conduction band requires good electronic coupling between the dye and TiO2. Therefore, sensitization of TiO2 semiconductor materials with organic dyes has been a subject of great interest. Numerous organic dyes such as coumarin, indoline, squaraine, xanthene, cyanine, hemicyanine, oligothiophene, perylene, carotenoid, porphyrin and phthalocyanine derivatives are capable of TiO2 sensitization. Among them, porphyrins are promising candidates for DSC, because of their strong Soret (400–450 nm) and moderate Q-band (550–600 nm) absorption properties as well as their natural role in photosynthesis. The steps involved in the photosensitization of TiO2 by dyes are shown in Scheme 1.
a
Department of Chemistry, University of Kalyani, Kalyani-741235Nadia, W.B., India. E-mail: [email protected]; Fax: +91-33-25828282; Tel: +91-33-25828750 b National Centre for Ultrafast Processes, University of Madras, Taramani Campus, Taramani, Chennai-600113, Tamil Nadu, India
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Scheme 1
Steps involved in the photosensitization of TiO2 by dyes.
Sensitization of colloidal TiO2 has been studied extensively in the past.8,9 Recently, Ramamurthy and co-workers performed spectroscopic and photovoltaic studies of protoporphyrin IX on a TiO2 electrode.10 In this work, we have used the protoporphyrin IX (PPIX) dye as a photosensitizer due to its significant visible light absorption and the presence of –COOH groups, which can serve as effective anchoring groups between the dye and the TiO2 surface [Scheme 2]. Another interesting part of the present work is the preparation of surface-modified TiO2 nanoparticles. Surface modification of semiconductor nanoparticles can change their optical, chemical and photocatalytic properties significantly.11 Surface modification of nanoparticles may enhance their excitonic and defect emission by blocking nonradiative electron/ hole (e−/h+) recombination at the defect sites (traps) on the surface of the semiconductor nanoparticles and may increase the photostability of semiconductor nanoparticles. Xagas et al.12 reported surface modification and photosensitization of TiO2 nanocrystalline films with ascorbic acid. Rabani et al.13
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Scheme 2
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Mode of attachment of PPIX on the TiO2 surface.
reported interfacial electron transfer dynamics in surfacemodified nanocrystallite layers using laser photolysis techniques. Qu et al.14 reported the enhancement of the photoinduced electron transfer from cationic dyes to colloidal TiO2 nanoparticles after modifying the surface with sodium dodecyl benzene sulfonate (DBS) molecules. Ghosh et al.15,16 studied interfacial electron transfer dynamics of alizarin-sensitized surface-modified and unmodified TiO2 nanoparticles using femtosecond transient absorption spectroscopy. The advantage with such surfactant-capped nanoparticles is that they can be readily dissolved in many nonaqueous solvents. It has been observed that electron injection dynamics does not change much with surface modification. However, they have noted a very interesting effect of surface modification on back electron transfer. They have observed that the back electron transfer dynamics is slow on the modified surface as compared to that on the unmodified TiO2 surface. In the present work we have synthesized AOT capped TiO2 nanoparticles through a phase transfer mechanism. The main motivation behind using AOT for preparing surface-modified TiO2 nanoparticles came from previous knowledge of the good stabilizing and charging properties of AOT.17–22 AOT has been commonly used for charging colloidal particles in applications such as motor oil,23 inkjet printing,24 liquid electrostatic developers25 and electrophoretic ink.26 For AOT solutions in nonpolar solvents, it has been shown that a small fraction of the
Scheme 3
reverse micelles is charged as a result of a disproportionation/ comproportionation mechanism, in which two neutral reverse micelles exchange a charge producing one negatively and one positively charged reverse micelle and vice versa [Scheme 3].19–21,24 Evidence for this mechanism comes from electrokinetic and adsorption measurements as well as surface force measurements.17,18 These charged reverse micelles increase the conductivity of a nonpolar solution and can induce an important electric field when they are separated. In applications such as electrophoretic inks,26 where movement of charged pigments is important, these charged micelles play an important role. This was our main driving force for the work. In DSC applications too, movement of charge plays an important role. If the presence of AOT capping on the TiO2 surface can enhance electron transfer from the dye to TiO2 and/or retard back recombination, then a distinct advantage will arise for DSC based applications.
2. Material and methods 2.1.
Materials
Titanium(IV) isopropoxide was purchased from Aldrich. The surfactant used was aerosol OT [AOT, sodium bis(2-ethylhexyl) sulfosuccinate] from SRL. Dimethylformamide (DMF) was purchased from Merck-India and n-heptane was purchased from Spectrochem. Protoporphyrin IX (PPIX) was obtained from Sigma and was used without further purification. PPIX solutions in DMF were used for the spectral studies. 2.2.
Synthetic procedure
A solution of Ti(IV) isopropoxide dissolved in isopropyl alcohol was added dropwise to distilled water at pH 1.5. The solution was continuously stirred for 10–12 hours until a transparent colloid formed. Then, AOT [sodium bis(2-ethylhexyl) sulfosuccinate] was added to n-heptane to make a 1 × 10−3 M solution. To the freshly prepared TiO2 colloids in water, the stock solution of AOT in n-heptane was added and the resulting mixture was stirred slowly for 3 hours. As the surface of the TiO2 nanoparticles is positively charged, AOT molecules can easily bind through the sulfonic group (SO3−) with the nanoparticles. The newly capped TiO2 nanoparticles look like reverse micelles and
Disproportionation and vice versa of AOT reverse micelles in nonpolar solvents.
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can be dissolved easily in organic solvents. Under this situation, TiO2 nanoparticles are extracted from the water to the organic phase. Now with the help of a separating funnel the organic phase was separated out. This phase was dried using CaCl2 and it finally got transformed into an optically clear solution. The organic phase was then refluxed for 2 h and the solvent was removed with the help of a rotary evaporator under a N2 atmosphere. Then the solid AOT-capped TiO2 nanoparticles were dried in an oven at 80 °C for 6 hours. 2.3.
Characterization techniques
The absorption spectra were recorded with a UV 2450-PC spectrophotometer (Kyoto, Japan). Transmission electron microscopic (TEM) studies of the nanoparticles were carried out at a resolution of 1.9 Angstrom with a JEOL, JEM-2100 Electron Microscope from Japan. TEM specimens were prepared by placing micro drops of solution on a carbon film supported by a copper grid. Scanning electron microscopy (SEM) studies were performed with a field emission SEM (JSM-6700F, Jeol, Japan) to observethe surface topology. Powder X-ray diffraction (XRD) was recorded using a Bruker D8 advanced powder X-ray diffractometer using CuKα at 1.5418 Å. Fluorescence spectra were recorded with a Perkin Elmer LS 55 spectrofluorimeter. For recording the emission spectra of protoporphyrin IX (PPIX), the excitation wavelength (λex) was fixed at 400 nm. For fluorescence quenching studies, the final dye concentration was 1 × 10−6 M. 2.4. Set-up for time-resolved fluorescence and transient absorption Flash photolysis was carried out using a Nd:YAG laser source producing nanosecond pulses (8 ns) of 355 nm light with the energy of the laser pulse being around 200 mJ. Dichroic mirrors were used to separate the third harmonic from the second harmonic and the fundamental output of the Nd-YAG laser. The monitoring source was a 150 W pulsed xenon lamp, which was focused on the sample at 90° to the incident laser beam. The beam emerging through the sample was focused onto a Czerny-Turner monochromator using a pair of lenses.
Fig. 1
Paper
Detection was carried out using a Hamamatsu R-928 photomultiplier tube. Transient signals were captured with an Agilent infinium digital storage oscilloscope and the data were transferred to a computer for further analysis. For laser flash photolysis studies, samples were purged with argon gas for 45 min prior to laser irradiation. Time-resolved fluorescence decays were obtained by the time correlated single-photon counting (TCSPC) technique. The samples were excited at 405 nm. Data analysis was carried out using the software provided by IBH (DAS-6), which is based on deconvolution techniques using the nonlinear least-squares method and the quality of the fit was ascertained with the value of χ2 < 1.2.
3. Results and discussion 3.1.
Nanoparticle characteristics
The absorption spectrum of the as-synthesized AOT-capped TiO2 nanoparticles is illustrated in Fig. 1(a). Compared with bulk TiO2,27 a shift of the absorption spectrum of nano-TiO2 towards the lower wavelength region was observed. The AOTcapped TiO2 nanoparticles show a much more blue-shifted λonset (358 nm) than bulk TiO2 (385 nm). We believe that this blue shifted λonset of the AOT-capped TiO2 nanoparticles relative to bulk TiO2 is due to excess negative charge provided by the sulphonate group of AOT onto the surface of the nanoparticles. These kinds of spectral shifts have been explained very nicely by earlier workers28 on the basis of an electrostatic model. They have proposed that injection of excess negative charge onto the nanoparticles creates a polarizing field which leads to an increase in energy and hence a blue shift of λonset. The band gap (Eg) of the as-synthesized TiO2 samples was determined using the Kubelka–Munk plot.29 The absorption edge of as-synthesized TiO2 was at 358 nm, corresponding to a band gap of 3.73 eV [Fig. 1(b)]. This corresponds to the energy of charge-transfer from the valence band (mainly formed by 2p orbitals of the oxide anions) to the conduction band (mainly formed by 3dt2g orbitals of the Ti4+ cations). The absorption edge at 385 nm of bulk anatase TiO2 corresponds
(a) Absorption-onset for the TiO2 nanoparticles. (b) Plot for determining the band gap.
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to a band gap of 3.2 eV. Thus the band gap energy of the assynthesized TiO2 is higher than that of bulk TiO2. This happens due to size quantization effects.30 The radius of the synthesized TiO2 nanoparticles was determined using the Brus equation, i.e. eqn (1):31
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ΔEg ¼
h2 1:8e2 þ polarization terms 2 8μR εR
ð1Þ
where h is Planck’s constant, R is the radius of the particle and ΔEg is the band gap shift. μ is the reduced mass of the exciton defined as (1/μ = 1/m*e + 1/m*h) where m*e and m*h are the reduced effective mass of the electron and the hole respectively, e is the electron charge and ε is the dielectric constant of the semiconductor. The value of μ is taken as μ = 1.63me (me is the electron rest mass).32 Since the optical dielectric constant of bulk titanium dioxide is very large (ε = 170), the polarization terms in eqn (1) are neglected. The band gap shift for colloidal TiO2 is 0.53 as compared to bulk anatase TiO2. Thus the radius (R) of the TiO2 nanoparticles determined using eqn (1) is 11.1 nm. The photoluminescence (PL) spectra of the AOT-capped TiO2 nanoparticles were measured for 364 nm excitation. Broad PL spectra in the range of 450–500 nm were obtained for the as-synthesized particles [Fig. 2]. This is assigned to the band-edge luminescence of TiO2 nanoparticles.33 The PL band of TiO2 nanoparticles comprises both direct and indirect transitions. The peaks in the PL spectra conform to spectra reported in the literature and also to the assignments based on calculations by Daude et al.33 TiO2 nanoparticles exhibit a shoulder in the PL signal at about 431 nm, possibly resulting from band-edge excitons, and this surface emission is attributed to an indirect transition X1a→Γ1b and is linked to exciton recombination in shallow trapped surface states.32,33 The occurrence of emission peaks in the visible region is mainly due to the presence of defect levels such as oxygen vacancies in the band gap. Serpone et al.30 reported the PL bands of anatase TiO2 nanocrystals in the long wavelength range
Fig. 2 Photoluminescence spectra of the TiO2 nanoparticles for 364 nm excitation.
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(465 nm) and attributed them to the oxygen vacancies. There are oxygen vacancies on the surface of TiO2 nanoparticles and the size of particles is small so that the average distance that the electrons can move freely is short. These factors make the oxygen vacancies bind electrons easily and thus form excitons. In general, the smaller the nanoparticle size, the larger is the oxygen vacancy content, thus the higher the probability of existence of excitons and hence the stronger is the PL signal.34 In order to determine the size and to study the crystalline nature of the AOT-capped TiO2 nanoparticles, powder XRD analysis was performed. From Fig. 3 it can be clearly observed that the diffraction peaks that appear in the powder XRD pattern of the AOT-capped TiO2 nanoparticles correspond to the anatase crystalline phase. The data are consistent with the standard powder diffraction pattern of anatase (as given by the Joint Committee on Powder Diffraction Standards, JCPDS). The XRD pattern exhibits prominent peaks at 2θ values of 25.23°, 37.66°, 47.96°, 53.86°, 62.69°, 68.75°, 75.09° and 82.15° corresponding to the [101], [004], [200], [105], [204], [116], [215] and [303] planes of anatase TiO2. The average diameter of the nanoparticulate samples determined using Scherrer’s equation i.e. eqn (2) is 9.42 nm: d ¼ 0:9λ=β cos θ
ð2Þ
where λ = wavelength of X-ray radiation [1.5418 Å], β = full width at half maximum (FWHM) of the XRD peak expressed in radians, and θ = angle of diffraction [2θ = 25.23°].
Fig. 3 Powder XRD pattern of (a) anatase TiO2 (reference) and (b) surfactant-modified TiO2 nanoparticles.
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Fig. 4
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(a) TEM image of the TiO2 nanoparticles; the inset shows the histogram. (b) SAED pattern.
TEM analysis was carried out to determine the actual size of the AOT-capped TiO2 particles, their growth pattern and distribution. It is evident from the histogram of the TEM image [Fig. 4(a)] that the average size of a majority of the particles is ∼5 nm. However, some secondary particles of size 10–12 nm are also observed from the micrograph. Thus, within experimental error limits, the particle sizes obtained from three different sets of experiments, i.e. absorption spectra, powder XRD and TEM, are almost similar. The selected area electron diffraction (SAED) pattern [Fig. 4(b)] shows distinct diffraction rings with characteristic spots corresponding to the planes of anatase TiO2. The SAED pattern indicates that the particles are highly crystalline. From the d-spacing, the rings are assigned to the [101], [004], [200], [105], [116] and [303] planes of anatase TiO2. Field emission scanning electron microscopy (FESEM) was used for a morphological study of the TiO2 nanoparticles. Fig. 5 shows the FESEM image of the as-prepared AOT-capped TiO2 nanoparticles. The inset shows the surface topology of the same at lower magnification. From the SEM image, it can be seen that the nanoparticles have a large surface area and Fig. 6
EDX pattern of the TiO2 nanoparticles.
form a porous network. A closer examination of these figures reveals a well-defined particle-like crystalline morphology. Energy dispersive X-ray spectroscopy (EDX) was employed to ascertain the composition of the AOT-capped nanoparticles [Fig. 6]. The EDX analysis indicates the presence of Ti and O proving that the as-synthesized nanomaterials are composed of TiO2 only. 3.2.
Fig. 5 FESEM image of the TiO2 nanoparticles; the inset shows the image at a lower resolution.
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Dye-sensitization studies
3.2.1. Absorption characteristics. The absorption spectrum of the PPIX solution in the UV-visible region shows an intense Soret band absorption at 405 nm (S0→S2 transition) [Fig. 7(a)] together with four weaker Q-bands at 506, 542, 577 and 630 nm (S0→S1 transition).10,35 In the presence of colloidal TiO2 the optical density of PPIX at 405 nm decreases. This may be due to adsorption of PPIX on the surface of surfactant-modified
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Fig. 7 (a) Emission spectra of PPIX (1 × 10−6 M) in the absence and presence of as-synthesized TiO2 nanoparticles: (i) 0 M, (ii) 5 × 10−5 M, (iii) 1 × 10−4 M, (iv) 5 × 10−4 M and (v) 1 × 10−3 M. (b) Plot to determine the apparent association constant (Kapp).
TiO2 nanoparticles through its anchoring group (–COOH) (Scheme 1). A similar type of adsorption of sensitizers on the surface of TiO2 nanoparticles was reported earlier.8,9 3.2.2. Fluorescence quenching characteristics. The fluorescence emission spectra of PPIX (for λex = 532 nm) in the absence and presence of increasing concentrations of AOTcapped TiO2 nanoparticles are shown in Fig. 7(a). Addition of the capped TiO2 nanoparticles to the solution of PPIX results in the quenching of its fluorescence emission. This quenching behaviour is similar to the previously reported fluorescence quenching of chlorophyllin,36 7-methoxynaphtho[1,2-b]thiophene-2-carboxylic acid,37 anthracene-9-carboxylic acid38 and erythrosine B39 by colloidal TiO2 nanoparticles. (This quenching is assigned to the electron injection from the excited state sensitizer molecules to the conduction band of colloidal TiO2 nanoparticles.) From the fluorescence quenching data, we can also calculate the Kapp using the following eqn (3): 1 1 1 ¼ þ F 0 F F 0 F′ K app ðF 0 F′Þ½TiO2
ð3Þ
where Kapp = apparent association constant, F0 = initial fluorescence intensity of PPIX, F′ = fluorescence intensity in the presence of adsorbed TiO2, and F = observed fluorescence intensity at its maximum. The plot of 1/(F0 − F) versus 1/[TiO2] is shown in Fig. 7(b). A linear plot was obtained. From the slope, the Kapp has been determined to be 3.3 × 103 M−1. The decrease in fluorescence emission may be attributed to electron transfer or energy transfer processes between the dye and the TiO2 nanoparticles. The band gap of TiO2 (3.73 eV) is greater than the excited state energy of PPIX (2.04 eV) and there is no overlap between the fluorescence emission spectrum of PPIX and the absorption spectrum of AOT-capped TiO2 nanoparticles. This excludes the possibility of energy transfer from PPIX to TiO2 nanoparticles. Thus the fluorescence
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quenching seen in Fig. 7(a) is due to electron transfer from the dye to the TiO2 nanoparticles. 3.2.3. Time resolved fluorescence characteristics. The fluorescence quenching data indicate that electron injection occurs from excited PPIX to the AOT-capped TiO2 nanoparticles. To get more insights into the electron injection process, time-resolved fluorescence experiments were performed. It was shown earlier that the sensitizer molecules adsorbed on the TiO2 surface have significantly shorter lifetimes than those in a homogeneous solution and this decrease in lifetime occurs due to the electron transfer process.8,9 In the absence of AOT-capped TiO2 nanoparticles, the fluorescence decay of PPIX can be fitted to a single exponential expressed as [F(t ) = A exp(−t/τ)] with a lifetime of 14 ns [Fig. 8(i)].
Fig. 8 Time resolved fluorescence decay of (i) PPIX and in the presence of surfactant-modified TiO2 nanoparticles: (ii) PPIX + 1 × 10−4 M TiO2 and (iii) PPIX + 1 × 10−3 M TiO2.
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Table 1 Fluorescence lifetimes and ket values for PPIX in the absence and presence of TiO2
Samples
τ1 (ns)
a1 (%)
τ2 (ns)
a2 (%)
ket × 109 (s−1)
PPIX PPIX–TiO2 (1 × 10−4 M) PPIX–TiO2 (1 × 10−3 M)
3.2 2.9
5 14
14 12 10
95 86
— 0.22 0.24
However, in the presence of surfactant-modified TiO2 nanoparticles, the fluorescence decay of PPIX [Fig. 8(ii) and (iii)] can be fitted to a bi-exponential [F(t ) = A1 exp(−t/τ1) + A2 exp(−t/τ2)], with short-lived and long-lived components [Table 1]. The longer component is assigned to free PPIX in solution while the shorter component is assigned to PPIX adsorbed onto TiO2. The contribution of the short component increases with the concentration of the TiO2 nanoparticles [Table 1]. The rate of electron transfer (ket) from the excited state PPIX to the conduction band of TiO2 can be calculated using eqn (4): ket ¼ 1=τads 1=τ
ð4Þ
where τ and τads are the lifetimes of PPIX in the absence and presence of surfactant-modified TiO2 nanoparticles respectively. The calculated ket values are shown in Table 1. As the ket values increase with an increase in TiO2 concentration the assumption of electron transfer from excited PPIX to TiO2 nanoparticles is valid. Table 2 shows the ket values and recombination rates (krec) obtained by other researchers who have used other dyes and bare TiO2.38,40–43 A comparison of the ket values obtained by us (Table 1) and those existing in the literature38,40–43 (Table 2) shows that our ket values are almost comparable (a little lower) to those reported for other dyes. The efficiency of electron injection from the excited state of PPIX to the conduction band of TiO2 is further verified by eqn (5): Es*=sþ ¼ Es=sþ Eð0;0Þ
ð5Þ
Es/s+ is the oxidation potential of PPIX and its value is 1.01 V vs. SCE, E(0,0) was calculated from the intersection of the PPIX absorption spectrum with the fluorescence emission spectrum in DMF at a normalized absorption/emission intensity and its value is 2.04 eV. Thus the excited state oxidation potential
Table 2
ket values and krec values reported in the literature
Samples
ket (s−1)
krec (s−1)
Anthracene-9-carboxylic acid/TiO2 Cyanine/TiO2 Squaraine/TiO2 Hypocrellin B/TiO2 Fluorescein/TiO2 PPIX–TiO2
4.8 × 108 a 4.0 × 109 b 1.9 × 108 c 4.9 × 109 d 1.7 × 108 e 2.4 × 108 f
5.5 × 107 a — 1.0 × 105 c 1.2 × 105 d — 8.3 × 103 f
The data have been taken from the following references: a Ref. 38. b Ref. 40. c Ref. 41. d Ref. 42. e Ref. 43. f The present work using AOTcapped TiO2.
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Scheme 4 Energy diagram justifying the electron transfer from the excited dye to the conduction band of TiO2.
(Es*/s+) of PPIX was determined to be −1.03 V vs. SCE. The conduction band potential of TiO2 is at −0.52 V vs. SCE.44 This suggests that electron transfer from the excited state of PPIX to the conduction band of TiO2 is energetically favored (Scheme 4). The above data and calculations conclusively prove that the fluorescence quenching of PPIX in the presence of TiO2 nanoparticles is due to the electron injection from the excited state of the dye to the conduction band of the TiO2 nanoparticle. Further, the driving force for electron injection (ΔGinj ) from the excited PPIX to the conduction band of TiO2 was calculated using eqn (6): ΔGinj ¼ Es*=sþ eCB ðTiO2 Þ
ð6Þ
The calculated ΔGinj value (−0.51 V) is negative, indicating the favorable driving force for electron injection from the excited PPIX to TiO2. 3.2.4. Transient absorption (TA) characteristics. To further probe the involvement of AOT-capped TiO2 nanoparticles in the recombination process, we carried out transient absorption (TA) studies. TA spectroscopy has earlier been used to provide insight into the recombination dynamics, i.e. back electron transfer process in the dye–TiO2 interface. If the observed fluorescence quenching of PPIX is due to electron injection into the conduction band of TiO2, one would expect to detect the presence of a cation radical of PPIX. Based on the above strategy, we carried out the TA measurements of PPIX with AOTcapped TiO2 nanoparticles. The TA spectrum of PPIX was recorded for excitation at 355 nm in the wavelength range of 400–550 nm [shown in Fig. 9(a)]. The absorption maximum at 440 nm with a lifetime of 8 μs can be ascribed to the triplet–triplet absorption of PPIX. The bleaching in the 400 nm region indicates the depletion of PPIX absorption. The transient absorption spectra of PPIX in the presence of AOT-capped TiO2 nanoparticles in DMF solution
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Fig. 9 (a) TA spectra of PPIX in DMF after excitation at 355 nm at different times. (b) TA decay monitored at 440 nm. (c) TA spectra of PPIX with TiO2 nanoparticles. (d) TA decay of PPIX in the presence of TiO2 nanoparticles monitored at 450 nm.
are shown in Fig. 9(c). Interestingly, in the presence of TiO2 nanoparticles, no new transient species was observed. However, TA gets broadened and shifts by around 10 nm. The TA spectra in the absence and presence of TiO2 nanoparticles are shown in Fig. 10. We assign this spectrum to the cation radical of PPIX produced due to electron transfer from PPIX to the TiO2 nanoparticles. Moreover, the measured decay at the peak (i.e. 450 nm) gives a lifetime of 120 μs [Fig. 9(d)] for PPIX in the presence of TiO2 nanoparticles. The rate constant for
Fig. 10
TA spectra of PPIX in DMF and on the TiO2 surface after 1 µs.
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electron recombination has been determined [krec = 1/τ = 1/120 μs] and was found to be 8.3 × 103 s−1. The krec value obtained by us is orders of magnitude lower than those previously reported for other dye–TiO2 systems (Table 2). This is a strong indication of the effectiveness of the AOT-capped TiO2 nanoparticles in retarding back recombination rates. As we know, reverse electron transfer has been found to be an important factor controlling the net charge-transfer efficiency in the photosensitization process. The much lower recombination rate compared to ket (shown in Table 1) is an indication of the weak interaction between the cation radical of the PPIX and AOT-capped TiO2. The rate of electron–cation recombination is four orders of magnitude lower than electron injection, which clearly reveals that electron injection is the dominant process in this system. Thus the interaction between PPIX and the AOT-capped TiO2 nanoparticles synthesized here offers a twofold advantage for future application in DSCs. First, electron transfer is very fast due to a favourable driving force. Second, back recombination is very slow. Now, the greatly repressed recombination for AOT-capped TiO2 nanoparticles arises due to two effects. First, once the electron is injected into the conduction band of AOT-capped TiO2 nanoparticle the enhanced charge mobility of the AOT reverse micelle encapsulating TiO2 [Scheme 5] will prevent back recombination. Herein lies the great potential of using AOT-capped TiO2 nanoparticles for DSC applications. Secondly, as reported by earlier workers,15,16,38,39,45 the Fermi
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Acknowledgements
Scheme 5 micelles.
TiO2
nanoparticles
encapsulated
within
AOT
reverse
S. De acknowledges the Department of Science and Technology (DST), New Delhi for the generous grant of SERC Fast Track Scheme no. SR/FT/CS-057/2008. A.K. thanks DST, India for the DST-Fast Track Project (ref. no. CS-316/2012, Dt. 04/12/2012). We acknowledge the Central Research Facility, Indian Institute of Technology Kharagpur, India for the TEM studies. We thank the Indian Association for the Cultivation of Science, Kolkata for providing the FESEM facility. The FIST grant to the Department of Chemistry, University of Kalyani from the Department of Science and Technology, India is acknowledged. Dr A. Gayen, Department of Chemistry, Jadavpur University is acknowledged for help with the powder XRD measurements. University of Kalyani is acknowledged for providing the basic infrastructure for research.
References level in surface-modified TiO2 colloids is pushed up in energy and as a result the overall free energy (ΔG0) of the back recombination process increases. Back recombination in dye sensitized TiO2 nanoparticle surfaces falls in the Marcus inverted regime. In this regime, the rate of back electron transfer (BET) decreases with increasing ΔG0. Thus, for AOT-capped TiO2 nanoparticles too, ΔG0 for back recombination increases and so the rate decreases.
4.
Conclusions
AOT-capped TiO2 nanoparticles have been synthesized based on a phase transfer mechanism. The TiO2 nanoparticles are relatively small (5–10 nm) and crystalline, the predominant crystalline phase being anatase. The nanoparticles are highly photoluminescent. They can bind effectively to PPIX. Calculations of the electron injection rate (ket) from time-resolved fluorescence studies indicate very efficient electron injection (ket = 2.4 × 108 s−1) from the PPIX excited state to the CB of TiO2. On the other hand, back recombination of the injected electron with the PPIX cation radical is four orders of magnitude slower than the rate of electron injection (krec = 8.3 × 103 s−1). More importantly, krec for the AOT-capped TiO2 nanoparticles (krec = 8.3 × 103 s−1) is orders of magnitude slower than those reported by earlier workers (krec ∼ 105–107 s−1). This is due to the additional influence of AOT which is known to enhance charge mobility of colloids and thus helps in moving the injected electron far away from the hole. Thus the conditions are perfect for the use of the PPIX–TiO2 system as the photoactive medium in DSCs. The high photoluminescence of the particles can also be utilized effectively to prepare devices. The ease of photosensitization and high photoluminescence of AOT-capped TiO2 nanoparticles can be effectively used in future for preparing DSCs.
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1 B. O’Regan, J. M. Anderson and M. Gratzel, J. Phys. Chem., 1990, 94, 8720. 2 B. O’Regan and M. Gratzel, Nature, 1991, 253, 737. 3 P. V. Kamat, Chem. Rev., 1993, 93, 267. 4 M. Grätzel, Acc. Chem. Res., 2009, 42, 1788. 5 F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-Baker, P. Wang, S. M. Zakeeruddin and M. Grätzel, J. Am. Chem. Soc., 2008, 130, 10720. 6 C. Y. Chen, M. Wang, J. Y. Li, N. Pootrakulchote, L. Alibabaei, C. Ngoc-le, J. D. Decoppet, J. H. Tsai, C. Grätzel, C. G. Wu, S. M. Zakeeruddin and M. Grätzel, ACS Nano, 2009, 3, 3103. 7 A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin and M. Grätzel, Science, 2011, 334, 629. 8 A. Kathiravan and R. Renganathan, Spectrochim. Acta, Part A, 2008, 71, 1106. 9 G. Ramakrishna, S. Verma, D. A. Jose, D. K. Kumar, A. Das, D. K. Palit and H. N. Ghosh, J. Phys. Chem. B, 2006, 110, 9012. 10 A. Kathiravan, V. Raghavendra, R. A. Kumar and P. Ramamurthy, Dyes Pigm., 2013, 96, 196. 11 G. Ramakrishna, A. Das and H. N. Ghosh, Langmuir, 2004, 20, 1430. 12 A. P. Xagas, M. C. Bernard, A. Hugot-Le Goff, N. Spyrellis, Z. Loizos and P. Falaras, J. Photochem. Photobiol., A., 2000, 132, 115. 13 J. Rabani, K. Ushida, K. Yamashita, J. Stark, S. Gershuni and A. Kira, J. Phys. Chem. B, 1997, 101, 3136. 14 P. Qu, J. Zhao, L. Zang, T. Shen and H. Hidaka, Colloids Surf., A, 1998, 138, 39. 15 G. Ramakrishna, A. K. Singh, D. K. Palit and H. N. Ghosh, J. Phys. Chem. B, 2004, 108, 1701. 16 G. Ramakrishna and H. N. Ghosh, Langmuir, 2003, 19, 505. 17 W. H. Briscoe and R. G. Horn, Langmuir, 2002, 18, 3945.
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18 C. E. McNamee, Y. Tsujii and M. Matsumoto, Langmuir, 2004, 20, 1791. 19 S. K. Sainis, V. Germain and F. R. Dufresne, Phys. Rev. Lett., 2007, 99, 018303. 20 M. Karvar, F. Strubbe, F. Beunis, R. Kemp, A. Smith, M. Goulding and K. Neyts, Langmuir, 2011, 27, 10386. 21 G. Seth Roberts, R. Sanchez, R. Kemp, T. Wood and P. Bartlett, Langmuir, 2008, 24, 6530. 22 S. K. Sainis, V. Germain, C. O. Mejean and E. R. Dufresne, Langmuir, 2008, 24, 1160. 23 I. F. Blagoridov, V. P. Lapin, G. L. Trofimpva and G. I. Shor, Chem. Technol. Fuels Oils, 1971, 7, 439. 24 I. D. Morrison, Colloids Surf., A, 1993, 71, 1. 25 K. A. Pearlstine, J. Imaging Sci., 1991, 35, 326. 26 X. Meng, F. Tang, B. Peng and J. Ren, Nanoscale Res. Lett., 2010, 5, 174. 27 H. Jin Zhai and L. Sheng Wang, J. Am. Chem. Soc., 2007, 129, 3022. 28 A. Henglein, A. Kumar, E. Janata and H. Weller, Chem. Phys. Lett., 1986, 132, 133. 29 H. Lin, C. P. Huang, W. Li, C. Ni, S. Ismat Shah and Y. H. Tseng, Appl. Catal., B., 2006, 68, 1. 30 N. Serpone, D. Lawless and R. Khairutdinov, J. Phys. Chem., 1995, 99, 16646. 31 L. Brus, J. Phys. Chem., 1986, 90, 2555. 32 E. Joselevich and I. Willner, J. Phys. Chem., 1994, 98, 7628.
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33 N. Daude, C. Gout and C. Jouanin, Phys. Rev. B: Solid State, 1977, 15, 3229. 34 L. Q. Jing, X. J. Sun, B. F. Xin, W. M. Cai and H. G. Fu, J. Solid State Chem., 2004, 177, 3375. 35 E. Balasubramaniam and P. Natarajan, J. Photochem. Photobiol., A, 1997, 103, 201. 36 P. V. Kamat, J. P. Chauvet and R. W. Fessenden, J. Phys. Chem., 1986, 90, 1389. 37 S. K. Pal, S. Bhattacharaya, S. K. Batabyal, T. K. Pradhan and T. Ganguly, J. Photochem. Photobiol., A, 2007, 189, 86. 38 P. V. Kamat, J. Phys. Chem., 1989, 93, 859. 39 P. V. Kamat and M. A. Fox, Chem. Phys. Lett., 1983, 102, 379. 40 C. Chen, X. Qi and B. Zhou, J. Photochem. Photobiol., A., 1997, 109, 155. 41 C. Wang, C. Liu, W. Wang and T. Shen, J. Photochem. Photobiol., A., 1997, 109, 159. 42 Z. X. Zhou, S. P. Qian, S. D. Yao and Z. Y. Zhang, Dyes Pigm., 2001, 51, 9. 43 J. He, F. Chen, J. Zhao and H. Hidaka, Colloids Surf., A, 1998, 142, 49. 44 G. Ramakrishna, H. N. Ghosh, A. K. Singh, D. K. Palit and J. P. Mittal, J. Phys. Chem. B, 2001, 105, 12786. 45 M. Jakob, H. Levanon and P. V. Kamat, Nano Lett., 2003, 3, 354.
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PAPER
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Ultraslow recombination in AOT-capped TiO2 nanoparticles sensitized by protoporphyrin IX Sudipta Biswas,a Swati De*a and Arunkumar Kathiravanb Aerosol OT (AOT) capped TiO2 nanoparticles have been prepared by a phase transfer mechanism. The TiO2 nanoparticles have a diameter of 5–10 nm, are highly crystalline (anatase) and show high photoluminescence. They are effectively sensitized by protoporphyrin IX (PPIX) and show high electron injection
Received 26th June 2014, Accepted 30th July 2014 DOI: 10.1039/c4dt01926a www.rsc.org/dalton
1.
rates while the rate of back recombination is much slower than those reported previously. Thus the AOT capped TiO2 nanoparticles synthesized in this work are highly effective not only in promoting ultrafast electron injection from PPIX to TiO2 but more importantly they lead to extremely slow back recombination rates. The significance of this work is in the synthesis of highly photoluminescent TiO2 nanoparticles which can be easily sensitized by a porphyrin dye, whereby ultraslow recombination is observed.
Introduction
Over the past few years sensitization of large band-gap semiconductor materials with organic dyes has been a field of extensive research because semiconductor materials such as TiO2 have been widely used as photocatalysts for solar energy conversion.1–3 In addition, one of the third-generation solar cells, i.e. the dye-sensitized solar cell (DSC) has received wide attention due to its low production cost and flexible production methods. Up to now, DSCs’ performance has reached power conversion efficiencies exceeding 12% by using ruthenium and porphyrin sensitizers.4–7 In DSC, the photoanodes play a vital role in the conversion of light into electrical energy. Dye sensitized TiO2 material has served as the frequently used material for DSC photoanodes. Effective electron transfer from the excited dye to the TiO2 conduction band requires good electronic coupling between the dye and TiO2. Therefore, sensitization of TiO2 semiconductor materials with organic dyes has been a subject of great interest. Numerous organic dyes such as coumarin, indoline, squaraine, xanthene, cyanine, hemicyanine, oligothiophene, perylene, carotenoid, porphyrin and phthalocyanine derivatives are capable of TiO2 sensitization. Among them, porphyrins are promising candidates for DSC, because of their strong Soret (400–450 nm) and moderate Q-band (550–600 nm) absorption properties as well as their natural role in photosynthesis. The steps involved in the photosensitization of TiO2 by dyes are shown in Scheme 1.
a
Department of Chemistry, University of Kalyani, Kalyani-741235Nadia, W.B., India. E-mail: [email protected]; Fax: +91-33-25828282; Tel: +91-33-25828750 b National Centre for Ultrafast Processes, University of Madras, Taramani Campus, Taramani, Chennai-600113, Tamil Nadu, India
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Scheme 1
Steps involved in the photosensitization of TiO2 by dyes.
Sensitization of colloidal TiO2 has been studied extensively in the past.8,9 Recently, Ramamurthy and co-workers performed spectroscopic and photovoltaic studies of protoporphyrin IX on a TiO2 electrode.10 In this work, we have used the protoporphyrin IX (PPIX) dye as a photosensitizer due to its significant visible light absorption and the presence of –COOH groups, which can serve as effective anchoring groups between the dye and the TiO2 surface [Scheme 2]. Another interesting part of the present work is the preparation of surface-modified TiO2 nanoparticles. Surface modification of semiconductor nanoparticles can change their optical, chemical and photocatalytic properties significantly.11 Surface modification of nanoparticles may enhance their excitonic and defect emission by blocking nonradiative electron/ hole (e−/h+) recombination at the defect sites (traps) on the surface of the semiconductor nanoparticles and may increase the photostability of semiconductor nanoparticles. Xagas et al.12 reported surface modification and photosensitization of TiO2 nanocrystalline films with ascorbic acid. Rabani et al.13
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Scheme 2
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Mode of attachment of PPIX on the TiO2 surface.
reported interfacial electron transfer dynamics in surfacemodified nanocrystallite layers using laser photolysis techniques. Qu et al.14 reported the enhancement of the photoinduced electron transfer from cationic dyes to colloidal TiO2 nanoparticles after modifying the surface with sodium dodecyl benzene sulfonate (DBS) molecules. Ghosh et al.15,16 studied interfacial electron transfer dynamics of alizarin-sensitized surface-modified and unmodified TiO2 nanoparticles using femtosecond transient absorption spectroscopy. The advantage with such surfactant-capped nanoparticles is that they can be readily dissolved in many nonaqueous solvents. It has been observed that electron injection dynamics does not change much with surface modification. However, they have noted a very interesting effect of surface modification on back electron transfer. They have observed that the back electron transfer dynamics is slow on the modified surface as compared to that on the unmodified TiO2 surface. In the present work we have synthesized AOT capped TiO2 nanoparticles through a phase transfer mechanism. The main motivation behind using AOT for preparing surface-modified TiO2 nanoparticles came from previous knowledge of the good stabilizing and charging properties of AOT.17–22 AOT has been commonly used for charging colloidal particles in applications such as motor oil,23 inkjet printing,24 liquid electrostatic developers25 and electrophoretic ink.26 For AOT solutions in nonpolar solvents, it has been shown that a small fraction of the
Scheme 3
reverse micelles is charged as a result of a disproportionation/ comproportionation mechanism, in which two neutral reverse micelles exchange a charge producing one negatively and one positively charged reverse micelle and vice versa [Scheme 3].19–21,24 Evidence for this mechanism comes from electrokinetic and adsorption measurements as well as surface force measurements.17,18 These charged reverse micelles increase the conductivity of a nonpolar solution and can induce an important electric field when they are separated. In applications such as electrophoretic inks,26 where movement of charged pigments is important, these charged micelles play an important role. This was our main driving force for the work. In DSC applications too, movement of charge plays an important role. If the presence of AOT capping on the TiO2 surface can enhance electron transfer from the dye to TiO2 and/or retard back recombination, then a distinct advantage will arise for DSC based applications.
2. Material and methods 2.1.
Materials
Titanium(IV) isopropoxide was purchased from Aldrich. The surfactant used was aerosol OT [AOT, sodium bis(2-ethylhexyl) sulfosuccinate] from SRL. Dimethylformamide (DMF) was purchased from Merck-India and n-heptane was purchased from Spectrochem. Protoporphyrin IX (PPIX) was obtained from Sigma and was used without further purification. PPIX solutions in DMF were used for the spectral studies. 2.2.
Synthetic procedure
A solution of Ti(IV) isopropoxide dissolved in isopropyl alcohol was added dropwise to distilled water at pH 1.5. The solution was continuously stirred for 10–12 hours until a transparent colloid formed. Then, AOT [sodium bis(2-ethylhexyl) sulfosuccinate] was added to n-heptane to make a 1 × 10−3 M solution. To the freshly prepared TiO2 colloids in water, the stock solution of AOT in n-heptane was added and the resulting mixture was stirred slowly for 3 hours. As the surface of the TiO2 nanoparticles is positively charged, AOT molecules can easily bind through the sulfonic group (SO3−) with the nanoparticles. The newly capped TiO2 nanoparticles look like reverse micelles and
Disproportionation and vice versa of AOT reverse micelles in nonpolar solvents.
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can be dissolved easily in organic solvents. Under this situation, TiO2 nanoparticles are extracted from the water to the organic phase. Now with the help of a separating funnel the organic phase was separated out. This phase was dried using CaCl2 and it finally got transformed into an optically clear solution. The organic phase was then refluxed for 2 h and the solvent was removed with the help of a rotary evaporator under a N2 atmosphere. Then the solid AOT-capped TiO2 nanoparticles were dried in an oven at 80 °C for 6 hours. 2.3.
Characterization techniques
The absorption spectra were recorded with a UV 2450-PC spectrophotometer (Kyoto, Japan). Transmission electron microscopic (TEM) studies of the nanoparticles were carried out at a resolution of 1.9 Angstrom with a JEOL, JEM-2100 Electron Microscope from Japan. TEM specimens were prepared by placing micro drops of solution on a carbon film supported by a copper grid. Scanning electron microscopy (SEM) studies were performed with a field emission SEM (JSM-6700F, Jeol, Japan) to observethe surface topology. Powder X-ray diffraction (XRD) was recorded using a Bruker D8 advanced powder X-ray diffractometer using CuKα at 1.5418 Å. Fluorescence spectra were recorded with a Perkin Elmer LS 55 spectrofluorimeter. For recording the emission spectra of protoporphyrin IX (PPIX), the excitation wavelength (λex) was fixed at 400 nm. For fluorescence quenching studies, the final dye concentration was 1 × 10−6 M. 2.4. Set-up for time-resolved fluorescence and transient absorption Flash photolysis was carried out using a Nd:YAG laser source producing nanosecond pulses (8 ns) of 355 nm light with the energy of the laser pulse being around 200 mJ. Dichroic mirrors were used to separate the third harmonic from the second harmonic and the fundamental output of the Nd-YAG laser. The monitoring source was a 150 W pulsed xenon lamp, which was focused on the sample at 90° to the incident laser beam. The beam emerging through the sample was focused onto a Czerny-Turner monochromator using a pair of lenses.
Fig. 1
Paper
Detection was carried out using a Hamamatsu R-928 photomultiplier tube. Transient signals were captured with an Agilent infinium digital storage oscilloscope and the data were transferred to a computer for further analysis. For laser flash photolysis studies, samples were purged with argon gas for 45 min prior to laser irradiation. Time-resolved fluorescence decays were obtained by the time correlated single-photon counting (TCSPC) technique. The samples were excited at 405 nm. Data analysis was carried out using the software provided by IBH (DAS-6), which is based on deconvolution techniques using the nonlinear least-squares method and the quality of the fit was ascertained with the value of χ2 < 1.2.
3. Results and discussion 3.1.
Nanoparticle characteristics
The absorption spectrum of the as-synthesized AOT-capped TiO2 nanoparticles is illustrated in Fig. 1(a). Compared with bulk TiO2,27 a shift of the absorption spectrum of nano-TiO2 towards the lower wavelength region was observed. The AOTcapped TiO2 nanoparticles show a much more blue-shifted λonset (358 nm) than bulk TiO2 (385 nm). We believe that this blue shifted λonset of the AOT-capped TiO2 nanoparticles relative to bulk TiO2 is due to excess negative charge provided by the sulphonate group of AOT onto the surface of the nanoparticles. These kinds of spectral shifts have been explained very nicely by earlier workers28 on the basis of an electrostatic model. They have proposed that injection of excess negative charge onto the nanoparticles creates a polarizing field which leads to an increase in energy and hence a blue shift of λonset. The band gap (Eg) of the as-synthesized TiO2 samples was determined using the Kubelka–Munk plot.29 The absorption edge of as-synthesized TiO2 was at 358 nm, corresponding to a band gap of 3.73 eV [Fig. 1(b)]. This corresponds to the energy of charge-transfer from the valence band (mainly formed by 2p orbitals of the oxide anions) to the conduction band (mainly formed by 3dt2g orbitals of the Ti4+ cations). The absorption edge at 385 nm of bulk anatase TiO2 corresponds
(a) Absorption-onset for the TiO2 nanoparticles. (b) Plot for determining the band gap.
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to a band gap of 3.2 eV. Thus the band gap energy of the assynthesized TiO2 is higher than that of bulk TiO2. This happens due to size quantization effects.30 The radius of the synthesized TiO2 nanoparticles was determined using the Brus equation, i.e. eqn (1):31
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ΔEg ¼
h2 1:8e2 þ polarization terms 2 8μR εR
ð1Þ
where h is Planck’s constant, R is the radius of the particle and ΔEg is the band gap shift. μ is the reduced mass of the exciton defined as (1/μ = 1/m*e + 1/m*h) where m*e and m*h are the reduced effective mass of the electron and the hole respectively, e is the electron charge and ε is the dielectric constant of the semiconductor. The value of μ is taken as μ = 1.63me (me is the electron rest mass).32 Since the optical dielectric constant of bulk titanium dioxide is very large (ε = 170), the polarization terms in eqn (1) are neglected. The band gap shift for colloidal TiO2 is 0.53 as compared to bulk anatase TiO2. Thus the radius (R) of the TiO2 nanoparticles determined using eqn (1) is 11.1 nm. The photoluminescence (PL) spectra of the AOT-capped TiO2 nanoparticles were measured for 364 nm excitation. Broad PL spectra in the range of 450–500 nm were obtained for the as-synthesized particles [Fig. 2]. This is assigned to the band-edge luminescence of TiO2 nanoparticles.33 The PL band of TiO2 nanoparticles comprises both direct and indirect transitions. The peaks in the PL spectra conform to spectra reported in the literature and also to the assignments based on calculations by Daude et al.33 TiO2 nanoparticles exhibit a shoulder in the PL signal at about 431 nm, possibly resulting from band-edge excitons, and this surface emission is attributed to an indirect transition X1a→Γ1b and is linked to exciton recombination in shallow trapped surface states.32,33 The occurrence of emission peaks in the visible region is mainly due to the presence of defect levels such as oxygen vacancies in the band gap. Serpone et al.30 reported the PL bands of anatase TiO2 nanocrystals in the long wavelength range
Fig. 2 Photoluminescence spectra of the TiO2 nanoparticles for 364 nm excitation.
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(465 nm) and attributed them to the oxygen vacancies. There are oxygen vacancies on the surface of TiO2 nanoparticles and the size of particles is small so that the average distance that the electrons can move freely is short. These factors make the oxygen vacancies bind electrons easily and thus form excitons. In general, the smaller the nanoparticle size, the larger is the oxygen vacancy content, thus the higher the probability of existence of excitons and hence the stronger is the PL signal.34 In order to determine the size and to study the crystalline nature of the AOT-capped TiO2 nanoparticles, powder XRD analysis was performed. From Fig. 3 it can be clearly observed that the diffraction peaks that appear in the powder XRD pattern of the AOT-capped TiO2 nanoparticles correspond to the anatase crystalline phase. The data are consistent with the standard powder diffraction pattern of anatase (as given by the Joint Committee on Powder Diffraction Standards, JCPDS). The XRD pattern exhibits prominent peaks at 2θ values of 25.23°, 37.66°, 47.96°, 53.86°, 62.69°, 68.75°, 75.09° and 82.15° corresponding to the [101], [004], [200], [105], [204], [116], [215] and [303] planes of anatase TiO2. The average diameter of the nanoparticulate samples determined using Scherrer’s equation i.e. eqn (2) is 9.42 nm: d ¼ 0:9λ=β cos θ
ð2Þ
where λ = wavelength of X-ray radiation [1.5418 Å], β = full width at half maximum (FWHM) of the XRD peak expressed in radians, and θ = angle of diffraction [2θ = 25.23°].
Fig. 3 Powder XRD pattern of (a) anatase TiO2 (reference) and (b) surfactant-modified TiO2 nanoparticles.
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Fig. 4
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(a) TEM image of the TiO2 nanoparticles; the inset shows the histogram. (b) SAED pattern.
TEM analysis was carried out to determine the actual size of the AOT-capped TiO2 particles, their growth pattern and distribution. It is evident from the histogram of the TEM image [Fig. 4(a)] that the average size of a majority of the particles is ∼5 nm. However, some secondary particles of size 10–12 nm are also observed from the micrograph. Thus, within experimental error limits, the particle sizes obtained from three different sets of experiments, i.e. absorption spectra, powder XRD and TEM, are almost similar. The selected area electron diffraction (SAED) pattern [Fig. 4(b)] shows distinct diffraction rings with characteristic spots corresponding to the planes of anatase TiO2. The SAED pattern indicates that the particles are highly crystalline. From the d-spacing, the rings are assigned to the [101], [004], [200], [105], [116] and [303] planes of anatase TiO2. Field emission scanning electron microscopy (FESEM) was used for a morphological study of the TiO2 nanoparticles. Fig. 5 shows the FESEM image of the as-prepared AOT-capped TiO2 nanoparticles. The inset shows the surface topology of the same at lower magnification. From the SEM image, it can be seen that the nanoparticles have a large surface area and Fig. 6
EDX pattern of the TiO2 nanoparticles.
form a porous network. A closer examination of these figures reveals a well-defined particle-like crystalline morphology. Energy dispersive X-ray spectroscopy (EDX) was employed to ascertain the composition of the AOT-capped nanoparticles [Fig. 6]. The EDX analysis indicates the presence of Ti and O proving that the as-synthesized nanomaterials are composed of TiO2 only. 3.2.
Fig. 5 FESEM image of the TiO2 nanoparticles; the inset shows the image at a lower resolution.
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Dye-sensitization studies
3.2.1. Absorption characteristics. The absorption spectrum of the PPIX solution in the UV-visible region shows an intense Soret band absorption at 405 nm (S0→S2 transition) [Fig. 7(a)] together with four weaker Q-bands at 506, 542, 577 and 630 nm (S0→S1 transition).10,35 In the presence of colloidal TiO2 the optical density of PPIX at 405 nm decreases. This may be due to adsorption of PPIX on the surface of surfactant-modified
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Fig. 7 (a) Emission spectra of PPIX (1 × 10−6 M) in the absence and presence of as-synthesized TiO2 nanoparticles: (i) 0 M, (ii) 5 × 10−5 M, (iii) 1 × 10−4 M, (iv) 5 × 10−4 M and (v) 1 × 10−3 M. (b) Plot to determine the apparent association constant (Kapp).
TiO2 nanoparticles through its anchoring group (–COOH) (Scheme 1). A similar type of adsorption of sensitizers on the surface of TiO2 nanoparticles was reported earlier.8,9 3.2.2. Fluorescence quenching characteristics. The fluorescence emission spectra of PPIX (for λex = 532 nm) in the absence and presence of increasing concentrations of AOTcapped TiO2 nanoparticles are shown in Fig. 7(a). Addition of the capped TiO2 nanoparticles to the solution of PPIX results in the quenching of its fluorescence emission. This quenching behaviour is similar to the previously reported fluorescence quenching of chlorophyllin,36 7-methoxynaphtho[1,2-b]thiophene-2-carboxylic acid,37 anthracene-9-carboxylic acid38 and erythrosine B39 by colloidal TiO2 nanoparticles. (This quenching is assigned to the electron injection from the excited state sensitizer molecules to the conduction band of colloidal TiO2 nanoparticles.) From the fluorescence quenching data, we can also calculate the Kapp using the following eqn (3): 1 1 1 ¼ þ F 0 F F 0 F′ K app ðF 0 F′Þ½TiO2
ð3Þ
where Kapp = apparent association constant, F0 = initial fluorescence intensity of PPIX, F′ = fluorescence intensity in the presence of adsorbed TiO2, and F = observed fluorescence intensity at its maximum. The plot of 1/(F0 − F) versus 1/[TiO2] is shown in Fig. 7(b). A linear plot was obtained. From the slope, the Kapp has been determined to be 3.3 × 103 M−1. The decrease in fluorescence emission may be attributed to electron transfer or energy transfer processes between the dye and the TiO2 nanoparticles. The band gap of TiO2 (3.73 eV) is greater than the excited state energy of PPIX (2.04 eV) and there is no overlap between the fluorescence emission spectrum of PPIX and the absorption spectrum of AOT-capped TiO2 nanoparticles. This excludes the possibility of energy transfer from PPIX to TiO2 nanoparticles. Thus the fluorescence
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quenching seen in Fig. 7(a) is due to electron transfer from the dye to the TiO2 nanoparticles. 3.2.3. Time resolved fluorescence characteristics. The fluorescence quenching data indicate that electron injection occurs from excited PPIX to the AOT-capped TiO2 nanoparticles. To get more insights into the electron injection process, time-resolved fluorescence experiments were performed. It was shown earlier that the sensitizer molecules adsorbed on the TiO2 surface have significantly shorter lifetimes than those in a homogeneous solution and this decrease in lifetime occurs due to the electron transfer process.8,9 In the absence of AOT-capped TiO2 nanoparticles, the fluorescence decay of PPIX can be fitted to a single exponential expressed as [F(t ) = A exp(−t/τ)] with a lifetime of 14 ns [Fig. 8(i)].
Fig. 8 Time resolved fluorescence decay of (i) PPIX and in the presence of surfactant-modified TiO2 nanoparticles: (ii) PPIX + 1 × 10−4 M TiO2 and (iii) PPIX + 1 × 10−3 M TiO2.
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Table 1 Fluorescence lifetimes and ket values for PPIX in the absence and presence of TiO2
Samples
τ1 (ns)
a1 (%)
τ2 (ns)
a2 (%)
ket × 109 (s−1)
PPIX PPIX–TiO2 (1 × 10−4 M) PPIX–TiO2 (1 × 10−3 M)
3.2 2.9
5 14
14 12 10
95 86
— 0.22 0.24
However, in the presence of surfactant-modified TiO2 nanoparticles, the fluorescence decay of PPIX [Fig. 8(ii) and (iii)] can be fitted to a bi-exponential [F(t ) = A1 exp(−t/τ1) + A2 exp(−t/τ2)], with short-lived and long-lived components [Table 1]. The longer component is assigned to free PPIX in solution while the shorter component is assigned to PPIX adsorbed onto TiO2. The contribution of the short component increases with the concentration of the TiO2 nanoparticles [Table 1]. The rate of electron transfer (ket) from the excited state PPIX to the conduction band of TiO2 can be calculated using eqn (4): ket ¼ 1=τads 1=τ
ð4Þ
where τ and τads are the lifetimes of PPIX in the absence and presence of surfactant-modified TiO2 nanoparticles respectively. The calculated ket values are shown in Table 1. As the ket values increase with an increase in TiO2 concentration the assumption of electron transfer from excited PPIX to TiO2 nanoparticles is valid. Table 2 shows the ket values and recombination rates (krec) obtained by other researchers who have used other dyes and bare TiO2.38,40–43 A comparison of the ket values obtained by us (Table 1) and those existing in the literature38,40–43 (Table 2) shows that our ket values are almost comparable (a little lower) to those reported for other dyes. The efficiency of electron injection from the excited state of PPIX to the conduction band of TiO2 is further verified by eqn (5): Es*=sþ ¼ Es=sþ Eð0;0Þ
ð5Þ
Es/s+ is the oxidation potential of PPIX and its value is 1.01 V vs. SCE, E(0,0) was calculated from the intersection of the PPIX absorption spectrum with the fluorescence emission spectrum in DMF at a normalized absorption/emission intensity and its value is 2.04 eV. Thus the excited state oxidation potential
Table 2
ket values and krec values reported in the literature
Samples
ket (s−1)
krec (s−1)
Anthracene-9-carboxylic acid/TiO2 Cyanine/TiO2 Squaraine/TiO2 Hypocrellin B/TiO2 Fluorescein/TiO2 PPIX–TiO2
4.8 × 108 a 4.0 × 109 b 1.9 × 108 c 4.9 × 109 d 1.7 × 108 e 2.4 × 108 f
5.5 × 107 a — 1.0 × 105 c 1.2 × 105 d — 8.3 × 103 f
The data have been taken from the following references: a Ref. 38. b Ref. 40. c Ref. 41. d Ref. 42. e Ref. 43. f The present work using AOTcapped TiO2.
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Scheme 4 Energy diagram justifying the electron transfer from the excited dye to the conduction band of TiO2.
(Es*/s+) of PPIX was determined to be −1.03 V vs. SCE. The conduction band potential of TiO2 is at −0.52 V vs. SCE.44 This suggests that electron transfer from the excited state of PPIX to the conduction band of TiO2 is energetically favored (Scheme 4). The above data and calculations conclusively prove that the fluorescence quenching of PPIX in the presence of TiO2 nanoparticles is due to the electron injection from the excited state of the dye to the conduction band of the TiO2 nanoparticle. Further, the driving force for electron injection (ΔGinj ) from the excited PPIX to the conduction band of TiO2 was calculated using eqn (6): ΔGinj ¼ Es*=sþ eCB ðTiO2 Þ
ð6Þ
The calculated ΔGinj value (−0.51 V) is negative, indicating the favorable driving force for electron injection from the excited PPIX to TiO2. 3.2.4. Transient absorption (TA) characteristics. To further probe the involvement of AOT-capped TiO2 nanoparticles in the recombination process, we carried out transient absorption (TA) studies. TA spectroscopy has earlier been used to provide insight into the recombination dynamics, i.e. back electron transfer process in the dye–TiO2 interface. If the observed fluorescence quenching of PPIX is due to electron injection into the conduction band of TiO2, one would expect to detect the presence of a cation radical of PPIX. Based on the above strategy, we carried out the TA measurements of PPIX with AOTcapped TiO2 nanoparticles. The TA spectrum of PPIX was recorded for excitation at 355 nm in the wavelength range of 400–550 nm [shown in Fig. 9(a)]. The absorption maximum at 440 nm with a lifetime of 8 μs can be ascribed to the triplet–triplet absorption of PPIX. The bleaching in the 400 nm region indicates the depletion of PPIX absorption. The transient absorption spectra of PPIX in the presence of AOT-capped TiO2 nanoparticles in DMF solution
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Fig. 9 (a) TA spectra of PPIX in DMF after excitation at 355 nm at different times. (b) TA decay monitored at 440 nm. (c) TA spectra of PPIX with TiO2 nanoparticles. (d) TA decay of PPIX in the presence of TiO2 nanoparticles monitored at 450 nm.
are shown in Fig. 9(c). Interestingly, in the presence of TiO2 nanoparticles, no new transient species was observed. However, TA gets broadened and shifts by around 10 nm. The TA spectra in the absence and presence of TiO2 nanoparticles are shown in Fig. 10. We assign this spectrum to the cation radical of PPIX produced due to electron transfer from PPIX to the TiO2 nanoparticles. Moreover, the measured decay at the peak (i.e. 450 nm) gives a lifetime of 120 μs [Fig. 9(d)] for PPIX in the presence of TiO2 nanoparticles. The rate constant for
Fig. 10
TA spectra of PPIX in DMF and on the TiO2 surface after 1 µs.
15072 | Dalton Trans., 2014, 43, 15065–15074
electron recombination has been determined [krec = 1/τ = 1/120 μs] and was found to be 8.3 × 103 s−1. The krec value obtained by us is orders of magnitude lower than those previously reported for other dye–TiO2 systems (Table 2). This is a strong indication of the effectiveness of the AOT-capped TiO2 nanoparticles in retarding back recombination rates. As we know, reverse electron transfer has been found to be an important factor controlling the net charge-transfer efficiency in the photosensitization process. The much lower recombination rate compared to ket (shown in Table 1) is an indication of the weak interaction between the cation radical of the PPIX and AOT-capped TiO2. The rate of electron–cation recombination is four orders of magnitude lower than electron injection, which clearly reveals that electron injection is the dominant process in this system. Thus the interaction between PPIX and the AOT-capped TiO2 nanoparticles synthesized here offers a twofold advantage for future application in DSCs. First, electron transfer is very fast due to a favourable driving force. Second, back recombination is very slow. Now, the greatly repressed recombination for AOT-capped TiO2 nanoparticles arises due to two effects. First, once the electron is injected into the conduction band of AOT-capped TiO2 nanoparticle the enhanced charge mobility of the AOT reverse micelle encapsulating TiO2 [Scheme 5] will prevent back recombination. Herein lies the great potential of using AOT-capped TiO2 nanoparticles for DSC applications. Secondly, as reported by earlier workers,15,16,38,39,45 the Fermi
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Acknowledgements
Scheme 5 micelles.
TiO2
nanoparticles
encapsulated
within
AOT
reverse
S. De acknowledges the Department of Science and Technology (DST), New Delhi for the generous grant of SERC Fast Track Scheme no. SR/FT/CS-057/2008. A.K. thanks DST, India for the DST-Fast Track Project (ref. no. CS-316/2012, Dt. 04/12/2012). We acknowledge the Central Research Facility, Indian Institute of Technology Kharagpur, India for the TEM studies. We thank the Indian Association for the Cultivation of Science, Kolkata for providing the FESEM facility. The FIST grant to the Department of Chemistry, University of Kalyani from the Department of Science and Technology, India is acknowledged. Dr A. Gayen, Department of Chemistry, Jadavpur University is acknowledged for help with the powder XRD measurements. University of Kalyani is acknowledged for providing the basic infrastructure for research.
References level in surface-modified TiO2 colloids is pushed up in energy and as a result the overall free energy (ΔG0) of the back recombination process increases. Back recombination in dye sensitized TiO2 nanoparticle surfaces falls in the Marcus inverted regime. In this regime, the rate of back electron transfer (BET) decreases with increasing ΔG0. Thus, for AOT-capped TiO2 nanoparticles too, ΔG0 for back recombination increases and so the rate decreases.
4.
Conclusions
AOT-capped TiO2 nanoparticles have been synthesized based on a phase transfer mechanism. The TiO2 nanoparticles are relatively small (5–10 nm) and crystalline, the predominant crystalline phase being anatase. The nanoparticles are highly photoluminescent. They can bind effectively to PPIX. Calculations of the electron injection rate (ket) from time-resolved fluorescence studies indicate very efficient electron injection (ket = 2.4 × 108 s−1) from the PPIX excited state to the CB of TiO2. On the other hand, back recombination of the injected electron with the PPIX cation radical is four orders of magnitude slower than the rate of electron injection (krec = 8.3 × 103 s−1). More importantly, krec for the AOT-capped TiO2 nanoparticles (krec = 8.3 × 103 s−1) is orders of magnitude slower than those reported by earlier workers (krec ∼ 105–107 s−1). This is due to the additional influence of AOT which is known to enhance charge mobility of colloids and thus helps in moving the injected electron far away from the hole. Thus the conditions are perfect for the use of the PPIX–TiO2 system as the photoactive medium in DSCs. The high photoluminescence of the particles can also be utilized effectively to prepare devices. The ease of photosensitization and high photoluminescence of AOT-capped TiO2 nanoparticles can be effectively used in future for preparing DSCs.
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