4 Photoinduced electron transfer reactions of meso...

4 Photoinduced electron transfer reactions of meso-tetrakis (4-sulfonatophenyl)porphyrin with colloidal TiO2 and metal-TiO2 nanoparticles

4.1. Introduction

TiO2 is a most useful semiconductor, but its wide band gap (3.2 eV) limits

its use in visible region as a sensitiser [1]. The main drawbacks are low quantum

yield and the lack of visible-light utilization which hinders its practical

applications [2]. To overcome these problems, numerous studies have been

performed to enhance the visible-light utilization of TiO2 which includes

metallization [3-7], sensitization [8,9] etc. The sensitization of TiO2 using visible

light absorbing organic dyes has been the topic of interest for the past few years

[10-16]. Recently we have reported the fluorescence quenching of riboflavin,

xanthene dyes and porphyrin by colloidal TiO2 through sensitization involving

electron transfer process [17-19]. The electron-transfer process at the

semiconductor-dye interface has been successfully utilized in the development of

solar cells, electronic devices, heterogeneous photocatalysis, and wastewater

treatments [20,21]. The efficiency of these devices depends on the properties of

the sensitizers, semiconductor and their interaction under photoexcitation. A

higher rate of electron injection from the excited dye into semiconductor and a

slower recombination rate are important in designing efficient solar cells.

Another possible method for improving the efficiency of electron-transfer

dynamics is through the modification of semiconductor nanoparticles with a noble

metal deposit. Semiconductor-metal nanoparticles have been widely used in

photocatalysis [22-24]. The deposition of metal on semiconductor particles

enhances the efficiency of photocatalytic redox processes [25,26]. The composite

films based on TiO2 and metal particles have shown higher photocurrent and

photovoltage because of improved charge separation [27-29]. Among such

Photoinduced electron … 74

nanocomposite structures, AgTiO2 and AuTiO2 have more attention because they

are extremely attractive noble metals to be investigated at the nanoscale due to

their remarkable catalytic activity [30-32], size and shape-dependent optical

properties [33] and their promising applications in chemical and biological

sensing [34]. AuTiO2 and AgTiO2 nanoparticles exhibit enhanced charge-transfer

efficiency by shifting the Fermi level of the composite to more negative potentials

[35-37]. Recently, Sudeep and co-workers [38] reported the electron injection

from excited state tricarbocyanine dye into TiO2 and AgTiO2 core-shell

nanoparticles and further they confirmed the suppression of back electron transfer

by means of the electrons injected into the TiO2 shell are quickly transferred to

metal core.

Based on the above backround, we have chosen meso-tetrakis

(4-sulfonatophenyl)porphyrin [scheme 1] as a sensitizer to study the interaction

with metal-semiconductor nanoparticles. Porphyrins find applications in many

fields such as photodynamic therapy for the treatment of cancer [39] and

photovoltaic conversion of solar energy due to their strong absorption in the

visible region (400-450 nm).

N

NH N

HN

SO3

SO3

O3S

SO3 Scheme 1: meso-tetrakis(4-sulfonatophenyl)porphyrin [TSPP]


Our interest is to investigate the process of electron transfer from excited

state TSPP to the conduction band of TiO2 by absorption, fluorescence and time

resolved spectroscopic measurements. From such studies, we can understand the

feasibility of flow of electrons from conduction band of TiO2 into the metal core

based on energetic calculations. On demand, electrons stored in the metal core

can be readily discharged or scavenged by electron acceptors as illustrated in

Scheme 2. Such core-shell structures provide an interesting strategy to improve

the charge separation in a dye sensitized semiconductor system. Since the

electrons injected into the TiO2 shell are quickly transferred to the metal core, it

should be possible to suppress the back electron transfer (Scheme 2). Further, the

mechanism of electron transfer process on the basis of energy level diagram has

also been proposed in this chapter.

TiO2

VB

CB

e

TSPP*

νhe

e_

acceptorsAu/Ag

Scheme 2: Photoinduced charge injection and charge separation

4.2. Experimental section 4.2.1. Materials Meso-tetrakis (4-sulfonatophenyl) porphyrin and titanium (IV) 2-propoxide

were purchased from Aldrich and used as such. Chloro auric acid (HAuCl4) and

sodium boro hydride (NaBH4) were purchased from LOBA chemicals, India.

Unless otherwise specified, all reagents used were of analytical grade and the

solutions were prepared using double distilled water.


4.2.2. Methods 4.2.2.1. Preparation of colloidal TiO2 nanoparticles

The colloidal TiO2 nanoparticles was prepared by the hydrolysis of

titanium(1V) 2-propoxide [40]. Typically, titanium(IV) 2-propoxide in 2-propanol

(l0%, 0.5 ml) was injected by syringe into 40 ml of water (pH 1.5, using nitric

acid) and kept stirred for 8 hours under N2 atmosphere. There is no stabilizing

agents were used. The colloidal TiO2 prepared by this method were stable for 3-5

days. The stock solution was diluted with water to obtain the desired

concentration of TiO2. No attempts were made to exclude the traces of 2-propanol

(~0.4%) present in the colloidal TiO2 and it was confirmed separately that the

presence of 2-propanol did not affect the photochemical measurements.

4.2.2.2. Preparation of colloidal AuTiO2 nanoparticles Colloidal AuTiO2 nanoparticles in water were prepared [Scheme 3] by

electrostatic adsorption of AuCl4− ions on TiO2 surface followed by its reduction

with NaBH4 [41]. AuTiO2 nanoparticles were prepared by adding 50 ml of 1mM

HAuCl4 solution to the 50 ml of 1mM colloidal TiO2 (acidic solution) solution

while stirring vigorously. The negatively charged AuCl4− ions adsorbed on

positively charged surface of TiO2 nanoparticles. The solution was stirred for an

additional 5 minutes to allow the complete adsorption of AuCl4− ions on the TiO2

surface. Reduction of the AuCl4− was achieved by the dropwise addition of

NaBH4 (1mM) until the solution changed into wine red colour.

++ + +++++++ + ++

TiO2 + [AuCl4]

+ ++

+ + +TiO2

++

++

+

[AuCl4]

NaBH4 TiO2

Au

_

_

+

Scheme 3: Preparation of colloidal AuTiO2 nanoparticles


4.2.2.3. Preparation of colloidal AgTiO2 nanoparticles The method of preparation of colloidal AgTiO2 nanoparticles in water was

similar to the one employed earlier [41]. AgTiO2 nanoparticles were prepared by

adding 50 ml of 1mM AgNO3 solution to the colloidal TiO2 (alkaline solution) in

water while stirring vigorously. The positively charged Ag+ ions adsorb on the

negatively charged surface of TiO2 nanoparticles. The solution was stirred for an

additional 5 minutes to allow complete adsorption of Ag+ ions on the TiO2

surface. Reduction of the Ag+ was achieved by the dropwise addition of NaBH4

(1mM) until a brownish colour was observed. The formation of colloidal AgTiO2

nanoparticles is illustrated in Scheme 4.

- - - - ---- - - - - -

TiO2 + [Ag]+

- --

- - -TiO2

--

--

-

[Ag]+

NaBH4 TiO2

Ag

-

Scheme 4: Preparation of colloidal AgTiO2 nanoparticles

4.2.2.4. XRD and TEM measurements

X-ray powder diffraction patterns were recorded on a Philips PW1710

model using CuKα radiation (λ= 0.154 nm) and a graphite monochromator in the

diffracted beam. TiO2 sample was in the form of powder. A scan rate of

0.05° min−1 was applied to record a pattern in the 2θ range of 20-80°.

Surface morphology, particle size, and various contours of the prepared TiO2,

AuTiO2 and AgTiO2 nanoparticles were analyzed by Transmission electron

microscopy (recorded using TECNAI G2 model).


4.3. Results and Discussion 4.3.1. Characterization of AuTiO2 nanoparticles In order to investigate the changes in the crystal structure due to gold

doping, X-ray diffraction measurements were taken in the range, 2θ = 10 - 80° for

the prepared AuTiO2 nanoparticles. Figure 1 shows the typical XRD patterns of

AuTiO2 nanoparticles. The 2θ values at which major peaks appear have been

found to be almost the same for AuTiO2 sample when compared to pure TiO2,

except the intensities of the peaks. This may be due to the fact that doping only

alters the crystallinity but not the crystal structure of AuTiO2 nanoparticles.

Diffractions that are attributable to anatase phase of TiO2 are clearly detectable at

2θ = 25o. A peak at 2θ = 44o in AuTiO2 can be assigned to (2 0 0) plane of gold

which proves that TiO2 surfaces are covered with gold particles.

Figure 1: XRD pattern of AuTiO2 nanoparticles


TEM image [Figure 2] permits easy differentiation of Au nanoparticles

(small bright areas) and TiO2 crystallites (large dark areas). That is, Au

nanoparticles are seen on the surface of the TiO2 particle as minute white dots. Also,

there is a possibility for gold to be incorporated into the interstitial positions of the

semiconductor particles. It is also clear that the crystallinity has been increased

which is indicated by the increased particle size with well developed faces. This

observation is also supported by the XRD pattern [Figure 1] where more intense

and sharp peaks are observed for AuTiO2 nanoparticles. Further, it is observed that

the average size of gold in AuTiO2 particles is in the range of 10-15 nm.

Figure 2: TEM image of AuTiO2 nanoparticles

4.3.2. Characterization of AgTiO2 nanoparticles To investigate the changes in the crystal structure affected by silver doping,

X-ray diffraction measurements have been taken in the range, 2θ = 10 - 80° for the

prepared AgTiO2 nanoparticles. Figure 3 shows XRD pattern of AgTiO2


nanoparticles. The 2θ values at which major peaks appear have been found to be

almost the same for AgTiO2 sample when compared to pure TiO2 except the

intensities of the peaks. This may be due to the fact that doping only alters the

crystallinity but not the crystal structure of AgTiO2. Diffractions that are

attributable to anatase phase of TiO2 particles (1 0 1) are clearly detectable at

2θ= 25o. A peak at 2θ = 38.4o, 44.5o and 64.6o in AgTiO2 can be assigned to

(1 1 1), (2 0 0) and (2 2 0) plane of silver which proves that TiO2 surfaces are

covered with silver particles.

Figure 3: XRD patterns of AgTiO2 nanoparticles

Transmission electron microscope picture was taken for AgTiO2

nanoparticles [Figure 4] to have an idea about the particle size and surface

modifications effected during doping. The TEM permits easy differentiation of Ag

nanoparticles (small dark areas) and TiO2 crystallites (large bright areas). That is,

Ag nanoparticles are seen on the surface of the TiO2 as dark dots. Also, there is a

possibility of silver to be incorporated into the interstitial positions of the


semiconductor particles. Further, it is observed that the average size of silver in

AgTiO2 particles is in the range of 10-20 nm. It is also clear that the crystallinity

has been increased which is indicated by the increased particle size with well

developed faces. This observation is also supported by XRD pattern [Figure 3]

where more intense and sharp peaks are observed for AgTiO2 nanoparticles.

Figure 4: TEM image of AgTiO2 nanoparticles

4.3.3. Characterization of TiO2 nanoparticles Figure 5 shows the XRD pattern of typical TiO2 nanoparticle obtained

from colloidal TiO2 by rotary evaporation. The XRD peaks are found to be broad

indicating fine size of the sample grains. The XRD pattern exhibits prominent

peaks at 2θ values of 25.6°, 38.4°, 48.4°, 54.8° and 63.2° which are similar to the

reported [42] values for anatase TiO2.

Transmission electron microscope picture was taken for the prepared TiO2

nanoparticles [Figure 6]. It is observed that the average size of TiO2 particles is in

the range of 10-20 nm.


Figure 5: XRD patterns of TiO2 nanoparticles

Figure 6: TEM image of TiO2 nanoparticles


4.3.4. Absorption characteristics of TSPP with TiO2 and metal TiO2 To study the dye-sensitized electron transfer reactions between sensitizer

and semiconductor nanoparticles in the excited state, it is very important to know

the type of ground state interaction of dye (sensitizer) molecules when they are

adsorbed on the nanoparticle surface. Figures 7a, 7b and 7c shows the absorption

spectra of TSPP in water and the effect of increasing concentration of colloidal

AuTiO2, AgTiO2 and TiO2 nanoparticles respectively. In figure 7a, it has been

observed that with increasing AuTiO2 concentration, the TSPP absorption around

432 nm decreases, accompanied with an increase in the absorption band

maximum around 413 nm. These changes are related to the complex formation

between TSPP and colloidal AuTiO2 nanoparticles. Similar type of spectral

behaviour has been observed for TSPP−AgTiO2 and TSPP−TiO2 systems

[Figures 7b & 7c]. The appearance of an isobestic point at 420 nm indicates that

TSPP existed in two states such as unadsorbed TSPP and adsorbed

TSPP…..M-TiO2. Similar type of isobestic point was reported by J. He et al [43].

(Note: Isobestic point is a specific wavelength at which two chemical species have

the same molar absorptivity)

In order to determine the complex formation between anchoring group of

TSPP with semiconductor nanoparticles, we have done similar experiments with

the model compound, meso-tetrakis(4-phenyl)porphyrin (TPP) which doesn’t

have any anchoring group in it’s structure. In this TPP−TiO2 system upon

increasing the concentration of colloidal TiO2 nanoparticles there is no change in

the absorption spectrum of TPP was observed as shown in figure 7d, indicating

that the interaction between TPP and the surface of TiO2 nanoparticles was not

occurred [Scheme 5]. Therefore we conclude that, TSPP adsorbed on the surface

of TiO2 and metal-TiO2 nanoparticle through its anchoring group (−SO3−).


TiO2N

NH N

HN N

NH N

HNTiO2

Interaction due to presence of anchoring group

No interaction, due to absence of anchoring group

....... ..........

TSPP TPP

SO3

SO3

SO3

O3S

Scheme 5: The adsorption model of TSPP & TPP on TiO2 surface.

The change in absorption spectra indicates that TSPP molecules adsorbed

on the surface of semiconductor nanoparticle to form a complex of the type

TSPP...M-TiO2.

TSPP + M-TiO2 TSPP.....M-TiO2Kapp

→ (1)

[TSPP.....M-TiO2][M-TiO2][TSPP]Kapp=

The apparent association constant for the formation of this type of surface

complex (Kapp), can be estimated from the changes in absorption intensity of the

band at 413 nm by using Benesi–Hildebrand equation [44].

Aobs = (1-α)C0εTSPPl + αC0εcl → (2)

where Aobs is the observed absorbance of TSPP solution containing different

concentrations of colloidal M-TiO2 at 413 nm; α is the degree of association

between TSPP and M-TiO2; εTSPP and εc are the molar extinction coefficients at

the defined wavelength for TSPP and the formed complex, respectively, ‘C0’ is

the initial concentration of free TSPP. Equation (2) can be expressed by

equation (3), where A0 and Ac are the absorbance of TSPP and the complex at 413

nm, respectively, with the concentration of C0:


Aobs = (1−α)A0 + αAc → (3)

At relatively high M-TiO2 concentrations, α can be equated to

(Kapp[M-TiO2])/(1+Kapp[M-TiO2]). In this case, equation (3) can be expressed as

equation (4):

Aobs-A0 Ac-A0 Kapp(Aobs-A0)[M-TiO2]+=1 1 1

→ (4)

The inset of figures 7a, 7b & 7c shows the Benesi-Hildebrand plot and there is a

good linear dependence of 1/(Aobs-A0) on the reciprocal concentration of TiO2 and

metal-TiO2 nanoparticles. The values of Kapp determined from the plots were

shown in Table 1. Further, the reason for higher association constant value for

TSPP−AuTiO2 compared to AgTiO2 and TiO2 may be due to the larger surface

area of AuTiO2. When the surface area is decreased, the effective electron

accepting nature should be decreased [45].

0

0.2

0.4

0.6

0.8

1

390 415 440 465 490

Wavelength (nm)

Abs

orba

nce 0

10

20

30

0 0.2 0.4 0.6 0.8 1

1/[AuTiO2] x 10-4 M

1/A

-A0

Figure 7a: Absorption spectrum of TSPP in the presence of colloidal AuTiO2 nanoparticles in the concentration range of 0−5 x 10−4 M in water. The inset is the straight line dependence of 1/Aobs−A0 on the reciprocal concentration of AuTiO2.


0

0.2

0.4

0.6

0.8

1

1.2

390 415 440 465 490

Wavelength (nm)

Abs

orba

nce 0

48

1216

0 0.2 0.4 0.6 0.8 1

1/[AgTiO2] x 10-4 M

1/A

-A0

Figure 7b: Absorption spectrum of TSPP in the presence of colloidal AgTiO2 nanoparticles in the concentration range of 0−5 x 10−4 M in water. The inset is the straight line dependence of 1/Aobs−A0 on the reciprocal concentration of AgTiO2.

0

0.2

0.4

0.6

0.8

1

1.2

390 410 430 450 470 490

Wavelength (nm)

Abs

orba

nce

02468

0 0.2 0.4 0.6 0.8 1

1/[TiO2] x 10-4 M

1/A

-A0

Figure 7c: Absorption spectrum of TSPP in the presence of colloidal TiO2 nanoparticles in the concentration range of 0−5 x 10−4 M in water. The inset is the straight line dependence of 1/Aobs−A0 on the reciprocal concentration of TiO2.


0

0.2

0.4

0.6

0.8

1

390 410 430 450 470 490

Wavelength (nm)

Abs

orba

nce

Figure 7d: Absorption spectrum of TPP in the presence of colloidal TiO2 nanoparticles in the concentration range of 0−5 x 10−4 M in water.

4.3.5. Fluorescence quenching of TSPP by TiO2 and metal-TiO2

Figure 8a, 8b and 8c shows the fluorescence emission spectra of TSPP in

the absence and presence of various concentrations of colloidal AuTiO2, AgTiO2

and TiO2 nanoparticles respectively. In figure 8a, we observed that the emission

intensity of TSPP is decreased regularly with increasing concentration of colloidal

AuTiO2 nanoparticles, which indicates that quenching has been occurred. There is

no significant emission peak shift with the addition of AuTiO2 was observed.

Similar type of spectral behaviour has been noticed for TSPP−AgTiO2 and

TSPP−TiO2 systems [Figure 8b & 8c].


0

150

300

450

600

570 595 620 645 670 695 720 745 770

Wavelength (nm)

Inte

nsity

Figure 8a: Fluorescence quenching of TSPP in the presence of colloidal AuTiO2 nanoparticles in the concentration range of 0−5 x 10−4 M in water.

0

130

260

390

520

650

570 595 620 645 670 695 720

Wavelength (nm)

Inte

nsity

Figure 8b: Fluorescence quenching of TSPP in the presence of colloidal AgTiO2 nanoparticles in the concentration range of 0−5 x 10−4 M in water.


0

130

260

390

520

650

570 595 620 645 670 695 720

Wavelength (nm)

Inte

nsity

Figure 8c: Fluorescence quenching of TSPP in the presence of colloidal TiO2 nanoparticles in the concentration range of 0−5 x 10−4 M in water.

According to the observation of absorption measurements, there are two

TSPP populations such as free TSPP and adsorbed TSPP. So we can expect two

quenching constants, one for the adsorbed TSPP with M-TiO2 (static quenching)

and another for free TSPP with M-TiO2 (dynamic quenching).

Stern-Volmer equation (5) for the combined static and dynamic quenching is,

= 1 + (KD + KS)[Q] + KD.KS [Q]2F0

F → (5)

= Kapp[Q]

F0F

_ 1 → (6)

where,

Kapp = (KD + KS) + KD.KS[Q]

KD is the dynamic quenching constant

KS is the static quenching constant

F0 is the fluorescence intensity of the fluorophore in the absence of quencher


F is the fluorescence intensity of fluorophore in the presence of

quencher Q is the concentration of quencher

F0F 1 1

[Q]Kapp = → (7)

A plot of [F0/F-1]/[Q] versus [Q] yields a straight line with an intercept of KD+KS

and a slope of KD.KS [figure 9]. The individual values were obtained from the two

solutions of the quadratic equation. The obtained KD and KS values were shown in

Table 1. The dynamic quenching constant (KD) is larger when compared to the

static quenching constant (KS). From the KD values we have calculated the

quenching rate constant (kq) by using equation 8 and it was found to be in

diffusion controlled limit, the values are shown in Table 1.

F0/F = 1+KD [Q] = 1+kq.τ0 [Q] → (8)

where, F0 and F are the fluorescence intensities of TSPP in the absence and

presence of quencher respectively, KD is the dynamic quenching constant which is

related to the quenching rate constant (kq), by KD = kq.τ0, and τ0 is the fluorescence

lifetime of TSPP (10.4 ns), in the absence of the quencher.

Table 1: Apparent association constant, dynamic and static quenching constants and quenching rate constants for TSPP-TiO2, AuTiO2 & AgTiO2.

S. No. Quenchers Kapp (x 104 M−1) KD (M−1) KS (M−1) * kq (x 109 M−1s−1)

1 AuTiO2 24.69 40.79 2.21 3.92

2 AgTiO2 10.05 28.63 2.07 2.75

3 TiO2 4.78 22.53 1.74 2.16

* calculated from dynamic quenching constant (KD)


0

70

140

210

280

350

0 1 2 3 4 5

[Q] x 10-4 M

[F0/F

-1]/[

Q]

AuTiO2

AgTiO2

TiO2

Figure 9: Plot of [F0/F-1]/[Q] vs [Q] for TSPP by AuTiO2, AgTiO2 and TiO2.

The quenching rate constant is decreased in the following order:

AuTiO2 > AgTiO2 > TiO2

Among the above three systems pure TiO2 shows lesser quenching rate

constant. While comparing AuTiO2 and AgTiO2 the former one shows more

quenching efficiency than the latter due to the higher Fermi level of Au (0.75 V)

than Ag (0.45 V).

According to the equation Es*/s+ = Es/s+ − Es, we have calculated the excited

state oxidation potential (Es*/s+) of TSPP which is −0.82 V vs SCE. where, Es/s+ is

the oxidation potential of TSPP 1.1 V vs SCE [46] and Es is the excited state

energy of TSPP 1.92 eV [46]. The conduction band energy level of TiO2 is

−0.1 V vs SCE. It suggests that the electron transfer from excited state TSPP to

the conduction band of TiO2 is energetically favorable [Scheme 6]. The electrons

injected into the conduction band of TiO2 are quickly transferred to the metal core

and it leads to the suppression of back electron transfer process [Scheme 2&6].

Hence, AuTiO2 is more efficient in suppressing the back electron transfer process

while compared to AgTiO2 and pure TiO2 nanoparticles.


1

2

3

0

-1

-2

νh

Es*/s+

Es/s+TiO2

CB

VB

(1.1 V)

(-0.82 V)

(- 0.1 V)

V vs SCE

(2.9 V)

0.75 VAu

0.45 VAg

Scheme 6: Schematic diagram describing the electron-donating energy levels of TSPP.

4.3.6. Calculation of free energy change (∆Get) for the electron transfer mechanism

The thermodynamic feasibility of the excited state electron transfer

reactions was calculated by employing the well known Rehm-Weller expression

[47].

∆Get = E½(ox)

− E½(red) − Es + C → (9)

where, E½(ox) is the oxidation potential of TSPP (1.1 V), E½

(red) is the reduction

(conduction band) potential of TiO2 (−0.1 V), Es is the excited state energy of

TSPP (1.92 eV) and C is the coulombic term. Since one of the species is neutral

and the solvent used is polar in nature, the coulombic term in the above

expression is neglected [48]. The ∆Get value thus calculated for TSPP-TiO2 is

negative (−0.72 eV). Hence, the ET process is thermodynamically favorable [49].


4.3.7. Fluorescence lifetime measurements As shown by previous reports [50-52], the dye molecules adsorbed on the

semiconductor particle surface had significantly shorter fluorescence lifetime than

the unadsorbed molecules. The fluorescence decay of TSPP (2 x 10−6 M) in the

absence and presence of metal-semiconductor nanoparticles were shown in

figure 10 (a, b and c for TSPP with AuTiO2, AgTiO2 and TiO2 respectively). In

the absence of metal-semiconductor nanoparticles, the fluorescence decay curve

of TSPP is fitted with single exponential decay (F(t) = A exp (−t/τ)) and it shows

the lifetime of 10.4 ns. Upon addition of metal-semiconductor nanoparticles

(5 x 10−4 M), the decay of TSPP is deviated from single exponential decay to

bi-exponential (F(t) = A1 exp (−t/τ1) + A2 exp (−t/τ2)). It shows shorter-lifetime

and longer-lifetime components. The shorter lifetime is attributed to the adsorbed

TSPP (τads) and the longer one is for free/unadsorbed TSPP (τ). The fluorescence

lifetime of TSPP with metal semiconductor nanoparticles are given in Table 2.

Further these lifetime data clearly indicates that more electron transfer is possible

in the case of Au-TiO2 system compared to pure TiO2 system because of the flow

of electrons from the conduction band of TiO2 into the metal core having low

lying Fermi level is energetically favorable (Scheme 6).


1

10

100

1000

10000

0 10 20 30 40 50

Time (ns)

Cou

nts

TSPPTSPP/AuTiO2

Figure 10a: Fluorescence decay of TSPP in absence and presence of colloidal AuTiO2 nanoparticles (5 x 10−4 M) in water.

1

10

100

1000

10000

0 10 20 30 40 50

Time (ns)

Cou

nts

TSPPTSPP/AgTiO2

Figure 10b: Fluorescence decay of TSPP in absence and presence of colloidal AgTiO2 nanoparticles (5 x 10−4 M) in water.


1

10

100

1000

10000

0 10 20 30 40 50

Time (ns)

Cou

nts

TSPPTSPP/TiO2

Figure 10c: Fluorescence decay of TSPP in absence and presence of colloidal TiO2 nanoparticles (5 x 10−4 M) in water.

The observed decrease in lifetime could be correlated with the electron transfer

process and it may be correlated by using equation (10),

ket = 1/τads – 1/τ → (10)

where, τ and τads are the lifetime of free TSPP molecules in aqueous solution and

adsorbed on the semiconductors surface respectively and ket is the specific rate of

electron transfer process from excited state TSPP to the conduction band of

semiconductors. By substituting the values of τ and τads in the above equation (10)

the values of ket were calculated and shown in Table 2. The presence of noble

metals promotes the interfacial electron transfer process in the excited state of

TSPP to metal-semiconductor nanoparticles [Scheme 2&6] and hence higher

ket was obtained for Au and AgTiO2 compared to pure TiO2.


Table 2: Fluorescence lifetime of TSPP (2 x 10−6 M) in the presence of metal-semiconductors (5 x 10−4 M) and the rate of electron transfer (ket).

S. No. Sensitizer τ (ns) τads(ns) ket (x 108 s−1)

1 TSPP 10.4 - -

2 TSPP+TiO2 9.9 2.82 2.54

3 TSPP+AgTiO2 9.4 2.29 3.40

4 TSPP+AuTiO2 8.0 1.81 4.56

4.3.8. Transient absorption characteristics The charge transfer interaction between TSPP and the TiO2 was further

probed using transient absorption spectroscopy. Nanosecond laser flash photolysis

experiments were carried out using 400 nm laser pulse as the excitation source.

The transient absorption spectra were recorded at 20 µs after laser pulse excitation

of TSPP in the range of 400-700 nm is shown in figure 11a. The absorption

maximum at 450 nm with a lifetime τ = 262 µs can be ascribed to the

triplet–triplet absorption of TSPP. The bleaching at 420 nm region indicates the

depletion of TSPP absorption.

The transient absorption spectra observed upon excitation of TSPP in the

presence of colloidal AuTiO2 and TiO2 nanoparticles in water solution were

shown in figure 11b and 11c. A new transient species with maximum absorption

around 500 nm (τ = 110 µs for AuTiO2 and τ = 65 µs for TiO2) was generated and

this peak can be assigned to cation radical of TSPP•+. In addition decrease in

lifetime is entirely due to electron transfer process. Therefore electron injection

occurred from the excited triplet state TSPP into the conduction band of TiO2 and

further the electrons accumulated within TiO2 layer get quickly transferred to Au.


-0.2

0

0.2

0.4

0.6

0.8

350 450 550 650 750

Wavelength (nm)

∆Α

-0.1

0.1

0.3

0.5

-20.00 80.00 180.00

Time (ms)

∆Α

Figure 11a: Transient difference absorption spectra of TSPP recorded at 20 µs after the laser flash (400nm) of water solution. The inset shows the decay of transient absorption at 480 nm.

-0.3

0.1

0.5

0.9

350 440 530 620 710 800

Wavelength (nm)

∆Α

-0.05

0.05

0.15

0.25

-20 30 80 130 180

Time (ms)

∆Α

Figure 11b: Transient difference absorption spectra of TSPP in the presence of AuTiO2 recorded at 20 µs after the laser flash (400nm) of water solution. The inset showed the decay of transient absorption at 500 nm.


-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

350 440 530 620 710 800

Wavelength (nm)

∆Α

-0.050

0.050.1

0.150.2

0.250.3

-20 30 80 130 180

Time (ms)

∆Α

Figure 11c: Transient difference absorption spectra of TSPP in the presence of TiO2 recorded at 20 µs after the laser flash (400nm) of water solution. The inset showed the decay of transient absorption at 500 nm.

TSPP*-AuTiO2 (S1)

TSPP+ + ecb (TiO2)AukET

TSPP*-AuTiO2 (T1)

TSPP*-AuTiO2 (T1)

kISC

ecb (TiO2)Au TiO2 Au(e)

If indeed such electron accumulation should occur, we can be able to see

its influence on the charge recombination (or back electron transfer) process. The

reverse electron transfer between TSPP•+ and the injected electron was monitored

from the decay of the transient absorption at 500 nm (TSPP•+). Therefore the rate

constant of reverse electron transfer kr = 1/τ = (1/110) µs = 9.09 x 103 s−1

(for AuTiO2) and 1.53 x 104 s−1 (for TiO2). The lower value of rate constant was

an indication of the weak interaction between cation radical of TSPP and AuTiO2

and which was five orders of magnitude slower than the value observed in the

sensitization of AuTiO2 colloidal semiconductor by singlet TSPP


(ket = 9.92 x 108 s−1, table 2). This further demonstrated that TSPP is a good

photosensitizer for AuTiO2 semiconductor. 4.3.9. Effect of pH

The effect of pH on the interaction between TSPP and TiO2 nanoparticles

has been studied by measuring absorption spectrum of TSPP in two different pH.

TSPP is a water-soluble compound and it exists as a monomer at natural pH

(λmax = 413 nm). If we change the pH of TSPP towards acidic (pH ~ 3),

protonation takes place only in the position of two pyrrolic nitrogen atoms

(λmax = 432 nm), not in the anchoring group (SO3−) as shown in scheme 7. The

absorption spectrum of TSPP at two different pH was shown in figure 12.

0

0.2

0.4

0.6

0.8

300 325 350 375 400 425 450 475 500

Wavelength (nm)

Abs

orba

nce

pH~3 natural pH

Figure 12: Absorption spectrum of TSPP at natural pH and pH~3

From this figure, we observed that while changing the pH from natural to

acidic the wavelength of TSPP is shifted from 413 nm to 432 nm. But in presence

of TiO2 there is no change in the absorption wavelength of TSPP (figure 7a-c). So

we confirmed that the change in absorption spectra of TSPP is mainly due to

adsorption on the surface of TiO2 and not due to pH effect. Lifetime measurement


also supports the results obtained from absorption study at natural pH. The

lifetime of TSPP at natural and acidic pH is 10.4 ns and 3 ns [53] respectively.

But addition of TiO2 shows the lifetime of TSPP around 10 ns [Table 2]. The

result shows that TiO2 does not altered the pH of TSPP solution which confirms

the interaction is not due to pH effect.

N

NH N

HN

SO3

SO3

O3S

SO3

NH

NH HN

HN

SO3

SO3

SO3

O3S

+

+

Absorption maximum at 413 nm(natural pH)

Absorption maximum at 432 nm(pH~3)

Scheme 7: Acid–base equilibrium of TSPP in aqueous solution.

4.4. Conclusions The interaction of TSPP with colloidal metal-semiconductor nanoparticles

has been studied by absorption, steady state and time-resolved fluorescence and

transient absorption spectroscopic methods. TSPP adsorbed on the surface of

metal semiconductor nanoparticles through its sulphonyl group, as evidenced by

the effects of colloidal metal semiconductor nanoparticles concentration on the

absorption study. The lifetime quenching data has been analyzed to determine the

rate of electron transfer process. Based on the results, it is suggested that the metal

core has effect on the electron transfer from excited state TSPP into conduction

band of TiO2. On the other hand, the presence of the metal core suppresses the


charge recombination process. Electron accumulation within the metal core is

likely to influence the overall charge separation in the composite system.

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