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UV-sensitized nanomaterial semiconductor catalytic reductionof CoIII(N–N)3

3+ /nm-TiO2 and Co:TiO2 formation: SEM-EDX

and HRTEM analyses

Krishnamoorthy Anbalagan

Lazor Devaraj Stephen

Received: 17 July 2009 / Accepted: 8 September 2009 / Published online: 25 September 2009

Springer Science+Business Media B.V. 2009

Abstract Interfacial electron transfer induced by 254 nm

light at nanomaterial (nm) titanium dioxide/CoIII(N–N)33?

interface in binary mixed solvent media such as water/

methanol (or 1,4-dioxane) has been probed. The distinct

photo reduction of cobalt(III) complexes, CoIII(N–N)33?;

(N–N)=(NH3)2, en (1,2-diamino ethane), pn (1,2-diamino

propane), tn (1,3-diamino propane), and bn (1,4-diamino

butane), by excited nm-TiO2 particles: CoIII? nm-TiO2 ?

hm ? TiO2 (h?;e-) ? CoIII ? nm-TiO2 (h) ? Co

II is

solvent controlled. The electron transfer from the conduc-

tion band of TiO2 (e-, CB) onto the metal centre of the

complex consists of (i) electron transport from CB into

surface-adsorbed species A: CoIII(N–N)33? (ii) solution

phase species B: CoIII(N–N)33?(sol.), accumulated at the

surface of the nanoparticle. In addition, UV irradiation of

CoIII(N–N)33? stimulates generation of CoIIaq ion, due to

charge transfer transition, in solution phase. After UV

irradiation, cobalt-implanted nm-TiO2 separated as gray

ultrafine particles, which were isolated. Photo efficiency of

the formation of CoII ion was estimated and the cobalt

implanted nanomaterial crystals isolated from the photolyte

solutions were subjected to SEM-EDX, X-ray mapping,

and HRTEM-SAED analyses. Solvent medium was found

to contribute in both the formation of CoII ion and inter-

stitial insertion of cobalt into the lattice of nm-TiO2.

Introduction

Nanocrystalline semiconductor catalysts have become

widely useful for many technological applications, mainly

due to their light transparency, charge separating proper-

ties, and electronic conductivity [1, 2]. Due to the highchemical stability and favorable energy band structure,

titanium dioxide has drawn attention for its potential

applications in photo catalysis in different fields. Semi-

conductors, especially TiO2, ZnO, and SnO2 are very

promising components in the development of solar cells

[3], electrochromic devices [4, 5], sensors [6, 7], photo-

electrocatalytic and photochromic cells [8, 9]. Recent

investigations have shown that Co(dbbip)22? /3? complexes

are promising candidates [10] for use as redox mediators in

dye-sensitized solar cells. Metal-doped semiconductors

find extensive applications in conjunction with the lattice

structures; for example, Chamber et al. [11, 12] reported

that the local structural environment for Co species in

anatase is similar to that of cobalt in CoTiO3 (highly

distorted octahedral coordination with oxygen ligands).

Nanosized metal-doped materials with novel morphologies

can have somewhat better performance than bulk materials.

From the point of view of kinetic investigations of elec-

tron transfer reactions of inorganic complexes in solution,

mixed solvents have a number of distinct advantages. These

include: (a) mixed solvents greatly influence the formation

and stability of transition states [13]/ion-pairs [14]/excited

states [15, 16], (b) they ensure definite rate of electron

transfer reactions even in dilute solutions through potential

matching, (c) the tuning of redox property is facile by the use

of mixed solvents of varying compositions. However,

despite these advantages, the potential of catalytic effect of

TiO2 as electron transfer mediator with cobalt(III) com-

plexes in binary-mixed solvents remains relatively unex-

plored. In this article, we have designed a novel solution

chemical reaction route to study the interfacial ET processes

and also report formation of surface-modified nm-TiO2particles due to cobalt implantation.

K. Anbalagan (&) L. D. StephenDepartment of Chemistry, Pondicherry University,

Pondicherry 605 014, India

e-mail: [email protected]

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Experimental

A series of CoIII(N–N)33?; (N–N)=(NH3)2, en (1,2-diamino

ethane), pn (1,2-diamino propane), tn (1,3-diamino propane),

and bn (1,4-diamino butane) were prepared as in literature

[17] and reagents used were generally of AnalaR grade

(Sigma-Aldrich) samples. For the UV excitation studies,

nm-TiO2 suspension in neat water/binary solvent mixturewas prepared by adding definite amounts of nanoparticu-

lates (surface area = 200–220 m2 /g, Sigma-Aldrich) and

pure crystals of CoIII(N–N)33?. In a typical solution,

complex concentration was 3.88 9 10-3–5.60 9 10-3 M

CoIII(N–N)33? (ionic strength: 0.1 M NaNO3) in neat water

or water/(methanol or 1,4-dioxane) binary mixtures:

(water:organic co-solvent = 95:5, 90:10, 85:15, 80:20,

75:25, 70:30), in which nm-TiO2 was introduced, stirred,

and ultrasonicated. Multiple photolysis experiments were

performed under identical conditions to confirm the

reproducibility. Light intensities were measured by ferri-

oxalate actinometry [18] and quantum yield, UCo(II), wascomputed by estimating Co(II) by Kitson’s method [19].

Solution absorption measurements were made using a

Shimadzu Model UV-2450 double beam UV–Vis spectro-

photometer. Complexes were photolysed by irradiation of

254 nm light source using 6 Watt Low Pressure Mercury

Vapor Lamp (Germicidal G4T5, 3H) Model 3006, in a

small Quartz Immersion Well Model 3210, 80 mL cap.

(Photochemical Reactors Ltd, UK). Photo irradiation was

carried out at definite time intervals. Pure semiconductor

catalyst, nm-TiO2, powder (labeled as sample: a), and

ultrafine crystals isolated from the photolyte solutions

before irradiation at 0 s (labeled as sample: b) and after

definite time interval at 120 s, (sample: c) were subjected

to SEM and HRTEM analyses.

Scanning electron microscopy and X-ray microanalyses

(EDX) of the samples were performed on a Hitachi S-

3400N, magnification: 59 to 300,0009, SE image resolu-

tion: 3.0 nm, Accln. voltage 0.3 kV to 30 kV, working

distance: 5–60 mm, maximum specimen size: 200 mm.

Prior to analysis, pressed powder samples were coated with

a thin layer of evaporated carbon for conduction and

examined at 25 kV accelerating voltage using a standard-

less procedure on a Hitachi S-3400N instrument equipped

with a energy-dispersive X-ray microanalysis system.

Three measurements were recorded for each sample over a

sample area of 2 mm. High-resolution transition electron

microscopy (HRTEM) scans of the samples were per-

formed on a JEOL 3010 instrument with a UHR polepiece.

This gave a lattice resolution of 0.14 nm and a point-to-

point resolution of 0.12 nm with standard probe and a

variable temperature probe (100–500 K). The instrument

has a Gatan digital camera. Droplets of aqueous suspen-

sions of the samples were placed on carbon-covered copper

grids for 30 s, supernatant fluid was blotted off and the

grids were left to air dry. Microscopy was carried out at

different primary nominal magnifications in the range of

M = 100 K up to 390 K.

Results and discussion

Photoproduction of cobalt-implanted nm-TiO2

The photo reduction of CoIII(N–N)33? complexes has been

investigated in water–methanol (or 1,4-dioxane) solutions

using nm-TiO2 as photo catalyst, which was excited with

254 nm light source. The cobalt(III) complex is also a good

UV light absorber, exhibiting two ligand to metal charge

transfer (LMCT) bands, one centered at 302.3–349.1 nm

and the second one at 466.9–501.4 nm (in water). It was

observed that the CoIII(N–N33? complexes are consider-

ably stable in neat water on exposure to light over long

periods of time [14, 15, 20]. However, addition of nm-TiO2provoked the complexes to degrade, and cobalt(II) was

generated in either pure water or binary solvent media.

Tables 1 and 2 illustrate the quantum yield of CoII for-

mation, which is strongly dependent on the mole fraction

of the organic co-solvent present in the mixture. Figure 1

exhibits the growth in photo efficiency (%), which is

altered by the increase in organic co-solvent content.

Surface survey using SEM and EDX analyses

Titanium dioxide semiconductor finds extensive use in a

number of applications, such as photoactive layers, super-

hydrophilic films, bifunctional membranes sensors, etc.

[21]. Therefore, the nano material crystalline powder from

the photolyte solution was isolated and characterized, since

nm-TiO2 illustrates an enhanced photo activity and porous

selectivity due to the organized internal structure. Figure 2

shows the SEM images of surface morphology, thickness,

and structural characteristics of ultrafine crystals of pure

nano material powder TiO2 (abbreviated as a), samples

isolated from the photolyte solution before irradiation (at

0 s, abbreviated as sample b), and after definite time of

irradiation (at 120 s, sample c). A significant difference of

the surface morphology and structure of nm-TiO2 of the

samples b and c could be seen, that is, significant aggre-

gation of particles is observed.

In a typical experiment, SEM micrographs of

CoIII(tn)33? /nm-TiO2 in water/methanol (90:10) at the

initial (at 0 s) and after 120 s irradiation using k = 254 nm

show the presence of disordered filaments leading to an

almost ‘fluffy’ structure due to cobalt deposition. A lower

coverage in the sample b and somewhat higher coverage in

sample c are visible. The particle size in sample b is

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5–10 nm but the size becomes 25–60 nm in sample c.

Figures 3 and 4 present the EDX profiles and X-ray map-

ping of the samples, in which Ti:Co = 33.33:0 (before

irradiation) and Ti:Co = 33.19:0.21 (%) (after irradiation

for 120 s). It indicates the implantation and homogeneous

distribution of cobalt on the lattice of titania. This is very

well confirmed by X-ray mapping, in which a minimum

percent of cobalt on the nm-surface is evident for all other

Table 1 Photoreduction efficiency of cobalt(III)–alkyl amine complexes in air equilibrated water–methanol mixtures (pH = 6.82)

Methanol Temperature (K) UCo(II)

% (v/v) x2 [CoIII(NH3)6]

3? [CoIII(en)3]3? [CoIII(pn)3]

3? [CoIII(tn)3]3? [CoIII(bn)3]

3?

0 0 293 0.345 ± 0.002 0.351 ± 0.004 0.361 ± 0.008 0.336 ± 0.001 0.291 ± 0.008

300 0.382 ± 0.003 0.361 ± 0.003 0.376 ± 0.002 0.342 ± 0.007 0.317 ± 0.003

Percentage increase in photoefficiency

5 0.0229 293 4.1 5.4 3.3 2.1 1.5

300 5.8 5.4 2.1 5.6 1.6

10 0.0471 293 8.6 6.4 1.9 1.7 3.9

300 7.9 7.5 1.5 3.6 3.6

15 0.0728 293 16.6 9.9 1.8 2.5 5.8

300 9.1 8.3 1.7 5.5 4.3

20 0.1001 293 18.2 12.2 4.3 4.8 14.5

300 12.1 11.9 4.7 5.7 7.2

25 0.1292 293 20.7 13.0 6.4 7.2 14.0

300 14.2 13.2 7.8 3.7 8.5

30 0.1602 293 21.9 17.5 11.9 8.1 12.1

300 17.1 17.2 9.4 7.2 10.4

The estimated values of UCo(II) for complexes in neat water and the increase in values in percentage

[Co(III) = 3.88 9 10-3

to 5.60 9 10-3

M, [NaNO3] = 0.1 M

Table 2 Photoreduction efficiency of cobalt(III)–alkyl amine complexes in air equilibrated water-1,4-dioxane mixtures (pH = 6.82)

1,4-dioxane Temperature (K) UCo(II)

% (v/v) x2 [CoIII(NH3)6]

3? [CoIII(en)3]3? [CoIII(pn)3]

3? [CoIII(tn)3]3? [CoIII(bn)3]

3?

0 0 293 0.345 ± 0.002 0.351 ± 0.004 0.361 ± 0.008 0.336 ± 0.001 0.291 ± 0.008

300 0.382 ± 0.003 0.361 ± 0.003 0.376 ± 0.002 0.342 ± 0.007 0.317 ± 0.003

Percentage increase in photoefficiency

5 0.0109 293 1.8 1.4 4.1 1.3 3.2

300 2.7 1.7 3.6 1.7 3.7

10 0.0229 293 4.8 4.8 5.1 2.5 5.8

300 4.2 4.7 5.7 4.1 4.9

15 0.0359 293 7.8 6.4 7.0 6.9 10.2

300 6.1 7.9 7.4 5.1 9.0

20 0.0502 293 9.6 9.7 10.2 8.4 12.1

300 8.4 9.5 10.1 8.6 10.8

25 0.0659 293 6.72 12.4 13.4 11.4 16.2

300 10.3 13.2 14.5 10.8 15.330 0.0831 293 15.1 13.3 15.4 12.5 18.4

300 14.6 15.6 17.1 13.6 17.1

The estimated values of UCo(II) for complexes in neat water and the increase in values in percentage

[Co(III) = 3.88 9 10-3

to 5.60 9 10-3

M, [NaNO3] = 0.1 M

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complexes. The metal centre reduction could be due to

(i) CoIII(N–N)33?

? nm-TiO2 (h?

;e-

) ? hm ? Co(II) or(ii) charge transfer to metal transition (LMCT) leading to

the generation of Co(II); CoIII(N–N)33?? hm (LMCT) ?

Co(II). Tables 1 and 2 illustrate that the photo efficiency

of Co(II) production which is dependent on the organic

co-solvent content of the medium. Therefore, reduction of

cobalt(III) centre of the complex was competitively initi-

ated by both nm-TiO2 (e-, CB) and solvent-influenced

LMCT transition of the complex as given in Eqs. 1–3.

TiO2 hþ

; eð Þ VB þ hm ! e CBð Þ ð1Þ

CoIII NNð Þ33þ þ e CBð Þ ! CoII NNð Þ3

2þ ! CoII

ð2Þ

CoIII NNð Þ33þ þS ! CoII NNð Þ3 S

þð Þ3þ

! CoII LMCTð Þ

ð3Þ

These reactions are taking place in competition with the

undesirable back electron transfer; however, the photo

efficiency is gradually enhanced as given in Tables 1 and 2.

We would now like to address the question of photo

reduction of CoIII by the nm-TiO2 surface (Eqs. 1, 2). Thedifferences observed in the quantum yields in the presence

of nm-TiO2 suspensions of CoIII complexes in binary

solvents could also be due to differences in the surface

chemistries, such as the mechanisms of adsorption,

desorption or the specific reactions of surface concentrated

species.

Surface morphology and cobalt implantation

According to HRTEM results, the cobalt-implanted nm-TiO2

isolated from the photolyte solutions are mainly nanosizedcrystallites and contain relatively wide distributions. Fig-

ure 5 presents the HRTEM images of the samples a and b

for a typical complex, CoIII(tn)33?, which illustrate segre-

gated but hexagonal patterns of the crystal lattice. The

weak fast Fourier transform (FFT) signals of Co for the

samples (iii) and (iv) indicate the presence of some Co in

the TiO2 matrix in the form of substituted particles. This

suggests the possibility of a small quantity of Co occupying

the substitutional sites of Ti of titania leading to solid

solution of anatase nm-TiO2. The lattice fringes due to

cobalt insertion are about 5 nm indicating the modified

anatase product. The selected area electron diffraction(SAED) pattern further confirms the formation of cobalt

Fig. 2 SEM images of pure and

isolated samples; a pure nm-

TiO2, b nm-TiO2 /CoIII

(tn)33?

(before irradiation, at 0 s), size:

10 lm (inset: 5 lm) and

(c) nm-TiO2 /CoIII

(tn)33?

(after

irradiation, at 120 s) size:

10 lm (inset: 5 lm) in

water:methanol = 90:10

0

4

8

12

16

20

0 0.05 0.1 0.1 0.2

Q u a n t u m E

f f i c i e n c y ( % )

x2

Fig. 1 Dependence of quantum efficiency (in %) versus mole

fraction of organic co-solvent ( x2) in water–methanol. Filled circle

CoIII(NH3)63?, filled triangle CoIII(en)3

3?, open triangle CoIII(pn)33?,

open square CoIII

(tn)33?

, filled square CoIII

(bn)33?

at 300 K


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insertion on the surface of nm-TiO2 (Co:nm-TiO2),

obtained from the photolyte solution after high dose (at

120 s) of 254 nm light. Moreover, there might not be

amorphous products on the surface as the image shows

regular lattice spacing. The cobalt phase is quite homoge-

neous with a diameter of about 5 nm with inter-cluster

distance generally greater than 5 nm.

Tuning of photoreduction

Thephotoredox processes of nm-TiO2:CoIII(N–N)3

3? can be

tuned in water–methanol/1,4-dioxane solutions, which also

ensures an efficient injection of electrons from the conduc-

tion band of titania to the metal centre of the complex ion

leading to the formation of Co(II) species; simultaneously,

Element Net

Counts

Weight % Atom % Formula

O 1259 40.05S 66.67

Ti 41503 59.95 33.33 TiO2Total 100.00 100.00

5µm

(i)

(ii)

Fig. 3 EDX spectrum of

Co:nm-TiO2 material isolated

from the photolyte solution of

nm-TiO2 /CoIII

(tn)33? complex

in water:methanol = 90:10.

(before irradiation, at 0 s,

sample b). X-ray mapping

of (i) Ti and (ii) O

Element Net

Counts

Weight % Atom % Formula

O 1615 39.94S 66.60

Ti 38516 59.59 33.19 TiO2

Co 159 0.47 0.21 CoOTotal 100.00 100.00

(i)

(ii)

(iii)

5µm

Fig. 4 EDX spectrum of

Co:nm-TiO2 material isolated

from the photolyte solution of

nm-TiO2 /CoIII(tn)3

3? complex

in water:methanol = 90:10.

(after irradiation, at 120 s,

sample c). X-ray mapping

of (i) Ti and (ii) O


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surface modified semiconductor species was generated at the

end of high dose of 254 nm irradiation. Evidently, addition

of methanol (or 1,4-dioxane, C4H8O2) in the medium

increases the photo efficiency of reduction of metal ion of the

complex mainly due to MLCT. On the other hand, it is

suggested that the density of CoIII(N–N)33? ion at the surface

of semiconductor is enhanced by the hydrophobic nature of the CH3OH/C4H8O2 organic co-solvent molecules. The

condition provides prominent forward ET compared to back

ET and the net effect is an enhanced-electron transfer rate.

Introduction of excess concentration of methanol/1,4-diox-

ane ensures uniform distribution of the complex ion on the

oxide surface and promotes electronic coupling of the

donor–acceptor levels.Moreover, the choice of the methanol

or 1,4-dioxane controls water content in the pores during the

loading of the metal complex due to surface adherence [22].

EDX analysis of cobalt-deposited TiO2 samples as illus-

trated in Figs. 3 and 4 indicate that enhancement of Co(II) on

the surface is more probable in solvent medium with higher

organic co-solvent content (methanol; when x2 = 0.1602/

1,4-dioxane; x2 = 0.0831), which results in efficient accu-

mulation of CoIII(N–N)33? near the active sites of nm-par-

ticles, thereby CoIII centers could capture the photo-excited

electrons; simultaneously, improved surface active sites of

TiO2 would increase the mobility of electrons resulting in

enhanced photo efficiency [23].

The EDX spot probe analysis shows that Co loading is

enhanced in all the samples after definite time (120 s) of

irradiation. These observations strongly illustrate that

adsorption/accumulation is a main factor responsible for the

photo activity enhancement. This suggests two features; (i)

the adsorbed molecules can efficiently reach light-activated

sites and (ii) electron transfer is efficient due to mobility of

the molecules near the surface at the interface. Photo catal-

ysis of nm-TiO2 in mixed solvents is more efficient; how-ever, the surface structure collapses readily leading to cobalt

insertion with some agglomeration of nm-particles from size

5–10 to 25–60 nm. An interesting observation is the unique

behavior of nm-photo catalysts in (i) enhanced activity, (ii)

agglomeration of particle size, and (iii) insertion of the

photo-generated Co(II) ion substitutionally into the lattice of

titanium dioxide retaining the crystalline phase. This implies

that a favourable interface exists in the electron transfer

process in TiO2 /CoIII(N–N)3

3? /water–methanol (1,4-dioxane)

system. Moreover, complex cation-binding ability on nm-

TiO2 is established as the solvent environment is varied,

which could affect (i) attachment of complex ion and (ii)

detachment of the metal centre after reduction (EDX shows a

small increase in Co on nm-surface). As shown in Fig. 5,

aggregates are visible both for TiO2 as well as CoII:nm-TiO2

in dried samples. A closer look at high-resolution images

shows that the aggregates appear with the same crystallo-

graphic orientation. Therefore, the shape and aggregation of

particles might be different in binary solvent mixtures.

Accordingly, the target molecules [24] are adhered on photo

catalyst particle surface. Thus, excitednm-TiO2 transfers the

Fig. 5 High-resolution TEM image of a CoII

:TiO2 (before irradia-

tion, at 0 s, sample b) and b CoII

:TiO2 (after irradiation, at 120 s,

sample c). c Lattice image taken from CoII:TiO2 away from

segregated area (before irradiation) (inset selected area diffraction

patterns (SADPs) taken from CoII

:TiO2 and fast Fourier transform for

CoII

:TiO2 complex). d Lattice image taken from CoII

:TiO2 away

from segregated area (after irradiation) (inset selected area diffraction

patterns (SADPs) taken from CoII

:TiO2 and FFT for CoII

:TiO2


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photo electron to surface as well as solution locations; ulti-

mately, it could be possible to apply some solvent designing

for photo catalytic systems to enhance their activity.

Sensitization by nm-TiO2

Direct band-gap excitation of the semiconductor is achieved

at k = 254 nm excitation (band gap & 3.2 ev) [25]; how-

ever, the electron-hole recombination rate retards the

product yield. Solvent environment could modify the

surface-solution species interaction due to hydrophobic/

hydrophilic contributions, for instance, Kusumoto et al. [26]

reported the photo catalytic activity of TiO2 in hydrogen

production from methanol/water solution. In this investi-

gation, photo reaction was carried out in water–metanol/1,4-

dioxane solutions, in which the efficiency of the photo

catalyst and the medium to pass the photon energy to the

reaction system is anticipated to be high. Mechanism

of solvent assisted [27] reduction of CoIII(N–N)3

3? on

nm-TiO2 is as presented in the following Eqs. 4–6.

CoIII NNð Þ33þ

;nm-TiO2

!hm

CoIII NNð Þ33þ

; nm-TiO2

ð4Þ

CoIII NNð Þ33þ

; nm-TiO2

! CoII NNð Þ32þ

; nm-TiO2

ð5Þ

CoII NNð Þ32þ

; nm-TiO2

! CoII NNð Þ32þ þ nm-TiO2

ð6Þ

Based on the observations, it can be concluded that photo-

induced reduction takes place (i) at the surface of semiconductor particle and (ii) at the solution phase of

the surface, where accumulation of the species is more

predominant. Scheme 1 represents the achieving of photo-

induced electron transfer at the surface (surface complex

ion; species A: CoIII(N–N)33?

(sol.S)) and at the solution

phase where the complex species accumulation is more

(solution phase complex ion; species B: CoIII(N–N)33?

(sol.)).

Light absorption by the semiconductor particles leads to the

generation of e-(CB), which is efficiently injected to the

metal center as given in Eqs. 7–9. However, efficiency of

reduction of CoIII(N–N)33?

(sol.) and CoIII(N–N)3

3?(sol.S)

species ions is more in binary solvent mixture containing

higher concentration of organic co-solvent than that of

solvent mixture containing less or in neat water.

nm-TiO2 þ hm ! e CBð Þ ð7Þ

e CBð Þ þ CoIII NNð Þ33þ

ðsol:SÞ ! CoII NNð Þ3 ðsol:SÞ

2þ

more efficientð Þ

ð8Þ

e CBð Þ þ CoIII NNð Þ33þ

ðsol:Þ ! CoII NNð Þ3 ðsol:Þ

2þ

less efficientð Þ

ð9Þ

That is, concerted geometrical and chemical environment

of solvent cage of binary mixed solvents, along with the

influence of the hydrophobic effect of –CH3 of CH3OH and

–C4H8 skeleton of 1,4-dioxane shows an enhancement in

photo efficiency as the organic co-solvent content in themedium increases. It means the efficiency of electron

transport is a function of densities of cationic complex and

the influence of solvent [28]. That is, heterogeneous sol-

vation-induced interfacial electron transfer dynamics can

be different between molecular donor and acceptors [28].

All the Franck–Condon factors of the electron transfer

reaction are available in parallel, and thus electron transfer

is controlled by the electronic coupling strength between

h ν

Co

III

Co

II

more effecient

TiO2(h+,e-) +

Surface Species (A)

Co

III

Co

II

less effecient

TiO2(h+,e-) +

Solution Species (B)

Co

III

e-

Co

IIIe-

Solution Species, B

Surface Species, A

TiO 2

Scheme 1 Reduction of target cobalt(III) complex ion as (i) surface

adsorbed complex ion (Species A): Co(N–N)33?

(sol.S) and (ii) solution

phase complex ion (Species B): Co(N–N)33?

(sol.) accumulated nearthe solution phase of nm-TiO2 surface (open circle water molecule,

filled circle organic co-solvent molecule)


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electron donation wave functions in TiO2 and the acceptor

orbitals of the metal complex [29].

It is clear, however, that only the very close contact

between the donor state of semiconductor and acceptor

orbital of the complex ion together with a strong chemical

bond, facilitated by solvation, can eliminate most of the

undesired slowing down effect. Binding in this way to the

surface of TiO2 in a selected binary solvent medium, thatcan accommodate the complex ion at the pores but without

slowing down the electron transport, can modify the rate of

reduction. Three important features can be discerned in this

investigation; first, efficient reduction of metal center,

second, charge recombination can be controlled and third,

insertion of cobalt ion in the lattice of the nm-titanium

dioxide due to reduction of CoIII(N–N)32?. The interfacial

electron-transfer processes are presumably associated with

the spatial heterogeneities of the nanoscale local environ-

ments and the inhomogeneous vibronic coupling between

the adsorbed molecules and the rough surfaces of the

semiconductors [30].

Conclusion

UV irradiation of nm-TiO2 /CoIII(N–N)3

3? complexes in

water–methanol/1,4-dioxane solutions stimulates genera-

tion of CoIIaq ion in solution phase and cobalt-implanted

anatase titanium dioxide as ultrafine particles. Photo effi-

ciency of formation of CoII ion was estimated and the

nanomaterial crystals isolated from the photolyte solutions

were subjected to SEM-EDX, X-ray mapping, and HRTEM-SAED analyses. It can be concluded that CoIII(N–N)3

3?

complexes show an improved photo efficiency of metal

centre reduction catalysed by nm-TiO2 particles in binary

solvent media, which is due to accumulation of CoIII(N–

N)33? at the pores due to solvation contributions. That is, the

attachment of complex ion is facilitated before photo

reduction and detachment after reduction [31, 32]. More

interestingly, after high dose of UV irradiation (k =

254 nm), cobalt implanted nm-TiO2 separated as gray

ultrafine particles. It indicates that Co(II) ion is inserted into

the lattice of the nm-particle leading to the formation of

CoII:nm-TiO2 with high order of crystallinity. The importantobservations are the unique behavior of nm-photo catalysts

in enhanced activity, agglomeration of particle size, and

insertion of the photo-generated Co(II) ion substitutionally

into the lattice of titanium dioxide retaining the crystalline

phase. This implies that a favorable interface exists in the

electron transfer process in TiO2 /CoIII(N–N)3

3? /water–

methanol (1,4-dioxane) system. Solvent medium was found

to contribute in both the formation of CoII ion and interstitial

insertion of cobalt in the lattice of nm-TiO2.

Acknowledgments Prof. K.A, Principal Investigator, records his

sincere gratitude to the Department of Science & Technology, New

Delhi and Council of Scientific and Industrial Research, New Delhi

for financial support through major research projects. The authors are

thankful to the Central Instrumentation Facility, Pondicherry

University for providing SEM instrumental facility.

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