PAPER www.rsc.org/materials | Journal of Materials Chemistry

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Self-assembled mesoporous TiO2 spherical nanoparticles by a new templatingpathway and its enhanced photoconductivity in the presence of an organic dye†

Astam K. Patra, Swapan K. Das and Asim Bhaumik*

Received 3rd September 2010, Accepted 10th December 2010

DOI: 10.1039/c0jm02937h

The supramolecular assembly of ionic/non-ionic surfactants has been extensively employed as

a template or structure directing agent (SDA) in the synthesis of a large variety of mesoporous materials

over the past one and a half decades. Herein, we report the first highly efficient synthesis strategy for

self-assembled mesoporous TiO2 materials with well-defined crystal morphologies using sodium

salicylate as a template. Mesoporous TiO2 nanoparticles showed a drastically enhanced

photoelectrochemical response under visible light irradiation after entrapping a photosensitizer

molecule (dye) inside the mesopores. The efficient synthesis strategy and enhanced photoresponse of

these mesoporous TiO2 materials could facilitate the design of other porous semiconductor oxides and

their applications in photon-to-electron conversion processes.

Introduction

Experimental methods for the design of self-assembled nano-

particles of definite size and shape are of major research interest

due to their numerous applications, including optoelectronics,1

catalysis,2 photovoltaics,3 magnetic memories4 and so on.

Different routes for the synthesis of metal oxide nanocrystals of

various size and shape have been reported in the literature.5 Sol–

gel, hydrothermal or solvothermal crystallization of metal oxides

from their respective metal salts/complexes attracted major

attention for the synthesis of metal oxide nanocrystals of various

shapes. Among the metal oxide semiconductors TiO2 is studied

most intensively because of its environmental,6 sensing,7,8 pho-

tocatalytic9,10 and optoelectronics applications.11

However, the major drawbacks of TiO2-based nanostructured

materials in optical/optoelectronics applications are their low

surface area and high band gap. One way to enhance the surface

area of a semiconductor oxide material is to introduce micro-

porosity or mesoporosity in the material. The supramolecular

assembly of ionic/non-ionic surfactants has been conventionally

employed as a template or structure directing agent to design

mesoporous materials for almost two decades.12 In this context,

some other reactive organic compounds like ionic liquids,13

dendrimers14 or polymers15 are often used as template molecules

for designing mesoporous materials. However, to the best of our

knowledge sodium salicylate and its supramolecular assembly

has never been utilized in the synthesis of mesoporous materials.

On the other hand, since UV irradiation cannot be applied on

a semiconductor surface for a long time in practical applications,

considerable research has been directed towards extending the

absorption edge of TiO2-based nanostructured materials towards

the visible region of the spectrum by doping titania with

Department of Materials Science, Indian Association for the Cultivation ofScience, Jadavpur, Kolkata, 700032, India. E-mail: msab@iacs.res.in;Fax: +91-33-2473-2805; Tel: +91-33-2473-4971

† Electronic supplementary information (ESI) available: Wide angleXRD patterns and N2 sorption isotherms. See DOI: 10.1039/c0jm02937h

This journal is ª The Royal Society of Chemistry 2011

metallic,16 non-metallic17 or organic species18 on the surface or in

the crystal lattice.

Herein, we report the first simple and generalized method for

the self-assembly of tiny mesoporous TiO2 nanospheres utilizing

sodium salicylate (SS) as a template. Electron injection from

a photosensitizer molecule trapped inside the pores of the semi-

conductor can facilitate its visible light-induced generation of

a photocurrent.19 In light of this photoelectrochemical applica-

tion potential, a photosensitizer molecule (organic dye) has been

entrapped inside the mesopores of our self-assembled meso-

porous TiO2 nanoparticles and their photoresponses have been

explored.

Experimental section

Self-assembled TiO2 nanoparticles were synthesized by the

following procedures: 2.0 g ammonium chloride (37.4 mmol,

Merck, 98.9% GR) was added to a 20 mL aqueous solution of

1.6 g sodium salicylate (SS: 10 mmol, Loba Chemie, 99.5%). The

solution was stirred for 15 min, then 4 mL ammonia solution

(25% aqueous) was added and the mixture stirred again for

30 min. 10 mmol titanium isopropoxide (Ti(OiPr)4, Aldrich) was

then taken in 5 g isopropyl alcohol and this solution was slowly

added to the first solution. The pH of the solution was then

adjusted to pH ¼ 10 by addition of ammonia solution (Merck,

25% aqueous GR) and stirred for 3 h. The mixture was kept at

freezing conditions (277 K) for 48 h. Additionally, the same

reactions were done hydrothermally at 320 and 350 K. The

resultant solids were collected by filtration and the materials are

designated as MT-1, MT-1A and MT-1B, respectively. In

another experiment, the pH was not adjusted externally using

ammonium chloride and ammonia, the concentration of the

template sodium salicylate was kept at 0.25 moles with respect to

the Ti(OiPr)4 and the synthesis gel was hydrothermally heated to

393 K for 48 h and same reactions were carried out at 320 and

350 K. These samples have been designated as MT-2, MT-2A

and MT-2B, respectively. All the synthesized solids were calcined

at 773 K for 6 h to obtain the desired mesoporous TiO2 nano-

crystals.

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Powder X-ray diffraction patterns of the samples were recor-

ded on a Bruker D-8 Advance diffractometer operated at a

voltage of 40 kV and a 40 mA current using Cu Ka

(l ¼ 0.15406 nm) radiation. TEM images were recorded on

a JEOL 2010 TEM operated at 200 kV. A JEOL JEM 6700F field

emission scanning electron microscope was used for the deter-

mination of the particle morphology. Nitrogen sorption

isotherms were obtained using a Beckmann Coulter SA3100

surface area analyzer at 77 K. Prior to the measurement, the

samples were degassed at 393 K for 12 h. UV-visible diffuse

reflectance spectra were recorded on a Shimadzu UV 2401PC

with an integrating sphere attachment. BaSO4 was used as

background standard. For the electrical measurements, firstly

pellets of each of the samples of 1 cm diameter were made and

two gold electrodes (of ca. 50 nm thickness) were thermally

evaporated on each pellet in the circular form of diameter 2 mm

through a shadow mask at a separation of 4 mm. The DC current

between the two electrodes was measured using a Keithley source

meter (model 2420). The photocurrents were measured by illu-

minating with white light from a 150 W Xenon lamp source

(Newport Corp. USA; model no. 69907).

Results and discussion

The small angle powder XRD patterns of the mesoporous TiO2

samples MT-2, MT-2-Cal, MT-2A and MT-2B are shown in

Fig. 1A and those of MT-1, MT-1-Cal, MT-1A and MT-1B are

shown in Fig. 1B. One broad peak signifying the average pore-

center-to-pore-center correlation length is observed for all the

samples. However, very interestingly, while decreasing the

Fig. 1 A: Small angle XRD patterns of MT-2 (as-synthesized, a); MT-

2B (b); MT-2A (as-synthesized, c); MT-2-Cal (calcined, d) and bulk

anatase TiO2 (e). B: Small angle XRD pattern of the synthesized MT-1-

Cal (calcined, a); MT-1 (as-synthesized, b); MT-1A (as-synthesized, c);

MT-1B (as-synthesized, d).

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synthesis temperature from 393 K to 277 K the particle-center-

to-particle-center distance drastically decreased from 14.9 nm to

2.9 nm, as seen from the powder diffraction pattern of sample

MT-1 in Fig. 1B. The decrease of this interparticle distance is

related to the increase in particle size from the high temperature

synthesis. At higher temperatures, the condensation and growth

of the particles/pore-wall is favored. On the other hand, during

calcination this d-spacing increases, in contrast to conventional

surfactant-templated mesoporous materials, where contraction

of the pore wall (and d-spacings) occurs during the removal of

the template molecules.12 The wide angle XRD patterns of the

TiO2 nanoparticles (Fig. 2 and ESI Fig. S1†) suggested highly

crystalline planes of anatase TiO2. Crystalline planes corre-

sponding to the peaks for anatase TiO2 have been indexed. Both

calcined samples show major peaks at 2q values of 25.3�, 37.8�,

48.0� and 54.2�, which correspond to anatase (101), (004), (200)

and (105) crystal planes (JCPDS 21-1272).2 Thus, these powder

XRD results revealed that we have synthesized highly stable and

crystalline spherical TiO2 nanoparticles through this new

synthesis method of employing SS as a templating agent.

In Fig. 3a, 3b and 3c, HR-TEM images of TiO2 nanoparticles

of a representative self-assembled mesoporous TiO2 material

(MT-2, MT-1A and MT-2B, calcined at 773 K) are shown. As

seen from the figures, pores of dimension ca. 5–7, 4–5 and around

3 nm (white spots) were observed throughout the images of the

samples synthesized at 393, 350 and 320 K. The selected area

electron diffraction (SAED) pattern shown in Fig. 3d suggested

the diffraction spots for anatase TiO2. These results suggested the

formation of self-assembled mesoporous TiO2 nanospheres with

well-defined lattice planes, and the diffraction spots are indexed

corresponding to an anatase structure.2 In Fig. 4, we have shown

the high resolution TEM image of representative as-synthesized

and calcined MT-2 samples. As seen from the figures, the pore is

expanded upon calcination. For the calcined sample, 5–7 nm

pores (white spots) are observed throughout the specimen.

Lattice fringes corresponding to the anatase TiO2 are also clearly

observed for the nanoparticles. The FE-SEM images (Fig. 5)

show that the samples are both composed of very small spherical

nanoparticles of 12–20 nm in size. For as-synthesized MT-2, the

crystal edges are sharper. This could be attributed to the high

temperature synthesis, where condensation and subsequent

crystallization is favored.

The N2 adsorption/desorption isotherms of samples MT-1,

MT-1A, MT-2B and MT-2 (Fig. 6A), and MT-1B and MT-2A

Fig. 2 Wide angle XRD patterns of MT-1 (a) and MT-2 (b).

This journal is ª The Royal Society of Chemistry 2011

Fig. 3 The TEM images of self-assembled TiO2 nanoparticles MT-2 (a),

MT-1A (b), MT-2B (c) and a selected area electron diffraction (SAED)

pattern of the as-synthesized mesoporous TiO2 sample MT-2 (d).

Fig. 4 The HR-TEM images of as-synthesized (a) and calcined (b,

773 K) MT-2 samples.

Fig. 5 The FE-SEM image of mesoporous TiO2 nanospheres MT-2 (a)

and MT-1 (b).

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(ESI†, Figs S2A and S2B) could be classified as type IV isotherm

characteristics of the mesoporous materials. In these isotherms,

between P/P0 of 0.05 and 0.20 the adsorption gradually increases

for MT-1, MT-1A, MT-1B and MT-2B, whereas for MT-2

a large increase in the adsorption occurred at higher P/P0 (0.60–

0.75), and for MT-2B a small increase in adsorption occurred at

P/P0 ¼ 0.40–0.65, corresponding to the mesoporous materials

having large mesopores. The latter isotherms are associated with

a desorption hysteresis such as is usually observed for large

mesopores.20 The BET surface areas for the calcined MT-1, MT-

1A, MT-1B, MT-2, MT-2A, and MT-2B samples were 285, 326,

270, 118, 262 and 294 m2 g�1, respectively. Their respective pore

volumes were 0.26, 0.33, 0.243, 0.11, 0.133 and 0.202 cc g�1. The

pore size distributions of the samples, measured using the Non

Local Density Functional Theory (NLDFT) method (using N2

adsorption on silica as a reference), suggested that MT-1

synthesized at 277 K has a much smaller average pore width (ca.

2.5 nm) than MT-2 (ca. 6.5 nm) synthesized at 393 K. The pore

widths and wall thicknesses obtained from powder XRD and N2

sorption data for both of the samples agree well with the values

obtained from the TEM analyses.

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Fig. 6 A: N2 adsorption (d)-desorption (B) isotherms of the calcined

MT-1 (a), MT-1A (b), MT-2B (c) and MT-2 (d) at 77 K. The y axes of

plots a, b and c have been enhanced by 90, 50 and 30 respectively for

clarity. B: The respective pore size distributions using NLDFT method

are shown.

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UV-visible spectroscopy is one of the most important analyt-

ical tools for characterizing the optical properties of the TiO2

nanocrystals. Titania nanocrystals usually show a broad

absorption in the wavelength range 300–350 nm.2 The UV-visible

diffuse reflectance spectra of different mesoporous TiO2 and dye-

entrapped samples are shown in Fig. 7. The as-synthesized TiO2

material showed an absorption maximum at ca. 328 nm and

a long absorption tail extended to 600 nm (the sample is pale

yellow in color), suggesting chemical binding of the salicylate

molecules at the TiO2 surface.18 For the mesoporous TiO2

materials MT-1 and MT-2, after template removal the absorp-

tion maximum is considerably blue shifted. The UV-visible

Fig. 7 UV-visible diffuse reflectance spectra of as-synthesized (a);

calcined mesoporous MT-1 (b); and dye-doped mesoporous TiO2 MT-1-

RB (c).

3928 | J. Mater. Chem., 2011, 21, 3925–3930

diffuse reflection spectrum of calcined MT-1 shows an absorp-

tion band at 334 nm, which corresponds to a band gap energy of

3.16 eV, whereas for calcined MT-2 the observed band gap was

3.05 eV. The large blue shift in the band gap vis-�a-vis anatase

TiO2 could be related to nanoscale porosity.21 On the other hand,

after dye loading, the MT-1-RB sample showed multiple

absorptions centered at 299, 522 and 562 nm (Fig. 7). The last

two absorptions could be attributed to the RB molecule and

charge transfer bands due to the coordination of the phenolic-

OH donor sites of RB to the Ti centers. The absorption for this

sample is ca. 562 nm, which corresponds to a band gap energy of

2.2 eV. This value is comparable to the band gap energy of Rose

Bengal dye absorbed onto a nanocrystalline TiO2 film (1.7 eV).22

It is interesting to note that the TiO2 band is blue-shifted after

doping with RB (Fig. 7). Upon dye-doping, the pores (empty

spaces) are filled with RB. Possibly as a result of this, the TiO2

band is blue-shifted.23 Furthermore, the reduction in the band

gap in MT-1-RB could be due to ligand to metal charge transfer,

and this could be very helpful for photocurrent generation, as

upon photoexcitation the dye molecule can easily inject the

electrons to the conduction band24 of the self-assembled TiO2

nanoparticles.

In Fig. 8 we have proposed a schematic model for the

synthesis of self-assembled mesoporous TiO2 nanospheres.

Titanium(IV) isopropoxide vigorously hydrolyzes under the

synthesis conditions and ambient temperature causes rapid

growth of titania nanoparticles. Under the synthesis conditions,

titania nanoparticles are positively charged and could interact

with the negative carboxylate groups of sodium salicylate

molecules through electrostatic interactions, as shown in Fig. 8.

The presence of the ortho phenolic-OH group in the salicylate

molecule under mildly acidic synthesis conditions in turn helps

to form a supramolecular assembly among the ligated salicylate

moieties via hydrogen bonding and hydrophobic interactions.25

This supramolecular assembly of the salicylate molecules helps

to form the cage-like structure inside the TiO2 nanocrystals.

Upon calcination, the template salicylate moieties get removed

and the mesopores become quite open. Aggregation or even

Fig. 8 The proposed templating pathway for the synthesis of meso-

porous TiO2 using the self-assembly of salicylate anions.

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close-packing of the tiny TiO2 nanospheres can also result in

a mesoporous structure.26 Thus, TiO2 nanocrystallites self-

assemble, and their crystalline structure effectively sustains the

local strain caused during the mesophase formation. Upon

calcination, the supramolecular assembly of salicylate molecules

breaks from the TiO2 nanostructure. Contrary to the previously

reported templating pathways for surfactants or block-co-

polymers, where the interaction between the template molecules

and the frameworks were ionic or H-bonding only,12 in our case

covalent bonding between the salicylate molecules and TiO2

exist in the as-synthesized material. As a result, on calcination,

instead of pore contraction (as conventionally observed for

surfactants or block-co-polymers), pore expansion occurs (d-

spacing increased). The interparticle repulsion generated therein

thus stabilizes 12–20 nm large particles with 5–7 nm pores. It is

interesting to note that there is a large difference in the pore and

particle sizes of the MT-1 and MT-2 samples. The main reason

for this difference is their synthesis temperatures. For high

temperature synthesis, condensation and crystallization are

favored, leading to larger particles. Furthermore, the supra-

molecular assembly of salicylate molecules could be stabilized at

higher temperature, thus the size of the pore as well as the TiO2

nanospheres gets enhanced on increasing the synthesis temper-

ature.

The calcined mesoporous TiO2 MT-1, MT-2 and Rose Bengal

entrapped MT-1-RB and MT-2-RB samples were kept in the

dark for several hours before the dark currents were measured.

Linear dark current I–V characteristics, as shown in Fig. 9,

suggested Ohmic behavior of the gold contacts in the samples.

From Fig. 9, it is clear that the dark current is much higher for

the Rose Bengal entrapped mesoporous TiO2 compared to that

in the absence of the sensitizer. We have prepared crystalline

mesoporous TiO2 using a conventional CTAB template.27 In this

case the sample is calcined at 723 K (surface area 176 m2 g�1) and

RB dye is loaded into it following the similar procedure described

earlier. The observed photogenerated current, DI for this RB

entrapped crystalline mesoporous TiO2 sample is 7.6 � 10�7 A,

while that in the absence of dye is 1.1 � 10�7 A. Thus the

observed enhancement in the photocurrent due to dye loading in

this case is ca. 7 times only. The low enhancement in the

Fig. 9 Dark currents I–V plots of template-free MT-1(- -) and MT-2

(- -) (b) And after impregnating rose bengal in mesoporous TiO2 MT-1-

RB (- -) and MT-2-RB (- -).

This journal is ª The Royal Society of Chemistry 2011

photocurrent in crystalline mesoporous TiO2 synthesized by the

conventional procedure could be attributed to small doping of

the dye (again due to low surface to volume ratio) in the latter

case.

Fig. 10 shows the photocurrent transients of dye loaded and

unloaded mesoporous TiO2 samples with a 10 V bias. As soon as

visible light is shone on the samples, the current increases, indi-

cating that the samples are sensitive to the white light. The

maximum current value after visible light illumination of the

Rose Bengal (RB) entrapped MT-1-RB sample reaches 1.54 �10�6 A, while the value for the MT-1 samples was only 5.05 �10�8 A. The value of the photogenerated current DI (photocur-

rent minus dark current) for the RB entrapped mesoporous TiO2

sample is 1.2 � 10�6 A, while for MT-1 it is only 3.1 � 10�8 A.

This indicates a large change in the photocurrent generation,

which is about 38 times more in the dye-doped composite

compared to the pure mesoporous TiO2. The mesoporous TiO2

sample MT-2 showed similarly enhanced photoresponses on

doping with RB dye (Fig. 10). On illuminating the TiO2–RB

nanocomposite with light energy greater than that of its

bandgap, electron-hole pairs are generated at the surface. The

photogenerated electrons can easily be transferred from the

conduction band of RB to the conduction band of TiO2.28 Thus,

the RB dye acts as a visible light sensitizer. Furthermore, the

higher surface area of these semiconductor nanoparticles29 could

be responsible for higher photon-to-electron conversion effi-

ciency. Thus, tunneling of these electron-hole pairs through the

highly crystalline self-assembled mesoporous TiO2 nanoparticles

in the presence of the dye sensitizer occurred, leading to a high

photogenerated current.

Conclusions

From the above experimental results, we can conclude that self-

assembled mesoporous spherical TiO2 nanoparticles can be

synthesized hydrothermally using the supramolecular-assembly

of sodium salicylate as a template. Salicylate anions ligated with

the positively charged Ti(IV) centers through covalent interac-

tion, and H-bonding interactions between the phenolic-OH

groups help to form the supramolecular structure of salicylate

moieties during synthesis, which on calcination generate

Fig. 10 The growth/decay of photocurrent with time over calcined MT-

1 and MT-2 on white light illumination. Light on/off points of those

samples are shown in calcined MT-1 (- -), calcined MT-2 (- -), MT-1-

RB (- -) and MT-2-RB (- -).

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mesopores of 2.5–6.5 nm dimensions, depending upon the

synthesis conditions. The synthesis strategy described herein for

mesoporous TiO2 showed good shape and size-control, high

surface area and can be extended to other metal oxide systems.

Our results on the drastic enhancement in photoconductivity on

entrapping a photosensitizer molecule (dye) in the mesopores vis-

�a-vis in the absence of any photosensitizer could motivate

researchers to explore the possibilities of designing novel porous

materials using the templating pathway described herein and

their application in photon-to-electron conversion processes.

Acknowledgements

AKP and SKD thank CSIR, New Delhi for their senior research

fellowships.

Notes and references

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