solvent-controlled synthesis of tio2 1d nanostructures growth mechanism and characterization

2a–c shows the SEM images of the samples S1–S3, respectively, which reveal that the reaction of TiO2precursor with highly alkaline aqueous solution and highly alkaline mixed solvents gav

Solvent-controlled synthesis of TiO 2 1D nanostructures: Growth mechanism and characterization

Kajari Das  , Subhendu K Panda, Subhadra Chaudhuri

Department of Materials Science and DST Unit of Nanoscience, Indian Association for the Cultivation of Science, Raja S C Mallick Road, Kolkata, WB 700 032, India

a r t i c l e i n f o

Article history:

Received 18 February 2008

Received in revised form

19 May 2008

Accepted 26 May 2008

Communicated by K Nakajima

Available online 3 July 2008

PACS:

78.67.Ch

Keywords:

A1 Growth models

A2 Growth from solutions

B1 Nanomaterials

B2 Semiconducting materials

a b s t r a c t

One-dimensional (1D) anatase TiO2nanostructures such as nanorods, nanowires and nanotubes with different aspect ratios were synthesized by a simple solvothermal process The influence of the different organic solvents and the reaction time on the morphology, size and the formation of the nanostructures were investigated The anatase TiO2precursor powder reacted with highly alkaline aqueous solution, yielding layered sodium titanate nanosheets These nanosheets transformed to different 1D sodium titanates nanostructures like nanorods, nanowires and nanotubes in the different solvents i.e highly alkaline aqueous solution, highly alkaline water–ethanol and highly alkaline water–ethylene glycol mixed solvent, respectively Acid treatment of these 1D sodium titanates resulted hydrated titanates and finally dehydration by calcinations at 500 1C in air gave the products retaining the morphology The synthesized samples were characterized with XRD, SEM and TEM All the 1D nanostructures showed intense and sharp absorption spectra indicated that the products were almost defect free Photoluminescence studies of the nanostructures showed photostable UV emission properties that arise from the band-edge free excitation

&2008 Elsevier B.V All rights reserved

1 Introduction

In the past few years, the design and fabrication of the

nanostructured semiconductors based on metal oxides have

received considerable attention due to their interesting physical

and chemical properties, and their potential applications in

industry and technology[1–5] TiO2is an n-type wide band-gap

oxide semiconductor used for variety of applications such as

dye-sensitized solar cell, environmental purification, nanodevices, gas

sensors, and photocatalysts [6–13] Many techniques such as

sol–gel processing with electrophoretic deposition, spin-on

process, sol–gel template method, metalorganic chemical vapor

deposition, anodic oxidative hydrolysis, sonochemical synthesis,

inverse microemulsion method, and molten salt-assisted pyrolysis

routes have been developed to synthesize different TiO2

nanos-tructures [14–21] Recently, the reaction between different TiO2

precursors and a concentrated NaOH solution under moderate

hydrothermal method[22]is observed to be an effective approach

to prepare 1D nanostructures of titania However, the main

attention is directed towards the control over structure and

morphology of titania only by varying the reaction temperature,

reaction time and pH of the solution during hydrothermal

treatment, while an important experimental parameter, solvent, has rarely been deliberately controlled to achieve different 1D nanostructures of titania Solvothermal process is the most useful technique to synthesize nanocrystalline materials with different morphologies and sizes where properties of the solvents like density, viscosity and diffusion coefficient change dramatically and the solvent behaves much differently from that expected at the normal conditions Consequently, the solubility, diffusion process and the chemical reactivity of the reactants are greatly enhanced The detailed studies of the effect of different organic solvents on the morphologies of the TiO2 nanocrystals under solvothermal conditions have not been reported so far

In this paper we have synthesized phase pure anatase TiO2in different nanoforms such as single-crystalline nanorods, nano-wires and nanotubes using a solvothermal route and investigate the effects of the different solvents and reaction time on the shape, size, and the optical properties of the nanostructures

2 Experimental section

All the reactants and the solvents were of analytical grade and were used without further purification In a typical procedure,

274 mg of pure anatase phase TiO2bulk-powder was mixed with

10 N NaOH (19.2 g of NaOH in 48 ml water) aqueous solution of

pH ¼ 12.77 under constant magnetic stirring for 1 h A milky

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Journal of Crystal Growth

0022-0248/$ - see front matter & 2008 Elsevier B.V All rights reserved.



Corresponding author Tel.: +91 33 2473 4971; fax: +91 33 2473 2805.

E-mail address: kajari26jul@rediffmail.com (K Das).

white solution was appeared, which was then transferred to a

Teflon-lined stainless steel autoclave with 60 ml capacity and heat

treated at 150 1C for 16 h The sample obtained was denoted by S1

Likewise, to investigate the effect of the co-solvent on the

morphology and the properties of TiO2nanostructures, 19.2 g of

NaOH was dissolved in 48 ml mixed solvent of water and an

organic solvent (ethylene glycol or ethanol or polyethylene

glycol-300 (PEG-glycol-300)) with a volume ratio 1:1 274 mg TiO2 precursor

powder was then mixed with the highly alkaline solvent mixer

under constant magnetic stirring for 1 h and the solution was heat

treated at 150 1C for 16 h in the autoclave The samples obtained

were named as S2, S3, and S4, respectively (Table 1) Since the

solubility of NaOH decreases in the organic solvents than the

water, the pH values of the mixed solvents were less than that of

pure alkaline aqueous solution For all the cases, the autoclave

chambers were air-cooled to room temperature after the

reac-tions The formed white precipitates were recovered by

centrifu-gation and washed several times with distilled water An

ultrasonic treatment of the products with 0.1 N HCl solution was

carried out at pH7, and the precipitates were finally calcinated at

500 1C for 4 h in air To investigate the formation mechanism of

the different nanostructures, some additional experiments were

also carried out at 150 1C for different time intervals, i.e for

30 min, 1, 8, 16 and 24 h in pure alkaline aqueous solution and for

30 min, 1, 8 and 16 h in different alkaline mixed solvents

The crystalline phases of the products were determined by

X-ray powder diffraction by using a Seifert 3000P diffractometer

with Cu Ka radiation (l ¼ 1.54 A˚) The morphologies of the

samples were studied by a scanning electron microscope (SEM;

Hitachi S-2300) Microstructural properties were obtained using

transmission electron microscope and high-resolution

transmis-sion electron microscope (TEM and HRTEM; JEOL 2010) For the

TEM observations, the powders were dispersed in 2-propanol and

ultrasonicated for 15 min A few drops of this ultrasonicated

solution were taken on a carbon-coated copper grid FTIR spectra

recorded in the range 4000–400 cm1 with a Shimadzu model

FTIR spectrometer using KBr wafer Optical absorbance of the

samples was recorded by a UV–vis–NIR spectrophotometer

(Hitachi, U-3410) Photoluminescence (PL) measurements were

carried out at room temperature with a Fluorescence

spectro-meter (F-2500) using 310 nm excitation wavelength

3 Results and discussion

The crystal structure, morphology and size of the synthesized

products determined by XRD, SEM, and TEM are listed inTable 1

Fig 1(a) shows the XRD pattern of all the samples, which revealed

ARTICLE IN PRESS

S4

S3

S2

S1

( 004)

FWHMS1 = 0.465° FWHMS2 = 0.485° FWHMS3 = 0.519° FWHMS4 = 0.586°

III

II

I

#

#

#

#

#

#

#

#

#H2Ti3O7

#

#

#

#

#

#

*Na2Ti3O7

*

*

*

*

2θ (in degree)

2θ (in degree)

Fig 1 XRD spectra of (a) samples S1–S4 synthesized in the different solvents and (b) the TiO 2 precursor powder (I), synthesized intermediate products washing with water (II) and obtained after ultrasonic acid treatment (III).

Table 1

The effect of experimental parameters on the morphologies, sizes and band gaps of

the TiO 2 nanoforms

Sample

name

Solvent Reaction

temperature (1C)

Reaction time (h)

Morphology Size (nm) Band

gap (eV)

L ¼ 120–170

3.69

ethylene

glycol

(1:1)

150 16 Nanotubes D ¼ 11,

L ¼ 100–140

3.67

ethanol

(1:1)

150 16 Ultralong

nanowires

D ¼ 4,

L ¼ several micrometers

3.73

PEG-300 (1:1)

K Das et al / Journal of Crystal Growth 310 (2008) 3792–3799 3793

that irrespective of the variation of the solvents, all the peaks

corresponding to the reflections from (1 0 1), (0 0 4), (2 0 0), (1 0 5),

(2 11), and (2 0 4) planes of anatase tetragonal TiO2 were

observed, which are well matched with the standard reported

values (JCPDS file No 21-1272) The (1 0 1) peaks of all the samples

were fitted to the Gaussian curves and the FWHM (full-width at

half-maximum) were estimated, as mentioned inFig 1(a) It is

noted that the FWHM were decreased in the order

S1oS2o-S3oS4, which may be attributed to the improvement of

crystal-linity of the samples in the order S14S24S34S4.Fig 1(b) shows

the XRD patterns of the TiO2precursor and the two intermediate

products obtained after washing the as synthesized products with

water and ultrasonicating with HCl The spectrum II indicates the

presence of the Na2Ti3O7 and H2Ti3O7 in the products washing

with water After treatment with 0.1 N HCl, the products were

completely transformed to the hydrated titanate (H2Ti3O7) by

substitution of the Na+by H+, which is clearly revealed from the

XRD spectrum III These two intermediate phases were appeared

for all the samples Final products were obtained after calcination

of the hydrated titanates at 500 1C for 4 h

The growth process of the different 1D TiO2 nanostructures

was monitored by SEM and TEM Fig 2(a–c) shows the SEM

images of the samples S1–S3, respectively, which reveal that the

reaction of TiO2precursor with highly alkaline aqueous solution

and highly alkaline mixed solvents gave 1D nanostructures

Although the shapes of these 1D nanostructures are not clear by

the SEM images, the TEM images later clearly show

nanostruc-tures’ morphologies Fig 2(d) shows the SEM image of the 1D

nanostructures obtained in highly alkaline aqueous solution at

150 1C for 24 h By comparing the images in Fig 2(a) and (d), it

may be concluded that the diameters of the 1D nanostructures

obtained in highly alkaline aqueous solution decrease with

increasing reaction time

Fig 3(a–c) shows the low magnification TEM images of the

TiO2 samples obtained in highly alkaline aqueous solutions at

150 1C for 8, 16 and 24 h, respectively, which clearly reveal that the

diameters of the nanorods decreased, whereas the lengths of the rods increased with the increasing reaction time Ultimately, the nanorods transformed to nanowires when the reaction time was greater than 24 h The possible explanation of this transformation

is given later in the formation mechanism part InFig 3(d), the low magnification TEM image of the sample S2 gives the nanotubes having inner diameter 6 nm and outer diameter

11 nm; and length 100–140 nm.Fig 3(e) shows the HRTEM image

of a nanotube The number of the walls counted from the two sides of the tubes was not identical Moreover, the wall thickness generally varied along the tube The left inset inFig 3(e) is an enlarged picture of the tube wall as marked by the white circle The periodicity of the fine fringes was 0.37 nm, which indicates the (1 0 1) plane of the anatase TiO2 The interspacing of the tube walls was 0.77 nm Crystal structures of the TiO2 nanorods (sample S1) were also studied through HRTEM, which is shown

inFig 3(f) The fringes parallel to the nanorod axis correspond to

an interplanar distance of about 0.35 nm, which is characteristic

of (1 0 1) plane of TiO2 in the anatase phase The clear lattice fringes confirm the nanorods are single-crystalline and defect free The inset ofFig 3(f) shows the FFT pattern of the nanorod, which also indicates the (1 0 1) plane of the anatase TiO2 and confirms the single-crystalline nature of the nanorod.Fig 3(g) shows the low-magnification TEM image of the ultralong nanowires of sample S3.Fig 3(h) gives the TEM image of the nanoparticles of

16 nm obtained for sample S4 The inset ofFig 3(h) is the HRTEM image of a nanoparticle showing lattice fringes (d ¼ 0.342 nm) corresponding to the (1 0 1) lattice plane of the anatase TiO2 The diameters and lengths of the different nanostructures calculated from TEM images are shown in Table 1 SEM and TEM images revealed that in all the solvents except PEG-300, 1D nanostruc-tures of different morphologies such as nanorods, nanowires and nanotubes with different diameters and lengths were formed

To understand the formation of the different 1D nanostruc-tures controlled experiments were carried out and the products at different time intervals were examined SEM images of the

Fig 2 SEM micrographs for (a–c) the samples S1, S2 and S3, respectively and (d) the product obtained in highly alkaline aqueous solution at 150 1C for 24 h.

intermediate products at different time intervals are shown in

Fig 4 The reaction of the TiO2precursor with the highly alkaline

aqueous solution at room temperature gave single layered titanate

nanosheets Fig 4(a) shows the single layered nanosheets and

some unreacted TiO2particles At high temperature (150 1C) and

pressure in autoclave, the splitting of the nanosheets occurred

within 30 min of the reaction, which is clearly shown inFig 4(b)

Fig 4(c) reveals that the formation of the network of the nanorods

began with the further splitted nanosheets at 150 1C for 1 h

Finally, the crystalline anatase nanorods were obtained; when the

reaction time was 8 h as shown in Fig 4(d) The bended

multilayered titanate nanosheets were obtained when TiO2

precursor reacted with the highly alkaline water–ethylene glycol

mixed solvent at room temperatures, shown in Fig 4(e) The

splitting and simultaneous rolling of these bended multilayered

nanosheets occurred at 150 1C for 30 min, which is clearly shown

in Fig 4(f) Fig 4(g) shows that the product obtained for the

reaction time 1 h gave nanotubes (arrow I) coexisting with

splitted and bended multilayered nanosheets (arrow II), left inset

of the figure shows the enlarged picture of the agglomerated

nanotubes; but the product for 8 h (Fig 4(h)) gave complete formation of the nanotubes Ultimately, well crystalline nanotubes were observed for the reaction time 16 h In Fig 4(i), the SEM image of the nanoparticles obtained in the highly alkaline water–PEG-300 mixed solvent at room temperature indicates that no sheet-like morphology was formed in this mixed solvent

Fig 5 shows the FTIR spectra of the samples S1–S4 All the spectra show broad bands at 3435 and 1635 cm1, which correspond to the presence of the OH group and water, absorbed

on the surface of the TiO2 samples [23–25] In addition, for samples S2–S4, two bands at 2860 and 2931 cm1are observed, which are attributed to the symmetric and asymmetric CH2

stretching vibrations, respectively [26], coming from the co-solvents The peak at 2348 cm1 was due to CO2 and was not related to the samples Also a band appears around 453 cm1for all the samples, which corresponds to a Ti–O band of anatase phase of TiO2[27,28] The intensity of this band increases in the order S14S24S34S4, revealing that the crystallinity of the products improve in this order, which is in good agreement with the XRD and TEM observations of the samples

ARTICLE IN PRESS

Fig 3 Low-resolution TEM images of the (a–c) TiO 2 samples obtained in highly alkaline aqueous solution at 150 1C for 8, 16 and 24 h, respectively, and (d) TiO 2 nanotubes (sample S2); (e) HRTEM image of the sample S2 and the inset shows the enlarged picture of the tube wall; (f) HRTEM image of sample S1 and the inset shows the FFT pattern; (g) low-resolution TEM image of the sample S3; (h) low-resolution TEM image of sample S4 and the inset shows the HRTEM image of a nanoparticle.

K Das et al / Journal of Crystal Growth 310 (2008) 3792–3799 3795

The growth process toward the formation of 1D nanostructures can be explained from the microstructural and crystallographic evidence.Fig 6is the simplified schematic model showing the different stages for the growth of different TiO2 nanostructures TiO6octahedron is the basic unit of the crystalline structure of the anatase TiO2.The three-dimensional framework of the oxide is build up by sharing the vertice edges of the octrahedra When TiO2

precursor reacts with highly alkaline NaOH solution, some of the Ti–O–Ti bonds of the raw material are broken and layered titanates composed of octahedral TiO6units with Na+metal ions are formed in the form of thin small sheets The formation of the intermediate nanosheets of sodium titanate (Na2Ti3O7) phase is confirmed by XRD spectrum and SEM image The titanate and anatase have common structural features: both crystal lattices consist of the octahedral sharing four edges and the zigzag ribbons[29] The strength of Na–O bonding is weaker than that of Ti–O bonding in Na2Ti3O7 layered structure These Na–O bonds may break at the high temperature and pressure in the autoclave and the single layered nanosheets split to form the nanorods As the reaction time increases, further splitting of the nanosheets may increase the aspect ratio of the nanorods and hence after

24 h reaction transformation of the nanorods to nanowires was observed After washing with water and ultrasonic acid treatment, hydrated titanates (H2Ti3O7) are formed by the substitution of Na+by H+, which is revealed by the XRD The final anatase TiO2 products are obtained when dehydration of the hydrated titanates are occurred at the time of the calcinations of the products at 500 1C for 4 h[30] The SEM images of the sodium

Fig 4 SEM micrographs for (a–d) the intermediate products obtained in highly alkaline aqueous solution at room temperature and at 150 1C for 30 min, 1 and 8 h, respectively; (e–h) the intermediate products in highly alkaline water–ethylene glycol mixed solvent at room temperature and at 150 1C for 30 min, 1 h, 8 h, respectively and inset of (g) shows the enlarged picture of the agglomerated nanotubes; (i) the intermediate product in highly alkaline water-PEG-300 mixed solvent at room temperature.

S1

S4 S3 S2

1635 2860

2931

Wave number (cm-1) 3435

and hydrated titanates also indicate that the morphology of the

intermediate products was almost same as the final anatase TiO2

products

It is well known that the solvent plays an important role to

control the morphology of the product In our experiments, the

polarity and coordinating ability of a co-solvent have strong effect

on the solubility, reactivity, and diffusion behavior of the

reactants, thus ultimately influencing the structure and

morpho-logical features of the resulting products Zhang et al [31]

considered that the bending of the multilayered titanate

nanosheets occurred in highly alkaline aqueous solution at

130 1C due to the imbalance of ion concentration on two different

sides of the nanosheets in an asymmetrical chemical

environ-ment In our experiment, when ethylene glycol was used as the

co-solvent with highly alkaline aqueous solution, multilayered

sodium titanate nanosheets were formed due to the chelating

property of ethylene glycol TiO6octrahedra may coordinate with

glycol to form chain-like structures[32], whereas the NaOH may

form titanate nanosheets by sharing the vertice edges of the

octrahedra The negative charge of the titanate layers coordinated

with glycol on the side underneath the surface is neutralized by

Na+ in the interlayer space The Na+ on the top surface of the

layers may undergo frequent collision with OHfrom the solution

The electronegativity of the titanate layers coordinated with

glycol is greater than that of titanate layers in normal alkaline

aqueous solution The Na+deficiency on the upper surface of the

multilayered titanates coordinated with glycol bends the

na-nosheets to minimize the excess surface energy The bending of

the multilayered nanosheets is confirmed by the SEM image At

high temperature and pressure in the autoclave, the splitting and

simultaneous rolling of the sheets give nanotube structures

Another reason for the rolling of the splitted multilayered

nanosheets at high temperature may be due to the mechanical

tensions that arise during the process of

dissolution/crystal-lization in nanosheets [33] Due to chelating property of the

ethylene glycol, the slow nucleation rate and the very fast growth

rate of the crystal may favor the rolling process of the sheets During spontaneous crystallization and rapid growth of layers, it

is possible that the width of the different layers varies, which gives rise to excess surface energy In order to decrease the excess surface energy, the rolling of splitted nanosheets occurs Bavykin

et al [34]described that the nanotubes were formed in highly alkaline aqueous solutions at low temperature (110–150 1C) and nanofibres were formed at the temperature 150 1C These nanotubes were transformed to the nanorods as calcinated at

500 1C These observations revealed that the nanotube structures are thermally unstable than the nanorods and nanofibres In our present work, the nanotubes were obtained at 150 1C using ethylene glycol as the co-solvent and the structures remained unchanged after calcination at 500 1C for 4 h, which indicate that the nanotubes formed by the ethylene glycol assisted method are more thermally stable than the normal alkaline aqueous solution The formation of ultralong nanowires may arise from the very fast growth rate and the rapid splitting of the sodium titanate nanosheets in highly alkaline water–ethanol mixed solvent The rapid growth rate and the fast splitting of the nanosheets may cause various defects in the crystals, which were clearly revealed from the optical absorbance and photoluminescence studies discussed later For the co-solvent PEG-300, the intermediate products were investigated and in this case no sheet-like morphology was observed and finally nanoparticles of 16 nm were obtained The SEM image shown in Fig 4(i) confirms the presence of nanoparticles instead of the nanosheets Pre-viously, Zhu et al.[35]reported the formation of TiO2mesoporous structures using PEG-200 as the solvent in a hydrothermal method, which destroyed and grains were appeared when the reactions time was increased Also, due to the presence

of the inorganic polymer, the degree of the crystallization was observed to be poor One-dimensional nanostructures were not formed, when we used only pure organic solvent without alkaline aqueous solution Thus it may be concluded that in highly alkaline aqueous solution formation of the 1D

ARTICLE IN PRESS

Highly

alkakline

aqueous

solution

Nanosheets &

unreacted particles

High temperature

& pressure

High temperature

& pressure Bended multilayered nanosheets

& unreacted particles

Nanosheets &

unreacted particles

Highly alkaline water-EG mixed soluiton

Highly alkaline

water-ethanol

mixed soluiton

TiO2 bulk

powder

nanosheets

Reaction time > 8h

Reaction time > 30 min

Reaction time > 24h

Nanorods

Nanowires

Reaction time > 16h

Nanotubes Splitting & simultaneous rolling

of the multilayred nanosheets High

temperature &

pressure Splitting

Ultra long nanowires Fig 6 Schematic diagram showing formation of different TiO 2 nanostructures under different synthesis conditions.

K Das et al / Journal of Crystal Growth 310 (2008) 3792–3799 3797

nanostructures takes place and the organic solvents used with

highly alkaline aqueous solution only control the shapes of the 1D

nanostructures

Fig 7(a) shows the optical absorption spectra of the samples

S1–S4 prepared in different solvents It is interesting to note that

the 1D nanostructures show intense and sharp absorption spectra

indicating the formed 1D nanostructures were almost defect free

and high crystalline in nature The spectra also show that the

sharpness of the absorption peaks near the band edge increased in

the order S14S24S34S4, indicates that well crystalline products

were obtained using water and water–ethylene glycol mixed

solvent.Fig 7(b) shows the differential absorbance spectra (dA/dl

versus l) of the TiO2 precursor powder and the sample S1 The

band gaps of all the other samples were also determined from

differential absorbance spectra, which are shown inTable 1 The

band gaps of the TiO2 nanostructures were varied from 3.63 to

3.73 eV, which are slightly blue shifted from the bulk value of

anatase phase of the TiO2 (3.21 eV) may be due to the nanosize

effect

The room temperature PL spectra of all the samples were recorded with 310 nm excitation.Fig 8shows the PL spectra of all the samples The signal at 372 nm could be attributed to emission peak from band edge free excitation[36] The red shift takes place for the samples S3 and S4, which may be due to the oxygen vacancies[37] In addition, another broad band between

400 and 510 nm was observed The origin of this broad band was already reported by Daude and co-workers [38] The lowest energy allowed phonon-assisted transitions of the anatase TiO2

from center to the edge of the Brillouin zone are the indirect transitions, namely G1b-X2b (406 nm) and G1b-X1a (426 nm) The emissions at 455 and 504 nm are due to the transitions from intragap energy levels implicating lattice defects and oxygen vacancy The PL results also reveal that the nanostructures showed photostable UV emission properties and also clearly indicate that the almost defect free, well crystalline TiO2samples were prepared at 150 1C using highly alkaline aqueous solution and highly alkaline water–ethylene glycol mixed solvent

4 Conclusions

Different 1D nanostructures such as nanorods, nanowires, and nanotubes of TiO2in anatase phase were synthesized by a simple solvothermal method We have demonstrated the control over the structures, sizes and morphologies of the products by controlling the reaction temperature, time and by using different mixed solvents In such synthesis, the reaction of the anatase TiO2

precursor powder with highly alkaline aqueous solution produced layered sodium titanate nanosheets at room temperature The splitting of the single layered nanosheets at high temperature and pressure in the autoclave gave the nanorods As the reaction time increases, further splitting of the nanosheets may increase the aspect ratio of the nanorods and hence after 24 h reaction transformation of the nanorods to nanowires was observed When ethylene glycol was used as the co-solvent with highly alkaline aqueous solution, multilayered sodium titanate na-nosheets were formed at room temperature and the splitting and simultaneous rolling of these sheets gave nanotube structures

at high temperature and pressure in the autoclave The ultralong nanowires were obtained in highly alkaline water–ethanol mixed solvent, whereas nanoparticles were formed in highly alkaline water–PEG-300 mixed solvent A plausible mechanism for the

S1 S2 S3 S4

TiO2 precursor

3.21eV 3.69eV

S1

300

λ (nm)

700

300

λ (nm)

700

Fig 7 (a) Optical absorbance spectra of all the samples, (b) differential absorbance

spectra (dA/dl versus l) of the TiO 2 precursor powder and the sample S1.

S4 S3

S1 S2

λ (nm)

Fig 8 Photoluminescence spectra of all the samples.

formation of different 1D nanostructures has been proposed The

final anatase TiO2 products were obtained by dehydration of

the hydrated titanates at 500 1C in air, where retention of the

morphologies of the products was observed during the phase

transformation TEM and SEM observations revealed that the

solvent is the most crucial factor in determining the morphologies

of the products and the reaction time is the most influential factor

in controlling the diameters and lengths of the products Due to

the phase purity and well crystalline nature of the TiO2 1D

nanostructures, they will be certainly applicable in the fields of

photocatalysis, electrocatalysis, lithium batteries, hydrogen

sto-rage, and solar-cell technologies

Acknowledgments

This paper is dedicated to the memory of Prof Subhadra

Chaudhuri whose continuous support and effective guidance have

made this work possible The authors thank Mr K K Das of IACS

for recording the SEM micrographs

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