New hydrothermal process for hierarchical TiO2 nanostructures

Arun Kumar Sinha, Subhra Jana, Surojit Pande, Sougata Sarkar, Mukul Pradhan, Mrinmoyee Basu, Sandip Saha, Anjali Pal and Tarasankar Pal*

Electronic Supplementary Information

ESI 1 Schematic representation for the formation of different shapes of TiO2

nanocrystals

R-H+

(R)4Ti+4

R-[Ti(O2)(OH)(H2O)n]+

Aqueous [Ti(O2)(OH)(H2O)n]+

Pyridine or Triethylamine

100 w bulb

H2O

100 w bulb

Pyridine or Triethylamine

Pyridine or Triethylamine

100 w bulb

Pyridine or Triethylamine

100 w bulb

H2O

100 w bulb

Nanoflower on rough glass surface

Nanopetal

Nanopillar

Nanopetal on smooth glass surface

Nanorod

Nanobundle30% H2O2

5% H2SO4

100 w bulb

Ti(SO4)2

- R-

R-H+

(R)4Ti+4

R-[Ti(O2)(OH)(H2O)n]+

Aqueous [Ti(O2)(OH)(H2O)n]+

Pyridine or Triethylamine

100 w bulb

H2O

100 w bulb

Pyridine or Triethylamine

Pyridine or Triethylamine

100 w bulb

Pyridine or Triethylamine

100 w bulb

H2O

100 w bulb

Nanoflower on rough glass surface

Nanopetal

Nanopillar

Nanopetal on smooth glass surface

Nanorod

Nanobundle30% H2O2

5% H2SO4

100 w bulb

Ti(SO4)2

- R-

ESI 2 Schematic representation of the set up for hydrothermal (MHT) reaction

Wooden Box

Test tube Holder

Screw Caped Test tube

100 Watt W-BulbThermometer

Stabilizer

In-put

Out-put

Wooden Box

Test tube Holder

Screw Caped Test tube

100 Watt W-BulbThermometer

Wooden Box

Test tube Holder

Screw Caped Test tube

100 Watt W-BulbThermometer

Stabilizer

In-put

Out-put

Physical parameters of simple set up

1. Screw capped test tube: (a) Length 10.5 cm

(b) Diameter 1.5 cm

(c) Volume 15 mL

2. Pressure: Autogenic created inside the screw capped test-tube

3. Temperature: 80º-90ºc

4. Wooden box:(a) Length (6×6 inch)

(b) Height 7 inch

5. Voltage of electric source: 220v

Physical parameters of simple set up

1. Screw capped test tube: (a) Length 10.5 cm

(b) Diameter 1.5 cm

(c) Volume 15 mL

2. Pressure: Autogenic created inside the screw capped test-tube

3. Temperature: 80º-90ºc

4. Wooden box:(a) Length (6×6 inch)

(b) Height 7 inch

5. Voltage of electric source: 220v

Screw capped test tubeScrew capped test tube

ESI 3 Photograph of TiO2 nanocrystals formation on glass surface at the air-water

interface

Reaction equation:

, 100 W bulb

30 hrs

, 100 W bulb

30 hrs

R-[Ti(O2)(OH)(H2O)n]+

TiO2

KHSO4Ti(SO4)2

R-H+

(R-)4Ti+4

30% H2O2

5% H2SO4

[Ti(O2)(OH)(H2O)n]+

R-H+

Pyridine

100W bulbGlass slide&

TiO2 Thin film

+

R-[Ti(O2)(OH)(H2O)n]+

TiO2

KHSO4Ti(SO4)2

R-H+

(R-)4Ti+4

30% H2O2

5% H2SO4

[Ti(O2)(OH)(H2O)n]+

R-H+

Pyridine

100W bulbGlass slide& 100W bulbGlass slide&

TiO2 Thin film

+

ESI 4 Photograph of TiO2 nanocrystals formation on resin surface at the solid-

liquid interface

Reaction equation:

30 hrs

Δ, 100 W bulb

30 hrs

Δ, 100 W bulb

R-H+ Ti(SO4)2 (R-)4Ti+4

(R-)4Ti+4 + 30% H2O2 R-[Ti(O2)(H2O)n]+

R-[Ti(O2)(H2O)n]+H2O

Or Triethylamine/Pyridine

R-[TiO2]

R-H+ Ti(SO4)2 (R-)4Ti+4

(R-)4Ti+4 + 30% H2O2 R-[Ti(O2)(H2O)n]+

R-[Ti(O2)(H2O)n]+H2O

Or Triethylamine/Pyridine

R-[TiO2]

+

ESI 5 Experimental Section Materials and instruments

All the reagents were of AR grade. Triple distilled water was used throughout the

experiment. Titanium dioxide was obtained from Loba-chemie Indoaustranal Co. Sulfuric

acid was purchased from E-Merck. SERALITE-SRC-120 standard grade strongly

cationic exchange resin (ion exchange capacity 4.5 meg/g dry resin), pyridine,

triethylamine and KHSO4 were obtained from Sisco Research Laboratories. Pt crucible

was purchased from Sigma Aldrich. All the reagents were used without further

purification. Glass slides and screw capped test tube were purchased from Blue Star India

Ltd. and were cleaned prior to thin-film formation.

All UV–Visible absorption spectra were recorded in a SPECTRASCAN UV 2600

digital spectrophotometer (Chemito, India). XRD was done in a PW1710 diffractometer,

a Philips, Holland, instrument. The XRD data were analyzed using JCPDS software.

Raman spectra of TiO2 nanocrystals were obtained with a Renishaw Raman Microscope,

equipped with a He–Ne laser excitation source emitting at a wavelength of 633 nm, and a

Peltier cooled (-70 °C) charge coupled device (CCD) camera. A Leica microscope with

20× objective lens was used. The holographic grating with 1800 grooves/mm and the 1

cm-1 slit enabled the spectral resolution. Laser power at the sample was 12 mW and the

data acquisition time was 30 sec. FESEM analysis was performed with a supra 40, Carl

Zeiss Pvt. Ltd instrument and an EDAX machine (Oxford link and ISIS 300) attached to

the instrument was used to obtain the nanocrystal morphology and composition. TEM

analysis was performed with an instrument H-9000 NAR, Hitachi, using an accelerating

voltage of 300 kV.

Preparation of Ti(SO4)2 and Titanium peroxo compound:

Titanic sulfate was prepared by fusing commercial titanium dioxide powder with

~12 fold excess of KHSO4. The clear melt was cooled, powdered and was extracted with

cold dilute sulfuric acid (5%) and then filtered. Next, titanium(IV) precursor ions (0.58

mM) were allowed to exchange with H+ ions of the neat acidic form of the cation-

exchange resin beads (R-H+) with occasional shaking and was kept overnight. The resin

beads on which titanium precursor ions were immobilized, were washed several times

with water to drain out the un-exchanged Ti(IV) species. Now, 30% H2O2 was added to

the resin beads and all the resin beads turned orange. Then the orange colored resin beads

were washed with copious amount of water for several times to remove excess H2O2.

Pure titanium peroxo compound (devoid of H2O2) was leached out from resin bead with

5% H2SO4 and the aqueous solution of peroxo compound was used for MHT reactions.

ESI 6a UV-Visible absorption spectra of TiO2 nanoflowers as film on quartz slide

surface after repetitive (a to c) deposition. The film bearing quartz slides were

immersed in the aqueous titanium peroxo solution and were subjected to MHT

reaction in presence of pyridine and subsequently analyzed in the

spectrophotometer placing the quartz slides in the cuvette

400 500 600 700 8000.0

0.3

0.6

0.9

1.2c

b

a

Abs

orba

nce

(a.u

.)

Wavelength (nm)

ESI 6b Preparation of colloidal suspension of TiO2 nanocrystals from resin beads

for UV-Visible study

Under sonication (~10 min) TiO2 nanocrystals leach out in ethanol from the resin

beads and form a sol. The sol has been exploited to study the optical property of TiO2

nanocrystals in suspension by UV-Visible absorption spectroscopy. All absorption

spectra were recorded in a Shimadzu UV-160 spectrophotometer (Kyoto, Japan) using a

1 cm quartz cuvette.

UV-Visible absorption spectra of TiO2 nanocrystals (a) nanopillars (b) nanorods (c) nanopetals and (d) nanobundles.

240 300 360 420 4800.0

0.6

1.2

1.8

2.4

d

c

b

a

Abs

orba

nce

(a.u

.)

Wavelength (nm)

ESI 7 Band position and band gap energy value of different TiO2 nanocrystals

Different shape of TiO2 nanocrystal

Band position (nm) Band gap energy value (eV)

Flower 327 3.23

Pillar 302 3.24

Rod 310 3.22

Petal 342 2.82

Bundle 246 ----

(αE p

)2

2.0 2.5 3.0 3.5 4.0-1

0

1

2

3

4

5

6

7

Band Gap= 2.84 ev

Ep

TiO2 nanopetal

2.0 2.5 3.0 3.50

10

20

30

40

Band gap = 3.23 ev

(αE

p)2

Ep

TiO2 nanoflower

Band Gap = 3.22 ev

2.0 2.5 3.0 3.5 4.0 4.5 5.005

10152025303540

(αE p

)2

Ep

TiO2 nanorod

TiO2 nanopillar

Band gap = 3.24 ev

(αE p

)2

2.0 2.5 3.0 3.5 4.0-1

0

1

2

3

4

5

6

7

Band Gap= 2.84 ev

Ep

(αE p

)2

2.0 2.5 3.0 3.5 4.0-1

0

1

2

3

4

5

6

7

Band Gap= 2.84 ev

Ep

TiO2 nanopetal

2.0 2.5 3.0 3.50

10

20

30

40

Band gap = 3.23 ev

(αE

p)2

Ep

TiO2 nanoflower

Band Gap = 3.22 ev

2.0 2.5 3.0 3.5 4.0 4.5 5.005

10152025303540

(αE p

)2

Ep

TiO2 nanorod

TiO2 nanopillar

Band gap = 3.24 ev

ESI 8 XRD pattern of TiO2 nanocrystals for (a) flowers (b) flowers after heat

treatment at 450 °C (c) rods (d) petals (e) pillars and (f) bundles

20 30 40 50 600

50

100

150

200b

Inte

nsity

(a.u

.)

2θ20 30 40 50 60

0

40

80

120

160a

Inte

nsity

(a.u

.)

2θ

20 30 40 50 600

120

240

360

480 c

Inte

nsity

(a.u

.)

2θ20 30 40 50 60

0

70

140

210

280d

Inte

nsity

(a.u

.)

2θ

20 30 40 50 600

40

80

120

160 e

Inte

nsity

(a.u

.)

2θ20 30 40 50 60

0

35

70

105

140 f

Inte

nsity

(a.u

.)

2θ

20 30 40 50 600

50

100

150

200b

Inte

nsity

(a.u

.)

2θ20 30 40 50 60

0

40

80

120

160a

Inte

nsity

(a.u

.)

2θ

20 30 40 50 600

120

240

360

480 c

Inte

nsity

(a.u

.)

2θ20 30 40 50 60

0

70

140

210

280d

Inte

nsity

(a.u

.)

2θ

20 30 40 50 600

40

80

120

160 e

Inte

nsity

(a.u

.)

2θ20 30 40 50 60

0

35

70

105

140 f

Inte

nsity

(a.u

.)

2θ

ESI 9 Raman spectra of TiO2 nanocrystals for (a) flowers (b) flowers at 450 °C (c)

rods and (d) petals

Raman spectroscopy helps us to know the different phases of the as-deposited

titania film before and after subsequent heat treatment. Rutile TiO2 is tetragonal and

belongs to D4h14 point group symmetry. The Raman peaks at 440 (for Eg mode) and 608

cm-1 (for A1g mode) (see the ESI 5a) are attributed to the rutile phase of TiO2 nanocrystals

deposited on glass surface. After subsequent heat treatment at 450 °C for 5 h, the titania

nanocrystals produce crystalline anatase phase retaining the signature of the rutile phase.

Annealed sample show peaks at 397 and 515 cm-1 which corresponds to the anatase phase

of TiO2 (see the ESI 5b). The Raman peaks at 444 and 607 cm-1 of TiO2 nanorods and

nanopetals, isolated from resin beads by sonication, correspond to rutile phase of TiO2

nanocrystals (see the ESI 53c, d). Isolated nanobundles and nanopillars do not show any

characteristic Raman band. However, the microcrystallinity of TiO2 nanobundles and

nanopillars has been authenticated from XRD analysis.

300 400 500 600 7000

6000

12000

18000

24000 a

Inte

nsity

(a.u

.)

Raman shift (cm-1)300 450 600 750

0

15000

30000

45000b

Inte

nsity

(a.u

.)

Raman shift(cm-1)

360 480 600 720

160000

180000

200000

220000

Inte

nsity

(a.u

.)

c

Raman shift (cm-1)300 450 600 750

40000

48000

56000

64000

Inte

nsity

(a.u

.)

d

Raman shift (cm-1)

300 400 500 600 7000

6000

12000

18000

24000 a

Inte

nsity

(a.u

.)

Raman shift (cm-1)300 450 600 750

0

15000

30000

45000b

Inte

nsity

(a.u

.)

Raman shift(cm-1)

360 480 600 720

160000

180000

200000

220000

Inte

nsity

(a.u

.)

c

Raman shift (cm-1)300 450 600 750

40000

48000

56000

64000

Inte

nsity

(a.u

.)

d

Raman shift (cm-1)

ESI 10 FESEM images to represent the step-wise formation of TiO2 nanoflowers at

the point of surface defect

ESI 11 TEM images of (a) nanoflowers (b) nanopetals (c) fringe spacing of TiO2

nanocrystals on glass surface and (d) spherical nanoparticles of the mother liquor

containing the decomposed peroxo compound and pyridine

d

a b

c d

ESI 12 TEM images of TiO2 nanorods at (a) low magnification (b) high

magnification and (c) fringe spacing of TiO2 nanocrystals on resin surface

a b

(110)

(001)

c

ESI 13 (a) TEM (b) HRTEM and (c) fringe spacing of TiO2 nanopetals on resin

surface

a b

(001) (110)

c

ESI 14 TEM images of (a) nanopillars and (b) nanobundles of TiO2 nanocrystals on

resin surface

ba

ESI 15 FESEM image of TiO2 nanoflowers on glass surface at the point of

deliberately created surface defect by HF etching for flower formation