二氧化钛的紫外拉曼光谱研究外文翻译

?

1

[

1

?

0

.8

84

（

A

ana

A

rut

）

]

where

A

ana and

A

rut represent the X

-

ray integrated intensities of

anatase (101) and

rutile (110) diffraction peaks, respectively.

To estimate the weight fraction of the rutile phase in the TiO

2

sample by Raman

spectroscopy, pure anatase phase and pure rutile phase of the TiO

2

sample, which have

been prepared by calcination of TiO

2

powder at 500 and 800 °C for 4 h, were

mechanically mixed at given weight ratio and ground carefully to mix sufficiently.

Figure 3A displays the visible Raman spectra of the mechanical mixture with 1:1,

1:5, 1:10, 1:15, 5:1, and 10:1 ratios of

anatase phase to rutile phase. The relationship

between the area ratios of the visible Raman band at 395 cm

-

1 for anatase phase to the

band

at

445

cm

-

1

for

rutile

phase

(

A

395

cm

-

1

/

A

445

cm

-

1

)

and

the

weight

ratios

of

anatase phase to rutile phase (

W

A

/

W

R

) is plotted in Figure 3B. It can be seen that a linear

relationship between the band area ratios and the weight ratios of anatase phase to rutile

phase in the mixture is obtained. The rutile content in the Degussa P25, which usually

consists of roughly

about 80% anatase and 20% rutile phase,

42

was estimated by

this

plot. Our Raman result indicates that the rutile content in

the Degussa P25 is about

18.7%, which is close to the known result. Thus, the above linear relationship based on

visible Raman spectroscopy can be used to estimate the rutile content in TiO

2

.

Figure 4A presents the UV Raman spectra of the mechanical

mixture with 1:1,

1:2, 1:4, 1:6, 1:10, and 1:15 ratios of anatase

phase to rutile phase with the excitation

line at 325 nm. Figure 4B shows the plot of the area ratios of the UV Raman band at

612 cm

-

1 for rutile phase to the band at 638 cm

-

1 for anatase

phase (

A

612 cm

-

1

/

A

cm

-

1

) versus the weight ratios of rutile phase to anatase phase (

W

R/

W

A). There is also a

linear relationship between the band area ratios and the weight ratios of rutile phase to

anatase phase.

3.3. Phase Transformation of TiO

2

at Elevated Calcination Temperatures.

3.3.1.

XRD

Patterns

and

Visible

Raman

Spectra

of

TiO

2

Calcined

at

Different

Temperatures.

Figure

5

shows

the

XRD

patterns

of

TiO

2

calcined

at

different

temperatures.

The

“A”

and

“R”

in

the

figure

denote

the

anatase

and

rutile

phases,

respectively. For the sample before calcination, diffraction peaks due to the crystalline

phase

are

not

observed,

suggesting

that

the

sample

is

still

in

the

amorphous

phase.

When the sample was calcined at 200 °C, weak and broad peaks at 2

?

=

25.5°, 37.9°,

48.2°,

53.8°,

and

55.0°

were

observed.

These

peaks

represent

the

indices

of

(101),

(004), (200), (105), and (211) planes of anatase phase, respectively.

[43]

These results

suggest that some portions of the amorphous phase transform

into the anatase phase.

The diffraction peaks due to anatase phase develop with increasing the temperature of

calcination. When the calcination temperature was increased to 500 °C, the diffraction

peaks due to anatase phase became narrow and intense in intensity. This indicates that

the crystallinity of the anatase phase is further improved.

[44]

When the sample was calcined at 550 °C, weak peaks were observed at 2

?

=27.6°,

36.1°, 41.2°, and 54.3°, which correspond to the indices of (110), (101), (111), and (211)

planes

of rutile phase.

[43]

This indicates that the anatase phase starts to

transform into

the rutile phase at 550 °C. The diffraction peaks of anatase phase gradually diminish in

intensity

and

the

diffraction

patterns

of

rutile

phase

become

predominant

with

the

calcination temperatures from 580 to 700 °C. These results clearly show that the phase

transformation

from

anatase

to

rutile

progressively

proceeds

at

the

elevated

temperatures.

The

diffraction

peaks

assigned

to

anatase

phase

disappear

at

750

°C,

indicating

that

the

anatase

phase

completely

changes

into

the

rutile

phase.

The

diffraction peaks of rutile phase became quite

strong and sharp after the sample was

calcined at 800 °C, owing to the high crystallinity of the rutile phase.

Figure

6

displays

the

visible

Raman

spectra

of

TiO

2

calcined

at

different

temperatures. For the sample before calcination, two broad bands at about 430 and 605

cm

-

1

are observed, indicating

that the sample is

in the amorphous phase.

[19]

For the

sample

calcined

at

200

°C,

a

Raman

band

at

143

cm

-

1

is

observed

and

the

high

-

frequency region

shows interference from the fluorescence

background, which

might

come

from

organic

species.

After

calcination

at

300

°C,

other

characteristic

bands of anatase phase appear at 195, 395, 515, and 638 cm

-

1

, but some portions of the

sample may exist in the amorphous phase because there is still a broad background in

Figure 6.

It

was

found

that,

when

the

sample

was

calcined

at

400

°C,

the

fluorescence

disappeared, possibly because the organic residues were removed by the oxidation. The

bands of anatase

phase increased in intensity and decreased in line width when

the

sample was calcined at 500 °C. This result suggests that the crystallinity of the anatase

phase is greatly improved,

[18]

which

is confirmed by XRD (Figure 5). The enlarged

section of Figure

6 shows the Raman spectrum of the high

-

frequency region of

the

sample calcined at 500 °C. Besides the bands at 395, 515, and 638 cm

-

1

, two very weak

bands

at

320

and

796

cm

-

1

are

observed.

These

two

bands

can

be

assigned

to

a

two

-

phonon scattering band and a first overtone of B1g at 396 cm

-

1

, respectively.

[37]

It

is noteworthy that a very weak band appears

at 445 cm

-

1

due to rutile phase for the

sample calcined at 550°C. This indicates that the anatase phase starts to change into the

rutile phase

at

550

°C.

This

result is

in

good agreement

with

that of XRD patterns

(Figure

5).

The

weight

percentage

of

the

rutile

phase

in

the

samples

calcined

at

different temperatures was estimated by visible Raman spectroscopy and XRD (shown

in Figure 7). As seen from Figure 7, the rutile content

estimated from visible Raman

spectrum

and

XRD

pattern

of

the

sample

calcined

at

550

°C

is

4.2%

and

5.7%,

respectively.

It

can

be

seen

that

the

rutile

contents

estimated

by

visible

Raman

spectroscopy and XRD are also in accordance with each other.

When the sample

was calcined

at

580

°C,

other two

characteristic bands were

observed at 235 and 612 cm

-

1 due to

Raman

-

active modes of rutile phase. Figure 7

shows the rutile content is 13.6% and 10.9% based on the visible Raman spectrum and

XRD pattern of the sample calcined at 580 °C.

The intensities of the bands of rutile

phase (235, 445, and 612

cm

-

1

) increased steadily while those of the bands of anatase

phase (195, 395, 515, and 638 cm

-

1

) decreased when the calcination temperatures were

elevated from 600 to 680 °C

(Figure 6). These results suggest that the TiO2 sample

undergoes the phase transformation from anatase to rutile gradually. The rutile content

was estimated for the samples calcined from 600 to 680 °C based on the visible Raman

spectra. The results show that the content of the rutile phase is increased from 33.1% to

91.2% respectively for the samples calcined at 600 and 680 °C

(Figure 7). The XRD

results

corresponding

to

the

above

two

samples

indicate

that

the

rutile

content

is

changed from 32.9%

to 90.7% (Figure 7). These results clearly show that the rutile

content in the sample estimated by visible Raman spectroscopy is agreement with that

estimated by XRD.

The

Raman

spectrum

of

the

sample

calcined

at

700

°C

shows

mainly

the

characteristic bands of rutile phase, but the very weak

bands of anatase phase are still

observed (Figure 6). When the

sample was calcined at 750 °C, the bands of anatase

phase disappeared and only the bands due to rutile phase (143, 235, 445, and 612 cm

-

1

)

were observed. These results indicate that the anatase phase completely transforms into

the rutile phase

and are consistent with the results from XRD (Figure 5). When

the

temperature

was

increased

to

800

°C,

the

characteristic

bands

due

to

rutile

phase

increased in intensity further.

Both the results of XRD and visible Raman spectra (Figures 5 and 6) show that

the anatase phase appears at around 200 °C

and perfect anatase phase is formed after

calcination at temperatures

of 400

-

500 °C. The rutile phase starts to form at 550

°C,

and the anatase phase completely transforms into the rutile phase at 750 °C. The signals

of visible Raman spectra come

mainly from the bulk region of TiO

2

because the TiO

2

sample

is

transparent

in

the

visible

region

(Figure

2).

[33]

XRD

is

known

as

a

bulk

-

sensitive method. Therefore, it is essentially in

agreement between the results of

visible Raman spectra and XRD patterns.

3.3.2. UV Raman Spectra of TiO

2

Calcined at Different

Temperatures.

UV

-

vis diffuse

reflectance spectra (Figure 2) clearly show that TiO

2

has strong electronic absorption in

the

UV region. Thus, the UV Raman spectra excited by a UV laser

line contain more

signal from the surface skin region than the bulk of the TiO

2

sample because the signal

from the bulk is attenuated sharply due to the strong absorption.

[33]

Therefore, if

a UV laser line in the absorption region of TiO

2

is used as the

excitation source of

Raman spectroscopy, the information from

UV Raman spectra is often different from

that of visible Raman spectra.

The laser line at 325 nm was selected as the excitation source

of the UV Raman

spectra. The UV Raman spectra and the content of the rutile phase of the TiO

2

sample

calcined at different temperatures are shown in parts A and B, respectively, of Figure 8.

When the sample was calcined at 200 or 300 °C,

the Raman band at 143 cm

-

1

with a

shoulder

band

at

195

cm

-

1

and

three

broad

bands

at

395,

515,

and

638

cm

-

1

were

observed, indicating that the anatase phase is formed in the sample. However, the low

intensity

and

the

broad

band

indicate

that

the

amorphous

phase

still

remains

in

the

sample. It can be seen that the fluorescence in the high

-

frequency region can be avoided

when

the

UV

laser

line

is

used

as

the

excitation

line.

However,

the

corresponding

visible Raman spectra (Figure 6) show interference from the fluorescence.

All bands assigned to anatase phase become sharp and strong

after calcination at

500 °C (Figure 8A). These results are in

agreement with those of XRD and visible

Raman

spectra

(Figures

5

and

6).

The

UV

Raman

spectra

of

the

sample

with

the

calcination temperatures from 550 to 680 °C are essentially

the same as those of the

sample calcined at 500 °C (Figure 8A).

However, according to the XRD patterns and

visible Raman spectra (Figures 5 and 6), the anatase phase starts to transform into the

rutile phase at only 550 °C and the anatase phase gradually changes into the rutile phase

in the temperature range of 550

-

680 °C.

After calcination at 700 °C, a new band at 612 cm

-

1 and two weak bands at 235

and 445 cm

-

1 due to rutile phase appear

while the intensities of the bands of anatase

phase begin to decrease (Figure 8A). On the basis of the UV Raman spectrum and XRD

pattern of the sample calcined at 700 °C, the rutile content in the sample is 56.1% and

97.0%, respectively (Figure

8B). It is found that the rutile content estimated by UV

Raman spectroscopy is far less than that estimated by XRD.

When the sample was calcined at 750 °C, the intensities of the bands due to rutile

phase increased, but the intensities of the bands due to anatase phase were still strong in

the UV Raman spectra (Figure 8A). The UV Raman spectrum of the sample calcined at

750 °C indicates that the rutile content is

84.3% (Figure 8B). However, the results of

XRD and visible

Raman spectrum (Figures 5 and 6) suggest that the anatase phase

totally transformed into the rutile phase after the sample was

calcined at 750 °C. The

characteristic bands due to anatase phase disappear and the sample is in the rutile phase

after calcination at 800 °C (Figure 8A). Obviously, there are distinct differences

between

the

results

from

the

UV

Raman

spectra,

visible

Raman

spectra,

and

XRD

patterns. It seems that the anatase phase remains at relatively higher temperatures when

detected by UV Raman spectroscopy than by XRD and visible Raman spectroscopy.

Another UV laser line at 244 nm was also selected as the excitation source of UV

Raman spectroscopy in order to get

further insights into the phase transformation of

TiO

2

. The results of the UV Raman spectra of TiO

2

calcined at different

temperatures

with the excitation line at 244 nm are presented

in Figure 9. When the sample was

calcined at 200 °C, four broad

bands were observed at 143, 395, 515, and 638 cm

-

1,

which clearly indicate that the anatase phase exists in the sample. The intensities of the

Raman bands due to anatase phase (143, 395, 515, and 638 cm

-

1

) become strong after

calcination at 500 °C. The UV Raman spectra hardly change for the sample calcined at

different temperatures even up to 680 °C. The characteristic bands (445 and 612 cm

-

1

)

of

rutile

phase

appear

only

when

the

calcination

temperature

exceeds

700

°C.

This

result

is

in

good

agreement

with

that

from

the

UV

Raman

spectrum

of

the

sample

calcined at 700 °C with 325 nm excitation (Figure 8A). The intensities of the bands due

to anatase phase (395, 515,

and 638 cm

-

1

) decrease while those of bands assigned to

rutile

phase (445 and 612 cm

-

1

) increase after calcination at 750 °C

(Figure 9). When

the sample was calcined at 800 °C, the Raman

bands due to anatase phase disappeared, while the bands of rutile phase developed. This

result indicates that the sample calcined at 800 °C is in the rutile phase. It is interesting

to note that the results of the UV Raman spectra with the excitation lines at

325 and 244 nm are in agreement with each other but are different from those of XRD

patterns and visible Raman spectra.

3.3.3.

TEM

of

the

TiO

2

Sample

Calcined

at

Different

Temperatures.

TEM

was

used to characterize the microstructure

of the TiO

2

sample calcined at 500, 600, and

800 °C (shown in

Figure 10). Most particles in the sample calcined at 500 °C

exhibit

diameters in a range between 10 and 30 nm (Figure 10a). On the other hand, remarkable

agglomeration is observed

for the TiO

2

sample calcined at 500 °C. The particle size

increases after calcination at 600 °C (Figure 10b). According to the results of XRD and

visible

Raman

spectra

(Figures

5

and

6),

the

sample

undergoes

the

phase

transformation from anatase to rutile gradually in the temperature range of 550

-

680 °C. These results imply that the phase transformation and

growth of the particle

size are interrelated. Many researchers

[45

-

49]

reported similar phenomena. Kumar et al.

45

attributed

this

particle

size

growth

to

the

higher

atomic

mobility

because

of

bond

breakage

during

the

phase

transformation.

When

the

calcination

temperature

was

increased to 800 °C, TiO

2

particles further grew and the particle size could be as large

as about 200 nm (Figure 10c).

3.4.

Visible

Raman

Spectra

and

UV

Raman

Spectra

of

La

2

O

3< /p>

/TiO

2

with

Increasing

La

2

O

3

Loading.

The

lite rature

[15,17,29]

proposed

the

mechanism

that

the

phase

transformation

of

TiO

2

might

start

at

the

interfaces

of

contacting

anatase

particles.

If

direct

contact

between

anatase

particles

of

TiO

2

is

avoided,

the

phase

transformation

of

TiO

2

from

anatase

to

rutile

could

be

retarded

or

prohibited.

This

assumption may be verified by covering the surface of anatase TiO

2

with an additive. In

this work, we used La

2

O

3

dispersed on anatase TiO

2

to prevent

the anatase particles

from contacting directly because it was reported that La

2

O

3

could be highly dispersed

on anatase Ti O

2

.

[34

-

36]

Figure

11A

displays

the

visible

Raman

spectra

of

La

2

O

3

/TiO

2

with

increasing

La

2

O

3

loading. TiO

2

support is in the anatase

phase because only characteristic bands

(143, 195, 395, 515, and 638 cm

-

1

) due to anatase phase are observed. When the TiO

2

support was calcined at 900 °C (TiO

2

-

900), the Raman spectrum gave the characteristic

bands of rutile phase, indicating that the TiO

2

-

900 sample was in the rutile phase. When

the sample with 0.5 wt % La

2

O

3

loading was calcined at 900 °C, the Raman spectrum

drastically changed, as compared to the TiO

2

-

900 sample. Only characteristic bands of

anatase phase were observed, suggesting that the TiO

2

sample retains its anatase phase

when La

2

O

3

loading is 0.5 wt % while the TiO

2

-

900 sample is in the rutile phase. The

spectra of the samples

with La

2

O

3

loadings from 1 to 6 wt % are almost the same as

those

for

the

sample

with

0.5

wt

%

La

2

O

3

loading,

except

for

a

small

decrease

in

intensity.

Figure 11B shows the UV Raman spectra of La2O3/TiO

2

with

increasing La

2

O

3

loading. The Raman spectrum of the TiO

2

support and the TiO

2

-

900 sample gives the

characteristic

bands

of

anatase

phase

and

rutile

phase,

respectively.

When

La

2

O

3

loading is varied from 0.5 to 6 wt %, the results of the UV Raman spectra show that all

the samples are in

the anatase

phase. These results are in

accordance with those of

visible Raman spectra (Figure 11A). The results from XRD patterns (the XRD patterns

for the above samples are not shown) agree

well with those of visible Raman spectra

and UV Raman spectra.

For both visible Raman spectra and UV Raman spectra, no

bands due to crystalline phase of La

2

O

3

are observed for all the

La

2

O

3

/TiO

2

samples,

showing that La

2

O

3

is highly dispersed

on the surface of the anatase phase of TiO

2

particles.

[35]

4. Discussion

It has been clearly shown that the results of visible Raman

spectra (Figure 6) are in

good agreement with the results from

XRD patterns (Figure 5) but are different from

those

of

UV

Raman

spectra

(Figures

8A

and

9)

for

TiO

2

calcined

at

elevated

temperatures.

The

inconsistency

in

the

results

from

the

three

techniques

can

be

explained by the fact that UV Raman

spectroscopy provides the information mainly

from

the

surface

region

while

visible

Raman

spectroscopy

and

XRD

provide

information from the bulk of TiO

2

. The discrepant results of UV Raman spectra, visible

Raman spectra, and XRD patterns are attributed to their different detectable depths for

TiO

2

particles.

[33]

The

disagreements

of

UV

Raman

spectra,

visible

Raman

spectra,

and

XRD

patterns suggest that the crystal phase in the surface region is different from that in the

bulk during the phase transformation of TiO

2

. Busca et al.

[20]

characterized the Degussa

P25

using

Fourier

transform

Raman

spectroscopy

and

XRD

technology.

They

estimated the rutile

-

to

-

anata se ratio by Raman

spectroscopy and XRD, and found that

the ratio estimated by Raman spectroscopy for the Degussa P25 was smaller than that

evaluated by XRD. Therefore, they assumed that the rutile phase

in the Degussa P25

was more concentrated in the bulk because

Raman spectroscopy excited by a near

-

IR

laser

line

should

be

more

surface

-

sensitive

than

XRD.

Different

from

their

investigations,

our experimental results give direct UV Raman

evidence to show that

the phases in the surface region are

generally different from that in the bulk region of

TiO

2

, particularly when TiO

2

is in the transition stage of the phase transformation.

As

presented

above,

the

anatase

phase

can

remain

at

relatively

higher

temperatures

as

observed

by

UV

Raman

spectroscopy

than

by

visible

Raman

spectroscopy

and

XRD.

These

facts

lead

us

to

the

conclusion

that

the

phase

transformation of TiO

2

takes place from its bulk region and then extends to its surface

region.

To further understand the process of the phase transformation of TiO

2

, we used the

TEM

technique

to

observe

the

microstructure

of

the

sample

calcined

at

different

temperatures

(Figure

10).

TEM

results

show

that

the

samples

are

composed

of

aggregated particles after the calcination. According to TEM measurements, Penn et al.

[17]

suggested

that

structural

elements

with

rutilelike

character

can

be

produced

at

a

subset of anatase interfaces, and these might serve as rutile nucleation sites. Lee et al.

[29]

investigated the growth and transformation in nanometersize

TiO

2

powders by in situ

TEM. The nucleation of rutile was found to occur at the amorphous interface of anatase

particles

where

there

are

strain

and

disorder.

On

the

basis

of

the

experimental

observations

and

combined

with

the

results

from

the

literature,

we

suggest

that

the

phase transformation of TiO

2

starts from the interfaces among the anatase particles of

the agglomerated TiO

2

particles. On the basis of kinetic data from XRD, Zhang et al.

15

Also

proposed

that

interface

nucleation

dominated

the

transformation

for

nanocrystalline anatase samples

with denser particle packing below 620 °C, or in the

temperature range of 620

-

680 °C.

If the direct contact between anatase particles is avoided, the phase transformation

could be retarded or prohibited because

the rutile phase nucleates at the interfaces of

contacting

anatase

particles.

We

prepared

the

anatase

particles

covered

with

highly

dispersed La

2

O

3

, and the results of visible Raman spectra and UV Raman spectra of the

La

2