Spectroscopic study of TiO2 (rutile)–benzophenone heterogeneous systems

Spectroscopic study of TiO2 (rutile)–benzophenone heterogeneous

systems

T. Bezrodnaa,*, T. Gavrilkoa, G. Puchkovskaa, V. Shimanovskaa, J. Baranb,M. Marchewkab

aInstitute of Physics, National Academy of Sciences of Ukraine, Pr. Nauki 46, 03028 Kyiv, UkrainebInstitute of Low Temperature and Structure Research, PAS, 2 Okolna Str., 50-950 Wroclaw, Poland

Received 15 December 2001; revised 20 February 2002; accepted 20 February 2002

Abstract

Heterogeneous systems based on TiO2 rutile powders and benzophenone (BP) as a dispersive medium have been investigated

by Raman and IR spectroscopy. Rutile particles have been obtained by thermal hydrolysis of titanium tetrachloride

hydrochloric acid solutions and subsequent temperature treatment at 573 K (Ru300) and 1173 K (Ru900). Loading with BP

results in changes of TiO2 polarizability. Porous and defective surface of Ru300 induces disordering to BP crystal which

becomes glassy-like. BP molecules form hydrogen bonds with the TiO2 surface active centers (Ti–OH) via phenyl ring p-

electron systems. In adsorbing on TiO2 particles, the BP displaces adsorbed water molecules. These changes in structure and

molecular interactions are reflected in the corresponding Raman and IR spectra which allow to compare two heterogeneous

systems (BP-Ru300 and BP-Ru900). q 2002 Elsevier Science B.V. All rights reserved.

Keywords: Benzophenone; TiO2 rutile particles; Raman and IR spectroscopy; Surface centers

1. Introduction

In recent years, titanium dioxide (TiO2) has

attracted considerable attention since it is known to

have strong activity as a photocatalyst which decom-

poses a large number of undesirable chemical

pollutants upon ultraviolet irradiation [1–3]. Besides,

this material is relatively cheap and can efficiently

operate at room temperature in a broad range of

sunlight spectrum. Specific applications of TiO2

crystalline particles are determined by their chemical,

structural and physical properties. In particular, its

surface state, the nature and concentration of surface

active centers and defects are considered to play

important roles. Point defects at the surface may act as

adsorption sites for certain molecules [4]. These

properties can be modified by the technological

procedure used in TiO2 synthesis.

Many methods, such as sol–gel and tetrabutyl

titanate hydrolysis processes, gas condensation tech-

nique, etc., have been successfully developed to

prepare titanium dioxide particles. Their microstruc-

tures, anatase–rutile structural transformation, and

lattice dynamics were studied by X-ray diffraction, IR

and Raman spectroscopy, respectively [5,6]. The

experimental results showed that the production

techniques and the processing conditions have a

strong influence on the microstructures and electronic

0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.

PII: S0 02 2 -2 86 0 (0 2) 00 2 66 -1

Journal of Molecular Structure 614 (2002) 315–324

www.elsevier.com/locate/molstruc

* Corresponding author. Tel.: þ380-44-265-1552; fax: þ380-44-

265-1589.

E-mail address: [email protected] (T. Bezrodna).

http://www.elsevier.com/locate/molstruc

and vibrational properties of these materials. How-

ever, it is not well understood yet what factors are

responsible for TiO2 reaction ability in different

chemical processes [7,8].

In the present paper, we use benzophenone

(C6H5)2CO (BP) as a dispersive medium. In our

heterogeneous systems based on rutile particles this

substance acts as a specific probe, which allows to

investigate the surface state of a solid and the

interactions occurred between molecules and different

surface active centers. BP itself has some interesting

characteristics (considerable piezoelectric effect, high

quantum yield of phosphorescence, conversion of

100% from the excited singlet state to the triplet one,

good chemical activity in hydrogen-containing sol-

vents) and its vibrational spectra have been repeatedly

investigated and interpreted before [9–11]. This

substance can exist in two crystal modifications, a-

stable and b-unstable, with the melting temperatures

of 322 and 300 K, respectively. The amorphous glass-

like phase can be created after abrupt cooling of the

BP melt and observed below 216 K [12]. Dipole

moment of BP molecules is about 1.9 D and

concentrated mainly on the CyO group.

The aim of this work is to investigate hetero-

geneous systems based on TiO2 (rutile) crystalline

powders loaded into benzophenone. These complex

materials possess a large area of the liquid–solid

surface boundary and so they are suitable for studying

surface phenomena. We used methods of Raman and

IR vibrational spectroscopy which are proved to be

particularly successful for identification of local

surroundings and changes in molecular interactions

and phase structure, being a powerful tool for

understanding processes of interactions between BP

molecules and active centers located on the solid

surface of TiO2 particles.

2. Experimental

Titanium dioxide is known to exist in three

crystalline modifications: rutile (tetragonal), brookite

(orthorhombic) and anatase (tetragonal). For our

experiments we selected TiO2 powders with rutile

crystalline constructions of particles, which have been

produced with a high degree of the chemical purity

(quantity of coloring admixtures—Fe, Co, Cr, Cu, Ni,

Mn, V—does not exceed 1 £ 1025 mass%) [13]. The

TiO2 preparation method includes thermal hydrolysis

of titanium tetrachloride (TiCl4) hydrochloric acid

solutions [14]. The synthesis is carried out under

normal pressure and temperature of 373 K in the

presence of specially prepared titanium nuclei which

are added to control a process of crystal growth.

Herewith, polycrystalline sediment—hydrated tita-

nium dioxide of different phase compositions and

hydrochloric acid, is obtained according to the

following scheme:

TiCl4 þ 3H2O ! TiOðOHÞ2 þ 4HCl

TiOðOHÞ2 ! TiO2 þ H2O

This technique allows to change physical–chemical

features of TiO2 materials—their phase composition,

the size and construction of particles, their surface

state, porous structure and other properties by

adjusting the synthesis conditions. Produced TiO2

particles had a spherical shape with a typical size of

10–30 mm in diameter and consisted of grown

together microcrystallite aggregates. Their rutile

structure has been confirmed by X-ray phase analysis

(DRON-2, Cu Ka radiation).

The samples used were additionally annealed in

air at different temperatures—at 573 K (Ru300)

and 1173 K (Ru900). Applying thermal treatment

to titanium dioxide results not only in the phase

transition from anatase to rutile, but also in the

occurrence of some structure changes that are

characteristic for the crystalline powder agglom-

eration: diffusion processes associated with the

increased mobility of lattice defects at high

temperatures; changes of oxygen vacancies in

TiO2 lattice, etc. [15,16]. Moreover, changes in

titanium dioxide porous structure, specific surface,

its surface chemistry (in the type and quantity of

surface sorption centers, i.e. in the hydroxide

covering) take place. In increasing the processing

temperature from 573 to 1173 K we observed that

the sizes of rutile crystallites also increased from

20 nm (Ru300) to 60 nm (Ru900) as was revealed

by the particle-size analysis. According to the

benzene adsorption data Ru300 sample was a

porous one with a ’free’ space value of

0.04 cm3 g21 and a specific surface area of

95 m2 g21, while Ru900 was non-porous with its

T. Bezrodna et al. / Journal of Molecular Structure 614 (2002) 315–324316

surface area of about 3 m2 g21. Besides Ru900

is considered to be more stoichiometric (its

Ti/O , 1.998) [13].

Benzophenone belongs to the class of aromatic

ketones. BP crystals (a-phase) were obtained by

means of the multiple zone melting method. Their

purity was controlled by the presence of exciton

phosphorescence. Heterogeneous systems were

obtained by loading TiO2 particles of chosen crystal-

line structure (Ru300 and Ru900) into BP melt,

mixing and subsequent temperature decreasing to the

room one.

Raman and IR absorption spectra have been

recorded and analyzed for the pure materials (TiO2

Fig. 1. Raman spectra of (1) benzophenone, (2) TiO2 powders and (3) BP–TiO2 heterogeneous systems in the cases of: (a) Ru300; (b) Ru900.

T. Bezrodna et al. / Journal of Molecular Structure 614 (2002) 315–324 317

and BP) and for their heterogeneous systems (BP-

Ru300 and BP-Ru900).

FT-Raman spectra were measured at room tem-

perature with Bruker IFS-88 spectrometer equipped

with the FRA-106 attachment (lexc ¼ 1.06 mm of

Nd:YAG laser with a power of 300 mW) in a region of

4000–80 cm21. IR absorption spectra for all samples

have been measured by the same Bruker IFS-88

spectrometer at room temperature in a region of

4000–400 cm21. Pure TiO2 powders have been

suspended in Nujol. The spectral slit width was

2 cm21 and the number of scan was set to 32.

Graphical separation and analysis of spectral

parameters for complex absorption bands have been

carried out with the help of PEAKFIT program.

3. Results and discussion

The unit cell of the TiO2 rutile structure (space

group D4h14(P42/mnm )) contains two TiO2 molecules in

the unit cell. From a group theoretical analysis for

rutile [17] it can be shown that 15 fundamental modes

belong to the following irreducible representations:

A1g þ A2g þ A2u þ B1g þ B2g þ 2B1u þ Eg þ 3Eu.

The theory reveals that four modes (A1g þ B1g þ

B2g þ Eg) are Raman active and other four modes

(A2u þ 3Eu) are infrared active. The other three

modes, A2g þ 2B1u, are neither Raman active nor

infrared active.

3.1. FT-Raman spectroscopy

The Raman spectrum of rutile was first recorded by

Narayanan [18]. Since then, a number of papers have

appeared in the literature concerning the lattice

dynamics of the rutile and the assignment of the

observed Raman bands for this structure [19,20].

There are some disagreements among different

authors concerning phonon frequencies and sym-

metries. They arise mainly due to the relatively strong

two phonon processes in TiO2.

Raman spectra of pure TiO2 powders (Ru300 and

Ru900) and BP, and Raman spectra of the BP-Ru300

and BP-Ru900 heterogeneous systems are presented

in Fig. 1. Our rutile spectra are in good agreement

with the spectra of TiO2 rutile single crystals [17,20]

and the powders obtained by slow evaporation of

tetraisopropyl titanate solution [21]. Moreover, it has

been shown that the powder spectrum of the rutile

does not change on heating to 1200 K and above [21].

Rutile Raman spectra at room temperature exhibit the

bands at 142, 235, 446 and 609 cm21 for both Ru300

and Ru900 (see Fig. 1). The bands at 142, 446,

609 cm21 can be assigned as B1g, Eg and A1g modes,

respectively [20,21]. There are several explanations

for the origin of a broad band observed at about

235 cm21. It was considered as a fundamental mode

[22], combination band [20], lattice disorder induced

band [23] or second-order band.

Fig. 1 shows that intensities of two strong TiO2

Fig. 2. Fragments of the Raman spectra for (1) benzophenone and (2) BP-Ru300 heterogeneous system.


Fig. 3. Fragments of the IR spectra in the phenyl ring deformation region for (a) benzophenone, (b) BP-Ru300 system and (c) BP-Ru900 system.


bands (446 and 609 cm21) significantly change after

loading TiO2 particles into benzophenone. Their

detailed analysis demonstrated that the intensity

redistribution occurs (from I446/I609 , 1 in pure

TiO2 powders to ,2 in both heterogeneous systems

BP-Ru300 and BP-Ru900, where I is an intensity of

the corresponding band at the constant half-widths).

The integrated intensity of these bands (I446 þ I609) is

considerably larger in the heterogeneous systems in

comparison to that of the 235 cm21 band, which

remains unaffected. This can be attributed to the

changes of polarizability of the TiO2 when BP

molecules approach the TiO2 surface. Moreover, the

same effect is observed in the TiO2 heterogeneous

systems with 5CB (4-pentyl-40-cyanobiphenyl) liquid

crystal. Those results will be presented elsewhere.

As it has been shown before [9], several vibrational

bands of the crystalline benzophenone are split

(,8 cm21), particularly in the regions of 80–150,

1170–1200, 3030–3090 cm21. This can result from

the resonance Davydov interaction of BP molecules,

because there are four translationally non-equivalent

molecules in the BP crystal cell. The BP melting is

accompanied with the disappearing of these splittings.

So there is only one band observed in each of the

mentioned spectral regions for the BP liquid [9]. The

similar Raman band splittings have been found in our

spectra for the benzophenone (Figs. 1 and 2a, c (curve

1)).

It should be noted that for the BP-Ru300 system

these Raman band splittings become small and in

some cases are not observed at all (Figs. 1 (curve 3)

and 2a,c (curve 2)). But for the BP-Ru900 system we

have the contrary situation. There is no difference

between BP bands in the spectrum of the crystal and

those ones in the spectrum of this heterogeneous

system. It can be explained in the following way.

Ru300 has a very defective surface due to the high

porosity of its TiO2 particles. These imperfections

destroy the crystalline alignment of BP near the TiO2

surface. At that benzophenone loses its long-range

ordering and becomes glassy-like. This conclusion is

in a perfect agreement with the results obtained for the

benzophenone confined into the porous glass which

was investigated by IR and phosphorescence spec-

troscopy [24]. In this disordered glassy BP state the

interaction between CyO groups of the neighboring

molecules becomes different due to the changes in

their positions. This results in the appearance of a new

band (1656 cm21) near the 1650 cm21 band which

corresponds to CyO stretching vibrations (Fig. 2b).

Besides, the hydration level of Ru300 is approxi-

mately by an order of magnitude higher than that of

Ru900 (see IR spectra below). Adsorbed water

molecules can also induce a disorder to the BP

structure.

Unlike Ru300 powder, Ru900 has a relatively

smooth surface and considerably greater size of its

rutile crystallites. Its hydration and a number of

surface defects are significantly smaller. So the

surface of Ru900 particles does not much affect the

BP structure. BP molecules layer on the particle

surface creating a covering ‘coat’ with the ordered

structure. And this reflects in the corresponding

Raman spectra for the BP-Ru900 complex system.

3.2. FT-IR spectroscopy

IR spectra of TiO2 rutile samples are characterized

by a broad strong band with transmittance minima at

722, 590, 525 and 471 cm21 assigned to the vibrations

of Ti–O and Ti–O–Ti framework bonds [25,26]. It

has been shown that the TiO2 particle shape,

aggregation state of the microcrystals and its surface,

temperature treatment have a big effect on IR spectra

of titanium dioxide. In our heterogeneous systems of

TiO2 particles loaded to BP we observed sharp and

intensive peaks corresponding to BP molecules. Some

of these bands were imposed on TiO2 broad bands,

which have approximately the same shape as in the

case of pure rutile samples.

The IR spectrum of BP was investigated and

interpreted before [9,27,28]. When this substance was

added to the rutile powders, we found that the most

significant changes are observed in the regions

corresponding to phenyl ring vibration bands. Fig. 3

shows the spectral region of 685–720 cm21 where the

phenyl ring deformation bands are presented. For the

pure benzophenone (Fig. 3a) the bands at 694 and

696 cm21 can be attributed to out-of-plane aromatic

ring g, R(CC) vibrations of the CC bonds; the bands at

706, 711 cm21—to out-of-plane r(CH) vibrations,

phenyl out-of-plane g(CC) and changes in g(CCC)

angles of the aromatic rings. In the heterogeneous

systems BP-Ru300 (Fig. 3b) and BP-Ru900 (Fig. 3c)

this spectral region considerably changes. New bands


(689 and 700 cm21 in the case of the BP-Ru300

system; 689 and 702 cm21 in the BP-Ru900) appear

in this region. These new bands have clearly different

spectral parameters (their intensity and width on a

half-maximum) for the two heterogeneous systems.

Fig. 4 demonstrates another two spectral regions

where we detected the intensity redistribution. The

bands at 991, 998 and 1002 cm21 correspond to

changes in phenyl angles g(CCC), out-of-plane r(CH)

and stretching n(CC) vibrations of aromatic rings.

Upon loading BP into TiO2 rutile particles, the

intensity of 1002 cm21 band significantly increases,

especially in the case of Ru900 powder (from I998/

I1002 , 3:2 for the bulk BP to ,3:4 for the BP-Ru900

heterogeneous system). These changes are seen in Fig.

4a. Considering another spectral region assigned to

stretching Q(CC) and deformation b(CCH), g(CCC)

vibrations (Fig. 4b), one can notice slight intensity

increasing for 1161 cm21 band. It is interesting to

note, that all mentioned changes for phenyl ring

vibration bands are clearly more pronounced for the

BP-Ru900 heterogeneous system. Most probable, this

is due to the fact that concentration of the adsorbed

water molecules on the surface and within pores of the

sample treated at lower temperature (Ru300 sample)

is higher, and therefore this causes a higher screening

effect on the interaction between the BP molecules

and TiO2 surface active centers. This interaction is

less seen in corresponding IR spectra for the BP-

Ru300 heterogeneous system. Besides, the Ru300

particles have a very defective surface structure which

simultaneously includes two charged oxygen

vacancies and internodal Ti atoms which have an

effective charge of þ3 or þ4. These defects assist in

water adsorption being specific surface sorption

centers for adsorbed molecules.

IR spectra in the range of the OH stretching

vibration bands are presented in Fig. 5a (for the Ru300

sample and the corresponding BP-Ru300 system) and

Fig. 5b (for the Ru900 and the BP-Ru900 system).

Graphical separation of these complex bands in the

pure TiO2 samples allows to select two main

components at about 3200, 3430 cm21 for both

Ru300 and Ru900, respectively. The bands at

3200 cm21 are observed in the absorption region of

liquid water. All authors assigned them to polymole-

cularly adsorbed and capillary condensed water

molecules which are hydrogen bonded with each

other, i.e. their associations (H2O· · ·H2O)n [29]. The

Ru300 and Ru900 samples have different amounts of

adsorbed water molecules, which is reflected in three

times lower intensity of this band in the Ru900 TiO2

powder than that in the Ru300 TiO2 spectrum. From

the IR spectra shown for both BP-Ru300 and BP-

Ru900 heterogeneous systems it is apparent that BP

molecules displace molecular water from the surface.

In these spectra ,3200 cm21 band practically

disappears. Jones and Hockey [30] derived the same

Fig. 4. Fragments of the IR spectra in the phenyl ring CC and CH vibration region for (1) benzophenone, (2) BP-Ru300 system and (3) BP-

Ru900 system.


conclusion when they studied the pyridine molecules

adsorbed on the rutile surface.

The broad band at ,3430 cm21 is usually

attributed to the –OH stretching vibrations of water

molecules, which take part in forming hydrogen

bonds on the surface H2O· · ·H2O and H2O· · ·HO–Ti

[29,30]. Its intensity also decreases with the high

temperature treatment, i.e. its intensity is lower for the

Fig. 5. The IR spectra in the OH-groups stretching vibration region for (1) pure TiO2 powders and (2) BP–TiO2 heterogeneous systems in the

cases of: (a) Ru300; (b) Ru900. p indicates bands of benzophenone.


Ru900 sample compared to that in the Ru300 (Fig. 5).

This band corresponds to the vibrations of water

molecules adsorbed on the TiO2 surface and addition-

ally stabilized by hydrogen bonds. Its shifting towards

the low frequency (Dn , 20–30 cm21) in both BP-

Ru300 and BP-Ru900 heterogeneous systems indi-

cates the creation of new hydrogen bonds with higher

energies. According to Iogansen rule for the enthalpy

change [31]

2DH ðkcal mol21Þ ¼ 0:3ðDnOH 2 40Þ1=2 ðcm21Þ

where DnOH ¼ nmOH 2 nOH; nm

OH ¼ 3750 cm21 (for

monomer of the water).

Energy increases by ,0.3 kcal mol21 in the new

network of hydrogen bonds. After loading, the

intensity of the 3400 cm21 bond decreases almost

1.5 times. For all systems studied we did not observe

bands of ‘free’ hydroxyl group (,3750 cm21).

Our experimental results suggest that a possible

mechanism of the interaction between BP molecules

and active centers located on the TiO2 surface is a

creation of hydrogen bonds via p-electron systems of

BP phenyl rings, i.e. of the p· · ·HO–Ti type. It should

be noted that spectral parameters of IR bands which

correspond to the CyO bond vibrations remain

unchanged. This seems to be an additional proof to

our conclusion. Besides, spectroscopic investigations

of 5CB liquid crystal filled with aerosil particles

revealed that in the 5CB–SiO2 heterogeneous system

hydrogen bonds were also created by means of

aromatic ring p-electron systems [32].

4. Conclusions

In summary, we have investigated heterogeneous

systems based on TiO2 rutile particles with different

surface states loaded into benzophenone. Raman

spectra showed, that two strong TiO2 bands change

their relative intensity ratio, and their integral

intensity significantly increases. This is due to

titanium dioxide crystal polarizability changes

induced by BP molecules near TiO2 surface. In the

case of the porous Ru300 sample BP ordering is

strongly affected by TiO2 surface defects and

adsorbed water molecules which result in BP

transformation into the glassy-like state. Considerable

changes have been observed in IR spectral region

corresponding to the BP phenyl ring vibrations in the

heterogeneous systems. Appearance of new bands in

the phenyl deformation region, intensity redistribution

in the phenyl stretching and in-plane bending

frequencies are reflected molecular interactions of

the BP molecules with surface active centers of rutile

particles. Detailed analysis of hydroxyl group stretch-

ing vibration region allows to select two broad bands

arising from the polymolecularly adsorbed water

molecules and from the hydrogen-bonded hydroxyl

groups in the case of pure TiO2 powders. Adding of

the BP molecules results in disappearance of the first

band and intensity decreasing and low frequency

shifting of the second one. This leads to a conclusion

that BP molecules destroy water associations and

displace water molecules at the TiO2 surface. This is

due to the formation of stronger hydrogen bonds by

the BP aromatic p-electron system (p· · ·HO–Ti) than

those of the water molecules. These processes are

more pronounced for the dehydrated Ru900 sample

where adsorbed water molecules almost do not have

any screening effect for the BP interactions with TiO2

surface.

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Spectroscopic study of TiO2 (rutile)–benzophenone heterogeneous systems