Improved photocatalytic activity of polymer-modified TiO2 filmsobtained by a wet chemical route

Jian Li • Petra Lommens • Els Bruneel •

Isabel Van Driessche

Received: 28 February 2012 / Accepted: 5 May 2012 / Published online: 26 May 2012

� Springer Science+Business Media, LLC 2012

Abstract Porous TiO2 films, exhibiting improved pho-

tocatalytic activity compared with commercial materials,

have been deposited on glass. The films were dipcoated

from a polymer-modified TiO2 precursor solution, con-

taining about 90 vol% water as solvent. The addition of

water-soluble polymers such as polyethyleneglycol and

polyvinylalcohol has produced TiO2 films with different

morphologies, exhibiting RMS roughnesses of up to 60 nm

and increased porosity. We studied the effect of the poly-

mers on the morphology and surface topography of a series

of polymer-modified TiO2 films and evaluated how their

presence in the precursor influences the crystallinity,

optical transmittance and most importantly, the photocat-

alytic activity of the films. X-ray diffraction analysis shows

that all films exhibit the anatase crystal structure after

calcining for 2 h at 500 �C. We find that the presence of

polyethyleneglycol inhibits the crystallization of the TiO2

films. Transmittance spectra show that most of the poly-

mer-modified TiO2 films obtained in this work are trans-

parent although high polymer content can lead to opaque

films because of increased porosity and surface roughness.

The surface morphology of the films was studied by

scanning electron microscopy and atomic force micros-

copy. Their photocatalytic efficiency was studied by fol-

lowing the decomposition of methylene blue under UV

irradiation. The activity of the reference TiO2 film obtained

from a precursor without polymers is comparable to that of

Saint-Gobain (SG) self-cleaning Bioclean glass, while

some of the polymer-modified films show efficiencies that

can be up to seven times higher.

Introduction

TiO2 has been proved to be an excellent photocatalyst

because it is non-toxic, cheap and capable of degrading

most of the organic and many inorganic compounds [1, 2].

As it is highly stable in aqueous media, it can degrade most

organic contaminants in water or air on activation by UV

light. Depending on the envisioned application, TiO2 has

been fabricated in different configurations such as powders

or films [3–5] and more recently fibers [6, 7] or aerogels

[8]. Among these, TiO2 films are widely applied for self-

cleaning coatings, photocatalytic desinfection, anticorro-

sion coatings and hydrogen generation from water and

solar cells [9–14].

Compared with TiO2 powder, the thin films show rela-

tively low photocatalytic activity because of the decrease

of accessible photoactive sites. Therefore, a concerned

research topic is to improve the photocatalytic activity of

TiO2 films. Considered that photochemical reactions

mainly take place on the surface of the TiO2 films, surface

modification has been recognized as one of the most

intriguing methods to develop an excellent photocatalyst.

One type of surface modifications is to produce rough or

porous surface structures. When using solution chemistry

or soft chemistry to create thin films [15–18], a polymer-

templating technique can improve the TiO2 photocatalytic

activity by generating porous TiO2 films with a large sur-

face area and without introducing other adventitious phases

in the TiO2 matrix. It is well-documented that the con-

ventional di- or tri-blockpolymer-assisted template tech-

nique using ionic-/non-ionic surfactants or block polymers,

J. Li � P. Lommens � E. Bruneel � I. Van Driessche (&)

SCRiPTs, Department of Inorganic and Physical Chemistry,

Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium

e-mail: Isabel.Vandriessche@UGent.be

URL: http://www.we06.ugent.be/

123

J Mater Sci (2012) 47:6366–6374

DOI 10.1007/s10853-012-6561-5

such as CATB/Brij or Pluronic-type polymers, can create

photocatalytically active ordered mesoporous TiO2 films

[19–23]. Yet the pore walls often lack crystallinity, suf-

fering from a trade-off between crystallization and pores

collapsing during heating at elevated temperatures where

the polymer matrix is long destroyed.

Recently, there has been a growing interest in employ-

ing aqueous precursor solutions and water-soluble poly-

mers to produce non-ordered porous TiO2 films with

improved photocatalytic activity. These polymers, such as

polyvinylalcohol (PVA) [24], polyethylene glycol (PEG)

[25, 26, 29] and hydroxypropyl cellulose [27, 28], are

environment-friendly and cost-effective. A number of

studies discuss the pore formation mechanism in the case

of PEG being added to ethanolic Ti-alkoxide sol–gel pre-

cursor systems. They describe how the macroscopic mor-

phology of the film is determined by the competition

between the polycondensation or gelation rate, the mac-

roscopic phase segregation [26, 29] and the interaction

between polymer and Ti-complex. The latter are dependent

on the molecular weight and concentration of the polymer,

the amount of H2O added to induce hydrolysis/condensa-

tion/gelation and the presence of chelating agents.

In this study, we prepared polymer-modified TiO2 pre-

cursor solutions via an environmental-friendly wet chemi-

cal process. Based on aqueous Ti-precursor solutions,

containing different types and concentrations of water-

soluble polymers, TiO2 films were deposited on glass

substrates by dip-coating. In this study, we use a com-

pletely water-based precursor, containing strongly com-

plexed Ti4? ions to avoid hydrolysis. This means one

cannot describe the pore formation in terms of hydrolysis

and condensation, because these reactions probably only

take place at much higher temperatures and certainly at

different velocities because of the strong chelating effect of

the complexing molecules present. We wanted to investi-

gate if in this different aqueous precursor system polymer

templating is a viable approach for improving the specific

surface of TiO2 layers and their final photocatalytic activ-

ity. These experiments focused on the effects of various

polymers on the crystal structure, morphology, transpar-

ancy and photocatalytic activity.

Experimental

Preparation of precursor solution

The formulation and synthesis protocol for the Ti-precursor

solution has been described earlier by Arin et al. in [5].

Tetrabutyl orthotitanate (C97.0 %, Fluka) and citric acid

monohydrate (CA, C99.5 %, Carl Roth GmbH? Co. KG)

were mixed with ethanol (absolute, Panreac), followed by

adding distilled water to the precursor solution. The molar

composition of chemicals in the final solution was fixed at

Ti/CA/ethanol/H2O = 1:2:7.5:82. Then ammonia (NH3 in

25 wt% water) was used to increase the pH to 5. Various

polymers were employed to modify this stock solution:

polyethylene glycol (PEG, MW 400, Carl Roth GmbH?

Co. KG), polyvinyl alcohol (PVA, MW 30,000–70,000,

Sigma) and polyvinylpyrrolidone (PVP, MW 8000, Alfa

Aesar). All precursor solutions are transparent and have a

shelf life of several months. The exact composition of all

polymer-modified precursors, as well as their viscosity, is

given in Table 1.

Deposition of TiO2 films

All the TiO2 films were deposited on glass by dip coating

(KSV Instruments) in a clean room environment (class

100). Before dip coating, Corning glass slides (soda lime

glass) were cleaned in an ultrasonic bath with ethanol and

Millipore water, respectively. The cleaned glass substrates

were coated with different TiO2 precursor solutions at a

withdrawal speed of 60 mm min-1. These wet films were

kept in a drying oven at 60 �C for 5 h. For obtaining a

triple-coated film (3CT), we repeated the above processing

three times; between each coating, the layer was heated at

200 �C for 20 min. The dried films were transferred into a

tube furnace and heated to 500 �C at 3 �C min-1 and then

held in O2 atmosphere for 1 h.

Characterization

Thermogravimetric and differential thermal analyses

(TG–DTA), through a NETZSCH STA 449 F3 Jupiter�,

were used to study the thermal decomposition behaviour

of various TiO2 precursors. To reduce important weight

losses below 100 �C and to increase the accuracy of the

Table 1 Serials of polymer-modified TiO2 precursor solutions

Sample name Polymer type Viscosity (cP) Polymer

content (wt%)

CT 0 3.5 0

3CT 0 3.5 0

PVA-1 PVA 7.7 2

PVA-2 PVA 16.0 4

PVA-3 PVA 56.6 8

PVP-1 PVP 4.4 2

PVP-2 PVP 4.9 4

PVP-3 PVP 6.9 8

PEG-1 PEG 3.7 2

PEG-2 PEG 4.0 4

PEG-3 PEG 4.9 8

J Mater Sci (2012) 47:6366–6374 6367

123

measurements, we used TiO2 precursor gels that were dried

for 24 h at 60 �C. All TGA/DTA experiments were con-

ducted using a dynamic oxygen atmosphere at heating rate

of 10 K min-1 from 25 to 1,000 �C.

The crystallinity and phase structure of the TiO2 films

were measured with a Bruker-AXS D8 X-ray diffractom-

eter (XRD) with Cu Ka radiation, equipped with a lynx eye

detector covering 3� and 192 channels. The XRD patterns

were recorded over the range of 22–66� with a step of 0.04�and 2 s per step.

A scanning electron microscope (SEM) equipped with an

energy dispersive X-ray (EDX) spectrometer (FEI Quanta

200F) was used to characterize the surface morphologies and

to evaluate the relative elemental concentrations in TiO2

films. The topography and RMS roughness data of the films

were recorded by atomic force microscopy (AFM, Molecular

Imaging, PicoPlus) at ambient environment. All data were

processed with the WSxM software [30].

The wavelength dependent transmittance (T%) of the

films on glass substrates was determined by UV–vis

spectrophotometry (Varian Cary 500).

The photocatalytic performance of all the TiO2 films

was evaluated by recording the maximum decrease in

absorption of a methylene blue solution (MB, Fischer

Scientific) after UV irradiation. A photocatalytic reactor in

a black box was equipped with three 15 W blacklight blue

lamps with a maximum emission at 365 nm. The UV

radiation intensity was kept at 1 mW cm-2 as measured on

the sample surface by a UV photometer (Newport 840

Model). The temperature of the photoreactor was con-

trolled at 23 ± 0.3 �C with an external water circulator.

For a typical photocatalytic experiment, the initial

absorption of the MB solution (C0 = 9 ± 0.4 lM) was set

to be about 0.7. The TiO2 film (2.0 cm2) was immersed in

10 mL of MB solution under stirring with UV irradiation

above. An aliquot of MB solution was withdrawn every

15 min for an absorption measurement, using a Cary

UV 50 Conc spectrophotometer. The absorbance at 665 ±

5 nm was used as a measure of the decomposition of MB

as a function of exposure time. Based on the Lambert–Beer

Law, the decrease of the MB absorption linearly reflects

the decrease of the MB concentration.

Results and discussion

Thermal analysis

The effect of the presence of the polymers on the thermal

decomposition behaviour of TiO2 gels was investigated by

TGA and DTA. As shown in Fig. 1, the polymer-modified

TiO2 gels present different decomposition behaviours

dependent on the polymer used. TGA results (not shown

here) show that in none of the samples, weight losses were

observed at temperatures higher than 570 �C. The number

of decomposition steps and the positions of the exothermic

peak maxima are different from those for both pure TiO2

gels. For the PEG-modified gel, it is clear that most of the

exothermic decomposition reactions shift to lower tem-

peratures (425 �C), whereas for PVP and PVA, the main

decomposition step lies at 470 �C. It means that although

there are strong chelating interactions between the carboxyl

ligands from the citric acid and titanium ions to inhibit

their hydrolysis, an interaction exists between the polymers

and titanium ions. PEG is built up of –C–C–O– repeating

units and the presence of oxygen atoms in the carbon chain

reduces its thermal stability. This clearly also influences

the decomposition behaviour of the Ti-itself, suggesting a

strong correlation between polymer and Ti source. PVP

and PVA consist of a pure carbon main chain and exhibit a

higher thermal stability.

Morphology characterization

The titania films obtained after full thermal treatment as

discussed in the experimental section exhibit a long-term

stability when exposed to air and humidity and do not peel

off of the substrate when handled in lab circumstances.

They can only be removed from the glass substrate by

severe scratching. To investigate the effect of various

polymers on the morphology of TiO2 films, we studied the

surface of the modified titania layers by SEM (Fig. 2). We

find that the different types of polymers result in signifi-

cantly different morphologies.

Figure 2a shows that a TiO2 film, obtained from a

polymer-free precursor, has a dense surface structure, with

some elongated particles and agglomerates on the surface.

The PVP-modified TiO2 films present equally dense sur-

faces and are therefore not further investigated in this

study. PVP consists of a carbon chain functionalized with

12

10

8

6

4

2

0

1000800600400200

(b)

(c)

(d)

(a)

DT

A (

µV/m

g)

Temperature (°C)

Exo

Fig. 1 DTA curves collected for a series of TiO2 precursor gels

containing 8 wt% of different polymers: a PEG-3, c PVA-3, d PVP-3

and b polymer-free Ti precursor for reference

6368 J Mater Sci (2012) 47:6366–6374

123

pyrrolidone groups connected to the carbon chain through

their nitrogen atom. This means that only a carbonyl group

is present to connect with the titania precursor. This is

expected to lead to much weaker links than in the case of

PEG and PVA both of which possess hydroxyl functions.

These weaker bonds between polymer and Ti source might

explain the absence of any pore formation in the case of

PVP. The inability of PVP to create pores in TiO2 systems

has been reported before [31]. Adding PVA creates a

distinctly different surface structure. Large holes with

diameters of greater than 5 lm are present in the films for

PVA-1 (not shown) and PVA-2 (Fig. 2b). Inside the holes,

small particles are present. Addition of 8 wt% of PVA

leads to a completely different surface appearance

(Fig. 2c). The higher magnification view shows that the

layer seems to consist of 1-lm particles, loosely assembled

into a layer [24]. This can suggest that at these high

polymer concentrations, phase separation plays a role [26].

Interestingly, an AFM image of the same sample shows

that each of the *1-lm island-shaped particles in Fig. 2c

Fig. 2 SEM images of TiO2 films with different quantities and types of polymers. a Polymer-free titania film, b PVA-2, c PVA-3, d PEG-1 and

e PEG-3. The insets are higher magnifications of the same films

J Mater Sci (2012) 47:6366–6374 6369

123

consists of many fine particles with a size of 25 nm. For

PEG-modified TiO2 films (Fig. 2d), the surface again looks

free of any holes, yet on further magnification, it is clearly

shown that adding 2 wt% of PEG generates a porous

sponge-like structure. This is in contrast with other litera-

ture reports, where it is claimed that the presence of citric

acid as complexing agent (which is also the case here)

completely hinders pore formation by slowing down the

condensation rates [26]. Furthermore, literature suggests

that PEG with a molecular weight smaller than 2,000 is not

creating any pores in the classical ethanol-based sol–gel

systems [29]. On the other hand, others do report the for-

mation of pores by addition of PVA to citratoperoxo–Ti

complexes in water [24]. Surprisingly, increased addition

of PEG to the precursor seems to reduce the porosity of the

titania layer (Fig. 2e). It has been reported before that there

is an optimum value for the weight percentage of polymer

to add. In our case, this optimum is less than 4 wt% of PEG

[26, 32]. Clearly, there is an important difference in the

nature of the pores for the PVA- and PEG-based layers and

their overall morphology. The PEG layers exhibit a more

granular structure with smaller crack-like pores, whereas in

the case of PVA, more organized macropores are present. It

might suggest that in the case of PEG, the pore formation

mechanism is different, not requiring the self-assembly of

the polymer into macroscopic domains. Yet, at this point,

we have not been able to clarify the unexpected influence

of the amount of PEG added on the morphology of the

layers.

Because the efficiency of a photocatalyst is essentially

determined by the accessibility of reaction spots, it is

evident that increased surface roughness will add to the

efficiency of the photocatalyst. Therefore, we used AFM to

determine the RMS roughness values for those samples

exhibiting the most promising surface morphology in SEM,

i.e., PVA-2, PVA-3 and PEG-1 (Fig. 3; Table 2). As shown

in Fig. 3, the surface topography for these three samples is

very different, as evidenced before from SEM.

Phase structure and crystallization

TiO2 mainly exhibits the following three crystallographic

phases, brookite, anatase and rutile. The anatase phase

generally shows the highest activity, although a number of

publications claim that mixtures of anatase and rutile per-

form even better. Normally, the anatase phase is being

formed from 400 �C, and the transition temperature for the

anatase to rutile transformation lies between 550 and

900 �C, depending on the nature of the sample, e.g., par-

ticle size. It is evident from previous reports that the

addition of polymers can influence the crystallization

behaviour and phase transformation temperatures for TiO2

[33–35]. Therefore, it is important to identify the crystal-

linity and phase structures of various polymer-modified

TiO2 films. Figure 4 shows the XRD spectra collected for

different TiO2 films, prepared from polymer-modified and

polymer-free aqueous titania precursor solutions. All films

were annealed for 1 h at 500 �C in O2 atmosphere. As

expected, all titania layers show solely reflections indica-

tive of anatase phase.

Taking the TiO2 film without polymer as a reference

(Fig. 4a), we categorize the crystallization statuses of all

polymer-modified TiO2 films into three groups, that is, less,

Fig. 3 AFM topography images measured on polymer-modified titania films obtained from precursor PVA-2 (a), PVA-3 (b) and PEG-1 (c). All

images were collected on a 5 9 5 lm2 surface

Table 2 Structural parameters of TiO2 films with and without

polymers

Sample Roughnessa RMS (nm) Thicknessb (nm)

CT 0.8 61

3CT – –

PEG-1 21 –

PVA-2 57 –

PVA-3 61 184

a AFM analysis, scanning area is 10 lm 9 10 lmb Cross-section measurement by SEM

6370 J Mater Sci (2012) 47:6366–6374

123

comparable and better crystallization, respectively. The

differences in XRD intensities relate to variations in film

thickness and crystalline quality. It is also important to note

that sodium ions from the soda lime glass substrate can

diffuse into the TiO2 films during annealing, which reduces

the anatase crystallization [36]. Yet this effect on all the

samples should be the same under identical experimental

conditions.

For a dense, fully crystallized thin film, the integrated

peak intensity should be proportional to the thickness under

the first-order approximation [37, 38]:

I ¼ I0Stk3F2A; ð1Þ

where I0, S, t, k, F and A are the intensity of the incident

X-ray beam, the area illuminated by the X-ray beam, the

thickness of the film, the wavelength of the X-rays, a

structure factor and a constant, respectively. Addition of

increasing weight percentages of polymers to the precursor

solutions leads to increased values for viscosity as can be

seen from Table 1. For example, for PEG modification, the

viscosity increases from 3.5 to almost 5 cP. Based on the

Landau–Levich equation [39], the viscosity increase of

the precursor solution is expected to lead to an increased

thickness; thus, leading to stronger intensity in the XRD

spectra (Eq. 1). Nevertheless, Fig. 4c, d shows that adding

PEG leads to much lower or similar diffraction intensity to

CT, which suggests that adding PEG inhibits the crystal-

lization development of TiO2 films.

PVA addition increases the solution viscosity from 3.5

to 56.6 cP and produces thicker films as proved by the

tripled integrated intensity ratio compared with CT and

FIB-SEM thickness determination (184 vs 60 nm for CT

layer).

Elemental analysis

To determine if there is any carbon residue in the films,

EDX was used for fast qualitative analysis. Figure 5 shows

the EDX spectra collected for two identical titania films

obtained from precursor PVA-3, after annealing for 1 and

6 h under pure O2 atmosphere. Both EDX spectra exhibit

the same peaks, representative for the elements present in

the film (C, O and Ti) and the substrate (Al, Ca, Mg, Na, O

and Si), yet the intensity ratios (Ti Ka/C Ka) of the EDX

peaks have changed.

For the sample annealed for 6 h, the Ti Ka/C Ka ratio is

two times higher than the one annealed for 1 h. As the two

films were prepared under the same conditions except

annealing time, we assume that the adventitious carbon

contamination on the film surface is the same for both films

so a decrease in intensity ratio can be caused by the loss of

carbon within the films. Meanwhile, this also implies that

after annealing at 500 �C, there are still certain quantities

of carbon within the films. This carbon will probably

segregate at the grain boundaries where it can interfere

with photochemical reactions and the carrier mobility. The

residual carbon may also affect the transmittance of the

TiO2 films (see following section).

Transparency study

From microscopy analysis, it is clear that various polymers

produce different morphologies of TiO2 films. Further-

more, EDX measurements have shown that residual carbon

is present in our films. This may affect the transparency in

the visible range of the electromagnetic spectrum, leading

to colorization of the films (yellowish, brown or black in

the worst case). Visual inspection of our films learns that

they all appear colourless, some of them completely

Fig. 4 XRD patterns obtained for a pure TiO2 film, b triple-coated

pure titania film (3CT), c PEG-1, d PEG-3, e PVA-1 and f PVA-2 and

g PVA-3. The Bragg positions for anatase titania (JCPDS01-0562) are

indicated at the bottom part of the graph

543210

TiC (b)

(a)

O Na Si

MgAl Ca

Energy (keV)

Fig. 5 Energy dispersive X-ray analysis images for a PVA-3

annealed for 1 h under O2 and b PVA-3 annealed for 6 h under O2

J Mater Sci (2012) 47:6366–6374 6371

123

transparent, others being more opaque and whitish in nat-

ure. To quantitatively study how the morphological struc-

tures influence the film transparency and appearance,

UV–Vis transmittance spectra were recorded (Fig. 6). Next

the typical band gap absorption in the UV range of the

spectrum, it is clear that in the visible region, the trans-

mittance of the TiO2 films with and without polymers

reduces at different levels. This phenomenon is caused by

different factors: roughness, porosity and thickness.

In Fig. 6a, spectra 1 and 2 are collected for a bare glass

substrate and for a polymer-free 60-nm thick titania layer.

As can be expected from this low thickness, no interference

fringes are visible in the spectrum until 800 nm. The layer

is transparent, almost undetectable with the naked eye, and

losses in the visible range compared with glass substrate

are limited to 20 %. Addition of 2, 4 and 8 wt% of PVA,

decreases the overall transmittance of the samples. This

decrease will partially relate to the increased layer

thicknesses (up to 180 nm) of these layers, yet the impor-

tant losses in the visible range of the spectrum can be

attributed to increased scattering on the surface and prob-

ably to a lesser extent also the increased porosity of the

layer. The layers containing 8 wt% PVA are opaque rather

than transparent to the eye.

In Fig. 6b, somewhat different results are found for the

addition of PEG. Also here, the transmittance decreases on

addition of the polymer. A substantial transmittance

decrease is observed as the amount of PEG increases from

4 to 8 wt%. This might suggest that under the quite smooth

surface of this layer as evidenced from SEM, a large

amount of pores is present.

Photocatalytic characterization

Photocatalytic reactions of MB solutions with UV-irradi-

ated TiO2 films were carried out in a photoreactor posi-

tioned in a black box with UV lamps. MB degradation

performance versus UV irradiation time is plotted in Fig. 7.

A blank measurement (trace a), in the absence of any

titania film, shows a weak concentration decrease for the

MB on exposure with UV light. Compared with the poly-

mer-free CT film (trace b), the photocatalytic efficiency of

the polymer-modified TiO2 films can be divided into three

groups. The layers modified with PVA exhibit the highest

photocatalytic efficiency. They perform better than the

commercial Saint-Gobain glass, which has an efficiency

similar to that of the CT- and PEG-modified layers.

As PEG-1 shows the highest porosity among all of the

samples, it was expected that it would show improved

photocatalytic efficiency [22, 40–43]. This is, however,

inverse to our results. The discrepancy probably stems

from the poor crystallization degree of the PEG-modified

samples. Their poor crystallization accompanies a high

fraction of amorphous TiO2 which contains imperfections

leading to electronic states in the band gap. They behave as

a recombination centre for photo-generated electrons and

holes, and further deteriorate the photocatalytic activity of

the films [44].

As for the PVA-modified group, they show significantly

improved photocatalytic MB degradation compared with

CT. Their MB degradation curves indicate linear slopes

and follow the apparent first-order kinetic model, the

Langmuir–Hinshelwood (L–H) mechanism [45–47]. As the

initial MB concentration is micromolar, the L–H kinetic

equation can be simplified to the following equation of an

apparent first-order kinetic reaction [45]:

lnC

C0

� �¼ kKt ¼ kappt;

where C0 is the initial concentration of the MB (mol/L),

C is the actual concentration of the MB (mol/L), t is the

Fig. 6 Transmittance spectra collected for polymer-modified TiO2

films on glass substrates. In graph a, spectra 1 and 2 are collected for

an uncoated microscopy slide and a polymer-free titania coating,

respectively. Spectra 3 to 5 are collected for titania layers containing

increasing amounts of PVA, i.e., PVA-1, PVA-2 and PVA-3. In graph

b, spectra 1 and 2 are again added as a reference together with spectra

6–8, collected for titania layers containing increasing amounts of

PEG, i.e., PEG-1, PEG-2, and PEG-3, respectively

6372 J Mater Sci (2012) 47:6366–6374

123

illumination time (min), k is the reaction rate constant

(mol/L min), K is the adsorption coefficient of the reactant

(L/mol) and kapp is the apparent rate (min-1). The calcu-

lated apparent reaction rate constant, kapp, was used to

compare the photocatalytic efficiency of the TiO2 films.

Based on the above equation, CT and Saint-Gobain self-

cleaning glass (SG) have similar kapp, 0.0022 and

0.0021 min-1, respectively; PVA-2 and PVA-3 have much

higher kapp, 0.0145 and 0.0119 min-1, respectively.

From AFM, we found that the RMS roughness is similar

for both samples (i.e. 48 nm for PVA-2 vs 60 nm for PVA-

3). Given the surface-driven nature of the photocatalytic

decomposition process, PVA-3 with higher specific surface

is expected to perform better; furthermore, this sample will

have the highest thickness as estimated from the precursor

viscosity. Yet PVA-2 shows a better photocatalytic effi-

ciency than PVA-3. This may be related to the very dif-

ferent morphology of these samples and their possibly

different surface functionality, as well as to high undesired

carbon contents in the higher polymer load films.

Conclusions

A flexible wet chemical route was developed to prepare

polymer-modified TiO2 precursor solutions, which can be

successfully utilized for depositing TiO2 films on glass via

dip coating. We use an aqueous titania precursor, based on

a Ti-alkoxide stabilized by addition of citric acid as che-

lating agent. By adding different water-soluble polymers,

we create templates for formation of porous titania layers

after calcination. By using SEM and AFM, we found that

the addition of polyvinylalcholol and polyethyleneglycol

can dramatically change the surface morphology of the

titania layers. Especially, PEG addition created highly

porous samples, when added in 2 wt% to the precursor.

XRD analysis showed that all PEG-modified samples suf-

fered from low crystallinity, whereas the PVA-modified

film showed increased intensities compared with the

polymer-free references, because of their increased thick-

ness based on the higher viscosity for polymer-loaded

precursor solutions.

The photocatalytic activity of the samples was deter-

mined by testing their efficiency in the decomposition of

MB solutions under UV irradiation. All our samples per-

form at least as well as a Saint-Gobain Bioclean reference

sample. As could be expected, the PEG-modified samples

performed worse than the PVA-modified ones, based on

their low crystallinity. Clearly, some effects of the polymer

modification on the photocatalytic activity still need further

research to fully control the approach.

Acknowledgements This research was funded by the European

Union, FP7-NMP-2007-SMALL-1 grant No. 205854. The authors

thank Olivier Janssens for XRD and SEM/EDX measurements. We

are grateful to thank Prof. Dirk Poelman for supplying UV–vis

spectrophotometer and UV photometer.

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