of

f-a

Fuj

culty o

ama,

ne 16

nopar

abric

useful optical, electrical, and chemical properties such as high

mechanical instability, and high cost. However layer-by-layer

(LBL) self assembly method using sequential adsorptions of

thickness of thin film can be controlled with a nanoscale [16].

thin film comprising polyelectrolyte and TALH solution

which is stable and double negatively charged inorganic

precursor [21] shows high refractive index (n =1.68 1.8),e average thickness

Thin Solid Films 499 (200refractive index [1], high relative dielectric constant [2],

remarkable solar energy conversion [3,4], and photocatalysis

[5]. Therefore TiO2 thin film can be a promising material for

an optical filter [6], antireflection film [7], a self-cleaning

coating [8], a high efficient dielectric [9], and a solar cell [10].

TiO2 thin film has been fabricated by several methods such

as a solgel synthesis [11], sputtering [12] and chemical vapor

deposition (CVD) [13]. However, these processes that require

a high temperature, a high qualitative vacuum system and

intricate equipments have a limit of film area or thickness,

In previous reports, TiO2 thin films composed of

positively charged TiO2 nanoparticles [17] and oppositely

charged polyelectrolyte [18] or consisted of polyelectrolyte

and negatively charged titanium (IV) bis(ammonium lac-

tato) dihydroxide (TALH) [19] have been successfully

fabricated by a LBL self-assembly method. However the

refractive index of the film composed of TiO2 nanoparticles

and polyelectrolytes is significantly decreased because of

the presence of pores and polyelectrolyte with low refractive

index relatively [20]. On the other hand, although the TiO2applications and photocatalytic properties by decomposing methyl orange molecules gradually according to UV irradiation time. In addition, as

the pH of TALH was decreased from pH 5.5 to 2.0, the thickness of (TiO2 /TALH)30 film was increased from ca. 85 nm to ca. 442 nm.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Layer-by-layer self-assembly method; TiO2 thin film; Polyelectrolyte; Quartz crystal microbalance

1. Introduction

In the past several years, there has been increasing interest

in the fabrication of TiO2 thin films because they present

ionized polyelectrolytes and oppositely charged materials in

aqueous solutions has lots of advantages such as a simple

process, low temperature deposition, no limit of thickness and

needless of complicated equipments [14,15]. In addition, thetemperature. Especially, by using the quartz crystal microbalance (QCM) to monitor the in-situ deposition phenomenon of TALH that is

saturated and then separated from the film before and after the deposition of 30 s at low pH, the anatase TiO2 multilayer films fabricated with 30

s deposition in TALH adjusted to pH 2.5 showed a higher refractive index (ca. n =1.75), a denser film growth, and a lower surface roughness

(ca. 10.5 nm) than those of the films deposited in different conditions. This film showed a high transmittance in visible range for opticalFabrication and characterization

layer-by-layer sel

Jin-Ho Kim *, Shiro

Department of Applied Physics and Physico-Informatics, Fa

Kohoku-ku, Yakoh

Available onli

Abstract

Titanium dioxide (TiO2) thin films assembled with TiO2 na

bis(ammonium lactato) dihydroxide (TALH) were successfully f0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2005.07.001

* Correspondi

E-mail addresses: kimjinho@appi.keio.ac.jp (J.-H. Kim)

shiratori@appi.keio.ac.jp (S. Shiratori).TiO2 thin film prepared by a

ssembly method

ita, Seimei Shiratori

f Science and Technology, Keio University, 3-14-1 Hiyoshi,

223-8522, Japan

August 2005

ticles and oppositely charged polyelectrolytes or titanium (IV)

ated via a layer-by-layer (LBL) self-assembly method at room

6) 83 89

www.elsevier.com/locate/tsfthis film has a small film growth with thng author. Fax: +81 45 566 1602.(ca.1.5 5 nm) of a bilayer [22,23]. In addition, the filmgrowth phenomenon between TALH and polyelectrolytes or

well as a gas sensor [25,26], because when material is

deposited on the electrode of QCM, frequency is decreased

used as a negatively charged polyelectrolyte. Titanium (IV)

bis(ammonium lactato) dihydroxide (TALH, 50 wt.%,

Aldrich) with double negative charges [21] was selected

as an inorganic precursor. The molecular structure of TALH

is shown in Fig. 1. The concentration of PPA and PDDA

were adjusted to 0.01 M using ultra pure water (>18 MVcm) and then the pH of solutions was adjusted with NaOH

or HCl. 0.1 wt.% of TiO2 colloidal solution was adjusted to

H HNH3

CH3

CH3

H

H

HH

C

C

C

CO

OO

OTi

OO

OO

J.-H. Kim et al. / Thin Solid Films 499 (2006) 838984in proportional to the mass change by the Sauerbrey

relation [27]:

df f2

NqAdm 1

In this study, we report that TiO2 thin films assembled

with TALH and TiO2 nanoparticles have been successfully

fabricated with a low surface roughness, high refractive

index, improved thickness growth and a high transmittance

by monitoring the deposition phenomenon depended on the

pH of solution as well as immersion time into TALH using a

QCM. In addition, the properties of these films were

compared with that of films comprising TiO2 nanoparticles

and polyelectrolyte.

2. Experimental details

2.1. Materials

The used starting materials were oppositely chargednanoparticles has not been sufficiently explored as functions

of the immersion time or the pH of solution [19].

Quartz crystal microbalance (QCM) has been used for

the monitoring of the in-situ deposition phenomenon of

materials on substrate in LBL assembly process [24] as

H3N

Fig. 1. Assumed molecular structure of the non-ionized form of TALH (the

arrows indicated electron pairs).poly(dimethyldiammonium chloride) (PDDA, 20 wt.%,

Aldrich) and TiO2 colloidal solution (ca. 7 nm in primary

particle size, anatase) obtained from Ishihara Sangyo, Ltd.

(Japan). Poly(phosphoric acid) (PPA, Wako) and poly(so-

dium 4-styrenesulfonate) (PSS, Mw=70,000, Aldrich) were

Fig. 2. Schematic illustration of the procedures to fabricatepH 2.5 and the pH of TALH solution was adjusted from 5.5

to 2.0 with HNO3. Glass, Si/SiO2, and QCM (AT-cut, 10

MHz) with a gold electrode were used as substrates.

2.2. Preparation of thin films

Negatively charged substrates, glass and Si/SiO2, were

obtained by KOH (1 wt.%) treatment in ultrasonication for 5

min and then rinsed in ultra pure water (pH 5.56.5). QCMwas also carried out KOH treatment and then the 3 bilayers

of (PDDA/PSS) were assembled on electrode to diminish

the surface influence of each QCM.

Substrates and QCM were immersed into TiO2 colloidal

solution and polyelectrolytes for 10 min and TALH solution

for 10 s 10 min. Fig. 2 depicts the schematic illustrationof the procedures to fabricate the (TiO2/TALH) multilayer

thin film. Rinse procedures (1 min3 times) weresubsequently carried out after immersion into solutions

using ultra pure water. These coating procedures were

repeated 30 times. When we deposit material B over A with

30 times, we describe as (A/B)30.

2.3. Characterization of multilayer thin films

The surface microstructure and the cross-section of thin

film deposited on glass substrate were investigated by a

field emission scanning electron microscope (FE-SEM,

Hitachi S-4700) and the root mean squared (RMS) surface

roughness was measured by an atomic force microscope

(AFM, Digital Instrument nanoscope ha) in a tapping mode.The RMS roughness was calculated according to the

following Eq. (2):

RMS ~ Zi Zave 2

N

s2

where Zi is the height on the Z-axis of feature i, Zave is the

average height of the entire image, and N is the number of

points in image.the multilayer thin film consisted of (TiO2/TALH).

The refractive index of film assembled on Si/SiO2substrate was determined by an ellipsometry (Film Tek

3000, Scientific Computing International) at the wavelength

of 632 nm. The absorbance and transmittance of prepared

TiO2 films deposited on glass were measured by a UV-vis

spectrophotometer (UV mini-1240, Shimadzu).

The photocatalytic properties of anatase TiO2 films to

decompose methyl orange molecules were measured.

Prepared thin film was put into the cell filled with methyl

orange solution that was adjusted to ca. 0.948 absorbance at

460 nm before UV irradiation and this cell was then exposed

to UV light (254 nm, 22 W, SUV-16, As One) for 30 min.

After taking the film out of the solution, the absorbance of

methyl orange solution was measured by UV-vis spectro-

photometer. These procedures were repeated until the total

irradiation time became 5.5 h.

resulted in the separation from TiO2 nanoparticles deposited

during previous procedure. We assume that this phenom-

larger clusters than that of Fig. 5 (ac). On the other hand,

the film fabricated using TALH adjusted to pH 5.5 showed

the formation of pores on film however the presence of pores

was remarkably decreased in TALH solution adjusted to pH

2.5 or 2.0 and these films contain ca. 70 100 nm clustersformed from the agglomeration of nanoparticles and TALH.

Fig. 5 (b) film showed a flat and dense surface morphology.

Fig. 6 shows the surface morphologies and cross-

sectional images of [(TiO2/TALH)(pH 2.5)]30 multilayers

as a function of immersion time into TALH. The prepared

thin films with 10 or 30 s immersion time present a dense

surface, however when the immersion time is increased to

10 min, the amount of pores is also increased on the film.

We consider that this result is caused by the separation of

TiO2 from film after 30 s from the film as shown in Fig. 3

(a). Fig. 6 (d) shows the cross-sectional image of Fig. 6 (b).

The thickness was ca. 275 nm, which was in good

accordance with an ellipsometry measurement.

J.-H. Kim et al. / Thin Solid F3. Results and discussion

3.1. Frequency shift of QCM

Fig. 3 shows the frequency shift of QCM deposited TiO2nanoparticles (pH 2.5) or PDDA (pH 2.5) in TALH solution

adjusted to pH 2.5 for 10 min. In this figure, the frequency

shift of QCM coated with PDDA that has a remarkably

larger charge density [28] than that of colloidal particles was

saturated in short time and then there is no large change of

frequency shift, however the frequency shift of QCM

deposited TiO2 nanoparticles was saturated after about 30

s and then decreased significantly. This decrease of

frequency shift after 30 s in TALH may be derived from

the separation of precipitated TiO2 on QCM because at low

pH, the destabilization of chelate results in TiO2 precipitate

or films and the TiO2 precipitates formed from titanium

lactate solution are charged positively [19], which hasFig. 3. Frequency shift of a QCM in TALH (pH 2.5) solution after the

deposition of TiO2 or PDDA: (a) TiO2 (pH 2.5) and (b) PDDA (pH 2.5).enon is gradually carried out after deposition for 30 s with

TiO2 nanoparticles. Therefore the deposition for 30 s in

TALH will be suitable for obtaining the reasonable TiO2thin film with high thickness growth.

The frequency shift for 30 s as a function of the pH of

TALH are given in Fig. 4. The frequency shift in TALH

adjusted to pH 5.5, 2.5, and 2.0 were 50, 145, and 210 Hz,

respectively. Hence, the thickest film can be assembled in

the pH 2.0 of TALH.

3.2. Surface morphology and thickness of films

Fig. 5 (a c) present the surface morphologies ofprepared (TiO2/TALH)30 multilayers thin films as a function

of the pH of TALH. Substrates were immersed into TALH

for 30 s. Fig. 5 (d) is the surface image of [(PDDA/

TALH)(pH 2.5)]30 thin film assembled using solutions

adjusted to pH 2.5. In this figure, the film was consisted of

Fig. 4. Frequency shifts of QCM in TALH solution as a function of the pH

of TALH after immersion into TiO2: (a) pH 5.5, (b) pH 2.5, and (c) pH 2.0.

ilms 499 (2006) 8389 85The RMS values of [(TiO2/TALH)(pH 2.5)]30 and

[(TiO2/PPA)(pH 2.5)]30 are shown in Fig. 7. In order to

Fig. 5. FE-SEM images of (TiO2/TALH)30 thin films as a function of the pH of TALH solution and (PDDA/TALH)30 film: (a) pH 5.5, (b) pH 2.5, (c) pH 2.0

pH 2

J.-H. Kim et al. / Thin Solid Films 499 (2006) 838986prepare films, glass substrates were immersed into TiO2colloidal and PPA solution for 10 min and TALH for 30 s.

The RMS of [(TiO2/TALH)(pH 2.5)]30 film as shown in Fig.

7 (a and b) was ca. 10.5 nm and the RMS of [(TiO2/

PPA)(pH 2.5)]30 as shown in Fig. 7 (c and d) was ca. 22.4

nm. From this result, it was turned out that the thin film

consisted of TiO2 nanoparticles and TALH has a lower RMS

value and the denser surface of TiO2 than that of the film

fabricated with nanoparticles and PPA. The refractive

and (d) [(PDDA/TALH)(pH 2.5)]30. TiO2 colloidal solution was adjusted toindices of thin films consisted of (TiO2/TALH) and (TiO2/

Fig. 6. Surface morphologies of [(TiO2/TALH)(pH 2.5)]30 thin films as a function o

(d) cross-sectional image of (b). Immersion time into TiO2 solution is 10 min.PPA) were ca. 1.70 1.75 and ca. 1.45 1.50 at 632 nm,respectively. These results will be important information for

optical applications.

Table 1 summarizes the properties of prepared (TiO2/

TALH)30 films as functions of the pH value and the

immersion time of TALH solution as well as a rinse process

after TALH assemble. As the immersion was increased from

10 to 30 s, the thickness of films was also increased from ca.

162 nm to ca. 275 nm in the pH 2.5 of TiO2 colloidal and

.5.TALH solutions and was then gradually decreased to ca. 50

f the immersion time into TALH solution: (a) 10 s, (b) 30 s, (c) 10 min, and

ng mo

10 A.

J.-H. Kim et al. / Thin Solid Fnm as the deposition was increased from 30 s to 10 min.

From this data, we suppose that at low pH, the negative

charge density of film formed from titanium lactate was

gradually increased until 30 s, which became the driving

force to deposit lots of positively charged TiO2 nano-

particles during next deposition. However, as the deposition

time in TALH was increased from 30 s to 10 min, the

formation of cationic TiO2 from TALH was gradually

carried out on the surface of film, which decreased the

negative charge of surface and then, the cationic TiO2complexes were finally separated from film deposited with

cationic TiO2 nanoparticles. This thickness decrease of film

Fig. 7. AFM images of (TiO2/TALH)30 and (TiO2/PPA)30 thin films in tappi

view, and (d) section analysis of (TiO2/PPA)30. Scan size and height is 10solution and 30 s into TALH solution. All solutions are adjusted to pH 2.5with the deposition over 30 s is consistent with the

frequency shift of QCM as shown in Fig. 3 (a).

The RMS values of films assembled with an immersion

for 20 or 30 s in TALH were lower than that of other films.

As the immersion time was increased from 30 s, the RMS

values were also gradually increased. In addition, as the pH

value of TALH was decreased from pH 5.5 to pH 2.0, the

Table 1

Properties of prepared (TiO2/TALH)30 thin films as functions of the pH and

immersion time of TALH solution as well as rinse process after TALH

deposition

Sample pH of

(TiO2/TALH)

Immersion

time into

TAHL

Rinse process

after TALH

deposition

Thickness

(nm)

Surface

roughness

(nm)

T1 (2.5/2.5) 10 s 1 min1 162 14.2T2 (2.5/2.5) 20 s 1 min1 170 10.2T3 (2.5/2.5) 30 s 1 min1 275 10.5T4 (2.5/2.5) 1 min 1 min1 155 12.7T5 (2.5/2.5) 5 min 1 min1 105 13.2T6 (2.5/2.5) 10 min 1 min1 50 16.2T7 (2.5/2.5) 30 s 1 min1 442 27.5T8 (2.5/2.5) 30 s 1 min1 85 17.5T9 (2.5/2.5) 1 min 1 min3 62 20.6film thickness was increased. Especially, the thin film

deposited in TALH adjusted to pH 2.0 was showed a high

thickness growth with ca. 442 nm. However the RMS value

was also largest than that of thin film fabricated in TALH

adjusted to pH 5.5 or 2.5.

Rinse process after TALH deposition also influenced the

thickness of films. Comparing the T4 and T9, the thickness

of film was decreased by the addition of rinse steps and the

RMS roughness was also increased because the cationic

TiO2 complexes were formed and separated from film

during rinse procedure.

de: (a) surface view, and (b) section analysis of (TiO2/TALH)30, (c) surface

m and 200 nm, respectively. Immersion time is 10 min into TiO2 and PPA

ilms 499 (2006) 8389 873.3. Absorbance, transmittance, and refractive index of

films

Fig. 8 shows the absorbance of prepared thin films as

functions of the pH of TALH and immersion time into

TALH solution. As shown in Fig. 8 (a), as the pH of

TALH was decreased from pH 5.5 to 2.0, the maximum

absorbance of TiO2 multilayer thin films was increased.

This result is in accordance with the thickness increase of

film as shown in Table 1. However, the absorbance of

films was not increased in proportional to the thickness of

film. Although the T7 film was fabricated with ca. 442 nm

in thickness, the absorbance was not remarkably higher

than that of T3 film. This means that the T7 film has a

high thickness growth with high RMS roughness during

deposition. Therefore some pores may be formed inside of

film.

Fig 8 (b) shows that the absorbance of films prepared

with deposition time from 10 to 30 s was gradually

increased and was then decreased from 30 s to 10 min at

285 nm. These data are in good accordance with the

thickness of films prepared in different conditions as shown

in Table 1.

Fig. 9. Transmittance of prepared (TiO2/TALH)30 thin films as a function of

the pH value of TALH with TiO2 solution adjusted to pH 2.5: (a) pH 2.5

and (b) pH 2.0. Immersion time is 10 min into TiO2 and 30 s into TALH,

respectively.

Fig. 8. UV-vis spectra of (TiO2/TALH)30 thin films prepared as functions as

(a) the pH of TALH solution with 30 s immersion time and (b) the

immersion time into TALH solution adjusted to pH 2.5. The pH and

immersion time of TiO2 solution are pH 2.5 and 10 min.

J.-H. Kim et al. / Thin Solid F88Fig. 9 presents the transmittances of sample T3 and T7

multilayers thin films. Sample T3 film showed a high

transmittance in a visible range, therefore this film may be

suitable for an optical application as a high refractive layer

because the refractive index of thin film was ca. 1.70 1.75at 632 nm. On the other hand, the transmittance of sample

T7 film was decreased in visible range. Even if it is clearly

difficult to identify the volume of pores and the roughness in

Fig. 5 (c) compared with Fig. 5 (b). When we compare the

thickness and absorbance of films, T7 film has a lower

density of TiO2 than that of T3 film by the formation of

pores, high roughness and thick film thickness. In fact, a

few cracks were shown on film. Therefore we think that the

T7 film may show a higher light reflectance in visible range

than that of T3 film.

3.4. Photocatalysis of film

Fig. 10. Absorbance at 460 nm in UV-vis spectra of aqueous methyl orange

solution decomposited by the (TiO2/TALH)30 thin film as a function of UV

irradiation time.ilms 499 (2006) 8389The absorbance changes of methyl orange solution by the

anatase TiO2 thin film (T3) were measured. The absorbance

of methyl orange solution at 460 nm as a function of UV

irradiation time was shown in Fig. 10. The absorbance of

methyl orange in UV-vis spectra was gradually decreased

from ca. 0.948 according to the increase of irradiation time

because methyl orange molecules were decomposed by

anatase TiO2 with UV irradiation. After 3 h, the decom-

position rate of methyl orange was slightly decreased since

the concentration of methyl orange remained was gradually

decreased. The film irradiated for 5.5 h still showed the

reproducible photocatalytic performance in new methyl

orange solution.

4. Conclusions

(TiO2/TALH)30 multilayer thin films consisted of TiO2nanoparticles and TALH were successfully fabricated via

LBL-SA method with an advanced growth in thickness ( ca.

275 nm) by adjusting the pH and the immersion time of

TALH solution. The in situ assembly of TALH that shows

the phenomenon to be saturated and separated before and

after 30 s was clearly monitored by a QCM. As the pH of

TALH was decreased, the thickness of films was increased.

Finally, by using TiO2 colloidal solution and TALH adjusted

to pH 2.5 with the immersion of 30 s into TALH, we

fabricated an anatase TiO2 thin film that has a lower RMS

roughness (ca. 10.5 nm), a higher refractive index (ca. 1.75)

than that of the film composed of TiO2 nanoparticles and

PPA. In addition, this film showed a high transmittance in

visible range for optical applications as well as good

photocatalytic performances to decompose the methyl

orange molecules with UV light irradiation.

Acknowledgements

This work was partially supported by Grant-in-Aid for

the 21st Century COE program KEIO Life Conjugate

Chemistry from the Ministry of Education, Culture, Sports,

6382.

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Fabrication and characterization of TiO2 thin film prepared by a layer-by-layer self-assembly methodIntroductionExperimental detailsMaterialsPreparation of thin filmsCharacterization of multilayer thin films

Results and discussionFrequency shift of QCMSurface morphology and thickness of filmsAbsorbance, transmittance, and refractive index of filmsPhotocatalysis of film

ConclusionsAcknowledgementsReferences