DC Magnetron Sputtering Deposition of Titanium Oxide Nanoparticles: Influence of Temperature,...

Full Paper

DC Magnetron Sputtering Deposition ofTitanium Oxide Nanoparticles: Influence ofTemperature, Pressure and Deposition Time onthe Deposited Layer Morphology, the Wettingand Optical Surface Properties

Laurent Dreesen,* Francesca Cecchet, Stephane Lucas

Titanium dioxide nanoparticles were prepared on glass substrates by reactive DC magnetronsputtering. As highlighted by the atomic force microscopy characterization, we were able tocontrol the nanoparticles’ surface coverage and diameter by varying the deposition time andthe total pressure, respectively. The titanium dioxide energy band gap, determined by usingultraviolet-visible spectroscopy, depends on the total pressure but is quite independent on thedeposition temperature. On the contrary, it is blue shifted when the pressure increases.Finally, the contact angles slightly decrease after ultraviolet illumination irrespective of thedifferent deposition parameters, indicating an improvement of the hydrophilic properties ofthe adsorbed layer. After 21 h in dark, the contact angles are nearly identical to the ones beforeexposure to UV light: the samples do not keep their hydrophilic behaviour.

Introduction

In 1972, Fujishima and Honda discovered the remarkable

photocatalytic properties of titanium dioxide (TiO2).[1]

Since then, the material has been intensively studied due

to its interesting properties. Indeed, TiO2 is, for instance,

promising for the development of powerful anti-bacterial

and self-cleaning coatings, or for building efficient solar

L. DreesenBiophotonics, Department of Physics, Institute of Physics, Uni-versity of Liege, B5, B-4000 Liege, BelgiumE-mail: [email protected]. CecchetLaboratoire Spectroscopies et Lasers (LLS), Centre de Recherche enPhysique de la Matiere et du Rayonnement (PMR), University ofNamur (FUNDP), rue de Bruxelles, 61, B-5000 Namur, BelgiumS. LucasLaboratoire d’Analyses par Reactions Nucleaires (LARN), Centre deRecherche en Physique de la Matiere et du Rayonnement (PMR),University of Namur (FUNDP), rue de Bruxelles, 61, B-5000Namur, Belgium

Plasma Process. Polym. 2009, 6, S849–S854

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

cells.[2–4] Its high refractive index also allows its use in the

development of optical waveguides, antireflection and

multilayer optical coatings.[5–7]

The photocatalytic applications are based on the ability

of the semiconductor to generate an electron-hole pair

across the band gap. This process requires UV light because

the band gap is close to 3 eV, while it is the visible radiation

which is mainly delivered by the sun. Moreover, the

efficiency of the photocatalytic process is limited by the

electron-hole pair lifetime which is directly correlated to

the optical band gap and to the crystalline quality of the

deposited layer.[8] Many research groups are therefore

working on the TiO2 doping as well as on ways to improve

its crystallinity.

TiO2 nanoparticles (NPs) may be an alternative

approach to solve the aforementioned problems. They

offer interesting advantages over thin films such as

increased active surface area and reduced electron-hole

pair recombination. TiO2 NPs can be synthesized by many

techniques, including sol-gel process, hydrothermal meth-

ods, sparking process, laser ablation, laser pyrolysis, spray

DOI: 10.1002/ppap.200932201 S849

L. Dreesen, F. Cecchet, S. Lucas

S850

deposition, MOCVD and RF induction plasma.[9–16] The

chemical methods of preparation suffer from an important

drawback: the use of organic precursors or solvents.

Indeed, these may lead to some unwanted impurities and

are not environment-friendly which limits the develop-

ment of efficient devices and the potential industrial

applications, respectively. The aforementioned physical

production techniques (laser ablation and RF induction

plasma), although environment-friendly, are limited by

the size of the produced samples which is not sufficient for

a large-scale production.

We recently showed the possibility of synthesizing TiO2

NPs by a more suitable physical method of preparation:

reactive direct current (DC)-magnetron sputtering. This is a

very interesting technique for industries formany reasons:

metal targets are used, the stoichiometry is controllable,

the environment is respected and a large-scale production

is possible.[17]

In this paper, we demonstrate that by adjusting process

deposition parameters such as the temperature, the

pressure or the deposition time, it is possible to control

the surface coverage, the diameter, the optical band gap

and the wetting properties of TiO2 NPs adsorbed on glass

substrates. This is made by performing atomic force

microscopy (AFM), ultraviolet-visible (UV-Vis) and contact

angle measurements on samples prepared by DC magne-

tron sputtering under various process parameters.

Experimental Part

Sample Preparation

The TiO2 NPs were made on cleaned glass plates by reactive DC

magnetron sputtering using an AJA system. The distance between

the target and the substratewas set at 15 cmand the base pressure

in the plasma chamber was 10�7 mbar. Titanium target (99.99%)

was sputtered with plasma (14 W�cm�2) of argon (99.99%,

flow¼12 sccm) and reactive oxygen (99.99%) was introduced

between the plasma and the substrate. An appropriate selection of

the oxygen flow was required. Indeed, without reactive gas, the

system showed metallic mode and only Ti atoms were sputtered

on the substrate. When the oxygen flowwas higher than a critical

value, a poisoning effect was established and the Ti target was

covered by an oxide layer.[18] A decrease in the deposition rate and

an increase in the target voltage were associated with the

transition between the two modes. For this work, we prepared

samples at three pressures (4, 20 and 40�10�3 mbar), two

temperatures (room and 400 8C) and two deposition times,

corresponding to equivalent film thicknesses of 1.5 and 5 nm.

The deposition rates were determined by using a quartz crystal

located in the deposition chamber near the substrate.

Figure 1. Cathode voltage versus O2 flow rate for the differentinvestigated pressures.

AFM Measurements

AFM images were recorded in air and in intermittent-contact

mode (IC-AFM) with a Nanoscope III from Veeco Instruments



(Santa Barbara, CA, USA). The cantilevers (OTESPAW model from

Veeco) were made from silicon. They were characterized by a

resonance frequency around 260 kHz, a nominal spring constant

of 42 N�m�1 and an integrated silicon tip with an apex radius of

curvature around 10 nm. The so-called soft-tapping conditions

were used, i.e., the ratio between the set-point amplitude and the

free amplitude of the cantilever vibration was always kept above

0.8.

UV-Vis Characterization

UV-Vis measurements were performed on the samples prepared

with the higher deposition time (higher equivalent film thickness)

using a CARY 500 UV-Vis-NIR spectrophotometer (VARIAN).

Contact Angles Measurements

A drop of 5 ml of water was deposited on the sample and the

contact angle was measured with a GBX-Digidrop apparatus.

Then, the sample was illuminated by UV light (0.68 W�m�2 at

340 nm) for 3 h and the contact angle was again recorded. Finally,

it was kept in the dark for 21 h, and a new measurement was

carried on. Each measurement was performed three to five times

in order to take into account possible sample non-homogeneity.

The precision on the contact angles was less than 58.

Results and Discussion

Sample Preparation

As mentioned previously, a precise determination of the

oxygen flow is required in order to get titanium oxide with

the appropriate stoichiometry, i.e. TiO2. We therefore

measure the evolution of the cathode voltage with the O2

flow rate for the different investigated pressures. The

results are represented by the curves a, b and c in Figure 1

for 4, 20 and 40� 10�3 mbar, respectively. The character-

istics do not depend on the temperature for a given

DOI: 10.1002/ppap.200932201

DC Magnetron Sputtering Deposition of Titanium Oxide Nanoparticles . . .

Table 1. Selected oxygen flow rates and sputtering rates for theinvestigated pressures. 1 sccm is 1 Standard cm3�min�1 at 0 8C and1 atm¼ 2.69� 1021 mol�min�1.

Pressure (mbar) 4T 10�3 20T 10�3 40T 10�3

Working O2 flow

rate (SCCM)

4.5 3.5 3.5

Sputtering deposition

rate (nm�min�1)

0.5 0.2 0.1

pressure (data not shown). The curves displayed in Figure 1

reveal the expected features.[18] First, the discharge voltage

increases when the oxygen flow rate increases from zero

up to a threshold. Between these two values, the films

deposited on the substrate are mainly a mix of the metal

and its oxides with low oxidation states. The threshold

occurs around 3 sccm (1 sccm is 1 standard cm3�min�1 at

0 8C, 1 atm¼ 2.69� 10�21 mol�min�1) when the pressure is

4� 10�3 mbar and around 2 sccm for the two other

pressures (20 and 40� 10�3 mbar). When it is reached, the

cathode voltage shows an abrupt increase due to the full

coverage of the target by the oxide. After that, the target

voltage decreases to a stable value. It is well known that

the required titanium stoichiometry, TiO2, is obtained after

the transition region. For our studies, the selected oxygen

flow and the related deposition rates are reported in

Table 1 for the investigated pressures. Table 2 summarizes

the names and the deposition parameters of the different

produced samples.

Table 2. Sample nomenclature with the corresponding depositionparameters.

Sample

name

Temperature

(-C)Pressure

(mbar)

Deposition

time (min)

TrP4S Room 4� 10�3 3

T400P4S 400 4� 10�3 3

TrP4L Room 4� 10�3 10

T400P4L 400 4� 10�3 10

TrP20S Room 20� 10�3 7.5

T400P20S 400 20� 10�3 7.5

TrP20L Room 20� 10�3 25

T400P20L 400 20� 10�3 25

TrP40S Room 40� 10�3 15

T400P40S 400 40� 10�3 15

TrP40L Room 40� 10�3 50

T400P40L 400 40� 10�3 50



AFM Characterization

The AFM images recorded for the lower deposition time

(equivalent film thickness of 1.5 nm) are shown in

Figure 2a–f. They are related to samples TrP4S, T400P4S,

TrP20S, T400P20S, TrP40S and T400P40S, respectively. Let

us briefly discuss their main characteristics. No deposited

layer is observed on the samples prepared at p¼ 4� 10�3

mbar, irrespective of the temperature (Figure 2a and b). An

adsorbed layer, with a very poor structure and a relatively

good surface coverage, begins to appear when the pressure

is set to 20� 10�3 mbar (Figure 2c and d). However, the

most interesting features appear with the highest studied

pressure (40� 10�3 mbar). Indeed, we observe a low

number of quite circular bright spots on the sample

prepared at room temperature (Figure 1e). They are

indicative of the presence of nanometre-size particles

whose diameters are not well defined. When the

temperature is selected to 400 8C, Figure 2f clearly high-

lights the coverage of the surface by NPs of a well-defined

size. A more precise data analysis reveals that their mean

diameter is around 11 nm (after tip-size deconvolution as

explained in Ref.[19]).

Figure 2. 1� 1 mm AFM images of the samples of 1.5 nm equivalentfilm thickness. Pictures a–f are related to TrP4S, T400P4S, TrP20S,T400P20S, TrP40S and T400P40S samples, respectively. Thesample deposition parameters are shown in Table 2.

www.plasma-polymers.org S851


Figure 3. 1� 1 mm AFM images of the samples of 5 nm equivalentfilm thickness. Pictures a–f are related to TrP4L, T400P4L, TrP20L,T400P20L, TrP40L and T400P40L samples, respectively. Thesample deposition parameters are shown in Table 2.

S852

For the longer deposition times (equivalent film thick-

ness of 5 nm), an adsorbed layer is observed, irrespective of

variations in temperature and pressure, as illustrated by

Figure 3a–f related to samples TrP4L, T400P4L, TrP20L,

T400P20L, TrP40LandT400P40L, respectively. Forapressure

equal to 4� 10�3 mbar (Figure 3a and b), it is difficult to

distinguishparticles of controlled size on the surface. This is

also the casewhen thepressure and the temperature are set

to 20� 10�3 mbar and room, respectively (Figure 3c).

However, when the temperature and the pressure are

400 8C and 20� 10�3 mbar, respectively, well-defined

bright spots are observed as one can see in Figure 3d. More

precisely, these features are characteristic of the deposition

ofNPswhosemeandiameter iscloseto7nm(againafter tip-

size deconvolution). For the highest pressure (40� 10�3

mbar), the surface is completely covered by a layer of NPs,

irrespective of the temperature variations (see Figure 3e

and f). It is difficult to determine theirmeandiameter using

an image treatment software because they are in close

contact with one another, but as an initial evaluation, it is

very similar to that deduced on sample T400P40S prepared

with a shorter deposition time (and represented in

Figure 2f). We can therefore conclude that the higher the



pressure is, the higher the mean NP diameter. We can

explain this behaviour as follows. When the pressure is

increased, the mean free path is reduced. The number of

collisions before reaching the substrate therefore increases,

giving rise to larger NPs. These observations strongly

suggest thatnucleationoccurs in thegasphase,which is the

basis of the theory of charged clusters (TCC). Let usmention

that TCC has been previously applied by Barnes et al. to

explain the mechanism of low temperature TiO2 thin film

growth.[20,21] The effects of the temperature on theTiO2NPs

are probably similar to those observed in Ref.[17]: the degree

of crystallinity increases when the temperature increases.

Inotherwords,at roomtemperaturetheNPsareamorphous

while their structure is a mix between anatase and rutile

when the temperature is raised. To verify the aforemen-

tioned assumptions, it would be necessary to perform

electron diffraction measurements in a TEM microscope.

Unfortunately, it was not possible to access such an

apparatus when the current studies were conducted.

UV-Vis Studies

Optical transmittance spectra are used to determine the

optical band gap of the samples prepared with the

deposition times corresponding to the highest equivalent

layer thickness (5 nm). For energy close to the optical band

gap, the absorption coefficient, a, is simply related to the

transmittance, T and layer thickness, d, by[22,23]

a ¼ � ln Tð Þ=d: (1)

As reported in the literature, the indirect-allowed

transitions dominate just above the absorption edge.[24]

The following equation can therefore be written[22]

ahnð Þ1=2¼ C hn� Eg� �

(2)

with Eg, hn and C, the optical band gap, the photon energy

and a constant independent of the photon energy,

respectively. By fitting the linear part of ðahnÞ1=2 versus

hn with a linear regression, Eg is deduced, for a¼ 0.

The ðahnÞ1=2-hn characteristics are reported in Figure 4a

(samples TrP4L, TrP20L and TrP40L) and 4b (samples

T400P4L, T400P20L and T400P40L) for the samples

prepared at room temperature and 400 8C, respectively.The experimental data and the fits of the linear parts are

represented by symbols and continuous lines, respectively.

The extrapolated band gaps are shown in Table 3 for the

different pressures and temperatures. Let us discuss

the main features. First, these values are in line with

those reported in the literature. Indeed, Eg lies generally

between 3 and 3.4 eV, depending on the TiO2 crystalline

structure. It is equal to 3, 3.2 or 3.4 eVwhen the TiO2 form is

DOI: 10.1002/ppap.200932201

DC Magnetron Sputtering Deposition of Titanium Oxide Nanoparticles . . .

Table 3. Optical band gaps at room temperature and 400 8C as afunction of the total pressure for the thicker samples.

Pressure (mbar) 4T 10�3 20T 10�3 40T 10�3

Sample name TrP4L TrP20L TrP40L

Band gap at RT8 (eV) 3.15 3.30 3.32

Sample name T400P4L T400P20L T400P40L

Band gap at

T¼ 400 8C (eV)

3.12 3.27 3.30

Table 4. Contact angles before (column 2) and after 3 h of UV light(column 3). Column 4 is the contact angle after 21 h in the dark ofthe sample previously irradiated by UV.

Sample

name

Initial

contact

angle (-)

Contact angle

after 3 h

of UV (-)

Contact angle

after 21 h

in dark (-)

Figure 4. Absorbance plots of the layers (5 nm) deposited at roomtemperature (a) and 400 8C (b) for the different investigatedpressures. In the upper panel circles, squares and triangles arethe experimental data related to TrP4L, TrP20L and TrP40Lsamples, respectively. In the lower panel circles, squares andtriangles are the experimental data related to T400P4L,T400P20L and T400P40L samples, respectively. The linear fitsare represented by continuous lines.

rutile, anatase or amorphous, respectively. Then, the band

gap does not vary significantly with the temperature,

irrespective of the pressure. On the contrary, it increases

from 3.15 to 3.3 eVwhen the pressure varies from 4 to 40�10�3 mbar. In other words, Eg is blue shifted with an

increase in the pressure. Combining this result with the

increase in the NPs’ diameter with the pressure as

observed in the AFM pictures, we can conclude that the

larger the NPs’ diameter is, the higher the optical band gap.

TRP4L 77 18 63

T400P4L 22 8 29

TrP20L 46 21 46

T400P20L 31 18 40

TrP40L 74 24 58

T400P40L 52 19 32

Contact Angles Measurements

The samples prepared with an equivalent film thickness of

1.5 nm do not show any significant evolution of their

contact angles with UV light. We therefore only show in

Table 4 the results obtained on the thicker samples



(equivalent film thickness of 5 nm). Let us highlight the

general trends. All the samples show a decrease in their

contact angles after UV illumination, revealing the

increase in the hydrophilicity. Unfortunately, none of

them is superhydrophilic, i.e., is characterized by a contact

angle close to zero. Moreover, they recover their initial

contact angle after having been stored in the dark for 21 h.

The contact angles after UV on the samples prepared at

400 8C (T400P4L, T400P20L and T400P40L) are lower than

those obtained at room temperature with the same

pressure (TrP4L, TrP20L and TrP40L). Heating the samples

during the process seems, therefore, interesting to improve

the hydrophilic properties. Such an observation has been

previously reported on thin films and is explained as a

modification of the TiO2 crystallinity with the tempera-

ture: at room temperature TiO2 is amorphous while it

becomes crystalline when the temperature increases.[25,26]

On the samples prepared at 400 8C, we also observed an

increase in the contact angles (before and after UV)with an

increase of the pressure. Combining this result with the

increase in the NPs’ diameter with the pressure as

illustrated by the AFM measurements, we can conclude

that the larger the NPs’ diameter is, the higher the contact

angle and therefore the lower the hydrophilicity. Such a

behaviour is not surprising and has been already observed

in thin films: small values of the crystallite size leading to

better hydrophilic properties.[27]

www.plasma-polymers.org S853


S854

Conclusion

Titanium dioxide NPs were deposited on glass substrates

using reactive DC magnetron sputtering technique. We

studied the effects of deposition time, substrate tempera-

ture and pressure chamber on the layer morphology,

optical and wetting properties. AFM measurements show

that the NP size increases from 7 to 11 nm when the

pressure varies from 20 to 40� 10�3 mbar. The optical

band gap, deduced from UV-Vis measurements performed

on the thicker samples (5 nm), is temperature independent

and slightly varies with the pressure, shifting from 3.1 to

3.3 eV when the pressure is increased from 4 to 40�10�3 mbar. Finally, the contact angle measurements,

carried on the 5 nm thickness samples, show an

improvement of the TiO2 hydrophilicity when the

deposited layer is illuminated by UV. Unfortunately, the

hydropholicity gain is lost after the samples are stored in

the dark. For the samples prepared at 400 8C, the

hydrophilicity also depends on the pressure being better

when the pressure decreases.

Acknowledgements: The authors thank A. Nonet and Y. Morciauxfor the technical support. This work is supported by the WalloonRegion. F. C. is Postdoctoral Researcher of the National Fund forScientific Research (FNRS).

Received: September 12, 2008; Accepted: March 3, 2009; DOI:10.1002/ppap.200932201

Keywords: nanostructures; optical properties; physical vapourdeposition; surface morphology; wettability

[1] A. Fujishima, K. Honda, Nature 1972, 238, 37.[2] N. P.Melott, C. Durucan, C. G. Pantano,M. Gugliemi, Thin Solid

Films 2005, 502, 112.[3] P. Evans, D. W. Sheel, Surf. Coat. Technol. 2007, 201, 9319.[4] K. Yeung, Y. Lam, Thin Solid Films 1983, 109, 169.



[5] K. L. Siefering, G. L. Griffin, J. Electrochem. Soc. 1990, 137,1206.

[6] K. Bange, C. R. Otterrnann, O. Anderson, U. Jeschkowski, M.Laube, R. Feile, Thin Solid Films 1991, 198, 211.

[7] Y. Sawada, Y. Taga, Thin Solid Films 1984, 116, L55.[8] X. H. Wang, J.-G. Li, H. Kamiyama, T. Ishigaki, Thin Solid Films

2006, 506–507, 278.[9] M. Schneider, J. Baiker, J. Mater. Chem. 1992, 2, 587.[10] C. Cheng, J. Ma, Z. Zhao, L. Qi, Chem. Mater. 1995, 7, 663.[11] W. Thongsuwan, T. Kumpika, P. Singjai, Curr. Appl. Phys. 2007,

8, 563.[12] E. Figgemeier, W. Kylberg, E. Constable, M. Scarisoreanu, R.

Alexandrescu, I. Morjan, I. Soare, R. Birjega, E. Opovici, C.Fleaca, L. Gavrila-Florescu, G. Prodan, Appl. Surf. Sci. 2007,254, 1037.

[13] T. Seto, Y. Kawakami, N. Suzuki, M. Hirasawa, S. Kano, N. Aya,S. Sasaki, H. Shimura, J. Nanoparticles Res. 2001, 3, 185.

[14] A. Ranga Rao, V. Dutta, Sol. Energ. Mater. Sol. Cell. 2007, 91,1075.

[15] Y. Sun, A. Li, M. Qi, L. Zhang, X. Yao,Mater. Sci. Eng. 2001, B86,185.

[16] J.-G. Li, M. Ikeda, Y. Moriyoshi, T. Ishigaki, J. Phys. D: Appl. Phys2007, 40, 2348.

[17] L. Dreesen, J.-F. Colomer, H. Limage, A. Giguere, S. Lucas, ThinSolid Films, revision submitted.

[18] R. Gouttebaron, D. Cornelissen, R. Snyders, J. P. Dauchot, M.Wautelet, M. Hecq, Surf. Int. Anal. 2000, 30, 527.

[19] M. Rasa, B. W. M. Kuipers, A. P. Philipse, J. Colloid Interf. Sci.2002, 250, 303.

[20] M. C. Barnes, S. Kumar, L. Green, N.-M. Hwang, A. R. Gerson,Surf. Coat. Technol. 2005, 190, 321.

[21] M. C. Barnes, A. R. Gerson, S. Kumar, N.-M. Hwang, Thin SolidFilms 2004, 446, 29.

[22] H. Tang, K. Prasad, R. Sanjines, P. E. Schmid, F. Levy, J. Appl.Phys. 1994, 75, 2042.

[23] D. Mardare, F. Iacomi, D. Luca, Thin Solid Films 2007, 515,6464.

[24] N. Daude, C. Gout, C. Jounain, Phys. Rev. B 1977, 15,3229.

[25] Q. Ye, P. Y. Liu, Z. F. Tang, L. Zhai, Vacuum 2007, 81, 627.[26] D. Glob, P. Frac, O. Zywitzki, T. Modes, S. Klinkenbeg, C.

Gottfried, Surf. Coat. Technol. 2005, 200, 967.[27] T. M. Wang, S. K. Zheng, W. C. Hao, C. Wang, Surf. Coat.

Technol. 2002, 155, 141.

DOI: 10.1002/ppap.200932201

DC Magnetron Sputtering Deposition of Titanium Oxide Nanoparticles: Influence of Temperature,...

Documents

Transcript of DC Magnetron Sputtering Deposition of Titanium Oxide Nanoparticles: Influence of Temperature,...