Sol–gel-derived super-hydrophilic nickel doped

TiO2 film as active photo-catalyst

Sunil Dutta Sharma *, Davinder Singh, K.K. Saini, Chander Kant,Vikash Sharma, S.C. Jain, C.P. Sharma

National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India

Received 13 June 2006; received in revised form 20 July 2006; accepted 31 July 2006

Available online 7 September 2006

Abstract

Pure and nickel doped TiO2 thin films on soda glass substrates were prepared by sol–gel dip coating process. The resulting films were annealed

at 500 8C for 1 h and characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and

UV–vis-near IR techniques. AFM reveals that doping of Ni ions suppresses the grain growth of crystal in the TiO2 film. The contact angle with

water of these films was measured by sessile drop method. The undoped films could be entirely wetted by water after 1 h UV illuminations, while

0.5 mol% Ni ion-doped films become entirely wetted after 20 min UV illumination. The photo-catalytic activity was characterized by photo-

catalytic degradation of aqueous methyl orange under UV radiation. It is found that, with a suitable amount (2–10 mol%), the Ni dopant increases

the photo-catalytic activity of TiO2 films. The mechanism can be attributed to these processes: (1) as the anatase grain sizes decrease with Ni doping

and the specific surface areas of doped TiO2 films increase, the charge transfer in TiO2 film is promoted; (2) by enhancing the electron–hole pair

separation and inhibiting their recombination, the Ni dopant enhances the charge pair separation efficiency for doped TiO2 films.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Titanium dioxide; Sol–gel; Hydrophilicity; Photo-catalyst

www.elsevier.com/locate/apcata

Applied Catalysis A: General 314 (2006) 40–46

1. Introduction

Nano-crystalline TiO2 thin films have attracted a great deal

of attention. They have many advanced functions in photo-

catalysis [1], solar energy cells [2], gas sensors [3], sterilizing

[4] and antifogging [5]. In 1997, Wang et al. [6] reported a new

phenomenon, denoted as super-hydrophilicity in TiO2 films.

When the films are irradiated by ultraviolet (UV) light, the

contact angle with water of these films decreases gradually and

finally becomes zero. There have been numerous research

papers reporting improvement in the photo-catalytic activity

and super-hydrophilicity of TiO2 thin films. Fujishima et al. [7]

reviewed the current progress in the area of TiO2 photo-

catalysis, observed the photo-induced super-hydrophilic phe-

nomenon in TiO2 and discussed its potential applications.

Navio et al. [8] studied the photo-catalytic efficiency of iron

doped titania containing Fe prepared from TiCl4 and Fe(III)

* Corresponding author.

E-mail address: sunil_vans@mail.nplindia.ernet.in (S.D. Sharma).

0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2006.07.029

acetylacetonate by sol–gel method. Watanabe et al. [9]

evaluated photo-induced hydrophilic conversion on different

crystal phases of single crystal rutile and polycrystalline

anatase titanium dioxide to clarify the dependence of the crystal

structure on the photo-induced hydrophilic conversion. Thanks

to this phenomenon, this discovery attracted much interest

to TiO2 films. However, there is still a problem that photo-

catalytic efficiency is not high, since TiO2 is active only under

ultraviolet (UV) light, because of its wide band gap �3.2 eV

and because the recombination of photo-generated electron–

hole pairs exhibits low photoquantum efficiency. The effective

separateness of electron–hole pairs, therefore, is one of the

important subjects for the increased utility of TiO2 as a photo-

catalyst. In recent years, impurity doping has been widely

performed by chemical synthesis and other methods in order to

improve photoactivity [10]. Because transition metal elements

have many valences and because trace transition metal ions

doped in the TiO2 matrix can be superficial potential traps

of photo-generated electron–hole pairs, then the process

lengthen the lifetime of electrons and holes and increase the

photo-catalytic activity.

S.D. Sharma et al. / Applied Catalysis A: General 314 (2006) 40–46 41

Many transition metal ion dopants have been investigated

for the TiO2 systems, among which iron has been most

examined [11]. In this paper, we reported the effect of different

amounts of nickel ions on photo-generated hydrophilicity and

photo-catalytic activity of TiO2 thin films.

2. Experimental

2.1. Preparation of TiO2 precursor sol

A 0.5 M TiO2 sol was prepared by the partial hydrolysis and

poly-condensation of titanium tetra-butoxide with water using

isopropyl alcohol (IPA) as a solvent and HNO3 as a catalyst.

Titanium tetrabutoxide and water have been taken in 1:2 molar

ratio. The hydrolysis and poly-condensation of titanium

tetrabutoxide proceeds according to the following scheme:

TiðOC4H9Þ4þ 2H2O ! TiðOC4H9Þ2ðOHÞ2þ 2C4H9OH

TiðOC4H9Þ2ðOHÞ2þTiðOC4H9Þ2ðOHÞ2! Ti2OðOC4H9Þ4ðOHÞ2þH2O

This reaction stops with the inclusion of two water

molecules:

TiðOC4H9Þ4þ 2H2O ! TiO2þ 4C4H9OH

All the chemicals used were of AnalaR grade and were used

as procured i.e. without further purification. The mixture was

stirred vigorously with a magnetic stirrer for 1 h. This results in

a colored transparent solution. The solution was kept overnight

before film deposition.

2.2. Preparation of pure TiO2 and Ni/TiO2 films

For the film deposition, the substrates were first degreased,

cleaned thoroughly and dried before deposition. Then the

substrate was dipped in the precursor solution bath and pulled

out with a constant speed of 12 cm/min to obtain the films of

uniform thickness. A very thin film of TiO2 that formed on the

substrate was first dried in air at room temperature, followed by

drying at 100 8C for 30 min in an electric oven. The films

formed were further heated at 500 8C for 1 h in an electric

furnace in air. The thickness of the films was adjusted by

repeating the above cycle.

For the preparation of the doped titania film, the procedure

was performed as described above; besides that, the appropriate

amount of nickel acetylacetonate from 2 to 10 mol% was

dissolved in the TiO2 precursor sol.

2.3. Characterization techniques

AFM and XRD studies were performed to get the structure

and final phase formation. XRD spectra of the samples have

been recorded in the 2u range 20–808 for crystal phase

identification with a SIEMENS D-500 diffractometer using

monochromatized Cu Ka radiation (l = 1.541 A). The surface

composition and chemical states of the films were analyzed by

X-ray photoelectron spectroscopy (XPS) using Al Ka on a

Perkin Elmer f model with hemispherical analyzer. The

nanoparticle size and surface morphology were studied using a

Nanoscope IIIa Atomic Force Microscope (AFM). Transmis-

sion and absorption spectra were recorded from 300 to 800 nm

with a SHIMADZU UV-3101 PC UV–visible spectrophot-

ometer at normal incidence. The hydrophilicity of the films was

studied in terms of contact angle measurement by a sessile drop

method using a CCD camera.

2.4. Photo-catalytic measurements

It has been well demonstrated that photo-degradation of an

organic compound will occur when TiO2 is illuminated by light

(l < 388 nm) in the presence of water containing dissolved

oxygen and organic compounds. Many organic compounds are

known to be degraded to CO2 under these conditions. The

primary step in photo-degradation is certainly the generation of

electrons and holes within the TiO2 particles.

TiO2 �!l< 388 nm

TiO2ðe� þ hþÞ (1)

In the presence of dissolved oxygen and an electron donor,�OH radical is formed by the reaction between the valence band

holes (h+) and the TiO2 surface-active OH� group or H2O.

hþ þ OH�!�OHad1 (2)

hþ þ H2Oad!�OHad þ Hþ (3)

The photogenerated conduction band electron (e�) is

trapped by the dissolved oxygen to form a super-oxide ion O2�.

e� þ O2!O2� (4)

Methyl orange is attacked by hydroxyl radicals formed in

the above equations and generates inorganic radicals or

some other intermediates. Eventually all the parent compounds

and intermediates are oxidized into CO2, SO42�, and NO3

�

[12].

TiO2 films are settled in aqueous methyl orange with a

concentration of 10 mg l�1 in a quartz cell (10 mm � 10 mm �30 mm). A high pressure mercury lamp (125 W) is used as a light

source. One face (10 mm � 30 mm) of TiO2 thin films is

irradiated along the normal direction. The averaged intensity of

UV irradiance was found to be 0.5 mW cm�2 by measuring with

a UV irradiance meter (Model UV-A); the wavelength range is

320–400 nm and the peak wavelength is 365 nm. The solution is

bubbled with air during irradiation. The concentration of aqueous

methyl orange is determined with a UV–vis spectrophotometer

by measuring the absorbance at 490 nm, because methyl orange

shows an intense band at approximately at 490 nm (A) and the

concentration of methyl orange (C) can be represented

empirically by the equation:

A ¼ 0:0725� C (5)

All the concentration profiles can be correlated by the

following exponential function with good agreement:

C ¼ C0 expð�ktÞ (6)

S.D. Sharma et al. / Applied Catalysis A: General 314 (2006) 40–4642

The apparent rate constant in the above equation decreases

with the increasing of initial concentration of methyl orange

when other parameters are kept unchanged. Therefore, the

photocatalytic decolorization of methyl orange is pseudo-first-

order reaction and its kinetic may also be expressed as follows

[12]:

ln

�C0

C

�¼ kt (7)

According to Eq. (5), Eq. (7) can also be expressed as

follows:

ln

�A0

A

�¼ kt (8)

where A0 is the initial absorbance of aqueous methyl orange, t

the reaction time and A is the absorbance of aqueous methyl

orange at t. k is determined by the linear regression method.

3. Results and discussion

3.1. XRD patterns

Fig. 1 shows XRD patterns of pure and Ni-ion doped TiO2

films annealed at 500 8C. Typical peaks in XRD pattern of TiO2

film are observed at 2u values of 25.288, 38.088, 48.048 and

55.028; these are assigned to 1 0 1, 1 1 2, 2 0 0 and 2 1 1 planes

respectively. Calculations show that the observed crystalline

peaks correspond to anatase TiO2 phase [13]. There is no

indication of rutile phase formation up to a sintering

temperature of 500 8C. For Ni ion-doped TiO2 films; the

crystalline phases of compounds formed by Ni ions were not

detected even when Ni ion concentration was as high as

10 mol%.

Fig. 1. XRD spectra of: (a) pure TiO2 film and (b) 10 mol% Ni/TiO2 film

annealed at 500 8C for 1 h.

3.2. X-ray photoelectron spectroscopy analysis

The photoelectron spectra of pure and Ni ion-doped TiO2

films annealed at 500 8C for 1 h is shown in Fig. 2. During XPS

Fig. 2. X-ray photoelectron spectra of: (a) 2 mol% (b) 5 mol% (c) 8 mol% and

(d) 10 mol% Ni-doped TiO2 thin films.

S.D. Sharma et al. / Applied Catalysis A: General 314 (2006) 40–46 43

Fig. 3. AFM images of: (a) pure TiO2 and (b) 10 mol% Ni-doped TiO2 films.

Fig. 4. UV–vis transmittance spectra of pure and Ni ion-doped TiO2 films.

measurements, development of static charge at the sample

surface created a problem in locating the actual binding energy

positions of Ti 2p and O 1s lines. To overcome this problem, the

amount of static charge was determined by keeping the binding

energy position of C 1s line fixed at 284.6 eV [14]. Thus the

amount of static charge that develops on the film surface was

estimated to be 3.95 eV. In the present study, the actual binding

energy positions corresponding to Ti 2p3/2 and Ti 2p1/2 lines for

pure TiO2 films were found at 457.95 and 463.6 eV; these

indicate the presence of Ti4+ in TiO2 film [15]. The binding

energies of Ti 2p region for the Ni-doped samples were almost

the same as those for pure TiO2 film, except that the XPS peaks

were slightly broadened in highly doped films containing more

than 4 mol% nickel oxide. Titanium was also in the Ti4+

oxidation state in all the Ni-doped TiO2 films.

Oxygen in the films was in the form of O2� in TiO2 and NiO.

The O 1s peak broadened to higher binding energy, which was

associated with the surface hydroxide, could be seen in sample

a and b with small amounts of nickel. The hydroxide might play

an important role in the degradation process. The redox

potential for photo-generated holes is +2.53 V versus the

standard hydrogen electrode (SHE) [7]. After reaction with the

hydroxide, these holes can produce hydroxyl radicals (�OH)

whose redox potential is only slightly decreased.

The NiO was characterized by high-intensity shake-up

satellites at the binding energy about 9 eV higher than the main

Ni 2p3/2 and Ni 2p1/2. In samples c and d with higher Ni

concentration, the Ni 2p peaks and their satellites had high

intensity, indicating the existence of fully oxidized nickel oxide

in the films [16]. Although the intensities of Ni 2p peaks were

low in sample a because of the small amount of Ni, the shake-up

satellites peaks with comparably high intensity could clearly be

seen in the spectra.

3.3. AFM observation

AFM was used to characterize the uniformity and particle

size of pure and doped films. As shown in Fig. 3, the TiO2

crystals of the two samples both exhibit spherical shape. The

TiO2 crystal of the pure film has a particle of 40–50 nm

(Fig. 3a), while TiO2 thin film with 10 mol% Ni ions added

(Fig. 3b) has TiO2 crystals whose particle size is as small as

about 20–30 nm. The crystal size of TiO2 in the film increases

significantly during the annealing process; for the Ni ion-doped

samples, the grain growth of TiO2 is inhibited and the particle

size is much smaller.

It has been observed that observed XRD peaks in Ni doped

TiO2 films are wider than in case of undoped TiO2 films. The

average crystallite size in TiO2 films has been calculated from

the widening of the XRD peaks using Scherer’s formula [17].

The crystallite size decreases from 40–50 nm in case of pure

TiO2 films to 20–30 nm in case of 10 mol% Ni doped TiO2

films. The reason for this reduction in crystallite size is that the

introduction of Ni2+ ions change the surface charge of the TiO2

sol particles and distances them from each other. In this way, the

TiO2 particles most likely formed in smaller size. This finding

is supported by AFM also.

3.4. UV–vis transmittance spectra

Fig. 4 shows the UV–vis transmittance spectra over the

wavelength range of 300–800 nm for pure and Ni ion-doped

TiO2 films deposited on soda glass. Pure TiO2 film shows up to

S.D. Sharma et al. / Applied Catalysis A: General 314 (2006) 40–4644

Fig. 6. The influences of Ni ions doping on hydrophilicity of TiO2 films.

85% transmittance in the visible region. The transmittance of

Ni ion-doped TiO2 films decreases as the concentration of

dopant increases because the metal absorbs electromagnetic

radiation in this region. For 10 mol% doped film, the

transmittance decreases up to 60%.

3.5. UV–vis absorption spectra

Fig. 5 shows the UV–vis absorption spectra over the

wavelength range of 300–450 nm for pure and Ni ion-doped

TiO2 films deposited on soda glass. The absorption edge shifts

to the longer wavelength direction in doped films. The reason

for this large bathochromic shift in optical energy gap is that the

energy level for Ni2+/Ni+ lies below the conduction band edge

(Ecb) and above the valence band edge (Evb) of TiO2.

Introduction of such energy levels in the band gap induces

the bathochromic shift in the band gap transition and the visible

light absorption through a charge transfer between a dopant and

CB (or VB) or a d–d transition in the crystal field according the

energy level [18].

3.6. Hydrophilicity

Fig. 6 shows the contact angle of water on pure TiO2 film and

on TiO2 films doped with different amounts of Ni ions under

0.5 mW cm�2 UV illumination. The contact angle of water on

pure TiO2 film before UV illumination was 528, indicative of its

relative hydrophobicity. After some time of UV exposure, the

contact angle of water decreases, which means the films convert

to a hydrophilic state. After 1 h of UV exposure, the pure TiO2

film shows a contact angle of ffi08. That is, water has

completely spread over the film surface and the film shows

good hydrophilicity. Ni ion-doped films have smaller contact

angles of water without being exposed in UV. The contact angle

of water goes on decreasing with increasing concentration of

dopands up to 5 mol%; after that it increases because the metal

Fig. 5. UV–vis absorption spectra of pure and Ni ions doped TiO2 films.

is not hydrophilic and this affects the hydrophilicity of the film

at high concentrations. Effects of UV illumination on the

contact angle for pure and doped films are shown in Fig. 6.

Without UV exposure, the smaller contact angle was observed

for 5 mol% Ni ion-doped film. However, after UV exposure,

10 mol% Ni ion-doped film shows the greater hydrophilicity

(smaller contact angle).

As for correlation of the hydrophilicity with the surface

properties, it is well known that a rough surface improves the

hydrophilicity. However, the improvement of hydrophilicity

was not due to the change of roughness in this research. It has

been reported that the generation of a highly hydrophilic TiO2

surface is related to the absorption of water on the photo-

generated surface defective sites [18]. Photo-generated

electrons holes could either recombine or move to the

surface to react with species adsorbed on the surface. Some of

the electrons react with lattice metal ions (Ti4+ and Ni2+) to

form Ti3+ and Ni+ defective sites. It was likely that the number

of defective sites increased with Ni ions doping and this led to

the improvement of hydrophilicity. Contact angle measure-

ments show that the amounts of defective sites increase with

increasing Ni ion doping up to 10 mol% after that it decrease.

The energy level for Ni2+/Ni+ lies below the conduction band

edge (ECB) and below the valence band edge (EVB). Doping

allows the vertical displacement of holes from one

semiconductor to another and retards the recombination of

the electron–hole pairs. Upon optical excitation, it was

believed that photo-generated electrons accumulated at the

conduction band of NiO, whereas holes accumulate at the

valance band of TiO2 [19]. Accumulated electrons can be

transferred to oxygen adsorbed on the surface, while

accumulated holes at the valance band of TiO2 lead to the

creation of oxygen vacancies, which are favorable for

dissociated water absorption [20]. As for the doped films,

photo-generated holes are effectively accumulated to TiO2

without recombining with electrons and the reaction of

hydrophilicity on TiO2 is greatly enhanced.

S.D. Sharma et al. / Applied Catalysis A: General 314 (2006) 40–46 45

3.7. Influence of Ni doping on the TiO2 photo-catalytic

activity

The photo-catalytic activity of TiO2 films was characterized

by the degradation test of methyl orange. The results are given

in Fig. 7. The TiO2 films as prepared have effectively degraded

the methyl orange. After 3 h low intensity irradiation, 25–60%

methyl orange was degraded. Total degradation of methyl

orange reaches 80% after 10 h irradiation. The TiO2 films thus

show desirable photo-catalytic activities. In Fig. 7, it can be

seen that after 3 h irradiation the photo-catalytic degradation of

methyl orange by pure TiO2 films is not more than 30%. But for

TiO2 films NiT-2 mol%, NiT-5 mol%, NiT-8 mol% and NiT-

10 mol%, after 3 h irradiation the degradation of methyl orange

reaches 48%, 51%, 36% and 29% respectively, which indicates

that, with a suitable amount, the Ni dopant has effectively

enhanced the TiO2 photo-catalysis.

According to the principle of TiO2 photo-catalysis, electron

(e�)-hole (h+) pairs are generated when TiO2 is irradiated with

UV light. Separated electrons (e�) and holes (h+) diffuse to the

surface of TiO2 and react with water, hydroxyl group and

molecular oxygen absorbed on TiO2 producing reactive

Fig. 7. The degradation of methyl orange over pure and nickel doped TiO2 films

after: (a) 3 h and (b) 15 h UV irradiation.

radicals, such as O2�, OH, H2O2, etc. These reactive radicals

further react with organic and inorganic compounds absorbed

on TiO2 and oxidize or reduce them. Generally, the photo-

catalytic activity of TiO2 is mainly dependent on three factors:

(1) the electron–hole generation capacity; (2) the separation

efficiency of the photo-generated charge pair; (3) the charge

transfer efficiency of holes and electrons to compounds

absorbed on TiO2.

In Ni ion-doped TiO2 films, nickel can provide a shallow trap

for photo-generated electron and hole so as to inhibit the

recombination and extend the lifetime of charge carrier.

Therefore, the photodegradation rate could be enhanced

consequently because more charge carriers are available.

3.8. Effect of pH on photo-degradation of methyl orange

The experiment was carried out to investigate the effect of

pH on the photo-degradation of methyl orange. At pH 10, the

concentration of methyl orange decreased from 10 through 9.8,

9.7, 9.6, 9.5 and 9.3 to 9.2 mg l�1 in 1–6 h where the rate

constant (K) was 0.01 h�1. At pH 7, the concentration of methyl

orange was 9.4, 8.2, 6.2, 5.1, 4.0 and 3.9 mg l�1 in 1, 2, 3, 4, 5

and 6 h respectively, where the rate constant was 0.16 h�1. At

pH 3, the methyl orange concentration decreased from 10,

through 8.3, 6.2, 3.8 and 0.8 to 0.4 mg l�1 in 1, 2, 3, 4, 5 and 6 h,

where the rate constant was 0.54 h�1. The rate constant was as

low as 0.01 h�1 in the basic region of pH 10; however it

increased to 0.54 h�1 in acidic region of pH 3. The removal

rates per immobilization area were estimated as 0.063, 0.041

and 0.006 mg methyl orange h�1 cm2 at pH 3, 7 and 10

respectively. The overall photo-reaction can be described as

follows

TiO2 ðcatalystÞ þ OH� þ 2Hþ þ �O2�

! 3�OH þ TiO2 ðcatalystÞ

The equilibrium constant (Ke) was estimated as

Ke ¼½�OH�3

½OH��½Hþ�2½�O2�

Since [OH�][H+] = Kw, the equation can be rearranged as

follows:

½�OH�3¼KeKw½Hþ�½�O2��

The concentration of [�OH] radical increases as the

concentration of [H+] increases. Thus the photo-degradation

of methyl orange was done more rapidly at pH 3 than at 10.

4. Conclusion

Pure and Ni ion-doped TiO2 thin films were prepared on

glass substrates by sol–gel method. There was only anatase

phase found in the XRD pattern for the films annealed at 500 8Cfor 1 h. This indicates the absence of Ni ions moving into the

lattice of TiO2 film. The AFM observation indicated that the

particle size of TiO2 in Ni ion-doped films was finer than in pure

films. The hydrophilicity of the films can be improved to a

S.D. Sharma et al. / Applied Catalysis A: General 314 (2006) 40–4646

limited extent by doping. It is found that with a suitable amount

(2–5 mol%), the Ni dopant effectively increases the photo-

catalytic activity of the TiO2 film.

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