ORIGINAL PAPER

Metal–metal charge transfer and interfacial charge transfermechanism for the visible light photocatalytic activity of ceriumand nitrogen co-doped TiO2

Rajashekhar K. Eraiah • Giridhar Madras

Received: 21 December 2013 / Accepted: 7 April 2014

� Springer Science+Business Media New York 2014

Abstract Nanosized cerium and nitrogen co-doped TiO2

(Ce–TiO2-xNx) was synthesized by sol gel method and

characterized by powder X-ray diffraction (PXRD), X-ray

photoelectron spectroscopy (XPS), FESEM, Fourier trans-

form infrared, N2 adsorption and desorption methods,

photoluminescence and ultraviolet–visible (UV–vis) DRS

techniques. PXRD analysis shows the dopant decreases the

crystallite sizes and slows the crystallization of the titania

matrix. XPS confirm the existence of cerium ion in ?3 or

?4 state, and nitrogen in -3 state in Ce–TiO2-xNx. The

modified surface of TiO2 provides highly active sites for

the dyes at the periphery of the Ce–O–Ti interface and also

inhibits Ce particles from sintering. UV–visible DRS

studies show that the metal–metal charge transfer (MMCT)

of Ti/Ce assembly (Ti4?/Ce3? ? Ti3?/Ce4?) is responsi-

ble for the visible light photocatalytic activity. Photolu-

minescence was used to determine the effect of cerium ion

on the electron–hole pair separation between the two

interfaces Ce–TiO2-xNx and Ce2O3. This separation

increases with the increase of cerium and nitrogen ion

concentrations of doped samples. The degradation kinetics

of methylene blue and methyl violet dyes in the presence of

sol gel TiO2, Ce–TiO2-xNx and commercial Degussa P25

was determined. The higher visible light activity of Ce–

TiO2-xNx was due to the participation of MMCT and

interfacial charge transfer mechanism.

Keywords Co-doped TiO2 � Metal–metal charge

transfer � Interface charge transfer � Visible light active

1 Introduction

Though titania is used for environmental remediation [1], it

is active primarily under ultraviolet radiation. Therefore,

investigators have tried to make titania more active under

visible light by doping the materials with either transition

metal or non-metal ions. The incorporation of several non

metal ions such as nitrogen [2, 3], carbon [4, 5] and fluorine

[6, 7] in the TiO2 lattice has been accomplished. Though

the modification of TiO2 by co-doping with metal ion and

non metal ion has been extensively studied [8–13], the

influence of lanthanide oxides on electron–hole pair sepa-

ration under visible light irradiation has not been widely

investigated.

Among the lanthanide oxides, ceria is a potentially good

catalyst because of its ability to shift between CeO2 and

Ce2O3 under oxidizing and reducing conditions as well as

the ease of formation of labile oxygen vacancies. The use

of metal-to-metal charge transfer (MMCT) of hetero-

bimetallic assemblies as a new class of visible-light

absorbing material has been recently demonstrated [14].

The anchored Zr (IV)–O–Cu(I) assembly on the pore of

mesoporous silica showed the photochemical splitting of

CO2 to CO by photoexcitation of the Zr(IV)/Cu(I) MMCT

band at 355 nm [15]. Similarly, MMCT has been reported

[16] for the combination of Ti4? and Ce3? ions as consti-

tuting elements [E0(Ce4?/Ce3?) = 1.72 V, E0(TiO2/

Ti3?) = -0.67 V] for the photocatalytic oxidative

decomposition of organics under visible light irradiation.

Ce has been employed as a catalyst in both stable Ce3?

(4f15d0) and Ce4? (4f05d0) oxide states due to its redox

property and oxygen storage capacity [17, 18]. There are a

few reports on cerium and nonmetal co-doped TiO2 [19,

20]. Cao et al. [19] prepared C, N and Ce co-doped TiO2

for the visible light activity of the catalyst. Xu et al. [20]

R. K. Eraiah � G. Madras (&)

Department of Chemical Engineering, Indian Institute of

Science, Bangalore 560012, India

e-mail: giridhar@chemeng.iisc.ernet.in

123

J Sol-Gel Sci Technol

DOI 10.1007/s10971-014-3350-4

studied the Ce and C co-doped TiO2 for the degradation of

dye Reactive Red X-38 under visible light.

In this investigation, cerium and nitrogen co-doped TiO2

(Ce–TiO2-xNx) with higher concentration (more than

1 at.%) of ceria was investigated for the visible light

degradation of dye. The surface area, crystallite size, sur-

face basicity, metal–metal charge transfer and interfacial

charge transfer mechanism was investigated. Significant

photocatalytic activity was observed for the degradation of

dyes like methylene blue (MB) and methyl violet (MV)

using this catalyst.

2 Experimental section

2.1 Materials

Analytical reagent (AR) grade chemicals were used in the

synthesis. Titanium isopropoxide (Lancaster Chemicals,

UK), Cerium Ammonium Nitrate (CAN), hexamethylene

tetraamine, Cationic dyes methylene blue (MB), methyl

violet (MV) dyes were obtained from SD Fine Chemicals,

India. Water was double distilled and filtered through a

Millipore membrane filter prior to use.

2.2 Catalyst preparation

Nanosized Ce–TiO2-xNx was prepared via the modified sol

gel method, using CAN as Ce and N precursor. 0.007 mol/l

of CAN was dissolved in 50 ml of distilled water and

cooled in ice cold bath to 4 �C. The pH of the solution was

adjusted to 8 using hexamethylene triamine. 5 ml titanium

isopropoxide (0.3 mol/l) was added drop wise to the ice

cooled solution to obtain a pale yellow colored precipitate.

The precipitate was stirred for 3 h at room temperature

using magnetic stirrer and washed repeatedly for 5 times

using distilled water. The obtained precipitate was filtered

using a vacuum filtration pump and dried for 12 h. The

powder was manually grinded for 1 h and calcined at

350 �C for 3 h to obtain 6 at.% cerium doped TiO2

(CTN06).

Similarly, 3 and 9 at.% cerium doped TiO2 (CTN03) and

(CTN09) samples were prepared by adding 0.0035 and

0.0106 mol/l solution of CAN to the 5 ml solution of

0.3 mol/l titanium isopropoxide separately. The pure ana-

tase TiO2 (ST) was prepared using same method without

adding ceria precursors.

2.3 Catalyst characterization

Powder X-ray diffraction (PXRD) patterns of catalysts

were recorded in a Phillips X’ Pert diffractometer using Cu

Ka radiation with a scan rate of 0.5�/min. The surface area

of the catalyst was determined with a standard Brunauer–

Emmett–Teller (BET) apparatus (model NOVA-1000,

Quantachrome). X-ray photoelectron spectroscopy (XPS)

using a Thermo Scientific Multilab 2000 instrument with

Al Ka source. Fourier transform infrared (FTIR) studies

were performed in the transmission mode in the frequency

range of 400–4,000 cm-1 (Perkin-Elmer, FTIR Spectrum-

1000). The photoluminescence measurements were per-

formed on a luminescence spectrometer (Perkin-Elmer LS

55) at room temperature under the excitation light of

285 nm. The ultraviolet–visible (UV–vis) absorption

spectra of TiO2 powders were obtained for the dry-pressed

disk samples using a UV–vis spectrophotometer (UV-1700

pharmaspec) with a wavelength range of 270–800 nm.

2.4 Photocatalytic experiments and sample analysis

Photocatalytic measurements were carried out in a cylin-

drical jacketed borosilicate glass container tube of 3.4 cm

inner diameter, 4 cm outer diameter and at the height of

21 cm where in the dye was degraded. The radiation was

provided by a 150 W metal halide lamp (with a cutoff

wavelength of 400 nm), which was placed inside a jacketed

quartz tube. The intensity was determined by o-nitro-

benzaldehyde actinometry and was found to be 1 W m-2

[21]. The quartz tube was dipped into the solution that was

stirred continuously and the temperature was maintained at

35 �C by cold water circulation. 100 mL of 50 ppm dye

solution and 25 mg of catalyst were stirred for 1 h in the

absence of light to check the initial surface adsorption. The

solution was then exposed to light and samples were col-

lected at regular intervals, which were then centrifuged to

remove the catalyst. These samples were then analyzed by

an UV-spectrophotometer (Shimadzu UV 1700 Pharma

Spec). The maximum absorption wavelengths (kmax) for

MB and MV were at 665 and 585 nm, respectively. The

change in the dye concentration with respect to time was

quantified based on the calibration of each dye by spec-

trophotometry. All experiments were repeated three times

and the error in concentration during degradation was less

than ±1 ppm. The stability of the catalyst was also

examined by conducting experiments with the spent cata-

lyst for at least seven cycles and no decrease in the rate of

degradation was observed.

3 Results and discussion

3.1 PXRD and BET studies

Figure 1a shows the typical PXRD patterns of ST, CTN-

03, CTN-06 and CTN-09 samples. The diffraction peaks at

2h = 25.3�, 37.9�, 48.4�, 53.9�, 62.7�, 69.0�, 70.2� are

J Sol-Gel Sci Technol

123

related to the anatase phase of TiO2 [22]. In addition, peaks

appear at 2h = 28.4� for the samples CTN-06 and CTN-09

due to the cubic cerium oxide (Ce2O3) [23] that is absent in

CTN-03 due to lower concentration of Ce2O3. The average

crystalline sizes were found to be 64, 62, 61 and 60 nm for

the samples ST, CTN-03, CTN-06 and CTN-09. This

indicates that an increase in % doping of Ce marginally

influences the crystallite size.

The ionic radii of Ce3? (1.11 A) Ce4? (1.01 A) and N3-

(1.71 A) are bigger than that of Ti4? (0.68 A) and O2-

(1.40 A), for Ce3?/Ce4? to replace Ti4? and N3- to replace

O2- in the crystal leading to change in the lattice param-

eters are as shown in Table 1 [24, 25]. This change is due

to the strain caused by the dopants in the lattice, Hence,

Ce3?/Ce4? and N3- can substitute in the TiO2 lattice,

without distorting the phase structure of TiO2.

On the basis of this, it can be inferred that Ce3?/Ce4? ions

has been substituted in the crystal lattice of the titania or cerium

oxide or exists as a highly dispersed polymorph over the titania

surface. N3- can substitute O2- and Ce3?/Ce4? can substitute

Ti4? in the TiO2 lattice. Therefore, some of the Ce3?/Ce4? ions

were incorporated into the TiO2 network while some were well

dispersed in the bulk and surface as Ce2O3. The average

crystallite sizes (D) were calculated using the Scherrer’s for-

mula D = kk/b cos h, where k is the wavelength of Cu Kaused, b is the full width at half maximum (fwhm) of the dif-

fraction angle under consideration, K is a shape factor (0.94)

and h is the angle of diffraction using a slow scanned spectra

(1/2�/min) of 101 plane of the samples are given in the Fig. 1b

and same is used to calculating the crystallite size.

The width of the (101) peak is broadened and its

intensity significantly decreased with an increase in the

10 20 30 40 50 60 70 80

Inte

nsit

y (a

.u)

CTN-09

CTN-03

CTN-06

ST

a

*

*

22 24 26 28 30

2θ scale (Degrees)2θ scale (Degrees)

*

Inte

nsit

y (a

.u)

CTN-09

CTN-06

CTN-03

ST

*b

Fig. 1 a PXRD patterns and

b enlarged spectra of 101 plane

of synthesized samples ST,

CTN-03, CTN-06, CTN-09 and

asterisk indicate Ce2O3

Table 1 Summary of data obtained by PXRD, UV–vis DRS absorption and BET measurements

Photo catalysts D (nm) (A´

) (A´

)3 kmax (nm) Eg (eV) Sarea (m2/g) Ti:Ce

ST 64 a = b = 3.7251

c = 9.2898

128.9014 380 3.3 63 1:0

CTN-03 62 a = b = 3.7165

c = 9.3854

129.5997 406 3.05 71 1:0.031

CTN-06 61 a = b = 3.7438

c = 9.3979

131.7213 409 3.03 84 1:0.062

CTN-09 60 a = b = 3.7117

c = 9.4072

129.6003 412 3.01 86 1:0.093

D: crystallite size in nm, A: lattice parameters, (A)3: unit cell volume, k max: absorption maxima in nm, Eg: band gap energies in eV, Sarea: surface

area obtained by N2 adsorption in m2/g, Ti:Ce ratio obtained from EDX data

J Sol-Gel Sci Technol

123

dopant concentration, as shown in Fig. 1b. This decrease

shows that the doping of the cerium ion inhibited the phase

transfer of titania from amorphous to anatase because the

addition of dopant suppresses the crystal growth. This also

indicates that Ce3?–TiO2 had higher thermal stability than

pure TiO2. Although the nitrogen content was increased up

to 4 at.%, no other crystalline phase like TiN could be

detected. Therefore, nitrogen ions can be assumed to be

incorporated in TiO2 [26]. The oxygen atom is replaced by

the nitrogen atom in the TiO2 lattice [27] and the marginal

shift may be due to the nitrogen incorporation in the TiO2

matrix [28].

This indicates a modification occurs in the electronic

states due to the replacement of oxygen by nitrogen on

doping [28, 29]. This leads to a substantial shift towards

the visible region in the optical absorption. The lattice of

the cell in doped samples expands in the c direction

because a small amount of Ce and nitrogen may enter

the TiO2 lattice. The interaction between Ce species and

Ti4? is because Ce2O3 has a strong basic property. Thus

partially cationic (e.g. Ce4?/Ti3?) exchanges between

Ce2O3 and TiO2 phases occur. This results in lattice

defects that enhance interaction between Ce2O3 and

TiO2.

860 870 880 890 900 910 920150000

155000

160000

165000

170000Ce 3da

Ce 3d5/2

Ce 3d3/2

Inte

nsit

y (c

ps)

Binding Energy (eV)

450 455 460 465 4700

2000

4000

6000

8000

10000

461.86 eV

456.12 eV

b

Ti 2p 1/2

Ti 2p 3/2

Ti 2p

Binding Energy (eV)524 528 532 536

2000

4000

6000

8000

10000

12000

14000 c

529.4 eV

527.4 eVO 1s

Inte

nsit

y (c

ps)

Inte

nsit

y (c

ps)

Inte

nsit

y (c

ps)

Inte

nsit

y (c

ps)

Binding Energy (eV)

375 380 385 390 395 400 405 410 415 420

21000

22000

23000

24000

25000

26000

27000

28000

29000d

396 eVN 1s

Binding Energy (eV)

1200 1000 800 600 400 200 00

50000

100000

150000

200000

250000

300000

350000

400000e

Ti 2s

Ti 3sTi 3p

Ti 2p

Ce 3d

O 1s

C 1s

N 1s

Binding Energy (eV)

Fig. 2 XPS spectra of a Ce 3d,

b Ti 2p, c O 1, d N 1s and

e CTN-06

J Sol-Gel Sci Technol

123

The BET surface area was measured for the synthesized

doped TiO2 samples. The values of the BET surface area

for the sample obtained for the samples are shown in

Table 1. It can be found that the co-doping of Ce3?/Ce4?

and nitrogen increase the surface area.

3.2 XPS analysis

Several studies have been devoted to Ce 3d XPS inter-

pretation for the two sets of spin–orbital multiplets corre-

sponding to the 3d3/2 and 3d5/2 by different researchers

[30–36]. The Ce 3d XPS CT-06 catalyst is shown in

Fig. 2a. The peaks at 881 and 885.2 eV represent Ce3?

while the peaks at 882.4, 887, and 895.2 eV represents

Ce4? in the Ce 3d5/2 spin–orbit split doublet. Corre-

spondingly, peaks at 899.6, 905.3, and 913.7 eV represent

Ce4?, while the peaks at 903.1 and 897.4 eV represent

Ce3? in the Ce 3d3/2 spin–orbit split doublet [31].

The Ti 2p3/2 peak was well fitted into the peaks of Ti4?.

From Fig. 2b, it can be seen that the Ti 2p3/2 spectra of

CTN-06 had an significant peak at 456.12 eV representing

Ti4?. The peaks at 461.86 eV represented Ti 2p1/2 peak.

Figure 2c shows the O 1s XPS spectra of the CTN-06

catalysts. Generally, the surface oxygen can be classified as

two types: the lattice oxygen (OI) and the chemisorbed

oxygen (OII) [31, 37, 38], which plays an important role in

the catalytic reaction. Furthermore, the oxygen in the gas or

that absorbed on the catalysts could easily exchange with

the chemisorbed oxygen. As shown in Fig. 2c, two peaks at

527.4 and 529.4 eV were observed on the CTN-06 catalyst,

which could be attributed to the OI type (527.4 eV) and OII

type (529.4 eV) oxygen on catalyst surface [31, 37, 38].

XPS reveals that the coexistence of Ce3?/Ce4? and N3-

results in charge imbalance leading to oxygen vacancies

and unsaturated chemical bonds that result in chemisorbed

oxygen on the catalyst surface. It could be seen from

Fig. 2d that the broad N 1s peak was found at 396–400 eV,

indicating nitrogen incorporation in the TiO2 lattice. The

broad XPS peak observed at 396–400 eV for the CTN-06

particles can be attributed to the substitutional O–Ti–N

sites in the TiO2 lattice [39–41]. The complete XPS spec-

trum of CTN-06 is shown in Fig. 2e.

3.3 UV–visible diffused reflectance spectral studies

for metal–metal charge transformation

The optical absorbance spectra of ST, CTN-03, CTN-06

and CTN-09 were measured in the UV–vis region and

shown in Fig. 3a. It shows that ST sample absorbs mostly

UV light, while all the prepared CTN-03, CTN-06 and

CTN-09 show absorbance in the region of 400–450 nm, the

absorbance increases with dopant concentration. The band

gap of the samples was determined from the bang edge [42]

as well as based on the Tauc plot. The respective calculated

band gap values are summarized in the Table 1. The co-

doping with nitrogen and cerium induces the narrowing of

the band gap of TiO2. In the metal ion-substituted TiO2, the

overlap of the conduction band due to Ti (d orbital) of TiO2

and the cerium (f orbital) orbital of the implanted cerium

ions could decrease the band gap of TiO2 to absorb visible

light (as shown in Scheme 1) [43].

Furthermore, nitrogen doping can form new states that

lie just above the valence band for the substitutional

nitrogen. This decreases the band gap of TiO2 and absorbs

200 300 400 500 600 700 800

1.6

1.4

1.2

1.0

0.8

0.4

0.2

Abs

orba

nce

(a.u

)

a

CTN-09

CTN-06

CTN-03

ST

Wavelength (nm)

0.6

750 1500 2250 3000 3750

b

Tra

nsm

itta

nce

(%)

Wavenumber (cm-1)

CTN-09

CTN-06

CTN-03

ST

1120

1424

16302853

29243438

Fig. 3 a UV–visible DRS absorption spectra and b FTIR spectra of

ST, CTN-03, CTN-06, CTN-09 samples

J Sol-Gel Sci Technol

123

the visible light (as shown in Scheme 1). Therefore,

nitrogen and cerium ion doping induces the formation of

new states close to the valence band and conduction band,

respectively. The interaction of the nitrogen and cerium ion

leads to the narrowing of the band gap and enhances the

photocatalytic activity in the visible light region. The shift

of the absorption edge in Ce3?/Ce4? and N3- doped titania

is due to the charge-transfer between the cerium ion

4f electrons and the TiO2 conduction or valence band

(tracks 1–6 in Scheme 1) [44, 45].

The absorbance of CTN-09 sample in the visible light

region is highest among these samples because the Ce3?

ion possesses an optically-active electron in the 4f orbital,

which have excited states of 4f7/2 and 4f5/2. The light

absorption in visible region cannot be due to 4f–4f transi-

tions because these 4f–4f transitions should be observed in

the infrared spectral region [46]. Thus the red shift of the

doped samples is attributed to the electron that can be

excited from the (1) valence band of Ce–TiO2-xNx into the

Ce 4f level (track 4 in Scheme 1) (2) ground state of CeO2

into the Ce 4f level (track 5 in Scheme 1) [47].

The UV–vis diffused reflectance spectrum of Ce–

TiO2-xNx exhibits the absorption that extends from the UV

to visible region (Fig. 3a). Since neither Ce3? nor the Ti4?

ion possess the 4f ? 5d transition in the spectral region,

the visible region absorption is due to the MMCT of Ti/Ce

assemblies i.e., Ti4?/Ce3? ? Ti3?/Ce4? [16, 47]. The

bimetallic Ti/Ce assembly shows intense MMCT absorp-

tion that can activate the catalyst (track 9 and 10 in

Scheme 1) [16].

The availability of charge is the sum of the detrapped

electrons and the conduction band electrons [48]. The

electrons in the Ce–TiO2-xNx increased indicating

increased electron injection from the Ce–TiO2-xNx con-

duction band. The charge density in the Ce–TiO2-xNx is

much smaller than the pure ST. Thus the charge transport

resistance of Ce–TiO2-xNx is higher due to the strong

recombination in Ti4?/Ce3? ? Ti3?/Ce4?. The increase of

the capacitance of Ce–TiO2-xNx indicates the enhance-

ment of the capability to accept or release electrons due to

the enhanced electron injection from the Ce4?. Ce4? trap

states grab the injected electrons from Ti3? and block the

interface recombination (track 6, 7, 8, 9 and 10). Ti3? trap

states in TiO2 determine the electron transport and result in

the suppression of interface recombination. The catalytic

activity is determined not only by the oxygen storage

capability but also the rate of the redox activity (MMCT

and interfacial charge transfer). Poor dispersion of ceria on

the surface of TiO2 as well as the transformation of Ce3?

and Ce4? lead directly to the disappearance of oxygen

vacancies in ceria particles. This slows down the reduction

rate of ceria and decreases its rate of degradation (oxida-

tion/reduction) in case of Ce–TiO2-xNx.

3.4 FTIR, SEM and EDX analysis

FT-IR of the ST, CTN-03, CTN-06 and CTN-09 were

analyzed and are shown in the Fig. 3b. All samples show a

relatively strong band at 1,630 cm-1, attributed to the O–H

bending vibration of chemisorbed/physisorbed H2O mole-

cule on the catalyst surface [49]. The peaks at

*3,600 cm-1 are attributed to the Ti–OH bond [31, 49].

The new peak at 1,120, 1,424 cm-1 was clearly observed

for cerium and nitrogen doped samples. This is due to the

bending vibration mode of the N–H bond [50] and this is

apparent because the intensity of the N–Ti absorption peak

increases with the increase in nitrogen concentration. The

peaks at 1,450 cm-1 correspond to N–Ti–N stretching and

N–Ti bending vibrations and are observed in all the doped

samples. FTIR shows that the N–Ti stretching and bending

Scheme 1 Schematic

illustration of interfacial charge

transfer between Ce3?–

TiO2-xNx and Ce2O3 and

Metal–metal charge transfer

between Ti3? and Ce4?

mechanism (highlighted) in

valence band structure of Ce3?–

TiO2-xNx and Ce2O3, the

mechanisms of photoresponse

under visible light and

photogenerated electron transfer

J Sol-Gel Sci Technol

123

vibration peaks were prominent and confirm the incorpo-

ration of nitrogen in the TiO2. The broad peak around

600 cm-1 was ascribed to the absorption bands of Ti–O/

Ce–O and O–Ti–O/O–Ce–O with no indication of Ce–O

bond vibration [51]. The increase in concentration of cer-

ium oxide species is due to expected competition between

the Ce and N with the hydroxyl group and accounts for the

difference in electronegativity of the supports. A sub

Fig. 4 SEM images of a ST, b CTN-03, c CTN-06, d CTN-09, and e EDX spectra of CTN-06

J Sol-Gel Sci Technol

123

monolayer of oxide is formed at similar electronegativity

values but a multilayer is formed when the latter values are

between Ti and Ce.

FESEM images of ST, CTN-03, CTN-06 and CTN-09

samples are shown in Fig. 4a–d. The morphology of the

sample prepared with water is made of many agglomera-

tions of smaller crystallites. The smaller crystallite sizes

are due to cerium ion, which are less reactive than the

titania precursor species. This retards the condensation and

crystallization and is consistent with the values obtained in

PXRD. The images clearly indicate that the particle was

homogeneous with agglomerates and agglomeration was

reduced in ST. The diameter of the particles was about

40–100 nm. These smaller crystallites are closely packed

in each particle, which reduces the porosity within each

agglomerated assembly. Figure 4e shows the EDX spectra

of CTN-06 samples, which confirm the doping of ceria and

nitrogen in the samples. The Ti:Ce ratio of the samples are

given in Table 1.

3.5 Mechanism for interfacial charge transformation

for the photocatalytic degradation

The projected contribution to the energy levels from O

(2s22p4), Ti (3s23p63d24s2), Ce (4f15d16s2) and N (2s22p3)

are given in Scheme 1. It is found that the valence band is

mainly composed of O 2p electric states strongly hybrid-

ized with Ti 3d and Ce 4f. The conduction band (CB) is

determined by the Ti 3d and Ce 4f. The CB is determined

by the Ti 3d electronic states strongly hybridized with O

2p. The energy level of introduced Ce is mainly composed

of Ce 4f and Ce 5d electronic states, hybridized with O

2p corresponding to Ce2O3. Other hybridized states are

valence band (VB) composed of N 2p electronic states

hybridized with Ti 3d and Ce 4f. The CB originates from Ti

3d cubic states hybridized with N 2p. The DRS (Sect. 3.3)

results demonstrated that while ST had no absorption in the

visible region ([400 nm), CTN had significant absorption

between 400 and 500 nm, which increased with increase of

cerium ion content. The absorption in the visible region has

been observed for Ce3?-doped Y2O3, Lu2O3, ZrO2 or

La2O3 wherein the onset of the absorption of Ce3? was

around 460 nm [51, 52]. CeO2 is an n-type semiconductor

with a band gap of about 3.2 eV [53, 54] and the absorption

at 400–500 nm by the CTN catalysts is not due to CeO2 but

because of Ce2O3 (as shown in Scheme 1).

Ce3? ion possesses a single optically active electron

with the ground state configuration in the 4f1 orbital. There

are only two electronic levels, an excited state of 2F7/2 and

a ground state of 2F5/2, in this configuration. The 4f–

4f transitions attributed to Ce3? can be observed only in the

infrared spectral region. However, Ce3? has the first state

configuration 5d1 that is rather close in energy. The

electronic dipole transitions 4f1$ 5d1 may occur in either

UV or visible region (track 1 and 2 in the Scheme 1).

Based on the valence band of Ce3?–TiO2-xNx from

XPS, electron–hole pairs could be generated in both types

of catalysts: Ce3?–TiO2-xNx and Ce2O3 in three ways

(Scheme 1). An electron can be excited from the (a)

valence band of Ce3?–TiO2-xNx into Ti3? defect level

(track 3 in Scheme 1), (b) valence band of Ce3?–TiO2-xNx

into Ce 4f level (track 4 in the Scheme 1) and (c) ground

state of Ce2O3 into Ce 4f level (track 5 in the Scheme 1).

Therefore, the red shift of absorption edge for Ce3?–

TiO2-xNx was expressed in the following Eqs. (1) and (2).

The Ce 4f levels play an important role in generating

electron–hole pairs.

Ce3þ�TiO2�xNx þ hv! e� þ hþ ð1Þ

Ce2O3�!hv

e� þ hþ ð2Þ

The charge transfer (metal-to-metal and interfacial) is the

rate determining step for photocatalytic reaction [55].

In this investigation, the PL emission spectra of catalysts

were investigated to determine the separation efficiency of

charge carriers. To enhance the photocatalytic activity, it is

critical to separate the photo generated electrons and holes

efficiently. The PL emission spectra of all samples were

examined in the range of 400–700 nm, and shown in

Fig. 5. It can be seen that the PL intensity of ST was sig-

nificantly higher than that of CTN samples. The lower PL

intensity of modified ST may be attributed to the efficient

capture of charge carriers by the energy levels of surface

states related to N–O species. The lower intensity for the

modified indicated a lower recombination rate. Hence, the

PL results imply the co-modified of Ce and N can effi-

ciently accelerate the separation of charge carriers.

For CTN, the Ce 4f level plays an important role in

interfacial charge transfer and elimination of electron–hole

Inte

nsit

y (a

.u)

CNT-09

CNT-06

CNT-03

ST

700650600550500450400

Wavelength in nm

Fig. 5 Photoluminescence spectra of ST and Ce–TiO2-xNX samples

J Sol-Gel Sci Technol

123

recombination. Cerium ions are superior to the oxygen

molecule in their ability to trap CB electrons of TiO2 (track

7 and 8 in the Scheme 1) [56, 57]. The electrons trapped in

Ce4?/Ce3? site are then transferred to the surrounding

adsorbed O2. Therefore, presence of Ce4? on TiO2 surface

may promote the processes indicated by Eqs. (3) and (4).

Equation (5) indicates the formation of OH�.

For CTN, the formation of labile oxygen vacancies and

particularly the relatively high mobility of bulk oxygen

species have been reported [57]. Below the conduction

band edge of TiO2 as defect levels of Ti3? (Scheme 1),

ceria had a high oxygen transport and storage capacity [58]

and thus the excited electrons can be easily transferred to

O2 on the surface of CTN catalysts.

Ce4þ þ TiO2ðe�Þ ! Ce3þ þ TiO2 ð3Þ

Ce3þ þ O2 ! Ce4þ þ O�2 ð4Þ

2Hþ þ�� O2 ! 2HO � ð5Þ

Ti3? can also form a defect level and act as hole-traps to

promote charge transfer (track 6 in Scheme 1). These

defects on the surface of titania can suppress the recombi-

nation of electron–hole pairs. The mechanism of interfacial

charge transfer [59, 60] can be expressed in Eqs. (6) and (7).

Ti4þ � O�Hþ e� ! Ti3þ � O�H ð6Þ

Ti3þ � O�Hþ hþ ! Ti4þ � �O�H ð7Þ

However, to certain degree, the higher content of Ti3? on

the CTN-03 and CTN-06 surface compared to that of TiO2

results in better interfacial charge transfer and superior

photocatalytic activity.

The photocatalytic activity of Ce–TiO2-xNx and P25

(used as a comparison) was evaluated (Fig. 6a, b) by

degradation of MB and MV as a model compound under

visible light. The photocatalytic properties of all the sam-

ples was in the following order; CTN-06 [ CTN-

03 [ CTN-09 [ ST [ P25. The photocatalytic degrada-

tion of MB and MV were fitted with first order kinetics

(log(C/C0) = kt), C is the concentration in the solution

after irradiation for time t and C0 is the initial concentra-

tion. The kinetics of degradation of MB and MV can be

expressed by Langmuir–Hinshelwood model. The kinetic

rate coefficient (k) values of CTN-06, CTN-03, CTN-09,

ST, P25 were calculated to be 0.0174, 0.0161, 0.0095,

0.0082, 0.0017 and 0.0454, 0.0278, 0.0242, 0.0183,

0.0005 min-1 for the MB and MV dyes respectively.

CTN-06 exhibits highest photocatalytic activity than pure

ST and P25, which is due to the narrow band gap [61] higher

surface area, interfacial charge transfer and MMCT. The

sample with higher concentration of Ce and N shows a lower

degradation rate of the dyes. This is due to defects on the

surface of Ce–TiO2-xNx that act as excess recombination

centers of photo induced electron–hole pairs [61]. As a result,

it leads to the decrease of photocatalytic activity.

Ce3? is incorporated to the system and this is considered

more effective than other lanthanides due to the 4f–

5d transition. Its Fermi energy level is very close to the

conduction band edge of TiO2. This contributes to the

accelerating the flow of electrons in the phenomenon of

interface charge transfer and MMCT within the Ce–

TiO2-xNx. For CTN-06, we can clearly observe that the

photocatalytic activity improved and the degradation effi-

ciency was 70 and 90 % for MB and MV dyes, respec-

tively. Therefore, the higher degradation rates of the dyes

in the presence of CTN-06 compared to other modified

materials is likely due to the synergistic effect of Ce and N

in addition to the interfacial charge transfer and MMCT.

4 Conclusions

The existence of cerium ion in ?3 or ?4 state, and nitrogen

in -3 state support the Ce–TiO2-xNx particles and provide

-40 -20 0 20 40 600.0

0.2

0.4

0.6

0.8

1.0

C/C

0C

/C0

Time (min)

WC DP25 ST CTN-03 CTN-06 CTN-09

a

-10 0 10 20 30 40 50 60 700.0

0.2

0.4

0.6

0.8

1.0

Time (min)

WC DP25 ST CTN-03 CTN-06 CTN-09

b

Fig. 6 Variation of normalized concentration with time for the

photocatalytic degradation of a MB and b MV by CTN-03, CTN-06,

CTN-09, DP25 and ST photocatalysts in the presence of visible light

J Sol-Gel Sci Technol

123

highly active sites for the degradation of dyes at the

periphery of the Ce–O–Ti interface. UV–visible DRS

studies shows that the MMCT of Ti4?/Ce3? ? Ti3?/Ce4?

and electron–hole pair separation between two interfaces of

Ce–TiO2-xNx and Ce2O3 are responsible for the visible

light activity. The degradation kinetics of two dyes, MB

and MV, in the presence of Ce–TiO2-xNx exhibited sig-

nificantly high visible light activity compared to commer-

cial Degussa P25.

Acknowledgments Authors thank to University Grants Commis-

sion (UGC), RKE thanks UGC for awarding Dr. D. S. Kothari Post

Doctoral Fellowship.

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