Nitrogen Doped TiO2 NRs

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Facile synthesis of nitrogen self-doped rutile TiO2 nanorods{

Shuan Wang, Junmin Xu, Hualin Ding, Shusheng Pan, Yunxia Zhang and Guanghai Li *

Received 25th May 2012, Accepted 26th June 2012

DOI: 10.1039/c2ce25827g

Nitrogen doping is a promising method to enhance the visible light absorption and photo-catalytic

activity of TiO2. A new method is reported for the synthesis of nitrogen self-doped rutile TiO2

nanorods, along with the formation study of V-shaped N-doped TiO2 nanorods, using TiN as a

precursor and using a hydrothermal method. Our synthesis method gives a facile and easy way to

control nitrogen doping in a TiO2 lattice. Two types of the V-shaped nanorods, with a (101) coherent

boundary of either 114.4u or 134.9u inner angle, were observed. The N-doped TiO2 nanorods exhibit

an enhanced visible light absorption and red-shift in band gap in comparison with pure rutile TiO2

nanopowders. The mechanisms of N doping and the formation of the V-shaped nanorods are

analyzed and discussed. The oriented attachment and Ostwald ripening are considered responsible for

the formation and growth of the straight and V-shaped N-doped TiO2 nanorods.

Introduction

Titanium dioxide (TiO2), with a wide band gap of 3.2 eV for

anatase or 3.0 eV for rutile, is an important semiconductor

photocatalyst and has attracted considerable attention. The

ability to control the band gap of TiO2 nanocrystals and to

enhance the utilization rate of the solar spectrum is essential for

applications in fields such as dye-sensitized solar cells1 and photo-

catalysis.2 Many methods have been explored to reduce the band

gap of TiO2, such as doping transition-metal (iron,3 vanadium,4

nickel5 and chromiump6) and non-metal (nitrogen,7 sulfur,8

fluorine,9 and carbon10) into the TiO2 host lattice. Among them,

the nitrogen doping is an effective method to enhance visible light

absorption and photo-catalytic activity.7,11–13

Different methods have been developed to incorporate

nitrogen in TiO2, and those methods can be generally classified

into three categories: (1) sputtering and implantation techniques,

mainly used to prepare single crystalline or polycrystalline

N-doped TiO2 thin films;7,14 (2) high temperature annealing

treatment under a N-containing atmosphere;15 and (3) wet

methods, including sol–gel,16 solvothermal and hydrothermal

methods.17–19 The first two methods need either a high

temperature or complicated and expensive equipment, while

the wet chemical method is simple and effective in controlling

both the nitrogen doping content and TiO2 nanocrystal size,

through changing the experimental parameters, such as reaction

temperature, solution pH value and solvent system.

It is well known that titanium nitride (TiN) is a metallic conductor

with a partially filled band and a chemical bond of simultaneously

metallic, covalent and ionic characters,20 which has a NaCl-like cubic

crystallization in the rock-salt structure, with N atoms occupying

interstitial positions in a close-packed arrangement of Ti atoms.21

The fact that TiO2

can be prepared via a simple oxidation process of

TiN22 promises an opportunity for nitrogen self doping.

TiO2 nanostructures with different morphologies, such as

nanowires,23 nanorods,24 nanotubes,19 and nanoflowers,25 have been

prepared in recent years. The V-shaped nanostructures have been

observed in SnO2,26 RuO2,27 and ZnSe.28 However, there is no report

about the V-shaped N-doped TiO2 nanostructures. The V-shaped

structures are of particular interest because the sudden break down in

lattice periodicity at the junction offers a good lateral confinement,

and thus can enhance the excitonic optical response.29 It is believed

that this complex nanorod derived structure could offer new

opportunities in tailoring the properties of discrete 1D nanostruc-

tures and in 3D organization of nanostructured materials.30

Here, a new method for the synthesis of nitrogen self-dopedrutile TiO2 nanorods is reported, along with the formation study

of V-shaped N-doped TiO2 nanorods, using TiN as a precursor,

and using hydrothermal method. The separation of N from TiN

provided a self doping in the formation process of TiO2

nanorods. An enhanced visible light absorption and red-shift

of the optical band gap were observed.

Experimental

Materials and synthesis

TiN nanopowders, provided by Hefei Kaier Nano Energy

Science and Technology Co., Ltd., Hefei, China, were produced

Key Laboratory of Materials Physics, Anhui Key Lab of Nanomaterialsand Nanostructure, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P.R. China. E-mail: [email protected];Fax: +86-551-5591437; Tel: +86-551-5591437 { Electronic Supplementary Information (ESI) available: [FESEMimages of TiN precursor powders, HRTEM image of the tapered tip inN-doped TiO2 nanorod and the corresponding schematic orientationrelations of the {111} facets on the tapered tip, HRTEM images of N-doped TiO2 nanocrystals at the early stage of hydrothermal treatment,FESEM image and XRD pattern of the product hydrothermally treatedwithout HCl and the determination of the band gap from the plots of (ahn)n vs. hn: with n=1/2 and n=2]. See DOI: 10.1039/c2ce25827g

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by the RF induction thermal plasma process using Titanium

tetrachloride as the precursor. In a typical synthesis process of

N-doped TiO2 nanorods, 62 mg TiN was added into 30 ml of

distilled water with magnetic stirring, then 6.0 ml of HCl (35%)

was added by drops into the above solution with continuous

stirring for 30 min. The final concentration of HCl is 2.0 M. The

solution was finally transferred to a 50 ml Teflon-lined autoclave

and heated at 180 u

C for different times. After cooling down toroom temperature, the precipitate was collected and washed with

distilled water and absolute alcohol several times, and then dried

at 80 uC for 24 h in air.

Characterization

The as-prepared products were characterized by X-ray diffrac-

tion (XRD, X’Pert Pro MPD), X-ray photoelectron spectro-

scopy (XPS, Thermo ESCALAB 250), field emission scanning

electron microscopy (FESEM, Sirion 200) and high resolution

electron microscopy (HRTEM, JEM 2010). Raman spectra were

recorded at room temperature using a confocal microprobe

Raman system (Renishaw, inVia) with the excitation wavelength

of 532 nm. UV-vis diffuse reflectance spectra were recorded on a

Shimadzu UV3600 spectrophotometer, equipped with an inte-

grating sphere attachment (Shimadzu ISR-260), using Ba2SO4

powder as an internal reference.

Results and discussion

Fig. 1 shows XRD patterns of the TiN precursor and the as-

prepared product that was hydrothermally treated at 180 uC for

12 h. One can see that all the diffraction peaks of the precursor

can be well indexed to cubic phase TiN (JCPDS card no. 38-

1420), see curve (1) in Fig. 1, while that of the as-prepared

product can be indexed to rutile phase TiO2

(JCPDS card no. 04-

0551) and no other phases or residual TiN can be detected, see

curve (2) in Fig. 1, indicating that all the TiN nanopowders have

been transformed into TiO2 after the hydrothermal treatment.

Fig. 2 shows the typical FESEM images of the as-prepared

product that was hydrothermally treated at 180 uC for 12 h. One

can see that the as-prepared product consists of the rectangle

nanorods with tapered tips and 20–50 nm in diameter and 500–

600 nm in length. It is worth noting that there exist some

branched V-shaped nanorods, as can be clearly seen in the

enlarged FESEM image in Fig. 2b, and the content of V-shaped

nanorods is over 30%. The TiN precursor exhibits an agglom-eration of nanoparticles with an average diameter of about

20 nm. (Fig. S1{)

To identify the element chemical state of the as-prepared

nanorods, XPS analysis was performed, as shown in Fig. 3 for

the nanorods that were hydrothermally treated at 180 uC for

12 h. The survey spectrum in Fig. 3a shows the existence of only

Ti, O, and N, without any impurities. The binding energy at

458.8 eV and 464.6 eV are attributed to the Ti 2p3/2 and Ti 2p1/2

in rutile phase TiO2, respectively, see Fig. 3b, which matches the

position for Ti4+ in TiO2, and is slightly lower than the pure

TiO2. The decrease in binding energy might be due to the

different electronic interactions between Ti and the doped

nitrogen, as pointed out by Satish et al.31

It was found that inthe TiN film, the binding energy of Ti 2p is at 455.2 eV, 22 which

is not present in Fig. 3b, indicating that no TiN is present in the

product. The O 1s peak shown in Fig. 3c is asymmetric and

broad, showing at least two kinds of oxygen species, including

crystal lattice oxygen and chemisorbed oxygen.32 Gaussian

fitting gives two peaks, one peak is at 530.0 eV assigned to

lattice oxygen in TiO2, and is well consistent with the previous

study,33 and another peak at 531.8 eV is attributed to

chemisorbed oxygen. The binding energy at 400.1 eV in

Fig. 1 XRD patterns of TiN precursor and N-doped TiO2, and that

from JCPDS card no. 38-1420 of TiN and JCPDS card no. 04-0551 of

rutile TiO2.

Fig. 2 (a) Low and (b) high magnification FESEM images of N-doped

TiO2 nanorods.

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Fig. 3d can be assigned to N 1s, which is considered to be related

to the nitrogen doping in TiO2, though there is some debate in

the literature. The binding energy of N 1s generally lies in the

range of 396–404 eV, depending strongly on the preparing

methods and conditions.34 In the thermal oxidation of titanium

nitride, Saha assigned N 1s peaks (at 400 and 402 eV) to

molecularly chemisorbed c-N2.22 This assignment is questionable

as it is widely known that molecular N2 is not chemisorbed on

metal oxides, such as TiO2, at room temperature. In the N–TiO2

powder prepared by an atmospheric plasma procedure and

posterior annealing, Chen attributed N 1s at 395.8–397.8 eV to

Ti–N.35

It was found that the binding energy of nitrogen insubstitution positions of the TiO2 lattice is at 397 eV.7,36 As there

is neither XPS peak at 397 eV nor TiN crystal phase in our

samples, we attribute binding energy at 400.1 eV to the

incorporated interstitial N in the TiO2 lattice, where nitrogen

simultaneously bonds to oxygen and to titanium in a defective

lattice site (i.e., in a kind of Ti–N–O or Ti–O–N local structure).

The quantification of Ti 2p and O 1s peaks gives average Ti:O

atomic ratio of nearly 1 : 2, while that of N 1s gives nitrogen

doping content of about 1.09 at %.

Raman spectroscopy is a very effective characterization

method in distinguishing different phase structures of TiO2 by

their characteristic vibration Raman peaks. Fig. 4 shows the

Raman spectra of N-doped TiO2 nanorods together with that of TiN nanopowders. The Raman bands at 190 and 536 cm21 in

Fig. 4a are the first-order scatterings of nonstoichiometric TiN,

which shows a red shift in comparison with bulk TiN (at 200 and

550 cm21).34 The Raman bands at 148, 249, 441, and 607 cm21

in Fig. 4b are ascribed to the B1g, two-phonon scattering, Eg,

and A1g modes of rutile phase TiO2, respectively.37 The peak

positions of the Eg and A1g modes (at 441 and 607 cm21,

respectively) exhibit a red shift in comparison with pure rutile

TiO2 (at 447 and 612 cm21, respectively), while that of the two-

phonon scattering mode at 249 cm21 shows a blue shift with

respect to pure rutile TiO2 (235 cm21).37 The size of the N-doped

TiO2 nanorods is in the nanometer scale, and the resulting

phonon confinement effect38 might result in the shift. The

interstitial N in TiO2 lattice is considered another reason that

induces the shift due to the nonstoichiometric effect.39 The

corresponding Gaussian fitting gives three more peaks situated

at 336, 543, and 689 cm21(Fig. 4b), and the bands at 336 and

543 cm21 are the first-order scattering, while that at 689 cm21 is

ascribed to the second-order scattering of nonstoichiometric

TiN,34 which are clearly different from that of pure TiN

nanopowders (Fig. 4a). The band at 200 cm21 corresponding

to the first-order scatterings of nonstoichiometric TiN cannot be

clearly observed, which might be due to its overlapping with the

two-phonon scattering of TiO2 at 249 cm21. The fact that the

bands at 336 and 689 cm

21

only exist in the N-doped TiO2nanorods and do not in the pure TiN nanopowders indicates that

the bands at 336, 543, and 689 cm21 come from the N doping,

and there is not any TiN remaining in the N-doped TiO2

nanorods. The appearance of the characteristic vibration bands

of Ti–N indicates that nitrogen substitutes for some oxygen

atoms in the TiO2 lattice, which is consistent with XPS results.

Fig. 5 shows TEM images of the N-doped TiO2 straight

nanorods. The edges of the nanorods are either smooth or have a

zigzag shape (see the arrows in Fig. 5a). The zigzag shape is due

to the inhomogeneous redeposition rate of the dissolved species

on the surfaces of larger crystals in the crystal coarsening

process. From the lattice fringe shown in Fig. 5b, it can be seen

that the crystal planes are perpendicular to the growth directionof the nanorod, and the measured inter-plane spacing (0.33 nm)

matches well with the literature reported value of the (110) plane

in rutile TiO2 (0.326 nm), indicating that the nanorod grows

along the [001] direction. The corresponding SAED pattern

shown in the inset of Fig. 5b can be indexed to the rutile phase

with a tetragonal structure, and the [110] axis is perpendicular to

the nanorod growth direction, which provides further evidence

that the nanorod grows along the [001] direction. The top end of

the nanorod has a four-sided tip with the {111} facets as

determined by measuring the inter-plane spacing and calculating

the angle between the top end facet and the (110) plane. (Fig.

S2{)

Fig. 3 XPS spectra of TiO2 nanorods hydrothermally treated at 180 uC

for 12 h with 2.0 M HCl: (a) the survey spectrum, the high resolution

scan of (b) Ti 2p, (c) O 1s and (d) N 1s.

Fig. 4 Raman spectra of (a) TiN nanopowders and (b) N-doped TiO2

nanorods. B1g, 2-p (two-phonon scattering), Eg and A1g modes come

from rutile phase TiO2.


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Fig. 6 shows TEM images of the V-shaped N-doped TiO2

nanorods with different inner angles. Detailed analysis revealed

that there are only two types of inner angles of the V-shaped

nanorods, one is about 114u and another is about 135u, as

measured from Fig. 6a and b. The V-shaped TiO2 nanorods with

an inner angle of 114u were reported recently,40 while the angle of

135u is first observed in the present study. The inner angle of the

V-shaped nanorod, h, also can be calculated from the HRTEM

image by the following equation:

h~2cos{1

h1h2zk 1k 2

a2 z

l 1l 2

c2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffih1

2zk 1

2

a2 z

l 12

c2

! h2

2zk 2

2

a2 z

l 22

c2

!v uut

(1)

where (h1 k 1 l 1) and (h 2 k 2 l 2) are the indices of the crystal lattice

plane and the boundary plane, respectively, a (= 0.459 nm) and c

(= 0.296 nm) are the lattice constants of rutile TiO2. The

calculation results show that the angle between plane (200)a and

(200)b is 114.4u (Fig. 7a), while that in Fig. 7b is 134.9u, which

are in good agreement with the measured results shown in Fig. 6.

From Fig. 7, one also can see that the grain boundaries of the

two branch crystals in both types of V-shaped nanorods are all

(101) lattice planes and the crystal lattice planes on both sides are

symmetrical indicating that the grain boundary is a coherent

twin boundary.

The time evolution of the hydrothermal process was analyzed

to further investigate the formation mechanism of the N-dopedTiO2 nanorods. Fig. 8 shows the XRD patterns of the products

at different hydrothermal times. One can see that there is only a

very small amount of rutile phase TiO2 (about 6 at %) when

hydrothermally treated for only one hour, and most of the

nanopowder is still TiN phase. The XRD peak intensity of the

TiN phase decreases while that of the rutile TiO2 phase increases

with increasing hydrothermal time, indicating the increase in the

content of rutile TiO2 phase. When the hydrothermal time was

increased to 12 h, all the TiN was transformed to TiO2. The

Fig. 5 (a) TEM and (b) HRTEM images of N-doped TiO2 straight

nanorods. The arrow in (a) shows the zigzag edge of the nanorods, and

the inset in (b) is the corresponding SAED pattern.

Fig. 6 TEM images of V-shaped N-doped TiO2 nanorods: (a) type I, (b)

type II and (c) a mass of V-shaped N-doped TiO 2 nanorods.

Fig. 7 HRTEM images of V-shaped N-doped TiO2 nanorods: (a) type I

and (e) type II. The insets are corresponding SEAD patterns of (b) grain

a, (c) junction and (d) grain b in type I V-shaped nanorod, (f) grain a, (g)

junction and (h) grain b in type II V-shaped nanorod.


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phase percentage of TiN and TiO2, as estimated from the

reference intensity ratio methods using corundum as an internal

standard, is listed in Table 1. It is worth to note that no anatase

or brookite phase was detected in all products, indicating that

the TiN precursor transforms directly to TiO2 rutile phase.

In the hydrothermal synthesis, the classic Ostwald ripening

(OR) growth mechanism, referring to a process where larger

particles grow at the expense of smaller ones, is a common

crystal growth mechanism. The growth rate of larger particles is

directly proportional to the solubility of the solid and the tension

of the solid–liquid interface, and is affected by particle size

distribution.41 Oriented attachment (OA) is an alternative

growth pathway, in which larger crystals form by crystal-

lographically controlled assembly of smaller nanocrystals.42

Fig. 9 shows the TEM images of the products synthesized at

different hydrothermal times. One can see that after hydro-

thermal reaction for 1 h, the product still consists of spherical

nanocrystals, and some nanocrystals increase in size in compar-

ison with the TiN precursor (Fig. S3{). It is worth noting that

some nanocrystals connect with each other, as shown in Fig. 9a,

implying the orientation attachment of some rutile TiO2

nanocrystals. With increasing hydrothermal time, the TiO2

nanocrystals grow in size and transform into nanorods (Fig. 9b

and c). The formation of rutile TiO2 grains is considered to be

derived directly from TiN nanocrystals where N separates out

with the assistance of HCl under the hydrothermal condition.

We found that the transformation from TiN to TiO2 can not

happen without the assistance of HCl (Fig. S4{). The chemical

reactions in the transformation from TiN to TiO2 can be

described by the following equations:43

TiN A Ti3+ + N32

Ti3+ + H2O A Ti(OH)2 + H+

Ti(OH)2 + O2 A Ti(IV)oxo species

Ti(IV)oxo species + N speciesA

TiO22xNx

where the Ti(IV) oxo species is an intermediate between TiO2+

and TiO2, consisting of partly dehydrated polymeric Ti(IV)

hydroxide.44,45 In the present case, Ti(OH)2 is derived from the

precursor solution (Ti3+N32) and O2 may come from either the

autoclave or the reaction solution. The N species separating from

TiN provides the nitrogen source for the self doping of the TiO2

nanocrystal.

It was found that the V-shaped TiO2 nanorod forms at the

early stage in the hydrothermal process, in which small grains

aggregate into some V-shaped cores by OA growth model and

then grow in size, see the white arrows in Fig. 9. In a nanoscale

system, the surface free energy accounts for a significant share intotal free energy, and the low energy surface will dominate,

accompanying the elimination of the high energy surface in the

crystal growth process. Due to much lower surface free energy

compared with other planes, the (110) plane will be inclined to

form to lower the system total free energy.46 Under the

hydrothermal condition, TiO2 nanocrystals have a high mobility.

The orientations of TiO2 nanocrystals can be adjusted to achieve

a thermodynamically stable state, and thus can lead to the

formation of V-shaped TiO2 by the OA growth model.

Based on above analysis, a possible formation mechanism of

the straight and V-shaped nanorods is schematically illustrated

in Fig. 10. Firstly, small primary TiO2 grains are anisotropically

Fig. 8 XRD patterns of the products obtained at different hydro-

thermal times (h). The JCPDS card no. 38-1420 is TiN and JCPDS card

no. 04-0551 is rutile TiO2.

Table 1 TiO2 contents after hydrothermal treatment for different times

Time (h) TiN (at %) Rutile TiO2 (at %)

1 94 62 56 444 14 868 2 98

12 0 100

Fig. 9 TEM images of products hydrothermally treated for the times of

(a) 1, (b) 2, (c) 4 and (d) 8 h. The black arrows in (a) show the connection

of some nanoparticles, and the white arrows show the formation of

V-shaped nanorods and increase in their size.


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derived from TiN, in which the (110), (111) and (200) facets

survive due to their relatively lower surface free energy.46 Other

facets with a high energy surface, such as (101), disappear and

are hard to expose. Then, these TiO2 grains aggregate to form

nanorods by the OA growth model, and then grow in size along

the [001] direction. The small V-shaped nanocrystal forms by the

OA mechanism through attaching two similar grains, in which

two (101) high energy planes join together and are eliminated, and

finally follows the OR coarsening process. The formation of the

V-shaped nanocrystal is essentially a facet-mediated aggregation

and there is sequential elimination of high energy surfaces, owing

to the significant contribution of surface energy to the total free

energy in a nanoscale system. The formation of the V-shaped TiO2

nanorods provides a typical case for a facet-mediated aggregation,

which has essentially significance for tailoring or constructing

complex nanorod derived nanostructures.

It was found that the hydrothermal temperature and HCl

concentration also affect the shape and size of the N-doped TiO2.

Fig. 11 shows the morphologies of the products hydrothermally

treated at different temperatures and HCl concentrations for 12 h.One can see that the size of TiO2 nanorods increases with

increasing temperature due to the accelerated OR process at

elevated temperatures, see Fig. 11a–d. At low HCl concentration,

there exist some irregular polygon nanocrystals among the TiO2

nanorods, see Fig. 11e and f, and these irregular polygon

nanocrystals almost disappear at high HCl concentration, see

Fig. 11g and h. It should be noted that the V-shaped TiO2

nanorods can be observed in all conditions, indicating the

formation of V-shaped nanorods is an inherent property of the

transformation from TiN to N-doped TiO2 nanocrystals. It is

reported that the roleof Cl2 can be two-fold:43 one is retarding the

formation of TiO2 by changing the composition or coordination

structure of the growing unit,47

and another is influencing themorphology through adsorption of Cl21 onto the (110) plane of

rutile TiO2.48 In our case, the high Cl2 concentration will facilitate

the nucleation and growth of rutile phase nanorods. This result

indicates that the shape and size of N-doped TiO2 nanorods can

be easily adjusted by changing the hydrothermal reaction

conditions. Further XPS analyses show that the N doping content

decreases with the increasing HCl concentration, and is about

1.49, 1.38 and 1.09 at % for 0.5, 1.0 and 2.0 M HCl, respectively.

The color of the N-doped TiO2 nanorods is slightly pale blue, in

contrast to the white color of rutile TiO2 nanopowders. Fig. 12

shows the diffuse reflectance spectra of N-doped TiO2 nanorods

synthesised by hydrothermal treatment at 180 uC with different

HCl concentrations and that of pure rutile TiO2 nanopowders. A

substantial enhancement in the visible light absorption can be seen

for N-doped TiO2 nanorods in comparison with pure rutile TiO2

nanopowders, which is consistent with the pale blue color of the

sample, and is considered to be due to the doping of N ions in

TiO2. Because of the sharp absorption edge of the N-doped TiO2

nanorods, the optical band gap energy can be calculated directly

by extrapolating the linear portion of the absorption edge to zero

of the absorbance, as shown in the inset in Fig. 12. Based on this,

the optical band gap is calculated to be about 2.98, 2.94 and

2.91 eV for the N-doped TiO2 nanorods with HCl concentrations

of 1.09, 1.38 and 1.49 at %, respectively. The optical band gap forpure rutile TiO2 nanopowders is about 3.01 eV, which is in a good

agreement with the reported value. The optical band gap, E g,

obtained by this method is more rational than that calculated by

extrapolating the linear portion of the (ahn)n vs. hn plot to a = 0

from the general relation of (ahn)n = B (hn2E g) (n = 1/2 or 2,

depending on whether the transition is indirect or direct,

respectively) (Fig. S5{). One can see the band gap of TiO2

nanorods decreases obviously upon N doping, and the higher the

doping content, the lower the optical band gap. This red-shift is

considered to be dueto the increased number of N ions in the TiO2

nanorods. From Fig. 12 one also can see that there is a tail-up in

the region of 500–800 nm. In fact, we found that this tail-up

Fig. 10 Schematic illustration of the formation mechanism of straight

and V-shaped N-doped TiO2 nanorods, the four-sided top is clearly

indicated.

Fig. 11 FESEM images of the products prepared at temperatures of (a)

160, (b) 180, (c) 200 and (d) 220 uC with 2.0 M HCl concentration, and at

HCl concentrations of (e) 0.25, (f) 0.5, (g) 1.0 and (h) 2.0 M at 180 uC.

The hydrothermal time is 12 h for all the conditions.


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disappeared if the N-doped TiO2 nanorods were annealed at

500 uC in air. As both XPS and Raman analyses denied the

existence of TiN in the N-doped TiO2 nanorods, it is thus

considered the tail-up is due to the relative high N doping content

in TiO2.

It was found that by adjusting the hydrothermal solvent and

pH value of the precursor solution, N-doped anatase or anatase/

rutile TiO2 nanopowders, nanorods and even nanospheres can be

obtained.

The decrease in band gap and the increase in visible light

absorption are beneficial in enhancing the performance in dye-

sensitized solar cells and photo-catalysis, further work isunderway.

Conclusion

N-doped TiO2 nanorods have been synthesized directly from a

TiN precursor by a facile hydrothermal method in the presence

of HCl solution. The nanorods are highly crystalline with a rutile

phase, and exhibit both straight and V-shaped morphologies. It

was found that the lower the HCl concentration, the higher the

N doping content and 1.09 at% N doping can be obtained for

2.0 M HCl concentration. There are two types of the V-shaped

N-doped TiO2 nanorods, one with a 114.4u

inner angle andanother with an angle of 134.9u, and these two types of V-shaped

nanorods have the same coherent boundary of the (101) plane.

The size and shape of the N-doped TiO2 nanorods can be

controlled by the hydrothermal conditions. The oriented

attachment and Ostwald ripening are considered responsible

for the formation and growth of the straight and V-shaped

N-doped TiO2 nanorods. The band gap decreases and visible

light absorption increases with increasing N doping content for

the N-doped TiO2 nanorods. Our results not only add a new

number in the V-shaped nanorod family but provide a simple

route to prepare N-doped TiO2 nanostructures, which will

benefit both basic research and practical applications.

Acknowledgements

This work was financially supported by the National Basic

Research Program of China (2012CB932303), and innovation

project of the Chinese Academy of Sciences (KJCX2-YW-H2O).

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Fig. 12 UV-vis diffuse reflectance spectra of N-doped TiO2 nanorods

with different HCl concentrations and pure rutile TiO2 nanopowders.

The inset is a plot of absorption versus energy in the absorption edge

region.


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