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Synthesis and characterization of well-aligned anatase TiO2 nanocrystalson fused silica via metal–organic vapor deposition

Chi-An Chen,a Yi-Min Chen,a Ying-Sheng Huang,*a Dah-Shyang Tsai,b Kwong-Kau Tiongc and Pei-Chen Liaod

Received 12th May 2009, Accepted 22nd June 2009

First published as an Advance Article on the web 13th July 2009

DOI: 10.1039/b909433d

Well-aligned anatase (A)–TiO2 nanocrystals (NCs) were grown by cold-wall metalorganic chemical

vapor deposition (MOCVD) on fused silica using titanium-tetraisopropoxide (Ti(OC3H7)4) as the

source reagent. Field emission scanning electron microscopy (FESEM) micrographs showed the

growth of vertically aligned NCs. X-ray diffractometry (XRD) pattern revealed the aligned A–TiO2

with a preferential orientation of (220). Raman spectrum confirmed the deposition of pure anatase

phase TiO2 on fused silica. Luminescence of self-trapped excitons and oxygen vacancies were observed

in anatase NCs. The indirect band gap of A–TiO2 was determined to be 3.14� 0.01 eV by analyzing the

surface photovoltage spectrum. Energy-dispersive X-ray spectroscopy (EDS) and X ray photoelectron

spectroscopy (XPS) analyses showed oxygen vs. titanium ratio of 2.0 � 0.1 for the as-deposited TiO2

NCs. Further structural characterization of the well-aligned A–TiO2 NCs was studied using

transmission electron microscopy (TEM) technique. The formation of building units bonded along

{112} facets with preferred (220) orientation of the well-aligned A–TiO2 NCs on fused silica were

presented and the probable growth mechanisms were discussed.

1. Introduction

Anatase (A)–TiO2 nanocrystals (NCs) have attracted much

attention due to the high potential applicability of the material as

catalysts,1 sensors,2 solar energy conversion3 and optical devices.4

Several techniques have been used for the growth of nano-

structured TiO2: sol-gel processing,5–7 hydrolysis,8,9 thermal

evaporation10–12 and metal-organic chemical vapor deposition

(MOCVD).13–16 MOCVD is known to be a technique that

possesses several advantages including better composition

control, high deposition rate, excellent step coverage and suit-

ability for scale-up.17,18 However, the information provided so

far is rather sketchy and a more complete picture on the growth

and characterization of A–TiO2 NCs is needed.

In this report, we have presented MOCVD growth of the well-

aligned A–TiO2 NCs on fused silica using titanium-tetraiso-

propoxide (Ti(OC3H7)4, TTIP) as the source reagent. The surface

morphology, structural, optical, and spectroscopic properties of

the as-deposited NCs were characterized in detail using field

emission scanning electron microscopy (FESEM), X-ray

diffractometry (XRD), micro-Raman scattering (RS), photo-

luminescence (PL), surface photovoltage spectroscopy (SPS),

X-ray photoelectron spectroscopy (XPS), transmission electron

microscopy (TEM), and selected-area electron diffractometry

aDepartment of Electronic Engineering, National Taiwan University ofScience and Technology, Taipei, 106, Taiwan. E-mail: ysh@mail.ntust.edu.twbDepartment of Chemical Engineering, National Taiwan University ofScience and Technology, Taipei, 106, TaiwancDepartment of Electrical Engineering, National Taiwan Ocean University,Keelung, 202, TaiwandDepartment of Electronic Engineering, Technology and Science Instituteof Northern Taiwan, Taipei, 112, Taiwan

This journal is ª The Royal Society of Chemistry 2009

(SAED). The probable growth mechanisms of the formation of

well-aligned A–TiO2 NCs were presented and discussed.

2. Experimental

2.1 Growth of well-aligned anatase–TiO2 nanocrystals

A vertical-flow and cold-wall MOCVD system was utilized for

the growth of the samples. The TTIP was used as the source

reagent for chemical vapor deposition of A–TiO2 NCs on fused

silica. There were two different flow paths connecting to the

growth chamber designed for gas transport. The first was

a by-pass flow path, which was designed for controlling the

steady-state chamber pressure prior to the deposition. The

second path was heated to the designated temperature to facili-

tate the transport of the source vapor to the growth chamber.

Three independent thermal couples were mounted on the source

transport line to control and monitor the temperature of the

shower head (Tsh), gas transfer line (Ttl) and the precursor

reservoir (Tpr), as indicated in (Fig. 1). During the source vapor

Fig. 1 Schematic diagram of cold-wall MOCVD apparatus.

CrystEngComm, 2009, 11, 2313–2318 | 2313

Fig. 2 The (a) typical FESEM images of 30� perspective-view, (b) cross-

sectional view, and (c) XRD pattern of the well-aligned A–TiO2 NCs

grown on fused silica.

Fig. 3 Raman spectrum of the well-aligned A–TiO2 NCs deposited on

fused silica.

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transport, Tsh, Ttl and Tpr were kept at a constant temperature

(50, 55 and 100 �C, respectively) by three independent controllers

to avoid the precursor condensation.

Pure oxygen gas was used to convey the source vapor and to

invoke the chemical reaction without adding any other inert gas.

The oxygen flow rate (FO2) of the reactive carrier gas was kept at

30 standard cubic centimeters per minute (sccm). During depo-

sition, the substrate temperature (Ts) and chamber pressure (Pc)

were controlled at 550 �C and 1.5 mbar, respectively. Changing

Pc and Ts could result in the deposition of materials with

different phases and morphologies.19 By keeping the controllable

CVD parameters constant, the deposition rate can be controlled

by directly changing the temperature of the precursor reservoir to

adjust the partial pressure of source vapor under the mass-

transport-limited condition.

2.2 Characterization of well-aligned anatase–TiO2

nanocrystals

The micrographs and the stoichiometry of well-aligned A–TiO2

NCs were studied using a JEOL-JSM6500F FESEM. XRD

patterns taken on a Rigaku D/Max-RC X-ray diffractometer

equipped with Cu Ka radiation source (l ¼ 1.5418 A) and Ni

filter were used to examine the phase and growth orientations of

the NCs. RS was used to characterize the structural phases of the

deposited NCs. RS spectra were recorded at room temperature

utilizing the back-scattering mode on a Renishaw inVia micro-

Raman system with 1800 grooves/mm grating and an optical

microscope with a 50x objective. The Ar-ion laser beam of the

514.5 nm excitation line with a power of about 1.5 mW was

focused onto a spot size � 5 mm in diameter. Prior to the

measurement, the system was calibrated by means of the

520 cm�1 Raman peak of a polycrystalline Si. X-ray photoelec-

tron spectroscopy using a thermo VG Scientific Theta Probe

system under the base pressure of 10�9 Torr. The XPS system

utilized Al Ka 1486.68 eV line as the X-ray source and the Ag

3d5/2 line at 368.26 eV as the calibration reference before the

measurement. XPS peak positions and integrated intensities were

obtained through curve fitting using Thermo VG Science:

Avantage v1.68 software.20 PL spectra were excited using the

325 nm line (�50 mW) of a He-Cd laser. The luminescence

signals were analyzed by using a Jobin-Yvon ‘‘TRIAX 550’’

spectrometer equipped with a ‘‘SIMPHONY’’ charge coupled

device (CCD) camera. The SPS measurement, which used

normalized incident light intensity, was performed at normal

incidence using a fixed grid and probe light chopped at

200 Hz.21,22 The contact potential difference between the sample

and a reference grid electrode was measured in a capacitive

manner as a function of the photon energy of the probe beam.

TEM images and SAED patterns were recorded to check the

nanostructure and preferential growth direction of the individual

A–TiO2 NC by a Phillips Tecnai G2 F20 FE-TEM at working

voltage of 200 kV.

3. Results and discussion

3.1 Well-aligned anatase–TiO2 nanocrystals on fused silica

3.1.1 FESEM and XRD. As illustrated in (Fig. 2), the

FESEM images show the deposition of well-aligned A–TiO2 NCs

2314 | CrystEngComm, 2009, 11, 2313–2318

on fused silica. The 30� perspective-view and cross-sectional view

show that the average edge size and length is about 150 nm and

5 mm, respectively, as depicted by (Fig. 2a and b). Energy-

dispersive X-ray spectroscopy (EDS) measurements indicate that

the NCs have an average atomic ratio of Ti to O of 1:2. A typical

XRD pattern of the A–TiO2 NCs on fused silica depicts in

(Fig. 2c), shows the preferred [110] directional growth of A–TiO2

NCs (A–TiO2 – 220 at 2q � 70.3�). A small fraction of A–TiO2 –

112, 200 and 211 orientations are also indicated by the much

weaker but distinct features located at 2q � 38.3�, 48�, and 54�,

respectively.

3.1.2 Raman scattering analysis. Raman spectroscopy can

sensitively identify the structural phases of TiO2 on the basis of

their characteristic Raman bands. (Fig. 3) shows the Raman

spectrum of as-deposited A–TiO2 NCs. Anatase is tetragonal

and belongs to the space group D194h

23 with each unit cell con-

taining two TiO2 chemical units. According to the factor group

analysis, there are six Raman active modes (1A1g + 2B1g + 3Eg).24

The Raman spectrum for anatase single crystal was investigated

by Ohsaka et al25 and six allowed bands in the first-order Raman

spectrum were identified as 142 cm�1 (Eg), 194 cm�1 (Eg),

393 cm�1 (B1g), 514 cm�1 (A1g + B1g), and 636 cm�1 (Eg). In

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addition several weak combination bands were also observed. As

can be seen in (Fig. 3), the spectrum exhibits five sharp peaks

located at 143 cm�1 (Eg), 196 cm�1 (Eg), 395 cm�1 (B1g), 514 cm�1

(A1g + B1g) and 637 cm�1 (Eg), as well as two weak broad features

at �302 cm�1 and 797 cm�1 (marked as *). The good agreement

of RS measurements with the literature25 confirms the anatase

phase of the as-deposited NCs.

3.1.3 X-ray photoelectron spectroscopy analysis. X-ray

photoelectron spectroscopy is frequently used as a complemen-

tary technique for assigning oxidation states and the stoichiom-

etry of the oxides. (Fig. 4a, b, and c) depict the slow scan XPS

spectra in the vicinity of C 1s, Ti 2p, and O 1s regions before

(curve I) and after (curve II) Ar ion bombardment. A mixed

Gaussian and Lorentzian line shape after the treatment of

background by Shirley function were used in the fitting proce-

dure to determine the peak positions accurately.

As shown in (Fig. 4a), a carbon C 1s peak at a binding energy

of 284.4 � 0.1 eV is observed before ion bombardment (curve I).

Fig. 4 The XPS spectra of (a) C 1s, (b) Ti 2p line, and (c) O 1s line for the

well-aligned A–TiO2 NCs grown on fused silica before (curve I) and after

(curve II) Ar ion sputtering bombardment.

This journal is ª The Royal Society of Chemistry 2009

The presence of this peak is related to surface pollution which

corresponds to the fact that the sample has been exposed to air

before the XPS measurements. The area of the C 1s peak

decreases upon ion bombardments (see curve II). This signature

persists even after a prolonged ion bombardment of duration

much longer than the one known to be necessary for removal of

the surface pollution on single crystal titanium dioxide

surfaces.26

The Ti 2p states consist of two features designated as Ti 2p3/2

and Ti 2p1/2. As can be seen in (Fig. 4b), before Ar ion

bombardment the Ti 2p3/2 curve is composed of a single peak at

a binding energy of 459.5 � 0.1 eV, with a full width at half

maximum of 1.4 � 0.1 eV. The separation between the Ti 2p3/2

and Ti 2p1/2 is 5.7 � 0.2 eV. The O 1s binding energy is 530.8 �0.1 eV. These results are in good agreement with that of A–TiO2

single crystal.27,28 The stoichiometry has been determined by the

relative areas of the total Ti 2p and O 1s XPS peaks with the

correction of the relative sensitivity factors. Oxygen vs. titanium

ratio of 2.0 � 0.1 is obtained. After argon bombardment,

a shoulder evolved on the low binding energy side of Ti 2p3/2

peak as evidenced by the presence of Ti3+.29,30 The argon

bombardment process induces a surface non-stoichiometry as

a consequence of the removal of oxygen from the surface caused

by the preferential sputtering phenomenon.31

In (Fig. 4c), two oxide states attributed to O2� and OH� species

are observed from the as-deposited NCs (curve I). However, the

OH� shoulder disappears after Ar ion sputtering (curve II) and

the main O 1s feature shifts its binding energy to the reference

value of O2� peak.31 This indicated that the OH� peak is just

a surface contamination peak probably due to water adsorption

in the air.

3.1.4 Photoluminescence and surface photovoltage spectros-

copy. PL spectrum of the as deposited A–TiO2 NCs is shown in

(Fig. 5). The spectrum can be deconvoluted into two broad

Gaussian-type bands with the peak positions located at around

2.50 and 1.94 eV. No emission near optical band edge can be

observed. In general, PL spectra of A–TiO2 could be attributed

to self-trapped excitons32,33 and oxygen vacancies.33,34

Comparing to the results of the previous study on the anatase

TiO2 single crystal32,35 and nanocrystals,36 the luminescence

band around 2.50 eV can be assigned to the recombination of

self-trapped excitons localized on TiO6 octahedra and the

Fig. 5 PL spectrum of the well-aligned A–TiO2 NCs grown on fused

silica.

CrystEngComm, 2009, 11, 2313–2318 | 2315

Fig. 6 (a) SPV spectrum of the well-aligned A–TiO2 NCs grown on

fused silica measured at room temperature, (b) SPV spectrum plotted in

coordinate (PVE)1/2 vs. E.

Fig. 7 (a) TEM image of a single A–TiO2 NC, (b) TEM image focused

on top of A–TiO2 NC, (c) the SAED pattern along [1�10]zone axis and (d)

a high-resolution TEM taken from the top of A–TiO2 NC marked in (b).

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dominate emission band at 1.94 eV is attributed to the oxygen

vacancies.

The room-temperature surface photovoltage spectrum for the

as grown A–TiO2 NCs is illustrated in (Fig. 6a). It is well known

that under low power optical excitation the photovoltage (PV)

signal is proportional to the absorption coefficient a multiplied

by the photon flux density F (PV faF).37,38 The photon flux

density F is equivalent to the light intensity I divided by the

photon energy E: F ¼ I/E. Thus, by keeping I constant one has

PV multiplied by the photon energy to be PVE fa. Therefore,

we analyze the SPV spectrum by the relation (PVE)1/2 vs E. The

band gap energy could be obtained by extrapolating the linear

part of the plot (PVE)1/2 vs. E to zero. As shown in (Fig. 6b), the

fit by (PVE)1/2 vs. E yields a satisfactory result which agrees well

with the indirect band character. The indirect band gap of the

A–TiO2 NCs is determined to be 3.14 � 0.01 eV, which is closed

to that of the A–TiO2 single crystal.39

3.1.5 TEM and SAED analysis. Further structural charac-

terizations of the A–TiO2 NCs are studied using the TEM

technique. Fig. 7a shows the TEM image of a single rod-like

A–TiO2 NC. Fig. 7b depicts the SAED pattern taken along the

[1�10] zone axis from the single NC. The TEM image focuses on

the top of A–TiO2 NCs, shown in Fig. 7c is an illustration of the

relationship of the growth plane of the as-deposited NCs. The

high-resolution TEM (HRTEM) image shown in (Fig. 7d) is

taken from the NCs marked by a white square in Fig. 7c. The

HRTEM image exhibits clear and well-defined lattice planes with

lattice spacing of d001 ¼ 0.48 nm for the (001) plane, d110 ¼ 0.28

nm for the (110) plane and d112 ¼ 0.46 nm for the (112) plane.

The lattice spacing of 0.28 nm in the longitudinal direction

corresponds to the d spacing of (110) crystal planes, confirming

the XRD analysis that the A–TiO2 NCs are preferentially

oriented along the [110] direction.

2316 | CrystEngComm, 2009, 11, 2313–2318

3.2 Growth mechanisms

The possible formation mechanisms of the well-aligned A–TiO2

NCs will be discussed as the following. The intrinsic growth

kinetics of A–TiO2 NCs, such as energetically favorable surface

for incoming atoms, can be well accounted for by the periodic

bond chain theory (PBC)40–42 which explains the growth behavior

at the molecular level. Other parameters such as growth condi-

tion are treated as the external factor. It has been determined that

the {112} facets of the A–TiO2 NCs have a larger surface energy

than that of the (110) and (001) planes.42 Accordingly, PBC

dictates that {112} facets will dominate the initial growth of the

well-aligned NCs. From ref. 42 the (001)-oriented plane has been

determined to be denser than the (110) and (112) planes with the

(112) plane being the least dense. The surface bond chain energy

between atoms obeys the relation of (001) plane < (110) plane <

(112) plane, resulting in a preferential growth of atoms bonded

on the (112) plane. In addition, the external factors such as

growth conditions (including growth chamber geometry with

oriented source delivery, growth temperature, pressure etc.) can

also influence the intrinsic factor resulting in the NCs growth

along the preferred orientation. The above mentioned mecha-

nism is quite similar to that of the so-called ‘‘oriented attach-

ment’’ model reported by Penn and Banfield.43 Penn and Banfield

presented the ‘‘oriented attachment’’ mechanism,43 in which the

formation of secondary mono-crystalline particles through

attachments of primary particles in a highly oriented fashion can

take place. This model provides a satisfactorily description of the

growth process of complex-shaped nanostructures using primary

nanoparticles themselves as building blocks. In this process, the

bigger particles are grown from small primary nanoparticles

through an oriented attachment mechanism, in which the adja-

cent nanoparticles are self-assembled by sharing a common

crystallographic orientation and attachment of these particles at

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a planar interface.44,45 The driving force for this spontaneous

oriented attachment is that the elimination of the pairs of high

energy surfaces will lead to a substantial reduction in the surface

free energy from the thermodynamic viewpoint. This viewpoint

agreed with the PBC description where the anisotropic growth of

nanoparticles is on crystallographic planes of a larger surface

energy.

The magnified FESEM images from Fig. 2a are shown in

Fig. 8a, b, c and d. From Fig. 8a and c, the columnar structure

consists of many building units with each building unit being

parallel to the C-plane on top of the A–TiO2 NCs. The cross

sectional view of the columnar structure is illustrated in Fig. 8b

and d. The alignment of the observed columnar layers can be

understood as follows: According to the PBC theory, higher

surface volume density leads to a lower growth rate. Under such

circumstances, the building unit of the same {112} facets will

bond to initiate the growth process. As the growth process

proceeds, the dominant (112) oriented building units can bond

periodically to complete the observed columnar structure.

Schematic diagrams illustrating the growth behavior as described

by the PBC theory are shown in Fig. 8e and f. The preferable

growth plane with {112} facets also explains the symmetric

Fig. 8 (a) Tilt view (30�), (b) cross-sectional view, (c) enlarged tilt view,

and (d) enlarged cross-sectional view of FESEM image focused on

a single A–TiO2 NC. (e) and (f) The schematic diagrams of the growth

behavior of A–TiO2 NC as described by the PBC theory.

This journal is ª The Royal Society of Chemistry 2009

columnar layers as observed along the [110] direction. The rela-

tionship as schematically illustrated in Fig. 8f is supported by the

XRD and the SAED analyses.

4. Conclusions

The well-aligned A–TiO2 NCs were grown on fused silica by

cold-wall MOCVD using Ti(OC3H7)4 as a source reagent.

FESEM micrographs showed that the growth of vertically

aligned NCs, XRD pattern revealed A–TiO2 aligned in a prefer-

ential orientation of (220), the Raman spectrum confirmed

deposition of pure anatase phase TiO2. Luminescence of self-

trapped excitons and oxygen vacancies were observed in anatase

NCs. The indirect band gap of A–TiO2 was determined to be

3.14 � 0.01 eV by SPS measurement. EDX and XPS analyses

showed an oxygen vs. titanium ratio of 2.0� 0.1 for the deposited

TiO2 NCs. The structural characterization of the well-aligned

A–TiO2 NCs via FESEM, XRD, TEM and SAED revealed the

formation of building units bonded along {112} facets with

preferred (220) orientation. The growth behavior, as described

by the PBC theory and the external factors such as growth

conditions resulting in the growth of NCs along a preferable

orientation, can also be well explained by the oriented attach-

ment mechanism.

Acknowledgements

The authors acknowledge the support of the National Science

Council of Taiwan under Nos. NSC 96-2112-M-011-001 and

NSC 97-2112-M-011-001-My3.

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