size dependency of nanocrystalline tio2 on its optical property and
TRANSCRIPT
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Applied Catalysis B: Environmental 68 (2006) 1–11
Size dependency of nanocrystalline TiO2 on its optical property and
photocatalytic reactivity exemplified by 2-chlorophenol
H. Lin a, C.P. Huang a,*, W. Li b, C. Ni b, S. Ismat Shah b,c, Yao-Hsuan Tseng d
a Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, United Statesb Department of Materials Sciences and Engineering, University of Delaware, Newark, DE 19716, United States
c Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, United Statesd Energy and Environment Laboratories, Industrial Technology Research Institute, Hsinchu 301, Taiwan, ROC
Received 19 January 2006; received in revised form 7 July 2006; accepted 18 July 2006
Available online 1 September 2006
Abstract
Anatase TiO2 nanocrystallines (17–29 nm) were successfully synthesized by the metal–organic chemical vapor deposition method (MOCVD).
Moderate manipulation of system parameters of MOCVD can control the particle size. The electro-optical and photocatalytic properties of the
synthesized TiO2 nanoparticles were studied along with several commercially available ultra-fine TiO2 particles (e.g., 3.8–5.7 nm). The band gap
of the TiO2 crystallines was determined using the transformed diffuse reflectance technique according to the Kubelka–Munk theory. Results
showed that the band gap of TiO2 monotonically decreased from 3.239 to 3.173 eV when the particle size decreased from 29 to 17 nm and then
increased from 3.173 to 3.289 eV as the particle size decreased from 17 to 3.8 nm. The results of band gap change as a function of particle size
agreed well with what was predicted by the Brus’ equation, i.e., the effective mass model (EMM). However, results of the photocatalytic oxidation
of 2-chlorophenol (2-CP), showed that the smaller the particle size, the faster the degradation rate. This is attributed in part to the combined effect of
band gap change relative to the spectrum of the light source and the specific surface area (or particle size) of the photocatalysts. The change of band
gap due to particle size represents only a small optical absorption window with respect to the total spectrum of the light source, i.e., from 380 to
400 nm versus>280 nm. Consequently, the gain in optical property of the larger particles was severely compromised by their decrease in specific
surface area. Our results clearly indicated the importance of specific surface area in controlling the photocatalytic reactivity of photocatalysts.
Results also showed that the secondary particle size grew with time due mainly to particle aggregation. The photocatalytic rate constants decreased
exponentially with increase in primary particle size. Primary particle size alone is able to predict the photocatalytic rate as it is closely related to the
electro-optical properties of photocatalysts.
# 2006 Published by Elsevier B.V.
Keywords: Size effect; TiO2; Photocatalyst; Particle size; 2-Chlorophenol; Size quantization effect; Electro-optical property
1. Introduction
Since the discovery of photovoltaic property of titanium
dioxide TiO2 by Fujishima and Honda [1], great efforts have
been focused on elucidating the electronic structure [2–5],
catalytic reactivity [6–8] and surface property [9] of TiO2.
Inexpensive and thermal-dynamically stable at room tempera-
ture, this semiconductor material has been widely used in
heterogeneous photocatalysis and proven to be capable of
decomposing a host of organic pollutants such as phenolic
* Corresponding author. Tel.: +1 302 831 8428.
E-mail address: [email protected] (C.P. Huang).
0926-3373/$ – see front matter # 2006 Published by Elsevier B.V.
doi:10.1016/j.apcatb.2006.07.018
compounds [10,11], metal ethylene diamine tetra acetate
(EDTA) complexes [12,13], airborne microbes [14] and
odorous chemicals [15]. Most of these studies involved
ultraviolet (UV) photons as the major exciting light sources.
Considering that there is only 5% of solar irradiation within the
UV range, intuitively it is desirable to enhance the photo-
catalytic performance of TiO2 by enabling it to utilize photons
from the near-visible to visible region. It has been suggested
that this can be achieved by manipulating the particle size of
photocatalyst [3,6,16] or doping the TiO2 with foreign ions
[17–19].
The initiation of a photocatalytic reaction requires a
minimum photon energy that exceeds the band gap of the
material in order to trigger the interband transition of electrons
H. Lin et al. / Applied Catalysis B: Environmental 68 (2006) 1–112
between the lowest unoccupied molecule orbital (LUMO) and
the highest occupied molecule orbital (HOMO); that is, the
incident wavelength needs to be smaller than the wavelength of
the band gap threshold, lbg. Thus, it is speculated that reducing
the band gap of TiO2 can enhance its photocatalytic
performance through more efficient utilization of lower energy
photons. TiO2 has three distinct crystalline structures: rutile,
anatase, and brookite. Most studies on the photocatalytic
reactivity were conducted with either rutile or anatase, which
reported that the indirect band gap was 3.0 eV (or lbg 413 nm)
and 3.2 eV (or lbg 387 nm), respectively. Although rutile has a
lower band gap than anatase, it has been demonstrated that
anatase-structure TiO2 exhibits a better photocatalytic perfor-
mance than that of rutile [20–22]. This is attributed in part to a
wider optical absorption band and smaller electron effective
mass of rutile than those of anatase, which leads to higher
mobility of charge carriers in rutile than anatase [5]. The band
gap of TiO2 is commonly believed to be indirect. In contrast to
direct interband transition, indirect transition requires phonons
(lattice vibration) to compensate for the change in wave vector
during electron transition.
It has been reported that the band gap of semiconductor
crystalline is a function of the particle size [3,4,23]. Below a
certain threshold, the density of point/surface defects of
semiconductor crystalline increases with decrease in particle
size. Due to mild delocalization of molecular orbitals on the
surface, defects in the bulk semiconductor create deep and
shallow traps near the band edge of its electronic state, which
brings about reduction in band gap, that is, red-shift in
absorption spectrum [3,4]. When the size of semiconductor
particle decreases from its bulk to that of Bohr radius, e.g., the
first excitation state, the size quantization (Q-size) effect arises
due to the spatial confinement of charge carriers. Consequently,
electrons and holes in the quantum sized semiconductor are
confined in a potential well and do not experience the
delocalization that occurs in the bulk phase. Therefore, the band
gap of ultra-fine semiconductor particle increases with the
decrease in particle size when it is smaller than the band gap
minimum [3,4]. This phenomenon has been described by the
Brus’ effective-mass model (EMM) [3]. Size quantization
effect has been studied using various semiconductors including
CdS [24], HgSe, PbSe, CdSe [25], ZnO [26], Cd3S2 [27], and
TiO2 [6,16,28]. The reported Q-size effect of semiconductor
clusters appears to be between 1 and 12 nm. Results of these
studies were mostly obtained from liquid phase UV–vis
absorption spectrum, and few had focused on the photode-
gradation of organic compound.
A good photocatalyst must have high photon conversion
efficiency in addition to high specific surface area. In fact, the
primary particle size of photocatalysts determines both the
specific surface area and the photon conversion efficiency.
Although the size dependency of band gap has been studied
theoretically, only a few investigations have been conducted on
the change of band gap as a function of particle size using TiO2
[6,16,29]. Anpo et al. [6] studied the change of band gap of
Ti02over a wide range of particle sizes (e.g., 3.8–200 nm) and
found significant blue shifts of the absorption edge by 0.093 and
0.156 eV for rutile and anatase crystalline, respectively, when
particle size was less than 12 nm. Kormann et al. [16] observed
the quantum confinement effect upon illumination of TiO2
colloids (e.g., <3 nm) and reported a blue-shift by 0.15–
0.17 eV in absorption spectrum. However, Serpone et al. [29]
did not observe a quantum size effect in the particles size range
between 2.1 and 26.7 nm of TiO2. It must be mentioned that
these authors depended on UV–vis absorbance measurements
to estimate the band gap energy. Obviously, this method cannot
exclude the light scattering effect during absorbance measure-
ments, which would lead to an over-estimation of the
absorbance, especially for large aggregates. It is known that
the scattering efficiency is proportional to the fourth-power of
particle radius (e.g., r4) according to Rayleigh scattering theory
[30]. During UV–vis absorbance measurements, the size of
aggregates will increase with time in suspensions. Furthermore,
most of these UV–vis absorbance measurements were
conducted during the agglomeration/reflux stage of particle
preparation using the sol–gel process [6,16,29]. The presence
of precursor intermediates can contribute light absorbance also,
which, in turn, might affect the reliability and the interpretation
of results observed.
As mentioned above, the particle size can affect the
photocatalytic reactivity. A few studies have been conducted to
assess the relationship between particle size and photocatalytic
reactivity [6,22,30–38]. Anpo et al. [6] observed an increase in
quantum yield in the hydrogenation of CH3CCH when the
particle size of anatase TiO2 decreased from 11 to 3.8 nm and
concluded that it was caused by the quantum size effect. Maira
et al. [30] studied the photocatalytic degradation of trichlor-
oethylene (TCE) in gas phase upon TiO2 in the size range of 2.3
and 27 nm and reported an optimum particle size of 7 nm. They
further concluded that the lower reactivity at particle size less
than 7 nm was due to changes in structure and electronic
properties of the ultra-fine particles. Almquist and Biswas [31]
compared the effect of particle size using both flame
synthesized and commercial TiO2 particles, on the photode-
gradation of phenol. They reported an optimal particle size in
the range of 25–40 nm with respect to photo-degradation rate
and optical responses. However, the TiO2 samples consisted of
a mixture of anatase and rutile crystallines at various
proportions that results may be affected by the anatase/rutile
ratio in addition to particle size. Hao et al. [39] and Jang et al.
[33] studied the effect of anatase particle size on the
photodegradation of rhodamine B and methylenen blue dye.
They observed that the photocatalytic reactivity monotonically
increased with decrease in particle size in the range of 8–15 and
15–30 nm. However, the effect of size-dependent band gap
change and its corresponding excitation source spectrum was
not considered in their work. Nam et al. [34] studied the
photocatalytic degradation of TCE (tetrachloroethene) using
TiO2 (anatase) thin film and found an increase in the
photocatalytic degradation rate as the primary particle size
decreased from 37 to 25 nm. Zhang et al. [35] examined the
effect of anatase particle size on the oxidation of trichlor-
omethene, CHCl3, and reported that TiO2 particles at 11 nm
yielded the highest photonic efficiency on the oxidation of
H. Lin et al. / Applied Catalysis B: Environmental 68 (2006) 1–11 3
CHCl3. They reported that photocatalytic efficiency did not
increase monotonically with decrease in particle size; rather it
increased with an increase in surface combination of the
electron–hole pairs. Xu et al. [36] and Gerischer [37] have
experimentally and theoretically studied the effect of particle
size of on the photocatalytic efficiency of TiO2. Both
researchers concluded that the photocatalytic reactivity of
TiO2 increases with decrease in particle size. Unfortunately, the
TiO2 used in the studies conducted by Xu et al. [36] and
Gerischer [37] had a particle size mainly in the range of
micronmeters rather than nanometers. As indicated above,
there appears to be no agreement on the effect of particle size on
the photocatalytic activities of TiO2; the optimal particles size
reported has covered a rather wide range, e.g., between 3.8 and
40 nm. Another issue related to the effect of particle size on the
photocatalytic reaction is the primary versus the secondary
particle size. This is of particular importance when dealing with
aqueous systems, as particle aggregation is inevitable in the
water environment [38]. Will particle aggregation affect the
photocatalytic reactivity during the course of water treatment
process? Maira et al. [30] reported that the degradation of TCE
in the gas phase was affected by both the primary and the
secondary particle sizes. Little information with regard to the
importance of secondary particle size on photocatalytic
reactivity in aqueous solution is available. There is a need
for more systematic studies on the effect of particle size, both
primary and secondary, on photocatalytic reactions in aqueous
solutions.
The objectives of this study were (1) to elucidate the effect of
primary particle size on the electro-optical property of TiO2 in
terms of band gap changes, (2) to assess the effect of primary
and secondary particle size on the photocatalytic reactivity of
TiO2 exemplified by 2-chlorophenol (2-CP), and (3) to
determine whether any relationship exists between the optical
property and photocatalytic reactivity. In order to better
evaluate the effect of particle size on the optical property of
TiO2 with minimum optical interference (e.g., aggregation in
the aqueous phase), band gap measurements were made using
the transformed diffuse reflectance technique according to the
Kubelka–Munk theory [40–42]. It is expected that this
Table 1
TiO2 particle size (nm) analyses by various techniques
Particle TEMa BETb XRDc DLSd
A3.8 3.8 3.94 9.22 51.1
A4.9 4.9 5 10.71 25.9
A5.7 5.7 5.53 11.55 71.6
A12 12 10.56 15.1 –
A17 17 16.95 – 161.5
A20 20 20.06 17.07 92.8
A29 29 28.05 24.3 183.5
a Gaussian fitting calculated particle size based on TEM bright field images.b Calculated particle size based on BET results.c Scherrer equation calculated particle size based on XRD results [anatase(1 0 1d Dynlamic light scattering measured particle size right after the application ofe Dynamic light scattering measurements during the experiments (smaller clustef Dynamic light scattering measurements during experiments (larger cluster).g Average particle size calculated by the weighted PSD function.
technique will enable better correction of interferences in
absorbance measurements caused by particle light scattering
and other factors such as the presence of precursor
intermediates. Results of band gap measurements were then
compared with those predicted by the Brus’ EMM model. The
effect of particle size on the photocatalytic reactivity of
TiO2was assessed using a 2-chlorophenol (2-CP) probe over a
wide range of primary particle sizes, e.g., from 3.8 nm
(reported known quantization effect) to 29 nm (approximated
bulk size). The next step was to compare the effect of primary
and secondary particle size on the photocatalytic degradation of
hazardous organic compounds exemplified by 2-CP. Finally, the
band gap change was compared with the photocatalytic
degradation rate of 2-CP for assessing the overall effect of
particle size.
2. Materials and methods
2.1. TiO2 particle preparation
Pure anatase TiO2 particles with diameter ranging from 12 to
33 nm were prepared by metal–organic chemical vapor
deposition (MOCVD). Details of the system configuration
and operation have been reported previously [43]. Titanium
tetraisopropoxide—Ti[OCH(CH3)2]4 (TTIP), 97% purity, pur-
chased from Aldrich, was selected as the precursor. The liquid
TTIP was placed in a Pyrex thermal well. Argon (99.999%
purity) was used as the carrier gas. The precursor flow rate was
adjusted by controlling the temperature of the TTIP solution
and/or the flow rate of the Ar gas. Ar gas flow at 5–10 sccm was
purged through the thermal well to carry the precursor to the
reactor. TTIP was pre-heated to just below its boiling point of
220 8C to increase its volatility and deposition rate. The total
pressure of the reactor was controlled at 10–15 Torr by purging
O2 at 15–35 sccm of flow rate to the Ar/TTIP mixture in a
baffle. The mixture was directly introduced to the reaction
chamber for the formation of TiO2 particles. Stainless mesh was
selected as the substrate, which was triple cleaned by methanol
and de-ionized water prior to experiments. Particles were
collected from the stainless mesh substrate placed in the middle
DLSe DLSf DLSg Circularity
126.9 505.5 387.7 0.838 � 0.021
121.9 400.9 255.8 0.842 � 0.017
140.7 560.4 391.3 0.847 � 0.020
– – – 0.846 � 0.037
125.3 – 125.3 0.833 � 0.05
103.7 – 103.7 0.828 � 0.083
167.3 – 167.3 0.827 � 0.072
)].
ultrasonic disintegration at an energy intensity of 18 kJ/L.
r).
H. Lin et al. / Applied Catalysis B: Environmental 68 (2006) 1–114
of the reaction chamber and perpendicular to the direction of
the TTIP/Ar flow. The temperature of the furnace was kept at
600 8C to assure the formation of anatase structured TiO2
polycrystallines. The post-decarbonation time was 30 min,
which was performed by constant purging of O2 after the TTIP/
Ar valve was turned off. For particles smaller than 10 nm, two
samples (3.8 � 0.3 and 4.9 � 0.8 nm) were purchased from
Reade1 Inc., and one sample with a size of 5.7 � 0.3 nm was
provided by Hombikat Inc. In addition, P25, which contains
approximately 75% of anatase and 25% of rutile, provided by
Degussa Co. was also used for comparison. Table 1 lists the
important physical–chemical properties of the TiO2 samples
used in this study.
2.2. TiO2 surface characterization
Particles were characterized by various techniques, includ-
ing transmission electron microscopy (TEM), X-ray diffraction
(XRD), scanning electron microscopy (SEM), BET specific
surface area measurement, dynamic light scattering (DLS),
diffuse reflectance spectra, and X-ray photoelectron spectro-
scopy (XPS). The particle size and shape of MOCVD
synthesized TiO2 particles were determined based on TEM
(JEOL 2000 FX) results (insets in Fig. 1) and further
Fig. 1. Particle size distribution histograms based on TEM images. Inset—
TEM bright field image of TiO2 particles: (a) A29 and (b) A4.9.
reconfirmed by BET, SEM, and XRD measurements
(Table 1). For the sake of simplicity, we designate Ax as
anatase TiO2 particles with ‘‘x’’ nm in diameter. TiO2 particles
were immersed in acetone solution after ultrasonic treatment
for 5 min (Branson model 1510, 150 W, 40 kHz). The dispersed
TiO2 particles were then deposited onto the lacey-carbon-
coated copper grid. A JEOL 2000 equipped with a LaB 6 field
emission gun, system was operated at 2.5 � 10�5 Pa pressure.
An acceleration voltage of 200 kV was used. Filament current
was controlled between 111 and 113 mA.
SEM images were taken by a JEOL scanning electron
microscope, model JSM 7400F. The working distance was kept
between 2.8 and 3.1 mm, and the acceleration voltage was set at
3.0 keV. Example images of A17 and A23 synthesized TiO2
particles are shown in Fig. 2. EDX results (data not shown) did
not show any impurity in these TiO2 samples studied regardless
of their source of origin. Structural characterization was done
using XRD scan with Cu Ka radiation in a Rigaku D-max B
diffractometer equipped with a graphite crystal monochroma-
tor. The diffraction pattern obtained showed that only anatase
Fig. 2. Example SEM images of MOCVD synthesized TiO2 particles: (a) A17
and (b) A23.
H. Lin et al. / Applied Catalysis B: Environmental 68 (2006) 1–11 5
TiO2 crystalline formation (with neither rutile nor brookite
structure) was observed in any of the samples analyzed. The
crystalline size along the (h k l) profile was calculated based on
a high-resolution XRD scan following the Scherrer formula
(Eq. (1)), i.e.:
Dðh k lÞ ¼kl
b cos u(1)
where k is the shape factor, l the wavelength of X-ray of Cu
Ka radiation, b the full width at half maximum (FWHM) of the
(h k l) peak, and u is the diffraction angle. Results of XRD
survey scan are shown in Fig. 3a. It is obvious that only anatase
was present in the TiO2 samples studied. Based on the high-
resolution scan, the particle size was calculated using the
anatase(1 0 1) peak (Fig. 3b).
The specific surface was measured based on the Brunauer–
Emmett–Teller multiplayer nitrogen gas adsorption (BET)
theory (NOVA2000 Gas Sorption Analyzer, Quantachrome
Corp.). Samples were degassed and calcined at 300 K for 24 h
before adsorption experiments using N2 as the adsorbate. The
relative pressure (P/P0) was within the range of 0.05–0.35. The
average pore radius was also recorded.
Secondary particle size (aggregates) measurements were
conducted with a zetameter in dynamic light scattering mode
(DLS), Zetasizer, model 3000 HSA, Malvern Instruments Ltd.,
Fig. 3. XRD spectra of TiO2 samples: (a) survey spectrum and (b) high
resolution scan on anatase(1 0 1) peak region.
UK. Particles were prepared in solutions containing 50 mg/L of
2-CP at pH 10. The solution was ultrasonicated (18 kJ/L)
immediately prior to size measurements. Readings were taken
immediately after ultrasonication and 5 min after photocatalytic
experiments were started. The real refractive index and
imaginary refractive index for both the solvent and the analytes
were obtained according to the user’s manual for the instrument.
Diffuse reflectance spectra of the powders were measured on a
double beam UV–vis–NIR scanning spectrophotometer (Shi-
madzu UV-3101PC, Japan), which was equipped with a diffuse
reflectance accessory. A given amount of TiO2 powder was
uniformly pressed in the tablet (provided by Shimadzu) and
placed in the sample holder on integrated sphere for the
reflectance measurements. The reflectance data was converted to
the absorption coefficient F(R1) values according to the
Kubelka–Munk equation (Eq. (2)) [40–42], i.e.:
FðR1Þ ¼ð1� R1Þ2
2R1(2)
where F(R1) is equivalent to the absorption coefficient.
A SSI-M probe XPS system employing Al Ka exciting
radiation was used for the general survey and high resolution
scans. Ti 2p, O 1s, and C 1s photoelectron peaks were recorded
in the high-resolution mode. These peaks were used to
determine the relative composition of the nanoparticles and the
valence states of Ti. XPS results of MOCVD-prepared TiO2
were reported earlier [43]. In addition, a comparison of
particles from different sources in our study is shown in Fig. 4.
(Note: A carbon peak was present in Fig. 4. This was from the
background carbon tape and not that of the TiO2 sample.) Our
XPS results did not show any impurity for samples collected
from difference sources at a detection limit of 0.05 at.%. Based
on the above surface analyses, namely, XPS and EDX, all TiO2
particles studied, regardless of its origin, appeared to be
homogeneous and compatible with each other in terms of
crystal structure, morphology and purity. It suffices to say that
particle size was the only observable difference among these
TiO2 samples in presented study.
Fig. 4. XPS results of TiO2 particles from different sources: (1) A3.8 (Reade1
Inc.), (2) A5.7 (Hombikat Inc.), and (3) A29 (MOCVD prepared TiO2).
H. Lin et al. / Applied Catalysis B: Environmental 68 (2006) 1–116
Fig. 5. (a) Diffuse reflectance spectra of various TiO2 anatase particles accord-
ing to the Kubelka–Munk equation; (b) the transformed Kubelka–Munk func-
tion vs. energy of the excitation source.
2.3. Photocatalytic reactivity experiments
Photodegradation experiments were performed in a photo-
catalytic reactor system. This bench-scale system consisted of a
cylindrical Pyrex-glass jacketed cooling cell with a reflective
interior surface. The system was cooled and maintained at
298 K. Photon source was provided by a 100-W medium
pressure mercury lamp (Hanova, NJ, USA). TiO2 (at a
concentration of 10 mg/L) nanoparticles were ultrasonicated
(at 18 kJ/L) before the experiment was started. The target
organic compound was 2-chlorophenol (2-CP) (>99.9% purity
from Aldrich). Constant air purging was provided at a flow rate
of 100 sccm using a porous diffuser. The initial pH value of the
mixture solution was set at 10.0 to minimize the evaporation of
2-CP during experiments (note that the pKa of 2-CP is 8.52).
The residual concentrations of the parent and intermediate
compounds were measured using high-performance liquid
chromatography (HPLC) (Model HP 2100, Agilent, CA, USA).
The instrument was equipped with a Zorbax Eclipse XDB-C18
column (4.6 mm � 250 mm) from Agilent. The eluent con-
sisted of 60% acetonitrile and 40% phosphate (pH 3) buffer.
The system flow rate was maintained at 0.5 ml/min with total
injection volume of 10-ml. Spectra were acquired by a diode
array detector (l = 210 nm).
3. Results and discussion
3.1. Optical properties
From the diffuse reflectance plot (Fig. 5a), we can see that
A20, A17 and A23 have better light absorbance than A3.8,
A4.9, A5.7, and A29 in the range of wavelength (l) between
400 and 350 nm. The damping of the F(R1) for A3.8 and A4.9
was observed when the wavelength was below 360 nm. We
suspect that is due to the size quantization effect and defects
induced deep energy traps that are more discretely created
(compared to A5.7). The damping of the diffuse reflectance
may be caused by attenuation of excited electron de-excited
during the scanning of different wavelength. Thus, the
luminescence is created during the de-excitation and amplified
the signal of the reflectance. The absorption coefficient of an
indirect semiconductor near the absorption threshold can be
expressed as
a ¼ Biðhn� EgÞ2
hn(3)
where Eg is the band gap of indirect allowed transition (eV), h
the Planck’ s constant (J s), Bi the absorption constant, and n is
the frequency of the light (s�1). Therefore, a transformed
Kubelka–Munk function can be constructed by plotting
[F(R1)]0.5 against the energy of excitation source to obtain
the band gap of TiO2 particles (Fig. 5b) [41,44]. The band gaps
for A3.8, A4.9, A5.7, A17, A20, A23, and A29 were deter-
mined to be 3.289 � 0.02, 3.251 � 0.02, 3.275 � 0.02,
3.173 � 0.02, 3.179 � 0.02, 3.224 � 0.02, and 3.239 �0.02 eV, respectively. Results showed that the band gap
decreased (e.g., red shifted) to a certain minimum value
(i.e., critical size) with decrease in particle size from an
estimated bulk value of 29 nm. Further decrease in particle
size from the critical size (corresponding to band gap mini-
mum) caused the band gap to increase (i.e., blue-shift). A
plausible explanation for this change in band gap with respect
to the size of the particle is that the bulk defects induce
delocalization of molecular orbitals in the conduction band
edge (e.g., LUMO) and create shallow/deep traps in electronic
energy, in turn causing the red-shift of the absorption spectra.
When crystalline size decreased below its size at the band gap
minimum, the traps shifted to higher energy, which resulted in
blue shifting of the absorption spectra (e.g., size quantization
effect) [3,4]. Based on the Bras’ EMM model, the first excitonic
energy of semiconductor cluster, E*, can be expressed as a
function of particle size, as given by the following expression:
E� ffiEg þ�h2p2
2R2
�1
me
þ 1
mh
�� 1:8e2
eR(4)
where Eg is the band gap of the bulk semiconductor, the second
term is the energy induced by the quantum confinement effect,
H. Lin et al. / Applied Catalysis B: Environmental 68 (2006) 1–11 7
Fig. 6. Band gap shift vs. the particle size of TiO2. Solid circles are experi-
mental results. Lines are predicted by the Brus model at different reduced
effective mass of exitions, m (1/m = 1/me + 1/mh) and dielectric constant
k = 184.
Fig. 7. Photocatalytic degradation of 2-CP under UV radiation: (a) changes of
normalized concentration as a function of time; (b) least square best fitting for
the first-order reaction rate constant k (s�1). Symbols: P25 (solid squares), A29
(solid circles), A20: (open circles), A17 (open squares), A5.7 (solid diamonds),
A4.9 (open triangles), and A3.8 (open diamonds).
and the third term is the shift of energy due to columbic
attraction between electron and hole pairs. £ is the Plank’ s
constant (J s), R the radius of the cluster (m), me the effective
mass of the electron (kg), mh the effective mass of the hole (kg),
e the charge of electron (C), and e is the electric permittivity
(C2 N�1 m�2) of the material (a product of dielectric constant k
and permittivity in free space e0). As shown in Eq. (4), the band
gap increased as particle size decreased to below a certain
threshold. The shifts in band gap were significantly affected by
the reduced mass of exciton, i.e., m�1 ¼ m��1e þ m��1
h and the
dielectric constant of the material k. The reported effective
mass of the electron, m�e was in the range of 5–13me [16,29,45].
The effective mass of the hole, m�h was reported to be �2me
[16]; the dielectric constant of TiO2 anatase crystalline was
reported to be between 23 and 184 [29,46–49]. Eq. (4) was
plotted by using the reported values for the dielectric constant f
of 184 and effective mass of exitons (m�e) from 5 to 13me and m�hof 2me (Fig. 6). This was equivalent to m values of between 1.43
and 1.73me. We can see the decrease (left shift) in size of band
gap minimum and increase in band gap shift (DEg) when m
increases from 1.43 to 1.73. In contrast, decrease in dielectric
constant from 184 to 23 will result in greater reduction in band
gap minimum and significantly enlarge the band gap shift in the
negative direction (result not shown). By comparing the
observed band gap values to those calculated by the EMM
model, it is seen that the observed values agreed very well with
those predicted by the model.
3.2. Photoreactivity
If the energy of the incident photon is greater than the band
gap, photoexcitation will occur and yield the electron and hole
pairs. The trapped charge carriers are formed within a 20-ps
pulse but have a life time in the nanosecond (ns)-range at Ti3+
sites within the semiconductor bulk [50]. This mechanism has
been confirmed by direct and indirect in situ electron
paramagnetic resonance (EPR) measurements (e.g., examina-
tion of paramagnetic species on hydrated TiO2 surfaces)
[6,51,52]. Generated electrons are readily trapped on Ti4+ sites
and form Ti3+. The trapped electron can be readily scavenged
by oxygen [51]. Localized holes can be scavenged by either
reacting with hydroxide ions or through electron transfer with
water to form hydroxyl radicals, which are strong oxidants that
can oxidize organic substances nonselectively [10,31,51,52].
It is generally accepted that photogenerated hydroxyl
radicals are primarily responsible for the heterogeneous
degradation of organic compounds, such as 2-CP
[10,31,53,54]. Fig. 7 shows the photocatalytic reactivity of
2-CP over TiO2 particles at various particle sizes. Results
indicated that the degradation reaction followed pseudo first-
order kinetics regardless of the particle size (Table 2). It also
showed that the reaction rate constants increased with decrease
in primary particle size. This is in contrast to the band gap
measured (Fig. 5b), where a minimum band gap was observed
with a specific primary particle size of �17 nm. That is, the
particles with size in the range of 17–29 nm, exhibited larger
lbg values than the smaller ones.
Photoreactivity under UV irradiation showed that the
smaller the particles, the higher the 2-CP decomposition rate
H. Lin et al. / Applied Catalysis B: Environmental 68 (2006) 1–118
Table 2
List of the least square best fitting parameters for different primary particle sizes
Diametera Specific
surface areab
k � 1000c R2 QE � 1000d
P25 33 47.7 � 5 16.36 � 0.17 0.9565 12.75
A29 29 55.7 � 5 17.47 � 0.02 0.9997 13.49
A20 20 77.9 � 5 20.31 � 0.03 0.9995 13.70
A17 17 92.2 � 5 21.17 � 0.04 0.9993 13.71
A5.7 5.7 282.5 � 5 24.55 � 0.10 0.9974 13.87
A4.9 4.9 312.7 � 5 30.03 � 0.11 0.9982 14.02
A3.8 3.8 396.1 � 5 29.23 � 0.15 0.9901 13.95
a Diameter based on TEM, primary size (nm).b BET specific surface area (m2/g).c First-order reaction rate constant obtained by least squares best fitting (s�1).d Quantum efficiency at 180 min of experiment.
under otherwise identical experimental conditions. In our study,
commercially available P25 appeared to have the worst
degradation rate of 2-CP (k = 0.01636 s�1), whereas A3.8
yielded the highest rate (k = 0.02923 s�1). The specific surface
area of particles increases with decrease in particle size
(Table 1). Obviously, the advantage in photocatalytic activity
gained through the enhancement in optical response from the
smaller band gap exhibited by larger particles (e.g., 17–29 nm)
was severely compromised by the decrease in specific surface
area. Furthermore, the variation in photon absorption due to
size effect was lbg between 381 and 395 nm. Considering the
emission spectrum of the excitation source (Fig. 8), utilizable
photons are in the range between 280 nm (note that the Pyrex
glass cuts off the photon emission at 280 nm, as indicated in
Fig. 8) and lbg of TiO2. Clearly, the change of light absorption
band due to size effect (e.g., 381–395 nm) is relatively small
compared to the fully utilizable light spectrum (e.g., 280–
395 nm) available in this study. In addition, the quantum
efficiency of the photocatalytic reactivity appeared to have an
Fig. 8. Photonflux spectrum of the medium pressure mercury lamp used. The
two vertical lines, between 381 and 395 nm, indicate the band gap changes
observed of the TiO2 samples used in this study. Generally, the band gap of the
bulk TiO2 was around 387.2 nm. The band gap red-shifted to ca. 395 nm as the
particle size decreased to about 17 nm then the band gap blue-shifted to ca.
381 nm as the particle size continued to decrease to about ca. 3.8 nm. Inset is the
plot of particle size vs. its corresponding wavelength of band gap threshold lbg
(nm).
exponential relationship to the incident wavelength near the
optical adsorption edge. Zang et al. [55] reported that the
incident photon conversion efficiency (IPCE) could vary by an
order of magnitude comparing excitation wavelength at 400
and 550 nm during the photocatalytic degradation of 4-
chlorophenol (4-CP). Puma and Yue [54] observed that the
reaction rate of 2-CP increased by 3.9 times (1.73 mM min�1
versus 6.78 mM min�1) when the system was under UV-A
versus UV-ABC (i.e., the full UV spectrum) irradiation. These
observations imply that even when the incident photon energy
is higher than the band gap of the photocatalyst, quantum
efficiency varies greatly below the wavelength of the band gap
threshold, lbg. This variation is especially large when the
incident wavelength is near the adsorption edge. Thus, size-
effect-enhanced optical absorption (381–395 nm in our study)
is likely to contribute an unnoticeable improvement on the
photocatalytic degradation of the 2-CP. As a result, it is not
surprising to see photocatalysts with lower band gap and
smaller specific surface area yield lower photodegradation
efficiency than those with higher band gap and larger specific
surface area. The quantum yield (F) was computed based on
Eq. (5) [48]:
Fl ¼mole of 2-CP degraded
Einstein of incident photons(5)
Fig. 9 shows the apparent quantum yield versus the primary
particle size at 180 min. It is seen that the quantum yield
increases almost monotonically with decreases in primary
particle size. Again, this is due to the advantage of the large
specific surface area of the ultra-fine particles, the spectrum of
the excitation source, and the wavelength-dependent nature of
photocatalytic reactivity.
In photocatalytic slurry systems, two major factors will
greatly impact the reaction: (a) specific surface area and (b) band
gap. The specific surface area determines the available sites for
reactions to take place, whereas, the band gap of the
semiconductor will define the amount of photons that are
available for quantum conversion. Under our experimental
conditions, results showed that ultra-fine particles (e.g.,<10 nm)
Fig. 9. Quantum yield as a function of primary particle size.
H. Lin et al. / Applied Catalysis B: Environmental 68 (2006) 1–11 9
Fig. 10. Secondary particle size measurements using the DLS method: (a)
measured immediately after the ultrasonic treatment; (b) measured 5 min after
the experiment was started.
have specific surface area nearly one order of magnitude larger
than that of larger particles (e.g., >10 nm). However, the ultra-
fine particles only exhibited a reaction rate roughly by a factor of
two and slightly higher apparent quantum efficiency (F) in
180 min than that of the larger particles. It seems that several
factors might offset the surface area advantage of ultra-fine
particles: (1) size quantization effect (e.g., less than 10 nm in our
case), (2) increase in surface electron–hole recombination, (3)
particle aggregation, and (4) spectrum of the excitation source.
Quantization effect yields a larger band gap as the particle size
decreases. As a result a light source with higher energy, e.g., blue-
shifted, is required to separate the excitons [3,4]. When the
particle size decreases, the density of recombination center
increases which encourages holes and electrons recombination
[35,38]. As discussed in Section 3.3, ultra-fine particles (e.g.,
<10 nm) can undergo rapid flocculation, which decreases the
availability of active surface sites. Finally, the energy band of
excitation source is also important to take advantage of the band
gap reduction due to size effect.
3.3. Effect of secondary particle size
Regardless of the primary particle size, it is inevitable that
there will be particle aggregation in aqueous solutions due to
factors such as the charge density/potential of the particle
surfaces and van der Waals forces. The effect of secondary
particle size (aggregates) was also assessed in this study. The
secondary particle sizes (aggregation) of TiO2 particles were
determined using dynamic light scattering (DLS) measure-
ments (Fig. 10) under two conditions: (1) right after the
ultrasonic treatment at an energy intensity of 18 kJ/L (herein
designated as ‘‘immediate’’ secondary particle size) and (2)
5 min (herein designated as ‘‘5-min’’ secondary particle size)
after the experiment was started. Fig. 10a shows the particle
size distribution (PSD) of the ‘‘immediate’’ secondary particles.
Results indicated the PSD was mono-dispersed. The average
particle size of the ‘‘immediate’’ secondary particles increased
from 3.8–29 to 51.1–183.5 nm, respectively, an increase by 13
to 6 times. However, as indicated in Fig. 10b, for the ‘‘5-min’’
secondary particles, the PSD was mono-dispersed when the
primary particle size was >10 nm and binomially distributed
when the primary particle size was <10 nm. If one calculates
the average ‘‘5-min’’ secondary particle size using the weighted
PSD function, that is, summing up the product of the particle
size of each size class and its corresponding distribution
frequency, the ‘‘5-min’’ secondary particle size increased from
3.8–29 to 387.7–167.3 nm, respectively, a 100 to 5 times
increase in particle size. This is expected as the flocculation of
smaller particles generally proceeds rapidly according to the
perikinetic flocculation rate expression [56]. During perikinetic
flocculation, Brownian diffusion is the major driving force. The
perikinetic flocculation rate is a second order expression with
respect to particle number concentration. Therefore, at identical
mass concentration, e.g., 10 mg/L, the smaller the primary
particles, the faster they aggregated. As the particle size
increased, perikinetic flocculation was overtaken by orthoki-
netic flocculation, which is driven by the extent of mixing and is
a first-order rate equation with respect to particle number
concentration. Our results agreed well with what would be
expected from the general principle of flocculation kinetics.
Fig. 11 shows the pseudo first-order 2-CP degradation rate
constants as a function of the primary and the secondary
particle size. It is seen that the photoreactivity decreased
exponentially with increase in primary particle size. As
indicated above, the primary particles underwent immediate
aggregation upon dispensed into the 2-CP solution at an ionic
strength of ca. 10�3 to 10�2 M. Generally the pseudo first rate
constant also decreased exponentially with the ’’immediate‘‘
secondary particle size. The rate constants remained relatively
unchanged at high values as the ‘‘5-min’’ secondary particle
size increased; the rate then decreased sharply as the ‘‘5-min’’
secondary particle size approached to a value of ca. 400 nm.
Further increase in the ‘‘5-min’’ secondary particle size resulted
in continuing decrease in rate constants. Results imply that
photocatalytic reactivity of each size-class of TiO2 particles
was better predicted by the primary particle size and the
‘‘immediate’’ secondary particle size. The specific surface area
advantage in terms of promoting photocatalytic reactions was
preserved to some extent during the aggregation process,
especially for the ultra-fine particles that underwent fast
flocculation. Our results agreed well with Maira et al. [30] who
H. Lin et al. / Applied Catalysis B: Environmental 68 (2006) 1–1110
Fig. 11. Photocatalytic reaction rate constants as a function of particle size.
TEM measured primary particle size (solid squares); size measured immedi-
ately after the ultrasonic treatment (open circles); size measured at 5 min after
the experiment was started (solid circles). Ultrasound energy = 18 kJ/L. Note
that the flocculation reaction occurred rapidly when the primarily particle size
was less than 10 nm. Immediately upon dispersion of the nano-TiO2 particles,
particle size grew by 6–13 times. The particle size continued to grow by 5–100
times for the ultra-fine particles (e.g., <10 nm) as the reaction proceeded.
reported similar effect of secondary particle size, immediately
measured, on the photocatalytic reaction rate in gas phase. No
information has been reported on the effect of secondary
particle size (e.g., ‘‘5-min’’ or longer) on photocatalytic
activities in aqueous solutions. Under otherwise similar
conditions, smaller particles (e.g., <10 nm) have the specific
surface area advantages but an increase in band gap which
limits their accessibility to the excitation energy due to blue-
shift in absorption spectrum. In aqueous solutions, although the
primary particle size and ‘‘immediate’’ secondary particle size
are able to predict the effect of particle size on the
photocatalytic reactivity in aqueous solutions, the ‘‘5-min’’
or longer time secondary particle size better represented the
actual temporal state of particles. Future research is needed to
couple the kinetics of particle flocculation and the photo-
catalytic reaction as to elucidate the total effect of particle size
on photocatalytic reactivity in aqueous solutions.
4. Conclusion
TiO2 crystallines of different particle size were successfully
synthesized by the MOCVD method (particle size ranged
between 12 and 29 nm). The particle size of TiO2 can be simply
manipulated by moderate control of the O2/precursor flow rate
and system pressure. Results from the transformed Kubelka–
Munk equation showed that the band gap of the TiO2
nanoparticles was a function of the primary particle size.
When the TiO2 particle size decreased from its bulk (i.e.,
=29 nm) to 17 nm, the band gap decreased. In contrast, for
ultra-fine TiO2 particles (e.g., 3.8, 4.9, and 5.7 nm), the band
gap increased by 0.05, 0.012 and 0.036 eV, respectively. This
could be attributed to both the delocalization of molecular
orbitals, which in turn creates energy traps and surface states on
the band edge, and the size quantization effect reported by other
researchers [3,6,16,30]. Obtained band gap values fitted the
Brus’ s EMM model well within the particle range of less than
30 nm. Although TiO2 particles in the size range of 17–23 nm
have lower band gap energy (0.01–0.06 eV lower than that of
the bulk), results showed that the photocatalytic reactivity of
TiO2 colloid in aqueous suspension increased with the decrease
in the primary particle size. This can be attributed in part to the
large specific surface area of smaller particles. Additionally,
UV lamp spectrum in the range of UV-B (290 nm > l >320 nm) and full UV-A (400 nm > l > 320 nm) provides
sufficient energy for exciton separation of higher band gap
energy in particles (e.g., A3.8, A4.9, and A5.7). We therefore
suggest that unless a well-confined light source is provided
(e.g., l = 380–400 nm), TiO2 particles in the size range of 17–
29 nm cannot perform better in terms of photocatalytic
reactivity than those in the 3.8–5.7 nm size range, under
otherwise identical experimental conditions. Furthermore, even
in the presence of light at wavelength between 380 and 400 nm,
the photocatalytic performance of large particles (e.g., 17–
29 nm) will be compromised due to their relatively low specific
surface area. Our results clearly indicated that particle
aggregation did take place readily in aqueous solutions. Unless
a constant energy input is provided throughout the entire
experiment, there is no way to maintain the primary particle
size. In light of the rapid aggregation among the ultra-fine
particles, especially those that are smaller than 10 nm, primary
particle size alone was able to predict the photocatalytic
reaction. Intuitively, one would reason that the primary particle
size holds key to the electro-optical property and photocatalytic
reactivity of photocatalysts due mainly to the change in band
gap energy (e.g., result in shifts in absorption spectrum) and its
specific surface area.
Acknowledgement
The authors wish to acknowledge our two anonymous
reviewers for their excellent comments. This work was
supported by a NSF grant NIRT #0210284.
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