Improved photocatalytic and antibacterial activities of three-dimensional polycrystalline anatase TiO2 photocatalysts

Article abstract of DOI:10.1016/j.apcata.2013.07.042

We report three-dimensional polycrystalline anatase TiO₂ photocatalysts (3D a-TiO₂) for environmental and bio-medical applications. The 3D a-TiO₂ was synthesized without thermal treatment by the growth of rod-like polycrystals on Degussa P25 (P25) via low temperature hydrothermal processing and ultrasound irradiation. X-ray diffraction and high-resolution transmission electron microscopic results showed that the sea-urchin-like TiO₂ structure consisted with anatase polycrystals. The photocatalytic degradation (reactive black 5 and rhodamine B solutions) rates of 3D a-TiO₂ were found to be 2.2 times higher than that of P25. The recyclability of the 3D a-TiO₂ was found to be high: the decolorization rate was 95% of the initial value after 15 cycles. In addition, 3D a-TiO₂ exhibited excellent antibacterial activities for the sterilization of gram-negative Escherichia coli (E. coli) and gram-positive Staphylococcus aureus (S. aureus). Even at the 10th recycled use, more than 98% of E. coli and S. aureus can be killed. These results indicated that 3D a-TiO₂ might have utility in several promising applications such as photocatalytic water/air purification and bactericidal agents.

Full text of DOI:10.1016/j.apcata.2013.07.042

Applied Catalysis A: General 467 (2013) 394–399
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Improved photocatalytic and antibacterial activities of
three-dimensional polycrystalline anatase TiO photocatalysts
2
a,∗
b
a
c
Hyun Uk Leea , Soon Chang Lee , Sang Moon Lee , Jae Won Lee ,
a,∗∗
Hae Jin Kim , Jouhahn Lee
Division of Materials Science, Korea Basic Science Institute, Daejeon 305-333, Republic of Korea
Department of Fine Chemical Engineering and Applied Chemistry, Chungnam National University, Daejeon 305-764, Republic of Korea
Department of Energy Engineering, Dankook University, Cheonan 330-714, Republic of Korea
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2013
Received in revised form 18 July 2013
Accepted 21 July 2013
Available online 13 August 2013
We report three-dimensional polycrystalline anatase TiO2 photocatalysts (3D a-TiO ) for environmental
and bio-medical applications. The 3D a-TiO2 was synthesized without thermal treatment by the growth of
rod-like polycrystals on Degussa P25 (P25) via low temperature hydrothermal processing and ultrasound
irradiation. X-ray diffraction and high-resolution transmission electron microscopic results showed that
the sea-urchin-like TiO2 structure consisted with anatase polycrystals. The photocatalytic degradation
2
(
reactive black 5 and rhodamine B solutions) rates of 3D a-TiO2 were found to be 2.2 times higher than
Keywords:
TiO2
Hydrothermal treatment
Ultrasound irradiation
Bacterial deactivation
Dye degradation
that of P25. The recyclability of the 3D a-TiO2 was found to be high: the decolorization rate was 95% of
the initial value after 15 cycles. In addition, 3D a-TiO2 exhibited excellent antibacterial activities for the
sterilization of gram-negative Escherichia coli (E. coli) and gram-positive Staphylococcus aureus (S. aureus).
Even at the 10th recycled use, more than 98% of E. coli and S. aureus can be killed. These results indicated
that 3D a-TiO2 might have utility in several promising applications such as photocatalytic water/air
purification and bactericidal agents.
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
traditional structured materials [15]. Variety of 1D TiO2 NSs,
including nanowires (NWs) [16,17], nanorods (NRs) [18,19],
nanotubes (NTs) [20], nanofibers (NFs) [21], nanoribbons [22], and
nanosheets [23] have been synthesized. Despite these successful
Considerable effort has been devoted in recent years to the
development of photocatalysts with high photocatalytic and
antibacterial activities for environmental and bio-medical appli-
synthetic methods, a new approach to the synthesis of 1D TiO NSs
2
cations [1–3]. Titanium dioxide (TiO ) is one of the most promising
semiconductor photocatalysts with a large band gap of 3.2 eV,
which can produce the hydroxyl radical and superoxide under
is essential for obtaining a larger surface area, which is important
for guaranteeing a high density of active sites available for surface
reactions, as well as high interfacial charge carrier transfer rates
[15,24–26]. Recently, three-dimensional TiO2 nanostructures (3D
TiO2 NSs) with 1D TiO2 NSs studies have progressed markedly
through a number of techniques, resulting in the formation, for
example, of rutile crystalline TiO2 NSs with favorable dendritic
architectures [27], rutile TiO2 nanoflowers [28], hollow TiO2 tubu-
lar hierarchical structures [29], and NW networks [30]. Several
key factors in the controllable synthetic techniques of 3D TiO2
NSs, such as the hydrolysis rate and/or the surfactant aggregation
structure, can affect the morphology of TiO2 NSs synthesized
by wet chemical methods (Table S1) [27–29,31]. Despite the
improvements in the controllable synthetic techniques of 3D TiO2
NSs achieved by wet chemical methods, a facile and inexpensive
process for generation of controlled morphologies as well as
formation of 3D TiO2 NSs with anatase polycrystals is still lacking
and will be of great interest to researchers in the field.
2
ultraviolet (UV)-light irradiation (<380 nm) [4–7]. TiO is especially
2
promising for use in the photoelectric conversion, energy storage,
bactericidal effect and photocatalytic treatment of water and air
pollutants, owing to its high and stable photocatalytic performance
under UV-light irradiation [8–11]. The photocatalytic activity of
TiO2 has been considered to be determined by the specific sur-
face area, crystallinity, crystallite size, crystal structure, surface
defects, and the ratio of the surface charge carrier transfer rate to
the electron-hole recombination rate [9–14].
One-dimensional TiO2 nanostructures (1D TiO2 NSs) have
attracted great interest due to their better performance than
∗
Corresponding author. Tel.: +82 42 865 3637; fax: +82 42 865 3610.
Corresponding author. Tel.: +82 42 865 3613; fax: +82 42 865 3610.
E-mail addresses: leeho@kbsi.re.kr, biolhu@hanmail.net (H.U. Lee),
∗
∗
In this study, three-dimensional polycrystalline anatase TiO2
jouhahn@kbsi.re.kr (J. Lee).
photocatalysts with rod-like crystals (3D a-TiO2) were synthesized
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.07.042

H.U. Lee et al. / Applied Catalysis A: General 467 (2013) 394–399
395
at a low temperature without thermal treatment by hydrothermal
processing (HP) and ultrasound irradiation (UI). UI aids in crys-
tallizing 3D a-TiO2 without thermal treatment, stabilizing its 3D
TiO NSs, and removing organic compounds [32–34]. The 3D a-TiO2
2
with high crystallinity (pure anatase phase) and large surface area
2
(
138.2 m /g) exhibited very high photocatalytic activity toward the
oxidation of azo dyes [reactive black 5 (RB 5) and rhodamine B (Rh
B)], and efficient sterilization (or killing) of bacteria [gram-negative
Escherichia coli (E. coli) and gram-positive Staphylococcus aureus (S.
aureus)]. The photocatalytic and antibacterial activities of 3D a-TiO2
samples were higher than those of the commercial Degussa P25
(
P25).
2
. Experimental
2.1. Synthesis of 3D a-TiO2
All reagents were analytical grade (Sigma Aldrich Co., USA) and
Fig. 1. XRD patterns of P25, as-grown and 3D polycrystalline anatase TiO2 (3D a-
used without further purification. The commercial P25 (P25) was
TiO ) photocatalysts.
2
used to form a sea-urchin structure of 3D a-TiO . In a typical syn-
thesis, P25 (150 mg) was dispersed in 300 mL of mixture solution
2
apparent reaction rate constant, and C0 and C are the initial and
the reaction concentrations of RB 5 or Rh B] of the dye solution
with reaction time (0, 20, 40, 60, 80, 100, and 120 min) was also
investigated [34]. Evolved CO2 was analyzed with a TM IGC120-
MB chromatograph equipped with a Q column. To demonstrate the
(
TiCl :distilled water: 10 M NaOH:30% H O = 10:70:15:5, v/v%).
4
2
2
This solution was injected into a 500 mL teflon-lined autoclave. The
◦
autoclave was sealed, and heated at ∼85 C for 4 h. After washing,
the TiO products (as-grown 3D a-TiO ) were dissolved in 0.05 M
2
2
stability of the photocatalysts, we recycled used 3D a-TiO . The
HCl, and the solution was treated with high-intensity ultrasound
at a frequency of 20 kHz, applied from the top of the glass reactor
2
recycling test for the photocatalytic activity of 3D a-TiO2 was per-
formed after washing with distilled water and drying in an oven
(
∼100 mL) using a high intensity probe (13 mm diameter, Son-
◦
(
60 C) for 6 h after every cycle.
ics/Materials VC750, USA) [34]. Ultrasound irradiation was applied
The antibacterial activities of the P25 and 3D a-TiO2 samples
were evaluated by the inhibition of E. coli and S. aureus under
for 40 min, while the electrical energy input was maintained at
2
1
00 W/cm . The TiO products (3D a-TiO ) were filtered, washed
2
2
UV-light irradiation [34]. Before these tests, all glassware and
with ethanol-deionized water mixed solution (ethanol:deionized
water = 30:70, v/v%), and finally dried under air at RT. The 3D a-TiO2
◦
samples were sterilized by autoclaving at 120 C for 15 min. Bac-
◦
terial cultures were grown in Luria-Bertani (LB) media overnight
photocatalysts were calcined in an electric furnace at 500 C under
◦
at 37 C with continuous shaking at ∼200 rpm. The bacterial cells
an air atmosphere. After being kept for 1 h at the desired calcina-
5
were diluted to a cell suspension of ∼2 × 10 colony forming units
tion temperatures, the TiO2 products (3D a-TiO -C) were cooled to
2
−
1
(
CFU mL ) with deionized water. The mass of the photocatalyst
room temperature in the electric furnace.
−
1
was adjusted to 50 ␮g mL . The suspensions were stirred with a
magnetic stirrer to prevent the samples from settling, and were
exposed to UV-light for various irradiation times (0, 5, 10, 15, 20,
2.2. Characterization
2
5, and 30 min). Then, 1 mL of the suspension was sampled, added
The crystalline structure of 3D a-TiO2 was investigated by X-
◦
to the LB plate, and incubated overnight at 37 C. After incubation,
the bacterial colonies were observed and quantified [34–37].
For the photocatalytic activity and antimicrobial tests, the
data were averaged and expressed as mean ± standard deviation.
Each test was repeated up to five times. Statistical analysis was
performed using analysis of variance (ANOVA), with p < 0.05 con-
sidered as significant.
ray diffraction (XRD; Rigaku RDA-cA X-ray diffractometer, Japan)
patterns using Cu K␣ radiation with a nickel filter. The mor-
phology, distribution, crystallinity and chemical composition of
3
D a-TiO2 were recorded by field emission-scanning electron
microscope (FE-SEM; Hitachi; S-4700, Japan), high resolution-
transmission electron microscope (HR-TEM; JEOL; JEM 2200, Japan)
and Fourier transform infrared spectrometer (FTIR; JASCO; JASCO-
6
000, Australia). Before the analyses, the samples were placed
onto the surfaces of the copper grids and dried under ambient
conditions. The Brunauer–Emmett–Teller (BET) surface areas, pore
3. Results and discussion
volumes, and pore diameters of the 3D a-TiO samples were deter-
3.1. Physicochemical and structural properties of 3D a-TiO2
2
mined using a BET analyzer (Micromeritics ASAP 2020, USA).
Fig. 1 depicts the XRD patterns of the P25, as-grown and the 3D
a-TiO2 samples. The P25 gradually underwent a phase transition
from the anatase/rutile crystal structure to amorphous during the
HP. Also, the as-grown 3D a-TiO2 notably underwent a phase tran-
sition from amorphous to the anatase crystal structure after UI. The
2
.3. Measurement of photocatalytic and antibacterial activities
The photocatalytic degradation of reactive black 5 (RB 5; 3 mg/L,
pH 6.67, Aldrich, USA) and rhodamine B (Rh B; 3 mg/L, pH 5.5,
Aldrich, USA) solutions by P25 and 3D a-TiO2 samples (0.5 g/L)
were carried out under a UV lamp (4 W, 365 nm, VSLAB VL-4CL,
Korea), and the absorbance of the solutions was measured by
using a UV-VIS-IR spectrometer (Varian, Cary5000, Australia) in
the region 200–800 nm [10,34]. The concentrations of the RB 5 and
Rh B solutions after UV-light irradiation were measured from the
peak intensities of the absorbances at 598 and 555 nm, respectively
high-intensity UI induces crystallization of 3D a-TiO because ultra-
2
sonic cavitations create a unique environment [32–34]. The main
peak of the rutile (1 1 0) plane of P25 disappeared completely after
growth of the rod-like polycrystals on P25 via HP and UI, indicating
that the 30% rutile phase in P25 was converted to the anatase phase
(3D a-TiO ), owing to anatase TiO NSs hang over the P25. Through
2
2
◦
thermal treatment at 500 C for 1 h (3D a-TiO -C), the intensities of
2
[
34]. The change in the concentration [ln(C /C) = kt, where k is the
the anatase peaks of 3D a-TiO samples became stronger and more
0
2

3
96
H.U. Lee et al. / Applied Catalysis A: General 467 (2013) 394–399
Fig. 2. (a) SEM and (b–d) EDS/HR-TEM images of 3D polycrystalline anatase TiO2 (3D a-TiO2) photocatalysts.
distinct than those of 3D a-TiO . The slight narrowing of the peak
ultrasonic cavitation created unique environments that allowed the
formation, growth, and implosive collapse of bubbles in the liquid
[32–34].
2
widths after thermal treatment could be attributed to the increased
degree of crystallinity and to a slight crystalline domain growth.
It is clear that the crystal phase of 3D a-TiO2 derived from our
method is anatase phase (JCPDS 73-1764). No characteristic peaks
were observed for other phase types (i.e., rutile or brookite), indi-
cating the high purity and crystallinity of 3D a-TiO2 formed via our
method.
The BET surface areas, pore volumes, and mean pore diame-
ters of P25 and the 3D a-TiO2 samples are summarized in Table 1.
2
The BET surface area and pore volume of P25 were 51.1 m /g
3
2
and 0.11 cm /g, respectively. The larger surface area (91.4 m /g)
3
and pore volume (0.17 cm /g) of as-grown 3D a-TiO could be
2
The FE-SEM images at different magnifications revealed that
D a-TiO2 with diameters in the range of 2.2–5.1 ␮m, was com-
posed of sea-urchin-like structures comprising of many rod-like
polycrystals (Fig. 2a). An EDS/HR-TEM images showed that the
rod-like polycrystals of 3D a-TiO2 were chemically composed of
Ti and O elements, as expected, and no other elements were noted
attributed to the inter-rod voids of the randomly stacked thin
2
3
nanostructures. The increase in the surface area (138.1 m /g) and
3
pore volume (0.20 cm /g) of 3D a-TiO2 were caused by the UI was
mainly attributed to the crystallization, stabilizing its nanostruc-
ture, and removing organic compounds [34]. Although 3D a-TiO -C
2
2
(88.7 m /g) had lower surface area than 3D a-TiO , it exhibited
2
(
Fig. 2b). HR-TEM images clearly showed that urchin-like struc-
higher crystallinity, which can be attributed to the increase in the
crystallite size by thermal treatment.
tures were assembled by rod-like polycrystals with stone-pillar
shapes (Fig. 2c). Electron diffraction patterns of rod-like polycrys-
tals were observed in a selected area of electron diffraction (Fig. 2d),
which were (1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 0 4) diffractions
of the polycrystals with the anatase phase. The HR-TEM image
and XRD pattern therefore indicated that 3D a-TiO2 consisted of
rod-like polycrystals with anatase phase. The lattice fringes with
inter-planar distances of 0.35 nm, which were in good agreement
with the d spacing values of the anatase (1 0 1) planes, were clearly
observed in the HR-TEM image.
3.2. Photocatalytic and antibacterial activities of 3D a-TiO2
photocatalysts
By UV-light irradiation on 3D a-TiO , the band-gap of TiO2 with
2
anatase crystalline is ∼3.2 eV with band-edge positions that allow
•
the formation of highly hydroxyl radicals (OH ) with water via the
•
reaction of the hole with the 3D a-TiO2 surface (Fig. S1). The OH
are able to decompose most hydrocarbon species of water pollut-
It is suggested that the formation of the 3D a-TiO2 originated
from the reaction of the P25, using a modified HP in the presence
of mixture solution, as shown in Fig. 3. When the starting P25 was
ants. The photocatalytic activities of the P25 and 3D a-TiO samples
were determined by the decomposition rates of RB 5 and Rho B
2
+
treated with aqueous mixture solution, Na was inserted into the
Table 1
space between the TiO6 octahedra. Ti
when some of the longer Ti O bonds were broken, while the shorter
O
Na bonds were formed
Physicochemical properties of P25 and 3D polycrystalline anatase TiO2 (3D a-TiO2)
photocatalysts.
Ti O bonds were preserved [38–42]. As a result, Na TinO2n+1 NSs
were first formed and then grew during the hydrothermal treat-
2
Sample
BET surface
area (m /g)
Pore diameter
(nm)
Pore volume
(cm /g)
2
3
ment. After Na TinO
formed by removal of the intercalated Na by washing with HCl
and distilled water [41–46]. Furthermore, the UI process induced
NSs growth, the H TinO
phase was
2
2n+1
2
2n+1
P25
51.1
91.4
138.1
88.7
13.68
6.81
4.41
8.74
0.11
0.17
0.20
0.15
+
As-grown 3D a-TiO2
3D a-Tio2
3D a-Tio2-C
crystallization of 3D TiO2 NSs with H TinO
phases because
2
2n+1

H.U. Lee et al. / Applied Catalysis A: General 467 (2013) 394–399
397
Fig. 3. The schematic diagram for the formation of 3D a-TiO2.
Fig. 4. Comparison of the photocatalytic activity of P25 and the 3D polycrystalline anatase TiO2 (3D a-TiO2) photocatalysts in (a and b) RB 5 and (c and d) Rho B dye solutions.
The data were fitted as dotted lines and the corresponding decomposition rates ([k]) are given.

3
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H.U. Lee et al. / Applied Catalysis A: General 467 (2013) 394–399
Fig. 5. (a) Evolution of CO2 from reactive black 5 (RB 5) as a function of UV-light irradiation time (min) by photocatalytic reaction with 3D polycrystalline anatase TiO2 (3D
a-TiO2) photocatalysts, and (b) recycling results of the 3D a-TiO2 in the decolorization of RB 5 solution.
solutions under UV-light irradiation (Fig. 4). In the case of 3D a-
120 min, the quantity of evolved CO2 for 3D a-TiO2 (0.207 mmol)
was 2.2 times higher than that for P25 (0.092 mmol). These results
show that a high degree of mineralization for RB 5 was achieved
[48]. The length of one cycle was 0–120 min (UV-light irradiation
time), and decolorization was measured at every time (Fig. 5b).
After fifteen cycles of the degradation reaction, the photocatalytic
conversion ratio of RB 5 for the 3D a-TiO2 remained approximately
95% under UV-light irradiation. The decrease in the decolorization
after every cycle was most likely due to the loss of the 3D a-TiO2
photocatalysts. However, we can clearly see that the removal
efficiency of organic contaminants using 3D a-TiO2 remained
approximately constant, even after 15 cycles. Undoubtedly, the 3D
a-TiO2 exhibits excellent photocatalytic stability.
The antibacterial activities of the 3D a-TiO2 in the disinfec-
tion of E. coli were compared with P25 and other 3D a-TiO2
samples, according to the decrease of the colony number formed
on a LB plate (Fig. S3). The dark controls (3D a-TiO2 suspen-
sions + bacteria in the dark) showed that all photocatalysts are not
toxic for the microorganisms under dark conditions. However, 3D
a-TiO2 inhibited the growth of E. coli and S. aureus efficiently, and
their antibacterial activities became stronger with increased sam-
ple concentrations and UV-light irradiation times (Fig. S3). Fig. 6
TiO , the RB 5 and Rho B solutions degraded almost entirely and
2
became transparent within 120 min under UV-light irradiation. The
as-grown 3D a-TiO , P25 and 3D a-TiO -C were still somewhat
2
2
bluish (RB 5 solution) or reddish (Rho B solution) after 120 min.
No change was observed in the absorbance of dye solutions under
UV-light irradiation only. The highest degradation rate ([k]) for 3D
−
1
a-TiO2 was 1.418 h , which was 2.2 times higher than that of P25
(
−1
[k] = 0.646 h ). The enhanced photocatalytic activity of 3D a-TiO2
may be attributed to its structural and photochemical properties
such as large surface area and surface hydroxyl groups (Fig. 3 and S2,
and Table 1) [13,47]. The 3D a-TiO2 with larger surface area, poly-
crystalline anatase phase and surface hydroxyl groups generally
induces higher photocatalytic activity because its structural and
physicochemical properties provides more active sites, and better
adsorptive and degradation capacity for azo dye particles. However,
a large surface area is not the unique factor affecting the enhanced
photocatalytic activity. High photocatalytic activity of 3D a-TiO2
photocatalysts can be achieved by the large specific surface area,
high crystallinity, low crystallite size, anatase crystal structure, high
surface defects, and low electron–hole recombination rate [34]. For
2
example, although 3D a-TiO had a larger surface area (138.1 m /g)
2
2
than 3D a-TiO -C (88.7 m /g), their photocatalytic activities were
shows that the antibacterial activities of the 3D a-TiO samples
2
2
−
1
the same. This result indicates that the degree of crystallinity is also
a significant factor that affects photocatalytic activity [47].
The quantity of evolved CO2 during the photocatalytic reaction
of RB 5 with 3D a-TiO2 photocatalysts under UV-light irradiation
is shown in Fig. 5a. The evolved CO2 increased gradually as the
photocatalytic reaction progressed. After UV-light irradiation for
(50 ␮g mL ) against E. coli and S. aureus under UV-light irradi-
ation and non-irradiation. The bacterial survival was calculated
by % survival = B/A × 100 (where the number of surviving micro-
bial colonies under the non-irradiated condition was A and under
the UV-irradiated condition was B) [34]. For the as-grown 3D a-
TiO , ∼14.8% bacterial inhibition of E. coli bacteria was observed
2
Fig. 6. Plot of survival (%) versus UV-light irradiation time (min) for P25 and the 3D polycrystalline anatase TiO2 (3D a-TiO2) photocatalysts against (a) E. coli and (b) S. aureus.

H.U. Lee et al. / Applied Catalysis A: General 467 (2013) 394–399
399
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•
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This research was supported by the Converging Research Center
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Appendix A. Supplementary data
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Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2013.
07.042.