Platinum decorated hierarchical top-porous/bottom-tubular TiO2 arrays for enhanced gas phase photocatalytic activity

Article abstract of DOI:10.1039/c4ra02153c

In this communication, Pt nanoparticles (NPs) were successfully loaded on hierarchical TiO₂ nanotube arrays (TiO₂ NTs) for efficient decomposition of gas phase pollutants. The loading of Pt NPs on TiO₂ NTs significantly enhanced the photocatalytic activity due to reduction of the recombination of photogenerated electrons and holes. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02153c

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Platinum decorated hierarchical top-porous/
bottom-tubular TiO arrays for enhanced gas phase
2
Cite this: RSC Adv., 2014, 4, 19533
photocatalytic activity
Received 12th March 2014
Accepted 15th April 2014
a
a
a
a
a
a
Dandan Yuan, Yang Gao, Hongjun Wu,* Tongxin Xiao, Yang Wang, Baohui Wang
and Zhonghai Zhang*b
DOI: 10.1039/c4ra02153c
www.rsc.org/advances
16
In this communication, Pt nanoparticles (NPs) were successfully Schottky junction with TiO2. As the electron affinity of anatase
loaded on hierarchical TiO
2
nanotube arrays (TiO
2
NTs) for efficient TiO (ꢀ4.2 eV) is signicantly lower than the work function of
2
decomposition of gas phase pollutants. The loading of Pt NPs on TiO
2
Pt, the higher work function will induce higher electronic
NTs significantly enhanced the photocatalytic activity due to reduction potential between Pt and TiO
of the recombination of photogenerated electrons and holes.
2
, which accelerates the electron
transfer and reduces the electron–hole recombination. Even the
photocatalytic studies on TiO with/without Pt NPs have been
extensively studied, however, previous studies were focused on
2
Photocatalytic technology has received signicant attention as a
promising way for remediating experimental pollution, and lots
of materials, especially metal oxide semiconductors, have been
17–20
photocatalytic activity in aqueous solution,
catalytic activity in gas phase has rarely studied. To our best
knowledge, the gas phase photocatalytic activity of noble metal
the photo-
1–3
widely explored as photocatalysts. Among them, titanium
dioxide (TiO ) is regarded as one of the most popular photo-
catalysts owing to its high resistance to photo-corrosion, phys-
decorated hierarchical TiO NTs has not been reported. In this
2
2
study, the gas phase photocatalytic activity of TiO
TiO NTs were estimated by decomposition of evaporated
methanol.
NTs and Pt/
2
2
4
–6
ical and chemical stability, ease of availability and low cost.
Notably, TiO nanotube arrays (TiO NTs), fabricated by elec-
2
2
The hierarchical TiO2 NTs were fabricated by a two-step
anodization process (Scheme 1(a)–(d)). Prior to anodization, the
Ti sheets were rst degreased by sonicating in ethanol and cold
distilled water, followed by drying in pure nitrogen stream. The
anodization was carried out using a conventional two-electrode
system with the Ti sheet as an anode and a Pt gauze (Aldrich,
trochemical anodization method, have been demonstrated to
be the most efficient photocatalysts for environmental protec-
tion and solar energy conversion because of their unique
morphological and electronic properties, especially their mon-
odirectional electron transfer, high electron mobility, and
enhanced light absorption induced by their one-dimensional
1
0
00 mesh) as a cathode respectively. All electrolytes consisted of
.5 wt% NH F in ethylene glycol (EG) solution with 2 vol%
7–9
nanostructure. In this communication, more interestingly,
4
the hierarchical TiO NTs with top-porous and bottom-tubular
2
structures, fabricated by a two-step anodization method, were
rationally selected as photocatalyst as it has been proven better
photocatalytic activity than conventional TiO
higher uniformity and enhanced light scattering activity.
For further enhancing photocatalytic activity of TiO
2
NTs due to its
10
2
NTs,
noble metal nanoparticle decoration was an effective option.
Many noble metals were investigated to couple with TiO ,
2
11,12
13
14
15
including Au,
Ag, Pd, and Ru. In this communication, Pt
nanoparticles (NPs) were selected due to its higher work func-
tion (5.65 eV), which would be more favorable for formation a
a
Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry &
Chemical Engineering, Northeast Petroleum University, Daqing 163318, China.
E-mail: hjwu1979@gmail.com
b
Department of Chemistry, East China Normal University, 500 Dongchuan Road,
Scheme 1 Two-step anodization processes for fabrication of hierar-
Shanghai 200241, China. E-mail: zhzhang@chem.ecnu.edu.cn
chical TiO
2
NTs and photocatalytic reduction for decoration of Pt NPs.
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water. All the anodization experiments were carried out at room
temperature. In the rst-step anodization, the Ti sheet was
anodized at 50 V for 30 min, then the as-grown nanotube layer
was ultrasonically removed in deionized water, leaving compact
two-dimensional hexagonal pattern on the surface of the Ti
sheet alone. The patterned Ti sheet then underwent the second
anodization at 20 V for 30 min, in which the hexagonal pattern
formed top-porous structure and subjective NTs grew below the
top-porous layer. Aer second-step anodization, the prepared
TiO NTs sample was cleaned with distilled water and dried off
2
with N gas. The as-anodized TiO NTs were annealed in air at
2
2
ꢁ
ꢁ
ꢂ1
4
50 C for 1 h with a heating rate of 5 C min
The Pt NPs were decorated on the TiO NTs by a photo-
catalytic reduction method using platinum(II) acetylacetonate
Pt(AcAc) ) as a precursor (Scheme 1(e) and (f)). The Pt(AcAc)
2
.
2
(
2
was diluted in deionized water and ethanol with a water–
ethanol volume ratio of 10 : 1, and the concentration of
Pt(AcAc)2 was xed at 1 mM. The Ti sheet containing the
prepared TiO NTs were soaked into the Pt precursor solution
2
for 24 h, then rinsed with deionized water, and subsequently
irradiated with simulated solar light for 30 min.
The morphology of nanotubular structures and distribution
of nanoparticles were determined by led-emission scanning
electron microscope (FESEM, Zeiss SigmaHV) and transmission
electron microscope (TEM, Tecnai T12). Fig. 1a shows a top view
SEM image of TiO NTs. An individual top hexagonally porous
2
structure is fairly observed, with an average diameter of the
hexagonal pores about 200 nm and a wall-thickness of 19 nm. A
cross-sectional view of the TiO NTs is presented in the bottom-
2
le inset of Fig. 1a, indicates a corresponding tubular structure
with a length of ꢀ600 nm. Fig. 1b is a high-magnication top-
2
view SEM image of Pt/TiO NTs, which shows the Pt NPs are
homogeneously decorated on the top porous TiO
2
structure
Fig. 1 (a) Top-view SEM image of hierarchical TiO
2
NTs, the bottom
with average diameter of ꢀ20 nm. The TEM image of Pt/TiO
2
left inset presents cross-sectional view SEM image; (b) top-view SEM
NTs is presented in the inset of Fig. 1b, which clearly showed Pt image of Pt/TiO
NTs, the inset shows the TEM image of Pt NTs inside
NTs; (c) XRD patterns of TiO NTs and Pt/TiO NTs; (d) EDS
spectra of Pt/TiO NTs (the top/yellow one) and TiO NTs (the bottom/
red one); (e) XPS survey; (f) core-level spectrum of Pt 4f of Pt/TiO NTs;
g) core-level XPS of Ti 2p; and (h) diffuse reflectance UV-vis
absorption spectra of TiO NTs, Pt/TiO NTs, P25 TiO NPs and Pt/P25
NPs.
2
the TiO
2
2
2
NPs are not only decorated on the outer side of NT but also into
the inter side of NTs with same diameter size of 20 nm as shown
in SEM image.
2
2
2
(
The crystal structures were characterized by grazing inci-
2
2
2
dence X-ray diffraction (GIXRD) analysis by an X-ray diffrac- TiO
2
tometer (Rigaku D/MAX-2200) with Cu Ka source in the range of
ꢁ
2
q ¼ 20–70 . The XRD patterns of the hierarchical TiO NTs and
2
source operating at 150 W. The survey and high-resolution
spectra were collected at xed analyzer pass energies of 160 and
Pt/TiO NTs are shown in Fig. 1c. Clearly, both of them pre-
sented pure crystalline anatase with strong preferential orien-
2
20 eV, respectively. Binding energies were referenced to the C1s
tation of (101). In addition, the Pt/TiO
extra Pt XRD pattern with strong orientation of (111), which
implies that the Pt NPs are successfully doped on the TiO NTs.
The chemical compositions of Pt/TiO NTs were analyzed by
energy dispersive spectrometer (EDS) equipped with FESEM.
The EDS spectra in Fig. 1d showed another evidence for Pt
existence on the TiO NTs, and the Pt showed an atomic ratio of
.92% in the Pt/TiO NTs composites.
The X-ray photoelectron spectra (XPS) were recorded to
determine the chemical components and the valence state of Pt
on the surface of the Pt/TiO NTs. The XPS data were collected
2
NTs sample shows an
binding energy of adventitious carbon contamination which
was taken to be 285.0 eV. As shown in Fig. 1e, the surface of the
2
2 2
TiO NTs and Pt/TiO NTs both contained Ti, O, and C
2
elements, but an additional Pt peak was observed in the survey
of Pt/TiO NTs. The core-level XPS spectrum of Pt, shown in
2
Fig. 1f, conrmed the existence of metallic Pt on the surface.
2
The core-level XPS spectra of Ti 2p in Fig. 1g shows doublet
0
2
21
peaks at 458.4 and 464.1 eV, corresponding to the Ti 2p3/2 and
Ti 2p1/2, respectively. Furthermore, a peak at 455.7 eV has been
3
+
dened as Ti both in TiO
2
NTs and Pt/TiO
NTs than in Pt/TiO
decreased intensity of Ti on Pt/TiO NTs can be ascribed to the
2
NTs, but showed
2
much higher intensity on TiO
2
2
NTs. The
by an Axis Ultra instrument (Kratos Analytical) under ultrahigh
vacuum (<10 Torr) and by using a monochromatic Al Ka X-ray
3
+
ꢂ8
2
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electron transfer from TiO to Pt as the formation of Schottky
22
junction.
Optical absorption is important factor for the PC perfor-
mance on TiO2 materials. The diffuse reectance UV-vis
absorption spectra were employed to characterize the optical
absorption properties TiO
Pt/P25 TiO NPs (Fig. 1h). The light in UV region can be
absorbed because its photons own higher energy than the band
gap of TiO , while the TiO NTs sample showed much stronger
2 2 2
NTs, Pt/TiO NTs, P25 TiO NPs and
2
2
2
light absorption in visible light region than P25 TiO2 NPs
sample, which can be ascribed to the unique hierarchical
structure of TiO NTs, induced strong light scattering. Aer Pt
2
2 2
NPs loading, the both of Pt/TiO NTs and Pt/P25 TiO NPs
samples did not show signicant difference to their pristine
samples.
The gas phase photocatalytic activities of TiO
2
NTs and
Pt/TiO NTs were estimated by decomposition of evaluated
2
methanol (5 mL). The concentration of methanol and corre-
sponding degraded by-products were measured by Fourier
transform infrared (FTIR) spectroscopy (Bruker Tensor27). All
the photocatalytic measurements were performed in a home-
made cylindrical cell with 5 cm length and 3.6 cm width, in
2 2
which the TiO NT and Pt/TiO NT samples were contained
respectively. The photocatalytic degradation of methanol was
carried out under irradiation with simulated solar light with
ꢂ2
intensity of 100 mW cm (model SET-140F, Seric LTD) with an
infrared blocking lter. Fig. 2a and b shows the FTIR spectra of
methanol decomposition as a function of irradiation time on
TiO NTs and Pt/TiO NTs respectively. Before the illumination,
2 2
the initial methanol processes the bands at 1031, 1055, 1340,
ꢂ1
2
860, 2950, and 3685 cm , which bands correspond to the C–O
3
stretching, CH rocking, O–H bending, C–H parallel symmetric
stretching, C–H out of plane asymmetric stretching, O–H
23–26
stretching, respectively.
As the increase of irradiation time,
all these bands started to decrease in intensity and new peaks
ꢂ1
appeared at 669, 1456, 1640, 1750, 2360 and 3440 cm . The
ꢂ1
band at 1456 cm can be assigned to the out of plane asym-
Fig. 2 Decomposition of methanol on TiO
2 2
NTs (a) and Pt/TiO NTs
ꢂ1
metric bending of C–H, 1640 cm proved the formation of
isolated C]C bonds, and 1750 cm can be assigned to the C]
O stretching vibration modes.
(b) respectively as a function of irradiation time.
ꢂ1
27,28
2
The gaseous CO is a linear
Fig. 3a. The experimental data of Fig. 3a were found to t
approximately a pseudo-rst-order kinetic model by the linear
transforms f(t) ¼ finf[1 ꢂ exp(ꢂkt)] (f is peak height, finf is peak
molecule with two infrared active absorption bands at
ꢂ1
ꢂ1
2
360 cm
(antisymmetric stretching mode) and 669 cm
29,30
(bending mode).
Hence, one can conclude that the primary
ꢂ1
31
height at innite time, and k is rate constant). The corre-
two peaks centered at 2360 and 669 cm imply the formation
sponding curves were presented in Fig. 3b, and the values of
rate constant, k, were also showed in Fig. 3b. Clearly, the Pt/TiO2
of CO
2
gas when methanol is decomposed. In the end, aer
ꢂ1
60 min, just CO
2
and H
2
O (3440 cm ) existence as the results of
NTs showed the highest k value, and our TiO
2
NTs sample
NPs.
completed decomposition of methanol on the Pt/TiO
sample.
2
NTs
showed higher k value than TiO NPs but lower the Pt/TiO
2
2
The enhanced photocatalytic activity aer Pt deposition may be
ascribed the possible formation of Schottky-junction between Pt
Based on the photocatalytic experimental data, we summa-
ꢂ1
rized the CO transmittance peak height (2360 cm , CO ) for
2
2
NPs and TiO
2 2
NTs. Fig. 3c presented the stability of Pt/TiO NTs
TiO NTs and Pt/TiO NTs respectively in Fig. 3a. In addition, for
2
2
during multiple cycles for methanol decomposition. Clearly,
comparing the photocatalytic activity of our TiO NTs samples
2
aer ve cycles, the Pt/TiO NTs did not exhibit any signicant
with commercial available photocatalyst, P25 TiO
photocatalytic lms, with similar thickness of TiO
2
NPs based
NTs, were
2
loss of their PC activities, indicating their high stability under
operation conditions, which is important for practical
applications.
2
also prepared on same Ti substrates and loaded Pt NPs with
same method. The photocatalytic data of CO transmittance
peak height on TiO NPs and Pt/TiO NPs were also presented in
2
2
2
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radicals, which stimulates chain reaction, and rst oxidation is
to form aldehyde then to formic acid, and then ultimately make
methanol deeply decompose to H O and CO . The reaction
2
2
process is shown below:
ꢂ
+
TiO
2
+ hn / e + h
(1)
(2)
(3)
(4)
(5)
(6)
(7)
ꢂ
ꢂ
e
+ Pt / Pt
ꢂ
2ꢂ
e
2
+ O / cO
ꢂ
2
Pt + O / cO
2ꢂ
+
+
H O + h /cOH + H
2
CH
3
2 2
OH + cOH /cCH OH + H O
+
2
CH OH + 2h / cCH OH + cCH + H O
3
2
3
2
2ꢂ
ꢂ
ꢂ
cCH OH + cO / HCHO + cOOH / HCOOH + OH (8)
2
+
+
HCOOH + h / CO
2
+ 2H
(9)
(10)
+
ꢂ
H + OH / H O
2
In summary, Pt NPs decorated hierarchical TiO
2
NTs were
successfully designed and estimated of gas phase photocatalytic
activity. It had been revealed that Pt modied the inherent
properties of TiO NTs as well as affected the photocatalytic
2
activity. The Pt/TiO2 NTs meaningfully proved the electron
transfer and reduced the recombination of photoelectrons and
holes. Compared with the pure TiO NT electrode, the Pt/TiO
2 2
NT electrode showed an outstanding enhancement on photo-
catalytic decomposition of methanol.
Acknowledgements
ꢂ1
H.W. thanks to the support from New Century Excellent Talent
Fig. 3 (a) Variation of the peak height of 2360 cm corresponding to
the normal vibration of CO
mittance spectra with the irradiation time; (b) comparison perfor-
mance of constant k on TiO NPs, TiO NTs, Pt/TiO NPs and Pt/TiO
NTs for methanol
2
molecules derived from the FTIR trans- Project of the Department of Education, Heilongjiang Province,
P. R. China, and Z.Z. thanks to the support from “Yingcai”
program of ECNU.
2
2
2
2
NTs respectively; (c) stability of Pt/TiO
decomposition.
2
Notes and references
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4
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6
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