Modified hierarchical TiO2 NTs for enhanced gas phase photocatalytic activity

Article abstract of DOI:10.1039/c5ra05981j

In this paper, three kinds of noble metal nanoparticles (NMNs) were successfully loaded on hierarchical TiO₂ nanotube arrays (TiO₂ NTs) to improve photocatalytic (PC) activity of gas phase pollutants. The hierarchical TiO₂ NTs, with unique top-nanoporous and bottom-nanotubular structure, were prepared through a facile two-step anodization method, and then the noble metal nanoparticles were loaded on the TiO₂ NTs by means of a photo-reduction method. The gas phase photocatalytic activity of TiO₂ NTs and NMNs/TiO₂ NTs were estimated by decomposition of gaseous methanol. The formation of Schottky junctions between TiO₂ NTs and NMNs significantly improved the PC due to they could significantly accelerate the electron transfer and thus reduction of the recombination of photogenerated electrons and holes.

Full text of DOI:10.1039/c5ra05981j

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Modified hierarchical TiO2 NTs for enhanced gas
phase photocatalytic activity†
Cite this: RSC Adv., 2015, 00, 57937
*
Di Gu, Hongjun Wu, Yanji Zhu and Baohui Wang
In this paper, three kinds of noble metal nanoparticles (NMNs) were successfully loaded on hierarchical TiO2
nanotube arrays (TiO2 NTs) to improve photocatalytic (PC) activity of gas phase pollutants. The hierarchical
TiO2 NTs, with unique top-nanoporous and bottom-nanotubular structure, were prepared through a facile
two-step anodization method, and then the noble metal nanoparticles were loaded on the TiO2 NTs by
means of a photo-reduction method. The gas phase photocatalytic activity of TiO2 NTs and NMNs/TiO2
NTs were estimated by decomposition of gaseous methanol. The formation of Schottky junctions
between TiO2 NTs and NMNs significantly improved the PC due to they could significantly accelerate the
electron transfer and thus reduction of the recombination of photogenerated electrons and holes.
Received 3rd April 2015
Accepted 29th June 2015
DOI: 10.1039/c5ra05981j
www.rsc.org/advances
electron affinity of anatase TiO2 is signicantly lower than the
work function of noble metal, which will induce higher elec-
Introduction
tronic potential between noble metal and TiO2, accelerating the
electron transfer and inhibits the electron–hole pairs recombi-
nation. The signicance of this work provide a fundamental
understanding not only to the fabrication but also the utility of
Schottky junctions for enhanced environmental remediation
method.
Even the PC studies on TiO2 with/without NMNs have been
extensively studied, however, previous studies were focused on
PC activity in aqueous solution,17–20 the PC activity in gas phase
has rarely studied. Compared with liquid phase reaction
process, gas phase photocatalytic oxidation reaction features
high reaction velocity, good exploitation rate of light, large
volume ow, which could oxidize reactant completely and free
of solvent inuence,21–24 and therefore lead to the wide appli-
cation. To the best of our knowledge, the gas phase PC activity
Photocatalytic (PC) technology has received widespread atten-
tion for pollutant decomposition, and a lot of materials, espe-
cially metal oxide semiconductors, have been widely utilized as
photocatalysts.1–3 Among them, titanium dioxide (TiO2) is
regarded as one of the most promising photocatalysts because
of its unique electronic properties, high resistance to photo-
corrosion and low cost.4–6 TiO2 nanotube arrays (TiO2 NTs),
fabricated by electrochemical anodization process, have been
demonstrated to be one of the most efficient materials of PC
applications for degradation of environmental pollutants and
conversion of solar energy because of their unique monodirec-
tional electron transfer, high electron mobility and one-
dimensional nanostructure.7–9 In this paper, we fabricated a
unique hierarchical TiO2 combined nanostructure of top-
nanorings and bottom-nanotubes by a two-step anodization
method, which showed better PC activity than pure TiO2 NTs
due to its higher hierarchical and light scattering activity.10
However, unmodied TiO2 as a wide bandgap semi-
conductor is limited to PC activity only under UV irradiation.
For further enhancing PC activity of TiO2 NTs and harvesting
long wavelength radiation, noble metal nanoparticles (NMNs)
modication were an effective option. Previous researches11–16
have shown that the deposition of noble metals in the surface of
TiO2 nanoparticles can improve the photocatalytic activity of
TiO2 signicantly. In this paper, three kinds of NMNs (Ag, Au,
Pt) were investigated to select one would be best favorable for
formation a Schottky junction with TiO2 (Scheme 1). As the
Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry &
Chemical Engineering, Northeast Petroleum University, Daqing 163318, China.
E-mail: hjwu@nepu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra05981j
Scheme 1 Schematic illustration of the energy bands between NMNs
and TiO2 NTs.
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of NMNs decorated hierarchical TiO2 NTs has not been repor- (Scheme 2e). The AgNO3 aqueous solution was 1.0 ꢀ 10ꢂ2 mol
ted. In this study, the gas phase PC activity of TiO2 NTs and Lꢂ1 (Fig. S1, ESI†), the HAuCl4 aqueous solution was 2.43 ꢀ
NMNs/TiO2 NTs were estimated by decomposition of evapo- 10ꢂ4 mol Lꢂ1 (Fig. S2†), the Pt(AcAc)2 (ref. 27) was diluted in
rated methanol.
deionized water and ethanol with a water–ethanol volume ratio
of 10 : 1, and the concentration of Pt(AcAc)2 was xed at 1.0 ꢀ
10ꢂ3 mol Lꢂ1 (Fig. S3†). The Ti foil containing the prepared TiO2
NTs were soaked into the precursor solution for 24 h and then
was irradiated in this solution with a 300 W high pressure
mercury lamp for 30 min to reduce the absorbed Ag+ to Ag0, Au3+
to Au0 and Pt2+ to Pt0 by photocatalysis at the expense of water
oxidation.
Experimental
Chemicals and materials
Titanium foil (0.2 mm thick, 99.6%, Strem Chemicals) was cut
into pieces of 25 ꢀ 10 mm2. Silver nitrate(I) (AgNO3, Sigma-
Aldrich Chemicals, 99%) was used as the precursor of Ag nano-
particles. Gold(III) acid chloride trihydrate (HAuCl4$4H2O, Sigma-
Aldrich Chemicals, 99%) was used as the precursor of Au nano-
particles. Platinum(II) acetylacetonate (Pt(AcAc)2, Sigma-Aldrich
Chemicals, 99%) was used as the precursor of Pt nanoparticles.
Ethylene glycol (EG), ammonia uoride (NH4F) and methanol
were purchased from Acros Organics and used as received.
Characterization of NMNs/TiO2 NTs
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). The chemical compo-
sitions of NMNs/TiO2 NTs were analyzed by energy dispersive
spectrometer (EDS) equipped with FESEM. The crystal struc-
tures were characterized by grazing incidence X-ray diffraction
(GIXRD) analysis by an X-ray diffractometer (Rigaku D/MAX-
2200) with Cu Ka source in the range of 2q ¼ 20–70ꢁ. UV-
visible diffuse reectance spectra was obtained on a UV-
visible spectrophotometer (Lambda 750) equipped with an
integrating sphere assembly. Fine BaSO4 powders were used as
reectance standard.
The photoelectrochemical measurements were carried out in
a three-electrode cell in 0.5 M Na2SO4 as the aqueous electrolyte
with a electrochemical analyzer (WaveNow, PINE). TiO2 NTs and
NMNs/TiO2 NTs were used as working electrodes, Ag/AgCl was
used as the reference electrode and Pt foil was used as the
counter electrode. The photoresponse was evaluated under
chopped light irradiation (light on/off cycles: 60 s) at a xed
electrode potential of 0.5 V. The photocurrent was measured
under irradiation from a 300 W high pressure mercury lamp. All
measurements were carried out under ambient conditions at
room temperature.
Preparation of hierarchical TiO2 NTs
The hierarchical TiO2 NTs were fabricated by a two-step anodiza-
tion process (Scheme 2a–d). Prior to anodization, the Ti foils were
rst ultrasonically cleaned with ethanol and room-temperature
distilled water, followed by drying in N2 gas. The anodization
was carried out using a conventional two-electrode system with
the Ti foil as an anode and a Pt gauze (Aldrich, 100 mesh) as a
cathode respectively. All electrolytes consisted of 0.5 wt% NH4F in
EG solution with 2 vol% water. All the anodization experiments
were carried out at room temperature. In the rst-step anodiza-
tion, the Ti foil 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 foil alone. The patterned Ti foil 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.25 Aer second-step anodization, the
prepared TiO2 NTs sample was cleaned with distilled water and
dried off with N2 gas. The as-anodized TiO2 NTs were annealed in
air at 450 ꢁC for 1 h with a heating rate of 5 ꢁC minꢂ1
.
Gas phase PC degradation experiments
Decoration of NMNs on TiO2 NTs
The gas phase PC activities of TiO2 NTs and NMNs/TiO2 NTs
were detected by decomposition of evaluated methanol (5 mL).
The decomposition rate of methanol was calculated through
Fourier transform infrared (FTIR) spectroscopy (Bruker
Tensor27). All the PC measurements were performed in a
homemade cylindrical cell with 5 cm length and 3.6 cm width
(Fig. S4†), in which the TiO2 NTs and NMNs/TiO2 NTs samples
were contained respectively. PC degradation of methanol was
carried out with the irradiation of a 300 W high pressure
mercury lamp (XC140408, Shanghai YaMing Lighting Co., Ltd).
Photocatalytic reduction method26 was used to deposit NMNs
on the TiO2 NTs using AgNO3, HAuCl4, Pt(AcAc)2 as precursor
Results and discussion
Fig. 1a shows the SEM image of TiO2 NTs in the rst step
anodization, it can be seen from the gure that the structure of
TiO2 NTs is very loose. Fig. 1b shows the compact two-
Scheme 2 Two-step anodization processes for fabrication of hierar-
chical TiO2 NTs and photocatalytic reduction for decoration of NMNs. dimensional hexagonal pattern le on the Ti foil aer the
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Fig. 1 Morphology of TiO2 NTs: (a) TiO2 NTs in the one-step anodization; (b) the Ti surface after the removal of one-step TiO2 NTs layer; (c)
hierarchical TiO2 NTs in the second anodization step; hierarchical Ag/TiO2 NTs (d), Au/TiO2 NTs (e) and Pt/TiO2 NTs (f), the inset of (f) shows the
TEM image of Pt nanoparticles inside the TiO2 NTs.
one-step TiO2 NTs were removed by ultrasonic treatment in
The photoresponse of the TiO2 NTs electrode and NMNs/
deionized water. Fig. 1c shows a top view SEM image of hier- TiO2 NTs electrode were investigated under illumination of 300
archical TiO2 NTs in the second step. Individual top hexagonally W high pressure mercury lamp with a bias of 0.5 V (vs. Ag/AgCl).
porous structure is fairly observed, with an average diameter of It can been seen from Fig. 4, all modied TiO2 NTs samples
ꢃ150 nm and a wall-thickness of ꢃ20 nm (Fig. S5†). A cross- depicted better reproducibility and stability as the light was
sectional view of the TiO2 NTs is presented in the bottom-le turned on and off. The photocurrent decreased almost to zero
inset of Fig. 1c, indicates corresponding tubular structure instantly as soon as the illumination was interrupted and the
with a length of 1 mm. It can be seen from Fig. 1d and e that Ag photocurrent came back close to the original value as the illu-
and Au nanoparticles are decorated on the surface of TiO2 NTs. mination was turned on. It is interesting to note that the Pt/TiO2
Fig. 1f is a high-magnication top-view SEM image of Pt/TiO2 NTs composite electrode exhibited a photocurrent density of
NTs. The TEM image of Pt/TiO2 NTs is presented in the inset of ꢃ20 mA cmꢂ2, which was about 4 times larger than that of the
Fig. 1f, which clearly showed Pt nanoparticles are not only unmodied TiO2 NTs electrode with a photocurrent density of
decorated on the outer side of NT but also into the inter side of ꢃ5 mA cmꢂ2. The remarkably enhanced photocurrent may be
NTs with same diameter size of ꢃ20 nm as shown in SEM ascribed to the formed Schottky junctions between TiO2 NTs
image.
Optical absorption and crystalline are two important
parameters for PC performance on TiO2 materials. Materials
with better optical absorption and higher crystallinity will
subsequently result in a better PC performance. Therefore, UV-
vis absorption spectra and XRD measurement were employed to
characterize the optical absorption properties and crystallinity
of NMNs/TiO2 NTs (Fig. 2 and 3). As shown in Fig. 2, all modi-
ed TiO2 NTs samples depicted better UV absorption than
unmodied TiO2 NTs samples, as the Schottky junctions facil-
itate the electron transfer and thus reduce the recombination of
electrons and holes, both contributing to enhanced PC perfor-
mance. The XRD patterns of the hierarchical TiO2 NTs and
NMNs/TiO2 are shown in Fig. 3. Clearly, both of them presented
pure crystalline anatase with strong preferential orientation of
(101). In addition, the NMNs/TiO2 sample shows an extra noble
metal XRD pattern with strong orientation of (111), which
implies that the noble metal nanoparticles are successfully
doped on the TiO2 NTs. The EDS spectra in Fig. 3 showed
another evidence for noble metal existence on the TiO2 NTs.
Fig. 2 Diffuse reflectance UV-vis absorption spectra of the NMNs/
TiO2 NTs.
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Fig. 3 XRD patterns and EDS spectra of TiO2 NTs and NMNs/TiO2 NTs: (a and b) TiO2 NTs and Ag/TiO2 NTs; (c and d) TiO2 NTs and Au/TiO2 NTs;
(e and f) TiO2 NTs and Pt/TiO2 NTs.
and NMNs in which have a fast electron transport and a more
efficient separation of the photogenerated holes and electrons.
The photocatalytic activity of NMNs/TiO2 NTs are evaluated
by decomposition of methanol. Fig. 5 shows the FTIR spectra of
methanol decomposition as a function of irradiation time
on TiO2 NTs, Ag/TiO2 NTs, Au/TiO2 NTs and Pt/TiO2 NTs
respectively.
Before the illumination, the initial methanol processes the
bands at 1031, 1055, 1340, 2860, 2950, and 3685 cmꢂ1, which
bands correspond to the C–O stretching, CH3 rocking, O–H
bending, C–H parallel symmetric stretching, C–H out of plane
asymmetric stretching, O–H stretching, respectively.28–31 As the
increase of irradiation time, all these bands started to decrease
in intensity and new peaks appeared at 669, 1456, 1640, 1750,
2360 and 3440 cmꢂ1. The band at 1456 cmꢂ1 can be assigned to
the out of plane asymmetric bending of C–H, 1640 cmꢂ1 proved
the formation of isolated C]C bonds, and 1750 cmꢂ1 can be
assigned to the C]O stretching vibration modes.32,33 The
Fig. 4 Amperometric I–t curves at an applied potential of 0.5 V under
illumination of 300 W high pressure mercury lamp with 60 s light on/
off cycles.
gaseous CO2 is a linear molecule with two infrared active
absorption bands at 2360 cmꢂ1 (antisymmetric stretching
mode) and 669 cmꢂ1 (bending mode).34,35 Hence, one can
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Fig. 5 Decomposition of methanol on TiO2 NTs (a), Ag/TiO2 NTs (b), Au/TiO2 NTs (c) and Pt/TiO2 NTs (d) respectively as a function of irradiation
time.
Fig. 6 (a) Variation of the peak height of 2360 cmꢂ1 corresponding to the normal vibration of CO2 molecules derived from the FTIR trans-
mittance spectra with the irradiation time; (b) comparation performance of constant k on TiO2 NTs and NMNs/TiO2 NTs respectively.
conclude that the primary two peaks centered at 2360 cmꢂ1 and same time in the valence band a positively charged hole (h+) is
669 cmꢂ1 imply the formation of CO2 gas when methanol is produced, those electron–holes generated aer excitation move
decomposed. In the end, aer 60 min, just CO2 and H2O exis- quickly to surface from inside and the electrons will transfer to
tence as the results of completed decomposition of methanol NMNs through the Schottky interface of TiO2 and NMNs. Under
on the Pt/TiO2 NTs sample.
the condition of PC oxidation on methanol, the oxygen
Based on the photocatalytic experimental data, we summa- adsorbing on the surface of the catalyst was reduced to cO2ꢂ by
rized the CO2 transmittance peak height (2360 cmꢂ1, CO2) for
TiO2 NTs, Ag/TiO2 NTs, Au/TiO2 NTs and Pt/TiO2 NTs respec-
tively in Fig. 6a. The experimental data of Fig. 6a 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 height at innite time, and k is rate constant).36 The cor-
responding curves were presented in Fig. 6b, and the values of
rate constant kTiO
¼ 0.0142 < kAg/TiO
¼ 0.0731 < kAu/TiO
NTs
NTs
2
2
2
¼ 0.1031 < kPt/TiO
¼ 0.1558. Clearly, the Pt/TiO2 NTs
NTs
NTs
2
showed higher k value than hierarchical TiO2 NTs and other
noble metal modied TiO2 NTs, which implied fastest decom-
position rate and highest gas phase PC activity. The enhanced
PC activity can be ascribed the formation of Schottky-junction
between NMNs and TiO2 NTs.
On the basis of the experimental results, the mechanism of
gas phase PC methanol was also discussed here. As illuminated
with energy higher than the band gap of TiO2, electrons (eꢂ) on
valence band will be excited to the conduction band, at the Scheme 3 Photocatalytic mechanism of NMNs/TiO2 NTs.
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Conclusions
In summary, noble metal nanoparticles decorated hierarchical
TiO2 NTs were successfully designed with a photocatalytic
reduction method and tested the PC activity of degradation of
gaseous methanol. It can know that the Pt/TiO2 NTs afforded
the best degradation effect of methanol into CO2 and have the
biggest degradation rate in all three kinds of NMNs/TiO2 NTs. It
had been revealed that noble metal modied the inherent
properties of TiO2 NTs as well as affected the PC activity. The
NMNs/TiO2 NTs meaningfully proved the electron transfer and
reduced the recombination of photoelectrons and holes.
Compared with the pure TiO2 NTs, the Pt/TiO2 NTs showed an
outstanding enhancement on PC decomposition of methanol.
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