Spaced Titania Nanotube Arrays Allow the Construction of an Efficient N-Doped Hierarchical Structure for Visible-Light Harvesting

Article abstract of DOI:10.1002/open.201700199

Regularly spaced TiO₂ nanotubes were prepared by anodizing a titanium substrate in triethylene glycol electrolyte at elevated temperature. In comparison to conventional TiO₂ nanotubes, spaced nanotubes possess an adjustable spacing between the individual nanotubes; this allows for controlled buildup of a hierarchical nanoparticle-on-nanotube structure. Here, we use this principle for layer-by-layer decoration of the tubes with TiO₂ nanoparticles. The hierarchical structure after N doping and NH₃ treatment at 450 °C shows a significant enhancement of visible-light absorption, although it only carries a low doping concentration of nitrogen. For optimized N-doped and particle-decorated spaced TiO₂ nanotubes, a considerable improvement in photocatalytic activity is obtained in comparison with conventional N-doped TiO₂ nanotubes or comparable N-doped nanoparticle films. This is attributed to an enhanced visible-light absorption through the N-doped nanoparticle shell and a fast charge separation between the shell and the one-dimensional nanotubular core.

Full text of DOI:10.1002/open.201700199

DOI: 10.1002/open.201700199
Spaced Titania Nanotube Arrays Allow the Construction of
an Efficient N-Doped Hierarchical Structure for Visible-
Light Harvesting
Nhat Truong Nguyen,[a] Selda Ozkan,[a] Ondrej Tomanec,[b] Radek Zboril,[b] and Patrik Schmuki*[a, b, c]
Regularly spaced TiO2 nanotubes were prepared by anodizing
a titanium substrate in triethylene glycol electrolyte at elevated
temperature. In comparison to conventional TiO2 nanotubes,
spaced nanotubes possess an adjustable spacing between the
individual nanotubes; this allows for controlled buildup of a hi-
erarchical nanoparticle-on-nanotube structure. Here, we use
this principle for layer-by-layer decoration of the tubes with
TiO2 nanoparticles. The hierarchical structure after N doping
and NH3 treatment at 4508C shows a significant enhancement
of visible-light absorption, although it only carries a low
doping concentration of nitrogen. For optimized N-doped and
particle-decorated spaced TiO2 nanotubes, a considerable im-
provement in photocatalytic activity is obtained in comparison
with conventional N-doped TiO2 nanotubes or comparable N-
doped nanoparticle films. This is attributed to an enhanced
visible-light absorption through the N-doped nanoparticle
shell and a fast charge separation between the shell and the
one-dimensional nanotubular core.
only be activated under UV-light irradiation, which contributes
only around 5% to the total energy of solar spectrum.[13]
Therefore, introducing modifications that allow the use of TiO2
in both UV and visible light to enhance the photocatalytic effi-
ciency is of great significance. Many efforts have been dedicat-
ed to establishing the absorption of TiO2 for visible light, in-
cluding sensitizing TiO2 with dyes[5,14] and doping TiO2 with
metal (V, Fe, Mn) or non-metal (N, C, S, B) elements that form
p-states near the valence band of TiO2.[15–23] Alternatively,
cation doping results in localized d-states deep in the band
gap of TiO2, but also leads to a lower activity through the for-
mation of recombination centers.[16]
The most widely and successfully used approach for visible-
light absorption is to dope TiO2 with nitrogen, which results in
photocatalytic activity under visible-light irradiation.[18,24] Asahi
et al. proposed that the TiO2 band gap becomes narrower be-
cause the delocalized N2p state of the dopant mixes with the
O2p valence band of TiO2.[24] Others proposed that N doping
induces a higher energy valence band caused by the high den-
sity of localized N2p states in the band structure of TiO2.[25,26] In
either case, low-energy photons in the visible region can
excite electrons from these occupied high-energy states to the
conduction band of TiO2. For photocatalytic reactions, after ex-
citation, generated electrons and holes migrate to the surface
and are then transferred to redox couples in the electrolyte. In
spite of the advantages of N doping, the process induces re-
combination states for electron–hole pairs that can significant-
ly reduce carrier lifetime, that is, N-doped TiO2 typically pro-
vides only short carrier diffusion lengths.[27]
Ever since the groundbreaking work of Fujishima and Honda[1]
on the photoelectrochemical splitting of water on TiO2 electro-
des, TiO2 has become the most investigated semiconductor
material for photoelectrolysis of water to hydrogen and
oxygen and for the photodegradation of organic compounds
in a liquid or gas environment.[1–12] However, owing to its wide
band gap (ca. 3.2 eV for anatase), photocatalysis in TiO2 can
[a] Dr. N. T. Nguyen, Dr. S. Ozkan, Prof. P. Schmuki
Department of Materials Science and Engineering WW4-LKO
University of Erlangen-Nuremberg
To overcome this drawback, herein we introduce a hierarchi-
cal structure based on the formation of a spaced TiO2 nano-
tube (NT) core as a current conductor decorated with suitable
TiO2 nanoparticle (NP) layers. Spaced NT layers can be formed
with a large and adjustable spacing between them that allows
for a defined layer-by-layer coating of TiO2 NPs on the wall of
the NTs (Figure 1a). These decorated NTs then can be annealed
in NH3 atmosphere to induce N doping of the TiO2 NPs on the
TiO2 core, that is, a hierarchical structure that shows an en-
hanced photocatalytic performance. The key here is that
spaced NTs allow for a controlled NP decoration; thus, the
structure provides both an enhanced surface area and facilitat-
ed electron-transport properties when optimally designed.
Figures 1b and 1c show SEM images of spaced TiO2 NTs
after anodization in a hot triethylene-glycol-based electrolyte
containing NH4F and H2O (see the Experimental Section in the
Supporting Information). The individual NTs have a length of
3 mm and a diameter of 170 nm. It is apparent that each NT is
Martensstrasse 7, 91058 Erlangen (Germany)
E-mail: schmuki@ww.uni-erlangen.de
[b] O. Tomanec, Prof. R. Zboril, Prof. P. Schmuki
Regional Centre of Advanced Technologies and Materials
Department of Physical Chemistry, Faculty of Science, Palacky University
Slechtitelu 11, 783 71 Olomouc (Czech Republic)
[c] Prof. P. Schmuki
Chemistry Department, Faculty of Sciences
King Abdulaziz University
80203 Jeddah (Saudi Arabia)
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
open.201700199.
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the outer part is filled up after the eighth layer (Figure S1). In
these structures, the TiO2 NPs have an average diameter of ap-
proximately 5–10 nm, and the thickness of each particle layer
is approximately 17 nm. The decorated tubes were then an-
nealed in air at 4508C in order to fully convert the tubes and
NPs into anatase. High-resolution transmission electron micros-
copy (HR-TEM) analysis also confirms that, after decoration, the
TiO2 NPs consist of anatase with a (101) lattice spacing of
3.48 ꢁ (Figures 1 f and 1g).[33] The high-resolution annular dark-
field (HAADF) image and elemental mappings in Figures 1h–k
reveal that the NPs consist of TiO2. Furthermore, fluoride ions
(from the anodization process) and spurious carbon remain on
the NT walls. It should be noted that it is particularly difficult
to distinguish N(Ka1) and Ti(La1) because of overlapping
peaks.[34]
The decorated spaced NTs were then annealed in the NH3
environment at 4508C for 3 h to induce N doping of the TiO2.
In the XRD patterns, the intense peak at 2q=25.38 (Figure 2a)
Figure 2. a) XRD patterns of as-formed spaced TiO2 NTs, annealed spaced
TiO2 (4508C air 1 h), spaced TiO2 NTs decorated with eight layers of NPs and
N-doped spaced TiO2 NTs decorated with eight layers of TiO2 NPs (A=ana-
tase, T=titanium). b) XPS spectrum and c) N1s peaks of N-doped spaced
TiO2 NTs decorated with eight layers of NPs. d) XPS depth profile of N-doped
spaced TiO2 NTs decorated with eight layers of NPs.
Figure 1. a) Fabrication of spaced TiO2 NTs decorated with layer-by-layer TiO2
NPs. SEM images of: b,c) as-formed spaced TiO2 NTs; d,e) spaced TiO2 NTs
decorated with eight layers of NPs. f,g) TEM images of spaced TiO2 NTs dec-
orated with eight layers of NPs. h) HAADF and i–k) elemental mappings of
spaced TiO2 NT decorated with TiO2 NPs.
remains present, corresponding to the anatase phase after the
previous thermal treatment in air at 4508C. That is, after NH3
treatment, the crystallography of the samples remains un-
changed although the intensity of anatase peaks slightly de-
creases. Successful N doping is confirmed by the appearance
of N1s peak at 396.5 eV in the XPS spectra (Figures 2b and
2c).[24,26,35,36] Furthermore, the concentration of nitrogen is ap-
proximately 2.4 at% and it remains unchanged even at a
depth of 100 nm (Figure 2d). This indicates that N doping
occurs not only on the surface, but also within the TiO2
structure.
separated from the others, leaving a space of approximately
200 nm between individual tubes.[28–30] This distance is used to
decorate the individual NT walls with TiO2 NPs by using a TiCl4
hydrolysis approach.[14,31] In fact, the inter-tube spacing is suffi-
ciently large that the TiCl4 treatment can be repeated several
times to decorate the NT walls in a layer-by-layer approach,
thereby maximizing the specific surface area.[29,30,32] Figures 1d
and 1e show SEM images of spaced TiO2 NTs after eight layers
of decoration. Accordingly, the outer diameter of particle-deco-
rated NTs increases after each layer of TiO2 particles, whereas
the inner diameter decreases. The inner part of the spaced NTs
is completely filled with NPs after five layers of decoration and
The N-doped samples were then investigated with photo-
current spectra (Figures 3a and 3b). The N-doped samples
show significant enhancement of incident photon-to-current
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work by intensity-modulated photocurrent spectroscopy
(IMPS) and intensity-modulated voltage spectroscopy (IMVS).
Herein, IMPS and IMVS measurements were carried out under
visible light (530 nm) (Figures 3c and 3d). It is found that the
spaced NTs decorated with TiO2 NPs show a significantly faster
electron transport time in comparison with conventional NTs,
and the transport rate is a hundred times faster than NP layers
on FTO. Furthermore, Figure 3d shows that the electron life-
time constants for the spaced NTs with NPs are one and two
orders of magnitude longer than for classic NTs and NP layers,
respectively. This indicates that the TiCl4 treatment associated
with NP decoration can passivate the defects on TiO2 NTs,
thereby clearly improving the electronic properties.[40]
To evaluate the efficiency of the hierarchical structure not
only on the photoelectrochemical properties—as described
above—but also on photocatalytic effects, we investigated
classic target models for photocatalysis in gas form (acetalde-
hyde) and in liquid form (methylene blue). To test for an acti-
vated visible-light response in the gas phase, we examined the
above hierarchical structures for the photodegradation of acet-
aldehyde under solar simulator AM 1.5 conditions with a
420 nm cut-off filter. Figure 4a shows the amount of photoca-
talytic CO2 generated for spaced NTs, compared to NTs and
TiO2 particle layers, after an 8 h irradiation. The results clearly
indicate that, as expected, no visible-light photocatalytic activi-
ty was detected for non-doped samples. However, all the in-
vestigated N-doped materials show visible-light activity, and
clearly the N-doped hierarchical spaced NTs show by far the
largest amount of generated CO2 in comparison with N-doped
NTs or NP layers on FTO (8 mmolcmÀ2 compared to 2 and
Figure 3. IPCE spectra of a) spaced TiO2 NTs with/without eight layers of TiO2
NPs and N-doped spaced TiO2 NTs with/without eight layers of TiO2 NPs;
b) N-doped reference NTs, spaced NTs with eight layers of TiO2 NPs and NP
layers on FTO. c) Electron transfer time constants from IMPS measurements
and d) electron lifetime constants from IMVS measurements under visible-
light illumination (l=530 nm) for different N-doped samples.
efficiency (IPCE) in the visible-light range. The results in Fig-
ure 3a also reveal that NP decoration can enhance the IPCE (at
450 nm) almost fivefold, in comparison with plain spaced NTs
(1 and 5%, respectively). We found that the maximum IPCE for
spaced NTs is obtained after eight layers of decoration and 3 h
of NH3 treatment, whereas lower or higher loadings of TiO2
particles led to a decrease of photocurrent under visible-light
irradiation (Figures S2 and S3). Figure S4 shows the IPCE of dif-
ferent lengths of spaced TiO2 NTs decorated with eight layers
of NPs. We found that the longer the spaced NTs, the higher
the IPCE in the visible range. A maximized IPCE was obtained
for 10 mm long tubes (Figure S5). The results of photoelectro-
chemical measurements under visible light show that the N-
doped hierarchical structures can be used to more efficiently
harvest photocurrent than corresponding non-doped layer
(Figure S6).
Figure 3b shows the IPCE results of different samples that il-
lustrate the main difference of the hierarchical NTs, compared
with classic close-packed TiO2 NTs (denoted NTs-NH3, Figure S7)
and TiO2 NP layers on FTO substrate (prepared through the
doctor blading method, denoted NP layers-NH3). In general, NP
layers-NH3 deliver a much lower photocurrent compared to NT
samples, owing to the high recombination of electrons and
holes in the NP layers.[37] Furthermore, under visible light (at
450 nm), the IPCE value of spaced NTs is 5.19%—that is over
seven times higher than that measured for classic TiO2 NTs
(0.66%). This is ascribed to the much higher particle loading of
the spaced NTs and the higher surface area resulting from sev-
eral layers of TiO2 NP decoration.[29,30,38]
Additionally, TiO2 NP decoration obtained from TiCl4 treat-
ments on TiO2 NTs has been reported to be able to passivate
surface states of NTs, thereby strongly reducing the recombina-
tion centers.[39] This benefit is also examined in the present
Figure 4. a) Photocatalytic CO2 evolution during the photodegradation of
acetaldehyde and b) photodegradation of methylene blue under visible-
light irradiation (solar simulator AM 1.5, 100 mWcmÀ2, 420 nm cut-off filter)
for different samples.
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6 mmolcmÀ2, respectively). Furthermore, these N-doped hier-
archical spaced-NTs exhibit a linear CO2 evolution rate, indicat-
ing that the structures and doping are stable over time under
these irradiation conditions (Figure S8).
funCOS and the Operational Programme Research, Development
and Education - European Regional Development Fund, project
no. CZ.02.1.01/0.0/0.0/15 003/0000416 of the Ministry of Educa-
tion, Youth and Sports of the Czech Republic for financial
support.
The photocatalytic activity of the N-doped samples in liquid
solution were further studied by means of photodecomposi-
tion of methylene blue aqueous solution under visible-light ir-
radiation (Figure 4b). Also in these experiments, the photoca-
talytic activity of hierarchical spaced NTs shows the highest ef-
ficiency of all N-doped samples. In all of these photocatalytic
experiments, the better performance of the hierarchical spaced
NTs can be attributed to two factors: the combination of high-
NP loading providing a high surface area and the electronic
features of the core–shell structure.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: hierarchical structures
·
nitrogen doping
·
photocatalysis
absorption
·
spaced TiO2 nanotubes
·
visible-light
Regarding the electronic properties, the photoelectrochemi-
cal experiments clearly show an enhanced electron mobility
for the core–shell structure compared to the particle structure.
This leads to swift electron transport away from the buried
particle/core interface, that is, a fast carrier separation is likely
the origin for the enhanced photocatalytic activity. Based on
thermal diffusion gradients during N doping, it is plausible that
the highest doping and, thus, the highest density of N states
above the TiO2 valence band will be reached in the outermost
part of the structure; this provides an additional graded n–n
junction within the hierarchical structure that may additionally
contribute to carrier separation and the significantly enhanced
activity observed of the hierarchical tube–particle structure
compared with plain NP layers.
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Received: December 19, 2017
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COMMUNICATIONS
N. T. Nguyen, S. Ozkan, O. Tomanec,
R. Zboril, P. Schmuki*
Construction site: A hierarchical struc-
ture that is constructed through the
combination of TiO2 nanoparticle deco-
ration on spaced TiO2 nanotubes with a
low nitrogen-doping concentration
shows enhanced visible-light absorption
and photocatalytic activity.
&& – &&
Spaced Titania Nanotube Arrays Allow
the Construction of an Efficient N-
Doped Hierarchical Structure for
Visible-Light Harvesting
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