Porous NiTiO3/TiO2 nanostructures for photocatatalytic hydrogen evolution

Article abstract of DOI:10.1039/c9ta04763h

We present a strategy to produce porous NiTiO₃/TiO₂ nanostructures with excellent photocatalytic activity toward hydrogen generation. In a first step, nickel-doped TiO₂ needle bundles were synthesized by a hydrothermal procedure. Through the sintering in air of these nanostructures, porous NiTiO₃/TiO₂ heterostructured rods were obtained. Alternatively, the annealing in argon of the nickel-doped TiO₂ needle bundles resulted in NiO_x/TiO₂ elongated nanostructures. Porous NiTiO₃/TiO₂ structures were tested for hydrogen evolution in the presence of ethanol. Such porous heterostructures exhibited superior photocatalytic activity toward hydrogen generation, with hydrogen production rates up to 11.5 mmol h^-1 g^-1 at room temperature. This excellent performance is related here to the optoelectronic properties and geometric parameters of the material.

Full text of DOI:10.1039/c9ta04763h

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Materials for energy and sustainability
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Zhang, J. Liu, T. Zhang, P. Tang, J. Arbiol, L. Soler, K. Sivula, N. Guijarro, X. Wang, J. Li, R. Du, Y. Zuo, A.
Cabot and J. Llorca, J. Mater. Chem. A, 2019, DOI: 10.1039/C9TA04763H.
Volume
Number 12
March 2018
Pages 4883-5230
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DOI: 10.1039/C9TA04763H
Journal Name
ARTICLE
Porous NiTiO /TiO Nanostructures for Photocatatalytic Hydrogen
3
2
Evolution
Received 00th January 20xx,
Accepted 00th January 20xx
ab
c
a
a
d
d
de
a
Congcong Xing, Yongpeng Liu, Yu Zhang, Junfeng Liu, Ting Zhang, Pengyi Tang, Jordi Arbiol,
b
c
c
a
a
a
Lluís Soler, Kevin Sivula, Néstor Guijarro, Xiang Wang, Junshan Li, Ruifeng Du, Yong Zuo,
DOI: 10.1039/x0xx00000x
ae
b
Andreu Cabot,* Jordi Llorca *
www.rsc.org/
We present a strategy to produce porous NiTiO
hydrogen generation. In a first step, nickel-doped TiO
Through the sintering in air of these nanostructures, porous NiTiO
Alternatively, the annealing in argon of the nickel-doped TiO needle bundles resulted in NiO
nanostructures. Porous NiTiO /TiO structures were tested for hydrogen evolution in the presence of ethanol. Such porous
heterostructures exhibited superior photocatalytic activity toward hydrogen generation, with hydrogen production rates
3
/TiO
needle bundles were synthesized by a hydrothermal procedure.
/TiO heterostructured rods were obtained.
/TiO elongated
2
nanostructures with excellent photocatalytic activity toward
2
3
2
2
x
2
3
2
-1
-1
up to 11.5 mmol·h ·g at room temperature. This excellent performance is related here with the optoelectronic properties
and geometric parameters of the material.
separation, hole-transportation and visible-light-driven
2
0-23
Introduction
Since first reports on photocatalytic water splitting, TiO has
2
been considered one of the most attractive photocatalysts.
Among others, its interest resides on its excellent
optoelectronic properties, high resistance to photocorrosion,
photocatalytic performance over TiO2.
To cite some examples in this direction, Wu et al.
demonstrated an enhancement of the charge separation and
/SrTiO nano-heterostructures to be
2 3
at the origin of an improved photoelectrochemical
1
-3
hole transportation in TiO
2
0
performance . When combined with TiO
2
, several other
4
-6
natural abundance, low-cost and safety.
However, TiO
2
oxides and chalcogenides have demonstrated to be able to
optimization for solar photocatalysis requires maximizing its
2
3-26
enhance its photocatalytic activities.
materials have been proven especially effective to improve
TiO photocatalytic activity. Rawool et al. demonstrated pn
heterojunctions in NiO/TiO produced by sol gel to promote
photocatalytic H
generation.27 Wei et al. showed Ni(HCO
In particular, Ni-based
7
surface area. It also requires the introduction of dopants,
additional phases or structural defects that allow this wide
band gap semiconductor to take advantage of a larger spectral
range of the solar radiation.
To maximize surface area, the production of hollow and
2
8
-13
2
2
3 2
)
produced through a hydrothermal method to be an excellent
co-catalyst to boost photocatalytic hydrogen evolution of
Enhanced photocatalytic H
was also reported with the introduction of Ni(OH)
porous TiO
2
4-16
nanostructures has recently drawn growing
Besides, Ti-based mixed oxides and particularly
attention.1
2
8
mesoporous TiO
2
.
2
production
clusters to
perovskite titanates MTiO
3
(M=Ba, Sr, Ca and Ni) have been
1
7-
2
pointed out as a new exciting class of photoactive materials.
2
9
commercially available TiO
2
and in Ni/NiO/N-TiO2-x
plasma treatment of
sol-gel derived Ti-Ni precursors. Besides, Qu et al. prepared
porous NiTiO nanorods with enhanced photocatalytic
performance toward the degradation of nitrobenzene.
1
9
Additionally heterostructured oxides, such as SrTiO
3 2
/TiO
heterostructures produced from a NH
3
and NiTiO /TiO have been demonstrated to promote charge-
3
2
3
0
3
2
1
a. Catalonia Institute for Energy Research (IREC), Sant Adrià de Besòs, 08930
Barcelona, Spain
While excellent results have been obtained to date, the
synthesis of such mixed oxides and oxide heterostructures in
b.
Institute of Energy Technologies, Department of Chemical Engineering and
Barcelona Research Center in Multiscale Science and Engineering, Universitat the form of high surface area materials remains as an
Politècnica de Catalunya, EEBE, 08019 Barcelona, Spain
Laboratory for Molecular Engineering of Optoelectronic Nanomaterials (LIMNO),
École Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne,
Switzerland
Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST,
Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain
ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Spain
Electronic Supplementary Information (ESI) available: additional TEM, HRTEM,
EELS analysis and other characterizations, photocatalytic hydrogen production
measurement. See DOI: 10.1039/x0xx00000x
extremely challenging endeavor, what has limited further
advances in this direction.
To overcome this limitation, we present here a simple, high
throughput and scalable hydrothermal-based strategy to
c.
3
1-33
d.
e.
produce porous NiTiO
based on the synthesis of needle bundles of Ni-doped TiO
TiO :Ni) and their posterior sintering in air to produce highly
3 2
/TiO heterostructures. This strategy is
†
2
(
2
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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
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crystalline NiTiO /TiO heterostructures with a significant level spectroscopy (EELS) detectors. Elemental analysisViewwaAsrticclearOrnileinde
3 2
of porosity reminiscent of the large density of interfaces within out using an Oxford energy dispersiveDOXI:-1r0a.y103s9p/eCc9tTrAo0m47e6t3eHr
the needle bundles. We further demonstrate here that these (EDX) combined with the Zeiss Auriga SEM working at 20.0 kV.
preliminarily optimized materials are excellent photocatalysts X-ray photoelectron spectroscopy (XPS) was examined on a
for hydrogen generation. We choose hydrogen generation as SPECS system equipped with a Phoibos 150 MCD-9 detector,
test reaction owing to the excellent performances of Ni-based working at 150 mW with an Al anode XR50 source. Fourier
2
7-30,34-36
photocatalysts in this direction.
transform infrared spectroscopy (FTIR, Alpha Bruker) was
carried with a platinum attenuated total reflectance single
reflection module. Nitrogen sorption measurements were
collected on a MicroActive 3.00 at 77.3 K.
Experimental
Chemicals
Photocatalytic Hydrogen Evolution Tests
Titanium(IV) isopropoxide (97%, Sigma-Aldrich), nickel(II)
nitrate hexahydrate (98%, Fluka), ethanol (96%, PanReac
AppliChem), hexadecylamine (HDA, 90%, Sigma-Aldrich),
potassium chloride (Sigma-Aldrich), and ammonium hydroxide
solution (28-30%, Sigma-Aldrich) were used without further
purification.
In a typical experiment, a cellulose paper impregnated with 2.0
mg of the photocatalyst was placed inside a photocatalytic
reactor that was equipped with a UV LED (Figure S1). An Ar gas
stream was saturated with a water:ethanol vapour mixture
(
90:10 ratio on a molar basis) by bubbling dry Ar gas at a flow
-1
rate of 20 mL·min through a saturator (Dreschel bottle)
containing a liquid mixture of 87.5 g of H O and 9.92 g of
This Ar stream was introduced into the
3 2
Synthesis of NiTiO /TiO Heterostructures
2
3
7,38
ethanol.
A solution of KCl (1.9 mg) in miliQ water (0.99 g) was added
dropwise to a HDA (0.45 g) solution in ethanol (20 mL) under
stirring at room temperature. To this solution, a proper
photoreactor and passed through the cellulose paper loaded
with the photocatalyst. The photoreactor effluent was
monitored on-line every 4 min using gas chromatography (GC)
3 2 2
amount of Ni(NO ) ·6H O was added (0 mg, 22.5 mg, 45 mg, 90
(Agilent 3000A MicroGC) using three columns: MS 5 Å, Plot U
mg, and 225 mg to reach 0, 0.5, 1, 2, and 5 mol%
concentrations, respectively). Then, 4.7 mL of titanium(IV)
isopropoxide was added. After stirring for 1 h at room
temperature, we kept the solution 16 h without stirring, and
afterward the solution was washed with ethanol three times
using centrifugation to recover the product. This product was
then dissolved in a solution containing 20 mL of MiliQ water,
and Stabilwax. The LED UV light source (from SACOPA, S.A.U.)
consisted of four LEDs at 365 ± 5 nm and a synthetic quartz
glass cylindrical lens that transmitted the light to the
photocatalyst. Light irradiation was measured directly with a
UV-A radiation monitor from Solar Light Co. and was 79.1± 0.5
−2
mW·cm at the sample position. At the beginning of each
experiment, the UV light was off, and the reaction system was
4
0 mL of ethanol and 2.2 mL of ammonium hydroxide. This
-1
purged by entering 20 mL·min of saturated Ar gas with the
water-ethanol vapour mixture, to remove oxygen in the line.
After 30 min, the UV light was turned on and we monitored all
photoreaction products during ca. 20-40 min by GC. Control
experiments were carried out with only the cellulose paper
support and no photoactivity was measured.
solution was stirred for 1 h and it was subsequently
transferred to a 100mL Teflon-lined hydrothermal reactor. The
reactor was heated at 170 °C and maintained at this
temperature for 17 h. Afterward, it was cooled down to room
temperature and the product was washed three times with
ethanol. Dried materials were finally annealed in a tube
furnace under synthetic air flow at 650 °C for 8 h.
Electrochemical Impedance Spectroscopy (EIS)
EIS analyses were performed using a Bio-Logic SP-300
potentiostat in a 3-electrode configuration employing a
Cappuccino-type PEC cell, wherein the photocatalyst was
Characterization
o
o
X-ray diffraction analyses (XRD, 2θ: 20 -80 ; scanning rate: deposited as a thin film in a conductive substrate (FTO) to be
/min) were carried out on a Bruker AXS D8 Advance X-ray used as working electrode, whereas a Ag/AgCl electrode and a
o
5
diffractometer with Ni-filtered Cu-Kα radiation (λ= 0.15406 Å), Pt wired were used as a reference and a counter electrode,
operating at 40 mA and 40 kV. The morphology and size of the respectively. The applied potential on the working electrode
particles were characterized by transmission electron was sinusoidally modulated with an amplitude of 25 mV using
microscopy (TEM, ZEISS LIBRA 120), working at 120 kV and frequencies ranging from 100 kHz to 50 mHz. The analysis of
field-emission scanning electron microscopy (SEM, Zeiss the impedance response was undertaken using ZView
Auriga) operating at 5.0 kV. High resolution transmission (Scribner Associates).
electron microscopy (HRTEM) images and scanning The apparent quantum yield (AQY) was estimated using the
transmission electron microscopy (STEM) studies were following equation:
2
푛퐻2
푛푁퐴
conducted on a FEI Tecnai F20 field emission gun microscope
operated at 200 kV with a point-to-point resolution of 0.19
nm, which was equipped with high angle annular dark field
퐴푄Y =
·100 =
·100
푛
퐸
푇
퐸
푝
푝
where 풏 ퟐ
is the number of molecules of H generated and np
푯
2
is the number of incident photons reaching the catalyst. The
(HAADF) and a Gatan Quantum electron energy loss
2
| J. Name., 2012, 00, 1-3
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ARTICLE
푬푻
number of incident photons can be calculated by 풏
where E
energy of a photon. 퐄
=
푬풑
,
EDX analysis showed the Ni atomic concentration to be
풑
View Article Online
DOI: 10.1039/C9TA04763H
T
is the total energy reaching the catalyst and Ep is the systematically higher than the nominal amount introduced,
-2
= 퐏퐒퐭, where P (W·m ) is the power pointing at a higher yield of reaction of the nickel(II) nitrate
density of the incident monochromatic light, S (m ) is the than the titanium(IV) isopropoxide under the hydrothermal
irradiation area and t (s) is the duration of the incident light conditions used for the synthesis (Table S1).
퐓
2
exposure. 퐄
=
퐡퐜
, where h is the Planck’s constant, c the
2
TiO :Ni nanopowders were annealed under synthetic air
퐩
speed of light and  (m) is the wavelength of the incident flow at 650 °C during 8 h. Upon this thermal treatment, the
monochromatic light. The number of hydrogen molecules can color of the material changed from the original blueish,
be calculated as 퐧퐇ퟐ = 퐧퐍
, where n are H
2
moles evolved characterizing TiO :Ni samples, to yellow (Figure S4). TEM
퐀
2
during the time of light exposure (t), and N
A
is the Avogadro micrographs of the annealed powder showed it to still consist
constant.3
9-41
In our experimental conditions, the wavelength on elongated nanostructures. However, on the contrary to the
of the incident light was  = 365 nm, the power density of the starting nanoneedle bundles, the annealed nanostructures
-2
incident light at the paper surface was P = 79.1mW·cm and displayed a crystallographic continuity (Figure 2c) and
2
the irradiation area was S = 2.27 cm .
presented numerous holes all over them (Figure 2d). In
contrast, undoped TiO nanopowders annealed in the same
2
conditions did not present such porous structure (Figure S5).
XRD analysis showed that the porous nanostructures
Results and discussion
obtained after the annealing of TiO
on anatase TiO (Figure 2a). With the annealing process, the
shift of the XRD peaks observed in the precursor TiO :Ni
material disappeared, pointing at the outward diffusion of the
Ni ions from the TiO lattice. Besides the anatase phase, XRD
patterns displayed the presence of a second crystal structure,
2
:Ni in air consisted mainly
Characterization of the Photocatalysts
2
2 2
Ni-doped TiO (TiO :Ni) nanostructures were produced from
2
the hydrothermal decomposition of titanium(IV) isopropoxide
in the presence of proper amounts of nickel(II) nitrate
hexahydrate, HDA as a shape-directing agent and KCl to
control the ionic strength of the solution (see experimental
section for details and Figure S2 for SEM images). As shown
by TEM characterization, the size and shape of the
nanostructures obtained from this hydrothermal reaction
strongly depended on the amount of Ni introduced (Figure S3).
In the absence of Ni, nanoparticles with an average size
around 50 nm and mostly irregular shapes, although some
elongated, were obtained. When introducing increasingly
higher amounts of Ni, more and more elongated
nanostructures were obtained. In the presence of a nominal
2
4
3
which was identified as NiTiO
peaks associated to this NiTiO
the nominal amount of Ni (Figure 2a). Rietveld analysis showed
the relative amount of the NiTiO phase to correspond to a 4.6
3
.
The relative intensity of the
4
2
3
phase clearly increased with
3
mol%, 9.0 mol% and 15 mol%, for samples obtained from
nominal Ni concentration of 1 mol%, 2 mol% and 5 mol%,
4
4
respectively (Figure S6 and table S1). Within its experimental
error, EDX analysis showed the Ni molar concentrations to be
maintained after the annealing process (Table S1).
HRTEM analysis confirmed the annealed material to
5
% of Ni, most structures resembled nanorods with a length in
the range from 100 nm to 200 nm and thickness of 30-60 nm
Figures 1b, 2b and S3). A closer look to these nanorods
4
5
2
contain mostly anatase TiO (Figures S7). EELS elemental
maps of the annealed materials showed Ni, Ti and O to be
evenly distributed throughout the nanostructures (Figure 2e).
(
allowed discerning that they consisted of needle-shaped
nanostructures assembled in bundles (Figure 2b).
XRD analyses showed the crystal structure of these needle
2
bundles to match that of anatase TiO . Diffraction peaks
shifted to lower angles with the introduction of increasing
amounts of Ni (Figure 1a), which pointed at the presence of Ni
2 2
ions within the TiO anatase lattice. Besides anatase TiO , no
additional crystallographic phase could be discerned from XRD
patterns.
The lack of HRTEM evidences of the presence of large NiTiO
3
crystal domains and the homogeneous distribution of Ni
throughout the material observed by EELS and EDX analyses
3
pointed at the growth of small NiTiO domains as a
discontinuous shell on the surface of the porous TiO
2
4
6
nanostructures.
XPS analyses of the annealed samples displayed only one
4
+
chemical state for Ti, assigned to Ti , and one for Ni, assigned
2
+
to Ni , consistently with XRD, HRTEM, EELS and EDX
4
7
characterization (Figures S8 and S9). XPS analyses also
showed the amount of Ni to be almost a twofold of that
measured by EDX, pointing at its preferential surface location
a
TiO2:Ni
b
5
%
2
%
(Table S1).
1
%
When Ni-doped materials were annealed under Ar instead
of air, the sample color changed from blue to black, instead of
yellow (Figure S4), but the elongated geometry of the initial
TiO2
TiO2 JCPDS:01-073-1764
structures was conserved. In contrast to the annealing in air,
the nanostructuration of the original nanoneedle bundles
2
00 nm
2
0
30 40 50 60 70 80 24.8
25.6
2 (Degree)
2
(Degree)
partially persisted after annealing in argon. The particles
Figure 1. a) XRD patterns of TiO
nanopowders. b) TEM image of TiO
2
and TiO
2
:Ni (1%, 2%, 5%) annealed in argon did not show full crystallographic continuity
:Ni (5%) nanopowder.
and contained a lower density of holes compared with the
2
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2
-1
material obtained from the annealing in air (Figure S10). XRD and 28.0 m ·g , respectively, supporting the higher porosity of
View Article Online
patterns of materials annealed under argon showed anatase the former (Figure S12). With the annDeOalIi:n10g.1p03ro9/cCe9sTsA0in476a3irH,
TiO
NiTiO
associated to a NiO
annealed under argon showed certain magnetization, which
2
as the main phase, with no evidence of the presence of pure TiO
, but with a small XRD peak at 2θ=28.6° that we those obtained from TiO
phase (Figure S11). The black nanopowder proper initial nanostructuration.
2
nanopowders did not result in porous materials as
3
2
:Ni because they did not present a
x
Photogeneration of Hydrogen
further pointed toward the formation of a NiO
S4).
x
phase (Figures
4
8
To test their photocatalytic properties toward hydrogen
evolution, the annealed materials were supported on a
conventional filter paper and placed on a reactor containing a
UV LED light source. An Ar flow saturated with a water-ethanol
vapor mixture was flowed through this reactor. Exhaust gases
were analyzed using GC, being acetaldehyde and hydrogen the
main reaction products (Figure S1).
a
NiTiO3/TiO2
NiTiO3
*NiTiO3
5%
*
*
*
*
2%
1%
TiO2
2 JCPDS:01-073-1764
Figure 3a shows the accumulated hydrogen production
TiO
during
nanostructures having different nominal Ni concentrations.
Data for bare TiO and pure NiTiO (Figures S13 and S14)
a
32 min reaction for NiTiO
3
/TiO
2
hetero-
2
0
30
40
50
60
70
80
24.8
25.6
32
34
36
2
 (Degree)
2 (Degree)
2 (Degree)
2
3
b
c
prepared following the same procedure (see experimental
section for details) were also plotted in Figure 3a as reference.
6
50oC
Air
NiTiO
3 2
/TiO hetero-nanostructures with a nominal 1% of Ni
systematically provided the highest hydrogen production
c
-1 -1
rates, with values up to 11.5 mmol·h g and an AQY of 11.6%
at 365 nm, well above those previously reported for other Ni-
Ti-O systems (Table S2). The hydrogen production rate of
1
00 nm
100 nm
NiTiO
bare TiO
b, S15). The presence of larger amounts of Ni did not improve
the evolution rate with respect to NiTiO /TiO (1%). We
associate this experimental evidence with the blocking of the
TiO surface active sites when too large amounts of NiTiO
were grown on the TiO surface. SEM characterization showed
the morphology of the NiTiO /TiO hetero-nanostructures to
3
/TiO
2
(1%) was almost a fourfold higher than that of
e
d
2
and 60 times larger than that of pure NiTiO
3
(Figure
3
3
2
0 nm200 nm
5
Ti
2
3
O
2
5
0 nm
0 nm
3
2
remain unmodified after the catalytic test (Figure 3c).
Ni
5
5
a
0.5%, 1%, 2%, 5%
b12
10
8
1%
2
00 nm
4
3
2
1
0
NiTiO3/TiO2
50 nm
Ti-O-Ni
Figure 2. a) XRD patterns of NiTiO
nanopowders obtained by annealing in air. b) TEM micrograph
of TiO :Ni (5%) needle bundles. c, d) TEM micrographs of
NiTiO /TiO (5%) porous structures obtained after annealing in
air. e) HAADF micrograph of a NiTiO /TiO (5%) rod and
3
/TiO
2
(0, 1%, 2%, 5%)
NiOx/TiO2 0.5%
TiO2
6
TiO :Ni
2
4
5%
2
2%
2
0
3
2
NiTiO3
3
2
0
10
20
30
40
Reaction Time (min)
corresponding EELS elemental maps for Ti, O, Ni and Ti-O-Ni.
NiTiO /TiO2
3
c
We hypothesize that the porous geometry of the
3 2 2
NiTiO /TiO structures obtained after annealing TiO :Ni in air
was created by the sintering of the nanoneedle bundles. The
large density of interfaces of the initial bundles translated in
the formation of a large density of holes on the sintered
material, which could be aided by the simultaneous outward
UV light
400 nm
3
diffusion of Ni ions to form NiTiO domains. On the contrary,
annealing in argon did not allow a similar level of sintering of
the nanostructures, which was promoted in air by the
ubiquitous presence of oxygen. Thus, the material obtained
from the annealing process in argon maintained smaller crystal
domains and presented a less porous structure. The BET
Figure 3. a) H
2
production of TiO
2
, NiTiO
production rate of TiO
(1%) and NiTiO . c) Optical
(5%) before and after 30 min
3 2
/TiO (1%, 2%, 5%),
NiO
x
/TiO
2
(1%) and NiTiO
3
. b) H
2
2
,
NiTiO
3
/TiO
2
(1%, 2%, 5%), NiO
x
/TiO
2
3
and SEM images of NiTiO
photocatalytic test.
3
/TiO
2
3 2 x 2
specific surface areas of NiTO /TiO and NiO /TiO were 40.8
4
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We believe the enhancement of the photocatalytic where 푞 is elementary charge, 퐴 is taken as the geometric area
View Article Online
2 3
properties of TiO with the NiTiO
introduction to be related (assuming a low surface roughness spin-coated thin film), 휀 is
DOI: 10.1039/C9TA04763H
5
1
with an improvement of three fundamental parameters: i) relative permittivity (taken to be 55 and 15 for TiO
2
and
is donor
estimate that the introduction of Ni had associated a factor 3 density, 푉 is applied potential, 푉푓푏 is flat band potential, 푘 is
5
2
surface area; ii) light absorption; and iii) charge separation. We NiTiO
3
,
respectively), 휀 is vacuum permittivity, 푁
0 퐷
increase of the surface area with respect to pure TiO
the formation of the porous NiTiO /TiO
2
through Boltzmann constant, and 푇 is absolute temperature. The
hetero- positive slope in the linear region (dashed line in Figure 4b)
3
2
nanostructures during the annealing process. Additionally, the unambiguously confirmed the n-type character of both NiTiO
presence of NiTiO provided a higher light absorption and TiO
, whereas 푉푓푏 values of –0.186 V and –0.276 V vs. RHE
coefficient at the used excitation wavelength. In case of using were obtained for TiO and NiTiO
, respectively. Likewise, 푁
3
3
2
2
3
퐷
1
8
–3
17
–3
solar radiation, the presence of NiTiO
larger improvement with respect to pure TiO
allow absorbing a larger portion of the solar spectra due to its al. on nanocrystalline anatase TiO
smaller band gap (Figure S16). Besides, the proper band workers on nanostructured NiTiO
alignment of TiO and NiTiO allowed an efficient spatial depletion width is estimated to be 20 nm in the linear region
charge separation that significantly reduced recombination of of M–S plot for both TiO and NiTiO so that M–S analysis can
the photogenerated charge carriers before reaction. be reasonably applied in the case of not fully depleted
To gain additional insight into the band structure alignment feature. Considering the density-of-state effective mass for
3
would provide an even shows values of 1.1×10 cm for TiO
2
and 5.5×10 cm for
2
, as it would NiTiO . Similar values have also been reported by Thimsen et
3
5
1
2
,
and by Trari and co-
.53 We note that the
3
2
3
2
3
5
4
–30
and the interfacial charge carrier dynamics at the NiTiO
heterostructure, EIS analyses were carried out on TiO
NiTiO /TiO (1%) and NiTiO
thin films in dark conditions. effective density of states in the conduction band 푁
Representative impedance responses in the form of Nyquist
plot for TiO and NiTiO /TiO (1%) are displayed in Figure 4a.
Responses typically feature a single semicircle. Interestingly,
the size of the semicircle obtained for the TiO sample was
3
/TiO
2
electrons 푚푑푒 for nanocrystalline anatase TiO
2
is 9.1×10
is 1.9×10–30 kg, the
5
1
55
2
,
kg, and for nanostructured NiTiO
3
5
4,56
3
2
3
퐶
is:
3
2휋푚 푘푇
2
푑푒
푁 ≡ 2
퐶
2
3
2
(
ℎ2
)
is estimated to be 7.7×10 cm–
20
where ℎ is Planck constant. 푁
퐶
2
3
19
–3
for TiO
between 푁
NiTiO could be treated as nondegenerate semiconductors,
2
, and 7.5×10 cm for NiTiO
3
. Since the ratio
reduced with the incorporation of Ni. A more quantitative
description of the EIS data can be provided by modelling the
electrical response with an equivalent circuit, such as a
Randles circuit (Figure 4a inset), which has been previously
and 푁
is in all cases less than 0.05, both TiO
and
퐷
퐶
2
3
5
4,56
and therefore, the Boltzmann statistics can be applied:
reported to successfully describe nanostructured TiO
2
and
푁퐶
퐸퐶퐵 ― 퐸 = 푘푇ln
2
2,50,54
퐹
other photocatalysts.
This circuit consists of a resistor 푅
푆
(
푁퐷
)
in series with a bulk capacitance 퐶푏푢푙푘, attributed to the space
charge region, and a second resistor 푅푐푡,푏푢푙푘 that represents
the charge transport resistance, both in parallel. At a first
glance, the decrease in the size of the semicircle with the
where 퐸퐶퐵 is the bottom of the conduction band (CB) and 퐸
is
퐹
the Fermi level position. 퐸퐶퐵 is found to be around 168 mV and
26 mV above 푉푓푏 for TiO
and NiTiO , respectively.
Based on the aforementioned results, Figure 4c displays a
scheme of the electronic band structures. The relative band
alignment between NiTiO and TiO allows the formation of a
direct Z-scheme heterojunction. In such Ni-TiO system, while
photogenerated electrons in the CB of TiO recombine with
photogenerated holes in the valance band (VB) of NiTiO
photogenerated electrons with strong reduction ability remain
in the CB of NiTiO ready to reduce water into hydrogen and
photogenerated holes with strong oxidation abilities in the VB
of TiO could drive ethanol dehydrogenation to acetaldehyde
and hydrogen. Therefore, the formation of a direct Z-scheme
NiTiO /TiO heterojunction can promote photogenerated
carriers with strong redox abilities to drive photocatalytic
reaction. As a result, NiTiO
3
loading condition shows a significantly increased
photocatalytic activity. Similar Z-scheme photocatalytic
systems have also been reported such as TiO
and CdS/cobalt–benzimidazole.
1
2
3
2
incorporation of Ni in TiO suggests the reduction in the charge
4
9,50
transport resistance,
which is further confirmed with the
decrease of 푅푐푡,푏푢푙푘 from 15.4 k to 9.8 k pointing out the
3
2
2
amelioration of the charge transport process. In addition, the
can be calculated as the inverse of the
angular frequency at the maximum of the Nyquist plot (
2
electron lifetime 휏
푛
3
,
2
2
2
휋푓푚푎푥), highlighted in Figure 4a. Results reveal that 휏푛
increases from 4.5 ms to 6.6 ms when the heterostructure is
formed. In general, 휏
is considered a metric of electron
recombination, i.e., the longer the value the lesser the charge
recombination. The lengthening of 휏
when TiO is interfaced
with NiTiO underpins the effective charge carrier separation
across the n-n heterojunction and thus the enhanced
photocatalytic activity of the composite with respect TiO
Similar behavior was reported by Wei and co-workers on
3
푛
2
푛
2
3
2
3
57
3
/TiO
2
composites at optimal
2
.
NiTiO
2
2
3
NiTiO /TiO
2
composite for inorganic sensitized solar cells.
58
59
2
/Rh, TiO
2
/CdS,
In an attempt to further interrogate the energy band
60
positions, Mott–Schottky (M–S) plots were constructed from
퐶푏푢푙푘 values extracted from EIS measurements on TiO
and
2
NiTiO and further analyzed using the M–S equation:
3
2
푘푇
푞
―
2
퐶
=
푉 ― 푉푓푏
―
푏푢푙푘
(
)
2
푞퐴 휀휀 푁
0
퐷
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Page 6 of 8
ARTICLE
Journal Name
b
3.5
3.0
c
1
4
2
0
8
6
4
2
0
a
View Article Online
DOI: 10.1039/C9TA04763H
1
1
TiO
2
35.5 Hz
2.5
2.0
1.5
1.0
0.5
0.0
TiO2
2
4 Hz
NiTiO
3
NiTiO3/TiO2 (1%)
10 12 14
0
2
4
6
8
-0.3 -0.2 -0.1 0.0 0.1 0.2
2
Z' (k cm )
V vs. RHE (V)
Figure 4. a) Representative impedance response of TiO
2
(black sphere) and NiTiO
3
/TiO
2
(1%) (orange sphere) with the
corresponding fit (solid lines) in Nyquist plot. The maximum point of the Nyquist plot is highlighted (hollow), and the equivalent
circuit used is included as an inset. b) Mott–Schottky plot of TiO (black sphere) and NiTiO (red sphere) including a linear fit
dashed lines). c) Schematics of the electronic band structures of a Z-scheme NiTiO /TiO heterojunction.
2
3
(
3
2
Conflicts of interest
Conclusions
Porous NiTiO
annealing in air of TiO
3
/TiO
2
heterostructures were prepared from the
:Ni nanoneedle bundles obtained from a There are no conflicts of interest to declare.
2
hydrothermal route. Pores were generated during the
sintering of the nanostructured bundles due to the presence of
a large density of interfaces on the precursor material.
Acknowledgements
NiTiO
3 2
/TiO heterostructures were used for photocatalytic
This work was supported by the European Regional
Development Funds and by the Spanish Ministerio de
hydrogen generation from ethanol-water solutions at room
temperature.
unprecedented H
NiTiO
3
/TiO
2
hetero-structures
provided
Economía
y
Competitividad through project SEHTOP
-
1 -1
2
production rates up to 11.5 mmol·h g and
(ENE2016-77798-C4-3-R and ENE2015-63969-R). Y.L. and N.G.
an AQY of 11.6%. This excellent performance was associated to
the improvement of three parameters, surface area, light
absorption and charge separation. We estimated a factor 3
increase of the surface area with the introduction of Ni
thank the Swiss National Science Foundation (SNF) for funding
under the Ambizione Energy grant no. PZENP2_166871. J.L. is a
Serra Hunter fellow and is grateful to ICREA Academia program
and GC 2017 SGR 128. T.Z., P.Y.T. and J.A. acknowledge
funding from Generalitat de Catalunya 2017 SGR 327 and the
through the formation of porous NiTiO
nanostructures during the annealing in air. Additionally, the
presence of NiTiO , with a lower band gap energy, provided a
higher light absorption coefficient at the excitation
wavelength. Besides, TiO and NiTiO presented a proper band
3
/TiO
2
hetero-
Spanish
acknowledges support from the Severo Ochoa Programme
MINECO, Grant no. SEV-2013-0295) and is funded by the
MINECO
project
ENE2017-85087-C3.
ICN2
3
(
2
3
CERCA Programme/ Generalitat de Catalunya. Part of the
present work has been performed in the framework of
Universitat Autònoma de Barcelona Materials Science PhD
program. C.C.X., Y.Z and T.Z. thank the China Scholarship
Council for scholarship support.
alignment that allows an efficient spatial charge separation of
the photogenerated charge carriers before reaction, as
confirmed by EIS analysis.
Author Contributions
Notes and references
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Journal Name
TOC graphical abstract
View Article Online
DOI: 10.1039/C9TA04763H
Porous NiTiO /TiO heterostructures prepared from the annealing in air of TiO :Ni nanoneedle bundles
3
2
2
displayed excellent photocatalytic activity toward hydrogen generation.
8
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