Long-lived photoinduced charge-carriers in Er3 + doped CaTiO3 for photocatalytic H2 production under UV irradiation

Article abstract of DOI:10.1016/j.catcom.2016.06.002

The incorporation of Er³⁺ ions in the CaTiO₃ structure provides an enhanced H₂ photoproduction due to the formation of long-lived charge-carriers in the semiconductor. Nitroblue tetrazolium reaction, photoluminescence and time resolved microwave conductivity techniques were used to investigate the generation of the superoxide species and the lifetime of the charge-carriers. The improved photoactivity has been explained in terms of the dopant agent, finding that phenomena such as a higher concentration of the superoxide radicals, a more efficient separation of the photogenerated charge-carriers, and a slower recombination process take place in the Er^{3 +} doped CaTiO₃ compared to the undoped CaTiO₃.

Full text of DOI:10.1016/j.catcom.2016.06.002

Catalysis Communications 84 (2016) 36–39
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Long-lived photoinduced charge-carriers in Er3+ doped CaTiO3 for
photocatalytic H2 production under UV irradiation
L.M. Lozano-Sánchez a, , M.G. Méndez-Medrano b,c, C. Colbeau-Justin c, J.L. Rodríguez-López b,
⁎
D.B. Hernández-Uresti d, S. Obregón d,
⁎
a
Tecnológico de Monterrey, Campus Monterrey, Eugenio Garza Sada 2501 Sur, Col. Tecnológico, C.P. 64849, Monterrey, Nuevo León, México
IPICYT, Instituto Potosino de Investigación Científica y Tecnológica. División de Materiales Avanzados, SLP, México
Laboratoire de Chimie Physique, CNRS, Univ Paris Sud, Université Paris-Saclay, 91405 Orsay Cedex, France
Universidad Autónoma de Nuevo León, Centro de Investigación en Ciencias Físico Matemáticas. Facultad de Ciencias Físico Matemáticas, Av. Universidad S/N, San Nicolás de los Garza, 66455,
b
c
d
Nuevo León, México
a r t i c l e i n f o
a b s t r a c t
The incorporation of Er3+ ions in the CaTiO3 structure provides an enhanced H2 photoproduction due to the for-
mation of long-lived charge-carriers in the semiconductor. Nitroblue tetrazolium reaction, photoluminescence
and time resolved microwave conductivity techniques were used to investigate the generation of the superoxide
species and the lifetime of the charge-carriers. The improved photoactivity has been explained in terms of the
dopant agent, finding that phenomena such as a higher concentration of the superoxide radicals, a more efficient
separation of the photogenerated charge-carriers, and a slower recombination process take place in the Er3+
doped CaTiO3 compared to the undoped CaTiO3.
Article history:
Received 7 March 2016
Received in revised form 18 May 2016
Accepted 1 June 2016
Available online 02 June 2016
Keywords:
Photocatalysis
Hydrogen
Erbium
© 2016 Elsevier B.V. All rights reserved.
CaTiO3
Charge-carriers
1. Introduction
preparation of Pt/CaTiO3 photocatalysts with a higher production rate
of hydrogen in a flowing mixture of water vapour and methane [9].
Global climate change has prompted the search for new technolo-
gies for the production of clean and renewable energy [1]. In this regard,
hydrogen can be considered as an interesting and attractive fuel due to
its zero carbon emission and renewable source [2]. Among the many re-
ported methods for hydrogen generation, the photocatalytic water
splitting process has been considered as one of the techniques with po-
tential large-scale production [3]. In this sense, metal oxides with crystal
structures such as pyrochlores (A2B2O7), spinels (AB2O4) and perov-
skites (ABO3) are interesting and promising materials for photocatalysts
[4–6].
CaTiO3 is one of the most important perovskite-related materials
and has received a lot of attention due its interesting catalytic, dielectric
and photoelectric properties [7]. This semiconductor has suitable con-
duction band (CB) and valence band (VB) positions, i.e., the CB potential
negative to the redox potential of H+/H2 and the VB potential positive to
the redox potential of O2/H2O. Due to its similar band structure to TiO2,
CaTiO3 and its doped derivatives have shown to exhibit photocatalytic
activity [8]. For instance, Shimura et al. have previously reported the
Other studies report its use as photocatalyst in the degradation of dyes
such as methyl orange, rhodamine B and methylene blue [10,11].
On the other hand, semiconductors usually exhibit low photocata-
lytic efficiencies probably because of their relatively low charge separa-
tion efficiencies and fast recombination of the photogenerated charge-
carriers [12]. Therefore, several strategies have been developed in
order to facilitate the separation of the charge-carriers. One of these
strategies is the addition of dopants to assist the trapping of specific
charge-carriers, thereby favoring their spatial separation. Among the
dopants we can find to the Er3+ ion, which has been reported in other
photocatalysts such as TiO2 and BiVO4 [13,14]. In this communication,
we describe the preparation of an Er3+ doped CaTiO3 photocatalyst
with enhanced H2 production under UV irradiation, where this im-
proved photoactivity can be result of the formation of long-lived
charge-carriers in the semiconductor.
2. Experimental
2.1. Preparation of the photocatalysts
⁎
Corresponding authors.
E-mail addresses: lmarcelo.lozano@gmail.com (L.M. Lozano-Sánchez),
First, TiO2 powder was placed in a Teflon vessel, while the stoichio-
metric amount of CaCl2 (Sigma Aldrich, ≥93%) was dissolved in 10 mL
sergio.obregonal@uanl.edu.mx (S. Obregón).
http://dx.doi.org/10.1016/j.catcom.2016.06.002
1566-7367/© 2016 Elsevier B.V. All rights reserved.

L.M. Lozano-Sánchez et al. / Catalysis Communications 84 (2016) 36–39
37
Fig. 1. (a–b) TEM and SEM images of undoped CaTiO3; (c) SEM image of Er3+-doped CaTiO3.
of distilled water. Separately, aqueous solutions of NaOH 5 M (80 mL)
and urea 2 M (10 mL) were prepared. Then, the three aqueous solutions
were poured in the Teflon vessel containing the TiO2. The suspension
was stirred and placed into a microwave reactor (Eyela MWO-1000
Wave Magic) and subsequently heated by microwave irradiation at
180 °C for 4 h, 400 rpm and a maximum variable microwave irradiation
power of 150 W. The mixture was cooled to room temperature, neutral-
ized with diluted HCl solution, filtered and washed with distilled water.
Finally, the solid was dried at 70 °C for 12 h and calcined at 850 °C for
1 h. The synthesis of the Er3+-CaTiO3 photocatalyst was performed in
the same way, where the ErCl3°6H2O precursor (Sigma Aldrich, 99.9%)
was dissolved in the NaOH aqueous solution. The nominal value of the
dopant ion was 1% at. This erbium amount was chosen because it is
the optimal value found in the photodegradation of methylene blue by
the Er3+-CaTiO3 system [8].
300 microscope operated at 300 kV. Erbium content was determined
by inductively coupled plasma-optical emission spectrometry (ICP-
OES) analysis. The Er3+ doped CaTiO3 sample was dissolved in acid di-
gestion (Hydrochloric acid, ACS reagent 37%) prior to the analysis
with
a Varian 700 ICP-OES Atomic emission spectrometer.
Photoluminescence spectra (PL) were recorded with a Hitachi FL-4500
spectrofluorometer. The charge-carrier lifetimes under UV excitation
of the photocatalysts were measured by Time Resolved Microwave Con-
ductivity method (TRMC). A pulsed laser source with an optical para-
metric oscillator (OPO) EKSPLA, NT342B, tunable in the range
between 220–2000 nm was used for TRMC measurements.
2.3. Photocatalysts tests
The samples were tested by means of the photocatalytic hydrogen
production using a homemade quartz inner-irradiation reactor connect-
ed to a closed glass gas system. In detail, 100 mg of the photocatalyst
was dispersed in 200 mL of an isopropyl alcohol-water solution
(1:10 v/v). A 0.5% wt. Pt as co-catalyst was photodeposited in situ on
the surface of the photocatalyst on the initial stage of the reaction
using H2PtCl6·6H2O (ACS reagent, ≥37.50% Pt basis) as precursor. Before
test, the reaction medium was evacuated in order to remove air
completely prior to irradiation. Then, the system was irradiated by
means of a Hg pen-lamp (254 nm, 100 W) encapsulated in a quartz
tube and placed inside the reactor. Hydrogen production was followed
by gas chromatography (Varian CP-3800) using a thermal conductivity
detector connected to a 5 A column molecular sieve. The temperature of
the reaction was maintained at 25 °C.
2.2. Materials characterization
X-ray diffraction (XRD) patterns were obtained using a D8 Bruker
Advance diffractometer with Cu Kα radiation. Diffuse reflectance spec-
troscopy was performed using a UV–vis spectrophotometer JASCO V-
570 equipped with an integrating sphere. BET surface area was mea-
sured with a Bel-Japan Minisorp II Surface Area & Pore Size analyzer.
The nitrogen adsorption-desorption isotherms were evaluated at
−196 °C after a pretreatment at 150 °C for 2 h. The morphology of the
samples was analyzed by field emission scanning electron microscopy
using a FEI XL30 microscope equipped with a field emission gun. Also,
transmission electron microscopy was performed through a Tecnai FEI
Fig. 2. (a) Photocatalytic H2 production and (b) production of superoxide ions (O–2) by undoped and Er3+ doped CaTiO3 photocatalysts.

38
L.M. Lozano-Sánchez et al. / Catalysis Communications 84 (2016) 36–39
the localized gap states induced by Ti3+ and/or oxygen vacancies in
the semiconductor [17]. Regarding the surface area, both samples
show low values (≤1 m2/g). The erbium content in the doped sample
has been supported by chemical analysis using the ICP-OES technique,
where the relative amount of erbium with respect to the titanium is in
accordance with the nominal value. Also, the photocatalysts showed a
well-defined prism-like morphology, similar to the previously reported
by our research group, see Fig. 1 [18]. The presence of urea and NaOH in
the preparation procedure provides the formation of CaTiO3 with ex-
posed (111) and (202) planes growing in the [121] direction.
Photocatalytic H2 production for undoped and doped CaTiO3 under
UV light irradiation is illustrated in Fig. 2a. The obtained results showed
that the Er3+ doped sample has a reaction rate of 5.72 mmol h−1 g−1
while pristine CaTiO3 exhibits a reaction rate of 4.53 mmol h−1 g−1
.
These values reveal that the addition of Er3+ ions provides an increase
in the photocatalytic behavior of CaTiO3. In order to evaluate the trans-
fer of the photogenerated electrons from the conduction band of the
photocatalyst to the reaction medium, the photocatalytic production
of superoxide ions (O–2) has been examined under the same UV irradia-
tion conditions, see Fig. 2b. As shown, Er3+ doped CaTiO3 exhibits a high
concentration of superoxide radicals at low irradiation time and a quite
stable concentration along higher reaction time. Furthermore, undoped
CaTiO3 shows a lower rate under same conditions, which can be consid-
ered as clear evidence of the higher photocatalytic behavior of the Er3+
doped material.
Fig. 3. Photoluminiscence emission spectra (λexc = 376 nm) for undoped and Er3+ doped
CaTiO3 photocatalysts.
The photocatalytic production of the superoxide ion (O–2) also was
evaluated under the same UV irradiation conditions as the hydrogen
production. The nitroblue tetrazolium (NBT) can be specifically reduced
by the superoxide ion (O–2) in order to form the insoluble purple
formazan (FZN), see Fig S1 [15]. The photocatalytic tests were made
with an initial concentration of NBT 5 × 10−5 mol L−1 and a 0.5 g L−1
concentration of the photocatalyst. The production of the superoxide
ion was quantitatively analyzed through the evolution of the absorption
maximum at 259 nm of the nitroblue tetrazolium.
Recently, an Er3+ doped TiO2 system has been reported with en-
hanced photocatalytic properties in the liquid-phase degradation of
phenol, methylene blue and gas-phase photo-oxidation of toluene
[13]. As well as our system, the improved photoactivity has been ex-
plained in terms of the dopant agent, where Er3+ ions could also be re-
duced to Er2+ by trapping an electron under UV excitation, and thus the
Er2+ species would react with the O2 providing a spatial separation of
the electron–hole pair (see Fig. S4). In this sense, Betenelli et al. argued
that a small fraction of the trapping-detrapping process would be re-
sponsible for the lowering of the diffusion coefficient of the
photogenerated electrons [19]. In order to confirm the above discussion,
the photoluminescence spectra were carried out at room temperature.
Photoluminescence analysis can be considered as a direct approach to
understand the separation efficiency of the photogenerated charge car-
riers [20,21]. In this regard, the higher PL intensity represents a higher
extent of an irradiative process associated with the electron–hole pair
recombination [22]. Fig. 3 shows the PL spectra of the samples at an ex-
citation wavelength near the absorption edge of the semiconductor
(λexc = 376 nm). Both samples exhibit two broad emission bands be-
tween 550–640 nm and 650–690 nm. As expected for Er3+ doped
CaTiO3, the PL intensity decreases respect to the undoped sample. In ad-
dition, the doped sample shows well-defined peaks between 520–
560 nm, which would correspond to the transitions of the 2H11/2 and
4S3/2 excited states to the 4I15/2 ground state of the lanthanide ion [8].
The understanding of the charge carrier dynamics in the early stages
(femtosecond to picosecond time scales) is a crucial step to design
photocatalysts with enhanced photocatalytic features. Thus, the
charge-carrier lifetimes in our photocatalysts were determined by
means of the Time Resolved Microwave Conductivity technique
(TRMC) [23]. It is a contactless method based on the measurement of
the change of the microwave power reflected by a sample induced by
pulsed laser illumination. The relative change of the reflected micro-
wave power is proportional to the conductivity of the sample. The
TRMC signal allows following directly the decay of the number of elec-
trons and of the holes after the laser pulse by recombination or trapping
of the charge-carriers. Excess energy facilitates the migration of the
charge-carriers to the surface providing a better response signal. There-
fore, we deliberately chose to excite the samples at 320 nm (above
bandgap excitation). Taking into account that trapped species have a
small mobility that can be neglected, the TRMC signal gives a direct
measurement of the lifetime of the free charge-carriers created in the
3. Results and discussion
Undoped and erbium doped CaTiO3 photocatalysts were identified
as orthorhombic CaTiO3 phase according the PDF 42–0423, see Fig. S2.
From Rietveld analysis, a slight increase of the cell volume was observed
for the erbium doped sample. This fact could indicate that the Er3+ ad-
dition can be carried out interstitially in the CaTiO3 crystalline structure,
as it was discussed in detail in a previous work [8]. As shown in Fig. S3,
the calculated band gap value for pristine CaTiO3 appears to be similar to
those reported previously, being at about 3.56 eV [16]. On the other
hand, the Er3+ doped photocatalyst exhibited a slightly lower value
around 3.46 eV. The decrease of the optical band gap would be due to
Fig. 4. TRMC signal for undoped and Er3+ doped CaTiO3 photocatalysts after excitation at
320 nm.

L.M. Lozano-Sánchez et al. / Catalysis Communications 84 (2016) 36–39
39
conduction band of the semiconductor [24]. From the corresponding
Appendix A. Supplementary data
TRMC data, it is noted that the undoped CaTiO3 shows a higher maxi-
mum intensity Imax, than the erbium doped sample, see Fig. 4 (inset
graph). This phenomenon may indicate that the presence of Er3+ ions
provides a fast capturing of the photogenerated electrons [25]. Further-
more, normalized data is displayed in the above Figure. The obtained
data show that both samples have different decay profiles. The Er3+
doped sample displays a slower signal decay compared to the undoped
sample. In this case, it would correspond to slower recombination phe-
nomena, it means to longer charge-carrier lifetimes, reinforcing the as-
sumption that the incorporation of erbium acts as an electron
scavenging. In this manner, in the doped phase the charge-carriers
have more time to migrate to the surface, increasing the photocatalytic
performance [24].
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2016.06.002.
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