2012-Photocatalytic activity of erbium-doped TiO2 nanoparticles immobilized in

Prévia do material em texto

Materials Research Bulletin 47 (2012) 290–295
Photocatalytic activity of erbium-doped TiO2 nanoparticles immobilized in
macro-porous silica films
J. Castañeda-Contreras a,*, V.F. Marañón-Ruiz a, R. Chiu-Zárate a, H. Pérez-Ladrón de Guevara a,
R. Rodriguez a, C. Michel-Uribe b
a C.U. de los Lagos, Universidad de Guadalajara, Lagos de Moreno, Jalisco, Mexico
b C. U. de Ciencias Exactas e Ingenierı́a, Universidad de Guadalajara, Guadalajara, Jalisco, Mexico
A R T I C L E I N F O
Article history:
Received 29 March 2011
Received in revised form 20 September 2011
Accepted 9 November 2011
Available online 19 November 2011
Keywords:
A. Thin films
B. Sol–gel chemistry
C. Atomic force microscopy
C. X-ray diffraction
D. Catalytic properties
A B S T R A C T
A macro-porous silica film served as mechanical support to immobilize TiO2 nanoparticles, which were
doped with erbium. The films and the nanoparticles were prepared by sol–gel route. The nanoparticles
exhibited photocatalytic activity under visible light. We obtained a degradation rate of methylene blue
that followed first order kinetics. The sensitization of the nanoparticles to visible light was attributed to a
red shift in the band-gap of the TiO2 due to the addition of erbium ions.
� 2011 Elsevier Ltd. All rights reserved.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u
1. Introduction
TiO2 has large potential as photo-catalyst due to its high
efficiency, chemical inertness, and photo-stability [1–5]. Two
crystalline phases of TiO2 have photocatalytic activity: anatase
and rutile. Anatase is the most active phase, but it requires UV
irradiation for photocatalytic activation. Currently, the photocata-
lytic action of TiO2 under visible light attracts much interest to
harness sunlight [6–12]. The addition of Er3+ produces a red shift of
the optical absorption edge of TiO2 as well as transitions of Er3+ intra-
f electrons, which lead to sensitization of TiO2 to visible light [13–
17]. Furthermore, the inclusion of Ti4+ in the lattice of Er2O3 leads to
higher adsorption and prevention of electron–hole recombination in
TiO2 [9]. On the other hand, it is preferable to have a large area of TiO2
since catalytic reactions usually occur on the surface of the material.
Therefore, TiO2 nanoparticles are an effective option to fulfill this
purpose. However, the utilization of nanoparticles as catalyst often
presents disadvantages such as stirring to avoid precipitation and
the difficulties to separate them from suspension. To overcome these
inconveniences, immobilization techniques exist such as chemical
vapor deposition [18], sputtering [19], immobilization with poly-
mers [20], sol–gel [21,22], and synthesis inside porous silica [23]. In
the present work, we choose macro-porous thin films as mechanical
* Corresponding author. Tel.: +52 014747423678; fax: +52 014747423678.
E-mail address: jcc050769@yahoo.com.mx (J. Castañeda-Contreras).
0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.11.021
support to immobilize the nanoparticles, because of the ability of
porous solids to interact with atoms, ions, and molecules in
applications that involve ion exchange, adsorption, and catalysis
[24]. A common approach to synthesize porous materials is self
assembly process, where the atoms, molecules, particles, and other
building blocks organize themselves into functional structures as
driven by the energetic of the system [5]. The self-assembly readily
occurs by the templated-surfactant micellar effect, which can be
accomplished in sol–gel materials. However, the surfactant must be
removed by calcinations at high temperatures [6–10]. Our study
attempts to simplify the latter process by use of phase separation.
This mechanism could be induced by strong interactions between
charged species in solution [25]. The phase separation allows the
formation of solvent templates, which could be easily removed by
evaporation at room temperature. We also describe the synthesis of
TiO2 nanoparticles doped with erbium (TiO2:Er3+) and their
immobilization in macro-porous silica films with ultrasonic
irradiation. To the best of our knowledge, this may be the first
report of TiO2:Er3+ immobilization inside macro-porous silica films
by the ultrasonic capillary effect.
2. Experimental
2.1. Synthesis of TiO2 nanoparticles doped with Er3+
All chemicals were purchased from Sigma–Aldrich and utilized
without further purification. The precursor for TiO2:Er3+ was
http://dx.doi.org/10.1016/j.materresbull.2011.11.021
mailto:jcc050769@yahoo.com.mx
http://www.sciencedirect.com/science/journal/00255408
http://dx.doi.org/10.1016/j.materresbull.2011.11.021
Fig. 1. XRD pattern of TiO2:Er3+ nanoparticles heated at 500 8C. The pattern is
related to the TiO2 crystalline phase of anatase.
J. Castañeda-Contreras et al. / Materials Research Bulletin 47 (2012) 290–295 291
titanium tetra-isopropoxide (TIPO) dissolved in a solution of
erbium nitrate (Er(NO3)3�6H2O), ethanol (ETOH), and distilled
water at room temperature. The TIPO molar content was 0.08, the
H2O/TIPO molar ratio was 120, and Er(NO3)3 molar content was
1 � 10�3. This mixture was stirred for 30 min. The reaction was
quenched by cooling the solution at 2–3 8C in an ice bath for 24 h.
Subsequently, the nano-particles were recovered by centrifugation
at 5000 rpm. The obtained material was washed with water twice
and then heated in an oven at 500 8C for 2 h.
2.2. Synthesis of macro-porous silica films
Macro-porous silica films were obtained from a solution of
tetraethyl-orthosilicate (TEOS, 98%) and rhodamine 6G (Rh6G).
The reaction began by dissolving TEOS and Rh6G in ETOH. This
mixture was kept under vigorous magnetic stirring during 30 min.
An aqueous solution of hydrochloric acid (HCl) was added to
hydrolyze the TEOS. The resulting solution is known as sol, with
molar ratios TEOS 1:H2O 4:HCl 0.05:ETOH 4:Rh6G S1:S2, where
S1 = 0.1 � 10�5 and S2 = 0.1 � 10�3. A pure-silica sol was prepared
without Rh6G, it was labeled as S3. Subsequently, 60 ml of the
corresponding sol were deposited on a Corning microscope slide
(plain) as substrate, which previously was cleaned with acetone
and isopropyl-alcohol. The substrate was fixed in a homemade
spin-coater. The spinning speed was set at 1200 rpm. The obtained
films underwent heating at 110 8C for 10 min on a hot plate, and
subsequently were illuminated with an ordinary spiral fluorescent
lamp (General Electric, 20 W) in order to photo-degrade the Rh6G
in the films.
2.3. Immobilization of TiO2:Er3+ on the films surface
The samples were sonicated during 10 min in an ultrasonic bath
(Branson 2000) filled with an aqueous solution of TiO2:Er3+ at
0.1 M. The ultrasonic energy promotes the infiltration of nano-
particles in the films by the ultrasonic capillary effect [26].
Subsequently, the samples were heated at 110 8C for 10 min on a
hot plate to remove the water.
2.4. Characterization of the films and the nanoparticles
The surface of the films was studied by atomic force microscopy
(AFM), with a microscope easyScan 2 from Nanosurf. We utilized a
standard silicon tip in AFM measurements, with imaging mode via
static force (contact). The tip voltage was set at �10 V in 5 mV
steps. On the other hand, the structure and size of the TiO2:Er3+ were
estimated by the patterns of X-ray diffraction (XRD), obtained with a
diffractometer Rigaku Miniflex coupled with a Cu source at 40 kV
(l = 1.54056 Å). The morphology of the particles was studied with a
Transmission Electron Microscope (TEM) JEOL JEM-2200FS, which
had an acceleration voltage of 200 kV.
2.5. Photocatalytic activity testing
The photocatalytic activity of TiO2:Er3+ was evaluated by the
degradation of methylene blue (MB). A sandwich-type cell was
formed with a film with immobilized TiO2:Er3+ and a microscope
slide as cover. It was filled with an aqueous solutionof MB at
0.01 M. The cell was illuminated with the same lamp utilized to
photo-degrade the Rh6G. A long-pass filter G62-981 from Edmund
Optics was utilized to block any UV from the lamp. The MB
degradation was studied by absorption spectra. They were
obtained by an experimental setup with a deuterium-tungsten
lamp DH-2000 from Ocean Optics as illumination source. The lamp
was coupled with fiber to a collimating lens holder 74-ACH from
Micropack. The samples were fixed to the aforementioned holder
and then illuminated with the lamp. The transmitted light was
collected from the samples with a collimating lens and coupled
with fiber to an Ocean-Optics QE65000 spectrometer.
3. Results and discussion
3.1. Catalyst characterization
Fig. 1 shows the XRD pattern of the TiO2:Er3+, which is indexed
as anatase (JCPDF file 75-1537). Neither other polymorphs of TiO2
nor erbium-oxide peaks were identified in the pattern. The
broadening of diffraction peaks indicates small size of the
nanocrystals. The average particle size (d) was calculated with
the Scherrer’s formula:
d ¼ 0:9l
b cos u
;
where l is the X-ray excitation wavelength (1.54056 Å), u is the
Bragg angle, and b is the experimental full width at half maximum of
the strongest reflection peak for anatase (1 0 1). Results indicated an
average particle size of 9 nm. TEM micrograph of Fig. 2 shows the
morphology of TiO2:Er3+. The size of the nanoparticles was in the
range of 6–14 nm, as calculated by the Feret’s diameter from the
TEM micrographs. Fig. 3 shows the absorption spectra for pure-TiO2
and TiO2:Er3+ nanoparticles. The spectrum of TiO2:Er3+ had a red
shift in the band gap transition, due to a charge transfer transition
between TiO2 and Er3+ intra-4f electrons [27]. The aforementioned
spectrum also shows an absorption band with peaks centered at
490 nm and 532 nm; they are related to Er3+ excited states 4F7/2 and
4S3/2, respectively. On the other hand, the absorption band from 4F9/2
state was negligible. The band gap energy (Eg) of the nanoparticles
was estimated by:
Eg ¼
1240
lg
;
where lg is the wavelength value corresponding to the intersec-
tion point of the horizontal and vertical parts of the absorption
spectra [28]. The calculated Eg for TiO2:Er3+ was 3.0 eV, which was
smaller than for pure-TiO2 (3.2 eV). Results indicate the sensitiza-
tion of TiO2:Er3+ to radiation wavelengths up to 415 nm.
Fig. 3. Absorption spectra of TiO2 nanoparticles and erbium-doped TiO2
nanoparticles.
Fig. 2. TEM micrograph of TiO2:Er3+ nanoparticles heated at 500 8C.
J. Castañeda-Contreras et al. / Materials Research Bulletin 47 (2012) 290–295292
3.2. Macro-porous silica films and immobilization of TiO2:Er3+
Self-assembly is a process to produce porous materials. This is
feasible due to interactions between charged species in solution
[29]. In the present work, a positively charged dye (Rh6G) was
added to the sol–gel reaction in order to promote the self-
assembly. Silica films were prepared with different molar ratios of
Rh6G, i.e., with S1 = 0.1 � 10�5 and S2 = 0.1 � 10�3, whereas pure-
silica films were obtained from S3 sol, as described in Section 2.2.
The AFM images of Fig. 4 show macro-pores on the films from S1
and S2. No macro-pores were observed in films from S3.
Furthermore, the rise of Rh6G content augmented the number
of macro-pores up to 700% in the films from S2 in comparison to S1.
The macro-pores had an average diameter of 800 nm. The action of
Rh6G as a template is an improbable explanation for the pore
formation, because the diameter of pores is away from these
reported from molecular templates, which are in the 2–50 nm
Fig. 4. AFM images of the surface of sol–gel silica films with Rh6G
range [30]. In addition, Rh6G was not removed from the films, i.e.
the dye was just photo-bleached. A probable mechanism for the
pore formation is phase separation. This process repels ETOH and
produces droplets of solvent at micron size due to the interactions
between the charged dye and the sol–gel species in solution,
similar of that reported by Fuentes et al. [25]. The droplets behaved
as pore templates [31], which were evaporated by the spin coating.
The random location of the pores in our films is characteristic of the
self-assembly due to phase separation [25,32,33]. However, the
effects of synthesis that determine pore characteristics such as the
diameter, shape, and depth are still unknown to us. Research is
currently in progress.
The films were sonicated in an aqueous solution of TiO2:Er3+ for
10 min to immobilize the nanoparticles, further sonication time
resulted in erosion and damage of the film. The effect of the
ultrasonic irradiation depends on acoustic cavitation: the forma-
tion, growth, and implosive collapse of bubbles that normally exist
in a liquid. The bubbles collapse produces high speed jets of liquid
against the film surface [34], leading to immobilization of the
 at increased molar ratios: (a) 0.1 � 10�5 and (b) 0.1 � 10�3.
Fig. 6. Absorption spectra of MB in a microscope slides cell irradiated with visible
light.
J. Castañeda-Contreras et al. / Materials Research Bulletin 47 (2012) 290–295 293
nanoparticles inside the macro-pores. The nanoparticle loading in
the films was estimated at 10 wt.%. It was calculated by the Beer’s
law with the absorption spectra of the samples and from the
spectra of solutions of TiO2:Er3+ at different contents.
3.3. Photocatalytic study
The photocatalytic activity of the TiO2:Er3+ was evaluated by
the absorption spectra of MB after visible light irradiation. Fig. 5(a)
points out that the MB was completely degraded after 60 min of
irradiation. Since no additional peaks appeared, MB is just
degraded and not photo-bleached. On the other hand, the plot
of log(C0/C) versus irradiation time is shown in Fig. 5(b), where C0 is
the MB initial concentration and C is the MB concentration at any
time. The linear relationship between the MB content and the
irradiation time indicates a first order kinetic in the MB
degradation. This behavior is similar to that reported in Ref.
[16] for TiO2 doped with rare earths. Nevertheless, it is well known
that the MB discoloration is also possible just with light irradiation.
In order to study this effect, a cell of plain microscope slides was
filled with the MB solution and was illuminated with the lamp.
Fig. 6 shows few degradation of MB after 180 min of irradiation.
Results indicate that the MB discoloration shown in Fig. 5 is due to
the photocatalytic action of TiO2:Er3+.
A study of photo-catalysis with pure-TiO2 nanoparticles
pointed out poor degradation of MB, as shown in the plot of C/
C0 in Fig. 7. Nevertheless, the same figure exhibits an improvement
in the photocatalytic properties of TiO2 with the addition of Er3+.
This behavior might be attributable to the red shift of the optical
absorption edge of TiO2 (Fig. 3), as well as to transitions of Er3+
intra-4f electrons [35]. The latter depends on absorption of energy,
and since the lamp had a strong emission peak (532 nm) that
matched with an absorption band of Er3+ (for 4S3/2 excited state),
Fig. 5. (a) UV–vis absorption spectra of MB on a silica film with immobilized
TiO2:Er3+ under visible light irradiation and (b) kinetic plot of the MB degradation.
then transitions of intra-4f electrons were expected to occur. This is
discussed in the next section.
3.4. Er3+ population and depopulation processes
Fig. 8 illustrates the excitation of Er3+ under visible light, as
reported by Wang et al. [17], where the absorption of 532 nm
photons promotes Er3+ from ground state to 4S3/2 excited state. The
lifetime of 4S3/2 was not measured in our samples, but on the basis
of known results for similar materials, we expected a value of
�1 mS. This lifetime could allow the population of upper state 4D7/2
by excited state absorption (ESA), where an excited erbium ion at
4S3/2 absorbs a 532 nm photon [36]. A no-radiative decay from 4D7/
2 populates next lower state 2P3/2. From there, radiative transition2P3/2! 4I15/2 produces UV photons, as shown in Fig. 8. These UV
photons could excite the immobilized TiO2, resulting in the
formation of electron–hole pairs in the TiO2 surface [17]. However,
we failed to detect the UV emission when TiO2:Er3+ were irradiated
with the fluorescent lamp. To clarify this point, the 4S3/2 excited
state was pumped directly in our samples by a Nd:YAG laser, which
Fig. 7. Comparison of the discoloration of MB under visible illumination for
immobilized TiO2:Er3+ and pure TiO2 nanoparticles.
Fig. 10. Proposed energy transfer mechanism from erbium 2P3/2 excited state to
TiO2 for excitation at 532 nm.
Fig. 8. Simplified energy-levels diagram of Er3+ and excitation path for the up-
conversion emission at 310 nm.
J. Castañeda-Contreras et al. / Materials Research Bulletin 47 (2012) 290–295294
delivered 100 mW of continuous light at 532 nm. No UV
luminescence was detected from the TiO2:Er3+ under such
pumping scheme, probably due to fast depopulation of the
intermediate excited states by multi-phonon relaxation. Never-
theless, the MB was degraded in a photocatalysis study with
TiO2:Er3+ and the 532 nm laser as illumination source, as shown in
the plot of C/C0 of Fig. 9. On the other hand, the degradation of MB
was absent in a similar experiment with pure TiO2. Results discard
the laser irradiation as only agent for the MB discoloration. The
photocatalytic activity of the TiO2:Er3+ and the lack of UV emission
suggest the excitation of TiO2 by an alternative depopulation
process of Er3+. A proposed mechanism is by an energy transfer
process from excited erbium ions at 2P3/2 to TiO2, as shown in
Fig. 10. This is feasible by non-radiative recombination, similar to
the back-transfer process for the excitation of silicon from erbium
[37]. However, the low absorption cross-section of Er3+ and the
relatively small lifetimes of the involved excited states are
potential drawbacks to the aforementioned process. This explains
the slower degradation rate of MB with monochromatic light at
532 nm (Fig. 9), in comparison to the results with the lamp (Fig. 5).
Fig. 9. Plot of the MB degradation on a macro-porous silica film with immobilized
TiO2:Er3+. The illumination source was a Nd:YAG laser at 532 nm.
Therefore, the primary mechanism for the visible light sensitiza-
tion was the red shift of the optical absorption edge of TiO2. This
affirmation is in agreement with that of Parida and Sahu [16]. The
visible-light sensitization promoted an electron from the TiO2
valence band to the conduction band. The generated charge
carriers produced oxidative species, such as O2� and hydroxyl
radicals that degraded MB in aqueous solution [38].
4. Conclusions
Macro-porous silica films were obtained from a phase separa-
tion process induced by the addition of Rh6G at early stages of sol–
gel route. The films served as mechanical support to immobilize
erbium-doped TiO2 nanoparticles. The samples exhibited photo-
catalytic activity under visible light, which was evaluated by the
degradation of MB in aqueous solution. The visible light
sensitization was attributed to a red shift in the TiO2 band-gap,
which was due to the addition of erbium to the catalyst and at less
extent to an energy transfer process from erbium ions to TiO2.
References
[1] M. Alvaro, C. Aprile, M. Benitez, E. Carbonell, H. Garcıa, J. Phys. Chem. B 110 (2006)
6661–6665.
[2] J. Zhang, F. Pan, W. Hao, Q. Ge, T. Wang, Appl. Phys. Lett. 85 (2004) 5778–5780.
[3] H. Tang, K. Parasad, R. Sanjines, P.E. Schmaidt, F. Levy, J. Appl. Phys. 75 (1994)
2042–2047.
[4] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–
96.
[5] K.P. Kühn, I.F. Chaberny, K. Massholder, M. Sticker, V.W. Benz, H.G. Sonntag, L.
Erdinger, Chemosphere 53 (2003) 71–78.
[6] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271.
[7] C.Y. Lee, J.T. Hupp, Langmuir 26 (5) (2010) 3760–3765.
[8] C. Burda, Y. Lou, X. Chen, A.S. Samia, J. Stout, J.L. Gole, Nano Lett. 3 (2003) 1049–
1051.
[9] A.W. Xu, Y. Gao, H.Q. Liu, J. Catal. 207 (2002) 151–157.
[10] D. Zhao, T. Peng, J. Xiao, C. Yan, X. Ke, Mater. Lett. 61 (2007) 105–110.
[11] M. Takeuchi, H. Yamashita, M. Matsuoka, M. Anpo, T. Hirao, N. Itoh, N. Iwamoto,
Catal. Lett. 67 (2000) 135–137.
[12] F. Yang, Y. Chu, L. Huo, Y. Yang, Y. Liu, J. Liu, J. Solid State Chem. 179 (2006) 457–
463.
[13] K.N.P. Kumar, A. Burggraaf, J. Mater. Chem. 3 (1993) 141–145.
[14] R. Gopalan, Y.S. Lin, Ind. Eng. Chem. Res. 34 (1995) 1189.
[15] G. Boschloo, A. Hagfeldt, Chem. Phys. Lett. 370 (2003) 381.
[16] K.M. Parida, N. Sahu, J. Mol. Catal. A: Chem. 287 (2008) 151–158.
[17] J. Wang, Y. Xie, Z. Zhang, J. Li, C. Li, L. Zhang, Environ. Chem. Lett. 8 (2010) 87–93.
[18] Q. Zhang, G.L. Griffin, Thin Solid Films 263 (1995) 65.
[19] W.D. Sproul, M.E. Graham, M.S. Wong, P.J. Rudnik, Surf. Coat. Technol. 89 (1997)
10.
[20] S. Malynych, I. Luzinov, G. Chumanov, J. Phys. Chem. B 106 (2002) 1280–1285.
[21] H. Hu, W. Xiao, J. Yuan, J. Shi, D. He, W. Shangguan, J. Sol–Gel Sci. Technol. 45
(2008) 1–8.
[22] C.J. Brinker, G.W. Scherer, Sol–Gel Science, Academic Press, Inc., San Diego, CA,
USA, 1990.
J. Castañeda-Contreras et al. / Materials Research Bulletin 47 (2012) 290–295 295
[23] S. Zhu, D. Zhang, X. Zhang, L. Zhang, X. Ma, Y. Zhang, M. Cai, Micropor. Mesopor.
Mater. 126 (2009) 20–25.
[24] M.E. Davis, Nature 417 (2002) 813–821.
[25] M.C. Fuentes, G.J.A. Soler, Chem. Mater. 18 (2006) 2109–2117.
[26] N.V. Dezhkunova, T.G. Leighton, J. Eng. Phys. Thermophys. 77 (1) (2004) 53–61.
[27] E. Borgarello, J. Kiwi, M. Gratzel, E. Pelizzetti, M. Viscald, J. Am. Chem. Soc. 104
(1982) 2996–3002.
[28] Y. Wang, G. Zhou, T. Li, W. Qiao, Y. Li, Catal. Commun. 10 (2009) 412–415.
[29] S.C. Glotzer, M.J. Solomon, N.A. Kotov, AIChE J. 50 (12) (2004) 2978–2985.
[30] A. Corma, Chem. Rev. 97 (1997) 2373–2419.
[31] C.J. Brinker, Y. Lu, A. Sellinger, H. Fan, Adv. Mater. 11 (1999) 579–585.
[32] P.C. Angelome, M.C. Fuertes, G. Soler-Illia, Adv. Mater. 18 (2006) 2397–2402.
[33] K. Nakanishi, Bull. Chem. Soc. Jpn. 79 (2006) 673–691.
[34] A. Gedanken, X. Tang, Y. Wang, N. Perkas, Y. Koltypin, M.V. Landau, L. Vradman, M.
Herskowitz, Chem. Eur. J. 7 (2001) 4547–4552.
[35] C.H. Liang, M.F. Hou, S.G. Zhou, F.B. Li, C.S. Liu, T.X. Liu, Y.X. Gao, X.G. Wang, J.L. Lü,
J. Hazard. Mater. 138 (2006) 471–478.
[36] P.C Becker, N.A. Olsson, J.R. Simpson, Erbium-Doped Fiber Amplifiers: Funda-
mentals and Technology, Academic Press, USA, 1999.
[37] A.J. Kenyon, Prog. Quant. Electron. 26 (2002) 225–284.
[38] P. Wilhelm, D. Stephan, J. Photochem. Photobiol. A: Chem. 185 (2006) 19–25.
	Photocatalytic activity of erbium-doped TiO2 nanoparticles immobilized in macro-porous silica films
	Introduction
	Experimental
	Synthesis of TiO2 nanoparticles doped with Er3+
	Synthesis of macro-porous silica films
	Immobilization of TiO2:Er3+ on the films surface
	Characterization of the films and the nanoparticles
	Photocatalytic activity testing
	Results and discussion
	Catalyst characterization
	Macro-porous silica films and immobilization of TiO2:Er3+
	Photocatalytic study
	Er3+ population and depopulation processes
	Conclusions
	References