Experimental Evidence of CO2 Photoreduction Activity of SnO2 Nanoparticles

Article abstract of DOI:10.1002/cphc.202000786

Tin dioxide (SnO₂) has intrinsic characteristics that do not favor its photocatalytic activity. However, we evidenced that surface modification can positively influence its performance for CO₂ photoreduction in the gas phase. The hydroxylation of the SnO₂ surface played a role in the CO₂ affinity decreasing its reduction potential. The results showed that a certain selectivity for methane (CH₄), carbon monoxide (CO), and ethylene (C₂H₄) is related to different SnO₂ hydrothermal annealing. The best performance was seen for SnO₂ annealed at 150 °C, with a production of 20.4 μmol g^?1 for CH₄ and 16.45 μmol g^?1 for CO, while for SnO₂ at 200 °C the system produced more C₂H₄, probably due to a decrease of surface ?OH groups.

Full text of DOI:10.1002/cphc.202000786

Accepted Article
Title: Experimental Evidence of CO2 Photoreduction Activity of SnO2
Nanoparticles
Authors: Juliana Arriel Torres, Gelson T. S. T. da Silva, Fernando
Barbosa de Freitas Silva, and Caue Ribeiro
This manuscript has been accepted after peer review and appears as an
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the VoR from the journal website shown below when it is published
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To be cited as: ChemPhysChem 10.1002/cphc.202000786
Link to VoR: https://doi.org/10.1002/cphc.202000786
0
1/2020

ChemPhysChem
10.1002/cphc.202000786
COMMUNICATION
Experimental Evidence of CO
2
Photoreduction Activity of SnO
2
Nanoparticles
#
[a]
# [a]
[a,b]
Juliana Arriel Torres , Gelson T. S. T. da Silva , Fernando Barbosa de Freitas Silva,
Ribeiro*[a,b]
and Caue
[
[
[
a]
b]
#]
Dr. J. A. Torres, Dr. G. T. S. T. da Silva, Dr. F. B. F. Silva, Dr. C. Ribeiro
Nanotechnology National Laboratory for Agriculture (LNNA)
Embrapa Instrumentation, Zip Code 13560-970, São Carlos, SP, Brazil
E-mail: caue.ribeiro@embrapa.com.br
Dr. F. B. F. Silva, C. Ribeiro
Department of Chemistry
Federal University of São Carlos (UFSCar), Zip Code 13565-905, São Carlos, SP, Brazil
E-mail: caue.ribeiro@embrapa.com.br
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Tin dioxide (SnO
2
) has intrinsic characteristics that do not
hydroxide in sol-gel synthesized SnO
reduced to Sn(OH)
2
surfaces, is expected to be
+
favor its photocatalytic activity. However, we evidenced that surface
2
at lower potentials than H /H
2
and CO
2
/CO. Other
modification can positively influence its performance for CO
photoreduction in the gas phase. The hydroxylation of the SnO
2
2
hydroxides can be formed depending on the synthesis condition,
which can be modified by crystallization routes
–
such as
[
8]
surface played a role in the CO
potential. The results showed that a certain selectivity for methane
CH ), carbon monoxide (CO), and ethylene (C ) is related to
different SnO hydrothermal annealing. The best performance was
seen for SnO
for CH and 16.45 µmol g for CO, while for SnO
2
affinity decreasing its reduction
hydrothermal annealing.
(
4
2 4
H
Therefore, herein we demonstrate that SnO
photocatalyst for CO
plays a role in the reduction potentials (as charge mediator) and the
CO affinity. We modified the hydroxylation degree of co-precipitated
SnO (sol-gel synthesis) by different hydrothermal annealing
2
is a
potential
2
o
-1
2
reduction, where the surface hydroxylation
2
annealed at 150 C, with a production of 20.4 µmol g
-
1
o
4
2
at 200 C the system
2
produced more C
groups.
2 4
H , probably due to a decrease of surface -OH
2
temperatures, which evidenced that the increase of strongly bonded -
OH groups promotes the reduction efficiency. On the other hand, the
higher annealing temperature is deleterious, probably due to the
higher crystallization of these hydroxides. However, this crystallization
modifies the product selectivity, slightly increasing the reduction to C2
Several semiconductors, highlighted by TiO
activity to promote redox reactions, such as the oxidation of organic
substances or reduction of molecules such as CO or N . These
2
, have photocatalytic
2
2
reactions occur through photoelectrochemical processes, driven by
the charge separation during illumination and specific electronic
products. These results reveal the potential of SnO
deeper investigation for these applications.
2
that merits
[
1]
2
structure of each material. SnO has an electronic structure similar
to TiO , but its electronic mobility is almost 100 times higher, leading
2
SnO
annealed via hydrothermal conditions at different temperatures. In
detail, the SnO was synthesized as follows (Fig. S1): 0.564 g of
SnCl .2H O (Sigma-Aldrich) was dissolved in 100 mL of ethanol
anhydrous (99.5% purity, Dinâmica) and then under vigorous stirring,
2
nanoparticles were synthesized at room temperature and
[
2]
to high efficiency in the photogenerated electrons transference. On
the other hand, the position of its conduction band minimum (CBM) is
+
2
located above the redox potential level of H /H
2
(-0.41 V vs. NHE, at
[
3]
2
2
2
pH 7), making SnO limited for application in reduction reactions.
This feature also limits its application in photocatalytic oxidation since
the reaction is controlled by the coupled reduction reaction.
2
2.5 mL of distilled water was added dropwise at room temperature
[
9,10]
for 12 h.
After that, the suspension was cleaned by dialysis in 5 L
of distilled water, often renewed, until removing all chloride ions, which
-
1 [11]
Electrochemical experiments indicate that SnO
2
nanoparticles are
was confirmed by tests with AgNO
solution inside the dialysis bag was dried at 70°C for 12 h, ground with
an agate mortar, and named “SnO -cop”. After that, the as-prepared
3
solution (0.1 mol L ). Then, the
efficient electrocatalysts of CO reduction due to the interaction of
2
[
4]
carbonate species with surface Sn hydroxides. In this case, the
energy for reduction is externally supplied, and analysis of reduction
2
sample was treated hydrothermally up to 200°C for 4 h.
potential for SnO
necessary potential range for CO
other hand, Sn hydroxides may have lower reduction potentials,
2
indicates that the oxide is not reduced to Sn in the
2
reduction, keeping stable. On the
The characterization procedure using different techniques, such as X-
ray diffractometry, scanning electron and high-resolution transmission
microscopy, Near-Infrared Spectroscopy, and diffuse reflectance
spectroscopy are shown in detail in the Supporting Information. The
[
5]
favoring the redox process. This surface interaction is expected to
occur in photocatalytic systems since no bias is necessary to promote
[
6]
it, but the reaction would be limited by the band's energy. Therefore,
to overcome the limitation, the hydroxylation of SnO can, in principle,
acts as a charge mediator, reducing the reduction potential to
catalysts have been characterized in CO
carried out in a gas-solid reaction using CO
under UV light. The catalysts were deposited over a glass substrate,
2
photoreduction experiments,
2
2
+ H O steam for 6 h
2
[
4,7]
favorable ranges for CO
2
reduction.
4
Sn(OH) , the most probable
1
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ChemPhysChem
10.1002/cphc.202000786
COMMUNICATION
2
making a thin film with 10 mg and 3.24 cm of the exposed area. The
reactants have been supplied as a moist CO by bubbling in water. A
scheme of the experimental setup is shown in Suppl. Fig. S2.
increase (in proportion) according to the annealing temperature. It is
attributed to the elimination of weak-bonded water, which is expected.
A deeper analysis of the bonded-water distribution is seen by
deconvoluting the crystalline water in four contributions, as seen in
Figure S4. Table 1 summarizes the proportion of each contribution,
2
The morphology and size of the SnO
by scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) (Fig. 1a-b). SnO -cop sample is precipitated in very
2
-cop particles were examined
which are attributed to different water vibration modes S
The temperature increases the contribution of strongly bonded water
) by reducing polymerized water (S ). It indicates that the water-
1 2 n
, S and S .
2
(S
1
n
massive agglomerates, consisting of large and faceted xerogel blocks,
water interaction diminishes with temperature, but one can see that
the distribution of water bonding modes is almost unaltered from
from 30 and 60μm (Fig. 1a). The xerogel structure is supported by the
2
-1
high surface area (188 m g ) and composed of small crystalline
particles (Figure 1b) of 4 nm, with adjacent lattice spacing (0.34 nm)
2 2
SnO -cop to SnO -100, indicating that, in this range, the only
modification is the elimination of weak-bonded water.
[
12,13]
referring to the planes (110).
Line defects (white arrows), typical
of oriented attachment growth, are seen evidencing the growth
mechanism. The X-ray diffraction patterns (Fig. 1c) indicate that the
hydrothermal annealing did not affect the phase, all indexed as
tetragonal cassiterite phase (JCPDS No. 41-1445). The wide peaks
and low intensity are characteristics of the amorphous nature of the
Table 1. Relative area (%) of the deconvolution band from NIR spectra of the
synthesized samples.
Samples
Vibration mode of the water molecules
[
14,15]
samples.
The peak sharpening for the sample at 200°C is
the particles, as
S
1
S
2
S
n
associated with the oriented growth of SnO
2
[
16]
expected. The bandgap value (Fig. S3) found for the commercial
sample (3.7 eV) agrees with the literature, while to the other samples,
[
17,18]
the values were lower.
SnO -200 are 3.2 and 3.5 eV, respectively, a slight blueshift in the
bandgap energy.
2
The bandgap values for the SnO -cop and
2
Peak
4
(
b)
Peak
1
Peak
2
Peak
+
1
2
Peak
3
Peak
SnO
SnO
SnO
SnO
2
2
2
2
-cop
-100
-150
-200
45.62
47.33
48.22
46.10
8.21
6.83
8.85
53.83
54.16
57.07
60.11
32.64
32.60
31.49
32.55
13.53
13.24
11.44
IFFT
14.01
7.34
SnO
SnO
SnO
SnO
2
2
2
2
-cop
-100
-150
-200
(
c)
(d)
S
1
– one active H-bond; S
2
– two active H-bond; S
n
- H
2
O with two active H-
SnO -200C
2
2
bond and one (or two) passive H-bound (polymeric chain of H O molecules).
SnO -100C
2
SnO -cop
2
A comparison of the total number of water groups at surface is done
by normalizing the FTIR spectrum by the O-Sn-O peak area (AO-Sn-O
,
JCPDS 41-1445
-
1 [20]
581 cm ), which is considered to be constant for all samples (since
2
0
30
40
50
60
70
80 4000
5000
6000
-17000
8000
2
 / degree
Wavenumber / cm
the composition is the same). Therefore, the intensity of H O and OH
2
Figure 1. (a) SEM images; (b) HRTEM images; (c) XRD patterns of the
synthesized SnO samples, and (d) Near-infrared spectrum (NIR) of the SnO
cop and SnO submitted to hydrothermal annealing at 100, 150, and 200°C.
peaks (AH2O and AOH) will vary according to the relative amounts of
water molecules, which are clearly seen by the quotient AH2O/ASn-O-Sn
and AOH/ASn (Table 2). Thus, the contribution of OH groups (weakly
2
2
-
2
bonded) for SnO
the water elimination. These data also reveal that, despite the
distribution of water group modes is similar for SnO -cop and SnO
00 (as seen in NIR experiments), the content of weakly bonded and
2 2
-cop is twice the observed for SnO -200, indicating
2
The SnO utilization for photocatalytic oxidation is barely described
2
2
-
but generally is reported as inactive. However, surface groups directly
influence photoactivity, as seen in our experiment (Fig. S7): the
1
2 2
crystalline water in SnO -cop is much higher. Besides, SnO -150
surface modification of SnO
effective since expressive results were obtained, mainly with the
SnO -200 sample. NIR analysis (Fig. 1d) contributes to elucidating the
mechanism involved during the oxidation process, being directly
related to the water molecules adsorbed on the surface of the SnO
samples. As seen in Figure 1d there are three characteristic peaks of
2
through hydrothermal treatment was
seem to be intermediary, showing almost the same weakly bonded
water content of SnO -100 but with more crystalline water, which is
probably from the surface crystallization of hydroxyl groups (Fig. 1d).
Finally, SnO -200 has lower content of both weakly-bonded and
2
2
2
2
crystalline water since the temperature is probably leading to the
crystallization of surface oxide (anhydrous).
-
1
st
water, i.e., ~7,000 cm corresponds to surface hydroxyl groups (1
-
1
st
overtone), ~5,000 cm attributed to crystalline water (hydroxide, 1
-
1
[19]
overtone) and 4,500 cm
attributed to xerogel structure.
This variation correlates with the CO
that the crystalline water plays a role in the reaction. SnO
has a reduction of crystalline water content, leaded to lower
2
reduction profiles, suggesting
Normalizing all the peaks by the surface hydroxyl groups, one can see
differences in the distribution of crystalline water, which tends to
2
-100, which
2
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ChemPhysChem
10.1002/cphc.202000786
COMMUNICATION
photoactivity compared to the others. SnO
2
-150, having the highest
Moreover, SnO
highest CO
an acidic surface, thus favoring CO
stages of the reaction. The production of CH
2
-150 has the more acidic surface, corresponding to its
content of crystalline water and less polymeric water, produced to the
best photoactivity, which is dramatically reduced in SnO -200 due to
2
conversion. These values show that the samples have
adsorption, one of the critical
and CO, in turn, was
close (20.4 µmol g , and 16.45 µmol g , respectively, for SnO -150).
This result is noteworthy when compared to the few reports in the
literature about SnO photoreduction activity, even also to
representative TiO systems (Table 3).
2
2
the lower content of water groups. Therefore, we suggest that the Sn
hydroxyl groups are the actual catalytic sites, which are influenced by
the bonding mode and by the total content. This hypothesis indicates
4
-
1
-1
2
that the photocatalytic trend SnO
2
-150 > SnO
2
-cop > SnO
2
-100 is due
2
to two factors, the crystallization water at surface and the total water
content.
2
However, in addition to CH
4
and CO, we also observed the production
Table 2. Absolute Areas (A) of the FTIR spectrum bands of the synthesized
samples.
of small amounts of C
2
H
4
(Fig. 2c). An increase in C selectivity is
2 4
H
o
seen for 200 C-annealed sample, considering the total catalytic
activity. We hypothesize that the higher annealing temperature has
led to higher growth and reduction of surface -OH groups, which is
Samples
A
O-Sn-O
A
H2O
A
OH
AH2O/AO-
A
OH
/
-1
-1
-1
(581 cm
)
(1630-1640cm
)
(2500-3500 cm
)
Sn-O
A
O-Sn-O
[
26]
consistent with our results. It is worth to mention that these results
SnO2-cop
42.00
65.00
91.77
100.76
4.44
44.90
0.11
0.06
0.06
0.04
1.07
0.44
0.51
0.42
were obtained with pristine SnO
2
and using a low-power lamp (18 W).
SnO
SnO
SnO
2
-100
2
-150
2
-200
3.65
28.92
22
20
18
16
14
12
10
8
6
4
2
0
(a)
SnO -cop
2
5.30
46.98
SnO -100
2
SnO -150
3.98
42.63
2
SnO -200
2
These molecules are possibly regarded to the surface hydroxylation
of SnO , coordinated as Sn(OH) , as a remnant of polycondensation
2
4
[
21]
step during coprecitation. These OH groups undergo oxidation by
+
the holes (h ) formed in the catalyst valence band when under UV,
leading to the formation of hydroxyl radicals (•OH) with high oxidizing
+
2+
[22]
power (-SnOH + h ↔ -Sn + •OH). Adsorbed OH- groups on the
Methane
Carbon monoxide
Ethylene
catalyst surface this potential to 1.6V vs NHE, favoring the radical
[
3]
formation. Therefore, the attack of these radicals leads to oxidation
reactions, indicating that the indirect photodegradation mechanism
1
60 (b)
SnO -cop
2
SnO -100
plays an important role in the photoactivity of the synthesized SnO
2
140
2
SnO -150
[
21,23]
2
samples.
1
20
00
SnO -200
2
1
Fig. 2a indicates the potential of SnO
2
samples to photoreduce CO
2
8
0
0
to C1 species. Although the intrinsic characteristics of SnO
2
are not
6
favorable to its photoactivity, these results reveal that the conditions
used in the reactions were probably essential for selective response
observed to certain products. The samples were selective for three
40
20
different products, CH
higher CH
4
, CO, and C
2
H
4
, respectively (Fig. 2a), with
0
o
4
selectivity for all samples except for 200 C-annealed (Fig.
Methane
Carbon monoxide
Ethylene
2
c). Fig. 2b shows the number of electrons transferred for each
[
24]
product according to Equation 1 (see in Supporting Information),
-
90
80
70
knowing that the transference of 8e are necessary for the production
(c)
SnO -cop
2
-
-
of CH
phase reactor implies that the CO
considering that humidified, high purity CO
system for 20 min. Thus, taking into account the acidic nature of CO
molecules, its adsorption will reduce the local pH, consequently
4
from CO
2
reduction, 2e for CO and 12e for C
concentration is extremely high,
is purged within the
2
H
4. The gas-
SnO -100
2
2
SnO -150
2
SnO -200
2
2
6
0
0
2
5
[
25]
40
30
lowering the potential for CO
base nature of the SnO surface, the isoelectric point of sample
suspensions at different pH values (Figure S6) was determined, which
predicts the adsorption behavior of CO molecules during the
2
reduction. To determine the acid-
2
2
0
0
0
2
1
photoreduction process. As expected, although the values found were
close for all samples and in accordance with the literature, a small
Methane
Carbon monoxide
Ethylene
deviation to alkaline behavior is seen to SnO
2
-100, due to the
Figure 2. (a) Amount of C-species produced from CO
by synthesized SnO
2
photoreduction driven
samples activated under UV light in a gas-solid reactor for
reduction of crystalline surface water (as evidenced by FTIR
measurements), which favors that this reaction can take place in lower
2
6
hours; (b) the corresponding photoelectron transfer number involved in the
2 4 2 4
CO photoreduction and (c) selectivity for CH , CO, and C H for the different
synthesized samples.
[
7,10]
local pH since below IEP its surface is positively charged.
.
3
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ChemPhysChem
10.1002/cphc.202000786
COMMUNICATION
Table 3. Comparison of CO
2
photoreduction production promoted by Sn-based
Improvement of Higher Education Personnel – Finance Code
catalysts and TiO
2
a standard catalyst used in photochemical reactions.
0
01; and CAPES/Embrapa Call 15/2014 Grant #166); Agronano
-
1
-1
Yield µmol g
h
Network (Embrapa Research Network); and SISNANO/MCTI,
FINEP. Caue Ribeiro also acknowledges Alexander von
Humboldt Foundation and CAPES by Experienced Research
Catalyst
CH
4
CO
⸺
CH
3
OH
C
2
H
4
Conditions
450 W
Ref
-
3
-5
[27]
SnO
SnO
2
2
3x10
6.5x10
⸺
⸺
⸺
⸺
Fellowship
(CAPES/Humboldt
Agreement
–
Process
8
8881.145566/2017-1). The authors also thank the Structural
[
28]
29]
1.37
1.14
48.01
300 W
Characterization Laboratory (LCE) for the technical support in the
transmission electron microscopy experiments.
[
SnS
2
/SnO
2
4
⸺
⸺
300 W
(
filter λ>400
nm)
Keywords: Gas-solid reaction • Photocatalysis • Surface
hydroxylation • Tin dioxide
[30]
[28]
[31]
[32]
SnO
P25
2 3
-C N
0.62
0.16
0.13
5.0
0.72
0.53
⸺
⸺
⸺
⸺
⸺
⸺
⸺
⸺
300 W Xe
300 W
5 W
[
[2]
1]
X. Li, J. Yu, M. Jaroniec, Chem. Soc. Rev. 2016, 45, 2603.
Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X.
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2
0.61
[
3]
V. R. de Mendonça, O. F. Lopes, A. E. Nogueira, G. T. S. T. da Silva, C.
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3148.
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2
/Fe
2
O
3
11.25
300 W (filter
λ>420 nm)
SnO
2
-150
3.41
2.74
⸺
0.03
18 W
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*
These data show the best results for the cited reports.
[
8]
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[
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Appl. Phys. Lett. 2003, 83, 1566.
[
4,26]
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4
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As
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[10]
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o
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[
[
11]
12]
o
SnO
2
-cop and 100 C, which is explained by possible large variations
of free water in these conditions, there is an increase in
o
[13]
photoreduction activity at 150 C coinciding with a higher amount of
[14]
[15]
[16]
hydroxyl groups. It is also noteworthy that the product selectivity is
almost identical for these three conditions, as seen in Fig. 2c, which
indicates that the reduction mechanism is probably the same.
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o
However, the higher crystallization for the sample annealed at 200 C
[17]
[18]
has probably affected the distribution of hydroxyl groups, leading to
more ordered hydroxides. These influenced the selectivity leading to
C-H bonds (methane and ethylene) rather than C-O. These features
are still a matter of investigation, but they evidence the role of surface
[
19]
20]
[
2
hydroxylation in SnO photoreduction activity.
[
[
21]
22]
In conclusion, we demonstrated that SnO
2
could be photocatalytically
-water gaseous
[
23]
24]
active for CO reduction in the solid-gas system (CO
2
2
[
mixture) under some synthesis conditions, easily modified by
hydrothermal annealing. Our results indicate that the structural
surface water (surface hydroxylation) plays a role in its photoactivity,
and product selectivity can be tuned by modifying the nature of these
surface hydroxides by crystallization. Moreover, the catalyst efficiency
was higher than previously reported papers, including compared to
[25]
1
1756.
[
[
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[28]
TiO
2
. These results demonstrate that SnO
2
should be more studied
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32]
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for CO
2
reduction due to its advantages, as the high chemical stability,
[
easy synthesis of small and uniform nanoparticles, and good
electronic properties.
[
1
0, 2321.
Acknowledgements
The authors are grateful to CNPq (grant #402287/2013-4 and
1
2
59866/2018-9) and FAPESP (grant #2016/21515-7 and
018/01258-5) for financial support. CAPES (Coordination for the
4
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.202000786
COMMUNICATION
CO2
CH4
CO
C H
e-
CB
2
4
VB
h+
OH-
OH
Oxidation
SnO
bonded -OH groups, which influenced the process of reducing the CO
mediators, reducing the potential for CO reduction. The semiconductors showed selectivity to C1 hydrocarbons and the sample
subjected to higher annealing showed selectivity for C2 products.
2
nanoparticles were synthesized and applied in the CO
2
photoreduction process. The synthesis methodology produced strongly
2
potential, as well as its affinity. The hydroxyls act as charge
2
5
This article is protected by copyright. All rights reserved.