Photocatalytic synthesis of silver-oxide clathrate Ag7 O 8 N O3

Article abstract of DOI:10.1149/1.3503581

Photocatalytic synthesis of silver-oxide clathrate Ag₇ O ₈ N O₃ was investigated. The process is based on photocatalytic enhanced oxidation of Ag+ in a AgN O₃ aqueous solution using a titanate-based photocatalyst (SrTi O₃) single crystal. By doping the crystal with Nb or La, the photocatalytic production of Ag ₇ O₈ N O₃ is enhanced even for relatively low concentrations of AgN O₃ aqueous solutions and more with higher Nb doping. This photocatalytic reaction can be understood in terms of selectivity of the oxidation reaction, favoring generation of O₂ or Ag ₇ O₈ N O₃, with the Nb doping as well as the concentration of the AgN O₃ aqueous solution having strong impact on the reaction selectivity.

Full text of DOI:10.1149/1.3503581

Journal of The Electrochemical Society, 157 ͑12͒ E181-E183 ͑2010͒
E181
0013-4651/2010/157͑12͒/E181/3/$28.00 © The Electrochemical Society
Photocatalytic Synthesis of Silver-Oxide Clathrate Ag7O8NO3
R. Tanaka,a S. Takata,a M. Katayama,a R. Takahashi,b J. K. Grepstad,b
T. Tybell,b and Y. Matsumotoa,z
aMaterials and Structures Laboratory, Tokyo Institute of Technology, Midori-ku, Yokohama 226-8503,
Japan
bDepartment of Electronics and Telecommunications, Norwegian University of Science and Technology,
Trondheim 7491, Norway
Photocatalytic synthesis of silver-oxide clathrate Ag7O8NO3 was investigated. The process is based on photocatalytic enhanced
oxidation of Ag+ in a AgNO3 aqueous solution using a titanate-based photocatalyst ͑SrTiO3͒ single crystal. By doping the crystal
with Nb or La, the photocatalytic production of Ag7O8NO3 is enhanced even for relatively low concentrations of AgNO3 aqueous
solutions and more with higher Nb doping. This photocatalytic reaction can be understood in terms of selectivity of the oxidation
reaction, favoring generation of O2 or Ag7O8NO3, with the Nb doping as well as the concentration of the AgNO3 aqueous solution
having strong impact on the reaction selectivity.
© 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3503581͔ All rights reserved.
Manuscript submitted July 9, 2010; revised manuscript received September 23, 2010. Published October 22, 2010.
The field of heterogeneous photocatalytic reactions on semicon-
ductor surfaces, such as TiO2 and related titanate compounds, has
attracted large attention in recent years.1-6 This is triggered by their
possible application in efficient energy conversion of sunlight into
chemical fuels, electricity, and in photooxidation of a variety of
pollutants and wastes. However, direct synthesis of compounds that
can form only under strong oxidizing conditions has been far less
explored.
One material system known to require strong oxidizing condi-
tions for its synthesis is silver-oxide clathrate Ag7O8NO3, a super-
conductor with a Tc of 1.04 K.7,8 It contains Ag ions of multiple
valence states, including not only Ag+, but also Ag2+ and Ag3+.9 In
order to attain such strong oxidizing conditions, there has so far
been reported two typical methods of synthesis. One is a chemical
deposition method, using oxidizing agents such as ͑NH4͒2S2O8 and
K2S2O8 for further oxidation of Ag+ in AgNO3.10 The other is an
electrochemical method based on anodic polarization of a Pt elec-
trode to a potential E in the range of 1.74–1.77 V vs the standard
hydrogen electrode potential ͑SHE͒ in a high concentration, e.g.,
5 M, AgNO3 aqueous solution with pH 1.45.11 The reaction for this
electrochemical deposition is described as
vendor and typically exhibit a regular step-and-terrace surface struc-
ture, with 0.4 nm unit-cell steps and TiO2 termination. This atomi-
cally well-defined surface allowed to rule out possible effects on the
photocatalytic reactions from surface area and unexpected defects.
AgNO3 aqueous solutions with varying concentration from
0.001 to 0.1 M were prepared as the electrolyte. If needed, KNO3
was added to attain concentrations of NO−3 up to 1 M. Here, the
oxidizing NO−3 ion acts as a sacrificial agent to suppress the reduc-
tion of Ag+. The irradiation time of the samples in the electrolyte
typically varied from 1 to 10 min. Photochemical deposition was
carried out with a high-pressure Hg lamp ͑28 mW/cm2͒. Following
the photochemical treatment, the particles deposited on the surface
were characterized using scanning electron microscopy ͑SEM͒ and
X-ray diffraction ͑XRD͒.
Results and Discussion
Figures 1a and b show SEM images of the SrTiO3 and
Nb:SrTiO3͑0.5 wt %͒ surfaces, respectively, after photochemical
treatment ͑5 min͒ in a 0.01 M AgNO3 aqueous solution. Precipi-
tates were found on both surfaces, but with distinctly different size
and shape, even after the same photochemical processing. Subse-
quent XRD measurements revealed that the precipitates were all
crystalline. The ␪ − 2␪ Bragg reflections are displayed in Fig. 1c
and d for SrTiO3 and Nb:SrTiO3, respectively. For SrTiO3, the ␪
− 2␪ Bragg reflections were found at 2␪ = 38.09° and 44.29°, cor-
responding to Ag ͑111͒ and ͑200͒ diffraction peaks,15 consistent
with previous reports17-19 on photochemical reduction of Ag+ to Ag
metal on SrTiO3 and related TiO2 semiconductors. For
Nb:SrTiO3͑0.5 wt %͒, the ␪ − 2␪ Bragg reflections were observed
at different diffraction angles, 2␪ = 31.28°, 36.27°, and 65.56°. This
suggests photochemical deposition of ͑111͒- and ͑100͒-oriented
Ag7O8NO3.16
7Ag+ + NO−3 + 8H2O → Ag7O8NO3 + 10e− + 16H+
͓1͔
where the equilibrium of the Ag7O8NO3 is at 1.59 V vs SHE.11
Since the equilibrium potential of Ag7O8NO3 is negative with
respect to the valence band, formed predominantly from O2p states
͑ϳ2.8 V vs SHE͒ in TiO2 and related semiconductors, thermody-
namics suggests that such material systems may serve as an oxidiz-
ing photocatalyst for Ag7O8NO3 formation when irradiated with
ultraviolet ͑UV͒ light in a AgNO3 aqueous solution. In fact, we
recently reported photocatalytic synthesis of Ag7O8NO3 on a
PbTiO3/Nb-doped SrTiO3 ͑001͒ epitaxial thin film structure.12,13
Traditionally, AgNO3 aqueous solutions have been commonly used
as a sacrificial agent for production of O2 in photocatalytic splitting
of water since oxidation of water is thermodynamically favored over
further oxidation of Ag+.
In this paper, we report on the photocatalytic synthesis of
Ag7O8NO3 on Nb-doped, as well as undoped, SrTiO3 ͑001͒ single
crystals for different concentrations of the AgNO3 aqueous solution.
Possible effects of the Nb doping on the photocatalytic synthesis of
Ag7O8NO3 is discussed.
The XRD intensity ratio of Ag7O8NO3 ͑222͒ to that of SrTiO3
͑002͒ is plotted as a function of the UV irradiation time, ranging
from 0 to 10 min, in Fig. 2. No diffraction lines due to Ag7O8NO3
was observed for SrTiO3 in 0.01 M AgNO3 irrespective of the UV
irradiation time. However, the XRD intensity from Ag7O8NO3 was
found
to
increase
with
UV
irradiation
time
for
Nb:SrTiO3͑0.5 wt %͒. Based on these results, the production of
Ag7O8NO3 is attributed to photocatalytic oxidation.
While Ag7O8NO3 was photochemically deposited on the
Nb:SrTiO3͑0.5 wt %͒ crystal surface, the production of Ag metal by
photocatalytic reduction was found simultaneously along the edges
of the crystal. Hence, the oxidation and reduction sites are spatially
separated. This is different from the case of SrTiO3, where O2 and
Ag are produced photochemically on the same crystal surface. When
the edges of Nb:SrTiO3͑0.5 wt %͒ were passivated with an insulat-
ing polymer, Ag7O8NO3 was replaced by Ag metal as the main
Experimental
The SrTiO3 ͑001͒ and Nb:SrTiO3 ͑001͒ ͑Nb: 0.05 wt %,
0.5 wt %͒ single crystals used in this work were wet-etched14 by the
z E-mail: matsumoto.y.ad@m.titech.ac.jp
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E182
Journal of The Electrochemical Society, 157 ͑12͒ E181-E183 ͑2010͒
(a) SrTiO3
(b) Nb:SrTiO3 (Nb:0.5wt%)
Table I. Dependence of photochemical reaction on electrolyte
components, Nb doping and “edge-passivation.” 1 M KNO3 was
added to suppress the reduction of Ag ions. The different photo-
catalytic behavior in a AgNO3 aqueous solution between the
SrTiO3 and Nb:SrTiO3 surfaces may be understood in terms of
different reaction selectivity for the oxidation products O2 or
Ag7O8NO3.
Electrolyte
1µm
1µm
(c)
(d)
0.01 M AgNO3
and 1 M KNO3
105
Crystal
Edge
0.01 M AgNO3
Nb:SrTiO3 ͑001͒
͑Nb = 0.5 wt %͒
͑Nb = 0.05 wt %͒
open
passivated
open
Ag7O8NO3
Ag7O8NO3
Ag7O8NO3
Ag7O8NO3
Ag
Ag
102
102
SrTiO3 ͑001͒
open
Ag
None
Ag-metal
20
Ag7O8NO3
was observed for 0.05 wt % Nb:SrTiO3; Ag7O8NO3 was predomi-
nately deposited on the surface only after adding the NO−3 oxidizing
agent. In contrast, Ag7O8NO3 was not produced on SrTiO3, though
the reduction of Ag+ to Ag metal was completely suppressed by
adding NO−3, different from the Nb:SrTiO3 cases. From these data,
summarized in Table I, the observed difference in photocatalytic
behavior in the AgNO3 aqueous solution for the SrTiO3 and
Nb:SrTiO3 surfaces can be understood in terms of different reaction
selectivities for the photocatalytic oxidation products O2 and
Ag7O8NO3. One requirement for the photocatalytic synthesis of
Ag7O8NO3 seems to be that reduction and further oxidation of Ag+
do not occur on the same crystal surface.
In order to investigate kinetic effects on the reaction selectivity,
the photocatalytic products were examined for different concentra-
tions of the AgNO3 aqueous solution. The XRD intensity of
Ag7O8NO3 ͑222͒ normalized to that of SrTiO3 ͑002͒ is plotted in
Fig. 3 as a function of the AgNO3 concentration for SrTiO3 and
Nb:SrTiO3͑0.05 wt %,0.5 wt %͒, respectively. For a AgNO3 con-
centration below 0.001 M, there was no detectable production of
Ag7O8NO3. Ag metal was deposited on all SrTiO3 and Nb:SrTiO3
crystal surfaces. However, for 0.5 wt % Nb:SrTiO3, the production
of Ag7O8NO3 was found to increase with the AgNO3 concentration.
Even for SrTiO3, Ag7O8NO3 was observed for concentrations above
0.05 M. This result suggests that the concentration of the AgNO3
aqueous solution is an important kinetic factor contributing to the
recorded differences in reaction selectivity of photocatalytic oxida-
tion products O2 or Ag7O8NO3. This underlines the benefit of highly
concentrated AgNO3 aqueous solutions for efficient anodic oxida-
tion of Ag+ to Ag7O8NO3.
We note that the higher the Nb doping, the more efficient the
production of Ag7O8NO3, confirmed for any concentration of the
AgNO3 aqueous solution. This result indicates that Nb-doped
SrTiO3 activates the production of Ag7O8NO3, which raises the
question of what is the role of the Nb doping in SrTiO3. One pos-
sibility, from a chemical perspective, is that Nb at the surface pro-
vides active sites for the production of Ag7O8NO3. However, this
hypothesis may be ruled out by the minuscule surface concentration
of Nb. With 0.5 wt % bulk concentration of Nb, the surface concen-
tration was too low to be detected, e.g., by Auger electron micros-
copy, without any annealing process. Thus, Nb cannot be expected
to contribute to a direct surface reaction. Furthermore, when Nb was
replaced by La as dopant in SrTiO3, similar activation for produc-
tion of Ag7O8NO3 was found in a 0.01 M AgNO3 aqueous solution.
It is most unlikely that Nb and La would play similar roles as active
sites on the surface, considering that Nb substitutes for Ti ͑B-site͒
whereas La substitutes for Sr ͑A-site͒. Thus, their chemical proper-
ties should be different.
100
40
60
20
40
60
2theta/theta (deg)
2theta/theta (deg)
Figure 1. SEM images ͓͑a͒ and ͑b͔͒ and XRD patterns ͓͑c͒ and ͑d͔͒ recorded
after photochemical treatment in 0.01 M AgNO3 on SrTiO3 and
Nb:SrTiO3͑Nb = 0.5 wt %͒, respectively. The irradiation intensity is
ϳ28 mW cm−2. The Nb doping effectively changed the reaction products
from Ag metal into Ag7O8NO3. ᭝’s designates substrate diffraction peaks.
reaction product. It is thus inferred that reduction and oxidation of
Ag+ are competitive processes on Nb:SrTiO3͑0.5 wt %͒ surface.
Adding KNO3 to the 0.01 M AgNO3 aqueous solution in order
to increase the concentration of the oxidizing agent NO−3 up to 1 M
completely suppressed the reduction of Ag+. As a result, Ag7O8NO3
was produced on 0.5 wt % Nb:SrTiO3 crystal surface even with the
edges passivated. A similar enhancement of Ag7O8NO3 production
10
Nb:SrTiO
3
8
6
4
2
0
SrTiO
5
3
0
10
Time (min)
Figure 2. The XRD intensity ratio of Ag7O8NO3 ͑222͒ to that of SrTiO3
͑002͒ plotted vs UV irradiation time from 0 to 10 min in 0.01 M AgNO3 for
SrTiO3 and 0.5 wt % Nb:SrTiO3, respectively. The production of Ag7O8NO3
is attributed to the photocatalytic oxidation on Nb:SrTiO3͑Nb = 0.5 wt %͒.
From the perspective of semiconductor physics, Nb and La act as
donors in bulk SrTiO3, thus making the SrTiO3 conductive. In gen-
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Journal of The Electrochemical Society, 157 ͑12͒ E181-E183 ͑2010͒
E183
effect of the space charge layer is to favor photogenerated hole
carriers to reach the surface so as to provide a high concentration of
free holes at the surface. This high concentration of holes may ex-
plain the enhanced oxidation of Ag+ in the photocatalytic reaction
even for relatively low concentration aqueous solutions of AgNO3.
Nb(=0.5wt%):SrTiO3
Nb(=0.05wt%):SrTiO3
SrTiO3
3
2
1
0
Conclusions
We have demonstrated photocatalytic oxidation of AgNO3 to sil-
ver oxide clathrate, Ag7O8NO3, on SrTiO3 single crystals. The se-
lectivity of the oxidation products O2 or Ag7O8NO3 depends not
only on the Nb doping level, but also on the concentration of the
AgNO3 aqueous solution and an additive constituent in the electro-
lyte. Further investigations will be needed in order to clarify the
origin of the enhanced oxidizability and the reaction selectivity for
the photocatalytic oxidation products O2 and Ag7O8NO3 on
Nb:SrTiO3 in AgNO3.
Acknowledgments
This work was partially supported by a grant-in-aid from the
Ministry of Education, Culture, Sports, Science and Technology of
Japan ͑Project no. 20360294͒ and by funding from the Research
Council of Norway under Contract no. 1628741/V00.
0.001 2 4 60.01
4 6 0.1
6
2
Tokyo Institute of Technology assisted in meeting the publication costs of
this article.
AgNO3 concentration (M)
Figure 3. The XRD intensity of Ag7O8NO3 ͑222͒ normalized to that of
SrTiO3 ͑002͒ plotted as a function of the concentration of the AgNO3 aque-
ous solution for SrTiO3 and Nb:SrTiO3 ͑0.05 wt %,0.5 wt %͒, respectively.
The concentration is a kinetic factor contributing to differences in reaction
selectivity for the photocatalytic oxidation to O2 or Ag7O8NO3.
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