A Facile Molecular Precursor-based Synthesis of Ag2Se Nanoparticles and Its Composites with TiO2 for Enhanced Photocatalytic Activity

Article abstract of DOI:10.1002/asia.201600157

The reactions of different silver(I) reagents AgX (X^?=iodide, trifluoroacetate, triflate) with selenoethers R₂Se (R=Me, tBu) in a variety of solvents were investigated in relation with their use as precursors for Ag₂Se nan

Full text of DOI:10.1002/asia.201600157

DOI: 10.1002/asia.201600157
Communication
Nanoparticles
A Facile Molecular Precursor-based Synthesis of Ag2Se
Nanoparticles and Its Composites with TiO2 for Enhanced
Photocatalytic Activity
Shashank Mishra,*[a] Dan Du,[a, b] Erwann Jeanneau,[c] Frederic Dappozze,[a] Chantal Guillard,[a]
Jinlong Zhang,[b] and StØphane Daniele[a]
interesting and useful properties.[4] There are two stable crys-
Abstract: The reactions of different silver(I) reagents AgX
(XÀ =iodide, trifluoroacetate, triflate) with selenoethers
R2Se (R=Me, tBu) in a variety of solvents were investigat-
ed in relation with their use as precursors for Ag2Se nano-
materials. Different reaction conditions led to different re-
activities and afforded either molecular complexes or
metal selenide nanoparticles. The reactions leading to in
situ formation of the metal selenide nanoparticles were
then extended in the presence of commercial TiO2 (P25)
to prepare silver selenide–titania nanocomposites with dif-
ferent Ag/Ti ratios. These nanocomposites, well character-
ized by elemental analysis (Ag, Se), PXRD, TEM, BET, XPS
and UV/Vis studies, were investigated as photocatalysts
for the degradation of formic acid (FA) solution. The
talline phases at atmospheric pressure: the high-temperature
a-Ag2Se with a body-centered cubic structure and low-temper-
ature b-Ag2Se with an orthorhombic structure.[5] The a-Ag2Se is
a well-known superionic conductor that is useful as the solid
electrolyte in photo-chargeable secondary batteries.[4] The b-
Ag2Se, on the other hand, is a narrow band-gap semiconductor
which has been widely used as a photosensitizer in photo-
graphic films or thermochromic materials.[6] Selection of a suita-
ble synthetic route to these nanomaterials is crucial as their
properties may evolve during the preparation. Several meth-
ods have been employed to obtain different forms of Ag2Se
nanomaterials. Liu et al. synthesized monodispersed Ag2Se
nanoparticles (NPs) of 60–80 nm in length and 30–40 nm in
width by a hydrothermal reaction using AgNO3 and Na2SeSO3
in the presence of poly(vinyl pyrrolidone) (PVP) and KI at
1808C for 20 h.[7] On the other hand, Li et al. used bulk seleni-
um (Se) powder and silver (Ag) foil in a solvothermal process
(10 h at 1608C in methanol or ethanol in an autoclave) to
obtain films of Ag2Se dendrites of highly oriented (001) nano-
crystals.[8] Zhu et al. used a hot injection method employing
TOPSe (Se dissolved in tri-octylphosphine) to prepare Ag2Se
quantum dots (QDs).[9] Yao et al. prepared an Ag2Se aerogel by
an ion-exchange reaction of a monolithic CdSe wet gel, fol-
lowed by its supercritical drying.[10] Soft chemical routes such
as chemical vapor deposition (CVD), metal–organic decomposi-
tion (MOD), and the sol–gel process are preferred for the elab-
oration of nanomaterials due to the number of advantages
they offer.[11] These procedures need well-characterized single-
source precursors (SSPs) with certain properties.[12] However,
well-characterized metal–organic single-source precursors for
Ag2Se nanomaterials are scarce.[13] Very often, these reported
SSPs require high temperature to be converted into Ag2Se
nanomaterials. Recently, selenoethers have been shown as
a simple but effective source for obtaining selenium-contain-
ing chalcopyrite semiconducting materials. Several reports
have appeared recently employing either metal complexes
with these selenoethers as single-source precursors,[14] or reac-
tions of these selenoethers R2Se (R=Me, Et, tBu) with metal
halides/oxohalides/alkyls using the CVD technique,[15–17] to
obtain high-quality metal solenoid films. However, so far the
selenoethers have been used only to obtain thin films, requir-
ing high temperature. There is no report available on the use
xAg2Se-TiO2 nanocomposites
(x=0.01,
0.13
and
0.25 mol%) exhibited a much higher catalytic activity as
compared to P25, which is an established benchmark for
the photocatalysis under UV light, and retained a good
photocatalytic stability after recycling for several times.
Silver chalcogenides with their narrow band gaps (0.9, 0.15
and 0.67 eV for Ag2S, Ag2Se and Ag2Te, respectively) form an
extremely important class of semiconductors with applications
not only in microelectronics but also in thermoelectric, solid
electrolytes in secondary batteries and as quantum dots in bio-
medical imaging.[1–3] Among these, silver selenide shows many
[a] Dr. S. Mishra, D. Du, F. Dappozze, Dr. C. Guillard, Prof. S. Daniele
UniversitØ Lyon 1
IRCELYON, CNRS-UMR 5256
2 Avenue A. Einstein, 69626 Villeurbanne (France)
Fax: (+33)472445399
E-mail: shashank.mishra@ircelyon.univ-lyon1.fr
[b] D. Du, Prof. J. Zhang
Key Laboratory for Advanced Materials and Institute of Fine Chemicals
East China University of Science and Technology
130 Meilong Road, Shanghai 200237 (China)
[c] Dr. E. Jeanneau
UniversitØ Lyon 1
Centre de DiffractomØtrie Henri Longchambon
5 rue de La Doua, 69100 Villeurbanne (France)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under http://dx.doi.org/10.1002/
asia.201600157.
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of these selenoethers to obtain NPs, particularly at room tem-
perature.
silver iodide derivative 2 shows limited (dichloromethane) to
poor solubility (THF, toluene, hexane). It should be noted that
the coordination chemistry of silver with neutral selenide-con-
taining ligands remains least explored.[25]
Due to its low cost, non-toxicity, chemical stability, and high
activity, TiO2 has been widely investigated as one of the most
promising photocatalysts. However, an easy recombination of
the photogenerated electron/hole (eÀ/h+) reduces its catalytic
efficiency.[18] To enhance the photocatalytic performance, ef-
forts have been made to modify the titania by doping it with
metals or non-metallic elements.[19] A considerable enhance-
ment in the photocatalytic efficiency of TiO2 under visible light
has been reported by coupling TiO2 with another low bandgap
semiconductor such as CdS,[20] CdSe,[21] AgX[22] and Ag2S.[23] To
the best of our knowledge, there is no report on the photoca-
talysis of Ag2Se-TiO2 composites, although Meng et al. recently
reported the preparation of an Ag2Se-graphene/TiO2 compo-
site which showed a red-shifted absorption and better visible-
light photocatalytic activity for the degradation of rhodamine
B (RhB) than pure TiO2.[24] In this paper, we describe interesting
and divergent reactivities of selenoethers R2Se (R=Me, tBu)
with different silver reagents which not only lead to the forma-
tion and isolation of new molecular complexes as SSPs for
silver selenide nanomaterials but, more importantly, also result
in the direct synthesis of b-Ag2Se NPs at room temperature.
This facile method of the synthesis of narrow band-gap semi-
conducting b-Ag2Se NPs was then used to obtain nanocompo-
sites with TiO2 which showed much improved photocatalytic
activity compared to that of pure TiO2 (P25) for the degrada-
tion of formic acid (FA) under UV irradiation. The commercially
available P25 is an established benchmark for photocatalysis
under UV light, and there are very few reports available pre-
senting improvements on its photocatalytic activity.
The isolated colorless precursors 1–3 were characterized by
the elemental analysis, FT-IR, and NMR (1H, 13C, 77Se) spectros-
copy as well as single-crystal X-ray diffraction. The FT-IR spectra
of 1 and 3 (Figure S1 in the Supporting Information) exhibit
characteristic bands for the trifluoroacetate and tBu2Se/Me2Se
ligands. The trifluoroacetate ligand may bind to the metal
center in a number of fashions, and the asymmetric and sym-
metric vibrations of the CO2 group can indicate its binding
modes.[26] The appearance of only a single band at 1670 cmÀ1
for nas CO2 is consistent with one type of TFA ligand present in
1, whereas an overlap of the same absorption due to three dif-
ferent bonding modes of the TFA ligand results in a strong
broad band centered around 1680 cmÀ1 in 3 (Figure 1). The de-
rivative 2 shows characteristic peak(s) for the tBu2Se ligand
only, as metal-iodide stretching usually appears below
1
400 cmÀ1. As expected, the H NMR spectra of 1–3 in CDCl3/
CD2Cl2 are simple and show only a singlet at d=1.57–
1.95 ppm for the tBu/Me groups of the selenoethers.
Reactions of selenoethers R2Se (R=Me, tBu) with different
silver(I) reagents were explored in different solvents in relation
with their use as possible precursors for Ag2Se nanomaterials.
Addition of tBu2Se to AgTFA (where TFA=trifluoroacetate) at
room temperature in diethylether resulted in a gradual change
in the color of the solution, from colorless to light brown and
then to dark brown. Finally, black Ag2Se NPs were precipitated
after 30 min of the reaction. An intermediate molecular spe-
cies, [Ag(TFA)(tBu2Se)2] (1), could be trapped within few mi-
nutes of the reaction and characterized spectroscopically and
by X-ray crystallography. The same species was isolated if the
reaction was carried out in anhydrous ethanol or tetrahydrofur-
an (THF). However, on prolonged stirring of 30–60 min, the
THF solution first turned brown and finally black to give Ag2Se
NPs. With silver triflate (AgOTf), the selenoether tBu2Se afford-
ed Ag2Se NPs immediately at room temperature, irrespective
of the nature of the solvent used (hexane, diethylether, THF).
By contrast, the reactions of tBu2Se with silver iodide (in di-
chloromethane) or that of Me2Se with silver trifluoroacetate (in
THF) were straightforward and led to the isolation of the mo-
lecular complexes [Ag4I4(tBu2Se)4] (2) and [Ag5(TFA)5(Me2Se)4]
(3), respectively. Unlike the derivative 1 which turns gradually
black even when kept in the dark and under inert atmosphere,
apparently due to the formation of Ag2Se NPs, the precursors
2 and 3 are stable at room temperature. While precursors
1 and 3 are highly soluble in common organic solvents, the
Figure 1. (a)–(c) Perspective view of the molecular structures of (1)–(3) with
30% probability ellipsoids. H atoms on alkyl group are omitted for clarity. (d)
Extended structure of (3). Selected bond lengths [Š] and angles [8]: (1):
Ag1ÀO1 2.524(9), Ag1ÀO2 2.495(9), Ag1ÀSe1 2.588(1), O1-Ag1-O2 52.0(3),
Se1-Ag1-Se2 124.41(5), O1-Ag1-Se1 110.4(3). (2): I1ÀAg1i 2.8693(4), I1ÀAg2
2.9367(4), Ag1ÀAg2i 3.3609(5), Ag1ÀAg1i 3.1566(6), Ag1ÀI2i 2.8705(4),
Ag1···Ag2 3.0719(5), Ag1ÀSe1 2.6362(5), Ag2···Ag2i 3.2328(7), Ag2ÀI2
2.8714(4), I1-Ag1-Se1 98.01(1), I1i-Ag1-Se1 118.38(1), I1-Ag2-I2i 114.39(1), I1-
Ag1-I1i 109.98(1). (3): O1ÀAg1 2.574(16), O7ÀAg2 2.218(14), O9ÀAg4
2.444(12), Se1ÀAg1 2.696(3), Se2ÀAg3 2.541(3), Ag1···Ag5 3.138(2), Ag2···Ag4
3.188(2), O1-Ag1-O3 98.8(5), O5-Ag2-O7 145.5(6), O9-Ag4-O8 75.4(4), Se4-
Ag2-Se2 161.71(9), Se2-Ag4-Se1 119.63(9), O1-Ag1-Se1 86.6(3). Symmetry ele-
ments, (2): (i) Àx+1, y, Àz+1/2, (3): (i) xÀ1/2, Ày+1/2, zÀ1/2.
The monomeric [Ag(TFA)(tBu2Se)2] (1) and tetrameric
[Ag4I4(tBu2Se)4] (2), which crystallize in the triclinic space group
ꢀ
P1 (with R1 =0.074 and wR2 =0.156) and the monoclinic space
group C2/c (with R1 =0.025 and wR2 =0.045), respectively, have
the tBu2Se ligand bonded in a terminal fashion (Figure 1). In 1,
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the TFA group is bonded in a chelating manner to afford a 4-
coordinate silver center. Except for a small bond angle, O1-
Ag1-O2 52.0(3)8 (due to small bite angle of the TFA ligand),
other Se-Ag1-Se and Se-Ag1-O angles are in the range
110.4(3)–124.41(5)8, thus indicating a distorted tetrahedral en-
vironment around the silver center. The tetrameric structure of
2 has the Ag and I atoms alternating in a distorted cubane-like
AgI core where the tBu2Se ligands cap the Ag corners. Each
face is distorted due to argentophillic Ag···Ag interactions
(3.072–3.361 Š) that makes the average Ag···Ag atomic separa-
tion shorter than the average I···I atomic separation by more
than 1 Š. The AgÀSe bond length, 2.588(1)–2.646(5) Š, is con-
sistent with the literature value on terminal-bonded selenium-
containing ligands.[27] The 2D sheet structure of 3, which crys-
tallizes in the monoclinic space group P21/n (with R=0.116
and wR=0.178), is composed of the [Ag5(m3-h1,h1,h1-TFA)2(m-
h2,h1-TFA)(h1-TFA)2(m-Me2Se)4] building block. This structure can
be seen formally as the association between [Ag4(m-Me2Se)2(m3-
h1:h1:h1-TFA)2(h1-TFA)2] and [Ag(m-Me2Se)2(m-h2:h1-TFA)2] units
via m-Me2Se and one oxygen atom of m,h2:h1-TFA ligands (Fig-
ure S2a in the Supporting Information). The silver atoms and
bridging Me2Se ligands form the core 2D sheet, whereas the
TFA ligands point out from both sides and are perpendicular
to the length of this 2D sheet, as shown in Figure S2b in the
Supporting Information. While the pseudo-tetrahedral geome-
try around Ag1 and Ag2 has an O3Se environment due to coor-
dination with one terminal TFA, two bridging TFA (m-h2,h1- and
m3-h1,h1,h1-), and one bridging Me2Se, the other silver atoms
are coordinated with either one h2-TFA and two m-Me2Se (Ag3)
or two h1-TFA and two m-Me2Se (Ag4 and Ag5) to have an
O2Se2 environment. Besides bridging trifluoroacetate and sele-
noether ligands, the silver atoms are also connected through
short Ag···Ag interactions of 3.14–3.19 Š, a range which is con-
sistent with the reported values on argentophilic interac-
tions.[28] The AgÀO bond lengths, 2.22–2.78 Š for bridging and
2.24 Š for the terminal trifluoroacetate ligand, compare well
with 1. The AgÀm-Se bond distances (2.51–2.70 Š) are slightly
longer than those found in 1 and 2 where the selenoether
ligand is terminal and has a tBu group. The O-Ag-O, Se-Ag-Se
and Se-Ag1-O angles in 3 are in the range 75.4–161.78, indicat-
ing a distorted tetrahedral environment around the silver
center.
Figure 2. XRD pattern of Ag2Se NPs obtained from the reaction of AgTFA
and tBu2Se at room temperature. Inset: SEM image of Ag2Se NPs.
are also available. This difference in behavior among sele-
noethers can be attributed to a decomposition path via b-hy-
drogen elimination as well as a weaker CÀSe bond in tBu2Se.
Indeed, the CÀSe bond becomes weaker with heavier alkyl li-
gands in the series: Me2Se>Et2Se>iPr2Se>tBu2Se.[30] Among
silver reagents, the trifluoroacetate (TFA) and triflate (OTf)
ligand appear to be much more labile than the iodide ligand.
Although the reaction of Me2Se with a silver salt did not give
Ag2Se
NPs
at
room
temperature,
its
complex
[Ag5(TFA)5(Me2Se)4] (3), which was isolated in excellent yield
and is stable at room temperature, can be used as a precursor
for the synthesis of silver selenide nanomaterials. The TG curve
of 3, recorded under N2 atmosphere, shows a single-step de-
composition in the temperature range 120–3158C with a resid-
ual mass (47.2%) that is consistent with the formation of
Ag2Se (47.8%) as the end product (Figure 3 inset). According
to the TGA data, 3 was decomposed in 1-octadecene at 3158C
for 1 h under argon atmosphere. The XRD pattern of the as-
prepared black powder, obtained after the usual work-up, ex-
hibited well-crystallized b-Ag2Se NPs (JCPDS file no. 00-024-
1041, Figure 3), although a small amount of silver NPs as con-
The XRD pattern of the as-prepared black powder obtained
from the reaction of AgTFA (or AgOTf) and tBu2Se at room
temperature (Figure 2 and Figure S3 in the Supporting Infor-
mation) exhibits well-crystallized b-Ag2Se NPs (JCPDS: 01-078-
7512). No other phase was observed. An average nanocrystal-
lite size of 35 nm was calculated from the Debye–Scherrer for-
mula. The direct synthesis of b-Ag2Se NPs at room temperature
is important because this phase is stable only at low tempera-
ture. The other observation is that only tBu2Se gives metal sel-
enide materials directly. Other selenoethers such as Et2Se and
Me2Se usually result in isolable molecular complexes which are
stable at room (and even higher) temperature.[29] tBu2Se[17] has
previously been used as a source of selenium for the deposi-
tion of metal selenide thin films by chemical vapor deposi-
tione, though few reports on the use of Et2Se[16] and Me2Se[15]
Figure 3. XRD pattern of Ag2Se NPs obtained from the decomposition of 3
at 3158C. Inset: TGA curve of 3 recorded under N2 atmosphere.
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tamination was also present (JCPDS file no. 04-001-2617, ~5%
as quantified using the reference intensity ratio (RIR) method).
As mentioned in the introduction section, only few single-
source precursors for Ag2Se nanomaterials are known in the lit-
erature.[13]
low, the BET surface area of the composite increased signifi-
cantly as compared to the pure TiO2. However, when the
mol% of Ag2Se increased to 15%, the surface area decreased
to 14.8 m2 gÀ1 due to the aggregation of Ag2Se on the TiO2.
The TEM images show that the composite is composed of
well-crystalline Ag2Se and TiO2 NPs (Figure 4 inset and Fig-
ure S6 in the Supporting Information). X-ray photoelectron
spectroscopy (Figure 4 and Figure S7 in the Supporting Infor-
mation) was used to analyze the valence states of the ele-
ments present on Ag2Se-TiO2 nanocomposites. The binding en-
ergies of Ag 3d5/2 and Se 3d were found to be 368.1 and
53.5 eV, respectively (Figure 4b and 4c), indicating the pres-
ence of these two elements as Ag+ and Se2À.[31] The Ti 2p3/2
and Ti 2p1/2 peaks at 458.5 eV and 464.4 eV, on the other hand,
indicated the presence of Ti4+ in the form of TiO2 (Figure S7 in
the Supporting Information). The Tauc plots for the determina-
tion of the band gap of the Ag2Se-TiO2 nanocomposites with
different% of Ag2Se are shown in Figure S8 in the Supporting
Information. TiO2 shows the characteristic spectrum indicating
a band gap of about 3.15 eV. However, as the mol% of Ag2Se
increased, the band edge was red-shifted considerably (band
gap in the region 3.1–2.9 eV). The increase in absorption is at-
tributed to the narrow band gap of Ag2Se NPs. With an in-
creased absorption in the visible wavelength, more electron–
hole pairs are expected to be generated in the composite
under UV/Vis light.
To prepare Ag2Se-TiO2 nanocomposites, the reaction of
AgTFA and tBu2Se was carried out at room temperature in the
presence of different amounts of commercial TiO2 (P25). After
general workup (see Figure S4 in the Supporting Information
for a schematic representation), the precipitated powders,
varying in color from off-white to black, were isolated by cen-
trifugation, washed twice with ethanol, and dried at room tem-
perature for 24 h. Elemental analysis of these Ag2Se-TiO2 sam-
ples showed 0.36, 0.68, 11.98 and 25.90% Ag, corresponding
to 0.13, 0.25, 5 and 15 mol% Ag2Se, respectively (Table S1 in
the Supporting Information). The XRD patterns of these sam-
ples show characteristic peaks of Ag2Se (JCPDS file no. 01-078-
7512) and anatase TiO2 (JCPDS file no. 01-021-1272; Figure S5
in the Supporting Information). These samples also had the
rutile TiO2 (JCPDS file no. 04-003-0648) as a minor phase,
which is common in the commercial TiO2 (P25). As the percent-
age of TiO2 increased, the peaks for the anatase phase became
more prominent. Next, Brunauer–Emmett–Teller (BET) measure-
ments were carried out on commercial TiO2 (P25) and Ag2Se-
TiO2 nanocomposites to investigate their surface characteristic
(Figure 4a). The obtained isotherms can be identified as
type IV which have a small hysteresis loop observed under
a relative pressure of 0.85 to 1.00. The surface areas, total pore
volumes, and average pore diameters are given in Table S2 in
the Supporting Information. When the mol% of Ag2Se was
Using the above-mentioned Ag2Se-TiO2 nanocomposites as
photocatalysts, photodegradation experiments of formic acid
(FA) were performed under UV and visible light by monitoring
with HPLC. All the samples were stirred in the dark for 30 min
to reach the adsorption–desorption equilibrium. There was no
apparent difference in the adsorption ability of formic acid
among the different samples. Pure Ag2Se showed almost no
activity while pure TiO2 (P25) had a moderate photoactivity
under our experimental conditions (Figure 5). The photocata-
lytic activity of Ag2Se-TiO2 composites depended strongly on
the percentage of Ag2Se. Generally speaking, the composites
with a low content of Ag2Se (0.01, 0.13 and 0.25%) showed
a better UV-induced catalytic activity than TiO2. In particular,
Figure 4. (a) BET isotherms of Ag2Se-TiO2 nanocomposites with different per-
centages of Ag2Se. Inset: Representative TEM image of 0.13% Ag2Se-TiO2
nanocomposite. XPS spectra showing binding energies of Ag 3d5/2 (b) and
Se 3d (c) at 368.1 and 53.5 eV, respectively.
Figure 5. Photodegradation of FA using TiO2, Ag2Se and Ag2Se-TiO2 with dif-
ferent percentages of Ag2Se under UV light.
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the degradation rate of FA increased as the percentage of
Ag2Se changed from 0.01 mol% to 0.13 mol% but then de-
creased significantly upon increasing the Ag2Se content from
0.25 mol% to 15 mol%. The nanocomposite with 0.13% Ag2Se
provided the best catalytic activity and photodegraded formic
acid (FA) completely within 30 min under UV-light irradiation.
Although these preliminary results do not provide any direct
proof of an improved separation of eÀ–h+ pairs as proposed
previously for the improved efficiency of Ag2S-TiO2 compo-
sites,[23] the better activity of the Ag2Se-TiO2 nanocomposites
(as compared to TiO2 alone) correlates with their light absorp-
tion property and high surface area (Figures S8 and S9 in the
Supporting Information).[32] The Ag2Se-TiO2 nanocomposite
generates eÀ–h+ pairs upon irradiation with UV/Vis light (Eq.
S1 and S2 in the Supporting Information). While the electrons
react with adsorbed oxygen molecules to form superoxide rad-
tion indicates that the Ag2Se NPs are formed via this inter-
mediate molecular species. This facile and direct synthetic
method of b-Ag2Se was then extended in the presence of
commercial TiO2 (P25) to prepare Ag2Se-TiO2 nanocomposites
with different Ag/Ti ratios. The as-prepared Ag2Se-TiO2 nano-
composites exhibited excellent photocatalytic activity for the
degradation of formic acid under UV light irradiation, the best
results being for 0.13% Ag2Se-TiO2 nanocomposite, which de-
graded 50 ppm of formic acid in 30 min.
Experimental Section
Details of the general experimental procedures and a scheme for
the synthesis of precursors and Ag2Se-TiO2 composites, X-ray crys-
tallography and structure refinement, FT-IR spectra, perspective
views on the extended structure of 3, powder XRD, XPS, diffuse re-
flectance spectra of Ag2Se and Ag2Se-TiO2 composites, experimen-
tal setup and graphical plots of photodegradation of formic acid
by Ag2Se-TiO2 catalysts under UV/Vis light, a correlation diagram
between the rate of FA degradation and the specific surface area
are given in the Supporting Information. CCDC 1442413 (1),
1442414 (2), and 1442415 (3) contain the supplementary crystallo-
graphic data for this paper. These data are provided free of charge
by The Cambridge Crystallographic Data Centre.
CÀ
icals (O2 ), the positively charged holes (h+) react with
C
HCOOH dissolved in H2O to generate HCOO radical (Eq. S3
and S4 in the Supporting Information). Formic acid is then con-
CÀ
C
verted to CO2 whereas O2 reacts with H+ to generate HO2
(Eq. S5 and S6 in the Supporting Information). Previous studies
have reported that formic acid degrades catalytically by first
forming formate and then CO2.[33] As the mol% of Ag2Se in-
creased, a large amount of Ag2Se distributed over the surface
of TiO2, leading to inefficient separation of electrons and holes.
Besides, because the surface area decreased from 61.3
(0.13 mol% Ag2Se) to 14.8 m2 gÀ1 (15 mol% Ag2Se), less active
sites were available. With high density of Ag2Se, the nanocom-
posites became darker which prevented the TiO2 to utilize the
UV light efficiently. This resulted in the generation of fewer
electrons and holes. All the Ag2Se-TiO2 composites studied
here had no activity under visible light (Figure S10 in the Sup-
porting Information). XPS studies on the 0.13% Ag2Se-TiO2
sample were also carried out to know whether there is any
change in the surface structure of the catalyst during photode-
gradation. A comparison of data shown in Figure S7a and S7b
in the Supporting Information shows that the amount of Ti3+
observed at 457.1 eV is actually increased after the photode-
gradation experiments. According to Wang et al.[34] and Fujishi-
ma et al.,[35] Ti4+ ions on TiO2 surfaces are converted into Ti3+
ions by UV light. The typical XPS peaks of Ag (Figure S7c and
S7d in the Supporting Information) did not show any differ-
ence before and after the photodegradation, indicating that
the as-prepared photocatalyst can be potentially recycled. This
was further confirmed by determining the photocatalytic activ-
ity of the recycled catalysts which showed good activity even
after recycling for 4 times (Figure S11 in the Supporting Infor-
mation). Detailed studies are currently underway to under-
stand further the synergistic effect between Ag2Se and TiO2
components for the aforesaid photocatalytic efficiency and to
establish a possible mechanism for it.
Acknowledgements
Authors are thankful to Dr. L. Cardenas (XPS), L. Burel (TEM), Y.
Aizac (PXRD), P. Mascunan & N. Cristin (elemental analyses) of
IRCELYON and O. Boyron and M. Taam (TGA) of C2P2, UCBL.
Keywords: formic acid · molecular precursor · photocatalysis ·
selenoether · silver selenide · titania
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Manuscript received: February 5, 2016
Revised: March 24, 2016
Accepted Article published: April 28, 2016
Final Article published: May 19, 2016
Chem. Asian J. 2016, 11, 1658 – 1663
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1663
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim