Heteroarchitectured Ag-Bi2O3-ZnO as a bifunctional nanomaterial

Article abstract of DOI:10.1039/c5ra27882a

Heteroarchitectured Ag-Bi₂O₃-ZnO was synthesized through a simple, cost effective, template free photodeposition-hydrothermal method. This composite material represents a prospective new class of novel catalysts wherein the improved light absorption behavior of a metal doped coupled oxide, Ag-Bi₂O₃-ZnO is anticipated to enhance with the observed benefits of a coupled system of Bi₂O₃-ZnO. Characterization of Ag-Bi₂O₃-ZnO revealed interesting optical and morphological characteristics. XRD patterns indicated that well-crystallized α-Bi₂O₃ was formed on the hexagonal wurtzite phase of ZnO along with Ag in the process. The HR-SEM images revealed the formation of organized intercrossed sheets and peanut shell like structure, which were composed of nanosheets and nanoparticles respectively. Elemental color mapping confirms the homogeneity of Zn, O, Ag and Bi in the catalyst. Ag-Bi₂O₃-ZnO has enhanced absorption in the UV and visible region when compared to Bi₂O₃ and ZnO. This catalyst exhibited enhanced photocatalytic activity for the degradation of Acid Red 1 (AR 1), Evens Blue (EB) and Acid Violet 7 (AV 7) under natural sunlight far exceeding those of the Bi₂O₃, Ag-ZnO, Ag-Bi₂O₃ and ZnO systems at neutral pH. Ag-Bi₂O₃-ZnO was found to be stable and reusable without appreciable loss of photocatalytic activity for up to five runs. Ag-Bi₂O₃-ZnO shows enhanced electrochemical methanol oxidation when compared to Ag-Bi₂O₃, Bi₂O₃-ZnO, Ag-ZnO and ZnO. Enhancement in methanol oxidation current demonstrates its high potential as an anode catalyst in direct methanol fuel cells.

Full text of DOI:10.1039/c5ra27882a

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Page 1 of 36
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DOI: 10.1039/C5RA27882A
Heteroarchitectured Ag-Bi O -ZnO as a bifunctional nanomaterial
2
3
a,d
b
Subramanian Balachandran, Natarajan Prakash and Meenakshisundaram
Swaminathanc*
a
Department of Chemistry, Annamalai University, Annamalainagar– 608 002, India
b
Graduate School of Science and Technology, Shizuoka University, 3ꢀ5ꢀ1 Johoku, Nakaꢀku,
Hamamatsu 432ꢀ8011, Japan
c
Nanomaterials Laboratory, International Research Centre, Kalasalingam University,
Krishnankoilꢀ 626126, Tamil Nadu, India
d
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics,
Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2,
Beijing 100190, P. R. China
∗Corresponding author. Tel./fax: +91ꢀ4144ꢀ220572.
Eꢀmail address: chemres50@gmail.com (M. Swaminathan)

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Heteroarchitectured AgꢀBi O ꢀZnO was synthesized through simple, cost effective, template free
2
3
photodepositionꢀhydrothermal method. This composite material represents a prospective new
class of novel catalysts wherein the improved light absorption behavior of a metal doped coupled
oxide, AgꢀBi O ꢀZnO is anticipated to enhance with the observed benefits of coupled system of
2
3
Bi O ꢀZnO. Characterization of AgꢀBi O ꢀZnO revealed interesting optical and morphological
2
3
2
3
characteristics. XRD patterns indicated that wellꢀcrystallized αꢀBi O was formed on the
2
3
hexagonal wurtzite phase of ZnO along with Ag in the process. The HRꢀSEM images revealed
the formation of organized intercrossed sheets and peanut shell like structure, which were
composed of nanosheets and nanoparticles respectively. Elemental color mapping confirms the
homogeneity of Zn, O, Ag and Bi in the catalyst. AgꢀBi O ꢀZnO has enhanced absorption in the
2
3
UV and visible region when compared to Bi O and ZnO. This catalyst exhibited enhanced
2
3
photocatalytic activity for the degradation of Acid Red 1 (AR 1), Evens Blue (EB) and Acid
Violet 7 (AV 7) under natural sun light far exceeding those of the Bi O , AgꢀZnO, AgꢀBi O and
2
3
2
3
ZnO systems at neutral pH. AgꢀBi O ꢀZnO was found to be stable and reusable without
2
3
appreciable loss of photocatalytic activity up to five runs. AgꢀBi O ꢀZnO shows enhanced
2
3
electrochemical methanol oxidation when compared to AgꢀBi O , Bi O ꢀZnO, AgꢀZnO and
2
3
2
3
ZnO. Enhancement in methanol oxidation current demonstrates its high potential as anode
catalyst in direct methanol fuel cells.

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Introduction
The design of efficient multifunctional catalysts will be a key to the present problems of
environmental pollution and energy crisis. Semiconductor oxides afford a solution to these
problems. ZnO, a semiconductor photocatalyst, has fascinated intense research awareness in a
broad range of applications, such as organic pollutants degradation, water splitting, and
photovoltaic devices. ZnO was reported to be the most active semiconductor for the solar
1
degradation of Acid Brown 14 among TiO , ZnO, SnO , ZrO , αꢀFe O , WO and CdS. The
2
2
2
2
3
3
optical properties of ZnO can be tuned by controlling not only the size and shape of the
2
–5
nanocrystals but also the type and concentration of appropriate doping atoms.
Bismuth oxide (Bi O ) is an important pꢀtype semiconductor with four main
2
3
crystallographic polymorphs denoted by αꢀ, βꢀ, γꢀ, and δꢀBi O . Bi O with a band gap of 2.8 eV
2
3
2
3
6
–10
in the visible range has been widely used for various applications,
and accounts for its ability
ꢀ
•
to generate highly reactive species, such as O2
• and OH radicals from water for mineralization
11, 12
of pollutants.
But, the singleꢀcomponent UV light sensitive photocatalysts for environmental
purification applications can only utilize photons below a wavelength of 450 nm, which
represents only part of the solar light spectrum. One effective way to induce solar light
absorption of a semiconductor is to couple it with another semiconductor having appropriate
13
band structure. A large number of binary coupled semiconductor materials such as TiO SiO ,
2
−
2
1
1
14
15
16
Bi O ꢀZnO, BiVO4 − ZnO, Pr O ꢀZnO, Bi S –ZnO had been reported with enhanced
2
3
6
11
2 3
photocatalytic activity. The surface adaption of semiconductors with noble metals (Ag, Pt, Au)
has fascinated considerable attention especially in heterogeneous photocatalysis. This can reduce
the recombination of the photogenerated electron/hole pairs and lengthen their lifetime through

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the conduction band electron trapping. Ag, as electron sinks, is well known due to the Schottky
1
7
18
barrier at the metalꢀsemiconductor interface. Nanocomposites such as TiO ꢀSiO ꢀAg, TiO ꢀ
2
2
2
1
9
20
21
22
CarbonꢀAg, TiO ꢀgrapheneꢀAg, TiO ꢀAgBrꢀAg, and TiO ꢀGrapheneꢀFe O
4
have been
2
2
2
3
synthesized. The development of new visible lightꢀactive photocatalysts is one of the most
2
3
important topics in photocatalysis research.
Methanol fuel cells (MFCs) are considered as significant power resource due to their low
2
4ꢀ28
emissions and high energy efficiencies.
However, the catalytic activity of high expensive of
Pt and Ptꢀbased electrocatalysts in MFCs diminish gradually due to chemisorption of
carbonaceous intermediate products on these catalysts and this is the main barrier to the
2
9–31
commercialization of direct methanol fuel cell (DMFC) technology.
Thus, it is necessary to
explore nonꢀPt catalysts to reduce costs and to improve the efficiency of the fuel cells.
A significant challenge in the development of DMFC technology is the need for highly active
anode catalysts for the methanol oxidation reaction.
Fabrication of heterostructred materials for multiple applications without using template
or catalyst in a cost effective, simple methodology is a highly challenging job for chemists and
material scientists. Herein, we report the preparation of AgꢀBi O ꢀZnO photocatalyst using
2
3
simple photodepositionꢀhydrothermal method. The electrocatalytic activity of the catalyst for the
electroꢀoxidation of methanol in alkaline media has been investigated.
EXPERIMENTS
Reagents
Analar grade zinc nitrate hexahydrate, oxalic acid dihydrate, bismuth nitrate pentahydrate,
silver nitrate, nitric acid, sodium hydroxide and absolute Methanol were obtained from Himedia
chemicals and used as such. Acid Red 1 (molecular formula C H N Na O S and molecular
1
8
13
3
2
8 2

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weight: 509.42), Evens Blue (molecular formula C H N Na O S and molecular weight:
3
4
24
6
4
14 4
9
60.8) and Acid Violet 7 (molecular formula C H N Na O S and molecular weight: 566.47)
20 16 4 2 9 2
2
from Colour Chem, Pondicherry and ZnO (surface area 5 m /g, particle size 4.80 ꢁm) from
Merck chemicals were used as received. Deionized distilled water was employed throughout
experiments.
Preparation of Ag-Bi O -ZnO
2
3
3
+
0
.97 g Bi(NO ) .5H O was dissolved in 10 mL 1.12 M nitric acid to avoid hydrolyzation of Bi
3 3 2
ions and before making up the solution to 100 mL, its pH was adjusted to 11 by the addition of
small amount of 0.2M NaOH. Then 2.0 mL of 0.1 M AgNO was quickly added to the above
3
solution. This solution was mixed with 100 mL solution of 11.90 g of Zn (NO ) .6H O (0.4 M)
3
2
2
in deionized water. Finally, 100 ml solution of oxalic acid dihydrate in deionized water (0.6 M)
was introduced into the above solution drop wise with stirring and stirring was continued for 2 h
to ensure complete precipitation of zinc oxalate and bismuth oxalate. The resulting suspension
was stirred for 1.0 h and then UV irradiation was carried out using four parallel medium pressure
mercury lamps emitting 365 nm wavelength for 3 h. The reaction mixture was transferred to the
Teflon lined stainless steel autoclave for the hydrothermal treatment at 120 °C for 12 h. During
the process the pressure was maintained at 18 psi. The final products were filtered, washed with
deionized water and ethanol several times, and dried at 80 °C for 12 h in an oven. The mixed
◦
precipitate was calcined in air at 450 C for 12 h, to get AgꢀBi O ꢀZnO. The preparation process
2
3
is given in scheme 1. This catalyst contains 9% Ag, 11.8% Bi O and 80.2% ZnO. Catalysts with
2
3
different percentages of Ag were prepared by a similar procedure with appropriate amount of
AgNO solution.
3

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Characterization
Xꢀray diffraction (XRD) patterns were recorded with a Siemens D5005 diffractometer
using Cu Kα (k=0.151 418 nm) radiation. Maximum peak positions were compared with the
standard files to identity the crystalline phase. The surface morphology of the AgꢀBi O ꢀZnO
2
3
was studied using a field emission scanning electron microscope (FEꢀSEM) (Model ULTRAꢀ55).
EDS analysis was performed on gold coated samples with FEꢀSEM (Model ULTRAꢀ55).
HRꢀTEM images were taken from 200 kV Ultra High Resolution Transmission Electron
Microscope (JEOLꢀ2010) having high resolution optical microscope (Leica microscope). Diffuse
reflectance spectra were recorded with Shimadzu UVꢀ2450. Photoluminescence spectra of the
samples were recorded on a Perkin Elmer LS 55 fluorescence spectrometer. The excitation
wavelength used to record PL was 320 nm. UV absorbance measurements were taken using
HitachiꢀUꢀ2001 spectrometer.
Photocatalytic experiments
All photocatalytic experiments were carried out under similar conditions on sunny days
of April–May 2013 between 11 am and 2 pm. An open borosilicate glass tube of 50 mL capacity,
40 cm height and 20 mm diameter was used as the reaction vessel. 50 mL of dye solution (5 ×
−
4
1
0 M) with 200 mg of AgꢀBi O ꢀZnO was magnetically stirred in the dark for 30 min to attain
2
3
adsorption–desorption equilibrium between the dye and AgꢀBi O ꢀZnO. After dark adsorption
2
3
the first sample was taken. Solar irradiation was carried out in the openꢀair condition. The
−
1
suspension was continuously aerated (airflow rate = 8.1 mL s ) by a pump to provide oxygen
and for the complete mixing of reaction solution. During the illumination time no volatility of the
solvent was observed. At specific time intervals, 2 mL of the sample was withdrawn and

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centrifuged to separate the catalyst. 1 mL of the centrifugate was diluted to 10 mL and its
absorbance was measured at 306, 314 and 285 nm for AR 1, EB and AV 7dyes respectively. The
absorbance at 306, 314 and 285 nm represent the aromatic content of AR 1, EB and AV 7
respectively and its decrease indicates extent of dye degradation. Solar light intensity was
measured for every 30 min and the average light intensity over the duration of each experiment
was calculated. The sensor was always set in the position of maximum intensity. The intensity of
solar light was measured using LT Lutron LX–10/A Digital Lux meter and the intensity was
1250x100 ± 100 lux. The intensity was nearly constant during the experiments.
Electrochemical studies
Cyclic voltammetry (CV) was performed with a CHI660 electrochemical workstation
CH Instruments, USA). Cyclic voltammetry was carried out with three electrodes, Ag/AgCl
(
electrode as the reference electrode, a counter electrode (Platinum wire) and unmodified or
modified glassy carbon electrode (GCE) as the working electrode. GCEs were polished before
the experiments in sequence with 1, 0.30 and 0.05 micron aluminium/water slurry on micro cloth
pads, followed by careful cleaning in 1:1 HNO –H O (v/v), ethanol, and water via ultraꢀ
3
2
sonication. The working electrode was prepared by spraying the catalysts dispersed in isopropyl
ꢀ1
alcohol. Cyclic voltammograms were obtained between +1.0 and ꢀ0.1 V at a scan rate of 50 mVs .
All potentials are reported with respect to the reversible hydrogen electrode. The CO stripping
voltammetry curves were obtained after adsorption of CO on catalysts at ꢀ0.2 V. To observe
methanol electroꢀoxidation, 0.5 M methanol solution in 0.5 M NaOH was used.

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Results and discussion
Preliminary study on the degradation of three azo dyes AR 1, EB and AV 7 with different
wt % of Ag in AgꢀBi O ꢀZnO catalysts was carried out. The percentages of AR1 degradation
2
3
with 3, 6, 9, 12 and 15% of Ag were found to be 67, 70, 75, 72 and 69% respectively for 60
minutes irradiation and the percentage of error is +1, or ꢀ1 in the consecutive measurements. The
maximum efficiency was observed with 9 wt% Ag loaded catalyst. Similar results were obtained
with EB and AV 7 degradation. Hence this 9 wt% Ag loaded Bi O ꢀZnO was characterized and
2
3
used for further experiments. Fig. 1 (a), (b), (c) and (d), show the typical Xꢀray diffraction
pattern of the prepared ZnO, 3wt%, 6wt%, 9wt% of Ag in Bi O ꢀZnO nanocatalysts respectively.
2
3
The 2θ values of prepared ZnO at 31.77, 34.49, 36.24, 56.60, 62.85, 66.38, 67.94, 69.08, 72.50
and 76.93 correspond to (1 0 0), (0 0 2), (1 0 1), (1 1 0), (1 0 3), (2 2 0), (1 1 2), (2 0 1), (0 0 4)
and (2 0 2) planes of wurtzite ZnO (Fig. 1a). The hexagonal wurtzite structure of ZnO consists of
alternating planes of tetrahedrally coordinated oxygen and zinc atoms arranged along the cꢀaxis.
This anisotropy results in the spontaneous polarization of certain planes, e.g., {001} or {101},
3
1
whereas other planes are nonpolar, e.g., {100}. The new peaks observed at 27.9, 30.11, 33.7,
4
6.6, 47.1 correspond to (121), (012), (ꢀ122), (041), (ꢀ104) planes of αꢀBi O (JCPDS # 71ꢀ
2
3
3
2
2
274) in Bi O ꢀZnO nanocatalysts (Fig. 1b, c and d). The crystallographic phase of the
2
3
prepared AgꢀBi O ꢀZnO is confirmed as monoclinic αꢀBi O form present in hexagonal wurtzite
2
3
2
3
ZnO base material. The relatively high intensity of the (101) peak is indicative of anisotropic
growth and implies a preferred orientation of the crystallites. The peaks at 2θ =38.2 and 46.23°
correspond to (111) and (200) planes of faceꢀcenteredꢀcubic (fcc) lattice of metallic Ag (JCPDS
3
3
file no. 04ꢀ0783). The increase in the concentration of Ag from 3wt% to 9wt% in AgꢀBi O ꢀ
2
3
ZnO increases the intensity of Ag peaks at 2θ =38.2 and 46.23°.

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The Field emission scanning electron microscope (FEꢀSEM) images of 9 wt%
AgꢀBi O ꢀZnO at three different magnifications are shown in Fig. 2. FEꢀSEM images show that
2
3
AgꢀBi O ꢀZnO has an ordered mixture of heterostructured intercrossed micro sheets and nano
2
3
particles (Fig. 2aꢀc) and the size of the particles are in the range of 20ꢀ35 nm (Fig. 2c). The
nanoparticles are highlighted in fig. 2a by green circle. The hydrothermal method reported here
is a simple process without rigorous conditions and hence it is a lowꢀcost and convenient method
to prepare nano sheet shaped AgꢀBi O ꢀZnO.
2
3
The surface morphology was further investigated by HRꢀSEM images (Fig. 3aꢀc). These
images confirm that AgꢀBi O ꢀZnO is composed of nanoparticles and nanosheets. During the
2
3
hydrothermal process nanoparticles are selfꢀassembled to form a sheet like structures (Fig. 3).
Nanosheets are attached with other nanosheets and give an intercrossed sheet like structure (Fig.
3c). This heterosturcture having a large number of cavities on surfaces may enhance the dye
adsorption on AgꢀBi O ꢀZnO and favor for active dye degradation.
2
3
To confirm the distribution of Zn, O, Bi and Ag in the catalyst, elemental mapping of
FEꢀSEM was carried out. Fig. 4a, b, c and d show the elemental mapping for zinc, oxygen,
bismuth and silver respectively. It is evident from the Fig.4a and b that Zn and O are higher in
density. There is a homogenous distribution of Bi and Ag at lower concentration (Fig. 4c and d).
Thus elemental mapping shows that the catalyst is composed of Zn, O, Bi and Ag. This also
indicates the purity of the catalyst AgꢀBi O ꢀZnO.
2
3
Typical TEM images of the AgꢀBi O ꢀZnO are shown in Fig. 5aꢀc. This exhibits
2
3
assembled nanoparticles with a large number of sheets. TEM images show a sheet with sphere
like structure of irregular morphology. Fig. 5d shows the highꢀresolution TEM image of the

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AgꢀBi O ꢀZnO. The lattice spacing of 0.328 nm corresponds to the interlayer spacing of the
2
3
(
012) plane of the αꢀBi O phase and lattice spacing of 0.256 nm is for (101) planes of wurtzite
2 3
3
3
ZnO. The lattice fringes with 0.236 nm spacing were assigned to the Ag (111) plane. The
results match exactly with XRD pattern discussed earlier.
In order to find out the presence of elements and to determine their valance states in Agꢀ
Bi O ꢀZnO, XPS study was carried out. Survey spectrum in Fig. 6a shows the presence of Zn, O
2
3
Bi and Ag. Carbon 1s peaks of the AgꢀBi O ꢀZnO are ascribed to adventitious hydrocarbon from
2
3
XPS instrument itself. In Fig. 6b, the O1s profile is asymmetric and can be fitted to two
symmetrical peaks α and β locating at 530.2 and 532.2 eV, respectively, indicating two different
kinds of O species in the sample. The peaks α and β should be associated with the lattice oxygen
3
4
(
O ) of ZnO and chemisorbed oxygen (O ) respectively. Fig. 6c presents the XPS spectra of
L
H
Zn 2p, and the peak positions of Zn 2p1/2 and Zn 2p3/2 locate at 1045.1 eV and 1021.9 eV.
Comparing the peak positions to those in the Handbook of X-Ray Photoelectron Spectroscopy,
2
+
we can conclude that: (i) Zn is in the state of Zn and (ii) the whole XPS spectra have a
3
5
downward shift of 0.5 eV since the standard peak position of Zn 2p3/2 is at 1022.4 eV. Two
signals ascribed to Bi 4f7/2 and Bi 4f5/2 at binding energies of 158.6 and 164.1 eV are observed in
Fig. 6d. There are two peaks locating at 368.4 and 374.5 eV, which are attributed to Ag 3d5/2 and
Ag 3d , respectively (Fig. 6e). According to Zhang et al, these two peaks can be attributed to
3
/2
0
36
metallic Ag (Ag ). The binding energy peaks of Ag 3d for the Ag/ZnO sample shifts upward
5
/2
when compared with the corresponding value of the synthesized pure metallic Ag (Binding
0
Energy value of Ag is 368.2).The shift must be due to the strong interaction between Ag and
ZnO.

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Optical Studies: The UV − visible diffuse reflectance spectra of the AgꢀBi O ꢀZnO
2
3
nanocomposite and ZnO were measured to determine their light absorption characteristics (Fig.
). The wavelength distribution of the absorbed light is a significant property of photocatalysts.
AgꢀBi O ꢀZnO nanocomposite has higher UV and visible light absorbance, as indicated by the
7
2
3
diffuse reflectance spectrum in absorption mode (Fig.7b). The AgꢀBi O ꢀZnO catalyst shows the
2
3
enhanced absorption in the visible region. The extended absorption of the AgꢀBi O ꢀZnO
2
3
nanocomposite in the visible region, is due to the typical surface Plasmon band exhibited by the
3
7
AgNPs.
Photoluminescence (PL) measurements were performed to determine the charge
recombination efficiency of the AgꢀBi O ꢀZnO nanocomposite and ZnO because the
2
3
photocatalytic activity is related closely to the recombination rate of photoexcited electrons and
holes. Fig. 8 shows the PL spectra of the AgꢀBi O ꢀZnO (Fig. 8b) nanocomposite and ZnO (Fig.
2
3
8a). Both spectra mainly consist of two emission bands: a band at 416 nm (2.98 eV) and a blueꢀ
green band at 485 nm (2.56 eV). The strong UV emission at 416 nm corresponds to the electronꢀ
3
8ꢀ 41
hole recombination.
Reduction of PL intensity at 416 nm ofAgꢀBi O ꢀZnO when compared
2 3
to prepared ZnO indicates the suppression of recombination of the photogenerated electron–hole
pair. The emission intensity of the AgꢀBi O ꢀZnO nanocomposite is lower than that of ZnO,
2
3
suggesting that the anchoring of Ag, and Bi O quench the fluorescence from the ZnO
2
3
nanoparticles and prolong the electronꢀhole pair lifetime enhancing the photocatalytic activity of
AgꢀBi O ꢀZnO.
2
3
Photocatalytic Activity of the heterostructured Ag-Bi O -ZnO
2
3
Photocatalytic degradation of Acid Red 1 (AR 1) under different conditions is shown in
Fig. 9. Dye is resistant to selfꢀphotolysis and for the same experiment with AgꢀBi O ꢀZnO in
2
3

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dark, a small decrease (8%) in dye concentration was observed due to the adsorption of dye on
the catalyst. AR 1 undergoes 98.6% degradation in the presence of AgꢀBi O ꢀZnO under natural
2
3
sunlight in 90 min. But, prepared AgꢀBi O , AgꢀZnO, Bi O and ZnO produced 74, 85, 62 and
2
3
2
3
8
1% degradations, respectively in 90 min. This shows that AgꢀBi O ꢀZnO is most efficient in
2 3
AR 1 degradation than other photocatalysts (Fig. 9). Based on the results, it is obvious that the
higher photocatalytic activity of AgꢀBi O ꢀZnO is due to the Ag and Bi O dopants. To test the
2
3
2
3
efficiency of the catalyst on the degradations of other dyes, we had carried out the experiments
on the degradation of EB and AV 7 under the same conditions. The degradation of three azo dyes
AR1, EB and AV 7 at different irradiation times are shown in figs. S2, S3 and S4 respectively.
There is no significant change in UV Vis spectra of dyes during irradiation, but the intensity
−
decrease gradually during degradation. This reveals that the intermediates do not absorb at
analytical wavelengths. Similar results were observed in the photocatalystic degradation of azo
1
4, 15
dyes.
Fig. S5 shows the percentages of degradation of EB and AV 7 along with AR1 using
AgꢀBi O ꢀZnO at different irradiation times. All the three dyes undergo almost complete
2
3
degradation in 90 min. The results reveal that this catalyst is efficient in the degradation of azo
dyes. In the case of EB dye, 44% of dye was absorbed by AgꢀBi O ꢀZnO. Experiments were
2
3
conducted to find out whether the adsorbed dye molecules had been completely degraded by the
4
2
reported procedure. The results revealed that the adsorbed dye underwent complete degradation
in 90 minutes.
The percentage of degradation in the solar process is affected by variables such as the pH
and catalyst loading. The effects of these variables were studied using the azo dyes (AR 1, EB
and AV 7) with AgꢀBi O ꢀZnO. The acidꢀbase property of the metal oxide surfaces can have
2
3
considerable implications on their photocatalytic activity. Percentages of degradation at different

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pH from 3ꢀ11 for three azo dyes are shown in Fig.10. It is observed that the degradation rate
increases with an increase in pH up to 7 and then decreases. After 90 min of irradiation, the
percentages of AR 1 degradation are 53, 71, 96, 78 and 62 at pH 3, 5, 7, 9 and 11, respectively.
The optimum pH is found to be 7 for AR1 degradation and a similar trend is observed for EB
and AV 7 dyes. Low removal efficiency at the acidic pH range may be due to the dissolution of
ZnO in AgꢀBi O ꢀZnO. To find out the reason for the effect of pH on degradation efficiency,
2
3
43
zero point charge (ZPC) of the catalyst was determined by the potentiometric titration method.
Zero point charge of AgꢀBi O ꢀZnOis found to be 7.1, which is less than ZPC of ZnO (9). When
2
3
the pH is above ZPC, the surface charge density of the catalyst becomes negative. This affects
the adsorption of dye molecules, which exist anions in the whole range of pH studied. Hence the
degradation efficiency is low at pH 9 and 11.
The reusability of AgꢀBi O ꢀZnO was tested for the degradation of azo dyes under
2
3
identical reaction conditions. After complete degradation, the catalyst was separated and washed
with large amount of deionized water. The recovered catalyst was dried in hot air oven at 100 ºC
for 90 min and used for a second run. Fig. S6 shows the results of dyes degradation for five runs.
AgꢀBi O ꢀZnO exhibits remarkable photostability as the AR 1 degradation percentages are 98,
2
3
96, 94.0, 94 and 94 for 90 min in the first, second, third, fourth and fifth run respectively. There
is no significant change in the degradation efficiency of AgꢀBi O ꢀZnO after thirdrun. After
2
3
3
+
completion of degradation reaction, the solution was tested for Bi leaching with sodium
sulfide. There is no precipitation of bismuth sulfide (Black color). As there is no leaching of
3
+
Bi this catalyst is nonꢀtoxic for the wastewater treatment.

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Mechanism of degradation
Mechanism of the photocatalytic activity of the AgꢀBi O ꢀZnO composite is proposed in
2
3
scheme 2. Three main reasons for the increase in the photocatalytic efficiency of metal doped
coupled oxide: (i) Visible absorbance AgꢀBi O ꢀZnO increased when compared to prepared
2
3
ZnO, (ii) Band energy levels of Bi O and ZnO are suitable for charge separation and (iii) Ag
2
3
metal can act as electron trap. The conduction band edges of ZnO and Bi O are situated at ꢀ0.38
2
3
and +0.32 eV, while the valence band edges of Bi O and ZnO are at +2.78 eV and 2.84 eV
2
3
respectively as shown in Scheme 2. The CB of Bi O is much more positive (+ 0.7 eV) than ZnO
2
3
whereas VB of Bi O is slightly more negative (ꢀ0.06 eV) than ZnO. Hence there is a greater
2
3
tendency for the flow of electrons from ZnO to Bi O As there is no significant difference in VB
2
3.
levels, the tendency for the holes to flow in to Bi O will be less. This makes the charge
2
3
separation effective. The enhanced charge separation is also revealed by the reduction of PL
intensity of the catalyst when compared to ZnO (Fig. 8). Further the separation of
photogenerated electronꢀhole may be due to the formation of heterojunciton between ZnO and
Bi O , leading to enhancement of photocatalytic activity. In addition to this ‘Ag’ metal can trap
2
3
the electron from ZnO and Bi O , inhibiting the recombination of electrons and holes in Agꢀ
2
3
Bi O ꢀZnO. Holes remained on ZnO valence band and electrons concentrated on Bi O
3
2
3
2
conduction band, enabled this type of three component system with strong oxidation and
•ꢀ
.
reduction capabilities to generate superoxide (O ), and hydroxyl radicals (OH ) for the dye
2
degradation.
In the case of EB dye, a dyeꢀsensitized mechanism is also possible for the degradation.
−
6
−1 −1
We carried out the degradation of EB with 365 nm UV light (IUV = 1.381 × 10 Einstein L s )
under the same conditions used for natural sun light. It was found that EB underwent 76.3%

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degradation with UV light, but under the same conditions 90% degradation occurred with solar
light in 75 min. The higher efficiency in solar light indicates the presence of a dyeꢀsensitized
mechanism in addition to AgꢀBi O ꢀZnOsensitization. This occurs when more dye molecules are
2
3
adsorbed on the semiconductor surface (42.6% EB adsorption in dark). Dye is sensitized by solar
irradiation and loses an electron (Eqn. 1) and this electron is transferred to the conduction band
of ZnO. This subsequently increases electron transfer to the adsorbed oxygen producing
superoxide radicals (Eqn. 2). Dye molecules are degraded by the superoxide radicals produced
by the dye sensitization mechanism (Eqn. 3). Further, to prove the dyeꢀsensitized mechanism, we
had also carried out an experiment for degradation of colorless 4ꢀnitrophenol by AgꢀBi O ꢀZnO
2
3
with UV and solar light. We found that degradation of colorless 4ꢀnitrophenol (absorbing only in
the UV region) was more efficient in UV light (81.9%) than in solar light (54.2%) in 75 min
under the same conditions, indicating the presence of only catalyst sensitized mechanism in the
degradation of 4ꢀnitrophenol. This confirms the presence of dyeꢀsensitized mechanism for
degradation of EB dye.
+
•
ꢀ
EB dye + hν
EB dye +ecb
(1)
(2)
(3)
ꢀ
e + O
O •−
cb
2
2
EB dye +• + O / O •−
degradation products
2
2
Electrocatalytic Methanol Oxidation
The structure of the electrode greatly influences its electrocatalytic activity. It has been
2
9, 30
proved that the electroꢀoxidation of methanol is a surfaceꢀsensitive reaction.
Pt wire and
Ag/AgCl electrodes were used as the counter and reference electrodes, respectively. Fig.11
shows the cyclic voltammograms (CVs) of glass carbon electrode coated with prepared ZnO (a),

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ꢀ1
AgꢀBi O ꢀZnO (b) taken in N saturated 0.5 M NaOH solution at a scan rate of 50 mV s . The
2
3
2
CVs (a and b) show typical potential regions for hydrogen adsorption/desorption and the
formation/reduction of surfaces of Metal oxide or MetalꢀOH ZnO and AgꢀBi O ꢀZnO.
ads
2
3
The smaller size and good dispersion of AgꢀBi O ꢀZnO modified GCE are favorable for
2
3
the oxidation of methanol. Two peaks of methanol oxidation under anodic condition are clearly
observed (Fig.11a and b).The oxidation peak in the forward scan corresponds to the oxidation of
freshly chemisorbed species coming from methanol adsorption and the reverse oxidation peak is
primarily associated with removal of carbonaceous species not completely oxidized in the
forward scan. The magnitude of the peak current on the forward scan indicated the
electrocatalytic activity of the coated catalysts for methanol oxidation. It is clearly seen that Agꢀ
Bi O ꢀZnO catalyst exhibits a current density of 3.13 mA, which is about 3 times higher than that
2
3
of and ZnO (1.12 mA).
It is well recognized that the stability of catalysts is essential for the application of fuel
cells. However, redox reactions on the surface of catalysts often cause inactivation of catalyst.
To find out the electrocatalytic stability of the catalysts under continuous operating conditions,
longꢀterm chronoamperometric experiments were carried out in 0.5 M NaOH + 0.5 M methanol
44
solution. Fig. S7 shows the curves of current densities versus time recorded at
0.1 V for 1000 s. All these catalysts initially show a rapid decrease in current for the oxidation
ꢀ
of methanol, and then a relatively steady current is achieved. The rapid current decay showed the
poisoning of the electrocatalysts, likely due to the formation of intermediate and some poisoning
4
5,46
species during the methanol oxidation reaction.
It is found that the current decay for the
reaction on the AgꢀBi O ꢀZnO(curve b) is significantly less than on the prepared ZnO (curve a).
2
3
This suggests that the AgꢀBi O loading plays a critical role in promoting the methanol
2
3

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oxidation, which could be attributed to the intrinsic electron transfer characteristic of AgꢀBi O ꢀ
2
3
ZnO.This reveals the higher stability of the AgꢀBi O ꢀZnO catalyst for methanol oxidation than
2
3
ZnO.
CONCLUSIONS
Heteroarchitectured AgꢀBi O ꢀZnOsemiconductor photocatalyst was synthesized by the
2
3
photodepositionꢀhydrothermal method and characterized. The HRꢀSEM images revealed the
formation of wellꢀorganized intercrossed sheets composed of nanosheets and nanoparticles
respectively. Elemental color mapping confirms the homogeneity of Zn, O, Ag and Bi in the
catalyst. AgꢀBi O ꢀZnO has a nanoparticlesꢀintercrossed Sheet like morphology and with high
2
3
porosity. The AgꢀBi O ꢀZnO has increased absorption in the UV and Visible region. AgꢀBi O ꢀ
2
3
2
3
ZnO was more efficient in dye degradation than the prepared ZnO, Ag–Bi O and TiO ꢀP25
2
3,
2
under natural sunlight. The optimum pH for the efficient removal of dye was found to be 7. This
Hydrothermal route for the fabrication of AgꢀBi O ꢀZnO is effective, fast, convenient, and ecoꢀ
2
3
friendly. A mechanism for high charge separation efficiency is proposed. This catalyst was found
to stable and reusable. Electrocatalytic activity of AgꢀBi O ꢀZnO nanocomposite exhibited
2
3
enhanced current production by methanol oxidation under room temperature.
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: chemres50@gmail.com ,m.swaminathan@klu.ac.in, . Tel./fax: +91ꢀ4144ꢀ220572.
Notes
The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS
We are grateful to the Council of Scientific and Industrial Research, New Delhi, for the
financial support through Research Grant no. 21 (0799) /10 / EMRꢀII. One of the authors
Dr. S. Balachandran is thankful to the UGC Networking Resource Centre, University of
Hyderabad for providing characterization facility and to Dr. Tushar Jana, School of Chemistry,
University of Hyderabad for the laboratory facility. Authors are grateful to Dr. P.V. Satyam,
Institute of Physics, Bhubaneswar for using HRꢀTEM facility.

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d
c
b
a
10
20
30
40
50
60
70
80
Position (°2 Theta)
Fig. 1 XRD patterns of (a) Prepared ZnO and (b) 3wt% AgꢀBi O ꢀZnO, (c) 6wt% AgꢀBi O ꢀ
2
3
2
3
ZnO and (d) 9wt% AgꢀBi O ꢀZnO.
2
3

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a
2
1
2
µm
b
µm
c
0 nm
Fig. 2 FEꢀSEM images of 9wt% AgꢀBi O ꢀZnO at different magnification (a) 2 µm (nano
2
3
particles are highlighted by green circle), (b) 1 µm and (c) 50 nm

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a
b
c
Fig. 3 HRꢀSEM images of 9wt% AgꢀBi O ꢀZnO at different magnification (a) 300 nm, (b) 1 µm
2
3
and (c) 1 µm

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a
b
c
d
Fig. 4 FESEM elemental colour mapping of 9 wt% AgꢀBi O ꢀZnO (a) Zn, (b) O, (c) Bi and (d)
2
3
Ag.

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0.236 nm
Ag
0.256 nm
ZnO
0.328 nm
Bi O3
2
Fig. 5 TEM images of 9 wt% AgꢀBi O ꢀZnO (a), (b), (c) particle distribution and (d) HRꢀTEM
2
3
image (lattice fringes)

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(
a)
1000
800
600
400
200
100
Binding Energy (eV)
8
0
0
160
Zn 2p
(
b)
(c)
O 1s
O-H
30.2
5
120
6
Zn 2p3/2
1021.9
Zn 2p1/2
1045.1
O-L
32.2
5
8
0
0
4
0
0
4
2
0
0
548
544
536
532 528 524
Binding Energy (eV)
1064 1056 1048 1040 1032 1024 1016
Binding Energy (eV)
540
2
00
(
e)
Bi 4f
(d)
Bi 4f 7/2
368.4 eV
Bi 4f 5/2
250
00
50
00
Ag 3d5/2
164.1
1
50
00
158.9
Ag 3d3/2
2
374.5 eV
1
1
1
50
50
0
0
384 380 376 372 368
Binding Energy (eV)
364
174
1
72 168 164 160 156 152
Binding Energy (eV)
Fig. 6 XPS analysis of 9wt% AgꢀBi O ꢀZnO (a) Survey spectrum, (b) O 1s ,
2
3
(c) Zn2p, (d) Bi 4f and (e) Ag 3d.

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b
a
2
00
300
400
500
600
700
800
Wavelength (nm)
Fig. 7 Diffuse reflectance spectra (a) prepared ZnO and (b) 9wt% AgꢀBi O ꢀZnO.
2
3