Fractal to monolayer growth of AgCl and Ag/AgCl nanoparticles on vanadium oxides (VO:X) for visible-light photocatalysis

Article abstract of DOI:10.1039/c7ta03321d

A facile and simple methodology was adopted for the trapping of highly crystalline AgCl and Ag/AgCl nanoparticles (NPs) into the interlayer spacings of vanadium oxides (VO_x). Self-organization of AgCl and Ag/AgCl-NPs on VO_x was found to be governed by the nature of the dicarboxylic acids used during the synthesis of the nanocomposites. A fractal-like morphology of the AgCl@VO_x nanocomposite was achieved in the presence of cis-1,2 cyclohexanedicarboxylic acid. Heating of the AgCl@VO_x nanocomposite above 68 °C resulted in the growth of polydispersed and ultrafine (3-4 nm) Ag/AgCl-NPs and its self-organization into monolayer formation on a partly crystalline VO_x matrix. Change in the conformation of the dicarboxylic acid to the trans-isomer resulted in the formation of a 'rod-like' structure of Ag/AgCl-NPs on a highly crystalline VO_x matrix. The band gaps of the nanocomposites were within the range of 1.8 to 2.9 eV. Because of such a low band gap, the synthesized nanocomposites were found to be highly active toward the photooxidation of methylene (MB) and methyl orange (MO) under sunlight.

Full text of DOI:10.1039/c7ta03321d

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10.1039/C7TA03321D.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Fractal to Monolayer Growth of AgCl and Ag/AgCl Nanoparticle on Vanadium Oxides
(VOx) for Visible Light Photocatalysis
Mukesh Sharma,†a Biraj Das,†b Jugal Charan Sarmahc, Anil Hazarikad, Biplab K. Dekae,
Young-Bin Parke and Kusum K. Bania*
a,b,* Department of Chemical Sciences Tezpur University, Assam, India, 784028
c Department of Physics, Tezpur University, Assam, India, 784028
d
Sophisticated Analytical Instrumentation Centre (SAIC) at Tezpur University, Assam, 784028,
India
e School of Mechanical, Aerospace and Nuclear Engineering, Ulsan National Institute of Science
and Technology, Ulsan, Republic of Korea, 44919.
† Both the authors contributed equally
Abstract: A facile and highly simple methodology was adopted for trapping of highly crystalline
AgCl and Ag/AgCl nanoparticles (NPs) into the interlayer spacing’s of vanadium oxides (VOx).
Self-organization of AgCl and Ag/AgCl NPs on VOx was found to be governed by the nature of
the dicarboxylic acids used during the synthesis of the nanocomposites. “Fractal like”
morphology of AgCl@VOx nanocomposite was achieved in presence of cis-1,2
cyclohexanedicarboxylic acid. Heating of the AgCl@VOx nanocomposite above 68 C resulted
in the growth of polydispersed and ultra-fine (3-4 nm) Ag/AgCl-NPs and its self-organization
into monolayer formation on partly crystalline VOx matrix. Change in the conformation of the
dicarboxylic acid to trans-isomer resulted in formation of ‘rod like’ structure of Ag/AgCl-NPs
on highly crystalline VOx-matrix. The band gaps of the nanocomposites were within the range of
1.8 to 2.9 eV. Because of such low band gap, the synthesized nanocomposites were found to be
highly active towards the photo oxidation of methylene (MB) and methyl orange (MO) under
sunlight.
Key Words: Ag/AgCl NPs, VOx, Low band gap, Photo catalytic oxidation

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Introduction
Semiconductors and nanomaterials with suitable band gap that can trap solar energy
emerged to be a class of fascinating materials for their large applications in solar cells,
photovoltaics, optoelectronics, Li-batteries etc.1-6 Apart from their application in energy storage
devices such kind of materials are also highly beneficial as photocatalyst for water treatment and
water splitting reactions.7-10 Photocatalyst have different ability to absorb solar light of different
wavelength based on the band gap.7-9 Semiconductors with small band gap like Cds, BiVO4
respond well to the visible light and acts as photocatalyst.11-13 But all kinds of semiconducting
materials with small energy barrier may not always serve the purpose. For example TiO2 a well-
known photocatalyst do not use visible light during photocatalysis.9 Sometimes, materials with
higher band gap can also act as photocatalyst and promote reactions.14 Nanoparticles with
surface plasmon resonance (SPR) also appear to be potential candidate for visible light
photocatalysis.15,16 However, single plasmon nanoparticle shows poor stability and limits their
application.17 Materials with visible light absorption ability are however considerably less
compared those that can use UV-light. Therefore, substantial amount of research work has been
devoted towards the development of materials for visible light carriers.7-13 Further, reacting
molecules usually do not absorb directly the visible light to drive the reaction. Visible light
photocatalyst thus can acts as a bridge to promote energy transfer between visible light and the
reacting substrate.18-22
Removal of organic contaminants such as coloured dye (e.g. methylene blue, methyl
orange) from water using visible light photocatalyst has become a thrust area of research due to
the increase in environmental pollution from industrial waste23 and stringent regulation imposed
by environmental laws.24 Among the different processes adopted by the researchers,13

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photocatalytic oxidation or degradation of such organic dye contents from water is considered to
be the greener approach.9-11 Hence, in recent progress a large number of catalysts have been
designed for photocatalytic decomposition of those organic pollutants.7-13 Very recently, Zhang
et al.25 found ultradispersed amorphous silver silicates/ultrathin g-C3N4 nanosheets
heterojunction composites (a-AgSiO/CNNS ) to be effective photocatalyst for dye
degradation.26,27 Silver doped in vanadium oxides (AgVOx) also shows good photo-redox
properties.28 For e.g. Ag0.68V2O5 has recently been found to exhibit good photocatalytic activity
with an experimentally observed band gap of 1.35 eV.29 But silver nanoparticles (Ag NPs) alone
do not have suitable band gap that can make them suitable photocatalyst. Ag/Ag-halides like
Ag/AgCl NPs because of the surface plasmon resonance (SPR) band makes them suitable
photocatalyst.30-34 They also possess comparable band gap (3.25 eV) to that of TiO2 (3.2 eV)
and hence can act as photocatalyst in ultra-violet spectrum of solar light. So various attempts
have been made to modulate the band gap of Ag/AgCl-NPs for photocatalytic reaction using the
visible light.30-32 Ag/AgCl-NPs either supported or doped in various metal oxides such as ZnO,
WO3 or TiO2 are also reported to have enhanced photocatalytic activity.31,35,36 However, one of
the limitations that restrict the synthesis of small or ultrafine Ag/AgCl nanocomposite lies in the
fast reaction between Ag+ and Cl- ion leading to agglomeration and increase in particle size.37
Zhang et al. also reported that ultrafine NPs could reduce the diffusion length for photogenerated
charge species and can enhance the photocatalytic activity.26,27 So, development of suitable
method that could suppress the agglomeration process, enhances the plasmon resonance would
be of high beneficial in designing an efficient photocatalyst. Further, if the nature of nucleation
process or self-assembling of the Ag/AgCl-NPs38 can be controlled and lead to “fractal” like

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structure or “monolayer” organization of NPs then they could find high applications in opto-
electronics and magnetic devices and could improve the photo-catalytic activity.39,40
So far a few studies have been made on Ag/AgCl NPs on various metal oxide support.
But to the best of our knowledge ultrafine Ag/AgCl NPs supported on vanadium oxides (VOx)
has not been synthesized and applied as visible light photocatalyst.38-40 Thus looking at the
benefits of Ag/AgCl NPs over Ag NPs and on limitation on the synthesis of Ag/Ag-halides NPs
doped on VOx, herein we adopted a new non-hydrothermal or solvothermal techniques for the
encapsulation of ultrafine nanoparticles of AgCl and Ag/AgCl NPs into VOx matrix. The
synthesized materials were well characterized by various spectrochemical and physicochemical
techniques and were found to be effective photocatalyst in the visible range of solar spectrum.
Experimental Section. Details of the materials used and physical characterization are provided
in supporting information.
Procedure for degradation of Methylene Blue (MB) and Methyl Orange (MO)
For monitoring the photocatalytic degradation of methylene blue (MB) and methyl
orange (MO) through UV-vis studies, we prepared a solution 2 × 10−5 (M) concentration of MB
and MO in 30 mL of water. To the respective solution we add 5 mg of the nanocomposite and
the entire solution was put under sunlight. UV-vis spectrum was recorded by collecting the
equivalent amount of the solution after an interval of 3 min. The dye degradation efficiency was
determined by using the following equation
(%) Dye degradation = (A0 – At)/A0 × 100
Where A0 and At were the initial and final absorbance of either MB or MO dye during the
photocatalytic reaction.

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TOC analysis was done to study mineralization. The degree of dye mineralization in the
photocatalytic processes was calculated by
(%) Mineralization efficiency = ((TOC0− TOC푡)/(TOC0)) × 100
where TOC0 and TOC푡 denote the TOC concentrations before and after the reaction,
respectively.41
Synthesis of AgCl@VOx and Ag/AgCl/VOx Nano-Composites. For synthesis of AgCl/VOx
and Ag/AgCl/VOx nanocomposite, solution of 5 mmol (0.86g) of cis 1,2 cyclohexane
dicarboxylic acid, C6H10(CO2H)2 was prepared in DMF and treated with 5 mmol of triethyl-
amine, and was stirred for 30 min. After that 5 mmol (0.768g) of anhydrous VCl3 was added and
stirred under refluxing condition. To this greenish solution, 5 mmol (0.85g) of AgNO3 was added
gently and the reaction mixture was further stirred for 1h at 68 C. After adding AgNO3, the
colour of the solution fades and an orange brownish precipitate was obtained which we named as
Compound A. This material on further heating upto 70 C resulted in a drastic colour change
from orange to purple, Compound B. After few minutes (2-3 min), Compound B was
transformed to blackish powder and we considered this as Compound C. It is pertinent to
mention herein that we have collected samples at each step for characterization. Similar
procedure was followed with trans 1,2 cyclohexanedicarboxylic acid, but in this case no any
other compounds were obtained rather the solution containing all the above substrates turns
black at 68 C. Further heating above 68 C did not result in any characteristic change in colour.
The overall synthetic approach is shown in Scheme 1.

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Scheme 1. Synthetic approach for the preparation of the nanocompoisites
Results and Discussion
PXRD pattern of Compound A was found to show clear and dominant peaks at 27.8,
32.2, 46.2, 54.9,57.6,67.4, 74.5 and 76.7 for (111), (200), (220), (311), (222), (400), (331)
and (420) plane, respectively characteristics of cubic phase of crystalline AgCl NPs42 (JCPDS
No 31-1238), Fig. 1a. Absence of any crystalline plane for VOx unit suggested for its amorphous
nature. Similar to our results Cronin and his co-workers44 also did not observed the signals for
VOx due to its amorphous nature. In Compound B, three additional peaks at 37.2, 43.4 and
63.6 were found for silver nanoparticles (Ag-NPs), Fig. 1b.45 However, the intensities of these
planes were much lower in comparison to AgCl planes, Fig. 1c and the peaks were found to be
slightly shifted (~by 1) from Compound A due to the interaction between AgCl NPs with Ag
NPs. Similar to Compound A, no crystalline planes for VOx was observed in Compound B. In
the PXRD pattern of Compound C, some additional peaks characteristics of V6O13 and V2O3 unit

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of VOx were observed, see the inset Fig. 1d.46 The appearance of such peaks suggested for the
change in the morphology of VOx matrix and gain of crystallinity.47
Fig. 1 PXRD pattern of a) Compound A b) Compound B c) comparison between Compound A
and B d) Compound C.
In the FTIR spectrum of Compound A, sharp signals for V=O V-O-V, V-O and VO2
stretching frequencies were seen at 920, 837, 635 and 517 cm-1, respectively, Fig. 2a .48 Absence
of sharp signals for carboxylates indicated for the formation of amorphous vanadium oxides
(VOx). Formation of carboxylate free VOx was further confirmed from the TGA analysis
showing no any degradation steps for carboxylate ions (Fig. 2b). Upto 400 C, a negligible
weight loss of 0.73% was only observed confirming the high stability of the nanocomposite.
Thus it was considered that the highly crystalline AgCl NPs were mostly located within the VOx

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matrix. Symmetric and asymmetric vibrational bands for –COO- with a difference of 147-149
cm-1 (Table S1) was however observed in the FTIR-spectrum of Compound B and Compound C,
Fig. 2c and 2d.49 Such difference in symmetric and asymmetric vibrational mode of -COO-
suggested for the presence of bridged carboxylate unit between Ag NPs and V-metal centre of
VOx and thereby stabilized the Ag/AgCl NPs. TGA analysis of Compound B and C also showed
minimal decomposition of 15-30% representing the high thermal stability of the material, Fig.
S1.
Fig. 2 a) FTIR spectrum of compound A b) TGA analysis of Compound A c) FTIR of
Compound B and d) Compound C.
XPS spectrum for Compound A is shown Fig. 3a. XPS band at 517.4 eV for V(2p3/2) and
V(2p1/2) at 524.8 eV appeared for +5 oxidation state of vanadium (Fig. 3b).50,51 The O(1s) signal
was found at 530.3 eV characteristics of oxygen atom of the metal oxide (VOx).52 The binding
energy values for silver was mostly of Ag+ ions with Ag (3d5/2) and Ag (3d 3/2) signals at

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367.7eV and 373.8 eV (Fig. 3c).31 The Ag:V ratio from XPS analysis was found to be 1:1. In
the voltammogram of Compound A, two oxidation peaks were found at 0.46 V and 0.69 V for
V(IV)/(V) couples. At the higher negative potential three cathodic signals were obtained at -0.84
+
V, -1.28 V and -1.86 V for VO22+/VO2 , [VO2]2-/[VO2]3- and [VO2]/[VO2]- reduction processes,
respectively. The redox potential values matched well with those determined by Bruyere et
al.53,54 Cyclic voltammogram (CV) of the Compound B was found to be almost similar to that of
Compound A with an absence of one anodic peak at 0.69 V. This clearly confirmed that some of
the V(IV) ions of VOx unit was involved in the reduction of Ag+ ions of AgCl NPs to Ag(0) NPs.
XPS spectrum of Compound B was also found to be same with Compound A (not shown here).
However, Ag:V ratio was found to be 1.8:1. In the CV of Compound C, two irreversible anodic
signal for V(III)/V(IV) couple and V(IV)/V(V) couple was found at -0.767 V and +0.59 V, Fig.
S2. Signals for V(IV)/V(V) was weak indicating its low concentration. Presence of sharp
oxidation peak for V(III)/V(IV) confirmed for the formation of V2O3 species and was in good
agreement with our PXRD. XPS analysis also confirmed for the mixed vanadium oxidation state
with the appearance of signal for V2p3/2 at 513.5 eV for V(III) and 516.6 eV for V(IV) Fig. 3b
(red line).50,52 Ag:V ratio in Compound C was found to be 2.1:1. The increase in Ag content in
Compound B and C was due to the loss of soluble vanadium oxides during washing. It is
pertinent to mention herein that Ag(0) species was not distinguishable in XPS spectrum of
Compound B and C due to high Ag+/Ag0 ratio.

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Fig. 3 a) Typical XPS-spectrum of Compound A, B and C b) XPS bands of Vanadium c) XPS
band of Silver and d) Cyclic Voltammogram of Compound A (black) and B (red).
The TEM analysis of the Compound A represented a fractal like morphology similar to
those observed by Trent et al.39 in case of Au-NPs on silica surface, Fig. 4(a-c). The average
particle size was found to be 7-10 nm. Ag:V ratio in Compound A from EDX analysis was found
to be 1:1 ratio. From the TEM analysis, the average pore diameter of the material was found to
be 74.3 nm. In the TEM-image of Compound B, Ag/AgCl NPs were found to be polydispersed
on the amorphous surface of VOx, Fig. 4 (d-f). However, the particle size almost remained the
same. EDX analysis of Compound B also confirmed for the increase in Ag:V ratio and

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correlated well with our CV-and XPS analysis. Apparently, in the TEM image of Compound C,
Ag/AgCl NPs were found to be almost spherical in shape with a particle size varying from 3-4
nm, Fig. 4(g-i). Monolayer growth of Ag/AgCl NPs on VOx matrix was clearly observed in the
TEM images of Compound C. The transformation of cubic to spherical shape was in accordance
with our PXRD analysis with the missing of two planes at 43.4 and 63.6 (see Fig. 1d). Ag:V
ratio was found to be more in Compound C compared to Compound A and B . Thus from the
above analysis we concluded that in Compound A, AgCl NPs with average particle size of ~7
nm was embedded inside the pores of porous vanadium oxide leading to the formation of AgCl
NPs doped vanadium oxide nanocomposite designated as AgCl@VOx. While on slight heating of
Compound A transformed the morphology and particle size of AgCl with concomitant reduction
of Ag(I) to Ag(0) and also enhanced the crystallinity of VOx matrix. Compound B and C were
thus considered to be nanocomposites of AgCl/Ag NPs and VOx designated as Ag/AgCl@VOx.
In our previous study we found the generation of cubic Ag particle of higher dimension
in nm to m range on treatment of AgNO3 with Fe(III)-gel of trans 1,2 cyclohexanedicarboxylic
acid.55 Interestingly with the same trans 1,2 dicarboxylic acid herein we obtained a different
morphology. TEM image of the black Compound D represented a rod like structure with the
NP’s size varying from 5-8 nm, Fig. 5(a-c). The diameters of the nanorods were found to vary
from 250-300 nm and lengths of the nanorods were in the range of 3-4 m (Fig. S3). The HR-
TEM images of the four compounds are shown in Fig. S4. All the four materials exhibited d-
spacing values varying from 0.20 nm to 0.43 nm revealing to the well-ordered layered structure
of silver nanoparticles (NPs) and vanadium oxide layers Fig. S4 (a-d). The intercalation of some
silver NPs molecules within the vanadium oxides might be responsible for the larger interlayer
spacing.44 The SAED patterns of the four compound matches well with those of AgCl NPs, Ag

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NPs, and vanadium oxides (VOx)(insets of Fig. S4(a-d).56 HRTEM image of Compound D also
confirmed for the adherence of NPs on the nanorod.
Fig. 4 TEM-images of Compound A (a-c), B (d-f) and C(g-i) in different scale. EDX spectrums
are shown as inset in respective cases.

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Fig. 5 TEM-images of Compound D (a-c) in different scale, EDX spectrum is shown as an inset
in image a).
PXRD pattern of Compound D was found to be completely different from all the three
compounds obtained with cis- 1, 2 dicarbixylic acids, Fig. 6a. Peaks were mostly dominated by
the signal of Ag(0). Signals for AgCl were observed to be very weak, and more crystalline peaks
for VOx were obtained in low angle region again comprising of VO2, V2O3 and V6O13, Fig. 6b
an Fig. 6c.50 It is pertinent to mention here in that we did not observe any phase transition
behavior in presence of trans isomer due to high stability associated with trans conformation.
XPS analysis of Compound D suggested for mixed valence oxidation state of vanadium with V
(2p3/2) signals appearing at 516.7 eV for V(IV) oxidation state and small shoulder peak at 517.8
eV for V(V), Fig. 7a (red line).50-52 The zero valent oxidation state of Ag was confirmed by the
presence of peak at 368.0 eV and 373.9 eV for 3d3/2 and 3d5/2 of Ag(0), Fig. 7b (black line).
Comparison of XPS bands for vanadium and silver with Compound A also showed shifting of
the bands to lower value for vanadium (2p3/2) and to higher value for silver (3d3/2) indicating
high concentration of low valent metal species, Fig. 7. CV of Compound D (Fig. S5) represented
a quasi reversible signals for V(III)/V(IV) couple at -1.08 V and -0.79 V54 confirming for high
concentration of V(IV) species and were in accordance with our XPS analysis. The FTIR

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spectrum and TGA analysis of Compound D are provided in Fig. S6 and S7 and indicated the
existence of VOx surface as well as the high thermal stability of the compound similar to the
other three compounds.
Fig. 6 PXRD pattern of Compound D showing the crystalline plane Ag/AgCl NPs and crystalline
VOx

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.
Fig. 7 Comparative XPS pattern of V and Ag for Compound D with Compound A
Thus we found that under the same reaction condition without varying the amount of Ag,
solvent amount, VCl3 and also the temperature, we could change the particle size as well as the
surface morphology of Ag/AgCl VOx nanocomposite simply by changing the conformational
isomers of the dicarboxylate.
BET surface area and the pore size distribution (PSD) of all the four compounds were
estimated and are given in Table 1. The corresponding N2-adsorption isotherms and PSD plots
for the four compounds are shown in Figure 8. All the four compounds represented Type-II
isotherms characteristics of mesoporous compounds. The surface areas of all the compounds
were found to be significantly high compared to other reported silver vanadates.38-40 The high
surface area of these four compounds clearly suggested for the highly porous nature of
amorphous vanadium oxide surface. The PSD analysis further confirmed the presence of
mesopores that are of suitable size to anchor AgCl and Ag/AgCl NPs thus leading to highly
hybrid nanocomposites.

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Table 1. BET analysis of the four Compounds
Compound
Pore volume (cm3/g) Pore radius (nm)
BET surface area
(m2/g)
912.6
A
B
C
D
0.462
0.442
0.432
0.342
76.3
63.3
65.4
54.3
560.6
654.5
664.5
Fig. 8 N2-adsorption isotherms and pore size distributions (inset) of compounds A, B, C and D.
Photocatalytic Dye Degradation
Since the Ag/AgCl NPs supported on metal oxides like TiO2, ZnO, WO3 etc. were found
to be effective photocatalyst for degradation of dye pollutants,37,57,58 so we attempted to examine
the photocatalytic activity of our nanocomposites. Interestingly, we found that all the four nano-
composite shows very good activity towards the photo-oxidation of methylene blue (MB) and

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methyl orange (MO) dye under sunlight. The catalytic activity of Compound A was highest
among the four compounds. Compound A could oxidize 210-5 (M) concentration of MB and
MO within 15 and 21 min, respectively (Fig. 9a and 9b). Compound B, C and D took slightly
more time compared to Compound A. Photocatalytic degradation of dyes was confirmed from
the decrease in absorbance band at 662 for MB and at 463 nm for MO in the UV-vis spectrum.
Representative spectrum of the two with Compound A is shown in Fig. 9.
Fig. 9 Uv-Vis spectrum for the catalytic degradation of a) MB and b) MO dye.
Complete decolourization of MB or MO solution however did not confirm that either dye
was completely mineralized into CO2 and H2O. So it was necessary to measure the
mineralization efficiency of the catalyst which was done by measuring the total organic
concentration (TOC) before and after photocatalysis.41 Plot of decolorization efficiency and TOC
loss for MB solution is shown in Fig. 10. The process of decolorization was found to be different
from that TOC removal. TOC values were found to decrease with reaction time. However the
time of decolorization and TOC decay does not correlate each other. There was high drop in
TOC value after 20 min however with all the catalyst decolorization of MB took place before
complete mineralization of the dye. Complete mineralization took nearly 30 min with Compound
A while nearly 40 min was required with other catalyst. Compound A showed some better result

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than the rest. Non-existence of the inflect point between MB degradation and TOC loss thus
confirmed that during the decolorization step (irradiation time 0–25ꢀmin) there was ~60%
decrease in organic carbon content and the rest ~40% of TOC was contributed from the
intermediate product which further get oxidized after prolonged irradiation upto ~30-40 min.
Similar behavior was also observed with MO solution and it took nearly 44 min for complete
mineralization. This suggested that complete mineralization of the dye was also dependent on
time of irradiation. It is pertinent to mention herein that in absence of the catalyst only with
exposure to the sunlight for nearly 50 min we could hardly see 2% loss in TOC. This clearly
confirmed for the photocatalytic activity of the synthesized materials.
Fig. 10 Decolorization efficiencies and TOC removal versus irradiation time. a), b), c) and d) are
plot of MB-degradation with A, B, C and D, respectively. e), f), g) and h) plot of TOC loss with
A, B, C and D, respectively.
In order to investigate whether the visible range or the UV range influence the photo
catalytic oxidation process, we also investigated same reaction under the UV lamp, but under
the UV light we did not see any colour change in dye upto three hours. This further confirmed

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that the photo catalytic oxidation process was truly assisted by the visible range spectrum of the
sunlight. So the synthesized nanocomposite material could be effectively used as visible light
driven photocatalyst. In order to differentiate the photocatalytic activity of the nanocomposites,
we recorded the diffuse reflectance spectra (DRS) of the compounds and predicted the band gap,
Fig. 11a and Fig. 11d. In Compound A, a broad band at 412 nm was mostly dominated by the
vanadium oxide surface.59 In case of Compound B two bands were observed at 268 and 496 nm.
The former was for AgCl NP’s while the later for Ag NP’s.57 In Compound C two bands for
AgCl NP’s appeared at 220 and 250 nm. Similarly, in Compound D bands were found for AgCl
NPs at 217, 255 and 285 nm.57 In Compounds C and D because of the smaller size of NP’s less
than the Bohr exciton radius (5.8 for Ag particle),60 the DRS spectrum represented a wide band
covering the whole visible range, Fig. 11a. This broadness of the spectrum could be attributed to
the quantum confinement effect usually observed when the particle sizes of the nanomaterial
significantly decreased below the Bohr exciton radius.59,61 The band gap of Compound A was
found to be significantly less (1.82) compare to Compound B (2.58), Compound C (2.91) and
Compound D (2.41), Fig. 11b. The change in the band gap was found to be indirectly related to
the particle size of NP’s. The inverse relationship between the band gap and particle size was
attributed to the reverse Surface Plasmon Resonance (SPR) effect.61 Recently Peng et al.,62
observed reverse SRP in case of AgNP’s having particle size less than 10 nm following the Mie
theory.63 Lowering of the band gap to 1.8 eV in Compound A suggested that the band gap of
AgCl NP’s could be tuned by combining with vanadium oxides.

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Fig. 11 a) DRS spectrum for Compound A, B, C, and D, b) and their corresponding band gaps.
The catalysts were recycled upto four consecutive cycles without any loss in catalytic
activity. The (%) of dye degradation efficiency as well as (%) efficiency of dye mineralization
was approximately same upto four cycles. Further, we did not observed any Ag or V leaching
during those consecutive cycles and also no any colour change in all the four photocatalyst
indicating high stability of the catalyst on exposure to sunlight. Retention of the structural
morphorlogy upto fourth cycle was also confirmed from the no change in PXRD (Figure S8)
pattern after the catalytic reaction. We studied the recyclability test only upto fourth cycle and
since the catalytic activities were well maintained upto that number of cycles so we did not opt
for further cycles. Recyclability test thus conferred for the high stability of all the catalyst.
Mechanism of Dye Degradation
Under solar irradiation, Ag/AgCl nanoparticles on the surface of porous vanadium oxides
+
react with visible light to produce electrons (푒cb) and holes (ℎvb ), Scheme 2. The holes are
trapped by H2O or O2 on the surface of the catalyst to generate H+ and OH∙ radicals and
-.
superoxide radical O2 . The holes, electron and radicals took part in a redox process and can

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degrade the organic pollutant into intermediates and the intermediates can be further degraded
into CO2 and H2O.
In order to elucidate the photo-degradation reaction mechanism of MB and MO under
visible light irradiation in presence of our catalyst, we further studied our reaction by adding
-.
ammonium oxalate that generally acts as a hole scavenging agent and p-benzoquinone as an O2
scavengers.27,64 Fascinatingly, the rate of photo degradation process in presence of all the four
catalyst was greatly suppressed in presence of both ammonium oxalate and p-benzoquinone.27,64
This suggested that both holes and superoxide radicals were responsible for photoxidation
process. Probably under visible light of solar spectrum e- got ejected to the conduction band (CB)
of both AgVOx and Ag/AgCl leaving behind the holes (h+) in their valence band (VB), Scheme
2. The e- in the CB of AgVOx can migrate to the CB of Ag/AgCl and at the same time holes
generated at the Ag/AgCl can move to the VB of AgVOx. The effective charge transfer between
AgVOx and Ag/AgCl is fascilated by the strong interfacial coupling existing between the hybrid
composites. Moreover, the SPR existing in ultrafine Ag/AgCl NPs further enhances this charge
migration but at the same it reduces the rate of electron-hole pair combination. This in turn
allows the positive holes to diffuse to the catalytic surface thereby directly oxidizing the dye
components. The electrons in the conduction band then combines with O2 to generate redox
-.
active O2 that further oxidizes the dyes.

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Scheme 2 Proposed schematic mechanism for photo catalytic reactions on the four synthesized
catalyst (A, B, C and D) surface under visible-light irradiation.
Conclusion
In summary, AgCl and Ag/AgCl NPs of very smaller dimension ranging from 5-10 nm
were encapsulated into amorphous vanadium oxides (VOx) to form nanocomposite of
AgCl@VOx and Ag/AgCl@VOx. The morphological change in the nanocomposites was found
to be controlled by conformational change and reaction temperature. The electrochemical
behavior as well as the electronic properties of the materials was found to vary with Ag and V
content. The materials with Ag:V ratio 1:1 possessed low band gap and was found to be highly
active for photochemical oxidation of MB and MO dyes under sunlight. Since the silver
vanadates nanocomposites are gaining high application in energy sector so the present work
could contribute in the development of Li-ion batteries and visible-light photocatalyst.
Supporting Informations: Materials, physical measurements, TGA of Compound B, C, D,
FTIR Spectrum of Compound D, Cyclic voltammograms of Compound C and D, TEM of
compound D, HRTEM image of four compounds.
Conflict of Interest: There are no conflicts of interest.

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Acknowledgement: MS and KKB thanks Department of Science and Technology (DST-SERB),
New Delhi India for financial grant (SB-EMEQ/463-2014). BD thanks UGC, New Delhi for
National Fellowship.
*Contact Author
Email: kusum@tezu.ernet.in or bania.kusum8@gmail.com,
Mob. No. 09859929360
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TOC
Photocatalytic oxidation of dye pollutant in natural sunlight using Ag/AgCl/VOx
nanocomposites.