-
Highly efficient photocatalytic reduction of Cr(VI) over
hierarchical–like In2S3 hollow microspheres
*Rengaraj Selvaraj1), Salma M. Z. Al-Kindy1), Tharaya Al
Fahdi1),Kholood Al
Nofli1),Iman Al Maadi1), Younghun Kim2)
1)Department of Chemistry, College of Science, Sultan Qaboos
University, P.C. 123, Al-Khoudh, Muscat, Sultanate of Oman
2)Department of Chemical, Engineering, Kwangwoon University,
Seoul 139-701, Korea
1)E-mail: [email protected]; [email protected]
ABSTRACT
Hexavalent chromium (Cr(VI)) is a common heavy metal pollutant
in the wastewaters, which mainly comes from electroplating,
pigments, mining and chromate manufacturing industries. Cr(VI) is
mobile and highly toxic, whereas Cr(III) is less toxic and can be
readily precipitated with alkaline or neutral solutions. Cr(VI) has
attracted considerable attention from society and regulation
authorities around the world because of its high acute toxicity and
strong carcinogenic activity to humans. The world health
organization (WHO) has regulated that the concentration of Cr(VI)
should be below 0.05 mg/L in drinking water. Hence, it is of great
importance to explore how to remove the Cr(VI) in water
effectively. Recently, the photocatalytic reduction of Cr(VI) using
the semiconductor photocatalysis technology has received
considerable attention. The development of high performance
photocatalysts is indispensable for the application of the
photocatalytic process to large scale Cr(VI) wastewater treatment.
Unfortunately, most of the semiconductors have a major drawback
stemming from its large band gap, which means it can absorb mainly
UV light. For overcoming this problem, intensive research efforts
have been recently focused on the development of visible light
active photocatalysts. In this study photocatalytic reduction of
Cr(VI) was investigated by using a indium sulphide (In2S3) hollow
microsphere under visible light in aqueous solution. In this work
we demonstrate that the one step solvothermal synthesis of
three-dimensional (3D) hierarchical like In2S3 hollow microspheres,
which are composed of two-dimensional (2D) nanosheets. The
synthesized products have been characterized by a variety of
methods, including X-ray powder diffraction (XRD), field-emission
scanning electron microscopy (FE-SEM), energy-dispersive X-ray
(EDX) analysis, and ultraviolet visible diffused reflectance
spectroscopy (UV-vis DRS). The optical properties of In2S3 were
also investigated by UV-vis DRS, which indicated that our In2S3
microsphere samples possess a band gap of 2.0 eV. Furthermore, the
photocatalytic activity studies revealed that the synthesized In2S3
hollow microspheres exhibit an excellent photocatalytic performance
in rapidly reducing aqueous Cr(VI) to Cr(III) under visible light
irradiation. These results suggest that In2S3 hollow microspheres
will be an interesting candidate for photocatalytic detoxification
studies under visible light radiation.
-
1. INTRODUCTION Chromium is one of the most abundant heavy
metals, causing pollutionof
groundwaters and soil due to its frequent industrial
application. Chromium occurs naturally mainly in the trivalent
Cr(III) and hexavalent Cr(VI) forms. The majority of its adverse
effects is caused by Cr(VI) because of its solubility, mobility and
high oxidizing potential leading to generally higher toxicity
causing health problems such as liver damage, pulmonary congestion,
vomiting and severe diarrhea(Nriagu and Nieboer, 1988). On the
other hand, Cr(III) is less reactive and toxic and can be readily
precipitated out of solution. Therefore, the majority of in situ
treatment methods employed at the present time utilize geofixation
of Cr(VI) by its reduction to Cr(III) and formation of insoluble
Cr(III) compounds (Jardineet al., 1999). A number of articles have
been published to date describing various applications of
individual biological or chemical approaches to precipitate
chromium into its insoluble Cr(III) form.
The methods employed for the removal of Cr(VI) includechemical
precipitation,
reverse osmosis, ion exchange, foam flotation, electrolysis,
photocatalytic reduction, adsorption, etc. However, most of these
methods require either high energy or large quantities of
chemicals, and in this respect the photocatalytic process is found
to be superior to other conventional treatment techniques.
An alternative clean route of treating Cr(VI) that has
receivedconsiderable
attention is photocatalytic reduction in the presence of a
semiconductor material such as ZnO, TiO2, CdS or WO3 under visible
or UV radiation (Rengarajet al., 2007; Sun et al., 2005; Wang et
al., 2004; Schranket al., 2002; Kabraet al., 2004). The
photocatalytic method is based on the reactive properties of an
electron–hole pair generated in the semiconductor when irradiated
by UV/vis light having energy greater than the band-gap energy of
the semiconductor. This method is widely used for treatment of the
drinking water and industrial wastewater by oxidizing the organic
pollutants. The reducing capacity of the semiconductor
photocatalyst which uses the electrons generated on the
semiconductor surface is, however, less explored. It is more
attractive compared to the conventional processes since it works at
near-neutral pH thereby requiring less alkali during
coagulation–precipitation. This method, if conducted with solar
radiation as the source of activation energy, can be more
economical and eco-friendly (Mohapatraet al., 2005). Among the
photocatalysts, WO3 is less available, cadmium itself is a toxic
heavy metal and CdS is prone to easy deactivation and
photocorrosion (Reuterglrdh and Langphasuk., 1997).
Application of the photocatalytic technique for remediation of
inorganic and
organic pollutants in wastewater has been reviewed by Kabraet
al. (2004). Cieslaet al. (2004)have made an exhaustive survey of
the work on environmental photocatalysis by transition metal
complexes. Photo-reduction of a metal ion is accelerated if it is
accompanied by simultaneous oxidation of an organic molecule that
plays the role of a ligand or a ‘sacrificial electron donor’.
Concurrent generation of one or more of the species O2•, HO2•, OH•,
H2O2 and HO2−during the photo reduction process has been reported.
This occurs because of the fact that the medium (water) acts as the
electron
-
donor simultaneously with the sacrificial organic species. The
phenomenon has been called ‘ligand to metal charge transfer’(LMCT).
Reduction of Cr(VI) to Cr(III) leads to drastic decrease in
bioavailability and toxicity of this element. The photocatalytic
reduction of Cr(VI) with semiconductor such as CdS, ZnS, WO3,
ZnOand TiO2 in UV light has been reported in literature (Wang et
al., 1992; Domenech and Munoz., 1987; Yoneyama., 1979).
So in the present work, we have studied the activity of
In2S3nanoflowers towards
the photocatalytic reduction of hexavalent chromium in aqueous
suspension using HCOOH as a hole scavenger in the presence of
visible light. 2. EXPERIMENTAL 2.1 Materials Analytical grade
indium nitrate (In(NO3)3) (Sigma-Aldrich, 99%) and
thiosemicarbazide (TSC, NH2NHCSNH2) (Sigma-Aldrich 99%) were used
as Indium and sufur source for the preparation of In2S3 hollow
microsphere photocatalyst. Potassium dichromate was obtained from
the Aldrich Chemical Company, USA, and Used without further
purification as the Cr(VI) source. Deionized , doubly distilled
water was used for the preparation of all solutions. 2.2
Preparation of In2S3 hollow microsphere
The hierarchical-like In2S3 hollow microsphere were synthesized
using analytical
grade indium nitrate In(NO3)3 (Sigma-Aldrich 99%) and
thiosemicarbazide (CSN3H5) (Sigma-Aldrich 99%) without further
purification. In a typical synthesis, 3.324 mmol of In(NO3)3 and
6.648 – 13.297 mmol (1: 2 - 4 molar ratio) of thiosemicarbazide
(TSC) were dissolved in 70 mL of ethanol/water (v/v = 1:1) and
continuouly stirred for 30 minutes to form a clear solution. Here
TSC serve both as a sulfur source and as capping ligand. The
solution was then transferred into an autoclave and maintained at
180oC for 10 h to 24h and then cooled to room temperature
naturally. The orange colour precipitate was harvested by
centrifugation and washed several times using deionized water and
ethanol to remove the possible remaining cations and anions, and
then dried in an airoven at 70oC for 24 h for further
characterisation purpose. It is noted that the post treatment of
the products after the reaction were carried out in fume hood to
keep from excess H2S (generated in the solvothermal process). 2.3
Characterization of In2S3 hollow microsphere
The structural analysis of the samples were performed using a
Bruker (D5005)
X-ray diffractometer equipped with graphite monochromatizedCuK
radiation ( = 1.54056 Å). An accelerating voltage of 40kV and
emission current of 30 mA were adopted for the measurements. The
morphology and microstructure were characterized by field-emission
scanning electron microscopy (Hitachi S-4800). The absorption
spectrum of the samples in the diffused reflectance spectrum (DRS)
mode was
-
recorded in the wavelength range between 200 and 1000 nm using a
spectrophotometer (Jasco – V670), with BaSO4 as reference. From the
absorption edge the band gap values were calculated by
extrapolation method. 2.4 Experimental Procedure
In order to investigate the photocatalytic activity of In2S3
hollow microsphere, the
photocatalytic reduction of chromium(VI) was carried out in the
In2S3 suspension under visible light irradiation. The reaction
suspensions were prepared by adding 0.100 g of the catalyst to 250
ml of aqueous Cr(VI) solution with an initial Cr(VI) concentration
of 10 mg L-1. Prior to the photoreaction, the suspension was
magnetically stirred in the dark for 30 min to establish an
adsorption/desorption equilibrium condition. The aqueous suspension
containing Cr(VI) and the photocatalyst was then irradiated by
visible light with constant stirring. At given time intervals,
analytical samples were taken from the suspension and immediately
centrifuged at 5000 rpm for 15 min, followed by filtering through a
0.45 mm millipore filter to remove any particles. The filtrate was
analyzed by means of a spectrophotometer. A blank experiment was
carried out using dichromate solution without a photocatalyst in
order to determine the extent of reduction of the concentration of
hexavalent chromium due to the visible radiation. After irradiation
and adsorption, the suspension was filtered and the Cr(VI) content
was analyzed quantitatively by measuring the absorption band at 350
nm using the spectrophotometer (Colon et al., 2002). 2.5 Analytical
methods
The chromium(VI) content was analysed quantitatively by
measuring the
absorption band at 350 nm using a SHIMADZU (UV2450) UV-Vis
spectrophotometer. In each test, the total chromium content
[Cr(III) +Cr(IV)] was measured by Atomic Absorption Spectrometer
(Varian – Spectra 220 FS) and it was observed that the total
chromium content remained constant as the reaction proceeded. This
indicates that there is no precipitation of Cr(III) and no
irreversible adsorption on the catalyst. 3. RESULTS AND DISCUSSION
3.1 XRD analysis of In2S3nano flowers.
The crystal structures of the samples were characterized by XRD.
Figure 1
shows the typical XRD pattern of the samples prepared by simple
template free solvothermal method. All the reflection peaks in
Figure 8 were carefully compared with the JCPDS data base and
identified as the formation of the tetragonal phase. The observed
peak positions, 2 = 11.8, 27.4, 33.5, 43.8 and 47.9 were
respectively indexed as (1 0 1), (1 0 9), (0 0 1 2), (1 0 15) and
(2 2 1 2) reflections of the tetragonal -In2S3 phase (a = 7.623Å, c
= 32.36 Å, JCPDS: 73-1366) (Liu et al., 2011; Lee et al., 2009).
The peaks are considerably strong and narrow, which indicate that
the product is well crystallized. No characteristic peaks arising
from the possible impurities are visible, such as InS, In2O3 and
other phases of In2S3. This clearly indicates that pure
-
crystalline -In2S3 tetragonal phase was formed via solvothermal
process. From Figure 8, it can also be seen that (1 0 9), (0 0 12),
and (2 2 12) planes show the three strongest diffraction peaks, and
the relative diffraction intensity of either (0 0 12)/(1 0 9) or (2
2 12)/(1 0 9) is unusually higher than the corresponding
conventional values (JCPDS: 73-1366). This observation revealed
that the resultant hollow In2S3 microspheres are grown
predominantely along the (0 0 12) and (2 2 12) directions. From
these XRD measurements it is interesting to note that similar XRD
patterns were observed for all In2S3 hollow microspheres
(In2S3-1:2, In2S3-1:3 and In2S3-1:4) prepared with different indium
nitrate to thiosemicarbazide ratio, indicating that the samples
possess similar crystalline structure and the structure does not
change due to change in precursors ratios.
Fig. 1.XRD patterns of hierarchical like In2S3 hollow
microspeheres prepared with
different indium nitrate and thiosemicarbazide ratio, (a) In2S3
- 1:2, (b) In2S3 - 1:3, (c) In2S3 - 1:4.
3.2 EDX analysis
In order to identify the components of the synthesized In2S3
hollow microsphere the EDX microanalysis and elemental mappings
have been carried out in a SEM. The EDX spectrum and the
corresponding elemental mappings recorded for Sample-In2S3 1: 4 are
shown in Figure 2, which illustrate the actual distribution of In
and S separately in the sample, with a In/S molar ratio of
approximately 2:3. The EDX analysis confirmed that there are no
elements other than In and S present in the sample. From the Figure
9 it is clear that the EDX spectrum displayed three intense peaks
between 2.8 and 3.5
10 15 20 25 30 35 40 45 50 550
20
40
0
20
40
0
15
30
45
60
75
Inte
nsity
Cps
.
(415
)(2
212)
(309
)
(001
2)(2
15)(1
09)
(116
)(10
3)(1
01)
2 (?
(a) In2S
3 - 1:2
(b) In2S
3 - 1:3
(c) In2S
3 - 1:4
-
KeV corresponding to indium LI (2.92 keV), L1 (3.29 keV) and L
(3.5 keV) and equally a strong sulfur K1 peak at 2.33 keV, which
are similar to the major constituents of the In2S3 hollow
microsphere. The peak observed at 1.5 KeVis originated from the Al
substrate holder on which the In2S3 hollow microsphere where
dispersed for the EDX measurements. The quantitative analysis
indicated that the atomic ratio of In and S in the sample is
approximately 2:3, which is closer to the stoichiometry of bulk
like In2S3, suggesting that the synthesized hollow microsphere
possess nearly stoichiometric composition.
Fig.2.EDX analysis of In2S3 hollow microsphere.
3.3 SEM analysis
The morphology of the samples were examined by SEM. Fig. 3 shows
the typical SEM images of the In2S3 prepared at 180oC for 24 h with
a molar ratio of 1:4. Low magnification SEM observations show that
the as-prepared In2S3 samples have spherical morphology with size
from 3 to 7 m in diameters (Figure 3). High magnification SEM
images reveal that In2S3 hollow micro-spheres are built from small
nanosheets with a thickness of 10 to 20 nm and nanofibers with a
length of 80 to 100 nm. The wall thickness of the hollow
microsphere is around 300- 500nm. These nanosheets and nonorods
construct the surface of the hollow In2S3 microspheres. The
surfaces of the hollow microspheres are analogous to hierarchical
structure.
The relationship between the observed morphology and the
synthesis time has
been investigated by SEM. The SEM image of the samples prepared
at 24 h is shown in Figure 3. Initially, after 10 hrs of synthesis
time, it has been observed that the spherical morphology is not
completely achieved. But after 15 h of synthesis clear regular
sphere morphology has been observed. These spheres were constructed
by
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00
2000
4000
6000
8000
10000 S
In
In
In
In
Inte
nsity
Cps
Energy (keV)
-
several hours) tchange that the the
molparametnanofiberipening
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Fig. 3
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Inwere alsthe wavreflectan
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On the basns of In2S3of surface , the
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Upon increaetained andSEM meaw microsphe results ospheres cce
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wth processsed. It is wfrom powdthe crystar source surface
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me further (thesis timelso been oile increas
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When the sselectivity
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aken at difre reveal thnofibers.
f the In2S3 es were recn in Figurethe Kubel
(up to 24 e did not observed sing both the main s and 1D
Ostwald
sopheres igher the urfactant y. Since ent, the 009).
fferent hat a
samples corded in e 4. The ka-Munk
-
theory(Rengarajet al., 2006). The Kubelaka-Munk function F(R) =
(1-R)2/2R, is used as the equivalent of absorbance (and is plotted
in Figure 4). The absorption edges of the In2S3 hollow microspheres
were found between 610 and 620 nm, corresponding to the absorption
edge of a semiconductor material. For bulk like In2S3, the band gap
(Eg) is reported to be vary between 2.0 and 2.2 eV which correspond
to 620 to 550 nm. For our samples, the absorption edges were found
vary between 610 and 620 nm, that correspond to the absorption edge
of a semiconductor material. It is also interesting to note that
the absorption edges for all samples were very close to each other
indicating similar optical properties. Furthermore, the steep
absorption edge is an indication of a narrow size distribution and
uniform crystallites of In2S3 microspheres (Revathiet al., 2008);
the particle size and uniform distribution is confirmed by SEM
(Figure 3). The band gap values of these samples were calculated by
extrapolating the absorption edge by a linear fit method. For all
samples the band gap values were found very close to each other
around 1.9 eV, indicating that there is no change in band gap
energy values while changing the molar ratio of the precursors. The
steep shape of the visible region reveals that the absorption band
of In2S3 is due to the transition from the valence band to the
conduction band (Liu et al., 2006), and is not due to the
transition from the metal impurity level to the conduction band, as
observed for the metal ion-doped semiconductors(Liu et al., 2009).
The band structure indicates that charge transfer upon
photoexcitation occurs from the S 3p orbital to the In 5p empty
orbital.Thus, the steep absorption edge implies single-phase In2S3,
which is in good agreement with our XRD studies. Since the In2S3
absorb significant amount of visible light, it can be used as a
visible-light active photocatalyst.Hence the photocatalytic
properties of these In2S3 microspheres have been systematically
studied and are discussed in the following sections.
Fig. 4. UV-Vis DRS spectrum of In2S3 microsphere.
300 400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
Band gap (Eg ) = ~ 1.96 eV
In2S3 - 1:2 In2S3 - 1:3 In2S3 - 1:4
Abs
orpt
ion
(arb
. uni
ts)
Wavelength (nm)
-
3.5 Photocatalytic reduction of Cr(VI) in aqueous solution 3.5.1
Photocatalytic activity for Cr(VI) reduction with In2S3 hollow
microsphere.
To determine the photocatalytic activity of the prepared
catalysts, two experiments were first carried out with HCOOH as a
hole scavenger to reduce Cr(VI) in aqueous solution with an initial
concentration of 10 mg/l at pH 3.00 using In2S3 catalyst. The
concentration of Cr(VI) with reaction time are shown in Fig. 5. The
concentration of formic acid used in this study is based on a molar
ratio of Cr(VI) : HCOOH = 1:12, which is slightly more than
theoretical demand of 1:6 as shown in latter part of discussion.
The experimental results shown in Fig. 5demonstrated that after 2
minutes reaction time, Cr(VI) in all the suspensions were reduced
by more than 90%. In2S3 catalysts achieved the highest efficiency
of Cr(VI) reduction by more than 99%. In this study, the In2S3
hollow microsphere sample under visible light irradiation
demonstrated a considerable reduction of Cr(VI) in aqueous
solution.
Fig. 5.pH Change and variation of Cr(VI) and Cr(III)
conentrations during reaction. [Conditions: Cr(VI) coentration =
10mg/L; pH = 3.00; amount of catalyst = 1 g/L]
3.5.2 Effect of reductants
In addition to the effect of catalysts dosoage, another way to
promote photocatalytic performance is to add sacrificial electron
donors (hole scavenger) to the reaction system. Distinct kinds of
sacrificial reagents were commonly found to have different effects
in various systems. Accordingly, choosing a suitable and effective
sacrificial reagent becomes especially important for the
improvement of catalytic performance. In this study, formic acid
with a simple one-carbon molecular structure
0 2 4 6 8 101.681.70
1.721.741.761.78
1.801.821.84
1.861.88
C/C
0
pH Cr(VI) Cr(III)
Time (min)
0.0
0.2
0.4
0.6
0.8
1.0
-
was chosen as a hole scavenger to investigate its effect on
Cr(VI) reduction. Hence, its oxidation to carbon dioxide is
straightforward and involves minimal intermediate products(Aguado
and Anderson., 1993). Also, formic acid is capable of forming
reducing radicals, which could help in the reduction
reaction(Kaiseet al., 1994). In the presence of formic acid, Cr(VI)
reduction can be expressed as follows:
Cr2O72- + 14H+ +6e- 2Cr3+ + 7H2O (1)
One mol of Cr(VI) requires 6 mol of electrons to be reduced to
Cr(III), which needs equivalent 6 mol of HCOOH to scavenge the
holes. Which is having strong agreement with the previous
literature(Papadamet al., 2007). The organic species such as HCOOH
accept holes from the valence band either directly or indirectly
and subsequently oxidized and thereby suppressing the electron –
hole recombination process, increasing the reduction efficiency.
Thus it can reduce Cr(VI) to Cr(III). Our experiments confirmed
that using a hole scavenger such as formic acid is an essential
condition to proceed this photocatalytic reduction reaction, since
no catalytic activity of In2S3 was observed in the Cr(VI)solution
without a hole scavenger. In the same time, it was found that the
pH increased from 1.69 to 1.85 due to the consumption of formic
acid as shown in Fig. 5. 3.5.3 Time dependence of photocatalytic
reduction of Cr(VI)
The time dependence of the photocatalyticCr(VI) reduction
reaction over In2S3
catalyst is shown in Fig. 5. At the beginning of reaction, the
concentration of Cr(VI) decreased rapidly, but only trace of Cr(VI)
appeared in the solution. With the proceeding of the reaction, it
had been found that the concentration of the Cr(III) increased
quickly, and the pH value of reaction solution also increased
accordingly. Fig. 5 describes that after certain reaction period,
the concentration of Cr(VI) was slowly reduced. Actually
photocatalysis is suitable for both oxidation and reduction
reactions, since e- and h+ are generated simultaneously on In2S3
under visible light illumination. In our experiments, reduction
reaction was dominated in the initial stage due to the presence of
formic acid as a hole scavenger. The pH rising might result from
the reduction reaction from loss of formic acid. These results
suggested that such a photocatalytic reduction reaction could
affect pH in the reaction solution significantly. Furthermore, such
a heterogeneous reaction in aqueous In2S3 suspension is also
dependent on its adsorption rate on the catalyst surface.
Therefore, the adsorption properties of catalysts can be greatly
changed at different pH values. In an acidic environment, H+ ions
are adsorbed onto the surface of In2S3, which was reported to have
a large surface proton exchange capacity. The photogenerated
electrons can be captured by the adsorbed H+ to form Hoads, which
is able to reduce Cr(VI)(Zhang et al., 2005). At high pH, the
surface of catalyst has a net negative charge. Photocatalytic
effectiveness was known to depend on how long the photo-generated
electrons are trapped on the conduction band and how efficiently
they are used in sub-sequent reduction reactions(Kominamiet al.,
2001). Moreover, the morphology of the catalyst are another crucial
factors in determining catalytic activity(Ranjit and Viswanathan.,
1997).
-
In this study, In2S3 hollow microspheres are not only should
prolong the life time of the photo-generated electrons very well,
but should also act as efficient reduction sites. Since equal
numbers of e- and h+ must be consumed in the photocatalytic
systems, the acceleration of reduction reaction with e- at the
homogeneously dispersed catalyst surface enhances the overall
reaction as observed. 3.5.4 Discussion about photocatalyticCr(VI)
reduction From the above studies it is observed that photocatalytic
reduction of Cr(VI) to Cr(III) takes place when Cr(VI) solution
containing In2S3 microsphere is illuminated in light having photon
energy greater than the band gap energy of the semiconductor.
During photocatalysis, adsorption occurred first when In2S3 was
dispersed in the aqueous solution containing metal ions. This
process is reversible and even takes place in absence of light
illumination. The reduction of Cr(VI) to Cr(III) occurs because
under illumination electron – hole pairs are created inside the
semiconductor particles. After migration of these species to the
surface of the particle, the photogenerated electrons reduce Cr(VI)
to Cr(III) and holes oxidize water and sacrificial electron
donor.
The possible mechanism of the photocatalytic reduction of
hexavalent chromium is as follows:
In2S3In2S3 (h+ + e-) (2) Cr2O72- + H+ + e- Cr3+ + H2O (3) H2O +
h+ O2 + H+ (4)
As the mentioned above, In2S3 hollow microsphere showed higher
photocatalytic
activity in Cr(VI) reduction. It has been generally believed
that the conversion of Cr(VI) into Cr(III) is structure-sensitive
and geometric effect of catalyst plays an important role in
catalytic reduction of Cr(VI). From the experimental results, it is
clear that In2S3 hollow microsphere shows very good candidate for
the reduction of Cr(VI). 3.6 Mechanism of the enhanced
photocatalytic activity
The photocatalytic activity of In2S3 for the reduction of Cr(VI)
may be enhanced by the electron-hole separation and the subsequent
transfer of the trapped electron to the adsorbed O2 or Cr(VI)
acting as an electron acceptor. After the photogeneration of
electrons and holes by photons of appropriate energy (h EG, EG =
~2.0 ev) as shown in Equation 5. Because of the shorter band gap in
the In2S3 nanostructure the electron-hole separation takes place.
This reaction enables the positive photoholes p+ to react with OH-
adsorbed species to create oOH as shown in Equation 8, which are
generally assumed to be the degradating oxidative agents. In the
same way photogenerated electron reduces Cr(VI) to Cr(III) as shown
in equation 8.
In2S3 + h e- + p+ (5)
OH- + p+oOH (6) oOH + HCOOH Intermediates CO2 +H2O (7)
Cr2O72- + 14H+ +6e- 2Cr3+ + 7H2O (8)
-
HconductHCOOHfollowingenhancenanostrureductio
Fig. 6.M
4. C Inprepare thiosemprecursomicrosp(Eg = 1.ratios.
Tmicrosplike In2Schromiu
Here, In2S3ion band t
H photodeg photocaed reductiucture incrn of Cr(VI)
Mechanism
CONCLUS
n conclusiohierarchi
icarbazideors. The Xheres. The9 eV) is saThe compoheres syntS3,
the In m in aque
3 nanostruthus improgradation talytic pation of oxreases
the).
m of the pho
ION
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e assisted X-ray diffrae UV-Vis Dame for allositional athesized
b
to S ratieous solut
ucture favooving the e
in aqueothways unxygen thro rates of H
otocatalytic
ve demonsn2S3 hollosolvothermction analy
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ors the mielectron – hous In2S3nder visibleough betteHCOOH
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strated a sow microsmal procesysis confir
surements and did noade by EDe free methund to bee effective
gration of hole separsuspensio
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on the In2S.
successful spheres bss with simrmed the teconfirmed
ot change DX measurhod mainta
e 2:3.In thely reduced
photo-proration. In aon has be shown in
on-hole sesimultane
S3 catalyst
and reproby a simp
mple inorgaetragonal that the oupon chanements coain the stoe
present d to the tr
oduced eleaddition to een to fon the Fig.eparation eously
incre
t under visi
oducible mple templa
anic compophase of t
observed bnging the ponfirmed thoichiometry
work, herivalent sta
ectron to this, the
ollow the . 6. The in In2S3 ease the
ible light
method to ate free ounds as the In2S3 band gap precursor hat
these y as bulk exavalent ate using
-
In2S3 as the semiconductor photocatalyst under visible light
radiation in the presence of sacrificial electron donor. The
enhancement of the photocatalytic activity of the In2S3 was
confirmed in the reduction of Cr(VI) in the presence of formic
acid. The improved separation of electrons and holes on the In2S3
surface allows more efficient channelling of the charge carriers
into useful reduction and oxidation reactions rather than
recombination reactions. The photocatalyticreduction of hexavalent
chromium is enhanced in the presence of sacrificial electron donors
such as HCOOH. The experiments demonstrated that Cr(VI) was
effectively degraded in aqueous In2S3 suspension by more than 99%
within 180 min, while the pH of the solution increased from 1.65 to
1.85 due to the reduction of Cr(VI) and consumption of formic acid.
ACKNOWLEDGEMENTS
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