The field of photocatalysis has been excellently
reviewed by Sahasi et al.1 Photocatalytic degradation of cetylpyridinium chloride over TiO2 has been
reported by Singhal et al.2 Similar photocatalytic reaction of xylidine
ponceau and orange-G dyes by ZnO
powder has been reported by Sharma et al.3 Yoneyama et al.4 studied the photocatalytic reduction of dichromate ions using WO3
powder in acidic range. Costa et al.5 proposed the mechanism
of gas phase degradation of cyclohexanol and methylcyclohexanol catalyzed by niobyum
pentaoxide and phosphite. Shyama and Arakawa6 have used zirconium dioxide
as photocatalyst. A composite system of
polycrystalline ZnO/TiO2 and its photocatalytic activity was studied by Marci et al.7 TiO2
sol-gel deposited over glass and its application as an effect of TiO2
acidic pretreatment on the photocatalytic phenol
degradation was reported by Colon et al.8 whereas
photodegradation of lignin from black liquor using a
UV/TiO2 system was investigated by Ksibi et
al.9 Enhanced efficiency of unsymmetrical versus symmetrical squaraine dyes sensitized nanocrystalline
TiO2 was reported by Alex et al.10 Morwetz and Selli11 investigated the effect of iron species in photocatalytic degradation of azo
dye in TiO2 suspension. Photocatalyst for
water decontamination has been reported by Gelover et
al.12 Kako et al.13 suggested some
preventive methods against catalytic poisoning of TiO2 photocatalyst by H2S. The Kim et al.14 used ZnO coated TiO2 nanoparticles
for the flexible dye-sensitized solar cells. Photocatalytic
degradation of brilliant red dye and textile waste water has been suggested by
Martins et al.15 Photocatalytic
degradation of acid blue-62 over CuO-SnO2 nanocomposite
photocatalyst under simulated sunlight has been
reported by Xia et al. 16 Degradation of some dyes using
zirconium phosphate photocatalyst have been studied
by Panwar et al.17 Use of semiconducting iron (II) oxide in photocatalytic degradation of some dyes (malachite green,
crystal violet and methylene blue) has been reported
by Ameta et al.18 Photoreduction of Congo red by ascorbic acid and EDTA over cadmium sulphide as photocatalyst was
carried out by Kothari et al.19 Photocatalytic
activity of antimony (III) sulphide in degradation of
Azure-B was carried out by Ameta et al.20
Experimental
procedure:
Cationic
dyes (methylene blue) and semiconducting bismuth
oxide or bismuth sulphide powder were used in the
present investigation. All the solutions were prepared in doubly distilled
water. The photocatalytic degradation of the dyes was observed by taking dye solution and
bismuth oxide or bismuth sulphide together.
Irradiation was carried out by keeping the whole assembly exposed to a 200W
Tungsten lamp (Philips; light intensity = 50.0 mW cm–2).
The intensity of light at various distances from the lamp was measured with the
help of a solarimeter. A water filter was used to cut
out thermal radiations. The pH of the solutions was measured with the help of
digital pH-meter.
0.3739 g of methylene blue
was dissolved in 100.0 mL of doubly distilled water
so that the concentration of dye solution was 1.0 ´ 10–3 M. It was used as a stock
solution. This stock solution was further diluted as
and when required. The absorbance of these dye solutions were determined with
the help of a spectrophotometer at λmax
= 660 nm for methylene blue. The solutions of the dyes were divided into four parts;
the first beaker containing only dye solution was kept in dark; the second
beaker containing only dye solution was kept in light; in the third beaker dye
solution and 0.10 g and 0.12 g of semiconductor bismuth oxide and bismuth sulphide respectively was kept in dark and in the fourth
beaker dye solution with 0.10 g and 0.12 g of semiconductor bismuth oxide and
bismuth sulphide respectively was exposed to light.
These beakers were kept for 4 hours and
then the absorbance of solution in each beaker was measured. It was observed
that the solutions in the first three beakers had the almost same initial
absorbance while the solution in the fourth beaker had a decrease in its
initial value of absorbance. Thus, by performing blank experiment it was
confirmed that the reaction between methylene blue
and semiconductor powder is neither thermal nor photochemical but it is a photocatalytic reaction. The progress of the reaction was
monitored spectrophotometrically by taking absorbance of the reaction mixture at different time interval. From these results, it is clear that reaction
requires both light and semiconductor to degrade dye, hence showing the photocatalytic nature of the reaction.
Figure:
- 1. A Typical Run (® -
[Methylene blue] = 6.0 ´ 10–6 M, Bi2S3 =
0.12g, pH = 9.00, Light Intensity = 50.0 mW cm–2, k = 3.20 ´ 10–5 sec–1 and ® [Methylene blue] = 4.0 x 10-6 M, Bi2O3 =
0.10g, Light Intensity = 50.0 mWcm–2, pH =
8.00, k = 3.83 x 10-5 sec-1)
Results
and discussion:
Photocatalytic degradation of methylene
blue is observed at lmax
= 660 nm. Degradation of the dyes in
absence of semiconductor (Bi2O3 or Bi2S3)
is negligible. Thus, photocatalytic
degradation is favorably affected by semiconductor. A plot of optical density (1 + log OD) versus
time is linear and hence, the reactions follow pseudo first-order kinetics
(Table 1 and Figure 1). The rate
constants are determined with the help of the curves.
Table 1 A TYPICAL RUN:
|
Time
(min.) |
Bismuth
oxide |
Bismuth
sulphide |
|
2
+ log O. D. |
2
+ log O. D. |
|
|
0 |
0.5051 |
0.6513 |
|
20 |
0.4771 |
0.6304 |
|
40 |
0.4594 |
0.6117 |
|
60 |
0.4346 |
0.5933 |
|
80 |
0.4166 |
0.5809 |
|
100 |
0.3979 |
0.5682 |
|
120 |
0.3820 |
0.5563 |
|
140 |
0.3617 |
0.5428 |
|
160 |
0.3424 |
0.5302 |
|
180 |
0.3284 |
0.5132 |
|
200 |
0.3118 |
0.4942 |
|
220 |
0.2900 |
0.4771 |
|
240 |
0.2718 |
0.4624 |
|
260 |
0.2577 |
0.4472 |
|
280 |
0.2380 |
0.4298 |
|
300 |
0.2175 |
0.4116 |
|
320 |
0.2041 |
0.3979 |
|
340 |
0.1875 |
0.3802 |
|
360 |
0.1673 |
0.3655 |
|
380 |
0.1430 |
0.3483 |
|
400 |
0.1206 |
0.3304 |
|
420 |
0.1004 |
0.3139 |
|
440 |
|
0.3010 |
|
460 |
|
0.2878 |
|
480 |
|
0.2695 |
|
500 |
|
0.2504 |
Effect
of pH:
The pH of the
solution is likely to affect the bleaching of the methylene
blue. The effect of pH on the rate of bleaching of methylene
blue was investigated in the pH range 5.00-10.0. The results are reported in
Table 2 and Figure 2. It is evident from the observed data that the rate of photocatalytic degradation of methylene
blue are optimum at pH 8.00 and 9.00 respectively and after that the rate
constant decreases on further increasing the pH of the solutions.
The increase in the rate of photocatalytic
bleaching with increase in pH may be due to more generations of •OH
radicals, which are produced from the interaction of OH- and hole (h+)
of the semiconductor. These •OH oxidize the dye molecules in their leuco forms, which ultimately degrade in the non-hazardous
products. But after pH 8.00 and 9.00 for methylene
blue, the dye molecules becomes neutral and feel less attraction to -OH
and hence the rate of the reaction decreases on further increasing pH of
the solutions.
Table 2 EFFECT OF pH
|
pH |
Bismuth
oxide |
Bismuth
sulphide |
|
k
x 105 (sec-1) |
k
x 105 (sec-1) |
|
|
5.00 |
1.91 |
1.00 |
|
5.50 |
2.13 |
1.09 |
|
6.00 |
2.55 |
1.64 |
|
6.50 |
2.90 |
1.91 |
|
7.00 |
3.10 |
2.00 |
|
7.50 |
3.29 |
2.13 |
|
8.00 |
3.83 |
2.25 |
|
8.50 |
3.71 |
2.55 |
|
9.00 |
3.62 |
3.20 |
|
9.50 |
2.55 |
2.79 |
|
10.00 |
2.10 |
2.36 |
Figure:
- 2. Effect of pH (® -
[Methylene blue] = 6.0 ´ 10–6 M, Bi2S3 =
0.12g, Light Intensity = 50.0 mW
cm–2and ® [Methylene blue] = 4.0 x 10-6 M, Bi2O3 =
0.10g, Light Intensity = 50.0 m Wcm–2,)
Effect
of dYE CONCENTRATION:
Effect of
concentration of methylene blue was studied by taking
different concentrations of this dye. The results are tabulated in Table 3 and
Figure 3. It was observed that the rate of photocatalytic
bleaching increases with an increase in the concentration of the dyes.
It may be due to
the fact that as the concentration of dye increases more dye molecules are
available for excitation and energy transfer and hence, an increase in the rate
of photocatalytic degradation of the dyes were
observed. The rate of photocatalytic degradation was
found to decrease with further increase in the concentration of the dyes i.e.
above 4.0 x 10–6 M and 6.0 ´ 10–6 M for methylene
blue. This may be attributed to the fact that after certain concentration, the
dye itself will start acting as a filter for the incident light and it will not
permit the desired light intensity to reach the semiconductor particles; thus,
decreasing the rate of photocatalytic degradation of
dye.
Table
3 Effect of dYE CONCENTRATION
|
[Methylene blue] x 106 M
|
Bismuth
oxide |
Bismuth
sulphide |
|
k
x 105 (sec-1) |
k
x 105 (sec-1) |
|
|
2.0 |
3.19 |
2.06 |
|
3.0 |
3.29 |
2.13 |
|
4.0 |
3.83 |
2.39 |
|
5.0 |
2.87 |
2.66 |
|
6.0 |
2.39 |
3.20 |
|
7.0 |
2.30 |
3.07 |
|
8.0 |
2.25 |
2.95 |
|
9.0 |
2.15 |
2.87 |
Figure: - 3. Effect of dye
concentrations (® -
Bi2S3 = 0.12g, pH = 9.00, Light
Intensity = 50.0 mW cm–2, and ®- Bi2O3 = 0.10g, Light Intensity = 50.0 mWcm–2, pH = 8.00)
Effect
of amount of SAMICONDUCTOR:
The amount of semiconductor is also likely
to affect the rate of photocatalytic bleaching of methylene blue hence; different amounts of photocatalyst were used. The results are reported in Table
4 and Figure 4. It was observed that the rate of photocatalytic
degradation of methylene blue increases with an
increase in the amount of semiconductor but ultimately, it became almost
constant after a certain amount i.e. 0.10 g a for both bismuth oxide and sulphide.
This may be
attributed to the fact that as the amount of semiconductor was increased, the
exposed surface area increased, which absorb more number of photons and as a
result the rate of photocatalytic degradation of the
dyes increased, but after a certain limit, if the amount of semiconductor was
further increased, then there will be no increase in the exposed surface area
of the photocatalyst. It may be considered like a
saturation point; above which any increase in the amount of semiconductor has
negligible or no effect on the rate of photocatalytic
degradation of the dyes, as any increase in the amount of semiconductor after
this saturation point will only increase the thickness of the layer at the
bottom of the reaction vessel. This was confirmed by taking reaction vessels of
different dimensions. The saturation point shifts to higher range for larger
vessels, while reverse was true for smaller vessels.
Table 4 Effect of amount of SAMICONDUCTOR
|
Semiconductor
amount (g) |
Bismuth
oxide |
Bismuth
sulphide |
|
k
x 105 (sec-1) |
k
x 105 (sec-1) |
|
|
0.02 |
2.74 |
1.05 |
|
0.04 |
2.87 |
1.08 |
|
0.06 |
3.19 |
1.38 |
|
0.08 |
3.48 |
1.52 |
|
0.10 |
3.83 |
1.62 |
|
0.12 |
3.90 |
1.60 |
|
0.14 |
3.80 |
1.58 |
|
0.16 |
3.48 |
- |
Figure:
- 4. Effect of amount of semiconductor (® -
[Methylene blue] = 6.0 ´ 10–6 M, pH = 9.00, Light Intensity = 50.0 mW cm–2 and ® [Methylene blue] = 4.0 x 10-6 M, Light Intensity = 50.0 mWcm–2,
pH = 8.00)
EFFECT OF LIGHT INTENSITY:
To
observe the effect of intensity of light on the photocatalytic
degradation
of the dyes, the light intensity was varied. The
results obtained are reported in Table 5 and Figure 5.
The data indicate that an increase in the
light intensity increases the rate of reaction and the optimum values were
found at 50 mW cm–2 for both
semiconductors.
It may be explained on the basis that as
the light intensity was increased, the number of photons striking per unit area
also increased, resulting into a higher rate of degradation. Further increase
in the intensity beyond the maximum limits result in decrease in the rate of
reaction. It may be probably due to thermal side reactions.
Table
5. EFFECT OF LIGHT INTENSITY
|
Intensity
of Light (mWcm–2 )
|
Bismuth
oxide |
Bismuth
sulphide |
|
k
x 105 (sec-1) |
k
x 105 (sec-1) |
|
|
10.0 |
2.83 |
1.59 |
|
20.0 |
3.01 |
2.02 |
|
30.0 |
3.24 |
2.55 |
|
40.0 |
3.51 |
2.87 |
|
50.0 |
3.83 |
3.20 |
Figure:
- 5. Effect of light intensity (® -
[Methylene blue] = 6.0 ´ 10–6 M, Bi2S3 =
0.12g, pH = 9.00, and ® [Methylene blue] = 4.0 x 10-6 M, Bi2O3 =
0.10g, pH = 8.00)
MECHANISM:-
1Dye0 
1 Dye 1 (Singlet excited state)
1 Dye 1 
3 Dye 1 (Triplet excited state)
SC e- (CB) + h+ (VB) or SC*
h+ + OH- 
•OH
•OH + 3 Dye 1 Leuco Dye
Leuco Dye Products
Methylene blue absorbs radiations of suitable wavelength and
gives rise to its excited singlet state. Then it undergoes intersystem crossing
(ISC) to give the triplet state of the dyes. The involvement of triplet state
was confirmed by using triplet state scavengers, where the reaction rate was
almost negligible. On the other hand, the semi-conducting bismuth oxide (SC)
also utilizes the radiant energy to excite its electron from valence band to
the conduction band; thus, leaving behind a hole. This hole abstracts an
electron from OH– ions to generate ·OH radicals. These radicals will oxidize the
dye to its leuco form, which may ultimately degrade
to products. The participation of ·OH radicals as an active oxidizing species
was confirmed by using hydroxyl radical scavenger isopropanol, where the rate of bleaching was drastically reduced.
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Received on 07.09.2012
Modified on 02.10.2012
Accepted on 09.10.2012
© A&V Publication all right reserved
Research J. Science and Tech. 4(5):
September –October, 2012:
208-212