M.R. Pradhan et al.
Inorganic Chemistry Communications 130 (2021) 108717
were synthesized by the researchers for different applications such a
photocatalytic oxidation and reduction of organic and inorganic pol-
lutants, supercapacitor, sensing, H2 energy production, antimicrobial
activities etc. [9–12]. g-C3N4/MnO2 and g-C3N4/SnO2 hybrid nano-
composites were fabricated by Kavil and his team for supercapacitor
application. They found a two times increase in specific capacitance
incase of composites than neat g-C3N4 [13]. Paul and his group fabri-
cated visible light driven photocatalyst (ZnO-modified g-C3N4) for
effective degradation of MB [14]. Development of heterojunction, better
separation of photo generated charge carriers increases the degradation
efficiency three times than neat pure g-C3N4. Cao et al. synthesized
SnO2/g-C3N4 composite applied for high performance ethanol sensing
[15]. Versatile boron doped g-C3N4 (B/g-C3N4) was fabricated by Flores
and his group for the selective photocatalytic production of acetone
from 2-propanol by utilizing the sustainable energy source [16].
The extensive utilization of g-C3N4 modified composites with
different metal oxides, such as one-dimensional MnO2 had built a center
of attraction because of its natural abundance, structural diversity, low
toxicity, superior electrical, optical, magnetic, catalytic, electrochemical
properties and more prominent multi valence nature. These unique
structural and physicochemical properties had taken MnO2, a great deal
of attention in the field of catalysis, sensing, supercapacitors. MnO2 exist
2. Material and methods
2.1. Synthesis of MnO2
For preparing MnO2 nanoparticles, 2 gm of Poly vinyl pyrolidone
(PVP) was dissolved in 150 mL of DI water and stirred vigorously for 60
min. To the above solution 8.4 gm of MnSO4 was added followed by the
addition of 11.4 gm of (NH4)2S2O8 with continuous stirring for 3–4 h. 2
mL of NH4OH was added to the above solution and stirred for 2 h and a
black precipitate was obtained. The precipitate was centrifuged with
minimum amount of DI water and ethanol and collected after drying
overnight at a temperature of 80 ◦C. To remove the extra impurities the
dried sample was calcined at 350 ◦C for 3 h and named as MnO2 (M).
2.2. Synthesis of exfoliated porous g-C3N4
Porous graphitic carbon nitride (g-C3N4) was prepared through a
semi-closed alumina crucible by taking 5 gm of melamine and minimum
amount of NH4HCO3 with a molar ratio (1:0.1) through thermal reaction
process. The crucible was heated to 500 ◦C and clasp for 4 h at a heating
rate of 5 ◦C minꢀ 1. After that deamination was performed at same
◦
temperature 500 C for 2 h, then the alumina crucible was cooled at
in different crystallographic arrangements (α, β and γ) were able to use
room temperature. The product was collected and grinded into powder.
1 gm of porous g-C3N4 and 135 mL distilled water were taken in a 250
mL beaker and thoroughly mixed and the solution was sonicated for 4 h.
The resultant solution was centrifuged at 2000 rpm for 5 min. After that
the obtaining residue was kept inside the hot air oven for 5 h at 70 ◦C.
The dried sample was powdered with help of a mortar and pestle to get
porous exfoliated g-C3N4 (G).
as a filler in the preparation of nano composites [17,18]. Also interest-
ingly, MnO2 is a narrow band semiconductor and can improve the utility
of visible light. Therefore, to improve its photocatalytic efficiency, dis-
tribution of MnO2 on the surface of g-C3N4 provides a better platform for
various applications. A number of works already been reported by sci-
entists using g-C3N4 grafted MnO2 for different applications. Wang and
his team synthesized MnO2/g-C3N4 heterojunction composite through
in-situ redox reaction process for enhanced reduction ability of CO2
[19]. The well matched band structure of MnO2 and g-C3N4 form a
hetero-interface between MnO2 and g-C3N4. This enhances the separa-
tion of photo-induced charge carriers and increases the production of CO
to 9.6 µmol gꢀ 1. A novel sandwich like MnO2/g-C3N4 was fabricated by
Chang et al. by incorporating MnO2 nano rod onto the 2D sheet of g-
C3N4. The MnO2/g-C3N4 nanocomposite electrode was shown its best
performance for supercapacitor [20]. Sanny Verma and Rajender Verma
synthesized MnO2-g-C3N4 for oxidation of aromatic amines to azo
compounds under visible light as source of energy at room temperature
[21].
2.3. Synthesis of MnO2 modified exfoliated porous g-C3N4
1 gm of porous exfoliated g-C3N4 was dissolved in minimum amount
of distilled water. Different weight percentage (1, 3, 5, and 7 wt%) of
MnO2 were added to the above solution and stirred continuously for 60
min at 80 ◦C. The resultant solid was calcined at the temperature 350 ◦C
for 3 h and renamed as GM1, GM3, GM5 and GM7.
2.4. Photocatalytic procedure
The prepared samples were used for the photocatalytic oxidation of
phenolic derivatives under the irradiation of visible light (λ > 420 nm).
20 mL of aqueous aromatic alcohol derivatives (100 ppm) were taken in
a conical flask with continuous stirring for 30 min in dark in presence of
above samples. To maintain the adsorption–desorption equilibrium, the
solutions were subjected to irradiate with 150 W xenon lamp with 420
nm cutoff filter. At regular time interval the solutions were discharged
and centrifuged to get a clear solution. The percentage (%) of oxidation
was calculated through % oxidation efficiency.
Huge tones of aromatic alcohols have been discharged from the
pharmaceutical industries. This increases the toxicity, and hazardous
level of aquatic ecosystem and thereby hampering the human life.
Different conventional treatment methods such as distillation, chemical
precipitation, and solvent extraction have been used for the removal of
aromatic alcohols. The partial oxidation of aromatic alcohols to alde-
hydes was traditionally done by CrO3/chromium complexes [22,23]. It
is a reaction bearing extreme importance in the industrial processes.
Aromatic aldehydes can be used as building blocks for the synthesis of
fine chemicals and pharmaceuticals. However the use of chromium
containing catalysts was carcinogenic and not environmentally friendly.
Use of visible light active photocatalysts play an important role in the
oxidation of aromatic alcohols. The conventional technique was not
suitable for large scale applications along with multiple drawbacks of
the like high operational cost and low removal efficiency. To overcome
this problem, photocatalytic degradation by the semiconductor based
photocatalyst was the most favorable sustainable green technique used
to mitigate the conventional methods [24–27].
% oxidation efficiency = A0-At/A0,
Where, A0 was the initial absorption intensity at t was 0, and At was
the final absorption intensity at time was denoted as t.
2.5. Plausible mechanism
α
-MnO2 a polymorphous of MnO2 was prepared by oxidation of
MnSO4 by (NH4)2S2O8 as per the following reaction:
2 MnSO4 + (NH4)2S2O8 + 4H2O → 2HMnO2 + (NH4)2SO4 + 3H2SO4
Herein, MnO2 was modified on the surface of porous g-C3N4 (2D)
nanosheet to improve the photocatalytic oxidation efficiency of aro-
matic alcohols. The proper light harvestation, electron-hole channeli-
zation, reduced in electron-hole recombination, charge transfer through
p-n junction mainly responsible in the enhancement of photocatalytic
oxidation efficiency of aromatic alcohols.
The intermediate manganous acid undergoes further oxidation by
using calculated amount of persulphate to MnO2 as follows:
2 HMnO2 + (NH4)2S2O8 → 2 MnO2 (blue ppt)↓ + (NH4)2SO4 + H2SO4
PVP acts as the binding glue for persulphate and MnSO4. The addi-
tion of NH4OH accelerates the rate of forward reaction by consuming
2