Efficient photocatalytic selective nitro-reduction and C-H bond oxidation over ultrathin sheet mediated CdS flowers

Article abstract of DOI:10.1039/c5cc01685a

We report here a visible light driven selective nitro-reduction and oxidation of saturated sp³ C-H bonds using ultrathin (0.8 nm) sheet mediated uniform CdS flowers as catalyst under a household 40 W CFL lamp and molecular oxygen as oxidant. Th

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Efficient photocatalytic selective nitro-reduction and
C-H bond oxidation over ultrathin sheet mediated CdS
flower
Cite this: DOI: 10.1039/x0xx00000x
*
Received 00th January 2012,
Accepted 00th January 2012
Sandip Kumar Pahari, Provas Pal, Divesh N. Srivastava, Subhash Ch. Ghosh and
Asit Baran Panda
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report here a visible light driven selective nitro reduction of photogenerated electron due to the transportation of electron,
3
originating from its long range lattice periodicity which provides
and oxidation of saturated sp C-H bonds using ultrathin (0.8
7, 9
better channel for electron transport. However, there are very few
reports on CdS sheets, with the thickness in a quantum confinement
nm) sheet mediated uniform CdS flowers as catalyst under
house hold 40W CFL lamp and molecular oxygen as oxidant.
The CdS flower was synthesized using a simple surfactant
7a
region. Xu et al. reported exfoliated CdS flower with ~ 4nm thick.
7b
Zhang et al. reported bulk CdS sheets without any confinement.
Best of our knowledge there are no reports on the synthesis of CdS
sheet as well as their assembly with thickness beyond its Bohr
assisted hydrothermal method
.
Reduction of organic nitroꢀcompounds to corresponding amine radius, i.e., ≤ Bohr radius (2.8nm).
without affecting the other reducible groups and oxidation of
saturated sp CꢀH bond using molecular oxygen as oxidant to value CdS sheet and their assembly towards uniform flowers with cubic
Here, we report for first time the synthesis of ultrathin (0.8 nm)
3
added products are really challenging and most important class of phase (ZB) in the presence of decanoic acid (DA) as a ligand,
1ꢀ2
reactions in organic synthesis as well as in chemical industry. Both potassium orthoethyl xanthate (XA) as sulphur source and water as
the reactions required harsh reaction condition, expensive reagents, solvent in hydrothermal condition. Furthermore, the synthesized
additional steps to protect other functional group and mostly ultrathin CdS flower showed excellent visible light, from 40W
3
generate hazardous waste. Thus, development of simple, green and simple house hold CFL lamp, driven photocatalytic activity towards
selective routes for these reactions are highly desirable. Owing to the selective nitro reduction and oxidation of saturated sp3 CꢀH bonds
only sustainable energy resource on earth, solar energy has great using molecular oxygen as oxidant in ambient condition. Generally,
potential as clean and economical energy source. Hence, efficient the photocatalytic reactions, specifically the oxidation of saturated
conversion and/or harvesting of solar energy to electrical energy or sp3 CꢀH bonds, were performed under Xe lamp for good activity and
chemical energy, i.e., visible light driven chemical conversion special precautions are needed. Here, for the first time we have used
(
photocatalysis) is promising and have been triggered worldwide 40W simple house hold CFL lamp to make the procedure simple and
during last two decade.
synthesized CdS flowers is too active to give excellent results under
Semiconductor based heterogeneous photocatalysts are the used CFL lamp.
advantageous over transition metal complex based homogeneous
In the developed synthetic strategy, use of aqueous ammonium
photocatalyst with respect to waste generation and reuse. Recent cadmium [{Cd(NH ) } ] complex solution enables the procedure
2+
3
4
studies revealed that CdS is one of the important visible light driven suitable for assembled CdS sheet in basic medium through the
4
ꢀ7
semiconductor photocatalysts. Efficiency of pure CdS, bulkꢀ/nanoꢀ desolation of decanoic acid in water by in-situ ammonium salt
particle, is not satisfactory due to the recombination of the photoꢀ formation. The whole synthesis comprise by the following steps;
8
generated electronꢀhole. Thus prolonging the recombination formation of ZB CdS nucleated nanoparticles through decomposition
through enhancement of lifetime the electron in conduction band and of in-situ generated cadmium xanthate formed by the reaction of
2+
efficient transfer of the electron to the reacting substrate are the ammonium complex of cadmium [{Cd(NH ) } ], simultaneous
3
4
effective path to improve photocatalytic efficiency. To improve the desolation of DA in aqueous reaction media by ammonium
lifetime of the photoꢀgenerated electron different attempts have been decanoate formation and stabilization of newly formed CdS
made through the formation of one dimensional (1D) rod and wire, particles, formation of CdS sheets by oriented attachment
†
composites of CdSꢀgraphene, CdSꢀTiO ꢀgraphene, CdSꢀ noble metal (Experimental section, see ESI ).
2
5ꢀ7
nanocomposits.
After the discovery of graphene, 2D
The synthesized flowers are uniform in size and selfꢀassembled
semiconductor nanosheets are getting interest for their high surface (Fig. 1a & S1, see ESI ) with a 100ꢀ125 nm. The flowers are
area, quantum confinement and most importantly improved lifetime constituted of assembled thin sheets. The sheets are overlapped on
†
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DOI: 10.1039/C C0168
Jou5Crnal Na5Ame
probable driving force for anisotropic growth through orientated
attachment of symmetric cubic (zinc blend) CdS dot is dipole
moments in the cubic crystal originating from the distribution of the
9d
chemically inhomogeneous plains. {111} facets of cubic CdS are
terminated by either Cd or S atoms with varying electronegativity
11
and contain dipole moments. In the present case, due to the high
surface energy of the {111} planes, originating from higher packing
density and the number of unsaturated atoms on the {111} facets, the
high index {111} facets grown faster. Thus, to reduce the surface
energy, the CdS dots aligned them self’s through chemically
inhomogeneous {111} facet and merged through orientated
attachment to give sheet structure (Fig. S4c, see ESI†). In the
intermediate stage of sheet formation some void space was also
observed and void space was filled by Ostwald ripening and finally
resulted uniform sheet. For the Ostwald ripening the width of the
sheets was less than that of parental spherical particles.
Fig. 1
TEM and HRꢀTEM images (aꢀc), XRD pattern (d), AFM image
with height profile (e) and UVꢀvis absorption and corresponding normalized
PL spectra (f) of synthesized CdS sheet assembled flower.
st
In the 1 set of reaction, reduction of pꢀnitrophenol to pꢀ
aminophenol was chosen as a model reaction to check the catalytic
each other and dark positions are probably the edgeꢀcurled nanoꢀ efficiency of synthesized sheet mediated CdS flower using hydrazine
sheets (Fig. 1b). In the highꢀresolution TEM images, the inter as a reducing agent and ethanol as solvent. A critical comparison of
planner distance of 0.33 ± 0.05 nm of sheets with the dꢀvalue of the reduction potential of both CdS flower and pꢀnitro phenol
(
111) plain of CdS ZB structure (JCPDS File No. 01ꢀ75–1546) indicates that the reduction of pꢀnitrophenol is thermodynamically
depict the sheets were grown through <111> direction (Fig. 1c). The feasible, as the potential of CdS flower is sufficiently less (– ve)
well resolved Xꢀray diffraction peaks of the synthesized CdS flower (Fig. S7, see ESI†). The reaction was performed systematically and
can be indexed to cubic structure (Zinc blend) of CdS (Fig. 1d) and the progress of the reaction was monitored using UVꢀVis absorption
support the TEM observation. The thickness of the individual sheets, spectroscopy. The characteristic peak of pꢀnitrophenol at 400 nm
obtained through AFM height profiling in the edge of the flower, are decreased gradually and a new peak of pꢀaminophenol at 304 nm
~
0.8nm. (Fig. 1e & S2, see ESI†). The observed compositional ratio appeared and the peak intensity increased gradually during the
of Cd and S (~1:1) from energyꢀdispersive Xꢀray spectroscopy progress of the reaction (Fig. 2, upper left panel). It takes 1.5 h for
EDX) (see ESI†, Fig. S3) confirmed the composition of synthesized the complete reduction of pꢀnitrophenol to pꢀaminophenol. We have
assembled sheet as CdS.
replaced the reducing agent hydrazine by NaBH4, but unfortunately
(
During progress of reaction, time resolved TEM analysis of did not show any significant improvement. The conversion was
intermediates revealed that the long range selfꢀassembled very small calculated by UVꢀVis spectroscopy (Fig. S8, see ESI†) and it was
nanoꢀdots (~1 nm) was formed in 30 min (see ESI†, Fig. S4a). Then, further supported by NMR spectroscopy indicates >99% conversion
the dots were reꢀassembled in a localized manner in a spherical of the reaction. Full conversion of pꢀnitrophenol in various solvents
shape with a diameter of ~ 100 nm in 30ꢀ45 min (see ESI†, Fig. other than ethanol, like 1,4ꢀdioxane, toluene, confirm that hydrazine
S4b). After 45 min, the dots within the spherical assembly aligned is working as source of electron and proton, and reduces the
themselves, merged through 2D oriented attachment and resulted probable photo corrosion of CdS as photo catalyst. Based on the
sheets. Finally, after 1.5 h, it gave a complete flower shape, where above mentioned observation, we have proposed a mechanism for
sheets are petal of flower (Fig. 1b, S4d see ESI†). All the above the effective conversion of the nitro compounds (Fig. 2b, upper right
described intermediates were of zinc blend structure (see ESI†, Fig. panel). According to proposed mechanism, the valence band electron
S5). The absorption spectra of assembled dots, formed in initial stage of CdS transfer to its conduction band, leaving a hole in the valence
(
in 30 and 45 min), clearly shows structured spectra with an band on the visible light irradiation. The high energy conduction
absorption maxima at 407 nm. Whereas the flower gave the fully band electron goes to the nitro group and subsequent amination took
+
unstructured spectra keeping the spectral position unchanged which place by taking H from hydrazine. The photoꢀgenerated hole in the
further confirmed the sheets were formed from dots through 2D VB of CdS was subsequently filled by the electron which was
oriented attachment (see ESI†, Fig. S6). Similar observation was formed from the decomposition of hydrazine.
9
d
reported by Weller and his group. for PbS sheets. Reasonable blue As this methodology was found highly effective for photocatalytic
shifted absorption maxima compared to the bulk CdS (490nm, reduction of pꢀnitrophenol, we extended the substrate scope to
2
.5eV) established the presence of strong quantum confinement different aromatic nitro compounds, e.g. pꢀnitro toluene and pꢀnitro
effect from its ultrathin feature.
benzonitrile (Fig. 2, lower left panel). To our delights it selectively
Based on the above mentioned experimental results a probable reduced the nitro groups in presence of other reducing functional
formation mechanism is proposed (see ESI†, Fig. S4). In the group like nitrile, alkene and ester group with excellent conversion
developed procedure water has a crucial role in the formation of and selectivity. We reused the catalyst for 4 cycles for the reduction
spherical assembly and inꢀturn sheet assembled flower shape. After of nitrophenol, no significant change in catalytic efficiency was
the formation of selfꢀassembled decanoic acid stabilized very small observed compared to fresh catalyst (Fig. S9, see ESI†).
(~1 nm) CdS dots (Fig. S4a see ESI†), reassembled in a spherical
As our catalyst works effectively for aromatic nitro reductions
assembly due to the hydrophobic interaction (repulsion) of the we realize that it would work on selective oxidation of primary CꢀH
aliphatic tail of DA attached to the CdS dot surface and polar solvent bonds. The synthesized ultrathin CdS flower showed excellent
water, and attraction force between hydrophobic tail (Fig. S4b, see photocatalytic activity towards oxidation of saturated sp3 CꢀH bonds
10
ESI†). In the favourable synthetic condition the dots are aligned using molecular oxygen as oxidant and benzotrifluoride as solvent in
themselves locally within the spherical assembly through a common ambient condition under visible light, from 40W simple house hold
crystallographic facet {111} and attached together to give the sheet CFL lamp. Results depict that the synthesized CdS ultraꢀthin sheet
structure in (111) direction through orientated attachment. The assembled flowers is too active to activate the sp3 CꢀH bonds with
2
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DOI: 10.1039/C5CC01685A
Notes and references
a
Central Salt and Marine Chemicals Research Institute (CSIRꢀCSMCRI)
and CSMCRIꢀ Academy of Scientific and Innovative Research, G. B.
Marg, Bhavnagarꢀ364002, Gujarat, India. Eꢀmail: abpanda@csmcri.org
Acknowledgement: CSIRꢀCSMCRI Communication No. 26/2014. The
Authors would like to acknowledge DST, India (SR/S1/ICꢀ33/2011) for
financial support. The authors also acknowledge the “ADCIF” of
CSMCRI for providing instrumentation facilities.
Electronic Supplementary Information (ESI) available: [detailed
experimental
procedure,
additional
TEM,
catalytic
results,
electrochemical data]. See DOI: 10.1039/c000000x/
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water as solvent make the procedure advantageous and greener.
During synthesis, water took a crucial role in short range spherical
assembly 1nm CdS dots and the dots were transformed to sheets in
the spherical assembly through oriented attachment. The synthesized
CdS flowers showed excellent visible light driven photocatalytic
activity for selective reduction of nitroarines to corresponding amino
compounds under simple house hold 40W CFL lamp. The CdS
3
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flower also active for oxidation of saturated sp CꢀH bonds in
molecular oxygen as oxidant. The photocatalytic activity is
comparable or sometime better with respect to the literature reported
pure CdS and CdS modified with costly graphene under hazardous 10 S. K. Pahari, A. Sinhamahapatra, N. Sutradhar, H. C Bajaj and A. B.
3
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1
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