Phase and shape dependent photoactivity of titania for nitroaromatics reduction under UV light irradiation

Article abstract of DOI:10.1166/jnn.2019.15744

This work describes importance of TiO2 nanostructures of different shapes (nanospheres and nanorods) and crystal phases (anatase and rutile) for the photoreduction of nitroaromatics into corresponding aminoaromatics under UV light irradiation. Anatase nanoparticles were most active of all the catalysts due to the superior stability of this phase and comparatively the rutile nanorods (L×W = 28-30 nm×3.5-3.8 nm) showed superior photoactivity (～3 times) as compared to rutile nanospheres of size 122 nm for the photoreduction due to electron transport along longitudinal length, larger surface area (69 m²g^-1), quenched PL emission, increased lifetime of charge carriers (1.8 ns) of rutile nanorods as compare to rutile nanospheres having lower surface area (18 m²g^-1) and charge carrier lifetime (1.1 ns). The overall rate of reduction/hour for m-NBA and m-NT photoreduction with all of these catalysts has been found to vary in the following order ANP > P25-TiO₂ > RNR > ANR > RNP.

Full text of DOI:10.1166/jnn.2019.15744

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Nanoscience and Nanotechnology
Vol. 19, 803–809, 2019
Copyright © 2019 American Scientific Publishers
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Phase and Shape Dependent Photoactivity of Titania for
Nitroaromatics Reduction Under UV Light Irradiation
Jaspreet Kaur and Bonamali Pal∗
School of Chemistry and Biochemistry, Thapar Institute of Engineering and Technology, Patiala 147004, Punjab, India
This work describes importance of TiO2 nanostructures of different shapes (nanospheres and
nanorods) and crystal phases (anatase and rutile) for the photoreduction of nitroaromatics into cor-
responding aminoaromatics under UV light irradiation. Anatase nanoparticles were most active of
all the catalysts due to the superior stability of this phase and comparatively the rutile nanorods
(L× W = 28–30 nm× 3ꢀ5–3ꢀ8 nm) showed superior photoactivity (∼3 times) as compared to rutile
nanospheres of size 122 nm for the photoreduction due to electron transport along longitudinal
length, larger surface area (69 m2g−1ꢁ, quenched PL emission, increased lifetime of charge carriers
(1.8 ns) of rutile nanorods as compare to rutile nanospheres having lower surface area (18 m2g−1
ꢁ
and charge carrier lifetime (1.1 ns). The overall rate of reduction/hour for m-NBA and m-NT photore-
duction with all of these catalysts has been found to vary in the following order ANP > P25-TiO2 >
RNR > ANR > RNP.
Keywords: Different Polymorphs of TiO2, Rutile and Anatase Nanorods, Photoluminescence
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Quenching, Enhanced Life Time of Excitons, Nitroaromatic Reduction.
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1. INTRODUCTION
It is reported that anatase TiO2 usually shows much
higher photocatalytic activity than rutile TiO2. However,
some recent reports show the improved selectivity of rutile
TiO2 nanoparticles21–24 for selective oxidation of aromatic
alcohols to aldehydes, photoreduction of nitroaromatics
to aminoaromatics such as m-nitrotoluene photoreduc-
tion to m-aminotoluene. Further, it is reported25 that
rutile nanoparticles show 100% yield and selectivity
of m-dinitrobenzene photoreduction to m-nitroaniline.
Also, metal (coinage and platinum group) loading also
improves the selectivity of P25-TiO2 for the photoreduc-
tion of m-dinitrobenzene26 while metal loading does not
shows significant improvement in rutile TiO2 nanoparticles
photoactivity.
Further, the photocatalytic activity of anatase and rutile
nanoparticles can also be improved by changing their
morphology27–30 from spherical to rod. The advantages
of one-dimensional nanorods over nanospheres as photo-
catalyst are that the one-dimensional geometry facilitates
fast and long distance electron transport, exhibit higher
specific surface area, light absorption increases because
of the larger length-to-diameter ratio. The improved pho-
toactivity of rutile nanorods31 has been observed for
Rhodamine B degradation (low temp) and gas-phase
decomposition of acetaldehyde. Herein, we investigated
the potential application of anatase and rutile titania
Titanium dioxide (TiO2ꢀ semiconductor nowadays is
used for greener synthesis of aminoaromatics by one
step reduction1–4 of nitroaromatics. The production of
aminoaromatics from nitroaromatics is of great interest
because aminoaromatics are important intermediates for
the synthesis5–8 of dyes, antioxidants, herbicides, poly-
mers, pharmaceuticals and other fine chemicals. These
can be prepared easily by the reduction of nitroaromat-
ics using catalytic hydrogenation, zinc, tin, iron, Au/SiO2,
Au/Al2O3, Pd/TiO2, Pt–Ne bimetallic nanoparticles8–15 and
Pt–TiO2 a variety of other reduction conditions. The draw
backs of all these methods include the requirement of
toxic solvents, strong reducing agents like NaBH4, expen-
sive metals like Au/Pt, reactions at high pressure and tem-
perature and generation of toxic byproducts. Whereas the
use of titania for the photoreduction of nitroaromatics is
highly advantageous over all these methods because it is
a cheap, non-toxic, generates no harmful byproduct and
require room temperature and pressure conditions. Also,
conduction band energy of TiO2 is −0.85 V in compar-
ison to −0.5 V versus SCE of the –NO2 group, so NO2
is preferably reduced16–20 over aceto, cyano, and aldehyde
functionality present in the same compound.
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 2
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doi:10.1166/jnn.2019.15744
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Phase and Shape Dependent Photoactivity of Titania for Nitroaromatics Reduction Under UV Light Irradiation
Kaur and Pal
nanorods and nanospheres for the improved photore-
duction of nitroaromatics such as m-nitrobenzoic acid
(m-NBA) and m-nitrotoluene (m-NT).
solution. Typically, 20 mL of aqueous TiCl4 (3 M) was
added added into 35 mL of nitric acid solution (15 M).
The mixture was refluxed for 24 h at 120 C. The residue
ꢀ
thus obtained was centrifuged, washed three times with
with dilute nitric acid solution at pH = 1. The centrifugated
solid was then redispersed in dilute nitric acid solution
(6 mL) at pH = 1.
2. EXPERIMENTAL DETAILS
2.1. Materials
Degussa P25 TiO2, titanium tetrachloride, tita-
nium(IV) isopropoxide, m-nitrobenzoic acid (C7NO4H5ꢀ,
m-aminobenzoic acid (C7NO2H7ꢀ, m-nitrotoluene
(C7NO2H7ꢀ, m-toluidine (C7NH9ꢀ, nitric acid, iso-
propanol, methanol were purchased from Loba Chemicals
and used without further purification. Deionized water
was obtained using an ultra filtration system (Milli-Q,
Milipore) with a measured conductivity of 35 mho cm−1
Then a growth solution of dilute TiCl4 (0.3 M) was pre-
pared and 20 mL of it was added into above seed solution
and refluxed for 24 h at 120 ꢀC. The residue thus obtained
was centrifuged, washed as mentioned above and dried in
ꢀ
hot air oven at 70–80 C for 3–4 h.
2.6. Characterization Techniques
The optical absorption spectra of as synthesized TiO2 sam-
ples were recorded with a diffuse reflectance spectropho-
tometer by Avantes using BaSO4 as reflectance standard.
Photoluminescence (PL) spectra were recorded by using
a spectrofluorimeter by Perkin-Elmer LS55 at room tem-
perature on excitation with xenon lamp (ꢂextꢀ at 320 nm
in ethanol suspension. Transmission electron microscope
(TEM) photographs were taken on the Hitachi 7500 model
with resolution 2 Å operating at voltage 120 kV. The
crystallographic studies have been carried out by using
X-ray diffractometer by PANalytical X’Pert PRO with
Cu-Kꢃ (k = 1ꢁ54060 Å) radiation operated at 45 kV. The
specific surface area was determined by N2 adsorption
ꢀ
at 25 C.
2.2. Synthesis of Anatase Nanorods (ANR)
ANR were synthesized by a hydrothermal method as
reported32 elsewhere. Briefly, 4.73 g of P25-TiO2 was
mixed with 72 ml of NaOH (10ꢀN) and then subjecting to
hydrothermal treatment at 130 C in a Teflon lined auto-
clave (80 ml capacity) for 20 h. The obtained slurry was
washed repeatedly with 0.1 N HNO3, followed byꢀwater
and methanol, and dried in a hot air oven at 70–80 C for
3 h. 3.2 g of obtained particles were further dissolved in
64 mL of water and the resulted slurry (pH = 5ꢁ6) was
ꢀ
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autoclaved at 175 C for 48 h. Thus obtained ANR were
Copyright: American Scientific Publishers
filtered, washed with distilled water and methanol, and
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ꢀ
dried in hot air oven at 70–80 C for 3 h.
2.3. Synthesis of Anatase Nanospheres (ANP)
ANS were synthesized by sol–gel33 method. Typically,
0.1 M titanium(IV) isopropoxide solution was prepared in
150 mL of iso-propanol. Another solution of measured
quantity of water and 150 mL iso-propanol was also pre-
pared. Both solutions were sealed and subjected to rapid
magnetic stirring so as to obtain homogeneous mixtures.
The water solution was then added drop wise to the 0.1 M
titanium(IV) isopropoxide solution under continous mag-
netic stirring for 5 h. This resulted into hydrolysis of tita-
nium(IV) isopropoxide and white colored precipitates were
formed. These as obtained precipitated were filtered and
washed thrice with deionized water and dried in hot air
ꢀ
oven at 80 C. The powder thus obtained was calcined at
ꢀ
450 C for 2 h and named as ANP catalyst.
2.4. Synthesis of Rutile Nanospheres (RNP)
RNS were synthesized by the calcination ofꢀcommercially
available Degussa P25 TiO2 powder at 800 C in a muffle
furnace for 2 h.
2.5. Synthesis of Rutile Nanorods (RNR)
RNR were synthesized through a seed-mediated growth
Figure 1. XRD patterns of as prepared titania nanoparticles of different
morphology.
process34 by hydrolysing TiCl4 in highly acidic aqueous
804
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Kaur and Pal
Phase and Shape Dependent Photoactivity of Titania for Nitroaromatics Reduction Under UV Light Irradiation
method using a Smart Sorb 92/93 instrument after pre-
heating 100 mg of samples at 150 C for 1 hour. The
(250 mm×4.6 mm, 5 ꢆm)] at ꢂ = 254 nm with a flow rate
of 1 mL/min using MeOH:H2O (70:30) as mobile phase
and by Gas chromatography-mass spectroscopy (GC-MS-
QP 2010 plus) using RTX-5Sil-MS column (15 m ×
0ꢁ25 mm ×0ꢁ25 ꢆm).
ꢀ
time resolve photoluminescence spectra was measured by
time-correlated single photon counting (TCSPC) set up by
Edinburgh FL920 model at room temperature on excita-
tion with diode lasers at 390 nm in ethanol suspension.
The average lifetime (ꢄavꢀ, is related to band edge lifetime
ꢄ1 and trapping or defect’s states given by the following
equation
3. RESULTS AND DISCUSSION
3.1. Structural Analysis
Crystalline phase of P25 TiO2 and as prepared samples
was analyzed by using XRD and are shown in Figure 1.
XRD profile of RNP shows highly intense peaks of rutile
titania whereas RNR exhibited comparatively less intense
peaks of rutile phase at 2ꢇ = 27ꢁ5ꢀ, 36.5ꢀ, 41ꢀ, 54.1ꢀ and
56.5ꢀ and are assigned to the (110), (101), (200), (111),
(210), (211) and (220) planes (JCPDS no. #21-1276).
However, ANP and ANR exhibited diffraction peaks at
2ꢇ = 25ꢁ4ꢀ, 37.9ꢀ, 47.9ꢀ, 53.9ꢀ and 54.9ꢀ are assigned
to (101), (004), (200), (105) and (211) planes (JCPDS
no. #21-1272) of anatase phase. BET surface area anal-
ysis exposed that surface of P25 is 45 m2g−1 while that
of ANP and ANR exihibited increased surface area of
ꢄav = ꢅa1ꢄ1 +a2ꢄ2ꢀ/a1 +a2
where, a1 and a2 denote the amplitude of band edge exci-
tonic and trapping state emission, respectively.
2.7. Photocatalytic Activity for
Nitroaromatic Reduction
The photoreduction was carried out in a test tube con-
taining 50 mg of TiO2 powder suspended in 5 ml
isopropanol (50 vol%) and 5 mM of nitroaromatic com-
pound (25 ꢆmol) in argon atmosphere under continuous
magnetic stirring and UV light irradiation. The reac-
tion solution after nitroaromatic photoreduction was cen-
trifuged, filtered through cellulose filter (0.22 ꢆm) and
product was analysed by HPLC [Agilent 1120 Com-
pact LC equipped with a Qualisil BDS C-18 column
89 m2g−1 and 71 m2g−1, respectively. The surface area
of RNP is 18 m2g
for RNR.
while it is increases to 69 m2g−1
−1
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Figure 2. TEM images of as prepared titania nanostructures (ANP = anatase nanoparticles, RNP = rutile nanoparticles, ANR = anatase nanorods,
RNR = rutile nanorods).
J. Nanosci. Nanotechnol. 19, 803–809, 2019
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Phase and Shape Dependent Photoactivity of Titania for Nitroaromatics Reduction Under UV Light Irradiation
Kaur and Pal
Figure 5. Photoluminescence spectra of various titania nanostructures.
plot using the equation
Figure 3. Optical absorption spectra of different titania nanostructures.
ꢃhꢈ = Aꢅhꢈ −Egꢀn
3.2. Morphological Analysis
where ꢃ = absorption coefficient, hꢈ = energy of photon,
A = constant, Eg = band gap of the material and n = index
and its value depends upon on the nature of the electronic
transition responsible for the absorption (n = 2 for indirect,
1/2 for direct band gap).
Morphology of the prepared samples was investigated
through TEM analysis. TEM image of P25-TiO2 exhibited
spherical nanoparticles of average size 25–30 nm, RNP
also exhibited spherical particles with the size of 120–
130 nm while RNR exhibited rod like morphology with the
size range (L×W = 28−30 nm×3ꢁ5−3ꢁ8 nm) as shown
in Figure 2. TEM micrographs of ANP shows spherical
nanoparticles of average size 8–10 nm and ANR shows
The band gap of the samples were calculated by extrap-
olating the straight-line portion of ꢃhꢈ versus Eg graph to
the x-axis as shown in Figure 4(a). The band gap values
varies from 3.0 eV for RNP to 3.21 eV for ANR (Fig. 4(b))
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the formation of rice like nanorods with size (L × W =
depending upon the phase and morphology. The low band
Delivered bygIanpgeexnhtaibited by RNP and RNR expected to facilitate the
efficient electron transfer and may serve as an efficient
photocatalyst.
Copyright: American Scientific Publishers
80−132 nm×8−13 nm).
3.3. Optical Properties
Optical properties of as prepared TiO2 samples were stud-
ied by recording UV-vis absorption spectra and emission
spectra. In Figure 3 absorption onset of P25-TiO2 appears
at 386 nm, whereas the absorption onset for ANP and
ANR is blue shifted to 380 and 384 nm respectively. How-
ever, the absorption onset of RNP occurs at 413 nm which
is further red shifted to 418 nm in RNR. Further, the band
gap values of all these samples were calculated from Tauc
The emission in nanocrystalline TiO2 is due to the
defects resulting from under coordinated Ti4+ ions, oxygen
vacancies, chemisorbed surface species, which result in the
localized intra-band gap states that serve as electron traps.
The photoluminescence (PL) spectra in Figure 5 showed
that TiO2 nanostructures exhibit the emission bands from
400 to 550 nm range which indicates the occurrence of
various surface defect sites. The emission peak, 403 nm,
Figure 4. (a) The plot of (ꢃhꢈꢀ1/2 function versus the band gap energy of various titania samples (b) variation in the band gap energy of titania
samples.
806
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Kaur and Pal
Phase and Shape Dependent Photoactivity of Titania for Nitroaromatics Reduction Under UV Light Irradiation
Figure 6. Time resolved decay curves of titania nanostructures.
corresponds to a direct band emission of the fundamental
band gap (3.2 eV) of anatase TiO2, whereas the emission
peak, 423 nm, is attributed to the self trapped excitons.
The peak, 446 nm, relates to the emission process of
oxygen vacancy defects in TiO2 and the emission signal,
485 nm corresponds to the charge-transfer from Ti3+ to
oxygen anion in a TiO86− complex, related with oxygen
vacancies at the TiO2 surface.35–38 The emission, 529 nm,
can be related with anion vacancies on the surface of
TiO2 nanoparticles. The strong quenching in PL emis-
decays (Fig. 6) has been found to be gradually increased
as 1.2, 1.5 and 2.1 ns for P25 TiO2 ANP, ANR and respec-
tively, confirming better delocalization of e−–h+ pairs in
the lengthy crystalline NR. Also, average lifetime of RNP
and RNR found to be 1.1 and 1.8 ns respectively.
3.4. Photocatalytic Study
The photocatalytic activity of as prepared titania cata-
lysts were evaluated by monitoring the photoreduction of
m-NBA and m-NT. Figure 7(a) showed the selective reduc-
tion of m-NBA to m-ABA after 6 h UV irradiation with
P25-TiO2, ANP and ANR catalysts. It found that smaller
ANP (8 nm) catalysts exhibits highest photoactivity for
m-NBA reduction and produces m-ABA with 42% yield
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sion, in the case of RNR nanoparticles, has been attributed
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to the decrease in recombination rate of photo-generated
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electron–hole pairs. The average lifetime39 of photoexcited
e−/h+ pairs is calculated by time resolved fluorescence
Figure 7. m-NBA reduction under UV light irradiation by (a) P25-TiO2, ANP and ANR for 6 h (b) RNP and RNR for 8 h.
J. Nanosci. Nanotechnol. 19, 803–809, 2019
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Phase and Shape Dependent Photoactivity of Titania for Nitroaromatics Reduction Under UV Light Irradiation
Kaur and Pal
Figure 8. m-NT reduction under UV light irradiation by (a) P25-TiO2, ANP and ANR for 6 h (b) RNP and RNR for 8 h.
as compared to 40% and 30% yield of P25 TiO2 and
ANR respectively. It could be probably due to increased
surface area of ANP (89 m2g−1ꢀ owing to its smaller
size as compare to 45 m2g−1 and 71 m2g−1 of P25 TiO
behave as an active site on the TiO2 surface and are nec-
essary for adsorption and the conversion of nitro group to
amine.
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Copyright: American Scientific Publishers
and ANR respectively. Figure 7(b) showed the significant
4. CONCLUSION
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higher photoactivity for RNR (∼3 times) as compared to
RNP for m-NBA reduction into ABA under 8 h UV irra-
diation. The yield of m-ABA significantly increases upto
48% with RNR as compare to 18% with RNP catalysts. It
could be attributed due to long distance electron transport
It is demonstrated that as prepared RNP, RNR, ANP, ANR
could be potentially utilized for nitroaromatics photore-
duction. RNR showed superior photoactivity (∼3 times)
as compared to RNP for the photoreduction of m-NT into
m-toluidine (m-TD) and m-NBA into m-aminobenzoic
acid (m-ABA) under 8 h UV irradiation. The obtained
results show that the long distance electron transport
along longitudinal length, larger surface area, quenched
PL emission, increased lifetime of charge carriers of RNR
as compare to RNP having lower surface area and charge
carrier lifetime collectively contribute to its enhanced pho-
tocatalytic activity. Futhermore, ANP also shows higher
photocatalytic activity than ANR owing to their smaller
size and larger surface area as compare to ANR and P25-
TiO2 for m-NBA and m-NT photoreduction under 6 h UV
light irradiation.
along longitudinal length, higher surface area (69 m2g−1
ꢀ
of RNR available for adsorbtion and reduction of m-NBA
as compare to lesser (18 m2g−1ꢀ surface area of RNP.
Also, by changing the morphology from RNP to RNR the
charge carrier time also enhances from 1.1 to 1.8 ns which
also contributes to its enhanced photoactivity. Also, the
obtained results shows that anatase titania nanostructures
shows higher photoactivity in lesser irradiation time as
compare to rutile nanostructures.
Further Figure 8(a) also shows the same trend for m-NT
reduction into m-toluidene (m-TD). It shows that ANP
showed highest photoactivity for the reduction of m-NT
and produces m-TD with yield 60% under 6 h of UV irra-
diation. Also, RNR gives yield 26 % (∼3 times) as com-
pare to 8% yield with RNP catalysts (Fig. 8(b)) during 8 h
UV irradiation. Figure 8 shows that rate of reduction/hour
for m-NBA and m-NT reduction with various catalysts fol-
lows the trend ANP > P25 TiO2 > RNR > ANR > RNP.
This can be attributed to the fact40–42 that reduction plane
{011} related with anatase nanostructures contains a rich
array of defects in comparison with reduction plane {110}
of R-TiO2. The defect sites are identified as Ti3+ which
Acknowledgment: The authors are thankful to Depat-
ment of Science and Technology, India (SR/NM/NS-
40/2008) and council of Scientific and Industrial Research
(CSIR), India for providing the finanacial support.
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Received: 20 March 2017. Accepted: 10 February 2018.
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