A facile and green strategy for the synthesis of 1-dimensional luminescent ZnO nanorods and their reduction behavior for aromatic nitro-compounds

Article abstract of DOI:10.1039/c5ra22908a

This article illustrates a facile microwave assisted synthesis of 1D ZnO nanorods using lauric acid. A fatty acid acts as a complexing and capping agent in the synthesis of the ZnO nanorods. The average diameter of the ZnO nanorods was ～5.5-10.0 nm. The Z

Full text of DOI:10.1039/c5ra22908a

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A facile and green strategy for the synthesis of 1-dimensional luminescent ZnO nanorods and its
reduction behavior for aromatic nitro-compounds
Archita Bhattacharjee and M. Ahmaruzzaman*
Department of Chemistry
National Institute of Technology, Silchar-788010, Assam, India
Abstract:
This article illustrates a facile microwave assisted synthesis of 1D ZnO nanorods using lauric acid. The fatty
acid acts as complexing and capping agent in the synthesis of ZnO nanorods. The average diameter of ZnO
nanorods was ~5.5ꢀ10.0nm. The ZnO nanorods were characterized using TEM, SAED, XRD, FTIR, UV
and PL spectra. The synthesized ZnO nanorods showed unusual luminescence properties. For the first time,
the reduction of toxic aromatic nitro compounds, such as paraꢀnitrophenol, paraꢀnitroaniline and 2,4,6ꢀ
trinitrophenol were carried out using synthesized ZnO nanorods as a catalyst in presence of NaBH4 in
aqueous medium.
Keywords: ZnO nanorods, lauric acid, PL, catalyst, reduction, aromatic nitro compounds.
* Corresponding author: md_a2002@rediffmail.com (M. Ahmaruzzaman)
1. INTRODUCTION
Aromatic nitroꢀcompounds are the most toxic organic compounds. Paraꢀnitrophenol is one of the hazardous
nitroꢀcompounds and it causes severe health hazards which impose threat to human, animal and aquatic
lives. The nitro compounds bearing more than one nitro group were generally used as an explosive such as,
2,4,6ꢀtrinitrophenol, which was widely used as an explosive in wars and industries. The presence of strong
electron withdrawing nitroꢀgroup in aromatic compounds makes it an explosophore, which is capable of
inducing explosive properties into a compound [1]. Hence, their disposal into the nearby water sources from
industries causes serious environmental pollution and harms the ecosystem. Various effective means were
developed for the treatment of such toxic compounds. The reduction of nitro compounds into their amino
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analogue is one of the most effective means for the treatment of such toxic compounds. The amino
derivatives were nonꢀtoxic and they were used as an important intermediate in the synthesis of various
analgesic, antipyretic drugs and in the synthesis of various dyes, as a photographicꢀdeveloper, corrosion
inhibitor [2]. Therefore, it is more suitable to develop an effective method to convert the nitro compounds
into their amino derivatives. The conventional methods of hydrogenation of nitro compound were associated
with serious environmental problems. In recent years, much attention was devoted towards the catalytic
reduction of aromatic nitro compounds by NaBH4 in presence of noble metal nanoparticles as a catalyst [3].
However, there is much less reports regarding the catalytic activity of metal oxide nanoparticles in the
reduction of aromatic nitroꢀcompounds. Therefore, in this paper, for the first time, we reported the catalytic
reduction of aromatic nitroꢀcompounds, such as paraꢀnitrophenol (PNP), paraꢀnitroaniline (PNA) and 2,4,6ꢀ
trinitrophenol (TNP) by NaBH4 in presence of ZnO nanorods as catalyst, in aqueous medium.
ZnO, a IIꢀVI semiconductor having a wide band gap of 3.37eV, has gained a lot of interest in recent years
because of its fascinating luminescence, optical, electrical, chemical and biological properties. ZnO
nanoparticles have wide range of promising applications in catalysis, antibacterial treatment, fabrication of
sensors and solar cells, optoelectronic devices namely LED and lasers, etc. [4ꢀ6]. Numerous methods and
techniques were employed for the synthesis of varied morphology of ZnO nanoparticles [5ꢀ7]. However,
most of the synthetic methods were associated with high temperature, expensive reagents, typical reaction
conditions, sophisticated instrument and tedious reaction strategies. In this paper, we report a facile
microwave heating method for the synthesis of 1D ZnO nanorods (NRs). This communication describes a
lauric acid mediated synthesis of ZnO NRs in ethanol. The synthesized ZnO NRs show enhanced
luminescence properties. Many researchers have reported ZnO nanoparticles having blue and green
emission bands [7]. However, this paper reports the yellow emission observed in case of synthesized ZnO
NRs which is very rare along with negligible blue and green emission.
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2. EXPERIMENTAL
Materials and Method:
All the reagents, zinc nitrate hexahydrate (Zn(NO3)2. 6H2O), methylamine, lauric acid, NaBH4, pꢀ
nitrophenol, pꢀnitroaniline and 2,4,6ꢀ trinitrophenol were procured from Merck and were of analytical grade
(AR). The reaction was carried out in a domestic microwave oven.
ZnO NRs were synthesized using zinc nitrate hexahydrate, lauric acid and methyl amine. A total of 0.01M
zinc nitrate hexahydrate was dissolved in 20ml ethanol and was treated with 20ml of 0.01M ethanolic
solution of lauric acid and 10ml of 40% methyl amine aqueous solution with constant stirring. The pH of the
reaction was maintained at 11. The reaction mixture was then irradiated with microwave waves for 180s.
This leads to the formation of white precipitate which was centrifuged and washed with alcohol and distilled
water several times. The precipitate was finally calcined at 120oC and collected for characterization.
ZnO NRs were characterized by powder Xꢀray diffraction (XRD) method using Phillips X’Pert PRO
diffractometer with CuKα radiation of wavelength 1.5418Å. The size, morphology and diffracted ring
pattern of ZnO NRs were determined by JEMꢀ2100 Transmission Electron Microscope. Infrared spectrum
was recorded by Bruker Hyperion 3000 FTIR spectrometer. Absorption spectra were recorded on Cary 100
BIO UVꢀvisible spectrophotometer.
Catalytic reduction of toxic aromatic nitro compounds:
For the reduction of aromatic nitro compounds, 6mM solution of aromatic nitro compound (pꢀnitrophenol,
pꢀnitroaniline and 2,4,6ꢀ trinitrophenol) and 0.1 M solution of NaBH4 were prepared. All the reactions were
carried out in a quartz cuvette and the absorbance was recorded in order to investigate the reduction process.
In the cuvette, 2ml of water was taken and then 60ꢁl of nitro compound and 350ꢁl of NaBH4 solution were
added and the absorption was recorded. To the above solution, 300ꢁl solution of ZnO NRs (0.005g) was
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added to initiate the reduction process. The absorbance was recorded immediately and at a regular interval
of time to study the reduction of the aromatic nitroꢀcompounds.
3. RESULTS AND DISCUSSION
The FTIR spectrum was recorded in order to investigate the formation of ZnO and the role of lauric acid in
the synthetic pathway. Fig. 1(a) represents the FTIR spectrum of synthesized ZnO NRs. The band observed
at 497 cmꢀ1 was attributed to the characteristic vibration of ZnꢀO bond [8]. This peak confirmed the
formation of ZnꢀO bond. The appearance of broad band around 3515cmꢀ1 corresponds to OH group of lauric
acid. The presence of lauric acid was also confirmed by the occurrence of sharp peaks at 2925 and 2854cmꢀ1
due to CꢀH asymmetric and symmetric stretching vibration. Therefore, the lauric acid gets adsorbed on the
surface of ZnO NRs. It was also evident that the band around 1710cmꢀ1 observed in case of lauric acid was
absent. The existence of bands around 1570 and 1415cmꢀ1 correspond to carboxylate asymmetric and
symmetric stretch. This confirmed the existence of lauric acid as laurate anion absorbed on the surface of
ZnO. Therefore, analysis of FTIR suggested that lauric acid act as a capping agent in the synthesis of ZnO
NRs.
Fig. 1(b) represents the XRD pattern of synthesized ZnO NRs. The crystalline structure, nature and purity of
a compound were investigated using XRD pattern. The occurrence of peaks at 2θ values of 31.86, 34.55,
36.33, 47.65, 56.69, 62.89, 66.33, 68.1, 69.13, 72.9, 77.14 and 89.71oC clearly reflects (100), (002), (101),
(102), (110), (103), (200), (112) (201), (004), (202) and (203) planes, respectively. The diffraction pattern
was well indexed to the hexagonal crystal structure of ZnO and also in good agreement with JCPDS card
No. 89ꢀ0511. The appearance of sharp peaks depict that the synthesized material is well crystallized. The
diffraction pattern did not show any peak for the precursor molecules or impurity which indicated the purity
and complete conversion of starting material into products. The average crystalline size of ZnO NRs
obtained using Deꢀbye Scherrer equation [6] was 18.08nm.
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The optical properties of ZnO NRs were investigated using UV and PL spectra. The UVꢀvisible spectrum
(Fig. 1c) showed a broad absorption band around 379 nm which confirms the formation of ZnO
nanoparticles and is associated with the transfer of electrons from valence band to conduction band [8]. The
optical band gap energy can be obtained from the absorption onset using Tauc’s plot [6]. The band gap
energy of synthesized ZnO nanoparticles can be obtained from the following equation [16]:
αhν = K (hνꢀEg)n ꢀꢀꢀꢀꢀꢀ (1)
where K is a constant, Eg is the band gap energy and the exponent ‘n’ depends on the type of transition
whose value is ½ for allowed direct transition. Therefore, by plotting (αhν)2 versus incident photon energy
(hν) and by extrapolating the linear portion of the curve to zero absorption coefficient, the band gap energy
can be estimated. The inset of Fig. 1(c) showed the plot of (αhν)2 versus (hν) for the synthesized ZnO
nanorods. The band gap energy of ZnO NRs was calculated from the plot of (αhν)2 versus photon energy
(hν) (inset of Fig. 1c) by extrapolating the linear region of the curve to zero absorption coefficient and was
found to be 3.28 eV [6].
The PL spectrum gives brief information regarding the purity and quality of the synthesized material. The
room temperature PL spectrum (Fig. 1d) of ZnO NRs showed a strong UV emission band centered at 387nm
and an emission band at 583nm when λex=300nm. The UV emission at 387nm is the bandꢀedge emission
arises due to the recombination of free excitons between the valence and conduction bands [6]. The yellow
emission band at 583nm occurs due to native deep level defects mainly attributed to oxygen interstitials (Oiꢀ
) [9]. Blue and green emission bands of very negligible intensity were also observed around 450 and 550
nm, respectively. The occurrence of blue and green emission peak was due to the radiative transition from
the extended Zni to valence band and from the conduction band to deep oxygen vacancies level, respectively
[7]. The intensity of UV emission reveals the crystallinity of the sample and the intensity of the bands in the
visible region reflects the concentration of defects in the sample [10]. Hence, the PL spectrum indicates the
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highly crystalline nature of synthesized ZnO NRs and the presence of defects also indicates an increase in
the optical and electrical properties of synthesized material. The synthesized ZnO nanorods showed unusual
luminescence properties, thereby showing a prominent yellow emission band. Roy et al. synthesized ZnO
nanorods with average length and diameter of 100–150 nm and 10–20 nm, respectively (aspect ratio=10),
showed blue and green emission bands [7]. The dependence of luminescence properties on the change in
size of ZnO nanoparticles were studied by Raoufi and in that case, he concluded that with an increase in
particle size, the intensity of visible deep level emission increases [11]. Wang et al. also showed that the
photoluminescence property of ZnO nanoparticles is sizeꢀdependent [12]. Herein, we reported the synthesis
of ZnO nanorods with a lower aspect ratio (length:diameter) of 1.3ꢀ1.8 and showed unusual yellow emission
along with blue and green emission bands. Therefore, the appearance of an unusual yellow emission band
may be attributed to the lower aspect ratio of the synthesized ZnO nanorods. Hence, the incorporation of
lauric acid leads to a decrease in the length and diameter of ZnO nanorods thereby decreasing the aspect
ratio which play an important role in determining the luminescence properties of ZnO nanoparticles.
The energy band diagram shown in Scheme 1 explains the probable mechanism of blue, green and yellow
emission in ZnO nanorods. The mechanism of blue, green and yellow emission bands were related with the
coexistence of Zni interstitials, oxygen vacancies and Oi interstitials in the synthesized ZnO nanorods. The
electrons from the valence band get excited to the conduction band only when the energy of incident photon
is greater than the band gap of the synthesized ZnO nanorods. The synthesized ZnO NRs showed a blue
emission having energy of 2.75 eV which was due to the transition from extended Zni to valance band. The
calculated energy gap between the conduction band and Zni was found to be 0.53 eV. The electrons excited
from valence band to the conduction band undergo nonꢀradiative transition to the extended Zni, wherein
further transition occurs directly to the valence band giving rise to the blue emission band at 450 nm. The
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electrons that were forced into the conduction band deꢀexcite to the deep levels of oxygen vacancies and this
give rise to green emission band at 550 nm. The calculated energy for green emission was 2.25 eV.
The synthesized ZnO nanorods showed unusual luminescence properties by giving rise to a prominent
yellow emission band along with negligible blue and green emission bands. The origin of yellow emission is
attributed to oxygen interstitials, Oi. The calculated yellow emission energy for the synthesized ZnO NRs
was 2.11 eV. The mechanism of yellow emission proceeds through the nonꢀradiative transition of excited
electrons from conduction band to the extended oxygen interstitial Oi, with energy difference of 1.17 eV.
From Oi, the electrons undergo further transition to the valence band thereby giving rise to an intense yellow
emission band at 583 nm. The intensity of blue and green emission bands are negligible in comparison to
the yellow emission band which indicate that most of the excited electrons from conduction band undergo
nonꢀradiative transition to the extended oxygen interstitials, Oi and then further undergoes transition to the
valence band thereby giving rise to a prominent high intensity yellow emission band.
Conduction
band (CB)
Zni
2.25 eV
Green
Oi
Eg= 3.28 eV
2.75 eV
Blue
2.11 eV
Yellow
Deep levels
Eg<Eex
Valence
band (VB)
Scheme 1. Schematic diagram for the probable mechanism of blue, green and yellow transitions
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The morphology and particle size distribution of synthesized ZnO nanoparticles were elucidated by Electron
Microscopic analysis. The TEM image (Fig. 2a) showed the formation of rodꢀlike ZnO nanoparticles with
diameter and length of 5.5ꢀ10.0 and 10ꢀ13 nm, respectively. The microstructure of ZnO nanorods were
analyzed by HRTEM image (Fig. 2b). The spacing between adjacent lattice planes obtained from HRTEM
image (Fig. 2b) was 0.122 and 0.165nm which corresponded to (202) and (110) lattice planes, respectively.
The SAED pattern (Fig. 2c) revealed the hexagonal crystal structure of ZnO NRs which was in resemblance
with the XRD pattern. The SAED pattern also reflects that ZnO NRs were well crystallized in nature.
In the synthesis of ZnO NRs, lauric acid plays the role of complexing as well as capping agent. Being basic
in nature, methyl amine is supporting the deprotonation of lauric acid thereby enhancing the formation of
ZnO NRs. However, for further confirmation of the role of lauric acid as capping agent in the synthesis of
ZnO NRs, a control experiment was carried out for the synthesis of ZnO nanoparticles in the absence of
lauric acid and the corresponding TEM image and FTIR spectrum was recorded (Fig. S1(a, b)†). The TEM
image (Fig. S1a†) showed the formation of ZnO nanorods with average diameter and length of 130 nm and
415 nm, respectively. This clearly indicates that the involvement of lauric acid in the synthesis of ZnO
nanoparticles leads to the formation of small sized ZnO nanorods. The adsorption of laurate anion on the
surface of ZnO nanorods terminate the growth of ZnO nanoparticles and prevent agglomeration by giving
rise to small sized ZnO nanorods with diameter and length 5.5ꢀ10 nm and 10ꢀ13 nm, respectively. The FTIR
spectrum (Fig. S1b†) of ZnO nanoparticles synthesized in the absence of lauric acid was also recorded and
the spectrum showed a band around 491 cmꢀ1. The appearance of this band confirmed the formation of ZnꢀO
bond. No bands were observed in the region 1700ꢀ1500 cmꢀ1 which clearly depicts the absence of laurate
anion on the surface of ZnO nanorods which inhibit the growth of ZnO nanorods thereby acting as a good
capping agent.
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Reduction of toxic aromatic nitro compounds using ZnO NRs as heterogeneous catalyst:
The catalytic reduction of aromatic nitroꢀcompounds by NaBH4 was taken as a model reaction to evaluate
the catalytic activity of ZnO nanorods. Fig. 3(a) represented the absorption spectra which indicate the
change of concentration of pꢀnitrophenol (PNP) with time in aqueous medium in presence of NaBH4 acting
as a reducing agent and ZnO NRs as a heterogeneous catalyst. It was evident that the absorption maximum
of PNP was centered at 317nm and after the addition of NaBH4 the absorption maximum was shifted to
403nm due to the formation of pꢀnitrophenolate ions (Fig. S2†). The color of the solution also changes to
intense yellow after the addition of NaBH4 due to the formation of pꢀnitrophenolate ions under alkaline
condition. The peak at 403nm remains unaltered after a couple of days in absence of any catalyst. It is a
kinetically restricted reaction in the absence of any catalyst. Therefore, PNP reduction doesn’t take place
using only the reducing agent NaBH4 in the absence of any catalyst [2]. However, after the addition of ZnO
NRs the absorption maximum at 403nm gradually decreases with time which indicates the reduction of PNP
and the color of the solution also fades away. A new peak centered at 300nm started generating with an
increase in reduction time. This peak arises due to the formation of pꢀaminophenol which is the final
reduced product [2, 3]. The complete reduction takes place within 120min. The rate of the reaction was
studied using first order kinetics by plotting lnAt versus time (Fig. 3b) and was found to be 0.026minꢀ1.
The absorption spectrum for the reduction of pꢀnitroaniline (PNA) using NaBH4 in presence of ZnO NRs as
heterogeneous catalyst is represented in Fig. 4(a). The disappearance of peak at 380nm and appearance of a
new peak centered at 300nm indicates the complete reduction of PNA to pꢀphenylenediamine [3].The rate of
the reaction was obtained using first order kinetics (Fig. 4b) and rate constant was found to be 0.044 minꢀ1.
Similarly, the reduction of 2,4,6ꢀtrinitrophenol (TNP) was carried out using NaBH4 as a reducing agent in
presence of ZnO NRs acting as a catalyst. From the UVꢀvisible spectrum (Fig. 5a), it was evident that TNP
showed an absorption maximum at 358 nm. The disappearance of the band around 357nm and the
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appearance of a new peak at 300nm showed the complete reduction of the nitro compound into their
corresponding amino derivative [3]. This reaction also followed first order kinetics. The rate of the reaction
obtained from the linear plot of lnAt versus time (Fig. 5b) was 0.141minꢀ1.
Moreover, to verify the role of ZnO NRs and NaBH4, control experiments were also performed for the
reduction of PNP, PNA and TNP using only NaBH4 in the absence of ZnO NRs. It was found that all the
aromatic nitroꢀcompounds were not reduced even after a couple of days. The reduction took place only
when ZnO NRs were added. Moreover, the reduction of nitro compounds carried out using ZnO NRs in the
absence of NaBH4 doesn’t take place successfully which further conferred that ZnO NRs cannot act as a
reducing agent. These control experiments confirmed the role of NaBH4 as a reducing agent and ZnO NRs
as a catalyst in the effective reduction of aromatic nitro compounds.
Therefore, ZnO NRs act as an effective catalyst in the reduction of pꢀnitrophenol, pꢀnitroaniline and 2,4,6ꢀ
trinitrophenol to their corresponding amino derivates. Table 1 represented the overall study for the reduction
of toxic aromatic nitro compounds in aqueous medium using ZnO NRs as a catalyst.
4. CONCLUSION
A facile microwave assisted route was developed for the synthesis of 1D ZnO NRs using lauric acid as
complexing and capping agent in ethanol medium. The ZnO NRs having an average diameter of 5.5ꢀ10.0nm
possesses hexagonal crystal structure and is well crystalline in nature. ZnO NRs showed unusual
luminescence properties wherein, high intensity UV and yellow emission bands were observed along with
negligible blue and green emission bands. The toxic nitroꢀcompounds, such as, PNP, PNA and TNP were
completely reduced into amino derivatives by NaBH4 in presence of ZnO NRs as a catalyst in aqueous
medium within 120, 45, and 18 min, respectively.
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†Supplementary Information: TEM image and FTIR spectrum of ZnO nanoparticles synthesized in the
absence of lauric acid, absorption spectrum of PNP and absorption spectrum of PNP after addition of
NaBH4 were available in Supplementary Information
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3. S. Mondal, U. Rana, R.R. Bhattacharjee, S. Malik, RSC Adv, 2014, 4, 57282ꢀ9.
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11. D. Raoufi, J Lumin 2013, 134, 213–9.
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Figure:
1.Figure 1. (a) FTꢀIR spectrum of synthesized ZnO NRs showing the formation of ZnO and analyzing the
role of lauric acid as capping agent in the synthesis of ZnO NRs; (b) XRD pattern of asꢀprepared ZnO
NRs predicting a hexagonal crystalline stucture; (c) Absorption spectrum of synthesized ZnO NRs
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showing a broad absorption band around 379 nm (upper right inset represents the corresponding plot of
(αhν)2 versus photon energy, hν); (d) PL spectrum of ZnO NRs (λex= 300nm).
2.Figure 2. (a) TEM microphotograph of synthesized ZnO NRs having average diameter and length of 5.5ꢀ
10.0 nm and 10ꢀ13 nm respectively, (b) HRTEM image of asꢀprepared ZnO NRs, (c) SAED pattern of
synthesized ZnO NRs having a hexagonal crystal structure.
3.Figure 3. (a) Absorption spectrum for conversion of PNP to PAP using ZnO NRs as catalyst in presence
of NaBH4 in aqueous medium, (b) Plot of ln[At] versus reduction time.
4.Figure 4. (a) UVꢀspectrum for reduction of PNA by NaBH4 in presence of ZnO nanorods as catalyst in
aqueous medium, (b) Plot of ln[At] versus reduction time.
5. Figure 5. (a) UVꢀspectrum of reduction for TNP by NaBH4 in presence of ZnO nanorods as catalyst in
aqueous medium, (b) Plot of corresponding ln[At] versus reduction time.
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1.0
0.8
0.6
0.4
0.2
0.0
(101)
(b)
2854
1415
1579
2925
3515
(a)
497
4000
3500
3000
2500
2000
1500
-1
1000
500
20
40
60
80
o
2θ( )
Wavenumber (cm
)
90000000
80000000
70000000
60000000
50000000
40000000
30000000
20000000
10000000
0
0.24
0.22
0.20
0.18
0.16
0.14
0.12
(c)
250
200
150
100
50
387nm
(d)
450nm
0
1
2
3
4
5
6
7
583 nm
550nm
hν
200
300
400
500
600
700
800
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 1 (a) FTꢀIR spectrum of synthesized ZnO NRs showing the formation of ZnO and analyzing the role of
lauric acid as capping agent in the synthesis of ZnO NRs, (b) XRD pattern of synthesized ZnO NRs
predicting a hexagonal crystalline stucture, (c) Absorption spectrum of synthesized ZnO NRs showing a
broad absorption band around 379 nm (upper right inset represents the corresponding plot of (αhν)2 versus
photon energy, hν), (d) PL spectrum of asꢀprepared ZnO nanorods (λex= 300nm).
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Fig. 2(a) TEM image of asꢀprepared ZnO nanorods having average diameter and length of 5.5ꢀ10.0nm and
10ꢀ13 nm respectively, (b) HRTEM image of synthesized ZnO nanorods and (c) SAED pattern of
synthesized ZnO nanorods having a hexagonal crystal structure.
1.0
2.0
0.5
0 min
(a)PNP
10 min
(b)
0.0
20 min
1.5
30 min
-0.5
40 min
50 min
60 min
80 min
100 min
120 min
-1.0
-1.5
-2.0
1.0
0.5
-2.5
-3.0
0.0
-3.5
0
50
100
300
400
500
600
700
Time (min)
Wavelength (nm)
Fig. 3 (a) Absorption spectra of conversion of PNP to PAP using ZnO nanorods as catalyst, (b) Plot of
ln[At] versus reduction time.
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1.0
0.8
0.6
0.4
0.2
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
(b)
(a) PNA
0 min
5 min
10 min
15 min
20 min
25 min
30 min
35 min
40 min
45 min
Rate constant, k= 0.044 min-1
300
400
500
600
700
0
20
40
Wavelength (nm)
Time (min)
Fig. 4 (a) UVꢀspectra of reduction of PNA by NaBH4 in presence of ZnO nanorods as catalyst, (b) Plot of
lnAt versus reduction time.
2.0
0.5
(b)
(a)
TNP
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
1.5
1.0
0.5
0.0
0 min
3 min
6 min
9 min
12 min
15 min
18 min
-1
Rate constant, k= 0.141 min
300
400
500
600
700
-2
0
2
4
6
8
10
12
14
16 18
20
Wavelength (nm)
Time (min)
Fig. 5 (a) UVꢀspectra of reduction of TNP by NaBH4 in presence of ZnO nanorods as catalyst, (b) Plot of
lnAt versus reduction time.
15

RSC Advances
Page 16 of 16
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DOI: 10.1039/C5RA22908A
Table 1: ZnO nanorods catalyzed reduction of toxic aromatic nitro compounds in aqueous medium
Entry
Substrate
Product
Time
(min)
Rate
constant, k
(min-1)
1
2
120
45
0.026
OH
NO2
OH
NH2
0.044
0.141
O2N
NO2
O2N
NH2
OH
OH
3
18
NH2
NH2
NO2
NO2
NH2
NO2
16