A ternary Cu2O-Cu-CuO nanocomposite: A catalyst with intriguing activity

Article abstract of DOI:10.1039/c5dt03859f

In this work, the syntheses of Cu₂O as well as Cu(0) nanoparticle catalysts are presented. Copper acetate monohydrate produced two distinctly different catalyst particles with varying concentrations of hydrazine hydrate at room temperature without using any surfactant or support. Then both of them were employed separately for 4-nitrophenol reduction in aqueous solution in the presence of sodium borohydride at room temperature. To our surprise, it was noticed that the catalytic activity of Cu₂O was much higher than that of the metal Cu(0) nanoparticles. We have confirmed the reason for the exceptionally high catalytic activity of cuprous oxide nanoparticles over other noble metal nanoparticles for 4-nitrophenol reduction. A plausible mechanism has been reported. The unusual activity of Cu₂O nanoparticles in the reduction reaction has been observed because of the in situ generated ternary nanocomposite, Cu₂O-Cu-CuO, which rapidly relays electrons and acts as a better catalyst. In this ternary composite, highly active in situ generated Cu(0) is proved to be responsible for the hydride transfer reaction. The mechanism of 4-nitrophenol reduction has been established from supporting TEM studies. To further support our proposition, we have prepared a compositionally similar Cu₂O-Cu-CuO nanocomposite using Cu₂O and sodium borohydride which however displayed lower rate of reduction than that of the in situ produced ternary nanocomposite. The evolution of isolated Cu(0) nanoparticles for 4-nitrophenol reduction from Cu₂O under surfactant-free condition has also been taken into consideration. The synthetic procedures of cuprous oxide as well as its catalytic activity in the reduction of 4-nitrophenol are very convenient, fast, cost-effective, and easily operable in aqueous medium and were followed spectrophotometrically. Additionally, the Cu₂O-catalyzed 4-nitrophenol reduction methodology was extended further to the reduction of electronically diverse nitroarenes. This concise catalytic process in aqueous medium at room temperature revealed an unprecedented catalytic performance which would draw attention across the whole research community.

Full text of DOI:10.1039/c5dt03859f

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A ternary nanocomposite Cu O-Cu-CuO: a catalyst for intriguing activity
2
Anup Kumar Sasmal, Soumen Dutta, and Tarasankar Pal*
Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India
E-mail: tpal@chem.iitkgp.ernet.in
Tel.: +913222283320; fax: +913222255303

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Abstract
In this work, the syntheses of Cu O as well as Cu(0) nanoparticle catalysts have been presented.
2
Copper acetate monohydrate produced two distinctly different catalyst particles with varying
concentrations of hydrazine hydrate at room temperature without using any surfactant or support.
Then both of them employed separately for 4-nitrophenol reduction in aqueous solution in
presence of sodium borohydride at room temperature. To our surprise, it was noticed that the
catalytic activity of Cu O was much higher than that of the metal Cu(0) nanoparticle. We have
2
confirmed the reason for the exceptionally high catalytic activity of cuprous oxide nanoparticle
over other noble metal nanoparticles for 4-nitrophenol reduction. A plausible mechanism has
been reported. The unusual activity of Cu O nanoparticle in the reduction reaction has been
2
observed because of the in situ generated ternary nanocomposite, Cu O-Cu-CuO which smartly
2
relay electrons and acts as better catalyst. Here in the ternary composite, highly active in situ
generated Cu(0) is proved to shoulder the responsibility for hydride transfer reaction. The
mechanism of 4-nitrophenol reduction has been established from TEM supported studies. To
further support our proposition, we have prepared compositionally similar Cu O-Cu-CuO
2
nanocomposite using Cu O and sodium borohydride which however displayed lower rate of
2
reduction than that of the in situ produced ternary nanocomposite. The evolution of isolated
Cu(0) nanoparticles for 4-nitrophenol reduction from Cu O under surfactant free condition has
2
also been taken into consideration. The synthetic procedures of cuprous oxide as well as its
catalytic activity on the reduction of 4-nitrophenol are very much convenient, fast, cost-effective,
and easily operable in aqueous medium and followed spectrophotometrically. Additionally, the
Cu O catalyzed 4-nitrophenol reduction methodology was extended further to the reduction of
2
electronically diverse nitroarenes. This concise catalytic account in aqueous medium at room

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temperature revealed an unprecedented catalytic performance which would draw the attention
across the whole research community.
Keywords: Ternary Cu O-Cu-CuO nanocomposite, reduction, nitroarenes, cuprous oxide
2
nanoparticles, copper nanoparticles.
Introduction
Reduction of nitro compounds towards the synthesis of amines is an imperative progression in
the chemical vicinity because of the resourceful existence of amines in various pharmaceuticals,
1
biologically active natural products, and dyes. Particularly, 4-aminophenol, an important
analogue of anilines, is an intermediate of several antipyretic and analgesic drugs such as
2
paracetamol, phenacetin, and acetanilide. Whereas, 4-nitrophenol is a common organic
pollutants in water which derived from industrial and agricultural resources. So the development
of efficient catalyst and facile synthetic procedure for the organic synthesis of amines from nitro
compounds is a major challenge and noteworthy task. There have been developed numerous
3
,4
methods to access amines from nitro substrates counting metals /acid reduction, electrolytic
5
,6
7,8
9-11
reduction, stoichiometric reducing agents, catalytic hydrogenation, etc. However, catalytic
hydrogenation has crafted much attention for its atomic efficiency, eco-friendliness, and
1
1
12,13
importance in the industries. Various heterogeneous catalysts of noble metals such as Ag,
14-16
16,17
18,19
Au,
Pd,
and Pt
nanomaterials have been utilized to access amines from nitro
compounds in presence of sodium borohydride or hydrazine, but these are unselective and costly.
Copper has been considered as an alternative and under-rated to noble metals because of its cost
effectiveness and wide array of potential applications in the area of nanoscience and

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2
0
nanotechnology. There exists a few reports for the preparation of copper nanoparticles (Cu
8
,21-25
NPs) catalyst and their application on conversion of nitro compounds into amines.
On the other hand, metal oxides have been considered as important functional materials because
2
6-28
of their diverse structure, properties, and technological applications.
So, they are considered
as future material to combat energy crisis after fine tuning their band gap energy. Explicitly,
cuprous oxide is such an important and widely recognized functional material which has
2
9
potential catalytic applications. There are available only few reports on cuprous oxide
2
1,30,31
nanoparticles (Cu O NPs) for its catalytic utility on nitroarene reduction.
However, the
2
authors exploited costly capping agent, instrumental manipulation in organic solvent, harsh
reaction condition (high temperature) and use of robust supporting substrate for the production of
Cu and or Cu O NPs. Additionally, the reduction of nitroarenes to amines were performed under
2
severe condition such as employing organic solvent, high temperature, time consuming etc.
Hence forth, it is challenging to develop new and facile method for the synthesis of effective
cuprous oxide and copper nanoparticles for the nitroarenes reduction under eco-friendly, and
ambient condition.
Generally, metal oxide nanoparticles have the tendency to clump and aggregate for their strong
3
1,32
interactions which relates to surface energy,
and thus nanostructured properties of the
particles depreciate. Therefore, it is difficult to control the utility of nanostructured properties of
the metal oxide nanoparticles without dispersion of the same on various surfactants or supporting
2
1,31,33,34
surfaces avoiding interparticle aggregation.
Herein, we have presented the synthetic methods of morphologically two different types of Cu O
2
and Cu(0) nanoparticles (NPs) from copper acetate reduction by hydrazine through careful
manipulation of hydrazine hydrate concentration. No surfactant or support is used for any one of

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the preparations at room temperature. We accessed also copper (0) nanoparticles from Cu O
2
through chemical reduction by hydrazine hydrate without using any surfactant. Then Cu O and
2
Cu(0) NPs catalysts have been exploited for 4-nitrophenol reduction with sodium borohydride.
In case of Cu O catalyzed 4-nitrophenol reduction, a ternary nanocomposite (Cu O-Cu-CuO) has
2
2
been proved to be evolved in situ during the course of 4-nitrophenol reduction while sodium
borohydride was used. We observed the very high reactivity of Cu O NPs catalyst for 4-
2
nitrophenol reduction due to the evolution of highly active in situ generated Cu(0) in Cu O-Cu-
2
CuO ternary nanocomposite which fetched admirable rate constant. The rate constant was found
to be higher than rate constants of the reported noble metal heterogeneous catalysts. The
reduction process of 4-nitrophenol by cuprous oxide is facile, economical, green, and convenient,
which was further applied to the reduction of electronically diverse nitroarenes. We utilized all
Cu O and Cu(0) NPs catalysts for 4-nitrophenol reduction study successfully overpowering their
2
35,36
tendency towards oxidation under aerobic or aqueous conditions.
The reductive
3
7,38
transformations were easily monitored by the UV visible spectroscopy.

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Experimental
Materials and instrument. The related information is available in the Supporting Information.
Scheme 1. Synthesis of cuprous oxide and copper nanoparticles from different reaction
conditions:
Synthesis of cuprous oxide nanoparticles from different reaction conditions:
Preparation of Cu O (S1) under stirring condition:
2
The preparation of cuprous oxide nanoparticles (Cu O NPs, sample no S1; Scheme 1, entry 2)
2
was accomplished through reduction of copper acetate by hydrazine hydrate at room temperature
under magnetic stirring in air. In a typical procedure, copper acetate monohydrate (0.25 mmol)
was taken in a 50 mL beaker and was dissolved in 20 mL water. Then hydrazine hydrate of 20
µL (80%) was introduced into the beaker under magnetic stirring. The stirring was continued for

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about 30 minutes. Then the solution of reddish Cu O NPs suspension was centrifuged at ̴7 000
2
rpm, and followed by washing five times with water followed by absolute ethanol washing to
obtain red precipitate. Finally, it was dried for 2 h in vacuum to furnish the red power of Cu O
2
NPs (Sample No. S1). Then the product was utilized for the characterization and nitroarenes
reduction.
Preparation of Cu O (S2) under free-standing condition:
2
Cu O NPs (sample no. S2; Scheme 1, entry 1) were prepared through the chemical reduction of
2
copper acetate using hydrazine hydrate under free standing condition at room temperature in air.
In a typical procedure, copper acetate monohydrate (0.25 mmol) was taken in a 15 mL test tube
and was dissolved with 5 mL water. Then hydrazine hydrate (20 µL of 80%) was added to it.
The whole reaction mixture was shaken gently and kept standing for 6 h at room temperature.
Cu O NPs of fine red color were deposited at the bottom of test tube. The product was carefully
2
washed with water for several times and finally with absolute ethanol. Next, it was dried in
vacuum for 2 h to furnish the cuprous oxide nanoparticles (sample no. S2). Then the product was
utilized for characterization and nitroarene reduction.
Synthesis of copper nanoparticles from different conditions:
Preparation of Cu(0) (S3) from copper acetate.
The synthesis of copper nanoparticles (Cu NPs) was also carried out through the reduction of
copper acetate using hydrazine hydrate under closed condition at room temperature (Scheme 1,
entry 3). In a typical procedure, copper acetate monohydrate (0.25 mmol) was placed in a 15 mL
vial and dissolved in water (4.8 mL). Subsequent to that hydrazine hydrate (200 µL of 80%) was
introduced to the reaction mixture drop wise and gently shacked. The whole reaction mixture
was kept standing under closed condition at room temperature for 12 h. Cu NPs with fine

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reddish-brown color deposited onto the inner surface and bottom of the vial. The solvent was
carefully decanted. Then 2 mL water was added and sonicated for few seconds (5-10 seconds)
carefully and the solution of fine reddish brown color was centrifuged, washed with water
several times and absolute alcohol, and finally dried in vacuum affording a fine reddish brown
powder of Cu NPs (sample no. S3). Then the product was utilized for characterization and
nitroarene reduction.
Preparation of Cu(0) (S4) from cuprous oxide Cu O (S1).
2
Copper nanoparticles (sample no. S4) was prepared from Cu O (S1) through the chemical
2
reduction by hydrazine hydrate under closed condition at room temperature (Scheme 1, entry 4).
In a typical procedure, 15 mg Cu O (S1) was placed in a 15 mL vial and dissolved in 5 mL water
2
with sonication for few seconds. Then hydrazine hydrate (180 µL of 80%) was introduced drop
wise by gentle shacking. The total reaction mixture was kept standing at room temperature under
closed condition for 12 h. Cu NPs with fine reddish-brown color deposited onto the inner surface
and bottom of the vial. Then the reaction mixture was sonicated for few seconds (~5-10 seconds)
carefully and the solution of fine reddish brown color was centrifuged, washed with water
several times and absolute alcohol. Then it was dried in vacuum affording a fine reddish brown
color powder of Cu NPs (sample no. S4). Then the product was utilized for characterization and
nitroarenes reduction.
Catalytic reduction of nitroarenes using Cu O and Cu(0) NPs.
2
−
4
In a typical reaction, aqueous solution of 4-nitrophenol (3 mL of 1 × 10 M) and NaBH (300
4
−
2
μL of 5 × 10 M) were mixed in a quartz cuvette at room temperature. Then, Cu O NPs (S1) of
2
1
mg was added into the reaction mixture, and the progress of the reduction reaction was
monitored by a UV−vis spectrophotometer. This experiment was also carried out using different

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catalysts (other cuprous oxide and copper nanoparticles) for the 4-nitrophenol reduction study.
Similarly, Cu O NPs (S1) was used for other nitroarenes reduction also.
2
Recycling of catalysts for 4-nitrophenol reduction.
After the first cycle of reduction of 4-nitrophenol, the catalyst was collected by centrifugation
and transferred into the cuvette immediately. Then the solution of 4-nitrophenol (3 mL of 1 ×
−
4
−2
1
0
M) and NaBH (300 μL of 5 × 10 M) [freshly prepared and monitored by UV−vis
4
spectrophotometer] was added immediately into that cuvette and the cuvette was placed into the
UV-vis spectrophotometer chamber quickly for monitoring. The next cycles were also performed
similarly.
Separation of Cu O-Cu-CuO nanocomposite on first cycle reduction.
2
After the first cycle of reduction of 4-nitrophenol by Cu O (S1), the black precipitate (Cu O-Cu-
2
2
CuO Nanocomposite) was collected by quick centrifugation and washing with water and ethanol,
and finally dried in vacuum affording a black powder (Cu O-Cu-CuO nanocomposite). Then the
2
product was used for characterization.
Synthesis of Cu O-Cu-CuO nanocomposite from Cu O (S1).
2
2
Freshly prepared and dried Cu O (S1) of 1 mg was taken in a centrifuge tube and 3 mL water
2
−
2
was added to it to get the Cu O suspended aqueous phase. Then NaBH (5 × 10 M) of 300 μL
2
4
was added to the solution. After 2 minute, quick centrifugation was performed washing with
water for several times and ethanol, and finally dried under vacuum affording a black powder
(
Cu O-Cu-CuO nanocomposite). Then the product was used for characterization and nitroarene
2
reduction.

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Results and discussion
Synthesis of cuprous oxide and copper nanoparticles.
Cuprous oxide (Cu O NPs) and copper (Cu NPs) nanoparticles were synthesized from copper
2
acetate (Cu(OAc) .H O) through reduction using hydrazine hydrate (N H .H O) just varying the
2
2
2
4
2
concentration ratio at room temperature without using any surfactant or capping agent. The ratio
3
9
of Cu(OAc) to N H was found to be very important. We established that the reduction
2
2
4
reaction between Cu(OAc) and low concentration of N H [Cu(OAc) of 0.25 mmol with N H
4
2
2
4
2
2
of 20-30 µL in 5 mL H O] leads to Cu O at room temperature under free-standing conditions
2
2
according to the equation (1). On the other hand, the reaction between Cu(OAc) and high
2
concentration of N H [Cu(OAc) of 0.25 mmol with N H of 200 µL in 5 mL H O] leads
2
4
2
2
4
2
exclusively to Cu(0) according to the equations (1) & (2) under the same conditions. It is
important to mention that both the Cu O NPs and Cu NPs synthetic procedure were devoid of
2
using any surfactants or capping agent.
4
Cu(CH COO) + N H + 2H O
2Cu O + N + 8CH COOH
(1)
(2)
3
2
2
4
2
2
2
3
2
Cu O + N H
4Cu + 2H O + N
2 2
2
2
4
Hence, initially we carried out the synthesis of Cu O NPs (sample no. S2) from copper acetate
2
(0.25 mmol) and hydrazine hydrate (20 µL) in water (5 mL) under the free-standing conditions
for 6 h at room temperature. We observed that there occurred aggregation of Cu O NPs (Figure
2
S1 in Supporting Information). Then we adopted the synthesis of Cu O NPs under magnetic
2
stirring (for 30 min) in water (20 mL) and we obtained fine red Cu O NPs (S1) with out any
2
aggregation (FESEM details are given in next section). The process was executed in water at
room temperature and with out using any surfactants. Stirring helps the dispersion and
subsequent electrostatic stabilization of Cu O particles.
2

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We chose hydrazine hydrate as reducing agent because of its advantage over sodium
borohydride. There will be no remnant during the copper acetate reduction by hydrazine hydrate
because the byproduct is nitrogen. Besides, hydrazine hydrate is economically cheap and stable
under normal condition. As hydrazine is a weaker reducing agent than NaBH , nucleation and
4
growth of the nanoparticles would be slower and also the electrostatic agglomeration would be
lower. Hence the tuning of nanoparticles synthesis would be under control. Use of NaBH is
4
always required fresh solution because aqueous solution of NaBH always degrades to NaBO2
4
4
0
under ambient conditions. During the reduction of copper acetate by sodium borohydride, there
would also be generation of NaBO . As NaBH is stronger reducing agent than N H , the
2
4
2
4
nucleation and growth of the nanoparticles would be faster and also the electrostatic
agglomeration would be higher.
On the other hand, Cu NPs were synthesized from copper acetate (0.25 mmol) with higher
concentration of hydrazine hydrate (200 µL) in water at room temperature under free standing
condition in a sample vial analogous to the conditions of Cu NPs fabrication on various
39
surfaces. The process was carried out under closed container for 12 h at room temperature
without using any surfactant in water. On completion of the reaction, copper of fine reddish-
brown color was deposited onto the bottom and inner surface on the vial. Then, solvent was
decanted carefully and water was added. To achieve the Cu NPs suspension, sonication of the
reaction mixture for few seconds (5-10 s) was performed. Cu NPs suspension was centrifuged,
washed (with water subsequently absolute alcohol), and dried in vacuum to furnish fine reddish-
brown color powder of Cu NPs (sample no. S3). We observed that synthesized Cu-nanoparticles
are stable for a month under closed condition. However, all these synthetic procedures for Cu O
2
NPs (or Cu NPs) were executed without using any surfactants at room temperature from the

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same cost-effective starting materials. Overall the processes were easy to operate and facile, and
does not require any stringent condition or fine control.
Structure and characterization of nanocatalysts.
The as-prepared cuprous oxide nanoparticles (Cu O NPs) have been analyzed by XRD. Figure 1a
2
illustrates the XRD spectrum of the as-prepared pure Cu O NPs (sample no. S1), which matched
2
very well with JCPDS Card No. 05-0667. The spectrum shows four characteristic peaks at 29.8°,
3
6.7°, 42.6°, 61.7°, and 73.9° for the indices (110), (111), (200), (220), and (311) respectively.
We executed also the XRD analysis of the as-prepared Cu O NPs (sample no. S2) having similar
2
39
pattern. The XRD analysis (Figure 1b) of pure copper nanoparticles (sample no. S3) represents
4
3.3°, 50.4°, 74.1°, 89.9°, and 95.1° for the indices (111), (200), (220), (311), and (222)
respectively which matched well with the JCPDS Card No. 04-0836.
Figure 1. XRD of (a) Cu O NPs (S1), and (b) Cu NPs (S3).
2

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The morphologies of the as-synthesized nanocatalysts were characterized by the FESEM. Figure
(a-b) shows the FESEM images of the cuprous oxide nanoparticles (Cu O NPs; sample no. S1).
2
2
The images exhibit that the Cu O NPs are spherical in nature having diameter of 50-100 nm and
2
devoid of aggregation. The purity of the as-prepared Cu O NPs (S1) was tested by EDX analysis
2
(Figure 2c) and matched very well with the results obtained from XRD.
a
b
cps/eV
5
Element:
Atomic%
62.7
4
c
Cu K
3
O
Cu
Cu
O K
32.3
2
1
0
2
4
6
8
10
12
14
keV
Figure 2. FESEM images (a, b) and EDX (c) at different magnification of as-synthesized Cu O
2
NPs (S1).
Cu O NPs, prepared from copper acetate using NaBH and alkaline ascorbic acid without using
2
4
any surfactant or capping agent with fine tuning of concentrations, were also analyzed which are
the particle size of 300-600 nm and 200-500 nm respectively (Figure S2a in Supporting
Information). The synthesized products were characterized by XRD also (Figure S2a in
Supporting Information).

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We analyzed also the FESEM images of the copper nanoparticles (Cu NPs) (S3) which are
polygonal spherical nanosphere with diameter of few hundred nanometers (Figure S2b in
Supporting Information). EDX analysis of Cu NPs (S3) was also carried out (Figure S2b in
Supporting Information) displaying the presence of Cu(0) only in the sample.
The figures of DRS (Diffuse Reflectance Spectra) of Cu O NPs (S1) and Cu(0) NPs (S3) have
2
been illustrated (Figure S2c in Supporting Information). They matched well with the earlier
41
report . The absorbance peaks were appeared at 448-452 nm and 547-550 nm due to the Cu O
2
NPs (S1) and CuNPs (S3), respectively.
Catalytic activities of nanocatalysts for nitroarenes reduction.
Reduction of nitroarenes to the corresponding amines is industrially important as mentioned
earlier. Additionally, nitro compounds do not easily undergo reduction by aqueous sodium
borohydride in general. The noble metal nanoparticles catalyzed direct reduction of 4-
4
2
nitrophenol towards 4-aminophenol is being considered as a green process. But the scarcity
and high cost of noble metals compel the scientists to the development of alternative catalysts for
4
3
the production of 4-aminophenol from 4-nitrophenol. In this work, we studied the catalytic
efficacy of the as-prepared catalysts (Cu O NPs & Cu NPs) on reduction of 4-nitrophenol to 4-
2
4
4
aminophenol with aqueous sodium borohydride as a model reaction. Under the mild conditions
at ambient condition and room temperature), the nanocatalysts were observed as highly active
towards nitro to amino transformation. It is noteworthy to mention that the initial concentrations
(
-2
-4
of sodium borohydride and p-nitrophenol were kept at 5x10 M and 1x10 M, respectively in all
the sets of reactions. The reactions were monitored by the UV-Vis absorbance spectroscopy
through evaluating the disappearance of peak for p-nitrophenol and generation of peak for p-

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aminophenol. Under an acidic or neutral aqueous conditions, solution of 4-NP exhibits strong
45
absorbance peak at 317 nm. Upon NaBH addition, there occurs a spectral shift to 400 nm due
4
4
2a,45,46
to the alkalinity and, thereby p-nitrophenolate ion generation.
However, as the reaction
passes, there occurs weakening of absorbance of the peak at 400 nm, and concomitantly gradual
11,16
enhancement of absorbance at
̴
297-300 nm because of the generation of p-aminophenol.
a
Table 1. Reduction of 4-nitrophenol using various catalysts
b
c
Rate constant (k)
Entry
Catalyst
Time(min)
-1 d
(
min )
e
1
Octahedral Cu O
36
40
45
7
0.1251
0.1112
0.1023
0.5212
2
e
2
Cubic Cu O
2
e
3
Spherical Cu O
2
4
Cu O (S2)
2
5
Cu O (S1)
3
0.9407
2
6
7
8
Cu (S3)
9
11
8
0.3105
Cu O-Cu-CuO
0.3235
0.4267
-4
2
Cu (S4)
a
-2
Conditions: catalyst (1.0 mg), NaBH (5×10 M of 300 µL ), and 4-NP (10 M of 3 mL) were
4
b
c
used in all reactions. Different catalysts were used for the screening studies. Time (min)
d
required for the completion of the reaction. Rate constants of the corresponding reactions using
e
catalysts, ref: 48.

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4
4,47
In continuation of our investigation on the reduction of 4-NP towards the generation of 4-AP,
4
8
we observed that Cu O was found to be an effective catalyst for the 4-NP reduction. It was
2
noticed that octahedral Cu O, cubic Cu O, and spherical Cu O nanoparticles, prepared from
2
2
2
-1
-1
CuSO .5H O, rendered the rate constant values (k) of 0.1251 min , 0.1112 min , and 0.1023
4
2
-1
min against the completion time of 36 minutes, 40 minutes, and 45 minutes, respectively (Table
4
8
1
, entry 1-3). Then, these time-consuming 4-NP reduction using Cu O nanoparticles prompted
2
us further to develop more efficient Cu O catalyst. Towards this end, we prepared the Cu O
2
2
(
sample no. S2) nanocatalyst from Cu(OAc) .H O and N H .H O under free standing condition
2 2 2 4 2
at room temperature. Pleasantly, this catalyst (S2) provided higher efficiency (rate constant value
-1
k = 0.5212 min ; see Table 1, entry 4). The reaction took only 7 min for the completion (Figure
3a is the UV-vis absorption spectrum). As from the FESEM image (Figure S1 in Supporting
Information), it was examined that the Cu O NPs (S2) nanocatalyst remained aggregated in a
2
point of fact. Then we prepared Cu O (S1) from Cu(OAc) .H O and N H .H O under magnetic
2
2
2
2
4
2
stirring condition at room temperature for 30 minutes only to restrain such aggregation. So in this
method, growth process of nanoparticles was considerably arrested. The Cu O (S1) particles
2
were obtained to be very small nanoparticles and remained separated. Then Cu O NPs (S1)
2
catalyst was executed for the 4-NP reduction, and we observed that the Cu O NPs (S1) catalyzed
2
-
1
the reduction more efficiently (k = 0.9407 min ; Table 1, entry 5, Figure 3b is the UV-vis
absorption spectrum).
We have observed also that the catalytic rates of cuprous oxides, obtained by reduction using
NaBH (Cu O(A)) and alkaline ascorbic acid (Cu O(B)), are lower than that of Cu O(S1). This is
4
2
2
2
due to the larger size of the nanoparticles (Figure S3 for UV-vis spectra in Supporting
Information).

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In all the reaction, the concentration of borohydride was much higher (approximate 50 times)
compared to the concentration of 4-NP, and thus the reactions followed through pseudo-first-
order kinetics. Consequently, the figure of ‘ln(A/A ) vs time’ of each reactions illustrates a
0
negative slope which identifies the rate constants (Figure 3c). The rate constants of Cu O NPs
2
-1
(
S1) catalyzed reaction was found to be 0.9407 min (k) using 1.0 mg catalyst (time required for
the reaction was 3 min.; Figure 3b is the UV-vis spectrum) with the rate catalyst activity
-1
-1
parameter of k ′ = 0.9407 min mg (k′ = k/m; k = rate constant & m = amount of catalyst). A
1
-1
lower catalyst dose (0.5 mg) offered the rate constant of 0.6232 min (time required for the
reaction was 5 min.; Figure S4a in Supporting Information is the UV-vis spectrum) with k ′ of
0
.5
-1
-1
1
.2464 min mg which is higher than k ′.
1

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Figure 3. UV-vis absorption spectra of the reduction of 4-nitrophenol by NaBH using (a) 1.0
4
mg Cu O NPs (S2), (b) 1.0 mg Cu O NPs (S1), (d) 1.0 mg Cu NPs (S3), and (c) plot of
2
2
logarithm of (A/A ) at 400 nm vs reduction time with varying amount of Cu O NPs (S1) catalyst.
0
2
On the other hand, the reaction on higher dose of catalyst (1.5 mg) completed within 2 minutes
(Figure S4b in Supporting Information: UV-vis spectrum) with the higher rate constant value of
-1
-1
-1
1
.5213 min encompassing k ′ = 1.0142 min mg . We tested also the reductive efficiency on
1.5
further recyclability after using Cu O (S1) catalyst in first cycle. It was found that the reactions
2
nd
rd
th
took 6 minutes, 10 minutes, 14 minutes, & 16 minutes to complete the reaction of 2 , 3 , 4 &
th
-1
-1
-1
-1
5
cycles with rate constant values of 0.5196 min , 0.3004 min , 0.2499 min , & 0.2348 min
respectively as expected (see Table S1 and Figure S5a for UV-vis spectra in Supporting
Information).
After successful employment of Cu O catalyst on 4-NP reduction, we used Cu(0) NPs (S3) to
2
test its catalytic efficiency on 4-NP reduction. It was accounted that Cu(0) (S3) was also amiable
to show good catalytic activity (reaction time 9 min; Table 1, entry 6) for the 4-NP reduction
-1
with rate constant value of k = 0.3105 min (much lower than k value of Cu O (S1)) using 1.0
2
mg catalyst (Figure 3d is the UV-vis absorption spectrum). However, Cu(0) was also found to
-1
show the recyclability but with very lower rate constant (k = 0.1276 min , time = 24 min. for the
second cycle; see Table S1 and Figure S5b in Supporting Information for UV-vis spectrum).
Mechanism of Cu O NPs catalyzed 4-nitrophenol reduction.
2
In this work, we explained the mechanistic pathway for the Cu O catalyzed nitroarene reduction
2
to amine. Like other metals, copper is a good electrical conductor and can transfer electron from
one adsorbed species to another on its surface easily. This remarkable property of copper makes
8
,21-25
it a good catalyst for nitro compound reduction.
On the basis of experimental results on as-
prepared Cu NPs catalyzed 4-NP reduction, we observed that 1.0 mg Cu NPs (S3) could reduce

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-1
4
-NP in 9 min. (k = 0.3105 min ; Table 1, entry 6). Furthermore, from the XRD investigation
(see Figure S6 in Supporting Information for XRD of Cu NPs after first cycle) it was observed
that the catalyst (Cu NPs) was not transformed to any oxide or oxide-metal composite.
Concurrently, we found also that XRD of Cu NPs did not change on NaBH treatment for 5 min.
4
(Figure S6 in Supporting Information). So, our as-prepared Cu NPs solely catalyzed the 4-
nitrophenol in aqueous medium in presence of sodium borohydride. Based on these results and
3
8
earlier reports along with classical Langmuir-Hinshelwood model, herein the mechanism of Cu
¯
NPs catalyzed 4-NP reduction pathway depicted (Figure 4a) which illustrates that BH and 4-
4
NP were sequentially adsorbed on Cu NP surface (adsorption process through B & C transient
species), followed by surface-hydrogen species

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a.
NO2
-NP
R
4
Reduction
+6e , +2H )
-
+
(
H N
2
R
4-AP
R
H
B
H
C
BH4
&
NO2
H
H
H
Adsorption
Desorption
Adsorption
H
H
D
A
Cu NP
b.
Reduction
(partly)
Cu O
2
+
NaBH4
Cu-Cu O
2
F
E
Partial oxidation of Cu
under alkaline pH  9
Cu O-Cu-CuO
2
G
Adsorption
Desorption
BH4
4
-NP reduction
NH2
NO2
R
R
H
4-AP
4-NP
H
NO2
B
R
H
H
Cu O-Cu-CuO
2
H
Hydride transfer
Figure 4. Plausible mechanism of 4-NP reduction over (a) Cu (0) and (b) Cu O.
2

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¯
and electrons transferred (reduction) from BH to 4-NP on its surface to furnish 4-aminophenol
4
(desorption process leading D & 4-aminophenol) through the 4-nitrosophenol and 4-
¯
hydroxylaminophenol intermediates (and even through hydrazine intermediate) with (+6e ,
+
37,38,44,49-50
+
2H ) transmittance.
3
0
On the other hand, Cu O, p-type semiconductor, has the band gap value of 2.2 eV, and thus it
2
has not the ability to transfer electron from one species to another on its surface easily under
ambient conditions. In this context, it is important to note that as-prepared Cu O (S1) rendered
2
-1
excellent catalytic activity towards 4-NP reduction (k = 0.9407 min ; Table 1, entry 5). To
investigate the mechanism, we analyzed the catalyst (black) obtained after first cycle of Cu O
2
NPs catalyzed reaction. We found that the black material was Cu O-Cu-CuO nanocomposite
2
after analysis (confirmed through XPS, XRD, and TEM analysis; Figure 5 and Figure S7 in
Supporting Information). XPS (Figure 5a) as well as fringe spacing (Figure 5b) described the
presence of Cu, Cu O, and CuO in the composite material. Furthermore, XRD and SAED pattern
2
(Figure S7 in Supporting Information) supported this consequence. To support this conjecture,
we executed also the reaction between sodium borohydride and Cu O for 2 min. keeping the
2
-2
same concentration and amount of NaBH (300 µL of 5 × 10 M) and Cu O (1.0 mg) in 3 mL
4
2
water. We obtained again black Cu O-Cu-CuO nanocomposite which was also characterized by
2
TEM. Thus the formation of a ternary complex (collected from the reaction medium) is
ascertained conclusively.
5
1
However, CuO (band gap value = 1.4 eV) is also a p-type semiconductor which cannot transfer
electron from one species to another on its surface easily under ambient condition like Cu O.
2
Hence forth, we postulated that highly active in situ Cu(0) was generated from Cu O through the
2
reduction by sodium borohydride. Then, the in situ generated Cu(0) catalyzed the reduction of 4-

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¯
NP to 4-AP through adsorption of 4-NP and BH , hydride transfer (i.e., reduction through
4
nitroso, and hydroxylamino intermediates), and desorption of 4-AP (depicted stepwise in Figure
4b).
b
Figure 5. XPS (a) and HR-TEM (b) images of Cu O-Cu-CuO nanocomposite.
2
However, the formation of CuO in Cu O-Cu-CuO is emblematic. The colors of the Cu O-Cu-
2
2
CuO powder (from Cu O/NaBH reaction and first cycle 4-NP reduction within 3 min.) were
2
4
black (Figure S7a in Supporting Information) which resembled the presence of CuO in the
composite. Herein, we discarded the possibility of acid assisted disproportionation reaction by
Cu O to generate Cu(0) and CuO because of the high alkalinity (pH = 9) of the reaction medium.
2
3
5,36
However, Cu O or Cu could be oxidized by oxygen dissolved in aqueous medium,
and or
But these processes for CuO generation are not so
fast. Even, we observed that our as-prepared Cu(0) NPs was unaffected by NaBH treatment for
2
5
2,53
because of the alkalinity of the solution.
4

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5
min. (confirmed by XRD (Figure S6); discussed earlier). However, it is reported that Cu(0),
generated from CuO by NaBH in the reaction medium (i.e., in situ), could be oxidized into CuO
4
4
8
under alkaline condition when borohydride used as a reductant. Therefore, we postulated that
CuO could be formed due to the oxidation of only highly active in situ generated Cu(0) may be
5
2,54
via Cu(OH) and subsequent dehydration.
2
Mechanistically, we elucidate on the Cu O catalyzed 4-nitrophenol reduction with NaBH .
2
4
5
5
Herein, highly active Cu(0) is generated in situ from Cu O by NaBH reduction, and
2
4
immediately forms Cu O-Cu composite (contains in situ Cu(0); F in Figure 4b) which quickly
2
transformed into Cu O-Cu-CuO (contains in situ Cu(0) too; G in Figure 4b). Because of the
2
alkalinity, the highly active in situ Cu(0) of Cu O-Cu oxidized in portion to CuO forming Cu O-
2
2
4
8
Cu-CuO composite (contains also in situ Cu(0)). So, based on direct evidence of Cu O-Cu-
2
CuO formation during 4-NP reduction/NaBH treatment on Cu O, band gap theory, and other
4
2
experimental results, we postulated that the very high reactivity of Cu O on 4-NP reduction can
2
only be due to the presence of in situ generated Cu(0) in the evoluted ternary nanocomposite
Cu O-Cu-CuO. The in situ generated Cu(0) catalyzed the reduction of 4-NP to 4-AP through
2
¯
adsorption of 4-NP and BH , hydride transfer (i.e., reduction through nitroso, and
4
hydroxylamino intermediates), and desorption of 4-AP (depicted stepwise in Figure 4b). After
desorption ternary nanocomposite Cu O-Cu-CuO could catalyze the reduction again. However
2
we observed also that the ternary Cu O-Cu-CuO nanocomposite (ex situ prepared), prepared
2
from Cu O NPs (S1) by NaBH treatment for 2 min (discussed earlier), displayed much lower
2
4
-1
activity with rate constant value of 0.3235 min and completion time of 11 min (see Table 1,
entry 7 and Figure S8 for UV-vis spectrum for 4-NP reduction). We observed also the fast
dropping of 4-nitrophenol reduction rate over Cu O (S1) sample on recycling (rate constant
2

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-1
value k = 0.5196 min in second cycle). It is worth mentioning that while catalyst, Cu O-Cu-
2
CuO, is generated from Cu O(S1) in the reaction mixture (first cycle rate constant value k = 0.
2
-1
9
407 min ) it imparts fresh Cu surface to drive the catalysis. On the other hand, same amount of
catalyst (preformed or used in first cycle) really provides decreased fresh Cu surface for the
reaction to proceed. On the other hand, the CuNPs (S4), prepared from Cu O (S1) through
2
chemical reduction by hydrazine hydrate at room temperature, displayed also lower catalytic
-1
activity towards 4-NP reduction (rate constant value k = 0.4267 min ) and completion time of 8
min (see Table 1, entry 8 and Figure S9 in Supporting Information for XRD and UV-vis
spectrum for 4-NP reduction). These results corroborated also the higher activity of Cu O NPs
2
(
S1) on 4-NP reduction because of the in situ Cu(0) in the Cu O-Cu-CuO matrix evoluted from
2
Cu O (S1) by NaBH during catalysis. Another additional aspect of the mechanism is that CuO
2
4
in the Cu O-Cu-CuO nanocomposite could generate also in situ Cu(0) by NaBH , which also
2
4
48
catalyzed the 4-nitrophenol reduction.
However, achieving of Cu O-Cu-CuO nanocomposite from the Cu O NPs (S1) catalyzed first
2
2
cycle reduction of 4-nitrophenol emerged as novel consequence. The Cu O-Cu-CuO
2
nanocomposite was characterized by XPS (Figure 5a; the deconvoluted graph defines the
presence of Cu(+1) at 931.71 eV, Cu(0) at 932.20 eV, and Cu(+2) at 933.81 eV for Cu2p ,
3
/2
5
6
which matched well with earlier report ), XRD (Figure S7b, which matched well with JCPDS
No. 05-0667 (Cu O), 04-0836 (Cu), & 48-1548 (CuO)), and TEM (Figure 5b for fringe spacing,
2
and Figure S7c-d for TEM image and SAED pattern in Supporting Information) successfully. In
the HR-TEM analysis (Figure 5b), the values of fringe spacing of 0.203 nm, 0.303 nm, 0.228
nm, and 0.254 nm are correspond to crystal planes (111) of Cu, (110) of Cu O, (111)/(200) and
2
4
1,57-58
(11-1)/(002) of CuO, respectively.
FESEM of Cu O-Cu-CuO nanocomposites and Cu O
2 2

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are similar in nature. However, it is very much significant to mention that Cu O-Cu-CuO
2
nanocomposite (characterized by TEM studies again) was also obtained by the sodium
borohydride treatment on Cu O, which is a new and facile method. It is anticipated that the new
2
Cu O-Cu-CuO nanocomposite could be a good photocatalyst to drive plasmonic catalysis.
2
Comparison study of catalytic activities between in situ Cu O-Cu-CuO (Cu O (S1)) and
2
2
reported catalysts.
After successful mechanistic elaboration of fast in situ Cu O-Cu-CuO (using Cu O(S1))
2
2
catalyzed 4-nitrophenol reduction we became interested to compare its activity with the reported
catalytic activities of heterogeneous nanocatalysts for 4-nitrophenol reduction. We illustrated a
comparison depiction in Table 2 in terms of rate activity parameter (k′ = k/m; k = rate constant &
m = amount of catalyst). It is noticed that the rate activity parameters (k′) by in situ Cu O-Cu-
2
CuO (using Cu O(S1)) presented in this work (Table 2; entries 8-9) are higher than rate activity
2
Table 2. Comparison of rate activity parameters (k´) by different reported Cu, Ag, Au, Pd and
Fe catalysts with our as-described Cu O(S1) (in situ Cu O-Cu-CuO) catalysts for the reduction
2
2
of 4-nitrophenol (4-NP).
Entry Catalyst
Rate constant
k)
Amount rate activity parameter (k/m)
(k´)
Ref
(
In situ
−
2
−1
−3 −1
-1
1
2
HNTs/Ag
In situ
1.03 × 10 s
2mg
2mg
5.15 × 10 s mg
59a
−
3
−1
−3 −1
-1
HNTs/Au 9.51 × 10 s
4.75 × 10 s mg
59a
24
In situ
Cu
−
4
−1
−3 −1
-1
3
3.7 × 10 s
3mg
0.123 × 10 s mg

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−
3
−1
−1
−3 −1
-1
4
5
Fe O @SiO -Ag 7.67 × 10 s
1 mg
2 mg
7.67× 10 s mg
59b
59c
3
4
2
−
2
−3 −1
-1
Fe O -Au
1.05 × 10 s
5.25× 10 s mg
3
4
−
3
−1
−3 −1
-1
6
7
Ag-NP/C
Pd-rGO
1.69 × 10 s
1 mg
1.69 × 10 s mg
59d
59e
−
3
−1
−3 −1
-1
4.5 × 10 s
0.3 mg
15 × 10 s mg
In situ
−3
−1
−3 −1
-1
8
9
Cu O-Cu-CuO 15.6 × 10 s
1mg
15.6 × 10 s mg
This work
This work
2
In situ
−3
−1
−3 −1
-1
Cu O-Cu-CuO 10.4 × 10 s
0.5mg
20.7 × 10 s mg
2
parameters of the in situ noble metal catalysts (Table 2; entries 1-2 for HNTs/Ag and HNTs/Au
5
9a
24
respectively) and in situ copper catalyst (Table 2; entries 3). Furthermore, the rate activity
parameters of supported costly noble metal heterogeneous catalysts such as Fe O @SiO -Ag,
3
4
2
5
9b-e
Fe O -Au, Ag-NP/C, and Pd-rGO (Table 2; entries 4-7)
are also lower than the rate activity
3
4
parameters of the as-described in situ Cu O-Cu-CuO (using Cu O(S1)). This unusual high
2
2
activity of the in situ Cu O-Cu-CuO (using Cu O(S1)) catalyzed 4-nitrophenol reduction reaction
2
2
prompted us to utilized the method further on other nitroarenes.
Scope of the in situ Cu O-Cu-CuO catalyzed 4-nitrophenol reduction.
2
To further broaden the scope of this fast reductive transformation of 4-nitrophenol, other
nitroarenes were executed under the same reaction conditions using most active catalyst Cu O
2
(
S1) (i.e., in situ Cu O-Cu-CuO). Pleasingly, like 4-NP (Table 3, entry 1) all the other
2
nitroarenes underwent smooth reduction to their corresponding amino derivatives. Nitrobenzene
displayed good activity towards reduction taking only 4 min (Table 3, entry 3), while 4-
aminonitrobenzene (here NH is strong electron donating group) took 5 minutes for the complete
2

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reduction (Table 3, entry 2). Para-nitronezaldehyde underwent smooth transformation to (4-
6
0
aminophenyl)methanol (through the reduction of aldehyde group by NaBH only) followed by
4
Cu O catalyzed nitro group reduction (within 3 minutes; Table 3, entry 4). N-(4-
2
nitrophenyl)acetamide, having strong electron withdrawing group (-NHCOMe), presented
significant sensitivity towards reduction to amino derivative (time for the complete conversion: 4
minutes; Table 3, entry 5). All these reductions were monitored by UV-vis spectroscopy also
(see Figure S10 for UV-vis spectra for entry 2-5) which matched well with reported
48,60
literature.
a
Table 3. Reduction of nitroarenes using Cu O NPs (S1).
2
Time (min)b
Entry
Reactant
Product
NO2
NH2
1
3
OH
OH
NO2
NH2
2
5
NH2
NO2
NH2
NH2
3
4
4
3
NO2
NH2
CHO
NO2
CH OH
2
NH2
5
4
NHCOMe
NHCOMe

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a
-2
-4
Conditions: Cu O NPs (S1) (1.0 mg), NaBH (5×10 M of 300 µL), and Ar-NH (1×10 M of 3
2
4
2
b
mL) were used in all reactions. Time (min) required for the completion of the reaction.
Conclusion
Cuprous oxide nanoparticles, accomplished from copper acetate monohydrate through chemical
reduction by hydrazine hydrate without using any surfactant or support at room temperature, was
found to provide an excellent catalyst with high rate constant for the reduction of 4-nitrophenol
in aqueous solution in presence of sodium borohydride under ambient condition. Copper
nanoparticle was also achieved from copper acetate monohydrate and hydrazine hydrate, but it
displayed lower catalytic activity. A plausible first hand mechanism on the high reactivity of
cuprous oxide catalyzed 4-nitrophenol reduction has been proposed. We have unequivocally
established very high reactivity of cuprous oxide nanoparticles on nitroarene reduction. This
happened because of the in situ formation of Cu(0) in the Cu O-Cu-CuO nanocomposite matrix
2
in the reaction medium. A new method of Cu O-Cu-CuO nanocomposite synthesis has also been
2
developed. It is envisaged that this new Cu O-Cu-CuO nanocomposite would be a plasmonic
2
catalyst in the days to come. The synthetic procedure of Cu O NPs and its catalytic activity for
2
reduction of 4-nitrophenol are very much convenient, cost-effective, easily operable and thus
further applied successfully for the reduction of other functional nitroarenes also.
Acknowledgement
The authors are thankful to the UGC, DST, and CSIR New Delhi for financial support and IIT
Kharagpur for research facilities.
Supporting information

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XRD, FESEM, TEM, EDX, UV-vis spectra, etc. of Cu, Cu O, and Cu O-Cu-CuO. These are
2
2
available free of charge via the Internet at http://pubs.acs.org.
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