Green synthesis of gold, silver, platinum, and palladium nanoparticles reduced and stabilized by sodium rhodizonate and their catalytic reduction of 4-nitrophenol and methyl orange

Article abstract of DOI:10.1039/c8nj01223g

Sodium rhodizonate was used as a bifunctional reducing as well as stabilizing agent for the single step synthesis of gold (Au), silver (Ag), platinum (Pt), and palladium (Pd) nanoparticles (NPs) in water. Transmission electron microscopy analysis revealed that Pt, Au, Ag, and PdNPs have average core diameters of about 2, 8, 26, and 39 nm, respectively. The ability of these nanoparticles towards the catalytic reduction of 4-nitrophenol (4-NP) with sodium borohydride (NaBH₄) and the dual-catalytic oxidation of formic acid followed by the reduction of methyl orange (MO) was studied. The apparent rate constants (k_app) of the catalytic reduction of 4-NP in the presence of Ag, Au, Pt, and PdNPs were calculated to be 2.1482, 1.1167, 1.088 × 10^-1, and 1.65 × 10^-2 min^-1, respectively. However, for the dual-catalytic oxidation of formic acid followed by the reduction of MO, the k_app values were calculated to be 4.145, 1.25 × 10^-2, 6.7 × 10^-3, and 9.0 × 10^-5 for the Pt, Pd, Au, and AgNPs, respectively.

Full text of DOI:10.1039/c8nj01223g

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The role of atropisomers on the photo-reactivity and fatigue of
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Green synthesis of gold, silver, platinum, and palladium nanoparticles reduced and
stabilized by sodium rhodizonate and their catalytic reduction of 4-nitrophenol and methyl
orange
Md. Tariqul Islam,a Ricardo Arana-Saenz,a Huiyao Wang,b Ricardo Bernal,a Juan C. Noveron.a*
a. Department of Chemistry, University of Texas, El Paso, 500 West University Avenue, El
Paso, Texas 79968, USA.
b. CURRL, New Mexico State University, 945 College Drive, Las Cruces, USA
* Eꢀmail: jcnoveron@utep.edu
Keywards: gold nanoparticles, silver nanoparticles, platinum nanoparticles, palladium
nanoparticles, sodium rhodizonate, catalytic reduction, 4ꢀnitrophenol (4ꢀNP), formic acid
oxidation, methyl orange (MO) reduction.
Abstract
Sodium rhodizonate was used as a bifunctional reducing as well as stabilizing agent for the
single step synthesis of gold (Au), silver (Ag), platinum (Pt), and palladium (Pd) nanoparticles
(NPs) in water. Transmission electron microscopy revealed that the Pt, Au, Ag, and PdNPs have
the average core diameter of about 2, 8, 26, and 39 nm, respectively. The ability of these
nanoparticles towards the catalytic reduction of 4ꢀnitrophenol (4ꢀNP) with sodium borohydride
(NaBH4) and the dualꢀcatalytic oxidation of formic acid followed by the reduction of methyl
orange (MO) was studied. The apparent rate constants (kapp), of the catalytic reduction of 4ꢀNP in
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the presence of Ag, Au, Pt, and PdNPs, were calculated to be 2.1482, 1.1167, 1.088 ✕10ꢀ1, and
1.65 ✕10ꢀ2 minꢀ1, respectively. However, for the dualꢀcatalytic oxidation of formic acid followed
by the reduction of MO, the kapp were calculated to be 4.145, 1.25 ✕10ꢀ2, 6.7 ✕10ꢀ3, and 9.0 ✕
10ꢀ5 for the Pt, Pd, Au, and AgNPs, respectively.
Introduction
Noble metal nanoparticles possess unique sizeꢀ and shapeꢀdependent electrical, optical,
chemical and physical properties.1 Therefore, they are useful in variety of applications including
chemical and biological sensing,2 catalysis,3 light harvesting,4 bioimaging,5 therapeutics,6 and so
on. In particular, their excellent catalytic properties, which is typically observed at the
nanoscopic size in contrast to the macroscopic size, has made them some of the most widely
studied nanomaterials.7 As compared to the bulk material, the nanoparticles have larger
proportion of atoms at the edges and the corners, which are surrounded by fewer atoms. The
exposed atoms at the edges and the corners are considered to be chemically more reactive than
the atoms in the bulk. Noble metal nanoparticles are reported to be used in a variety of catalytic
reactions including, but not limited to the hydrogenation,8 oxidation,9 reduction,10 coupling.11
Recently, the catalytic reduction of 4ꢀnitrophenol (4ꢀNP) to 4ꢀaminophenol (4ꢀAP) and the dualꢀ
catalytic oxidation of the formic acid followed by the reductive degradation of organic pollutants
have drawn much attention.12,13 The catalytic reduction of 4ꢀNP to 4ꢀAP is not only a model
reaction in academia but also the reduced product (4ꢀAP) has industrial applications.14 For
example, 4ꢀAP is used as a photographic developer, corrosion inhibitor, antioxidant, precursor
for the manufacture of analgesic and antipyretic drugs, an intermediate in the synthesis of
paracetamol, etc.15 On the other hand, the dualꢀcatalytic oxidation of the formic acid followed by
the reductive degradation of organic pollutants is becoming an intriguing method in the area of
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hydrogen storage and the environmental remediation.15 The catalytic oxidation of formic acid
generates H2 and CO2 gases. This reaction could be utilized as a clean and stable source of
generating H2 gas. Moreover, the in situ generated H2 gas could be utilized to reductively
degrade organic and inorganic pollutants in water viz. methyl orange, nitrate, and hexavalent
chromium in water.16 The formic acid induced reductive degradation of organic pollutants has
benefit over other reducing agents viz. NaBH4 as it does not produce any secondary pollutants.
However, the catalytic reduction of the 4ꢀNP and the dual catalytic oxidation of formic acid
followed by the reductive degradation of MO commonly require noble metal nanoparticles as the
catalysts.17,18
There are number methods for the synthesis of noble metal nanoparticles, where a wide variety
19
reducing and/or stabilizing agents are utilized. For example, sodium citrate,
sodium
20
borohydride,
thiols, sodium rhodizonate,21 phosphine,22 tripolyphosphate,23 chitosan,24
pyridine,25 bipyridene,26 amines,27 fullerenes,28 phthalocyanine,29 carboxylic acid,30 polyethylene
glycol,31 dendrimers,32 plant extract,33 etc. have been utilized as the reducing and/or stabilizing
agents for the NPs synthesis. However, most of these methods cannot produce Au, Ag, Pt, and
Pd nanoparticles following a common synthetic procedure. Further, some methods require
multiple steps, harsh reducing agent viz. NaBH4, organic solvents, prolonged stirring/incubation
time, and separate reducing and stabilizing agents. Additionally, in the biological and medical
drug delivery systems, chemical sensing of aqueous species, bioꢀimaging, and catalysis the
requirement for the simplicity in the synthesis and water solubility is of the prime choice.
Therefore, a generic and simple method for synthesis of noble metal nanoparticles using limited
number of reagents is highly desired in the field of the noble metal nanoparticles research.
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Herein, we report a general oneꢀstep method for the synthesis of Au, Ag, Pt, and PdNPs in
water that are both reduced and stabilized by sodium rhodizonate. To the best of our knowledge,
the ability of the rhodizonate ions for the synthesis of Au, Ag, Pt and PdNPs following a generic
method has not been reported yet. The rhodizonate ion acts as a bifunctional reducing as well as
stabilizing agent for the nanoparticles. UVꢀVis spectroscopy, transmission electron microscopy
(TEM), energy dispersive Xꢀray (EDX) spectroscopy, and the zeta potential measurements were
used to characterize the NPs. The catalytic activities of the NPs were evaluated by two types of
catalytic reactions. The first reaction studied here is the reduction of 4ꢀNP to 4ꢀAP in water in the
presence of excess NaBH4. The second reaction we studied here is the dualꢀcatalytic oxidation of
the formic acid followed by the reduction of MO.
Materials and Methods
Materials
.
Sodium rhodizonate dibasic (97%), hydrogen tetrachloroaurate trihydrate (HAuCl4 3H2O,
.
99.9%), hydrogen hexachloroplatinate (IV) hydrate (H2PtCl6 3H2O ≥99.9%), silver nitrate
(AgNO3), Palladium nitrate [Pd(NO3)2], 4ꢀnitrophenol (O2NC6H4OH ≥99%), sodium
borohydride (NaBH4, 99%), formic acid (CH2O2) and methyl orange (MO), were purchased from
SigmaꢀAldrich, USA. MilliꢀQ water (>18.20 Mꢁ cm resistivity) was obtained from MilliꢀQ
(Advantage Aꢀ10) water filter system.
Characterization techniques
UVꢀVis spectrum was obtained by using Agilent Cary 50 Conc UVꢀVisible Spectrophotometer
using quartz cuvette of path length 10mm. Zeta potential measurement was carried out using
Zetasizer Nano ZS90 with Disposable folded capillary cell. For the Zeta potential and UVꢀVis
analyses, 6 times diluted NPs solution in water was used. Lowꢀresolution Transmission electron
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microscopy (TEM) experiments were performed using a Hitachi Hꢀ7650 transmission electron
microscope with an acceleration voltage of 80 kV. Highꢀ resolution Transmission electron
microscopy (TEM) experiments were performed using a JEOL JEM3200FS Electron Microscope
with an acceleration voltage of 300 kV. Carbon coated copper grids with 200 mesh (Electron
Microscopy Science) were used for TEM imaging. 30 microliters of sample were deposited onto
the grid and allowed to dry. IMAGEJ software was used to analyze average diameter and size
distribution of AuNPs from the TEM images. Energy dispersive Xꢀray (EDX) analysis was
carried out on dried AuNPs, which was centrifuged out at a speed of 15,000 rpm for 30 min.
Hitachi Sꢀ3400N Type II scanning electron microscope (SEM) equipped with an energy
dispersive Xꢀray spectrometer with accelerating voltage of 15 kV was used to obtain EDX
spectrum. Glass substrate was used for the EDX analysis of AuNPs.
Synthesis of the NPs in water
All the nanoparticles were prepared following a same synthetic procedure. Separate 0.5 mM
stock solutions of metal precursors were prepared by dissolving a required amount metal salt in
water. A separate 9.7 mM aqueous solution of sodium rhodizonate was prepared by dissolving
83.04 mg (0.388 mmol) of sodium rhodizonate in 40 ml of water. For a typical synthesis, in a
100 mL roundꢀbottomed flask 20 mL of 0.5 mM aqueous solution of metal precursor was
allowed to boil. Afterwards, 4 mL of 9.7 mM sodium rhodizonate was injected while vigorous
stirring of the solution. The reaction mixture was further boiled for 20 min to make sure a
complete reduction of the metal ions. The molar ratio of the sodium rhodizonate to the noble
metal ions was 3.88 in all the syntheses. The NPs solutions were preserved under ambient
conditions for further characterization and catalytic applications.
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Scheme 1. Synthesis of the Au, Ag, Pt, and PdNPs in water that are reduced and stabilized by
sodium rhodizonate.
Catalytic activity of the NPs
The catalytic activity of the nanoparticles was evaluated by two different types of reduction
reactions, Scheme 2 and 3. The first reaction was the reduction of 4ꢀnitrophenol (4ꢀNP) to 4ꢀ
aminophenol (4ꢀAP) in presence of excess NaBH4, whereas the second reaction was the dualꢀ
catalytic oxidation of formic acid followed by the reduction of methyl orange.
Scheme 2. Metal nanoparticles catalyzed reduction of 4ꢀNP to 4ꢀAP in presence of NaBH4.
For the reduction of 4ꢀNP, in a standard quartz cuvette 1 mL of 4ꢀNP aqueous solution (0.2
mM) was mixed with 50 µL of NPs stock solution. Afterwards, 0.5 mL of freshly prepared
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NaBH4 aqueous solution (5mg NaBH4 in 0.5 ml of water) was added into the quartz cuvette and
the reaction course was monitored, every 30 second, by using the UVꢀVis spectrophotometer.
The UVꢀVis spectrophotometer monitored the timeꢀdependent lowering of the absorbance of 4ꢀ
nitrophenolate at 400 nm. It is worth mentioning that the 4ꢀNP reacts with NaBH4 to yield 4ꢀ
nitrophenolate, which shows strong absorption band centred at 400 nm.
For the dualꢀcatalytic oxidation of formic acid followed by the reduction of MO, in a standard
quartz cuvette, 50 µL of NPs stock solution was added to 1.5 mL of 26.7 ppm aqueous solution
of MO. Afterwards, 50 µL of formic acid was added and mixed quickly. The reaction course was
monitored by the UVꢀvis spectroscopy, which monitored the lowering of the absorbance of MO
at 515 nm in every 30 s interval of time.
Scheme 3. Metal nanoparticles induced dualꢀcatalytic oxidation of formic acid followed by the
reduction of methyl orange (MO).
The percent reduction of 4ꢀAP and MO was calculated using the following equation.34
Percent reduction of 4ꢀAP or MO = ꢀ ꢂꢀ ꢄ 100 %
ꢁ
ꢃ
ꢀ
ꢁ
where A0 and At represent the absorbance of 4ꢀNP and MO at 400 and 515 nm, respectively at
the beginning and at time (t) of the catalytic reduction.
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The reaction kintics was analyzed by the linear form of the pseudoꢀfirstꢀorder kinetic model,
which is expressed by the following equation.
ꢅ
ꢆ
Pseudoꢀfirstꢀorder reaction rate equation, ꢀln ꢅ = kt
ꢇ
where k (1/min) reprsents the pseudoꢀfirstꢀorder rate constant of the catalytic reduction; Co and
Ct represent the initial and timeꢀdependent concentrations of 4ꢀNP and MO, respectively,
Results and discussion
The Au, Ag, Pt, and PdNPs were facilely synthesized by mixing solutions of sodium rhodizonate
and metal precursor solutions in water at an elevated temperature. The reaction mixture changed
color immediately after the solutions were mixed together. This indicated the formation of NPs
by the naked eye visualization. The rhodizonate ions reduced the metallic ion to metallic NPs.
Afterwards, the excess rhodizonate ions bound to the metallic ions situated on the surface of the
nanoparticles by the coordination bond. As a result the rhodizonate ions capped the NPs from
further growth. Therefore, the rhodizonate ions acted as the bifunctional reducing as well as
capping agent. The ability of rhodizonate ion to form coordination complexes with different
metal ions has been reported before.35,36 However, its ability toward the reduction of Ag, Pt, and
Pd ions to their metallic nanoparticles followed by their stabilization is not investigated until we
reported here. Therefore, sodium rhodizonate could potentially be utilized as a soft reducing
agent for various other organic syntheses, where reducing agent viz. NaBH4 is too strong to use.
Furthermore, the catalytic activity of the nanoparticles towards the reduction of 4ꢀNP and the
reductive degradation of MO is systematically studied and compared. The catalytic reduction of
4ꢀNP is not only a model reaction that is commonly studied in academia but also the reduction
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product has some industrial applications. The catalytic oxidation of formic acid followed by the
reductive degradation of MO, on the other hand, could be utilized as a green method to degrade
organic pollutants in water, which could be an attractive alternative method in the area of
environmental remediation.
Characterization of the NPs
UV-Visible spectroscopy of the NPs
The typical UVꢀVis spectrum of the Au, Ag, Pt, and PdNPs are shown in Figure 1. Strong
absorption bands centered at about 530 and 416 nm were observed for the Au and AgNPs,
respectively. This type of absorption band is also known as the surfaceꢀplasmon resonance (SPR)
absorption band, which is usually exhibited by the Au and Ag nanoparticles.37,38 The SPR band
of the nanoparticles originates from the collective oscillation of conduction electrons upon
interaction with electromagnetic radiation.39 It could be observed that the SPR band generated by
AgNPs is about 100 nm blue shifted compared to the AuNPs. This is because of the fact that the
Ag and Au have completely different atomic size, electronic structure and conductivity.
1
0.8
0.6
0.4
0.2
0
AuNPs
AgNPs
PtNPs
PdNPs
370
470
570
670
770
Wavelength (nm)
Figure 1. UVꢀVisible absorption spectrum of the Au, Ag, Pt, and PdNPs in water.
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On the other hand, the Pt and PdNPs did not show any SPR band in the visible region of the
electromagnetic radiation, which is similar to other reports.40,41 The SPR band of platinum and
palladium nanoparticles are usually found in the ultraviolet range, unlike the Au and AgNPs,
which usually display SPR band in the visible region. Additionally, the size of the nanoparticles
and their degree of aggregation usually shift the surface SPR band. For example, gold
nanoparticles less than 2 nm in size usually do not exhibit the SPR band in the visible region.42
TEM, HRTEM, EDS, and Zeta Potential of the NPs
The typical TEM images of the NPs with the corresponding size distributions are shown in
Figure 2. The TEM images show that the Au, Ag, and PtNPs are fairly spherical and well
dispersed in in water although some degree of aggregations could also be observed. It could be
observed that some of the AuNPs look like nanoprism and nanocube. The AgNPs are seen to be
larger in diameter and have more faced morphology compared to the other NPs.
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Figure 2. Typical TEM images of the a) PtNPs, b) AuNPs, c) AgNPs, and d) PdNPs.
Corresponding size distributions of the NPs (eꢀh). Scale bar = 20, 20, 200 and 100 nm for the Pt,
Au, Ag and PdNPs, respectively.
The PdNPs are observed to be somewhat less spherical and amorphous in shape. Moreover, the
PdNPs were found to be more aggregated compared to the Au, Ag, and PtNPs in solution. The
corresponding size distribution histograms of the NPs reveal that the Pt, Au, Ag, and PdNPs have
the average core diameter of about 2, 8, 26, and 39 nm, respectively. The change in particles size
could be attributed to the difference in the reduction potential of the metals and the stabilization
ability of the rhodizonate ion towards the different noble metal ions studied here.
In order to determine the crystallinity as well as to analyze the nanoparticles qualitatively, highꢀ
resolution TEM analysis was carried out, Figure 3. HRTEM image can reveal the crystalline
faces of the nanoparticles with atomic resolution. Moreover, the interlayer spacing, obtained
from the HRTEM images, is usually utilized to characterize the different lattice planes of
crystalline materials, which in turn is used for the qualitaive analysis of the material.
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Figure 3. HRTEM images of the a) PtNPs, b) AuNPs, c) AgNPs, and d) PdNPs showing the
characteristic (111) interlattice spacing
The interlayer spacing, shown in figure 3a, is measured to be about 0.22 nm, which is the
characteristic lattice spacing of the platinum Pt(111) planes.43 Similarly, the characteristic
interlayer spacing of the Au(111), Ag(111), and Pd(111) planes were measured to be about 0.23,
0.24, and 0.22 nm, respectively, figure 3bꢀd.44,45,46 Therefore, the HRTEM images confirmed the
nanoparticles are crystalline and metallic in nature.
Energyꢀdispersive Xꢀray (EDX) analysis was conducted to determine the elemental composition
of the nanoparticles. As a representative, the EDX spectrum was obtained from the AuNPs
sample only, Figure 4a. The EDX spectrum revealed the presence of gold, carbon, and oxygen in
the nanoparticles sample. The presence of carbon and oxygen indicate the presence of
rhodizonate ions that are bound to the AuNPs surfaces. This further indicates that the
stabilization of the AuNPs by the excess rhodizonate ions present in the solution.
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Figure 4 a) Energy dispersive Xꢀray (EDX) spectrum of AuNPs and b) Zeta potential of the
AuNPs in water.
The zeta potential measurements were carried to determine the net charge on the NPs surface,
Figure 4b. The net zeta potential was determined to be ꢀ32.7, ꢀ34.0, ꢀ20.5, and ꢀ7.46 for the Au,
Ag, Pt, and PdNPs, respectively. The net negative values of the zeta potential further suggests
the binding of the negatively charged rhodizonate ions on the nanoparticles surface and thereby
yield a negative zeta potential. This indicates the dispersion stability of the NPs in water due to
the interparticles electrostatic repulsive forces. Moreover, it could be observed that the PdNPs
have the least net zeta potential value. This in turn indicates the poor dispersion stability of
PdNPs in water in compared to the other NPs. This result is similar to the TEM studies, where
the PdNPs were seen highly aggregated compared to the other NPs.
Catalytic reduction of 4-nitrophenol by the NPs
The timeꢀdependent decrease in the absorbance of 4ꢀNP centered at 400 nm with the
corresponding kinetics of the catalytic reductions is shown in Figure 5. Figure 5a shows the
continuous decrease in the absorbance of 4ꢀNP at 400 nm during its catalytic reduction in
presence of the AgNPs. The timeꢀdependent percent reduction of the 4ꢀNP, catalyzed by
different NPs, is shown in Figure 5b. It could be observed that the Ag and AuNPs were faster
catalyzing the reduction of 4ꢀNP compared to the Pd and PtNPs. The unꢀcatalyzed reaction,
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where no NPs were used, shows negligible percent reduction of the 4ꢀNP. This signifies the
robust nature of 4ꢀNP to undergo the reduction by NaBH4 only. The slow catalytic activity of the
PtNPs could be attributed to its strong stabilization by the rhodizonate ions in solution. However,
the slow catalytic activity of the PdNPs could be attributed to its large particle size and high
degree of aggregations in solution.
Figure 5 a) Timeꢀdependent UVꢀvis absorption spectrum of the 4ꢀNP reduction catalysed by
the AgNPs, b) Timeꢀdependent percent reduction of the 4ꢀNP catalysed by different
nanoparticles, and c) Pseudoꢀfirst order kinetics of the corresponding catalyzed and unꢀcatalyzed
reductions.
The kinetics of the catalytic reduction of 4ꢀNP, catalysed by different NPs, is shown in Figure
5c. A linear relationship between the –ln(Ct/C0) and reaction time (t) was observed. This
suggests that the kinetics of the 4ꢀNP reduction follows the Langmuir–Hinshelwood (LH) model.
Moreover, the linear increase of −ln(Ct/C0) with respect to the reaction time indicates that the
reduction follows pseudoꢀfirstꢀorder kinetics. The apparent rate constants (kapp), of the catalytic
reduction of 4ꢀNP in the presence of Ag, Au, Pt, and PdNPs were found to be 2.1482, 1.1167,
1.088 ✕10ꢀ1, and 1.65 ✕10ꢀ2 minꢀ1, respectively. Whereas, the apparent rate constant of the unꢀ
catalyzed reaction is calculated to be 8.0 ✕10ꢀ4 minꢀ1. Moreover, it could be observed that the
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induction time for the Ag and AuNPs catalyzed reaction is about zero, which indicate their fast
catalytic activity.
Catalytic oxidation of formic acid followed by the reduction of MO
Figure 6 shows the timeꢀdependent decrease in the absorbance of MO centered at 515 nm with
the corresponding kinetics of the reduction. Likewise the 4ꢀNP reduction, the –ln(Ct/Co) vs time
(t) is a straight line, which indicates that the reactions follow pseudoꢀfirstꢀorder kinetics. The
apparent rate constants of the reactions are calculated to be 4.145, 1.25 ✕10 ꢀ2, 6.7 ✕10 ꢀ3, 9.0 ✕
10 ꢀ5 and 7.0 ✕10ꢀ5 minꢀ1, for PtNPs, PdNPs, AuNPs, AgNPs catalysed and control experiments,
respectively. The unꢀcatalyzed reaction, where no NPs were used, showed negligible percent
reduction of the MO. The PtNPs demonstrated very fast and the Pd and AuNPs demonstrated
moderate catalytic activity, whereas the AgNPs showed negligibl activity under the experimental
conditions.
Figure 6. a) Timeꢀdependent UVꢀvis absorption spectrum of the MO reduction catalysed by the
PtNPs; b) Timeꢀdependent % reduction of the MO catalysed by different nanoparticles, and c)
Pseudoꢀfirst order kinetics [−ln(Ct/C0) vs. reaction time (t)] curves for the catalytic reduction of
MO
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The Pt and the PdNPs are wellꢀknown catalysts for the oxidation of formic acid to generate CO2
and H2 gases.47 Afterwards, the H2 gas gets chemisorbed on the NPs surface, which eventually
reduces the MO and break the conjugation through the diazo (N=N) bond. As a result the MO
looses its color gradually. The slow catalytic activity of the PdNPs, in this study, could be
attributed to their large particle size and high degree of aggregations in solution. Moreover, it
could be observed that the induction period for the PtNPs catalyzed reaction is about 30 sec. This
may happen due to the two steps catalysis reactions viz. the oxidation of formic acid and the
reduction of the MO. Induction period is commonly observed in heterogeneous catalysis and it is
a measure of the time that takes to reach the adsorptionꢀdesorption equilibrium between the
reactants and products with the catalyst.
Conclusions
In conclusion, we report a facile method for the generation of Au, Ag, Pt and Pd nanoparticles in
water that are both reduced and stabilized by sodium rhodizonate. The performance of these
nanoparticles towards the catalytic reduction of 4ꢀnitrophenol (4ꢀNP) and methyl orange (MO)
was systematically studied and compared. The results indicated that AgNPs followed by AuNPs
exhibited significantly faster kinetics for the reduction of the 4ꢀNP with NaBH4 as compared to
the Pt and the PdNPs. In contrary, the reduction of MO with formic acid catalyzed by the Pt and
PdNPs was found to be faster as compared to the AuNPs and AgNPs. Sodium rhodizonate
mediated synthesis of the Au, Ag, Pt and Pd nanoparticles represent a general route for
functionally active catalysts with a wide range of potential applications.
Author Information
Corresponding Author: Juan C. Noveron
*Eꢀmail: jcnoveron@utep.edu
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Notes
The authors declare no conflicting financial interest.
Acknowledgement
Financial support from NSF grants CHEꢀ0748913, MRIꢀ0923437, DMR PREMꢀ1205302, the
NEWTꢀ1449500, and USDA 2014ꢀ38422ꢀ22078 are gratefully acknowledged. We thank Dr.
Jorge GardeaꢀTorresdey research group for their kind assistance with the Zetaꢀpotential
measurements.
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Graphical abstract
Sodium rhodizonate mediated green synthesis of gold, silver, platinum, and palladium
nanoparticles and their catalytic reduction of 4-nitrophenol and methyl orange