Au-Ag and Pt-Ag bimetallic nanoparticles@halloysite nanotubes: Morphological modulation, improvement of thermal stability and catalytic performance

Article abstract of DOI:10.1039/c8ra00423d

In this study, Au-Ag and Pt-Ag bimetallic nanocages were loaded on natural halloysite nanotubes (HNTs) via galvanic exchange based on Ag@HNT. By changing the ratio of Au to Ag or Pt to Ag in exchange processes, Au-Ag@HNT and Pt-Ag@HNT with different nanostructures were generated. Both Au-Ag@HNT and Pt-Ag@HNT systems showed significantly improved efficiency as peroxidase-like catalysts in the oxidation of o-phenylenediamine compared with monometallic Au@HNT and Pt@HNT, although inert Ag is dominant in the composition of both Au-Ag and Pt-Ag nanocages. On the other hand, loading on HNTs enhanced the thermal stability for every system, whether monometallic Ag nanoparticles, bimetallic Au-Ag or Pt-Ag nanocages. Ag@HNT sustained thermal treatment at 400 °C in nitrogen with improved catalytic performance, while Au-Ag@HNT and Pt-Ag@HNT maintained or even had slightly enhanced catalytic efficiency after thermal treatment at 200 °C in nitrogen. This study demonstrated that natural halloysite nanotubes are a good support for various metallic nanoparticles, improving their catalytic efficiency and thermal stability.

Full text of DOI:10.1039/c8ra00423d

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Au–Ag and Pt–Ag bimetallic
nanoparticles@halloysite nanotubes:
Cite this: RSC Adv., 2018, 8, 10237
morphological modulation, improvement of
thermal stability and catalytic performance†
Siyu Li, Feng Tang, Huixin Wang, Junran Feng and Zhaoxia Jin
*
In this study, Au–Ag and Pt–Ag bimetallic nanocages were loaded on natural halloysite nanotubes (HNTs)
via galvanic exchange based on Ag@HNT. By changing the ratio of Au to Ag or Pt to Ag in exchange
processes, Au–Ag@HNT and Pt–Ag@HNT with different nanostructures were generated. Both Au–
Ag@HNT and Pt–Ag@HNT systems showed significantly improved efficiency as peroxidase-like catalysts
in the oxidation of o-phenylenediamine compared with monometallic Au@HNT and Pt@HNT, although
inert Ag is dominant in the composition of both Au–Ag and Pt–Ag nanocages. On the other hand,
loading on HNTs enhanced the thermal stability for every system, whether monometallic Ag
ꢀ
nanoparticles, bimetallic Au–Ag or Pt–Ag nanocages. Ag@HNT sustained thermal treatment at 400 C in
Received 15th January 2018
Accepted 1st March 2018
nitrogen with improved catalytic performance, while Au–Ag@HNT and Pt–Ag@HNT maintained or even
ꢀ
had slightly enhanced catalytic efficiency after thermal treatment at 200 C in nitrogen. This study
DOI: 10.1039/c8ra00423d
demonstrated that natural halloysite nanotubes are a good support for various metallic nanoparticles,
rsc.li/rsc-advances
improving their catalytic efficiency and thermal stability.
12
monometallic Ag NPs are inert. However, coarsening of metal
or bimetallic nanoparticles in catalytic processes under
Introduction
Metal nanoparticles (NPs) have great applications in the different reaction conditions induces a dramatic decrease of
chemical industry and environmental science as heterogeneous their catalytic efficiency. To avoid it, anchoring metal or bime-
catalysts. Compared with monometallic nanoparticles, bime- tallic nanoparticles on supports with high surface area is
tallic NPs have improved catalytic activity and durability in a general solution. This introduces a new inuential factor for
many reactions owing to the combined advantages of the catalyst efficiency, that is, the interaction between NP catalysts
1,2
13,14
bimetallic components at the atomic level. In particular, and their support.
For example, the strain between nano-
bimetallic nanocrystals display plenty of scope for modulation particles and substrates may assist the catalytic process via
15
of their morphologies and compositions, providing a wide reducing the energy barrier of reactions. Impressively, the
1
,3–6
platform of specic catalysts.
demonstrated a variety of applications, based its catalytic may signicantly enhance the thermal stability of metallic
The Ag nanoparticle itself has interaction between metal nanoparticles and support surface
16
activity, surface-enhanced Raman scattering and antibacterial catalysts. Mesoporous supports with different Si/Al ratios have
7,8
action. Ag-based bimetallic nanocrystals Ag–M (M ¼ Au, Pd or demonstrated distinct catalytic activities due to their different
Pt) have shown signicantly improved catalytic activity and densities of active defects which play a key role in stabilizing
9–11
17
extended the application landscape.
Au–Ag NPs have demonstrated peroxidase-like activity, whereas
For example, bimetallic and activating metal nanoparticles.
Halloysite, a kind of natural clay (chemical composition:
Al
2 2 5 4 2
Si O (OH) $nH O, 1 : 1 layer aluminosilicate) with tubular
18–21
nanostructure, is promising as a catalyst support.
Halloysite
Department of Chemistry, Renmin University of China, Beijing 100872, P. R. China. has many advantages, such as being environmentally benign,
E-mail: jinzx@ruc.edu.cn
low cost and having large surface area. However, to anchor
†
Electronic supplementary information (ESI) available: The preparation method
metal nanoparticles, specic surface modication of halloysite
of Au NPs, Pt NPs, Au@HNT, and Pt@HNT samples. Tables S1–S3 and Fig. S1–S10.
The experimental parameters in the preparation of bimetallic NPs@HNT; TEM
images of M@HNTs; UV-vis spectra of Au–Ag@HNT; the content of Ag and
Au(Pt) in Au(Pt)–Ag@HNT measured by ICP-OES; peroxidase-like catalytic
22–24
is generally required,
a surface modier.
and organosilane is oen used as
2
5–33
But the introduction of small molecules
as surface modier on halloysite surface may disfavor the
performance of Au(Pt)–Ag@HNT; time-dependent UV-vis absorption spectra and thermal stability of the metal@halloysite system owing to its
the 4-NP reduction rate constant k* for the catalytic reduction of 4-NP by NaBH
4
thermal decomposition at high temperature, and its
in the presence of M@HNT. See DOI: 10.1039/c8ra00423d
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susceptibility in some organic solvents or varied pH conditions solution was diluted to 80 mL with deionized water. Cleaned
may also induce detachment of modier molecules. These halloysite nanotubes (0.16 g), glucose (6.40 g) and PVP (1.60 g)
weaknesses have limited the application of metal@halloysite were dispersed in EG (80 mL). Aer mixing, 10 mM freshly
catalysts. Moreover, because of the elegant control of experi- prepared Tollens' reagent (80 mL) was dropped into the above
ꢀ
mental conditions required in the fabrication of bimetallic solution. The mixture was vigorously stirred in a 40 C water bath
34
nanoparticles, decorating bimetallic nanoparticles on halloy- for 30 min. The reaction time was shortened to 10 min when the
ꢀ ꢀ
site nanotubes has seldom been achieved.
preparation temperature was changed to 60 C or 80 C. Then
Herein, Ag@HNT was fabricated through a modied silver- the suspension was centrifuged (8000 rpm, 10 min) and washed
mirror-reaction without pretreatment of HNT surface. Then three times with deionized water and ethanol, respectively. The
ꢀ
porous bimetallic Au–Ag and Pt–Ag nanocages were successfully as-prepared Ag@HNT was dried at 65 C in an oven overnight.
generated on halloysite via galvanic exchange of Ag@HNT.
The preparation methods of Au NPs, Pt NPs, Au@HNT, and
Structural and morphological characterizations showed that Au Pt@HNT samples are shown in the ESI.†
or Pt domains intertwine with Ag domains, and Ag is dominant
in the composition of these Au–Ag and Pt–Ag nanocages on HNT. Preparation of Au–Ag@HNT and Pt–Ag@HNT
Bimetallic Au–Ag@HNT and Pt–Ag@HNT demonstrated signif-
The galvanic exchange method was followed for the synthesis of
icantly improved activity as peroxidase-like catalysts in the
catalytic oxidation of o-phenylenediamine (OPD) in the presence
of H O compared with monometallic Au@HNT and Pt@HNT,
where Ag@HNT is almost inert. In addition, the thermal
stabilities of Ag@HNT, Au–Ag@HNT and Pt–Ag@HNT were
carefully studied. The peroxidase-like catalytic ability of Au–
bimetallic nanoparticles based on Ag@HNT. Ag@HNT (5.2 mg)
ꢁ1
ꢀ
was dispersed in PVP solution (1 mg mL , 100 mL) in a 50 C
water bath. Then, HAuCl (10 mM) aqueous solution (200 mL)
2
2
4
was slowly dropped into the above solution under vigorous
magnetic stirring, such that the added Au : Ag atomic ratio was
0.1. The color of the solution changed from brown to brownish
Ag@HNT and Pt–Ag@HNT systems was maintained, or slightly
red within several seconds, indicating the formation of Au–
Ag@HNT via galvanic exchange. The composition of Au–
Ag@HNT was tuned by adjusting the ratio of Ag@HNT to
HAuCl (Table S1†). The reaction solution was stirred for 5 min
4
and centrifuged at 10 000 rpm for 10 min.
ꢀ
improved, aer thermal treatment at 200 C for 20 min in N
2
environment. Ag@HNT can resist thermal treatment at higher
ꢀ
temperature (400 C), to give even better catalytic efficiency in
the reduction of 4-nitrophenol by NaBH . The combination of
4
Ag-based bimetallic nanostructures and natural halloysite will
allow the development of catalysts with low cost and high effi-
ciency, which are highly desirable for industrial applications.
For the synthesis of Pt–Ag@HNT, a similar procedure was
followed except H PtCl $6H O (10 mM, 200 mL) was used as
2 6 2
precursor, instead of HAuCl . The color of the solution changed
4
from brown to grayish black within several minutes, showing
the formation of Pt–Ag@HNT. The composition of Pt–Ag@HNT
Experimental section
Materials
was adjusted by changing the ratio of Ag@HNT to H
2 6
PtCl
(Table S1†). The reaction mixture was stirred for 10 min and
Halloysite mineral (HNT) was purchased from Zhengzhou centrifuged at 12 000 rpm for 10 min.
Jinyangguang Ceramics Co. Ltd. Pristine HNT was cleaned with
Finally, both Au–Ag@HNT and Pt–Ag@HNT were washed
fresh water, dried in a freeze-drier, and then ground into ne twice with ethanol, saturated sodium chloride solution and
ꢀ
powder in a mortar before use. Analytical grade chloroauric acid then deionized water. Then the samples were dried at 65 C in
(
HAuCl
silver nitrate (AgNO
AA) were purchased from Sinopharm Chemical Reagent Co. Thermal treatment
4
), chloroplatinic acid hexahydrate (H
2 6 2
PtCl $6H O), an oven overnight.
3
), 4-nitrophenol (4-NP) and ascorbic acid
(
Ltd. Sodium borohydride (NaBH , AR) was purchased from
4
The dried powders of Ag@HNT, Au–Ag@HNT and Pt–Ag@HNT
Tianjin Fuchen Chemical Reagent Technologies Co. Ltd.
Glucose (AR) was purchased from Tianjin Fengchuan Chemical
Reagent Technologies Co. Ltd. Ammonium hydroxide
3
ꢁ1
ꢀ
were heated under nitrogen ow (40 cm min ) from 25 C to
ꢀ
ꢀ
ꢀ
ꢁ1
2
2
00 C (or 400 C) at a heating rate of 10 C min , and kept at
ꢀ
ꢀ
00 C (or 400 C) for 20 min.
(
NH
hydrogen peroxide (H
purchased from Beijing Chemical Works. Polyvinylpyrrolidone
PVP, MW 58 000, AR) was purchased from Alfa Aesar. o-Phe-
3
$H
2
O, 25%), ethanol (AR), ethylene glycol (EG, AR),
2 2
O
, 30%) and sodium chloride (AR) were
Characterizations of metallic nanoparticles and their
composites with HNTs
(
Nanoparticles and M@HNTs samples were dispersed in water
to form suspensions by sonication. Then a drop of each
suspension was placed onto a copper grid for transmission
electron microscopy (TEM) characterization (Hitachi TEM, H-
nylenediamine (OPD, 99.9%) was purchased from Sigma. All
chemicals were used as received without further purication.
Preparation of Ag@HNT
7650B) at an accelerating voltage of 100 kV. Scanning electron
Tollens' reagent (10 mM) was prepared as follows. First, microscopy (SEM) characterization was conducted by using an
NH $H O (2%) was added into AgNO solution (160 mM, 5 mL) SU8010 scanning electron microscope. High-resolution trans-
3
2
3
to induce precipitation; then extra aqueous ammonia was added mission electron microscopy characterization (HRTEM) was
until the precipitate was completely dissolved; nally the performed by using a Tecnai G2 F20 (FEI) microscope at an
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accelerating voltage of 200 kV. Energy-dispersive X-ray analysis
(EDX) element mapping was conducted in high-angle annular
dark eld (HAADF) mode with the same microscope.
UV-vis absorption spectra of metallic nanoparticles and their
composites with halloysites were measured by using a Varian
Cary 50 spectrometer. Wide-angle X-ray diffraction was per-
formed with an XRD-7000 diffractometer (Shimadzu) in
the reection mode using a Cu target to produce incident X-rays
ꢀ
ꢁ1
(
l ¼ 0.15418 nm). The scan speed was 2 min , and the scan
ꢀ
ꢀ
scope was from 10 to 80 . The Ag, Au and Pt contents of these
samples were determined by using inductively coupled plasma
optical emission spectroscopy (ICP-OES) (Agilent ICP/700). Before
characterization, samples (1 mg) were dissolved in an aqua regia
3
solution (1 : 3 v/v HNO : HCl, 10 mL) for 3 days. Then, 0.5 mL of
the aqua regia solution was diluted to 10 mL with deionized
ꢁ
1
water. The nal concentration of the test liquid was 5.0 mg L
.
Scheme 1 Schematic illustration of the fabrication of Ag@HNT, Au–
Ag@HNT, and Pt–Ag@HNT.
Catalytic performance
Catalytic reduction of 4-NP by NaBH
of M@HNT in the reduction of 4-NP in the presence of NaBH
was investigated by using UV-vis spectrometry. It was observed
that a red shi of the absorption peak due to 4-NP from 317 to
4
. The catalytic efficiency
Ag@HNT, we added PVP to the reaction medium. Glucose was
the reducing agent for Ag ions. In addition, we noticed that
4
ꢀ
ꢀ
ꢀ
different temperatures (40 C, 60 C or 80 C) showed signicant
inuence on the size distribution of Ag nanoparticles on HNTs.
Higher temperature accelerated the growth of Ag NPs, resulting
in large size and wide size-distribution of Ag NPs (Fig. S1†). The
4
00 nm occurred immediately aer the addition of NaBH . This
4
was because of the formation of 4-nitrophenolate ion in alkaline
conditions caused by NaBH . Aer adding M@HNT to this
4
ꢀ
modied silver-mirror process at 40 C showed great success in
solution, the intensity of the peak (400 nm) of the nitro
compound was successively decreased, and a new peak at
95 nm appeared due to the formation of 4-aminophenol (4-AP).
loading Ag nanoparticles on untreated halloysite nanotubes
(
1
(
Fig. 1). The mean diameter of fabricated Ag NPs was about
4 nm (Fig. 1b). HRTEM and selected area electron diffraction
SAED) showed that the obtained Ag nanoparticles on HNT were
2
The catalytic efficiency was quantitatively calculated based on
the successively decreased intensity of the peak at 400 nm with
35
well crystallized (Fig. 1c and d).
time. In a typical reaction, NaBH (1.70 mg) was added to the
4
4-NP aqueous solution (0.1 mM, 15 mL) in a round ask with
vigorous magnetic stirring, then Ag@HNT (7.5 mg) or bimetallic
NPs@HNT (4.5 mg) was added. The mixture was sampled aer
every 2 min and the absorption spectra of the samples were
recorded aer centrifugation (14 000 rpm, 2 min).
Peroxidase-like catalytic oxidation of OPD. Ag-based bime-
tallic NPs can quickly catalyze the oxidation of OPD, a typical
horseradish peroxidase (HRP) substrate, in the presence of
H O . The oxidation rates were evaluated by monitoring the
2
2
absorbance at 418 nm originating from DAP (2,3-dia-
minophenazine), the oxidized product of OPD. The catalytic
oxidation of OPD by bimetallic NPs@HNT in the presence of
2 2
H O was performed as follows: 10 mL of 0.1 M OPD aqueous
solution (freshly prepared) and bimetallic NPs@HNT (0.2 mg)
ꢀ
were added to 3 mL of H
2
O at 40 C. Then, H
2
O
2
(30%, 100 mL)
was added to the mixture. The pH of the mixture was 7. The
oxidation reaction progress was monitored by recording the UV-
vis absorption spectra of the solution at 2 min intervals.
Results and discussion
The fabrication and catalytic performance of Ag@HNT, Au–
Ag@HNT and Pt–Ag@HNT
Fig. 1 (a) SEM image of Ag@HNT with densely decorated Ag NPs. (b)
TEM image of Ag@HNT, demonstrating Ag nanoparticles with well-
controlled diameter. (c) HRTEM image of Ag@HNT, showing well-
To decorate Ag nanoparticles on raw halloysite, the silver-mirror
process was modied (Scheme 1). For improving the stability of crystallized Ag with cover layer. (d) SAED pattern of Ag@HNT.
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We noticed that the presence of halloysite signicantly added atomic ratio of Au to Ag in this galvanic exchange
improved the size control of Ag nanoparticles. Under the same intensied the etching of solid Ag by HAuCl . The morphology
4
experimental conditions only without halloysite, the size of Ag of Au–Ag nanostructures on halloysite nanotubes can be
nanoparticles was about 40–50 nm in diameter and they modulated from nanocages to network-like nanostructures by
aggregated heavily (Fig. S2†). Using these Ag@HNT samples changing the atomic ratio of Au to Ag from 0.1 to 1 (Fig. 2, S3a,
ꢀ
(
generated at 40 C), we fabricated bimetallic Au–Ag@HNT and S3b†). The corresponding SPR absorption of bimetallic nano-
Pt–Ag@HNT. Scheme 1 presents the fabrication procedure of particles also changed with the atomic ratio of Au : Ag, from
Ag@HNT and bimetallic Au–Ag@HNT and Pt–Ag@HNT nano- 450 nm (0.1, corresponding to nanocages) to 527 nm (1, corre-
composites. Galvanic exchange is commonly used in the sponding to network-like nanostructure). A further increase of
generation of bimetallic nanoparticles or even trimetallic the ratio of Au to Ag induced broken nanostructure (Fig. S3c†).
nanostructures, in which the morphology of bimetallic nano- ICP-OES characterizations showed the weight percentage of
36–39
particles varies with experimental conditions.
PtCl was added separately to Ag@HNT suspension to atomic ratios from 0.1 to 1 (Fig. S4a†).
generate galvanic exchange. The shi or disappearance of The morphological and structural characterizations of Pt–
HAuCl4 or Au : Ag in the bimetallic nanostructures obtained with added
H
2
6
surface plasmon resonance (SPR) of the original Ag nano- Ag@HNT are shown in Fig. 3. In the case of the fabrication of
particles indicated the formation of Au–Ag bimetallic nano- the Pt–Ag system, Pt–Ag nanocages were generated with added
particles (Fig. 2a). The etching of HAuCl
4
induced a slight atomic ratio of Pt : Ag ꢂ0.1 (Fig. 3c) in which the weight
decrease of the loading amount of silver nanoparticles on HNT percentage of Ag measured by ICP-OES was nearly double the
and signicant hollowing of solid silver nanoparticles (Fig. 2b, percentage of Pt (Fig. S4b†). HRTEM characterization presented
c). Silver nanoparticles on halloysites became hollow Au–Ag crystallized Pt domains intertwined with Ag domains (Fig. 3d).
nanocages. The content of gold and silver in the Au–Ag@HNT Combining the morphological and composition information,
was checked by EDX (Fig. 2e). Silver remained dominant in the we observed that when the weight percentage of Pt is over that
composition of bimetallic Au–Ag nanocages. Increasing the of Ag in bimetallic nanostructures, the complete Pt–Ag
Fig. 2 (a) UV-vis spectra of Ag@HNT and Au–Ag@HNT. (b) SEM image Fig. 3 (a) UV-vis spectra of Ag@HNT and Pt–Ag@HNT. (b) SEM image
of Au–Ag@HNT. (c) TEM image of Au–Ag@HNT. (d) HRTEM image of of Pt–Ag@HNT. (c) TEM image of Pt–Ag@HNT. (d) HRTEM image of
Au–Ag@HNT, identifying the co-existence of Au and Ag. (e) EDX Pt–Ag@HNT, identifying the co-existence of Au and Ag. (e) EDX
spectrum of Au–Ag@HNT and the corresponding atomic percentages spectrum of Pt–Ag@HNT and the corresponding atomic percentages
of Au and Ag. The added atomic ratio of Au : Ag was 0.1.
of Pt and Ag. The added atomic ratio of Pt : Ag was 0.1.
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nanocages lose their cage-like morphology (Fig. S5†). XRD the catalytic performance of Au–Ag@HNT and Pt–Ag@HNT
patterns of Au–Ag@HNT and Pt–Ag@HNT also supported the compared with Ag@HNT. Although Ag@HNT had nearly no
existence of bimetallic nanoparticles (Fig. S6†).
catalytic effect in this reaction, both Au–Ag@HNT and Pt–
The formation of bimetallic nanocages on halloysite nano- Ag@HNT demonstrated high catalytic efficiency. Using the re-
tubes signicantly extended and improved the catalytic ability ported molar absorption coefficient of the neutral form of DAP,
ꢁ
1
ꢁ1 43
of Ag@HNT. Bimetallic Au–Ag and Pt–Ag demonstrated
3
418 ¼ 16 700 M cm
, it can be estimated that 74.3% of the
peroxidase-like catalytic activity in the oxidation of OPD in the OPD was transformed into DAP in the rst 20 min in the pres-
40
presence of H
2
O
2
, in which Ag itself was almost inert.
2 2
H O
ence of Pt–Ag@HNT, which showed much higher catalytic effi-
can oxidize OPD to form 2,3-diaminophenazine (DAP, yellow ciency than Au–Ag@HNT. The surface properties of Pt–
41
color), showing a new absorbance peak at 418 nm (Fig. 4a).
Ag@HNT would inuence the absorption of H O and the
2 2
Monitoring the absorbance change at 418 nm is the common particle-mediated electron transfer, promoting H O to break
2
2
42
way to investigate this reaction. We propose the reaction up into HOc radicals. Because halloysite and Ag have nearly no
process from OPD to DAP in the presence of H on the catalytic effect, for comparing the reaction rate in different
surfaces of mono- or bi-metallic NPs as shown in eqn (1) and (2) catalyst systems, the reaction rate constant (k) in each system
2 2
O
42
in Fig. 4a based on the literature. Monometallic Au and Pt was divided by the molar value of the active elements (calcu-
nanoparticles, loaded on HNT or not, showed a catalytic effect lated on the basis of ICP-OES results), which are Au in both
in the oxidation of OPD. Pt@HNT showed higher catalytic effi- Au@HNT and Au–Ag@HNT, and Pt in both Pt@HNT and Pt–
ciency than Au@HNT, but the performances of their unloaded Ag@HNT, to generate k* shown in Table 1.
nanoparticles were nearly the same (Fig. 4b). Fig. 4c presents
The improvement in catalytic performance produced by
loading on a support can be assigned to the inuence of
morphology and electronic structure of metallic nanoparticles
44,45
being changed by the support's properties.
In the prepara-
tion of Ag NPs and Ag@HNT, we found that HNT supports were
favorable to the size control of loaded Ag NPs. On the other
hand, it is reported that Au nanoparticles are easily faceted
while anchored on the support surface compared with in solu-
46
tion. Loading on halloysite improved the catalytic perfor-
mance of monometallic Au or Pt nanoparticles (Table 1), and
forming bimetallic Au–Ag@HNT and Pt–Ag@HNT further
enhanced their catalytic efficiency. The DAP formation rate in
the case of Au–Ag@HNT was over 13 times that in the Au@HNT
system, and k* for Pt–Ag@HNT was 6 times that in the Pt@HNT
case (Table 1). This indicated the active components were much
more efficient while they were separated by the Ag spacer.
Separated Au or Pt may favor the most exposure of the active
facets (Fig. 4d). The Au–Ag@HNT and Pt–Ag@HNT catalysts
were recycled 4 times, and the corresponding DAP formation
rate constants are shown in Fig. S7.† The k* value decreased
slightly aer each cycle, which might partially be attributed to
the sample loss in the recycling process.
Table 1 The DAP formation rate constant k* (normalized based on the
molar content of active elements) in different catalytic systems
k* (mol-DAP
per min per mol-M)
Fig. 4 (a) The proposed reaction mechanism of the transformation of
OPD to DAP. (b) Peroxidase-like catalytic performance of Au, Pt,
Sample
Au@HNT, and Pt@HNT. The purple line shows the data from the Au NPs
control experiment. (c) Peroxidase-like catalytic performance of Au@HNT
Ag@HNT, Au–Ag@HNT and Pt–Ag@HNT. Absorbance at 418 nm was Pt NPs
monitored as a function of time after adding catalysts (0.2 mg). Pt@HNT
0.03
0.04
0.03
0.16
0.39
0.23
0.12
0.99
0.95
0.48
Reaction conditions: H
2
O
2
(0.3 M), OPD (0.3 mM), and catalyst (0.2 mg) Au–Ag@HNT
Au : Ag ¼ 0.1
Au : Ag ¼ 0.5
Au : Ag ¼ 1
Pt : Ag ¼ 0.1
Pt : Ag ¼ 0.5
Pt : Ag ¼ 1
ꢀ
at 40 C. Au(Pt)–Ag@HNT was prepared with the added atomic ratio of
Au(Pt) : Ag of 0.1. (d) Schematic diagram of monometallic catalyst (with
limited exposure of the active facets of metallic nanoparticles) and Pt–Ag@HNT
bimetallic catalyst (with enhanced exposure of the active facets of
metallic nanoparticles) in the peroxidase-like catalytic oxidation of
OPD.
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The catalytic efficiencies of Au–Ag@HNT and Pt–Ag@HNT structural information concerning Ag@HNT nanocomposites
ꢀ
generated with different experimental parameters were also aer thermal treatment at 400 C in N atmosphere for 20 min.
2
compared (Table 1, Fig. S8†). An increase of Au or Pt amount
Thermal treatment induced a change of size distribution of
induced a decrease of catalytic efficiency, which may be because Ag on halloysite. The number of larger particles increased,
of the damage to perfect bimetallic nanocages at higher atomic which was supported by the sharper peak of the (111) Ag
ratios of Au(Pt) : Ag (Fig. S3, S5†), and the decrease of Ag crystal plane in the XRD pattern of heated Ag@HNT (Fig. 5a).
amount (Fig. S4†). If the Ag spacer is lost, the active domains of But the loading amount of silver on HNT showed no signi-
Au or Pt may aggregate together, leading to a reduction in cant change (Table 2). HRTEM characterization revealed that
catalytic efficiency. Bimetallic nanocages of Au–Ag@HNT or Pt– carbon was derived from pyrolysis of polymeric stabilizer
Ag@HNT, in which there are enough Ag spacers between Au or covering Ag NPs and halloysites, and it may behave as a buffer
Pt domains, showed better performance than those bimetallic to stop the merging of Ag NPs on halloysite (Fig. 5b). The
nanostructures with less Ag. In addition, the catalytic perfor- composition of Ag@HNT, Au–Ag@HNT and Pt–Ag@HNT
mances of Ag@HNT, Au–Ag@HNT and Pt–Ag@HNT were demonstrated no distinct change before and aer heating to
ꢀ ꢀ
compared in the reduction of 4-NP in the presence of NaBH
4
,
200 C (400 C) based on the measurement by ICP-OES (Table
presenting a universal improvement for bimetallic systems 2). In the case of bimetallic Au–Ag@HNT, Au–Ag nanocages
(
Fig. S9†).
became solid bimetallic nanoparticles, whereas Pt–Ag@HNT
showed no distinct morphological change aer thermal
ꢀ
treatment at 200 C. Increasing the temperature of the thermal
The inuence of thermal treatment of metal NPs-HNT
composites
ꢀ
treatment (400 C) showed signicant inuence on loading
densities for both Au–Ag and Pt–Ag systems (Fig. 6). The
The thermal stabilities of Ag@HNT, Au–Ag@HNT and Pt–
Ag@HNT were explored. Fig. 5 presents morphological and
polymer layer covering Ag nanoparticles protected against the
ꢀ
merging of Ag NPs during heating to 400 C, but this protec-
tion was lost in the case of bimetallic Au–Ag or Pt–Ag nano-
structures. The severely decreased loading density of
bimetallic NPs on HNT compared with that of Ag@HNT may
be caused by damage to the polymer cover during the fabri-
cation of Au–Ag@HNT or Pt–Ag@HNT. On the other hand,
bimetallic nanoparticles experienced reconstruction during
ꢀ
thermal treatment at 400 C owing to the different surface
4
7
energies of the two metals. For Au–Ag@HNT, the bimetallic
nanoparticles changed from intertwined bimetallic nanocages
to Ag-shell–Au-core nanostructures because of the lower
surface energy of Ag, which could also induce damage of the
covering layer of the original Ag NPs.
ꢀ
Surprisingly, Ag@HNT aer thermal treatment at 400
C
showed enhanced catalytic performance (Fig. 7). The conver-
sion of 4-NP to 4-AP was faster than that with the original
Ag@HNT even though the added amount of treated Ag@HNT
ꢀ
was smaller. Aer thermal treatment (400 C), the 4-NP reduc-
ꢁ1
tion rate constant k* (normalized as k mol ) was approximately
7 times the original one (Table S2,† Fig. 7). All heated Ag@HNT
1
Table 2 The content of Ag, Au and Pt in Ag@HNT, Au–Ag@HNT and
ꢀ
ꢀ
Pt–Ag@HNT before and after heating to 200 C (or 400 C) under
nitrogen, measured by ICP-OES
a
Temperature
Ag@HNT
(wt%)
Au–Ag@HNT
(wt%)
Pt–Ag@HNT
(wt%)
ꢀ
(
C)
b
RT
Ag: 39.3 ꢃ 0.9
Ag: 39.1 ꢃ 1.0
Ag: 38.8 ꢃ 0.8
Au: 5.7 ꢃ 0.8
Ag: 19.9 ꢃ 0.6
Au: 6.2 ꢃ 1.0
Ag: 19.6 ꢃ 0.6
Au: 7.9 ꢃ 1.1
Ag: 18.8 ꢃ 0.6
Pt: 4.9 ꢃ 0.7
Ag: 17.5 ꢃ 0.7
Pt: 3.6 ꢃ 1.3
Ag: 14.4 ꢃ 0.1
Pt: 4.0 ꢃ 0.7
Ag: 18.9 ꢃ 0.2
2
00
Fig. 5 (a) Powder X-ray diffraction (XRD) patterns of HNT and of
ꢀ
400
Ag@HNT before and after heating to 400 C under nitrogen. (b)
ꢀ
HRTEM image of Ag@HNT heated to 400 C under nitrogen. TEM
ꢀ
images of Ag@HNT before (c) and after (d) heating to 400 C under
a
Au(Pt)–Ag@HNT was prepared with the added atomic ratio of
Au(Pt) : Ag of 0.1. RT, Room temperature.
nitrogen. Size distributions of Ag nanoparticles before (e) and after (f)
b
ꢀ
heating to 400 C under nitrogen.
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Fig. 8 (a) Peroxidase-like catalytic performance of Au@HNT and Au–
ꢀ
ꢀ
Ag@HNT before and after heating to 200 C (or 400 C) under
nitrogen. (b) Peroxidase-like catalytic performance of Pt@HNT and Pt–
ꢀ
ꢀ
Ag@HNT before and after heating to 200 C (or 400 C) under
nitrogen. The black line shows the control experiment. Absorbance at
418 nm was monitored as a function of time after adding catalysts (0.2
mg). Reaction conditions: H
2
O
2
(0.3 M), OPD (0.3 mM), and catalyst
ꢀ
(0.2 mg) at 40 C. Au(Pt)–Ag@HNT was prepared with the added
atomic ratio of Au(Pt) : Ag of 0.1.
Bimetallic nanoparticles (Ag–Au and Au–Cu) with porous
nanostructure and larger cavities have shown better catalytic
performance than those with solid morphology in the literature
Fig. 6 TEM images of Au–Ag@HNT (a) and Pt–Ag@HNT (b) after
ꢀ
48–50
heating to 200 C under nitrogen, and Au–Ag@HNT (c) and Pt– reports.
Based on the morphological characterization of
ꢀ
Ag@HNT (d) after heating to 400 C under nitrogen. Au(Pt)–Ag@HNT
treated Au–Ag@HNT, we noticed that porous Au–Ag@HNT
was prepared with the added atomic ratio of Au(Pt) : Ag of 0.1.
ꢀ
became solid Au–Ag@HNT aer heating at 200 C. On
comparing the peroxidase-like catalytic efficiency, we found
ꢀ
that solid Au–Ag@HNT (200 C) maintained the same catalytic
ꢀ
samples, no matter whether generated at 40, 60 or 80 C, efficiency as porous Au–Ag@HNT, although solid Au–Ag@HNT
ꢀ
showed improved catalytic efficiency compared with untreated (400 C) showed lower catalytic efficiency (Fig. 8). The inuence
systems (Table S2†). We suppose that the partly carbonized PVP of thermal treatment of Pt–Ag@HNT catalyst presented
on Ag nanoparticles aer thermal treatment may contribute to a similar result, which was that catalyst performance was
ꢀ
this improvement through accelerating the charge transfer.
slightly enhanced aer heating to 200 C whereas it declined
4
Fig. 7 Time-dependent UV-vis absorption spectra for the catalytic reduction of 4-NP by NaBH in different catalytic systems. (a) In the presence
of Ag@HNT (the amount of catalyst was 7.5 mg). (b) In the case of Ag@HNT 200. (c) In the case of Ag@HNT 400. The amounts of Ag@HNT 200
and Ag@HNT 400 catalyst were 4.5 mg. (d, e, f) The corresponding plots of ln(A
t
/A
0
) against time for the reduction of 4-NP in different catalytic
systems.
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ꢀ
aer higher temperature treatment (400 C). The catalytic effi-
5 C. F. Calver, P. Dash and R. W. J. Scott, ChemCatChem, 2011,
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6 A. K. Singh and Q. Xu, ChemCatChem, 2013, 5, 652–676.
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ciency of corresponding monometallic Au@HNT and Pt@HNT
systems before and aer thermal treatment at 200 C was also
ꢀ
compared (Fig. 8). Both Au@HNT 200 and Pt@HNT 200 pre-
sented decreased catalytic efficiency, which clearly highlighted
the positive effect of bimetallic systems. Since the catalytic
performance of metal nanoparticles directly relates to their
specic crystalline facets, improved crystallization of bimetallic
ꢀ
nanostructure aer 200 C treatment may contribute to their
improved catalyst performance; but the loss of Ag spacers due to
ꢀ
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Res., 2017, 50, 1774–1784.
1
1 T. Shirman, J. Lattimer, M. Luneau, E. Shirman, C. Reece,
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halloysite nanotubes were generated on the basis of Ag@HNT, Soc., 2015, 137, 15882–15891.
which was fabricated via a modied silver-mirror-reaction 13 N. J. Divins, I. Angurell, C. Escudero, V. Perez-Dieste and
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2
6
Ag@HNT systems presented an improved and extended cata-
lytic effect compared with Ag@HNT. The thermal stability of
Ag@HNT, Au–Ag@HNT and Pt–Ag@HNT was explored as
C. A. Muryn, F. Maccherozzi, S. S. Dhesi, R. Perez and
G. Thornton, Proc. Natl. Acad. Sci. U. S. A., 2015, 112, 7903–
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regards morphology, composition and catalytic performance. 16 W. L. Fu, Y. Q. Dai, J. P. H. Li, Z. B. Liu, Y. Yang, Y. B. Sun,
The Ag@HNT system had improved catalytic performance even
aer thermal treatment at 400 C for 20 min as no obvious
Y. Y. Huang, R. W. Ma, L. Zhang and Y. M. Sun, ACS Appl.
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ꢀ
agglomeration and exfoliation of Ag nanoparticles on HNT were 17 A. Wang, Y. Hsieh, Y. Chen and C. Mou, J. Catal., 2006, 237,
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mer covering on Ag nanoparticles during thermal treatment. 18 S. Riela, M. Massaro, C. G. Colletti, G. Lazzara, S. Milioto and
However, the galvanic exchange process in the fabrication of R. Noto, J. Mater. Chem. A, 2017, 5, 13276–13293.
bimetallic Au–Ag and Pt–Ag on halloysite may damage this 19 T. Zhou, Y. Zhao, W. Han, H. Xie, C. Li and M. Yuan, J. Mater.
polymer cover, resulting in a signicant decrease of loading
Chem. A, 2017, 5, 18230–18241.
density of bimetallic nanostructures aer thermal treatment at 20 R. Y. Joshua Tully and Y. Lvov, Biomacromolecules, 2016, 17,
ꢀ
4
00 C, and lowering catalytic performance. But moderate
615–621.
ꢀ
thermal treatment at 200 C did not decrease catalytic efficiency 21 G. K. Dedzo, G. Ngnie and C. Detellier, ACS Appl. Mater.
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Interfaces, 2016, 8, 4862–4869.
2
2
2
2
2 J. Feng, H. Fan, D. Zha, L. Wang and Z. Jin, Langmuir, 2016,
32, 10377–10386.
Conflicts of interest
3 W. O. Yah, A. Takahara and Y. M. Lvov, J. Am. Chem. Soc.,
2012, 134, 1853–1859.
4 W. O. Yah, H. Xu, H. Soejima, W. Ma, Y. Lvov and
A. Takahara, J. Am. Chem. Soc., 2012, 134, 12134–12137.
5 S. Jana and S. Das, RSC Adv., 2014, 4, 34435–34442.
There are no conicts to declare.
Acknowledgements
The authors acknowledge nancial support from the National 26 H. Zhu, M. Du, M. Zou, C. Xu and Y. Fu, Dalton Trans., 2012,
Natural Science Foundation of China (Grant 51673210).
41, 10465–10471.
2
2
7 Y. Zhang, X. He, J. Ouyang and H. Yang, Sci. Rep., 2013, 3,
2948.
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