Article
Cite This: Langmuir 2019, 35, 14651−14658
pubs.acs.org/Langmuir
Pt−Cu Bimetallic Nanoparticles Loaded in the Lumen of Halloysite
Nanotubes
Xiaobin Gao, Feng Tang, and Zhaoxia Jin*
Department of Chemistry, Renmin University of China, Beijing 100872, P. R. China
Downloaded via JOHNS HOPKINS UNIV on February 21, 2023 at 18:25:06 (UTC).
See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information
*
ABSTRACT: In this study, we demonstrate that Pt−Cu
bimetallic nanoparticles with different compositions (Pt3Cu,
PtCu, PtCu3) can be loaded in the lumen of halloysite
nanotube (HNT) via a simple one-pot reduction. Increasing
the pH of metallic precursor (H2PtCl6 and CuCl2)/HNT
solutions enhances the dissociation of H2PtCl6, advancing the
association of [PtCl6]2− with the positively charged inner
surface (Al−OH) of HNT. Moreover, the shrinkage of bond
length from Pt−Cl in [PtCl6]2− to Pt−O in [PtCl4(OH)2]2−
due to pH-modulated ligand exchange may also assist Pt(IV)
being trapped inside the halloysite. In the meantime, Cu(II)
cations may complex with Pt(IV) anions via electrostatic force
that would help the formation of Pt−Cu bimetallic nanoparticles inside the halloysite. The obtained PtCu3@HNT system shows a significantly enhanced catalytic performance in the
reduction of 4-nitrophenol by sodium borohydride, with a mass activity approximating 60 times higher than that of unloaded Pt
nanoparticles. The high catalytic efficiency can be maintained after thermal treatment at 200 or 400 °C.
■
INTRODUCTION
Bimetallic nanoparticles show significantly improved catalytic
activity, adaptability, and durability via the combination of two
different metals at atomic level and the miscellaneous
distribution of each metal in nanoparticles with different
shapes.1−3 Pt−Cu bimetallic nanoparticles have shown broad
applications as a cost-effective catalyst.4−10 Compared with
pure Pt nanoparticles, Pt−Cu bimetallic nanoparticles show
enhanced catalytic performance because of the optimization of
electronic structure and surface strains.10 However, the
structural stability of Pt−Cu bimetallic nanoparticles is
sensitive to thermal treatment. Simonovis et al. observed that
Pt/Cu(111) surface showed poor thermal stability when the
temperature was above 400 K.6 Epron et al. have found that
the post-treatment at 400 °C would decrease the catalytic
activity of Pt−Cu/γ-Al2O3 catalyst for nitrate reduction in
water.11 The catalytic activity of Pt−Cu bimetallic catalysts
supported on activated carbon also decreases with increasing
calcination temperature for the reduction of nitrates.12 Finding
an effective strategy to improve the thermal stability is a crucial
issue for Pt−Cu bimetallic catalyst.
Anchoring bimetallic nanoparticles on host substrates can
enhance their catalytic performance and stability by modulating the property of metallic nanoparticles through substrates.13−17 As a natural nanomaterial, halloysite nanotubes
have been widely studied as support substrates to immobilize
catalysts, enzymes, and other functional compounds, or for
controlled release of biocides, drugs or anticorrosion
agents.16,18−30 Bimetallic nanoparticles loaded on the outer
© 2019 American Chemical Society
surface of halloysite nanotubes have been produced
recently,16,31 and Au−Ag or Pt−Ag bimetallic nanoparticles
loaded on halloysite nanotubes show an improved thermal
stability.16 It is expected that if Pt−Cu bimetallic nanoparticles
are immobilized inside halloysite nanotubes, their thermal
stability will be improved further under the protection of
halloysite nanotubes as reported in Cu−Ni@halloysite nanotube (HNT) case.32 Although halloysite nanotubes have lumen
with a diameter of 10−20 nm, immobilization of nanoparticles
inside lumen is hard to achieve.32−35 Thermal decomposition
of silver acetate inside the halloysite has been used to generate
Ag nanoparticles or nanorods in the interior of halloysite,35 but
a solution-based fabrication process may have broad versatility
for various metal nanoparticles. In solution-based processes,
complex chemical modifications of the inner surface of
halloysite are required for anchoring metal nanoparticles inside
halloysite.33 Recently, Rostamzadeh et al. have reported a rapid
and low-temperature synthesis for the growth of Au or Ag
nanoparticles in the interior of halloysite nanotubes by using
ethanol/toluene as a solvent, oleic acid and oleylamine as
surfactants, and ascorbic acid as a reducing agent.34 The size of
gold nanoparticles can be adjusted by changing the reaction
time and the amount of reducing agent. A straightforward
strategy for loading bimetallic nanoparticles inside halloysite
Received: August 22, 2019
Revised: October 5, 2019
Published: October 18, 2019
14651
DOI: 10.1021/acs.langmuir.9b02645
Langmuir 2019, 35, 14651−14658
Article
Langmuir
Figure 1. Schematic illustration of the immobilization process of Pt−Cu bimetallic nanoparticles inside the halloysite.
was deposited on a copper grid and dried in air. High-resolution TEM
(HRTEM) characterization was performed by using a Tecnai G2 F20
(FEI) microscope at an accelerating voltage of 200 kV. High-angle
annular dark-field-scanning transmission electron microscope
(STEM) and energy dispersive X-ray mapping were performed on a
Nion U-HERMES200 scanning transmission electron microscope
with an accelerating voltage of 300 kV. The suspension (10 μL) of
PtCu@HNT was deposited on a molybdenum grid and dried in air.
The crystal structures of samples were determined by using XRD7000 diffractometer (Shimadzu) in the reflection mode using a Cu
target (λ = 0.15418 nm). UV−vis absorption spectra were collected
by using a UV-3600 spectrometer (Shimadzu).
Inductively coupled plasma-optical emission spectroscopy (ICPOES, Agilent, ICP/700) was used to measure the contents of Pt and
Cu in these samples. Before characterization, moderate Pt−Cu@
HNT and Pt−Cu nanostructures were dissolved in aqua regia (1:3 v/
v HNO3/HCl, 1 mL) for 2 days. Then, the solution was diluted to 8
mL with aqua regia solution (10%) and used in measurement.
Pretreatment of Halloysite Nanotubes. Halloysite nanotubes
(10 g) were dispersed in water (200 mL) under vigorous stirring for 5
min. The dispersion was made to stand for 5 min, then the sediments
and the impurities in the upper part of suspensions were discarded
and the part in middle containing well-dispersed halloysite nanotubes
was collected. After repeating the above-mentioned washing step 3
times, the pH of the obtained dispersion was adjusted to about 8.5 by
adding NaOH, then the suspension was stirred for 9 h and stood for 3
h. Finally, the collected suspension was centrifuged at 10 000 rpm for
3 min, and the sediment was washed by fresh water three times and
then freeze-dried. These halloysite nanotubes were used in our
experiment.
Preparation of Pt−Cu@HNT. In a typical process for generating
PtCu3@HNT, pretreated halloysite nanotubes (40 mg) were
dispersed in water (150 mL) by ultrasonic treatment of 30 min.
The dispersion was purged by N2 for 10 min under stirring. Then,
H2PtCl6 (100 mM, 250 μL), CuCl2 (100 mM, 750 μL), and NaOH
(500 mM, 300 μL) were injected into the above dispersion under N2
atmosphere and stirring for 10 min. Then, freshly prepared NaBH4
(500 mM, 1 mL) was added into the mixture under N2 atmosphere
with continuous stirring for 20 min. The resultant solution was
allowed to settle for several minutes to remove the byproducts (Pt−
Cu connected gels) at the container’s bottom. Then, the suspension
was centrifuged at 8000 rpm for 3 min, washed by water three times,
and freeze-dried in vacuum to produce PtCu3@HNT. Pt@HNT,
Pt3Cu@HNT, and PtCu@HNT were synthesized by varying the
mole ratio of Pt and Cu precursors and following the same procedure,
and the dosage of NaOH (500 mM) was changed to 600, 500, and
400 μL, respectively.
Preparation of Pt−Cu Nanostructure. Pt−Cu metallic
connected nanostructures with different chemical compositions were
obtained by keeping the same mole proportion of Pt and Cu
precursors and same procedure as that for generating Pt−Cu@HNT,
but only without adding HNT.
Thermal Treatment. Pretreated HNT was heated from 20 to 800
°C at a heating rate of 10 °C min−1 in a nitrogen flow (40 cm3 min−1).
PtCu3@HNT catalyst was heated from room temperature to 200 °C
(or 400 °C) at a heating rate of 10 °C min−1 and kept at 200 °C (or
400 °C) for 20 min in a nitrogen flow (40 cm3 min−1).
nanotubes in the aqueous solution is highly desirable for
extending their applications.
In this study, we demonstrate a simple method for loading
Pt−Cu bimetallic nanoparticles with different compositions
inside the halloysite. The critical step for generating Pt−Cu
nanoparticles inside the halloysite is adjusting the pH of
mixture solutions in which the halloysite and Pt−Cu
precursors with different ratios are mixed. Pt3Cu, PtCu, and
PtCu3 bimetallic nanoparticles can be located inside the
halloysite following this approach, as well as Pt nanoparticles.
The structural and morphological characterizations of Pt−
Cu@HNT were conducted by using X-ray diffraction,
transmission electron microscopy, and high-resolution transmission electron microscopy. The composition of Pt−Cu@
HNT was determined by using inductively coupled plasma
optical emission spectroscopy (ICP-OES). The reduction of 4nitrophenol (4-NP) by NaBH4 with the assistance of a metal
catalyst is a model reaction to assess the catalytic efficiency of
metal nanoparticles because its dynamic process can be easily
monitored by using UV−vis spectroscopy.36 Thus, we
evaluated the catalytic performance of different Pt−Cu@
HNT by using this model reaction. The catalytic performance
of Pt−Cu@HNT shows a composition-dependent feature,
with the highest activity in PtCu3@HNT, over 20 times higher
than that of Pt@HNT or nearly 60 times higher than that of
unloaded Pt nanoparticles. We have compared the structures
and catalytic performance of PtCu3@HNT before and after
thermal treatment at 200 and 400 °C. Its high catalytic activity
was maintained after thermal treatment, showing a significant
improvement of thermal stability of Pt−Cu@HNT. To
understand why the solution pH is critical for trapping
bimetallic nanoparticles inside the halloysite, we have analyzed
the coordination chemistry of Pt(IV) and Cu(II) in this
process. A series of structural and coordinative changes of
Pt(IV) and Cu(II) complexes may help the formation of Pt−
Cu@HNT inside the halloysite.
■
EXPERIMENTAL SECTION
Materials. Halloysite nanotubes (HNTs) were purchased from
Zhengzhou Jinyangguang Ceramics Co., Ltd. Chloroplatinic acid
hexahydrate (H2PtCl6·6H2O, Pt > 38%) was obtained from Beijing
HWRK Chem Co., Ltd. Copper dichloride (CuCl2, ≥99.0%) was
purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. 4Nitrophenol (4-NP, ≥99.5%) and sodium borohydride (NaBH4,
≥98.0%) were from Tianjin Fuchen Chemical Reagent Technologies
Co. Ltd. Sodium hydroxide (NaOH, AR) was obtained from Beijing
Chemical Works Co. Ltd. All chemicals were used as received without
further purification, except for HNT. Water was purified (18.2 MΩ
cm) using Mingche-D 24UV system (Merck Millipore).
Characterization. Transmission electron microscopy (TEM,
Hitachi, H-7650B) measurements were conducted at an acceleration
voltage of 80 kV. The samples were dispersed in water under
ultrasound bath for several minutes. Then, the suspension (10 μL)
14652
DOI: 10.1021/acs.langmuir.9b02645
Langmuir 2019, 35, 14651−14658
Article
Langmuir
Figure 2. (a)−(d) HRTEM images of Pt@HNT, Pt3Cu@HNT, PtCu@HNT, and PtCu3@HNT. (e) STEM image of PtCu@HNT. The observed
part in the STEM image is highlighted by a white circle in its HRTEM image shown in the inset. (f) EDS mapping of Pt and Cu, (g) EDS mapping
for Pt, (h) EDS mapping for Cu, (i) EDS mapping for Al, and (j) EDS mapping for Si. These EDS mapping results present a good distribution of Pt
and Cu in bimetallic PtCu nanoparticles located inside HNT. (k) The atomic composition of Pt and Cu in Pt3Cu@HNT, PtCu@HNT, and
PtCu3@HNT measured by ICP-OES. (l) X-ray powder diffraction (XRD) patterns of Pt@HNT, Pt3Cu@HNT, PtCu@HNT, and PtCu3@HNT.
Table 1. Loading Capacity and Diameters of Pt or Pt−Cu Nanoparticles inside HNT
samples
Pt@ HNT
Pt3Cu@ HNT
PtCu@ HNT
PtCu3@ HNT
PtCu3@ HNT-200
PtCu3@ HNT-400
loading capacitya
diameter (nm)
0.267
2.5 ± 0.4
0.301
2.5 ± 0.5
0.252
3.0 ± 0.6
0.197
3.1 ± 0.6
0.193
2.8 ± 0.4
0.204
2.7 ± 0.6
a
The loading capacity was calculated by WPt−Cu/WSample. WPt−Cu and WSample are the weights of loading metals (Pt and Cu) and the total weight of
the corresponding samples (Pt, Cu, and HNT), respectively.
■
Catalytic Performance in 4-NP Reduction. We conducted the
4-NP reduction by NaBH4 in the presence of various kinds of Pt−
Cu@HNT. Because of the formation of 4-nitrophenolate ion in
alkaline conditions, its UV−vis absorption peak immediately red-shifts
from 317 to 400 nm after adding NaBH4 solution into 4-NP solution.
The catalytic efficiency of the 4-NP reduction can be quantitatively
calculated based on the decreased intensity of the peak at 400 nm
with time. In a typical reduction reaction, fresh NaBH4 (0.8 mmol)
was added to the 4-NP aqueous solution (0.1 mM, 8 mL) under
vigorous stirring (1500 rpm). The catalytic reaction started by adding
appropriate amount of Pt−Cu@HNT or Pt−Cu catalysts. The peak
absorption at 400 nm was recorded every 30 s.
RESULTS AND DISCUSSION
Figure 1 presents the illustration of our method for loading
Pt−Cu nanoparticles in the interior of halloysite nanotubes.
Before the reduction of Pt(IV) and Cu(II), a small amount of
NaOH was added in the mixture solution containing HNT and
Pt(IV)−Cu(II) precursor (H2PtCl6 and CuCl2) to adjust the
solution pH to 10, and the mixture solution was stirred for 10
min. Then, NaBH4 was added to conduct the reduction of
Pt(IV) and Cu(II). A following settlement after reaction is an
effective step to separate unloaded Pt−Cu from Pt−Cu@HNT
because unloaded Pt−Cu nanoparticles connected together
14653
DOI: 10.1021/acs.langmuir.9b02645
Langmuir 2019, 35, 14651−14658
Article
Langmuir
O−C bonds.33 Aiming at loading chemicals inside HNT, Lvov
and Takahara et al. developed an elegant method to selectively
modify the inner lumen of the halloysite with octadecylphosphonic acid,43 or catecholic group.44 Lazzara and Riela et al.
have modified the inner surface using the click reaction.42
However, in our process, we have not functionalized the inner
surface of the halloysite using special agents as usually
conducted. Therefore, what happens after adjusting the
mixture solution pH to 10?
like a gel and precipitated, while Pt−Cu@HNT was
concentrated in the suspension. Figure 2a−d shows HRTEM
images of the obtained Pt@HNT and Pt−Cu@HNT with
different compositions (Pt3Cu@HNT, PtCu@HNT, and
PtCu3@HNT). Pt−Cu nanoparticles were evenly located
inside most of the halloysite tubes (Figure S1). The decoration
of Pt or Pt−Cu nanoparticles on the outer surface of the
halloysite is rare, which is likely due to the agglomeration of Pt
or Pt−Cu nanoparticles without the confinement of the
halloysite, resulting in precipitating in sediment (Figure S2).
The loading amount of Pt or Pt−Cu bimetallic nanoparticles is
high (Table 1), and the size of Pt nanoparticles loaded inside
the inner lumen of HNT is well-controlled (2−3 nm in average
diameter; Table 1). The incorporation of CuCl2 in the
precursor solutions generates bimetallic Pt−Cu with similar
diameter, indicating that the size of metal particles is not
disturbed by Cu alloying. In literature, it is reported that the
addition of NaOH brings a benefit in controlling the size of Pt
nanoparticles.37 With the increase of Cu component, the
loading yield of metal increases gradually (Table S1). STEM
elemental mapping shows the homogeneous distribution of
platinum and copper in bimetallic PtCu nanoparticles inside
halloysite nanotubes (Figure 2e−j), confirming the well-mixed
platinum and copper in bimetallic nanoparticles. The
composition of bimetallic Pt−Cu nanoparticles was also
determined by using ICP-OES; the atomic ratios of Pt and
Cu in Pt−Cu@HNT basically follow the molar ratios of
H2PtCl6 and CuCl2 in the preparation (Figure 2k). In their Xray diffraction patterns (Figure 2l), the characteristic peaks of
the (111) and (200) planes for Pt (2θ = 40.14 and 46.42°)
reveal the face-centered cubic structure of Pt or Pt−Cu
nanoparticles. It is noticed that the 2θ values for Pt (111) and
(200) planes in Pt@HNT are slightly shifted compared with
the value in JCPDS#04-0802 (39.765 for Pt(111) and 46.244
for Pt(200), respectively). We suppose that it is probably
because of the distortion of crystalline Pt inside the halloysite
induced by the interaction between Pt and Al−O surface. For
bimetallic Pt3Cu@HNT and PtCu@HNT, the diffraction
angles of the (111) plane slightly shifted toward higher
diffraction angles (2θ = 41.16° for PtCu@HNT) relative to the
Pt@HNT, showing the lattice contraction with the incorporation of Cu (Figure 2l), which is in accordance with the PtCu3
bulk alloy.38,39 The characteristic diffraction patterns of pure
Cu or its oxides have not been observed. We noticed that
without the addition of NaOH, Pt−Cu nanoparticles mainly
formed connected Pt−Cu gels separated from halloysite
nanotubes, and it is difficult to find Pt−Cu loaded inside the
inner lumen of the halloysite (Figure S3). On the other hand,
it is hard to generate pure Cu nanoparticles loaded inside the
lumen of halloysite even with the addition of NaOH (Figure
S4), showing that in the decoration of Pt−Cu bimetallic
nanoparticles inside halloysite, Pt works as a conductor to
introduce Cu into the halloysite.
Why the addition of NaOH becomes the crucial factor in
anchoring Pt or Pt−Cu inside halloysite? What makes Pt a
conductor in the formation of Pt−Cu bimetallic nanoparticles
inside the halloysite? It is well-known that halloysite nanotubes
have different surface chemistries in their inner surface (Al−
OH) and outer surface (Si−OH).40 The different chemistry
inside/outside of halloysite nanotubes is often used to modify
their inner lumen.33,41,42 For loading Pd nanoparticles inside
the halloysite, Dedzo et al. grafted 1-(2-hydroxyethyl)-3methylimidazolium onto the halloysite to generate stable Al−
pH = 10
H 2PtCl4 ⎯⎯⎯⎯⎯⎯→ [PtCl4]2 − → [PtCl4(OH)2 ]2 −
(1)
[Cu(OH) (H 2O)n ]+ + [PtCl4(OH)2 ]2 −
→ [Cu(OH) (H 2O)n ][PtCl4(OH)2 ]− (n
= 1 − 4)
(2)
In this condition, the hydrolyzation of Cu(II) will take place.45
The positive charge of the Al−OH surface attracts negative
species.40 However, the dissociation of H2PtCl6 is weak at a
lower pH or near neutral condition, resulting in a low fraction
of negative [PtCl6]2− in a solution. With the change of the
solution pH to 10, there are more [PtCl6]2− ions because of
the enhanced dissociation of H2PtCl6 (eq 1). These negatively
charged [PtCl6]2− ions are strongly attracted by the Al−OH
surface. On the other hand, [PtCl6]2− may change to
[PtCl4(OH)2]2− in weak basic condition.46 Previous study
showed that the Pt−Cl distance in [PtCl6]2− is 2.32 Å and the
Pt−O distance in [PtCl4(OH)2]2− is 2.03 Å; thus, a steric
shrinkage of Pt(IV) complex happens while Cl− ligand is
replaced by OH− ligand.47 In the HNT suspension, the
structural shrinkage may also help [PtCl4(OH)2]2−, the
dominant species of Pt(IV) at pH 10,46 go through the
channel of halloysite and get trapped inside it. Moreover,
[PtCl4(OH)2]2− species can be anchored on the Al−OH
surface via hydrogen bonds, and chloride may also bind to the
alumina surface,17 all resulting in a tight binding of Pt complex
on alumina, and the subsequent formation of Pt nanoparticles
after reduction by NaBH4.
In the bimetallic mixture solution, we use CuCl2 as a
precursor for Cu(II), which will become [Cu(OH)(H2O)n]2+
species in a weak basic aqueous solution.45 The positive change
of the Cu(II) complex prohibits it from going into the
halloysite. As a result, we have not observed the formation of
Cu or CuO inside the halloysite. Why Cu can be dragged into
the inner lumen of halloysite in the presence of [PtCl6]2−? In
the previous study, Ding et al. utilized heterometallic double
complex salts, formed from a cation [Pd(NH3)4]2+ and an
anion [PtCl6]2−, to generate supported bimetallic alloy
nanoparticles.17 Considering [Cu(OH)(H2O)n]+ cations and
[PtCl4(OH)2]2− anions in our precursor mixture, we suppose
that a series of a heterometallic double complex salt, such as
[Cu(OH)(H2O)n][PtCl4(OH)2]−, may be formed and work as
a carrier to pull Cu(II) through the channel of halloysites (eq
2). As a consequence, Pt−Cu bimetallic nanoparticles are
located inside the halloysite. Without the help of [PtCl6]2−, Cu
ions cannot be imbibed into the halloysite due to the repulsive
force between the positive [Cu(OH)(H2O)n]+ and alumina
surface. A similar complex of Cu−Ni precursor and sodium
citrate has been used in loading Cu−Ni nanoparticles inside
halloysite nanotubes.32
Figure 3 presents HRTEM images of Pt@HNT, Pt3Cu@
HNT, PtCu@HNT, and PtCu3@HNT. Pt and Pt−Cu
14654
DOI: 10.1021/acs.langmuir.9b02645
Langmuir 2019, 35, 14651−14658
Article
Langmuir
These peaks become sharper, probably because of the growth
of PtCu3 crystals outside the halloysite in heating. The
composition of PtCu3@HNT is basically preserved before and
after thermal treatment (Figure 4f).
The catalytic activities of Pt@HNT, Pt3Cu@HNT, PtCu@
HNT, and PtCu3@HNT in 4-nitrophenol reduction were
assessed (Figure 5). Compared with Pt-based catalyst without
HNT, Pt and Pt−Cu bimetallic nanoparticles anchored inside
the halloysite nanotubes show a significant improvement in
their catalytic performance (Figure S5), which likely is the
result of structural distortion of Pt inside the halloysite.
PtCu3@HNT exhibits best catalytic activity, even after thermal
treatments at 200 and 400 °C. The apparent rate constants
(kapp) for all catalyst systems are listed in Table S2. The
increasing amount of Cu in Pt−Cu bimetallic nanoparticles
may induce the decrease of Pt crystalline size in PtCu3@HNT,
resulting in a composition-dependent catalytic performance in
these catalysts. Moreover, loading in halloysite brings benefit
to catalyst recycling. The catalyst system shows good catalytic
consistency in 5 cycles (Figure S6). Because halloysite
nanotubes show a structural transformation in the range of
470−490 °C depending on their different origins (Figure
S7),20 their interstitial dimension shrinks from 10 to 7 Å due to
the loss of interstitial water molecules. Therefore, we limited
our study to the thermal stability of Pt−Cu@HNT below 400
°C. The stability of Pt−Cu@HNT after thermal treatment at a
higher temperature (>500 °C) is under confirmation. Other
synthetic microporous substrates such as zeolite have shown
higher thermal stability, so they may be suitable as loading
substrates for bimetallic nanoparticles.49,50 However, compared
with expensive synthesized porous substrates, the low cost of
natural halloysite makes it a very promising substrate for
catalyst loading. On the other hand, the unique inner lumen of
halloysite nanotubes gives spatial confinement not only for
catalysts but also for all reactants. The catalyst-inside-halloysite
is an ideal platform to study the influence of spatial
confinement to catalytic process, on which this simple process
locating Pt−Cu bimetallic nanoparticles inside the halloysite
Figure 3. HRTEM images of Pt@HNT (a), Pt3Cu@HNT (b),
PtCu@HNT (c), and PtCu3@HNT (d).
nanocrystals show well-defined lattice fringes with the lattice
spacing of 0.225−0.216 nm, confirming the (111) facets of Ptbased nanocrystals.48 We have further characterized the
nanostructure of PtCu3@HNT after the thermal treatment at
200 and 400 °C (Figure 4). After the thermal treatment, the
size of PtCu3 nanoparticles located inside halloysite nanotubes
is maintained (Figure 4a,c), while that for PtCu3 nanoparticles
outside the halloysite increases clearly due to the thermalinduced aggregation (Figure 4c, highlighted by a white circle),
confirming the significant confinement of inner lumen of the
halloysite. Most of the diffraction peaks of PtCu3@HNT
remained unchanged after heating (Figure 4e), indicating that
the crystal structure of bimetallic nanoparticles is basically
maintained. The diffraction peak for Pt(111) slightly shifted
toward a lower 2θ value that may be originated from the Pt−
Cu outside the halloysite as we observed in HRTEM images.
Figure 4. (a)−(d) HRTEM images of PtCu3@HNT after thermal treatment at 200 °C (a, b) and 400 °C (c, d). The diameters of bimetallic
nanoparticles located inside the halloysite did not change after the thermal treatment. (e) XRD patterns of PtCu3@HNT, PtCu3@HNT-200, and
PtCu3@HNT-400, compared with those of HNT. (f) Compositions of PtCu3@HNT before and after the thermal treatment at 200 and 400 °C
obtained by ICP-OES.
14655
DOI: 10.1021/acs.langmuir.9b02645
Langmuir 2019, 35, 14651−14658
Article
Langmuir
confinement of halloysite endows bimetallic nanoparticles with
good thermal stability. The resulted PtCu3@HNT-200 and
PtCu3@HNT-400 catalysts demonstrated excellent catalytic
performance in the 4-nitrophenol reduction similar to PtCu3@
HNT, over 20 times higher than that of Pt@HNT and nearly
60 times higher than that of unloaded Pt nanoparticles. This
study provides us a good method to generate Pt−Cu bimetallic
nanoparticles with the protection of halloysite nanotubes,
which will be useful in the further application of Pt−Cu
bimetallic catalysts.
■
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.langmuir.9b02645.
TEM images of Pt@HNT, Pt−Cu@HNT; sediment
after standing in the preparation of Pt3Cu@HNT;
product without adding NaOH in the preparation of
PtCu@HNT and Cu@HNT; catalytic performances of
Pt, Pt−Cu-connected nanostructures; PtCu3@HNT in 5
catalytic cycles; TGA and derivative TGA curves of
halloysite nanotubes; loading yields of metal in these
preparations; apparent rate constants of different
catalysts (PDF)
Figure 5. Catalytic performance. (a) Time-dependent absorbance
change at 400 nm in Pt@HNT, Pt3Cu@HNT, PtCu@HNT, and
PtCu3@HNT systems. (b) Corresponding plots of ln (At/A0) against
time in Pt@HNT, Pt3Cu@HNT, PtCu@HNT, and PtCu3@HNT
systems. (c) Time-dependent absorbance change at 400 nm in
PtCu3@HNT, PtCu3@HNT-200, and PtCu3@HNT-400 systems.
(d) Corresponding plots of ln (At/A0) against time in PtCu3@HNT,
PtCu3@HNT-200, and PtCu3@HNT-400 systems, where At and A0
represent the absorbance of 4-NP at time t and at the initial stage,
respectively. (e) Mass activities of Pt@HNT, Pt3Cu@HNT, PtCu@
HNT, and PtCu3@HNT before and after thermal treatment at 200
and 400 °C.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: jinzx@ruc.edu.cn.
ORCID
Zhaoxia Jin: 0000-0002-6108-0636
Notes
The authors declare no competing financial interest.
provides us a good chance to investigate. A further assessment
of the catalytic feature of Pt−Cu@HNT using different
reactions will be conducted.
■
■
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (Grant 51673210).
■
CONCLUSIONS
Herein, we demonstrated a simple process to anchor Pt
nanoparticles or Pt−Cu bimetallic nanoparticles inside the
halloysite. The coordination chemistry of Pt(IV) and Cu(II)
with the change of solution pH contributes to the formation of
Pt−Cu@HNT. The change of solution pH to 10 first
stimulates the dissociation of H2PtCl6, resulting in a large
portion of negative species [PtCl6]2− that can be attracted by
the positively charged Al−OH in the inner surface of
halloysite; in addition, it also induces a ligand exchange from
[PtCl6]2− to [PtCl4(OH)2]2−, in which a longer Pt−Cl is
replaced by a shorter Pt−O, leading to a shrinkage of this
complex, thus favoring Pt(IV) being trapped in the interior of
the halloysite as well. On the other hand, the electrostatic force
between Cu(II) cations ([Cu(OH)(H2O)n]+) and Pt(IV)
anions may induce the formation of a heterometallic double
complex that helps Cu(II) go into the halloysite with Pt(IV)
and form bimetallic nanoparticles via the reduction of NaBH4.
The compositions of Pt/Cu bimetallic nanoparticles are
adjusted by the added amount of Pt and Cu in precursors.
The obtained Pt or Pt−Cu bimetallic nanoparticles with
different compositions demonstrated a well-controlled size
distribution because of the confinement of halloysite nanotubes. Moreover, the thermal treatment of PtCu3@HNT was
conducted at 200 and 400 °C, separately. The size of bimetallic
nanoparticles was maintained after heating, showing that the
REFERENCES
(1) Gilroy, K. D.; Ruditskiy, A.; Peng, H.-C.; Qin, D.; Xia, Y.
Bimetallic nanocrystals: syntheses, properties, and applications. Chem.
Rev. 2016, 116, 10414−10472.
(2) Singh, A. K.; Xu, Q. Synergistic catalysis over bimetallic alloy
nanoparticles. ChemCatChem 2013, 5, 652−676.
(3) Zaleska-Medynska, A.; Marchelek, M.; Diak, M.; Grabowska, E.
Noble metal-based bimetallic nanoparticles: the effect of the structure
on the optical, catalytic and photocatalytic properties. Adv. Colloid
Interface Sci. 2016, 229, 80−107.
(4) Ge, X.; Chen, L.; Kang, J.; Fujita, T.; Hirata, A.; Zhang, W.;
Jiang, J.; Chen, M. A core-shell nanoporous Pt-Cu catalyst with
tunable composition and high catalytic activity. Adv. Funct. Mater.
2013, 23, 4156−4162.
(5) Marcinkowski, M. D.; Darby, M. T.; Liu, J.; Wimble, J. M.; Lucci,
F. R.; Lee, S.; Michaelides, A.; Flytzani-Stephanopoulos, M.;
Stamatakis, M.; Sykes, E. C. H. Pt/Cu single-atom alloys as cokeresistant catalysts for efficient C-H activation. Nat. Chem. 2018, 10,
325−332.
(6) Simonovis, J. P.; Hunt, A.; Palomino, R. M.; Senanayake, S. D.;
Waluyo, I. Enhanced stability of Pt-Cu single-atom alloy catalysts: in
situ characterization of the Pt/Cu(111) surface in an ambient
pressure of CO. J. Phys. Chem. C 2018, 122, 3−4488.
(7) Wan, X.-X.; Zhang, D.-F.; Guo, L. Concave Pt-Cu nanocuboctahedrons with high-index facets and improved electrocatalytic
performance. CrystEngComm 2016, 18, 3216−3222.
14656
DOI: 10.1021/acs.langmuir.9b02645
Langmuir 2019, 35, 14651−14658
Article
Langmuir
(8) Qiu, H.-J.; Shen, X.; Wang, J. Q.; Hirata, A.; Fujita, T.; Wang, Y.;
Chen, M. W. Aligned nanoporous Pt-Cu bimetallic microwires with
high catalytic activity toward methanol electrooxidation. ACS Catal.
2015, 5, 3779−3785.
(9) Wei, X.; Wang, A.-Q.; Yang, X.-F.; Li, L.; Zhang, T. Synthesis of
Pt-Cu/SiO2 catalysts with different structures and their application in
hydrodechlorination of 1,2-dichloroethane. Appl. Catal., B 2012,
121−122, 105−114.
(10) Chaudhari, N. K.; Hong, Y.; Kim, B.; Choi, S.-I.; Lee, K. Pt-Cu
based nanocrystals as promising catalysts for various electrocatalytic
reactions. J. Mater. Chem. A 2019, 7, 17183−17203.
(11) Epron, F.; Gauthard, F.; Barbier, J. Influence of oxidizing and
reducing treatments on the metal-metal interactions and on the
activity for nitrate reduction of a Pt-Cu bimetallic catalyst. Appl.
Catal., A 2002, 237, 253−261.
(12) Soares, O. S. G. P.; Ó rfão, J. J. M.; Ruiz-Martínez, J.; SilvestreAlbero, J.; Sepúlveda-Escribano, A.; Pereira, M. F. R. Pd-Cu/AC and
Pt-Cu/AC catalysts for nitrate reduction with hydrogen Influence of
calcination and reduction temperatures. Chem. Eng. J. 2010, 165, 78−
88.
(13) Wong, A.; Liu, Q.; Griffin, S.; Nicholls, A.; Regalbuto, J. R.
Synthesis of ultrasmall, homogeneously alloyed, bimetallic nanoparticles on silica supports. Science 2017, 358, 1427−1430.
(14) Divins, N. J.; Angurell, I.; Escudero, C.; Pérez-Dieste, V.;
Llorca, J. Influence of the support on surface rearrangements of
bimetallic nanoparticles in real catalysts. Science 2014, 346, 620−623.
(15) Liu, X.; Wang, A.; Yang, X.; Zhang, T.; Mou, C.-Y.; Su, D.-S.;
Li, J. Synthesis of thermally stable and highly active bimetallic Au-Ag
nanoparticles on inert supports. Chem. Mater. 2009, 21, 410−418.
(16) Li, S.; Tang, F.; Wang, H.; Feng, J.; Jin, Z. Au-Ag and Pt-Ag
bimetallic nanoparticles@halloysite nanotubes: morphological modulation, improvement of thermal stability and catalytic performance.
RSC Adv. 2018, 8, 10237−10245.
(17) Ding, K.; Cullen, D. A.; Zhang, L.; Cao, Z.; Roy, A. D.; Ivanov,
I. N.; Cao, D. A general synthesis approach for supported bimetallic
nanoparticles via surface inorganometallic chemistry. Science 2018,
362, 560−564.
(18) Tully, J.; Yendluri, R.; Lvov, Y. Halloysite clay nanotubes for
enzyme immobilization. Biomacromolecules 2016, 17, 615−621.
(19) Lvov, Y. M.; Shchukin, D. G.; Möhwald, H.; Price, R. R.
Halloysite clay nanotubes for controlled release of protective agents.
Acs Nano 2008, 2, 814−820.
(20) Li, L.; Fan, H.; Wang, L.; Jin, Z. Does halloysite behave like an
inert carrier for doxorubicin? RSC Adv. 2016, 6, 54193−54201.
(21) Lvov, Y.; Wang, W.; Zhang, L.; Fakhrullin, R. Halloysite clay
nanotubes for loading and sustained release of functional compounds.
Adv. Mater. 2016, 28, 1227−1250.
(22) Massaro, M.; Colletti, C. G.; Lazzara, G.; Milioto, S.; Noto, R.;
Riela, S. Halloysite nanotubes as support for metal-based catalysts. J.
Mater. Chem. A 2017, 5, 13276−13293.
(23) Wang, L.; Chen, J.; Ge, L.; Zhu, Z.; Rudolph, V. Halloysitenanotube-supported Ru nanoparticles for ammonia catalytic decomposition to produce COx-free hydrogen. Energy Fuels 2011, 25, 3408−
3416.
(24) Zhang, Y.; He, X.; Ouyang, J.; Yang, H. Palladium nanoparticles
deposited on silanized halloysite nanotubes: synthesis, characterization and enhanced catalytic property. Sci. Rep. 2013, 3, No. 2948.
(25) Massaro, M.; Cavallaro, G.; Colletti, C. G.; D’Azzo, G.;
Guernelli, S.; Lazzara, G.; Pieraccini, S.; Riela, S. Halloysite nanotubes
for efficient loading, stabilization and controlled release of insulin. J.
Colloid Interface Sci. 2018, 524, 156−164.
(26) Cavallaro, G.; Danilushkina, A. A.; Evtugyn, V. G.; Lazzara, G.;
Milioto, S.; Parisi, F.; Rozhina, E. V.; Fakhrullin, R. F. Halloysite
nanotubes: controlled access and release by smart gates. Nanomaterials 2017, 7, No. 199.
(27) Stavitskaya, A. V.; Novikov, A. A.; Kotelev, M. S.; Kopitsyn, D.
S.; Rozhina, E. V.; Ishmukhametov, I. R.; Fakhrullin, R. F.; Ivanov, E.
V.; Lvov, Y. M.; Vinokurov, V. A. Fluorescence and cytotoxicity of
cadmium sulfide quantum dots stabilized on clay nanotubes.
Nanomaterials 2018, 8, No. 391.
(28) Klitzing, R. V.; Stehl, D.; Pogrzeba, T.; Schomäcker, R.;
Minullina, R.; Panchal, A.; Konnova, S.; Fakhrullin, R.; Koetz, J.;
Möhwald, H.; Lvov, Y. Halloysites stabilized emulsions for hydroformylation of long chain olefins. Adv. Mater. Interfaces 2016, 4,
No. 1600435.
(29) Zhu, T.; Qian, C.; Zheng, W.; Bei, R.; Liu, S.; Chi, Z.; Chen, X.;
Zhang, Y.; Xu, J. Modified halloysite nanotube filled polyimide
composites for film capacitors: high dielectric constant, low dielectric
loss and excellent heat resistance. RSC Adv. 2018, 8, 10522−10531.
(30) Massaro, M.; Colletti, C. G.; Fiore, B.; Parola, V. L.; Lazzara,
G.; Guernelli, S.; Zaccheroni, N.; Riela, S. Gold nanoparticles
stabilized by modified halloysite nanotubes for catalytic applications.
Appl. Organomet. Chem. 2018, No. e4665.
(31) Liu, Y.; Zhang, J.; Guan, H.; Zhao, Y.; Yang, J.-H.; Zhang, B.
Preparation of bimetallic Cu-Co nanocatalysts on poly (diallyldimethylammonium chloride) functionalized halloysite nanotubes for
hydrolytic dehydrogenation of ammonia borane. Appl. Surf. Sci.
2018, 427, 106−113.
(32) Sanchez-Ballester, N. M.; Ramesh, G. V.; Tanabe, T.;
Koudelkova, E.; Liu, J.; Shrestha, L. K.; Lvov, Y.; Hill, J. P.; Ariga,
K.; Abe, H. Activated interiors of clay nanotubes for agglomerationtolerant automotive exhaust remediation. J. Mater. Chem. A 2015, 3,
6614−6619.
(33) Dedzo, G. K.; Ngnie, G.; Detellier, C. Pd NP decoration of
halloysite lumen via selective grafting of ionic liquid onto the aluminol
surfaces and catalytic application. ACS Appl. Mater. Interfaces 2016, 8,
4862−4869.
(34) Rostamzadeh, T.; Khan, M. S. I.; Riche, K.; Lvov, Y. M.;
Stavitskaya, A. V.; Wiley, J. B. Rapid and controlled in situ growth of
noble metal nanostructures within halloysite clay nanotubes. Langmuir
2017, 33, 13051−13059.
(35) Abdullayev, E.; Sakakibara, K.; Okamoto, K.; Wei, W.; Ariga,
K.; Lvov, Y. Natural tubule clay template synthesis of silver nanorods
for antibacterial composite coating. ACS Appl. Mater. Interfaces 2011,
3, 4040−4046.
(36) Wunder, S.; Polzer, F.; Lu, Y.; Mei, Y.; Ballauff, M. Kinetic
analysis of catalytic reduction of 4-nitrophenol by metallic nanoparticles immobilized in spherical polyelectrolyte brushes. J. Phys.
Chem. C 2010, 114, 8814−8820.
(37) Yang, Z.; Fujigaya, T.; Nakashima, N. NaOH-aided platinum
nanoparticle size regulation on polybenzimidazole-wrapped carbon
nanotubes for use as non-humidified polymer electrolyte fuel cell
catalyst. ChemCatChem 2016, 8, 268−275.
(38) Weihua, W.; Xuelin, T.; Kai, C.; Gengyu, C. Synthesis and
characterization of Pt-Cu bimetallic alloy nanoparticles by reverse
micelles method. Colloids Surf. A 2006, 273, 35−42.
(39) Zhao, Y.; Wu, Y.; Liu, J.; Wang, F. Dependent relationship
between quantitative lattice contraction and enhanced oxygen
reduction activity over Pt-Cu alloy catalysts. ACS Appl. Mater.
Interfaces 2017, 9, 35740−35748.
(40) Bretti, C.; Cataldo, S.; Gianguzza, A.; Lando, G.; Lazzara, G.;
Pettignano, A.; Sammartano, S. Thermodynamics of proton binding
of halloysite nanotubes. J. Phys. Chem. C 2016, 120, 7849−7859.
(41) Lvov, Y.; Panchal, A.; Fu, Y.; Fakhrullin, R.; Kryuchkova, M.;
Batasheva, S.; Stavitskaya, A.; Glotov, A.; Vinokurov, V. Interfacial
self-assembly in halloysite nanotube composites. Langmuir 2019, 35,
8646−8657.
(42) Arcudi, F.; Cavallaro, G.; Lazzara, G.; Massaro, M.; Milioto, S.;
Noto, R.; Riela, S. Selective functionalization of halloysite cavity by
click reaction: structured filler for enhancing mechanical properties of
bionanocomposite films. J. Phys. Chem. C 2014, 118, 15095−15101.
(43) Yah, W. O.; Takahara, A.; Lvov, Y. M. Selective modification of
halloysite lumen with octadecylphosphonic acid: new inorganic
tubular micelle. J. Am. Chem. Soc. 2012, 134, 1853−1859.
(44) Yah, W. O.; Xu, H.; Soejima, H.; Ma, W.; Lvov, Y.; Takahara,
A. Biomimetic dopamine derivative for selective polymer modification
14657
DOI: 10.1021/acs.langmuir.9b02645
Langmuir 2019, 35, 14651−14658
Article
Langmuir
of halloysite nanotube lumen. J. Am. Chem. Soc. 2012, 134, 12134−
12137.
(45) Sweeney, A. F.; Armentrout, P. B. Guided ion beam studies of
the collision-induced dissociation of CuOH+(H2O)n (n = 1−4):
comprehensive thermodynamic data for copper ion hydration. J. Phys.
Chem. A 2014, 118, 10210−10222.
(46) Spieker, W. A.; Liu, J.; Miller, J. T.; Kropf, A. J.; Regalbuto, J. R.
An EXAFS study of the co-ordination chemistry of hydrogen
hexachloroplatinate(iv) 1. speciation in aqueous solution. Appl.
Catal., A 2002, 232, 219−235.
(47) Chen, X.; Chu, W.; Wang, L.; Wu, Z. Geometry of Pt(IV) in
H2PtCl6 aqueous solution: an x-ray absorption spectroscopic
investigation. J. Mol. Struct. 2009, 920, 40−44.
(48) Lu, B.-A.; Sheng, T.; Tian, N.; Zhang, Z.-C.; Xiao, C.; Cao, Z.M.; Ma, H.-B.; Zhou, Z.-Y.; Sun, S.-G. Octahedral PtCu alloy
nanocrystals with high performance for oxygen reduction reaction and
their enhanced stability by trace Au. Nano Energy 2017, 33, 65−71.
(49) Liu, L.; Lopez-Haro, M.; Lopes, C. W.; Li, C.; Concepcion, P.;
Simonelli, L.; Calvino, J. J.; Corma, A. Regioselective generation and
reactivity control of subnanometric platinum clusters in zeolites for
high-temperature catalysis. Nat. Mater. 2019, 18, 866−873.
(50) Weckhuysen, B. M. Stable platinum in a zeolite channel. Nat.
Mater. 2019, 18, 778−779.
14658
DOI: 10.1021/acs.langmuir.9b02645
Langmuir 2019, 35, 14651−14658