Tongjie Yaoab,
Quan Zuoa,
Hao Wanga,
Jie Wuc,
Xiao Zhangab,
Jianmin Sun*ab and
Tieyu Cui*b
aState Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China. E-mail: sunjm@hit.edu.cn
bThe Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150080, China. E-mail: cuit@hit.edu.cn
cKey Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China
First published on 9th October 2015
In this paper, we have introduced a one-step method to prepare PdxAuy bimetallic nanostructures supported on reduced graphene oxide (rGO) nanosheets and wrapped in a polypyrrole (PPy) layer. By using a pyrrole monomer as a special reducing agent for metal salts, the morphologies of PdxAuy bimetallic nanostructures could be easily turned to be spherical, coral-like and porous cluster-like via simply changing dosage or molar ratio of PdCl2 and HAuCl4·4H2O. The roles of the pyrrole monomer and rGO support in formation of rGO/PdxAuy/PPy composites were investigated in detail. Transmission electron microscopy, elemental mapping analysis, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier-transform infrared spectra were used to characterize their morphologies, structures and compositions. Compared with corresponding rGO/Pd/PPy and rGO/Au/PPy composites, the as-prepared rGO/PdxAuy/PPy composites displayed enhanced catalytic activity towards the reduction of 4-nitrophenol.
For catalysts support, it is usually required to provide high surface area, good stability, robust surface chemistry and excellent dispersion characteristic, which is important for optimizing the synergistic nanoparticle–support interaction and maximizing the reactive activity of metal catalysts. Based on the principle, graphene nanosheets are ideal candidates due to their high surface area, mechanical strength and conductivity. Many groups have combined the bimetallic nanoparticles and reduced graphene oxide (rGO) nanosheet together to further improve catalytic activity, and the most commonly used procedure is reduction of the mixture of graphene oxide (GO) nanosheets and bimetallic salt by chemical or thermal method.8–12 The approach is simple; however, the as-prepared bimetallic nanoparticles on rGO nanosheet surface usually only exhibit spherical morphology. It is well known the catalytic reactions take place on the nanostructure surface, slight changes in the sizes, structures, compositions or morphologies can influence catalytic activity. Therefore, the development of bimetallic nanostructures with non-spherical morphologies, especially the controllable morphologies, is of great importance.
In the past two decades, conducting polymers have stimulated increasing interest owing to their good conductivity, redox property, stability and biocompatibility.13,14 The applications of conducting polymer in the field of catalysis have also aroused considerable attentions. On one hand, the coordination interaction generated between functional groups on their backbone and metal salt can effectively hinder the aggregation of metal nanoparticles;15 on the other hand, their good conductivity is beneficial for transformation of electron in the catalytic reaction.16 Moreover, conducting polymer monomer can be used as reducing agent for metal salt, which leads to synthesis of conducting polymer and metal nanoparticles in one-step.17–19 Based on the aforementioned statements, herein, we selected pyrrole monomer as a special reducing agent to prepare rGO/PdxAuy/polypyrrole (PPy) composites with PdxAuy bimetallic nanostructure supported on rGO nanosheets and wrapped in PPy layer. With the help of both pyrrole monomer and rGO support, the resulting bimetallic nanostructures not only exhibited high dispersity, but also had good stability. Importantly, their morphologies could be easily turned to be spherical, coral-like and porous cluster-like via simply changing the molar ratio or dosage of PdCl2 and HAuCl4·4H2O. The catalytic property of rGO/PdxAuy/PPy composites was investigated by reducing the 4-nitrophenol (4-NP) with NaBH4 as the reducing agent. Compared with monometallic composites, such as rGO/Pd/PPy and rGO/Au/PPy composites, they showed higher catalytic activity.
For comparison purposes, other catalysts were prepared by the same procedure, except some parameters were slightly changed. To prepare rGO/Au/PPy, rGO/Pd34Au66/PPy, rGO/Pd75Au25/PPy and rGO/Pd/PPy composites, the total dosage of PdCl2 and HAuCl4·4H2O was fixed at 2.1 × 10−5 mol, while their molar ratio was changed to be 0:
4, 1
:
3, 3
:
1 and 4
:
0, respectively. To prepare rGO/(Pd48Au52)0.5/PPy, rGO/(Pd48Au52)2/PPy and rGO/(Pd48Au52)4/PPy composites, the molar ratio of PdCl2 to HAuCl4·4H2O was fixed at 1
:
1, while the total dosage of PdCl2 and HAuCl4·4H2O was changed to be 1.05 × 10−5, 4.2 × 10−5 and 8.4 × 10−5 mol, respectively. Two reference samples PdAu/PPy and rGO/PdAu composites were prepared via above approach except GO nanosheets were not used or pyrrole monomer was replaced by NaBH4 solution (10 mL, 10 mM).
Fig. 2 exhibits the representative TEM images of rGO/Pd48Au52/PPy composites at different magnifications. In low magnifications (Fig. 2a and b), Pd48Au52 bimetallic nanostructures uniformly disperse on the scrolled rGO nanosheet surface without aggregations in large area. No nanostructures outside of nanosheets can be found. The above phenomenon implied: first, a good combination existed between the rGO support and Pd48Au52 nanostructures; second, the coverage of PPy layer effectively prevented nanostructures from aggregations and falling off. In high magnifications (Fig. 2c and d), the Pd48Au52 nanostructures can be seen more clearly. Every nanostructure comprises of several spherical nanoparticles with diameter of about 4.5 nm, which makes them have many branches and look like “coral”. When the molar ratio of PdCl2 to HAuCl4·4H2O is 1:
1, “coral-like” nanostructures occupy the most of TEM image view; however, some single nanoparticles still can be found, and their size is nearly the same as the nanoparticles that constitute the “coral”, further suggesting “coral-like” nanostructures originate from assembly of nanoparticles.
To determine the distributions of element Pd and Au in “coral-like” nanostructures, HAADF-STEM measurements and corresponding elemental mapping analysis were carried out. Fig. 2e and f clearly show both Pd and Au have homogeneous distributions in “coral-like” nanostructure, and their distributions almost overlap with each other (Fig. 2h), suggesting the formation of Pd48Au52 alloy, rather than individual Pd or Au nanoparticles.24,25 The alloy structure could be further confirmed through HRTEM image and XRD patterns. Fig. 2i reveals good crystallization of the Pd48Au52 nanostructures with a (111) lattice spacing of 2.29 Å, which is between the (111) lattice spacing of face centered cubic Pd (2.25 Å) and Au (2.36 Å) nanoparticles.26 The XRD patterns of the rGO/Au/PPy, rGO/Pd/PPy and rGO/Pd48Au52/PPy composites are shown in Fig. 3. For rGO/Au/PPy composites (Fig. 3a), the peaks located at 38.16, 44.3 and 64.69° are corresponding to of (111), (200) and (220) lattice planes of the Au nanoparticles (JCPDS no. 04-0784). For rGO/Pd/PPy composites (Fig. 3c), the peaks at 40.14, 46.33 and 68.50° are assigned to the (111), (200) and (220) lattice planes of the Pd nanoparticles (JCPDS no. 05-0681). The XRD pattern of the rGO/Pd48Au52/PPy composites is presented in Fig. 3b. The peaks of Pd48Au52 nanostructures are located between the diffraction peaks of Au and Pd nanoparticles, indicating that Pd is incorporated into the face centered cubic Au structure to form an alloy phase with a concomitant lattice contraction.27
![]() | ||
Fig. 3 XRD patterns of (a) rGO/Au/PPy composites; (b) rGO/Pd48Au52/PPy composites; (c) rGO/Pd/PPy composites. |
Fig. 4a shows the FT-IR spectra of GO nanosheets, their characteristic absorption peaks emerge at 3404, 1739, 1620, 1228 and 1047 cm−1, which is in a good agreement with previous study.28 After redox reaction, rGO/Pd48Au52/PPy composites were prepared and their FT-IR spectrum is shown in Fig. 4b. As PPy is a strong absorber in infrared region, the feature peaks belonged to rGO nanosheets are covered. The PPy characteristic peaks can be distinguished clearly: the bands at 1538 and 1441 cm−1 are attributed to the stretching mode of the C–C and C–N in the pyrrole ring. The peaks at 1297 and 1165 cm−1 are related to the in-plane vibrations of C–H. Besides these, ring deformation at 896 cm−1 is also observed.29 Obviously, FT-IR spectrum reveals PPy is one of components in rGO/Pd48Au52/PPy composites. Further evidences of PPy existence and its distribution were provided by elemental mapping analysis. As shown in Fig. S1,† the element N uniformly disperses on the rGO nanosheet surface, implying PPy layer covers on the whole rGO nanosheets.
To reveal the reduction of GO nanosheets to rGO nanosheets during the redox reaction, the XPS measurements were carried out. Fig. S2a† shows the survey XPS spectra of GO nanosheets before and after redox reaction. The spectrum of original GO nanosheets displays two major components at 282 and 529 eV corresponding to the signals of C 1s and O 1s, respectively. In spectrum of rGO/Pd48Au52/PPy composites, besides C 1s and O 1s signals, three new peaks located at 400, 337 and 85 eV are assigned to Pd 3d, N 1s and Au 4f, respectively. In Fig. S2b,† the carbon species in original GO nanosheets are divided into four peaks: C–C/CC (284.5 eV), C–O (286.5 eV), C
O (287.2 eV) and O–C
O (288.5 eV).30 In rGO/Pd48Au52/PPy composites (Fig. S2c†), they can be fitted into five peaks, as an additional C–N (285.6 eV) peak originated from PPy layer appears.31 Compared with GO nanosheets, the content of oxidized carbon species decreases from original 64% to 45%, suggesting oxygen containing groups are reduced by pyrrole monomer. Based on the TEM images, XRD patterns, XPS and FT-IR spectra, we could conclude that rGO/PdxAuy/PPy composites had been successfully prepared.
In previous study, the bimetallic nanostructures deposited on the rGO support via chemical or thermal reduction usually only exhibited spherical morphology.9,10,26,27 Seldom paper reported non-spherical nanostructures on rGO nanosheet surface.32,33 In our study, the “coral-like” Pd48Au52 alloy appeared on the rGO nanosheet surface by using pyrrole monomer as reducing agent. It was necessary to reveal the role of pyrrole monomer in formation of “coral-like” nanostructure. Fig. 5a and b show the TEM images of rGO/PdAu composites prepared by using NaBH4 instead of pyrrole monomer. Similar to previous report,9,10 only spherical nanoparticles can be observed on rGO nanosheet surface. Although most of nanoparticles are uniform with an average diameter of 4.0 nm, the large aggregations still can be seen. Additionally, some nanoparticles appear outside of rGO nanosheets. These results suggested that the pyrrole monomer helped to control the dispersity of the nanostructures and the formation of the “coral-like” geometry. Besides pyrrole monomer, the influence of rGO nanosheets in formation of rGO/Pd48Au52/PPy composites was also investigated. Fig. 5c and d show the SEM and TEM images of PdAu/PPy composites prepared in absence of rGO nanosheets. Only core/shell spherical composites with rough surface and diameter of tens of nanometers can be obtained. Their aggregations are very severe. When metal salts were reduced to the metal nanoparticles, owing to high surface energy, PPy chains immediately coated on their surface. Because of the strong hydrogen bond and π–π* interactions between the PPy chains, severe aggregations inevitably took place.34 The phenomenon indicated rGO nanosheets played important roles in uniformly dispersing both PdxAuy nanostructures and PPy layer.
![]() | ||
Fig. 5 (a and b) TEM and magnified TEM images of rGO/PdAu composites; (c and d) SEM and TEM images of PdAu/PPy composites, insets show the corresponding magnified images. |
The morphologies of PdxAuy nanostructures could be easily adjusted via simply varying molar ratio of PdCl2 to HAuCl4. Fig. 6 shows the TEM images of PdxAuy nanostructures prepared with different molar ratio of PdCl2 to HAuCl4·4H2O. Without PdCl2 (Fig. 6a and b), although few single Au nanoparticles can be observed, most of Au nanostructures appear as the “coral”. With the ratio of PdCl2 to HAuCl4·4H2O increasing from 1:
3 to 3
:
1 (Fig. 6c–f and Fig. 2a–d), the amount of “coral-like” nanostructures gradually decreases; while more and more single nanoparticles appear in the TEM image. For rGO/Pd/PPy composites (Fig. 6g and h), PdCl2 tends to form Pd nanoparticles in absence of HAuCl4·4H2O. In addition to the molar ratio of PdCl2 to HAuCl4·4H2O, the dosage of metal salt also had influence on morphologies of bimetallic nanostructures. Compared with Fig. 2, when the amount of metal salt decreases by half (Fig. 7a and b), the number of spherical nanoparticles dramatically increases. When the usage of metal salt is doubled (Fig. 7c and d), the number of “coral-like” nanostructures obviously increases, and it is hard to find single nanoparticles in the view of TEM image. When the dosage of metal salt is four times larger than that of Fig. 2 (Fig. 7e and f), the alloys assemble to the porous clusters with diameter of about 70 nm. It is necessary to mention that no aggregations occurs in these nanostructures with the help of rGO nanosheets and PPy layer. According to aforementioned study, we could conclude that the morphologies of PdxAuy bimetallic nanostructures were easily turned from spherical to coral-like and to porous cluster-like via simply adjusting both molar ratio and usage of PdCl2 and HAuCl4·4H2O.
![]() | ||
Fig. 6 TEM images of (a and b) rGO/Au/PPy composites; (c and d) rGO/Pd34Au66/PPy composites; (e and f) rGO/Pd75Au25/PPy composites; (g and h) rGO/Pd/PPy composites. |
![]() | ||
Fig. 7 TEM images of (a and b) rGO/(Pd48Au52)0.5/PPy; (c and d) rGO/(Pd48Au52)2/PPy; (e and f) rGO/(Pd48Au52)4/PPy composites. |
As commonly known, the 4-NP was organic pollutant in industrial and agricultural waste water, whereas 4-aminophenol (4-AP) had a numerous applications including those in analgesic and antipyretic drugs and photographic developers.35,36 It was of great significance that 4-NP could be efficiently reduced to 4-AP. Therefore, reduction of 4-NP to 4-AP was selected as a probe reaction to evaluate the catalytic activity of rGO/PdxAuy/PPy composites. Typically, this reduction reaction could be catalyzed by metal catalysts at room temperature. The catalytic activity depended on the electron transformation from the BH4− donor to the acceptor 4-NP and the diffusion and adsorption of 4-NP on the catalyst surface. The reduction process of 4-NP was monitored by measuring the UV-Vis absorption spectra of the reaction solutions. Fig. 8a shows the spectrum of original 4-NP solution, a strong absorption peak emerges at 317 nm (curve a). After the addition of NaBH4 into aqueous 4-NP solution, the color quickly turns from light-yellow to bright-yellow. The wavelength of absorption peak shifts from 317 to 400 nm due to the formation of 4-nitrophenolate anions under alkaline condition. Without catalysts, this reaction proceeds very slowly, even after reaction for 20 h, the intensity of peak at 400 nm only reduces 6.1% (curve c). In contrast, if 0.2 mg rGO/Pd48Au52/PPy catalysts are added into the reaction system, the peak at 400 nm quickly vanishes within 3.0 min, and a new peak emerged at 300 nm is attributed to the 4-AP (curve d), indicating the rGO/Pd48Au52/PPy composites have good catalytic property.
Fig. 8b displays the time-dependent UV-Vis absorption spectrum during the reduction reaction in the presence of the as-prepared rGO/Pd48Au52/PPy composites. In this study, the dosage of rGO/Pd48Au52/PPy composites was reduced to only 3.5 μg. The absorption of the 4-nitrophenolate anions at 400 nm decreases accompanied by a concomitant increase in the 300 nm peak of 4-AP. The bright yellow color of the 4-NP solution is completely bleached within 18.5 min (inset of Fig. 8b), suggesting the completion of the reaction. Based on the evolution of the absorbance data, the reaction rate constant k was calculated.37 A linear relationship of ln(At/A0) vs. reaction time t is observed in Fig. 8c, which matches well with the pseudo-first-order equation, and the k value is calculated to be 0.269 min−1.
The catalytic activities of other catalysts with different molar ratio were also investigated. As shown in Fig. 9a, for rGO/Pd/PPy, rGO/Pd34Au66/PPy, and rGO/Pd75Au25/PPy composites, the catalytic reactions finish within 40 min, and their reaction rate constant is calculated to be 0.15, 0.18 and 0.23 min−1, respectively. Among all these catalysts, the catalytic activity of rGO/Au/PPy composites is the lowest, and their value is 0.036 min−1 (Fig. 9b). Based on these data, it was easily concluded that compared with monometallic nanoparticles, the strong synergistic effect of the PdxAuy bimetallic nanostructures dramatically improved catalytic property.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17081h |
This journal is © The Royal Society of Chemistry 2015 |