plate-like ZnO nanostructures and their self-assembly mechanism

Accepted Manuscript The Synthesis of Cu/Plate-Like ZnO Nanostructures and Their Self-assembly Mechanism Hongmei Wang, Zhufeng Lu, Dingze Lu, Chunhe Li, Pengfei Fang, Xiao Wang PII:

S1293-2558(16)30026-7

DOI:

10.1016/j.solidstatesciences.2016.02.009

Reference:

SSSCIE 5287

To appear in:

Solid State Sciences

Received Date: 15 November 2015 Revised Date:

4 February 2016

Accepted Date: 15 February 2016

Please cite this article as: H. Wang, Z. Lu, D. Lu, C. Li, P. Fang, X. Wang, The Synthesis of Cu/PlateLike ZnO Nanostructures and Their Self-assembly Mechanism, Solid State Sciences (2016), doi: 10.1016/j.solidstatesciences.2016.02.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract The opper oleate firstly decompose at low temperature, and is self-reduced into irregular Cu nanoparticles. Meanwhile, zinc oleate remains unchanged and the oleate

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ion acts as protection agent for the Cu nanoparticles. When the reaction temperature is raised up to 290 oC, zinc oleate begins to decompose and ZnO nanoparticles are generated with regular size and shape and wrapped by Cu nanoparticles. Finally, the

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small ZnO particles further self-assemble into hexagonal disc-like structures with Cu

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nanoparticles on their surfaces. In this growth process, oleate ions play two important

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roles: 1) reducing Cu ion and 2) wrapping Cu nanoparticles to make them stable.

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The Synthesis of Cu/Plate-Like ZnO Nanostructures and Their Self-assembly Mechanism Hongmei Wang

a,*

, Zhufeng Lu a, Dingze Lu b, Chunhe Li b, Pengfei Fang b, Xiao

Wang c,* College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001, P.R. China

b

Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education,

c

School of Science, East China University of Science and Technology, Shanghai 200237, P.R. China

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a

a,*

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Wuhan University, Wuhan 430072, P.R. China

Corresponding author: Hongmei Wang, College of Biological, Chemical Sciences and

Engineering, Jiaxing University, Jiaxing 314001, P.R. China Tel.: 86-573-83646045 Fax: 86-573-83640107 E-mail address: hongmei256163.com c,*

China

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Corresponding author: East China University of Science and Technology, Shanghai 200237, P.R.

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E-mail address: [email protected]

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ACCEPTED MANUSCRIPT Abstract A composite Cu/ZnO nanostructure with Cu nanoparticles supported on ZnO hexagonal nanoplates has been successfully fabricated by a heating approach, using their metal oleate salts as the precursors without any additives. Combined Fourier

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transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and other examination technologies, the structural properties and formation mechanism of as-synthesized Cu/ZnO nanocomposites are studied in detail. The results reveal that the nanostructures are plate-like with uniform

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shape and size, and Cu nanoparticles exhibit specific (111) plane matching with the (002) facet of ZnO, indicating a surface-induced interaction mechanism. Further

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characterization demonstrates that copper nanoparticles can be generated by a decomposition/self-reduction route of copper salts, and the oleate ions act as dual roles in the process: reducing and protecting agents. The difference of decomposition temperature between metal oleates also plays important roles in the formation of Cu/ZnO nanostructure. In addition, the catalytic performance of these nanocomposites

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is evaluated and it can be found that compared with Cu/rod-like ZnO, as-synthesized samples are highly selective for methanol.

Keywords: Cu/ZnO nanocomposites; Decomposition/self-reduction mechanism;

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Self-assembly; Metal oleates

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1 Introduction In recent decades, functional hybrid nanomaterials with uniform shape and size have inspired more and more attentions in many industrial fields, not only because

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these materials provide insight into the association between the nanostructure or even atom arrangement and their performances, but also because their enhanced and usually novel properties over their each components generate versatile solutions for [1]

. Up to date, great progress

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various applications in optics, magnetics, catalysis etc.

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has been made in their synthesis studies, from traditional immersion route to surface-controlled growth and even in-situ method. In those processes, the shape and size control of hybrid nanomaterials has been usually performed by the bonding surfactants or other ions, and in most cases, been generated by involving the

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pre-synthesized each component on the whole, rather than a really synthetic or self-assembly routes. These have greatly handicapped the material applications in various fields, due to the existence of surfactants and other ion on the surface leading

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to a loose attachment for the different components of the hybrid nanomaterials, and

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also poor performance. Therefore, the development of new coating methods which can improve the stability and dispersion of metal nanoparticles are frequently adopted to facilitate morphology characterization and optimize their performance. Shen et al. have successfully synthesized Au-Pt/TiO2 by a single-step borohydride

reduction method

[2]

. Uniform Au@TiO2 double-shelled octahedral nanocages have

been fabricated by a Cu2O-templated strategy combined with spatially confined galvanic replacement [3]. Huang et al. have reported that photochemical reduction can

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ACCEPTED MANUSCRIPT improve the stability and dispersion of metal on the surface of TiO2 without external additives

[4]

. Liu et al. have also successfully achieved metal/metal oxide

nanoparticles using in-situ method without other reducing agents or surfactants in [5]

.

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which metal content in metal/metal oxide nanoparticles can be monitored

Core-shell Pt@porous SiO2 with high stability at high temperature have been prepared using in-situ growth method by Yang et al [6]. Wang et al. have reported that Ag (111)

[7]

. Taeghwan and coworkers have synthesized Pd/Fe3O4 and Rh/Fe3O4 with

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adjusted

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can grow on ZnO nanorod by in-situ method and the exposed Ag faces can be

metal acetylacetone compound as raw material in the oleic acid and oleylamine mixture solvent by thermal method

[1, 8]

. However, most of the reported synthesis of

hybrid nanomaterials involved complex steps.

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Herein, through using metal-oleate as precursors, we introduced a novel method for the fabrication of Cu/plate like ZnO nanocomposites, which are evaluated to be the most efficient catalysts for the direct hydrogenation of CO2. By simply heating the

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precursors at a high temperature, Cu nanoparticles can be generated by a

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decomposition/self-reduction mechanism, and oleate ions acted as dual roles: reducing and protecting agents. It is very interesting to find that uniform-shape hybrid nanostructures can be generated by our routes, and the size of Cu nanoparticles can be also successfully controlled.

2 Experimental section 2.1

Raw materials

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ACCEPTED MANUSCRIPT Cupric acetate (Cu(CH3COO)2·H2O, AR, Sinopharm Chemical Reagent Co., Ltd), Zinc acetate dehydrate (Zn(CH3COO)2·H2O, AR, Sinopharm Chemical Reagent Co., Ltd), Absolute alcohol (CH3CH2OH, Sinopharm Chemical Reagent Co., Ltd),

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n-Hexane (C6H14, AR, Sinopharm Chemical Reagent Co., Ltd), 1-octadecene (C18H36, 95+%, Aldrich). All chemicals in our work were used as received without further

2.2

The synthesis of metal-oleate complex

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purification.

method

[9]

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In our present report, metal-oleate complex was prepared using a phase transfer . In a typical experiment, 6 mmol of copper acetate monohydrate and 12

mmol of sodium oleate were dissolved in a solvent mixture composed of water, ethanol and n-hexane (volume ratio 3:4:7) by continuous stirring. The obtained

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mixture was then left at 70 oC for another 4 h under argon atmosphere to produce a two-phase system. Copper-oleate solution can be obtained by separating the upper blue solution and washing it with excess distilled water to remove the unreacted

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chemicals. Then, the solvent was evaporated under low pressure with heating to

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generate a blue waxy solid-copper oleate. By a similar way, zinc-oleate can be also prepared. 2.3

The synthesis of Cu/plate-like ZnO nanocomposites

Cu/plate-like ZnO nanocomposites can be produced using a simple heating method.

In a typical case, certain quantities of copper-oleate and zinc-oleate complexes were added into a three-necked bottle with 15 ml 1-octadecene to generate a clear solution. After replacing air atmosphere with an argon flow for 20 mins at room temperature,

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ACCEPTED MANUSCRIPT the reaction system was slowly heated to 290 oC for 1 h to generate a red turbid system. As-synthesized Cu/ZnO nanocomposties can be separated from the solution by centrifugation at 4000 rpm for 5 min, and the collected products were washed with

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acetone and cyclohexane for several times by filtering, and dried at 80 oC for 10 h in a vacuum oven finally. 2.4

HNMR spectra were recorded in chloroform-d (CDCl3) on a Varian Mercury

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Characterization

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VX-600M spectrometer with tetramethylsilane (TMS) as internal reference. FT-IR spectra were obtained by Nicolet 5700 Fourier transform infrared spectroscopy in the wavenumber range of 400-4000 cm-1 at room temperature. Samples were mixed with Potassium bromide (KBr) and pressed to be a pellet before

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analysis.

Powder X-ray diffraction (XRD) patterns were recorded by a Bruker D8 Advanced diffractometer using Cu Kα radiation for 2θ ranging from 20° to 80°.

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Transmission electron microscopy (TEM) images were taken with a Philips G2,

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transmission electron microscope operating at 200 kV. High resolution transmission electron microscope (HRTEM) images were obtained using a JEOL JEM-2010 field emission transmission electron microscope. The samples were prepared by drop casting the nanoparticle suspension onto a carbon-coated copper grid, and allowing the solvent, hexane, to evaporate. Thermogravimetric (TG) analysis was performed with TA instrument Q600 system. All the processes were carried out with a heating rate of 3.3 oC/min under nitrogen

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Catalytic activity test

Carbon dioxide hydrogenation to methanol reaction was performed in a fixed-bed

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micro-reverse meteorological chromatography coupled device (WFSM-3060, Tianjin Xianquan Industry and Trade Development Co., LTD). The reaction conditions used in activity test were 4.5 MPa, total flow rate of 40 stp·ml·min-1 and molar feed

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composition of CO2/H2 = 1/2.2. All post-reactor lines and valves were heated to 150 o

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C to prevent product condensation. The products were analyzed on-line by a gas

chromatograph equipped with a thermal conductivity detector, in which Porapak-Q column was used to separate reaction products.

3 Results and discussion

Structural characterization of metal-oleate complexes

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3.1

With CDCl3 as the solvent, the structure information of as-synthesized metal-oleate complexes is examined by 1HNMR spectroscopy. As labeled from a to d in Figure 1,

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the four peaks in the 1HNMR spectrum can be attributed to various organic moieties in copper oleate one by one. Not any impurity was detected in the 1HNMR

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characterization. The conclusion is further confirmed by FT-IR examination. As seen from Figure 2, it can be found that the typical absorption peaks of C=C

bond located at 3100 cm-1 and 1740 cm-1, corresponding to its in-plane stretching and the bending vibration, respectively. The existence of carboxylate ions can also be found from the peak of 1586 cm-1. Based on these results and the excellent solubility of solid products in non-polar solvent, the products can be determined as copper-oleate complex. The spectra of zinc oleate are the same as that of copper oleate, and can be found in Figure S1 and S2 of supporting information. 7

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Figure 1. 1HNMR spectra of copper oleate and its structural relationship with

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chemical shift (solvent is CDCl3).

Figure 2. FT-IR spectrum of copper oleate and the structure relationship with

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absorption peak.

Additionally, the decomposition behavior of as-synthesized metal-oleate complexes

is studied by TG analysis. As shown in Figure 3A, zinc oleate exhibits major loss beginning from 250 oC. According to the reports by Li et al.

[10]

, metal oleate salts

have revealed different stages of weight loss. In our cases, the onset peak found at 290 o

C in the TG curve can be attributed to the detachment of the first oleate ion and the

maximum weight loss peak located at 353 oC should correspond to the rapid

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ACCEPTED MANUSCRIPT decomposition of all the oleate ions. Finally, the zinc oleate can fully decompose to zinc oxide at around 490 oC according to 84.7% weight loss. On the basis of the formula weight of zinc oleate, if the organic moiety is completely lost and final product is only ZnO, the mass loss amounts to 87.0% in total. The 2.3% mass

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difference should be attributed to the formation of oleate capped oxide particles [11]. Similar to zinc oleate, copper oleate reveals three decomposition stages as shown in TG curve (Figure 3B). It is found that copper oleate can decompose at the relatively

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low temperature, and the three peaks are located at 246, 287, and 407 oC, respectively. It is interesting to find that the copper oleate can be totally conversed to copper

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according to the weight change of 87.2%, which is close to the theoretic total weight loss 87.3% from copper oleate to copper. From the TG results mentioned above, it is obvious that copper oleate can decompose at the lower temperature than zinc oleate, and the final composites for the two metal oleates are distinguished. To completely

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290 oC.

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decompose copper oleate and zinc oleate, the synthesis temperature should be set at

Figure 3. Thermogravimetric curves of zinc oleate (A) and copper oleate (B) from room temperature to 500 oC with a heating rate of 3.3 oC /min. 3.2

Microstructure analysis of Cu/plate-like ZnO nanocomposites

In a typical synthesis of Cu/ZnO nanocomposties, copper oleate and zinc oleate at a 9

ACCEPTED MANUSCRIPT molar ratio of 1:6 were added into octadecene, and then the system was heated to 290 o

C under argon atmosphere and kept for 1 h. The TEM images in Figures 4A and 4B

at various magnifications clearly demonstrate a uniform hexagonal plate-like structure

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of the product. Further XRD characterization reveals that the combination of typical diffraction peaks of ZnO and Cu (JCPDS Card No. 36-1451 and 04-0836), indicating that the as-prepared nanoparticles are Cu-ZnO nanocomposites. HRTEM examination

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in Figure 4C also indicates that there exist two different materials in our product.

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XRD results combined with HRTEM images suggest that the lattice spacing with 0.26 nm should be attributed to the (002) plane of ZnO [12-14] and the lattice spacing of 0.21 nm corresponds to (111) plane of Cu

[14, 15]

. Through the above analysis results, it is

clear that the plate-like structures are ZnO, while the Cu nanoparticles are located on

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its surface, revealing a dark color. Both Cu nanoparticles and ZnO nanoplates are uniform. The average lateral size of ZnO nanoplates can be determined as 42.97 nm, and that of Cu nanoparticles is 8.43 nm (Figures 4E and 4F). When the molar ratio of

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copper oleate and zinc oleate was changed from 1:6 to 1:12 by decreasing the copper

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oleate concentration, it is found from Figures 4 and 5 that the size of plate-like ZnO has almost no change, while the size of Cu particles increases from 8.43 nm to 10.93 nm. The unusual phenomenon of increasing size of nanoparticles with decreasing concentration of precursor is contrary with the traditional nucleation growth theory. It may be primarily caused by steric effect induced by the increasing of Cu nanoparticles on the surface of ZnO.

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Figure 4. (A-B) TEM images, (C) HRTEM, (D) XRD patterns and (E-F) size

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distribution of the Cu/ZnO nanocomposites prepared at the copper oleate/zinc oleate

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molar ratio of 1:6.

Figure 5. TEM images and size distribution of Cu/ZnO prepared at the copper oleate/zinc oleate molar ratio of 1:12. To further understand the role of oleate ions during the synthesis of Cu/ZnO nanocomposites, the morphologies of a series Cu/ZnO nanostructures which were prepared by adjusting the concentration of copper oleate but keeping the 11

ACCEPTED MANUSCRIPT concentration of zinc oleate unchanged were examined by TEM in Figure 6. When the ratio of copper oleate and zinc oleate increases from 1:6 to 1:3, large Cu nanoparticles together with small disc-like Cu/ZnO nanoparticles can be observed as shown in

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Figure 6A. By contrast, only disc-like Cu/ZnO nanoparticles with uniform size exist in the product for the molar ratio of copper oleate to zinc oleate ranging from 1:6 to 1:18 (Figure 6B to 6D). Possibly, at the higher molar ratio of 1:3, oleate ion existed in

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the system is insufficient to stabilize Cu nanoparticles, resulting in uncontrolled

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growth of Cu nanoparticles. Considering the higher decomposition temperature of zinc oleate, we believe that the oleate ions of zinc oleate act as the protection agents

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for the Cu nanoparticles at the early stage.

Figure 6. TEM images of Cu/ZnO prepared at various molar ratios of copper-oleate with zinc-oleate: A) 1: 3; B) 1: 6; C) 1: 12; D) 1: 18. 3.3 3.3.1

The formation mechanism of Cu/plate-like ZnO nanocomposites The formation mechanism of Cu nanoparticles 12

ACCEPTED MANUSCRIPT From the above mentioned, it is obvious that Cu nanoparticles can be generated in the absence of additional reducing agents, probably indicative of a self-reducing process as observed from TG analysis. To better understand the formation mechanism

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of Cu nanoparticles, the decomposition behavior of copper oleate solution in octadecene at high temperatures has been inspected as shown in Figure 7. As shown in Figure 7 (A) and (B), irregular nanoparticles were produced when copper oleate in

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octadecene were heated at 248 oC or 290 oC under argon atmosphere. It is interesting

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to find that the nanoparticles generated at 248 oC are wrapped with a thin shell (originated from oleate), while no protection shell exists for those produced at 290 oC. This phenomenon is probably ascribed to the reaction temperature difference, since oleate ion can be wholly decomposed at 290 oC. Further XRD examination reveals

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that the generated materials exhibit three peaks at 2θ=43.3o, 50.4o and 74.2o, corresponding to the diffraction of (111), (200) and (220) faces of Cu, respectively.

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nanoparticles.

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These results indicated that copper oleate can decompose into irregular Cu

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Figure 7. TEM images and XRD patterns for copper nanoparticles synthesized at two

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different temperatures: A, C) 248 oC, octadecene; B, D) 290 oC, octadecene. To further understand the self-reduction process of copper oleate, temperatureprogrammed desorption-mass spectrometry techniques (TPD-MS) are used to analyze

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gas components generated by the decomposition of copper oleate at different

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temperatures (see Figure S3). It can be found that carbon dioxide and carbon monoxide begin to be generated at about 150 oC, and the generation of these two kinds of gases becomes rapid at about 190 oC. When the temperature continues to rise to near 290 oC, hydrogen began to form. However, the main components are still CO and CO2 gases. The XRD results and the observation of a brick red color appearing at 250 oC, suggest that CO gas generated in copper oleate decomposition process can react with Cu(II) ion at high temperatures and induce the formation of Cu

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copper nanoparticles is proposed as in Scheme 1.

Scheme 1. The mechanism for decomposition/self-reduction of copper oleate. The study of plate-like Cu/ZnO self-assembly process

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3.3.2

The TG analysis results of the metal oleate salts suggest a great decomposition

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temperature difference existed between zinc oleate and copper oleate. Cu nanoparticles are generated from copper oleate at a relatively lower temperature. Thus, a Cu-nanoparticle-induced self-assembly process for the growth of Cu/plate-like ZnO nanocomposites can be proposed. To better investigate the self-assembly process,

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typical Cu/ZnO samples at three reaction stages were analyzed using TEM technology. As shown in Figure 8(A), irregular nanoparticles were produced at the first reaction

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stage, and a wrapping surfactant layer is clearly observed at the surface of Cu particles. These images suggest that zinc oleate acts as the surfactants to handicap the

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aggregation of Cu nanoparticles. At the second stage, ZnO nanoparticles with uniform size were generated and Cu nanoparticles were found around the ZnO nanoparticles (shown in Figure 8(B)). At this stage, the surfactant shell appears to be thinner and not easily observed, providing a further proof for the decomposition of zinc oleate. At the final stage, Cu/hexagonal plate-like ZnO nanomaterials were produced just as shown in Figure 8(C). In the present work, the discussions are mostly based on the TEM images, and more experiment should be carried out to fully uncover the mechanism. 15

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Further study is on the way and will be reported elsewhere.

Figure 8. TEM images of Cu/ZnO prepared in different reaction stages with the molar

The mechanism for Cu/ZnO nanoparticles

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3.3.3

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ratio of copper oleate to zinc oleate is 1:6 in octadecene.

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Scheme 2. Formation mechanism of Cu/plate-like ZnO nanostructures.

Combined with the self-reduction mechanism of copper oleate and TEM characterization of the reaction process, the formation mechanism of Cu/plate-like ZnO nanoparticles is proposed as shown in Scheme 2. Based on TG analysis, copper oleate decomposes at low temperature firstly, and then is self-reduced into irregular Cu nanoparticles. Meanwhile, zinc oleate remains unchanged and the oleate ion acts

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particles further self-assemble into hexagonal disc-like structures with Cu nanoparticles on their surfaces. In this growth process, oleate ions play two important roles: 1) reducing Cu ion and 2) wrapping Cu nanoparticles to make them stable. The catalytic performance of Cu/plate-like ZnO nanocomposites

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3.4

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Moreover, the catalytic activities of these catalysts for carbon dioxide hydrogenation to methanol are investigated by mixing Cu/ZnO and Al2O3 with 2:1 mass ratio and presented in Table 1. It can be clearly found that Cu/plate-like ZnO catalyst exhibits better activity for CO2 hydrogenation with a 73.1% selectivity of [16, 17]

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methanol comparing with the catalytic performance of Cu/rod-like ZnO According to the previous study

[17]

, the morphology of ZnO had a significant effect

on its interaction with Cu in the hydrogenation of CO2 to methanol. The exposed

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polar (002) face in our plate-like ZnO showed a much stronger interaction with Cu

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and more amounts of surface oxygen vacancies existed at the materials’ interface. Thus CO2 was likely to be activated by the generated vacancies and the Cu phase at the interface assisted molecular rearrangement (formate) and hydrogenation to obtain methanol.

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ACCEPTED MANUSCRIPT Table 1. Catalytic performance of Cu/plate-like ZnO nanocomposites mixed with

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alumina in the synthesis of methanol from hydrogenation of CO2.

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4 Conclusions

In the present work, we have developed a novel synthesis of regular Cu/plate-like

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ZnO nanoparticles by simply heating the mixture of copper oleate and zinc oleate in octadecene. The method can be also used to tailor the particle size of Cu nanoparticles. It is very interesting that copper oleate can decompose and be self-reduced to Cu,

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which can induce the self-assembly of ZnO and further formation of Cu/plate-like ZnO nanoparticles. Our reports have also presented a possibility to use oleate ion as reductant and protecting agent for the synthesis of metal/metal oxide hybrid

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nanostructures. Furthermore, the Cu/plate-like ZnO nanostructures show stronger

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interactions between ZnO and Cu, and more oxygen vacancies due to the exposed polar (002) face in plate-like ZnO, which exhibits the higher activity for CO2 hydrogenation. It is believed that the reports should be advisable for the materials preparation, and will contribute to their application in the fields of catalysis, sensors, optics and magnetics.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (No.51402126) and Natural Science Foundation of Zhejiang Province (No. 18

ACCEPTED MANUSCRIPT LQ13B010003).

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Supplementary Information

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shift (solvent is CDCl3).

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Figure S1. 1HNMR spectra of zinc oleate and its structural relationship with chemical

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Figure S2. FT-IR spectrum of zinc oleate and the structure relationship with

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adsorption peak.

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Figure S3. TPD-MS signals for gas mixture generated by copper oleate from room

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temperature to 400 oC.

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Highlights

1) Cu/plate-like ZnO nanocomposites have been fabricated by a simple heating method.

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2) Copper oleate can be decomposed and self-reduced to Cu. 3) A self-decomposition-induced assembly mechanism has been proposed.

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4) The oleate ions act as multiple roles in the process: reducing and protecting agents.